Biodiesel production from vegetable oil by using modified CaO as solid basic catalysts

Journal of Cleaner Production 42 (2013) 198e203

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Biodiesel production from vegetable oil by using modified CaO as solid basic catalysts Ying Tang a, *, Jingfang Xu a, Jie Zhang a, Yong Lu b a b

College of Chemistry and Chemical Engineering, Xi’an Shiyou university, Second Dianzi Road No. 18, Xi’an, Shaanxi 710065, China Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 August 2011 Received in revised form 27 October 2012 Accepted 1 November 2012 Available online 26 November 2012

A high efficient production of fatty acid methyl ester (FAME) from soybean oil and rapeseed oil was carried out using modified CaO as solid basic catalyst by connecting bromooctane to the surface of CaO chemically in a simple way. It was found that 99.5% yield of the FAME over modified CaO was obtained from soybean oil using 15:1 molar ratio of methanol to oil after 3 h at reaction temperature of 65
C, which is much higher than the yield of 35.4% over commercial CaO at the same reaction conditions. For the transesterification between rapeseed oil and methanol, the reaction time to its highest yield, 99.8%, was shortened to 2.5 h. The physical and chemical properties of catalysts were characterized by using techniques of X-ray diffraction (XRD), scanning electron microscope (SEM), BET surface area measurement (BET), Fourier transform-infrared (FT-IR) spectroscopy and thermogravimeter (TG). The results indicated that well dispersed CaO with relatively small particle sizes and high surface areas were obtained after modification. Furthermore, the thermal stability of modified CaO is improved and the amount of Ca(OH)2 formed during the modifying process is very little. Influence of the amount of modifier and various reaction conditions, such as mass ratio of catalyst to oil, reaction temperature and molar ratio of methanol to oil, were investigated in detail. Furthermore, water-tolerance of the modified CaO was tested by adding water in the reaction system. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Biodiesel Basic catalyst CaO Modification

1. Introduction Insufficient fossil storage makes urgent search for alternate fuels, which have the same properties as diesel, to supply or replace fossil fuel (Ma and Hanna, 1999; Srivastava and Prasad, 2000; Upham et al., 2009). Biodiesel, fatty acid methyl esters of seed oils and fats, has already been found suitable for using as an alternative fuel in diesel engine (Lynd, 1996). Besides, in the view of environmental protection, biodiesel is a cleaner burning fuel for its essentially free of sulfur. The conventional method for biodiesel production is transesterification of vegetable oils or animal fats with short-chain alcohols (generally methanol) in the presence of catalysts (Marchetti et al., 2007; Meher et al., 2006a). Normally, both strong base and strong acid can be used as catalysts for transesterification between triglycerides and short alcohol in a homogeneous system. Liquid base is preferred to liquid acid catalysis like sulfuric or sulphonic acids for its low corrosion to equipment and high reaction activity. However, homogeneous base

* Corresponding author. E-mail address: [email protected] (Y. Tang). 0959-6526/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jclepro.2012.11.001

catalysts offer several process disadvantages including the waste water produced during the product cleaning and no opportunity to operate a continuous process. Therefore many types of solid bases, such as alkali earth oxides and hydroxides, have been reported in the preparation of biodiesel to develop an eco-friendly and efficiently biodiesel produce (Kawashima et al., 2008; Cantrell et al., 2005; Dorado et al., 2004; Ngamcharussrivichai et al., 2008). Among these heterogeneous basic catalysts, calcium oxide (CaO) is a potential one for its low cost, high basic strength (H_ ¼ 26.5) and low methanol solubility (Nair et al., 2012). While the relatively low reaction rate of CaO baffles its wide application. Many works have been devoted to enhance the reaction rate of CaO catalyst. Zhu et al. (2006) obtained a 93% conversion of jatropha curcas oil using ammonium carbonate solution treated CaO. Watkins et al. (2004) promoted the basicity of CaO by doping CaO with lithium. Reddy et al. tested nanocrystalline CaO in biodiesel productiond under room temperature (Reddy et al., 2006), and the results showed that the first three cycles provided >99% conversions, but the conversion decreased in the fourth and fifth cycles obviously. However, the preparations of these catalysts are quite expensive or complicated because the avoidance the rapid hydration and carbonation of CaO during the process of contacting with moisture air or in

