Acid Catalysis in Confined Channels of Metal-Organic Frameworks: Boosting Orthoformate Hydrolysis in Basic Solutions

摘要

Enzymes use a confined docking cavity and residual groups in the cavity to regulate substrate selectivity and catalytic activity. By mimicking enzymes, we herein report that metal-organic framework (MOF) KLASCC-1, with channels and inside-channel pyridyl groups, can promote orthoformate hydrolysis in basic solutions. By studying pH-dependent hydrolysis and using an analogue MOF that lacks inside-channel pyridyl groups, we proved protonated pyridyl groups as acid catalytic sites for orthoformate hydrolysis. By using MOFs with only open pyridyl groups, we demonstrated the necessity of the confined channels. X-ray diffraction structures of KLASCC-1 with encapsulated substrates confirmed that these channels can regulate activity and size selectivity. Recycling tests and crystallographic studies confirmed that KLASCC-1 kept its framework structure in catalysis. This work shows the potentials of using MOFs for host-guest catalysis that cannot be otherwise completed and underlines the advantages of using crystal engineering to identify active sites.