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CHAPTER 1

INTRODUCTION

1.1 SYNTHESIS OF NITROGEN CONTAINING

HETEROCYCLES

The chemistry of heterocyclic compounds is one of the interesting

branches of organic chemistry for the diversity of its synthetic procedures,

physiological and industrial significance. Nitrogen containing heterocyclic

compounds, such as quinolines, indoles, pyrroles, pyridines, pyrazines and

pyrrolidines are of high industrial interest for application as intermediates to

produce pharmaceuticals, herbicides, fungicides and dyes. Currently most of

these compounds are recovered by distillation of coal tar. Alternatively, they

are also obtained by well-known liquid-phase syntheses such as Skraup’s,

Fischer, Madelung’s, Chichibabin’s reactions. However, they have many

important drawbacks, such as, use of hazardous reaction conditions,

expensive or toxic feeds and large waste production (Kartritzky et al 1997,

Koleter et al 1973, Watanabe et al 1990). Recently, novel reactions and/or

processes have been investigated, shifting the balance from an exclusive focus

on yield to the one that places economic value on minimizing wasteful

byproducts and operating in more safer conditions. Increasing attention is

being focused on the vapor phase synthesis of nitrogen containing compounds

with heterogeneous catalysts that exhibit significant advantages in comparison

to homogeneous catalysts. The major advantages include continuous

production, simplified product recovery, regeneration of catalyst and

reduction in liquid waste streams (Hardt 1983).

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Synthesis of pyrazine and its derivatives has received growing

interest because of their application in production of perfumery, dyes, drugs

and pharmaceutical industries. The first recorded synthesis of pyrazine was on

tetraphenylpyrazine. It was also prepared from the reaction of ammonia on

benzoin reactants. A vapor phase catalytic process for the manufacture of

substituted pyrazines by reacting diols and diamines in the temperature range

300 to 600°C was reported (Okada et al 1974, Sato et al 1978). The catalysts

used in the patented process contain zinc oxide as the major ingredient with

silicon dioxide and aluminium oxide as carriers. Catalytic systems such as

copper-chromium (Koei Chemical Co 1978), copper-zinc-chromium, zinc-

phosphoricacid-manganese and silver (Koei Chemical Co 1997) have been

patented as catalysts for the preparation of methylpyrazine from

ethylenediamine and propylene glycol. Other metals like cobalt, nickel, iron,

aluminium were also used in combination with zinc oxide. On the other hand,

in the case of liquid-phase reaction, Nippon Soda Co. (1986) established an

industrial manufacturing process for the preparation of pyrazines from

diaminomaleonitrile (DAMN). It was reported that, a mixture of 70% zinc

oxide and 30% chromium oxide catalyst yielded desired products without any

byproducts. However, extensive work was done only on the synthesis of 2-

methyl pyrazine (MP), which is a preliminary step to prepare 2-

amidopyrazine (AP), a well-known bacteriostatic and antitubercular drug. The

process is schematically represented in Figure 1.1.

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NH2

NH2

+OH

OH

CH3

N

N CH3

N

N CN

N

N CONH2

ethylenediamine propylene glycol methylpyrazine cyanopyrazine

amidopyrazine

Figure 1.1 Schematic representation of formation of amidopyrazine

from ethylenediamine and propylene glycol

In general three different types of catalysts have been employed for

the synthesis of MP, they are

1. Acidic solids such as alumina and silica-alumina.

2. Chromites and/or oxides of zinc and

3. Unpromoted and ZnO-promoted zeolites.

These catalysts were identified based on the behavior of some

already available commercial catalysts as such or with some modification.

Forni et al (1987) worked with several types of catalysts in a fixed

bed catalytic microreactor, at atmospheric pressure and 430°C, by feeding an

equi-molar mixture of ethylenediamine (ED) and propylene glycol (PG),

diluted to 50 wt% with deionized water and the following recommendations

were given:

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ZnO is the key substance for good performance in all these

catalysts.

Addition of palladium sulphate improves the selectivity, but

reduces the conversion.

When zinc oxide alone is used the catalyst is prone to sinter

due to low Huttig temperature.

Mixture of zinc oxide and zinc chromite is suitable for the

reaction.

The ratio of zinc oxide to zinc chromite affects the conversion.

Catalyst particle size less than 20 mesh and slight excess of

PG with respect to the stoichiometric value is required for

minimizing the formation of undesired byproducts.

The catalyst experiences a higher decay rate due to coking

particularly at temperatures higher than 430°C.

Regeneration of the catalyst was carried out first with a short

purging in nitrogen, followed by oxidizing the carbonaceous deposits in

flowing air at temperatures less than 450°C and finally reducing in a stream of

4 vol % hydrogen at temperature less than 320°C. The activity of the catalyst

was restored after regeneration revealing the fact that deactivation was mainly

due to coking and not due to sintering. However, the reaction system of ED

and PG to yield methyl pyrazine was considered as complex, than one can

forecast, due to the formation of many intermediates and byproducts derived

either from one reactant or from both (Forni and Pollesel 1991). The optimum

reaction temperature leading to the highest selectivity of the desired product is

about 660 K and the main reaction products show a strong tendency to remain

adsorbed on the catalyst surface. On the lower energy sites of the catalyst,

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reactants and products adsorb and desorb unaltered while only the higher

energy sites activate the reactants for both the main reaction and the reactions

leading to undesired byproducts. The rate determining step of the process is of

the Rideal-Eley type between adsorbed PG and gaseous ED and the reaction

proceeds through the formation of an intermediate, fully hydrogenated species

methylpiperazine which quickly dehydrogenates and aromatizes to MP.

