Carbon dioxide methylation with toluene to para-xylene process: technical, economic and environmental evaluation

The production of para-xylene (PX) through carbon dioxide (CO2) methylation is a rapidly developing research field, although there are still several uncertainties. This study proposes and simulates two processes for PX production using both direct and indirect CO2 methylation routes. The indirect route involves CO2 hydrogenation to produce methanol, followed by methylation of toluene to PX. In contrast, the direct route entails rapid activation of CO2 into H3CO* species, the C1 intermediate for methylation, on the metal oxide solid solution of the bifunctional catalyst. The effects of operating conditions on feedstock conversion and PX selectivity are investigated, along with technical, economic, and environmental analyses conducted for both processes. The results show that the direct process has better utilization of benzene ring, carbon element and hydrogen element, with percentages of 96.39 %, 96.63 %, and 32.21 % respectively, compared to the indirect process. Furthermore, the direct process also demonstrates advantages such as low energy consumption, high sensitivity to feedstock and product prices, and good techno-economic benefits, with low investment and high annual sales revenue. Additionally, the direct process exhibits the lowest CO2 emission of 0.97 kg-CO2/kg-PX.