Desirable catalyst properties in selective oxidation reactions

Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 171-178 171

O ~ N - O - F , - N - HO C H ~ 0”-COONGO 0 I li

t O ~ N O H O C N ~ C H ? ~ N C O H O N ~ O

I t HOGNO

I t O N ~ O H

/ CROS2LINK / 1 Alkal, +usion

H P - Q C H Z O”* F i g u r e 14. Mechanism of cross-linking w i th urethane cross-linking agenta and alkaline fusion.

Guise and Smith,4O and alkali fusion would seem to be applicable as a more detailed method of analysis.

Other possible amide type applications concern the analysis of poly(l,4-benzamides) and their chlorine-sub- stituted derivatives41 and a wide variety of polyamides that exhibit liquid-crystalline proper tie^.^^

The increasing complexity of polymer produds is further illustrated by the recent commercial development of a transparent engineering nylon and a thermoplastic elas- tomer based in part on 4,4‘-methylenediphenyl diiso- cyanate by the Upjohn Co. The analysis of such products is very likely to be successfully conducted by using alkali fusion technique^.^^

A slightly different application has been reportedu with a compounded natural rubber cross-linked with a pro- prietary polyurethane cross-linking agent, i.e., Novor 924. The polyurethane and its method of cross-linking and subsequent cleavage are shown in Figure 14. Here the aromatic nature of the cross-linker present as a very minor component was readily established. The presence of other minor additives in rubber is possible as should be the recently introduced aliphatic polyurethane cross-linker, Le., Novor 950.

Conclusion The use of alkaline fusion as the preliminary step in the

analysis of condensation polymers as conducted in these laboratories is described; other studies are tabulated. Current studi- on other systems are indicated, and further utility of the technique with several additional systems is suggested.

References (1) Haken, J. K. Pfog. Org. Coat. 1979, 7 , 209-252. (2) Whltlock, L. R.; Siggla, R. Sep. furif . Methods 1974, 3, 299-337. (3) Smets, G.; De Loecker. W. J. pOrVm. Sci. 1959, 41, 375-380. (4) Siggla, S.; WhRlock, L. R.; Tao, J. C. Anal. Chem. 1989, 47,

(5 ) Ettre, K.; Varadl, P. F. Anal. Chem. 1983, 35, 69-73. (6) Frankoskl, S. P.; Siggla, S. Anal. Chem. 1972, 4 4 , 507-511. (7) Frankoskl, S. P.; Slggla, S. Anal. Chem. 1972, 44. 2078-2088. (8) Williams, R. J.; Siggia, S. CRed In Sep. furif. Methods 1974, 3 ,

(9) Rahn, P. C.; Slggia, S. Anal. Chem. 1973, 45, 2336-2341. (10) Schlueter, D. D. Thesis, University of Massachusetts, Amherst, MA,

1976. (11) Schlueter, D. D.; Slggia, S. Anal. Chem. 1977, 49, 2343-2348. (12) Williams, R. J.; Slggla, S. Anal. Chem. 1977, 49, 2337-2342. (13) Schlueter, D. D.; Slggia, S. Anal. Chem. 1977, 49, 2349-2353. (14) Gibian, D. 0. Thesis, University of Massachusetts, Amherst, MA, 1979. (15) Sasto, L. G., Jr. Thesis, University of Massachusetts, Amherst, MA.

1982. (16) Anton, A. Anal. Chem. 1988, 40 , 1116-1118. (17) Morl, S.; Furusawa, M.; Takeuchl, T. Anal. Chem. 1970, 42,

(18) Morl, S.; Furusawa, M.; Takeuchl. T. Anal. Chem. 1970, 42,

(19) Gladlng, G. J.; Haken, J. K. J. Chromatogr. 1978, 757, 404-409. (20) O’Nelll, L. A.; Christensen, G. J . Oil Colour Chem. Assoc. 1978, 59,

(21) Haken, J. K.; Obita, J. A. J. Oil Colow Chem. Assoc. 1980, 63 , 200-209.

(22) Haken, J. K.; Ob&, J. A. J. Chromatogr. 1981, 213, 55-62. (23) Lee, H.; Stoffey, D.; Neville, K. ”New Linear Polymers”; McGraw-HIII,

New York, 1967; Chapter 6-6. (24) Haken, J. K.; Obita, J. A. J. Chromatogr. 1982, 244, 265-270. (25) Haken, J. K.; Rohanna, M. A. J. Chromatogr. 1984, 298, 263-272. (26) Haken, J. K.; Obita, J. A. J. Chromatogr. 1982, 244, 259-263. (27) Preston, J. US. Patent 3376269, 1966. (28) Preston, J. US. Patent 3484407, 1969. (29) Haken, J. K.; Oblta, J. A. J. Chromatogr. 1982, 239, 377-384. (30) Haken, J. K. “The Gas ChromatoaraDhy of Coating Materials”; Dekker.

1387-1392.

299-337.

138- 140.

959-961.

285-290.

New York, 1974. (31) Vlmaiaslrl, P. A. D. T.; Haken, J. K.; Burford, R. P. J . Chromatogr.

(32) Matuszak, M. L.; Frisch, K. C.; Reegen, S. L. J. f o k m . Sci. 1973, 1 7 , 1985, 379, 121-130.

1683- 1690. (33) Barringer, C. M. Teracel30 Polyalkylene Ether Glycol Bulletin No. 11R-

1-1956, Du Pont, Wllmlngton, DE, 1956. (34) Haken, J. K.; Burford. R. P.; Vlmalaslrl, P. A. D. T. Advances in Chro-

matography; Elsevier: Amsterdam, 1985; pp 347-356. (35) McFadden, J.; Scheulng, J. Chromatcgr. Sci. 1984, 2 2 , 310-312. (36) Haken, J. K., unpubllshed results, 1984. (37) Hercules Inc. U.S. Patent 2926 154, 1960. (38) Earle, R. H., Jr.; Saunders, R. H.; Kangas, L. R. Appi. folym. Sci.