Y. Tang et al. / Journal of Cleaner Production 42 (2013) 198e203

199

2. Experimental

temperature range from 25
C to 800
C with a ramping rate of 10
C min1. X-ray diffraction patterns were recorded on a D/Max-3C X-ray powder diffractometer (Rigalcu Co., Japan) using a Cu-Ka source fitted with an Inel CPS 120 hemispherical detector. An FT-IR infrared spectrophotometer (Nicolet 6700, USA) was used to identify the surface group of the CaO. SEM photographs were taken by Quanta 200 scanning electron microscope equipped with an energy dispersive spectrometer (Philips-FEI Co., the Netherlands). The nitrogen adsorptionedesorption isotherms at 77 K of the solid catalysts were measured by the static method in an automatic volumetric Micrometritics ASAP 2010 adsorption analyzer. The leaching of the catalyst into the reaction medium was determined using inductively coupled plasma mass spectrometry (ICP-MS, Thermo Jarrell Asch IRIS Advange 1000) technique.

2.1. Materials

3. Results and discussion

Soybean oil and rapeseed oil were purchased from Oil and Fats Company of Xi’an (Shaanxi, China). The oil was refined by neutralization with Na2CO3 (105 g/kg, Na2CO3/oil) as described (Liu et al., 2008b). Calcium oxide (CaO) was purchased form Keou Fine Chemical Co., Tianjing. Methanol (99.8% purity), bromooctane and heptadecanoic acid methyl ester were obtained from Sigma Chemcial Co.

3.1. Characterization of the catalyst

2.2. Modification of CaO The commercial CaO was modified as follows: 2.1 g of commercial CaO particles without elimination of its hydroxide and carbonates were added into 30 ml bromooctane/hexane solution with bromooctane concentration (weight to CaO) ranged from 5 mg/g to 105 mg/g under stirring at room temperature for activation. After 24 h, the particles were separated and washed with hexane; then, the modified CaO was obtained after a vacuum drying process. 2.3. The procedure for the synthesis of biodiesel The catalytic activities for the transesterification of soybean oil with methanol were measured using typical procedure as follows: a given amount of modified CaO and methanol were placed in a three necked flask equipped with a reflux condenser and a thermometer. Then, the soybean oil was added into the mixture and heated to set temperature. Samples (about 0.05 g) were withdrawn at various intervals, and then the solid catalyst was separated by filtration and excess methanol was distilled off under vacuum. The products were analyzed by the GC (HP-6890) using inner standard method as described by Granados et al. (2007). The yield was defined as a ratio of the weight of fatty acid methyl esters to the weight of fatty acid methyl esters in theory. 2.4. Reusability The reusability of modified CaO was investigated at same react condition by repeating the transesterification reaction several times with used catalysts. Catalysts were separated from the previous reaction mixture by centrifugation, washed with hexane, and then dried at 60
C. The sample was collected and was subjected to GC analysis for the fatty acid methyl ester determination as described earlier. 2.5. Catalyst characterization

