**Pyridinium-Based Ionic Liquids — Application for Cellulose Processing**

Elena S. Sashina and Dmitrii A. Kashirskii

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/59286

#### **1. 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 [1-3], especially for cellulose dissolution [4-13], 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 [16-18] 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].

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

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.

#### **2. Experimental**

#### **2.1. 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, Ep, was calculated as a difference: the energy of the cation-anion complex, Eca, minus the sum of the energies of the cation, Ec, and the anion, Ea:

$$\mathbf{E\_{p} = 2625.5 \bullet \{E\_{ca} \cdot (E\_{c} + E\_{a})\}} \text{ .}$$

where 2625.5 is the conversion factor of the Hartree energy (atomic units) in kJ mol-1.

#### **2.2. 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 CDCl3 (0.65 mL, δ=7.240 ppm). Solutions of cellulose with ILs were investigated using the CDCl3 (δ=7.240 ppm). TMS (δ=0) was used as the internal standard [19].

#### **2.3. Thermal analysis**

Differential scanning calorimetry (NETZSCH-Gerätebau GmbH Thermal Analysis DSC 204), thermogravimetry methods (F. Paulik J.Paulik L. Erdey system) and Boёtius heated stage (VEB Wagetechnik Rapido) were used for the thermal analysis [20]. The conditions of the experiment are shown in Table 1. For the Boёtius heated stage: the samples were placed on a microscope slide and the cover glass, and these were added to the heated stage, and the melting process was observed with the aid of a microscope PHMK 05 (with an increasing of 16 times). For the melting point, the state of the total loss of the sharp edges of the salts' crystals is taken.

#### **2.4. Gas chromatography–mass spectrometry**

The IL samples were identified on the Shimadzu GCMS-QP2010 Plus. For this equipment the FFNSC 1.2 and NIST08 libraries were used. The main analysis conditions involved a capillary


**Table 1.** Conditions of the thermal analysis of ionic liquids [20].

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 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, Ep, was calculated as a difference: the energy of the cation-anion complex, Eca, minus the

Ep =2625.5∙{Eca - (Ec + Ea)},

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 CDCl3 (0.65 mL, δ=7.240 ppm). Solutions of cellulose with ILs were investigated

Differential scanning calorimetry (NETZSCH-Gerätebau GmbH Thermal Analysis DSC 204), thermogravimetry methods (F. Paulik J.Paulik L. Erdey system) and Boёtius heated stage (VEB Wagetechnik Rapido) were used for the thermal analysis [20]. The conditions of the experiment are shown in Table 1. For the Boёtius heated stage: the samples were placed on a microscope slide and the cover glass, and these were added to the heated stage, and the melting process was observed with the aid of a microscope PHMK 05 (with an increasing of 16 times). For the melting point, the state of the total loss of the sharp edges of the salts' crystals is taken.

The IL samples were identified on the Shimadzu GCMS-QP2010 Plus. For this equipment the FFNSC 1.2 and NIST08 libraries were used. The main analysis conditions involved a capillary

H-NMR spectra obtained with the

where 2625.5 is the conversion factor of the Hartree energy (atomic units) in kJ mol-1.

using the CDCl3 (δ=7.240 ppm). TMS (δ=0) was used as the internal standard [19].

**2. Experimental**

390 Ionic Liquids - Current State of the Art

**2.2. 1**

**2.1. Quantum-chemical calculations**

**H-NMR spectroscopy**

**2.3. Thermal analysis**

sum of the energies of the cation, Ec, and the anion, Ea:

An analysis of the chemical shifts was carried out by the 1

**2.4. Gas chromatography–mass spectrometry**

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.

#### **2.5. 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.

#### **2.6. 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 temper‐ ature control using the magnetic stirrer. The calculation of the dissolved cellulose concentra‐ tion was given by the formula:

$$D = \frac{m\_1}{m\_1 + m\_2} \bullet (100 - W)\_{\prime\prime}$$

where D is the concentration of the cellulose solution with ILs, wt%; m1 and m2 are the mass of the cellulose and ILs, respectively, g; W is the water content of the cellulose, wt% [7].

#### **2.7. 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.

#### **2.8. 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 U15C MLW (bath temperature: 20 °C). Using the time of the expiration (in sec.) of pure cadoxene (τ0) and the solution (τ<sup>i</sup> ) the relative viscosity was calculated:

$$\eta\_{\mathbf{r}} = \frac{\mathbf{r}\_{\mathbf{r}}}{\mathbf{r}\_{\mathbf{r}}}.$$

The specific viscosity was determined from the relationship: ηs=η<sup>r</sup> – 1, and the reduced viscosity: ηi=ηr/D. The intrinsic viscosity [η] found by graphical extrapolation to the value of D=0 using the software package Origin 9.0 was:

$$\lim\_{\mathcal{D}\to 0} \frac{\eta\_\*}{\mathcal{D}} = \lim\_{\mathcal{D}\to 0} \frac{\ln \eta\_\*}{\mathcal{D}}.$$

The intrinsic viscosity is related to the DP value for a given diameter of the viscometer capillary as [η]=7 10-3 DP0.9, therefore

$$\text{DP} = \left(\frac{\text{L}\text{v}}{0.007}\right)^{\frac{1}{0.9}}\text{J}$$

#### **3. Physical and chemical properties of pyridinium-based ionic liquids**

#### **3.1. 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.

#### *3.1.1. 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, Ep, the distances between the cation and anion r(H2···Cl), the torsion angle Θ between the atoms N, C2 , H2 of 1-methylpyridinium rings, and the chloride-anion Cl for [C1Py]Cl [11].

**Figure 2.** PES for [C1Py]Cl

**2.8. Degree of polymerization of cellulose**

392 Ionic Liquids - Current State of the Art

) the relative viscosity was calculated:

D=0 using the software package Origin 9.0 was:

as [η]=7 10-3 DP0.9, therefore

**salts, and their structural features**

solution (τ<sup>i</sup>

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 U15C MLW (bath temperature: 20 °C). Using the time of the expiration (in sec.) of pure cadoxene (τ0) and the

> <sup>η</sup>r<sup>=</sup> <sup>τ</sup><sup>i</sup> τ0 .

The specific viscosity was determined from the relationship: ηs=η<sup>r</sup> – 1, and the reduced viscosity: ηi=ηr/D. The intrinsic viscosity [η] found by graphical extrapolation to the value of

The intrinsic viscosity is related to the DP value for a given diameter of the viscometer capillary

0.007 ) <sup>1</sup> 0.9 .

DP=( <sup>η</sup>

**3. Physical and chemical properties of pyridinium-based ionic liquids**

**3.1. Theoretical studies of the 1-alkylpyridinium- and 1-alkyl-3-methylpyridinium-based**

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

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, Ep, the distances between the cation

, H2

of 1-methylpyridinium rings,

the numbers of the substituents of the ring have different anions.

*3.1.1. Features of the structures of the 1-substituted pyridinium-based ILs*

and anion r(H2···Cl), the torsion angle Θ between the atoms N, C2

and the chloride-anion Cl for [C1Py]Cl [11].

ln η<sup>r</sup> <sup>D</sup> .

lim D→0 ηs <sup>D</sup> =lim D→0

> The minimum of the Ep value corresponds to the location of the anion near the atom H2 (the most electron-deficient atom) of the ring and H6 of the aliphatic chain (Fig. 3A). At the point of the global minimum, the torsion angle between atoms N, C2 , H2 , 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 Еp= -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 Еp=+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 Ep 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 Н1') is reduced. So, in a series of C1–C6 the distance between the chloride anion and the nearest H2 atom of the ring increases (2.190 → 2.225 Å). Starting from C3 the anion approaches the substituents, and the distance between the anion and the nearest atom H2 of the cationic substituent is shortened (2.624 ← 2.525 Å). In the C0–C4 series, as in the case of the imidazolium-based ILs [22], there is a tendency for a gradual decrease in the

 Figure 3. Models of [C1Py]Cl with different arrangements of the anion around the cation, calculated by HF/3-21G(d) **Figure 3.** Models of [C1Py]Cl with different arrangements of the anion around the cation, calculated by HF/3-21G(d)

The 1-substituted formation from pyridine molecule (Py) leads to minimal changes in the bond lengths and angles on

interaction energy between the cation and the anion; this trend correlates with the experimen‐ tally 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 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 Н1') is reduced. So, in a series of C1–C6 the distance between the chloride anion and the nearest H2 atom of the ring increases (2.190 → 2.225 Å). Starting from C3 the anion approaches the substituents, and the distance between the anion and the nearest atom H<sup>2</sup> of the cationic substituent is shortened (2.624 ← 2.525 Å). In the C0–C4 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.** The main geometrical parameters, the effective charge values on the atoms, q, the cation-anion interaction energy, Ep (kJ mol-1), and the dipole moment, (D), of the models Py, 1-substituted pyridinium-based chlorides, calculated by HF/6-31G(d) [11]


**Table 3.** The solubility biomasses with 1-alkylpyridinium-based salts, according to the works [25,26]

interaction energy between the cation and the anion; this trend correlates with the experimen‐ tally 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

anion approaches the substituents, and the distance between the anion and the nearest atom H<sup>2</sup>

differences were noted only in the angle values of α and β for the 1-substituted pyridinium compared with Py.

