The catalytic reforming of bio-ethanol over SiO2 supported ZnO catalysts: The role of ZnO loading and the steam reforming of acetaldehyde
In this study, the activity and the product distributions of sol-gel made SiO2 supported ZnO catalysts in the steam reforming of ethanol and acetaldehyde is presented as a function of ZnO loading and temperature. We show that although highly dispersed ZnO in SiO2 (upto 50% ZnO loading) can be prepared using a single step sol-gel method, a precise control of crystallite size could not be achieved. From CO2 TPD measurements, we found that the basic site densities of ZnO/SiO2 catalysts stays < 0.05 μ mol / m2 and do not increase linearly with ZnO loading. The highest basic site density among the catalysts occurs on pure ZnO. All ZnO/SiO2 catalysts are active at 350 °C whereas pure ZnO catalyst is active at 450 °C. Iso-conversion activity tests show that ethanol steam reforming activities of the catalysts seem to be dependent on the ZnO crystallite size rather than the basic site density of the catalysts when the surface coverage of the basic site density is < 0.32 % but acetone is not formed only on catalysts with ZnO crystallite size < 5 nm regardless of their basic site densities. Interestingly, we found that ethanol was mostly dehydrogenated to acetaldehyde and hydrogen although H2O/C2H5OH molar ratio in the feed was 12. CO was not also produced in the steam reforming of ethanol over all the catalysts. Acetone and propene are produced from acetaldehyde as observed in the steam reforming of acetaldehyde. The steam reforming of acetaldehyde as compared to its decomposition was found to be more favorable over the catalysts with small ZnO crystals, such as 30% and 50% ZnO catalysts.