Cellulose Crystal Dissolution in Imidazolium-Based Ionic Liquids: A

Article Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX

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Cellulose Crystal Dissolution in Imidazolium-Based Ionic Liquids: A Theoretical Study Takuya Uto, Kazuya Yamamoto, and Jun-ichi Kadokawa* Department of Chemistry, Biotechnology, and Chemical Engineering, Graduate School of Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan S Supporting Information *

ABSTRACT: The highly crystalline nature of cellulose results in poor processability and solubility, necessitating the search for solvents that can efficiently dissolve this material. Thus, ionic liquids (ILs) have recently been shown to be well suited for this purpose, although the corresponding dissolution mechanism has not been studied in detail. Herein, we adopt a molecular dynamics (MD) approach to study the dissolution of model cellulose crystal structures in imidazolium-based ILs and gain deep mechanistic insights, demonstrating that dissolution involves IL penetration-induced cleavage of hydrogen bonds between cellulose molecular chains. Moreover, we reveal that in ILs with high cellulose dissolving power (powerful solvents, such as 1-allyl-3-methylimidazolium chloride and 1-ethyl-3-methylimidazolium chloride), the above molecular chains are peeled from the crystal phase and subsequently dispersed in the solvent, whereas no significant structural changes are observed in poor-dissolving-power solvents. Finally, we utilize MD trajectory analysis to show that the solubility of microcrystalline cellulose is well correlated with the number of intermolecular hydrogen bonds in cellulose crystals. The obtained results allow us to conclude that both anions and cations of high-dissolving-power ILs contribute to the stepwise breakage of hydrogen bonds between cellulose chains, whereas this breakage does not occur to a sufficient extent in poorly solubilizing ILs.

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INTRODUCTION The energy crisis of the past few decades has highlighted the need of shifting from fossil fuels to renewable energy sources such as lignocellulosic biomass, which is a carbon-neutral and nonfood raw material.1−3 Lignocellulose mainly contains cellulose, a linear β(1→4)-D-glucan biopolymer that is abundantly present in nature and is a promising precursor of new green and sustainable materials.4,5 However, the highly crystalline structure of native cellulose causes the formation of hierarchical fiber composites in which hydrogen bonding between molecular chains induces their assembly into microfibrils and thus results in poor solubility and processability.6 Native cellulose forms two crystalline phases, namely cellulose Iα and Iβ.7−9 The latter form is thought to be more stable than the former,10,11 with both phases featuring stacked molecular chain sheets connected by hydrogen bond networks (Figure 1).12,13 Moreover, all of the molecular chains in these phases are aligned with the same polarity, which is denoted as parallel chain packing. Recently, ionic liquids (ILs)14 existing in the liquid state at temperatures below the boiling point of water have attracted attention as solvents for structural polysaccharides, as exemplified by the pioneering work of Rogers et al., who reported that ILs comprising chloride ions (e.g., 1-butyl-3methylimidazolium chloride (BMIMCl)) can dissolve cellulose.15 Subsequently, a number of other ILs have also been © XXXX American Chemical Society

found to dissolve cellulose, with the corresponding solubilities listed in Table 1.16−18 For example, 1-allyl-3-methylimidazolium chloride (AMIMCl)19 and 1-ethyl-3-methylimidazolium chloride (EMIMCl)20 are good solvents for cellulose, with several studies also indicating that imidazolium acetates (1butyl-3-methylimidazolium acetate (BMIMOAc) and 1-ethyl-3methylimidazolium acetate (EMIMOAc)) are also well suited for this purpose.21,22 Biotechnological studies have demonstrated that pretreatment of cellulose by ILs enhances its hydrolysis during enzymatic saccharification,23−26 with other IL applications involving the synthesis of functional cellulosic materials (e.g., cellulose regeneration and derivatization).27−31 Previously, we reported that the dissolution of cellulose in BMIMCl upon heating followed by cooling to room temperature for several days affords a cellulose ion gel and can thus be used for the further development of novel polysaccharide-based materials,32−34 which, however, requires a detailed understanding of the cellulose dissolution mechanism. In earlier studies, it has been presumed that the oxygen and hydrogen atoms of cellulose hydroxyl groups engage in donor− acceptor interactions with IL cations and anions.16,35,36 In Received: September 25, 2017 Revised: November 10, 2017

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DOI: 10.1021/acs.jpcb.7b09525 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 1. Molecular chain sheet corresponding to the (100) plane of cellulose Iβ, with blue dotted lines indicating intermolecular and intramolecular hydrogen bonds.

