Integration of Salt-Induced Phase Separation with Organosolv

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Research Article pubs.acs.org/journal/ascecg

Integration of Salt-Induced Phase Separation with Organosolv Pretreatment for Clean Fractionation of Lignocellulosic Biomass Zhanying Zhang,*,†,‡ William O. S. Doherty,‡ and Ian M. O’Hara†,‡ †

Syngenta Centre for Sugarcane Biofuels Development, Queensland University of Technology, Brisbane, Australia Centre for Tropical Crops and Biocommodities, Queensland University of Technology, Brisbane, Australia

‡

S Supporting Information *

ABSTRACT: In this study, salt-induced phase separation was integrated with n-propanol-based pretreatment for clean fractionation of sugar cane bagasse. First, biomass solid residue and pretreatment hydrolysate were separated by filtration following pretreatment. Lignins with high molecular weights (HMWs) were separated through precipitation and filtration after diluting hydrolysate with water. The filtrate was added with (NH4)2SO4 at a saturated concentration, which induced the formation of a biphasic solution: the top layer was n-propanol phase containing sugar degradation products and lignins with low molecular weights (LMWs); the bottom layer was aqueous phase, rich in salt and soluble sugars. The solvent phase was directly reused for pretreatment, while the aqueous phase was distilled to recycle both salt and water. Direct recycling solvent phase for pretreatment four times only led to a slight decrease in glucan digestibility from 100% (fresh solvent) to 96%. Direct reuse of solvent phase also led to a significant increase in furfural concentration from 2.4 g/kg to 15.1 g/kg, while recycling water and salt in aqueous phase by distillation led to a substantial increase in xylose concentration from 3.2 g/kg to 13.5 g/kg after five batches of pretreatments. In addition, the total yield of HMW lignins increased gradually from 38% to 49%. KEYWORDS: Organosolv pretreatment, Phase separation, Fractionation, Recycling, Lignocellulosic biomass

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INTRODUCTION Lignocellulosic biomass is an abundant and low-cost sustainable carbon source for the production of biofuels (ethanol, nbutanol, etc.) and biochemicals (lactic acid, succinic acid, etc.) through fermentation processes.1,2 Pretreatment of lignocellulosic biomass is required to improve cellulase accessibility to cellulose for enhanced fermentable sugar production.3,4 Organosolv-based biomass fractionation processes are one of the leading biomass pretreatment processes.3,5 The fractionation processes make the maximal use of each biomass component (i.e., cellulose, hemicellulose, and lignin) possible. The commonly studied organic solvents include low boiling point solvents such as ethanol (78.4 °C) and tetrahydrofuran (THF, 66.0 °C) and middle boiling point solvents such as nbutanol (117.7 °C) and methyl isobutyl ketone (MIBK, ∼117− 118 °C).3,5−15 Ethanol has been used to effectively fractionate biomass in the acid-catalyzed process.8,9 In the ethanol process, cellulose-rich pulp is separated from pretreatment solution following pretreatment and is hydrolyzed to produce glucose by cellulase enzymes. The pretreatment solution was diluted to precipitate lignins with high molecular weights (HMWs). With this process, ethanol is recovered from the diluted pretreatment solution by distillation. However, recovery and separation of soluble sugars, sugar degradation products such as furfural, and soluble lignins/phenolics with low molecular weights (LMWs) are not economically feasible because of the low product © 2017 American Chemical Society

concentrations. The acid-catalyzed THF process is similar to the acid-catalyzed ethanol pretreatment process.6,15 THF can be distilled directly from aqueous solution, leading to the precipitation of lignin. n-Butanol and MIBK are the other two commonly used solvents for biomass fractionation.10−14 Because of the low solubility of n-butanol in water (73 g/L at 25 °C), the pretreatment solution forms a biphasic solution after the removal of cellulose-rich pulp by filtration.10,11 The organic solvent phase is rich in n-butanol and lignin, while the aqueous phase contains soluble sugars and a low concentration of nbutanol. With MIBK processes, biomass is first cooked in an aqueous MIBK-alcohol (e.g., ethanol) solution.12−14 After cooking and solid/liquid separation, the liquid solution forms a biphasic solution with tuning the solvent ratio by adding water. The organics phase is rich in MIBK and lignin, while the aqueous phase contains soluble sugars and alcohol. With these n-butanol and MIBK processes, solvent and lignin are recovered and separated by distillation of the solvent-rich phase. Despite the progress in the fractionation of biomass using these organic solvents, recovery and separation of pretreatment Received: February 27, 2017 Revised: May 8, 2017 Published: May 17, 2017 5284

