The water gas shift reaction catalyzed by Rh₆(CO)₁₆ adsorbed on alumina or [Rh(CO)₂Cl]₂ adsorbed on alumina, Na-Y zeolite and H-Y zeolite

Rh₆(CO)₁₆ chemisorbed on η-alumina or [Rh(CO)₂Cl]₂ chemisorbed on η-alumina, Na-Y zeolite and H-Y zeolite catalyze the water gas shift reaction to various degrees. The following order of activity was observed: alumina > Na-Y > H-Y zeolite. With alumina the reaction occurs between 25 and 100°C. Turnover numbers as high as 255/Rh atom/h are obtained at 50°C and under 50 atm. These turnover numbers are the same whether the precursor complex is Rh₆(CO)₁₆ or [Rh(CO)₂Cl]₂. The mechanism of such a reaction has been deduced from infrared studies, mass balance and labeling experiments. It involves three steps: electrophilic attack by surface protons on the metallic frame with formation of Rh^I(CO)₂(OAl{A figure is presented})(HOAl{A figure is presented}) and possibly Rh^III(H)(H)(OAl{A figure is presented})HOAl{A figure is presented}); reductive elimination of H₂ from Rh(H)(H)(OAl{A figure is presented})(HOAl{A figure is presented}) under CO pressure; and nucleophilic attack by water on CO coordinated to rhodium(I) with formation of CO₂, H⁺ and regeneration of Rh₆(CO)₁₆. If the CO pressure is too low or if liquid water is used, aging of the catalyst occurs which seems to be due to the formation of metallic rhodium. The intermediacy of [Rh(CO)₄]^- is also suspected in the step of metal-metal bond formation.