The Template Synthesis And Catalytic Application Of Hierarchical ZSM-5 Zeolite

Hierarchical zeolites are a kind of artificial zeolite with high hydrothermal stability,good shape selectivity and abundant acid active sites,which overcome the diffusion and mass transfer resistance of micropores.It is widely used in petrochemical,environmental protection and pharmaceutical intermediate production.In recent years,there are many reports on the preparation of hierarchical zeolites.The soft template method which has attracted much attention on hierarchical zeolite synthesis,utilizes functional quaternary ammonium salt templates to simultaneously form micropores and mesopores,and tune their performance according to changing the number of ammonium bases,the length of alkyl chains and so on.However,the preparation of hierarchical zeolites by soft template method generally has problems such as long synthesis time,high cost and difficulty in industrialization.In previous studies,hierarchical ZSM-5 zeolite with three-dimensional interactive MFI topology can be prepared with most organic structure-directing agents?OSDAs?.Even if not OSDAs,the inorganic SDA sodium ions also can guide hierarchical ZSM-5 zeolite synthesis.Therefore,the synthesis strategies of hierarchical ZSM-5 zeolite can universally promote other hierarchical zeolite synthesis.In this paper,we chose the soft template method to prepare hierarchical ZSM-5 zeolite through adding assisted templates,simplifying bifunctional quaternary ammonium salt templates to shorten synthetic process,reduce synthesis cost and tune performance.What?s more,we also used small molecule template to prepare hierarchical ZSM-5 zeolite to shorten synthesis time in alkaline sodium ions environment,details as follows:In this paper,the single quaternary ammonium surfactant C_ph-ph-10-6h-ph-10-6 as main template induced preparing a series of hierarchical ZSM-5 zeolite with different structure by adding and tuning assisted template TPAOH.The properties were characterized by XRD,SEM,TEM,N₂ adsorption and desorption method to study the effects of TPAOH on crystal morphology,pore structure,acid distributions and catalytic performance of samples.It suggested that the structure-directing effect between C_ph-ph-10-6 and TPA⁺was affected by the content of assisted template TPAOH,agreed withthe DFT calculation results.As the molar ratio of C_ph-ph-10-6/TPAOH changed from 5/0 to 5/8,the morphology of samples gradually converted from ultra-thin naosheets to three-dimensional blocks,the mesoporous volume gradually reduced,the BET surface area and the acidity were all first increased and then decreased.In the alkylation of benzyl alcohol with mesitylene,the highest conversion of benzyl alcohol was53.6%,it was extra 40%compared with CZSM-5 zeolite due to S_ext/S_BET and Br?nsted/Lewis shortened the diffusion resistance and provided many acid active sites,respectively.The work showed that the dual template method can adjust the morphology,textural properties(such as S_ext/S_BET)and acidity?Br?nsted/Lewis?of hierarchical zeolites by regulating the content of TPAOH,easily and effectively.In this paper,Gemini-type template C_ph-10-6-6 and Bola-type template BC_ph-10-6-6h-10-6-6 were used as prototypes to simplify template synthesis process,obtaining C_ph,C_ph-10,C_ph-10-6,Cph-10-6-6,BCph,BCph-10-6,BCph-10-6-6 to synthesize hierarchical ZSM-5 zeolite.A series of characterization techniques,such as XRD,SEM,TEM,was used to explore the performance of samples.It found that the morphology changed from three-dimensional blocks to two-dimensional nanosheets structure with OSDAs length increasing.C_ph,C_ph-10 and BC_phh without quaternary ammonium group N⁺could only fabricate microporous structure,while C_ph-10-6 and BC_ph-10-6 could induce hierarchical structure fabrication even without alkyl tail chains.The DFT simulation results also showed that the effective group N⁺is pivotal to fabricate micro-and mesoporous structure,and C_ph-10-6 and BC_ph-10-6 had the greatest interaction with inorganic precursors.In my studies,the intrinsic inducing mechanism of quaternary ammonium salt template to form micro-and mesoporous structure by tailoring templates was explored.It was unnecessary to synthesize complex functional quaternary ammonium salt templates,just synthesize simple quaternary ammonium salt with structure-directing groups.Simplifying the preparation process of templates and reducing the cost of hierarchical zeolites.The hierarchical M-C_ph-10-6 and M-BC_ph-10-6h-10-6 samples had the best catalytic performance in the alkylation of benzyl alcohol and mesitylene,of which the methanol conversion rates reached 78.9%and 67.6%,and the selectivity of alkylation product were close to 100%and 67.7%.In this paper,hierarchical ZSM-5 zeolite was synthesized by inexpensive small molecule template TPAOH under NaOH alkaline environment,of which the performance was investigated by the influence of synthesis time,silicon-aluminum ratio and alkali concentration through XRD,SEM,TEM,and so on.The hierarchical ZSM-5 samples were successfully synthesized in 1 d under the premise of TPAOH inducing hierarchical structure growth with NaOH.However,the adjustable ranges were all narrow.The best synthesis condition was 1 d,silicon-aluminum ratio of 50,and alkali concentration ratio of 1.In the alkylation of benzyl alcohol and mesitylene,the hierarchical ZSM-5 samples synthesized in optimal condition catalyzed the conversion of benzyl alcohol to 52.6%and the selectivity of alkylation product close to 100%.The hierarchical ZSM-5 zeolite was successfully synthesized by small molecule template TPAOH,which effectively saved original material cost,reduced time cost,and made samples apply for industrial production.