Structure and Carbon Dioxide Adsorption Properties of a Nanosized Aluminum l‑Aspartate Metal-Organic Framework

Herein, we report on the green synthesis of a nanosized (40 nm average size platelet crystals) aluminum-based metal-organic framework (MOF) [Al-(OH)-(l-Asp)-(HNO₃)_0.31]·1.5H₂O (l-H₂Asp = l-aspartic acid; Al-l-Asp), composed of nanometric crystals and isostructural to Al-fumarate. The synthesis was performed under mild conditions using γ-valerolactone (GVL), a solvent derived from biomass valorization. The MOF structure, modeled through DFT calculations, is constituted of 1D-infinite Al-hydroxo chains connected by the carboxylic groups of l-aspartic acid with the same framework topology as that of Al-MIL-53. Al-l-Asp displays thermal stability up to 240 °C, and no breathing effects are observed upon thermal activation. Solid-state NMR was used to investigate the activation process and chemical details of the pore structure and activation dynamics. The activated MOF has been exploited in carbon dioxide adsorption. The BET surface area is about 600 m²·g^-1, and the MOF CO₂ capture capacity reaches 3.2 mmol·g^-1 at T = 0 °C and p _CO2 = 1 bar. Remarkably, compared to the Al-fumarate analogue, the CO₂ heat of adsorption is increased from about 20 kJ·mol^-1 to a value of 30 kJ·mol^-1, proving the beneficial effect of the presence of an amino group on the linker skeleton on the CO₂-MOF interaction. CO₂/N₂ IAST selectivity is relatively high, ranging from 60 to more than 80 at increasing pressure range (from 0.1 to 1 bar). These values overcome those measured for the analogue fumarate MOF, and they are comparable to the benchmark compounds for CO₂ in postcombustion adsorption applications.