Symp. 1971, 18, 707-714. (39) Smith, P.; Mills, J. H. CH€M€CH 1973, 3 , 748-755. (40) Guise, 0. B.; Smith. G. C. J. Chromatogr. 1982, 235, 365-376. (41) Morgan, P. W. U.S. Patent 3943 110, March 9 1976. (42) Morgan, P. W. CH€MECH 1979, 9 , 316-326. (43) Chem. Eng. News July 9, 1984, 62(28), I O . (44) Burford, R. P.; Haken, J. K.; Obita, J. A. J. Chromatogr. 1983, 268,

515-521,

Receiued for review December 31, 1984 Accepted December 27, 1985

Desirable Catalyst Properties In Selective Oxidation Reactions

Harold H. Kung

Chemical Engineering Department and the Ipatieff Laboratoty, Northwestern University, Evanston, Illinois 6020 1

Heterogeneous oxide-catalyzed selective oxidation reactions can be classified into dehydrogenation and dehydrogenation with oxygen insertion. The oxide properties that are important In each of the steps in these reactions are discussed. The breaking of the C-H bonds in alkanes is facilitated by weakly adsorbed oxygen. The C-H bond breaking, of alkenes is enhanced by strongly basic surface lattice oxygen and cations that

are soft acid and undergo redox readily. Desorption of alkenes and dienes is enhanced by cations that are hard acid. The selective CO bond formation Is controlled by the number and the ease of removal of the available lattice oxygen, while the combustion reaction can be minimized by shortening the residence time of the surface intermediates, weakening the adsorption of the desired products and minimizing the amount of

0 1986 American Chemical Society 01 96-4321/86/1225-017 1$01.50/0

172 Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986

weakly adsorbed oxygen or the density of combustion sites. The function of a promoter is to enhance the rate of the rate- and selectivitydetermining steps. Thus the desirable influence of a promoter on the soli depends on the nature of the critical step.

Introduction Selective oxidation of hydrocarbons is the largest class

of reactions catalyzed by transition-metal oxides. It is also among the most important and most studied catalytic reactions. From the industrial point of view, a desirable process must produce the desired products in high selectivity and high yield. The requirement of high selectivity is extremely challenging in partial oxidation processes where the desired products are produced instead of carbon monoxide and dioxide, which are the combustion products that are thermodynamically much more favorable, even though the chemical transformations involved in partial or total oxidation share many common features. In this paper, the oxide properties that are known to affect selectivity will be discussed, using as examples some of the systems that have been relatively well studied.

A number of excellent recent review articles have ad- dressed the question of selectivity from various points of view.l-14 A monograph summarizing most of the literature before 1974 is also available.1s I t is clear that the overall activity and selectivity depend on the chemical nature of the catalyst, especially the composition and distribution of the components; the physical nature of the catalyst such as texture, pore size, and pore volume; and the process variables such as conversion, temperature, hydrocarbon- to-oxygen ratio, and the presence of water. It is also ev- ident that many of these factors are interrelated. For example, the gas-phase hydrocarbon-to-oxygen ratio affects the oxygen content of the catalyst. At any particular in- stance, the steady-state composition of the catalyst depends on the relative rates of reduction of the oxide by the hydrocarbon and reoxidation by the gaseous oxygen. The long-term stability of the catalyst depends on the fugacity of the oxygen in the gas phase, since the oxide must gradually attain a state that is in a thermodynamically stable point with the gas. For a given oxygen fugacity, the steady-state oxygen content of the solid also depends on the temperature.

It is common that products of partial oxidation are intermediates in the combustion process that produces carbon oxides. These intermediates can undergo secondary reactions for further oxidation. The extent of such secondary reactions increases with increasing conversion and contact time of the intermediate with the catalyst. It increases if the desorbed partial oxidation products need to diffuse in the long and narrow pores of the catalyst before leaving the catalyst. Thus the selectivity depends on the texture of the catalyst. In addition, the texture of the catalyst also affects the heat-transfer characteristics, which determines the temperature profile of the catalyst.

Water is often added to the feed in partial oxidation processes. It could have two functions. Often it is added to aid in the removal of the heat released in the oxidation reaction. Its presence may also affect the acidity and basicity of the catalyst by changing the degree of surface hydroxylation, which may be important in the reaction.

In addition to these physical and process variables, the selectivity critically depends on the chemical composition of the catalyst. It is often found that selective partial oxidation catalysts are multicomponent oxides that are based on a surprisingly few number of oxides as basic components. These basic components include oxides of

Table I. Common Oxide-Catalyzed Selective Oxidation Reactions

reaction catalyst Dehydrogenation

ethylbenzene - styrene Fe-Cr-K-0 isopentane, isopentene - Sn-Sb-0

butane, butene - butadiene promoted V-0

methanol - formaldehyde Fe-Mo-0, MOO,

Dehydrogenation and Oxygen Insertion butane, butene - maleic v-P-0

propene - acrolein Bi-Mo-0 (propene and NH, - Bi-Mo-0, U-Sb-0,

propene - acrolein, acrylic acid, Co-Mo-Te-0, Sb-V-Mo-0

benzene - maleic anhydride V-P-0, V-Sb-P-0 o-xylene, naphthalene - phthalic promoted V-0

isoprene Bi-Mo-0, promoted Fe-0,

anhydride

acetonitrile) Fe-Sb-0, Bi-Sb-Mo-0

acetaldehyde

anhydride

formaldehyde methane - methanol, Mo-0, V-0

ethylene - ethylene oxide

methyl ethyl ketone - biacetyl

Fe-Mo-0 (also catalyzed by

Co-0 (promoted by Ni, Cu) promoted Ag)

methyl ethyl ketone - V-Mo-0 acetaldehyde, acetic acid

molybdenum, vanadium, copper, iron, antimony, tin, and uranium. To these basic components other components, often referred to as promoters, are added to result in enhanced activity and selectivity. The exact chemical interaction of the promoters with the basic components and among themselves is a subject of strong research interest. Several types of interactions are identified: formation of new compounds, increase in acidity or basicity, and gen- eration of defect sites for hydrocarbon or oxygen activation.

Categorically, a promoter enhances catalytic activity by facilitating processes involved in the rate-determining step. It enhances selectivity by facilitating processes leading to the desired product and/or inhibiting processes leading to the undesired product. Therefore, to better understand catalytic selective oxidation, it appears helpful to consider each of the chemical steps involved in the reaction as to what chemical properties would facilitate that step. When the rate-determining step or the critical branching step that determines selectivity is known, such a consideration could lead to more systematic search for promoters and better understanding of why some oxides are better basic components than others. This is the purpose of this paper. Here, we first discuss classification of selective oxidation reactions. This is followed by considerations of the chemical interactions involved in each of the steps of the reactions.

Types of Selective Oxidation Reactions Selective oxidation reactions can be classified into two

types: one involves only dehydrogenation, the other involves both dehydrogenation and oxygen insertion into the hydrocarbon molecule. Table I summarizes the common oxide-catalyzed selective oxidation reactions and the catalysts.

