ARTICLE pubs.acs.org/IECR
Homogeneous Synthesis and Characterization of Cellulose Acetate Butyrate (CAB) in 1-Allyl-3-Methylimidazolium Chloride (AmimCl) Ionic Liquid Yan Cao,† Huiquan Li,*,† and Jun Zhang*,‡ † ‡
Key Laboratory of Green Process Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China 100190 Key Laboratory of Engineering Plastics (KLEP), Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China 100190 ABSTRACT: Cellulose acetate butyrate (CAB) with butyryl content of 647 wt % was homogeneously synthesized in 1-allyl-3methylimidazolium chloride (AmimCl) in a single step without using any catalysts. The effects of different acylating agents (acetic anhydride and butyric anhydride) addition methods and reaction conditions on the acyl content of CABs were investigated. Synthesized CABs were characterized by FTIR, NMR, solubility, and thermal analysis. The acylating agents addition method influences the butyryl content, substituent distribution within the anhydroglucose units (AGU), and the properties of CAB. The CAB obtained with butyryl content greater than 38% is completely soluble in 2-methylethyl ketone, 1,2-dichloromethane, ethyl acetate, and butyl acetate. After the mix-acylation, AmimCl can be easily recycled and reused. This study provides a novel way for the clean production of CAB under mild conditions for future industrial applications.
1. INTRODUCTION Cellulose esters are some of the most commercially important cellulose derivatives. The annual world production of cellulose esters can reach as high as 9 105 ton.1 Among these cellulose esters, cellulose acetate (CA) is the most highly produced because of its applications in coating, film, membrane, textile, and cigarettes.2 Although CA shows very good physical properties such as burning resistance, definition, and tenacity, CA with DS value from 0 to 3 is not water resistant and has a poor solubility in some common solvents, such as 2-methyl-ethyl ketone, 1,2-dichloromethane, ethyl acetate, and butyl acetate, which limit its application. To overcome these limitations of CA, studies were conducted to identify alternative cellulose esters, such as cellulose acetate butyrate (CAB). By introducing the butyryl group into CA, the mix-cellulose ester CAB does not only have the excellent properties of CA but also exhibits better solubility in organic solvents due to its hydrophobicity. Further, CAB shows notable characteristics in flexibility, weatherability, and light and cold resistance that makes it widely used in the paint industry for top grade cars and printing inks.3 The emergence of CAB provides a possibility for further improving the properties of cellulose esters. The strong inter- and intramolecular hydrogen bonding network of cellulose causes its poor solubility in common solvents. Achieving a homogeneous solution of cellulose without using harsh conditions is often difficult. Thus, commercial CAB is mainly produced by acylating cellulose with excess acylation reagent in the presence of an acid catalyst to a degree of substitution (DS) of 3.0 under heterogeneous system. Partial hydrolysis to the product of desired (average) DS and viscosity then follows the acylation.4 However, this process often lowers the degree of polymerization (DP), results in the formation of different hydrogen-bonding patterns, affects the reactivity of the cellulose, and often leads to nonuniform DS values because of r 2011 American Chemical Society
heterogeneous modification reactions. On the other hand, the heterogeneous process is very complicated and energy-consuming, and the use of excessive acid leads to serious environmental problems. To overcome problems concerning the synthesis of CAB under heterogeneous conditions, the production of CAB under homogeneous conditions is demonstrated. In the latter condition, cellulose becomes completely soluble in the solvent. By employing a homogeneous reaction, the whole cellulose chain is accessible for derivatization, and many of the problems associated with the two-phase (industrial) reactions are avoided.5 The main advantages of acylation in the homogeneous phase are excellent control of the DS values and a uniform distribution