Table 1. Solubility of Cellulose in Imidazolium-Based ILs16−18

research milestones, there is not enough universal understanding of cellulose dissolution in ILs. In particular, the higherorder structure of cellulose has not been correlated with its solubility in ILs. Herein, we studied the solubility of model cellulose Iβ crystal structures in imidazolium-based ILs by MD simulation. Before performing the above simulation, the performance of force field parameters was verified for each IL and cellulose because the selection of appropriate force field parameters and calculation models strongly influences calculation results.57−59 The dissolution of several crystal models in ILs with different solubilizing power was investigated in terms of structural changes in cellulose crystals and cellulose−IL interactions. To

addition to experimental studies utilizing NMR spectroscopy,36−40 X-ray diffraction,41−44 and neutron-scattering techniques,40,42 several computational studies relying on density functional theory (DFT)44−48 and molecular dynamics (MD)47−56 approaches have shown that cellulose dissolution in ILs involves the breakage of hydrogen bonds, significantly contributing to the mechanistic understanding of this process. Importantly, recent DFT and MD calculations indicate that efficient cellulose dissolution is promoted by IL cations due to their structural (π-electron delocalization) and dynamic (cation volume) effects.47 It was also reported that the solubility of cellulose in ILs can be successfully modeled by quantitative structure−activity relationships.18 However, despite these B


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Figure 2. ab-Projection of cellulose Iβ crystal models and the corresponding unit cells.

Figure 3. Introduction of nick parts into the surface chain sheets of the 30-chain × 20-mer model.

same simulations were carried out using the original charge scale (charge × 1.0) and different van der Waals parameters optimized on ion-oxygen distant (IOD set)63 for monovalent ions. Cellulose crystal model/IL systems were optimized and subsequently equilibrated upon heating. In the next step, cellulose dissolution was simulated for 500 ns at constant temperature (400 or 450 K) and pressure (1 bar), with the torsional restraint of the pyranose ring backbone and O5−C1− C2−C3/C3−C4−C5−O5 fragments harmonically restrained by a force of 20 kcal/(mol Å2) to maintain the 4C1 chair form. MD calculations were performed using a 2 fs integration time step coupled with the SHAKE64 option. The particle mesh Ewald method was adopted65 for long-range interactions, and the cutoff for nonbonding interactions in the coordinate space was fixed at 12.0 Å. Quantum calculations were performed using Gaussian09 software.66 All of the minimization and MD calculations for solution-phase systems were carried out utilizing the PMEMD and PMEMD.CUDA modules67,68 of the AMBER 14 package69 with an NVIDIA Kepler GPU system. Partial charge assignment and MD trajectory analysis were performed using the Antechamber70 and CPPTRAJ modules71 of the AMBERTools 15 package, respectively. Cellulose crystal model/ILs systems were generated using Packmol 16.103 package.72 Molecular graphics were constructed using PyMOL 1.7.1.0 (Schrödinger, LLC)73 and VMD 1.9.3 software.74

support the proposed dissolution mechanism, we also attempted to quantitatively reproduce the solubility of cellulose in ILs by MD trajectory analysis.

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COMPUTATIONAL METHODS Three crystal models (9-, 10-, and 30-chain ones) were constructed for cellulose Iβ,12 with the ab-base planes of these models featuring regularly arranged cello-oligomers with a polymerization degree of 10 or 20 (Figure 2). In addition, nick parts were introduced into the surface chain sheet of the 30chain × 20-mer model (Figure 3). The above crystal models were placed in a rectangular periodic box containing imidazolium-based ILs, with the number of ion pairs ranging from 750 to 5000. Figure 4 shows the nine imidazolium-based ILs (obtained by combining three cations and three anions) used as solvents.

Figure 4. ILs used as solvents in the MD simulation.

