Pairwise Parahydrogen Addition Over Molybdenum Carbide Catalysts

Implications for the Hydrogenation of Propyne and Propene with Parahydrogen due to the in situ Transformation of Rh2C to Rh0/C

NMR spectroscopy studies using parahydrogen-induced polarization have previously established the existence of the pairwise hydrogen addition route in the hydrogenation of unsaturated hydrocarbons over heterogeneous catalysts, including those based on rhodium (Rh0). This pathway requires the incorporation of both hydrogen atoms from one hydrogen molecule to the same product molecule. However, the underlying mechanism for such pairwise hydrogen addition must be better understood. The involvement of carbon, either in the form of carbonaceous deposits on the surface of a catalyst or as a metal carbide phase, is known to modify catalytic properties significantly and thus could also affect the pairwise hydrogen addition route. Here, we explored carbon's role by studying the hydrogenation of propene and propyne with parahydrogen on a Rh2C catalyst and comparing the results with those for a Rh0/C catalyst obtained from Rh2C via H2 pretreatment. While the catalysts Rh2C and Rh0/C differ notably in the rate of conversion of parahydrogen to normal hydrogen as well as in terms of hydrogenation activity, our findings suggest that the carbide phase does not play a significant role in the pairwise H2 addition route on rhodium catalysts.

Contrasting Metallic (Rh0) and Carbidic (2D-Mo2C MXene) Surfaces in Olefin Hydrogenation Provides Insights on the Origin of the Pairwise Hydrogen Addition

Kinetic studies are vital for gathering mechanistic insights into heterogeneously catalyzed hydrogenation of unsaturated organic compounds (olefins), where the Horiuti-Polanyi mechanism is ubiquitous on metal catalysts. While this mechanism envisions nonpairwise H2 addition due to the rapid scrambling of surface hydride (H*) species, a pairwise H2 addition is experimentally encountered, rationalized here based on density functional theory (DFT) simulations for the ethene (C2H4) hydrogenation catalyzed by two-dimensional (2D) MXene Mo2C(0001) surface and compared to Rh(111) surface. Results show that ethyl (C2H5*) hydrogenation is the rate-determining step (RDS) on Mo2C(0001), yet C2H5* formation is the RDS on Rh(111), which features a higher reaction rate and contribution from pairwise H2 addition compared to 2D-Mo2C(0001). This qualitatively agrees with the experimental results for propene hydrogenation with parahydrogen over 2D-Mo2C1-x MXene and Rh/TiO2. However, DFT results imply that pairwise selectivity should be negligible owing to the facile H* diffusion on both surfaces, not affected by H* nor C2H4* coverages. DFT results also rule out the Eley-Rideal mechanism appreciably contributing to pairwise addition. The measurable contribution of the pairwise hydrogenation pathway operating concurrently with the dominant nonpairwise one is proposed to be due to the dynamic site blocking at higher adsorbate coverages or another mechanism that would drastically limit the diffusion of H* adatoms.

Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Advancing homogeneous catalysis for parahydrogen-derived hyperpolarisation and its NMR applications

Parahydrogen-induced polarisation (PHIP) is a nuclear spin hyperpolarisation technique employed to enhance NMR signals for a wide range of molecules. This is achieved by exploiting the chemical reactions of parahydrogen (para-H2), the spin-0 isomer of H2. These reactions break the molecular symmetry of para-H2 in a way that can produce dramatically enhanced NMR signals for reaction products, and are usually catalysed by a transition metal complex. In this review, we discuss recent advances in novel homogeneous catalysts that can produce hyperpolarised products upon reaction with para-H2. We also discuss hyperpolarisation attained in reversible reactions (termed signal amplification by reversible exchange, SABRE) and focus on catalyst developments in recent years that have allowed hyperpolarisation of a wider range of target molecules. In particular, recent examples of novel ruthenium catalysts for trans and geminal hydrogenation, metal-free catalysts, iridium sulfoxide-containing SABRE systems, and cobalt complexes for PHIP and SABRE are reviewed. Advances in this catalysis have expanded the types of molecules amenable to hyperpolarisation using PHIP and SABRE, and their applications in NMR reaction monitoring, mechanistic elucidation, biomedical imaging, and many other areas, are increasing.

Recent advances in the application of parahydrogen in catalysis and biochemistry

Nuclear Magnetic Resonance (NMR) spectroscopy and Magnetic Resonance Imaging (MRI) are analytical and diagnostic tools that are essential for a very broad field of applications, ranging from chemical analytics, to non-destructive testing of materials and the investigation of molecular dynamics, to in vivo medical diagnostics and drug research. One of the major challenges in their application to many problems is the inherent low sensitivity of magnetic resonance, which results from the small energy-differences of the nuclear spin-states. At thermal equilibrium at room temperature the normalized population difference of the spin-states, called the Boltzmann polarization, is only on the order of 10-5. Parahydrogen induced polarization (PHIP) is an efficient and cost-effective hyperpolarization method, which has widespread applications in Chemistry, Physics, Biochemistry, Biophysics, and Medical Imaging. PHIP creates its signal-enhancements by means of a reversible (SABRE) or irreversible (classic PHIP) chemical reaction between the parahydrogen, a catalyst, and a substrate. Here, we first give a short overview about parahydrogen-based hyperpolarization techniques and then review the current literature on method developments and applications of various flavors of the PHIP experiment.

