Identity and chemical function of gallium species inferred from microkinetic modeling studies of propane aromatization over Ga/HZSM-5 catalysts

Ga/HZSM-5 catalysts, synthesized by the incipient wetness impregnation technique, showed a steady decline in Brønsted acidity with gallium addition. A maximum in propane conversion and aromatics selectivity at a Ga/Al ratio of about 0.5 suggests synergy between proton and gallium sites. A microkinetic model using 312 elementary steps and 25 rate and equilibrium parameters to describe the aromatization of propane over HZSM-5 with Si/Al of 16 is the base case against which the effects of Ga are compared. Kinetic models based on two different Ga active sites, including GaH2+ and GaH2+, were first used individually to describe the diverse dataset that includes conversion to 10 different products as a function of temperature (510–540 °C), space time (2–8 gcat h/mol), and Ga/Al (0–1) variations. An evaluation of these models based on an assigned catalytic functionality for these sites and the associated parameters showed that both sites are required to provide a unified description of the catalytic behavior across gallium content with monohydridic Ga-sites being predominantly prevalent at low Ga/Al ratio and dihydridic Ga-sites at high Ga/Al ratios. In this paper, we address the ability to discriminate between the models and their implications for the primarily dehydrogenation nature of the Ga active sites.

Unified gallium monohydride–gallium dihydride model predictions, indicating the regions where each active site dominates. All aluminum assumed to be in pairs, Si/Al = 16, T = 520 °C, space time = 1.514 gcat h/mol.Figure optionsDownload high-quality image (46 K)Download as PowerPoint slide