Chapter -1 Introductionshodhganga.inflibnet.ac.in/bitstream/10603/27786/6/06_chapter_1.pdf · reactions. Numerous important applications are found in the pharmaceutical and ... Aniline

Chapter -1 Introduction

Chapter 1 General Introduction

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1.0. General Introduction

Heterogeneous catalysts are philosopher’s stones, of fundamental importance for

both the industry and academia in the production of fuels and chemicals, both bulk and fine

globally [1, 2]. Due to rising of world population, the answer for escalating universal

demand for energy and feedstock, a steer towards sustainability, and the cost factor of rare

and noble metals, development of new catalysts is vital. It is necessary to develop new

catalysts which display highest selectivity towards required products at high rates.

Preferably, such development can be achieved through design of particularly made systems

to suit various applications. Present heterogeneous catalysts are based on transition metal

oxide systems and zeolites etc. These catalysts are useful in a wide range of reactions such

as cracking, hydrogenation and oxidation reactions, to produce bulk chemicals at a large

scale. Heterogeneous catalysts offer advantages in terms of easy separation from the

product, high stability and excellent life time compared to homogeneous catalysts.

Homogeneous catalysts, with some exceptions, are mostly applied for special challenging

catalytic problems, e.g., for the production of fine chemicals, as they show very high

selectivity towards desired product. The structural environment can be easily and in some

cases rationally tuned by means of variation of ligands attached to the single atom active

site.

Over the past decades, most of the studies on heterogeneous catalysts focus on

understanding the interaction of the substrate with the catalysts and the parameters deciding

the performance of a heterogeneous catalyst [3, 4]. A classical example of tailor making the

catalytic properties of a heterogeneous catalyst by means of (surface) modification has been

commercially applied for many years; the famous one, in this category is Lindlar catalyst.

Surface modification of Pd/CaCO3 not only selectively hydrogenates triple bonds to double

bonds, but also shows regioselectivity of the resulting double bond [5,6]. Further it was

reported that surface poisoning with quinoline during the processing additionally increases

the selectivity towards the double bond [7]. Even after six decades, the structure of this

catalyst is still under examination and improvements are being proposed. To understand the

structure-activity relationship of heterogeneous catalysts, a deep understanding of all the

parameters which influence the reaction is necessary. It is important to know the

mechanism of a reaction, which involves the information of intermediates during the


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catalytic process and the nature of the active sites, i.e. the electronic and geometric

structure of the metal and the support. To gain an insight into deep understanding of all

these issues appropriate individual reactions must be carried out to obtain information

under practical working conditions [4, 8 , 9].

The main aim of this thesis is to develop efficient supported non-noble metal

catalysts for few industrially important hydrogenation reactions, determining the structure

of the catalyst under reaction conditions and predicting the reaction mechanism by spotting

the reaction intermediates. The reactions of interest are the hydrogenation of nitrobenzene

& levulinic acid and the decisive goal is to design a good catalyst systems.

1.1. Supported metal catalysts

Supported metal catalysts are widely employed in many industrial processes such as

petroleum refining and petrochemical industries. These catalysts play an important role in

the conversion of hydrocarbons to synthesis gas via reforming, isomerization of paraffins,

cycloalkanes, reduction of various organic compounds (hydrogenation), oxidation,

reduction of NOx gases, hydrodechlorination and Fischer-Tropsch synthesis. Supported

metal catalysts differ from bulk metal catalysts in the aspect of consisting of smaller metal

particles, which are separated from one other to certain extent. In supported catalysts, the

catalyst usually consists of a support, an active component and promoter (if needed). The

catalytically active material is usually dispersed on a support and the role of support is to

expose maximum amount of the catalyst material, to provide a large interfacial area and to

stabilize the active phase. There are number of advantages in depositing catalytically active

metals on supports like alumina, titania, zirconia, zinc oxide, silica, and mesoporous silica’s

(SBA-15, SBA-16, COK-12 and KIT-6). The use of support is to enhance the mechanical

strength, thermal stability and to disperse the active component in the form of smaller

particles. In most of the catalytic reactions, the combined characteristics of active phase and

support can drastically enhance the catalytic functionalities. Although the support material

only serves as a medium for keeping the catalytically active species supported, these

species interacts to some degree with the support. Due to the active phase-support

interactions, the catalytic performance strongly depends on a complex mix-up of


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contributions of the morphology and dispersion of active metal particles. Therefore, besides

the active component and promoter the role of support also needs to be considered. It is

possible to disperse metal particles throughout the porous structure of the support. As a

result, large active metal area is produced relative to the weight of the metal used. The

surface to volume ratio of active component is especially advantageous in case of precious

metals. The support is quite useful in the dissipation of reaction heat and retards the

sintering of metal crystallites, hence maintaining the active surface and increase the poison

resistance, which promotes a longer catalyst life. These advantages make supported metal

catalysts preferable over bulk metal catalysts for chemical processing.

