Recycling and Reuse of Ionic Liquid in Homogeneous Cellulose Acetylation

MATERIALS AND PRODUCT ENGINEERING Chinese Journal of Chemical Engineering, 21(5) 577—584 (2013) DOI: 10.1016/S1004-9541(13)60524-8

Recycling and Reuse of Ionic Liquid in Homogeneous Cellulose Acetylation* HUANG Kelin (黄科林)1,2, WU Rui (吴睿)2, CAO Yan (曹妍)3, LI Huiquan (李会泉)3,** and WANG Jinshu (王金淑)1 1 2 3

School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China Guangxi Branch of China Academy of Science and Technology Development, Nanning 530022, China Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

Abstract Molecular distillation was used to recover ionic liquid (IL) 1-allyl-3-methylimidazolium chloride (AmimCl) in homogeneous cellulose acetylation. The five factors that affect the separation efficiency of molecular distillation, namely, feed flow rate, distillation temperature, feed temperature, wiper rotating speed, and distillation pressure, are discussed. The optimal recovery condition was determined via orthogonal experiments using an OA9(34) design. The IL was recycled and reused 5 times in the homogeneous cellulose acetylation system under optimal conditions. The purity of recycled IL the 5th time reached 99.56%. FT-IR (Fourier transform infrared spectroscopy) and 1H NMR (nuclear magnetic resonance) spectroscopy showed that the structure of the recovered IL is not changed. This work proves that AmimCl has excellent reusability, and that molecular distillation is an effective method for recovering IL in homogeneous cellulose acetylation. Keywords cellulose, ionic liquid, molecular distillation, recycling

1

INTRODUCTION

Ionic liquid (IL), also known as room-temperature ionic liquid, melt below 100 °C and are purely composed of ions [1]. Considered as promising environmentfriendly solvents, IL have received significant attention for their low vapor pressure, good chemical and thermal stability, non-flammability, and their easy reusability and recyclability. They have been widely used in organic compound synthesis and extractive separation, as media for electrochemical reaction, and so on. Recently, some ILs, such as 1-allyl-3-methylimidazolium chloride (AmimCl) [2], 1-butyl-3-methylimidazolium chloride (BmimCl), and 1-ethyl-3-methylimidazolium acetate (Emim-Ac), have been used to dissolve cellulose and they have been found to be good media for homogeneous cellulose derivatization because of their good capacity for dissolving cellulose. Many cellulose esters, such as cellulose acetate (CA) [3, 4] and cellulose acetate butyrate (CAB) [5], have been successfully synthesized in IL. Industrially, cellulose acetates are often produced by reaction of cellulose with an excess of acetic anhydride in the presence of acid catalysts [6, 7]. Due to the heterogeneous nature of the reaction, it is impossible to synthesize partially substituted cellulose acetates directly. Thus, hydrolysis to the product of desired (average) degree of substitution (ds) and viscosity then follows. Compared with the above complex two-step, environmentally polluting heterogeneous process, the homogeneous process requires only one step, has a higher reaction efficiency without any catalyst, easier solvent recycling, and allows the structural control of the

products, among others. These features make it more competitive than the currently used heterogeneous methods. Although research on homogeneous cellulose acetylation using IL has progressed rapidly, the high cost of IL hinders their industrialization. One method to resolve this problem is large-scale IL production, which has already been realized. Another method is to effectively recycle and reuse IL, which is also necessary for environmental protection because of their potential toxicity and non-biodegradability. Many techniques and methods have been reported for the recycling of IL from waste liquid, such as liquidliquid extraction [8] and supercritical carbon dioxide extraction [9]; membrane extraction technology [10]; aqueous biphasic systems [11]; and distillation [12, 13]. However, these technologies have many limitations, such as cross-contamination, high costs, complex processes, high energy and time consumption, and so on. When AmimCl is used for cellulose acetylation, at present, the methods of recycling of which are generally common vacuum distillation, for the IL easily forms hydrogen bonds with water and acetic acid in the reaction system made the thorough removal of dissolved liquid impurities from the IL filtrate is very difficult. Consequently, finding other ways to recycle IL is necessary. Wiped film molecular distillation (MD) [14] is highly efficient and is widely used in the separation and purification of heat-sensitive organic substances. Compared with conventional vacuum distillation, MD is suitable for separating small quantities of impurities because the separation is based on the differences in the mean free path of gas molecules [15, 16].

