Fluoroacrylate copolymer-grafted rhodium catalysts for hydrogenation reactions in supercritical fluids

Organometallic catalysts in homogeneous media offer high reactivity, selectivity, and tunability. Nevertheless, most solvents used in organic synthesis have a negative effect on the environment due to their toxicity. Supercritical carbon dioxide (scCO₂) has shown significant potential for replacing the common toxic organic solvents as reaction media because of its unique properties, which near the critical region are easily tunable by small variation in pressure and/or temperature allowing the possibility of engineering the reaction environment to improve the reactivity and selectivity. Limitations of scCO₂ as solvent are associated with its weak solvent strength. Efforts have been directed to the design of catalyst ligands that are soluble in scCO₂ under mild conditions. Particularly, flouroacrylate copolymers and silicones have been found to be moderately soluble in scCO₂. Based on that, a novel catalyst soluble in scCO₂ was synthesized by grafting rhodium ligands to a fluroacrylate copolymer. To determine the performance of the novel catalyst, hydrogenation of 1-octene and cyclohexene were used as model reactions. The main objective of this research was to evaluate the catalytic activity of the novel catalyst for hydrogenation reactions in scCO₂. The effect of catalyst composition, operating conditions, hydrogen concentration, and substrate to catalyst molar ratio on the reactivity and selectivity were analyzed. It was found that 1-octene was not only hydrogenated to n-octane, but also it was isomerized to (E)2-octene and (Z)2-octene. Also, no hydrogenation of the 2-octene isomers was observed in the presence of 1-octene. On the other hand, cyclohexene was only hydrogenated to cyclohexane, and no side products were detected. Moreover, deactivation of the catalyst was observed during the reaction time. Catalyst composition only affected the reactivity of the catalyst, but no effect was observed on the selectivity. The same trend was observed when analyzing the effect of pressure and temperature. Finally, a kinetic model that represents the hydrogenation and isomerization processes with the new catalyst was developed and its parameters determined from experimental data.