Thermally Insulating Cellulose Nanofiber Aerogels from Brewery Residues

Brewery spent grain (BSG), the main waste of the brewery industry, is often disposed of or sold as an animal feed at low prices, neglecting the potential for further valorization. In this work, BSG was used for the first time to produce cellulose nanofiber aerogels for green thermal insulating applications. Cellulose nanofibers (CNFs) were extracted from BSG through microfluidization with and without TEMPO-oxidation pretreatment to obtain bio-based building blocks for aerogels. The TEMPO-treated CNFs exhibited a finer diameter (5.4 nm) with a more homogeneous size distribution (2–10 nm) than that of nonoxidized samples. All of the produced aerogels demonstrated low densities (0.009–0.031 g/cm3), low thermal conductivities (32.3–37.7 mW/mK), and a compressive Young's modulus in the range of 4–311 kPa. Unidirectional freezing and TEMPO-oxidation showed a distinct improvement in the thermal conductivity and mechanical properties of the aerogels compared to the random frozen samples. Specifically, the unidirectional frozen aerogel with the highest investigated CNF concentration of 3 wt % exhibited the lowest thermal conductivity (32.4 mW/mK), highest Young's modulus (311 kPa), and good thermal stability. These results show that the BSG-derived nanocellulose is a valuable resource for producing eco-friendly, biodegradable, and thermally insulating aerogels.