Mixtures of tetrabutylammonium chloride salt with different glycol structures: Thermal stability and functional groups characterizations

Journal of Molecular Liquids 294 (2019) 111588

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Mixtures of tetrabutylammonium chloride salt with different glycol structures: Thermal stability and functional groups characterizations Mohd Faridzuan Majid a,⁎, Hayyiratul Fatimah Mohd Zaid a,b, Chong Fai Kait a, Noraini Abd Ghani a,c, Khairulazhar Jumbri a,c a b c

Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak Darul Ridzuan, Malaysia Centre of Innovative Nanostructures & Nanodevices (COINN), Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak Darul Ridzuan, Malaysia Centre for Research in Ionic Liquids (CORIL), Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak Darul Ridzuan, Malaysia

a r t i c l e

i n f o

Article history: Received 4 July 2019 Received in revised form 14 August 2019 Accepted 19 August 2019 Available online 20 August 2019 Keywords: Deep eutectic solvent Ethylene glycol Thermal analysis Hydrogen bonding Tetrabutylammonium chloride FTIR

a b s t r a c t Deep eutectic solvent (DES) is an alternative solvent which has the basic characteristics as conventional ionic liquid. The uniqueness lies on its easy synthesis, less toxic and low-cost production. Three glycol-based DES (ethylene glycol, tetraethylene glycol and poly (ethylene glycol)) with tetrabutylammonium chloride (TBAC) as the hydrogen bond acceptor were successfully prepared. Unusually, all DES was achieved their eutectic mixing as low as 50 °C within 60 min. Thermal analysis study revealed that the most stable DES was the one with higher ethoxy chains. Structure elucidation study via Fourier transformation infrared (FTIR) spectroscopy confirmed the formation of eutectic mixture based on the intermolecular interaction from decreasing hydroxy group stretching frequency. Modifying ethoxy repeating unit in hydrogen bond donor could tailor the design of DES for numerous applications. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Ionic liquid (IL) is known as molten salt that made up from cation and anion which bonded via electrostatic interaction with low vapor pressure and recyclability benefit. Due to its tailor-made properties, the synthesis of IL can be achieved to perform specific task. IL is widely used as electrolyte, solvent, adsorbent and catalyst in oil and gas industry, such as biomass conversion into chemical platforms, enhanced oil recovery, transformation of CO2 into fuels, fuel oil desulfurization and solar and energy storage [1–8]. Utilization of IL could yield an excellent performance in different area of applications, however the cost production of IL is too high and the synthesis procedure is quite complex [9–11]. To overcome this, low-cost IL is preferred with simple synthesis steps. Deep eutectic solvent (DES) is known as the new generation IL which made up with cheaper raw materials and easy to biologically degrade. It composed of hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) which mutually interact via hydrogen bonding, to form a eutectic mixture which melts from its constituent's component in room temperature. DES was firstly tested for enzyme catalysis [12], followed by the molecular purification via DES which had been discovered in 1995. Huge amount of reports on DES had been received since ⁎ Corresponding author. E-mail address: [email protected] (M.F. Majid).

https://doi.org/10.1016/j.molliq.2019.111588 0167-7322/© 2019 Elsevier B.V. All rights reserved.

2004, with more focus on biocatalysis, electrochemistry and zeolite synthesis [13]. Recently, DES is actively progressed in multidisciplinary research such as carbon dioxide capture [14], separation of bioactive compounds [12,15], sulfur removal from fuel oil [14,16,17] and water osmosis [18]. Glycol is a type of organic compound which has two hydroxyl groups attached to the carbon chain. Ethylene glycol is the simplest form of glycol which commercially used as antifreeze in automobile cooling systems. When the glycol chain increased, it forms a macromolecule that resembles polymer properties with a different morphology [19]. Poly (ethylene glycol) (PEG) is example of glycolic polymer which is used in skin creams, dispersant agent in toothpastes, anti-foaming agent in beverages and ink solvent for paper printing [20,21]. Critical solution temperature could be observed for a mixture of PEG in different medium, which could be achieved by engineering the desired solution temperature for specific application [22]. Recently, glycol-based DES had been widely studied for numerous applications and fundamental investigation. Lima et al. [23] synthesized a series of glycol-based DES using several tetrabutylammonium salts for dibenzothiophene (DBT) removal from oil. As high as 85% of DBT could be removed when tetrabutylammonium chloride:poly(ethylene glycol) (TBAC:PEG) used as extractor. Shu and Sun [24] synthesized tetrabutylammonium chloride:ethylene glycol (TBAC:EG) to remove sulfur from model oil and 99.5% of extraction efficiencies was reported. Most of the literature survey reported the performance of DES in specific applications, however

