Preparation,Characterization And Catalytic Performances Of Small Pore Zeolite-Encapsulated Noble Metal Bifunctional Catalysts

With the aggravation of inferior and heavy crude oil in the world,the catalytic diesel obtained by FCC process has low cetane number,high sulfur and nitrogen content,high density,high polycyclic aromatic hydrocarbon content and poor complete combustion performances,which can not meet the requirements of vehicle diesel.Therefore,upgrading and optimizing the quality of catalytic diesel oil is an important task for petroleum processing in China.Hydrogenation and ring-opening of polycyclic aromatic hydrocarbons(PAHs)is an important way to improve the quality of diesel,which can not only reduce the content of PAHs in diesel components,but also increase the cetane number of diesel.Zeolite supported metal bifunctional catalysts have been widely studied because of their excellent hydrogenation and ring-opening catalytic performances,in which metal components are used as active sites for hydrogenation and dehydrogenation,and isomerization,ring-opening and cracking are carried out on the acid sites of zeolite.The catalytic performances of zeolite supported metal bifunctional catalysts greatly depends on the synergistic effect between metal and acid sites,which can be controlled by the ratio of metal to acid sites,the distance between metal and acid sites and the type of metal and supports.Zeolite-encapsulated noble metal catalysts have attracted much attention due to their high activity,selectivity,sintering resistance and sulfur resistance in recent years.Zeolite-encapsulated noble metal catalyst not only exhibits the shape-selectivity for reactants and products,but also can control the reaction pathway by adjusting the properties of microporous channels.However,the catalytic applications of zeolite-encapsulated noble metal catalysts mainly involve small molecule reactions,and the reactions involving bulky reactants are rarely studied.At present,zeolite supported metal bifunctional catalysts are usually prepared by the traditional impregnation method in the hydrogenation reaction of PAHs.Compared with traditional zeolite supported metal bifunctional catalysts,zeolite-encapsulated noble metal catalysts can accurately control the metal location,which contribute to the regulation of the synergistic effect between metal and acid sites.This paper is based on the characteristics of zeolite-encapsulated noble metal catalysts and aims at the key problem that the reaction path of zeolite supported metal bifunctional catalyst is difficult to be accurately optimized and controlled in the hydrogenative conversion of PAHs.Three kinds of bifunctional catalysts based on small pore zeolite encapsulation of noble metals were prepared.The synergistic effect between metal active sites and acidity(medium strong acid sites,strong acid sites and external surface acid sites)can be enhanced by precise control of metal location and optimization of catalysts structure and properties,which improve the catalytic activity,selectivity to target products and good sulfur resistance in methylcyclopentane hydrogenative conversion and naphthalene hydrogenation and ring-opening reaction.The main research contents are as follows:1.PtNPs were successfully encapsulated in SOD cages(Pt@SOD)by organosilane assisted crystallization method,which can be further confirmed by TEM and toluene hydrogenation probe reaction.The Pt@SOD+HZSM-5 coupling catalyst was prepared by coupling Pt@SOD with HZSM-5 zeolite at high temperature.It can be found thatNa⁺in Pt@SOD were exchanged for H⁺in HZSM-5 in solid-state by EDS,NH₃-TPD and Py-FTIR,which not only increases the hydroxyl concentration around PtNPs in Pt@SOD,but also the medium strong acid can be formed on HZSM-5.The Pt@SOD+HZSM-5 coupling catalyst shows high catalytic activity and high selectivity to C6 cyclization products in the hydrogenative conversion of methylcyclopentane,which is attributed to the synergistic effect between PtNPs in SOD cages and the medium strong acid on HZSM-5.In the reaction process,H₂ dissociates on the surface of PtNPs to form H species,which migrate to HZSM-5via hydroxyl group and form a catalytic interface with the medium strong acid.The hydrogenation/dehydrogenation of the spillover H species plays a key role in the reaction process,and medium strong acid can inhibit the cracking reaction effectively.In addition,this new type of coupling catalyst also shows good sulfur resistance.2.PtNPs were successfully encapsulated in NaA zeolite cages by in-situ crystallization method(Pt@NaA),and its pore sizes and surface