High solubility of cellulose in slow-cooling alkaline systems and interacting modes of alkali and urea at the molecular level

doi:10.21203/rs.3.rs-3251961/v1

Download PDF

Research Article

High solubility of cellulose in slow-cooling alkaline systems and interacting modes of alkali and urea at the molecular level

https://doi.org/10.21203/rs.3.rs-3251961/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this latest preprint version

The dissolution of microcrystalline cellulose (MCC) in a urea-NaOH system is beneficial for its mechanical processing, but the solubility has been low. The MCC solubility was greatly improved to 14 wt% under a slow-cooling condition with a cooling rate of − 0.3°C/min. The cooling curve or thermal history played a crucial role in the dissolution process. An exotherm (31.9 ± 1.6 J/g MCC) was detected by DSC only under the slow-cooling condition, and the cryogenic dissolution of MCC is attributed to the exothermic interaction between MCC and solvent. More importantly, the low cooling rate promoted the dissolution of MCC by providing enough time for the diffusion of OH⁻ and urea into MCC granules at higher temperatures. The Raman spectral data showed that the intramolecularly and intermolecularly hydrogen bonds in cellulose were cleavaged by NaOH and urea, respectively. XPS and solid-state ¹³C NMR results showed that hydrogen bonds were generated after dissolution, and a dual-hydrogen-bond binding mode between urea and cellulose was confirmed by DFT calculations. The increase of entropy dominated the spontaneity of cryogenic dissolution of MCC, and the decrease of enthalpy played a minor role. The high solubility of MCC in the slow-cooling process and the dissolution mechanism are beneficial for the studies on cellulose modification and mechanical processing.

Cellulose

Cryogenic dissolution

Hydrogen bond

Urea

Thermal history

Microcrystalline cellulose (MCC) is a natural polymer comprising anhydro-D-glucose units covalently linked by β-1, 4-glycosidic bonds (Wang, et al. 2021; Zhao, et al. 2022). The MCC polymeric chains are linked by inter-chain or inter-molecule hydrogen bonds (Li, et al. 2018a; Parveen, et al. 2017), so MCC is recalcitrant in mechanical processing such as injection molding and extrusion and chemical conversions such as hydrolysis (Sun, et al. 2015; Tian, et al. 2022). Dissolution aims to break these hydrogen bonds without the crack of β-1, 4-glycosidic bonds. A traditional cellulose dissolution system is the solution of cuprammonium hydroxide (Dong, et al. 2016), but the solubility is lower than 8 wt% (Jia, et al. 2012; Jia, et al. 2014b). Currently, the prevalent industrial cellulose-dissolution technique is the viscose process (Li, et al. 2021), where caustic soda and toxic explosive carbon disulfide are consumed. Ionic liquids (ILs) are the common candidates for the dissolution of MCC. For instance, Xu and co-workers (Xu, et al. 2010) reported the dissolution of cellulose in 1-butyl-3-methylimidazolium-based ionic liquids. The MCC solubility was up to 11.5 wt% at 40°C and 15.5 wt% at 70°C with the solvent [C₄mim][CH₃COO]. However, the high cost and difficult recovery of ILs limit their applications in the dissolution of MCC.

In the year of 1998, Isogai et al (Isogai and Atalla 1998) reported a freeze–thaw method (Wang, et al. 2016) for cellulose dissolution, in which the mixture of aqueous NaOH solution and cellulose was frozen into solid in the first place, and the MCC solubility was not high. Inspired by this work, Zhang et al (Li, et al. 2015; Li, et al. 2012; Qi, et al. 2011) developed a NaOH-urea-water solvent with maximum MCC solubility of 5.2 wt% at below − 12°C. Furthermore, the solubility was improved to 5.5‒7 wt% with LiOH (Cai, et al. 2010; Liu, et al. 2010; Quanling, et al. 2011). Norgren et al (From, et al. 2020) reported the dissolution of 4 wt% dissolving pulp in a LiOH-urea system. 6‒7.5 wt% solubility was achieved in NaOH-thiourea-water systems under moderate conditions (− 8 to 5°C) (Chang and Zhang 2011; Liang, et al. 2008). The relatively low solubility of cellulose in these alkaline systems leads to excessive consumption of alkali, urea or thiourea, large volumes of containers, and high transportation fees.

In these previous studies (Liu, et al. 2010; Qi, et al. 2011; Ran, et al. 2010), cellulose was instantly cooled to ‒5 – ‒20°C. The effect of cooling rate on the dissolution of cellulose has been underappreciated, and the crack and generation of bonds in the cryogenic dissolution process are still unclear. The cooling rate is the reflection of thermal history (temperature profile) of a temperature-varying process. In addition, the deep reason for the cryogenic dissolution of cellulose has not been clearly elucidated by these studies. Fausto et al. (Jarmelo, et al. 2000) reported that different crystalline phases of methyl glycolate were separately generated with different cooling rates. Therefore, we speculated that the cooling rate or thermal history had influence on the solubility of MCC, and the reason for this high solubility was investigated in the present paper. The heat during the dissolution process was measured by differential scanning calorimetry (DSC). The changes in hydrogen bonds and functional groups before and after the dissolution of MCC were monitored by Raman spectroscopy, and the chemical shifts of vicinal carbon atoms due to these changes were measured by solid-state ¹³C-CP-MAS NMR spectroscopy (solid-state ¹³C NMR). The chemical state of C and N elements on the surface of regenerated cellulose was probed by X-ray photoelectron spectroscopy (XPS) to unravel the interaction between urea and cellulose. Based on the DSC, Raman, and XPS results, a plausible and reasonable dissolution mechanism was proposed in the last section, and this mechanism was validated by density functional theory (DFT) calculations.

