Moisture-Resistant and Superhydrophobic Metal−Organic Frameworks Obtained via Postsynthetic Modification

Abstract

Postsynthetic covalent modification of metal-organic frameworks (MOFs) with long alkyl substituents is shown to protect these materials against moisture. These MOFs, which normally display hydrophilic properties, are readily converted into hydrophobic or superhydrophobic materials. Overcoming the inherent sensitivity of MOFs to water represents a major step forward in their widespread use in technology applications.Metal-organic frameworks (MOFs) are porous, crystalline materials that are attractive for applications in gas adsorption, 1 separations, 2 and catalysis 3,4 because of their chemical tunability, high porosities, and good thermal stability. 5 Unfortunately, some of the most promising materials are limited because of an instability with respect to moisture. [6][7][8] Stabilizing these MOFs against ambient humidity would make these porous materials more suitable for specialized and industrial applications. The postsynthetic modification (PSM) approach, which has become an important tool in developing functionally diverse systems and introducing new physical and chemical properties into MOFs, 9 offers an opportunity to address this challenge.Amine-containing MOFs can readily undergo PSM to form amide-functionalized MOFs. 10,11 It was hypothesized that the introduction of hydrophobic alkyl chains via PSM could improve the moisture resistance and change the physical properties (i.e. hydrophobicity) of these MOFs. The effect of PSM on two different MOF systems was examined (Figure 1). Isoreticular metal-organic frameworks (IRMOFs) are cubic frameworks comprised of Zn 4 O clusters and dicarboxylate ligands. In this study, IRMOFs constructed of 1,4-benzenedicarboxylate (BDC, IRMOF-1), 2-amino-1,4-benzenedicarboxylate (NH 2 -BDC, IRMOF-3), and PSM derivatives of the latter were investigated. The second MOF systems examined belong to the Material Institut Lavoisier (MIL) class of materials, 12 specifically, MIL-53(Al)-NH 2 a flexible MOF comprised of infinite Al 3+ clusters and NH 2 -BDC. The IRMOFs are generally considered sensitive to moisture, while the MILs are stable to water and other polar solvents (indeed MIL-53(Al)-NH 2 is synthesized in water). In order to assess changes in the moisture stability and hydrophobicity/philicity of the materials upon PSM, each material was exposed to ambient air or immersed in water and then characterized using scohen@ucsd.edu. Supporting Information Available: Experimental details, Table S1, Figures S1-S17. NIH Public AccessAuthor Manuscript J Am Chem Soc. Author manuscript; available in PMC 2011 April 7. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript contact angle measurements, powder X-ray diffraction (PXRD), and scanning electron microscopy (SEM).Contact angle measurements are commonly used to examine the hydrophobic/philic properties of materials. Hydrophilic materials display contact angles less than 90°. Contact angles between 90° and 150° are found for hydrophobic materials, and materials with contact angles >150° are considered ...

Coating sponge with a hydrophobic porous coordination polymer containing a low-energy CF3-decorated surface for continuous pumping recovery of an oil spill from water

“…By comparison, different organic hydrocarbons can be rapidly adsorbed by USTC-6, suggesting its strong oleophilic properties. To date, some PCPs have been reported to be hydrophobic upon a postsynthetic modification with alkyl or fluoroalkyl groups; 41,42,46,47 however, very few pristine PCPs have been reported to display such a high hydrophobicity. 33,43,45 To our knowledge, USTC-6 is a very rare highly hydrophobic PCP bearing a low-energy rough surface decorated with -CF 3 groups.…”

“…Of the various strategies that have evolved to produce water-stable hydrophobic PCPs, the incorporation of fluorine into PCPs is particularly effective. [41][42][43][44][45][46][47] Therefore, we are interested in developing fluorous PCPs for oil spill cleanup and recovery as corrugated PCP surfaces with -CF 3 alignment may render it very hydrophobic. 33 Figures S2 and S4-S6).…”

Design and Construction of Chemically Stable Metal-Organic Frameworks

“…In 2010, Nguyen et al were able to increase the hydrophobic character of isoreticular metal-organic framework (IRMOF)-3 and MIL-53(Al)-NH 2 by conducting postsynthetic modifications on the amine groups to add alkyl chains into the frameworks (Figure 16). 35 In these experiments, IRMOF-1 is used as the control group. The untreated IRMOF-1 and -3, as well as IRMOF-3-AM1, -AM2, and -AM3 (-AMn = amide with an n carbon chain) exhibit water contact angles close to 0°.…”

“…For instance, the required conversion ratios are 25% and 50% for IRMOF-3-AM15 and IRMOF-3-AM4, respectively. 35 Water stability of these IRMOFs is determined by placing the crystals in water for approximately 30 min, after which the water is quickly removed and the samples are air-dried for 1 h before scanning electron microscopy (SEM) images of the crystals are obtained. IRMOF-1 is unstable even in ambient air and the collapse of the MOF could be seen on the SEM image.…”

“…These examples demonstrate the beneficial role of alkyl groups that are added postsynthetically for the preparation of water stable MOFs. 35 Apart from alkyl groups, fluoroalkane residues can also serve as hydrophobic fragments to increase the water stability of frameworks. For example, a highly fluorinated silver azolate framework, denoted fluorinated metal-organic framework (FMOF)-1, was reported by Yang et al in 2011 (Figure 17).…”