Han Ung
Kim
,
Jong Wha
Kim
,
Sumin
Seo
and
Jungho
Jae
*
School of Chemical Engineering, Pusan National University, Busan 46241, Republic of Korea. E-mail: jh.jae@pusan.ac.kr; Tel: +82-51-510-2989
First published on 13th March 2023
The efficient hydrolysis of cellulose into its monomer unit such as glucose or valuable cello-oligosaccharides is the critical step for the cost-effective production of biofuels and biochemicals. However, the current cellulose hydrolysis process involves high energy-demanding pretreatment (e.g., ball-milling) and long reaction times (>24 h). Herein, we investigated the feasibility of the dissolution/regeneration (DR) of cellulose in ionic liquids (ILs) and deep eutectic solvent (DES) as an alternative to ball-milling pretreatment for the effective hydrolysis of cellulose. Because chlorine-based solvents were reported to be the most active for cellulose pretreatment, [EMIM]Cl and [DMIM]DMP were selected as the IL molecules, and choline chloride–lactic acid and choline chloride–imidazole were selected as the DES molecules. The level of the crystallinity reduction of the regenerated cellulose were analyzed using XRD and SEM measurements. The hydrolysis kinetics of the regenerated cellulose from ILs and DES were examined at 150 °C using sulfonated carbon catalysts and compared with those of the ball-milled cellulose. Overall, the cellulose pretreatment using the ILs and the DES had superior kinetics for cellulose hydrolysis to the conventional ball milling treatment, suggesting a possibility to replace the current high energy-demanding ball-milling process with the energy-saving DR process. In addition, the utilization of supercritical carbon dioxide-induced carbonic acid as an in situ acid catalyst for the enhanced hydrolysis of cellulose was presented for the first time.
As an alternative to the high energy-demanding ball-milling process, an ionic liquid (IL) pretreatment of cellulose has been studied by many researchers.16 In the presence of ionic liquids, numerous hydrogen bonds in the cellulose can be broken or reduced due to their strong interaction with cations and anions of the ionic liquids, weakening the solid crystalline structure. Previous studies have mainly focused on the utilization of ionic liquid as a reaction medium to convert the dissolved cellulose in an ionic liquid directly into monosaccharides or other derivatives. This approach demonstrated the excellent conversion and yield.17,18 However, the hydrolysis of cellulose in ionic liquids as a solvent has the separation problem of sugar products and ionic liquids due to their high boiling points, which greatly increases the process cost. To solve this problem, a method of regeneration of cellulose by adding anti-solvents such as water to the dissolved cellulose in ILs was proposed by other researchers.19–24 It has been shown that the addition of excess water results in the destruction of cellulose–ionic liquid bonds and subsequent reformation of cellulose–cellulose hydrogen bonding, thereby precipitating as a solid cellulose again. Due to the distorted conformation resulting from the destruction/reformation of the H-bondings between glucose monomers, the regenerated cellulose has amorphous form with a reduced degree of polymerization (DP).25
Multiple researchers have shown that regenerated celluloses from ionic liquids have enhanced hydrolysis kinetics than untreated cellulose. For instances, Zhao et al. reported that the cellulose regenerated from various chloride- and acetate-based ILs such as [BMIM]Cl exhibited 2 to 10 times faster rates for enzymatic hydrolysis than untreated cellulose.24 Morales-delaRosa et al. compared the acid hydrolysis of cellulose regenerated from [EMIM]Cl with that of the untreated cellulose.21 They found that the use of the homogeneous acid catalyst, phosphotungstic acid, resulted in the complete conversion of the regenerated cellulose within 5 h at 413 K with a high glucose yield of 87%, while the untreated cellulose exhibited only 24% cellulose conversion under the identical condition. Meanwhile, Lai et al. studied the solid acid-catalyzed hydrolysis of regenerated cellulose using sulfonated SBA-15 catalysts.26 Compared to homogeneous acids, the relatively low glucose yield ∼50% was obtained at 433 K. Kim et al. also reported the low glucose yield of ∼35% when the cellulose treated in [BMIM]Cl is hydrolyzed over Nafion® NR50 catalyst presumably due to the limited accessibility of cellulose to the solid catalyst surface.27