Figure 3. Models of [C1Py]Cl with different arrangements of the anion around the cation, calculated by HF/3-21G(d) 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

shortened (2.624 ← 2.525 Å). In the C0–C4 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.

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 Н1') is reduced. So, in a series of C1–C6

atom of the ring increases (2.190 → 2.225 Å). Starting from C3 the

of the cationic substituent is

<sup>C</sup> <sup>D</sup> 

**Figure 3.** Models of [C1Py]Cl with different arrangements of the anion around the cation, calculated by HF/3-21G(d)

<sup>A</sup> <sup>B</sup>

*a* 1.321 1.335 1.325 1.331 1.333 1.334 1.334 1.331 1.333 1.331 1.331 *b* 1.385 1.381 1.377 1.388 1.386 1.384 1.384 1.387 1.385 1.386 1.386 *c* 1.384 1.379 1.387 1.374 1.376 1.377 1.377 1.375 1.377 1.375 1.375 *α* 117.704 117.670 122.744 121.34 121.21 121.19 121.06 121.06 121.21 121.28 121.14 *β* 123.607 122.990 120.268 119.96 120.08 120.17 120.20 120.21 120.11 120.07 120.19 *γ* 118.229 118.840 118.285 119.48 119.44 119.42 119.41 119.41 119.41 119.39 119.39

**[НРу]Cl [С1Py]Cl [С2Py]Cl [С3Py]Cl [С4Py]Cl [С5Py]Cl [С6Py]Cl [APy]Cl [BzPy]Cl**

component.

394 Ionic Liquids - Current State of the Art

**Paramet ers:**

Geometry

**Ру**

the distance between the chloride anion and the nearest H2

**Calc. Ref. [21]**

Regardless of the kind of anion (we examined 1-methylpyridinium cation-based models of salts with chloride-anion, bromide-anion and acetate-anion, Figure 4),these have been placed near the pyridinium ring H<sup>2</sup> atom, which is characterized by low electron density (Table 4). The bond lengths of the pyridinium rings with chloride-, and bromide-based anions have identical values. Effective negative charges on the nitrogen atoms are virtually identical to chloride-, bromide-, and acetate-based salts which have a 1-methylpyridinium cation. Acetate anion has the greatest negative charge on the O<sup>1</sup> and O<sup>2</sup> atoms. The main calculated parameters of the optimized complexes are presented in Tables 4 and 5 [11].

presented in Tables 4 and 5 [11].

\* by dilution in DMSO (1:2).

Charges

[11]

qH1<sup>ʹ</sup>

Table 4. The bond lengths (Å), angles (deg.), interaction energies Ep (kJ mol-1) and atomic charges of 1-methylpyridinium chloride and bromide, calculated by HF/6-31G (d). X = anion [11]

*β*  123.607 122.990 120.268 119.96 120.08 120.17 120.20 120.21 120.11 120.07 120.19 *γ*  118.229 118.840 118.285 119.48 119.44 119.42 119.41 119.41 119.41 119.39 119.39 *δ*  118.624 118.650 120.149 119.46 119.44 119.41 119.40 119.39 119.45 119.48 119.39 r(H2···Cl) 2.190 2.201 2.203 2.205 2.221 2.225 2.221 2.208 r(Cl…H1ʹ) 1.821 2.464 2.575 2.624 2.623 2.525 2.592 2.537 2.564

qCl ‐0.746 ‐0.852 ‐0.862 ‐0.864 ‐0.864 ‐0.852 ‐0.866 ‐0.866 ‐0.852 qN ‐0.515 ‐0.685 ‐0.603 ‐0.604 ‐0.605 ‐0.605 ‐0.603 ‐0.611 ‐0.607 ‐0.602 qH2 0.202 0.293 0.363 0.364 0.363 0.363 0.357 0.365 0.359 0.361

 0.431 0.325 0.305 0.304 0.305 0.317 0.299 0.314 0.314 Еp ‐436.965 ‐369.919 ‐367.434 ‐364.033 ‐362.158 ‐363.133 ‐365.211 ‐365.149 ‐866.474 �, D 2.31 2.20 [24] Table 2. The main geometrical parameters, the effective charge values on the atoms, q, the cation‐anion interaction energy, Ep (kJ mol‐1), and the dipole moment, � (D), of the models Py, 1‐substituted pyridinium‐based chlorides, calculated by HF/6‐31G(d)

Solvents Dissolving power

[С2Py]Cl cellulose ~ 5 %, at 90 – 100 °С [26] [С4Py]Cl biomass poplar 30 mg g‐1\* at 60 °С [25]

[APy]Br biomass poplar 5 mg g‐1\* at 60 °С [25] [BzРу]Cl cellulose ~ 5 % at 110 – 115 °С [26] Table 3. The solubility biomasses with 1‐alkylpyridinium‐based salts, according to the works [25,26]

[НPy]Cl biomass poplar 80 mg g‐1\*, pine 60 mg g‐1\* at 60°С [25]

[APy]Cl biomass poplar mg g‐1\* at 60 °С [25]; cellulose ~ 5 % at 105 – 110 °С [26]

Regardless of the kind of anion (we examined 1‐methylpyridinium cation‐based models of salts with chloride‐anion, bromide‐anion and acetate‐anion, Figure 4),these have been placed near the pyridinium ring H2 atom, which is characterized by low electron density (Table 4). The bond lengths of the pyridinium rings with chloride‐, and

greatest negative charge on the O1 and O2 atoms. The main calculated parameters of the optimized complexes are

Figure 4. Optimized models of 1-methylpyridinium bromide (A), and 1-methylpyridinium acetate (B) **Figure 4.** Optimized models of 1-methylpyridinium bromide (A), and 1-methylpyridinium acetate (B)



**Table 4.** The bond lengths (Å), angles (deg.), interaction energies Ep (kJ mol-1) and atomic charges of 1 methylpyridinium chloride and bromide, calculated by HF/6-31G (d). X=anion [11]

*β*  123.607 122.990 120.268 119.96 120.08 120.17 120.20 120.21 120.11 120.07 120.19 *γ*  118.229 118.840 118.285 119.48 119.44 119.42 119.41 119.41 119.41 119.39 119.39 *δ*  118.624 118.650 120.149 119.46 119.44 119.41 119.40 119.39 119.45 119.48 119.39 r(H2···Cl) 2.190 2.201 2.203 2.205 2.221 2.225 2.221 2.208 r(Cl…H1ʹ) 1.821 2.464 2.575 2.624 2.623 2.525 2.592 2.537 2.564

qCl ‐0.746 ‐0.852 ‐0.862 ‐0.864 ‐0.864 ‐0.852 ‐0.866 ‐0.866 ‐0.852 qN ‐0.515 ‐0.685 ‐0.603 ‐0.604 ‐0.605 ‐0.605 ‐0.603 ‐0.611 ‐0.607 ‐0.602 qH2 0.202 0.293 0.363 0.364 0.363 0.363 0.357 0.365 0.359 0.361

 0.431 0.325 0.305 0.304 0.305 0.317 0.299 0.314 0.314 Еp ‐436.965 ‐369.919 ‐367.434 ‐364.033 ‐362.158 ‐363.133 ‐365.211 ‐365.149 ‐866.474 �, D 2.31 2.20 [24] Table 2. The main geometrical parameters, the effective charge values on the atoms, q, the cation‐anion interaction energy, Ep (kJ mol‐1), and the dipole moment, � (D), of the models Py, 1‐substituted pyridinium‐based chlorides, calculated by HF/6‐31G(d)

