(670e) Synthesis and Catalytic Effects of Acid and Metal Siting in Zeolites

Siting of active sites in microporous zeolite crystals has significant ramifications on reactivity, selectivity, and stability in complex reaction systems due to coupled reaction-diffusion and transition state-selectivity phenomena. This is further accentuated in bifunctional catalytic systems where proximity between catalytic functions are also relevant. Encapsulating metal nanoparticles within zeolite micropores is one method to lever acid-metal proximity in addition to maximizing metal surface area and thermal stability. Although techniques such as incipient wetness and ion-exchange can be viable metal encapsulation techniques for some zeolites, universal synthetic strategies applicable to a wide range of zeolites and metal identities remain elusive, especially for highly reducible metals (e.g., Au) and intimately mixed bimetallics (e.g., AuPd). Here, encapsulated metal (Au, Pd, AuPd) in MFI and BEA are prepared via hydrothermal syntheses utilizing sulfur-containing silanes that ligate metal precursors during zeolite crystallization. Zeolite crystallinity is not drastically affected by encapsulated metals, nor the silane ligand as shown via powder X-ray diffraction. Scanning transmission electron microscopy reveals d_STEM ~ 1.5 nm nanoparticles that are alloyed in the case of AuPd nanoparticles, indicated by the absence of the bulk gold localized surface plasmon resonance band in diffusive reflectance UV-vis. Intracrystalline and extracrystalline metal surface areas are determined through oxidation rates of small alcohols (benzyl alcohol) relative to bulky alcohols (1‑pyrenemethanol) that cannot access zeolitic micropores, as well as through oxidation rates in the presence of bulky titrants. Similar reaction-based techniques assess active site location (internal versus surface) in monofunctional Brønsted acid zeolites, which can be used as descriptors for polyethylene conversion. Insights from these evaluations of active site location can be applied to a myriad of catalytic systems to enable understanding of underlying mechanistic phenomena that drive both conversion rates and selectivity, especially in the presence of diffusion limitations.