Introduction
Lignocellulosic biomasses have emerged as promising feedstock for the development of value-added products, and their abundancy, renewability, and biodegradability are some of their most attractive characteristics. Wheat straw, corn stover, sugarcane bagasse, cornhusk, cotton, oil palm empty fruit bunch, agave bagasse, rice husk, cereal straw, and different types of wood mainly comprise cellulose, hemicelluloses, and lignin, and have been used as raw materials for the production of chemicals, energy, and various materials.
Cellulose is the most abundant natural polymer on earth and the predominant component of lignocellulose (Wan Daud and Djuned
2015; Chen et al.
2016; Ruiz-Cuilty et al.
2018; Candido et al.
2017). Cellulose has been typically described as a linear chain of β-
d-glucopyranosyl units linked by 1–4-β-glucosidic bonds. Each anhydroglucose unit contains three reactive hydroxyl groups, which are susceptible to replacement by other chemical groups. Moreover, cellulose can be chemically modified to obtain materials with different physical and chemical properties for specific purposes.
Cellulose acetates (CAs), which are some of the most important derivatives of cellulose, are widely used for manufacturing textile fibers, cigarette filters, coats, films, composites, and membranes for filtration. CAs are synthesized via the acetylation of cellulose, where acetyl groups substitute the hydroxyl groups during esterification reactions where acetic acid, acetic anhydride, and sulfuric acid are typically used as the solvent, acetylating agent, and catalyst, respectively (Candido and Gonçalves
2016; Filho et al.
2005; El Nemr et al.
2015; Cerqueira et al.
2007; Das et al.
2014; Cao et al.
2018). CAs are typically produced from materials such as wood pulps or cotton, which contain highly pure cellulose, but are rather expensive (Cao et al
2018). Therefore, in the past decades, researchers focused their efforts toward the preparation of CAs from less expensive lignocellulosic biomasses, including corn fiber, rice hulls, and wheat straw (Biswas et al.
2006), rice husk (Das et al.
2014), agave bagasse (Soto-Salcido et al.
2018), sugarcane bagasse (Candido et al.
2017; Nakanishi et al.
2011), newspaper paper and mango seeds (Meireles et al.
2010), and
Pinus sp. sawdust (Ballinas-Casarrubias et al.
2015).
The interactions of cellulose, hemicelluloses, and lignin result in the highly complex tridimensional structure of biomasses. Each of these compounds could be used for manufacturing value products. However, to be able to use them separately, biomasses need to be fractionated first (Li et al.
2018). Fractionation changes the micro- and macrostructure of biomasses and is typically performed by dissolving lignin and hemicellulose while leaving cellulose relatively unchanged and suitable for derivation (Chen et al.
2017). Kraft (sulphate) and sulfite cooking processes are the most commonly used pulping methods in the industry. Delignification occurs when lignin reacts with the sodium hydroxide in white liquor. The mass yields of these processes are considerably low, namely 50% or even lower, owing to the degradation and solubilization of carbohydrates during alkaline pulping at high temperature. The dominant pulping process, i.e., Kraft pulping, utilizes sodium sulfide under strongly alkaline conditions, and generates undesired sulfide derivatives and lignin, which contains sulfur (Idarraga et al.
1999; Santos et al.
2012; Robles et al.
2018; De La Torre et al.
2013). The Kraft cooking process is carried out in large cooking plants. When pure cellulose is the target compound, the pulp requires pre- and/or post-treatment. The large plants do not allow tailoring the pulp quality, and therefore, novel solvents for its fractionation have been developed. The fractionation processes studied and developed to date have aimed to minimize the loss and degradation of sugars, avoid the generation of harmful byproducts and the use of environmentally harmful reagents, and reduce the process cost (Chen et al.
2017). During the last decades, novel pretreatment methods using ionic liquids (IL) or organic solvents have been developed for the fractionation of biomasses. IL-based pretreatments have been demonstrated to be effective for the fractionation of several types of feedstock (Perez-Pimienta et al.
2016) however, their large-scale use is limited, mainly owing to the high cost of ILs and the need for complex and expensive recycling processes (Berglund et al.
2017).
Recently, a novel type of ionic solvents, known as deep eutectic solvents (DESs) have attracted the attention of researchers owing their high biodegradability and facile preparation process; moreover, DESs need not be purified after they are produced. Previous studies mostly defined DES as special mixtures of at least one hydrogen-bonding donor (HBD) and at least one hydrogen-bonding acceptor (HBA). The ability of DESs to donate and accept protons allows them to form hydrogen bonds with other compounds, and therefore, it improves their solvation properties. When natural compounds, such as amino acids, organic acids, sugars, urea, and choline derivatives, are used to synthesize DESs, the obtained solvents are commonly called natural deep eutectic solvents (NADESs). The natural origin of the components of NADESs prevails over some of the main drawbacks of common organic solvents, including their toxicity and high volatility (Santana et al.
2019; Faggian et al.
2016).
NADESs are prepared using heating, grinding, vacuum evaporation, and freeze-drying methods. When NADESs are prepared via heating, which is the simplest and most commonly used method, the components are mixed and heated until eutectic solvents form. The vacuum evaporation method requires that the components are first dissolved in water followed by their evaporation in vacuum. For the grinding method, the solid components are grinded in a mortar under nitrogen atmosphere until a transparent and homogeneous liquid is formed. When NADESs are prepared via freeze drying, the solid components are separately dissolved in water, and then, the solutions are mixed, frozen, and freeze-dried until a clear mixture is obtained (Van Osch et al.
2017).
The NADES that consists of choline chloride (CC) and lactic acid (LA) as the HBA and HBD, respectively, has been used to extract highly pure lignin from wood and is considered to be a promising solvent for the fractionation of biomasses and recovery of celluloses, because it requires milder cooking conditions than those used for conventional methods (Chen and Mu
2019; Mamilla et al.
2019; Satlewal et al.
2018; Li et al.
2018). According to the literature, different type of waste biomasses and pulps treated with NADESs present a wide range of delignification yields. For example, pretreatment using the NADES that consisted of a mixture of CC and LA with a molar ratio of approximately 1:9 in the temperature range of 60–90 °C was reported to decrease the lignin content of wheat straw to 14.6% (Jablonsky et al.
2015), potato peels and apple residues to 33% and 62%, respectively (Procentese et al.
2018), hardwood Kraft pulp to 37.8% (Jablonsky et al.
2018), and corncob to 93.1% (Zhang et al.
2016). The waste biomass used in this study was agave bagasse, which is a solid waste generated during the production of tequila. Annually, approximately 350 kiloton of agave bagasse is produced in Mexico. This waste stream is typically discarded to landfills or is burnt. Furthermore, the usability of the agave bagasse waste biomass for the production of CA using the new NADES-based process was compared with that of silver birch (
Betula pendula). The effect of the NADES pretreatment of agave bagasse on its delignification, crystallinity index (CrI), degree of polymerization (DP), hemicellulose removal, and cellulose accessibility for further chemical modification is currently unknown.
Therefore, the feasibility of the novel NADES-based fractionation process aimed at purifying cellulose, for further synthesis of CAs, from waste biomass, was studied. We hypothesized that the waste biomass could be valorized into CAs via NADES pretreatment followed by conventional acetylation. It was assumed that the NADES pretreatment of the waste biomass could be an efficient and mild fractionation process for producing cellulose-enriched pulp, which would be suitable for valorization into CAs.
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