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Thermal stability and FT-IR analysis of Phosphonium-based deep eutectic solvents with different hydrogen bond donors
Accepted Manuscript Thermal stability and FT-IR analysis of Phosphonium-based deep eutectic solvents with different hydrogen bond donors
Hosein Ghaedi, Muhammad Ayoub, Suriati Sufian, Bhajan Lal, Yoshimitsu Uemura PII: DOI: Reference:
S0167-7322(17)31671-9 doi: 10.1016/j.molliq.2017.07.016 MOLLIQ 7593
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
19 April 2017 3 July 2017 6 July 2017
Please cite this article as: Hosein Ghaedi, Muhammad Ayoub, Suriati Sufian, Bhajan Lal, Yoshimitsu Uemura , Thermal stability and FT-IR analysis of Phosphonium-based deep eutectic solvents with different hydrogen bond donors, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.07.016
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ACCEPTED MANUSCRIPT
Thermal stability and FT-IR analysis of Phosphonium-based deep eutectic solvents with different hydrogen bond donors Hosein Ghaedia, Muhammad Ayouba,*, Suriati Sufiana, Bhajan Lala, Yoshimitsu Uemuraa
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Department of Chemical Engineering, Universiti Teknologi PETRONAS,
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32610- Bandar Seri Iskandar, Perak, MALAYSIA *
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Corresponding author: Email: [email protected]; Telephone/fax: +6053687623
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ACCEPTED MANUSCRIPT ABSTRACT Recently, deep eutectic solvent (DES), a relatively new type of solvent, has received a considerable amount of attention from researchers in different fields of research. DESs have a high potential to be an alternative to ionic liquids (ILs) and traditional solvents. It is important
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the knowledge of thermal stability and interaction of functional groups on both salt as a
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hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) in order to use of DESs as
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alternative solvents for numerous industrial applications. In this work, four DESs have been selected with the same salt but four different HBDs. Salt was allyltriphenyl phosphonium
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bromide (ATPPB) and HBDs were glycerol (GL), ethylene glycol (EG), diethylene glycol
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(DEG) and triethylene glycol (TEG). DESs were prepared easily into the same molar ratio of 1:4 salt to HBDs. Fourier transform infrared spectroscopy (FT-IR) as an effective technique was
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conducted to gain information about hydrogen-bonding and vibration modes as well as
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investigate the functional groups of DESs. Finally, the thermal stability of DESs was studied
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under nitrogen at a temperature range of 30 to 800oC.
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Keywords: DES; salt; HBD; FT-IR; functional group; thermal stability.
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ACCEPTED MANUSCRIPT 1. Introduction
Ionic liquids (ILs) are salts that composed of large organic cations and organic/inorganic anions that cannot easily form an ordered crystal and thus remain liquid at or near room temperature [1]. ILs have some unique properties, such as low melting point, high solvency
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power for both polar and non-polar solvents, high ionic conductivity, high thermal and chemical
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stability, tunable physicochemical character, low flammability and volatility [2-11].
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However, despite these unique properties of ILs, there have been drawbacks, such as costly large scale industrial applications, complex reaction steps and purification procedures for
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synthesizing, high viscosity, potentially toxic, limited biodegradability, and low CO2 loading
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capacity (in comparison with traditional alkanolamines) [12, 13]. Deep eutectic solvents (DESs) are derived from two or more salts as the hydrogen bond
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acceptors (HBAs) and hydrogen bond donors (HBDs) including amides, amines, alcohols, and
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carboxylic acids. This is why researchers are trying to find new solvents as an alternative to ILs. Recently, DESs have gained popularity between researchers for many purposes Because they
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have unique properties, for example, non-volatile, thermally stable, highly conductive and also
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composed of low-cost, nontoxic, natural, biodegradable constituents which are important from the environmental and economical perspective. They not only have an easy preparation and
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synthesizing way but also a much lower melting point than the constituents of mixture. Also they have a wide liquid range and high solvation capacity [14-20]. Therefore, DESs can be regarded as the alternative to ILs in many applications, for instance, CO2 capture [15, 21, 22], electrochemistry and metal extraction [23], nanotechnology [24], separation processes [25], stabilization of DNA [26], materials chemistry [27], organic synthesis [28], synthesis of polymers [29], transition metal catalyzed reactions [30] and biotransformations [31].
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ACCEPTED MANUSCRIPT It has been reported that the cost of synthesizing a DES was only about 20% less than that of ILs [32]. Kareem et al. [16, 33], Luo et al. [34], Shahbaz et al. [35], Ghareh Bagh et al. [36] and Hayyan et al. [37] measured the physical properties of phosphonium-based DESs and studied their potential use in industrial applications. Aissaoui et al. [38] conducted the FT-IR
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investigation, and investigated the functional groups of some phosphonium-based DESs. Hayyan
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et al. [39] investigated the toxicity of several phosphonium-based DESs using the Gram positive
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and negative bacteria. The results indicated that DESs have a potential application to be
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antibacterial agents.
Thermogravimetric analysis (TGA) is a method of thermal analysis in which changes in
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physical and chemical properties of materials are measured as a function of increasing
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temperature (with constant heating rate), or as a function of time (with constant temperature or mass loss). The thermogravimetric analysis of the DESs used for CO2 capture is of utmost
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importance. This provides information about the decomposition of the solvents. The solvent
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which is being used for CO2 absorption should be thermally stable over a wide range of absorption temperatures for its effective usage. There is very rare information regarding the
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thermal stability of DESs in literature especially phosphonium-based DESs. Zhao et al. [40]
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analyzed the thermal stability of DESs consisting of mixtures of a choline salt (chloride or acetate form) and glycerol. Their results showed that these DESs are generally stable up to nearly 200oC and the decomposition temperature (Tdcp) values for choline chloride/urea (1:2), choline chloride/glycerol (1:2), choline acetate/glycerol (1:1.5), and choline acetate/glycerol (1:2) fall in the range from 205 to 216oC. Florindo et al. [41] measured the decomposition temperature of choline chloride-based DESs with several carboxylic acids (levulinic, glutaric, malonic, oxalic, and glycolic). Their results showed that the DESs exhibit a very close decomposition
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ACCEPTED MANUSCRIPT temperature. The lowest value was pertinent to choline chloride/ malonic acid (397.83 K) and the highest was for choline chloride/glutaric acid (512.20 K). Abbas and Binder [42] studied the thermal stability of choline chloride-based DESs with ethylene glycol (EG), glycerol (PG), DMSO, DMF and urea as HBDs. Their results showed that DESs are stable up to 300°C, except
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for choline chloride/DMSO mixture that shows decomposition below 100°C. Francisco et al.
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[43] reported the thermal stability of choline chloride/lactic acid. Their results showed that this
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type of DES is stable up to about 400K.
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Therefore, in the present work, four DESs were prepared at a molar ratio of 1:4 by mixing HBDs such as glycerol (GL), ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol
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(TEG) with allyltriphenyl phosphonium bromide (ATPPB) salt as a HBA. The functional groups and vibration modes were investigated through the FT-IR. The thermal stability of DESs was
2. Material and method
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2.1.Chemicals
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conducted at the temperature range of 30oC to 800oC under nitrogen.
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Allyltriphenyl phosphonium bromide (ATPPB) was purchased from Angene International Limited. All HBDs including glycerol (GL) (> 99.8 percent purity), ethylene glycol (EG)
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(>99.5% purity), diethylene glycol (DEG) and triethylene glycol (TEG) (> 99 % purity) were supplied by R & M Chemicals. Table 1 displays the abbreviation of these chemicals and DESs along with their molar ratio, symbol and molecular weight. In order to prevent moisture and any contamination, all materials were kept in a controlled environment. Fig. 1 displays the molecular structure of ATPPB and HBDs.
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Table 1 The abbreviation, symbol, molecular weight and molar ratio of components and DESs. Salt
HBD
Molar ratio
MDESb
Abb.c
Msaltd
Abb.c
MHBDe
Salt
HBD
DES1
150.327
ATPPB
383.26
GL
92.094
1
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DES2
126.308
ATPPB
383.26
EG
1
4
DES3
161.548
ATPPB
383.26
DEG
106.12
1
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DES4
196.788
ATPPB
383.26
TEG
150.17
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DES
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62.07
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Symbol. MDES molecular weight of DES in g·mol−1 was calcultaed according to Ghaedi et al. [44, 45]. c Abbreviation d Molecular weight of salt in g·mol−1. e Molecular weight of HBD in g·mol−1.
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b
Glycerol (GL)
Ethylene glycol (EG)
Diethylene glycol (DEG)
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Allyltriphenyl phosphonium bromide (ATPPB)
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Triethylene glycol (TEG) Fig. 1. A schematic of chemical structure of the salt and HBDs.
