Application of kaolin-based catalysts in biodiesel production via transesterification of vegetable oils in excess methanol

Bioresource Technology 145 (2013) 175–181

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Application of kaolin-based catalysts in biodiesel production via transesterification of vegetable oils in excess methanol Tan Hiep Dang a, Bing-Hung Chen a,⇑, Duu-Jong Lee b a b

Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan

h i g h l i g h t s " Natural kaolin clay was successfully transformed into an effective solid catalyst, zeolite LTA. " Zeolite LTA successfully transesterifies soybean and palm oils in excess methanol for biodiesel. " Conversion efficiency of soybean and palm oils to biodiesel using zeolite LTA can reach 97% and 95%.

a r t i c l e

i n f o

Article history: Available online 13 December 2012 Keywords: Biodiesel Catalyst Kaolin Zeolite LTA Vegetable oil

a b s t r a c t Biodiesel production from transesterification of vegetable oils in excess methanol was performed by using as-prepared catalyst from low-cost kaolin clay. This effective heterogeneous catalyst was successfully prepared from natural kaolin firstly by dehydroxylation at 800 °C for 10 h and, subsequently, by NaOH-activation hydrothermally at 90 °C for 24 h and calcined again at 500 °C for 6 h. The as-obtained catalytic material was characterized with instruments, including FT-IR, XRD, SEM, and porosimeter (BET/BJH analysis). The as-prepared catalyst was advantageous not only for its easy preparation, but also for its cost-efficiency and superior catalysis in transesterification of vegetable oils in excess methanol to produce fatty acid methyl esters (FAMEs). Conversion efficiencies of soybean and palm oils to biodiesel over the as-prepared catalysts reached 97.0 ± 3.0% and 95.4 ± 3.7%, respectively, under optimal conditions. Activation energies of transesterification reactions of soybean and palm oils in excess methanol using these catalysts are 14.09 kJ/mol and 48.87 kJ/mol, respectively. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Heavy use of non-renewable fossil resources, resulting in global environmental impacts and severe climate changes, has been globally concerned in recent decades. Biodiesel, a mixture of long-chain fatty acid methyl esters (FAMEs) or ethyl esters (FAEEs), is mostly produced by either the esterification of free fatty acids or the transesterification of animal fats, vegetable oils, or even waste frying oils, with short-chain alcohols, typically methanol or ethanol, in presence of suitable catalysts (Ma and Hanna, 1999; Sharma et al., 2011; Yin et al., 2012; Zhang et al., 2012, 2003; Kumar et al., 2011; Wang et al., 2012; Leung et al. 2010). Hence, biodiesel has been generally considered as an environmentally friendly alternative energy and one promising remedy to alleviate the aforementioned severe environmental issues stemmed from the overconsumption of unsustainable fossil fuels (Ma and Hanna, 1999; Sharma et al., 2011; Semwal et al., 2011). ⇑ Corresponding author. Tel.: +886 6 275 7575x62695; fax: +886 6 234 4496. E-mail addresses: [email protected], [email protected] (B.-H. Chen). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.12.024

Among several methods for biodiesel production, catalyzed transesterification of triglycerides is of the most popularity, as it possesses many advantages such as higher performance and less facility requirements in biodiesel production, etc. (Ma and Hanna, 1999) Thus, more and more studies on transesterification processes of fats and oils in methanol or ethanol for biodiesel production have been recently conducted. Triglycerides, triesters of saturated/unsaturated monocarboxylic acids with glycerol, are catalytically transesterified with short-chain aliphatic alcohols in excess, usually methanol due to its low-cost (Leung et al., 2010; Zhang et al., 2003). In general, transesterification processes of triglycerides in excess methanol are expressed as follows:

Triglyceride þ 3CH3 OH catalyst

ƒƒƒƒ! Fatty acid methyl esters ðBiodieselÞ þ Glycerol

ð1Þ

Nowadays, researchers have successfully achieved in biodiesel production over various catalysts, including enzymes (Huang et al., 2012), homo-/heterogeneous acid or base catalysts (Zhang

