Crystal structure and thermal decomposition kinetics of 1-(pyridinium-1-yl)propane-(1-methylpiperidinium)bi[bis(trifluoromethanesulfonyl)imide],[PyC3Pi][NTf2]2☆

Jun Wang*,Manman Wang,Xuzhao Yang,Wenyuan Zou,Xiang Chen

Henan Provincial Key Laboratory of Surface&Interface Science,Zhengzhou University of Light Industry,Henan 450001,China

Keywords:Asymmetrical dicationic ionic liquid Crystal structure Decomposition kinetics

A B S T R A C T The structure of a room temperature asymmetrical dicationic ionic liquid(ADIL),1-(pyridinium-1-yl)propane-(1-methylpiperidinium)bi[bis(trifluoromethanesulfonyl)imide]([PyC3Pi][NTf2]2),was studied by the X-ray diffraction method.Meanwhile,thermal analysis of[PyC3Pi][NTf2]2 was also studied using non-isothermal thermogravimetric analysis(TGA).The title crystal belongs to the triclinic with space group Pī and unit-cell parameters a=0.95217(8)nm,b=1.05129(11)nm,c=1.70523(14)nm,α =89.759(8)°,β =80.657(7)°,γ =68.007(9)°,and F(000)=792.Thermal stability and thermal decomposition kinetics of the title compound were also investigated using TGA under the atmosphere of highly pure nitrogen.Heating curves at different rates were correlated with kinetic equations Friedman and ASTM(also called is o-conversion method).The values of average activation E(k J·mol?1)and pre-exponential constant lg A are 149.58 k J·mol?1 and 8.83,respectively,which were obtained by the two methods.The kinetic model function,activation energy and pre-exponential constant of this reaction using the multivariate non-linear-regression method were f(α)=(1?α)(1+4.1870α),151.04 kJ·mol?1 and 8.81,respectively,which were basically consistent with is o-conversion methods.

1.Introduction

Ionic liquids(ILs)are a class of organic molten salts consisting of only cations and anions[1].In recent years,ILs have extensive applications because of their unique physicochemical properties such as negligible vapor pressure,high thermal stability,high thermal conductivity,wide electrochemical window and low melting point[2–4].Compared to traditional monocationic ionic liquids,dicationic ionic liquids(DILs),especially asymmetrical dicationic ionic liquids(ADILs),have shown to possess superior physicochemical properties in the field of volatility and thermal stability[5].Therefore,they can be used as solvents of high temperature organic synthesis[6,7]or as a gas chromatography stationary phase[8,9].Because these applications were often conducted at relatively high temperatures,it is essential to study the thermal stability and thermal decomposition kinetics of ADILs,which have not been studied.In addition,most ADILs are liquids at room temperature.Thus,it is technically difficult to grow single crystals and only few studies have so far been attempted to illustrate the structures of ADILs.

In this paper,the crystal structure of[PyC3Pi][NTf2]2(Fig.1)was analyzed by X-ray diffraction.In addition,non-isothermal decomposition of[PyC3Pi][NTf2]2was carried out at five different heating rates(5,10,15,20 and 25 K·min?1).Thus,kinetic triplet viz.activation energy,pre-exponential constant and mechanism function were obtained.

Fig.1.Molecular structure of[PiC3Py][NTf2]2.

2.Experimental

2.1.Materials

The asymmetrical dicationic ionic liquid[PiC3Py][NTf2]2was synthesized by our laboratory as Fig.2.1,3-Dibromopropane,pyridine,1-methylpiperidine and bistrifluoromethane-sulfonimidate lithium salt(LiNTf2)were all purchased from Aladdin(Shanghai).The target asymmetrical dicationic ionic liquid was then dried under vacuum at 343 K for 2–3 days aimed to remove trace amounts of water.Its purity was greater than 99%in mass fraction.The melting point was 119.2–119.8°C.Water contamination was analyzed using the Karl Fischer technique for[PiC3Py][NTf2]2which showed that the mass fractions were less than 0.005%.

Fig.2.Synthesis routes of[PiC3Py][NTf2]2.

Table 1Crystallographic data for[PiC3Py][NTf2]2①

2.2.Apparatus and experimental method

The1H NMR spectra of the purified asymmetrical dicationic liquid were recorded in DMSO on a Bruker Avance 400 spectrometer at a temperature of 25°C.The IR determination of ionic liquid adopted the KBr pellet method on a Tensor-27 FT-IR Spectrometer.

