Synthesis of Ca-Doped Three-Dimensionally Ordered Macroporous Catalysts for Transesterification

-e novel three-dimensionally ordered macroporous (3DOM) CaO/SiO2, 3DOM CaO/Al2O3, and 3DOM Ca12Al14O32Cl2 catalysts for biodiesel transesterification were prepared by sol-gel method. -e 3DOM catalysts were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). -e 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. -e result showed that 99.0% RPO conversion was achieved using the 3DOMCa12Al14O33Cl2 as a catalyst under the optimal condition of 12 :1 methanol to oil molar ratio and 6wt.% catalyst with reaction time of 3 hours at 65°C.


Introduction
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 [1].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.
is clean diesel provides low carbon monoxide emission, low greenhouse gases emission, noncombustion of hydrocarbon, and nonsulfur dioxide content compared to those of fossil fuel.e 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 [2].e catalysts for transesterification are categorized into two groups: homogeneous and heterogeneous.e 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.e solid catalysts or heterogeneous catalysts, on the other hand, are easily separated from the produced biodiesel.For example, KNO 3 /Al 2 O 3 , La 2 O 3 /ZrO 2 , and K 2 CO 3 on alumina/silica support are highly active for transesterification of vegetable oils [3][4][5].
Recently, a new hierarchical material named threedimensionally 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.is may enhance biodiesel production.In this work, the threedimensionally ordered macroporous (3DOM) CaO/SiO 2 , 3DOM CaO/Al 2 O 3 , and 3DOM Ca 12 Al 14 O 32 Cl 2 were synthesized by the sol-gel method (SG).e obtained 3DOM catalysts were characterized by various techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR).
e catalytic efficiency of the 3DOM catalysts for transesterification was investigated.Optimal conditions for transesterification using the obtained 3DOM catalysts were studied.e property of the produced biodiesel using the 3DOM catalysts was reported herein.e monodispersed poly(methyl methacrylate) (PMMA) spheres were synthesized by emulsifier-free emulsion polymerization as previously described by Phumthiean [6].A mixture of water and methyl methacrylate was stirred at 75 °C under nitrogen (N 2 ) atmosphere.

Experimental Procedures
en, 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.
e PMMA spheres were self-assembled by gravitation until a clear solution and a colloidal crystal arrays were observed.e obtained PMMA arrays were dried at 60 °C for 24 h.

Preparation of 3DOM Catalysts by Sol-Gel Method.
e Ca-doped 3DOM catalysts were synthesized by the sol-gel method using calcium nitrate tetrahydrate (Ca(NO 3 ) 2 e 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.
e materials were dried at 80 °C for 24 h and calcined at 700 and 800 °C.e heating rate was set at 2 °C/min.e atomic mol percentage is shown in Table 1.

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 °.  e 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.
e surface morphology of the catalysts was observed by a FEI Quanta 450 scanning electron microscope (SEM) using an acceleration voltage of 20 kV.
e 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.
e 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.e 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.e reaction temperature was fixed at 65 °C.

Refined Palm Oil (RPO) Conversion Analysis and Biodiesel
Analysis.Nuclear magnetic resonance (NMR) spectra were recorded on a VARIAN NMR spectrometer.e spectrum was obtained at 400 MHz for 1 H, using CDCl 3 as a solvent.e conversion of fatty acid methyl ester was measured using peak areas of the 1 H NMR signals from methyl ester at 3.6 ppm and that of glycerol at 2.3 ppm [8].e percentage of the fatty acid methyl ester conversion was calculated as follows: where C is the percentage of fatty acid methyl ester conversion, A 1 is peak of the methyl esters, and A 2 is peak of the methylene in glycerol.e 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).
e average diameter was approximately 338 ± 38 nm.ese PMMA colloidal crystals were used as templates for syntheses of 3DOM CaO/SiO 2 , 3DOM CaO/Al 2 O 3 , and 3DOM Ca 12 Al 14 O 32 Cl 2 catalysts for transesterification reaction.
For 3DOM CaO/Al 2 O 3 , the e ect 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 re ned palm oil (RPO) conversion increased with the increase of the catalyst amount (Figure 6(a)).e 12 wt.% of catalyst gave the highest RPO conversion of 58%.Although the stoichiometric ratio of methanol to palm oil for transesteri cation 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/Al 2 O 3 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)).e 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].e e ect of reaction time is shown in Figure 6(c).e 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.e optimal conditions using the 3DOM CaO/Al 2 O 3 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 Ca 12 Al 14 O 32 Cl 2 catalyst, the e ects 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 transesteri cation because the high catalyst amount led to high viscosity in mixture.e 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)).e increasing methanol to oil molar ratio of 12 : 1 and 18 : 1 gave the maximum highest RPO conversion of 99%.e e ects of reaction time are shown in Figure 6(c).It obtained the highest RPO conversion at 3 h.e optimal conditions for transesteri cation were obtained as follows: by the catalyst amount 6 wt.%, methanol to oil molar ratio 12 : 1, and reaction time of 3 h.e RPO conversion was obtained at 99% under the optimal condition.