The IR spectra of the commercial CaO and modified CaO with 5 mg/g bromoocatane are shown in Fig. 1. The bottom spectrum represents the modified CaO. The spectra bands at 867 cm1 and 1477 cm1 are detected over two catalysts that can be assigned to the vibration modes of mono and bidentate carbonates. Besides, the characteristic absorption of C]O between 2000 and 1500 cm1 indicates the presence of calcium carbonate over these two catalysts. Apparently, the intensity increased after modifying, suggesting the modified CaO still has strong basic property which causes the adsorption of CO2 at room temperature. The bands at 1621 and 3460 cm1 are associated with eOH mode of water physisorbed on the surface of the CaO. The important feature stretching of alkane at the range from 2800 to 3000 cm1 and the bending alkane at 1440 cm1 is observed over modified CaO (Granados et al., 2007). Furthermore, the characteristic stretching of eCeO appeared at the band from 1050 cm1 to 1085 cm1 indicates the modifier connects CaO surface chemically. Fig. 2 shows the XRD pattern of the CaO before and after modifying with 5 mg/g bromoocatane. A series of reflections at 32.1, 37.3 and 53.9 is consistent with X-ray diffractograms of CaO. Minor reflections at 17.9, 28.6, 34.1, 46.9 and 50.7 of commercial CaO are attributed to Ca(OH)2 phases, indicating the hydration of commercial CaO (Ngamcharussrivichai et al., 2007). The peaks at 29.2 and 38.9 are assigned to the reflection of CaCO3 due to the exposure of fresh CaO in air during modification process.

Transmission [%]

reaction system is very difficult. So the deactivation and stability of CaO catalyst are still the unsolved problems for biodiesel production. Recently, the research of accelerating catalytic activity of CaO for biodiesel production was aimed at the activation of CaO by pretreatment with methanol (Kawashima et al., 2009; Liu et al., 2008a) or under supercritical condition (Kiwjaroun et al., 2009), while the activation mechanism was still controversial. The interesting of our research group is to develop a new CaO derivate catalyst for the transesterification between vegetable oil and methanol with excellent performance and good stability in watercontained reaction system. Both the CaO activation condition and various reaction conditions for transesterification were investigated.

4000

3500

3000

2500

2000

1500

1000

-1

TGA experiments were carried out using Q600 SDT thermal analysis machine (TA Instruments, USA) under a flow of air in the

Frequence [cm ] Fig. 1. IR pattern of commercial CaO and modified CaO.

500

200

Y. Tang et al. / Journal of Cleaner Production 42 (2013) 198e203 Table 1 BET surface area, total pore volume and average pore diameter of commercial CaO and modified CaO.

CaO Ca(OH)2 CaCO3

Intensity(a.u.)

Modified CaO

Commercial CaO

15

20

25

30

35

40

45

50

55

60

65

70

75

80

o

2 Theta [ ] Fig. 2. XRD pattern of commercial CaO and modified CaO.

Comparing the main peak area, it can be seen that the diffraction peaks corresponding to the CaO phases of commercial CaO particles are slightly less intense than that of the modified CaO. The high intensity observed in the XRD pattern of modified CaO points to the larger dimensions of the modified crystallite. Scanning microscopy was extensively been used for examination of the morphology of catalyst surfaces. The SEM micrographs of commercial CaO particles and modified CaO with 5 mg/g bromoocatane are shown in Fig. 3. Great difference can be found between these two samples. It can be seen that the surface of commercial CaO particle is built in aggregates, while the modified CaO presents a more exfoliated morphology, which can be regarded as one aspect to its higher catalytic activities. The BET surface area, total pore volume and pore diameter of commercial CaO and modified CaO with 5 mg/g bromoocatane were measured and summarized in Table 1. From the result, it can be seen that the modified CaO processes a higher surface area (68.6 m2/g) as a result of surface exfoliation. On the other hand, large pores (0.03 cm3/g) on modified CaO indicate that there are few pores located between the exfoliated sheets, which is favorable to be used in the liquidesolid heterogeneous phase reactions due to the enough large reaction area in stirred reactor. Fig. 4 shows the TG/DSC thermogram of modified CaO with 5 mg/g bromoocatane. There are two steps in the TG curve in the

Catalyst

Commercial CaO

Modified CaO

BET area (m2/g) Total pore volume (cm3/g) Average pore diameter (nm)