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there is limited publications on the thermal stability data and the structure elucidation of DES. Thermal analysis will help scientist and engineer to evaluate the optimum heat required when concerning for upscaling purpose. Structure elucidation in the other hand will help researcher to reconfirm the formation of DES, as physical appearance solely does not give much information on molecular interaction inside the material. Allyltriphenyl phosphonium bromide-based DES with different length of glycol molecules were successfully prepared by Hosein Ghaedi and co-workers for CO2 capturing application [25]. He stated that the nature of HBD has an impact on the thermal stability of DES. Delgado-Mellado et al. [26] published their work on FTIR-TGA of glycol-based DES using choline chloride as HBA. It was reported that some DES shows only single mass loss during decomposition procedure. In this study, three glycol-based DES using TBAC as HBA were synthesized with mole ratio HBA:HBD of 1:1 to 1:4. The thermal analysis of all synthesized DES was evaluated via TGA and the formation of eutectic mixture corresponds to specific molecular vibrations was confirmed using FTIR. 2. Materials and method 2.1. Chemicals Tetrabutylammonium chloride (TBAC) salt was purchased from Acros, while ethylene glycol (EG) and tetraethylene glycol (TEG) were purchased from Merk. Poly(ethylene glycol) was supplied by Sigma Aldrich with an average molecular weight of 400. All chemicals were used without further purification steps. Table 1 shows the list of chemicals together with their abbreviation, chemical structure and percentage of purity. 2.2. DES synthesis The DESs were prepared by mixing TBAC as hydrogen bond acceptor (HBA) with different glycolic hydrogen bond donors (HBD). In brief, the amount pure components were weighed according to the mole ratio of HBA:HBD ranging from 1:1 to 1:4. Due to the hygroscopic nature of TBAC, it should be kept tightly in the glass vial before mixed with the desired HBD. The mixture was then heated to 50 °C with stirring speed of 500 rpm for 60 min. A clear homogeneous liquid should appear to indicate the successful interaction between the components. The DESs were cooled to room temperature and the physical appearance was recorded. The synthetic route of the formation of DES is shown in Fig. 1. 2.3. Thermogravimetry analysis Thermal analysis of each pure component and synthesized DES were run via Simultaneous Thermal Analyzer (STA 6000, Perkin Elmer). Approximately 7–15 mg of DES sample was settled in a small pan and

nitrogen gas was chosen as the gas atmosphere with flow rate of 20 mL min−1. Temperature programme was fixed from 50 °C to 650 °C and the temperature hold measurement was set for 1 min. 2.4. Fourier transformation infrared spectroscopy (FTIR) Functional group characterizations were done via Thermo-Nicolet FTIR spectrometer. Before the sample was analysed, background spectrum was collected first to eliminate unwanted residue peak from the sample spectrum. Number of scans was set to 16 and the wavenumbers were recorded from 400 to 4000 cm−1. 3. Results and discussion 3.1. Formation of DES Three DESs, namely TBAC:EG, TBAC:TEG and TBAC:PEG was successfully synthesized with different mole ratios starting from 1:1 to 1:4. Surprisingly, the formation of DES can be achieved as low as 50 °C in shorter time compared to previous studies reported by literature. Table 2 shows the parameter comparison of EG-based DES synthesis. The physical appearance of the DES can be seen from Figs. S1–S3 in the Supplementary information. All mixture of TBAC-based DESs were appeared as clearly homogenous liquid after the heating procedure. Fig. 2 shows the intermolecular interaction of HBA and HBD in the formation of eutectic mixture. The formation of DES began as heat being applied to melt the combination of both pure components. Hydrogen bond was formed from the interaction of chloride anion and hydrogen atom which bonded to the electronegative oxygen in the HBD, subsequently enhance the complexation of the HBA to form the eutectic mixture [27]. Molecular dynamic simulation performed by several authors also confirmed the formation of such interactions in quaternary salts with respectives complexing agents [28–31]. It is reported that the chloride ion itself plays an important role for the neutralization of charges via electrostatic interaction with nitrogen atom from quaternary salt, thus providing the coordination shell in DES. Furthermore, the formation of DES is determined by the nature and molecular structure of HBD. In a report provided by A. R. Harifi-Mood et al., it was known that EG has a high preferential to solvate with the chloride anion of choline cation with an average interaction energy of −64.85 kJ mol−1 compared to glycerol (−81.83 kJ mol−1) and urea (−83.76 kJ mol−1) [29]. EG itself is smaller in size compared to other HBDs, therefore we also expect an increase in interaction energy of TBAC with TEG and PEG due to their longer repeating ethoxy chains. After being left to room temperature, all DESs remained in liquid state except for TBAC:EG with the mole ratio of 1:1. Porofiri et al. also found that the TBAC:EG 1:1 mol ratio system was solid even at 50 °C [32]. Most of the researchers reported that the homogeneous mixing for TBAC:EG system could be achieved at a minimum ratio of 1:2

