Dissolution behavior of microcrystalline cellulose in DBU-based deep eutectic solvents: Insights from spectroscopic investigation and quantum chemical calculations

Journal Pre-proof Dissolution behavior of microcrystalline cellulose in DBU-based deep eutectic solvents: Insights from spectroscopic investigation and quantum chemical calculations

Hui Fu, Xinyu Wang, Haina Sang, Yunpeng Hou, Xihai Chen, Xiang Feng PII:

S0167-7322(19)35209-2

DOI:

https://doi.org/10.1016/j.molliq.2019.112140

Reference:

MOLLIQ 112140

To appear in:

Journal of Molecular Liquids

Received date:

17 September 2019

Revised date:

11 November 2019

Accepted date:

14 November 2019

Please cite this article as: H. Fu, X. Wang, H. Sang, et al., Dissolution behavior of microcrystalline cellulose in DBU-based deep eutectic solvents: Insights from spectroscopic investigation and quantum chemical calculations, Journal of Molecular Liquids(2018), https://doi.org/10.1016/j.molliq.2019.112140

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2018 Published by Elsevier.

Journal Pre-proof

Dissolution behavior of microcrystalline cellulose in DBU-based deep eutectic solvents: Insights from spectroscopic investigation and quantum chemical calculations Hui Fua,*, Xinyu Wanga, Haina Sanga, Yunpeng Houb, Xihai Chenb, Xiang Fengb a

College of science, China university of petroleum, Qingdao 266580, China

b

of

College of Chemical Engineering, China University of Petroleum, Qingdao 266580, China

ro

ABSTRACT

New solvents have been developed to dissolve microcrystalline cellulose by

-p

easily prepared DBU-based deep eutectic solvents (DESs), which have excellent

re

solvent power for biopolymers. cellulose/[DBU][MTU] as a typical example for

lP

detailed analysis, which was characterized (by means of FT-IR and NMR) and studied as potential cellulose dissolution media. Furthermore, selecting cellobiose as a model

na

for Density functional theory (DFT) calculation. The hydrogen bonds were explored to evaluate the strength of interaction between DESs and cellulose, which is by means

Jo ur

of Atoms in molecules (AIM), Natural bond orbital (NBO) and Reduced density gradient (RDG). Spectroscopic and theoretical analysis are conductive to in-depth understanding of the dissolution mechanism: both of hydrogen bond acceptor and donor in DESs interact with cellulose, contributed to the dissolution behavior. Intermolecular interactions in the complexes have been identified as non-covalent interactions, especially hydrogen bonds, which were considered to be the driving force of dissolution.1

Keywords: Cellulose; Deep eutectic solvent; Dissolution; Hydrogen bond; Mechanism

*Corresponding authors. E-mail addresses: [email protected] (H. Fu). 1

Journal Pre-proof 1. Introduction Cellulose is one of the most ubiquitous abundant natural biopolymer at present, and exhibits excellent properties such as being renewable, biocompatible and biodegradable. In recent decades, more emphasis has been given on developing cellulose conversion processes for commercialization. This will introduce new bio-based products for sustainable use while improving the economic returns of biorefinery facilities. To characterize, process, and modify cellulose, it often needs to be dissolved first, explaining the necessity of good cellulose solvents. Due to the

of

strong hydrogen bonds between cellulose chains, cellulose is difficult to dissolve in

ro

water and general organic solvents, such as ethanol, ether, etc. This obviously restricts the further processing and application of cellulose. According to reports, various

-p

solvent systems have been developed to promote the dissolution of cellulose, e.g.,

re

N-methylmorpholine-N-Oxide (NMMO), LiCl/N,N-dimethylacetamide (DMAc),

lP

cuprammonium [1-3]. However, the above solvent systems are often accompanied by environmental pollution and energy consumption.

na

In search of a better biomass processing strategy, biomass pretreatments using green solvents, such as ionic liquid (IL) and deep eutectic solvent (DES) have

Jo ur

attracted much attention. As a novel ionic liquid analogue, DES may also have an ionic character but consist of a mixture of organic compounds having a melting point significantly lower than that of either individual component. It is a mixture of hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA), with different structure to ILs. It has many similar properties to ionic liquids [4,5]. Its advantages of easy preparation, low production cost [6,7] make it more attractive in the fields of metal deposition [8-10], compound extraction [11], CO2 capture [12], etc. DES is also known as “Solvent for the 21st Century” [13] because of its better performance and biocompatibility. It should be noted that DESs perform well in terms of biomass dissolution. In 2013, chitin was found effectively dissolved in a mixed deep eutectic system [14]. DBU/MEA/SO2 also presented great potential [15] in the process of seeking the best delignification solvent for lignocellulosic biomass. Loow et al [7] further pointed out the effectiveness of DESs in breaking down the stubborn structure 2

Journal Pre-proof in lignocellulosic. Recently, Malaeke [16] reported dissolution behavior of lignin in four types of DESs. In terms of the dissolution of cellulose in DES, Chen [17] has systematically reported the relative researches as review form. While the latest research of Sirvio [18] has successfully dissolved cellulose in DES composed of guanidine hydrochloride and anhydrous phosphoric acid and regenerated it for the preparation

of

nanoparticles.