DOI: 10.1021/acssuschemeng.7b00617 ACS Sustainable Chem. Eng. 2017, 5, 5284−5292

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Sugar cane bagasse fractionation process using aqueous n-propanol solution. Solvent Screening. 100 g/kg to 300 g/kg alcohol solutions were prepared by mixing the required amount of alcohol solvents with water. One group of the alcohol solutions contained glucose (2.5 g/ kg), xylose (10.0 g/kg), and arabinose (2.5 g/kg). The other group of alcohol solutions contains sugar dehydration products HMF (1.0 g/ kg) and furfural (10.0 g/kg). 30.0 g of each alcohol solution was transferred to a 50 mL Falcon centrifuge tube, and (NH4)2SO4 was added to the solution gradually at room temperature (24 °C) until saturated concentrations were reached. Afterward, the centrifuge tubes were centrifuged at 1000 rpm for 5 min in a Beckman centrifuge (GS6R, Beckman, US). Except for the ethanol solution containing 100 g/ kg ethanol, all the other solutions formed biphasic solutions. The bottom phase (aqueous phase) was removed by a syringe, and the weights of both aqueous phase and organic solvent phase were recorded. The concentrations of alcohols in the bottom phase and sugars in both organic solvent and aqueous phases were determined by a high performance liquid chromatography (HPLC) system as described in the HPLC Analysis section. The concentrations of alcohols in solvent phase could not be determined by the HPLC system because of the high alcohol concentrations affecting the accuracy of the test. However, they were calculated based on the mass of alcohol solvent phase, the mass of total alcohol, and the mass of alcohol in aqueous phase. The phase separation experiment was conducted in duplicate, and the results were the means of the duplicate data. Biomass Fractionation. Sugar cane bagasse was fractionated in a process shown in Figure 1. For pretreatment, 5.0 g of sugar cane bagasse was transferred to a 100 mL pressure resistant glass flask (Ace Glass Inc., US) containing 50.0 g of aqueous n-propanol solution with 588 g/kg n-propanol, 400 g/kg water, and 12 g/kg H2SO4. The flask was sealed with a screw lid and immersed in an oil bath preheated to 140 °C. The reaction was conducted at 140 °C for 90 min under magnetic stirring at 250 rpm. A total of five flasks were used in the first batch pretreatment to collect enough materials for later use. Biomass fractionation by acidified aqueous ethanol (588 g/kg ethanol, 400 g/kg water, and 12 g/kg H2SO4) was included as control. The pretreatment conditions of biomass fractionation were the same as those with npropanol. After pretreatment, the flasks were cooled in iced water, and all of the reaction solutions from the same batch pretreatment (five flasks) were mixed and filtered by a Whatman 541 filter. The solid residue was washed with 500 mL of water (5 × 100 mL) and was filtered. The wash solution and the original pretreatment solution were mixed to precipitate lignin followed by filtration to separate the diluted pretreatment solution and lignin. Lignin was collected and washed with 200 mL (2 × 100 mL) of water. The diluted pretreatment solution was saturated with adding 400 g of (NH4)2SO4 to promote the formation of a biphasic solution. Undissolved (NH4)2SO4 was filtered and collected. After phase separation, the weights of solvent phase and aqueous phase were recorded. One milliliter of sample from the solvent phase and 1.0 mL of sample from the aqueous phase were withdrawn for HPLC analysis to determine the concentrations of n-

byproducts like soluble sugars (e.g., hemicellulose sugars), sugar degradation products (e.g., 5-hydroxymethyl furfural (HMF) and furfural), and organic acids (e.g., acetic acid) in aqueous phase are costly because of the low concentrations. Especially, the publications on biomass pulping/prereatment by aqueous n-propanol solutions are limited,16 which is due to the similar boiling points between n-propanol (97 °C) and water (100 °C), making the separation of n-propanol from water difficult. In this study, a salt-induced phase separation step was integrated with an alcohol solvent-based pretreatment process, and the whole process was developed for clean fractionation of lignocellulosic biomass. n-Propanol was selected as a model solvent for process development though this process is also adaptable to other solvents such as iso-propanol. Separation of n-propanol from water was not an issue in this process because n-propanol was recovered and recycled through phase separation without distillation. More importantly, this process led to the substantial increases in the concentrations of soluble sugars in the aqueous phase as well as the accumulation of sugar degradation products and lignins with LMWs in the solvent phase.

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METHODS

Materials. Sugar cane bagasse was collected from Racecourse Sugar Mill (Mackay Sugar Limited) in Mackay, Australia. Sugar cane bagasse was washed with hot water at 90 °C to remove residual sugars to a negligible amount. The washed sugar cane bagasse was air-dried and gently shaken on a sieve having an aperture size of 1.0 cm to remove the pith, and the residues were ground to fine particles by a cutter grinder (Retsch SM100, Retsch GmBH, Germany). The milled bagasse was screened, and particles having a width range of 250 to 500 μm were collected and stored for pretreatment. The moisture of the sieved bagasse particles was 7.1 wt %. Bagasse particles mainly consisted of 41.6% glucan, 19.5% xylan, 3.5% arabinan, 28.3% lignin, 2.4% acetyl, and 2.1% in terms of mass fraction. Ethanol, iso-propanol, n-propanol, 2-butanol, n-butanol, glucose, xylose, arabinose, (NH4)2SO4, and sulfuric acid (mass fraction of 98%) were purchased from Sigma-Aldrich (US). Accellerase 1000 (Batch no. 1600877126), a Danisco product (Genencor Division, Danisco Inc., US), was purchased through Enzymes Solutions Pty. Ltd. (Australia). Accellerase 1000 contained 30.4 mg protein/mL enzyme solution, which was measured using Bradford Protein Assay Kit purchased from Bio-Rad (US). The filter paper activity of Accellerase 1000 was ∼40 FPU/mL, which was measured using a method developed by the National Renewable Energy Laboratory (NREL, US).17 All of the chemicals used in this study were of analytic grade or above. 5285

