Activation of Cellulose by Supercritical ... - ACS Publications

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Activation of Cellulose by Supercritical Tetrafluoroethane and Its Application in Synthesis of Cellulose Acetate Chen Meng, Hui Lu, Gui-Ping Cao,* Chen-Wei Yao, Yue Liu, Qi-Ming Zhang, Yun-Bo Bai, and Hua Wang UNILAB, State Key Lab of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: Cellulose is the most ancient and abundant polysaccharide on earth. However, the strong hydrogen bond networks and the high crystallinity strongly prevent cellulose from reacting with other compounds to produce higher performance materials. In this work, the investigation via molecular dynamics simulation showed that supercritical 1,1,1,2tetrafluoroethane (SC R134a) could effectively destroy the strong hydrogen bond networks of cellulose and activate cellulose. Furthermore, experiments were also conducted to support the simulation results, where cellulose was activated via introducing SC R134a and subsequently used in the synthesis of cellulose acetate. The experimental results revealed that SC R134a exhibited a prominent effect on activating cellulose. The crystallinity of cellulose decreased from 0.926 to 0.686 after being activated by SC R134a. In addition the morphology (SEM) of cellulose suggested that the average diameter increased by 25% and numerous cracks appeared on the outer surface of cellulose fibers. Moreover, the consumption of reagents for synthesizing cellulose acetate decreased sharply, especially for acetic acid, which was only 25% of that used in industry, therefore, making the process much more green and energy-saving.

1. INTRODUCTION Nature cellulose is the most ancient and abundant biomass resource, with a trillion tons produced per year by photosynthesis.1 Because of the shortage of fossil fuel, the applications of cellulose have reattracted considerable attention both in academic and industry in recent years.2−5 Cellulose acetate (CA), known as one of the most important cellulose derivatives,6,7 has a wide range of applications, including textile,4 membrane separation,8 coatings,9 plastics,10 and cigarette industries and also food,11 medicine,12 filters, and living goods.13 As the traditional heterogeneous synthesis of CA employed in industry, acetic acid, acetic anhydride, and sulfuric acid act as solvent, acylation reagent, and catalyst, respectively.14 A thorny, long-standing problem of this industrial technology is the huge consumption of acetic acid (8 kg/kg cellulose5,15) and sulfuric acid (0.10−0.15 kg/kg cellulose16), which causes serious acid wastewater pollution and high production cost. This problem is mainly caused by the strong intra- and intermolecular hydrogen bond networks and the high crystallinity of cellulose,17,18 which leads to the poor reactivity of cellulose and also prevents its further derivatization and limits its applications. Therefore, many research works have been reported to improve cellulose reactivity. In Hong et al’s research,19 a NaOH solution was used to activate cellulose before the synthesis of cellulose carbamate. However, being activated by a NaOH solution destroys the chain structure and results in serious degradation of cellulose. Some other physical activation methods have also been investigated by researchers, such as microwave heating,10,20,21 steam explosion,22−24 etc. Besides the heterogeneous synthesis of CA using activated cellulose, dissolution and modification of cellulose in solvents like ionic liquid (IL) have been reported frequently in recent years.7,25−27 However, Zhang28 pointed out that several urgent questions, such as high cost and difficult © 2015 American Chemical Society

recovery of IL, still remained to be explored before IL was large-scale utilized in the cellulose industry. Therefore, the heterogeneous synthesis method used in this work is still the main technology to produce CA in industry, before which cellulose has to be activated. Among these methods, activation using supercritical fluid (SCF) might be an effective one. SCF has widely been applied in polymer science and technology, such as chemical reaction, polymerization, swelling, and production of porous materials or particles.29,30 The synthesis of cellulose carbamate, which was assisted by supercritical CO2 (SC CO2), was reported in Yin’s publications.2,31,32 SC CO2, acting as both solvent and carrier, was used to expand the structure of cellulose, making it easier for urea to impregnate. They reported that a remarkable increase of the nitrogen content of cellulose carbamate was obtained with the aid of SC CO2. A similar function was also pointed out by Yu et al.1 that SC CO2 influenced the interaction between the chains of cellulose. With the penetration and swelling effect of SC CO2, the titania particles were facilitated to access and impregnate into not only the amorphous but also the crystalline structure of cellulose. SCF possesses many specific properties,31 such as its low viscosity and high diffusivity closer to gas, relatively high density closer to liquid, no surface tension, etc. Due to these advantages, SCF can easily penetrate into the polymer. Besides SC CO2 mentioned above, supercritical water was also employed for swelling and dissolving cellulose.33,34 In a previous investigation, cellulose crystallites were destructed and cellulose II was observed which resulted from the swelling and rearrangeReceived: Revised: Accepted: Published: 12204

September 14, 2015 November 12, 2015 November 14, 2015 November 14, 2015 DOI: 10.1021/acs.iecr.5b03418 Ind. Eng. Chem. Res. 2015, 54, 12204−12213

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Industrial & Engineering Chemistry Research

Figure 1. Unit cell of cellulose crystal viewed on the planes of (a) AB and (b) BC. (Color code: C, gray; O, red; H, white. The length dimension is Å.)

