Synthesis and Characterization of Calcium Oxide Impregnated on Silica from Duck Egg Shells and Rice Husks as Heterogeneous Catalysts for Biodiesel Synthesis

: Biodiesel can be prepared from various types of vegetable oils or animal fats with the aid of a catalyst. Calcium oxide (CaO) is one of the prospective heterogeneous catalysts for biodiesel synthesis. Modification of CaO by impregnation on silica (SiO 2 ) can improve the performance of CaO as catalyst. Egg shells and rice husks as biomass waste can be used as raw materials for the preparation of the silica modified CaO catalyst. The present study was directed to synthesize and characterize CaO impregnated SiO 2 catalyst from biomass waste and apply it as catalyst in biodiesel synthesis. The catalyst was synthesized by wet impregnation method and characterized by x-ray diffraction, x-ray fluorescence, nitrogen adsorption-desorption, and basicity density. The activity of the catalyst in biodiesel synthesis was assayed at different molar ratios of palm oil to methanol ranging from 1:6, 1:9, 1:12 and 1:15. The biodiesel composition was determined by gas chromatography-mass spectroscopy and the properties of the biodiesel were also characterized. The results showed that the CaO impregnated SiO 2 catalyst was successfully synthesized based on confirmation by XRD and XRF. The catalyst has a surface area, average pore diameter, total pore volume, and basicity density of 19.38 m 2 /g, 3.22 nm, 0.0122 cm 3 /g, and 3.4 mmol/g, respectively. The catalyst activity assay indicates that the molar ratio of palm oil to methanol of 1:12 is the optimum condition for biodiesel synthesis. At this molar ratio, 81.4% biodiesel yield was achieved, and it met the quality standards according to ASTM D 6751.


Introduction:
The world's energy consumption, especially in developing countries, is currently still dependent on the type of energy from oil and gas.For example, in Indonesia the energy consumption was 989.9 million Barrel Oil Equivalent (BOE), of which 42%, 17%, and 10% was met by gasoline, coal, and gas, respectively, with an average growth rate of fuel oil demand is 2.8% per year 1 .The greenhouse effect and global climate change as a result of fuel combustion are critical issues that affect the energy industry, public policy makers, and society.The increase in global warming and the impact of other environmental hazards have prompted almost all countries to reduce dependence on fossil fuel 2 .
Biomass is one of the most important renewable and sustainable energy sources 3 .The processing of this biomass will produce one type of renewable fuel, for example biodiesel.Biodiesel in general is a methyl ester of long chain fatty acids so it is often known as fatty acid methyl ester (FAME).Biodiesel can be made from oil extracted from fungi, algae, vegetable oil, algae, animal fat, or other sources such as waste cooking oil [4][5][6][7] .
Application of homogeneous catalysts in transesterification stage during biodiesel production has potency to cause several problems, including the occurrence of saponification reactions, increased viscosity, non-optimal yield of biodiesel, and separation of the catalyst at the end of the reaction is relatively difficult 8 .The main disadvantage of homogeneous catalysts is that they cannot be reused or regenerated 9 .Therefore, the application of a heterogeneous catalyst can be considered to reduce these problems.Calcium oxide (CaO) has received much attention as a solid heterogeneous catalyst for the synthesis of biodiesel because it requires mild reaction conditions, relatively inexpensive, and has less negative impact on the environment 10 .
Calcium oxide without modification from various sources has been prepared with different characteristics and applied as heterogeneous catalyst in the synthesis of biodiesel 11,12 .Improvement of the characteristics of solid heterogeneous catalysts can be carried out with various efforts.One of them is by expanding its specific surface by impregnation on silica (SiO2).Lani et al. succeeded in improving the characteristics of the CaO catalyst through the impregnation method on SiO2 from rice husk 13 .
CaO impregnated on SiO2 (CaO/SiO2) catalyst can be synthesized from various types of biomass.CaO can be prepared from egg shell calcination, while rice husk is a potential biomass as silica source.Rice husk has a high content of amorphous SiO2 14 .Heterogeneous catalysts synthesized from biomass have varied characteristics, depending on various factors, such as: the type and characteristics of the biomass, as well as the method and conditions of catalyst preparation.The optimal performance of heterogeneous catalytic reactions in biodiesel synthesis is influenced by the type and characteristics of the catalyst, reaction conditions, and physico-chemical properties of the oil 15 .
Therefore, the present study was directed to synthesize and characterize CaO/SiO2 from biomass waste commodities, egg shells and rice husks, and evaluate its performance as catalysts in the transesterification of palm oil into biodiesel.

