Efficient and rapid direct transesterification reactions of cellulose with isopropenyl acetate in ionic liquids

Abstract

We describe a conceptually novel protocol that allowed the facile transesterification reaction of cellulose without any additional catalysts and corrosive chemicals. The key concept in this cellulose modification is the dual functionalities of ionic liquids, namely a solvent for reactants and an activating reagent for transesterification reactions.

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Driven by the growing fear of the depletion of fossil resources, naturally occurring polymers, especially polysaccharide derivatives, are now spotlighted once again in order to achieve greener chemical production^1–3 as well as functionalized materials by taking advantage of the nature of polysaccharides.^4–7 Among the polysaccharides, cellulose is one of the most important candidates as a chemical feedstock. Although the chemistry of cellulose is expected to achieve a sustainable polymer production, cellulose chemistry has faced a natural and intrinsic solubility limitation because cellulose is well-known to be insoluble in many common organic and aqueous media due to the strong hydrogen-bonding networks within its molecular structures. This has limited the synthetic and thus industrial possibilities of cellulose based polymers.

In 2002, Rogers and co-workers reported a pioneering and ground-breaking study in the field of the cellulose utilization based on ionic liquids (ILs).⁸ They have successfully demonstrated that ILs showed a significant ability to dissolve cellulose under mild conditions. This finding sparked intense interests in not only the creation of ILs with the efficient ability to dissolve cellulose,^9–15 but also the chemical modification of the cellulose^16–21 once again. For example, Heinze and co-workers intensively focused on the homogeneous cellulose modification reactions in ionic liquids to produce diverse cellulose derivatives.²² To be precise, they have successfully demonstrated that homogenous cellulose solutions in ILs were ideal for cellulose chemistry where cellulose hydroxyl groups reacted with a range of reactants such as acid chlorides, acid anhydrides, isocyanates, etc. Despite that the cellulose modification with acid chlorides or acid anhydrides is advantageous from the view point of reactant loadings,²² highly reactive and thus corrosive characters of the above-mentioned chemicals also need to be considered. Recently, Meier and Barner-Kowollik demonstrated that organocatalytic transesterification reactions of cellulose in ILs with non-activated esters occurred to afford modified cellulose derivatives with a limited degree of substitution values (DS, less than 0.7).²³ As discussed, the cellulose features only hydroxyl groups as a reacting functionality, cellulose modification requires either a highly reactive reagent or efficient catalysts. Thus, it still remains a challenging task for chemists to propose a synthetic strategy for the cellulose modifications without using the corrosive chemicals and additional activating reagents while still maintaining high hydroxyl group conversions.

When considering the synthetic potentials of ILs, ILs show not only the unique property of dissolving cellulose, but also tailored functionalities empowered by the precise molecular design of cations and anions of the ILs. Especially, ILs featuring precisely tailored cations and anions have been found to behave as a catalyst for organic transformation reactions. In this context, such tailored ILs are rationally expected to offer dual functionalities for cellulose chemistry, namely an efficient solvent for cellulose and an activating species for the organic reactions of cellulose.^24,25 For example, Abbott et al. accomplished a successful cellulose acetylation reaction by using Zn-based acidic ionic liquids featuring Zn ion as a Lewis acid.²⁴ Very recently, Kilpeläinen and King demonstrated that the methyltrioctylphosphonium acetate ([P₈₈₈₁][OAc]) and DMSO mixed solvent can accomplish the facile reaction of cellulose with dimethyl carbonate with [P₈₈₈₁][OAc] being an organocatalyst to afford modified cellulose with fairly low DS values (less than 1.0).²⁵ Thus, it is still a challenging task for chemists to take full advantage of the dual functionalities of ILs, namely a solvent for cellulose and activating species for the organic transformation reactions, which is expected to provide a simple but highly reliable synthetic strategy for cellulose modifications.

During the course of our study, we have found that the transesterification reaction (TER) of cellulose with isopropenyl acetate (IPA) in 1-ethyl-3-methyl-imidazolium acetate (EmimOAc) smoothly occurred to produce cellulose triacetate without any additional catalyst or corrosive chemicals, with which we can take full advantage of the dual functionalities of ILs, namely a solvent for cellulose and an activating agent for TER. In this study, we describe (1) the characterization and optimization of the direct transesterification of cellulose with IPA in EmimOAc without any additional chemicals, and (2) the kinetic insight into the direct TER of the cellulose with EmimOAc being a solvent and an activating reagent (Scheme 1).

