Fast Disassembly of Lignocellulosic Biomass to Lignin and Sugars by Molten Salt Hydrate at Low Temperature for Overall Biorefinery.

Lignocellulose is a complex of cellulose, hemicellulose, and lignin, whose overall conversion is still a challenge, especially by a fast and efficient method. Here, a very simple method has been developed using acidic molten salt of zinc chloride hydrate as the solvent and catalyst for complete disassembly of lignocellulose at 95 °C and atmospheric pressure in 12 min. The major products are lignin and monosaccharides, such as glucose and xylose. It was found that high-purity lignin in yield of about 20 wt % can be obtained with various biomass, and the maximum yield of glucose from bamboo is 40.56 wt % and that of xylose from wheat straw is 16.82 wt %. Importantly, zinc chloride can be recovered through precipitation by ammonia and reused for next cycles. It provides a simple route to separate and efficiently convert lignocellulose, especially high-grade feedstock for biorefinery.

Introduction

Lignocellulosic biomass, a kind of renewable carbon resource, has attracted much attention to produce biomass-based fuels and other platform chemicals or materials.[1] Lignin, cellulose, and hemicellulose are the three major component biopolymers of lignocellulosic biomass and potential feedstock that can serve as alternative to fossil resources in the near future. It is still a challenge to find a new and efficient method to convert lignocellulose and its components into valuable chemicals or materials. Nowadays, two routes have been employed for this conversion: (i) depolymerization of lignocellulose directly without distinguishing its components, such as thermal pyrolysis and gasification, which usually regard biomass as a mixture of C, H, and O elements, and the degraded products are so complex that upgrade is generally required,[2] and (ii) fractionation of lignocellulose to cellulose, hemicellulose, or lignin and then their conversion to the desired chemicals or materials, with the major obstacle being efficient disassembly of the robust complex.[3] A similar problem was also faced by paper-making industries, especially for the nonwoody biomass.

From a chemical perspective, the interactions among the three major components include hydrogen bonding between the cellulose and lignin, as well as between the cellulose and hemicellulose, and covalent bonds (mainly ether bonds) between cellulose and lignin. Therefore, disassembly of lignocellulose usually requires rigorous chemical or special physical treatment, with plenty of pollutants formed during the processes. Recently, green solvents and technologies have been developed to solve this problem, and ionic liquids are known for their ability to directly dissolve cellulose, hemicellulose, lignin, and even raw lignocellulose at low temperature, and have been used as solvents for biomass green conversion.[4] However, ionic liquids are still expensive, and their recovery and reuse is still a challenge. As a kind of ionic liquid-like system, inorganic molten salt hydrates are inexpensive and easily obtained. In addition, they are environmentally friendly due to their nontoxicity and nonvolatility. Interestingly, it is well known that molten salt hydrates have excellent ability to dissolve polysaccharides, such as cellulose and hemicellulose, and can be used as media for their hydrolysis,[5] such as cellulose hydrolysis in acidified LiBr molten salt hydrate media[6] and synthesis of furfurals from biopolymers in molten salt hydrate.[7] However, they showed limited ability to dissolve lignin, which provides an opportunity to quantitate lignin in lignocellulosic biomass, such as acidic lithium bromide trihydrate.[8] The key point of efficient utilization of lignocelluloses is their disassembly by a simple and mild method and then conversion of the relative fractions to valuable building block molecules, intermediates, or materials. So, molten salt hydrates may be a suitable medium for efficient and direct production of lignin and relative monosaccharides in a one-pot process, especially with various molten salt hydrates by recovery and reutilization.

