Performance Comparison of Heterogeneous Catalysts based on Natural Bangka Kaolin for Biodiesel Production by Acid and Base Activation Processes

This study is aimed to evaluate e?ciency low-cost natural kaolin from Bangka island as a catalyst to produce biodiesel (fatty acid methyl ester, FAME) via the transesterification reaction using the cooking oil as a model of free fatty acids (FFA) source. Heterogeneous catalysts of the natural kaolin were prepared by an activation process using base (1-2 M NaOH) and acid (1 M HCl) solution. The base and acid activated kaolin are labelled as Kb1M, Kb2M, and Ka1M, respectively. The quality of biodiesel was analyzed according to the SNI 04-7182-2015 method, the American Society for Testing and Materials (ASTM) D 6751, and the Europäische Norm (EN) 14214, while the composition of biodiesel was determined using gas chromatography-mass spectrometry (GC-MS) analysis. Among the activated kaolin, the Kb2M showed the best heterogenous catalyst performance, producing 96.3% methyl ester with a yield of 69.4%. The highest FAME conversion was achieved using the Kb1M catalyst at 79.1% with a mole ratio of cooking oil to methanol being 1:3, whereas the lowest FAME conversion, 72.0%, was obtained using the Kb2M catalyst with a mole ratio of cooking oil to methanol being 1:6. Overall, the Kb2M showed the best efficient catalyst, while the Ka1M showed the lowest catalytic performance.

Acid; Base; Biodiesel; Heterogenous Catalyst; Kaolin

         Heterogeneous catalysts possess different phases between reactants and products. These catalysts have various advantages such as being environmentally compatible, non-corrosive, easy to separate from reactants, and facile to regenerate (Guan et al., 2009). It is noted that heterogeneous catalysts can promote the transesterification of triglycerides for biodiesel production (Aziz et al., 2017; Yan et al., 2010). Efforts to reduce the cost production of biodiesel rely on the availability of abundant and cheap raw material catalysts, sources of free fatty acid, proper reactor design, method, and type of reaction process (transesterification or esterification).
        Natural kaolin occurs over several regions of Indonesia, such as West Kalimantan, South Kalimantan, Bangka Belitung, Sulawesi, and Java with a total deposit of approximately 66.21 million tons (Subari, Wenas and Suripto, 2008). Natural Bangka kaolin has been used as an adsorbent for the adsorptive removal of antibiotic rifampicin from an aqueous solution (Majid et al., 2023). On the other hand, the application of Indonesian kaolin from Bangka has also been reported as an adsorbent for adsorption of negatively charged acid blue 25 and acid 1 (Asbollah et al., 2022). Natural kaolin has also been reported as a catalyst for transesterification reactions (Ali et al., 2018; ??ng, Chen, and Lee, 2017). With its high surface area, porosity, and composition, along with its low cost, the natural kaolin highlights its potential as an effective raw catalyst material. For instance, graphene oxide (GO) enriched natural kaolinite clay as catalyst for biodiesel production has been reported by Syukri et al. (2020). GO is a single atomic layer of graphite oxide that has a high specific surface area with a complex mixture of oxygen at the edges and basal planes (Kusrini et al., 2020a; Nasrollahzadeh et al., 2014). GO has a Bronsted acid side which is important for esterifying the free fatty acid (FFA) content in oil (Atadashi et al., 2013). It was reported that metal oxide/GO composites increased the mechanical strength of heterogeneous catalysts (Marso et al., 2017).
        To increase the quality and ability of natural Bangka kaolin as a heterogeneous catalyst for biodiesel production, activation of kaolin using base and acid treatments is needed to enhance the active sites of the natural kaolin. This process is able to enlarge its surface area and remove impurities on the surface of natural kaolin. Base treatment for the activation can improve crystallinity of the natural kaolin (Belver, Bañares-Muñoz, and Vicente, 2002). The catalyst that was activated using base treatment showed 4,000 times faster than those found for the acid catalyst under a similar amount of catalyst for the transesterification reaction (Fukuda, Kondo, and Noda, 2001). New catalyst materials for energy production and/or other applications have attracted many researchers to address the need for clean energy (Arnas, Whulanza, and Kusrini, 2024; Whulanza & Kusrini, 2024).
          Biodiesel is an alkyl ester with other non-toxic compounds. It is a mixture of long-chain fatty acid methyl esters (FAMEs) or ethyl esters (FAEEs). Biodiesel can be produced by either the transesterification of animal fats, vegetable oils, or used cooking oil or esterification of free fatty acids (FFAs) (Dang, Chen and Lee, 2013) in the present of alcohol compound such as methanol (CH₃OH) or ethanol (C₂H₅OH). Usually, production of biodiesel through esterification and/or transesterification reactions that are assisted by an acid or base catalyst (Maneerung et al., 2016). Biodiesel can be obtained by a transesterification reaction, whereas the green diesel as the second generation of diesel can be produced through hydrodeoxygenation reaction (Aisyah et al., 2023). Apart from containing esters, vegetable oils and animal fats also contain small amounts of FFA. The presence of FFA in the transesterification reaction with an alkali catalyst needs to be considered. The maximum FFA content in vegetable oil when using a base catalyst is approximately 3%, if it exceeds this level the reaction cannot occur, as reported by Atadashi et al. (2013). This is because free fatty acids will react with an alkali catalyst to form soap. Thus, before the transesterification reaction was carried out, the oil must be first pretreated to reduce the FFA content. After the transesterification reaction was complete, two products will be obtained, namely biodiesel (methyl ester) and glycerol.
            Although sources of biodiesel can be vegetable oils such as palm oil, coconut oil, corn oil, soybean oil, sunflower seed oil, and rapeseed oil, non-edible oils such as jatropha curcas, pongamina pinnata, sea mango, palanga, and/or tallow oil are more preferred (Leung, Wu and Leung, 2010). Many countries use vegetable oils as the main ingredient for making biodiesel because the properties of the biodiesel produced are close to those of diesel fuel (Gui et al., 2008). Biodiesel has advantages compared to diesel fuel from petroleum. The advantages of biodiesel are an environmentally friendly fuel because it produces much better emissions (free sulfur and smoke number), and higher cetane number (>50). Thus, the combustion efficiency of biodiesel is better than that of crude oil, displays lubricating properties for engine pistons, and improves vehicle life, safe storage, and transport, including its non-toxic and biodegradable properties (Balat and Balat, 2010). Biodiesel is an ideal fuel for the transportation industry because it can be used in various diesel engines, including agricultural machines.
            Thus, to observe the potential of Bangka kaolin as a heterogeneous catalyst for biodiesel production, this natural kaolin was activated using a base solution (1–2 M NaOH) and in acid (1 M HCl) media. Both heterogeneous catalysts were evaluated and tested for biodiesel production, where cooking oil was used as a source of free fatty acid (FFA) for biodiesel production. Kaolin was chosen as the model clay for the development of the e?cacious heterogeneous catalyst in production of biodiesel through the catalyzed transesterification reaction.

