Directed substitution and evolution of active sites in quaternary ammonium resin catalyst for oxalate transesterification

An acetate-functionalized heterogeneous catalyst, N + -(Ac), was successfully synthesized via directed substitution of an inert site in quaternary ammonium resin by a weak-base active center and applied to the transesterification of dimethyl oxalate (DMO) with ethanol to prepare diethyl oxalate (DEO). When the optimal N + -(Ac) catalyst catalyzed the DMO transesterification with ethanol for 2 h, the DMO conversion, DEO yield, and corresponding turnover frequency (TOF) reached 93.4%, 49.7%, and 13.04 h −1 , respectively. Additionally, the synthesized N + -(Ac) exhibited catalytic universality for DMO transesterification with different alcohols, and the catalytic efficiency decreased with increasing difficulty of cleaving the hydrogen–oxygen bond from C 2 to C 4 and steric hindrance of isomeric alcohols. The pretreatment experiments of catalyst with different water content and raw materials were designed and the resin matrix, fresh catalyst, deactivated catalyst and regenerated sample were systematically analyzed using various characterization techniques to investigate the catalytic deactivation mechanism of the transesterification reaction between DMO and ethanol to synthesize DEO. The results showed that DMO played a key role in catalyst deactivation and the catalyst was deactivated upon replacement of the active sites by monomethyl oxalate anions from hydrolysis side reaction of DMO. Finally, an efficient raw material regeneration strategy for the catalyst was proposed. Herein, new theoretical support is introduced for the structural evolution of weak-base catalysts and will aid in developing heterogeneous catalysts for oxalate transesterification.