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Industrial eggshell wastes as the heterogeneous catalysts for microwave-assisted biodiesel production
Catalysis Today 190 (2012) 112–116
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Industrial eggshell wastes as the heterogeneous catalysts for microwave-assisted biodiesel production P. Khemthong, C. Luadthong, W. Nualpaeng, P. Changsuwan, P. Tongprem, N. Viriya-empikul, K. Faungnawakij ∗ National Nanotechnology Center (NANOTEC), National Science and Technology, Development Agency, 111 Thailand Science Park, Paholyothin Rd., Patumthani 12120, Thailand
a r t i c l e
i n f o
Article history: Received 30 September 2011 Received in revised form 29 November 2011 Accepted 1 December 2011 Available online 5 February 2012 Keywords: Biodiesel production Microwave irradiation Eggshell Palm olein oil Fatty acid methyl esters CaO
a b s t r a c t Active biodiesel production catalysts were derived from waste eggshells by simple calcination in air. The physicochemical properties of the activated catalysts were characterized by XRD, N2 sorption, CO2 -TPD, TGA–DTG, XRF, and SEM, while the catalytic activity was tested in producing biodiesel via transesterification on palm oil with methanol under microwave conditions. The effect of microwave power, reaction time, methanol-to-oil ratio, and catalyst loading was investigated. The experimental results revealed that the catalysts exhibited a high content of CaO (99.2 wt%) with a high density of strong base sites. The catalytic testing demonstrated a remarkable enhancement for biodiesel production using microwaves compared to conventional heating. The maximum yield of fatty acid methyl esters reached 96.7% under the optimal condition of reaction time of 4 min with 900 W microwave power, methanol-to-oil ratio of 18:1, and catalyst loading of 15%. The results indicated that the CaO catalysts derived from eggshells showed good reusability and had high potential to be used as biodiesel production catalysts under microwave-assisted transesterification of palm oil. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Development of alternative fuel technology for diesel engines has become an important issue due to diminishing fossil fuel reserves, rising crude oil price and the environmental crisis. Biodiesel, a renewable biofuel has been considered as an alternative diesel fuel. It can be easily produced through transesterification of oil or esterification of fats using basic or acidic catalysts with heating functions, respectively [1,2]. The conventional processes are currently operated using homogeneous catalytic systems [3–6]. However, the homogeneous catalysts result in corrosion problems with nonreusability and multiple processing steps, including twostep reaction, residue catalyst removal, and product separation. The heterogeneous catalysts have been attached to overcome the drawbacks from the homogeneous system since this heterogeneous process would provide the significant advantages of lower amounts of wastewater production and catalyst reusability [7–9]. The heterogeneous catalysts of alkaline earth metal oxides [10–12], heteropolyacids [13,14], and zeolites [15,16] have been investigated for biodiesel production. Among these, alkaline earth metal oxides, and in particular CaO, have been shown to possess good performance of commercial significance for a wide spectrum
∗ Corresponding author. Tel.: +66 2 564 7100x6638; fax: +66 2 564 6981. E-mail address: [email protected] (K. Faungnawakij). 0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2011.12.024
of environmentally important reactions [17–19]. Alba-Rubio et al. [20] has reported that the transesterification reaction of oil and methanol can be effectively catalyzed by CaO with a biodiesel yield higher than 90%. This research has also suggested that the basic strength of CaO is sufficient for this reaction. Notwithstanding, sources for the preparation of Ca-based catalysts are mainly commercial precursors, including nitrate, carbonate, acetate, and hydroxide [21–24], and hence would increase the cost of biodiesel production. Thus, much effort has been directed towards finding cheap precursors for the catalysts. In Thailand, there are a number of industrial food factories, producing several tons of waste eggshells a day. This huge amount of wastes could be a promising resource of the catalysts which has high potential for industrial application. Recently, the utilization of natural calcium sources from waste materials has been considered as a new trend for biodiesel production. Some researchers have found that the derived CaO catalysts from waste shells of freshwater mussel [25], cockle (Anadara granosa) [26], mollusk [27], egg [27], Turbonilla striatula [28], oyster [29], and Rohu fish (Labeo rohita) scale [30] not only exhibit a high potential use as biodiesel synthesis catalysts but also add value to the green biodiesel process due to their eco-friendly characteristics and cheap cost. In our previous work, we have investigated the transesterification of palm olein oil with methanol catalyzed by CaO derived from the wastes of mollusk shell, bone, and eggshell [10,31] and found that these catalysts are active to biodiesel production
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resulting in fatty acid methyl ester (FAME) above 90%. Among the derived catalysts, an eggshell-derived catalyst displayed the highest catalytic performance when compared to others. However, those reactions require a longer time (>60 min) to achieve a satisfactory conversion of oil to biodiesel because of heat and mass transfer limitations. Therefore, a microwave-assisted production of biodiesel was applied in this research to expedite the chemical reaction and give a high product yields in a short time. Microwaves are basically electromagnetic radiation, transferring energy directly to the reactants and thereby giving rise to intense localized heating. Consequently, the preheating step is eliminated and the reaction can be completed in a shorter time. Some studies on microwave assisted synthesis of biodiesel reported that the microwave radiation can accelerate the reaction, reduce the reaction time, and increase the product yield [13,32–34]. The aim of this investigation is to examine the effect of the microwave irradiation generated from a household microwave for the transesterification of palm olein oil when a solid CaO catalyst derived from waste eggshells was employed. Furthermore, the effects of operating conditions, including reaction time, microwave power, methanol-to-oil ratio, amount of catalyst loading, and the catalyst reusability were thoroughly studied.
