Porous Media NMR Analysis

Recent years have seen a significant progress in the study of porous media of natural and industrial sources. This paper provides a brief outline of the recent technical development of NMR in this area. These progresses are relevant for NMR application in material characterization.

The wettability conditions in a porous media containing two or more immiscible fluid phases determine the microscopic fluid distribution in the pore network. Nuclear magnetic resonance measurements are sensitive to wettability because of the strong effect that the solid surface has on promoting magnetic relaxation of the saturating fluid. The idea of using NMR as a tool to measure wettability was presented by Brown and Fatt in 1956. The magnitude of this effect depends upon the wettability characteristics of the solid with respect to the liquid in contact with the surface.Their theory is based on the hypothesis that molecular movements are slower in the bulk liquid than at the solid-liquid interface. In this solid-liquid interface the diffusion coefficient is reduced, which correspond to a zone of higher viscosity. In this higher viscosity zone, the magnetically aligned protons can more easily transfer their energy to their surroundings. The magnitude of this effect depends upon the wettability characteristics of the solid with respect to the liquid in contact with the surface.

NMR Cryoporometry (NMRC) is a recent technique for measuring total porosity and pore size distributions. It makes use of the Gibbs-Thomson effect : small crystals of a liquid in the pores melt at a lower temperature than the bulk liquid : The melting point depression is inversely proportional to the pore size. The technique is closely related to that of the use of gas adsorption to measure pore sizes (Kelvin equation). Both techniques are particular cases of the Gibbs Equations (Josiah Willard Gibbs): the Kelvin Equation is the constant temperature case, and the Gibbs-Thomson Equation is the constant pressure case.

To make a Cryoporometry measurement, a liquid is imbibed into the porous sample, the sample cooled until all the liquid is frozen, and then warmed slowly while measuring the quantity of the liquid that has melted. Thus it is similar to DSC thermoporosimetry, but has higher resolution, as the signal detection does not rely on transient heat flows, and the measurement can be made arbitrarily slowly. It is suitable for measuring pore diameters in the range 2 nm–2 μm.

Nuclear Magnetic Resonance (NMR) may be used as a convenient method of measuring the quantity of liquid that has melted, as a function of temperature, making use of the fact that the {\displaystyle T_{2}} T_{2} relaxation time in a frozen material is usually much shorter than that in a mobile liquid. The technique was developed at the University of Kent in the UK.It is also possible to adapt the basic NMRC experiment to provide structural resolution in spatially dependent pore size distributions, or to provide behavioural information about the confined liquid.porous media NMR


Multinuclear and hypersensitive MRM in heterogeneous catalysis 

In many heterogeneous catalytic processes, heat transport is an important factor which, if not properly controlled, can lead to the formation of hot spots in the catalyst bed, degradation of reaction conversion and selectivity, reactor runaway and even explosion. The development of non-invasive thermometry techniques for the studies of operating reactors is necessary to advance our understanding of heat transport processes in the catalyst bed and is essential for the development of efficient and environmentally safe industrial reactors and processes.

NMR analyzer and MRI techniques are known to be able to evaluate local temperatures of liquids. However, for a multiphase gas-liquid-solid reactor the available techniques based on the liquid phase benchtop NMR signal detection are not applicable since the local liquid content in the catalyst pores varies with space and time.

We have demonstrated earlier that the direct 27Al MRI of industrial alumina-supported catalysts (e.g., Pd/Al2O3) is a potential way toward the spatially resolved thermometry of an operating packed bed catalytic reactor. Recently, we were able to implement this approach and to obtain 2D temperature maps of the catalyst directly in the course of an exothermic catalytic reaction. The images obtained clearly demonstrate the temperature changes with the variation of the reactant feed and also the existing temperature gradients within the catalyst at a constant feed.

One of the obstacles in developing novel applications of MRM in porous media is its fairly low sensitivity even if 1H signal detection is used. A number of hyperpolarization techniques are currently being developed that can enhance the NMR signal by 4-5 orders of magnitude even at intermediate (3-7 T) magnetic fields, and even more in low and ultra-low magnetic field applications that are currently gaining popularity. Parahydrogen-induced polarization (PHIP) is the only hyperpolarization technique of relevance to catalysis as PHIP effects are observed in hydrogenation reactions when parahydrogen is involved.

We have shown that PHIP can be generated not only in homogeneous hydrogenation reactions but also in heterogeneous catalytic processes catalyzed by a broad range of different heterogeneous catalysts. Thus, the development of the novel hypersensitive NMR/MRI techniques for heterogeneous catalysis becomes possible.

Also, this approach can provide hyperpolarized gases and catalyst-free hyperpolarized liquids for a wide range of novel applications of NMR and MRI in, e.g., materials science, chemical engineering and in vivo biomedical research. Applications of this hypersensitive approach to the studies of gas flow in microfluidic chips and of the hydrogenation reaction in a packed bed microreactor will be demonstrated.

This work was supported by the following grants: RAS 5.1.1, RFBR 11-03-00248-a and 11-03-93995-CSIC-a, SB RAS integration grants 9, 67 and 88, NSh-7643.2010.3, FASI 02.740.11.0262 and МК-1284.2010.3.