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

Abstract

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°C), and the overall acidity (measured in terms of the pyridine chemisorbed at 400°C and also in terms of the activity in o-xylene isomerization at 400°C and in toluene disproportionation at 500°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°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 I-T 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 (C₁ + C₂) 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.