Ionic liquid mediated technology for synthesis of cellulose acetates using different co-solvents

Carbohydrate Polymers 135 (2016) 341–348

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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

Ionic liquid mediated technology for synthesis of cellulose acetates using different co-solvents Olatunde Jogunola a,b,∗ , Valerie Eta a,b , Mattias Hedenström a , Ola Sundman a , Tapio Salmi b , Jyri-Pekka Mikkola a,b a b

Technical Chemistry, Department of Chemistry, Chemical-Biological Centre, Umeå University, 90187 Umeå, Sweden Laboratory of Industrial Chemistry and Reaction Engineering, Johan Gadolin Process Chemistry Centre, Åbo Akademi University, 20500 Åbo/Turku, Finland

a r t i c l e

i n f o

Article history: Received 17 June 2015 Received in revised form 27 August 2015 Accepted 30 August 2015 Available online 2 September 2015 Keywords: Cellulose acetylation Cellulose dissolution Ionic liquid Dispersing agents Cellulose acetates Recycling

a b s t r a c t In this work, cellulose acetate was synthesized under homogeneous conditions. Cellulose was first dispersed in acetone, acetonitrile, 1,5-diazabicyclo(4.3.0)non-5-ene (DBN) or dimethyl sulphoxide (DMSO) and the resulting suspension was dissolved in an ionic liquid, 1,5-diazabicyclo(4.3.0)non-5-enium acetate [HDBN][OAc] at 70 ◦ C for 0.5 h. It was possible to dissolve more than 12 wt% cellulose with a degree of polymerization in the range of 1000–1100. The dissolved cellulose was derivatized with acetic anhydride (Ac2 O) to yield acetylated cellulose. As expected, the use of the co-solvents improved the acetylation process significantly. In fact, cellulose acetates with different properties could be obtained in half an hour, thus facilitating rapid processing. When DBN was used as the dispersing agent (the precursor of the ionic liquid), the problems associated with recycling of the ionic liquid were significantly reduced. In fact, additional [HDBN][OAc] was obtained from the interaction of the DBN and the by-product, acetic acid (from Ac2 O). However, the cellulose acetate obtained in this manner had the lowest DS. Consequently, the native cellulose and acetylated celluloses were characterized by means of 1 H- and 13 C-NMR, FT-IR, GPC/SEC and by titration. The cellulose acetates produced were soluble in organic solvents such as acetone, chloroform, dichloromethane and DMSO which is essential for their further processing. It was demonstrated that the ionic liquid can be recovered from the system by distillation and re-used in consecutive acetylation batches. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Cellulose is an abundant and widely available natural polymer with a well-defined structure used in numerous applications such as paper products, consumables, energy crops, biofuels, etc. It does not melt and is insoluble in conventional solvents due to its crystalline structure, thus limiting its wider applicability and making it an under-utilized bio-resource. Today, the emphasis is on investigating and understanding the cell-wall of the biopolymer as a vital biomaterial and environmental benign substance through the development of a multi-disciplinary approach (Shafizadeh, 1973). The challenges associated with the insolubility can be overcome by derivatization of the cellulose. Cellulose esters, one of the derivatives of cellulose, are traditionally produced in a heterogeneous

∗ Corresponding author at: Laboratory of Industrial Chemistry and Reaction Engineering, Johan Gadolin Process Chemistry Centre, Åbo Akademi University, 20500 Åbo/Turku, Finland. E-mail address: jolatund@abo.fi (O. Jogunola). http://dx.doi.org/10.1016/j.carbpol.2015.08.092 0144-8617/© 2015 Elsevier Ltd. All rights reserved.

state, in which cellulose reacts with excess acylating agent in the presence of catalyst like pyridine or sulfuric acid. The cellulose typically remains in dispersed state, consequently retarding the transformation rates to various functionalized derivatives due to slow mass transfer in the cellulose slurry. The processes are associated with limitations such as instability of the process (side reactions), uneven substitution patterns, longer time, corrosion and appreciable cellulose degradation (Heinze, Liebert, & Koschella, 2006). In addition, it is difficult to synthesize partially substituted cellulose esters. Homogeneous functionalization of cellulose, as an alternative process, has generated interest for many years and is still subject of ongoing research. It has the advantages of producing cellulose esters with uniform substitution pattern along the polymer chain and controllable degree of substitution. Furthermore, it is possible to have better control over the synthesis process and the structural features of the emerging products (Heinze et al., 2006). Homogeneous cellulose functionalization requires solvents capable of interacting with the anhydroglucose units and deconstructing the multifaceted interaction network