Materials

Microcrystalline cellulose (MCC, 99 wt%, granule diameter = 20‒100 µm) was purchased from Tianjin Guangfu Fine Chemical Co., Ltd (Tianjin, China). The viscosity-average polymerization degree of MCC was 214, calculated by the Mark-Houwink equation [η] = 0.0372 × molecular weight^0.77 documented in the literatures (Cai, et al. 2006; He, et al. 2015). In the determination experiment of polymerization degree, MCC was dissolved in an aqueous solution of 4.6 wt% LiOH and 15 wt% urea at − 10°C. The data was measured with a Ubbelohde viscometer at 25°C (Fig. S1). Analytical-grade sodium hydroxide, ammonium carbamate and urea were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Pure water was prepared with a Millipore pure-water generator (the United States). Absolute ethanol was purchased from Kemiou Chemical Reagent Co., Ltd. (Tianjin City, China).

Slow-cooling cryogenic dissolution procedures

In a typical slow-cooling process, at 26°C (room temperature), 2.7 g of NaOH and 3.6 g of urea were dissolved in 23.7 g of water. The solution was thoroughly mixed with 2.3 g of MCC. Then, the mixture was placed into a BCD-256KT-type refrigerator (Haier Group Co., Ltd., China). The system temperature was monitored with a calibrated thermometer with an accuracy of 0.1°C. The cooling rate was controlled by the regulator of refrigerator and insulation jackets (tin foil, plastic, and cotton) with different thickness. About 2.5 h later, when the temperature reached the value expected, the mixture was withdrawn and warmed to room temperature naturally. In an instant-cooling process, after the NaOH-urea solution was cooled to a certain temperature in less than 5 min, MCC was added to the cold solution with stirring, and the low temperature was maintained for a long time in the refrigerator. After cooling, the mixture was sampled and dispersed on glass slide at room temperature. After the sample was sealed with cover glass, the morphology of mixture and dissolution situation were observed with a DM2000 optical microscope (Leica Microsystems Co., Ltd., Germany). If the MCC granules were not completely dissolved, the residue would be observed.

Characterizations

After the cryogenic dissolution processes, the MCC solution was immersed in 20 mL of ethanol overnight for the precipitation or regeneration of cellulose in the form of thick film. After the removal of ethanol via vacuum filtration, the thick film was rinsed with water, which was then removed through vacuum filtration. The rinse step was repeated until the pH of filtrate reached 7 and electric conductivity of filtrate was lower than 1 µs/cm. After drying naturally in air, a regenerated-cellulose sample in the form of xerogel was derived for measurements.

For X-ray diffraction (XRD) analysis, powdered MCC and regenerated-cellulose xerogel film were separately placed on glass disks and scanned from 5 to 80° (2θ) at a rate of 8°/min with an X’Pert PRO X-ray diffractometer (40 kV, 40 mA; PANalytical B.V., Netherlands) equipped with a CuKα source. The DSC sample was a mixture of 0.8 mg of MCC and 3.5 mg of the NaOH-urea solution (see Section 2.2). The heat during the dissolution process was quantified by DSC from 5 to − 20°C with an STA449F3-type apparatus (NETZSCH Co., Ltd., Germany), under the conditions of different cooling rates in a N₂ ambience. The Raman spectra of MCC and regenerated cellulose in the form of xerogel (drying in air) were recorded with an RM2000-type spectrometer (532-nm wavelength, Renishaw, UK) with resolution of 1 cm^− 1. The chemical shifts of carbon atoms in the original and regenerated cellulose were determined by solid-state ¹³C-CP-MAS NMR spectroscopy (solid-state ¹³C NMR, AVANCE III, 400 M, Bruker, Switzerland).

Determination of viscosity and calculation of diffusion coefficient

First, 38.5 g of water was added to a thick-walled flask, and 3.33 g of NaOH was added to water with stirring until NaOH was completely dissolved. Then, 5.7 g of urea was added to the solution until the solvent was homogeneous and transparent. To prepare a 5 wt% cellulose solution, 2.5 g of MCC was added to the solvent. After stirring, the suspension was placed in the refrigerator. With the thick wall of flask and appropriate refrigerator conditions, the mixture was cooled to − 15°C within about 2.5 h, and the MCC was completely dissolved. The viscosity of this cellulose solution at different temperatures was determined with a rotational viscometer (Brookfield, USA). The diffusion coefficients of urea and OH⁻ in this cellulose solution were calculated according to the Wilke-Chang equation (Rossi, et al. 2015; Wilke and Chang 1955) (Eq. (1)).

$${D_{{\text{A,B}}}}{\text{=7}}{\text{.4}} \times {\text{1}}{{\text{0}}^{ - 8}} \times \frac{{\sqrt {{\psi _{\text{B}}}M{W_{\text{B}}}} T}}{{{\eta _{\text{B}}}{V_{\text{A}}}^{{0.6}}}}$$

where A and B stand for the solute and solvent, respectively; D_A,B stands for the diffusion coefficient, cm²/s. ψ_B is the association parameter reflecting solvent/solvent interactions (2.6 for water); MW_B is the molecular weight of the solvent (g/mol); T represents the temperature, K. η_B is the viscosity of solvent, mPa · s; V_A denotes the molar volume (mL/mol) of the solute. V_A of urea and OH⁻ was calculated with the group-contribution method (Li and Carr 1997a), and MW_B of the multicomponent solvent was calculated by following Eq. (2) (Li and Carr 1997b).

$$M{W_{\text{B}}}{\text{=}}\sum\limits_{{i=1}}^{4} {{\text{molecular weight of }}i \times {\text{molar fraction of }}i}$$

where i = 1, 2, 3, and 4 stand for the components cellulose, NaOH, urea, and water.

DFT calculations

The binding mode between functional groups in cellulose and urea in the aqueous alkaline system was explored by calculating the bonding possibility between urea and cellulose in water at 298.15 K. The calculations were performed with the Forcite module of the Materials Studio software. The change in Gibbs free energy (ΔG) was evaluated by calculating the total energies before and after the interaction at 298.15 K. The calculations were conducted with the DMol3 module of the Materials Studio software.

