Physicochemical properties of amide–AlCl3 based ionic liquid analogues and their mixtures with copper salt☆

Pengcheng Hu*,Wei Jiang,Lijuan Zhong,Shufeng Zhou

College of Chemical Engineering,HuaQiao University,Xiamen 361021,China

Keywords:Ionic liquid analogues Conductivity Density Viscosity Isobutane solubility

ABSTRACT The physicochemical properties of three different amide–AlCl3 based ionic liquid(IL)analogues and their mixtures with copper salt,such as conductivity,viscosity,density and isobutane solubility were determined over a wide range of temperatures.The effects of amide structure,amide/AlCl3 molar ratio and CuCl modification on these physicochemical properties were investigated.Results showed that the conductivity of amide–AlCl3 based IL analogues was much lower than that of traditional Et3NHCl–AlCl3 IL with same ligand/AlCl3 molar ratio due to incomplete splitting of AlCl3,whereas the density and viscosity of amide–AlCl3 based IL analogues were slightly higher.The viscosity of amide–AlCl3 based IL analogues was closely related to the amide structure,and followed the order of DMA–AlCl3＞ AA–AlCl3＞ NMA–AlCl3 with same amide/AlCl3 molar ratio.Meanwhile,the density of amide–AlCl3 based IL analogues ranked in the following order:AA–AlCl3＞ NMA–AlCl3＞ DMA–AlCl3.Increasing the amide/AlCl3 molar ratio decreased the conductivity and density,while increased the viscosity.The solubility experiment indicated that the isobutane solubility in NMA–AlCl3 was highest than that in two other IL analogues.Under the modification of CuCl,the conductivity,viscosity and density of these IL analogues increased,whereas the isobutane solubility decreased.These results provide the foundation for the development of a suitable IL analogue catalyst for isobutane alkylation.

1.Introduction

Alkylate obtained from isobutane alkylation is an ideal blending component for motor gasoline[1,2].Various ionic liquids(ILs)with more effective and environment-friendly are used as catalysts in isobutane alkylation to replace the harmful concentrated sulfuric acid and hydro fluoric acid[3–5].In recent years,we had developed a new type ofliquid catalyst,namely amide–AlCl3based IL analogues and theirmixtures with copper salt[6],which show good catalytic performance for isobutane alkylation because oftheir tunable Lewis acidity and high catalytic activity and selectivity.Amide–AlCl3-based IL analogues with metal-containing anions and cations were formed by the asymmetric splitting of AlCl3dimer with the assistance of amides.Under the catalysis of these IL analogues,the C8 selectivity,molar ratio of trimethylpentanes to dimethylhexanes and the research octane number of alkylate can reach up to 94.65 wt%,14.98 wt%and 98.40 wt%,respectively[7].

An important attractiveness of amide–AlCl3based IL analogues is that their physicochemical properties can be finely tuned by amide structure and amide/AlCl3molar ratio[7,8],which are similar to traditional imidazolium and pyridinium ILs.Therefore,these IL analogues are also considered better alternatives to traditional ILs because they can overcome the disadvantages of traditional ILs,such as high toxicity,poor biodegradability,and complex preparation[9–11].Many literatures have reported the physicochemical data of different ILs,their change rules and in fluencing factors.For example,Xue et al.[12]and Andresova et al.[13]studied the relationship between alkyl chain branching and physicochemical properties of imidazolium ILs by means of molecular dynamics simulation and experimental method,and found that the viscosity of branched ILs was higher than that of linear ILs,whereas the boiling and melting point of branched ILs were slightly lower.Liu et al.[14]found that the relationship between molar conductivity and dynamic viscosity of[Cnpy][NTf2]could be described by the Walden rule.Okoturo et al.[15]determined that the viscosity of[Bmim]Cl–AlCl3ILs increased with the increased[Bmim]Cl/AlCl3molar ratio and decreased temperature because of the formation of weaker hydrogen bonds.

The attractions foramide–AlCl3based IL analogues mainly atpresent focus on the structural characterization[16–18]and their application in catalytic reactions[19–21].However,the effects of amide structure,amide/AlCl3molar ratio and CuCl modification on the physicochemical properties ofthese ILanaloguesstilllack an in-depth recognition.Determination ofphysicochemicalproperties ofthese IL analogues before and after CuCl modification is essential for understanding the relationships among catalyst structure,physicochemical properties and catalytic performance[22],meanwhile,it also provides the important fundamental engineering data for developing and scaling-up the IL analoguecatalyzed alkylation process[23].