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in crystalline cellulose by cleaving inter- and intra-molecular hydrogen interactions, rendering it soluble since solvent–glucan interactions are dependent on the deconstruction state of cellulose (Cho, Gross, & Chu, 2011). Solvent systems and metal complexes such as N,N-dimethylacetamide (DMAc)/LiCl, dimethylsulfoxide (DMSO)/tetrabutylammonium fluoride trihydrate (TBAF), dimethylformamide (DMF)/N2 O4 , N-methylmorpholine-N-oxide monohydrate (NMMO), cuprammonium hydroxide (cuoxam), cadmiumethylenediamine (cadoxen) and molten salt hydrates (LiClO4 ·3H2 O, or LiSCN·2H2 O) have been used for the dissolution and chemical modification of cellulose to novel products (Edgar, Arnold, Blount, Lawniczak, & Lowman, 1995; Fischer, Thümmler, Pfeiffer, Liebert, & Heinze, 2002; Liebert & Heinze, 2001; Regiani, Frollini, Marson, Arantes, & El Seoud, 1999; Rosenau, Potthast, Sixta, & Kosma, 2001). For more than a decade, room temperature ionic liquids (RTILs) have received significant attention as promising green solvents (Wasserscheid & Keim, 2000) that can dissolve cellulose (Swatloski, Spear, Holbrey, & Rogers, 2002; Zhang, Wu, Zhang, & He, 2005) due to their unique physico-chemical properties and allow efficient, homogeneous synthesis of cellulose esters. Wu and co-workers (2004) reported the homogeneous acetylation of cellulose in 1-allyl-3-methylimidazolium chloride ([Amim][Cl]) using acetic anhydride in the absence of a catalyst. Cellulose acetates (DS = 0.94–2.74) were produced by varying the reaction conditions such as time and temperature as well as molar ratio of the acetylating agent (ACA)-to-anhydroglucose unit (AGU). Also, it was stipulated that the ionic liquid (IL) was recyclable. In another study, Abbott, Bell, Handa and Stoddart (2005) studied the acetylation of cellulose in a Lewis acidic deep eutectic solvent (choline chloride + zinc chloride). Furthermore, esterification of cellulose was investigated in four imidazolium-based and a pyridinium-based ILs under mild conditions using acyl chloride or acyl anhydride by Barthel and Heinze (2006) and Heinze, Schwikal and Barthel (2005). The influence of pyridine and increasing molar ratio of the ACA-to-AGU were studied accordingly and DS values from 2.5 to 3.0 were obtained, irrespective of the cellulose types. However, the cellulose acetylation in the pyridinium-based IL was unsuccessful. Cao and co-workers (2010) reported homogeneous cellulose acetylation in [Amim][Cl] at relatively higher cellulose concentration (8–12 wt%) without a catalyst and whereupon cellulose acetates with different DS values were obtained by varying the reaction conditions. Also, acetylation of cellulose in imidazolium-based ILs using a laboratory kneader system was investigated by Kosan, Dorn, Meister and Heinze (2010). Here, the cellulose acetate produced was subsequently processed to fibers by dry/wet spinning. Finally, low-energy microwave was utilized for dissolution and resultant acetylation of cellulose in 1-allyl-3-butylimidazolium chloride [Abim][Cl] by Possidonio, Fidale and El Seoud (2009). In order to expand the scope of ILs used for cellulose esterification and explore the possibility of using non-imidazoilium type of ILs, Yang and co-workers (2014) demonstrated rapid and efficient acetylation of cellulose using reversible ionic liquid [HDBU][O2 COCH3 ]–DMSO mix under mild conditions. Also, Stepan et al. (2013) completely acetylated xylan (a hemicellulose) in 1,5diazabicyclo(4.3.0)non-5-enium acetate [HDBN][OAc]. Herein, we report fast dissolution and homogenous acetylation process for cellulose in [HDBN][OAc] with a dispersing agent such as acetone, acetonitrile, DBN or DMSO under mild conditions. This protic IL has the advantages of being distillable and miscible with reagents and cellulose derivatives. It does not react with cellulose and is non-toxic in contrast to ILs based on imidazole. In addition, to the best of our knowledge, there has not been a report of esterification of cellulose in any IL using acetone as a co-solvent. The chemical structure of [HDBN][OAc] is depicted in Fig. 1

H N

O CH3

+

N

O

-

Fig. 1. Molecular structure of 1,5-diazabicyclo(4.3.0)non-5-enium acetate.

