What is a green solvent? A comprehensive framework for the environmental assessment of solvents

TLDR

Solvents dominate the environmental performance of chemical processes, influencing cost, safety, and health, and the concept of green solvents seeks to reduce their environmental impact. The study asks how to quantify a solvent’s greenness. The authors present a life‑cycle framework that integrates substance‑specific hazard assessment with emissions and resource‑use quantification, demonstrated on 26 organic solvents and a case study of alcohol–water mixtures for solvolysis. The framework identifies simple alcohols and alkanes as environmentally preferable, flags dioxane, acetonitrile, acids, formaldehyde, and THF as unsuitable, and shows methanol–water or ethanol–water mixtures outperform pure alcohols or propanol–water mixtures, proving the tool useful for selecting green solvents and assessing new technologies.

Abstract

Solvents define a major part of the environmental performance of processes in chemical industry and also impact on cost, safety and health issues. The idea of "green" solvents expresses the goal to minimize the environmental impact resulting from the use of solvents in chemical production. Here the question is raised of how to measure how "green" a solvent is. We propose a comprehensive framework for the environmental assessment of solvents that covers major aspects of the environmental performance of solvents in chemical production, as well as important health and safety issues. The framework combines the assessment of substance-specific hazards with the quantification of emissions and resource use over the full life-cycle of a solvent. The proposed framework is demonstrated on 26 organic solvents. Results show that simple alcohols (methanol, ethanol) or alkanes (heptane, hexane) are environmentally preferable solvents, whereas the use of dioxane, acetonitrile, acids, formaldehyde, and tetrahydrofuran is not recommendable from an environmental perspective. Additionally, a case study is presented in which the framework is applied for the assessment of various alcohol–water or pure alcohol mixtures used for solvolysis of p-methoxybenzoyl chloride. The results of this case study indicate that methanol–water or ethanol–water mixtures are environmentally favourable compared to pure alcohol or propanol–water mixtures. The two applications demonstrate that the presented framework is a useful instrument to select green solvents or environmentally sound solvent mixtures for processes in chemical industry. The same framework can also be used for a comprehensive assessment of new solvent technologies as soon as the present lack of data can be overcome.