Aprotic vs Protic Ionic Liquids for Lignocellulosic Biomass

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11928−11936

Aprotic vs Protic Ionic Liquids for Lignocellulosic Biomass Pretreatment: Anion Effects, Enzymatic Hydrolysis, Solid-State NMR, Distillation, and Recycle Md. Mokarrom Hossain,† Aditya Rawal,‡ and Leigh Aldous*,†,§ †

School of Chemistry, UNSW Sydney, Sydney, NSW 2052, Australia NMR Facility, Mark Wainwright Analytical Centre, UNSW Australia, Sydney, NSW 2052, Australia § Department of Chemistry, King’s College London, Britannia House, 7 Trinity Street, London SE1 1DB, United Kingdom Downloaded via GUILFORD COLG on July 24, 2019 at 22:11:57 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Imidazolium-based ionic liquids (ILs) with acetate, formate, and chloride anions have been investigated for lignocellulosic biomass pretreatment in both their (conventional) aprotic and (largely unexplored) protic forms. Notably, the protic IL 1-ethylimidazolium chloride has been demonstrated to reversibly dissolve whole wood ( Pinus radiata) and significantly enhance enzymatic hydrolysability of the pretreated biomass. The acetate-based and formate-based protic IL could only extract lignin. Solid-state 13 C NMR and thermogravimetric analysis demonstrate that the protic IL physically pretreats the biomass in a different manner to the aprotic ILs; however, the extent of lignin sidereactions such as methoxy group loss was dominated by the nature of the anion. The protic IL 1-ethylimidazolium chloride could be recycled by vacuum distillation and undergo several rounds of wood pretreatment followed by distillation recycle without a decrease in effectiveness. KEYWORDS: Wood, Pinus radiata, Cellulase, Glucose, Lignin side-reactions, Acetate, Chloride

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INTRODUCTION Lignocellulosic biomass has been considered as an alternative source of liquid fuel and feedstock chemicals1 to offset our heavy reliance upon finite fossil fuel sources.2 Cellulose, lignin, and hemicellulose make up the vast majority of lignocellulosic biomass. Lignin blocks accessibility to the holocellulose components and is a major obstacle in biomass processing.3 Furthermore, the high crystallinity of cellulose resists chemical and biological treatment and thus limits conversion to final products.4 Numerous approaches to biomass pretreatment have been introduced, mostly aimed at lignin removal, reduction of cellulose crystallinity, improving cellulose accessibility, and component fractionation.5 Ionic liquids (IL), defined as organic salts with a melting point below 100 °C, are strong potential candidates for biomass pretreatment.1 Strong interactions between ILs and lignocellulosic biomass (primarily stemming from the hydrogen bonding basicity of the ILs’ anions6) can result in partial and/or complete dissolution of lignocellulosic components.7 ILs have been employed with biomass as diverse as wood,8,9 grasses,10 nut shells,11 chilli,12 feathers,13 etc. For lignocellulosic biomass, typically lignin and cellulose can dissolve in certain ILs, and a cellulose-rich fraction can be precipitated from the IL by addition of acetone/water, acetonitrile, methanol, etc. as an antisolvent.11 © 2019 American Chemical Society

However, although a majority of the dissolved material (by mass) is precipitated, a plethora of smaller biomolecules and compounds remain dissolved in the IL; these represent a major recycling challenge. The development of protic ILs (ionic species formed by reversible proton transfer) predates the development of aprotic ILs (e.g., ion formation arising due to covalent bond formation and breaking). Despite this, aprotic ILs dominate studies relating to lignocellulosic biomass processing. Strong hydrogen-bonding basicity is known to be a critical parameter for ILs in achieving biomass dissolution (in addition to other interactions).6 The labile proton inherent to protic ILs is therefore an impediment in lignocellulosic biomass dissolution. As such, whole lignocellulosic biomass dissolution has not been widely demonstrated in a protic IL, although other biomass (such as keratin13) has been fully dissolved. Recently, certain protic ILs have been labeled as “cost-effective” biomass pretreatment media, largely via delignification processes;14 biomass solubility in ILs has been extensively discussed and reviewed elsewhere.1,2,15−17 Received: November 16, 2018 Revised: May 30, 2019 Published: June 11, 2019 11928

DOI: 10.1021/acssuschemeng.8b05987 ACS Sustainable Chem. Eng. 2019, 7, 11928−11936

