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Perspective Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX pubs.acs.org/journal/ascecg Metrics of Green Chemistry and Sustainability: Past, Present, and Future Roger A. Sheldon* Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, PO Wits 2050, South Africa Department of Biotechnology, Delft University of Technology, Section BOC, van der Maasweg 9, 2629 HZ Delft, The Netherlands ABSTRACT: The first green chemistry metricsthe E factor (kgs waste/kg product) and atom economy (mol wt of product/sum of mol wts of starting materials)were introduced in the early 1990s and were actually green chemistry avant la lettre. In the last two decades, these two metrics have been adopted worldwide by both academia and industry. The E factor has been refined to distinguish between simple and complete E factors, for example, and to define the system boundaries. Other mass-based metrics such as process mass intensity (PMI) and reaction mass efficiency (RME) have been proposed. However, mass-based metrics need to be augmented by metrics which measure the environmental impact of waste, such as life cycle assessment (LCA), and metrics for assessing the economic viability of products and processes. The application of such metrics in measuring the sustainability of processes for the manufacture of pharmaceuticals and other fine chemicals is discussed in detail. Mass-based metrics alone are not sufficient to measure the greenness and sustainability of processes for the conversion of renewable biomass vs fossil-based feedstocks. Various metrics for use in assessing sustainability of the manufacture of basic chemicals from renewable biomass are discussed. The development of a sustainable biobased production of chemicals meshes well with the concept of a circular economy, based on resource efficiency and waste minimization by design, to replace traditional linear, take−make−use−dispose economies. KEYWORDS: E factor, Atom economy, Carbon economy, Step economy, Circular economy, Biobased economy, Ethanol equivalent, Life cycle assessment ■ INTRODUCTION: ORIGINS OF GREEN CHEMISTRY The essence of Green Chemistry can be paraphrased as follows:1 • Efficient utilization of (preferably renewable) raw materials, including energy resources in the manufacture and application of chemicals. • Elimination of waste and the use of toxic and or hazardous solvents and reagents in the manufacture and application of chemicals. In the 1980s, there was a growing concern regarding the generation of waste and the use of toxic and hazardous materials in chemicals manufacturing, in particular, in the fine chemical and allied industries. A pertinent example is provided by the fine chemical phloroglucinol, a pharmaceutical intermediate and reprographic chemical with a global production of ca. 200 tons per annum. The major producer in 1980s was Océ Andeno, based in Venlo in The Netherlands, a company that was later acquired by DSM. The manufacturing process, which had been known since the 19th century,2 involved the conversion of the high explosive 2,4,6- trinitrotoluene (TNT) in three steps (Scheme 1). In the first step, the TNT is dissolved in fuming sulfuric acid (oleum) followed by the addition of potassium dichromate to oxidize the TNT to trinitrobenzoic acid. The latter is subsequently reduced, with iron powder and hydrochloric © XXXX American Chemical Society A acid (the Bechamp reduction), to triamino benzoic acid which undergoes spontaneous thermal decarboxylation to afford 1,3,5- triaminobenzene. Refluxing the aqueous acidic solution results in the hydrolysis of the triaminobenzene to phloroglucinol. The process is highly selective, affording phloroglucinol in an overall yield of ca. 90%. Hence, a cursory examination would suggest that it is a selective, efficient process. However, phloroglucinol is not the only product formed. As shown in the overall stoichiometry of the process, in addition to phloroglucinol, substantial amounts of the following inorganic salts are formed: Cr2(SO4)3, NH4Cl, FeCl2, and KHSO4. Indeed, for every kilogram of phloroglucinol produced ca. 40 kg of solid, chromium containing waste were formed. Based on the reaction stoichiometry one would expect ca. 20 kg of waste. The fact that double that amount is formed can be attributed to the use of more than stoichiometric amounts of reagents and the fuming sulfuric acid is used in large excess, that is, as a solvent, and has to be neutralized with base. Although organic chemists would, certainly in 1980, have considered this to be an efficient process, it is actually the antithesis of green chemistry. The substrate (TNT) is a hazardous material, the oxidizing agent consists of a Received: September 29, 2017 Revised: November 1, 2017 Published: November 8, 2017 DOI: 10.1021/acssuschemeng.7b03505 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXXPDF Image | Metrics of Green Chemistry and Sustainability
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