Dissolution situations in the instant- and slow-cooling processes

The mixture of MCC (10 wt%) and NaOH-urea solution (90 wt%) was separately treated with instant- and slow-cooling processes from 26 to − 20°C. The NaOH-urea solution contained 7 wt% NaOH, 12 wt% urea and 81 wt% water. As shown in Fig. 1A, after instant cooling, the mixture still contained a large quantity of white solid (no freeze and thaw), and the mixture was slightly viscous, indicating low solubility of MCC. In contrast, as shown in Fig. 1B, the slow-cooling mixture was transformed into a very viscous and transparent solution (also no freeze and thaw). This slow-cooling process took 1.5 h. The small white spots in the solution were tiny bubbles rather than MCC. The optical microscopy images showed that the instant-cooling mixture contained many MCC granules (Fig. 1C), and 10 wt% MCC also could not be dissolved in the NaOH-urea solution with a freeze-thaw procedure from 26 to − 22°C (Fig. S2), even if the freeze procedure took 1.5 h. In contrast, the slow-cooling mixture was homogeneous without granules (Fig. 1D), showing the complete dissolution of 10 wt% MCC. The rationale for the criterion of cellulose dissolution by using optical microscopy is supported by many literatures (Chen, et al. 2020; Kostag and El Seoud 2019; Rosca, et al. 2005; Xu, et al. 2019). The optical and microscopic images directly demonstrated that the solubility of MCC could be improved through the regulation of cooling rate. It was confirmed that the cooling rate or thermal history affected the solubility of MCC.

The low efficiency of instant-cooling-dissolution method was also proven at other terminal temperatures. The MCC-NaOH-urea-water mixtures were instantly cooled from 26 to − 4, −6, − 8, −10, − 12, and − 14°C, separately, and the dissolution situations are shown in Fig. 2A‒F, respectively. In the microscopic images of all these samples, the edges of un-dissolved granules were clear, and a few cracks inside MCC granules were observed in Fig. 2C and E (− 8 and − 12°C). Regardless of the terminal temperature, the MCC solubility in these instant-cooling processes could not be improved to the high level of 10 wt%. In other words, the slow-cooling process exhibited a promotion effect on the cryogenic dissolution of MCC compared to all these instant-cooling processes.

Effects of thermal history and solvent composition on the MCC solubility

In this section, the slow-cooling conditions were further optimized for higher solubility. The maximum solubility was quantified on the basis of the facts that no granule was observed in this solution by optical microscopy, and more MCC would not be dissolved in this system. The solubility increased with the decrease of terminal temperature (Fig. 3A), implying that low temperatures were favorable to the dissolution process. The solubility reached a high level of ca. 10 wt% at ≤ − 14°C (terminal temperature). More importantly, as shown in Fig. 3B, with the same terminal temperature of − 14°C, higher solubility of 11.8 wt% was realized under the cooling-rate condition of − 0.286°C/min. The average cooling rate was calculated from the overall temperature decrease divided by cooling time. It reflects the slope of cooling curve (Fig. 4), which represents the thermal history of MCC dissolution. However, with the further decrease of cooling rate, the solubility declined to a small extent. A possible reason is that a proportion of urea was decomposed in the alkaline solution during the very long dissolution period, and the decreased urea concentration was unfavorable to the dissolution of MCC.

Table 1

Cellulose solubility values reported in the literatures and this paper
Cellulose type	Solvent	Temperature (°C)	Solubility (wt%)	Reference
Flax cellulose	NaOH, urea, and water	−12	2	(Tang, et al. 2019)
Dissolving pulp	LiOH, urea, and water	−12	4	(From, et al. 2020)
Spruce cellulose	H₂SO₄ (aq)	−20	5	(Huang, et al. 2016)
Dissolving pulp	1-ethyl-3-methylimidazolium diethyl phosphate (IL)	100	5	(Zhang, et al. 2019)
Microwave-treated cellulose/urea	NaOH/ZnO (aq)	−12	5.5	(Fu, et al. 2014)
MCC	LiOH, urea, and water	−12	6	(Yang, et al. 2011a)
Cotton linter pulp	NaOH, thiourea, and water	−8	6	(Weng, et al. 2004)
Cotton linter pulp	NaOH, urea, ZnO, and water	−13	8	(Yang, et al. 2011b)
Cotton linter pulp	Cuprammonium hydroxide (aq)	Not reported	8	(Jia, et al. 2014a)
MCC	Tetra(n-butyl)ammonium acetate (IL)	60	15	(Kostag and El Seoud 2019)
MCC	1-allyl-3-methylimidazolium methyl phosphonate (IL)	80	22	(Xu, et al. 2019)
MCC	NaOH, urea, and water	−14	14	This paper

Under the conditions of preferable terminal temperature and cooling rate, the effects of NaOH and urea contents in the solvent were investigated. As shown in Fig. 3C and D, low contents of NaOH and urea led to significantly low solubility values, demonstrating that both NaOH and urea were indispensible and involved in the interactions during dissolution. Under the preferable conditions (terminal temperature = − 4.7°C; cooling rate = − 0.3°C/min; 9 wt% NaOH and 12 wt% urea), the solubility reached a maximum of 14 wt%, which is the highest cellulose solubility reported in the studies using non-ionic-liquid solvents (Table 1). In addition, the operation conditions are relatively mild compared to those reported in the literatures, and the dissolution of MCC can be easily realized in winter (minimum operation temperature = − 14°C).

Crystal structure of the regenerated cellulose

According to the literatures (Isogai and Atalla 1998; Qi, et al. 2011; Ran, et al. 2010), after the dissolution of cellulose I, cellulose II can be precipitated through solvent-adding or heating. Therefore, in the case of complete dissolution, the diffraction peaks corresponding to cellulose I should not be detected in the XRD pattern of precipitated or regenerated cellulose (Lethesh, et al. 2020). As depicted in Fig. 5, diffraction signals at 2θ of 15.1, 16.5, and 23.0° were detected in the XRD pattern of pristine MCC (left inset), indicating a typical I-type crystalline form (Wang, et al. 2022). After the dissolution of 14 wt% MCC (maximum solubility in Section 3.2), white and loose solid was precipitated with the addition of absolute ethanol. Diffraction peaks at 2θ of 12.3, 20.2, and 22.2° were observed in the pattern of white solid (Wang, et al. 2022), indicating a II-type crystalline form. The absence of signals at 15.1 and 16.5° confirmed that the precipitated or regenerated solid did not contain cellulose-I crystals. Thereby, 14 wt% MCC was completely dissolved in the solvent (9 wt% NaOH, 12 wt% urea, and 79 wt% water) under the condition of appropriate thermal history, in accordance with the result of optical microscopy (right inset in Fig. 5). Although the intrinsic viscosity of cellulose could be taken as another index to the complete dissolution of MCC (Lethesh, et al. 2020), most of the solutions prepared in the present work were too viscous to be measured with a Ubbelohde viscometer due to their high concentrations. For instance, a 7 wt% solution could not flow in the viscometer within 12 h (Fig. S3).