Figure 2. (1, −1, 0), (1, 1, 0), and (2, 0, 0) surfaces of crystalline cellulose Iβ.

molecules and crystalline cellulose.41−43 Based on the simulation results of the interaction between water and cellulose, Matthews et al.44 pointed out that the layers of water played a role in the barrier for cellulose enzymes in enzyme-catalyzed hydrolysis. The interaction between urea and cellulose was investigated recently by Wernersson45 using MD simulation, which provided an explanation on the effect of urea on the rising of cellulose solubilization in water. This work contained two parts, i.e., the simulation one and the experimental one, which were performed to investigate the feasibility of the new process of cellulose activation using supercritical R134a (SC R134a). In the simulation part, the interaction between cellulose and R134a was calculated by molecular dynamics (MD) simulations. The results noted that the hydrogen bonds of cellulose could be partly destroyed by SC R134a. Based on the simulation results, we thereby proposed a novel activation method of cellulose using SC R134a to improve its reactivity. The morphology, crystallinity, and structure of activated cellulose were characterized by SEM, XRD, and FT-IR, respectively. Furthermore, the performance of this activation method was tested by the heterogeneous acetylation of activated cellulose.

ment of cellulose chains. However, the problem of serious degradation still accompanied the dissolution process, with the degree of polymerization of cellulose even lower than 30. In the present work, 1,1,1,2-tetrafluoroethane (R134a) is introduced to activate cellulose on account of its lower cost, lower toxicity, and nonflammability. In addition, the critical pressure and the critical temperature of R134a are 4.07 MPa and 101.1 °C, respectively, which are appropriate for industrial application. Moreover, its molecular polarity, with dielectric constant of 9.5 and dipole moment of 2.05 D, is much higher than that of CO2, therefore, making R134a much easier to break the hydrogen bonds of cellulose. After the key step of effective activation, cellulose can be easily modified via derivatization reaction. Over the past few decades, the stronger computational power has made molecular simulations become an excellent method in polymer research. Several studies on cellulose using this method have been reported recently,35,36 in which the structure of cellulose has been investigated particularly.37,38 The unit cell dimensions of two crystal lattices of cellulose, Iα and Iβ, were precisely predicted by Viëtor et al.,39 which were demonstrated by recent experimental data. Heiner et al.40 reported the full atomic details of the crystalline cellulose Iα and Iβ by molecular dynamics (MD) simulation. The calculation results of energetics demonstrated that the Iβ phase was more stable than the Iα phase, which was consistent with the experimental data. Computer simulations, on the other hand, are also performed to investigate the interaction between small

2. SIMULATION It is well-known that cellulose I is divided into two distinct crystalline phases, Iα and Iβ. Cellulose Iα mainly exists in bacteria and cellulose Iβ primarily exists in animals or tunicate 12205

DOI: 10.1021/acs.iecr.5b03418 Ind. Eng. Chem. Res. 2015, 54, 12204−12213

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Industrial & Engineering Chemistry Research cells.43 In the present work, only cellulose Iβ, the more stable phase, was considered in the following simulation, whose atomic coordinates were obtained from Nishiyama’s study.18 Cellulose Iβ was the monoclinic system with lattice dimensions of a = 7.784 Å, b = 8.201 Å, c = 10.380 Å, α = β = 90°, γ = 96.5°, which were determined by synchrotron X-ray and neutron fiber diffraction. Figure 1 shows the unit cell of the cellulose Iβ crystal from AB and BC planes. The (1, −1, 0), (1, 1, 0), and (2, 0, 0) surfaces of crystalline cellulose Iβ, shown in Figure 2, were cleaved to construct a cellulose super cell. Periodic boundary conditions were employed in the structure models of these cellulose surface systems, where each constructed super cell consisted of 24 cellooctaose. 300 molecules of R134a were then added into these systems to construct mixed systems, which were given in Figure 3.

