Recyclable Choline Nicotinate and Ferulate Aqueous Solutions as Efficient Lignin Solvents

Four novel choline carboxylate aqueous solution systems were developed by mixing H2O with choline nicotinate [Ch][Na], choline ferulate [Ch][Fa], choline vanillate [Ch][Va] and choline syringate [Ch][Sa]. The solubility of lignin in the four solvents was determined at 25 °C. The influence of the molar ratio of H2O to [Ch][Na] ([Ch][Fa], [Ch][Va] and [Ch][Sa]) and the anionic structure on lignin solubility were systematically investigated. It was found that, the anionic structure and H2O content significantly affected lignin dissolution. Interestingly, H2O/[Ch][Na] and H2O/[Ch][Fa] solvents show efficient capacity for lignin dissolution even at room temperatures. The dissolution of lignin in H2O/[Ch][Na] and H2O/[Ch][Fa] solvents is mainly ascribed to the interaction of lignin with the alkyl chain in the anion and cation dissociated from [Ch][Na]([Ch][Fa]) by H2O. In addition, the recycling of the lignin solvent was examined, and the structure and thermostability of the lignin regenerated from the solvent were also estimated.


Introduction
With the rapid depletion of fossil resources, the production of chemicals and materials from renewable lignocellulosic biomass in place of fossil resources is attracting much attention [1]. Lignin is one of the three principal components (lignin, cellulose and hemicellulose) of lignocellulosic biomass [2,3]. At the same time, lignin is also the second most abundant biopolymer in nature next to cellulose and the only native biopolymer on the Earth that contains aromatic phenolpropanoid monomers. Lignin has been regarded as a promising alternative to fossil resources. Lignin is a cross-linked amorphous copolymer synthesized from random polymerization of three primary phenylpropane monomers, coniferyl alcohol, paracoumaryl alcohol and sinapyl alcohol, which are bonded together through several different C-O-C and C-C interunit linkages [4]. The complex structure results in low solubility or insolubility in classical organic solvents and water [5], which is one of the main challenges for the efficient utilization of lignin. Therefore, many efforts have been made for improving lignin dissolution.
Over the past years, ionic liquids (ILs) have been utilized to dissolve/process lignocellulosic biomass in view of their outstanding properties such as non-detectable vapor pressure, physicochemical tunabilities, recoverability, and so on [6][7][8][9][10][11][12][13][14][15][16]. The reported ILs for dissolving/processing lignin include imidazolium-based ILs [17][18][19], ammonium (phosphonium and pyrrolidinium) based ILs [20], pyridinium carboxylate ILs [21], and bio-derived ILs which are composed of ions derived from naturally occurring bases (e.g., choline) and acids (e.g., amino acids and carboxylic acids) [22,23]. However, the ILs are usually expensive, very viscous or toxic and the bio-derived ILs exhibit poor dissolution capacity for lignin. In this context, some IL-water solutions were developed as lignin solvents. Wang et al. found that some aqueous dialkylimidazolium-based IL solution systems could dissolve lignin at 60 • C, and 39.8 g/100 g of the maximum lignin solubility was obtained in aqueous 1-ethyl-3-methylimidazolium acetate ([C 2 mim][CH 3 COO]) solution containing 70 wt % of [C 2 mim][CH 3 COO] [24]. Recently, binary solvent systems consisting of γ-valerolactone + water (dimethyl sulfoxide, N,N-dimethylformamide or ILs) have been reported to efficiently dissolve various types of lignin [25]. It has also been reported that two novel solvents including eco-friendly polysorbate aqueous solvents and aqueous glycol solvents display excellent dissolving capacity for lignin [26,27]. Very recently, Xu et al. developed 13 novel kinds of choline carboxylate/H 2 O solvents for lignin [28]. The authors find that the solubility of lignin increases with increasing alkyl chain length in the carboxylate anions, and a substitution of H in carboxylate anions by the OH or NH 2 group as well as the use of choline di-/tri-carboxylates leads to the decrease of lignin solubility, or even makes the lignin insoluble in the solvents.
Although previous investigations revealed some important aspects in the development of lignin solvents, the dissolution performance of lignin is unknown in H 2  are derived from nicotinic acid, ferulic acid, vanillic acid and syringic acid, respectively. Nicotinic acid is vitamin B3, and ferulic acid, vanillic acid and syringic acid are often used as medicine, spice and food additives.