DSC measurement

The reason for the effect of thermal history on MCC solubility was studied by DSC, in which the exotherm or endotherm during the dissolution processes with different cooling rates was measured. As shown in Fig. 6A, with the cooling rates of − 20 to − 0.5°C/min, no exothermic or endothermic peaks were observed. In contrast, when the cooling rate was decreased to − 0.3°C/min, an obvious exothermic phenomenon was observed at circa − 4.7°C, implying that the structures of components were altered or interactions took place. Under this cooling-rate condition, this exothermic phenomenon was not observed in the NaOH-water and cellulose-NaOH-water systems (Fig. 6B). The intense exothermic peak in the profile of cellulose-water system was attributed to the freeze of water in the absence of NaOH. Therefore, urea participated in the exothermic process, which merely occurred under this slow-cooling condition. This cooling rate (− 0.3°C/min) was close to the condition adopted in the experiment with the maximum solubility of 14 wt% (Fig. 3D). The saturated mass fraction of urea in water is as high as 28.6 wt% at 0°C (vs 12 wt% urea in the aqueous phase of sample), and thereby this exothermic phenomenon should not be ascribed to the supersaturation and crystallization of urea (Samuel and Yan 2003). Considering the principle that the formation of new bonds is an exothermic process, the cryogenic environment promoted the exothermic dissolution of cellulose through the generation of new bonds. Furthermore, according to the maximum solubility (14 wt%) and quantity of dissolved cellulose (13.2 wt%) in the sample, the heat was calculated to be 31.9 ± 1.6 J/g MCC based on the integral area of the exothermic peak. The exothermic phenomenon during the dissolution of cellulose indicated that the cryogenic environment was favorable to the interaction or reaction between the solvent and cellulose from the thermodynamic point of view. The efficient cryogenic dissolution of MCC (Fig. 1) can be explained from this point of view. In contrast, other well-known dissolution processes of substances (e. g. sucrose dissolved in water) are promoted at higher temperatures under most circumstances.

Reason for the high cellulose solubility under the slow-cooling condition

After the verification of the promotion effect of cryogenic environment on the dissolution of cellulose by DSC, the indispensability of slow cooling (Fig. 1) was explored from the aspect of diffusion and mass transfer of urea and NaOH. As shown in Fig. 7A, the viscosity of 5 wt% cellulose solution was evidently increased with the decrease of temperature. The fit result showed that the data points were consistent with an exponential relationship (the inset of Fig. 7A), which usually implies a drastic variation trend. The viscosity was increased from 0.4 (20°C) to 2.25 Pa · s (− 15°C). For comparison, the viscosity of viscous glycerol is 1.5 Pa · s (20°C). Under the instant-cooling conditions, the outer shells of MCC granules could be dissolved due to the thermodynamic factor. However, the viscosity of dissolved cellulose in the shell would be instantly increased at low temperatures in the instant-cooling process, and a viscous barrier would be generated over the granules. The diffusion of urea and NaOH into the cores of granules would be impeded by this barrier. In contrast, the viscosity of dissolved cellulose was slowly decreased in the slow-cooling process, so the urea and NaOH in the bulk could diffuse into the granules for complete dissolution of cellulose at higher temperatures. Meanwhile, because the high-temperature condition continued for a long time in the slow-cooling process, the dissolved cellulose with relatively low viscosity could diffuse into the bulk due to the low viscosity and high diffusion rate.

The diffusion coefficients of urea and OH⁻ in this solution were calculated according to the Wilke-Chang equation (Rossi, et al. 2015; Wilke and Chang 1955). As depicted in Fig. 7B, from 20 to − 15°C, both diffusion coefficients were decreased by an order of magnitude. The diffusion rates of urea and OH⁻ would be decreased proportionally under the same concentration-gradient condition according to the Fick’s law (Qin, et al. 2022). In the instant-cooling process, the duration of high-temperature dissolution was very short, so the high diffusion rates of urea and OH⁻ merely took place in this short period. In contrast, the low viscosity and high diffusion rate could be maintained for a long time in the slow-cooling process, and thereby urea and OH⁻ could smoothly diffuse into the cores of MCC granules for dissolution. Meanwhile, the dissolved cellulose could diffuse into the solution, so the viscous barrier that prohibited the diffusion of urea and OH⁻ disappeared. In summary, due to the low viscosity and weak barrier effect of dissolved cellulose in the slow-cooling procedure, the efficient diffusion of solvent components ensured the smooth solvent-cellulose interaction and complete dissolution of MCC granules. Therefore, the cooling rate or thermal history does have an impact on the solubility of cellulose by controlling the diffusion rates of solvent and dissolved cellulose, and the speculation of this paper (thermal history affects the solubility) was confirmed.