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Cellulose molecules were described by the GLYCAM06 force field,60 and ILs were modeled using the general AMBER force field (GAFF).61 The atomic partial charge parameters for ILs were determined as follows. Geometry optimization and energy calculations for each anion and cation were performed using Hartree−Fock theory and the 6-31G(d) basis set, and partial charges assigned using the restrained electrostatic potential method62 were adjusted to 80% scale (charge × 0.8) for accurate physical property prediction.57,59 To evaluate the force field parameters, MD simulations of pure IL systems (500 ion pairs) were carried out at constant temperature (300 K) and pressure (1 bar). As a control, the

RESULTS AND DISCUSSION As a force field for carbohydrates, GLYCAM06 is one of the most suitable. The combination of GLYCAM and AMBER force field has affinity and has been used for considerable MD simulation studies. If the force field parameter of cellulose selects GLYCAM06, it would be reasonable to use the AMBER force field for imidazolium-based ILs. The force field parameters for the ILs are developed based on the general AMBER force field (GAFF) (Table S1). Before simulating cellulose dissolution, we verified the performance of force field parameters by calculating the C


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The Journal of Physical Chemistry B Table 2. Calculated and Experimental Data70−76 for Density of Ionic Liquids density of the solvent (g/cm3) at 300 K

a

ionic liquid

GAFF (charge × 0.8)

GAFF (charge × 1.0)

GAFF + IOD (charge × 0.8)

GAFF + IOD (charge × 1.0)

exp

AMIMCl AMIMBr AMIMOAc BMIMCl BMIMBr BMIMOAc EMIMCl EMIMBr EMIMOAc

1.158 1.445 1.086 1.075 1.322 1.033 1.149 1.462 1.080

1.195 1.490 1.129 1.021 N/Da 1.046 1.153 N/Da 1.117

1.080 1.309

1.117 1.347

b

b

1.017 1.222

1.014 1.229

b

b

1.069 1.317

1.107 1.353

b

b

1.166 N/D 1.111 1.100 1.300 1.055 1.136 N/D 1.099

b

Unable to calculate due to potential energy diverged. Excluded because it was evaluation of monoatomic ion parameters.

Figure 5. Representative MD structures of cellulose crystals (10-chain × 10-mer model, central six residues) in various ILs after 500 ns, with each molecular chain color-coded.

Figure 6. Dissolution process for the (100) surface of the 10-chain × 10-mer crystal model in AMIMCl showing Cl− (yellow spheres) and AMIM+ (blue sticks) near the chains. (a) Time = 3 ns, (b) time = 19 ns, (c) time = 63 ns, and (d) time = 331 ns at 400 K.

conformation change in BMIMCl.82,83 Moreover, although GLYCAM06 is known to be an ideal choice for solvated glucose,84,85 it has recently been reported to severely underestimate the stability of higher-energy nonchair forms of α- and β-glucose, being much more accurate in the case of the lower-energy chair form.86 Because the chair-to-non-chair conversion of the pyranose ring complicates the accurate determination of potential energy, we investigate the optimal conditions for maintaining the 4C1 chair form and simulating vibrations based on this conformation, which result in the harmonic restraints for the pyranose ring backbone. For cellulose dissolution simulation, we classify ILs into those having powerful (AMIMCl and EMIMCl), good (BMIMOAc, BMIMCl, and EMIMOAc), and poor (1-butyl-

densities of pure ILs at 300 K using GAFF-adopted MD simulations (Table 2). In the above simulations, the charge scale is adjusted to 80% (charge × 0.8) to achieve a reasonable agreement with experimental densities75−81 (deviation between experimental and calculated values = −2.3−1.7%) because the original charge scale (charge × 1.0) and the introduction of IOD set parameters result in a poor density prediction (deviations of −7.2−2.5% and −7.8 to −2.6%, respectively). Besides IL parameters, we also consider the force field parameters of cellulose. Initially, cellulose dissolution was simulated without any torsional restraint, with the pyranose ring being freely convertible from the chair form into the boat form. However, research performed in the past three years suggests that the pyranose ring can undergo a chair-to-boat D


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the extent of hydrogen bonding, evaluating the number of hydrogen bonds in cellulose crystals after dissolution in ILs. Figure 7 illustrates the relationships between the reported

