Saudi Cultural Missions Theses & Dissertations
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Item Restricted Microwave Assisted Synthesis of Au/TiO2 Catalyst for Photocatalysis and Oxidation Processes(Cardiff University, 2024) Alazmi, Abdullah Mubarak S; Edwards, JenniferHeterogeneous catalysis using supported metal nanoparticles has attracted tremendous interest for applications including photocatalysis and selective oxidations. This work focuses on synthesis and characterization of Au nanoparticles supported on TiO2 (Au/TiO2), which exhibits unique photocatalytic and catalytic properties arising from quantum and plasmonic effects of Au as well as strong metal-support interactions. A facile microwave-assisted polyol approach was used to prepare Au/TiO2 with 0.1-3 wt% Au loading. Structural characterization by XRD revealed anatase TiO2 support was retained after Au deposition. Au particle size increased from sub nm clusters at 0.1 wt% to 13.5 nm nanoparticles at 3 wt% loading, demonstrating tunable nanoparticle dimensions. CO oxidation testing showed highest mass activity for 0.5 wt% Au/TiO2, with declining performance at higher loadings attributed to loss of low-coordination sites through agglomeration. Photocatalytic degradation of methyl orange dye was most efficient with 0.5 wt% Au/TiO2 under simulated solar irradiation. The higher photoactivity is ascribed to improved visible light harvesting and minimized charge recombination from uniform deposition of small plasmonic Au nanoparticles. The results demonstrate optimized Au loading and dispersion governs both catalytic and photocatalytic efficiency of Au/TiO2 synthesized via microwave polyol method.29 0Item Restricted Modifying the Local Environment of Supported Metal Catalysts for the Valorization of Biomass and Renewables-derived Compounds(University of Colorado Boulder, 2024) Al-Khulaifi, Faysal; Medlin, J Will; Holewinski, AdamIn this thesis, we investigate the modification of supported metal catalysts for the biomass upgrading of model compounds in the liquid phase via two methods: either by using thiolate self-assembled monolayers (SAMs) or by alloying Au host metals with dilute dopant metals to form bimetallic catalysts. The aim is to gain a better understanding of the catalytic activity and how it is influenced by these catalyst modifications. In the first study, we investigated the liquid-phase partial oxidation of glutaraldehyde (GA) to glutaric semialdehyde (GSA) as a model reaction using Pd/Al2O3 as catalyst. The Study explored the impact of various thiolate SAM modifiers on the catalyst, with and without reductive pretreatment. The treated catalyst exhibited enhanced activity and a reduced apparent induction period compared to the untreated catalyst. The selectivity for GSA was influenced by the identity of the SAM layers, showing that hydrophilic coatings such as thioglycerol (TG) and thiolactic acid (TLA) enhanced selectivity to GSA. The TG coating increased selectivity from approximately 60% (on uncoated and hydrophobic thiol-coated catalysts) to about 80% at equivalent conversion (~30%). However, the TG-coated catalyst showed lower catalytic rates compared to uncoated (UC) and hydrophobically coated materials. Various materials characterization techniques ruled out morphological and electronic causes. Density functional theory (DFT) calculations suggested that hydrogen bonding contributed to the stabilization of GSA adsorption on the Pd surface compared to the uncoated surface. DFT predicted a significant stabilization of glutaric acid (GAC) adsorption for TG-coated Pd, potentially resulting in the accumulation of this overoxidation product and lower overall rates. Experimental evidence corroborated the inhibition of the rate of GA partial oxidation by GAC adsorption. The applicability of using near-surface non-covalent interactions between thiol ligands and reactants was further explored in the second study. The investigation focused on the liquid-phase oxidation of cinnamyl alcohol (COH) to cinnamaldehyde (CAD) using Pd/Al2O3 modified with different aromatic thiol SAMs. The aim was to examine the impact of pi stacking between the aromatic moiety of the thiol and surface reaction intermediates. Under the specified reaction conditions, the primary products identified were CAD, methylstyrene (MS), and 3-phenyl-1-propanol (PHP). The incorporation of aromatic thiols on the catalyst led to an increased yield of CAD, with the highest yield observed when employing 3-phenylpropane-1-thiol (3-PT), a thiol featuring a three-carbon atom spacer between its head sulfur atom and its benzene ring. Alkyl spacer length did not significantly affect the yields of MS and PHP. However, MS exhibited notably higher yields with thiol coatings compared to the uncoated catalyst, suggesting that thiol coatings discourage flat-lying adsorption orientations associated with PHP formation. Control reactions and temperature programmed desorption (TPD) experiments indicated that enhancements in