21.1 0.012 1.96

68.6 0.033 1.93

temperature range from 400
C to 700
C due to the loss of H2O and CO2 from hydrated and carbonated CaO. Furthermore, the DSC curve of the modified CaO presents two broad peaks at 450
C and 700
C corresponding to the decomposition of Ca(OH)2 and CaCO3 formed by the hydration and carbonation of CaO in the storage and preparation (Yang et al., 2009), which are lower than those reported (Vujicic et al., 2010) due to the deposition of organic modifier over CaO surface (Ngamcharussrivichai et al., 2008). However, decomposition temperatures of Ca(OH)2 and CaCO3 over modified CaO are still higher than those of commercial CaO (339
C and 614
C), suggesting that the thermal stability of modified CaO is enhanced. Furthermore, from the percentage of weight loss corresponding to the decomposition of Ca(OH)2 in the two samples, 2.305% for commercial CaO and 2.414% for modified CaO, it can be concluded that the amount of Ca(OH)2 formed during modifying procedure is relatively less. 3.2. Catalytic activities test 3.2.1. Transesterification reaction The transesterification process conducted by GC at the catalysis of modified CaO with 5 mg/g bromoocatane were introduced and discussed. Fig. 5 gives the compositions of produced biodiesel, wherein soybean oil was successively converted into diglyceride and monoglyceride, and finally into glycerin and FAMEs as same as reported results (Xie and Li, 2006; Barakos et al., 2008). It can be seen that the obtained biodiesel consisted of six FAMEs: palmitic acid (C16:0), palmitoleic acid (C16:1), stearic acid (C18:0), oleic acid (C18:1), linoleic acid (C18:2), and linolenic acid (C18:3). 3.2.2. Influence of amount of modification reagent The amount of modifier is a key factor to the catalytic activity of modified CaO. In this work, the influence of modifier amount on the catalytic activity was investigated by using the commercial CaO particles treated with different amount of bromooctane from 105 mg/g to 5 mg/g (the mass ratio of modifier to CaO). The transesterification activity results are summarized in Fig. 6.

Fig. 3. SEM photographs of commercial CaO (A) and modified CaO (B).

Y. Tang et al. / Journal of Cleaner Production 42 (2013) 198e203 20

9

90 80

8

7

TG(%)

-20

-30

Weight loss [mg]

-10

6

70

FAME yield [%]

0

DSC [mV]

100

DSC(mV/mg)

10

201

60 50 40

5 mg/g 1 mg/g 0.1 mg/g -3 10 mg/g -5 10 mg/g

30 -40

20 -50 100

200

300

400

500

600

700

5 800

10

o

1

Temperaure [ C]

2

3

4

5

Reaction time [h] Fig. 4. TG/DSC thermogram of modified CaO.

These results show that after 1 h the catalyst that treated with 1 mg/g gives FAME yield of 27.12% while after 5 h the conversion of triglycerides into FAME increases to an optimal value of 99.5%. Further modifier incensement leads to lower yield of FAME because of the occupation of active sites on CaO surface by excessive modifier. 3.2.3. Influence of catalyst concentration The effect of modified CaO concentration on the transesterification was investigated with the concentration varying from 1 wt% to 15 wt% (weight to oil) at 65
C with methanol/oil molar ratio of 30:1. From the results (Fig. 7), it can be found that low concentration of catalyst is insufficient to catalyze the reaction for completion (Eevera et al., 2009), initially increasing the amount of catalyst results in enhancement of the yield. No further enhancement of FAME yield is gained as excessive catalyst is employed. The reason for this decreasing trend is due to the saponification in presence of high amount of catalysts, which increases the viscosity of the reactants and lowers the yield of FAME as suggested by Yang et al. (2007). Therefore the 5 wt% modified CaO is the optimal catalyst amount in this reaction.

