Pyridinium-Based Ionic Liquids — Application for Cellulose Processing

the extreme groups: from 2' to 9' ) groups following changes of the alkyl chain length and/or anion species. This indicates the relatively minimal interaction of the anion with the hydrocarbon atoms of the alkyl chain. It may be noted that for [АМРу]Cl, the H 3' atom is more upfield shifted in comparison with the same atom for [С 3 МРу]Cl. These effects can be explained by the greater sensitivity due to the π-electron contribution to the allylic substituent, and the presence of two doublets splitting for the group 3' from two magnetically inequivalent protons in the cis- and trans-


Introduction
Over the last two decades new direct solvents for cellulose and other natural polymers have been actively explored; these are ionic liquids (ILs). The unique properties of this new class of compounds allow the use of ILs in various fields of science and technology, among which the use of ILs as the reaction medium for biomass processes occupies an important place [1][2][3], especially for cellulose dissolution [4][5][6][7][8][9][10][11][12][13], and the dissolution of other natural polymers [14], and produces from their solutions new biodegradable materials, including films, fibres and membranes [15].
On the whole, imidazolium-, ammonium-and pyridinium-based ILs can be used for cellulose dissolution ( Figure 1). Imidazolium ILs are studied here in great detail, but the comparisons between the cost of synthesis, toxicity [16][17][18] and dissolving power [4,6] of the three classes of ILs shows pyridinium-containing salts to be of most benefit. The most effective salts have a pyridinium ring with position 1,3 of alkylic substituents [6,7]. The important feature of ILs is the possibility of selecting the cation-anion pairs for the purposeful synthesis of solvents with desirable properties in technology. In view of the large numbers of ILs available, the selection of ionic solvents out of all the possible cation-anion pairs is a big problem. To make a reasonable selection it is important to understand the relationship between the structure, physicochemical properties, and dissolving power of ILs.

Quantum-chemical calculations
Quantum-chemical calculations were performed using the Gaussian 03 program by HF/ 3-21G(d) and HF/6-31G(d) methods [7]. As the IL models were used the ionic pairs which they contained associated cation and anion. For the cellulose model, we took the cellotetraose (i.e., saccharide consisting of four glycoside units). The interaction energy of the IL's cation-anion pairs, E p , was calculated as a difference: the energy of the cation-anion complex, E ca , minus the sum of the energies of the cation, E c , and the anion, E a : E p = 2625.5 • {E ca -(E c + E a )}, where 2625.5 is the conversion factor of the Hartree energy (atomic units) in kJ mol -1 .

1 H-NMR spectroscopy
An analysis of the chemical shifts was carried out by the 1 H-NMR spectra obtained with the Bruker Avance II Plus 700 MHz spectrometer (at an operating frequency of 700 MHz) at a temperature of 27 °C. For the preparation of the samples, a sample of the IL (19 -27 mg) was dissolved in CDCl 3 (0.65 mL, δ=7.240 ppm). Solutions of cellulose with ILs were investigated using the CDCl 3 (δ=7.240 ppm). TMS (δ=0) was used as the internal standard [19].
chromatographic column (30 m), a stationary phase SLB-5ms, the carrier gas was helium, and the evaporator temperature was 200 °C. The thermostat columns had a temperature range of 40 °C (for five minutes) to 280 °C, a heating rate of 10 deg/min and exposure for 15 minutes. The detector of the mass spectrometer with the quadrupole mass analyser had a detection range of M/Z 40 -450 Da. The delay of the start of the registration (for the cut-off of the solvent) was 2.2 min. For the sample preparation, IL crystals (5 mg) were dissolved in ethanol (1:200), 1 μL of solution was injected into the evaporator chromatograph with a microsyringe.

Fourier transform infrared spectroscopy
The FTIR spectra of the samples were recorded on a Nicolet 6700 FT-IR from Thermo Scientific. The resolution was 0.5 cm -1 . The spectra were recorded at room temperature using an NXR FT-Raman module.

Dissolution of cellulose with ILs
Cellulose was added to the solvent after the complete dissolution of the previous sample (in this case the solution was transparent to light and did not contain any undissolved cellulose fibres visible under the microscope). To determine the maximum dissolving power of the ILs, the dissolution was carried out for not less than 12 hours with a constant stirring and temperature control using the magnetic stirrer. The calculation of the dissolved cellulose concentration was given by the formula: where D is the concentration of the cellulose solution with ILs, wt%; m 1 and m 2 are the mass of the cellulose and ILs, respectively, g; W is the water content of the cellulose, wt% [7].