Solvents Dissolving power

[С2Py]Cl cellulose ~ 5 %, at 90 – 100 °С [26] [С4Py]Cl biomass poplar 30 mg g‐1\* at 60 °С [25]

[APy]Br biomass poplar 5 mg g‐1\* at 60 °С [25] [BzРу]Cl cellulose ~ 5 % at 110 – 115 °С [26] Table 3. The solubility biomasses with 1‐alkylpyridinium‐based salts, according to the works [25,26]

[НPy]Cl biomass poplar 80 mg g‐1\*, pine 60 mg g‐1\* at 60°С [25]

[APy]Cl biomass poplar mg g‐1\* at 60 °С [25]; cellulose ~ 5 % at 105 – 110 °С [26]

Regardless of the kind of anion (we examined 1‐methylpyridinium cation‐based models of salts with chloride‐anion, bromide‐anion and acetate‐anion, Figure 4),these have been placed near the pyridinium ring H2 atom, which is characterized by low electron density (Table 4). The bond lengths of the pyridinium rings with chloride‐, and bromide‐based anions have identical values. Effective negative charges on the nitrogen atoms are virtually identical to chloride‐, bromide‐, and acetate‐based salts which have a 1‐methylpyridinium cation. Acetate anion has the greatest negative charge on the O1 and O2 atoms. The main calculated parameters of the optimized complexes are

A B

Parameters [С1Ру]

**Figure 4.** Optimized models of 1-methylpyridinium bromide (A), and 1-methylpyridinium acetate (B)

Geometry

Figure 4. Optimized models of 1-methylpyridinium bromide (A), and 1-methylpyridinium acetate (B) Table 4. The bond lengths (Å), angles (deg.), interaction energies Ep (kJ mol-1) and atomic charges of 1-methylpyridinium chloride and bromide, calculated by HF/6-31G (d). X = anion [11]

> *a* 1.339 1.331 1.331 *b* 1.372 1.387 1.387

**Parameters [С1Ру]<sup>+</sup> [С1Py]Cl [С1Py]Br**

*a* 1.339 1.331 1.331 *b* 1.372 1.387 1.387 *c* 1.388 1.374 1.374 α 120.68 121.36 121.24 *β* 121.13 119.95 120.02 *β'* 121.13 121.27 121.32 *γ* 118.80 119.49 119.48 *γ'* 118.80 118.49 118.50 *δ* 119.46 119.46 119.43 r(X…H2) 2.191 2.348 r(X…H1') 2.463 2.580

q<sup>Х</sup> -0.852 -0.839 qN -0.655 -0.603 -0.604 q<sup>С</sup> <sup>2</sup> 0.163 0.185 0.174 q<sup>С</sup> <sup>3</sup> -0.271 -0.301 -0.301 q<sup>С</sup> <sup>4</sup> -0.087 -0.097 -0.097 q<sup>С</sup> <sup>5</sup> -0.271 -0.284 -0.283 q<sup>С</sup> <sup>6</sup> 0.163 0.156 0.165 q<sup>С</sup> 1' -0.332 -0.353 -0.356

<sup>+</sup> [С1Py]Cl [С1Py]Br

Charges

\* by dilution in DMSO (1:2).

presented in Tables 4 and 5 [11].

396 Ionic Liquids - Current State of the Art

Geometry

Charges

[11]

qH1<sup>ʹ</sup>



**Table 5.** Bond lengths (Å), angles (deg.), interaction energies Ep (kJ mol-1) and atomic charges of 1-methylpyridinium acetate, calculated by HF/6-31G (d). (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<sup>1</sup> and O<sup>2</sup> atoms), and it decreases the positive charges on the cations which interact due to the H<sup>2</sup> and H1' 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 experi‐ mental results [22, 27].

#### *3.1.2. Features of the structures of 1,3-disubstituted-pyridinium-based ILs*

Figure 5 shows the potential energy surface for the 1,3-disubstituted pyridinium IL from the example of [C4MPy]Cl. For [C4MPy]Cl we observed two structures of ion pairs in mutual arrangement. These structures correspond to the minima of Ep: chloride-anion is located near the protons H2 (Figure 5, point I) and H6 (Figure 5, point II). The Ep values of both the cationanion pairs with variable positions (Figure 6) for these ions are not significantly different. 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]. **3.1.2. Features of the structures of 1,3‐disubstituted‐pyridinium‐based ILs**

Figure 5 shows the potential energy surface for the 1,3‐disubstituted pyridinium IL from the example of [C4MPy]Cl. For [C4MPy]Cl we observed two structures of ion pairs in mutual arrangement. These structures correspond to the

**Parameters Values**

Е<sup>p</sup> -418.464

*3.1.2. Features of the structures of 1,3-disubstituted-pyridinium-based ILs*

parentheses) [11].

398 Ionic Liquids - Current State of the Art

mental results [22, 27].

H<sup>2</sup>

**Table 5.** Bond lengths (Å), angles (deg.), interaction energies Ep (kJ mol-1) and atomic charges of 1-methylpyridinium acetate, calculated by HF/6-31G (d). (The values of the parameters for isolated acetate-anions are shown in

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<sup>1</sup> and O<sup>2</sup> atoms), and it decreases the positive charges on the cations which interact due to the

 and H1' 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 experi‐

Figure 5 shows the potential energy surface for the 1,3-disubstituted pyridinium IL from the example of [C4MPy]Cl. For [C4MPy]Cl we observed two structures of ion pairs in mutual arrangement. These structures correspond to the minima of Ep: chloride-anion is located near

q<sup>С</sup> 2' 0.760 (0.723) q<sup>С</sup> 3' -0.554 (-0.554) q<sup>Н</sup> <sup>2</sup> 0.442 q<sup>Н</sup> <sup>3</sup> 0.298 q<sup>Н</sup> <sup>4</sup> 0.252 q<sup>Н</sup> <sup>5</sup> 0.248 q<sup>Н</sup> <sup>6</sup> 0.258 q<sup>Н</sup> 1' 0.342 q<sup>Н</sup> 1'' 0.200 q<sup>Н</sup> 1''' 0.200 q<sup>Н</sup> 3' 0.155 (0.116) q<sup>Н</sup> 3'' 0.160 (0.114) q<sup>Н</sup> 3''' 0.155 (0.116) q<sup>О</sup> <sup>1</sup> -0.789 (-0.758) q<sup>О</sup> <sup>2</sup> -0.759 (-0.758) Σ (CH3COO) -0.872 (-1.000)

**Figure 5.** PES for [C4MPy]Cl. The Roman numerals indicate the points of the energy minima on the PES

Figure 5. PES for [C4MPy]Cl. The Roman numerals indicate the points of the energy minima on the PES

The results of the quantum chemical calculations for the other IL models give similar geometries with the mutual **Figure 6.** arrangement of the anion around the cation. The optimized models of [ Two optimized models of [C4MPy]Cl: on a PES to point I corresponds to structure A, and for point II – struc‐ С4МРу]CH3COO and [С4МРу]Br are shown in Figure 7. ture B

Figure 6. Two optimized models of [C4MPy]Cl: on a PES to point I corresponds to structure A, and for point II – structure B

The results of the quantum chemical calculations for the other IL models give similar geome‐ tries with the mutual arrangement of the anion around the cation. The optimized models of [С4МРу]CH3COO and [С4МРу]Br are shown in Figure 7.

Figure 7. Optimized models of [C4MPy]CH3COO (A), and [C4MPy]Br (B)

ring proton, this model was adopted for all subsequent calculations of geometry. The main results of the quantum

Since we observed the global minimum of the potential energy on the PES for the model with the anion located near to the H<sup>2</sup> **Figure 7.** Optimized models of [C4MPy]CH3COO (A), and [C4MPy]Br (B)

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 H2 , C<sup>2</sup> , and N, and the distance r(H2…X) between the anion X and the H<sup>2</sup> atom. Thus, for 1,3-disubstituted chloride-containing pyridinium models, the torsion angle Θ has a range from 0.2º (for [C1MPy]Cl) to 11.1º and 14.2º (for the substituted pyridinium-based chlorides with chain lengths of C2 to C10). 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 Cl– to Br– makes 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 Ep 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<sup>2</sup> 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<sup>2</sup> , C<sup>2</sup> , and N, and the distance r(H2…X) between the anion X and the H<sup>2</sup> atom. Thus, for 1,3-disubstituted chloridecontaining pyridinium models, the torsion angle Θ has a range from 0.2º (for [C1MPy]Cl) to 11.1º and 14.2º (for the substituted pyridinium-based chlorides with chain lengths of C2 to C10). 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 Cl– to Br– makes 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 Ep 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.


