Quasi-HKUST Prepared via Postsynthetic Defect Engineering for Highly Improved Catalytic Conversion of 4-Nitrophenol.

ABSTRACT

HKUST-1 [Cu3(BTC)2(H2O)3]n·nH2OMeOH was submitted to thermolysis under controlled conditions at temperatures between 100 and 300 °C. This treatment resulted in partial ligand decarboxylation, generating coordinatively unsaturated Cu2+ sites with extra porosity on the way to the transformation of the initial HKUST-1 framework to CuO. The obtained materials retaining in part the HKUST-1 original crystal structure (quasi-MOFs) were used to promote 4-nitrophenol conversion to 4-aminophenol. Because of the partial linker decomposition, the quasi-MOF treated at 240 °C contains coordinatively unsaturated Cu2+ ions distributed throughout the Q-HKUST lattice together with micro- and mesopores. These defects explain the excellent catalytic performance of QH-240 with an apparent rate constant of 1.02 × 10-2 s-1 in excess of NaBH4 and an activity factor and half-life time of 51 s-1g-1 and 68 s, respectively, which is much better than that of the HKUST parent. Also, the induction period decreases from the order of minutes to seconds in the presence of the HKUST and QH-240 catalysts, respectively. Kinetic studies fit with the Langmuir-Hinshelwood theory in which both 4-nitrophenol and BH4- should be adsorbed onto the catalyst surface. The values of the true rate constant (k), the adsorption constants of 4-nitrophenol and BH4- (K4-NP and KBH4-), as well as the activation energy are in agreement with a rate-determining step involving the reduction of 4-nitrophenol by the surface-bound hydrogen species.