Table 1 List of chemicals for DES synthesis. Name

Abbreviation

Chemical structure

Purity (%)

Supplier

TBAC

95

Acros

Ethylene glycol

EG

99.5

Merck

Tetraethylene glycol

TEG

98

Merck

Poly(ethylene glycol) 400

PEG

Not available

Sigma Aldrich

Tetrabutylammonium chloride

M.F. Majid et al. / Journal of Molecular Liquids 294 (2019) 111588

3

Fig. 1. Synthetic route of the formation of DES.

[33–35]. Increasing the mole ratio for HBD can provide more hydrogen bonding sites to the TBAC. Differential scanning calorimetry study by Mjalli et al. revealed that an increase amount of HBD will shift the melting point towards the eutectic point of the mixture, which was located at the optimum ratio of 1:3 for TBAC:EG system [36]. In the case of TEG and PEG, the clear mixing of DES could be achieved as minimum as 1:1 mol ratio and this might be originated by the increasing repeating units of their ethoxy chains. In industrial application, an optimum mole ratio should be taken into consideration as it will affect the rate of reaction. Lima et al. addressed that the optimum mole ratio for the synthesis of TBAC:TEG and TBAC:PEG was 1:2 [23]. In regard of standardization purpose, TBAC:EG (1:3), TBAC:TEG (1:2) and TBAC:PEG (1:2) were selected for further characterizations. 3.2. Thermal stability The study of thermal analysis of a substance is crucial as it will provide information on how a tested material behaves as heat is flow. When considering for industrial scale, the thermostability of the substance will become a prerequisite to ensure the material could withstand a high temperature for reaction and separation process [38]. One of the pragmatic analytical tools to obtain the required data is via thermogravimetry analysis (TGA). TGA measures the amount and rate of weight loss of a material as a function of temperature or time in a fixed atmosphere. It gives information on the chemical analysis as well as the prognosis of thermal stability for up to 1000 °C. TGA analysis of DES will give an insight on how they behave especially when employed in industrial process such as separation. TGA was performed first for the pure components before analysing the synthesized DES. As can be shown in Fig. 3, the thermal stability of each components was following this order: EG b TBAC

b TEG b PEG. By looking at the HBD components, increasing the ethoxy group units has a great effect on the thermal stability of the substance. More energy is required to destruct the C\\O and C\\C bond as it involves a series of competitive intramolecular and intermolecular process [39]. The viscosity of pure components also contributes to their thermal stability. Viscosity values of EG, TEG and PEG at room temperature were 16.617 mPa.s, 58.3 mPa.s and 120 mPa.s respectively, which correlated with the thermal trends of HBD. Hosein Ghaedi and his coworkers [25] also reported the same trend for the thermal stability and viscosity of glycerol, triethylene glycol and diethylene glycol. Higher frictional forces between the fluids might lowering the heat transfer, stabilized the integrity of molecules and eventually increases the energy to be supplied for the bond breaking process [40]. Figs. 4–6 show the TGA thermogram of TBAC:EG, TBAC:TEG and TBAC:PEG respectively where the weight percentage and derivative weight percentage curves were plotted against the temperature. Based on the TGA results, two significant derivative peaks appeared for TBAC:PEG and TBAC:EG, while for TBAC:TEG, single peak was recorded. The presence of these peaks indicates the degradation stages of a substance while the temperature reported on the peak represent the maximum degradation temperature. The decomposition steps of individual EG and PEG together with their TBAC were occurred in two single steps, however for TBAC:TEG, it was appeared in single step. The reason behind this was due to the near mass loss initiation around 180 °C. This phenomenon was also reported by Delgado-Mellado et al. [26] where single step degradation occurred in choline chloride:glycerol DES based on the TGA curve. Meanwhile, the degradation steps of individual components in TBAC:EG and TBAC:PEG occurred in a different sequence. For TBAC:EG, EG decomposed first, followed by TBAC; for TBAC:PEG, initial degradation was TBAC and later by PEG. A simple explanation on this was due to the difference in molecular weight of

Table 2 EG-based DES synthesis parameter comparison.

