FACTORS CONTROLLING ISO-SELECTIVITY N-SELECTIVITY AND PARA-SELECTIVITY IN THE ALKYLATION OF TOLUENE WITH ISOPROPANOL ON MOLECULAR-SIEVES/

Toluene alkylation with isopropanol was investigated over molecular si eves possessing different acidity (Al- and Fe-silicates) and structura l 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 pro pyltoluenes only cymenes were found with H-mordenite, and H-Y yielded n-propyltoluenes only at temperatures above 550 K, while up to 520 K c ymenes were exclusively produced. On the other hand, with isomorphousl y substituted molecular sieves with MFI structure, having different ac idities, a substantial concentration of n-propyltoluenes besides cymen es was found over the whole temperature range investigated (470-620 K) . The data indicate that the dominating factor for formation of n-prop yltoluenes (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 act ivity of the molecular sieves, controlled by the number and strength o f 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 mo lecular sieves tend to produce n-propyltoluene. However, simultaneousl y this structure prefers an over-equilibrium concentration of para-alk yltoluenes. As a compromise between these factors, the highest yield o f the desired product, p-cymene, can be found with molecular sieves ha ving a MFI structure possessing a low number of bridging OH groups of a lower acid strength, (H-(Fe)ZSM-5), and by employing short contact t imes and a reaction temperature below 570 K.