Elucidation of Catalytic Propane Dehydrogenation Using Theoretical and Experimental Approaches: Advances and Outlook

Propylene is an important chemical used in the production of polypropylene, propylene oxide, propylene carbonate, and many more useful chemical compounds. High propylene demand and low industrial production motivate propane dehydrogenation (PDH) technology for propylene production. CrOx and Pt–Sn catalysts are employed for industrial PDH, occurring via the nonoxidative route. Cr-based catalysts face serious health and environmental issues. Moreover, the nonoxidative route used industrially faces thermodynamic limitations and catalyst deactivation occurring, because of recrystallization, sintering, agglomeration, and coking. The nonoxidative routes favor homolytic dissociation of the bonds (because of positive values of ΔG and ΔH of reaction), while both homolytic and heterolytic dissociation are observed in the oxidative route. Nonoxidative PDH follows the Langmuir–Hinshelwood mechanism, whereas oxidative PDH can occur via the Langmuir–Hinshelwood mechanism (with soft oxidants such as CO2, S2, SO2, and NOx) or the Mars–van Krevelen mechanism (with O2). First-order kinetics of nonoxidative PDH and cracking is observed at low partial pressures of propane, whereas, at high partial pressures, it becomes zero-order, along with standard Gibbs free energy of the reaction (ΔGR0) and enthalpy change of the reaction (ΔHR0) equal to 86.2 kJ mol–1 and 124.3 kJ mol–1, respectively, at 25 °C. ΔGR0 values for all the steps of oxidative PDH are negative, showing the thermodynamic feasibility of the oxidative route for PDH. Using H2 as a cofeed for nonoxidative PDH removes the precursor of coke, which increases catalyst stability and decreases the coke formation rate. Water addition results in an increase in the COx selectivity with an increment in the amount of water. The effect of promoters, reaction conditions, and support on Pt and Pt-based catalysts in terms of selectivity and yield of propylene, conversion of propane, changes in binding and bond dissociation energies, and activation energy is discussed with the help of DFT calculations and characterization techniques, such as X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), etc. Limitations of coking and regeneration cycles and possible theoretical improvements through mathematical calculations and simulations are discussed for further research in PDH.