However, the byproducts were from the PG and other alkylpyrazines were

from diamines. A detailed reaction mechanism for the formation of the

products from PG alone was disclosed by Forni and Miglio (1991). They

proved that the byproducts are formed as a result of dehydration, hydro

dehydrogenation, double bond isomerization and migration, condensation and

steam dealkylation, disproportionation and oxidation processes which are

accomplished well on the Zn-Cr-O (Pd) catalyst.

N-(2-hydroxyethyl)ethylenediamine undergo dehydrocyclization on

the acidic zeolites to yield methylpiperazine, whereas the same reactant

yielded MP over copper chromite catalyst (Subrahmanyam et al 1993), thus

indicating that dehydrocylization along with dehydrogenation is favored on

the chromite catalyst. Later, Kulkarni et al (1993) carried out the synthesis of

MP and piperazine over HZSM-5 and modified copper chromite catalysts.

The reaction was carried out in a fixed bed reactor at atmospheric pressure

and in the temperature range 350-450°C. They have also reported that the

dehydrocylization occurs on the acid centers. The Bronsted acid centers were

found to be particularly responsible for the conversion of PG and propylene

–hydroxypropyl-ethylenediamine. The authors disclosed

the mechanism for the formation of N-containing heterocycles. In the case of

ZnO-Cu-chromite catalyst, the active centers were particularly ZnO and

elemental or ionic copper species. The role of chromium was to stabilize the

small ZnO crystallites by forming Zn-Cr spinel, which acted as a support

offering high dispersion to ZnO.

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A good number of catalytic compositions were studied by

Subrahmanyam et al (1995). The reactivity of these catalysts towards MP

formation was explored and the authors opined that MP could be synthesized

mainly by three methods.

1. Vapor phase catalytic reaction of ED with PG.

2. Synthesis of methylpiperazine and its dehydrogenation

to MP and

3. Reacting ED and propylene oxide to get MP or

–hydroxypropylethylenediamine and the subsequent

cyclization of the latter to MP.

-diamines to its

linear and cyclic oligomers, in the presence of steam over acid form of

pentasil zeolites. The major products were piperazine and DABCO with

excellent efficiencies (90%). He also revealed that the minor byproducts

originate from the reductive cleavage of the reactant. When oxygen is co-

injected, the reaction products were almost exclusively alkylpyrazines. When

1,3 diaminopropane was used about half of the products were alkylpyridines

and the balance were methylamine and ethylamine. It was interesting to note

that virtually no reaction occurred when diamines with four carbons were

used and the reason for this could not be explained. However, diamines with

5, 6 or 7 carbons yielded cyclic, secondary amines and ammonia as the major

reaction products.

A comparative study on the catalytic activity of ZnO modified

zeolites for the transformation of ethanolamine to a variety of products was

reported by Anand et al (2001). Ethylpyrazine was the major product over

ZSM-5 and Ferrierite (FER) catalysts and no appreciable selectivity towards

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any particular compound was observed on Mordenite (MOR). However

formation of DABCO was appreciable over the ZnO modified MOR catalyst.

They attributed that the narrow pore channels inside the ZnO-MOR as a

reason for the formation of different low boiling compounds like

methylamine, ethylamine, aziridine and ethylenediamine. Pyrazine was

observed as the major byproduct in the intermolecular cyclization of EA to

yield DABCO over modified pentasil zeolites (Subrahmanyam et al 2002).

Anand and Rao (2002) studied the synthesis of MP on Ferrierite

supported ZnO catalysts. Reviewing the available data from the literature,

they reported the application of several other catalysts among which

MnSO4-H3PO4–ZnO; Ag-La/Al2O3; ZnO–WO3 mixture; modified ZSM–5

and chromite catalysts were found to be interesting. ZnO was observed to be

the most important ingredient in all these catalysts. Reaction of ED and PG

over FER zeolites resulted in the formation of MP, the ZnO modified catalyst

showing more selectivity towards MP. However, the catalyst got deactivated

in a period of 7 h. The authors believed that the deactivation due to coking

was the reason for shorter life cycle of the catalyst. The coke was removed by

air oxidation at 500°C for 5 h. They have also revealed that a little higher

concentration of ED, than the stoichiometric quantity, improves the MP

selectivity.

Baiker et al (1986) reported on a continuous process for the

synthesis of cyclic amines, using alumina supported copper in a fixed-bed

reactor. They tested the copper catalyzed amination of alcohols (Baiker et al

1983, 1984) in a series of model reactions, including the cyclization of

5–amino–pentanol-(1), 4-amino-butanol-(1), and 2-(o-aminophenyl)-ethanol.

High activity and excellent selectivity to the desired product were observed.

