From the perspective of bio-refinery, sulfurous acid (H2SO3) treatment of lignocellulosic biomass is attractive because of its ability to act both as an acid catalyst and as a sulfonation agent. Therefore, its capability to hydrolyze polysaccharides (especially glucan) into monosaccharides was compared with two other acids, hydrochloric and sulfuric acids. The decomposition of methyl α-D-glucopyranoside (MGPα) in these three acids, hydrochloric, sulfuric, and sulfurous acids were studied. In addition, p-creosol and vanillyl alcohol were introduced to check whether it is possible to convert polysaccharides (such as hemicelluloses) into monosaccharides during the sulfurous acid treatment. The results showed that the decomposition of MGPα is much slower in H2SO3 than in HCl and H2SO4. The ligninsulfonic acid resulting from the lignin sulfonation reaction can be expected to improve the efficiency of hydrolysis of polysaccharides into monosaccharides during sulfurous acid treatment. Moreover, a higher actual yield of glucose was obtained in this case than in the other two acids.
Decomposition Study of Methyl α-D-Glucopyranoside (MGPα) and Lignin Model Compounds for better Glucose Yield during Sulfurous Acid Treatment
Yan Shi,a,* Jinglei Xie,b Jinbao Kou,a Rui Kong,a Nan Sun,a and Miaoli Bai
From the perspective of bio-refinery, sulfurous acid (H2SO3) treatment of lignocellulosic biomass is attractive because of its ability to act both as an acid catalyst and as a sulfonation agent. Therefore, its capability to hydrolyze polysaccharides (especially glucan) into monosaccharides was compared with two other acids, hydrochloric and sulfuric acids. The decomposition of methyl α-d-glucopyranoside (MGPα) in these three acids, hydrochloric, sulfuric, and sulfurous acids were studied. In addition, p-creosol and vanillyl alcohol were introduced to check whether it is possible to convert polysaccharides (such as hemicelluloses) into monosaccharides during the sulfurous acid treatment. The results showed that the decomposition of MGPα is much slower in H2SO3 than in HCl and H2SO4. The ligninsulfonic acid resulting from the lignin sulfonation reaction can be expected to improve the efficiency of hydrolysis of polysaccharides into monosaccharides during sulfurous acid treatment. Moreover, a higher actual yield of glucose was obtained in this case than in the other two acids.
Keywords: Methyl α-d-glucopyranoside (MGPα); Sulfurous acid hydrolysis; Monosaccharide; Hemicelluloses; Cresol; Vanillyl alcohol
Contact information: a: Department of Packaging Engineering, Tianjin University of Commerce, Tianjin 300134, People’s Republic of China; b: Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, People’s Republic of China;
* Corresponding author: email@example.com
The acid hydrolysis of glycosidic bonds during the conversion of wood polysaccharides into monosaccharides is of importance in many technical processes (Sjostrom 1993). The efficiency of the hydrolysis reaction is closely related to the recovery yield of each monosaccharide. It has been reported that hemicelluloses in lignocellulosic biomass are first hydrolyzed by acids such as HCl, H2SO4, or H2SO3 into monosaccharides (Taherzadeh and Karimi 2007). Subsequently the fermentation process was employed to convert the monosaccharides into useful compounds, such as ethanol and others (Tanifuji et al. 2011; Gaur et al. 2016; Kapoor et al. 2016). As catalyst, acid catalyzes not only the hydrolysis of polysaccharides into monosaccharides, but also further degradation reactions of monosaccharides into furans and carboxylic acid. Thus the yield of monosaccharides from hemicelluloses during acid treatment can be affected by both the efficiency of polysaccharide hydrolysis and the preservation of monosaccharides produced. Many researchers