Product distribution in the aromatization of dilute ethene over H-GaAlMFI zeolite: effect of space velocity

Abstract

Influence of space velocity on the aromatization of dilute ethene (5 mol% in N₂) over H-GaAlMFI zeolite catalyst, having high acidity (0.46 mmol g^-1, measured in terms of the pyridine chemisorbed at 400 °C) and high concentration of non-framework Ga-oxide species (0.32 mmol g^-1), at atmospheric pressure covering a wide temperature range (300-500°C) has been thoroughly investigated. The selectivity of aromatics, propene, propane and C₄ hydrocarbons and alkane/aromatics and H₂/aromatics mole ratios are strongly influenced by the space velocity. The results indicate that the aromatization involves H₂ transfer reactions predominantly at the lower temperatures and/or higher space velocities whereas dehydrogenation reactions become predominant at higher temperatures and/or lower space velocities. The distribution of aromatics and C₈-aromatic isomers depends strongly upon the amount (i.e. yield) of aromatics and C₈-aromatics, respectively, formed in the process. The primary aromatics produced in the process are found to be mainly p- and o-xylenes. The aromatics distribution is, however, controlled by the aromatics inter-transformation (viz. isomerization, alkylation/dealkylation and disproportionation) reactions. The p-xylene/m-xylene ratio is decreased as expected, but the p-xylene/o-xylene ratio is increased with increasing both the space velocity and temperature. The increase of p-xylene/o-xylene ratio is found to be unusual, much above the equilibrium value.