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ACS Sustainable Chemistry & Engineering Perspective Scheme 3. Mass-Based Metrics for Measuring Greenness metric, including water in addition to solvent in the input of materials and renamed it Process Mass Intensity (PMI), to benchmark the environmental footprint of processes for APIs and to use this data to drive the greening of the pharmaceutical industry.34,35 In our opinion, none of these alternative metrics offer any particular advantage over the E factor for describing how wasteful a process is, and waste elimination was always the major driving force behind the development of green chemistry. Using the E factor places emphasis firmly on designing cleaner, waste-free processes, and the ideal E Factor of 0 more clearly reflects the ultimate goal of zero waste manufacturing plants. In contrast, using the PMI focuses on reducing the costs of the raw materials input, and it has been argued34 that “waste and waste reduction doesn’t come anywhere near to capturing management attention to the extent that the cost of high-value materials does”. Perhaps this says more about the perception of senior managers in the pharmaceutical industry than the Scheme 4. Pfizer’s Commercial Process for Sildenafil (Viagra) relative merits of the E factor and PMI as sustainability metrics. It is not necessarily the viewpoint of the chemical industry at large that tends to experience and understand this completely differently. The major driver for the introduction of green chemistry was and still is waste prevention at source that was seen as not only beneficial for the environment but also good for economic competitiveness by circumventing the costs of waste treatment and, thus, providing for more efficient usage of raw materials. When all is said and done, PMI and the E factor represent two sides of the same coin, one focusing on optimizing resource utilization and the other on reducing waste generation. An additional advantage of the E factor is that, in evaluating a multistep process, E factors of individual steps are additive but PMIs are not because PMI does not discount step products from the mass balance. Thus, for a three-step process, the overall E factor is Etotal = E1 + E2 + E3 whereas the overall PMI is PMItotal = PMI1 + E2 + E3. Christensen and co-workers36 proposed the use of the C factor, defined as the total mass of CO2 emitted divided by the mass of product formed, as a metric for comparing the CO2 burdens of different processes to a particular product. It can also be used to compare biomass-based vs fossil resource-based processes and as one facet of a Life Cycle Assessment study (see below). System Boundaries and Intrinsic E Factors. E factors, indeed all mass-based metrics, are very much dependent on the starting point of the synthesis; that is, it is necessary to define the boundary conditions for calculation of E factors.23 In our original development of the concept, E factors were calculated on a gate-to-gate basis,3 that is, the starting point is the raw material entering the factory gate and the end point is the product leaving it. The E factor relates only to those processes carried out at the manufacturing site. However, a raw material used in an API synthesis may itself be an advanced intermediate, prepared in a multistep process from readily available raw materials. Indeed, E factors can be dramatically reduced overnight by purchasing an intermediate rather than producing it on site. However, this can easily lead to inconsistencies in measuring the greenness of pharmaceutical processes. Hence, the intrinsic E factors associated with E DOI: 10.1021/acssuschemeng.7b03505 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXXPDF Image | Metrics of Green Chemistry and Sustainability
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