Research Article | Open Access
Synthesis of Ca-Doped Three-Dimensionally Ordered Macroporous Catalysts for Transesterification
The novel three-dimensionally ordered macroporous (3DOM) CaO/SiO2, 3DOM CaO/Al2O3, and 3DOM Ca12Al14O32Cl2 catalysts for biodiesel transesterification were prepared by sol-gel method. The 3DOM catalysts were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The hierarchical porous structure was achieved; however, only 3DOM CaO/Al2O3 and 3DOM Ca12Al14O32Cl2 catalysts were used for transesterification due to high amount of active CaO. Various parameters such as methanol to oil molar ratio, catalyst concentration, reaction time, and their influence on the biodiesel production were studied. The result showed that 99.0% RPO conversion was achieved using the 3DOM Ca12Al14O33Cl2 as a catalyst under the optimal condition of 12 : 1 methanol to oil molar ratio and 6 wt.% catalyst with reaction time of 3 hours at 65°C.
In recent years, the demand of fossil energy is increasing by a rapid growth of global transportation and industrial evolution, thus driving world economic. In addition, the fossil energy becomes expensive due to limited resources. It is predicted that the fossil energy would be exhausted by 20 centuries . Hence, more researchers are focusing on new alternative energy resources. Biodiesel is one of the alternative energies consisting of monoalkyl ester that was derived from recycled cooking oil, vegetable oil, and animal fats. In addition, it is renewable, clean-combustion diesel replacement. Biodiesel is produced by transesterification from either vegetable oil or animal fat with methanol in the presence of a catalyst, resulting in glycerol and biodiesel. This clean diesel provides low carbon monoxide emission, low greenhouse gases emission, noncombustion of hydrocarbon, and nonsulfur dioxide content compared to those of fossil fuel. The physical properties and energy content of biodiesel are similar to fossil fuel; therefore, it can be used to function conventional diesel engines efficiently without modification .
The catalysts for transesterification are categorized into two groups: homogeneous and heterogeneous. The homogeneous catalysts such as sodium hydroxide or potassium hydroxide are most often used commercially because of their high catalytic activity and high productivity. However, the product must be neutralized, preventing corrosion in engine. The solid catalysts or heterogeneous catalysts, on the other hand, are easily separated from the produced biodiesel. For example, KNO3/Al2O3, La2O3/ZrO2, and K2CO3 on alumina/silica support are highly active for transesterification of vegetable oils [3–5].
Recently, a new hierarchical material named three-dimensionally ordered macroporous (3DOM) has been extensively studied because of its unique ordered structure with the interconnected wall. Owning to this structure, the refined palm oil (RPO) may not only enter the 3DOM catalyst pores easily but also transfer into the inner area of the catalyst. This may enhance biodiesel production. In this work, the three-dimensionally ordered macroporous (3DOM) CaO/SiO2, 3DOM CaO/Al2O3, and 3DOM Ca12Al14O32Cl2 were synthesized by the sol-gel method (SG). The obtained 3DOM catalysts were characterized by various techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The catalytic efficiency of the 3DOM catalysts for transesterification was investigated. Optimal conditions for transesterification using the obtained 3DOM catalysts were studied. The property of the produced biodiesel using the 3DOM catalysts was reported herein.
2. Experimental Procedures
2.1. Synthesis of PMMA Colloidal Crystal
The monodispersed poly(methyl methacrylate) (PMMA) spheres were synthesized by emulsifier-free emulsion polymerization as previously described by Phumthiean . A mixture of water and methyl methacrylate was stirred at 75°C under nitrogen (N2) atmosphere. Then, 1.2 g of 2,2′-azobis(2-amidinopropane) dihydrochloride, an initiator, was added into the mixture and stirred for 2 h until reaction was completed. After reaction, the mixture was cooled down to room temperature and filtrated through glass wool to remove large particles. The PMMA spheres were self-assembled by gravitation until a clear solution and a colloidal crystal arrays were observed. The obtained PMMA arrays were dried at 60°C for 24 h.
2.2. Preparation of 3DOM Catalysts by Sol-Gel Method
The Ca-doped 3DOM catalysts were synthesized by the sol-gel method using calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) and tetraethoxysilane (TEOS) for 3DOM Cao/SiO2, calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) and aluminium nitrate nonahydrate (Al(NO3)3·9H2O) for 3DOM CaO/Al2O3, and aluminium isopropoxide (AIP) and calcium chloride dihydrate (CaCl2·2H2O) for 3DOM Ca12Al14O32Cl2. The precursor of each catalyst was mixed in ethanol and was stirred for 30 min at room temperature. Subsequently, each precursor solution was added on the PMMA arrays until the PMMA template was saturated with the precursor solution. The materials were dried at 80°C for 24 h and calcined at 700 and 800°C. The heating rate was set at 2°C/min. The atomic mol percentage is shown in Table 1.