Two-dimensional molybdenum carbide 2D-Mo2C as a superior catalyst for CO2 hydrogenation

Early transitional metal carbides are promising catalysts for hydrogenation of CO₂. Here, a two-dimensional (2D) multilayered 2D-Mo₂C material is prepared from Mo₂CT_x of the MXene family. Surface termination groups T_x (O, OH, and F) are reductively de-functionalized in Mo₂CT_x (500 °C, pure H₂) avoiding the formation of a 3D carbide structure. CO₂ hydrogenation studies show that the activity and product selectivity (CO, CH₄, C₂–C₅ alkanes, methanol, and dimethyl ether) of Mo₂CT_x and 2D-Mo₂C are controlled by the surface coverage of T_x groups that are tunable by the H₂ pretreatment conditions. 2D-Mo₂C contains no T_x groups and outperforms Mo₂CT_x, β-Mo₂C, or the industrial Cu-ZnO-Al₂O₃ catalyst in CO₂ hydrogenation (evaluated by CO weight time yield at 430 °C and 1 bar). We show that the lack of surface termination groups drives the selectivity and activity of Mo-terminated carbidic surfaces in CO₂ hydrogenation.

The development of robust and efficient catalysts for CO₂ hydrogenation to value-added chemicals is an urgent task. Here the authors report two-dimensional carbide catalyst based on earth-abundant molybdenum that hydrogenates CO₂ with high activity, stable performance and tunable selectivity.

Recent Progress in Emerging Two-Dimensional Transition Metal Carbides

As a new member in two-dimensional materials family, transition metal carbides (TMCs) have many excellent properties, such as chemical stability, in-plane anisotropy, high conductivity and flexibility, and remarkable energy conversation efficiency, which predispose them for promising applications as transparent electrode, flexible electronics, broadband photodetectors and battery electrodes. However, up to now, their device applications are in the early stage, especially because their controllable synthesis is still a great challenge. This review systematically summarized the state-of-the-art research in this rapidly developing field with particular focus on structure, property, synthesis and applicability of TMCs. Finally, the current challenges and future perspectives are outlined for the application of 2D TMCs.

Heterogeneous Catalysis and Parahydrogen-Induced Polarization

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.

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

Demonstration of parahydrogen-induced polarization effects in hydrogenations catalyzed by heterogeneous catalysts instead of metal complexes in a homogeneous solution has opened an entirely new dimension for parahydrogen-based research, demonstrating its applicability not only for the production of catalyst-free hyperpolarized liquids and gases and long-lived non-equilibrium spin states for potential biomedical applications, but also for addressing challenges of modern fundamental and industrial catalysis including advanced mechanistic studies of catalytic reactions and operando NMR and MRI of reactors. This essay summarizes the progress achieved in this field by highlighting the research contributed to it by our colleague and friend Kirill V. Kovtunov whose scientific career ended unexpectedly and tragically at the age of 37. His role in this research was certainly crucial, further enhanced by a vast network of his contacts and collaborations at the national and international level.

Catalytic hydrogenation with parahydrogen: a bridge from homogeneous to heterogeneous catalysis

One of the essential themes in modern catalysis is that of bridging the gap between its homogeneous and heterogeneous counterparts to combine their individual advantages and overcome shortcomings. One more incentive can now be added to the list, namely the ability of transition metal complexes to provide strong nuclear magnetic resonance (NMR) signal enhancement upon their use in homogeneous hydrogenations of unsaturated compounds with parahydrogen in solution. The addition of both H atoms of a parahydrogen molecule to the same substrate, a prerequisite for such effects, is implemented naturally with metal complexes that operate via the formation of a dihydride intermediate, but not with most heterogeneous catalysts. Despite that, it has been demonstrated in recent years that various types of heterogeneous catalysts are able to perform the required pairwise H2 addition at least to some extent. This has opened a major gateway for developing highly sensitive and informative tools for mechanistic studies of heterogeneous hydrogenations and other processes involving H2. Besides, production of catalyst-free fluids with NMR signals enhanced by 3-4 orders of magnitude is essential for modern applications of magnetic resonance imaging (MRI), including biomedical research and practice. The ongoing efforts to design heterogeneous catalysts which can implement the homogeneous (pairwise) hydrogenation mechanism are reported.