Self-aldol condensation of aldehydes over Lewis acidic rare-earth cations stabilized by zeolites

Abstract

The self-aldol condensation of aldehydes was investigated with rare-earth cations stabilized by [Si]Beta zeolites in parallel with bulk rare-earth metal oxides. Good catalytic performance was achieved with all Lewis acidic rare-earth cations stabilized by zeolites and yttrium appeared to be the best metal choice. According to the results of several complementary techniques, i.e., temperature-programmed surface reactions, in situ diffuse reflectance infrared Fourier transform spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, the reaction pathway and mechanism of the aldehyde self-aldol condensation over Y/Beta catalyst were studied in more detail. Density functional theory calculations revealed that aldol dehydration was the rate-limiting step. The hydroxyl group at the open yttrium site played an important role in stabilizing the transition state of the aldol dimer reducing the energy barrier for its hydration. Lewis acidic Y(OSi)(OH)2 stabilized by zeolites in open configurations were identified as the preferred active sites for the self-aldol condensation of aldehydes.