As we mark the 20 th anniversary after the introduction of the twelve principles of green chemistry, sustainable modification of cellulose, being the most abundant biobased polymer, is worth consideration. Many researchers work on this renewable polymer, however the use of non-sustainable solvents, reactants and modification approaches simply shifts the environmental burden to other stages of the life cycle. Therefore, to achieve true sustainable modification of cellulose, its renewability combined with mild and efficient reaction protocols is crucial in order to obtain sustainable materials that will reduce the overall negative effect of the fossil-based resources they are replacing.

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.

The direct transesterification of cellulose with high oleic sunflower oil, without any activating steps, was achieved in a DBU-CO2 solvent system to obtain fatty acid cellulose esters (FACEs). Optimization of the reaction parameters (i.e., concentration, temperature, plant oil equivalents, as well as reaction time) was performed using microcrystalline cellulose (MCC) and followed by Fourier-transform infrared spectroscopy (FT-IR). Further confirmation of the FACEs structures was achieved via 1H and 13C NMR, and 31P NMR revealed DS (degree of substitution) values of up to 1.59. The optimized conditions were successfully applied to filter paper (FP) and cellulose pulp (CP). Characterization of the FACEs showed improved thermal stability after transesterification reactions (up to 30 °C by TGA) and a single broad 2θ peak around 19.8° by XRD, which is characteristic of a more amorphous material. In addition, films were prepared via solvent casting and their mechanical properties obtained from tensile strength measurements, revealing an elastic modulus (E) of up to 478 MPa with elongation of about 35% and a maximum stress of 22 MPa. The film morphology was studied by scanning electron microscopy (SEM) and showed homogeneous surfaces. In this report, we thus demonstrated a more sustainable approach toward FACEs that combines cellulose and plant oil (two renewable resources) directly, resulting in fully renewable polymeric materials with appealing properties.

An effective and sustainable succinylation of cellulose is described. The thus introduced carboxylic acids groups allowed a straightforward modification of cellulose via multicomponent reactions in a unprecedented manner.

We report a sustainable and easy approach for the preparation of cellulose-based aerogels from the DBU–CO2 switchable solvent system via a solubilization and coagulation approach followed by freeze-drying. The easy, fast, and mild solubilization step (15 min at 30 °C) allows for a rapid preparation procedure. The effect of various processing parameters, such as cellulose concentration, coagulating solvent, and the superbase, on important aerogel characteristics including density, porosity, pore size, and morphology, were investigated. Density values obtained ranged between 0.05 and 0.12 g/cm3, with porosity values between 92% and 97%. The morphology of the obtained cellulose aerogels was studied using scanning electron microscopy (SEM) showing a random and open large macroporous cellulose network with pore sizes ranging between 1.1 and 4.5 μm, depending on the processing conditions. In addition, specific surface areas determined by N2 adsorption applying the BET equation ranged between 19 and 26 m2/g. The effect of the coagulating solvent and superbase on the crystallinity was investigated using X-ray diffraction (XRD) showing an amorphous crystal structure with a broad 2θ diffraction peak at 20.6°. In addition, no chemical modification was observed in the prepared aerogels from infrared spectroscopy. Finally, the recovery and reuse of the solvent system was demonstrated, thus making the process more sustainable.

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We report the use of the DBU-CO2 switchable solvent system for the direct electrospinning of cellulose. Two cellulose types were investigated, i.e. microcrystalline cellulose (MCC) and cellulose pulp (CP). The morphologies of the obtained cellulose fibers were studied using scanning electron microscopy and optical microscopy. Results obtained showed that only particles with mean diameter about 1.2 μm could be obtained when MCC was used, even at high concentration (10 wt%). In the case of CP, an optimized concentration of 4 wt% resulted in standing fibers with a mean diameter of about 500nm. In order to improve the spinnability of the cellulose, different concentrations and ratios of PVA in combination with cellulose were investigated. The combination of cellulose (both MCC and CP) resulted in the formation of a unique fiber morphology, characterized by a homogeneous bead-like structure. An in-depth study of the fiber structure was carried out using Raman spectroscopy and showed that both cellulose and PVA were present in the formed beads. Finally, the challenge observed remained a complete removal of the solvents, which are not volatile enough, as well as explore a coagulation collection process for the fiber recovery in order to recover and re-use the employed solvent. Graphic abstract

We introduce a novel isocyanide-based multicomponent reaction, the Passerini four component reaction (P-4CR), by replacing the carboxylic acid component of a conventional Passerini three component reaction (P-3CR) with an alcohol and CO2.