Biomass Pretreatment Using Dilute Aqueous Ionic Liquid (IL

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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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b00480 • Publication Date (Web): 28 Mar 2017 Downloaded from http://pubs.acs.org on March 29, 2017

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ACS Sustainable Chemistry & Engineering

Biomass pretreatment using dilute aqueous ionic liquid (IL) solutions with dynamically varying IL concentration and its impact on IL recycling XuemingYuana, Seema Singhb,c, Blake A. Simmonsb,d and Gang Chenga,b,d* a College of Life Science and Technology, Beijing University of Chemical Technology, North 3rd Ring East, # 15, Beijing, 100029, China. b Deconstruction Division, Joint BioEnergy Institute (JBEI), 5885 Hollis Street, Emeryville, CA 94608, USA. c Sandia National Laboratories, 7011 East Ave, Livermore, CA 94551, USA d Biological Systems and Engineering Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA. Corresponding author’s [email protected] Tel: +86 13693276690

email:

[email protected];

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 3h. 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 of [C2C1Im][OAc]: use the liquor collected after biomass pretreatment in neat [C2C1Im][OAc] without firstly 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. 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 Synopsis: Dilute aqueous ionic liquid (IL) solutions were used for biomass pretreatment and its impact on IL recycling and reuse was studied

ACS Paragon Plus Environment


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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 year’s 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 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 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


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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 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 ILs8, 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 DMSO32, 42, which are cheaper and less viscous. One early study discovered that addition

of

small amounts of

water (5-10 wt.%) into

1-butyl-3-methylimidazolium 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 faction of lignin and hemicellulose from the biomass samples during pretreatment,16,

37, 39, 40

and in some cases affected cellulose crystalline

structure37, 40 or its accessibility38. 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/anti-solvents from the ILs.26, 30 In all of the previous pretreatment studies that used aqueous IL solutions, the water



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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.

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 2mm.Non-structural materials were extracted with a Soxhlet extractor using water and ethanol for 12h, 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/TP-510-42618). It removes non-structural components in biomass samples such as wax, proteins, minerals, etc. It was not required for biomass pretreatment. The ionic liquid, [C2C1Im][OAc]c, were purchased from Lanzhou Institute of Chemical Physics, China. Cellulase (NS50013, 137.3mg

protein/ml),

β-glucosidase

(NS50010,

208.7mg

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


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Cellulose and hemicellulose contents were determined by a two-step acid hydrolysis and subsequent HPLC analysis, based on the standard NREL procedure (NREL/TP-510-42618). 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 80oC with ultrapure water as the eluent at a flow rate of 0.5mL/min. The lignin content was determined with the acetyl bromide method using an averaged extinction coefficient of 23.007L /g· cm.45 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 3hrs. 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 from7 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 24h. After that they were stored in sealed plastic bags at 5ºC 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:



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Component loss = 1-

% ×% %

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(1)

XRD measurement The samples were scanned on a D8 ADVANCE diffractometer equipped with a sealed tube Cu Kαsource. The operating voltage and current were 40 kV and 40mA and the x-ray wavelength was 0.15406nm. Scans were collected from 2θ = 5 to 60º with step size of 0.03at 4 s per step. To minimize the uncertainty introduced by biomass pretreatment, samples obtained from 3 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., SI-1402) 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, β-glucosidase (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 72h hydrolysis was separated from the enzymatic residue by high speed centrifugation (10,000g for 10 min) and analyzed with DNS assay against a glucose standard. All assays were performed in triplicate.


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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 wt.% 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 wt.% 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 72h 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 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 sample pretreated with 10 wt.% [C2C1Im][OAc] aqueous solution at 115°C for 1.5h.16 In another work, glucose yield from switchgrass pretreated using 20 wt.%[C2C1Im][OAc] aqueous solution at 160°C for 3h accounted



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for 70% of that pretreated with neat [C2C1Im][OAc].40 Compared with previous studies, the current process benefit from elimination of pressure cells and exposing biomass to a dynamically varying [C2C1Im][OAc] concentration.

Figure 1. Effect of pretreatment liquor reuse and [C2C1Im][OAc] concentration in aqueous solutions on sugar conversion of pretreated white poplar samples. 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 (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 3h.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


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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.

Figure 3 Effect of pretreatment liquor reuse and [C2C1Im][OAc] concentration in aqueous solutions on cellulose crystalline structures in white poplar samples.

Changes to cellulose crystalline structures are presented in Figure 3. Addition of water retarded the transformation of cellulose crystalline structure from cellulose I to II, consistent 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 native cellulose I structure with reduced biomass crystallinity of 0.38.



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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 area10, 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 anti-solvents.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 increasing number of recycling and reuse.46 Depending on the pretreatment severity, different impacts of


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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 0.5h to precipitate dissolved cellulose from [C2C1Im][OAc] and to wash remaining [C2C1Im][OAc] away 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 second and third time recycling 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 third time



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recycling 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. 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 Table 1 Recycling and reuse of pretreatment liquor

Neat IL Recycling/Reuse , 1st Recycling/Reuse e, 2nd Recycling/Reuse , 3rd Recycling, 4th

Biomass used for pretreatment(g)

Mass of recycled pretreatment liquor(g)

Anti-solvent water (g)

0.3

1.2

19.2

0

0.3

17.5±0.3

19.2

14.4±0.1

86

0.3

18.4±0.1

19.2

15.7±0.1

83

0.3

18.0±0.1

19.2

15.9±0.1

82

17.3±0.2

Liquor Water collected by recovery distillation(g) (wt.%)

81

Figure 1 presents the number of recycling and reuse on the sugar conversion of white poplar samples. The 72h sugar conversion decreases from 95 to 65 % with increasing number of recycling and reuse. This decreased biomass pretreatment efficacy is


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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 are consistent with this conclusion (Figure 3). Native cellulose crystalline structure of white poplar sample pretreated with the liquor after first time recycling 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 second and third time reuses contains native cellulose I lattice with biomass crystallinity of 0.43 and 0.62, respectively. Higher biomass 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, five times higher than that of 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 increasing number of recycling and reuse. 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



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It is noted that the sugar conversion of the sample pretreated with the liquor after first time recycling is higher than that of the one pretreated with 7 wt.% aqueous solution. This is also attributed to changes of 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 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.

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


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[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 pretreatment was 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.

Acknowledgements 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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TOC associated synopsis

A

distillation

apparatus

was

used

in

the

biomass

pretreatment

with

1-ethyl-3-methylimidazolium acetate ([C2C1Im][OAc]) solutions as well as recycling and reuse


aqueous

Page 21 of 21

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182x129mm (144 x 144 DPI)


Biomass Pretreatment Using Dilute Aqueous Ionic Liquid (IL

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