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.
Highlights
Parahydrogen-induced polarization (PHIP) is a nuclear spin hyperpolarization technique based on the use of parahydrogenenriched H2 in catalytic reactions.[1−3] Upon hydrogenations with para-H2, strong NMR signals of specific shape amplified by several orders of magnitude can be observed, revealing the high nuclear spin hyperpolarization in the reaction products
It was found that the catalyst provides the formation of hyperpolarized propane in propene hydrogenation by means of parahydrogen-induced polarization (PHIP), demonstrating significant NMR signal enhancements
Article of more than two orders of magnitude compared to thermal polarization. This is the first observation of PHIP effects over vanadium-based catalysts
Summary
Parahydrogen-induced polarization (PHIP) is a nuclear spin hyperpolarization technique based on the use of parahydrogenenriched H2 (para-H2) in catalytic reactions.[1−3] Upon hydrogenations with para-H2, strong NMR signals of specific shape amplified by several orders of magnitude can be observed, revealing the high nuclear spin hyperpolarization in the reaction products. Thermally polarized ethylene was detected (Figure 6b) Such observations imply that like in propane dehydrogenation the side reactions leading to the C4 and C2 hydrocarbons took place, meaning that propyne hydrogenation over the VCAT catalyst is accompanied either by oligomerization and hydrogenolysis processes or metathesis because the initial feed contained only the C3 hydrocarbon. ZrCAT immobilized complex was used as an example of group 4-based catalyst in propene hydrogenation with para-H2 This complex became active at 150 °C reaction temperature, and hyperpolarized propane was detected in 1H NMR spectra of the reaction mixture under continuous flow conditions (Figure S4 in Supporting Information). This assumption is supported by the production of hyperpolarized 1-butene with a 1H NMR signal shape similar to that detected in propene hydrogenation when 1,3-butadiene hydrogenation with para-H2 was tested at 300 °C with the same catalyst (Figure S5 in Supporting Information)
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