Aromatization of dilute ethylene over Ga-modified ZSM-5 type zeolite catalysts

Aromatization of dilute ethylene (5 mol% ethylene in N-2) over different Ga -modified H-ZSM-5 type zeolite catalysts (viz. Ga/H-ZSM-5 (Ga loading = 0%, 1.4% and 4.7%), H-GaMFI and H-GaAlMFI (with different bulk and framework c ompositions of Si, Ga and Al)) at 400 degreesC and atmospheric pressure was investigated at different space velocities (3100-24,600 cm(3)g(-1)h(-1)), The product selectivity and the distribution of aromatics and xylene isomer s are strongly influenced by the space velocities and consequently show a s trong dependence on the ethylene conversion. With decreasing the conversion , the selectivity for propylene is increased but that of aromatics is decre ased, the concentration of Cs-aromatics and that of p- and o-xylene in xyle nes are increased but that of benzene and toluene are decreased. Based on t hese observations a simplified reaction path for aromatization process is p roposed. Because of their uniform distribution of non-framework (non-FW) Ga -species in the zeolite channels, the H-GaMFI and H-GaAlMFI zeolite catalys ts show higher aromatics selectivity and/or higher ethylene-to-aromatics co nversion activity in the ethylene aromatization than the Ga/H-ZSM-5 zeolite catalysts. The high zeolitic acidity is essential for the high ethylene co nversion activity of the zeolite catalysts, but the high acidity alone is n ot enough for obtaining high aromatics selectivity or high ethylene-to-arom atics conversion activity. The aromatics selectivity does not show direct d ependence on the acidity or non-FW Ga of the zeolite catalyst. For a Ga-mod ified ZSM-5 zeolite catalyst to be highly active and also highly selective in the ethylene aromatization, it should have both high acidity and an opti mum ratio of non-FW Ga to strong acid sites, which is close to 1.0. (C) 200 1 Elsevier Science B,V. All rights reserved.