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ACS Sustainable Chemistry & Engineering Perspective procured raw materials need to be considered. One possibility is to define the starting point as a commodity-type, commercially available, raw material. This can be illustrated by reference to the commercial process for the manufacture of sildenafil citrate (Viagra) shown in Scheme 4a. This process afforded23 a (traditional) E factor, including 10% of solvents used and excluding water, of 6.4 kg/kg, corresponding well with Pfizer’s reported 6 kg/kg.37 The sEF (excluding solvents and water) is 3.9 kg/kg, and the cEF is 50.3 kg/kg. However, one of the primary raw materials, 1-methyl-4- nitro-3-propyl-1H-pyrazole-5-carboxylic acid (1), does not meet the starting point criterion of a readily available commodity-type chemical, defined as being commercially available at a price not exceeding $100 mol−1 for the largest quantity offered. Hence, it was argued23 that the intrinsic E factor of (1), which is derived from readily available diethyl oxalate and 2-pentanone in a five-step process (Scheme 4b), should be included in the calculation. Indeed, its inclusion afforded significant increases in the overall E factors: the sEF increases from 3.9 to 9.9 kg/kg, the cEF from 50.3 to 85.5 kg/ kg, and the E factor from 6.4 to 13.8 kg/kg. It is immediately clear, from this example, that agreement is needed on an industry-wide standardized starting point concept for analyzing ■process greenness. SUSTAINABILITY METRICS AND THE ENVIRONMENTAL IMPACT OF WASTE The mass-based metrics discussed in the preceding section give a good initial indication of the greenness and sustainability of a process, particularly from the viewpoint of waste prevention and resource utilization, but this constitutes only a part of the analysis of the sustainability of a chemical process. The concept of sustainable development was first introduced in 1987 and is defined as development that meets the needs of the present generation without compromising the ability of future generations to meet their own needs.38 Sustainability is dependent on the rates of both resource utilization and waste generation.39 A sustainable technology needs to fulfill two conditions: (i) Natural resources should be used at rates that do not unacceptably deplete supplies over the long-term. (ii) Residues should be generated at rates no higher than can be readily assimilated by the natural environment. It is evident, for example, that nonrenewable fossil resourcesoil, coal, and natural gasare being used at a much higher rate than they are replaced by natural geological processes. Consequently, their use is unsustainable in the long term. By the same token, the extraction and use of fossil resources is generating carbon dioxide at rates that cannot be assimilated by the natural environment. This is widely accepted to be a root cause of climate change.40 This marked discrepancy between the time scales for the formation of natural resources and their exploitation is referred to41 as the “ecological time-scale violation”. In order to preserve the planet’s resources, to enable future generations to fulfill their own ambitions regarding living standards, the current rate of resource extraction has to be restrained. A balance needs to be found between economic development, environmental impact, and societal equity, referred to as the triple bottom line. This is reflected in the three types of sustainability metrics or indicators: economic, ecological, and societal, or the three Ps, profit, planet, and people, represented by three overlapping circles in Scheme 5.42−44 Two-dimensional metricssocio-economic, eco-effi- F Scheme 5. Sustainability metrics Venn Diagram ciency, and socio-ecologicalare obtained where two of the circles intersect and a fully sustainable technology where three circles overlap, corresponding to all three aspects of sustainability being fulfilled. Mass efficiency metrics pertain to five of the 12 principles (numbers 1, 2, 5, 8, and 9) of Green Chemistry and are, therefore, at the heart of green process design. However, for a holistic analysis of the greenness of a chemical process, more aspects need to be delineated, namely, the energy efficiency of the process (principle 6), the renewability of the raw materials (principle 7), health and hazard risks for workers involved (principles 3, 11, and 12), and the environmental impact of the chemicals used and the waste generated (principle 3). [Principles 4 and 10 are associated with the product rather than the process for making it.] Moreover, in order to assess its overall sustainability, the economic and societal metrics, that is, the socio-economic indicators, of the process need to be considered. In short, green chemistry is an integral part of sustainable, benign-by-design processes for chemicals manu- facture. Energy Efficiency Metrics. The inclusion of the energy requirements of a process in the E factor was always implicit since energy consumption generates waste, mainly in the form of carbon dioxide. However, since many fine chemicals and pharmaceutical intermediates are produced in campaigns in multipurpose production facilities, energy usage is often not allocated to particular processes. In the production of commodity chemicals, in contrast, energy consumption plays a prominent role in comparing, for example, biomass-based with petrochemical-based routes (see below). As is the case with mass efficiency metrics, energy efficiency metrics can be based either on raw materials consumed or waste generated, in methane equivalents consumed or in CO2 equivalents generated, respectively, for example. In the context of energy consumption in chemicals manufacture, it is worth noting that manufacturing plants use steam, in many cases “waste” heat generated by other exothermic reactions, for heating reactions. This means that heating reactions often does not involve any “extra” energy input. In contrast, cooling reaction mixtures consume substantial amounts of “extra” energy. Hence, reactions at say −20 °C should be avoided. Cumulative Energy Demand (CED), defined as the total amount of primary energy potential used during the production cycle, is an often used energy metric in environmental DOI: 10.1021/acssuschemeng.7b03505 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXXPDF Image | Metrics of Green Chemistry and Sustainability
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