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This article was downloaded by: [UNAM Ciudad Universitaria]On: 24 February 2012, At: 11:19Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK
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Nickel Molybdate Catalysts and Their Use in theSelective Oxidation of HydrocarbonsL. M. Madeira a , M. F. Portela b & C. Mazzocchia ca LEPAE, Departamento de Engenharia Química, Faculdade de Engenharia, Universidadedo Porto, Porto, Portugalb GRECAT (UQUIMAF, ICEMS, Lisboa), Departamento de Engenharia Química, InstitutoSuperior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049‐001, Lisboa,Portugalc Dipartimento di Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, Milano,Italy
Available online: 17 Oct 2008
To cite this article: L. M. Madeira, M. F. Portela & C. Mazzocchia (2004): Nickel Molybdate Catalysts and Their Use in theSelective Oxidation of Hydrocarbons, Catalysis Reviews: Science and Engineering, 46:1, 53-110
To link to this article: http://dx.doi.org/10.1081/CR-120030053
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Nickel Molybdate Catalysts and Their Use in theSelective Oxidation of Hydrocarbons
L. M. Madeira,1 M. F. Portela,2,* and C. Mazzocchia3
1LEPAE, Departamento de Engenharia Quımica, Faculdade de Engenharia,
Universidade do Porto, Porto, Portugal2GRECAT (UQUIMAF, ICEMS, Lisboa), Departamento de Engenharia Quımica,
Instituto Superior Tecnico, Universidade Tecnica de Lisboa, Lisboa, Portugal3Dipartimento di Chimica, Materiali e Ingegneria Chimica,
Politecnico di Milano, Milano, Italy
CONTENTS
ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
2. Preparation of Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
2.1. Coprecipitation Techniques . . . . . . . . . . . . . . . . . . . . . . 55
2.2. Other Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
2.2.1. Molybdenum-Enriched Catalysts . . . . . . . . . . . . . . 60
2.2.2. Nickel-Enriched Catalysts . . . . . . . . . . . . . . . . . . 61
2.3. Supported and Doped Catalysts . . . . . . . . . . . . . . . . . . . . 61
3. Thermal Activation—Transition of Phases . . . . . . . . . . . . . . . . . 63
4. Characterization of Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . 66
53
DOI: 10.1081/CR-120030053 0161-4940 (Print); 1520-5703 (Online)
Copyright # 2004 by Marcel Dekker, Inc. www.dekker.com
*Correspondence: M. F. Portela, GRECAT (UQUIMAF, ICEMS, Lisboa), Departamento de
Engenharia Quımica, Instituto Superior Tecnico, Universidade Tecnica de Lisboa, Av. Rovisco Pais,
1049-001, Lisboa, Portugal; Fax: þ351-21-8477695; E-mail: [email protected].
CATALYSIS REVIEWS
Vol. 46, No. 1, pp. 53–110, 2004
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4.1. Composition of Phases for Catalysts with Different Ni :
Mo Ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.2. Other Physicochemical Characterizations . . . . . . . . . . . . . . 67
4.2.1. Stoichiometric Nickel Molybdate . . . . . . . . . . . . . . 67
4.2.2. Catalysts with Excess Molybdenum or Nickel . . . . . . . 71
4.2.3. Catalysts Prepared Using Organic Precursors and
Sol–Gel Methods . . . . . . . . . . . . . . . . . . . . . . . 75
4.2.4. Doped and Supported Nickel Molybdates . . . . . . . . . 76
4.3. Characterization of the High Temperature b-Phase . . . . . . . . . 79
5. Applications of Ni–Mo–O Catalysts . . . . . . . . . . . . . . . . . . . . 80
5.1. Oxidation of Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . 81
5.2. Oxidative Dehydrogenation of Light Alkanes . . . . . . . . . . . . 85
5.2.1. Undoped Ni–Mo Catalysts . . . . . . . . . . . . . . . . . . 85
5.2.2. Doped and Supported Catalysts . . . . . . . . . . . . . . . 89
5.2.3. Kinetics and Mechanism . . . . . . . . . . . . . . . . . . . 93
5.3. Nature of Active Sites . . . . . . . . . . . . . . . . . . . . . . . . . 97
6. Conclusions and Future Trends . . . . . . . . . . . . . . . . . . . . . . . 98
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
ABSTRACT
This paper reviews the preparation techniques, characterization, and use of nickel
molybdate catalysts in the selective oxidation of hydrocarbons, particularly of light
alkanes. Catalysts with different Ni :Mo ratios, unsupported and supported, undoped
and doped, were considered. Particular attention is given to the thermal activation
process for the transition of the low temperature a-phase into the metastable b-phase,
which was shown to be more selective in some cases. Special reference is also made to
the results of kinetic studies performed, to the mechanisms proposed for some
important reactions, and to the nature of the active sites. Finally, after some general
conclusions, future trends are analyzed.
Key Words: Nickel molybdate; Preparation; Characterization; Selective oxidation;
Hydrocarbons; Oxidative dehydrogenation; Light alkanes.
1. INTRODUCTION
Olefins, aromatics, and many oxygenates are widely used as important raw materials
in industrial processes,[1,2] and thus the strong pressure of international markets has led
to constant optimization of production processes. Cost reduction can be achieved by
using cheaper raw materials (for instance alkanes), combined in some cases with the use
of more sophisticated catalysts. Indeed, in the last years a clear trend has been obser-
ved for the use of light alkanes for the direct production of oxygenates—through
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partial oxidation[3–5]—or to manufacture olefins through dehydrogenation or oxidative
dehydrogenation (ODH) processes,[5–7] due to the ready availability and low price of
natural gas. However, this is a challenging problem for the chemical industry because
alkanes are less reactive than the products obtained, such as alkenes, dienes, or aldehydes
and acids, which are easily totally oxidized at the high temperatures required to activate
alkanes properly.
Therefore, around the world much effort has been put into developing new catalytic
systems providing selective oxidation of hydrocarbons, particularly light alkanes with
useful yields. However, the search for better and more effective catalyst compositions,
preparations, and processes continues and, up to now, few promising catalysts were found
for these applications. For instance, metal molybdates were successfully employed in
selective oxidation reactions and are quite versatile catalysts for important industrial
processes.[5] Among them, nickel molybdates show very interesting potential for oxidation
reactions, and particularly for ODH of light alkanes. A large number of papers and
patents is found in the literature regarding these applications (mentioned throughout this
text). But nickel (Ni)–molybdenum (Mo) catalysts are also very important for other
processes, such as the hydrodesulfurization and hydrodenitrogenation of petroleum
distillates;[8–17] the water–gas shift reaction;[18] the steam reforming, hydrogenolysis, and
cracking of n-butane;[19] the oxidative coupling of methane;[20] and other industrially
important hydrogenation and hydrotreating reactions.[8,21–24] Despite this large number of
important industrial applications, a review that systematically analyzes the preparation
techniques used, the more important characterization results, and the main catalytic
studies performed for oxidation reactions with Ni–Mo–O catalysts, is not found in the
open scientific literature. With this review we intend to fill this gap. We should remark
that we will only consider reaction investigations involving selective oxidation of
hydrocarbons.
2. PREPARATION OF CATALYSTS
2.1. Coprecipitation Techniques
During the preparation, through the coprecipitation method, of nickel–molybdenum
catalysts with different Ni :Mo ratios, Andrushkevich et al.[25] found in 1973 that both the
chemical composition and composition of phases of the obtained precipitate depend
strongly on the precipitation conditions (concentration of reactant ions in solution,
temperature, and duration of the aging process). The unsatisfactory aspects of the
coprecipitation method, involving direct mixture of the solutions, and particularly the lack
of reproducibility in the results, were eliminated by using an experimental setup that
allowed continuous preparation of the catalysts by precipitation.[25] The nickel nitrate and
ammonium paramolybdate solutions were mixed at constant flow rate at 868C. The NH4þ
ion concentration in the molybdenum-solution was equal to the NO32 ion concentration in
the nickel solution. The Ni :Mo ratio in the solution was varied by changing the respective
ratio in the original solutions. When steady-state conditions for precipitation were
established, the pH in the reaction volume was 5.4. The obtained precipitates were air
dried at room temperature and calcined at 5008C.[26]
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Andrushkevich et al. also knew that, for preparation of nickel molybdates, the pH of
the medium during precipitation has a significant influence on the composition of the
precipitates. Thus, they decided to investigate the problem thoroughly.[27] They found that
by increasing the ammonia concentration in the paramolybdate, while the other
precipitation conditions were kept constant, an increase in the nickel concentra-
tion in the final precipitate was recorded due to solubilization of molybdenum with
ammonia.
After the 1980s, several works were published in which the NiO–MoO3 system was
studied because of its use as a hydrodesulfurization catalyst. However, the preparation
methods adopted varied slightly from one group to another:
Vagin et al.[28] prepared NiO–MoO3 samples with various compositions by
coprecipitation of analytical salts [Ni(NO3)2 and (NH4)6Mo7O24], from the
corresponding solutions at 908C and pH ¼ 6.0–6.5. The solutions containing the
precipitates were then evaporated in a water bath, dried at 1108C, and calcined at
6008C for 6 hr.
Brito et al.[29] also prepared a series of Ni–Mo mixed oxides by coprecipitation
(either in continuous or discontinuous mode), always controlling the precipitation
conditions in order to change the Ni :Mo ratio of the final product, namely by the
pH of the medium.
The hydrated precursor of the hydrodesulfurization catalysts[11] was synthesized by
coprecipitation of nickel nitrate (pH ¼ 4.7) and ammonium heptamolybdate
(pH ¼ 5.6) aqueous solutions. The methodology used to obtain the phase that is
stable at high temperatures (b-NiMoO4) will be described later (see Section 3).
The investigations carried out at the Polytechnic of Milan, Italy, have helped, among other
aspects, to clarify the experimental conditions that determine the formation of oxides with
different compositions in the NiO–MoO3 system. In two preliminary studies,[30,31] special
attention is given to the methodology that enables the precursor of the catalytically active
phase to be obtained. The solvated precursor was prepared by mixing, with stirring,
equimolar solutions of ammonium molybdate and nickel nitrate [Ni(NO3)2 . 6H2O] at pH
5.6 and at a temperature of 858C. The molybdenum solution was prepared by dissolution
of molybdic acid (H2MoO4. H2O) in an ammoniacal solution at 858C and pH of 6.2. The
precursor obtained, partially crystalline and with a pale yellow color, was dried at 1208Cand thermally activated at 5508C for 2 hr. It is noteworthy that the NiMoO4 prepared by
coprecipitation was also patented;[32] however, nickel molybdate was also synthesized by
the dry mode, from NiO and MoO3.[30,31]
As shown by Mazzocchia et al.,[30–34] the precipitation process can lead to different
precursors with the general formula:
xNiO yMoO3 nH2O mNH3 (1)
Small changes in experimental conditions such as pH, precipitation temperature,
H2MoO4/NH4OH ratio, filtration temperature, duration of aging of the precipitate in the
mother liquor, and duration and temperature of drying may lead to precursors with
different x, y, n, and m values. The effect of some of these parameters is shown in Fig. 1.
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Attention should be paid to the fact that each arrow refers to the effect of a given parameter
on the type of precursor obtained.
For formation of the precursors, the following equilibria are established between the
different species:[33]
7MoO2�4(aq:) þ 8Hþ(aq:) !Mo7O
6�24(aq:) þ 4H2O (2)
Ni2þ(aq:) þMoO2�4(aq:) !NiMoO4(s) (3)
Ni2þ(aq:) þMo7O6�24(aq:) !NiMo6O
10�24(aq:) þ (Mo6þ) (4)
Ni2þ(aq:) þ 2OH�(aq:) !Ni(OH)2(s) (5)
The conditions required for preparation of pure a-NiMoO4 are highly critical.[31] In order
to avoid polymerization of the molybdate ions it is not sufficient to control the
environmental conditions (Fig. 1). The rate at which the nickel solution is added is also a
determining factor, probably because the rate of Eq. (4) is a critical condition.
For formation of the precursor with Formula (1), the main reactions involved are the
following:
1
6,
x
y, 1)Eqs. (3)þ (4); x ¼ y)Eq. (3);
1 ,x
y, 1)Eqs. (3)þ (5)
Figure 1. Effect of the precipitation parameters on the type of precursor obtained. Each arrow
refers to the effect of a given parameter. (Information adapted from Refs.[30,33].)
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 57
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Thus, at pH 6 the G precursor is obtained (see Fig. 1), which becomes more green as pH is
increased due to coprecipitation of nickel hydroxide [cf. Eq. (5)], yielding an x/y ratio
higher than 1. In fact, in the patented preparation method, in which the temperature was
maintained at 858C, but with a pH of 6, the final catalyst had the following composition:
Ni1.5MoO4.5.[32] On the other hand, if room temperature is used, without changing the
other experimental conditions, a pale blue (B) precursor is obtained for which y/x ¼ 6
(cf. Fig. 1). After thermal activation, the sample exhibits the infrared spectrum and
x-ray diffraction (XRD) pattern characteristic of excess MoO3. The yellow precursor
(S—of stoichiometric), where x ¼ y ¼ 1, is obtained in the conditions already mentioned.
If, during filtration, the solution is allowed to cool (between 658C and 858C), a pale yellowprecursor (E) is obtained with a ratio y/x . 1. For this precursor in particular, both the
time of aging in the mother liquor and the temperature of the solution determine the y/xratio. Using, for instance, a pH of 5.6 but mixing the solutions at 708C, a precursor is
obtained, which after calcination (during 2 hr at 5508C) yields a catalyst with the formula
NiMo1.5O5.5.[32]
The stoichiometric catalyst has been used in several studies, but the preparation
procedure employed was not always exactly the same. For instance, 0.5M solutions of
Ni(NO3)2 . 6H2O and H2MoO4 were employed, with a final pH of 5.1 and at 858C.[35] Inother cases 0.25M solutions were used, with precipitation at pH 5.2 and a temperature of
908C,[36] with filtration of the precipitate at 858C and drying for 4 hr at 1108C, followed bycalcination for 2 hr at 5508C.[37]
The parameters m and n in the precursor with Formula (1) are affected by slight
variations in the temperature or in the amount of ammonia. For instance, in a study where
the yellow precursor (S) was prepared by using 0.25M solutions of molybdic acid and
nickel nitrate, at 858C and pH 5.4, thermogravimetric analyses revealed the following
composition for the precursor: NiMoO4. 3/4 H2O . 3/4 NH3.
[33] A green precursor (G)
was also prepared with the following composition: Ni1þdxMoO4. 1/3 H2O . 5/3 NH3,
from a 0.25M solution of ammonium heptamolybdate [(NH4)6Mo7O24. 4H2O] at 858C,
the pH of the solution being adjusted with ammonia in order to yield an NH3/Mo ratio of
1.5. The 0.25M solution of nickel nitrate, at the same temperature, was added at a
controlled rate (7mL/min). The green precipitate was immediately formed and the pH
dropped from its initial value of 8.48 to 7.09 in 1 hr.
2.2. Other Techniques
New techniques have been developed for preparation of catalysts that enable
clarification of specific aspects in multicomponent catalytic systems. Better control of
the contact between phases is achieved when compared with catalysts prepared by
precipitation or impregnation. In these situations, it is also difficult to control the thickness
and structure of the superficial layer. With these goals in mind, Zou and Schrader[38] used
reactive sputtering, an advanced technique for materials processing, in order to produce
catalysts of the NiMoO4–MoO3 system with controlled compositions and structures,
especially thin films with well-defined architectures. Another advantage of the samples
prepared in this way, compared to materials obtained by precipitation, is that they are more
easily characterized by several techniques. They prepared very thin films of MoO3, of
NiMoO4, and of combinations of both oxides over different supports (including SiO2),
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which were later characterized by various techniques. In the samples containing both
components, the films were prepared through a sequential process, the NiMoO4 being
deposited over the predeposited MoO3. More recently, they have examined in detail the
deposition parameters for the reactive sputtering technique and found that the multilayer
films of NiMoO4 on a-MoO3 include an interfacial material identified as b-NiMoO4,
which was detected at relatively low temperatures in the bilayer structures.[39]
The use of organic salts for preparation of active catalytic systems, such as
Ni–Mo–O, has the advantage of providing a lower crystallization temperature. Using an
oxalic precursor (a product that decomposes at a lower temperature than ammonia) and
different thermal treatments, Mazzocchia et al. have obtained several catalysts with
different compositions.[34] The NiC2O4. 2H2O and MoOC2O4
. 4H2O mixture was
prepared by adding ammonium heptamolybdate to 250mL of a solution containing a large
excess of oxalic acid. After dissolution, nickel nitrate is added at room temperature such
that the Ni :Mo ratio is 1. The solution (0.14M in Ni and in Mo) is then slowly warmed
under vacuum to 408C. Precipitation starts immediately and increases as the water
evaporates. The precursor is finally dried at 1208C for 15 hr.
Another method of catalyst preparation that has been recently used resorts to natural
substances or polymers. A polymeric network is created, containing the ionic compounds
of the active catalyst inside the organic matrix. In this context, Anouchinsky et al. tested
a new methodology for NiMoO4 preparation in which the precursor is an organic gel (agar-
agar) containing the Ni and Mo ions in a 1 : 1 atomic ratio.[40] This approach offers several
advantages: it is cheap and simple and multicomponent catalytic systems can be prepared
by simple dissolution of the desired elements, at the appropriate concentrations, in the
aqueous solution. In this way one may change, for instance, the Ni :Mo ratio with the
simultaneous presence of promoters. The gel was prepared from 0.5M solutions of nickel
nitrate and ammonium heptamolybdate and mixed at room temperature with continuous
stirring. A load of 1% (by weight) of powdered agar-agar is added and the solution warmed
at 808C to solubilize the agar-agar. Rapid cooling of the solution yields the gel, which is
subsequently dried by slow heating (108C/hr) from room temperature up to 1208C. Thistemperature is then maintained for 4 hr and finally the gel is calcined.
The agents that control pH in the synthesis of precursors of mixed oxides must be
easily removed from the precipitate. From this point of view, oxalic precursors have been
shown to be advantageous since they crystallize at low temperatures.[34] It would then be
expected that, for the Ni–Mo–O system, the use of the sol–gel technique could be
beneficial.
The sol–gel technique offers a low-temperature method for synthesizing materials
that are either totally inorganic in nature or both inorganic and organic. The process
offers many advantages, including the use of simple and inexpensive equipment,
excellent control of the stoichiometry of precursor solutions, and ease of compositional
modifications.
Good control of the stoichiometry may be very useful for fine control in the
preparation of Ni–Mo–O catalysts, as their composition is crucial for catalytic app-
lications. With this goal in mind, as well as the fact that precursor decomposition can be
achieved at low temperatures, several authors decided to apply the sol–gel route for
preparation of nickel molybdate catalysts. Although papers on this subject are somewhat
scarce, we should mention the work of Anouchinsky et al., who have prepared several
catalysts by the sol–gel method.[40] As expected, homogeneous dispersion of the Ni and
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 59
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Mo ions in the precursor was achieved, which led to formation of the NiMoO4 phases,
whose crystallization occurs at temperatures lower than those prepared by coprecipitation.
This process also leads to the stabilization of the b-phase at room temperature. Lezla
et al.[41] have also adopted the sol–gel methodology to prepare the stoichiometric catalyst
using citric acid (1mol/Ni), which was added to a solution of nickel nitrate (0.4M). Then
a solution of (NH4)6Mo7O24. 4H2O was added very slowly so as to avoid precipitation.
The solution was evaporated until a gel and then a solid were obtained. Finally, the solid
was ground and heated in air at 5008C for 24 hr.[41]
The sol–gel technique also was used recently for the preparation of supported catalysts,
with particular advantages in the case of the Ni–Mo–O system (see Section 2.3).
Nanocrystalline NiMoO4, among other molybdates, was also recently prepared from
the complete evaporation of a polymer-based metal-complex precursor solution.[42,43]
These fine-grained materials (with particle diameters less than 100 nm) are expected to
have potential applications in many technological areas.
2.2.1. Molybdenum-Enriched Catalysts
Catalysts containing excess MoO3 are often prepared by drying the final solution,
after mixing the reactants in appropriate ratios. However, this method gives rise to
numerous problems regarding the nature of the dried precursor, because the concentration
of the solution changes during the drying process, leading to precipitation of hetero-
polymolybdates or to a mixture of molybdate and molybdic acid.[44] Thus, Mazzocchia et
al. decided to prepare several catalysts with excess Mo, all with the same composition but
obtained from different precursors. They used decomposition of the heteropolymolybdate
or dry-mixing of nickel molybdate and excess of molybdenum trioxide. A catalyst studied,
derived from the heteropolymolybdate, was NiMoO4. 5MoO3, obtained through thermal
decomposition of (NH4)4H6NiMo6O24. 5H2O.
[44]
In other works a certain excess of MoO3 was introduced by cooling the solution of
the a-NiMoO4 precursor, with consequent coprecipitation of (NH4)4H6NiMo6O24. nH2O
(cf. Fig. 1).[31] Both the temperature and the cooling period depend on the excess of
molybdenum desired.
In a very interesting work published by Ozkan and Schrader, the synthesis of nickel
molybdates containing an excess of molybdenum through several methods is described in
detail.[45] The excess of Mo (relative to the stoichiometric) is present as a new phase,
MoO3, substantially increasing the complexity of the system. The authors report the fol-
lowing methods for incorporation of MoO3 into the catalyst: precipitation, solid state
reaction, and impregnation. Nickel molybdates prepared through precipitation were
obtained from aqueous solutions of ammonium heptamolybdate [(NH4)6Mo7O24. 7H2O]
and nickel nitrate [Ni(NO3)2 . 6H2O], the pH being changed with ammonium hydroxide or
nitric acid solutions. In order to obtain pure nickel molybdate, pH during addition and
reaction was kept at 6 (with a temperature of 638C). The catalysts with excess MoO3 are
obtained by acidification of the medium during addition of the solutions, the pH depending
on the excess of MoO3 desired. It should be stressed that with this procedure the catalyst
composition is insensitive to both concentration and composition of the reactants, the pH
of the medium during the precipitation being the key factor. The solid-state synthesis
basically consists of heating NiO together with MoO3, or MoO3 mixed with NiMoO4.
