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

Monocyclic arenes (C6H5X, X = H, CH3, OH, OCH3, Cl) are efficiently hydrogenated to their corresponding aliphatic hydrocarbons under relatively mild condition (T = 393 K, pH2 = 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 γ-titanium or zirconium hydrogen phosphate, (γ-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, γ-Zr(PO4)2HxRhy, or with Rh-complexes, γ-Zr(PO4)2Hx(Rh-L)y (L = 2,2′-bipyridyl; 1,10-phenanthroline), proved to be more active than the titanium analogous compounds γ-Ti(PO4)2HxRhy and γ-Ti(PO4)2Hx(Rh-L)y. For both series of materials the M(PO4)2HxRhy catalysts, (M = Ti, Zr), are more active than the corresponding M(PO4)2Hx(Rh-L)y system. For the most active catalysts, γ-Zr(PO4)2HxRhy and γ-Zr(PO4)2Hx(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 H2, 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.