The E factor 25 years on: the rise of green chemistry and sustainability

The global impact, over the last 25 years, of the principles of green chemistry and sustainability, and the pivotal role of the E factor concept in driving resource efficiency and waste minimisation, in the chemical and allied industries, is reviewed. Following an introduction to the origins of green chemistry and the E factor concept, the various metrics for measuring greenness are discussed. It is emphasised that mass-based metrics such as atom economy, E factors and process mass intensity (PMI) need to be supplemented by metrics, in particular life cycle assessment, which measure the environmental impact of waste and, in order to assess sustainability, by metrics which measure economic viability. The role of catalysis in waste minimisation is discussed and illustrated with examples of green catalytic processes such as aerobic oxidations of alcohols, catalytic C-C bond formation and olefin metathesis. Solvent losses are a major source of waste in the pharmaceutical and fine chemical industries and solvent reduction and replacement strategies, including the possible use of neoteric solvents, such as ionic liquids and deep eutectic solvents, are reviewed. Biocatalysis has many benefits in the context of green and sustainable chemistry and this is illustrated with recent examples in the synthesis of active pharmaceutical ingredients. The importance of the transition from an unsustainable economy based on fossil resources to a sustainable bio-based economy is delineated, as part of the overarching transition from an unsustainable linear economy to a truly green and sustainable circular economy based on resource efficiency and waste minimisation by design.