Nickel molybdate, obtained by precipitation, was also impregnated with ammonium
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heptamolybdate to provide catalysts with an excess of MoO3 between 2% and 55%.
Molybdenum trioxide was also impregnated with NiMoO4. The experimental procedure
used for these syntheses has been described in detail.[45]
More recently, Lezla et al. have also prepared Ni–Mo–O catalysts with Mo :Ni ratios
between 0.90 and 2.15 using several methods, which include precipitation, evaporation to
dryness, sol–gel, impregnation, and mechanical mixing. They analyzed the influence of
the preparation method on the catalytic performances for propane ODH.[41]
2.2.2. Nickel-Enriched Catalysts
Catalysts with an excess of Ni have usually been prepared in two ways: (i)
precipitation at 858C and pH 6.2, using 0.25M solutions of H2MoO4 and Ni(NO3)2, which
yields an Ni :Mo ratio of 1.40 and provides formation of NiO together with the a- and
b-phases of NiMoO4; and (ii) mechanical mixing of NiMoO4. H2O and Ni(OH)2, which
provides Ni :Mo ratios in the range 1.1–1.6, followed by activation at various
temperatures.[46] Chemical impregnation of a-NiMoO4, using an aqueous solution of
nickel acetate, was also adopted for preparation of Ni-enriched catalysts.[47] Using moly-
bdenum oxalate instead provided catalysts with excess molybdenum.[47]
2.3. Supported and Doped Catalysts
The use of supported nickel molybdate catalysts in ODH reactions is not very
common. Some exceptions are recent works in which TiO2 (anatase)[48] and SiO2
[49–52]
were used to support the active phase. In the first case, the catalysts were prepared using
two procedures: (i) wet impregnation of the support with an aqueous suspension of
NiMoO4, and (ii) direct precipitation of NiMoO4 on the support surface at 858C, usingsolutions of nickel nitrate and ammonium heptamolybdate.[48] The SiO2-supported
catalysts were also prepared by wet impregnation[52] or by direct precipitation of NiMoO4
on the support,[49,52] or even by sol–gel routes.[50,51]
Nickel–molybdenum catalysts are frequently used as supported catalysts in important
industrial processes like hydrodesulfurization or hydrogenation, so many works exist in
the literature regarding these issues. Conventional methods of preparation of hydro-
treatment catalysts usually consist of depositing transition-metal salts onto the support,
usually g-Al2O3, followed by calcination to produce stable oxidic materials that must be
sulfided either prior to or during the start-up of the hydrotreatment process. In a pioneering
work, Laine et al.[12] found that it is advantageous to impregnate alumina with nickel
before molybdenum. Later, Brito and Laine[53] prepared nickel–molybdenum catalysts
supported over g-Al2O3 through impregnation of commercial g-Al2O3. First Mo was
added—using an ammonium heptamolybdate solution—followed by drying (overnight at
1208C) and calcination (2 hr at 4008C). Portions of this solid were then submitted to dry
impregnation with nickel nitrate solutions with the purpose of obtaining solids with
several NiO loads. After drying, the samples were calcined at different temperatures
between 4008C and 8008C.It is well known that for the preparation of silica-supported catalysts, the sol–gel route
allows very good control of the composition, homogeneity, and textural properties of
the final products. In fact, the nanoscale chemistry involved in sol–gel methods appears to
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be the most straightforward way to prepare tailored nanocomposites, including organic–
inorganic hybrid materials. Moreover, sol–gel methods have been found to be effective for
dispersing small metal oxide particles in nonmetallic matrices. With these features in mind,
Cauzzi et al.[51] prepared NiMoO4/SiO2 composites by the sol–gel process via silicon
alkoxides, involving Si(OMe)4 (tetramethoxysilane), Ni(NO3)2, and (NH4)6Mo7O24 as
starting materials. The dried gels were treated at increasing temperatures until crystalline
grains of nickel molybdate highly dispersed in the amorphous silica matrix were formed
(6758C). Xerogels with different Ni :Mo ratios were synthesized and the preparation
procedures are described in detail.[51] It is noteworthy that besides leading to the support of
catalytic materials, the xerogel plays the important role of stabilizing b-nickel molybdate,
which otherwise would turn into the a-phase at room temperature.
Many other interesting investigations dealing with the preparation of Ni–Mo
supported catalyst could be mentioned. However, they are directed for use in processes
other than oxidation of hydrocarbons, which is outside the scope of the present review. It is
nonetheless important to stress that different materials have been used as supports for Ni–
Mo catalysts, namely alumina,[17,54,55] magnesia-alumina mixed oxides,[13] zeolites,[21,24]
titania-alumina mixed oxides,[14,56] activated-carbon,[15,16,57] or zirconia.[58]
In order to improve the catalytic performance of the Ni–Mo–O system in the
selective ODH of alkanes, particularly n-butane, nickel molybdate was doped with several
alkali (lithium, sodium, potassium, or cesium)[59] or alkaline-earth (calcium, strontium,
and barium)[60] promoters. The samples were prepared through wet impregnation of the
pure a-NiMoO4 material, using different loads of the respective nitrate solutions, followed
by filtration, drying, crushing, and calcination in dry air for 2 hr at 5508C. For propaneODH, promoters such as K, Ca, and P were frequently used, the catalysts being prepared
through an incipient wetness impregnation technique starting from an a-NiMoO4 calcined
pure catalyst.[61–63] Still for application in oxydehydrogenation processes, we should note
the preparation of catalyst compositions that contain other elements like phosphorus,
antimony, bismuth, or arsenic, and that are effective in converting paraffins or monoolefins
to a higher degree of unsaturation.[64] Methods described therein include coprecipitation,
impregnation, dry mixing, and similar methods the final catalyst compositions being
unsupported or supported.
While with the conventional impregnation method the doping element only stays on
the catalyst surface, the sol–gel route simultaneously produces surface and structural
modifications. In addition, better dispersion of the active species on the support can
frequently be achieved, as well as appropriate compositional homogeneity. This led Soares
et al.[65] to prepare, by the citric acid method, mixed Ni–Mg molybdates, which were
calcined under air flow at 5508C for 8 hr. These catalysts were tested for n-butane ODH
and exhibited a noteworthy catalytic performance.[65]
Dopants such as tellurium (Te) and phosphorus (P) were also added to Ni–Mo
catalysts, particularly for application in the direct oxidation of propane to acrolein and
acrylic acids. Reported techniques for preparation of the Te-doped catalysts include the
mechanical mixing of Te2MoO7 with NiMoO4–MoO3, the mixing of telluric acid with
NiMoO4–MoO3,[66,67] and the impregnation of nickel molybdate with ammonium
telluromolybdate.[68] For the P-doped system, the incipient wetness technique was used,
with (NH4)2HPO4.[66,67]
The preparation of doped nickel–molybdenum catalysts can also be found in US
Patent No. 3,968,054, by Cherry et al.,[69] who described an improved coprecipitation
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method for the preparation of antimony-doped Ni–Mo catalysts, useful for oxidation of
n-butane to maleic anhydride. Finally, Ferlazzo et al.[70] claimed a process for preparation
of a complex molybdenum-based catalytic system, which is comprised by one or two
crystalline phases (including beta nickel molybdate) and at least one modifying agent
(promoter element), useful, for instance, for the selective conversion of unsaturated
hydrocarbons into unsaturated aldehydes or diolefins.
3. THERMAL ACTIVATION—TRANSITION OF PHASES
The structure of some molybdates changes with temperature, while for others it
remains unchanged. For instance an irreversible structural conversion in Bi2MoO6 was
observed at temperatures higher than 5508C. Transformation is complete at 6408C.[71]
A reversible conversion in CoMoO4 occurs at 5008C while for NiMoO4 a temperature of
about 6908C is needed.[72,73]
In fact, as long ago as in 1973 Plyasova et al.[26] identified two polymorphic forms in
the nickel molybdate. One of them has a monoclinic crystalline network with the
molybdenum with number of coordination six and is stable at room temperature (then
named the P-phase). When heated to ca. 6508C, this form was converted into another
(then named the N-phase)—isomorphic with a-MgMoO4 and a-MnMoO4—in which the
molybdenum has number of coordination four, but which is not stable at room
temperature. After cooling a transition into P-phase was observed. The thermograms of
samples with Ni :Mo ratios close to 1 that were previously calcined at 5508C, showed an
endothermic effect at ca. 6508C when the solid was heated and an exothermic one at 508Cwhen the solid was cooled.[26] The former was attributed to conversion of the low
temperature into the high temperature phase (P! N), and the latter to the reverse
transformation, i.e., N! P.
The transformations of phases that occur when the precipitates are heated were
studied for the first time by Andrushkevich et al.[27] Differential thermal analysis showed
an endothermal effect due to the removal of crystallization water at about 1808C and
another at 4208C due to the decomposition of the hydrated molybdate. These results are in
very good agreement with the data shown in Fig. 2, obtained recently by Zavoianu et al.[48]
We can see that the thermal analysis performed over the precursor of NiMoO4 shows a loss
of weight below 473K, which corresponds to the desorption of water and ammonia. The
strong exothermic processes occurring at 723–773K are attributed to the decomposition
of NH4(NiMoO4)2OH .H2O and ammonium nitrate present in the structure.[48]
According to Andrushkevich et al.,[27] in samples with excess Mo the peak that
appears at 7808C coincides with the melting point of the molybdenum oxide present in the
catalyst. In their studies they still concluded that the crystallization temperature of the
fresh precipitate (4308C for Ni :Mo ratio ¼ 0.7) increases with the nickel content,
probably because the nonstoichiometric molybdate or the formed solid solution crystallize
at higher temperatures than the stoichiometric molybdate. Indeed, the infrared spectrum of
a sample with an Ni :Mo ratio ¼ 2.3 heated at 6508C shows the characteristic bands of the
N-phase (now named b-phase), showing that crystallization occurs at 6258C and is
accompanied by the exothermal effect observed in the thermal analyses. It should also be
noted that these authors found that in samples with a great excess of Ni (Ni :Mo ¼ 2.3) the
N! P transformation was not recorded when the sample was cooled.
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The Ni–Mo–O system indeed presents certain particularities. It is now well known
that NiMoO4 can have three different structures, two of them stable at atmospheric
pressure, while the other is observed at higher pressures. The two atmospheric pressure
isomorphs are now commonly named the a-phase—stable at room temperature and with
octahedral coordination of the Mo6þ ions—and the b-phase—high temperature phase,
metastable, and with tetrahedral coordination of the molybdenum.[74] The b-phase is
formed after heating the a-phase to ca. 7208C and undergoes reverse transition at low
temperature on cooling to ca. 2008C.[63] Figure 3 shows the differential thermal analysis
(DTA) cycle of phase transitions in the stoichiometric NiMoO4 system.
Figure 2. Thermal analysis of the precursor of NiMoO4. (Adapted from Ref.[48], with the kind
permission of Elsevier Science.)
Figure 3. The DTA cycle of stoichiometric NiMoO4 phase transitions. (From Ref.[63], with the
kind permission of Kluwer Academic Publishers.)
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The coordination of the molybdenum atoms in both phases was confirmed more
recently by Rodriguez et al. using x-ray absorption near-edge spectroscopy (XANES),
which has also shown that the Ni atoms are in octahedral sites.[75] However, in a
subsequent paper these authors reported that in the a-phase the molybdenum exhibits a
pseudo-octahedral coordination with two very long Mo–O distances (2.3–2.4 A).[76]
Regarding the stability of the phases, calculations of first-principles density functional
theory (DFT) have evidenced that the a-phase is �9 kcal/mol more stable than the
b-phase, with an energy barrier for the a to b transition of �50 kcal/mol, while time-
resolved XRD experiments point to an apparent activation energy of �80 kcal/mol.[76]
The phase stable at high temperature, b-NiMoO4, is frequently formed by heating the
precalcined a-NiMoO4 sample in situ, for instance at 7608C for 5min. The sample is then
quickly cooled to the desired temperature (which must always be higher than 2508C) forother treatments (e.g., sulfiding), characterization, or catalytic runs. It should be noted that
when more severe treatments were applied for b-phase formation (temperatures higher than
7608C or for more than 5min), after cooling to room temperature the sample exhibited not
only the a-phase, but also a more complex diffractogram, with peaks characteristic of both
phases. In this case the b-phase is stabilized at room temperature, which would be due, as
detailed below, to an excess of NiO as a result of the decomposition of the mixed compound
and sublimation of MoO3. When the normal treatment is applied for transition of phases,
b! a conversion is recorded when the sample is cooled to room temperature.
To use the high temperature b-phase of NiMoO4 in catalytic runs, Mazzocchia et al.
performed the a- to b-phase conversion in the reactor, by thermal activation of nickel
molybdate. The reactor was usually heated in 25min to 7008C under oxygen, and then this
temperature wasmaintained for 5–15min before cooling to the reaction temperature,[32,33,37]
but always avoiding excessive cooling to prevent the b to a-phase transition. The
temperature selected for transition of phases is in agreement with the high temperature XRD
data that show that at 5958C the b-phase is already present, but a temperature of about 7008Cis required to obtain full conversion into a pure b-phase.[37]
The data found in the literature reveal some discrepancies regarding the temperature
for phase transition in the NiMoO4 system. According to Di Renzo and Mazzocchia,[36]
this is due to the strong influence of the preparation conditions of the sample. Therefore, it
was decided to investigate, by differential thermal analysis, how thermal treatment of the
precursor affects the transition of phases in NiMoO4. It was found that the transition
temperature of the a- to the b-phase increases, and that for the b! a transition it
decreases, due to a temperature-induced relaxation. Thus, when the activation temperature
of the sample is increased, the activation energy for the a- to b-phase transition is
increased. It should, however, be noted that the endothermic peak that corresponds to this
transition (a! b) was not detected when the previous calcination thermal treatment was
performed at a temperature below 5508C. The temperature at which the exothermic peak
(relative to the b! a conversion) began ranged between 2578C and 2008C, depending onwhether the previous heating temperature was 7008C or 9008C, respectively.[36] Later, theuse of a high temperature diffraction camera showed that when activating NiMoO4 at
temperatures between 7008C and 9008C the temperature of the b to a transition at no point
differed significantly from 1808C, but the transition rate in the sample heated to 9008C was
slower than in the samples heated to lower temperatures. This effect was attributed to the
loss of MoO3 in the NiMoO4 with formation of a nickel-rich solid solution and with the
structure of the b-phase of NiMoO4.[46]
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4. CHARACTERIZATION OF CATALYSTS
4.1. Composition of Phases for Catalysts with Different Ni :Mo Ratios
Plyasova et al. studied, by XRD and infrared spectroscopy, the composition of phases of
the Ni–Mo–O system with Ni :Mo ratios from 0.2 to 2.0.[26] As described in the previous
section, two polymorphic forms have been identified in nickel molybdate catalysts: one that
is stable at room temperature (then named the P-phase), which when heated to about 6508Cis transformed into another (then named the N-phase), Which is unstable at room
temperature. After cooling, the transition to the P-phase is observed. It has been reported that
the high temperature modification reacts with excess nickel (relative to the stoichiometric),
forming a solid solution that is stable at room temperature. The existence of the N-phase at
room temperature and the absence of peaks characteristic of the P-phase or of NiO in the
x-ray diffractogram for samples with well-defined compositions indicates that, in certain
conditions, nickel is dissolved in the structure of the N-phase, stabilizing it at room
temperature, thereby forming a solid solution of nonstoichiometric composition.[26] It was
subsequently found that in samples containing excess nickel, relative to the stoichiometric
NiMoO4, the solid solution formed has a solubility limit in Ni in the range Ni :Mo ¼ 1.10–
1.20 (atomic).[77] A solid solution of the vacancy type is formed, i.e., the excess of dissolved
Ni ions occupies the normal octahedral positions in the structure of the N-phase, while some
tetrahedral positions of the Mo remain empty.
Other investigations have also been carried out in which differential thermal analysis
and XRD techniques were used to investigate the composition of the Ni–Mo–O system in
a wide range of Ni :Mo ratios (from pure NiO up to pure MoO3). The four phases detected
and identified were the following: nickel oxide, molybdenum trioxide, normal nickel
molybdate, and nonstoichiometric nickel molybdate.[78] In all the samples, with the
exception of the pure oxides, thermal analyses showed an irreversible exothermal effect at
about 430–4408C (which corresponds to crystallization of the nickel molybdate with
composition NiMoO4) and another at 620–6708C (which is presumed to be due to the
transition of phases of nickel molybdate). It was found that samples with m , 1
(m ¼ Ni :Mo ratio) present two phases: MoO3 and NiMoO4; the species with m close to 1
are mainly composed of normal nickel molybdate; the species with m . 1 are mixtures of
three phases: nickel oxide and normal and nonstoichiometric nickel molybdates.[78]
However, form . 1.6, only nickel oxide and the high temperature phase were detected.[26]
This identification of the phases that are present in Mo- or Ni-rich catalysts was
subsequently confirmed.[35] While in the stoichiometric catalyst, at room temperature, only
the low temperature phase was detected (with Mo in octahedral coordination), in Ni-rich
catalysts (Ni :Mo ¼ 1.0–1.3) a solid solution of nickel and both phases (high and low
temperature) were found, and the presence of nickel oxide was not detected in the x-ray
diffractograms or in the IR spectra. Therefore, the b-phase is stabilized at room temperature
due to the excess of nickel in the crystalline lattice of the molybdate. In catalysts where
Mo :Ni . 1, both the low temperature phase and MoO3 are present. For example, the com-
pound [(NH4)4H6NiMo6O24], prepared in well-defined conditions (see Fig. 1), at the drying
temperature presents very well-defined XRD diffraction patterns but at the thermal
activation temperature (5508C), only the a-NiMoO4 and MoO3 peaks were identified.[31]
For catalysts with excess nickel, in 1958 Corbet et al.[79] observed that, after heating
to 5008C the precursor obtained by precipitation of the solution containing Ni2þ and Mo6þ
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ions at pH higher than 6, a new phase was formed. This phase, then called the N-phase
with Ni :Mo . 1, according to Di Renzo et al.[46] corresponds to the high temperature
phase (b-NiMoO4), whose stabilization at room temperature is achieved by insertion of
excess Ni in the NiMoO4 lattice. The formation of the nickel-rich solid solution (NiO in
b-NiMoO4) entails an increase in the lattice parameters and leads to an increase in the
reducibility of the system. In fact, a catalyst with composition Ni :Mo . 1.40 is more
easily reduced than NiMoO4, which agrees with some results that will be described later
concerning the reducibility of this system.
The formation of the solid solution of NiO in NiMoO4 is demonstrated by the
stabilization of the b-phase at room temperature when the nickel-rich samples are
activated at temperatures around 550–7508C. The XRD data indicate that the solid
solution is responsible for the enlargement of the lattice parameters of the structure of the
b-NiMoO4 phase compared with the parameters of the stoichiometric phase.[46]
The Ni-rich samples prepared by coprecipitation, when heated to 5508C, mainly
showed the high temperature phase (b-NiMoO4), even when cooled to room temperature.
The longer this treatment lasts, the higher the percentage of that phase, and the smaller the
amount of crystalline NiO. However, when the temperature was increased, the percentage
of b-phase stabilizing at room temperature decreased and the proportion of NiO increased.
At very high temperatures the solid solution separates. Indeed, x-ray data showed that NiO
precipitation reached a significant rate at 8008C, contraction of the b-NiMoO4 cell
occurred at 9008C, and that after heating to 10008C the sample was composed, at room
temperature, only of a-NiMoO4 and NiO. This was confirmed by the results of diffuse
reflectance spectroscopy: the sample activated at 5508C showed the characteristic band of
the tetrahedral coordination of molybdenum (b-NiMoO4 phase) at 35,500 cm21, while
when activated at 9008C it presented the spectrum characteristic of stoichiometric
NiMoO4 with a band at 30,000 cm21 (typical of the octahedral coordination of
molybdenum in the a-phase) with the additional band of NiO at 14,000 cm21.[46]
4.2. Other Physicochemical Characterizations
4.2.1. Stoichiometric Nickel Molybdate
Stoichiometric nickel molybdate has been characterized by several research groups.
Usually the bulk composition of the catalyst is determined by inductively coupled plasma
spectroscopy, for molybdenum, and atomic absorption, for nickel. A typical BET surface
area for the stoichiometric catalyst is 44.1m2/g.[59] Since the material is crystalline, XRD
analysis (including high temperature XRD) has often been used to study its structure. A
typical diffractogram for both phases is presented in Fig. 4, which shows the characteristic
peak of the a-phase located at 2u ¼ 28.78 (JCPDS powder diffraction file card no. 33-948)
and of the b-phase at 26.48.The structure of the solid has also frequently been analyzed by infrared spectroscopy
because it provides useful information regarding, for instance, the stoichiometry of the
material. The Fourier transform infrared (FTIR) spectra of a-NiMoO4 (Ni :Mo ¼ 1.00) is
shown in Fig. 5, which is in good agreement with others found in the literature.[26,37] It is
characterized by bands at 608, 934, and 958 cm21. When, through stabilization, the
b-phase is also present, the spectrum at room temperature reveals a band at 950 cm21, and
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two new characteristic bands are also visible at 800 and 880 cm21 as a consequence of the
change in the Mo coordination from 6 to 4. This is an important feature in order to ensure
that the obtained nickel molybdate has a well-defined octahedral structure. Moreover, the
absence in the FTIR spectra of the characteristic MoO3 bands (at 980 cm21—attributed to
the vibration of the Mo–O bond—and at 870 and 812 cm21—attributed to the Mo–O
bond),[37] and the absence of those characteristic of the b-phase, are a good indication that
the prepared nickel molybdate does not contain excess of either Mo or Ni.