Figure 4. Models of (a) amorphous cellulose with two chains and (b) mixed system of amorphous cellulose and 100 molecules of R134a. (Color code: C, gray; O, red; H, white; F, blue. Style: stick style, cellulose with DP = 8; ball and stick, cellulose with DP = 20.).

Figure 3. Different mixed systems of (a) cellulose Iβ (1, − 1, 0)/ R134a system, (b) cellulose Iβ (1, 1, 0)/R134a system, and (c) cellulose Iβ (2, 0, 0)/R134a system. (Color code: C, gray; O, red; H, white; F, blue.).

consistent force field (pcff),46 which was usually applied to polymers and organic materials. All of the structure model systems should be initially subjected to annealing and energy minimization, followed by the MD simulation. The MD simulation was conducted under the constant volume and constant temperature ensemble (NVT) and propagated for 1 ns, using the time step of 1 fs. The temperature was kept at 383 K which was consistent with that set in the following experiments. Long-range electrostatics was calculated by the Ewald method, while van der Waals was handled using Atom based method with the cutoff of 12.5 Å. The results of the simulation were stored for each 1 ps. The mean square displacement (MSD), a quantitative measurement of the average magnitude of atomic motions, was analyzed according to the equation, ⟨Δr2⟩ = ⟨|r(t) − r(0)|2⟩, where r denotes the atomic position.41 The interaction between cellulose and R134a could be analyzed by the radial distribution function (RDF), which expressed the probability of finding a particular atom from the reference atoms in a thin spherical shell with thickness of dr and radius of r. The bond type between the two atoms could be determined by the first peak in RDF. Hydrogen bond network of cellulose was the key point in the activation process. Two geometric criteria were employed to identify the hydrogen bond: (1) the distance between a donor and an acceptor should be less than 3.5 Å43,47 and (2) the donor-hydrogen-acceptor angle should be greater than 110°.18 The numbers of hydrogen bonds in cellulose systems were calculated by a self-edit perl script.

A periodic amorphous cellulose model was constructed as shown in Figure 4a, with the lattice dimensions of a = b = c = 17.13 Å, α = β = γ = 90°. The model contained two different lengths of cellulose chains with degree of polymerization (DP) of 8 and 20. The mixed system of cellulose/R134a contained the two amorphous cellulose chains and 100 molecules of R134a, as shown in Figure 4b, with the lattice dimensions of a = b = c = 32.70 Å, α = β = γ = 90°. In addition, the mixed system of cellulose/CO2 with those two amorphous cellulose chains and 100 molecules of CO2 was also constructed for comparison. In all of the three systems, the DP of cellulose constructed in amorphous models did not affect the results of simulation analysis. In this work, MD simulation was carried out using Material Studio 5.5 program with the force field of the polymer

3. EXPERIMENTAL SECTION Materials. The cotton sample, of which the DP was about 2000, was produced in Xinjiang, China. It contained about 0.99 kg/kg of α-cellulose after pretreatment. High purity nitrogen, sulfuric acid, acetic acid, acetic anhydride, sodium hydroxide, hydrochloric acid, sodium carbonate, sodium bicarbonate, sodium hypochlorite, and perchloroethylene (PCE) were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Phenolphthalein, methyl red, and anhydrous ethanol were supplied by Sinopharm Chemical Reagent Co., Ltd. R134a was bought from Shanghai Rui Yi Trading Co., Ltd. All these chemicals were analytical reagent grade and used without further purification. Pretreatment of Cotton. The fat and PCE soluble impurities were extracted from raw cotton in a Soxhlet 12206


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cooled to room temperature, the excess alkaline in the solution was titrated using 0.2 mol/L of HCl standard solution, with phenolphthalein as indicator. The titration was conducted again overnight so that the excess NaOH could spread out. Meanwhile, a blank experiment was carried out. The DS was calculated as eqs 2 and 3.

