Investigation into the reaction mechanism underlying the atmospheric low-temperature plasma-induced oxidation of cellulose

Journal Pre-proof Investigation into the reaction mechanism underlying the atmospheric low-temperature plasma-induced oxidation of cellulose Yizhong Cao, Haiming Hua, Pei Yang, Minzhi Chen, Weimin Chen, Siqun Wang, Xiaoyan Zhou

PII:

S0144-8617(19)31300-1

DOI:

https://doi.org/10.1016/j.carbpol.2019.115632

Reference:

CARP 115632

To appear in:

Carbohydrate Polymers

Received Date:

1 September 2019

Revised Date:

1 November 2019

Accepted Date:

16 November 2019

Please cite this article as: Cao Y, Hua H, Yang P, Chen M, Chen W, Wang S, Zhou X, Investigation into the reaction mechanism underlying the atmospheric low-temperature plasma-induced oxidation of cellulose, Carbohydrate Polymers (2019), doi: https://doi.org/10.1016/j.carbpol.2019.115632

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Investigation into the reaction mechanism underlying the atmospheric low-temperature plasma-induced oxidation of cellulose Yizhong Cao a, b, c, Haiming Hua d, Pei Yang a, b, Minzhi Chen a, b, Weimin Chen a, b, Siqun Wang c, Xiaoyan Zhou a, b, e* a

College of Materials Science and Engineering, Nanjing Forestry University, Nanjing, 210037, China

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Fast-growing Tree & Agro-fibre Materials Engineering Center, Nanjing, 210037, China

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Center for Renewable Carbon, University of Tennessee, Knoxville, TN, 37996, USA

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College of Science, Nanjing Forestry University, Nanjing, 210037, China

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Dehua TB New Decoration Material Co., Ltd., Deqing 313200, China

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Corresponding author: E-mail addresses: [email protected] (X.Y Zhou) Tel (Fax): (025)-85428506

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Highlights:

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Graphical abstract

Plasma possesses energetic particles and strong capacity to deconstruct cellulose

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Plasma-induced cleavage of β-glucosidic bond and hydrogen bond are

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pronounced

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Cleavage of C4-O covalent bond is the first-step reaction

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Subsequent pyranose ring-breaking reaction is dominant generating carbonyl

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Carboxyl can be generated via an endothermic oxidation reaction 1

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Abstract: Atmospheric low-temperature plasma has been widely applied in surface modification of lignocellulose for manufacturing lightweight, strong composites. This study is aimed at elaborating the structural changes of cellulose after plasma treatment and further understanding the mechanism underlying plasma-induced oxidation of cellulose. Experiments suggested that atmospheric low-temperature plasma exhibits strong capacity to cleave covalent bonds, leading to oxidation and degradation of cellulose. Theoretical analysis revealed that cleavage of C4-O covalent bond is the first-step reaction during plasma-induced oxidation due to its low bond dissociation energy (229.2 kJ·mol-1). Subsequent pyranose ring-breaking reaction dominates dynamically and thermodynamically. Obtained outcomes are vital for fundamentally understanding the plasma-lignocellulose interaction. On that basis, plasma treatment for activation and oxidation of lignocellulose can be optimized and designed for improved efficiency. Wettability of lignocellulose can be thus improved in a short time, providing an opportunity to manufacture lignocellulose-based composites with enhanced efficiency and mechanical properties in future. Graphic abstract: Key words: Reaction pathway, Cellulose, Oxidation, Atmospheric low-temperature plasma, Density functional theory (DFT)

