Microwave-induced high dispersion anchoring of MOFs on SiC surfaces: Built-in electric field dominated process

Abstract

Abstract Microwave irradiation has emerged as a promising method for the preparation of highly dispersed metal-organic frameworks (MOFs) catalysts, which can improve their dispersion and catalytic efficiency. However, the underlying mechanism behind this "microwave effect" and the enhanced accessibility of anchored MOFs catalysts remain poorly understood. In this study, we employed microwave-absorbing SiC as a substrate material to enhance the specific recognition ability of MOFs precursors under microwave irradiation. The synthesized UIO-66-NH2@SiC catalyst was applied to the esterification reaction of cyclohexene. The catalyst with an anchored structure can achieve a conversion rate of 72% of cyclohexene. In comparison, the conversion rate obtained with pure MOFs as the catalyst was merely 63%. Subsequently, we proposed a mechanism of microwave-induced anchoring process, i.e. this process was attributed to the built-in electric field effect. To investigate this mechanism, we used a thermosensitive fluorescent material, Eu/Tb-MOF, to reveal that the microwave-induced anchoring of UIO-66-NH2 on the SiC surface does not originate from the commonly believed "hotspots". Instead, numerical simulations were conducted to analyze the electric field distribution, revealing that the electric field intensity between SiC particles was 7 times higher than in other regions. Furthermore, we validated the simulation results by using fluorescent powder as an electric field tracer with a thin film of naphthalene diamide. Our findings demonstrate that electric fields can be employed to control the approach of metal ions towards a functionalized SiC ceramic surface, leading to spontaneous and irreversible immobilization when the metal ions make contact with the surface. Overall, our research provides a theoretical foundation for understanding the microwave-assisted synthesis of anchored catalysis materials.