Received 2012-03-07, accepted 2012-09-15. * Supported by the Major Project of the State Key Development Program for Basic Research of China (2009CB219901) and the National Natural Science Foundation of China (21006118). ** To whom correspondence should be addressed. E-mail: [email protected]

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Consequently, the crude feed can be separated extremely rapidly at much lower temperature and lower pressure. Considering the physical process for MD separation, the target components could be protected from contamination, especially natural plant extracts. However, there are few literatures reported for recycling of IL with it. In this study, MD was used to recover IL from homogeneous cellulose acetylation. The reuse of recovered IL is also discussed. 2 2.1

MATERIALS AND METHOD Materials and apparatus

Materials: AmimCl was synthesized as described in our previous work [17]. Acetic anhydride (purity 99.0%) and dimethyl sulfoxide (DMSO, purity 99.0%) were purchased from the Beijing Chemical Reagents Company and were of analytical grade. Microcrystalline cellulose (MCC), with a degree of polymerization of 200-340 was prepared by degradation of sugarcane bagasse cellulose as described in our previous work [18]. Apparatus: A Pope 2# wiped-film molecular still (Pope Scientific, USA) was used in this study. The schematic diagram of this instrument is shown in Fig. 1.

AmimCl solution, and the solution was stirred 4 h at 100 °C. Then, the CA solution was precipitated with a four fold volume of distilled water. The resulting mixture was separated by filtration. The solid residue was washed thrice with distilled water and dried in a vacuum oven at 80 °C for about 24 h. Then, the filtrate, which contains IL, water, and acetic acid, was collected for IL recovery [19, 20]. 2.2.2 Pretreatment of the IL filtrate Activated carbon (2%-5%, by mass) was added into the filtrate, and the solution was stirred for 1 h at 50 °C. After filtration, the decolorized mixture-solvent was placed into a rotary evaporator (IKA RV10, Germany) and dehydrated at 110 °C for 4 h to remove most of the water and obtain the pretreated IL (PRIL). 2.2.3 Molecular distillation for PRIL After decolorization and vacuum distillation, the PRIL was poured into the dosing vessel of a molecular still with the needle valve closed, and it was preheated to temperatures ranging from 30 °C to 90 °C. The cold trap was then filled with liquid nitrogen to protect the vacuum system, and the cooling water was simultaneously turned on. The vacuum pressure was regulated to 13.3 Pa to 133 Pa using the vacuum modulator valve of the rotary vane vacuum pump. Meanwhile, the compact thermostat was switched on, and the distillation temperature was set to between 70 °C and 110 °C. Then, the motor drive was switched on and adjusted to the proper rotation speed (from 1 ml·min−1 to 6 ml·min−1) with the needle valve switched on. The feed was distilled when it flowed into the hot zone, the volatile impurities were distilled as distillate, and the recovered IL (RIL) remained as residue. After distillation, the RIL was weighed and ready for reuse. 2.3

Figure 1 Diagram of the Pope 2# short-path still 1—rotary vane vacuum pump; 2—vacuum modulator valve; 3—exhaust-valve; 4—cold trap; 5, 11—condenser; 6—motor drive; 7—dosing vessel; 8—needle regulator valve; 9—heating jacket; 10—wiper; 12, 14—distillate receiver; 13—residue receiver; 15, 17—cooling water inlet; 16, 18—cooling water outlet

2.2

Cellulose acetylation and IL recycling

2.2.1 Cellulose acetylation Before use, the MCC sample dried for 3 h to 5 h at 70 °C in a vacuum oven. Then, 20 g of cellulose (8% based on mass of IL) was dispersed in 250 g of AmimCl and the mixture was heated with mechanical stirring at 80 °C for 4 h. Finally, a clear and viscous cellulose solution was obtained. Then, the temperature was increased to 100 °C and acetic anhydride, with a molar ratio of 8︰1 [acetic anhydride/anhydro-glucose units (AGU)], was carefully added to the cellulose/

RIL reuse

The RIL was used in cellulose acetylation as described in Section 2.2.1, and then recycled as in Sections 2.2.2 and 2.2.3. The reuse-recycling process was repeated five times. 2.4

