Functional Comparison of Polar Ionic Liquids and Onium

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Functional comparison of polar ionic liquids and onium hydroxides for chitin dissolution and deacetylation to chitosan Mizuki Shimo, Mitsuru Abe, and Hiroyuki Ohno ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00368 • Publication Date (Web): 18 May 2016 Downloaded from http://pubs.acs.org on May 24, 2016

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ACS Sustainable Chemistry & Engineering

Functional comparison of polar ionic liquids and onium hydroxides for chitin dissolution and deacetylation to chitosan Mizuki Shimo,†,‡ Mitsuru Abe,†,‡,║ and Hiroyuki Ohno *,†,‡ †Department

of Biotechnology, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho,

Koganei, Tokyo 184-8588, Japan ‡Functional

Ionic Liquid Laboratories, Graduate School of Engineering, Tokyo University of

Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan KEYWORDS: hydrogen bond, proton-accepting ability, hydroxide anion, ethylenediamine, mild condition, deacetylation

ABSTRACT: A series of ionic liquids (ILs) have been synthesized and their dissolution ability of chitin has been evaluated under mild condition. We have focused on tris(2-hydroxyethyl)methylammonium (THEMA) type ILs. These [THEMA]-type ILs partly dissolved chitin, and addition of ethylenediamine (EDA) was confirmed to improve the chitin dissolution. Relatively high hydrogen bond accepting ability of the IL/EDA mixtures was required to dissolve chitin. Among these mixtures, [THEMA][OAc]/EDA mixture dissolved chitin completely without heating. As the other potential solvents, a series of tetraalkylammonium hydroxides/water mixtures was also found to dissolve chitin without heating. Conversion of chitin to chitosan during dissolution process has been found in such tetraalkylammonium hydroxides/water mixtures in spite that almost no deacetylation was found in IL/EDA mixture. Effect of ILs and onium hydroxides on both chitin dissolution and deacetylation ability was discussed. ACS Paragon Plus Environment

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INTRODUCTION Chitin is a polymer of N-acetylglucosamine (2-(acetylamino)-2-deoxy-D-glucose) and it is the second most plentiful polysaccharide after cellulose on earth. Chitin is easily found in shells of crabs and shrimps as well as insect exoskeleton. Due to their quantity and quality as renewable raw materials, there are strong requests to use chitin effectively. Recycling of chitin also helps to minimize a large quantity of disposed shells of crabs and shrimps. However, the use of chitin is not popular due to very poor solubility in common solvents. Generally, it requires both polar solvents and harsh condition such as high temperature.1 Recently, some polar ionic liquids (ILs) have been reported to dissolve several poorly soluble biomaterials such as cellulose.2-7 There are also a few reports on ILs such as 1-butyl-3methylimidazolium chloride8 and 1-ethyl-3-methylimidazolium acetate to dissolve chitin.9 However, it was more difficult to dissolve chitin than cellulose under mild condition. For example, pure chitin was dissolved in 1-ethyl-3-methylimidazolium acetate when the chitin/IL mixture was heated up to 100 °C,9 even though cellulose was dissolved in the IL under milder condition, e.g., 40 °C.10 The significant difficulty of the chitin dissolution might be attributed to the characteristic hydrogen bond networks in chitin crystals. The unit structure of chitin has an acetamide group on the C2 position as compare with that the repeating unit of cellulose has a hydroxyl group on the C2 position. This acetamide group leads a strong hydrogen bond between C=O and NH groups of the adjacent chains.11 Accordingly, a large amount of energy is needed to dissolve chitin. There is a strong demand to dissolve chitin at room temperature. However potential factors to dissolve chitin under mild condition have not been clarified yet, polarity of ILs should be of primary parameter to discuss the dissolution of chitin. Kamlet-Taft parameters are frequently used to discuss the polarity of these ILs. Kamlet-Taft α and β values show hydrogen bonding donating and accepting ability of the ILs.12 For example, ILs showing large β value are semi-empirically known to dissolve cellulose.13-15 On the other hand, chitin has an acetamide group 2 ACS Paragon Plus Environment

