Preparation,Characterization And Catalytic Performance Of Layered Double Hydroxides Supported Pd Catalysts

Layered Double Hydroxides(LDHs)are a class of layered inorganic materials composed of positively charged two-dimensional(2D)layers with interlayer charge compensating anions and water molecules.Their unique properties,such as tunable acidity and basicity of layer surface,tunability of the chemical composition of inorganic layers and the interlayer gallery anions,make LDHs a kind of promising support in preparation of supported catalysts for heterogeneous catalysis.The supported LDHs catalysts have been used in a wide range of applications.However,owing to heavily aggregated sheets,narrow spacing distance,the conventional LDHs usually have low surface utilization,which severely hinders the dispersion of supported active species on their surface.Thus,in order to solve the above problems and prepare LDHs supported Pd catalysts with high dispersion,this paper mainly focused on the following two aspects: 1)LDH nanosheets with high specific surface area were prepared by delamination and APTS grafting,then the nanosheets were used as supportfor loading Pd;2)by changing the solvent used in Pd loading process.Prepare the LDHs supported Pd catalysts in ethanol with small Pd particle size and high dispersion.The specific results are as follows:1)Preparation,characterization and catalytic performance of Pd nanoparticles supported on LDH nanosheets functionalized with APTSA novel functionalized LDH support was prepared by modifying the high surface porous LDH nanosheets obtained by AMOST method with aminopropyltriethoxysilane(APTS),and the Pd supported catalyst Pd/APTS-LDH-del was prepared.The obtained catalysts were systematically characterized by XRD,FT-IR,N2 adsorption-desorption,TEM and EDS.The results showed that grafting APTS onto the LDH nanosheets significantly improved Pd nanoparticle dispersion,decreased the Pd particle size and narrowed the range of size distribution of supported Pd nanoparticles.In the solvent-free oxidation of benzyl alcohol,Pd/APTS-LDH-del catalyst exhibited higher activity,selectivity to benzaldehyde as well as an improved stability compared with the Pd supported on non-silylated LDH nanosheets(Pd/LDH-del).This study indicates that modification of LDH nanosheets with APTS is an effective approach for finely tuning surface basicity of the LDH support,improving supported Pd dispersion,decreasing the Pd particle size and enhancing metal-support electronic interaction.2)Preparation of LDHs supported Pd catalysts with high dispersion in ethanolsolutionLDH-co synthesized by conventional coprecipitation method was use as the support.The Pd supported catalysts were prepared using sol-immobilization and impregnation method in aqueous and ethanol solution.TEM characterization results show the catalysts prepared in ethanol have higher Pd dispersion,smaller mean size of Pd nanoparticles and narrower size distribution than the catalysts prepared in aqueous solution no matter which method was used.The Pd nanoparticles supported on LDH-co in aqueous solution seriously aggregated into larger particle size.3)Pd supported on LDHs synthesized by tradition method,LDH nanosheets obtained via AMOST method and APTS silylated LDH nanosheets catalysts were prepared using sol-immobilization method in ethanol solvent,and the prepared three kinds of catalysts are denoted as Sol-Pd/LDH-co,Sol-Pd/LDH-del and Sol-Pd/APTS-LDH-del,respectively.In solvent-free benzyl alcohol oxidation,the catalyst Sol-Pd/LDH-co exhibited equivalent even better activity compared with Sol-Pd/LDH-del and Sol-Pd/APTS-LDH-del catalysts.This result indicates that adjusting modes of supported Pd is an efficient method for preparing highly dispersed LDH supported Pd catalysts.This method can simplify the catalyst preparing procedure,reduce the use of organic solvents,and is in accordance with the concept of green chemistry and sustainable development.