Article pubs.acs.org/IECR
Transesterification of Triolein to Biodiesel Using Sodium-Loaded Catalysts Prepared from Zeolites Yu-Yuan Wang,† Tán̂ Hiêp Đăng,† Bing-Hung Chen,*,† and Duu-Jong Lee‡ †
Department of Chemical Engineering, National Cheng Kung University, 1 University Road, Tainan 70101, Taiwan Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 10607, Taiwan
‡
ABSTRACT: Transesterification of triolein in excess methanol over sodium-loaded catalysts prepared from Na-ion exchange with zeolites at a relatively lower reaction temperature, namely, at 60 °C, is studied in this work. Two zeolites, i.e., as-synthesized Zeolite MCM-22 and commercial Zeolite HY (CBV-780), are employed in this study. Although both zeolites show to some extent the capability of catalyzing transesterification of triolein in methanol to biodiesel, the resulting yields are only ca. 90% for CBV-780 after a 40-h reaction and 16.3% for MCM-22 after a 90-h reaction. In contrast, the conversion yields are much improved with Na-ion exchange to the surface of zeolite catalysts. For example, the yields of triolein to biodiesel have reached 98% and 99% within a 5.5-h reaction, respectively, using the NaOH-treated CBV-780 and MCM-22 catalysts, even if these NaOH-treated catalysts become amorphous and have suffered a loss of the Brunauer−Emmett−Teller surface area. Furthermore, no saponification was observed using these NaOH-treated catalysts.
1. INTRODUCTION Nowadays, there is an increasing worldwide concern for the conservation of nonrenewable natural resources and for environmental protection. Therefore, it is necessary to develop an alternative and environmentally friendly energy resource to reduce the overconsumption of nonrenewable fossil fuel.1,2 Biodiesel, a mixture of long-chain fatty acid methyl esters (FAMEs), is regarded as one kind of promising renewable energy resources. It is produced by either esterification3−5 of free fatty acids or transesterification6,7 of animal fats, vegetable oils, or even waste frying oil, with short-chain alcohols.8 There are several approaches to convert them to biodiesel, such as pyrolysis,9,10 microemulsion,10,11 or transesterification.3,5,8,10 In comparison with the methods mentioned above, the transesterification process has been shown to possess some advantages, such as higher performance in biodiesel production, less facility requirements in biodiesel production plants, etc.3,5,8,10 Therefore, researches on transesterification reactions of oils for biodiesel production have become more and more popular recently. Transesterification is, in general, a reaction of triglycerides, either esters of saturated or unsaturated monocarboxylic acids with glycerol, with short-chain alcohols, commonly methanol6 because of its relatively lower price.12 The general equation of this reaction is expressed as follows:
and give a better conversion efficiency of oil to biodiesel. Moreover, both homogeneous6 and heterogeneous2−4 catalysts, in acid or base form,2 can catalyze the transesterification reaction. Nevertheless, solid catalysts are preferably used to avoid some disadvantages associated with the use of homogeneous ones, such as a difficulty to separate or purify products, consuming more energy to remove produced water from the reaction mixture.2 Hence, more works with the application of heterogeneous catalysts have been reported recently.1,2,10,11 However, heterogeneous catalysts still have some obstacles to overcome in application. For example, lower yields or longer reaction times are often encountered by using heterogeneous catalysts like ion-exchange resins with high cross-linking density.14 Moreover, most ion-exchange resins are usually of poor thermal stability and swelling behavior.15 Compared with other solid acid catalysts, zeolites with/without any surface treatment are promising for acid−base-catalyzed transesterification reactions because of the diversity in their structures, desired pore sizes and Si-to-Al ratios, and ion-exchange capacity.15−17 Zeolites are microporous aluminosilicate minerals. There are several types.18 However, most of the synthetic ones have received more attention in catalysis than those of naturally occurring forms because of their various advantages, such as better controllability in an undesired impurity phase and chemical composition as well as possession of optimal properties for catalytic applications, all leading to their better catalysis.18 Although they often function as active catalysts, they
triglyceride + alcohol catalyst
HooooooI glycerol + fatty ester (biodiesel)
(1)
From a theoretical point of view, 1 mol of triglycerides reacts with 3 mol of alcohol. However, an excess amount of alcohol is usually employed to drive the reaction equilibrium to the product side and, thus, to speed up the transesterification reaction in favor of biodiesel production.13 Additionally, catalysts are often introduced to increase the reaction rate © 2012 American Chemical Society
