Low acid hydrothermal fractionation of Giant Miscanthus for production of xylose-rich hydrolysate and furfural

Bioresource Technology 218 (2016) 367–372

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Low acid hydrothermal fractionation of Giant Miscanthus for production of xylose-rich hydrolysate and furfural Tae Hyun Kim a, Hyun Jin Ryu b, Kyeong Keun Oh c,⇑ a

Department of Environmental Engineering, Kongju National University, Cheonan, Chungnam 31080, Republic of Korea R&D Center, SugarEn Co., Ltd., Cheonan, Chungnam 31116, Republic of Korea c Department of Applied Chemical Engineering, Dankook University, Cheonan, Chungnam 31116, Republic of Korea b

h i g h l i g h t s
LAH (low acid hydrothermal) fractionation process was optimized.
Xylan degradation, xylose recovery, and xylose decomposition were evaluated.
Furfural production rate was high at early LAH fractionation stage, then decreased.
Xylose-rich hydrolyzate and glucan-rich residual solid were obtained.

a r t i c l e

i n f o

Article history: Received 26 April 2016 Received in revised form 23 June 2016 Accepted 25 June 2016 Available online 27 June 2016 Keywords: Xylan Pretreatment Sulfuric acid Furfural Degradation Decomposition

a b s t r a c t Low acid hydrothermal (LAH) fractionation was developed for the effective recovery of hemicellulosic sugar (mainly xylose) from Miscanthus sacchariflorus Goedae-Uksae 1 (M. GU-1). The xylose yield was maximized at 74.75% when the M. GU-1 was fractionated at 180 °C and 0.3 wt.% of sulfuric acid for 10 min. At this condition, the hemicellulose (mainly xylan) degradation was 86.41%. The difference between xylan degradation and xylose recovery yield, i.e., xylan loss, was 11.66%, as indicated by the formation of decomposed products. The furfural, the value added biochemical product, was also obtained by 0.42 g/L at this condition, which was 53.82% of furfural production yield based on the xylan loss. After then, the furfural production continued to increase to a maximum concentration of 1.87 g/L, at which point the xylan loss corresponded to 25.87%. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Lignocellulosic biomass has been suggested as an abundant and promising source for various biorefinery industries since its three main constituents are cellulose, hemicellulose, and lignin (Huijgen et al., 2012; Snelders et al., 2014; Cherubini, 2010). These components can be used as precursors and intermediates for biofuels and value-added chemicals, which promise to render such a biorefinery process economically feasible. The success of biorefineries depends on the availability of renewable, consistent, high volume feedstock supplies, which can be provided by dedicated energy crops, such as switchgrass or miscanthus (Werpy and Petersen, 2004). Miscanthus sp. is high yielding crop biomass which grows rapidly, produces an annual crop without the need for replanting with very low nutrient requirements and is therefore a promising ⇑ Corresponding author. E-mail address: [email protected] (K.K. Oh). http://dx.doi.org/10.1016/j.biortech.2016.06.106 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.

energy crop (Eliana et al., 2014; Heaton et al., 2010). For example, a grass native to temperate Asia, Goedae-Uksae 1 (M. GU-1), which is a member of Miscanthus sp., has been reported to grow up to 4 m height and 10 mm stem thickness producing high biomass yield of 30 t/ha (Yan et al., 2015; Kang et al., 2013). It is a lignocellulosic biomass with high cellulose and low lignin content that can be converted to bioenergy and/or biochemicals by biochemical conversion, which is sequenced pretreatment, hydrolysis and fermentation (Wu et al., 2013). Efficient fractionation of lignocellulosic biomass into its three main constituents can improve overall efficiency in the biorefining industry. For effective utilization of each component in lignocellulosic biomass, fractionation has become the prerequisite and central unit operation in the biorefinery process (Li et al., 2013; Mosier et al., 2005). Hemicellulose (in grasses) consists of a linear xylose polymer with short side chains comprised of sugars and organic acids. The major hydrolysis product is xylose, which may further be dehydrated to form furfural. The xylose in biomass

