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

Abstract

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 ongoing pilot-scale process design.

An ecofriendly and reusable solid acid catalyst ZrO2‐SIEC for highly selective alkylation of 2,4/2,5‐xylenol

“…On the one hand, the steric hindrance effect of 4‐2,5‐xylenol was greater than that of 6‐2,4‐xylenol. On the other hand, the alkylation of 2,4/2,5‐xylenol was a reversible reaction, especially the dealkylation of 4‐2,5‐xylenol was easier, 7 thus its overall yield was relatively lower.…”

“…During the alkylation of 2,4/2,5‐xylenol, the reaction temperature not only affects the reaction rate, but also has an important effect on product yield and selectivity 7,8 . With ZrO 2 ‐SIEC, the effect of varying temperature, ranging from 60°C to 90°C, on the yield was investigated with a catalyst dosage of 9.0%.…”

“…However, due to the very close boiling points (b.p.) of 2,4‐xylenol and 2,5‐xylenol (i.e., 211.0°C and 211.2°C, respectively) at atmospheric pressure, it is difficult to efficiently separate the mixed 2,4/2,5‐xylenol through traditional vacuum distillation 7 . This has still remained currently as a significant technical bottleneck for industrial application, restricting the efficient utilization of this by‐product as a resource for high‐value chemicals.…”

“…In our recent studies, we have adopted a three‐step strategy through alkylation‐distillation‐dealkylation to successfully demonstrate the feasibility of separating 2,4/2,5‐xylenol mixture 7,8 . In this process, the mixed 2,4/2,5‐xylenol isomers were first converted by alkylation to 6‐tert‐butyl‐2,4‐xylenol (or 6–2,4‐xylenol, b.p.…”

“…Also, the suspension of solid particles is favorable for fluid mixing and heat transfer in multiphase systems. However, the overall catalytic efficiency of solid acid catalysts has been found to be lower than that of liquid acids 7 . This is largely associated with a number of factors such as the mass transfer process, especially involving gas–liquid–solid multiphase, the loading efficiency of acid, the possible deactivation of catalyst resulting from mechanical, thermal, and/or chemical instabilities 22 …”