| A chemical reaction involving carbon chain elongation from C1 building blocks is a promising alternative to petroleum-based chemical production because it allows a wider range of carbon sources, including renewable biomass. Methanol reductive carbonylation (MRC) produces ethanol from synthesis gas, a mixture of H2 and CO, and methanol, which is also produced from synthesis gas. Prior to the 1990s, industry-scale MRC was inhibited by the ineffectiveness of the existing catalysts. Wegman and Moloy reported that a Rh(dppp) complex (dppp = 1,3-bis(diphenylphosphino)propane) catalyzed MRC to above 80% selectivity at only 1000 psi of synthesis gas - relatively milder conditions than typical Co-based catalysts. Their proposed mechanism for Rh-catalyzed methanol carbonylation is not consistent with the recent observations that the catalytic rate is inhibited by iodide ions and CO. Additionally, dppp-based ligands modified with ortho-alkoxy groups on the phenyl rings have been found to perform better than the original dppp. Herein, we propose and explore a mechanism based on a Rh-catalyzed hydroformylation via a Rh-H intermediate. Interception of the proposed intermediates was not successful, but characterization of related complexes allows us to conclude that the reaction at ambient pressure and temperature occurs via formation of Rh-dppp dimeric species. That the essential hydrogenolysis step of this hydride mechanism does not occur under high pressure of synthesis gas further precludes the proposed mechanism from being operational. We then explore an alternative mechanism involving a cationic intermediate formed from dissociation of the iodide ligand. In order to study the reaction under high pressure, a recently developed high pressure NMR apparatus was utilized. Kinetic measurements of the reaction agree with the model where the reaction was inhibited by accumulated iodide ions produced in the reaction. The rate measurement also demonstrates that the hydrogenolysis of the Rh-complex is rate-determining step and is where the Rh-dppp and Rh-dpppOEt (dpppOEt = 1,3-bis(di-2-ethoxyphenylphosphino)propane) catalysts are different, in accordance to the hypothesis that the ortho-alkoxy group promotes the ionization. Finally, we were able to directly observe Rh-dpppOEt ionized species with coordinating alkoxy group and propose a complete ionization-based mechanism based on these observations. |