have shown that sulfurous acid treatment seems to be beneficial to bio-refinery for hemicelluloses processing (Zhu et al. 2009; Yu et al. 2010; Shi et al. 2012; Shi 2014). In a way, sulfurous acid treatment of lignocellulosic biomass is attractive because it not only removes lignin effectively but also hydrolyzes hemicelluloses to monosaccharides (Takahashi et al. 2010; Shi et al. 2013). Usually the temperatures used in acid hydrolysis of lignocellulosic materials are between 110 °C to 160 °C. Previous results by the authors suggested that sulfurous acid is a weak acid to hydrolyze hemicelluloses compared with H2SO4and HCl, but the hydrolysis of cellulose in sulfurous acid treatment at 140 °C is not more severe than that in H2SO4 and HCl treatments at 120 °C (Shi et al. 2012), indicating that the use of sulfurous acid retains the yield of cellulose, while the hemicelluloses of wood are equally well hydrolyzed to monosaccharides in comparable yield with strong mineral acids such as HCl and H2SO4. Such sulfurous acid treatment is well suited to biorefinery operation of an existing sulfite-based pulping facility for the recovery of hemicelluloses-derived monosaccharides, especially xylose and glucose. Therefore, in order to understand the difference in abilities to hydrolyze polysaccharides (especially glucan) into monosaccharides using H2SO3 and two other acids, MGPα was selected as the carbohydrate model compound (Bragd et al. 2000; Ciriminna et al. 2000; Nakagawa et al. 2015). Creosol and vanillyl alcohol as lignin model compounds (Carvalho et al. 1998; Nie et al. 2014) were employed to check their effect on glucose yield during acid hydrolysis. The molecular structures of three models are shown in Fig. 1.
Fig. 1. Molecular structures of three model compounds
It was presumed that one of the lignin model compounds, the creosol, would not react with HSO3– ion but vanillyl alcohol would react with HSO3– to form vanillyl alcohol sulfonic acid via the carbonium ion intermediate in acidic solution as shown in Fig. 2.
Fig. 2. Reaction of vanillyl alcohol in sulfurous acid
The adducts thus produced from the reaction with HSO3– ion may affect the glucose yield from hemicelluloses or cellulose of wood after sulfurous acid treatment.
All monosaccharides including d-glucose, d-xylose, l-arabinose, d-mannose, and d-galactose, sodium borohydride (NaBH4), sodium bisulfite (NaHSO3), and other reagents were obtained from Wako Pure Chemical Industries Ltd. (Japan). The monosaccharides were used without further purification. Methyl α-d-glucopyranoside (MGPα) was purchased from Tokyo Chemical Industry Co. Ltd. (>98.0%). 2-Methoxy-4-methylphenol (Cresol (98%)) was supplied by Hubei Xin Ming Tai Chemical Co. Ltd. (China).
The following instruments were used for performing the experiments and analysis:
(1) Teflon inner tube sealed container (TAF-SR type; total volume of Teflon inner tube: 50 mL; maximum working pressure: 10 MPa; maximum working temperature: 180 °C; and seal O-ring: Fluorinated gum) from Taiatsu Techno Corp., Japan)
(2) Oil bath with shaker RSO-200TE (Riko, Japan)
(3) Gas chromatograph (GC-14B, Shimadzu, Japan)
Three acid solutions of the same pH containing the same amount of MGPα were prepared, as shown in Scheme 1. It was assumed that commercially available SO2 solution is 5% with pH = 0.93 (25.9 °C). Therefore, before heat treatment in oil bather shaker, the initial pH of the reaction mixture was determined to be more than 0.93. Similarly HCl, pH = 1.04 (25.7 °C); H2SO4, pH = 1.02 (25.9 °C); and 4% solution of SO2, pH = 1.01, respectively. Approximately 4 mL of the diluted solution was put in a reactor and subjected to heat treatment at 140 °C. Acetylation reaction was then conducted with the addition of sodium acetate (CH3COONa) and acetic acid at 120 °C for 3 h.
