Aryloxy Ionic Liquid-Catalyzed Homogenous Esterification of Cellulose with Low-Reactive Acyl Donors

Abstract

Ionic liquids (ILs) are recyclable, non-volatile, and can dissolve cellulose, a natural polymer that is insoluble in versatile solvents. Therefore, ILs have been used to modify cellulose. However, 1-ethyl-3-methylimidazolium acetate (EmimOAc), a commercially available IL often used to dissolve and modify cellulose to prepare cellulose-based materials, causes the undesired introduction of an acetyl group derived from the acetate anion of EmimOAc onto the hydroxy group of cellulose during esterification. In this study, for cellulose esterification, we prepared aryloxy ILs as non-carboxylate-type and basic ILs, which can theoretically prevent the undesired introduction of an acyl group from the IL onto the hydroxy group of cellulose. The optimized 1-ethyl-3-methylimidazolium 2-pyridinolate (Emim2OPy) and mixed solvent system achieved rapid cellulose esterification (within 30 min) with an excellent degree of substitution (DS) value (up to >2.9) derived from the employed low-reactive vinyl esters and bio-based unsaturated aldehydes, without any undesired substituent introduction from side reactions.

Investigation of cellulose dissolution in morpholinium-based solvents: impact of solvent structural features on cellulose dissolution

“…Nuclear magnetic resonance (NMR) spectroscopy. NMR spectra of the synthesized MorBr, MorOAc and MorCl (in D 2 O) were obtained with Bruker Avance 500 MHz spectrometers with 16 scans for 1 H NMR (500 MHz) and up to 1024 scans for 13 C NMR (125 MHz) at 25 °C.…”

“…To assess the stability of cellulose solutions in MorOH(aq) as they were prone to color change over time (Fig. 1), freshly dissolved cellulose (4 wt%) in 1.3 M BMMorOH(aq) and 1.3 M MPMorOH(aq) were subjected to 13 C NMR measurements and spectra were collected every 1 h over 22 h on the same sample.…”

“…The light yellow color has been observed before in NDMMOH(aq) but not as dark as with the synthesized MorOH(aq) in this study. However, monitoring the 4 wt% cellulose solutions in 1.3 M BMMorOH(aq) and MPMorOH(aq) over 22 h by 13 C NMR showed no detectable indications of degradation (Fig. 2a and S18 in ESI †).…”

“…4) Fig. 2 (a) 13 C NMR spectra of 4 wt% freshly dissolved cellulose in 1.3 M MPMorOH(aq) (orange) and an aged solution for 22 h post dissolution (purple) (due to perfect overlap of both spectra the orange spectrum cannot be seen). (b) Molecular weight distribution (determined by SEC in DMAc/LiCl) for cellulose as the reference (red) and cellulose dissolved and precipitated after 24 h storage from MPMorOH(aq) (blue) and BMMorOH(aq) (green).…”

“…6,7 But this solvent is prone to exothermic and oxidative reactions since it is a strong oxidant and is sensitive to alkylating and acylating agents and redox-active transition metal ions. 8 Development of ionic liquids capable of dissolving cellulose [9][10][11] should also be mentioned in this context as it has opened new avenues for shaping and modifying cellulose, 12,13 with the recycling of the solvent remaining as one of the main challenges. N,N-dimethylacetamide (DMAc)/LiCl is another well-studied solvent mainly used in gel permeation chromatography (GPC) for evaluating molecular weight distribution of cellulose 14 as well as for its homogeneous functionalization.…”