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.

通过发酵制备的短链醇，因其可用作燃料添加剂、燃料或用于转化为烯烃，将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，需进行分离操作——例如可借助纳米多孔材料(nanoporous materials)中的液相吸附(liquid phase adsorption)技术实现。然而，偏好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，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(adsorption isotherms)，以及其混合物的多组分液相吸附等温线，以此验证前述假设。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——二者均会促进水的吸附，进而使得通过吸附分离发酵产物的过程复杂化；而CAU-10的内疏水性对此过程的影响相对有限。