Mechanism of Alcohol–Water Separation in Metal–Organic Frameworks

Abstract

The metal–organic framework Zn2(BDC)2(TED) (1) has been reported to be water-stable and highly selective toward the adsorption of water and alcohols, suggesting the application of this material as a separation membrane for the production of bioethanol. We have studied the adsorption mechanism of water, methanol, ethanol, and dimethylether in this framework by means of density-functional theory with corrections for London dispersion. We show that the combination of the hydrogen bond between the hydroxyl group in ethanol with the oxy group in 1 and the van der Waals interaction between the ethanol alkyl chain with the phenyl ring in 1 is responsible for the preferential adsorption of ethanol over water in the framework. The calculated enthalpy of adsorption for the four compounds in 1 is in excellent agreement with experimental results. We further note that the computational approach has to be chosen with care: It is essential to account for London dispersion interactions, as well as the use of large models, preferably the full periodic structure, to obtain correct adsorption geometries and energies.

The role of C–H⋯π interactions in modulating the breathing amplitude of a 2D square lattice net: alcohol sorption studies

“…19 In similar alcohol systems reported previously, it has been suggested that the alcohol molecules interact through their hydroxyl groups with the oxygen atoms of the framework while for longer alcohol chains the alkyl groups are expected to bend towards the phenyl rings to enhance hydrophobic van der Waals attractions. 41 In our study, we found that the hydroxyl moiety on n-propanol does not interact with the host network, but the propyl chain does indeed "hug" the inside of the network cavity, making a number of weak dispersion contacts with the aromatic rings of the host network (Fig. S2 †).…”

“…Absorption of alcohols by the framework may be attributed to the presence of the phenyl rings in the framework, providing a hydrophobic effect which attracts the alkyl chains of the alcohols. 41 Absorption of water leads to poisoning of the framework as evidenced by the formation of a new phase (Fig. 5).…”

A Chemical Route to Activation of Open Metal Sites in the Copper-Based Metal–Organic Framework Materials HKUST-1 and Cu-MOF-2

“…However, in contrast to HKUST-1, the architecture of MOF-2 is based on stacking lamellar phase sheets of Cu 2 (BDC) 2 in two dimensions. The "extracoordination" (solvent coordination) ability of the OCSs in both HKUST-1 and MOF-2 enables them to be excellent adsorbents or molecule separators for applications such as carbon dioxide sorption, 11,[28][29] water sorption, 30 amine sorption, 31 nitric oxide capture, 32 water/ethanol separation, 33 and others. 9,[19][20][22][23][24][34][35][36] To utilize that type of MOF for the aforementioned applications, an activation process to remove both pre-coordinating solvent molecules from the OCS and pore-filling guest molecules (typically the solvents used during the synthesis process) from the pores is a prerequisite step.…”

Hybrid and Mixed Matrix Membranes for Separations from Fermentations

“…For example, De Lima et al . investigated the mechanism for alcohol-water separation in the MOF Zn 2 (bdc) 2 (TED) (TED = 1,4-diazabicyclo[2.2.2]octane) [ 38 ]. From the density functional theory (DFT) calculations conducted, they deduced that the combination of hydrogen bonding between the alcohol hydroxyl group and oxy group of Zn 2 (bdc) 2 (TED) and the van der Waals interactions between the alcohol alkyl chain with the phenyl ring is the reasoning for the framework exhibiting an increased adsorption of ethanol over water.…”