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

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 C[dbnd]O and C[sbnd]O, 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).