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as mentioned above, and it strongly contributes the hydrogen bonding networks in the chitin crystal.11 Therefore, ILs with both large α and β values are strongly suggested to be discussed for chitin solubilization. On the other hand, ILs have no power to dissolve cellulose and other polysaccharides when water was added.16 We clarified that this is due to the decrease of hydrogen bonding accepting ability of ILs by the addition of water. We then reported onium hydroxide aq. solutions as potential solvents for cellulose in the presence of water.17 In the present paper, we compare power and mechanism of the chitin dissolution in these two different solvent groups

EXPERIMENTAL SECTION Materials Chitin powder and chitosan powder (degree of deacetylation of over 80%) were purchased from Wako Pure Chemical Industries, Ltd. These samples were passed through a sieve with 50 to 300 mesh and were dried in vacuo for 6 hours at room temperature before use. Tetramethylammonium hydroxide ([N1,1,1,1]OH) aqueous (aq.) solution (15%), tetraethylammonium hydroxide ([N2,2,2,2]OH) aq. solution (10%), tetra-n-butylammonium hydroxide ([N4,4,4,4]OH) aq. solution (25%), and ethylenediamine (EDA) were purchased from Wako Pure Chemical Industries, Ltd.

Methanesulfonic acid (H[MeSO3]),

trifluoromethanesulfonic acid (H[CF3SO3]), tetra-n-propylammonium hydroxide ([N3,3,3,3]OH) aq. solution (10%), and tetra-n-hexylammonium bromide ([N6,6,6,6]Br) were purchased from Tokyo Chemical Industry Co., Ltd. Water content of all tetraalkylammonium hydroxide aq. solutions was adjusted by evaporation at 30 °C or addition of water before use. The water content was confirmed with Karl Fischer coulometric titration system (Kyoto Electronics; MKC-510N).

[N6,6,6,6]Br was

recrystallized before use. Tris(2-hydroxyethyl)methylammonium methyl sulfate ([THEMA][MeOSO3]) was purchased from Sigma-Aldrich, and it was dried in vacuo at 80 °C until water content reached less than 0.2%.

Acetic acid (H[OAc]) and dimethyl sulfoxide-d6 containing 0.03% tetramethylsilane 3 ACS Paragon Plus Environment


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(Me4Si) were purchased from Kanto Chemical Co. Ltd. Bis(trifluoromethane)sulfonylimide (H[Tf2N]) was purchased from Morita Chemical Industries Co. Ltd. These acids and organic solvents were used as received unless otherwise stated.

Synthesis of ILs THEMA type ILs were synthesized by the same method as previously reported.18

First

[THEMA][MeOSO3] was dissolved in water, and the resulting solution was passed through a column filled with anion exchange resin (Amberlite® IRN 78) to give an aq. solution of [THEMA]OH. Equimolar amount of acetic acid was then slowly added to the aq. solution of [THEMA]OH. The resulting solution was stirred for 2 h at room temperature. After removal of water by evaporation, the IL solution was dried in vacuo at room temperature for 6 h. The temperature was gradually increased to 80 °C, and the IL was further dried at the temperature until water content reached less than 0.2 wt%. Through these steps, [THEMA][OAc] was obtained as a colorless liquid. Similarly, [THEMA][MeSO3], [THEMA][CF3SO3], and [THEMA][Tf2N] were prepared using the same procedures as mentioned above with the corresponding acids.

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H NMR data of THEMA-type ILs Tris(2-hydroxyethyl)methylammonium acetate ([THEMA][OAc]) 1

H-NMR (400 MHz; DMSO-d6; Me4Si) δH (in ppm) = 1.62 (3H, s, CH3COO), 3.16 (3H, s, NCH3),

3.56 (6H, t, J = 5.4 Hz, NCH2CH2OH), 3.83(6H, brs, NCH2CH2OH), 6.47 (1H, brs, NCH2CH2OH). 13CNMR(100MHz; DMSO-d6; Me4Si) δC = 26.00 (CH3COO), 50.20 (NCH3), 55.31 (NCH2CH2OH), 64.66 (NCH2CH2OH), 174.73 (CH3COO). Tris(2-hydroxyethyl)methylammonium methanesulfonate ([THEMA][MeSO3]) 1

H-NMR (400 MHz; DMSO-d6; Me4Si) δH = 2.33 (3H, brd, J = 6.0 Hz, CH3COO), 3.14 (3H, s,

NCH3), 3.53 (6H, t, J = 5.0 Hz, NCH2CH2OH), 3.84 (6H, brd, J = 1.5 Hz NCH2CH2OH), 5.29 (1H,

