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Biocompatible choline-based deep eutectic solvents enable one-pot production of cellulosic ethanol Feng Xu, Jian Sun, Maren Wehrs, Kwang Ho Kim, Sameeha S. Rau, Ann M. Chan, Blake A. Simmons, Aindrila Mukhopadhyay, and Seema Singh ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01271 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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ACS Sustainable Chemistry & Engineering

Biocompatible choline-based deep eutectic solvents enable one-pot production of cellulosic ethanol Feng Xu, Jian Sun, Maren Wehrs, Kwang Ho Kim, Sameeha S. Rau, Ann M Chan, Blake A. Simmons, Aindrila Mukhopadhyay, and Seema Singh 1,2#

1,2#

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1

3, 4, 5

1,2+

3, 4, 6, 7

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1,2*

Deconstruction Division, Joint BioEnergy Institute, 5885 Hollis Street, Emeryville, California

94608, USA Biological and Engineering Sciences Center, Sandia National Laboratories, 7011 East Avenue, Livermore, California 94551, USA 2

3

Biological Systems and Engineering Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA

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Biofuels and Bioproducts Division, Joint BioEnergy Institute, 5885 Hollis Street, Emeryville,

California 94608, USA 5

Institut für Genetik, Technische Universität Braunschweig, Spielmann Str.7, Braunschweig

38106, Germany 6

Environmental Genomics and Systems Biology Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA

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College of Letters & Sciences, University of Berkeley, 101 Durant Hall, Berkeley, California 94720, USA

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Equal contribution to this work; *Corresponding author: E-mail: [email protected]

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Current affiliation: Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, South Korea.

ABSTRACT Previous configurations of biomass conversion technologies based on the use of ionic liquids (ILs) suffer from problems such as high operating costs and large amounts of water used. There have been recent efforts on process intensification and integration to realize a one-pot approach for biofuel production using certain ILs, but these typically still require pH adjustment and/or dilution after pretreatment and before saccharification and fermentation. Deep eutectic solvents (DESs) were investigated as an alternative to ILs to address these challenges, and the results

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obtained suggest that certain DESs are compatible with hydrolytic enzymes and common biofuel producing microorganisms such as Saccharomyces cerevisiae. Among the DESs investigated, choline chloride:!glycerol (Ch12) achieved the highest rates of lignin extraction and pretreatment efficiency in terms of sugar yields (>80%) after enzymatic hydrolysis. Most importantly, the DES-Ch12-based “one-pot” biomass conversion process does not require any pH adjustment before commencing with saccharification and fermentation. Degradation compounds generated from polysaccharides (e.g., furfural) and lignin (e.g., ferulic acid) during biomass conversion were characterized and evaluated for their potential inhibitory effect on yeast growth and biofuel production. We conclude that this DES can be used to achieve biofuel (e.g., ethanol) production with a theoretical yield of 77.5% based on the initial glucan present in the biomass in a consolidated one-pot process configuration and re-defines biomass conversion using DESs. KEYWORDS: Deep eutectic solvents, Biocompatible, Lignocellulosic biomass, Biofuel, Onepot INTRODUCTION Biofuels and bioproducts derived from sustainable feedstocks are considered a potential solution to address the challenges associated with human population growth. For efficient biofuel 1

production, the biochemical conversion of lignocellulosic biomass has been frequently discussed in terms of process optimization as well as the reaction mechanism of various thermochemical processing (e.g., pretreatment) and biochemical conversion (e.g., enzymatic hydrolysis and fermentation). Current challenges to the realization of an affordable and scalable biomass 2-4

conversion technology are those associated with complicated process designs, difficulties associated with efficient solvent recycle, and water consumption.

5-6

Process intensification by minimizing separations has the potential to significantly reduce energy and water usage. Process intensification and integration within a biorefinery context can be challenging because of the typical discrepancy between the conditions used for pretreatment and those used downstream for saccharification and fermentation. For example, pretreatment usually employs acidic or basic conditions to disrupt the lignocellulosic plant cell wall and/or decrystallize cellulose for improved enzyme accessibility. Reagents used in pretreatment are usually not compatible with downstream processing (e.g., enzymatic saccharification and microbial fermentation) because of the differences in pH optima or the toxicity of the reagents

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and byproducts from pretreatment process. The recent advent of a certain class of biocompatible ionic liquids (ILs, such as cholinium lysinate) presents a very promising development in pretreatment due to their effectiveness and low toxicity to the enzymes or microbes after pH adjustment.

