Transesterification of palm oil to biodiesel using Br?nsted acidic ionic liquid as high-efficient and eco-friendly catalyst☆

Yaoyao Feng ,Ting Qiu ,Jinbei Yang ,2,Ling Li,Xiaoda Wang ,*,Hongxing Wang

1 School of Chemical Engineering,Fuzhou University,Fuzhou 350108,China

2 School of Ocean Science and Biochemistry Engineering,Fuqing Branch of Fujian Normal University,Fuzhou 350300,China

1.Introduction

In the wake of increasing energy crisis and climate change issue,it is an urgentneed to develop earth-abundantgreen energy[1–3].The ideal one,biodiesel fuel,has attracted significant attention as a promising alternative fuel due to its remarkable merits,such as sulfur-free,renewable,nontoxic and biodegradable[4].Compared with fossil fuels,biodiesel has been proved to be cleaner and can reduce the emissions,especially ofparticulate matter,greenhouse gases,oxides ofnitrogen and sulfur dioxide[5,6].Transesterification is the most common way to produce biodiesel from triglycerides,where the reaction needs short-chain alcohol(methanol and alcohol)to produce fatty acid methyl esters(FAMEs)[7].Given that the raw material account for almost 75%of total cost in biodiesel production process,the non-food sources should be utilized to decrease the cost in order to achieve the commercialization of biodiesel[8,9].The palm tree,with the name of“the king of world's oil tree”,is the world famous tropical high oilcontained tree.Palm oil,is an agro-industrial inedible and cheap raw materialexacted frompalm fruit,which isthe idealfeedstock forbiodiesel production[10–12].Therefore,the production of biodiesel by using palm oil as feedstock can be worthy and meaningful.

The catalysts are widely used in traditional biodiesel industry synthesis including homogeneous alkali and acid due to its high catalytic activity[13,14].Nevertheless,many inedible biodiesel feedstocks contain plentiful free fatty acids(FFAs)which would obviously lessen the effect of base catalysts due to the saponification reaction[15–17].By contrast,acid catalysts which can tolerate high FFA contents seem to be preferable for preparing biodiesel[13,18].The traditionally acidic catalysts,such as sulfuric acid and p-toluenesulfonic acid,are commonly used as the catalysts for both the transesterification and esterification[19].However,traditional acid catalysts have some disadvantages such as separation difficulty,equipment corrosion and sulfur contamination in biodiesel.Normally,transesterification also can be carried out in the presence of an enzyme-catalyst[20–22].Whereas,the high costand unstable activities are the major drawbacks thatrestrictfurther application of enzyme catalysts for industrial scale.

As one of the most promising catalysts in biodiesel synthesis,ionic liquids(ILs)have attracted tremendous scientific and industrial interests in past decades owing to their virtually negligible vapor pressure,remarkable thermal stability,superior miscibility,and also the acidity and basicity of ILs can be designed or controlled[23–27].According to previous researches,ILs,especially the one with acidity nature,are preferred for biodiesel synthesis[28].The traditional Br?nsted acid ILs based on the imidazole type have shown benign catalytic activity and great water-stable ability[29–31].However,it is a pity that the expensive cost limits the large-scale application for biodiesel production.Therefore,we aimed to synthesize an IL based on cheap materials and use them in the synthesis of biodiesel with high conversion.

Herein,four types of eco-friendly Br?nsted acidic ILs based on quaternary ammonium were synthesized,which possessed cheap cost relative to the imidazolium-based ILs.And then,they were used for the transesterification of palm oil for biodiesel production.A comprehensive investigation of various parameters was accomplished for biodiesel production,and a kinetic study for the conversion process was also studied to determine the kinetic parameters.What's more,we elucidated the reuse performance of ILs,as well as the quality of the re fined biodiesel.

2.Experimental

2.1.Materials

Palm oil as raw materials was purchased from the Hainan,China.Palmitic acid,oleic acid(85%),N,N-dimethylcyclohexylamine(89 CNY·100 ml?1),N,N-dimethylbenzylamine(88 CNY·100 ml?1),N-methyl-imidazole(168 CNY·100 ml?1),1,3-propanesulfonate,p-toluenesulfonic acid,sulfuric acid,ethyl acetate,and methanol were supplied by Aladdin.All chemicals(AR grade)were used without any further purification.Waste cooking oil was obtained from the school restaurant.The oil was filtrated to remove the insoluble impurities and distillated to remove water,followed by decolorization with active carbon.

