Application of acid-treated bentonite in catalytic synthesis of alkylated diphenylamine

Alkylated diphenylamines are amine antioxidants with excellent properties, widely used in lubricating oils or rubber products to enhance their antioxidant properties. Herein, hydrochloric acid treated bentonite was chosen as the catalyst for the synthesis of diisononyl diphenylamine (DNDPA) due to its excellent surface acidity, more suitable pore structure for this reaction, and easy separation from the product. In addition, the optimal concentration of hydrochloric acid during the acid treatment process was investigated. X-ray fluorescence analysis (XRF), X-ray diffraction (XRD), Pyridine-adsorbed Fourier transform infrared spectroscopy (Py-IR), N₂ adsorption and desorption isotherms, and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were employed to analyze the crystal composition, acid properties, and pore structure of the catalyst samples. The experimental findings revealed that bentonite from the first mineral belt of China, activated through hydrochloric acid treatment, demonstrated the best catalytic performance in alkylated diphenylamine synthesis. Under optimally hydrochloric acid concentrations, the Brønsted acidity and specific surface area of bentonite catalysts were significantly enhanced. The optimal acid-treated bentonite achieved a remarkable 96% conversion of diphenylamine (DPA) in the reaction, with a corresponding 63% yield of DNDPA. Meanwhile, excessive hydrochloric acid concentrations adversely affected the structure of the bentonite, thereby impeding the catalytic reaction. Based on these reaction outcomes, the mechanism underlying the catalyst’s role in diphenylamine alkylation was investigated.