Synthesis Of Hierarchical Mordenite And Its Catalytic Performance In The Carbonylation Of Dimethyl Ether

As the crucial step of preparing ethanol from syngas,the carbonylation of dimethyl ether（DME）to synthesize methyl acetate（MA）has aroused the research interest of scientists.However,due to the diffusion limitation of micropores,the catalyst for this reaction,mordenite（MOR）,is prone to coke deposition and deactivation during carbonylation of DME.Introducing mesopores is an effictive solution to enhance the catalytic performance of zeolites.However,the synthesis of hierarchical MOR still has some problems,such as complex synthesis methods and long synthesis cycle.Besides,the synthesized zeolites are mostly in the form of powder and they must be processed to certain shape using binders in industry applications.But the introduction of binders will affect the acidity or block part of the pores of zeolites.The present work mainly focuses on these two problems.Hierarchical MOR was synthesized by hydrothermal method and binderless method respectively,and the catalytic performance of the synthesized samples in the carbonylation of DME was studied.（1）Firstly,the structural growth inducer（SGI）was prepared by using tetraethylammonium hydroxide（TEAOH）as template.Subsequently,hierarchical MOR was obtained with the assistance of a small amount of SGI and cetyltrimethylammonium bromide（CTAB）.This strategy successfully reduces the crystallization temperature to 140°C,shortens the crystallization time to 48 h,and greatly reduces the amount of TEAOH.Through changing the amount of CTAB and SGI,the morphology,acidity and porosity of the synthesized samples can be adjusted.The formation mechanism of hierarchical MOR was studied.It was found that SGI is helpful to promote the nucleation of zeolite,while CTAB,as a structure growth inhibitor and surfactant,can inhibit the growth of BEA structure and adjust the morphology of MOR.The synthesized samples and commercial MOR were used for the carbonylation of DME.It was found that after 24 h on stream,the conversion of DME on the hierarchical MOR still maintained 45%-50%of its maximum value,while the conversion on the commercial MOR was only 9.4%of its maximum value,suggesting that the catalytic stability of the hierarchical MOR is much higher than that of commercial MOR.Combined with the characterization results,the reason for the improved stability of the synthesized samples can be attributed to the abundant mesopores.（2）With MOR as seed,the silicon source,aluminum source,alkali source and seed are shaped,and then liquid-solid transformation was carried out in an aqueous solution containing template to prepare hierarchical binderless MOR.The effect of Si O₂/Al₂O₃ ratio on the crystallinity,morphology and acidity of the samples was studied by changing the amount of aluminum source added in the synthesis.Finally,a series of synthesized samples were used in the carbonylation of DME.It was found that when Si O₂/Al₂O₃ ratio was 25,the obtained sample showed the best catalytic activity in the carbonylation of DME and the highest DME conversion on this sample can reach 44.7%.Combined with the characterization results,it can be attribute to the highest crystallinity,the largest mesoporous volume and the largest number of strong acid sites.