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hydrolyzate can be recovered and used as a feedstock for xylitol production, which has huge market demand (Franceschin et al., 2011; Cheng and Zhu, 2009). However, hydrolyzing cellulose and hemicellulose to monosaccharides (hexose and pentose) remains a bottleneck in the production of bio-based products from lignocellulosic biomass and, therefore, hinders its industrial application. Compared to ethanol production, the recovery of xylose from biomass hydrolyzate as feedstock for xylitol production is much more economically profitable. Xylose is the cheapest pentose, with possible application as secondary products, food additives, and detergents after esterification, as well as a sweetener in the form of crystalline powder. Hydrothermal fractionation targeted at the degradation of hemicellulose in a solid phase has been successfully applied to different feedstocks (Aita and Kim, 2010). The depolymerization of hemicellulose is catalyzed by sulfuric acid. The hydronium ions in acids catalyze the cleavage of glycosidic bonds within the xylan, leading to the release of xylose into the liquid phase (Garrote et al., 1999; Nabarlatz et al., 2004; Grenman et al., 2011; Felix and Tilley, 2009). The fractionation of biomass using dilute sulfuric acid is one of the most cost-effective methods. To achieve a dilute acid fractionation, the mixture of biomass and dilute acid solution is usually controlled at a moderate acid concentration and temperature (Chen et al., 2010a,b). Xylan-rich hydrolysate is produced by hydrolysis of hemicellulose using hydrothermal fractionation, which can further break xylose in hydrolysate down to furfural (Huijgen et al., 2012). The thermal degradation of biomass and the decomposition of carbohydrates are complex processes that include many different simultaneous reactions such as dehydration, depolymerization, rehydration, rearrangement, condensation and carbonization (Patwardhan et al., 2011). Commonly, acid hydrothermal fractionation (low pH) uses various acids to increase the hydrolytic reactivity. The liquid hydrolyzate from acid hydrothermal fractionation contains various compounds, including sugar decomposed products (furfural and hydroxymethylfurfural (HMF)), organic acids, and phenolic compounds, which are toxic to microorganisms in the fermentation (Tsubaki et al., 2013; Li et al., 2014). Therefore, the formation of inhibitors should be monitored and minimized (Larsson et al., 1999; Palmqvist and Hahn-Hägerdal, 2000; Kim et al., 2015). In this study, low acid hydrothermal (LAH) fractionation (generally, no more than 1 wt.% of acid concentration) was developed for the effective recovery of hemicellulosic sugar (mainly xylose) from M. GU-1. Sulfuric acid can provide a strong catalytic effect for the degradation of xylan with low formation of by-products. The LAH fractionation was designed for efficient degradation of xylan and conversion of xylose in the hydrolyzate into furfural without further decomposition. The effects of three process variables, i.e., reaction temperature, reaction time, and catalyst concentration, were optimized for maximum recovery of xylose and minimum of by-product formation. The xylan loss during LAH fractionation was evaluated by measurement of xylose recovery yield and furfural formation. The mass balance of M. GU-1 through LAH fractionation was determined on the basis of glucose and xylose. 2. Materials and methods 2.1. Feedstock M. GU-1, Miscanthus sacchariflorus Goedae-Uksae 1 was harvested from the banks of the Geum River, Korea in 2013 by the Bioenergy Crop Research Center, National Institute of Crop Science, Rural Department Administration (Muan, Jeollanamdo, Korea). Samples were chopped, ground and sieved to obtain 40–14 mesh

(0.43–1.40 mm) sized particles. The screened straw particles were air-dried for 24 h at 45 ± 5 °C and then used directly in LAH fractionation studies. The moisture content of milled straw was 5.17% based on total wet biomass weight. The chemical composition of the raw M. GU-1 is 41.9 ± 0.1% glucan, 22.1 ± 0.2% XMG, the total sum of three oligomeric sugars xylan, mannan, and galactan, 1.7 ± 0.2% arabinan, 22.0 ± 0.4% Klason lignin, 1.5 ± 0.1% organosolv extractives (ethanol), 8.2 ± 0.2% watersoluble extractives, and 2.2 ± 0.1% ash (average number (number of replica = 20) with standard deviations). The mass closure of raw M. GU-1 was reached 99.6 ± 1.3% on oven-dried biomass. 2.2. Low acid hydrothermal (LAH) fractionation The LAH fractionation experiments were performed using sealed bomb tubular reactors, 180 mm long with an inner diameter of 10.7 mm, constructed out of stainless steel tubing (SS 316 L) and capped at either end with Swagelok fittings to give an internal volume of 16.2 mL. The reactors were loaded with 500 mg of ovendried M. GU-1. The residual moisture in the dried M. GU-1 sample was accounted for when quantifying the amount of solution to be added, which gave a solid/liquid ratio of 1:10. The tubular reactors were submerged in the first bath (molten salt) at 240 °C for rapid preheating to the target temperature in about 1.5 min. The reactors were then quickly transferred into the second bath of silicone oil set at the desired reaction temperature of 160–200 °C. After the desired reaction time, the reactors were quickly transferred to an ice–water bath to quench the reaction for 10 min. The tubes were removed from the water bath, and the end caps and Teflon plugs were removed. The contents were separated into liquid and solid phases by filtration. 2.3. Analytical methods The chemical compositions of the solid and liquid samples were determined following the procedures of the National Renewable Energy Laboratory (NREL; Golden, CO, USA) laboratory analytical procedures (LAP) (NREL/TP-510-42618 for structural carbohydrates and lignins; NREL/TP-510-42623 for sugars in the liquids or in the hydrolyzates) (Sluiter et al., 2011, 2008). The sugars were determined using high performance liquid chromatography (HPLC; Breeze model, Waters Co., Milford, MA, USA) equipped with a refractive index detector (Waters 2414, Waters Co., Milford, MA, USA). A Bio-Rad Aminex HPX-87H column (300 mm length  7.8 mm internal diameter) and Cation H micro-guard cartridge (30 mm length  4.6 mm internal diameter) (Bio-Rad Laboratories Inc., Hercules, CA, USA) were used for sugar analysis. The sample was filtered using a syringe filter (Whatman, 0.45 mm pore size, Fisher Scientific Cat # 67791304) before analysis and the mobile phase was 5.0 mM sulfuric acid. The HPLC analysis conditions were a column temperature of 60 °C and a mobile phase flow rate of 0.6 mL/min. The sugars in the liquid samples were determined after secondary acid hydrolysis to account for the oligomer content (XMG). The obtained xylose and furfural were weighed and their purity was analyzed. The xylose and furfural recovery yield and hemicellulose conversion were calculated as follows:

