Research Article pubs.acs.org/journal/ascecg
Biomass Pretreatment Using Dilute Aqueous Ionic Liquid (IL) Solutions with Dynamically Varying IL Concentration and Its Impact on IL Recycling Xueming Yuan,† Seema Singh,‡,§ Blake A. Simmons,‡,∥ and Gang Cheng*,†,‡,∥ †
College of Life Science and Technology, Beijing University of Chemical Technology, North Third Ring East, # 15, Beijing, 100029, China ‡ Deconstruction Division, Joint BioEnergy Institute (JBEI), 5885 Hollis Street, Emeryville, California 94608, United States § Biomass Science and Conversion Technology Department, Sandia National Laboratories, 7011 East Ave, Livermore, California 94551, United States ∥ Biological Systems and Engineering Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States ABSTRACT: A distillation apparatus was used in the pretreatment of white poplar samples with aqueous 1-ethyl-3-methylimidazolium acetate ([C2C1Im][OAc]) solutions. During biomass pretreatment, the concentration of [C2C1Im][OAc] was increasing due to removal of water via distillation. This allowed utilization of aqueous IL solutions with initially low IL concentrations that were not considered as effective pretreatment media previously. Aqueous [C2C1Im][OAc] solutions with initial concentrations of 7 and 15 wt % were tested to pretreat white poplar samples at 130 °C for 3 h. The biomass loading in [C2C1Im][OAc] was 20 wt %. After pretreatment, sugar conversion was found to account for 72−77% of the sample pretreated in neat [C2C1Im][OAc]. The encouraging results of this operation prompted a new approach to recycle and reuse [C2C1Im][OAc]: use the liquor collected after biomass pretreatment in neat [C2C1Im][OAc] without first separating water from it. In a similar fashion, biomass pretreated with the recycled liquor was exposed to dynamically increasing [C2C1Im][OAc] concentration, combining biomass pretreatment and water evaporation into one step. Three cycles of recycling and reuse were performed in this work, and the sugar conversion was found to decrease with the number of recycling cycles. It was believed that process optimization could improve sugar conversion further. The potential of this approach of biomass pretreatment, IL recycling, and reuse may stimulate the design of new IL pretreatment technologies. KEYWORDS: Ionic liquid, Biomass pretreatment, Recycling, Dilute IL solution, Distillation
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INTRODUCTION Pretreatment of lignocellulosic biomass using certain ionic liquids (ILs) to enhance enzymatic digestibility has become one of the leading pretreatment technologies after about 10 years of research and development.1−4 IL pretreatment was originally developed based on one of the unique properties of certain ILs: solubilization of cellulose under mild conditions.5,6 Dissolution and regeneration of cellulose in biomass from ILs lead to transformation of a recalcitrant native cellulose crystalline structure into cellulose II of reduced recalcitrance.7 However, it is not necessary to achieve full dissolution of biomass samples for improved digestibility since swelling and regeneration also reduce cellulose crystallinity or even lead to completely amorphous cellulose.8,9 In addition to cellulose crystalline structures, pretreated biomass samples often contain different contents of cellulose, hemicellulose, and lignin compared with untreated samples, which results in improved specific surface area.10,11 © 2017 American Chemical Society
The presence of lignin has been proposed as being detrimental to the enzymes12 used for saccharification after pretreatment, although correlations between lignin content/ structure and biomass digestibility have been controversial and vary widely.13,14 Removal of lignin during pretreatment changes the specific surface area of the substrate produced and improves accessibility of enzymes to polysaccharides present in plant cell walls.11 The relative contributions of specific surface area, lignin content, and cellulose crystalline structure to final sugar conversion vary with pretreatment conditions and biomass types.15 Mechanistic studies of the interactions between ILs and biomass components have improved our understanding of the IL pretreatment process;1,2,16 however one of the challenges Received: February 14, 2017 Revised: March 22, 2017 Published: March 28, 2017 4408
DOI: 10.1021/acssuschemeng.7b00480 ACS Sustainable Chem. Eng. 2017, 5, 4408−4413
Research Article
ACS Sustainable Chemistry & Engineering
