Metrics of Green Chemistry and Sustainability: Past, Present, and Future

The first green chemistry metrics - the 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.