Based

on

the

above

research,

1,8-Diazabicyclo[5.4.0]undec-7-enium methylthiourea ([DBU][MTU]) has been selected as candidate DES to dissolve cellulose in our work. To the best of our

of

knowledge, it is the first time that [DBU][MTU] was employed to study the

ro

mechanism of cellulose dissolution.

In terms of biomass dissolution, related mechanism in ionic liquids has been

-p

studied [19-21], however, few research focused on dissolution mechanism in DESs.

re

As HBA and HBD coexisting, DES structure is different from ionic liquid. How does

lP

this structure affect its use as a biomass solvent? How does the presence of hydrogen bond network formed between HBA and HBD affect the biomass structure itself? Is

na

the contribution of HBA/HBD positive or negative? Are the contributions of HBA and HBD equal? In brief, the effects of HBA/HBD on the dissolving ability of the DESs

Jo ur

remains unexplored.

In this work, experimental characterizations (e.g., NMR and FT-IR) together with theoretical calculations (e.g., AIM, NBO and RDG) were employed to elucidate the cellulose dissolution mechanism in DES. All the above analyses were discussed the influence of HBA (DBU) and HBD (MTU) in DES on the dissolution process. Meanwhile, the multiple interaction sites between DES and cellulose molecular chains cannot be ignored. Notably, this is the first study to explore the dissolution mechanism in depth by [DBU][MTU] through experiment and theoretical calculations. Adequate understanding of dissolution mechanism in DES is of prime scientific significance and is of referential importance to the design of efficient deep eutectic solvent. 2. Experimental section 2.1. Materials 3

Journal Pre-proof Microcrystalline cellulose with particle size of 250 μm was purchased from Aladdin Reagent (Shanghai) Co., Ltd. The DP of cellulose is 220. The cellulose samples were dried in vacuum at 70°С for 24h before dissolution. N-Methylthiourea (25g, 98%) used in this experiment was bought from Sinopharm Chemical Reagent (Shanghai) Co., Ltd. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU, 500g, 98%) was provided by Sun Chemical Technology (Shanghai) Co., Ltd. 2.2. Preparation of DES DBU and methylthiourea (MTU) were added to round bottom flask at a molar

of

ratio of 4:1, and the reaction was carried out in a 40 °C water bath under N 2

ro

atmosphere for 1 hour. After the reaction was completed, the products were placed in

2.3. cellulose dissolution and regeneration

-p

a vacuum drying oven and dried for 4 hours to obtain [DBU][MTU].

re

The dried [DBU][MTU] (ca. 10g) was located in a round-bottom flask, which

lP

was immersed in an oil bath (SZCL-3A, Henan Yuhua Instrument Factory). Subsequently, 0.05g cellulose powder was slowly added into the flask. After the

na

cellulose powder was fully dissolved, it was added again until the last added cellulose was observed to be incompletely dissolved. This process is completed under the

Jo ur

judgment of polarizing microscope (Moticam2206, Fig. S1-S2). The regeneration of the dissolved cellulose was obtained through the addition of 100ml ethanol to the DES. The precipitate was separated using a centrifuge at 5000rpm for 10 minutes. Subsequently, the regenerated cellulose was washed three times via centrifugation in order to remove any ionic liquid. Then, it was dried in vacuum oven for 24h at 70°C. 2.4. Measurements of FT-IR spectra Using KBr as a blank tablet, the liquid sample was uniformly applied to the tablet and measured using a TENSOR II Fourier infrared spectrometer (Bruker, Germany). The scan rate was 40 cm/s and the recorded wave number range was 8000-350 cm-1. Baseline correction and peak acquisition were performed using OPUS 5.5 software. Infrared spectra before and after the dissolution of cellulose were compared to investigate the differences. 2.5. Measurements of NMR spectra 4

Journal Pre-proof The 13C NMR nuclear magnetic resonance spectra of [DBU][MTU] and cellulose dissolved in [DBU][MTU] were recorded on a Bruker Ascend TM 400 MHz spectrometer. The sample was added to a 5 mm nuclear magnetic tube and all research systems used deuterated DMSO (DMSO-d6) as an external standard. 2.6. Computational methods Density functional theory (DFT) was employed to further investigate the interactions between cellulose and DESs. Since cellulose is macromolecular polysaccharide, cellobiose was chosen as a model (Fig. 8) for estimating the

of

electronic properties of the hydrogen bond and its chemical environment to simplify

ro

the calculation. M062X exchanges correlation functions [22] and 6-311++G(d,p) basis set were used to fully optimize geometries of DESs and cellulose, which have

-p

been proved can correctly describe non-covalent interactions, especially hydrogen

re

bonding. By comparing electron energies of several initial structures, the most stable

lP

geometry could be obtained in this work. Frequency calculations were carried out to further determine the rationality of the optimized structure. The interaction energy

na

(ΔE) was estimated including the basis set superposition errors (BSSE) correction using the counterpoise (CP) methods [23]. Furthermore, the interactions between