2.2.Preparation and characterization of catalysts

2.2.1.Catalyst preparation

The four Br?nsted acidic ILs were prepared through a simple twostep method,referring to the previous works of our group[32].The structure ofILs waspresented in Fig.1.The synthesis route ofthe typical[CyN1,1PrSO3H][p-TSA]as shown in Fig.2,and the detailed preparation process are as follows:

1,3-Propanesulfonate(0.1 mol)and ethyl acetate(100 ml)were mixed together in a 250 ml flask with magnetic stirring at an icebath,and then equal-moleN,N-dimethylcyclohexylamine was added dropwise into the flask.After dropping,the solution was slowly heated up to 50°C and stirred for 8 h to form the white solid zwitterion.The zwitterion was separated by filter and washed repeatedly with ethyl acetate.After being dried in vacuum(80 °C,?0.1 MPa),the zwitterion was obtained in good yield(91.0%).Subsequently,the zwitterion was dissolved in deionized water under magnetic stirring and equal molep-toluenesulfonic acid was dropped slowly at cold temperature.The mixture was slowly heated up to 80°C and stirred for 5 h.Then,the water contained in colorless mixture was removed by distillation under vacuum condition at 90°C,and washed with ethyl acetate thrice.Finally,the light yellow viscous liquid[CyN1,1PrSO3H][p-TSA]was dried at 80°C under vacuum overnight,and obtained in good yield(97.0%).

2.2.2.Catalyst characterization

Nuclear magnetic resonance(NMR)spectra were measured using Bruker AV500M spectrometer(Bruker Corporation,Switzerland)in D2O and calibrated with tetramethylsilane(TMS)as the internal reference.Fourier transform infrared(FT-IR)was recorded between 4000 and 400 cm?1by using Nicolet 510P FT-IR(Nicolet Inc.,American)to analyze the structure,by spreading the sample on KBr pellets.Thermal gravimetric analysis was performed on Netzsch STA449F5 instrument(Netzsch Co.,Ltd.,Germany).

Fig.1.Structures of the four Br?nsted acidic ionic liquids.

2.3.Preparation of biodiesel from palm oil and analysis

2.3.1.Properties of palm oil

For determination of acidic value(A)and saponification value(S)of palm oil,the methods described by national standards PRG GB/T 5530-2005 and GB/T 5534-2008 were used.Additionally,the main physicochemical properties of the palm oil were analyzed to determine its quality and the results were listed in Table S1.An Engler glass viscometer was used to determine the viscosity while a 25 ml pycnometer was used to determine the density of palm oil.Product analysis was determined by gas chromatography(GC2014,Shimadzu Corporation,Japan)equipped with a TM-5 capillary column(0.32 mm(ID)× 50 m × 0.5 μm)and a flame ionization detector.Column temperature:started at 150 °C for 1 min,increased to 240 °C at 30°C·min?1and maintained for 2 min,then increased to 280 °C at 5 °C·min?1and maintained for 2 min,and finally increased to 310 °C at 40 °C·min?1and maintained for 8 min;injector temperature:310 °C,and detector temperature:310 °C.

Fig.2.Route of synthesis of[CyN1,1PrSO3H][p-TSA].

The molecular weight(MW)of the palm oil was calculated using Eq.(1).The peaks of the components in the GC were clearly presented in Fig.S1.And the composition of the palm biodiesel was given in Table S2.The highest content(48.14%)is the peak of methyl oleate.

