Rh-ions and Rh-complexes intercalated in γ-titanium or γ-zirconium hydrogen phosphate as highly efficient catalysts for arene hydrogenation

Monocyclic arenes (C6H5X, X = H, CH3, OH, OCH3, Cl) are efficiently hydrogenated to their corresponding aliphatic hydrocarbons under relatively mild condition (T = 393 K pH(2) = 1-4 MPa) using rhodium(III) (Rh(III)) ions or Rh(III) N,N'-complexes (N,N' = 2,2'-bipyridyl, 1,10-phenanthroline) intercalated into gamma-titanium or zirconium hydrogen phosphate, (gamma-M(PO4)(H2PO4); M = Ti, Zr), acting as ionic exchange supports. The hydrogenation rate depends on the catalyst, the substrate and their molar ratio, the temperature and the hydrogen pressure. Zirconium materials charged with naked Rh-ions, gamma-Zr(PO4)(2)HxRhy, or with Rh-complexes, gamma-Zr(PO4)(2)H-x(Rh-L)(y) (L = 2,2'-bipyridyl; 1,10-phenanthroline), proved to be more active than the titanium analogous compounds gamma-Ti(PO4)(2)HxRhy and gamma-Ti(PO4)(2)H-x(Rh-L)(y). For both series of materials the M(PO4)(2)HxRhy catalysts, (M = Ti, Zr), are more active than the corresponding M(PO4)(2)H-x(Rh-L)(y) system. For the most active catalysts, gamma-Zr(PO4)(2)HxRhy and gamma-Zr(PO4)(2)H-x(Rh-L)(y), the influence of the hydrogen pressure on the turnover frequency (TOF) and the stability over recycling was studied in the benzene hydrogenation. The activity increases with the pressure and at 4 MPa of H, a TOF of 4019 h(-1), expressed as mol of benzene hydrogenated per mol of Rh per hour, was found, using a Rh/benzene molar ratio of 1/8045. The catalysts remained stable for six runs in the case of benzene hydrogenation, while with toluene, a loss of activity was observed after the third cycle. The high activity and stability of Rh-zirconium materials used for benzene hydrogenation is also evidentiated by the fact that. under a hydrogen pressure of 2 MPa with a molar ratio Rh/benzene of 1/30171, 975.3 mmol of benzene were hydrogenated in 11 h, corresponding to a turnover number (TON) of 43538 mol of benzene hydrogenated per mol of Rh. (c) 2005 Elsevier B.V. All rights reserved.