3-methylimidazolium bromide (BMIMBr) and 1-ethyl-3methylimidazolium bromide (EMIMBr)) solvents depending on dissolving power (Table 1). Figure 5 shows the representative MD cellulose crystal structures (10-chain × 10-mer model) in AMIMCl, BMIMCl, and BMIMBr, with a smaller crystal model (9-chain × 10-mer) also exhibiting a similar dissolution behavior (Figure S1). The above models exhibit disordered structures in accordance with the solubility of cellulose in each IL, with dispersed molecular chains and collapsed molecular chain sheets observed in powerful solvents (AMIMCl and EMIMCl). Although the arrangement of molecular chains on the crystal surface changed in good solvents (BMIMOAc, BMIMCl, and EMIMOAc), the molecular chain sheets inside the crystal remained intact. On the other hand, no significant structural changes are observed in poor solvents (BMIMBr and EMIMBr). We focus on the structural changes in molecular chain sheets constituting cellulose crystals, as exemplified by cellulose dissolution in AMIMCl, with Movie S1 showing an animation of the MD trajectory calculated for the 10-chain × 10-mer model in the above IL. The obtained results show that surface molecular chain sheets parallel to the (100) plane underwent continuous splitting starting from the crystal terminus, which results in a disordered crystal structure. Figure 6 illustrates the dissolution of the surface molecular chain sheet in AMIMCl (3−331 ns at 400 K), showing that chloride ions are initially concentrated around the polar groups of cellulose molecular chains, inducing the cleavage of intermolecular hydrogen bonds. Subsequently, imidazolium cations penetrate the gaps between molecular chains to prevent the reformation of these hydrogen bonds, thus inducing the splitting of molecular chain sheets. In addition, the above cations coordinate pyranose rings under the layer. Occasionally, the cleaved hydrogen bonds are recovered by the desorption of imidazolium cations from the crystal surface. The repeated breakage and reformation of hydrogen bonds between molecular chains cause a gradual collapse of the corresponding sheets on the crystal surface, and these chains are finally peeled off from the crystal and irreversibly dispersed in AMIMCl. Notably, imidazolium cations are observed to orient themselves between the peeled chains and the crystal phase, promoting dissolution by successively destroying the molecular chain sheets. Another powerful solvent, EMIMCl, shows similar dissolution behavior (Figure S2). Figures S3 and S4 show the dissolution behavior of cellulose in good solvents (BMIMCl, BMIMOAc, and EMIMOAc), revealing that although the anions of these ILs are concentrated between molecular chains, the slow uptake of cations into molecular chain sheets resulted in insufficient hydrogen bond cleavage, and these sheets are continuously maintained throughout the simulation time. Previously, based on the distribution of BMIMCl on the crystal surface, the synergistic polar and stacking interactions between cellulose and ILs were suggested to stabilize detached cellulose chains.55,87 Consequently, we herein observe that both anions and cations stepwise contribute to the dissolution of cellulose in AMIMCl and EMIMCl, whereas the cations of good solvents (BMIMCl, BMIMOAc, EMIMOAc) could not efficiently penetrate spaces between cellulose chains, as mentioned above. As the dissolution of cellulose in ILs is thought to involve the collapse of molecular chain sheets accompanied by hydrogen bond cleavage, we attempt to correlate cellulose solubility with

Figure 7. Relationship between experimentally determined solubility of microcrystalline cellulose and the number of intermolecular (top) and intramolecular (bottom) hydrogen bonds in cellulose crystals (10chain × 10-mer model; 201−250 ns at 400 K).

solubilities of microcrystalline cellulose in ILs and the number of intermolecular and intramolecular hydrogen bonds (O2··· O6/O3···O6 and O3···O5/O2···O6, respectively; Figure S5) in the 10-chain × 10-mer model, with the observed correlation (R2 = 0.93 and 0.70 for inter- and intramolecular hydrogen bonds, respectively), indicating that these parameters are intimately related. Based on the above correlation, AMIMBr and AMIMOAc, for which no cellulose solubility data are available, are predicted to exhibit microcrystalline cellulose solubilities of 3.1 and 8.5 wt %, respectively, with similar results obtained for 9-chain × 10-mer models (Figure S6; R2 = 0.71 and 0.74 for inter- and intramolecular hydrogen bonds, respectively). To further estimate the cellulose-dissolving power of ILs, we calculate the cellulose−IL interaction energy (ΔEbind) as the difference between total system energies (Etotal) and their separate parts (Ecellulose and Eionic liquid) ΔE bind = Etotal − (Ecellulose + E ionic liquid)

Figure 8 shows the ΔEbind values (kcal/mol) obtained from the MD trajectory of the 10-chain × 10-mer model in ILs (201− 250 ns at 400 K), revealing that these values are in qualitatively good agreement with experimental solubilities. From the breakdown of the interaction for each cation and anion, the ΔEbind values between cellulose and anion correspond to the cellulose solubility (Figure S7). Powerful solvents (AMIMCl and EMIMCl) exhibit large ΔEbind values for both cations and anions, suggesting their interactions with cellulose are stronger than those of other ILs. Moreover, the cellulose chains are fully detached, and the inside chains could directly contact with ILs E


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of intermolecular hydrogen bonds followed by chain peeling from the crystal surface.

