Structural characterization of cellulose nanofibers isolated from spent coffee grounds and their composite films with poly(vinyl alcohol): a new non-wood source

Abstract

The waste valorization of spent coffee grounds (SCGs), which are obtainable in large amounts worldwide for new non-wood source has been considered. Cellulose nanofibers derived from SCGs have been successfully produced by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation of SCGs containing 10% cellulose (dry weight). The TEMPO-oxidized cellulose nanofibers (TOCNFs) are 20–35 nm wide observed by scanning electron microscopy. X-ray diffraction showed that TOCNFs are present in a cellulose crystal form I. The average crystal size corresponding to a fiber width was 4.2 nm, as determined from the diffraction pattern. Solid-state NMR shows that hemicellulose and lignin were mostly removed from SCGs via TEMPO-mediated oxidation, but small amounts of triacylglycerols remained in the TOCNFs. Thermogravimetric analysis of TOCNFs showed two major steps of thermal decomposition at 251 °C and 267 °C, which were higher than the coffee roasting temperature range. Furthermore, in order to investigate an interaction of these TOCNFs with a polymer, a SCG-derived TOCNF composite film with poly(vinyl alcohol) as a water-soluble polymer was prepared. We found the TOCNFs were successfully integrated into the polymer. The outcome of this study indicated that SCGs could be used as well as wood as an alternative source for producing TOCNFs, thus contributing to the development of sustainable green chemistry.