Optical microscopy
A microscopic examination of the samples was carried out using a laboratory microscope Euler Science 670TD. A digital camera was used DCM 510 (5-MPix, 1/2'' CMOS) with a maximum resolution of 2592×1944, and a sensitivity of 0.90 V/lux-sec at 550 nm.  [20].

Degree of polymerization of cellulose
To determine the degree of polymerization (DP) of the initial cellulose and the regenerated polymer (i. e. the film obtained from solution of cellulose with ILs) from an aqueous solution of cadmium-ethylenediamine complex (cadoxene), containing 29 wt% ethylenediamine. The cellulose was used at the following concentrations (D): 0.1, 0.2, 0.3, and 0.4 wt%. We used an Ostwald viscometer (the capillary of which had an internal diameter of d=0.99 mm), analytical balance (with a precision of four decimal places) and a thermostat at U15 C MLW (bath temperature: 20 °C). Using the time of the expiration (in sec.) of pure cadoxene (τ 0 ) and the solution (τ i ) the relative viscosity was calculated: The specific viscosity was determined from the relationship: η s =η r -1, and the reduced viscosity: η i= η r /D. The intrinsic viscosity [η] found by graphical extrapolation to the value of D=0 using the software package Origin 9.0 was: The intrinsic viscosity is related to the DP value for a given diameter of the viscometer capillary as [η]=7 10 -3 DP 0.9 , therefore DP = ( η 0.007 ) 1 0.9 .

Theoretical studies of the 1-alkylpyridinium-and 1-alkyl-3-methylpyridinium-based salts, and their structural features
Quantum chemical calculations can give an insight into geometry and the charge distribution of organic compounds. We studied pyridinium derivatives with different alkylic lengths and the numbers of the substituents of the ring have different anions.