1. Dehydrogenation Reactions. These are reactions in which a hydrocarbon molecule is converted into a more unsaturated hydrocarbon by breaking carbon-hydrogen bonds and forming C=C bonds. In the absence of oxi- dants, hydrogen is a byproduct. In such cases, the reactions are run at rather high temperatures (above 500 O C )

because the thermodynamic equilibrium normally favors the reactants a t low temperatures. At these high temperatures, undesired coking takes place readily, and water

Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 173

the insertion into the hydrocarbon are the same type of oxygen. A recent report by Ueda et al. shows that this is false.16 They find that on a Bi2Mo06 catalyst a t 400 "C the conversion of propene pulses decreases rapidly in the absence of gaseous oxygen. However, the conversion of butene pulses decreases much more slowly. From the total amount of propene reacted, the extent of reduction of the catalyst that corresponds to the deactivated state with respect to propene oxidation can be calculated. If the catalyst is first prereduced to the same extent by the butene dehydrogenation reaction or by the oxidation of hydrogen before the propene pulses are administered, the conversion of propene is found to be essentially the same as for a fresh catalyst. Thus the reduction by butene or H2 does not remove the oxygen important in the oxidation of propene. This conclusion is further confirmed by l80- labeling experiments. In these experiments, Ueda et al. first reduce the catalyst to a certain extent either by butene pulses or by propene pulses. The catalyst is then reoxidized with 1802. The reoxidized catalyst is then used for propene oxidation, and the l80 content in acrolein is monitored. If the reduction of the catalyst is achieved with propene, 180-labeled acrolein appears immediately in the first propene pulse. The amount of labeled acrolein produced steadily decreases for subsequent propene pulses. If the reduction is achieved using butene, the acrolein formed initially contains little 180-labeling. The degree of labeling increases steadily with pulse number before falling again. Thus there must be two types of oxygen: one for water formation in dehydrogenation and one for insertion into the hydrocarbon. It is possible (and likely) that the optimal properties for the two processes are different.

Reduction of Bi2Mo06 followed by reoxidation with 1802 also causes shifts in Raman bands related to the Mo=O bonds at 725, 803, and 844 cm-'. The magnitude of the band shifts are much larger if the reduction is with propene rather than with l -b~tene, '~J ' consistent with the above conclusion that different oxygens are involved in insertion and in water formation.

is often added to reduce coking. The more common dehydrogenation processes are con-

ducted with oxygen as oxidant to yield water as a byproduct. Sometimes iodine is used instead of oxygen. The formation of water provides the thermodynamic driving force for the reaction. Thus the reaction can be conducted at a lower temperature than without oxygen, and deacti- vation due to coking is less severe.

In these dehydrogenation reactions, the carbon skeletons of the hydrocarbon molecules remain intact.

2. Dehydrogenation and Oxygen Insertion. There are many examples of this type of oxidation reaction. Oxygen is needed as oxidant both for incorporation into the hydrocarbon molecules and in the formation of water in the dehydrogenation steps. The general features of these reactions are that C-H bonds are broken and C-0 bonds are formed. Exceptions to these are the oxidation of ethylene to ethylene oxide, in which no C-H bonds are broken, and the ammoxidation reactions such as propene to acrylonitrile, in which C-N bonds are formed. In some cases, such as the oxidation of benzene to maleic anhydride, the carbon skeleton is broken. In others, the carbon skeleton remains intact. The selectivity is determined in part by the ability of the oxide to catalyze the formation of C-O bonds without breaking (or breaking only a desired number) of C-C bonds. Excessive breaking of the C-C bonds, of course, leads to combustion.

Chemical Factors Affecting Selectivity A large variety of factors have been investigated in at-

tempts to correlate changes in catalytic activity and selectivity with properties of the oxide. Factors such as the presence of cation vacancies, the nature of the metal-oxygen (M-0) bond, the strength of the M-0 bond, the crystal structure, and the surface acidity and basicity have been investigated, and successful correlations with limited groups of reactions have been obtained. Suffice to say, however, that there is not yet one single correlation that is successful for the entire class of selective oxidation reactions. This is not unexpected because different chemical processes are involved in different types of oxidation reactions, such as those classified in Table I.

The oxidation of butane to maleic anhydride can be used as a generic illustrative example to discuss the oxide properties important in selective oxidation. This reaction can be expressed in a stepwise manner:

0 II

I 2 C,H,, - C,H8 - C4H6 0 9

CO. CO,, H 2 0

The first step is the activation of alkane, and the second step is the activation of alkene that leads to the formation of butadiene. They involve only oxidative dehydrogenation. The third and fourth steps lead to the formation of maleic anhydride. They involve both oxidative dehydrogenation and oxygen insertion. Thus this overall reaction includes both common types of selective oxidation. From it, it is also easy to see why there are catalysts that only catalyze dehydrogenation to produce dienes, while there are catalysts that when used under different conditions catalyze either oxygen insertion to produce oxygenates or dehydrogenation to produce dienes.

There had been a common perception that the oxygen in the formation of water during dehydrogenation and in

Activation of Alkane The first step in activating alkane is to break a C-H

bond. (In catalysis by superacids, the first step is to protonate the hydrocarbon, followed by release of a hydrogen molecule to produce a carbenium ion; this will not be discussed.) This step can be expressed as

RCHZR' + M-0 -+ RCHR'-M-OH (1)

In considering the rate of this step, it is relevant to question whether the transition state more closely resem- bles a situation where the dissociation of the C-H bond is accompanied by real charge separation to form H+ and RCH,- or it is without charge separation. The heats of reaction, AHo of the C-H bond dissociation process with and without charge separation in the gas phase have been determined for some hydrocarbons. These values are listed in Table 11. Roughly speaking, the heats of reaction of the homolytic processes are about 400 kJ/mol, while those of the heterolytic processes are about 1600 kJ/mol. The much larger value for the heterolytic processes is due to the energy required for charge separation. The catalytic processes would require less energy than the gas-phase AH" values because the hydrogen atom or proton would form 0-H bonds with the surface oxygen and the hydrocarbon fragment would interact with a surface cation (or site). These interactions should be so strong that step 1 eventually becomes thermodynamically favorable.


Table 11. AHo of Breaking C-H Bonds in Hydrocarbons dissociation AHo, k J/mol

Homolytic" CH, -+ CH3 + H

C3H8 + s - C ~ H ~ + H

n-CIH,o + S-CqHg + H

t-C,H,o + t-C4H9 + H

435 410 410 395 410 395 405 375

C2H6 - CZH5 + H C3Hs -+ n-C3H7 + H

n-CIH,o + n-C4Hg + H

l'-C4H1,3 - i-C,Hg + H

Heterolyticb CzH2 - CZH- + H+ C3H6 - C3Hc + H+ C2H4 -+ CpHC + H+ CHI - CH3- + H+

from ref 86. bFrom ref 87.