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2.2.Preparation of DESs
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DESs were prepared with the same ratio of 1:4 ATPPB to HBDs. ATPPB was mixed with
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HBDs using a magnetic stirrer. The weight measurement of pure components of all DESs (ATPPB + HBD) was performed in a digital balance (Sartorius, model BSA 224S-CW) with an
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accuracy of ±0.1 mg. The combinations of salt and HBDs were mixed at 350 rpm under
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atmospheric pressure and temperature range of 343.15 to 373 K for 1-3 h in a fume hood until a homogeneous and uniform liquid without any precipitates formed. The finalized DESs were
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found to have a yellowish-brown color. The DESs were kept in tight bottles to prevent any
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contamination from outside atmosphere. Since DESs are known as the hygroscopic solvents, the water contents of the DESs were measured by using a Mettler Toledo coulometric Karl-Fischer
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further purification.
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titrator (C30) and found to be less than 0.003 mas fraction. The DESs were used without any
2.3.Fourier transform infrared spectroscopy (FT-IR)
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A Thermo Scientific™ Nicolet™ iS 10 FT-IR spectrometer using KBr disc with high standard resolution was used to investigate the structure and functional groups of the DESs at ambient temperature. For collecting the spectra of solid and liquid samples, the spectrometer was adjusted in resolution 4 and by selecting the Norton-Beer (N-B) strong apodization function. The range of all spectra was between the wavenumbers of 4000 and 400 cm−1. Peak intensity and wavenumber values of salt, HBDs and DESs are compared. 2.4.Thermal stability 7
ACCEPTED MANUSCRIPT A Simultaneous Thermal Analyzer Perkin-Elmer (STA 6000) was employed to determine the thermal decomposition temperatures of DESs. The samples were placed in a small pan under nitrogen atmosphere with a flow rate 20 mL.min−1 and heated at the temperature range from 30°C to 800°C with the heating rate of 10°C.min−1. The accuracy of the temperature control was
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found to be ±0.03°C.
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3. Results and discussion
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3.1. FT-IR analysis
It is important to study the functional groups on new solvents, the combinations of various
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constituents, for instance DESs, and the observed changes in their structure. For infrared
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spectroscopy, a known method is Fourier transform infrared spectroscopy (FT-IR) that can be used for the field identification of unknown constituents. In order to calculate the spectrum in the
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frequency domain, this method uses Fourier Transformation formula when the mathematical
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form of the interferogram is known. This effective analytical technique involves the interaction of infrared radiation (IR) with a constituent by either transmitted or reflected IR beam through
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the constituents.
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A sensor measures the strength of light transmittance or absorbance as a function of its wavenumber or wavelength. Wavenumber is the reciprocal of the wavelength. Examination of
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the transmitted light reveals how much energy was absorbed at each wavenumber or wavelength. The absorbance of certain wavelengths of the IR beam by the component can be associated with the presence of particular functional groups existing in that component. Therefore, the chemical information, functional groups, chemical bonds and structure can be determined according to the wavelengths and strengths of IR bands absorbed by the component. There is a broad consensus that for analyzing qualitatively and quantitatively and investigating the structure of polyatomic and complex molecules like DESs, infrared spectroscopy is useful and valuable method. 8
ACCEPTED MANUSCRIPT In general, there are six vibration modes including symmetric stretching vibrations, asymmetric stretching vibration. The remaining vibrations are bending vibrations such as scissoring, rocking (in-plane or in- phase vibrations), wagging, and twisting vibrations (out-ofplane or out-of- phase vibrations) [46-48]. Asymmetric stretching vibration always takes place at
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the higher wavenumber compared with symmetric vibration. Bending vibrations generally
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required less energy and occur at longer wavelength (or lower wavenumber) than stretching
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vibrations [47, 48]. The vibration modes of a methylene group (-CH2) in a molecule are shown
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(a)
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Fig. 2.
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Symmetric stretching vibration
(d)
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(c)
Asymmetric stretching vibration
Rocking bending vibration
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Scissoring bending vibration
(e)
(f)
Wagging bending vibration
Twisting bending vibration
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ACCEPTED MANUSCRIPT Fig. 2. Different vibration modes of -CH2 group; stretching vibrations including: (a), symmetric stretching vibration; (b), asymmetric stretching vibration; and bending vibrations including: (c), scissoring bending vibration; (d), rocking bending vibration; (e), wagging bending vibration; (f), twisting bending vibration. Dark circles shapes display hydrogen (H)
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atoms and green circle shapes represent the carbon (C) atoms.
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3.1.1. Vibrational mode of hydroxyl group (OH)
Bands in the region of 3700-3100 cm-1 can be typically attributed to the various hydroxyl
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(OH) stretching vibrations. The witnessed OH stretching vibrations can be largely affected by hydrogen bonding, hence they depend on the strength of the hydrogen bonds. Usually, a broad
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hydrogen bond at wavenumbers between 3550 and 3230 cm-1 demonstrates the existence of OH
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stretching band in the condensed phase. In the case of water, alcohols and phenols, the OH
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stretching bands can be usually witnessed at 3400 cm-1 [48, 49]. Fig. 3 illustrates FT-IR spectra of ATPPB (salt), GL, EG, DEG and TEG (HBDs) and Fig. 4
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depicts FT-IR spectra of DESs formed by these components with mole ratio of 1:4 salt to HBDs. As can be seen clearly from Fig. 4, there are no intensive hydroxyl stretching bands at
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wavenumber between 3800 and 3400 cm-1. This indicates that the formed DESs have a very low water content. The main feature of Fig. 4 is the formation of hydrogen bonds between ATPPB and HBDs and demonstrates the effect of hydrogen bonding on the OH stretching frequency. From Figs. 3 (b), (c), (d) and (e), it can be seen that absorption bands in pure GL, EG, DEG and TEG were 3282.90, 3285.44, 3335.95 and 3386.22 cm-1, respectively. Pure GL had the weaker OH stretching band and TEG had the strong OH stretching band. By comparison of
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ACCEPTED MANUSCRIPT structure of DESs, it can be found that TEG and DEG has the additional C-O-C group in their structure (see Fig. 1). In fact, C-O-C functional groups are considered as the electronegative groups and tend to attract electrons (or electron density) on hydrogen in OH bands end towards
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themselves and increase the strength of OH bands.
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Fig. 3. FT-IR spectra for the single components studied in this work; (a), ATPPB; (b), GL; (c), EG; (d) DEG, (e), TEG.
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Fig. 4. FT-IR spectra for the DESs studied in this work; (▬), ATPPB-GL (DES1); (▬),
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ATPPB-EG (DES2); (▬) ATPPB-DEG (DES3); (▬) ATPPB-TEG (DES4).
the OH stretching bands at the wavenumbers between 3353 and 3326 cm-1 in the spectrum
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of Fig. 4 can be attributed to hydroxyl telescopic vibrations at the wavenumber of 3300 cm-1 [18]. As a matter of fact, the interactions of hydrogen bonding can be expected, since the hydroxyl group of HBDs favors the hydrogen bonding between the halide anion of salt and the moiety of HBDs. To be more precise, the force constant was reduced by transferring a portion of the cloud of electrons of the oxygen atom to the hydrogen bonding. Consequently, the main indication of hydrogen-bonding formation between HBDs and ATPPB can be this shifting of
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ACCEPTED MANUSCRIPT OH stretching vibration. The region of OH band is shown in Table 2. These results are in agreement with the results reported by Aissaoui et al. [38] and Hayyan et al. [18] for methyltriphenyl phosphonium bromide and glycerol (MTPPB-GL). After formation of ATPPBGL (DES1) and ATPPB-EG (DES2), the OH stretching bands changed to the higher bandwidth at
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wavenumbers of 3335.07 cm-1 and 3326.47 cm-1, respectively (see Fig. 4). Indeed, the formed
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hydrogen bonds in this region are strong in comparison with those of pure GL and EG. On the
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other hand, it is clear from Fig. 4 that the OH stretching bands in pure DEG and TEG changed to the weaker OH stretching bands at the frequencies of 3333.60 cm-1 and 3352.71 cm-1 after
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formed DESs ATPPB-DEG (DES3) and ATPPB-TEG (DES4), respectively. Amongst all DESs,
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DES2 had the lower frequency of OH stretching bond and DES4 had the higher frequency of OH stretching bond in spectra.
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Generally, the OH wagging vibrations appear at the wavenumbers between 900-500 cm-1
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[48] [32]. Here, in the region of 840-800 cm-1, there is a peak at about 816 cm-1 that may possibly involve the OH wagging vibration. According to the spectra of DES1, DES2, DES3 and DES4,
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these vibrations were investigated at 816.09, 815.61, 816 and 816.34 cm-1, respectively.
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3.1.2. Vibrational mode of aromatic and aliphatic groups
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C-H stretching vibrations include aromatic and aliphatic groups namely methyl (CH3 symmetric stretching) and methylene (CH2 asymmetric and symmetric stretching). Peaks at 3100-3000 cm-1 include the ring stretching vibrations. As can be seen from Fig. 4 for DES1, DES2, DES3 and DES4, the aromatic C-H stretching vibrations occurred at the wavenumbers of 3023.22, 3058.05, 3057.45 and 3058.05 cm-1, respectively. Many of aromatic and aliphatic stretching vibrations were detected in the region of 16001000 cm-1. Overall, the wavenumbers near to 1600 and 1580 cm-1 indicate the quadrant
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ACCEPTED MANUSCRIPT stretching vibrations, whereas the semi-circle stretching vibrations will bring about bands at wavenumbers between 1500 and 1460 cm-1. These may occur as single bands or as a multiple component envelop of bands [48]. There were three peaks at about 1610, 1586 and 1484 cm-1 in spectra of all DESs. These peaks may possibly indicate the aromatic ring quadrant and semi-
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circle stretching vibrations [48].