Biomass Pretreatment. A distillation apparatus was used to pretreat biomass samples. White poplar samples were exposed to neat [C2C1Im][OAc] and two aqueous [C2C1Im][OAc] solutions at 130 °C for 3 h. The biomass loading with respect to [C2C1Im][OAc] was kept at 20 wt % in all solutions. The initial [C2C1Im][OAc] concentrations in the aqueous solutions were 7 and 15 wt %. At the end of pretreatment, the [C2C1Im][OAc] concentrations increased from 7 and 15 wt % to 55 and 64 wt %, respectively. After pretreatment, the reaction was quenched with the amount of water that was 16 times of the [C2C1Im][OAc] used in the pretreatment and vigorously stirred for half an hour. No additional washes were performed. The biomass solids were recovered by centrifuge and lyophilized for 24 h. After that, they were stored in sealed plastic bags at 5 °C for analysis. For each pretreatment condition, three replicates were performed. The losses of biomass components, cellulose, hemicellulose, and lignin, during pretreatment are calculated by the following equation:
has been the cost of this process relative to other pretreatment technologies.17 Different strategies have been proposed to reduce the cost of IL pretreatment: developing low-cost ILs,18−20 reducing usage of ILs,8,21−25 and recycling and reuse of ILs.26−36 A cost-effective and scalable IL pretreatment technology has not yet been developed which calls for continuing research in this field. An alternative to neat ILs used in biomass pretreatment has been mixtures of ILs with water10,37−41 and DMSO,32,42 which are cheaper and less viscous. One early study discovered that addition of small amounts of water (5−10 wt %) into 1-butyl-3methylimidazolium acetate [C4C1Im][OAc] and [C2C1Im][OAc] decreased their cellulose solvating ability, and the glucose and xylose yields after saccharification decreased as much as 50%.43 Later, studies using different ILs and/or pretreatment conditions (higher temperatures and longer pretreatment time) showed that pretreatments using aqueous ILs solutions could achieve comparable sugar yields after saccharification as compared to neat ILs.37−40 These aqueous IL solutions primarily removed a fraction of lignin and hemicellulose from the biomass samples during pretreatment16,37,39,40 and in some cases affected cellulose crystalline structure37,40 or its accessibility.38 The studies suggest that it is not necessary to use dry ILs to get decent biomass digestibility; this is particularly interesting in IL recycling which often involves separation of water/antisolvents from the ILs.26,30 In all of the previous pretreatment studies that used aqueous IL solutions, the water concentration was fixed in pressure cells during pretreatments, and it was less than 80 wt %.37−40 In some of the studies, the pretreatment temperatures were increased to 150−160 °C, which were on the borderline of liquid hot water pretreatment.44 In this work, we proposed a process to pretreat biomass using aqueous IL solutions at 130 °C in a distillation apparatus. One of the most studied cellulose solvents used in biomass pretreatment, [C2C1Im][OAc],3,4 was used in this work.
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Component loss % comonent in treated biomass × % recovered solids =1− % comonent in untreated biomass (1) XRD Measurement. The samples were scanned on a D8 ADVANCE diffractometer equipped with a sealed tube Cu Kα source. The operating voltage and current were 40 kV and 40 mA, and the Xray wavelength was 0.15406 nm. Scans were collected from 2θ = 5 to 60° with step size of 0.03 at 4 s per step. To minimize the uncertainty introduced by biomass pretreatment, samples obtained from three parallel pretreatments under the same condition were mixed for the XRD measurement. To ensure the reproducibility of the XRD data, microcrystalline cellulose (Avicel PH101) was measured each time along with other biomass samples. Enzymatic Hydrolysis. Enzymatic hydrolysis of the samples was carried out in a reciprocating shaker (Scientific Industries, Inc., SI1402) at 50 °C and 30 rpm. All samples were diluted to 5 g substrates per liter in a 50 mM sodium acetate buffer with a pH of 4.8 supplemented with 0.08 g/L tetracycline. The total volume was 10 mL with cellulase (NS50013) concentration of 50 mg protein/g glucan, βglucosidase (NS50010) concentration of 5 mg protein/g glucan, and hemicellulase (NS22002) concentration of 34 mg protein/g xylan. The hydrolysate liquid after 24, 48, and 72 h hydrolysis was separated from the enzymatic residue by high speed centrifugation (10,000g for 10 min) and analyzed with a DNS assay against glucose standard solutions. All assays were performed in triplicate.
