Abstract

The cis-[Rh(CO) 2(2-picoline) 2](PF 6) complex is active for the catalytic reduction of nitrobenzene to aniline in aqueous 2-picoline solution under water gas shift reaction conditions. The catalytic activities for this system were studied as functions of the reaction parameters (P(CO), [Rh](tot), and T). The nitrobenzene reduction rate displays a first-order dependence on CO pressure, P(CO), over the range 0-1.9 atm at 80, 100 and 120°C. The kinetics behavior with respect to P(CO) leads to the proposal that addition of CO to the catalytic species precedes the rate-limiting step. The catalytic activity proved to be non-linear in total rhodium concentration, [Rh](tot), over the range 5-60 mM. This observation was interpreted in terms of the presence of active mononuclear and polynuclear rhodium species in the system, the mononuclear species being the more active for nitrobenzene reduction. In situ FT-IR spectroscopic study confirms the presence of rhodium species with terminal and bridging carbonyls. (C) 2000 Elsevier Science Ltd.