The mechanism of the phosphine-modified nickel-catalyzed acetic acid process

Abstract

The nickel-catalyzed methanol (MeOH) carbonylation reaction was studied with an in situ infrared technique using a high pressure cylindrical internal reflectance reactor (CIR-reactor). The role of phosphine ligands was investigated in order to determine the relationship between the structural and electronic properties of the ligand and catalytic properties. It was found that the highest carbonylation rates occurred for the phosphine ligands having the greatest cone angles. Altering the electronic properties of substituted triarylphosphines resulted in systematic changes in the carbonylation rate, and a Hammett treatment of the rate data using normal sigma constants led to a volcano plot. A modified Hammett plot using Taft polar sigma constants for various trialkylphosphines led to a linear relationship in which the rate increased as the electron-donating properties of the ligand increased. The carbonylation activity was correlated with the steric size of trialkylphosphines by the observation of a linear relationship to the ligands cone angle. The in situ reaction monitoring studies showed that the phosphine ligand was substantially converted to the corresponding phosphonium salt through reaction with excess methyl iodide in the system. The in situ reaction monitoring studies, conducted at typical process conditions, showed that the phosphonium salt reversibly dissociated to differing amounts of the free phosphine depending on the electronic and steric properties of the phosphine. The reaction monitoring studies, using phosphines of widely differing electronic and steric properties showed that the reaction rates increased linearly as the concentration of free PR 3 in solution increased. The results of this ligand study and a prior process parameter study led to a reaction mechanism in which phosphine is mainly transformed to [P(CH 3)R 3] +I −. It was shown that this soluble salt partially dissociates to provide sufficient free phosphine to coordinate to Ni° to form the active catalyst. A low partial pressure of hydrogen was found essential to provide the catalytic cycle with reduced nickel. The Ni° combines with free PR 3, forming Ni(PR 3) 2 which is located within the active catalytic cycle. The kinetic data and in situ reaction monitoring observations are consistent with a reversible slow step in the active cycle consisting of CH 3I reacting with Ni(PR 3) 2, forming an oxidative addition product which is rapidly carbonylated. All other subsequent steps are much faster than the oxidative addition reaction.