Hydrogen Bonding Network Disruption in Mesoporous Catalyst Supports Probed by PFG-NMR Diffusometry and NMR Relaxometry

The pulsed-field gradient (PFG)-NMR technique has been applied to study molecular diffusion of organic liquids within mesoporous materials used in heterogeneous catalysis, in order to assess the effect of chemical functionalities on the effective self-diffusivity of the probe molecule within the pore space. True tortuosity values of the porous matrix can be calculated from the ratio of the unrestricted free self-diffusivity to the self-diffusivity within the pore space only when the small liquid-phase probe molecules do not have any chemical functionality that interacts within the solid phase (e.g., alkanes). The use of molecules with reactive chemical functionalities gives values heavily dependent on the physical and chemical interactions within the porous medium; hence, these values cannot be defined as tortuosity. Polyols showed an interesting behavior of enhanced rate of self-diffusion within the confined pore space, and this is attributed to the ability of the porous medium to disrupt the extensive intermolecular hydrogen bonding network of polyols.