[PyC3Pi][NTf]2:1H NMR(400 MHz,DMSO)d(ppm):1.527–1.586 δ(m,2H,11-CH2),1.776–1.833 δ (m,4H,10,12-CH2),2.366–2.445 δ (m,2H,7-CH2),2.995 δ(s,3H,14-CH3),3.308 δ(t,J1,3=0.033 Hz,4H,9,13-CH2),3.409 δ (t,2H,J1,3=0.020 Hz,8-CH2),4.65 δ (t,J1,3=0.038 Hz,2H,6-CH2),8.22 δ (t,J1,3=0.036 Hz,2H,2,4-CH2),8.65 δ (t,J1,3=0.040 Hz,1H,3-CH2),9.08 δ (d,2H,1,5-2H).IR(KBr):3030 cm?1,2962 cm?1,2167 cm?1,1491 cm?1,781 cm?1,686 cm?1,567 cm?1.

X-ray single-crystal diffraction data for[PiC3Py][NTf2]2was collected on a Bruker Smart 1000 CCD diffractometer at 293(2)K with Mo-Kα radiation(λ =0.071073 nm)by the ω scan mode.The program SAINT[10]was used for integration of the diffraction profiles.Semi-empirical absorption corrections were applied using the SADABS program[11].All the structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by full-matrix least-squares methods with SHELXL[12].Non-hydrogen atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F2.All hydrogen atoms were included in calculated positions and refi ned with fixed thermal parameters riding on their parent atoms.CCDC-980152 contained the supplementary crystallographic data for[PiC3Py][NTf2]2.Crystallographic data and experimental details for structural analyses are summarized in Table 1.Selected bond lengths and angles as well as hydrogen-bonding geometries are listed in Tables 2–4.

A thermogravimetric analyzer(TGA Instruments,Netzsch STA 449 C)was utilized to measure the thermal decomposition kinetics.The conventional non-isothermal thermogravimetric runs were carried out at different heating rates of 5,10,15,20 and 25 K·min?1.In this research,the Friedman and ASTM(American Society for Testing and Materials)methods were used to study the kinetics.In addition,the multivariate non-linear-regression was employed to determine the kinetic model and mechanism function.

3.Results and Discussion

3.1.Crystal structure of[PiC3Py][NTf2]2

Single-crystal X-ray diffraction analyses reveal that[PiC3Py][NTf2]2crystallizes in the triclinic system,Pī space group,and the asymmetric unit of which contains two[NTf2]?anions and one[PiC3Py]+cation.The ORTEP diagram of the asymmetric unit is shown in Fig.3.There into,N(3)possesses 100%position probability,which only 50%for both N(4)and N(5),giving the charge neutral structure of the title compound.Besides the cation–anion interactions between[PiC3Py]+and[NTf2]?,in the crystal structure of[PiC3Py][NTf2]2,[PiC3Py]+and[NTf2]?serve as hydrogen-bond donor and acceptor,respectively.And thus,a 3-D extended supramolecular architecture(Fig.4)is formed by C–H…N and C–H…O hydrogen bonds betw een[PiC3Py]+and[NTf2]?(Table 4).There is no doubt that these strong hydrogen-bonding interactions play an important role in the formation of the 3-D supramolecular architecture.

Table 2Selected bond lengths(nm)for[PiC3Py][NTf2]2

Table 3Selected bond angles(°)for[PiC3Py][NTf2]2

Table 4Hydrogen bonding geometries for[PiC3Py][NTf2]2

3.2.Thermogravimetric curve analysis of[PiC3Py][NTf2]2

Non-isothermal decomposition of the title compound was performed at five different heating rates(5,10,15,20 and 25 K·min?1).The corresponding thermogravimetric(TG)and differential thermogravimetric(DTG)curves for decomposition are shown in Fig.5.There is only one stage during the heating process.Providing a case study of 10 K·min?1,the onset decomposition temperature(Tonset:mass loss is defined as 1%)and the maximum mass loss rate temperature(Tmax)are 630.45 K and 706.55 K,respectively.The thermal stability of similar ionic liquids has been reported elsewhere[13–16].For example,the Tonsetof the symmetrical dicationic ionic liquid[PyC5Py][NTf2]2was 605.85 K[15].By comparison,it is found that the thermal stability of the title compound is relatively good.The TG/DTG curve indicates that the mass loss is about 1%at lower temperature(＜630.45 K),which is ascribed to the evaporation and a very small proportion of decomposition of[PiC3Py][NTf2]2[17].The mass loss ends up at about 817.15 K with a mass loss of about 97.8%in the range of 630.45 to 817.15 K,which is due to the decomposition of[PiC3Py][NTf2]2.The residue in an open Al2O3crucible is carbon with a mass of about 1.2%.