2.3.2.General procedure for biodiesel synthesis

The solubility ofthe ILs in methanol,palm oil,fatty acid methylester,and glycerin was performed before experiment.The results indicated that ILs can only be homogeneous with methanol.The whole reaction was performed in a 50 ml cylindrical stainless steel high-pressure reactor equipped with a temperature sensor and magnetic stirrer,which allows the homogeneity of reactants.In brief,the calculated amount of ILs and methanol was mixed together and added to the palm oil,and then the mixture was heated quickly to the ideal temperature under 1.0 MPa.The optimization study was performed always using 20 g palm oil.The reaction was carried out under various conditions to study the effect of the types of ILs,the amount of catalysts(1%,2%,3%,4%,5%,based on the mass of palm oil),the molar ratio of methanol to oil(12:1,15:1,18:1,21:1,24:1),the reaction temperature(90,100,110,120,130°C),the reaction time(60,90,120,150,180 min)and the agitation speed(0,100,200,300,400,500,600 r·min?1).When the reaction finished,the excess of methanol in reaction solution was removed through vacuum rotary evaporation and then poured into a separating funnel and left standing for 30 min to be separated into two phases.The upper phase was FAMEs(palm biodiesel)with lighter color and analyzed by GC.The lower phase was the mixture of ILs and glycerol.After washing the mixture sequentially with hexane and ethyl acetate three times and then dried in vacuum at 60°C for 180 min,the ILs were separated from glycerol by centrifuging for 30 min and kept to be reused for the next procedure.

2.3.3.Determination of yield

The conversion yield was calculated by measuring the mass of byproduct glycerin.And the glycerin can be quantitatively analyzed by a way of simple and accurate periodic acid oxidation titrate[33].

The total transesterification equation can be described as follows:

The molecular mass of both the synthesized biodieseland palm oil is almost equal according to Eq.(3),which can be illustrated as:

Therefore,the yield of biodiesel was calculated through the following equation:where,η is the yield of biodiesel;Wbis the sample mass of biodiesel,g;Wtis the sample mass of palm oil,g;Wgis the mass of glycerin andMgis the molecular mass of glycerin,92.09 g·mol?1.

3.Results and Discussion

3.1.Characterization of the acidic ionic liquid

The1H and13C NMR spectral data for four ionic liquids were as follows:

(A)[PhCH2N1,1PrSO3H][p-TSA]:1H NMR (500 MHz,D2O):δ 7.41–7.31(m,2H),7.25–7.12(m,5H),6.90(d,J=7.9 Hz,2H),4.04(s,2H),3.09–3.00(m,2H),2.65(d,J=7.3 Hz,2H),2.63(s,6H),2.02–1.90(m,5H).13C NMR(126 MHz,D2O):δ 141.93,140.00,132.73,130.74,129.32,129.13,126.70,125.31,67.96,62.26,49.39,47.31,20.45,18.21.

(B)[PhCH2N1,1PrSO3H][HSO4]:1H NMR(500 MHz,D2O):δ 7.3–7.27(m,1H),7.26(d,J=1.5 Hz,2H),7.25(s,2H),4.18(s,2H),3.17–3.11(m,2H),2.74(s,6H),2.69(t,J=7.2 Hz,2H),2.06–1.98(m,2H).13C NMR(126 MHz,D2O):δ 132.74,130.69,129.08,126.77,67.95,62.11,49.46,47.24,18.12.

(C)[CyN1,1PrSO3H][p-TSA]:1H NMR(500 MHz,D2O):δ 7.58(d,J=

8.2 Hz,1H),7.25(d,J=8.2 Hz,1H),3.32–3.25(m,2H),3.18(tt,J=11.8,3.0 Hz,1H),2.84(s,6H),2.82(d,J=7.2 Hz,2H),2.27(s,2H),2.10–2.02(m,2H),1.98(d,J=13.0 Hz,3H),1.79(d,J=13.5 Hz,2H),1.52(d,J=13.1 Hz,1H),1.37–1.28(m,2H),1.25–1.16(m,2H),1.06–0.94(m,1H).13C NMR(126 MHz,D2O):δ 176.43,142.26,139.99,129.47,125.43,72.42,60.66,47.87,47.40,25.51,24.89,24.26,20.52,20.44,17.89.(D)[CyN1,1PrSO3H][HSO4]:1H NMR(500 MHz,D2O):δ 3.32–3.26(m,2H),3.22–3.15(m,1H),2.85(s,6H),2.79(t,J=7.2 Hz,2H),2.07–1.96(m,4H),1.77(d,J=13.5 Hz,2H),1.49(d,J=13.2 Hz,1H),1.40–1.28(m,2H),1.26–1.14(m,2H),1.04–0.92(m,1H).13C NMR(126 MHz,D2O):δ 72.42,60.63,47.89,47.37,25.51,24.88,24.25,17.85.