Features of the structures of the 1-substituted pyridinium-based ILs
Theoretically, the optimal geometry of the studied pyridinium derivatives can be determined based on the results of quantum chemical calculations (method HF/3-21G(d)) as coordinates of the minimum (minima) on the potential energy surface, PES. As an example, Figure 2 shows the function of a cation-anion pair's interaction energy, E p , the distances between the cation and anion r (H2···Cl) , the torsion angle Θ between the atoms N, C 2 , H 2 of 1-methylpyridinium rings, and the chloride-anion Cl for [C 1 Py]Cl [11]. The minimum of the E p value corresponds to the location of the anion near the atom H 2 (the most electron-deficient atom) of the ring and H 6 of the aliphatic chain ( Fig. 3A). At the point of the global minimum, the torsion angle between atoms N, C 2 , H 2 , and Cl becomes 0°, since the ring of pyridine and its derivatives have a rigid and almost planar structure, and the anion is located on the same plane. The energy of the complex is -519 kJ mol -1 , r (H2···Cl) 2.17 Å. There are local minima with energy Е p = -367.06 kJ mol -1 at r (H2···Cl) =5.87 Å, and Θ=23.38° (Fig. 3B) and-363.45 kJ mol -1 at r (H2···Cl) =2.73 Å and Θ=10.87° (Fig. 3C). The least probable relative positions of anion and cation correspond to the maximum positive value of Е p =+948.00 at r (H2···Cl) =3.08 Å, Θ=90.61°, (Fig. 3D). In this case the anion is located above the plane of the ring. A positive value of E p is due to the repulsion of the anion by the π-bonds of the pyridinium ring saturated with electron density.
The 1-substituted formation from pyridine molecule (Py) leads to minimal changes in the bond lengths and angles on the pyridinium ring (Table 2). It indicates that these structures have rigid and high symmetry geometry [21]. Appreciable differences were noted only in the angle values of α and β for the 1-substituted pyridinium compared with Py.
Calculations show that the distance between the anion and the cation increases slightly with the increasing length of the cationic substituent, but the distance between the anions and cationic substituents (atom Н 1' ) is reduced. So, in a series of C 1 -C 6 the distance between the chloride anion and the nearest H 2 atom of the ring increases (2.190 → 2.225 Å). Starting from C 3 the anion approaches the substituents, and the distance between the anion and the nearest atom H 2 of the cationic substituent is shortened (2.624 ← 2.525 Å). In the C 0 -C 4 series, as in the case of the imidazolium-based ILs [22], there is a tendency for a gradual decrease in the interaction energy between the cation and the anion; this trend correlates with the experimentally observed decrease in the dissolving power toward cellulose and other higher plants (Table  3). It can be assumed that this leads to changes in the degree of the ordering of the ionic liquid (density fluctuation) manifested as the appearance of polar and nonpolar domains [23]. In any case, increases in the length of the substituent leads to a decrease in the density of the polar component.   (Table 2). It indicates that these structures have rigid and high symmetry geometry [21]. Appreciable differences were noted only in the angle values of α and β for the 1-substituted pyridinium compared with Py.
Calculations show that the distance between the anion and the cation increases slightly with the increasing length of the cationic substituent, but the distance between the anions and cationic substituents (atom Н 1' ) is reduced. So, in a series of C 1 -C 6 the distance between the chloride anion and the nearest H 2 atom of the ring increases (2.190 → 2.225 Å). Starting from C 3 the anion approaches the substituents, and the distance between the anion and the nearest atom H 2 of the cationic substituent is shortened (2.624 ← 2.525 Å). In the C 0 -C 4 series, as in the case of the imidazolium-based ILs [22], there is a tendency for a gradual decrease in the interaction energy between the cation and the anion; this trend correlates with the experimentally observed decrease in the dissolving power toward cellulose and other higher plants (Table 3). It can be assumed that this leads to changes in the degree of the ordering of the ionic liquid (density fluctuation) manifested as the appearance of polar and nonpolar domains [23]. In any case, increases in the length of the substituent leads to a decrease in the density of the polar component.
(The values of the parameters for isolated acetate-anions are shown in parentheses) [11].
It is important to note that in the series of acetate > chloride > bromide the negative charges of the anion have been decreasing (for acetate-anion is the total value of the charges on the O 1 and O 2 atoms), and it decreases the positive charges on the cations which interact due to the H 2 and H 1' atoms with the anions. This is due to a decrease in the electron density transfer during interionic interactions. As a result, in the same series, the interaction energy between the cation and anion decreases, and the distance between the anion and cation increases. In practice, this could be the reason for the decrease in the solubility of the cellulose in these salts. For example, [APy]Cl dissolves in seven times more biomass poplar than [APy]Br (Table 3). It is also important that the imidazolium-based ILs produce similar theoretical and experimental results [22,27].   (Table 3). It is also important that the imidazolium-based ILs produce similar theoretical and experimental results [22,27].    Tables 6 and 7. The mutual arrangement of the anion around the cation is characterized by the torsion angle Θ between the anion X, and the pyridinium ring atoms H 2 , C 2 , and N, and the distance r (H2…X) between the anion X and the H 2 atom. Thus, for 1,3-disubstituted chloride-containing pyridinium models, the torsion angle Θ has a range from 0.2º (for [C 1 MPy]Cl) to 11.1º and 14.2º (for the substituted pyridinium-based chlorides with chain lengths of C 2 to C 10 ). We can compare, for monosubstituted 1-methylpyridinium chloride the Θ ≈ 0º. The Θ angle for bromide-containing models is 7 -15º more than the same chloride-based models. Having the anion located further away from the plane of the pyridinium ring, and the increase in the Θ between the cation and anion in the homologous series, and finally the changing of the anion Clto Brmakes the cation-anion pair still less associated. The reason for this is the reducing of the ion's packing density in the IL crystal, and thus their melting point was changed. A decrease in the degree of the association is also confirmed by a characteristic change in the E p values. The length of the С-С bonds, and angles with the pyridinium ring for the calculated models in general are not very different from each other. Monosubstituted and disubstituted salts have different geometric parameters: the long distances of b and c, and the relatively large angles of α and β have a 1-alkyl-3-methylpyridinium ring relative to the 1-alkylpyridinium ring. These differences are caused by relatively strong interactions with the anions. Since we observed the global minimum of the potential energy on the PES for the model with the anion located near to the H 2 ring proton, this model was adopted for all subsequent calculations of geometry. The main results of the quantum chemical calculations are shown in Tables 6 and 7. The mutual arrangement of the anion around the cation is characterized by the torsion angle Θ between the anion X, and the pyridinium ring atoms H 2 , C 2 , and N, and the distance r (H2…X) between the anion X and the H 2 atom. Thus, for 1,3-disubstituted chloridecontaining pyridinium models, the torsion angle Θ has a range from 0.2º (for [C 1 MPy]Cl) to 11.1º and 14.2º (for the substituted pyridinium-based chlorides with chain lengths of C 2 to C 10 ). We can compare, for monosubstituted 1-methylpyridinium chloride the Θ ≈ 0º. The Θ angle for bromide-containing models is 7 -15º more than the same chloride-based models. Having the anion located further away from the plane of the pyridinium ring, and the increase in the Θ between the cation and anion in the homologous series, and finally the changing of the anion Clto Brmakes the cation-anion pair still less associated. The reason for this is the reducing of the ion's packing density in the IL crystal, and thus their melting point was changed. A decrease in the degree of the association is also confirmed by a characteristic change in the E p values. The length of the С-С bonds, and angles with the pyridinium ring for the calculated models in general are not very different from each other. Monosubstituted and disubstituted salts have different geometric parameters: the long distances of b and c, and the relatively large angles of α and β have a 1-alkyl-3-methylpyridinium ring relative to the 1-alkylpyridinium ring. These differences are caused by relatively strong interactions with the anions.  For the IL which has an allylic substituent we observed a relatively more larger in the Θ angle relative to [C 3 MPy]Cl. This may be the result of the interaction of the anion with the π-electrons of the allylic substituent. Because the anion for all of the salts slightly shifted toward the substituent on the nitrogen atom (including [C 1 МРу]Cl), then in the comparison of these two compounds the closest anion is located in the more polar allylic substituent than the proplic substituent (the distance between the chloride-anion and H 1' atom are 2.617 Å and 2.675 Å, respectively). This may indicate a more dense atomic packing for [AМРу]Cl. The Φ angle value for [AМРу]Cl on average is less than the other models.

