Separation of close‐boiling 2,4‐/2,5‐xylenol isomers from coal tar

It is challenging to separate 2,4‐/2,5‐xylenol isomers with very close boiling points. In the present work, a multistep process for the separation of 2,4‐/2,5‐xylenol isomers from coal tar was investigated, involving alkylation, distillation, and dealkylation. Starting from the selection of suitable catalyst, the operating conditions for alkylation, distillation, and dealkylation were optimized by varying key parameters systematically. Among the four catalysts examined, p‐toluenesulfonic acid (PTSA) showed best performance in terms of conversion and selectivity. Under the optimized alkylation conditions, the yields of 6‐tert‐butyl‐2,4‐xylenol and 4‐tert‐butyl‐2,5‐xylenol were 97.3% and 86.2%, respectively. Following pH adjusted to 7–8 with 10% sodium carbonate and water washing, the minimum reflux ratios during distillation for obtaining 6‐tert‐butyl‐2,4‐xylenol and 4‐tert‐butyl‐2,5‐xylenol were 4 and 12, with product purity of 99.8% and 99.5%, respectively. Under the optimized dealkylation conditions, both yields of 2,4‐xylenol and 2,5‐xylenol reached 99.9%. After recrystallization, the final product purity was over 99.5%. This lab‐scale investigation provides key information for an on‐going pilot‐scale process design.