DOI: 10.1021/acssuschemeng.7b00617 ACS Sustainable Chem. Eng. 2017, 5, 5284−5292

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ACS Sustainable Chemistry & Engineering Table 1. Alcohol Distribution in Biphasic Solutions

alcohol mass distribution (%)e

alcohol concentration, g/kg solvent

a

b

D

solvent phase

6.3 ± 0.3 38.4 ± 1.7 76.6 ± 3.8 114.1 ± 5.6 475.3 ± 20.1

ethanol iso-propanol n-propanol 2-butanol n-butanol

470.6 772.4 842.4 901.3 950.6

± ± ± ± ±

c

aqueous phase

d

74.6 ± 2.2 20.1 ± 0.4 11.0 ± 0.4 7.9 ± 0.3 2.0 ± 0.1

14.3 13.2 10.8 15.3 14.0

solvent phase

aqueous phase

± ± ± ± ±

48.2 ± 1.7 13.4 ± 0.6 7.6 ± 0.3 5.4 ± 0.2 1.3 ± 0.0

51.8 86.6 92.4 94.6 98.7

1.7 0.6 0.3 0.2 0.0

a

Initial solvent concentration before phase separation was 20 wt %. bD, distribution coefficient: component concentration (g/kg) in solvent phase/ component concentration (g/kg) in aqueous phase. cCalculated values: (total amount of a component (g) − the component amount in aqueous phase (g, component concentration in aqueous phase (g/kg) × the amount of aqueous phase (kg))/total amount of solvent phase (kg)). dResults determined by HPLC analysis. eAlcohol mass distribution: total amount of alcohol in each phase (g)/total amount of alcohol in both phases (g) × 100%.

Table 2. Sugar Distribution in Biphasic Solutions glucose mass distribution (%)c solventa ethanol iso-propanol n-propanol 2-butanol n-butanol

glucose Db 0.43 0.04 0.02 0.01 0.01

± ± ± ± ±

0.02 0.00 0.00 0.00 0.00

solvent phase 7.9 0.9 0.4 0.2 0.1

± ± ± ± ±

0.5 0.0 0.0 0.0 0.0

aqueous phase 92.1 99.1 99.6 99.8 99.9

± ± ± ± ±

0.5 0.0 0.0 0.0 0.0

arabinose mass distribution (%)c

xylose mass distribution (%)c xylose Db

solvent phase

± ± ± ± ±

10.8 ± 0.5 2.4 ± 0.1 1.1 ± 0.0 0.4 ± 0.0 0.3 ± 0.0

0.61 0.11 0.05 0.03 0.02

0.02 0.00 0.00 0.00 0.00

aqueous phase 89.2 97.6 98.9 99.6 99.7

± ± ± ± ±

0.5 0.1 0.0 0.0 0.0

arabinose Db

solvent phase

± ± ± ± ±

10.6 ± 0.6 1.8 ± 0.0 0.9 ± 0.0 0.3 ± 0.0 0.2 ± 0.0

0.59 0.08 0.04 0.02 0.01

0.03 0.00 0.00 0.00 0.00

aqueous phase 89.4 98.2 99.1 99.7 99.8

± ± ± ± ±

0.6 0.0 0.0 0.0 0.0

a

Initial solvent concentration before phase separation was 20 wt %. bD, distribution coefficient: component concentration (g/kg) in solvent phase/ component concentration (g/kg) in aqueous phase. cSugar mass distribution: the amount of a sugar in each phase (g, sugar concentration in each phase (g) × the amount of each phase (kg))/total amount of the sugar in both phases × 100%). pulp was stored at 4 °C for enzymatic hydrolysis. The pretreatment was conducted in duplicate, and the results were the means of the duplicate data. Enzymatic Hydrolysis of Bagasse Pulp. Enzymatic hydrolysis was carried out in a 20 mL glass vial containing 5 g of solution. The cellulose loading was 2 wt % and the reaction solution also contained 0.05 M citrate buffer to maintain the pH at 4.8 and 0.02 wt % sodium azide to prevent the growth of microorganisms. The dosage of Accellerase for enzymatic hydrolysis was ∼20 FPU/g cellulose (0.5 mL of Accellerase solution/g cellulose). The reaction was carried out at 50 °C for 72 h in a rotary incubator (Ratek OM 11 Orbital Mixer, Australia) with shaking speed of 150 rpm. The sampling times were 0, 6, 24, and 72 h, and sample volume was 0.2 mL each time. The samples were centrifuged at 9,000g for 5 min. 0.1 mL of supernatant was diluted 10 times by deionized water. The diluted sample was filtered through a 0.22 μm disk filter prior to sugar analysis by HPLC. All of the enzymatic hydrolysis experiments were conducted in duplicate. HPLC Analysis. A HPLC system with a Bio-Rad Aminex HPX87H column and Waters refractive index detector and UV detector (280 nm) was used to quantify solvent and sugar degradation products such as 5-hydroxymethylfurfural (HMF) and furfural in samples. The mobile phase was 5 mM H2SO4 at a flow rate of 0.6 mL/min. The column temperature was 65 °C. A Phenominex RPM monosaccharide column was used to determine the sugars during enzymatic hydrolysis. The column temperature was 85 °C, and the mobile phase was water at a flow rate of 0.5 mL/min.