Regarding the electrical conductivity (s) of the solid, it is known that a-NiMoO4 is an
n-type semiconductor when prepared in quasistoichiometric conditions.[37,81] Studies
performed with this catalyst have shown that when the oxygen partial pressure in the
gas phase is lowered, at sufficiently high temperatures, the electrical conductivity
increases according to s / PO2
21/5.8.[37] The value of the exponent is close to 21/6,thus demonstrating that the main surface defects are compatible with the model of
doubly ionized vacancies,[82] whose formation can be described by the following
equilibria:
(OO)s !1
2O2(g)þ VO (6)
VO !VoO þ e� (7)
VoO !Voo
O þ e� (8)
Figure 4. X-ray diffraction patterns of a-NiMoO4 at 218C (A) and b-NiMoO4 at 7108C (B).
(Adapted from Ref.[80], with the kind permission of Elsevier Science.)
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where
(OO)s ¼ surface anion;
VO ¼ anionic vacancy with the two electrons trapped (neutral entity);
VOo and VO
oo ¼ singly and doubly ionized anionic vacancies, respectively.
From the equilibria of Eqs. (6)–(8), and taking into account that the corresponding
equilibrium constants follow Van’t Hoff’s law [Ki ¼ Koiexp (2DH i/RT)], it can be easily
deduced (e.g., Ref.[83]) that:
s ¼ A� e�½ � ¼ A� 2Ko6Ko7Ko8e�(DH6þDH7þDH8)=3RTP
�1=6O2
(9)
Thus, the exponent21/6 affecting the oxygen partial pressure is indicative of the existenceof doubly ionized anionic vacancies for a-NiMoO4, whose overall enthalpy of formation is
DH ¼ DH6 þ DH7 þ DH8 ¼ 3Ec. Mazzocchia et al.[37] found a value of 134 kJ/mol for Ec
(DH ¼ 402 kJ/mol), while Madeira et al. recorded an activation energy of 124.5 kJ/mol.[83]
Steinbrunn et al. also reported a value for Ec of 125.4 kJ/mol (at temperatures in the range
450–6508C), but unlike the previous authors, they found that the a-phase has two
conduction regimes: (1) in the range 450–6508C it is a p-type semiconductor; (2) at higher
temperatures (650–7008C) it is a n-type semiconductor with a higher activation energy of
conduction (Ec ¼ 182.4 kJ/mol).[73] Other studies concerning the electrical conductivity of
the Ni–Mo–O system can be found in the literature (e.g., Ref.[81,84–86]).
Due to the use of nickel–molybdenum catalysts in the hydrodesulfurization of
petroleum, the reducibility of nickel molybdate has been the subject of several studies. In
Table 1 some of them are summarized, which clearly illustrates that the mechanism of
Figure 5. The FTIR spectra of a-NiMoO4. (Adapted from Ref.[60], with the kind permission of
Academic Press, Inc.)
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Table
1.
Mechanismsandinterm
ediate
productsofNiM
oO4reductionbyhydrogen.
1ststep
2ndstep
Reference
NiM
oO
4����!
(4758C
)NiþNi-MoalloysþMoO
2þinterm
etallics
(e.g.,Ni 4MoÞ
MoO
2����!
(7008C
)Mo
[29]
NiM
oO
4������!
(400–5008C
)Ni-Moalloyþam
orphousmolybdenum
lower
oxide
Amorphousmolybdenum
lower
oxidephase������!
N2(6008C
)MoO
2
[87]
NiM
oO
4������!
(300–4508C
)NiþMoO
2[28,88]
NiM
oO
4������!
(300–5008C
)NiM
oxalloyþMoO
2NiM
oxalloyþMoO
2����!
(7008C
)MoþNi 3Mo
[22]
NiM
oO
4������!
(500–6008C
)NiþMo2O
3NiþMo2O
3�!
Interm
etallideofNiandMo
[89]
Source:
Ref.[87] .
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NiMoO4 reduction is far from being unambiguously established. More recently, Madeira
et al.[90] used the temperature-programmed reduction (TPR) technique followed by XRD
analysis in order to clarify the mechanism of NiMoO4 reduction by hydrogen. The TPR
profile is shown in Fig. 6 and is similar to those found in the literature,[9,11,22,29,46] with two
maxima at 5458C and 7258C. It was proposed that nickel molybdate reduction starts at low
temperatures (3008C), leading to metals (Ni and probably Ni4Mo) and amorphous
MoO2. Ni2þ, after being reduced to metallic nickel, activates molecular hydrogen, thus
favoring Mo6þ reduction. The amount of MoO2 formed becomes significant only at ca.
6208C. In this way, the peak at lower temperatures can be attributed to reduction of all
Ni2þ to metallic nickel and of Mo6þ to Mo4þ or to Ni4Mo. The second TPR peak would be
due to Mo4þ reduction. Metallic nickel activates hydrogen and induces molybdenum
reduction with formation of metallic Mo and an Ni–Mo alloy. Finally, at temperatures
higher than 7258C, a mixture of Mo, Ni–Mo alloy, and intermetallic Ni3Mo was found.[90]
Other techniques have also been applied for the characterization of NiMoO4, as will be
seen in the following sections. The use of surface techniques such as electron spin resonance
(ESR) and x-ray photoelectron spectroscopy (XPS) also deserves special mention, since they
have been very useful for identification of the active sites of the Ni–Mo–O catalytic system
for selective oxidation reactions. This will be dealt with in Section 5.3.
4.2.2. Catalysts with Excess Molybdenum or Nickel
Nickel molybdates with excess molybdenum have been characterized in detail by
Ozkan and Schrader,[45] who have particularly established that Mo in excess appears as a
new phase: MoO3. The solids were carefully characterized by several techniques,
Figure 6. The TPR profile of a-NiMoO4 with 5% H2 in argon. (Adapted from Ref.[90], with the
kind permission of Elsevier Science.)
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including BET surface area, Raman spectroscopy, XRD, x-ray fluorescence, photoelec-
tronic spectroscopy, and scanning electron microscopy. Curiously, an association of
particles was found, with crystallites of MoO3 covered superficially by NiMoO4 particles.
Other important results of this study were as follows:[45]
The BET surface areas of the catalysts decrease when the percentage of MoO3 excess
increases, varying between 37m2/g for pure NiMoO4 and 3m2/g for MoO3.
The percentage of MoO3 in the final product increases with the acidity of the medium
during precipitation (confirmed by x-ray fluorescence).
Two forms of crystallites, an irregular, round and porous form, attributed to NiMoO4,
and a hexagonal form, due to MoO3 were identified.
Ni–Mocatalystswith an atomic ratio ofMo :Ni . 1 are suitably characterized byFTIRandFT
Raman because new bands appear, typical of MoO3. As stated above, the infrared spectrum
exhibits additional bands at about 810, 860, and 990 cm21,[35] while the Raman peaks, char-
acteristic of MoO3, also become evident (shown in Table 2). This new phase is also clearly
visible inx-raydiffractograms.For instance, Fig. 7 shows theXRDpatterns at roomtemperature
of nickelmolybdates with different Ni :Mo ratios. It was found that all the Ni–Mo–O catalysts
show the diffraction patterns of the a-phase, and the x-ray diffractograms are practically
identical forNi :Mo ¼ 0.92or 1.00.[68]On the other hand, the strongpeaks corresponding to the
MoO3 phase are found in the diffraction patterns of samples with Ni :Mo ¼ 0.38.
Surface-sensitive techniques have also been used in order to investigate possible
variations in the oxidation states of samples with nonstoichiometric compositions. Ozkan
and Schrader[45] found that the band positions and bandwidths are identical for NiMoO4
samples containing excessMoO3, independently of the preparation technique used. Table 3
lists the observed binding energies (+0.2 eV) for such samples, showing that the Mo 3d
binding energies for NiMoO4 samples are identical to those of MoO3, while the Ni 2p band
positions are also very similar for the Ni–Mo–O catalysts, but entirely different from
those of NiO.[45] These results are important for demonstrating that the oxidation states of
molybdenum and nickel do not change in samples with different Mo :Ni ratios, and that
there is no NiO present in the observed samples. Furthermore, they can also exclude the
possibility of an entirely new compound in samples with excess MoO3.
All the precursors and corresponding catalysts presented in Fig. 1 were characterized
in detail by Mazzocchia et al. The E precursor, which contains excess Mo, deserved
special attention given the excellent catalytic properties revealed in the oxidation of
1-butene to maleic anhydride.[30] In this case, the presence of excess molybdenum of the
MoO3 type was revealed by the bands at 995, 865, 820, and 370 cm21 in the infrared
spectrum, and by the corresponding reflections at d ¼ 6.96, 3.82, 3.47, and 3.26 A in the
Table 2. Raman bands (cm21) of NiMoO4 and MoO3.
Sample
NiMoO4 963vs 916s 709s 494m 420m 389m 373m 179m
MoO3 998s 822vs 670s 381m 339m 293m 285s 160m
Source: Ref.[45].
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XRD patterns. When activated at 7608C the excess MoO3 is eliminated, and the resulting
pattern is characteristic of stoichiometric molybdate.[30]
Still using catalysts with excess Mo, but obtained from different precursors,
Mazzocchia et al. observed that in spite of an identical global atomic composition, the
physical–chemical properties of the solids and their catalytic behavior in propene
oxidation were completely different.[44] The differences in the electrical conductivities
suggested the existence of different phases in the two typical samples used. The sample
obtained by decomposition of the heteropolymolybdate presented a smaller activation
energy of conduction, a well-defined exothermal peak between 310–4308C in the
differential thermal analysis, due to the crystallization of MoO3; and the MoO3 showed a
preferential orientation of the crystalline planes in the (010) direction, compared with the
sample obtained by the dry mixture of NiMoO4 and MoO3.
Subsequently, several solids presenting more significant amounts of molybdenum than
the original product (in which Mo :Ni ¼ 0.98) were investigated. The materials exhibited
different concentrations of defects whose nature could be explained by electrical conductivity
measures and thermoluminescence experiments.[47,84] The Mo concentration varied between
Figure 7. The XRD diffraction patterns of nickel molybdates with various Ni :Mo atomic ratios.
(From Ref.[68], with the kind permission of Elsevier Science.)
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1% and 10% of the total number of Mo atoms in the initial product (a-NiMoO4). The
electrical conductivity measurements showed that the original product, containing nickel
vacancies, presents p-type conductivity. A small addition of Mo fills those vacancies.
However, at higher loads the Mo atoms occupy an interstitial position and free electrons
appear in the solid, responsible for the n-type conductivity. The recorded thermolumines-
cence emission was attributed to some of the atoms being in interstitial position.[84]
It should also be stressed that in catalysts containing excess Mo (or even Ni), Vagin
et al. found that the samples with the largest surface areas were those in which the
dominant phase was NiMoO4, with values varying between 22.0 and 29.4m2/g.[28] When
the NiO, and particularly the MoO3 content is increased, the surface area decreases.
Later Brito et al. investigated the reducibility of nickel and molybdenum catalysts, but
supported over g-Al2O3, and analyzed the effects of Ni concentration and calcination
temperature in the TPR profiles.[53] The acid character of these catalysts is noteworthy
(demonstrated by ammonia adsorption), especially when enriched in molybdenum, due to the
higher acidity of Mo6þ oxides compared to the alumina support. Characterization of the
catalysts showed a superficial interaction between Ni and Mo in catalysts calcined at
temperatures lower than 8008C, probably with formation of Ni–Mo–O phases. However, the
metastable phase (b-NiMoO4) was detected in diffractograms at room temperature of the
catalysts calcined at 8008C (characteristic line at 2u ¼ 26.88), which suggests that the supportplays a part in the stabilization of this phase. Regarding the reducibility of the catalysts, the
existence of a synergetic effect between Ni andMo should be noted since the reduction of any
of the species is facilitated by the presence of the other.[53]
Quite recently, Kaddouri et al. have also studied the reduction behavior of Ni–Mo–O
catalysts, and analyzed the effect of MoO3 (as well as the effect of tellurium and
phosphorous compounds) on the reducibility of NiMoO4 catalysts.[67] It is noteworthy that
when excess MoO3 is present, specifically above 0.5 (Mo : catalyst ratio by weight),
oxygen depletion from the solid decreases. When the MoO3 load is limited, the overall
reduction rate increases, compared to the stoichiometric NiMoO4 catalyst. It was found
that the 0.5 MoO3–NiMoO4 catalyst has the highest reduction rate.[67]
For catalysts with excess nickel, and particularly for Ni :Mo ¼ 1.7, Mazzocchia et al.
found that the thermal analyses did not show the exothermal peak that results from
NiMoO4 crystallization and, in addition, the XRD patterns showed that the degree of
crystallinity of the catalyst is lower, due to the formation of a NiMoO4 solid solution with
excess NiO.[31] It is known that excess Ni allows the stabilization of the b-phase at room
Table 3. Photoelectron spectra binding energies for pure compounds and catalysts (eV).
Sample Mo 3d5/2 Mo 3d3/2 Ni 2p3/2 Ni 2p1/2
MoO3 232.7 235.8
NiMoO4 232.6 235.7 855.7 873.3
NiO 853.3 871.3
NiMoO4 with 15% excess MoO3
(precipitation)
232.7 235.8 855.8 873.4
NiMoO4 with 15% excess MoO3
(impregnation)
232.7 235.9 855.7 873.4
Source: Ref.[45].
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temperature, which is confirmed by XRD (Ib/Ia ¼ I3.33/I3.09) and IR (presence of the
b-phase characteristic bands at 880 and 800 cm21) data.[31] For Ni-enriched catalysts,
a p-type semiconductor behavior has also been reported.[47]
The possibility of distinguishing the beta stoichiometric phase from the NiMoO4dNiO
phase, which is a solid solution, may lead to confusion in distinguishing the catalytic
properties of the NiMoO4 phases. Mazzocchia et al. considered this possibility and came
to the conclusion that there are indeed reasons to confuse the solid solution and the b-
NiMoO4 phase. One of these reasons is that an excess of NiO, even if very small, involves
the formation of an Ni1þ1MoO4þ1 solid solution, which is not very active for propane
ODH.[32–34] Another reason depends on how the thermal cycles were performed in order
to obtain the b-phase. If both the time and the temperature to which the catalyst is brought
are not strictly controlled, a loss of xMoO3 may occur with the consequent formation of an
Ni1þxMo12xO423x solid solution.
4.2.3. Catalysts Prepared Using Organic Precursors and Sol–Gel Methods
The agents that control pH in the synthesis of precursors of mixed oxides must be
easily eliminated from the precipitate. From this point of view, ammonia is usually
adopted instead of, for instance, alkaline or alkaline-earth hydroxides. However, the ease
of ammonia oxidation necessitates careful control of the conditions used for precursor
activation in order to avoid hot spots that induce heterogeneity in the properties of the final
oxide. In this context, the role of ammonium ions in the thermal activation of several
precursors of nickel molybdates, with Ni :Mo ratios smaller or larger than 1, has been the
subject of study by thermal and gravimetric analyses. Particularly, the different processes
(endothermal and exothermal) that occur in the activation of each precursor were clearly
defined.[33,91] It should be noted that elimination of ammonium ions can follow two
different pathways: ammonia removal or oxidation. The competition between the two
mechanisms depends on the oxygen partial pressure, kinetic factors, and on the heating
rate and the material of the cell. This last factor is related to the catalytic role of metals in
ammonia oxidation. While the decomposition of NH4NO3 in air is endothermal in alumina
cells, it becomes exothermal with metallic cells and occurs at a high rate if platinum is
used, which is also able to oxidize NH3 at temperatures of the order of 1508C.[91]
Later, Mazzocchia et al. decided to prepare an oxalic precursor, because organic
precursors have the advantage of crystallizing at lower temperatures, particularly the
oxalic precursor compared with ammonia.[34] The analyses performed revealed the
following processes in the precursor decomposition (when heated in O2):
MoOC2O4 � 4H2O����!1008C
MoOC2O4 þ 4H2O (10)
NiC2O4 � 2H2O����!2108C
NiC2O4 þ 2H2O (11)
MoOC2O4 þ1
2O2����!
2808CMoO2 þ 2CO2 (12)
MoO2 þ1
2O2����!
.2808CMoO3 (13)
NiC2O4 þ1
2O2����!
3508CNiOþ 2CO2 (14)
NiOþMoO3����!.4508C
b-NiMoO4 (15)
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The endothermal processes recorded are due to the loss of water and to oxalate
decomposition. The strong exothermal effect found was attributed to molybdenum
oxidation and to crystallization of MoO3. The final exothermal peak observed corresponds
to NiMoO4 crystallization. It should be stressed that the final composition of the catalyst
depends on the thermal treatment applied, and particularly on the heating rate and time.
For instance, to obtain NiMoO4 it is necessary to decompose the oxalate as quickly as
possible, usually introducing the precursor directly in a preheated oven at 5508C. It isnoteworthy that the catalyst obtained presented considerable stabilization of the b-phase at
room temperature.[34]
More recently, Anouchinsky et al. prepared several catalysts by the sol–gel method,
applying different thermal activations to the dry powder.[40] X-ray analysis indicated that
the dry gel is already a partially crystallized compound, several peaks of the a- and
b-phases of NiMoO4 were identified. Neither NiO nor MoO3 were found in the samples,
which confirms that the homogeneous dispersion of Ni and Mo ions in the precursor has
specifically led to the formation of the NiMoO4 phases, whose crystallization occurs at
temperatures lower than with precursors prepared by coprecipitation. Indeed, the main
exothermal effect recorded in the thermal analyses occurs at slightly over 2008C (either in
air or in nitrogen), and corresponds to the elimination of volatile compounds and to a
degree of crystallization of NiMoO4, concluded at 4708C. Water elimination is the
predominant phenomenon during thermal activation and the composition of phases of the
formed oxide varies as follows: fast heating (by direct introduction into the previously
warmed oven) favors b-NiMoO4 stabilization at room temperature, while slow heating
leads to the preferential formation of the more stable a-phase. This behavior was
explained based on the availability of Ni ions to form the solid solution with the
b-NiMoO4 phase, because when the sample is heated slowly, and therefore crystallization
occurs more slowly, the availability of Ni ions is reduced.[40]
4.2.4. Doped and Supported Nickel Molybdates
Alkali and alkali-earth metals have been widely and successfully used as promoters of
mixed oxides. In several works published by Portela et al. the effects of either
alkali[59,80,83,90] or alkaline-earth[60] promoters in the physical–chemical properties of the
stoichiometric nickel molybdate were analyzed. As already mentioned, these catalysts
were prepared by wet impregnation. In spite of the high metal content of the treating
solutions, in the bulk of the solids only traces of promoters were found when using alkali
metal salts. Consequently, they remain only on the catalyst surface, curiously in the overall
expected concentration, mainly affecting the NiMoO4 surface properties. The surface
promoter content was quantified by XPS, and will be herein denoted as X% (X is the
nominal metal/Mo atomic ratio in solution, which is equal to the surface content for
alkali-doped nickel molybdate). For alkali-earth elements (Ca, Sr, or Ba), the nominal
promoter content was found in the catalyst bulk.[60]
The above-mentioned promoters, particularly alkali metals, affect the NiMoO4 BET
surface area. It was found that higher promoter loadings lead to a greater decrease in SBETand that for a given loading of promoter the surface area decreases with increasing
promoter ionic radius. Surface areas as low as 26.7 and 26.6m2/g were found for 6%
Cs-a-NiMoO4[59] and 12% Ba-a-NiMoO4,
[60] respectively. However, the binding ener-
gies recorded for nickel and molybdenum showed no major changes after promoter
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addition (as compared to a-NiMoO4), demonstrating that the oxidation state of the
catalysts may remain unchanged.[59]
The use of cesium (Cs) deserved special attention because of its high selectivity in the
ODH of n-butane.[59,80] It is particularly noteworthy that Cs-doping does not affect the
nickel molybdate structure at room temperature (neither does Li, Na, or K), as revealed by
FTIR or XRD, but strongly affects the a- to b-phase transition. Indeed, high temperature
x-ray diffraction (HTXRD) analyses have shown that with a surface cesium loading of 3%
or 6% (atomic ratio Cs :Mo), the transition of phases at 7108C is only ca. 50%.[80] In our
opinion this behavior is probably related to the size of the promoter, which has an ionic
radius (r) of 1.67 A, because with either K or Ba (r ¼ 1.33 and r ¼ 1.34 A, respectively),
the a! b transition is complete, even when using higher promoter loadings.[60]
The reason for the choice of these kinds of promoters concerns the importance of the
adsorption bond strength of hydrocarbons to the surface in ODH reactions. It has been
proposed that for the selective oxidation of alkanes into alkenes, which are considered to
be nucleophilic molecules, a basic surface is crucial for obtaining higher selectivities
because it significantly decreases the chances of further oxidation into carbon oxides. This
led to doping NiMoO4 with basic promoters and using carbon dioxide as the probe
molecule in order to characterize the basicity of the surface of some Cs-doped catalysts
through temperature-programed desorption (TPD).[80] Figure 8 shows the TPD profiles for
a-NiMoO4 doped with different Cs loadings. It should be noted that Cs-doping
significantly increases the surface basicity, shown particularly by the area of the first peak,
with a maximum for a surface Cs loading of 3% (atomic ratio Cs :Mo). However, an
overdoping effect was recorded for the 6% Cs-NiMoO4 sample.
