Detailed Understanding of the DBU/CO₂ Switchable Solvent System for Cellulose Solubilization and Derivatization

Abstract

In this article, we present an optimization study of the switchable solvent system DBU/CO2 for cellulose solubilization and derivatization via online Fourier transform infrared spectroscopy (FT-IR). By varying temperature, CO2 pressure, and solubilization time, we succeeded in achieving cellulose solubilization within 10–15 min at 30 °C. Compared to traditionally used ionic liquids, the system presented here is cheaper, is easier to recycle, and enables a very fast cellulose solubilization under mild conditions. The efficiency of our optimized mild conditions were further confirmed by X-ray diffraction (XRD) experiments showing the typical transformation from cellulose I to II upon regeneration. In addition, we prove the existence of the in situ formed carbonate anions by trapping them with benzyl bromide or methyl iodide as electrophiles, leading to the successful synthesis of cellulose benzyl carbonate and cellulose methyl carbonate, respectively, under utilization of CO2 as a renewable building block for cellulose derivatization. The synthesized cellulose carbonates were characterized by FT-IR, 1H NMR, and 13C NMR spectroscopy. A degree of substitution (DS) value of 1.06 was achieved for the cellulose benzyl carbonate as determined by 31P. This study thus provides deep insight into the possibilities of the studied switchable solvent system for cellulose solubilization and offers unprecedented possibilities for novel derivatization protocols of cellulose.

Preparation of Cellulose Films from Sustainable CO2/DBU/DMSO System

“…It is evident that the roughness of films regenerated in an ethanol bath (Figure 6B) was much lower than the ones regenerated using the other three approaches, which was consistent with the results from the SEM. It can also be observed that the films prepared in aqueous baths exhibited rougher appearances, which may be due to the re-crystallization of cellulose induced by water [38] and the reaction between the cellulose-based carbonate anion [39] and acid/base.…”

On the direct use of CO2in multicomponent reactions: introducing the Passerini four component reaction

“…As expected and to our delight, the conversion of butanol increased from 56% (1.84 M with respect to butanol) to 73% (3.68 M with respect to butanol) in DCM. Presumably, the mechanism of the P-4CR proceeds via the in situ formation of a carbonate anion initiated by a base catalyst 16,[20][21][22] (see Scheme 1b). The carbonate then reacts as the acid component with the isocyanide and aldehyde component in the typical fashion of the conventional Passerini 3-CR.…”

“…20,21 We recently reported an optimization of the DBU-CO 2 switchable solvent system and could unambiguously proof that carbonate anions are indeed formed in situ. 22 The formation of this in situ carbonate not only led to a mild solubilization of cellulose in DMSO, but also allowed an activation of the cellulose hydroxyl groups leading to a milder modication such as succinylation. 23 In this regard, a high DS value of 2.6 was reported for reaction carried out at room temperature in 30 min.…”

Regeneration of Cellulose from a Switchable Ionic Liquid: Toward More Sustainable Cellulose Fibers

“…Scientists worldwide are investigating efficient ways to convert biomass into valuable materials in order to reduce their environmental footprint and replace, for instance, petrochemically derived polymers. [ 2,3 ]…”

“…Generally, green chemistry has become one of the most commonly used concepts towards a sustainable production of chemicals and the prevention of waste. [ 3,7 ]…”