UC Davis

When supported metal catalysts contain metal components existing at or near atomic dispersion, the support surface largely controls the properties of the highly dispersed metal species by determining the local chemical bonding environments of the metals. The framework structures of zeolites afford unique bonding environments for supported metals which can result in catalysts having unusual properties. The research described in this dissertation was aimed at identifying potential advantages of using zeolites as supports for catalysts containing chromium or platinum ‒ two industrially applied catalyst metals whose intra-zeolite chemistries are not fully resolved in the literature. Chromium was dispersed on HZSM-5. Samples were characterized using X-ray absorption near edge structure (XANES) and infrared (IR) spectroscopies and evaluated for ethane dehydrogenation. At low chromium loadings, chromium was located at zeolite aluminum sites and Cr/HZSM-5 samples displayed stable ethane dehydrogenation activity with time on stream. Higher chromium loadings resulted in catalysts with higher dehydrogenation activity per chromium atom but that deactivated quickly, and this was correlated to higher fractions of electron-rich or multinuclear chromium present in these samples. The results represent an attempt to assess the potential for catalytic application of Cr/ZSM-5, taking into account the speciation of chromium among various anchoring sites on the zeolite surface. Platinum was dispersed onto HZSM-5 and characterized using X-ray absorption and IR spectroscopies. During exposure of Pt/ZSM-5 to high-temperature, oxidizing conditions, Pt2+ ions were stabilized at six-membered rings in the zeolite that contained paired-aluminum sites. This interpretation was informed by a theory-guided analysis of X-ray absorption fine structure spectroscopy (EXAFS) data. These Pt2+ ions formed highly uniform platinum gem-dicarbonyls, and the steps leading toward formation of platinum clusters were monitored through the evolution of IR spectra during exposure of platinum carbonyls to reducing conditions. Platinum clusters in HZSM-5 were redispersed into Pt2+ cations under high-temperature, oxidizing conditions, with the Pt2+ cations returning to paired-aluminum, six-membered ring sites. Similar platinum gem-dicarbonyl complexes formed in several commercially used zeolites (ZSM-5, Beta, mordenite, and Y), demonstrating the generality of the chemistry across zeolite frameworks. The findings connect catalyst structural properties to critical performance outcomes for an industrially-relevant catalyst material system. Chromium was dispersed onto a series of MFI zeolites with various support compositions. The catalysts were characterized by IR or X-ray absorption spectroscopies and evaluated for ethane dehydrogenation with or without CO2. The copresence of Cr2+ and Cr3+ in siliceous or borosilicate MFI zeolites was correlated with significant enhancements in the rates ethane dehydrogenation when CO2 was added to the reaction mixture. The aluminosilicate MFI zeolite, in contrast, stabilized chromium in the +2 oxidation state during reaction, resulting in a catalyst that exhibited low rates of CO2 reduction to CO by H2 and no enhancement of ethane dehydrogenation by CO2. The mechanistic role of CO2 is discussed in the context of ethane dehydrogenation with Cr/MFI catalysts. Additional experiments characterizing Cr/zeolite or Pt/zeolite samples provided insights into the local environments of the supported metals. Platinum carbonyl complexes in HZSM-5 and Y zeolite were characterized by XAS in order to complement the results of IR spectroscopy. Similar chromium or platinum species existed in zeolites of identical framework structure but different heteroatom identity. Supported chromium or platinum species similar to those found in HZSM-5 were found to exist in zeolites other than HZSM-5.