Such an overdoping effect was also noted in the electrical conductivity data recorded
for the same catalysts by Madeira et al.[83] (Fig. 9). It was found that cesium-doped
catalysts are much more conductive than unpromoted a-NiMoO4 due to surface Csþ ions
Figure 8. The TPD profiles of CO2 adsorbed at 308C for a-Ni :MoO4 doped with different Cs
loadings. (Adapted from Ref.[80], with the kind permission of Elsevier Science.)
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and associated oxygen species (ionic conductivity) and also exhibit smaller activation
energies of conduction (Ec), with values in the range 83–94 kJ/mol.[83] As noted above,
for the nickel molybdate catalyst the value obtained was Ec ¼ 124.5 kJ/mol, very close to
that obtained by other authors (Ec ¼ 134 kJ/mol).[37] This was, at the time, the main
evidence that led Madeira et al. to assume that under similar conditions the catalyst
exhibits the same electrical behavior with the same type of defects as those suggested by
Mazzocchia et al.,[37] i.e., doubly ionized anionic vacancies [cf. Eqs. (6)–(9)]. Such n-type
conductivity was confirmed subsequently by in situ electrical conductivity runs under
different atmospheres.[92]
Another interesting effect resulting from Cs-doping was the increase of the resistance of
nickel molybdate to reduction,[90] similarly to the effects recorded when doping molybdenum
catalysts with Li, Na, or K[93] or vanadium oxide with Cs.[94] Although the TPR profile
(cf. Fig. 6) was almost unaffected by the addition of Cs to the catalyst, an increase in the
temperature of onset of reduction was recorded when the surface Cs loading was increased,
with a value of 3008C for undoped a-NiMoO4 and 3508C for 6% Cs-NiMoO4.[90]
Concerning the characterization of alkali earth-doped a-NiMoO4, particularly
noteworthy is the formation of new oxygen species, detected as oxide and peroxide
compounds through XRD, FTIR, and FT Raman analyses.[60] In addition, CO2-TPD
results also showed an increase in the basicity of barium (Ba)-doped catalysts with
promoter loading, with an overdoping effect for Ba concentrations higher than 9%.[60]
Similar results were also recently reported by Liu et al. when doping an Ni0.9MoO4
catalyst with various barium loadings (molar ratio of Ba/Mo between 1% and 15%).[95] In
this paper, the XRD patterns and IR spectra demonstrate the formation of BaO2, the
concentration of which increases with barium loading up to 9%, decreasing for Ba
contents in the range 9–15% due to formation of BaMoO4.
Figure 9. Change of electrical conductivity at 3908C (B) and 4508C (W) as a function of surface
cesium contents on a-NiMoO4. (From Ref.[83], with the kind permission of Elsevier Science.)
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The deposition of coke over nickel molybdate has also been the subject of study and
has revealed interesting features, particularly the stabilization of the high temperature
b-phase at room temperature, as shown by XRD and FTIR analyses.[96] The role of coke
in this stabilization was demonstrated by completely eliminating it from the catalyst. After
this gasification, the solid again shows the typical a-phase structure at room temperature,
thus demonstrating the instability of the b-phase in the absence of coke. As mentioned
below, catalyst deactivation was not found after such forced coke deposition. On the
contrary, the effect of b-phase stabilization markedly improved catalyst performance,
particularly its selectivity for oxydehydrogenation of n-butane.[96]
The stabilization of b-NiMoO4 at low temperatures is very important because there is
much evidence that, for some reactions, and particularly for ODH of light alkanes, this
phase shows more interesting catalytic properties than the a-phase, especially selectivity
to dehydrogenation products. Thus, special reference should also be made to recent studies
that showed stabilization of b-NiMoO4, even at room temperature, when using TiO2[48] or
SiO2[49] as supports. Moreover, this effect was found both when preparing the cata-
lysts through precipitation[48,49] and through sol–gel routes.[50,51] The formation
and stabilization of this phase was also found over PNiMo :Al2O3 catalysts,[97] but in
this case it turned out to be a disadvantage because these catalysts are used in hydro-
desulfurization reactions in which the b-phase is much less active. Published data
on oxidic Ni-Mo :Al2O3 also point to a role of the alumina support in stabilizing
b-NiMoO4.[98]
The acid–base properties of SiO2-supported nickel molybdate catalysts were also
evaluated and compared with those of unsupported stoichiometric NiMoO4.[52,99]
Temperature-programed desorption experiments of NH3 and CO2 have showed that
supported catalysts with ca. one monolayer of the active phase are less acidic than the
unsupported nickel molybdate, but acidity increases with the number of monolayers.
The use of Ni–Mo-supported catalysts is widely reported in the literature, mainly for
applications in petroleum hydrotreatment processes, and so there are many examples of
interesting works in which the characterization of such materials is mentioned. A simple
example is the combination of TPR and ESR techniques, which have helped to clarify that
in Al2O3-supported Ni–Mo catalysts the good catalytic properties in the hydrode-
nitrogenation reaction of pyridine can be attributed to the improvement in the reducibility
of Mo, the formation of an Ni–Mo–O phase, and the creation of more anionic vacancies
when using Ni or W as promoters.[100,101] For the interested reader, other references are
provided, merely illustrative, to show that different supports have been used and the
catalytic systems characterized, namely alumina,[97] titania-alumina mixed oxides,[14,56]
alumina-magnesia mixed oxides,[13] activated-carbon,[15,16] and zirconia.[58]
4.3. Characterization of the High Temperature b-Phase
It is well known that the XRD signal at interplanar spacing around 3.33 A is
considered to be characteristic of b-NiMoO4 in Ni–Mo catalysts.[98] Thus, the use of a
heating camera has enabled several researchers, including the present authors, to record
the XRD patterns of both a- and b-phases of NiMoO4 at different temperatures, as well
as to study the transition of phases and their stability.[37,80] For instance, from Fig. 4 it is
clear that the a! b transformation is practically complete after 10min at 7108C in air, as
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shown by the relative intensities of the characteristic peaks (2uI2100 ¼ 28.78 and
2uI2100 ¼ 26.48, for a and b-phase, respectively). The HTXRD data have also provided
evidence that the a-phase is stable up to at least 6258C and that the b-phase is stable down
to at least 4258C,[80] the range of temperatures typical for ODH reactions.
The characterization of the b-phase through other techniques is not easy due to its
instability at low temperatures. The BET surface area, for instance, is estimated to be of the
same order of magnitude as the value obtained with the a-phase after thermal treatment
(e.g., 15min at 7008C), followed by quenching to room temperature. Mazzocchia et al.
obtained specific surface areas for the a- and b-phases of 40 and 15m2/g, respectively,[32,37]
very close to those obtained by Martin-Aranda et al. (44.1 vs. 16.0m2/g).[59]
Regarding electrical conductivity (s), it has been observed that the b-phase is also a
semiconductor of the n-type, like the a-phase, and it therefore obeys the same law (for
PO2. 19.7 kPa):
s ¼ so exp �DHa
RT
� �P�1=nO2
(16)
with a conduction enthalpy (DHa) of 125.4 kJ/mol.[37] For the b-phase the exponent n is
close to 4, demonstrating that the main surface defects are singly ionized anionic
vacancies, while with a-NiMoO4 a value close to 6 was obtained, indicative of doubly
ionized anionic vacancies [cf. Eqs. (6)–(8)].[37] Steinbrunn et al. also found that the
electrical transport for both nickel molybdate phases takes place via a classical intrinsic
band conduction mechanism, but they concluded that the b-phase behaves as a p-type
semiconductor in the temperature range 450–6508C, with an activation energy of
125.4 kJ/mol.[73]
Another property of the high temperature phase that was studied by Mazzocchia et al.
was its reduction rate by H2. A significant increase was reported in the reduction rate of
b-NiMoO4 phase compared to the a-phase.[31,67]
Among several results obtained by Brito et al.,[11] it should be noted that the BET
surface area of a-NiMoO4 (38m2/g)—obtained by calcination of the hydrated precursor
at 5508C—also decreases considerably after cooling b-NiMoO4 to room temperature
(26m2/g). However, the TPR profiles show clear differences in the reducibility of both
phases, with higher reduction temperatures for the b-phase due to the greater difficulty in
reducing Mo6þ in tetrahedral than in octahedral coordination.
5. Applications of Ni–Mo–O Catalysts
The multifunctional character of the Ni–Mo–O system is demonstrated by the wide
variety of reactions in which it is applied and through the great diversity of products
obtained with a given reactant. An example is butane oxidation, in which the possible
reactions involved include dehydrogenation, isomerization, oxidation with oxygen
insertion, partial oxidation with rupture of carbon–carbon bonds, and total oxidation.[35]
Nickel–molybdenum-based catalysts have been used in several reactions. As already
mentioned, this review essentially concerns oxidation reactions, and particularly ODH of
light alkanes. However, their use in reactions such as hydrogenation and hydrogeno-
lysis of toluene,[22] hydrodesulfurization of thiophene,[8–15] hydrodenitrogenation of
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pyridine,[16,17] water–gas shift,[18] steam reforming of hydrocarbons,[19] oxidative
coupling of methane,[20] COx hydrogenation,[55] and in other important hydrogenation and
hydrotreating reactions[8,21,23,24] should also be mentioned.
5.1. Oxidation of Hydrocarbons
According to Ozkan and Schrader,[45] there is strong evidence that the presence of
excess MoO3 is a key factor that determines the catalytic behavior of simple molybdates in
selective oxidation reactions. In this context, they decided to prepare nickel molybdates
with excess MoO3 through several techniques (see Section 2), and used these catalysts in
the conversion of some C4 hydrocarbons into maleic anhydride. The hydrocarbons used
were 1-butene,[102] butadiene, and furan.[103] The results recorded show that specific
concentrations of MoO3 are necessary in order to obtain high selectivities to maleic
anhydride (with a maximum for 15% molar excess of MoO3), and that pure NiMoO4 and
MoO3 are not selective. The most selective prepared catalyst for maleic anhydride
production was an MoO3 phase superficially covered with NiMoO4, which exhibited
stability in reaction conditions for 200 hr. Characterization of the catalysts used after such
long runs did not reveal any chemical or structural alteration, with no change in the
oxidation states of Mo 3d and Ni 2p (as found by XPS), but only a change of color (from
yellow to dark gray) due to carbon deposition on the surface.[102] Regarding the role of
each phase, it was clearly established that selectivity for maleic anhydride is determined
by competition between the processes of carbon oxides and maleic anhydride formation,
both occurring at different MoO3 sites. NiMoO4 is the component responsible for 1-butene
ODH, and moreover this phase selectively blocks the MoO3 sites that lead to total
oxidation, thus favoring selectivity to maleic anhydride.[103]
Zou and Schrader then decided to develop a technique to prepare thin films
(150–300 A) of NiMoO4 on the surface of (0 1 0) MoO3 previously deposited over a
support.[38] The catalysts thus prepared revealed excellent catalytic behaviors in the
oxidation of 1-butene to furan and maleic anhydride. It is noteworthy that when only
NiMoO4 was deposited a high selectivity to butadiene was obtained, with yields of about
48%, which was attributed to the presence of defects, probably in Ni–O–Mo sites.[38]
A synergetic effect between the a-phases of NiMoO4 and MoO3 was also detected in
1997 byMagaud et al. during propane oxidation.[104] Indeed, activity and selectivity to acetic
acid and acrylic acid are maxima when the ratio a-MoO3/(a-NiMoO4 þ a-MoO3) is close to
0.25, due to a specific arrangement of the two species, especially a reciprocal covering.
Mazzocchia et al. have also frequently used the Ni–Mo–O system in hydrocarbons
oxidation reactions. In a pioneering work, 1-butene oxidation to maleic anhydride was
studied with several catalysts.[30] Once again, the catalyst with excess Mo, relative to the
stoichiometric one (precursor E in Fig. 1), was shown to be particularly active and
selective. Moreover, pulsed-feed experiments have shown that ODH to butadiene is
possible due to the intervention of reticular oxygen (through a redox mechanism), while
the formation of partial and total oxidation products involves different forms of adsorbed
oxygen. In the conditions studied, the greatest selectivity to maleic anhydride was 64%,
which was attributed to the presence of Mo(V) sites that are able to activate the oxygen
molecule and that exist in catalysts with MoO3 present in an NiMoO4 matrix.
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 81
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The oxidation of propene has also been the subject of study, in particular the influence
of the Mo :Ni ratio on the behavior of the catalysts.[31] While all the catalysts tested have
produced acrolein, only catalysts with excess MoO3 enabled acrylic acid to be obtained. In
any case, the maximum yield was always obtained with catalysts containing excess MoO3,
which increases the superficial acidity of the catalyst and thus favors formation of acrylic
acid. In addition, the b-phase of NiMoO4 enabled propene to be converted into acrylic acid
and also oxidized acrolein into acrylic acid, while the NiMoO4 a-phase is practically
inactive in both reactions.
From the above-mentioned results it is clear that excess MoO3 is crucial in oxidation
reactions, the same being true for propene oxidation. However, the method of preparation
of the precursor significantly affects the catalytic behavior. Preparation of precursors with
identical atomic composition, but through different methods, leads to catalysts with quite
different properties.[44] For instance, the catalyst prepared by coprecipitation presents a
smaller electrical conduction activation energy and is catalytically more active than the
one prepared by a simple mixture of NiMoO4 and MoO3. The former is also more selective
to acrolein and to acrylic acid and presents a preferential orientation of the MoO3
crystalline planes in the (0 1 0) direction.[44]
Oxidation of propene with Ni–Mo-based catalysts has also been the subject of some
patents. For instance, US Patent No. 4,388,223, by Ferlazzo et al.,[70] describes the
preparation of a complex molybdenum-based catalytic system, which is formed of one
or two crystalline phases and at least one promoter element. When one of the phases
involves beta-nickel molybdate, runs performed using propene as the unsaturated
hydrocarbon resulted in a conversion of 95.3%, with a selectivity of 95.6% for the
acrolein and acrylic acid produced (feed containing the following percent volume
composition: propene/oxygen/steam ¼ 6.64/12.5/34.0, with a contact time of 2.3 sec at
3708C).[70] Finally, Umemura et al. reported a yield to acrolein as high as 91.5% using
an Mo–Co–Ni–Bi–Fe–Al–Ti–O type catalyst, which also has an excellent crushing
strength.[105]
The great interest in oxidizing propene directly to acrylic acid led Mazzocchia et al. to
study this reaction further. After determination of the optimum proportion between MoO3
and NiMoO4 in the binary system, i.e., NiMoO4. 2MoO3, the catalyst was promoted with
Te (Te2MoO7), which considerably increased the performance of the solid. This catalyst
was then used in a preliminary kinetic study.[106] As regards the effects of each oxide,
molybdic anhydride by itself dramatically increased the selectivity (and also activity) of
NiMoO4, while the presence of tellurium, in spite of decreasing conversion, increased the
selectivity to acrylic acid. It is possible that the decrease in conversion results from the fact
that tellurium molybdate accelerates the a! b-NiMoO4 transformation,[107] that leads to
a less active catalyst. The synergistic effect between MoO3 and a-NiMoO4 was related to
the large amount of Ni found by XPS on the surface of the catalyst, which was in
agreement with the above-mentioned results obtained by Ozkan and Schrader[45] that
demonstrated the presence of NiMoO4 on the surface of MoO3 crystals. With regard to the
effect of tellurium, it was considered that this promoter keeps Ni and Mo in high oxidation
states (e.g., Mo5þ þ Te4þ! Mo6þ þ Te3þ), necessary for easy desorption of acrolein,
whose formation probably represents the rate-determining step.[106]
More recently, the partial oxidation of propene was carried out with Ni–Mo–Te–O
ternary catalysts by Kaddouri et al.[67] It was concluded that the catalytic behavior is
governed by both the synergetic effect generated by combining NiMoO4 with Mo- and
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Te-oxides and by the oxygen partial pressure rather than lattice oxygen. Indeed, Te-doped
Ni–Mo–O catalysts have a high potential for partial propene oxidation, which is linked
also to the increase in catalyst reducibility induced by the presence of tellurium, although
the reaction seems to be governed by the molecular oxygen partial pressure. This was
concluded after a preliminary kinetic study, in which a power-law rate expression of the
form r ¼ kPmO2PC3H
n6showed a reaction order with respect to oxygen, m, of 0.39, 0.23, and
0.48 for CO, CO2, and acrolein formation, respectively. The order with respect to propene,
n, was 0.43, 0.49, and 0.32.[67] Thus, a conventional redox mechanism does not seem to
operate, and the yield of acrolein can be improved by lowering propene and increasing
oxygen partial pressures.
Even more interesting than the direct oxidation of propene to acrylic acid is the direct
use of propane, for which the Ni–Mo–Te–O system doped with P has been successfully
tested.[66] It was found that the catalytic system provides a wide product distribution,
leading to propene, acrolein, and acrylic acid formation according to a reaction pathway as
shown in Sch. 1. However, the reported yields were low, as compared to the more
promising study published by Fujikawa et al.[68]
Sautel et al. also tested nickel–molybdenum catalysts for such a reaction and
compared the results obtained by the almost stoichiometric catalyst (Mo :Ni ¼ 0.98) with
another one 5% enriched in Mo.[84] It was found that at 500 or 5308C, and with any phase,propene and acrolein formation rates were higher with the Mo-enriched sample, while the
CO2 formation rate was much lower. Therefore, the Mo-enriched compound was a better
catalyst for both conversion and selectivity, and this behavior was attributed to Mo atoms
in interstitial positions (demonstrated by electrical conductivity and thermoluminescence
measurements). While the higher conversion was attributed to increased propane
adsorption, the lower rate of CO2 formation was explained on the basis of smaller
availability of lattice oxygen due to the formation of interstitial Mo atoms.[84]
Investigating the oxidation of butane to butadiene and maleic anhydride, Mazzocchia
et al. found that significantly different results were obtained with respect to product
distribution by changing contact time, temperature, or the butane : oxygen ratio.[35] It was
also interesting that an excess of MoO3 is responsible for higher activity, despite reducing
selectivity to dehydrogenation products and increasing formation of carbon oxides. The
fact that practically the same yield in butenes was observed when the number of pure
butane pulses was increased suggested that butane dehydrogenation occurs without
the intervention of lattice oxygen. On the other hand, maleic anhydride formation is
related to the activation of gaseous oxygen in sites that disappear with strong reduction and
that cannot be regenerated by reoxidation. Such sites probably correspond to Mo(V)
Scheme 1. Reaction pathway proposed for propane oxidation with Ni–Mo–O based systems.
(From Ref.[66], with the kind permission of Elsevier Science.)
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sites.[35] The nickel molybdate catalysts used were prepared by coprecipitation and exhi-
bited low hydrocarbon conversions: at 4758C, 19% conversion of n-butane was recorded
with low selectivity to maleic anhydride.[35]
The great interest in producing maleic anhydride is due to its use as raw material for
products ranging from agricultural chemicals, paints, paper sizing, and food additives to
synthetic resins. To meet the high demand for this valuable chemical, a variety of
commercial processes and efficient catalysts have been developed, particularly from
n-butane oxidation. With this goal in mind, Cherry et al.[69] announced the preparation of a
catalyst composition useful for the vapor phase oxidation of butane to maleic anhydride.
The catalytic system used—an oxide composition containing Sn–Ni–Mo—was found to
be highly selective, stable, and long-lasting, providing selectivities for the desired maleic
anhydride in the range of about 25–35%. Surprisingly it was found that these catalysts are
more effective when unsupported. Table 4 shows some typical results obtained with
Ni–Mo catalysts, undoped or doped with antimony. For the Sb : Ni :Mo catalytic system,
the constant selectivity recorded with increasing conversion up to 70% is particularly
noteworthy, while with nickel molybdate the selectivity for maleic anhydride decreases
markedly above about 50–60% conversion.
In the United States patent by Kourtakis and Sullivan, several molybdenum-
containing oxides (including nickel–molybdenum-based materials) are described, which
can be used in a wider context for catalyzing other C4 oxidation processes.[108] They state
that such catalysts can be used advantageously with regard to conversion and selectivity in
a wide variety of conventional techniques and reactor configurations (i.e., fixed or
fluidized bed reactors or recirculating solids reactors) to perform the oxidation of C4
hydrocarbons to maleic anhydride.[108]
Other important oxidation processes in which Ni–Mo containing catalysts have been
successfully used include: (i) the oxidation of toluene, where a 70% yield of benzaldehyde
Table 4. Catalytic data of oxidation of butane to maleic anhydride.
Catalytic
system
T
(8C)Contact
time (sec)
Conversion
(%)
Selectivity
(%)
Yield
(%)
Sb/Ni/Moa 400 0.32 29 33 10
403 0.46 39 31 12
400 0.83 58 27 16
400 0.98 68 25 17
450 0.09 37 27 10
450 0.13 48 26 13
450 0.24 65 29 19
453 0.23 70 26 18
Ni/Mob 500 0.46 36 27 10
506 0.86 52 29 15
500 1.51 70 21 15
499 2.10 81 17 14
aAtomic ratio ¼ 1 : 0.24 : 0.14. Experimental conditions: butane/air ¼ 0.8/99.2 (mol%); W ¼ 29 g.bExperimental conditions: butane/air ¼ 0.9/99.1 (mol%); W ¼ 18.4 g.
Source: Ref.[69].
