ROLE OF CATALYSIS IN SUSTAINABLE DEVELOPMENT

Sustainable development is generally defined as “development, which meets the needs of the present without compromising the ability of future generations to meet their own needs”

The implication is that human development should be such as to “enable all people to meet their basic needs and improve their quality of life, while ensuring that the natural systems, resources and diversity upon which they depend are maintained and enhanced both for their benefit and for that of future”

A major impediment in achieving sustainable development is the environmental damage being caused by rapid population growth and industrialization.

The continuous improvement in the living standards of the human race, especially during last few centuries, has been mainly due to a rapid industrial growth. Technological improvements have also improved human life expectancy resulting in rapid population growth . The growth in population and the consequent demand for fuels and chemicals has had a major negative impact on the environment. It is now believed that catalysis can play a major role in environment protection (if not in reversing the damage already done) and enable sustainable development by a number of ways. Basically, catalysis can help in (i) primary pollution control through non-polluting processes that are atom efficient and produce negligible waste, (ii) secondary pollution control through end-of-pipe solutions, (iii) use of economically attractive alternate feedstocks, (iv) use of renewable feedstocks, (v) producing bio-degradable products, (vi) development of energy efficient processes and (vii) routes to alternate energy.

The world catalyst business today is about US$11 billion, of which nearly 30 % is in the area of environment catalysts (auto-exhaust, de-NOx etc). The rest of the business is shared nearly equally between refining, chemical and polymer industries. The many ways catalysis can be used to decrease pollution or damage to the environment and contribute to sustainable development will now be examined.

The major atmospheric pollutants are green house gases, NOx, CO and hydrocarbon emission. The major green house gases are CO2 and methane. CO2 is generated from power generation, transport vehicles and industries, and its production cannot be prevented. It is estimated that about 21 % of world CO2 emissions come from transport, 8 % from the oil and gas industry and 3.4 % from cement production; essentially about 80% from fossil fuel burning . Since the eighteenth century, approximately a trillion tons of carbon dioxide has been released into the atmosphere, nearly 50% of it during the last 3 or 4 decades . CO2 emissions from transport vehicles can be decreased in the short term by increasing the fuel efficiency of vehicles, but numerous long-term options such as the use of more efficient fuel-cells and H2 are also possible. CO2 emissions can be effectively controlled only by alternate power (non-fossil fuel based) generation methods. Another approach is to convert CO2 into useful materials, though this is not mostly possible for energy considerations. A trivial amount of CO2 is already being sequestered into chemicals, which ultimately end up again as CO2

The other important atmospheric pollutants, viz. NOx and halocarbons that both together are believed to be responsible for the destruction of the protective ozone layer (besides their deleterious effects on living beings) are being now effectively controlled through the use of catalysts. NOx emission is controlled by the use of catalytic converters for mobile sources and SCR catalysts for industrial plants. Halocarbon emissions are being (or will be) controlled through the use of catalysts for the destruction of existing stockpiles of the unwanted halocarbons, the transformation of these into benign ones and in the synthesis of newer gases for refrigeration and other applications.

Carbon dioxide

NH3 Phenol Epoxide Epoxide CH4 Butadiene

Urea Hydroxy benzoic acids cyclic carbonates Poly carbonates Syn gas Butene dicarboxylic acids

Fig. .1. Some examples of the use of CO2 in chemicals production

During the last 3 decades, there has been an effort to decrease pollution from transport vehicles by eliminating the use of lead in gasoline and lowering S levels in gasoline, diesel and other fuels. Reduction of S in petroleum fuels is mostly achieved through hydrotreating that consumes hydrogen. The production of hydrogen by steam reforming of hydrocarbons entails the co-production of CO2 (5 times on weight basis), this partially offsetting the benefits of reducing the S content in fuels. The S specifications in diesel and gasoline are being limited to 50 ppm in Europe by January 2005, and to 15 ppm for diesel and 30 ppm for gasoline by 2006 in USA, other countries following different S-reduction plans. The reduction of S from diesel is typically carried out by catalytic hydrodesulfurization (HDS). It is estimated that the HDS catalyst has to be at least 400 % more active to desulfurize a typical diesel feed to 50 ppm S (compared to 500 ppm). Such super-active catalysts are now available and processes are being offered to desulfurize diesel to about 10 ppm at moderate operating conditions (less than 50 bar pressure and 350ºC). Other novel processes such as biodesulfurization and oxidative desulfurization have also been proposed. The different processes available and under development for S-reduction in diesel are presented in Table 2.1