In the current study,three amide–AlCl3based IL analogues were synthesized through a one-step method using acetamide(AA),N-methylacetamide(NMA),and N,N-dimethylacetamide(DMA)as donor molecules.A general reaction for these IL analogue formation was shown in Fig.1.The in fluences of the amide structure,amide/AlCl3molar ratio,CuCl modification and temperature on the physicochemical properties,such as conductivity,viscosity,density and isobutane solubility were investigated in details.

2.Experimental

2.1.Materials

All reagents were used as received without further treatment unless stated otherwise.Acetamide(AA;99.5%),N-methylacetamide(NMA;99.5%),N,N-dimethylacetamide(DMA;99.5%),triethylamine hydrochloride(Et3NHCl;99.5%),anhydrous AlCl3(99.5%),and CuCl(99.5%)were purchased from Aladdin Chemistry Company.Isobutane was obtained froma commercialalkylation unit.Then,isobutane was dried by a molecular sieve to ensure that the water content was less than 10 mg·L-1,measured by a Karl-Fischer moisture meter with the gas sampler.

2.2.Synthesis of IL analogues

Amide–AlCl3based IL analogues with different amide/AlCl3molar ratios were prepared in a glovebox under N2protection.The synthesis of 0.65AA–1.0AlCl3was used to describe the general procedure for the synthesis of the amide–AlCl3-based IL analogues.0.65AA–1.0AlCl3was synthesized in a 250 ml two-necked flask placed in a thermostatic oil bath equipped with a stirrer.Anhydrous AlCl3(26.68 g,0.2 mol)was placed in the flask;then,AA(7.68 g,0.13 mol)was added slowly while stirring for 30 min.The mixture was then heated to 80°C and maintained at that temperature until all solids “dissolved”(approximately 4 h).The method to modifying the structure of these IL analogues with CuCl was similar to the patent application US20040133056A1[24].All amide–AlCl3-based IL analogues were maintained free from moisture.

2.3.Physicochemical properties measurement

2.3.1.Conductivity

The conductivity of amide–AlCl3based IL analogues and their mixtures with CuCl in the temperature range of 288.15 K–353.15 K was carried out with a Mettler-Toledo SevenEasy?conductivity instrument calibrated with standard aqueous KCl solutions.A platinum conductivity probe was placed in a septum-capped dram vial containing 10 mL of the sample.During measurements,the vial was immersed in a thermostatic oil-bath.The oil-bath temperature was kept constant to within±0.05 K.The experimental errors were controlled within ±0.005 mS·cm-1.The conductivity measurement was performed in a glovebox free from the moisture.

2.3.2.Density

The density of amide–AlCl3based IL analogues and their mixtures with CuCl in the temperature range of 288.15 K–353.15 K was determined by using an Anton Paar densitometer(DMA-4200)with a digital oscillating-tube.The temperature in the oscillating tube was monitored with a digital thermometer,in which the temperature could be held constant with±0.05 K.The densitometer was calibrated with deionized water and dry air.The experimental errors were controlled within±0.0001 kg·m-3.The density measurement was performed in a glovebox to prevent moisture contamination.

2.3.3.Viscosity

The dynamic viscosity of amide–AlCl3based IL analogues and their mixtures with CuCl was measured using an Ostwald viscometer in the temperature range of 288.15 K–353.15 K.The viscometer containing about 30 ml of the sample was immersed in a thermostatic oil-bath.The attaining thermal equilibrium time in the viscometer was at least 30 min to get a more accurate and stable sample temperature.Each datumpointofthe viscosity is the average value ofthree measurements.The viscosity measurement was performed in a glovebox to prevent moisture contamination.

2.4.Apparatus for isobutane solubility

The schematic diagram of the absorption apparatus is presented in Fig.2.The isobutane absorption procedure was as follows:the absorption vessel with the added absorbent(approximately 30 g)was placed in a constant-temperature water bath.With opened valves 8 and 9,the absorption and storage vessels were degassed by vacuum pumping.Valves 8 and 9 were closed,and valve 7 was opened;isobutane was transferred into the storage vessel until the pressure reached a certain value(P1).Valve 7 was closed,and valve 9 was adjusted slowly;isobutane was transferred from the storage vessel to the absorption vessel.Valve 9 was closed,and the absorption vessel was stirred to accelerate the absorption.The above-mentioned stepswere repeated continuously until the pressure in the absorption vessel reached a set absorption pressure(P2);meanwhile,the final pressure in the storage vessel(P3)was recorded.All the pressure data were collected continuously by pressure sensors.According to the state equation of ideal gas,the mole amount of absorbed isobutane is as shown in Eq.(1).

