Thermal properties of 1-alkyl-3-methylpyridinium halide-based ionic liquids

Article abstract of DOI:10.1016/j.tca.2013.06.022

This study was carried out for the comparison of thermal properties that determine various conditions and methods, including the differential scanning calorimetry (DSC), thermogravimetry (DTG) and the Bo?tius heated stage. Series of 1-alkyl-3-methylpyridinium chloride- and bromide-based ionic liquids having alkylic chain lengths ranging from C₂ to C₁₀ were synthesized, and their thermal characteristics were defined: glass transitions, melting and decomposition. Detailed thermal properties of ionic liquids, obtained in this work, can be useful in a variety of chemical engineering processes that are related with their application.

Full text of DOI:10.1016/j.tca.2013.06.022

Accepted Manuscript
Title: Thermal properties of 1-alkyl-3-methylpyridinium
halide-based ionic liquids
˙
Author: Elena S. Sashina Dmitrii A. Kashirskii Grazyna
Janowska Marian Zaborski
PII:
S0040-6031(13)00333-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.tca.2013.06.022
TCA 76531
To appear in:
Thermochimica Acta
Received date:
Revised date:
Accepted date:
7-5-2013
17-6-2013
19-6-2013
Please cite this article as: E.S. Sashina, D.A. Kashirskii, G. Janowska, M.
Zaborski, Thermal properties of 1-alkyl-3-methylpyridinium halide-based ionic liquids,
Thermochimica Acta (2013), http://dx.doi.org/10.1016/j.tca.2013.06.022
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1
Thermal properties of 1-alkyl-3-methylpyridinium halide-based ionic liquids
2
3
4
5
6
7
Elena S. Sashina,1* Dmitrii A. Kashirskii,1 Grażyna Janowska,2 Marian Zaborski2
1 Faculty of Applied Chemistry and Ecology, St. Petersburg State University of
Technology and Design, Bol’shaya Morskaya str., 18, 191186, St. Petersburg,
Russia
2 Institute of Polymer and Dyes Technology, Technical University of Lodz,
Stefanowskiego str., 12/16, 90-924 Łódź, Poland
8
9
* Author for correspondence – e-mail: organika@sutd.ru, tel.: +7812-3151092,
fax: +7812-3160665
10 Abstract
11 This study was carried for the compare of thermal properties that determinate in
12 various conditions and methods, including of the differential scanning calorimetry
13 (DSC), thermogravimetry (DTG) and the Boёtius heated stage. Were synthesized
14 of series 1-alkyl-3-methylpyridinium chloride- and bromide-based ionic liquids
15 having alkylic chain lengths ranging from C2 to C10, and defined their thermal
16 characteristics: glass transitions, melting and decomposition. Detailed thermal
17 properties of ionic liquids, which obtained in this work, can be useful in a variety
18 chemical engineering processes that are related with their application.
19 Keywords: ionic liquids; pyridinium salts; DSC; TGA; melting point;
20 decomposition.
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21 1. Introduction
22
Ionic liquids (ILs) defined as organic salts with melting points up to 100 °C
23 and consisting of organic cation and organic or inorganic anion, in recent years
24 found wide application in various fields of science and technology. Their unique
25 properties such as high thermal stability, ability of regeneration, low flammability,
26 low vapor pressure, electric conductivity make them promising for use in processes
27 of separation and extraction [1 – 2], catalysis [3 – 4], electrochemistry [5],
28 analytics [6], and use ILs as a solvents for many different reactions [7 – 8]. In the
29 last two decades researchers focused attention on ILs as the solvents in biomass
30 processing [9 – 11], especially for dissolution of cellulose [12 – 22] and other
31 natural polymers [23], and preparation from this solutions new biodegradable
32 materials, including films, membranes, and fibers [24]. Thus ILs represents a new
33 class of direct solvents for natural polymers.
34
Cellulose can be dissolved with the ILs containing such cations as
35 imidazolium, ammonium and pyridinium, and the following anions: bromide,
36 chloride, acetate, etc [25].
37
Pyridinium cation-based ionic liquids have proved to be effective solvents
38 for cellulose [13 – 15]. The first patent of the use of pyridinium chloride-based
39 salts for cellulose dissolution was published by Graenacher [16]. However, at
40 present one can find only few works devoted to investigation of pyridinium ILs
41 dissolving power and other physical and chemical properties.
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42
Dissolving power of pyridinium ILs depends on their chemical structure,
43 especially on the type of anion, the length, number and the position of alkylic
44 substituent in the pyridinium rings. It was revealed that 1-alkyl-3-
45 methylpyridinium salts had the greatest dissolving power [14]. It is also known
46 that with increasing of the alkylic chain length the dissolving power toward the
47 cellulose is decreased.
48
It is necessary to select conditions of natural polymers dissolving within the