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brs, NCH2CH2OH). 13C-NMR(100MHz; DMSO-d6; Me4Si) δC = 50.21 (NCH3), 55.42 (NCH2CH2OH), 64.68 (NCH2CH2OH) Tris(2-hydroxyethyl)methylammonium trifluoromethanesulfonate ([THEMA][CF3SO3]) 1

H-NMR (400 MHz; DMSO-d6; Me4Si) δH = 3.14 (3H, s, NCH3), 3.52 (6H, t, J = 5.5 Hz,

NCH2CH2OH), 3.84(6H, brs, NCH2CH2OH), 5.23 (1H, brd, J = 3.5 Hz, NCH2CH2OH).

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C-

NMR(100MHz; DMSO-d6; Me4Si) δC = 50.19 (NCH3), 55.42 (NCH2CH2OH), 64.70 (NCH2CH2OH) Tris(2-hydroxyethyl)methylammonium bis(trifluoromethanesulfonyl)imide ([THEMA][Tf2N]) 1

H-NMR(400 MHz; DMSO-d6; Me4Si) δH = 3.14 (3H, s, NCH3), 3.52 (6H, t, J = 5.3 Hz,

NCH2CH2OH), 3.84(6H, brt, J = 4.0Hz, NCH2CH2OH), 5.23 (1H, brs, NCH2CH2OH).

13

C-

NMR(100MHz; DMSO-d6; Me4Si) δC = 50.18 (NCH3), 55.42 (NCH2CH2OH), 64.71 (NCH2CH2OH), 118.73, 121.29 (F3CS)

Preparation of onium hydroxide aqueous solution [N6,6,6,6]OH aq. solution was prepared by the anion exchange reaction of corresponding halide salt, [N6,6,6,6]Br. This solution was passed through a column filled with anion exchange resin (Amberlite® IRN 78) using water/methanol mixture. After anion exchange, methanol was removed by evaporation, and the water content of these aq. solutions was analyzed and adjusted by moderate evaporation or addition of water. Anion exchange was confirmed by the generation of no precipitation by adding silver nitrate solution to the prepared onium hydroxide aq. solution. Water content of the hydroxide aq. solution was adjusted by evaporation at 30 °C. The cation species of those synthesized hydroxide aq. solutions were quantitatively confirmed with NMR measurements.

Structure of these ILs was

confirmed with both 1H- and 13C-NMR spectroscopies. These spectra were recorded with a JEOL ECX400. [N6,6,6,6]OH aq. solution (water/hydroxide = 15/1 (molar ratio))


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H-NMR (400 MHz; D2O; DSS) δH = 0.874 (12H, t, J = 6.4 Hz, CH3), 1.317 (24H, m,

CH2CH2CH2CH2CH3), 1.601 (8H, m, NCH2CH2CH2), 3.181 (8H, m, NCH2). 13C-NMR (100 MHz; D2O; DSS) δC = 16.476 (CH3), 23.970 (CH2CH2CH3), 24.809 (CH2CH2CH2CH3), 28.137 (NCH2CH2CH2), 33.629 (NCH2CH2CH2), 60.774 (NCH2).

Kamlet-Taft parameters Kamlet-Taft parameters of ILs were determined by the reported method3 as briefly mentioned below. Three spectroscopic grade dyes, (2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio)phenolate (Reichardt’s dye #33, from Fluka), 4-nitroaniline (from Tokyo Chemical Industries Co. Ltd), and N,N-diethyl-4nitroaniline (from Kanto Chemical Co. Inc.), were used as received. Methanol solutions containing these dye (10 µl) were added to 0.25 g of ILs, individually. The methanol was then carefully removed by drying under reduced pressure, and then these IL solutions were placed into quartz cells with 1.0 mm light-path length. From the maximum absorption wavelength (λmax), three Kamlet-Taft parameters such as α, β, and π* values were calculated with the following equations:

ν(dye) = 1/(λmax(dye)10–4) ET(30) = 0.9986 (28 592/λmax (Reichardt’s dye #33)) – 8.6878

π* = 0.314(27.52 – ν (N,N-diethyl-4-nitroaniline)) α = 0.0649ET(30) – 2.03 – 0.72π* β = (1.035ν (N,N-diethyl-4-nitroaniline) + 2.64 – ν (4-nitroaniline))/2.80

Dissolution test of chitin Commercially available chitin powder was added to ILs, IL/EDA mixtures, or tetraalkylammonium hydroxide aq. solutions. The concentration of the added chitin was adjusted to 0.1 wt% for IL or IL/EDA mixture and 0.2 wt% for tetraalkylammonium hydroxide aq. solutions, respectively. The


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mixtures were gently stirred at room temperature up to two weeks. Chitin dissolution was confirmed by naked eyes and optical micrograph observations (Olympus BX51).