Although previous studies have demonstrated the effectiveness of the dissolution/regeneration (DR) pretreatment for cellulose hydrolysis, there are still several unanswered questions as follows. First, there is no study on the direct comparison of the hydrolysis kinetics between the ball-milled and DR pretreated celluloses, making it difficult to estimate the level of efficiency of the DR process over the conventional ball-milling process. Second, the solid acid-catalyzed hydrolysis of DR pretreated cellulose was not optimized, having the relatively low yield of glucose (<50%). For instance, the most active solid acid catalyst for the cellulose hydrolysis was reported to be the sulfonated carbon catalyst, but this type of catalyst has not yet been applied to the DR-pretreated cellulose.28 Third, although ILs can be recycled after the DR process, they are expensive, corrosive, and toxic, which limits their large-scale application. Thus, it would be interesting to expand the scope of the ILs for the DR process into an inexpensive and greener solvent system, e.g., deep eutectic solvent (DES). DES is a eutectic mixture that has a lower melting point than its individual components. It is generally obtained by the complexation of a quaternary ammonium salt with a hydrogen bond donor (HBD) in which the latter can form intermolecular hydrogen bonds with the anions of the former, resulting in the decrease in the melting point. While ILs are formed from one type of discrete anion and cation, DESs can be formed from a eutectic mixture of a variety of an ionic and/or cationic species, rendering the wide tunability for their solvent properties.29 Although the efficiency of the breaking of the hydrogen bonds of cellulose (i.e., cellulose dissolution) is still lower with DESs than with ILs, choline chloride (ChCl) based DESs have shown the promising performance. Specifically, ChCl paired with the HBD of imidazole (Im) exhibited the best cellulose dissolution efficiency up to 4.5 wt%, while the toxicity of Im might be rather problematic.30,31 ChCl paired with lactic acid is the greener DES system investigated widely for the delignification of a range of biomass feedstock.31 Although its cellulose solubility is rather low (<2 wt%), it could be a desirable alternative to the toxic ILs. Thus, two DESs of the ChCl–Im and ChCl–LA were selected for the comparison with ILs in this study.
The objective of this paper is to investigate the hydrolysis of regenerated cellulose from ILs and DES using sulfonated carbon catalysts and compare their activity with the conventional ball-milled cellulose. For this purpose, two representative ILs for cellulose dissolution, 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) and 1,3-dimethylimidazolium dimethyl phosphate ([DMIM]DMP), and two choline chloride-based DESs were selected as the test samples. The sulfonated carbon catalysts were prepared by the thermal oxidation of the commercial activated carbon followed by sulfonation. The hydrolysis of cellulose was conducted at 150 °C for 12 h. In addition, the hydrolysis experiments in the presence of 100 bar-CO2 was conducted to see if the carbon dioxide-induced carbonic acid can act as an acid co-catalyst to boost the hydrolysis of cellulose.
Fig. 1 Chemical process of the DR treatment (left) and chemical structures of ILs and DESs used in this study (right). |
The AC sample was then used as a parent material for introducing a sulfonic group (–SO3H). The synthesis method is as follows. First, 15 mL of 98% concentrated sulfuric acid was slowly added to 1 g of AC in water. Since –OH and –COOH groups can react with the oxygen and moisture in the atmosphere to cause a substitution reaction, all the synthesis experiments were conducted in a nitrogen atmosphere. The sample solution was stirred in a nitrogen atmosphere and heated to 200 °C for 24 h. After filtration, the carbon powder was washed with hot distilled water to remove the physically adsorbed SO42−. Finally, it was dried in an oven at 110 °C for 12 h followed by calcination at 200 °C for 2 h in a nitrogen atmosphere. The final sample was then directly used for the hydrolysis experiments of cellulose.
In case of the CO2 addition experiment, the liquid CO2 was introduced to the reactor using the HPLC pump at a constant feeding rate after charging the reactor with the reactant and water. The amount of CO2 injected into the reactor was 100 bar at 35 °C. The final reactor pressure exceeded 200 bar when the reactor temperature was raised to 150 °C. Thus, the CO2 in the reactor was in the supercritical state.
Product analysis was performed by High Performance Liquid Chromatography (HPLC). Agilent's HPLC 1200 Infinity instrument was used to analyze soluble sugar compounds and by-products, i.e., sugar-decomposed products. For the column, Hi-Plex Ca (Duo), 300 × 6.5 mm was used, and the product yield was quantified based on an external calibration method. For the estimation of the cellulose conversion, the carbon content of the reactant solution before and after reactions were monitored by Total Organic Carbon (TOCs) analyzer. The cellulose conversion is defined as the total carbon amount of water-soluble products in the product solution divided by the amount of cellulosic carbon in the reactant. The glucose yield was estimated by the quantitative glucose data from HPLC analysis. The formula for calculating the cellulose conversion and glucose yield are as follows.