Jo ur

cellobiose and DESs were studied by AIM [24] and NBO theory [25]. Reaction sites were predicted by using the electrostatic potential method (ESP) [26]. At the same time, RDG [27] was applied to analyze the strength of non-covalent interactions. All DFT calculations were performed at the Gaussian 09 software package [28]. 3. Results and discussion 3.1. FT-IR analysis

5

of

Journal Pre-proof

Fig. 1. FT-IR spectra of the [DBU][MTU] before (black) and after(red) dissolving cellulose.

ro

The FT-IR spectra of the cellulose/[DBU][MTU] and pure [DBU][MTU] are

-p

given in Fig. 1. As shown in the figure, there is an absorption peak of 3450 cm-1 in pure DES. After cellulose dissolution, the absorption peak moves to 3511 cm-1. This

re

means that blue shift occurs on the absorption peak of the N-H bond in the MTU. It

lP

can be explained by the change of intermolecular hydrogen bonding. In Fig. S3, there are double hydrogen bonding between N27 of DBU and H atoms of MTU. When

na

cellulose is dissolved, one hydrogen bonding disappears and the other is weakened. This result is consistent with vibration frequency analysis. In the range of 3000-2700

Jo ur

cm-1, saturated hydrocarbon-based vibration occurs, and weak interaction exists between methylene in DBU and cellulose. Besides, the bending vibration of -NH2 in [DBU][MTU] and the stretching vibration of C=N cause an absorption peak at 1624 cm-1 in the infrared spectrum. Contrasting the infrared spectra, it can be found that a sharp new peak is generated at 2052 cm-1. This may be due to the stretching vibration of thiocyanate (-SCN), which indicates the weak interaction between sulfur in DES and hydrogen in cellulose [8]. Furthermore, the spectra of untreated cellulose and regenerated cellulose are shown in Fig. S4. the general similarity of two spectra, with no evidence of degradation products. In addition, there is no –SCN band in regenerated cellulose spectrum, indicating that [DBU][MTU] is indeed a non-derivative solvent for cellulose. 3.2. NMR analysis 6

Journal Pre-proof To further explore the mechanism of dissolution, the

13

C NMR spectra of 8wt%

cellulose/[DBU][MTU] solutions and pure [DBU][MTU] liquids at 298K were determined, corresponding to Fig. 2a and Fig. 2b, respectively. The following analysis is based on pure [DBU][MTU] as a starting reference to investigate the chemical shifts in the system after dissolving cellulose. Comparing the

13

C NMR spectra of [DBU][MTU] before and after the

dissolution of cellulose, it can be found that C1, C2, and C3 generate downfield chemical shifts with Δδ of 0.372, 0.079, and 0.02 ppm, respectively. The explanation

of

for the above results is as follows. The hydroxyl hydrogen atom in cellulose interacts

ro

with the N12, N13 atoms in DBU to form hydrogen bonds. The density of electron clouds around the N atom increases, leading to the reduction of electron clouds 13

C NMR signal. Simultaneously,

-p

density around C1, C2, C3 and thereby enhancing

re

the chemical shift of C1 is significantly higher than that of the other carbon atoms.

Jo ur

na

hydroxyl hydrogen atom.

lP

This is attributed to the fact that the N13 atom forms strong hydrogen bonds with the

7

ro

of

Journal Pre-proof

-p

Fig. 2. 13C NMR spectra of (a) cellulose/[DBU][MTU] and (b) pure [DBU][MTU].

re

The strength of the hydrogen bonds formed on the amino groups of the complexes is lower than that of pure DES, and the hydrogen bond on the imino group

lP

disappears. As a result, the electron cloud density of C10 increases and an upfield chemical shift occurs (Δδ= -0.073 ppm). Although the hydrogen on hydroxyl forms a

na

new hydrogen bond with the sulfur on the MTU, the bond energy is still very low and this effect can be ignored. The results declare that [DBU][MTU] is a good solvent

Jo ur

during the dissolution of cellulose, which has a vital significance on the destruction of the hydrogen bonds of cellulose. 3.3. ESP analysis

8

-p

ro

of

Journal Pre-proof

Fig. 3. 3D plots of the electrostatic potential (ESP) surface of four stable conformers. (a) DBU (b)

re

MTU (c) cellobiose (d) [DBU][MTU] (e) cellobiose/[DBU][MTU]. Hydrogen: white; Carbon:

lP

gray; Oxygen: red; Nitrogen: blue; Sulfur: yellow

Electrostatic potential (ESP) is the definition of a positive charge that moves

na

from infinity to the surface of the molecule to measure everywhere properties. It has profound theoretical and practical significance in predicting different reactive sites.

