Isolated and Paired Metal Sites in Zeolites Using Solid-State Ion Exchange.

Isolated and paired extraframework transition metal cations in zeolites are emerging as top candidates for numerous applications, including, but not limited to, selective methane oxidation to methanol, selective catalytic reduction of nitrogen oxides, propane dehydrogenation, propylene epoxidation, and direct air capture of carbon dioxide. Importantly, these well-defined heterogeneous catalysts offer parallels with molecular and metalloenzyme catalytic active sites. Aqueous-phase ion exchange (APIE) is the most common synthesis technique to obtain these catalysts. Solid-state ion exchange (SSIE) is an often overlooked technique that offers synthetic advantages compared to APIE. Thus, recent advances in solid-state synthesis strategies merit contemporary contextualization. In this minireview, we describe the basic principles, methods, mechanisms, challenges, and advances in solid-state ion exchange in the context of well-defined transition metal cation active sites located in extraframework positions of the zeolite.