L. P.
Hinner
,
J. L.
Wissner
,
A.
Beurer
,
B. A.
Nebel
and
B.
Hauer
*
Institut für Technische Biochemie, Allmandring 31, Universtität Stuttgart, D-70569 Stuttgart, Germany. E-mail: bernhard.hauer@itb.uni-stuttgart.de
First published on 15th August 2016
A homogeneous acylation of cellulose with different vinyl esters in the biodegradable and less toxic ionic liquid 1-ethyl-3-methyl-imidazolium acetate ([EMIM]OAc) is described for the first time. The reaction proceeds in the absence of any additional catalyst and glucose- and cellulose-esters with chain lengths of C8 to C16 are accessible by using equimolar amounts of acyl donor. Cellulose esters with a degree of substitution (DS) in the range of 0.9–3.0 were synthesised successfully. Different reaction parameters like reaction time, temperature and amount of substrate were systematically changed and analysed by NMR, IR and HPLC-GPC. The highest DS was achieved at 80 °C and a reaction time of 2 hours. Taking into consideration the literature, the DS and degree of polymerisation (DP) of fatty acid chloride and vinyl ester-based synthesis routes were compared. Similar DS-values were obtained, but the DP was significantly reduced during the synthesis using fatty acid chlorides in [BMIM]Cl. As an undesirable side reaction, acetates from [EMIM]OAc are bound to the cellulose backbone. The quantity of bound acetate groups during vinyl ester-based synthesis rose with decreasing polarity of the substrates but overall proved to be much lower compared to the literature described anhydride or fatty acid chloride based synthesis routes in [EMIM]OAc. This novel process was extended by using further acyl donors like vinyl benzoate, pivalate and 2-ethylhexanoate to demonstrate the applicability of the vinyl ester-based cellulose modification in [EMIM]OAc. [EMIM]OAc was recycled with an efficiency of ∼90% and reused for subsequent syntheses.
Cellulose consists of several anhydroglucose units (AGU), which are β-(1→4)-glycosidically linked, thereby forming long polymer chains and a very strong hydrogen bond network with both intra- and inter-chain hydrogen bonds. This strong hydrogen bond network makes it very difficult to process cellulose because cellulose is not soluble in common organic solvents.2
The modification of cellulose, among others, can be achieved by esterification of the free cellulose hydroxy groups. However, the present technologies for cellulose modifications are mainly limited to heterogeneous reaction processes, where cellulose is not solubilised and an excess of acyl donor is required.3 Due to the strong steric hindrance of non-solubilised cellulose, such strategies are limited to the use of short acyl chains (C2–C4). In contrast, a homogeneous modification procedure of cellulose, where cellulose is solubilised, leads to benefits like control of the amount and distribution of substituents along the polysaccharide backbone, where even longer, sterically challenging acyl chains can be introduced.4 A homogeneous modification of cellulose can be realised in ionic liquids capable of solubilising cellulose.5–7
Parameters like degree of substitution (DS) and degree of polymerization (DP) are important to characterize the modified cellulose. The DS defines how many alcohol groups of cellulose are modified (n = 0–3). For thermoplastic applications of cellulose acetate, a DS of 2.5 is often applied because this value allows thermoplastic processability. However, the desired DS is dependent on the particular applications of produced materials and thus a good synthesis should permit a huge variety of modifications. The, e.g., heterogeneous process for cellulose acetate production is a multi-step process where cellulose is first fully acetylated (DS = 3) followed by a partial deacetylation step. A homogeneous synthesis protocol would allow an active DS adjustment by using different amounts of acyl donors.
