Non-Aqueous Liquid Phase Synthesis of Acetic Acid via Ionic Liquid Promoted Homogeneous Carbonylation of Methanol over Ir(III) Catalysts

Carbonylation of methanol to acetic acid is an aqueous homogeneous catalytic process widely used in industry. In the existing methanol carbonylation industry, a large amount of water (14-15 wt.%) is required in the Monsanto process to inhibit catalyst deactivation, which also consumes a lot of energy to separate the water from the acetic acid product. Here, non-aqueous liquid phase synthesis of acetic acid was carried out by ionic liquid promoted homogeneous carbonylation of methanol over Ir(III) catalysts. It was found that 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Bmim]Tf2N) and N-butyl pyridinium bis(trifluoromethanesulfonyl)imide ([BPy]Tf2N) could promote the acetic acid selectivity (>98%) and methanol conversion (>99%) under a relatively mild reaction condition of 160 degrees C and 3.0 MPa. In the reaction, Tf2N- formed an Ir*[Tf2N] complex with Ir to promote the stability of Ir and enhance the activation of CO. From DFT calculation results, the CO insertion was a rate controlling step on the Ir-base catalyst in the reaction cycle, determining the conversion and selectivity of the reaction. The oxygen and nitrogen groups from Tf2N- interacted with Ir to form an Ir*[Tf2N] complex, which could effectively reduce the energy barrier of the CO insertion step, enhancing the selectivity of HAc. Moreover, the catalyst system could be easily recycled and reused with the methanol conversion of 89.66% after five cycles. The methanol carbonylation based on the ionic liquid promoted catalyst is a promising non-aqueous liquid-phase sustainable process.

Automated summary

In this study, a non-aqueous liquid phase synthesis of acetic acid was achieved by ionic liquid promoted homogeneous carbonylation of methanol. The catalyst system demonstrated high selectivity and conversion rate under mild reaction conditions. The Tf2N- in the ionic liquid formed a complex with Ir, enhancing the stability of the catalyst and the activation of CO. The research results suggest that the ionic liquid promoted catalyst system has potential for a non-aqueous liquid phase sustainable process.