2. Experimental 2.1. Catalyst preparation and characterization The waste eggshells were washed by tap water to remove dust and impurities, and were then dried and calcined at 800 ◦ C in air atmosphere with a heating rate of 10 ◦ C/min for 4 h. The product was obtained as white powder. The decomposition and phase change of waste eggshells were studied by thermogravimetric analysis (Mettler Toledo; TGA/SDTA 851e ). The crystalline phase of the calcined sample was confirmed using a powder X-ray diffractometer (XRD – Bruker; D8 Advance) coupled with Cu K˛ radiation. The BET surface area and pore volume were measured by N2 adsorption–desorption isotherm apparatus (BEL; BELSORP-max). The morphology was investigated by scanning electron microscopy (SEM – Hitachi; S-3400). The chemical compositions were analyzed by energy dispersive X-ray fluorescence spectroscopy (XRF – Shimadzu; EDX-720). The basic property was evaluated by temperature-programmed desorption of CO2 (CO2 -TPD – Quantachrome; ChemBETTM Pulsar) carried out in the U-shaped reactor equipped with a thermal conductivity detector (TCD) and a mass spectrometer (ThermoStarTM , Gas analysis system).
Fig. 1. The characteristic patterns of the calcined eggshells according to TGA/DTG curve (A), XRD pattern (B) and CO2 -TPD curve (C).
2.2. Catalytic testing The reactions were carried out in a Teflon-line autoclave reactor (Parr; Microwave Digestion Bomb), placed inside a household microwave oven (Samsung; GE107Y). The fixed 10.2 ml of palm olein oil (acid value of 1.1 mg KOH/g oil) and the desired amount of the derived CaO catalysts (5, 10 and 15 wt%) were added to the autoclave (43 ml), and then the methanol was introduced to the oil at various mole ratios of 12:1, 15:1, 18:1, 21:1 and 24:1. The transesterification was operated at 450–900 W with varied reaction time of 1–4 min under microwave irradiation, and it was instantly stopped by rapid cooling in an ice bath. In some cases, the reaction was allowed to proceed for a period of time after the microwave irradiation was stopped. The amounts of FAME in the product were analyzed by gas chromatography (Shimadzu; GC-2014) equipped with capillary column of DB-WAX (30 m × 0.15 mm) using a flame ionization detector (FID). Methylheptadecanoate was used as an
internal standard to quantify FAME content, according to the EN14103 standard method. 3. Results and discussion 3.1. Catalyst characterization The physicochemical properties of the eggshell-derived catalyst, which was calcined at 800 ◦ C for 4 h, were characterized by nitrogen sorption, and ED-XRF, and CO2 -TPD. As a result, the BET surface area and total pore volume were determined as 1 m2 g−1 and 0.005 cm3 g−1 , respectively. The %Ca composition was 99%, while the concentration of basic sites was calculated as 194 mol m−2 . The thermal analysis results in Fig. 1A along with the first derivative of weight loss demonstrated weight loss of eggshells and phase formation of CaO when the temperature was raised from
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Fig. 3. Effect of microwave power on the transesterification reaction over eggshellderived catalyst. Conditions: MeOH/oil molar ration of 18:1; reaction time of 3 min; catalyst loading of 10 wt%.
