Comparison of Conventional and HF-Free-Synthesized MIL-101 for CO₂ Adsorption Separation and Their Water Stabilities

Abstract

The preparation of MIL-101 was simplified to avoid the use of hazardous hydrofluoric acid (HF). The potential application of MIL-101, prepared by both this new method and the conventional approach, for CO2 isolation from multicomponent mixtures such as flue gas is evaluated with regard to its relative adsorption capacity for different flue gas components and retention of the CO2 capacity in the presence of H2O, because flue gas contains gases other than CO2 and moisture. CO2, CH4, and N2 isotherms were determined for temperatures in the range 25–105 °C and pressures up to 15 bar. CO2 adsorption capacities of 15.4 and 16.8 mol/kg at 25 °C and 15 bar were determined for conventional (1) and HF-free-synthesized MIL-101 (2), respectively. The shape of the CO2 adsorption isotherm of 1 is slightly different from that of 2. Material 1 showed higher CO2 adsorption at low CO2 pressure at 25 °C than 2 but lower CO2 adsorption at CO2 pressures above 8 bar and almost identical adsorption for H2O. Both materials adsorb more CH4 than N2, and the heat of CO2 adsorption is low. The selectivity of material 2 for CO2 relative to N2 is higher than that of 1, but for CO2 relative to CH4, it is similar to that of 1. The structures of the two materials remain intact after exposure to H2O vapor at 100 °C and 50 mbar H2O vapor pressure.

Mixed-Solvothermal Synthesis of MIL-101(Cr) and Its Water Adsorption/Desorption Performance

“…When the DMF/H 2 O ratio equaled 0.14–0.5, MIL-101­(Cr) crystals were synthesized. The diffraction peaks matched exactly with those of MIL-101­(Cr)-T synthesized by traditional hydrothermal method . With the increase of DMF/H 2 O ratio, the crystallinity of MIL-101­(Cr) crystals increased first and then decreased, and the strongest diffraction peaks appeared at the ratio of 0.2.…”

Molecular Level Understanding of the Free Energy Landscape in Early Stages of Metal–Organic Framework Nucleation

“…Fluoride anions have an affinity to the chromium at the metal center of MOF building units and their interaction assists the stability of the crystal structure 22 . Nevertheless, recent studies [23][24][25][26] have used alternative synthetic methods by replacing fluoride with hydroxide anions using deionized water [23][24][25] . In this work, the self-assembly of MIL-101 (Cr) SBU from its constituent half-SBUs is studied as a prototypical example to highlight the most important stages of nucleation.…”

“…More precisely, metal precursors are introduced in the form of metal nitrate hydrates (e.g., Cr­(NO 3 ) 3 ·9H 2 O in the case of chromium) and fluoride has been used by experimentalists in the synthesis of MIL-101 (Cr), as it promotes rapid crystal growth of the material. , Fluoride anions have an affinity to the chromium atoms in the metal center of MOF building units, and their interaction assists the stability of the crystal structure . Nevertheless, recent studies − have used alternative synthetic methods by replacing fluoride with hydroxide anions using deionized water. − …”

Adsorption on Mesoporous Metal–Organic Frameworks in Solution: Aromatic and Heterocyclic Compounds

“…The MIL‐101(Cr)(F) and an “HF‐free” MIL‐101(Cr) retain their structure under 50 mbar water vapor and at 100 °C due to the hydrophobicity of the pore surface 24. Adsorption of water on MIL‐101(Cr) and MIL‐101(Cr)(F) was compared by the density functional theory (DFT)‐simulated adsorption isotherms;19 the difference for the small water loadings was due to the different partial charge on Cr III .…”