Novel efficient procedure for biodiesel synthesis from waste oils with high acid value using 1-sulfobutyl-3-methylimidazolium hydrosulfate ionic liquid as the catalyst☆

S huanglan Hu ,Ying Li,Wenyong Lou ,3,*

1 School of Biosciences and Bioengineering,South China University of Technology,Guangzhou 510640,China

2 School of Food Sciences and Engineering,South China University of Technology,Guangzhou 510640,China

3 State Key Laboratory of Pulp and Paper Engineering,South China University of Technology,Guangzhou 510640,China

1.Introduction

Biodiesel(long-chain monoalkyl fatty acid esters),as an alternative diesel fuel,is a promising nontoxic,biodegradable,and renewable fuel[1,2],which is prepared from oil plants and bioengineering microalgae and other aquatic plant oils or animal fat or waste cooking oils though the transesterification of triglyceride with methanol or esterification of FFAs with methanol,has been attracted considerable attentions during the past decade[3,4].On the other hand,biodiesel technology developed countries mostly use vegetable oil as the main raw material for the production.Our rape planting area and the total rape output rank first in the world,but the unit area yield is still lower than the average level of the world.Therefore,fully developing and utilizing non-edible oil for biodiesel production has momentous significance on our country as well as can reduce the biodiesel cost[5,6].

Waste oils containing large amount of FFAs can be used as raw materialto prepare biodieselcatalyzed by an acidic catalyst,such as sulfuric acid and benzene sulfonic acid[7].These homogeneous catalysts have suffered from other problems,such as equipment corrosion,complex procedure and pollution.Solid acid catalyst can overcome some of the above mentioned problems,however,low reaction rate and catalytic activity are disadvantages of these catalysts[8].Owing to the detrimental effects of these catalysts,great efforts directed toward the development of environmental friendly catalysts have been made.

Ionic liquid,usually know as room-temperature ionic liquid[9],is a new type of polarsolvent,and has importantattributes,such as high catalytic activity,excellent chemical and thermal stability,design possibilities,potential recoverability and ease of separation of the products from reactants.Therefore,ionic liquid is considered to be the mostpromising green solvent and catalyst in the twenty- first century,which has been applied to biological catalysis,separation science and electrochemical fields[10].With the development of ILs,various types of ILs are prepared and used for esterification or transesterification as reaction media or as catalyst with satisfactory yield[11-16].Hanet al.prepared a novel Br?nsted acidic ionic liquid with an alkane sulfonic acid group and used for the preparation of biodiesel from waste oil,and the achieved yield was as high as 93.5%after a 4 h reaction,moreover his group[17,18]has studied the method for biodiesel preparation using ionic liquid as catalyst in earlier years,and those results indicated that the ionic liquid catalyst exhibited well an acid catalysis property.To date,the use of ILs as catalysts for biodiesel synthesis from waste oil has still remained unexplored largely,with only a few reports[19-22].Our group[23]has screened a new kind of acidic IL([BHSO3MIM][HSO4])as catalyst for the synthesis of methyl oleate from esterification of oleic acid with methanol,displaying the excellent catalytic activities and operational stability.In this present work,such acidic IL[BHSO3MIM][HSO4]as catalyst was explored for biodiesel synthesis from waste oils with high acid value.The effects of varying reaction conditions on the production of biodiesel were investigated.

2.Experimental

2.1.Materials

The raw material used in this work is waste oils with high acid value collected from the restaurant of South China University of Technology(Guangzhou).[BHSO3MIM][HSO4]was purchased from Lanzhou AoKe Chem.Co.,Ltd.(China)and was over98%purity.Methyl oleate(＞99%purity),methyl hexadecanoate(＞99%purity),methyl linoleate(＞99%purity),methyl linolenate(＞99%purity)and methyl stearate(＞99%purity)were purchased from Sigma-Aldrich(USA)and methyl heptadecanoate(＞97%purity)was purchased from TCI(Japan).Oleic acid,potassium hydroxide(KOH)and other chemicals were also from commercial sources and were of the highest purity available.

2.2.Pretreatment of waste oils with high acid value

The waste oils with high acid value were found to be smelly,deep color in appearance and contaminated with water and other impurities.The waste oils with high acid value were pre-treated before the reaction by heating to 60°C,then adding 10%(based on the mass of waste oils with high acid value)active florid in over stirring.The mixture was beaten well for25 min at100 °C-105 °C,then filtered while the solution is hot.Its acid value and saponification value were determined by a standard titrimetry method,and water content was tested by constant weight method.Fatty acid composition of the pretreated waste oils with high acid value was then analyzed by gas chromatography.

