The Role Of Catalysis

The increasing use of catalytic processes can substantially reduce waste at the source, resulting in primary pollution prevention. The theoretical process efficiency

Stoichiometric:

3 PhCH(OH)CH3 + 2 Cr03 + 3 H2S04 -- 3 PhCOCH3 + Cr2(S04)3 + 6 HzO

Catalytic:

Catalyst

Atom efficiency = 120/138 = 87%

By-product: HzO

Figure 9.1 Acetophenone synthesis by stoichiometric and catalytic oxidation.

can be quantified by the atom efficiency, the ratio between the molecular weight of the product, and the sum of the molecular weights of all substances produced in the stoichiometric equation. It should be pointed out, however, that the atom efficiency only takes the chemicals appearing in the stoichiometric equation into account.

Figure 9.1 compares the synthesis of acetophenone by classic oxidation of 1-phenylethanol with stoichiometric amounts of chromium oxide and sulphuric acid, with an atom efficiency of 42%, with the heterogeneous catalytic oxidation with O2, with an atom efficiency of 87%, and with water as the only by-product. This is especially important if we consider the environmental unfriendliness of chromium salts: the potential environmental impact of reactions can be expressed by the environmental quotient (EQ), where E is the E-factor (kg waste/kg product) and Q is the environmental unfriendliness quotient of the waste. If Q is

Hydrogénation: 9 Catalyst H\ /0H

100% atom efficient

Carbonylation: H OH Catalyst

100% atom efficient

Oxidation: H OH Catalyst V

87% atom efficient

Figure 9.2 Atom-efficient catalytic processes.

AICI3 -1

solvent

AICI3 -1

solvent

+ CH3C02H

+ CH3C02H

Homogeneous

Heterogeneous

AICI3 >1 equivalent Solvent (recycle) Hydrolysis of products 85-95% yield

H-Beta, catalytic, and regenerable No solvent No water necessary >95% yield/higher purity

4.5 kg aqueous effluent per kg 0.035 kg aqueous effluent per kg Figure 9.3 Friedel-Crafts acylation of anisole.

1 for NaCl, for example, then for chromium salts Q could be arbitrarily set at, say 100 or 1000. Similarly, clean catalytic technologies can be utilized for hydrogenation of acetophenone and carbonylation of 1-phenylethanol (Figure 9.2), with 100% atom efficiency in both cases.

One way to significantly reduce the amount of waste is to substitute traditional mineral acids and Lewis acids with recyclable solid acid catalysts. A good example of this is the Rhodia process for the synthesis of 4-methoxy acetophenone by Friedel-Crafts acetylation of anisole (Figure 9.3) with acetic anhydride, catalyzed by the acid form of zeolite beta.8 This replaced a traditional Friedel-Crafts acylation using acetyl chloride in combination with more than one equivalent of aluminium chloride in a chlorinated hydrocarbon solvent. The new process requires no solvent and avoids the generation of HCl in both the acylation and the synthesis of the acetyl chloride. The original process generated 4.5 kg of aqueous effluent (containing AlCl3, HCl, chlorinated hydrcarbon residues, and acetic acid) per kg of product. The catalytic alternative generates 0.035 kg of aqueous effluent (i.e., >100 times less), consisting of 99% water, 0.8% acetic acid, and <0.2% other organics per kg of product. Workup consists of catalyst filtration and distillation of the product. Because of the simpler process, a higher chemical yield is obtained (>95% vs. 85-95%) and higher product purity is obtained. Moreover, the catalyst is recyclable and the number of unit operations is reduced from 12 to 3. The conclusion is clear: The new technology is not only cleaner and greener, it also leads to lower production costs than the classic process. An important lesson indeed.

Other important successes have been achieved in developing clean, "green," methods to oxidize alcohols, for example, the Ru/TEMPO (tetramethylpiperidiny-loxyl radical) catalysis, shown in Figure 9.4, for the aerobic oxidation of alcohols.

Conv. (%)

Sel. (%)

85

95

96

>99

91

>99

95

>99

85

Figure 9.4 Aerobic oxidation of primary and secondary alcohols catalyzed by RuCl2 (Ph3P)3/TEMPO in PhCl at 100°C.

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