The FT-IR spectral data of four designed ionic liquids and the recovered[CyN1,1PrSO3H][p-TSA](G)were as follows,[CyN1,1PrSO3H][p-TSA]and recovered catalyst was showed in Fig.3.

Fig.3.FT-IR spectra of(a)[CyN1,1PrSO3H][p-TSA]and(b)recovered[CyN1,1PrSO3H][p-TSA].

(B)[PhCH2N1,1PrSO3H][HSO4]:IR(cm?1):3449,1703,1476,1230,1165,1031,880,733,576.

(C)[CyN1,1PrSO3H][p-TSA]:IR(cm?1):3452,2946,2863,1704,1454,1223,1009,852,727,576.

(D)[CyN1,1PrSO3H][HSO4]:IR(cm?1):3449,1703,1476,1230,1165,1031,880,733,576.

Fig.3a showed featured bands for organic group,in which the C--H and CH2bend stretching vibrations of cyclohexylamine were observed at 2946 and 1454–1704 cm?1.2863 and 1454 cm?1were from the C--H stretching vibration of CH2and C--N stretching vibration of side chain,respectively.Beyond that,the featured bands at 1223 and 1009 cm?1were assigned to the asymmetric stretching vibrations of S=O,which were attributed to the presence of sulfonic groups.Among other ILs,the same characteristic peaks can be found in the corresponding FT-IR spectra.

The results of NMR and FT-IR indicated that the ILs were consistent with their designed structures(Fig.1).The purity peak of1H NMR spectra indicated the high purity of ILs.It also can be concluded that the synthesis and purification methods were practicable.

TG–DTG analysis was performed to investigate the thermal-stability of the four ionic liquids.When temperature increased to 292°C,the initial mass loss of 8.23%of[CyN1,1PrSO3H][p-TSA]was mainly attributed to the evaporation of physical absorbed water.When the temperature ranged between 292 and 360°C,the significant weight loss of 32.99%of[CyN1,1PrSO3H][p-TSA]was mainly due to the organic structure decompose.This phenomenon revealed benign thermostability of[CyN1,1PrSO3H][p-TSA],which were attributed to the stable structural composition ofthe IL.Among other ILs,the same regularthermostability can be found.Therefore,the Br?nsted acidic ILs we synthesized were suitable as catalysts even under high temperature.

3.2.Performance of IL catalysts

TheN-methyl-imidazole based ILs have been investigated by our group and exhibited benign catalytic activity in the transesterification reaction[29].Therefore,the catalytic activity of four ILs in the transesterification of palm oil with methanol was tested in comparison to[Ps-mim][p-TSA](E),[Ps-mim][HSO4](F)and concentrated sulfuric acidic(H).The reaction was conducted at 110°C for 90 min using 20 g palm oil,10.6 g methanol(molar ratio of methanol to oil=15:1),0.6 g IL catalyst(3 wt%based on the mass of palm oil)and 400 r·min?1agitation speed.

As shown in Table 1,the catalytic activity of various catalysts for the transesterification of palm oil increased in the following order:G＞C＞E＞D＞A＞F＞B＞H.Compared with the imidazolium-based ILs,the four ILs gave a higher or considerable yield of biodiesel for transesterification.The catalytic performance of the ILs containing anion with different acidity of the conjugate acid([HSO4]?,[p-TSA]?)was compared to choose the better performing anion.These results indicated that for both,imidazolium and ammonium ILs,the ILs containing[p-TSA]?anion showed the better catalytic activity,gaining the higher biodiesel yield.The IL anion plays a key role in the Br?nsted acidic nature of the ILs;the more acidic the conjugate acid of the anion,the stronger the IL's acidity[34–36].However,this does not necessarily result in a higher catalytic activity because the cation also has an important effect on the acidity of IL.The ILs with the cation ofN,N-dimethylcyclohexylamine showed superior efficiency,while the anions of the ILs were the same.It can be observed from Table 1 that the IL[CyN1,1PrSO3H][p-TSA]showed higher conversion,and 83.51%conversion of palm oil was achieved at mild conditions.What's more,the price of ionic liquid with theN,N-dimethylcyclohexylamine-cation was cheaper thanN-methyl-imidazole and the preparation process of the[CyN1,1PrSO3H]+based ILs was easy and obtained high yield.Despite the 85.15%conversion obtained with conventional catalyst H2SO4,equipment corrosion and unrecyclable nature limit the use of concentrated sulfuric acidic.Hence,in this study,[CyN1,1PrSO3H][p-TSA]was selected as the best catalyst for the transesterification of palm oil with methanol and was used in the following experiments to optimize various reaction parameters.