Fig. 2. Typical SEM image (A) and N2 adsorption–desorption isotherm of calcined eggshell (B).
particles. In other words, there were various sizes (1–10 m) and shapes of particles, such as rod, dumbbell, and rod-dumbbell connected shapes; therefore, these assembled particles constructed a termite nest-like structure (i.e. macro-porous structure). The N2 adsorption–desorption isotherm of the derived CaO catalyst is shown in Fig. 2B. A type II isotherm (based on IUPAC’s classification) was produced with a low slope in the middle region of the isotherm and a desorption line almost overlapping with an adsorption line. In particular, this isotherm is represented unrestricted monolayer–multilayer adsorption and attributed to a nonporous or macroporous material. The low values of BET surface area and total pore volume were very consistent with the sorption measurement and SEM analysis. The result revealed that this derived eggshell is a dense material due to its very low surface area and pore volume with very high basic site density which should make it active for the transesterification reaction [10,31]. 3.2. Catalytic testing
room temperature to 1000 ◦ C. When the eggshell was heated, the removal of water and organic compounds from the eggshell sample was sequentially observed at temperatures below 600 ◦ C, whereas the dominant decomposition with ca. 50% weight loss at around 700–830 ◦ C (peak at 810 ◦ C) was attributed to the production of CaO due to the loss of carbon dioxide [35]. Viriya-empikul et al. [10,31] reported that calcination of eggshells at 800–1000 ◦ C would completely convert CaCO3 (a major content in eggshell) to CaO, while the calcination at 800 ◦ C displayed the most active catalyst. The major component of the calcined eggshell was CaO species, as demonstrated in the XRD pattern. The result reveals sharp XRD reflections (Fig. 1B) with (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) orientations, implying that the calcined sample was well crystallized during the heat treatment process. Furthermore, the chemical composition of the calcined catalyst mainly contains a high purity of CaO with a large basic strength and a variety of basic sites (i.e. weak to strong base sites were observed as shown in Fig. 1C) as indicated by ED-XRF and CO2 -TPD, respectively. Generally, the basicity on the catalyst surface is of key importance in biodiesel production. The CO2 -TPD profile of CaO eggshell-derived catalyst (Fig. 1C) clearly shows three distinct basic sites with peak maxima at 244, 400 and 688 ◦ C, ascribed to weak, medium and strong bases, respectively, when the strong basic site of calcined CaO catalyst was observed as a major part of total base site. The morphology of the calcined sample was investigated by SEM as shown in Fig. 2A (5000× magnification). According to the SEM images, the calcined eggshell typically comprises irregular shape of
3.2.1. Effect of microwave power In order to determine the optimum parametric conditions for production of biodiesel (represented as FAME) assisted by microwave irradiation, the effect of power supplied by the microwave oven was studied. Firstly, the experiments were carried out on the palm olein oil without pretreatment at 18:1 MeOH/oil molar ratio with a supplied energy of 900 W for 3 min and catalyst loading of 10 wt%. The %FAME result shown in Fig. 3 elucidated the effect of microwave power at 450 W (2.6%FAME), 600 W (5.5%FAME), and 900 W (40.4%FAME). It was clear that higher microwave power gave rise to higher biodiesel yield. Thus the microwave power of 900 W would be chosen for further investigation. The microwave-assisted transesterification reaction over eggshell-derived catalysts could proceed with a rapid reaction time. It was considered that the related chemical reactions are accelerated by microwave energy, giving rise to intense localized heating and thereby accelerating the chemical reaction and giving high product yields in a short time [33,36,37]. Without catalyst, the %FAME obtained was extremely low being below 1% under microwave conditions. 3.2.2. Effect of reaction time and holding time The performance of the derived CaO catalyst under microwave conditions was further conducted to study the influence of reaction time as well as holding time (without microwave irradiation) on the %FAME. The experiment was performed as a two-step process,
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Fig. 4. Effect of reaction time on the transesterification reaction over eggshell-derived catalyst in microwave irradiation. Conditions: MeOH/oil molar ratio of 18:1; catalyst loading of 10 wt%; microwave power of 900 W.