Physical and chemical properties of 1-alkyl-3-methylpyridinium-based ILs
A series of 1-alkyl -3-methylpyridinium-based ILs (Table 8) was examined experimentally. The synthesis of these solvents was performed as described in paper [7].

Physical and chemical properties of 1-alkyl-3-methylpyridinium-based ILs
A series of 1-alkyl-3-methylpyridinium-based ILs (Table 8) was examined experimentally. The synthesis of these solvents was performed as described in paper [7].
A feature common to most hydrophilic IL crystals (shown in the micrographs on Figure 9) is the gradual saturation of water from the atmosphere. Two of the synthesized ILs, [С 4 МРу]Br and [С 4 МРу]CH 3 COO, are liquid at room temperature, i.e. they belong to a subclass RTIL.

Physical and chemical properties of 1-alkyl-3-methylpyridinium-based ILs
A series of 1-alkyl -3-methylpyridinium-based ILs (Table 8) was examined experimentally. The synthesis of these solvents was performed as described in paper [7]. A feature common to most hydrophilic IL crystals (shown in the micrographs on Figure 9) is the gradual saturation of water from the atmosphere.   1

H-NMR spectroscopy
The experimental data of the chemical shifts and spin-spin coupling (J-couling) are presented in Table 9 [19]. Figure 10 shows the general scheme of the 1-alkyl-3-methylpyridinium ring and acetate-anion numbered to correlate with corresponding the chemical shifts (δ) of protons and proton groups.

