Add time:08/11/2019         Source:sciencedirect.com

Consideration is given to the various parameters which affect the yield and selectivity of propanoic acid formation in the hydrocarboxylation of ethene using a rhodium/I− catalyst. The source of rhodium, the solvent, the form of iodide promoter, the composition of the carbon monoxide/ethene mixture, and the effect of additives are all considered. Rhodium(III) chloride trihydrate rather than the iodide produces the highest reaction rate for the iodoethane promoted reaction. When dichloromethane is used as a diluent, the reaction rate and yield of acid are lowered by ca. 20–30% without affecting the nature of the products. However, diethylether and water are the major products when ethanol is the reaction solvent, but hydrocarboxylation continues, the reaction rate being enhanced 3-fold when ethene is present. Throughout, iodoethane is a better promoter than hydrogen iodide, but iodoethane alone is a poor solvent. CO:alkene mixtures in the range 1:2 to 2:1 are preferred and give similar yields and rates of acid production. In CH2Cl2 solvent, additives enhance the reaction in the order H2O > C2H50H > C2H5COOH, and when the solvent is glacial acetic acid, very small amounts of water have a dramatic promotional influence on the reaction rate. Iodine, hypophosphorous acid and lithium propanoate additives have an adverse effect on catalysis.

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