1580 1640 1690 1690

" Calculated from AHof of the alkanes and the radicals, the latter

Because saturated hydrocarbons are rather inert and they interact relatively weakly with other species, it is most likely that for this reaction step the transition state is reactant-like. In that case, the activation energy would parallel the AHo values of the gas-phase process, unless the species in the charge separation process is stabilized substantially by electrostatic interaction with the solid. Since the electrostatic potential above an ionic solid is less than 20% of the electrostatic potential a t a bulk ion

stabilization by electrostatic interaction with the solid is less than 3 eV (or 300 kJ/mol). Thus this interaction is insufficient to substantially stabilize the charge separation process. I t is concluded that for saturated hydrocarbons the first step in activating the molecule is the dissociation of the C-H bond in a manner similar to the production of hydrocarbon free radicals. Since a tertiary C-H bond is weaker than a secondary C-H bond, which is in turn weaker than a primary C-H bond, it further suggests that the probability of bond dissociation also follows this order.

Nonetheless, this step must be thermodynamically favorable for the reaction to proceed at any reasonable rate. The AH" of reaction 1 is given by

AHo = EC-H - ( E M 4 - EM-L) - ( E 0 - H - E 0 - L ) (2)

In this equation, EC-H is the C-H bond energy. It is about 380 kJ/mol. EM< is the energy of the cation-alkyl bond, and EM-L is the cation-lattice interaction energy that is lost when the cation-alkyl bond is formed. E c H and EsL are the energies of the 0-H bond and the oxygen-lattice interaction that is lost when the 0-H bond is formed. Since an alkyl species is "soft" and a cation is "hard", EM* is usually small (<lo0 kJ/mol) and is smaller for cations of higher oxidation states.23 For ionic solids the major contribution to EM-L is through electrostatic interaction, which is changed little by the formation of the cation-alkyl bond. Thus it is expected that (EMx - I 3 M - L ) is small. In particular, this term varies relatively little from one oxide to another. The value of E c H is also expected to be roughly constant independent of the oxide. Thus when the values of AHo among oxides are compared, the important term is E0-L. This step is thermodynamically favorable if EcL is small. In other words, this step requires a very reactive oxygen that is bound weakly to the solid for it to be thermodynamically favorable.

The importance of the strength of binding of the reactive oxygen in alkane activation has been observed. It has been reported that the activation energy for methane combustion increases linearly with increasing surface oxygen bond energy.24 Methane combustion is a reaction in which ac-

tivating the first C-H bond is rate limiting: the more stable is the surface oxygen species, the more difficult is the activation of the methane C-H bond. A similar correlation is also observed in H2 oxidation, where the dissociation of the H-H bond is likely as difficult as the dissociation of a C-H bond.24 A number of reactive adsorbed oxygen species have been identified by EPR spec t ro~copy.~~~ ' They include 0- and 0;. These species are usually generated by adsorption of N20 or O2 on oxides that have been irradiated by y-ray or UV light. Oxides including Si02-supported V205 and Moo3, MgO, ZnO, and Co-Mg-0 have been used. These species are very reactive. They react with CO, CH,, C2H6, C2H4, and H2 at room temperature and There are also other reactive oxygen species not detected by ESR that are very reactive.% They could be adsorbed atomic oxygen, perturbed oxygen molecule, or 0- and 0; that are coupled so strongly with the solid that they have fast spin-lattice relaxation. N20 has been used as a source of reactive oxygen because it decomposes readily on many oxides. By the use of N20, ethane has been dehydrogenated to e t h ~ l e n e ~ ~ , ~ ~ and methane has been oxidized to methanol and form- a l d e h ~ d e . ~ ~ - ~ ~

In summary, this step is enhanced by reactive surface oxygen species that bind weakly to the solid. These species can be produced by the decomposition of N20 or the adsorption of oxygen molecules at surface defects. Since it is not desirable to have a high surface density of such reactive surface oxygen to prevent extensive degradation of the hydrocarbon molecule leading to combustion, it is appealing to attempt to control its production by controlling the nature and density of surface defects. This is particularly so if the defects can be generated by utilizing support and/or promoters.

Following reaction 1, a second C-H bond breaking could lead to the formation of adsorbed alkene. The energetics of this process can be similarly expressed by eq 2, with the modification that the term (EM4 - EM-L) is to be corrected for the energy difference of a cation-to-alkene bond and a cation-alkyl bond. If it is assumed that this difference is small, the AHo of this step would again be primarily determined by the reactivity of the surface oxygen species involved, as measured by their strength of adsorption.

Activation of Alkene The first step in the activation of alkene is again the

dissociation of a C-H bond. There is strong experimental evidence, especially in the case of propene, that the allylic C-H bond is preferably broken to form an adsorbed r-allyl specie^.^'-^^ This process can be represented by

The ability of the C=C and the r-allyl to delocalize the charge to the metal ion, which, being in the solid, can be substantially stabilized by the lattice electrostatic potential, makes it possible and likely that this C-H bond breaking proceeds via an acid-base type of bond dissociation. The degree of charge transfer is so extensive that a-allyl radical, and sometimes even cation, has been suggested as the surface intermediate on the basis of the reactions of model corn pound^.'^^^^^^^

Following this argument, the rate of this step would be enhanced if (1) the cation can readily undergo reduction, (2) the surface oxygen is a strong Brmsted base, and (3) the cation is a soft acid. The first two points follow from the discussion above. The third point comes from the fact


correlation is not observed for octan-2-ole55 To effectively increase the electron density of lattice

oxygen by promoters, it is important that the promoter is in close contact with the oxygen, preferably by direct bonding. Formation of a compound is one method to achieve close contact. Addition of Bi203 to Moo3 is known to result in compounds such as Bi2MoOP9J5 Bi203 is less acidic than Moo3. Thus the oxygen bonded to Mo and Bi simultaneously is likely to be more basic than oxygen in Moo3. This is perhaps one of the functions of a Bi promoter, since the slow step in propene oxidation is the formation of r-allyl and is associated with the presence of Bi.1951619v48356 Other possible functions of Bi will be discussed later.

The slow step in the oxidation of butene to butadiene at over 300 "C on iron oxide is also the activation of alkene.3 It has been reported that the addition of acidic oxides such as oxides of Mo, P, and V increases the ac- tivityF7@ The effect was interpreted as due to the increase in acidity of the oxide, which results in increased activity to activate the basic butene molecules. Following the discussion above, another possible explanation is that the Mo, P, and V cations of these oxides are in higher oxidation states than the Fe cation. If they are incorporated into the Fe203 lattice, cation vacancies are formed to maintain charge balance. This results in an increase in basicity of the oxygen near the vacancies and an enhanced activity. Unfortunately, in the absence of detailed characterization of the oxides as to the acidity and the formation of solid solutions or new compounds, the explanations cannot be tested.

Inspection of Table I11 does indicate some correlations with known facts. For example, V2O5, Moo3, and Fez03 are important components in many selective oxidation catalysts. They are neither strongly basic nor strongly acidic. TiO, and Sb205 are often used as promoters. These cations are of high oxidation states. La203, CeO,, and SnO, are basic oxides. Their addition to a catalyst could increase the basicity of the oxygen, in addition to providing redox cation sites in some cases.