Hydrogen bonds determined by Raman spectroscopy

For the understanding of the bonds and functional groups during the dissolution process, the cellulose dissolved in the 14 wt% solution was precipitated with ethanol, and the regenerated dry cellulose powders were measured by Raman spectroscopy. Figure 8 shows that the pristine MCC contained a large number of inter- and intra-chain hydrogen bonds corresponding to the bands at 1100 and 1120 cm^− 1, respectively (Morssli, et al. 1991; Schenzel, et al. 2005). After the dissolution and regeneration procedures in the NaOH solution without urea (green profile), the intramolecularly and intermolecularly hydrogen bonds were shifted to a low-frequency zone by 15 cm^− 1, approximately. More importantly, most of the intramolecularly hydrogen bonds were cracked, while a small portion of the intermolecularly hydrogen bonds were cracked. In contrast, both hydrogen-bond-related bands almost disappeared in the spectrum of cellulose dissolved in the solution of urea and NaOH (red profile). Before the Raman measurement, all the samples were rinsed with water thoroughly, and the intense bands corresponding to urea at 1525 and 1600 cm^− 1 indicate that the urea molecules strongly interacted with the functional groups on the surface of regenerated cellulose (Mafy, et al. 2015; Shen, et al. 2020). Due to the strong interactions, the locations of bands were different from the values of pure urea (1525 vs 1468 cm^− 1, 1600 vs 1622 cm^− 1). Noticeably, the overlapping bands at 1525 and 1600 cm^− 1 may be assigned to carbamate ions (possible decomposition of urea in the alkaline solution) according to the literature (Foo, et al. 2016), but the absence of bands at 1700–1770 cm^− 1 excluded the possibility of carbamate existing on the regenerated cellulose (Ekin and Webster 2006; Lachenmeier 2005; Yadav, et al. 2021). These results indicate that the intramolecularly and intermolecularly hydrogen bonds in cellulose were cleavaged during its dissolution in the NaOH-urea-water system, and new bonds were formed between urea and cellulose. The crack of bonds is an endothermic process while the formation of bonds is exothermic. In the light of the obvious exothermic peak detected by DSC, it is rational to believe that new and stable bonds were generated between MCC, NaOH, and urea after the crack of original hydrogen bonds. And, the apparent exothermic phenomenon was observed by DSC (Fig. 6).

Cleavage and generation of hydrogen bonds

Solid-state ¹³C NMR spectroscopy is a powerful tool to study the properties of biomass carbon atoms (Amaral, et al. 2019). The numbering of anhydroglucose unit (AGU) in cellulose is based on a common law (Amaral, et al. 2019). As illustrated in Fig. 9, the resonance of C₄ and C₆ atoms, especially the latter, was shifted upfield. Kamide et al. reported that inter-molecule hydrogen bonds were generated at the hydroxyl groups neighboring C₃ and C₆ atoms in cellulose-I crystals, i. e. MCC (Kamide, et al. 1992). The Raman measurement result (Fig. 9) showed that the inter-molecule hydrogen bonds in MCC were cracked in the presence of urea, so the C₃ and C₆ atoms would be affected by the dissolution-regeneration process. The resonance of C₂, C₃, and C₅ overlapped, so it is impossible to clearly analyze the change in C₃ atom. Instead, the C₄ atom was in the vicinity of C₃ atom, and the shifted resonance (upfield) of C₄ and C₆ atoms could reflect the shielding effect of urea on the resonance of C₄ and C₆ atoms by forming hydrogen bonds with hydroxyl groups (Liu, et al. 2012; Radula-Janik, et al. 2013). In this section, the crack of original inter-molecule hydrogen bonds and formation of new hydrogen bonds are understood.

Status of elements on the surface of the regenerated cellulose

Figure 10A shows the XPS spectrum of regenerated cellulose. Only C, O, and N elements were detected, and the ratio of N on the surface was very low. The N 1s spectrum (Fig. 10B) shows that the binding energy was increased by 0.2 eV, which is attributed to the bonding between urea and cellulose (Raman result in Section 3.6). These new bonds are probably hydrogen bonds, because urea is well recognized as a strong hydrogen donor (Cai, et al. 2012; Li, et al. 2018b; Paul and Paul 2015; Singh, et al. 2011). The peak at 287.7 eV in the C 1s spectrum (Fig. 10C) is assigned to the C = O groups in the urea molecules binding with regenerated cellulose. Urea might decompose into carbamate in the alkaline solutions, so it is necessary to analyze the possibility of carbamate binding with regenerated cellulose. As depicted in Fig. 10C, the peak at 288.8 eV corresponds to the carboxyl group in pure ammonium carbamate. Its intensity is much lower than the peak at 287.7 eV in the spectrum of regenerated cellulose. Therefore, the peak at 287.7 eV should not be assigned to the carboxyl group in carbamate. In addition, the Raman result also denied the existence of carbamate on the surface of regenerated cellulose (Section 3.6). The C 1s binding energy (287.7 eV) of the urea over regenerated cellulose is evidently smaller than that (289 eV) of C = O in pure urea, indicating that the paired electrons in the oxygen atoms of cellulose were transferred to the proton and adjacent carbonyl groups in urea (Stevens, et al. 2020). This shift is so obvious (1.3 eV) that it can only be ascribed to the formation of chemical or hydrogen bonds. The peak at 286.4 eV is assigned to C–OH and C–O–C groups, which are the intrinsic moieties of cellulose (Titirici, et al. 2007). The XPS analysis result proved the strong hydrogen-bonding interaction between urea and regenerated cellulose.