2. Experimental 2.1. Materials Cellulose powder (ca ∼ 20 ␮, DPw ∼ 1000, Aldrich), 1,5diazabicyclo[4.3.0]non-5-ene; DBN (98%, Fluka), dimethylacetamide; DMAc (99.5%, VWR), lithium chloride; LiCl (emsure, Merck) and Pullulan standards for GPC (Polymer lab/Agilent Technologies) were used as received. Acetic anhydride (98%), dimethyl sulfoxide; DMSO (99%), acetonitrile (99.8%), acetone (99%), acetic acid (99.7%), chloroform (99.5%), dichloromethane (99.5%), lithium cbromide; LiBr (ReagentPlus grade) and methanol were purchased from Sigma–Aldrich. These materials were used without further purification. 2.2. Synthesis and thermal analysis of 1,5-diazabicyclo(4.3.0)non-5-enium acetate Equimolar amount of acetic acid (7 ml) was drop-wisely added to 15 g of stirred, pre-cooled DBN under nitrogen atmosphere. As the acetic acid was added, the viscosity and temperature of the solution increased upon formation of the ionic liquid (solid at room temperature) as a result of the neutralization reaction. The purity of the synthesized [HDBN][OAc] was confirmed by NMR spectroscopy (Fig. 2). After the synthesis, simultaneous thermogravimetric analysis (TGA) and differential thermal analysis (DTA) was performed on a SDT Q600 (V20.9 Build 20) under nitrogen atmosphere. The TA instrument software was used for data evaluation. The instrument has two ceramic beams; both of them are attached to their own balance and thermocouple. The sample holder containing the sample is placed in the first beam, while the second beam acts as a reference. The sample was heated from 20 ◦ C to 400 ◦ C with a ramp of 10 ◦ C/min with a gas flow of 100 ml/min. The weight changes of the sample were recorded as a function of temperature (or time) under controlled atmosphere. 2.3. Cellulose dissolution Cellulose powder (1.2 g) was dispersed in 6 ml acetone, acetonitrile, DBN or DMSO and added to the freshly prepared [HDBN][OAc] at 25 ◦ C. The weight ratio of the cellulose to IL to co-solvent was about 0.2:4:1. The reactor was heated, when necessary until a clear transparent cellulose solution of about 5% (w/w) was obtained under vigorous stirring. The solution containing the dissolved cellulose was observed by optical microscopy. In order to ensure proper dissolution prior to acetylation, the cellulose solution was heated to 70 ◦ C for 0.5 h. 2.4. Acetylation of cellulose The acetylation of the dissolved cellulose was performed in a Radleys Tonado IS6 reactor system equipped with six 250 ml flasks, an overhead stirrer and heating plate. In a typical acetylation procedure, the solution containing the dissolved cellulose was transferred to the flasks at room temperature before

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Fig. 2.

1

343

H NMR analysis of synthesized [HDBN][OAc].

assembling the reactor. Typically, 1.75, 3.50 or 5.00 ml acetic anhydride were added drop-wise to the cellulose solution and then heated to 80 ± 0.2 ◦ C for 0.25, 0.5, 1, 2 or 3 h. The homogeneity in the reaction medium was maintained during the process by an impeller stirring at 200 rpm. After acetylation, the mixture was dispersed in methanol and filtered to recover the precipitated cellulose acetate. The recovered material was washed repeatedly with distilled water to neutrality in order to ensure removal of any traces of the IL, the dispersing agents and other chemicals used prior to this process. Finally, the product was freeze-dried, ground to a fine powder and then characterized.

2.6. Nuclear magnetic resonance (NMR) spectroscopy

2.5. Degree of substitution determination

where I(CH3) and I(AGU) denote the integrals for the acetate methyl groups and the AGU region respectively. Methyl peaks from positions C-2, C-3 and C-6 were also integrated individually to determine the DS at each site. 13 C NMR spectra were recorded on a 600 MHz Bruker Avance II HD spectrometer equipped with a 5 mm BBO cryoprobe. 5900 scans were added with a relaxation delay of 2 s. A 30◦ excitation pulse was used and waltz-16 proton decoupling was performed during both relaxation delay and acquisition. Processing of spectra was performed in Topspin 3.1 (Bruker Biospin, Rheinstetten, Germany).

2.5.1. Titration The degree of substitution (DS) was calculated based on the average number of acetyl groups required to displace the hydroxyl groups in the anhydroglucose units of the cellulose polymers. The content of the acetyl groups was determined by saponification of cellulose acetate with sodium hydroxide followed by neutralization of the excess alkali with hydrochloric acid and the subsequent titration against sodium hydroxide (Filho et al., 2008). About 0.1 g of acetylated cellulose powder previously dried in vacuum was transferred to a 250 ml conical flask. 5 ml of 0.25 M NaOH and 5 ml ethanol were added to the sample and the mixture was allowed to stand for 24 h. This was followed by neutralization of the alkali with 10 ml of 0.25 M HCl. After 0.5 h, the mixture was titrated against 0.25 M NaOH using phenolphthalein as indicator. The samples were analyzed in triplicate. Commercial cellulose acetate (CCA) with known DS was used as the reference. The percentage of acetyl groups (% Ac) and the DS were computed using Eqs. (1) and (2), respectively. %Ac =

[(VNaOH + VNaOH,t )CNaOH − VHCl × CHCl ] × 4.3 ms

162 × %Ac DS = 4300 − (42 × %Ac)

(1)

Approximately 10 mg of cellulose acetate sample was dissolved in 600 ␮L DMSO-d6 and the 1 H NMR spectra were recorded at 25 ◦ C on a 400 MHz Bruker Avance III spectrometer equipped with a 5 mm BBO smart probe. 64 scans were used with a relaxation delay of 5 s in between to allow for full relaxation. The AGU region and the acetate CH3 region were then integrated and DS calculated using Eq. (3) (Goodlett, Doughert, & Patton, 1971). DS =