Study of 1-alkyl-3-methylpyridinium-based ILs by 1 H-NMR spectroscopy
The experimental data of the chemical shifts and spin-spin coupling (J-couling) are presented in Table 9 [19]. Figure 10 shows the general scheme of the 1-alkyl-3-methylpyridinium ring and acetate-anion numbered to correlate with corresponding the chemical shifts (δ) of protons and proton groups.   The experimental data of the chemical shifts and spin-spin coupling (J-couling) are presented in  For the peaks of the resonance signals of the ring protons, we observed singlet (H 2 ), duplet (H 4 and H 6 ) and triplet (H 5  We observed the greatest changes in the values of the chemical shifts during the alkyl chain lengthening for H 2 and H 6 atoms (labelled peaks 2 and 6, respectively) of the 1-alkyl-3methylpyridinium ring (to Δδ=0.30 and 0.24 ppm, respectively) [19]. According to the quantum-chemical calculations, these provisions in the ion pairs are the most optimal. The changing of the δ value confirms the calculations made on the basis of the findings of the interaction of these atoms with an anion Cl -, as well as to change the distance between the ion pair in the homologous series of ILs.
With an increase of the alkylic chain length, the δ signals for the H 2 and H 6 atoms begin to move upfield (shielding), while the resonance frequency of the H 4 and H 5 protons are practically unchanged. The upfield δ gives an increase in the distance between the chloride-anion and the corresponding hydrogen atoms of the aromatic ring. For example, according to calculations (see Table 6), the distance r (Н2…Cl) for [С 2 МРу]Cl is 2.192 Å, and Θ angle=11.148º, whereas [С 10 МРу]Cl is 2.216 Å, and 14.192º, this corresponds to a change δ 2 of 9.617 ppm to 9.323 ppm.
The replacement of the Cl − -anion with the Br − -anion leads to a significant move upfield for the IL hydrogen atoms. This can be attributed to differences in the effective charges on the atoms and the distances between the cations and the Cl − /Br − -anions. According to quantum chemical calculations, the r (Н2…Cl) distances for some of the chloride-based models are less than the corresponding bromides.
For almost all the pyridinium-based salts, we did not observe any significant changes in Δδ for the hydrogen atoms of the methyl (labelled 3" and extreme in the ring from 2' to 10') and methylene (except for the extreme groups: from 2' to 9') groups following changes of the alkyl chain length and/or anion species. This indicates the relatively minimal interaction of the anion with the hydrocarbon atoms of the alkyl chain. It may be noted that for [АМРу]Cl, the H 3' atom is more upfield shifted in comparison with the same atom for [С 3 МРу]Cl. These effects can be explained by the greater sensitivity due to the π-electron contribution to the allylic substituent, and the presence of two doublets splitting for the group 3' from two magnetically inequivalent protons in the cis-and trans-positions ( Figure 11). J=14.7 s Table 9. Chemical shifts δ (ppm), and spin-spin coupling J (Hz) from the 1 H-NMR spectra for ILs with CDCl 3 [19] Pyridinium-Based Ionic Liquids -Application for Cellulose Processing http://dx.doi.org/10.5772/59286 In this way, the 1 H-NMR signals of the spectra for ILs depend strongly on the structure, and this firstly depends on the distances between the cation and anion.

Thermal analysis of 1-alkyl-3-methylpyridinium-based ILs
The phase transition for ILs was investigated using DSC calorimetry and the heated stage [20]. Table 10 shows the melting points and glass transition values for IL.  In this way, the 1 H-NMR signals of the spectra for ILs depend strongly on the structure, and this firstly depends on the distances between the cation and anion.

Thermal analysis of 1-alkyl-3-methylpyridinium-based ILs
The phase transition for ILs was investigated using DSC calorimetry and the heated stage [20]. Table 10 shows the melting points and glass transition values for IL.
It should be noted that the glass transition was not revealed for all the ILs. It appeared that the DSC-method was not useful for finding the melting points of some of the ILs, mainly with an odd number of carbon atoms in the alkylic chain. Most of the melting peaks obtained by DSC were not clearly defined. It was impossible to determine the melting point of [C 4 MPy]Br by means of DSC, but that IL was a greenish-brown viscous liquid at room temperature, and therefore it could be related to the room temperature ionic liquid (RTIL) species. In contrast, the salt [C 2 MPy]Br, due to the high value of its melting point, was not considered as an ionic liquid.
In general, it was noticed that there is a tendency that the melting point values of 1-alkyl-3methylpyridinium chloride-based ILs are reduced with a lengthening of the alkylic chain. This trend could be explained by a decrease in the packing density of the ion pairs of IL following an increase in the number of the alkylic chain carbon atoms.
A complete decomposition of the IL sets is seen at a temperature of about 250 °C [20]. The decomposition temperature values slightly decreased with the growth of the alkylic chain length. Furthermore, again the quaternary pyridinium salts, [С 2 МРу]Cl and [С 3 МРу]Cl, appeared to be the exceptions, as well as with respect to the melting point tendency. A change of the anion ambiguously affected the decomposition temperature, thus [С 2 МРу]Br decomposed at a higher temperature than [С 2 МРу]Cl but the decomposition temperature of bromides and chlorides with the [С 4 МРу] + cation was comparable.
With the use of a gas chromatography-mass spectroscopy, we can identify the fragments resulting from the decomposition [С 4 МРу]Cl (at temperatures up to 280 °C). In the chromatogram shown in Figure 12A, there are two significant peaks. Peak 1 (M/Z 93, 94, Figure 12B) corresponds to 3-methylpyridine, CAS №108-99-6 (with a similarity of 94%). This result is expected, since the 3-methylpyridine is a natural result of the fragmentation of these salts. Peak 2 (M/Z 150, 151) when compared with the libraries FFNSC 1.2 and NIST08 does not give a satisfactory result. The said peak value may correspond to an isolated cation (C 10 H 16 N) with a molecular weight of 150.2417 g mol -1 . A peak with different M/Z values is less intense, but the molecular weight can suggest some probable fragments ( Figure 13). From the analysis of