DFT calculation and plausible dissolution mechanism

According to the results of Raman, XPS, and solid-state ¹³C NMR, the NH₂ group in urea interacts with the O-containing functional groups in cellulose via hydrogen bonds during the dissolution process. The binding sites on the cellulose chain were theoretically probed by DFT calculations. The calculation and simulation results are illustrated in Fig. 11. The intramolecularly and intermolecularly hydrogen bonds in MCC are cracked by NaOH and urea, respectively, in the cryogenic dissolution process (Fig. 11A). After the dispersion of cellulose chains, urea remains on the chain. The poor dissolution effect of NaOH solution and almost intact inter-molecule hydrogen bonds of NaOH-dissolved cellulose (Fig. 8) indicate that the crack of inter-molecule hydrogen bonds by urea is the primary reason for the high solubility of MCC such as 14 wt%. Therefore, we focused on the interaction or binding between cellulose and urea in the DFT calculations, of which the results are illustrated in Fig. 11B. In the cryogenic dissolution process, a stable individual chain composed of urea and cellulose is generated, and urea and cellulose are linked by two hydrogen bonds. The change in Gibbs free energy (ΔG) was calculated to be − 39.2 kJ/mol urea. On the other hand, the Raman analysis result (Fig. 8) proved that most of the intramolecularly hydrogen bonds were cracked in the NaOH-water system, while the majority of intermolecularly hydrogen bonds were sound. The DSC measurement result showed that the slow-cooling process of MCC in the NaOH-water system did not exhibit an obvious exothermic effect (Fig. 6B), indicating that the thermal effect of crack of intramolecularly hydrogen bonds was too weak to be determined. Therefore, the crack of inter-molecule hydrogen bonds by urea was taken as the dominant exothermic contributor, and the isobaric heat measured by DSC (Fig. 6), − 10.4 kJ/mol urea, was taken as the enthalpy change (ΔH) during the interaction between urea and MCC. ΔH accounts for 26.5% of ΔG, implying that the heat during the urea-MCC interaction contributed to the spontaneity of MCC dissolution to a small extent. And, because TΔS accounts for 73.5% of ΔG, the dominant contributor is the increase of entropy (ΔS, chaos degree), which should be attributed to the dispersion and random distribution of those individual cellulose chains.

The solubility of MCC in the NaOH-urea-water system was greatly improved from 5 to 14 wt%, under the condition of an appropriate thermal history with low cooling rates. This important factor of thermal history has been neglected in previous studies. The complete dissolution of MCC was confirmed by optical microscopy and XRD. This MCC dissolution technique has decided advantages of high solubility, mild operation conditions, low cost and low energy consumption. In the slow-cooling dissolution process, a moderate exotherm was detected, showing that a low temperature was indispensable for the dissolution of MCC from a thermodynamic perspective. In contrast, the instant cooling of the system led to the instant increase of local viscosity and low diffusion rates of solvent and dissolved cellulose, resulting in the low MCC solubility. Therefore, the hypothesis that cooling rate or thermal history determines the solubility of MCC in the NaOH-urea-water system has been verified. The Raman, XPS, and NMR measurement results indicate that the intramolecularly and intermolecularly hydrogen bonds in MCC were attacked by NaOH and urea, respectively, and urea bound with regenerated cellulose in a dual-hydrogen-bond manner. The crack of inter-molecule hydrogen bonds by urea determined the efficient dissolution of MCC. The cryogenic dissolution of MCC in this NaOH-urea-water system was a spontaneous course with ΔG of − 39.2 kJ/mol urea. The exotherm promoted the dissolution process by decreasing the Gibbs free energy, but the increase of entropy due to the dispersion and chaotic distribution of dissolved cellulose chains contributed to most of the spontaneity. This work sheds light on the molecular-level reason for the cryogenic dissolution of MCC and interactions between solvent components and cellulose, especially under the slow-cooling condition.

Supporting Information

The Supporting Information is available free of charge on the Cellulose website.

Determination of viscosity-average polymerization degree of MCC; Dissolution situation of 10 wt% MCC in a solvent containing 7 wt% NaOH, 12 wt% urea and 81 wt% water in a freeze-thaw process from room temperature to −22 °C; Viscous cellulose solution (7 wt%) in a Ubbelohde viscometer.

Acknowledgments

The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 22268011 and 31960492).

Author contributions

Conceptualization, writing - original draft, and wring- review & editing were performed by SA. Data curation was performed by ZH. Formal analysis was performed by WY and CH. All the authors read and approved the final manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (Grant No. 22268011 and 31960492).

Data availability

The corresponding author will provide the datasets generated for this study on request.

Conflict of interest

The authors have no relevant financial or non-fnancial interests to disclose.

Ethical approval

We declare that there is no financial or personal relationship between us and other people or organizations that may have an inappropriate impact on this work. Consent has been taken from all the authors for participation in this study.

Consent for publication

Consent has been taken from all the authors for publication of this manuscript.

