Covalently Functionalized Graphene for Chemical Separation Membranes

Chemical modifications of graphene oxide (GO) afford functionalized graphene with versatile characteristics. A Johnson-Claisen rearrangement is one of [3, 3] sigmatropic rearrangements to synthesize covalently functionalized graphene. Specifically, allylic alcohols on the GO surface are applied to the Claisen rearrangement to form ethyl ester groups anchored to the GO sheet through a robust carbon-carbon bond. These functional groups and robust carbon bonds survive in thermal and reductive conditions, indicating this functionalized graphene’s significant potential in applications in harsh conditions.<br/><br/>GO-based membranes are promising candidates as a pressure-driven non-thermal method to mitigate global energy use and to cope with environmental concerns in water purification and chemical separations because of their favorable characteristics: high water permeability, two-dimensional structures, and mechanical strength. There is a particular need for energy-intensive weak black liquor (WBL) concentration in the kraft pulping process. WBL is a high alkaline corrosive fluid with polymers, other organic molecules, inorganic salts, and total solids of ~18% and is processed at high temperatures of up to 95 °C. Although WBL is concentrated to ~75% solids via the evaporation processes to create biofuels, even partial concentration with non-thermal technology to ~30% solids significantly reduce energy requirements and are beneficial for economics and environments.<br/><br/>We herein focused on further chemical modifications of the robust graphene material prepared by the Claisen rearrangement of GO to apply its membrane for WBL concentration that is processed in the harsh conditions. To increase the hydrophilicity and water permeability of the graphene, we saponified the ester groups under base conditions to generate carboxylic acid groups. A narrowed interlayer spacing of the graphene sheet also lowers water permeation; therefore, we then amidated the hydrophilic graphene under mild conditions using linear diamines. These diamines can contribute to not only the expansion of the interlayer spacing but also the cross-linking between the graphene sheets. Water-dispersible graphenes with various interlayer spacings were successfully synthesized, and we prepared membranes from each graphene material. Large molecules (~1 kDa) were selectively excluded, and WBL was concentrated at a stable rate and water permeation thanks to the expanded interlayer spacing and hydrophilicity. These versatile water-dispersible graphenes represent promising membrane materials in WBL concentration and other chemical separations.