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Engineering
Procedía
ELSEVIER
Procedía Engineering 42 (2012) 1278 - 1287
www.elsevier.com/locate/procedia
20th International Congress of Chemical and Process Engineering CHISA 2012 25 - 29 August 2012, Prague, Czech Republic
Mechanochemically synthesized CaO-ZnO catalyst for
biodiesel production
Z. Kesic3a*, I. Lukic3, M. Zdujicb, H. Liuc, D. Skala3d
a University ofBelgrade, Faculty of Technology andMetallurgy, Karnegijeva 4, HOOOBelgrade, Serbia; bInstitute ofTechnical Sciences of the Serbian Academy of Sciences and Arts, Knez Mihailova 35, 11000 Belgrade, Serbia;
cChina University of Geosciences, School ofEnvironmental studies, Wuhan 430074, PR China dUniversity ofBelgrade, IChTM Centerfor Catalysis and Chemical Engineering, Njegoseva 12, 11000 Belgrade, Serbia
The transesterification of triglycerides (vegetable oil) with methanol using CaO-ZnO mixed oxides catalyst were conducted to produce FAME (Fatty Acid Methyl Esters, i.e. biodiesel). Catalyst was synthesized by ball milling of CaO and ZnO powder mixture (using molar ratio of Ca and Zn of 1:2 and 1:4) with the addition of water, as well as solely by ball milling (molar ratio of 1:2) of mentioned powders and subsequent. After ball milling prepared mixtures were calcinatied at 700 °C in air atmosphere. The samples of formed catalysts were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy. The particle size distribution as well as the base strength using Hammett indicator was determined, too. CaO.ZnO catalysts were used in the methanolysis of sunflower oil, at 60 °C and 1 bar, using molar ratio of sunflower oil to methanol of 1:10 and with 2 wt% of catalyst based on oil weight. All catalysts exhibited good activity in the methanolysis of sunflower oil, with the yield of FAME above 90 % after 4 hours of reaction.
© 2012 Published by Elsevier Ltd. Selection under responsibility of the Congress Scientific Committee (Petr Kluson)
Keywords-. Biodiesel; heterogeneous catalyst; CaO-ZnO catalyst; ball milling
1. Introduction
a* Corresponding author. Tel.: +381 11 3370 473; fax: +381 11 3370 473. E-mail address', kesiczeljka@gmail.com: skala@tmf.bg.ac.rs.
Abstract
1877-7058 © 2012 Published by Elsevier Ltd. doi : 10. 1016/j .proeng.2012.07.509
Fatty acid methyl esters (biodiesel) are produced by the transesterification of triglycerides, which is one of the main constituents of vegetable oils, and methanol. The homogeneous base catalysts (NaOH and KOH) are generally used for the industrial production ofbiodiesel. However, their utilization in vegetable oils, and moreover in the case of waste vegetable oil transesterification, very often forms soaps as undesirable byproducts, which in turn generates large amounts of wastewater during the separation of the catalyst and formed products [1].
A successful heterogeneous process of biodiesel synthesis will solve most of the economical and environmental drawbacks of a classical and to-day mainly applied in industry homogeneous process. Very important characteristic of the heterogeneous catalyst for potential application in the process of biodiesel synthesis is its leaching in contact with methanol. One of the most widely used catalysts in many recently performed researches is CaO due to its availability, low cost and high catalytic activity for synthesis ofbiodiesel [2]. However, it was proved that leaching of this catalyst was also observed [3]. Thus it is important to improve CaO properties which can be done by using some support material or other metal oxides [4-9].
Several researchers have shown that the combination of CaO with other metal oxides such as ZnO [48] and MgO [9] can provide higher yield of FAME (Table 1). Ngamcharussrivichai et al [5] synthesized Ca/Zn mixed oxide catalyst with different Ca/Zn ratio and reported that amount of active compound (CaO) plays an important role in catalyst activity. Catalyst obtained by coprecipitation of the corresponding mixed metal nitrate solution in the presence of a soluble carbonate salt and calcined at 800 °C showed excellent catalytic activity (the FAME yield > 94%, the methanol/oil ratio 30:1, the amount of CaOZnO catalyst 10 wt % with the Ca/Zn ratio of 0.25, and the reaction time 1 h), but it requires higher amount of catalyst (10%).
