For homogeneous hydrogenation reactions catalyzed by transition-metal complexes in solution, utilization of the nuclear spin isomers of molecular hydrogen has become an established tool for studies on reaction mechanisms and kinetics.[1] Parahydrogen-induced polarization [2] (PHIP) can enhance the NMR spectroscopy signals of reaction intermediates and products by several orders of magnitude and provides the high sensitivity essential for such studies. It was demonstrated recently [3,4] that PHIP effects can also be observed in hydrogenation reactions catalyzed by metal complexes immobilized on a solid support. Industrial hydrogenation processes are predominantly heterogeneous and utilize supported metal catalysts. Such catalysts are not expected to produce PHIP effects, [5] since the reaction mechanism involved should destroy the original correlation of the two nuclear spins of parahydrogen. Herein we demonstrate for the first time that, contrary to these expectations, supported metal catalysts such as Pt/Al 2 O 3 and Pd/ Al 2 O 3 do exhibit PHIP effects. This fact can be used for the production of spin-polarized fluids for MRI applications and for developing new research tools for mechanistic and kinetic studies on heterogeneous hydrogenation processes.

Substantial NMR signal enhancements provided by parahydrogen-induced polarization (PHIP) are associated with the ability of a catalyst to incorporate both H atoms of a dihydrogen molecule into the same product molecule. Therefore, PHIP can provide valuable information about the mechanisms and kinetics of catalytic hydrogenation reactions as well as produce hyperpolarized molecules for sensitivity enhancement in NMR. In this work, the PHIP technique was applied to study the structure sensitivity and the support effects on the degree of pairwise H 2 addition in propene hydrogenation over supported platinum catalysts. Four series of Pt catalysts supported on Al 2 O 3 , SiO 2 , ZrO 2 , and TiO 2 were examined. A nontrivial dependence of the selectivity toward pairwise H 2 addition on the Pt particle size was found. Its analysis indicates that at least three types of different active sites coexist on the catalysts surface. Among them, the major one is responsible for the nonpairwise H 2 addition to the double bond, whereas pairwise addition can proceed on the other two minor active sites. An explanation of the nature of these active sites is proposed. A substantial increase in the pairwise addition selectivity was found for Pt/TiO 2 catalysts as compared to other catalyst series, possibly due to a strong metalÀsupport interaction taking place even after low temperature catalyst reduction.

For homogeneous hydrogenation reactions catalyzed by transition-metal complexes in solution, utilization of the nuclear spin isomers of molecular hydrogen has become an established tool for studies on reaction mechanisms and kinetics.[1] Parahydrogen-induced polarization [2] (PHIP) can enhance the NMR spectroscopy signals of reaction intermediates and products by several orders of magnitude and provides the high sensitivity essential for such studies. It was demonstrated recently [3,4] that PHIP effects can also be observed in hydrogenation reactions catalyzed by metal complexes immobilized on a solid support. Industrial hydrogenation processes are predominantly heterogeneous and utilize supported metal catalysts. Such catalysts are not expected to produce PHIP effects, [5] since the reaction mechanism involved should destroy the original correlation of the two nuclear spins of parahydrogen. Herein we demonstrate for the first time that, contrary to these expectations, supported metal catalysts such as Pt/Al 2 O 3 and Pd/ Al 2 O 3 do exhibit PHIP effects. This fact can be used for the production of spin-polarized fluids for MRI applications and for developing new research tools for mechanistic and kinetic studies on heterogeneous hydrogenation processes.