5-6

These features allow for a consolidated one-pot biomass-to-biofuel conversion

process that combines pretreatment, saccharification and fermentation in one vessel, but the 5-6

process requires the use of additional reagents and steps that can be challenging, especially in a high biomass loading process. To overcome these challenges, there is a need to identify alternative solvent systems that do not require pH adjustment after pretreatment. One example of an alternative IL is found in the use of certain protic alkylammonium ILs that do not require pH adjustments, water-washes and solid-liquid separations after pretreatment.

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However, these protic ILs were toxic to the microbes used in biofuel production and required significant dilution of the pretreatment effluent to reach acceptable concentrations. There has been recent attention on the use of certain deep eutectic solvents (DESs) many characteristics and properties with ILs,

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pretreatment solvents.

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because they share

and can be powerful lignocellulosic biomass

DESs are usually prepared by mixing a hydrogen bonding donor (HBD)

with a salt, a relatively convenient and inexpensive process as compared to the synthesis of most conventional ILs.

16,19

The numerous combinations of DES precursors also provide an opportunity

to identify a biocompatible DES that is also effective at biomass pretreatment. To the best of our knowledge, there have been no reports describing the use of DESs in a consolidated biofuel production process. This work introduces a set of biocompatible DESs that appear promising for use in the conversion of biomass into biofuels and bioproducts using a one-pot process. Since choline chloride is a relatively inexpensive, biodegradable and non-toxic compound that can be extracted from biomass or readily synthesized from fossil reserves, it was used as an organic salt to 15

produce DESs. The mechanism of biomass pretreatment in the presence of the synthesized DESs was identified and the processing conditions were optimized to decrease energy inputs and time. Potential cytotoxic by-products, such as furfurals, that may be formed during biomass pretreatment were monitored, and their impact on saccharification and fermentation is evaluated and discussed. To our knowledge, this is the first report that uses a biocompatible DES that integrates biomass pretreatment, saccharification, and fermentation in a single vessel without any solid/liquid separation and/or pH adjustment.

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MATERIAL AND METHODS All of the chemicals were reagent grade and purchased from Sigma-Aldrich (St. Louis, MO) if not specified otherwise. Corn stover was supplied by Michigan State University and prepared as reported. The enzymes (Cellic® CTec 2 and HTec 2) were a gift from Novozymes North 6

America (Franklinton, NC), containing 188 mg protein per mL. DESs preparation: Choline chloride and nine hydrogen bond donor molecules were mixed in the ratios listed in Table 1. The mixture was heated and stirred at 30, 60 or 80 C in a conical flask o

with plug to reduce volatilization until a homogenous colorless liquid was formed. Afterwards, the synthesized DESs were kept in a vacuum desiccator with silica gel until further use. Biomass pretreatment: A corn stover solid loading of 10% was used. For example, 0.5 g corn stover was mixed thoroughly with 4.5 g DES in a pressure tube (50 mL, Ace Glass Inc., Vineland, NJ), and the tube was then heated in an oil bath at a certain temperature for a few hours. The pretreated biomass was separated by centrifugation for compositional analysis. Briefly, the pretreated biomass was washed by de-ionized water for at least three times, and the solid fraction was collected after each wash by centrifugation. The solid fraction was then lyophilized for composition. The composition analysis was conducted according to the published NREL procedure.

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Enzymatic saccharification: The digestibility test was conducted with either a one-pot approach or a conventional approach in which the solid fraction was washed and separated before saccharification. Particularly, in a one-pot process of saccharification, 1M citric acid/citrate buffer was added to the pretreated biomass slurry for a final buffer concentration of 50 mM. The mixture of DES and biomass (e.g., 40 mL, in which DES is 4.5g, corn stover is 0.5g) was then tested for sugar yield in a 50-mL screwcap Falcon tube. The saccharification was carried out at 50 °C for 3 days (saccharification only) and pH 5 at 48 rpm in a rotary incubator (Enviro-Genie, Scientific Industries, Inc.) using commercial enzyme mixtures, Cellic® CTec2 and HTec2, with an enzyme dosage of 20 mg protein per gram glucan and 2 mg protein per gram xylan, respectively. Fermentation: For ethanol production assays, Saccharomyces cerevisiae strain BY4741 (MATa his3!0 leu2!0 met15!0 ura3!0), a derivative of S288C was cultivated according to the

