Preparation And Catalytic Properties Of Pd@NH₂-UiO-66 As A Bifunctional Catalyst

Metal-organic frameworks(MOFs)are a new class of porous materials,which are assembled from inorganic nodes and organic linkers.Due to their high surface area and pore volume,high porosity,tunable shape,and modifiable surface properties,the applications of MOFs in heterogeneous catalysis have recently attracted tremendous attention,particularly,those using MOFs as supports for noble metal(e.g.,Pt,Ru,Au,and Pd)nanoparticles.There are a lot of reports on the MOFs' role in catalysis,however,there is few studies involving in the bifunctional metal@MOF system.Based on the excellent properties and structure features of NH2-UiO-66,in this thesis we used two different methods to synthesize the Pd@NH2-UiO-66 bifunctional heterogeneous catalyst,in which the noble metal sites and Lewis acid active centers were combined in the cavities and/or frameworks of the MOF,respectively.Then we applied this bifunctional catalyst in the liquid-phase hydrodeoxygenation of biomass-based model compounds(vanillin)and the hydrogenation of 2,3,5-trimethylbenzoquinone(TMBQ)to 2,3,5-trimethylhydroquinone(TMHQ),a key vitamin-E intermediate,respectively.The relationship between the physicochemical properties and the catalytic performances of the developed catalyst were discussed.The main results are summarized as follows:(1)Ultrasmall palladium nanoparticles(1.5-2.5 nm)encapsulated in MOFs have been prepared by introducing a palladium precursor into a highly porous and hydrothermally stable amine-functionalized UiO-66(NH2-UiO-66)via a direct anionic exchange and subsequent H2 reduction.The prepared Pd@@NH2-UiO-66 catalyst was then applied in the hydrodeoxygenation of vanillin(a typical model compound of lignin)at a low H2 pressure in aqueous media.Excellent catalytic results(100%conversion of vanillin with exclusive selectivity for 2-methoxy-4-methylphenol)could be achieved over the developed 2.0 wt%Pd@NH2-UiO-66 catalyst under mild conditions in water.Furthermore,the catalytic activity and selectivity were not affected after six reaction cycles indicating that the catalyst has excellent stability and highly reproducibility in this reaction system.It was found that the free amine groups presented in the framework of NH2-UiO-66 plays a key role in the formation of uniform,well-dispersed and leaching resistant palladium nanoparticles within the MOF host.Moreover,the developed Pd@NH2-UiO-66 exhibits a novel synergetic catalysis in the hydrodeoxygenation due to the cooperation between the well-dispersed metallic Pd sites and the amine-functionalized MOF support,in which Pd offers hydrogenation activity and the MOF support facilitates hydrogenolysis of the intermediate vanillin alcohol to the 2-methoxy-4-methylphenol product.(2)Pd nanoparticles encaged within an amine-fimctionalized MOF,2.0 wt%Pd@NH2-UiO-66,was successfully prepared via one pot direct construction of the MOF framework,cooperative in situ metal precursor incorporation followed by on-site moderate reduction process,in which N,N'-dimethylformamide was used both as the solvent for the efficient assembly of NH2-UiO-66 frameworks and the reductant for the effective fabrication of palladium nanoparticles.Owing to the large specific surface area,numerous of nanocages accessible through small pore windows,as well as excellent coordination capability of metal ions sites from the amine groups within the NH2-UiO-66 matrix,the Pd nanoparticles were homogeneously confined within its cavities.Intriguingly,the prepared Pd@NH2-UiO-66 exhibits a high catalytic activity and exclusive selectivity in the liquid-phase hydrogenation of 2,3,5-trimethylbenzoquinone(TMBQ)to 2,3,5-trimethylhydroquinone(TMHQ),a key vitamin-E intermediate in a short period under relatively mild reaction conditions.Additionally,this catalyst was recycled up to 8 times without any loss in both selectivity and activity.Furthermore,the reaction mechanism suggested that the superior performance of Pd@NH2-UiO-66 could be ascribed to the cooperation between the Pd nanoparticle and the amine-functionalized UiO-66.This MOF-based composite shows great potential as heterogeneous catalyst for the catalytic hydrogenation reactions due to its high stability,excellent catalytic properties and simple preparation.