1. Introduction

Awareness of environmental protection encourages many researchers to study in biodiesel production as an alternative to fossil fuels. Fossil fuel can give negative impact on the environment through carbon oxides emissions, unburned hydrocarbons, sulphur dioxide and particulates [1,2,3,4]. Biodiesel can minimize greenhouse gas emissions because carbon dioxide from biodiesel combustion is offset by the carbon dioxide sequestered while growing oil palm or other feedstock [5]. Biodiesel usually obtained from vegetable oil such as palm oil [6], soybean oil [7], canola oil [8], and rapeseed oil [9] or sometimes from animal fats [10] by transesterification (Scheme 1) and molecules in biodiesel are primarily known as Fatty Acid Methyl Esters (FAME). Triglyceride are the composition of vegetable oils and animal fats, which are esters containing three free fatty acids. During transesterification, triglycerides are reacted with methanol which is low molecular weight alcohol to produce methyl esters of fatty acids and glycerol.

Triglycerides to methyl esters in complete reaction involves three sequential reactions with a monoglyceride (MAG) and a diglyceride (DAG) as intermediates. During transesterification, triglycerides (TAG) in the oil react with alcohol in the presence of a catalyst such as sodium hydroxide, NaOH or potassium hydroxide KOH to produce biodiesel. Transesterification occurs in three sequential reversible steps: (a) TAG reacts with methanol to produce a diglyceride (DAG), liberating a single fatty acid methyl ester, (b) DAG reacts with methanol to produce a monoglyceride (MAG) and another FAME, and (c) MAG reacts with methanol to produce a FAME, liberating the glycerol by-product. However, MAG and DAG are formed and remain in the final biodiesel product [11].

Scheme 1. Transesterification

Biodiesel produced by transesterification of vegetable oil with methanol is possible with both homogeneous acid or base catalyst and heterogeneous acid, base, or enzymatic catalysts [12, 13]. The base catalysts are more often used commercially than acid catalysts, which are corrosive. The development of the heterogeneous catalyst alleviates most of the economic and environmental weaknesses of the homogeneous catalyst problems such as the neutralization of the base catalyst during reaction and the difficulties when washing and separating the final product which will produce large amounts of wastewater [14,15].

Scheme 2. Three consecutive reactions during transesterification

Ideal heterogeneous catalysts are highly stable and mesoporous, have strong active sites and a low cost [16]. A common technique to prepare heterogeneous catalyst is impregnation method which very simple, clean and environmentally friendly process. The development of a solid catalyst loaded on a support or carrier and can be called as impregnated catalyst, is very promising with good conversion results in the transesterification of vegetable oil. Previous research includes, K₂CO₃/Al-O-Si aerogel catalyst [3], F/CaO [14], and KOH supported on palm shell activated carbon catalysts [17]. Catalyst support with porous materials are favoured due to their high surface [2]. Other heterogeneous catalysts that showed good performance in the transesterification of vegetable oil are Na/NaOH/γ- Al₂O₃ [7], commercial hydrotalcite [18], zeolites and modified zeolites [19,20] but complicated preparation has limited their industrial applications.

Transesterification of vegetable oils catalysed by various heterogeneous catalysts have been developed. Silica loaded with base metal salts or different potassium compounds are efficient solid-base catalysts [21]. K₂CO₃/Al-O-Si aerogel catalyst prepared by the sol-gel method exhibits good activity with a high yield of over 92%. However undesired leaching of the active components was also observed [3]. It was found that the K₂CO₃ catalyst exhibited much higher performance, proving that potassium plays different roles in catalysis. Pure K₂CO₃ proved to be remarkably active in biodiesel production by transesterification of sunflower oil with 90% conversion of oil in 108 min [22].

This work aims to produce biodiesel through a heterogeneous system with silica loaded potassium carbonate as a solid base catalyst in the transesterification of palm oil. The catalyst was prepared by impregnation method. To study the physicochemical properties of the catalyst prepared by X-ray Diffraction (XRD), Brunauer – Emmet - Teller (BET) surface area, Scanning Emission Microscopy (SEM), and Temperature Programmed Desorption of Carbon Dioxide (CO₂-TPD). The product will be analysed by ¹H-Nuclear Magnetic Resonance (¹H-NMR) to confirm the existence of biodiesel and Gas Chromatography (GC) to determine composition of methyl esters.

