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 m² g^–1), pore volume (0.61 cm³ 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.