1.2. Hydrogenation

Selective hydrogenation of aromatic nitro compounds is useful to synthesize

organic chemicals which are key products in fine chemical synthesis. This is an important

and interesting area of research especially if there are two or more C=C bonds. The reaction

becomes challenging if the aromatic compounds possess functional groups, since the

hydrogenation of C=C is thermodynamically more favorable than hydrogenation of the

functional groups. Getting the selectivity towards a particular functionality is not easy and

requires specific reaction conditions and catalyst system.

Chemical reaction between molecular hydrogen and an element or compound,

generally takes place in the presence of a catalyst. The reaction may be one in which

hydrogen simply adds to a double or triple bond connecting two atoms in the structure of

the molecule or one in which the addition of hydrogen results in dissociation (breaking up)

of the molecule (called hydrogenolysis, or destructive hydrogenation). Nearly all organic

compounds containing multiple bonds connecting two atoms can react with hydrogen in the

presence of a catalyst. Especially, hydrogenation of -NO2 an –C=O are of particular interest

in the present thesis.

1.2.1. Common features of hydrogenation reactions

There are two types of hydrogenation reactions i.e., chemeoselective hydrogenation

and regioselective hydrogenation. The chemeoselective hydrogenation catalysts are mostly


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based on supported metals involving Pt, Ru, Rh, Cu and Pd. The physico-chemical

properties of a support, like its acid-base character, reducibility and the extent of metal-

support interaction play an important role in the complex chemistry of supported metal

catalysts. Synthesis of catalysts with basic support is an interesting and challenging job. It

is useful to study the nature of metal species present on the support from the point of view

of reduction characteristics, dispersion and metal support interaction.

1.2.2. Industrial applications of hydrogenation reactions

Hydrogenation of organic compounds (hydrogenation and hydrogenolysis) is a

reaction of great industrial importance. The addition of hydrogen is used in the production

of edible fats from liquid oils. In the petroleum industry, plentiful processes involve the

conditioning of gasoline and manufacture of petrochemical products which are based on the

destructive hydrogenation of hydrocarbon. Nickel, platinum, and palladium based catalysts

are commonly used catalysts for hydrogenation reactions. Copper chromate and nickel

supported on kieselguhr are comprehensively used for high pressure hydrogenations

reactions.

Numerous important applications are found in the pharmaceutical and

petrochemical industries. Complete hydrogenation converts unsaturated fatty acids to

saturated ones. In practice the process is not usually carried out to completion. Since the

original oils typically contain more than one double bond per molecule (that is, they are

poly-unsaturated), the result is usually described as partially hydrogenated vegetable oil,

i.e., some, but usually not all, of the double bonds in each molecule have been reduced.

Hydrogenation results in the conversion of liquid vegetable oils to solid or semi-solid fats,

such as margarine. Changing the degree of saturation of the fat changes important physical

properties such as melting point. That’s why semi-solid fats are preferred for baking

because the fat mixes well with flour to produce a more desirable texture in the baked

product. Since partially hydrogenated vegetable oils are much less expensive than most

other fats with similar characteristics, and because they have other desirable characteristics

leading to longer shelf life, they are predominantly used in most commercial baked goods.

Processes accomplishing the reverse are called "dehydrogenation" or "partial

dehydrogenation." A side effect of incomplete hydrogenation that has implications for


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human health is the isomerization of the remaining unsaturated carbon bonds. The cis

configuration of these double bonds predominates in the unprocessed fats of most foods.

The catalytic hydrogenation process favors the conversion of cis to trans forms because the

trans conformation is lower energy than the natural cis conformation.

Hydrogenation typically uses hydrogen gas as a reactant and an undissolved (or

"heterogeneous") metal catalyst, such as copper, nickel, palladium or platinum. If not, the

"homogeneous" rhodium-based catalyst commonly known as Wilkinson's catalyst is

frequently used. The reaction is usually carried out at high temperature and pressure. The

French chemist, Paul Sabatier greatly facilitated the industrial use of hydrogenation. In

1897, he discovered that the introduction of a trace of nickel as a catalyst facilitated the

addition of hydrogen to molecules of carbon compounds.

1.2.3. Selective hydrogenation of nitrobenzene

Aniline is the one of the most significant key compound in organic chemistry. Many

commodity chemicals including cyclohexylamine, benzoquinone, akylanilines etc., are

manufactured from aniline. Aniline is mainly produced from the catalytic hydrogenation of

nitrobenzene at 300-475 °C in fixed bed reactor. The hydrogenation of nitrobenzene to

aniline selectively is shown in Scheme 1.1.

Scheme1.1. Hydrogenation of nitrobenzene.