with and without microwave irradiation, respectively. In the first phase, the reaction was carried out under microwave irradiation for 1–4 min, and subsequently stopped the transesterification reaction immediately by quenching in an ice bath. In the next phase, the reaction after microwave irradiation was stopped at 4 min, and the reaction was allowed to proceed (representing at holding time of 5, 10 and 30 min). As shown in Fig. 4, within the first 3 min, the %FAME rapidly increased to 40.4% and at a reaction time of 4 min, the high conversion of 72.2%FAME was achieved. Since the accumulation of microwave irradiation energy inside the system (solution inside the autoclave) could maintain the reaction; hence, the reaction system was held in the autoclave without further microwave supply. As a result, the yield of biodiesel production further increased from 72.2 to 90.3, 92.2, and 94.2%FAME when the holding time was expanded to 5, 10, and 30 min, respectively. It should be noted that the temperature of oil irradiated by microwave for 4 min was 122 ◦ C.
the conversion of palm olein oil because the excessive methanol diluted the reaction between catalyst and reactant [13]. On the other hand, the lowest molar ratio (12:1) could not generate a practical production due to the limited formulation of methoxy species on CaO surface, resulting in the low reaction rate [31]. The appropriate MeOH/oil molar ratio was found to be in a range of 15:1 (72.1%FAME) to 21:1 (77.8%FAME). However, the separation between biodiesel and glycerol was very difficult at the high MeOH/oil molar ratio. Thus, the most suitable molar ratio was found to be in a range of 15:1–18:1, and was used for further study on the effect catalyst loading. In comparing with the conventional process at the same conditions, the biodiesel production assisted by microwave irradiation reached 72.2%FAME after 4 min, while for the conventional heating method at a reaction temperature of 60 ◦ C only ca. 50%FAME resulted after 30 min.
3.2.3. Effect of MeOH/oil molar ratio In this study, the effect of MeOH/oil ratio, one of the most important variables in biodiesel production, was studied. As indicated in Fig. 5, the highest molar ratio (24:1) had less significant effect on
3.2.4. Effect of catalyst loading and reusability The effect of catalyst loading into the system on the biodiesel production is illustrated in Fig. 6. The reaction condition was controlled at MeOH/oil ratio of 18:1 and a microwave power of 900 W for 4 min. The %FAME sequentially increased from 0.5 to 96.7%,
Fig. 5. Effect of methanol-to-oil ratio the transesterification reaction over eggshellderived catalyst in microwave irradiation. Conditions: Reaction time of 4 min; catalyst loading of 10 wt%; microwave power of 900 W.
Fig. 6. Effect of the catalyst amount on the transesterification reaction over eggshellderived catalyst in microwave irradiation. Conditions: MeOH/oil molar ratio of 18:1; reaction time of 4 min; microwave power of 900 W.
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Acknowledgments Authors acknowledged the financial support from National Nanotechnology Center, National Science and Technology Development Agency. References
Fig. 7. Effect of the reusability of eggshell-derived catalysts on the transesterification reaction over eggshell-derived catalyst in microwave irradiation. Conditions: MeOH/oil molar ratio of 18:1; reaction time of 4 min; catalyst loading of 10 wt%; microwave power of 900 W.
when the amount of catalyst increased from 0 to 15 wt%. According to the transesterification reaction rate, 15 wt% of catalyst presented the suitable amount to catalyze this biodiesel production process. In contrast, the catalyst loading at 5 wt% could not produce biodiesel yield higher than 10%FAME because this less amount of catalyst was not sufficient to catalyze the transesterification reaction. Reusability of the catalysts was evaluated in the transesterification, and the results are shown in Fig. 7. The used catalyst was separated by centrifugation, and reused for the next reaction test without pretreatment or regenerating. The results indicated that the catalyst could be reusable at least 5 times with a 10–15%FAME drop from the 90.3%FAME obtained over fresh catalyst. After the reaction, the crystalline phase of the used catalyst was confirmed by XRD measurement as shown in Fig. 1B. It turned out that the used catalyst has still maintained the characteristic CaO phase after being reused 5 times. As a result, the activated eggshell catalyst exhibited a high potential to be used for catalyzing biodiesel production in heterogeneous process with good reusability. 4. Conclusions The CaO catalysts derived from eggshells possessed mainly the strong base sites with average base site density of 194 mol m−2 which exhibited an excellent performance in producing biodiesel under microwave conditions. The assistance of microwave irradiation enhanced the rate of transesterification reaction compared to the conventional heating method. The highest biodiesel yield (96.7%FAME) was observed by addition of 15 wt% of the catalyst to a mixture of MeOH/oil via microwave irradiation at 900 W for 4 min. In the process, the CaO catalyst derived from eggshell still maintained the catalytic activity and crystalline phase after being reused 5 times.
