Structured Aluminum Phosphonate Metal–Organic Framework for Efficient Hydrogen Purification: Reversible HCl Adsorption and Water-Triggered Regeneration

Abstract Efficient removal of acidic impurities such as hydrogen chloride (HCl) is essential for hydrogen purification, yet most conventional adsorbents are nonregenerable and require frequent replacement, underscoring the need for scalable, regenerable alternatives. This study evaluates the aluminum-based metal–organic framework (MOF) Al-CAU-60 as a regenerable HCl adsorbent. A reproducible, scalable reflux synthesis was developed for phosphonate-based Al-CAU-60·6HCl, enabling 10 L scale production with high yield (151 g, >96%) and preserved crystallinity. Following neutralization, the MOF was shaped into mechanically robust pellets (Al-CAU-60/PVF) using 10 wt % polyvinyl formal (PVF) as a binder. Structural and chemical integrity were confirmed through scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM-EDX), powder X-ray diffraction (PXRD), attenuated total reflectance–Fourier transform infrared (ATR-FTIR), and thermogravimetric (TGA) analyses. Under dynamic breakthrough tests (10 bar, 25 °C, 100 ppm of HCl), Al-CAU-60/PVF showed an average HCl uptake of 1.38 mmol/g at the 1 ppm breakthrough and 1.73 mmol/g at saturation. The material retained its performance over seven adsorption–regeneration cycles using only water-triggered desorption, demonstrating an exceptional example of fully regenerable, phosphonate-based MOF sorbents. In contrast, zeolite 13X, while initially more active, lost ∼90% capacity after the first cycle, confirming the superior cyclic stability of Al-CAU-60/PVF. Mechanistic analysis revealed that HCl adsorption proceeds via reversible protonation–deprotonation of phosphonate groups during water-based regeneration.