Synthesis Of HZSM-5 Zeolite-based Catalysts And Their Catalytic Performances In Aromatization Of Methanol

Aromatics,especially light aromatics?benzene?B?,toluene?T?and xylene?X?,known as BTX?are important basic organic chemical raw material,its derivatives are widely used for many industries such as synthetic resins,synthetic detergents,synthetic rubber,medicines,pesticides,explosives and specialty chemicals.However,With the increasing shortage of petroleum resources in our country,the production of aromatics by traditional petroleum route has been unable to meet the demand of our country's consumption in recent years.The catalytic conversion of methanol to aromatics?MTA?,which is an important route for the production of high quality aromatic hydrocarbons by non-petroleum route,has very important practical value and considerable market prospects.Methanol molecules are very lively,and the methanol conversion could reach more than 95%at 400-600°C.As a result,the key of methanol to aromatics reaction is to develop high-efficient catalysts with high BTX selectivity and long lifetime.In order to solve this problem,four kinds of HZSM-5 based catalysts were prepared,moreover,the relationship between the properties of catalysts and the catalytic performance of MTA was studied.In addition,the mechanism of MTA reaction was also explored.In particular,the main contents and the results are as follows:1.The“Nanoscale Kirkendall effect”was utilized to synthesize mesoporous ZSM-5 catalysts?D-HZSM-5-4?without the involvement of additional templates.The resultant ZSM-5 catalysts were characterized by XRD,FT-IR,N₂adsorption-desorption,HRTEM,NH₃-TPD,²⁷Al MAS-NMR,SEM and TG technologies,moreover,they were also used for the MTA reaction,the results indicated that:?1?The dry gel conversion method had higher usage efficiency of templates?TPA⁺?compared with hydrothermal synthesis procedure;?2?The formation mechanism of mesopores was closely related to the concentration of TPA⁺?SiO₂/TPA⁺?,“Nanoscale Kirkendall effect”was existed under higher SiO₂/TPA⁺ratio?SiO₂/TPA⁺=13.5?,and thus HZSM-5 catalysts with intracrystalline mesopores could be successfully synthesized.The introduction of mesopores could effectively enhance the coke capacity and decrease the average rate of coke formation;?3?The higher concentration of TPA⁺was beneficial to increase the amount of strong acid sites and micropore volume,the synergistic effect of them greatly increases the selectivity of BTX hydrocarbons in MTA reaction;?4?The methanol conversion of D-HZSM-5-4 catalyst was still more than 95%after 60 h reaction at T=400°C,P=0.1 MPa,WHSV=1 h^-1,and the initial selectivity of BTX was up to 32.75%.In addition,the methanol conversion and BTX selectivity of regenerated D-HZSM-5-4 catalysts?R-D-HZSM-5-4?were slightly decreased compared with fresh D-HZSM-5-4catalysts.2.Nanocrystallite self-assembled hierarchical ZSM-5 zeolite microsphere?NSHZ?waspreparedbybond-blockingmechanismusing3-glycidoxypropyltrimethoxysilane?KH-560?as blocking templates.The resultant ZSM-5 catalysts were characterized by XRD,SEM,HRTEM,N₂adsorption-desorption,ICP,NH₃-TPD,Py-IR,FT-IR,TG and DRIFTS technologies,moreover,they were also used for the MTA reaction,the results indicated that:?1?The addition of KH-560 could effectively inhibit the further growth of nano-ZSM-5 crystals,the construction of nanocrystals could promote the rapid removal of macromolecular hydrocarbons from zeolite micropores,thereby inhibiting the blockage of micropores in ZSM-5 zeolite and improving the catalytic lifetime of catalysts;?2?The catalytic performances and product distributions were closely related to the acid properties of catalysts,large amount of strong acid sites and proper B/L ratio were beneficial to improve the selectivity of BTX;?3?The methanol conversion of NSHZ catalyst was still more than 95%after 100 h reaction at T=400°C,P=0.1 MPa,WHSV=1.2 h^-1,and the initial selectivity of BTX was up to 36.56%.In addition,the methanol conversion and BTX selectivity of NSHZ catalysts still remained after two regeneration test.3.The hierarchical ZSM-5 zeolites were modified by different molarities of the ZnSiF₆·6H₂O solution(0.025 mol L^-1,0.05 mol L^-1,0.1 mol L^-1).The resultant ZSM-5 catalysts were characterized by XRD,In-situ FT-IR,²⁹Si MAS-NMR,SEM,N₂ adsorption-desorption,ICP,NH₃-TPD,Py-IR,UV-Vis,XPS,Potentiometric titration and TG technologies,moreover,they were also used for the MTA reaction,the results indicated that:?1?SiF₆^2-could selectively passivate the acidic sites on the outer surface of zeolites,which slowing down the formation rate of the“carbon membrane”on the outer surface of the catalyst,and thus the catalyst lifetime was obviously increased;?2?The Si species produced by SiF₆^2-hydrolysis could effectively repair the partial lattice defects of the zeolite framework,and the stability of catalyst was further improved;?3?A large number of^?ZO…H…O-Zn^?species?Zn-Lewis acid sites?were produced with the introduction of Zn species,which effectively improved the dehydroaromatization activity of catalyst,and thus the selectivity of BTX substantially increased;?4?The methanol conversion of NZ3 catalyst(modified by 0.05 mol L^-1 ZnSiF₆·6H₂O)was still more than 95%after 174 h reaction at T=400°C,P=0.1 MPa,WHSV=0.8 h^-1,and the initial selectivity of BTX was up to 48.27%.4.Zn/HZSM-5?NZ2?and Zn-Ni/HZSM-5?NZ3?catalysts were prepared by equal volume impregnation followed by ultrasonic.They were characterized by XRD,HRTEM,NH₃-TPD,Py-IR,XPS and XRF technologies,moreover,they were also used for the MTA reaction,the results indicated that:?1?The cracking and hydrogen transfer reactivity of catalyst were inhibited,and the dehydroaromatization activity of catalyst was promoted with the introduction of Zn species,which effectively improved the selectivity of BTX hydrocarbons;?2?Partial ZnO nanocrystals were transformed into^?ZO…H…O-Zn^?species with the introduction of Ni species,which significantly enhance the strong interaction between Zn species and ZSM-5 catalyst.As a result,the loss rate of Zn species was greatly reduced,and the stability of catalyst was significantly improved;?3?The methanol conversion of Zn-Ni/HZSM-5 catalyst was still more than 90%after 65 h reaction at T=400°C,P=0.1 MPa,WHSV=1.2 h^-1,and the initial selectivity of BTX was up to 42.6%.