Synthesis And Catalytic Performance Of Hierarchical Metal-zeolite Core-shell Materials

Based on the principle of green chemistry,it is of high value to prepare high efficiency and low cost multi-functional catalyst.The core-shell structure could integrate different active sites,which provides new sight for the design synthesis of multi-functional materials.The hierarchical structure is beneficial for enhancing mass transfer efficiency and improving the reaction activity.Considering the feature of target reaction,rational design of special hierarchical core-shell catalysts has important industry prospects.On basis of widely used USY zeolite,we developed novel methods to prepare hierarchical metal-zeolite core-shell catalysts,which integrated multi-functions,improved the mass transfer efficiency,enhanced the catalyst activity and stability,and ultimately facilitated the selectivity of target products.The detailed work was listed as follows.?1?It is a great challenge to prepare supported metal catalysts with high activity and stability in heterogeneous catalysis filed.Herein,by virtue of dealumination-growth method,the adscititious Ni²⁺ions chemoselectively interacted with Al species dissolved from the zeolite framework to form a NiAl-LDHs structure anchored robustly on the surface of zeolite crystals,giving rise to USY@Ni Al-LDHs core-shell material.This method not only realized the dealumination of the low-silica zeolite and the immobilization of Ni metal species in one-pot,but also lowered the reduction temperature of Ni²⁺ions.The resultant USY@Ni material upon reduction possessed remarkably increased mesoporous volume and surface area than pristine USY zeolite,and highly dispersed Ni nanoparticles?5 nm?,leading to excellent hydrogenation activity and stability for the condensation of acetone and the conversion of m-nitroaniline to m-phenylenediamine.?2?According to the Si/Al ratios of pristine zeolites,the adscititious metal salt chemoselectively interacted with silica or aluminum species extracted from USY zeolites and formed NiSiO₂ or NiAl-LDHs precursor,through optimizing the synthesis condition of in situ growth method.Upon reduction,highly dispersed Ni nanoparticles?NPs?were precisely confined in the SiO₂ or Al₂O₃ matrix anchored on the USY zeolites,giving rise to unique hierarchical core-shell structure.Compared with the NiSiO₂ nanosheets on USY zeolite with high Si/Al ratio,the NiAl-LDHs nanosheets on USY zeolite with low Si/Al ratio produced highly reductive,dispersed and stable metallic Ni species.Through investigating the hydrogenation ammoniation of octanol or ether alcohol over the series USY@Ni catalysts with different Si/Al ratios,the USY@Ni catalyst with low Si/Al ratio exhibited the highest activity,which is much higher than that of Ni based catalysts prepared by impregnation or physical mixing method.Moreover,the corresponding selectivity of primary amine over the USY@Ni?Si/Al=3?catalyst is much higher than the raney nickel or supported noble metal catalysts.?3?On basis of aforementioned method,the CoAl-LDHs nanosheets were coated on USY zeolite and formed CoAl-LDHs@USY core-shell material.Upon calcination,the precursor turned into unique CoO_x@USY material,of which the shell was composed of strong interacted Co-Al oxide and highly dispersed Co₃O₄species.The open three-dimensional structure of CoO_x@USY catalyst maximally exposed the Co-Al active sites and acid sites of zeolite.Serving as a selective catalytic reduction of NO,the designed CoO_x@USY yielded a remarkably elevated NO conversion efficiency and resistance to H₂O and SO₂.The method featured easy operation and high atom efficiency,providing new insights into the design and synthesis of efficient catalysts for reducing NO emission.?4?After investigating the synthesis conditions of above method,the adscititious metal method was furtherly expanded to zinc.The Al³⁺ions extracted from USY zeolite with low Si/Al ratio migrated to the surface and interacted with Zn²⁺ions which produced ZnAl-LDHs nanosheets with high aspect ratio coated on USY zeolite and fabricated USY@ZnAl-LDHs precursor.Various characterization demonstrated the migration of aluminum and the change of coordinated status,confirming the reaction pathway of as-formed ZnAl-LDHs phase.Upon calcination,the Lewis acidity as well as mesopore volume of USY@ZnO/Al₂O₃ material were significantly increased.Acting as catalysts in Baeyer-Villiger oxidation of2-adamantanone,USY@ZnO/Al₂O₃ materials exhibited excellent performance with the conversion of ketones reached up to 90%and the selectivity of lactone almost100%,which endowed the catalyst with promising performance using H₂O₂ or bulky tert-butyl hydroxide as oxidant.?5?The alkaline-assisted desilication of zeolite encountered the loss of framework atoms.The desilication process of USY zeolite with the presence of NaOH and surfactant inhibited the collapse of zeolite framework.Moreover,the dissolved silica acted as silica source and self-assembled into mesoporous SiO₂ on the surface of USY zeolite,giving rise to the hierarchical USY@MSA core-shell material.The pH value is of vital importance for the formation of core-shell structure and the thickness of shell could be adjusted by the desilication conditions.A series of characterizations indicated the micro-mesoporous structure of USY@MSA material with the mesopore size of³0 nm.The bifunctional Co/USY@MSA catalyst was fabricated by supporting cobalt through impregnation method,of which the hierarchical porosity and appropriate acidity facilitated selectivity of diesel in the Fisher-Tropsch synthesis.