Special Issue: APCChE 2012 Received: Revised: Accepted: Published: 9959
December 1, 2011 April 12, 2012 April 16, 2012 April 16, 2012 dx.doi.org/10.1021/ie202782q | Ind. Eng. Chem. Res. 2012, 51, 9959−9965
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h, and then heated at 2 K/min to 500 °C for calcination for 8 h. These NaOH-treated catalysts are referred to as CBV-780Na, MCM-22Na, and a-SiAlNa, respectively, in this report. 2.4. Transesterification Reaction (Biodiesel Production). Triglycerides are a family of chemical compounds formed from three molecules of fatty acids esterified on a molecule of glycerin. If all of these three fatty acids are oleic acids, it is called triolein. Triolein is commonly one of the most abundant triglycerides in various edible vegetable oils, like olive oil, canola oil, and almond oil. Therefore, triolein is selected as the model vegetable oil in this work. Transesterification of triolein in excess methanol was carried out in a three-necked, 250-mL flask provided with a thermometer, a reflux condenser, a mechanical stirrer, and a drop funnel. The three-necked batch reactor was placed in a constant-temperature oil bath over a magnetic stirrer/hot plate (Ika C-Mag HS7 connected with an Ika ETS-D5 temperature controller; Staufen, Germany), which could provide good temperature control within 0.5 °C. The reaction temperature was always maintained constant at 60 °C. Predetermined amounts of catalysts, expressed as a weight percentage of triolein, and methanol were preheated to 60 °C in the reactor prior to the introduction of triolein. In brief, transesterification of triolein is carried out at (1) methanol/triolein = 15 by weight in an initial feed, (2) catalyst/triolein = 0.33, 1, or 1.7 by weight, (3) reaction temperature at 60 °C, and (4) constant stirring at 700 rpm. After reaching a preset reaction time, the reaction mixture was quickly filtered to separate the solid catalyst. Methanol was then removed by a rotary evaporator (model R-2000, Panchun Scientific, Taiwan) under reduced pressure at 30 °C. Biodiesel was obtained by decantation from coexisting glycerin after the methanol-free product was allowed to set for 24 h to phaseseparate in a separatory funnel. Transesterification experiments were conducted in duplicate or triplicate to ensure good empirical reproducibility. In general, the standard deviation of the conversion yield (%) of triolein is estimated empirically from 0.7% to 2.5% for a CBV-780Na catalyst and at about 1.0% for MCM-22Na. 2.5. Characterization of Biodiesel. The conversion of triolein to biodiesel was mainly calculated from 1H NMR measurements on a mixed biodiesel obtained from duplicate or triplicate experiments, although gas chromatography with flame ionization detection (GC-FID) was also employed to monitor provisional batch-to-batch variation in triolein conversion to biodiesel. The gas chromatograph equipped with a flame ionization detector (Shimadzu GC-2400) and a DB-5 separation column with nitrogen at 30 mL/min as the carrier gas was applied to quantify methyl fatty esters (biodiesel), according to the procedures described in ASTM D6584-08. All 1H NMR experiments were performed at 299 K on a Bruker Avance 500 spectrometer at a proton resonance frequency of 500.13 MHz. NMR spectra were recorded in chloroform-d. Assignments of the chemical shifts of protons for glycerides and FAMEs could be found elsewhere.22−24 In general, the chemical shifts of protons could be found at about 3.66 ppm for methoxy groups in FAMEs and 2.35 ppm for αcarbonyl methylene groups present in all fatty esters and triglycerides.22−24 Furthermore, glyceryl protons on triglycerides are known to have chemical shifts near 4.1−4.4 ppm. Hence, the presence of chemical shifts in this range indicates the existence of unreacted triolein. In brief, the conversion of triolein to biodiesel (Y) could be estimated from the integral
still suffer with some downsides as effective catalysts in biodiesel production from transesterification of oils of either animal or vegetable origins, e.g., higher reaction temperature,15−17 longer reaction times,15 costly or complicated processes to prepare them,19 and lower conversion efficiency.17 It is, therefore, imperative to develop zeolites made from costeffective methods and with better catalysis and higher conversion efficiency to lower the production cost of biodiesel. To overcome these drawbacks, synthesis of Zeolite MCM-22 from economical materials was attempted in this work, along with the development of proper surface modification procedures on a commercial zeolite and as-synthesized MCM-22. Furthermore, their catalytic capability of biodiesel production via transesterification of vegetable/microalgal oils was also investigated.
