Preparation Of Modified ZSM-5 Zeolite And Its Performance Studies Of Aromatization

The ZSM-5 zeolite is a main component of aromatization catalyst.Its physicochemical properties are closely connected with catalytic performance.Therefore,from the catalyst point of view,the key to improve the efficiency of the reaction is to tune the acidity and pore structure of the current molecular sieve catalyst.On the one hand,acidity had to ensure the full activation and directional conversion of feed gas.On the other hand,channel structure should not only guarantee the raw material has full contact with the active centers of the catalyst,effectively restrain formation of coke precursors and achieve the shape selective catalysis,but also make sure the product moleculars leave the active centers of the catalyst in time by diffusion.This paper utilized different methods to tune the acidity and channel structure of ZSM-5 zeolite.The aromatization catalytic performance of modified ZSM-5 zeolite was investigated in the MTA or MDA reaction,respectively.There is significant guiding for the development a more efficient aromatization catalyst on the industrial.The main research contents and results are as follows:(1)The thermodynamic parameters of the MTA reaction were calculated by equilibrium constant method.The results show that the each reaction in MTA system are not restricted by thermodynamic equilibrium,and its can be regarded as an irreversible reaction with large equilibrium constant.The reaction temperature has a great influence on the distribution of product,so there is an optimal temperature range for the formation of aromatic hydrocarbons.The thermodynamic parameters and equilibrium compositions of the MDA reaction were calculated by minimum Gibbs free energy method(HSC 6.0 thermodynamic analysis software).The results showed that the MDA reaction is an endothermic system.The temperature has a significant influence on the MDA reaction system,in which the balance conversion fo methane is in line with the reaction temperature.The high temperature is beneficial to improve the conversion of methane and the formation of aromatic hydrocarbons,but there are large amounts of coke produced in the high temperature.Therefore,a reasonable temperature range(600-900 ?)is needed to reach a max yield of aromatic hydrocarbons.(2)A facile approach was developed for the preparation of nano-sized HZSM-5 with a hierarchical mesoporous structure by adding imidazole into conventional zeolite synthesis precursor solution.The physicochemical properties of modified HZSM-5 were characterized by X-ray diffraction(XRD),N2 adsorption-desorption isotherms,scanning electron microscopy(SEM)and NH3-temperature-programmed desorption(NH3-TPD).The coke in spent catalysts was characterized by thermogravimetric(TG).The results showed that hierarchical HZSM-5 zeolites with excellent textural properties,such as abundant porous structure,uniform particle size and suitable acidity,could be synthesized by the recipe of one-pot synthesis route.Moreover,the obtained HZSM-5 exhibited higher selectivity of total aromatics as well as longer lifetime in the catalytic conversion of methanol to aromatics,comparing with conventional HZSM-5.It is expected that the synthesis approach demonstrated here will be applicable to other zeolites with particular textural properties and controllable particle sizes,facilitating the emergence of new-type porous materials and their related applications in catalysis and separation.(3)A Mo-promoted Zn/HZSM-5 catalyst was prepared by isometric impregnation method and the physicochemical properties of catalysts were characterized by X-ray diffraction,N2 adsorption-desorption isotherms,transmission electron microscopy(TEM),NH3-temperature-programmed desorption and pyridine adsorption infrared spectroscopy(Py-IR).These analyses showed that the doping of Mo species could tune the microstructures and acidic of catalyst.The l%Mo(IM)-5%Zn(IE)/HZSM-5 catalyst has high catalytic activity and stability for methanol to aromatics reaction.The yield of aromatics reached 77.3%at 450 ? and TOS = 3 h.When the TOS = 98 h,the yield of total aromatics remains at 60.4%.The modification of Mo may bring a opportunities for prolong the lifetime of Zn/HZSM-5 catalyst in the MTA reaction.(4)HZSM-5 zelolites were successfully synthesized within 1 h with organic template free system by the seed-assisted method.This synthetic method not only could reduce the production cost of HZSM-5 zeolite,but also prevent the pollutiorn of environment compared to the conventional synthesis process.Characterization of the zeolite by XRD,SEM,NH3-TPD,Py-IR and N2 adsorption/desorption confirmed that the ZSM-5 crystal structures with a high crystalline and revealed that the rapid synthesis process in accordance with seed surface crystallization mechanism.The catalysts with different Zn and Mo content were prepared by ion-exchange and incipient wetness impregnation to improve the aromatics selectivity of HZSM-5 zeolite.A similar catalytic performance and lifetime were observed for the Mo-Zn/HZSM-5(lh)and the Mo-Zn/HZSM-5(seed)in the methanol to aromatics reaction.However,the 6%Mo/HZSM-5(lh)exhibited a lower catalytic lifetime than that of 6%Mo/HZSM-5(seed)catalyst in methane to benzene reaction under the same reaction condition.(5)The morphology of zeloite and the assembly of zeloite particles have important influence on the catalytic performance.Therefore,the ZSM-5 zeloite with different morphological structure were prepared by different post-treatment.First,the core-shell composite materials with ZSM-5 zelotie as the core and Silicalite-1 zeolite as shell were prepared by silylation.Second,the hollow structure ZSM-5 zeolite microspheres with rich silicon surface were prepared by the weak organic alkali(TPAOH)etching method,which effectively overcomes the disadvantages of hard template or secondary growth method.Finally,according to the zeolite framework Si are dissolved with the order principle dissolution process,the ZSM-5 zeolite microsphere with hierarchical pore structure and rich silicon surface were prepared by the weak organic alkali(n-butylamine)etching method.The catalytic performance of ZSM-5zeolite with different morphological structure were investigated,the experimental results show that the catalyst lifetime could be improved and the distribution of the product has a significant influence by tuning the morphological structure of zeolite in MTA and MDA reaction.