Modifying the Local Environment of Supported Metal Catalysts for the Valorization of Biomass and Renewables-derived Compounds

Abstract

In 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.