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1. Introduction

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Natural lignocellulosic materials and further-derived cellulose nanomaterials have attracted significant interests and they have been successfully applied into manufacture of lightweight, strong composites (Kusano, Madsen, Berglund, & Oksman, 2019, Scalici, Fiore, & Valenza, 2016). A commonly supported concept is that mechanical performance of lignocellulose composites highly depends on interfacial adhesion of lignocellulose and polymer matrix (Nakagaito & Yano, 2005). Various surface modification technologies, including surface sanding (Hiziroglu, Zhong, & Tan, 2013), steam explosion treatment (Han, Deng, Zhang, Paul, & Wu, 2010), and acid or alkali treatment (Hafezi, Enayati, Hosseini, Tarmian, & Mirshokraii, 2016, Han, Umemura, Kawai, & Kajita, 1999), have been successfully applied for achieving fine interfacial adhesion and subsequently fine mechanical properties of lignocellulose composites. The aforementioned pretreatments can be divided into physical and chemical pretreatments. Some of these methods are utilized in combination to achieve the finest modification effects. However, as shown in Tab.S1 (Supplementary material), these technologies usually involve physical or chemical processes, which are energy- and time-consuming and can easily cause environmental pollution (Balu, Breedveld, & Hess, 2008, Inbakumar et al, 2010). Thereby, interest in sustainable and green surface modification technologies has dramatically emerged in recent decades. Recent studies have pinpointed that atmospheric low-temperature plasma treatment exhibits evident advantages against aforementioned physical or chemical pretreatments (Vanneste, Ennaert, Vanhulsel, & Sels, 2016). A characteristic feature of plasma treatment is that it can precisely deliver energy or energetic particles to the target surface (Liu et al, 2013, Szili, Bradley, & Short, 2014, Szili, Oh, Hong, Hatta, & Short, 2015), thus dramatically improving the treatment efficiency. Plasma treatment can improve surface polarity of material in a short time. As summarized in Tab.S1, plasma treatment has substantial advantages with regard to time- and energy-saving compared to conventional surface modification methods of lignocellulose. Moreover, the 2

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physiochemical reactions during plasma treatment are solvent-free and the whole modification procedure does not generate pollution (Scalici et al., 2016, Vanneste et al., 2016). Plasma treatment requires no additional chemicals, thus reducing the cost. When using electricity from renewable energy sources (solar and wind, et al), plasma treatment is more promising and interesting to be considered in future (Graves, Ebbesen, Mogensen, & Lackner, 2011). Another important feature of plasma treatment is that it only affects the outmost layer of the materials and leaves the inner layer of the materials unaffected, which makes plasma treatment ideally suitable for modification of material surfaces (Acda, Devera, Cabangon, & Ramos, 2012). Apart from the aforementioned advantages, atmospheric plasma treatment is known as an easily-controlled and flexible technology. By changing the feeding gas (i.e., O2, N2, air, NH3, et al), different active species can be produced and surface properties of the materials can be changed in a corresponding manner (Busnel, Blanchard, & Pregent, 2010, Poaty, Riedl, Blanchet, Blanchard, & Stafford, 2013). At present, atmospheric dielectric barrier discharge (DBD) plasma is used more frequently due to its no need of vacuum systems and subsequent excellent maneuverability and efficiency. Recently, we applied atmospheric DBD plasma to improve the wettability of poplar veneer (Chen, M. et al, 2016). Recent publication also demonstrated that interfacial adhesion of wheat straw with phenol-formaldehyde (PF) resin can be enhanced by atmospheric DBD plasma (Chen, W.M. et al, 2016). Plasma-induced oxidation on the surface of lignocellulose is reported to be the main-observed phenomenon after plasma treatment (Inbakumar et al., 2010, Král et al, 2015, Kröpke, Akishev, & Holländer, 2001). By using oxygen-containing feeding gas (i.e., O2, and air et al), oxygen-containing functional groups, including carbonyl and carboxyl, can be grafted on plasma-treated lignocellulosic materials.(Chen, M. et al., 2016, Chen, W.M. et al., 2016, Tang, Zhang, Wang, Yang, & Zhou, 2015) These functional groups are polar due to the electronegativity of oxygen, thus increasing the polarity of material surface. Improvement of wettability of lignocellulose can be attributed to the elevated polarity. Another important phenomenon is that plasma affects the topography of materials surface, which elevates the superficial area of materials and thus improves the wettability and interfacial adhesion (Jamali & Evans, 2011). As discussed previously, plasma treatment exhibits evident advantages against chemical and physical pretreatments in aspect of energy/time-saving and environmental friendly. In recent decades, plasma treatment is widely adopted for surface modification of varies lignocellulose, including wood, straw, plant fibers and cellulose nanomaterials, et al, to manufacture lignocellulosic composites with fine mechanical performance (Acda et al., 2012, Balu et al., 2008, Busnel et al., 2010, Chen, W.M. et al., 2016, Kusano et al., 2019, Scalici et al., 2016). Plasma treatment is also adopted for improving bio-refinery as it can break down the physical obstacle in lignocellulose and reduce degree of polymerization (Jonsson & Martin, 2016, Klarhöfer, Viöl, & Mausfriedrichs, 2010). On that basis, accessibility of carbohydrate polymers can be improved, thus leading to an improved bio-refinery. The application of plasma treatment on lignocellulose is widening, while there is still lacking in the fundamentally discussion about the changes in molecular structures and the underlying mechanism. Fundamentally understanding the oxidation mechanism of lignocellulose during plasma treatment is desired and critical for developing and popularizing plasma treatment. Improved insights into the plasma-lignocellulose interaction can promote designing more efficient plasma treatment. Moreover, elaborating the chemical mechanism during plasma modifying lignocellulose is beneficial for tuning the chemical properties on lignocellulose surface and thus for achieving the optimum modification effects. 3