extractor with refluxed PCE. After the reflux extraction, the cotton was washed by ethanol for several times to remove residual PCE, followed by being dried at 80 °C for 10 h. The dried cotton was immersed in 0.5 × 10−2 kg/kg of NaHCO3 solution and alkaline boiled at 100 °C for 90 min. The impurities of protein and ash in cellulose were removed during this process. Then the sample was washed by deionized water until the pH of washing was about 7, followed by being dried at 105 °C for 10 h. The dried cotton was bleached in 0.005 kg/kg NaClO solution at 25 °C for 20 min. Finally, the cotton sample was washed and dried at 105 °C for 10 h. Activation of Cellulose Using SC R134a. The activation of cotton using SC R134a was conducted in an autoclave with a volume of 50 mL and a maximum working pressure of 10 MPa. The autoclave with 1 g of cotton in it was enclosed and all of the bolted connections were secured. After the air in the autoclave was replaced, a fixed mass of R134a was injected from a buffer vessel into the autoclave, followed by heating the autoclave to the activation temperature (110 °C) while the pressure reached 5.0 MPa. R134a exists in liquid state at 25 °C with the saturated vapor pressure of 661.9 kPa. In order to inject enough mass of R134a into the autoclave, the buffer vessel was heated to obtain a higher pressure of R134a. After the sample was exposed to SC R134a for 2 h, an explosive release in three seconds was done. For comparison, cellulose activated by SC CO2 was also carried out for 2 h in this typical system and the temperature and the pressure were 110 °C and 15.0 MPa, respectively. Synthesis of CA Using Activated Cellulose. In the synthesis process of CA, acetic acid was used as the solvent, acetic anhydride as the acylation reagent, and sulfuric acid as the catalyst, respectively. Certain amounts of those reagents and activated cellulose were quantitatively added into the autoclave. All the mass of those reagents were based on the mass of cellulose and the detailed conditions of reaction were given in Table 4. The acetylation was conducted at 110 °C for 2 h. After the reaction, cellulose completely converted into CA, which was soluble in acetic acid. The product CA was precipitated by dropping the reaction solution into 500 mL of deionized water, the antisolvent of CA, then filtered, and washed for several times to remove the residual acetic acid, followed by being dried at 105 °C for 10 h. Characterization of Cellulose before and after Activation. The morphology of cellulose samples was investigated using NOVA Nano Scanning Electron Microscopy 450 (FEI, America). The samples were coated with gold by a vacuum sputter-coater to improve the conductivity of the samples. Wide angle X-ray diffraction patterns of cellulose samples were performed using an X-ray diffractometer (D/ max2550 V, Rigaku, Japan). The XRD was conducted by Cu− Kα radiation. Diffraction patterns were collected in the 2θ range of 10° - 40°. The FT-IR spectra of cellulose were recorded with a Fourier-transform infrared spectrophotometer (Nicolet 6700, Thermo Fisher Scientific Inc., America). The spectra of cellulose samples were analyzed by Attenuated Total Refraction Method. Determination of Degree of Substitution (DS) of CA. The DS of the product was determined by titration method with the theory of saponification reaction,5,10,13,48 as shown in eq 1. CA sample (0.2 g) was added into a round-bottom flask, and after that 25.00 mL of 0.1 mol/L NaOH ethanol solution and 40.0 mL of 0.75 mL/mL ethanol solution were added. The mixture was refluxed at 90 °C for 0.5 h. After the mixture

cell‐(OH)3 ‐ x (OCOCH3)x + x NaOH → cell‐(OH)3 + xCH3COONa

n = (V1 − V2)c HCl × 10−3 DS =

n m − 42.04n 162.14

=

(1) (2)

162.14n m − 42.04n

(3)

where n represents the mole of acetyl in CA, mol. V1 is the volume of HCl solution consumed in the blank experiment, mL. V2 is the volume of HCl solution consumed in the titration of CA mixture, mL. cHCl is the concentration of HCl standard solution, mol/L. m is the mass of CA, g. DS is the degree of substitution of sample (x in eq 1). The number 162.14 is the molecular mass of anhydroglucose unit (AGU, C6H10O5), g/ mol, and 42.04 the molecular mass of acetyl (CH3CO−) minus the atomic mass of H, g/mol. Since each AGU in cellulose possesses three hydroxyls, the maximum theoretical value of DS is equal to 3.

4. RESULTS AND DISCUSSION MD Simulation of the Interaction between Amorphous Cellulose and R134a. Strong inter- and intramolecular hydrogen bonds in cellulose restrain its reaction with other reagents. Whether the addition of R134a could break the hydrogen bonds of cellulose was verified by calculating the interaction between cellulose and R134a via MD simulation. The simulative atomic mean square displacement (MSD) of cellulose in the amorphous cellulose/R134a mixed system was shown in Figure 5, in which the MSDs of cellulose in both the pure amorphous cellulose system and the amorphous cellulose/ CO2 mixed system were also shown for comparison. It could be obviously observed that the MSD of cellulose in the mixed systems displayed a relatively higher slope than that in the pure amorphous cellulose system without SCF,

Figure 5. MSDs of cellulose chains with different DP in amorphous cellulose system, amorphous cellulose/CO2 mixed system and amorphous cellulose/R134a mixed system. The simulation temperature was 383 K. 12207