Research Article

ACS Sustainable Chemistry & Engineering

ethyl-3-methylimidazolium chloride ([Emim]Cl), 1-ethyl-imidazolium chloride ([Eim]Cl), and 1-ethyl-imidazole were all from IoLiTec, Germany, and used as received. Glacial acetic acid (Fisher Chemicals), sulfuric acid (98%, Ajax Finechem), hydrochloric acid (32%, Ajax Finechem), formic acid (Ajax Finechem), sodium acetate (Ajax Finechem), deuterium oxide (Cambridge Isotope), cycloheximide (Sigma-Aldrich), tetracycline hydrochloride (Sigma-Aldrich), cellulase (Cellucast 1.5L, 700 EGU/g, Novozymes) were used as received. All thermal gravimetric analysis (TGA) experiments were performed using a TGA/DSC 1 STARe system (Mettler-Toledo, Switzerland). Approximately 10 mg of the sample was placed in a platinum crucible and heated at a rate of 10 °C min−1 under a continuous flow of 30 mL min−1 of argon. Water content was measured by Karl Fischer titration using an 831 KF Coulometer (Metrohm, Switzerland). UV−vis spectra were acquired using a Cary 100 Bio (Varian, Australia) UV−vis spectrophotometer. pH was measured using a Mettler Toledo pH meter (Switzerland). Density and viscosity were measured using a combined set of density meter, DMA 4100 M, and microviscometer, Lovis 2000ME (Anton Paar, Austria). All the 1H NMR spectra were acquired using Bruker AVANCE III 300 spectrometer. The 13C solid-state NMR experiments were also carried out on Bruker AVANCE III 300 spectrometer. All the NMR data were processed using Bruker TopSpin 3.1 software. Distillations of the ILs were performed using a Kugelrohr apparatus, Buchi glass oven B-585 (Switzerland). Synthesis of Ionic Liquids. 1-Ethyl-imidazolium acetate ([Eim][OAc]) and 1-ethyl-imidazolium formate ([Eim][HCOO]) were prepared in the lab by stoichiometric mixing of 1-ethyl-imidazole and glacial acetic acid or formic acid, respectively. 1-Ethylimidazolium chloride ([Eim]Cl) was synthesized by mixing equimolecular 1-ethylimidazole and hydrochloric acid (32%), which was in liquid form. The excess water introduced by the HCl was then removed by overnight heating at 70 °C under strong vacuum. Solubility and Fractionation Tests. Solubility of wood powder in different ILs was performed in a round-bottomed flask to which 1 g of IL and a known quantity of biomass were added. The mixture was placed into an oil bath and heated at 115 °C (for wood) for 18 h, stirring using a Teflon-coated stirrer bar at 700 rpm. The neck of the flask remained open to the atmosphere or was connected to a Schlenk line and maintained under continuous high vacuum (different treatments specified in the text; where not specified, the experiments were open to the atmosphere). As a general fractionation process, 10 mL (ca. 10 times the volume of the IL) of an acetone−water mixture (1:1) was added to solutions of biomass in the IL in the flask and stirred for 20 min. Cellulose-rich solid residue was recovered via simple filtration using a Millipore nylon filter (0.22 μm) and dried overnight at 70 °C under vacuum. All other fractionation tests followed the same general volumes and processes, with the different solvents employed detailed in the main text. For the extraction of lignin from wood flour in [Eim][OAc] and [Eim][HCOO], the same dissolution parameter (115 °C for 18 h) as above was maintained with an extra reflux setup, and fractionation was done via simple filtering of an IL−wood mixture without adding any antisolvent. Quantification of Lignin and Glucose Content of Wood Flour. The lignin content of untreated biomass and IL-regenerated cellulose-rich solid was quantified according to the TAPPI method T222om-11.1 A dried sample was placed in a 20 mL sample tube, and 72% H2SO4 was added using 1.5 mL for 100 mg of untreated biomass or 4 mL for 200 mg of regenerated solid. A small magnetic stirrer bar was placed in the mixture and stirred for 2 h at room temperature. The mixture was then transferred into a round-bottomed flask followed by the addition of water (56 mL for untreated or 150 mL for regenerated solid) and refluxed for 4 h. The refluxed solution was filtered using a Millipore nylon filter (0.22 μm). The residue was collected and dried, and the weight was used to quantify the acid insoluble lignin content. The filtrate was diluted with water to make up 100 mL for pine wood flour or 200 mL for the regenerated