Scheme 1. Preparation of MGPα in three acids with the same pH
Preparation of monosaccharides in sulfurous acid containing lignin model compound
The experimental procedure is shown in Scheme 2. To begin with, 0.5 mol/L acetone solutions of cresol and vanillyl alcohol were prepared. About 4 mL of this solution was taken in the reactor and blown dried with nitrogen gas. Then 5 mL of monosaccharides solution in sulfurous was added, and subjected to heat treatment at 140 °C.
Scheme 2. Preparation of monosaccharides in sulfurous acid with and without addition of base
GC-Analysis of monosaccharide yield by alditol-acetate method (Borchardt & Piper 1970)
The resultant reaction mixture from heat treatment was allowed cool. An internal standard (inositol, 3 mmol/L), was then added. The reaction mixtures from acid treatments was neutralized to pH 5.5 with NaOH solution for HCl treatment and Ba(OH)2 solution for sulfuric acid treatment (for the neutralization of H2SO3, see next section). The supernatant liquid of neutralized samples were reduced by NaBH4 for 24 h. The excess NaBH4 present in the reaction medium was quenched by the addition of acetic acid, and the borate was then removed by repeated methanol addition and followed by evaporation. The alditols thus produced were converted into acetates by the addition of acetic anhydride. The gas chromatographic analysis was performed (GC-14B, Shimadzu, Japan) under the following conditions: Column used – TC-17, 30 m × 0.25 mm; Column temperature, 220 °C; Injection temperature, 220 °C; Detector temperature, 230 °C.
Determination of monosaccharides from sulfurous acid treatment
The resultant reaction mixture was exposed to the same conditions outlined in Scheme 3 for estimating the monosaccharide from sulfurous acid treatment. As described earlier, after the addition of the inositol (internal standard), the reaction mixture was further acidified to pH 1 by the addition of dilute sulfuric acid solution, and then kept under reduced pressure for about 1 h to eliminate SO2 from the medium. After this step, the reaction mixture was neutralized to pH 5.5 with Ba(OH)2 solution. The supernatant liquid was converted into alditol acetates in the same manner as described previously and subjected to GC analysis.
Scheme 3. Sample preparation to determine monosaccharides after sulfurous acid treatment
RESULTS AND DISCUSSION
Calibration Curve of α-D-Glucopyranoside (MGPα)
A standard solution containing 50 mmol/L of MGPα and 3 mmol/L of inositol was prepared for calibration. The calibration curve for α-d-glucopyranoside is shown in Fig. 3. Thereafter, the actual glucose yield resulted from the decomposition of MGPα at 140 °C was determined by GC analysis, and specific calculation methods are as follows:
where, Wactual glucose is the weight of actual glucose from decomposition of MGPα, calculated based on calibration curve yglucose ; W MGPα is the weight of remained MGPα, calculated based on calibration curve y MGPα; Woriginal MGPα is the weight of MGPα added to Teflon inner tube sealed container; yglucose= 1.0042 xglucose + 0.0503 (here, x is the area ratio of glucose to inositol, y is the Molar ratio of glucose to inositol), y MGPα=1.1171 xMGPα (here, x is area ratio of MGPα to inositol, y is the Molar ratio of MGPα to inositol )
Fig. 3. Calibration curve prepared for α- d-glucopyranoside (MGPα)
Decomposition of Methyl α-D-Glucopyranoside (MGPα) in HCl, H2SO4, and H2SO3Solutions
Decomposition of MGPα was investigated at 140 °C, and the results showed that the decomposition of MGPα was much slower in SO2 aqueous solution than in HCl and H2SO4 (as shown in Fig. 4) solutions. The yield of glucose formed in SO2 aqueous solution was almost the same as that formed in HCl and H2SO4 solutions, nevertheless at the expense of treatment time. At short treatment times (less than 15 min), the yield of glucose formed in SO2 aqueous solution was about the half of that formed in HCl and in H2SO4 solutions. Although the hydrolyzing efficiency of MGPα to form glucose was lower in H2SO3 than by the other two acids, the preservation of glucose that are produced seemed to be better in H2SO3 than in the other two acids after full degradation of MGPα.