Lignin is a complex aromatic polymer with network structure that consists of three phenylpropane units, p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S), linked through β-O-4 and C–C bonds in different combinations.[9] The ratio of phenylpropane units and the amount of linkages and functional groups are significantly different, depending on plant species.[10] Many researchers have conducted a great deal of experiments to study the isolation and quantitation of lignin from lignocellulose to enhance the utilization of lignin for chemicals and biofuels in the biorefinery.[11] However, the formation of humins from dehydration products of sugars under acidic condition caused lignin overestimation.[12] The key step of isolation of lignin from lignocellulose is the degradation of polysaccharides, especially cellulose. It is well known that cellulose can be dissolved in some inorganic molten salt hydrates, including LiClO4·3H2O, LiI2·2H2O, LiBr·3H2O, LiSCN·2H2O, ZnCl2·3H2O, Ca(NCS)2·3H2O, and a eutectic mixture of NaSCN/KCN/LiSCN·3H2O.[4,13] Among them, zinc chloride hydrate is most frequently used as a medium to dissolve and hydrolyze cellulose, but it can never be used for lignocellulose directly.

Here, we developed a facile and quick one-step method to isolate high-purity lignin and degrade cellulose and hemicellulose to relative monosaccharides from lignocellulosic biomass, and this process is the basis for efficient conversion of lignocellulose to valuable chemicals and materials (Scheme ). In brief, acidic zinc chloride (AZC) hydrate molten salt was used as a solvent, which could swell and dissolve cellulose and hemicellulose quickly and completely. Meanwhile, cellulose and hemicelluloses can be hydrolyzed under acidic conditions and isolate lignin as an insoluble residue (IR). The structure, composition, and purity of lignin were characterized by means of two-dimensional (2D) heteronuclear single quantum correlation (HSQC) NMR spectroscopy, energy-dispersive spectroscopy (EDS), and Fourier transform infrared (FT-IR) spectroscopy. In addition, an environmentally friendly and efficient method to recycle and reuse zinc chloride from filtrate is developed.

Scheme 1

Schematic of the Method for Lignin Quantitation

Results and Discussion

Isolation and Quantitation of Lignin

Zinc chloride hydrate is a general inorganic salt, whose molten salt is an excellent solvent for cellulose and hemicellulose. This is because the zinc cation (Zn2+) and the chloride anion (Cl–) are able to interact with the oxygen atoms (O) and hydrogen atoms (H) of the cellulose hydroxyl groups, respectively, and form hydrogen bonds with cellulose.[14] The cellulose–Zn2+ and cellulose–Cl– interactions will lead to the breakage of the intra- and intermolecular hydrogen bonds of cellulose at elevated temperature. In addition, they could destroy the rigorous crystalline structure of cellulose and make it swollen, which can be dissolved.[5,13,14] Thereby, the glycosidic linkages of the carbohydrates and lignin were exposed fully and cleaved under mild dilute acid conditions. The homogeneous system generated on the basis of dissolution is conducive to facilitate the hydrolysis of hemicelluloses and cellulose. As a result, all kinds of corresponding monosaccharides, such as glucose, xylose, etc., could be obtained. In brief, as shown in Scheme , it provides a one-step method to isolate high-purity lignin and monosaccharides (such as glucose, xylose, mannose) from lignocellulosic biomass using acidic zinc chloride. The obtained lignin and monosaccharides can be used as platform feedstock for efficient biorefinery, and valuable chemicals or materials can be obtained from them, which makes the overall utilization of lignocellulose possible.[10]

Figure shows the photographs of the processes of fast disassembly of lignocellulose (wheat straw) in acidic molten salt of zinc chloride hydrate and isolation of pure lignin and monosaccharides with the recovery of zinc chloride by ammonia. First, a powder of wheat straw and zinc chloride hydrate was mixed homogeneously and then acidic solution (HCl) was added under stirring at 95 °C. The disassembly of straw was quick and formed a black liquid after 12 min, indicating the complete disassembly of lignocellulose. After adding ethyl acetate, some solid formed and the product was separated into solid and liquid by filtration. The solid is insoluble lignin, whereas the liquid contains soluble zinc chloride and monosaccharides degraded from both cellulose and hemicellulose. After decolorization and addition of ammonium hydroxide, plenty of precipitates of zinc compounds formed. After separating and dissolving the precipitates in hydrochloric acid aqueous solution, a clear transparent solution was obtained, which was heated at 400 °C to obtain white zinc chloride, indicating the successful recovery of the salt. In addition, when the filtrate was cooled, plenty of white sugars formed, as shown in Figure . Clearly, wheat straw was quickly disassembled into lignin and sugars and zinc chloride hydrate can be well recovered.