The esterification of cellulose is usually carried out with highly reactive and corrosive carboxylic acid chlorides or anhydrides.4 The usage of anhydrides leads to high conversion rates but large amounts of acyl donor react to yield undesirable carboxylic acid side products. In addition, especially long-chain fatty acid anhydrides are highly nonpolar, which results in a minor solubility and a reduced mass transfer in polar cellulose solvent systems. In the case of carboxylic acid chlorides, acid scavengers are needed to avoid accumulation of highly reactive hydrogen chloride.8 Despite this, the usage of acetyl chloride in combination with some halogen containing ionic liquids leads to significant degradation of the cellulose backbone and the ionic liquid.9 In this context, the ionic liquid [EMIM]OAc displays a halogen free alternative with a high cellulose dissolution capability. In addition, [EMIM]OAc is, in comparison with halogen containing ionic liquids, less toxic and even biodegradable.10 [EMIM]OAc shows further benefits like non-volatility and a low melting point, but is, in this combination, not a complete inert solvent because it leads to a slight acetylation of 1.5 mol% of all cellulose hydroxy groups.11
Additionally, it was shown that the usage of anhydrides or chlorinated substrates as an acetylation agent led to the activation of the ionic liquid acetates and thereby to an increased acetylation of cellulose.10,12 This effect hampers the control of the substitution pattern of cellulose in the environmentally friendly ionic liquid [EMIM]OAc and limits its industrial application.
To reduce the acetylation effect induced by acyl donors and catalysed by [EMIM]OAc, the usage of less reactive acyl donors is recommended. A promising alternative to anhydride or acyl chloride based syntheses is the transesterification of cellulose originating from vinyl esters. A homogeneous vinyl ester-based synthesis of cellulose ester was already applied in a DMSO/tetrabutylammonium fluoride (TBAF) system, yielding cellulose esters with a high DS.13 As a significant drawback, the reaction suffers from long reaction times, up to days, and high substrate excess, necessary to accomplish high DS-values. In addition, the difficult recycling process of the DMSO/TBAF system and its flammability hamper industrial scale applications.12
Recently, a vinyl ester-based cellulose modification was carried out in a heterogeneous reaction system in DMSO with NaOH or KOH being used as catalysts. This cellulose ester process requires high substrate concentrations and is limited to short acyl chains of C2 to C4. However, due to low steric hindrance, synthesis was achieved in only five minutes.14 In contrast to this heterogeneous process, the vinyl ester-based synthesis of starch acetate in alkaline ionic liquids like [BMIM]OAc was described to be homogeneous.15 The catalytic property of some low molecular salts like acetates for transesterification forming starch acetates is a well-known principle.16 The catalytic principle in the context of ionic liquids was first described by Shogren and Biswas and recently used for the synthesis of cellulose acetate in [EMIM]OAc based on isopropenyl acetate as the acyl donor.15 This reaction was shown to proceed in minutes, but still high acyl donor concentrations are required.
In the present study, the first vinyl ester-based synthesis of cellulose derivatives in [EMIM]OAc is demonstrated. A variety of cellulose esters using long chain fatty acids, aromatic, branched and sterically challenging substrates, like pivalate, as acyl donors are accessible. This process leads to a high DS requiring a low amount of acyl donor, it ensures a constant high degree of polymerisation and allows significantly lower acetate activation in comparison with the anhydride based synthesis in [EMIM]OAc. Apart from these facts the vinyl ester-based synthesis shows benefits like high conversion rates, volatile side products, no need for acid scavengers, reusability of [EMIM]OAc, mild reaction conditions, low toxicity and biodegradable solvents.
Equimolar concentration of acyl donor (3 mol per AGU) resulted in a DS of 2.4, representing a conversion of 80% of cellulose hydroxy groups. Increasing the amount of acyl donor to 4 mol per AGU led to a higher DS of 2.9, while a further increase of substrate to 5 mol per AGU did not result in a higher DS (Table 1). All produced cellulose laurates were soluble in [EMIM]OAc and product separation was achieved by the addition of water. In comparison with the DMSO/TBAF reaction system, based on 10 mol per AGU vinyl laurate to obtain a DS of 2.6, the present process with significantly lower acyl donor concentrations resulted in even higher DS-values.13 Furthermore, the vinyl ester-based synthesis in [EMIM]OAc is as efficient with regard to the amount of acyl