2.3. Characterization of the 3DOM Catalysts
All 3DOM catalysts were characterized by XRD, FTIR, and SEM techniques in order to investigate the crystal structure, functional group, and morphology of the synthesized materials, respectively. X-ray diffraction measurements were performed by a Bruker D8 advance diffractometer using Cu Kα radiation (λ = 0.154 nm) with current of 40 mA and voltage of 40 kV. Data were collected in range of 10–70° 2θ with step size of 0.02°.
The infrared spectra were recorded at room temperature in the range of 400–4000 cm−1 with 32 scans and 4 cm−1 resolution using a Bruker Equinox 55 FTIR spectrometer. The surface morphology of the catalysts was observed by a FEI Quanta 450 scanning electron microscope (SEM) using an acceleration voltage of 20 kV.
2.4. Transesterification: Catalytic Study of the Synthesized 3DOM Catalysts
The transesterification condition, adapted from Suwanthai , was employed to study catalytic efficiency of two Ca-doped 3DOM materials, 3DOM CaO/Al2O3 and 3DOM Ca12Al14O32Cl2. The reaction was carried out in a 100 mL three-neck round-bottom flask equipped with a magnetic stirrer. A 10 g of palm oil was added into the flask, and then the temperature of oil was raised to the designated temperature. The 3DOM catalyst and methanol were added into the flask using 8 wt.% of catalyst amount and 12 : 1 of molar ratio of methanol to oil. The reaction was then stirred under 750 rpm at the temperature of 65°C for 3 h. After the reaction was completed, the 3DOM catalyst was separated by centrifugation.
An optimal condition for transesterification using the 3DOM catalyst was achieved by varying the amount of the 3DOM catalyst (6–12 wt.%), molar ratio of methanol to oil (9 : 1–24 : 1), and reaction time (3–5 h), respectively. The reaction temperature was fixed at 65°C.
2.5. Refined Palm Oil (RPO) Conversion Analysis and Biodiesel Analysis
Nuclear magnetic resonance (NMR) spectra were recorded on a VARIAN NMR spectrometer. The spectrum was obtained at 400 MHz for 1H, using CDCl3 as a solvent. The conversion of fatty acid methyl ester was measured using peak areas of the 1H NMR signals from methyl ester at 3.6 ppm and that of glycerol at 2.3 ppm . The percentage of the fatty acid methyl ester conversion was calculated as follows:where is the percentage of fatty acid methyl ester conversion, is peak of the methyl esters, and is peak of the methylene in glycerol.
The biodiesel product from transesterification has been purified and used to study its properties using the following standard condition: acid value (ASTM D664), kinematic viscosity at 40°C (ASTM D445), density (ASTM D1298), and flash point (ASTM D93).
3. Results and Discussions
3.1. Characterization of the 3DOM Materials
The morphology of the synthesized PMMA templates revealed monodispersed PMMA spheres with face-centered cubic (FCC) arrangement  (Figure 1(a)). The average diameter was approximately 338 ± 38 nm. These PMMA colloidal crystals were used as templates for syntheses of 3DOM CaO/SiO2, 3DOM CaO/Al2O3, and 3DOM Ca12Al14O32Cl2 catalysts for transesterification reaction.
The morphology of the 3DOM CaO/SiO2 calcined at 600, 700, and 800°C was exhibited in Figures 1(b)–1(d). It can be seen that a highly ordered porosity was achieved at the temperature of 600 and 700°C (Figures 1(b) and 1(c)), respectively. However, the 3DOM CaO/SiO2 at 800°C was a nonordered structure due to high calcination temperature (Figure 1(d)). Increasing the calcination temperature may result in accumulation and sintering of Ca, resulting in bigger particles and decreasing in surface area [10, 11].
The SEM images of the synthesized 3DOM CaO/Al2O3 at 700 and 800°C were shown in Figures 2(a) and 2(b). The 3DOM structures were obtained after calcination at 700°C with average porous diameter of 231 ± 17 nm. At 800°C, the failures of the hierarchical structures of the 3DOM CaO/Al2O3 were similar to the 3DOM CaO/SiO2 at 800°C. The morphology of 3DOM Ca12Al14O32Cl2 at 700°C was also achieved with a pore diameter of 165 ± 9 nm with irregular particles on the surface (Figure 2(c)).