propanol, sugars, and sugar degradation products. The solvent phase was equally divided into five portions and transferred to flasks for the next batch pretreatment. Water was distilled from aqueous phase at 80 °C under vacuum pressure and collected. Fresh water was added to offset the water distilled from aqueous phase to make the final mass of 500 g, which was used for washing solid residues generated in the next batch pretreatment. Stillage ((NH4)2SO4, sugars, and ∼25% water) and the previous undissolved (NH4)2SO4 was reused to induce biphasic solution in processing the next batch pretreatment solution. The second batch pretreatment was also conducted in five flasks with the same reaction conditions (bagasse loading, preteratment temperature, and time) as those for the first batch pretreatment. Prior to the second batch pretreatment, recovered n-propanol was replenished with a small amount of n-propanol depending on the npropanol concentration in solvent phase determined by HPLC to make the final concentration of n-propanol in the solvent solution the same as that (588 g/kg) in the first batch pretreatment. Twelve g/kg H2SO4 (final acid concentration) was added to the solvent solution. A small amount of fresh water was also added to maintain the solvent composition ratio to be the same as that for the first batch pretreatment. Following the second batch pretreatment, the pretreatment mixture was processed in the same manner as that for the first batch pretreatment. After lignin separation, (NH4)2SO4 collected from the first batch pretreatment plus 50 g of fresh (NH4)2SO4 was used to saturate and dilute the pretreatment solution from the second batch pretreatment. The pretreatments were repeated four times, and the procedure for processing the pretreatment solution following each pretreatment was similar to that for processing the second batch pretreatment solution. Another 50 g of fresh (NH4)2SO4 was added in processing the pretreatment solution from the third batch pretreatment. In the fourth and fifth batch pretreatments, no more fresh (NH4)2SO4 was used since the amount of (NH4)2SO4 recovered previously was sufficient to saturate the pretreatment solution and to promote biphasic solution. A portion of biomass pulp collected after each batch pretreatment and all the lignins recovered were vacuum-dried at 45 °C for 48 h for biomass compositional analysis by a method developed by the National Renewable Energy Laboratory.18 The remaining biomass

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RESULTS AND DISCUSSION Comparison of Sugar Distribution with Different Biphasic Solutions. Five aqueous alcohol solutions were tested for their capacity to form biphasic solutions in the presence of saturated (NH4)2SO4. All of the alcohol solutions formed biphasic solutions with alcohol concentrations of 100− 300 g/kg except for the one with 100 g/kg ethanol. Table 1 shows solvent distribution at an initial alcohol concentration of 200 g/kg containing sugars. All of the alcohol solvents were 5286

DOI: 10.1021/acssuschemeng.7b00617 ACS Sustainable Chem. Eng. 2017, 5, 5284−5292

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ACS Sustainable Chemistry & Engineering Table 3. HMF and Furfural Distribution in Biphasic Solutions HMF mass distribution (%)c solvent

a

ethanol iso-propanol n-propanol 2-butanol n-butanol

b

HMF D 14.7 44.2 45.4 38.5 35.7

± ± ± ± ±

0.6 2.1 1.3 1.6 1.3

solvent phase 76.2 89.1 88.9 86.6 85.7

± ± ± ± ±

1.1 0.4 0.4 0.5 0.4

furfural mass distribution (%)c b

aqueous phase

furfural D

± ± ± ± ±

21.6 ± 0.7 79.5 ± 3.6 101.5 ± 3.8 88.4 ± 3.2 95.5 ± 3.1

23.8 10.9 11.1 13.4 14.3

1.1 0.4 0.4 0.5 0.4

solvent phase

aqueous phase

± ± ± ± ±

17.6 ± 0.9 6.4 ± 0.3 5.3 ± 0.2 6.3 ± 0.2 5.9 ± 0.0

82.4 93.6 94.7 93.7 94.1

0.9 0.3 0.2 0.2 0.0

a

Initial solvent concentration before phase separation was 20 wt %. bD, distribution coefficient: component concentration (g/kg) in solvent phase/ component concentration (g/kg) in aqueous phase. cMass distribution: the amount of a component in each phase (g, component concentration in each phase (g) × the amount of each phase (kg))/total amount of the component in both phases ×100%).

Although pretreatments by biphasic organic solvent systems have been reported previously, the detailed information on how the soluble sugars and sugar degradation products distributed in organic solvent phase and aqueous phase is very limited. This study shows that the distributions of these compounds vary depending on the solvents. The results also show it is possible to recover water miscible alcohols, soluble sugars, and sugar degradation products through the formation of biphasic solutions induced by a salt. Compared to the ethanol biphasic system, iso-propanol, n-propanol, and 2-butanol systems (1) have high concentrations and recoveries of solvents in solvent phase, (2) have recoveries of high sugar degradation products in alcohol solvent phase, and (3) have high sugar recoveries in aqueous phase. Compared to n-butanol (solubility of ∼73 g/kg water at 25 °C) and 2-butanol (solubility of ∼184 g/kg water at 25 °C), iso-propanol and n-propanol are miscible with water. If a 60% alcohol solution is used for pretreatment, HMW lignins can be precipitated out with diluting iso-propanol or npropanol pretreatment solution with any ratio of water prior to adding salt. However, when n-butanol and 2-bunaol are used, large amounts of water are required to prevent self-formation of biphasic solutions to separate lignins prior to the formation of biphasic solutions by adding salts. Iso-propanol and n-propanol were only occasionally used for biomass pulping,16,23 possibly due to the high cost of solvent. In addition, the boiling point of n-propanol (97 °C) is too close to that of water, which makes its recovery more difficult by distillation. With the salt-induced biphasic solutions, isopropanol and n-propanol can be recovered efficiently from their aqueous solutions. In this study, further investigations have been focused on n-propanol as it is expected that isopropanol will have properties similar to those of n-propanol in the formation of biphasic solutions and biomass fractionation. The effects of (NH4)2SO4 concentrations on the distribution of n-propanol and sugars were also examined with 200 g/kg npropanol solution (Tables S1 and S2). At a salt concentration of 50 g/kg, biphasic solutions did not form. At salt concentrations of 100 g/kg and above, a biphasic solution formed, and recoveries of solvent (in solvent phase) and sugars (in aqueous phase) increased with increasing salt concentrations. Saturated salt concentration resulted in the highest recoveries of solvent (in solvent phase) and sugars (in aqueous phase). Fractionation of Sugar Cane Bagasse. Fractionation Process Description. Figure 1 shows the process for fractionation of sugar cane bagasse using aqueous n-propanol. In this process, pulp was separated following pretreatment and was converted to glucose by enzymatic hydrolysis. Lowering solution pH and dilution are well-known methods for lignin precipitation following soda pulping and alcohol pulping of