2. EXPERIMENTAL SECTION 2.1. Materials. Triolein, hexane, sodium hydroxide, a ammonium hydroxide solution, hexamethyleneimine (HMI), AS-40 colloidal silica, and a silica−alumina catalyst support (grade 135) were obtained from Fluka-Sigma-Aldrich (St. Louis, MO). Anhydrous methanol (HPLC grade) was purchased from Mallinckrodt (Phillipsburg, NJ). Sodium aluminate (50% Al2O3 and 50% Na2O) was supplied by Alfa Aesar (Heysham, Lancashire, U.K.). Zeolite HY (CBV 780) was bought from Zeolyst (Conshohocken, PA) in powder form (1−2 μm diameter crystals). All chemicals were of reagent grade and were used as received. Deionized water from a Millipore Milli-Q ultrapurification system having resistivity greater than 18.2 MΩ cm was used in the sample preparation. 2.2. Synthesis of Zeolite MCM-22. Zeolite MCM-22 was prepared by the procedure modified from that originally published in the literature.20,21 Sodium hydroxide (1.6 g) and sodium aluminate (1 g) were dissolved in enough deionized water at room temperature to ensure their complete dissolution. HMI (9 g) and AS-40 colloidal silica (30 g) were, subsequently, added to the solution that was previously heated to 50 °C with continuous stirring at 300 rpm. The resulting slurry was transferred to a Teflon-lined autoclave made of SUS 316 (Parr Instruments, Moline, IL), aged for 24 h at 50 °C, and allowed to proceed with further reaction at 150 °C for 5 days. Afterward, the autoclave was cooled to room temperature to harvest the product by vacuum filtration. The pasty product was washed thoroughly with 2 L of deionized water. Further, this pasty product, namely, the as-synthesized Zeolite MCM-22 sample, was calcined under dry air with a heating rate of 3 K/min to 600 °C for 8 h and then treated twice with a 2 M NH4OH solution in excess under mechanical stirring at 80 °C for 48 h. Finally, Zeolite MCM-22 was obtained by repeating the calcination process again in dry air at 500 °C for 8 h. 2.3. Surface Modification of the Catalysts. Sodium was doped to the surface of zeolites and a silica−alumina catalyst support (labeled as a-SiAl) by an ion-exchange method. NaOH solutions in two different concentrations were prepared: 6 g/L for CBV-780 and MCM-22 and 3.5 mg/L for a-SiAl. These catalyst supports were soaked in NaOH solutions for 24 h under mechanical stirring. Noticeably, the loadings of NaOH solutions to catalyst support differ and are 5 mL of NaOH/g of CBV-780, 15 mL of NaOH/g of MCM-22, and 10 mL of NaOH/1 g of a-SiAl, respectively. Subsequently, the NaOHtreated catalyst samples were rinsed with deionized water (twice with 20× catalyst mass), dried in an oven at 120 °C for 1 9960
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Figure 1. SEM micrographs of CBV-780 (Zeolite HY) and as-synthesized Zeolite MCM-22, (a) CBV-780 (Zeolite HY) and (b) CBV-780Na, (c) assynthesized Zeolite MCM-22 and (d) MCM-22Na, and (e) amorphous silica−alumina catalyst support (a-SiAl) and (f) a-SiAlNa.
