Transformation Of Bioethanol Into C₂-C₄ Light Olefins Over Nanoscale HZSM-5 Molecular Sieve Catalysts

Due to the energy crisis and the effect of green-house gas, more and more researchers have paid much attention to the utilization of biomass which is well accepted as an alternative to the fossil resource. In the dehydration of bioethanol (from the fermentation of biomass) into ethylene, catalysts with high activity and stability below 300℃are desireable, and in the transformation of bioethanol into bulk chemicals (light olefins), the stability of catalyst and the distribution of products are important. In this study, nanoscale HZSM-5 zeolite catalysts and Ce-modified nanoscale HZSM-5 zeolite catalysts were prepared, and their catalytic performance in the dehydration of bioethanol into ethyene (240-290℃) and the transformation of bioethanol (20v%) into C2-C4 light olefins (400℃) were investigated in a fixed-bed reactor under the atmospheric pressure.In the dehydration of bioethanol into ethylene catalysed by nanoscale HZSM-5 zeolite catalysts at temperatures ranging 240-290℃, it was found that for the higher concentration of ethanol feedstock (95v%), carrier-gas was favourable for the selectivity of ethene and the stability of catalyst, while for the lower concentration of ethanol feedstock (37v%), cerium modification with 3wt% CeO2 loading was propitious to the stability of catalyst. Under the same reaction conditions, catalytic stability of the nanoscale HZSM-5 zeolite catalyst is better than that of the microscale HZSM-5 zeolite catalyst. Combined with various characterizations, the possible reasons for the pronounced differences on the catalytic stability between nanoscale and microscale HZSM-5 zeolite catalysts were discussed.In the transformation of bioethanol (20v%) into C2-C4 light olefins at 400℃, it was illustrated that cerium modifications not only modulated the acidic properties of catalysts, but also regulated the distribution of products (the selectivity of C3+C4 olefins); the composition of cerium species changed with the cerium content and the calcination temperature; for the optimal catalytic performance (the best stability and selectivity of C3+C4 olefins), there existed a best cerium content (5wt%) and a best calcination temperature (520℃). The results of in-situ H2 reducion proved that the increase of the ratio of Ce3+/Ce4+ was favorable for the conversion of ethanol, the selectivity of propene, and the ratio of propene to propane. Combined with the results of kinetic runs and various characterizations, the possible reasons were discussed from the changes of Bronst acid sites caused by the cerium modification. Moreover, from the functions of metal cation, a new viewpoint was also presented, which the metathesis of ethene and cis-2-butyene could be promoted to form the additional C3 olefin over the active sites (L acid sites) due to the introduction of cerium, regulating the distribution of C2-C4 light olefins further.