Recycling of solvents Chitin was treated with [THEMA][OAc]/EDA mixture at 40 °C for 48 h, then the insoluble part was filtered to collect homogeneously dissolved chitin solution. An excess amount of water was slowly added to the resulting sample solutions under stirring, and then the dissolved chitin was precipitated. After filtration of the precipitated chitin, the used IL solution was dried in vacuo at room temperature for 6 h. The treatment temperature was gradually increased up to 80 °C, and the IL was further dried until water content reached less than 0.2 wt%. The yield of the recycled IL was calculated from the weight of the dried [THEMA][OAc] obtained. Similarly, chitin was treated with [N2,2,2,2]OH aq. solution (molar ratio of onium hydroxide to water is 1:7.5) under the same condition. After filtration of the precipitated chitin, the [N2,2,2,2]OH aq. solution was dried by evaporation at 30 °C until the water content reached the same value as before treatment. The water content was confirmed with Karl Fischer coulometric titration system. The yield of the recycled [N2,2,2,2]OH aq. solution was calculated from the volume and water content of the [N2,2,2,2]OH aq. solution recovered.

FT-IR measurement for degree of deacetylation of chitin After the chitin dissolution, the insoluble part was filtered off to collect the chitin dissolving solution. An excess amount of methanol was slowly added to the resulting sample solutions with stirring, and then the dissolved chitin was precipitated in powdered state. After repeating wash with methanol to remove residual IL or onium hydroxide aq. solution, the regenerated chitin powder was collected by filtration.


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IR spectra of chitin, chitosan, and the regenerated chitin were recorded with a JASCO FTIR-4200 spectrometer using a KBr pellet as a reference and baseline presented previously.19 Both absorbance at 1560 cm-1 attributed to amide II band of both amino group and acetamide group and that at 1070 cm-1 attributed to C-O stretching band of acetamide group were measured and the degree of deacetylation (DDA) of these samples was calculated from the absorbance ratio (A1560/A1070). DDA of the purchased chitin and chitosan was also determined by the same method as mentioned in the reference.19

RESULTS AND DISCUSSION Dissolution of chitin with polar ionic liquids In the present study, we have selected THEMA cation to design polar ILs, because this THEMA cation provides ILs with high proton donating ability as expressed by a large α value (e.g., α =1.02 for [THEMA][Tf2N] and α = 0.86 for [THEMA][(MeO)(H)PO2]). Table 1 summarizes the results of chitin dissolution test in five different ILs at room temperature. As shown in Table 1, chitin was partially soluble in ILs with high β value. By contrast, the α values seemed to affect much smaller on the chitin dissolution. ILs having very high α values of over 1.0 have no power to dissolve chitin as long as their

β value was lower than 0.5. Similar to the dissolution of cellulose with ILs, β value was found to be the dominant parameters for chitin dissolution. All these data strongly suggest that strong interaction (hydrogen bonding) between chitin and ILs is the key to dissolve chitin in these ILs.

Table 1. Anion structure, Kamlet-Taft parameters (α and β ), and chitin dissolving ability of THEMAbased ILs. Entry

Anions

α

β

Chitin dissolution

1

[OAc]

0.81

0.67

□

2

[MeSO3]

0.88

0.52

□

3

[MeOSO3]

1.01

0.41

×


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4

[CF3SO3]

1.05

0.22

×

5

[Tf2N]

1.02

0.09

×

□: partially soluble, ×: insoluble

Effect of ethylenediamine addition Recently, X-ray diffraction analysis revealed that ethylenediamine (EDA) penetrates inside the αchitin crystals and produced an α-chitin-EDA complex.20 Although the EDA did not dissolve the chitin crystals, it was suggested that the EDA strongly interacted with chitin crystals. This inspired us that the additional EDA enhanced hydrogen bonding ability of IL/EDA mixture and weaken the interchain hydrogen bonds in chitin. Then, EDA was added to our polar ILs (mixing ratio of 2/1 (mol/mol)) and the chitin dissolving ability of these IL/EDA mixtures has been examined. As a result, it was found that the EDA addition positively affected the chitin dissolution for several ILs.

