Preparation Of Silicalite-1 Encapsulated Pd-based Catalyst And Investigation Of Catalytic Oxidation Performance Of Toluene

A series of issues such as air pollution,photochemical smog and inducing human cell carcinogenesis caused by volatile organic compounds（VOCs）have received widespread attention.Catalytic oxidation technology is the preferred method for degrading VOCs due to its high efficiency and energy-saving benefits.The core of this technology is the design and preparation of catalysts.noble metal-based catalysts have excellent performance in catalytic activity,stability and tolerance,especially Pd,Pt noble metal-based catalysts,however,with the deepening of research,it has been found that precious metals are easy to sinter and aggregate during use,so that the activity of the catalyst is reduced or even inactivated,which greatly limits its industrial application.By introducing precious metals into the inside of the carrier with rich pore structure,the pores and skeleton of the carrier can be fully utilized to achieve the purpose of inhibiting the sintering and aggregation of precious metals.Silicalite-1 zeolite has a rich microporous pore structure and rigid skeleton,and its excellent hydrothermal stability has also made it used in many catalytic fields,and a variety of effective methods have been developed to realize the encapsulation of precious metal nanoparticles.Herein,the relationship between the structure and properties of the prepared catalyst was systematically studied by designing and preparing a catalyst with an in-situ introduction of the noble metal Pd-base by different encapsulation methods and used for the catalytic oxidation reaction of toluene.The specific research results are as follows:（1）The catalyst of Silicalite-1 zeolite（S-1）encapsulated Pd Co bimetallic nanoparticles（1Pd-y Co O_x@S-1,（y=0,2,4,6））with rich pore structure was successfully prepared by an economical one-pot two-step method.Compared to precursor samples,the catalyst after dry gel conversion treatment has a large specific surface area.The HRTEM image shows that the metal Pd in the 1Pd-4Co O_x@S-1catalyst has a small particle size（2.1 nm）and the metal Pd and Co are uniformly dispersed.The catalytic performance of the prepared catalyst was explored using toluene as a model molecule,and the results showed that the catalytic activity of the catalyst was significantly improved after the dry gel conversion treatment,which was attributed to the increase of the specific surface area that exposed more active components of the catalyst.The results of catalyst tolerance experiments show that1Pd-4Co O_x@S-1 catalyst has excellent cycle stability,long-term stability and water resistance,which is mainly attributed to the rich pore structure that inhibits the sintering and aggregation of precious metals during the reaction,and the shell of the molecular sieve can shield part of the water vapor.The results of in situ DRIFTS showed that the degradation mechanism of toluene on the 1Pd-4Co O_x@S-1catalyst followed the Mv K mechanism,the lattice oxygen species from the Pd Co components participated in the entire oxidation reaction,and the gas-phase oxygen species rapidly replenished and reconstructed the consumed lattice oxygen species through oxygen vacancy defects.（2）Using organic amines as ligand protectors,the catalyst of Silicalite-1 zeolite encapsulated Pd Ag alloy（Pd Ag@S-1）with high crystallinity was prepared by a facile one-pot hydrothermal method.Compared with Silicalite-1 zeolite encapsulated single noble metal catalyst and supported catalyst,Pd Ag@S-1 catalyst can show excellent toluene catalytic performance in a long-term harsh environment due to the strong interaction between Pd Ag components,and the temperature of complete toluene conversion is 180°C.Due to the shielding effect of the molecular sieve shell and the stable alloy structure,Pd Ag@S-1 catalyst exhibits excellent cycle stability,water resistance and high temperature sintering resistance,and can still achieve low-temperature degradation of toluene at high weight hourly space velocity.The results of in situ DRIFTS revealed that the oxidation process of toluene on the surface of the solid catalyst followed the order:benzyl alcohol→benzaldehyde→benzoic acid→maleic anhydride→CO₂ and H₂O,and the rapid breaking of the C=C bond of the benzoic acid species was the rate-determining step of toluene oxidation.