Hydrogen bonding network disruption in nanoporous catalyst supports probed by PFG-NMR diffusometry and NMR relaxometry

Pulsed field gradient nuclear magnetic resonance (PFG-NMR) has been applied to study molecular self-diffusion of several classes of organic compounds within different inorganic supports (TiO2, γ-Al2O3 and SiO2,) commonly used in heterogeneous catalysis to assess the effect of chemical functionality on the effective self-diffusion coefficient of the probe compound within the pore space [1]. A new parameter, ξ, the PFG-NMR interaction parameter, is introduced and is defined as the ratio of the PFG-NMR derived free bulk liquid self-diffusivity to that measured within the pore space. Such a ratio is reminiscent of the tortuosity factor [2], τ, but it is theoretically different [3]. True tortuosity values of porous media from PFG-NMR measurements can only be calculated when the small liquid-phase probe molecules (such as liquid alkanes) do not interact with the solid phase. In contrast, diffusion measurements of polar liquids with chemical functionalities yield values of the self-diffusivity ratio that reflect the physico-chemical interactions between the probe molecules themselves and their interaction with the surrounding pore space (see Figure 1).