The formation of adsorbed butadiene requires the removal of a second hydrogen from the alkene at the methyl group that is not part of the a-allyl moiety. It is reasonable that because of the similarity between this C-H bond breaking and the second C-H bond breaking in the formation of alkene from alkane this step is enhanced by weakly adsorbed oxygen. There is little pertinent information available on this step to test the validity of this assumption. In a somewhat related system of the oxidation of propene to acrolein, it has been reported that the first C-H bond breaking, which forms x-allyl, is not affected by the presence of gaseous oxygen, while the second C-H bond breaking is.59 This observation is consistent with the analysis presented here. Because of the high acidity of the a-C-H the breaking of this bond is related to the basicity of the lattice oxygen. The second C-H bond is more al- kanic and is less acidic, and its breaking requires weakly adsorbed oxygen.

Desorption of Alkene, Diene, and Water If alkene or diene is the desired product, they must be

desorbed rapidly once formed. These species interact with the surface cations by ?r-bonding. Thus desorption is facilitated if such bonding is weak. Weak bonding occurs if the cation is hard, since the polarizable 7r-electrons in the alkene or diene are soft. Thus the solid is preferably ionic, and the cation is in a high oxidation state.23

Binding of water to the surface is likely by the electron lone pairs of the oxygen. In terms of Lewis acidity, water

Table 111. Acidic and Basic Oxides Determined by Adsorption of Probe Moleculesa

acidic: ZnO, A1203, SiOz, Tho2, CeOz, TiOz, ZrOz, SnOz, VzOs, Moo3, WOs, Cr203, AszOs, Biz03, SbzOs

basic: ZnO, Al2O3, Si02, Thoz, Ce02, Ti02, ZrOz, SnOz, BaO, CaO, SrO, MgO, BeO, La203

Determined by Isoelectric Pointb 8 < pH: ZnO, MgO, NiO, CdO, Zr02, Tho2

5 < pH < 8: Fez03, Crz03, A1203, MOO,, UO,, SnOZ pH < 5: SbzOs, WO,, MnOz, UO,, TiOz

nFrom ref 48 and 49. bFrom ref 50.

that a diene is a softer base than an alkene. Thus, a soft-acid cation would make the energetics of diene formation more favorable. However, we shall see later that there is an optimal softness. Similarly, there is an optimal degree of ease of reduction of the cation.

In Table I11 the binary oxides are classified according to whether they are acidic or basic. The classification according to Tanabe is determined by adsorption of in- dicator r n o l e c ~ l e s . ~ ~ ~ ~ ~ The acidic and basic strengths so determined are found to depend on the pretreatment of the oxide, especially the degree of dehydroxylation. I t is most likely that the distribution and quantity of acid and base centers depend on the presence and nature of im- purities and surface lattice defects. When sufficiently understood, this type of information can be very useful, especially in considering the action of promoters.

Since water is almost always a byproduct in selective oxidation, it seems also appropriate to classify the oxides by their isoelectric points, which are determined with aqueous suspensions of the oxides.@-' This is also listed in Table 111.

Finally, the ease of an oxide to accept electrons can be estimated from the electron affinity of the oxide. This results from the fact that the conduction bands of most transition-metal oxides are mostly cationic in character. However, the importance of surface states and surface defects must also be eonsidered.

When an oxidation reaction is conducted under conditions where alkene activation is rate limiting, the effective promoter would be one that increases the Bransted basicity of the oxygen, which is equivalent to increasing the electron density a t the oxygen. For example, in the oxidation of butene and propene on PbMo04, it was found that the rate of production of the selective oxidation product increases with the amount of cation vacancies introduced by the incorporation of BiS5l Since the oxygen atoms surrounding the cation vacancies are electron rich, they are more basic than other lattice oxygen atoms. Similar dependence of cation vacancies has also been observed on the Pb-La- Bi-Mc-0 system.52 In this system it is further concluded that these vacancies are important for low Bi concentrations. This may indicate that oxygen associated with Bi is basic. At high enough Bi concentrations, the basicity of oxygen is sufficient for this reaction so that changes in basicity due to the presence of cation vacancies are no longer important.

A linear correlation between the electron-binding energy of lattice oxygen in a series of mixed antimony oxide and the rate of acrylonitrile production from propene has also been found.53 If it is true that the electron density varies inversely with the binding energy, the correlation also supports the concept that basicity of the oxygen is important. The validity of the assumption that the electron-binding energy of the oxygen is an indication of its basicity is supported by the good correlation observed for the dehydration/dehydrogenation selectivity of 1-butanol with the binding energy of although such a


would bind to surface Lewis acid sites. Thus desorption is facilitated if the surface site is a weak Lewis acid. In terms of hard-soft acid, water is a hard base, and desorption is facilitated if the cation is a soft acid.

The discussion on the role of hard-soft acid-base properties of cations in the activation of alkene and in the desorption of dienes and water points to the fact that changes in hardness and softness affect different steps in different (sometimes opposite) manners. Thus there is an optimal condition for optimal activity and selectivity. It is important to determine the rate-limiting step so as to modify the catalyst appropriately. It should be emphasized that the rate-limiting step for a reaction depends on the reaction conditions. For example, at high temperature the activation of butene is rate limiting in its oxidation to butadiene on iron oxide.3 But at low temperature the desorption of butadiene is rate limiting. The latter is concluded from temperature-programmed desorption results.@' In fact, conversion of adsorbed butene to adsorbed butadiene has been observed at as low as -10 0C,61 Similar results are observed in propene oxidation to acrolein on U-Sb-0 catalyst.59

Oxygenate Formation 1. Oxygen Insertion. This is the step where C-0

bonds are formed, which may take place subsequent to, simultaneous with, or prior to C-H bond breaking. In this step, it is important that only the correct number of oxygen atoms are incorporated into the molecules. Excess incorporation or incorporation at the wrong position leads to undesirable combustion. There are a t least two important oxide properties that affect the oxygen insertion step: geometric effect and metal-oxygen bond energy.

The geometric effect has been explored by Grasselli and co-worker~.~,~ The idea is that the active site must not be surrounded by too many or too few active oxygens. Too many would lead to excessive oxidation, and too few would lead to an inactive catalyst. Using copper oxide catalyzed oxidation of propene to acrolein as an example, Grasselli and co-workers show that the selective production of acrolein depends on the degree of reduction of copper oxide.62 The optimum was observed with an intermedi- ately reduced oxide. Although the structure of the copper oxide could have been changed due to extensive reduction, the idea of an optimum number of active oxygen surrounding the active site remains relevant.

