1-Ethyl-3-methylimidazolium acetate ([C2mim]OAc) is the most commonly used cellulose-dissolving ionic liquid and has been extensively studied for biorefinery applications. Although [C2mim]OAc had been considered to be a liquid, we here found [C2mim]OAc crystals at room temperature (around 25 °C) with a melting point of 44 °C. A plausible reason for the crystallization is shearing stress. [C2mim]OAc seed crystals led to the crystallization of liquid [C2mim]OAc.

# Introduction

Ionic liquids and room-temperature ionic liquids are defined as liquid salts below 100 °C and at room temperature, respectively (Welton, 1999, 2018). They are referred to as green solvents and cannot be replaced by water or organic solvents due to their unique properties, such as low volatility and non-flammability. Furthermore, the physicochemical properties (including hydrophobic/hydrophilic balance, viscosity, and polarity) of ionic liquids can be widely modified by altering the structures of the cation and anion. Consequently, a diverse array of potential applications is currently being explored. —In 2002, ionic liquids composed of imidazolium cations and halide anions (chloride and bromide) were developed as effective cellulose solvents (Swatloski et al., 2002). This discovery inspired many researchers to seek better ionic liquids for dissolving cellulose (Fukaya et al., 2007; Kosan et al., 2007; Pinkert and Marsh;, 2009; Wang et al., 2012; Endo et al., 2017; Li et al., 2018). From previous studies, it is understood that carboxylate anions with the high hydrogen bonding acceptability and imidazolium cations can dissolve biomass under milder conditions and are now considered the most suitable structures for cellulose solvents (Kilpeläinen et al., 2007; Sun et al., 2009). Among these, 1-ethyl-3-methylimidazolium acetate ([C2mim]OAc: Fig. 1
                     ) is particularly recognized as the most common cellulose solvent and has been intensely studied for biorefinery applications (Brennan et al., 2010; Stark, 2011; Moyer et al., 2018; Husson et al., 2018; Usmani et al., 2020; Sun et al., 2009). [C2mim]OAc had been recognized as a liquid at room temperature, or grass at lower temperatures (−78 °C) (Vitz et al., 2009; Komori et al., 2021). However, we recently have discovered [C2mim]OAc crystals at room temperature. Thus, we investigated the true melting point and crystallinity of [C2mim]OAc.

# Materials and methods

## Materials

[C2mim]OAc was purchased from Iolitech GmbH and was used as received.

## Measurement of thermal properties

Water content was measured by the Karl Fischer titration method using an AQ2200 instrument (HIRANUMA Co., Ltd.). Thermogravimetric analysis was performed at a heating rate of +10 °C/min using a DTG-60AH instrument (Shimadzu Co., Ltd.). Differential scanning calorimetry was conducted at a heating rate of +10 °C/min and a cooling rate of −10 °C/min using a DSC-60A plus instrument (Shimadzu Co., Ltd.).

## Analysis of [C2mim]OAc purity

1H and 13C NMR (600 MHz) spectra were recorded using an ECZ-600R spectrometer (JEOL). Elemental analysis was performed using a MicroCorder JM10 (J-Science Lab Co., Ltd.) and analyzed with ChemDraw software. The elemental composition was calculated as [C2mim]OAc ·2H2O, because [C2mim]OAc crystals absorb water from the air during sample preparation and measurement of elemental analysis, which is unavoidable. Mass spectrometry was conducted using a JMS-700 (JEOL) and analyzed with ChemDraw. Wide-angle XRD was performed with a NANO-Viewer (IP system, Rigaku Co., Ltd., Japan) using a Cu Kα radiation source at a voltage of 40 kV and a filament current of 30 mA. The [C2mim]OAc crystals were placed in a capillary (diameter: 1 mm) under a dry nitrogen atmosphere for measurement.

# Results and discussion

[C2mim]OAc was vacuum dried with heating to reduce the water content to 0.14 wt%. The dried [C2mim]OAc was promptly transferred to a glove box with a dry nitrogen atmosphere and stored in a vial. After one month, [C2mim]OAc was crystallized (Fig. 2
                     ). The melting point of the [C2mim]OAc crystals was 44 °C (Table 1
                     , Fig. S1). The grass transition temperature was −81 °C once it melted and the decomposition temperature was 227 °C. The grass transition and decomposition temperatures were similar to those reported previously (Komori et al., 2021).

