PREPARATION AND CHARACTERIZATION OF MESOPOROUS SBA-15 SILICA SUPPORTED GOLD CATALYSTS FOR SELECTIVE OXIDATIONS

Zengran Sun

doi:10.7273/000006527

The aim of this research is to design, prepare, and characterize gold catalysts that combine the active surface of gold nanoparticles with the confinement effects afforded by the highly ordered mesoporous SBA-15 silica. These gold catalysts show promising catalytic efficiency in facilitating the selective oxidation of benzyl alcohol, which produces benzaldehyde, a valuable product for various industrial applications. However, traditional methods for preparing these catalysts require modification of the SBA-15 surface with ligands to stabilize the gold nanoparticles inside the SBA-15 pores. High-temperature calcination (≥ 500 ℃) is then needed to remove the anchored ligands for catalyst activation. This calcination causes the deterioration of the gold nanoparticles’ size and morphology, reducing catalytic efficiency. We demonstrate a novel reversible ionic liquid (RevIL) technique for preparing gold catalysts, eliminating the need for chemical modification of the silica surface. RevILs can be switched between their molecular and ionic forms by reacting with CO2. Upon removal of CO2 via sparging with inert gas or gentle heating, RevILs revert to their molecular form and leave the gold surface. We use the RevIL technique to prepare and deposit the size-controlled gold nanoparticles inside the pores of SBA-15 silica. The gold nanoparticles’ location inside the pores is confirmed by a microscopic technique and the nanoparticles’ improved thermal stability compared to those deposited on the surface of nonporous SiO2. Owing to the bare gold surface, the porous gold catalysts prepared with the RevIL technique are highly active in the selective oxidation of benzyl alcohol without calcination. We confirm the bare gold surface by the disappearance of the RevILs’ chemical signals via Fourier-transform infrared spectroscopy analysis. The porous SBA-15 supported gold catalysts show improved catalytic activity over the nonporous SiO2 supported gold catalysts’, implying that the SBA-15 pores impact catalysis. By controlling the geometry of the gold catalysts, we alter the pore environment near the gold surface to tune the reactive molecules’ mass transport, thus influencing catalytic activity. We hypothesize that the SBA-15 pores have no impact on the translational diffusion of benzyl alcohol molecules but restrict the random rotational diffusion. To test this hypothesis, we use the NMR-based DOSY and T1 relaxation techniques to measure the translational and rotational diffusion coefficients of benzyl alcohol in three distinctive environments: bulk, pore, and non-pore. These diffusion coefficients are measured at varying temperatures and are then correlated to Arrhenius behavior to determine the activation energy associated with temperature-dependent diffusion processes. A high activation energy indicates difficult diffusion processes. We demonstrate that the trend of activation energy of translational diffusion is bulk (difficult)> non-pore > pore (easy), while for rotational diffusion, the trend is pore (difficult) > bulk > non-pore (easy). These findings indicate that the SBA-15 pores do not induce translational mass-transport limitation but hinder the free rotation of benzyl alcohol molecules near the gold surface. Given that rotational diffusion governs the adsorption behavior of reactive species, we further investigate the impact of SBA-15 pores on the adsorption process. Consequently, we hypothesize that the SBA-15 pores fundamentally alter the adsorption behavior of benzyl alcohol. To verify this hypothesis, we use three probe molecules—hexane, cyclohexanol, and toluene—to examine the effects of dilution, hydroxyl group adsorption, and phenyl ring adsorption, respectively, on the catalytic activity of the porous and nonporous gold catalysts. The results reveal that while dilution has no impact on catalytic activity, the presence of cyclohexanol’s hydroxyl group and toluene’s phenyl ring greatly reduces the porous and nonporous gold catalysts’ catalytic activities. To explain these effects, we construct a kinetic model to elucidate the catalytic dynamics and quantify the kinetic parameters, providing a quantitative assessment of how the porous catalyst structure influences the adsorption process compared to the nonporous catalyst structure. We demonstrate that the SBA-15 pores significantly enhance the adsorption rate of reactive species, as shown by a 200% higher adsorption constant of benzyl alcohol in the porous gold catalysts than the nonporous. This enhancement improves the catalytic activity of the porous SBA-15 supported gold catalysts. In addition, the SBA-15 pores promote the adsorption of hydroxyl groups while hindering the adsorption of phenyl rings, as evidenced by a 107% higher adsorption constant of cyclohexanol’s hydroxyl group and a 110% lower adsorption constant of toluene’s phenyl ring in the nonporous gold catalysts than the porous. These findings suggest unique adsorption behavior of benzyl alcohol within the SBA-15 pores, characterized by two processes: 1) the SBA-15 pores hinder the adsorption of the phenyl ring in benzyl alcohol through restricted rotational diffusion, reducing the likelihood of active site blockage; and 2) the SBA-15 pores facilitate the alignment of the hydroxyl group in benzyl alcohol with the gold surface, thus facilitating chemical reactions and, consequently, catalytic activity. Based on the findings in this dissertation, we conclude that, in the absence of translational mass transport limitations, the SBA-15 pores can modify the adsorption behavior of benzyl alcohol molecules through tuned rotational mass transport, enhancing the catalytic performance of the porous SBA-15 supported gold catalysts.

PREPARATION AND CHARACTERIZATION OF MESOPOROUS SBA-15 SILICA SUPPORTED GOLD CATALYSTS FOR SELECTIVE OXIDATIONS

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