of the functional groups along the polymer chains. Moreover, selectivity of the functionalization reaction within anhydroglucose units (AGU) may appear. The first attempts of the chemical modification of cellulose under totally homogeneous condition are the esterification of cellulose in DMAc/LiCl using carbonic acid anhydride and chlorides as the acylating reagent.68 Even now, other cellulose solvents such as 1,3-dimethyl-2-imidazolidinone (DMI)/LiCl,9 dimethyl sulfoxide (DMSO)/tetrabutylammonium fluoride trihydrate (TBAF),10,11 and some molten salt hydrate12,13 have also been employed on homogeneous cellulose esterification. These solvent systems, however, have some drawbacks, such as expensive reagents, complex cellulose dissolution process, and difficult solvent recycling. Furthermore, most publications focused on monosubstituted cellulose esters. The mix-substituted cellulose esters CAB is only mentioned in the DMAc/LiCl solvent system Received: March 4, 2011 Accepted: May 14, 2011 Revised: May 14, 2011 Published: May 14, 2011 7808
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Industrial & Engineering Chemistry Research without a detailed synthesis and characterization data. The role of the reaction rule in homogeneous systems is also not quite clear.1416 Ionic liquids (ILs) are composed only of ions and are liquids at temperatures, T < 100 °C. ILs are an attractive alternative to conventional volatile organic solvents because of their nonvolatility and nonflammability, which is due to their negligible vapor pressure at ambient temperature. Further, an ionic liquid can be recycled and reused because of their immiscibility with a range of organic solvents. Rogers et al. found that the chloride-containing imidazole IL, such as 1-butyl-3-methylimidazolium chloride (BmimCl), showed good solubility for cellulose.17 Many ionic liquids can dissolve cellulose; examples of these include 1-allyl-3methylimidazolium chloride (AmimCl),1821 1-(2-hydroxylethyl)-3-methyl imidazolium chloride (HemimCl),22 1-ethyl-3methylimidazolium acetate (EmimAc),18 1-allyl-3-(1-butyl)imidazolium chloride (AbimCl),23,24 and so on. Wu et al. first employed IL in homogeneous cellulose derivatization. AmimCl was successfully utilized as reaction media in synthesizing CA in one step without using any catalyst, the results of which appear very promising.25 Ionic liquids have been widely used for cellulose dissolution and homogeneous derivatization in preparing many kinds of cellulose esters.2629 Considering the disadvantage of the heterogeneous synthesis of CAB in the industry and the predominance of using IL for homogeneous cellulose modification, this study explored new paths for the preparation of CAB. In this work, the homogeneous mix-acylation of cellulose in ionic liquid AmimCl was studied. Three methods for the addition of acylating agents (acetic anhydride and butyric anhydride) were chosen. Various parameters of acylation, such as reaction time, temperature, and the molar ratio of the acetic (and butyric) anhydride/AGU in cellulose were investigated. As a result, samples with butyryl content ranging from 6 to about 47 wt % were obtained in one step. The structure and properties of acylated cellulose, such as the substituent distribution, solubility, and thermal properties, were characterized.
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anhydride was added to the solution simultaneously under vigorous stirring and react for required times (t1). 2.4.2. Adding Acetic Anhydride First and Butyric Anhydride after (Method B, Samples B1B4). The MC/AmimCl solution was held at the desired temperature for 30 min. The required amount of acetic anhydride was added to the solution first. A few hours later (t1), butyric anhydride was carefully added under vigorous stirring and reacted for the required times (t2). 