2.3.Optimization of[BHSO3MIM][HSO4]-catalyzed conversion of waste oils with high acid value to biodiesel

Waste oils(2.82 g)were added to a 50 mlround bottom flask,followed by adding certain amount of methanol and[BHSO3MIM][HSO4]catalyst.The mixture was heated at a predetermined temperature in oil bath(re flux condensation,magnetic stirring at 500 r·min-1).After the completion of the reaction,the reaction mixture became the biphasic system,and the desired product(biodiesel)stayed mainly in the upper phase.Samples(100 μl)were withdrawn from the upper phase and centrifuged,and then the supernatant(5 μl)was mixed with 200 μl methyl heptadecanoate(internal standard)prior to GC analysis.

2.4.[BHSO3MIM][HSO4]-catalyzed conversion of waste oils with different acid values to biodiesel

Waste oils with different acid values(2.82 g),methanol(0.77 g)and catalysts(0.28 g)were mixed in a 50 ml round bottom flask,and then the mixture waskeptat140°C in oilbath(re flux condensation,magnetic stirring at 500 r·min-1)for 8 h.Aliquot(100 μl)was withdrawn and centrifuged,and then the supernatant(5 μl)was mixed with 200 μl methyl heptadecanoate(internal standard)prior to GC analysis.

2.5.Operational stability of[BHSO3MIM][HSO4]catalyst

Waste oils(2.82 g),methanol(0.77 g)and 0.28 g[BHSO3MIM][HSO4]catalystwere mixed in a 10 mlround bottom flask at140°C for6 h(re flux condensation,magnetic stirring at 500 r·min-1).After the completion of the reaction,the by-product water and excess methanol were removed from the reaction mixture by evaporation,and then the IL catalyst[BHSO3MIM][HSO4]was further separated from the product by centrifugation.After thoroughly washed with n-hexane and air-dried,the obtained IL catalyst was used in the next cycle.The activity of[BHSO3MIM][HSO4]catalyst in the first reaction cycle was defined as the relative activity of 100%.Sample(100 μl)was withdrawn at a specified time from the reaction mixture in each batch and centrifuged,and then the supernatant(5 μl)was mixed with 200 μl methyl heptadecanoate(internal standard)prior to GC analysis.

2.6.GC analysis

The reaction mixtureswere assayed with a Shimadzu GC 2010(Japan)instrument equipped with an HP-5 capillary column(0.53 mm×15 m,USA)and a flame ionization detector.The column temperature was held at 180 °C for 1 min,raised to 186 °C at 0.8 °C·min-1,then kept at 186 °C for 1 min,and further raised to 280 °C at 20 °C·min-1.Nitrogen was used as the carrier gas at a flow rate of 12.5 ml·min-1.The split ratio was 1:25(v/v).The injector and the detector temperatures were set at 250 and 280°C,respectively.The retention times for methyl hexadecanoate,methyl heptadecanoate,methyl oleate and methyl stearate were 3.118,4.488,5.734 and 6.515 min,respectively.The average error for this determination was less than 1%.All reported data were averages of experiments performed at least in duplicate.The yield of methyl oleate was calculated as follows:ME YieldMME,the amount of methyl ester obtained;Moil,the amount of initial waste oils,N,dilution of the sample,Cs,the concentration of internal standard solution,ki,the standard curve slope of fatty acid methyl ester oficomponent,Ai,the peak area of fatty acid methyl ester oficomponent,As,the peak area of internal standard.

3.Results and Discussion

3.1.Detection of physicochemical properties of the pretreated waste oils with high acid value

The major components of fatty acids,shown in Table 1,were oleic acid,linoleic acid,palmitic acid,linolenic acid and stearic acid.The physicochemical properties of the pretreated waste oils with high acid value were determined and shown in Table 2.

Table 1Composition of fatty acid of pretreated waste oils

Table 2Main parameters of pretreated waste oils

3.2.Effect of molar ratio of methanol to waste oils

Both transesterification and esterification are reversible reactions,and excessive methanol content will move the equilibrium of the reactions to the products.It can be seen in Fig.1 that within the range of ratio value tested ME yield increased obviously with an increase of molar ratio and arrived at a maximum value when molar ratio was 8:1.On the other hand,conversion of waste oil slightly decreased when the ratio of methanol to waste oils was raised.This is probably because so much excess methanolwould dilute the ILconcentration mainly dissolved in methanol(upper phase)and thus reduce the possibility of[BHSO3MIM][HSO4]catalyst contacting with the substrates in the oil phase(bottom phase).

3.3.Effect of catalyst amount

The catalyst amount was also of great importance for the reactions.The effect of the mass ratio of catalyst to waste oil on the reaction illustrated that there were not enough active sites for the reaction when the catalyst amount was low(Fig.1).The yield increased with the catalyst amount from 4 wt%to 10 wt%.Therefore,10%was selected as the best catalyst amount considering economic bene fits(Fig.2).

Fig.1.Effect of molar ratio of methanol to waste oils on IL-catalyzed conversion of waste oils with high acid value to biodiesel.Reaction conditions:2.82 g waste oils,0.28 g[BHSO3MIM][HSO4](10%based on the mass of waste oils),120 °C,500 r·min-1.

