Impacts of Small SBA-15 Mesopores on Translation and\nRotational Diffusion of Benzyl Alcohol

Small\nSBA-15 pores can enhance the catalytic activity of gold catalysts\nin the selective oxidation of benzyl alcohol reaction. The goal of\nthis work is to probe the impact of SBA-15 pores on the translational\nand rotational diffusion of the reactive species (e.g., benzyl alcohol).\nHerein, we demonstrate the use of nuclear magnetic resonance-based\ndiffusion-ordered spectroscopy (DOSY) and <i>T</i><sub>1</sub> relaxation techniques to measure the translational and rotational\ndiffusion coefficients of liquid benzyl alcohol molecules in three\ndistinct environments: <i>bulk</i>, <i>pore</i>, and <i>near-surface</i>. The DOSY and <i>T</i><sub>1</sub> relaxation techniques render the routine diffusion measurements\nfor liquid molecules confined in small pores possible. Furthermore,\nwe measure the translational and rotational diffusion coefficients\nof benzyl alcohol molecules in the <i>bulk</i>, <i>pore</i>, and <i>near-surface</i> environments at\nvarying temperatures. These measurements are then correlated via the\nArrhenius equation to determine the activation energy associated with\ntheir translational (<i>E</i><sub>t</sub>) and rotational\n(<i>E</i><sub>r</sub>) diffusion. A higher activation energy\nsuggests a more difficult diffusion process, while a lower activation\nenergy implies a facile diffusion process. Our findings demonstrate\nthat the ranking of <i>E</i><sub>t</sub> follows the order\nof <i>bulk</i> (difficult) > <i>near-surface</i> > <i>pore</i> (easy), whereas the ranking of <i>E</i><sub>r</sub> follows the order of <i>pore</i> (difficult)\n> <i>bulk</i> > <i>near-surface</i> (easy).\nThis\nresult suggests that the small SBA-15 pores can facilitate the translational\ndiffusion but hinder the rotational diffusion of benzyl alcohol molecules.