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Reactions involving catalysts for hydrogenation seems to fall into four groups based

on nature of the catalyst, pressure and temperature ranges, economy and selectivity. These

are (i) Platinum-palladium series, (ii) Nickel series, (iii) Copper-mixed oxide series and (iv)

Molybdenum-Tungsten sulfide series. Platinum–palladium catalysts are useful at low

temperatures and pressures. Nickel catalysts prepared by varied processes are particularly

helpful. Raney nickel, prepared by leaching a nickel-aluminium alloy with caustic, finds

widespread application in pressure reductions; Copper chromium oxide is an exceptionally

smart catalyst for the reduction of nitro group. The nickel catalysts were applied to

hydrogenation of aromatic compounds such as nitrobenzene to aniline, benzene to

cyclohexane etc.

Around 85% of aniline is obtained by catalytic hydrogenation of nitrobenzene.

However, the transformation is extremely facile and is administrated under relatively mild

conditions. For this reason, hydrogenation occurs rapidly over most of the metals and is

often used as a standard reference reaction for scrutinizing the activities of other

hydrogenation catalysts [10 -15]. The hydrogenation of nitrobenzene has been reported to

occur over carbon nanotube supported platinum catalyst [16]. Platinum nanoparticle core-

polyaryl ether trisacetic acid ammonium chloride dendrimer shell nanocomposites were

employed for hydrogenation of nitrobenzene to aniline with petro chemically derived H2

under mild conditions [17]. Active carbons were used as supports for palladium in the

liquid phase hydrogenation of nitrobenzene to aniline [18]. Hydrogenation of nitrobenzene

was studied over Pt/C catalyst in supercritical carbon dioxide and ethanol [19]. Polymer

anchored metal complex catalyst has been used for the hydrogenation of nitrobenzene [20].

Liquid phase hydrogenation of nitrobenzene was studied over Pd-B/SiO2 amorphous

catalyst [21].

1.2.3.1. Synthesis of Aniline

Aniline is an important chemical that is used extensively in laboratory organic

synthesis and possesses larger scale industrial applications [22-24]. A reliable method of

aniline production provides a constant feed to many wide ranging industries including

pharmaceutical, automotive and construction industries.


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There are several methods of synthesizing aniline using a variety of starting

materials but nitrobenzene is the classical and the most frequently used feedstock. On the

small scale, aniline can be produced by the chemical reduction of nitrobenzene in a process

known as the Bechamp reaction [25-27] (Scheme 1.2). This traditional method involves the

use of iron turnings and water in presence of hydrochloric acid to reduce the nitro-group to

amine. This was also the first method used in the industrial production of aromatic amines

way back in 1854 and had the advantage of getting iron oxide pigments from the iron oxide

residues, as a side product during the reaction [27]. The Bechamp reaction is still currently

utilized by Bayer to produce a wide range of iron oxide pigments in batch processes but not

aimed for the commercial production of aniline.

NO2

Nitrobenzene (NB)

Fe

HCl

NH2

Aniline (AN) Scheme 1.2. The Bechamp Reaction

More recently, the Bechamp reaction was surpassed by a more economically viable

route, by a catalytic hydrogenation of nitrobenzene. In this process the aromatic nitro group

is reacted with three mole equivalents of hydrogen gas, in the presence of suitable catalyst

to produce the amine and water. This hydrogenation is very easily, carried out under

relatively mild conditions and produces only low level of by-products. This reaction occurs

rapidly over most of the metals and is often employed as a reference reaction to compare

the activity of other hydrogenation catalysts in different reactor systems [28-39]. Currently

using heterogeneously catalysed hydrogenation of nitrobenzene, aniline can be produced

with greater than 99% selectivity. Details of commercial reactions in operation and

industrial catalysts are provided in the subsequent sections.

Although utilized to a lesser extent, three other catalytic routes have been used in

the production of aniline. The first route involves the amination of chlorobenzene [25]. For

instance, the Kanto Electrochemical Co. Process involves of the ammonolysis of

chlorobenzene using aqueous ammonia over a Niewland catalyst: a mixture of copper (I)


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chloride and ammonium chloride, to produce aniline and hydrochloric acid as reaction

products. This process yields 91% selectivity towards amine and was employed by Bayer

until 1986 when it was discontinued for economic reasons [25].

A second route towards aniline uses phenol as a starting material and again involves

ammonia in the amination process [25, 27]. The hydroxyl group on phenol reacts with

gaseous ammonia over an A12O3-SiO2 catalyst to produce aniline and water as products.