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per pass was achieved in a fixed bed reactor with NiMoO4 activated at 4508C;[109] (ii) theconversion of alcohols to aldehydes with an Ni–Mo catalyst containing a complex iron
molybdate;[110] (iii) methacrolein production by catalytic oxidation of isobutene;[105] and
(iv) the ammoxidation of olefins to unsaturated nitriles, notably acrylonitrile and metha-
crylonitrile production, using propene and isobutene as olefin reactants, respectively.[111]
5.2. Oxidative Dehydrogenation of Light Alkanes
5.2.1. Undoped Ni–Mo Catalysts
The search for catalysts with good performance in butadiene production has been
studied for a long time because of its use as a monomer in the production of synthetic
rubber. Although references exist since the 1960s on the use of the nickel–molybdenum
catalytic system in n-butane to butadiene ODH,[112] it was only after 1974 that a
considerable number of studies began to appear on this subjects. One of the pioneering
patents is that of Bertus et al., in which a stationary bed of nickel molybdate provided a
yield of butadiene of 4.2–13.5% by weight, with a selectivity of about 33% (at a
temperature within the range 550–5908C, a space velocity of the butane supply of
50–500 hr21 and at a molar ratio n-butane : oxygen : steam of 1 : 1 : 20).[113]
Around the same time, Pilipenko et al.[114] investigated the effect of the composition
of the nickel–molybdenum system in that reaction. n-Butane conversion at 6008C was
about 30–40% in samples with Ni :Mo atomic ratios m ¼ 17.3–0.48, drastically
decreasing when the MoO3 concentration was increased, reaching values lower than 0.6%
with the pure oxide. The pure nickel oxide is not also interesting for this ODH reaction
because its selectivity to butadiene was practically nil, although conversion is high. The
data obtained show that catalysts with compositions in the range m ¼ 1.92–1.28 present
the highest selectivities and yields butadiene, with values up to 54% and 17.1%,
respectively, decreasing strongly when the MoO3 loading is increased. Therefore,
catalysts with three phases, normal nickel molybdate, nickel oxide, and nonstoichiometric
nickel molybdate (phase N, now called b-phase), are the most efficient in butane to
butadiene ODH. The authors have proposed that an oxygenated nickel compound, which
may exist in several forms, is responsible for the catalytic activity. Finally, by performing
some runs from 1-butene and butadiene, they have suggested that nickel oxide is the
species responsible for the butane to butenes conversion step.[114]
The assignment of the active component of this system caused some controversy. Two
years later Itenberg et al. again studied this reaction with nickel–molybdenum
catalysts.[115] Although catalytic tests with the individual oxides have shown that
molybdenum trioxide and nickel oxide are active in n-butane ODH, the low recorded
conversions and selectivities to butadiene led them to conclude that they were not the
agents responsible for the catalytic activity of the NiO–MoO3 system. Then they
performed some runs with catalysts of different compositions and found that the maximum
selectivity (to butenes or butadiene), at equal butane conversion levels, was obtained with
samples with Ni :Mo atomic ratios between 1.0 and 1.2. A study of the composition of
phases led them to conclude that the active component was nickel molybdate or a solid
solution of nickel oxide in the molybdate lattice. A shift of the ratio for m . 1.2 or
m , 1.0 led to a decrease in selectivity, apparently due to the presence of the individual
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 85
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oxides. It was also reported that both nickel molybdate modifications (P and N, where Mo
presents octahedral and tetrahedral coordination, respectively) presented practically the
same activity and selectivity. That is, the crystalline structure of the nickel molybdate
lattice would not affect the behavior in butane ODH. Similar conclusions were also
presented by Cavani and Trifiro,[116] who reported that both crystalline forms of the
molybdate (now called a and b) present similar catalytic behaviors in butane ODH (for
T ¼ 5008C, butane conversion ¼ 12%, yield to butadiene ¼ 4.2%, yield to
butenes ¼ 4.8%). In addition, they reported that in the presence of excess MoO3 or NiO
relative to the stoichiometric molybdate, activity increases but selectivity decreases
considerably.[116]
The claim that a- and b-phases of NiMoO4 exhibit similar performances in ODH is
another subject of controversy. Indeed, neither the results obtained by Mazzocchia et al.
(mentioned in Table 5) nor those obtained by Madeira and Portela seem to support this.
For instance, in the first study reported by these authors it was reported that the a-phase is
more active (see Fig. 10) while the b-phase is much more selective for dehydrogenation
products at comparable conversions and similar temperatures.[59]
Most papers published on ODH reactions with undoped Ni–Mo–O catalysts concern
propane conversion. For catalysts with Mo :Ni ratios .1/1, Lezla et al. considered the
a-phase as the active one.[41] The addition of molybdenum oxide to nickel molybdate
significantly improved the behavior of the catalyst and the most effective composition found
had a Mo :Ni ratio ¼ 1.27/1, with which a selectivity to propene of 63% was obtained, at a
propane conversion level of 22% (at 5008C, t ¼ 3.8 sec, C3/O2/H2O/N2 ¼ 20/10/30/40).[41] They found that interfacial synergetic effects exist between the planes (0 1 0) of a-
NiMoO4 and (1 0 2) of MoO3, as already noticed in some oxidation reactions.
Thomas et al.[47] also tested nickel molybdates with different Ni :Mo ratios in propane
oxydehydrogenation. It was noticed that selectivity to propene is enhanced for Mo-rich
catalysts, which also have better catalytic activity as measured by reaction rates; although,
the surface areas were lower than those of Ni-rich products. Mo-rich catalysts were five
times more efficient than Ni-rich products and the high kinetic constants for propene
formation shown in Fig. 11 counterbalance the low values of their surface areas. This
effect was attributed to the existence of Mo atoms in interstitial positions, revealed by
electrical conductivity and thermoluminescence measurements.[47,84]
Table 5. Catalytic data of oxidative dehydrogenation of propane.a
Catalyst
T
(8C)Conversion of
propane (%)
Selectivity to
propene (%)
Yield to
propene (%)
a-NiMoO4 560 23.3 50.6 11.8
a-NiMoO4 600 37.1 33.8 12.5
b-NiMoO4 560 16.8 80.3 13.5
b-NiMoO4 600 29.0 62.5 18.1
NiMo1.5O5.5 600 37.2 28.6 10.6
Ni1.5MoO4.5 600 34.0 18.5 6.3
aExperimental conditions: Qt ¼ 15 LPTN/hr (25% propane, propane/O2 molar ratio ¼ 0.9);
W ¼ 0.5 g.
Source: Ref.[32].
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In another very interesting work the possibility of using nickel molybdate in
circulating bed reactors for propane ODH was tested.[117] In these reactors the catalyst is
regenerated by oxygen (in a regeneration zone) after leaving the reaction zone where it
oxidizes the hydrocarbon. The maximum amount of oxygen that can be reversibly
removed from a catalyst is a parameter that determines its applicability in these reactors. In
the case of NiMoO4 it corresponds to 2% of the total oxygen content, excessive reduction
leads to the irreversible transformation into MoO2 and Ni (NiO).[117]
More recently, Stern and Grasselli[72] have also studied propane to propene
conversion with several molybdates supported over SiO2. The results showed that the
Figure 10. Conversion and selectivity for n-butane ODH with the a (A) and b (B) phases of
NiMoO4. Experimental conditions: W ¼ 0.5 g; C4H10/O2/N2 molar ratio ¼ 4/9/87; W/F ¼
20 gcat hr/molbutane. (Adapted from Ref.[59], with the kind permission of Elsevier Science.)
Figure 11. Change of the kinetic constant for the formation of propene during propane ODHwith the
atomic composition in Ni–Mo–O catalysts. Experimental conditions:W ¼ 0.25 g; T ¼ 5008C; C3H8/O2/He% ¼ 15/18/67; QT ¼ 15L/hr. (From Ref.[47], with the kind permission of Elsevier Science.)
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reaction is catalytic and is not initiated with formation of radicals in the gas phase, the rate
controlling step being the breaking of the C–H bond with abstraction of a hydrogen from a
methylene group of the propane molecule. Among the simple molybdates tested of the
type AMoO4 (with A ¼ Ni, Co, Mg, Mn, or Zn), the nickel one presented the greatest
activity, even when normalized per unit of surface area (which was the highest: 39m2/g).The molybdenum–oxygen bond, which is influenced by the nature of the adjacent metal,
A, is probably responsible for propane activation and, consequently, bonds of the type
Ni–O–Mo–O are the most active. It is noteworthy that at equal conversion levels, the
highest selectivity was also found with the NiMoO4 catalyst (60% for propene at a
conversion of 27%).
Mazzocchia et al. have also studied the ODH of propane to propene with catalysts
containing Ni :Mo ratios below and above 1, as well as the behavior of the two NiMoO4
phases.[32] In Table 5 some of the data are presented. It is clear that the two phases of the
Ni–Mo–O system exhibit quite different catalytic behaviors. Further investigation
confirmed that although the a-phase is slightly more active, the b-phase is almost twice as
selective to propene at comparable conversions and identical temperatures.[37] This
selectivity difference was accounted for on the basis of the different types of oxygen bonds
at the active sites. Since the presence of M55O bonds is usually associated with the
formation of oxygen-containing products, and because in the b-phase the character of this
bond is weaker than in the a-phase (band at lower frequencies in the infrared spectrum),
the high temperature phase should be more selective to dehydrogenation products. This
different behavior of the two phases was very recently attributed by Kaddouri et al. to the
different reducibility of the two phases.[67] In fact, a relationship was found between the
reducibility of the catalysts and the catalytic behavior in propane ODH, demonstrating that
lattice oxygen plays an important role in the reaction, i.e., the process is governed by a
redox or Mars–van Krevelen mechanism.[118]
The formation of propene by reaction of propane with superficial O22 anions was
proposed, leading to the formation of anionic vacancies (VOoo) shown by electrical
conductivity measurements [Eq. (17)].[37] Regeneration of the oxygen species active in
propane ODH occurs through spontaneous reoxidation of the surface by gaseous O2
[Eq. (18)] according to a Mars–van Krevelen mechanism:
C3H8 þ O2�s �! C3H6 þ H2Oþ Voo
O þ 2e� (17)
1
2O2(g)þ Voo
O þ 2e� �! O2�s (18)
The fact that the b-phase is more selective than the a-phase led Mazzocchia et al.[33] to try
to prepare the former phase at lower temperatures than those generally used (about 7008C)in order to avoid sintering, which drastically reduces the surface area. It would also be
convenient to stabilize this phase but without excess NiO, because the precursor with
excess nickel (green), which stabilizes the b-phase at room temperature, does not exhibit
good performance in propane ODH. It was found that when calcining the yellow precursor
of stoichiometric nickel molybdate in situ, after thermal decomposition at 5008C, the b-
phase of NiMoO4 crystallizes.[33] The catalytic runs were quite interesting because the
precursor calcined in the reactor at 5508C showed a selectivity to propene at 5308C, whichwas almost as high as the b-phase (65.7% against 78.5%), which had been obtained
starting from a-NiMoO4 and heating from 258C to 7008C, then cooling later to the
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reaction temperature. Furthermore, the first catalyst presented a higher level of
propane conversion (18.3% against 10.0%), leading to a larger yield to propene (12.0%
against 7.8%).
The previous situation still presented a technical problem because the reactor had to
be continuously heated to keep the catalyst as the more selective b-phase for a long period,
and it could not be cooled to room temperature in order to prevent transition to the
a-phase. To overcome this drawback, an oxalic precursor was prepared which, after
thermal activation, led to stabilization of the NiMoO4 b-phase at room temperature.[34] In
this way the need for continuous heating of the reactor is avoided in possible industrial
applications. Moreover, the catalyst presented a specific surface area of 34m2/g, similar to
that of a-NiMoO4 (32m2/g).[34]
Later Anouchinsky et al. immobilized the Ni and Mo ions inside an organic gel (agar-
agar), which led to NiMoO4 crystallization at lower temperatures and to possible
stabilization of the b-phase at room temperature, depending on the thermal treatment
applied in the precursor activation.[40] Such stabilization may be due to the presence of
organic residues because, as mentioned above, stabilization of the b-phase at room
temperature was also found after forced coke deposition over NiMoO4.[96] The catalyst
obtained starting from the gel presented selectivity to propene similar to that obtained with
the pure b-phase, but it was less active.[40] Propene productivities (in mmol/hr) at 5008Cwere the following: 4.7 (a-NiMoO4), 2.5 (b-NiMoO4), and 1.9 (a þ b NiMoO4, obtained
from the gel). The catalytic results in propane ODH do not appear very promising,
although the preparation method has not yet been completely optimized.
5.2.2. Doped and Supported Catalysts
Due to the great industrial interest in butadiene, butenes have also been converted into
this product by ODH using Ni–Mo doped catalysts. Some interesting results are shown in
Table 6, obtained with catalysts claimed by Bertus in one of the pioneering patents dated
1978.[64] The effective conversion and yield of butadiene with both catalytic systems is
noteworthy, and it is also apparent that contact times must be short.
The use of promoted Ni–Mo catalysts in n-butane oxydehydrogenation is not recent.
For instance, US Patent No. 4,094,819 of 1978[64] presents good results using several
Ni–Mo doped-catalysts, those obtained with arsenic being particularly interesting
(Table 7). Even more efficient was a catalyst of oxides of molybdenum, cobalt, and nickel,
which provided a conversion of butane of about 20%, with a selectivity for n-butenes of
about 26% and of about 35% for butadiene, thus providing a total yield for C4s of
12.2%.[119]
Some studies on the use of promoted Ni–Mo catalysts for n-butane ODH have been
published by Madeira and Portela. In a preliminary study it was shown that all the alkali
metals studied (Li, Na, K, or Cs) significantly improve the selectivity of the NiMoO4
catalyst,[59] when promoting either the a- or b-phase. As shown in Fig. 12, this effect
increases in the sequence unpromoted , Li-doped , Na-doped , K-doped , Cs-doped,
the b-phase of this catalyst providing a yield to C4 products of 14.5% (for a conversion
level of 28.2%).[59] The fact that higher selectivities were obtained with Cs-doped nickel
molybdate led the authors to study the effect of this promoter in more detail.[80,83]
The effect of doping with alkali metals, i.e., the increase of selectivity to ODH
products, can be easily understood if one takes into account their basic nature. Thus, the
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 89
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more basic surface facilitates desorption of the nucleophilic olefins/di-olefins from the
catalyst surface (butenes and butadiene that have high electron densities at the p bonds),
thus avoiding their overoxidation into carbon oxides. Indeed, a good correlation was
recorded between surface basicity, measured by CO2-TPD experiments, and selectivity for
C4 products as shown in Fig. 13,[80] in which an overdoping effect is also visible.
Alkali-earth metals, namely Ca, Sr, and Ba, were also used by Madeira et al. as
promoters of the NiMoO4 catalyst in n-butane ODH.[60] In this case, and due to the
moderate basicity of such promoters (as compared with alkali metals and revealed by
CO2-TPD), a less pronounced increase in selectivity to C4s was recorded. However, and
because there is much interest in obtaining butadiene directly from butane, this “tuned”
basicity seems to be very interesting because a very high selectivity to butadiene
was achieved. In fact, alkali-earth doped catalysts were almost twice as selective to
butadiene as was undoped NiMoO4. Nonetheless, an overdoping effect was also detected
Table 6. Catalytic data of oxidative dehydrogenation of butenes to butadiene.
Catalytic
system
T
(8C)Contact
time (min)
Conversion
(%)
Selectivity
(%)
Yield
(%)
Ni/Mo/Pa 538 15 38.8 72.2 28.0
538 60 36.0 74.8 27.0
538 180 35.7 74.0 26.4
482 15 26.3 82.8 21.8
482 60 25.5 77.4 19.7
482 180 24.2 74.0 17.9
Ni/Mo/Sbb 538 15 28.3 83.0 23.5
538 60 19.1 92.2 17.6
538 180 16.2 94.3 15.3
aCatalyst composition ¼ 38.3/26.1/5.3 (wt%). Experimental conditions: butene/oxygen/steamfeed rate ¼ 300/264/5780GHSV.bCatalyst composition ¼ 35.7/24.3/15.2 (wt%). Experimental conditions: butene/oxygen/steamfeed rate ¼ 300/264/5400GHSV.Source: Ref.[64].
Table 7. Catalytic data of oxidative dehydrogenation of n-butane to butenes and butadiene.a
Catalytic
system
Conversion
(%)
Total
selectivity
(%)
Yield C4H8
(%)
Yield C4H6
(%)
Total yield
(%)
Ni/Mo/Bib 10.0 33.6 1.7 1.6 3.3
Ni/Mo/Sbc 15.0 23.0 3.6 0.3 3.9
Ni/Mo/Asd 15.9 59.0 6.0 3.4 9.4
aExperimental conditions: butane/oxygen/steam feed rate ¼ 50/50/500 GHSV; temperature ¼ 5668CbComposition ¼ 36.0/24.4/15.8 (wt%).cComposition ¼ 35.7/24.3/15.2 (wt%).dComposition ¼ 35.7/24.3/13.8 (wt%).
Source: Ref.[64].
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for barium-doped catalysts that exhibit a maximum of selectivity (as well as a maximum of
basicity) at a 9% promoter loading (atomic ratio Ba :Mo).[60] Similar conclusions were
also recently reported by Liu et al. for propane oxydehydrogenation.[95] They found that
selectivity to propene increased with the barium load from 1% to 9%, but decreased with
the increase of its content from 12% to 15%, i.e., a maximum was found once again for a
molar ratio Ba :Mo of 9%. The 9% Ba–Ni0.9MoO4 catalyst is thus the most interesting
one, with a very good yield to propene of 30.5%.[95]
Although alkali (and also alkaline-earth) metals improve selectivity in n-butane ODH,
the conversion level usually decreases, and this becomes more pronounced as the promoter
loading (or size) increases. This effect results mainly from the decrease in the BET surface
area after doping.[59,60]
Stern and Grasselli[72] have tested some nickel-containing binary molybdates with the
formulas Ni0.5A0.5MoO4 (with A ¼ Co, Mg, Mn, or Zn) in propane ODH. Although these
catalysts have not shown better performance than NiMoO4, the most active and selective
one (Ni0.5Co0.5MoO4) was used as a reference to investigate in more detail the Ni–Co–
Mo–O system due to its great ease of preparation. With this catalyst a kinetic study was
then performed and a mechanism proposed for propane ODH,[120] as mentioned below.
These authors decided to study in detail the following systems in propane ODH:
Ni12xCoxMoO4 (with x between 0 and 1), AMo1+xOy (where A ¼ Ni or Co and x is
between 0 and 0.1), and promoted Ni0.5Co0.5MoOx.[72] It is worth noting that the results of
the variation of molybdenum content (x) between 0.9 and 1.1 in catalysts of the type
NiMoxOy have shown that catalytic activity to partial oxidation products is strongly
sensitive to Mo content. The catalyst of stoichiometric composition, NiMoO4, presented
Figure 12. Effect of different alkali promoters (1% loading, surface atomic ratio metal/molybdenum) on the catalytic behavior of a- and b-phases of NiMoO4 for n-butane ODH.
Experimental conditions as in Fig. 10. (Adapted from Ref.[59], with the kind permission of Elsevier
Science.)
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by far the greatest activity and the highest yield, both activity and yield significantly
decreasing with increasing or decreasing Mo content.
Mazzocchia et al. have also used doped-nickel molybdate catalysts in ODH reactions,
namely K-, Ca-, and P-doped for propane[61–63] or isobutane conversion.[62] It is
particularly noteworthy that with calcium and potassium addition, propane conversion
decreases but selectivity to propene increases, showing once again that basic sites
(assessed by catalytic decomposition of isopropanol in the absence of oxygen) are crucial
to the oxydehydrogenation process. On the other hand, promotion with phosphorous
(a typical acidic element) has led to enhancement of propane conversion, which was
ascribed to an increase in alkane adsorption on the more acid catalyst surface.[61,63]
Similarly, to previous results obtained by Madeira et al.,[90] it was found that these
promoters significantly increase the reduction resistance of nickel molybdate, indicating
that the mobility of the lattice oxygen has become less important.[61,63] It seems that the
catalytic activity of Ni–Mo–O-based catalysts for ODH of light alkanes is related to
catalyst reducibility, while selectivity to dehydrogenation products depends mainly on the
acid–base character of the catalyst surface. However, Kaddouri et al. concluded that
selectivity also depends on the degree of catalyst reduction, since propene formation is
improved with the pulse period in a periodic-flow reactor operation.[61]
With regard to isobutane conversion, it was found that the catalytic performances of
stoichiometric nickel molybdate, in terms of selectivity for isobutene, can be improved by
the addition of potassium oxide, which avoids subsequent overoxidation of the reactive
isobutene formed. However, methacrolein formation is negatively affected (see Table 8).
The great interest in producing methacrolein, which is widely used to produce methacrylic
Figure 13. Selectivity to dehydrogenation products (at an n-butane conversion level of 5%) as
a function of catalyst surface basicity (amount of adsorbed CO2 at 308C) for unpromoted and
Cs-promoteda-NiMoO4 catalysts. (Adapted fromRef.[83], with the kind permission of Elsevier Science.)
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acid for the polymer industry, led Kaddouri et al. to try to avoid this negative effect.[62]
This was achieved by using low oxygen partial pressure in the feed, which also increases
isobutene selectivity, due to a decrease in formation of carbon oxides.
The use of supported catalysts in the oxydehydrogenation of light alkanes is
uncommon. Recent studies have been performed on isobutane ODH, using TiO2[48] or
SiO2[49,52] as supports. Such catalysts have been shown to be more selective to isobutene
than unsupported NiMoO4, which was attributed to the acid–base properties of the
surface[52] or to b-phase stabilization at low temperature.[48,49] This is very important
because the b-phase is metastable at room temperature and, in industrial practice, it is not
practical to keep reactor operation above 2508C to avoid the transition to the a-phase.
Furthermore, incorporation of excess nickel to stabilize the b-phase at low temperatures
favors competitive side reactions. Silica-supported nickel molybdate catalysts, prepared
through sol–gel procedures, were also tested in isobutane ODH,[50,51] but no significant
yields were obtained.