Protic ILs can be distilled, i.e., by the application of heat and vacuum, to encourage dissociation of the acid−base pair.18 The topic of distillable ionic liquids and biomass processing has been reviewed;15 as an example, protic ILs based upon a “superbase” have been demonstrated to dissolve pure cellulose and are also distillable.19 In another example, protic ILs have been demonstrated to extract lignin from corn stover and can be distilled to recover the lignin.20 A major goal of whole biomass pretreatment is to make the biomass more amenable to downstream processing, such as acid or enzymatic hydrolysis to yield sugars.21 Recently, inherently acidic protic ILs have been investigated, such as those prepared from H2SO4. Initial proton transfer forms the cation in the protic IL, and the resulting [HSO4]− anion is inherently acidic due to its second, labile proton. These were demonstrated to be cheap (relative to aprotic ILs) and 20%− 75% as effective as a benchmark aprotic IL at pretreating switchgrass prior to enzymatic hydrolysis.14 Recycle was not demonstrated, and they underwent side-reactions before distilling. In this study, we have investigated three imidazolium-based protic ILs, 1-ethylimidazolium acetate ([Eim][OAc]), 1ethylimidazolium formate ([Eim][HCOO]), and 1-ethylimidazolium chloride ([Eim]Cl) (structures displayed in Figure 1), for lignocellulosic biomass processing. It is

Figure 1. Structure of the imidazolium-based protic ([Eim][OAc], [Eim][HCOO], and [Eim]Cl) and aprotic ([Emim][OAc] and [Emim]Cl) ILs used.

important to note that 1-methylimidazolium acetate has been previously reported.15 Two direct aprotic analogues were chosen, [Emim][OAc] and [Emim]Cl, which have been very extensively studied for biomass processing, and their physical properties are known.22−24 This allowed direct protic vs aprotic comparison. Biomass dissolution capabilities and the enzymatic hydrolysis of pretreated lignocellulosic biomass (using Pinus radiata wood) were investigated for the five ILs; the biomass recovered from the IL after pretreatment was analyzed in detail by solid-state NMR. Distillation-based recycle and reuse of the protic IL [Eim]Cl was also investigated.

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EXPERIMENTAL SECTION

Materials and Instruments. Pine (Pinus radiata) wood flour was received from Micro Milling Pty Ltd. (NSW, Australia) and used as received. 1-Ethyl-3-methylimidazolium acetate ([Emim][OAc]), 111929


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Figure 2. 1H NMR spectra for H6 on the imidazolium ring (ca. 8.5 ppm, left) and D2O (ca. 4.7 ppm, right) for the various ratios of Eim and HCl; 1:0 (a), 1:0.5 (b), 1:1 (c), 1:1.5 (d), and 1:2 (e). material. The UV absorbance of the diluted sample was measured at 205 nm. The acid soluble lignin content was calculated using an extinction coefficient of 110 L g−1 cm−1.1 A total of 1 mL of the filtrate was then neutralized to pH 6 with CaCO3. The supernatant layer was filtered, and its glucose content was measured using a portable glucose meter (Accuchek Active, Roche), which had been calibrated prior to use and demonstrated to be selective to glucose over other sugars. Enzymatic Hydrolysis. Enzymatic hydrolysis of the various biomass samples were performed by slightly modifying and scaling down the standard method.2 Enzymatic hydrolysis was conducted in centrifuge vials (1.5 mL) using 973.3 μL of 50 mM acetate buffer of pH 4.8, 40 μg of cycloheximide (4 μL from 10 mg mL−1 in 70% ethanol solution), 30 μg of tetracycline hydrochloride (3 μL from 10 mg mL−1 in distilled water solution), and 25 FPU g−1 cellulase (7.14 μL of as received Cellucast 1.5L). The sample vials were placed in a heating block to maintain a constant temperature of 50 °C, and the whole block was then placed onto a plate shaker (Maxi Rotator, USA). Moreover, the entire sample was vigorously shaken 5 min before measuring the glucose concentration. The converted glucose concentration was measured over time using a portable glucose meter (Accuchek Active, Roche) for selective quantification, which was calibrated earlier in the lab for accuracy. Each experiment was performed in triplicate. Recycling of ILs. Recycling of ILs was carried out using a Kugelrohr apparatus, Buchi glass oven B-585 (Switzerland) consisting of a three bulb setup which was connected to vacuum. The first bulb (inside the oven) contained the contaminated ILs, and the temperature was increased in 20 °C increments, up to 200 °C, and left at this temperature until all of the ILs vaporized from first bulb (∼ 1 h). The distilled IL was collected in the second bulb, while the water and other impurities were collected in the third bulb (cooled in an ice bath). Solid-State 13C Nuclear Magnetic Resonance (13C sNMR). The samples of untreated and ionic liquid-treated wood powder were finely ground and packed into 4 mm zirconia rotors with Kel-F caps. The 13C NMR experiments were carried out on Bruker AVANCE III 300 spectrometer with a 7 T superconducting magnet operating at frequencies of 300 and 75 MHz for the 1H and 13C nuclei, respectively. The quantitative 13C NMR spectra of the materials were acquired by the MultiCP technique3 at 13 kHz MAS with a 13C-90° and 1H-90° pulse length of 4 and 3.2 μs, respectively. 1H SPINAL64 decoupling with a field strength of 80 kHz and a Hahn-echo before signal detection were used to eliminate baseline distortion. Here, 1.5 s recycle delays, seven loops of 0.9 s repolarization time periods, and 1 ms cross-polarization contact time after each repolarization time period were used for acquiring the data.