2. Materials and Method

2.1. Materials

Refined palm oil was purchased from Giant Hypermarket, Malaysia and all chemicals such as potassium carbonate (99.99%), silica (99.99%), and methanol were obtained from Merck, Germany. The given properties of silica were pore size of 100 Å and surface area 330 m²/g, making it suitable for use as a catalyst support.

2.2. Methods

2.2.1. Catalyst Preparation

The catalysts were synthesized by using a wet impregnation method. Potassium carbonate powder, K₂CO₃ was mixed with 10 mL deionized water to form a K₂CO₃ solution. The solution was poured onto 5 g silica which acted as a support for the catalyst with weight percentage of 5 wt%. The mixture was stirred for 30 min. The solids were dried overnight in an oven at 373 K and calcined in air at 1273 K for 4 h. The calcined catalyst was then crushed into powder and sieved. The preparation process was repeated for other catalyst loadings of 10 wt%, 15 wt%, and 20 wt% K₂CO₃/SiO₂.

2.2.2. Catalyst Characterization

The crystalline phase of the catalyst was assessed by X-ray diffraction (XRD) analysis using a Shimadzu XRD-600 Diffractometer by employing CuKα radiation (λ = 1.541 Å) generated by Philips glass diffraction x-ray tube broad focus 2.7 kW type, operated at ambient temperature (30 kV and 100 mA). The samples were scanned at a range of 2θ = 10^o – 60^o with a scanning rate of 2^o/min. The obtained diffractograms were matched against the Joint Committee on Powder Diffraction Standards (JCPDS) PDF1 database version 2.6. The surface area of the samples were determined by the nitrogen (N₂) adsorption-desorption technique, using Thermo Finnigan Sorptomatic Instrument model 1900. The BET surface area was calculated and total pore volume was determined by the estimation from the N₂ uptake The morphology and surface structure of the sample (support and catalysts) were identified using SEM with LEO 1455 Variable Pressure scanning electron microscopy. The analysis was performed at an accelerated voltage of 20kV. The samples were coated with a thin layer of gold as the conducting material using a BIO-RAS sputter coater. Basicity of the catalysts was determined using CO₂-TPD analysis and was performed using Thermo Finnigan TPD/R/O 1100. For each experiment, 0.002 g of catalyst was pre-treated in nitrogen to remove all water vapour and any impurities in the pipeline, which was then heated up to 523 K at 30 K min^-1. The samples were cooled to adsorption temperature 300 – 450 K and loaded with carbon dioxide. Prior to analysis, the pre-treated samples were flushed with helium and heated up to 1173 K at 10 K min^-1.

2.2.3. Transesterification Reaction

Synthesized catalysts were tested for transesterification of palm oil. Commercial palm oil (cooking oil Cap Buruh) was used and the experiments were performed in 50 mL round bottom flasks with a water-cooled condenser. 5 g of palm oil, methanol (MeOH) to oil molar ratio of 20:1, catalyst amount of 4 wt% with 5 wt% K₂CO₃/SiO₂ was put into the round bottom flask. Reflux was performed at 333 K for 1 h. The experimental design consisted of five factors and four levels (Table 1). At the end of the reaction, the mixtures were centrifuged. Three phases formed, the upper layer was methanol, the middle layer was a mixture of biodiesel and little amount of glycerol, and the bottom layer was solid catalyst. The catalyst was easily removed after centrifugation. Methanol was then removed from the biodiesel mixture by heating the mixture at 343 K for 1h to completely evaporate the methanol. The biodiesel mixture was left overnight for self-separation of biodiesel from glycerol. A separating funnel was used, resulting in an upper layer of biodiesel and bottom layer of glycerol. The biodiesel obtained was analyzed using ¹H-nuclear magnetic resonance (¹H-NMR) and gas chromatography (GC).

Table 1. Factors and their levels employed in orthogonal array

3. Results and Discussion

3.1. Characterization of the Catalyst

3.1.1. X-ray Diffraction (XRD)

The XRD pattern of SiO₂ support (Figure 1) shows three main peaks at 2θ = 20.31^o, 21.50^o, and 35.5^o which correspond to (100), (002), and (110) planes, respectively. All of these peaks are well matched to the reference peaks from SiO₂ references peaks from SiO₂ JCPDS file no. 18-1169. However, the peak at 2θ = 23.01^o is not clearly seen. This is due to the semicrystalline nature of the calcined SiO₂ support resulting from the irregular arrangement of atoms and/or molecules. Impregnating the pure SiO₂ solid with 5wt% K₂CO₃ produced the effective progressive increase in the degree of crystallinity of the above mentioned phase and intensity. The appearance of peak at 2θ = 23.01^o is proportional to the amount of K₂CO₃. The presence of K₂CO₃ was observed according to the JCPDS file no. 01-07-0292.

