Adsorption of Nitrous Oxide in Ultramicroporous Magnesium Gallate (Poster)

Nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting substance, yet remains understudied in terms of materials-based capture.1 Here, we show that magnesium gallate (MgGal), a sustainable ultramicroporous metal-organic framework (MOF), effectively captures N2O through strong host–guest interactions and a unique phase transition. Gas sorption isotherms revealed a characteristic inflection point, whose position shifted to lower pressures with increasing temperature, indicating temperature-dependent sorption behavior. A significant uptake was observed well below atmospheric pressure, reaching pore saturation, and the isotherms displayed pronounced hysteresis during desorption, reflecting strong host-guest interactions and indicating potential for gas capture applications. To uncover the sorption mechanism, we performed in situ synchrotron X-ray powder diffraction under isobaric conditions. These measurements enabled real-time tracking of structural changes in MgGal and identification of the N2O adsorption sites. Sequential Rietveld refinements during activation revealed a transformation from a dihydrated structure (phase I) to an anhydrous activated form (phase II), accompanied by a symmetry change from space group P31 to a P3121 superstructure, without loss of framework connectivity. This activation process was fully reversible and could be triggered by vacuum treatment. However, when performed in air, phase II was observed but proved to be metastable and gradually decomposed via decarboxylation of the organic linker.2 Upon N2O loading, the activated phase II initially adsorbed a small amount of gas before undergoing a sharp transformation back to the phase I. In this final structure, the N2O molecules occupy different positions within the pores than in phase II. Volumetric and diffraction-based uptake measurements were in excellent agreement, indicating a total uptake of one N2O molecule per linker. Sorption isobars extracted by Rietveld refinement further revealed a large adsorption–desorption hysteresis (~37°C), highlighting the kinetic stability of the loaded state. The strong affinity for N2O was attributed to hydrogen bonding between the gas molecules and hydroxyl groups of the gallate linker, as previously observed for CO2 by ex situ measurements.3 Our results suggest that MOFs with proven CO2 sorption properties may offer a promising starting point for selective N2O capture, with implications for environmental remediation and gas separation technologies.