Figure 1

Photographs of lignocellulose (wheat straw) disassembly by acidic zinc chloride hydrate molten salt to lignin and sugars (95 °C, 12 min), with recovery of zinc chloride that can be reused by ammonia.

In addition, under such condition, a small fraction of monosaccharides could be dehydrated to furfural (from pentoses) and 5-hydroxymethylfurfural (HMF, from hexoses). Only a trace amount of the lignin residue could be dissolved and quantitated by UV spectrophotometry.

Factors Affecting Lignin Quantitation Using the AZC Method

Overall, to disassemble lignocellulose, it is vital to remove and hydrolyze cellulose and hemicellulose completely. Although cellulose could be dissolved in zinc chloride molten salt efficiently, its hydrolysis needs Brønsted acid as a catalyst to destroy glycosidic bond. First, acid concentration plays a key role in the efficient and complete hydrolysis of cellulose and hemicellulose. As illustrated in Figure a,b, the yield curves of insoluble wheat straw residue dropped to the lowest value of 18.62 wt % at 1.5 M HCl aqueous solution. When the acid concentration is less than 1.5 M, hemicellulose and cellulose could not be hydrolyzed completely, which can be proved by the low yield of glucose and xylose. However, when the acid concentration reached 2 M, even 3 M, a high total insoluble residue (2 M for 20.15 wt %, 3 M for 40.39 wt %) was observed, resulting from the carbonization of lignocellulose under high acidic concentration, and it facilitated the dehydration of sugars to HMF and furfural and the formation of humins. Reaction temperature is another crucial factor in the efficient and complete hydrolysis of hemicelluloses and cellulose. Cellulose could hardly be hydrolyzed completely at room temperature even with a high concentration of acid and a long reaction time. A positive reduce toward the insoluble residue was observed when the temperature reached 95 °C. However, when temperature was elevated to 115 °C, the local overheating also caused carbonization of biomass and then dehydration of sugars to HMF and/or furfural was accelerated at the high reaction temperature, which probably leads to the formation of more humus under the acidic condition. As shown in Figure c,d, increase of reaction temperature from 75 to 95 °C led to significant reduction of the insoluble residue content from 24.45 to 16.87 wt % and increase of glucose content from 6.5 to 23.88 wt %, indicating that high temperature is conducive to remove more carbohydrates. It is worth reminding that further elevating the temperature to 115 or 125 °C increased the yield of insoluble residue up to 20.34 or 21.17 wt %, which resulted from the carbonization of biomass and the formation of humins. The color of the insoluble precipitate became darker gradually with the extension of reaction time. Meanwhile, the content of xylose and glucose reduced slowly. Therefore, it was important to control the reaction time so that cellulose and hemicellulose were hydrolyzed completely and carbonization or sugar dehydration was avoided. As shown in Figure , the content of insoluble residue decreased sharply to 16.87 wt % in 12 min and then increased slowly up to 20.15 wt % at 20 min and 25.34 wt % at 30 min, suggesting that the majority of cellulose and hemicellulose were removed from the biomass within 12 min. Then, the content of insoluble residue increased slowly, probably because the carbonization of residual carbohydrates and lignin carbohydrate complex interrelating with lignin was resistant to the dissolution and hydrolysis due to interaction with lignin. In addition, the yield of glucose and xylose obviously increased, but long reaction time will cause monosaccharide dehydration and the accumulation of furans and humins with increasing time.

Figure 2

Effects of reaction conditions on yields of insoluble lignin (IL), soluble lignin (SL), glucose, xylose, furfural, and HMF: HCl concentration (a, b), reaction temperature (c, d), and reaction time (e, f) (lignocellulose: wheat straw).

HMF and Furfural Formation from Sugar Dehydration

During the process, a small portion of HMF and furfural (λa = 240 nm) was generated by the dehydration of sugars, which caused interference with lignin quantitation because of their relatively large extinction coefficients overlap of absorbance. Therefore, it is necessary to correct the acid-soluble lignin absorbance at 240 nm according to eq where Abs′ is the corrected absorbance of SL at 240 nm, and AbsT and Absf are the absorbances of the hydrolysate and furfural/HMF at 240 nm, respectively.