The results of the quantum chemical calculations for the other IL models give similar geome‐ tries with the mutual arrangement of the anion around the cation. The optimized models of

A B

Figure 7. Optimized models of [C4MPy]CH3COO (A), and [C4MPy]Br (B) Since we observed the global minimum of the potential energy on the PES for the model with the anion located near to

(for [C1MPy]Cl) to 11.1º and 14.2º (for the substituted pyridinium-based chlorides with chain lengths of C2 to C10). 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

Since we observed the global minimum of the potential energy on the PES for the model with

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

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 Ep 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.

 to Br– makes 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 Ep 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

containing pyridinium models, the torsion angle Θ has a range from 0.2º (for [C1MPy]Cl) to 11.1º and 14.2º (for the substituted pyridinium-based chlorides with chain lengths of C2 to C10). 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

increase in the Θ between the cation and anion in the homologous series, and finally the changing of the anion Cl–

ring. These differences are caused by relatively strong interactions with the anions.

torsion angle Θ between the anion X, and the pyridinium ring atoms H<sup>2</sup>

 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

atom. Thus, for 1,3-disubstituted chloride-containing pyridinium models, the torsion angle Θ has a range from 0.2º

, C<sup>2</sup>

ring proton, this model was adopted for all subsequent

, and N, and the distance r(H2…X) between the anion X

, C<sup>2</sup>

atom. Thus, for 1,3-disubstituted chloride-

to Br– makes

, and N, and the

[С4МРу]CH3COO and [С4МРу]Br are shown in Figure 7.

400 Ionic Liquids - Current State of the Art

torsion angle Θ between the anion X, and the pyridinium ring atoms H2

distance r(H2…X) between the anion X and the H<sup>2</sup>

the anion located near to the H<sup>2</sup>

**Figure 7.** Optimized models of [C4MPy]CH3COO (A), and [C4MPy]Br (B)

These differences are caused by relatively strong interactions with the anions.

the H<sup>2</sup>

and the H<sup>2</sup>

Cl–

\* The distance from the O<sup>1</sup> oxygen atom of acetate-anion to H<sup>2</sup> of pyridinium ring; distance from O<sup>2</sup> to H<sup>2</sup> is 2.526 Å.

**Table 6.** The main geometrical parameters: bond lengths (Å), and angles (degrees) for 1,3-disubstituted pyridinium salts, calculated by HF/6-31G (d) (An explanation of the symbols is given in the text and in figure 7)


\* A partial charge of the O<sup>1</sup> oxygen atom; partial charges to other atoms of the acetate-anion:-0.749 for O<sup>2</sup> , 0.753 for C',-0.556 for C'', 0.160 for H', 0.156 for H'', 0.156 for H''', the total charge of the acetate-anion *q*(*CH* <sup>3</sup>*COO* - ) is-0.854e.

**Table 7.** The partial charges q for atoms and the substituents by the Mulliken value, energy Ep (kJ mol-1) and dipole moments (*μ* <sup>→</sup> ) for the models of 1,3-disubstituted pyridinium salts, are calculated by HF/6-31G(d).

For the IL which has an allylic substituent we observed a relatively more larger in the Θ angle relative to [C3MPy]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 [C1МРу]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 H1' 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.

With respect to the values of the electron density distribution in the ion pair, an increase in the alkyl chain length of the cation clearly affects only the length of C4. A further increase in the length of the substituent does not lead to significant changes in the charge of the atom H<sup>2</sup> . This leads to a decrease in the solvent polarity as a result of increasing the volume portion of the non-polar components. The electric dipole moment *μ* → values, which are characteristic of the electron density asymmetry, are gradually reduced in chloride-containing ILs, where the length of the substituents are increased. The displacement dipole of the moment vector along the alkyl chain length takes place gradually, as shown in Figure 8. The calculated values of μ for bromide-containing ILs are also subject to a similar downward trend of the alkyl chain elongation.

Figure 8. The displacement vector of the dipole moments for [C2MPy]Cl (A) and [C10MPy]Cl (B) **3.2. Physical and chemical properties of 1-alkyl-3-methylpyridinium-based ILs Figure 8.** The displacement vector of the dipole moments for [C2MPy]Cl (A) and [C10MPy]Cl (B)

#### solvents was performed as described in paper [7]. Table 8. List of investigated solvents: 1-alkyl-3-methylpyridinium-based ILs, and their molecular weight **3.2. Physical and chemical properties of 1-alkyl-3-methylpyridinium-based ILs**

No. ILs Symbol М, g mol-1 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 series of 1-alkyl -3-methylpyridinium-based ILs (Table 8) was examined experimentally. The synthesis of these

1 1-Ethyl-3-methylpyridinium chloride [С2МРу]Cl 157.6415 2 1-Propyl-3-methylpyridinium chloride [С3МРу]Cl 171.6682 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, [С4МРу]Br and [С4МРу]CH3COO, are liquid at room temperature, i.e. they belong to a subclass RTIL.

3 1-Butyl-3-methylpyridinium chloride [С4МРу]Cl 185.6949

4 1-Pentyl-3-methylpyridinium chloride [С5МРу]Cl 199.7215

5 1-Hexyl-3-methylpyridinium chloride [С6МРу]Cl 213.7482

6 1-Heptyl-3-methylpyridinium chloride [С7МРу]Cl 227.7759

7 1-Octyl-3-methylpyridinium chloride [С8МРу]Cl 241.8016

8 1-Nonyl-3-methylpyridinium chloride [С9МРу]Cl 255.8282

9 1-Decyl-3-methylpyridinium chloride [С10МРу]Cl 269.8549

10 1-Allyl-3-methylpyridinium chloride [AМРу]Cl 169.6523

11 1-Ethyl-3-methylpyridinium bromide [С2МРу]Br 202.0924

12 1-Butyl-3-methylpyridinium bromide [С4МРу]Br 230.1458

13 1-Butyl-3-methylpyridinium acetate [С4МРу]CH3COO 209.2859

they belong to a subclass RTIL.

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, [С4МРу]Br and [С4МРу]CH3COO, are liquid at room temperature, i.e.


For the IL which has an allylic substituent we observed a relatively more larger in the Θ angle relative to [C3MPy]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 [C1МРу]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 H1' 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

With respect to the values of the electron density distribution in the ion pair, an increase in the alkyl chain length of the cation clearly affects only the length of C4. A further increase in the length of the substituent does not lead to significant changes in the charge of the atom H<sup>2</sup>

leads to a decrease in the solvent polarity as a result of increasing the volume portion of the

electron density asymmetry, are gradually reduced in chloride-containing ILs, where the length of the substituents are increased. The displacement dipole of the moment vector along the alkyl chain length takes place gradually, as shown in Figure 8. The calculated values of μ for bromide-containing ILs are also subject to a similar downward trend of the alkyl chain

<sup>A</sup> <sup>B</sup>

No. ILs Symbol

A series of 1-alkyl-3-methylpyridinium-based ILs (Table 8) was examined experimentally. The

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, [С4МРу]Br and [С4МРу]CH3COO, are liquid at room temperature, i.e. they belong to a subclass RTIL.

**3.2. Physical and chemical properties of 1-alkyl-3-methylpyridinium-based ILs**

**3.2. Physical and chemical properties of 1-alkyl-3-methylpyridinium-based ILs**

synthesis of these solvents was performed as described in paper [7].

**Figure 8.** The displacement vector of the dipole moments for [C2MPy]Cl (A) and [C10MPy]Cl (B)

solvents was performed as described in paper [7].

they belong to a subclass RTIL.