They found that N-(2-hydroxy-ethyl)ethylenediamine yields piperazine,

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pyrazine, pyrrolidine and 2-ethylpyrazine on the alumina supported copper

catalyst at a temperature of 220°C, with a feed ratio of 5x10-6 moles/s and

hydrogen 1.5x10-5 moles. At first hour on stream the catalyst showed 95%

selectivity towards piperazine at virtually total conversion. Severe

deactivation of the catalyst was observed on longer time on stream. The major

byproducts were pyrazine and pyrrolidine. Regeneration of the deactivated

catalyst by hydrogen treatment was not successful, indicating an irreversible

deactivation. For all the reactions hydrogen was fed along with the reactant,

and when hydrogen was replaced with nitrogen, the initial dehydrogenation of

the reactants and piperazine to pyrazine has become prominent.

Hence various catalysts like zeolites, ZnO modified zeolites and

chromites of zinc and copper were extensively used for the heterogenous

vapor phase synthesis of pyrazine and its derivatives from various reactants

like ED, PG, EA, N-(2-hydroxyethyl)ethylenediamine etc., However,

synthesis of MP was focused much due to its wide range application in

pharmaceutical industry.

1.2 EVALUATION OF VARIOUS CATALYTIC SYSTEMS FOR

THE SYNTHESIS OF NITROGEN CONTAINING

HETEROCYCLES

A catalyst is according to its definition a substance that influences

the rate of a chemical reaction but is itself unchanged at the end of the

reaction. The efficiency of a catalyst is dependent on the activity and

selectivity of the catalyzed reaction and the catalyst life. Although a catalyst,

according to its definition, should remain unchanged, deactivation occurs

during its lifetime. Continuing efforts are being made to understand and

utilize the catalytic processes for practical purposes. This has resulted in

numerous inventions during the last century, which have been responsible for

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improving the quality of human life. Catalyst not only reduces the cost of

production but also improves the quality of the products. Catalytic reforming,

aromatization, isomerization, cyclization and other processes such as

cracking, alkylation, acylation, oxidation, polymerization, hydrogenation,

dehydrogenation, etc., are directed primarily in improving the quality of

products. Applications of catalytic processes are evident in petrochemical

industry for the manufacture of fine and specialty chemicals. Catalysts claim a

significant role in the economic development and growth of chemical

industries and contribute around 20% of Gross National Product (GNP).

1.3 CATALYST SUPPORTS

The support acts as a vehicle for the active phase and any promoter

that is present on its surfaces. The traditional support materials are Al2O3,

ZrO2, TiO2, SiO2, SiO2-Al2O3, etc. These supports have been employed for

active phase containing oxides of transition metals like copper, chromium,

vanadium and molybdenum. The selection of a support is based on certain

desirable characteristics such as

1. Inertness

2. Mechanical properties like attrition resistance, hardness and

compressive strength

3. Stability under reaction and regeneration conditions

4. High surface area and low cost and porosity, including

average pore size and pore size distribution.

Recently, the physical and chemical properties of support have also

been recognized as major contributors to the resultant catalytic activity.

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1.3.1 Clay and clay based systems

Recent years have witnessed a phenomenal growth in the use of

inorganic solids as reaction media for organic transformations. Laszlo (1987)

claimed that the future of synthetic organic chemistry rests with

heterogeneous media rather than the currently predominant homogeneous

systems. Clay minerals constitute one such medium (Laszlo 1993). Clay

minerals occur abundantly in nature. Their high surface area, sorptive and

ion-exchange properties have been exploited for catalytic applications through

decades. Solid clay catalysts (Pinnavia 1983) have a broad range of functions

including

Use as catalytically active agents (usually as solid acids).

As bifunctional or inert supports.

As fillers to give solid catalysts with required physical

properties (e.g. Attrition resistance, density and specific heat

capacity).

1.3.2 Structure of clay minerals

Clay minerals are made up of layered silicates. They are crystalline

materials of very fine particle size ranging from 150 to less than 1 micron

(colloidal form). There are two basic building blocks viz. tetrahedral and

octahedral layers, which are common to clay minerals .

1.3.3 Layer structure

Tetrahedral layers consist of continuous sheets of silica tetrahedral

linked via three corners to form a hexagonal mesh and the fourth corner of

each tetrahedron (normal to the plane of the sheet) is shared with octahedral

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in adjacent layers. Octahedral layers in clay mineral, on the other hand,

consist of flat layers of edge-sharing octahedral, each formally containing

cations at its centre (usually Mg2+ or Al3+) and OH- or O2- at its apices.

Octahedral layers may be trioctahedral or dioctahedral depending on the

degree of occupancy of the octahedral sites.

These different classes of clay minerals, namely the 1:1, 2:1, etc.

have a different arrangement of tetrahedral and octahedral layers. Structural

units of clays therefore consist of alternating tetrahedral or octahedral sheets

(OT or 1:1 structure), e.g. kaolinite group. A sandwich of one octahedral sheet

between two tetrahedral sheets (TOT or 2:1 structure), e.g. smectite clay

minerals, of which, the most common member is montmorillonite. An

arrangement in which, the three layer TOT units alternate with a brucite layer

(2:1:1 structure), e.g.chlorite.

