SUPPORTED ORGANOACTINIDE COMPLEXES AS HETEROGENEOUS CATALYSTS - A KINETIC AND MECHANISTIC STUDY OF FACILE ARENE HYDROGENATION

This contribution reports a kinetic and mechanistic study of arene hydrogenation by the supported organoactinide complexes Cp'Th(benzyl)3/DA (1/DA), Th(1,3,5-CH2C6H3Me2)4/DA (2/DA), and Th(eta3-allyl)4/DA (3/DA) where Cp' = eta5-Me5C5 and DA = dehydroxylated gamma-alumina. In slurry reactions (90-degrees-C, P(H-2) = 180 psi), the activity for benzene hydrogenation follows the order 1/DA < 2/DA < 3/DA with an N(t) value for 3/DA of approximately 25 000 h-1 active site-1. This approaches or exceeds most conventional platinum metal catalysts in efficacy for benzene reduction. Benzene hydrogenation by 3/DA at 90-degrees-C, P(H-2) = 180 psi, follows the rate law N(t) = nu[benzene]0[p(H-2)]1 with N(t)(H-2)/N(t)(D2) = 3.5 +/- 0.3 and E(a) = 16.7 +/- 0.3 kcal mol-1. Partially hydrogenation products cannot be detected at partial conversions, and there is no D2 incorporated in the unconverted benzene. D2 is not delivered to a single benzene face, but rather a 1:3 mixture of all-cis and cis,cis,trans,cis,trans isotopomers is formed. Active site characterizations using D2O poisoning, hydrogenolysis, and CH3Cl dosing indicate that less-than-or-equal-to 8 +/- 1% of the Th surface sites are responsible for the bulk of the benzene hydrogenation. EPR and XPS studies provide no evidence for surface Th oxidation states less than +4. As a function of arene, the relative rates of Th(eta3-C3H5)4/DA-catalyzed hydrogenation are benzene > toluene > p-xylene > naphthalene, with the regiochemistry of p-xylene reduction similar to that for benzene. Experiments with 1:1 benzene-p-xylene mixtures reveal that benzene is preferentially hydrogenated with almost complete exclusion of p-xylene (approximately 97:3), inferring that the benzene binding constant to the active sites is approximately 6.7X that of p-xylene. It is possible to propose a mechanism for arene hydrogenation which involves single Th(IV) sites, includes inoperativity of oxidation addition/reductive elimination sequences, and passes among established metal-ligand structures via precedented pathways.