TBAC:EG TBAC:EG TBAC:EG TBAC:EG TBAB:EG ATPPB:EG TBAC:EG

Temperature (°C)

Time (minutes)

Stirring speed (rpm)

Mole ratio

Reference

90–120 50 80 50 70 70–100 50–60

240 60 120 180 80 180 60

– 800 400 – 180 350 500

1:3 1:3 1:2 1:2 1:2 1:4 1:2

[24] [23] [36] [32] [37] [25] This work

4

M.F. Majid et al. / Journal of Molecular Liquids 294 (2019) 111588

Fig. 2. Intermolecular interactions of synthesized DES. (a) TBAC:EG (b) TBAC:TEG (c) TBAC:PEG.

each individual component where it follows this order: EG b TBAC b PEG. A brief comparison of the maximum temperature (Tmax) of weight loss and rate of decomposition for each DES is presented in Table 3. It is clearly known that the highest Tmax is TBAC:PEG, followed by TBAC:TEG and TBAC:EG. The weight loss rate of individual

components in TBAC:PEG was almost the same, while the decomposition rate of TBAC:EG from first to second step was increased by a factor of five and this is corresponded to the sharp derivative peak of TBAC. To determine at what temperature a substance starts to reduce its weight, an onset temperature can be calculated via intersection of

100 TBAC

90

PEG

TEG

EG

80

Weight % (%)

70 60 50 40 30 20 10 0 30

130

230

330

430

Temperature (°C) Fig. 3. TGA thermograms for pure components.

530

630

M.F. Majid et al. / Journal of Molecular Liquids 294 (2019) 111588

5

TBAC:EG 5

120

-5

Weight % (%)

80 -10 60 -15 40 -20 20

Derivative Weight % (%/min)

0

100

-25

0

-30 50

150

250

350

450

550

650

Temperature (°C) Fig. 4. TGA thermogram of TBAC:EG.

followed by TBAC:TEG (T10% = 213.21 °C, T90% = 259.21). The main factor for this trend was due to the longer ethoxy chain which slower the decomposition process of the DES. Based on the results interpreted by the TGA thermograms, it can be concluded that TBAC:PEG was the most stable DES in terms of its degradation resistance. A higher unit of ethoxy group increase the amount of heat required to break the intermolecular forces formed in the eutectic mixture. The least stable DES was TBAC:EG as its rate of decomposition is high at lower temperature.

tangent of weight against temperature curve and baseline weight. The value of all onset temperatures of DES can be referred to Table 3. TBAC:PEG had the highest onset temperatures (first Tonset = 210.81 °C and second Tonset = 320.52 °C), followed by TBAC:TEG and TBAC:EG. Note that TBAC:TEG only show one onset temperature (214.92 °C) which indicate the simultaneous mass loss of both pure components. By contrasting the TGA curve in Figs. 4 and 6, the decomposition steps of TBAC:EG was occurred faster than TBAC:PEG. This was due to the strong intermolecular forces present in TBAC:PEG, which requires a high amount of energy to cleave the neighbouring bonds. The thermal stability of a material can also be determined by looking at the temperature when the DES loss its certain percentage of weight. In this study, the degradation temperature for 10% and 90% weight loss (T10% and T90%) of DES were reported for better correlation. By referring to Table 3, again, TBAC:PEG portrayed the highest degradation temperature at 219.15 °C and 378.15 °C for T10% and T90% respectively. TBAC: EG degraded at a lower temperature (T10% = 132.13 °C, T90% = 222.13)

3.3. Functional group elucidations In chemistry point of view, determination of molecular structure of a material will clarify the existence of a compound. FTIR is one of the spectroscopy techniques which can help to identify the functional groups through molecular vibrations. It has two major types of vibrations, which are the stretching and bending. Confirmation of the

5

100

0

80

-5

60

-10

40

-15

20

-20

-25

0 50

150

250

350

450

Temperature (°C) Fig. 5. TGA thermogram of TBAC:TEG.

550

650

Derivative Weight % (%/min)

Weight % (%)

TBAC:TEG 120

6

M.F. Majid et al. / Journal of Molecular Liquids 294 (2019) 111588

TBAC:PEG400 120 10

Weight % (%)

0 80 -10 60 -20 40 -30

20

0

Derivative Weight % (%/min)

100

-40 50

150

250

350

450

550

650

Temperature (°C) Fig. 6. TGA thermogram of TBAC:PEG.

Table 3 Maximum temperature (Tmax), weight loss rate, onset temperature (Tonset) and decomposition temperature of DES at 10% and 90% weight loss.

