Acetylene Oligomerization over Pd Nanoparticles with Controlled Shape: A Parahydrogen-Induced Polarization Study

Substantial signal enhancements achieved by using parahydrogen in catalytic hydrogenations are powerful tools for mechanistic studies of chemical reactions involving molecular H-2. Potentially, this technique can be extended to other reaction classes, providing new information about reaction mechanisms. Moreover, this can lead to new substances with highly polarized spins. Herein, we report strong signal enhancements of oligomerization reaction products observed during the selective hydrogenation of acetylene over Pd nanoparticles of different shapes and sizes supported on SiO2. C4 oligomeric products (1,3-butadiene, 1-butene, 2-butene) demonstrated a high degree of nuclear spin polarization, with the highest degree observed for 1-butene [more than 1.7% vs (2.4 X 10(-3))% at thermal equilibrium], which was an order of magnitude larger than that of the triple C-C bond hydrogenation products. No dependence of polarization on the metal surface statistics Or, generally, on the nanoparticle morphology (cubic, octahedral, cuboctahedral) could be observed. In contrast, the particle size effect was such that larger particles provided higher signal enhancements. This observation is in line with the increased activity over larger Pd nanoparticles observed during acetylene hydrogenation over the same catalysts

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

Parahydrogen-Induced Polarization Study of the Silica-Supported Vanadium Oxo Organometallic Catalyst

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

Manipulating Stereoselectivity of Parahydrogen Addition to Acetylene to Unravel Interconversion of Ethylene Nuclear Spin Isomers

Symmetric molecules exist as distinct nuclear spin isomers (NSIMs). A deeper understanding of their properties, including interconversion, requires efficient techniques for NSIMs enrichment. Selective hydrogenation of acetylene with parahydrogen (p-H2) was used to achieve the enrichment of ethylene NSIMs and to study their equilibration processes. The effect of stereoselectivity of H2 addition to acetylene on the imbalance of ethylene NSIMs was experimentally demonstrated by using different heterogeneous catalysts (an immobilized Ir complex and two supported Pd catalysts). The interconversion of NSIMs with time during ethylene storage was studied with NMR spectroscopy by reacting ethylene with bromine water which renders the p-H2-derived protons in the produced 2-bromoethan(2H)ol (BrEtOD) magnetically inequivalent, thereby revealing the non-equilibrium nuclear spin order of ethylene. A thorough analysis of the shape and transformation of the 1H NMR spectra of hyperpolarized BrEtOD allowed us to reveal the initial distribution of produced ethylene NSIMs and their equilibration processes. Comparison of the results obtained with different catalysts was key to properly attributing the derived characteristic time constants to different NSIMs interconversion processes: ~ 3-6 s for interconversion between NSIMs with the same inversion symmetry (i.e., within g or u manifolds) and ~ 1700-2200 s between NSIMs with different inversion symmetries