enriched in the top layers (organic solvent phase) but to different extents. The distribution coefficient was used to indicate the solubility difference of a compound in solvent phase and aqueous phase, which was the ratio of concentrations of the compound in solvent phase and aqueous phase at equilibrium. In addition, the mass distribution of each compound in solvent phase and aqueous phase was calculated considering the concentration of the compound in each phase and the amount of the corresponding phase. The solvent distribution coefficient of n-butanol was the highest, followed by 2-butanol, n-propanol, and iso-propanol, while it was the lowest for ethanol. The trend of phase separation abilities of ethanol, n-propanol, and iso-propanol was in line with the previous study.19 As a result, n-butanol concentration and recovery in the solvent phase was the highest, followed by npropanol, iso-propanol, and ethanol. 2-Butanol and n-propanol concentrations in the solvent phase were 901.3 g/kg with a recovery of 94.6% and 837.4 g/kg with a recovery of 92.4%, respectively. Table 2 shows sugars distribution in the solvent phase and aqueous phase. Distribution coefficients of sugars were much lower than those of alcohols because of the much lower solubilities of sugars in alcohol solvent phase than those in aqueous phase. The majority of each sugar was distributed to aqueous phase. Recoveries of sugars in aqueous phase with ethanol solvent were the lowest but still close to 90%, while with n-butanol they were the highest, close to 100%. The recoveries of sugars in aqueous phase with n-propanol and 2butanol solvents were greater than 98%. It was also observed that the recovery of glucose in aqueous phase with each alcohol was the highest, followed by arabinose and xylose at the given initial concentrations. Alcohol-based aqueous biphasic systems have been used to extract bioproducts such as proteins,20 nucleic acids,21 and 2,3-butanodiol.22 However, the information on distribution of sugars, especially C5 sugars, in alcohol-based biphasic systems is limited. The present results clearly show that salt-induced alcohol-based biphasic systems can be used to extract sugars (to aqueous phase). Aqueous alcohol solutions containing sugar degradation products such as HMF and furfural were also induced to biphasic solutions. The alcohol solvent distribution in the biphasic solutions were similar to those containing sugars (data not shown). In contrast to sugar distribution, the majorities of HMF and furfural were distributed to alcohol solvent phases after in the biphasic solutions (Table 3). In general, distribution coefficient of furfural was higher than that of HMF in each biphasic solution at the given initial concentrations. The distribution coefficients of HMF and furfural in ethanol biphasic solution were much lower than those in other alcohol biphasic solutions. 5287

DOI: 10.1021/acssuschemeng.7b00617 ACS Sustainable Chem. Eng. 2017, 5, 5284−5292

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ACS Sustainable Chemistry & Engineering Table 4. Bagasse Pulp Yield, Composition, Glucan Digestibility, and Glucose Yield content (%)b batch no. first second third fourth fifth ethanol (first) untreated bagasse

pulp yield (%)a 40.9 41.5 41.0 41.6 42.1 44.0 100

± ± ± ± ± ±

0.6 1.0 1.0 2.1 0.8 0.5

recovery (%)c

glucan

xylan

lignin

± ± ± ± ± ± ±

1.8 ± 0.1 2.0 ± 0.0 1.6 ± 0.1 1.7 ± 0.0 2.0 ± 0.2 4.0 ± 0.3 19.5 ± 0.6

8.3 ± 0.3 8.7 ± 0.3 9.6 ± 0.2 10.2 ± 0.5 10.8 ± 0.6 9.2 ± 0.3 28.3 ± 1.1

87.1 86.8 87.6 86.4 86.1 85.3 41.6

1.1 2.3 1.8 0.7 1.3 2.1 0.9

glucan 85.6 86.3 85.9 85.9 86.7 89.8 100

± ± ± ± ± ±

2.3 1.9 1.0 0.9 2.1 0.3

xylan 3.7 ± 4.2 ± 3.5 ± 3.5 ± 4.3 ± 9.0 ± 100

0.2 0.2 0.1 0.0 0.1 0.4

lignin 12.0 12.7 13.9 15.0 16.1 14.3 100

± ± ± ± ± ±

0.5 0.8 0.4 0.8 0.5 0.6

72 h glucan digestibility (%)d

glucose yield (%)e

100.6 ± 0.1 100.3 ± 1.0 97.3 ± 2.1 96.4 ± 0.4 96.3 ± 2.0 98.4 ± 2.4 6.5 ± 0.2

86.1 ± 0.5 86.6 ± 1.4 83.6 ± 1.6 82.9 ± 0.5 83.5 ± 1.9 88.4 ± 1.5 6.5 ± 0.2

Pulp yield (%) = the amount of solid residue after pretreatment (g)/the amount of bagasse used for pretreatment (g) × 100%. bComponent content was calculated based on the dry mass. cComponent recovery (%) = component content in solid residue after pretreatment (%) × pulp yield (%)/component content in biomass prior to pretreatment (%). dGlucan digestibility (%) = total amount of glucose after enzymatic hydrolysis (g)/ (total amount of glucan in solid residue after pretreatment (glucan content (%) × the amount of solid residue (g)) × 1.11) × 100%. eGlucose yield (%) = glucan recovery (%) × glucan digestibility (%)/100. a

