Process Design Strategies To Produce p-Xylene via Toluene Methylation: A Review

It is envisaged that the future refineries will be reconfiguring for the direct conversion of crude oil to chemicals to improve their profitability. In this direction, the toluene methylation process offers a promising route for the production of high-value p-xylene from low-cost feedstocks such as toluene and methanol. There is a growing commercial interest in the toluene methylation process, because it can theoretically produce double the amounts of p-xylene with high selectivities, when compared with the other existing processes (such as disproportionation and transalkylation), and it can be the next-generation method of producing p-xylene. Past and current research approaches were mainly focused on improving the catalyst activity (high toluene conversion and para-selectivity) by introducing several modifying agents. However, still the major challenge of this process lies in the development of an efficient catalyst without compromising on the methylation activity and para-selectivity. In addition, catalyst deactivation is another major issue that is mainly due to the coking precursors derived from methanol conversion. To address these issues, a good understanding of the mass transfer plays a vital role in the catalyst design, as well as process design and optimization strategies. This review summarizes the fundamental aspects of possible reaction mechanisms that occur in the process, useful reaction kinetics data for the process optimization studies and scaleup, with emphasis on recent developments in the catalyst design, deactivation mechanism, and process development strategies. A future perspective is also provided related to the catalyst design and process design strategies for addressing the gaps in this field.