Synthesis of acetic acid via methanol hydrocarboxylation with CO2 and H2

Acetic acid is an important bulk chemical that is currently produced via methanol carbonylation using fossil based CO. Synthesis of acetic acid from the renewable and cheap CO2 is of great importance, but state of the art routes encounter difficulties, especially in reaction selectivity and activity. Here we report a route to produce acetic acid from CO2, methanol and H2. The reaction can be efficiently catalysed by Ru–Rh bimetallic catalyst using imidazole as the ligand and LiI as the promoter in 1,3-dimethyl-2-imidazolidinone (DMI) solvent. It is confirmed that methanol is hydrocarboxylated into acetic acid by CO2 and H2, which accounts for the outstanding reaction results. The reaction mechanism is proposed based on the control experiments. The strategy opens a new way for acetic acid production and CO2 transformation, and represents a significant progress in synthetic chemistry.

Notes: X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300W AlKα radiation. The base pressure was about 3×10 -9 mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon.
The preparation of imidazole-Ru 3 (CO) 12 compound was conducted at room temperature. In the experiment, 0.0085 g Ru 3 (CO) 12 (40 µmol Ru) was dissolved in 20 mL dioxane, 0.05 g imidazole (750 µmol) was dissolved in 20 mL methanol. The above solutions were mixed and stirred for 5 h, then 40 mL diethyl ether was added to precipitate the target compound. The compound was washed with additional 40 mL diethyl ether for 2 times and dried in vacuum before the XPS analysis. The preparation of imidazole-Rh 2 (OAc) 4 compound was conducted at room temperature. In the experiment, 0.0088 g Rh 2 (OAc) 4 (40 µmol Rh) and 0.05 g imidazole (750 µmol) was dissolved in 20 mL methanol respectively. The above solutions were mixed and stirred for 5 h, and then 40 mL diethyl ether was added to precipitate the target compound. The compound was washed with additional 40 mL diethyl ether for 2 times and dried in vacuum before the XPS analysis.
Note: The molecular weight of acetic acid generated in the reaction was still 60 Daltons. This result supports three deductions. 1. The CH 3 and OD group broke away during the reaction. Otherwise, the molecular weight of acetic acid should be 61 Daltons. 2. The CO 2 directly participated in the reaction. If methanol carbonylation with CO dominated in the reaction, the OD group generated in situ would take part in the formation of acetic acid with the CH 3 CORh*I intermediate and the molecular weight of acetic acid should be 61 Daltons. The mechanism of rhodium catalyzed methanol carbonylation was reported elsewhere (Ref 3).
3. H atom in the COOH group of acetic acid was from the reactant H 2 . Otherwise, the molecular weight of acetic acid should be 61 Daltons.
Note: The molecular weight of acetic acid formed in the reaction was 61 Daltons. This demonstrates that the two C atoms in the acetic acid product were from 13 C of 13 CH 3 OH and C of CO 2 respectively.
Note: In the 1 H NMR spectrum, the proton signal of 13 CH 3 group on the acetic acid molecule splits into two peaks by the coupling with 13 C atom. In the 13 C NMR spectrum, the signal of carbonyl group became weaker and splits into dual peaks, which is caused by the coupling with the adjacent 13 C atom in the 13 CH 3 group. Both 1 H NMR and 13 C NMR spectra confirmed that the CH 3 group in acetic acid molecule is from methanol, i.e., CH 3 group of CH 3 OH is transferred into the acetic acid product in the reaction.