49 temperature range in which the pyridinium-based ILs exist in liquid phase, melting
50 and decomposition points of these compounds become important characteristics for
51 processing of biopolymers by means of dissolving. Literature concerning these
52 data is not numerous and the results described in these works are conflicting, their
53 differences in the value may be more than 10 °C [13, 18, 20].
54
It is known that the thermal properties of many substances may be highly
55 dependents from the method and the measurement conditions. However, detailed
56 information on these properties is very important in the chemical engineering
57 processes.
58
In order to find out the detailed thermal characteristics of the series of 1-
59 alkyl-3-methylpyridinium halide-based ionic liquids ([CnMPy]X) were
60 synthesized. These compounds had different alkylic chain length from C2 to C10 as
61 well as allylic substituent and bromide- and chloride-anions. Thermal
62 investigations of these ILs were conducted using different methods.
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63 2. Materials and Methods
64 2.1. Reagents
65
The following chemicals were used in our work: allyl chloride (Fluka, 98
66 %), acetonitrile (Lab-scan analytical science, 99.9 %), 1-bromoethane (Aldrich, 98
67 %), 1-brombutan (Aldrich, 99 %), diethyl ether (Aldrich), 3-picoline (Aldrich, 99
68 %), toluene (Aldrich), 1-chloroethane (Aldrich, ≥ 99.7 %), 1-chloropropane
69 (Aldrich, 98 %), 1-chlorobutane (Aldrich, 99 %), 1-chloropentane (Fluka, ≥ 99 %),
70 1-chlorohexane (Fluka, > 99 %), 1-chloroheptane (Aldrich, 99 %), 1-chlorooctane
71 (Aldrich, 99 %), 1-chlorononane (Aldrich, 98 %), 1-chlorodecane (Aldrich, 98 %).
72 2.2. Methods
73 2.2.1. Synthesis of 1-alkyl-3-methylpyridinium halide-based ILs
74
1-Alkyl-3-methylpyridinium halide salts were prepared by the alkylation
75 reaction of 3-picoline with required alkyl halide (R ― X) using standard procedure
76 [17] according to the following scheme:
77
78 The used method was modified to increase the product yield: alkyl halide (0.2000
79 mol) was added into two-neck round-bottom flask with the solution of 3-
80 substituted pyridine (0.1665 mol) in toluene (20 ml). The solution was then stirred
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81 using a magnetic stirrer under a nitrogen atmosphere for 1 h at room temperature.
82 After that the reaction mixture has been stirred at 110 - 130 °C for 24 - 48 hours to
83 reflux condenser plant without access to nitrogen. After the reaction completion
84 the obtained product delaminated; the dense layer of ionic liquid was decanted
85 from the toluene layer. The product was washed with toluene. Recrystallization
86 was carried out by dissolution in acetonitrile followed by precipitation in excess of
87 diethyl ether. Volatile organic compounds have been removed for 6 – 8 h using the
88 rotary evaporator (Rotavapor R-215, Büchi), at pressure gradual decrease from
89 atmospheric one to 20 – 120 mbar, and temperature rise from 60 °C to 90 °C – 140
90 °C.
91
92 different alkylic substituents in the cation: 1-ethyl-3-methylpyridinium
93 ([С2МРу]Cl), 1-propyl-3-methylpyridinium ([С3МРу]Cl), 1-butyl-3-
94 methylpyridinium ([С4МРу]Cl), 1-pentyl-3-methylpyridinium ([С5МРу]Cl), 1-
95 hexyl-3-methylpyridinium ([С6МРу]Cl), 1-heptyl-3-methylpyridinium
96 ([С7МРу]Cl), 1-octyl-3-methylpyridinium ([С8МРу]Cl), 1-nonyl-3-
Twelve ionic liquids were synthesized. It is chloride-based ILs containing
97 methylpyridinium ([С9МРу]Cl), 1-decyl-3-methylpyridinium ([С10МРу]Cl), 1-
98 allyl-3-methylpyridinium ([AМРу]Cl); and bromide-based ILs with two cation: 1-
99 ethyl-3-methylpyridinium
([С2МРу]Br),
and
1-butyl-3-methylpyridinium
100 ([С4МРу]Br).
101 2.2.2. Characterization of ILs
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102
The obtained salts were identified using nuclear magnetic resonance with
1
103 respect to H nuclei (spectrometer Bruker Avance II Plus 700 MHz), Fourier
104 transform infrared spectroscopy (Nicolet 6700 FT-IR), and liquid chromatography-
105 mass spectrometry (spectrometer GCMSQP2010 Plus, Shimadzu). Water content
106 was determined by thermogravimetric analysis (STARe TGA/DSC 1 system,
107 Mettler Toledo). As an example 1H NMR spectrum of 1-ethyl-3-methylpyridinium
108 chloride is given in Fig. 1. The absence of peaks corresponding to initial reagents
109 and external impurities indicated the correct choice of synthesis and purification
110 methods. The water content of the ILs was determine by TGA using argon as the
111 protective gas, it is did not exceed 0.1 wt%.
112 2.2.3. Thermal study of ILs
113
Differential scanning calorimetry (NETZSCH-Gerätebau GmbH Thermal
114 Analysis DSC 204), thermogravimetry methods (F. Paulik J. Paulik L. Erdey
115 system) and Boёtius heated stage (VEB Wägetechnik Rapido) with an optical
116 microscope of visual device RHMK 05 (with increasing in 16×) were used for
117 thermal analysis.
118
The conditions of the experiment are shown in Table 1.
119 3. Results and Discussion
