Factors controlling iso-/n- and para-selectivity in the alkylation of toluene with isopropanol on molecular sieves

Abstract

Toluene alkylation with isopropanol was investigated over molecular sieves possessing different acidity (Al- and Fe-silicates) and structural type (Y, mordenite and MFI structure) to elucidate factors playing a decisive role in iso-/n- and para-selectivity in propyltoluenes. The desorption/transport of bulky propyltoluenes from the zeolite channel systems was found to be the reaction rate controlling step. Of the propyltoluenes only cymenes were found with H-mordenite, and H-Y yielded n-propyltoluenes only at temperatures above 550 K, while up to 520 K cymenes were exclusively produced. On the other hand, with isomorphously substituted molecular sieves with MFI structure, having different acidities, a substantial concentration of n-propyltoluenes besides cymenes was found over the whole temperature range investigated (470–620 K). The data indicate that the dominating factor for formation of n-propyltoluenes (via a bimolecular mechanism involving the reaction between isopropyltoluene and toluene molecule) is the zeolite structural type. Further, the n-propyltoluene formation is enhanced by the acidic activity of the molecular sieves, controlled by the number and strength of the acid sites and by the reaction temperature, as well as by longer contact time. Thus, both the MFI structure and high acidity of the molecular sieves tend to produce n-propyltoluene. However, simultaneously this structure prefers an over-equilibrium concentration of para-alkylto-luenes. As a compromise between these factors, the highest yield of the desired product, p-cymene, can be found with molecular sieves having a MFI structure possessing a low number of bridging OH groups of a lower a id strength, (H-(Fe)ZSM-5), and by employing short contact times and a reaction temperature below 570 K.