Kinetic study of carbonylation of ethanol to propionic acid using homogeneous rhodium complex catalyst in the presence of diphosphine ligand

Carbonylation of ethanol is a potentially attractive route for propionic acid production, while its industrial practice is greatly hampered by the low space-time yield. To improve the reaction rate of ethanol carbonylation, a series of diphosphine ligands were investigated in the homogeneous rhodium complex catalyst system. The catalyst activity and stability were enhanced by using bis(diphenylphosphino)methane monosulfide (dppmS) as hemilabile diphosphine ligand and the space-time yield of propionic acid was increased significantly. In the presence of dppmS, not only the effect of ligand addition, the content of ethyl iodide, lithium iodide, and rhodium catalyst on catalytic performance were carried out, but also the reaction conditions were systematically investigated in a titanium alloy autoclave reactor. Consequently, the carbonyl space-time yield reached 6.21 mol.L-1.h(-1) under the optimal reaction conditions. Additionally, the corresponding mechanism of ethanol carbonylation with addition of dppmS was proposed. A kinetic model of the reaction was established in the temperature range of 433-473 K. The reaction orders of catalyst, ethyl iodide, and iodide ion concentrations were determined to be 0.86, 0.36, and 0.20, respectively. The activation energy was found to be 25.23 kJ.mol(-1). Residual error distribution n and a statistical test showed that the kinetic model is reasonable and acceptable.