The [C2mim]OAc crystals obtained in this study were confirmed to be pure [C2mim]OAc. The 1H and 13C NMR spectra, which do not contain peaks of impurities, indicated that the [C2mim]OAc crystals were highly pure (Figs. 3 and 4
                     
                     ). The high purity of [C2mim]OAc was further confirmed through elemental analysis and mass spectrometry (Table 2
                     ).

The wide-angle XRD spectrum of [C2mim]OAc (Fig. 5
                     ) reveals that it is crystalline, as evidenced by the sharp peaks. On the other hand, the crystal structure was not analyzed since it requires special equipment, techniques, and insights.

To date, [C2mim]OAc crystals have not been reported, to the best of our knowledge. Initially, we discuss our discovery of these crystals. We stored liquid [C2mim]OAc in a glass vial in a glove box. Upon removal from the glass vial, we observed crystallization at the interface between the rim of the glass vial and the lid, suggesting that shearing stress may have triggered the crystallization.

Another possible factor is water content. In this study, the water content of [C2mim]OAc was reduced to 0.14 wt%, typically which is challenging to achieve below 0.3 wt% technically due to strong hydrogen bonding basicity (Jessop et al., 2012). We examined whether the [C2mim]OAc crystal melts when containing water. Given its hygroscopic nature, the crystal was expected to melt in the atmosphere. Half and most part of the [C2mim]OAc crystals liquefied within one and two hours, respectively (Fig. 6
                     ).

We also investigated the minimum amount of water required to liquefy the crystals. One mg of water was added to 200 mg of [C2mim]OAc crystals until the crystals completely liquidized. The crystals liquified at water content of 3.6 wt% detected by the Karl Fischer titration method. This finding suggests that a water content of less than 3.6 wt% can support crystallization. Reducing the water content to 3.6 wt% is feasible; therefore, we can conclude that the lack of reports on [C2mim]OAc crystals is due more to shearing stress than to water content. This hypothesis is supported by literature reporting liquid [C2mim]OAc with low water content (Vitz et al., 2009).

The [C2mim]OAc crystals can serve as a seed for the crystallization of liquid [C2mim]OAc. When seed crystals were introduced into liquid [C2mim]OAc dried under vacuum, all liquid [C2mim]OAc crystallized within 30 min (Fig. 7
                     ). The [C2mim]OAc crystals effectively acted as seeds for the liquid [C2mim]OAc.

In biorefineries, the recycling of ionic liquids through a purification process is essential. However, purification has been challenging due to the presence of many unknown compounds, including decomposed lignin, and salts in the used [C2mim]OAc (Liu et al., 2012; Mai et al., 2014). Since recrystallization is an effective purification method against such contaminants, crystallization of [C2mim]OAc potentially contribute to biorefinery.

# Conclusions

[C2mim]OAc, the most common cellulose-dissolving ionic liquid, typically exists as a liquid at room temperature. However, we observed [C2mim]OAc crystals at room temperature. The melting point of the [C2mim]OAc crystal was determined to be 44 °C. [C2mim]OAc crystal demonstrates high crystallinity and can serve as a crystalline seed for the crystallization of liquid [C2mim]OAc. The [C2mim]OAc crystals indicated the potential for purifying [C2mim]OAc via recrystallization, an effective purification method.

# CRediT authorship contribution statement

Ayumi Hachisu: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Akio Ohta: Validation, Methodology. Kenji Takahashi: Writing – review & editing, Resources. Kosuke Kuroda: Writing – review & editing, Writing – original draft, Validation, Supervision, Methodology, Funding acquisition, Data curation, Conceptualization.

The authors thank Takatsugu Endo (Doshisha University) for discussions on the XRD spectra. This study was also partly supported by KAKENHI (Grant No. 18K14281 from the Japan Society for the Promotion of Science) and the Kanazawa University SAKIGAKE project 2020/2022.