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.

Scheme 1. VCAT Structure

Figure 1

Experimental setup and procedure. The reagent gas flow controlled by a flowmeter was supplied either to the packed bed reactor outside and then to the empty sample tube inside the NMR magnet (right branch, ALTADENA experiment), or directly into the sample tube with the catalyst inside the NMR magnet (left branch, PASADENA experiment).

Figure 2

¹H NMR spectra of the gaseous reaction mixture from propene hydrogenation catalyzed by VCAT. Spectrum (a) was detected while the propane was produced in the reactor at 350 °C and the reagent gas flow rate of 4.1 scc/s and the mixture was flowing through the sample tube, whereas spectrum (b) was detected 5 s after the gas flow was abruptly stopped. The initial propene/para-H₂ ratio in the reagent gas mixture was equal to 1/4.

Figure 3

¹H NMR signal enhancements measured as a function of the reactor temperature for hyperpolarized propane produced in propene hydrogenation with para-H₂ over VCAT. Average enhancement values over CH₂ and CH₃ groups are presented. The bar charts shown in (a–c) correspond to the reagent mixtures with 1/3, 1/4, and 1/6 propene/para-H₂ ratios, respectively. The reagent gas flow rates in the reactor are indicated in the legends.

Figure 4

Propene conversion as a function of temperature in propene hydrogenation over VCAT. The results are shown for the reagent gas mixtures of (a) 1/4 and (b) 1/6 propene/p-H₂ ratios and two different gas flow rates (2.1 and 4.1 scc/s).

Scheme 2. Formal Mechanism of Pairwise Replacement Leading to the Formation of Hyperpolarized Propane

Figure 5

¹H NMR spectra of reaction mixture formed in propane dehydrogenation on VCAT when the propane/para-H₂ (1/4) gas mixture was passed through the reactor at different rector temperatures of (a) 400, (b) 450, (c) 500, (d) 550, (e) 600, and (f) 650 °C.

Figure 6

¹H NMR spectra of the reaction mixture produced in propyne hydrogenation with para-H₂ over VCAT at 500 °C. Spectrum (a) was detected in the continuous flow mode using a single scan, whereas (b) is detected using 256 scan accumulations after the flow was abruptly stopped. The reagent mixture of 1/4 propyne/para-H₂ ratio was used, which was supplied to the reactor at the flow rate of 2.1 scc/s.

Figure 7

Enhancement factors and propene conversions obtained in propene hydrogenation with para-H₂ over TaCAT in the temperature range of 200–400 °C. The gas reagent flow rate was 2.1 scc/s. The enhancement factors are calculated for protons of CH₃ group of propane.