In situ DRIFTS study of CO coupling to dimethyl oxalate over structured Al-fiber@ns-AlOOH@Pd catalyst

- Catalytic mechanism of CO coupling to dimethyl oxalate was investigated using in situ DRIFTS.
- Intermediate with stretching vibrations of CO and CO, and rocking vibration of CH3 was captured.
- As-captured intermediate was identified as COCOOCH3∗ by using CH3OCOCOCl probe molecule.
- Consecutive CO insertion into OCH3∗ to form COCOOCH3∗ was confirmed.
- Coupling of COCOOCH3∗ with OCH3∗ was the rate-determining step.

Pd-catalyzed CO coupling to dimethyl oxalate (DMO) process has been commercialized but its reaction mechanism is still open for debate between COCOOCH3∗ and COOCH3∗ (∗, a surface site). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies were performed to clarify such controversy on a high-performance Al-fiber@ns-AlOOH@Pd catalyst. Intermediate species consisting of stretching vibrations of CO and CO, and rocking vibration of CH3 was indeed captured, which could be assigned to either COCOOCH3∗ or COOCH3∗. To make discrimination between COCOOCH3∗ and COOCH3∗, methyl oxalyl chloride (CH3OCOCOCl) and methyl chloroformate (CH3OCOCl) were employed to form COCOOCH3∗ and COOCH3∗ species on the catalyst surface for DRIFTS analyses. Interestingly, the characteristic bands of the as-observed intermediate species in the real reaction matched the obtained COCOOCH3∗ species from dissociative adsorption of CH3OCOCOCl. A double carbonylation reaction pathway was thus confirmed for the CO coupling to DMO, i.e., consecutive insertion of two CO molecules into OCH3∗ to form COCOOCH3∗ followed by combining OCH3∗ to yield DMO (CH3OCOCOOCH3).

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