Parahydrogen-Induced Polarization in Heterogeneous

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Parahydrogen-Induced Polarization in Heterogeneous Hydrogenations Catalyzed by an Immobilized Au(III) Complex Kirill V. Kovtunov,† Vladimir V. Zhivonitko,† Avelino Corma,‡ and Igor V. Koptyug*,† †

International Tomography Center, SB RAS, 3A Institutskaya Street, Novosibirsk 630090, Russia, and Instituto de Tecnologia Quimica UPV-CSIC, Avda. de los Naranjos, s/n, 46022 Valencia, Spain

‡

ABSTRACT Hydrogenation of unsaturated compounds with parahydrogen can lead to an enhancement of the NMR signals of reaction products by several orders of magnitude. Parahydrogen-induced polarization (PHIP) of nuclear spins is useful for developing both novel MRI applications as well as hypersensitive tools for operando studies in catalysis, but until recently, PHIP was observed only in homogeneous hydrogenations. To assess the potential of combining PHIP with heterogeneous catalysis, heterogeneous gas-phase hydrogenation of propene and propyne with parahydrogen was carried out using a Au(III) Schiff base complex immobilized within a metal-organic framework material IRMOF-3. Observation of PHIP in the 1H NMR spectra of reaction products implies that both hydrogen atoms of a single H2 molecule are transferred as a pair to the same product molecule and supports the conclusions made earlier that the well-defined isolated Au(III) centers of this catalyst are the active sites involved in hydrogen activation. SECTION Surfaces, Interfaces, Catalysis

B

ridging the gap between homogeneous and heterogeneous catalysis is a very active area of research, driven by the desire to combine the well-defined structure and performance of homogeneous catalysts with the facile recyclability of their heterogeneous counterparts.1,2 Efforts in this area can provide a deeper understanding of the fundamental aspects of catalytic events and yield more efficient and even entirely new catalysts and catalytic processes based on the synergism of the support and the supported species. Furthermore, there are other good reasons to bridge this gap. It has been demonstrated recently that catalytic hydrogenation processes can be used to dramatically increase sensitivity in the applications of magnetic resonance imaging (MRI) to lab animal studies.3-5 In this context, hydrogenation of unsaturated substrates with parahydrogen can lead to parahydrogen-induced polarization (PHIP) of nuclear spins ; an enhancement of the NMR signals of reaction products by several orders of magnitude. Apart from dramatically improving the image quality in MRI, this can also advance a number of novel applications, for example, molecular and cellular imaging in living organisms. Until recently, PHIP-MRI was based exclusively on the homogeneous hydrogenation with transition-metal complexes.6-11 As in industrial catalysis, a facile removal of a heterogeneous (HET) catalyst would be beneficial in these applications since biocompatibility will be the key issue in extending such studies to humans. Applications of MRI in, for example, materials science and engineering also benefit greatly from the utilization of various strategies of the NMR signal enhancement of various fluids.

r 2010 American Chemical Society

Application of HET-PHIP in this context is an emerging field,12,13 and the development of the appropriate catalysts is under way.14,15 In the mean time, it is worth exploring the ability of HET-PHIP to provide useful information on the nature of the active catalytic sites. Indeed, the studies of homogeneous processes that involve hydrogen-activating metal complexes in solution have benefited greatly from the NMR signal enhancement provided by PHIP,10 while no such studies have been reported to date for heterogeneous hydrogenations. So far, HET-PHIP effects were reported only for immobilized Rh(I) complexes14 and for supported Pt and Pd metal nanoparticles.15 Further development of HET-PHIP, both as a highly sensitive tool for catalytic research and as a source of hyperpolarized fluids for novel NMR and MRI applications, requires a much deeper understanding of HETPHIP fundamentals. It would thus greatly benefit from extending PHIP observations to a much broader range of different heterogeneous catalysts. It is essential that such catalysts are well-characterized and remain stable under reactive conditions. In recent years, a lot of attention was attracted by supported gold catalysts.16,17 Supported Au nanoparticles catalyze various reactions, including oxidation and hydrogenation processes, but the nature of the active sites involved and the role of the support are often unclear. Recent studies add to the

Received Date: March 25, 2010 Accepted Date: May 12, 2010 Published on Web Date: May 14, 2010

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DOI: 10.1021/jz100391j |J. Phys. Chem. Lett. 2010, 1, 1705–1708

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Figure 2. 1H NMR spectrum detected during the hydrogenation of propyne using the IRMOF-3-SI-Au catalyst and H2 enriched with parahydrogen. The inset shows the low-field part scaled by a factor of 6. The methyne and methylene hydrogens of propene and the corresponding NMR lines are labeled X, Y, and Z.

product by the Au(III) reaction center. This may serve as a direct confirmation of the reaction mechanism for this type of catalysts, showing that, most likely, the hydrogen molecule is activated on a well-defined localized reaction center and that in any case, both hydrogen atoms of a single H2 molecule are transferred together (as a pair) to the same product molecule. Comparison of the two spectra shown in Figure 1 allows one to estimate the signal enhancement factor provided by PHIP. The ratio of the signal intensities gives an enhancement of ∼16, which is at least 30 times lower than what could be expected under ideal conditions. There are several reasons for that. First, each polarized multiplet has the positive and the negative components that partially cancel each other because of a finite width of the individual NMR lines. Second, the relaxation times of liquids and gases in porous materials characterized by large surface-to-volume ratios of the pore space (i.e., in small pores) are often significantly reduced as compared to bulk fluids. Therefore, it is likely that substantial polarization losses are caused by the interaction of a propane molecule with the pore walls of the support as it travels in the MOF pores from the reaction center out into the bulk gas phase. Because of the concurrent broadening of the NMR signals of the occluded propane molecules, only molecules in the bulk gas phase contribute to the detected spectra. Hydrogenation of an unsymmetrically substituted alkyne (e.g., propyne) allows one to use PHIP to address the stereoselectivity with respect to the syn and anti addition of the two H atoms to the substrate upon its hydrogenation. Indeed, in propene, the product of the partial hydrogenation of propyne, the two H atoms of the methylene group have different chemical shifts, and therefore, it is possible to tell from the PHIP spectra if one or both of them is polarized. As can be seen from the spectrum in Figure 2, only the signal of the H atom trans to the methyl group is polarized, which corresponds to the selective syn hydrogenation of propyne to propene by the IRMOF-3-SI-Au catalyst. In contrast, for supported Pt/Al2O3 catalysts, both methylene hydrogens were observed to exhibit PHIP enhancement,19 presumably because propene can undergo isomerization on supported metal catalysts.

