Molecular Active Sites in Heterogeneous Ir–La/C-Catalyzed Carbonylation of Methanol to Acetates

Abstract

We report that when Ir and La halides are deposited on carbon, exposure to CO spontaneously generates a discrete molecular heterobimetallic structure, containing an Ir-La covalent bond that acts as a highly active, selective, and stable heterogeneous catalyst for the carbonylation of methanol to produce acetic acid. This catalyst exhibits a very high productivity of ∼1.5 mol acetyl/mol Ir·s with >99% selectivity to acetyl (acetic acid and methyl acetate) without detectable loss in activity or selectivity for more than 1 month of continuous operation. The enhanced activity can be mechanistically rationalized by the presence of La within the ligand sphere of the discrete molecular Ir-La heterobimetallic structure, which acts as a Lewis acid to accelerate the normally rate-limiting CO insertion in Ir-catalyzed carbonylation. Similar approaches may provide opportunities for attaining molecular (single site) behavior similar to homogeneous catalysis on heterogeneous surfaces for other industrial applications.

Dual‐Metal Interbonding as the Chemical Facilitator for Single‐Atom Dispersions

“…Noble metals (e.g., Pd, Rh, Ir) have been reported to be stabilized by other metal species with higher valence (e.g., La, and Re in oxides, and La in halides) to form hetero-pairs. [37][38][39] Here, first-principles calculations based on density functional theory (DFT) combined with experimental results are conducted to reveal the origin of the interactive facilitation effect between Fe and Rh. The experimental studies show that the formation of the RhFe chemical bond significantly enhances single atom dispersion with no requirement for high-temperature atomization.…”

Acid-promoted Ir-La-S/AC-catalyzed methanol carbonylation on single atomic active sites

“…The H2-TPR results confirm that the addition of a suitable amount of sulfuric acid to Ir-La-S/AC during the co-impregnation procedure favors the reduction of more Ir 3+ species to active states at a comparatively lower temperature. The data from in situ XPS demonstrates that more Ir 3+ can be reduced to active Ir + over Ir-La-20%S/AC than over Ir-La/AC under the same reaction conditions, which is thought to be beneficial for the reductive elimination of AcI from Ir 3+ species as one of the essential elementary reaction steps during methanol carbonylation [34]. Furthermore, the HAADF-STEM measurements show that the components loaded on Ir-La-S/AC are dispersed better than those on Ir-La/AC.…”

“…The preparation of Ir-La/AC was similar to the process reported by Kwak et al [34]. First, 0.0862 g of IrCl3 was dissolved in 17.16 g of deionized water at 65 °C.…”

“…Binding energies, full widths at half-maximums (FWHMs), oxidation states, and peak areas of Ir 3+ and Ir + species obtained by in situ XPS are listed in Table 2. The binding energies of Ir 4f7/2 appear at 62.7 and 61.35 eV, which are assigned to Ir 3+ and Ir + species, respectively [34,44]. There is no doubt that only Ir 3+ species exist in fresh Ir-La-20%S/AC and Ir-La/AC.…”

“…Research by the Eastman Chemical Company has shown the effectiveness of the Ir-La/AC catalyst, which has proved to be active for vapor-phase heterogeneous methanol carbonylation. A mechanism of reaction over Ir-La/AC catalyst was proposed by Kwak et al [34][35][36]. to explain the model of molecular active sites, as well as the promotional effect of La species acting as Lewis acids to accelerate CO insertion as the rate-limiting step.…”