Synthesis And Catalytic Performance Of MFI Zeolite-Based Hierarchically Structured Core-Shell Porous Materials

Synthetic zeolites with unique catalytic properties have been widely used in the fields of petrochemical industry.However,their relatively small micropore channels result in unfavorable mass transfer constraints for the reaction involving bulky molecules,which leads to low catalytic activity and rapid deactivation.The hierarchically core/shell-structured materials not only effectively improve the mass transfer,but also allow to construct different active sites either in core or in shell,endowing zeolites with new catalytic functionalitiesBased on widely used zeolites with MFI topology,we developed novel methods to prepare hierarchical core-shell catalysts,which improved the mass transfer eff-iciency,and enhanced the activity and stability.The detailed results are listed as follows(1)We first successfully prepared a core-shell TS-1@Si/C material using TEOS as a silica precursor and resorcinol and formaldehyde(RF)as a carbon precursor via a self-assembly and nanocasting route,onto the surface of TS-1 zeolite.This material has amphiphilic characteristics due to the hydrophilicity of Si species and the hydrophobicity of carbon species in the shell.The hydrophobicity of the material can be controlled by adjusting the ratio of Si/C in shell.The l-hexene/H2O2 Pickering emulsion was only stabilized via TS-1@Si/C materials with suitable hydrophilic/hydrophobic properties.Without solvent and stirring,the TS-1@Si/C catalyst showed higher catalytic activity and good stability under Pickering interface catalysis conditions than TS-1 under conventional reaction conditions(2)Starting from MFI-type Silicalite-1(S-1),the needle-like Cu2Si2O5(OH)2 precursors were generated and coated on the outer surface of S-1 crystals through a base-assisted chemoselective interaction between zeolite Si species and the corresponding Cu salt source.The H2 reduction led to the formation of Cu+/Cu0 nanoparticles and SiO2 as shell covering S-1.This method,good controllability,realized the in-situ construction of mesopores and high dispersion of Cu NPs.The ultrafine Cu NPs were well dispersed among the SiO2 matrices and their size distribution was centred at about 5.0 nm even at a Cu loading up to 31.7 wt%.The S-1@Cu hybrid,possessing the co-existing Cu0/Cu+ active species with a suitable ratio,served as a highly active,selective and robust catalyst for selective ethylene carbonate hydrogenation,providing a lifetime>350 h together with>99%ethylene carbonate conversion,>99%ethylene glycol yield,and more importantly 93%methanol yield at a relatively low Cu loading of 21.4 wt%(3)On basis of aforementioned base-assisted in situ growth method,the Cu2Si2O5(OH)2 nanorods were coated on ZSM-5 aluminosilicate,resulting in a ZSM-5@CuSiO3 core-shell material as highly efficient zeolite supported metal catalyst.The open three-dimensional structure of ZSM-5@CuSiO3 material greatly exposed the Cu active sites and acid sites of zeolite.The designed ZSM-5@CuSiO3 exhibited a remarkably elevated NO conversion with high resistance to H2O and SO2 in the selective catalytic reduction(SCR)of NO by NH3.This method is featured of easy operation and is expected to provide a new idea for the design of new SCR catalyst for reducing NO emission(4)The in-situ growth synthesis was further expanded to supporting zinc on zeolites.In alkaline synthesis system,the Si species released from the surface of ZSM-5 zeolite interacted with Zn2+ ions to produce Zn4Si2O7(OH)2 species coated on ZSM-5 zeolite,which fabricated a ZSM-5@Zn2SiO4 core-shell material.After introducing Zn into ZSM-5,the Lewis acidity increased greatly from 0.09 mmol g-1 for ZSM-5 to 0.26 mmol g-1 for ZSM-5@Zn2SiO4;meanwhile the mesopore volume increased from 0.09 to 0.35 cm3 g-1.Acting as a catalysts in methanol to aromatization,ZSM-5@Zn2SiO4 materials exhibited a good performance with high methanol conversion and aromatics selectivity.