Parahydrogen-induced polarization with a metal-free P–P biradicaloid

Abstract Metal-free H₂ activations are unusual but interesting for catalytic transformations, particularly in parahydrogen-based nuclear spin hyperpolarization techniques. We demonstrate that metal-free singlet phosphorus biradicaloid, [P(μ-NTer)]₂, provides pronounced ¹H and ³¹P hyperpolarization while activating the parahydrogen molecules. A brief analysis of the resulting NMR signals and the important kinetic parameters are presented

Quantifying the adsorption of flowing gas mixtures in porous materials by remote detection NMR

Abstract Traditional adsorption measurements are carried out at static conditions for a single gas component, and multi-component adsorption measurements are challenging and time-consuming. Here we introduce an efficient remote detection NMR method for in situ analysis of adsorption of flowing gas in mesoporous materials. We investigated adsorption of continuously flowing propane and propene gases as well as their mixture in packed beds of controlled pore glass and silica gel materials. Remote detection provided from 300 to 700 -fold sensitivity enhancement as compared to a direct experiment carried out by a large coil around the packed bed. The unique time-of-flight information obtained using this method was utilized in flow velocity determination, and the velocities were converted into amount of adsorbed gas. As the detection was performed outside the packed bed region, the spectra were not influenced by the porous materials. Because resonances of each gas component were well-resolved in the spectra, the amount of adsorption of each gas component could be determined from the same data, measured in a few minutes

Parahydrogen-induced polarization study of the silica-supported vanadium oxo organometallic catalyst

Abstract Parahydrogen can be used in catalytic hydrogenations to achieve substantial enhancement of NMR signals of the reaction products and in some cases of the reaction reagents as well. The corresponding nuclear spin hyperpolarization technique, known as parahydrogen-induced polarization (PHIP), has been applied to boost the sensitivity of NMR spectroscopy and magnetic resonance imaging by several orders of magnitude. The catalyst properties are of paramount importance for PHIP because the addition of parahydrogen to a substrate must be pairwise. This requirement significantly narrows down the range of the applicable catalysts. Herein, we study an efficient silica-supported vanadium oxo organometallic complex (VCAT) in hydrogenation and dehydrogenation reactions in terms of efficient PHIP production. This is the first example of group 5 catalyst used to produce PHIP. Hydrogenations of propene and propyne with parahydrogen over VCAT demonstrated production of hyperpolarized propane and propene, respectively. The achieved NMR signal enhancements were 200−300-fold in the case of propane and 1300-fold in the case of propene. Propane dehydrogenation in the presence of parahydrogen produced no hyperpolarized propane, but instead the hyperpolarized side-product 1-butene was detected. Test experiments of other group 5 (Ta) and group 4 (Zr) catalysts showed a much lower efficiency in PHIP as compared to that of VCAT. The results prove the general conclusion that vanadium-based catalysts and other group 4 and group 5 catalysts can be used to produce PHIP. The hydrogenation/dehydrogenation processes, however, are accompanied by side reactions leading, for example, to C4, C2, and C1 side products. Some of the side products like 1-butene and 2-butene were shown to appear hyperpolarized, demonstrating that the reaction mechanism includes pairwise parahydrogen addition in these cases as well

Efficient catalytic microreactors with atomic-layer-deposited platinum nanoparticles on oxide support

Abstract Microreactors attract a significant interest for chemical synthesis due to the benefits of small scales such as high surface to volume ratio, rapid thermal ramping, and well-understood laminar flows. The suitability of atomic layer deposition for application of both the nanoparticle catalyst and the support material on the surfaces of channels of microfabricated silicon microreactors is demonstrated in this research. Continuous-flow hydrogenation of propene into propane at low temperatures with TiO₂-supported catalytic Pt nanoparticles was used as a model reaction. Reaction yield and mass transport were monitored by high-sensitivity microcoil NMR spectroscopy as well as time-of-flight remote detection NMR imaging. The microreactors were shown to be very efficient in propene conversion into propane. The yield of 100 % was achieved at 50 °C with a reactor decorated with Pt nanoparticles of average size of roughly 1 nm and surface coverage of 3.2 % in 20 mm long reaction channels with a residence time of 1100 ms. The activity of the Pt catalyst surfaces was on the order of several to tens of mmol s−1 m−2

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

ISSN:1867-3880ISSN:1867-389

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

Magnetic resonance-based methods such as nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are widely used to provide in situ/operando information of chemical reactions. However, the low spin density and magnetic field inhomogeneities associated with heterogeneous catalytic systems containing gaseous reactants complicate such studies. Hyperpolarization techniques, in particular parahydrogen-induced polarization (PHIP), increase significantly the NMR signal intensity. In this study, we test 16 glass tube reactors containing Pd, Pt, Rh or Ir nanoparticles dispersed on a thin layer of TiO2, CeO2, SiO2 or Al2O3 for the hydrogenation of 1,3-butadiene using parahydrogen. The catalytic coatings of Ir and Rh gave hydrogenation products with the highest nuclear spin polarization while the coatings of Pd are the most selective ones for the semihydrogenation of 1,3-butadiene to 1- and 2-butenes. Spatially resolved NMR spectroscopy of the reagent and the product distribution along the reactor axis provided further mechanistic insight into the catalytic function of these reactive coatings under operando conditions.ISSN:2044-4753ISSN:2044-476

