Synthesis of 1 , 2 , 3-Triazolium-Based Ionic Liquid and Preliminary Pretreatment to Enhance Hydrolysis of Sugarcane Bagasse

Synthesis of Azidoacetic Acid 5 To a solution of sodium azide (8.2 g, 126 mmol, 2 equiv.) in 42 mL of water bromoacetic acid 2 (10 g, 63 mmol, 1 equiv) was slowly added. The solution was stirred at room temperature overnight. The reaction mixture was acidified to pH 1 by the addition of concentrated HCl, and subsequently extracted with Et2O (3 × 75 mL). The combined organic phases were dried over Na2SO4 and concentrated in vacuo to afford azidoacetic 5 acid as an oil (59 mmol, 95% yield).


Introduction
Recently, the growing awareness of environmental issues has focused attention on the need for greener and more sustainable technologies in the chemical industry. 1,23][4] Considering that solvents are often necessary in chemical reactions, alternative solvents have been developed.The ideal solvent should have very low volatility, and it should be chemically and physically stable, recyclable, reusable and easy to handle.One such candidate is ionic liquid (IL). 5nlike volatile organic compounds, ILs have a low vapor pressure, which results in safer chemical processes, thereby preventing explosions and fires. 4][11][12][13][14] Among the various classes of ILs, those containing N-heterocyclic cations are most widely used. 12Imidazolium salts (Figure 1) represent the most prominent subclass in this area and a number of them are commercially available. 15Their solvent properties, such as melting points, solubility, and viscosity can easily be tuned in a wide range by varying the substituent at the nitrogen atoms, as well as by varying the counter-ions.3][14][15][16] Even if imidazolium salts have found very wide application in organic synthesis and catalysis, 17,18 they have some limitations. 122,3-triazolium-based ILs are prepared in a two-step procedure in good yield.In the first step, azide and alkynes are transformed using Cu(I)-catalyzed click reaction and after alkylation affords the ILs.This reaction is an ideal synthesis platform to systematically probe ILs properties due to its excellent molecular control, ease of synthesis, benign reaction conditions, and fidelity.6,18,19 In short, the Cu(I)-catalyzed click reaction allows easy access to regioisomers and simple variation of functional groups, which enables us to study the effect of structural changes on properties.6,7,19 The development of a chemically inert 1,2,3-triazoliumbased IL is reported in this work.The stereotype of our novel 1,2,3-triazolium IL is shown in Figure 2.
In Brazil, ethanol is largely produced from sugar cane juice, known as first generation ethanol (1G).The residual lignocellulosic biomass from the 1G ethanol industry is currently, for several reasons, the most promising resource for the production of lignocellulosic ethanol, also called second generation (2G). 20Biofuels produced from lignocellulosic biomass minimize competition with the food chain and increase overall yields in comparison to biofuels from the first generation. 21Lignocellulosic biomass is mainly composed of cellulose, hemicellulose and lignin.The predominant component of lignocellulosic biomass is cellulose, a linear β (1,4)-linked chain of glucose molecules.The main steps for ethanol production from lignocellulosic biomass are pretreatment, hydrolysis, fermentation and distillation/purification. The pretreatment should enhance the fiber accessibility and consequently facilitate the subsequent steps of enzymatic hydrolysis and fermentation. 22Ls have been reported to be capable of dissolving cellulose and lignocellulosic materials such as rice straw, wheat straw, sugarcane bagasse and woody biomass. 14,23,24retreatment with ILs can reduce cellulose crystallinity and partially remove hemicelluloses and lignin, not generating degradation products which are inhibitory to enzymes or fermenting microorganisms. 19][27][28][29] Lee et al. 25 have reported a set of ILs which can be used to selectively extract lignin from wood flour and have provided a new route for fractioning lignocellulosic biomass. 4Cellulose dissolved in ILs can be precipitated by the addition of anti-solvents, like water.A solutedisplacement takes place.The ions of the ILs are extracted into the aqueous phase through hydrogen bonding, dipolar and Coulombic forces. 30,31Water molecules form hydrodynamic shells around the ILs ions.Therefore, the direct interactions of ILs ions with cellulose are shielded.Thus, intra-and inter-molecular hydrogen bonds are rebuilt and cellulose precipitates.However, the structure changes significantly, which can be observed by scanning electron microscopy (SEM). 32he cellulose-rich fraction can be precipitated with water, and lignin and other extractives can be removed through multiple washing and solvent evaporation.It is important to mention that the saccharification time can be drastically reduced and the yields of nearly 100% can be achieved after biomass pretreatment using ILs.Li et al. 23 have reported that a 12 h saccharification time was sufficient for switchgrass and more than 90% of hydrolysis yield was achieved, whereas the use of dilute sulfuric acid for pretreatment required 72 h for saccharification and resulted in 80% of hydrolysis yield.Although the feasibility of ILs for a large-scale application is still hindered by ILs costs, the use of ILs improved the enzymatic digestibility of biomass significantly in comparison to other known pretreatment methods.Moreover, the use of ILs can provide a better understanding of the interaction mechanisms of ILs with biomass and can be an interesting research topic. 20 This study reports the synthesis of three noncommercial ILs readily obtained from low-cost reactants.These compounds were assessed as solvents for the pretreatment of sugarcane bagasse.Moreover, their effects on the surface morphology of the pretreated biomass were evaluated by scanning electron microscopy (SEM).
Instrumentation1 H NMR spectra were recorded on Bruker AC-300 and 500 spectrometers at 300.13 and 500.13MHz and 13 C NMR spectra were recorded on a Bruker AC-300 at 75 MHz.The chemical shifts are given in parts per million relative to tetramethylsilane.Mass spectra were recorded on LC-MS/MS-TOF API QSTAR PULSAR spectrometer, and samples were introduced by infusion method using electro spray ionization technique.