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et al., 2003; Leung et al. 2010), oxides or mixed oxides (Semwal et al., 2011; Yan et al., 2009), or even from modified natural materials (Birla et al., 2012). However, there are still some obstacles to be overcome such as a very low conversion efficiency of oils to biodiesel if enzyme is used as catalysts, as well as significantly additional energy consumption for separation or purification of biodiesel products from by-products if homogeneous catalysts are employed. Hence, heterogeneous solid catalysts have been more and more focused in biodiesel production (Ma and Hanna, 1999; Kim et al., 2008; Sharma et al., 2011). Nevertheless, most solid catalysts still pose some drawbacks. For example, swelling behavior, a longer duration of reaction required, a lower conversion efficiency or costly pre-treatments is often encountered by utilization of ion exchange resins with high cross-linking density and zeolites as heterogeneous catalysts (Kim et al., 2008; Xie et al., 2007). Additionally, using the uneconomical catalysts could lead to an unfavorably increasingly total cost of biodiesel production process. Therefore, it is necessary to find out cost-effective catalytic materials with/without further treatments to facilitate the catalyzed transesterification processes for biodiesel production. Compared with other heterogeneous materials, unmodified/modified natural clays have been proven promising for acid-base-catalyzed biodiesel production via esterification reaction (de Oliveira et al., 2012) or organic syntheses (do Nascimento et al., 2011). Notably, most of the modified ones have received more attention in catalysis than those of naturally occurring counterparts. In our preliminary work, we have found that heterogeneous catalyst prepared from kaolin clay after proper treatment could really render a higher conversion efficiency of triolein in excess methanol to methyl oleate. Coincidentally, this kaolin-based catalyst possesses characteristics of zeolite LTA. In this work, we extend the use of such a solid catalyst prepared from cost-effective natural clay, kaolin, in catalyzed transesterification of vegetable oils for biodiesel production. Performance, characteristics and kinetics of these as-prepared catalysts in transesterification of vegetable oils have been studied to shed light on development of biodiesel production from vegetable oils using these catalysts. 2. Methods 2.1. Materials Kaolin (Al2Si2O5(OH)4) was purchased from J.T. Baker Company. Food grade of soybean oil supplied by Uni-President Enterprises Corp. (Tainan, Taiwan) and palm oil produced by Chang Guann Co. Ltd. (Kaohsiung, Taiwan) were bought in a supermarket in Tainan, Taiwan. Sodium hydroxide, n-hexane, n-heptane were obtained from Fluka-Sigma–Aldrich (St. Louis, MO). Anhydrous methyl alcohol (GC grade) was purchased from Mallinckrodt (Phillipsburg, NJ). All chemicals were of reagent grade and used as received. Deionized water from a Millipore Milli-Q ultrapurification system having resistivity greater than 18.2 MX cm was used in the sample preparation.

Various instruments such as XRD, SEM and FT-IR were applied to evaluate the change in structure and morphology of fresh and used catalysts. XRD was performed on Rigaku Ultima diffractometer with Cu Ka radiation at 40 kV and 20 mA, and recorded in a 2h range of 5–50° with a scanning rate at 4°/min and a scan step of 0.01°/2h. The powder samples were loaded to glass tray/holder of XRD. Afterwards, a piece of microslide was employed to wipe off the excess of the powder. Fourier transform infrared (FT-IR) spectra of catalyst samples diluted in KBr pellets were recorded in the 4000–400 cm1 range with a resolution of 4 cm1 by accumulating 128 scans using a Varian 2000 FT-IR (Scimitar Series) to identify the functional groups on catalysts. Furthermore, the morphology was observed on a ultra-resolution field-emission scanning electron microscope (FESEM) (Hitachi SU8000). The solid-state 27Al and 29Si NMR spectra of the harvested solid samples were performed on a Bruker Avance 400 NMR spectrometer (Rheinstetten, Germany). The chemical shifts were referenced to tetramethylsilane for 29Si and AlCl3(aq,1000ppm) for 27Al. Thermogravimetric analyses (TGA) of the catalysts were also carried out by using a Perkin–Elmer TGA-7 instrument in which the samples were heated with 10 °C/min under a nitrogen flow and with a temperature range of 25–1000 °C. Furthermore, N2 adsorption–desorption isotherms were carried out on a SA 3100 Surface Area and Pore Volume analyzer instrument to determine the Brunauer–Emmett–Teller (BET) surface area and to estimate the pore size distribution using the Barrett– Joyner–Halenda (BJH) procedure. 2.3. Biodiesel synthesis The biodiesel was synthesized via transesterification of soybean/palm oils in excess methanol with a procedure modified from our previous work (Wang et al., 2012). Desired amount of methanol and as-prepared catalyst were separately pre-heated to a proper reaction temperature before being transferred to a vial sealed by a screw-Teflon cap containing 1.5 g soybean or palm oils. The reactor vessel was placed in a temperature controlled water bath on a magnetic controlled stirrer/hot plate (Ika C-Mag HS7 connected with an Ika ETS-D5 temperature controller; Staufen, Germany), which could provide a good temperature control within 1 K. The reacting mixture was continuously transesterified to a preset duration of transesterification reaction under predetermined reaction conditions, including reaction temperatures, feeding ratios of methanol-to-oil and catalyst loadings. A constant stirring rate was maintained at 600 rpm. The reaction was quickly stopped by cooling in icy water after reaching predetermined reaction duration. This reacting mixture was washed with n-hexane to remove any absorbed fatty acid methyl esters (FAMEs) out of the solid catalyst. It was, subsequently, centrifuged at 4000 rpm for 15 min to ensure separation of solid catalyst from liquid layer containing biodiesel and n-hexane. For NMR analysis, the decanted liquid phase was transferred to a rotary vacuum evaporator to remove n-hexane and any byproducts from biodiesel. Notably, transesterification process for biodiesel production was performed in duplicate or triplicate. 2.4. Biodiesel characterization