radiation (40 kV and 20 mA) filtered by a monochromator. The morphology of the catalysts was observed by using a Jeol JSM-6700F scanning electron microscope. The BET surface area analysis and the Barrett−Joyner− Halenda (BJH) pore-size distribution and volume analysis on the catalysts were carried out with a Micromeritics ASAP 2000 gas porosimeter using N2 at 77 K as the adsorbate. The catalyst components were examined with an inductively coupled plasma atomic emission spectrometer (Varian Vista Pro). The solid samples of catalysts have to be dissolved completely in nitric acid or even in aqua regia and, subsequently, diluted in deionized water prior to ICP-AES analysis. The content of leached sodium from the catalyst in deionized water and methanol was measured with an atomic absorption spectrometer (model SensAA, GBC Scientific Equipment, Braeside, Victoria, Australia).
area of methoxy protons (A3.66) and protons on α-carbonyl methylene groups (A2.35), respectively, expressed as follows: production yield of biodiesel (FAME), Y (%) A /3 = 3.66 × 100 A 2.35 /2 2A3.66 = × 100 3A 2.35
(2)
2.6. Characterization of Catalysts. The catalysts were mainly examined by using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer−Emmett− Teller (BET) surface area and porosimetry analysis, inductively coupled plasma atomic emission spectroscopy (ICP-AES), and atomic absorption spectroscopy (AAS). Identification of the catalysts was performed with XRD analysis (Rigaku D/Max-2000/PC) with a high-linearity scintillation counter over a range of diffraction angle (θ) from 2θ = 2° to 50° at a 2θ rate of 2°/min with Cu Kα 9961
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Table 1. Metal Contents and BET Surface Areasa of NaOH-Treated Catalysts before/after the Transesterification Reaction CBV-780Na Na (wt %) Al (wt %) Si (wt %) Si/Al (by mole) BET surface area (m2/g) a
MCM-22Na
before rxn
after rxn
1.64 1.15 41.0 34.37 91.183
1.24 1.01 42.24 40.2 79.495
before rxn 4.74 2.65
104.274
a-SiAlNa after rxn
before rxn
after rxn
3.02 2.67 38.84 13.98 44.101
2.72 5.53 32.84 5.71 373.089
2.63 5.22 30.95 5.70 373.594
The BET surface areas of CBV-780, MCM-22, and a-SiAl are 622.285, 307.176, and 464.524 m2/g, respectively.
3. RESULTS AND DISCUSSION 3.1. Characterization of As-Synthesized Zeolite MCM22 and Other Catalysts. Zeolites have been widely used as catalysts in different applications, for example, fluid catalytic cracking of heavy petroleum distillates, octane number enhancement of light gasoline by isomerization, etc.18 However, not until very recently have zeolites been attempted in biodiesel production from either esterification of fatty acids or transesterification of oils, commonly appearing as triglycerides, diglycerides, and monoglycerides, with short-chain alcohols, like methanol.16 One disadvantage in the application of zeolite to catalyze the transesterification process of triglycerides in alcohols is reflected in the need of an elevated reaction temperature, for example, 200−476 °C reported for Zeolite HY to produce biodiesel from used vegetable oil with methanol.18 Therefore, we have undertaken the proper procedures to modify the surface property of synthetic zeolites in order to render them with better catalysis in transesterification of triolein for biodiesel production. The morphology of zeolites and catalyst support before and after NaOH ion-exchange treatment are shown in Figure 1. Two zeolites and one catalyst support are used in this work. They are CBV-780, namely, Zeolite HY (Figure 1a), in-houseprepared Zeolite MCM-22 (Figure 1c), and an amorphous SiO2−Al2O3 catalyst support, labeled as a-SiAl (Figure 1e). Both CBV-780 and a-SiAl are cubelike particulates with dimensions of less than 1 μm, while flakelike Zeolite MCM22 has an average size of 1−2 μm. All of them are microporous and have large BET surface areas (Table 1). For example, the BET surface areas of CBV-780, MCM-22, and a-SiAl are 622.285, 307.176, and 464.524 m2/g, respectively. The BJH measurements also indicate that their pore sizes are mainly 6, 4.5, and 5 Å, respectively, for CBV-780, MCM-22, and a-SiAl. Moreover, the as-synthesized Zeolite MCM-22 has a very sharp pore-size distribution from BJH measurements, compared to the other two. The structures of these catalysts and supports were examined with powder XRD. Figures 2 and3 show the XRD patterns of CBV-780 and Zeolite MCM-22 prepared in-house. Clearly, the commercial CBV-780 sample matches the characteristics of ultrastable Zeolite HY in dehydrated and dealuminated form (Figure 2).25 Likewise, most of the characteristic peaks shown in Figure 3 could be attributable to the calcined MCM-22, although there are still some peaks, for example, the XRD peaks at 2θ = 6.6° and 20°, that could not be properly assigned. Certainly, the XRD evidence indicates a successful synthesis of Zeolite MCM-22 in this work from a cost-effective and alternative source of silica, i.e., colloidal silica commercially available. Figures 2 and 3 also reveal that, after ion exchange with Na ions, both CBV-780 and Zeolite MCM-22 lose their
Figure 2. XRD patterns of CBV-780 (Zeolite HY) and CBV-780Na before/after reaction.