The importance of metal-oxygen bond strength has long been recognized. It is generally believed that selective oxygen insertion requires an optimal metal-oxygen bond strength. Too weak bonds result in nondiscriminative C-O bond formation, which leads to combustion eventually, while too strong bonds lead to inactive oxygen for the insertion process. The weakly adsorbed oxygen important in alkane activation would not be suitable for this step. In practice, there is generally a lack of reliable information on the M-0 bond strength. Part of the difficulties in generating such information is the fact that this bond strength depends on the degree of reduction of the oxide and the geometric environment of the oxygen atom. In general, the M-0 bond strength increases with the degree of reduction. Under steady-state reaction conditions, an oxide attains a certain oxidation state that represents a balance of the energetics of the bonds which are broken and formed, which in turn determines the rates of the individual processes. Deviation from this steady state is then energetically unfavorable.

Sachtler and co-workers have explored this concept and conjectured that there is an optimal value for the rate of increase in the heat of reduction of an oxide with reduction

L I 6

c 0 4- u 3 3 a,

CT

Degree o f Reduction, x Figure 1. Dependence of heat of reduction on the degree of reduction, showing the region of optimal selectivity.

Table IV. Structure-Sensitive Oxidation Reactions" catalysts reactions

Vz05, V-Ti-0, benzene to maleic anhydride

butene, butadiene to maleic anhydride CO, C2H4, Hz oxidation

ethanol to acetaldehyde methanol to formaldehyde

V205/A1203 o-xylene to phthalic anhydride

Moo3, MoO,/graphite propene oxidation

Fe203 butene to butadiene

"Extracted from the summary in ref 3.

near the steady state of the oxide.@ This is schematically shown in Figure 1. The optimal dAHr/dx (where AHr is the heat of reduction and x is the degree of reduction) is represented by the indicated region, where the steady state of a selective catalyst should be. If the steady state of an oxide is to the left of this region, the oxide can supply an excess number of oxygen atoms readily and combustion results. If the steady state is to the right, the oxygen atoms are too inactive for the insertion reaction. Sachtler and co-workers test this concept with three oxides; only V206/Si02 with an intermediate dAHr/dx value is selective for benzene oxidation to benzoic a ~ i d . 6 ~ This concept and the geometric effect described earlier share a common feature that a selective oxidation catalyst has an optimum number of the right type of oxygen atoms. The two con- cepts explore different methods of controlling this number.

Both the geometric and the M-0 bond strength effects are short ranged. An efficient way for a promoter to affect them is by the formation of a compound. Part of the beneficial effects in the formation of bismuth molybdate, vanadium pyrophosphate, iron vanadate, iron antimonate, etc. could be due to these reasons. Another way is to try to make use of oxide-support interaction, which promotes the formation of an oxide layer along a particular orien- tation such as by epitaxial growth. The specific requirement of anatase TiOz and not rutile as support for vanadium oxide for the oxidation of o-xylene to phthalic anhydride may be due to this reason.64

The idea that some crystallographic surface planes are more selective than others in catalytic oxidation is interesting. This follows from the fact that the metal and oxygen ions in different crystallographic planes usually have different environment, which includes the number, nature, and separation of the surrounding ions. Indeed, a number of oxidation reactions (both selective and non- selective) have been found to be structure sensitive, which is a phenomenon because the activity and selectivity of a catalyst depend on the surface structure, the crystallite


size, and/or the method of preparation.& Table IV briefly summarizes the known examples of structure-sensitive oxidation reactions.

The importance of the metal-oxygen bond distance has been explored by Ziolkowski,66 making use of the correlation between the metal-oxygen bond strength and the bond distance. That the M-0 bond strength is important has been discussed earlier. The nature of the M-0 bond is also important. Terminal M = O bonds are found on the basal planes of molybdates, Moo3, and V2O5, which are selective catalysts. It has long been suggested that the presence of such M=O groups is and that the effect of promoters is to weaken these bonds.6g Re- cently, direct observation of the correlation between activity and the number of V=O groups has been made on Vz05 ~ a t a l y s t s . ~ ~ ~ ~ ~ ~ ~ I t is yet unclear, however, how the V=O participates in the oxidation process. Since there are selective oxidation catalysts (e.g., cuprous oxide, U- Sb-0, ferrites) that do not possess M=O groups, the presence of these groups is not a necessary condition for selective catalysts.

After the removal of an oxygen atom from the lattice, the catalyst needs to be reoxidized to attain a steady state. Such a reduction-oxidation process can be facilitated if the oxide forms shear structure readily.73 The shear structure allows high oxygen mobility in the lattice. Dissociative adsorption of gaseous oxygen leading to reoxidation of the catalyst takes place readily on reduced transition-metal oxides and on oxides that possess 3d electron^.^^-^^ Thus the reoxidation process normally should have little activation barrier.

2. Desorption of Oxygenates. Factors that affect the desorption of oxygenates are the same as those discussed for the desorption of water. Most oxygenates (such as aldehydes and anhydrides) would interact with the surface via the oxygen electron lone pair. These oxygenates are usually Lewis bases and hard bases. Thus desorption is facilitated by weak Lewis acid sites and soft-acid sites.

In certain systems, the desorption of oxygenates is found to be the slow step. For example, propene is oxidized to acrolein on Zn0,'8v79 yet the acrolein remains adsorbed on the surface. It is further oxidized before desorption.

In addition to acidity and basicity, the strength of adsorption of oxygenates can be affected also by the atomic structure of the oxide surface. For example, adsorption of alcohols, aldehydes, and their oxygenates is much weaker on the oxygen-polar surface of ZnO, where the Zn cations are recessed into the surface relative to the oxygen anions such that close approach of the adsorbate to the Zn ion is prevented, than on the other surfaces where the Zn ions are on the surface.s0 If oxygenates can be made to form on these oxygen-polar surfaces, their desorption would be much enhanced.

Combustion Reaction The degradation of the hydrocarbon into carbon mon-

oxide or carbon dioxide and water can be initiated at any point along the selective oxidation reaction pathway. For example, it has been established that ferrite catalysts possess separate selective oxidation and combustion sites for butene oxidation.60ys1 Butene adsorbed on the combustion sites only forms combustion products. Thus degradation takes place on adsorption on these sites. On the selective oxidation sites, degradation takes place by the reaction of weakly adsorbed oxygen with either the adsorbed n-allyl or adsorbed butadiene.82 It is also common to find that when the selective oxidation product is used as a reactant, it can be readily further oxidized to the combustion products.

Table V. Classification of Oxides Based on Weakly Adsorbed Oxygen"

oxides containing moderately weakly adsorbed 02: CuO, Ti02, ZnO weakly adsorbed 02: Mn02, a-Fe2O3, Co3O4, NiO, Sn02 no weakly adsorbed 02: V205, Moo3, BizO3, W03, Bi2Mo06

"From ref 84.

