In Situ Determination of Carbon Number Distributions of Mixtures of Linear Hydrocarbons Confined within Porous Media Using Pulsed Field Gradient NMR

T 1 Relaxation of Methane in Mixtures with Gaseous Water

Synthetic, ecofriendly fuels and chemicals can be produced through Power-To-X (PtX) processes. To study such catalytic processes operando and spatially resolved, magnetic resonance imaging (MRI) is a versatile tool. A main issue in the application of MRI in reactive studies is a lack of knowledge about how the gathered signals can be interpreted into reaction data like temperature or species concentration. In this work, the interaction of methane and gaseous water is studied regarding their longitudinal relaxation time T 1 and the chemical shift. To this end, defined quantities of methane-water mixtures were sealed in glass tubes and probed at temperatures between 130 and 360 °C and pressures from 6 to 20 bar. From the obtained T 1 relaxation times, the collision cross section of methane with water σ j,CH4-H2O is derived, which can be used to estimate the temperature and molar concentration of methane during the methanation reaction. The obtained T 1 relaxation times can additionally be used to improve the timing of MRI sequences involving water vapor or methane. Further, details about the measurement workflow and tube preparation are shared.

Experimental Determination of H2 and CO Diffusion Coefficients in a Wax Mixture Confined in a Porous Titania Catalyst Support

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 H₂ 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 H₂ and CO within wax-saturated porous titania in the range (1.00-2.43) × 10^-8 and (6.44-8.50) × 10^-9 m² s^-1, respectively, were measured in the temperature range of 140-240 and 200-240 °C for H₂ 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 n-hexatriacontane (n-C₃₆) 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 H₂ 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 H₂ and CO diffusion within bulk and confined wax to within 3%.