Selective production of xylenes from alkyl-aromatics and heavy reformates over dual-zeolite catalyst

Disproportionation of 1,2,4-trimethylbenzene (1,2,4-TMB), its transalkylation with toluene as well as dealkylation of ethyltoluenes (ETs) over MOR and MFI zeolites were investigated in a fluidized-bed reactor at 300-400 °C. MFI exhibited good ET conversion (60 wt.%), high dealkylation selectivity (60%), but low disproportionation and transalkylation activities. MOR showed high ET conversion (80 wt.%) with low dealkylation selectivity (33%), and high disproportionation and transalkylation activities. Based on these results, a dual-zeolite catalyst consisting of MOR and MFI was proposed to promote the diverse reactions leading to selective production of xylenes via dealkylation-transalkylation of alkylaromatics. Single-zeolite (MOR-based) and dual-zeolite (MOR-and MFI-based) catalysts were prepared by addition of binder, characterized and evaluated for the conversion of C₈+ and C₉+ heavy reformates (either alone or with the addition of various amounts of toluene). The catalysts exhibited little differences in disproportionation and transalkylation activities. However, the dealkylation of ETs was enhanced over dual-zeolite catalyst, which can be attributed to its higher acid site strength. The reduction in microporous volume in dual-zeolite catalysts prevented some undesired secondary reactions, which require formation of bimolecular reaction intermediates. Compared with the single-zeolite catalyst, the dual-zeolite catalyst exhibited an increase in the xylenes yield from 16.6 to 19.8 wt.% and an enhancement in xylene selectively from 37.1 to 43.3% in the conversion of C₉+ heavy reformate at 400 °C. Addition of about 20 wt.% toluene to heavy reformate feed resulted in further increase in xylenes yield to 21.2 wt.% over the dual-zeolite catalyst.