Characterization

2.4.1 Fourier transform IR (FTIR) The CA prepared using both fresh (FIL) and recycled IL were ground into powder, and dried under a vacuum for 24 h. The IR spectra of the samples were recorded with an FTIR spectrometer (Perkin-Elmer spectrometer GXII, USA). The test specimens were prepared via the KBr-disk method. 2.4.2 1H NMR The structures of IL and CA were determined using 1 H NMR spectroscopy (Bruker AV-400 or DMX-300 spectrometer, at ambient temperature) after the samples were dissolved in D2O and DMSO-d6. A drop of deuterated trifluoroacetic acid was added into the CAs/DMSO-d6 solution to shift the active hydrogen to

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3

the low field area. The degree of substitution of the CA (ds) was calculated according to Goodlerttl et al. [21]: ds =

I CH3 × 7

3.1

(1)

I AGU × 3

2.4.3 RIL purity The Mohr method [22] was adopted to measure the purity of the recycled IL, which was calculated as

VAgNO3 × 10−3 × CAgNO3 × 158.63 mIL

× 100%

(2)

where VAgNO3 is the volume AgNO3 solution used, CAgNO3 is the AgNO3 concentration, and m is the mass of the sample. 2.4.4 Acetic acid content of the recycled RIL The GB/T 1628-2008 [23] was adopted to measure the acetic acid content of the recycled IL: CHAc =

VNaOH × 10−3 × CNaOH × 60.05 × 100% mIL

(3)

where VNaOH is the volume of NaOH solution used in titration, CNaOH is the NaOH concentration, and mIL is the mass of the sample. 2.4.5 Water content of the RIL The water content in RIL was measured using Karl Fischer titration (Metrohm 787KF, Switzerland).

3.2 Levels of single factor experiments for molecular distillation

2.4.6 RIL viscosity and density The viscosity and density of the RIL was measured with a viscometer (Anton Paar AMVN, Austria) and a densitometer (Anton Paar DMA5000, Austria), respectively.

Five factors were proven to greatly affect the separation efficiency of MD, feed flow rate, distillation temperature, feed temperature, wiper rotating speed, and distillation pressure. The influence of these factors on the recycling of IL through MD is discussed in detail below.

2.4.7 Viscosity of CA Based on the standard [24], the viscosity of CA was measured with an Ubbelohde viscosity meter and calculated as t −t η= s 0 (4) t0CCA

3.2.1 Effect of feed flow rate The feed flow rate affects the residence time of raw materials on the evaporation surface, which is crucial to the separation. The slower the feed flow rate, the longer the time the raw materials stay on the evaporation surface, thus, the higher the separation efficiency. As Fig. 2 shows, the purity of RIL increased with decreasing feed flow rate, and the increase was initially

where η is the viscosity of CA, ts is the exudation time of the CA solution, t0 is the exudation time of the solvent, and CCA is the CA concentration. Table 1

The composition of the RIL before and after pretreatment

IL

HAc

Filtrates

PRIL

Filtrates

Mass/g Percentage/%

Mass/g Percentage/%

Mass/g Percentage/%

50

19.3

Pretreatment of IL

At the end of cellulose acetylation, the filtrate was first decolorized with activated carbon. Considering plenty of water was used during the washing of CA, water accounted for most of the filtrate, which also contained small amounts of HAc generated during the acetylation. The conventional vacuum distillation and MD techniques were combined during the treatment of the filtrate to save energy and to maximize the advantages of MD. In other words, most of the water was initially removed using conventional vacuum distillation pretreatment, and then the small amounts of HAc and water [less than 10% (by mass) in total] that were difficult to remove were disposed through molecular distillation. The composition of the RIL before and after pretreatment is listed in Table 1. After vacuum distillation pretreatment for 4 h at 110 °C, the purity of the IL quickly increased from 19.3% to 92.8%, and the mass of water in the PRIL dropped sharply from 200 g to 1.88 g, and that of acetic acid from 9.6 g to 2.0 g. However, as the distillation progressed, the increase in IL purity became increasingly slower. A water content exceeding 1% seriously blocks the dissolution of cellulose in IL and consumes the acetic anhydride used during acetylation. Simultaneously, the acid exit in IL led to some cellulose degradation. Therefore, a more efficient treatment for removing the water and acetic acid as thoroughly as possible is imperative.

where I CH3 is the peak integral of the methyl protons of the acetyl moiety, and I AGU is the peak integral of all the protons in the anhydroglucose unit.

