Design and Application of Layered Double Hydroxide Catalysts for Tandem Transformations
Open Access DepositedThe principles of green chemistry have increasingly guided the development of more sustainable and efficient chemical processes. Among these principles, catalysis plays a pivotal role by enabling enhanced reactivity and selectivity while minimizing waste and energy input. This work focuses on the design and application of heterogeneous catalysts, specifically palladium (Pd) supported hydrotalcites (HTs), as multifunctional platforms for both high-value synthetic transformations and the valorization of renewable feedstocks.We present a reliable one-step synthetic protocol for preparing Pd-supported hydrotalcite (Pd/HT) catalysts, utilizing an inexpensive and robust layered double hydroxide (LDH) material. In contrast to conventional commercial supports, hydrotalcites offer exceptional structural and electronic tunability through the controlled substitution of divalent and trivalent metal dopants. These materials also feature both acidic and basic active sites and can act as non-innocent supports, directly influencing catalytic behavior. This unique combination of properties provides enhanced control over reactivity and selectivity while maintaining excellent thermal and chemical stability. A comprehensive suite of characterization techniques was employed to investigate catalyst composition, metal dispersion, oxidation states, and thermal properties. These analyses allowed for the establishment of clear structure-activity relationships across a diverse catalyst library. Correlating these properties with reactivity trends reveals the influence of support composition on key catalytic transformations, including but not limited to dehydrogenation, hydrogenation, decarbonylation, aldol condensation, and hydrogenolysis. Building on these mechanistic insights, Pd/HT catalysts have been shown to effectively promote multi-step reaction sequences within a single vessel. By enabling tandem or cascade transformations without the need for intermediate isolation, these systems reduce the number of reaction and purification steps, decrease material consumption, and minimize waste, supporting the advancement of more sustainable synthetic methodologies. This one-pot approach to complex molecule construction represents a strategic shift in reaction planning and synthesis, particularly in the context of biomass upgrading and circular chemical manufacturing. Overall, this dissertation illustrates how rational catalyst design, guided by detailed characterization, experimental validation, and computational modeling, can lead to efficient, scalable, and green chemical processes. The findings set the stage for further development of modular heterogeneous catalyst systems tailored for integrated and sustainable synthesis. Chapter 1 provides an overview of green and sustainable chemistry, including its historical development, key challenges, and emerging opportunities. Fundamental principles commonly applied in this work are presented, along with metrics used to evaluate and compare the sustainability of chemical processes. The chapter also introduces the general structural and functional features of hydrotalcites, establishing their relevance as tunable, multifunctional catalyst supports. Chapter 2 outlines the synthetic procedure for the base hydrotalcite material and describes the incorporation of alternative metal dopants into its layered structure. A range of characterization techniques is introduced, each selected for its ability to provide specific insights into structural, compositional, and electronic properties. The rationale behind each method is discussed, highlighting the type of information it contributes to understanding the catalyst system. Chapter 3 investigates the catalytic performance of Pd/HT materials in the decarbonylation of aryl aldehydes. A diverse library of metal-doped Pd/HT catalysts is examined to evaluate how dopant identity influences decarbonylation activity. The most active catalysts are then tested with a variety of electronically diverse aryl aldehydes to assess the impact of substituent effects on reactivity. A host of computational parameters are calculated to help elucidate mechanistic insights and how substrate electronics effect activity. Additionally, it is demonstrated that benzylic alcohols can undergo selective dehydrogenation followed by decarbonylation, highlighting the potential for expanding the substrate scope to include alternative starting materials. Chapter 4 builds upon the aldehyde decarbonylation studies by exploring the activity of Pd/HT catalysts for the decarbonylation of thioesters, a transformation that remains largely unexplored in the context of heterogeneous catalysis. The influence of dopant metals is systematically evaluated, and the most active catalysts are further examined through an extended substrate scope. Differences in reactivity between symmetric and asymmetric thioesters are analyzed, providing insight into structural effects. Thermodynamic enthalpies of decarbonylation are calculated and correlated with experimental data to support mechanistic interpretations. Additionally, the performance of Pd/HTs is compared to that of traditional commercial supports. These studies demonstrate that high conversion and selectivity can be achieved across a broad range of thioester substrates. Chapter 5 focuses on the depolymerization of lignocellulosic biopolymers, with particular emphasis on lignin. As a proof of concept, lignin model compounds are employed to study key transformations, including transfer hydrogenolysis using a renewable hydrogen-donating solvent, as well as decarbonylation. These simplified models enable mechanistic insight into the performance of Pd/HT catalysts for lignin valorization. The chapter also investigates catalyst recyclability and provides an in-depth analysis of the role of copper as a dopant, highlighting its influence on activity and selectivity. Chapter 6 builds on insights gained from model compound studies by employing Pd/CuHT in a tandem process designed to both extract lignin from the cellulosic fraction of whole biomass and carry out its subsequent depolymerization within a single vessel. This integrated approach enables the direct production of valuable phenolic monomeric platform chemicals, demonstrating the potential of Pd/CuHT for streamlined biomass valorization. In addition, this chapter addresses some of the challenges associated with complex multi-component systems and the difficulty of achieving high selectivity towards desired products Chapter 7 explores a multi-step process for upgrading primary alcohols into energy-dense hydrocarbons suitable as drop-in fuel substitutes. This integrated strategy draws on findings from previous chapters, as it requires a sequence of key transformations—dehydrogenation, aldol condensation, decarbonylation, and rehydrogenation—all carried out using a single catalyst in a one-pot system. The discussion highlights the importance of multifunctional active sites on the catalyst, which are essential for enabling such a complex transformation. Particular attention is given to the role of dopant metals in influencing each individual reaction step. By elucidating structure–activity relationships, the process is fine-tuned to maximize selectivity and efficiency at each stage. Chapter 8 outlines key challenges encountered in this work, while also emphasizing the significant opportunities they present. These insights are framed within the broader goal of expanding current methodologies to new chemical spaces. As industries increasingly prioritize sustainability, the demand for efficient and cost-effective processes grows. The adoption of flow and continuous systems, paired with durable solid-state catalysts, offers a promising path forward. Moreover, the integration of computational chemistry and data-driven tools is becoming essential for rational catalyst design and process optimization. The interplay between experimental strategies and predictive modeling highlights a powerful route for advancing sustainable catalysis.
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