Figure 3. XRD patterns of as-synthesized Zeolite MCM-22 and MCM-22Na before/after reaction.
crystallinity and become amorphous. Similarly, both silica− alumina catalyst supports (a-SiAl) with/without Na-ion exchange exhibit a broadening peak from 2θ = 15° to 33°, indicating their amorphous nature. However, both amorphous CBV-780Na and MCM-22Na are more effective in catalyzing transesterification of triolein in methanol than their unmodified analogues. Noticeably, even with such high BET surface areas, these catalysts without Na-ion-exchange treatment do not render better catalysis in the transesterification reaction of triolein. For example, a-SiAl shows almost no catalysis after the reaction proceeds for 90 h at 60 °C. Zeolite MCM-22 gives a slightly 9962
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improvement in catalyzing the transesterification reaction of triolein. Figure 5 implies that, with MCM-22Na as the effective catalyst, the conversion of triolein could easily achieve over 95%
better catalysis, namely, 16.3% production yield of triolein to biodiesel over 90 h of reaction at 60 °C. In contrast, CBV-780 (Zeolite HY) possesses the best catalysis and results in a yield of 92.6% after a 40-h reaction at 60 °C with more CBV-780 used. Consequently, to increase the production yield of biodiesel from triolein and to shorten the required duration of the transesterification reaction, surface modification on these catalysts and supports is performed by Na-ion exchange in a NaOH solution. Satisfactory outcomes, in terms of the production yield of biodiesel, have been attained and presented in the next section. 3.2. Transesterification of Triolein in Methanol. Catalytic transesterification of triolein in excess methanol was performed using a fixed weight ratio of methanol-to-triolein at 15 in initial feeds. Figure 4 indicates that unmodified CBV-780
Figure 5. Transesterification yield of triolein to biodiesel using MCM22Na catalysts. The initial feed ratio of methanol to triolein by weight is kept constant at 15.
at a reaction time of less than 1 h. When the catalyst loading is further reduced by 2/3 to 1/3 of the original loading, namely, with catalyst/triolein = 1/3 by weight, the conversion of triolein is increased from 80.5% at 1 h from the start of the transesterification reaction to 96.6% at the end of the third hour of the transesterification process. Practically, reducing the catalyst loading from 1 to 0.33 does not seem to obviously influence the transesterification reaction of triolein. On the contrary, only 16.3% of triolein has been converted to biodiesel over the as-synthesized MCM-22 during 90 h of the transesterification reaction of triolein. It has to be mentioned that the BJH pore-size distributions of both CBV-780Na and MCM-22Na become broader and have drifted to a larger value after the transesterification reaction, e.g., 8 and 7.5 Å for CBV780Na and MCM-22Na, respectively. Table 1 also indicates that some sodium on CBV-780Na and MCM-22Na might be lost during transesterification. Additionally, the Na-ionexchange method seems to turn the crystalline CBV-780 and MCM-22 into amorphous structures in CBV-780Na and MCM-22Na (Figures 2 and 3). Importantly, no saponification was observed in transesterification experiments of triolein using CBV-780Na and MCM-22Na catalysts. In order to check whether the Na-ion-exchange method could improve catalysis, a similar procedure for surface treatment was carried out on amorphous silica−alumina catalyst support (a-SiAl). As a result, transesterification of triolein did not seem to occur until 5 h from the commencement of the reaction. A poor conversion of triolein, explicitly only 9%, was realized after 9 h of the transesterification reaction. Therefore, it is not always good to conclude that the presence of sodium on the catalyst surface could help improve catalytic transesterification. Interestingly, the BET surface area of a-SiAlNa is relatively larger than those of CBV-780Na and MCM-22Na, i.e., 373 versus 91 m2/g (CBV-780Na) and 104 m2/g (MCM-22Na), respectively (Table 1).
