The gas-phase dehydration of glycerol was examined for the production of acrolein over solid acid catalysts, viz. zeolites, metal oxides and supported heteropoly acids (HPAs). Several techniques: N2-physisorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature-programmed desorption of ammonia, carbon dioxide and water (NH3-TPD, CO2-TPD and H2O-TPD) with mass spectroscopy, temperature-programmed oxidation (TPO) with mass spectroscopy, infrared spectroscopy (FT-IR) after pyridine adsorption and glycerol adsorption, thermogravimetric analysis (TG), differential scanning calorimetry (DSC), CHNS analysis, 29Si and 27Al MAS NMR and Raman analysis were employed to characterize the catalysts.
The metal oxides viz. g-Al2O3, SiO2-Al2O3, TiO2, ZrO2, SiO2, AC, CeO2 and MgO on the catalytic performance is intensively examined. SiO2-Al2O3 showed the highest glycerol dehydration activity at 315 ℃ among the tested metal oxides because of its the larger amount of strong acid sites [52]. CeO2 showed the highest 1-hydroxyacetone selectivity at 315 ℃ among the various metal oxides [52].
The silica-aluminas with the variation of Al content were examined over the dehydration of glycerol. For comparison, SiO2 and h-Al2O3 were also examined. Si0.4Al0.6Ox showed the highest glycerol conversion, whereas Si0.8Al0.2Ox showed the highest acrolein selectivity at 315 ℃ among the tested catalysts.
The H(Na)-zeolites viz. H(Na)-ZSM-5, H-b, H-ferrierite, H-Y and H-mordenite were carried out over the dehydration of glycerol. For comparison, g-Al2O3 and SiO2-Al2O3 were also examined. H-Zeolites such as H-ZSM-5, H-b and H-ferrierite with various SiO2/Al2O3 ratios were also examined over the dehydration of glycerol. It is implies that the catalytic performance can be closely influenced by the SiO2/Al2O3 ratio, which changed the physicochemical properties for H-zeolites [39, 40]. Among all of the tested H-zeolites, H-ZSM-5 with SiO2/Al2O3 ratio of 150 showed the highest glycerol conversion and acrolein selectivity at 315 ℃ [39].
The metal oxide supported HSiW catalysts were carried out over the dehydration of glycerol. HSiW/ZrO2 and HSiW/SiO2-Al2O3 showed the highest acrolein selectivity at 315 ℃ among the tested HSiW catalyst [52].
The silica-alumina supported HSiW catalysts were carried out over the dehydration of glycerol. HSiW/Si0.8Al0.2Ox showed the highest glycerol dehydration activity at 315 ℃ among the tested HSiW catalysts.