Xylose yield ð%Þ ¼

xylose ðgÞ  0:88  100 xylan in untreated biomass ðgÞ

ð1Þ

Furfural yield ð%Þ ¼

fufural ðgÞ  1:375  100 xylan in untreated biomass ðgÞ  xylose recov ered ðgÞ  0:88 ð2Þ

T.H. Kim et al. / Bioresource Technology 218 (2016) 367–372

where 0.88 is the ratio of molecular weight of C5H8O5 over C5H10O5 and 1.375 is the ratio of molecular weight of C5H8O4 over C5H4O2 (Chen et al., 2012). The conversion factor for hemicellulosic sugar ignores the factor for mannose and galactose, because xylose is the dominant sugar of the hemicellulose of M. GU-1. 3. Results and discussion 3.1. Effects of LAH fractionation conditions on the high xylose recovery LAH fractionation was conducted by varying one of the following three operating parameters: reaction temperature, reaction time, and acid concentration. The following three different experimental sets were investigated: (1) five different treatment temperatures (160, 170, 180, 190, and 200 °C) while maintaining acid concentration of 0.3 wt.% and reaction time of 10.0 min, (2) five different reaction times (10.0, 15.0, 20.0, 25.0, and 30.0 min) while maintaining temperature of 180 °C and acid concentration of 0.3 wt.%, and (3) three different concentrations (0.1, 0.3, and 0.5 wt.%) while maintaining temperature of 180 °C and reaction time of 10.0 min. Fig. 1 summarizes the effects of reaction temperature and acid concentration on the xylose and glucose recovery to the LAH fractionation at fixed reaction time of 10 min. As the LAH fractionation severity at fixed reaction time increased, the xylose concentration increased but then decreased with further increase in temperature and acid concentration, even though the glucose concentration in the liquid hydrolyzate continued to increase. The dashed line in Fig. 1 represents the xylose yield of 70%. The hemicellulosic sugar, represented by xylan, recovery concentration peaked at 11.9 g/L, which corresponds with a recovery yield of 74.8% at the following operation conditions: acid concentration of 0.3 wt.%, reaction temperature of 180 °C, and reaction time of 10 min. As the fractionation severity, i.e., reaction temperature and reaction time, increased, the xylose yield was decreased rapidly, which was attributed to the degradation of xylan into not only xylose but also furfural and formic acid by the hasher fractionation severity. A little glucose was also released from the M. GU-1 into the liquid hydrolyzate during the fractionation process, due to the degradation of cellulose. Since the main goal of fractionation is the clean separation of the hemicellulosic sugar from M. GU-1, the glucose release is the major cause not only of the sugar loss from the solid residues but also of the reduced selectivity in the liquid hydrolyzate.

