Reductive carbonylation of methanol for ethanol production in Rh-Ru-dppp-methyl iodide catalytic system under mild conditions – The effect of lithium salts and catalyst composition

Abstract

Reductive carbonylation of methanol is a potential process for ethanol production, but the reaction temperature is always higher than 190°C and reaction pressure higher than 19MPa. Due to the corrosions of the catalytic system, the reaction conditions, especially the pressure limited commercial applications of this process. The catalytic system consists of rhodium, ruthenium, dppp and methyl iodide, and was investigated for reductive carbonylation of methanol to ethanol under 6.0MPa at 120°C, and it exhibited an attractive catalytic activity. By comparing the products selectivity and the turnover frequency (TOF) of the catalytic systems, the role of each component in the catalytic system has been investigated. Rhodium catalyst was found to catalyze methanol reductive carbonylation for acetaldehyde formation, and ruthenium catalyst was responsible to catalyze hydrogenation of acetaldehyde to ethanol. The dppp can coordinate to rhodium and form a molecule rhodium active species, which enhanced the stability and solubility of the rhodium catalyst under the relatively low reaction pressure. The methyl iodide can promote the split of a carbon‑oxygen bond of methanol, thus accelerate the reaction process even at 120°C. The synergy effects of these catalyst compositions give rise to the ethanol formation under the relatively mild condition. Additionally, reaction conditions, catalytic system proportional and presence of lithium salts were combined to tune the TOF and ethanol production. Properly increasing initial methyl iodide addition, dppp/Rh mole ratio, H2/CO mole ratio, reaction temperature and pressure can accelerate the TOF. Raising Ru/Rh ratio and reaction temperature were in favor of acetaldehyde hydrogenation to ethanol. High H2/CO and dppp/Rh ratio can suppress the acetic acid formation. This work could provide a deeper understanding for further optimization to enhanced ethanol production.