Zeolite Encapsulation Of Sub-nano Pd For Direct Synthesis Of Hydrogen Peroxide

Hydrogen peroxide as a clean and strong oxidant was one of the commonly used chemicals in various fields of chemical industry.More importantly,it can be applied to the synthesis and degradation of pesticides and has excellent prospects in the field of environment protection.Compared to the traditional anthraquinone process,the direct synthesis of hydrogen peroxide（DSHP）from hydrogen and oxygen with advantages of cleanliness and low consumption was a potential alternative.In DSHP process,Pd supported catalysts was a highly active DSHP catalyst.Theoretical studies and experimental results indicated that subnano Pd particles possess the best performance in direct synthesis of hydrogen peroxide,and the activity and selectivity of hydrogen peroxide were also affected by the valence state of Pd particles.However,sub-nano Pd particles were prone to agglomerate during the regulation of Pd valence,which restricts its catalytic performance.In addition,because of the internal configuration diffusion limitations of reactants and products in micropores,the reaction rate was slow,and the residence time of H₂O₂ was too long to decompose.In this paper,the introduction of mesopore in microporous zeolite can improve yield and selectivity of H₂O₂.In this parper,subnano-size Pd particles were encapsulated within titanium silicate voids via the mercaptosilane-assisted dry gel conversion（DGC）synthesis method,and the regulation of the valence state of Pd particles was achieved by sequent heat treatment.These catalysts were used for direct synthesis of hydrogen peroxide catalytic reaction and its in-situ selective oxidation reaction.The effects of Pd metal particle size,valence state and confinement distribution were studied,and catalytic performance was correlated with the catalyst characterization results.The main conclusions and results of this parper:（1）Encapsulated metal catalysts generally had excellent shape-selective catalytic properties.In the hydrogenation experiment of a mixture of nitrobenzene and1-nitronaphthalene,Pd@HTS-1 can hydrogenate the two to corresponding amine,and the hydrogenation rate of nitrobenzene was 16 times that of nitronaphthalene,which proved the encapsulation of Pd particles and its excellent shape-selective catalytic properties.（2）The encapsulated Pd@TS-1 and Pd@HTS-1 catalysts had excellent thermal stability resulted from their structural characteristics.After oxidation-reduction-reoxidation three sequential treatments,TEM images indicated that Pd particles did not grow significantly and remained in sub-nanometer level,while particles on Pd/HTS-1 prepared by impregnation method increased obviously.Meanwhlie,the change of Pd valence state after heat treatments(Pd²⁺（1）Pd⁰（1）Pd²⁺+Pd⁰)was detected by XPS characterization.（3）After heat treatments,the dual-encapsulation of active Pd⁰ by PdO and TS-1 was achieved,and the catalysts achieved highest hydrogen peroxide yield(35010 mmol·g_Pd^-1·hour^-1)and selectivity（48.35%）.In addition,the productivity of hierarchical Pd@HTS-1 increased by nearly 10 times compare to microporous Pd@TS-1,which proved that the hierarchical pore structure influenced the diffusion efficiency and the hydrogen peroxide yield.（4）Hydrogen peroxide synthesized by hydrogen and oxygen could be used as an oxidant to selectively oxidate benzyl alcohol to benzaldehyde with 100%selectivity.However,most of H₂O₂ has not be consumed,and its conversion rate was only about 1%,which was attributed to the low temperture of direct synthesis of hydrogen peroxide.This in situ oxidation of hydrogen peroxide demonstrates its possibility and potential in applications of organic synthesis such as pesticides,and also provides a new idea for the synthesis of benzaldehyde.