Catalysis is vital to industrial chemistry, and the optimization of catalytic reactors attracts considerable resources. It has proven challenging to correlate the active regions in heterogeneous catalyst beds with morphology and to monitor multistep reactions within the bed. We demonstrate techniques, using magnetic resonance imaging and para-hydrogen (p-H2) polarization, that allow direct visualization of gas-phase flow and the density of active catalyst in a packed-bed microreactor, as well as control over the dynamics of the polarized state in space and time to facilitate the study of subsequent reactions. These procedures are suitable for characterizing reactors and reactions in microfluidic devices where low sensitivity of conventional magnetic resonance would otherwise be the limiting factor.

For homogeneous hydrogenation reactions catalyzed by transition-metal complexes in solution, utilization of the nuclear spin isomers of molecular hydrogen has become an established tool for studies on reaction mechanisms and kinetics.[1] Parahydrogen-induced polarization [2] (PHIP) can enhance the NMR spectroscopy signals of reaction intermediates and products by several orders of magnitude and provides the high sensitivity essential for such studies. It was demonstrated recently [3,4] that PHIP effects can also be observed in hydrogenation reactions catalyzed by metal complexes immobilized on a solid support. Industrial hydrogenation processes are predominantly heterogeneous and utilize supported metal catalysts. Such catalysts are not expected to produce PHIP effects, [5] since the reaction mechanism involved should destroy the original correlation of the two nuclear spins of parahydrogen. Herein we demonstrate for the first time that, contrary to these expectations, supported metal catalysts such as Pt/Al 2 O 3 and Pd/ Al 2 O 3 do exhibit PHIP effects. This fact can be used for the production of spin-polarized fluids for MRI applications and for developing new research tools for mechanistic and kinetic studies on heterogeneous hydrogenation processes.

Parahydrogen-induced polarization of nuclear spins provides enhancements of NMR signals for various nuclei of up to four to five orders of magnitude in magnetic fields of modern NMR spectrometers and even higher enhancements in low and ultra-low magnetic fields. It is based on the use of parahydrogen in catalytic hydrogenation reactions which, upon pairwise addition of the two H atoms of parahydrogen, can strongly enhance the NMR signals of reaction intermediates and products in solution. A recent advance in this field is the demonstration that PHIP can be observed not only in homogeneous hydrogenations but also in heterogeneous catalytic reactions. The use of heterogeneous catalysts for generating PHIP provides a number of significant advantages over the homogeneous processes, including the possibility to produce hyperpolarized gases, better control over the hydrogenation process, and the ease of separation of hyperpolarized fluids from the catalyst. The latter advantage is of paramount importance in light of the recent tendency toward utilization of hyperpolarized substances in in vivo spectroscopic and imaging applications of NMR. In addition, PHIP demonstrates the potential to become a useful tool for studying mechanisms of heterogeneous catalytic processes and for in situ studies of operating catalytic reactors. Here, the known examples of PHIP observations in heterogeneous reactions over immobilized transition metal complexes, supported metals, and some other types of heterogeneous catalysts are discussed and the applications of the technique for hypersensitive NMR imaging studies are presented.

Substantial (31)P NMR signal enhancement of more than two orders of magnitude at 7 T for free and bound PPh3 species was observed under reversible interaction of (PPh3)3Ir(H2)Cl with parahydrogen. The large improvement in sensitivity made single-shot (31)P NMR imaging of a model object possible. The observed effects are temperature and magnetic field dependent as shown experimentally and theoretically.

We demonstrate an analytical model for the description of the signal amplification by reversible exchange (SABRE) process. The model relies on a combined analysis of chemical kinetics and the evolution of the nuclear spin system during the hyperpolarization process. The presented model for the first time provides rationale for deciding which system parameters (i.e. J-couplings, relaxation rates, reaction rate constants) have to be optimized in order to achieve higher signal enhancement for a substrate of interest in SABRE experiments.