7I(CH3 ) 3I(AUG)

(3)

2.7. Bond stretching by Fourier transform infrared (FTIR) spectroscopy The characterization of cellulose acetates by FTIR was performed on an ATI Mattson Infinity series spectrometer. About 3 mg of sample was homogenously dispersed in 150 mg of KBr and pressed into a pellet. The sample was placed into the sample holder and allowed to stabilize for 30 min prior to measurement. Infrared spectra were recorded by taking 64 scans using a detector of 4 cm−1 resolution at a frequency range between 4000–400 cm−1 in transmission mode. 2.8. Gel permeation and size exclusive chromatography (GPC/SEC)

(2)

where %Ac indicates the acetyl group content in %; VNaOH = the amount of NaOH used for saponification (5 ml); VNaOH,t = the amount of NaOH used for back titration (x ml); CNaOH = the concentration of sodium hydroxide (0.25 M); VHCl = the amount of HCl used for neutralization (10 ml); CHCl = the concentration of hydrochloric acid (0.25 M); ms = the weight of the sample.

The degree of polymerization was performed on a polymer laboratories PL-GPC 50 Plus instrument (Agilent) equipped with two PLgel MIXED-A 300 mm × 7.5 mm columns and a 50 mm × 7.5 mm PLgel guard column (Agilent Santa Clara, CA, USA), and a refractive index (RI) detector. The eluent used were either DMSO containing 0.1% (w/w) LiBr or DMAc containing 0.5% LiCl, and the flowrate was set to 0.5 ml/min. The system was calibrated using 8

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pullulan standards with molecular masses ranging from 9.6 to 708 kDa. The cellulose sample was dispersed in Mill-Q H2 O. The solvent was exchanged repeatedly with methanol, followed by DMAc. Thereafter, the sample was treated at 5 mg/ml in DMAc containing 8% LiCl until it dissolved and then diluted with DMAc to a final concentration of 1 mg/ml. For cellulose acetate, the samples were dissolved in DMSO containing 0.1% (w/w) LiBr, and then heated at 70 ◦ C for 2 h to increase the dissolution. All the samples were filtered using 0.45 ␮m Teflon filters before injection into the SEC instrument.

3. Results and discussion For academic research and industrial application, the knowledge of both the chemical and physical properties of an ionic liquid is essential in order to design a suitable IL and optimize its use. Therefore, a study of the thermal properties of the solvent is very important in the choice of experimental conditions. It was discovered from TGA/DTA data that [HDBN][OAc] is stable at 100 ◦ C and no decomposition of [HDBN][OAc] occurred at the derivatization temperature (80 ◦ C). Furthermore, the melting point of the IL is 65 ◦ C, which corresponds to value obtained by Parviainen et al. (2013). More information on the TGA/DTA of the IL is provided in the supporting information.

3.1. Dissolution of cellulose in 1,5-diazabicyclo(4.3.0)non-5-enium acetate In order to increase cellulose dissolution efficiency at lower cellulose-to-IL solution viscosity, organic electrolyte solutions or co-solvents have been used at ambient temperature with a relatively low IL load (Andanson et al., 2014; Rinaldi, 2011; Tian, Fang, Jiang, & Sun, 2011; Xu, Zhang, Zhao, & Wang, 2013). Our IL is solid at room temperature and attempts to dissolve cellulose completely in [HDBN][OAc] without a co-solvent were challenging due to poor dispersion caused by high viscosity of the IL. In fact, the attempts resulted in the agglomeration of some of the cellulose particles into lumps. Henceforth, cellulose was dispersed in acetone, acetonitrile, DBN or DMSO before being added to the IL at 25 ◦ C and this tactics prevented the agglomeration of the cellulose. Consequently, the IL dissolved the cellulose and the co-solvent remained miscible in the IL as neutral, non-reactive spectator/dispersant. This is in line with the report of Gericke, Liebert, El Seoud and Heinze (2011) about the miscibility of DMSO and acetonitrile in [Amim][Cl], [Bmim][Cl], and [Emim][Cl] during the dissolution of cellulose in these ILs. However, in contrast to our report, acetone was not miscible with any of their ILs. Nevertheless, they predicted that it is possible to find a compatible ionic liquid that will dissolve cellulose and at the same time be miscible with acetone due to its excellent solvent properties. There are reports of ILs that are miscible with acetone in the literature (Ruiz, Ferro, Palomar, Ortega, & Rodriguez, 2013; Zhai, Wang, Zhao, Tang, & Wang, 2006). [HDBN][OAc] is polar; possess a high dipolarity/polarizability, relatively strong basicity due mainly to the acetate ion and moderate acidity mainly influenced by the [DBNH]+ cation. Acetone with its carbonyl oxygen atom might form a favorable hydrogen bonding with the acidic hydrogen atom on the cation of the ionic liquid. In addition, the hydrophilic and hydrophobic nature of acetone might enhance its solubility in the IL. Nevertheless, a detailed study of the interaction between [HDBN][OAc] and the co-solvents is outside the scope of this paper. However, the bond interactions of the dispersing agents can be described by empirical scales developed by Catalan (2009), which is made up of acidity (SA), basicity (SB), dipolarity (SdP) and polarizability (SP). These scales, together with Kaft-Taft parameters of

