Water is a major product of Fischer-Tropsch synthesis, and hence the behaviour of water within Fischer-Tropsch synthesis catalysts and its potential influence on catalyst rate and selectivity are questions of long-standing interest. The present work applies three different magnetic resonance techniques to study how water interacts with a model wax, n-octacosane, within the pore space of a porous silica of mean pore size ~18 nm. 1 H magnetic resonance spectroscopy, spin-lattice relaxation time and pulsed-field gradient measurements were performed at 195 C, and for water pressure in the range 3-13.6 bar, conditions relevant to low temperature Fischer-Tropsch synthesis. The uptake of water within this system is shown to be very similar to that observed for capillary condensation of water within the empty pore space of the same porous silica under the same experimental conditions; suggesting that capillary condensation of water within the wax-saturated pores is occurring. The behaviour of water is characterised by two regimes. At low water relative pressures of ~0.3 ≤ P/P0  ~0.8 water moves into the pore space, displacing wax from the pore surface and existing as a water-rich layer between the pore surface and an oil-rich phase in the centre of the pore; the strong interaction with the pore surface is evidenced by the short nuclear spin relaxation time values of water at the lowest pressures which then increase slightly as multi-layer adsorption at the pore surface occurs with increase in pressure. In the water relative pressure range ~0.8 ≤ P/P0  ~0.97, condensation of water within the pores is observed, characterised by increases in both spin-lattice relaxation time and molecular diffusivity. Analysis of the data suggests that as much as ~40% of the pore surface is occupied by condensed water after condensation has occurred. It is suggested that these two regimes of water behaviour inside initially wax-filled pores might explain previously reported aspects of the behaviour of Fischer-Tropsch catalyst performance as a function of pore size and amount of co-fed water.

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.

Spatially-resolved and unresolved magnetic resonance measurements are used in combination with a partial least squares regression (PLSR) method to measure chemical composition within catalyst pellets during the 1-octene hydrogenation reaction occurring in a fixed bed of 0.3 wt% Pd/Al 2 O 3 catalyst pellets. The PLSR method is used to discriminate between chemical species within and external to the void space of the catalyst pellets. The spatiallyresolved data show that the hydrogenation and isomerisation reactions are dominant in the upper and lower region of the reactor, respectively. The local intra-pellet compositions also show product accumulation inside catalyst pellets consistent with reaction occurring under conditions of mass transfer limitation. An average measure of the intra-pellet composition within the whole bed was then used to estimate the liquid-solid mass transfer coefficient during the course of the reaction. The values of LS obtained from the NMR measurements were in the range 0.15 × 10-5 m s-1 < LS < 0.25 × 10-5 m s-1 , for reactor operating conditions characterised by gas and liquid Reynolds numbers 0.2 < L < 0.6 and 0.1< G <0.2; these values are shown to be consistent with those predicted by existing literature correlations. Closest agreement was found with values predicted from dissolution experiments performed under similar hydrodynamic conditions in trickle flow. In addition to introducing a method for the direct measurement of LS , the data presented also confirm that estimates of LS are more accurate when performed in an environment in which the hydrodynamics and fluidsolid contacting conditions are representative of the system of interest.

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.

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.

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Pulsed field gradient NMR experiments are used to probe the diffusion mechanism of liquid n-alkanes, with carbon numbers in the range of 8−40, confined in five mesoporous silicas of average pore diameters 6.9, 13.9, 19.9, 25.0, and 38.7 nm. A novel method, based on the Cohen−Turnbull−Bueche model of diffusion, employs mixtures instead of pure liquids to achieve separation of free volume and diffusion mechanism effects on diffusion coefficients. Changes in the diffusion mechanism as a function of confinement, defined as the ratio of average pore diameter to average molecular size (C f ), are observed. The Zimm mechanism is identified in bulk liquids, while for all C f values less than 9, diffusion occurs by the reptation mechanism. Between C f values of 9 and 34, the data are consistent with the Rouse mechanism for chain lengths in the range of 8−16 and the coexistence of both Rouse and reptation mechanisms for longer chain lengths of 21−40. The critical entanglement chain length, where the dominant mechanism changes from Rouse to reptation in nalkanes, is shown to significantly reduce in confined liquids relative to bulk. The critical entanglement chain length reduces by a factor of at least 12 in pore diameters of 6.9 nm.

The analysis of 1D anti-diagonal spectra from the projections of 2D double-quantum filtered correlation spectroscopy NMR spectra is presented for the determination of the compositions of liquid mixtures of linear and branched alkanes confined within porous media. These projected spectra do not include the effects of line broadening and therefore retain high-resolution information even in the presence of inhomogeneous magnetic fields as are commonly found in porous media. A partial least-square regression analysis is used to characterize the mixture compositions. Two case studies are considered. First, mixtures of 2-methyl alkanes and n -alkanes are investigated. It is shown that estimation of the mol % of branched species present was achieved with a root-mean-square error of prediction (RMSEP) of 1.4 mol %. Second, the quantification of multicomponent mixtures consisting of linear alkanes and 2-, 3-, and 4-monomethyl alkanes was considered. Discrimination of 2-methyl and linear alkanes from other branched isomers in the mixture was achieved, although discrimination between 3- and 4- monomethyl alkanes was not possible. Compositions of the linear alkane, 2-methyl alkane, and the total composition of 3- and 4-methyl alkanes were estimated with a RMSEP <3 mol %. The approach was then used to estimate the composition of the mixtures in terms of submolecular groups of CH 3 CH 2 , (CH 3 ) 2 CH, and CH 2 CH(CH 3 )CH 2 present in the mixtures; a RMSEP <1 mol % was achieved for all groups. The ability to characterize the mixture compositions in terms of molecular subgroups allows the application of the method to characterize mixtures containing multimethyl alkanes. The motivation for this work is to develop a method for determining the mixture composition inside the catalyst pores during Fischer–Tropsch synthesis. However, the method reported is generic and can be applied to any system in which there is a need to characterize mixture compositions of linear and branched alkanes.