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.
Bibliographical noteFunding Information:
The authors would like to thank Dr. Yogesh Joshi and Prof. Kendall Thomson for valuable discussions on the density functional theory calculations on these materials. Dr. Shuo-Huan Hsu is acknowledged for advancing the reaction modeling suite (RMS) used in this work. The authors are also grateful to Prof. Gary Blau for providing statistical insights on the models and Steve Stamatis for advice on the AIC calculations. This work was supported by the Department of Energy, Office of the Basic Energy Sciences , through the Catalysis Science Grant No. DE-FG02-03ER15466 . We are grateful to ExxonMobil for supplying the HZSM-5 sample used in this study and Dr. James Rekoske from UOP for valuable discussions.
- Alkane aromatization
- Kinetic modeling