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The aromatic in-plane C-H bending vibrations appeared between 1300 and 1000 cm-1. This
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vibration may possibly occur between 1300-1200 cm-1 in spectra of DESs. While the aromatic out of plane C-H bending vibrations appear at the wavenumbers between 900 and 690 cm-1. In
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all DESs, two strong peaks at around 749 and 720 cm-1 may involve the five adjacent C-H
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wagging vibrations in the mono-substituted benzenes. These out-of-plane bends were investigated in DES1, DES2, DES3 and DES4 at the higher wavenumbers of 748.79, 748.02,
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749.45, 750.25 cm-1, and the lower wavenumbers of 721.59, 720.87, 721.88 and 722.20 cm-1,
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respectively. Most of the time, the aromatic in-plane C-H bending vibrations overlap the stronger bands in this region. Their comparison with aromatic out-of-plane C-H bending vibrations shows
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that these are less useful in spectra [47, 48].
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The CH3 stretching vibrations appear in the vicinity between 2950 - 2975 cm-1 and 28852865 cm-1. Since components do not include the methyl groups in their structures, CH3 vibrations
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did not occur in spectra. Normally, the wavenumbers for CH2 stretching bands are between 2940 - 2915 cm-1 and 2875 - 2840 cm-1 [17, 18, 34, 48, 50]. As mentioned earlier, the asymmetric stretching vibrations have the higher frequencies, so the appearance of these vibrations is expected at the left of symmetric stretching vibrations in the spectra. For all DESs, the region of CH2 stretching vibrations was between 2920 and 2876 cm-1 along with two positive peaks for all DESs except for ATPPB-TEG (DES4) with single positive peak. The CH2 asymmetric stretching
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ACCEPTED MANUSCRIPT vibrations appeared at the wavenumbers of 2934.43, 2933.11 and 2920.85 cm-1 in DES1, DES2, and DES3, respectively. This peak did not appear in the case of DES4 (see Fig. 4 for DES4). The CH2 symmetric stretching vibrations occurred at the wavenumbers of 2876.23, 2869.36, 2866.39 and 2867.33 cm-1 in DES1, DES2, DES3, and DES4, respectively.
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The CH2 bending vibrations in all DESs studied here, had a wavenumber of between 1500-
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1100 cm-1 excluding the CH2 rocking which occurred near to wavenumber of 880 cm-1 in spectra
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of the DESs. The CH2 scissoring vibrations were witnessed at 1450 and 1420 cm-1. In DES1, DES3, DES3 and DES4, these vibrations may possibly be assigned to the bands at 1437.98,
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1437.22, 1438.17 and 1438.43 cm-1, respectively. The CH2 wagging at 1320 and 1300 cm-1, the
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CH2 twisting at 1300 and 1200 cm-1 were observed in the spectra of DESs. Sometimes these vibrations are accompanied with more than one peak.
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The CH=CH2 stretching vibrations usually occur at 1660–1610 cm-1 [47, 48]. Here, peaks at
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wavenumbers between 1640 and 1611 cm-1 may possibly show this vibration. The C=C and =CH2 wagging vibrations appeared between 1000-900 cm-1.
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The observed peaks in the region of 1610 to 1480 cm-1 show the C=C and C-C ring
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stretching in DES arising from an aromatic group. As can be seen from Fig. 4, there was a ring stretch with two positive peaks at the wavenumbers between 1484-1587 cm-1 for the formed
[48, 49].
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DESs. The aromatic C-H wagging vibrations appeared in wavenumbers between 800-700 cm-1
3.1.3. Vibrational modes of the C-OH and C-O-C According to Fig. 4, the wavenumbers that occur between 1111.98 and 1023.63 cm-1 involve the C-OH and C-O-C stretching of components with two and three positive peaks [51]. By comparing all figures for DESs, it is evident that there is a peak at wavenumber of 1084.64 cm-1 16
ACCEPTED MANUSCRIPT for DES2 which was not observed for other DESs (see Fig. 4 for DES2 with red circle peak). This investigation is in agreement with the results reported by Aissaoui et al. [18] for MTPPB-EG. This peak may be attributed to an overtone of a peak at the wavenumber of 535.02 cm-1 which resulted in a lower intensity peak at 1084.64 cm-1. The C-OH bending vibration usually occurs at
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the right of and near to CH2 bending vibration. In all probability, a peak between 1410 and 1400
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cm-1 indicates C-OH bending vibration. This peak was observed in spectra of all DESs.
3.1.4. Other vibrational modes
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In general, the analysis of curves which are often affected by overlapping bands is more
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complex. Here, for all DESs (i.e., ATPB-GL, ATPB-EG, ATPB-DEG, ATPB-TEG), the P–H stretching bands may be overlapped with CH2 stretching vibrational bands at wavenumbers of
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between 3000-2800 cm-1 at the lower frequencies of 3000 cm-1 [18, 34]. There were several
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weak peaks in the region between 1700 and 2250 cm-1 that no useful information can be obtained from the analysis of the region. These peaks are the combination and overtone absorptions at
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about 1100-800 cm-1 and summation bands of ring CH waging vibrations [47, 48]. The C-C
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stretching vibrations may possibly be investigated at 875- 840 cm-1 in spectra of DESs [52]. Usually, the wavenumbers between 650-500 cm-1 indicate the existence of bromide in
study.
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components [18, 48]. Table 2 represents all possible vibrational modes of groups in DESs in this
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Table 2 Assignments of the IR of the DES1, DES2, DES3 and DES4. Assignment
3400-3200 3000-3100
OH stretching vibrations Aromatic CH stretching vibrations =CH2 allyl stretching vibrations
3000-2800
Aliphatic stretching vibrations P–H stretching vibration. C=C allyl stretching vibrations Quadrant and semi-circle ring stretching vibrations C=C aromatic stretching vibrations CH2 scissoring vibration C-OH bending vibration =CH2 bending CH2 wagging vibration CH2 twisting vibration Aromatic in-plane C-H bending vibrations
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Wavenumber (cm-1)
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1660-1610 1610-1480
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1200-1180 1150-1000
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1450-1420 1410-1400 1370-1320 1320-1300 1300-1200
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1000-900 900-840 800-840 780-700 700-680 650-500
P-aryl stretching C-O stretching vibration C-O-C stretching vibration C=C allyl wagging vibration =CH2 wagging vibration -CH2 rocking vibration C-C stretching vibration OH wagging vibration Aromatic C-H wagging vibration Aromatic ring pucker Bromide
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3.2. Thermal stability
One of the significant parameters related to the solvents used for CO2 capture is the thermal
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stability. When the materials are heated, their structures and chemical compositions can undergo changes, for example, fusion, melting, crystallization, oxidation, decomposition, transition,
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expansion and sintering. The thermal stability provides with information about the extent of
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temperature a solvent can withstand. This obtained information is very useful in both quality control and problem solving. For safe transportation and storage of these solvents, understanding
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the thermal stability of DESs is important. Therefore, the thermal stability of pure DESs used for
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CO2 solubility was analysed in this study. Mostly TGA is employed to obtain the relative stability of materials [53]. It should be mentioned that as the DESs transfer into the TGA pan, a
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small amount of water may be absorbed from the atmosphere. DESs were heated at the constant
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rate (10oC.min-1) until a weight loss events from 30oC to 800oC. Fig. 5 depicts the weight loss of single components against temperature. Totally, HBD components of DES have the lower the
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decomposition temperature compared with salt components [42]. It is clear from Fig. 5, ATPPB
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salt had the higher decomposition temperature rather than GL, EG, DEG and TEG components. Amongst HBDs, GL had the higher thermal stability and EG had the lower stability. DESs followed this trend according to the thermal stability: GL>TEG>DEG>EG. This may be attributed to the viscosity. The viscosity of GL was about 1400 mPa.s at 298.15 K, while that of TEG, DEG and EG was 38.254, 28.893 and 16.617 mPa.s at the same temperature, respectively. Therefore, it is more likely that ATPPB-GL (DES1) has the higher decomposition temperature
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ACCEPTED MANUSCRIPT and ATPPB-EG (DES4) has the lower thermal decomposition temperature. The thermal stability of DESs will be discussed in the following paragraphs.
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pure ATPPB
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pure GL
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pure EG
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pure DEG
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Weight loss (%)
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40
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30 20 10
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30 100 170 240 310 380 450 520 590 660 730 800
Temperature /oC
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Fig. 5. Dynamic TGA curves for salt and HBDs. The heating rate is 10oC.min-1 under N2.