2.2. Synthesis and characterization of kaolin-based catalyst Calcined kaolin (CK) was obtained by thermal treatment on raw kaolin (RK) at 800 °C for 10 h in air with a heating rate at 5 °C/min. The calcined kaolin is also known as metakaolin, a dehydroxylated form of kaolin. Subsequently, the as-synthesized catalyst, transformed kaolin (TK), was achieved by activating 3.5 g calcined kaolin (CK) with 7.0 g NaOH in 250 mL deionized water hydrothermally at 90 °C, followed by calcination at 500 °C for 6 h.

Biodiesel was analyzed mainly by using gas chromatography with flame ionization detector (GC-FID, Shimadzu GC-2014 with a SGE HT-5 capillary column in 12 m  0.32 mm  0.1 lm). In this case, GC-grade nitrogen was used as carrier gas, while other operating conditions were fulfilled by following procedures described in ASTM D6584-08. Finally, the conversion yield (Y) of vegetable oils to biodiesel was calculated using the following relationship (Kamath et al., 2011; Soetaredjo et al., 2011).

T.H. Dang et al. / Bioresource Technology 145 (2013) 175–181

Yð%Þ ¼

W B  xB  100 WT

ð2Þ

where Y stands for conversion yield, WT for total mass of vegetable oils, WB for mass of biodiesel layer, and xB for weight percentage of FAMEs in the biodiesel layer, respectively. The biodiesel yields obtained from GC-FID analyses were regularly validated by 1H NMR analysis (Wang et al., 2012). 2.5. Kinetics study According to the stoichiometric relationship of the reactants and the products (Eq. (1)), a general equation of reaction rate could be presented as follows (Eq. (3)):



dC TG ¼ k  C aTG  C bMe dt

ð3Þ

where the consumption of triglycerides (TG) per unit time, reaction rate constant, concentrations of triglycerides and methanol (Me) after an interval of reaction time t are  dCdtTG , k, CTG, CMe, respectively. Additionally, the reaction order of triglycerides is a, while that of methanol is b. The concentration changes of reactants are, consequently, described as bellow:

C TG ¼ C TG0 ð1  XÞ

ð4Þ

C Me ¼ C TG0 ðh  3XÞ

ð5Þ

h ¼ C Me0 =C TG0

ð6Þ

where CTG0 and CMe0 are initial concentrations of triglycerides and methanol, respectively, h the ratio of CMe0 to CTG0, and X the conversion of triglycerides to methyl esters. In this work, some assumptions for determination of the reaction kinetics are made. Firstly, only methanol molecule absorbs onto the surface of catalyst. Secondly, the surface reaction is the rate-limiting step (RLS). Moreover, Eley–Rideal mechanism is employed to explain the collision between absorbed molecule and impinging molecule (Yan et al., 2009). Accordingly, the change of catalyst concentration during the process and the reverse reaction could be neglected. Furthermore, the transesterification of vegetable oils is assumed as a three-single-step reaction. Subsequently, the reaction order of triglycerides and methanol are, respectively, a = 1 and b = 3. However, a pseudo-first order reaction could well describe Eq. (8) because an excess amount of methanol over that of triglycerides is used (Birla et al., 2012). Notably, it should be understood that the real reaction order does not change by feeding an excess amount of methanol. Therefore, Eq. (3) could be simplified as follows:



