Reactivity Aspects of SBA15-Based Doped Supported Catalysts: H₂O₂ Direct Synthesis and Disproportionation Reactions

Pd and PdAu catalysts supported on SBA15 and SiO₂ were prepared and investigated for H₂O₂ direct synthesis in a batch autoclave (10 °C and 17.5 bar) and in the absence of halides and acids. The SiO₂ supported catalysts exhibited inferior performances compared to the mesoporous ordered SBA15. A good control of both the catalysts dispersion and nanoparticle stability was achieved using SBA15. Catalysts were doped with bromine, a promoter in the H₂O₂ direct synthesis. Productivity and selectivity decreased when bromine was incorporated in the catalysts, thus indicating a possible poisoning due to the grafting process. A synergetic effect between Pd and Au was observed both in presence and absence of bromopropylsilane grafting on the catalyst surface. Three modifiers of the SBA15 support (Al, CeO₂ and Ti) were chosen to elucidate the influence of the surface properties on metal dispersion and catalytic performance. Higher productivity and selectivity were achieved incorporating Al into the SBA15 framework, whereas neither Ti nor CeO₂ improved H₂O₂ yields. The enhanced performance observed for the Prau/Al–SBA15 catalysts was attributed to the increased number of Brønsted acid sites. A modification of this catalyst with bromine was confirmed to impair both productivity and selectivity, possibly due to the broader particle size distribution and the poor stability of the metal nanoparticles, as demonstrate by transmission electron microscopy (TEM) images. H₂O₂ disproportionation was also investigated. A much slower reaction rate was observed compared to the H₂O₂ production, suggesting that the major contributor in the process of H₂O₂ destruction must be connected to the hydrogenation reaction.