Abstract
This study focuses on the extraction of cellulose nano-whiskers (CNWs) from the leaves of Adansonia kilima (AK), usually known as African baobab, using a combination of a microwave-assisted alkali (KOH) pre-treatment with subsequent bleaching process prior to ultra-sonication. Ultra-sonication was carried out using the ionic liquid (IL) 1-butyl-3-methylimidazolium hydrogen sulfate (Bmim-HSO4). Process parameters for ultra-sonication were optimized using a two-level factorial Box–Behnken design (BBD). Process variables such as ultra-sonication power (x1), hydrolysing time (x2) and temperature (x3) were varied. Responses selected were percentage crystallinity index, CrI% (y1) and yield% (y1) for the finally procured CNWs sample. Regression analysis was carried out to develop quadratic model to analyze the effect of process variables on IL-assisted ultra-sonication process. Analysis of variance (ANOVA) showed that ultra-sonication power was the most influential aspect for hydrolyzing the amorphous segments of crude cellulose extracted from baobab leaves. A relative study of the physio-chemical properties of the starting lignocellulosic substrate (AK), KOH pre-treated, bleached and IL-assisted ultra-sonicated CNWs was conducted. The synthesized samples were characterized using Fourier transform infrared spectroscopy, Scanning electron microscopy, atomic force microscopy, high resolution transmission electron microscopy, X-ray diffraction and thermo-gravimetric and zeta potential analysis. Under optimum condition, the extracted CNWs showed an average width of 15–20 nm; with high crystallinity index of 86.46%. This research provides an insight about the delignification of Adansonia kilima (AK) leaves and its effective conversion to CNWs having high crystallinity.
Highlights
Application of renewable lignocellulosic feedstock to produce nanocellulose has gained extensive interest over the last few decades
Crystallinity Index (CrI)% (y1 ) and yield% (y2 ) of cellulose nano-whiskers (CNWs), two polynomial equations were suggested after regression analysis based on the data obtained from the basic design matrix (Table 2)
Presence of ionic liquid (IL) under optimum condition of ultrasonication can selectively degrade the amorphous domain of crude cellulose obtained from Adansonia kilima (AK)
Summary
Application of renewable lignocellulosic feedstock to produce nanocellulose has gained extensive interest over the last few decades. Successful extraction has been carried out by following those methods, some practical obstacles remain for their large-scale production, such as prolonged reaction time with lower yield percentages, use of concentrated corrosive mineral acids and environmental pollution. In this regard, some novel protocols consisting of microwave-assisted heating in presence of alkali (NaOH, KOH, Ca(OH)2 ) and peroxide bleaching process with subsequent hydrolysis using ionic liquids (ILs)-as green solvent during the ultrasonication have been recognized as eco-friendly approaches [16,17,18]. The output of this research clearly shows the role of Bmim-HSO4 IL in extracting highly purified crystals of CNWs having high crystallinity index (CrI%)
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