Table 1. Literature review of the activity of mixed oxides of Ca and Zn or Mg as heterogeneous catalysts for biodiesel production
Catalyst Oil wt% Reaction conditions Yield, % Ref.
T, °C Molar Time, h
atmosphere ratio
CaOZnO sunflower 1.3 60, air 12:1 2 >90 [4]
CaOZnO palm kernel 10 60, air 30:1 1 >94 [5]
Ca/Zn ratio of 0.25
CaZn2(0H)6-2H20 sunflower 4 60,N2 12:1 0.75 >90 [6]
CaOZnO Jatropha 4 65, air 15:1 6 >80 [7]
curcas
CaOZnO sunflower 2 60, air 10:1 4 97.5 [8]
CaO-MgO Jatropha 3 120, air 25:1 3 90 [9]
curcas oil
In order to study potential catalyst for biodiesel production the mixture of CaO-ZnO was prepared and characterized. It was synthesized by ball milling of CaO and ZnO powder mixture with or without addition of water followed by subsequent calcination at 700 °C in air atmosphere. Such procedure of mechanochemical treatment of different powders is widely used for the activation and synthesis of a broad class of materials due to its relative simplicity-solid phase reactions without usage of solvents. Mechanochemical process is also characterized with reduced energy requirements and much easier workup procedure [10].
Different molar ratio of CaO and ZnO (1:2 and 1:4) with addition of water, as well as CaO and ZnO (molar ratio of 1:2) without addition of water, were used as starting materials for mechanochemical treatment. Mechanochemical treatment of CaO and ZnO with addition of water led to formation of
calcium zinc hydroxide dihydrate [11], while it is not the case when procedure without water was used. After calcination at 700 °C, mixture of CaOZnO was obtained.
2. Materials and methods
Commercial edible sunflower oil (Dijamant, Zrenjanin, Serbia; molecular weight 876.6 g mol"1, acid value of 0.202 mg NaOH g_1) and methanol (99.5% purity, Fluka, Switzerland) were used for experimental studies. Catalysts were synthesized by mechanochemical treatment. CaO (obtained after calcination of Ca(OH)2) and ZnO (Kemika, Zagreb, Croatia) were used for catalyst synthesis. Mechanochemical treatment was carried out in the planetary ball mill Fritsch Pulverisette 5, in air atmosphere. Two zirconia vials of 500 cm3 volume each charged with 500 g zirconia 10 mm diameter balls were used as milling mediums. The balls to powder mass ratio was approx. 30. A powder mixture of CaO and ZnO, with the molar ratio of 1:2 and 1:4 with stoichiometrically required addition of water necessary for formation calcium zinc hydroxide dihydrate (CaZn2(0H)6-2H20) were used as starting materials for mechanochemical treatment.
A mixture of CaO and ZnO powder, with the molar ratio ofl:2 was also used as starting materials for mechanochemical treatment and preparation of mixed oxide of CaO.ZnO. Angular velocity of supported (basic) disc, measured by tachometer, was 250 rpm (26.2 rad s_1).
Prepared catalysts are listed in Table 2 and denoted as follows: CZH for ball-milled with addition of H20 and CZ for ball-milled without addition of H20 (subscript number represents the molar ratio of CaO:ZnO (2 for 1:2 and 4 for 1:4) while after dash line in subscript temperature of calcination, i.e. 700 °C, is also signed).
3. Catalyst characterization
XRD patterns were recorded with Ital Structure APD2000 X-ray diffractometer in Bragg-Brentano geometry using CuKa radiation (k = 1.5418 A) and step-scan mode (range: 10-70° 20, step-time: 0.50 s, step-width: 0.1°).
