Synthesis Of Hierarchical ZSM-5 Zeolites And Their Catalytic Performance In Methane Dehydroaromatization

Methane dehydraromatization(MDA)is one of the most promising reactions for direct conversion of methane,which converts methane directly to aromatic hydrocarbons and hydrogen.Due to the thermodynamic properties of MDA reaction,there are some problems,such as low methane conversion rate and rapid carbon deposition of catalyst which lead to deactivation.Mo/HZSM-5 is the most used catalyst for MDA reaction,and the structure optimization of catalyst support ZSM-5 has always been the focus.Replacing the conventional microporous ZSM-5 support with a hierarchical one can increase the dispersion of Mo active components in the channel and improve the anti-coking performance of the catalyst,which is an effective method to improve the reaction performance of the catalyst.However,the preparation of hierarchical ZSM-5 zeolite is faced with many problems,such as complex synthesis process(requires mesopore-forming agent or desilication,dealumination),long synthesis cycle(requires several days),and not environmental-friendly(requires roasting to remove organic template agent).To solve the above problems,three corresponding schemes were proposed to synthesize hierarchical ZSM-5,and the MDA reaction performance of the catalysts prepared by them was evaluated respectively.A nano-assembled hierarchical ZSM-5 zeolite with controllable size was directly synthesized by TPAOH without a second template.The primary nanocrystalline size is20?100nm,the secondary stacking particle size is 0.2-2?m,and the mesopore volume is(0.264cm³/g).Methane conversion rate and benzene yield of Mo/HZSM-5 catalyst prepared with nano-assembled ZSM-5 support are about 9%and 5%after 24h MDA reaction(compared with 3%and 1.8%for conventional catalyst).Based on the synthesis experiments above,the ultrafast synthesis of hierarchical zeolite was realized in a stainless-steel tubular reactor.At 250?,the highly crystalline hierarchical ZSM-5 zeolite was obtained in only 20min(the conventional synthesis method requires tens of hours to several days).The zeolite has a high specific surface area(S_BET=416.27m²/g)and a high mesoporous volume(0.222cm³/g).Mo/HZSM-5 catalyst prepared by fast-synthesized ZSM-5 zeolite still has good performance in MDA reaction.After 24h on stream,methane conversion rate is 8%,benzene yield is 4%(compared with 3%and 1.8%for conventional catalyst).Green synthesis of a bundle-like hierarchical ZSM-5 zeolite without template was realized by seed induction.Na F was used to regulate the one-dimensional growth of the zeolite along the c-axis,which significantly shortened the diffusion path of a-axis and b-axis,improved the mass transfer efficiency and optimized the selectivity of aromatic hydrocarbon products.The Mo/HZSM-5 catalyst prepared by bundle-like zeolite has no toluene and xylene selectivity,and a benzene selectivity as high as 60%-70%(the conventional catalyst is 30%-40%).