Enhancing the Water Stability of Al‐MIL‐101‐NH₂ via Postsynthetic Modification

Abstract

The resistance of metal-organic frameworks towards water is a very critical issue concerning their practical use. Recently, it was shown for microporous MOFs that the water stability could be increased by introducing hydrophobic pendant groups. Here, we demonstrate a remarkable stabilisation of the mesoporous MOF Al-MIL-101-NH2 by postsynthetic modification with phenyl isocyanate. In this process 86 % of the amino groups were converted into phenylurea units. As a consequence, the long-term stability of Al-MIL-101-URPh in liquid water could be extended beyond a week. In water saturated atmospheres Al-MIL-101-URPh decomposed at least 12-times slower than the unfunctionalised analogue. To study the underlying processes both materials were characterised by Ar, N2 and H2 O sorption measurements, powder X-ray diffraction, thermogravimetric and chemical analysis as well as solid-state NMR and IR spectroscopy. Postsynthetic modification decreased the BET equivalent surface area from 3363 to 1555 m(2) g(-1) for Al-MIL-101-URPh and reduced the mean diameters of the mesopores by 0.6 nm without degrading the structure significantly and reducing thermal stability. In spite of similar water uptake capacities, the relative humidity-dependent uptake of Al-MIL-101-URPh is slowed and occurs at higher relative humidity values. In combination with (1) H-(27) Al D-HMQC NMR spectroscopy experiments this favours a shielding mechanism of the Al clusters by the pendant phenyl groups and rules out pore blocking.

Anchorage of Au3+ into Modified Isoreticular Metal–Organic Framework-3 as a Heterogeneous Catalyst for the Synthesis of Propargylamines

“…Postsynthetic modification of MOFs has recently been employed as a general approach for incorporating a wide range of functional groups into MOFs 32–37 . Topologically diverse MOFs can be customized by postsynthetic modification with different functional groups 38–42 . Wittmann et al .…”

“…Wittmann et al . 42 synthesized the remarkable stabilisation of the mesoporous MOF Al-MIL-101-NH 2 by postsynthetic modification with phenyl isocyanate. Moreover, UMCM-1-NH 2 was modified with two metal-binding groups that were metallated with Fe 3+ and Cu 2+ to yield UMCM-1-AMFesal and UMCM-1-AMCupz, respectively 33 .…”

Synthesis, Structure, and Selected Properties of Aluminum-, Gallium-, and Indium-Based Metal-Organic Frameworks

“…This was demonstrated by cyclic voltammetric measurements in organic solvents. Recently a PSM reaction was used to improve the hydrolytic stability of Al-MIL-101-NH 2 [162].…”

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

“…The reported stability28 of MIL‐101(Al)‐NH 2 in liquid water at <50 °C for 12 h was challenged by the recent finding30 that it converts to MIL‐53(Al)‐NH 2 in water at room temperature in <5 min. Post‐synthetic modification of microporous MIL‐53(Al)‐NH 2 results in moisture‐resistant MOFs 31.…”

“…Post‐synthetic modification of microporous MIL‐53(Al)‐NH 2 results in moisture‐resistant MOFs 31. The NH 2 group of mesoporous MIL‐101(Al)‐NH 2 was modified with phenylisocyanate C 6 H 5 NCO, and the obtained hydrophobic MIL‐101(Al)‐URPh (URPh=phenylurea) was stable in liquid water for 7 days 30…”