Studies On Direct Conversion Of Syngas To Aromatics Or C₂₊ Oxygenated Compounds Via Relay Catalysis

Syngas is the key hub for converting the non-petroleum carbon resources,such as coal,nature gas,shale gas and biomass,to liquid fuels and basic chemical.Highly selective conversion of syngas to target products is the core of C1 chemistry.However,by the classical Fischer-Tropsch synthesis(FTS)route,the product selectivity is limited by the Anderson-Schulz-Flory(ASF)distribution,which results in the low selectivity of target products.In this thesis,bifunctional or multifunctional catalysts are designed for directly transforming syngas into aromatics and C2+oxygenate compunds via relay catalysis.The key factors for influencing the catalytic performance and reaction mechanism have been studied in detail.The Zn-ZrO2/H-ZSM-5 bifunctional catalyst was employed for direct conversion of syngas to aromatics.It was found that the Zn/Zr molar ratio,density of Br?nsted acid site of H-ZSM-5 and the proximity between two functional components have profound effect on the catalytic performance.Zn-ZrO2/H-ZSM-5 catalyst provided a 20%CO conversion and 80%aromatics selectivity through the optimization of composition of catalyst and reaction condition.Moreover,this catalyst showed excellent stability after the reaction of 1000 h.The hydrogenation ability of ZrO2-based oxide was regulated by doping different metal elements.Studies display that introducing a small amount of Mo can significantly improve the space-time yield(STY)of aromatics.Under a high space velocity,the STY of aromatics can reach 0.17 g gcat-1 h-1 with 76%aromatics selectivity over Mo-ZrO2/H-ZSM-5 catalyst.The detailed reaction mechanism on direct conversion of syngas into aromatics has been studied.It is indicated that the CO,adsorbed on ZnOZrO2 oxide acting as a "hydrogen accepter",reacts with the H species formed from the dehydrogenative aromatization to enhance the aromatics selectivity.Based on these findings.a CO self-promotion mechanism is proposed,which has been verified by 13CO isotope labeling experiment.By conducting the WGS reaction on the metal oxide-zeolite bifunctional catalyst,it is demonstrated that the CO2 is generated from the WGS reaction.In addition,the formation CO2 can be effectively impeded by co-feeding a definite amount of CO2.In the direct conversion of syngas to methyl acetate,the low-temperature and hightemperature multifunctional catalysts have been developed.It was confirmed that the DME is the key intermediate for selective production of methyl acetate from syngas.For the Cu-Zn-Al/H-ZSM-5 | H-MOR catalyst conducted at low temperature,the formed water can be elegantly removed through WGS reaction,which ensures the efficiency of successive carbonylation of DME.In addition,selective removal the Br?nsted acid site of 12-MR of H-MOR can markedly improve the stability of catalyst.Catalytic conversion of syngas to different products,including methyl acetate,ethanol and ethylene,can proceed on the high-temperature ZnAl2O4 | H-MOR catalyst by various coupling modes.Furthermore,the syngas can be directly transformed into ethanol via methyl acetate or acetic acid by introducing the hydrogenation functional component.By optimization of the composition of catalyst and reaction condition,the selectivity of methyl acetate,acetic acid and ethanol via relay catalysis can reach to 85%,81%and 59%,respectively,which breaks the limitation of ASF distribution.