Robust In Situ Magnetic Resonance Imaging of Heterogeneous Catalytic Hydrogenation with and without Hyperpolarization

Abstract

Magnetic resonance imaging (MRI) is a powerful technique to characterize reactors during operating catalytic processes. However, MRI studies of heterogeneous catalytic reactions are particularly challenging because the low spin density of reacting and product fluids (for gas phase reactions) as well as magnetic field inhomogeneity, caused by the presence of a solid catalyst inside a reactor, exacerbate already low intrinsic sensitivity of this method. While hyperpolarization techniques such as parahydrogen induced polarization (PHIP) can substantially increase the NMR signal intensity, this general strategy to enable MR imaging of working heterogeneous catalysts to date remains underexplored. Here, we present a new type of model catalytic reactors for MRI that allow the characterization of a heterogeneous hydrogenation reaction aided by the PHIP signal enhancement, but also suitable for the imaging of regular nonpolarized gases. These catalytic systems permit exploring the complex interplay between chemistry and fluid-dynamics that are typically encountered in practical systems, but mostly absent in simple batch reactors. High stability of the model reactors at catalytic conditions and their fabrication simplicity make this approach compelling for in situ studies of heterogeneous catalytic processes by MRI.Catalytic processes (e. g., hydrogenation, reforming, oxidation, etc.) are at the cornerstone of chemical and petrochemical industry [1] and, to make these processes more sustainable, control over reaction selectivity, conversion rates, mass and heat transport inside the working reactor (that is under operando conditions) is required. Magnetic resonance imaging (MRI) is a tool applicable to operando studies that provide information for example about the distribution of liquids in the catalyst pellet during hydrogenation of α-methylstyrene [2] or heptene, [3] the coke profiles within a catalyst pellet, [4] the flow of water [5] or propane [6] through a packed bed, the structure of a porous medium and the water flow during the deposition of fines, [7] or the velocities of particles in a fluidized bed. [8][9][10] MRI of reactors utilizing gases are not as developed as studies of liquids because the spin density in the gas phase is ca. 3 orders of magnitude lower relative to the condensed phase, which significantly limits the deployment of MRI for studying gases. [11] Nevertheless, a recent report showed that the distribution of the reactants and products inside a monolith type reactor can be measured even in the gas phase during ethylene hydrogenation reaction using two different catalyst supports, thereby revealing an effect of the support on the mass and heat transport. [12] Overall, to date, there are only a handful of MRI studies of the gas phase reactions with regular non-hyperpolarized gases. [13,14] The limitation of low NMR sensitivity spurs the implementation of hyperpolarization techniques, which can potentially increase the intensity of NMR signals by up to several orders of magnitude, including spin-ex...

Spatially resolved NMR spectroscopy of heterogeneous gas phase hydrogenation of 1,3-butadiene with parahydrogen

“…We have described recently a convenient approach to obtain glass tube reactors with Rh/TiO 2 coatings that allowed for MRI visualization of the distribution of reagents and products in the heterogeneous hydrogenation of propene to propane. 46 Here, we extend this approach to prepare reactive coatings based on CeO 2 , SiO 2 , and Al 2 O 3 oxide supports ( Fig. 1a) with the aim of identifying and studying catalysts for the selective semihydrogenation of 1,3-butadiene ( Fig.…”

“…0.5 μmol of these complexes onto each coating. 46 Since trisIJdibenzylideneacetone)dipalladium and trisIJdibenzylideneacetone)platinum were less soluble in acetonitrile than [(COD)RhCl] 2 and [(COD)IrCl] 2 complexes, we used a 1 : 1 acetonitrile-DCM solvent mixture and deposited ca. 0.4 μmol of Pd and Pt (on the metal basis) on each reactor tube.…”

“…Hydrogenation was performed in the 7.1 T magnetic field of the NMR spectrometer at atmospheric pressure and 130°C. The fabrication of the catalytic reactors was similar to the procedure described previously 46 except that drop-casting on a spinning borosilicate glass tube was used instead of dip-coating. One of the produced reactors was placed at the bottom of a 10 mm NMR tube, and the gas mixture was supplied through a 1.6 mm Teflon capillary placed concentrically along the reactor.…”

“…We reported recently that model glass reactors with catalytically active Rh/TiO 2 coatings can minimize the magnetic field inhomogeneity over the reactor for the catalytic hydrogenation of propene to propane. 46 Here, we extend the reactive coating approach to TiO 2 , CeO 2 , SiO 2 , and Al 2 O 3 which are used to support nanoparticles of Rh, Pt, Pd and Ir, giving a total of 16 reactors with a thin layer of reactive coating designed for NMR and MRI investigations of catalytic hydrogenation reactions. We chose 1,3-butadiene as a substrate with the objective of identifying selective semihydrogenation catalysts, which are materials for the formation of 1-or 2-butenes from 1,3-butadiene, as well as identifying catalysts giving the highest polarization of the hydrogenation products.…”

Bridging the Gap: From Homogeneous to Heterogeneous Parahydrogen‐induced Hyperpolarization and Beyond

“…HET-PHIP was also successfully used to enhance MR imaging experiments, for instance to address gas flow in various geometries (Figure 4), and to obtain 2D and 3D maps of product distribution in a model catalytic reactor. [31] While supported metals do work in PHIP experiments, at present the achieved polarizations normally do not exceed a few percent in experiments where significant product yields have been achieved. Therefore, an active search for better heterogeneous catalysts continues.…”