Preparation Of MFI Zeolite Membrane Encapsulated Core-shell Materials For Shape-selective Hydrogenation Of Xylene

MFI zeolite membranes possess a specific three-dimensional channel system and have been used for xylene shape separation because their pore openings are close to the size of p-xylene.However,zeolite membrane separation is of limited practical potential due to prohibitive low fluxes.In this paper,a novel method was proposed to separate p-xylene by combining membrane separation and catalytic reaction.MFI zeolite membrane?Silicalite-1?coated core-shell catalyst was designed and prepared.Meanwhile,in order to increase p-xylene production,a xylene isomeration-selective hydrogenation process was proposed and its feasibility was explored.Silicalite-1 zeolite membranes were prepared on defective porous Al₂O₃ spheres by a seeding-pre-crystallization-crystallization process.The results show that a layer of continuous and thin MFI zeolite seed is coated on the surface of Al₂O₃ sphere by wetting-rolling seeding method.The pre-crystallization process can protect Al₂O₃ supports and induce the growth of zeolite crystals.For dynamic hydrothermal synthesis method,the prepared zeolite membranes after the crystallization process are dense,continuous and homogeneous when the TPAOH content is 0.17 and the H₂O content is 160.The thickness of Silicalite-1 zeolite membrane is about 3?m.Compared with dynamic hydrothermal synthesis method,larger zeolite crystal grains are observed and the thickness of the membrane is about 8?m for static hydrothermal synthesis method.However,the zeolite membranes are uneven on the surface of Al₂O₃spheres.Ni/Al₂O₃@Silicalite-1 core-shell catalysts were prepared by dynamic synthesis method and static synthesis method,respectively.The shape-selective hydrogenation performances of core-shell catalysts were tested with binary mixtures of xylene as feeds.The results indicate that the core-shell catalysts prepared by dynamic synthesis method have better performance for the selective hydrogenation of p-xylene.At 220 ^oC,the p/o and p/m hydrogenation selectivities are 15.5 and 11.2,respectively.However,for the core-shell catalysts prepared by static synthesis method,the p/o and p/m hydrogenation selectivities are 7.2 and 4.1,respectively.In addition,for the core-shell catalysts prepared by dynamic synthesis method,the p/o and p/m hydrogenation selectivities increase with the increasing temperature.Xylene isomerization-selective hydrogenation process was studied using fixed bed reactor.HZSM-5 zeolites and Ni/Al₂O₃@Silicalite-1 core-shell catalysts were prepared,respectively.The effect of reaction temperature on the isomerization performance of HZSM-5zeolite was investigated.The optimum reaction temperature is 350 ^oC.The shape-selective hydrogenation reaction was performed at 220 ^oC with isomerization reaction products as feeds.The results indicate that the p/o and p/m hydrogenation selectivities are 7.6 and 3.3,respectively.Core-shell catalyst exhibits a high para-selectivity for xylene hydrogenation.The results confirm that the xylene isomerization-selective hydrogenation process has high feasibility.