2.4.3. Addition of Butyric Anhydride and Then Acetic Anhydride (Method C, Sample C1C4). The MC/AmimCl solution was heated to the desired temperature and held for 30 min. The required amount of butyric anhydride was added to the solution first. A few hours later (t1), acetic anhydride was carefully added under vigorous stirring and reacted for the required times (t2). Each resulting product obtained from Methods A, B, and C was then precipitated in a 5-fold amount of methanol and filtered to separate the precipitate. The solid was washed 3 times with methanol and dried in vacuum at 80 °C for 24 h. To recycle IL, the collected filtrate was distilled under reduced pressure to remove water, acetic acid, and butyric acid. The recovered IL was dried in vacuum oven. 2.5. FTIR Spectrometry. The underivatized cellulose and CABs were grounded into powder and dried in vacuum for 24 h, and the KBr-disk specimens were prepared. The IR spectra of the samples were recorded using a Fourier Transform IR spectrometer (FTIR PE-2000, United States). 2.6. NMR. The DS of CABs was determined by 1H NMR spectroscopy (Bruker AV-400 or DMX-300 spectrometer at ambient temperature) after dissolving the samples in DMSOd6 containing a drop of deuterated trifluoroacetic acid, whose function is to shift the active hydrogen to low-field area. The DS of CABs was calculated using the following formulas:
2. MATERIAL AND METHODS 2.1. Materials. The cellulose sample used is microcrystalline cellulose (MC) with DP of 200. CAB 381-0.5 (38% butyryl, 13.5% acetyl content) and CAB551-0.2 (51% butyryl, 2% acetyl content) were purchased from Eastman Kodak products from TianJin DJ International Trading Co. Ltd. The other chemical reagents were purchased from commercial resources in China and were used as received. 2.2. Synthesis of AmimCl. 1-Methylimidazole (400 mL) and allyl chloride (515 mL) at a molar ratio of 1:1.25 were reacted for 8 h at 55 °C in a round-bottom flask fitted with a reflux condenser with stirring. Unreacted chemical reagents and water were removed by vacuum distillation, to obtain AmimCl, which is a liquid of light amber color. 2.3. Dissolution of Cellulose in AmimCl. The cellulose sample was dried at 70 °C for 35 h in vacuum oven prior to use. The cellulose was then dispersed in AmimCl and heated at 80 °C for 1 h under vigorous mechanical stirring. Finally, a clear and viscous cellulose solution was obtained. 2.4. Mix-Acylation of Cellulose in AmimCl. 2.4.1. Acetic Anhydride and Butyric Anhydride Added Simultaneously to the Cellulose/AmimCl Solution (Method A, Samples A1A7). The MC/AmimCl solution was hold at desired temperature for 30 min. The required amount of acetic anhydride and butyric
DSA ¼
ðICH3 ÞA 7 IAGU 3
ð1Þ
DSB ¼
ðICH3 ÞB 7 IAGU 3
ð2Þ
DSTotal ¼ DSA þ DSB
ð3Þ
where DSA, DSB, and DSTotal are the DS of the acetyl, butyryl, and acyl groups, respectively. (ICH3)A, (ICH3)B, and IAGU are the peak integral of the methyl protons of the acetyl moiety, the methyl protons of the butyryl moiety, and all protons of the AGU. Seven is the amount of protons on the AGU; 3 is the amount of protons on the methyl. The acetyl (A) and butyryl (B) content of CAB could be defined as Að%Þ ¼
DSA 43 100% ð4Þ 162 DSTotal þ DSA 43 þ DSB 71
Bð%Þ ¼
DSB 71 100% ð5Þ 162 DSTotal þ DSA 43 þ DSB 71
where 43, 71, and 162 are the molecular weight of acetyl group (CH3CO), butyryl group (CH3(CH2)2CO), and the AGU (C6H10O5), respectively. The substituent distribution of CAB was determined by 13C NMR spectroscopy (Bruker AV-400 spectrometer at ambient temperature, overnight) after dissolving the samples in DMSO-d6. 7809
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Table 1. Homogeneous Synthesis of CAB in AmimCl (Conditions and Results)
a
DSA
A (%)
DSB
B (%)
DSTotal
1
0.68
14
0.32
11
1.00
3
0.96
17
0.56
16
1.52
5
5
1.25
20
0.69
18
1.94
5
3
3
1.10
19
0.54
15
1.64
80
5
7
3
0.92
16
0.63
18
1.55
A6
80
5
9
3
0.77
14
0.67
20
1.44
A7
100
5
5
3
1.48
22
0.91
22
2.39
A8a B1
100 80
5 5
5 5
3 1
3
1.52 1.80
22 31
0.94 0.20
23 6
2.46 2.00
B2
100
5
5
1
5
2.36
36
0.27
7
2.63
B3
100
5
5
1
3
1.93
32
0.26
7
2.19
B4
100
3
9
1
5
1.62
26
0.57
15
2.19
C1
100
5
5
3
0.5
0.73
10
1.63
38
2.36
C2
100
5
5
3
1
0.48
7
1.69
40
2.17
C3
100
5
5
3
3
0.27
4
2.15
47
2.42
C4 CB
100 100
5 0
5 5
3 0
5 3
0.27 0
4 0
2.11 1.84
47 45
2.38 1.84
CA
100
5
0
0
3
2.37
39
0
0
2.37
sample
temp (°C)
acetic anhydride/AGU
butyric anhydride/AGU
t1 (h)
A1
80
5
5
A2
80
5
5
A3
80
5
A4
80
A5
t2 (h)
Using recycled ionic liquid AmimCl as reaction medium.