Jo ur

The ESP plot distinguishes the strength of the electrostatic potential on the surface of the molecule by color. As depicted in Fig. 3, blue represents the positive electrostatic potential (low electron density) region, red represents the negative electrostatic potential (high electron density) region, and the remaining colors represent the intermediate level of the electrostatic potential. VS,max and VS,min depict the configuration corresponding local maximum and minimum of ESP. In the 3D image of the ESP plot, it can be seen that a large red region (VS,min= -36.38kcal/mol) is found near the N atom in the DBU, which is easily combined with the blue region of MTU (VS,max= 41.57kcal/mol). Reaction sites are shown in Fig. 3a and 3b. In Fig. 3c, the ESP distribution on the surface of cellobiose is relatively uniform. The positive region is around the hydroxyl hydrogen in cellobiose and the negative region is associated with hydroxyl lone pair electrons. This analysis can be 9

Journal Pre-proof extended to possible reaction sites on the surface of cellobiose molecules. It is suspected that the local maximum of ESP (VS,max= 45.57kcal/mol) is found near the hydrogen in the hydroxyl group. Meanwhile, VS,min (-35.92kcal/mol) of [DBU][MTU] is located at the sulfur atom (Fig. 3d), which means that it is easy to attack the hydroxyl hydrogen of cellobiose and generate electrophilic reaction. In summary, on the one hand, the negative electrostatic potential region in DBU is combined with the positive region in cellobiose and MTU. On the other hand, the sulfur in the yellow region of the MTU interacts with the hydroxyl hydrogen in the

of

cellobiose, and the amino hydrogen forms hydrogen bond with the framework oxygen.

ro

That is to say, HBA (DBU) and HBD (MTU) can work with cellobiose together, and the formed DES ([DBU][MTU]) can be clearly seen from Fig. 3d. Based on the above

-p

prediction, we obtain the most stable structure (Fig. 3e).

re

3.4. Geometries and interaction energy analysis

lP

The interaction energy can be defined as follows: ΔE = EC/DES − (EC + EDES )

na

Where EC, EDES and EC/DES stand for the energies of cellobiose, deep eutectic solvents and cellobiose/DES solvent system, respectively. By calculation, ΔE of the

Jo ur

cellobiose/[DBU][MTU] is -87.74kJ/mol. Negative values indicate that dissolution can occur. What’s more, the solubility is positively correlated with interaction energy. Considering these two aspects, [DBU][MTU] is a better solvent. Meanwhile, the true driving force for dissolution behavior by analyzing stable configuration is due to hydrogen bond between DES and cellulose.

10

ro

of

Journal Pre-proof

-p

Fig. 4. Geometry of the cellobiose/[DBU][MTU]. Hydrogen: white; Carbon: gray; Oxygen: red;

re

Nitrogen: blue; Sulfur: yellow.

The optimized structure and parameters of complex (cellobiose/[DBU][MTU])

lP

can be seen in Fig. 4, where the bond distances of O79-H80···N27 and N28-H29···O60 are 1.951 Å and 2.015 Å, respectively. It can be seen the hydrogen bond strength is strong.

na

In addition, the bond length of N28-H30···N27 in [DBU][MTU] is extended from 2.151 Å (Fig. S3a) to 2.20 Å, accompanied by the disappearance of N33-H34···N27 hydrogen

Jo ur

bond. This means that cellobiose interacts with [DBU][MTU], while the H-Bond strength between DBU and MTU is weakened. When cellobiose is close to DBU, its internal structure changed. At the same time, there formed a new hydrogen bond of O79-H80···N27 between cellobiose and [DBU][MTU], with the bond length of O79-H80 changed from 0.961Å to 0.973Å, which declares that the new hydrogen bond formation reduces the bond strength of hydroxyl in cellobiose. In brief, the formation of hydrogen bonds between DES and cellobiose affects the strength of the H-bond network between HBA and HBD. From another point of view, the structure of cellobiose becomes looser, which effectively explains the occurrence of dissolution behavior. 3.5. Vibration frequency analysis Table 1 11

Journal Pre-proof The experimental infrared data and calculated frequencies of cellobiose/[DBU][MTU], [DBU][MTU] and cellobiose. Species

Bonds

Assignments

Experiment

Cellobiose/[DBU][MTU]

[DBU][MTU]

(cm-1)

νas

3511

3569

β

1701

1677

N33-H34

ν

-

3626

O69-H70

ν

3670

3648

N28-H30

νas

3450

3469

β

1624

1641

ν

-

3531

ν

-

N28-H30

N33-H34 Cellobiose

O41-H42

a

3901

O41-H42 and O69-H70 represent the same bond in different system.

ro

a

DFT calculations

(cm )

of

-1

The main vibration frequencies of DES and cellobiose/[DBU][MTU] are listed in

-p

Table 1. It can be easily found that there is consistency between the experimental and

re

theoretical aspects. Taking the N28-H30 bond in cellobiose/[DBU][MTU] as an example, the corresponding stretching vibration and bending vibration values for the

lP

experiment are 3511 cm-1 and 1701 cm-1, respectively, which are consisted with the theoretical results of 3569 cm-1 and 1677 cm-1. Notably, the frequency value at 1677

na

cm-1 is formed by the coupling of bending vibration of N28-H30 and stretching vibration of C6=N27. Furthermore, the hydrogen bond of N33-H34···N27 (Fig. 4)

