Multilayered graphene oxide membranes for bioethanol purification: Microscopic insight from molecular simulation

Abstract

Graphene oxide (GO) membranes have received considerable interest for separation. The channels in GO membranes are in an atomic scale and thus it is challenging to unravel underlying separation mechanism via experiment. In this study, we model multilayered GO membranes with various channel widths (W) and oxidation degrees (O/C), subsequently simulate their performance for bioethanol (an ethanol/water mixture) purification. In the sub-nm membranes with W = 0.75 nm, ethanol-selective permeation occurs at a low O/C but it changes to highly water-selective at a high O/C (with separation factor αH2O/EtOH of 5 and ∞ at O/C = 15% and 35%, respectively). In the wider membranes with W = 1.00 and 1.50 nm, moderate ethanol-selective permeation with αEtOH/H2O of 1–3 is observed. Detailed microscopic analysis reveals that ethanol and water differ in their intercalation and diffusion. In the wider membranes, both ethanol and water exhibit similar intercalation behavior and adopt Fickian diffusion, particularly at a low O/C ratio. In the sub-nm channels, both components feature sub-diffusion but ethanol has negligible diffusion at a high O/C = 35%. Depending on W, ordered structures with different layers are formed for ethanol and water. In the sub-nm channels, hydrogen bonds between water and channel are found to increase in number with increasing O/C ratio but their stability decreases. The microscopic insight provides quantitative understanding of bioethanol purification in GO membranes, and might facilitate the development of new GO membranes for important separation processes.