2.7. Thermal Analysis. The differential scanning calorimetry (DSC) analysis of CAB was conducted on a PE DSC-7. To provide the same thermal history, each sample was heated from 50 to 250 at 20 °C/min, and all the Tg reported are observed in the second scan. The thermogravimetric analysis (TGA) was performed on a PE TGA-7 from 50 to 600 °C for all the samples, keeping a constant heating rate at 20 °C/min. All measurements were made under nitrogen atmosphere.
3. RESULTS AND DISCUSSION 3.1. Influence of the Reaction Conditions and Method of Addition of Acylating Agents on the Acyl Content of CAB.
Reaction time, temperature, and molar ratio of reagent were investigated, and the results are listed in Table 1. The DS value of CAB is calculated by 1H NMR as shown in Figure 2, and the detailed peaks are described as δ = 1.0 ppm (methyl proton of butyryl); δ = 1.5 and 2.2 ppm (methylene proton of butyryl); δ = 2.0 ppm (methyl proton of acetyl); δ = 35.5 ppm (proton of cellulose backbone). As seen from the table, both acetyl content and butyryl content increased with the reaction temperature (comparing sample A2 with A7, B1 with B3). In addition, the molar ratio of the two acylating agents to AGU is also important in the reaction. Increasing the ratio of butyric anhydride/AGU gives more butyryl content but less acetyl content. For example, at a reaction time of 3 h, temperature of 80 °C, molar ratio of acetic anhydride/AGU fixed at 5, and increasing molar ratio of butyric anhydride/AGU from 3 (sample A4) to 5 (sample A2), the butyryl content slightly increased from 15% to 16%, whereas the acetyl content decreased from 19% to 17%. When the molar ratio of butyric anhydride/AGU is increased to 7 (sample A5) and even to 9 (sample A6), the butyryl content increased to 18% and 20%, and the acetyl content continued to decrease to 16% and then 14%, respectively.
Figure 1. FTIR spectra of CB and CAB, samples B1, A2, and C2, and Eastman CAB 381-0.5 in Table 1.
Reaction time also plays an important role in the cellulose mixacylation. Under method A, increasing the reaction time t1 will result in both increasing of the acetyl content and the butyryl content. For example, keep other conditions unchanged, increasing t1 from 1 h (A1) to 3 h (A2) and then to 5 h (A3), the acetyl content increased from 14% to 17% and then to 20%, and the butyryl content increased from 11% to 16% and then to 18%, respectively. However, increasing the reaction time of t2 will lead to an increment of butyryl content but the decrement of acetyl content under method C. For instance, with the increase in the reaction time t2 from 0.5 h to 1 h, and then to 3h (sample C1 to C3), the DSA decreased (from 10% to 7% and then to 4%), whereas DSB increased (from 38% to 40% and then to 47%). Increasing the time to 5 h (sample C5), acetyl content (4%) and butyryl content (47%) did not significantly change, which indicates a nearly complete functionalization of all the hydroxyl groups. These may be explained by the strong stereohindrance effect between 7810
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Figure 2. 1H NMR spectrum of the CAB sample C4.