Fig.1.General synthetic process of amide–AlCl3 base IL analogues.

Fig.2.Schematic diagram of the experimental apparatus(1—absorption vessel;2—storage vessel;3—isobutane cylinder;4—buffer vessel;5—vacuum pump;6,7,8—needle valves;9,10—ball valves;11,12—pressure sensors;13—pressure gauge).

In Eq.(1),X is the mole amountof absorbed isobutane(mol);P1and P3are the initialand finalpressures(Pa)in the storage vesselbefore and after isobutane absorption;P2is the set absorption pressure(Pa);V1is the volume of the absorption vessel(m3);V2is the volume of the absorbent(m3);V3is the volume of the storage vessel(m3);nLis the mole amount of absorbent(mol);R is the ideal gas constant;and T is the absorption temperature(K).

3.Results and Discussion

3.1.Conductivity

Amide–AlCl3-based IL analogues have a different structure than ILs constituted of only discrete cations and anions[6,16],a large difference in the conductivity will exist between these IL analogues and ILs.Fig.3 showed the conductivity of three different amide–AlCl3based IL analogues and Et3NHCl–AlCl3IL(a traditional chloroaluminate IL)with ligand/AlCl3molar ratio of 0.65.The conductivity of Et3NHCl–AlCl3IL was 6–7 times higher than that of these IL analogues at room temperature while the conductivity gap increased gradually with increasing temperature.These phenomena were attributed to fact that Et3NHCl–AlCl3IL was entirely constituted by ions,while amide–AlCl3based IL analogues were a mixture ofanionic,cationic,and neutralcomplexes in equilibrium[25,26].Therefore,the number of conductive ions in Et3NHCl–AlCl3IL was more than that in amide–AlCl3based IL analogues.Furthermore,the conductivity of these IL analogues could be detected,and followed the change rule of temperature.This phenomenon in turn supported the fact that AlCl3underwent the incomplete asymmetric splitting to form the conductive metal-containing anion and cation in the presence of amides[9,17].

Fig.3.Conductivity of three amide–AlCl3 based IL analogues and Et3NHCl–AlCl3 IL with ligand/AlCl3 molar ratio of 0.65 at different temperatures.

The conductivity of three amide–AlCl3based IL analogues followed the order of NMA–AlCl3＞ DMA–AlCl3＞ AA–AlCl3under the same temperature,as shown in illustration on the top left of Fig.2.Our research group investigated that AA–AlCl3presented mainly in the form of monodentate coordination via the O atom,by contrast,bidentate coordination through both the O and N atoms was dominant in other IL analogues because of the inductive effect of the methyl group substituted on the amide structure[6].Moreover,bidentate coordination was more favorable for asymmetric splitting of AlCl3to generate more conductive ions compared with monodentate coordination.Thus,the conductivity of AA–AlCl3was lowest.More methyl groups on the DMA would lead to a strong steric effect,which did not favor the asymmetric splitting of AlCl3to generate more conductive ions.Therefore,the conductivity of NMA–AlCl3was higher than that of DMA–AlCl3under the same temperature.

The conductivity ofNMA–AlCl3based ILanalogue decreased with increasing NMA/AlCl3molar ratio,as shown in Table 1,because the asymmetric splitting degree of AlCl3decreased with the increase of NMA/AlCl3molar ratio,and more conductive ion transformed into nonconductive molecule.The same phenomena were also observed in DMA–AlCl3and AA–AlCl3,as shown in Tables S1 and S2.The conductivity of NMA–AlCl3had a slightincrease after CuCl modification,as shown in Table S3.CuCl could react with[AlCl4]-to generate[AlCuCl5]-with higher electric mobility[27,28].Meanwhile,CuCl modification also broke out the intrinsic balance between conductive ion and nonconductive molecule,and accelerated the asymmetric splitting of AlCl3generating more conductive ions.

