How Important Is the Internal Hydrophobicity of Metal–Organic Frameworks for the Separation of Water/Alcohol Mixtures?

Short-chain alcohols obtained by fermentation will play a key role in the industrial transformation toward green chemistry because of their use as fuel additives and fuels or for their conversion into olefins. The fermentation broth is often a highly diluted aqueous solution that requires separation, for instance, by liquid phase adsorption in nanoporous materials. However, entropy effects that prefer the adsorption of water might significantly reduce the separation efficiencyeven in nanoporous materials with internal hydrophobicity. In this paper, we investigate this assumption by a case study on the separation of aqueous alcohol mixtures by liquid phase adsorption in CAU-10an ultramicroporous metal–organic framework with internal hydrophobicityusing adsorption experiments and grand canonical Monte Carlo simulations to predict both the unary gas adsorption isotherms of ethanol, n-butanol, or water as well as the multicomponent liquid phase adsorption isotherms of their mixtures. It was observed that separation from the liquid phase is commonly driven by entropy effects and strong interactions between the guest moleculesboth favoring the adsorption of water and thus complicating the separation of fermentation product by adsorptionwhile the internal hydrophobicity of CAU-10 is of comparatively little importance.经发酵制备的短链醇可作为燃料添加剂、燃料使用，或用于转化为烯烃，因此在面向绿色化学的工业转型中发挥关键作用。发酵液通常为高度稀释的水溶液，需进行分离处理，例如借助纳米多孔材料中的液相吸附法实现分离。然而，优先吸附水的熵效应可能会显著降低分离效率——即便在具备内疏水特性的纳米多孔材料中亦是如此。本文以CAU-10——一种具备内疏水特性的超微孔金属有机框架（metal–organic framework）——中的液相吸附分离含水醇混合体系为案例研究对象，通过吸附实验与巨正则蒙特卡洛（grand canonical Monte Carlo）模拟，分别预测乙醇、正丁醇或水的单组分气体吸附等温线（unary gas adsorption isotherms）以及其混合物的多组分液相吸附等温线（multicomponent liquid phase adsorption isotherms），以此验证上述假设。研究发现，液相分离过程通常由熵效应与客体分子间的强相互作用主导——这两类因素均会促进水的吸附，从而使通过吸附法分离发酵产物的过程变得复杂；而CAU-10的内疏水特性对此的影响相对较小。