Measuring green chemistry: methods, models, and metrics

This cross-journal themed collection intends to create a virtual and live place to collect scientific publications dedicated to measuring improvements in terms of sustainability, including contributions that propose and apply green metrics and that underline their importance in terms of their teaching and dissemination.

The so-called “short twentieth century” (1914–1991) has profoundly shaped our society and, among the many changes that define the current millennium, green and sustainable chemistry stand out. The formal birth of green chemistry was in the 90s with, for example, the definition of the Pollution Prevention Act and the 12 principles of Green Chemistry (https://doi.org/10.1039/C8GC00482J). The industrial policies developed in previous decades that were aimed at reducing both waste production and use of precious resources were formally realized during these years. These realizations were underscored by the critical need for scientific and technological innovation, wherein academia and funding agencies began to recognize that industrial competitiveness and efficiency requires high performance with minimal environmental impact.

Discussing green and sustainable chemistry has become commonplace, and the importance of teaching these topics in order to equip a new generation of scientists with the skills necessary to address the growing challenge of sustainability is now widely recognized. Just as an example, this themed collection includes contributions highlighting the importance and the dedication of our authors to teaching and defining green chemistry (https://doi.org/10.1039/D0GC03313H; https://doi.org/10.1039/D1GC02623B; https://doi.org/10.1039/D3GC00800B).

Besides the terminological difference between green chemistry and sustainable chemistry, which may be considered related sciences, a process or a substance can only be defined as green or greener after comparing its alternatives. In fact, comparison is key to assessing any advance in terms of sustainability and adequately developing green chemistry. For example, the choice of the “ideal” solvent, the catalyst and the choice of the reaction conditions strongly depends on the area of application. The best balance between cost and efficiency (environmental and chemical) may result in selecting different solvents for each stage, from the preparation of reactants to the isolation of the pure product. For efficient green chemistry research, comparison is the key, and a priori assumptions are not truly possible, i.e. It can be questionable, and perhaps not even useful, to assign the term “green” or “non-green” to a substance or process without defining its context of use. The community has demonstrated the importance of these comparisons through different contributions (https://doi.org/10.1039/C9GC02775K, https://doi.org/10.1039/D3GC04293F, https://doi.org/10.1039/B807379A). More generally, the comparative approach is crucial when planning a contribution to sustainability. For example, the tremendous utility of cross-coupling chemistry is undebatable and is used for the preparation of numerous materials. It often requires the use of an aryl halide, a catalyst, a medium, and so on. The scientific community looks at investigating this chemistry by trying to use an apparently simpler chloride, sometimes at the expense of the complexity of the catalytic system and without considering the higher toxicity of chlorides (https://doi.org/10.1039/D3GC01896B). Applying a life cycle assessment (LCA) to this chemistry clearly shows that the best combination depends on the target. In the case of coupling partners, and in contrast with historical perception, chloride reactants can be significantly worse than bromide or iodide counterparts, and the overall assessment of the process, including catalyst, solvent, and technology used, is not obvious, as it varies case by case.

This clearly shows that assumptions of the past can still be discussed and that an efficacious contribution to green chemistry requires strong systems thinking. For these reasons, authors should compare their processes in a broader context of alternatives and avoid reporting “self-improvements” that may lead to a self-referential green vision. It is immediately evident that for green chemistry, new methods of measuring progress in terms of sustainability are needed to supplement the existing metrics, such as atom economy and E-factor, which were introduced back at the beginning of the 90s. Today, it is of fundamental importance to be able to analyze processes and substances in a multidisciplinary and multivariable manner (https://doi.org/10.1039/D3GC02516K) in order to compare and achieve the quantification of sustainability in the most effective way possible using simplified metrics (https://doi.org/10.1039/D2GC04747K, https://doi.org/10.1039/D3GC02516K), technoeconomic analysis (https://doi.org/10.1039/D2GC00843B, https://doi.org/10.1039/D2GC01840C) and/or LCA (https://doi.org/10.1039/D2GC04860D, https://doi.org/10.1039/D4GC00394B). The use of a multi-metric comparison can be very valuable. For example, C–H functionalization processes are proposed to be among the most efficient, atom-economical, and are potentially less wasteful. Although this assumption is true in general, simple C–H bonds can be directly modified without pre-functionalization (i.e., fewer steps and less waste). A case-by-case comparison that considers different parameters for different syntheses for active pharmaceutical ingredients led to a non-obvious result, indicating that the opposite can also be true (https://doi.org/10.1039/D3GC02516K). Single and more complex metrics are useful to identify, make progress, and achieve specific goals on a specific issue. For example, atom economy offers a unique tool to plan the ideal process and mass metrics, such as E-factor (https://doi.org/10.1039/D2GC04747K), offer an effective manner to quantify the advance in terms of waste produced once the type of process is selected. Multivariable analysis, including yields, reaction mass efficiency (RME), and so on, help the visual identification of all the mass aspects of the process. Additionally, safety and toxicity parameters can significantly shed light on the wider greenness of a process by considering the mass and properties of the materials used. More articulated metrics appear every day, such as the eco scale, which try to include mass, economic, technical and safety parameters in a simplified manner accessible to many. The ultimate and most reliable/comprehensive environmental assessments can only be done with the use of LCA.

“In Green Chemistry, authors are encouraged to supply, discuss, or establish metrics to assess the sustainability of their processes. This practice will ensure the rigorousness of published claims and facilitate the publication's acceptance by the community.” – Javier Pérez-Ramírez, Editorial Board Chair for Green Chemistry

The enormous attention that the academic and industrial scientific community has paid to green chemistry has allowed the involvement of an ever-increasing number of researchers working to address various problems and improve the quality and quantity of the substances and processes used. Scientists from every sector of research and production address the problems associated with making the use and access to a substance, a process or a technology more sustainable. Energy, fuels, drug discovery, pharmaceuticals or fine chemicals synthesis, etc.; in every field the concept of sustainability and environmental efficiency is discussed and treated.

The need to evolve the perception of sustainability, the feeling that a substance or process is green, and quantifying the real advantages and disadvantages of using the substance or process remains widely relevant. Both in the fields of basic and applied research, such quantification cannot be taken for granted. It must become an increasingly consolidated practice that accompanies each study, be it academic or industrial. We can then highlight the possible advantages in numerical terms through green metrics, from the simplest or qualitative to the most technical and complete.

The use of metrics in developing a green and sustainable process can also be crucial at the preliminary stage of a project in order to adequately direct research and process optimization in compliance with one of the synonyms of green chemistry: “benign-by-design”. The application of green metrics allows us to highlight the major strengths but, above all, to identify the weaknesses of a given substance or technology.

Green metrics are therefore essential in order to be able to define the areas of future development needed to solve existing problems.

Contributions reporting and quantifying the advance in term of sustainability in all the different areas of research will be always welcome and will find a home in this thematic collection and in Green Chemistry.

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