Kinetics of dealkylation-transalkylation of C₉ alkyl-aromatics over zeolites of different structures

Dealkylation of methylethylbenzenes as well as the conversion of mixtures trimethylbenzenes (TMBs) and methylethylbenzenes (MEBs) over medium- and large-pore zeolites with different topologies and acid-site concentrations were investigated in a fluidized-bed reactor. MEB conversion, its dealkylation selectivity and transalkylation selectivity were profoundly influenced by the topology of the zeolites. Zeolite beta shows a highest MEB conversion (70-80%) while ZSM-5 exhibited very high dealkylation selectivity. The results of kinetic study indicate that MEB conversion is influenced by zeolite type and by SiO₂/Al₂O₃ ratio. The extent of TMB and MEB conversions over different zeolites as well as xylene yields indicate that while ZSM-5 could substantially convert MEBs (mainly by dealkylation), it could not effectively catalyze TMB conversion. On the other hand, the conversions of MEBs as well as TMBs were quite high over mordenite and zeolite beta, resulting in much higher xylene yields. These results indicate that while the dealkylation of MEBs is a necessary condition, it is not a sufficient condition to obtain higher xylene yield. Kinetic modeling results show that the transalkylation reaction was significantly faster than the disproportionation reaction indicating that the transfer of methyl group from TMB to toluene is a preferred route under the reaction conditions studied.