Preparation Of MFI Zeolite@Pt/Al₂O₃ Core-Shell Catalysts For Isomerization-Selective Hydrogenation Of Xylene

In the last two decades,MFI membrane has been widely developed in the field of membrane science and technology.MFI-type zeolite has a three-dimensional pore system with straight channels in the b-direction?0.54 × 0.56 nm?and sinusoidal channels in the a-direction?0.51 × 0.55 nm?.P-xylene with a kinetic diameter of 0.58 nm can enter and diffuse through the zeolite channels more readily than the bulkier m-and o-xylene isomers,which have kinetic diameter of 0.68 nm.Due to molecular sieving through zeolitic pores,MFI membrane has been one of the most promising candidates for a new approach of xylene separation.However,membrane separation is of limited practical potential because of prohibitive low fluxes.In this paper,we propose a core-shell structure catalyst coupling separation and catalysis,which has great potential to develop into a novel approach for xylenes separation.The impact factors in the synthesis of MFI zeolite membrane coated core-shell catalyst were investigated.The experimental results indicate that after using the small size crystals as seeds and perfoming the adsorbing process twice,the distribution of the seeds on the support is uniform.In the crystallization process of zeolite membrane,with the TPAOH/TEOS molar ratio of 0.4 and reaction time of 72 h,the synthesized membrane is well-intergrown and uniform.Besides,the influences of Si/Al ratio on the morphology and thickness of the ZSM-5 membrane are investigated as well.One layer ZSM-5 was successfully coated onto the surface of Al₂O₃ pellets using a hydrothermal synthesis method to obtain the ZSM-5@Al₂O₃ core-shell catalyst.Based on the adjustable Si/Al ratio and acidity of ZSM-5,the isomerization performances of ZSM-5@Al₂O₃ catalyst under different Si/Al ratio,reaction temperature and xylene feed were investigated.The research results show that in the range of 300-500 oC,the isomerization activities of m-and o-xylene increased with the increasing temperature.In the case of binary p/m mixture as the feed,isomerization activity was relative higher than using p/o mixture as the feed.In addition,p-xylenes is prone to form at relative low Si/Al ratio due to the increase of medium acid sites.To combine molecular sieving and hydrogenation performance,a Silicalite-1@Pt/Al₂O₃?PA-S?core-shell catalyst was synthesized.The selective hydrogenation performance of xylene isomers under different reaction temperatures and feeds were investigated.While a Pt/Al₂O₃ catalyst had no selectivity in the hydrogenation of one-component xylene isomers,the PA-S catalyst showed much higher efficiency for the hydrogenation of p-xylene than for that of m-and o-xylene.The p/o and p/m hydrogenation selectivity of 94.5 and 34.5,respectively were obtained at 200oC.A H-ZSM-5@Silicalite-1@Pt/Al₂O₃?PA-SZ?core-shell catalyst that combined selective hydrogenation and isomerization was synthesized.The binary xylene mixtures were performed as feeds to evaluate the selective hydrogenation property of this catalyst.For comparison,the bare Pt/Al₂O₃ catalyst as well as the monolayer Silicalite-1@Pt/Al₂O₃?PA-S?and H-ZSM-5@Pt/Al₂O₃?PA-Z?catalysts were also performed under the same reaction conditions.In the hydrogenation of xylene isomers,the p/o and p/m selectivity over the PA-SZ catalyst strikingly exceeded that over the bare Pt/Al₂O₃ core catalyst as well as the monolayer PA-S and PA-Z core-shell catalysts.The p/o and p/m hydrogenation selectivity increased with the increasing temperature ranging from 200 to 350 oC and reached as high as 61.4 and 43.7,respectively,at 350 oC.