Jo ur

disappears, which reasonably explains the result for the blue shift(3531cm-1 → 3626cm-1) of data in the table. The frequency of O69-H70 is reduced from 3901 cm-1 to 3648 cm-1. The reason for this result is that the strength of hydrogen bonds of O69-H70···O79 increases (1.971Å → 1.791Å) after dissolution. 3.6. AIM analysis AIM is a tool widely employed to investigate non-covalent interactions, which is based on the electron density distribution between two atoms to define the chemical structure of the system. Its core is topological analysis. Table 2 lists the topological parameters at the bonding critical point (BCP) of interaction between [DBU][MTU] and cellobiose, such as electron density(ρ), laplacian density (∇2ρ), potential density (Vcp), curvature (λ), and the energy of hydrogen bond (EHB). These parameters are calculated by MultiWFN code [29]. Using the Espinosa-Molins-Lecomte equation [30] 12

Journal Pre-proof the relationship between Vcp and EHB is EHB=Vcp/2. Moreover, ∇2ρ=λ1+λ2+λ3, where λi is an eigenvalue of the Hessian matrix. A Bond Critical Point (BCP) [31] is estimated that one of three eigenvalue is positive and the others are negative. This proves that there is a chemical interaction between the two atoms. Molecule plots are depicted by MultiWFN code and Visualized Molecular Dynamics (VMD) program [32] in Fig. S5, and BCPs are depicted by orange points. Table 2

[DBU][MTU]. Species

Hydrogen bonds

ρBCP(a.u.)

∇2ρBCP(a.u.)

Vcp(a.u.)

Cellobiose/

O79-H80···N27

0.0273

0.0899

-0.0204

[DBU][MTU]

N28-H29···O60

0.0226

0.0851

O81-H82···N17

0.0199

0.0691

N28-H30···N27

0.0187

0.0597

O61-H62···S32

0.0212

0.0508

N28-H30···N27

0.0203

0.0662

λ1

λ3

EHB(kcal/mol)

-0.0335

-0.0353

0.1588

-6.40

-0.0172

0.1417

-0.0272

-0.0295

-5.40

-0.0137

0.1137

-0.0227

-0.0219

-4.30

-0.0115

-0.0192

0.0990

-0.0202

-3.61

-0.0121

0.0951

-0.0226

-0.0217

-3.80

-0.013

0.1113

-0.0232

-0.022

-4.08

-p

ro

λ2

re

[DBU][MTU]

of

Topological parameters at bond critical points (BCPs) for cellobiose/[DBU][MTU] and pure

lP

According to the AIM theory, ρ and ∇2ρ are used to describe the properties of hydrogen bond. There is a positive correction between the electron density and the

na

strength of hydrogen bond. When ∇2ρ is greater than 0, it represents the presence of hydrogen bonds or ionic bonds or van der Waals forces. In contrast, if it is less than 0,

Jo ur

it is a covalent bond. In addition, the topological parameters of closed shell interaction have a certain range of parameters, that is, the electron density ranges from 0.002 to 0.035 a.u. and the laplacian density ranges from 0.024 to 0.139a.u. The data listed in Table 2 are all within this range. For example, ρ=0.0273 a.u. and ∇2ρ=0.0899 a.u. of O79-H80···N27 show the strongest intermolecular hydrogen bonds. This can be confirmed from the geometric analysis. From DES to the complex, the distance of N28-H30···N27 bond changes from 2.15Å to 2.20Å, and the bond angle changes from 151.0° to 151.2°. Such small geometrical changes lead to little energy change of EHB (-3.61 kcal/mol to -4.08 kcal/mol). Simultaneously, by comparing the ρ and EHB of N28-H29···O60 and O81-H82···N17, it is clear that N28-H29···O60 is stronger. Therefore, it can be concluded that HBA and HBD interact with cellobiose, indicating that two parts of DES contribute to the dissolution process, and the results are consistent with 13

Journal Pre-proof the following analysis.

of

3.7. NBO analysis

na

lP

re

-p

ro

LP(1)N27→σ*(1)O79-H80 E(2)= 8.01kcal/mol

Jo ur

LP(2)O60→σ*(1)N28-H29 E(2)= 5.46kcal/mol

Fig. 5. Natural bond orbital 3D overlap images for donor-acceptor orbital interactions in intermolecular hydrogen bonds.