Fig. 6. Effect of amount of modifier on the FAME yield from soybean oil. Reaction conditions: temperature 65
C, concentration of modified CaO 5 wt%, methanol/oil molar ratio 30:1.

methanol to oil from 5 to 15, and further increase of methanol amount effect on the yield of FAME slightly, which is due to the dilution effect by too much alcohol, and moreover high alcohol amount slows down the separation of the glycerin phase and the methyl ester phase (Kawashima et al., 2009). Therefore, the optimum ratio of methanol/oil is 15:1. 3.2.5. The comparison of modified CaO and commercial CaO The comparison on the catalytic activities of the modified CaO and commercial CaO was carried out under the optimum reaction condition obtained from above study: 65
C, 5 wt% catalyst of catalyst (weight to oil) and 15:1 molar ratios of methanol/oil. It can be noticed that the difference in FAME yield for these two processed is obvious. Fig. 9 shows that the yield of FAME over the modified catalyst was enhanced near to 99.2% in the presence of the modified CaO, while 35.4% for commercial CaO at the same reaction time when soybean oil was used as resource. Only larger surface area and pores of modified CaO cannot cause the great enhance of

3.2.4. Influence of methanol/oil ratio An excessive alcohol is used in biodiesel production to ensure the oils completely converting to methyl esters in a short time. In our work, the investigation of methanol/oil ratio on the FAME yield (Fig. 8) showed that the yield of FAME increases with the ratio of

100 90

FAME yield [%]

80 70 60 50

1% 3% 5% 10% 15%

40 30

1

2

3

4

5

Reaction time [h]

Fig. 5. GC graph for biodiesel.

Fig. 7. Effect of modified CaO concentration on the FAME yield from soybean oil. Reaction conditions: temperature 65
C, amount of modifier 1 mg/g, methanol/oil molar ratio 30:1.

202

Y. Tang et al. / Journal of Cleaner Production 42 (2013) 198e203 Table 2 Water content test for biodiesel production over modified CaO.

100 90

FAME yield [%]

80 70

Water content (%)

Biodiesel yield (%)

0.5 1.0 2.0 5.0 6.0

98.8 99.3 99.1 91.4 70.8

60 50

5.0:1 10.0:1 15.0:1 30.0:1

40 30 1

2

3

4

5

Reaction time [h] Fig. 8. Effect of molar ratio of methanol to oil on the FAME yield from soybean oil. Reaction conditions: temperature 65
C, amount of modifier 1 mg/g, catalyst concentration 5%.

FAME yield but the hydrophobic alkyls of modifier greatly promote the diffusion of oil to CaO surface and then enhance the internal diffusion. Therefore, the heterogeneity of the three-phase reaction system is weakened. In order to investigate the activity of modified CaO to other kind of oil, rapeseed oil was used for comparison. As same as the activity results for transesterification between soybean oil and methanol, modified CaO has shown promising results in the case of rapeseed oil. The FAME yield from rapeseed oil is little higher than that when using soybean oil during the total reaction time. This should be contributed to the different composition of the two oils. 3.2.6. Water tolerance of modified CaO Previous study for the transesterification between methanol and oil showed that trace water increases the reaction rates for the generation of more methoxide anions (Meher et al., 2006b). However, too much water leads to saponification of FAME and greatly reduces the hydroxyl groups since the hydration of CaO will

