Zinc-modified ZSM-5 Molecular Sieve Catalyzed Methanol To Aromatics

Aromatics,especially benzene,toluene and xylene?BTX?,are important basic chemical raw materials and are generally obtained through petroleum routes.However,China's energy status of“multi-coal,lean oil,and low gas”makes aromatics rely heavily on imports.The development of a non-oil route aromatics route is particularly important for easing China's dependence on imported aromatics.Methanol to hydrocarbons?MTH?reaction have received widely attention since their emergence in the 1980s.Methanol to aromatics?MTA?is an effective route to alleviate the pressure of aromatics demand.This study used zinc modified ZSM-5 molecular sieves to catalyze the reaction of methanol to aromatics.Firstly,ZSM-5 molecular sieves with different ratios of silicon to aluminum were synthesized by adjusting the amount of aluminum source added and the pH of the solution.The relationship between ZSM-5 molecular sieves with different acidity was compared.It was found that the aromatics yield reached 19%when the ratio of silicon to aluminum was 30.Then,different amounts of zinc were impregnated on the commercial catalyst,and the relationship between the ZSM-5 molecular sieves with different metal loadings and the reaction performance was compared.It has been found through experiments that the yield of aromatic hydrocarbons reaches 22.5%when the loading of zinc is 1wt%.Next,the effect of the diffusion properties of the catalyst on the MTA reaction was investigated.The first is to synthesize a precursor ZSM-5 molecular sieve having a silica to alumina ratio of 60,and the precursor is subjected to alkali treatment and then modified with zinc and the pore structure and acidity of the catalyst after alkali treatment and zinc modification are characterized.It was found that the alkali treatment can improve the pore structure of the molecular sieve and improve the stability of the catalyst.Secondly,a thin-sheet ZSM-5 zeolite was successfully synthesized with urea addition.The obtained samples were characterized by XRD,SEM,TEM,Ar physical adsorption and NH₃-TPD and evaluatd in MTA reaction.The results showed that the thin-sheet structure and Zn modification facilitated the stability and selectivity to aromatics of the sample,respectively.Compared to traditional ZSM-5 catalyst,the lifetime of the zinc-modified thin-sheet ZSM-5 reaches 120h,and the aromatic selectivity reaches 59%.Finally,a unique method of encapsulating zinc nanoparticles in ZSM-5 zeolite was developed.First,extensive intracrystalline mesoporosity in zeolites was obtained through alkali treatment,after which Zn nanoparticles are introduced to hierarchical ZSM-5 by impregnation.This method make zinc nanoparticles to be distributed inside the zeolites rather than on the outer surface.The outer surface of the zeolite is then covered with SiO₂ by extruding.Finally,the Zn-encapsulated HZSM-5 zeolites is obtained by dry gel conversion.After a series of characterizations,the zinc species is effectively encapsulated in the ZSM-5 zeolite,which exhibits more Lewis acid sites and higher Zn?OH?⁺/ZnO ratios than the Zn-impregnated ZSM-5and shows higher aromatic selectivity and catalyst life in the MTA reaction.