CAD yield were likely attributed to pi interactions introduced by modifying the Pd catalyst with aromatic thiols. In the third study, we investigated the liquid-phase crotonaldehyde (CrA) hydrogenation reaction, utilizing synthesized dilute bimetallic alloys where Au serves as the host, doped with dilute amounts of Ni, Pd, and Pt at varying metal weight loadings and ratios. Catalysts with 1wt% (comprising 1% dopant and 99% Au) and 10wt% (5% dopant and 95% Au) were tested under optimized reaction conditions to explore their reactivity and selectivity. CrA hydrogenation finds wide applications in the synthesis of fine chemicals and pharmaceuticals. The reaction can take two primary pathways. Hydrogenation of either the C=C or the C=O bond in CrA can result in the formation of butanal (BuA) or the more challenging crotyl alcohol (CrOH), respectively. Both molecules can undergo further hydrogenation to produce butanol (BuOH), a product of significant importance in the liquid fuel industry, by reducing their remaining unsaturated double bond. The study findings revealed that all alloyed catalysts exhibited enhanced rates compared to the pure Au catalyst, with the most significant improvements observed in the case of the 10wt% catalysts. However, evidence of crotyl alcohol (CrOH) formation was only noted in the 10wt% AuNi catalyst and the 10wt% AuPt catalyst (≤ 2% selectivity). Despite the low selectivity, these results align with previous DFT calculations and literature reports, indicating that CrA adsorption orientation facilitates the pathway toward CrOH. Alloying Au in this context created an environment that not only significantly enhanced rates compared to the monometallic case but also allowed for the possibility of sampling a different reaction pathway.9 0Item Restricted Selective dehydroisomerisation of cyclic monoterpenes to p-cymene over bifunctional metal-acid catalysts(Saudi Digital Library, 2023-10-02) Alsharif, Aliyah; Kozhevnikov, IvanThe dehydroisomerisation of α-pinene and limonene, naturally occurring abundant monoterpenes, is a clean sustainable route to produce p-cymene, an important intermediate in organic synthesis and ingredient in cosmetics and medicinal products. Currently, p-cymene is produced in a mixture with o- and m-isomers by the Friedel-Crafts alkylation of toluene with propene, followed by isomer separation, with an adverse effect on the environment. The aim of this work is to study the dehydroisomerisation of cyclic monoterpenes, such as α-pinene, β-pinene, limonene, α-terpinene, γ-terpinene and terpinolene, in the gas phase using silica-supported ZnO and CdO as new bifunctional metal-acid catalysts. It is demonstrated that all these monoterpenes can be converted to p-cymene with excellent yields of 90–100% using ZnO/SiO2 and CdO/SiO2 as the catalysts. The catalysts were prepared by wet impregnation of silica with metal nitrates from an aqueous solution followed by drying and calcination at 400–500 oC and characterised by BET, TGA, XRD, DRIFTS, H2-TPR and ICP-OES. The dehydroisomerisation reactions were carried out in a continuous flow fixed-bed microreactor with online GC analysis. It was found that dehydroisomerisation of α-pinene over ZnO/SiO2 produces p-cymene with 90% yield at 100% conversion at 370 oC and WHSV = 0.020 h-1. The reaction with limonene gives a 100% p-cymene yield at 325 oC and WHSV = 0.080 h-1. ZnO/SiO2 catalyst shows stable performance for over 70 h without co-feeding hydrogen to the reactor. The reaction over silica-supported ZnO catalysts with β-pinene produces a 100% p-cymene yield at 400 oC and WHSV = 0.080 h-1. The reaction with monocyclic terpenes such as limonene, α-terpinene, γ-terpinene and terpinolene gives a 100% p-cymene yield at 300–325 oC and WHSV = 0.16–0.08 h-1. The dehydroisomerisation of bicyclic monoterpenes, such as α-pinene and β-pinene, over CdO/SiO2 gives 91–95% p-cymene yields at 325–375 oC and WHSV = 0.010–0.020 h-1, whereas the more reactive monocyclic terpenes, such as limonene, α-terpinene, γ-terpinene, and terpinolene, give a 100% yield at 200–250 oC and WHSV = 0.040–0.080 h-1. This catalyst shows stable performance for over 25 h without co-feeding hydrogen. To the best of our knowledge, CdO/SiO2 has the highest efficiency in monoterpene-to-p-cymene dehydroisomerisation among the catalysts reported to date. The proposed mechanism of monoterpene dehydroisomerisation to p-cymene on bifunctional ZnO/SiO2 and CdO/SiO2 catalysts involves two steps: fast isomerisation of monoterpene reactant on Brønsted acid sites (silanol groups of silica support) to form p-menthadiene intermediates followed by their slow dehydrogenation on oxo-metal sites to p-cymene. The dehydrogenation is suggested to proceed through the abstraction of allylic hydrogen from the substrate by an oxo-metal site followed by the elimination of another hydrogen atom to form p-cymene π-bonded to metal ion (Zn(II) or Cd(II)). Then, the elimination of the p-cymene molecule and H2 closes the catalytic cycle.13 0