E Factors and Atom Efficiency

Two useful measures of the potential environmental acceptability of chemical processes are the E factor [12-18], defined as the mass ratio of waste to desired product and the atom efficiency, calculated by dividing the molecular weight of the desired product by the sum of the molecular weights of all substances produced in the stoichiometric equation. The sheer magnitude of the waste problem in chemicals manufacture is readily apparent from a consideration of typical E factors in various segments of the chemical industry (Table 1.1). 

A higher E factor means more waste and, consequently, greater negative environmental impact The ideal E factor is zero. Put quite simply, it is kilograms (of raw materials) in, minus kilograms of desired product, divided by kilograms 

Industry segment Product tonnagea) kg waste b /kg product 

Other metrics have also been proposed for measuring the environmental acceptability of processes. Hudlicky and coworkers [19], for example, proposed the effective mass yield (EMY), which is defined as the percentage of product of all the materials used in its preparation. As proposed, it does not include so-called environmentally benign compounds, such as NaCl, acetic acid, etc. As we shall see later, this is questionable as the environmental impact of such substances is very volume-dependent. Constable and coworkers of GlaxoSmithKline [20] proposed the use of mass intensity (MI), defined as the total mass used in a process divided by the mass of product, i.e. MI = E factor+1 and the ideal MI is 1 compared with zero for the E factor. These authors also suggest the use of so-called mass productivity which is the reciprocal of the MI and, hence, is effectively the same as EMY. 

In our opinion none of these alternative metrics appears to offer any particular advantage over the E factor for giving a mental picture of how wasteful a process is. Hence, we will use the E factor in further discussions. 

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An interesting example, to further illustrate the concepts of E factors and atom efficiency is the manufacture of phloroglucinol [23]. Traditionally, it was produced from 2,4,6-trinitrotoluene (TNT) as shown in Fig. 1.2, a perfect example of nineteenth century organic chemistry. 

Quantifying Environmental Impact Efficiency, E-factors, and Atom Economy... 

In contrast to the E factor, it is a theoretical number, i.e. it assumes a yield of 100% and exactly stoichiometric amounts and disregards substances which do not appear in the stoichiometric equation. A theoretical E factor can be derived from the atom efficiency, e.g. an atom efficiency of 40% corresponds to an E factor of 1.5 (60/40). In practice, however, the E factor will generally be much higher since the yield is not 100% and an excess of reagent(s) is used and solvent losses and salt generation during work-up have to be taken into account. 

Explain the concepts of atom efficiency and environmental friendliness. What is an E-factor Which processes usually have the highest E-factors Explain what catalysis means on the different length scales indicated in Eig. 1.8. 

Similarly, a catalytic route to indigo was developed by Mitsui Toatsu Chemicals (Inoue et al, 1994) to replace the traditional process, which dates back to the nineteenth century (see earlier), and has a low atom efficiency/high E factor (Fig. 2.15). Indole is prepared by vapour-phase reaction of ethylene glycol with aniline in the presence of a supported silver catalyst. The indole is selectively oxidised to indigo with an alkyl hydroperoxide in the presence of a homogeneous molybdenum catalyst. 

The same reasoning applies to the synthesis of pure enantiomers as to organic synthesis in general processes should be atom efficient and have low E factors, i.e. involve catalytic methodologies. This is reflected in the increasing attention being focused on enantioselective catalysis, using either enzymes or chiral metal complexes. 

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... 

The limit of detection is a useful figure which takes into account the stability of the total instrumental system. It may vary from instrument to instrument and even from day to day as, for example, mains-borne noise varies. Thus, for atomic absorption techniques, spectroscopists often also talk about the characteristic concentration (often erroneously referred to as the sensitivity—erroneously as it is the reciprocal of the sensitivity) for 1% absorption, i.e. that concentration of the element which gives rise to 0.0044 absorbance nnits. This can easily be read off the calibration curve. The characteristic concentration is dependent on such factors as the atomization efficiency and flame system, and is independent of noise. Both this figure and the limit of detection give different, but useful, information about instrumental performance. 

Renewable raw materials can contribute to the sustainability of chemical products in two ways (i) by developing greener, biomass-derived products which replace existing oil-based products, e.g. a biodegradable plastic, and (ii) greener processes for the manufacture of existing chemicals from biomass instead of from fossil feedstocks. These conversion processes should, of course, be catalytic in order to maximize atom efficiencies and minimize waste (E factors) but they could be chemo- or biocatalytic, e.g. fermentation [3-5]. Even the chemocatalysts themselves can be derived from biomass, e.g. expanded com starches modified with surface S03H or amine moieties can be used as recyclable solid acid or base catalysts, respectively [6]. 

Quantitative evaluation of chemical processes in terms of environmental impact and eco-friendliness has gradually become a topic of great interest since the original introduction of the atom economy (AE) by Trost [1], and the E-factor by Sheldon [2]. Since then, other indexes have been proposed for the green metrics of chemical processes, such as effective mass yield (EMY) [3], reaction mass efficiency (RME) [4] and mass intensity (MI) [5], along with unification efforts [6, 7] and comparisons among these indexes [8]. 

The fundamental advance represented by the RCH/RP process (as a prototype of a biphasic technique) in terms of the environment, conservation of resources, and minimization of environmental pollution can be demonstrated by various criteria and proved very convincingly by means of the environmental factor, E [19] (which is far more suitable and constructive than Trosfs atom efficiency [20]). Sheldon defined the E factor as the ratio of the amount of waste produced per kilogram of product and specified the E factor for every segment of industry (Table 1). 

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