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3.2. By-products formations during LAH fractionation of M. GU-1 As shown in Fig. 2, more by-products were formed with increasing fractionation severity. In particular, acetic acid production was increased significantly to 2.41 g/L at 200 °C and 0.3 wt.% acid concentration. Fig. 2 also shows the generation of decomposed products such as formic acid and furfural in terms of the M. GU-1 hydrolyzate concentration (g/L) as a function of fractionation severity. Furfural is a well-known decomposition product formed from xylose in the acid hydrolysis of lignocellulosic biomass. The level of 5-HMF in the hydrolyzate remained below the detectable limit, which was attributed to the very low level of glucose releasing in the M. GU-1 fractionated hydrolyzate and its further decomposition. The furfural concentration in the hydrolyzate increased with increasing fractionation severity to 1.25 g/L at the severe fractionation conditions of 180 °C and 0.5 wt.%, and continued to increase with further xylose decomposition to 3.55 g/L at 200 °C and 0.5 wt.%. The concentration of formic acid formed by the decomposition of furfural (Panagiotopoulos et al., 2012) did not exceed 0.5 g/L under any fractionation condition. The formation of formic acids during the LAH fractionation at even higher fractionation severity reached a maximum of 0.49 g/L at 200 °C and acid concentration of 0.3 wt.%. 3.3. Monitoring the xylan degradation and xylose decomposition Xylose is formed by the hydrolysis of hemicellulose and the pretreatment and/or fractionation can further break xylose down to form furfural (Huijgen et al., 2012). To determine the formation characteristics of sugars and degradation products, the yields of hemicellulosic sugar (i.e., xylose) and decomposed products (furfural and formic acid etc.) were evaluated because furfural and formic acid are well known decomposed products formed from xylose during the acid hydrolysis (Qi and Xiuyang, 2007). Fig. 3 shows the changes the sugar recovery and the formation of degradation products such as formic acid and furfural in the M. GU-1 hydrolyzate as a function of reaction time at low acid fractionation conditions (0.3 wt.% of sulfuric acid). The xylose yield was maximized at 74.75% when the M. GU-1 was fractionated at 180 °C and 0.3 wt.% sulfuric acid for 10 min. At this condition, the hemicellulose (mainly xylan) degradation yield was 86.41%. The difference between xylan degradation and xylose recovery, i.e., xylan loss, was 11.66%, as indicated by the formation of decomposed products. As the reaction time exceeded 10 min., the xylan

Fig. 1. Concentration profiles of degradation of cellulose and hemicellulose in liquid hydrolyzate after LAH fractionation of M. GU-1 under various reaction conditions.

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Fig. 2. Concentration profiles of decomposition products in liquid hydrolyzate after LAH fractionation of M. GU-1 under various reaction conditions.

During the LAH fractionation of hemicellulose, furfural is generated by the decomposition of xylose in parallel to the formation of sugars. The furfural production was 0.42 g/L at 10 min. and continued to increase to a maximum of 1.87 g/L at 30 min. Fig. 3. also shows the changes of furfural yield. At the point of maximum xylose recovery, the furfural yield was 53.82% on the basis of xylan loss, after which the furfural yield was decreased rapidly to 43.30% at 15 min and increased again to the maximum yield of 64.39%. The reasonable estimation of low furfural yield at time 15 min. was attributed to the high xylan loss by increasing the amount of unrecovered xylose, i.e., the large denominator in the furfural yield equation. As a byproduct, furfural has economic value in terms of the efficiency of bio-refining, including xylose and bioethanol production. Furfural produced from the hemicellulosic fraction of lignocellulosic residues is considered a promising bio-based chemical (Werpy and Petersen, 2004) for use in applications such as fungicide, specialist adhesives, flavor compound, and a precursor for Fig. 3. Percentage changes in compositional contents in solid and liquid hydrolyzates through LAH fractionated M. GU-1, xylan degradation and xylose decomposition yield as a function of reaction time.

degradation yield increased to 91.99%. However, despite this increase in hemicellulose degradation, the xylose yield gradually decreased from 74.75% to 64.78%, which was attributed to the partial xylose conversion for the generation of decomposed products such as furfural and formic acid. The formation of formic acid increased linearly from 0.10 g/L to 0.32 g/L until 10 min. of reaction time. But as the reaction time was increased from 10 min., formation of formic acid was almost constant at 0.33–0.36 g/L. Meanwhile, the formation of furfural continued to increase throughout the reaction period up to 1.87 g/L, at which point the xylan loss corresponded to 25.87%. These results are compared to the NREL report, which claimed that overall xylan-to-xylose conversion of dilute acid pretreatment with corn stover was 79.6% with 6.4% loss to furfural, 9.0% of remaining oligomers, and 5% of unreacted xylan (Sievers et al., 2008). The results were in good agreement with the findings of Liu et al. (2012), which suggested that the furfural yield increased at reaction times when dilute sulfuric acid was used to hydrolyze the hemicellulose in sweet sorghum bagasse and the maximum furfural yield reached approximately 15.8% in 120 min at 150 °C.