This method has become greater importance in recent years, once the as phenol technology

was commercially established. The development of a novel single-stage phenol

transformation of benzene to phenol has promoted to produce large quantities of phenol as

a feedstock [40]. In addition, Du Pont has explored a third route for aniline production

using benzene as starting material to produce aniline directly without the need of producing

nitrobenzene intermediate [25]. Benzene is reacted with concentrated ammonia over a

NiO/Ni catalyst promoted with zinc oxide. An aniline selectivity of 97 % was observed in

this route at a maximum benzene conversion of only 13 % and hence proved to be a barrier

to the industrial scale production. Despite the intensive investigations into these routes, the

method of choice for the production of aniline is the hydrogenation of nitrobenzene using a

heterogeneous catalyst because of its simplicity in operation. A summary of all four

methods can be seen in the scheme. 1.3

(i) The catalytic hydrogenation of nitrobenzene

NO2 NH2

3 H2 (g) 2 H2OCatalyst

(ii) The amination of chlorobenzene


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Cl

NH3

NH2

HClCatalyst

(iii) The amination of phenol

OH

NH3

NH2

H2OCatalyst

(iv) The amination of benzene

NH3

NH2

Catalyst

H2O

Scheme 1.3. Various routes for the synthesis of aniline

1.2.3.2. Commercial production of aniline

As already stated the commercial production of aniline is based on using a

heterogeneous catalyst through the effective hydrogenation of nitrobenzene. However the

reaction conditions widely vary depending on the type of catalyst used.


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Figure-1.1. Aniline capacity share by company in Western Europe

During 2001, the global capacity for aniline stood at 3.0 million tonne/year [27].

The majority of global aniline production is centered in the U. S, Western Europe and Asia.

Western Europe and Asia alone contributes 42.2 % to the world’s aniline stocks [41-43]. In

recent years the major companies producing aniline are Bayer, Hustsman, BASF, DOW

and Quimigal and the percentage contribution by each company is shown in Figure-1.1.

The global aniline market is projected to reach 6.2 million tons by the year 2015,

due to the increasing demand of aniline from various end-user markets. In particular, the

rising demand for methylene diphenyl diisocyanate (MDI), the chief ingredient in

polyurethane foam, prompts to consume more quantities of aniline.

The nitrobenzene hydrogenation process can be carried out in either the gas phase

or liquid phase and both are used in commercial production. Of the aniline producers using

a gas phase process, Bayer utilizes a fixed-bed of NiS catalyst that has been activated using

copper or chromium [26]. Reactions are performed at temperatures ranging from 573-748

K and aniline selectivity’s greater than 99 % are achieved. The catalyst bed is prone to

catalyst deactivation due to carbon deposition but can be regenerated in air at 250-350 °C

followed by H2 (g) passivation. On the other hand, a fluidized bed is utilized for the

hydrogenation process operated by BASF using catalysts containing copper - chromium,

barium and zinc oxides on a SiO2 support [23]. Reaction conditions are in the temperature

range of 270-290 °C and 1-5 bar pressure with a large excess of hydrogen. This method

exhibits a high selectivity of 99.5% towards aniline but needs regeneration of deactivated

catalyst in air at regular intervals throughout production process.

33%

27%

19%

11%

10%

BayerHuntsmanBASFDOW chemicalsQuimigal


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The liquid phase hydrogenation of nitrobenzene is also performed industrially with

Huntsman technology employing a semi-continuous batch process using a stirred tank

reactor to produce aniline [41]. A typical catalyst consists of 55 % by weight of nickel

supported on Kieselguhr that has been pre-reduced and stabilized before use [45]. The

catalyst is re-activated by hydrogen when the reactor vessel reaches reaction temperature

(usually between 70- 150 °C). Hydrogenation is generally performed under a hydrogen

pressure of 20-40 bar and a nitrobenzene conversion in excess of 99.7 % can be achieved.

1.2.3.3. Uses of Aniline

Aniline is used as a feedstock in a number of different industries leading to a wide

range of applications.

Figure-1.2. Uses of Aniline in Global Market

Aniline is primarily used in MDI foams for the automotive and construction

industries. Many chemicals can be made from Aniline, including:

Isocyanaates for the urethane industry

Antioxidants, activators, accelerators, and other chemicals for the rubber industry

Indigo, acetoacetanilide, and other dyes and pigments for a variety of applications

Diphenylamine for the rubber, petroleum, plastics, agricultural, explosives, and

chemical industries

Various fungicides and herbicides for the agricultural industry

MDI Pharmaceuticals Rubber Dyes others


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Pharmaceutical, organic chemical, and other products

In the synthesis of sildenafil (Viagra), the antibiotic linezolid (Zyvox) and the HIV

ptotease inhibitor amprenavir (agenerase) etc.

(i) Synthesis of Methylene di- phenylene diisocyanate (MDI)

The huge amount of global aniline ( 80-85%) is consumed in the production of

methylene di-phenylene diisocyanate (MDI) [26,27,42]. MDI is synthesized in a two step

process, as shown in Scheme 1.4 [46,47], which includes condensation with formaldehyde

to produce a, dimeric species followed by phospenation to transform the amine

functionality to isocyanate groups. MDI is then polymerized and used to synthesize

extremely versatile materials known as polyurethanes. This is a growing industry and

presently, the demand for MDI is increasing steadily at the rate of 6-8% per year [27].

