LCA as a tool to detect environmental “hot spots” in early-stage mechanochemical synthesis of organic dyes

Mechanochemical methods are currently under investigation as alternative approaches to conventional solvent-batch synthetic chemistry. Environmental life cycle assessment (LCA) is needed to evaluate and compare chemical products made through mechanochemical processes with those from conventional production routes, using “cradle-to-gate” system boundaries. However, chemical supply chains involved in product manufacturing are often unknown, and synthesis tracing of these chemicals is needed to create life cycle environmental profiles. Herein, we trace the synthesis of the chemicals required to produce Pigment Black 31, a perylene diimide (PDI) organic dye, based on two main principles: maximum reaction yield and minimum synthetic steps. This approach facilitates comparing the environmental life cycle impact of producing Pigment Black 31 via twin-screw extrusion (TSE) with a conventional solvent-batch process, wherein similar reactants are used in both systems. The production of Pigment Black 31 via the TSE process affords a roughly order-of-magnitude reduction across global warming and human and ecological health life cycle environmental impacts following ReCiPe 2016 methods, mainly due to solvent reduction and elimination compared with production via the solvent-batch process. Interestingly, this reduction is found despite the need for a methanol extraction step in the purification of Pigment Black 31 made by TSE. We conclude that while LCA data sets may not be available for other organic dyes produced similarly through TSE processes, the life cycle environmental impacts for Pigment Black 31 can be used to approximate those of similar organic dyes, such as Pigment Black 32.