Preparation Of Ga-Zn Bifunctional Catalyst And Its Application In CO₂ Oxidative Dehydrogenation Of Propane To Propylene

As an important chemical raw material,propylene has a rich downstream industry chain.China is a major consumer of propylene,and the supply and demand imbalance is becoming increasingly prominent.Propane is widely available but is generally used as fuel,resulting in significant resource waste.The CO₂ oxidative dehydrogenation of propane to propylene（CO₂-OPDH）process can effectively reduce the thermodynamic heat absorption requirement,and the weak oxidizing property of CO₂ can avoid deep oxidation of the products,reduce carbon deposition,and achieve the resource utilization of greenhouse gas CO₂,which is a"green chemistry"pathway.However,most of the reported CO₂-OPDH catalysts have low activity,and the promoting effect of CO₂ on the propane dehydrogenation process is not significant.Moreover,they tend to deactivate quickly,and their stability needs to be improved urgently.Therefore,the development of highly efficient catalysts remains the core of this process.In this study,Ga and Zn species were used as active components,and both mesoporous MCM-41 and microporous Silicalite-1 molecular sieves were selected as supports to construct Ga-Zn bifunctional catalysts for the CO₂-OPDH reaction.By using XRD,nitrogen-sorpyion,SEM,TEM,XPS,TG-MS,Raman,and CO₂-TPD characterization techniques,the effect of the catalyst’s phase composition and the support’s pore structure on the propane dehydrogenation reaction was revealed,and the role of CO₂ in the reaction was thoroughly investigated.The main research content and results of this paper are as follows:（1）GaN-ZnO solid solution was prepared using the"solid-state nitridation method"with gallium nitrate and zinc nitrate as the gallium source and zinc source,respectively,and hexamethylenetetramine as the nitrogen source.The effect of calcination temperature on the formation process of the solid solution was investigated in detail.Based on this,GaN-ZnO/MCM-41 was prepared using MCM-41 as the support by the equal volume impregnation method,and its catalytic performance in the CO₂-OPDH reaction was studied.The influence of the solid solution structure in the catalyst on its catalytic performance was investigated in detail under reaction conditions of 600°C and propane space velocity of 1.77h^-1.The results show that（GaN）_0.5（ZnO）_0.5/MCM-41 can achieve the propane conversion rate of 24%and the propylene selectivity of 88%.The formation of the GaN and ZnO solid solution structure is beneficial to the dispersion of the active components,which can provide more active sites,thereby improving the propane dehydrogenation activity.At the same time,the stability of the solid solution structure can effectively reduce the loss of Zn species.In addition,CO₂ molecules can limit the depth of dehydrogenation reaction,reduce carbon deposition,and improve the selectivity and stability of the catalyst.（2）In order to further improve the performance of the catalyst,reduce the difficulty of catalyst preparation,and increase its industrialization potential,Zn Ga O_x was first prepared by co-precipitation method.Subsequently,it was physically mixed with fully silicon microporous molecular sieve Silicalite-1 in a certain proportion for use in catalyzing the CO₂-OPDH reaction,and further investigated the effect of the ratio of Ga and Zn species on the catalyst performance.The results showed that（ZnO）_0.67（Ga₂O₃）_0.33/Silicalite-1 could achieve the propane conversion rate of 50%,propylene selectivity of 94%,and good stability during the reaction.Moreover,the catalytic performance of（ZnO）_0.67（Ga₂O₃）_0.33/Silicalite-1 was superior to that of single-component ZnO/Silicalite-1 and Ga₂O₃/Silicalite-1 catalysts.This may be due to the fact that the Zn Ga O_x structure can generate more active sites,which is beneficial for the adsorption and activation of propane molecules.Meanwhile,the microporous molecular sieve Silicalite-1 can accelerate the desorption of propylene from the active sites of the catalyst,change the reaction equilibrium,and promote the forward dehydrogenation of propane.