NMR methods for the characterization of mass transport and reaction processes in porous materials

Abstract

NMR imaging (MRI) and localized NMR spectroscopy are powerful techniques for non-invasively characterizing fluids in opaque porous materials, particularly those used for gas phase catalytic reactions. Transport processes of gases such as diffusion, dispersion, and flow can be investigated by fast NMR methods despite short effective transverse relaxation times in porous media. This project aims at developing novel and optimizing existing NMR measurement methods for the non-invasive characterization of liquids and gases in porous materials. The project comprises the development and application of MRI, in particular diffusiometry and velocimetry techniques that allow investigations required for optimizing heterogeneous catalytic gas phase reactions. Thus, optimized NMR methods were applied to determine the local velocity fields, molecular diffusion, dispersion, and temperature. Two major components of mass transport, diffusion and flow of gases, were investigated as preparatory studies for the analysis of gas phase reactions. To perform these investigations, measurement techniques were developed and optimized based on the specific demands in catalytic gas phase reactions taking place in monolithic structures. The measurements enabled the spatially resolved characterization of mass transport in such opaque systems by determining vital engineering parameters including temperature, composition of substances, velocity fields as well as molecular diffusion and dispersion of gas in catalyst supports. A 7-Tesla NMR imaging system (Bruker Biospec 70/20 USR) was used to develop methods and to perform measurements. A spatially resolved NMR method for measuring the probability function of molecular displacement was developed to characterize diffusion and dispersion of thermally polarized gases in open-cell foams with different pore densities. The apparent diffusion coefficients and dispersion coefficients of thermally polarized methane were measured under off-flow and flow conditions, respectively. Additionally, the influence of mechanical and diffusional dispersion at various flow rates (0.1-2.25 L·min-1, sample diameter: 25 mm) was investigated. The 3D MR velocimetry (MRV) measurements of gas flow in regular and irregular monolithic catalyst supports were conducted using an optimized spin-echo based phase-contrast MRV sequence. The obtained MRV data of thermally polarized methane gas were compared to numerical simulations performed for the identical samples. Finally, an optimized diffusion-weighted (DW) MRI technique was used for the in situ analysis of temperature in the catalyst supports. Using 3D DW-MRI to measure the temperature dependent diffusion coefficients of ethylene glycol, glycerol, and the temperature stable ionic liquid Pyr13 [TFSI] allowed to use capillaries filled with these liquids as NMR thermometers for a broad temperature range. Measurements were performed in a temperature range from 20 to 160 °C and allowed to obtain the axial temperature profiles in catalyst supports.