NH2

CH2O (aq)

HCl

Aniline Formaldehyde

H2CH2N NH2

H2COCN NCO

COCl2

Methylene di-para-phenylene-isocyanate (MDI)

Scheme 1.4 Synthesis of MDI from aniline.

(ii) Polyurethanes

R OHHO R NCOOCN

Dihydroxy compound Diisocyanate

R O CO

NH R OCO

HN

Polyurethane

Scheme 1.5 Formation of polyurethane

Polyurethanes are synthesized by diisocyanate addition polymerization, a method

discovered by Bayer in 1937 [46]. This involves a reaction of a diisocyanate with a

polyhydroxyl compound in the presence of a suitable catalyst and additives [24, 46, 48, and

49]. A variety of diisocyanates and hydroxylated compounds can be used in the reaction

leading, to a range of polymeric products with different physical properties. The most

common commercial methods utilize a diisocyanate and hydroxyl terminated polyester or

polyether. The production of polyurethane is shown in scheme 1.5. MDI polyurethanes are


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versatile polymers used in the manufacture of rigid and semi- rigid foams, elastomers and

coating resins. These materials are in general used in the construction industry for building

insulation, white goods industry for insulation in the automotive industry for car interiors

and in the sportswear industry for flexible trainer soles [47, 50 and 51].

The remainder of the aniline is used in the production of a number of other products

namely dyes, rubber additives and pharmaceuticals. In addition, it is also utilized in

agricultural industry to synthesize pesticides, herbicides, photographic developers,

explosives and speciality fibres [52].

1.2.3.4. Production of aniline from nickel catalysts

An assortment of catalytic systems was prepared for the synthesis of aniline from

the hydrogenation of nitrobenzene. All the heterogeneous catalytic systems require under

dynamic conditions such as high pressure, high temperatures and solvent systems etc.

NO2 NH2

3 H2

Ni/SBA-15 2 H2O

Nitrobenzene (NB) Aniline (AN)

Scheme 1.6 Hydrogenation of nitrobenzene over Ni/SBA-15 catalysts

However synthesis of aniline from nitrobenzene can be operated at normal

conditions like atmospheric pressure, solvent free conditions over nickel based catalysts.

Scheme.1.6 shows the hydrogenation of nitrobenzene over Ni/SBA-15 catalyst, which

appears to be a highly economically viable procecess [53].


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1.2.4. Conversion of biomass to levulinic acid

1.2.4.1. Methods and catalytic system

Figure 1.9. Conversion of lignocellulosic biomass to levulinic acid [54, 55].

Extensive R & D work to convert renewable, reliable and abundant lignocelulosic

biomass to promising building blocks have been conducted by National Renewable Energy

Laboratory (NREL) and Pacific Northwest National Laboratory (PNNL) [54, 55]. Among

the promising candidates, levulinic acid resides at the top 12 building blocks which is

obtained through agriculture waste and forestry [54, 55].

Levulinic acid (LA) is a water-soluble acid (pKa = 4.59) with a high-boiling point

(248 °C), that crystallizes at room temperature (melting point 38 °C). The molecular

structure of LA contains two reactive functional groups (–C=O and –COOH) that affords

the prospect for a range of synthetic transformations [55, 56].

Isomerisation of D-glucose to D-fructose in the presence of acidic species produces

a number of compounds such as 5-hydroxymethylfurfural (HMF), formic acid, including

LA [57-59] (Figure 1.10). HMF is prone to recombine with sugars or itself through aldol-

condensation in acidic solution, resulting in polymers with undefined structures and

stoichiometry [60, 61]. It is thus expected that by changing the solvent as well as the


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amount and/or the nature of the employed catalyst, the reaction rates of diverse steps can be

altered to obtain variety of products [62-65].

Figure 1.10. Possible reaction pathways from cellulose to levulinic acid

To take advantage of the large potential applications of LA as key platform

chemical, chemical industry firms and researchers devoted numerous activities to address

the above mentioned issues. The first pioneering study performed by Mulder in early 1840s

on the preparation of LA was reported [66]. His group tried to prepare LA by heating a

mixture of sucrose with mineral acid such as HCl. However the details on the reaction

conditions and the LA yield are unknown. The first commercial scale was set up for the

production of levulinic acid in an autoclave in United States by Stanley in 1940 [66].

Hanna [67] and Heeres [68] along with their co-workers have performed the reaction by

using kernel grain sorghum and starch as starting material for the manufacture of levulinic

acid. Particularly, the maximum yield of levulinic acid obtained was above 50%. Hawley

and co-workers [57] have been studied the conversion of HMF into levulinic acid in the

eighties wherein a LZY zeolite catalyst was selected and later on Heeres [68] reported a

maximum yield of 60% by using sulphuric acid as a catalyst. Horváth and fellow workers

[69] reported 54% yield of levulinic acid by the conversion of sucrose and rehydration of

HMF to levulinic acid in 2008. For these processes they have used sulphuric acid or Nafion

NR50 – a solid acid catalyst with water as a reaction medium which facilitates easy

separation.


