Superabsorbent Aerogels from Cellulose Nanofibril Hydrogels

Abstract

Deep eutectic solvents (DESs) are promising green chemicals that can function as solvents, reagents, and catalysts in many applications because of their biodegradability, ready availability, and low toxicity. Here, a DES of choline chloride-urea was used as a nonhydrolytic pretreatment medium to obtain cellulose nanofibril (CNF) hydrogels from recycled cellulose pulps (boxboard, milk containerboard, and fluting) and virgin birch cellulose pulp using a mechanical Masuko grinder. The mechanical disintegration of DES pretreated cellulose fibers resulted in highly viscous, gel-like cellulose nanofibril hydrogels with shear thinning behavior. According to transmission electron microscope (TEM) imaging, the nanofibrils had widths from 2 nm to 80 nm, possessed the initial cellulose I crystalline structure, and had a crystallinity index of 53-56%. The nanofibril hydrogels obtained were further used to produce low-cost, ultralight, highly porous, hydrophobic, and reusable superabsorbing aerogels that were used as efficient sponges to absorb oil and chemicals. The nanofibril sponges prepared by the consequent hydrophobic modification (silylation) of CNF hydrogels and freeze-drying had ultralow density (0.003 g/cm 3) and high porosity (up to 99.8%). The sponges exhibited excellent oil/water absorption selectivity and ultra-high oil (marine diesel oil, kerosene, gasoline, motor 2 oil, castor oil, or linseed oil) and organic solvent (dimethyl sulfoxide, chloroform, n-hexane, toluene, acetone, or ethanol) absorption capacity. The nanofibril aerogels showed particular selectivity for marine diesel oil absorption from an oil-water mixture and possessed ultra-high absorption capacities of up to 143 g/g, which were much higher than the commercial absorbent materials (i.e., polypropylenes) (9-27 g/g) used as references. Additionally, the absorbed oil could be recovered by means of simple mechanical squeezing, and the superabsorbent could be reused for at least 30 cycles.