Effect of hot-water extraction of sugar maple on organosolv delignification and lignin recovery

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1 Journal of Bioresources and Bioproducts. 2016, 1(1): ORIGINAL PAPER Effect of hot-water extraction of sugar maple on organosolv delignification and lignin recovery Chen Gonga,b* and Biljana Bujanovica a) Department of Paper and Bioprocess Engineering SUNY-ESF, Syracuse, NY b) China National Pulp and Paper Research Institute, Beijing, China *Correspondence author: Chen Gong ABSTRACT: This paper reported a gradual disassembly of the chemical components of hardwood, starting with hot-water extraction (HWE) for the removal of hemicelluloses, followed by organosolv delignification to remove the lignin. Under mild acid conditions, in addition to hemicelluloses, lower molecular weight lignin fractions were removed (~15% of the total lignin) in the HWE pre-treatment; also, the cleavage of the acid-labile lignin-carbohydrate bond took place to some extent. As a result, the HWE pretreatment promoted the subsequent delignification process and facilitated the lignin recovery from the spent liquor, in terms of higher delignification efficiency and higher purity of the lignin recovered from the spent liquor. The effects of the HWE pre-treatment prior to the delignification process were investigated in this study for both the oxygen-pressurized acetone-water (AWO) and the ALCELL processes, with focuses on the delignification efficiency and the properties of the lignin recovered from the process spent liquor. Key words: Hot water extraction; wood; Organosol; Delignification; Lignin recovery; Sugar maple; Acetone; ALCELL 1. INTRODUCTION With the increasing demand for resources and energy, it has been realized that we need to find alternatives to reduce our dependency on petroleum before it is exhausted, and to explore efficient approaches to produce chemicals, energy and materials to fulfill the needs of the world. For this purpose, a hardwood-based biorefinery was developed in our previous study. The idea was to fully utilize the woody biomass (especially for hardwood species) by efficiently recovering all lignocellulosic constituents, including hemicelluloses, lignin, and cellulose, to achieve an integrated lignocellulosic-based biorefinery. As shown in Figure 1, hot-water extraction (HWE) was adopted as the first step of biorefinery. Without any chemical input but using just water, this autohydrolysis process is very preferable for no need for chemical regeneration or disposal cost.1 More importantly, the advantageous effects were shown with hot-water extracted wood chips. In kraft pulping, half less of H-factor was sufficient to produce pulp of similar kappa number with the extracted chips than with regular woodchips, which presumably shortened the reaction time and reduced the chemical usage.2 Organosolv processes with HWE and byproducts from hardwood biorefinery, such as peracetic acid, tetrahydrofurfural alcohol, acetone and ethanol, have been investigated in our previous studies using sugar maple (Acer saccharum) as a model species for the potential of integration of biorefinery. Among several investigated organosolv processes, the oxygen-pressurized acetonewater process (AWO) exhibited the highest efficiency of delignification when performed on hot-water extracted sugar maple (~96% of lignin removed from extracted wood and retaining most of the carbohydrates).3 In this study, the AWO process was compared with a well-developed organosolv process in industry, the ALCELL process (ethanol/water), to further evaluate the profitability of the AWO process. Furthermore, due to the potential cleavage of lignin-lignin bonds and lignincarbohydrates bonds during the pretreatment, the recovered organosolv lignin from the extracted wood is expected to be of higher purity and lower polydispersity than nonextracted one. Therefore, the benefits of the inclusion of HWE on these two processes will be examined. The experiments will focus on evaluating the efficiency of delignification and the recovery of lignin extracted from sugar maple with/without HWE pretreatment during the AWO and ALCELL delignification procedures. 2. MATERIALS AND METHODS 2.1 Materials Sugar maple (Acer saccharum) (SM) wood was supplied from the SUNY-ESF Lafayette Road Experimental Station in Syracuse, NY, USA. The SM was chipped on a Carthage chipper and screened using a Williams Classifier with 3/8", 5/8", and 7/8" accepts. Screened chips were stored at 8 C. Hot water extraction (HWE): SM woodchips were extracted using hot water (160 C; time to temperature: 45 min) for two hours, at a L/W ratio of 4 to 1 with 500 g OD wood in an M/K digester. After the HWE, woodchips were washed twice with water at 80 C for 15 min, and the yield of hot-water extracted wood chips (ESM) accounted for ~77% yield of OD wood. For the delignification experiments, the un-extracted and 22