an IL (Parviainen et al., 2013) with an analogous structure similar to [HDBN][OAc] are collected in Table 1. It was discovered that DMSO as a co-solvent in comparison to acetone and acetonitrile, has the largest miscibility threshold before inducing precipitation of the cellulose in the cellulose-ILdispersing agent system. For example, large amounts of acetone resulted in the precipitation of the dissolved cellulose. Thus, less than 30 wt% of the co-solvent was used to disperse the cellulose before introducing the cellulose-co-solvent mixture into the IL in all the cases. However, we should emphasize here that we did not investigate the extend limits in which co-solvents could be added in the ternary cellulose solution. Nevertheless, native cellulose with degree of polymerization of about 1000 (i.e. DPw = 1056 ± 40) was readily dissolved in the solvent system within 15 min. However, in order to enhance the homogeneity of the solution and ensure complete dissolution, 0.5 h of dissolution time was selected. In addition to dispersing cellulose, a co-solvent with high hydrogen bond basicity (cf. SB of Table 1) accelerates the dissolution process and at the same time decreases the viscosity of the resultant solution (Stoppa, Hunger, & Buchner, 2008). The dissolution of cellulose in ionic liquid may be ascribed to the accessibility of anion of the ionic liquid (CH3 COO− ) to the hydroxyl proton of cellulose or high polarity of the electrolyte solution (Fawcett, Brooksby, Verbovy, Bako, & Palinkas, 2005; Moulthrop, Swatloski, Moyna, & Rogers, 2005; Remsing, Swatloski, Rogers, & Moyna, 2006; Wang, Gurau, & Rogers, 2012; Xu, Wang, & Wang, 2010). The polarity of the IL enables interaction and cleavage of hydrogen bonds in cellulose polymers to [HDBN]+ due to its high hydrogen bond basicity. Also, it has been proposed that the addition of a co-solvent to the ionic liquid separates the [HDBN][OAc] ions, thereby improving the solubility of the cellulose in the ionic liquid system (Zhao, Liu, Wang, & Zhang, 2013). As observed by optical microscopy, dissolution was suggested by the disappearance of the optically visible, individual fibers, while insoluble samples consisted of dense fiber networks. Upon addition of an anti-solvent e.g. ethanol, cellulose precipitated due to the disruption of the stabilized cellulose fragments, thus re-establishing the hydrogen bonding network. It is important to mention here that the cotton linters cellulose was neither soluble in acetone, acetonitrile nor DMSO but was soluble in DBN, which is one of the precursors of the ionic liquid. DBN dissolved more than 10 wt% cellulose at 70 ◦ C. At room temperature, the cellulose/DBN mixture was cloudy but homogeneous. After dissolution, the cellulose remained in the dissolved state at room temperature. 3.2. Acetylation of cellulose The acetylation of the dissolved cellulose was investigated by studying the average number of OH groups substituted by acetyl groups per a monomer unit. The results revealed that the acetylated celluloses have different properties due to varying reaction conditions (Table 2). In the absence of any dispersing agent (SCA2), acetylation of cellulose resulted in a product with very low DS value due to poor mass transfer and limited cellulose solutionacetylating agent interaction. Also, it was observed that upon use of [HDBN][OAc] without the acetylating agent (Ac2 O in this case), acetylated cellulose was not produced (SCA1), indicating that no exchange of the anion of the ionic liquid with the cellulose hydroxyl groups took place (control experiment). This was also confirmed by FTIR. Thus, the ionic liquid is stable at the reaction conditions. Furthermore, it was observed that products whose weights were equal or less than that of the native cellulose (1.20 g) underwent minor or no acetylation, whereas products with a clear weight increase (20% or more) displayed high DS values. As expected, the use of a co-solvent improved the acetylation process significantly. For example, the degree of acetylation of

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345

Table 1 Solvent parameters of 1,5-diazabicyclo(4.3.0)non-5-enium propionate [HDBN][CO2 Et] and the dispersing agents. Empirical scalesa

Solvent

SA

SB

SdP

SP

Acetone Acetonitrile DMSO DBN

0.000 0.044 0.072 –

0.475 0.286 0.647 –

0.907 0.974 1.000 –

0.651 0.645 0.830 –

Ionic liquid

Kaft-Taft parametersb

[HDBN][CO2 Et]

˛

ˇ



EN T

0.64

1.11

1.04

0.68

Miscibility with IL (1:1)

Cellulose solubility

+ + + +

− − − +*

+, soluble; −, insoluble; [a] Catalan (2009); [b] solvent parameters obtained from Parviainen et al. (2013); IL = 1,5-diazabicyclo(4.3.0)non-5-enium acetate; +*, soluble at 70 ◦ C.

