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

Abstract

Aromatization of dilute ethylene (5 mol% ethylene in N₂) 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 compositions of Si, Ga and Al)) at 400°C and atmospheric pressure was investigated at different space velocities (3100-24,600 cm³ g^-1 h^-1). The product selectivity and the distribution of aromatics and xylene isomers are strongly influenced by the space velocities and consequently show a strong dependence on the ethylene conversion. With decreasing the conversion, the selectivity for propylene is increased but that of aromatics is decreased, the concentration of C₈-aromatics and that of p- and o-xylene in xylenes are increased but that of benzene and toluene are decreased. Based on these observations a simplified reaction path for aromatization process is proposed. Because of their uniform distribution of non-framework (non-FW) Ga-species in the zeolite channels, the H-GaMFI and H-GaAlMFI zeolite catalysts show higher aromatics selectivity and/or higher ethylene-to-aromatics conversion activity in the ethylene aromatization than the Ga/H-ZSM-5 zeolite catalysts. The high zeolitic acidity is essential for the high ethylene conversion activity of the zeolite catalysts, but the high acidity alone is not enough for obtaining high aromatics selectivity or high ethylene-to-aromatics conversion activity. The aromatics selectivity does not show direct dependence on the acidity or non-FW Ga of the zeolite catalyst. For a Ga-modified ZSM-5 zeolite catalyst to be highly active and also highly selective in the ethylene aromatization, it should have both high acidity and an optimum ratio of non-FW Ga to strong acid sites, which is close to 1.0.