Scheme 1 Schematic representation of the transesterification reaction of cellulose in EmimOAc with EmimOAc as a solvent and an activating reagent.

As envisioned by significant properties of EmimOAc as an activating agent for organic transformation reactions such as the spontaneous generation of carbene species,²⁶ TER was selected because cellulose features hydroxyl moieties as chemically reactive groups. In order to confirm whether or not the EmimOAc served as both a solvent for the cellulose and an activating agent for the TER, the reaction of cellulose with stable esters was conducted. To be precise, a 3 wt% cellulose solution in EmimOAc was treated for 24 hours with an excess amount of IPA as an ester donating component at 80 °C under an Ar atmosphere. Surprisingly, the TER of the cellulose without additional chemicals was obviously confirmed by the clear transition from the biphasic reaction mixture to a homogeneous reaction solution, suggesting that the cellulose was chemically modified and thus the strong hydrogen bonding between the hydroxyl groups was switched off. The obtained polymer was purified by reprecipitation into methanol to afford a pale-colored solid as the product. IR measurements of the obtained polymers before and after the TER revealed that the band at 1738 cm⁻¹ due to the ester stretching clearly developed after the reaction, supporting the facile TER of the cellulose (Fig. 1). Along with C=O peak at 1738 cm⁻¹, C–H absorption at 1365 cm⁻¹ and C–O one at 1211 cm⁻¹ were also observed in Fig. 1.^27,28 In the ¹H NMR spectrum of the obtained product, peaks due to the cellulose backbone appeared in the region ranging from 3.0 to 5.3 ppm and peaks due to the acetyl groups appeared at 2.0 ppm (Fig. 2A). Based on these results, the obtained product was assigned as the cellulose acetate.^29–35 In order to provide a detailed structural insight into the obtained cellulose acetate, the degree of substitution (DS) value was determined for the obtained cellulose acetate. The DS was determined based on the ¹H NMR measurements of the cellulose derivative that was obtained by the per-benzoylation reaction of the obtained cellulose acetate with benzoyl chloride in CHCl₃ with NEt₃ as a proton scavenger. As supported by the ¹H NMR measurement of the obtained polymer (Fig. S-1, ESI†), the hydroxyl group conversion of the cellulose was determined to be 98.7%, corresponding to a DS of 2.96, showing that the TER of cellulose in EmimOAc without any additional reagents proceeded with a high efficiency. In addition to the ¹H NMR measurements of the obtained product, the ¹³C NMR of the recovered polymer further guaranteed high DS values because a clearly unimodal peak due to the anomeric carbon appeared at 100 ppm and only three distinct peaks due to the carbonyl carbons were observed in the region ranging from 168 to 171 ppm (Fig. 2B). In addition, a size exclusion chromatography measurement of the obtained polymer showed the fairly high weight average molecular weight of 62 000 g·mol⁻¹ with the polydispersity index of 3.1 being fair for naturally-occurring polymers.

Fig. 1 ATR-mode FT-IR spectra of original cellulose (upper) and the obtained cellulose acetate with DS of 2.96 (lower).

Fig. 2 ¹H (upper) and ¹³C (lower) NMR spectra in CDCl₃ of the obtained cellulose acetate with DS of 2.96 measured at 55 °C. The symbol (*) refers to a peak due to the NMR solvent.