Table 2.1. Catalysts / processes for deep desulfurization of diesel fuel

TechnologyProcess licensor / catalyst manufacturer Principle

Conventional;Available

MAK-fining (Akzo & others); Haldor-Topsoe; Criterion; UOP; IFP; Japan energy Corporation

Novel catalyst / process improvements

Development CCI; N I M & C Res. (Japan) Novel Pt-Pd-zeolite catalyst

Emerging CNRS (Lyon, France) Chelation of alkyl- DBT

Emerging Petro Star Inc.; Unipure Oxidize S to sulfones and solvent extract the sulfones

Emerging Philips – S-Zorb Adsorption and oxidation

Emerging Sulphco Ultrasonic oxidation

Emerging Enchira Biotechnology corporation Biodesulfurization

Gasoline (petrol) is manufactured by blending of various refinery streams and the component that contributes most to S is cracked (FCC) naphtha that may contain upto 2000 ppm of S. The removal of S from FCC naphtha by typical hydrotreatment processes is not attractive due to the hydrogenation of the octane rich olefins into paraffins that possess much lower octane numbers. One option is to desulfurize the VGO feed to the FCC unit. Though this is practiced at present by some refiners, it is expensive and does not always fully solve the problem. A number of novel processes have recently been commercialized or are under development for removing S from FCC naphtha. The novel ones involve adsorption / decomposition of s-compounds, fractionation of the FCC naphtha and HDS of the heavier cut containing more S and less olefins and isomerizing / cracking the n-paraffins after hydrotreating.

The alternate fuels of importance are Fischer-Tropsch (FT) liquids (hydrocarbons in the C5 – C2O range; naphtha, kerosene and diesel), dimethylether (DME) and methanol. These are obtained by reacting CO and H2 (syngas). Syngas is produced from natural gas or coal by reacting with water at high temperature (steam reforming of gas or gasification of coal). The various products of syngas are shown in Fig. 2.2.

DME is a non-polluting substitute for diesel. FT liquids can be used as clean (S-free) kerosene and diesel fuels and naphtha. Methanol and hydrogen can be directly used as fuels in internal combustion engines or converted into electricity using a fuel cell. All the above conversion processes such as the steam refining of natural gas, production of DME, FT liquids, methanol and hydrogen and fuel cell operation are all catalytic processes. Though some of these are established processes like steam-reforming, FT and MeOH synthesis, many recent improvements have been reported in all these processes. An excellent example is the conversion of natural gas (or hydrocarbons up to naphtha) into CO free H2 for fuel cell applications. Novel noble metal monolith catalysts (Pt-Rh or Re loaded on monoliths wash-coated with mixed oxides) have been developed for auto-thermal reforming of natural gas into CO, CO2 and H2. The CO is converted into CO2 using a shift catalyst and the H2 is then purified free of CO (less than 10 ppm CO) H2 using a PROX (preferential oxidation) catalyst. The PROX catalyst is generally a supported metal (Au or Pt). Sometimes, H2 is also separated using molecular sieves.

Natural gas

CO2 or H2O

Syn gas

DME FT liquids Methanol

Fuel cell

Hydrogen

Coal

Water

Bio-mass

Fermentation Vegetable oils Gasification

Ethanol MethanolTransesterification

Fuels, olefins

FT

Fuel hydrocarbons

Fuel; ETBE Biodiesel, biolubricants

The most preferred source of alternate fuels is biomass, such as cellulosic materials like bagasse, wood chips, straw, and vegetable oils. Effective use of these materials and discontinuing the use of fossil fuels should decrease the overall CO2 load in the atmosphere as the production of these raw materials will help in depleting atmospheric CO2. These raw materials can be converted into fuels and chemicals as shown in Fig. 3. Again all the above processes involve catalysts; fermentation is carried out using bio-catalysts while all the other steps use mainly heterogeneous catalysts.Catalysis in green chemistry An important source of global pollution is the chemicals manufacturing industry. Many of the steps involved in the synthesis of fine chemicals and pharmaceuticals are based on reactions and reactants developed many decades ago when environmental concerns were absent. A large number of these reactions are based on the use of stoichiometric amounts of reagents producing large volumes of (often hazardous) byproducts.

Table 2.2. Concepts that define the enviro-soundness of processes [4]

1. The E-factor Industry Product tonnage

Kg byproduct / Kg product (E-factor)Petroleum 106-108 <0.1Bulk Chemicals 104-106 <1 – 5Fine Chemicals 102-104 5 - >50Pharmaceuticals 10-103 25 - >100

2. Environmental Quotient (EQ) = (E-factor x unfriendliness quotient, Q).Q can be 1 for NaCl and 100 – 1000 for heavy metal salts etc.