Amaral HR, Cipriano DF, Santos MS, Schettino MA, Ferreti JVT, Meirelles CS, Pereira VS, Cunha AG, Emmerich FG, Freitas JCC (2019) Production of high-purity cellulose, cellulose acetate and cellulose-silica composite from babassu coconut shells. Carbohyd Polym 210:127‒134
Cai J, Kimura S, Wada M, Kuga S, Zhang L (2010) Cellulose aerogels from aqueous alkali hydroxide-urea solution. ChemSusChem 1:149‒154
Cai J, Liu Y, Zhang L (2006) Dilute solution properties of cellulose in LiOH/urea aqueous system. Journal of Polymer Science Part B: Polymer Physics 44:3093‒3101
Cai L, Liu Y, Liang H (2012) Impact of hydrogen bonding on inclusion layer of urea to cellulose: study of molecular dynamics simulation. Polymer 53:1124‒1130
Chang C, Zhang L (2011) Cellulose-based hydrogels: present status and application prospects. Carbohyd Polym 84:40‒53
Chen K, Xu W, Ding Y, Xue P, Sheng P, Qiao H, He J (2020) Hemp-based all-cellulose composites through ionic liquid promoted controllable dissolution and structural control. Carbohyd Polym 235:116027
Dong Y, Jia B, Fu F, Zhang H, Zhang L, Zhou J (2016) Fabrication of hollow materials by fast pyrolysis of cellulose composite fibers with heterogeneous structures. Angew. Chem. Int Ed 128:13702‒13706
Ekin A, Webster DC (2006) Synthesis and characterization of novel hydroxyalkyl carbamate and dihydroxyalkyl carbamate terminated poly(dimethylsiloxane) oligomers and their block copolymers with poly(ε-caprolactone). Macromolecules 39:8659‒8668
Foo GS, Lee JJ, Chen CH, Hayes SE, Sievers C, Jones CW (2016) Elucidation of surface species through in situ FTIR spectroscopy of carbon dioxide adsorption on amine-grafted SBA-15. ChemSusChem 10:266‒276
From M, Larsson PT, Andreasson B, Medronho B, Svanedal I, Edlund H, Norgren M (2020) Tuning the properties of regenerated cellulose: effects of polarity and water solubility of the coagulation medium. Carbohyd Polym 236:116068
Fu F, Guo Y, Wang Y, Tan Q, Zhou J, Zhang L (2014) Structure and properties of the regenerated cellulose membranes prepared from cellulose carbamate in NaOH/ZnO aqueous solution. Cellulose 21:2819‒2830
He M, Duan B, Xu D, Zhang L (2015) Moisture and solvent responsive cellulose/SiO₂ nanocomposite materials. Cellulose 22:553‒563
Huang W, Wang Y, Zhang L, Chen L (2016) Rapid dissolution of spruce cellulose in H₂SO₄ aqueous solution at low temperature. Cellulose 23:3463‒3473
Isogai A, Atalla RH (1998) Dissolution of cellulose in aqueous NaOH solutions. Cellulose 5:309‒319
Jarmelo S, Maria TMR, Leitão MLP, Rui F (2000) Structural and vibrational characterization of methyl glycolate in the low temperature crystalline and glassy states. Phys Chem Chem Phys 2:1155‒1163
Jia B, Dong Y, Zhou J, Zhang L (2014a) Constructing flexible cellulose–Cu nanocomposite film through in situ coating with highly single-side conductive performance. J Mater Chem C 2:524‒529
Jia B, Mei Y, Cheng L, Zhou J, Zhang L (2012) Preparation of copper nanoparticles coated cellulose films with antibacterial properties through one-step reduction. ACS Appl Mater Interfaces 4:2897‒2902
Jia B, Yu L, Fu F, Li L, Zhang L (2014b) Preparation of helical fibers from cellulose-cuprammonium solution based on liquid rope coiling. RSC Adv 4:9112‒9117
Kamide K, Okajima K, Kowsaka K (1992) Dissolution of natural cellulose into aqueous alkali solution: role of super-molecular structure of cellulose. Polymer Journal 24:71‒86
Kostag M, El Seoud OA (2019) Dependence of cellulose dissolution in quaternary ammonium-based ionic liquids/DMSO on the molecular structure of the electrolyte. Carbohyd Polym 205:524–532
Lachenmeier DW (2005) Rapid screening for ethyl carbamate in stone-fruit spirits using FTIR spectroscopy and chemometrics. Anal Bioanal Chem 382:1407‒1412
Lethesh KC, Evjen S, Venkatraman V, Shah SN, Fiksdahl A (2020) Highly efficient cellulose dissolution by alkaline ionic liquids. Carbohyd Polym 229:DOI: 10.1016/j.carbpol.2019.115594
Li H, Kang Z, Zhang X, Cao Q, Jin LE (2018a) Contributions of ultrasonic wave, metal ions, and oxidation on the depolymerization of cellulose and its kinetics. Renew Energy 126:699‒707
Li J, Carr PW (1997a) Accuracy of empirical correlations for estimating diffusion coefficients in aqueous organic mixtures. Analytical Chemistry 69:2530‒2536
Li J, Carr PW (1997b) Estimating diffusion coefficients for alkylbenzenes and alkylphenones in aqueous mixtures with acetonitrile and methanol. Analytical Chemistry 69:2550‒2553
Li J, Lu S, Liu F, Qiao Q, Na H, Zhu J (2021) Structure and properties of regenerated cellulose fibers based on dissolution of cellulose in a CO₂ switchable solvent. ACS Sustainable Chem Eng 9:4744‒4754
Li Q, Sun J, Zhuang L, Xu X, Sun Y, Wang G (2018b) Effect of urea addition on chitosan dissolution with [Emim]Ac-urea solution system. Carbohyd Polym 195:288‒297
Li R, He M, Li T, Zhang L (2015) Preparation and properties of cellulose/silver nanocomposite fibers. Carbohyd Polym 115:269‒275
Li R, Zhang L, Xu M (2012) Novel regenerated cellulose films prepared by coagulating with water: structure and properties. Carbohyd Polym 87:95‒100
Liang S, Wu J, Tian H, Zhang L, Xu J (2008) High-strength cellulose/poly(ethylene glycol) gels. ChemSusChem 1:558‒563
Liu S, Jian Z, Tao D, Zhang L (2010) Microfiltration performance of regenerated cellulose membrane prepared at low temperature for wastewater treatment. Cellulose 17:1159‒1169
Liu X, Pan X, Shen W, Ren P, Han X, Bao X (2012) NMR study of preferential endohedral adsorption of methanol in multiwalled carbon nanotubes. J. Phys. Chem. C 116:7803‒7809
Mafy NN, Afrin T, Rahman MM, Mollah MYA, Susan MABH (2015) Effect of temperature perturbation on hydrogen bonding in aqueous solutions of different urea concentrations. RSC Adv 5:59263‒59272
Morssli M, Cassanas G, Bardet L, Pauvert B, Terol A (1991) Vibrational analysis of sodium α-, β- and γ-hydroxybutyrates. inter- and intramolecular hydrogen bonds. Spectrochim Acta Part A 47:529‒541
