Research On Modification And Catalytic Performance Of Zn/ZSM-5 For Methane And Methanol Co-reaction

Methane is a clean energy source with high abundance,and it is an important raw material for the synthesis of high value-added fuels and chemicals.Nevertheless,the high C-H bond energy of 439 kJ/mol brings it a huge challenge to methane utilization.Traditional oxidative coupling reactions can convert methane,but a large amount of CO and CO2 produced greatly reduces the carbon utilization efficiency.Currently in an anaerobic environment,the converting of pure methane requires severe temperature conditions(>600?)under thermodynamic limitations.High temperature leads to large energy consumption and poor catalyst stability.However,when co-fed with alcohols or hydrocarbons,methane can be converted under mild conditions of 400?600?.Among them,methanol is a basic organic chemical raw material with mature production technology.Co-reaction of methanol and methane can realize the mild conversion of methane while alleviating the current situation of methanol overcapacity.Besides it can effectively neutralize the heat of methanol reaction and methane reaction,which has certain advantages in energy and economy.However,few researches about the co-reaction of methane and methanol are published,and the research on catalyst modification and catalytic performance optimization is still blank,which seriously hinders the development and application of the co-reaction.To solve the above problems,the dissertation was carried out on the co-reaction of methane and methanol on Zn/ZSM-5 zeolite catalyst.Focusing on the mechanism of methane activation and pathway of co-reaction,the effects of catalyst pore structure,acidity and supported metals on its catalytic performance were systematically discussed,and the reaction and deactivation laws of co-reaction were further investigated.It provides references and guidance for in-depth understanding of the methane and methanol co-reaction,and the designation of high-performance catalyst and optimization of reaction conditions.The main achievements of this research are as follows:(1)The effects of zeolite framework and acid modification on the catalyst physicochemical properties and catalytic performance were investigated,and the framework and acidity of catalyst were the key factors affecting the co-reaction.The results showed that among the Zn/ZSM-35,Zn/ZSM-5,Zn/ZSM-11,and Zn/Y,Zn/ZSM-5 reached the highest methane conversion and BTX selectivity,which meant it was the most suitable catalyst for the co-reaction.Then,the ZSM-5 was treated with 1 mol/L acetic acid,citric acid,nitric acid and hydrofluoric acid solution.Zn-loaded ZSM-5 with nitric acid treated had the largest proportion of strong and medium strong acid,and showed the highest methane conversion of 13.69%,with the highest BTX selectivity of 45.54%.Acidity was the key factors affecting the catalytic performance of catalyst.(2)Through Si/Al control and steam treatment,series of catalysts with varying acidity were obtained.The relationship between the acidity of ZSM-5 and catalytic performance for methane and methanol co-reaction was studied.Combined with the effect of methanol WHSV on methane conversion,a mechanism was proposed about methanol and methane co-activation on the ZSM-5 catalyst.The results showed that,compared with the pore structure of the catalyst,its acid property was closely related to methane conversion.With the increase of acid amount the methane conversion showed a trend of increasing first and then decreasing.Besides,the greater the acid amount of catalyst was,the higher methanol space velocity corresponding to the maximum methane conversion would be.The results showed that methane conversion was influenced by both methanol and catalyst acid.On ZSM-5,methane was activated by the acid site which adsorbed methanol molecule.(3)The co-reaction activity of Zn-loaded ZSM-5 with different Si/Al ratios was investigated,and the interaction between Zn species and ZSM-5 and its intrinsic relationship with catalyst reaction behavior was revealed.The results showed that there were two Zn species on the catalyst,which were ZnO and Zn(OH)+.Zn(OH)+was formed by strong interaction of Zn and ZSM-5 and was an efficient active center for methane conversion.So the methane conversion increased after Zn was loaded on catalyst.Increasing the acidity of the ZSM-5 can promote the presence of Zn in the form of Zn(OH)+ thereby increasing the methane conversion.The study also found that Zn(OH)+sintered to form ZnO during the reaction,so the reaction stability of Zn/ZSM-5 became worse compared to ZSM-5.(4)On Zn/ZSM-5,the effects of reaction temperature,reactant space velocity,and feed ratio on co-reaction were investigated.In addition,a comparative carbon deposition behavior study of methanol-to-hydrocarbon and co-reaction was conducted to guide the regeneration process of cataly st.The results showed that compared with methanol to hydrocarbons,co-reaction was beneficial to increase BTX production.Taking the maximum methane conversion as evaluation index,the optimal reaction conditions were 450?,reactant space velocity 2 gmethaol/(gcatalyst·h)and methanol/methane molar ratio 1?1.25.The coked catalysts obtained from methanol to hydrocarbon and the co-reaction had the same carbon deposit species outside the pores.However,the latter had a higher content of condensed aromatics in the carbon deposits inside the pores,which was 59.92 wt.%,and it meant more severe regeneration conditions were required.(5)The temperature programmed surface reactions of methane,methanol and methane-methanol were studied by online mass spectrometry.Besides,butene or xylene,as the representative of methane and methanol co-reaction products,its co-reaction with methane was investigated.And then,the pathway of methane and methanol co-reaction was proposed.The results showed that methane would not convert when it flowed alone.In co-reaction,when the aromatic substance was produced,methane began to convert,and system still contained a small amount of methanol.The products of co-reaction can promote the activation of methane,and the activation of methane by aromatics was stronger.On Zn/ZSM-5,the co-reaction of methane and methanol was divided into three stages:1.methanol dehydration to dimethyl ether and the equilibrium conversion of the two substances;2.low-carbon olefin generation stage;3.methane conversion and the converting of few methanol and the generation stage of high-carbon hydrocarbons.