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Fig. 6 depicts the TGA and derivative of TGA (DTGA) curves of DESs along with the onset
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and maximum temperatures. The DTGA curve provides the decomposition rate and is helpful for evaluating the mass-loss steps accurately. Fig. 6 shows the weight loss together with derivative weight loss (red solid line, DTGA) versus temperature. According to Fig. 6, there are two significant peaks in derivative of the weight loss. These peaks of derivative indicate the point of greatest rate of change on the weight loss curve. The first significant weight loss belongs to HBDs and was accompanied with a first significant peak of derivative weight loss curve (Tmax,1). By continuing the heating with constant rate, the next significant weight loss takes place which 20
ACCEPTED MANUSCRIPT can be observed by second significant peak of derivative weight loss curve (Tmax,2). This second weight loss which was accompanied with a sharp peak is pertinent to the salt (ATPPB) because the salt is decomposed later than HBDs. In all probability, the second decomposition step is the main decomposition step in DESs. Table 3 lists the Tmax in derivative weight loss curves for all
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DESs.
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Several research groups reported and applied the Tdcp or T10% for analysis of stability of materials [54, 55]. This temperature reflects the temperature at which a weight loss of 10% is
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observed. Also, the T90% are reported for all DESs for better comparison. The T10% and T90% of DESs are listed in Table 3. As can be observed, amongst the DESs, DES1 (ATPP-GL) had the
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higher T10% about 209.13oC and the lower value of T10% was pertinent to DES2 (ATPP-EG) about
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122.54oC. Although all DESs had a close value of T90%, the DES1 (ATPP-GL 1:4) had the higher T90% value about 361.55oC and the lowest value was pertinent to DES4 (ATPP-TEG) about
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350.53oC.
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Another advantageous way to analyze the thermal stability of DESs is the onset temperature
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(Tonset). Indeed, the Tonset indicates the upper decomposition range for the DESs. It is easy to determine the Tonset in TGA curves. As shown in Fig. 6, the Tonset is the point of intersection of
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the baseline weight and the tangent of the weight against temperature curve when the decomposition of DESs happens [56]. Depend on the liquids, the Tonset may occur more than one time. Here, there were two onset temperatures. The first one indicates the decomposition temperature of HBDs which is an indication of starting the degradation on DES and second one is pertinent to the decomposition temperature of salt. The Tonset of all DESs is listed Table 3. As can be evident, the DES1 had the highest first Tonset value about 247.24oC in comparison with other DESs, while DES2 had the lower Tonset value about 132.35oC. The second Tonset value of 21
ACCEPTED MANUSCRIPT DESs was close to each other. The DES1 had the highest second Tonset value about 338.35oC and the lowest second Tonset value was pertinent to DES2. According to Fig. 6 for TGA results, it can be clearly seen that the first decomposition step happened slowly in DES2 and the second decomposition step in DES4 was slower. This matter can help us to gain better insight to prepare
(b)
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(a)
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the stable DESs by considering the best amount and type of HBA or HBD.
(d)
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(c)
Fig. 6. Results of the TGA/DTGA of DESs along with the onset and maximum temperatures. The heating rate is 10oC.min-1. (a), DES1; (b), DES2; (c), DES3; (d), DES4. The dark solid lines represent the TGA curves and the red solid lines display the DTGA curves.
22
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Table 3 The maximum
temperatures
(Tmax.),
onset
temperatures
(Tonset)
and
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decomposition temperatures at 10 and 90 percent weight loss for the DESs in
DES2
DES3
DES4
Tmax.,1 / oC
285.04
170.06
214.50
243.50
Tmax.,2 / oC
359.15
349.39
358.52
353.22
First Tonset / oC
247.42
132.35
178.65
204.25
Second Tonset / oC
338.35
322.29
336.56
326.64
T10% / oC
209.13
122.54
154.92
177.19
T90% / oC
361.55
354.89
359.39
350.53
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DES1
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Parameters
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this work.
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The interpretation of thermal stability results of different DESs is difficult. The type of HBD, nature of salt or HBD, alkyl chain length, viscosity, hydrogen bonding interactions, electrostatic
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interactions, dispersion interactions and hydrogen bonding sites on HBD can be considered as important factors in changing the properties especially the thermal stability of DESs. Zhu et al. [57] studied comprehensively the various type of interactions and analyzed the strengths of hydrogen bonding in DESs. It should be noted that a few of the weak hydrogen bonds together can compete with a stronger hydrogen bonding between ion of salt and hydroxyl group of HBD [58], and also interactions such as Van der Waals hydrogen bonding interactions and other interactions may coexist for noncovalent interaction systems [57]. 23
ACCEPTED MANUSCRIPT It has been well established that alkyl chain length has an effect on the thermal stability. There is a discrepancy in this matter and literature reports point out that the thermal stabilities implemented under N2 produce the different results. Villanueva et al. [59] studied the thermal stability of alkyl imadazolium-based ILs with the same anion under N2 atmosphere. Their results
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revealed that an increasing alkyl chain length on cation decreased the thermal stability of ILs.
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Wu et al. [54] investigated the thermal stability of morpholin-based ILs with alkyl sulfate anion
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under N2 atmosphere. They stated that ILs with methyl sulfate anion has the higher thermal stability. However, Kosmulski et al. [60] studied that the thermal stability of imidazolium-based
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ILs increased when the alkyl chain length on cation increased under N2. In this work, in order to
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investigate the effect of alkyl chain length on the thermal stability, three DESs such as DES2, DES3, and DES4 were considered because of a close similar structure and difference in ethylene
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group. According to data represented in Table 3, it was found that by increasing the alkyl chain
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length on HBDs, the thermal stability of DESs increases. This result is in a well agreement with
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result reported by Kosmulski et al. [60].
It is interesting to mention that the thermal stability trend on DESs was similar to that of the
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HBDs. That is if the HBDs followed this trend on the thermal stability: GL>TEG>DEG>EG, the
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thermal stability of DESs was ATPPB-GL (DES1)> ATPPB-TEG (DES4)> ATPPB-DEG (DES3)> ATPPB-EG (DES2) which was similar to trend on HBDs. Indeed, it can be said that the nature of HBD can have a great effect on the thermal stability of DES. It is worthwhile to compare thermal stability of DESs in this work and literatures. Tdcp of DES1 (ATPPB-GL) is higher than that of choline chloride/glycerol (1:2), choline acetate/glycerol (1:1.5) reported by Zhao et al. [40]. This DES has almost the same stability as choline chloride/glutaric acid (Tdcp =246.85 oC) reported by Florindo et al. [41]. Moreover, all of these 24
ACCEPTED MANUSCRIPT DESs in this work are more stable than choline chloride/lactic acid with Tdcp of 126.85oC measured by Francisco et al. [43].
4. Conclusion
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In this work, DESs including ATPPB-GL (DES1), ATPPB-EG (DES2), ATPPB-DEG
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(DES3) and ATPPB-TEG (DES4) were prepared into the same molar ratio of 1:4 salt to HBDs. The functional groups of these phosphonium-based DESs were investigated and analyzed via
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FT-IR. The findings confirmed that the selected DESs exhibit the similar spectra and the similar chemical compositions with different levels of transmittance. The interesting result of this work
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can be the similar behavior of DESs with their HBDs. Indeed, DESs behaved like their
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corresponding HBDs especially at the higher wavenumbers. The thermal stability of HBDs and DESs was analyzed. It was found that the type of HBD has a great effect on the thermal stability
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of DESs. Amongst HBDs and DESs, the highest thermal stability was pertinent to GL and
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ATPPB-GL (DES1), respectively and EG and ATPPB-EG (DES2) had the lowest thermal stability. It was found that as the alkyl chain length on HBD increases, the thermal stability of
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DESs increases. Last but not least, it was found that the nature of HBD can play a pivotal role in
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thermal stability of DES.
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Figure captions Fig. 1. A schematic of chemical structure of the salt and HBDs.
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Fig. 2. Different vibration modes of -CH2 group; stretching vibrations including: (a), symmetric
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stretching vibration; (b), asymmetric stretching vibration; and bending vibrations including: (c),
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scissoring bending vibration; (d), rocking bending vibration; (e), wagging bending vibration; (f), twisting bending vibration. Dark circles shapes display hydrogen (H) atoms and green circle
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shapes represent the carbon (C) atoms.
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Fig. 3. FT-IR spectra for the single components studied in this work; (a), ATPPB; (b), GL; (c), EG; (d) DEG, (e), TEG.
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Fig. 4. FT-IR spectra for the DESs studied in this work; (▬), ATPPB-GL (DES1); (▬), ATPPBEG (DES2); (▬) ATPPB-DEG (DES3); (▬) ATPPB-TEG (DES4).
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Fig. 5. Dynamic TGA curves for salt and HBDs. The heating rate is 10oC.min-1 under N2. Fig. 6. Results of the TGA/DTGA of DESs along with the onset and maximum temperatures.
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The heating rate is 10oC.min-1. (a), DES1; (b), DES2; (c), DES3; (d), DES4. The dark solid lines
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represent the TGA curves and the red solid lines display the DTGA curves.
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Highlights: - Spectra of phosphonium based DESs and their single components was investigated via FT-IR. Spectra of DESs and their HBDs were almost similar to each other.