3. Atom Efficiency = Weight of desired product / weight of all products.

While in the very large volume petroleum refining industry the byproduct yield (weight) per unit weight of product (called E- factor) is generally small, it is unacceptably large in the fine chemical and pharmaceutical sectors. Some of the concepts that define the enviro-soundness of processes (according to Sheldon) are outlined in Table 2 [4]. The large volumes of by-products lower the atom efficiencies of many of the present processes and often necessitate expensive waste treatment lowering the overall economics of the processes. Therefore, newer and more appropriate processes and reaction steps are continuously being developed, many of these developments taking place in the fine and specialty chemical sectors and involving the use of catalysts. Many such recently discovered applications of catalysts involving green processes have gone into commercial practice in the fine and specialty chemical industries.

Green synthesis of chemicals will need to consider many aspects besides product selectivity. The 12 fundamental guidelines for the green synthesis of chemicals as outlined by Anastas are listed in Table 3.

Importance of selective catalysis

Minimize by-products

Better economics Less Pollution Better Product save non-renewable raw materials

Savings

Raw material cost Smaller unit; Less purification LITY

Fig. 2.4. Importance of selective catalysts in sustainable development

Chemo-selectivity: This is the ability of catalysts to produce different chemical entities from the same substrate (Fig. 2.5)

OH

O

OH

ZnO

Pt/Na-Al2O3

Pt/Al2O3

NiO2 ; Al2O3

Regio-selectivity: This arises when catalysts are able to carry out the desired change in a substrate molecule at the desired location or place (Fig. 2.6).

CH3

CH2 CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

+ +

H-Y

K-Y

H-ZSM-5

Enantio-selectivity: Enantio-selectivity is best observed when homogeneous catalysts are used. Different types of enatio-selective reactions have been reported over catalysts. An example of an oxidation reaction (Sharpless) is presented in Fig. 2.7

H

CH2OH H

H

+ CMe

Me

MeCOOH

OHC6H5OOC

C6H5OOC OH OH

CH2OHcatalyst

Ti (DET)

(R)-glycidol 95%

Fig. 2.8. Examples of shape selectivity in catalysis over zeolites: a) shape-selective cracking of n-paraffins, b) selective production of p-xylene by methylation of toluene and c) selective disproportionation of m-xylene to 1,2,4-trimethylbenzene

Chemicals are typically classified, according to the volume of their production and application, as bulk, fine and specialty chemicals. Bulk chemicals are produced in large volumes. These are mostly petrochemicals or derivatives. Invariably, catalysts are widely used in petrochemical production. Typical examples are the alkylation of aromatics over solid-acid catalysts and the selective oxidation of hydrocarbons over mixed oxides in the vapour-phase or transition metal catalysts in the liquid phase. As already mentioned, pollution (the E-factors) in these processes is rather small (Table 2.2).

Fine chemicals are those whose global production does not exceed about 10,000 tons per annum. Fine chemicals may also be intermediates for many specialty chemicals, such as pesticides, fragrances etc. Presently, most fine chemicals are manufactured through highly polluting processes using stoichiometric amounts of reagents. For example Friedel Crafts alkylation is generally carried out with AlCl3 as the catalyst (used in more than stoichiometric quantities). At the end of the reaction, the catalyst is destroyed to recover the product. Similarly, methylation of phenolic compounds is done with Me-sulfate and subsequent neutralization of the acid. Many oxidations are carried out at present with dichromates with attendant difficulty in disposal of the co-products, the Mn and Cr salts. For the past few decades, the above environmentally unsafe and atom in efficient processes are being slowly replaced with newer catalytic processes that are less polluting and more atom efficient . Some illustrative examples of green processes practiced at present (or being developed) in the bulk and fine chemical industries are presented below.

Bulk Chemicals:Alkylation of aromatic compounds

Typical alkylation reactions of interest in the chemicals industry are the alkylation of benzene to produce ethylbenzene, cumene and linear alkyl benzene. In the past, alkylation reactions were carried out with mineral acids such as, AlCl3, HF, BF3 and H2SO4 as catalysts. Presently, most alkyations are carried out over solid acid catalysts, typically zeolites. The earliest commercial use of a zeolitic solid-acid as an alkylation catalyst was in the Mobil-Badger process for the production of ethylbenzene [10]. A number of processes for the production of alkyl aromatics such as ethylbenzene and cumene by the alkylation of benzene with olefins over solid acid catalysts are now avialable (Dow, C D Tech., Mobil, UOP and others [11]. Zeolites posses very strong acidities and shape-selective properties. The combination of these two properties can be exploited for greater economies and improved product qualities. For example, in the production of cumene, the zeolite catalysts produce less poly alkyl products (di and tri diisopropyl benzenes) than the supported phosphoric acid catalysts. The small amounts of di and tri isopropyl benzenes produced in the zeolite catalyzed processes are separated and transalkylated with benzene in another reactor to produce more cumene. Usually, another zeolite catalyst is used in the transalkylation reactor.

