Acidity, Catalytic Activity, and Deactivation of H-Gallosilicate (MFI) in Propane Aromatization: Influence of Hydrothermal Pretreatments

Effect of various hydrothermal pretreatments [at different temperatures (400–800°C) and concentrations of steam (13–80 mol%) or with liquid water at 150°C under autogenous pressure] to H-gallosilicate (MFI) zeolite (bulk Si/Ga = 33 and Na/Ga = 0.1) on its acidity/acid strength distribution (determined by chemisorption and step-wise thermal desorption of pyridine from 100–400°C), acid function (studied by acid catalyzed model reactions viz. isooctane cracking for external acidity ando-xylene isomerization and toluene disproportionation for internal or intracrystalline acidity), and deactivation due to coking in propane aromatization (at 550° and time-on-stream of 8.5 ± 0.5) has been thoroughly investigated. With the increase in the severity of hydrothermal treatment to the zeolite, its crystallinity, framework (FW) Ga (observed by71Ga and29Si MAS NMR and FTIR), acidity (measured in terms of the pyridine chemisorbed at 400°C and activity in the model reactions), and catalytic activity in the propane aromatization are decreased, but its deactivation due to coking and shape selectivity are increased appreciably. These effects are attributed to the extensive degalliation of the zeolite due its hydrothermal treatments. Its product selectivity, dehydrogenation/cracking (D/C) activity ratio, and aromatics/(methane + ethane) mass ratio in the propane aromatization are also influenced by its hydrothermal treatments, depending upon the conversion. The product and shape selectivity of the zeolite are also affected by its deactivation due to coking. The influence of hydrothermal treatments on the activity/selectivity and catalyst deactivation are attributed to a combined/complex effect produced by the decreased zeolitic acidity (i.e., FW Ga) and increased non-FW Ga oxide species in the zeolite channels, depending upon the severity of hydrothermal treatment. The hydrothermal stability of H-gallosilicate (MFI) is much lower than that H-ZSM-5.