Rh 3+ and Rh 3+–diamine complexes intercalated in γ-titanium hydrogen phosphate. Synthesis, characterisation and catalytic activity towards aniline oxidative carbonylation processes

Abstract

New materials containing Rh(III) ions or Rh(III)–diamine complexes [(L=2,2′-bipyridyl (bipy); 1,10-phenanthroline (phen); 2,9-dimethyl-1,10-phenanthroline (dmp)] intercalated in γ-titanium phosphate (γ-TiP) are described. The compounds have formula γ-TiPH x Rh y · tH 2O ( x=2–3 y; 0< y<0.18; 1.5< t<2.2), hereafter indicated as γ-TiPRh, and γ-TiPH x Rh y L z · nH 20 ( x=2–3 y; 0< y<0.25; 0< z<0.35; 2.5< n<4.2), hereafter indicated as γ-TiPRhL. They were prepared by contacting samples of γ-TiP and γ-TiPL with appropriate volumes of Rh(III) solution, at 45°C. The materials were characterised by X-ray diffraction pattern and TG-DTA techniques. Their catalytic activity towards aniline carbonylation was tested. The compounds catalyse the oxidative carbonylation of aniline to diphenylurea (DPU) and/or methylphenylcarbamate (MPC). When the reaction is carried out in methanol or CH 3CN, at 70–90°C under moderate pressure of a CO/O 2 mixture ( P tot=0.5÷5 MPa; P(0 2)=0.1÷0.8 MPa), aniline is converted to DPU with a yield of 90% with respect to CO. The catalysts recovered and reused do not show any decrease of activity. When more drastic temperature conditions (110–150°C) are used, the carbonylation is not selective and the yield of carbon monoxide fixation is strongly dependent on the solvent. In CH 3CN, aniline is converted into DPU with a selectivity of 90% and the yield of CO fixation is still high (85%). Conversely, in methanol, N-phenylmethylcarbamate (MPC) is the main product with relevant amounts of CO 2 formed and a CO fixation yield not higher than 70%. Under these conditions, the catalysts when reused have shown a decrease of activity. On the basis of XPS analysis, it has been ascertained that the decrease in activity is due to an irreversible reduction of Rh 3+ to Rh o that causes the release of the metal from the support and the consequent deactivation of the catalyst.