SiO2 deposition on metal oxides to tune acid catalyzed reaction behavior

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Mixed oxides including SiO2', 'and supported oxides on SiO2', 'comprise the majority of', 'heterogeneous catalysts employed in the chemical industry. SiO2', '-Al2O3', 'and microporous,', 'crystalline variants called zeolites have been used widely since the 1960s in petroleum', 'processing, commodity chemical production, and fine chemical synthesis [1]. SiO2', 'supported', 'TiOx catalysts TS-1 and nonporous Ti-SiO2', 'were patented in the 1970s and have been used since', 'in alkene oxidation and ketone ammoxidation processes [2-3]. Regardless of catalyst type, the', 'prevailing synthetic approach has been to rely on black box sol-gel processes to synthesize the', 'catalyst structure and active sites in one pot. This dissertation supports an alternative approach', 'where sol-gel deposition of SiO2', 'is used to tune active site identity and sterics while confining', 'catalytic sites to the external surface of the catalyst.', 'Preliminary work was conducted to develop a controlled, tunable, liquid phase SiO2', 'deposition technique on bulk metal oxides. NanoDur Al2O3', 'and Evonik P25 TiO2', 'were used as', 'example nonporous substrates for SiO2', 'deposition. The deposition technique was found to result', 'in ~0.4 nm/cycle SiO2', 'growth, conformal to the surface of the support [4]. This value can be', 'brought down to the expected monolayer deposition rate of 0.3 nm/cycle by adding TEOS/cycle', 'according to its kinetic diameter. Grafting of a carbonaceous template, 4-tert-Butylcalixarene,', 'before SiO2', 'deposition allowed for nanocavity formation in the resulting overcoat. Higher', 'external surface area was observed, which corresponds to nanocavity walls. A high heating ramp', 'rate during calcination posttreatment was found to impart microporosity on the overcoat [4].', '2', 'Then, the SiO2', 'deposition technique was extended to supported Lewis acid catalysts to', 'enact confinement around external surface sites. Liquid phase limonene oxidation was identified', 'as a reaction that probes for acid site strength and steric effects. The reaction network and', 'kinetics were explored using Ti, Nb, and Ta supported on wide pore SiO2', '. Calixarene and other', 'bulky templates Cp and Cp* were used to synthesize predominantly site isolated,', 'extraframework O3MO(H) on SiO2', 'using a surface loading of ~0.20 M/nm2', '. In a test of oxidation', 'activity, direct selectivity, and external regioselectivity, the use of Ti-SiO2', 'with anhydrous tert', 'butyl hydroperoxide (TBHP) showed promise with high activity (58 hr-1', '), direct selectivity (95', 'mol %), and low external regioselectivity (25 mol %) to limonene 8,9-oxide. I hypothesized that', 'templated SiO2', 'deposition on Cp*Ti-SiO2', 'would increase oxidation rate and regioselectivity.', 'An in depth investigation was conducted using SiO2', 'overcoated Ti-SiO2', 'along with', 'conventional mesoporous material Ti-SBA-15 and zeolite Ti-Beta. These 7 catalysts seemed to', 'possess nominally similar TiOx sites because their DRUV-vis spectra and Ti K-edge XANES', 'spectra result in similar indirect edge energies (3.5-4.5 eV) and average coordination numbers', '(4.2-4.6). However, their limonene oxidation activity spans a wide range of values between', '14-60 hr-1', '. In-situ titration with phosphonic acids per prior work from the group did not resolve', 'differences in oxidation rate. Then, limonene adsorption behavior was assessed using a literature', 'GCMS SIM mode analysis which tracked ions with m/z = 68, 93, and 67 [5]. One SiO2', 'overcoated Ti-SiO2', 'material was found to strongly bind limonene (ΔHads', '= -39 kJ/mol) and thus', 'have the lowest apparent activation enthalpy (9 kJ/mol). It was the top performer under industry', 'conditions which require large catalyst particle sizes and low processing temperatures. These', 'materials can find future use in contaminant removal and conversion of bulky reactants.

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  • 10/08/2019
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