While the processes mentioned above mainly utilize the super acidic and hydrophobic nature of zeolites, it is possible to make additional use of the shape selective (molecular sieving) properties of these materials to prepare selectively certain alkylaromatics in high yields. Typical examples are the alkylation of naphthalene with cumene to produce 2,6 diisopropyl naphthalene (DIPN) and the alkylation of ethylbenzne to produce p-diethylbenzene (DEB). DIPN is readily converted into the dihydroxy or dicarboxylic compounds used for making liquid crystal polymers. DEB is used as a solvent in the Parex processes used for separating m- and p-xylenes. To achieve product shape selectivity, the pores in the zeolite are tailored to be slightly larger than the diameter of the p-isomer and a trifle smaller than the dimensions of the o- and m-isomers. As a result the o- and m- isomers cannot diffuse out of the reaction zone (pore intersection) even though all the three isomers may be formed; only the narrower p-dialkyl isomer diffuses out of the pores and is obtained as the major product. The other isomers present in the reaction zone equilibrate to produce more p-isomer. Mordenite is the catalyst of choice for DIPN production (DOW), while the processes commercialized by NCL (Pune) and IPCL (Vadodara) for DEB production use a ZSM-5 type material. ZSM-5 has been used by Mobil to produce selectively 2,6 dimethyl naphthalene which can replace DIPN. Similarly 4,4’-diisopropyl biphenyl, another polymer precursor can also be prepared selectively using mordenite catalysts.

Another beneficial use of solid acid catalysts is in manufacture of linear alkyl benzenes (LAB), which are the precursors for detergents. The conventional processes are based on the use of anhydrous HF (UOP) or AlCl3 (Enichem) as the catalyst. UOP has recently commercialized a novel process (DETAL) using a solid acid catalyst replacing HF [12]. The catalyst is believed to be a non-zeolite. The major benefits for the above solid acid processes (apart from environmental ones) are lower construction and operating costs. Besides, the 2-phenyl alkane content in the product is higher over solid catalysts making the product more suited for use in liquid detergents. The LAB product manufactured using solid acid catalysts is also highly biodegradable with > 95 % linearity of the alkyl group. The changes that have occurred in the LAB process during the past three decades is presented in table Table 1.4 [13]. Not only has the process become cleaner and greener, the quality of the product (LAB) has also improved.

Table 1.4. Evolution of LAB processes [13].

Alkylating agentCatalyst LAB production; thousand metric tons per year

Years

1970 1980 1990 2000

Chloro-paraffins AlCl3 400 400 240 180

High purity olefins AlCl3 0 100 280 120

Olefin/paraffin mixture HF 260 600 1280 1850

Olefin/paraffin mixture Solid acid 0 0 0 260

Total 660 1100 1800 2410

Selective oxidation reactionsSelective oxidation processes account for about 25 % of all the chemical processes. Oxidation processes are based on various catalyst types and methodologies, from fixed vapour phase processes to liquid-multiphase processes involving solid, liquid and gaseous catalysts. These reactions are at times hazardous, eco-unfriendly and involve raw material waste due to poor selectivity for the desired product. Over the years, constant innovations in catalyst and process design have resulted in a number of new developments improving their economics and eco-friendliness. In the case of adipic acid manufacture, for example, new developments should avoid the co-production of N2O or should enable its use in another process. This is discussed below.

Adipic acid manufacture: Adipic acid (AA) is commercially manufactured at present by the oxidation of cyclohexanol with nitric acid. The process generates equimolar amounts of N2O as the byproduct. N2O is an ozone-depleting agent and is eco-unfriendly. Besides, the use of corrosive HNO3 is also undersirable for many ressons. Cyclohexanol itself is produced (conventionally) from cyclohexane by a low yield process through liquid phase oxidation using Co/Mn salts [14

OOH

+

COOH

COOH

Thomas

H2

Conventional

HNO3

Thomas

COOH

COOHD-Glucose

E-coli

Frost's route

Muconic acid

Adipic acid

Noyori's route

. Different routes to adipic acid production

A number of innovative routes to the production of AA have been reported recently. Noyori et al. have described a route to its manufacture by the oxidation of cyclohexene with H2O2 in the liquid phase [15]. Cyclohexene is easily manufactured by selective hydrogenation of benzene over Ru catalysts. The major advantage of this route is its simplicity and eco-friendliness, though the use of H2O2 makes it a little less economically attractive at present. However, as new processes for cheaper H2O2 are under development, it could be a viable process in the next few years when the price of H2O2 becomes less.

