Hydrophobic and moisture-stable metal–organic frameworks

Abstract

Metal-organic frameworks (MOFs) have proved to be very attractive for applications including gas storage, separation, sensing and catalysis. In particular, CO(2) separation from flue gas in post-combustion processes is one of the main focuses of research among the scientific community. One of the major issues that are preventing the successful commercialization of these novel materials is their high affinity towards water that not only compromises gas sorption capacity but also the chemical stability. In this paper, we demonstrate a novel post-synthesis modification approach to modify MOFs towards increasing hydrophobic behaviour and chemical stability against moisture without compromising CO(2) sorption capacity. Our approach consists of incorporating hydrophobic moieties on the external surface of the MOFs via physical adsorption. The rationale behind this concept is to increase the surface hydrophobicity in the porous materials without the need of introducing bulky functionalities inside the pore which compromises the sorption capacity toward other gases. We herein report preliminary results on routinely studied MOF materials [MIL-101(Cr) and NiDOBDC] demonstrating that the polymer-modified MOFs retain CO(2) sorption capacity while reducing the water adsorption up to three times, with respect to the un-modified materials, via an equilibrium effect. Furthermore, the water stability of the polymer-functionalized MOFs is significantly higher than the water stability of the bare material. Molecular dynamic simulations demonstrated that this equilibrium effect implies a fundamental and permanent change in the water sorption capacity of MOFs. This approach can also be employed to render moisture stability and selectivity to MOFs that find applications in gas separations, catalysis and sensing where water plays a critical role in compromising MOF performance and recyclability.

Enhanced Water Stability in Zn‐Doped Zeolitic Imidazolate Framework‐67 (ZIF‐67) for CO2 Capture Applications

“…In spite of the numerous potential applications, there has been a limited success in reality, including for CO 2 capture in the post‐combustion process. One of the major drawbacks of MOFs is they are sensitive to moisture/water that causes structure instability and breakdown . A large number of MOFs, especially those constructed by Zn 4 O cluster nodes, quickly decompose upon contact with humid air .…”

Trapping gases in metal-organic frameworks with a selective surface molecular barrier layer

“…While previous work has considered high molecular weight compounds such as polydimethysiloxane10 or copolymer Pluronic P-123 (ref. 11) to coat MOFs external surfaces by high temperature vapour-phase deposition or liquid immersion, these methods have not been used for and are not compatible with gas storage and release. Ethylenediamine (EDA) molecules, on the other hand, have low-molecular weight, relatively high vapour pressure (∼10 Torr, 20 °C) at ambient condition and contain terminal amine groups in EDA molecules, which are known to interact more strongly with a variety of MOFs, particularly those with open or unsaturated metal sites (for example, found in MOF-74) by forming metal-amine complexes121314.…”

Probing the Water Stability Limits and Degradation Pathways of Metal–Organic Frameworks

“…The general concept is mainly directed towards rendering the MOF surface hydrophobic, in an attempt to enhance the resistance to moisture/water. This can be achieved through coating of the MOF, postsynthetically, with hydrophobic polymers . Alternatively, the incorporation of hydrophobic graphite oxide during MOF synthesis has been shown to enhance the hydrothermal stability and catalytic activity of HKUST‐1 .…”