Explanation of the Adsorption of Polar Vapors in the Highly Flexible Metal Organic Framework MIL-53(Cr)

Abstract

A comparison of the adsorption of water, methanol, and ethanol polar vapors by the flexible porous chromium(III) terephthalate MIL-53(Cr) was investigated by complementary techniques including adsorption gravimetry, ex situ X-ray powder diffraction, microcalorimetry, thermal analysis, IR spectroscopy, and molecular modeling. The breathing steps observed during adsorption strongly depend on the nature of the vapor. With water, a significant contraction of the framework is observed. For the alcohols, the initial contraction is followed by an expansion of the framework. A combination of IR analysis, X-ray diffraction, and computer modeling leads to the molecular localization of the guest molecules and to the identification of the specific guest-guest and host-guest interactions. The enthalpies of adsorption, measured by microcalorimetry, show that the strength of the interactions decreases from ethanol to water. Differential scanning calorimetry experiments on an EtOH/H(2)O mixture suggest a selective adsorption of ethanol over water.

A synthetic route to ultralight hierarchically micro/mesoporous Al(III)-carboxylate metal-organic aerogels

“…Generally, all sorptions onto aerogels are reversible, and the isotherms are close to type IV but show steep initial uptakes and significant hysteresis loops in a wide P / P 0 range after the micropore filling. Such stepwise adsorption and the remarkable hysteresis loop obviously can be attributed to the unique features of the hierarchical MOAs, which combined the mesoporous nature with the flexible nature that is characteristic of microporous MIL-53(Al) (4552). With respect to the adsorption capabilities, the aerogels adsorb benzene up to 247 cm 3  g −1 (86 wt%), n -hexane up to 122 cm 3  g −1 (47 wt%), and methanol up to 672 cm 3  g −1 (96 wt%).…”

Water Stability and Adsorption in Metal–Organic Frameworks

“…In subsequent work, Bourrelly et al applied a variety of approaches including gravimetric adsorption, PXRD, microcalorimetry, and IR spectroscopy to further understand the MIL-53(Cr) breathing behavior. 114 The water adsorption and desorption isotherms of the material along with PXRD patterns at different water vapor pressures at 298 K are shown in Figure 16. As mentioned in section 1.2, the adsorption and desorption isotherms of flexible materials cannot be fully described by the existing IUPAC classifications.…”

Investigating adsorption of organic compounds in metal-organic framework MIL-53

“…DFT calculations have previously been used to analyze the arrangement of methanol and ethanol molecules within the pores of MIL-53, indicating that the longer carbon chains within ethanol molecules allow stronger interactions with the framework as a result of van der Waals interactions. 59 It follows that the differences in desorption temperatures between n-butanol and isopropanol are due to the relatively long four-carbon chain of n-butanol, which can interact with the hydrophobic rings of the organic linker through van der Waals interactions, resulting in stronger adsorption and a higher desorption temperature.…”