Table VI. Desirable Surface Properties in Selective Oxidation Reactions

reaction alkane activation

C-H bond dissociation

alkene activation C-H bond dissociation

diene (or alkene) desorption water desoprtion

oxygenate formation oxygen insertion

oxygenate desorption prevention of combustion

properties

highly reactive surface oxygen (weakly adsorbed oxygen and/or surface lattice defects)

(1) surface oxygen strong

(2) cation readily undergoes

(3) cation soft acid cation hard acids cation soft acids

Brernsted base

reduction

(1) limited number of available

(2) dAHJdr large for further

cation soft acids (1) short residence time of

surface intermediates (2) weak adsorption of desired

product (3) no weakly adsorbed oxygen (4) no combustion site

oxygen

reduction

The extent of combustion degradation can be minimized by minimizing the surface residence time of the hydrocarbon reaction intermediates, the strength of adsorption of the desired products relative to the reactants, the amount of adsorbed oxygen active for combustion, and the ratio of the combustion to selective oxidation sites. The surface residence time of the reaction intermediates and the strength of adsorption of the desired products are determined by the interaction of the hydrocarbon species with the surface. Factors affecting this interaction have been discussed in the previous sections. Those factors should be optimized accordingly.

The oxygens active for combustion are the weakly adsorbed oxygens. As discussed in the alkane activation step, these oxygens are the most reactive. That they cause combustion has been directly observed in butene oxidation on iron o ~ i d e . ~ ~ ~ ~ ~ Using a temperature-programmed desorption technique, Iwamoto et al. determined the strength and quantity of various forms of adsorbed oxygen on a number of oxides.& From their results, common oxides can be classified into three groups depending on whether they contain moderately weakly adsorbed oxygen, weakly adsorbed oxygen, or no weakly adsorbed oxygen, as shown in Table V. It is interesting to note that the common selective oxidation catalysts, which include V,05, MOO,, and Bi2MoQ6, do not contain weakly adsorbed oxygen. Unfortunately, little is known on how to control the oxide properties to remove the weakly adsorbed oxygen, which may well be adsorbed on the surface defect sites. Indeed, it is observed that oxygen vacancy sites adsorb oxygen that desorbs at a lower temperature than lattice oxygen. These oxygen are also more reactive toward H2 and butane oxidation than is lattice oxygen.85

Finally, there is also little understanding on the dif- ferences between combustion and selective oxidation sites that is useful in catalyst design considerations.

178

Conclusion The selective oxidation and combustion reactions involve

a series of consecutive chemical transformations. The rate of each transformation depends on the particular interaction of the intermediate with the surface and the particular bonds that are broken or formed. Thus different catalyst properties affect different transformation steps, and different properties need to be optimized to improve activity and selectivity. In any case, knowledge of the rate-limiting step that determines reaction rate and the critical branching step that determines selectivity will be very helpful in deciding which promoter should be used to affect the desired oxide properties. The discussion presented, which is summarized in Table VI, should serve as a helpful guide for catalyst development and research.

Acknowledgment Support of this work by the Department of Energy,

Basic Energy Sciences Division, is gratefully acknowledged. References

Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986

Dadyburjor, D. B.; Jewur, S. S.; Ruckenstein, E. Catal. Rev. 1979, 19, 293. Centi, G.; Trlfiro, F. Appl. Catal. 1984. 12, 1. Kung, H. H.; Kung, M. C. Adv. Catal. 1985, 33, 159. Haber, J. I n Surface Propertles and Catawsls by Nonmetals; Bon- nelle, J. P. et al., Ed.; D. Reidel: Hingham, MA, 1983; p 1. Grasselli, R. K.; Burrington, J. D. Adv. Catal. 1980, 30, 133. Grasselli, R. K. I n Surface Properties and Catawsis by Non-metals; Bonnelle, J. P. et ai.. Ed.; D. Reidel: Hlngham, MA, 1983; p 273. Grzybowska, B.; Mazurklewlcz, A.; Sloczynski, J. Appl. Catal. 1985, 13, 223. Haber, J. Int. Chem. Eng. 1975, 15, 21. Keulks, G. W.; Krenzke, L. D.; Notermann, T. M. Adv. Catal. 1978. 27, 183. Grasselli, R. K. Appl. Catal. 1985, 15, 127. Grasselli, R. K.; Burrington, J. D. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 393. Cavani, F.; Centi, G.; Trlflro, F. Appl. Catal. 1985, 15, 151. Hodnett, B. K.; Delrnon, B. Appl. Catal. 1985, 15, 141. Cavani, F.; Centi, G.; Manenti, 1.; Trlfiro, F. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 221. Hucknall, D. J. Selective OxMation of Hydrocarbons ; Academic: New York, 1974. Ueda, W.; Moro-oka. Y.; Ikawa, T. J. Chem. SOC., faraday Trans. 1 1982, 78, 495. Grasselli, R. K.; Brazdil, J. F.; Burrington, J. D. Pmc. Int . Congr. Ca- tal., Bth, 1984 1984, 5 , 369. Kung, H. H. J. Catal. 1982, 73 , 387. Parry, D. E. Surf. Sci. 1975, 49, 433. Parry, D. E. Surf. Sci. 1978, 54, 195. Mark, P. Catal. Rev. 1987, 1 , 165. Mark, P. Catal. Rev. 1975, 12, 71. Pearson, R. G. Hard and Soft Acids and Bases; Dowden, Hutchinson, and Ross: Stroudsburg, PA, 1973. Sazonov. B. A.; Popovskii, V. V.; Boreskov, G. K. Kinet. Katal. 1988, 9 , 312. Bieianski, A.; Haber, J. Catal. Rev. 1979, 19, 1. Lunsford, J. H. Catal. Rev. 1973, 8 , 135. Lunsford, J. H. Adv. Catal. 1972, 22, 265. AMa, K.; Tajima, M.; Isobe, M.; Onishi, T. Proc. Int. Congr. Catal.. 8th, 1984 1984, 3 , 335. Shvets, V. A.; Kazanskii. V. B. Kinet. Catal. (Engl. Trans/.) 1979, 19, 784. Kaliiguine, S. L.; Sheiimov, B. N.; Kazansky, V. B. J. Catal. 1978, 55, 384. Volodin. A. M.; Cherkashin, A. E. React. Kinet. Catal. Lett. 1982, 20, 347. Ward, M. B.; Lin, M. J.: Lunsford, J. H. J. Catal. 1977, 50. 306. Yang, T. J.; Lunsford, J. H. J. Catal. 1980, 63, 505. Liu, R. S.; Iwamoto, M.; Lunsford, J. H. J. Chem. SOC., Chem. Com- mun. 1982, 78. Khan, M. M.; Somorjai, G. A. J. Catal. 1985, 91, 263.

