Structural deactivation mechanisms of zeolite-supported magnesium chloride with water: Ammonia sorption stability for intensified scalable hydrogen storage

MgCl 2 is a highly effective ammonia sorbent due to its strong and reversible interaction with ammonia, making it attractive for storage and separation applications. However, its strong tendency to absorb moisture presents a major practical challenge, and the effects of water exposure on supported MgCl 2 remain poorly understood. This work investigates MgCl 2 supported on ultra-stable Y zeolite, quantitatively linking material exposure to relative humidity with ammonia sorption capacity. FTIR and XPS analyses show that water exposure decreases ammonia uptake capacity by forming oxygenated Mg species such as MgO and MgOHCl, which replace active MgCl 2 sites. Pre-ammoniation does not prevent water uptake, as NH 3 displacement by H 2 O is energetically favourable. DFT calculations also reveal that ligand exchange is thermodynamically more favourable than water intercalation, providing new mechanistic insight into material degradation. Furthermore, temperature-programmed desorption results show that simultaneous exposure to NH 3 and H 2 O alters the coordination environment of the Mg 2+ centre, reducing the stability of ammonia complexes. Cycling experiments, along with XRD, demonstrate that while thermal treatment alone does not alter the MgCl 2 structure, repeated ammonia absorption and desorption induce irreversible changes. These include the formation of defect-rich crystalline domains, which reduce sorption capacity over time. High-temperature dynamic sorption experiments show that although sorption capacity decreases at higher temperatures, MgCl 2 /USY retains a measurable capacity of up to 65 mg/g sorbent at 300 °C and 8 bar. This indicates that the sorbent may still operate under integrated ammonia synthesis-separation conditions. This study provides the first quantitative, mechanistic understanding of moisture- and dry NH 3 cycling-induced degradation of supported MgCl 2 sorbents, defining operational conditions for their integration into ammonia synthesis-separation process.