Rhodium solvent effect

Table 7 Solvent effect on the generation of rhodium species...

The rate law determined by Kaduk and Ibers (236) is rate = k [Rh]1/2-[NO] 1/2. It is also found that water accelerates the rate of the reaction while addition of PPh3 suppresses the rate. The reaction rate is independent of the pressure of CO and is unaffected by the addition of acid as HPF6. The authors consider the suppression of the rate by PPh3 to be due to an equilibrium formation of the inactive [Rh(NO)2(PPh3)2]+, and propose the dependence on water to be only a solvent effect. The observed dependences on the total rhodium concentration and the pressure of NO may at first glance seem curious, and indeed the authors state (236) that the exact functional forms of these dependences are not unambiguously established since the kinetic studies were run over a limited range of conditions. A possible explanation for the observed fractional dependences can be developed,... 

Solvents also influence the stereoselectivity of the phenol reduction, however, the solvent effect is also dependent on the catalyst used14-18. In contrast to ruthenium, which shows little dependence on the solvent, hydrogenations over rhodium are much more solvent sensitive. In this case the cis/trans ratio tends to decrease with increasing dielectric constant of the solvent. 

Alkali moderation of supported precious metal catalysts reduces secondary amine formation and generation of ammonia (18). Ammonia in the reaction medium inhibits Rh, but not Ru precious metal catalyst. More secondary amine results from use of more polar protic solvents, CH OH > C2H5OH > Lithium hydroxide is the most effective alkah promoter (19), reducing secondary amine formation and hydrogenolysis. The general order of catalyst procUvity toward secondary amine formation is Pt > Pd Ru > Rh (20). Rhodium s catalyst support contribution to secondary amine formation decreases ia the order carbon > alumina > barium carbonate > barium sulfate > calcium carbonate. 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

---