Synergistic Effect of Mo–Fe Bimetal Oxides Promoting Catalytic Conversion of Glycerol to Allyl Alcohol

Abstract

In this study, KIT-6 silica with well-ordered 3-D mesoporosity was developed as support to prepare Fe/KIT-6, Mo/KIT-6, and MoFe-x/KIT-6 (x = 0.25, 0.3, and 0.35) oxide catalysts for catalytic conversion of gas-glycerol into allyl alcohol. The catalysts were also characterized by XRD, BET, XPS, H2-TPR, and NH3-TPD. The catalytic conversion of glycerol showed a positive correlation with the surface moderate acid density of catalysts, following the order of Fe/KIT-6 < MoFe0.25/KIT-6 < MoFe0.35/KIT-6 < MoFe0.3/KIT-6 < Mo/KIT-6. Differently, the production of allyl alcohol was closely related with the moderate redox sites following a hydrogen transfer mechanism. The MoFe-x/KIT-6 showed much higher selectivity than the Fe/KIT-6 and Mo/KIT-6, which resulted from the strong synergistic effect between Fe2O3 and MoO3 altering the surface moderate acid strength, surface acid amounts, and reducibility of catalysts. The MoFe-0.3/KIT-6 exhibited a remarkable yield of 26.8% of allyl alcohol at 94.0% conversion of glycerol without external hydrogen donors supplied to the system, which benefits from the good balance between moderate acidity and weak reducibility of catalysts. The developed cubic Ia3d meso-structure was also benefit for improving the catalytic stability of MoFe0.3/KIT-6. Allyl alcohol can produce from gas–solid catalytic conversion of glycerol over the weekly acidic Fe2O3 and MoO3 supported on SiO2. The yield of allyl alcohol can be significantly improved over the MoO3–Fe2O3/SiO2 composite oxide catalysts, because of the strong interaction between MoO3 and Fe2O3. The catalytic conversion of glycerol was positively related with the surface weak acid site density of catalysts, while the allyl alcohol seems to form over the redox sites. Comparing with the single component Fe2O3/SiO2and MoO3/SiO2 catalysts, the strong synergistic effect of MoO3 with Fe2O3 guarantee the MoO3–Fe2O3/SiO2 having relatively high surface week acid site density and certain reducibility, which showed a good balance between weak acidity and reducibility thus obviously increasing the allyl alcohol yield from 26.8 to 94% catalytic conversion of glycerol through gas–solid catalytic reaction without any additives.