It is noteworthy that

[THEMA][OAc] (entry 1 in Table 1) completely dissolved chitin without heating when EDA was added. This is the first report for complete dissolution of chitin at room temperature.

In addition,

[THEMA][MeOSO3] (entry 3) and [THEMA][CF3SO3] (entry 4) gained the power to partly dissolve chitin after the addition of EDA. It was found that the EDA addition is effective to dissolve chitin for some ILs. The EDA addition had however no improvements on the chitin dissolution for two ILs, [THEMA][MeSO3] (entry 2) and [THEMA][Tf2N] (entry 5).

In the case of [THEMA][Tf2N]/EDA

mixture, it is understandable not to dissolve chitin due to fairly low β value of the [THEMA][Tf2N] (see Table 1, entry 5), but it could not be fully understood that there was no effect of EDA addition on the chitin dissolution in [THEMA][MeSO3] in spite that the [THEMA][MeSO3] showed moderate power to dissolve chitin partly. To understand the effect of EDA addition, we have measured the Kamlet-Taft parameters of these ILs after addition of EDA. As shown in Figure 1, the β value of every IL increased by the EDA addition. On the other hand, the α values were found to decrease in several ILs (see entry 1, 2, and 3 in Figure 1). To examine the effect of further EDA addition, differing amounts of EDA was added to an IL, 9 ACS Paragon Plus Environment


[THEMA][MeOSO3] (entry 3), and its Kamlet-Taft parameters were evaluated. As a result, with increasing the mixing ratio of the IL/EDA from 1/0.5 to 1/2.0 (by molar ratio), α value decreased and β value increased monotonically (see entry 3E, 3E1, and 3E2 in Figure 1). Additionally, [THEMA][MeOSO3] acquired an ability to dissolve chitin with the increase of the EDA addition (entry 3E1 and 3E2).

After addition of EDA, [THEMA][MeOSO3] (entry 3E1 and 3E2) dissolved chitin

completely, although they have lower β values than [THEMA][OAc] (see entry 1) which dissolved chitin partially.

Considering the changes of these parameters into account, we suggest here that the ILs

having high β value (and mixtures of these ILs and EDA) are favorable for chitin dissolution at room temperature. However, these data strongly suggested that the β value is certainly a considerable factor, but this is not the only factor to govern the chitin dissolution.

1.2

4E

4

5

3

5E

1.0

3E 2 3E1

α

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0.8

1

2E 3

E2

1E

0.6 0 0

0.2

0.4

0.6

0.8

β

Figure 1. Correlation between Kamlet-Taft parameters of ILs (entry 1-5 and 1E-5E are ILs without or with EDA, respectively) and their chitin dissolving ability at 25 °C. Entry 3E1 and 3E2 were IL and EDA mixture with mixing ratio of 1:1 and 1:2 (by mol), respectively. ○: soluble, □: partially soluble, ×: insoluble.

On the other hand, the α value was suggested to affect the chitin dissolution ability of the solvents, and chitin was less soluble in ILs having higher α value. To our knowledge, this is the first report to ACS Paragon Plus Environment

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clarify the influence of both the hydrogen bond donating (α value) and accepting abilities (β value) of solvents for complete dissolution of chitin. In the ILs having large α value, there are many hydrogen bonding among component ions and accordingly these ILs with large α value generally show relatively large viscosity. This sometimes gave a negative effect on the dissolution process of target materials. For sustainable use of solvents, high recycle rate is essential.

We then tried to recover

[THEMA][OAc] after dissolving chitin. The recycle rate of 90% was obtained after chitin dissolution. A small scale experiment might be the reason for 10% loss. Since [THEMA][OAc] is a hydrophilic IL, drying needs considerable energy.

Both water and EDA were also recovered by the cooled trap, but

separation of EDA from water also needs energy. Considering these data into account, mixture of IL and EDA is good to dissolve chitin but is not the best solvent for sustainable chitin dissolution.