Materials
Lignin (alkali) with a moisture content of 5% was purchased from Sigma-Aldrich (St. Louis, MO, USA). The enzymatically hydrolyzed lignin, isolated from cellulolytic enzyme hydrolysis of corncob, was from Shandong Longlive Bio-Technology Co., Ltd. (Shandong, China) [30]. The two lignins were dried before use under vacuum at 60 • C; choline hydroxide aqueous solution (46%, w/w) was from Alfa Aesar (Haverhill, MA, USA); nicotinic acid (98.0%), ferulic acid (99.0%), vanillic acid (98.0%), and syringic acid (98.0%) were purchased from Aladdin Industrial Corporation (Shanghai, China); deuterated DMSO (DMSO-d 6 ) used for nuclear magnetic resonance (NMR) spectra examination was purchased from Qingdao Weibo Tenglong Technol. Co. Ltd (Qingdao, China). These materials were used as received. Distilled water was used throughout the experiments. Additional lignin was added after the lignin in the tube was completely solubilized, based on observations using a polarizing microscope. When lignin became saturated to the point that no more lignin was dissolved further, its solubility (expressed by gram per 100 g of solvent) at 25 • C was calculated from the amount of the solvent and lignin added.

Characterization of the Regenerated Lignin
Fourier transform infrared (FT-IR) spectra were determined on a Necolet Nexus spectrometer (Nicolet iN10, Thermo Fisher Scientific, Waltham, MA, USA) to analyze the functional groups in the regenerated lignin samples with KBr pellets. The FT-IR spectra for each sample were collected for a total of 16 scans at a resolution of 2 cm −1 within the wavenumber range from 4000 to 400 cm −1 . Thermogravimetric analysis (TGA) was conducted on a NETZSCH STA 449 C thermal analyser (Netzsch Corporation, Freistaat Bayern, Germany) using alumina crucibles under flowing N 2 at a heating rate of 10 • C min −1 . The sample mass for each measurement was ca. 10-15 mg. The number averaged (M n ) and weight averaged molecular weight (M w ) were examined on a Waters e2695 chromatographic instrument (Waters, MA, USA). N,N-Dimethylformamide was used as mobile phase. Calibration of lignin molecular weight was achieved using polystyrene calibrants.

Measurements of 13 C NMR Spectra
Measurements of 13 Table 1).  At the same time, as a representative, we also determined the solubility of the enzymatically hydrolyzed lignin which has a similar structure to lignin in untreated lignocellulosic biomass [18], and the solubility data of the lignin are given in Table 3. Apparently, [Ch][Fa]/H2O solvents still exhibit powerful dissolution capacity for the enzymatically hydrolyzed lignin.

Solubility (g/100 g Solvent) [Ch][Na] [Ch][Fa] [Ch][Va] and [Ch][Sa
R is the mole ratio of H2O to choline carboxylate. a Insoluble at the given molar ratio. b Not measured.
Polymers 2018, 10, x FOR PEER REVIEW 4 of 9  At the same time, as a representative, we also determined the solubility of the enzymatically hydrolyzed lignin which has a similar structure to lignin in untreated lignocellulosic biomass [18], and the solubility data of the lignin are given in Table 3. Apparently, [Ch][Fa]/H2O solvents still exhibit powerful dissolution capacity for the enzymatically hydrolyzed lignin.    At the same time, as a representative, we also determined the solubility of the enzymatically hydrolyzed lignin which has a similar structure to lignin in untreated lignocellulosic biomass [18], and the solubility data of the lignin are given in Table 3. Apparently, [Ch][Fa]/H2O solvents still exhibit powerful dissolution capacity for the enzymatically hydrolyzed lignin.

Solubility (g/100 g Solvent) [Ch][Na] [Ch][Fa] [Ch][Va] and [Ch][Sa
R is the mole ratio of H2O to choline carboxylate. a Insoluble at the given molar ratio. b Not measured.   At the same time, as a representative, we also determined the solubility of the enzymatically hydrolyzed lignin which has a similar structure to lignin in untreated lignocellulosic biomass [18], and the solubility data of the lignin are given in Table 3. Apparently, [Ch][Fa]/H2O solvents still exhibit powerful dissolution capacity for the enzymatically hydrolyzed lignin.   At the same time, as a representative, we also determined the solubility of the enzymatically hydrolyzed lignin which has a similar structure to lignin in untreated lignocellulosic biomass [18], and the solubility data of the lignin are given in Table 3. Apparently, [Ch][Fa]/H2O solvents still exhibit powerful dissolution capacity for the enzymatically hydrolyzed lignin.