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Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Industrial eggshell wastes as the heterogeneous catalysts for microwave-assisted biodiesel production P. Khemthong, C. Luadthong, W. Nualpaeng, P. Changsuwan, P. Tongprem, N. Viriya-empikul, K. Faungnawakij ∗ National Nanotechnology Center (NANOTEC), National Science and Technology, Development Agency, 111 Thailand Science Park, Paholyothin Rd., Patumthani 12120, Thailand
a r t i c l e
i n f o
Article history: Received 30 September 2011 Received in revised form 29 November 2011 Accepted 1 December 2011 Available online 5 February 2012 Keywords: Biodiesel production Microwave irradiation Eggshell Palm olein oil Fatty acid methyl esters CaO
a b s t r a c t Active biodiesel production catalysts were derived from waste eggshells by simple calcination in air. The physicochemical properties of the activated catalysts were characterized by XRD, N2 sorption, CO2 -TPD, TGA–DTG, XRF, and SEM, while the catalytic activity was tested in producing biodiesel via transesterification on palm oil with methanol under microwave conditions. The effect of microwave power, reaction time, methanol-to-oil ratio, and catalyst loading was investigated. The experimental results revealed that the catalysts exhibited a high content of CaO (99.2 wt%) with a high density of strong base sites. The catalytic testing demonstrated a remarkable enhancement for biodiesel production using microwaves compared to conventional heating. The maximum yield of fatty acid methyl esters reached 96.7% under the optimal condition of reaction time of 4 min with 900 W microwave power, methanol-to-oil ratio of 18:1, and catalyst loading of 15%. The results indicated that the CaO catalysts derived from eggshells showed good reusability and had high potential to be used as biodiesel production catalysts under microwave-assisted transesterification of palm oil. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Development of alternative fuel technology for diesel engines has become an important issue due to diminishing fossil fuel reserves, rising crude oil price and the environmental crisis. Biodiesel, a renewable biofuel has been considered as an alternative diesel fuel. It can be easily produced through transesterification of oil or esterification of fats using basic or acidic catalysts with heating functions, respectively [1,2]. The conventional processes are currently operated using homogeneous catalytic systems [3–6]. However, the homogeneous catalysts result in corrosion problems with nonreusability and multiple processing steps, including twostep reaction, residue catalyst removal, and product separation. The heterogeneous catalysts have been attached to overcome the drawbacks from the homogeneous system since this heterogeneous process would provide the significant advantages of lower amounts of wastewater production and catalyst reusability [7–9]. The heterogeneous catalysts of alkaline earth metal oxides [10–12], heteropolyacids [13,14], and zeolites [15,16] have been investigated for biodiesel production. Among these, alkaline earth metal oxides, and in particular CaO, have been shown to possess good performance of commercial significance for a wide spectrum
∗ Corresponding author. Tel.: +66 2 564 7100x6638; fax: +66 2 564 6981. E-mail address: [email protected] (K. Faungnawakij). 0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2011.12.024
of environmentally important reactions [17–19]. Alba-Rubio et al. [20] has reported that the transesterification reaction of oil and methanol can be effectively catalyzed by CaO with a biodiesel yield higher than 90%. This research has also suggested that the basic strength of CaO is sufficient for this reaction. Notwithstanding, sources for the preparation of Ca-based catalysts are mainly commercial precursors, including nitrate, carbonate, acetate, and hydroxide [21–24], and hence would increase the cost of biodiesel production. Thus, much effort has been directed towards finding cheap precursors for the catalysts. In Thailand, there are a number of industrial food factories, producing several tons of waste eggshells a day. This huge amount of wastes could be a promising resource of the catalysts which has high potential for industrial application. Recently, the utilization of natural calcium sources from waste materials has been considered as a new trend for biodiesel production. Some researchers have found that the derived CaO catalysts from waste shells of freshwater mussel [25], cockle (Anadara granosa) [26], mollusk [27], egg [27], Turbonilla striatula [28], oyster [29], and Rohu fish (Labeo rohita) scale [30] not only exhibit a high potential use as biodiesel synthesis catalysts but also add value to the green biodiesel process due to their eco-friendly characteristics and cheap cost. In our previous work, we have investigated the transesterification of palm olein oil with methanol catalyzed by CaO derived from the wastes of mollusk shell, bone, and eggshell [10,31] and found that these catalysts are active to biodiesel production
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resulting in fatty acid methyl ester (FAME) above 90%. Among the derived catalysts, an eggshell-derived catalyst displayed the highest catalytic performance when compared to others. However, those reactions require a longer time (>60 min) to achieve a satisfactory conversion of oil to biodiesel because of heat and mass transfer limitations. Therefore, a microwave-assisted production of biodiesel was applied in this research to expedite the chemical reaction and give a high product yields in a short time. Microwaves are basically electromagnetic radiation, transferring energy directly to the reactants and thereby giving rise to intense localized heating. Consequently, the preheating step is eliminated and the reaction can be completed in a shorter time. Some studies on microwave assisted synthesis of biodiesel reported that the microwave radiation can accelerate the reaction, reduce the reaction time, and increase the product yield [13,32–34]. The aim of this investigation is to examine the effect of the microwave irradiation generated from a household microwave for the transesterification of palm olein oil when a solid CaO catalyst derived from waste eggshells was employed. Furthermore, the effects of operating conditions, including reaction time, microwave power, methanol-to-oil ratio, amount of catalyst loading, and the catalyst reusability were thoroughly studied.
