Aerogel is a porous material with a mesh or honeycomb structure, low density, and large surface area. Due to its unique structure and physical properties, aerogel has become an attractive material and received increasing attention in the fields of aviation, petroleum, medicine, and construction (Du, Zhou, Zhang, & Shen, 2013; Labudek & Martiník, 2011; J. Li et al., 2011a; F. Lu & Ralph, 2010; Ma et al., 2019). At present, most aerogels are prepared from petroleum-based polymeric materials, such as polyvinyl alcohol (H. B. Chen, Wang, & Schiraldi, 2014), polyimide (Ghaffari Mosanenzadeh, Alshrah, Saadatnia, Park, & Naguib, 2020; F. Yang et al., 2020), polyurethane (Perrut, 1998), and polypyrrole (Xie et al., 2015). However, petroleum-based raw materials have the disadvantages of a high cost, slow biodegradability, and poor biocompatibility. Therefore, biomass-based aerogels have attracted more attention. Compared with petroleum-based aerogels, biomass-based aerogels are prepared from abundant and renewable resources, and they are environmentally friendly and biocompatible and have shown broad applications in different areas.
Cellulose is the most abundant natural macromolecular material with excellent structural properties. However, cellulose is embraced in the cell wall of plants, and there are extensive hydrogen bonds between the cellulose chains, which form a rigid cellulose matrix and make it very difficult to dissolve in common solvents, such as water and ethanol. Many solvent systems have been developed to dissolve cellulose for the preparation of cellulose-based materials, including aerogels, such as N-methyl-morpholine-N-oxide (NMMO), lithium chloride/N, N-dimethylacetamide (LiCl/DMAc), sodium hydroxide/urea or thiourea systems (Jie, Kimura, Wada, Kuga, & Zhang, 2010; F. Li & Chen, 2008; Lindman, KarlstroM, & Stigsson, 2010; Vlachos, Quiroz, Norton, Nguyen, & Vasiliadou, 2019), but some of these methods are time-consuming, less green and costly. Hence, finding new solvents for cellulose is still of great interest.
Inorganic molten salt hydrates (MSH) are water-salt systems in which the molar ratio of water to salt is close to the coordination number of the salt cation. At this concentration, only ion-water interactions occur, with almost no water-water and ion-ion interactions (Rodriguez Quiroz, Norton, Nguyen, Vasileiadou, & Vlachos, 2019). It was found that some MSH, such as Zn(NO3)2·6H2O, ZnCl2·(3–4)H2O, FeCl3·6H2OZnCl2·(3–4)H2O, LiCl·(2–5)H2O, and LiBr·3H2O could effectively solubilize cellulose (Dealmeida et al., 2010; Fischer, Leipner, Thümmler, Brendler, & Peters, 2003; Liao, Pang, & Pan, 2019; Ning et al., 2018; Penfield & Axelson, 1999; Sen, Martin, & Argyropoulos, 2013; Y. J. Yang et al., 2014; C. Yoo, N. Li, M. Swannell, & X. Pan, 2017). Compared with other cellulose solvents, MSH has unique properties, such as low viscosity, low vapor pressure, high boiling point, low cost, non-toxic, and easy recovery. Many studies have been carried out on using MSH for biomass processing and conversion. For example, Liu (G. Liu et al., 2022) et al. converted cellulose to 5-hydroxymethylfurfural in the MSH system without an additional catalyst, yielding 44.1% 5-HMF using ZnCl2·4H2O (64.9 mol%) at 160°C and 90 min. Liu (C. Liu, Wei, Yin, Pan, & Wang, 2021) et al. synthesized furfural in the γ-valerolactone/MSH two-phase system. Xylan was almost completely converted (99.9%) at 140°C in 2 h with a furfural yield of 77.2 mol%. The solvent could be recovered, and the furfural yield was above 73.0 mol% after three recycles. Yoo (C. G. Yoo, N. Li, M. Swannell, & X. Pan, 2017) et al. converted glucose to fructose in an MSH system. The results showed that up to 31% of glucose was isomerized to fructose in lithium bromide trihydrate at 120°C for 15 min, and an alternative method for producing fructose from glucose was provided. Li (N. Li et al., 2019) et al. synthesized glucooligosaccharides from glucose via non-enzymatic glycosylation in an acidic lithium bromide trihydrate with a yield of 75%. These results show that MSH can be used as a promising solvent for biomass.
Lithium bromide trihydrate (LiBr·3H2O) is an MSH that has been studied for biomass processing. For example, Deng (Deng, Kennedy, Tsilomelekis, Zheng, & Nikolakis, 2015) et al. achieved the hydrolysis of cellulose under mild conditions (85°C, 30 min) by acidified LiBr-3H2O and obtained more than 90% yield. Pan et al used it as a solvent in lignin isolation and depolymerization (X. Yang, Li, Lin, Pan, & Zhou, 2016), lignin demethylation (Z. Li, Sutandar, Goihl, Zhang, & Pan, 2020) and preparation of cellulose nanocrystals (N. Li, Bian, Zhu, Ciesielski, & Pan, 2021). Recently, the lignin-containing aerogels were prepared from Douglas fir wood for energy and environmental applications, where LiBr·3H2O was used as a solvent (Liao et al., 2019). Prior to dissolving Douglas fir with LiBr·3H2O, alkali treatment of Douglas fir was required to remove biomass-based acid from Douglas fir [22]. Despite the report on the adverse impact of biomass-based acid on aerogel formation in this system, it is still unclear how alkali would impact acid removal and aerogel formation. Therefore, it is crucial to investigate how alkali affects the formation and properties of aerogels and to determine the optimal alkali treatment conditions. In addition, the presence of lignin in the lignin-containing aerogel decreases the overall homogeneity and increases the minimum biomass content to form the gel (J. Li et al., 2011b; Y. Lu et al., 2012). It is not clear how the presence of lignin affects the aerogel production and properties in the MSH system.
The objectives of this study are to investigate 1) the effects of the alkali pretreatment on the removal of the acidic substances on the physical properties of the aerogel, such as surface morphology, pore structure, and specific surface area, and 2) the fate of lignin in lignin-containing biomass on the hydrogel and aerogel production and properties in molten salt systems.