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The nature of HBDs had a great effect on the thermal stability of DESs.
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As the alkyl chain length on HBD increased, thermal stability of DESs increased.
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Hosein Ghaedi, Muhammad Ayoub, Suriati Sufian, Bhajan Lal, Yoshimitsu Uemura PII: DOI: Reference:
S0167-7322(17)31671-9 doi: 10.1016/j.molliq.2017.07.016 MOLLIQ 7593
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
19 April 2017 3 July 2017 6 July 2017
Please cite this article as: Hosein Ghaedi, Muhammad Ayoub, Suriati Sufian, Bhajan Lal, Yoshimitsu Uemura , Thermal stability and FT-IR analysis of Phosphonium-based deep eutectic solvents with different hydrogen bond donors, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.07.016
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ACCEPTED MANUSCRIPT
Thermal stability and FT-IR analysis of Phosphonium-based deep eutectic solvents with different hydrogen bond donors Hosein Ghaedia, Muhammad Ayouba,*, Suriati Sufiana, Bhajan Lala, Yoshimitsu Uemuraa
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Department of Chemical Engineering, Universiti Teknologi PETRONAS,
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32610- Bandar Seri Iskandar, Perak, MALAYSIA *
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Corresponding author: Email: [email protected]; Telephone/fax: +6053687623
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ACCEPTED MANUSCRIPT ABSTRACT Recently, deep eutectic solvent (DES), a relatively new type of solvent, has received a considerable amount of attention from researchers in different fields of research. DESs have a high potential to be an alternative to ionic liquids (ILs) and traditional solvents. It is important
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the knowledge of thermal stability and interaction of functional groups on both salt as a
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hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) in order to use of DESs as
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alternative solvents for numerous industrial applications. In this work, four DESs have been selected with the same salt but four different HBDs. Salt was allyltriphenyl phosphonium
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bromide (ATPPB) and HBDs were glycerol (GL), ethylene glycol (EG), diethylene glycol
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(DEG) and triethylene glycol (TEG). DESs were prepared easily into the same molar ratio of 1:4 salt to HBDs. Fourier transform infrared spectroscopy (FT-IR) as an effective technique was
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conducted to gain information about hydrogen-bonding and vibration modes as well as
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investigate the functional groups of DESs. Finally, the thermal stability of DESs was studied
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under nitrogen at a temperature range of 30 to 800oC.
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Keywords: DES; salt; HBD; FT-IR; functional group; thermal stability.
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ACCEPTED MANUSCRIPT 1. Introduction
Ionic liquids (ILs) are salts that composed of large organic cations and organic/inorganic anions that cannot easily form an ordered crystal and thus remain liquid at or near room temperature [1]. ILs have some unique properties, such as low melting point, high solvency
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power for both polar and non-polar solvents, high ionic conductivity, high thermal and chemical
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stability, tunable physicochemical character, low flammability and volatility [2-11].
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However, despite these unique properties of ILs, there have been drawbacks, such as costly large scale industrial applications, complex reaction steps and purification procedures for
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synthesizing, high viscosity, potentially toxic, limited biodegradability, and low CO2 loading
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capacity (in comparison with traditional alkanolamines) [12, 13]. Deep eutectic solvents (DESs) are derived from two or more salts as the hydrogen bond
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acceptors (HBAs) and hydrogen bond donors (HBDs) including amides, amines, alcohols, and
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carboxylic acids. This is why researchers are trying to find new solvents as an alternative to ILs. Recently, DESs have gained popularity between researchers for many purposes Because they
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have unique properties, for example, non-volatile, thermally stable, highly conductive and also
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composed of low-cost, nontoxic, natural, biodegradable constituents which are important from the environmental and economical perspective. They not only have an easy preparation and
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synthesizing way but also a much lower melting point than the constituents of mixture. Also they have a wide liquid range and high solvation capacity [14-20]. Therefore, DESs can be regarded as the alternative to ILs in many applications, for instance, CO2 capture [15, 21, 22], electrochemistry and metal extraction [23], nanotechnology [24], separation processes [25], stabilization of DNA [26], materials chemistry [27], organic synthesis [28], synthesis of polymers [29], transition metal catalyzed reactions [30] and biotransformations [31].
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ACCEPTED MANUSCRIPT It has been reported that the cost of synthesizing a DES was only about 20% less than that of ILs [32]. Kareem et al. [16, 33], Luo et al. [34], Shahbaz et al. [35], Ghareh Bagh et al. [36] and Hayyan et al. [37] measured the physical properties of phosphonium-based DESs and studied their potential use in industrial applications. Aissaoui et al. [38] conducted the FT-IR
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investigation, and investigated the functional groups of some phosphonium-based DESs. Hayyan
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et al. [39] investigated the toxicity of several phosphonium-based DESs using the Gram positive
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and negative bacteria. The results indicated that DESs have a potential application to be
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antibacterial agents.
Thermogravimetric analysis (TGA) is a method of thermal analysis in which changes in
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physical and chemical properties of materials are measured as a function of increasing
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temperature (with constant heating rate), or as a function of time (with constant temperature or mass loss). The thermogravimetric analysis of the DESs used for CO2 capture is of utmost
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importance. This provides information about the decomposition of the solvents. The solvent
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which is being used for CO2 absorption should be thermally stable over a wide range of absorption temperatures for its effective usage. There is very rare information regarding the
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thermal stability of DESs in literature especially phosphonium-based DESs. Zhao et al. [40]
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analyzed the thermal stability of DESs consisting of mixtures of a choline salt (chloride or acetate form) and glycerol. Their results showed that these DESs are generally stable up to nearly 200oC and the decomposition temperature (Tdcp) values for choline chloride/urea (1:2), choline chloride/glycerol (1:2), choline acetate/glycerol (1:1.5), and choline acetate/glycerol (1:2) fall in the range from 205 to 216oC. Florindo et al. [41] measured the decomposition temperature of choline chloride-based DESs with several carboxylic acids (levulinic, glutaric, malonic, oxalic, and glycolic). Their results showed that the DESs exhibit a very close decomposition
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for choline chloride/DMSO mixture that shows decomposition below 100°C. Francisco et al.
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[43] reported the thermal stability of choline chloride/lactic acid. Their results showed that this
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type of DES is stable up to about 400K.
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Therefore, in the present work, four DESs were prepared at a molar ratio of 1:4 by mixing HBDs such as glycerol (GL), ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol
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(TEG) with allyltriphenyl phosphonium bromide (ATPPB) salt as a HBA. The functional groups and vibration modes were investigated through the FT-IR. The thermal stability of DESs was
2. Material and method
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2.1.Chemicals
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conducted at the temperature range of 30oC to 800oC under nitrogen.
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Allyltriphenyl phosphonium bromide (ATPPB) was purchased from Angene International Limited. All HBDs including glycerol (GL) (> 99.8 percent purity), ethylene glycol (EG)
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(>99.5% purity), diethylene glycol (DEG) and triethylene glycol (TEG) (> 99 % purity) were supplied by R & M Chemicals. Table 1 displays the abbreviation of these chemicals and DESs along with their molar ratio, symbol and molecular weight. In order to prevent moisture and any contamination, all materials were kept in a controlled environment. Fig. 1 displays the molecular structure of ATPPB and HBDs.
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Table 1 The abbreviation, symbol, molecular weight and molar ratio of components and DESs. Salt
HBD
Molar ratio
MDESb
Abb.c
Msaltd
Abb.c
MHBDe
Salt
HBD
DES1
150.327
ATPPB
383.26
GL
92.094
1
4
DES2
126.308
ATPPB
383.26
EG
1
4
DES3
161.548
ATPPB
383.26
DEG
106.12
1
4
DES4
196.788
ATPPB
383.26
TEG
150.17
1
4
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Symb.a
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DES
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62.07
a
Symbol. MDES molecular weight of DES in g·mol−1 was calcultaed according to Ghaedi et al. [44, 45]. c Abbreviation d Molecular weight of salt in g·mol−1. e Molecular weight of HBD in g·mol−1.
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b
Glycerol (GL)
Ethylene glycol (EG)
Diethylene glycol (DEG)
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Allyltriphenyl phosphonium bromide (ATPPB)
6
ACCEPTED MANUSCRIPT
Triethylene glycol (TEG) Fig. 1. A schematic of chemical structure of the salt and HBDs.
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2.2.Preparation of DESs
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DESs were prepared with the same ratio of 1:4 ATPPB to HBDs. ATPPB was mixed with
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HBDs using a magnetic stirrer. The weight measurement of pure components of all DESs (ATPPB + HBD) was performed in a digital balance (Sartorius, model BSA 224S-CW) with an
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accuracy of ±0.1 mg. The combinations of salt and HBDs were mixed at 350 rpm under
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atmospheric pressure and temperature range of 343.15 to 373 K for 1-3 h in a fume hood until a homogeneous and uniform liquid without any precipitates formed. The finalized DESs were
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found to have a yellowish-brown color. The DESs were kept in tight bottles to prevent any
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contamination from outside atmosphere. Since DESs are known as the hygroscopic solvents, the water contents of the DESs were measured by using a Mettler Toledo coulometric Karl-Fischer
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further purification.