acid:base in protic ionic liquids is a critical determining factor in their physical properties.25 Investigation of the PILs in D2O by 1H NMR was found to be a convenient method to probe the acid:base ratio. For example, the 1H NMR spectra for [Eim]Cl indicated that excess base (Eim) in the PIL resulted in a proton on the imidazolium ring (H at the C-2 position, ca. 8.5 ppm) shifting upfield, whereas the presence of excess HCl resulted in water (H2O inherent in the D2O solvent, ca. 4.7 ppm) shifting downfield in Figure 2 (full spectra in Figure S2 in the Supporting Information (SI)). Thus, the stoichiometry could be monitored, and as long as the concentration was kept constant, achieving a 1.0:1.0 (±0.05) ratio could be accurately confirmed using this technique. The physical and thermal properties of the protic and aprotic ILs used in this study were characterized (SI Table S1, SI Figures S3 and S4) and compared with literature values. Notably, the chloride-based systems had melting points above room temperature ([Eim]Cl, 69 °C; [Eim]Cl, 82 °C), while the others were liquid at room temperature. Also important to note is the fact that (when measured as 10 wt % aqueous solutions) none of the ILs were found to be strongly acidic, with [Emim]Cl having the lowest pH value of 5.0. The thermal properties of the ILs were investigated by TGA (SI Figure S4), with the thermal stability increasing in the order [Eim][HCOO] ∼ [Eim][OAc] < [Emim][OAc] < [Eim]Cl < [Emim]Cl. All could be volatilized, although this was irreversible for [Emim][OAc] and [Emim]Cl. Wood Solubility in Protic and Aprotic Ionic Liquids. The solubility of pine wood flour (average diameter ∼500 μm, water content 5.5 ± 0.5 wt %) in the ILs was investigated at 115 °C, open to the atmosphere, and under constant stirring; the results are numerically summarized in SI Table S2. Briefly, [Emim][OAc] could dissolve up to 5.5 ± 0.5 wt % wood flour, consistent with a previous report for softwood dissolution in [Emim][OAc] under similar conditions (ca. 93% dissolution of 5 wt % wood, at 110 °C and 16 h).8 The IL [Emim]Cl showed a maximum dissolution capability of 2.5 ± 0.5 wt % under the same conditions. When the temperature was increased to 150 °C, a maximum of 4.5 ± 0.5 wt % wood could be dissolved, which matches previously reported solubility values for pine wood flour using similar parameters (4 wt %, 150 °C, 24 h).26 The protic IL [Eim]Cl demonstrated a maximum dissolution capability of 3.5 ± 0.5 wt % of wood flour at 115 °C; dissolution was confirmed by optical microscopy. This is interesting as a rare example of a protic IL demonstrating complete solubility of whole biomass. Furthermore, wood solubility in [Eim]Cl exceeded that observed in its more widely

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RESULTS AND DISCUSSION Synthesis and Characterization of Protic Ionic Liquids. The protic ILs [Eim][OAc], [Eim][HCOO], and [Eim]Cl were prepared in the lab by simple stoichiometric mixing of 1-ethylimidazole (Eim) and either acetic, formic, or hydrochloric acids, respectively. However, the ratio of 11930


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ACS Sustainable Chemistry & Engineering investigated aprotic analogue [Emim]Cl (only 2.5 ± 0.5 wt %, as noted above). The protic ILs [Eim][HCOO] and [Eim][OAc] dissociated at 115 °C; hence, dissolution experiments were performed using a reflux condenser. These ILs were unable to dissolve wood, but did turn yellow; after adding 5 wt % wood to [Eim][OAc], only 95% of the initial mass of wood could be recovered by filtration. It was noted at this stage that the behavior of [Eim][HCOO] and [Eim][OAc] was largely indistinguishable; only [Eim][OAc] is discussed hereafter. Characterization of the [Eim][OAc] post-wood pretreatment by UV−vis spectrometry (SI Figure S5) confirmed the presence of dissolved noncondensed phenolic groups, consistent with lignin extraction.27 Increased wood loading (up to 10 wt %) demonstrated increased lignin extraction (SI Figure S5(b), left y-axis) although with decreasing efficiency per gram of IL (SI Figure S6(b), right y-axis). Standard TAPPI analysis of the undissolved biomass confirmed partial delignification (from 29.6 wt % to 25.1 wt %), consistent with the reported delignification of corn stover by the protic IL [Eim][OAc].20 Enzymatic Hydrolysis of Wood Pretreated with Ionic Liquids. The wood dissolved in [Emim][OAc], [Emim]Cl, and [Eim]Cl could be recovered using the widely reported8 1:1 acetone:water antisolvent. Similar procedures were performed for wood after stirring in [Eim][OAc]. Wood loading was fixed at 2 wt %, and the recovered material was subjected to enzymatic hydrolysis using cellulase enzymes as a probe for the degree of disruption of the biomass. The glucose yield was monitored as a function of time and is displayed in Figure 3 (numerical data is displayed in Table S3 in the SI).