occur (Reddy et al., 2006). In our work, the effect of water content on FAME yield over the modified CaO was carried out. From Table 2, it can be seen that the reaction maintains 91.4% yield of biodiesel even 5% water contained in the reaction system, which is much higher than the reported water tolerance limit, 3.2%. The possible reason may be attributed to the hydrophobicity of alkyl groups of modifier, which protect the water molecular away from CaO surface and keep its stability. 3.2.7. Repeated experiments and properties of biodiesel In general, the tests of reusability are important factors to heterogeneous catalysts for industry application. Therefore, unless the stability of catalysts can be improved significantly, none will be suitable for industrial use. The reusable property of commercial CaO and modified CaO was investigated under the optimum reaction condition (Fig. 10). The results showed that, in the commercial CaO catalyzed reactions, the yield maintains on a low level, no more than 56%, and the deactivation occurs after reused in 4 cycle. The FAME yield reaches to 95% over modified CaO and maintains sustained activity even after being used for 15 cycles with slight decrease which is higher than previous report (Granados et al., 2009) and provides the opportunity to operate a continuous process in industrial application. The filtrates of the reaction mixture were measured by ICPeAES to evaluate the leaching of calcium. Less calcium of 26 ppm and 10 ppm was detected after first and second cycles indicating that the dissolve amount of calcium oxide into the reaction medium is insignificant. Furthermore, some properties of prepared biodiesel produced by modified CaO including viscosity, density, flash point and ester content are listed in Table 3. Obtained values over commercial CaO and modified CaO are similar and coherent with EN14214 and the literature values (Demirbas, 2003). However, the time required to

100

100

80

60

FAME yield %

FAME yield [%]

80

40

modified CaO+soybean oil commercial CaO+soybean oil modified CaO+rapeseed oil commercial CaO+rapeseed oil

20

0

60

modified CaO commercial CaO

40

20

0

1

2

3

4

5

Reaction Time [h] Fig. 9. Comparison the catalytic activity over commercial CaO and modified CaO. Temperature: 65
C concentration of modified CaO: 5 wt% molar ratio of methanol to oil: 15:1 Amount of modifier: 1 mg/g.

0

2

4

6

8

10

12

14

16

Number of repetitions Fig. 10. Effect of repeated use on FAME yield over commercial CaO and modified CaO from soybean oil: 65
C temperature, 15:1 molar ratios of methanol to oil, 5 h and 5 wt % catalyst amount.

Y. Tang et al. / Journal of Cleaner Production 42 (2013) 198e203 Table 3 Fuel properties of biodiesel. Properties

Commercial CaO

Modified CaO

EN14214

Relative density, 298 K Viscosity, 313 K (mm2/s) Flash point (K) Ester content (%)

0.89 4.6 435 89.2e99.8

0.89 4.6 437 91.8e99.8

0.86e0.90 3.5e5.0 393 (min) 96.5 (min)

achieve a maximum conversion over modified CaO is greatly reduced from 5 h to 3 h, which indicating that the reaction rate is greatly improved most probably due to the efficient diffusion over surface of modified CaO. 4. Conclusion In this study, an efficient solid basic catalyst for biodiesel production for transesterification of soybean oil with methanol was prepared by modification of commercial CaO particles chemically with bromooctane in a simple way. The physicalechemical characteristics showed that the surface area and thermal stability of modified CaO have been greatly improved without changing its solid phase. The yield of transesterification reaction reached to 99.2% after 3 h when the reaction was catalyzed by the modified CaO under the optimum condition, which is much higher than that over commercial CaO without any pretreatment. This is probably due to the great enhancement of grease diffusion to catalyst surface, which has been changed to be hydrophobic by the modifier. From this view, this novel efficient catalyst has the great potential for practical biodiesel production so much as other liquidesolid heterogeneous base catalytic reactions. Acknowledgments This work was financially supported by grants from, Natural Science Research Plan Projects of Shaanxi Science and Technology Department (No. 2011JQ2014), Scientific Research Plan Projects of Shaanxi Education Department (No. 11JK0591) and PetroChina Innovation Foundation (2012D-5006-0405). References Barakos, N., Pasias, S., Papayannakos, N., 2008. Transesterification of triglycerides in high and low quality oil feeds over an HT2 hydrotalcite catalyst. Bioresour. Technol. 99, 5037e5042. Cantrell, D.G., Gillie, L.J., Lee, A.F., Wilson, K., 2005. Structureereactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis. Appl. Catal. A 287, 183e190. Demirbas, A., 2003. Chemical and fuel properties of seventeen vegetable oils. Energy Sources 25, 721e728.

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