2 hot-water extracted wood chips were milled with a Wiley Mill to 30-mesh before use. AWO and ALCELL delignification processes were carried out in a 300 ml Parr reactor (4560 Mini bench top reactor) using exactly the same conditions for both the SM and ESM wood mill samples. The AWO process conditions were: acetone to water : 6:4, v/v; time to maximal temperature (Tmax): 60 minutes; Tmax: 150 C; time at Tmax: 120 minutes; wood mill concentration: 2%; oxygen pressure: 1.38 MPa (200 psi). 4 The ALCELL process conditions were: ethanol to water: 6:4, v/v; time to maximal temperature (Tmax): 30 minutes; Tmax: 195 C; time at Tmax: 120 minutes; wood mill concentration: 2%. 5 The digester yields (% of the starting material) and residual lignin contents (% of the delignified wood) were determined. The corresponding levels of delignification degree and delignification selectivity were calculated. In the present study, delignification degree refers to the amount of lignin removed, expressed in percentages of the amount of lignin present in wood before delignification. The delignification selectivity expressed as the ratio of the amounts of lignin and carbohydrates removed, expressed in percentages of the original amount of lignin and carbohydrates, respectively; a larger value indicates a higher level of selectivity toward delignification than toward the removal of carbohydrates. Figure 1. A proposed integrated Biorefinery based on hot-water extraction of hardwood. 2.2 Procedures for Isolating Lignin from the AWO and ALCELL spent liquors The AWO spent liquor was evaporated under reduced pressure in a rotary evaporator (Buchi, Rotavapor R-210, Switzerland) to remove acetone. The ph of the resulting aqueous liquor was reduced to 2 with 20 % sulfuric acid. Then lignin precipitate was collected by centrifugation and vacuum-dried and designated as precipitate L 1. The supernatant was collected for further treatment. In contrast, the ALCELL lignin was precipitated via diluting spent liquor by adding distilled water (three times the volume of the spent liquor) and collected by centrifugation, and vacuum-dried as organosolv lignin (designated in this work as L 1 ). 6 Both supernatants from the AWO and ALCELL spent liquors were sequentially extracted with chloroform and ethyl acetate. Based on these experiments, the yield of low molecular weight lignin degradation compounds (chloroform extract: L 2 ; ethyl acetate extract: L 3 ) was determined Chemical Characterization of Wood, Hot-Water 23

3 Extracted Wood, Pulp, and Lignin Before lignin analysis in accordance with a modified Klason lignin method, 8 native wood was extracted with acetone/water (9:1, v/v; AW), whereas hot-water extracted wood, pulp, and lignin samples were extracted with DCM. After hot-water extraction, lignin becomes soluble in AW, 3 and therefore DCM was used instead for the extraction of the hot-water extracted wood to prevent loss of lignin prior to the lignin determination. The Klason and acid-soluble lignin contents were tested with the extracted samples. 2.4 Characterization of Lignin by SEC The lignin was purified and then acetylated before characterization following the procedure described in literature. 9 The isolated precipitate was purified by dissolving ~500 mg into 10 ml of acetone and then filtered through a 0.45 μm syringe filter to remove insoluble impurities. The acetone was removed by evaporation, and the purified lignin was dried in a vacuum oven. Purified lignin (~50 mg) was dissolved in 2 ml of pyridine-acetic anhydride (1:1, v/v) and kept in the dark at room temperature for 24 h. The reaction was stopped by transferring the sample into a container with a large quantity of ice and water mixture. The precipitate was filtrated to remove the pyridine. The precipitated lignin acetate was dried in a vacuum oven. The molecular weight distribution was determined using size exclusion chromatography (SEC). The columns used were Waters Styragel HR 0.5, HR 3 and HR 4E. The solvent used was THF and the concentration of the sample was 1 mg/ml. The dissolved samples were then filtered through 0.45μm PVDF filters. The detection was performed with UV spectrophotometry at 280 nm. The flow rate was 0.8 ml/min. Polystyrene standards (266 to 2,520,000 Da molar mass at the peak maximum) were used for calibration and a 3rd order polynomial equation was used for quantification. 10 The control for these experiments was milled wood lignin (MWL) isolated from ESM at the PBE Department in SUNY-ESF. 11 This MWL was isolated at the yield of more than 30 % of the total lignin in wood, which suggests it was a good representative of the lignin in wood RESULTS AND DISCUSSION 3.1 Effect of HWE Pre-treatment on Organosolv Delignification The delignification results of two processes on sugar maple (SM) and extracted sugar maple (ESM) are shown in Table 1. For the un-extracted SM, the ALCELL process exhibited superior delignification, with more than 80% of the lignin removed from original wood. The delignification degree of the ALCELL process was 1.4 times higher than the AWO process. However, the AWO process had a relatively high selectivity for the SM wood than the ALCELL process (2.9 vs. 2.2). For the ESM wood, the AWO process demonstrated higher delignification efficiency due to the effect of HWE treatment. This beneficial effect of HWE on the delignification in the AWO process was consistent with previous findings, attributed to an increase of porosity in wood, 13 a lower content of acetyl groups, 1 a higher content of PhOH groups, 8 and more likely, a weaker association between lignin and carbohydrates. 14 However, these beneficial effects of HWE may not be so advantageous in the ALCELL delignification process, and the possible reasons can be explained as follows. Table 1 AWO vs. ALCELL delignification results Delignification Sample Digester yield, % Residual Lignin, % Delignification, % Method Klason AS a Total DD b DS c AWO SM ESM ALCELL SM ESM a- Acid soluble lignin, % OD delignified wood; b- Lignin removed, % the total lignin, delignification degree is calculated based on the lignin content measured after different types of pre-extraction: for SM, pre-extraction with acetone/h 2 O (9:1) the lignin content (21.9 %); for ESM, pre-extraction with DCM, the lignin content (25.9 %); c- Based on the % of the total lignin removed divided by the % of the total carbohydrates removed; According to McDonough 15, the major cleavage of lignin-lignin bonds occurs in benzyl ether position during the ALCELL process (shown in Figure 2). However, the cleavage of aryl ether bonds has also been proposed (including α-o-4 and possible β-o-4) during autohydrolysis, 16,17 as indicated with a significantly higher PhOH content in the remaining lignin. 18 As a result, the benzyl ether bonds are either absent or in short supply in hot-water extracted wood. Meanwhile, the acidity provided by deacetylation of hemicelluloses can enhance the delignification efficiency in the ALCELL process. Due to the predominant removal of hemicelluloses during HWE, the acidity is also deficient in hot-water extracted wood. 19 While for the AWO delignification process, it has been found that the rate-determining step is oxygen transfer. 20 Oxygen may cause the cleavage of intra-molecular linkages (especially for the lignin with free phenolic hydroxyl groups, PhOH), generating muconic acid, which then 24