Another interesting route that has been proposed is a bio-catalytic one involving the enzymatic oxidation of D-glucose to muconic acid and its hydrogenation to adipic acid [16]. This route thoughunattractive at present due to the expensive raw material (D-glucose), could however become viable once new processes for the transformation of biomass to cheap D-glucose are developed.

However, the most economic processes for production of adipic acid will be those that use n-hexane or cyclohexane as the raw materials and O2 as the oxidant. This has just been achieved to a limited extent very recently by Thomas et al., who have described the use of metal aluminophosphates for the aerial oxidation of cyclohexane and n-hexane directly into adipic acid in two recent publications [17]. As the raw materials are cheap, these inventions could become commercially viable soon inspite of their relatively low yields. In the case of cyclohexane oxidation, the authors report selectivities for cyclohexanol, cyclohexanone and adipic acid of 21.7, 32.3 and 19.8%, respectively, at a conversion of 19.8% over a FeALPO catalyst at 130ºC and 15 atm of air. The process assumes importance as the coproducts, cyclohexanone and cyclohexanol are also commercially valuable. The various routes to adipic acid are shown in Fig. 2.9

Phenol manufacture: Phenol is at present manufactured from benzene by alkylation into cumene, oxidation of the cumene to the hydroperoxide and its hydrolysis to phenol and acetone. Obviously, this is a circuitous route and the direct insertion of (O) into benzene should be much more desirable. A process recently developed by Solutia makes use of the byproduct N2O from adipic acid plants to oxidize benzene to phenol over a Fe-ZSM-5 catalyst [18]. This process is especially suited for integration with adipic acid plants as the product phenol can again be converted into adipic acid by hydrogenation into cyclohexanol and subsequent oxidation. It is also possible to hydroxylate benzene with H2O2 over TS-1 to produce phenol [19, 20], though this route is not yet commercially viable. The different routes to the manufacture of phenol are shown below in Fig. 2.10.

H3PO4/zeolite[O]

N2OFeZSM-5

TS1

O

OOH

OH

H2O2/

+

(Benzene) (Cumene) (Cumene hydroperoxide)

(phenol)

Different routes for phenol production

Production of caprolactam The conventional process for the production of caprolactam involves first the

synthesis of hydroxylamine, reacting it with cyclohexanone to make the oxime and then rearranging the oxime with oleum to produce caprolactam. The synthesis of hydroxylamine sulfate (NH2OH.H2SO4) is a lengthy and environmentally unsafe process (shown below).

In the conventional process, 4.5 Kg of the byproduct (NH4)2SO4 is produced for every Kg of lactam. The (NH4)2SO4 byproduct is formed from the neutralization of the sulfuric acid released during oxime formation with hydroxylamine sulfate and the oleum used in the rearrangement of the oxime. However, very recently the process has become totally different (Fig. 2.11). In the new process, cyclohexanone is reacted with NH3 and H2O2 to the oxime over the titanosilicate catalyst, TS-1 [19]. The oxime is then coverted in the vapour-phase over B-MFI or siliceous ZSM-5 to yield caprolactam [21, 22]. The new process does not produce any (NH4)2SO4 and is a good example of a green process.

NH3 + Air → NOxS + O2 → SO2

NH3 + CO2 + H2O → (NH4)2CO3(NH4)2CO3 + NOx → NH4NO2

H2O

NH4NO2 + SO2 + NH3 + H2O → HON(SO3NH4)2 → (NH4)2SO4 + NH2OHH2SO4

Fig. 2.11. Green process for caprolactam production.