NBO theory is a set of the theories developed from the concepts of natural spin orbit, natural bond orbit and natural hybrid orbit. It is often applied to characterize hydrogen bond properties. The values of second-order perturbation energies denote the strength of the orbital interaction between the electron donor and the electron acceptor. Formula denotes second-order perturbation energy [19]: 𝐹 2 (𝑖, 𝑗) E(2) = ∆𝐸𝑖𝑗 = 𝑞𝑖 𝜀𝑗 − 𝜀𝑖 Where qi represents the occupancy of the donor orbit, 𝜀 i and 𝜀 j denote the diagonal elements, respectively. F(i , j) is the off-diagonal NBO matrix element. 3D images of the orbital interaction are depicted in Fig. 5. 14

Journal Pre-proof The influence of HBA and HBD on dissolution behavior also can be seen from the NBO data. Table S2 displays the main donor-acceptor interactions in cellobiose, [DBU][MTU] and complex. We can know that the E(2) of the intermolecular orbital formed by the lone pairs of N27 atom in DBU and the anti-bonding orbital σ*(1)O79-H80 in cellobiose is 8.01 kcal/mol, which can be more visually seen from Fig. 5. It is obvious that there is a track overlap between DBU and cellobiose. Overlaps of interaction orbitals are increased along with the increments of E(2). The corresponding E(2) values of LP(1)O79→σ*(1)O69-H70 in the cellobiose and complex

of

are located at 4.73 kcal/mol and 12.84 kcal/mol, respectively. Consequently, this

ro

effectively illustrates the change of intramolecular hydrogen bonds in cellobiose. By comparing the E(2) values between the different donors and acceptors listed in Table

-p

S2, the intensities of the hydrogen bonds reflected are in accordance with the above

re

analysis.

Jo ur

na

lP

3.8. RDG analysis

Fig. 6. The RDG scatter plot (the left) and sign(λ2)ρ mapped RDG isosurfaces (the right) of cellobiose/[DBU][MTU].

RDG analysis is used as another useful method to further discuss non-covalent interaction [33]. The product of sign(λ2)ρ is projected onto RDG isosurfaces with different colors to revel the type and intensity of non-covalent interactions as well as to visualize weak interactions using VMD program. Fig. 6 depicts RDG scatter plot and colored RDG isosurfaces of cellobiose/[DBU][MTU]. The same pictures of pure [DBU][MTU] and cellobiose are shown in Fig. S6. Here, the blue region indicates 15

Journal Pre-proof strong interaction force (hydrogen bond), the green region shows weak interactions like Van der Waals forces, and the steric effect is distinguished by red. From RDG isosurface map, we can clearly see that there are non-covalent interactions between HBA, HBD and cellobiose. The regional color reflects intensity of interaction. It can be found that HBA and HBD have similar effects. Table 3 The value of sign(λ2)ρ of different hydrogen bond in cellobiose, [DBU][MTU] and cellobiose/[DBU][MTU].

Cellobiose/[DBU][MTU]

O69-H70···O79 O79-H80···N27

-0.0355

-p

N28-H29···O60

Sign(λ2)ρ

of

Hydrogen bonds

ro

Species

-0.0212

O81-H82···N17

-0.0199

N28-H30···N27

-0.0188

re

Cellobiose a

-0.0226

O61-H62···S32

lP

[DBU][MTU]

-0.0273

N28-H30···N27

-0.0203

O31-H32···O41a

-0.0219

Jo ur

na

O31-H32···O41 and O69-H70···O79 represent the same H-bonds in different system.

Fig. 7. Visual drawing of main discussed interaction in cellobiose/[DBU][MTU] and cellobiose. Isosurfaces are colored for s=0.5 a.u. and colored over the range -0.04
For cellobiose/[DBU][MTU] system, Table 3 shows the sign(λ2)ρ of different H-bonds, and the typical visual drawing of the interaction can be seen in Fig. 7. For double H-bonds, the sign(λ2)ρ of O79-H80···N27 and N28-H30···N27 are located at -0.0273 and -0.0188 a.u., respectively. It is obvious that the former is stronger than the 16

Journal Pre-proof latter. For the same intramolecular H-bond (O69-H70···O79) in cellobiose, the strength becomes stronger after dissolution, with the value of sign(λ2)ρ corresponding to -0.0355 and -0.0219 a.u, respectively. The transition of RDG isosurfaces from green to blue illustrates the trend of stronger interactions. Additionally, the change process of N28-H30···N27 is similar to the above results, and the green isosurfaces are proved to be weak hydrogen bonds. Hence, the cellulose can be well dissolved in DES.

Jo ur

na

lP

re

-p

ro

of

3.9. Supposed dissolution mechanism

Fig. 8. Graphic mechanism of cellulose in DES. Blue ball: DBU (HBA); Pink ball: MTU (HBD); Green dotted line: inherent hydrogen bonds in cellulose; Red dotted line: new hydrogen bond between DES and cellulose.

Based on the above analysis, both of HBA and HBD have interaction sites with cellulose chains, which greatly promotes the occurrence of dissolution behavior. Since the H-bonds between DBU and MTU is stable, they tend to coexist in the form of DES during cellulose dissolution (Fig. 8). Obviously, H-bond network of cellulose chains is destroyed. HBA forms new hydrogen bonds with –OH and –CH2OH of cellulose, while HBD interacts with both of skeletal oxygen and hydroxyl hydrogen of cellulose, which is due to the simultaneous presence of methylene and sulfur in MTU. Accordingly, the synergistic effect of HBA and HBD destroys the hydrogen bonds in 17

