H.Na-Gallosilicate (MFI) propane aromatization catalyst: Influence of H+ exchange on acidity, activity, and deactivation due to coking

The effect of H+ exchange of H.Na-GaMFI (bulk Si/Ga = 33 and framework Si/Ga = 48 +/- 3) On its external acidity (measured in terms of the activity in iso-octane cracking at 400 degrees C), and the overall acidity (measured in terms of the pyridine chemisorbed at 400 degrees C and also in terms of the activity in o-xylene isomerization at 400 degrees C and in toluene disproportionation at 500 degrees C) has been investigated. The initial activity and product selectivity (at the same conversion) and time-on-stream activity (or deactivation due to coking) in propane aromatization (at 550 degrees C) over the zeolite with different degrees of H+ exchange has also been thoroughly investigated. Both the external and overall acid sites and the initial activity and selectivity for aromatics in the propane aromatization are increased with increasing H+ exchange. The dehydrogenation/cracking activity ratio and aromatics/(methane + ethane) mass ratio are also increased with increasing the HC exchange. However, the rate of catalyst deactivation due to coking is higher for the zeolite with higher H+ exchange because of its higher activity. The shape selectivity of the zeolite is increased with its deactivation due to coking, the effect being stronger for the lower H+ exchange. The selectivity for cracking products (C-1 + C-2) is increased, and consequently dehydrogenation/cracking and aromatization/cracking activity ratios are decreased due to the catalyst deactivation. The initial activity and selectivity for aromatics in propane aromatization shows a strong dependence on the zeolite acidity. (C) Elsevier Science Inc. 1997.