Mesostructured aluminosilicate alkylation catalysts for the production of aromatic amine antioxidants

The conversion of diphenylamine (DPA) and α-methylstyrene (AMS) to the antioxidants mono- and dicumenyldiphenylamine was carried out over mesostructured aluminosilicate catalysts with hexagonal (2% Al-MCM-41), wormhole (2% Al-HMS), and lamellar/vesicular (2% Al-MSU-G) framework structures. A commercial acid-treated clay catalyst, Engelhard F-20, was included for comparison perposes. The yields of the desired antioxidant, namely dicumenyldiphenylamine (DCDPA), increased in the order F-20 (∼57%)<2% Al-MCM-41 (∼85%)<2% Al-HMS, 2% Al-MSU-G (∼90%) when the reaction was carried out under stoichiometric reaction conditions at 90 °C. The DCDPA yields obtained with the mesostructured catalysts are the highest reported to date for this technologically important antioxidant. A heteropolyacid catalyst, H3PW12O40·xH2O (PW12) supported on mesostructured wormhole HMS and lamellar/vesicular MSU-G silica, also was examined as a catalyst for DCDPA production. The supported catalyst systems, however, afforded lower maximum yields of DCDPA (∼73–80%) in comparison to the mesostructured aluminosilicate catalysts. The exceptionally high yields of alkylated products obtained with the mesoporous aluminosilicate catalysts in comparison to the F-20 clay and the supported PW12 catalysts are attributable in part to intermediate acid strengths that minimize completing dimerization reactions of the AMS alkylating agent. Also, the pore structures of the mesostructured catalysts facilitate access to active sites on the framework walls and provide more efficient transport of reagents to framework reaction centers. Also, the regular mesoporosity of the aluminosilicate catalysts makes these structures less prone to pore plugging and to the masking of acidity through the adsorption of the high molecular weight reaction products.