Facile synthesis of MIL-101(Cr) for enhanced gas sorption at room temperature

• Elucidated the role of modulator in MIL-101(Cr)'s gas adsorption mechanism. • Modulator enhances the crystallinity and facilitates more open metal sites. • Nitric acid is identified as a potential modulator. • Nitric acid as a modulator permits higher gas uptake. Among the several metal organic frameworks (MOFs) investigated for gas sorption applications, MIL-101(Cr) stands as a potential material due to its superior pore characteristics. The commonly used modulator for its synthesis was hydrofluoric acid (HF), a highly corrosive and toxic reagent. Of the several modulators explored for replacing the HF, acetic acid and nitric acid are considered advantageous, as they offer high yield and better pore characteristics. This study aims to understand the role of a modulator on (i) structural properties and (ii) gas adsorption of MIL-101(Cr). Towards this, acetic acid and nitric acid were employed as modulators. The synthesized samples were characterized using various tools and examined for their adsorption performance of different gases (CO 2 , CH 4 , H 2 , and water vapour) at 25°C temperature. Incorporating a modulator during the synthesis of MIL-101(Cr) helped achieve higher crystallinity and more open metal sites (OMS) resulting in enhanced gas sorption. Under the tested conditions, the nitric acid-based sample exhibited CO 2 , CH 4 , H 2 , and water vapour adsorption capacities of 439.86, 183.08, 43.56, and 1384.96 cm 3 g -1 , respectively. This is attributed to its higher crystallinity, higher chromium content, and smaller pore diameter. Overall, this study elucidates the role of the modulator and correlates effectively the underlying chemistry on the sorption capacities of various gases.