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Mascal [70] recently reported the dehydration of glucose to levulinic acid with 79%

yield using hydrochloric acid as a catalyst and dimethyl chloride as a solvent. As well, 5-

(chloromethyl) furfural was dehydrated into LA at 190 °C in water toget 91% yield [70].

Jin and coworkers converted carbohydrate biomass to HMF and levulinic acid. The highest

yield of LA was 55%, which was obtained with HCl at a pH of 1.5 in 5 min reaction time

[71]. Zhuang and coworkers investigated the conversion of cellulose to levulinic acid by

different metal chlorides including alkali metals (Li, Na and K), alkaline earth metals (Mg

and Ca), transition metals (Cr, Mn, Fe, Co, Cu and Zn) and Al as a group IIIA metal.

Among those metal chlorides, chromium chloride was found to be exceptionally effective

for the conversion of cellulose to levulinic acid, affording yield of 67 mol% after a reaction

time of 180 min at 200 °C [72]. Lucht investigated the conversion of cellulose to glucose

and LA by means of a solid catalyst system based on Nafion SAC 13 or FeCl3/silica and

obtained 5% yield of LA [73].

Likewise, a comprehensive overview of biomass based synthetic protocols for LA

formation, representative studies were cited. Based on literatures, HMF was traditionally

thought as an intermediate for the formation of levulinic acid under acidic conditions. A

large number of researches have been focused on the production of HMF using different

solvents and different catalysts including homogeneous and heterogeneous catalysts. In

order to better understand the production of HMF and levulinic acid, the representative

results of HMF are also cited in the literatures and patents.

Although major achievements in HMF or LA production was reached, the main

disadvantages still exists, especially for production of LA, which can be shortly

summarized as following:

(1) Often mineral acids such as hydrochloric acid or sulphuric acid were used which

enables serious drawbacks such as corrosiveness leading to deterioration of steel based

high pressure equipment.

(2) The usage of high boiling point solvents such as DMF or DMSO might be an alternative

because it allows normal pressure applications. However the necessary separation and

isolation of the gained products is tedious and not cost effective.

(3) If bio-catalysts are used for the transformation, their separation from the reaction

mixture is difficult. Overall, the need for more selective and active catalysts, which are


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easily separable, robust, and which are compatible with earlier and following

transformation steps is still a major goal in this field.

1.2.4.2. Mechanistic aspects of the formation of levulinic acid

Numerous studies have contributed to investigate possible reaction mechanisms of

sugar dehydration to LA. However, until now a fully comprehensive reaction network able

to explain the variety of product distributions obtained, is still not clearly understood [74-

78]. The available information implies that C6-sugars initially would be dehydrated to form

5- hydroxymethylfurfural (HMF) as an intermediate which was subsequently hydrated to

give the final products levulinic and formic acid. Figure 1.11 shows the proposed acidic

mechanism for the conversion of C6 sugars, such as D-fructose to HMF. A study by Antal

and coworkers suggested that HMF is formed from dehydration of fructose in its furanose

form and occurs through a series of cyclic furan intermediates (Figure1.11, pathway b)

[75]. Moreau [59, 78] and coworkers postulated that HMF is formed via an enediol

pathway (Figure 1.11, pathway a) in which the enediol is the decisive intermediate in the

isomerization of glucose to fructose. The conversion of HMF results the formation of

water in addition to the C-2 and C-3 bond of the furan ring to give the final products

levulinic and formic acid (see Figure 1.12) [59, 78].

Figure-1.11. Possible dehydration mechanisms for formation of HMF [55]. The

acyclic route is labeled with an “a,” the cyclic route with a “b”[57].


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O

O O

OHOO

OHOO

O

O

OH

OH OH

OO

OH

OH

O O O

CH(OH)2O

O

OH

O

+H2O/H+ -H2O +H2O

HCOOH 5,5- dihydroxypent-3-ene-2-one

HMF

-H2O

Figure 1.12 Proposed mechanisms for formation of levulinic acid from HMF [79].

1.2.5. Potential applications of LA and its derivatives

1.2.5.1. Building blocks derived from levulinic acid

Figure-1.13. Overview of important LA derivatives. This figure was adapted from PNNL

report [55].

LA can be generated at least in principle from almost all C6 sugars manufactured in

the biorefinery, and for that reason, have frequently been suggested as a starting material


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for a wide number of compounds [56]. Reductions, oxidations and condensations reactions

could give access to potential derivatives (Figure-1.13).

1.2.5.2. Economical and ecological considerations

The family of compounds available from LA is quite broad, and addresses a number

of large volume chemical markets [56].