antisolvent treatment.30 In this study, we were unable to probe the composition of the material still dissolved in the antisolvent, but this merits further study. Interestingly, the protic IL [Eim]Cl was also effective, resulting in the release of ca. 75% of glucose. This makes [Eim] Cl a protic IL that is effective at both whole biomass dissolution and disruption (cf. distinct from strongly acidic ILs, which digest biomass by chemical destruction)31 and is almost on par with [Emim][OAc]. In comparison to other systems, pretreatment of switchgrass with a series of [HSO4]−-based protic ILs resulted in less than 50% glucose release upon enzymatic hydrolysis.14 The mechanism of [Eim]Cl pretreatment was also different from that of its aprotic analogues, which can be demonstrated by (i) the kinetics of the glucose release in Figure 2, (ii) visual observation of the pretreated biomass and thermogravimetric analysis of the pretreated biomass, and (iii) solid-state 13C NMR of regenerated biomass material. Addressing point (i), maximum glucose yields could be consistently reached for aprotic IL-treated biomass after 24 h enzymatic hydrolysis, whereas [Eim]Cl-treated biomass required 48 h (cf. Figure 2). Slower glucose release is consistent with a more crystalline substrate, which is separately confirmed below. Addressing point (ii), the wood precipitated from [Eim]Cl by antisolvent addition was visually volumous when compared to the starting material, having undergone significant volume expansion. After drying, this structure collapsed to a much smaller, denser powder than the initial wood powder, with an associated drop in the enzymatic hydrolysis yield. This is known for most pretreatments, and warned against in standardized methodology,32 but was noted to be extremely significant for [Eim]Cl. Thermogravimetric analysis (TGA) of the wood (SI Figure S8) revealed a distinct inverse correlation between the decomposition temperature of the pretreated material and its enzymatic hydrolysis yield; e.g., untreated wood was indistinguishable from [Eim][OAc]-treated wood in terms of both enzymatic hydrolysis and TGA, whereas [Emim][OAc]-treated wood released more glucose and decomposed ca. 30 °C earlier. Once again, [Eim]Cl is the exception to the trend, with the “wet” [Eim]Cl pretreated wood having two distinct decomposition profiles and the dried material being more thermally stable that the original wood. Solid-State NMR before and after Pretreatment. Figure 4 displays the 13C solid-state nuclear magnetic resonance (sNMR) spectrum for untreated, as-received pine wood powder; this spectra clearly identifies the various fractions inherent in the biomass, particularly cellulose, hemicellulose, and lignin. Assignment of the various functional groups was achieved based upon prior published chemical shift values33 and the known chemical composition of the pine wood; these are summarized in detail in the SI in Table S4. Broadly, the region between 60 to 110 ppm represents the cellulose and hemicellulose (the combined material being known as holocellulose), while the chemical shift between 110 to 160 ppm represents the aromatic fractions present as lignin. Moreover, two distinct peaks of hemicellulose can be identified at 172 and 22 ppm for carboxyl and methyl groups, respectively. Some overlap between the holocellulose fraction and the nonaromatic components of the lignin fraction occurs in the 60−110 ppm region; however, analysis of the 13C sNMR of pure lignin (1,4-dioxane-extracted from pine wood) indicates the lignin has 13C sNMR signals in the 60−110 ppm region ca. 10% of the size of the total 13C sNMR signal

Figure 3. Enzymatic conversion of untreated and various IL-treated pine wood over time.

For untreated wood, only ∼16% of the available glucose could be released, even after 96 h enzymatic hydrolysis. Treatment with the protic IL [Eim][OAc] (e.g., only lignin extraction) had no positive effect upon the hydrolysability of the regenerated biomass. This demonstrates that despite some lignin extraction this protic IL is incapable of beneficial biomass disruption. As expected from prior studies,28,29 the conventional aprotic ILs [Emim][OAc] and [Emim]Cl were effective pretreatment media, allowing the enzymatic release of ca. 80% and 57% of the available glucose, respectively. Quantitative yields of glucose (100%) could not be obtained in the enzymatic broth due to the known retention of glucosecontaining hemicellulose fractions in the IL despite the 11931


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Figure 4. 13C sNMR for untreated pine wood powder where red, green, and blue colors highlight the cellulose, lignin, and hemicellulose fractions, respectively.

