A Multifaceted Photocatalytic Exploration: From Alkyne Semi-Hydrogenation to Plasmonic Nanostructures
Creators: name: Cely Pinto, Melissa Julieth
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2024
Subjects: Photocatalysis
Catalysts
Light
Nanostructures
Heterogeneous Catalysis
Nanomaterials

description: In this dissertation, we summarize the effect of metal nanoparticles (NPs) and metal nanoparticle-decorated titanium dioxide (M@TiO₂) materials in processes such as photoenolization, and semi-hydrogenation of alkynes. We also studied the photochemistry of substituted aromatic ketones such as tetralones and capping ligands such as lipoic acid for the synthesis of metal nanostructures. The primary objective of this multifaceted research was to identify optimal conditions and mechanisms that enhance both reaction efficiency and the fast synthesis of nanostructures. By focusing on molecules that can revert to their original state, we aim to increase their availability for further chemical reactions. This work also contributes to the development of more effective catalytic systems, facilitating innovative applications in organic synthesis. Furthermore, our research addresses practical challenges faced at the laboratory level, where students often find difficult to monitor reactions in a timely manner. Timely monitoring is crucial for successful organic transformations, and photocatalysis offers a significant advantage by allowing easy control of the reaction environment through the simple on/off manipulation of light, since light serves as the initial factor that initiates the reactions. This approach not only enhances reaction management but also leads to the development of cost-effective catalysts that can serve as an alternative materials, reducing reliance on those that are currently depleting, such as palladium. Firstly, we investigated the photoenolization process as a convenient driver for the synthesis of gold (Au) nanostructures using the substituted ketone 3,3,6,8-tetramethyl-1-tetralone which undergoes photoenolization to produce a photoenol excited state with a lifetime of around~3 μs, which involves the carbonyl triplet state of the ketone (τ ~1.9 ns), as a precursor. For this, we used the laser flash photolysis (LFP) technique to evaluate the absorbance signal over time after the excitation of the sample with a laser pulse. The change in absorbance can result in decay or growth signals from transient species created in the sample from the laser excitation and can be monitored through time leading us to conclusions on the structure of the species. In this case, we monitored the excited states of the photoenol and the ketone itself. In the case of excited photoenol like the one from the selected tetralone, if metal ion trapping fails, it returns to the original ketone precursor and remains available for future events that can lead to the synthesis of target nanoparticles (NPs). This study also includes the characterization of the photochemistry of the substituted tetralone, and the dual behaviour of reaction intermediates, as biradicals and excited states, in energy and electron transfer processes. Furthermore, we studied the photopolymerization of α-lipoic acid (LA) as a novel approach to produce a cross-linked polymeric matrix of lipoic acid monomers (PALA) which helps to control the size of plasmonic gold nanostructures when using the same substituted ketone 3,3,6,8-tetramethyl-1-tetralone, previously mentioned, as the photo-initiator for the reduction of Au(III) to Au⁰. A complete characterization of the polymer is included, and the dual behaviour of LA as an in-situ stabilizer for the nanostructures and reducing agent is investigated. These findings are relevant to the understanding of the photochemical transformation of this biologically relevant compound and would benefit the increasing use of LA and PALA for the synthesis of various nanomaterials. Likewise, to further explore the role of photocatalysts in organic chemistry processes, we explored the semi-hydrogenation reaction of alkynes. This reaction is particularly important in the fine chemicals and pharmaceutical industries, and it is thus important to find catalytic processes that will drive the reaction efficiently and at a low cost. The real challenge is to drive the alkyne-to-alkene reaction while avoiding over-hydrogenation to the saturated alkane moiety. The problem is more difficult when dealing with aromatic substitution at the alkyne center. M@TiO₂ materials were used in this research as heterogeneous catalysts, offering the advantage of an ease-catalyst separation. Particularly, systems such as Pd@TiO₂, Cu@TiO₂ and CuPd@TiO₂. Photocatalysts based on Palladium (Pd) tend to proceed to the alkane moiety, and stopping at the alkene with good selectivity requires very precise timing with basically no timing tolerance when the reaction times are measured in hours or overnight. However, the goal of high conversion with high selectivity could be achieved with TiO₂-supported copper (Cu@TiO₂), although with slower kinetics than for Pd@TiO₂. Therefore, by combining the features of both metals in one catalyst, we discovered that the novel bimetallic catalyst, namely, CuPd@TiO₂ (0.8% Cu and 0.05% Pd), could improve the kinetics by 50% with respect to Cu@TiO₂, while achieving a selectivity over 95% and with exceptional timing tolerance when methanol was used as the hydrogen source in the system. Further, the low Palladium content minimizes its use, as Palladium is regarded as an element at risk of depletion. Additionally, in order to facilitate catalyst separation from batch reaction and develop a suitable catalytic system we employed a magnetic catalyst named Fe₃O₄@TiO₂ to explore the synthesis of Schiff Bases from Benzyl Alcohol and Nitrobenzene. Schiff bases are condensation products of primary amines with carbonyl compounds like aldehydes or ketones and are extensively used in different fields. They serve multiple purposes such as dyes, catalysts, and intermediate species in organic synthesis. They have also demonstrated diverse biological activities including antibacterial and antifungal activities. In this study, we compared the efficiency of the magnetic catalyst with respect to Pd@TiO₂, Cu@TiO₂, and TiO₂ under ultraviolet light (UV) and inert atmosphere. Finally, moving into another technique of catalysis, we investigated the mechanistic and kinetic studies of polarity reversal catalysis involving amino-boranes compounds that serve as catalysts generating nucleophilic radicals that can abstract hydrogen from electron-poor substrates such as nitriles. This ongoing research allows us to form electrophilic radicals from electron-deficient molecules. By using LFP we have successfully monitored both radicals from these molecules and boryl radical species.