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Utilizing electron spin resonance (ESR) analysis, Chen et al (Chen, M. et al., 2016) and Hardy et al (Hardy, Levasseur, Vlad, Stafford, & Riedl, 2015) revealed the elevated concentration of free radicals on the wood surface after plasma treatment. This finding suggested the presence of free radical reactions during plasma treatment. Previous publication also reported an electron spectroscopy investigation which clarified the loss of the hydroxyl group of cellulose after plasma treatment in synthetic air (Klarhöfer et al., 2010). Although aforementioned literatures proposed improved understanding into mechanism of oxidation of cellulose in plasma, the reaction pathway of plasma-induced oxidation of lignocellulose remains unclarified. Herein, we report an investigation of the plasma-induced oxidation of cellulose macromolecules via experimental and theoretical analysis. The basic hypothesis of this work is that plasma-induced oxidation of cellulose involves oxygen-based radical reactions and it will affect molecular structure of cellulose. The main objective of this work is to investigate the structural changes of cellulose after plasma treatment and to clarify reaction mechanism of plasma-induced oxidation of cellulose molecules. Understanding the reaction pathway of plasma-induced oxidation of cellulose is beneficial for elucidation of the entire reaction mechanism of plasma treatment of lignocellulose surface.

2. Materials and Methods

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2.1 Chemicals Cellulose derived from cotton (Sigma S3504) was purchased from Sigma-Aldrich. The surface characterization of cotton is provided in the Supplementary materials. No further purification was performed before plasma treatment. Acetic acid, acetic anhydride, sulfuric acid (97 %), and tetrahydrofuran (THF) were purchased from Sigma-Aldrich. Acetone-d6 was purchased from Cambridge Isotope Laboratories, Inc. 2.2 Plasma treatment Plasma treatment was performed by atmospheric DBD plasma, which is manufactured by Suman plasma technology Co. Ltd (Nanjing, China). The schematic diagram of plasma generator is displayed in Fig.S2 (Supplementary materials). As displayed, plasma was generated between four pairs of quartz tube, which acts as plasma generators. Air was utilized as plasma feeding gas. The applied input power of plasma generator is 4.5 kW, and the processing parameters were set to 4.5 kW and 8 m·min-1, which correspond to the optimized processing parameters of wood surface modification described in the literatures (Cao et al, 2018, Chen, M. et al., 2016). For one plasma treatment, cellulose powders (20 mg) were placed on thin quartz plate to pass through the plasma at a feeding speed of 8 m·s-1. The width of plasma generator is 20 cm, thus the total time of plasma treatment is approximately 1.5 s. 2.3 Experimental and computational setup Optical Emission Spectroscopy (OES) was utilized to monitor the component and corresponding emission intensity of plasma. X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared spectroscopy (FT-IR) and Grazing Incidence X-Ray Diffraction (GIXRD) were performed for chemical composition and crystal structure of cellulose after plasma treatment. For detailed information of structural changes of cellulose molecule, Gel Permeation Chromatography (GPC) and Two Dimensional-Nuclear Magnetic Resonance Heteronuclear Single Quantum Coherence (2D-NMR HSQC) were performed. Detailed information of experimental setup was expatiated in Supplementary material. Computational investigation was performed using Density Functional Theory (DFT). The detail information was also provided in 4

Supplementary material.