Table 2 Synthesis of cellulose acetate at 80 ◦ C under different conditions.a Entry

Dispersing agent

Ac2 O/AGU molar ratio

Time (h)

DSb

SCA1 SCA2 SCA3 SCA4 SCA5 SCA6 SCA7 SCA8 SCA9 SCA10 SCA11 SCA12 SCA13 SCA14 SCA14r SCA15 SCA16

DMSO – DMSO DMSO DMSO DMSO DMSO Acetone ACN DBN DMSO DMSO ACN ACN ACN Acetone Acetone

0.0 5.0 2.5 2.5 5.0 5.0 5.0 5.0 5.0 5.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0

3.0 3.0 0.5 2.0 0.25 0.5 1.0 0.5 0.5 0.5 0.5 2.0 0.5 2.0 2.0 0.5 2.0

0.0 0.9 1.28 1.66 2.01 2.56 2.57 2.58 2.68 2.16 2.80 2.82 2.84 2.89 2.85 2.78 2.83

Solubility (CH3 )2 CO

DMSO

CH2 Cl2

CHCl3

− − − − + + + + + + − − − − − − −

− − + + + + + + + + + + + + + + +

− − − − − + + + + + + + + + + + +

− − − − − + + + + + + + + + + + +

DMSO, dimethylsulfoxide; DBN, 1,5-diazabicyclo(4.3.0)non-5-ene; ACN, acetonitrile; +, soluble; −, insoluble; 14r, cellulose acetate from the recycled ionic liquid. a 5 wt% of cellulose concentration was for all reactions. b DS, degree of substitution obtained from 1 H NMR.

synthesized cellulose acetate increased from 0.9 (reached at 3.0 h) to 2.16 (reached at 0.5 h) when DBN was added as the dispersing agent (SCA10) under similar experimental conditions. On the other hand, when DBN was replaced with any of the three co-solvents (SCAs 6, 8 or 9), appreciably higher DS values were obtained. Among the co-solvents, the use of DBN resulted in the lowest DS value simply because DBN scavenged the acetic acid by-product whereupon, more [HDBN][OAc] was formed. At the same time, it led to a reduction in the amount of acetic anhydride available to react with cellulose. Consequently, it is evident that the nature of the cosolvent is important not only for dissolution but also for cellulose acetylation. Although, the system diluted with ACN yielded cellulose acetates with the highest DS values under the same reaction conditions, but it is premature to generalize that one co-solvent is better than the other in terms of the differences in the rate of cellulose acetylation (DS values). Therefore, more experiments and precise analyses are needed for further conclusions to be drawn. Nevertheless, the preferred co-solvent should possess high hydrogen bond basicity (cf. SB of Table 1) capable of increasing the dissociation of the ionic liquid and at the same time providing suitable reaction environment for cellulose acetylation. However, if problems associated with recycling is to be taken into consideration, DBN as a co-solvent is a good option. In this case, no new component is added into the system and additional [HDBN][OAc] is generated during the processing. Interestingly, it was observed that the reaction proceeded much faster during the first 15 min. For example, acetylated cellulose with a DS of 2.01 was obtained

within 15 min (SCA5), whereas further increase in DS value to 2.56 occurred at 0.5 h (SCA6) and 2.57 at 1 h (SCA7). Therefore, 15–30 min was enough in order to obtain acetone-soluble cellulose acetates, which are of industrial interest. In effect, our process if compared to e.g. the procedure of Wu et al. (2004) is faster and produced cellulose acetates with a higher DS, even though the cellulose we used had a higher degree of polymerization. In addition, our procedure does not require the activation of cellulose before dissolution unlike other cellulose solvents (El seound, Marson, Ciacco, & Frollini, 2000; Takaragi, Minoda, Miyamoto, Liu, & Zhang, 1999). As expected, it was possible to control the DS value by the amount of the Ac2 O added to the reaction mixture. For example, as the ACAto-AGU molar ratio increased from 2.5 (SCA3) to 5.0 (SCA6), there was a corresponding linear increase in the DS. However, the increment decreased as the molar ratio increased to 7.0 (SCA11) under identical experimental conditions and it was observed that instead, cellulose triacetate was produced at this molar ratio in 0.5 h without any added acid or base. Using an Ac2 O-to-AGU molar ratio of 5.0, cellulose acetate produced in [Bmim][Cl] system (Heinze et al., 2005) from Avicel (DP = 286) yielded a higher DS (DS = 2.72 in 2 h) compared to our system, where cotton linters cellulose (DP ∼ 1000) was used to obtain acetylated celluloses with DS of 2.01 and 2.56 in 0.25 h and 0.5 h, respectively. However, the cellulose acetate produced (Heinze et al., 2005) was not soluble in chloroform or acetone in contrast to cellulose acetate synthesized in the [HDBN[OAc] system. Furthermore, the synthesis of [HDBN][OAc] is simple, with no added auxiliaries and more rapid processing times are achieved

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Fig. 3.

