Structural factors controlling selectivity of supported metal catalysts for partial and semi-hydrogenation

Acetylene semi-hydrogenation and 1,3-butadiene selective hydrogenation are commonly used as model reactions for studying partial hydrogenation reactions and purifying impurities in olefin products in the petrochemical field. Therefore, catalyst design must not only consider the catalytic activity for acetylene and 1,3-butadiene but also the selectivity of the target product to avoid over-hydrogenation and polymer formation while maintaining catalytic stability. While previous research has significantly advanced catalyst development and our understanding of the catalytic process, it has mainly focused on studying Pd-based catalysts. Although Pd catalysts have shown promising catalytic performance, there is a lack of studies on other metal catalysts, particularly regarding the reaction mechanism of their active sites and the factors influencing their catalytic performance. This dissertation aims to supply this intermission by focusing on studying and discussing two non-palladium-based metals, copper and rhodium, catalysts for partial hydrogenation reactions. First, a series of CunCeMgOx catalysts with various copper nanoparticle sizes and surface defect densities were synthesized and tested for partial hydrogenation of 1,3-butadiene. Data demonstrated a reaction pathway involving the dissociation of molecular hydrogen on the peripheral oxygen vacancies (Ov-Cu+) before reacting with 1,3-butadiene adsorbed on the corresponding Cu+ atoms. Analysis of the performance data indicated that the turnover frequency of these Cu+ sites is approximately five-times higher than those of the surface Cu0 sites. Among the catalysts considered, Cu0.5CeMgOx with the smallest copper nanoparticle size showed a comparable stability, while the others were easily deactivating because of carbon deposition. Furthermore, different from the conventional copper-based catalyst, the Cu0.5CeMgOx catalyst achieved a complete suppression of total hydrogenation even at space velocities offering a complete 1,3-butadiene conversion. The findings offer a broad potential for the rational design of low-cost, highly selective, and stable copper-based partial hydrogenation catalysts for reactions that are prone to coke formation. The analysis and discussion of catalytic active sites and reaction pathways for partial hydrogenation in copper-based catalysts have been carried out through experimental design, leading to reasonable speculations. However, because of the complexity of supported nanoparticle catalysts, the focus of research has shifted towards supported mononuclear catalysts, aiming to achieve a deeper and more accurate understanding of catalytic reaction mechanisms. In the case of supported mononuclear metal catalysts, their catalytic performance is primarily influenced by their coordination environment. Nonetheless, the facile sintering of supported atomically dispersed precious metal catalysts under high temperatures, particularly in reducing conditions, poses a challenge for their practical applications. The carbonyl ligand of mononuclear rhodium complexes supported on HY zeolite can mildly inhibit the sintering of rhodium under reducing conditions, but severely suppress the catalytic activity of the hydrogenation reaction. In this study, we demonstrate that substitution of the carbonyl ligand with acetylene ligand can maintain the atomic dispersion of the supported mononuclear rhodium complex under harsh reducing condition (> 573 K), as confirmed by in-situ X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS) spectroscopies, and in-situ Infrared (IR) spectroscopies. In contrast, the supported rhodium carbonyl complex aggregates into nanoclusters under the same conditions. Furthermore, our results indicate that the acetylene ligand can provide improved anti-sintering ability while retaining a limited ethylene hydrogenation activity towards ethane. Meanwhile acetylene ligands can be applied to the high temperature stability of oxide-supported rhodium complexes in acetylene semi-hydrogenation reactions. After addressing the stability-limitation of supported mononuclear rhodium catalysts under reducing conditions by changing the ligand type, supported rhodium catalysts were investigated for potential application in partial hydrogenation reactions. Supported rhodium catalysts are known to be unselective for semi-hydrogenation reactions. Now, by tuning the electronic structure of the active sites determined by the metal nuclearity and the electron-donor properties of the support, we demonstrate that atomically dispersed HY zeolite-supported rhodium with reactive acetylene ligands affords a stable ethylene selectivity >90% for acetylene semi-hydrogenation, even while ethylene present in a large excess over acetylene at 373 K and atmospheric pressure. IR and XAS complemented with calculations at the level of density functional theory and kinetics measurements show how the catalyst performance depends on the electronic structure of the rhodium, influenced by the support as a ligand. After investigating the influence of electron density on supported mononuclear rhodium centers on their catalytic performance, we conducted further experiments using ionic liquids to encapsulate HY zeolite-supported mononuclear rhodium complexes. The objective was to modulate electron densities on rhodium centers and analyze the effect of the ionic liquid structure on their electron-donating ability. The results reveal that the structure of the cations in the ionic liquids, especially the imidazolium cations, has a relatively minor impact on the electron-donating capacity of the ionic liquids when compared to the type of anions, which have a more significant effect on the electron-donating capacity. This finding serves as a valuable reference for controlling the effect of electronic structure on supported mononuclear rhodium. Moreover, the weak effect of cations on electron-donating capacity can also be combined with the filtration properties of ionic liquids to control the catalytic performance of the catalyst. The results presented in this dissertation demonstrate the potential application of copper-based catalysts in selective hydrogenation reactions. Additionally, it highlights the promising prospects of supported mononuclear rhodium catalysts in semi-hydrogenation reactions, containing the ability to enhance mononuclear rhodium's resistance to sintering under reducing conditions, the capacity to modulate catalytic performance for semi-hydrogenation through the support's electron-donor capacity and the potential use of ionic liquids for modifying mononuclear rhodium catalysts.