Characterizing Solid–Liquid Interactions in a Mesoporous Catalyst Support Using Variable-Temperature Fast Field Cycling NMR

Variable-temperature fast field cycling NMR measurements are used to probe the surface dynamics of different liquids imbibed within a γ-alumina catalyst support material. The imbibed liquids were grouped as nonpolar, polar aprotic, and polar protic; the surface dynamics were found to depend strongly on the chemical properties of the adsorbate. For almost all liquids, a single relaxation environment was observed. However, for methanol, the relaxation behavior of the hydroxyl and methyl groups showed distinct temperature- and frequency-dependent behavior, demonstrating the occurrence of a functionality-specific relaxation process. The nuclear magnetic relaxation dispersion (NMRD) profiles of the protic functionalities were renormalized to a single NMRD profile, indicating a common relaxation process. Further insight was achieved by modeling the NMRD profiles. For nonpolar species, translational diffusion in the presence of paramagnetic impurities dominated the relaxation, and the solid–liquid interaction strength ranked as toluene > cyclohexane ≈ n -heptane. For polar aprotic liquids, the data were consistent with relaxation being dominated by an intermittent surface binding mechanism, and the adsorbates ranked as methanol > dimethyl sulfoxide (DMSO) > tetrahydrofuran (THF). Finally, for polar protic liquids, the relaxation was controlled by a chemical exchange process that resulted in a surface diffusion of the labile 1 H species in the presence of paramagnetic impurities.