Surface solid frustrated Lewis pair constructed on alumina for selective hydrogenation reaction

Abstract

The design of solid frustrated Lewis pairs (FLPs) in heterogeneous systems is crucial to achieve the desired catalytic behavior in vital heterogeneous processes such as hydrogenation reactions. Herein, we report a synthesis approach for FLPs via calcination of AlOOH under different atmospheres (air and N2 atmospheres), in which unsaturated Al sites near OH vacancies and surface O atoms act as Lewis acidic sites and Lewis basic sites, respectively. A study combining 1H nuclear magnetic resonance (NMR), 27Al NMR, and electron paramagnetic resonance (EPR) demonstrates the presence of different types of OH vacancies, which results in spatial hindrance in generating various FLP sites. Notably, the selectivity of the hydrogenation of unsaturated aldehydes can be switched depending on the catalyst: the Al2O3–air catalyst with AlⅣ–O FLP sites exhibits over 90% selectivity to unsaturated alcohols via CO hydrogenation, while the Al2O3–N2 catalyst with AlⅤ–O FLP sites shows a unique selectivity toward saturated alcohols via hydrogenation of CC and CO bonds with a yield above 90%. Structure–property relationship studies based on in situ Fourier transform infrared spectroscopy (FT-IR) and density functional theory (DFT) calculations reveal that the different adsorption modes, i.e., vertical adsorption configuration at the AlⅣ–O FLP sites and flat adsorption configuration at the AlⅤ–O FLP sites, are responsible for the different selectivity in the hydrogenation reactions. This in-depth study on the structure–selectivity relationship provides new insights for the development of selective hydrogenation reactions heterogeneously catalyzed by solid FLPs.