(511h) Silicalite-1 Supported Metal Catalyst for Non-Oxidative Propane Dehydrogenation in a Carbon Membrane Reactor

Non-oxidative propane dehydrogenation (PDH) is a critical process enabling the on-purpose production of propylene and H₂. However, it poses challenges stemming from its thermodynamic limitations, high endothermicity, and the rapid catalyst deactivation at elevated temperatures. In commercial PDH processes, catalysts undergo continuous reaction-regeneration cycles, diminishing process efficiency. Addressing these challenges, a sub-nanometer bimetallic cluster encapsulated within a pure siliceous zeolite (Pt-Zn/S1) has been developed for PDH reactions, exhibiting remarkable stability of up to 110 hours without deactivation. High-angle annular dark field scanning transmission electron microscopy reveals the presence of small metal clusters effective for alkane activation, suppressing side reactions involving hydrogenolysis (C-C cleavage) and/or coke formation (C-C coupling) on large ensembles of metal atoms. The confinement of zeolite micropores, as evidenced by X-ray photoelectron spectroscopy, prevents metal site sintering and consequent deactivation. Proposed Langmuir–Hinshelwood kinetic model for PDH on Pt-Zn/S1 is based on reaction mechanisms considering a two-step dehydrogenation process evaluated through experimental fitting. The model suggests that propane is initially adsorbed on the Pt surface, undergoing C-H activation, followed by a surface reaction involving a second C–H activation, and subsequent desorption of propylene and hydrogen. Experiments conducted at varying temperatures and partial pressures of reactant and product gases yield forward and backward reaction rate constants, as well as desorption equilibrium constants. The obtained rate constants are utilized to calculate activation energies, adsorption energies, and pre-exponential factors, forming the basis for a broadly applicable rate equation for PDH under diverse reaction conditions, facilitating process simulation. Finally, coupling the catalyst with a carbon molecular sieve membrane enables the development of a low-temperature PDH membrane reactor (MR), significantly enhancing C₃H₈ conversion (nearly three times the equilibrium conversion) and C₃H₆ selectivity at reaction temperatures at least 150°C lower than commercial PDH reactors, with outstanding stability surpassing any known MR reported in PDH literature.