Table 1 Conductivity of NMA–AlCl3 based IL analogue at different NMA/AlCl3 molar ratios

3.2.Density

Fig.4 showed the density of three different amide–AlCl3based IL analogues and Et3NHCl–AlCl3IL with ligand/AlCl3molar ratio of 0.65.The density of these IL analogues was effected by amide structure,and followed the order of AA–AlCl3＞ NMA–AlCl3＞ DMA–AlCl3under the same temperature.This result indicated that the density of amide–AlCl3based IL analogues decreased with increasing number of methyl groups substituted on the N atom in amides,which was similar with the density variation ofimidazolium IL.Forexample,increasing the numberofcarbon atom in the alkylside chains could decrease the density ofimidazolium IL[12,29].Furthermore,the density of three amide–AlCl3based IL analogues was higher than that of Et3NHCl–AlCl3IL,which could be attributed to the fact that Et3NHCl–AlCl3with entirely ion structure had more free volume than amide–AlCl3based IL analogues according to the Furth hole theory[30,31].The densities of three amides itself were 1.1590 g·cm-3(AA)and 0.9571 g·cm-3(NMA),0.9366 g·cm-3(DMA)at 25°C,respectively[32],which was consistent with the density rules of amide–AlCl3based IL analogues.This finding indicated that the density of these IL analogues might be related to that of amide itself.

Fig.4.Density of three amide–AlCl3 based IL analogues and Et3NHCl–AlCl3 IL with ligand/AlCl3 molar ratio of 0.65 at different temperatures.

The effect of NMA/AlCl3molar ratio on the density of NMA–AlCl3based IL analogue was investigated,as shown in Fig.5.The density of NMA–AlCl3decreased with increasing NMA/AlCl3molar ratio under the same temperature.The increase of NMA/AlCl3molar ratio was equivalentto decrease the moles ofAlCl3with high density and increase the moles of NMA with low density,because the density of AlCl3(2.44 g·cm-3,25 °C)was much higher than that of NMA(0.9571 g·cm-3,25 °C)[32].The same phenomena were also observed in DMA–AlCl3and AA–AlCl3,as shown in Figs.S1 and S2.Furthermore,the density of NMA–AlCl3decreased linearly with increasing temperature.As the temperature increased,the vibration of cation and anion accelerated,the Coulomb force weakened and the free volume increased,which resulted in the decreasing density of NMA–AlCl3.

Fig.5.Density of NMA–AlCl3 based IL analogue at different NMA/AlCl3 molar ratios.

Fig.6 showed the density of amide–AlCl3based IL analogues with amide/AlCl3molar ratio of 0.65 before and after CuCl modification.The CuCl modification increased the density of these IL analogues,which was attributed to factthat CuCl coordinated with the anion had a higher density(3.53 g·cm-3,25 °C)[32].

Fig.6.Density of three amide–AlCl3 based IL analogues with amide/AlCl3 molar ratio of 0.65 before and after CuCl modification.

3.3.Viscosity

The viscosity of three different amide–AlCl3based IL analogues and Et3NHCl–AlCl3IL with ligand/AlCl3molar ratio of 0.65 was measured,as shown in Fig.7.The viscosity of these IL analogues was higher than that of Et3NHCl–AlCl3IL under the same temperature,and followed the order of NMA–AlCl3＞ AA–AlCl3＞ DMA–AlCl3.Compared with NMA and AA,the methyl side chain substituted on the N atoms for DMA was most,thus the mobility of DMA–AlCl3was weakest.Moreover,DMA–AlCl3had highest relative molecular mass,the Van der Waals force was also strongest,which resulted in a highest viscosity in DMA–AlCl3.Although no methyl group substituted on the N atoms for AA,the ionicity of AA–AlCl3with monodentate structure was lowest under the same amide/AlCl3molar ratio,thus the flowable “free holes”was least[33,34].This was the main reason for the phenomenon that the viscosity of AA–AlCl3was higher than that of NMA–AlCl3.By contrast,Et3NHCl–AlCl3IL constituted entirely by ions had the highest ionicity(100%),thus the viscosity of Et3NHCl–AlCl3was lowest.

Fig.7.Viscosity ofthree amide–AlCl3 based IL analogues and Et3NHCl–AlCl3 IL with ligand/AlCl3 molar ratio of 0.65.

Table 2 showed the effect of NMA/AlCl3molar ratio on the viscosity of NAM–AlCl3.The viscosity of NMA–AlCl3increased with increasing amide/AlCl3molar ratio under the same temperature.This phenomenon can be attributed to the fact that the bulky[Al2Cl7]-was gradually replaced by compact[AlCl4]-with the increase of NMA/AlCl3molar ratio[9],resulting in an increasing Coulomb force between cation and anion.Another reason was that the asymmetric splitting degree of AlCl3decreased with increasing NMA/AlCl3molar ratio,and the ionicity of NMA–AlCl3decreased,thus the number of flowable free holes decreased.The same phenomena were also observed in DMA–AlCl3and AA–AlCl3,as shown in Tables S4 and S5.