O

NH3 + H2O2

Ti-Silicate

NOH

Molecular Sieves NH

O

CaprolactamCyclohexanone oxime Yield = 90 %

Route 2

CO O

O

CH3H3C H2O

DMC+ OH2Transesterification

DPC+ CH3OH

2 CH3OH + CO + 1/2 O2+

DMC

OHHO

BPA

+ CO O

O473 - 593 K

Catalyst BPC + 2

DPC

OH

Route 1

OONa Na + COCl2

NEt3CO O

O

( )n

Bisphenol-A (BPA) (Na salt)

Bisphenol-A Polycarbonate (BPC)

The conventional routes to polycarbonate production

An interesting example of a green process that has been commercialized recently is the Asahi-Kasei process for polycarbonates [23]. The different processes used today for the manufacture of polycarbonates are presented in Fig. 12 a. Much of present day production of polycarbonates uses phosgene, a highly toxic chemical (Route 1; Fig. 12 a). The phosgene process is based on interfacial polycondensation of phosgene (in methylene chloride) and Na-bisphenol A (in water). The process uses highly toxic reagents and solvents besides requiring much water for washing of the polycarbonate product. It also produces NaCl as the byproduct. Another process involves the polymerization of diphenyl carbonate (DPC) with bisphenol-A (BPA) (Route 2: Fig. 12 a). DPC is prepared by the reaction of dimethyl carbonate (DMC) that is prepared by the oxidative carbonylation of methanol.

The novel green Asahi-Kasei process is based on the condensation of DPC and BPA in a melt-polymerization process including a pre-polymerization step. The various steps in this process are presented in Fig. 12 b. The process is very clean producing valuable ethylene glycol as the byproduct. The starting material is ethylene oxide (EO) (Step 1; Fig. 12 b). The byproduct of ethylene oxide manufacture, CO2 is itself consumed in the process (173 tons per 1000 tons of BPC); ethylene oxide is converted into ethylene carbonate (EC) by reaction with CO2 (Step 2; Fig. 12 b). Step 3 is the conversion of EC into DMC by reaction with methanol and the production of high purity monoethylene glycol (MEG). The process is claimed to satisfy nearly all the tenets of green chemistry proposed by Anastas [

CH2 CH2 + 1/2 O2

H2C CH2 1

(EO)

(EO) + CO2

CH2 CH2

O O

C

O(EC)

2

CH2 CH2

O O

C(EC)

+ 2 MeOH

MeOCOMe

(DMC)

+ HOCH2CH2OH

(MEG)

3

2 MeOCOPh PhOCOPh

(DPC)

+

MeOCOMe

OOO

O

O

(MPC) (DMC)

PhOCOPh

O

(DPC)

+ HO C OH

CH3

CH3

O C O

CH3

CH3

* C

O

*OPhH + PhOH 5

4

PC prepolymer (n = 10 to 20)

n

O

The green Asahi-Kasei process for polycarbonate productions

Fine-chemicals Hydroxylation of phenol

The dihydroxybenzenes, catechol and hydroquinone are valuable in the fine-chemical industry, and have been commercially manufactured by many ways (Fig. 13), though the main route is the hydroxylation of phenol with H2O2 (Fig. 13 c).

The reaction is carried out in a homogeneous liquid phase, the catalysts used being mostly metals salts and metal complexes and the processes are not very clean.

The reaction over these homogeneous catalysts yields more catechol than hydroquinone, the catechol / hydroquinone (CAT/HQ) ratio being mostly around 2. A recently developed clean hydroxylation process uses TS-1 as the catalyst [19].

Due to the use of the molecular sieve catalyst, the hydroquinone yield is more, the CAT/HQ ratio being about 1.

TS-1 has been found to catalyze the clean oxidation of many substrates with H2O2 (Table 2.5) producing water as the byproduct. The selective oxidation of

propylene to the epoxide with H2O2 over TS-1 is expected to become a commercial reality soon.

Different processes for manufacturing catecol and hydroquinone

Table 2.5. .Selective oxidation by H2O2: reactions catalyzed by TS-1

Reactants Product

1. Benzene Phenol

2. Phenol Catechol and hydroquinone

3. Olefins Epoxides

4. Cyclohexane Cyclohexanol

5. Alkanes Alcohols

6. Alcohols Aldehydes and ketones

7. Ketones + NH3 Oximes

8. Sulfides Sulfoxides

Synthesis of vanillinVanillin is an important chemical used in flavouring and perfumery. It is mostly manufactured by condensation of guiacol with glyoxylic acid in an alkaline medium to [3-methoxy, 4-hydroxy phenyl]-glyoxylic acid, its oxidation and decarboxylation by neutralizing with acid. The raw material guiacol is manufactured by the methylation of catechol with methyl sulfate and neutralization of the acid. As already mentioned, the production of catechol by the hydroxylation of phenol itself is not a clean process. In effect, the overall process for manufacturing vanillin produces much waste that needs expensive cleaning up. Very recently a process that uses only solid catalysts has been put into commercial practice by Rhodia [8]. The process shown in Fig. 14 is very clean and highly atom efficient, the overall reaction being PhOH + CH3OH + HCHO + H2O2 vanillin + 3H2O

Green synthesis of vanillin using heterogeneous catalysts.