1

H NMR spectra (DMSO-d6) of cellulose acetates with (a) DS = 2.01, (b) DS = 2.89.

Table 3 Distribution of acetyl groups at position 2-, 3- and 6- of the AGU of cellulose acetate. Samples

Cellulose SCA2 SCA3 SCA4 SCA5 SCA6 SCA7 SCA8 SCA9 SCA10 SCA11 SCA12 SCA13 SCA14 SCA14r SCA15 SCA16

1

H NMR

DPw ± ␴

Titration

C-2

C-3

C-6

Aver. DS

DS

0.00 – 0.45 0.60 0.68 0.90 0.91 0.88 0.91 0.78 0.93 0.94 0.96 0.98 0.96 0.94 0.94

0.00 – 0.26 0.37 0.55 0.69 0.71 0.71 0.79 0.52 0.87 0.92 0.88 0.91 0.89 0.84 0.85

0.00 – 0.57 0.69 0.79 0.97 0.95 0.99 0.99 0.86 1.00 0.97 1.00 1.00 1.00 1.00 1.00

0.00 0.90 1.28 1.66 2.01 2.56 2.57 2.58 2.68 2.16 2.80 2.82 2.84 2.89 2.85 2.78 2.83

0.00 0.80 ± 0.1 1.30 ± 0.05 1.60 ± 0.05 1.94 ± 0.08 2.32 ± 0.20 2.31 ± 0.18 2.50 ± 0.10 2.70 ± 0.03 2.14 ± 0.06 2.69 ± 0.11 2.53 ± 0.22 2.71 ± 0.08 2.68 ± 0.10 2.77 ± 0.03 2.91 ± 0.04 2.75 ± 0.04

1056 ± 40 – – 780v 699 ± 49 759 ± 116 – – – – 599 ± 16 588 ± 8 593 ± 18 638 ± 43 698 ± 131 624 ± 43 620 ± 50

v = weak signal, aver. = average.

than in the case of other IL-based processes (Heinze et al., 2005; Wu et al., 2004; Yang, Xie, & Liu, 2014). The processing times include both the acetylating and dissolution times.

synthesized CAs in both organic and inorganic solvents is important for their subsequent processing. 4.2. Nuclear magnetic resonance (NMR)

4. Properties of synthesized cellulose acetate 4.1. Solubility The solubility of the synthesized acetylated cellulose in acetone, chloroform, dichloromethane (DCM) and DMSO was partially dependent on the degree of substitution (Table 2). All synthesized cellulose acetates were soluble in DMSO at room temperature. Cellulose acetates with DS range of 2.01–2.68 were soluble in acetone as well as the three other solvents. It was only in one case (SCA5) that the resulting cellulose acetate was insoluble in CH2 Cl2 or CHCl3 due to partial substitution. These observations are partly consistent with literature data (Edgar, 2004). According to the previous investigations; cellulose acetates with DS <2.7 are supposedly insoluble in CH2 Cl2 or CHCl3 . Nevertheless, in our case, SCAs 6–10 were exceptions and indeed soluble in the abovementioned solvents. The reason is likely due to the fact that the solubility of cellulose derivatives does not depend on the average DS alone but also on the distribution of the acetyl substituents in the AGUs and along, as well as among the cellulose chains (Kamide, Okajima, Kowsaka, & Matsui, 1987). Thus, a small difference in substitution patterns can lead to large differences in solubility in certain cases (Kern et al., 2000). The synthesized triacetate celluloses (SCAs 11–16), as expected, are insoluble in acetone. Generally, the solubility of the

1 H NMR was used to determine the total degree of substitution as well as individual contribution of the different position available in the AGU of cellulose, i.e. C-2, C-3 and C-6. Luckily, clearly resolved signals of the methyl carbon atoms of the acetyl groups for both partially and highly substituted cellulose acetates could be recorded. The acetyl moieties of the three hydroxyl groups were estimated by integrating the carbonyl carbon area of the 1 H NMR spectra (Sun, Lu, Zhang, Tian, & Zhang, 2013; Luo & Sun, 2006). Thus, representative 1 H NMR spectra of some of the partially substituted synthesized CA with annotations for the major peaks are shown in Fig. 3. Signals from the AGU can be seen in the region 2.8–5.2 ppm and signals from the acetyl CH3 -groups appeared at 1.6–2.3 ppm. The solvent peak from DMSO is located at 2.5 ppm. Furthermore, a peak from residual water at 3.33 ppm is clearly seen in all spectra. The water peak partially overlapped with the AGU signals and this has to be taken into consideration upon determination of DS either by omitting the water peak region in the integration (Cao et al., 2014) or by adding a small amount of TFA-d (Ass, Ciacco, & Frollini, 2006; Chen, Chen, Zhang, Liu, & Sun, 2014). The water peak was fitted using a Lorentzian peak shape and it was subtracted from the spectra prior to integration of the AGU region. Total DS, as well as distribution of acetyl moieties on the different OH-groups are listed in Table 3. C-6 position,

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Fig. 4.