donor as the synthesis using fatty acid chloride in [BMIM]Cl. For instance, it has been reported that 5 mol steroyl chloride per AGU led to DS values of 2.6.8 In this case it has to be mentioned that a potentially higher steric hindrance in the case of a longer chain length (C18) may cause a lower synthesis efficiency. For a better understanding we synthesised cellulose laurate in [BMIM]Cl at 80 °C using lauroyl chloride and achieved a DS-value of 2.3, showing comparable potency of vinyl ester-based synthesis in [EMIM]OAc (DS 2.4) (Table 2). We further investigated the influence of the reaction temperature and time under equimolar reaction conditions. At reaction temperatures of 40–60 °C and 4 h reaction time the vinyl ester-based synthesis in [EMIM]OAc created DS-values lower than 2.1. The highest DS in the range of 2.3 to 2.4 was achieved at 70 °C or 80 °C, respectively, while a further increase to 90 °C and 110 °C resulted in lower DS values in the range of 1.9 to 2.2 (Fig. 3). The temperature optimum between 70 °C and 80 °C differed from the previously investigated temperature optimum in the synthesis of glucose laurate synthesis at 60 °C (S1†). Those differences can be explained by a higher viscosity during the reaction, which can result in lower mass transfer rates of solutions of cellulose in [EMIM]OAc compared to glucose in [EMIM]OAc. The optimised reaction temperature of 80 °C has already been applied for the synthesis of cellulose esters using fatty acid chlorides.8,9 The vinyl ester-based synthesis in [EMIM]OAc could be carried out at lower temperatures between 40 °C and 50 °C, enabling even more mild reaction conditions. However, this requires a longer reaction time and/or a higher acyl donor concentration to achieve a comparable degree of esterification (Fig. 3). The influence of the reaction time was investigated at an optimum temperature of 80 °C. During the first 30 minutes, the reaction proceeded fast and led to a DS-value of 2.1. After 60 minutes, the reaction was finished and DS-values between 2.1 and 2.4 were measured. No further increase in the DS of cellulose laurate could be detected up to 4 hours (Fig. 4). The reaction was more than 10-fold faster compared to vinyl ester-based synthesis in DMSO/TBAF where a reaction time of 70 hours was needed.13 We conclude that the reaction temperature and time have a great influence on the reaction speed and degree of substitutions of the produced cellulose ester.
Product | Vinyl laurate [mol per AGU] | DS |
---|---|---|
Cellulose laurate | 2 | 1.6 |
3 | 2.4 | |
4 | 2.9 | |
5 | 2.8 |
Compound | Ionic liquid | Vinyl laurate [mol per AGU] | Temp. [°C] | Time [h] | DS | M w [g mol−1] | M n [g mol−1] |
---|---|---|---|---|---|---|---|
Vinyl laurate | [EMIM]OAc | 3 | 80 | 4 | 2.4 | 280368 | 89936 |
Lauroyl chloride | [BMIM]Cl | 3 | 80 | 2 | 2.3 | 8743 | 5630 |
We compared the DSacetate after syntheses of cellulose laurates using lauroyl chloride, lauric anhydride and vinyl laurate in [EMIM]OAc. In accordance with the literature, we detected a majority of DSacetate towards DSlaurate using lauroyl chloride and lauric anhydride, respectively.10 The highly reactive lauroyl chlorides created 88.2% acetylation while lauric anhydride gave 83.3% acetylation of all substituted hydroxyl groups (DStotal) (Table 3). Vinyl laurate induced an acetylation of only 11.1%, which implies that the majority of 88.9% of all OH-groups is modified by the introduced acyl donor. These results show that the usage of vinyl esters allows a better control of the substitution pattern of cellulose in [EMIM]OAc in comparison with fatty acid chloride or anhydride based syntheses. In addition, the usage of fatty acid chlorides and anhydrides with equimolar substrate concentrations caused a significantly reduced DSacyl donor of 0.3 compared to the vinyl laurate synthesis, where the DSacyl donor is higher than 2.1. Furthermore, the amount of additional acetate groups varied not only between fatty acid chlorides, anhydrides and vinyl esters, but also different side chains resulted in differences in %-DSacetate. Esterification with sterically challenging and highly nonpolar vinyl palmitate gave a higher additional acetylation of 20.0%, while the application of shorter and less nonpolar vinyl laurate or vinyl octanoate resulted in significantly lower acetylation, of 11.1% and 6.3%, respectively (Fig. 5 and Table 3). In addition, a %-DSacetate of 4.2% to 4.5% of cellulose was achieved using the shorter and less nonpolar vinyl pivalate and vinyl benzoate (Table 3). However, the moderate nonpolar but branched and sterically demanding vinyl 2-ethylhexyloate leads to a significantly smaller DSacyl donor of 0.9 and a combined elevated %-DSacetate of 18.1% (Table 3). Such results suggest that polarity and steric properties have a great influence on additional acetylation.