2. Experimental 2.1. Catalyst preparation and characterization The waste eggshells were washed by tap water to remove dust and impurities, and were then dried and calcined at 800 ◦ C in air atmosphere with a heating rate of 10 ◦ C/min for 4 h. The product was obtained as white powder. The decomposition and phase change of waste eggshells were studied by thermogravimetric analysis (Mettler Toledo; TGA/SDTA 851e ). The crystalline phase of the calcined sample was confirmed using a powder X-ray diffractometer (XRD – Bruker; D8 Advance) coupled with Cu K˛ radiation. The BET surface area and pore volume were measured by N2 adsorption–desorption isotherm apparatus (BEL; BELSORP-max). The morphology was investigated by scanning electron microscopy (SEM – Hitachi; S-3400). The chemical compositions were analyzed by energy dispersive X-ray fluorescence spectroscopy (XRF – Shimadzu; EDX-720). The basic property was evaluated by temperature-programmed desorption of CO2 (CO2 -TPD – Quantachrome; ChemBETTM Pulsar) carried out in the U-shaped reactor equipped with a thermal conductivity detector (TCD) and a mass spectrometer (ThermoStarTM , Gas analysis system).
Fig. 1. The characteristic patterns of the calcined eggshells according to TGA/DTG curve (A), XRD pattern (B) and CO2 -TPD curve (C).
2.2. Catalytic testing The reactions were carried out in a Teflon-line autoclave reactor (Parr; Microwave Digestion Bomb), placed inside a household microwave oven (Samsung; GE107Y). The fixed 10.2 ml of palm olein oil (acid value of 1.1 mg KOH/g oil) and the desired amount of the derived CaO catalysts (5, 10 and 15 wt%) were added to the autoclave (43 ml), and then the methanol was introduced to the oil at various mole ratios of 12:1, 15:1, 18:1, 21:1 and 24:1. The transesterification was operated at 450–900 W with varied reaction time of 1–4 min under microwave irradiation, and it was instantly stopped by rapid cooling in an ice bath. In some cases, the reaction was allowed to proceed for a period of time after the microwave irradiation was stopped. The amounts of FAME in the product were analyzed by gas chromatography (Shimadzu; GC-2014) equipped with capillary column of DB-WAX (30 m × 0.15 mm) using a flame ionization detector (FID). Methylheptadecanoate was used as an
internal standard to quantify FAME content, according to the EN14103 standard method. 3. Results and discussion 3.1. Catalyst characterization The physicochemical properties of the eggshell-derived catalyst, which was calcined at 800 ◦ C for 4 h, were characterized by nitrogen sorption, and ED-XRF, and CO2 -TPD. As a result, the BET surface area and total pore volume were determined as 1 m2 g−1 and 0.005 cm3 g−1 , respectively. The %Ca composition was 99%, while the concentration of basic sites was calculated as 194 mol m−2 . The thermal analysis results in Fig. 1A along with the first derivative of weight loss demonstrated weight loss of eggshells and phase formation of CaO when the temperature was raised from
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Fig. 3. Effect of microwave power on the transesterification reaction over eggshellderived catalyst. Conditions: MeOH/oil molar ration of 18:1; reaction time of 3 min; catalyst loading of 10 wt%.
