(581c) Synthesis and Performance of Encapsulated Au/Pd Nanoparticles in Zeolites for Oxidation Catalysis

To circumvent energy-intensive syngas-mediated methane (CH₄) upgrading routes to liquid chemicals, CH₄ partial oxidation (MPO) to methanol with subsequent methanol-to-hydrocarbons (MTH) presents as an enticing pathway. However, MPO with O₂ often leads to overoxidation to CO and CO₂. Gold-palladium (AuPd) nanoparticles are promising catalysts for CH₄ upgrading, as they are active for MPO by hydrogen peroxide (H₂O₂) and H₂O₂ synthesis from H₂ and O₂. Encapsulation of AuPd metal within microporous zeolite frameworks is attractive for maximizing metal surface area and enabling tandem reactions (MTH) on zeolitic acid sites, but requires development of synthesis strategies for zeolite-encapsulated AuPd nanoparticles. Here, encapsulated metal in MFI and BEA (Au, Pd, AuPd@Zeolite) are prepared via direct hydrothermal syntheses utilizing 3-mercaptopropyl trimethoxysilane to ligate metal precursors during zeolite crystallization. These samples are compared with metal supported post-synthetically on BEA zeolite. Powder X-ray diffraction indicates that encapsulated metal nanoparticle-zeolite catalysts are well-crystallized, and transmission electron microscopy reveals well-dispersed nanoparticles (d_TEM = 1-3 nm). Further, diffuse reflectance UV-vis suggests a prevalence of alloy AuPd nanoparticles, as the localized surface plasmon resonance band indicative of bulk gold (~500 nm) is absent from AuPd@Zeolite spectra. Encapsulation efficiencies were measured through oxidation rates of small alcohols relative to bulky alcohols that cannot access zeolitic micropores as well as small alcohol oxidation in the presence of bulky titrants. In MPO experiments, BEA-supported Pd catalysts exhibited enhanced selectivity toward methanol versus Au, but unproductive H₂O₂ disproportionation also increased. Relative to Au and Pd monometallic catalysts, the bimetallic AuPd catalyst yielded increased selectivity to methanol, which is attributed to Au-Pd electronic interactions mitigating overoxidation of liquid oxygenates. Finally, Brønsted acidic HBEA supports inhibit unproductive H₂O₂ decomposition over NaBEA analogs. Insights from these tailored catalyst syntheses can be applied to other metals and zeolite frameworks for more precise syntheses of thermally stable bifunctional catalysts.