Table 1Catalytic activity of ILs on the transesterification of palm oil with methanol①

3.3.Optimization study of different parameters

3.3.1.Effect of molar ratio of methanol to oil

It is known to us that excess methanol is necessary to shift the reaction equilibrium toward the side of the product because the reaction is reversible.As shown in Fig.4a,the biodiesel yield increased gradually from 63.8%to 75.4%by increasing the mole ratio of methanol to oil from 15:1 to 21:1.Further increase in the mole ratio did not result in an increase in the yield,probably because the excess methanol might dilute the mixture of palm oil and IL[34,37].What's more,the larger amount of methanol would complicate the separation process between methanol and IL.Therefore,the lightly increment in conversion beyond the mole ratio of 21:1 was ignorable and 21:1 was selected as the optimal mole ratio of methanol to oil.

3.3.2.Effect of reaction temperature

Temperature performance on production of palm biodiesel was showed in Fig.4b.Only 39.4%of the palm oil was converted into biodiesel when the reaction temperature was 90°C.And the yield of biodiesel increased sharply with a rise in temperature,reaching 98.2%at 110°C.As is known to us,the miscibility of the reactant and molecular collision were bene fited from the increasing reaction temperature.Furthermore,the increasing temperature would reduce the viscosity ofoil,which was favorable to lessen the mass transfer limit.However,further increase in temperature had little effect on the yield of biodiesel probably because of the fast evaporation of methanol.So,the optimum reaction temperature was 110°C.

3.3.3.Effect of catalyst dosage

The main factor,namely the amount of catalyst,is likely to have an important in fluence on the conversion of the palm oil.As shown in Fig.4c,the biodiesel yield increased quickly from 0 to 98.2%with increasing the amount of catalyst from 0 to 3.0 wt%,which was mostly due to the more acidity site exposure to reactants.However,when the amount of catalyst exceeded 3.0 wt%,only a lightly increase can be observed for the conversion of biodiesel(98.9%).Hence,in view of the reaction rate and the economic aspects,the optimum amount of catalysts was taken to be 3.0 wt%.

3.3.4.Effect of reaction time

The effect of reaction time on the transesterification of palm oil was investigated.Fig.4d showed that the yield of biodiesel was enhanced as the reaction time increased from 30 to 180 min.At first,the reaction happened rapidly and 98.2%of biodiesel production was obtained within 120 min.Then,the reaction moved closer to equilibrium after 120 min,and the yield of biodiesel increased slightly when the reaction time continued to extend.Therefore,the preferred reaction time was 120 min,taking into consideration the reaction efficiency and energy consumption.

Fig.4.Effect of various reaction parameters on the yield of biodiesel:(a)molar ratio of methanol to oil(3.0 wt%[CyN1,1PrSO3H][p-TSA],100 °C,400 r·min?1,and 120 min),(b)reaction temperature(3.0 wt%[CyN1,1PrSO3H][p-TSA],molarratio of21:1,400 r·min?1,and 120 min),(c)catalystdosage(molarratio of21:1,110 °C,400 r·min?1,and 120 min),(d)reaction time(3.0 wt%[CyN1,1PrSO3H][p-TSA],molar ratio of 21:1,110 °C,and 400 r·min?1),and(e)agitation speed(3.0 wt%[CyN1,1PrSO3H][p-TSA],molar ratio of 21:1,110 °C,and 120 min).

