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

Abstract

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.

Low-Field Functional Group Resolved Nuclear Spin Relaxation in Mesoporous Silica

“…While previous analyses of confined methanol suggest the existence of two distinct relaxation populations, ,− the correlation data in Figure exemplifies the first observation of such phenomena in longer chain primary alcohols, as well as the first such data for both secondary alcohols and carboxylic acids. While high and intermediate field measurements have failed to evidence the existence of multiple relaxation environments in the presence of longer-chain (>C 1 ) liquid hydrocarbons, , we attribute the clear and persistent observation of hydroxyl group relaxation phenomena across our range of polar-protic adsorbates to the combination of short experimental echo times and low-magnetic-field strength (see the Methods section); these experimental conditions significantly reduce the impact of magnetic susceptibility contrast effects on the measurement of short T 2 times, facilitating the accurate measurement of relaxation data associated with rapidly relaxing hydroxyl protons (further details are provided in Supporting Information Note 2).…”

Extending NMR Tortuosity Measurements to Paramagnetic Catalyst Materials Through the Use of Low Field NMR

“…[18] When weakly interacting species are adsorbed in porous media the decrease in T 1 with respect to frequency is small, as was observed previously for alkanes imbibed within γ-alumina, which showed a maximum change in the relaxation rate, 1/T 1 , of 1.2 s À 1 between 10 MHz and 10 kHz. [19] In the present work, as expected, a small decrease in T 1 was observed for ndodecane imbibed within Ti-I, Ti-II, and Ti-III. To within experimental error, no decrease in T 1 was observed for ndodecane imbibed in T-IV and Ti-V but it is noted that changes in the relaxation rate consistent with those observed for Ti-I, Ti-II and Ti-III were below the resolution of the measurement due to the increased contribution to the relaxation from the presence of paramagnetic species in Ti-IV and Ti-V. A similar frequency dependent effect occurs for spin-spin relaxation, T 2 , however the decrease in T 2 at low frequencies is offset by reduced internal gradient effects, [20] associated with magnetic susceptibility differences at solid-liquid interfaces giving rise to internal gradients within the material.…”

“…Typically, the T 1 time constants of bulk and adsorbed liquids decrease when measured at lower NMR frequencies [18] . When weakly interacting species are adsorbed in porous media the decrease in T 1 with respect to frequency is small, as was observed previously for alkanes imbibed within γ‐alumina, which showed a maximum change in the relaxation rate, 1/ T 1 , of 1.2 s −1 between 10 MHz and 10 kHz [19] . In the present work, as expected, a small decrease in T 1 was observed for n ‐dodecane imbibed within Ti‐I, Ti‐II, and Ti‐III.…”

Dynamical Susceptibilities of Confined Water from Room Temperature to the Glass Transition

“…In FCR experiments, the spin–lattice relaxation (SLR) time T 1 is measured in a broad range of (Larmor) angular frequencies ω utilizing a rapidly switchable electromagnet. − Exploiting the time-honored relations between spin relaxation and molecular dynamics, the measured T 1 (ω) dispersions provide straightforward access to the shapes of spectral densities and dynamical susceptibilities. As a result, FCR not only notably extends the time window of SLR studies of water reorientation in confinements to 10 –11 ≤ τ ≤ 10 –5 s but also removes some ambiguities in the interpretation of conventional SLR data. , Previous FCR approaches to confined liquids were mainly restricted to experimental conditions under which the SLR was governed by effects other than the rotational dynamics of the liquid molecules. − …”