Figure 8. The displacement vector of the dipole moments for [C2MPy]Cl (A) and [C10MPy]Cl (B)

Table 8. List of investigated solvents: 1-alkyl-3-methylpyridinium-based ILs, and their molecular weight

1 1-Ethyl-3-methylpyridinium chloride [С2МРу]Cl 157.6415

2 1-Propyl-3-methylpyridinium chloride [С3МРу]Cl 171.6682

3 1-Butyl-3-methylpyridinium chloride [С4МРу]Cl 185.6949

4 1-Pentyl-3-methylpyridinium chloride [С5МРу]Cl 199.7215

5 1-Hexyl-3-methylpyridinium chloride [С6МРу]Cl 213.7482

6 1-Heptyl-3-methylpyridinium chloride [С7МРу]Cl 227.7759

7 1-Octyl-3-methylpyridinium chloride [С8МРу]Cl 241.8016

8 1-Nonyl-3-methylpyridinium chloride [С9МРу]Cl 255.8282

9 1-Decyl-3-methylpyridinium chloride [С10МРу]Cl 269.8549

10 1-Allyl-3-methylpyridinium chloride [AМРу]Cl 169.6523

11 1-Ethyl-3-methylpyridinium bromide [С2МРу]Br 202.0924

12 1-Butyl-3-methylpyridinium bromide [С4МРу]Br 230.1458

13 1-Butyl-3-methylpyridinium acetate [С4МРу]CH3COO 209.2859

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, [С4МРу]Br and [С4МРу]CH3COO, are liquid at room temperature, i.e.

A series of 1-alkyl -3-methylpyridinium-based ILs (Table 8) was examined experimentally. The synthesis of these

. This

→ values, which are characteristic of the

М,

g mol-1

for [AМРу]Cl on average is less than the other models.

non-polar components. The electric dipole moment *μ*

elongation.

402 Ionic Liquids - Current State of the Art

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, [С4МРу]Br and [С4МРу]CH3COO, are liquid at room **Table 8.** List of investigated solvents: 1-alkyl-3-methylpyridinium-based ILs, and their molecular weight

<sup>A</sup><sup>B</sup>

Figure 9. Crystals of [С2МРу]Cl (A) and [С4МРу]Cl (B) when magnified 10 times

shows the general scheme of the 1-alkyl-3-methylpyridinium ring and acetate-anion numbered to correlate with corresponding

**Figure 9.** Crystals of [С2МРу]Cl (A) and [С4МРу]Cl (B) when magnified 10 times

temperature, i.e. they belong to a subclass RTIL.

#### **3.2.1. 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 *3.2.1. Study of 1-alkyl-3-methylpyridinium-based ILs by 1 H-NMR spectroscopy*

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 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.

204 the chemical shifts (δ) of protons and proton groups.

196

199 they belong to a subclass RTIL.

A B

200 Figure 9. Crystals of [С2МРу]Cl (A) and [С4МРу]Cl (B) when magnified 10 times

**3.2.1. Study of 1-alkyl-3-methylpyridinium-based ILs by 1** 201 **H-NMR spectroscopy** 

14

12 1-Butyl-3-methylpyridinium bromide [С4МРу]Br 230.1458 13 1-Butyl-3-methylpyridinium acetate [С4МРу]CH3COO 209.2859

197 A feature common to most hydrophilic IL crystals (shown in the micrographs on Figure 9) is the gradual saturation of 198 water from the atmosphere. Two of the synthesized ILs, [С4МРу]Br and [С4МРу]CH3COO, are liquid at room temperature, i.e.

acetate-anion CH3COO<sup>−</sup> 206 (B) **Figure 10.** The numbering of the protons and the proton groups for the 1-alkyl-3-methylpyridine cation [СnМРу]<sup>+</sup> (A) and the acetate-anion CH3COO<sup>−</sup> (B)

Figure 10. The numbering of the protons and the proton groups for the 1-alkyl-3-methylpyridine cation [СnМРу] <sup>+</sup> 205 (A) and the

For the peaks of the resonance signals of the ring protons, we observed singlet (H2 ), duplet (H4 and H6 ) and triplet (H5 ). Here, the *J* for H4 peak are 7.7 or 8.4 Hz, for H5 *J*=14, 11.9 or 14.7 Hz (for [С4МРу]CН3СОО), and for H<sup>6</sup> *J*=5.6, 6.3 or 4.2 Hz (for [С4МРу]CН3СОО).

We observed the greatest changes in the values of the chemical shifts during the alkyl chain lengthening for H2 and H6 atoms (labelled peaks **2** and **6**, respectively) of the 1-alkyl-3 methylpyridinium ring (to Δδ=0.30 and 0.24 ppm, respectively) [19]. According to the quan‐ tum-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 H2 and H6 atoms begin to move upfield (shielding), while the resonance frequency of the H<sup>4</sup> and H5 protons are practi‐ cally 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 [С2МРу]Cl is 2.192 Å, and Θ angle=11.148º, whereas [С10МРу]Cl is 2.216 Å, and 14.192º, this corresponds to a change δ**<sup>2</sup>** of 9.617 ppm to 9.323 ppm.

The replacement of the Cl<sup>−</sup> -anion with the Br<sup>−</sup> -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<sup>−</sup> /Br<sup>−</sup> -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 H3' atom is more upfield shifted in comparison with the same atom for [С3МРу]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).


14

12 1-Butyl-3-methylpyridinium bromide [С4МРу]Br 230.1458 13 1-Butyl-3-methylpyridinium acetate [С4МРу]CH3COO 209.2859

A B

A B

). Here, the *J* for H4 peak are 7.7 or 8.4 Hz, for H5 *J*=14, 11.9 or 14.7

atoms (labelled peaks **2** and **6**, respectively) of the 1-alkyl-3-

, as well as to change the distance between the ion pair in the

and H5



(A)

), duplet

and H6 atoms begin to

protons are practi‐

of 9.617 ppm to

202 The experimental data of the chemical shifts and spin-spin coupling (*J*-couling) are presented in Table 9 [19]. Figure 10 203 shows the general scheme of the 1-alkyl-3-methylpyridinium ring and acetate-anion numbered to correlate with corresponding

Figure 10. The numbering of the protons and the proton groups for the 1-alkyl-3-methylpyridine cation [СnМРу] <sup>+</sup> 205 (A) and the

For the peaks of the resonance signals of the ring protons, we observed singlet (H2

Hz (for [С4МРу]CН3СОО), and for H<sup>6</sup> *J*=5.6, 6.3 or 4.2 Hz (for [С4МРу]CН3СОО).

With an increase of the alkylic chain length, the δ signals for the H2

whereas [С10МРу]Cl is 2.216 Å, and 14.192º, this corresponds to a change δ**<sup>2</sup>**


move upfield (shielding), while the resonance frequency of the H<sup>4</sup>

**Figure 10.** The numbering of the protons and the proton groups for the 1-alkyl-3-methylpyridine cation [СnМРу]<sup>+</sup>

We observed the greatest changes in the values of the chemical shifts during the alkyl chain

methylpyridinium ring (to Δδ=0.30 and 0.24 ppm, respectively) [19]. According to the quan‐ tum-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

cally 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 [С2МРу]Cl is 2.192 Å, and Θ angle=11.148º,

IL hydrogen atoms. This can be attributed to differences in the effective charges on the atoms

calculations, the r(Н2…Cl) distances for some of the chloride-based models are less than the

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 H3' atom is more upfield shifted in comparison with the same atom for [С3МРу]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

/Br<sup>−</sup>

200 Figure 9. Crystals of [С2МРу]Cl (A) and [С4МРу]Cl (B) when magnified 10 times

**3.2.1. Study of 1-alkyl-3-methylpyridinium-based ILs by 1** 201 **H-NMR spectroscopy** 

acetate-anion CH3COO<sup>−</sup> 206 (B)

(B)

and H6

and the distances between the cations and the Cl<sup>−</sup>

protons in the *cis-* and *trans-*positions (Figure 11).

204 the chemical shifts (δ) of protons and proton groups.

and the acetate-anion CH3COO<sup>−</sup>

404 Ionic Liquids - Current State of the Art

) and triplet (H5

(H4

and H6

9.323 ppm.

lengthening for H2

these atoms with an anion Cl–

homologous series of ILs.

The replacement of the Cl<sup>−</sup>

corresponding bromides.

197 A feature common to most hydrophilic IL crystals (shown in the micrographs on Figure 9) is the gradual saturation of 198 water from the atmosphere. Two of the synthesized ILs, [С4МРу]Br and [С4МРу]CH3COO, are liquid at room temperature, i.e.

196

199 they belong to a subclass RTIL.

