Effects of acidification and basification of impregnating solution on the performance of Mo/HZSM-5 in methane aromatization

The effects of acidification and basification of ammonium heptamolybdate solution during impregnation of HZSM-5 on the catalytic performance of Mo/HZSM-5 in methane dehydro-aromatization have been investigated. At reaction temperature of 700 °C and GHSV = 1600 ml h^-1 g^-1, a 2 wt.% Mo/HZSM-5 prepared with H₂SO₄-acidified solution gave a high aromatics yield of 7.2% but the catalytic activity declined drastically with time on-stream. A 2 wt.% Mo/HZSM-5 prepared with NH₃-basified solution showed both high activity (7.1% aromatics yield) and good stability. With a rise in reaction temperature, the 2 wt.% Mo/HZSM-5 prepared with NH₃-basified solution gave increased initial aromatics yields of 10.2 and 15.5% at 750 and 800 °C, respectively. The extent of catalyst deactivation with on-stream time increased with increasing reaction temperature or reactants space velocity but was less severe compared to that over a similar catalyst prepared without pH adjustment of the impregnation solution. The catalyst stability could be further improved by adding an appropriate concentration of CO₂ or O₂ in the reactant stream. With 5.6 vol.% O₂ or 9.6 vol.% CO₂ in the methane feed, steady aromatics yield of ca. 8.4% at 800 °C and 1600 ml h^-1 g^-1 was obtained for a period of 6 h.