GaN/Qx Catalyzed CO₂ Oxidative Dehydrogenation Of Propane To Propylene

Currently,the industrial production of propene via the conventional oil routes presents a downward trend in China,but the demand for propene steadily increases,which causes a significant contradiction between the supply and demand of propene.The high-valueadded propene prepared by CO2 oxidative dehydrogenation(CO2-ODHP)with rich and cheap propane as raw material would not only alleviate the shortage of propene but also achieve the utilization of CO2,which is expected to be a more ideal approach to produce propene.GaN can effectively activate the C-H bonds in low alkanes,and exhibits higher activity in the dehydrogenation of propane.However,bulk GaN typically shows low activity and poor stability since it has large particle size,small surface areas,high crystallinity and easy adhesion.Increasing the specific surface area and dispersion is significantly important to improve the catalytic performance of GaN.Therefore,preparation of supported gallium-based catalysts with large specific surface areas and suitable pore structures may be the most promising approach to achieve better performance as well as effective utilization of GaN active components.In the present thesis,GaN/Q-x catalysts were prepared by dispersing GaN nanoparticles over porous Q-x(the main composition is SiO2,x=3,15,30,50)in a wet impregnation method.The physicochemical properties of these catalysts were analyzed by means of XRD,nitrogen-sorption and UV-vis DRIFTS,and their catalytic properties were evaluated.Furthermore,the mechanism for CO2-ODHP over GaN/Q-x catalysts was investigated by TPSR and Raman.The main contents and conclusions were presented as follows:(1)Preparation and characterization of catalysts.GaN nanoparticles were prepared by the "one-step solid-phase reaction",and the GaN/Q-x catalysts with different GaN loading amounts were prepared by using various Q-x(SiO2)supports in a wet impregnation method.The catalytic results of catalysts indicated that GaN/Q-x catalysts gave smaller-sized GaN nanoparticles and higher dispersion and effective utilization of active components.This resulted from their larger specific surface areas,more suitable pore structures and more basic sites on the surfaces of catalysts in contrast to that of bulk GaN(the results of CO2-TPD),which caused stronger adsorption capacity for CO2.These properties are all favorable for catalyzing the CO2-ODHP.(2)Catalytic performance evaluation.The dehydrogenation properties of these catalysts were evaluated by using a fixedbed reactor at 1.0 MPa and 600? with the total gas flow rate of 30 mL.min-1.The catalytic performances of propane dehydrogenation over bulk GaN and GaN/Q-x catalysts with various GaN loading amounts and Q-x supports were compared by testing with different molar ratios of reactants.It could be concluded that the activities and selectivity of the GaN/Q-x catalysts in the CO2-ODHP are much higher than those of bulk GaN,in which the 5-GaN/Q-3 catalyst displays the best catalytic performance(propane conversion of 31%with a selectivity of 93%)at 600? with the molar ratio of C3H8/CO2/N2=1/2/7.(3)Reaction mechanism for CO2-ODHP.The reaction mechanism for CO2-ODHP over GaN/Q-x catalysts were studied by means of TPO,TPSR and Raman analyses.The results show that the surface coordinately-unsaturated Ga3+species in GaN/Q-x,acting as the active species,could activate the C-H bonds in propane.The synergetic interaction between GaN and Q supports could promote the dehydrogenation of propane.In addition,the role of CO2 in CO2-ODHP was elucidated.CO2 can reduce the amount of carbon species by removing the amorphous carbon formed at the early stage of the reaction.Furthermore,CO2 can react with H2 formed in the reaction by the reverse water gas shift reaction(RWGS),thus shifting the equilibrium of the dehydrogenation reaction towards higher propane conversion.Finally,a large number of micropores on Q-x supports are beneficial to the enrichment of CO2 molecules which promotes the reaction.