for lignin in the aromatic region (110−160 ppm). Considering all these facts, the carbon signal was integrated which gave an estimation of carbon content of ca. 30.0% for lignin and ca. 70.0% for holocellulose. This is in excellent agreement with the values determined using conventional techniques, where lignin content was quantified using the standardized TAPPI method32 as 29.6 wt % lignin, and holocellulose was measured as 69.9 wt % (52.4 wt % cellulose; using a reported chlorite method).34 All components of the cellulose could be clearly identified, as well as crystalline and amorphous components. The crystallinity index of the cellulose fraction can be estimated in whole biomass by 13C sNMR using the C4 peak separation method.35,36 The crystallinity index of the pine wood was quantified as 34.8%, which is in excellent agreement with the value of 34% previously reported for the same species by comprehensive X-ray diffraction analysis.37 Pine wood powder was then treated with [Emim][OAc], [Eim][OAc], [Emim]Cl, and [Eim]Cl under similar conditions, namely, stirring 2 wt % of wood in the ILs at 115 °C for 18 h, and then regenerated with an acetone/water mixture. Figure 5 displays the resulting 13C sNMR spectra, comparing the recovered fraction as a function of IL vs the original wood sample. It is notable that for all pretreated samples, the 13C sNMR of the recovered cellulose fractions were not well resolved enough to allow quantitative determination of cellulose cystallinity using the C4 peak separation method. The 13C sNMR spectra for [Emim][OAc]-pretreated biomass mirrored previously reported observations. For example, the cellulose fraction became more amorphous, and the quantity of lignin was reduced.38,39 There was also evidence of significant reaction between the [Emim]+ cation and the C1, C2, and C6 positions of cellulose28,40,41 due to the base-catalyzed side-reaction between the IL and cellulose.40 This reaction has been widely noted for pristine cellulose samples, but this is a notable observation of it occurring during pretreatment of whole biomass. For [Emim]Cl, the cellulose became more amorphous,17,42 but when considering mass balance, a significant fraction of the holocellulose had not been regenerated (consistent with prior reports for whole

Figure 5. 13C sNMR for (a) untreated pine wood powder and the fractions recovered from this after after IL pretreatment, then water− acetone addition to the IL for (b) [Emim][OAc], (c) [Emim]Cl, (d) [Eim]Cl, and (e) [Eim][OAc].

biomass8,43). Extending pretreatment time or increasing the temperature exacerbated this; these observations (using whole wood) are consistent with prior reports where the depolymerization of pure cellulose and subsequent repolymerization (to form black char) was observed in [Emim]Cl at high temperature.44,45 Compared to [Emim]Cl, the protic [Eim]Cl-treated material demonstrated higher holocellulose recovery. Interestingly, the crystallinity of the cellulose fraction actually increased upon [Eim]Cl treatment, as demonstrated in the 85−90 ppm region in Figure 5. This is consistent with the selective removal of the amorphous fractions but is surprising given the prior (visual) dissolution of the whole biomass. It is not currently 11932


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Therefore, with respect to cellulose fractions, clear differences were observed across protic vs aprotic systems and between different anions. With respect to side-reactions of lignin and selective dissolution of lignin fractions, the trend was dominated by anions, rather than the (a)protic nature of the ILs. Distillation, Recovery, and Recycle of [Eim]Cl. After dissolving and regenerating the wood in [Eim]Cl, only ca. 58% of the initial mass of the dissolved biomass was recovered. This value is typical for ILs, with some smaller fragments of the lignocellulose biomolecules (ca. 32% by mass) remaining dissolved, not to mention oils, waxes, metal salts, hormones, proteins, and a vast plethora of other chemical compounds found in (recently living) biomass. These represent a major challenge for the recycling of ILs and will inevitably accumulate during multiple cycles. Distillation is one clear route to periodic purification but typically results in irreversible decomposition at higher temperatures in aprotic ILs.50,51 However, distillation of protic ILs is typically facile and was investigated here for [Eim]Cl. Under semioptimized conditions, it was found that gram scales of [Eim]Cl could be distilled in 30 min under vacuum (8−10 mbar) at 200 °C using a Kugelrohr oven. It was initially assumed that only the 1-ethylimidazole would be recovered, with the anhydrous HCl gas lost to vacuum. However, surprisingly, a total of 98.6 (±0.2) wt % of the original mass could be consistently recovered after distillation under these conditions, with the IL recovered in its original 1.0:1.0 (±0.05) acid:base ratio (vide infra) with no appreciable loss of either component. Aprotic ILs are known to distill as ion pairs, whereas protic ILs typically dissociate.52 The relative ease and rapidity with which [Eim]Cl could be distilled suggests the latter process, but the stoichiometric recovery of both the Eim (base) and the significantly more volatile HCl (acid) is surprising in this case. Either the density of the volatilized material was sufficient that the [Eim]Cl could quantitatively recombine via acid−base neutralization when cooled slightly or a hydrogen-bonded pair (as opposed to ion pair) was distilled. Regardless of the mechanism, [Eim]Cl could be rapidly distilled and recovered in high yield. Conversely, fractional distillation of [Eim][OAc] was observed, whereby heating to 115 °C at atmospheric pressure resulted in evaporation of only the acetic acid component. Distillation was also investigated for biomass-contaminated [Eim]Cl. Thus, a sample of wood was dissolved in [Eim]Cl (2 wt % loading), treated with the acetone−water antisolvent mixture and the precipitated biomass removed by filtration. The acetone−water was removed from the filtrate using a rotary evaporator to yield a heavily discolored [Eim]Clresidual biomass mixture. This was distilled (200 °C, 8−10 mbar), and 93% of original [Eim]Cl (by mass) was recovered. While pure by NMR, the recovered [Eim]Cl was slightly yellow (SI Figure S9(a)), hence was distilled again using same conditions to yield colorless [Eim]Cl (SI Figure S9(b)). The 1 H NMR spectra (Figure 6) for the fresh and recovered [Eim] Cl after the first and second distillations confirmed no structural change in the [Eim]Cl after the distillations and that the original 1.0:1.0 (±0.05) acid:base ratio [Eim]Cl employed was recovered in an identical ratio. Similar experiments were repeated with [Eim]Cl and 2 wt % dissolved wood, except distillation was performed directly from the biomass−IL solution without prior antisolvent addition. A

