Green room-temperature HF-free synthesis of an amorphous chromium terephthalate framework with open Cr sites for highly efficient cyclohexene oxidation.

Cyclohexene oxidation has been the subject of extensive studies as the products obtained are important synthetic intermediates to produce fine chemicals and can be utilized in polyether polymer synthesis. Different research groups have focused on the development of heterogeneous catalysts for cyclohexene oxidation with environmentally benign oxidants. In this work, we synthesized and characterized a chromium-based MOF, referred to as amorphous Cr-MIL-LT, in an aqueous medium at room temperature without using hydrofluoric acid. The obtained amorphous Cr-MIL-LT possesses high porosity, small particle size and coordinatively unsaturated Cr sites in its structure. As a catalyst, amorphous Cr-MIL-LT exhibited superior catalytic activity in the oxidation of cyclohexene with a high conversion of 97%. The enhancement of the catalytic performance could be attributed to its amorphous structure and furnished open metal sites (Cr-sites), indicating that it can have potential applications in catalytic oxidation reactions. In addition, density functional theory (DFT) calculations show that both H2O2 and cyclohexene form stable adsorbed complexes with the MIL-101(Cr) active sites, with the H2O2 complex being significantly more stable. The reaction profile for the homolytic O-O bond cleavage of H2O2 on Cr3+ reveals a kinetically accessible process with modest endothermicity. The formation of oxygen-centered radical species is confirmed by spin-density and Bader charge analyses, which show substantial unpaired electron density on the dissociated ˙OH radical and Cr-bound OH.