Propane aromatization over HZSM-5 and gallium/HZSM-5 catalysts: A catalyst design perspective

Quantitative modeling of propane aromatization over HZSM-5 and Ga/HZSM-5 catalysts in this study has focused on addressing the kinetic parameters associated with the Bronsted acid and gallium sites as well as the chemical role of gallium. Microkinetic modeling was supplemented by DFT and experimental studies in an effort to develop fundamental mechanistic understanding. Reaction kinetic studies of propane conversion to aromatics were conducted on an HZSM-5 sample at a pressure of 1 atmosphere, temperatures in the range 520--550°C and different space times (5--30 gm-cat hr/mol). The more than 300 elementary steps included in the HZSM-5 kinetic model were based on the following chemistry: C--H and C--C bond activation in alkanes via the Haag-Dessau mechanism, hydride transfer and alkylation via rearrangement of carbonium-ion-like species, carbenium-ion-like transition states for olefin adsorption/desorption, beta-scission/oligomerization, cyclization, alkylation and dealkylation of aromatic molecules. Alkoxide surface intermediates were considered and reactions were grouped into reaction families considering similar reactants forming similar products via similar transition states with the nature of the transition state being the driver for the kinetics. The number of parameters was further reduced based on relative rates for beta-scission and hydride transfer from the literature. Pre-exponential factors were bounded by the transition state theory and activation barriers were bounded by values from the literature. For gallium-modified materials, each gallium atom was considered to replace one acidic site, and independent site balances for gallium and acidic sites were employed. For the gallium model, the kinetic parameters obtained for HZSM-5 were not re-optimized; each rate constant however, was scaled to account for the reduced number of acidic sites. Gallium sites were considered to introduce additional reaction pathways corresponding to alkane and napthene dehydrogenation as well as C--C bond cleavage in alkanes, however, no oligomerization or cyclization functionality was considered for gallium sites. Good agreement between experimental results and model predictions over a wide range of space times and hence conversions suggest that we have developed a model that describes the activity of Bronsted acid and gallium sites in HZSM-5 and Ga/HZSM-5 materials. In addition, the DFT study of olefin adsorption on cluster based models of HZSM-5 demonstrates the effect of local zeolite structure on the energetics of olefin adsorption and proposes a novel mechanism for olefin adsorption on a single zeolite lattice oxygen.