MRI of Heterogeneous Hydrogenation Reactions Using Parahydrogen Polarization

Abstract

The power of magnetic resonance imaging (MRI) is its ability to image the internal structure of optically opaque samples and provide detailed maps of a variety of important parameters, such as density, diffusion, velocity and temperature. However, one of the fundamental limitations of this technique is its inherent low sensitivity. For example, the low signal to noise ratio (SNR) is particularly problematic for imaging gases in porous materials due to the low density of the gas and the large volume occluded by the porous material. This is unfortunate, as many industrially relevant chemical reactions take place at gas-surface interfaces in porous media, such as packed catalyst beds. Because of this severe SNR problem, many techniques have been developed to directly increase the signal strength. These techniques work by manipulating the nuclear spin populations to produce polarized (i.e. non-equilibrium) states with resulting signal strengths that are orders of magnitude larger than those available at thermal equilibrium. 2This dissertation is concerned with an extension of a polarization technique based on the properties of parahydrogen. Specifically, I report on the novel use of heterogeneous catalysis to produce parahydrogen induced polarization and applications of this new technique to gas phase MRI and the characterization of micro-reactors. First, I provide an overview of nuclear magnetic resonance (NMR) and how parahydrogen is used to improve the SNR of the NMR signal. I then present experimental results demonstrating that it is possible to use heterogeneous catalysis to produce parahydrogen-induced polarization.These results are extended to imaging void spaces using a parahydrogen polarized gas. In the second half of this dissertation, I demonstrate the use of parahydrogen-polarized gasphase MRI for characterizing catalytic microreactors. Specifically, I show how the improved SNR allows one to map parameters important for characterizing the heat and mass transport in a heterogeneous catalyst bed. This is followed by appendices containing detailed

Thermal maps of gases in heterogeneous reactions

“…The catalysts were prepared according to reported procedures (29)(30)(31)(32). Details about the motional-averaging technique calibration, temperature mapping, T 1 and T 2 relaxometry, and microreactor imaging (30), can be found in Methods. Hz/K.…”

“…This is because the gradient only needs to be about 5-10 times larger than the linewidth over the voxel in the absence of a gradient. Microreactor Imaging: The catalyst, Wilkinson's catalyst supported on silica gel, and microreactor were prepared as described in (30). Parahydrogen induced polarization, PHIP (with hydrogen 50% para-enriched), was used to provide signal enhancement from the microreaction.…”