Acyl donor | Acyl donor [mol per AGU] | DSacyl donor | DSacetate | DStotal | %-DSacetate [%] |
---|---|---|---|---|---|
Vinyl octanoate (C8) | 2 | 1.5 | 0.1 | 1.6 | 6.3 |
Vinyl laurate (C12) | 2 | 1.6 | 0.2 | 1.8 | 11.1 |
Vinyl palmitate (C16) | 2 | 1.6 | 0.4 | 2.0 | 20.0 |
Vinyl benzoate | 3 | 2.3 | 0.1 | 2.4 | 4.2 |
Vinyl pivalate | 3 | 2.1 | 0.1 | 2.2 | 4.5 |
Vinyl 2-ethylhexyloate | 3 | 0.9 | 0.2 | 1.1 | 18.1 |
Lauroyl chloride | 3 | 0.04 | 0.3 | 0.34 | 88.2 |
Lauric anhydride | 3 | 0.06 | 0.3 | 0.36 | 83.3 |
Regarding the reaction time, it can be speculated that the activation of acetates in [EMIM]OAc using vinyl esters is much slower compared to anhydrides showing a reduced acetylation of cellulose. The acylation of cellulose with vinyl esters is faster compared to the undesirable activation of acetate. Decreasing the polarity of introduced vinyl esters resulted a low solubility as well as reduced mass transfer in polar [EMIM]OAc and therefore a slower reaction. Assuming that the vinyl ester-based acetate activation proceeds at the phase interface and is faster than the mass transfer of nonpolar acyl donor, an increased acetylation of the cellulose backbone, as observed, is consequential. For sterically demanding substrates like vinyl 2-ethylhexyloate, the acylation is hindered which supports acetylation. Apart from these assumptions, it can be pointed out that the vinyl ester-based synthesis allows a gentle modification of cellulose with a comparably low acetylation. Especially if unbranched, relatively polar vinyl esters are used, acetylation is limited to low DSacetate values.
The cellulose 2-ethylhexyloate with a DS2-ethylhexyloate of 0.9 and a DSacetate of 0.1 was also prepared. 1H-NMR confirmed the formation of cellulose 2-ethylhexyloate. The two terminal methyl groups (ε,γ’) displayed signals at δ = 0.95–0.70 ppm (6H), the protons of the γ- and δ-carbon of hexyl groups gave δ = 1.35–1.10 ppm (4H) and the protons of the β-carbon of the hexyl group and the β’-carbon of the ethyl side chain led to a peak at δ = 1.65–1.35 ppm (4H) (Fig. 6B). At δ = 2.10–1.85 ppm a signal from the α-carbon of the additional bound acetate appeared in the 1H-NMR spectrum (Fig. 6B). The IR spectrum of cellulose 2-ethylhexyloate showed distinctive signals at 3500 cm−1 (O–H), signals between 2872 cm−1 and 2958 cm−1 (C–H) and a band at 1732 cm−1 (CO in OC–O–R) (S3†). The vinyl ester-based synthesis in [EMIM]OAc enabled the gentle synthesis of a huge variety of cellulose esters with high and moderate degrees of substitution. The products were soluble in the ionic liquid and product separation was achieved by the addition of water. Not only fatty acid esters with different chain lengths, even aromatic, bulky and branched cellulose modifications were successfully synthesised.
Reactor run | [EMIM]OAc input [g] | [EMIM]OAc recycled [g] | Recycling efficiency [%] |
---|---|---|---|
1. Synthesis | 200 | 186 | 93 |
2. Synthesis (1. recycl.) | 186 | 163 | 88 |
3. Synthesis (2. recycl.) | 163 | 156 | 96 |
4. Synthesis (3. recycl.) | 156 | 137 | 88 |
In general, the recycling of the ionic liquid is a key parameter for an ecologically friendly and economically feasible process. In that context, King et al. discussed the possibility of distilling [EMIM]OAc under special conditions, but they discussed this option very sceptically regarding the purity and corrosiveness of the recycled ionic liquid.22 Therefore, evaporation strategies for ionic liquids seem inappropriate for [EMIM]OAc.
