Preparation And Catalytic Performance Of Metal Doped Hierarchical HZSM-5 Zeolite

Transition metal oxide nanoparticles are widely used in many reaction systems because of their low cost and high catalytic activity.However,the high surface free energy always leads to agglomeration and sintering of metal species in the process of preparation and catalytic reaction,which would significantly reduce the atomic utilization rate and conversion frequency（TOF value）.HZSM-5 zeolite as metal carrier can not only improve the metal dispersion,reduce the probability of agglomeration and sintering,but also provide acidic sites to promote catalytic reaction.However,the diffusion limitation often occurs in the catalytic reactions involving macromolecular reactants.The construction of hierarchical HZSM-5 zeolite can not only solve the problem of diffusion limitation,but also improve its specific surface area and provide more attachment sites for metal species.In this paper,metal doped hierarchical HZSM-5 zeolite catalysts were constructed by various strategies,and the catalytic performance was evaluated by NO oxidation-removal and catalytic combustion of chlorobenzene（CB）as model reaction,in order to explain the influence of the structure and properties of catalysts on the catalytic performance.Using[3-（trimethoxysilyl）propyl]octadecyldimethylammonium chloride（TPOAC）and tetrapropylammonium hydroxide（TPAOH）as meso-porogen and micropore structure-directing agent,respectively,Fe-ZSM-5 catalysts was prepared by hydrothermal method.The influence of ethanol concentration on the structure,porosity,Fe species and catalytic performance of catalysts was investigated,and the mechanism of ethanol action was speculated.Appropriate ethanol dosage in synthesis solution probably stabilized TPOAC micelles during MFI crystallization,facilitated the integration of mesopores into zeolite crystals,benefited Fe²⁺and oxygen vacancy formation,accelerated·OH radicals generation,and thereby enhanced NO oxidation-removal efficiency.The hierarchical Fe-ZSM-5 prepared with ethanol/H₂O ratio of 1 in synthesis gel displayed the excellent catalytic activity and anti-sulfur performance,with the optimal NO conversion up to 35.9%enhancement compared to the microporous Fe-ZSM-5.The suitable catalytic reaction conditions were determined and the oxidation removal mechanism of NO over hierarchical Fe-ZSM-5 was obtained by·OH scavenger test.Furthermore,amphiphilic organosilane（TPOAC）was used as mesoporous template for preparing single-crystal hierarchical Fe-ZSM-5 catalysts by dry gel conversion method,and the effect of ethanol steam on the catalyst properties was exployed.It was found that ethanol inhibited the crystallization of the zeolite,stabilized the TPOAC micelles,promoted the formation of homogeneous mesoporous phase,and improved the specific surface area of the catalytst.At the same time,moderate amount of ethanol also interacted with iron components and promoted the formation of oxygen vacancies.Catalytic combustion of chlorobenzene was regarded as an activity evaluation reaction,and the results showed that hierarchical Fe-ZSM-5 prepared with ethanol/H₂O ratio of 0.5displayed the higher CB conversion and CO₂selectivity due to the synergistic effect of acid site and oxygen vacancy.Subsequently,hierarchical Mn/Fe-ZSM-5（MDG-0.5E）was prepared by impregnation of meso-Fe-ZSM-5 with manganese,which further improved the catalytic performance,due to the higher concentration of oxygen vacancy.In addition,the reaction mechanism of catalytic chlorobenzene combustion was revealed by in-situ DRIFTS and DFT simulation.