UC Davis

Supported single-site metal catalysts that have nearly uniform structures are drawing increasing attention because of prospects for increased application and elucidation of structure-catalytic activity relationships. The research summarized in this dissertation was carried out with the goal of fundamental understanding of the structures, reactivities, and catalytic properties of highly uniform solid catalysts with well-defined structures. Catalysts were synthesized by using organometallic complex precursors, Rh(C2H4)2(acac) or Pt(acac)2 (where acac is acetylacetonato), and the porous crystalline material SAPO-37 as a support. The samples were characterized by infrared (IR) spectroscopy, X-ray absorption spectroscopy (including X-ray absorption near edge structure and extended X-ray absorption fine structure (EXAFS) analysis), X-ray diffraction (XRD) crystallography, N2 adsorption, and thermogravimetric analysis (TGA). The catalytic performance was measured for ethylene hydrogenation. SAPO-37-supported rhodium diethylene complexes formed in the synthesis were anchored by two Rh−O bonds at SAPO framework tetrahedral sites, as shown by IR and EXAFS spectra. The ethylene ligands were readily replaced with CO, giving sharp νCO bands indicating highly uniform supported species. Comparing the spectra with those of rhodium complexes on zeolite HY shows that the SAPO- and zeolite-supported complexes are isostructural. The two catalysts had similar initial room-temperature activities per Rh atom for ethylene conversion in the presence of H2, but the SAPO-supported catalyst was selective for ethylene hydrogenation and the zeolite-supported catalyst selective for ethylene dimerization; correspondingly, the catalyst on the SAPO was more stable than that on the zeolite during operation in a flow reactor. These results show how isostructural SAPO-37 and zeolite HY are similar in structure yet different in reactivity, corresponding to the different bonding environments for rhodium. SAPO-37-supported rhodium clusters were synthesized by exposure of SAPO-37-supported rhodium diethylene complexes to H2 at 373 K for 1 h. EXAFS data indicate an average Rh–Rh coordination number of 3.0 and an average Rh–Rh distance of 2.66 Å. XANES spectra show that the transformation of rhodium diethylene complexes to rhodium clusters is stoichiometrically simple. At 303 K, the SAPO-37-supported rhodium clusters are similar in selectivity for hydrogenation of ethylene (fed to a flow reactor with a H2:C2H4 molar ratio of 1:4) to DAY zeolite-supported rhodium clusters but their initial room-temperature activity per rhodium atom was found to be substantially less than that of the zeolite-supported rhodium clusters. Atomically dispersed supported platinum catalysts were synthesized by the reaction of Pt(acac)2 with SAPO-37. EXAFS spectra show that, after heating in air to 623 K, each platinum atom on average was bonded to approximately four light scatterer atoms (such as support oxygen atoms), with no evidence of a Pt–Pt contribution that would have indicated platinum clusters. XANES data indicate a platinum formal oxidation state of +2. IR data show that, upon exposure of the sample to CO, the non-support ligands on the platinum were CO in various coordinations, with platinum in various oxidation states. The supported platinum was characterized as a catalyst for ethylene hydrogenation. Within a few minutes of the start of flow of ethylene + H2, the EXAFS-determined Pt−Pt coordination number increased from essentially zero to 1.8 ± 0.4; the XANES white line intensity decreased; and, after 2 h of continuous reactant flow, the value had increased to 2.7 ± 0.5 as the XANES white line intensity slightly increased—with these data taken together indicating the almost instantaneous formation of platinum clusters of only a few atoms each (with average diameters in the range of about 0.4–0.8 nm). Subsequent exposure of the catalyst to flowing ethylene led to a decrease in the Pt−Pt coordination number to 1.6 ± 0.3 and an increase in the XANES white line intensity, indicative of partial oxidative fragmentation of the clusters by ethylene. Platinum clusters in SAPO-37 that were formed by exposure to H2 prior to catalysis were found to be catalytically active for ethylene hydrogenation, and a comparison of the activities of the catalyst initially containing atomically dispersed platinum versus that containing platinum clusters as a function of time on stream leads to the inference that the clusters are the catalytically active species, with no evidence of catalysis by the atomically dispersed platinum.