Preparation And Properties Of Molecular Sieve-Based Platinum Catalysts

The hydrogenation of levulinic acid toγ-valerolactone is a crucial reaction in converting carbohydrates into renewable fuels or chemicals.Noble metals have become efficient catalysts for the hydrogenation of levulinic acid due to the high activity,selectivity and stability.However,noble metal nanoparticles are prone to agglomeration and sintering during the catalytic process,resulting in a decrease in the catalytic activity.Therefore,rationally controlling the size of nanomaterials and designing noble metal-based catalysts with high dispersity are of great significance to improve the overall catalytic activity and anti-sintering ability.Encapsulating noble metal nanoparticles on the surface or inside of inorganic carriers with high specific surface area is an effective way to construct highly dispersed and highly activated catalysts.Among them,zeolite molecular sieve is a kind of crystalline material composed of silicon and aluminum elements,which are considered as one of the ideal inorganic carriers owing to the rigid framework and porous structure.In this paper,we aimed to enhance the catalytic activity of Pt-based catalysts.The porous zeolite encapsulated metal nanoparticle catalysts were designed using MFI-type pure silica Silicalite-1 molecular sieves as the carriers.The structure-effect relationship was revealed between the structural characteristics and catalytic performance of catalysts.The specific researches are as follows:（1）Preparation and catalytic properties of in-situ encapsulated Pt Zn bimetallic catalysts on Silicalite-1 molecular sievesIn this work,the Pt Zn_x@S-1 catalyst with Pt Zn bimetallic nanoparticles encapsulated by Silicalite-1 molecular sieve was directly synthesized through in-situ hydrothermal method.The metal complexes were added to the initial gel of molecular sieves based on the ligand-stabilized protection strategy.Due to the electrostatic interaction,the bimetals were encapsulated in the zeolite framework by self-assembly during the crystallization process.The catalysts of Pt Zn@S-1 can be obtained after hydrogen reduction treatment.The Silicalite-1 zeolite was successfully prepared,and the Pt Zn nanoparticles were uniformly dispersed inside the zeolite according to the XRD,SEM,TEM,Raman and other characterizations.The preparation method can prevent metal nanoparticles from precipitating in the form of hydroxide in an alkaline solution,and the highly dispersed metal nanoparticles can effectively alleviate the agglomeration phenomenon in the catalysis process.Therefore,the series of Pt Zn_x@S-1catalysts with different metal ratios exhibited excellent catalytic performance in the hydrogenation of levulinic acid.The conversion rate of Pt Zn₂@S-1 catalyst to levulinic acid reached 73.5%,and the maximum yield ofγ-valerolactone was 70.2%.（2）Preparation and catalytic properties of secondary crystallization encapsulated Pt Cu bimetallic hollow catalysts on Silicalite-1 molecular sievesIn this study,a series of Pt Cu_x@H-S-1 catalysts with various metal ratio were prepared by encapsulating Pt Cu nanoparticles in Silicalite-1 zeolite with a hollow porous structure through a secondary crystallization process.This method selectively dissolved Silicalite-1 with metal precursor in tetrapropylammonium hydroxide solution and performd external recrystallization.The etching effect of the alkaline solution resulted in the hollow and hierarchical pore structure of the Silicalite-1,which dispersed Pt Cu bimetallic nanoparticles into the channels of zeolite uniformly.For one thing,the hollow pore structure can provide abundant sites for catalytic reactions and promote the diffusion of organic reactants/intermediates/products.For another,the highly dispersed Pt Cu nanoparticles can effectively alleviate the aggregation and thermal migration during the reaction process,improving the catalytic activity and anti-sintering ability of catalyst.The theoretical calculations manifested that the introduction of Cu species can optimize the atomic coordination environment.The Pt Cu₃@H-S-1 catalyst obtained higher adsorption energy of reactant molecules,delivering outstanding catalytic performance for levulinic acid hydrogenation.The conversion to levulinic acid reached 98.5%and the yield ofγ-valerolactone was 95.7%.The Pt Cu₃@H-S-1 catalyst also exhibited good cycling stability due to the protection of the recrystallized shel l.