**Table 9.** Chemical shifts δ (ppm), and spin-spin coupling *J* (Hz) from the <sup>1</sup> H-NMR spectra for ILs with CDCl3 [19]

Figure 11. Fragment of the 1 H-NMR spectrum, indicating the peaks of the allylic substituent in [АМРу]Cl (A), and the structure of the [АМРу] <sup>+</sup> cation (B). Here δ = 5.459 (d, *J* = 10.5 Hz, 1Н) – **3'**, *-cis*; δ = 5.639 (d, *J* = 17.5 Hz, 1Н) – **3'**, *-trans*; δ = 5.682 (d, *J* = 6.3 Hz, 2Н) **– 1'**; δ = 6.123 (ddt, *J* = 39.9 Hz, 1Н) **– 2'** In this way, the <sup>1</sup> H-NMR signals of the spectra for ILs depend strongly on the structure, and this firstly depends on the **Figure 11.** Fragment of the 1 H-NMR spectrum, indicating the peaks of the allylic substituent in [АМРу]Cl (A), and the structure of the [АМРу]<sup>+</sup> cation (B). Here *δ=*5.459 (*d, J=*10.5 Hz, 1Н) *–* **3'***,-cis; δ=*5.639 *(d, J=*17.5 Hz, 1Н) *–* **3'***,-trans; δ=*5.682 (*d, J=*6.3 Hz, 2Н) **– 1'***; δ=*6.123 (*ddt, J=*39.9 Hz, 1Н) – **2'**

distances between the cation and anion. **3.2.2. 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.

Table 10. ILs glass transition temperature and melting obtained by differential scanning calorimetry (DSC) and use of the Boetius

#### heated stage (Bo.), ºC [20] Glass trans. (DSC) Melting (DSC) Melting *3.2.2. Thermal analysis of 1-alkyl-3-methylpyridinium-based ILs*

ILs (Bo.) Onset Tg Endpt Tm Endpt Ref. 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.

[С2МРу]Cl 25 29 33 130 [С3МРу]Cl 9 18.5 28 92 [С4МРу]Cl 17 35 53 93 113 95 [4], 111.35 [9] 97 [С5МРу]Cl 16 21.5 27 92 [С6МРу]Cl 62 84 81.95 [9] 77 [С7МРу]Cl 8 16.5 25 72 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 [C4MPy]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 [C2MPy]Br, due to the high value of its melting point, was not considered as an ionic liquid.


A

406 Ionic Liquids - Current State of the Art

B

Figure 11. Fragment of the 1

In this way, the <sup>1</sup>

(*d, J=*6.3 Hz, 2Н) **– 1'***; δ=*6.123 (*ddt, J=*39.9 Hz, 1Н) – **2'**

distances between the cation and anion.

points and glass transition values for IL.

ILs

*3.2.2. Thermal analysis of 1-alkyl-3-methylpyridinium-based ILs*

**3.2.2. Thermal analysis of 1-alkyl-3-methylpyridinium-based ILs**

this firstly depends on the distances between the cation and anion.

Table 10 shows the melting points and glass transition values for IL.

the [АМРу]

**Figure 11.** Fragment of the 1

structure of the [АМРу]<sup>+</sup>

In this way, the 1

liquid.

H-NMR spectrum, indicating the peaks of the allylic substituent in [АМРу]Cl (A), and the structure of

H-NMR signals of the spectra for ILs depend strongly on the structure, and this firstly depends on the

Glass trans. (DSC) Melting (DSC) Melting (Bo.) Onset Tg Endpt Tm Endpt Ref.

<sup>+</sup> cation (B). Here δ = 5.459 (d, *J* = 10.5 Hz, 1Н) – **3'**, *-cis*; δ = 5.639 (d, *J* = 17.5 Hz, 1Н) – **3'**, *-trans*; δ = 5.682 (d, *J* =

The phase transition for ILs was investigated using DSC calorimetry and the heated stage [20]. Table 10 shows the melting

6.3 Hz, 2Н) **– 1'**; δ = 6.123 (ddt, *J* = 39.9 Hz, 1Н) **– 2'**

cation (B). Here *δ=*5.459 (*d, J=*10.5 Hz, 1Н) *–* **3'***,-cis; δ=*5.639 *(d, J=*17.5 Hz, 1Н) *–* **3'***,-trans; δ=*5.682

H-NMR signals of the spectra for ILs depend strongly on the structure, and

The phase transition for ILs was investigated using DSC calorimetry and the heated stage [20].

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 [C4MPy]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 [C2MPy]Br, due to the high value of its melting point, was not considered as an ionic

H-NMR spectrum, indicating the peaks of the allylic substituent in [АМРу]Cl (A), and the

Table 10. ILs glass transition temperature and melting obtained by differential scanning calorimetry (DSC) and use of the Boetius heated stage (Bo.), ºC [20]

> [С2МРу]Cl 25 29 33 130 [С3МРу]Cl 9 18.5 28 92 [С4МРу]Cl 17 35 53 93 113 95 [4], 111.35 [9] 97 [С5МРу]Cl 16 21.5 27 92 [С6МРу]Cl 62 84 81.95 [9] 77 [С7МРу]Cl 8 16.5 25 72

**Table 10.** ILs glass transition temperature and melting obtained by differential scanning calorimetry (DSC) and use of the Boetius heated stage (Bo.), ºC [20]

In general, it was noticed that there is a tendency that the melting point values of 1-alkyl-3 methylpyridinium 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, [С2МРу]Cl and [С3МРу]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 [С2МРу]Br decom‐ posed at a higher temperature than [С2МРу]Cl but the decomposition temperature of bromides and chlorides with the [С4МРу]<sup>+</sup> cation was comparable.

With the use of a gas chromatography-mass spectroscopy, we can identify the fragments resulting from the decomposition [С4МРу]Cl (at temperatures up to 280 °C). In the chroma‐ togram 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 (C10H16N) 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 Figures 12B and 13, it becomes obvious that the peaks with an *M/Z* of less than 79 – 77 may represent fragments of the ILs, formed in the destruction of the pyridinium ring.

Figure 12. Chromatogram: the function of the time input sample [С4МРу]Cl, and the values of the detector signal (A); mass spectrum: the *M/Z* function and the peaks' intensity (B) **Figure 12.** Chromatogram: the function of the time input sample [С4МРу]Cl, and the values of the detector signal (A); mass spectrum: the *M/Z* function and the peaks' intensity (B) B Figure 12. Chromatogram: the function of the time input sample [С4МРу]Cl, and the values of the detector signal (A); mass

spectrum: the *M/Z* function and the peaks' intensity (B)

Figure 13. Series of hypothetical fragments formed during the decomposition of [С4МРу]Cl. Numerals showing the molecular weight of these fragments Figure 13. Series of hypothetical fragments formed during the decomposition of [С4МРу]Cl. Numerals showing the molecular weight of these fragments **Figure 13.** Series of hypothetical fragments formed during the decomposition of [С4МРу]Cl. Numerals showing the molecular weight of these fragments

alkyl-3-methylpyridinium-based ILs. More detailed information will published in other works.

alkyl-3-methylpyridinium-based ILs. More detailed information will published in other works. **4. Dissolution mechanism of cellulose in pyridinium-based ionic liquids 4.1. Dissolution of cellulose with 1-alkyl-3-methylpyridinium-based ILs**

**4. Dissolution mechanism of cellulose in pyridinium-based ionic liquids 4.1. Dissolution of cellulose with 1-alkyl-3-methylpyridinium-based ILs**

Thus, the decomposition fragments of [С4МРу]Cl are 1-alkyl-3-methylpyridinium cations, 3-methylpyridine, *etc*. **3.2.3. 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 Thus, the decomposition fragments of [С4МРу]Cl are 1-alkyl-3-methylpyridinium cations, 3-methylpyridine, *etc*. **3.2.3. 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)](ClO4). The obtained data of the β parameter for ILs (from 0.70 to 0.99) indicate a relatively high basicity of the 1- Thus, the decomposition fragments of [С4МРу]Cl are 1-alkyl-3-methylpyridinium cations, 3 methylpyridine, *etc*.

[Cu(tmen)(acac)](ClO4). The obtained data of the β parameter for ILs (from 0.70 to 0.99) indicate a relatively high basicity of the 1-

Our study of the solubility of cellulose in the synthesized ILs showed that for the same cation the 1-alkyl-3 methylpyridinium 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 [25-29] and 1-alkyl-3-methylimidazolium [27] halides.