Table 2 Viscosity of NMA–AlCl3 based IL analogue at different NMA/AlCl3 molar ratios

The viscosity of amide–AlCl3based IL analogues with amide/AlCl3molar ratio of 0.65 before and after CuCl modification was investigated,as shown in Fig.8.The viscosity of CuCl modified IL analogues was higher than that of no modified IL analogues under the same temperature.The relative molecular mass of these IL analogues increased after CuCl modification,thus Van der Waals force increased.Moreover,the bulky[Al2Cl7]-was gradually replaced by new compact[AlCuCl5]-after CuCl modification,thus Coulomb force between cation and anion increased.These two factors resulted in an increasing viscosity of amide–AlCl3based IL analogues after CuCl modification.

Fig.8.Viscosity of three amide/AlCl3 based IL analogues with amide/AlCl3 molar ratio of 0.65 before and after CuCl modification.

Fig.9.Isobutane solubility(mol·mol-1)in amide–AlCl3 based IL analogues and Et3NHCl–AlCl3 IL at 15 °C.

3.4.Isobutane solubility

Isobutane solubility is an important factor in fluencing the isobutane alkylation.A high solubility of isobutane favored the hydrogen transfer reaction(main reaction)and inhibited the oligomerization(side reaction)in the phase interface[35,36].Fig.9 showed the isobutane solubility in three differentamide–AlCl3based ILanalogues and Et3NHCl–AlCl3IL with ligand/AlCl3molar ratio of 0.65.The isobutane solubility under the same pressure(absolute pressure)ranked in the following order:Et3NHCl–AlCl3＞ NMA–AlCl3＞ DMA–AlCl3＞ AA–AlCl3when that was calculated by dividing moles of isobutane by moles of IL analogue.Furthermore,the isobutane solubility in amide–AlCl3based IL analogues was decreased with increasing amide/AlCl3molar ratio,whereas the viscosity of these IL analogues was just the opposite.The larger the viscosity was,the slower the mass transfer was,and the lower the isobutane solubility was.

However,when the unitofisobutane solubility wasmol100 g-1,the isobutane solubility in NMA–AlCl3was higher than that in Et3NHCl–AlCl3,as shown in Fig.10.This result was attributed to fact that NMA(74.12 g·mol-1)had a far lower relative molecular mass than Et3NHCl(137.65 g·mol-1).The isobutane solubility in these IL analogues had a slight decrease after CuCl modification,as shown in Table 3.Because CuCl modification increased the viscosity of these IL analogues,thus the number of flowable “free holes”decreased.This result led to the decrease of“physical dissolution”of isobutane in these IL analogues.

Fig.10.Isobutane solubility(mol·(100 g)-1)in amide–AlCl3 based IL analogues and Et3NHCl–AlCl3 IL at 15 °C.

Table 3 Isobutane solubility in amide–AlCl3 based IL analogues before and after CuCl modification

4.Conclusions

In this study,several amide–AlCl3based IL analogues were synthesized through a one-step method using three differentstructure amides as donor molecules.The in fluences of the amide structure,amide/AlCl3molar ratio,and CuCl modification on the physicochemical properties,such as conductivity,viscosity,density and isobutane solubility were investigated for these IL analogues.The results showed that the conductivity of Et3NHCl–AlCl3IL entirely constituted by ions was higher than that of three amide–AlCl3based IL analogues.The amide structure effected the coordination mode of amide and the asymmetric splitting degree of AlCl3,resulting in the conductivity of these IL analogues following the order of NMA–AlCl3＞ DMA–AlCl3＞AA–AlCl3.Moreover,a high amide/AlCl3molar ratio led to the decreasing number of conductive ions in these IL analogues.

The density of amide–AlCl3based IL analogues was related to the number of methyl side chain substituted on the N atom,and followed the order of AA–AlCl3＞ NMA–AlCl3＞ DMA–AlCl3.The density of these IL analogues decreased with increasing amide/AlCl3molarratio because AlCl3had a higher density.The number of free holes were affected by both amide structure and amide/AlCl3molar ratio,which resulted in the phenomena that the viscosity of amide–AlCl3based IL analogues followed the order of NMA–AlCl3＞ AA–AlCl3＞ DMA–AlCl3,together with that increased with increasing amide/AlCl3molar ratio.The isobutane solubility in amide–AlCl3based IL analogues was affected by the mass transfer of isobutane,and ranked in the following order:NMA–AlCl3＞ DMA–AlCl3＞ AA–AlCl3.In addition,the conductivity,viscosity and density of these IL analogues increased after CuCl modi fication,whereas isobutane solubility decreased.

Supplementary Material

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.cjche.2018.06.018.