Biodiesel Properties.
e properties of biodiesel obtained from transesteri cation using 3DOM CaO/Al 2 O 3 and 3DOM Ca 12 Al 14 O 32 Cl 2 were studied following the biodiesel standard of USA (ASTM) and Europe (EN) as shown in Table 2. e density of the produced biodiesel was acceptable, but the acid value and viscosity did not meet the standard value.is may be due to the high acid value (3.02) of raw material.erefore, the raw material should be esteri ed to reduce free fatty acid Advances in Materials Science and Engineering [7].e high viscosity may be due to the Ca contaminant which causes side reaction (saponi cation) producing soap which then increases the viscosity of the biodiesel.), and hydroxyl (OH − ) groups, depending on the composition of the 3DOM materials.e 3DOM CaO/SiO 2 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/Al 2 O 3 and Ca 12 Al 14 O 32 Cl 2 were used for transesteri cation, because the ordered structure of both catalysts was maintained with an active CaO phase after calcination.

Conclusion
For 3DOM CaO/Al 2 O 3 , 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.
e optimal condition of the novel 3DOM Ca 12 Al 14 O 32 Cl 2 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   e viscosity and acid value of biodiesel product were slightly out of standard range (ASTM D445 and ASTM D664, resp.). is may be due to the Ca contaminant causing saponification.e density of biodiesel, however, was within specification (EN 14214).

Figure 6 :
Figure 6: RPO conversion: (a) e ect of % wt.catalyst, (b) e ect of MeOH : oil molar ratio, and (c) e ect of reaction time.

Table 1 :
Atomic mol percentage used for Ca-doped 3DOM material synthesis.
1 was a C O vibration of the carbonate group.It is noted that di erent calcination temperatures did not signi cantly alter the FTIR pattern of the 3DOM CaO/SiO 2 .FTIR spectra of both 3DOM CaO/Al 2 O 3 and 3DOM Figure 3: XRD patterns of the 3DOM CaO/SiO 2 .H Ca(OH) 2 , O CaO, S Ca 2 SiO 4 , and C CaCO 3 .Figure 4: XRD patterns of the 3DOM CaO/Al 2 O 3 at 700-800 °C and the 3DOM Ca 12 Al 14 O 32 Cl 2 .H Ca(OH) 2 , O CaO, M Ca 12 Al 14 O 32 Cl 2 , and C CaCO 3 .4 Advances in Materials Science and Engineering Ca 12 Al 14 O 32 Cl 2 exhibited characteristic bands at 500-700 cm −1 and 700-900 cm −1 which corresponded to Al-O vibration of octahedral and tetrahedral Al 2 O 3 , respectively [3]. e strong board band centered at 1460 cm −1 was attributed to C O vibration of the carbonate group.e band at 3645 cm −1 indicated OH stretching vibration of the absorbed water molecules onto CaO giving rise to Ca(OH) 2 [16].Al 2 O 3 and 3DOM Ca 12 Al 14 O 32 Cl 2 were used as a solid catalyst for transesteri cation of palm oil with methanol at 65 °C and stirring rate at 750 rpm.

Table 2 :
Properties of RPO biodiesel.Advances in Materials Science and Engineering temperature at 65 °C, giving 99% of RPO conversion.e 3DOM Ca 12 Al 14 O 32 Cl 2 catalyst had more efficiency than conventional solid and 3DOM CaO/Al 2 O 3 catalysts.e density of biodiesel was 0.86 g/cm 3 which was in the ASTM specification.