The Synthesis Of Hierarchical Pore GaZSM-5 Molecular Sieve And Its Catalytic Performance Of Methanol Conversion Reaction

With the depletion of petroleum resources and the increase of demand,the conversion of methanol to hydrocarbons?MTH?as a low pollution/low cost non petroleum route has attracted the attention of researchers.High efficient methanol to hydrocarbon catalyst is one of the important factors affecting the technical development of coal chemical industry.Compared with ZSM-5 molecular sieve,the hierarchical GaZSM-5 molecular sieve has more excellent catalytic life and product selectivity in the conversion of methanol to hydrocarbons.In this paper,in-situ synthesis method and post-treatment method are used to prepare hierarchical GaZSM-5 molecular sieves.XRD,SEM,TEM,XRF,NMR,N₂adsorption-desorption and other test methods are used to characterize structure.NH₃-TPD,Py-IR and hydroxyl infrared were used to analyze acid characterization.The influence of the structure and acidity of hierarchical GaZSM-5 molecular sieves on methanol conversion performance was studied.Two series of GaZSM-5 and Al ZSM-5 molecular sieves,with different Si/Me ratios,were synthesized in situ by hydrothermal method.The influence of Si/Me ratio on the acidity and pore structure of zeolite was investigated.With the decrease of Si/Me?Me is gallium or aluminum?,the amount of Br?nsted acid increased.Compared with Al ZSM-5,GaZSM-5 has lower acid strength due to the formation of Br?nsted acid by Si-OH-Ga rather than Si-OH-Al.The samples of GaZSM-5 and Al ZSM-5 were used in the conversion of methanol to hydrocarbons.The effects of Si/Ga or Si/Al ratio on the catalyst life and product distribution were investigated.Compared with Al ZSM-5samples with the same Si/Me,the presence of a certain amount of ex-Ga O⁺species in the GaZSM-5 zeolite promoted the dehydrogenation reaction,making the aromatics selectivity of the GaZSM-5 zeolite higher.The molecular sieves of GaZSM-5 and Al ZSM-5 were treated with different concentrations of sodium hydroxide solution,and two series of molecular sieves were used for MTH reaction respectively.The effects of acidity and pore structure of molecular sieves on the product distribution of methanol conversion to hydrocarbons were studied.The mesoporous ratio of the two series of molecular sieves treated by Na OH is obviously increased.The appropriate concentration of alkali treatment can reduce the amount of strong acid in the molecular sieve and inhibit the formation of coke in the MTH reaction.When the alkali concentration is excessive,although the mesoporous rate of the obtained molecular sieve is high,the relative crystallinity decreases and the acid amount increases,which promotes the formation of aromatics products and accelerates the coking rate,leading to the shortening of catalytic life.GaZSM-5 zeolite with hierarchical pore structure was prepared by post-treatment of different concentrations of hydrochloric acid.The influence of hydrochloric acid modification on the acidity and pore structure of GaZSM-5 zeolite was studied,and the influence on the catalytic performance of methanol conversion to hydrocarbons was studied.Hydrochloric acid treatment can increase the content of metal oxide species in the outer framework,increase the volume of mesopores and decrease the number of strong acid centers,which is beneficial to inhibit the formation of carbon deposition and improve the catalytic performance.By adding glucose into the synthesis system of molecular sieves,GaZSM-5 and Al ZSM-5 with multiple pores were synthesized.The effect of glucose content on the structure and acidity of zeolite was investigated.The crystal size of the molecular sieve increased after adding glucose,which indicated that the introduction of glucose inhibited the growth of crystal particles.In addition,the introduction of glucose resulted in the formation of mesoporous structure and the inhibition of metal atoms?gallium or aluminum?entering the molecular sieve framework.When the amount of glucose increased,the Si/Me ratio of the molecular sieve increased,resulting in the decrease of the acidity of the molecular sieve.Glucose assisted synthesis of hierarchical GaZSM-5and Al ZSM-5 samples has better stability in MTH reaction due to their rich mesoporous structure and lower acid strength.The effects of different sucrose content on the structure,acidity and catalytic MTH performance of the two molecular sieves were studied.With the increase of sucrose content,the hierarchical GaZSM-5 and Al ZSM-5 zeolites have lower acid sites,larger BET specific surface area,mesoporous volume and better catalytic activity.The product distribution of MTH reaction catalyzed by zeolite is affected by the concentration of Ga and sucrose.The mesoporous structure introduced by sucrose accelerated the molecular mass transfer rate to a certain extent,and the formation of outer framework gallium species promoted the conversion of low-carbon olefins to aromatics.