Construction And Catalytic Applications Of Isolated Metal Active Sites Confined In Zeolite

Zeolite-supported metal（cation）catalysts represent a type of solid catalysts widely emplyed in petrochemical and fine chemical industry.For zeolite-supported metal catalysts prepared via conventional wet impregnation,the diverse metal（cation）sites and poor sintering resistance unavoidably lead to low selectivity toward target product as well as low carbon utilization rate in the catalytic reactions.In this context,promoting the catalytic selectivity and the controllability of chemical transformations is very challenging when dealing with the applications of zeolite-supported metal catalysts.In this paper,various strategies of post-synthesis modification and dirct hydrothermal synthesis were developed to synthesize a variety of zeolite-supported catalysts with well-defined isolated metal sites.The structure of metal sites and the structure-activity relationship in certain catalytic reaction were focused.The detailed content was described as follows:1.On the basis of previous studies from our group,we prepared Pb-Beta zeolite via a simple and facile post-synthesis modification route,and conducted a detailed study on the incorporation of Pb species into the framework of BEA zeolite.The characterization results demonstrated that associated silanols were created from the removal of framework Al sites in H-Beta through acid treatment.In the process of wet impregnation,the Pb precursors could react with the silanols and Pb species were then incorporated into the framework of Si-Beta zeolite.The results from UV-Vis and XPS analyses indicated that the Pb species existed in well isolated form,probably in four-coordinated configurations.In addition,compared with the classical hetero-atom zeolites,i.e.Sn-beta,Ti-beta and Zr-beta,the structure of Pb sites should be more open due to its larger atomic size.NH₃-TPD analysis confirmed the presence of abundant Lewis acid sites in Pb-Beta zeolites.In the reaction of epoxide aminolysis,the as-prepared Pb-Beta zeolite showed unique catalytic activity as well as good recycling ability.2.With the protection of Pd species by–SH group,we successfully synthesized Pd@MFI parent material via hydrothermical route in the mixed solution of water and ethanol.Different samples of Pd_n@S-1,Pd_n@H-ZSM-5 and Pd_n@Na-ZSM-5 were obtained through subsequent processes of ion exchange and hydrogen reduction at elevated temperatures.According to HAADF-STEM observations,Pd nanoclusters with an apparent diameter of about 1.8 nm were encapsulated in the channels of MFI zeolites,and these Pd nanoclusters wer formed by multiple atomic clusters along the two-dimensional channels of zeolite.FTIR spectra of CO adsorption further confirmed that the apprant Pd nanoclusters were composed of atomic clusters.Through global optimization simulation,two most stable structural models,i.e.Pd₄@MFI and Pd₆@MFI,were obtained.The as-preapred Pd_n@S-1 samples were applied as catalysts in the hydroconversion of furfural.Under optimized reaction conditions,Pd_n@S-1 preferred to catalyze furfural decarbonylation to furan.Pd_n@Na-ZSM-5 could catalyze the selective hydrogenation of furfural to furfural alcohol,with a high furfural conversion rate of 9.2 mmol/g/h and furfural alcohol selectivity up to 90.2%.While for Pd_n@H-ZSM-5,the abundant Br?nsted acid could attack furan ring and therefore resulted in the formation of ring opening products like1,5-pentanediol.The modulation of product from furfural hydroconversion was finally explained in detail from the view of reactant adsorption and activation.3.We successfully synthesized Ni@CHA zeolites by hydrothermal method using cheap Ni-DETA as template,and subsequently prepared Li-Ni@CHA,Na-Ni@CHA and K-Ni@CHA through replacing a small amount of Ni²⁺at the ion-exchange sites by Li⁺,Na⁺and K⁺,respectively.A series of techniques including H₂-TPR,XAS,STEM and in-situ UV-Vis were employed to reveal the existence of four coordination Ni species in the six-member ring of CHA zeolite framework,and the stable coordination environment of Ni（II）species in zeolite was also confirmed by DFT calculations.Ni@CHA zeolites were applied in the selective hydrogenation of acetylene,and remarkable catalytic performance was achived with Na-Ni@CHA.Typically,100%acetylene conversion with 97%ethylene selectivity was achieved employed reaction conditions.The reaction mechanism of acetylene selective hydrogenation over Na-Ni@CHA was investigated in detail by means of TPD,in situ FTIR and DFT calculations.The adsorption of H₂molecules on the four-coordinated Ni（II）followed by heterolylic activation was found to be the key point of the rection,which fully followed the mechanism of coordination catalysis.