Characterization of the AZC Lignin

To prove that the solid residue from lignocellulose is high-purity lignin by the AZC method, the solid residue was characterized by elemental analysis, thermogravimetric analysis (TGA), FT-IR spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM)–EDS, gel permeation chromatography (GPC), and 2D NMR (HSQC) spectroscopy. First of all, the solid residues were collected and characterized by a Thermo Scientific Flash 2000 series elemental analyzer. Thermal behaviors of solid residues were characterized by thermogravimetric analysis (TGA). As shown in Figure , solid residues eventually lose 97.45, 95.24, and 97.39% of their original weight when the temperature increases up to 550 °C in air. When the temperature reaches 350, 400, and 550 °C, the lignin residues of rice husk, wheat straw, and bamboo powder lose their mass bluffly until the pyrolysis was almost completed, respectively. The residual solid was ash, whose main components were silicon and inorganic salt. Further, the solid residues adopted two-step sulfuric acid hydrolysis (the NREL method) to detect the trace of residual carbohydrates. As shown in Table S2, only trace amounts of monosaccharides were detected in the filtrate collected from hydrolysis of different species of biomass, suggesting that carbohydrates could be removed from biomass nearly completely through the AZC method.

Figure 3

TGA (a) and differential thermogravimetry (b) curves of lignin isolated from various lignocelluloses.

Then, the three kinds of lignin residue generated from different feedstock were examined by Fourier transform infrared (FT-IR) spectroscopy with a Bruker vector-2 spectrophotometer using the KBr disk method in the spectral range of 400–4000 cm–1. In the structure of lignin, there are many bonds and functional groups. Although the IR spectrum cannot detect some specific information about each band, it can be used to speculate the structure of lignin and can provide information about its purity. FT-IR spectra of lignin are shown in Figure a, and the band assignments are summarized in Table S3. It can be concluded that the as-prepared lignin belongs to the HSG-type lignin from the information of functional group region. The relatively strong and broad peak at 3470 cm–1 contributes to the hydroxyl group, and the band at 1140 cm–1 is characteristic of herbal lignin, which can be explained by the presence of the S units in lignin. Comparing the three kinds of lignin, it can be found that their spectra were almost identical, implying that their structures are close to each other.[15] The characteristic peaks of 2933 and 2854 cm–1 are attributed to the presence of C–H stretching. Considering the structure of lignin, the peaks in the range of 700–850 cm–1 can be assigned to aromatic C–H. Typically, from 1400 to 1650 cm–1, multiple peaks for C=C stretching in aromatics rings skeleton are observed. The presence of a peak at 1713 cm–1 verifies the C=O stretching in an α,β-unsaturated aldehyde or carboxylic acid. In addition, the peak at 1115 cm–1 for C–O stretching was assigned to alcohols, esters, and ethers. The molecular weight distributions of lignin were determined by means of GPC (N,N-dimethylformamide, DMF) analysis, and the values of the number-average (Mn) and weight-average (Mw) molecular weights were calculated from the GPC curves (related to polystyrene standard). As shown in Figure b, wheat straw lignin (WSL) and bamboo powder lignin (BPL) exhibited similar molecular weights, 10 872 and 9642 g mol–1, respectively, and the rice husk lignin (RHL) has a relatively lower molecular weight, which is summarized in Table , as well as the polydispersity index (Mw/Mn).

Figure 4

FTIR (a) and GPC (b) spectra of lignin isolated from various lignocelluloses.

