Oxidative carbonylation of toluene to p-toluic acid catalyzed by rhodium in the presence of vanadium and oxygen

The mechanism and kinetics of the liquid phase, oxidative carbonylation of toluene to toluic acid were investigated. The catalyst system consisted of Rh(acac)3, NH4VO3, trifluoroacetic acid (TFAH), and trifluoracetic anhydride (TFAA). Liquid toluene together with gaseous CO and O2 was used as the reactant. The effects of temperature, the partial pressures of O2 and CO, the concentrations of TFAH and TFAA, the concentrations of Rh(acac)3 and NH4VO3, and time on the activity and selectivity of the catalyst for forming p-toulic acid were investigated. The conversion of toluene to toluic acid increased with temperature but the p/m and p/o isomer ratios decreased. A conversion maximum occurred with CO partial pressure, and conversion increased monotonically with O2. The conversion of toluene to toluic acid increased with time. Under optimal conditions, 700 mole of toluic acid were produced per mole of Rh in 4 h. During the course of reaction, Rh(III) is reduced to Rh(I). The latter species is reoxidized by reaction with V(V) in the form of VO2+. The V(IV) produced in this process is then reoxidized by O2. The concentration of water, produced during the reoxidation of Rh(I) to Rh(III), in the reaction system must be controlled by reaction with TFAA, since the accumulation of excess water can lead to the generation of H2 via the water-gas-shift reaction leading to a reduction of Rh(III) to Rh(I). A mechanism for the overall catalytic process is proposed and discussed.

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