dC TG 0 ¼ k  C TG  C 3Me ¼ k  C TG dt

ð7Þ

where k’ = k(CMe)3  constant, if methanol is used in excess. Integrating Eq. (7) from t = 0 to t = tf and CTG0 to CTG, the conversion of biodiesel could be rearranged as follows: 0

lnð1  XÞ ¼ k  tf

ð8Þ

Moreover, the activation energy (EA) is often expressed by the Arrhenius equation (Eq. (9)):         EA EA EA 0 k ¼ A  exp  or k ¼ A  C 3Me0  exp  ¼ A1  exp  RT RT RT ð9Þ where A is a frequency factor, and T the temperature. Consequently, the activation energy could be obtained from ln(k) vs. the reciprocal of temperature, T1, (Eq. (9)):

lnðkÞ ¼ lnðAÞ 

EA EA 0 or lnðk Þ ¼ lnðA1 Þ  RT RT

177

ð10Þ

Alternatively, the activation energy (EA) can be calculated from the linear relationship of the logarithm of the obtained apparent rate constant k’ vs. the reciprocal of temperature, T1. 3. Results and discussion Transformed kaolin (TK) catalyst could catalyze transesterification of soybean oil and palm oil in excess methanol for biodiesel production, while calcined kaolin (CK) and raw kaolin powder possess negligible catalysis (Supplementary materials). 3.1. Catalyst: preparation and characterization 3.1.1. Fourier transform infrared spectroscopy (FT-IR) FT-IR analyses were carried out in the range of 4000–400 cm1 to investigate the characteristics of surface chemistry, especially functional groups, on raw, calcined and activated kaolin (Table 1). Not only the OH group (ca. 3659 and 3620 cm1), but also other feature of raw kaolin (1114.9 cm1) disappears during calcination at 800 °C for preparation of CK. Once again, significant changes in surface features from CK to TK in the range 1200–400 cm1 were also observed. For instance, the broad shoulder peak of CK near 1076.3 cm1 has been shifted to 997.2 cm1, corresponding to the characteristics of tetrahedral cations linked by bridging oxygen atoms. Notably, the FTIR spectra of transformed kaolin catalyst (TK) before its use in transesterification and TK harvested after the fourth batch cycle of transesterification reaction are not obviously discernible (Supplementary materials). 3.1.2. X-ray diffraction (XRD) The XRD patterns of RK, CK and TK were provided in the Supplementary materials of this article. The XRD characteristics of RK almost disappeared after thermal treatment leading to formation of CK (aka metakaolin). Explicitly, disordered metakaolin was formed in this work after dehydroxylation of kaolin at 800 °C for 10 h. A new broad band with increasing background in 2h range from 20–30°, which is commonly assigned to amorphous phase of SiO2, could be observed on the XRD pattern of CK (Lenarda et al., 2007; Worasith et al., 2011). The remaining peaks in this range could be ascribed to traces of mica and quartz in calcined kaolin (Lenarda et al., 2007; Worasith et al., 2011), and possibly some long-range order due to layer stacking. After NaOH-activation of calcined kaolin (CK), transformed kaolin (TK) with crystallinity is resulted. Coincidently, the XRD characteristics of transformed kaolin catalyst are quite similar to those of zeolite Linde Type A (LTA, dehydrate form), (Treacy and Higgins, 2007; Ríos et al., 2009). Durability of the transformed kaolin catalyst was studied by its repeated usage in batch transesterification reaction. TK recycled and harvested after serving four batch cycles of catalyzed transesterification reaction of vegetable oils in excess methanol still possesses similar XRD patterns to those of fresh TK. Alternatively, crystallinity of TK is retained after four cyclic batch transesterification. 3.1.3. Scanning electron microscopy (SEM) The morphology of RK, CK and TK was examined with SEM (Supplementary materials). The raw kaolin mainly consists of stacking of flaky particles with rough surface. Despite of being calcined at 800 °C, the morphology of CK is insignificantly different from that of RK. Interestingly, after NaOH-activation treatment, flaky CK turned to cubic TK of ca. 2 lm in size. The cubic appearance and dimension of TK catalysts remained relatively unchanged after serving four batch cycles of transesterification. Again, the observed