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Industrial & Engineering Chemistry Research indicating a greater mobility of cellulose in mixed systems. In the cellulose/CO2 mixed system, the slope of MSD was about 200 times that in the pure cellulose system. Furthermore, the slope of MSD in the cellulose/R134a mixed system was nearly 300 times higher than that in the pure cellulose system. The above results clearly demonstrated that the addition of R134a could dramatically increase the mobility of cellulose and promote its modification, which shows more effectiveness than the addition of CO2. The radial distribution functions (RDF) were analyzed, and the results were shown in Figure 6. The respective RDFs

Table 1. Numbers of Hydrogen Bonds per AGU in Three Cellulose Systemsa model system

n

n1

n2

n3

amorphous cellulose amorphous cellulose/CO2 amorphous cellulose/R134a

3.01 2.00 1.46

1.13 0.43 0.07

1.88 1.57 1.39

0.00 0.25 1.14

a

n represents the total number of inter- and intramolecular hydrogen bonds per AGU, n = n1+ n2. n1 represents the number of intermolecular hydrogen bond per AGU in cellulose systems. n2 represents the number of intramolecular hydrogen bond per AGU in cellulose systems. n3 represents the number of hydrogen bond per AGU between cellulose and R134a or CO2.

In the system of amorphous cellulose with two chains (DP = 8, DP = 20), there were 3.01 hydrogen bonds in each AGU, which was similar to Gupta’s research.43 Among these H-bonds, 1.13 of them were intermolecular hydrogen bonds, while others were intramolecular hydrogen bonds. After the addition of CO2, the number of cellulose H-bonds in each AGU decreased to 2.00, and only 0.25 of H-bonds were formed between cellulose and CO2. In addition the numbers of inter- and intramolecular hydrogen bonds dropped to 0.43 and 1.57, respectively. In comparison, a significant variation on the number of hydrogen bonds was observed after the addition of R134a. 1.14 of H-bonds in each AGU were formed between cellulose and R134a. The total number of H-bonds dropped to 1.46, only 48.5% of that in cellulose system without R134a. The number of intramolecular hydrogen bonds decreased to 1.39, only 74% of that in the cellulose system. Moreover, the intermolecular hydrogen bonds were almost destroyed during the process, which would be the critical factor of cellulose activation. The above observation demonstrated once again that the activation effect of R134a on cellulose is much stronger than that of CO2. These results combined with the analysis of MSD and RDF provide strong evidence of the effective activation process using SC R134a, during which the hydrogen bonds between cellulose chains are almost destroyed. The destruction of both intra- and intermolecular hydrogen bonds in cellulose will greatly improve the reactivity of cellulose. MD Simulation of the Interaction between Crystalline Cellulose Iβ and R134a. The simulative atomic MSDs of cellulose chains in the crystalline cellulose Iβ system and cellulose Iβ/R134a mixed systems are shown in Figure 7. In the cellulose/R134a mixed systems, the MSDs of cellulose chains in

Figure 6. RDFs between (a) F atoms in R134a and O atoms in cellulose in the amorphous cellulose/R134a mixed system and (b) O atoms in CO2 and O atoms in cellulose in the amorphous cellulose/ CO2 mixed system.

between F atoms in R134a and one kind of hydroxyl oxygen atoms, i.e., O2, O3, and O6 in the AGU of cellulose chains were calculated. The RDFs between O atoms in CO2 and three different types of O atoms in cellulose were also calculated for comparison. Three kinds of hydroxyl oxygen atoms displayed quite similar RDFs in both cellulose/CO2 and cellulose/R134a mixed systems, as shown in Figure 6. The first peak of RDF in the cellulose/CO2 mixed system (Figure 6b) appeared at 3.25 Å. In contrast, the interaction between R134a and cellulose was much stronger, which could be indicated by the first sharp peak appearing at the distance of 3.09 Å (Figure 6a), corresponding to the formation of a hydrogen bond between F atoms in R134a and O atoms in cellulose. The position of the first peak in RDF represents the distance between the O atoms in cellulose and the F atoms in R134a, which have the largest probability around the cellulose. The first peak position of RDF between cellulose and different anions of ILs was investigated by Zhao et al.49 In addition the shift of the first peak in RDF toward higher value directly corresponded to the decrease of chain packing of cellulose.41,50 Therefore, it can be concluded from the results of RDF that R134a has a greater tendency than CO2 to form hydrogen bonds with the oxygen atoms in cellulose. A possible explanation is that F atoms in R134a possess stronger polarity than O atoms in CO2, facilitating them to become the acceptors of hydrogen bonds. The further investigation was focused on the statistics of hydrogen bond in cellulose systems, which was calculated quantitatively. The numbers of hydrogen bonds in each AGU in the amorphous cellulose system, amorphous cellulose/CO2 mixed system, and amorphous cellulose/R134a mixed system were calculated and presented in Table 1.