120 3.1. Glass transition and melting of ILs
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121
Solid-liquid phase transitions of the investigated ILs (Table 2) were defined
122 using the DSC-method. Melting points were not identified for all ILs samples
123 because of the absence of distinct peaks in some cases.
124
After glass transition substances began to melt, as it followed from
125 appearance of a broad endothermic peak at DSC curve, for instance, for melting of
126 the salt [С4МРу]Cl (Fig. 2a).
127
It should be noted that the glass transition were not revealed for all ILs. As
128 appeared the DSC-method was not useful to find out melting points of some ILs,
129 mainly with an odd number of carbon atoms in the alkylic chain. Most of the
130 melting peaks obtained by DSC were not clearly defined. It was impossible to
131 determine the melting point of [С4МРу]Br by means of DSC, but that IL was a
132 greenish-brown viscous liquid at room temperature, and therefore it could be
133 related to room temperature ionic liquid (RTIL) species. In contrast, the salt
134 [С2МРу]Br, due to the high value of its melting point, was not considered as an
135 ionic liquid.
136
In general, there was noticed the tendency to reducing the melting point
137 values of 1-alkyl-3-methylpyridinium chloride-based ILs with the lengthening of
138 the alkylic chain. This trend could be explained by the decrease in packing density
139 of ion pairs of IL under increase in number of the alkylic chain carbon atoms [15,
140 19]. The existing data concerning melting points of imidazolium-based ILs [26 –
141 31] and ammonium-based ILs [32 – 33] demonstrated the same tendency.
7
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142 Conducted investigations showed that melting points identified by DSC were
143 different from those obtained by use of a heated stage. These probably could be
144 explained by the difference in applied heating techniques: in an atmosphere of inert
145 nitrogen or air. Taking into consideration that these ILs were highly hygroscopic
146 their samples could absorb air moisture during heating on the Boёtius stage, and
147 that could affect the results. There were published data with different melting point
148 values for dehydrated and water-containing ILs [26]. The values of melting points
149 obtained under different atmospheric conditions might be useful for processes
150 running either in nitrogen atmosphere or air.
151 3.2. Decomposition of ILs
152
Table 3 represents values of the salts decomposition temperature obtained by
153 different methods. Complete decomposition of ILs occurred at temperature about
154 250 °C (fig. 2b). The decomposition temperature values slightly decreased with
155 growth of the alkylic chain length. And again quaternary pyridinium salts,
156 [С2МРу]Cl and [С3МРу]Cl, appeared to be the exceptions, as well as they were in
157 the melting point tendency. Change of the anion ambiguously affected the
158 decomposition temperature, thus [С2МРу]Br decomposed at higher temperature
159 than [С2МРу]Cl but the decomposition temperature of bromides and chlorides
160 having the [С4МРу]+ cation were comparable.
161 4. Conclusions
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162
A comparative study of the thermal properties of 1-alkyl-3-
163 methylpyridinium-based IL was performed. This study included variety methods
164 that allowed to get detailed of the thermal characteristics of series of quaternary
165 pyridinium salts. 1-Alkyl-3-methylpyridinium chlorides with alkylic chain length
166 C2-C10 were in liquid state at temperatures ranging from 70 – 130 °C till their
167 decomposition at 230 – 250 °C. Bromide of the same cation with substituent C2
168 was liquid at 165 °С, it’s decomposition occurred at 284 °C, and with substituent
169 C4 was liquid at room temperature (it’s decomposition temperature was 251 °C).
170 The melting points of quaternary pyridinium salts were higher than those of
171 imidazolium quaternary salts. The values of melting points and decomposition
172 temperatures of the salts decreased with lengthening of the alkylic chain. As well
173 replacement of the alkylic substituent on the allyl one reduced the values of the salt
174 melting point and decomposition temperature. Detailed thermal characteristics of
175 the pyridinium salts, which obtained in this work, can be useful in a variety
176 chemical engineering processes that are related with their application.
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14
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Table 1.
Table 1. Conditions of thermal analysis of ionic liquids.
Temperature
range, °С
20 ÷ 200
Heating rate,
Sample mass,
mg
5.000 ÷ 6.000
7.800 ÷ 8.500
~ 90.000
Method
Atmosphere
deg/min
TGA
DSC
DTG
10
10
7.9
10
Argon
Nitrogen
Air
–80 ÷ 350
20 ÷ 400
Boёtius heated stage
20 ÷ mpa
7.000 ÷ 10.000
Air
a - For the melting point is taken the state of total loss of sharp edges of the
salts’ crystals.
Page 15 of 20