3. Results and discussion

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3.1 OES investigation Components of atmospheric DBD plasma can be identified using OES, and the results are displayed in Fig.1. Concentration of components can also be revealed by the optical emission intensity. As shown in Fig.1, multiple peaks located in the wavelength region of 300 nm-450 nm can be assigned to nitrogen molecular ion (N+2 ) (Chien et al, 2017). Existence of N+2 suggests that nitrogen can only be partially ionized in the plasma, which can be attributed to the noble nature of nitrogen. Apparently, N+2 has the highest intensity due to the fact that nitrogen occupies approximately 78% in air. We can speculate that the majority of the particles interact with cellulose in the atmospheric DBD plasma are N+2 . N+2 is low in reactivity, while it possesses enough energy to cleave chemical bonds and thus promote post-oxidation of cellulose. Emission peaks located at 297 nm, 778 nm and 844 nm are associated with atomic oxygen (Dufour, Hubert, Vandencasteele, & Reniers, 2012). Oxygen in air is more reactive and complete ionization can be achieved in atmospheric DBD plasma. Reactions that generate atomic oxygen are also displayed in Fig.1. Due to the electronegativity of oxygen, electrons are hard to be attracted to oxygen. Thus, the direct collision of oxygen and electrons (Reaction D) is unlikely (Khan & Al-Jalal, 2008). On the same basis, the oxygen molecular ion (O+2 ) is difficult to be generated via direct collision ionization of oxygen and electrons (Reaction C). The corresponding reaction rate constant of Reaction C is 3.3×10-15 cm3·(mol·s)-1 (Lee, Graves, Lieberman, & Hess, 1993). A previous research pinpointed the dominance of collision between O+2 and electrons (Reaction A) and the corresponding reaction rate constant is 4.8×10-7 cm3·(mol·s)-1 (Dufour et al., 2012). Herein, we can conclude that atomic oxygen can be generated through collision of O+2 and electrons. Previous literatures also evidenced the dominance of penning ionization (Reaction B) in generation of O+2 . Excited noble gas atoms (i.e., argon and helium) collided with oxygen, thus generating O+2 at a faster reaction rate (2.4×10-10 cm3·(mol·s)-1) as compared with Reaction C (Lee, Jin, Sang, & Kim, 2005). Atomic oxygen possesses strong capacity and high reactivity to directly oxidize cellulose. Meanwhile, a weak signal of oxynitride (NO) can be observed in the region corresponding to the ultraviolet system (200 nm-290 nm), indicating the reactions between reactive plasma components (Chien et al., 2017). 3.2 Spectroscopy investigation Cellulose is known as a polysaccharide macromolecule composed of cellobiose units. Optimized 2D and 3D structures of cellobiose are shown in Fig.2a. Two pyranose rings are bonded via the β-1, 4-glucosidic bond. Information on the surface chemical composition of cellulose after plasma treatment can be provided via XPS analysis. As listed in Tab.1, relative content of oxygen substantially elevated after plasma treatment, thus increasing the O/C ratio. Additional information about plasma-induced oxidation can be revealed via high-resolution XPS C1s spectra (Fig.2b-c). Carbon-related groups of cellulose can be divided into four well-resolved peaks (Klarhöfer et al., 2010, Tang et al., 2015). The peak at 284.8±0.1 eV represents aliphatic carbon (C-C/C-H) while peaks located at 285.6±0.1 eV, 287.8±0.1 eV and 289.0±0.1 eV are associated with C-O, C=O/O-C-O and O-C=O, respectively. Obtained results revealed that relative content of aliphatic carbon significantly reduced after plasma treatment, indicating the apparent cleavage of the C-C/C-H covalent bonds during plasma treatment. Collison and bombardment between energetic 5

particles with cellulose can be the main reason underlying the cleavage of C-C/C-H covalent bonds. Also, Plasma-induced degradation of contaminants on cellulose surface is another possible reason. The relative contents of C-O, C=O/O-C-O and O-C=O were dramatically increased, suggesting substantial oxidation of cellulose by plasma treatment. It also revealed that oxygen can be incorporated into the cellulose molecule in the forms of C-O, C=O/O-C-O and O-C=O after plasma treatment.