Table 2 lists the full width at half maximum (FWHM) values of the catalyst corresponding to 2θ = 21.50^o of the (002) plane. It is found that the FWHM values increases with K₂CO₃ loading. It can be seen from Figure 1 that the peak intensity decreases suggesting that at low loadings, K₂CO₃ might be mainly residing on the catalyst surface. With increased amounts, K₂CO₃ might also be incorporated into the SiO₂ lattice causing disarray in the support atomic arrangement. The K₂CO₃ not only disrupts SiO₂ crystallinity but also affects the size of the particles. The crystallite sizes of K₂CO₃/SiO₂ corresponding to 2θ = 21.50^o were 82.39, 58.96, 51.51, and 30.71 nm with K₂CO₃ loadings of 5, 10, 15, and 20 wt% K₂CO₃/SiO₂, respectively. The crystallite size of SiO₂ was calculated by using the Debye-Scherrer formula:

Figure 1. X-ray patterns of SiO2 and K2CO3 supported SiO2 catalysts ( ) K2CO3 ( ) SiO2

(1)

Where

t = crystallite size for the (hkl) phase

λ = X-ray wavelength of radiation for CuKα (1.5438 Å)

β_hkl = full-width at half maximum (FWHM) at the (hkl) peak

θ_hkl = diffraction angle for the (hkl) phase

Table 2. XRD data for all catalysts

3.1.2. BET Surface Area Measurement

The BET surface area, pore volume, and pore diameter of the supported catalysts are presented in Table 3. As shown in the table, the BET surface area of unsupported SiO₂ is 333.3 m²/g. However, the surface area decreases significantly to 71.9, 37.2, 28.1 and 12.3 m²/g for 5, 10, 15, 20 wt% of K₂CO₃ loading, respectively. The BET surface area values further decreased when the samples were calcined, in which 0.53, 0.49, 0.45, and 0.40 m²/g were obtained for 5, 10, 15, 20 wt% supported samples, respectively. The huge differences in the BET surface area values for the calcined samples as compared to the uncalcined ones are due to the granulation of the originally powdery solid when the samples were calcined at high temperatures.

Table 3. Surface area, pore diameter, and pore volume for silica and K2CO3/SiO2 for (A) before calcination and (B) after calcination

As can be seen in Table 3, the pore volume of the catalysts ranged from 0.0030-0.0041 cm³ whereas the pore size diameter ranged from 30.5 to 32.1 nm. According to Fernandez et al., (2007) [22], they defined that the diameter of triglyceride molecules (around 2 nm), as the diameter of the smallest cylinder where the molecule can pass without distortion. All of the catalysts synthesized in this study could easily accommodate the bulky triglyceride. Furthermore, the presence of K₂CO₃ on the surface and in the voids allows for sufficient contact between the reactant and the catalyst’s active sites. There may also be some very small pores that cannot be occupied by triglycerides [23].

3.1.3. Scanning Electron Microscopy (SEM)

Details of the surface morphology of the K₂CO₃/SiO₂ catalyst were obtained from scanning electron microscopy (SEM) with magnification of 3000x and at a scanning voltage of 15kV are shown in Figure 2. All of the supported catalysts showed similar morphology and there is no particular shape can be derived from the image except for the rough surfaces of the materials. The images also shows were well distributed on SiO₂.

Figure 2(a). SEM micrograph of 5 wt% K2CO3/SiO2

Figure 2(b). SEM micrograph of 10 wt% K2CO3/SiO2

Figure 2(c). SEM micrograph of 15 wt% K2CO3/SiO2

Figure 2(d). SEM micrograph of 20 wt% K2CO3/SiO2

3.1.4. Temperature Programmed Desorption of Carbon Dioxide (CO₂-TPD)

The basicity of the catalyst was evaluated using CO₂-temperature programmed desorption (CO₂-TPD). Figure 3 shows the CO₂-TPD profiles of K₂CO₃/SiO₂ catalysts. The CO₂-TPD curves demonstrate the base strength of K₂CO₃-supported silica with different weight percentages. The quantification of desorbed CO₂ is accomplished by calculating the area under the peak and the amount obtained for each sample (Table 4).

