Enzymatic pretreatment for cellulose nanofibrils isolation from bagasse pulp: Transition of cellulose crystal structure

Highlights

• Enzyme and cold alkali assisted mechanical production of CNF were studied.

• Cold alkali pretreatment could change cellulose structure from type I to type II.

• Environment-friendly method for preparing high thermal stable CNF was obtained.

Abstract

In this work, cellulase, low-concentration cold alkali and cellulase combined with cold alkali were used to pretreat unbleached bagasse pulp from which cellulose nanofibrils (CNFs), about 30 nm in diameter, were successfully prepared through ultrafine grinding and high-pressure homogenization. X-ray diffraction analysis showed that cellulase pretreatment increased the crystallinity of CNFs. After low-concentration cold alkali pretreatment, the crystallinity of CNFs significantly reduced and the crystal structure of the cellulose changed from type I to type II. Thermogravimetric analysis showed that CNFs prepared by cellulase combined with cold alkali treatment produced more regenerated cellulose and had lower thermal stability. The use of cellulase and low-concentration cold alkali pretreatments combined with ultrafine grinding and high-pressure homogenization is an environment-friendly method for preparing CNFs. The use of low-concentration cold alkali reduces the consumption of alkali and clean water.

Introduction

As human civilization continues to advance, people’s awareness of environmental protection increases, and the search for new, environment-friendly materials to replace petrochemical-based counterparts becomes increasingly important (Dong et al., 2019; Fan et al., 2017; Nie, Zhang et al., 2018; Wang, Liu et al., 2018; Wang, Fu et al., 2018). Lignocellulosic biomass is an important substitute for petrochemical-based materials because of its renewability, degradability abundance, and low cost (Nie et al., 2014; Yao, Nie, Yuan, Wang, & Qin, 2015; Yu et al., 2018; Zhang, Nie, Qin, & Wang, 2019; Zhang, Tao, Zhang, Liao, & Nie, 2019). The global annual synthesis of cellulose through photosynthesis has reached 1.5 × 10¹² tons, making it one of the most abundant renewable resources on the planet (Klemm, Heublein, Fink, & Bohn, 2005; Nie, Yao, Wang, & Qin, 2016; Wang, Liu et al., 2018; Wang, Fu et al., 2018; Yao et al., 2017). Biomass fibers can be prepared by chemical, enzymatic, or mechanical methods to prepare cellulose nanofibrils (CNFs) of diameters in the range 1–300 nm and lengths of several hundred nanometers to several micrometers (Hayashi, Kondo, & Ishihara, 2005; Lin, Wu, Zhang, Liu, & Nie, 2018). CNFs have the advantages of high values of aspect ratio, specific surface area, degree of polymerization, crystallinity, and mechanical strength, alongside low values of density and low coefficient of thermal expansion (Nie, Zhang, Lin et al., 2018; Zhang, Zhang, Yan, Zhang, & Nie, 2018). Composite materials developed based on the advantages of CNFs can be used in applications such as flexible electronic devices, solar panels, energy storage materials, medical materials, sensors, and catalyst carriers (Dai et al., 2019; Li, Wang, Wang, Qin, & Wu, 2019; Xing et al., 2019; Zhu et al., 2016).

The thermal stability of CNFs is affected by factors such as raw materials, preparation methods, and drying methods (Peng et al., 2013). Improving the thermal stability of CNFs can further expand its range of applications. The thermal stability of CNFs is related to the content of cellulose, hemicellulose, and lignin in the different raw materials (Zhang, Nie et al., 2019; Zhang, Tao et al., 2019). CNFs prepared from fibers with high cellulose and low amorphous polymer content, such as cotton, have higher thermal stability than those prepared from other raw materials (W. Chen et al., 2014). The content of lignin in the fiber has a significant effect on the thermal stability of CNFs. CNFs with a high lignin content exhibits high thermal stability. Nair et al. prepared CNFs with lignin contents of 21 wt.% and 5 wt.% whose initial thermal degradation temperatures were 306 ℃ and 278 ℃, respectively, a difference of more than 20 ℃ (Nair & Yan, 2015). CNFs prepared by the TEMPO oxidation method can witness a significant reduction in thermal stability owing to the introduction of a large number of carboxyl groups during oxidation (Herrera et al., 2018). Variations in the crystal structure of cellulose also affect the thermal stability of CNFs. The crystal structure of CNFs prepared by pretreatment with a high concentration of sodium hydroxide changes from type I to type II, which improves its thermal stability of CNFs (Ouajai & Shanks, 2005). After the fiber is mechanically ground and homogenized by high pressure, the thermal stability is appropriately improved in the initial stage. The thermal stability of CNFs prepared after long-term grinding and homogenization is slightly lower (Zhang et al., 2018). In addition, acetylation and phosphating of CNFs can improve the thermal stability of nanocellulose (Agustin, Nakatsubo, & Yano, 2015).

Pretreatment with different kinds of biological enzymes can cause cellulose to swell to different degrees and effectively reduce energy consumption during grinding, which improves grinding efficiency and affects the thermal stability of the CNFs prepared (Dai et al., 2016; Nie et al., 2015; Pei et al., 2016). Following high-concentration alkali pretreatment, the crystal structure of cellulose changes (Du, Qin, Huang, Nie, & Song, 2015). At room temperature, when the alkali concentration is higher than 17.5 wt.%, the structure of cellulose changes from type I to type II (Wang, Li, Yano, & Abe, 2014). Compared with the cellulose of type I structure, the thermodynamic properties of the anti-parallel chain of the type II cellulose cell are relatively stable (Sarko & Muggli, 1974). However, pretreatment with high-concentration alkali increases production costs because of the large amount of alkali required. At the same time, cleaning the fibers after alkali pretreatment also results in wasteful consumption of a significant quantity of clean water, and the alkali used is treated as waste liquid. Hence, this process is neither economical nor environment-friendly. Lee et al. found that under conditions of cooling, low-concentration alkali also changes the crystal structure of cellulose. The short fibers of the fluff were pretreated with 4 wt.% alkali. The resultant CNFs consisted of a mixture of cellulose with type I and type II. When alkali concentration is increased to 6 wt.%, the crystal structure of the CNFs is completely converted to type II (Lee, Sundaram, Zhu, Zhao, & Mani, 2018).

This work mainly studies the effects of cellulase and cold alkali pretreatment on the thermal stability of the CNFs prepared. The effects of different pretreatment methods combined with superfine grinding and high-pressure homogenization on the morphology, crystal structure, crystallinity, and thermal stability of CNFs were analyzed by transmission electron microscopy (TEM), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray powder diffraction (XRD), and thermogravimetric analysis (TGA). An environment-friendly method of cellulase and cold alkali-assisted pretreatment for the isolation of CNFs from sugarcane bagasse pulp was obtained. This study provides scientific guidance for reducing the amount of chemicals used in the preparation of CNFs and expanding the application range of CNFs.