Factors And Atom Efficiency

Two useful measures of the potential environmental impact of chemical processes are the E-factor,5-7 defined as the mass ratio of waste to desired product, and the

Methods and Reagents for Green Chemistry: An Introduction, Edited by Pietro Tundo, Alvise Perosa, and Fulvio Zecchini

Copyright © 2007 John Wiley & Sons, Inc.

TABLE 9.1 E-factors for Different Segments of the Chemical Industry

Industry Segment

Production (tons)

E-Factor kg Waste/kg Product

Oil refining

106-108

ca. 0.1

Bulk chemicals

104-106

<1-5

Fine chemicals

102-104

5-50

Pharmaceuticals

10-103

25->100

atom efficiency,1'2 calculated by dividing the molecular weight of the product by the sum of the molecular weights of all substances produced in the stoichiometric equation.

A prime cause of high E-factors is the use of stoichiometric inorganic reagents. Fine chemicals and pharmaceuticals manufacture, for example, is rampant with classic stoichiometric technologies that generate copious amounts of inorganic salt as waste. Examples that readily come to mind are stoichiometric reductions with metals (Zn, Fe) and metal hydrides (NaBH4, LiAlH4, and derivatives thereof) and stoichiometric oxidations with permanganate, dichromate, periodate, and so forth. Similarly, processes employing mineral acids (H2SO4, HF), Lewis acids (AlCl3, ZnCl2, BF3), or inorganic bases (NaOH, K2CO3), often in stoichiometric amounts, represent a major source of inorganic waste that cannot easily be recycled. Reactions of this type, widely employed in the fine chemical industry, include Friedel-Crafts acylation mediated by Lewis acids such as AlCl3, sulfona-tions, and diazotizations, to name but a few.

The workup for such reactions involves neutralization and concomitant generation of salts such as NaCl, Na2SO4, and (NH4)2SO4. The elimination of such waste streams and a reduction in the dependence on the use of hazardous chemicals, such as phosgene, dimethyl sulfate, peracids, sodium azide, halogens, and HF, are primary goals in green chemistry.

Table 9.1 contains the values of E-factors (mass ratio of waste to desired product) for different industry segments; most of the processes for fine chemicals and pharmaceuticals, with a very large E-factor, use reagents in stoichiometric quantities, often in combination with environmentally unfriendly solvents. The E-factor is the actual amount of waste formed in the process and includes everything except the desired product, not only the raw materials and reagents involved in the stoichiometric equation but also chemicals used in the workup, for example, acids and bases for neutralization, and solvent losses. Strictly speaking, it should also include the fuel used to generate the energy required to operate the process, but this is often difficult to quantify. Process water is not included, as this leads to E-factors that are not generally meaningful.

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