Table 4. Total number of CO2 Molecules from Each Catalyst by CO2-TPD

Figure 3. CO2-TPD profiles of catalyst with different K2CO3 loading

The strength of basic sites was deduced from the work by Pasupulety et al., (2013) [24], where it was suggested that desorption temperature between 673-873 K indicates basic sites of weak and medium strength, and desorption temperature range of 873-1123 K indicates strong basic sites. According to Figure 3, it was discovered that 5 wt% K₂CO₃/SiO₂ catalyst produces a strong base as stipulated by the high desorption temperature of more than 600 K. In this study, the differences in the distribution of basic sites for each catalyst indicates that the basicity and base strength distributions are significantly influenced by the amount of K₂CO₃ loading on SiO₂. Figure 3 also revealed that the loading of 5 wt% K₂CO₃ on SiO₂ results in the creation of a large number of basic sites of weak and medium strength at T_max 612, 818, and 946 K. High amounts of K₂CO₃ (15 wt% and 20 wt%) increases the basic strength as the strong basic sites were revealed again by 10wt% K₂CO₃/SiO₂ at T_max = 694, and 929 K. Hence, high desorption activation energy is needed i.e 109.5 and 145.4 kJ/mole respectively.

From Table 4, it can be seen that the total amount of CO₂ desorbed increases from 1.6x10²⁰, 1.9x10²⁰, 4.1x10²⁰, and 9.6x10²⁰ atom/g with increased amounts of K₂CO₃; 5, 10, 15, and 20wt% K₂CO₃/SiO₂, respectively. It is noteworthy that the amount of carbon dioxide molecules available thermally was directly proportional to the K₂CO₃ loadings. It is possible that more active basic sites are present in the supported catalysts and therefore, there would be a higher availability of carbon dioxide that could be thermally desorbed. The order of basicity was as follows: 20wt% K₂CO₃/SiO₂ > 15wt% K₂CO₃/SiO₂ > 10wt% K₂CO₃/SiO₂ > 5wt% K₂CO₃/SiO₂. The trend suggests that higher K₂CO₃ loading in the case of silica-supported catalyst would achieve higher activity and selectivity in biodiesel production. This phenomenon indicated that the basicity of the catalysts became stronger as was reported by previous studies, Gao et al., (2008) [25] and Jing et al., (2004) [26]. The enhanced basic sites of the catalysts enabled high yield reactions to occur.

As was previously discussed, the number of basic sites were quantified by the integration of the desorption curves (Figure 3), which is proportional to the number of moles of CO₂ desorbed from the surface, which in turn is proportional to the number of monolayers. As reported by Zãvoianu et al., (2001) [27], desorbed CO₂ would increase the basicity, which would also increase the number of monolayers. It was reported that supported samples with a monolayer of the active phase generally has a higher number of adsorption sites per weight of active phase. This is attributed to the higher dispersion of the active phase and its interaction with the support. In this study, the number of monolayers increased by 0.8, 0.9, 1.8, and 4.3 as the loading of K₂CO₃ on the support increased from 5, 10, 15, and 20 wt% respectively.

3.2. Analysis of Biodiesel

3.2.1. ¹H-Nuclear Magnetic Resonance (¹H-NMR)

¹H-Nuclear Magnetic Resonance (¹H-NMR) was used to quantify fatty compounds in biodiesel based on the fact that the amplitude of a proton nuclear magnetic resonance (¹H-NMR) signal is proportional to the number of hydrogen nuclei contained in the molecule [28]. Figure 4 shows the spectrum of the biodiesel obtained at optimum condition by using K₂CO₃/SiO₂ catalyst. However, without using K₂CO₃/SiO₂ catalyst, biodiesel cannot be produced. The signal for methylene protons appears at 2.3 ppm which together with ester group in triglycerides. After transesterification, the methoxy protons of methyl esters appear at 3.7 ppm (Figure 4 and Table 5), which describes the typical chemical structures related to the ¹H-NMR spectrum.