EXPERIMENTAL SECTION
Materials. Wood chips were prepared from white poplar trees harvested from a local farm in Beijing, China. Biomass samples were ground and sieved to retain particulates with sizes equal to or less than 2 mm. Nonstructural materials were extracted with a Soxhlet extractor using water and ethanol for 12 h, respectively. This extraction was for the purpose of analyzing biomass components as well as losses of them during pretreatment as started in the NREL’s protocol (NREL/TP510-42618). It removes nonstructural components in biomass samples such as wax, proteins, minerals, etc. It was not required for biomass pretreatment. The ionic liquid, [C2C1Im][OAc]c, was purchased from Lanzhou Institute of Chemical Physics, China. Cellulase (NS50013, 137.3 mg protein/mL), β-glucosidase (NS50010, 208.7 mg protein/mL), and hemicellulase (NS22002, 21.5 mg protein/mL) were provided by Novozymes, China. The cellulose activity was measured to be 49 FPU/mL by the protocol of NREL (TP-510-42628, 1996). Compositional Analysis. Cellulose and hemicellulose contents were determined by a two-step acid hydrolysis and subsequent HPLC analysis based on the standard NREL procedure (NREL/TP-51042618). The sugar composition of the hydolysates was determined by high performance liquid chromatography (HPLC) using a refractive index detector (Hitachi, Tokyo, Japan). A Sugar-pak1 column (Waters, Milford, MA, USA) was used at 80 °C with ultrapure water as the eluent at a flow rate of 0.5 mL/min. The lignin content was determined with the acetyl bromide method using an averaged extinction coefficient of 23.007L/g cm.45
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RESULTS AND DISCUSSION
Pretreatment Using Aqueous [C2C1Im][OAc] Solutions. White poplar samples were pretreated in aqueous solutions with dynamically increasing [C2C1Im][OAc] concentrations, as described in the Biomass Pretreatment section. At the end of pretreatment, the water recoveries from 7 and 15 wt % solutions were 94 ± 2 and 89 ± 2 wt %, respectively. Consequently, the [C2C1Im][OAc] concentrations increased from 7 and 15 wt % to 55 and 64 wt %, respectively. Control experiments were performed where water was distilled from the aqueous [C2C1Im][OAc] solutions without biomass samples. The water recoveries from 7 and 15 wt % solutions were 92 ± 2 and 88 ± 2 wt %, respectively. Figure 1 presents the impact of the pretreatments on total sugar conversion of white poplar samples. The main purpose here was to correlate total sugar conversion to biomass pretreatments using dilute aqueous [C2C1Im][OAc] solutions. The 72 h sugar conversion, measured by the DNS assay, of the sample pretreated by neat [C2C1Im][OAc] is around 95%, which suggests that the residual [C2C1Im][OAc] in the biomass samples had little effect on the hydrolysis. The sugar 4409
DOI: 10.1021/acssuschemeng.7b00480 ACS Sustainable Chem. Eng. 2017, 5, 4408−4413
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Figure 3. Effect of pretreatment liquor reuse and [C2C1Im][OAc] concentration in aqueous solutions on cellulose crystalline structures in white poplar samples.