Other processesSome well-known examples of green-chemistry already practiced in the fine chemical industry using heterogenous catalysts are briefly described below. Production of citral: Citral is an intermediate in the synthesis of vitamin A and ionones used in perfumery. The earlier route for its manufacture was from -pinene involving its transformation into geraniol and nerol through pyrolysis, chlorination and hydrolysis, and the further stoichiometric oxidation using MnO2 (or dehydrogenation over a Cu-catalyst) with poor yields. The BASF process for its synthesis uses formaldehyde and isobutene and involves the reaction of the product isoprenol with molecular oxygen at high temperatures over a silver catalyst (Fig. 2.15). The yield of citral is reported to be 95 % [24].

Production of citral by the BASF process

Green route for lazabemide

Production of Lazabemide In this process invented by Hoffmann-La Roche, the anti-parkinson drug Lazabemide is synthesized in a single step through the amido-carbonyation of 2,5 dichlropyridine over a Pd-catalyst (palladium dichloro-bis (triphenyl phosphine)) at moderately high yields [25] (Fig. 16). The alternate process using 2-methyl-5-ethyl pyridine is a multistep one

Synthesis of aziridine: Aziridine (ethylimimine) is used in polymer and pharmaceutical industries. The process for its preparation involves the dehydration of ethanolamine (Fig. 17). The conventional route involves the use of sulfuric acid with the attendent difficulties and excessive byproduct (sodium sulfate) production. The process developed by Nippon Shokubai uses silica supported mixed metal oxides as the catalyst (see below) [26]. The reaction is carried out at temperatures above 400oC and short contact times (<1sec). Many weakly acidic zeolite-type molecular sives have also been recently reported to be useful for this reaction

Enantio-selective reduction in the production of S-naproxen

Again, another area where catalysts are finding new applications is in the synthesis of chiral compounds with applications in the drug and agrochemical sectors. While most drugs derived from natural sources are chiral, the synthetic products are often achiral or racemic mixtures. As, in general, only one enantiomer in the racemic mixture is the active component, the other enantiomer is unnecessarily introduced into the human body, often leading to increased side-effects. Therefore, the present trend is to market only the active enantiomer. A number of catalytic processes to synthesize chiral compunds have recently been commercialized [26, 27]. The general method used in the preparation of these chiral molecules is to prepare a mixture of enantiomers and resolve them into the desired isomers through various techniques often involving the destruction of one of the isomers. A more economic and cleaner route is the direct preparation of the desired isomer through asymmetric catalysis. A number of commercial processes are now in practice based on asymmetric catalysis. Two interesting processes are the production of S-naproxen by the reduction of a 2-arylacrylic acid derivative over Ru-BINAP [26] (Fig. 18) and the manufacture of (-)-menthol wherein the key intermediates neryldiethyamine and geranyldiethylamine are synthesized from isoprene and myrcene using a chiral Rh-BINAP catalyst [28]. Other examples of enantoselective hydrogenations are the hydrogenation of 4-chloroacetate ester to the (R)- hydroxy compound over Ru-BINAP [29] and the reduction of pyruvic esters over dihydrochinchonidine loaded Pt-alumina catalyst [30].

Epoxidation of allylic alcohols in greater than 90%ee (enantiomeric excess) can be obtained by Sharpless oxidation using tertiarybutyl hydroperoxide (TBHP) as the oxidant and titanium isopropoxide – diethyl tartarate (DIPT) as the catalyst (Fig. 7) [6].

Enzymes and enzyme mimics

Enzymes, which are so abundant in nature are the most efficient catalysts known. They exhibit high chemo-, regio- and enantio-selectivities. Traditionally, human beings have exploited enzyme catalysis in the production of fermented drinks, alcohol and many pharmaceutical products. Recently, many new processes based on enzyme catalysis have been put into practice. Some of these are the oxidation of glycolic acid into glyoxalic acid [31] and the hydration of acrylonitrile and 3-cyanopyridine into the corresponding amides [32].

O2 +CH3OH

COOH

1/2 O2 H2O2

catalase

H2O

glycolate oxidaseEC1.1.3.15

CHO

COOH

EDA

(complex etc.)

