Metal organic frameworks as catalysts in solvent-free or ionic liquid assisted conditions
Metal organic frameworks (MOFs) are being intensively studied as solid catalysts for organic reactions in liquid media. This review focuses on those reports in which these materials have been used as catalysts in the absence of solvents or embedding ionic liquids (ILs). One of the major roles of solvents in liquid phase reactions is to desorb reagents and products from the active sites, facilitating the turnover of the active sites. For this reason, it is a general observation that most solid catalysts undergo strong deactivation and poisoning in the absence of solvents. In the present review, examples are presented showing that, due to their large porosity and framework flexibility, MOFs can be used as catalysts in the absence of solvents for several reaction types including cyanosilylations, condensations, cyloadditions and CO2 insertions, among others, and that they show even better performance than in the presence of conventional organic solvents. This review also describes the synergy that arises from the combination of ILs, frequently with suitable task-specific chains, and MOFs due to the cooperation with the catalysis when two centres in MOF and ionic liquid are present and due to the change in the microenvironment of the active sites. By embedding an ionic liquid in the MOF pores or by synthesising the ionic liquid covalently attached to the ligand in satellite positions, reusable and efficient catalysts requiring the minimum amount of ionic liquid can be obtained. Both complementary strategies increase the greenness of MOFs as heterogeneous catalysts and have advantages from the environmental point of view. Finally, the last section describes the catalytic activity of hierarchical porous MOFs in some selected reactions.