(532cp) The Optimization in Catalysis Due to Better Adsorption to the Gold Surface within the SBA-15 Pores

Mesoporous silica (e.g., SBA-15) has been shown to optimize catalysis via the in-pore orientation. We have leveraged the pulse field gradient (PFG) NMR technique to demonstrate that the SBA-15 pores do not impact the translational diffusion, implying the absence of mass-transfer limitation. The rotational NMR studies using d-toluene demonstrate the reduction in the anisotropic rotation within the SBA-15 pores, in which the rotation about the major axis was reduced less than the orthogonal rotation, indicating that the reactive species more likely to align with the gold surface for better adsorption. In the absence of mass-transfer limitation, we have hypothesized that the initial adsorption of reactive species to the gold surface is the rate-limiting step. To test this hypothesis, we have conducted the selective oxidation of ortho-, meta-, or para-methoxy benzyl alcohol using the porous SBA-15 supported and the non-porous SiO₂ supported gold catalysts under the solvent of methanol. The catalytic activity is evaluated based on the surface turnover frequency (TOF_s) that has normalized the differences in the gold loadings, surface area, and particle size. The porous gold catalysts (TOF_s = 8.0 s^-1) are significantly more active than the non-porous gold catalysts (TOF_s = 0.8 s^-1) in the oxidation of para-methoxy benzyl alcohol, indicating that the SBA-15 pore benefits the adsorption of alcohol to the gold surface. However, both porous and non-porous gold catalysts are inactive in the oxidation of ortho-methoxy and meta-methoxy benzyl alcohol, indicating that the SBA-15 pores do not benefit the surface chemistry. This work clearly demonstrates the impact of pore geometry on facilitating the adsorption of reactive species to the catalyst surface, ultimately improve catalytic activity.