The XRD patterns of the 3DOM CaO/SiO2 catalysts at 600–800°C are shown in Figure 3. At 600°C, an amorphous SiO2 band centered at 30° and CaCO3 at 29° (104) were presented. No calcium oxide was observed because a decomposition of CaCO3 to CaO occurs at temperature higher than 600°C . The diffraction peaks at 32° (222), 41° (081), and 44° (281) corresponded to calcium silicate (Ca2SiO4) (JPCDS 33-303) and CaCO3 at 23° (012), 29° (104), and 47° (018) (JPCDS. 5-586) were observed after calcination at 700 and 800°C. The 3DOM CaO/SiO2 at 800°C exhibited similar XRD patterns with Ca2SiO4 as a major phase and CaO, CaCO3, and Ca(OH)2 as minor phases (Figure 3).
The XRD patterns of the 3DOM CaO/Al2O3 after calcination at 700 and 800°C are shown in Figure 4. Both samples exhibited diffraction peaks at 32° (111), 37° (200), 54° (220), 64° (311), and 67° (222) corresponding to CaO and weak diffractions peaks of Ca(OH)2 and CaCO3. The XRD pattern of the 3DOM Ca12Al14O32Cl2 at 700°C exhibited mixed phase of Cl-bearing mayenite (JCPDS 1-77-4045)  with CaO and Ca(OH)2.
The FTIR spectra of all 3DOM CaO/SiO2 exhibited characteristic bands at 1000 cm−1 and 480 cm−1, corresponding to stretching and bending vibration of Si–O bond, respectively  (Figure 5). The band at approximately 3600 cm−1 was associated with the OH stretching vibrations of surface hydroxyl groups (Ca–OH) due to moisture absorption . The band at 1460 cm−1 was a C=O vibration of the carbonate group. It is noted that different calcination temperatures did not significantly alter the FTIR pattern of the 3DOM CaO/SiO2. FTIR spectra of both 3DOM CaO/Al2O3 and 3DOM Ca12Al14O32Cl2 exhibited characteristic bands at 500–700 cm−1 and 700–900 cm−1 which corresponded to Al–O vibration of octahedral and tetrahedral Al2O3, respectively . The strong board band centered at 1460 cm−1 was attributed to C=O vibration of the carbonate group. The band at 3645 cm−1 indicated OH stretching vibration of the absorbed water molecules onto CaO giving rise to Ca(OH)2 .
Numerous 3DOM catalysts were prepared. However, not all 3DOM catalysts were suitable for transesterification; for example, the 3DOM CaO/SiO2 calcined at 600°C was achieved but no active CaO was found. At 700–800°C, CaO was produced in the 3DOM CaO/SiO2, but preliminary transesterification studied showed no RPO conversion. Therefore, all 3DOM CaO/SiO2 catalyst were not used for transesterification.
Both 3DOM CaO/Al2O3 and 3DOM Ca12Al14O32Cl2 were used as a solid catalyst for transesterification of palm oil with methanol at 65°C and stirring rate at 750 rpm. The basicity of the catalysts was previously studied using the Hammett indicator [16–18]. The basicity of 3DOM CaO/Al2O3 was in the range of 10.0 < H_ < 12.2, whereas that of 3DOM Ca12Al14O32Cl2 is 11 < H_ < 15.
For 3DOM CaO/Al2O3, the effect of catalyst amount was investigated ranging from 6 to 12 wt.% with molar ratio of methanol to oil of 12 : 1 at 65°C for 3 h. It was found that the refined palm oil (RPO) conversion increased with the increase of the catalyst amount (Figure 6(a)). The 12 wt.% of catalyst gave the highest RPO conversion of 58%. Although the stoichiometric ratio of methanol to palm oil for transesterification is 3 : 1, additional of methanol makes equilibrium moving forward to produce more biodiesel. In this study, it was found that the reaction using 3DOM CaO/Al2O3 catalyst at 12 : 1 MeOH : oil ratio resulted in 47% RPO conversion. However, the higher MeOH : oil ratio at 18 : 1 and 24 : 1 only produced 40% and 17% RPO conversion, respectively (Figure 6(b)). The decreasing of RPO conversion may be due to an alternation of the reaction equilibrium. Excess methanol may increase the solubility of glycerol; therefore, the equilibrium of the reaction shifted backward, resulting in a reduction of biodiesel [16, 19]. The effect of reaction time is shown in Figure 6(c). The reaction time of 3 h gave the lowest RPO conversion of 47% because reaction was incomplete. When the reaction time was increased, the RPO conversion increased to the highest value of 94% at 5 h. The optimal conditions using the 3DOM CaO/Al2O3 catalyst were as follows: 12 wt.% catalyst, methanol to oil molar ratio of 12 : 1, and reaction time 5 h under the temperature of 65°C.
