Conversion of Light Alkanes into Aromatic Hydrocarbons .VII. Aromatization of Propane on Gallosilicates: Effect of Calcination in Dry Air

The aromatization of propane at 530°C on three MFI gallosilicates (Si/Ga = 30, 50 and 180) and on a MFI aluminosilicate (Si/Al = 30) was investigated. After pretreatment at 530°C under dry air flow, 30 Al MFI was seven times more active than 30 Ga MN. This can be related to the greater strength of the acid sites of the aluminosilicate sample, which was shown by a calorimetric study of ammonia adsorption and by ammonia thermodesorption. On all the catalysts propene and methane + ethene in quasi-equimolar amounts appeared as primary products, whereas ethane, butenes, butanes, C+5 olefinic and naphthenic compounds, and C6-C8 aromatics were secondary products. However, large differences were found in the product distribution, namely a lower initial cracking/ dehydrogenation ratio on 30 Ga MFI, greater selectivity to aromatics, and lower selectivity to alkanes (C1, C2, C4) at high conversion, which can be explained by a dehydrogenating effect of gallium. The product distribution depended also on the Si/Ga ratio of the gallosilicate, a better selectivity to aromatics being found with 30 Ga MFI. The selectivity differences are mainly due to differences in the acid site density. The treatment of gallosilicates at 700-800°C under dry air flow caused a significant increase in their activity and in their selectivity for propane aromatization. This could be related to the formation of a small amount of extraframework gallium species, well dispersed in the crystallites and highly active for dehydrogenation of propane and of naphthene intermediates. On these catalysts propane aromatization occurred mainly through a bifunctional scheme and the formation of unwanted C1-C2 products, through acid cracking was very limited. The effect of the pretreatment temperature on the acid and dehydrogenating activity of the 30 Ga MFI sample showed that framework gallium species were much less active for dehydrogenation than extraframework species and indeed are probably inactive.