Figure 7. MSDs of cellulose chains in the crystalline cellulose system and cellulose Iβ/R134a mixed systems. The simulation temperature was 383 K. 12208


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Industrial & Engineering Chemistry Research three mixed systems displayed an obvious difference, which suggested the different mobility of cellulose chains on various surfaces. Cellulose chains on the (1, −1, 0) surface possessed the highest mobility, while those on the (1, 1, 0) surface came second, and chains on the (2, 0, 0) surface exhibited the lowest mobility. This order of mobility was also pointed out by Velioglu et al.51 in their research on the mobility of cellulose chains in IL. They found that certain chains on the (1, −1, 0) surface exhibited the highest mobility, which were more mobile than those on the (1, 1, 0) surface. In agreement with the MSD of cellulose chains in amorphous cellulose systems, all the MSD curves of crystalline cellulose appeared to have higher slopes after the addition of R134a, which was powerful evidence of the improvement of cellulose reactivity after SC R134a activating. The RDFs in crystalline systems were analyzed and the results are illustrated in Figure 8. The respective RDFs between

Table 2. Numbers of Hydrogen Bonds per AGU in Crystalline Cellulose Iβ Systemsa model system cellulose cellulose cellulose cellulose

Iβ Iβ (1, −1, 0)/R134a Iβ (1, 1, 0)/R134a Iβ (2, 0, 0)/R134a

n

n1

n2

n3

3.00 2.56 2.52 2.59

1.12 0.78 0.77 0.95

1.88 1.78 1.75 1.64

0.00 0.19 0.13 0.10

a

n represents the total number of inter- and intramolecular hydrogen bonds per AGU, n = n1 + n2. n1 represents the number of intermolecular hydrogen bond per AGU in crystalline cellulose. n2 represents the number of intramolecular hydrogen bond per AGU in crystalline cellulose. n3 represents the number of hydrogen bond per AGU between cellulose and R134a.

amorphous cellulose system, 37.5% of the hydrogen bonds were intermolecular, while others were intramolecular H-bonds. After the addition of R134a, the formation of hydrogen bonds between F atoms and O atoms could be clearly confirmed by the data in Table 2, which explicated the strong interaction between the R134a and cellulose. Cellulose chains on (1, −1, 0), (1, 1, 0), and (2, 0, 0) surfaces formed 0.19, 0.13, and 0.10 hydrogen bonds with R134a in each AGU, respectively. Meanwhile, with the addition of R134a, the total number of hydrogen bonds of cellulose in these three systems decreased by 14.8%, 16.0%, and 13.7%, respectively. Cellulose chains on (1, −1, 0) and (1, 1, 0) surfaces revealed similar circumstances, with the decreases of 6% for intramolecular and 30% for intermolecular H-bonds. In addition, on the surface of (2, 0, 0), the number of hydrogen bonds decreased by 0.17 in each AGU for inter- and 0.24 for intramolecular hydrogen bonds. As discussed above, R134a can effectively destroy the intraand intermolecular hydrogen bond networks of cellulose through forming new hydrogen bonds between R134a and cellulose, which can activate cellulose for its further modification. The destruction of H-bond networks may lead to the changes of morphology and crystallinity of cellulose, which can be characterized using SEM and XRD, respectively. Morphology of Cellulose before and after Being Activated by SCF. SEM was carried out to characterize the morphology of cellulose. The photographs of original cellulose as well as cellulose activated respectively by SC CO2 and SC R134a are illustrated in Figure 9. The surface of original cellulose (Figure 9a) was smooth without any defect. It had a compact structure with an average diameter of 13.8 μm, resulting in poor accessibility to reagents and low reactivity. The surface of cellulose activated by SC CO2 (Figure 9b) became rough with slight cracks, and the average diameter increasing to 14.7 μm. After cellulose was activated by SC R134a, the surface (Figure 9c) exhibited a rough and loose texture, with the average diameter increasing to 17.2 μm. A clear morphology of cellulose activated by SC R134a at larger magnification (20 000×) showed that numerous cracks appeared on its outer surface. These variations on the diameter and surface of activated cellulose are mostly attributed to the high diffusivity and polarity of SC R134a, which can break the hydrogen bonds of cellulose. Crystallinity Changes of Cellulose During Activation. Figure 10 showed the X-ray diffraction patterns of cellulose before and after activation. All of these three samples displayed the typical structure of cellulose I crystal form. The characteristic peaks at 2θ = 14.7°, 16.7°, and 22.6° assigned to the cellulose I specific diffraction planes of (1, −1, 0), (1, 1, 0), and

Figure 8. RDFs between F atoms and O atoms in the crystalline cellulose Iβ/R134a mixed system. Black solid lines represent the RDF between O6 and F. Red dash-dot lines represent the RDF between O2 and F. Green dashed lines represent the RDF between O3 and F.