Montmorillonite is the most important smectite used in catalytic

applications. High cation-exchange capacity (CEC) and good swelling

properties allow a wide variety of catalytically active forms of

montmorillonite to be prepared (e.g. containing acidic cations, metal

complexes, photocatalytically active cations, etc.). Montmorillonites are most

frequently used as Bronsted acid catalysts, where the exchangeable cations are

either protons or polarizing cations (e.g. Al3+, Cr3+ or Fe3+). Strength of acid

sites depends upon the type of interlayer cations present (H3O+ >

Al3+>Ca2+>Na+). Higher acid strength generally leads to greater catalytic

activity, but poorer product selectivity. Controlling the acid site strength by

choice of interlayer cations proves to be useful for ‘fine-tuning’ the catalyst

selectivity. Montmorillonites undergo leaching of aluminum and to a lesser

extent, silicon, when treated with mineral acids under harsh conditions

(e.g. refluxing). This leads to increased surface area and concentration of

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weak acid sites, but a decrease in concentration of strong acid sites. Acid

leached montmorillonite is particularly useful for catalytic applications

requiring only weak acid sites, where strong acid sites give rise to poor

selectivity.

1.3.4 Acidity

Clay minerals have layer surface and edge defects, which would

result in weaker Bronsted and/or Lewis acidity, generally at low

concentrations. The acid strength is usually expressed by the Hammett scale.

On this scale, the acidity of clay minerals can be comparable to that of

concentrated sulfuric acid. The surface acidity of natural clays with Na+ or

NH4+ as interstitial cations ranges from +1.5 to –3. Washing of the clay with

mineral acid, such as HCl, brings down the Hammett (Ho) function from

-6 to -8, which is between conc. HNO3 (-5) and conc. H2SO4 (-12).

1.3.5 Clayfen and claycop

Impregnation of Fe(III) and Cu(II) nitrates onto

K10–montmorillonites produced a novel class of multipurpose reagents

termed clayfen and claycop, respectively by Laszlo and co-workers (1980,

1982).

Claycop is relatively less reactive than clayfen but enjoys a greater

stability (Laszlo et al 1988). It has been used for the oxidation of

dihydropyridines to pyridines (Laszlo et al 1984), quantitative regeneration of

carbonyl groups from the protective bisthioacetals, selenoacetals (Laszlo et al

1986) and also from thiocarbonyls. Though the clay catalysts were replaced

by more thermo stable zeolites, the modified montmorillonites are versatile

heterogeneous catalysts for a wide variety of organic reactions.

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Commercially available clays (or their modified forms) can provide

environment friendly alternatives for liquid Bronsted acids such as

concentrated nitric or sulfuric acids and substitution of Lewis acids such as

AlCl3. The relative ease of modification of clay materials by exchanging or

pillaring with various metal-cations, their reuse, and simple recovery from

reaction mixtures bodes well for the future of clay based catalysts in which

their properties can be fine-tuned for specific chemical transformations. For

success in the industrial context, more studies need to be conducted in

continuous or vapor-phase reactors using clay catalysts.

1.4 SILICA BASED SYSTEMS

Silica gel is a high surface area form of silica. It is amorphous and is

formed by polymerization of silicic acid. The silanol groups [-Si-OH]

condense to form siloxane [-Si-O-Si]. The individual particles are spherical.

Since a large number of silanol groups are present on the surface, the particles

interact forming inter particle siloxane linkages, forming aggregates that

ultimately form the gel.

The small primary particles (3-30 nm) are linked in the gel to form a

highly porous structure. The surface area of silica gel ranges between

200-800 m2g-1 and it is stable towards sintering up to 500°C. A nonporous

highly pure form of finely divided silica is sometimes used as support. These

are prepared by flame hydrolysis of silicon tetrachloride. Their particle size

vary from 5-40 nm and specific surface between 50 – 400 m2g-1. They carry

the names cabosil, aerosil etc.

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1.4.1 Structure of silica gel

Silica gel contains physically adsorbed water molecules that are

removed at about 120°C. At higher temperatures, the chemically bound

hydroxyl groups are gradually removed, more from within the particle than

from the surface. Heating at 300°C still retains about 5 OH per nm2. The

surface silanol groups are weakly acidic. Silica is an important catalyst

support, but a poor catalyst itself.

1.4.2 Silica supported copper catalysts

The literature available on copper oxide impregnated silica is very

scanty. The available literature dealt with the characterization of the metallic

copper on silica and other supports like zirconia, titania and alumina. Infrared

spectroscopy was extensively used for studying this type of supported copper

catalysts. Wainwright and co workers (1986) studied the surface species in the

hydrogenolysis of methyl formate over copper on silica using in situ IR

spectroscopy. Van Der Grift et al (1990) characterized silica supported copper

catalysts prepared by means of deposition – precipitation methods. Rochester

et al (1991) reported the infrared study of the adsorption of methanol on

oxidized and reduced Cu/SiO2 catalysts. Bell and coworkers (1994)

investigated the mechanism of methanol decomposition on silica-supported

copper using IR spectroscopy and TPD spectroscopy. Boccuzzi et al (1999)

studied copper supported on different carriers using various techniques like

TEM, DR UV-VIS-NIR and FTIR spectroscopy. The authors concluded that

the morphological and surface properties of the copper phase are essentially

the same for all the systems investigated. This indicates that the features of

the supported phase produced by following the adopted preparation and

activation methods do not depend on the nature of the carrier. During the

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mechanistic study of methanol decomposition over Cu/SiO2, ZrO2/SiO2, and

Cu/ZrO2/SiO2 Bell and Fisher (1999) proposed that the hydrogen atoms

formed during the dehydrogenation of species are located on zirconia, can

recombine efficiently on metallic copper sites and desorb as H2. Thus,

methanol decomposition over Cu/ZrO2/SiO2 is envisioned to occur primarily

on ZrO2, with the primary function of copper being the removal of hydrogen.

