Pulsed field gradient (PFG) NMR measurements, combined with a novel optimization method, are used to determine the composition of hydrocarbon mixtures of linear alkanes (C7–C16) in both the bulk liquid state and when imbibed within a porous medium of mean pore diameter 28.6 nm. The method predicts the average carbon number of a given mixture to an accuracy of ±1 carbon number and the mole fraction of a mixture component to within an average root-mean-square error of ±0.036 with just three calibration mixtures. Given that the method can be applied at any conditions of temperature and pressure at which the PFG NMR measurements are made, the method has the potential for application in characterizing hydrocarbon liquid mixtures inside porous media and at the operating conditions relevant to, for example, hydrocarbon recovery and heterogeneous catalysis.

Optimisation of a heterogeneous catalytic process requires characterisation of the catalyst at industrially-relevant conditions and lengthscales. Here we use magnetic resonance imaging to gain insight into Fischer-Tropsch synthesis occurring in a pilot-scale fixed-bed reactor operating at 220 °C, 37 bar, and for three H2/CO feed ratios. Molecular diffusion and carbon number of hydrocarbon products are spatially-resolved within both the reactor and individual 1 wt% Ru/TiO2 catalyst pellets. These data highlight the importance of mass transfer, in addition to nanoscale catalyst activity, on catalyst performance. In particular, a start-up time of up to 3 weeks is required for steady-state to be achieved in the catalyst pores. Further, the average carbon number present in the pores can be as much as double that in the product wax.

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The ability to measure and predict molecular diffusion coefficients in multicomponent mixtures is not only of fundamental scientific interest but also of significant relevance in understanding how catalytic processes proceed. In the present work, the direct measurement of the molecular diffusion of H 2 and CO gas-phase species diffusing in n-alkane mixtures using pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) methods is reported. The work is of direct relevance to Fischer−Tropsch (FT) catalysis, with the measurements being made of the gas−wax system with the wax in both the bulk liquid state and when confined within a titania catalyst support, at temperatures and pressures typical of low-temperature FT synthesis. Molecular diffusion coefficients of H 2 and CO within wax-saturated porous titania in the range (1.00−2.43) × 10 −8 and (6.44− 8.50) × 10 −9 m 2 s −1 , respectively, were measured in the temperature range of 140− 240 and 200−240 °C for H 2 and CO, respectively, at a pressure of 40 bar. The wax mixture was typical of a wax produced during FT catalysis and had a molar average carbon number of 36. It is shown that the hydrogen diffusion coefficient within this wax mixture is consistent, to within experimental error, with the hydrogen diffusion coefficient measured in pure single-component nhexatriacontane (n-C 36 ) wax; this result held with the waxes in the bulk liquid state and when confined within the porous titania. The tortuosity of the porous titania was also measured using PFG NMR and found to be 1.77; this value is independent of temperature. The ability of existing correlations to predict these experimentally determined data was then critically evaluated. Although the Wilke−Chang correlation was found to underestimate the molecular diffusion coefficients of both H 2 and CO diffusing in the wax in both the bulk state and when confined within the porous titania, parameterized correlations based on the rough hard sphere model, having accounted for the experimentally determined tortuosity factor, predicted the H 2 and CO diffusion within bulk and confined wax to within 3%.