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titrator (C30) and found to be less than 0.003 mas fraction. The DESs were used without any
2.3.Fourier transform infrared spectroscopy (FT-IR)
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A Thermo Scientific™ Nicolet™ iS 10 FT-IR spectrometer using KBr disc with high standard resolution was used to investigate the structure and functional groups of the DESs at ambient temperature. For collecting the spectra of solid and liquid samples, the spectrometer was adjusted in resolution 4 and by selecting the Norton-Beer (N-B) strong apodization function. The range of all spectra was between the wavenumbers of 4000 and 400 cm−1. Peak intensity and wavenumber values of salt, HBDs and DESs are compared. 2.4.Thermal stability 7
ACCEPTED MANUSCRIPT A Simultaneous Thermal Analyzer Perkin-Elmer (STA 6000) was employed to determine the thermal decomposition temperatures of DESs. The samples were placed in a small pan under nitrogen atmosphere with a flow rate 20 mL.min−1 and heated at the temperature range from 30°C to 800°C with the heating rate of 10°C.min−1. The accuracy of the temperature control was
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found to be ±0.03°C.
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3. Results and discussion
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3.1. FT-IR analysis
It is important to study the functional groups on new solvents, the combinations of various
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constituents, for instance DESs, and the observed changes in their structure. For infrared
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spectroscopy, a known method is Fourier transform infrared spectroscopy (FT-IR) that can be used for the field identification of unknown constituents. In order to calculate the spectrum in the
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frequency domain, this method uses Fourier Transformation formula when the mathematical
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form of the interferogram is known. This effective analytical technique involves the interaction of infrared radiation (IR) with a constituent by either transmitted or reflected IR beam through
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the constituents.
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A sensor measures the strength of light transmittance or absorbance as a function of its wavenumber or wavelength. Wavenumber is the reciprocal of the wavelength. Examination of
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the transmitted light reveals how much energy was absorbed at each wavenumber or wavelength. The absorbance of certain wavelengths of the IR beam by the component can be associated with the presence of particular functional groups existing in that component. Therefore, the chemical information, functional groups, chemical bonds and structure can be determined according to the wavelengths and strengths of IR bands absorbed by the component. There is a broad consensus that for analyzing qualitatively and quantitatively and investigating the structure of polyatomic and complex molecules like DESs, infrared spectroscopy is useful and valuable method. 8
ACCEPTED MANUSCRIPT In general, there are six vibration modes including symmetric stretching vibrations, asymmetric stretching vibration. The remaining vibrations are bending vibrations such as scissoring, rocking (in-plane or in- phase vibrations), wagging, and twisting vibrations (out-ofplane or out-of- phase vibrations) [46-48]. Asymmetric stretching vibration always takes place at
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the higher wavenumber compared with symmetric vibration. Bending vibrations generally
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required less energy and occur at longer wavelength (or lower wavenumber) than stretching
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vibrations [47, 48]. The vibration modes of a methylene group (-CH2) in a molecule are shown
(b)
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(a)
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Fig. 2.
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Symmetric stretching vibration
(d)
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(c)
Asymmetric stretching vibration
Rocking bending vibration
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Scissoring bending vibration
(e)
(f)
Wagging bending vibration
Twisting bending vibration
9
ACCEPTED MANUSCRIPT Fig. 2. Different vibration modes of -CH2 group; stretching vibrations including: (a), symmetric stretching vibration; (b), asymmetric stretching vibration; and bending vibrations including: (c), scissoring bending vibration; (d), rocking bending vibration; (e), wagging bending vibration; (f), twisting bending vibration. Dark circles shapes display hydrogen (H)
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atoms and green circle shapes represent the carbon (C) atoms.
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3.1.1. Vibrational mode of hydroxyl group (OH)
Bands in the region of 3700-3100 cm-1 can be typically attributed to the various hydroxyl
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(OH) stretching vibrations. The witnessed OH stretching vibrations can be largely affected by hydrogen bonding, hence they depend on the strength of the hydrogen bonds. Usually, a broad
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hydrogen bond at wavenumbers between 3550 and 3230 cm-1 demonstrates the existence of OH
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stretching band in the condensed phase. In the case of water, alcohols and phenols, the OH
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stretching bands can be usually witnessed at 3400 cm-1 [48, 49]. Fig. 3 illustrates FT-IR spectra of ATPPB (salt), GL, EG, DEG and TEG (HBDs) and Fig. 4
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depicts FT-IR spectra of DESs formed by these components with mole ratio of 1:4 salt to HBDs. As can be seen clearly from Fig. 4, there are no intensive hydroxyl stretching bands at
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wavenumber between 3800 and 3400 cm-1. This indicates that the formed DESs have a very low water content. The main feature of Fig. 4 is the formation of hydrogen bonds between ATPPB and HBDs and demonstrates the effect of hydrogen bonding on the OH stretching frequency. From Figs. 3 (b), (c), (d) and (e), it can be seen that absorption bands in pure GL, EG, DEG and TEG were 3282.90, 3285.44, 3335.95 and 3386.22 cm-1, respectively. Pure GL had the weaker OH stretching band and TEG had the strong OH stretching band. By comparison of
10
ACCEPTED MANUSCRIPT structure of DESs, it can be found that TEG and DEG has the additional C-O-C group in their structure (see Fig. 1). In fact, C-O-C functional groups are considered as the electronegative groups and tend to attract electrons (or electron density) on hydrogen in OH bands end towards
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themselves and increase the strength of OH bands.
11
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Fig. 3. FT-IR spectra for the single components studied in this work; (a), ATPPB; (b), GL; (c), EG; (d) DEG, (e), TEG.
12
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Fig. 4. FT-IR spectra for the DESs studied in this work; (▬), ATPPB-GL (DES1); (▬),
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ATPPB-EG (DES2); (▬) ATPPB-DEG (DES3); (▬) ATPPB-TEG (DES4).
the OH stretching bands at the wavenumbers between 3353 and 3326 cm-1 in the spectrum
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of Fig. 4 can be attributed to hydroxyl telescopic vibrations at the wavenumber of 3300 cm-1 [18]. As a matter of fact, the interactions of hydrogen bonding can be expected, since the hydroxyl group of HBDs favors the hydrogen bonding between the halide anion of salt and the moiety of HBDs. To be more precise, the force constant was reduced by transferring a portion of the cloud of electrons of the oxygen atom to the hydrogen bonding. Consequently, the main indication of hydrogen-bonding formation between HBDs and ATPPB can be this shifting of
13
ACCEPTED MANUSCRIPT OH stretching vibration. The region of OH band is shown in Table 2. These results are in agreement with the results reported by Aissaoui et al. [38] and Hayyan et al. [18] for methyltriphenyl phosphonium bromide and glycerol (MTPPB-GL). After formation of ATPPBGL (DES1) and ATPPB-EG (DES2), the OH stretching bands changed to the higher bandwidth at
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wavenumbers of 3335.07 cm-1 and 3326.47 cm-1, respectively (see Fig. 4). Indeed, the formed
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hydrogen bonds in this region are strong in comparison with those of pure GL and EG. On the
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other hand, it is clear from Fig. 4 that the OH stretching bands in pure DEG and TEG changed to the weaker OH stretching bands at the frequencies of 3333.60 cm-1 and 3352.71 cm-1 after
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formed DESs ATPPB-DEG (DES3) and ATPPB-TEG (DES4), respectively. Amongst all DESs,
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DES2 had the lower frequency of OH stretching bond and DES4 had the higher frequency of OH stretching bond in spectra.
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Generally, the OH wagging vibrations appear at the wavenumbers between 900-500 cm-1
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[48] [32]. Here, in the region of 840-800 cm-1, there is a peak at about 816 cm-1 that may possibly involve the OH wagging vibration. According to the spectra of DES1, DES2, DES3 and DES4,
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these vibrations were investigated at 816.09, 815.61, 816 and 816.34 cm-1, respectively.
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3.1.2. Vibrational mode of aromatic and aliphatic groups
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C-H stretching vibrations include aromatic and aliphatic groups namely methyl (CH3 symmetric stretching) and methylene (CH2 asymmetric and symmetric stretching). Peaks at 3100-3000 cm-1 include the ring stretching vibrations. As can be seen from Fig. 4 for DES1, DES2, DES3 and DES4, the aromatic C-H stretching vibrations occurred at the wavenumbers of 3023.22, 3058.05, 3057.45 and 3058.05 cm-1, respectively. Many of aromatic and aliphatic stretching vibrations were detected in the region of 16001000 cm-1. Overall, the wavenumbers near to 1600 and 1580 cm-1 indicate the quadrant
14
ACCEPTED MANUSCRIPT stretching vibrations, whereas the semi-circle stretching vibrations will bring about bands at wavenumbers between 1500 and 1460 cm-1. These may occur as single bands or as a multiple component envelop of bands [48]. There were three peaks at about 1610, 1586 and 1484 cm-1 in spectra of all DESs. These peaks may possibly indicate the aromatic ring quadrant and semi-
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circle stretching vibrations [48].