Journal Pre-proof cellulose chains, which makes the inherent compact structure loosen and eventually leads to complete cellulose dissolution. 4. Conclusions In general, DESs have been extensively employed as solvents for biopolymer so as to enhance their solubility. This work mainly explores the cellulose/[DBU][MTU] system from both experimental and theoretical perspectives. Spectroscopic investigations reveal that the dissolution of cellulose in DES cause changes in the chemical environment around carbon, which is due to the H-bonds destruction in

of

cellulose, and the H-bonds formation between DES and cellulose. Moreover, detailed

ro

theoretical analyses have shown that every interaction between [DBU][MTU] and cellulose involves a combination of multiple interactions that makes the interaction

-p

strong enough to maintain the stability of the complex structures. Compared with

re

covalent bonds, non-covalent interactions are weak, highly susceptible to the

lP

environment and diversified. Among them, the magnitude of hydrogen bonding is larger than that of most other non-covalent interactions. The macroscopic dissolution

na

behavior is accompanied by the microscopic hydrogen bond change. In other words, when HBA and HBD interact with cellulose forming non-covalent interaction, the

Jo ur

H-bonds strength of DES is reduced. Meanwhile, H-bond network of cellulose molecule chains is destroyed, and chains structure becomes loosen, resulting in dissolution. Therefore, hydrogen bonds play an important role in promoting the dissolution of cellulose in [DBU][MTU], illuminating its dissolution mechanism. The experimental facts also confirm that the deep eutectic solvent is a promising solvent for dissolving biomass. Thus in depth understanding of these weak interactions may lead to design of efficient functional materials.

Acknowledgement This work was supported by Shandong Provincial Natural Science Foundation (ZR2017MB030), the Fundamental Research Funds for the Central Universities（17CX02068）and the National Nature Science Foundation of China (21606254). 18

Journal Pre-proof

Reference

Jo ur

na

lP

re

-p

ro

of

[1] A.J. Sayyed, L.V. Mohite, N.A. Deshmukh, D.V. Pinjari, Structural characterization of Cellulose pulp in aqueous NMMO solution under the process conditions of Lyocell slurry, Carbohyd. Polym. 206 (2019) 220-228. [2] Y. Ono, K. Furihata, N. Isobe, T. Saitom, A. Isogai, Solution-state structures of the cellulose model pullulan in lithium chloride/N,N-dimethylacetamide, Int. J. Biol. Macromol. 107 (2018) 2598-2603. [3] A.J. Sayyed, N.A. Deshmukh, D.V. Pinjari, A critical review of manufacturing processes used in regenerated cellulosic fibres: viscose, cellulose acetate, cuprammonium, LiCl/DMAc, ionic liquids, and NMMO based lyocell, Cellulose 26 (2019) 2913-2940. [4] T.L. Greaves, C.J. Drummond, Protic ionic liquids:  properties and applications, Chem. Rev. 108 (1) (2008) 206-237. [5] Y. Chen, T. Mu. Application of deep eutectic solvents in biomass pretreatment and conversion. Green Energy & Environment 4 (02) 2019 95-115. [6] M. Zdanowicz, K. Wilpiszewska, T. Spycha, Deep eutectic solvents for polysaccharides processing.A review, Carbohyd. Polym. 200 (2018) 361-380. [7] Y.L. Loow, E.K. New, G.H. Yang, L.Y. Ang, T.Y. Wu, Potential use of deep eutectic solvents to facilitate lignocellulosic biomass utilization and conversion, Cellulose 24 (9) (2017) 1-28. [8] J. Wang, P. Wang, Q. Wang, H. Mou, B. Cao, D. Yu, D. Wang, S. Zhang, T. Mu, Low Temperature Electrochemical Deposition of Aluminum in Organic Bases/Thiourea-Based Deep Eutectic Solvents, ACS Sustain, Chem. Eng. 6 (11) (2018) 15480-15486. [9] E.A. MernissiCherigui, K. Sentosun, P. Bouckenooge, H. Vanrompay, S. Bals, H. Terryn, J. Ustarroz, Comprehensive study of the electrodeposition of nickel nanostructures from deep eutectic solvents: self-limiting growth by electrolysis of residual water, J. Phys. Chem. C 121 (17) (2017) 9337-9347. [10] J. Wang, H. Mou, R. Li, Y. Li, D. Wang, Z. Xue, T. Mu, Solution processing of V2VI3 chalcogenides with a deep eutectic solvent for enhanced visible-light-driven hydrogen production, Green Chem. 20 (2018) 5266-5270. [11] M.W. Narm, J. Zhao, M.S. Lee, J. H. Jeong, J. Lee, Enhanced extraction of bioactive natural products using tailor-made deep eutectic solvents: Application to flavonoid extraction from Flossophorae, Green Chem. 17 (3) (2015) 1718-1727. [12] T.J. Trivedi, J.H. Lee, H.J. Lee, Y.K Jeong, J.W. Choi, Deep Eutectic Solvents as Attractive Media for CO2 Capture, Green Chem. 18 (9) (2016) 2834-2842. [13] A. Paiva, R. Craveiro, I. Aroso, M. Martins, R. L. Reis, A. R. C. Duarte, Natural deep eutectic solvents-solvents for the 21st century, ACS Sustain. Chem. Eng. 2 (5) (2014) 1063-1071. [14] M. Sharma, C. Mukesh, D. Mondal, K. Prasad, Dissolution of α-chitin in deep eutectic solvents, RSC Adv. 3 (39) (2013) 18149-18155. [15] I. Anugwom, L. Rujana, J. Waran, M. Hedenstrom, J.P. Mikkola, In quest for the optimal delignification of lignocellulosic biomass using hydrated, SO2 switched DBU MEASIL switchable ionic liquid, Chem. Eng. J. 297 (2016) 256-264. [16] H. Malaeke, M.R. Housaindokht, H. Monhemi, M. Izadyar, Deep eutectic solvent as an efficient molecular liquid for lignin solubilization and wood delignification, J. Mol. Liq. 263 (2018) 193-199. 19