Figure 4. Transesterification yield of triolein to biodiesel using CBV780 and CBV-780Na catalysts. The initial feed ratio of methanol to triolein by weight is kept constant at 15.
has little catalytic effect on transesterification of triolein. With a catalyst loading equal to the weight of triolein, the conversion of triolein to biodiesel reaches only 13.1% after a transesterification reaction lasting 9 h, 19.5% after 24 h of reaction, and 77.5% after 40 h of reaction. Increasing the catalyst loadings by 70% to 1.7 times that of triolein by weight, the conversion of triolein is slightly higher and reaches 38.5% and 92.6%, respectively, at 24 and 40 h after the onset of the transesterification reaction. With Zeolite HY as the catalyst for transesterification of used vegetable oil, Brito et al.17 also reported a lower conversion efficiency attained nearly 26.6% at 476 °C in a continuous tubular reactor with a space time of ca. 22 min. In contrast, with NaOH-treated CBV-780, namely, CBV780Na, as the catalyst, the conversion of triolein to biodiesel has been greatly improved. For example, with a loading of catalyst to triolein by weight at 1, the conversion of triolein increases from 9.5% at 1 h from the onset of the transesterification reaction of triolein dramatically to 78.4% at 2.5 h, 91.3% at 3.5 h, and 98.0% at 5.5 h. Compared to its unmodified analogue CBV-780, CBV-780Na could significantly shorten the reaction time required to reach the same level of conversion. For instance, it takes only 3.5 h to reach 91.3% of triolein conversion over CBV-780Na, in contrast to nearly 40 h required for CBV-780. Similarly, surface modification with Naion exchange on Zeolite MCM-22 also gives substantial 9963
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been continuously replenished, once if lost to methanol, is proposed. It is known that zeolites are widely used as molecular sieves and for ion exchange.29,30 Sorption of guest molecules could occur in the cages of zeolites,29 where they function as a molecular depot of sodium supplied from the Na-ion-exchange process. Upon extended treatment of Na-ion exchange, the structure of zeolite collapsed and the interconnected aluminosilicate units break up into individual components or much smaller blocks. Even so, these Na-filled cages still exit, evident with the BET surface area of the NaOH-treated zeolites of around 100 m2/g before reaction (Table 1). In addition, the surface diffusivity becomes much larger at a lower temperature.29,30 Hence, sodium could possibly be ferried from the interior of the cage in these aluminosilicate building blocks to Na-depleted active sites by a surface diffusion mechanism. In summary, more work may be warranted to investigate the effect of the surface chemistry and internal structure of zeolite to biodiesel production from transesterification of triolein/ vegetable oils in excess methanol to test our hypothesis and to shed light on detailing mechanisms underlain in the transesterification reaction.
