Metal-organic frameworks-based heterogeneous single-atom catalysts (MOF-SACs) - Assessment and future perspectives

The architecture of heterogeneous single-atom catalysts (SACs) is deemed as an ultimate goal for the chemical industry and for materials catalysis research for many years. The design of these materials is intended in the first place to maximize the cost-efficiency of precious metals used in catalytic applications and related fields. Furthermore, SACs are highly interesting from a fundamental point of view because of their better-defined active site ensembles which facilitate the investigation of reaction mechanisms on heterogeneous catalysts, a highly debated subject since the introduction of the concept 'active sites' by H.S. Taylor. Metal-organic frameworks (MOFs) are highly attractive platforms for this purpose not only because of their exceptionally large specific surface area and structural diversity, but also because they are crystalline materials with regular structure which facilitate the elucidation of the structure of "active sites". Having these features combined in one single material is very meritorious for both fundamental and applied aspects. This review highlights the distinctive attributes of MOFs in the context of heterogeneous catalysis and outlines strategies for their use to prepare heterogeneous SACs. We discussed selected examples focusing on their catalytic applications, limitations, and future perspectives for the expansion of these materials in catalytic applications. First, we discussed concepts related to SACs, active sites, and surface reactivity. Afterwards, we gave a concise description of the buildup of MOFs and approaches to use them for the synthesis of MOF-SACs. This is followed by discussing key examples of MOF-SACs including non-noble (Cu, Fe, Ni) and noble (Pt, Rh and Au) metal centers. In addition, we highlighted strategies and examples for the use of MOF as sacrificial templates for the synthesis of highly dispersed metal-SAC composites. Finally, we discussed general perspectives and considerations for the development of these materials.