When woody biomass was hydrolyzed by H2SO3, the ligninsulfonic acid formed from lignin sulfonation reaction can be expected to improve the hydrolyzing efficiency of polysaccharides into monosaccharides in H2SO3. The decomposition of MGPα in H2SO3 with vanillyl alcohol at 140 °C, 15 min was also investigated.
As shown in Table 1, in the presence of vanillyl alcohol in H2SO3, the hydrolyzing efficiency of MGPα into glucose was considerably improved, moreover with higher actual yield of glucose than in other two acids. However, the presence of cresol in H2SO3, the hydrolyzing efficiency of MGPα into glucose was not distinctly enhanced but there is a slight increase in actual yield of glucose.
Table 1. Comparison of Decomposition of MGPα and Formation of Glucose at 15 min in Various Acidic Systems
Effect of Formation of α-Sulfonic Acid on Degradation of Monosaccharides in Sulfurous Acid
Introduction of base to SO2 aqueous solution leads to the formation of more HSO3– ions. In the presence of p-vanillyl alcohol (or cresol), the degradation experiments of monosaccharides in sulfurous with and without base were investigated at 140 °C. Para-Vanillyl alcohol as lignin model compound reacted with HSO3– ions to produce vanillyl alcohol sulfonic acid.
Fig. 5. Effect of lignin model compound on degradation of monosaccharides in 4% SO2solution with and without base at 140 °C (■ l-arabinose; ●d-xylose; ▲d-mannose ;▼d-glucose; ◆d- galactose)
As shown in Fig. 5, the degradation of each monosaccharide in sulfurous system without base and with base was lower in the absence of p-vanillyl alcohol than in the presence of p-vanillyl alcohol. Because in acidic condition, the decrease in recovery yield may be due to the formation of vanillyl alcohol-sulfonic acid, which lowered the pH of system and accelerated the degradation of each monosaccharide in sulfurous system because of the reaction between benzyl cation and hydroxyl group in monosaccharides. However, when base was added, sometimes the recovery yield of xylose and mannose were higher in the sulfurous system with base and with vanillyl alcohol than in the sulfurous system without base and without vanillyl alcohol. The addition of base seemed to prevent the degradation of arabinose, xylose, and mannose. Differently, the degradation of glucose and galactose was faster in sulfurous system with base than in sulfurous system without base. In the presence of p-vanillyl alcohol, mannose was most stable in sulfurous system with base. This showed that the decomposition of bisulfite adducts was slower from mannose than from other monosaccharides, suggesting that the binding power of SO2 to the monosaccharide was the highest. This result was in agreement with the findings by other researchers (Voegele et al. 2002).
- Decomposition of MGPα was much slower in SO2 aqueous solution than in HCl and H2SO4 solutions. The yield of glucose formed in SO2 aqueous solution was almost the same as that formed in HCl and H2SO4 solutions, nevertheless at the expenses of treatment time. Monosaccharides degraded at different rates in different acids even when the molar concentrations of the various acids were kept constant.
- Lignin-sulfonic acid produced in H2SO3 improved the hydrolyzing efficiency of polysaccharides into monosaccharides in H2SO3.
- Although the hydrolyzing efficiency of MGPα into glucose was lower in H2SO3 than in the other two acids, the preservation of glucose seemed to be better in H2SO3 than in the other two acids after the full degradation of MGPα during H2SO3 treatment.
We would like to sincerely thank the Japanese Government for providing the Japanese Monbu-Kagakusho scholarship which enabled this research at the Wood Chemistry Lab of Graduate School of Agricultural and Life Sciences in the University of Tokyo. Part work of this study was supported by the National Natural Science Foundation for Young Scientists of China (Grant No.51403156) and the Tianjin Research Program of Application Foundation and Advanced Technology (Grant No. 13JCYBJC38800).
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Article submitted: March 13, 2017; Peer review completed: May 22, 2017; Revised version received and accepted: May 31, 2017; Published: June 14, 2017.