Table 1

Molecular Weights (Mw and Mn) and Polydispersity Index (Mw/Mn) of Lignin Isolated from Different Kinds of Lignocellulose

	WSL	BPL	RHL
Mw	10 872	9642	6726
Mn	4854	5297	3778
Mw/Mn	2.24	1.82	1.78

Two-dimensional 1H–13C NMR (2D NMR) techniques have been used to estimate the purity, structures, and the main linkages of the as-prepared lignin. Although the Klason lignin is indomitable as it is insoluble in any organic solvent, the lignin isolated from lignocellulosic biomass using acidic zinc chloride was found to be soluble in some organic solvents, such as N,N-dimethylformamide (DMF), formic acid acid/water (98:2, v/v), and dimethyl sulfoxide (DMSO).[16] Therefore, about 50 mg of lignin was dissolved directly in 600 μL of solvent for the 2D NMR characterization. The side chains (δC/δH 50–80/2.5–6.0) and the aromatic regions (δC/δH 100–135/5.5–8.5) of the HSQC spectra of WSL are shown in Figure . It is reported that β-O-4′ (structure A) is the predominant type of linkage in native lignin, with small quantified contents of β-5′ (structure B) and β–β′ (structure C) linkages.[17] According to previous publications, the amounts of β-O-4′ are about 25–40 per 100 units, and β-5′ (structure B) and β–β′ (structure C) account for 2–5 and 7–10 per 100 units in native lignin, respectively. The information of the interunit linkages in lignin can be obtained from the side-chain region (δC/δH 50–90/2.5–6.0) of WSL in the 2D-HSQC NMR spectra. As shown in Figure (left), the most prominent signals correspond to methoxy group at δC/δH 56.13/3.74 ppm and several kinds of β-O-4′ aryl ether linkages. Among these, the Cα–Hα correlations in structures A linking to G and S units were observed at δC/δH 71.66/4.86 and 71.82/5.13 ppm, respectively. Besides, a small signal and a relatively large signal were identified at δC/δH 85.10/4.67 and 86.76/5.62 ppm, respectively, and the Cβ–Hβ correlations corresponding to the S-type β-O-4′ (Aβ(S)) and G-type β-O-4′ (Aβ(G)) substructures at δC/δH 85.9/4.12 and 86.8/3.99 ppm. The Cβ–Hβ correlations of structures B and C were overlapping with the signal of methoxy group, and they can be discovered at δC/δH 53.15/3.80 and 53.18/3.27 ppm, respectively. The Cγ–Hγ correlations in structures A, B, and C can be distinguished at δC/δH 60.65/3.40, 63.12/3.43, and 71.66/4.08, respectively. Another signal region is the aromatic region in Figure (right), which corresponds to the aromatic rings of guaiacyl (G), p-hydroxyphenyl (H), and syringyl (S) units of lignin. The C2,6–H2,6 correlations of S units were prominently observed at δC/δH 103.8/6.71 ppm, and the Cα-oxidized S units (structure S′) showed a relatively weak signal at δC/δH 106.68/7.24 ppm. Different correlations for the C5–H5 and C6–H6 of G units were observed at δC/δH 113.46/6.76 and 115.37/6.66 ppm, respectively, whereas the correlations for the C2–H2 and C6–H6 in α-ketone structures oxidized of G units were observed at δC/δH 111.24/9.39 and 119.66/6.66 ppm, respectively. In addition, the signals for the C2,6–H2,6 and C3,5–H3,5 correlations of substructures H were clearly observed at δC/δH 115.37/6.66 and 128.65/7.07 ppm, respectively, in the aromatic region of the HSQC spectrum of WSL. It should be pointed out that the position of C3,5–H3,5 correlations in H units overlapped with guaiacyl 5-positions.[18] A little fly in the ointment is an unexpected polysaccharide peak that was detected for the β-5 linkage at δC/δH 66.36/4.04 ppm. However, the above observations suggest that the purity of lignin was very high and the traces of carbohydrates were negligible, which scarcely had any effect on lignin quantitation.[15,19]

Figure 5

2D 1H–13C correlation (HSQC) spectra of WSL.

In addition, wheat straw and its lignin residue were examined by means of SEM–EDS. As shown in Figure , the SEM image indicates that the fiber and sheet structures of the original biomass disappeared and the particle size of the lignin residue was less than 10 μm, suggesting that carbohydrates have been removed completely from the original lignocellulose. In addition, only a small zinc peak was observed in the energy-dispersive spectra, indicating that there is less zinc residue. Furthermore, the C/O molar ratio of the lignin residue was 1.83, which was similar to the consequence of elemental analysis. These evidences indicate that lignin with excellent purity was separated and prepared successfully.