1.5 ZEOLITES

Zeolites are crystalline aluminosilicates with uniform pore structure.

The structural formula of the zeolite can be represented as

Mn+ x/n[(AlO2-)x (SiO2)y]z H2O

where ‘Mn+’ represents a cation, which balances the negative charge

associated with framework aluminium ions. The framework ions, Si4+ and

Al3+ are each tetrahedrally coordinated to four oxygen anions. The periodic

three dimensional network, which is characteristic of zeolites, is formed by

linking the (SiO4) and (AlO4)–tetrahedral units through shared oxygen ions.

These tetrahedra tend to form rings, containing four to twelve tetrahedral

units. Such rings normally form the entrances to channels or cages in zeolites

and thus define the pore diameter for a particular structure.

1.5.1 Zeolites for the synthesis of heterocyclic compounds

Zeolites are found to be excellent catalysts for the production of

N-containing heterocyclic compounds. The combination of acidity, shape

selectivity and eco-friendly nature of zeolite are important factors for organic

synthesis of fine and intermediate chemicals. The possibilities offered by

zeolites to introduce metal ions for tuning essential catalytic properties like

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hydrophilicity and hydrophobicity balance (via variation in the Si/Al ratio)

and the type and strength of acid sites, make them widely applicable as

catalysts.

1.5.2 Advantages of zeolites

Zeolites offer advantages of having high thermal stability, insoluble

versions of Brönsted and Lewis acids and shape selectivity by possessing

identical, well-defined reaction cavities. Through the processes of cation

exchange, metal framework substitution, covalent modification and organic

templating, the chemist can fine-tune the zeolites dimensions, acidity and

electrostatistics. The latter is particularly important for electron and energy

transfer processes, or for stabilizing reactive cationic intermediates.

1.6 COPPER OXIDE BASED SYSTEMS

Oxides are one of the seminal solid catalysts used for the industrial

processes involving cyclization, dehydrogenation, oxidation, ammoxidation,

polymerization etc. Alumina, silica, transition metal oxides and silica-alumina

in their various modified forms were evaluated for their catalytic properties.

The transition metal oxides possess both Lewis and Bronsted acidity. Lewis

acidity arises due to the ability of the metal ion to exist in various oxidation

states. Chromia is a well known Lewis solid acid (Auroux et al 1990).

Bronsted acidity in the bulk oxides is due to the loosely bound protons

associated with the oxide ions. Copper oxide and CuO/Cr2O3 systems have

found several applications and hold greater promise in a number of reactions

of industrial importance such as cyclization, condensation, hydrogenation and

dehydrogenation.

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Copper chromite (CuCr2O4) is a tetragonally distorted normal spinel

oxide with c/a<1. The tetragonal distortion in this spinel is due to the

cooperative Jahn-Teller effect of Cu2+ ion on the tetrahedral sites (Ghose et al

1989). Copper chromite, is a very versatile catalyst, used in hydrogenation,

dehydrogenation, deamination, dehydrocyclization, propellant combustion,

decomposition of alcohols, nonaromatic hydrocarbon reforming, conversion

of CO with H2O or NO, as well as a number of oxidation reactions. No

attempt to understand the mechanism of the selected reaction on copper

chromite has appeared in the literature since the early work of Adkins (1931)

to the recent.

Extensive studies on the mechanism of its catalytic activity have,

however, not been undertaken, and the few reports published until now appear

to be rather contradictory. The opposing views in the literature may partly

depend on the fact that these catalysts are prepared, pretreated and used

differently, and that changing the copper to chromium ratio may also

contribute to their vivid nature.

Copper chromite was first prepared by Adkins (1931) by

decomposition of copper ammonium chromate and used for hydrogenation

reactions. Adkins et al (1932) reported that the catalysts prepared were black

or dark brown in color and apparently contain copper in the divalent state.

The catalysts turned reddish when the divalent copper is reduced to the

monovalent or elementary state at 360°C. The resulting reddish copper

compound is rather inactive for hydrogenation although it has high catalytic

activity for some other type of reactions like the conversion of aldehydes into

esters. It was found advantageous to have barium, calcium or magnesium

compounds in the copper-chromium oxide catalyst in order to retard or inhibit

reduction of the catalyst. In another paper Adkins and co workers (1950)

reported that the divalent copper was the active species in the high pressure

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hydrogenation of esters and ketones, and that the deactivation was caused by

reduction to the monovalent and metallic states.