CIL =

RESULTS AND DISCUSSION

50

92.8

9.6

3.7

H2O PRIL

Filtrates

Mass/g Percentage/% Mass/g Percentage/% 2.0

3.7

200

77.0

PRIL Mass/g Percentage/% 1.88

3.5

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Chin. J. Chem. Eng., Vol. 21, No. 5, May 2013

Figure 2 Effect of feed flow rate on the purity of RILs at distillation temperature of 70 °C, feed temperature of 50 °C, wiper speed of 275 r·min−1, and pressure of 133 Pa

fast, and then becomes slower. Although the purity continued to increase with the flow rate below 1 ml·min−1, the distillation efficiency was low. Hence, the appropriate feed flow rate was from 1 ml·min−1 to 3 ml·min−1.

Figure 4 Effect of feed temperature on the purity of RIL at a feed flow rate of 1 ml·min−1, distillation temperature of 90 °C, wiper speed of 275 r·min−1, and pressure of 133 Pa

the RIL increased only slightly from 98.3% to 98.8%, illustrating the minor influence on the separation. To avoid energy loss at higher temperature, the feed temperature was maintained at 80 °C.

3.2.2 Effect of distillation temperature Based on the mean free path theory of gas molecules [15], the mean free path is inversely proportional to pressure, and directly proportional to temperature, so which increased with increasing distillation temperature, thereby increasing the separation efficiency. Fig. 3 shows the effect of distillation temperature on the purity of IL. The purity of the IL increased with distillation temperature, from 60 °C to 80 °C, an almost linear relationship was observed between IL purity and distillation temperature. When the temperature was higher than 80 °C, the increment in IL purity decreased. Moreover, when the temperature exceeds 100 °C with higher vacuum, the color of RIL would darkened slightly, Hence, the appropriate distillation temperature was between 80 °C and 100 °C.

3.2.4 Effect of wiper rotating speed The wiper rotation speed affects the heat and mass transfer of materials on the internal surface of the evaporator. The higher the speed of the wiper, the thinner is the liquid film; thus, the higher the separation capacity. Fig. 5 shows that the purity of the recycled IL increased with rotation speed at gradually decreased rate. This finding indicates that the distillation was more rapid at lower rotation speeds. However, at rotation speed exceeding 440 r·min−1, the raw materials would splash onto the condensing surface, which would lead to the loss of more desired components. Hence, the appropriate rotation speed was from 385 r·min−1 to 440 r·min−1.

Figure 3 Effect of distillation temperature on the purity of RIL at a feed flow rate of 1 ml·min−1, feed temperature of 50 °C, wiper speed of 275 r·min−1, and pressure of 133 Pa

Figure 5 Effect of rotation speed on the purity of RIL at a feed flow rate of 1 ml·min−1, distillation temperature of 90 °C, feed temperature of 80 °C, and pressure of 133 Pa

3.2.3 Effect of feed temperature Feed temperature affects the viscosity of raw materials, and therefore the separation efficiency. Hence, the effect of feed temperature on molecular distillation was studied and Fig. 4 shows that as the feed temperature increased from 30 °C to 90 °C, the purity of

3.2.5 Effect of distillation pressure As mentioned above, the mean free path is inversely proportional to the distillation pressure. The stronger the vacuum, the larger the difference between mean free paths of species. Fig. 6 shows that the purity of the RIL increased clearly with decreasing distillation

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Purity of RIL in OA9 (34) and range analysis

Table 3

Factors Trial No. Feed flow Temperature Pressure Rotating rate (A) (B) (C) speed (D)

Figure 6 Effect of distillation pressure on the purity of RIL at a feed flow rate of 1 ml·min−1, distillation temperature of 90 °C, feed temperature of 80 °C, and wiper speed of 385 r·min−1

pressure. When the pressure was decreased to 133 Pa, the purity increased sharply to 98.80%. The further reduction in pressure to 13.3 Pa resulted in a slight increase in purity to 99.40%. Hence, the appropriate distillation pressure chosen was within from 13.3 Pa to 133 Pa. 3.3 Orthogonal test design and analysis of molecular distillation