butyric anhydride and acetic anhydride. As there are two acid anhydride exits in the solution, they will compete for the hydroxyl group of cellulose. If increasing the reaction time or addition of one acid anhydride, the substituent of it will undoubtedly increase. However, it will certainly block the reaction of cellulose with the other acid anhydride thus resulting in the relatively lower substituent of the other acid anhydride. Furthermore, the composition of the cellulose mixed ester can be controlled by the order of introduction of the acylating agents. Methods A and B resulted in a relatively low butyryl content of CAB (11%23% for Method A; 6%15% for Method B). The smaller stereohindrance in acetic anhydride, which makes it easier to react with most of the hydroxyl groups in cellulose, exists in the cellulose/IL solution as the cellulose reacts with butyric anhydride for these two methods. In Method C, adding butyric anhydride first and making it react fully with cellulose in the absence of acetic anhydride, the butyryl content suddenly increased to 38%47%, which is close to the commercial CAB with the butyryl content in the range of 3252%.3 3.2. FTIR Spectra. The FTIR spectra of CB, the three CAB samples synthesized under different methods of addition of the acylating agents with different butyryl content (samples A2, B1, and C2), and the Eastman CAB 381-0.5 are given in Figure 1. All the cellulose esters show a single peak at 1745 cm1 (CdO ester) for the two acyl moieties. As the total DS value of CAB increased, the ester peak intensity increased, whereas the OH stretching band at 3496 cm1 decreased. Further, sample B1 does not show the characteristic peak at 1309 and 1420 cm1 (CH2 stretching in butyryl group) due to its low butyryl content (6 wt %). When the butyryl content increased to 16% (sample A2), these two peaks began to emerge and became evident as the butyryl content increased to 40% (sample C2 and Eastman CAB 381-0.5). 3.3. 13C NMR Spectra. In this work, 13C NMR analysis was used to determine not only the molecular structure of CAB but also the distribution of the substituent in the AGU. The full-range 13 C NMR spectra are listed in Figure 3a, in which δ = 50 110 ppm is assigned to the signal of the AGU carbonate region.19,21,30,31 An expanded 13C NMR AGU carbonate region of CAB samples is shown in Figure 3b. The peak at 62.8 ppm is attributed to C6 carbons bearing a substituted hydroxyl group. Peaks at 80.1 and 102.5 ppm are assigned to C4 and C1 carbon bearing an unsubstituted hydroxyl group, respectively. The peak
Figure 3. (a) 13C NMR spectrum of CAB: samples A2, B4, and C2 and Eastman CAB 381-0.5 in Table 1. (b) The AGU carbonate region in (a). (c) The carbonyl carbonate region in (a).
around 99.5 ppm designated as C10 is assigned to C1 carbons adjacent to C2 carbons bearing a substituted hydroxyl group and a peak around 76.0 ppm designated as C40 is assigned to C4 carbons adjacent to C3 carbons bearing a substituted hydroxyl group. The resonance peaks of C2, C3, and C5 carbons in both of the four samples heavily overlap, and a strong cluster around 70 to 75 ppm in the case of a high DS sample is observed. All of the three synthesized CAB samples show signals for C1 and C4 that 7811
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Figure 4. Distribution of substituent in CAB samples.
are different from the commercial CAB 381-0.5. This indicates that the hydroxyl groups in the C2 and C3 positions are partially substituted for the homogeneously synthesized CAB, which is nearly fully substituted in the Eastman CAB 381-0.5 synthesized heterogeneously. δ = 165175 ppm are assigned to the signal of the carbonyl carbon region. Clearly seen from the expanded Figure 3c, δ = 171174 ppm are attributed to the butyryl carbonyl carbon region (marked as “c”), in which the three peaks are assigned to the c-C6 (173.0 ppm), c-C3 (172.2 ppm), and c-C2 (171.7 ppm) positions in AGU. δ = 168171 ppm are attributed to the acetyl carbonyl carbon region (marked as “a”), in which the three peaks are assigned to the a-C6 (170.6 ppm), a-C3 (169.7 ppm), and a-C2 (169.3 ppm) positions in AGU. The substituent distribution