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Table 1 FT-IR peak assignments for functionality of raw, calcined and transformed kaolin. Materials

Peak (cm1)

Assignment/Descriptions

Raw kaolin (RK)

3620.4 & 3695.6 1114.9 1006.8 & 1033.8 912.3 & 937.4 700.2 & 752.2 536.2 & 472.6 420.5 3700–3000; 1650 1413 & 1450.5

Vibrations of -OH stretching of water Sharp but not intense, indicating not a high crystallinity In-plane Si-O stretching Bending of -OH groups inside and on surface of materials, respectively Stretching vibrations of symmetric Al-O and Si-O, perpendicular Double rings; Al-O-Si and Si-O-Si deformation Bending vibrations of tetrahedral Al-O and Si-O Broad O-H stretching and an adsorption of water Carbonate group (less intense and sharp peak around 1413 cm1, disappearing after calcination at 500 °C) Asymmetric Al-O vibrations More substitution of Al in tetrahedral sites

Calcined kaolin (CK) and transformed kaolin (TK)

1076.3 (CK) shifted to 997.2 (TK) 557.4 & 474.5 of CK shifted to 555.5 & 466.8 of TK, respectively

morphology of TK resembles that of zeolite LTA (Kugbe et al., 2009), consistent with the findings by XRD and FT-IR. Hence, it is rational to conclude that kaolin has been successfully transformed into zeolite LTA after NaOH activation treatment of calcined kaolin. That is, transformed kaolin (TK) catalyst used in this work is zeolite LTA in dehydrated form. 3.1.4. Solid-state nuclear magnetic resonance (solid-state NMR) The 27Al and 29Si solid-state NMR analysis were carried out in raw, calcined and transformed kaolin catalyst. For raw kaolin without any thermal treatment undergone, there is a sharp 27Al NMR peak with a chemical shift at 1.4 ppm, which could be assigned to octahedral aluminum, namely 6-coordinated Al. In contrast, the dehydroxylation of kaolin at an elevated temperature gives rise to significant structural changes, especially in the loss of structure hydroxyl groups close to the octahedral Al–O layer (Mackenzie et al., 1985). Consequently, broad overlapping resonance at 3.7, 27.6 and 57.5 ppm, attributable to 6-, 5-, and 4-coordinated Al local environments, could be observed on the 27Al solid-state NMR spectrum of calcined kaolin (aka metakaolin). Furthermore, broader 27Al NMR peaks, instead of a sharp resonance in raw kaolin, also implied the nature of disordered structure in the calcined kaolin (metakaolin). Activation of calcined kaolin (aka metakaolin) by NaOH has led to formation of ordered structure, as shown with sharp 27Al NMR peaks at 67.2 ppm and 59.8 ppm. Both NMR peaks could be attributed to tetrahedrally bound aluminum in different T-sites in the zeolite LTA framework (van Bokhoven et al., 2000). A sharp intense 29Si nuclear magnetic resonance with a chemical shift at 91.7 ppm was found on raw kaolin, possibly arising from silica in raw kaolin (Mackenzie et al., 1985). The nature of the sharp NMR peak on raw kaolin indicates existence of crystal structure in it. A broad shoulder of the 29Si NMR peak also indicates variations in the Si–O–T bond angles, implying amorphous nature of the calcined kaolin (Mackenzie et al., 1985). The broad 29Si NMR peak centered at 101.4 to 107.5 ppm is not only characteristic of metakaolinite, but also implies formation of an amorphous metakaolin as the calcined kaolin (Mackenzie et al., 1985). The broad 29Si NMR peak is mainly attributed to Si linked to four other Si atoms in silica polymorphs, and the presence of amorphous silica (Mackenzie et al., 1985; Ríos et al., 2009). It is also in good agreement with XRD results about the existence of amorphous Si phase in calcined kaolin. Furthermore, the characteristic NMR peaks of fresh TK have chemical shifts at 77.2, 82.4, 89.1 and 94.6 ppm, which could be, respectively, assigned to Si(4Al) in the chains and rings, Si(3Al) and Si(2Al) units in framework silicates and aluminosilicates of zeolite A (Klinowski, 1988; Colin et al., 1986). Ríos et al. (2009) also observed it. Notably, similar to bifurcation in 27Al NMR peaks on transformed kaolin, i.e. zeolite LTA, the splitting of the 29Si NMR peaks into two main overlapping

