The Production of Acetic Acid RHODIUM CATALYSED CARBONYLATION OF METHANOL By James F. Roth Monsanto Co., St. Louis, Missouri The new Monsanto Company acetic acid process employs a rhodium- based homogeneous liquid-phase catalyst which promotes methanol carbonylation in high selectivity and at low pressures. This article des- cribes some of the chemistry involved in this process, with particular emphasis on the role played by rhodium complexes in the catalytic cycle. The new acetic acid plant built by the halogen ligands. A wide variety of rhodium Monsanto Company at Texas City is designed compounds and iodine compounds have been to operate at lower pressures than previous found to give similar reaction rates and plants. The output of 300 million pounds product distribution. This suggests that per year of acetic acid derived from methanol these tend to form the same active catalytic is the result of the successful development species. Solvent effects, e.g. faster rates in of a homogeneous catalyst based on rhodium. more polar solvents, indicate that the active This new catalyst was reported recently species may be ionic. (I, 2) for the low pressure liquid-phase synthesis of acetic acid from methanol and Kinetics of Methanol Carbonylation carbon monoxide. The overall stoichiometry of the acetic Previously, workers at B.A.S.F. described acid synthesis is represented simply by a commercial acetic acid process, based on CH,OH 1 CO-rCH,COOH methanol carbonylation and an iodine- promoted cobalt catalyst, that employs The results of kinetics studies of the carbon monoxide pressures of about 7500 rhodium-catalysed homogeneous reaction lb/in2 and gives a molar selectivity to acetic have been reported (2) and have several acid of about go per cent (3). The new interesting and unusual features. The reaction rhodium catalyst can be used at much lower was found to be zero order with respect to pressures and affords reaction rates essentially both reactants-methanol and carbon monox- independent of carbon monoxide partial ide. An obvious consequence of the zero pressure in the range between 200 and 1800 order dependence on carbon monoxide lb/in2. It effects methanol carbonylation pressure is the ability to obtain high reaction in 99 per cent selectivity and displays similar rates at low reaction pressures. With respect selectivity even when the carbon monoxide to catalyst components, it was found that feed contains as much as 50 per cent hydrogen. reaction rate was directly proportional to both the rhodium and iodide concentrations. Catalyst Composition Another key observation made relates to the The carbonylation catalyst is comprised of fate of the methyl iodide (formed via reaction two components, a soluble rhodium complex of CH,OH+HI) concentration during the and an iodide promoter. The catalytic species reaction. This was determined by analysis of is believed to consist of a coordination com- timed liquid samples removed during the plex of rhodium with carbon monoxide and course of the reaction. It was observed that Platinum Metals Rev., 1975, 19, (l), 12-14 12 The Monsantu Co. acetic acid plant at Texas City uses a new proress bused on a rhodium homogeneons liquid- phase catalyst which gims higher selectivity ,for the car- bonylation of methanol ut lower pressures than earlier processes methyl iodide reached a steady-state concentration very rapidly, that this con- centration remained con- stant over most of the reaction, and that the rate of formation of acetic acid was constant during the same period. Mechanism of the Reaction All of the kinetics results can be rationalised by as- suming that the reaction proceeds in a stepwise manner, as follows : The initial step in the catalytic cycle consists of a rapid conversion of metha- nol to methyl iodide, for example, via : the rhodium-alkyl bond occurs to yield an acyl-rhodium(II1) complex. This is ac- CH,OH+HI%CH,I +H,O (1) companied by subsequent or simultaneous Then, an oxidative addition of methyl replacement of co in the rhodium co- iodide to a Rh(1) complex (d8 square planar) ordination sphere. occurs [Equation (z)]. This results in forma- tion of a