13

347

C NMR spectra (DMSO-d6) of (a) SCA5, (b) SCA14.

which is the least sterically hindered hydroxyl group of the AGU, is more or less fully substituted in most of the synthesized CAs (DS ∼ 1). Some of the 13 C NMR spectra of the acetylated cellulose are given in Fig. 4. For all the samples, it can be seen that the reactivity of the OH-groups differ with C-3 being the least substituted. In essence, C6-OH > C2-OH > C3-OH: this trend is similar to observations made upon homogenous acetylation of cellulose in 1-allyl-3-methylimidazolium chloride (Cao et al., 2007), LiCl/1,3-dimethyl-2-imidazolidinone (Takaragi et al., 1999) and LiCl/N,N-dimethylacetamide (Marson & El Seoud, 1999). However, this trend is different from observations reported in previous studies (Cao, Zhang, He, Li, & Zhang, 2010; Wu et al., 2004), where C6-OH > C3-OH > C2-OH. The DS values of cellulose acetates obtained from 1 H NMR analysis were controlled by comparing them with titration result and a good agreement between the values was noted. The DP values of the cotton linters cellulose and some of the synthesized cellulose acetates were determined by GPC. Degradation of the cellulose acetates took place during the acetylating process (Table 3). The Fourier Transform Infrared Spectroscopy (FTIR) results also confirmed the formation of cellulose acetates. The spectra and discussion are depicted in the supporting information. 4.3. Recycling of 1,5-diazabicyclo(4.3.0)non-5-enium acetate Ionic liquids are generally more expensive than conventional solvents and thus the industrial uses of ionic liquids require efficient recycling and re-use strategies. Maximizing recycling is one of the ways of contributing to sustainability and sustainable development of industrial processes; therefore, the recyclability of the ionic liquid was studied since DBN, one of the precursors, is relatively expensive. After the cellulose acetylation reaction, the cellulose acetate was precipitated with methanol leaving the filtrate. The residual ionic liquid was subjected to vacuum treatment to recover the low-boiling solvents such as acetone, methanol (used for precipitation), and acetonitrile (if acetone or acetonitrile was used as a co-solvent), thus leaving pure [HDBN][OAc]. If DMSO was used as the co-solvent, it could be collected together with the ionic liquid. Preliminary studies revealed that the recycled ionic liquid-DMSO system was able to dissolve and acetylate cellulose, which indicated that the solvent system could be re-used. The DS of the cellulose acetate produced from the recycled ionic liquid is 2.85 (SCA14r, Table 2), while that of the fresh ionic liquid is 2.89 (SCA14). It is important to mention here that if DBN is used as the dispersing agent, the numbers of unit operations required upon recycling of the IL are reduced; in essence, no new chemical species are added

into the system. The cost of the DBN can be reduced by bulk purchase.

5. Conclusions 1,5-diazabicyclo(4.3.0)non-5-enium acetate [HDBN][OAc]based solvent systems were found to be very efficient in dissolving cellulose in the presence of dispersing agent such as acetone, acetonitrile, DBN or DMSO under mild conditions. This ternary cellulose/ionic liquid/dispersing agent system was treated with acetic anhydride under different conditions and cellulose acetates with different degrees of substitution were obtained. It was deduced that 0.5 h is enough for the completion of the acetylation reaction. Cellulose acetate produced when using DBN as the dispersing agent, resulted in the lowest DS under identical experimental conditions, but additional [HDBN][OAc] was produced in the process. The acetylated celluloses synthesized were soluble in acetone, chloroform, dichloromethane and DMSO depending on their average DS and the distribution of the acetyl substituents in the AGUs of the cellulose chains. The degree of substitution was determined by titration and 1 H NMR and further confirmed by FTIR. The NMR results demonstrated that the distribution of the acetyl moiety among the three OH groups of the anhydroglucose unit shows a preference for the C-6 position. Furthermore, the ionic liquid and co-solvents can be recovered from the filtrate after the precipitation of the acetylated cellulose and re-used in consecutive acetylation batches. The solvent system can be used to synthesize other cellulose esters.

Acknowledgements This work is a part of activities at the Åbo Akademi Johan Gadolin Process Chemistry Centre (PCC), a Centre of Excellence financed by the Åbo Akademi University in collaboration with the activities of the Wallenberg Wood Science Center (WWSC). Financial support from the Kempe Foundations, the Knut and Alice Wallenberg Foundation, the Bio4Energy programme and the Academy of Finland are gratefully acknowledged.

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:10.1016/j.carbpol.2015.08.092.

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