Study Of CO And CO₂ Hydrogenation To Lower Olefins Over A Bifunctional Catalyst Via Methanol Intermediate

Based on the concept of bifunctional catalysis,this work was aiming at highly selective production of lower olefins from CO/H2 and CO2/H2.The role factors of catalytic performance,active sites of catalysts,kinetics studies and reaction mechanism were systematically studied.Application of Zn-ZrO2/SSZ-13 bifunctional catalyst on CO hydrogenation was studied in this work.By means of regulating and controlling the Zn/Zr molar ratio,acidity of SSZ-13,contact distance of Zn-ZrO2 and SSZ-13,the selectivity of lo2wer olefins reaches as high as 78%at 29%CO conversion.The kinetic studies strongly indicate that methanol/DME are the key reaction intermediates on our bifunctional catalyst and the formation of methanol/DME is the rate-determining step.Langmuir-adsorption model was occurred on CO/H2 adsorption with H2 dissociatively adsorbed on surface sites.The in situ DRIFT spectrum have demonstrated methanol was formed through hydrogenation of formate species to methoxide species.This work also suggest that CO2,a major by-product,is formed by the water-gas shift reaction.The applied of bifunctional catalysts composed of spinel oxides and SAPO-34 on CO2 hydrogenation was studied in this work.It is found that various performance were displayed with different catalysts,ZnAl2O4/SAPO-34 and ZnGa2O4/SAPO-34 showed a outstanding performance with a selectivity of lower olefins as high as 87%and 86%at 15%and 13%CO2 conversion,the CO selectivity,a major by-product,was inhibited at 49%and 46%.This work employed EPR spectroscopy to investigate the oxygen vacancy,H2-D2 exchange reaction was performed to inspect the ability of H2 activation,Correlation of the formation rate of target products with oxygen vacancy,We speculated that CO2 activates on the oxygen vacancy of oxides,while the-Zn-O-domains or-Ga-O-domains are responsible for the activation of H2 possibly through heterolytic dissociation.Kinetic studies showed that CH3OH/DME are the key reaction intermediates.In situ DRIFT spectrum have demonstrated that CO2 activates on the oxygen vacancy to form CO2*,methanol was formed through hydrogenation of CO2*and formate species successively to methoxide species.Aiming to study the similarities and differences of CO hydrogenation and CO2 hydrogenation,Zn-Al-O/SAPO-34 bifunctional catalyst was performed in this work.ZnAl2O4/SAPO-34 showed a high lower olefins selectivity in CO/H2 and CO2/H2 reactions.The WGS reactions on Zn-Al-O indicate that CO2,a major by-product in CO/H2 system,is formed by the water gas shift reaction.CO,a major by-product in CO2/H2 system,is formed by the reversible water gas shift reaction.The CO2 selectivity and CO selectivity are inhibited at 0%by adding corresponding gas in the feed gas.EPR spectroscopy results and CO&CO2 adsorption experiments indicate CO and CO2 activate on the oxygen vacancy of Zn-Al-O catalysts.H2-D2 exchange reactions showed that-Zn-O-domains are responsible for the activation of H2 possibly through heterolytic dissociation.Kinetic studies and in situ DRIFT spectra indicate that CH3OH/DME are the key reaction intermediates in CO/H2 and CO2/H2 systems.