4 facilitates the dissolution of lignin (Figure 4). 4 Meanwhile, with the assistance of oxygen, the cleavage of Cα-Cβ bond may also occur, leading to the depolymerization of lignin (Figure 3). 4 As HWE prompted partial cleavage of lignin, an increased amount of PhOH content was found in the extracted wood. 3 Therefore, the AWO process was more effective with the ESM wood. Fig.2 The proposed delignification mechanism for the ALCELL process. 15 Fig.3 The proposed delignification mechanism for the ALCELL process

5 Fig.4 The proposed delignification mechanism in the AWO process. 4 Even though the HWE treatment did not improve the ALCELL delignification rate (the difference in the delignification degree between SM and ESM was minor), it resulted in a higher selectivity. Thus, the HWE led to higher retention of polysaccharides during the ALCELL delignification process due to higher selectivity, but it did not affect the amount of lignin removal. 3.2 Effect of HWE Pre-treatment on Lignin recovery Lignin was isolated from the spent liquor of the two processes for both the extracted wood and un-extracted wood to study the effect of HWE on the subsequent organosolv delignification process. The results are shown in Figure 5. Based on the mass of original wood (left y axis as shown in Figure 5), a higher amount of precipitate (higher molecular weight fraction, HMF) and a higher amount of the low-molecular weight lignin degradation products (LMW) were isolated from the AWO spent liquor of the extracted wood, compared with those for the un-extracted wood. These results correlate well with the delignification results observed in Table 1, indicating that an increased efficiency for the AWO process with the HWE extracted wood, which in turn led to a higher recovery yield of lignin from the spent liquor. Different from the AWO process, a slightly higher amount of a HMW fraction and a lower amount of LMW fractions were observed for the ESM wood than for the SM wood in the ALCELL process, which suggests that less lignin degradation and more condensation occurred due to the HWE treatment, and the rate of the condensation reactions (Figure 3) exceeded the rate of the degradation reactions. The lignin recovery results also indicate that the delignification mechanism for the AWO and ALCELL processes are different. For the AWO process, under acidic conditions with the presence of oxygen, lignin tends to degrade into more carboxylic compounds as the evidence of the cleavage of the aromatic 26