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published NREL procedure. Yeast was inoculated directly into concentrated hydrolysates from 20

saccharification. For an integrated one-pot ethanol SSF, the temperature was decreased after a 1 6

day pre-saccharification (50 °C), and the SSF was then conducted with yeast loading of 3g/L (based on cell weight) under fermentative conditions at 120 rpm at 37 °C for 2-3 days. Analysis of sugars, ethanol, and other degradation compounds: The concentration of sugar, ethanol, HMF, and furfural was measured by HPLC (Agilent HPLC 1200 Series) equipped with a Bio-Rad Aminex HPX-87H column and a Refractive Index detector. The solid fraction after saccharification or fermentation in a dilute solution is below 1 wt% after dilution and its volume displacement could then be negligible. Glucose yield and ethanol yield were calculated based on the glucan content in corn stover, as 1.11 g glucose per gram glucan and 0.568 g ethanol per gram glucan, respectively. The phenolic compounds derived from lignin were determined using a LC-MSD according to the previously reported method.

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RESULTS AND DISCUSSION DESs as biocompatible solvents for biomass conversion One-pot biomass conversion can reduce the operating costs of biofuel production because it simplifies process design and reduces the energy input for the mass transfer between reactors that is typically required in traditional biomass process. One of the key elements for a successful 6, 9

one-pot process is the use of biocompatible reagents at all steps of the process, and it is important to screen DESs and determine their relative biocompatibility. Choline-based DESs are promising in this regard because they are bio-derived and can be produced in bulk. All DESs prepared in 17

this study are liquids at room temperature. A number of DES candidates based on [Ch][Cl] and various HBDs were selected and mixed in a certain molar ratio of HBD to salt (Table 1). Another key element for the successful consolidation process is a suitable pH value of DES that could enable downstream bioconversion without pH adjustment. Table 1 shows that the measured pH 5-7

values of the as-synthesized DESs in their 10wt% aqueous solutions vary in a broad range from 0.7 to 9.5. These varieties of pH values are mainly ascribed to the nature of HBD since [Ch][Cl] solution is a weak acidic salt with a pH value of around 5.3 in a 10wt% concentration. Saccharomyces cerevisiae is a well-studied and established host for the industrial production of ethanol,

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and was thus employed as production host in this study. Figure 1 shows the S.

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cerevisiae BY4741 growth profile in the presence of 5 wt% of various DES aqueous solutions. Six DES candidates were prepared by mixing [Ch][Cl] with urea, ethylene glycol, xylitol, isosorbide and glycerol in different molar ratios (abbreviated as Ch1, Ch5, Ch6, Ch9, Ch11, and Ch12 respectively), and were identified as promising in terms of biocompatibility. with yeast growth reaching similar cell density as those grown without DESs present (Figure 1), and in particular the DES-Ch12 showed excellent biocompatibility (Figure 1). Figure S1 further shows the growth profiles of the yeast strain in the presence of some of the synthesized DESs at different concentrations. Impact of delignification on sugar production The selected biocompatible DESs were further investigated in terms of biomass pretreatment efficiency as measured by sugar yield after saccharification, as well as lignin extraction efficiency, under a variety of processing conditions. Figure 2 shows the effect of selected process conditions on sugar yield and mass loss. As shown in Figure 2A, the selected biocompatible DESs showed different yields of sugar production, with DES-Ch5, -Ch6, and -Ch12 generating >70% yields following pretreatment and saccharification. The xylose yields are relatively low (below 50%), compared to other biomass processing methods. Compositional 23

analysis indicates that 30-40 % of xylan is hydrolyzed into the liquid phase after pretreatment, resulting in a significant mass loss.

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Based on these initial results, we then studied the performance of DES-Ch12 using different combinations of process temperatures and pretreatment times. As shown in Figure 2B, glucose yields increased with increases in either temperature or time and are generally attributed to increased delignification, which increases the accessible area of polysaccharides and reduces the absorption of enzymes by lignin. The results show that a significant portion (>60%) of lignin 25

was removed at 180 C, whereas the lignin extraction was ~10% at 160 C (Figure 2B). The o

o

significant improvement in lignin removal with the increasing of temperature is consistent with previous studies,

7, 26-27

and is most likely ascribed to the enhanced cleavage of ether bonds of lignin

facilitating lignin extraction from the biomass under higher temperature. The change in 27

pretreatment time, however, does not significantly affect delignification. At 160 C, the glucose o

yields increased significantly with increases in time despite minimal delignification, and is attributed to expansion of the cellulose fibers through DES penetration into the fiber bundles.