Figure 1. Effect of pretreatment liquor reuse and [C2C1Im][OAc] concentration in aqueous solutions on sugar conversion of pretreated white poplar samples.
conversions of the samples pretreated by the aqueous [C2C1Im][OAc] solutions are lower, accounting for 72% and 77% of that pretreated by neat [C2C1Im][OAc], respectively. It is believed that the sugar yields can be improved further by increasing water recovery through optimization of the distillation apparatus. In a recent study, the sugar yield dropped more than 50% for beech wood samples pretreated with 10 wt % [C2C1Im][OAc] aqueous solution at 115 °C for 1.5 h.16 In another work, glucose yield from switchgrass pretreated using 20 wt %[C2C1Im][OAc] aqueous solution at 160 °C for 3 h accounted for 70% of that pretreated with neat [C2C1Im][OAc].40 Compared with previous studies, the current process benefits from elimination of pressure cells and exposing biomass to a dynamically varying [C2C1Im][OAc] concentration. It is interesting to notice that losses of biomass components during aqueous [C2C1Im][OAc] solution pretreatment are similar to those of the sample pretreated by neat [C2C1Im][OAc], as shown in Figure 2. Around 75 wt % hemicellulose
with previous studies.16,40,43 The samples pretreated by neat [C2C1Im][OAc] has the less recalcitrant cellulose II structure (indicated by the peak at ∼12°), while the one pretreated by 15 wt % aqueous [C2C1Im][OAc] solution have both cellulose II and distorted cellulose I structures (as indicated by the shoulder peaks at ∼12° and ∼16°, respectively7). The sample pretreated by 7 wt % aqueous [C2C1Im][OAc] solution still has the native cellulose I structure with reduced biomass crystallinity of 0.38. On a relative scale, biomass crystallinity of cellulose I was evaluated by comparing the ratio of the peak intensities at 18° and 22°.7 The untreated sample has a biomass crystallinity of 0.59. Since the sample pretreated by 15 wt % aqueous [C2C1Im][OAc] solution contains a mixture of cellulose I and II, a comparison of biomass crystallinity with the other samples is not reasonable. However, the presence of less recalcitrant cellulose II and distorted cellulose I render the biomass samples more degradable, as shown in Figure 1. These changes of cellulose crystalline structures correlate nicely with sugar conversions of the samples. Removal of biomass components often leads to increased specific surface area,10,11 which also contributes to the improved sugar conversion. [C2C1Im][OAc] Recycling and Reuse. In order to make the IL pretreatment process more practical for industrial applications, ILs need to be recycled and reused for several consecutive cycles. Studies of recyclability and reusability of ILs have primarily focused on [C2C1Im][OAc] where it was concentrated by evaporation of the antisolvents.28,32,33,46−49 The recycled [C2C1Im][OAc] is usually contaminated with biomass components and degraded products which were produced during the pretreatment process.32,33,36,47,49 In several reports, biomass samples were pretreated with recycled [C2C1Im][OAc] without further purification, and diverse effects of impurities on pretreatment efficacy and extractability of lignin from biomass were reported.32,33,36,47,49 The most notable impurity has been shown to be lignin, which is enriched with an increasing number of recycling and reuse cycles.46 Depending on the pretreatment severity, different impacts of lignin in [C2C1Im][OAc] on pretreatment efficacy have been presented.32,33,49 This is similar to the effects of water in [C2C1Im][OAc] on pretreatment efficacy, as discussed above.16,37−40,43 After pretreatment, the amount of water added to the pretreatment slurry was 16 times of that of the [C2C1Im][OAc] in this work. The obtained biomass mixture was then stirred vigorously for 0.5 h to precipitate dissolved cellulose from [C2C1Im][OAc] and to wash remaining [C2C1Im][OAc] away
Figure 2. Effect of pretreatment liquor reuse and [C2C1Im][OAc] concentration in aqueous solutions on losses of biomass components in white poplar samples during pretreatment.