The activation and conversion of hydrocarbons is one of the most important challenges in chemistry. Transition-metal ions (V, Cr, Fe, Co, etc.) isolated on silica surfaces are known to catalyze such processes. The mechanisms of these processes are currently unknown but are thought to involve C-H activation as the rate-determining step. Here, we synthesize well-defined Co(II) ions on a silica surface using a metal siloxide precursor followed by thermal treatment under vacuum at 500 °C. We show that these isolated Co(II) sites are catalysts for a number of hydrocarbon conversion reactions, such as the dehydrogenation of propane, the hydrogenation of propene, and the trimerization of terminal alkynes. We then investigate the mechanisms of these processes using kinetics, kinetic isotope effects, isotopic labeling experiments, parahydrogen induced polarization (PHIP) NMR, and comparison with a molecular analog. The data are consistent with all of these reactions occurring by a common mechanism, involving heterolytic C-H or H-H activation via a 1,2 addition across a Co-O bond.

We demonstrate the creation and observation of para-hydrogen-induced polarization in heterogeneous hydrogenation reactions. Wilkinson's catalyst, RhCl(PPh3)3, supported on either modified silica gel or a polymer, is shown to hydrogenate styrene into ethylbenzene and to produce enhanced spin polarizations, observed through NMR, when the reaction was performed with H2 gas enriched in the para spin isomer. Furthermore, gaseous phase para-hydrogenation of propylene to propane with two catalysts, the Wilkinson's catalyst supported on modified silica gel and Rh(cod)(sulfos) (cod = cycloocta-1,5-diene; sulfos = -O3S(C6H4)CH2C(CH2PPh2)3) supported on silica gel, demonstrates heterogeneous catalytic conversion resulting in large spin polarizations. These experiments serve as a direct verification of the mechanism of heterogeneous hydrogenation reactions involving immobilized metal complexes and can be potentially developed into a practical tool for producing catalyst-free fluids with highly polarized nuclear spins for a broad range of hyperpolarized NMR and MRI applications.

The photochemistry of two phosphine oxides and the rate constants of reaction of their daughter radicals with several alkenes, halocarbons, and oxygen have been determined. Photolysis of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (1) and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (4) in each case affords a phosphinoyl and a benzoyl radical. The phosphinoyl radicals are readily detected by laser flash photolysis and exhibit absorption maxima at 325 and 450 nm for the diphenylphosphinoyl (3) and 2,6-dimethoxybenzoyl-2,4,4-trimethylpentylphosphinoyl (6) radicals, respectively. The rate constants for reaction of the phosphinoyl radicals with alkyl halides, alkenes, and oxygen range from 104 to 109 M-1 s-1. Radical 3 is 2−6 times more reactive than radical 6. For example, 3 adds to methyl methacrylate with a rate constant of (11 ± 2) × 107 M-1 s-1 whereas 6 has an addition rate constant for the same reaction of (2.3 ± 0.3) × 107 M-1 s-1. The rate constants for reaction with alkyl halides decrease with increasing C−X bond strength, while the rate constants for quenching by acrylates decrease with increasing methyl substitution on the β-carbon. The 2,6-dimethoxybenzoyl (5) and phosphinoyl (6) radicals derived from 4 are readily detected by time-resolved ESR (TR ESR); benzoyl radical 5 appears as a singlet and phosphinoyl radical 6 appears as a doublet of triplets (A(P) = 285 G, A(H) = 4.8 G). The CIDEP patterns of 5 and 6 indicate that the radicals are formed from α-cleavage of the triplet excited state of 4. TR ESR has also proved useful in the direct detection of the polarized benzyl radicals formed from addition of phosphinoyl radicals 3 and 6 to styrene and 2,4,6-trimethoxystyrene. The lower reactivity of 6 compared to 3 is attributed to its more planar structure and lower degree of spin localization in a s-orbital on phosphorus.