Figure 1. 1H NMR spectra detected during the hydrogenation of propene using the IRMOF-3-SI-Au catalyst and (a) normal H2 or (b) H2 enriched with parahydrogen. The enhanced antiphase lines of the methylene and the methyl groups of propane are labeled A and B, respectively.

debate by providing evidence that cationic gold species rather than the metal particles are the active sites of catalytic transformations. A detailed understanding of the mechanisms involved can be sought by bridging the gap between the homogeneous and the heterogeneous gold catalysis.2 In particular, in a recent study, a metal-organic framework (MOF) material with a Au(III) Schiff base complex attached to the framework pore walls was prepared and characterized.18 This material, designated as IRMOF-3-SI-Au, was shown to be an active and stable catalyst for a number of catalytic transformations, including the heterogeneous gasphase hydrogenation of 1,3-butadiene. It was also ascertained that the Au(III) complex remains intact after the reaction and thus represents a well-defined isolated Au(III) active site that can activate molecular hydrogen. This is exactly what is needed for PHIP observation. Indeed, the correlated state of the nuclear spins of a parahydrogen molecule can lead to a NMR signal enhancement only if the two H atoms of the same H2 molecule end up in the same product molecule. Therefore, the well-defined localized active centers are of particular interest if PHIP effects are sought. We therefore tested the IRMOF-3-SI-Au catalyst in the gasphase hydrogenation of propene using parahydrogen to see if PHIP can be useful in the studies of reaction mechanisms. The results show that the IRMOF-3-SI-Au catalyst hydrogenates propene into propane (Figure 1a). Under the experimental conditions, the conversion was relatively low. When a parahydrogen-enriched mixture was used instead of normal H2, two strongly enhanced antiphase multiplets were observed at the chemical shifts corresponding to the methyl and methylene groups of propane (Figure 1b). This observation demonstrates that the nuclear spin order of two hydrogen atoms of the same hydrogen molecule is preserved in the hydrogenation


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(RUC1-2915-NO07), and the Council on Grants of the President of the Russian Federation (ΜK-1284.2010.3).

Finally, if cationic Au species are also the suspected active centers for reactions catalyzed by supported gold catalysts, then such catalysts should be also active in PHIP production, at least to the extent of the contribution of such centers to the overall hydrogenation reaction. Preliminary results show that hydrogenation of propene with parahydrogen on Au/Al2O3 supported catalysts does indeed demonstrate PHIP effects. This observation may be yet another indication that even for supported Au nanoparticles, the actual catalytic centers could be the cationic gold species. To summarize, parahydrogen-induced polarization of nuclear spins was observed in the products of heterogeneous gas-phase hydrogenations carried out with the Au(III) Schiff base complex immobilized within a metal-organic framework material. This is the first time that PHIP effects were observed for a gold catalyst. As it was established earlier, the immobilized Au(III) complex remains intact under reactive conditions.18 This is thus the first observation of HET-PHIP in a gas-phase hydrogenation where the nature of the active catalytic centers has been confirmed. The results obtained support the conclusions made earlier that the well-defined isolated Au(III) centers of this catalyst are the active sites involved in hydrogen activation and in other catalytic processes. This is an important step toward a new hypersensitive NMR-based tool for operando20 studies of heterogeneous catalytic hydrogenations.

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EXPERIMENTAL SECTION (9)

The NMR spectra were recorded on a 300 MHz Bruker AV300 NMR spectrometer. The IRMOF-3-SI-Au catalyst18 (50 mg) was placed in a 10 mm NMR tube, which was then positioned in the rf probe inside of the NMR spectrometer magnet and maintained at 130 °C throughout the hydrogenation experiment. Normal H2, for which the ratio of ortho and para isomers o/p is ∼3:1, was used as a reference. For PHIP experiments, H2 was enriched with parahydrogen to o/p ≈ 1:1 by passing it through a bed of FeO(OH) powder (SigmaAldrich) maintained at liquid N2 temperature. A 3:1 mixture of hydrogen and propene (or propyne) was supplied to the catalyst through a Teflon capillary that extended to the bottom of the NMR tube. The gas flow set at ∼300 mL/min induced the motion of the solid catalyst particles in the tube, thereby enhancing mass-transport processes. As a result of the catalyst bed fluidization, the number of catalyst particles in the sensitive region of the rf coil was relatively small, and the contribution of the occluded gas to the observed NMR signal was negligible. The 1H NMR spectra were detected without interrupting the gas flow using rf pulses with a π/4 flip angle.

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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: koptyug@ tomo.nsc.ru.

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ACKNOWLEDGMENT This work was partially supported by the

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grants from RAS (5.1.1), RFBR (08-03-00661, 08-03-00539), SB RAS (67, 88), the program of support of leading scientific schools (NSh-7643.2010.3), FASI (State Contract 02.740.11.0262), CRDF


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