Solvatochromism of 1-alkyl-3-methylpyridinium-based ILs solutions
To determine the Kamlet-Taft hydrogen bond acceptor (basicity, β parameter) for the series of ILs we used indicator [Cu(tmen)(acac)](ClO 4 ). The obtained data of the β parameter for ILs (from 0.70 to 0.99) indicate a relatively high basicity of the 1alkyl-3-methylpyridinium-based ILs. More detailed information will published in other works.

Dissolution mechanism of cellulose in pyridinium-based ionic liquids 4.1. Dissolution of cellulose with 1-alkyl-3-methylpyridinium-based ILs
Our study of the solubility of cellulose in the synthesized ILs showed that for the same cation the 1-alkyl-3methylpyridinium bromides show a lower dissolving ability than chlorides, the latter providing more concentrated solutions of cellulose (Table 11). This is consistent with the results of studies on 1-alkyl-3-methylimidazolium halides [7]. An increase in the temperature from 110 to 165 °C leads to a twofold increase in the maximum concentration of cellulose in the solutions of the bromide-containing ILs. With the increasing length of the substituent in the pyridinium chloride ionic liquids, their dissolving power

Solvatochromism of 1-alkyl-3-methylpyridinium-based ILs solutions
To determine the Kamlet-Taft hydrogen bond acceptor (basicity, β parameter) for the series of ILs we used indicator [Cu(tmen)(acac)](ClO 4 ). The obtained data of the β parameter for ILs (from 0.70 to 0.99) indicate a relatively high basicity of the 1alkyl-3-methylpyridinium-based ILs. More detailed information will published in other works.

Dissolution mechanism of cellulose in pyridinium-based ionic liquids 4.1. Dissolution of cellulose with 1-alkyl-3-methylpyridinium-based ILs
Our study of the solubility of cellulose in the synthesized ILs showed that for the same cation the 1-alkyl-3methylpyridinium bromides show a lower dissolving ability than chlorides, the latter providing more concentrated solutions of cellulose (Table 11). This is consistent with the results of studies on 1-alkyl-3-methylimidazolium halides [7]. An increase in the temperature from 110 to 165 °C leads to a twofold increase in the maximum concentration of cellulose in the solutions of the bromide-containing ILs. With the increasing length of the substituent in the pyridinium chloride ionic liquids, their dissolving power toward cellulose decreases. A similar trend was found for 1-alkylpyridinium [25][26][27][28][29] and 1-alkyl-3-methylimidazolium [27] halides.   Table 11. The maximum achieved concentration (D) of cellulose Alicell Super (DP 599) in chloride-and bromide-based ILs [7] Thus, the results obtained suggest that 1,3-disubstituted pyridinium cation-based ILs are the most effective solvents for cellulose. Pyridinium salt can dissolve in 1.5 times more cellulose than the imidazolium salt with the same substituents and anion. This is confirmed by the data of [4,6].  Figure 14).
[С4МРу]Br 110 3.5 165 7 [C4MIm]Cl 120 17 Table 11. The maximum achieved concentration (D) of cellulose Alicell Super (DP 599) in chloride-and bromide-based ILs [7] Thus, the results obtained suggest that 1,3-disubstituted pyridinium cation-based ILs are the most effective solvents for cellulose. Pyridinium salt can dissolve in 1.5 times more cellulose than the imidazolium salt with the same substituents and anion. This is confirmed by the data of [4,6]. We can obtain the 5 The influence of the temperature during the dissolution is accompanied by the degradation of the natural polymer, as exemplified by the reduction in the degree of polymerization (DP) ( Table 12). In the FTIR spectra of regenerated cellulose samples of IL, we observed the appearance of characteristic peaks at ~1000 cm -1 corresponding predominantly to the С 6 Н2-О 6 Н 6 group. Their presence indicates the modification of cellulose II [30]. For the modification of cellulose I (Alicell Super) stretching vibrations of υ С 6 -О 6 were observed at 1031 cm -1 . The appearance of two bands at 3440 cm -1 and 3484 cm -1 (the weak hydrogen bond group of С 2 -О 2 Н 2 ) also indicate the modification of cellulose II (Figure 15). The influence of the temperature during the dissolution is accompanied by the degradation of the natural polymer, as exemplified by the reduction in the degree of polymerization (DP) ( Table 12).  In the FTIR spectra of regenerated cellulose samples of IL, we observed the appearance of characteristic peaks at ~1000 cm -1 corresponding predominantly to the С 6 Н 2 -О 6 Н 6 group. Their presence indicates the modification of cellulose II [30]. For the modification of cellulose I (Alicell Super) stretching vibrations of υ С 6 -О 6 were observed at 1031 cm -1 . The appearance of two bands at 3440 cm -1 and 3484 cm -1 (the weak hydrogen bond group of С 2 -О 2 Н 2 ) also indicate the modification of cellulose II (Figure 15).  protons of the pyridine ring, which shifted upfield. This could be related to the distancing of the anion interacting with the cellulose at the formation of the hydrogen bond. The chemical shifts of H 2 and H 6 changed insignificantly for [С 4 МРу]CH 3 COO after adding cellulose to the solution. It could be assumed that this was related to the ability of the acetate anion to interact simultaneously with two electron-acceptor centres (cellulose hydroxyls and the IL cation). The δ а downfield shift of the acetate hydrogen's atoms of the -СН 3 group confirmed that the anion of the IL interacted with the cellulose.   Table 9). The solutions were obtained in CDCl 3 [19] Therefore, the interaction of cellulose with pyridinium-based ILs is caused by the ability of the IL anion to exist as the hydrogen bond acceptor.

