ABSTRACT

Industrially, petroleum naphtha reforming and steam cracking are two major multistep and energy-intensive sources for p-xylene production. Additionally, p-xylene is also produced by selective toluene disproportionation, heavy (C₉) aromatics trans-alkylation and isomerization of xylenes. Rapidly changing market dynamics from fuels to petrochemicals coupled with growing petrochemicals demand has led to the further exploration of economical and highly p-xylene selective processes by using surplus low-value aromatics. Methylation of toluene with methanol has been envisaged as one such potential opportunity to produce p-xylene. For the purpose, significant efforts have been made in the past forty years to develop suitable shape selective zeolite catalysts and technologies to make toluene methylation a highly p-xylene selective process. Current paper briefly highlights various advancements in the toluene methylation process development and aims to provide an extensive review of several works put up in the past to design suitable catalysts, catalyst’s limitations and developmental challenges for toluene methylation. Further, this paper provides an useful insight for two competing chemistries of toluene alkylation and side-by-side methanol reactions. Finally, motivated by competitive chemistry of methanol conversion reactions, existing catalytic limitations and novel synthesis opportunities in engineering shape selective zeolite catalysis, a way forward is proposed to arrive at a logical and futuristic catalyst design strategy for p-xylene production via toluene methylation. The proposed strategy is envisaged to offer low-to-moderate catalyst acidity, improved toluene methylation activity, high p-xylene selectivity and improved catalyst life which, otherwise, up till now have been the bottlenecks to develop distinctly economical toluene alkylation to p-xylene process.

摘要

在工业上，石脑油重整和蒸汽裂解是对二甲苯生产的两个主要的多步骤和能源密集型来源。此外，对二甲苯还通过选择性甲苯歧化、重质（C ₉) 芳烃的烷基转移和二甲苯的异构化。从燃料到石化产品的快速变化的市场动态，加上不断增长的石化产品需求，导致通过使用剩余的低价值芳烃来进一步探索经济且对二甲苯选择性高的工艺。甲苯与甲醇的甲基化被认为是生产对二甲苯的潜在机会之一。为此，在过去的四十年中，人们做出了巨大的努力来开发合适的形状选择性沸石催化剂和技术，以使甲苯甲基化成为一种高度对二甲苯选择性的过程。目前的论文简要强调了甲苯甲基化工艺开发的各种进展，旨在对过去为设计合适的催化剂所做的几项工作进行广泛的回顾，甲苯甲基化催化剂的局限性和发展挑战。此外，本文为甲苯烷基化和并行甲醇反应这两种相互竞争的化学反应提供了有用的见解。最后，受甲醇转化反应的竞争化学、现有的催化限制和工程择形沸石催化中新的合成机会的推动，提出了一种前进的方向，以达到通过甲苯甲基化生产对二甲苯的合乎逻辑和未来的催化剂设计策略。所提出的策略旨在提供低至中等的催化剂酸度、改进的甲苯甲基化活性、高对二甲苯选择性和改进的催化剂寿命，否则，到目前为止，这一直是开发明显经济的甲苯烷基化制对二甲苯的瓶颈过程。