PHIP with Supported Metal Catalysts

The ability of transition metal complexes to produce metal dihydride species upon activation of pHj seemed for a long time to be a necessary prerequisite for the pairwise PH2 addition to a substrate and for the observation of the effects of PHIP in 

It was found in subsequent studies that PHIP effects observed experimentally depend dramatically on numerous factors including the type of metal and the metal particle size, the type of support, the substrate, and so on [15,90]. Therefore, 

D-g is defined as reaction dimension it may be used to deduce the nature of active sites involved in a catalytic process. 

Hydrogenation of other gaseous substrates on supported Pt catalysts was addressed using pH2 as well. Hydrogenation of propyne over Pt/y-Al203 catalyst was shown to produce PHIP in the products of both syn and anti addition of H2 to the substrate [90]. Thus, supported Pt metal catalysts achieve a non-stereoselective pairwise addition of H2 to propyne. The likely reason for this is the ability of supported metal catalysts to readily isomerize propylene. Non-stereoselective pairwise Hj addition to produce 1-butene was also observed in the hydrogenation of 1-butyne [15]. For this substrate, other products are possible. They include 2-butene (cis- and tram-) and butane, and all were shown to exhibit PHIP effects. A detailed scheme describing the formation of all these hyperpolarized products has been proposed [15]. 

The same hyperpolarized products are observed upon hydrogenation of 1,3-butadiene. It was suggested that the reaction proceeds via the semihydrogenation of 1,3-butadiene to produce a butenyl intermediate that can either give polarized 

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Supported metal catalysts can produce PHIP effects in liquid-phase hydrogenations as well, as was demonstrated successfully in several publications [15, 126, 127]. Bubbling of a mixture of propylene and pH2 through an NMR tube with a solvent (toluene, acetone) and solid catalyst (Rh/Ti02 or Rh/AlO(OH)) produced PHIP effects for the dissolved reaction product propane in both PASADENA and ALTADENA experiments [15,126]. The same two supported metal catalysts were used in the hydrogenation of styrene into ethylbenzene in acetone-d6, producing... 

What was not known at the time when these first PHIP experiments with heterogeneous catalysts were performed was the fact that metal clusters and particles can produce PHIP effects as well. In fact, it was widely believed that the mechanism of catalytic hydrogenation on metal surfaces was incompatible with the requirement of the pairwise H2 addition to a substrate. Therefore, the possibility cannot be excluded that the immobilized Rh complexes used in the early studies were precatalysts rather than the actual catalysts, especially in some of the gas - solid hydrogenations. Extended catalyst activation periods may have resulted in the reduction of supported metal complexes and the production of nanoparticulate Rh catalysts. However, the main conclusion that PHIP effects can be produced in heterogeneous processes is still valid. 

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