For the 3DOM Ca12Al14O32Cl2 catalyst, the effects of the catalyst amounts varied from 3 to 12 wt.%. It can be seen that increasing catalyst amount gives higher RPO conversion (Figure 6(a)). However, a high catalyst amount (12 wt.%) was not suitable for transesterification because the high catalyst amount led to high viscosity in mixture. The optimum catalyst amount is 6 wt.% by weight. At MeOH : oil molar ratio of 9 : 1, the RPO conversion of 96% was observed (Figure 6(b)). The increasing methanol to oil molar ratio of 12 : 1 and 18 : 1 gave the maximum highest RPO conversion of 99%. The effects of reaction time are shown in Figure 6(c). It obtained the highest RPO conversion at 3 h. The optimal conditions for transesterification were obtained as follows: by the catalyst amount 6 wt.%, methanol to oil molar ratio 12 : 1, and reaction time of 3 h. The RPO conversion was obtained at 99% under the optimal condition.
3.3. Biodiesel Properties
The properties of biodiesel obtained from transesterification using 3DOM CaO/Al2O3 and 3DOM Ca12Al14O32Cl2 were studied following the biodiesel standard of USA (ASTM) and Europe (EN) as shown in Table 2. The density of the produced biodiesel was acceptable, but the acid value and viscosity did not meet the standard value. This may be due to the high acid value (3.02) of raw material. Therefore, the raw material should be esterified to reduce free fatty acid . The high viscosity may be due to the Ca contaminant which causes side reaction (saponification) producing soap which then increases the viscosity of the biodiesel.
The 3DOM CaO/SiO2, 3DOM CaO/Al2O3, and 3DOM Ca12Al14O32Cl2 were successfully fabricated. The 3DOM CaO/SiO2 catalysts calcined at 700°C obtained the crystalline phase of CaCO3, Ca(OH)2, CaO, and Ca2SiO4. The 3DOM CaO/Al2O3 catalysts calcined at 700 and 800°C obtained crystalline of CaO and Ca(OH)2. The results of FTIR spectra were corresponded with functional groups of silica (SiO2), alumina (Al2O3), carbonate (CO32−), and hydroxyl (OH−) groups, depending on the composition of the 3DOM materials. The 3DOM CaO/SiO2 was not suitable for the reaction due to no active CaO. In addition, 3DOM materials that calcined at 800°C exhibited collapsed 3DOM structure. Only 3DOM CaO/Al2O3 and Ca12Al14O32Cl2 were used for transesterification, because the ordered structure of both catalysts was maintained with an active CaO phase after calcination.
For 3DOM CaO/Al2O3, the optimal condition was 12 wt.% of catalyst, the methanol to oil molar ratio of 12 : 1, and reaction time of 5 h under stirring rate of 750 rpm and reaction temperature at 65°C, giving rise to 93% of RPO conversion. The optimal condition of the novel 3DOM Ca12Al14O32Cl2 catalyst was as follows: the catalyst amount 6 wt.%, the methanol to oil molar ratio of 12 : 1, and reaction time of 3 h under stirring rate of 750 rpm and reaction temperature at 65°C, giving 99% of RPO conversion. The 3DOM Ca12Al14O32Cl2 catalyst had more efficiency than conventional solid and 3DOM CaO/Al2O3 catalysts. The density of biodiesel was 0.86 g/cm3 which was in the ASTM specification. The viscosity and acid value of biodiesel product were slightly out of standard range (ASTM D445 and ASTM D664, resp.). This may be due to the Ca contaminant causing saponification. The density of biodiesel, however, was within specification (EN 14214).
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The authors would like to thank the Kasetsart University Research and Development Institute (KURDI) and Faculty of Science, Kasetsart University, for financial supports. The authors also express gratitude to the Center of Excellence-Oil Palm, Kasetsart University, for instrumental assistance.
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