Selective hydrogenation of benzene to cyclohexene with zirconium hydride catalysts

Abstract

Hydrides of the early transition metals have been investigated as possible catalysts for the hydrogenation of aromatics. Bis(η-cyclopentadienyl)-zirconium dihydride, [(η-C5H5)2ZrH2]n catalyzes the hydrogenation of naphthalene (80–90 °C, 860 kPa H2) to tetralin and dihydronaphthalene. The solid state binary hydrides of the Group IVA, VA elements and (to a lesser extent) lanthanum and uranium hydrides, after suitable activation, catalyze the hydrogenation of benzene, a property previously associated only with Group VI to VIII metal catalysts. Under the conditions employed (liquid phase reactor, 80–180 °C, ~900 kPa H2) the relative activity of the hydrides as catalysts is as follows: ZrH2 ~ HfH2 > TiH2 > NbH ⪢ LaH3 ~ UH3 Cyclohexane is the major reaction product; with ZrH2 and HfH2, however, some cyclohexene is formed at low conversions of benzene.Solid, reduced zirconium materials isolated from reaction of excess n-butyllithium with ZrCl4 or HfCl4 in hydrocarbons catalyze the hydrogenation of benzene at 100 °C and 860 kPa H2 to yield mostly cyclohexane. However, treatment of these materials with hydrogen (and with hydrogen together with triethylamine or diethyl ether) gives modified zirconium hydride catalysts which hydrogenate benzene with relatively high (up to ~ 90%) initial selectivities for cyclohexene.