Dissolution and deacetylation of chitin It was reported that the proton accepting ability of ILs decreased considerably by the addition of water.16 In this study, dissolution ability of these ILs also dropped by the addition of water. In nature, chitin source such as shrimp shells contain water, and we have to dry them to dissolve in polar ILs. Similar to the case of dissolution of cellulose in the native state, we have to choose new materials as solvents to dissolve chitin even in the presence of water. According to our previous strategy, we examined a series of onium hydroxide aq. solutions.17 Similarly, we expected another effect of these onium hydroxide aq. solutions i.e., “deacetylation”. Chitin is expected to be useful materials after deacetylation, called chitosan. It is important to detect the degree of deacetylation of chitin, because physicochemical properties of chitosan deeply depended on the degree of deacetylation of chitin. For the deacetylation of chitin, strong alkaline conditions are known to be effective.1,21 From the viewpoint of the basicity, most ILs are not suitable to provide such strongly basic conditions. As we have already reported that a series of tetraalkylphosphonium and tetraalkylammonium hydroxides aq. solutions dissolve cellulose and other natural polymers at room temperature,17,22-24 and they are sufficiently stable ACS Paragon Plus Environment

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at ambient temperature.22,25 It should be noted here that used onium hydroxide aq. solutions are reported to be recyclable because diluted solution loosed water by evaporation to reach reasonable concentration for biomass dissolution.22 These onium hydroxide aq. solutions are expected to be a potential candidate as solvents for chitin dissolution and subsequent deacetylation. Due to simple synthesis of a series of onium hydroxides with different alkyl chain length, we have selected tetraalkylammonium hydroxides in this study.

Table 2 summarizes the results of the dissolution test of chitin in a series of

tetraalkylammonium hydroxide aq. solutions. As we have already reported that there is a suitable water content of the tetraalkyl-onium hydroxide aq. solutions for the dissolution of cellulose at room temperature,17,22 we examined the chitin dissolving ability of these tetraalkylammonium hydroxides as the function of water content. In all the tetraalkylammonium hydroxide aq. solutions containing 15 water molecules per ion pair or more, no chitin were dissolved. On the other hand, when the number of water molecules per ion pair was lower than 10, chitin was dissolved in these easily. The most important result was that some of these tetraalkylammonium hydroxide aq. solutions containing 7.5 water molecules per ion pair completely dissolved chitin as shown in Table 2. Both [N2,2,2,2]OH and [N3,3,3,3]OH with 7.5 water molecules per ion pair were the best for chitin dissolution among the tested solutions. In the case of cellulose dissolution, [N4,4,4,4]OH and tetra-n-butylphosphonium hydroxide aq. solutions were known to be better than other hydroxide aq. solutions.17,22 However, it was suggested that the shorter alkyl chains were suitable for chitin dissolution. The difference of the hydrogen bonding networks between cellulose and chitin as well as their unit structure should be taken into consideration.

Discussion on the alkyl chain length of the most effective tetraalkylammonium

hydroxide aq. solution should be provided after collecting some data on the detailed experiments on the interaction of these ions (and hydrated ions) with polysaccharides. Most importantly, however, we also succeeded in dissolving chitin at room temperature with a kind of alkaline solvents, tetraalkylammonium hydroxide aq. solutions.


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Table 2. Chitin dissolving ability of tetraalkylammonium hydroxide aq. solutions under different water content. Number of water molecules per ion pair

Cation 7.5

10

15

20

30

[N1,1,1,1]

―

□

×

×

×

[N2,2,2,2]

○

□

×

×

×

[N3,3,3,3]

○

□

×

×

×

[N4,4,4,4]

□

□

×

×

×

[N6,6,6,6]