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Additional information on the macromolecular structure of plasma-treated cellulose was provided by FT-IR and GIXRD analysis (Fig.2d-f). The broad peak located in the wavenumber region from 3000 cm-1 to 3700 cm-1 in the FT-IR spectra can be assigned to the absorption of hydroxyl. The wide peak can be divided into three peaks associated with the hydrogen bonds. Peaks at 3351 cm-1 and 3473 cm-1 correspond to the O3H---O5 and O2H---O6 intermolecular hydrogen bonds, respectively. A blue shift of these peaks indicates the lengthening of intermolecular hydrogen bonds (Prabhu, Vaideki, & Anitha, 2017). The peak at 3243 cm-1 is associated with O6H-O3 intramolecular hydrogen bond. A slight red shift of peak at 3243 cm-1 to 3203 cm-1 can also be observed after plasma treatment. Results obtained from FT-IR spectra can be attributed to the elongation of the flanking hydrogen bond, leading to the weakened bond force (Altaner, Horikawa, Sugiyama, & Jarvis, 2014, Nishiyama, Langan, & Chanzy, 2003). Plasma treatment has been demonstrated to cause a strain in the cellulose unit cell, which may lead to cleavage of hydrogen bond and destruction of the cellulose chain (Prabhu et al., 2017). According to previous researches, hydrogen bond is the weakest bonding in cellulose, which will be easily dissociated (Kang X. Y, 2016, Yang et al, 2017). As demonstrated by GIXRD (Fig.2f), three crystalline peaks can be observed at 2θ of 14.9°, 22.8° and 33,4°, which correspond to the (110), (002) and (040) lattice planes of cellulose, respectively (Wen, Wang, Wei, Wang, & Liu, 2017). Due to the limited detection depth of GIXRD (Kontturi et al, 2011, Nakayama et al, 2018), obtained results suggested that relative crystallinity of cellulose significantly reduced from 58.0 % to 38.8 % after plasma treatment, indicating the destruction of the crystalline region on the cellulose surface. Cleavage of the flanking hydrogen bonds and destruction of the cellulose chain can be the reasons for reduction in relative crystallinity.

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Detailed information about the changes in the molecular structure of cellulose after plasma treatment can be revealed by Nuclear Magnetic Resonance (NMR) investigation and the results are shown in Fig.3. Solution-state 13C-NMR spectra of acetylated cellulose are illustrated in the inset image of Fig.3. Acetyl carbonyl carbons are detected at 169.0 ppm-170.3 ppm (Chen, Xu, Wang, Cao, & Sun, 2016). The carbon signals at 102.4 ppm and 99.2 ppm can be assigned to C1 and C1s due to the substitution of the hydroxyl group at the C2 position (Chen, J.H. et al., 2016, Wang, Wen, Zhang, & Liu, 2017) . The NMR signals at 79.9 ppm and 76.0 ppm correspond to the C4 and C4s (the hydroxyl group at the C3 position is substituted). In addition, the carbon signals at 73.0 ppm-70.0 ppm correspond to the combination of C2, C3, and C5, and the signal at 62.1 ppm is associated with C6 (Wang et al., 2017). Additional information about the structure of the cellulose skeleton can be well-revealed in 2D-NMR HSQC spectra. The 13C-1H correlation signals of all carbons in cellulose can be assigned using chemical shifts described in the literatures (Chen, J.H. et al., 2016, Holding et al, 2016, Kono, 2013, Labafzadeh, Helminen, Kilpelainen, & King, 2015) and are summarized in Tab.2. The 13C-1H correlation signals at δH/δC 4.5-5.3 6

ppm/90.1-104.2 ppm correspond to the C1 position from the reducing end (RE), non-reducing end (NRE) and internal (int) cellulose of acetylated cellulose. The 13C-1H correlation signals at δH/δC 4.7 ppm/72.8-77.3 ppm, 5.0-5.4 ppm/70.6-75.6 ppm, 3.2-5.0 ppm/69.2-82.5 ppm, 3.6-4.1 ppm/69.2-76.5 ppm and 3.8-4.4 ppm/60.8-63.4 ppm can be assigned to the C2, C3, C4, C5 and C6 positions of acetylated cellulose, respectively.