but sharp ones may be mainly attributable to Si atoms in different T-sites, namely Si(4Al) and Si(3Al) units, in zeolite framework. 3.1.5. BET Surface area and BJH pore volume Active sites and specific surface area play important roles in catalyst activity. In this case, the specific surface area of the fresh and the recycled as-synthesized TK catalysts were examined. The used TK catalysts were recycled after serving four batch cycles of transesterification reaction. The BET and Langmuir surface areas of the fresh TK catalyst were 6.82 and 9.28 m2/g, respectively, whereas those of the recycled TK catalyst were 4.21 and 5.74 m2/g, respectively. With regard to specific area of zeolitic materials, these values of specific area are relatively lower, compared to those at 7–100 m2/g reported for synthetic zeolites (de Kimpe, 1976). Furthermore, BET surface area analysis was measured ex-situ. Adsorbed moisture to these zeolite LTA samples during transportation could interfere the accuracy of these BET measurements. However, their catalytic performance, to be presented and discussed later, is not derogatory, probably because the transesterification reaction takes place in liquid phase, rather than gas phase. The BJH porosimetric analysis indicates that fresh TK catalyst possess an averaged pore volume of 0.015 mL/g, and an averaged pore diameter of 12.2 nm. Likewise, the BJH pore volume and pore diameter of the recycled TK catalyst were around 0.012 mL/g and 18.8 nm. Accordingly, the size of voluminous triglycerides is estimated at around 3–5 nm, much smaller than pores of the activated catalyst (Granados et al., 2007). Hence, significant diffusion restriction of triglyceride molecules into pores and cages of transformed kaolin could be avoided. Certainly, pores with larger diameters are desirable for better diffusion of reactants/product molecules to the active sites in the pores of zeolites (Sharma et al., 2011). 3.2. Transesterification of vegetable oils using as-prepared catalyst The catalytic performance of raw kaolin in catalyzing transesterification of both soybean oil and palm oil in excess methanol for biodiesel production was far from satisfaction. Respectively, the conversion efficiencies were only around 2.2% and 2.6% after 40 h of transesterification reaction. Hence, it is imperative to work on transformation of raw kaolin to suitable catalysts for transesterification of vegetable oils. As a result, the transformed kaolin catalyst was found to be really able to facilitate the catalyzed transesterification of soybean oil and palm oil in excess methanol. As both soybean oil and palm oils are mixtures of triglycerides, biodiesel made from both oils comprise of mixtures fatty acid methyl esters (FAMEs). For example, methyl oleate is the dominant FAME component in biodiesel made from soybean oil, while other major FAMEs include methyl palmitate, methyl palmitoleate, and methyl myristate. Likewise, the most abundant FAME in biodiesel made from palm oil is

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methyl palmitate (ca. 42.3%), followed by methyl oleate, methyl palmitoleate, methyl stearate, and methyl linoleate. Several factors that could influence the conversion yield of transesterification reaction of triglycerides in excess methanol to fatty acid methyl esters (FAMEs) have been studied, accordingly. According to our preliminary study, the process parameters with the most significance in affecting biodiesel production from catalyzed transesterification of vegetable oils in excess methanol include, in general, (1) the feeding ratio of methanol to oil (methanol/oil, R), (2) reaction temperature (T), (3) reaction time (t) and (4) catalyst loadings (C), while the agitation speed is kept constant at 600 rpm due to its insignificance. In general, a higher reaction temperature, more catalyst loadings, and a longer reaction time would favor transesterification of vegetable oils (Fig. 1). Out of these influential factors, the reaction temperature imparts the most effect on the conversion yield of vegetable oils to biodiesel. Though stoichiometry of transesterification reaction of triglycerides with methanol predicts that one mole of triglyceride molecules require three moles of methanol, methanol in excess could favor more biodiesel produced (Fig. 1a and d). The effect of methanol-to-oil feed ratio on conversion efficiency is more prominent

