Exceptional Hydrophobicity of a Large-Pore Metal–Organic Zeolite

Abstract

Porous materials combining high hydrophobicity, large surface area, as well as large and uniform pore size are very useful but rare. The nanoporous zeolitic metal azolate framework, RHO-[Zn(eim)2] (MAF-6, Heim = 2-ethylimidazole), is an attractive candidate but thought to be unobtainable/unstable. In this work, the supramolecular isomerism of [Zn(eim)2] is thoroughly studied using a rapid solution mixing reaction of [Zn(NH3)4](OH)2 and Heim, which enables MAF-6 with high crystallinity, purity, and thermal/chemical stabilities to be synthesized in large quantity. Gas and vapor adsorption isotherms, gas chromatography, and water contact angle measurements, as well as transient breakthrough and molecular dynamics simulations show that MAF-6 exhibits large surface area (langmuir surface area 1695 m(2) g(-1)), pore volume (0.61 cm(3) g(-1)), pore size (d = 18.4 Å), and aperture size (d = 7.6 Å) with high hydrophobicity on both the internal pore and external crystal surfaces. It can barely adsorb water or be wetted by water (contact angle 143°) but readily adsorb large amounts of organic molecules including methanol, ethanol, mesitylene, adamantane, C6-C10 hydrocarbons, xylene isomers, and saturated/unsaturated analogues such as benzene/cyclohexene/cyclohexane or styrene/ethylbenzene. It can also separate these organic molecules from each other as well as from water by preferential adsorption/retention of those having higher hydrophobicity, lipophilicity, or oil/water partition coefficient. These properties are very different with other porous materials such as SOD-[Zn(mim)2] (Hmim = 2-methylimidazole, MAF-4/ZIF-8) with a hydrophobic pore surface but a hydrophilic crystal surface and small aperture size.

A Macroporous Metal–Organic Framework with Enhanced Hydrophobicity for Efficient Oil Adsorption

“…Among them, a survey of MOF databases suggests that zeolitic imidazolate frameworks with high stability (ZIFs, a subfamily of MOFs) are excellent candidates for such purpose and they have been used for alcohol/water separation . However, taking ZIF‐8 as an example, the micropore surface is hydrophobic but the crystal surface is hydrophilic because of the existence of the defects, as well as hydrophilic groups on the crystal surface . Although ZIFs with superhydrophobic crystal surface can be obtained through postsynthetic modifications, these approaches suffer from their respective limitations, such as reduced porosity or tedious synthetic procedure .…”

Hydrophobic Metal–Organic Frameworks: Assessment, Construction, and Diverse Applications

“…Hydrophobic microporous and mesoporous adsorbents commonly show type V isotherms, where the uptakes are low at the low‐relative‐pressure ( P / P 0 ) range, indicating the presence of relatively weak adsorbent–adsorbate interactions, and molecular clustering is followed by pore filling at the high‐ P / P 0 range. For the water adsorption isotherms of some highly hydrophobic MOFs, such as [Zn(2‐mim) 2 ] (ZIF‐8), Zn(2‐eim) 2 (MAF‐6), Zn(4,5‐dcim) 2 (ZIF‐71), and {Ag 2 [Ag 4 (3,5‐tftz) 6 ]} (FMOF‐1), the uptakes are very low, even at the pressures near the saturation pressure ( P 0 ), as with the type VII adsorption isotherm shown in Figure 2b . As the hydrophilicity of MOFs increases, their water adsorption isotherms gradually transform into type I isotherms (Figure 2b).…”

“…These results all demonstrated the high hydrophobicity of the internal pore surface of ZIF‐8, although, there were some differences among the water vapor adsorption isotherms, which should originate from the differences in those ZIF‐8 samples obtained by distinct synthesis and/or activation methods. However, it should be mentioned that most ZIF‐8 samples reported in the literature have hydrophilic exterior crystal surfaces, as indicated by their water contact angles (0°–56°) …”

“…He et al reported a zinc 2‐ethylimidazolate with the RHO zeolitic topology, MAF‐6, showing a large surface area (Langmuir surface area: 1695 m 2 g −1 ), high pore volume (0.61 cm 3 g −1 ), large pore ( d = 18.4 Å), and aperture size ( d = 7.6 Å), and highly hydrophobic internal pore and external crystal surfaces . The water uptake of MAF‐6 at P / P 0 = 0.97 and 298 K was only 0.90 mmol g −1 (16.2 mg g −1 ).…”

Adsorptive removal of herbicides from water over nitrogen-doped carbon obtained from ionic liquid@ZIF-8

“…ZIF-8, which is prepared from 2-methylimidazole, is one of the highly porous and hydrophobic materials [58,59] among the ZIFs reported, and it is widely used for the adsorptive removal of various organics from water such as benzotriazole and p-arsanilic acid [60,61]. Using high-temperature pyrolysis, ZIF-8 has been converted to highly porous MDCs with acidic and basic functionalities as well as high surface areas [42].…”