Heterogeneous Catalysis and Parahydrogen‐Induced Polarization

Abstract

Parahydrogen‐induced polarization with heterogeneous catalysts (HET‐PHIP) has been a subject of extensive research in the last decade since its first observation in 2007. While NMR signal enhancements obtained with such catalysts are currently below those achieved with transition metal complexes in homogeneous hydrogenations in solution, this relatively new field demonstrates major prospects for a broad range of advanced fundamental and practical applications, from providing catalyst‐free hyperpolarized fluids for biomedical magnetic resonance imaging (MRI) to exploring mechanisms of industrially important heterogeneous catalytic processes. This review covers the evolution of the heterogeneous catalysts used for PHIP observation, from metal complexes immobilized on solid supports to bulk metals and single‐atom catalysts and discusses the general visions for maximizing the obtained NMR signal enhancements using HET‐PHIP. Various practical applications of HET‐PHIP, both for catalytic studies and for potential production of hyperpolarized contrast agents for MRI, are described.

Pd on Nanodiamond/Graphene in Hydrogenation of Propyne with Parahydrogen

“…Pd–Au, , Pd–Ag, Pd–In, and Pt–Sn catalysts were recently investigated in terms of PHIP; for Pt–Sn catalysts, a pairwise selectivity of 10.9% was observed, but the overall conversion was estimated only as 0.5% . The progress of catalyst design in the context of HET-PHIP is covered in recent reviews. , …”

“…34 The progress of catalyst design in the context of HET-PHIP is covered in recent reviews. 28,35 In addition to the requirement of pairwise addition of H 2 to an unsaturated substrate, the observation of enhanced 1 H NMR signals of hydrogenated products as a result of the PHIP effect 35 also requires that the equivalence of p-H 2 -derived protons must be broken in the reaction. For instance, for ethylene produced in hydrogenation of acetylene, all its hydrogen atoms are equivalent so that no NMR signal enhancement is produced even if addition of parahydrogen to acetylene is pairwise.…”

Recent advances in the application of parahydrogen in catalysis and biochemistry

“…, by dipolar interactions, 177 and result in a PHIP. An overview about these variants of PHIP was recently published by Pokochueva et al 178 …”

Mechanisms of Methylenecyclobutane Hydrogenation over Supported Metal Catalysts Studied by Parahydrogen‐Induced Polarization Technique

“…Parahydrogen‐induced polarization (PHIP) [16,17] is a method which can provide unique information about mechanisms of reactions involving hydrogen [18–21] . PHIP is based on the use of parahydrogen (p‐H 2 ), a nuclear spin isomer of H 2 molecule with zero nuclear spin.…”

“…[1,[11][12][13][14][15] Parahydrogen-induced polarization (PHIP) [16,17] is a method which can provide unique information about mechanisms of reactions involving hydrogen. [18][19][20][21] PHIP is based on the use of parahydrogen (p-H 2 ), a nuclear spin isomer of H 2 molecule with zero nuclear spin. When p-H 2 is added to an asymmetric unsaturated substrate in a pairwise manner (i. e., when two atoms from the same p-H 2 molecule end up in the same molecule of reaction product), the corresponding signals in 1 H NMR spectrum (i) are significantly enhanced in intensity due to hyperpolarization of nuclear spins (i. e., a non-equilibrium population of nuclear spin energy levels is created) and (ii) have a characteristic antiphase line shape.…”