Parveen F, Gupta K, Upadhyayula S (2017) Synergistic effect of chloro and sulphonic acid groups on the hydrolysis of microcrystalline cellulose under benign conditions. Carbohyd Polym 159:146‒151
Paul S, Paul S (2015) Exploring the counteracting mechanism of trehalose on urea conferred protein denaturation: a molecular dynamics simulation study. J Phys Chem B 119:9820‒9834
Qi H, Yang Q, Zhang L, Heinze T (2011) The dissolution of cellulose in NaOH-based aqueous system by two-step process. Cellulose 18:237‒245
Qin G, Zhang R, Yang C, Lv X, Pei K, Yang L, Liu X, Zhang X, Che R (2022) Magnetic-field-assisted diffusion motion of magnetic skyrmions. ACS Nano 16:15927‒15934
Quanling Y, Hayaka F, Tsuguyuki S, Akira I, Lina Z (2011) Transparent cellulose films with high gas barrier properties fabricated from aqueous alkali/urea solutions. Biomacromolecules 12:2766‒2771
Radula-Janik K, Kupka T, Ejsmont K, Daszkiewicz Z, Sauer SPA (2013) Halogen effect on structure and ¹³C NMR chemical shift of 3,6-disubstituted-N-alkyl carbazoles. Magnetic Resonance in Chemistry 51:630‒635
Ran L, Chang C, Zhou J, Zhang L, Kuga S (2010) Primarily industrialized trial of novel fibers spun from cellulose dope in NaOH/urea aqueous solution. Ind Eng Chem Res 49:11380‒11384
Rosca ID, Watari F, Uo M, Akasaka T (2005) Oxidation of multiwalled carbon nanotubes by nitric acid. Carbon 43:3124‒3131
Rossi F, Cucciniello R, Intiso A, Proto A, Motta O, Marchettini N (2015) Determination of the trichloroethylene diffusion coefficient in water. AIChE J 61:3511‒3515
Samuel YH, Yan H, Handbook of Aqueous Solubility Data. CRC Press: 2003.
Schenzel K, Fischer S, Brendler E (2005) New method for determining the degree of cellulose I crystallinity by means of FT Raman spectroscopy. Cellulose 12:223‒231
Shen Y, Lin H, Gao W, Li M (2020) The effects of humic acid urea and polyaspartic acid urea on reducing nitrogen loss compared with urea. Journal of the Science of Food and Agriculture 100:4425‒4432
Singh K, Bharose R, Singh VK, Verma SK (2011) Sugar decolorization through selective adsorption onto functionalized accurel hydrophobic polymeric support. Ind Eng Chem Res 50:10074‒10082
Stevens JS, Coultas S, Jaye C, Fischer DA, Schroeder SLM (2020) Core level spectroscopies locate hydrogen in the proton transfer pathway – identifying quasi-symmetrical hydrogen bonds in the solid state. Phys Chem Chem Phys 22:4916‒4923
Sun B, Peng G, Lian D, Xu A, Li X (2015) Pretreatment by NaOH swelling and then HCl regeneration to enhance the acid hydrolysis of cellulose to glucose. Bioresour Technol 196:454‒458
Tang L, Feng Y, He W, Yang F (2019) Combination of graphene oxide with flax-derived cellulose dissolved in NaOH/urea medium to generate hierarchically structured composite carbon aerogels. Ind Crops Prod 130:179‒183
Tian S, Jiang J, Zhu P (2022) Fabrication of a transparent and biodegradable cellulose film from Kraft pulp via cold alkaline swelling and mechanical blending. ACS Sustainable Chem Eng 10:10560‒10569
Titirici MM, Thomas A, Yu S-H, Müller J-O, Antonietti M (2007) A direct synthesis of mesoporous carbons with bicontinuous pore morphology from crude plant material by hydrothermal carbonization. Chemistry of Materials 19:4205‒4212
Wang C, Xia S, Yang X, Zheng A, Zhao Z, Li H (2021) Oriented valorization of cellulose and xylan into anhydrosugars by using low-temperature pyrolysis. Fuel 291:120156
Wang H, Kataoka H, Tsuchikawa S, Inagaki T (2022) Terahertz time-domain spectroscopy as a novel tool for crystallographic analysis in cellulose: cellulose I to cellulose II, tracing the structural changes under chemical treatment. Cellulose 29:3143‒3151
Wang S, Lu A, Zhang L (2016) Recent advances in regenerated cellulose materials. Progress in Polymer Science 53:169‒206
Weng L, Zhang L, Ruan D, Shi L, Xu J (2004) Thermal gelation of cellulose in a NaOH/thiourea aqueous solution. Langmuir 20:2086‒2093
Wilke CR, Chang P (1955) Correlation of diffusion coefficients in dilute solutions. AIChE J 1:264‒270
Xu A, Wang J, Wang H (2010) Effects of anionic structure and lithium salts addition on the dissolution of cellulose in 1-butyl-3-methylimidazolium-based ionic liquid solvent systems. Green Chem 12:268‒275
Xu K, Xiao Y, Cao Y, Peng S, Fan M, Wang K (2019) Dissolution of cellulose in 1-allyl-3-methylimidazolium methyl phosphonate ionic liquid and its composite system with Na₂PHO₃. Carbohyd Polym 209:382–388
Yadav T, Brahmachari G, Karmakar I, Yadav P, Prasad AK, Pathak A, Agarwal A, Kumar R, Mukherjee V, Pandey GN, Bento RRF, Yadav NP (2021) Conformational and vibrational spectroscopic investigation of N-n‑butyl, S-2-nitro-1-(p-tolyl)ethyl dithiocarbamate – a bio-relevant sulfur molecule. Journal of Molecular Structure 1238:130450
Yang Q, Fukuzumi H, Saito T, Isogai A, Zhang L (2011a) Transparent cellulose films with high gas barrier properties fabricated from aqueous alkali/urea solutions. Biomacromolecules 12:2766‒2771
Yang Q, Qi H, Lue A, Hu K, Cheng G, Zhang L (2011b) Role of sodium zincate on cellulose dissolution in NaOH/urea aqueous solution at low temperature. Carbohyd Polym 83:1185‒1191
Zhang J, Kitayama H, Gotoh Y, Potthast A, Rosenau T (2019) Non-woven fabrics of fine regenerated cellulose fibers prepared from ionic-liquid solution via wet type solution blow spinning. Carbohyd Polym 226:115258
Zhao H, Li C-F, Yu X, Zhong N, Hu Z-Y, Li Y, Larter S, Kibria MG, Hu J (2022) Mechanistic understanding of cellulose β-1,4-glycosidic cleavage via photocatalysis. Appl. Catal., B 302:120872

No competing interests reported.

Download PDF

Version 1

posted

You are reading this latest preprint version

High solubility of cellulose in slow-cooling alkaline systems and interacting modes of alkali and urea at the molecular level

Status:

Version 1

Abstract

Figures

Introduction

Materials and methods

Results and Discussion

Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1