Materials and Methods: Preparation of Catalyst
Calcium oxide as the active ingredient of the catalyst was prepared from the shells of Java duck eggs.The egg shells were washed with demineralized water, dried, ground with a mortar, and sieved to obtain a 50 mesh size powder.Egg shell powder was calcined at a temperature of 900°C for 1 hour with a temperature rise rate of 10°C/minute.Silica as the support of calcium oxide was prepared from rice husks.Rice husks were obtained from the milling process of IR-64 rice varieties from Sumedang, West Java.Rice husks were washed with demineralized water to remove impurities, and then dried in an oven at 60°C for 24 hours.Furthermore, rice husks were soaked in 1 N hydrochloric acid solution for 1 hour, washed with distilled water until neutral pH was reached and dried.Rice husks were ashed in a kiln at 600°C for 6 hours.Rice husk ash was suspended in 0.5 N sodium hydroxide solution with an ash to solution ratio of 1:100 (g/mL) while heated and stirred at 100°C for 4 hours to extract silica in the form of sodium silicate.The extracted mixture was filtered through Whatman filter paper no.41 to take the filtrate.Then 10% sulfuric acid was added to the filtrate until the pH reached 7, and sodium silicate was converted to silica gel.The reaction was carried out for 24 hours, followed by silica gel precipitation stage for 48 hours.The silica gel was filtered, washed with distilled water, and dried at 105°C for 24 hours.
Calcium oxide impregnated on silica catalyst was prepared using the wet impregnation method.5.0 g of calcium oxide is dissolved in 100 mL of distilled water.Then 3% (w/w) silica was mixed with calcium oxide solution, and refluxed for 4 hours at 80°C.The reflux product was filtered, and the residue was dried at 60°C for 24 hours.The dry residue from the impregnation was then it is calcined in a furnace at 800°C for 3 hours.

Characterization of CaO/SiO2
The x-ray diffraction (XRD) pattern was collected using a Philips Analytical Diffractometer by using a Cu-Kα radiation.The elemental composition in the form of metal oxides of CaO/SiO2 was collected using fluorescent x-ray (XRF) method.NOVA 3200e Quantachrome TouchWin v1.0 was used to obtain the surface area, total pore volume, and pore size distribution of the catalyst through the adsorption-desorption of nitrogen gas method using the Brunauer-Emmet-Teller (BET) and Barret-Joyner-Hallenda (BJH) equations.While the basicity density of the catalyst was determined by the acid-base titration method according to Kim et al. 16 .

Synthesis and Characterization of Biodiesel
A mixture of methanol and 3% (w/w) catalyst was heated in the reactor at 50°C for 30 minutes.Furthermore, oil was added to the reactor to reach final ratio of oil to methanol of 1:6, 1:9, 1:12 and 1:15.The transesterification was carried out for 2 hours at 60°C.The reaction product was transferred to a separator funnel to separate the catalyst from the biodiesel mixture, residual methanol and glycerol.The biodiesel and residual methanol were then separated from the glycerol by centrifugation at 7000 rpm for 15 minutes.Biodiesel was purified from methanol by distillation.The biodiesel yield was determined using Eq. 1. Extraction of silica from rice husk with NaOH and H2SO4 solution occurs based on the following serial reaction 19 Eq. 5 to 7.
SiO2(s) + 2NaOH(aq) → Na2SiO3(aq) + H2O(l) … 5 Na2SiO3(aq)+H2SO4(aq) →H2SiO3(l)+ Na2SO4(aq) …6 H2SiO3(l) → SiO2.H2O(s) … 7 The average yield of silica gel from rice husk ash after drying is 77.31%.The silica gel yield from the three extraction stages are shown in Table 3.The silica yield obtained varies depending on the washing step with distilled water to remove sulfate salts.The relatively low silica yield is thought to be because the silica gel also escapes in the washing step.Calcined duck egg shell, silica gel, and dry silica are presented in Fig. 1.

Characteristics of CaO Impregnated on Silica Catalyst
CaO from calcined duck eggshell after impregnation on silica from rice husk has distinctive characteristics, both surface characteristics and activity as a catalyst.The temperature factor in the wet impregnation method is one of the important factors.The dispersion of oxides (CaO and silica) as the active phase will occur if the thermal treatment of the oxide mixture is carried out at a temperature high enough to induce the mobility of the active phase 20 .