(1) Conversion of LA to 2-methyltetrahydrofuran (2-MTHF) [80,81] and various levulinate

esters in the Cluster of Excellence ˝Tailor-Made Fuel from Biomass (TMFB)˝ which

address ˝fuel markets as gasoline and biodiesel additives, respectively˝[82]. 2-MTHF is

a highly flammable mobile liquid which currently acts as a replacement for THF in

special applications. In comparison with THF, 2-MTHF dissolves only small amounts

of water which allows easier separations [83,84].

(2) Levulinate esters have been considered as potentially renewable diesel fuels [85-87].

Besides, these ketoesters are good substrates for a variety of condensation and addition

reactions [88-90]. A levulinate ester can also be efficiently converted into a glassy

polymer via reaction with primary alkyl amines. This polymer finds application in

coatings and films [87-91].

(3) γ-Amino-levulinic acid is ˝a herbicide and a precursor for porphyrins and hemoglobin.

It targets a market of 90 – 140 thousand tons per year and is produced at a cost in the

range of 4 to 6 $/kg.˝ This material find application in the production of new acrylate

polymers [92], ˝at a market size of 1.1 billion t/a with production costs of about 2.8

$/kg˝ [55].

(4) Diphenolic acid is of particular interest because it can serve as a replacement for

bisphenol A in the production of polycarbonates [55, 93]. It can be prepared by the

condensation reaction between phenol and levulinic acid in the presence of

hydrochloric acid. ˝The polycarbonate resin market is almost 2 million t/a, at a rate of

about 5 $/kg˝ [94].

(5) Novel technology also suggests oxidative processes for production of acrylic acid from

LA [95-96].

(6) LA is also a potential starting material for production of succinic acid, which is now

used within ˝the food and beverage industry, primarily as a sweetener. Global


20

production is estimated at 16.000 to 30.000 tons a year, with an annual growth rate of

10%.˝ [97].

(7) Production of LA derived lactones offers the opportunity to enter a large solvent

market, as these materials could be converted into analogs of N-methylpyrrolidinone

[98, 99]. Reduction of LA leads to 1,4-pentanediol [98], which could be used for

production of new polyesters [100,101].

1.2.6. Uses of Gamma valerolactone

Gamma valerolactone is a promising candidate for production of fuels and

chemicals. Various products are manufactured from the gamma valerolactone such as 2

methyl tetrahydrofuran,1,4 pentane diol, α-methylene γ-valerolactone, methyl pentenoate,

dimethyl adepate, butene, C8 alkanes, pentenoic acid, pentanoic acid, pentanoate esters, 5-

nonanone. Furthermore, Gamma valerolactone is used as a solvent due to its renewable

nature. The following Figures represent the transformation of GVL to variety of chemicals (

Figure-1.15) and as a solvent for many reactions (Figure-1.14).

Figure-1.14. Lignocellulosic biomass derived product obtained using GVL as solvent.


21

Figure-1.14. Reaction pathways for the conversion of GVL into fuels, fuel additives and

chemicals [102].

1.3. Aims & Objectives

Reactions of hydrogen play a vital role in the chemical reactions with about 40% of

the industry directly depends on hydrogen in the synthesis of bulk and fine chemicals. Most

part of hydrogen is utilized in the production of NH3 and the remaining of it is utilized in

various hydrogenation reactions counting from hydrogenation of oils to synthesis of fuels,

from the manufacture of bulk chemicals such as methanol to making of various drugs and

pharmaceuticals, in the transformation of nitro compounds to amines compounds, levulinic

acid to γ-valerolactone. Hydrogen can also be produced in small way in reactions like

secondary alcohols to carbonyl compounds. Based on the importance of hydrogen, the

following reactions have been selected for study in the present work.


22

a) Hydrogenation of Nitro benzene to aniline

b) Hydrogenation of levulinic acid to γ-valerolactone

Nickel catalysts supported on various supports are used for hydrogenation reactions.

The support plays an important role in dispersing the active metal and influences the

electronic and adsorption properties of the metal thereby changing the catalytic properties.

Various conventional supports such as Al2O3, SiO2, and MgO are commonly used in

dispersing the non-noble metals. Even though, many inorganic materials have been used for

dispersing the metal; the metal support interaction is also an important aspect for increasing

the activity. So the search for new and improved supports is still on, for better dispersion of

the metal, strong metal-support interaction which helps in enhancing the catalytic activity

involving the metal function. Using these catalysts for the selective synthesis of chemicals

and chemical intermediates at optimum conversion levels is, of course, the final objective.

The aim of the present investigation is to study systematically and exploit the

various supports such as conventional to latest mesoporous materials as supports for Ni.

Many variables can affect the structural and surface properties, which in turn affect the

adsorption, and catalytic properties of the supported metals. By controlling these variables

one can tailor the catalyst to suit a particular reaction.

