Synthesis of Benzoxazole via the Beckmann Rearrangement of Salicylaldoxime on Protonated Zeolites: A Green Continuous Process

Benzoxazole was prepared through the Beckmann rearrangement of salicylaldoxime using a series of H-zeolites, K-10 montmorillonite clay, and some common oxide catalysts under well-optimized reaction conditions of temperature, weight hourly space velocity, and catalyst amount. Salicylaldoxime underwent a facile 1,2-o-hydroxyphenyl shift followed by an intramolecular cyclization to yield benzoxazole. Syn-anti isomerization of the oxime on acid catalysts is a key step in the reaction. o-Hydroxybenzonitrile, o-hydroxybenzamide, and salicylaldehyde were the main byproduct. We have observed definite correlations between acid sites distribution of the catalysts and different products formed during the reaction. Catalysts were susceptible for deactivation and the decrease in the percentage conversion of oxime with time is associated with a corresponding increase in the acid hydrolysis producing salicylaldehyde at later stages of the reaction. However, the deactivated catalysts can be regenerated without considerable loss of catalytic activity through an oxidative treatment.