Thermogravimetric analysis (TGA) were carried out on SDT Q600 instrument in air atmosphere, with the flow rate of 100 mL min1 at the 20 °C min1 heating rate ranging from 25 to 800 °C.
Fourier-transform infrared (FTIR) spectra were recorded using BOMEM (Hartmann & Braun) spectrometer. Measurements were conducted in wave number range of 4000 400 cm-1, with 4 cm-1 resolution.
The particle size distribution was measured by laser particle size analyzer (PSA) on Mastersizer 2000 (Malvern Instruments Ltd, UK), which covers the particle size range of 0.02-2000 ^m.
Hammett indicator experiments were conducted to determine the basic strength of catalysts. The following Hammett indicators were used: phenolphthalein (H- = 9.3), thymolphthalein (H- = 10.0), thymolviolet (H- = 11.0) and 4-nitroaniline (H- = 18.4). Typically, 500 mg of the catalyst was mixed with 1 mL of Hammett indicators solution that was diluted in 20 mL methanol. After 2 hours of equilibration the color of the catalyst was noted. The basic strength of the catalyst was observed to be higher than the weakest indicator that underwent the color change, and lower than the strongest indicator that underwent no color change. To measure the basicity of solid bases, the method ofHammett indicator-benzene carboxylic acid (0.02 mol L_1 anhydrous ethanol solution) titrationwas used.
Synthesized catalysts were tested in the methanolysis of sunflower oil. Experiments were performed in the 300 cm3 batch autoclave (Autoclave Engineers) equipped with a heater and a mixer. Defined amounts of sunflower oil, methanol and catalyst were used for synthesis performed in a batch autoclave. All catalyst samples were tested at 60 °C and 1 bar, with molar ratio of sunflower oil to methanol of 1:10 and with 2 wt % of catalyst (based on oil). The reaction samples were withdrawn periodically. The reaction samples were taken out from the reactor at different reaction times, and after filtration and
separation of the residual methanol using rotational evaporator, analyzed by gas chromatography (Varian 3400) with a FID detector and MET-Biodiesel capillary GC column (14 m x 0.53).
4. Results
4.1.XKD analysis
The X-ray diffraction analysis was conducted to investigate the structure and crystallinity of the catalysts. The XRD pattern of CZ2 showed characteristic peaks of ZnO (JCPDS 36-1451), and none of characteristic peaks of calcium species (CaO) was observed (Fig. 1).
° CaZn2(0H),;2H,0 - ZnO "
1-1-1-1-.-,-r
20 40 60
Fig. 1. XRD patterns of synthesized catalysts : (a) CZ2, (b) CZH2 and (c) CZH4
XRD pattern of the CZH2 resulted in peaks that corresponded mainly to calcium zinc hydroxide hydrate CaZn2(0H)6-2H20 (JCPDS 25-1449). The presence of ZnO peaks in CZH2 indicates that the formation of calcium zinc hydroxide hydrate was not completed. Small peak at 29.4 28 is assigned to CaC03 (JCPDS 5-586), which is obtained by the reaction of CaO with C02 from air at the beginning of the milling process. It can be concluded that mechanochemical reaction of CaO and ZnO powders with added stoichiometrical amount of water yielded a mixture of CaZn2(0H)6-2H20, ZnO and CaC03 phases. Ball milling of CaO and ZnO powders with molar ratio of 1:4 with water addition (CZH4) resulted in weakening of the XRD peaks of calcium zinc hydroxide hydrate and the appearance of more intense peaks of ZnO.
4.2. TGA analysis
The TGA diagram (Fig. 2) of calcium zinc hydroxide hydrate is characterized by two-step decomposition [12]. The first dominant step of weight loss could be observed from 120 to 180 °C, which
may be attributed to the elimination of hydrated water and dehydration of Zn(OH) 2 to form ZnO. In respect to the initial composition of used powders, it should be approximately 23.3%. A second region, from 350 to 400 °C corresponds to the dehydratation of Ca(OH)2 (5.8%) [12]. Finally, above 650 °C and up to 700 °C there is a weight loss which indicates the presence of calcium carbonate.