Fig. 2. Typical SEM image (A) and N2 adsorption–desorption isotherm of calcined eggshell (B).
particles. In other words, there were various sizes (1–10 m) and shapes of particles, such as rod, dumbbell, and rod-dumbbell connected shapes; therefore, these assembled particles constructed a termite nest-like structure (i.e. macro-porous structure). The N2 adsorption–desorption isotherm of the derived CaO catalyst is shown in Fig. 2B. A type II isotherm (based on IUPAC’s classification) was produced with a low slope in the middle region of the isotherm and a desorption line almost overlapping with an adsorption line. In particular, this isotherm is represented unrestricted monolayer–multilayer adsorption and attributed to a nonporous or macroporous material. The low values of BET surface area and total pore volume were very consistent with the sorption measurement and SEM analysis. The result revealed that this derived eggshell is a dense material due to its very low surface area and pore volume with very high basic site density which should make it active for the transesterification reaction [10,31]. 3.2. Catalytic testing
room temperature to 1000 ◦ C. When the eggshell was heated, the removal of water and organic compounds from the eggshell sample was sequentially observed at temperatures below 600 ◦ C, whereas the dominant decomposition with ca. 50% weight loss at around 700–830 ◦ C (peak at 810 ◦ C) was attributed to the production of CaO due to the loss of carbon dioxide [35]. Viriya-empikul et al. [10,31] reported that calcination of eggshells at 800–1000 ◦ C would completely convert CaCO3 (a major content in eggshell) to CaO, while the calcination at 800 ◦ C displayed the most active catalyst. The major component of the calcined eggshell was CaO species, as demonstrated in the XRD pattern. The result reveals sharp XRD reflections (Fig. 1B) with (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) orientations, implying that the calcined sample was well crystallized during the heat treatment process. Furthermore, the chemical composition of the calcined catalyst mainly contains a high purity of CaO with a large basic strength and a variety of basic sites (i.e. weak to strong base sites were observed as shown in Fig. 1C) as indicated by ED-XRF and CO2 -TPD, respectively. Generally, the basicity on the catalyst surface is of key importance in biodiesel production. The CO2 -TPD profile of CaO eggshell-derived catalyst (Fig. 1C) clearly shows three distinct basic sites with peak maxima at 244, 400 and 688 ◦ C, ascribed to weak, medium and strong bases, respectively, when the strong basic site of calcined CaO catalyst was observed as a major part of total base site. The morphology of the calcined sample was investigated by SEM as shown in Fig. 2A (5000× magnification). According to the SEM images, the calcined eggshell typically comprises irregular shape of
3.2.1. Effect of microwave power In order to determine the optimum parametric conditions for production of biodiesel (represented as FAME) assisted by microwave irradiation, the effect of power supplied by the microwave oven was studied. Firstly, the experiments were carried out on the palm olein oil without pretreatment at 18:1 MeOH/oil molar ratio with a supplied energy of 900 W for 3 min and catalyst loading of 10 wt%. The %FAME result shown in Fig. 3 elucidated the effect of microwave power at 450 W (2.6%FAME), 600 W (5.5%FAME), and 900 W (40.4%FAME). It was clear that higher microwave power gave rise to higher biodiesel yield. Thus the microwave power of 900 W would be chosen for further investigation. The microwave-assisted transesterification reaction over eggshell-derived catalysts could proceed with a rapid reaction time. It was considered that the related chemical reactions are accelerated by microwave energy, giving rise to intense localized heating and thereby accelerating the chemical reaction and giving high product yields in a short time [33,36,37]. Without catalyst, the %FAME obtained was extremely low being below 1% under microwave conditions. 3.2.2. Effect of reaction time and holding time The performance of the derived CaO catalyst under microwave conditions was further conducted to study the influence of reaction time as well as holding time (without microwave irradiation) on the %FAME. The experiment was performed as a two-step process,
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Fig. 4. Effect of reaction time on the transesterification reaction over eggshell-derived catalyst in microwave irradiation. Conditions: MeOH/oil molar ratio of 18:1; catalyst loading of 10 wt%; microwave power of 900 W.