General procedure for synthesis of ILs
In a flask, a mixture of 1,4-dissubstituted-1,2,3triazole (7, 8 or 9) (5 mmol) and methyl iodide (20 mmol) was stirred at 80 °C for 24 h (procedure used by Y. Jeong and J. S. Ryu). 15Upon completion of the reaction, the residues formed were removed by filtration, then the reaction mixture was concentrated in vacuo to afford

Pretreatment
The synthesized ILs (3,0 g) (10, 11 and 12) were mixed with biomass at a 20:1 (ILs:biomass) ratio and heated to 120 °C for 2 h.After pretreatment, 10 mL of distilled water was added into the pretreatment vessel, under ice bath, to recover the biomass.The ILs/water mixture and biomass were separated by vacuum filtration.The solids were repeatedly washed with distilled water to remove any remaining ILs from the samples until the washing solution appeared colorless.Experiments were run in triplicate with three separate batches.

Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM -FEI/FEG-450 model) was used to investigate the morphology of the untreated and treated materials.Samples were adhered to carbon tape, sputter coated with 28 nm gold using an Emitech/K550 model and observed in the SEM through the use of an acceleration voltage of 20 KV and a working distance of approximately 14.5 mm.Several images were obtained on different areas of the samples (at least 20 images per sample) to guarantee the reproducibility of the results.The n-propyl azide 4 was generated in situ from n-propyl bromide 1 and NaN 3 , where it was captured by propargyl alcohol via a Cu I -catalised azide-alkyne 1,3-dipolar cycloaddition (CuAAC) "click" reaction.The desired triazole product 7 was obtained as a sole product in 80% yield. 33,34Azidoacetic acid 5, synthesized by reaction between sodium azide and bromoacetic acid 2 in water, 35 was treated with propargyl alcohol in the presence of copper sulfate and sodium ascorbate in The three types of 1,2,3-triazolium salts 10, 11 and 12 are liquids at room temperature.Therefore, all 1,2,3-triazolium salts may be classified as ILs.The thermal stabilities of the 1,2,3-triazoliun compounds might be a concern because they contain three nitrogens.Thermal decomposition temperatures of 1,2,3-triazolium salts were determined by thermogravimetric analysis and are described in Table 1.The solubility of ILs in organic solvents is an important factor for recycling.Thus, we tested the solubilities of 1,2,3-triazolium ILs in several common organic solvents (Table 2).The solubilities increase with the dielectric constant of the solvents.The ionic salts do not dissolve in hexane, AcOEt, Et 2 O or CH 2 Cl 2 but their solubilities increase in EtOH or H 2 O.

Synthesis
The structures of the new ILs are in agreement with their 1 H NMR data.

Pretreatment and scanning electron microscopy
The effect of pretreatment of sugarcane bagasse with the synthesized ILs was analyzed by SEM. Figure 3 shows the morphological characteristics of the pretreated bagasse with ILs 12 (C and D), 11 (E and F) and 10 (G and H), as well as of the untreated material (A and B), obtained by SEM.Untreated bagasse sample (Figure 3) presents a compact morphology, while the ones submitted to pretreatment with ILs exhibited a more disorganized morphology, with greater exposure of the fibers.The purpose of the pretreatment step is to improve fiber exposure and increase the accessibility to hydrolytic enzymes.Further studies to evaluate sugarcane bagasse pretreatment with novel ILs should include enzymatic hydrolysis. 14,37

Figure 3 .
Figure 3. SEM images of the sample of sugarcane bagasse untreated (in natura) (A and B), pretreated with ILs 12 (C and D), 11 (E and F) and 10 (G and H).In the left column are the images with magnification of 1000× and in the right column magnification of 2000×.