Fig.2.Effect of catalyst amount on IL-catalyzed conversion of waste oils with high acid value to biodiesel.Reaction conditions:2.82 g waste oils,0.77 g methanol,120 °C,500 r·min-1.

3.4.Effect of reaction time

Fig.3.Time course of IL-catalyzed conversion of waste oils with high acid value to biodiesel.Reaction conditions:2.82 g waste oils,0.77 g methanol,0.28 g[BHSO3MIM][HSO4](10%based on the mass of waste oils),120 °C,500 r·min-1.

The effect of reaction time on[BHSO3MIM][HSO4]catalyzed conversion of waste oilis shown in Fig.3.Initially,an increase of the yield could be observed with the increase of reaction time from 0 to 6 h.Further increase in reaction time showed little improvement of yield suggesting that the reaction has reach equilibrium at 6 h.Hence,6 h was the optimum reaction time for[BHSO3MIM][HSO4]catalyzed conversion of waste oil into biodiesel.

3.5.Effect of reaction temperature

As shown in Fig.4,the catalystactivity was sensitive to temperature.The yield increased sharply when the temperature increased from 60 to 120°C.Further increase of the temperature had no significant effect on the transesterification and esterification reactions.Therefore,we choose 120°C as the optimum reaction temperature compared to higher temperature(140 °C and 160 °C)to save energy.

3.6.[BHSO3MIM][HSO4]-catalyzed conversion of waste oils with different high acid values to biodiesel

Fig.4.Effect of reaction temperature on IL-catalyzed conversion of waste oils with high acid value to biodiesel.Reaction conditions:2.82 g waste oils,0.77 g methanol,0.28 g[BHSO3MIM][HSO4](10%based on the mass of waste oils),500 r·min-1.

Under the optimized reaction conditions,the acid values of the waste oils were varied in a series of separate experiments designed to develop a deeper understanding of the influence of acidity of the waste oil on the performance of[BHSO3MIM][HSO4]catalyst,and the results are shown in Fig.5.The results show that the addition of a small amount of FFAs led to significant increase in initialrate of ME formation.However,further increase in the FFA content from 48%to 72%only led to a slight increase in ME yield and initial reaction rate.It is known to us that the reaction rate of transesterification was much slower than esterification.When the waste oil contained both FFA and triacylglycerol,esterification reaction occurred earlier compared to transesterification reactions.So the greater the acidity of the waste oil is,namely,the higher the FFA content is,the faster of the conversion rate is.As expected,the maximum yield of ME could be observed when the reaction reached the equilibrium after an enough long reaction time for waste oils with different FFA values.With a great increase of reaction time from 8 to 44 h,the obtained yield of ME increased somewhat from around 46%to 54%for the waste oil with a relatively lower FFA content of 10%.Obviously,the reaction rate of ME formation for the waste oil with a higher FFA content significantly surpassed that for the waste oil with a lower FFA content,mainly in that the rate of esterification reaction was faster than that of transesterification reaction.

Fig.5.IL-catalyzed conversion of waste oils with different high acid values to biodiesel.Reaction conditions:2.82 g waste oils,0.77 g methanol,0.28 g[BHSO3MIM][HSO4](10%based on the mass of waste oils),140 °C,500 r·min-1.

3.7.The operational stability of[BHSO3MIM][HSO4]catalyst

It is well known that the stability and reusability of a catalystare two key factors to identify whether it can be applied in industry.The catalyst was recycled five times without the inclusion of any additives,and the results are shown in Fig.6.The results show that the ME yield was stable above 92%,which indicated that the catalyst was stable.Therefore,the result shows that the[BHSO3MIM][HSO4]catalyst has the potential to be used in industrial application.

As shown in Table 3,in comparison with the IL catalysts reported previously,the novel IL catalyst[BHSO3MIM][HSO4]showed high activity toward the esterification of oil acid with methanol,the reaction conditions were relatively moderate.

4.Conclusions

The conversion of waste oils with high acid value to biodiesel was investigated in the presence of water-stable SO3H-functional Br?nsted acidic IL,namely,[BHSO3MIM][HSO4].The performance of the catalyst was evaluated in terms of molar ratio of methanol to waste oil,catalyst amount,reaction time,reaction temperature,recycles and ME yield.The results revealed that under the optimized reaction conditions the yield of the ME would reach up to 94.9%and after reused for five times,the catalyst still retained about 97%of its original catalytic activity.Furthermore,satisfactory yields(80.2%-94.9%)have been obtained for the conversion of waste oils with different acidity under the optimum reaction conditions,which rendered the method as a potential application in the production of biodiesel using waste oils with high acid value as raw material.

Fig.6.Operational stability of the IL catalyst[BHSO3MIM][HSO4].Reaction conditions:2.82 g waste oils,0.77 g methanol,0.28 g[BHSO3MIM][HSO4](10%based on the mass of waste oils),140 °C,6 h,500 r·min-1.

Table 3Results of esterification of different catalysts

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