Liu, H. F.; Liu, R.-S.; Liew, K. Y.; Johnson, R. E.; Lunsford, J. H. J. Am. Chem. SOC. 1984, 106, 4117. Adarns, C. R.; Jennings, T. J. J. Catal. 1983, 2 , 63. Adarns, C. R.; Jennings, T. J. J. Catal. 1984, 3 , 549. Voge, H. H.; Wagner, C. D.; Stevensen, D. P. J. Catal. 1963, 2 , 58. McCain, C. C.; Gough, G.; Godin, G. W. Nature (London) 1983, 198, 989.

(41) Sachtler, W. M. H.; DeBoer, N. K. Proc. Int. Congr. Catal. 3rd, 1964, 1985, 1 , 252.

(42) Grasselli, R. K.; Suresh, D. D. J. Catal. 1972, 25, 273. (43) Godin, G. W.; McCain, C. C.; Porter, E. A. Proc. Int. Congr. Catal.,

4th, 1968 1971, 1 , 271. (44) Martir, W.; Lunsford, J. H. J . Am. Chem. SOC. 1981, 103, 3728. (45) Burrington, J. D.; Grasselli, R. K. J. Catal. 1979, 59, 79. (46) Grzybowska, B.; Haber, J.; Janas, J. J. Catal. 1977, 49, 150. (47) Sokolovskii, V. D.; Bulgakov, N. N. React. Kinet. Catal. Lett. 1977, 6 ,

Rr, "-. (48) Tanabe, K. Solid Acids and Bases; Academic: New York, 1974. (49) Tanabe, K. I n Catawsis, Science and Technology; Anderson, J. R.,

Boudart, M., Ed.; Springer-Verlag: Berlin, 1981; Voi. 2, p 231. (50) Parks, G. A. Chem. Rev. 1965, 65, 177. (51) Sleight, A. W.; Linn, W. J. Ann. N.Y. Acad. Sci. 1976, 272, 22. (52) Brazdil. J. F.; Kartisek, L. C.; Grasselli, R. K. J. Catal. 1983, 81, 142. (53) Zhdan, R. A.; Shepelin, A. P.; Osipova, 2. G.; Sokolovskii, V. D. J.

Catal. 1979, 58, 8. (54) Vinek, H.; Noiler, H.; Ebei, M.; Schwarz, K. J. Chem. Soc., faraday

Trans. 11977. 73, 734. ( 5 5 ) Davls, B. H.; Russell, S. N.; Reucroft, P. J.; Shalvoy, R. B. J. Chem.

Soc., faraday Trans. 11980, 76, 1917. (56) Driscoll. D. J.; Lunsford, J. H. J. Phys. Chem. 1983, 87, 301. (57) Ai, M. J. Catal. 1978, 52, 16. (56) Ai, M. J. Catal. 1979, 60, 306. (59) Keulks, G.; Yu, 2. Proc. Int. Congr. Catal., Bth, 1984 1984, 3 , 289. (60) Kung, M. C.; Cheng, W. H.; Kung, H. H. J. Phys. Chem. 1979, 83,

1737. (61) Kung, M. C.; Kung, H. H., results to be published. (62) Calbhan, J. L.; Grasselli, R. K. AIChE J. 1983, 9 , 755. (63) Sachtler, W. M. H.; Dorgels, G. J. H.; Farenfort, V.; Voorhoeve, R. J.

H. Proc. lnt. Congr. Catal., 4th, 1988 1971, 1 , 355. (64) Wachs, I. E.; Saleh, R. Y.; Chan, S. S.; Chersich, C. C. Appl. Catal.

1985, 15, 339. (65) Boudart, M. Adv. Catal. 1989, 20, 153. (66) Zlolkowski, J. J. Catal. 1983, 80, 263. (67) Trlfiro, F.; Centola, P.; Pasquon, I.; Jiru, P. Proc. Int. Congr. Catal.,

4th, 1988 1971, 1, 310. (68) Mltcheli, P. C. H.; Trifiro, F. J. Chem. Soc. A : 1970, 3183. (69) Tamaru, K.; Teranishi, S.; Yoshida, S.; Tamura, N. Proc. Int. Congr.

Catal., 3rd, 1964 1985, 1 , 266. (70) Mori, K.; Inomata, M.; Miyamoto, A,; Murakami, Y. J. Phys. Chem.

1983, 87, 4560. (71) Mori, K.; Miyamoto, A.; Murakami, Y. J. Phys. Chem. 1984, 88,

274 1. (72) Mori, K.; Miyamoto, A.; Murakami, Y. J. Phys. Chem. 1984, 88,

2735. (73) Haber, J.; Janas, J.; Schiavelio, M.; Tilley, R. J. J. Catal. 1983, 82,

395. (74) Kurtz, R. L.; Henrich, V. E. Phys. Rev. B : Condens. Matter 1982, 25,

3563. (75) Kurtz, R. L.; Henrich, V. E. Phys. Rev. B : Condens. Matter 1983, 28,

6699. (76) Henrich, V. E.; Dresselhaus, G.; Zeiger, H. J. J. Vac. Sci. Techno/.

1978, 15, 534. (77) Henrich, V. E.; Dresselhaus, G.; Zeiger, H. J. Phys. Rev. Lett. 1978,

36, 1335. (78) Kugler, B. L.; Kokes, R. J. J. Catal. 1974, 32, 170. (79) Kugler, B. L.; Gryder, J. W. J. Catal. 1978, 44, 126. (80) Akhter, S.; Lui, K.; Kung, H. H. J. Phys. Chem. 1985, 89, 1958. (81) Kung. H. H.; Kundalkar, B.; Kung, M. C.; Cheng, W. H. J. Phys. Chem.

1980, 84, 382. (82) Yang, B. L.; Kung, H. H. J. Catal. 1982, 77, 410. (83) Yang, B. L.; Kung, M. C.; Kung, H. H. J. Catal. 1984, 89, 172. (84) Iwamoto, M.; Yoda, Y.; Tamazoe, N.; Seiyama, T. J. Phys. Chem.

1978, 82, 2564. (85) Seiyama, T.; Yarnazoe, N.; Eguchi, K. Ind. Eng. Chem. Prod. Res.

Dev. 1985, 24, 19. (86) Kerr, J. A. Chem. Rev. 1986, 66, 465. (87) Bartmess, J. T.; McIver, R. T. I n Gas Phase Ion Chemistty; Bower,

M. T., Ed.; Academic: New York, 1981; Vol. 2, p 87.

Received for review August 5, 1985 Accepted January 2, 1986

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Desirable catalyst properties in selective oxidation reactions