Table 2.
Table 2. ILs glass transition temperature and melting obtained by differential
scanning calorimetry (DSC) and use of the Boёtius heated stage (Bo.), °С
ILs
Glass transition
(DSC)
Melting (DSC)
Melting
(Bo.)
onset Tg endpt Tm endpt
Ref
[С2МРу]Cl
[С3МРу]Cl
[С4МРу]Cl
[С5МРу]Cl
[С6МРу]Cl
[С7МРу]Cl
[С8МРу]Cl
[С9МРу]Cl
[С10МРу]Cl
[AМРу]Cl
[С2МРу]Br
[С4МРу]Br
25
9
17
16
29
18.5 28
35 53
21.5 27
a
16.5 25
a
a
a
33
a
a
93
a
62
a
69
a
130
92
97
92
77
72
74
64
64
113
84
95 [13], 111.35 [18]
81.95 [18]
8
83
67.1 [20], 80.05 [18]
79.35 [18]
55
a
82
18
27.5 37
a
a
108
163
RTIL
146 165
a
a – Not detected.
Page 16 of 20

Table 3.
Table 3. Thermal characteristics of decomposition for ILs obtained by differential
scanning calorimetry (DSC), thermogravimetry (DTG) and literature values (ref)
obtained by thermogravimetric analysis (TGA), °С.
(DSC)
(DTG)
ILs
ref (TGA)
onset endpt onset endpt
[С2МРу]Cl 194
[С3МРу]Cl 200
[С4МРу]Cl 202
[С5МРу]Cl 196
[С6МРу]Cl 196
[С7МРу]Cl 193
[С8МРу]Cl 190
[С9МРу]Cl 189
[С10МРу]Cl 183
[AМРу]Cl 170
[С2МРу]Br 190
[С4МРу]Br 190
249
245
251
248
244
244
242
241
241
229
284
251
175
187
254
245
242 [26]
238 [26]
232 [26]
227 [26]
190
192
245
245
199 –235 [34]
Page 17 of 20

Figure 1. 1H NMR spectrum of [C2MPy]Cl (in CDCl3).
Page 18 of 20

▶ 1-Alkyl-3-methylpyridinium halide-based ionic liquids were synthesized. ▶ A
comparative study allowed to get detailed of the thermal characteristics of ILs. ▶
The melting points of pyridinium ILs decreased with lengthening of the alkyl
chain. ▶ Detailed thermal characteristics can be useful in variety chemical
processes.
Page 19 of 20

Figure 2. Regions of DSC curve for [C4MPy]Cl: (a) glass transiti
Page 20 of 20