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According to the 2D-NMR HSQC spectra, the 13C-1H correlation signals of all carbons in the acetylated cellulose molecule reduced after plasma treatment. Apparently, all carbon-related covalent bonds in cellulose can be cleaved by the plasma due to the high energy of the plasma components (Zhou, Chen, & Du, 2017). As discussed previously, atmospheric DBD plasma used in this work consists of energetic particles. Apart from that, plasma is believed to consist of negatively charged electrons and other energetic particles (Edwards, Buschle-Diller, & Goheen, 2006). Cleavage of the carbon-related bonds in the cellulose molecule can be attributed to the impact of high-energetic particles (including N+2 , atomic oxygen and electrons, et al) in the plasma. Liu et al (Liu et al, 2015) experimentally suggested the activation energy of weak chemical bond of cellulose (C-O bond in glucoside bond) is 129.7 kJ·mol-1. They also demonstrated that β-1, 4-glucosidic bonds have relatively low activation energies compared to that of the other chemical bonds, suggesting the higher likelihood of cleavage. Herein, we found that the reduction of the 13 1 C- H correlation signal of C4 was more pronounced than that of the other carbon positions. This finding indicates that the cleavage of the C4-related chemical bonds, including β-1,4-glucosidic bonds and two C-C covalent bonds in the pyranose ring (C3-C4 and C4-C5), is more pronounced than that of other carbon-related chemical bonds. It can be speculated that the cleavage of the β-1, 4-glucosidic bonds and C-C covalent bonds in the pyranose ring are the major reactions during plasma treatment. The cleavage of the β-1, 4-glucosidic bond is the first step of these reactions due to the relatively low activation energy. This finding is consistent with the results of previous literatures, which have demonstrated that the cleavage of the β-1,4-glucosidic bonds is the first step in a series of reactions during pyrolysis of cellulose in hydrogen plasma (Huang, Liu, Wei, Huang, & Hao-Jie, 2011, Huang, Cheng, Chen, & Zhan, 2013). Previous literature have also emphasized that the pyranose ring-breaking reactions are the major consequent reaction after cleavage of glucoside bond (Zhang, Geng, & Yu, 2015). Investigation into the molecular weight of cellulose can provide further evidence into the structural changes of cellulose after plasma treatment. As listed in Tab.3, the weight-average molecular weight (Mw) and number-average molecular weight (Mn) of cellulose substantially reduced after plasma treatment. Additionally, the polydispersity of cellulose decreased, suggesting that plasma treatment shortens the length of the cellulose macromolecules. This finding confirms and complements previously mentioned oxidation and degradation of plasma-treated cellulose, which can be attributed to the cleavage of β-1, 4-glucosidic bonds and pyranose ring-breaking reactions. However, degradation induced by plasma is rather limited due to the limited penetrability of plasma. It has been demonstrated that plasma treatment can only affect the surface and subsurface of materials. The penetration depth of plasma into wood surface is around 330 nm (Král et al., 2015). 3.2. Computational investigation As discussed previously, cleavage of the β-1, 4-glucosidic bond is the first-step reaction during plasma treatment of cellulose. For further clarify the reaction pathway of oxidation of cellulose, DFT was utilized in this work. We used cellobiose as the model compound. The 7

optimized structure and charge distribution of cellobiose are shown in Fig.4a. The Van der Waals electronic surface potential (ESP) image is displayed in Fig.4b.

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As shown in Fig.4a and b, the oxygen atom in the β-1, 4-glucosidic bond carries a massive negative charge (-0.54) and exhibits evident negative electronic potential. The adjacent carbon atoms have asymmetric positive charge, which can be attributed to the asymmetric structure of cellobiose. Detail information of chemical bonds in cellubiose is listed in Tab.S3. The bond dissociation energy (BDE) of C4-O is the lowest (229.2 kJ·mol-1) compared to other chemical bonds and the Mayer bond order of C4-O (0.83) suggests the higher reactivity of C4-O. Hence, the C4-O covalent bond can be easily affected by energetic particles. Furthermore, the ESP image of cellobiose suggests that oxygen atoms in β-1, 4-glucosidic bond possess strong negative electronic potential and positive particles in plasma can be attracted to oxygen atoms. Therefore, the collision probability of β-1, 4-glucosidic bond and particles in plasma is rather high, thus leading to evident cleavage of C4-O covalent bonds. Combining results of charge distribution and ESP, cleavage of the C4-O bonds is the first step in a series of reactions during plasma treatment. Consequent reactions after cleavage of C4-O initial with reaction between carbon radicals and atomic oxygen. Carbon radicals generated after cleavage of C4-O covalent bonds can react with atomic oxygen in the plasma and the Gibbs free energy (ΔG) of this reaction is -341.7 kJ·mol-1, indicating this reaction is spontaneous. We mainly investigated three consequent reaction pathways which are proposed by previous literatures (Biella, Prati, & Rossi, 2002, Hao, Liu, Feng, Zhang, & Wang, 2014, Pasta, La Mantia, & Cui, 2010). Oxygen-containing groups can be grafted on cellulose molecule through these reaction pathways. Gibbs free energy profiles of these pathways are shown in Fig.5a and 5b. All reaction pathways are listed in Fig.5c. It should be noted that the concentration of atomic oxygen in the plasma is limited. Hence, these pathways are competitive with each other. In Pathway A, the hydrogen atom at C4 is transferred to the oxygen atom through hydrogen transfer reaction and forms into another carbon radical (intA1) and a hydroxyl group. The ΔG value of this reaction is -28.4 kJ·mol-1. However, the energy barrier (ΔG≠) of this reaction is 125.9 kJ·mol-1 suggesting limited feasibility of this reaction. In contrast, carbonyl and carbon radicals (intB1 and intC1) can be generated through Pathway B and Pathway C. The corresponding ΔG≠ values of these reactions are 32.2 kJ·mol-1 and 15.3 kJ·mol-1, respectively. It suggests that Pathway B and C are more dominant than Pathway A. Moreover, the carbon radicals in intB1 and intC1 can react with atomic oxygen and form intB2 and intC2, respectively. A reaction subsequent to the formation of intB2 can involve the cleavage of the C3-C2 covalent bond producing intB3. The ΔG of this reaction is 47.4 kJ·mol-1 indicating that this reaction cannot be spontaneous. One subsequent reaction after the formation of intC2 leads to cleavage of C5-C6 covalent bond and pyranose ring-breaking, which produces intC3. The ΔG value of corresponding reaction is -57.6 kJ·mol-1. In addition, the ΔG≠ of this reaction is close to 0 kJ·mol-1, suggesting that this reaction may be dominant thermodynamically and dynamically. Another subsequent reaction after the formation of intC2 is the cleavage of the C5-O covalent bond, resulting in the production of intC4 and subsequent destruction of the pyranose ring. The ΔG and ΔG≠ values of this reaction are -5.0 kJ·mol-1 and 59.8 kJ·mol-1, respectively. Comparison of these two reactions reveals that the cleavage of C5-C6 covalent bond is the dominant reaction from the point of thermodynamics and dynamics. To sum up, carbonyls can be successfully generated on the cellulose molecule through both Pathways B and C. Pathway C is more spontaneous and has lower energy barrier and thus, is the main mechanism of grafting carbonyl in 8