lignocellulosic biomass, respectively.24,25 In this study, the pretreatment solution was diluted approximately three times, and the pH was not adjusted since the pH of the dilute solution was already low (∼2.5) (Figure S1). After dilution, the npropanol concentration was about 200 g/kg solution. After removal of HMW lignins by dilution and filtration, (NH4)2SO4 was added to promote the formation of a biphasic solution (Figure S1). n-Propanol was enriched in the top phase and was directly reused in the next batch pretreatment. The aqueous phase was almost colorless, indicating a good extraction of lignins with low molecular weight (soluble lignin/phenolics) into the solvent phase. Instead of recovering n-propanol by distillation following each pretreatment like other organosolv processes,9,26 water and salt were separated by distillation, recovered, and reused in the present process, which led to a gradual increase in concentrations of soluble sugars. After a number of recycles, the concentrated sugar solutions with salt can be separated through ion chromatography technology, e.g., simultaneous moving bed chromatography (SMBC), which is similar to the process for the separation of sugars and ion liquid using SMBC.27 Alternatively, salt-rich xylose solution may be used for the production of furfural in a biphasic system.28 Fractional distillation of solvent can be used to recover the accumulated LMW lignins, acetic acid, and furfural after a number of solvent recycles. Characterization of Bagasse Pulp. Table 4 shows bagasse pulp yield, composition, and glucan digestibility with five batches of pretreatments. Bagasse pretreated by aqueous ethanol solution was also included as a control. Slightly higher pulp yield, xylan, and lignin contents but lower glucan content were found with bagasse sample pretreated by acidified aqueous ethanol compared to those with bagasse pretreated by acidified aqueous n-propanol (first batch), indicating that acidified aqueous n-propanol was slightly more effective in the removal of xylan and lignin. As a result, glucan recovery in bagasse pretreated with aqueous n-propanol was lower than that in bagasse pretreated with ethanol. From the first batch to the fifth batch, lignin content and recovery in pretreated bagasse increased gradually. The color of pretreated bagasse also darkened with increasing pretreatment batch numbers. The increased lignin content is likely caused by lignin condensation due to the content of soluble lignins in recycled n-propanol solution, lignin precipitation due to water wash of the pulp, and/or reduced pretreatment efficiency with the use of recycled lignin-containing n-propanol solutions.

Figure 2 shows the glucan digestibility at 6, 24, and 72 h. At 6 h, glucan digestibility decreased with increasing pretreatment

Figure 2. Glucan digestibility at 6, 24, and 72 h. Pretreatment was conducted at 140 °C for 90 min using pretreatment solutions containing 58.8% alcohol and 1.2% H2SO4.

batch number. This is possibly due to the increased lignin content, which negatively affects cellulose accessibility by the formation of a lignin barrier and/or nonproductive adsorption of cellulase by lignin.29−31 Nevertheless, after 24 h of hydrolysis, the gaps in glucan digestibility between the first batch pulp and subsequent batch pulps were narrowed. At 24 h, glucan in the first batch pretreated biomass was completely converted to glucose. Further increase in enzymatic hydrolysis time to 72 h only slightly increased the glucan digestibilities of subsequent batches of pulps. After 72 h of hydrolysis, glucan in bagasse samples from the first and second batch pretreatments were completely hydrolyzed to glucose, while glucan digestibility decreased slightly to 96% with increasing pretreatment batch number to five. Glucan digestibility of bagasse pretreated with n-propanol (1st batch) was slightly higher than that of bagasse pretreated with ethanol, indicating that pretreatment by acidified aqueous n-propanol was slightly more efficient in improving glucan digestibility than that by aqueous ethanol. Table 4 shows that glucose yields were the highest with bagasse pretreated in the first batch and second batch but decreased slightly in the subsequent pretreatments due to the decrease in glucan digestibility. In general, the glucose yields of the five pretreatments with n-propanol and recycled n-propanol were lower than that of the ethanol pretreatment due to the low 5288

DOI: 10.1021/acssuschemeng.7b00617 ACS Sustainable Chem. Eng. 2017, 5, 5284−5292

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ACS Sustainable Chemistry & Engineering