Figure 4. 1H-NMR spectrum of biodiesel produced at reaction time 3 h, reaction temperature of 333 K, methanol to oil molar ratio of 20:1 with 20 wt% catalysts loading and 4wt% of catalyst amount

Table 5. Assignment of 1H-NMR peaks of contained in biodiesel

Peaks are noted for saturated structures. Most of the peaks have a chemical shift if the resonance is between 0 and 5 ppm. Additionally, the resonance of the unsaturated structures are between 5 and 9 ppm (alkene proton between 5 and 7 ppm, and aromatic protons between 7 and 9 ppm). Peak areas are measured by electronic configuration of the response signals in a spectrum. The yield of methyl esters was determined by areas of the signals of methylene and methoxy protons, according to the following equation [29]:

(2)

Where Y_FAME is the yield (in percentage) of methyl esters, A_ME is signal area of the equivalent hydrogen singlet from the methyl ester methoxyl group (strong singlet); A_CH2 is the signal area from the methylenic protons. In addition, derivation of factor 2 and 3 from the fact that the methylene carbon possesses two protons and the alcohol (methanol derived) carbon has three attached protons. Confirmation of the existence of methyl esters in biodiesel can be determined by these two distinct peaks. Other observed peaks are at 0.85 ppm of terminal methyl protons, a strong signal at 1.26 ppm related to methylene protons and at 5.28 ppm due to olefinic hydrogen [30]. A broad superposition of triplets between 1.0 and 0.8 ppm might also be considered as it appears as the terminal methyl group.

3.2.2. Component Determination in the Biodiesel by Gas Chromatography (GC)

The synthesized biodiesel products were analysed by gas chromatography to determine the composition of fatty acid methyl esters (Figure 5). Each peak corresponds to a fatty acid methyl ester component of palm oil and was identified using the library match software. The identities of the fatty acid methyl esters (FAME) were verified by comparing the respective retention time data with mass spectroscopic analysis.

Figure 5. Chromatogram analysis for biodiesel produced at reaction time 3 h, reaction temperature of 333 K, methanol to oil molar ratio 20:1, with 20wt% catalyst loading and 4wt% of catalyst amount. FAME: C12 = methyl laureate; C14 methyl myristate; C16: methyl palmitate; C18 (C18:2 methyl linoleate; C18:1 methyl oleate; C18:0 methyl stearate)

The presence of three saturated and two unsaturated FAMEs in palm oil by GC analysis confirmed as they having similar impact spectra. FAME was in the range of 4 – 6 min [22]. Table 6 presents the results obtained for the biodiesel samples. The main methyl esters for palm oil biodiesel are palmitate (C16:0) and oleate (C18:1). This data agrees with published literature [31, 32].

Table 6. Fatty acid composition of biodiesel produced

4. Conclusions

Silica loaded with K₂CO₃ catalyst was successfully prepared by the impregnation method and improved biodiesel production. The influence of the different K₂CO₃ loadings on silica, which was varied from 5, 10, 15, and 20 wt% were studied comprehensively. The addition of K₂CO₃ on SiO₂ significantly influenced the crystallinity of the supported catalyst but also affected the particle size of the catalyst, which was found to decrease with increased amounts of K₂CO₃. Additionally, BET surface area analysis revealed that K₂CO₃-supported SiO₂ has a small surface area but its high pore diameter allows it to accommodate bulky triglyceride molecules. According to the CO₂-TPD results, the addition of K₂CO₃ on SiO₂ results in the creation of strong basic sites which is needed as the basicity of the catalyst plays an important role in base-catalyzed biodiesel production which in this case 20wt % catalyst which has shown having the highest number of basic site which is determined as the prime contributor to the highest activity for the transesterification reaction as opposed to other loadings.

The transesterification reaction of palm oil catalyzed by silica loaded with K₂CO₃ prepared in this study revealed the efficiency of the catalyst for the reaction. It was found that up to 98.10% yield of biodiesel was obtained when palm oil was transesterified with methanol in the presence of K₂CO₃/SiO₂ catalysts. The existence of the biodiesel was confirmed by ¹H-NMR analysis in which the distinct peaks of methyl esters methoxyl group at signal 3.67 ppm, α methylene groups of esters at signal 2.31 ppm and methyl ester compositions were successfully quantified.

ACKNOWLEDGEMENTS

This research was supported by Graduate Research Fellowship (GRF). The authors gratefully acknowledge the contribution from Malaysian Palm Oil Board (MPOB) for helping with the yield analysis.