Deciphering the Nature of Ru Sites in Reductively Exsolved Oxides with Electronic and Geometric Metal–Support Interactions

The reductive exsolution of metallic Ru from fluorite-type solid solutions Ln2Ru0.2Ce1.8O7 (Ln = Sm, Nd, La) leads to materials with metal–support interactions that influence the electronic state and the catalytic activity of Ru. In situ X-ray absorption spectroscopy at the Ru K-edge identified that with increasing temperature, the exsolution of Ru from Sm2Ru0.2Ce1.8O7 in a H2 atmosphere proceeds via an intermediate Ruδ+ state, that is, Ru4+→Ruδ+→Ru0. X-ray photoelectron spectroscopy (XPS) established that, in parallel (H2 atmosphere at ca. 500 °C), also Ce4+ ions reduce to Ce3+, which is accompanied by an electron transfer from the reduced host oxide to the exsolved Ru0 clusters, creating Ruδ− states. Low-temperature diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) using CO as a probe molecule reveals a red shift of the CO adsorption bands by ca. 18 cm–1 when increasing the temperature during the H2 treatment from 300 to 500 °C, consistent with an increased π-backdonation from more electron-rich Ru species to CO. However, at a lower reduction temperature of ca. 100 °C, a blue-shifted CO band is observed that is explained by a Lewis-acidic Ruδ+–CO adduct. Nuclear magnetic resonance (NMR) signal enhancement in parahydrogen-induced polarization experiments was used as a structure-sensitive probe and revealed a decreasing propene hydrogenation rate with increasing exsolution temperature, accompanied by a notable enhancement of propane hyperpolarization (ca. 3-fold higher at 500 °C than at 300 °C). These data suggest that the exsolved, subnanometer-sized Ru species are more active in propene hydrogenation but less selective for the pairwise addition of p-H2 to propene than Ruδ− sites engaged in a strong metal–support interaction. © 2020 American Chemical Society.ISSN:1932-7455ISSN:1932-744

Bimetallic Pd–Au/Highly Oriented Pyrolytic Graphite Catalysts: from Composition to Pairwise Parahydrogen Addition Selectivity

The model Pd and Au mono- and bi-metallic (Pd–Au) catalysts were prepared using vapor deposition of metals (Au and/or Pd) under ultrahigh vacuum conditions on the defective highly oriented pyrolytic graphite (HOPG) surface. The model catalysts were investigated using the X-ray photoelectron spectroscopy and scanning tunneling microscopy at each stage of the preparation procedure. For the preparation of bimetallic catalysts, different procedures were used to get different structures of PdAu particles (Au_shell–Pd_core or alloyed). All prepared catalysts showed rather narrow particles size distribution with an average particles size in the range of 4–7 nm. Parahydrogen-enhanced nuclear magnetic resonance spectroscopy was used as a tool for the investigation of Pd–Au/HOPG, Pd/HOPG, and Au/HOPG model catalysts in propyne hydrogenation. In contrast to Au sample, Pd, PdAu_alloy, and Au_shell–Pd_core samples were shown to have catalytic activity in propyne conversion, and pairwise hydrogen addition routes were observed. Moreover, bimetallic samples demonstrated the 2- to 5-fold higher activity in pairwise hydrogen addition in comparison to the monometallic Pd sample. It was shown that the structures of bimetallic Pd–Au particles supported on HOPG strongly affected their activities and/or selectivities in propyne hydrogenation reaction: the catalyst with the Au_shell–Pd_core structure demonstrated higher pairwise selectivity than that with the PdAu_alloy structure. Thus, the reported approach can be used as an effective tool for the synergistic effects investigations in hydrogenation reactions over model bimetallic Pd–Au catalysts, where the active component is supported on a planar support

Ultrafast multidimensional Laplace NMR for a rapid and sensitive chemical analysis

Abstract Traditional nuclear magnetic resonance (NMR) spectroscopy relies on the versatile chemical information conveyed by spectra. To complement conventional NMR, Laplace NMR explores diffusion and relaxation phenomena to reveal details on molecular motions. Under a broad concept of ultrafast multidimensional Laplace NMR, here we introduce an ultrafast diffusion-relaxation correlation experiment enhancing the resolution and information content of corresponding 1D experiments as well as reducing the experiment time by one to two orders of magnitude or more as compared with its conventional 2D counterpart. We demonstrate that the method allows one to distinguish identical molecules in different physical environments and provides chemical resolution missing in NMR spectra. Although the sensitivity of the new method is reduced due to spatial encoding, the single-scan approach enables one to use hyperpolarized substances to boost the sensitivity by several orders of magnitude, significantly enhancing the overall sensitivity of multidimensional Laplace NMR