understood if the crystallinity survived the dissolution process or was recovered during precipitation; in situ XRD studies are ongoing. However, these observations allow us to conclude that [Eim]Cl is a milder solvent for biomass than [Emim]Cl, accounting for the dramatic reduction in charring in [Eim]Cl. The amorphous sections are preferentially lost, resulting in an extensively swollen biomass sample that collapses when dried but when taken directly for enzymatic hydrolysis allows good accessibility to the cellulase enzymes. This accessibility accounts for the high glucose yield, while the higher crystallinty accounts for the slower hydrolysis yields. Comparison between these results for protic [Eim]Cl (increases accessibility and crystallinity) and the protic [Eim][OAc] (only delignifies; crystallinity and accessibility largely unchanged) confirms prior statements derived from non-IL studies; increased cellulose accessibility is significantly more influential for biomass pretreatment than mere lignin removal.46 However, this is only speculated, and at present, it cannot be quantitatively demonstrated. The effect of IL pretreatment upon biomass tends to be multifaceted, with both chemical and physical modifications of the biomass occurring. In a side-by-side comparison of the protic ILs [Eim][OAc] and [Eim]Cl, [Eim][OAc] only delignifies, whereas [Eim]Cl swells and physically alters the biomass structure, suggesting that the latter is far more significant in terms of enhancing enzymatic hydrolysis yields when pretreating with protic ILs. Also of note is the spectra corresponding to the lignin fractions. The methoxy group was observed at 56 ppm in the 13 C sNMR spectra for pine wood powder and IL-regenerated materials; a significant reduction of the methoxy group (compared to the C−O peak) was observed after treating with acetate-based ILs (both protic and aprotic). In contrast, less significant reduction of the methoxy group was observed for Cl-based ILs. The reduction of the methoxy group signal has also been reported for both acetate- and chloride-based ILs using industrial lignin.47 These prior studies suggested that the reduction might originate from the conversion of aromatic rings to quinonoid structures.47,48 In addition, after treating with acetate-based ILs, a reduction of C−H aromatic species in the lignin fraction was observed. This decomposition of aromatic C−H in acetate-based IL might be due to dehydration reactions known to occur in alkaline environments.28,49 On the other hand, less significant changes in C−C and C−H bonds were observed for Cl-based IL-treated wood. This variation of interactions between lignin (in a whole wood sample) and ILs is consistent with a previous study where a microstructural change of industrial lignin using [Emim][OAc] was much more significant compared to [Bmim]Cl.47 Being a softwood, it is assumed that the pine wood used in our study mostly contained guaiacyl (G) units and only a trace amount of syringyl (S) units.33 This is supported by the NMR data, where the peak at 148 ppm in the 13C sNMR (Figure 5) is associated with C3 and C4 in G units, and the small shoulder at 153 ppm is associated with the C3 and C5 position of the S unit.33,35 After treating milled wood with acetate-based protic and aprotic ILs [Emim][OAc] and [Eim][OAc], the shoulder peak at 153 ppm disappeared; this confirmed the quantitative removal of the S unit. However, the reverse trend was observed for both Cl-based protic and aprotic ILs, where the shoulder peak became more prominent indicating preferential loss of the G unit instead. 11933


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Figure 7. Relative enzymatic hydrolysis efficiency for [Eim]Cl-treated wood flour (2 wt % loading each time, 110 °C, 18 h) over four successive pretreatments with the same batch of [Eim]Cl. The initial efficiency (glucose yield from freshly synthesized [Eim]Cl) taken as 100%; error bars represent triplicate hydrolysis measurements after 48 h.

