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

The nickel-catalyzed methanol (MeOH) carbonylation reaction was studied wit h an in situ infrared technique using a high pressure cylindrical internal reflectance reactor (CIR-reactor). The role of phosphine ligands was invest igated in order to determine the relationship between the structural and el ectronic properties of the ligand and catalytic properties. It was found th at the highest carbonylation rates occurred for the phosphine ligands havin g the greatest cone angles. Altering the electronic properties of substitut ed triarylphosphines resulted in systematic changes in the carbonylation ra te, and a Hammett treatment of the rate data using normal sigma constants l ed to a volcano plot. A modified Hammett plot using Taft polar sigma consta nts for various trialkylphosphines led to a linear relationship in which th e rate increased as the electron-donating properties of the ligand increase d. The carbonylation activity was correlated with the steric size of trialk ylphosphines by the observation of a linear relationship to the ligands con e angle. The in situ reaction monitoring studies showed that the phosphine ligand was substantially converted to the corresponding phosphonium salt th rough reaction with excess methyl iodide in the system. The in situ reactio n monitoring studies, conducted at typical process conditions, showed that the phosphonium salt reversibly dissociated to differing amounts of the fre e phosphine depending on the electronic and steric properties of the phosph ine. The reaction monitoring studies, using phosphines of widely differing electronic and steric properties showed that the reaction rates increased l inearly as the concentration of free PR, 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(CH3)R-3]I-+(-). I t was shown that this soluble salt partially dissociates to provide suffici ent free phosphine to coordinate to Ni degrees to form the active catalyst. A low partial pressure of hydrogen was found essential to provide the cata lytic cycle with reduced nickel. The Ni degrees combines with free PR3, for ming Ni(PR3)(2) which is located within the active catalytic cycle. The kin etic data and in situ reaction monitoring observations are consistent with a reversible slow step in the active cycle consisting of CH3I reacting with Ni(PR3)(2), forming an oxidative addition product which is rapidly carbony lated. All other subsequent steps are much faster than the oxidative additi on reaction. (C) 1999 Elsevier Science B.V. All rights reserved.