of the homogeneously synthesized CAB under the three different addition methods and the commercially available CAB are quite different from each other. The partial DS values of the acyl moiety among the three OH groups were calculated from the integration of the carbonyl carbon area of the 13C NMR spectra. The three hydroxyl groups at C2, C3, and C6 positions exhibit different reaction activities, and the results are presented in Figure 4. The substituent distribution for the acetyl and butyryl groups is C6A (0.63) > C3A (0.25) > C2A (0.08) and C6B (0.41) > C3B (0.09) > C2B (0.06) (sample A2), respectively, for method A. This can be due to the possible reactivity order of hydroxyl group on cellulose in the system of AmimCl being C6 > C3 > C2, which has been proven recently.19,21,25 For method B (sample B4), the substitution order is C6A (0.77) > C3A (0.51) > C2A (0.34), C3B (0.26) > C2B (0.21) > C6B (0.10), respectively. Because acetic anhydride, having small stereohindrance, reacted quickly with most of the highly active hydroxyl group of C6 in the AGU as soon as it was added into the homogeneous solution, thus, most of the butyric anhydride can only react with the relatively low reactivity but abundant hydroxyl group at the C2 and C3 positions. The acetyl group prevents the substitution of butyryl at the C6 position by reacting with it quickly and “protecting” it. And that the substituent distribution for sample C2 (method C) is C3A (0.19) > C2A (0.17) > C6A (0.12) and C6B (0.94) > C3B (0.48) > C2B (0.27), just the opposite with the sample B4 (method B) that the butyryl group “protects” the hydroxyl group of C6 from reacting with acetic anhydride this time. However, because of the whole heterogeneous process starts with the full substitution, followed by partial
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hydrolysis, the substituent distribution of CAB 381-0.5 is C3> C6 > C2 for both of the acetyl and butyryl group as well as the total partial substitution, unlike that of homogeneous synthesized CAB in our experiment, whose total partial substitution still satisfies the order of C6 Total > C3 Total > C2 Total as illustrated in Figure 4. 3.4. Solubility. For CAB, widely applicable solvents are needed. In the present work, the solubility of the synthesized CABs in 2-methyl-ethyl ketone, 1,2-dichloromethane, ethyl acetate, and butyl acetate was investigated with Eastman CAB 381-0.5 and CAB 551-0.2. The results are given in Table 2. In a typical dissolution process, about 1.0 g of CAB was used to be dissolved in 8.0 g of chosen solvent. The introduction of the butyryl group can effectively improve the solubility of CAB. For example, CABs with butyryl content less than 20% (sample CA, A1A6, B1B4) are insoluble in the four solvents. Sample A7 with butyryl contents of 22% began to dissolve in 2-methyl-ethyl ketone and 1,2-dichloromethane but remain insoluble in the highly polar ethyl acetate and butyl acetate. Further, the solubility of CAB increased for relatively higher butyryl contents (>38%) (in series C, Table 2). Samples CB, C1C4, Eastman CAB 381-0.5, and CAB 551-0.2 are soluble in all solvents. Thus, the solubility of CAB homogeneously synthesized in AmimCl can also be controlled by the method of addition of the acylating agents. 3.5. Thermal Analysis. The underivatized cellulose, CA, five synthesized CAB samples with different butyryl content ranging from 640%, commercial Eastman CAB 381-0.5, and CAB 551-0.2 were characterized by DSC and TGA in an N2 atmosphere. Table 3 summarizes the onset temperature of the thermal decomposition (Ton) and the glass transition temperature (Tg) for the underivatized cellulose and cellulose esters. The Tg of the underivatized cellulose is 230 °C which is detected by using more sensitive methods as the literature reported.32,33 In our experiments, the DSC curve of underivatized cellulose did not display any endothermic or exothermic peaks until it decomposed. On the contrast, cellulose acetate butyrates exhibited a clear Tg in their DSC curves. The thermal behavior of cellulose ester samples in the TGA thermograms was similar with each other. The Ton of the CABs (334348 °C) is