cellulose molecule.

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It has been demonstrated that grafting of carbonyl is the critical process (Ishimoto, Hamatake, Kazuno, Kishida, & Koyama, 2015, Ishimoto, Kazuno, Kishida, & Koyama, 2014). In this work, we found that carboxyl can be generated by subsequent oxidation of carbonyl through the hydrogen transfer reaction as confirmed in the literatures (Hao et al., 2014, Knill & Kennedy, 2003, Pasta et al., 2010). As shown in Fig.5b, the ΔG value of this reaction is -510.2 kJ·mol-1, indicating this reaction is more spontaneous. However, the corresponding ΔG≠ of this reaction is relatively high (76.5 kJ·mol-1), indicating the yield of carboxyl is associated with input power of plasma. This finding is in agreement with our previous researches, which confirms that an rather evident increasing in carboxyl can be observed after 4.5 kW plasma treatment as compared to 1.5 kW and 3 kW plasma treatment (Chen, M. et al., 2016). XPS analysis performed in our previous work revealed that content of carboxyl on 4.5 kW plasma-treated wood surfaces is 2.46%, which elevated by 67.35% as compared to untreated sample (1.47%). In addition, 1.5 kW and 3 kW plasma-treated wood samples exhibits contents of carboxyl of 1.82% and 1.87%, respectively. Comparison between different input powers of plasma reveals that 4.5 kW plasma exhibits the strongest oxidation capacity.

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4. Conclusion

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A series of experimental and theoretical analyses were performed to investigate the reaction pathway of plasma-induced oxidation of cellulose. Experimental analysis demonstrated that nitrogen molecular ion and atomic oxygen assist with destruction of the chemical bonds and promote the oxidation of cellulose. FT-IR and GIXRD analysis demonstrated that plasma treatment leads to the cleavage of the hydrogen bonds in the cellulose macromolecules and to a reduction in relative crystallinity (from 58.0 % to 38.8 %). Cleavage of the β-1, 4-glucosidic bond is believed to be the first-step reaction during oxidation of cellobiose due to its low bond dissociation energy (229.2 kJ·mol-1) and high reactivity. DFT calculation suggested that the cleavage of the β-1, 4-glucosidic bond followed by the cleavage of the ether bond of the pyranose ring is the dominant reaction pathway during oxidation of cellobiose in plasma. The energy barrier of this reaction pathway is rather lower than that of other potential reactions, suggesting that this reaction pathway is thermodynamically and dynamically dominant. As demonstrated by DFT calculation, the carbonyl is the important intermediat product during plasma treatment. Carboxyl can be subsequently generated via oxidation of carbonyl. However, the energy barrier of this reaction is 76.5 kJ·mol-1 suggesting the yield of carboxyl strongly depends on the input power of plasma. This finding provides a mechanistic investigation into the oxidation reaction pathway during plasma treating cellulose. Efforts are performed on elaborating the structural changes of cellulose and interaction between plasma and cellulose. The outcomes we obtained clarified the oxidation reaction pathway of cellulose during plasma treatment. It brings about important sights into fundamental understanding the plasma-lignocellulose interaction. On that basis, researches can optimize and design plasma treatment for surface modification of lignocellulose. Lignocellulose-based composites can be manufactured with improved efficiency and properties in future. 9