Our study of the solubility of cellulose in the synthesized ILs showed that for the same cation the 1-alkyl-3 methylpyridinium 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 [25-29] 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

Table 11. The maximum achieved concentration (D) of cellulose Alicell Super (DP 599) in chloride- and bromide-based ILs [7] ILs Temperature, °С D, wt% [С2МРу]Cl 140 26 [С3МРу]Cl 110 22

ILs [7] ILs Temperature, °С D, wt% [С2МРу]Cl 140 26 [С3МРу]Cl 110 22

#### *3.2.3. Solvatochromism of 1-alkyl-3-methylpyridinium-based ILs solutions*

Figures 12B and 13, it becomes obvious that the peaks with an *M/Z* of less than 79 – 77 may

A

A

B

B

Figure 12. Chromatogram: the function of the time input sample [С4МРу]Cl, and the values of the detector signal (A); mass spectrum: the *M/Z* function and the peaks' intensity (B)

Figure 12. Chromatogram: the function of the time input sample [С4МРу]Cl, and the values of the detector signal (A); mass spectrum: the *M/Z* function and the peaks' intensity (B)

**Figure 12.** Chromatogram: the function of the time input sample [С4МРу]Cl, and the values of the detector signal (A);

150 120 108

150 120 108

94 93 80 79

94 93 80 79

Figure 13. Series of hypothetical fragments formed during the decomposition of [С4МРу]Cl. Numerals showing the molecular weight of these fragments Thus, the decomposition fragments of [С4МРу]Cl are 1-alkyl-3-methylpyridinium cations, 3-methylpyridine, *etc*.

**Figure 13.** Series of hypothetical fragments formed during the decomposition of [С4МРу]Cl. Numerals showing the

Thus, the decomposition fragments of [С4МРу]Cl are 1-alkyl-3-methylpyridinium cations, 3-

Figure 13. Series of hypothetical fragments formed during the decomposition of [С4МРу]Cl. Numerals showing the molecular weight of these fragments Thus, the decomposition fragments of [С4МРу]Cl are 1-alkyl-3-methylpyridinium cations, 3-methylpyridine, *etc*.

[Cu(tmen)(acac)](ClO4). The obtained data of the β parameter for ILs (from 0.70 to 0.99) indicate a relatively high basicity of the 1-

To determine the Kamlet-Taft hydrogen bond acceptor (basicity, β parameter) for the series of ILs we used indicator

To determine the Kamlet-Taft hydrogen bond acceptor (basicity, β parameter) for the series of ILs we used indicator [Cu(tmen)(acac)](ClO4). The obtained data of the β parameter for ILs (from 0.70 to 0.99) indicate a relatively high basicity of the 1-

Our study of the solubility of cellulose in the synthesized ILs showed that for the same cation the 1-alkyl-3 methylpyridinium 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 [25-29] and 1-alkyl-3-methylimidazolium [27] halides.

Our study of the solubility of cellulose in the synthesized ILs showed that for the same cation the 1-alkyl-3 methylpyridinium 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 [25-29] 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

Table 11. The maximum achieved concentration (D) of cellulose Alicell Super (DP 599) in chloride- and bromide-based ILs [7] ILs Temperature, °С D, wt% [С2МРу]Cl 140 26 [С3МРу]Cl 110 22

ILs [7] ILs Temperature, °С D, wt% [С2МРу]Cl 140 26 [С3МРу]Cl 110 22

**3.2.3. Solvatochromism of 1-alkyl-3-methylpyridinium-based ILs solutions**

molecular weight of these fragments

408 Ionic Liquids - Current State of the Art

methylpyridine, *etc*.

**3.2.3. Solvatochromism of 1-alkyl-3-methylpyridinium-based ILs solutions**

mass spectrum: the *M/Z* function and the peaks' intensity (B)

**4. Dissolution mechanism of cellulose in pyridinium-based ionic liquids 4.1. Dissolution of cellulose with 1-alkyl-3-methylpyridinium-based ILs**

alkyl-3-methylpyridinium-based ILs. More detailed information will published in other works.

alkyl-3-methylpyridinium-based ILs. More detailed information will published in other works. **4. Dissolution mechanism of cellulose in pyridinium-based ionic liquids 4.1. Dissolution of cellulose with 1-alkyl-3-methylpyridinium-based ILs**

represent fragments of the ILs, formed in the destruction of the pyridinium ring.

To determine the Kamlet-Taft hydrogen bond acceptor (basicity, β parameter) for the series of ILs we used indicator [Cu(tmen)(acac)](ClO4). The obtained data of the β parameter for ILs (from 0.70 to 0.99) indicate a relatively high basicity of the 1-alkyl-3-methylpyridinium-based ILs. More detailed information will published in other works.

#### **4. 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-3-methylpyridinium 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 [25-29] 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]. We can obtain the 5 – 6 wt% cellulose solutions in [С2МРу]Cl, [С3МРу]Cl, [С4МРу]Cl, [С5МРу]Cl, [С6МРу]Cl during 7 – 15 min (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 – 6 wt% cellulose solutions in [С2МРу]Cl,

[С3МРу]Cl, [С4МРу]Cl, [С5МРу]Cl, [С6МРу]Cl during 7 – 15 min (Figure 14).

Figure 14. Dissolution of 5 wt% of cellulose with [С4МРу]Cl for 1 min (A), and 10 min (B) at 120 °C. Increased by four times The influence of the temperature during the dissolution is accompanied by the degradation of the natural polymer, as **Figure 14.** Dissolution of 5 wt% of cellulose with [С4МРу]Cl for 1 min (A), and 10 min (B) at 120 °C. Increased by four times

exemplified by the reduction in the degree of polymerization (DP) (Table 12). Table 12. DP changing for regenerated cellulose films from their solution in [С4МРу]Cl at 120 °C, and its influence on dissolution time: Dissolution time, min 0 20 120 180 DP 734 419 137 <100 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).


**Table 12.** DP changing for regenerated cellulose films from their solution in [С4МРу]Cl at 120 °C, and its influence on dissolution time:

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<sup>6</sup> Н2–О<sup>6</sup> Н<sup>6</sup> group. Their presence indicates the modification of cellulose II [30]. For the modification of cellulose I (Alicell Super) stretching vibrations of υС 6 –О6 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<sup>2</sup> –О<sup>2</sup> Н<sup>2</sup> ) also indicate the modification of cellulose II (Figure 15).

**ILs Temperature, °С D, wt%**

[AМРу]Cl 130 24 [С2МРу]Br 165 6.5

[C4MIm]Cl 120 17

**Table 11.** The maximum achieved concentration (D) of cellulose Alicell Super (DP 599) in chloride-and bromide-based

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 – 6 wt% cellulose solutions in [С2МРу]Cl, [С3МРу]Cl, [С4МРу]Cl,

[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 – 6 wt% cellulose solutions in [С2МРу]Cl,

<sup>A</sup> <sup>B</sup>

Figure 14. Dissolution of 5 wt% of cellulose with [С4МРу]Cl for 1 min (A), and 10 min (B) at 120 °C. Increased by four times The influence of the temperature during the dissolution is accompanied by the degradation of the natural polymer, as

**Figure 14.** Dissolution of 5 wt% of cellulose with [С4МРу]Cl for 1 min (A), and 10 min (B) at 120 °C. Increased by four

Table 12. DP changing for regenerated cellulose films from their solution in [С4МРу]Cl at 120 °C, and its influence on dissolution time: Dissolution time, min 0 20 120 180

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)

DP 734 419 137 <100

*Dissolution time, min* 0 20 120 180

**DP** 734 419 137 <100

group. Their presence indicates the modification of cellulose II [30]. For the

<sup>6</sup> were observed at 1031 cm-1. The appearance of two bands at

) also indicate the modification of cellulose II (Figure 15).

In the FTIR spectra of regenerated cellulose samples of IL, we observed the appearance of characteristic peaks at ~1000

6 –О

–О<sup>2</sup> Н2

**Table 12.** DP changing for regenerated cellulose films from their solution in [С4МРу]Cl at 120 °C, and its influence on

[С4МРу]Br

410 Ionic Liquids - Current State of the Art

[С5МРу]Cl, [С6МРу]Cl during 7 – 15 min (Figure 14).