and estimated LMW lignin concentration in solvent phase (Figure 3b). It is worth noting that the total lignin distribution was calculated based on the amount of accumulated lignins at the end of each batch. For example, the total lignin in the biomass for the first batch was 28.3 g with the use of 100 g of bagasse, while it is 56.6 g (28.3 × 2) with the use of the same amount of bagasse in the second batch pretreatment. The recovered HMW lignin yield after first batch pretreatment was 38% and increased to 49% after five batches of pretreatments. Although increasing dilution times with water could lead to the precipitation of more lignin,32 more salt will be required to induce the formation of a biphasic solution, and more energy will be consumed to recover water from the aqueous phase. Therefore, in this study three times the dilution was used to precipitate a reasonable amount of lignin. In contrast, the yield of soluble lignins/phenolics with LMWs in solvent phase decreased from 50% to 35% (Figure 3a), but the concentration increased from 19 g/kg to 60 g/kg (Figure 3b) after five batches of pretreatments. The increase in recovered HMW lignin yield is likely attributed to the increase in soluble lignin concentration in solvent phase, which limits the dissolution of more lignin (the increase in soluble lignin concentration was 16 g/kg solvent from the first batch to second batch and was 7 g/ kg solvent from the fourth batch to fifth batch). Lignin in solid residue increased slightly from 12% to 16% after five batches of pretreatmens (Figure 3a and Table 4), which explained the slight drop of glucan digestibility followed by repeated pretreatment (Figure 2 and Table 4). Removal of HMW lignin from solvent solution reduces the viscosity of the solvent and thus increases the mixing, mass, and heat transfer during pretreatment, leading to improved pretreatment effectiveness.33 Although lignin concentration has increased from 19 g/kg to 60 g/kg after five batches of pretreatments, the results show that pretreatment effectiveness in terms of glucan digestibility only decreased slightly (Table 4). It is expected that the solvent can be recycled for more batches without significantly decreasing glucan digestibility. After a number of pretreatment batches, the high concentration of LMW lignins can be recovered by fractional distillation. Solvent Concentration and Distribution. Table 5 shows the concentration and mass distribution of n-propanol in the biphasic solution after each batch pretreatment. The initial npropanol concentration in the dilute solution before phase separation was about 196 g/kg, close to that previously used in the synthetic alcohol solutions (200 g/kg). However, compared to the synthetic solution, the concentration of n-propanol in the solvent phase of the biphasic solution following pretreatment was 170 g/kg to 200 g/kg lower (632−672 g/kg vs 842 g/kg).

glucan recoveries. This indicates that the pretreatment conditions with n-propanol were a little bit severe, leading to the hydrolysis of higher portions of glucan to pretreatment solutions than those with ethanol. Nevertheless, the glucose yield did not decrease significantly and was greater than 83% after five batches of pretreatments. Optimization of pretreatment conditions will be able to improve the glucose yield in future studies. Lignin Yield. Figure 3 shows the distribution and yield of lignin after each batch pretreatment concentration (Figure 3a)

Figure 3. Lignin distribution after each batch pretreatment (a) and concentration of LMW in solvent phase after phase separation (b). LMW lignin concentration = (total amount of lignin in untreated bagasse (g) − the amount of lignin in solid residue after pretreatment (g) − the amount of HMW lignin recovered from precipitation (g))/ the amount of solvent phase (kg).

Table 5. Concentration, Distribution, and Yield of n-Propanol in Biphasic Solutions after Each Batch Pretreatment alcohol mass distribution (%)d

alcohol concentration, g/kg batch no. first second third fourth fifth

D 71.9 73.9 77.6 73.8 67.2

a

± ± ± ± ±

solvent phase 3.5 4.0 2.6 3.6 2.9

654.7 671.6 664.3 646.1 631.9

± ± ± ± ±

b

aqueous phase

13.0 15.8 10.2 14.4 15.7

9.1 9.1 8.6 8.8 9.4

± ± ± ± ±

0.4 0.6 0.2 0.6 0.7

c

solvent phase 93.8 92.6 92.8 92.9 92.4

± ± ± ± ±

0.2 0.3 0.3 0.2 0.3

aqueous phase 6.2 7.4 7.2 7.1 7.6

± ± ± ± ±

0.2 0.3 0.3 0.2 0.3

a

D, distribution coefficient: component concentration (g/kg) in solvent phase/component concentration (g/kg) in aqueous phase. bCalculated values: (total amount of n-propanol (g) − n-propanol amount in aqueous phase (g, n-propanol concentration in aqueous phase (g/kg) × the amount of aqueous phase (kg))/the amount of solvent phase (kg)). cResults determined by HPLC analysis. dAlcohol mass distribution: total amount of alcohol in each phase (g)/total amount of alcohol in both phases (g) × 100%. 5289

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Arabinose was not detected in solvent phase. bD, distribution coefficient: component concentration (g/kg) in solvent phase/component concentration (g/kg) in aqueous phase. cComponent mass distribution: the amount of a component in each phase (g, component concentration in each phase (g) × the amount of each phase (kg))/total amount of the component in both phases × 100%. a

0.3 0.3 0.3 0.1 0.2 ± ± ± ± ± 5.1 5.4 5.7 5.6 6.1 0.3 0.3 0..3 0.1 0.2 ± ± ± ± ±

solvent phase

94.9 94.6 94.3 94.4 93.9 89.9 ± 1.6 103.2 ± 3.9 100.9 ± 3.2 95.5 ± 3.0 84.6 ± 2.8 0.6 0.4 0.5 0.4 0.3 ± ± ± ± ± 15.6 13.3 12.5 11.4 11.2 0.6 0.4 0.5 0.4 0.3 ± ± ± ± ± 84.4 86.7 87.5 88.6 88.8 1.1 1.6 1.2 1.4 1.7 ± ± ± ± ± 25.9 38.6 42.3 43.9 43.7 0.2 0.1 0.1 0.1 0.1 ± ± ± ± ± 95.6 97.0 97.3 97.3 97.5 0.2 0.1 0.1 0.1 0.1 ± ± ± ± ± 4.4 3.0 2.7 2.7 2.5 0.01 0.01 0.00 0.01 0.01 ± ± ± ± ± 0.22 0.18 0.16 0.16 0.14 0.2 0.1 0.1 0.0 0.1 ± ± ± ± ± 95.0 96.1 96.8 97.1 97.3 0.2 0.1 0.1 0.0 0.1 ± ± ± ± ± 5.0 3.9 3.2 2.9 2.7 ± ± ± ± ± 0.25 0.24 0.20 0.17 0.16 first second third fourth fifth

