Hydrogen homogenous splitting

The ability to catalyze certain reactions of molecular hydrogen homogeneously in solution has been demonstrated for many transition metal ions and complexes (34)—among them complexes of Cu Cu Ag Hg Hgi, Col, Coll, pdii, Ptii, Rhi, Rh i, Ru i Ruiii, and Ir. In each case it appears that H2 is split by the catalyst with the formation of a reactive transition metal hydride complex (which may or may not be detected) as an intermediate. Three distinct mechanisms by which this can occur have been recognized (34), which are exemplified by the following reactions. 

Platinum. Rate laws have been determined for homogeneous hydrogenation of ethylene and acetylene in the presence of platinum(n)-tin(ii) catalysts. This kinetic evidence and ancillary results from deuterium tracer experiments suggest a mechanism in which the molecular hydrogen is split heterolytically to give a Pt—H bond plus H+ ethylene or acetylene then... 

Scheme 3.4 The heterolytic splitting of dihydrogen at Ru(ll) to give a hydridic-protonic bond, as proposed by Chu et al. [55] in the mechanism of the homogeneous hydrogenation of carbon dioxide.

The explanation for the apparent correlation between catalytic activity and electron affinity of metals cannot be as simple as that which has been advanced for the homogeneous catalysts. This is because chemisorption on metals (unlike the splitting of hydrogen by metal ions in solution ) is an exothermic process and, hence, as shown earlier, catalytic activity depends not only on a low activation energy of adsorption but also on a low heat of adsorption. The interpretation applied earlier to homogeneous catalysts can account for an inverse dependence of Ea on the work function, but does not suggest any obvious reason why Q should show a similar dependence. 

In favorable cases, homogeneous catalytic hydrogenation of organic substrates such as olefins also may be achieved by transfer of hydrogen from the hydrido-transition metal complex. The following examples, all discovered within the last few years, illustrate how this can be realized for each of the three mechanisms of splitting of hydrogen (heterolytic, homolytic, and insertion) described above. 

Ruthenium (II) Chloride. The homogeneous catalysis of the hydrogenation of fumaric acid in aqueous solution by ruthenium (II) chloride has been interpreted (36) in terms of the following mechanism in which the heterolytic splitting of H2 by a Ru -olefin complex is the rate-determining step. 

In a very early study Patat (1945) investigated the hydrolysis of aniline to phenol in a water-based acidic solution in near-critical and supercritical water (Tc = 374.2°C, Pc = 220.5 bar). Phosphoric acid and its salts are used as the catalyst for this reaction. The reaction proceeds extremely slowly under normal conditions and reaches equilibrium at low conversion levels. For these reasons, Patat chooses to study the reaction in supercritical water to temperatures of 450°C and to pressures of 700 bar in a flow reactor. He finds that the reaction follows known, regular kinetics in the entire temperature and pressure space studied and the activation energy of the hydrolysis (approximately 40 kcal/mol) is the same in the supercritical as well as in the subcritical water. He suggests that the reaction is catalyzed by hydrogen ions formed from dissolution of phosphoric acid in supercritical steam. Very small amounts of phosphoric acid and the salts of the phosphoric acid are dissolved in the supercritical steam and are split into ions. Patat lists several dissolution constants for primary ammonium phosphates in supercritical steam. In this instance, the reaction performance is improved when the reaction is operated homogeneously in the mixture critical region and, thus, in intimate contact between the reactants and the catalyst. 

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