Efficient one-pot synthesis of 3-methylindole from biomass-derived glycerol with aniline over Cu/SiO2 modified with ZnO and Fe2O3 and deep insight into the mechanism

An efficient catalyst of Cu/SiO2 modified with ZnO and Fe2O3 was constructed for the one-pot synthesis of 3-methylindole originating from biomass-derived glycerol with aniline. The structure and property of as-prepared Cu/SiO2–ZnO–Fe2O3 composite were characterized by temperature programmed reduction of H2 (H2-TPR), X-ray diffraction (XRD), transmission electron microscope (TEM), temperature programmed desorption (TPD) of NH3 and CO2, inductively coupled plasma (ICP) emission spectroscopy, thermogravimetric and differential thermal analysis (TG–DTA). The results indicated that the interaction between copper and support was improved after adding ZnO to Cu/SiO2, as a result, Cu particles could be firmly anchored on the surface of SiO2–ZnO, which not only increased the dispersion of active component but also inhibited the aggregation or sintering of copper particles effectively. Meanwhile, the acid or base sites on the ZnO modified Cu/SiO2 catalyst significantly increased, which was beneficial to the hydrogenolysis of glycerol to acetol and 1,2-propanediol and promoted the synthesis of 3-methylindole. The introduction of Fe2O3 to Cu/SiO2–ZnO could promote the reduction of CuO and decrease the acidity of the catalyst, thereby the activity and stability of the catalyst were further improved. Under the optimized ZnO or Fe2O3 content of 0.100 or 0.030 mmol/g-SiO2, Cu/SiO2–ZnO–Fe2O3 exhibited excellent catalytic activity and long-term stability, which the yield of 3-methylindole was up to 73% and it was still more than 68% when the catalyst was reused four times. Moreover, the catalytic mechanism for the one-pot synthesis of 3-methylindole from glycerol and aniline over Cu/SiO2–ZnO–Fe2O3 was investigated in depth and a probable synthetic route was proposed based on the research of the catalytic reactions of glycerol, 1,2-propanediol or acetol with aniline as well as the hydrogenolysis of glycerol and the catalytic conversion of acetol, which both acetol and 1,2-propanediol were the intermediates for the production of 3-methylindole.