six-coordinate alkyl-rhodium(I1 I) (d6)complex. slow CH,I + [Rh complex] Rapid insertion of carbon monoxide, Finally, the d6acyl-rhodium(II1) complex present as a ligand in the Rh complex, into reacts to produce acetic acid and to regenerate Platinum Metals Rev., 1975, 19, (l), 13 the original d8 rhodium(1) complex. Several results in introduction of the methyl group different reaction modes can be proposed to into the coordination sphere and two subse- describe this step. These include a direct quent rapid steps involving first an excep- hydrolysis of the acyl-rhodium(II1) complex tionally facile carbon monoxide insertion by water, a specific methanol-assisted sol- reaction and then a facile reductive elimina- volysis of this complex, or reductive elimina- tion step. This sequence leads to the estab- tion of acetyl iodide from the complex lishment of a catalytic cycle. This multiple followed by immediate hydrolysis of the step process all occurs with a degree of acetyl iodide to yield acetic acid, the: starting selectivity that is remarkable even for a rhodium(1) complex and HI. Of these homogeneous catalytic reaction. possibilities, we favour the latter, as shown: It has been shown (4) that a heterogeneous 1 analogue of the homogeneous catalyst can be CH, 0 prepared and that the kinetics of methanol I1 +[Rh complex] + CH,C-I (4) carbonylation are identical for the solid I supported rhodium complex and the homo- Rh complex I I i Hzo geneous liquid-phase catalyst systems. This CH,COOH +HI suggests that, in spite of the obvious physical .i _I differences between these catalysts, a similar This overall reaction scheme is cclnsistent reaction mechanism is probzbly operative. with the observed reaction order depen- dencies and with the observed conslancy of References CHJ concentration during most of the I F. E. Paulik and J. F. Roth, J. Cherrr. SOC., Chem. Commwz., 1968, 1578 reaction, if one assumes that reaction (2) 2 J- F. Roth, J. H. Craddock, A. Hershman and is the rate-determining step. Accoxding to F. E. Paulik, Chem. Techno!., 1971,600-605 3 H. Hohenschultz, N. von Kutepow and W. this scheme, the role played by rhodium Himmele, Hydrocarbon Proces., 1966, 45, 141 complexes is threefold, consisting of the rate- 4 K. K. Robinson, A. Hershman, J. H. Craddock determining oxidative addition step, which and J. F. Roth, J. Caralysis, 1972,27, (3), 389 Superconductivity in Palladium-Hydrogen Alloys Superconducting properties have not gen- the H:Pd ratio on the surface of palladium erally been associated with the platinum foils to more than I, had also shown that the group metals. However, in I972 T. substitution of deuterium for hydrogen can Skoskiewicz of the Polish Academy of raise T, to 11 I<. It is suggested that these Sciences reported that palladium enriched additions may be modifying the lattice with hydrogen possesses superconductivity, vibrations, or phonons, in the solids, thereby with a transition temperature T, of -5 K changing their coupling to the electrons, when H:Pd=o.g. since it is the electron-phonon interaction Recent research in this field has been which is believed to lead to superconductivity. briefly summarised by R. Taylor (New Stritzker has now shown that alloying of Scientist, 1974, 64, (923, Nov. 14), 473), who suitable amounts of copper, silver or gold to comments on work by B. Stritzker et al. of Pd-H can raise T, still more (to 16.6 K for the Nuclear Research Centre at Julich aimed 45 per cent Cu-Pd when H:Pd-0.7; the at raising T, (Z. Physik, 1974, 268, (z), optimum silver and gold concentrations are 261-264) and by C. A. Mackliet et al. at the smaller). Naval Research Laboratory at Washington, Mackliet and his co-workers have dis- D.C., on the specific heat of superconducting covered that, whereas the peaks in the specific Pd-H alloys (Solid State Commun., 1974, 15, heat of superconductors usually drop sharply (t),207-210). above T, to the normal state, i.e. the super- The Julich team, which had first raised T, conducting state is more ordered, in Pd-H the to 9.4 K by H+ ion bombardment to increase drop is less marked. F. J. S. Platinum Metals Rev., 1975, 19, (l), 14-14 14 .