In-silico design of covalent organic framework membranes for efficient water/ethanol separation

Abstract

Covalent organic frameworks (COFs) have recently emerged as a promising class of nanoporous materials for membrane separation. At present, COF membranes are not efficient for separation of water from ethanol (ethanol dehydration) owing to pervasive challenge to separate small-sized molecular mixture. In this proof-of-concept study, we have computationally designed a series of novel COFs as pervaporation (PV) membranes for efficient ethanol dehydration. We utilized TpPa-1 as a model COF and its framework was rationally modified by attaching several hydrophobic and hydrophilic functional groups, resulting in new COF membranes namely TpPa-1-C5H8, TpPa-1-COOC2H5, TpPa-1-OC3H6OH, TpPa-1-C2Ph, TpPa-1-OC4H8OH and TpPa-1-OC2H4NHOCH3. These membranes possess different functionality and aperture size. Molecular dynamics (MD) simulations demonstrated that the permeation flux of water and ethanol is mostly governed by pore aperture size. The COF with the largest pore aperture holds the highest flux. In contrast, polarity of the pore functional groups governs the separation factor. COF membranes with hydrophobic pore functionalities display a lower separation factor than the membranes with hydrophilic pore functionalities. Among the designed COF membranes, TpPa-1-OC4H8OH has shown superior performance compared to the state-of-art membranes. Also, activation energies for permeation of water and ethanol through the two promising TpPa-1-OC3H6OH and TpPa-1-OC4H8OH membranes are lower than the reported PV membranes. Interestingly, water and ethanol molecules indicate backward movement before leaving the membrane surface at permeate side. The potential of mean force (PMF) shows that free energy barrier for water to enter the pore of highly selective COF membrane is ∼18.84 kJ/mol and the driving force for the water transfer from feed side to permeate side is ∼ −10.5 kJ/mol at PV condition. The current study not only reveals the underlying mechanism for solvent permeation and separation, but also suggests a highly selective COF membrane for water/ethanol separation. We envisage potential applications of the designed COFs as novel materials for sustainable solvent recovery.