Figure 6

SEM images and EDX spectra of wheat straw (a, b) and insoluble lignin (c, d) after acidic molten salt treatment.

During the disassembly of lignocellulose, cellulose and hemicellulose can also be efficiently degraded into relative monosaccharides, such as glucose, xylose, etc., and their yield strongly depends on the kind of lignocellulose. As shown in Table , the maximum yield of glucose can reach up to 40% for bamboo, indicating that it is an efficient method to prepare glucose and xylose, which are the source of various kinds of chemicals and can also be used as nutrition if necessary. For example, they can be used for producing HMF, furfural, hydrocarbon, and alcohols, and lignin can also be used for producing phenols, BTX, dicarboxylic acids, and polymers. During the disassembly processes, high-purity lignin can be obtained with high yields (Table S4), which can be used as feedstock of lignin for further refinery in the near future.

Table 2

Yields of Monosaccharide from Various Lignocelluloses via Acidic Zinc Chloride Molten Salt Treatment

biomass	solvent	rhamnose (%)	xylose (%)	mannose (%)	fructose (%)	glucose (%)
wheat straw	f-ZnCl2	2.98	16.82	1.52	0	26.97
	r-ZnCl2-1	0	13.48	1.68	0	24.46
	r-ZnCl2-2	1.04	14.19	1.77	0	24.80
	r-ZnCl2-3	0	14.03	1.47	0	25.88
bamboo powder	f-ZnCl2	0	9.86	0	0.21	40.56
	r-ZnCl2-1	0	8.87	0	0.13	38.87
	r-ZnCl2-2	0	10.64	0	0.17	40.32
	r-ZnCl2-3	0	9.13	0	0.09	39.64
rice husk	f-ZnCl2	0	5.31	0	0.18	28.15
	r-ZnCl2-1	0	4.89	0	0.07	26.43
	r-ZnCl2-2	0	5.97	0	0.11	27.68
	r-ZnCl2-3	0	6.21	0	0.13	26.75

Recovery and Reuse of Zinc Chloride

There are two methods for ZnCl2 recovery. First, ZnCl2 can be extracted from an aqueous solution with an organic phase and is subsequently stripped into a solution from which ammine chloride can be crystallized. For example, ethylene glycol is employed as an organic phase and then the mixed solution is contacted with gaseous ammonia to crystallize zinc ammine chloride, followed by heating to recover precipitate and remove ammonia. The second method is a simple hydrometallurgical process for producing anhydrous zinc chloride. The zinc ammine chloride precipitation could be easily achieved by the aqueous solution of ammonium hydroxide, which can be obtained from the hydrolysate by filtering or centrifuging to separate carbohydrates and zinc chloride. The structures and purity of the recycled zinc chloride were characterized by acetic acid salt spray test, XRD, and significant phenomenon of deliquescence in the air. As shown in Figure a, the fresh and recycled zinc chloride has the same peaks at 2θ values, indicating a successful recovery of zinc chloride with a high recovery rate (∼86 wt %). Meanwhile, the effect of pH on the solubility of Zn(NH3)2Cl2 has been studied, and as shown in Figure b, when the pH value was increased from 5 to 7, the concentration of Zn2+ significantly reduced from 63 to 10 g L–1 and reached its lowest point (5 g L–1) at pH = 7.5. But it began to increase at pH = 8, indicating that excess ammonium hydroxide will lead to the dissolution of zinc ammine chloride precipitates. Therefore, the optimum condition to precipitate zinc ions as complete as possible is adding ammonium hydroxide until pH = 7.5. Zinc chloride could be recycled successfully based on these studies so that the zinc ion pollution is avoided and a green chemical circle is achieved. Interestingly, the recycled zinc chloride can be reused for disassembly of fresh lignocellulose.

Figure 7

XRD spectra of fresh and recycled ZnCl2 (a) and effect of pH on solubility of Zn(NH3)2Cl2 (b).