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CONCLUSIONS Herein, we systematically evaluated the dissolution of crystal cellulose models in imidazolium-based ILs, revealing that in ILs with high dissolving power (powerful solvents, AMIMCl and EMIMCl), this process comprises three stages (Figure 10).

Figure 8. ΔEbind between cellulose crystals (10-chain × 10-mer model; 201−250 ns at 400 K) and ILs. Numbers in green brackets indicate ΔEbind values, with red and blue areas showing their breakdown into cellulose−cation and cellulose−anion interaction energies, respectively.

for the powerful solvents. On the other hand, the inside chains would be far from the ILs for good solvents and poor solvents, lowering the interaction energy. Furthermore, we extend the MD simulation to a larger cellulose crystal model (30-chain × 20-mer), revealing that dissolution takes place only from the crystal terminus, as observed for 9- and 10-chain models, probably reflecting the very high degree of cellulose fiber polymerization. Because the transmission electron microscopy imaging suggests that enzymatic hydrolysis can proceed from the nick portion of cellulose microfibrils,88 we introduced such a portion at the middle part of the 30-chain × 20-mer model surface (Figure 3) to observe the dissolution of cellulose fiber inner parts. Figure 9 illustrates the MD structure of the 30-chain × 20-mer model in AMIMCl (75 ns, 450 K), revealing that intermolecular hydrogen bonds are continuously cleaved by the IL along the fiber axis direction starting from the nick portion, with molecular chains consequently peeled off from the crystal phase. Movie S2 shows the MD trajectory animation for the 30chain × 20-mer model in AMIMCl during 51−75 ns. Finally, the peeled molecular chains were shown to be molecularly dispersed in solution, confirming that the dissolution behavior of the 30-chain × 20-mer model was similar to that of 9- and 10-chain crystal models. Thus, the dissolution of cellulose microfibrils in ILs is demonstrated to proceed via the cleavage

Figure 10. Dissolution of crystal cellulose models in powerful solvents (AMIMCl and EMIMCl).

Initially, IL anions penetrate spaces between molecular chains and induce the cleavage of hydrogen bonds between these chains, with imidazolium cations subsequently orienting themselves between molecular chains and increasing interchain distances. Finally, molecular chains are dispersed and molecular chain sheets inside the crystal models collapse. Thus, IL cations and anions cooperatively contribute to continuous intermolecular hydrogen bond cleavage in powerful solvents (Figure S8). In the case of good solvents (BMIMCl, BMIMOAc, and

Figure 9. Structure of the 30-chain × 20-mer crystal model in AMIMCl showing Cl− (spheres) and AMIM+ (translucent sticks) in the vicinity of cellulose chains (75 ns, 450 K). F


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EMIMOAc), the anions are concentrated between molecular chains, with the slow uptake of cations into molecular chain sheets resulting in insufficient hydrogen bond cleavage. Finally, cellulose crystals do not exhibit significant structural changes in poorly solubilizing ILs (BMIMBr and EMIMBr). MD trajectory analysis revealed that the solubility of microcrystalline cellulose is fairly well correlated with the number of hydrogen bonds in cellulose crystals, suggesting that cellulose solubility in different ILs can be easily predicted from the number of hydrogen bonds obtained by the MD simulation. Thus, theoretical methods based on the present approach are expected to enable the design of ILs with high cellulosesolubilizing power, with additional detailed mechanistic studies of cellulose dissolution in ILs thought to act as a guide for efficient lignocellulose decomposition and the design of new cellulosic materials.

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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/acs.jpcb.7b09525. Movies, figures, and plots of additional results for the cellulose dissolution and hydrogen binding of the crystal models obtained from MD trajectories (PDF) An animation of the MD trajectory calculated for the 10chain × 10-mer model in AMIMCl (AVI) An animation of the MD trajectory calculated for the 30chain × 20-mer model in AMIMCl (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-99-2857743. Fax: +81-99-285-3253. ORCID

Jun-ichi Kadokawa: 0000-0001-8813-1950 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Number JP16J10411 (Grant-in-Aid for JSPS Research Fellow). The calculations were partly performed using the Research Center for Computational Science, Okazaki, Japan.

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Cellulose Crystal Dissolution in Imidazolium-Based Ionic Liquids: A

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