Based on the single factor experiments discussed above, the conditions for the separation using MD was optimized through orthogonal array (OA) experiments 4 [25]. An OA9 (3 ) matrix, an OA with four factors and three levels, was employed, and the considered factors and levels were assigned as shown in Table 2. Table 2

The factors and levels affecting IL recovery Factor

Level Feed flow rate (A)/ml·min−1

Temperature (B)/°C

Pressure (C)/Pa

Rotating speed (D)/r·min−1

1

3

85

13.3

385

2

2

90

66.5

413

3

1

95

133.0

440

Nine experimental run were carried out according to the OA9 matrix. Then, a data analysis was carried out through range analysis to determine the optimal reaction conditions and their magnitudes, as shown in Table 3. As shown in the range analysis in Table 3, Ki is the sum of the evaluation indices (i = 1, 2, 3) of all the levels for each of the factors, and ki (mean value of Ki) is used to determine the optimal level and the optimal combination of factors. The optimal level for each factor was the level obtained when ki was highest. Range (Rj) is defined as the range between the maximum and minimum values of ki and it is used to evaluate the importance of the factors. The results show that the level of importance of the factors was as

Results Purity of IL /% (by mass)

1

1

1

1

1

99.10

2

1

2

2

2

98.25

3

1

3

3

3

98.15

4

2

1

2

3

98.65

5

2

2

3

1

97.60

6

2

3

1

2

99.58

7

3

1

3

2

96.70

8

3

2

1

3

99.70 99.36

9

3

3

2

1

K1

295.50

294.45

298.38

296.06

K2

295.83

295.55

296.26

294.53

K3

295.76

297.09

292.45

296.50

k1

98.500

98.150

99.460

98.687

k2

98.610

98.517

98.753

98.177

k3

98.587

99.030

97.483

98.833

Rj

0.110

0.880

1.977

0.656

follows: system pressure (1.977)>distillation temperature (0.880)>wiper rotating speed (0.656)>feeding rate (0.110). The RC value of the distillation pressure was much larger than that of the other three factors, which indicates that distillation pressure has the biggest influence on the purity of the recycled IL, and a small change in the distillation pressure produces significant changes in RIL purity. However, the RIL purity also changed slightly with the changes in feeding flow rate because the RC range of the reaction time (RA) was so small. The optimum conditions for the separation process using MD was used to recover IL in homogeneous cellulose acetylation was determined as follows: distillation pressure, 13.3 Pa, distillation temperature, 95 °C; feeding rate, 1 ml·min−1; and wiper rotation speed, 440 r·min−1. 3.4

The recycling and reuse of RIL

The RIL was recycled five times in cellulose acetylation under the optimum conditions obtained through the orthogonal array experiments above. The properties of the RIL and CA obtained from the first and the fifth cycles were characterized and compared. 3.4.1 Properties of RIL As shown in Table 4, with the first and the fifth RIL, the purity of the RIL recycled five times (5th RIL) still reached as high as FIL, the viscosity and densities both were similar to that of the fresh one. Moreover, as shown in Fig. 7, the colors of the RIL (faint yellow or yellow) were close to that of the FIL. However, the

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Table 4

The properties of RIL Content/%

Color

Viscosity /mPa·s−1

Density /kg·cm−1

IL

HAc

H2O

FIL

faint yellow

1179

1.144

100

0

0

1st RIL

yellow

1135

1.143

99.60

0.363

0.05

5th RIL

yellow

1100

1.142

99.56

0.400

0.05

FIL 1st RIL 5th RIL Figure 7 Color of FIL and RIL used for CA preparation

color darkened slightly as the number of cycles increased. These results indicate that the physical properties of the recycled IL remained unchanged. 3.4.2 FTIR spectra of RIL The IR spectra of the FIL and RIL are characterized and compared in Fig. 8, in Spectra 2 and 3. The absorbance peaks were observed at 1715 cm−1 for C O of residual acetic acid, this indicates that small amounts of acetic acid remained in the RIL, apart from that, all other absorbance peaks of the RIL were consistent with the FIL, and no other byproducts were generated. 3.4.3 1H NMR spectra of RILs The purity of the RIL was also confirmed through 1 H NMR spectroscopy, as shown in Fig. 9. The 1H NMR