F atoms in R134a and one kind of hydroxyl oxygen atoms, O2, O3, and O6, in cellulose chains on different surfaces were calculated. The interaction between R134a and cellulose could be indicated by the first sharp peak in RDF. On the surface of (1, − 1, 0), the first peaks of RDF between F atoms in R134a and O2, O3 atoms in cellulose emerged at the distances of 2.91 and 3.25 Å, respectively, while the RDF between F and O6 did not show an obvious peak. The RDFs of cellulose on the (1, 1, 0) surface showed similar peaks, which exhibited the first peaks at 2.99 and 3.21 Å for F atoms and O2, O3 atoms, respectively. The above results suggested the formation of hydrogen bonds between F atoms and O2, O3 atoms in cellulose, and that F atoms in R134a acted as the acceptors of hydrogen bond because of the strong polarity. However, the RDFs of cellulose on the (2, 0, 0) surface appeared to have no apparent peaks, indicating that the formation of hydrogen bonds between F atoms in R134a and O atoms in cellulose on this surface was harder than that on (1, −1, 0) and (1, 1, 0) surfaces. The main reason was that all the hydroxyl groups on the (2, 0, 0) surface were equatorially orientated with pyranose rings, which encumbered the donation of hydrogen bonds to R134a. According to the results of RDFs, it could be confirmed that new hydrogen bonds were formed between F atoms in R134a and O atoms in cellulose on surfaces of (1, −1, 0) and (1, 1, 0) after the addition of R134a. Thus, the numbers of hydrogen bonds were further calculated, and the results are given in Table 2. There were 3.00 of hydrogen bonds in each AGU in the crystalline cellulose Iβ system, and consonant with those in the 12209


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Figure 9. SEM photographs of (a) original cellulose before activation, (b) cellulose activated by SC CO2, and (c) cellulose activated by SC R134a at different magnifications.

further shown quantitatively by the following theoretical calculation. The crystallinity of cellulose (Ic) was calculated, and the results are listed in Table 3. The crystallinity of cellulose before Table 3. Crystallinity of Cellulose before and after Activation material

Ic

cellulose before activation cellulose activated by SC CO2 cellulose activated by SC R134a

0.926 0.895 0.686

activation was 0.926, while that of cellulose after being activated by SC CO2 was 0.895. It was found that Ic decreased by 3% during activation process using SC CO2, which was in agreement with the previous work. Yu1 pointed that the highly ordered crystalline structure prevented SC CO2 from penetrating into cellulose, resulting in the poor activation effect. However, the crystallinity of cellulose considerably dropped from 0.926 to 0.686, almost 25.9% of the crystalline area was destroyed after being activated by SC R134a. It is the higher molecular polarity and the most electronegative atom of fluorine that break the H-bonds of cellulose and, therefore,

Figure 10. X-ray diffraction patterns of cellulose before and after activation.

(2, 0, 0), respectively, which pointed by previous literature.1,21,52−55 However, the peak intensities of cellulose after activation were weaker than those of cellulose before activation, indicating the decrease of crystallinity. This assumption was 12210


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reactivity of cellulose. The product synthesized without activation only obtained the DS of 1.47, representing an incompletely reaction, which was due to the strong hydrogen bonds and high crystallinity of cellulose. In contrast, cellulose activated by SC R134a completely converted into CA with the DS of 2.98. Moreover, in comparison with the experiments of cellulose activated by SC CO2 (nos. 3 and 5) and SC R134a (nos. 1 and 4), it revealed that the activation effect of SC R134a was much better than that of SC CO2, for cellulose activated by SC CO2 react with acetic anhydride more difficult under the same conditions. In addition several experimental results (nos. 1, 4, 6, and 7) suggested that CA was successfully synthesized using cellulose activated by SC R134a which could partly destroy the hydrogen bonds of cellulose. The products in those experiments were all dissolved in acetic acid, and the values of DS were approximately equal to 3.0. Like the experimental data discussed above, the minimum consumptions of reagents were 2.0 kg/kg for acetic acid, 2.1 kg/kg for acetic anhydride, and 0.5 × 10−2 kg/kg for sulfuric acid (no. 7). Thus, in view of the reagent consumption applied in the CA industry, especially for acetic acid (8 kg/kg cellulose), the activation method using SC R134a can greatly reduce the consumption of reagents and contribute to environmental protection.

contribute to the much more effective activation of cellulose. The decrease of the crystallinity combined with the looseness of morphology observed by SEM can significantly improve the reactivity of cellulose, which can promote its further modification. Structure of Cellulose before and after Activation. As shown in Figure 11, the FT-IR spectra of cellulose before and

Figure 11. FT-IR spectra of cotton cellulose before and after activation by SC R134a.

