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.