Insight into the stability of binuclear Ir–La catalysts for efficient heterogeneous methanol carbonylation

Abstract

Heterogeneous single-site catalysts are ideal alternatives to their homogenous counterparts. Methanol carbonylation to produce acetic acid is a typical homogeneous catalytic process. Here, we investigate the stability of a heterogeneous single-site catalyst for vapor methanol carbonylation constituted by an Ir–La binuclear complex on activated carbon (Ir1-La1/AC). Through the structure of the binuclear complex on the Ir1-La1/AC catalyst, the La promoter could protect the Ir+ species from reduction by H2, favor the single-atom dispersion of Ir metal, and also increase the reaction rate of methanol carbonylation. Furthermore, the apparent activation energy of the Ir1-La1/AC catalyst was 12.2 kJ/mol lower than that of the Ir1/AC catalyst. A density functional theory calculation showed that reductive elimination of acetyl iodide was the rate-determining step for the Ir1/AC catalyst, leading to a severe carbon deposition problem, while CO migration insertion was believed to be the rate-determining step on the Ir1-La1/AC catalyst, which can reduce the carbon deposition to a great extent.