Mechanistic Study Of The Tandem Reaction Of CO₂ And Ethane Aromatization Over Zn/P-ZSM-5 Catalyst

With the vigorous development of shale gas technology,ethane production has increased and prices are reduced.Converting ethane into value-added aromatic hydrocarbons has high economic value.Due to its unique pore channel selectivity and acid properties,the ZSM-5zeolite has been widely used in catalyzing the aromatization of ethane.The tandem reaction of CO₂ and ethane aromatization is a combination of oxidation and reduction reaction which provides a new and more environmental friendly process to generate high value chemicals.In this thesis,density functional theory（DFT）calculations are used to study the mechanism of tandem reaction of CO₂ reduction and ethane aromatization over Zn/P-ZSM-5 from four aspects,as detailed below.1.By means of DFT calculations,thermodynamically stable catalyst models of HZSM-5,Zn/ZSM-5 containing（Zn-O-Zn）²⁺and Zn²⁺species,and Zn/P-ZSM-5 containing isolated zinc phosphate groups were constructed with good accessible active sites.The Lewis acid properties of Zn/ZSM-5 and Zn/P-ZSM-5 are proposed to be different in these models.2.DFT resuls revealed that the rate-limiting steps of ethane dehydrogenation and ethene aromatization both are the dehydrogenation of zinc-alkyl species over（Zn-O-Zn）²⁺active site of Zn/ZSM-5.The energy barriers of the rate-limitin steps for the two reactions are close,but the effective barrier of benzene formation from ethene aromatization is higher.The Zn²⁺and（Zn-O-Zn）²⁺active sites have similar catalytic performance and reaction route toward ethene aromatization,but the activity of（Zn-O-Zn）²⁺active site toward ethane dehydrogenation to ethene is better.Zn replaces the Br?nsted acid site of HZSM-5 and creates new Lewis acid site,which changes the reaction pathway and makes the aromatization reaction more facile to occur.The introduction of Zn can significantly suppress the formation of methane and improve the selectivity to aromatics.DFT calculations indicated that the coke formation is also a main side reaction during the aromatization of ethane over Zn/ZSM-5.3.The reaction mechanism of ethane dehydrogenation and aromatization over Zn/P-ZSM-5 catalyst was inveatigated and compared with that over Zn/ZSM-5.The results indicated that the catalytic performance of Zn/ZSM-5 is better than Zn/P-ZSM-5 toward ethane dehydrogenation to ethene,while the performance of subsequent ethene aromatization is similar over the two catalysts.The extra-framework cations formed by the introduction of P can occupy a certain space in the zeolite pore channel,making the active sites more difficult to access for ethane reacant,thus resulting in a decrease of the catalytic ability of Zn/P-ZSM-5 for ethane conversion.Comparing Zn/ZSM-5 for methane formation from ethane and ethene,due to the synergistic effect between P and Zn in the P-O-Zn active species,the generation of methane by-product over Zn/P-ZSM-5 is largely suppressed,thereby positively affecting the main reaction of ethane aromatization.4.H₂O produced from the reverse water gas shift（RWGS）reaction of CO₂ can hinder the conversion of ethane through differnet manners such as occupying adsorption active sites,hydrolyzing the（Zn-O-Zn）²⁺active sites,forming H-bonds,and participating the reaction via H-transfer,which are detrimental to the main reaction of ethane conversion.Due to the influence of P,the Zn/P-ZSM-5 catalyst ensures the stability of the active sites of the catalyst.When CO₂is cofeeded with ethane reactant,the competive adsorption of CO₂ molecule leads a reduction of available sites for catalyzing ethane and negatively impacts the conversion of ethane.However,CO₂ can promote the formation of P-O-Zn active species of Zn/P-ZSM-5 and decrease the rate-limiting barrier of ethane dehydrogenation so that add kinetic advantages to the conversion of ethane and promote the reaction toward ethane aromatization over Zn/P-ZSM-5.Therefore,it is necessary to balance the competitive adsorption of CO₂ with the advantage of its improving the kinetics of ethane conversion.CO₂ can consume coke and hydrogen through Boudouard and RWGS reactions,playing a positive role in prolonging the life of catalyst and shifting the reaction toward forming aromatic products.Based on DFT calculations,it is predicted that the tandem reaction of CO₂ and ethane aromatization is likely achieved over zinc-based bimetal modified ZSM-5 catalysts such as Zn-Pt/ZSM-5 and Zn-Fe/ZSM-5.