for palm oil at lower reaction temperatures (Fig. 1d). Especially, palm oil containing more saturated acids is more viscous than soybean oil. Dilution by excess methanol could facilitate transesterification reaction. Fig. 1b and e show the effect of catalyst loadings to the resulted yield of biodiesel. With enough catalyst present in reaction systems, the production yield of biodiesel is not significantly affected by reaction temperature. On the contrary, when reaction temperature is not high enough, for example at 50 °C, the effect of catalyst loading on conversion efficiency is observable especially with less catalyst present (Fig. 1b and e). In general, reaction temperature has landed significant effect on production yield of biodiesel (Fig. 1c and f). Notably, it has to mention that the reaction temperature is always under control, so that the transesterification reactions of both soybean oil and palm oil in presence of TK catalyst proceed only in liquid phase, not in gas phase. In regard of reaction temperature effect, the transesterification reaction could preferentially proceed at a temperature as high as possible, but less than boiling point of reaction system. Consequently, for palm oil, the higher yield near 95% was gained after 2 h of reaction at 63 °C. Approximately 90% of triglycerides in palm

100 80

80

Yield, %

Yield, %

100

60 40

0

5

10

15

20

25

40 63 oC 50 oC

20

63 oC 50 oC

20

60

0 0

30

CH 3OH/Oil, w/w (a) t = 3 h; C=50 wt% based on mass of oil

5

10

15

20

25

30

35

CH 3OH/Oil, w/w (d) t = 4 h; C=5 wt% based on mass of oil

100

105

Yield, %

Yield, %

80 60 40

0

0

10

20

30

40

50

60

70

80

75 60

63 oC 50 oC

20

90

45

90

63 oC 50 oC

0

5 10 15 20 25 30 35 40 45 50 55

Catalyst loadings, wt% (e) R= 25; t = 4 h

Catalyst loadings, wt% (b) R= 20; t = 3 h 105

105

90 75

75

Yield, %

Yield, %

90

60 o

30

0

1

2

3

4

5

6

45 30

63 C 50 oC 40 oC

45

60

63 oC 50 oC

15 7

Time, h (c) R=20; C = 50 wt% based on mass of oil

0

0

1

2

3

4

5

6

7

Time, h (f) R = 10; C = 5 wt% based on mass of oil

Fig. 1. Transesterification of soybean oil (a–c) and palm oil (d–f) in excess methanol using transformed kaolin (TK) catalyst. R () = mass ratio of methanol-to-oil in feed; C (wt.%) = catalyst loading based on mass of oil; t (h) = reaction time.

T.H. Dang et al. / Bioresource Technology 145 (2013) 175–181

oil were eventually converted to biodiesel after 6 h of reaction at 50 °C (Fig. 1f). Therefore, the optimal condition for transesterification reaction of palm oil could be found as follows: (1) a mass ratio of methanol-to-palm oil in feed at 10, (2) at 63 °C for reaction temperature, and (3) with a catalyst dosage at 10 wt.% of palm oils initially present in reaction system. Similarly, for catalyzed transesterification of soybean oil over as-prepared zeolite LTA catalyst, the optimal conditions could be summarized as follows: (1) 20 as the mass ratio of methanol-to-soybean oil in feed, (2) one hour of reaction, and (3) a catalyst loading at 50–55 wt.% based on initial mass of soybean oil (Fig. 1c). Under these optimal conditions, the conversion efficiencies from soybean/palm oils to biodiesel could be obtained as high as 97.0 ± 3.0% and 95.4 ± 3.7%, respectively. 3.3. Kinetics of transesterification reaction