3.3.5.Effect of agitation speed

Agitation speed is also an important factor that affects the contact area in homogeneous reaction.The effect of agitation speed on the yield of biodiesel was investigated in the agitation speed range of 0–600 r·min?1.As shown in Fig.4e,the yield of biodiesel increased from 40.3%to 85.2%as the agitation speed increased from 0 to 100 r·min?1.The results suggested thatthe reaction only can be carried out at the oil–methanol interface when the agitation speed was 0 r·min?1.When the agitation speed was slow,it was not enough to provide the homogenization of the reactants.The high yield 98.2%was obtained at agitation speed 400 r·min?1.Further increase in agitation speed led to mild increase of the yield of biodiesel.Thus,considering the energy consumption,the optimal agitation speed was 400 r·min?1.

3.3.6.Orthogonal experiment

A standard three-level,four factor(L9(34))orthogonal array matrix was designed with four major in fluencing factors[38]and presented in Table 2.The factors(molar ratio of methanol to oil(factor A),the amount of catalyst(factor B),reaction time(factor C),and reaction temperature(factor D))were chosen on the basis of the results of a single factor experiment for further optimization.

The importance levels of different factors on the yield of biodiesel were illustrated by range analysis.Ki,jwas the summation of optimizing index which agrees with No.jlist factor andilevel;ki,jwas the average value ofKi,j.Rjwas the range of No.jlist factor and difference value of the maximum and minimum values of levels of No.jlist factor.The largervalue ofRjled to a biggerin fluence ofNo.jlistfactoron optimizing index.The results were listed in Table 3.Based on the result of range analysis,the importance sequence offactorsin the yield ofbiodiesel increased in the following order:reaction temperature＞reaction time＞molar ratio of methanol to oil＞catalyst dosage.The resultindicated that the reaction temperature,has prominent effect on the yield ofbiodieselfor the transesterification ofpalm oiland the optimum condition is A3B2C3D3.In consequence,the highest biodiesel yield of 98.4%was obtained at the following optimum conditions:catalyst dosage,3.0 wt%;the initial molar ratio of methanol to oil,24:1;reaction temperature,120°C;reaction time,150 min;and agitation speed,400 r·min?1.

Table 2Factors and levels in the orthogonal experiment design for optimizing the synthesis of biodiesel

Table 3Arrangement and experimental results of the orthogonal experiment design for optimizing the synthesis of biodiesel

3.4.Kinetic study

In order to investigated the dependence of reaction rate on temperature,the transesterification of palm oil with methanol on the kinetic study was carried out with the different temperatures(i.e.T=90,95,100,105,110°C).According to the previous study,it was proved that acid catalyzed transesterification process was a first order reaction and the expression for methanol can be treated as a constant because of the excess methanol concentration(the molar ratio of methanol to oil is 21:1)[39–42].The reaction rate for palm oil transesterification can be simplified to describe as in the following Eq.(5):

whereXis the yield of biodiesel at timet(min),andk′is the reaction rate constant.

After obtaining the results of biodiesel yield at different temperatures,reaction rate constant for relative temperature was decided by plotting graph of ln(1?X)versustime and the corresponding plot was shown in Fig.S2.Meanwhile,the calculated reaction rate constants and each coefficient of determination(R2)were displayed in Table S3.The reaction rate constant increased gradually with a rise in reaction temperature,while the minimum and maximum reaction rate constants appeared at 90 °C and 110 °C,respectively.Moreover,the assumption that the transesterification process followed first-order kinetics was demonstrated by the highR2.

The relationship between reaction rate constantand activation energy(Ea)was illustrated by Arrhenius equation:

whereRis the ideal gas constant andAis the pre-exponential factor.

Hence,the linearequation ofln(k)versus1/T(K)plotas presented in Fig.S3,was used for the determination of the value of activation energy(Ea)and pre-exponential factor(A).As a result of calculation,EaandAare 122.93 kJ·mol?1and 1.83 × 1015,respectively.