Fig. 2.19. Manufacture of glyoxylic acid by an enzymatic routeThe present routes for the production of glyoxylic acid are the low yield eco-unsafe nitric acid oxidation of acetaldehyde or glyoxal and the multi step process starting from dimethyl maleate. A novel microbial process produces glyoxylic acid in nearly 98% yield [31]. The critical step is the incorporation of the catalase enzyme into the microbe to destroy the coproduct H2O2 to suppress further oxidation of glyoxylic acid. Besides, ethylene diamine (EDA) is added to the reaction mixture to trap the glyoxylic acid to suppress over oxidation and inhibition by the product. The raw material glycolic acid is readily obtained

The present routes for the production of glyoxylic acid are the low yield eco-unsafe nitric acid oxidation of acetaldehyde or glyoxal and the multi step process starting from dimethyl maleate. A novel microbial process produces glyoxylic acid in nearly 98% yield [31]. The critical step is the incorporation of the catalase enzyme into the microbe to destroy the coproduct H2O2 to suppress further oxidation of glyoxylic acid. Besides, ethylene diamine (EDA) is added to the reaction mixture to trap the glyoxylic acid to suppress over oxidation and inhibition by the product. The raw material glycolic acid is readily obtained from the acid-catalyzed carbonylation of formaldehyde.

CH3

N

CN

N

CONH2

N

O2/NH3

oxide catalyst nitrile hydratase

H2O

Enzymatic method for the production of nicotinamide

Another example of an enzyme based process in commercial operation is the transformation of acrylonitrileinto acrylamide using bacterial nitrile hydratase (Nitto Chemical Co., Japan). Using a similar enzyme, Lonza (Switzerland) has developed a process to convert 3-cyanopyridine into byproduct-free nicotinamide [32].

Enzymes are composed of an active centre (a transition metal complex) encaged inside a large protein molecule. One of the main reasons for the fragility of enzymes is the easy denaturing of the protein molecules. Attempts are now being made to encapsulate metal complexes inside zeolite cages so as to mimic enzymes. The most common enzymes being mimicked are the monooxygenases. A number of beneficial effects such as the enhancement of dispersion and activity due to cage effects has been reported. Examples of some studies are given in Table 2.6. Table 2.6. Some examples of oxidation using encapsulated complexes

CatalystReactant Oxidant Products Referenc

e

Mn-Salen /zeoliteX Styrene O2 PhCHO + Styrene oxide

33 (a)

CuPc(X)/MCM-41 /Y Cyclohexane TBHP/H2O2 Cyclohexanol, -one 33 (b)

Co(DMG)/X propene O2 Acetone 33 (c)

CoPcSO3/ HT Di-t-bu-phenol O2 Quinones 33 (d)

Fe-carboxylate/silica

Hexane, Cyclohexane

O2 alcohols 33 (e)

CoPc/EMT Ethylbenzene O2 Acetophenone 33 (f)

Alternate raw materialsAn important aspect of sustainable growth is the use of renewable raw materials for the production of chemicals. The conversion of cellulose and starch into value added products using enzymes and the use of biomass derived alcohol for chemical manufacture are typical examples. These new routes require new catalysts. In some processes, it may be possible to change over to cheaper feedstocks, provided catalysts are available that can selectively transform these into the desired products.

Table 2.7. Selected literature information on selective oxidation of light alkanes

Reaction Catalyst Temperature (C)

Yield (mol %)

CH4 CH3OH Cu/SiO2 350 5.7

CH4 HCOH V/SiO2 600 2.9

C2H6 CH3COOH W-V-Re-Nb-Sb-Ca oxides

277 10.9

C2H6 CH3CHO FePO4 400 2.5

C3H8 CH2=CHCHO Ca-Bi-Mo oxides 550 3.2

C3H8 CH2=CHCOOH Mo-V-Te-Nb oxides 380 48.5

C3H8 CH3CH2COOH CsFeHPVMo11O40 380 13

A typical case is the replacement of olefins by cheaper paraffins in the manufacture of many industrial chemicals. When olefins are used as the feedstock for oxygenates, the raw material (olefin) cost is nearly 60 – 70 % of the value of the product. Thus, the need for the transformation of much cheaper alkanes directly into oxygenates has become very important in recent years for economic reasons. Despite the large amount of research in this area over the last 10 - 15 years, no commercial process has gone on stream except the conversion of butane to maleic acid. Important processes, such as those for the conversion of propane to acrylic acid (or acrylonitrile), isobutane to methacrylic acid and ethane to acetic acid are still under development or pilot plant evaluation. Some typical published examples of direct alkane conversions are presented in Table 7 [34, 35].

Among all the above processes listed, the manufacture of acetic acid from ethane appears to be the most promising in that it is reportedly to be under commercialization by SABIC. Acetic acid is an important commodity chemical used in the preparation of vinyl acetate (VA) monomer and acetic anhydride, and as a solvent in PTA manufacture.