0.01 0.01 0.01 0.01 0.00

aqueous phase solvent phase aqueous phase solvent phase glucose Db batch no.

solvent phase

aqueous phase

xylose Db

xylose mass distribution (%)c glucose mass distribution (%)c

Table 6. Soluble Sugars Distribution in Biphasic Solutions after Each Batch Pretreatmenta

HMF Db

HMF mass distribution (%)c

furfural Db

furfural mass distribution (%)c

The solvent distribution coefficient of the biphasic solution formed from the pretreatment solution was generally lower than that of the synthetic solution. These results indicate that the presence of other impurities such as soluble lignin and phenolics may have affected the solvent distribution. Other components which may also have affected solvent distribution were possibly propyl glycosides. Alkyl glycosides are the major products when alcohols and sugars are incubated in the presence of acid catalysts at elevated temperatures.34,35 Nevertheless, the concentration of n-propanol in the solvent phase was in a range of 632 g/kg to 672 g/kg, higher than the initial solvent concentration (588 g/kg) used for pretreatment. Therefore, the solvent phase can be reused directly for the next batch pretreatment with slight dilution. After five batches of pretreatments, n-propanol concentration was 632 g/kg, still higher than that used for the first pretreatment. In addition, the glucan digestibility after five batches of pretreatments was still high (96%), indicating that more recycling can be conducted before the recovery of n-propanol from the solvent phase by distillation. The aqueous phase contained only 9 g/kg n-propanol. Since the aqueous phase is distilled to separate water (and the residual n-propanol) and salt for recycling, the loss of npropanol will be negligible. Table 5 shows that after five pretreatments the majority of n-propanol was still distributed to the solvent phase, while only a small portion of n-propanol was distributed to the aqueous phase. Distribution of Soluble Sugar and Sugar Degradation Products. Table 6 shows the soluble sugar and sugar degradation product distribution in biphasic solution after each pretreatment. Arabinose was not detected in the solvent phase due to its low concentration. Compared to the synthetic solution, sugar distribution coefficients with pretreatment solutions were higher, indicating that the solvent phase has stronger affinity to sugars. This is likely due to the formation of propyl glycosides, which mainly existed in solvent phase and led to an increased sugar distribution coefficient. Nevertheless, the total amounts of glucose and xylose present in aqueous phase with aqueous n-propanol solution after pretreatment were still very high (>95%). Mass factions of HMF and furfural in alcohol solvent phases were also high, 89% and 94% respectively, after five batches of pretreatments. Figure 4 shows accumulations of sugars in aqueous phase and sugar degradation products (HMF and furfural) in solvent phase. Soluble glucose concentration increased from 0.7 g/kg to 2.7 g/kg, xylose from 3.2 g/kg to 13.5 g/kg, and arabinose from 0.4 g/kg to 1.6 g/kg with increasing pretreatment batches from one to five due to the recycle use of salt stillage (containing sugars). Higher sugar concentrations are expected with increasing numbers of recycles. Sugars and salts can be separated and recovered by SMBC after a number of recycles. Alternatively, these xylose- and salt-rich solutions can be used to produce value added chemicals such as furfural in saltinduced biphasic systems.28 HMF concentration in solvent phase increased from 0.15 g/kg to 0.30 g/kg, whereas furfural concentration increased from 2.4 g/kg to 15.1 g/kg with increasing pretreatment batches from one to five. The significant increase in furfural is attributed to the presence of xylose and significant amounts of n-propyl xylosides in solvent phase, which were dehydrated to furfural during the recycle of solvent and repeated pretreatment. The high concentration of furfural in solvent phase could reduce the recovery cost. The HMF and furfural concentrations in aqueous phase after five

aqueous phase

ACS Sustainable Chemistry & Engineering

DOI: 10.1021/acssuschemeng.7b00617 ACS Sustainable Chem. Eng. 2017, 5, 5284−5292

ACS Sustainable Chemistry & Engineering

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00617. Solvent and sugar distribution at different salt concentrations and pretreatment solution process (PDF)

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AUTHOR INFORMATION

Corresponding Author

*GPO Box 2432, 2 George St., Brisbane, QLD 4001, Australia. Tel: +61 7 3138 7792. Fax: +61 7 3138 4132. E-mail: jan. [email protected]. ORCID

Zhanying Zhang: 0000-0002-8041-0389 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support by Syngenta Biotechnology Inc. (US) and Sugar Research Australia. REFERENCES

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Figure 4. Accumulation of sugars in aqueous phase (a) and sugar degradation products in solvent phase (b).

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CONCLUSIONS A clean biomass fractionation process consisting of biomass pretreatment using aqueous n-propanol solution and saltinduced phase separation was developed and demonstrated. Unlike the other organosolv fractionation processes, this clean fractionation process did not recover the organic solvent by distillation; instead, the solvent was recovered and reused directly after each pretreatment following phase separation. In contrast, in the present process, water and salt used to induce the formation of the biphasic solution are recovered and recycled by distillation following each pretreatment. As a result, this novel clean fractionation process leads to the accumulation of sugars, especially hemicellulose sugars in aqueous phase as well as the accumulation of LMW lignins and sugar degradation products in solvent phase. After a number of repeated pretreatments, these components can be recovered at higher concentrations, which improving the process economics for the recovery of byproducts. 5291

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