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ACS Sustainable Chemistry & Engineering Perspective produce the equivalent mass of a carbon-based chemical using molar equivalency. Since the “ethanol equivalent” can be produced by well-known fermentation, the required mass of biomass feedstock, the land area, and even the volume of water can be calculated. The calculations were based on the first- generation corn-based bioethanol technology commercially practiced in the United States and the stoichiometry shown in Scheme 6. Based on their calculations of ethanol equivalents, Scheme 6. Stoichiometry of Carbon Dioxide to Ethanol via Glucose the authors concluded, inter alia, that replacement of the 387 × 106 tons of gasoline used in the United States in 2008 is not a viable proposition by a long way. In contrast, the conversion of biomass to basic chemicals, such as ethylene, propylene, and xylenes, could be a sustainable option in the near future, especially with second-generation bioethanol from waste lignocellulose. More recently, the same group130 developed a novel set of metrics for assessing the sustainability of biomass-based carbon chemicals based on ethanol equivalents (EE) as the common currency. A sustainability value of resource replacement (SVrep) and a sustainability value of the fate of waste (SVwaste) are determined and used to establish a sustainability indicator (SUSind). The latter is calculated with the followingequation: SUS =[SV ×SV ]/[SV +SV ] ind rep waste rep waste Sustainability is achieved when all resources are replaced (SVrep = 1) and all waste can be recycled or the remaining parts treated within a reasonable time frame. SUSind > 0.5 is better than required sustainability. The SUSind values of six bioethanol-based basic petrochemicalsethylene, propylene, toluene, p-xylene, styrene, and ethylene oxidewere deter- mined based on the availability of bioethanol in 2008 and 2014. If the global production of all of these chemicals was from bioethanol, then the total amount of bioethanol feedstock needed would be 151.93 mio tonnes in 2008 and 150.26 mio tonnes in 2014. This is substantially more than the total bioethanol production of 27.8 and 42.8 in 2008 and 2014, respectively. The SUSind values of these chemicals were between 0.1 and 0.429 indicating that the global demand of none of these chemicals could be met with bioethanol-based production. The authors suggested that bioethanol-based carbon products should be labeled “sustainable” only when the necessary land is available to produce the required bioethanol. The calculations are all based on the state-of-the-art technology for producing bioethanol from corn starch that requires 1 kg of additional bioethanol to produce 2.3 kgs of bioethanol. The authors noted that production of the bioethanol from second-generation lignocellulose in agricultural residues and food supply chain wastes could significantly i■mprove this scenario. SUMMARY AND FUTURE OUTLOOK The green and sustainable manufacture of chemicals is an essential component of the transition from a linear economy M that is devouring the planet’s natural resources and degrading the ecosphere to a circular economy that is resource efficient and waste free by design. In order to facilitate this development, it is necessary to have reliable metrics for comparing the greenness and sustainability of competing technologies. The original green metrics, the E factor and atom economy, were focused on the elimination of waste and the maximization of resource efficiency, two sides of the same coin. Other mass- based metrics, such as process mass intensity and reaction mass efficiency, have also been developed. However, not only the amount but also the nature of the raw materials and waste are important in determining greenness. Hence, mass-based metrics need to be supplemented by metrics that measure the environmental impact of raw materials and waste. The life cycle assessment methodology has been adapted by many pharmaceutical and fine chemical companies to make it more suitable for the evaluation of process greenness. In order to assess the sustainability of a process, these metrics need to supplemented with energy efficiency, economic, and societal metrics. It is also clear that different segments of the chemical industry focus on different aspects of green chemistry and sustainability. In the pharmaceutical and fine chemical industries, for example, solvent use is an important contributor to waste and has associated toxicity/hazard issues. Processes for commodity chemicals, on the other hand, are often solvent free. For some industry segments, the origin of the raw materials, including solvents, is important; for example, for cosmetics companies, they should preferably be renewable. The transition from an unsustainable economy based on fossil resources to a sustainable biobased economy driven by renewable biomass is another important cornerstone of sustainable development Hence, reliable sustainability metrics are needed for comparing biobased and petrochemical routes to commodity chemicals, materials, and fuels. Another important development is that downstream manufacturers such as pharmaceutical and cosmetic ingredient producers increasingly realize that the sustainabilities of their processes are also influenced by the greenness of the raw materials they are using. Sustainability spans the whole gamut from resources extraction and conversion to raw materials on to their further elaboration to end products and disposal of waste. Hence, many companies now require their suppliers to guarantee the greenness of the materials they are delivering. Cosmetic ingredient manufacturers, such as L’Oreal and Mane, may require that a supplier guarantees that a particular material is derived from renewable biomass rather than being of fossil origin. Looking to the future, there is still a great need for simple and reliable metrics for quick evaluation of processes in an early stage of development. In particular, a simple, back-of-the- envelope method for evaluating the process costs based on the initial technical data would be very useful. From a circular economy viewpoint, there is a definite need for metrics that readily identify and define “critical elements”.131 Finally, as we already mentioned, there is a pressing need for new economic indicators that incorporate the currently externalized costs of repairing the environmental damage caused by industrial activities.132 It would be wise to heed the words of Kenneth Boulding, the British-American economist and philosopher: “If the society toward which we are developing is not to be a nightmare of exhaustion, we must use the interlude of the present era to develop a new technology which is based on a circular flow of DOI: 10.1021/acssuschemeng.7b03505 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXXPDF Image | Metrics of Green Chemistry and Sustainability
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