There is an array of methodologies to prepare nanocellulose (NC) and its fibrillated form (CNF) with enhanced physicochemical characteristics. However, acids, bases or organosolv treatments on biomass are far from green, and seriously threaten the environment. Current approach to produce NC/CNF from biomass should be revised and embrace the concept of sustainability and green chemistry. Although hydrothermal process, high-pressure homogenization, ball milling technique, deep eutectic solvent treatment, enzymatic hydrolysis etc., are the current techniques for producing NC, the route designs remain imperfect. Herein, this review highlights the latest methodologies in the pre-processing and isolating of NC/CNF from lignocellulose biomass, by largely focusing on related papers published in the past two years till date. This article also explores the latest advancements in environmentally friendly NC extraction techniques that cooperatively use ball milling and enzymatic hydrolytic routes as an eco-efficient way to produce NC/CNF, alongside the potential applications of the nano-sized celluloses.
This study explores an eco-friendly delignification technique for raw oil palm leaves (OPL), highlighting the optimized conditions of choline chloride-lactic acid deep eutectic solvent (DES)-mediated ball milling pretreatment to maximize the co-production yields of highly crystalline cellulose and lignin. Our five-level-four-factor Taguchi design identified the optimal reaction settings for cellulose production (85.83 % yield, 47.28 % crystallinity) as 90-minute milling, 1500 rpm, mill-ball size ratio of 30:10, ball-to-sample mass ratio of 20:1, DES-to-sample mass ratio of 3:1. Conversely, the maximal lignin extraction yield (35.23 %) occurred optimally at 120-minute milling, 600 rpm, mill-ball size ratio of 25:5, ball-to-sample mass ratio of 20:1 and DES-to-sample mass ratio of 9:1. Statistical results showed that milling frequency (p-value ≤ 0.0001) was highly significant in improving cellulose crystallinity and yield, while DES-to-sample mass ratio (p-value ≤ 0.0001) was the most impacting on lignin yield. The thermogravimetric method affirmed the elevated cellulose thermal stability, corroborating the enhanced cellulose content (40.14 % to 73.67 %) alongside elevated crystallinity and crystallite size (3.31 to 4.72 nm) shown by X-ray diffractograms. The increased surface roughness seen in micrographs mirrored the above-said post-treatment changes. In short, our optimized one-pot dual-action pretreatment effectively delignified the raw OPL to produce cellulose-rich material with enhanced crystallinity and lignin solidity.
Enzymatic treatment on lignocellulosic biomass has become a trend in preparing nanocellulose (NC), but the process must be optimized to guarantee high production yield and crystallinity. This study offers insights into an innovative protocol using cultivated fungal cellulase and xylanase to improve NC production from raw oil palm leaves (OPL) using five-factor-four-level Taguchi orthogonal design for optimizing parameters, namely substrate and enzyme loading, surfactant concentration, incubation temperature and time. Statistical results revealed the best condition for producing NC (66.06 % crystallinity, 43.59 % yield) required 10 % (w/v) substrate, 1 % (v/v) enzyme, 1.4 % (w/v) Tween-80, with 72-h incubation at 30 °C. Likewise, the highest sugar yield (47.07 %) was obtained using 2.5 % (w/v) substrate, 2.0 % (v/v) enzyme, 2.0 % (w/v) Tween-80, with 72-h incubation at 60 °C. The auxiliary enzymes used in this study, i.e., xylanase, produced higher crystallinity NC, showing widths between 8 and 12 nm and lengths >1 μm and sugars at 47.07 % yield. Thus, our findings proved that optimizing the single-step enzymatic hydrolysis of raw OPL could satisfactorily produce relatively crystalline NC and sugar yield for further transformation into bio-nanocomposites and biofuels. This study presented a simple, innovative protocol for NC synthesis showing characteristics comparable to the traditionally-prepared NC, which is vital for material's commercialization.