A special mention should be made of the unexpected effects found after coke deposition
on nickel molybdate catalyst.[96] First, and surprisingly, it was observed that, with this depo-
sition, deactivation was not found during n-butane ODH. On the contrary, conversion
increased (more than 40%), as well as selectivity to dehydrogenation products, and
particularly to butadiene. These effects were attributed to the stabilization of the more
selective b-phase at low temperatures and to the presence of catalytically active coke.[96]
5.2.3. Kinetics and Mechanism
One of the few kinetic and mechanistic studies found in the literature performed with
Ni–Mo–O catalysts for propane oxydehydrogenation was published by Stern and
Grasselli.[120] It was carried out with the above-mentioned Ni0.5Co0.5MoO4/SiO2 catalyst
and the results showed that the reaction proceeds through ODH, propene being formed as
Table 8. Typical results of oxidative dehydrogenation of isobutane to isobutene and methacrolein,
at 8% conversion level.a
Catalysts
Selectivity Surface
area
(m2/g)i-C4H8 CH255CCH3CHO CO CO2 (CH3)2CO
a-NiMoO4 25.5 12.5 27.7 34.3 0.0 32.2
a-NiMoO4/0.20%K 44.4 11.5 14.7 27.8 1.6 30.7
a-NiMoO4/0.25%K 52.2 9.5 11.8 24.6 1.9 29.4
b-NiMoO4 41.3 15.5 17.9 22.7 2.6 13.1
b-NiMoO4/0.20%K 66.0 6.4 5.7 20.9 1.0 10.6
b-NiMoO4/0.25%K 69.2 6.3 6.2 12.6 5.7 9.8
a0-NiMoO4/0.20%Kb 60.1 8.4 8.3 22.6 0.6 10.6
a0-NiMoO4/0.25%Kb 69.1 11.0 7.2 12.1 0.5 9.8
aExperimental conditions: W ¼ 0.5 g; %i-C4H10 ¼ 15; 4 , %O2 , 7; 4208C , T , 4808C;10 , F , 20L/hr.ba0-NiMoO4 is obtained after cooling the b phase to room temperature.
Source: Ref.[62].
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the primary and exclusive product. Formed propene is first oxidized to acrolein, which is
then oxidized to carbon oxides and acrylic acid. However, small amounts of COx are also
formed directly from propene. The selective oxidations of propane to propene and of
propene to acrolein are both zero-order in oxygen (a common behavior found in the
oxidation of hydrocarbons over metallic oxides and consistent with the redox mechanism
of Mars and van Krevelen)[118] and first-order with respect to the hydrocarbon (consistent
with a rate-limiting reaction between the hydrocarbon and an active site on the catalyst
surface). The deep oxidation of propane into CO and CO2 has an order of 1/2 in oxygen
and is of first-order with respect to propane, while the deep propene oxidation shows
dependence on the hydrocarbon concentration of the type r / kx/(1 þ Kx), that is, the
rate-limiting step involves a surface species that is in adsorption equilibrium with the gas
phase and is also of order 1/2 in oxygen. To conclude, the work by Stern and Grasselli[120]
showed that the partial and deep oxidation of the hydrocarbons on the Ni–Co–Mo–O
system occurs through two different mechanisms. The partial oxidation of propane to
propene and of this product to acrolein can be described by the Mars–van Krevelen
mechanism,[118] in which the adsorbed hydrocarbon reacts with the lattice oxygen
(nucleophilic). On the other hand, deep oxidation of propene to COx can be described by a
Langmuir–Hinshelwood mechanism in which the adsorbed hydrocarbon reacts with
adsorbed and dissociated oxygen (electrophilic). Finally, it should be noted that isotopic
studies in propane and propene activation have revealed that hydrogen abstraction from a
methylene group and in allylic position (a-hydrogen) are the respective rate-controlling
steps.[120]
The greater selectivity presented by the b-phase in some oxydehydrogenation
reactions was the reason that impelled Sautel et al. to perform a detailed kinetic study on
propane to propene ODH, trying likewise to clarify some important aspects of the reaction
mechanism.[121] For the conditions used, it was found that the propene formation rate
presents partial orders to propane and to oxygen close to one and zero, more precisely
0.95 + 0.01 and 0.03 + 0.02, respectively (see Fig. 14), and therefore gaseous oxygen is
not directly responsible for the ODH reaction. This is indicative of a Mars–van Krevelen
type mechanism. Over a-NiMoO4, Del Rosso et al. also found that a change in the oxygen
partial pressure does not affect propene formation, which is instead dependent on the
propane partial pressure. Indeed, for the propene formation rate partial orders with respect
to propane and oxygen of 1.2 + 0.01 and 0.04 + 0.01, respectively, were found.[122] The
involvement of lattice oxygen was then confirmed using continuous, transient, and
periodic operating systems.[122]
For the more selective b-NiMoO4, it was proposed that the global reaction:
C3H8 þ1
2O2 ! C3H6 þ H2O (19)
can be written in six steps:[121] (1) propane adsorption on the catalyst surface; (2) oxidation
of the adsorbed propane by the oxygen of the NiMoO4 lattice; (3) and (4) desorption of the
products (propene and water); (5) and (6) oxygen adsorption on the catalyst surface and
filling of the oxygen vacancies in the solid. Assuming a rate-determining step and that
the others are at near-equilibrium conditions, the theoretical rate equations were deduced
and were compared with the experimental data. It was possible to conclude that the
rate-limiting step that controls the overall reaction rate can be either propane adsorption
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(step 1) or the reaction between adsorbed propane and the oxygen of the NiMoO4 lattice
(step 2).[121]
A kinetic study with b-phase nickel molybdate was also reported for ethane ODH, in
which interesting and even uncommon results were obtained.[123] First, it was found that in
ethane oxydehydrogenation the a-phase is not only more active but also more selective
than the b-phase, therefore behaving differently from the case of propane ODH. Secondly,
it was found that for the b-phase the overall rate of ethane conversion can be described by:
rC2H6¼
d(C2H6)
dt¼ k(C2H6)
1:15(O2)0:21 (20)
with a reaction order of 0.16 for C2H4 formation with respect to oxygen. In addition, tests
performed under continuous flow in the absence of gas phase oxygen have led to the
conclusion that no ethylene is formed. Thus, and unlike the case of propane ODH, lattice
oxygen does not seem to guide the reaction towards dehydrogenation. For this case, a
mechanism that considers the intervention of surface O2 species was proposed, in which the
activation of ethane involves hydrogen abstraction by these species to give ethyl radicals.[123]
Regarding butane conversion, a kinetic study of the selective oxidation and
degradation of n-butane over undoped and cesium-doped nickel molybdates was recently
reported, covering a wide range of experimental conditions.[90] The rate data were fitted to
power-law rate equations, i.e.:
ri ¼ kPn1butaneP
n2O2
(21)
The computed reaction orders n1 and n2 showed that Cs doping only affects the partial
order with respect to butane, which increases for dehydrogenation products and decreases
for CO and CO2. The partial order with respect to oxygen is almost unaffected, in both
cases, i.e. with undoped or Cs-doped NiMoO4, showing zero-order dependence for
formation of C4s, which suggested the existence of a Mars–van Krevelen process.[90]
In a Mars–van Krevelen (or redox) model[118] it is assumed that hydrocarbon reacts
with the lattice oxygen of an oxidation catalyst, which becomes reduced. The reduced
catalyst then reacts with molecular oxygen from the gas phase to complete the catalytic
cycle.
Figure 14. Evolution of the rate of formation of propene with the partial pressure of propane (for
PO2¼ 18 � 103 Pa) and oxygen (for Ppropane ¼ 15 � 103 Pa) introduced with b-NiMoO4. (From
Ref.[121], with the kind permission of Elsevier Science.)
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A generalized Mars–van Krevelen model was applied by Madeira and Portela[124,125]
to the results of the above-mentioned kinetic study of the selective oxidation and
degradation of n-butane over undoped and cesium-doped nickel molybdates. By nonlinear
regression analysis, the following rate equations were obtained:
ri ¼kokrPbutane
ko þ akrPbutane
(22)
for the products of selective butane conversion (butenes and butadiene)[124] and
ri ¼kokrP
1=2O2
P2butane
koP1=2O2þ akrP
2butane
(23)
for the degradation products of butane (CO and CO2), over the pure NiMoO4 catalyst,[125]
where
ri ¼ butane conversion rate to specified products;
ko, kr ¼ kinetic constants for the reoxidation and reduction steps, respectively;
PO2, Pbutane ¼ oxygen and butane partial pressure, respectively;
a ¼ stoichiometric number of oxygen moles required in the reaction.
Other studies performed with nickel molybdate catalysts also supported a Mars–van
Krevelen mechanism for n-butane ODH. Evidence for such a mechanism includes: (i) the
zero-order dependence on the oxygen partial pressure for C4 formation,[90] (ii) the
relationship between catalytic activity and reducibility (inferred by the temperature of
onset of reduction) of several Cs-doped catalysts,[90] (iii) the similar apparent activation
energies for butane conversion with and without gas phase oxygen,[126] (iv) the fact that no
oxygen adsorption was observed by O2-TPD,[124] and (v) catalytic tests without O2 in the
feed, which showed that butane can be converted to C4 products with high selectivity even
without gas phase oxygen.[126] The tests performed in the absence of oxygen showed that
lattice oxygen plays a crucial role in selectivity during butane conversion, as is also the
case for propane ODH.[122] In addition, the very low butane conversion levels achieved
(typically below 1% as compared to runs with oxygen in the gas phase—between 2.9%
and 7.9%),[126] indicate that alkane conversion is limited by the reducibility of the catalyst
or by lattice oxygen availability and mobility. However, such low conversion levels
(,1%) are typically consistent with an oxygen consumption that corresponds to a small
percentage of the mobile oxygen content of the catalyst monolayer, as shown by Del Rosso
et al.[122] Therefore, the reaction seems to be controlled by the reducibility of the catalyst
(or by the oxygen diffusion within the solid).
More recently, the existence of a redox mechanism in n-butane ODH over nickel
molybdate catalysts was further supported by an in situ study of the catalyst’s electrical
conductivity.[92] When the catalysts were subjected to a sequence of gaseous atmospheres
of the type: oxygen–butane–oxygen-reaction mixture, a reversible redox process was
observed, for both undoped and Cs-doped NiMoO4 (Fig. 15). The sharp increase in the
electrical conductivity (s) recorded when pure butane is introduced in the cell is attributed
to the release of electrons into the conduction band during surface reduction:
C4H10 þ (OO)S �! C4H8 þ H2Oþ VooO þ 2e� (24)
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When oxygen is reintroduced, s reversibly decreases and practically returns to its initial
value, this process corresponding to the filling of the vacancies by gas phase oxygen in
accordance with the following equation:
VooO þ 2e� þ
1
2O2(g) �! (OO)S (25)
In the steady state, the electrical conductivity of the NiMoO4 catalyst under the reac-
tion mixture is much closer to that of the oxidized state than to that of the reduced one (see
Fig. 15), which agrees with the faster reoxidation as compared to reduction of the
catalyst.[92] The n-type semiconductor behavior of nickel molybdate was also
demonstrated, because (@s/@t)O 2, 0 and (@s/@t)C4H10
. 0.
The kinetics and mechanism of isobutane ODH over nickel molybdate were also
recently investigated.[127] It was concluded that isobutene is formed via a redox
mechanism with the participation of lattice oxygen, while the formation of carbon oxides
occurs with the participation of chemisorbed oxygen.
5.3. Nature of Active Sites
The nature of the active sites in the selective oxidation of hydrocarbons has been
widely investigated for molybdenum-containing catalysts,[7] and the use of surface-
sensitive techniques has made a crucial contribution. We should mention, for instance, the
very recent study by Watson and Ozkan,[128] in which ESR was used to investigate
changes in Mo(V) species upon contact with propane.
Figure 15. Kinetics of the changes in electrical conductivity of unpromoted and Cs-promoted
a-NiMoO4 catalysts under different atmospheres (at 3758C). (From Ref.[92], with the kind
permission of Elsevier Science.)
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The role of Mo5þ as an active species is well recognized by several authors. For
instance, Abello et al.[129,130] suggest the involvement of Mo5þ species (for Mg–Mo–O
catalysts) in propane ODH. Characterization by several techniques, particularly XPS and
EPR, gave clues that the active site would be a coordinatively unsaturated form of Mo5þ,
which could be generated on the surface by propane reduction. Khan and Somorjai[131]
proposed that the active sites in molybdenum-based catalysts are coupled pairs of Mo(V)
and Mo(VI) species, which are responsible for the redox mechanism. More recently, the
importance of the Mo(V)/Mo(VI) redox pair in propane ODH over molybdenum
phosphates was also pointed out.[132]
Although some consensus seems to exist regarding the active species for molybdenum-
containing catalysts, other active species are also suggested. For instance, Harlin et al.
claim that the oxidation state of molybdenum active in the dehydrogenation of n-butane
is either Mo5þ or MO4þ, which upon reduction to lower oxidation states leads to a
catalyst responsible for increased selectivity to cracking and coke formation.[133] Other
authors found instead that catalyst performance (Mg–Mo–O and Co–Mo–O systems)
during propane ODH increases in parallel with the increase of surface weak acid sites,
suggesting that these sites are involved in propane activation.[134] An excess of octa-
hedral molybdenum, partially covering the surface, seems to be responsible for the weak
acid sites.
For Ni–Mo–O catalysts, it is also generally accepted that the active site involves
Mo5þ species. Mazzocchia et al., while investigating butane to butadiene and maleic
anhydride oxidation, found practically the same yield of butenes when the number of
butane pulses was increased, suggesting that butane dehydrogenation occurs without the
intervention of lattice oxygen.[35] Moreover, maleic anhydride formation is related to
gaseous oxygen activation over sites that disappear by deep reduction and that cannot be
regenerated by reoxidation. Such sites probably correspond to Mo(V) sites. Although no
unequivocal and definitive proof exists that establishes the nature of the active and/orselective sites involved in selective oxidation reactions, other authors also seem to agree
that the redox couple Mo6þ/Mo5þ is necessary for the ODH step, particularly for propane
conversion.[41] However, further research using surface sensitive techniques for
characterization of NiMoO4 catalysts, of which there have so far been rather few, is urgent.
6. CONCLUSIONS AND FUTURE TRENDS
In this paper, the main scientific publications regarding the Ni–Mo–O catalytic
system and its applications for the selective oxidation of hydrocarbons were reviewed.
Particular attention was also dedicated to the preparation techniques that have been used
and to the main physical–chemical characterization data.
With regard to the preparation techniques, most studies found in the literature have
focused on the use of the coprecipitation method. These techniques have been
progressively refined considering that their preparation involves a sequence of unit
operations, each requiring the optimization of its specific parameters. In this context it is
important to note the work of Mazzocchia et al., which led to significant scientific and
technical results detailed in Refs.[32,37,106,121,135,136]. At the same time the application of
increasingly sophisticated characterization techniques was essential for fundamental
results contributing to the optimization and applicability of the prepared catalysts.[33,91,137]
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The use of supported Ni–Mo–O catalysts is now the subject of more inves-
tigation,[48,49] particularly catalysts prepared by sol–gel techniques.[50,51] This is a topic
that, in our opinion, should be further explored because interesting results have already
been obtained, particularly the stabilization of the high temperature b-phase at room
temperature. Since products obtained by sol–gel techniques depend strongly on all
experimental conditions, which play an important role in determining their physi-
cochemical characteristics and consequently their catalytic behavior, further fundamental
research on this subject is required.
Mixed Ni–Mg molybdates were also recently synthesized by Soares et al.[65] via the
sol–gel technique and revealed interesting catalytic behavior in n-butane ODH. C4 yields
were significantly improved using mixed molybdates, particularly for an Mg/Ni atomic
ratio of 0.31. But this behavior has not been fully clarified.
Nanosized materials are another promising field for research. It is known that the
catalytic properties of mixed metal oxides are largely dependent on their microstructure.
In the nanoparticle phase, the surface to volume ratio increases drastically and the surface
atoms include an increasing fraction of the total particulate volume with high defect
structures. Thus, they may exhibit interesting new or improved catalytic properties.
Nanocrystalline NiMoO4, with particle size of about 20 nm, has been prepared,[42,43] but
no catalytic data have been reported.
Regarding the application of Ni–Mo–O catalysts in oxidation reactions, the direct
oxidation of propane deserves special mention. It is known that the most important
industrial process for synthesis of acrylic acid involves two steps, the first one being
oxidation of propene to acrolein, which is further oxidized in the second stage to acrylic
acid. The most promising route would be the direct oxidation of propane to acrylic acid in
a single step. This was successfully tested by Kaddouri et al.[66] but their yields were not
sufficient for industrial application. More recently, Fujikawa et al.[68] found that nickel
molybdates modified with telluromolybdate are good catalysts for this reaction, with a
maximum yield of 20% (acrylic acid and acrolein). However, because of the instability
and toxicity of tellurium compounds, other catalysts should be tested in the future.
Several nickel–molybdenum-based catalytic formulations have already proved
promising for hydrocarbon oxidation reactions. The continuous screening of new classes
of catalytic materials and their optimization has been a great challenge, but it is very time
consuming. However, high-throughput experimentation methodology has been success-
fully used for other applications,[138] and is an important approach to be considered for
accelerated catalyst design, evaluation, and development. Besides, this technique is
particularly promising for the development of novel multicomponent mixed metal oxide
catalysts through rapid microscale synthesis and catalytic screening, using combinatorial
methods.
Although the data found in the literature for ODH reactions with Ni–Mo–O catalysts
do not point to very high levels of paraffin conversion, it seems that these oxides can operate
at low conversions, at which selectivity to dehydrogenation is quite favorable. Indeed,
according to Cavani and Trifiro,[116] the various molybdates (among them nickel) present a
common feature in ODH reactions, which is a decrease of selectivity to olefins when the
hydrocarbon conversion level increases (e.g., n-butane). For this reason these catalysts need
to operate at low conversions, with recycling of the nonconverted paraffin. But among the
required properties for heterogeneous oxidation catalysts (activity, selectivity, and stability),
selectivity to desired product(s) is crucial since the production of undesirable products
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increases costs at the industrial level.[5] Therefore, many authors have investigated the
applicability of nickel molybdates in such reactions and, among other aspects, have tried to
increase the selectivity of the catalyst to the desired oxidation products, particularly by the
use of promoters. Promising results have been achieved, but for very low hydrocarbon
conversion levels, and the yields obtained do not yet recommend the use of such processes
on an industrial scale, as an alternative to conventional dehydrogenation processes.
Despite the large number of studies found in the literature concerning the use of
Ni–Mo–O catalysts for the selective oxidation of hydrocarbons, and particularly for
the ODH of light alkanes, some important aspects, such as the nature of the active sites, the
hydrocarbon activation process, the kinetics and mechanism of the reaction, and the
factors that determine selectivity, are not yet sufficiently clear.
It has been shown that nickel–molybdenum catalysts are efficient for several oxidation
processes. Their use for isobutane conversion into isobutene is an application that should be
explored further in the future. Isobutene is a key reactant for the production of methyl t-butyl
ether (MTBE) and ethyl t-butyl ether (ETBE), which are used as lead-free octane boosters
for gasoline.[139,140] The use of MTBE has given rise to significant pollution problems not
found with ETBE, which is nowmuch in demand as a lead-free gasoline additive, improving
the combustion process, and decreasing carbon monoxide emissions. So, to produce the
amounts of ETBE required, additional sources of isobutene will be needed. Recently
some studies have been performed using TiO2-[48] and SiO2-supported
[49,52] NiMoO4 for the
oxydehydrogenation of isobutane. Although these catalysts have proved to be more selective
to isobutene than unsupported NiMoO4, which was attributed to b-phase stabilization at low
temperature[48,49] and to the acid–base properties of the surface,[52] the yields obtained were
not sufficiently high and the formation of many undesirable products was reported, namely
carbon oxides, products from cracking reactions (light alkanes), and heavy organic
compounds that condense at the reactor outlet or remain on the catalyst surface (coke). Other
studies have also been recently reported, but the isobutene yields attained were even
lower.[50,51,62] This subject is worth further investigation to achieve higher performances in
isobutane to isobutene conversion.
In the ODH of light alkanes, some studies suggest that, in practice, it would be
advantageous to operate with a high alkane/oxygen ratio in the gas-phase feed in order to
favor selectivity for dehydrogenation products. This agrees with the fact that a higher
partial order in oxygen was observed for formation of carbon oxides (CO and CO2)
compared with that found for the desired selective oxidation process (see Refs.[62,90]).
However, various groups have found difficulties in this operation. For instance, Madeira
et al.[90] found catalyst reduction and strong coke deposition when using high butane
concentrations in the feed of a tubular reactor containing undoped a-NiMoO4. Similar
problems were also found by Del Rosso et al. when operating a periodic flow reactor under
severe conditions (i.e., high temperature, long pulse periods, high propane partial pressure,
etc.) during propane ODH.[122,141] In this case, the deep reduction depleted more than the
surface oxygen monolayer, leading to irreversible deactivation of the catalyst with
formation of metallic nickel and coke filaments. A possible way to avoid this drawback
would be the use of promoters to inhibit such reduction, or the use of accelerators in the
gas phase. For instance, it was recently reported that Cs partially inhibits NiMoO4
reduction under pure butane, but completely inhibits it under a reaction mixture containing
butane and oxygen.[92] Thus, the promoter keeps the catalyst in a higher oxidation state,
allowing the use of higher alkane concentrations. Cs-doped NiMoO4 catalysts, therefore,
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seem to be promising for reactor designs in which operation is achieved in the absence of
gas-phase oxygen, with the consumed lattice oxygen periodically restored by exposure to
an oxygen-containing gas, i.e., in a cyclical process similar to that proposed by Boutry
et al.[142] This can be done in practice using multiple reactors or catalytic reactors with
mobile or fluidized beds. Concerning the use of accelerators in the gas phase, which are
applied in some industrial processes, Dury et al.[143] found very recently that the addition
of CO2 to the gas stream helps to maintain the catalytic surface of NiMoO4 in a high
oxidation state, under conditions for which the catalyst undergoes reduction with
consequent deactivation. In specific conditions it can also increase propene formation
from propane, thus suggesting the possible utilization of CO2 in industrial applications.
