Selective synthesis of cumene and p-cymene over Al and Fe silicates with large and medium pore structures

The alkylation of benzene and toluene with isopropanol was studied in relation to time-on-stream (TOS) at temperatures of 520 and 570 K with the large pore zeolites Y, beta, mordenite and ZSM-12 and medium pore MFI silicates with isomorphously substituted Al and Fe in the framework. It has been found that the conversion of toluene and benzene does not correlate with the number and strength of bridging OH groups, as it does for alkylation reactions leading to xylenes and ethyltoluenes. The TOS behaviour of the conversion as well as the product composition indicated that the reaction rate is controlled by the desorption/transport rate of the bulky propylbenzenes and propyltoluenes being affected by the acidity and channel geometry of the zeolite. Formation of the undesired n-propylbenzene and n-propyltoluenes via bimolecular transalkylation reactions is enhanced by the acidity (strength and number of OH groups) and narrow intersecting channels of the molecular sieves. Thus, on acid catalysts with a more open structures (Y, beta, AlCl3, H3PO4) the formation of branched isomers is preferred. However, as these zeolites do not exhibit a high para-selectivity needed for synthesis of p-cymene, ferrisilicates of MFI structure possessing less acidic Si-OH-Fe groups are favoured.