□

□

×

×

×

○: soluble , □: partially soluble, ×: insoluble, ―: not measured

Degree of deacetylation (DD) of the once dissolved chitin in these onium hydroxide aq. solutions was determined to assess the availability to obtain chitosan. The dissolved chitin was precipitated by the addition of an anti-solvent, i.e., methanol in this study. The DD of the regenerated chitin was estimated by FT-IR measurement using two peaks associated with C-O stretching band of acetamide group (1070 cm-1) and amide-II band of amino groups and acetamide groups (1560 cm-1).19 Figure 2 shows the relationship between absorbance ratio (A1560/A1070) and DD of chitin samples. The DD of the commercially available chitin before dissolution was 15%, and that of the regenerated chitin from the [THEMA][OAc]/EDA mixture (see Figure 1, entry 1E) was 17%. The chitin dissolution with polar [THEMA][OAc]/EDA mixture was found not to proceed the deacetylation of chitin. Against these, DD of chitin once dissolved in [N2,2,2,2]OH aq. solution (molar ratio of onium hydroxide to water is 1:7.5) for 2 weeks was found to be 91%. This value was higher than that of commercially available chitosan (80%) as seen in Figure 2. The DD of the chitin dissolved in [N2,2,2,2]OH aq. solution for 1 week was only 57%. This means that deacetylation proceeded after dissolved in the onium hydroxide aq. solution.


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These data strongly suggested that the complete deacetylation is not an essential requirement to dissolve chitin. Polar ionic liquids could dissolve chitin without deacetylation, and DD of chitin decreased with time in the onium hydroxide aq. solution.

The DD of chitin is the function of treatment time in the

onium hydroxide aq. solution as also shown in Fig. 2, and accordingly the DD would be controllable. Further studies on the fine control of deacetylation of chitin in ILs will be reported in the near future. We also tried to recover [N2,2,2,2]OH aq. solution after dissolving chitin. From the volume and concentration (or water content), we calculated the amount of [N2,2,2,2]OH and compared it with the initial value. The obtained recycle rate was 87 ± 5 %. Around 10% loss should also be due to small scale experiments for the dissolution test. However, this [N2,2,2,2]OH aq. solution is based on simple components; [N2,2,2,2]OH and water. Furthermore, precipitation of chitin or chitosan was easily formed by adding excess amount of water. So that recycling is quite easy and efficient, just by adjusting water content.

chitin chitin after [THEMA][OAc]/EDA treatment

0.8 0.7

chitin after [N2,2,2,2]OH : H2O = 1 : 7.5 treatment (1 week)

0.6

A1560/A1070

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0.5 0.4 0.3

chitosan chitin after [N2,2,2,2]OH : H2O = 1 : 7.5 treatment (2 weeks)

0.2 0.1 10

20

30

40

50

60

70

80

90

100

Degree of Deacetylation (%)

Figure 2.

DD of chitin before and after dissolution in either [THEMA][OAc]/EDA mixture or

[N2,2,2,2]OH aq. solutions.


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CONCLUSIONS ILs having high proton accepting ability were confirmed to be potential solvents for chitin at room temperature. Addition of EDA to these polar ILs was found to be very effective to improve the dissolution of chitin due to the improvement of the proton accepting ability of the ILs. Some ILs/EDA mixtures with high Kamlet-Taft β value successfully dissolved chitin without heating. On the other hand, tetraalkylammonium hydroxide aq. solutions were found to be effective solvents to produce chitosan at room temperature even in the presence of water. [N2,2,2,2] and [N3,3,3,3] hydroxide aq. solution dissolved chitin and gradually deacetylated it to chitosan at room temperature. This procedure will open easy and valuable recovery method of chitin and further deacetylation to obtain chitosan with desired degree of deacetylation. Tetraalkylammonium hydroxide aq. solutions are based on simple components, ammonium hydroxide and water. Chitin or chitosan was easily precipitated by adding excess amount of water to the solution.

From viewpoint of sustainability, tetraalkylammonium

hydroxide aq. solutions should be better solvent for chitin dissolution and preparation of chitosan.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +81-42-388-7024. Fax: +81-42-388-7024. Present Addresses ║

Present address: Division of Forest and Biomaterials Science, Graduate School of Agriculture, Kyoto

University, Kyoto 606-8502, Japan Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACS Paragon Plus Environment

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ACKNOWLEDGEMENT Financial support for this work was provided by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (KAKENHI).

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Functional comparison of polar ionic liquids and onium hydroxides for chitin dissolution and deacetylation to chitosan . Mizuki Shimo, Mitsuru Abe, and Hiroyuki Ohno*

Mixtures of polar ionic liquids and ethylenediamine are good solvents for chitin. Mixtures of water and tetraalkylammonium hydroxides are good solvents to dissolve chitin and deacetylate to produce chitosan.


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Functional Comparison of Polar Ionic Liquids and Onium

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