High Performance of Carbon-Coated MgO–Al₂O₃ Composite Catalysts in Transfer Hydrogenation of Cinnamaldehyde to Cinnamyl Alcohol

Mg and Al layered double hydroxide (LDH) was allowed to react with glucose at 180 °C for 3 h, and subsequent pyrolysis in N2 at 600 °C for 4 h led to a carbon-coated MgO–Al2O3 composite catalyst. The catalytic performance was tested in the transfer hydrogenation of cinnamaldehyde (CAL) to selectively produce cinnamyl alcohol (COL). The catalyst with a Mg/Al ratio of 3 (Mg3–Al1@C) exhibited 97.2% conversion with 99.9% selectivity to COL in 3 h at 100 °C using isopropanol (IPA) as the solvent and hydrogen donor, and, in contrast, the counterpart without carbon coating (Mg3–Al1) gave 50.3% conversion with 88.2% selectivity. Characterization by means of X-ray diffraction (XRD), CO2-TPD, NH3-TPD, and transmission electron microscopy (TEM) revealed that the carbon layer on Mg–Al mixed metal oxide was able to markedly reduce the undesired sites of strong acid and strong base. It was found that the improved selectivity to COL was related to the reduction of the strong basic sites on the surface, which promoted the side reaction of aldol condensation of cinnamaldehyde. The adsorption of IPA, CAL, and COL, as well as the measurements of contact angle, revealed that the high performance of Mg3–Al1@C was closely related to the enhanced desorption of COL and improved adsorption of both IPA and CAL due to the presence of the external carbon layers. In addition, the presence of carbon layers helps to prevent the unfavorable deposition of bulky molecules produced in the course of the reaction. After seven consecutive runs, the conversion and selectivity of Mg3–Al1@C hardly changed, whereas those of Mg3–Al1 declined from 43.3% to 3.9%. In addition, Mg3–Al1@C exhibited high performance in the transfer hydrogenation of other unsaturated aldehydes, including furfural, geranialdehyde, benzaldehyde, and syringaldehyde.