Temperature effects in benzene alkylation with ethane into ethylbenzene over a PtH-MFI bifunctional catalyst

Benzene alkylation with ethane into ethylbenzene (EB) over a PtH-MFI bifunctional catalyst was thoroughly studied at six different temperatures between 290 and 490 °C. Our work demonstrates that temperature affects in a different way the thermodynamic equilibrium of two major reaction steps: (i) ethane dehydrogenation into ethene over Pt sites and (ii) benzene alkylation with ethene over acid sites. This, in turn, leads to significant differences in the product distribution and catalyst performance at different temperatures. It is shown that with increasing temperature ethane dehydrogenation is highly accelerated, while the EB formation rate increases in much lesser extent. As a consequence, high concentration of ethene is observed at high temperatures (450–490 °C), while the maximum concentrations of EB are very similar at all temperatures. Ethene, which is formed in excess at high temperatures, is converted via oligomerization and cracking steps into higher alkenes that alkylate benzene (or aromatic products). These alkylation reactions are followed by cyclization and dehydrogenation steps leading to formation of polyaromatics (coke precursors) and subsequent catalyst deactivation. On the other hand, at lower temperatures (290–410 °C) ethene reacts preferably with benzene forming EB. Hence, side reactions are suppressed and high EB selectivity and catalyst stability are observed at these temperatures. Based on the analysis of the catalyst activity, selectivity and stability, it is concluded that 370–410 °C can be considered as the optimum temperature range for the direct benzene alkylation with ethane into EB. Further improvement of the process may be achieved by selective removal of hydrogen from the reactor and by optimization of the ethane/benzene ratio in the feed.