Stroupe (1949) proposed that prior to reduction copper chromite

catalysts consist of intimate mixtures of copper oxide and copper chromite

and after reduction at an unspecified pressure, the catalyst consisted of copper

metal and cuprous chromite. Miya et al (1966) in simultaneous liquid phase

hydrogenation and catalyst reduction at 200 atm proposed that copper

supported on chromia is the true active catalyst and that no other oxide phases

were present after reduction.

The copper chromium oxide system in partially reduced state is

known as an excellent catalyst for the selective hydrogenation of dienes into

mono olefins in vegetable oils and ketones or fatty acids into alcohols

(Miya et al 1966). Bonnelle et al (1982) used thermogravimetry, wide-line 1H NMR and catalytic hydrogenation and revealed the presence of some

reactive hydrogen species in a reduced cubic copper chromite spinel catalyst

and quantitatively evaluated these hydrogen species. However it was not

possible to be precise about the location and the charge of this species hence it

was suggested that it is located in the bulk of the catalyst closely associated

with the spinel phase.

Mixed copper chromium oxides were found to be excellent catalysts

for selective hydrogenation of unsaturated compounds. The kinetic model for

the hydrogen extraction that was occluded in the cubic spinel phase of the

copper chromite catalyst that hydrogenates dienes even in absence of gaseous

hydrogen was disclosed by Bonnelle et al (1985). They have also established

the quantitative relationships between the structural characteristics of the

catalyst and its activities. The combination of a cuprous cation in an

octahedral environment and the occluded hydrogen species (H*) are identified

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as the catalytic sites for hydrogenation. The Cr3+ ions are responsible for the

isomerization process and the hydroxyl groups on the surface of the catalyst

interact with the gaseous phase. However, the active phase of copper chromite

is the spinel oxide and not the metallic copper particles. The similarities in the

properties of the copper chromite catalyst and the cuprous ion in homogenous

catalysts were studied by Bonnelle and Daage (1985) during the

hydrogenation of dienes in the absence of molecular hydrogen. They followed

the general reaction scheme represented by Siegel (1973).

Copper catalysts were found to be particularly active for the

disproportionaion of dimethylamine compared to nickel and cobalt. The

byproducts like methane or other hydrocracking products were not observed,

on this catalyst indicating that hydrocracking does not take place to a

significant extent (Baiker et al 1984). Temperature Programmed Desorption

(TPD) and Surface Reaction (TPSR) studies revealed the nature of

deactivation of the catalyst. The deactivation of the copper catalyst appears to

be reversible and even a small loss in activity of the copper catalyst after

regeneration with hydrogen could be completely attributed to sintering of the

copper particles, which was evidenced by measurements of X-ray diffraction

line broadening. However, two deactivation processes were considered to

explain the deactivation of the metal catalysts during disproportionation in the

absence of hydrogen. They are, on one hand, the formation of inactive metal

nitrides and on the other, the formation of carbonaceous deposits and metal

carbides. Unfortunately the authors could not give any decisive conclusion on

the structure of the carbonaceous deposits hydrogenated at higher

temperatures. Cunningham et al (1981) proposed a mixed-valance model for

the active sites on CuO. These sites would involve the contemporary presence

of Cu(II) and Cu(I) in a study of 2-propanol decomposition. Copper based

catalysts appear to be uniquely efficient for the dehydrogenation of methanol,

efforts have been made in the last few years in order to develop this type of

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catalyst. Moreover, attention has also been focused on the type of carrier on

which copper is dispersed and on the influence of small amounts of promoters

on the catalytic properties for the dehydrogenation of methanol (Wainwright

et al 1984).

Wainwright et al (1984 a) studied the nature of the active catalyst

sites, the distribution of copper on the catalyst surface and the reasons for the

catalyst deactivation during dehydrogenation of methanol to methylformate

on copper chromite catalyst. During these studies a relatively high degree of

copper dispersion after reduction was observed, being supported by the XRD

studies. The peaks corresponding to CuO were lost and the peaks

corresponding to copper had emerged. Although the maxima were poorly

defined and overlapped throughout the pattern, a significant transition from

the cupric to the cuprous phase had clearly taken place. A relatively high

degree of copper dispersion is achieved by incorporation of the bulk of the

copper on cuprous chromite.

The dehydrogenation activity of copper was found to be highly

enhanced by the incorporation of amphoteric or basic metal oxide promoters

to the catalyst (Ai 1984). Cunningham et al (1986) studied the selective

dehydrogenation of isopropanol using unsupported powdered samples of pure

CuO, Cu2O and Cu metal, to clarify the importance of these three valence

states, in the catalysis of reactions involving hydrogen handling reactions by

copper catalysts. The low exposure activity profiles for CuO, Cu2O and Cu

metal and their modification by mild surface pre-reduction, confirmed the

development of significant ongoing dehydrogenation activity only when

oxidized and metallic copper locations were present simultaneously.

Ruiz coworkers (1991) investigated the dehydrogenation of

methanol to methylformate over supported copper catalysts in a fixed bed

21

microreactor. The copper/silica and copper/zirconium oxide catalysts were

found to be very active and selective towards methylformate formation. The

authors found that the copper/zinc oxide catalyst was less active and the

copper/magnesium oxide was less selective towards methylformate formation.