hydroxyl concentrations can be controlled by ammonium exchange and calcination treatment to prepare Pt@HA material.It can be demonstrated that PtNPs are uniformly dispersed in the Pt@HA crystal and the crystal structure of Pt@HA collapsed to some extent by XRD,TEM,XPS,NH₃-TPD and toluene hydrogenation,which reduced the pore size effectively.Pt@HA shows excellent deep hydrogenation ability and good sulfur resistance in naphthalene hydrogenation reaction because of its abundant hydroxyl concentration and unique structure.Furthermore,the Pt@HA+HBeta coupling bifunctional catalyst was prepared by coupling Pt@HA with HBeta acidic zeolite.It can be confirmed that surface properties and acidities of Pt@HA and HBeta have not changed by EDS and NH₃-TPD analysis.Pt@HA+HBeta coupling bifunctional catalyst still has abundant strong acid sites and shows excellent catalytic activity and high selectivity for alkylbenzene in hydrogenation and ring-opening reaction naphthalene,which was attributed to the enhanced the synergistic effect between PtNPs and strong acid by accurately controlling the location of PtNPs and regulating the ratio and distance between PtNPs and strong acid sites.In addition,Pt@HA+HBeta coupling catalyst also shows good sulfur resistance due to its unique structure.3.In view of Pt@SOD and Pt@NaA have low Si/Al ratio,weak acidity and both need to be coupled with acidic zeolites in the hydrogenation process before they can be applied.Therefore,in this part,a kind of hierarchical Pt/SSZ-13 bifunctional catalysts were constructed,which could not only eliminate the coupling with acidic zeolite,but also can regulate the synergistic effect between PtNPs and acid sites by tuning the pore structure and acidity of silica-rich SSZ-13 zeolite:1)PtNPs was supported on microporous SSZ-13 zeolite by a simple impregnation-calcination-reduction method.The location of PtNPs in microporous SSZ-13zeolite can be regulated by changing different calcination temperatures,and the synergistic effect PtNPs and the external surface acid sites can be explored in the reaction process by controlling position of PtNPs precisely.The catalyst(Pt/SSZ-13-350)with PtNPs located in the cages at the crystal edge of microporous SSZ-13 zeolite shows excellent catalytic performance and good sulfur resistance in hydrogenation and ring-opening reaction of naphthalene.2)A series of hierarchical SSZ-13-MT were prepared by using Y zeolite as a single Al source and adding TPOAC template in the transformation-crystallization system.According to N₂ adsorption,NH₃-TPD and Py-FTIR analysis,the number of acid sites on the external surface of SSZ-13-MT can be effectively controlled by the amount of TPOAC.Hierarchical Pt/SSZ-13-MT with rich mesopores and external surface acid sites exhibits excellent hydrogenation performance in naphthalene hydrogenation.3)Aiming at the problem that the hierarchical SSZ-13 prepared with Y zeolite as a single Al source has high Si/Al ratios,which is not conducive to the capture of Pt by CHA pore cages.NaAl O₂ was added in the synthesis system as the second Al source to successfully construct the low-Si hierarchical SSZ-13-MT in the mode of double Al sources.Low-Si hierarchical SSZ-13-MT not only has abundant external surface acid sites,but also can capture the PtNPs in optimal position,which enhances the synergistic effect of PtNPs and external surface acid sites in the reaction process,thus,low-Si hierarchical Pt/SSZ-13-MT catalyst shows high catalytic activity and high selectivity for the isomerized and ring-opening products of decalin in hydrogenation ring-opening reaction,and also shows good sulfur resistance.In addition,the advantages of the low-Si hierarchical Pt/SSZ-13 bifunctional catalyst prepared by the double Al source mode are highlighted by comparing with the hierarchical Pt/SSZ-13-MC catalyst prepared by the traditional hydrothermal crystallization method.In sum,three kinds of bifunctional catalysts based on small pore zeolite encapsulation of noble metals were prepared.The synergistic effect between metal active sites and acidity(medium strong acid sites,strong acid sites and external surface acid sites)can be enhanced by precise control of metal location and optimization of catalysts structure and properties,which improve the catalytic activity,selectivity to target products and good sulfur resistance in methylcyclopentane hydrogenative conversion and naphthalene hydrogenation and ring-opening reaction,with a view to providing an effective theoretical basis and approach for the design and development of bifunctional catalyst for hydrogenation and ring-opening of PAHs.