Mixed-metal or mixed-linker metal organic frameworks as heterogeneous catalysts

Metal organic frameworks (MOFs) are among the most studied heterogeneous catalysts that have been applied to promote a wide range of reactions. Most of the initial studies on the catalytic activity of MOFs were based on the use of materials containing a single metal and a single linker. However, the most recent trend in the field is to exploit the synthetic flexibility offered by MOFs to obtain new MOFs possessing two different metals in their structure, or the same metal in different oxidation states (mixed metals) or different linkers (mixed linkers), resulting in materials with a superior catalytic activity over the corresponding single metal or single linker MOFs. This review is aimed to address the possible advantages of the use of mixed metal or mixed linker strategies to increase the activity of MOFs in some selected reactions. After some general sections introducing the structural features of MOFs, the nature of possible active sites, different ways to characterize mixed-metal or mixed-ligand MOFs and good practices, the main body of the review describes the current state of the art in the use of this type of MOF as heterogeneous catalysts, classified depending on the presence of more than one metal or more than one ligand. The final concluding remarks include some future targets in the area.