Preparation Of Zeolite-Based Core-Shell Material And Its Shape-Selective Catalytic Effect

Metal core-shell catalyst coated with microporous membrane material of zeolite is a kind of multifunctional catalyst which combines the advantages of metal materials,nanomaterials and crystal materials.This type of catalyst can effectively prevent the active components of the core from contacting with toxic molecules or selectively passing through some small organic molecules by using the screening selectivity of zeolite pores,thereby improving the activity,selectivity and stability of the catalytic reaction.In this paper,a new type of zeolite-based metal core-shell catalytic material was designed and prepared,and the hydrogen spillover effect of the new zeolite@metal core-shell structure and its application in hydrogenation catalysis were explored by adjusting the type,thickness,pore size of the shell zeolite and the type of core metal.The purpose was to explore the mechanism of hydrogen spillover,and to use hydrogen spillover to improve the activity and selectivity of the catalyst.Pt/Al₂O₃@LTA core-shell catalytic material was prepared by gel hydrothermal method with LTA zeolite as shell material and precious metal Pt as core metal component.A series of characterizations such as XRD,SEM,EDS,IR,TG and TPR confirmed that the core metal dispersion was high,the core-shell catalyst film was dense,and the zeolite phase was uniform.This material was used in the selective hydrogenation catalytic evaluation of olefin model compounds with 1-hexene（0.17nm）,cyclohexene（0.42 nm）and cyclooctene（0.55 nm）as raw materials.When the reaction temperature was 200℃,LHSV was 2 h^-1,and the hydrogen-oil ratio was 200（Vol.）,compared with Pt/Al₂O₃ catalyst,Pt/Al₂O₃@LTA catalyst had high selectivity for small-size 1-hexene（7.1）due to the pore screening selectivity of LTA（0.42 nm）zeolite shell,while Pt/Al₂O₃ had no screening effect on cyclohexene and cyclooctene.SOD zeolite was used as the shell material,and transition metal Ni was used as the core metal component.Ni/Al₂O₃@SOD core-shell catalytic materials were prepared by seed hydrothermal method and gel hydrothermal method,respectively.In the process of preparing core-shell catalyst by gel method,the viscosity of the gel is controlled by adjusting the type of silica source for synthesizing SOD.The continuous and dense SOD zeolite film can be obtained by using high viscosity gel,and the bonding strength between the zeolite film and the support alumina ball can be improved.The characterization results show that the zeolite particles on the surface of the film are uniform,closely arranged and have high degree of intergrowth.The controllability of the thickness of the SOD membrane was effectively realized by controlling the content of the gel wrapped on the spherical surface and the crystallization time.The Ni/Al₂O₃@SOD core-shell catalyst coupled with the traditional supported Mo S2catalyst was used for the evaluation of fixed bed hydrodesulfurization.The study confirmed that the Ni/Al₂O₃@SOD core-shell catalyst effectively limited the contact between the sulfur-containing compound H2S and the inner metal component due to the effect of the outer zeolite.In the coupled catalyst,the Ni/Al₂O₃@SOD core-shell catalyst was used as the active hydrogen donor in the hydrogen spillover,and Mo S2/Al₂O₃ was used as the acceptor.The active hydrogen was overflowed from the Ni/Al₂O₃@SOD core-shell catalyst to the Mo S2/Al₂O₃ catalyst for hydrogenation reaction.Due to the participation of active hydrogen,the activity of traditional Mo S2/Al₂O₃ catalyst was greatly improved.The catalytic model confirmed that the active hydrogen could be remotely transferred from one catalyst active center to another to improve the catalytic efficiency and reduce the reaction temperature.The catalytic data showed that the reaction temperature of Ni/Al₂O₃@SOD+Mo S2/Al₂O₃ catalyst was significantly lower than that of Mo S2/Al₂O₃ catalyst at the same conversion of DBT.