Humidity-responsive molecular gate-opening mechanism for gas separation in ultraselective nanocellulose/IL hybrid membranes

Abstract

Nanofibrillated cellulose (NFC) represents an important class of bio-based nanomaterials that possess favorable properties including hydrophilicity, 1D structure, biodegradability, and surface tunability. Although widely known for its effective gas-barrier attributes due to its inherent crystallinity and hydrogen-bonding capability, the ability of NFC to form dense films has been of considerable interest in selective gas-separation applications as a viable replacement for synthetic polymers. With precise control of targeted properties at the nanoscale, NFC can likewise be used to enable selective removal of greenhouse gases such as CO₂. Herein we report a class of “green” hybrid membranes composed of NFC and an ionic liquid (IL), 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]), that together exhibit exceptional separation properties arising from controllable nanoscopic design. With this new class of green membranes, CO₂/N₂ selectivities as high as ∼370 and CO₂ permeabilities as high as ∼330 Barrer have been obtained at optimal IL loadings and/or humidity levels. The current work demonstrates that size exclusion of a molecular penetrant in a water-swollen NFC membrane matrix relies on the network architecture of partially swollen nanocellulose fibrils to selectively permeate CO₂ through enhanced diffusive pathways. Additionally, the gas-transport and rheological properties of these NFC-fabricated membranes can be precisely tuned through the independent use of humidity as an external control parameter.