Acknowledgements

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This work was supported by the National Natural Science Foundation of China (Grant No. 31870549), the Program for 333 Talents Project in Jiangsu Province (Grant No. BRA2016381), the Postgraduate Research &Practice Innovation Program of Jiangsu Province (KYCX17_0838), the Doctorate Fellowship Foundation of Nanjing Forestry University (2017) and the Advanced Analysis and Testing Center of Nanjing Forestry University. The authors also would like to thank for the finance support by project of construction of First-Class disciplines.

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Fig.1 OES spectrum of atmospheric DBD plasma with an input power of 4.5 kW. Reactions that generates atomic

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oxygen are also displayed.

Fig.2 (a) Optimized 2D and 3D structures of cellobiose. (b-c) High resolution XPS C1s spectra of untreated and

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plasma-treated cellulose. (d-e) Magnification of FTIR spectra at the wavenumber region of 3000–3700 cm−1. (f)

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GIXRD pattern of cellulose before (black) and after (red) plasma treatment.

Fig.3 Partial expansion of the 2D-NMR HSQC spectrum of untreated (a) and plasma-treated (b) acetylated cellulose recorded in acetone-d6. Insert: Representative structure and 13C spectrum of untreated and plasma-treated acetylated cellulose. Chemical bonds colored in red represent the cleaved bonds.

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Fig.4 (a) calculated charge distribution and (b) Van der Waals ESP image of cellobiose, which is used as the model compound in this work.

Fig.5 (a) Gibbs free energy profiles (ΔG0298.15) of potential reactions of cellobiose after cleavage of β-1, 4-glucosidic

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bond (C4-O). (b) Reaction pathway of carboxyl via further oxidation of carbonyl.

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Tab.1 Elemental composition and relative content of carbon-related groups obtained from XPS analysis

Samples

Theoretical celluloseb Untreated cellulose Plasma-treated cellulose

O/C

C-C/C-

Ratio

H (%)

n/a

0.83

68.6

25.1

0.04

0.33

37.4

0.02

0.60

C (%)

O (%)

N a (%)

54.6

45.7

74.9 62.6

calculated by difference

b

calculated based on theoretical model of cellulose

O-C=O

C-O (%)

(%)

25.7

5.7

n/a

50.7

33.0

16.3

n/a

21.9

40.0

20.2

17.9

δH/δC (ppm)

Assignment

δH/δC (ppm)

5.3/90.1

C1 (RE)

3.3/77.3

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a

C=O/O-

C-O (%)

4.7/101.4

C1 (int/NRE)

3.2/74.8

C4 (int)

4.5/104.2

C1 (int)

3.6/69.3

C5 (RE)

4.7/77.3

C2 (RE)

4.6/74.6

C2 (int/NRE)

4.7/72.8

C2 (int)

5.4/70.6

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Tab.2 Assignment of 13C-1H correlation signals in the HSQC spectra of acetylated cellulose

Assignment C4 (int)

C5 (RE)

4.1/69.2

C5 (NRE)

3.8/73.6

C5 (int)

C3 (NRE)

3.5/76.5

C5 (int)

5.0/75.6

C3 (int/NRE)

4.4/63.5

C6 (RE/NRE)

5.1/73.7

C3 (int)

3.9/61.3

C6 (NRE)

C4 (RE)

4.4/63.4

C6 (int)

C4 (NRE)

4.2/63.4

C6 (int)

C4 (NRE)

3.9/60.8

C6 (int)

3.8/60.8

C6 (int)

5.0/69.2

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3.7/77.5

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3.8/75.4

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3.6/75.4

C4 (int)

3.6/82.5

C4 (int)

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3.8/77.3

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Tab.3 Relative molecular weight of untreated and plasma-treated cellulose according to GPC Mw (g·mol-1)

Mn(g·mol-1)

Polydispersity (Mw/Mn)

Untreated Cellulose

138000

61700

2.23

Plasma-treated Cellulose

118000

50400

2.34

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Sample

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