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The aromatic in-plane C-H bending vibrations appeared between 1300 and 1000 cm-1. This
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vibration may possibly occur between 1300-1200 cm-1 in spectra of DESs. While the aromatic out of plane C-H bending vibrations appear at the wavenumbers between 900 and 690 cm-1. In
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all DESs, two strong peaks at around 749 and 720 cm-1 may involve the five adjacent C-H
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wagging vibrations in the mono-substituted benzenes. These out-of-plane bends were investigated in DES1, DES2, DES3 and DES4 at the higher wavenumbers of 748.79, 748.02,
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749.45, 750.25 cm-1, and the lower wavenumbers of 721.59, 720.87, 721.88 and 722.20 cm-1,
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respectively. Most of the time, the aromatic in-plane C-H bending vibrations overlap the stronger bands in this region. Their comparison with aromatic out-of-plane C-H bending vibrations shows
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that these are less useful in spectra [47, 48].
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The CH3 stretching vibrations appear in the vicinity between 2950 - 2975 cm-1 and 28852865 cm-1. Since components do not include the methyl groups in their structures, CH3 vibrations
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did not occur in spectra. Normally, the wavenumbers for CH2 stretching bands are between 2940 - 2915 cm-1 and 2875 - 2840 cm-1 [17, 18, 34, 48, 50]. As mentioned earlier, the asymmetric stretching vibrations have the higher frequencies, so the appearance of these vibrations is expected at the left of symmetric stretching vibrations in the spectra. For all DESs, the region of CH2 stretching vibrations was between 2920 and 2876 cm-1 along with two positive peaks for all DESs except for ATPPB-TEG (DES4) with single positive peak. The CH2 asymmetric stretching
15
ACCEPTED MANUSCRIPT vibrations appeared at the wavenumbers of 2934.43, 2933.11 and 2920.85 cm-1 in DES1, DES2, and DES3, respectively. This peak did not appear in the case of DES4 (see Fig. 4 for DES4). The CH2 symmetric stretching vibrations occurred at the wavenumbers of 2876.23, 2869.36, 2866.39 and 2867.33 cm-1 in DES1, DES2, DES3, and DES4, respectively.
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The CH2 bending vibrations in all DESs studied here, had a wavenumber of between 1500-
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1100 cm-1 excluding the CH2 rocking which occurred near to wavenumber of 880 cm-1 in spectra
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of the DESs. The CH2 scissoring vibrations were witnessed at 1450 and 1420 cm-1. In DES1, DES3, DES3 and DES4, these vibrations may possibly be assigned to the bands at 1437.98,
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1437.22, 1438.17 and 1438.43 cm-1, respectively. The CH2 wagging at 1320 and 1300 cm-1, the
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CH2 twisting at 1300 and 1200 cm-1 were observed in the spectra of DESs. Sometimes these vibrations are accompanied with more than one peak.
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The CH=CH2 stretching vibrations usually occur at 1660–1610 cm-1 [47, 48]. Here, peaks at
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wavenumbers between 1640 and 1611 cm-1 may possibly show this vibration. The C=C and =CH2 wagging vibrations appeared between 1000-900 cm-1.
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The observed peaks in the region of 1610 to 1480 cm-1 show the C=C and C-C ring
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stretching in DES arising from an aromatic group. As can be seen from Fig. 4, there was a ring stretch with two positive peaks at the wavenumbers between 1484-1587 cm-1 for the formed
[48, 49].
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DESs. The aromatic C-H wagging vibrations appeared in wavenumbers between 800-700 cm-1
3.1.3. Vibrational modes of the C-OH and C-O-C According to Fig. 4, the wavenumbers that occur between 1111.98 and 1023.63 cm-1 involve the C-OH and C-O-C stretching of components with two and three positive peaks [51]. By comparing all figures for DESs, it is evident that there is a peak at wavenumber of 1084.64 cm-1 16
ACCEPTED MANUSCRIPT for DES2 which was not observed for other DESs (see Fig. 4 for DES2 with red circle peak). This investigation is in agreement with the results reported by Aissaoui et al. [18] for MTPPB-EG. This peak may be attributed to an overtone of a peak at the wavenumber of 535.02 cm-1 which resulted in a lower intensity peak at 1084.64 cm-1. The C-OH bending vibration usually occurs at
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the right of and near to CH2 bending vibration. In all probability, a peak between 1410 and 1400
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cm-1 indicates C-OH bending vibration. This peak was observed in spectra of all DESs.
3.1.4. Other vibrational modes
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In general, the analysis of curves which are often affected by overlapping bands is more
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complex. Here, for all DESs (i.e., ATPB-GL, ATPB-EG, ATPB-DEG, ATPB-TEG), the P–H stretching bands may be overlapped with CH2 stretching vibrational bands at wavenumbers of
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between 3000-2800 cm-1 at the lower frequencies of 3000 cm-1 [18, 34]. There were several
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weak peaks in the region between 1700 and 2250 cm-1 that no useful information can be obtained from the analysis of the region. These peaks are the combination and overtone absorptions at
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about 1100-800 cm-1 and summation bands of ring CH waging vibrations [47, 48]. The C-C
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stretching vibrations may possibly be investigated at 875- 840 cm-1 in spectra of DESs [52]. Usually, the wavenumbers between 650-500 cm-1 indicate the existence of bromide in
study.
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components [18, 48]. Table 2 represents all possible vibrational modes of groups in DESs in this
17
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Table 2 Assignments of the IR of the DES1, DES2, DES3 and DES4. Assignment
3400-3200 3000-3100
OH stretching vibrations Aromatic CH stretching vibrations =CH2 allyl stretching vibrations
3000-2800
Aliphatic stretching vibrations P–H stretching vibration. C=C allyl stretching vibrations Quadrant and semi-circle ring stretching vibrations C=C aromatic stretching vibrations CH2 scissoring vibration C-OH bending vibration =CH2 bending CH2 wagging vibration CH2 twisting vibration Aromatic in-plane C-H bending vibrations
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Wavenumber (cm-1)
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1660-1610 1610-1480
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1200-1180 1150-1000
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1450-1420 1410-1400 1370-1320 1320-1300 1300-1200
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1000-900 900-840 800-840 780-700 700-680 650-500
P-aryl stretching C-O stretching vibration C-O-C stretching vibration C=C allyl wagging vibration =CH2 wagging vibration -CH2 rocking vibration C-C stretching vibration OH wagging vibration Aromatic C-H wagging vibration Aromatic ring pucker Bromide
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3.2. Thermal stability
One of the significant parameters related to the solvents used for CO2 capture is the thermal
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stability. When the materials are heated, their structures and chemical compositions can undergo changes, for example, fusion, melting, crystallization, oxidation, decomposition, transition,
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expansion and sintering. The thermal stability provides with information about the extent of
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temperature a solvent can withstand. This obtained information is very useful in both quality control and problem solving. For safe transportation and storage of these solvents, understanding
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the thermal stability of DESs is important. Therefore, the thermal stability of pure DESs used for
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CO2 solubility was analysed in this study. Mostly TGA is employed to obtain the relative stability of materials [53]. It should be mentioned that as the DESs transfer into the TGA pan, a
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small amount of water may be absorbed from the atmosphere. DESs were heated at the constant
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rate (10oC.min-1) until a weight loss events from 30oC to 800oC. Fig. 5 depicts the weight loss of single components against temperature. Totally, HBD components of DES have the lower the
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decomposition temperature compared with salt components [42]. It is clear from Fig. 5, ATPPB
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salt had the higher decomposition temperature rather than GL, EG, DEG and TEG components. Amongst HBDs, GL had the higher thermal stability and EG had the lower stability. DESs followed this trend according to the thermal stability: GL>TEG>DEG>EG. This may be attributed to the viscosity. The viscosity of GL was about 1400 mPa.s at 298.15 K, while that of TEG, DEG and EG was 38.254, 28.893 and 16.617 mPa.s at the same temperature, respectively. Therefore, it is more likely that ATPPB-GL (DES1) has the higher decomposition temperature
19
ACCEPTED MANUSCRIPT and ATPPB-EG (DES4) has the lower thermal decomposition temperature. The thermal stability of DESs will be discussed in the following paragraphs.
110
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100
pure ATPPB
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90
pure GL
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pure EG
70
pure DEG
60
pure TEG
50
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Weight loss (%)
80
40
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30 20 10
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0
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30 100 170 240 310 380 450 520 590 660 730 800
Temperature /oC
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Fig. 5. Dynamic TGA curves for salt and HBDs. The heating rate is 10oC.min-1 under N2.