(mostly xylan) was lost during pretreatment. In one prior study, neat [C2C1Im][OAc] and its aqueous solutions were used to pretreat switchgrass at 160 °C for 3 h.40 The losses of biomass components decreased with increasing water content up to 80 wt %.40 Around 79 wt % xylan was lost after pretreatment using neat [C2C1Im][OAc], and around 70 wt % xylan was lost using neat water under that pretreatment condition.40 Changes to cellulose crystalline structures are presented in Figure 3. Addition of water retarded the transformation of the cellulose crystalline structure from cellulose I to II, consistent 4410
DOI: 10.1021/acssuschemeng.7b00480 ACS Sustainable Chem. Eng. 2017, 5, 4408−4413
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ACS Sustainable Chemistry & Engineering Table 1. Recycling and Reuse of Pretreatment Liquor biomass used for pretreatment (g) neat IL recycling/reuse, first recycling/reuse, second recycling/reuse, third recycling, fourth
0.3 0.3 0.3 0.3
mass of recycled pretreatment liquor (g) 1.2 17.5 ± 18.4 ± 18.0 ± 17.3 ±
0.3 0.1 0.1 0.2
antisolvent water (g)
water collected by distillation (g)
19.2 19.2 19.2 19.2
0 14.4 ± 0.1 15.7 ± 0.1 15.9 ± 0.1
liquor recovery (wt %) 86 83 82 81
crystallinity than that of untreated samples (0.59) is due to removal of amorphous material during pretreatment, similar to what occurs in dilute acid pretreatment.50 The results suggest that significant amount of [C2C1Im][OAc] was lost during recycling. This was mainly due to the fact that the liquor recovery was around 80−85% for each cycle. However, the sample still exhibits 65% sugar conversion, 5 times higher than that of the untreated sample, which is attributed to the increased specific surface area as a result of extraction of biomass components, especially hemicellulose, shown in Figure 2. Losses of biomass components during pretreatment stayed on similar levels with an increasing number of recycling and reuse cycles. This indicates that pretreatment liquors maintained their solvating power under current experimental conditions. Previous studies also demonstrated that lignin extractability by recycled [C2C1Im][OAc] did not change due to its higher solubility in [C2C1Im][OAc] under certain pretreatment conditions.46,49 It is noted that the sugar conversion of the sample pretreated with the liquor after the first recycling cycle is higher than that of the one pretreated with 7 wt % aqueous solution. This is also attributed to changes in the cellulose crystalline structure. The white poplar sample pretreated with recycled liquor contains a low-order structure which is less recalcitrant than that of the one pretreated with 7 wt % aqueous solution. The maximum possible concentration of [C2C1Im][OAc] in the pretreatment liquor at the end of pretreatment was calculated as 39 wt % based on the amount of water removed during pretreatment via distillation (Table 1). In comparison, the sample pretreated with 7 wt % aqueous solution still contains the native cellulose I structure, and the [C2C1Im][OAc] concentration increased to 55 wt % at the end of pretreatment. Although a prior study concluded that the presence of lignin in [C2C1Im][OAc] decreased pretreatment efficiency for cellulose and rice straw,32 another work demonstrated that lignin accumulation in [C2C1Im][OAc] did not affect the efficiency of the pretreatments of wood sawdust.49 The role of dissolved lignin on the pretreatment efficacy in a dynamic environment requires further study.