Fig. 4. Changes in the rate of xylose decomposition and furfural formation yields and time profile of unidentified decomposed products.

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T.H. Kim et al. / Bioresource Technology 218 (2016) 367–372 Table 1 Comparison of mass closure for LAH fractionated M. GU-1 and liquid hydrolyzate. Process sample

Untreated LAH Fractionation Fractionated Fractionatedd Component Retention (%)

Solid remaining (%)

Solid (%)

Mass balance (%)a

Liquid (%) b

Cellulose

XMG

K. Lignin

Ash

100

41.9

22.1

22.0

2.2

59.9

64.4 38.6 92.1

5.0 3.0 13.6

29.3 17.6 80.4

2.6 1.6 72.8

c

Glucose

xmg

Cellulose

XMG

2.2

17.4

97.4 (2.6)

92.3 (7.7)

a Extraction Mass Balance (EMB) = (RCLi + RCSi)/RRi); where Ci is the mass of each sugar component as CLi determined though HPLC chromatography, the subscripts L, S and R refer to the extracted liquid, extracted solids and raw M. GU-1 fractions, respectively. b XMG: sugar polymer (Xylan + Mannan + Galactan). c xmg: sugar monomer (xylose + mannose + galactose). d Data are based on the oven dry raw M. GU-1.

5-methyl furfural, furfuryl alcohol, tetrahydrofurfuryl alcohol and tetrahydrofuran. Therefore, a higher xylose recovery and higher furfural yield, which is relevant to xylan loss, should be selected for hemicellulose hydrolysis.

on the unrecovered xylose. In addition, a glucan–rich residual solid, with a glucan content of 64.4%, was obtained from the LAH fractionation.

3.4. Unidentified decomposition products in liquid hydrolyzate of M. GU-1

To describe the degradation behavior of hemicellulose in the LAH fractionation of lignocellulosic biomass, the reaction conditions should be varied while monitoring the decomposition of xylose. Approximately, 50% of unrecovered xylose seemed to be converted to furfural, however, the conversion of the hemicellulose to monosaccharides with a degree of further decomposition is an unavoidable consequence. Although a few studies have reported on furfural yield through the acid hydrolysis of hemicellulose, the xylan loss could be reduced using developed fractionation technique. The obtained results in this study can be used for further optimization the conditions and reactor design to maximize the worthwhile xylan degradation products.

An experiment with xylose dissolved solution was conducted to describe the xylose decomposition, furfural production and other unidentified decomposed products during LAH fractionation of M. GU-1 at the optimized conditions obtained through the previous experiment: acid concentration of 0.3 wt.% and fractionation temperature of 180 °C . Fig. 4 presents the composition of decomposed xylose solution over the range of reaction times. The xylose content decreased monotonically over the reaction time, whereas the furfural content increased rapidly at the early stage of reaction but then increased only slowly with increasing reaction time. Xylose could not be used for production of furfural. Fig. 4 shows that the aqueous solutions of xylose was decomposed very easily, which led to full xylose conversion, but a limited amount of furfural. Other products were not detected by HPLC. To identify the decomposition products in the fractionated liquid hydrolyzate, analytical inquiry was conducted with GC/MS chromatograms but unfortunately the decomposition products could not be identified and they were merely assumed to be xylose derivatives such as methyl furan, methyl propanoic acid, and carbamic acid. 3.5. Mass balance on glucose and xylose through LAH fractionation Table 1 summarizes the solid remaining after LAH fractionation, the contents of the fractionated solid and liquid, and their degradation mass balance at the aforementioned optimum conditions. The percentage of solid remaining after the LAH fractionation was 59.9%, and the glucan content was 64.4%, which corresponds to a glucan content of 38.6% on the basis in the fractionated M. GU-1. Glucan hydrolyzing (7.9%) during the LAH fractionation was not significant, whereas 86.4% of xylan and 19.6% of lignin were hydrolyzed after the LAH fractionation. Consequently, 2.2% of glucan and 17.4% of xylan were found as monomeric sugars such as glucose and xylose in the liquid hydrolyzate. This result equates to a xylan recovery of 78.7% for the untreated M. GU-1. However, only 2.2% of the glucan was determined in the liquid hydrolyzate, which was attributed to the release of glucan from M. GU-1 both as monomeric forms of glucose and also as oligomeric forms of glucose such as cello-oligomers (oligomers were not measured in this study). The xylose-rich hydrolyzate was obtained, with a xylose recovery yield of 74.8% and a high furfural yield of 53.8% based

4. Conclusion

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