Cellulose conversion (%) = 100 × B/A
Glucose yield (%) = 100 × C/A
A: total amount of cellulosic carbon analyzed by TOCs.
B: total amount of water-soluble product carbon analyzed by TOCs.
C: total amount of carbon in the glucose product analyzed by HPLC.
Fig. 2 XRD graph of pretreated cellulose: DES (ChCl:LA) (red), DES (ChCl:Im) (blue), [EMIM]Cl (green), [DMIM]DMP (pink) and ball-milling (yellow). |
FE-SEM analysis was performed to characterize the particle size of cellulose after treatment with the ILs and DES. Fig. 3 shows the FE-SEM images of regenerated cellulose samples as well as the native and ball-milled celluloses. The native cellulose sample had an average particle size of 300 µm. In general, the average particle size of cellulose decreased largely after the treatment of ILs and DES. Specifically, cellulose treated with [EMIM]Cl and [DMIM]DMP had average particle sizes of 35–40 µm and 50–60 µm, respectively. On the other hand, DES-treated celluloses had relatively large particle sizes of 80–90 µm, compared to two IL treated samples. These particle size analysis results are consistent well with those of XRD measurements, in which the DES-treated sample has the higher crystallinity than IL-treated samples. It is though that large cellulose particles are initially fragmented to small cellulose molecules after the dissolution with ILs and then reaggregate with the adjacent cellulose during the regeneration process of adding an excess of water, thereby forming the reduced particle size compared to untreated cellulose.21 Meanwhile, the particle size of the ball-milled cellulose was much smaller than those of IL and DES-treated samples, showing ∼10 µm average particle size. All of the pretreated cellulose samples underwent a significantly reduction in the size of particles compared to the original one (300 µm to 80–10 µm). Overall, the average particle size of the cellulose samples increases in the following order, ball-milling < [EMIM]Cl < [DMIM]DMP < DES (ChCl:Im) < DES (ChCl:LA). In addition, the morphology of cellulose underwent conspicuous changes depending on the pretreatment method. Untreated cellulose exhibited rough and irregular surfaces, retaining its long stick shape (average length of 300–500 µm). The ball-milled cellulose comprised ellipsoidal fine particles resulting from fine cleavage through mechanical grinding. The IL-treated cellulose samples consisted of various rod-shaped, plate-shaped, or irregular elliptical fine particles, while the DES-treated samples lost their particle characteristics due to fibrillation. It appeared that a huge lump-like cellulose was formed by the random regeneration of hydrogen bonds between adjacent cellulose molecules.
Fig. 3 FE-SEM image of cellulose treated with ILs and DES: [EMIM]Cl (a), [DMIM]DMP (b), DES (ChCl:LA) (c), DES (ChCl:Im) (d), ball-milling (e) and untreated cellulose (f). |
Fig. 5 Reaction results of pretreated cellulose (reaction condition: cellulose 0.05 g, catalyst 0.05 g, D. I. water 40 mL, temp. 150 °C, time 12 h). |
Specifically, the conventional ball-milled cellulose exhibited a low conversion of ∼58% and a low glucose yield of ∼40%. When [EMIM]Cl was used, the highest cellulose conversion of ∼97% and the highest glucose yield of ∼68% were achieved. In contrast, [DMIM]DMP showed a ∼85% conversion a ∼62% monosaccharide yield, which are slightly lower than those of [EMIM]Cl. Since the phosphate-based anion exhibits lower electronegativity than the chloride anion, the decomposition rate of hydrogen bonds in cellulose would be lower, which, in turn, results in the lower hydrolysis efficiency with phosphate.27 In the case of the cellulose sample treated with DES (ChCl:LA), it has conversion (∼60%) and monosaccharide yield (∼40%) much lower than those of IL treatment. However, when the cellulose sample was treated with the DES (ChCl:Im), it showed improved conversion (∼85%) and enhanced monosaccharide yield (∼53%). The observed hydrolysis activity trend is well correlated with the degree of the crystallinity reduction of cellulose and, in turn, the solvent power of the DES. In general, the solvation performance depends on the various properties of DES, including the hydrogen bond basicity, hydrogen bond acidity, dipolarity/polarizability, etc.37 According to Ren et al., the cellulose dissolution efficiency is mainly controlled by the hydrogen bond basicity of the DES rather than other parameters because the essence of the cellulose dissolution is the disruption of the hydrogen bonds in cellulose.30 Thus, the higher hydrogen bond basicity of the DES–Im induced by π–π conjugative effect of the imidazole ring may be the reason for the higher solubility of cellulose, resulting in the higher hydrolysis activity toward glucose production.
