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

Pulsed Field Gradient (PFG) NMR is recognised as an analytical technique used to characterise the tortuosity of porous media by measurement of the self-diffusion coefficient of a fluid contained within the pore space of the material of interest. Such measurements are usually performed on high magnetic field NMR hardware (>300 MHz). However, many materials of interest, in particular heterogeneous catalysts, contain significant amounts of paramagnetic species, which make such measurements impossible due to their characteristic short spin-spin relaxation times. Here it is demonstrated that by performing PFG NMR measurements on a low field magnet (2 MHz), tortuosity measurements can be obtained for a range of titania (TiO₂) based carriers and catalyst precursors containing paramagnetic species up to a 20 wt.% loading. The approach is also used to compare the tortuosity of two catalyst precursors of the same metal loading prepared by different methods.