5. CONCLUSIONS It was illustrated at a molecular level via MD simulation that the mobility of cellulose chains was both enhanced in amorphous and crystalline cellulose systems after the addition of R134a, which would provide more opportunities for reagent attacking. Moreover, SC R134a could partly destroy the intra- and intermolecular hydrogen bonds of cellulose through forming new hydrogen bonds between R134a and cellulose, which could effectively improve the reactivity of cellulose. In the following experiments, it has been proved that SC R134a does play a key role in the activation of cellulose. The marked decrease of the crystallinity and the damage of the morphology of cellulose activated by SC R134a, characterized by XRD and SEM respectively, considerably improved the reactivity of cellulose. It was demonstrated by FT-IR that activation using SC R134a was only a physical method for cellulose activation, during which no chemical reaction happened. Moreover, in the synthesis of CA, cellulose activated using SC R134a exhibited higher reactivity than cellulose without activation or activated using SC CO2. In addition, the consumption of reagents decreased sharply when using the activated cellulose for the synthesis of CA. Especially, the amount of the solvent, acetic acid, decreased to 2.0 kg/kg cellulose, only 25% of that used in industry. Therefore, this novel activation method of cellulose would be considerably beneficial for environmental protection and economic advantages.

after activation by SC R134a were quite similar. All the characteristic peaks of cellulose existed both in cellulose samples before and after activation; that is, no new peaks were observed in the activated sample, which indicated that no chemical reaction happened during the activation process. In other words, activation by SC R134a was only a physical activation method for cellulose. Properties of CA Synthesized by Cellulose after Activation. To further testify to the performance of activation, cellulose activated by SC R134a was used for the synthesis of CA. The influences of the mass ratio of these reagents to cellulose on the properties of the product were investigated. The solubility of CA in acetic acid was recorded and the DS of the product was determined. The results as well as the detailed reaction conditions are listed in Table 4. Compared with the experimental results of CA synthesized with and without activation (nos. 1 and 2) under the same reaction conditions, it can be concluded that the activation process before synthesis of CA obviously promoted the Table 4. Reaction Conditions and the Solubility and DS of the Productsa no.

SCF

rrs, kg/kg

rAc, kg/kg

rc, 10−2 kg/kg

solubility

DS

1 2 3 4 5 6 7 8

R134a

4.0 4.0 4.0 2.0 2.0 2.0 2.0 2.0

3.2 3.2 3.2 3.2 3.2 3.2 2.1 2.0

1.0 1.0 1.0 1.0 1.0 0.5 0.5 0.5

O + + O × O O +

2.98 1.47 1.76 3.00

CO2 R134a CO2 R134a R134a R134a

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2.97 2.99 1.22

AUTHOR INFORMATION

Corresponding Author

*Tel: 0086-21-6425 3934. Fax: +86-21-6425 3934. E-mail: [email protected].

Ta = 110 °C, ta = 2 h, pa = 5.0 MPa, Tr = 110 °C, tr = 2 h. rrs represents the mass ratio of acetic acid to cellulose added before reaction, kg/kg cellulose. rAc represents the mass ratio of acetic anhydride to cellulose, kg/kg cellulose. rc represents the mass ratio of catalyst to cellulose, kg/kg cellulose. O represents that the product was completely dissolved. + represents that after reaction, there was still cotton left. × represents that the product was cotton-like. a

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by National Undergraduate Training Programs for 12211


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Innovation and Entrepreneurship (201310251016) as well as Shanghai Undergraduate Training Programs for Innovation and Entrepreneurship (S14008).

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ABBREVIATIONS AGU = anhydroglucose unit CA = cellulose acetate DP = degree of polymerization DS = degree of substitution IL = ionic liquid MD = molecular dynamics MSD = mean square displacement R134a = 1,1,1, 2-tetrafluoroethane RDF = radial distribution function SCF = supercritical fluid PCE = perchloroethylene

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