Fig.3.ORTEP drawing of[PiC3Py][NTf2]2.

Fig.4.Packing diagram of[PiC3Py][NTf2]2.

Fig.5.TG(a)and DTG(b)curves of[PiC3Py][NTf2]2 at different heating rates d T/d t.

As can be seen from Fig.5,both Tonsetand Tmaxare shifted to higher temperature as the heating rate increases,which can be seen from the mass loss rate at different heating rates as well.It is probably caused by the residence time of the sample in the furnace which decreases with the increase of heating rate[18].

3.3.Kinetics of thermal decomposition

In order to obtain activation energy E and pre-exponential factor A,kinetic analysis was carried out on the heating curves at different heating rates with ASTM E698 and Friedman.The data of E and lg A were shown in Table 5.

3.3.1.ASTM method[19–21]

In this approach,pre-exponential factor was evaluated on the assumption of a single step reaction,which was introduced by Kissinger.The standard equation can be shown as follow s:

w here E is the activation energy,A is the pre-exponential factor,R is the universal gas constant,β is the heating rate and Tmaxis the maximum temperature.

Table 5Arrhenius parameters in Friedman analysis and ASTM method

Fig.6.ASTM plot of[PiC3Py][NTf2]2.

Table 6Arrhenius parameters in multivariate non-linear-regression methods

From Table 6,the C1model is the best fit model with the correlation coefficient of 0.9993,and Fexpand Fcritare 1.00 and 1.13,respectively.The mechanism function f(α)is(1 ? α)(1+4.1870α).In addition,the corresponding E and lg A are 153.98 kJ·mol?1and 8.77,respectively.The values of E and lg A are very similar with the calculated values from the Friedman method and ASTM E698 method.activation energy E is obtained(152.53 kJ·mol?1),and the corresponding average logarithmic pre-exponential factor lg A is 8.98.

3.3.2.Friedman method[22–25]

The Friedman analysis follow s the mathematical equation:

w here α stands for a degree of conversion and ?(α)is the function mechanism of the reaction.

The plot of ln[(dα/d t)β]versus1/T gives a group of straight line.The E can be obtained from the slope.

The Friedman analysis as shown in Fig.7 indicates that the average activation energy E is 146.62 kJ·mol?1.The corresponding average logarithmic pre-exponential factor lg A is 8.67.

3.4.Determination of thermal decomposition kinetic model

The probable kinetic model of the thermal degradation process was estimated through the multiple linear regression method.In this study,Netzsch TA4 thermokinetics provides all the commonly used models which were used for the determination of mechanism function[26,27].

On the basis of correlation coefficient and simulates to a single step reaction,the mentioned models are obtained and listed in Table 6.In Table6,the C1model corresponds to the1st-order reaction with autocatalysis and the mechanism function f(α)is(1 ?α)(1+Kcatα),the Fnmodel fi ts to the n th-order reaction and the f(α)in Fnis(1 ? α)n,the F2model is related to the second-order reaction and the f(α)is(1 ?α)2,and the F1model corresponds to the 1st-order reaction and the f(α)is(1?α).

Fig.7.Curves of E and A values in Friedman method.

4.Conclusions

The single crystal structure of[PyC3Pi][NTf2]2was analyzed by X-ray diffraction.The crystal belonged to the triclinic with space group Pī.There were strong hydrogen-bonding interactions in the molecular structure.Besides,thermal stability and thermal decomposition kinetics of the title compound were investigated using TGA to obtain kinetic triplet.Iso-conversional methods show ed that the average activation energy and pre-exponential constant were 149.58 kJ·mol?1and 8.83,respectively.Multivariate non-linear regression methods indicated that the expanded C1model was the best fit model.The kinetic model function was f(α)=(1?α)(1+4.1870α).In addition,the responding activation energy and pre-exponential constant of this reaction were 153.98 kJ·mol?1and 8.77,respectively.And the activation energy value and pre-exponential constant were in good agreement with isoconversion methods.

Nomenclature

A pre-exponential factor,s?1

E activation energy,kJ·mol?1

lg A average logarithmic pre-exponential factor

R the universal gas constant,J·mol?1·K?1

Tmaxthe maximum mass loss rate temperature,K

Tonsetthe onset decomposition temperature,K

α reaction progress

β heating rate,K·min?1