Modeling propane aromatization on HZSM-5 and Ga/HZSM-5

The effect of gallium addition to HZSM-5, a method known to enhance the dehydrocyclization ability of the catalyst, was studied. The aromatization of propane over this catalyst was described by 311 elementary steps, including the activation of alkanes via the carbonium ion chemistry, hydride transfer, dehydrogenation, olefin adsorption/desorption, oligomerization, β-scission, and cyclization. XRD confirmed the crystalline nature of the catalyst while elemental analysis was used to obtain accurate Ga exchange ratios. FTIR spectroscopy, performed after gallium introduction, indicated a steady decline in the strength of the 3610/cm band with increase in Ga content, thereby suggesting that gallium sites compensate for a fraction of the Bronsted acid sites. The propane conversion as well as selectivity to aromatics went through a maximum with the Ga/Al ratio, appearing to maximize at about 0.5. The experimental observations thus pointed towards a synergistic mechanism between the proton and gallium sites. The mechanistic implications of models of this synergism, obtained by fitting the data over the full Ga composition range, were discussed. This is an abstract of a paper presented at the AIChE Annual Meeting and Fall Showcase (Cincinnati, OH 1/04/2005).