More intriguingly, both NaOH-treated zeolites, viz., CBV780Na and MCM-22Na, suffer losses of BET surface area and crystallinity after surface treatments. Nonetheless, even with smaller BET surface areas, both CBV-780Na and MCM-22Na could still catalyze the transesterification reaction of triolein more effectively than their unmodified analogues having larger BET surface areas. Noticeably, sodium is intrinsically present in all zeolites, including those unmodified ones, such as CBV-780 and MCM-22, because sodium oxide or sodium aluminate is the starting material required in zeolite synthesis. Probably, the Na-ion-exchange method could leave sodium present on the surface of the catalyst as an active site, which is known to be able to catalyze transesterification. In addition, the amorphous nature of both CBV-780Na and MCM-22Na does not seem to hamper the conversion of triolein. It is known that heterogeneous catalysts often suffer in deactivation of catalysis because of leaching of the active species.1,26−28 Consequently, sodium concentrations in methanol and in produced biodiesel after triolein transesterification lasting 7−8 h were measured with AAS. There was no AASdetectable sodium in produced biodiesel. In contrast, free sodium was found in methanol, which possibly exists as sodium methoxide. For example, a level of leached sodium at 0.18 and 0.34 wt %, based on the initial catalyst mass and triolein mass (catalyst/triolein = 1), was found in the methanol phase with CBV-780Na and MCM-22Na catalysts used, respectively. Equivalently, the contents of sodium leached out from CBV780Na and MCM-22Na catalysts were about 10.8% and 7% of total sodium, including structural and surface sodium, on the catalysts. Because the leached sodium could be responsible for the catalytic activity in triolein transesterification, the effect of homogeneous NaOH on catalyzed transesterification was checked. Two NaOH loadings, 0.2 and 0.5 wt % based on the initial triolein mass, were adopted in transesterification experiments of triolein carried out at (1) methanol/triolein = 15 by weight in the initial feed, (2) reaction temperature at 60 °C, (3) constant stirring at 700 rpm, and (4) reaction for 8 h. The biodiesel production yields were negligible with 0.2 wt % NaOH and stayed almost constant at 12.5 ± 3.5% in the presence of 0.5 wt % NaOH. Certainly, this indicates that the leached sodium in methanol in this work may have little impact on transesterification of triolein, in contrast to those found by Ramos et al.,26 who utilized Zeolite X impregnated with sodium acetate to transesterify sunflower oil for biodiesel. Alternatively, surface sodium on CBV-780Na and MCM-22Na catalysts must have contributed substantially to transesterification of triolein in this work. Preliminary experiments to test the cyclic endurance of these NaOH-treated catalysts were performed as well. After each cycle, the catalysts were filtered, rinsed twice with hexane (10 mL/g of catalyst), and dried in an oven at 120 °C for 3 h. The CBV-780Na catalyst does not exhibit discernible deactivation in catalysis for the first three cycles totaling 30 h of transesterification reaction. Afterward, an apparent lag time in catalysis exists on the CBV-780Na catalyst. For example, it takes 5 and 10 h to reach ca. 95% in biodiesel production yield on the fourth and fifth cycles, compared to less than 1 h required for the first three cycles. The NaOH-treated catalysts prepared from zeolites inevitably lost surface sodium to methanol. However, the preliminary experiment indicates that they could still catalyze transesterification at least for a few cycles. Therefore, based on our conjecture, a hypothesis that surface sodium must have
4. CONCLUSIONS The successful application of sodium-loaded catalysts prepared from Na-ion exchange with zeolites in catalyzing the transesterification of triolein in excess methanol at 60 °C for biodiesel production has been demonstrated in this work. Triolein is taken as the model vegetable oil. Moreover, Zeolite MCM-22 is successfully synthesized with colloidal silica as the source of silicon and sodium aluminate. Ion exchange with Na ions on the zeolite surface has been proven to be effective in catalyzing the transesterification of triolein to biodiesel. As a result, nearly 98% and 99% of triolein could be transformed into biodiesel within 5.5 h of the transesterification reaction over the NaOH-treated CBV-780 and Zeolite MCM-22. More strikingly, these effective catalysts possess less BET surface areas and less crystalline nature, compared to their unmodified analogues. Sodium is inevitably leached out from these sodiumloaded catalysts during transesterification of triolein. After the transesterification reaction proceeded for 7 h, nearly 10.9% of total sodium, including structural and surface sodium, on the CBV-780Na catalyst is leached out to the methanol phase. In contrast, no detectable level of sodium could be found in the layer of produced biodiesel. However, the catalytic effect of this free sodium to triolein transesterification is still negligible.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +886-6-275-7575, ext. 62695. Fax: +886-6-234-4496. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The work was financially supported by the National Science Council of Taiwan (NSC 100-3113-E-006-016). Furthermore, the authors thank Jenny Wu of the NCKU Instrument Center for her assistance in NMR operations and Hsin-Yu Chou for performing cyclic endurance tests of catalysts and AAS analysis. 9964
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