Solubility (g/100 g Solvent) [Ch][Na] [Ch][Fa] [Ch][Va] and [Ch][Sa
R is the mole ratio of H2O to choline carboxylate. a Insoluble at the given molar ratio. b Not measured.
a −1.6 170.6 a Not observed. At the same time, as a representative, we also determined the solubility of the enzymatically hydrolyzed lignin which has a similar structure to lignin in untreated lignocellulosic biomass [18], and the solubility data of the lignin are given in Table 3 Figures S1 and S2), and the corresponding 13 C NMR data are given in Table 4. At the same time, Figure 1 gives the schematic structure and the numbering of the C atoms in [Ch][Na] to help understanding.

Effect of Anionic Structure
Based on the 13 C NMR data in Table 4 that, after lignin was dissolved in H 2 O/[Ch][Na] solvent, the C9 signal in nicotinate anion slightly shifts upfield (chemical shift decreases). This is mainly ascribed to the strong interaction of the carboxyl group in nicotinate anion with H 2 O, which disables the carboxyl group to interact with lignin. At the same time, it is noted that the signals of the carbon atoms C1-C8 except for C7 shift upfield (a decrease in the chemical shifts). This suggests that the aromatic ring or/and alkyl units in cations and anions interact with lignin, resulting in the increase of the electron cloud density of these atoms. Therefore, the dissolution of lignin in H 2 O/[Ch][Na] solvent mainly results from the interaction of the alkyl chain instead of the carboxyl group in nicotinate anions, which is similar to the results reported in the literature [26]. To verify this speculation, the solubility of lignin in aqueous choline chloride (

Recovery of Solvent and Structure and Thermal Properties of the Regenerated Lignin
Recovery of H2O/choline carboxylate solvent was estimated. After the complete dissolution of lignin in the H2O/choline carboxylate solvent, lignin can be regenerated and the H2O/choline carboxylate solvent can be recovered by adding additional water. In a typical recovery trial, 4.0 g of H2O/[Ch][Na] (R = 8:1) solvent and 40.0 wt % lignin solution were used. The lignin solution was filtered using a 60 mL sand-core filter funnel to obtain lignin, and the lignin was then washed 4-5 times by 50 g H2O to ensure that [Ch][Na] had been washed out. The recovered lignin was about 89 wt %. All filtrate was collected together. The H2O/[Ch][Na] solvent recovered could be obtained by evaporating off H2O. Interestingly, after three dissolving-recovering cycles, the solvent still displayed the same dissolution capacity of lignin as the original solvent.
The averaged molecular weight is shown in Table 5. The molecular weight of the regenerated lignin from [Ch][Fa]/H2O (R = 10:1)/lignin solution is close to that of original lignin, indicating that the molecular structure of the regenerated lignin is hardly disrupted. The molecular weight of the regenerated lignin from [Ch][Na] (R = 8:1)/H2O/lignin solution is slightly higher than that of the original lignin. This is mainly ascribed to the fact that in the regeneration processes, the lignins of some small number weight are washed off.   The averaged molecular weight is shown in Table 5. The molecular weight of the regenerated lignin from [Ch][Fa]/H 2 O (R = 10:1)/lignin solution is close to that of original lignin, indicating that the molecular structure of the regenerated lignin is hardly disrupted. The molecular weight of the regenerated lignin from [Ch][Na] (R = 8:1)/H 2 O/lignin solution is slightly higher than that of the original lignin. This is mainly ascribed to the fact that in the regeneration processes, the lignins of some small number weight are washed off.            O solvents can be recovered and reused, and the recovered solvent still displays the same dissolution capacity of lignin as the original solvent after three dissolving-recovering cycles. Moreover, the regenerated lignin exhibits good thermal stability and hardly disrupted molecular structure according to FT-IR, TGA and molecular weight investigations. Author Contributions: A.X., L.C., X.X., Z.X. and R.L. designed the experiments; L.C., R.G., M.Y. and L.Z. performed the experiments and analyzed the data; A.X. and L.C. wrote the manuscript.