with and without microwave irradiation, respectively. In the first phase, the reaction was carried out under microwave irradiation for 1–4 min, and subsequently stopped the transesterification reaction immediately by quenching in an ice bath. In the next phase, the reaction after microwave irradiation was stopped at 4 min, and the reaction was allowed to proceed (representing at holding time of 5, 10 and 30 min). As shown in Fig. 4, within the first 3 min, the %FAME rapidly increased to 40.4% and at a reaction time of 4 min, the high conversion of 72.2%FAME was achieved. Since the accumulation of microwave irradiation energy inside the system (solution inside the autoclave) could maintain the reaction; hence, the reaction system was held in the autoclave without further microwave supply. As a result, the yield of biodiesel production further increased from 72.2 to 90.3, 92.2, and 94.2%FAME when the holding time was expanded to 5, 10, and 30 min, respectively. It should be noted that the temperature of oil irradiated by microwave for 4 min was 122 ◦ C.
the conversion of palm olein oil because the excessive methanol diluted the reaction between catalyst and reactant [13]. On the other hand, the lowest molar ratio (12:1) could not generate a practical production due to the limited formulation of methoxy species on CaO surface, resulting in the low reaction rate [31]. The appropriate MeOH/oil molar ratio was found to be in a range of 15:1 (72.1%FAME) to 21:1 (77.8%FAME). However, the separation between biodiesel and glycerol was very difficult at the high MeOH/oil molar ratio. Thus, the most suitable molar ratio was found to be in a range of 15:1–18:1, and was used for further study on the effect catalyst loading. In comparing with the conventional process at the same conditions, the biodiesel production assisted by microwave irradiation reached 72.2%FAME after 4 min, while for the conventional heating method at a reaction temperature of 60 ◦ C only ca. 50%FAME resulted after 30 min.
3.2.3. Effect of MeOH/oil molar ratio In this study, the effect of MeOH/oil ratio, one of the most important variables in biodiesel production, was studied. As indicated in Fig. 5, the highest molar ratio (24:1) had less significant effect on
3.2.4. Effect of catalyst loading and reusability The effect of catalyst loading into the system on the biodiesel production is illustrated in Fig. 6. The reaction condition was controlled at MeOH/oil ratio of 18:1 and a microwave power of 900 W for 4 min. The %FAME sequentially increased from 0.5 to 96.7%,
Fig. 5. Effect of methanol-to-oil ratio the transesterification reaction over eggshellderived catalyst in microwave irradiation. Conditions: Reaction time of 4 min; catalyst loading of 10 wt%; microwave power of 900 W.
Fig. 6. Effect of the catalyst amount on the transesterification reaction over eggshellderived catalyst in microwave irradiation. Conditions: MeOH/oil molar ratio of 18:1; reaction time of 4 min; microwave power of 900 W.
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Acknowledgments Authors acknowledged the financial support from National Nanotechnology Center, National Science and Technology Development Agency. References
Fig. 7. Effect of the reusability of eggshell-derived catalysts on the transesterification reaction over eggshell-derived catalyst in microwave irradiation. Conditions: MeOH/oil molar ratio of 18:1; reaction time of 4 min; catalyst loading of 10 wt%; microwave power of 900 W.
when the amount of catalyst increased from 0 to 15 wt%. According to the transesterification reaction rate, 15 wt% of catalyst presented the suitable amount to catalyze this biodiesel production process. In contrast, the catalyst loading at 5 wt% could not produce biodiesel yield higher than 10%FAME because this less amount of catalyst was not sufficient to catalyze the transesterification reaction. Reusability of the catalysts was evaluated in the transesterification, and the results are shown in Fig. 7. The used catalyst was separated by centrifugation, and reused for the next reaction test without pretreatment or regenerating. The results indicated that the catalyst could be reusable at least 5 times with a 10–15%FAME drop from the 90.3%FAME obtained over fresh catalyst. After the reaction, the crystalline phase of the used catalyst was confirmed by XRD measurement as shown in Fig. 1B. It turned out that the used catalyst has still maintained the characteristic CaO phase after being reused 5 times. As a result, the activated eggshell catalyst exhibited a high potential to be used for catalyzing biodiesel production in heterogeneous process with good reusability. 4. Conclusions The CaO catalysts derived from eggshells possessed mainly the strong base sites with average base site density of 194 mol m−2 which exhibited an excellent performance in producing biodiesel under microwave conditions. The assistance of microwave irradiation enhanced the rate of transesterification reaction compared to the conventional heating method. The highest biodiesel yield (96.7%FAME) was observed by addition of 15 wt% of the catalyst to a mixture of MeOH/oil via microwave irradiation at 900 W for 4 min. In the process, the CaO catalyst derived from eggshell still maintained the catalytic activity and crystalline phase after being reused 5 times.
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