6 rings; 21 these compounds are difficult to be recovered from the spent liquor. On the other hand, the ALCELL process favors the cleavage of lignin linkages without further degradation, resulting in a higher yield of lignin precipitate. Accordingly, the ALCELL spent liquor had a higher end ph (~4), while the AWO spent liquor had a lower end ph (~3). Based on the amount of lignin removed (right y axis as shown in Figure 5), only ~75% of the removed lignin was recoverable from the AWO spent liquor for both the SM and ESM wood samples, which provides further evidence for the notion of lignin degradation during in the AWO process. As for the ALCELL process, the total recovery yield of lignin based on the lignin removed was over 100 %. This may be explained by the ethanolysis in the ALCELL process, leading to the addition of ethoxy groups to the Cα position (Figure 3). The isolated organosolv lignin was also determined for purity. The AWO lignin purity ranged from 81% to 85 %, as Klason lignin based on the recovered precipitate, while a slightly higher purity (~89 %) was observed for the ALCELL lignin. More importantly, a consistently higher lignin content was found in the L 1 fraction from the extracted wood than from the un-extracted wood, demonstrating the beneficial effect of the HWE treatment on the purity of the recovered lignin. Figure 5. Fractions of recovered lignin from the AWO and ALCELL spent liquors 3.3 Lignin Characterization by SEC The isolated lignin precipitates were further analyzed by size exclusion chromatography (SEC) in comparison to the milled wood lignin (MWL) of the SM wood, as shown in Figure 6. The results of lignin molecular weight are expected to reflect the variations in chemical reactions, such as the depolymerization or repolymerization. They may also reflect the topochemical effects, meaning the morphological area from which lignin is released (such as middle lamella and secondary wall). 22 In Figure 6, all of the recovered lignin fractions exhibit bimodal distribution profiles in contrast to the unimodal curve of the MWL, indicating a wide polydispersity after treatment. For the AWO process, more severe lignin degradation might take place in the case with the HWE pre-treatment, and resulted in a lower molecular weight of the AWO lignin from the ESM wood compared to that from the SM wood. In contrast, there was little difference between the recovered ALCELL lignin for the SM and ESM wood samples, which correlates well to the delignification results discussed in previous section. In general, the lignin isolated from both the AWO and ALCELL had a much lower molecular weight in comparison to the control lignin (MWL), which may be explained by the pronounced degradation of the macromolecular lignin in the treatments. In addition, the organosolv lignin from both processes had lower polydispersity than the MWL lignin, indicating their better uniformity for potential applications. 27

7 Fig.6 Molecular weight distribution of the lignin samples (SM: AWO lignin from the SM wood; ESM: AWO lignin from the ESM wood; SM_ALCELL: ALCELL lignin from the SM wood; ESM_ALCELL: ALCELL lignin from the ESM wood; SM_MWL: mill wood lignin from the SM wood) 4. CONCLUSION Organosolv delignification processes were investigated in this study using sugar maple (Acer saccharum) as a model species for the potential of integrated biorefinery. The oxygen-pressurized acetone-water procedure (AWO) resulted in a much higher delignification degree when performed on the hot-water extracted sugar maple wood (~96% of lignin removed from the extracted wood vs. ~53% of original lignin removed from the un-extracted wood). The ALCELL delignification process also benefited from the HWE pre-treatment, but to a lesser extent than the AWO process. The organosolv lignin was recovered from the spent liquor of the two processes. The results demonstrated that the HWE pre-treatment prior to the AWO delignification process resulted in a higher amount of lignin degradation products of lower molecular weight, which were recovered from the spent liquor by liquid-liquid extraction. These lignin degradation products were also more hydrophilic than those recovered from the ALCELL process, and they could not be fully recovered from the spent liquor, indicating a possible cleavage of the aromatic rings. Furthermore, it was found that the lignin recovered from the organosolv liquor in the delignification of the hot-water extracted wood had higher purity, and all the organosolv lignin exhibited a lower polydispersity than that the MWL lignin. REFERENCES 1. Amidon, T.E., Wood, C.D., Shupe, A.M., Wang, Y., Graves, M., and Liu, S. Biorefinery: conversion of woody biomass to chemicals, energy and materials. Journal of Biobased Materials and Bioenergy, 2008, 2(2), Hasan, A., Bujanovic, B., and Amidon, T. Strength properties of kraft pulp produced from hot-water extracted wood chips within the biorefinery. J. Biobas. Mat. Bioen. 2010, 4(1), Gong, C., Goundalkar, M., Bujanovic, B., and Amidon, T. Evaluation of different sulfur-free delignification methods for hot-water extracted hardwood. J. Wood Chem. Technol., 2012a, 32, Zarubin, M.Y., Dejneko, I. P., Evtuguine, D.V., and Robert, A. Delignification by oxygen in acetone-water media. TAPPI J. 1989, 72 (11), Pye, E.K. and Lora, J. H. The ALCELL TM process: a proven alternative to kraft pulping. Tappi J., 1991, 74(3), Botello, J. I., Gilarranz, M. A., Rodríguez, F., and Oliet, M. Recovery of solvent and by-products from organosolv black liquor. Separation Science and Technology, 1999, 34(12), Bujanovic, B., Hirth, K.C., Ralph, S.A., Reiner, R.S., and Atalla, R.H. Composition of the Organic Components in Polyoxometalate (POM) Liquors from Kraft Pulp Bleaching. Presented at the 14th ISWFPC, Durban, South Africa,

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