Hydrogenation reactions are generally studied on Ru , Pd and Ni based catalysts as

these materials are active components for these reactions. However, the expensive nature of

Ru and Pd catalyst limit their application in industry. It is therefore important to design and

develop inexpensive non-noble metal catalysts such as Ni supported on various supports for

these reactions.

The following are the objectives/highlights of the thesis:

i) A comparative study on the oxide carriers like SBA-15 and MgO as supports for

Ni catalysts for aniline synthesis from nitrobenzene hydrogenation.

ii) A detailed study of the aniline synthesis over Ni supported on ZrO2 and TiO2

prepared by reductive deposition method and comparison of their activity with

impregnated catalysts. Ni dispersion studies to confirm the activity differences

between the catalysts prepared by reductive deposition and impregnation

methods.


23

iii) A detailed study of the γ-valerolactone synthesis over Ni supported on HZSM-5

in vapour phase at atmospheric pressure. Studies on optimization of Ni loading

to get good LA conversion and GVL selectivity.

iv) A comparative study on hydrogenation of levulinic acid using various supported

Ni catalysts like N/iAl2O3, Ni/MgO, Ni/SiO2,Ni/TiO2, Ni/ZnO and Ni/ZrO2

prepared by impregnation method.

v) A comparative and detailed study of hydrogenation of levulinic acid using Ni

supported on mesoporous silica (SBA-15) with different Ni loadings.

vi) A study on the hydrogenation of levulinic acid using various types of

mesoporous silica’s with 2D and 3D architechtures supported Ni catalysts, with

more emphasis on the influence of nature of porosity on the catalytic

performance.

1.4. Scope of thesis

1. The Ni/SBA-15 catalyst is more active in nitrobenzene hydrogenation than

the Ni/MgO catalyst. Influence of water released during the nitrobenzene

hydrogenation is studied for the first time which is not reported in the

literature earlier.

2. TiO2 supported Ni catalyst prepared by reductive deposition method is

found to be more active in aniline synthesis than impregnated catalyst.

3. The vapour phase hydrogenation of levulinic acid over non-noble metal

catalysts such as Ni/HZSM-5 prepared by conventional impregnation

method is a highly efficient catalyst under additive free conditions.

4. Influence of support on the levulinic acid hydrogenation over Ni catalysts

prepared by impregnation method, reveal that Ni/SBA-15 is a better catalyst.

5. Porosity plays crucial role in hydrogenation of levulinic acid over Ni

supported on various mesoporous silica’s with 2D and 3D architectures than

the conventional supports.

There is a lot of scope to extend these studies to other hydrogenation reactions.


24

1.5. Organization of Thesis

The thesis has been organized into six chapters:

Chapter-I: It presents the introduction part which contains the importance of

hydrogenation, in particular, hydrogenation of Nitrobenzene and levulininc acid. The aims,

objectives and scope of thesis in carrying out the hydrogenation of Nitrobenzene and

levulininc acid over various supported Ni catalysts are presented in this chapter.

Chapter-II: This chapter deals with the literature review on hydrogenation of Nitro

benzene and levulinic acid.

Chapter-III: This chapter describes the different experimental methods employed for the

preparation, characterization and activity evaluation of the catalysts.

Results and Discussion: Chapter- IV and V present the results and discussion of the work

over different catalysts studied for the hydrogenation under different conditions. The

structure-activity relationship has been discussed with the help catalyst characteristics

derived from the characterization techniques such as XPS, SEM-EDAX, TEM-SAED,

XRD, TPR, BET-surface are measurements, H2 chemisorption, TG-DTA, AAS, FT-IR,

pyridine adsorbed FT-IR and NH3 TPD.

Chapter-IV: This chapter has been divided into two sections.

Section-A: This section deals with the advantage of support for the nitrobenzene

hydrogenation and also role of water on its catalytic activity released during the reaction.

The catalysts employed for NB hydrogenation are Ni/MgO and Ni/SBA-15.

Section-B: A comparative study of hydrogenation of nitrobenzene hydrogenation over

Ni/ZrO2 and Ni/TiO2 catalysts prepared by conventional impregnation method and

reductive deposition precipitation methods is discussed.

Chapter-V: This chapter has been divided into four sections highlighting the

hydrogenation of levulinic acid to γ-valerolactone.


25

Section-A: Studies on the hydrogenation of levulinic acid hydrogenation using

Ni/HZM-5 catalysts prepared by conventional impregnation method have been discussed.

Section-B: This section deals with the influence of various oxidic supports for the

levulinic acid hydrogenation over Ni catalysts.

Section-C: This section describes hydrogenation of levulinic acid using SBA-15 supported

Ni catalysts prepared by impregnation method with various Ni loadings.

Section-D: This section deals with the advantage of support having 2D and 3D

architectures for the LA hydrogenation over Ni catalysts prepared by impregnation method.

Chapter-VI: This chapter concludes and summarizes all the observations made in the

results and discussion chapters viz, chapter- IV, and V.

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