Zeolite-supported metal chloride sorbent materials under sorption-enhanced ammonia synthesis reaction conditions for H2 storage: Mechanisms, micro-kinetics and modelling

Efficient ammonia separation is essential for the intensification of ammonia synthesis processes. Metal halides are promising ammonia sorbents because of strong and reversible interaction with ammonia and possibility to tune sorption strength through variation of metal cation and the support. In this work, MgCl₂-, CaCl₂-, and SrCl₂-supported ZSM-5 materials were investigated under an ammonia-containing atmosphere at absolute pressures of 1–8 bar and temperatures of 50–400 °C. Among the studied sorbents, MgCl₂/ZSM-5 showed the best performance, attributed to the higher charge density of Mg 2+ , which promotes ammonia coordination. Lower salt loadings gave slightly higher apparent coordination numbers, indicating improved accessibility of active sites. During the first formation of the hexaammine complex, MgCl₂/ZSM-5 underwent activation, suggesting structural rearrangement and accelerated ammonia uptake. A mean-field microkinetic model describing stepwise ammonia absorption and desorption was developed, explicitly accounting for the underlying sorption mechanism governed by both kinetic effects and thermodynamic equilibrium. The model was coupled with continuous-flow reactor formulations – continuous stirred-tank reactor (CSTR) or axially-dispersed plug-flow reactor (AD-PFR) – depending on the Péclet number, which characterises the gas-phase transport regime, thereby bridging meso- and reactor-scale phenomena. The model successfully reproduced breakthrough and temperature-programmed desorption experiments, including three distinct desorption peaks associated with progressively stronger ammonia binding. These results identify supported MgCl₂ as a promising sorbent for high-temperature ammonia separation, relevant for hydrogen storage and transport applications, while highlighting the need for mechanically stable supports to accommodate structural expansion during ammine formation.