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Tracking the formation and impact of inhibitory compounds It is known that sugars can generate inhibitory compounds such as hydroxymethylfurfural (HMF) and furfural at high pretreatment temperatures, and are considered toxic to the yeast at certain concentrations.

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The reported inhibition of HMF for yeast growth is significant at 3 g L and -1

decreases ethanol yield from 99% to 89%. We monitored the production of these compounds 29

during DES pretreatment by using high-performance liquid chromatography (HPLC) and the results are shown in Figure 3A. The concentrations of HMF (e.g., 14.9 mg L ) and furfural (e.g., -1

36.4 mg L ) are significantly below the reported inhibitory concentrations for yeast fermentation. -1

The results suggest that the DES process can provide hydrolysates with minimal inhibition of yeast growth and biofuel production. As DES-Ch12 showed high levels of delignification, it is expected that inhibitory phenolic compounds such as p-coumaric acid and ferulic acid might be formed and the hydrolysates were analyzed using liquid chromatography-mass selective detector (LC-MSD). An increase in 18

pretreatment severity did increase the concentration of the phenolic compounds generated, and this finding is consistent with increased lignin extraction efficiency (Figure 3B). Benzoic acid and p-coumaric acid are the dominant lignin degradation compounds detected in the hydrolysates, but their concentrations are very low and are not above the ~1mM required for inhibition of yeast growth (Figure 3B). One-pot biomass conversion to biofuel A two-stage temperature controlling strategy was employed for saccharification and fermentation, based on our previous configuration for ethanol fermentation with bionic liquids.

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Presaccharification of the pretreated slurry was firstly conducted at 50 °C for 24 hrs, and then followed by a simultaneous saccharification and yeast fermentation at 37 °C for 48 hrs. A multistep ethanol conversion from corn stover was then successfully demonstrated in a single vessel. Compared to conventional configurations, the one-pot process with DES-Ch12 eliminated all solid/liquid separation steps (Figure 4), and did not require any pH adjustment. The process generated 149 g of ethanol from 1 kg of corn stover, which is equal to a conversion yield of 77.5% based on the glucose present. Feasibility of C6-C5 sugars co-fermentation

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Next, in order to test the feasibility of C5/C6 sugar co-fermentation and thus to further improve the economic feasibility of the process, the yeast strain JBEI-9009, developed in our lab for 22

optimized xylose consumption was used for further experiments. 30

The effect of DES Ch12 supplementation on growth and carbon utilization of JBEI-9009 was analyzed in a plate reader based assay (Figure S2, ESI). JBEI-9009 as well as the unmodified control strain can tolerate 10 wt% Ch12 in rich media (YPD) without showing significant growth limitations. To test if the presence of DES Ch12 in the media impairs the ability of the strain JBEI-9009 to efficiently utilize xylose, the strain was cultivated in CSM media with 2 wt% xylose containing 10 wt% Ch12 and final samples were taken for HPLC measurements. It was observed that even though the strain exhibits a longer lag phase when cultivated in the presence of DES Ch12, it eventually reaches a similar final OD600 (Figure S3A, ESI) and shows similar consumption of xylose in the media (Figure S3B, ESI).

CONCLUSIONS Biocompatible DESs were prepared by mixing choline chloride and a range of salts. Some of the DESs studied were demonstrated to be effective biomass pretreatment solvents and were found to be biocompatible and did not inhibit yeast growth. The generation of inhibitory degradation compounds from polysaccharides and lignin during pretreatment was also monitored, and the levels of these compounds detected were below reported toxicity thresholds. DES-Ch12 was demonstrated to be an effective pretreatment solvent that enabled the consolidation of saccharificiation and fermentation into a one-pot process that generated high yields of ethanol from corn stover. This promising approach offers significant advantages over other IL and DES biomass conversion technologies in that it does not require pH adjustment or dilution between pretreatment, saccharification and fermentation unit operations. In addition, the use of inexpensive renewable chemicals as the precursors for DESs may minimize the operational costs and environmental footprint of the entire process, providing a more affordable, sustainable and scalable biorefinery. Future work should be directed at the development of biofuel hosts that can convert all types of sugars produced, and/or the conversion of engineered bioenergy crops with enhanced C6 content, using this DES one-pot process. SUPPORTING INFORMATION

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Yeast growth profile with DESs; Sugar and lignin degradation; Xylose fermentation in DES. ACKNOWLEDGEMENTS This work conducted by the Joint BioEnergy Institute was supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research (DE-AC0205CH11231). The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paidup, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. REFERENCES 1.

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