Water Cluster Confinement and Methane Adsorption in the Hydrophobic Cavities of a Fluorinated Metal–Organic Framework

Abstract

Water cluster formation and methane adsorption within a hydrophobic porous metal organic framework is studied by in situ vibrational spectroscopy, adsorption isotherms, and first-principle DFT calculations (using vdW-DF). Specifically, the formation and stability of H2O clusters in the hydrophobic cavities of a fluorinated metal-organic framework (FMOF-1) is examined. Although the isotherms of water show no measurable uptake (see Yang et al. J. Am. Chem. Soc. 2011 , 133 , 18094 ), the large dipole of the water internal modes makes it possible to detect low water concentrations using IR spectroscopy in pores in the vicinity of the surface of the solid framework. The results indicate that, even in the low pressure regime (100 mTorr to 3 Torr), water molecules preferentially occupy the large cavities, in which hydrogen bonding and wall hydrophobicity foster water cluster formation. We identify the formation of pentameric water clusters at pressures lower than 3 Torr and larger clusters beyond that pressure. The binding energy of the water species to the walls is negligible, as suggested by DFT computational findings and corroborated by IR absorption data. Consequently, intermolecular hydrogen bonding dominates, enhancing water cluster stability as the size of the cluster increases. The formation of water clusters with negligible perturbation from the host may allow a quantitative comparison with experimental environmental studies on larger clusters that are in low concentrations in the atmosphere. The stability of the water clusters was studied as a function of pressure reduction and in the presence of methane gas. Methane adsorption isotherms for activated FMOF-1 attained volumetric adsorption capacities ranging from 67 V(STP)/V at 288 K and 31 bar to 133 V(STP)/V at 173 K and 5 bar, with an isosteric heat of adsorption of ca. 14 kJ/mol in the high temperature range (288-318 K). Overall, the experimental and computational data suggest high preferential uptake for methane gas relative to water vapor within FMOF-1 pores with ease of desorption and high framework stability under operative temperature and moisture conditions.

Thermally robust and porous noncovalent organic framework with high affinity for fluorocarbons and CFCs

“…Hydrophobic behaviour was also confirmed by contact angle measurements with H 2 O, which revealed a contact angle of 132±1°. While this hydrophobicity has little precedent among the porous molecular crystals, fluorinated organic ligands were employed by Omary et al [36][37][38] in the construction of very hydrophobic fluorinated MOFs, which were in turn used for hydrocarbon capture. We have subsequently expanded on this work by developing routes to larger perfluorinated ligands 39 , while Nguyen and Cohen 40 used postsynthetic functionalization of amine-bearing MOFs with long-chain acyl chlorides to create superhydrophobic MOFs.…”

Fluorine-functionalized metal–organic frameworks and porous coordination polymers

“…A hydrophobic space derived from fluorine atoms was also suitable for investigating properties of water clusters themselves due to the negligible interaction of water molecules with the pore walls. Omary and colleagues 16 reported water cluster confinement in the hydrophobic cavities using the same fluorinated MOF, [Ag 6 (tz) 6 ]. From Raman and IR spectroscopy and theoretical calculations, it was suggested that a small number of pentamer water clusters were formed in the large pores and the binding energy between the water clusters and the CF 3 -decorated walls was weak.…”

Structural, elastic, thermal, and electronic responses of small-molecule-loaded metal–organic framework materials

“…31,32 We have already successfully applied vdW-DF to investigate the adsorption of small molecules in MOFs and nano-structures in numerous other studies. 7,8,[33][34][35][36][37][38][39][40] Projector augmentedwave theory, 41,42 combined with a well-converged planewave cutoff of 600 eV, were used to describe the wave functions. The total energy was sampled on a 2×2×2 k -point grid, resulting in four irreducible k -points, necessary to fully converge the stress tensor.…”