100

60

40

20

0

With the temperature-dependent reaction yields (Fig. 1c and f), the activation energies of transesterification reaction could be deduced. According to results shown in Fig. 1c and f, the rate constant and the activation energy of transesterification reaction in excess methanol of both soybean and palm oils are estimated and tabulated in Table 2. The activation energies for transesterification reaction of soybean oil and palm oils are 14.09 kJ/mol and 48.87 kJ/mol, respectively. The higher EA is the larger energy barrier and the more sensitivity in rate constant with regard to temperature are (Rothenberg, 2008). These values are ca. 1.3– 2.4 times higher than those reported in the literature by using sodium hydroxide as the homogeneous catalyst (Noureddini and Zhu, 1997). The discrepancy in activation energy could be attributable to the use of different kind of catalysts (Yan et al., 2009). Furthermore, the activation energy of transesterification reaction of palm oil higher than that of soybean oil oils could be resulted from more saturated fatty acids in palm oil than soybean oil. It is known that palmitic acid is the most abundant species in palm oils, in contrast to oleic acid in soybean oil. Furthermore, fatty acids with more unsaturated bonds are more prone to thermochemical damage than saturated fatty acids. That is, because of more saturated fatty acids present, palm oil is more chemically and thermally stable than soybean oil. Therefore, transesterification reaction of palm oil required more energy to proceed. Furthermore, the transesterification reaction of palm oil is rather chemically controlled, since their EA value is greater than 20 kJ/mol (Rothenberg, 2008). Finally, the activation energy of the zeolite LTA catalyzed transesterification reaction of soybean oil is much smaller than that reported for transmethylation of soybean oil, 33.6–84 kJ/mol (Freedman et al., 1986). 3.4. Re-utilization of catalysts For the sake of process economics, durability of transformed kaolin catalysts, namely zeolite LTA, was studied. Used catalyst was recycled, rinsed by n-hexane to remove any remaining substances on catalyst, followed by thermal treatment again at 110 °C for one day to get rid of absorbed water on the surface of materials. Finally, it was readily available for next batch cycle of transesterification reaction. Table 2 Apparent rate constant and activation energy at different temperatures. R () = mass ratio of methanol-to-oil in feed; C (wt.%) = catalyst loading based on mass of oil. Oils

Reaction conditions

Temperature, °C

k, min1

EA, kJ/mol

Soybean

R = 20; C = 50 wt.%

R = 10; C = 5 wt.%

0.117 0.278 0.177 3.903 7.886

14.09

Palm

40 50 63 50 63

48.87

Soybean oil Palm oil

80

Yield, %

180

1st

2nd Cycle

3rd

4th

Fig. 2. Durability of transformed kaolin (TK) catalyst on transesterification of soybean oil and palm oil in excess methanol at 60 °C. R () = mass ratio of methanol-to-oil in feed; C (wt.%) = catalyst loading based on mass of oil; t (h) = reaction time; T (°C) = reaction temperature. For transesterification of soybean oil: R = 20; t = 2.5 h; C = 55 wt.%; T = 60 °C. For transesterification of palm oil: R = 10; t = 4 h; C = 10 wt.%; T = 60 °C.

In general, there was not any discernible difference in XRD patterns, morphology checked by SEM micrographs and FT-IR spectra between fresh and recycled TK catalysts, (Supplementary materials). That is, that the structure and morphology of catalyst were still conserved. Fig. 2 showed the conversion yields of vegetable oils to biodiesel at different batch cycles of transesterification reaction. In both cases, performance of catalyst is slightly deteriorated one cycle after one cycle. For instance, the conversion efficiency of soybean oil to biodiesel drops from 97% in the first cycle to about 75% after the 4th cycle, whereas that of palm oil decreases from 95.4% to ca. 80%.

4. Conclusions An effective solid catalyst was successfully synthesized from natural kaolin mineral. Analyses by various modern instruments reveal that transformed kaolin are zeolite Linde type A (zeolite LTA, dehydrate form). The as-prepared zeolite LTA could effectively catalyze transesterification of vegetable oils in excess methanol at a temperature less than 60 °C for biodiesel production. Under optimal conditions, conversion efficiencies as high as 97.0 ± 3.0% and 95.4 ± 3.7%, respectively, from soybean/palm oils to biodiesel could be attained. Moreover, the activation energies of transesterification reaction of soybean oil and palm oil in excess methanol are 14.09 kJ/mol and 48.87 kJ/mol, respectively.

Acknowledgements The work was financially supported by the National Science Council of Taiwan (NSC 100–3113–E–006–016 and NSC1012221-E-006-242). Furthermore, authors would like to thank Ms. Jenny Wu of the NCKU Instrument Center for her assistance in NMR operations.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2012.12. 024.

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