Free-lignin cellulose obtained from agar industry residues using a continuous and minimal solvent reaction/extraction methodology

Abstract

Herein we present a green procedure to obtain cellulose (Cel) polymers with different physicochemical properties at high purity, and underline its two major aspects: sustainability and efficiency. In the first place, regarding sustainability, the source of Cel was residues from agar industries, which are based on red seaweed and hence free of lignin, thus facilitating the extraction of Cel. In the aspect of efficiency, a continuous extraction/reaction system was used to obtain pure Cel from these residues. The extraction/reaction device used in this study normally works in a liquid–liquid extraction fashion, but in this particular case it was successfully employed as a liquid–solid system. This methodology is important, because it concomitantly reduces the time of extraction/reaction procedures in the same flask and also minimizes the amount of solvent used. Thus high purity Cel was obtained using a continuous and minimal solvent extraction/reaction system in neutral/acidic/basic conditions leading to Cel_n/Cel_a/Cel_b polymers in 42/34/43.3% yield. These materials were characterized by ¹³C cross-polarization magic-angle spinning (CP-MAS) NMR, Fourier transform infrared spectroscopy (FT-IR), CHNS elemental analyses, X-ray diffraction (XRD), size exclusion chromatography (SEC) and compared against microcrystalline cellulose (MCC), confirming chemical integrity. Crystallinity index (CI [%]), was obtained from XRD/CP-MAS NMR data. All samples had slightly higher crystallinity than that of MCC. Molecular weight (M_W, g mol⁻¹), polydispersity index (PDI) and degree of polymerization (DP) for Cel_n, Cel_a, Cel_b polymers were all higher than those in MCC. Compared to MCC, the physicochemical characteristics of the isolated Cel polymers varied depending on the treatment, neutral being the mildest. The greener procedures developed herein provide Cel suitable for research and development of Cel derived substances.