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

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 Zn(4)O(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 Zn(4)O(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 Zn(4)O(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 Zn(4)O(bdc)(bpz)(2) is about 75 at PV condition, and Zn(4)O(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.