UC Davis

Zeolites are commonly used as a support for transition metal catalysts due to their high surface area, ability to stabilize isolated cations, and porous structure. The basic building block of a zeolite is the TO4 tetrahedra where T is usually Si4+ but can be substituted with other elements such as Al3+, Fe3+, and B3+. The charge disparity that results from these substitutions creates the need for an extraframework cation that can be a transition metal. The compositional flexibility of certain zeolite frameworks, such as BEA and MFI, make them a good candidate to use a modular support whose elemental composition can be modified without changing the crystallographic structure of the support. The research in this dissertation was focused on using the flexibility in elemental composition of the BEA zeolite framework to tune the properties of a supported metal. The electron density, as well as reactivity, of a supported metal catalyst can be influenced by changes in zeolite elemental composition. This approach resulted in an increased understanding of how zeolite composition influences highly dispersed, supported metals. Zeolite Beta (BEA framework) supported Ni catalysts were synthesized and tested for catalytic activity using ethylene dimerization. The naming convention that will be used is M-[X]-Beta, where M is the extra-framework cation and X is the heteroatom composition. The catalysts were characterized using Fourier Transform Infrared Spectroscopy (FTIR) with various probe molecules, N2 adsorption, x-ray diffraction (XRD), catalysis, and thermogravimetric analysis (TGA). Ni was dispersed onto H-[X]-Beta (X = Al, Ga, and Fe) via anhydrous deposition by using n-pentane solvent and Ni(acac)2 as the metal source. N2 adsorption, as well as XRD, demonstrated that there is no significant change to the crystallinity or pore structure of the zeolite after Ni deposition. CO and NO adsorption onto the cationic Ni sites showed the presence of Ni in extraframework exchange positions and the relative electron density of the Ni cation increases in the order Ni-[Fe]-Beta > Ni-[Ga]-Beta > Ni-[Al]-Beta. C2H4 adsorption shows that the Ni cations on Ni-[Ga]-Beta and Ni-[Al]-Beta have alkyl ligands bonded to them of various length, while after exposure to C2H4 the adsorbed species on Ni-[Fe]-Beta are butenes. As C2H4 dimerization catalysts, operating at 180 °C and 2.16 kPa C2H4, the activity towards the formation of butene is in the order: Ni-[Fe]-Beta > Ni-[Ga]-Beta > Ni-[Al]-Beta. The activation energy was measured to be 44.8 kJ/mol and 32.4 kJ/mol for Ni-[Al]-Beta and Ni-[Fe]-Beta respectively, indicating that Ni-[Fe]-Beta is more effectively stabilizing the TOF determining transition state. Ni was atomically dispersed into the vacant silanol nests of dealuminated Beta zeolite and used as a C2H4 hydrogenation catalyst. NH4-[Al]-Beta was dealuminated with HNO3 to create silanol nests, then Ni was deposited using Ni(acac)2 in n-pentane. After evacuation of the solvent the sample was calcined in air to remove the acac ligands. The Ni sites were probed by solid-state FTIR with CO and NO adsorption, which confirmed the presence of cationic Ni2+ in silanol nests. X-ray Absorption Spectroscopy (XAS) of the oxidized Ni-[DeAl]-Beta was used to determine the geometry and average local environment of the Ni sites. Wavelet analysis of Ni-[DeAl]-Beta and NiO was used to show the absence of Ni – Ni scattering in the oxidized Ni-[DeAl]-Beta extended x-ray absorption fine spectrum (EXAFS). The pre-edge feature of the XANES region suggested that Ni is in a tetrahedral geometry. EXAFS fitting found a Ni – O coordination number of 4 and Ni – Si coordination number of 4, consistent with Ni dispersed into silanol nests. Ni-[DeAl]-Beta catalysts were activated by reduction in 10% H2 at 300°C, then used for C2H4 hydrogenation. XANES after reduction shows approximately 50% of Ni sites are reduced to a metallic state. EXAFS analysis shows Ni metallic clusters of approximate 1 nm in size based on Ni – Ni coordination numbers. STEM images also indicate that there is a lack of large Ni clusters (> 1 nm) after reduction of Ni-[DeAl]-Beta. Ni-[DeAl]-Beta was found to be 20-fold more active than Ni-[Al]-Beta and 2.9-fold more active than NiO-SiO2 as a C2H4 hydrogenation catalyst. The work in this dissertation furthers knowledge in the field of heterogenous catalysis forward by demonstrating how zeolites can be used as a modular support for transition metals. The compositional flexibility in zeolite Beta is leveraged to influence the active metal site without changing the crystallographic structure of the support. The fine control over the electronic properties and geometry of active Ni sites represents progress towards more creating more tunable sites for a heterogenous catalyst.