In our case we have chosen the often applied anti-solvent strategy where the addition of a solvent leads to product precipitation and enables separation.9,23,24 Afterwards the evaporation of the volatile anti-solvent recovers the ionic liquid. The performed recycling experiments using water as an anti-solvent demonstrate a recycling efficiency of about 90%. In contrast to the synthesis efficiency, the recycling efficiency is not hampered by each recycling step. In addition, the ability of the ionic liquid to solubilize cellulose is not negatively affected. The usage of other anti-solvents like ethanol or acetone might be cost saving alternatives due to lower boiling points24 but these solvents enhance the hazardousness of the process due to their flammability. The loss of about 10% w/w ionic liquid can be explained by the residual ionic liquid, which remained in the crude product after the water induced precipitation and was washed out during the methanol purification step.
[BMIM]Cl and [EMIM]OAc were received from IoLiTec-Ionic Liquids Technologies GmbH, Heilbronn, Germany. ReadyCal-Kit Poly(styrene) Mp 474–2520000 Da and gel permeation column SDV, linear M 10 μM, 8 × 300 mm were purchased from Polymer Standard Service GmbH, Mainz, Germany. DMSO-d6 was purchased from Euriso-Top GmbH, Saarbrücken, Germany.
In the case of cellulose 2-ethylhexyloate the integral of the two terminal methyl groups (I2methyl) at 0.78–0.92 ppm, for cellulose pivalate the integral of the three terminal methyl groups (I3methyl) at 0.90–1.35 ppm and for cellulose benzoate the area of the phenyl protons (Iphenyl) at 6.90–8.10 ppm were used to determine the corresponding DS-values consistent with the following equations:
For cellulose pivalate, ethylhexyloate, benzoate and for low to medium substituted cellulose fatty acid esters also the DSacetate was calculated by using the area of the acetate methyl groups (Iacetate) at 1.85–2.07 ppm in relation to the integral of AGU (IAGU) at 3.50–5.50 ppm according to the equation:
The FT-IR spectra were recorded with a FT-IR spectrometer (Bruker, Vector 22, equipped with a MKII golden gate single reflection diamond ATR-system).
Gel permeation chromatography was performed using an Agilent Technology 1260 Infinity ELSD-HPLC system (Agilent, Santa Clara, USA) using a SDV, linear M 10 μM, 8 × 300 mm, column. The flow rate and temperature were 1.5 mL min−1 and 40 °C. THF was used as the eluent and the solvent for cellulose ester dissolution. The method stayed isocratic for 10 minutes. For molecular weight calibration, ReadyCal-Kit Poly(styrene) Mp 474–2520000 Da from PSS (Mainz, Germany) was used. The calculation of molecular weight distribution (Mw, Mn) was carried out using Agilent Cirrus GPC multi detector software.
Glucose laurates (mono-, di-, tri-, tetra-) were separated using an Agilent Eclipse XDP (CN-column, 150 × 4.6 mm, 5 μm). The flow rate and temperature were 0.8 mL min−1 and 35 °C. The method is isocratic at 5% ACN and 95% water (0.1% v/v formic acid) for the first 3 minutes. Afterwards an ACN–water (0.1% v/v formic acid) gradient increased from 3% ACN to 98% ACN in 15 minutes and stayed isocratic for 5 minutes. Glucose laurates were analysed by HPLC-ELSD (Agilent Technology 1260 Infinity HPLC-ELSD system) and identified by HPLC-MS (Agilent Technologies 6130 Quadrupole MS). The LC-MS was operated in API-ES mode under the following conditions: drying gas flow 10 L min−1, nebuliser 40 psig, drying gas temperature 300 °C, and +/− Vcap 3000. The system was used in positive and negative scan modes with a fragmentor voltage of 70 eV to cover values between m/z 100 and 1500. The ELSD was operated at a nebuliser temperature of 90 °C, an evaporator temperature of 40 °C and a gas flow of 1 SLM. Quantification of glucose laurates was performed by using the peak area of the detector response because no standard of glucose laurates was available.
Footnote |
† Electronic supplementary information (ESI) available: Influence of temperature and time on the production of glucose laurates in [EMIM]OAc. IR-spectra of cellulose 2-ethylhexyloate, cellulose pivalate, and cellulose benzoate. See DOI: 10.1039/c6gc02005d |
This journal is © The Royal Society of Chemistry 2016 |