Journal Pre-proof

Jo ur

na

lP

re

-p

ro

of

[17] Y.L. Chen, X. Zhang, T.T. You, F. Xu, Deep eutectic solvents (DESs) for cellulose dissolution: a mini-review, cellulose 26 (2019) 205-213. [18] J.A. Sirvio, Fabrication of regenerated cellulose nanoparticles by mechanical disintegration of cellulose after dissolution and regeneration from a deep eutectic solvent, J. Mater. Chem. A 7 (2) (2019) 755-763. [19] B. Cao, J. Du, Z. Cao, X. Sun, H. Sun, H. Fu, DFT study on the dissolution mechanisms of α-cyclodextrin and chitobiose in ionic liquid, Carbohyd. Polym. 169 (2017) 227-235. [20] Y. Zhang, H. He, K. Dong, M. Fan, S. Zhang, A DFT study on lignin dissolution in imidazolium-based ionic liquids, RSC Adv. 7 (21) (2017) 12670-12681. [21] X. Sun, Q. Tian, Z. Xue, Y. Zhang, T. Mu, The dissolution behaviour of chitosan in acetate-based ionic liquids and their interactions: from experimental evidence to density functional theory analysis, RSC Adv. 4 (57) (2014) 30282-30291. [22] Y. Zhao, D.G. Truhlar, Density functionals with broad applicability in chemistry, Accounts Chem. Res. 41 (2) (2008) 157-167. [23] F.B. Van Duijneveldt, van Duijneveldt-van de Rijdt., G.C.M. Jeanne, J.H. Van Lenthe, State of the art in counterpoise theory, Chem. Rev. 94 (7) (1994) 1873-1885. [24] R.F.W. Bader,Atoms in Molecules: A Quantum Theory. [25] A.E. Reed, L.A. Curtiss, F. Weinhold, Intermolecular interactions from a natural bond orbital, donor-acceptor viewpoint, Chem. Rev. 88 (6) (1988) 899-926. [26] B. Cao, S. Liu, D. Du, Z. Xue, H. Fu, H. Sun, Experiment and dft studies on radioiodine removal and storage mechanism by imidazolium-based ionic liquid, J. Mol. Graph. Model. 64 (2015) 51-59. [27] V. Tognetti, P. Cortona, C. Adamo, A new parameter-free correlation functional based on an average atomic reduced density gradient analysis, J. Chem. Phys. 128 (3) (2008) 034101. [28] M. Frisch, G. Trucks, H. Schlegel, G. Scuseria, M. Robb, J. Cheeseman, Wallingford, CT: Gaussian, Inc., 2009, Gaussian 09, revision A. 02 (1988). [29] T. Lu, F. Chen, Multiwfn: a multifunctional wavefunction analyzer, J. Comput. Chem. 33 (5) (2012) 580-592. [30] E. Espinosa, E. Molins, C. Lecomte, Hydrogen bond strengths revealed by topological analyses of experimentally observed electron densities, Chem. Phys. Lett. 285 (3) (1998). [31] O. Knop, K.N. Rankin, R.J. Boyd, Coming to grips with N−H···N bonds. 1. distance relationships and electron density at the bond critical point, J. Phys. Chem. A 105 (26) (2001) 6552-6566. [32] W. Humphrey, A. Dalke, K. Schulten, VMD: visual molecular dynamics, J. mol. Graph. Model. 14 (1) (1996) 33-38. [33] J. Wang, Z. Xue, C. Yan, Z. Li, T. Mu, Fine regulation of cellulose dissolution and regeneration by low pressure CO2 in DMSO/organic base: dissolution behavior and mechanism, Phys. Chem. Chem. Phys. 18 (48) (2016) 32772-32779.

20

Journal Pre-proof Conflict of interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the submitted work.

Jo ur

na

lP

re

-p

ro

of

Hui Fu

21

Journal Pre-proof

Jo ur

na

lP

re

-p

ro

of

Highlights: 1. Cellulose dissolution mechanism in [DBU][MTU] was studied in experiment and DFT simulation. 2. IR and NMR spectra indicated hydrogen bonds were the main driving force for dissolution. 3. Theoretical analyses indicate HBA and HBD in DES can affect the dissolution of cellulose.

22