Synthesis of 3-Methylindole from Glycerol Cyclization with Aniline over CuCr/Al2O3Catalysts Modified by Alkali Earth Oxides

Abstract

In the present work, CuCr catalysts supported on γ-Al2O3 are prepared and modified with alkali earth elements by impregnation, characterized by N2 adsorption–desorption, XRD, H2-TPR (temperature-programmed reduction by H2), CO2-TPD and NH3-TPD (temperature-programmed desorption of NH3 or CO2), and applied in the synthesis of 3-methylindole (3-MI) with a N-heterocycle from glycerol and aniline in the fixed-bed reactor. The results show that the introduction of alkali earth elements into the CuCr/Al2O3 catalyst can improve the yield of target 3-MI in the order of Mg < Ca < Sr < Ba. CuCr-Ba/Al2O3 gives rise to a high 3-MI yield of 39.09% and 65.17% in N2 as a carrier gas and 20%H2–N2 mixture gas, respectively. According to catalysts characterization and catalytic tests, the reaction pathway of glycerol cyclization with aniline is proposed, the formation of 3-MI and 3H-indol-3-yl methanol is hypothesized to be through the aniline cyclization with 2,3-hydroxypropanal from glycerol dehydrogenation over Cu0 centers and basic sites. The acidic sites mainly play a role on activating aniline, which interacts with glycerol to form 3-MI or quinoline through cyclization and dehydration.