The overall hydrolysis efficiency of the DES-treated cellulose is lower than that of IL. This is due to the lower cellulose dissolution efficiency of DES as confirmed by XRD and SEM measurements. The hydrolysis activity of the DES (ChCl:Im) is, however, comparable to that of the second place IL, i.e., [DMIM]DMP, although its toxic characteristic nullifies the environmental benefit over ILs. The eco-friendly natural DES (ChCl:LA) exhibited much lower activity for cellulose hydrolysis, suggesting that the new DES system for cellulose dissolution is needed. In addition, it is worth paying attention that the DES-treated sample had the advantages over the IL-treated samples in terms of product selectivity; it has higher yields of valuable cellobiose and oligosaccharides and lower production of sugar decomposed products, including 5-hydroxymethylfurfural (5-HMF), formic acid and levulinic acid.38 The detailed distribution of these decomposed products are displayed in Fig. S2.† In particular, when compared to the ball-milled cellulose, the DES-treated samples still exhibited lower selectivity to sugar decomposed products even at similar conversion (∼60%), suggesting their structural resistance to the secondary decomposition reactions.
In order to analyze the detailed composition of the oligosaccharides, MALDI-TOF analysis was also performed. Fig. 6 shows MALDI-TOF analysis spectra of hydrolysis products obtained from IL- and DES-treated cellulose samples. All samples mainly included tri-, tetra- and pentasaccharide as oligosaccharide products. The relative abundance of individual oligosaccharides differed depending on the cellulose samples. Especially, the selectivity of tetra- and pentasaccharide for the DES (ChCl:LA)-treated samples were relatively higher than those for IL-treated samples. Recently, those short-chain cello-oligosaccharides are receiving increasing attention due to their wide application in pharmaceutics, agriculture, food and chemicals. In this respect, the selectivity enhancement toward the oligosaccharides in cellulose hydrolysis is of growing importance.39,40 Especially, compared to the ball-milling treatment, IL- or DES treatment allows for more production of valuable oligosaccharides which can be considered as an important benefit of the DR treatment.
Fig. 6 MALDI-TOF analysis of pretreated cellulosic hydrolysis products: [EMIM]Cl (top), [DMIM]DMP (middle), DES (ChCl:LA) (bottom). |
Fig. 7 shows hydrolysis reaction results in the presence of supercritical CO2, while Fig. 8 shows MALDI-TOF spectra of the hydrolysis products. The reaction condition was identical to the previous experiment except that supercritical carbon dioxide was injected at room temperature to 100 bar. We have previously shown that supercritical carbon dioxide-induced carbonic acid is sufficiently acidic to catalyze the hydrolytic depolymerization of lignin to biocrude oils.41 According to the calculation based on the thermodynamic model reported by Duan et al.,42 the pH of carbonic acid at 150 °C and 100 bar-CO2 is nearly 3.0. Thus, we envisioned that the added CO2 can act as an acid co-catalyst for the enhanced hydrolysis of the regenerated cellulose, particularly for the water-insoluble fraction of cellulose due to the homogeneous nature of the carbonic acid. Although several researchers have reported on the use of high-pressure CO2 as an acid catalyst for biomass conversion such as the dehydrative cyclization of biomass-derived polyols,43,44 there is no report on the use of supercritical CO2 for the hydrolysis of cellulose.
