Role of Catalysis in Sustainable Development4th

8/13/2019 Role of Catalysis in Sustainable Development4th

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ROLE OF CATALYSIS INSUSTAINABLE DEVELOPMENT


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Sustainable development is

generally defined as development,

which meets the needs of thepresent without compromising the

ability of future generations to

meet their own needs


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The implication is that human development

should be such as to enable all people to meettheir 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


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A major impediment in achieving sustainable

development is the environmental damagebeing caused by rapid population growth and

industrialization.


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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.


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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 wayscatalysis can be used to decrease pollution or damage to the

environment and contribute to sustainable development will now be

examined.


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The major atmospheric pollutants are green house gases, NOx, CO andhydrocarbon 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 beenreleased 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 isnot mostly possible for energy considerations. A trivial amount of CO2 is already

being sequestered into chemicals, which ultimately end up again as CO2


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The other important atmospheric pollutants, viz. NOx andhalocarbons 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 ofcatalytic 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 refrigerationand other applications.


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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


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During the last 3 decades, there has been an effort to decrease pollution from transport vehiclesby 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 2006in 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 % moreactive 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 350C). Other novel processes such asbiodesulfurization and oxidativedesulfurizationhave also been proposed. The different processes available and under

development for S-reduction in diesel are presented in Table 2.1


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Table 2.1. Catalysts / processes for deep desulfurization of diesel

fuel

Technology

Process licensor / catalyst manufacturer Principle

Conventional;

Available

MAK-fining (Akzo & others); Haldor-

Topsoe; Criterion; UOP; IFP; Japanenergy 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 PhilipsS-Zorb Adsorption and oxidation

Emerging Sulphco Ultrasonic oxidation

Emerging Enchira Biotechnology corporation Biodesulfurization


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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 ispracticed 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.


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The alternate fuels of importance are Fischer-Tropsch (FT) liquids

(hydrocarbons in the C5C2O range; naphtha, kerosene and diesel),

dimethylether (DME) and methanol. These are obtained by reactingCO 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.


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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 bedirectly 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 isthe 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.


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Natural gas

CO2 or H2O

Syn gas

DME FT liquids Methanol

Fuel cell

Hydrogen

Coal

Water


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Bio-mass

Fermentation

Vegetable oils Gasification

EthanolMethanolTransesterification

Fuels,

olefins

FT

Fuel hydrocarbons

Fuel; ETBEBiodiesel, biolubricants


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The most preferred source of alternate fuels is biomass, such as cellulosic

materials like bagasse, wood chips, straw, and vegetable oils. Effective use ofthese 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.


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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 106-108 100

2. Environmental Quotient (EQ) = (E-factor x unfriendliness quotient, Q).

Q can be 1 for NaCl and 1001000 for heavy metal salts etc.

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


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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 isunacceptably 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 beingdeveloped, 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.


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Importance of selective catalysis

Minimize by-products

Better economics Less Pollution Better Product save non-renewable

raw materials

Savings

Raw material

costSmaller unit; Less purification LITY

Fig. 2.4. Importance of selective catalysts in sustainable development


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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


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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

CH2CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

+ +

H-Y

K-Y

H-ZSM-5


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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

Me

COOH

OHC6H5OOC

C6H5OOC OH O

H

CH2OH

catalyst

Ti (DET)

(R)-glycidol 95%


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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


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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 ofaromatics 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).


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Fine chemicals are those whose global production does not exceed about 10,000

tons per annum. Fine chemicals may also be intermediates for many specialtychemicals, 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.


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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 thepast, 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 benzenewith 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 smallamounts 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.


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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 thep-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.


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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.


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Table 1.4. Evolution of LAB processes [13].

Alkylating agent

Catalyst 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


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Selective oxidation reactions

Selective 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 andinvolve 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.


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Adip ic 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 agentand 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


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O

OH

+

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


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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 couldbecome 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 130C 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


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Phenol m anufacture: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 acidplants 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.


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H3PO4/zeolite

O

N2O

FeZSM-5

TS1

O

OOH

OH

H2O2/

(Benzene) (Cumene) (Cumene hydroperoxide)

(phenol)

Different routes for phenol production

Product ion o f caprolactam


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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 titanosilicatecatalyst, 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 NOx

S + 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.


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O

NH3+ H2O2

Ti-Silicate

NOH

Molecular SievesNH

O

CaprolactamCyclohexanone oxime Yield = 90 %


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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

CatalystBPC + 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


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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(inwater). 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.


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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 CH21


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(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


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Fine-chemicals

Hydroxylat ion of phenolThe 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 thanhydroquinone, 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 ofpropylene to the epoxide with H2O2 over TS-1 is expected to become a

commercial reality soon.


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(a)

NH2O

O OH

MnO 2

H2SO4

Fe/HCl

(b)

OOH

OOH

OH

OH

O2

(c)

OH OH

OH

OH

OH

H2O2

catalyst

Different processes for manufacturing catecol and hydroquinone


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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


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Synthesis of vanillin

Vanillin 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. Asalready 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


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OH

H2O2

OH

OH

CH3OH

La-phosphate

OH

OH

OMe

CH2OH

OH

OMe

CHO

TS-1

Supported metal

HCHO Zeolite

Vanillin

Green synthesis of vanillin using heterogeneous catalysts.


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Other pro cesses

Some 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 andhydrolysis, 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].


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OHH2CO

CHO

H+ O2, 5000C

citral

+

Production of citral by the BASF process


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Cl

N Cl

+ CO H2N(CH2)2NH2+

Cl

N

NH

NH2HCl

O

Lazabemide

Pd

catalyst

Green route for lazabemide

Production of LazabemideIn 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


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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 (sodiumsulfate) 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 (


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H3CO

COOH

H3CO

COOH

(S)-Naproxen92% yield, 97%ee

P

P

Ru

O

O

O

O

Ru(OAc)2 (S)-BINAP

(Catalyst)

H2

catalyst

Enantio-selective reduction in the production of S-naproxen


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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 hydrogenationsare 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 isopropoxidediethyl tartarate (DIPT) as the catalyst (Fig. 7) [6].


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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 oxidase

EC1.1.3.15

CHO

COOH

EDA

(complex etc.)

Fig. 2.19. Manufacture of glyoxylic acid by an enzymatic route

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 H2O2to 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


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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 ofthe 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


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Another example of an enzyme based process in commercial operation is the transformation of acrylonitrile

into 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 arethe 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

Catalyst

Reactant 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)


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Alternate raw materials

An important aspect of sustainable growth is the use of renewable raw materials for the production of chemicals. The conversion of cellulose andstarch 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


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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 rawmaterial (olefin) cost is nearly 6070 % 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.

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