First Tmax (°C) Second Tmax (°C) First Tmax weight loss rate (%/min) Second Tmax weight loss rate (%/min) First Tonset (°C) Second Tonset (°C) T10% (°C) T90% (°C)

TBAC:EG

TBAC:TEG

TBAC:PEG

154.02 214.40 −4.599 −25.051 119.30 199.42 132.13 222.13

247.89 – −20.465 – 214.92 – 213.21 259.21

225.00 376.75 −9.057 −9.140 210.81 320.52 219.15 378.15

substitutional groups and organic chain can be verified via infrared radiation. If the certain group excites as infrared being transmitted, it means it is infrared actives and the stretching or bending characteristics of

them can be seen on FTIR spectrum. Moreover, the intramolecular and intermolecular interactions in a mixture can also be determined by looking on the characteristic peak shifting [41]. To investigate the formation of DES, characterization of pure components and synthesized sample was performed to compare the shifting effect which indicates a change in the molecular bonding. Fig. 7 shows the individual FTIR spectrum for each pure component of HBA and HBD. OH stretching vibrations can be seen for all HBD indicated by (b), (c) and (d) at 3448.28 cm−1, 3408.28 cm−1 and 3286.04 cm−1 respectively. The wavenumber shows a decreasing value for PEG b TEG b EG. As the ethoxy group decreases, formation of intermolecular forces between HBD become easier as the molecule can approach to themselves more frequent. FTIR investigation by Caccamo et al. also revealed that the intermolecular interaction in EG-PEG1000 mixture was dominated by EG [42]. Hydrogen bonding will lengthen the bond thus decreasing the energy of vibration. In the case of TBAC, since there is no OH group in the structure, the broad stretching at 3423.70 cm−1

Transmittance (displaced scale)

(a)

(b)

(c)

(d)

3900

3400

2900

2400

1900

1400

Wavenumbers (cm-1) Fig. 7. FTIR spectrum for pure components. (a) TBAC (b) PEG (c) TEG (d) EG.

900

400

M.F. Majid et al. / Journal of Molecular Liquids 294 (2019) 111588

7

Transmittance (displaced scale)

(a)

(b)

(c)

3400

2900

2400

1900

1400

900

400

Wavenumbers (cm-1) Fig. 8. FTIR spectrum for synthesized DES. (a) TBAC:PEG (b) TBAC:TEG (c) TBAC:EG.

might resulted from the moisture while transferring for analysis since TBAC is a hygroscopic substance. Two significant stretching vibrations were identified from 2980 cm−1 to 2800 cm−1 which corresponds to asymmetric and symmetric stretching of sp3 –CH2 and –CH3 for all components. Besides stretching vibration, the bending of sp3 –CH2 and –CH3 can also be observed at the region between 1465 cm−1 to 1375 cm−1. The C-O-C stretching vibration were detected at 1098.85 cm−1 1092.68 cm−1 for TEG and PEG respectively, while strong C-OH stretching was identified at 1029 cm−1, 1059.56 cm−1 and 1092.68 cm−1 for EG, TEG and PEG respectively. Due to hydrogen bonding, shorter ethoxy chain shows characteristic stretching at lower wavenumber. Fig. 8 shows the FTIR spectrum of synthesized DES. By comparing DES spectrum with the individual component, transformation of spectrum was observed due to the interaction between salt and glycolic components. The –OH broad stretching for TBAC:PEG, TBAC:TEG and TBAC:EG was 3295.68 cm−1, 3298.47 cm−1 and 3317.80 cm−1 respectively. The stretching had shifted to lower wavenumber due to the formation of hydrogen bonding between chloride anion and –OH group, however in the case of TBAC:EG, the stretching was shifted to a higher frequency. It was suspected that the site of hydrogen bonding was less vacant as it only had single ethoxy unit, compared to TEG and PEG which can interact easily to the halide ion of TBAC. Another research also reported the increase frequency of OH stretching after EG had interacted with allyltriphenyl phosphonium bromide as HBA, while in the case of longer chain HBD such as diethylene glycol and triethylene glycol, the peak was shifted to a lower frequency [25]. Aliphatic -CH2 and -CH3 were observed around 2980 cm−1 and 2860 cm−1 with sharp intense peak for all synthesized DES, which indicates asymmetry and symmetry stretching of TBAC and glycolic compound. -CH2 scissoring bending of TBAC at 1473 cm−1 had also diminished which arises from the complexation effect of HBD.

4. Conclusion In summary, TBAC:EG, TBAC:TEG and TBAC:PEG was successfully synthesized at lower temperature and time. TGA analysis confirmed that the thermal stability of DES increased according to the following trend: TBAC:EG b TBAC:TEG b TBAC:PEG. Ethoxy repeating unit plays a vital role on the decomposition resistance of DES where longer chain required much more energy to break the intermolecular force. The formation of hydrogen bonding in DES was confirmed via FTIR spectrum,

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