1-1-1-1-1-1-1
0 300 600 900
Temperature/ °C
Fig. 2. TGA curves of (a) CZ2,(b) CZH„and (c) CZH2
For CZH2 the first weight loss up to 200 °C was 18.9%, which is less than theoretical one and in agreement with XRD results that mechanochemical synthesis of calcium zinc hydroxide dihydrate was not completed, while for CZH4 it was 13.8 wt%. Also, the weight loss of 4.7 % and 6.2% for CZH2 and CZH4, respectively, at 650 °C indicates that during mechanochemical treatment the reaction of C02 and CaO and formation of CaC03 occurred
For CZ2 up to 400 °C, there was weight loss corresponding to H20 release (4.5%). A weightloss of 5 wt% at about 660 °C is attributed to a decomposition of CaC03 to form CaO.
From TGA diagrams (Fig. 2), it was concluded that for mechanochemical treatment with water addition formation of calcium zinc hydroxide hydrate took place, in comparison to sample prepared without addition of water.
4.3. FTIR analysis
FTIR spectra were recorded for CZH2, CZ2 and CZH4 samples and are presented in Figure 3. Characteristic bands for calcium zinc hydroxide hydrate CaZn2(0H)6-2H20 are: two sharp bands at 3615 and 3505 cm-1, assigned to (OH) stretching vibrations, band which is characteristic of the O-H stretching vibration of Ca(OH)2 at 3643 cm-1, bridging OH bending mode is visible at 940 cm-1, the HOH
bending mode of lattice water appears at 1600 cm1, the stretching bands at 3150, 3034 and 2880 cm-1 are attributed to the O-H groups from H20 molecules and the bending vibration of Zn-O-H is noticed at 1070 cm1 [13]. The presence of carbonates was determined for all the samples, which is confirmed by the broad band centered at 1465 cm"1. The fundamental bands of CaC03 can be seen at 1465, 874, 712 cm1 and the band at 2350 cm1 from the vibrations in C02 molecule [14].
-1-1-1-1-1-1-1-1-1
4000 3000 2000 1000 0
Wavenumber/ cm 1
Fig. 3. FTIR spectrum of (a) CZH„, (b) CZH2 and (c) CZ2
4.4. The particle size distribution
The particle size distribution of the prepared catalysts CZH2, CZ2 and CZH4 is shown in Fig. 4a. Difference is notable between the catalyst obtained by ball milling with and without addition of water with the molar ratio of CaO:ZnO of 1:2. The particle size distribution of the CZ2 catalyst is uniform with the size range of 0.2-40 ^m, for the CZH2 bimodal distribution is obtained: a larger fraction of the powder particles is within the size range of 0.2-40 ^m and the rest is within the range of 40-300 ^m, while for CZH4 particle size distribution is between CZ2 and CZH2. After calcination at 700 °C, particle size distribution shifts to smaller values for CZH2.7oo and CZH4-7oo, while for CZ2.7oo remained almost the same (Fig. 4b). Such an effect could be expected because calcination process of CZH2 and CZH4 causes removal of H20 and C02 from the CaZn2(0H)6-2H20 and CaC03, inducing particle crushing and diminution.
Particle size juii Particle size jim
Fig. 4. Particle size distribution of the prepared samples (a) before calcination and (b) after calcination at 700 °C
4.5. The basic strength of prepared catalysts
The basic strength of calcinated catalysts, which were prepared by ball milling is given in Table 2. A higher basicity was found for catalyst prepared with water addition. Better dispersion of CaO on the surface of ZnO could remarkably increase the basicity of the catalyst. Obviously, the basicity of CZH 2-700 is the highest, and it could influence the catalytic activity for the biodiesel synthesis. Namely, the reaction activity mainly depends on the number of basic sites present at the catalyst surface as well as on their strength [15]. The difference in the distribution of basic sites for each catalyst indicates that the basicity and base strength distributions are influenced by the presence of CaO in the CaOZnO mixed oxides.