In order to provide a preliminary insight into the mechanism of the TER of cellulose in EmimOAc, control experiments were carried out. Thus, a TER of cellulose with IPA was conducted in 1-butyl-3-methyl imidazolium chloride, in which the counter anion was known to be very weakly basic. As expected, no distinct TER of cellulose was confirmed by the IR measurements of the recovered polymer (Fig. S-2, ESI†). Furthermore, a TER of cellulose in EmimOAc with IPA in the presence of water failed to produce any chemically modified cellulose derivatives (Fig. S-3, ESI†). As already reported, the catalytic ability of the ILs was expected to be due to the spontaneous generation of the N-heterocyclic carbenes (NHCs) from the parental ILs^36,37 or cooperative molecular recognition by both the anions and cations.³⁸ When the counter anion of the ILs showed a sufficient basicity to interact with the acidic species, both mechanisms are plausible. However, when taking into account that this TER process occurred with a high reactivity and fast reaction kinetics at a fairly elevated temperature, the generation of NHC is rationally expected to have played a role in the TER in EmimOAc because NHC is known to be one of the most reactive organocatalysts.³⁹ In addition, catalytic activation via molecular recognitions rather requires the reaction temperature to be low because the hydrogen bonding ability is known to be significantly weakened at elevated temperature. Although the precise TER mechanism in the EmimOAc is still under discussion, we have established a new synthetic protocol that enabled the efficient modification of the cellulose using EmimOAc as a solvent and an activating reagent.

Next, detailed kinetic investigations of the direct TER of cellulose in EmimOAc were carried out. Fig. 3A shows the kinetic evolution of the DS values for a cellulose acetate obtained by the TER of cellulose in EmimOAc at 80 °C in the presence of an excess amount of IPA. The DS values of cellulose reached 2.33 even 15 minutes after the reaction was initiated and reached a practically constant DS value (∼2.9) 30 minutes after the reaction initiation. This rapid and efficient kinetic nature of the TER of cellulose in EmimOAc with IPA encouraged us to investigate the effect of the IPA concentration on the reaction kinetics. In this context, in order to further confirm the high reactivity of the TER of cellulose in EmimOAc, kinetic studies of the effects of the IPA concentration were conducted. As depicted in Fig. 3B, the TER of cellulose with IPA in EmimOAc smoothly proceeded within 1 hour with [IPA]/[OH] being 1.08, which essentially afforded cellulose acetate with a DS of 2.36. These kinetic features of the TER of cellulose in EmimOAc guaranteed highly efficient and rapid cellulose modification reactions without any additional catalysts and corrosive chemicals. To be fair, the employment of acid chlorides and acid anhydrides usually requires lower reactant loadings,²² which is important when the employed reactants are precious or expensive. However, from the view point of reaction handling, the present new synthetic protocol should provide a simple but robust chemical strategy for cellulose modification,⁴⁰ with which non-experts do not need to handle corrosive and easily-hydrolysed chemicals such as acid chlorides and acid anhydrides. This advantage of the newly developed synthetic method should be beneficial when considering the growing potential of cellulose for many applications where not only chemists but also a range of interdisciplinary scientists need to conduct chemical reactions.

Fig. 3 (A) Kinetic evolution of DS values for the cellulose acetate synthesized by TER in EmimOAc (line; guidance). (B) Effect of IPA to hydroxyl groups ratio on the TER efficiency in EmimOAc (line; guidance). The reaction conditions are as follows; EmimOAc as a solvent and activating reagent; initial cellulose concentration was adjusted to 3 wt%; Ar atmosphere; initial [IPA]/[OH] for (A) = 16/1; the reactions in (B) were conducted for 1 hour. The chemical compositions of the recovered polymers were determined by ¹H NMR measurements in DMSO-d₆ of the products obtained by the benzoylation reaction of the cellulose acetates.

Conclusions

In conclusion, we have successfully established a new cellulose modification protocol that allows the facile TER of cellulose without using additional catalysts and corrosive chemicals. Investigation into the reaction kinetics of the TER of cellulose in EmimOAc revealed that the direct TER in EmimOAc smoothly and rapidly proceeded, affording cellulose acetate with high DS values. All in all, we successfully took full advantage of the dual functionalities of EmimOAc, namely a solvent for cellulose and an activating agent for the TER, which should lead to a new synthetic concept not only for biomass-derived polymers, such as the cellulose, but also for a wide range of synthetic polymers. The precise insights into the reaction mechanism of the above-established TER and TER of cellulose with other stable esters are also highly interesting and are currently under investigation.

Acknowledgements

This research was promoted by COI program “Construction of next-generation infrastructure using innovative materials – Realization of a safe and secure society that can coexist with the Earth for centuries – supported by MEXT and JST”. This study was also supported in part by an Advanced Low Carbon Technology Research and Development Program (ALCA) of the JST and the Cross-ministerial Strategic Innovation Promotion Program (SIP) from the JST.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra14408f

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