Some of the most important works concerning the ODH of alkanes have indicated the
possibility of using riser reactors in order to obtain results of industrial interest to alkenes.
The above-reviewed data have clearly demonstrated the redox properties of NiMoO4. Also,
tests performed by the periodic operation technique have determined the degree of optimal
reduction of NiMoO4 for its regeneration and also for the deposit conditions of coke. These
results make it possible to propose technologies based on riser reactors. The mechanical
properties of NiMoO4 catalytic systems are a fundamental factor for the success of these
technologies. In fact for such technologies the mechanical properties of the catalyst are
essential, since it has to resist both abrasion and circulation, because it has to be moved from
the reaction reactor to the regeneration reactor and back to the reaction reactor.
For the improvement of the mechanical properties of the catalysts there are some
noteworthy works in which microwave technology has been used for unit operations of
drying and calcination. The solids obtained after these operations, at the end of the
microwave thermal treatments, have better mechanical properties than those that have
undergone conventional thermal treatments.[144,145]
For technologies based on riser reactors there are two steps requiring optimization of
the reaction parameters:
1. During the first step a reaction between the alkane and lattice oxygen takes place
and, therefore, the contact time, temperature, and alkane/catalyst ratio have to beoptimized in the reactor to ensure maximum reduction of the catalyst allowing
both its reoxidation and optimal control of coke formation.
2. During the second step catalyst reoxidation occurs and it is essential to establish
the temperature, contact time, and oxygen partial pressure precisely, to prevent
harmful structural and morphological changes during reoxidation.
The use of catalytic membrane reactors (CMRs) is also a promising alternative since they
enable control of oxygen distribution, maintaining its partial pressure sufficiently low
within the catalytic bed.[146] In addition, generated heat is also more regularly distributed
along the bed, considerably decreasing hot-spot formation and leading to safer and more
stable operation. The catalyst charge can also be increased in this way.
The application of CMRs for oxidation reactions, particularly for partial oxidation of
hydrocarbons, was very recently patented by Schwartz et al.[147] Their invention relates to
the use of a gas-impermeable membrane for transport of oxygen anions, the membrane
separating oxidation and reduction zones. An oxygen-containing gas is reduced at the
membrane in the reduction zone (thus generating oxygen anions) while a species in a
reactant gas is oxidized in the oxidation zone of the reactor. This technology is claimed to
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be useful for several reactions, including partial oxidation of hydrocarbons to oxygenated
species and ODH of alkanes, using several catalysts and different configurations.[147] In
this approach, Ni–Mo–O based catalysts could also be considered. But for successful
industrial application, both catalyst development and process engineering, particularly
reactor design, must be considered simultaneously.
REFERENCES
1. McKetta, J.J. Chemical Processing Handbook; Marcel Dekker: New York, 1993.
2. Bielanski, A.; Haber, J. Oxygen in Catalysis; Marcel Dekker: New York, 1991.
3. Arpentinier, P.; Cavani, F.; Trifiro, F. The Technology of Catalytic Oxidations. 1.
Chemical, Catalytic and Engineering Aspects; Editions Technip: Paris, 2001; Vol. 1.
4. Hodnett, B.K. Heterogeneous Catalytic Oxidation: Fundamental and Technological
Aspects of the Selective and Total Oxidation of Organic Compounds; John Wiley &
Sons: Chichester, 2000.
5. Centi, G.; Cavani, F.; Trifiro, F. Selective Oxidation by Heterogeneous Catalysis;
Kluwer Academic/Plenum Publishers: New York, 2001.
6. Kung, H.H. Oxidative dehydrogenation of light (C2 to C4) alkanes. In Advances in
Catalysis; Eley, D.D., Pines, H., Haag, W.O., Eds.; Academic Press: New York,
1994; Vol. 40, 1–38.
7. Madeira, L.M.; Portela, M.F. Catalytic oxidative dehydrogenation of n-butane.
Catal. Rev. Sci. Eng. 2002, 44 (2), 247–286.
8. Vasudevan, P.T.; Fierro, J.L.G. A review of deep hydrodesulfurization catalysis.
Catal. Rev. Sci. Eng. 1996, 38 (2), 161–188.
9. Brito, J.L.; Barbosa, A.L. Effect of phase composition of the oxidic precursor on the
HDS activity of the sulfided molybdates of Fe(II), Co(II), and Ni(II). J. Catal. 1997,
171 (2), 467–475.
10. Silva, V.L.S.T.; Frety, R.; Schmal, M. Activation and regeneration of a NiMo/Al2O3
hydrotreatment catalyst. Ind. Eng. Chem. Res. 1994, 33 (7), 1692–1699.
11. Brito, J.L.; Barboza, A.L.; Albornoz, A.L.; Severino, F.; Laine, J. Nickel molybdate
as precursor of HDS catalysts: effect of phase composition. Catal. Lett. 1994, 26,
329–337.
12. Laine, J.; Pratt, K.C.; Trimm, D.L. Factors affecting the preparation of supported
nickel-molybdenum hydrodesulphurization catalysts. J. Chem. Technol. Biot. 1979,
29 (7), 397–403.
13. Klimova, T.; Casados, D.S.; Ramırez, J. New selective Mo and NiMo HDS catalysts
supported on Al2O3–MgO(x) mixed oxides. Catal. Today 1998, 43 (1–2), 135–146.
14. Wei, Z.B.; Yan, W.; Zhang, H.; Ren, T.; Xin, Q.; Li, Z. Hydrodesulfurization activity
of NiMo/TiO2-Al2O3 catalysts. Appl. Catal. A: Gen. 1998, 167 (1), 39–48.
15. Brito, J.L.; Severino, F.; Delgado, N.N.; Laine, J. HDS activity of carbon-supported
Ni–Mo catalysts derived from thiomolybdate complexes. Appl. Catal. A: Gen. 1998,
173 (2), 193–199.
16. Vazquez, P.; Pizzio, L.; Blanco, M.; Caceres, C.; Thomas, H.; Arriagada, R.;
Bendezu, S.; Cid, R.; Garcıa, R. NiMo(W)-based hydrotreatment catalysts supported
on peach stones activated carbon. Appl. Catal. A: Gen. 1999, 184 (2), 303–313.
Madeira, Portela, and Mazzocchia102
Dow
nloa
ded
by [
UN
AM
Ciu
dad
Uni
vers
itari
a] a
t 11:
19 2
4 Fe
brua
ry 2
012
ORDER REPRINTS
17. Chu, Y.; Wei, Z.; Yang, S.; Li, C.; Xin, Q.; Min, E. NiMoN/g-Al2O3 catalyst for
HDN of pyridine. Appl. Catal. A: Gen. 1999, 176 (1), 17–26.
18. Andreev, A.A.; Kafedjiysky, V.J.; Edreva-Kardjieva, R.M. Active forms for water-
gas shift reaction on NiMo-sulfide catalysts. Appl. Catal. A: Gen. 1999, 179 (1–2),
223–228.
19. Borowiecki, T.; Giecko, G.; Panczyk, M. Effects of small MoO3 additions on the
properties of nickel catalysts for the steam reforming of hydrocarbons. II. Ni–Mo/Al2O3 catalysts in reforming, hydrogenolysis and cracking of n-butane. Appl. Catal.
A: Gen. 2002, 230 (1–2), 85–97.
20. Driscoll, S.A.; Zhang, L.; Ozkan, U.S. Oxidative coupling of methane over alkali-
promoted simple molybdate catalysts. InCatalytic Selective Oxidation; Oyama, S.T.,
Hightower, J.W., Eds.; ACS Symposium Series No. 523; American Chemical
Society: Washington, DC, 1993; 340–353.
21. Li, D.; Nishijima, A.; Morris, D.E. Zeolite-supported Ni and Mo catalysts for
hydrotreatments. I. Catalytic activity and spectroscopy. J. Catal. 1999, 182 (2),
339–348.
22. Tsurov, M.A.; Afanasiev, P.V.; Lunin, V.V. Composition and catalytic properties of
products from the reduction of NiMoO4. Appl. Catal. A 1993, 105 (2), 205–221.
23. Yasuda, H.; Higo, M.; Yoshitomi, S.; Sato, T.; Imamura, M.; Matsubayashi, H.;
Shimada, H.; Nishijima, A.; Yoshimura, Y. Hydrogenation of tetralin over sulfided
nickel–tungstate/alumina and nickel–molybdate/alumina catalysts. Catal. Today
1997, 39 (1–2), 77–87.
24. Sarbak, Z. NiMo Catalysts supported on chromium modified zeolites of type
X and Y—their structure and HDS activity. Appl. Catal. A: Gen. 2001, 207 (1–2),
309–314.
25. Andrushkevich, M.M.; Buyanov, R.A.; Sitnikov, V.G.; Itenberg, I.Sh.;
Khramova, G.A. Kinet. Katal. 1973, 14, 464.
26. Plyasova, L.M.; Ivanchenko, I.Yu.; Andrushkevich, M.M.; Buyanov, R.A.;
Itenberg, I.Sh.; Khramova, G.A.; Karakchiev, L.G.; Kustova, G.N.;
Stepanov, G.A.; Tsailingol’d, A.L.; Pilipenko, F.S. Study of the phase composition
of nickel-molybdenum catalysts. Kinet. Catal. 1973, 14 (4), 882–886.
27. Andrushkevich, M.M.; Buyanov, R.A.; Khramova, G.A.; Sitnikov, V.G.;
Itenberg, I.Sh.; Plyasova, L.M.; Kustova, G.N.; Stepanov, G.A.; Tsailingol’d, A.L.;
Pilipenko, F.S. The production of nickel-molybdenum catalysts. Kinet. Catal. 1973,
14 (4), 887–891.
28. Vagin, A.I.; Burmistrova, N.V.; Erofeev, V.I. Reduction kinetics of NiO–MoO3
catalysts. React. Kinet. Catal. Lett. 1985, 28 (1), 47–52.
29. Brito, J.L.; Laine, J.; Pratt, K.C. Temperature programmed reduction of Ni–Mo
oxides. J. Mater. Sci. 1989, 24, 425–431.
30. Mazzocchia, C.; Del Rosso, R.; Centola, P. Oxidacao em Fase Vapor do Buteno-1
a Anidrido Maleico em Presenca de Molibdato de Nıquel (vapor phase oxidation
of 1-butene to maleic anhydride over nickel molybdate). Rev. Port. Quim. 1977,
19 (1–4), 61–66.
31. Mazzocchia, C.; Di Renzo, F.; Centola, P.; Del Rosso, R. Correlations between
propene oxidation selectivity and physico-chemical features of NiO–MoO3
catalysts. In Chemistry and Uses of Molybdenum; Barry, H.F., Mitchell, P.C.H.,
Eds.; Golden (CO.), 1983; 406–410.
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 103
Dow
nloa
ded
by [
UN
AM
Ciu
dad
Uni
vers
itari
a] a
t 11:
19 2
4 Fe
brua
ry 2
012
ORDER REPRINTS
32. Mazzocchia, C.; Tempesti, E.; Aboumrad, C. Catalyst for Oxidative Dehydrogena-
tion of Propane. US Patent 5,086,032, February 4, 1992 (Norsolor); European Patent
90-400137, 1990 (Norsolor); French Patent 89-00522, January 18, 1989 (Norsolor).
33. Mazzocchia, C.; Anouchinsky, R.; Kaddouri, A.; Sautel, M.; Thomas, G. Thermal
activation of typical oxidative dehydrogenation catalyst precursors belonging to the
Ni-Mo-O system. J. Therm. Anal. 1993, 40 (3), 1253–1265.
34. Mazzocchia, C.; Kaddouri, A.; Anouchinsky, R.; Sautel, M.; Thomas, G. On the
NiO, MoO3 mixed oxide correlation between preparative procedures, thermal
activation and catalytic properties. Solid State Ionics 1993, 63–65, 731–735.
35. Mazzocchia, C.; Del Rosso, R.; Centola, P. Oxidacion Selectiva del Butano en
Presencia de Catalizadores del Sistema NiO–MoO3 (selective oxidation of butane in
the presence of NiO-MoO3 catalysts). An. Quim. 1983, 79 (1), 108–113.
36. Di Renzo, F.; Mazzocchia, C. How thermal treatment influences the phase transition
of NiMoO4. Thermochim. Acta 1985, 85 (1), 139–142.
37. Mazzocchia, C.; Aboumrad, C.; Diagne, C.; Tempesti, E.; Herrmann, J.M.;
Thomas, G. On the NiMoO4 oxidative dehydrogenation of propane to propene: some
physical correlations with the catalytic activity. Catal. Lett. 1991, 10 (3–4),
181–192.
38. Zou, J.Y.; Schrader, G.L. Selective oxidation over structured multicomponent
molybdate catalysts. In New Developments in Selective Oxidation II; Corberan, V.C.,
Bellon, S.V., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam,
1994; Vol. 82, 19–26.
39. Zou, J.Y.; Schrader, G.L. Deposition of multiphase molybdate thin films by reactive
sputtering. Thin Solid Films 1998, 324 (1–2), 52–62.
40. Anouchinsky, R.; Kaddouri, A.; Mazzocchia, C. Effects of thermal treatments on a
NiMoO4 precursor prepared by immobilizing Niaq and MoO4aq in an organic matrix.
J. Therm. Anal. 1996, 47 (1), 299–309.
41. Lezla, O.; Bordes, E.; Courtine, P.; Hecquet, G. Synergetic effects in the Ni-Mo-O
system. Influence of preparation on catalytic performance in the oxidative
dehydrogenation of propane. J. Catal. 1997, 170 (2), 346–356.
42. Sen, A.; Pramanik, P. Low-temperature synthesis of nano-sized metal molybdate
powders. Mater. Lett. 2001, 50 (5–6), 287–294.
43. Sen, A.; Pramanik, P. A chemical synthetic route for the preparation of fine-grained
metal molybdate powders. Mater. Lett. 2002, 52 (1–2), 140–146.
44. Mazzocchia, C.; Di Renzo, F.; Mari, C. Correlations between structure, electrical
properties and reactivity of a nickel molybdate catalyst. In Reactivity of Solids;
Barret, P., Dufour, L.-C., Eds.; Elsevier: Amsterdam, 1985; 1061–1066.
45. Ozkan, U.; Schrader, G.L. NiMoO4 selective oxidation catalysts containing excess
MoO3 for the conversion of C4 hydrocarbons to maleic anhydride. I. Preparation and
characterization. J. Catal. 1985, 95, 120–136.
46. Di Renzo, F.; Mazzocchia, C.; Thomas, G.; Vernay, A.M. Formation and properties
of the solid solution of NiO in NiMoO4. React. Solids 1988, 6 (2–3), 145–155.
47. Thomas, G.; Sautel, M.; Kaddouri, A.; Mazzocchia, C. Comparison between
electrical conductivity properties and catalytic activity of nickel molybdate. Solid
State Ionics 1997, 101–103 (2), 775–780.
48. Zavoianu, R.; Dias, C.R.; Portela, M.F. Stabilisation of b-NiMoO4 in TiO2-
supported catalysts. Catal. Commun. 2001, 2 (1), 37–42.
Madeira, Portela, and Mazzocchia104
Dow
nloa
ded
by [
UN
AM
Ciu
dad
Uni
vers
itari
a] a
t 11:
19 2
4 Fe
brua
ry 2
012
ORDER REPRINTS
49. Dias, C.R.; Zavoianu, R.; Portela, M.F. Isobutane oxydehydrogenation on SiO2-
supported nickel molybdate catalysts: effect of the active phase loading. Catal.
Commun. 2002, 3 (2), 85–90.
50. Tempesti, E.; Kaddouri, A.; Mazzocchia, C. Sol-gel processing of silica supported
Ni and Co molybdate catalysts used for IsoC4 alkane oxidative dehydrogenation.
Appl. Catal. A: Gen. 1998, 166 (2), L259–L61.
51. Cauzzi, D.; Deltratti, M.; Predieri, G.; Tiripicchio, A.; Kaddouri, A.; Mazzocchia, C.;
Tempesti, E.; Armigliato, A.; Vignali, C. Synthesis of MMoO4/SiO2 catalysts
(M ¼ Ni or Co) by a sol-gel route via silicon alkoxides. Stabilization of b-NiMoO4
at room temperature. Appl. Catal. A: Gen. 1999, 182 (1), 125–135.
52. Dias, C.R.; Zavoianu, R.; Portela, M.F. Study of the acid-base properties of SiO2-
supported NiMoO4 catalysts by temperature-programmed desorption: effect of the
support. React. Kinet. Catal. Lett. 2002, 77 (2), 317–324.
53. Brito, J.L.; Laine, J. Reducibility of Ni-Mo/Al2O3 catalysts: a TPR study. J. Catal.
1993, 139 (2), 540–550.
54. Yoshimura, Y.; Matsubayashi, N.; Sato, T.; Shimada, H.; Nishijima, A. Molybdate
catalysts prepared by a novel impregnation method. Effect of citric acid as a ligand
on the catalytic activities. Appl. Catal. A: Gen. 1991, 79 (2), 145–159.
55. Aksoylu, A.E.; Isli, A.I.; Onsan, Z.I. Interaction between nickel and molybdenum in
Ni-Mo/Al2O3 catalysts: III. Effect of impregnation strategy. Appl. Catal. A: Gen.
1999, 183 (2), 357–364.
56. Martin, C.; Martin, I.; Rives, V.; Damyanova, S.; Spojakina, A. Characterization
and fourier transform infrared spectroscopic study of surface acidity in NiMo/TiO2-Al2O3 catalysts. Spectrochim. Acta A-M 1995, 51 (11), 1837–1845.
57. Sakanishi, K.; Nagamatsu, T.; Mochida, I.; Whitehurst, D.D. Hydrodesulfurization
kinetics and mechanism of 4,6-dimethyldibenzothiophene over NiMo catalyst
supported on carbon. J. Mol. Catal. A: Chem. 2000, 155 (1–2), 101–109.
58. Sotiropouloua, D.; Yiokari, C.; Vayenas, C.G.; Ladas, S. An x-ray photoelectron
spectroscopy study of zirconia-supported Mo and Ni-Mo hydrodesulfurization
catalysts. Appl. Catal. A: Gen. 1999, 183 (1), 15–22.
59. Martin-Aranda, R.M.; Portela, M.F.; Madeira, L.M.; Freire, F.; Oliveira, M. Effect of
alkali metal promoters on nickel molybdate catalysts and its relevance to the
selective oxidation of butane. Appl. Catal. A: Gen. 1995, 127 (1–2), 201–217.
60. Madeira, L.M.; Martın-Aranda, R.M.; Maldonado-Hodar, F.J.; Fierro, J.L.G.;
Portela, M.F. Oxidative dehydrogenation of n-butane over alkali and alkaline earth
promoted a-NiMoO4 catalysts. J. Catal. 1997, 169 (2), 469–479.
61. Kaddouri, A.; Del Rosso, R.; Mazzocchia, C.; Gronchi, P.; Centola, P. On the
reactivity of K2O-, CaO-, and P2O5-doped nickel molybdate catalysts in a periodic-
flow reactor. Catal. Lett. 1999, 63 (1–2), 65–71.
62. Kaddouri, A.; Mazzocchia, C.; Tempesti, E. Propane and isobutane oxidative
dehydrogenation with K, Ca and P-doped a-, and b-nickel molybdate catalysts.
Appl. Catal. A: Gen. 1998, 169 (1), L3–L7.
63. Kaddouri, A.; Del Rosso, R.; Mazzocchia, C.; Fumagalli, D. Isothermal reduction
behaviour of undoped and Ca-, K-, and P-doped NiMoO4 phases used for selective
propane oxydehydrogenation. J. Therm. Anal. Calorim. 2001, 63 (1), 267–277.
64. Bertus, B.J. Catalyst and Process for Oxidative Dehydrogenation. US Patent
4,094,819, June 13, 1978 (Phillips Petroleum Company).
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 105
Dow
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Uni
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itari
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t 11:
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4 Fe
brua
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ORDER REPRINTS
65. Soares, A.P.V.; Dimitrov, L.D.; Oliveira, M.; Portela, M.F. n-Butane oxidative dehy-
drogenation over Ni12xMgxMoO4 catalysts, Proceedings of the 4th World Congress
on Oxidation Catalysis, Berlin, Germany, Sept. 16–21, 2001, Vol. I, 179–185.
66. Kaddouri, A.; Mazzocchia, C.; Tempesti, E. The synthesis of acrolein and acrylic
acid by direct propane oxidation with Ni–Mo–Te–P–O catalysts. Appl. Catal. A:
Gen. 1999, 180 (1–2), 271–275.
67. Kaddouri, A.; Del Rosso, R.; Mazzocchia, C.; Gronchi, P.; Fumagalli, D. Isothermal
reduction behaviour of some metal molybdates. Selective light alkane oxy-
dehydrogenation and/or olefins partial oxidation. J. Therm. Anal. Calorim. 2001,
66 (1), 63–78.
68. Fujikawa, N.; Wakui, K.; Tomita, K.; Ooue, N.; Ueda, W. Selective oxidation of
propane over nickel molybdate modified with telluromolybdate. Catal. Today 2001,
71 (1–2), 83–88.