Copper/graphite catalyst whose performance was similar to the massive

copper, showed the highest reduction temperature, as the graphite support is

not only characterized by its low surface area (12m2/g) but also by the

absence of surface groups on which copper oxide would be anchored. The

reaction was also studied on CuM/SiO2 promoted catalysts. The state of

copper in the catalysts used for the reaction was determined by X-ray

photoelectron spectroscopy. Additional catalytic experiments were carried out

on unreduced silica-supported copper catalyst using carbon dioxide as a

carrier feed. Poisoning experiments with carbon monoxide led to the

conclusion that metallic copper is likely to be the active copper species in this

reaction. Although the copper containing catalysts used in the

dehydrogenation were studied in detail, the debate concerning the nature of

the active copper centers was still not clear. Various possible active centers

have been considered as active and selective towards methylformate

formation, including Cuo (Halasz 1989) as well as Cu2+ ions (Ikawa et al

1985) or intermediate oxidation state Cu+ (Wachs and Madix 1978,

Whan et al 1987).

Murthy and Ghose (1994) replaced both Cu and Cr of copper

chromite progressively with relatively inert Mg2+ and revealed that the

activity below 500 K is due to Cu2+ and a reduction in activity was observed

when Cu2+ reduced to Cu+.

Sachtler et al (1996) proposed that H2 reduces Cu2+ ions and the first

detectable product is Cu+ because Cuo is thermodynamically unstable in the

presence of Cu2+. The Cu0 species was found only after all the detectable Cu2+

22

was used up. Vannice et al (1997) suggested two possible reasons for the

deactivation of the copper chromite catalyst,

1. The formation of coke and/or poisoning of the catalyst by

either adsorbed reactant or some product that is formed.

2. Change in the oxidation state of the copper during the course

of the reaction.

However, there have not been many studies characterizing these

copper chromite systems, especially the state of copper on the surface.

Although Cu+ species was most frequently associated with activity in various

hydrogenation reactions, the participation of Cu0 and even Cu2+ was also

proposed. Chudinov et al (1988) investigated changes in the oxidation state of

a copper-chromium-zinc alumina catalyst during reduction between 323 and

673 K and reported that significant amounts of Cu0 began to form only above

573 K. Pillai (1994) examined the influence of a variety of pretreatments on

copper chromite and concluded that Cu+ species were the active sites in the

reaction of reductive alkylation of aniline with acetone to N–isopropylaniline.

Herman and co-workers (1979) also suggested that Cu+ was the active species

in copper chromite used for methanol synthesis and similar conclusions were

reached by Monnier et al (1985) who proposed that the Cu+ species existed in

a crystalline CuCrO2 phase. Hubaut and co-workers (1986) studied the

hydrogenation of dienes on copper chromite and declared that Cu+ species

constituted the active sites. Makarova and co-workers (1993) suggested that,

after pretreatment in hydrogen between 453 and 643 K, two phases coexisted

in the catalyst, i.e., metallic Cu and a cation-deficient, controlled-disordered

spinel. In contrast, Imura et al (1983) have stated that the active form of

copper for methanol decomposition can vary and they proposed that Cu2+,

Cu+, Cu0 species are present in different catalysts. Capece and co-workers

(1982) determined the oxidation states and surface composition of copper

23

chromite at various stages of catalytic use and after reductive pretreatments,

they concluded that Cu+ is the active species for double-bond isomerization,

while CuO is required for hydrogenation of conjugated dienes.

Nishimura et al (1975) have reported that olefin hydrogenation proceeds on

metallic copper and Yurchenko et al (1990) used XPS and XRD to observe

Cu0 formation during reduction under hydrogen. Thus, despite these

characterization studies of copper chromite catalysts, uncertainity still

remains on the relationships between pretreatment procedures, composition

and active sites.

Satoshi Sato et al (1997) described the physical and catalytic

properties of CuO-Al2O3 systems, particularly CuAl2O4 prepared by the

amorphous citrate process. In addition, they discussed effective role of Cu(0)

or Cu(II) in the methyl alcohol dehydrocoupling reaction. CuO-Al2O3

systems, especially CuAl2O4 prepared by the amorphous citrate process

exhibits an effective catalytic activity for the dehydrocoupling of

methylalcohol to methylformate. CuO–Al2O3 samples calcined at

temperatures below 1000°C are partially reduced during the catalytic reaction,

and the selectivity to methylformate decreases with increasing Cu(0) content.

Although Cu(0) is active and selective at temperatures below 210°C on the

basis of the fact that the CuO-Al2O3 samples calcined at 1100°C are not

reduced at 310°C, it has been summarized that Cu(II) species in the CuAl2O4

plays an important role as active sites in the dehydrocoupling at temperatures

above 250°C and that Cu(I) species in the CuAlO2 catalyst is also active up to

290°C.

The structural promotion effect of chromia has been mentioned

previously and chromia was found to stabilize the surface area and structure

of skeletal copper (Wainwright et al 1998). Laine et al (1988) determined the

structure and activity of a number of chromium promoted Raney copper

24

catalysts, prepared by leaching Cu-Cr-Al alloys of various compositions using

20% aqueous sodium hydroxide solution and reported that chromium

contributed to a greater extent to the surface area than did copper. It has been

suggested that the role of chromia is to maintain very fine crystallite sizes in

the surface of active metal and to provide a large specific surface area

(Tu et al 1994).

