Kinetic Study of Propylene Hydrogenation over Pt/Al 2 O 3 by Parahydrogen-Induced Polarization
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Appl Magn Reson DOI 10.1007/s00723-012-0400-3
Magnetic Resonance
Kinetic Study of Propylene Hydrogenation over Pt/ Al2O3 by Parahydrogen-Induced Polarization Oleg G. Salnikov • Kirill V. Kovtunov • Danila A. Barskiy • Valery I. Bukhtiyarov Robert Kaptein • Igor V. Koptyug
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Received: 20 July 2012 / Revised: 28 August 2012 Ó Springer-Verlag 2012
Abstract Parahydrogen-induced polarization has been successfully used for a kinetic study of propylene hydrogenation over a Pt/Al2O3 catalyst. It was shown that the reaction orders with respect to hydrogen are different for the pairwise and the non-pairwise hydrogen addition and are equal to 0.7 and 0.1, respectively. This observation of different reaction orders confirms the coexistence of different types of active sites which are responsible for the overall and the pairwise hydrogen addition to the propylene C=C double bond. Moreover, 0.7 reaction order with respect to H2 for pairwise hydrogen addition indicates that the contribution of pairwise addition depends on the concentration of molecular hydrogen. Therefore, this observation can be developed into a practical tool for producing fluids with highly polarized nuclear spins by changing the hydrogen concentration.
1 Introduction Parahydrogen-induced polarization (PHIP) is a very informative method for the investigation of hydrogenation reactions [1–7] and can also be used as a source of a O. G. Salnikov K. V. Kovtunov D. A. Barskiy I. V. Koptyug International Tomography Center, Siberian Branch, Russian Academy of Sciences, Institutskaya St. 3A, Novosibirsk 630090, Russia O. G. Salnikov K. V. Kovtunov D. A. Barskiy R. Kaptein I. V. Koptyug (&) Novosibirsk State University, Pirogova St. 2, Novosibirsk 630090, Russia e-mail: [email protected]; [email protected] V. I. Bukhtiyarov Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, 5 Acad. Lavrentiev Pr., Novosibirsk 630090, Russia R. Kaptein Bijvoet Center, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands
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significant enhancement of nuclear magnetic resonance (NMR) signals which is very important for advanced magnetic resonance imaging applications [8–11]. As a rule, this method works only when both atoms from one parahydrogen molecule are added to the same substrate molecule (the only known exception is the hydroformylation reaction in which polarization may be observed even though only one hydrogen atom from the H2 molecule ends up in the product [12]). If the two hydrogen atoms end up in magnetically non-equivalent positions in the product molecule, a significant enhancement of 1H NMR signals is observed. During homogeneous hydrogenation processes, many catalysts produce dihydride complexes that are normally invisible in the NMR spectra due to their low concentration; however, these complexes may be revealed by PHIP. The strong PHIP signal enhancement also allows one to study the kinetics of homogeneous reactions [13–16]. In these studies, kinetic data were obtained in exp
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