Biofuel purification by pervaporation and vapor permeation in metal–organic frameworks: a computational study

Abstract

We report a computational study for the purification of biofuel (water–ethanol mixtures) in two metal–organic frameworks (MOFs), hydrophilic Na-rho-ZMOF and hydrophobic Zn₄O(bdc)(bpz)₂ at both pervaporation (PV) and vapor permeation (VP) conditions. In Na-rho-ZMOF, water is preferentially adsorbed over ethanol due to its strong interaction with nonframework Na⁺ ions and ionic framework, and the adsorption selectivity of water–ethanol is higher at a lower composition of water. With increasing water composition, water diffusivity in Na-rho-ZMOF increases but ethanol diffusivity decreases, and the diffusion selectivity of water–ethanol increases. In contrast, ethanol is adsorbed more in Zn₄O(bdc)(bpz)₂ as attributed to the favorable interaction with methyl groups on the pore surface, and ethanol–water adsorption selectivity is higher at a lower composition of ethanol. With increasing water composition, the diffusivities of water and ethanol in Zn₄O(bdc)(bpz)₂ increase and the diffusion selectivity of ethanol–water decreases slightly. The permselectivities in the two MOFs at both PV and VP conditions are largely determined by the adsorption selectivities. The maximum achievable permselectivity in Na-rho-ZMOF is approximately 12 at VP condition, and Na-rho-ZMOF is preferable to remove a small fraction of water from water–ethanol mixtures and enrich ethanol at the feed side. The maximum permselectivity in Zn₄O(bdc)(bpz)₂ is about 75 at PV condition, and Zn₄O(bdc)(bpz)₂ is promising to extract a small fraction of ethanol and enrich ethanol at the permeate side. This study presents microscopic insights into the separation of water–ethanol mixtures in hydrophilic and hydrophobic MOFs at both PV and VP conditions, and provides atomistic guidelines toward the selection of an appropriate MOF and operating condition for biofuel purification.