Conclusions

Here, we have developed a facile one-step method to isolate lignin from lignocellulosic biomass. Acidic zinc chloride molten salt was used as a reaction medium, which could dissolve and hydrolyze hemicellulose and cellulose quickly and completely, but not lignin. The optimal reaction conditions for this method were zinc chloride molten salt with 1.5 M HCl at 110 °C for 12 min, which ensured the complete dissolution and hydrolysis of hemicellulose and cellulose, prevented the carbonization of biomass, and reduced the formation of humus to avoid the overestimation of lignin. The procedure can be finished within 12 min, which is much shorter than the NREL method (3–6 h). In addition, this method was conducted under mild conditions. The reaction medium (acidic zinc chloride) can be easily recycled with no hazardous heavy-metal pollution of zinc ions. Furthermore, the method could be applied to different species of lignocellulose, including wheat straw, bamboo powder, and rice husk, and high-purity lignin can be obtained with high yield of glucose and xylose, which provides a key step to disassemble lignocellulose for green overall utilization in the near future.

Experimental Section

Materials

Lignocellulosic biomass, including rice hull, bamboo powder, and wheat straw, were obtained from Shandong, Anhui, and Shanxi provinces, respectively, and their elemental contents are listed in Table S1. Deionized water with resistivity of 18 MΩ·cm was produced from Milli-Q water (Millipore). Zinc chloride (ZnCl2·4H2O, 98%) was purchased from Sinopharm Chemical Reagent Co. Ltd. In addition, analytical-grade H2SO4 (98%) and hydrochloric acid (37%) aqueous solutions were purchased from Shanghai Chemical Reagents Company and used directly without further purification. Other reagents, such as chromatographic acetonitrile (99.99%), furfural (99%), and 5-hydroxymethylfurfural (HMF, 98%), were purchased from Aladdin Chemical Reagent Co. Ltd.

Biomass Sample Preparation

Lignocellulosic biomass, such as rice hull, bamboo powder, and wheat straw, were washed, dried, and smashed, and biomass powders between 40 and 80 mesh were collected.

Lignin Isolation by the AZC Method

Lignocellulosic biomass powder (1.5 g, weighed to the nearest 0.1 mg) was mixed with solid zinc chloride and stirred completely in a 50 mL flask. Then, it was heated in an oil bath at 95 °C for 10–20 min under magnetic stirring (400 rpm). Simultaneously, 5 mL of hydrochloric acid (1.5 mol L–1) should be dropped slowly over scheduled time. At the end of dropping, the flask was kept sealed until the solid mixture was immersed into the liquid completely. The mixture was agitated using a magnetic stirring bar (400 rpm) at the preset reaction time. The reaction was quenched by cooling the reactor by ice–water mixture. The native lignin was extracted from the homogeneous solution through liquid–liquid extraction using ethyl acetate. The inorganic solution was collected for the subsequent analysis of sugars and their decomposition products by means of high-performance liquid chromatography (HPLC) within 6 h. All of the organic mixture was filtered under reduced pressure. The solid residues were dried at 85 °C overnight and then gravimetrically quantitated. Hemicellulose and cellulose were dissolved and hydrolyzed from solid residues, leaving lignin as an insoluble residue to be quantitated gravimetrically. The insoluble residue (IR) was calculated using eq , and the insoluble lignin (IL) was calculated using eq , which is quantitated by removing the ash. Soluble lignin (SL) was quantitated using eq , according to the Beer–Lambert law on a UV–visible spectrophotometer.where ms, mc+r, mc, and ma are the oven dry weight of sample, weight of the crucible with the insoluble solid residue, the crucible weight, and ash weight, respectively; Abs is the average UV–vis absorbance for the sample at the appropriate wavelength; Vf is the volume of filtrate; δ = (volume of sample + volume of diluting solvent)/volume of sample; ε = absorptivity of biomass at specific wavelength (25 g–1 L cm–1 for woody biomass at 240 nm and 30 g–1 L cm–1 for herbaceous biomass at 320 nm, adopted from the NREL method); and l = path length of the UV–vis cell in centimeters (1.0 cm).