3.5.Comparison of catalytic activity over various catalysts for(trans)esteri fication

Catalytic results of both transesterification and esterification over various acid catalysts were shown in Table 4.Previously,[BHSO3MIM][HSO4]exhibited benign conversion(97.7%)for esterification reaction of oleic acid and methanol,but a high catalyst amount of 10 wt%was required[43].[mimC4SO3H][SO3CF3]presented good catalytic activity(90%)for production of biodiesel from sewage sludge lipids,which expanded the application of Br?nsted acidic ILs[34].What's more,SO3H-functional acidic ionic liquid was used for the synthesis of biodiesel from waste oils.It was found that the biodiesel yield could reach at 95%at a high reaction temperature of 170°C[44].Despite the 95.1%conversion obtained by using Br?nsted acidic ILs based on 1-benzyl-1H-benzimidazole as catalysts at 60°C,it was not easy to prepare the catalyst[45].While[CyN1,1PrSO3H][p-TSA]based on cheap cation exhibited higher catalytic activity for production biodiesel under the condition of low catalyst dosage and short reaction time.

To expanded the catalytic performance of[CyN1,1PrSO3H][p-TSA]for different feedstocks,the conversion of high acid value feedstocks(such asoleic acid,palmitic acid,and waste oils)wasalso investigated,and the results were showed in Table 5.Clearly,[CyN1,1PrSO3H][p-TSA]was capable of catalyzing the esterification of oleic acid and palmitic acid with the yield of 97.1%and 96.8%,respectively.As is known to us,non-edible oils,such as waste oils,were more suitable for biodieselproduction due to their low price.While[CyN1,1PrSO3H][p-TSA]achieved the yield of 95.9%for catalyzing the waste oils to biodiesel under mild conditions.Our investigation showed that[CyN1,1PrSO3H][p-TSA]could be a good candidate for both transesterification and esterification because of its high catalytic activity for various feedstocks under mild conditions.

Table 5Conversion of different feedstocks to biodiesel with[CyN1,1PrSO3H][p-TSA]

3.6.Catalyst recovery

One of the reasons that identify ILs as environmental-friendly catalysts is their recyclable nature.In order to investigated the stability and reusability of[CyN1,1PrSO3H][p-TSA],seven runs of experiments were performed under optimum conditions.After each run,the catalyst was separated from the biodiesel product by washing,drying and centrifuging as described in the experimental Section 2.3.2.

As shown in Fig.5,[CyN1,1PrSO3H][p-TSA]could be reused after seven runs and the yield of biodiesel was slightly decreased( first run=98.4%,seventh run=86.1%).Additionally,the FT-IR spectrum of the recovered ILs(Fig.3b)was matched with the fresh ILs,which could be due to the benign stability of the IL.The catalyst activity of ILs decreased marginally which may be due to the weight loss of ILs during the recycling process.Therefore,the result indicated that the[CyN1,1PrSO3H][p-TSA]as catalyst for transesterification was thermally stable and recyclable.

3.7.Quality of the re fined product

When the reaction finished,the biodiesel product was collected and washed with hot deionized water repeatedly to remove the impurities.After being dried in vacuum for 120 min,we can get the final re fined biodiesel.The main physicochemical properties of the re fined biodiesel product were investigated carefully to evaluate its quality(Table 6).The results indicated that the measured values of the re fined biodiesel product met the requirement of biodiesel standard EN 14214.There,it can be certified that the biodiesel prepared by palm oil was of good quality and palm oil can be developed as a feedstock for biodiesel production.

Table 4Comparison of biodiesel conversion with reported catalyst

Fig.5.Reusability of[CyN1,1PrSO3H][p-TSA]catalyst under optimal conditions.

Table 6Quality of the re fined biodiesel

4.Conclusions

In this paper,four eco-friendly Br?nsted acidic ILs were synthesized and used as catalysts,presenting the advantages of simple preparation process,remarkable catalytic activity,low cost,thermal stability and steady recyclability.[CyN1,1PrSO3H][p-TSA]was demonstrated to be a more active catalystfor the transesterification ofpalmoiland methanol.The highestyield of 98.4%has been obtained under the optimum conditions.In addition,[CyN1,1PrSO3H][p-TSA]exhibited good potential for transforming several high acid value feedstocks into biodiesel.The refined palm biodiesel matched well the biodiesel standard EN 14214.Furthermore,the activation energy was obtained from the kinetic study and displayed promising result for the improvement of efficient and environmental-friendly technology for industrial scale.

Supplementary Material

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.cjche.2017.06.027.

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