Characteristics of Catalyst with XRD and XRF
The diffraction pattern of the prepared CaO/SiO2 is presented in Fig. 2. Typical diffraction peaks of CaO were observed at 2θ of 32.2°; 37.4°; 53.9°; 64.2°; 67.4°; 79.7° and 88.6°.Meanwhile, there is a peak with a fairly high intensity at 32.0°; 32.2° and 32.6° which indicate the presence of a new phase formed, namely Ca2SiO4.This agrees with results described by Yamaguchi et al. who found that Ca2SiO4 has several forms of crystal types where in the range 2θ of 32°-33° is the crystal peak of Ca2SiO4 with monoclinic crystal type 21 .The presence of H2O during the wet impregnation process between CaO and SiO2 allows the formation of dicalcium silicate hydrate (Ca2SiO4.H2O), which is then dehydrated to form Ca2SiO4 compounds.The crystal peak of Ca3SiO5 (hatrurite or tricalcium silicate) was observed at 2θ of 29.5°.Ca3SiO5 is formed from the reaction between Ca2SiO4 and calcium hydroxide 22 .The chemical composition of the catalyst using the XRF method showed that after CaO was impregnated on SiO2, the CaO content in the catalyst was 72.8%, while the silica content as a buffer was 26.4% (Table 4).The more CaO content in the CaO/SiO2 catalyst, the higher the basicity, so that the catalyst activity in the transesterification process is higher.The amount of CaO and SiO2 in the hybrid catalyst depends on the success of the impregnation process.5, shows the results of surface area, total volume, and pore diameter of Ca/SiO2 catalysts, as well as their comparison with research results from Lani et al. 13 .The two features have relatively significant different characteristics even though they are synthesized from the same type of raw material and method.Impregnation of CaO on silica is expected to increase the activity of the catalyst by increasing the accessibility of the active site and providing sufficient attachment area 23 .In comparison, the pore diameter of CaO as catalyst 33-41 nm, is slightly larger than that of the molecular reaction, where the dimensions of glycerol and biodiesel product are about 0.10 nm and 0.64 to 1.52 nm 24 , respectively.The larger average pore diameter and the interconnection between the catalyst pores will limit the molecular diffusion of the reactants, so that the molecular reactants can easily infiltrate the interior of the catalyst, and most of the active sites will be used during the transesterification reaction 25 .The result of the basicity density of the catalyst by acid-base titration was obtained at 3.4 mmol/g of catalyst.

Biodiesel Yield and Characteristics
Biodiesel is synthesized from RBDPO (refined, bleached, and deodorized palm oil) with a free fatty acid content of 0.11%, at molar ratio variation of oil to methanol 1:6, 1:9, 1:12 and 1:15, the application of 3% CaO/SiO2 produced biodiesel and the yield are presented in Fig. 3, while the kinematic viscosity and biodiesel density values of each molar ratio variation and its comparison with quality standards according to ASTM D 6751 are presented in Table 6.Transesterification at a molar ratio of 1:12 resulted in the highest biodiesel yield, which was 81.4%.The density of biodiesel from all oil to methanol molar ratios has met the standard of ASTM D 6751, which is 860-894 kg/m 3 .Meanwhile, ASTM D 6751 requires biodiesel to have a kinematic viscosity of 1.9-6.0mm 2 /s.Therefore, the viscosity of biodiesel from transesterification with a molar ratio of oil to methanol of 1:6 does not meet the maximum limit value requirement.Based on the consideration of yield value and biodiesel characteristics in the density and viscosity parameters, a molar ratio of oil to methanol of 1:12 was decided as the optimum condition for transesterification.For comparison, Table 7, shows the characteristics of CaO-based catalysts and biodiesel yields produced by several researchers.Biodiesel yield is influenced by the characteristics of the catalyst and the conditions of the transesterification reaction.Further characterization of biodiesel from the optimum molar ratio obtained the results as shown in Table 8.Of all the measured biodiesel quality parameters, biodiesel from the research results fulfilled the biodiesel quality standard according to ASTM D 6751.Analysis with GC-MS showed that biodiesel or FAME is dominated by methyl oleate and methyl palmitate with composition of 47.88 and 38.61%, respectively (Table 9), while the chromatogram of biodiesel is shown in Fig. 4.

Conclusion:
CaO impregnated on SiO2 catalyst has been successfully synthesized from local biomass waste in Indonesia in the form of duck egg shells and rice husks of IR-64 variety.The catalyst has certain characteristics based on XRD, XRD diffraction patterns, surface area, total pore volume, average pore diameter, and basicity density.The CaO/SiO2 catalyst has optimum activity in transesterification of pure palm oil into biodiesel at a molar ratio of oil to methanol of 1:12 with biodiesel yield of 81.4% and has met the ASTM D 6751 standard.

Figure 2 .
Figure 2. XRD diffraction pattern of CaO impregnated on SiO2 catalyst

Figure 3 .Figure 4 .
Figure 3.The relationship between the molar ratio of oil to methanol and biodiesel yield in the transesterification of RBDPO with 3% CaO/SiO2 catalyst

Table 1 . Percentage of mass lost from duck egg shell during calcination stage at 900°C for 1 hour
During the reaction, CO2 was released.The formation of CaO is characterized by a reduction in the mass of the calcined egg shell.The mass reduction in the form of mass lost at four times of calcination of duck egg shells is shown in Table1, while the composition of metal oxides determined by XRF is shown in Table2.Calcined duck egg shells contain about 98.5% of CaO and various other metal oxide impurities in relatively small levels.