Preparation of Monosaccharide and Recycling of Zinc Chloride

The filtrate obtained from the procedure of lignin isolation was collected, and some activated carbon was added to decolorize it. Then, the transparent filtrate was collected and ammonium hydroxide aqueous solution was added to form precipitate of zinc chloride ammine by controlling pH from 7 to 8. The clear transparent filtrate can be obtained through separating the filtrate and precipitate. The mixture of monosaccharides can be crystallized from the filtrate by standing it for a period of time. On the other hand, the white precipitate is extremely stable in air atmosphere and can be easily dissolved by adding hydrochloric acid aqueous solution. Anhydrous zinc chloride could be generated by heating in a tube furnace at 400 °C for 90 min under nitrogen atmosphere.

Klason Lignin Quantitation by the NREL Method

Quantitation of lignin by the two-stage sulfuric acid hydrolysis was conducted following the NREL standard protocol. In brief, 0.3 g of biomass (weighed to the nearest 0.1 mg) was treated in 72% H2SO4 at 30 °C for 60 min. The slurry was diluted to 4% H2SO4 and autoclaved at 121 °C for 60 min. After filtration, the acid-insoluble lignin and the acid-soluble lignin were quantitated gravimetrically and spectrophotometrically, respectively.

Chromatographic Analysis

Monosaccharides in the filtrates were quantitatively analyzed by means of high-performance ion chromatography (HPLC) on a SHIMADZU-GL system equipped with a refractive index detector and a general 1 M NH2 column (4.6 × 250 mm2) at 40 °C. The mixed solution of acetonitrile and deionized water (72:28, v/v) was used as the mobile phase at a flow rate of 1.0 mL min–1. The yields of monosaccharides were calculated from the carbon mole ratio after normalizing the monitored concentration of products. The dehydration products (HMF and furfural) of the monosaccharides were determined using HPLC with a Wonda Cr act ODS-2 column (4.6 × 250 mm2) at 60 °C and an UV–vis detector at 210 nm. The solution of 0.1% phosphoric acid was used as the mobile phase at 1.0 mL min–1. The molecular weights of the three lignin preparations were determined by gel permeation chromatography (GPC) on a SHIMADZU-GL system equipped with a refractive index detector and a Shodex GPC KD-804 column at 60 °C. The lignin was dissolved in N,N-dimethylformamide (DMF) (∼10 mg mL–1) and filtered through a 0.45 μm nylon membrane. DMF was used as the mobile phase at a flow rate of 1.0 mL min–1. The number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (Mw/Mn) were calculated using monodispersed polystyrene standards.

NMR Analysis

The NMR spectra were recorded on a Bruker AVIII 400 MHz spectrometer equipped with a DCH cryoprobe using dimethyl sulfoxide-d6 (DMSO-d6) as the solvent. For the 1H–13C heteronuclear single quantum correlation (HSQC) NMR spectroscopy, around 50 mg of lignin was dissolved in 0.5 mL of DMSO-d6. The spectral widths were 12 ppm (from 11 to −1 ppm) and 220 ppm (from 200 to −20 ppm) for the 1H and 13C dimensions, respectively. The peak of solvent (DMSO-d6) was used as a reference point of internal chemical shift. Data of spectra were processed using the standard Bruker Topspin-NMR software.

SEM–EDS Analysis

The morphology and elemental distribution of the lignin residue were observed and detected by a field emission scanning electron microscope (Leo Co., Oberkochen, Germany) coupled with an energy-dispersive spectrometer. Lignin residue was tightly fixed on the surface of the conductive tape stuck to the aluminum mount and sputter-coated with a thin layer of platinum before microscopic observation. The accelerating voltage was 10.0 kV for morphology observation and 15.0 kV for EDS analysis.

Other Analysis

The elementary compositions of the lignin residue were recorded by CHONS elemental analysis using the Elementar vario EL cube at 1150 °C. The chemical structure of lignin was examined by means of Fourier transform infrared (FT-IR) spectroscopy with a Bruker vector-2 spectrophotometer (Germany) using potassium bromide (KBr) assay method in the spectral range of 400–4000 cm–1. The thermal stability of the lignin residue was recorded by thermogravimetric analysis (TGA).