lower than that of the underivatized cellulose (376 °C) and CA (354 °C). TGA analysis reveals that the introduction of butyryl groups slightly decreased the thermal stability of cellulose. Tg is a critical temperature at which the material properties of a polymer dramatically change. The Tg of all CABs is lower than that of the underivatized cellulose (230 °C) and CA (185 °C). Higher butyryl content leads to less hydrogen bonding and a lower Tg. Increasing the butyryl content from 6% to 40%, the Tg of CABs gradually decreased from 178 to 141 °C, as reported in the literature.34 Introducing hydrophobic butyryl groups to the cellulose chain breaks the hydrogen bonds and makes the cellulose chains more flexible under the thermal effect. When commercial CAB 381-0.5 and 551-0.2 were tested for comparison, their Tg (124 and 103 °C) was similar to the synthesized CAB. 3.6. Recycling of the Ionic liquid. Our process is economically attractive since we could recycle and reuse the IL. In our study, at the end of each acylation of cellulose, CAB was precipitated with a large excess of methanol. The polymer was filtered off, and the residual ILs in the filtrate were recovered by evaporation. The high purity of the IL was characterized by 1H NMR as shown in Figure 5. The mix-acylation of cellulose in the 7812
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Table 2. Solubility of CABs Homogeneously Prepared in AmimCla solvents B (%)
2-methyl-ethyl ketone
ethyl acetate
butyl acetate
1,2-dichloromethane
CB
45
þ
þ
þ
þ
CA
0
A1
11
A2
16
A3
18
A4
15
A5
18
A6 A7
20 22
þ
þ
A8
23
þ
þ
B1
6
B2
7
B3
7
B4
15
C1
38
þ
þ
þ
þ
C2 C3
40 47
þ þ
þ þ
þ þ
þ þ
C4
47
þ
þ
þ
þ
CAB 381-0.5
38
þ
þ
þ
þ
CAB 551-0.2
51
þ
þ
þ
þ
samples
a
“þ” stands for soluble, “-” stands for insoluble.
Table 3. Thermal Properties of CAB samples
DSTotal DSA A (%) DSB B (%) Tg (°C) Ton (°C)
cellulose
0
0
0
0
0
230
376
CA A2
2.37 1.52
2.37 0.96
100 17
0 0.56
0 16
185 175
354 348
A7
2.39
1.48
22
0.91
22
169
345
B1
2.00
1.80
31
0.20
6
178
334
C2
2.17
0.48
7
1.69
40
141
338
CAB 381-0.5
2.75
1.00
13
1.75
38
124
363
CAB 551-0.2
2.75
0.01
2.0
2.74
51
103
361
4. CONCLUSIONS Cellulose mixed-esters containing acetyl and butyryl were successfully synthesized in the absence of any catalyst in the ionic liquid AmimCl. FTIR and NMR confirmed the structure of CAB. 13C NMR indicates that the substituent distributions of the homogeneously synthesized CAB under the three different addition methods are different from that of commercial CAB. CAB with butyryl content >38% shows good dissolving ability in 2-methylethyl ketone, 1,2-dichloromethane, ethyl acetate, and butyl acetate. TGA results indicate that the introduction of butyryl groups slightly decreased the thermal stability of cellulose. The Tg of all CABs are between 178 to 141 °C, which gradually decreased with increasing butyryl content from 6 to 40%. After the mix-acylation, used ionic liquid AmimCl was easily recycled and reused. Compared with the complex production process of Eastman CAB 381-0.5 and 551-0.2, the homogeneous way is easier to control, and the whole reaction is moderate without any catalyst and cosolvent. Consequently, this study provides not only a new idea for the synthesis of cellulose mix-esters in IL but also a new way for the clean production of CAB under mild conditions for future industrial applications. ’ AUTHOR INFORMATION Corresponding Author
*Phone: þ86-10-82544825. Fax: þ86-10-62621355. E-mail:
[email protected] (H.Q.L.). Phone: þ86-10-62613251. Fax: þ86-10-62613251. E-mail:
[email protected] (J.Z.). Figure 5. The 1H NMR spectrum of fresh and recycled IL.
recovered AmimCl was also carried out under the same reaction conditions, and CAB with a comparable DS value was obtained; these results are listed in Table 1.
’ ACKNOWLEDGMENT This work was supported by the National Science Foundation of China (No. 21006118) and the National Basic Research 7813
dx.doi.org/10.1021/ie2004362 |Ind. Eng. Chem. Res. 2011, 50, 7808–7814
Industrial & Engineering Chemistry Research Program of China (973 Program) (Grant No. 2009CB219901) from Ministry of Science and Technology.
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