Ma et al (1999) developed a novel method of preparing skeletal

copper chromia catalyst. The presence of sodium chromate in the leaching

solution was found to prevent rearrangement of copper through deposition of

chromia on skeletal copper and to improve the pore structure and active

surface. The addition of chromia in the skeletal copper catalysts improved the

bulk activity and selectivity of the desired product. Plyasova et al (2000)

studied the structure transformations of copper chromite under reduction-

reoxidation conditions. Shreiber et al (2000) studied the methanol

dehydrogenation in a slurry reactor and evaluated copper chromite and

iron/titanium catalysts.

Wainwright and co workers (2005) determined the kinetics of the

dehydrogenation of methanol to methylformate for a commercial copper

chromite catalyst as well as a skeletal copper catalyst that has undergone

deactivation to a steady state activity. The activation energy is more for

skeletal copper compared to commercial copper chromite catalyst.

Methylformate and hydrogen, the reaction product, significantly inhibit

dehydrogenation. Recent studies have shown that the addition of chromia to

the skeletal copper system improves the structure of skeletal copper and

thereby promotes the stability of the catalysts for dehydrogenation of

methanol. The improvement of performance was attributed to the presence of

chromia on the surface of copper, which minimized polymerization of

adsorbed species on active copper sites.

25

1.6.1 Advantages of copper chromite catalysts

Copper chromite catalysts possess certain advantages over the

conventional catalysts used for the hydrogenation/dehydrogenation reactions.

No special apparatus such as a reduction furnace is needed and the catalyst

need not be freshly prepared before use. Less labor is involved in its

preparation and smaller quantities may be used for the reduction compared to

the conventional nickel catalyst. The catalyst is always ready for use and does

not change on standing in contact with air or moisture. A sample of the

catalyst may be used repeatedly, that is to say, it is not rapidly deactivated

during use. It has been shown to be quite superior in several respects to any

catalysts hitherto used.

1.7 SCOPE

From the literature survey, it can be viewed that most of the reports

are related to the synthesis of methylpyrazine, which is a precursor of

amidopyrazine, an anti-tubercular drug. The percentage yield of

methylpyrazine reported in the literature is very less, due to enormous

byproducts emerging from either one of the reactants or from both. The

catalysts used in the reported literature were found to facilitate many side

reactions. The other pyrazines like, pyrazine, dimethyl pyrazine and

trimethylpyrazine also find importance in pharmaceutical, perfumery and

food industries. Hence there is requirement to synthesize pyrazine,

methylpyrazine and dimethylpyrazine with high yields using substrates that

do not give rise to many byproducts. Similarly, a suitable catalytic system,

which facilitates the formation of the desired product in high yields, should

also be selected.

The ever-growing number of patents being filed on the use of

heterogeneous copper catalysts witnesses their renewed interest due to their

selectivity and low eco-toxicity. However, some aspects have not been fully

26

elucidated, particularly the nature and structure of the copper intermediate

species and the role of pretreatment. In spite of these studies, an in-depth

characterization of the catalysts subjected to the reaction conditions is not yet

available, which adds to the difficulties of understanding these catalyst

systems.

Thus, the present work was taken up in an attempt to understand the

catalytic behavior of copper chromite spinel oxide for the synthesis of

pyrazine and its derivatives by novel routes using various reactants like

ethylenediamine, diaminopropane, ethylene glycol and ethanol amine.

1.8 OBJECTIVES

The main objectives of the present investigation are Synthesis of copper chromite catalyst with different copper to

chromium ratios, from the respective nitrates of copper and chromium by co-precipitation method.

Preparation of TiO2, Silica gel, Zeolite HY and

Montmorillonite K10 supported copper oxide catalysts by wet impregnation method.

Characterization of the synthesized and supported catalysts by

employing XRD, nitrogen adsorption, TGA, FT-IR, AAS, n-butyl amine titration and ESCA.

Synthesis of pyrazine from single reactant ethylenediamine

(ED) over copper chromite catalyst and identifying optimum

ratio of copper to chromium. Optimization of reaction parameters such as the effects of temperature, contact time,

feed ratio and time on stream for the proven catalyst.

Correlation of results with the physio-chemical properties of the catalyst.

27

To identify the active copper species for deamination and dehydrogenation with the help of product pattern and confirm with the physio-chemical characterization.

To propose plausible mechanism for the deaminocyclization reaction occurring on the surface of the catalyst.

Synthesis of methylpyrazine from diaminopropane and

ethylene glycol over the copper chromite and other supported catalysts.

Synthesis of methylpyrazine from ethanol amine and diaminopropane over synthesized and supported catalysts.

Synthesis of dimethyl pyrazine from single reactant

diaminopropane over the copper chromite and the supported catalysts.

Optimization of the reaction parameters such as the effects of

temperature, feed ratio and contact time to achieve better conversion and selectivity.

Correlation of the results with the physio-chemical properties of the catalyst.

To propose plausible reaction mechanism for the dehydrocyclization and the deaminocyclization reactions occurring on the surface of the catalyst.

Study of the stability and activity of the promising catalyst for all the reactions by performing time on stream.