69. Cherry, W.E.; Dickason, A.F.; Hedge, J.A. Catalyst for the Oxidation of Butane to
Maleic Anhydride. US Patent 3,968,054, July 6, 1976 (Sun Ventures, Inc.).
70. Ferlazzo, N.; Bertolini, N.; Ghirga, M. Catalyst for the Conversion of Unsaturated
Hydrocarbons Into Diolefins or Unsaturated Aldehydes and Nitriles, and Process for
Preparing the Same. US Patent 4,388,223, June 14, 1983 (Euteco Impianti S.p.A.).
71. Portela, M.F.; Pinheiro, C.; Dias, C.; Pires, M.I. A comparison between low and high
temperature Bi2O3.MoO3 phases for 1-butene reactions. In Structure-Activity and
Selectivity Relationships in Heterogeneous Catalysis; Grasselli, R.K., Sleight, A., Eds.;
Studies in Surface Science and Catalysis, Elsevier: Amsterdam, 1991; Vol. 67, 77–85.
72. Stern, D.L.; Grasselli, R.K. Propane oxydehydrogenation over molybdate-based
catalysts. J. Catal. 1997, 167 (2), 550–559.
73. Steinbrunn, A.; Tahri, A.; Colson, J.C. Electrical transport in polycrystalline nickel
molybdate NiMoO4. Solid State Ionics 1991, 49, 99–103.
74. Sleight, A.W.; Chamberland, B.L. Transition metal molybdates of the type AMoO4.
Inorg. Chem. 1968, 7 (8), 1672–1675.
75. Rodriguez, J.A.; Chaturvedi, S.; Hanson, J.C.; Albornoz, A.; Brito, J.L. Electronic
properties and phase transformations in CoMoO4 and NiMoO4: XANES and time-
resolved synchrotron XRD studies. J. Phys. Chem. B 1998, 102 (8), 1347–1355.
76. Rodriguez, J.A.; Hanson, J.C.; Chaturvedi, S.; Maiti, A.; Brito, J.L. Phase
transformations and electronic properties in mixed-metal oxides: experimental and
theoretical studies on the behaviour of NiMoO4 and MgMoO4. J. Chem. Phys. 2000,
112 (2), 935–945.
77. Plyasova, L.M.; Andrushkevich, M.M.; Buyanov, R.A.; Itenberg, I.Sh. Determi-
nation of the type of solid solution of nickel in nickel molybdate. Kinet. Catal. 1973,
14 (5), 1190–1191.
78. Pilipenko, F.S.; Tsailingol’d, A.L.; Levin, V.A.; Tuktarova, L.S.; Stepanov, G.A.;
Boreskov, G.K.; Buyanov, R.A.; Andrushkevich, M.M. Phase composition of the
NiO-MoO3 system. Kinet. Catal. 1973, 14 (3), 649–651.
79. Corbet, F.; Stefani, R.; Merlin, J.C.; Eyraud, C. Comportement thermique de
molybdates de nickel hydrates. C. R. Hebd. Acad. Sci. 1958, 246 (11), 1696–1698.
80. Maldonado-Hodar, F.J.; Madeira, L.M.; Portela, M.F.; Martın-Aranda, R.M.;
Freire, F. Oxidative dehydrogenation of butane: changes in chemical, structural and
catalytic behavior of cs-doped nickel molybdate. J. Mol. Catal. A: Chem. 1996,
111 (3), 313–323.
Madeira, Portela, and Mazzocchia106
Dow
nloa
ded
by [
UN
AM
Ciu
dad
Uni
vers
itari
a] a
t 11:
19 2
4 Fe
brua
ry 2
012
ORDER REPRINTS
81. Tahri, A.; Steinbrunn, A. An experimental study of nickel molybdate electrical-
conductivity. J. Chim. Phys. Pcb. 1991, 88 (1), 129–143.
82. Les Techniques Physiques d’Etude des Catalyseurs; Herrmann, J.M., Imelik, B.,
Vedrine, J.C., Eds.; Technip: Paris, 1988; Ch. 22, 753.
83. Madeira, L.M.; Herrmann, J.M.; Freire, F.G.; Portela, M.F.; Maldonado, F.J.
Electrical conductivity, basicity and catalytic activity of Cs-promoted a-NiMoO4
catalysts for the oxidative dehydrogenation of n-butane. Appl. Catal. A: Gen. 1997,
158 (1–2), 243–256.
84. Sautel, M.; Thomas, G.; Iacconi, P.; Kaddouri, A.; Anouchinsky, R.; Mazzocchia, C.
Study of the physical and catalytic properties of several powders of nickel molybdate
prepared in different ways. In New Horizons for Materials; Vincenzini, P., Ed.;
Advances in Science and Technology: Techna, 1995; No. 4, 443–450.
85. Sautel, M. Influence du Mode de Preparatin de Molybdates de Nickel sur la Vitesse
d’Oxydation du Propane. Ecole Nationale Superieure des Mines de Saint-Etienne,
INPG-EMSE: Saint-Etienne, France, 1995; PhD Dissertation Thesis.
86. Levin, D.; Ying, J.Y. The structure and defect chemistry of non-stoichiometric
nickel molybdates. J. Electroceram. 1999, 3 (1), 25–36.
87. Kipnis, M.A.; Agievskii, D.A. Phase-composition of products from the reduction of
NiMoO4. Kinet. Catal. 1981, 22 (6), 1252–1257.
88. Samigov, K.A.Tashkent State University: Tashkent, 1973; PhD Dissertation Thesis.
89. Ismailov, T.S.; Talipov, G.Sh.; Inoyatov, N.Sh. Catalytic Processing of
Hydrocarbon Feedstocks; Tashkent, 1971; No. 5, 91.
90. Madeira, L.M.; Portela, M.F.; Mazzocchia, C.; Kaddouri, A.; Anouchinsky, R.
Reducibility of undoped and Cs-doped a-NiMoO4 catalysts: kinetic effects in the
oxidative dehydrogenation of n-butane. Catal. Today 1998, 40 (2–3), 229–243.
91. Di Renzo, F.; Mazzocchia, C.; Anouchinsky, R. The role of ammonium ions in the
activation of nickel molybdate precursors. Thermochim. Acta 1988, 133, 163–168.
92. Madeira, L.M.; Herrmann, J.M.; Disdier, J.; Portela, M.F.; Freire, F.G. New
evidences of redox mechanism in n-butane oxidative dehydrogenation over undoped
and Cs-doped nickel molybdates. Appl. Catal. A: Gen. 2002, 235 (1–2), 1–10.
93. Driscoll, S.A.; Gardner, D.K.; Ozkan, U.S. Characterization, activity, and
adsorption/desorption behavior of alkali-promoted molybdate catalysts for the
oxidative coupling of methane. J. Catal. 1994, 147 (2), 379–392.
94. Owens, L.; Kung, H.H. Effect of cesium modification of silica-supported vanadium-
oxide catalysts in butane oxidation. J. Catal. 1994, 148 (2), 587–594.
95. Liu, Y.; Wang, J.; Zhou, G.; Xian, M.; Bi, Y.; Zhen, K. Oxidative dehydrogenation
of propane to propene over barium promoted Ni–Mo–O catalyst. React. Kinet.
Catal. Lett. 2001, 73 (2), 199–208.
96. Maldonado-Hodar, F.J.; Madeira, L.M.P.; Portela, M.F. The effects of the coke
deposition on NiMoO4 used in the oxidative dehydrogenation of butane. J. Catal.
1996, 164 (2), 399–410.
97. Atanasova, P.; Cordero, R.L.; Mintchev, L.; Halachev, T.; Agudo, A.L. Temperature
programmed reduction of the oxide form of PNiMo/Al2O3 catalysts before and after
water extraction. Appl. Catal. A: Gen. 1997, 159 (1–2), 269–289.
98. Brito, J.L.; Laine, J. Detection of b-NiMoO4 in oxidic nickel-molybdenum catalysts.
Appl. Catal. 1991, 72 (2), L13–L15.
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 107
Dow
nloa
ded
by [
UN
AM
Ciu
dad
Uni
vers
itari
a] a
t 11:
19 2
4 Fe
brua
ry 2
012
ORDER REPRINTS
99. Zavoianu, R.; Dias, C.R.; Portela, M.F. Study of the acid-base properties of SiO2-
supported NiMoO4 catalysts by temperature-programmed desorption. Effect of the
active phase loading. React. Kinet. Catal. Lett. 2001, 72 (2), 201–208.
100. Park, Y.C.; Oh, E.S.; Rhee, H.H. Characterization and catalytic activity of WNiMo/Al2O3 catalyst for hydrodenitrogenation of pyridine. Ind. Eng. Chem. Res. 1997,
36 (12), 5083–5089.
101. Park, Y.C.; Rhee, H.H. The role of nickel in pyridine hydrodenitrogenation over
NiMo/Al2O3. Korean J. Chem. Eng. 1998, 15 (4), 411–416.
102. Ozkan, U.; Schrader, G.L. NiMoO4 Selective oxidation catalysts containing excess
MoO3 for the conversion of C4 hydrocarbons to maleic anhydride. II. Selective
oxidation of 1-butene. J. Catal. 1985, 95, 137–146.
103. Ozkan, U.; Schrader, G.L. NiMoO4 Selective oxidation catalysts containing excess
MoO3 for the conversion of C4 hydrocarbons to maleic anhydride. III. Selective
oxidation of 1,3-butadiene and furan. J. Catal. 1985, 95, 147–154.
104. Magaud, L.; Batiot, C.; Barrault, J.; Ganne, M. Selective oxidation of propane into
acrolein over nickel–molybdenum catalysts—effect of bismuth, Proceedings of the
Third European Congress on Catalysis (EuropaCat-3), Krakow, Poland, Aug. 31–
Sept. 6 1997, Vol. 1; 198.
105. Umemura, S.; Ohdan, K.; Asada, H. Process for the catalytic preparation of acrolein
and methacrolein. US Patent 4,267,385, May 12, 1981 (UBE Industries, Ltd.).
106. Mazzocchia, C.; Anouchinsky, R.; Kaddouri, A.; Tempesti, E. Optimization of
NiO/MoO3/TeO2 catalytic systems for direct oxidation of propene to Acrylic Acid. In
NewFrontiers in Catalysis; Guczi, L., Solymosi, F., Tetenyi, P., Eds.; Studies in Surface
Science and Catalysis, Elsevier: Amsterdam, 1993; Vol. 75; Part C, 1975–1978.
107. Bart, J.C.J.; Bossi, A.; Petrini, G.; Battiston, G.; Castellan, A.; Covini, R. Preparation
and optimization of (Ni, Mo, Te) oxide acrylic acid catalysts. Appl. Catal. 1982,
4 (2), 153–164.
108. Kourtakis, K.; Sullivan, J.D. Molybdenum based oxidation catalysts. US Patent
6,271,169, August 7, 2001 (E.I. du Pont de Nemours and Company).
109. Madhok, K.L.; Srivastava, K.P.; Yadav, S. Heterogeneous vapour phase oxidation
and sorption-desorption studies of toluene over nickel molybdate catalysts. Indian J.
Technol. 1982, 20 (5), 184–189.
110. Aglietti, G.; Baratella, P.; Lugo, L.; Reni, C. Oxidation catalysts. US Patent
3,464,931, January 1, 1967 (Italiana Resine Sir Spa Soc. It.).
111. Li, T.P. Production of (Amm)oxidation catalyst. US Patent 4,168,246, September 18,
1979 (Monsanto Company).
112. Stepanov, G.A.; Tsailingol’d, A.L.; Pilipenko, F.S.; Sobolev, A.M.; Boreskov, G.K.;
Buyanov, R.A.; Veniaminov, S.A. British Patent 1,197,537.
113. Bertus, B.J. et al. US Patent 3,793,225, February, 1974.
114. Pilipenko, F.S.; Tsailingol’d, A.L.; Stepanov, G.A. Catalytic activity of nickel-
molybdenum catalysts during oxidative dehydrogenation of n-butane to 1,3-
butadiene. Kinet. Catal. 1976, 17 (4), 842–846.
115. Itenberg, I.S.; Andrushkevich, M.M.; Buyanov, R.A.; Khramova, G.A.; Sitnikov, V.G.
Catalytic activity of an NiO–MoO3 system in a single-stage oxidative dehydrogena-
tion of n-butane into divinyl. Kinet. Catal. 1976, 17 (4), 867–870.
116. Cavani, F.; Trifiro, F. Selective oxidation of C4 paraffins. In Catalysis, Specialist
Periodical Report, Royal Society of Chemistry: Cambridge, 1994; Vol. 11, 246–317.
Madeira, Portela, and Mazzocchia108
Dow
nloa
ded
by [
UN
AM
Ciu
dad
Uni
vers
itari
a] a
t 11:
19 2
4 Fe
brua
ry 2
012
ORDER REPRINTS
117. Mahmoud, M.L.O.M.; Bechara, R.; Czernicki, M.; Vanhove, D.; Pietrzyk, S.
Oxidative dehydrogenation of propane on NiMoO4 catalyst, Proceedings of the
Second European Congress on Catalysis (EuropaCat-II), Maastricht, The Nether-
lands, Sept, 3–8, 1995; 200.
118. Mars, J.; van Krevelen, D.W. Oxidations carried out by means of vanadium oxide
catalysts. Chem. Eng. Sci. 1954, 3 (Spec.Suppl.), 41.
119. Boutry; et al. US Patent 3,577,477, May, 1971.
120. Stern, D.L.; Grasselli, R.K. Reaction network and kinetics of propane
oxydehydrogenation over nickel cobalt molybdate. J. Catal. 1997, 167 (2), 560–569.
121. Sautel, M.; Thomas, G.; Kaddouri, A.; Mazzocchia, C.; Anouchinsky, R. Kinetics of
oxidative dehydrogenation of propane on the b-phase of nickel molybdate. Appl.
Catal. A: Gen. 1997, 155 (2), 217–228.
122. Del Rosso, R.; Kaddouri, A.; Anouchinsky, R.; Mazzocchia, C.; Gronchi, P.;
Centola, P. Oxidative dehydrogenation of propane by continuous and periodic
operating flow reactor with a nickel molybdate catalyst. J. Mol. Catal. A: Chem.
1998, 135 (2), 181–186.
123. Kaddouri, A.; Anouchinsky, R.; Mazzocchia, C.; Madeira, L.M.; Portela, M.F.
Oxidative dehydrogenation of ethane on the a and b phases of NiMoO4. Catal.
Today 1998, 40 (2–3), 201–206.
124. Madeira, L.M.; Portela, M.F. Effects of cesium doping on the kinetics and
mechanism of the n-butane oxidative dehydrogenation over nickel molybdate
catalysts. In 3rd World Congress on Oxidation Catalysis; Grasselli, R.K.,
Oyama, S.T., Gaffney, A.M., Lyons, J.E., Eds.; Studies in Surface Science and
Catalysis, Elsevier: Amsterdam, 1997; Vol. 110, 797–806.
125. Madeira, L.M.; Portela, M.F. Kinetics and mechanism of the selective oxidation and
degradation of n-butane over nickel molybdate catalysts. In Natural Gas Conversion
V; Parmaliana, A., Sanfilippo, D., Frusteri, F., Vaccari, A., Arena, F., Eds.; Studies in
Surface Science and Catalysis, Elsevier: Amsterdam, 1998; Vol. 119, 611–616.
126. Maldonado-Hodar, F.J.; Madeira, L.M.; Portela, M.F.; Martın-Aranda, R.M.
Oxidacao desidrogenante do n-butano sobre NiMoO4 dopado com cesio: estudo dos
efeitos fısicos e quımicos da dopagem e cinetica da reaccao, Proceedings of the XVth
Ibero-American Symposium on Catalysis, Cordoba, Argentina, Sept, 16–20, 1996,
Vol. 1; 251–256.
127. Agafonov, Y.A.; Nekrasov, N.V.; Gaidai, N.A. Kinetic and mechanistic study of the
oxidative dehydrogenation of isobutane over cobalt and nickel molybdates. Kinet.
Catal. 2001, 42 (6), 821–827.
128. Watson, R.B.; Ozkan, U.S. Propane and propylene adsorption effects over MoOx-
based catalysts induced by lower levels of alkali doping. J. Mol. Catal. A: Chem.
2003, 194 (1–2), 115–135.
129. Abello, M.C.; Gomez, M.F.; Cadus, L.E. Oxidative dehydrogenation of propane
over molybdenum supported on MgO-gamma-Al2O3. Ind. Eng. Chem. Res. 1996,
35 (7), 2137–2143.
130. Cadus, L.E.; Abello, M.C.; Gomez, M.F.; Rivarola, J.B. Oxidative dehydrogenation
of propane over Mg-Mo-O catalysts. Ind. Eng. Chem. Res. 1996, 35 (1), 14–18.
131. Khan, M.M.; Somorjai, G.A. A kinetic study of partial oxidation of methane with
nitrous oxide on a molybdena–silica catalyst. J. Catal. 1985, 91 (2), 263–271.
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 109
Dow
nloa
ded
by [
UN
AM
Ciu
dad
Uni
vers
itari
a] a
t 11:
19 2
4 Fe
brua
ry 2
012
ORDER REPRINTS
132. Costentin, G.; Studer, F.; Savary, L. Oxidative dehydrogenation of propane over
molybdenum phosphates characterized by XRD and x-ray absorption, Proceedings
of the Third European Congress on Catalysis (EuropaCat-3), Krakow, Poland, Aug
31–Sept 6, 1997, Vol. 1; 103.
133. Harlin, M.E.; Backman, L.B.; Krause, A.O.I.; Jylha, O.J.T. Activity of molybdenum
oxide catalyst in the dehydrogenation of n-butane. J. Catal. 1999, 183 (2), 300–313.134. Ueda, W.; Yoon, Y.-S.; Fujikawa, N.; Lee, K.-W.; Moro-oka, Y. Catalytic oxidative
dehydrogenation of propane to propylene, Proceedings of the Second European
Congress on Catalysis (EuropaCat-II), Maastricht, The Netherlands, Sept, 3–8,
1995, S3 01; 96.
135. Mazzocchia, C.; Tempesti, E.; Anouchinsky, R.; Kaddouri, A. Catalyseur et Procede
d’Oxydation Menagee et Selective d’Alcanes. French Patent 92-08641, July 10,
1992 (Elf Atochem S. A.).
136. Mazzocchia, C.; Kaddouri, A.; Tempesti, E.; Del Rosso, R. Propane oxidative
dehydrogenation by continuous and periodic operating flow reactor with a nickel
molybdate catalyst. In 12th International Congress on Catalysis; Corma, A.,
Melo, F.V., Mendioroz, S., Fierro, J.L.G., Eds.; Studies in Surface Science and
Catalysis, Elsevier: Amsterdam, 2000; Vol. 130, 785–790.
137. Mazzocchia, C.; Di Renzo, F.; Thomas, G.; Aboumrad, C. Stability of beta nickel
molybdate. Solid State Ionics 1989, 32–33, 228–233.
138. Derouane, E.G.; Parmon, V.; Lemos, F.; Ribeiro, F.R. Principles and Methods for
Accelerated Catalyst Design and Testing; Derouane, E.G., Parmon, V., Lemos, F.,
Ribeiro, F.R., Eds.; NATO ASI, Vol. 69. Kluwer Academic Publishers: Dordrecht,
2002.
139. Logsdon, J.E. Ethanol. In Kirk–Othmer Encyclopedia of Chemical Technology;
John Wiley & Sons; 1994.
140. Schadlich, K.; Schug, P. Octane enhancers. In Ullmann’s Encyclopedia of Industrial
Chemistry, Electronic Release,Wiley-VCHVerlagGmbH:Weinheim, Germany, 2002.
141. Del Rosso, R.; Kaddouri, A.; Fumagalli, D.; Mazzocchia, C.; Gronchi, P.; Centola, P.
Deactivation of alkane oxidative dehydrogenation catalyst by deep reduction in
periodic flow reactor. Catal. Lett. 1998, 55 (2), 93–95.
142. Boutry, P.; Daumas, J.C.; Montarnal, R. Cyclical process for the dehydrogenation of
saturated hydrocarbons. US Patent 3,692,860 (Institut Francais du Petrole).
143. Dury, F.; Gaigneaux, E.M.; Ruiz, P. The active role of CO2 at low temperature in
oxidation processes: the case of the oxidative dehydrogenation of propane on
NiMoO4 catalysts. Appl. Catal. A: Gen. 2003, 242 (1), 187–203.
144. Bond, G.; Moyes, R.B.; Whan, D.A. Recent applications of microwave heating in
catalysis. Catal. Today 1993, 17 (3), 427–437.
145. Bond, G.; Moyes, R.B.; Pollington, S.D.; Whan, D.A. The advantageous use of
microwave radiation in the preparation of supported nickel catalysts. In New Frontiers
in Catalysis (Proceedings of the 10th International Congress on Catalysis); Guczi, L.,
Solymosi, F., Tetenyi, P., Eds.; Studies in Surface Science and Catalysis, Elsevier:
Amsterdam, 1993; Vol. 75, 1805 (also Akademiai Kiado, Budapest, Hungary).
146. Marcano, J.G.S.; Tsotsis, T.T. Catalytic Membranes and Membrane Reactors;
Wiley-VCH Verlag GmbH: Weinheim, 2002.
147. Schwartz, M.; White, J.H.; Sammells, A.F. Two Component–Three Dimensional
Catalysis. US Patent 6,355,093, March 12, 2002 (Eltron Research, Inc.).
Madeira, Portela, and Mazzocchia110
Dow
nloa
ded
by [
UN
AM
Ciu
dad
Uni
vers
itari
a] a
t 11:
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4 Fe
brua
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