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 g_cat 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.