Preparation And Catalytic Applications Of UiO-66 Coated Metal Nanoparticle Catalysts

Metal organic frameworks?MOFs?materials are a kind of organic-inorganic hybrid materials formed by bridging metal center ions with organic ligands.Metal nanoparticles?MNPs?with high activity are prone to aggregation and difficult to be recycled on account of high surface energy.The preparation of MNP@MOF composites materials using MNPs loaded or coated within MOFs not only promotes the reuse of materials,but also produces unexpected performance due to their synergetic effect.As a downstream product of biomass energy,furfural can be converted into a series of C₄ and C₅ molecules through the hydrogenation reaction.In this paper,UiO-66 materials with excellent stability of MOFs were chosen as the shell to coat MNPs and the core-shell catalysts were prepared which were applied in the selective hydrogenation of furfural.The main contents and results are as follows:Pd@UiO-66 core-shell catalysts in which Pd nanoparticles?NPs?were coated within UiO-66 carrier were prepared via a one-step encapsulation method and applied in the hydrogenation of furfural.The synthesis route of catalysts involved the two-step procedure including firstly the formation of UiO-66 skeleton and simultaneously the encapsulation of Pd NPs within UiO-66 at the lower temperature?75°C?,and then subsequently the reduction of Pd²⁺at the elevated temperature?120°C?by the weak reducibility of N,N-dimethylformamide?DMF?solvent.The structure and crystallinity of UiO-66 remained unchanged before and after the loading of Pd NPs.Pd NPs with an average particle size of ca.1.8 nm were evenly dispersed inside UiO-66 carrier.The hydrogenation of furfural with solvent water at low temperature?40??could maintain high catalytic activity while suppressing the formation of by-products.The 100%yield of furfuryl alcohol was attained over Pd@UiO-66 catalysts with the Pd loading of 2.4 wt%at 40?under 2.0 MPa H₂ pressure for 6 h.In addition,the 100%yield of tetrahydrofurfuryl alcohol could be obtained by increasing the Pd content,increasing the H₂pressure and prolonging the reaction time.Compared with the Pd/UiO-66 supported catalysts,the core-shell catalysts demonstrated more excellent catalytic performance,because the confinement effect of MOFs framework structure was conducive to the formation of uniformly dispersed Pd NPs with small particle size.In order to improve the utilization of noble metal Pd,using the strategy of additional small amounts of non-noble metal,Pd-Co and Pd-Cu nanoparticles were encapsulated within UiO-66 to form bimetallic Pd-Co@UiO-66 and Pd-Cu@UiO-66 core-shell catalysts,which were applied in the hydrogenation of furfural to cyclopentanone.In XPS spectrum,the Pd 3d characteristic peaks transferred to higher binding energy over the bimetallic catalysts,indicating the formation of bimetallic alloy.Pd-Co alloy was uniformly dispersed inside UiO-66 with an average particle size of ca.2.1 nm in TEM images.The bimetallic core-shell catalysts could demonstrate excellent synergistic catalytic properties with the 96.4%and95.0%cyclopentanone yield at 120°C under the 3.0 MPa H₂ pressure for 12 h,respectively.Compared with monometallic Pd@UiO-66 catalysts,the same cyclopentanone yield was obtained over the bimetallic catalysts with less Pd loading under lower reaction temperature,indicating that the addition of non-noble metals could reduce catalyst costs and energy consumption.Adding trace amounts of Co and Cu could reduce the hydrogenation activity and inhibit the formation of by-product?tetrahydrofurfuryl alcohol?.The core-shell catalysts showed more outstanding recyclability than the supported catalysts,which could maintain high cyclopentanone yield after 5^th runs.