Environmental Impact Assessment of Green Protocols for the Production of Zeolitic Imidazolate Frameworks

Zeolitic imidazolate frameworks (ZIFs) represent a unique class of reticular materials that are important for gas separation, energy storage, biomedicine, and catalysis. As a general trend in materials science, the production of ZIFs currently adopts the 12 Green Chemistry principles. However, an important question arises at this stage: how "green" are the developed synthetic protocols, and what is their actual quantitative impact on the environment? To address this question, here, we report a cradle-to-gate environmental impact assessment of recently developed green synthesis protocols for the most important ZIF materials: ZIF-8, ZIF-67, and ZIF-90. To identify potential environmental hotspots, the complete protocols were decoded into separate stages, and 19 environmental impact indicators were calculated. Among them, a novel criticality indicator was determined for reticular materials for the first time. The revealed dominant contributors are: linker production for ZIF-8, cobalt salt production for ZIF-67, and waste solvent treatment for ZIF-90. As a general trend, the environmental impacts of green protocols are either comparable to or exceed those of commonly produced inorganic industrial materials such as copper or indium tin oxide. This work creates a roadmap toward the next step of reticular materials production: the development of sustainable synthesis protocols based on the quantitative evaluation of their environmental impacts.