[С3МРу]Cl, [С4МРу]Cl, [С5МРу]Cl, [С6МРу]Cl during 7 – 15 min (Figure 14).

exemplified by the reduction in the degree of polymerization (DP) (Table 12).

modification of cellulose I (Alicell Super) stretching vibrations of υС

3440 cm-1 and 3484 cm-1 (the weak hydrogen bond group of С<sup>2</sup>

Н2–О<sup>6</sup> Н6

cm-1 corresponding predominantly to the С<sup>6</sup>

[С4МРу]Br

ILs [7]

times

(Table 12).

dissolution time:

120 7.5

110 3.5 165 7

110 3.5 165 7

**Figure 15.** FTIR spectra of Alicell Super and regenerated cellulose from a 23 wt% solution in [С4МРу]Cl

#### **4.2. Study of cellulose solutions with 1-alkyl-3-methylpyridinium-based ILs by 1 H-NMR spectroscopy**

Table 12 presents 1 H-NMR chemical shifts values for cellulose solutions in [С4МРу]Cl and [С4МРу]CH3COO. The Δδ values for cellulose varied insignificantly for each of the peaks. We observed the greatest change of Δδ (Δδ=δsolution – δIL) for cellulose in [С4МРу]Cl for H<sup>2</sup> and H6 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 H2 and H6 changed insignificantly for [С4МРу]CH3COO 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 –СН3 group confirmed that the anion of the IL interacted with the cellulose.


**Table 13.** Chemical shifts (δ), taken from the <sup>1</sup> H-NMR spectra of cellulose solutions in [С4МРу]Cl, [С4МРу]CH3COO, and their changing Δδ (shown in parentheses) of relative solvents (see Table 9). The solutions were obtained in CDCl<sup>3</sup> [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.

#### **4.3. 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-methylpyri‐ dinium 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<sup>6</sup> Н<sup>6</sup> , and the secondary О2'Н2' 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 1.8 to 2.7 Å, and increases in the series of [[С4МРу]CH3COO < [С4МРу]Cl < [С4МРу]Br.

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 Csp3 –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<sup>6</sup> Н2–О<sup>6</sup> Н<sup>6</sup> .

The interaction of [С4МРу]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 cellote‐ traose hydroxyl is larger than it is for the chloride-anion (2.445 Å for O6 Н<sup>6</sup> …Br, 2.710 Å for O2'Н2'…Br and 2.295 Å for O6 Н<sup>6</sup> …Cl, 2.514 Å for O2'Н2'…Cl), because the length of the hydrogen bond between the halide and H-donor increases with the radii of the halide-ions [31].

**ILs** 2 4 5 6

8.199 (0.007)

8.117 (-0.002)

4.943 (0.015)

4.814 (-0.004)

A. IL ring protons

B. IL protons of –СН3, and –СН2 groups

and their changing Δδ (shown in parentheses) of relative solvents (see Table 9). The solutions were obtained in CDCl<sup>3</sup>

Therefore, the interaction of cellulose with pyridinium-based ILs is caused by the ability of the

**4.3. Study of the mechanism of cellulose solvation by 1-alkyl-3-methylpyridinium-based**

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-methylpyri‐ dinium X' model, wherein X are acetate-, chloride-, and bromide-anions. The geometry of the

The formation of the solvate complex is accompanied by the appearance of two hydrogen

Н<sup>6</sup>

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 1.8 to 2.7 Å, and

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

> 140º. The anion of the solvent model is positioned closer to the hydrogen atom of the primary

The interaction of [С4МРу]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

in ranges from ~2 Å to ~2.4 Å, and a bond angle (between O, H and Cl)

complexes with optimum energy characteristics are shown in Figure 16.

increases in the series of [[С4МРу]CH3COO < [С4МРу]Cl < [С4МРу]Br.

**ILs** 3" 1' 2' 3' 4' а

7.977 (0.009)

7.926 (-0.002)

1.986 (0.012)

1.931 (-0.003)

9.425 (-0.025)

9.202 (0.004)

1.367 (0.019)

1.340 (-0.004)

H-NMR spectra of cellulose solutions in [С4МРу]Cl, [С4МРу]CH3COO,

, and the secondary О2'Н2' hydroxyl from the

0.903 (0.023)

0.917 (0.017)

3.780 (0.240)

acetate (C)

primary О<sup>6</sup>

Distances

Angles

Н6

Х–О<sup>6</sup> –Н<sup>6</sup>

9.573 (-0.060)

9.202 (0.004)

2.601 (0.017)

2.569 (-0.004)

**[С4МРу]Cl**

412 Ionic Liquids - Current State of the Art

**[С4МРу]CH3COO**

**[С4МРу]Cl**

**[С4МРу]CH3COO**

[19]

**Table 13.** Chemical shifts (δ), taken from the <sup>1</sup>

IL anion to exist as the hydrogen bond acceptor.

**ILs using a quantum-chemical calculation**

bonds between the anion and the primary О<sup>6</sup>

Н2–О<sup>6</sup> Н<sup>6</sup> .

distance Csp3 –OH···Cl–

hydroxyl group С<sup>6</sup>

Table 16. The main calculated parameters for the solvation complexes of cellotetraose with [С4МРу]CH3COO, [С4МРу]Cl, and [С4МРу]Br. (In parentheses are the values of the changing parameters relative to the IL pairs, see. Tables 6 and 7) Parameters [С4МРу]CH3COO [С4МРу]Cl [С4МРу]Br **Figure 16.** Fragments of optimized complexes of cellotetraose with 1-butyl-3-methylpyridinium chloride (A), bromide (B), and acetate (C)

r(H2…О1), Å 2.403 (0.562)

r(H2…О2), Å 2.252 (-0.274)

r(H2…X), Å 2.531 (0.316) 2.640 (0.263)

r(H2'-X), Å 1.853 2.514 2.710

r(H6-X), Å 1.806 2.295 2.445

, deg. 165.33 163.94 158.09

, and the secondary О2'Н2' hydroxyl from the neighbouring glycosidic link of the cellotetraose. The distance between

The formation of the solvate complex is accompanied by the appearance of two hydrogen bonds between the anion and the

Х–О2'–Н2', deg. 160.17 166.52 167.46


**Table 14.** The main calculated parameters for the solvation complexes of cellotetraose with [С4МРу]CH3COO, [С4МРу]Cl, and [С4МРу]Br. (In parentheses are the values of the changing parameters relative to the IL pairs, see. Tables 6 and 7)

### **5. Conclusion**

Thus, using quantum-chemical calculations, the features of the interaction of cellulose with 1 alkyl-3-methylpyridinium-based ILs were identified. This shows a decrease in the intensity of the binding of the hydroxyl anions in the series CH3COO– , 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.

**Figure 17.** Schematic representation of the interaction mechanism of cellulose with pyridinium-based ILs

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).

#### **Acknowledgements**

*Parameters [С4МРу]CH3COO [С4МРу]Cl [С4МРу]Br*

r(H2…О1), Å 2.403 (0.562) r(H2…О2), Å 2.252 (-0.274)

the binding of the hydroxyl anions in the series CH3COO–

r(H2…X), *Å* 2.531 (0.316) 2.640 (0.263)

*r(H2'-X), Å* 1.853 2.514 2.710 *r(H6-X), Å* 1.806 2.295 2.445

Х–О2'–Н2' *, deg.* 160.17 166.52 167.46

**Table 14.** The main calculated parameters for the solvation complexes of cellotetraose with [С4МРу]CH3COO, [С4МРу]Cl, and [С4МРу]Br. (In parentheses are the values of the changing parameters relative to the IL pairs, see.

Thus, using quantum-chemical calculations, the features of the interaction of cellulose with 1 alkyl-3-methylpyridinium-based ILs were identified. This shows a decrease in the intensity of

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

**Figure 17.** Schematic representation of the interaction mechanism of cellulose with pyridinium-based ILs

cation, which at solvation was weakened (the distance between the ions increases).

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

, Cl– , Br–

, which explains the

–Н<sup>6</sup> *, deg.* 165.33 163.94 158.09

Distances

*Angles*

Tables 6 and 7)

**5. Conclusion**

Х–О<sup>6</sup>

414 Ionic Liquids - Current State of the Art

difference in the solvent power of ILs.

pyridinium ILs with cellulose, shown in Figure 17.

This work was supported by the Ministry of Education and Science of the Russian Federation (State Task No. 2014/186).

#### **Author details**

Elena S. Sashina\* and Dmitrii A. Kashirskii

\*Address all correspondence to: organika@sutd.ru

Institute of Applied Chemistry and Ecology, St. Petersburg State University of Technology and Design, St. Petersburg, Russia

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