Efficient purification of bioethanol by an ethanol-trapping coordination network

Bioethanol is an essential energy source and is produced from agricultural feedstocks. However, raw bioethanol contains much water and methanol, and the traditional distillation approach has hardly removed it. Adsorptive separation of ethanol from water and methanol by adsorbents is an energy-efficient and promising method to produce fuel-grade ethanol. In this work, by regulating the pore surface in a new coordination network, [Cd(4-pmntd)(tpa)] (4-pmntd = N,N’-Bis(4-pyridymethy) naphthalene diimide; tpa = terephthalic acid), we target ultrahigh ethanol uptake and efficient ethanol/water separation, surpassing most benchmark materials. Explicitly, the ethanol uptake of Cd(4-pmntd)(tpa) (62.5 wt% at 100 kPa and 298 K) has the second-highest adsorption capacity among all the metal–organic frameworks (MOFs). In addition, Cd(4-pmntd)(tpa) gives ethanol/water selectivity of 32 toward a mixture of ethanol and water. Dynamic breakthrough measurements confirmed the excellent separation of ethanol and water. Under different ethanol/water ratios, Cd(4-pmntd)(tpa) extracts a low ethanol concentration from an ethanol/water mixture and produces ethanol in 99.1% purity in one step. Spectroscopic measurement and grand canonical Monte Carlo (GCMC) simulations provide critical insight into the adsorption/separation mechanism. The framework can form rich intermolecular C-H_guest…π_host, C-H_guest…O_host and C-H_host…O_guest interactions with ethanol molecules because of the suitable pore size and pore surface set in Cd(4-pmntd)(tpa). Thus, the adsorption enthalpies for ethanol in Cd(4-pmntd)(tpa) reached 66.16 kJ/mol, but only 29.44 kJ/mol and 49.21 kJ/mol for water and methanol, respectively. Moreover, the porous material exhibits good stability and can be easily combined with chitosan to form filter film, which is vital for potential industrial applications.