2-Propanol Dehydration on the Nodes of the Metal–Organic Framework UiO-66: Distinguishing Catalytic Sites for Formation of Propene and Di-isopropyl Ether

Abstract

2-Propanol dehydration was used as a test reaction to probe the catalytic properties of metal–organic framework (MOF) UiO-66. Experiments were performed with a flow reactor operated at atmospheric pressure and 510 K, showing (a) how the catalytic activity increased and then decreased, depending on the nature of ligands on the Zr6O8 MOF nodes (such as formate, acetate, hydroxyl, or alkoxy groups); and (b) how the selectivity changed with changing node ligands, which were characterized by IR spectroscopy, 1H NMR spectroscopy of digested MOF samples, and other techniques. The selectivity is sensitive to the node ligand composition, with the dehydration reaction initially facilitated by the removal of adventitious node formate and acetate ligands formed in the MOF synthesis and concomitant formation of node OH ligands from water formed in the catalysis. Node pair sites consisting of a node Zr-μ1-OH site and a neighboring node zirconium vacancy site are inferred to be active for propene formation. The ether formation rate increased with an increasing density of node 2-propoxy ligands, leading to the suggestion that these ligands at a paired zirconium defect site react with adjacent 2-propanol molecules to form di-isopropyl ether in a bimolecular nucleophilic substitution mechanism. These results show how the selectivity of UiO-66 can be modulated simply by changing the node ligands though postsynthetic modifications, without changing the node motif, oxidation state of the node metal atoms, pore structure, MOF topology, or linker chemistry.