Fig. 8 MALDI-TOF analysis of hydrolysis products using pretreated cellulose and supercritical CO2: [EMIM]Cl (top), [DMIM]DMP (middle), DES (ChCl:LA) (bottom). |
The initial experiments on the conversion of cellulose with and without 100 bar-CO2 confirmed the catalytic effect of CO2, showing the increased glucose yield (see Fig. S3†). Notably, in the presence of the sulfonated carbon catalyst, the addition of supercritical CO2 enhanced the cellulose conversion and glucose yields significantly. In case of the [EMIM]Cl treated cellulose, the cellulose conversion reached ∼100%, and the glucose yield increased to ∼82%, suggesting the promoting role of carbon dioxide-induced carbonic acid in the cellulose hydrolysis. Similarly, [MMIM]DMP-treated sample exhibited the enhanced hydrolysis kinetics, having ∼78% glucose yield at nearly ∼100% conversion. The obtained glucose yield of the [MMIM]DMP-treated cellulose was, however, still lower than that of the [EMIM]Cl treated cellulose as in the case of the reaction without CO2. An increase in the hydrolysis activity was also significant in the case of the DES-treated sample. The conversion of the DES (ChCl:LA)-treated cellulose increased from 60% to 100%, and the glucose yield increased from 40% to 62% with an addition of CO2. Similarly, the conversion of the cellulose treated with DES (ChCl:Im) increased from 83% to 100%, and the glucose yield also increased from 53% to 70%. Compared to two IL-treated samples, the DES-treated samples showed the lower glucose yields, but higher cellobiose (∼5%) and oligosaccharide yields (∼20%), indicating its slower hydrolysis rate. The MALDI-TOF analysis results were also consistent with the yield data. The peak intensity of oligosaccharides in the DES (ChCl:LA)-treated sample was much higher than those in IL-treated samples, as shown in Fig. 7. In addition, high-carbon-number hexasaccharide was also newly detected as the major product. This result suggests that the DES-treatment can be the good option if the target products from the hydrolysis of cellulose are cellobiose or oligosaccharides.
The ball-milled cellulose also showed the enhanced hydrolysis kinetics with an addition of CO2, giving ∼100% conversion and ∼70% glucose yield. Notably, the glucose yield of the ball-milled cellulose is higher than those of the DES-treated ones. In the case of the ball-milled cellulose, there was no formation of cellobiose and oligosaccharides, and the sugar-decomposed products such as levulinic acid were produced with a very high yield (∼25%), while the DES-treated samples have very high yields of oligosaccharides and low yields of levulinic acid. This distinct product selectivity of the ball-milled cellulose likely arises from its structural differences from the DES-treated cellulose, thereby impacting the hydrolysis chemistry. Specifically, Chen et al. reported that the activation energy required to cleave the glycosidic bonds increases with the decrease in the molecular size of the cello-oligosaccharides.45 Thus, in the case of the DES-treated sample, the formation of glucose appears to proceed via the formation of cello-oligosaccharide intermediates by the preferential cleavage of the middle chains of the cellulose, resulting in the slower hydrolysis kinetics toward glucose production. In contrast, in the ball-milled sample, glucose may be produced directly from cellulose due to the preferential attack of the end chains of the cellulose. The ball-milled cellulose has more condensed structure so that the interaction of the carbon catalyst surface can be limited to the end chains of the cellulose. Once glucose is formed, it can easily undergo the secondary dehydration reaction to 5-HMF and LA.
Finally, Fig. 9 shows the by-product distribution obtained from the cellulose hydrolysis in the presence of supercritical CO2. Compared to the reaction without CO2, the formation of 5-HMF was significantly reduced, while the formation of levulinic acid and formic acid were increased substantially for all cellulose samples. This suggest that supercritical CO2-derived carbonic acids accelerated the rehydration of 5-HMF to levulinic acid and formic acid, confirming its Brønsted acidity.
Overall, this study demonstrated that the cellulose DR pretreatment using the ILs and the DESs have superior kinetics for cellulose hydrolysis to the conventional ball milling treatment, suggesting a possibility to replace the current high energy-demanding ball-milling process with the energy-saving IL treatment. Moreover, compared to the results reported by other researchers using either ball-milling or IL pretreatments, the glucose yield obtained with the DR pretreatment is higher (see Table S1†). Between DES and IL-treated samples, the IL treatment shows the faster hydrolysis kinetics toward the monosaccharide production than the DES treatment due to their effective destruction of cellulose crystallinity induced by high solvation ability. The toxic DES (ChCl:Im) have comparable hydrolysis activity to the ILs, while the green and natural DES (ChCl:LA) exhibits relatively low hydrolysis activity due to their poor solvation efficiency, suggesting that the development of new DES system with high efficiency for cellulose dissolution is needed to realize the eco-friendly and energy-saving hydrolysis process for cellulose. Importantly, the DES treatment also demonstrated the advantage over the ball-milling treatment in terms of product selectivity, having the higher selectivity toward valuable short-chain cello-oligosaccharides.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ra08224a |
This journal is © The Royal Society of Chemistry 2023 |