Study of the mechanism of cellulose solvation by 1-alkyl-3-methylpyridinium-based ILs using a quantum-chemical calculation
A study of the formation of 'cellulose -ionic liquids' complexes were performed by HF/ 6-31G(d) quantum-chemical calculation method for the 'cellotetraose -1-butyl-3-methylpyridinium X' model, wherein X are acetate-, chloride-, and bromide-anions. The geometry of the complexes with optimum energy characteristics are shown in Figure 16.
The formation of the solvate complex is accompanied by the appearance of two hydrogen bonds between the anion and the primary О 6 Н 6 , and the secondary О 2' Н 2' hydroxyl from the neighbouring glycosidic link of the cellotetraose. The distance between the anion and the hydrogen atoms of the cellotetraose hydroxyls, r (H6-X) , r (H2'-X) , ranges from 1. The geometric characteristics obtained meet the criteria for the formation of the hydrogen bonds [31]: for example, the formation of a hydrogen bond with the chloride-anion has a distance C sp 3 -OH···Cl -in ranges from ~2 Å to ~2.4 Å, and a bond angle (between O, H and Cl) > 140º. The anion of the solvent model is positioned closer to the hydrogen atom of the primary hydroxyl group С 6 Н 2 -О 6 Н 6 .
The interaction of [С 4 МРу]Br with the model of the cellulose ( Figure 16B), as shown by quantum-chemical calculations, is similar: anion forms hydrogen bonds with both the primary and secondary hydroxyl groups. The distance between the bromide-anion and the cellotetraose hydroxyl is larger than it is for the chloride-anion (2.445 Å for O 6 Н 6 …Br, 2.710 Å for O 2' Н 2' …Br and 2.295 Å for O 6 Н 6 …Cl, 2.514 Å for O 2' Н 2' …Cl), because the length of the hydrogen bond between the halide and H-donor increases with the radii of the halide-ions [31].

Conclusion
Thus, using quantum-chemical calculations, the features of the interaction of cellulose with 1alkyl-3-methylpyridinium-based ILs were identified. This shows a decrease in the intensity of the binding of the hydroxyl anions in the series CH 3 COO -, Cl -, Br -, which explains the difference in the solvent power of ILs.
According to the theoretical studies (quantum-chemical calculations) and experimental studies (analysis of the chemical shifts), we propose a probable mechanism of the solvation of pyridinium ILs with cellulose, shown in Figure 17. In the first stage, the interaction of the polymer with the solvent's anion forms hydrogen bonds with primary and secondary cellulose hydroxyls. The anion maintains ionic bonding with the cation, which at solvation was weakened (the distance between the ions increases).
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