Flowing scalable production of sulfenamides by active site-tuned lacunary polyoxometalate foams

Abstract

The flowing production of sulfenamide rubber additives with performance beyond industry norms is inspiring, yet it remains challenging to conquer the generally low efficiency, harsh reaction conditions, and complex equipment needed in the traditional industrial synthesis process. Herein, four transition-metal lacunary polyoxometalates (TM-LPOMs) were fabricated into ultralow density foams and used to construct TM-LPOMs@CMC (TM = TM = Cu, Co, Ni and Mn; CMC = carboxymethylcellulose) foam materials for the efficient flowing catalytic production of sulfenamide rubber additives under mild conditions. Noteworthily, the optimal Cu-LPOMs@CMC could successfully catalyze the efficient coupling of 2-mercaptobenzothiazole and cyclohexylamine with excellent yield (96%) and could be scaled up for the production of sulfenamides (production rate, ∼430 mg h⁻¹ in a batch experiment). The as-obtained sulfenamide rubber additives could be used as rubber-vulcanization accelerators with superior rubber plastic properties (e.g., 5.8% reduced permanent deformation rate and 16.2 °C decreased internal temperature rise) compared to industrial rubber additives used in industrial rubber molding processes. This might open a new avenue to apply POM-based foams in continuous catalytic applications in the rubber industry.