from the biomass. This was followed by a centrifuge step where biomass solids and the liquor were separated. The liquor collected after biomass pretreatment in the neat [C2C1Im][OAc], named as pretreatment liquor, was used for next cycle of biomass pretreatment without evaporation of water and purification. The same amount of biomass samples were added to the pretreatment liquor (Table 1); the actual weight percentage of biomass in [C2C1Im][OAc] should be larger than 20 wt % considering [C2C1Im][OAc] loss during recycling. As shown in Table 1, the pretreatment liquor recovery was 86 wt % after the first recycle. Therefore, the upper limit of [C2C1Im][OAc] concentration in the pretreatment liquor was calculated as 1.2/17.5 = 6.9 wt % if there was no [C2C1Im][OAc] loss. Similarly, the maximum [C2C1Im][OAc] concentrations in the pretreatment liquors after the second and third recycling cycle were estimated to be 1.2/18.4 = 6.5 wt % and 1.2/18.0 = 6.7 wt %, respectively. Determination of the [C2C1Im][OAc] concentration in the pretreatment liquor by UV−vis was complicated by the presence of solubilized lignin. Future work includes evaluation of different approaches. For example, the water content in the pretreatment liquor after the third recycling cycle was measured to be around 97.3 wt % by evaporation of water at 80 °C. Therefore, it is certain that the [C2C1Im][OAc] concentration decreases in the pretreatment liquor with increasing the number of recycling and reuse cycles. Biomass pretreatment was done in the distillation apparatus, similar to that of using aqueous [C2C1Im][OAc] solutions. Pretreatments using 7 and 15 wt % aqueous [C2C1Im][OAc] solutions provide experimental support for this mode of recycling and reuse. One difference is that the pretreatment liquor contains dissolved biomass components. Recycling and reuse were repeated three times in this work. The recovery of pretreatment liquor varied from 81 to 86 wt % in each cycle. In comparison, the IL recovery was reported to be in the range of 80 to 95 wt % in each cycle in previous studies.33,48,49 Figure 1 presents the number of recycling and reuse cycles on the sugar conversion of white poplar samples. The 72 h sugar conversion decreases from 95 to 65% with an increasing number of recycling and reuse cycles. This decreased biomass pretreatment efficacy is mainly due to diminishing [C2C1Im][OAc] concentration in the pretreatment liquor. Changes in cellulose crystalline structures (0.35, 0.43, and 0.62) with the number of recycling cycles are consistent with this conclusion (Figure 3). The native cellulose crystalline structure of the white poplar sample pretreated with the liquor after the first recycling cycling was severely distorted, as indicated by the weak shoulder peaks around 12° and 16° as well as the broader main peak at around 22°. The XRD data suggests a lower ordered structure with reduced recalcitrance.42 The samples pretreated with the liquor after the second and third reuse cycles contains native cellulose I lattice with biomass crystallinity of 0.43 and 0.62, respectively. Higher biomass
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CONCLUSIONS A new method of biomass pretreatment using aqueous IL solutions was proposed. Using this approach, dilute aqueous [C2C1Im][OAc] solutions (7 and 15 wt % IL) can produce reasonable pretreatment efficiencies. Pretreatments of white poplar with dilute aqueous [C2C1Im][OAc] solutions of dynamically increasing [C2C1Im][OAc] concentration were successfully performed. The developed method allowed obtaining decent sugar conversion from the biomass samples pretreated in aqueous [C2C1Im][OAc] solutions using a distillation setup. More importantly, this mode of pretreatment was further exploited in the design of new IL recycling/reuses processes. Aqueous IL solutions recycled after biomass 4411
DOI: 10.1021/acssuschemeng.7b00480 ACS Sustainable Chem. Eng. 2017, 5, 4408−4413
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pretreatment were used directly to pretreat biomass. Further studies, such as improving water recovery, minimizing IL loss, and purifying IL after several cycles, are needed to optimize this process, and this approach could result in a highly efficient IL pretreatment platform.
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AUTHOR INFORMATION
Corresponding Author
*E-mails:
[email protected];
[email protected]. cn. Tel: +86 13693276690. ORCID
Gang Cheng: 0000-0003-2014-7838 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Gang Cheng acknowledges support for this research by the National Natural Science Foundation of China (U1432109) and China Scholarship Council (201606885004). We acknowledge support by the DOE Joint BioEnergy Institute (http:// www.jbei.org) through the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, through contract DE-AC02-05CH11231 between Lawrence Berkeley National Laboratory and the U.S. Department of Energy.
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DOI: 10.1021/acssuschemeng.7b00480 ACS Sustainable Chem. Eng. 2017, 5, 4408−4413
Research Article
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DOI: 10.1021/acssuschemeng.7b00480 ACS Sustainable Chem. Eng. 2017, 5, 4408−4413
