Effect and regeneration of heterogeneous palladium/charcoal catalysts poisoned by tetramethylthiourea–quinoline in the stereoselective hydrogenation of methacycline to α-doxycycline

Abstract

For the synthesis of clinically important α-doxycycline by hydrogenation of methacycline, some parameters were controlled for optimization: (a) the dosage of 0.4 wt.% Pd/C catalyst poisoned by tetramethylthiourea–quinoline (TQ), which has the lowest Pd content till now; (b) the reaction temperature and reaction time; (c) the effect of TQ. Under optimized conditions, the best stereoselectivity (94.5%) and excellent conversion (90.7%) were obtained. After being employed, the catalyst showed great reduction in conversion activity and stereoselectivity. Compared to fresh values, the total pore volume that remained was only 1.48% and the total BET surface area that remained was only 0.52%. Some short-chain aliphatic alcohol has been examined for the catalyst regeneration, and successful results were obtained by methanol refluxing treatment. After four times regeneration, the conversion and stereoselectivity only decreased 4 and 1%, respectively.