Figure 8 FTIR spectra of FIL and RIL used in the preparation of CA 1—FIL; 2—1st RIL; 3—5th RIL

spectra of the RIL (Spectra 2 and 3) was consistent with the RIL, no other impurities were detected except acetic acid residues with methyl protons at δ = 2.0. 3.4.4 The physical properties of RIL-CA The physical properties of the CA prepared using the 1st RIL (CA-1st RIL) and the 5th RIL (CA-5th RIL) were compared with that of the CA prepared using fresh IL (CA-FIL), as presented in Table 5 and discussed below. The degree of substitution (ds) and viscosity (η) are two important parameters for evaluating the quality of CA. Hence, ds and η of the CA prepared using FIL and the RIL were measured and calculated as described in Section 2.4. As shown in Table 5, ds and η of CA-1st RIL and CA-5th RIL are close to that of CA-FIL. Moreover, as shown in Fig. 10, the appearance

Figure 9 1H NMR spectra of FIL and RIL used in the preparation of CA 1—FIL; 2—1st RIL; 3—5th RIL

Chin. J. Chem. Eng., Vol. 21, No. 5, May 2013

Table 5

The properties of CA prepared using FIL and RIL

Sample

ds

η

CA-FIL

2.85

43.62

CA-1st RIL

2.80

43.68

CA-5th RIL

2.85

43.55

583

3.4.6 1H NMR spectra of RIL-CA The 1H NMR spectra of the CA prepared using RIL and FIL are shown in Fig. 12 and described as follows: δ = 2.0 (methyl protons of acetyl group, 3H), δ = 2.8-5.9 (protons of cellulose backbone, 7H). No other peaks were generated by the byproducts in the 1 H NMR spectra of CA prepared using RIL, which confirmed that the CA was consistent with that prepared using FIL.

of the CA-RIL were also consistent with that of CA-FIL, although the color darkened slightly as the number of cycles increased, their colors also corresponded to the standard color, white or grayish-white.

CA-FIL CA-1st RIL CA-5th RIL Figure 10 External appearance of the CA prepared using FIL and RIL

In summary, the physical properties of the CA prepared using RIL recycled multiple times were consistent and similar to the properties of the CA prepared using the FIL. 3.4.5 FTIR spectra of RIL-CA The IR spectra of the CA prepared using RIL and FIL were characterized and compared. As shown in Fig. 11, two important ester bonds at 1757 cm−1 (C O) and 1235 cm−1 ( CO stretching of acetyl group) indicate that CA was obtained. No other peaks were generated by the byproducts seen in the spectra of CA obtained using RIL, which indicates that the macroscopic structure of the CA obtained using RIL were the same as that obtained using FIL.

Figure 12 1H NMR of CA prepared by FIL and RIL 1—CA-FIL; 2—CA-1st RIL; 3—CA-5th RIL

4

CONCLUSIONS

In this paper, wiped-film molecular distillation was used to recycle AmimCl used in homogeneous cellulose acetylation. Orthogonal array experiments [OA9 (34)] determined that the order of importance of the parameters that affect MD was as follows: system pressure>distillation temperature>rotation speed>feeding rate. The optimal level for each factor was determined as follows: distillation pressure, 13.3 Pa; distillation temperature, 95 °C; feeding rate, 1 ml·min−1; rotation speed, 440 r·min−1; and feed temperature, 80 °C. AmimCl was effectively recycled and reused five times under the optimal conditions, and the purity of the RIL reached up to 99.5% after being recycled five times. Thus, these results indicate that the IL used in cellulose acetylation has exceptional reusability, and molecular distillation is an effective method for recovering IL in homogeneous cellulose acetylation. NOMENCLATURE

Figure 11 FTIR spectra of CA prepared using FIL and RIL 1—CA-FIL; 2—CA-1st RIL; 3—CA-5th RIL

CAgNO3 CCA CIL CNaOH I CH3 IAGU mIL P T ts

concentration of AgNO3, mol·L−1 concentration of cellulose acetate, mol·L−1 purity of recycled IL, % concentration of NaOH used in titration, kg·L−1 the peak integral of methyl protons of acetyl moiety the peak integral of all protons of anhydroglucose unit mass of IL, g distillation pressure, Pa distillation temperature, °C exude time of cellulose acetate solution, s

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t0 VAgNO3 VNaOH δ

η

exude time of solvent, s volume of used AgNO3, L volume of NaOH used in titration, L chemical shift viscosity of cellulose acetate

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