Figure 6. 1H NMR spectra of [Eim]Cl before use (a) and recovered from [Eim]Cl−wood fraction mixture after first (b) and second (c) distillation at 200 °C using Kugelrohr apparatus.

is effective at pretreating wood (75% of glucose released upon enzymatic hydrolysis), on par with the aprotic [Emim][OAc] (80%) and exceeding that possible by its aprotic analogue [Emim]Cl (57%). Two other protic ILs, [Eim][OAc] and [Eim][HCOO], were effective only at lignin extraction but ineffective at biomass disruption. Characterization of the regenerated biomass demonstrated that pretreatment by [Eim] Cl operates by a different mechanism to that of [Emim][OAc] and [Emim]Cl. Detailed solid-state NMR analysis highlighted that [Eim]Cl appears to selectively dissolve and retain amorphous cellulose over crystalline cellulose. Compared to the chloride-based ILs, the acetate-based ILs were more reactive to various functional groups of lignin fractions, resulting in a significant reduction in the number of methoxy groups. Significantly, the protic nature of [Eim]Cl allowed its distillation and could be easily recovered by distillation in its original 1:1 acid:base ratio. This allowed repeated uses of the IL, with no decrease in pretreatment effectiveness upon recycle.

total of 95% of the original [Eim]Cl was recovered in the receiving bulb, but the 2 wt % dissolved wood was recovered as a dense char (cf. earlier observations about recovered, dried biomass from [Eim]Cl collapsing and charring under mild drying conditions). The resulting char only yielded 5% of the possible glucose content upon enzymatic hydrolysis (SI Figure S10) compared to [Eim]Cl-treated then antisolvent-treated samples (75% glucose), untreated wood (16%), and untreated wood heated at 200 °C (9%). Therefore, in terms of enzymatic digestibility and IL recovery, the best outcomes could be achieved by dissolving wood in [Eim]Cl, recovering the solid via acetone−water addition, and distilling the [Eim]Cl to recover pure [Eim]Cl. The pretreatment effectiveness of recycled [Eim]Cl was also investigated. A total of 2 wt % wood was dissolved in [Eim]Cl then treated with the antisolvent. The regenerated biomass was subjected to enzymatic hydrolysis, and the recovered [Eim]Cl was distilled and reused to pretreat more wood. This was repeated three times, and pretreatment effectiveness was consistently maintained (Figure 7) demonstrating the potential of [Eim]Cl to perform comprehensive biomass pretreatment followed by recycle. Notably, tetramethylguanidinium acetate has previously been highlighted as a cellulose−dissolving acetate-based protic ionic liquid.19 In this study, [Eim]OAc was quite ineffective, likely because acetic acid is too weak an acid and ethylimidazole too weak a base; hence, it did not form a true protic IL. An interesting comparison is that of tetramethylguanidinium acetate vs [Eim]Cl, where the former has a strong base and the latter has a strong acid; both form true protic ILs which are capable of dissolving cellulose and whole biomass and are both recoverable via distillation. Likely there are certain conditions where employing one over the other would be preferred; however, ideally, a side-by-side comparison is required under identical conditions before the “best” system for a specific task can be identified.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b05987. Tables: physical properties of the ILs, maximum wood flour solubility, enzymatic conversion, and NMR chemical shifts. Figures: chemical structures, NMR spectra as a function of acid:base ratio, DSC curves, TGA curves, UV−vis spectra, glucose calibration plot, photographs of distillation, and enzymatic conversion of wood postdistillation. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ORCID

Aditya Rawal: 0000-0002-5396-1265 Leigh Aldous: 0000-0003-1843-597X

CONCLUSIONS The protic IL [Eim]Cl has been demonstrated to be capable of whole biomass dissolution (up to 3.5 ± 0.5 wt % pine wood). Enzymatic hydrolysis experiments demonstrated that [Eim]Cl

Notes

The authors declare no competing financial interest. 11934


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ACKNOWLEDGMENTS L.A. acknowledges the Australian Research Council (ARC DECRA DE130100770) for research funding. M.M.H. acknowledges the School of Chemistry, UNSW, for a Ph.D. scholarship. Micro Milling Pty Ltd. is thanked for the kind donation of wood flour.

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