Table 2. Synthesized catalysts used working condition, molar ratio of CaO:ZnO, calcinations procedure and basic strength
Catalyst Molar ratio of Preparation method Calcination Basic strength Basicity
denotation Ca(OH)2 and temperature, °C (h-) (mmol g"1)
ZnO, medium
CZH2 CaO:ZnO 1:2 ball-milling / / /
+h2o 7h
czh2.7OO CaO:ZnO 1:2 ball-milling 700 11.0-18.4 0.072
+h2o 7h
CZ2 CaO:ZnO 1:2 ball-milling / / /
cz2-7OO CaO:ZnO 1:2 ball-milling 700 10.0-11.0 0.09
CZH„ CaO:ZnO 1:4 ball-milling / / /
+h2o 7h
czh4.700 CaO:ZnO 1:4 ball-milling 700 9.3-10.0 0.38
+h2o 7h
Catalysts prepared with water addition showed high activity after 4h of reaction. Taking both the base strength and the catalytic activity into account, it might be concluded that the observed activities of catalysts seem to be related to their base strengths, i.e. the higher base strengths of the catalysts result in the higher conversions of triglycerides. The CZ2-700 sample possessed the weakest base strength, consequently exhibiting weaker catalytic activity.
4.6Activity of prepared catalyst
Among the catalysts prepared by ball milling, sample CZH2-7oo possessed the highest catalytic activity due to the small particle size and base strength. The catalysts prepared by ball milling with water addition have higher activities and faster reaction rates than that obtained by ball milling without water addition. The particle size effect is attributed to the formation of small CaO particles at the surface of the large ZnO particles, since small particles had the properties of well accessible basic sites. Finally, one can conclude that more accessible basic sites facilitated the transesterification reaction.
The obtained experimental results indicate that the activity of synthesized catalysts increases with addition of water to initial powder mixture, while the reaction rate increases with increasing the CaO to ZnO molar ratio from 1:4 to 1:2.
Time h
Fig. 5. FAME yields for catalysts synthetized by ball milling and calcined at 700 °C, experimental conditions 60 °C and 1 bar, molar ratio of methanol to sunflower oil of 10:1 and with 2 wt% of catalyst
Fig. 6 shows the comparison of the experimental data obtained in this study for catalyst CZH2-7oo with the results published by other authors who have also used CaOZnO as a catalyst in the transesterification reaction. Reaction conditions for the transesterification are shown in Table 1.
Although carbonates formation is reduced when CaO is used as strating material instead of Ca(OH)2 [8], this change of initial compounds used for catalyst preparation didn't have influence to increase the rate of transesterification reaction.
—O— this study
* Z.Kesic
— ■ — J.M.Rubio-Caballero
Y.H.Taufiq-Yap —CNgamcharussiivichai
• A.C.Alba-Rubio
—I—
—I— 150
—I—
—1— 250
—I—
—I— 350
time, min
Fig. 6. Comparison of the experimental data obtained in this study with the results of the studies in which CaO-ZnO has been used as catalyst
Conclusion
Experiments performed in this study showed that CaO-ZnO mixture as a heterogeneous catalyst exhibited good activity in the methanolysis of sunflower oil. Different samples of CaO-ZnO precursor were obtained by ball milling of CaO and ZnO powders with or without the addition of water.
After the calcination at 700 °C, catalyst obtained by ball milling with water addition was more active compared to the catalyst prepared without water addition. The reasons for the different activity of synthesized catalysts could be explained by the difference of their basicity. The highest catalytic activity exhibits the catalyst obtained by mechanochemical treatment of CaO and ZnO powders (molar ratio of CaO:ZnO of 1:2) with added water, and subsequent calcination at 700 °C. Such prepared catalyst gave FAME formation of almost 99 wt% after 3 h reaction of sunflower oil and methanol (1:10 molar ratio) in a batch reactor at 60 °C.
Acknowledgements
Financial support of the Ministry ofEducation and Science of the Republic of Serbia (Grant No. 45001) is gratefully acknowledged.
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