Catalyst in Sight: The Use of Benchtop NMR Spectrometers to Maintain the Activity of Pd-PPh₃ Catalysts

Carbonylation reactions often suffer from catalyst deactivation. Operando spectroscopy has been proven to be a suitable tool for both understanding deactivation mechanisms and suppressing deactivation. In this work, the use of spectral information obtained from operando studies for a process control aiming at maintaining catalytic activity is shown for the first time using a benchtop NMR spectrometer. In the methoxycarbonylation of ethylene catalyzed by a system of [Pd(OAc)2], PPh3, and TsOH, a PPh3 signal in the aromatic region could be isolated that is crucial for catalytic activity in the reaction. This signal could be identified as a superposition of the signal of complexed PPh3 and free PPh3 in exchange with this complex. The integration of this Pd-bound PPh3 signal over time was introduced as a key reaction parameter. The final methanol conversion of all experiments performed depends linearly on this parameter, the Pd-bound PPh3 lifetime. Phosphonium-ion formation was found to be the main cause of ligand degradation and could be detected by a benchtop and high-field NMR. Through tracking of the Pd-bound PPh3 signal in real time together with manual dosing of fresh PPh3 and TsOH, the catalytic activity could be maintained and methanol conversion increased.