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Fig. 6 depicts the TGA and derivative of TGA (DTGA) curves of DESs along with the onset
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and maximum temperatures. The DTGA curve provides the decomposition rate and is helpful for evaluating the mass-loss steps accurately. Fig. 6 shows the weight loss together with derivative weight loss (red solid line, DTGA) versus temperature. According to Fig. 6, there are two significant peaks in derivative of the weight loss. These peaks of derivative indicate the point of greatest rate of change on the weight loss curve. The first significant weight loss belongs to HBDs and was accompanied with a first significant peak of derivative weight loss curve (Tmax,1). By continuing the heating with constant rate, the next significant weight loss takes place which 20
ACCEPTED MANUSCRIPT can be observed by second significant peak of derivative weight loss curve (Tmax,2). This second weight loss which was accompanied with a sharp peak is pertinent to the salt (ATPPB) because the salt is decomposed later than HBDs. In all probability, the second decomposition step is the main decomposition step in DESs. Table 3 lists the Tmax in derivative weight loss curves for all
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DESs.
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Several research groups reported and applied the Tdcp or T10% for analysis of stability of materials [54, 55]. This temperature reflects the temperature at which a weight loss of 10% is
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observed. Also, the T90% are reported for all DESs for better comparison. The T10% and T90% of DESs are listed in Table 3. As can be observed, amongst the DESs, DES1 (ATPP-GL) had the
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higher T10% about 209.13oC and the lower value of T10% was pertinent to DES2 (ATPP-EG) about
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122.54oC. Although all DESs had a close value of T90%, the DES1 (ATPP-GL 1:4) had the higher T90% value about 361.55oC and the lowest value was pertinent to DES4 (ATPP-TEG) about
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350.53oC.
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Another advantageous way to analyze the thermal stability of DESs is the onset temperature
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(Tonset). Indeed, the Tonset indicates the upper decomposition range for the DESs. It is easy to determine the Tonset in TGA curves. As shown in Fig. 6, the Tonset is the point of intersection of
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the baseline weight and the tangent of the weight against temperature curve when the decomposition of DESs happens [56]. Depend on the liquids, the Tonset may occur more than one time. Here, there were two onset temperatures. The first one indicates the decomposition temperature of HBDs which is an indication of starting the degradation on DES and second one is pertinent to the decomposition temperature of salt. The Tonset of all DESs is listed Table 3. As can be evident, the DES1 had the highest first Tonset value about 247.24oC in comparison with other DESs, while DES2 had the lower Tonset value about 132.35oC. The second Tonset value of 21
ACCEPTED MANUSCRIPT DESs was close to each other. The DES1 had the highest second Tonset value about 338.35oC and the lowest second Tonset value was pertinent to DES2. According to Fig. 6 for TGA results, it can be clearly seen that the first decomposition step happened slowly in DES2 and the second decomposition step in DES4 was slower. This matter can help us to gain better insight to prepare
(b)
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(a)
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the stable DESs by considering the best amount and type of HBA or HBD.
(d)
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(c)
Fig. 6. Results of the TGA/DTGA of DESs along with the onset and maximum temperatures. The heating rate is 10oC.min-1. (a), DES1; (b), DES2; (c), DES3; (d), DES4. The dark solid lines represent the TGA curves and the red solid lines display the DTGA curves.
22
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Table 3 The maximum
temperatures
(Tmax.),
onset
temperatures
(Tonset)
and
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decomposition temperatures at 10 and 90 percent weight loss for the DESs in
DES2
DES3
DES4
Tmax.,1 / oC
285.04
170.06
214.50
243.50
Tmax.,2 / oC
359.15
349.39
358.52
353.22
First Tonset / oC
247.42
132.35
178.65
204.25
Second Tonset / oC
338.35
322.29
336.56
326.64
T10% / oC
209.13
122.54
154.92
177.19
T90% / oC
361.55
354.89
359.39
350.53
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DES1
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Parameters
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this work.
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The interpretation of thermal stability results of different DESs is difficult. The type of HBD, nature of salt or HBD, alkyl chain length, viscosity, hydrogen bonding interactions, electrostatic
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interactions, dispersion interactions and hydrogen bonding sites on HBD can be considered as important factors in changing the properties especially the thermal stability of DESs. Zhu et al. [57] studied comprehensively the various type of interactions and analyzed the strengths of hydrogen bonding in DESs. It should be noted that a few of the weak hydrogen bonds together can compete with a stronger hydrogen bonding between ion of salt and hydroxyl group of HBD [58], and also interactions such as Van der Waals hydrogen bonding interactions and other interactions may coexist for noncovalent interaction systems [57]. 23
ACCEPTED MANUSCRIPT It has been well established that alkyl chain length has an effect on the thermal stability. There is a discrepancy in this matter and literature reports point out that the thermal stabilities implemented under N2 produce the different results. Villanueva et al. [59] studied the thermal stability of alkyl imadazolium-based ILs with the same anion under N2 atmosphere. Their results
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revealed that an increasing alkyl chain length on cation decreased the thermal stability of ILs.
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Wu et al. [54] investigated the thermal stability of morpholin-based ILs with alkyl sulfate anion
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under N2 atmosphere. They stated that ILs with methyl sulfate anion has the higher thermal stability. However, Kosmulski et al. [60] studied that the thermal stability of imidazolium-based
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ILs increased when the alkyl chain length on cation increased under N2. In this work, in order to
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investigate the effect of alkyl chain length on the thermal stability, three DESs such as DES2, DES3, and DES4 were considered because of a close similar structure and difference in ethylene
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group. According to data represented in Table 3, it was found that by increasing the alkyl chain
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length on HBDs, the thermal stability of DESs increases. This result is in a well agreement with
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result reported by Kosmulski et al. [60].
It is interesting to mention that the thermal stability trend on DESs was similar to that of the
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HBDs. That is if the HBDs followed this trend on the thermal stability: GL>TEG>DEG>EG, the
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thermal stability of DESs was ATPPB-GL (DES1)> ATPPB-TEG (DES4)> ATPPB-DEG (DES3)> ATPPB-EG (DES2) which was similar to trend on HBDs. Indeed, it can be said that the nature of HBD can have a great effect on the thermal stability of DES. It is worthwhile to compare thermal stability of DESs in this work and literatures. Tdcp of DES1 (ATPPB-GL) is higher than that of choline chloride/glycerol (1:2), choline acetate/glycerol (1:1.5) reported by Zhao et al. [40]. This DES has almost the same stability as choline chloride/glutaric acid (Tdcp =246.85 oC) reported by Florindo et al. [41]. Moreover, all of these 24
ACCEPTED MANUSCRIPT DESs in this work are more stable than choline chloride/lactic acid with Tdcp of 126.85oC measured by Francisco et al. [43].
4. Conclusion
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In this work, DESs including ATPPB-GL (DES1), ATPPB-EG (DES2), ATPPB-DEG
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(DES3) and ATPPB-TEG (DES4) were prepared into the same molar ratio of 1:4 salt to HBDs. The functional groups of these phosphonium-based DESs were investigated and analyzed via
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FT-IR. The findings confirmed that the selected DESs exhibit the similar spectra and the similar chemical compositions with different levels of transmittance. The interesting result of this work
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can be the similar behavior of DESs with their HBDs. Indeed, DESs behaved like their
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corresponding HBDs especially at the higher wavenumbers. The thermal stability of HBDs and DESs was analyzed. It was found that the type of HBD has a great effect on the thermal stability
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of DESs. Amongst HBDs and DESs, the highest thermal stability was pertinent to GL and
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ATPPB-GL (DES1), respectively and EG and ATPPB-EG (DES2) had the lowest thermal stability. It was found that as the alkyl chain length on HBD increases, the thermal stability of
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DESs increases. Last but not least, it was found that the nature of HBD can play a pivotal role in
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thermal stability of DES.
25
ACCEPTED MANUSCRIPT References
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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[7]
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[6]
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[5]
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[4]
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[3]
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Figure captions Fig. 1. A schematic of chemical structure of the salt and HBDs.
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Fig. 2. Different vibration modes of -CH2 group; stretching vibrations including: (a), symmetric
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stretching vibration; (b), asymmetric stretching vibration; and bending vibrations including: (c),
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scissoring bending vibration; (d), rocking bending vibration; (e), wagging bending vibration; (f), twisting bending vibration. Dark circles shapes display hydrogen (H) atoms and green circle
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shapes represent the carbon (C) atoms.
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Fig. 3. FT-IR spectra for the single components studied in this work; (a), ATPPB; (b), GL; (c), EG; (d) DEG, (e), TEG.
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Fig. 4. FT-IR spectra for the DESs studied in this work; (▬), ATPPB-GL (DES1); (▬), ATPPBEG (DES2); (▬) ATPPB-DEG (DES3); (▬) ATPPB-TEG (DES4).
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Fig. 5. Dynamic TGA curves for salt and HBDs. The heating rate is 10oC.min-1 under N2. Fig. 6. Results of the TGA/DTGA of DESs along with the onset and maximum temperatures.
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The heating rate is 10oC.min-1. (a), DES1; (b), DES2; (c), DES3; (d), DES4. The dark solid lines
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represent the TGA curves and the red solid lines display the DTGA curves.
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Highlights: - Spectra of phosphonium based DESs and their single components was investigated via FT-IR. Spectra of DESs and their HBDs were almost similar to each other.
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The nature of HBDs had a great effect on the thermal stability of DESs.
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As the alkyl chain length on HBD increased, thermal stability of DESs increased.
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