2.1.   Materials

        Natural kaolin was originated from Bangka, Belitung Island, Indonesia. Palm cooking oil is a commercial product containing FFA <2%. NaOH and HCl were purchased from Merck (Germany). All the chemicals were used without any further purification.

2.1.1.    Pretreatment of natural kaolin

50 g of Bangka natural kaolin (150 mesh) was mixed with 400 mL of distilled water and stirred using a magnetic stirrer until the mixture became homogeneous. The natural kaolin was then separated from this mixture using the centrifugation technique. The clean natural kaolin was dried in an oven at 105°C for 2 h. Furthermore, a dried natural kaolin was calcined at 500°C for 6 h to form a metakaolin. Then, a metakaolin was used to produce a heterogeneous catalyst in subsection 2.3.

2.2. Activated heterogeneous catalysts using base and acid treatments

      Each sample of metakaolin (3.7 g) was mixed with at 1 M or 2 M NaOH solution, along with magnetic stirring at 500 rpm for 6 h at room temperature. Then, the base metakaolin obtained by using 1 M and 2 M NaOH in the respective order were separated from the NaOH solution using a centrifugation process for 10 minutes at a speed of 2500 rpm. The solid base metakaolin was washed using distilled water, and it was dried in an oven at 105°C for 2 h. Each base metakaolin product was calcined in the furnace at 500°C for 6 h. Finally, both heterogeneous catalysts were produced and named as follows: base-activated kaolin using 1 M NaOH (Kb1M) and base-activated kaolin 2 M NaOH (Kb2M). A similar procedure was used to produce acid-activated kaolin using 1M HCl solution, which was named as acid-activated kaolin 1 M (Ka1M).

2.3. Performance test of heterogeneous catalysts for biodiesel production

     The palm cooking oil was used as a model source of oil. This oil was heated at 65°C. A mixture of methanol and catalyst with a ratio of 1:3 was added into the heated cooking oil and stirred using magnetic stirring at 500 rpm and 60°C for 60 minutes. The ratio of catalyst to cooking oil is 3 wt.%. Then, the mixture was put in the separated funnel and kept for 6 h until 2 layers formed, where the biodiesel (fatty acid methyl ester, FAME) product was obtained. FAME was washed using a warm water until the color of the water was clean. A product of biodiesel was heated at 110°C to remove the remaining water. Finally, the biodiesel product was kept for further characterization according to the SNI 04-7182-2015 method.

2.4. Characterizations

     The heterogeneous kaolin-based catalysts were characterized using a scanning electron microscopy-energy dispersive X-ray (SEM-EDX), X-ray diffraction (XRD), and Fourier transform infra-red (FTIR) spectroscopy. XRD characterization was carried out to determine the size and crystalline structure of the catalyst. FT-IR spectral characterization was carried out to determine the types of functional groups of samples. The crystallite sizes of catalysts were determined using the Debye-Scherrer (Equation (1)). Due to limited access to instruments and funding for this study, spectral characterization (FTIR, XRD, and SEM-EDX) were conducted for both Kb1M and Kb2M heterogeneous catalysts. 

where L is crystallite size,  is wavelength of diffraction light (0.15406 nm),  full width at half maximum (FWHM) in radians, K is constant (0.89), and  is diffraction angle.

3.1.   Preparation of kaolin to metakaolin

        Pre-treatment of kaolin aims to reduce impurities and eliminate the water content present in natural kaolin. No color change was observed in the kaolin before and after calcination at 500°C. At a calcination temperature of 500°C, the bond between the hydroxyl groups attached to the octahedral alumina on the kaolin surface weakens and breaks which is called pre-dehydroxylation. A dehydroxylation reaction occurred, where the hydroxyl group bonds in natural kaolin absorb energy which then decomposes. Removal of nearly all hydroxyl groups from natural kaolin causes the crystal structure of kaolin to break down, which then becomes amorphous. This amorphous phase of kaolin is called a metakaolin. A comparison of the color of kaolin (150 mesh), heated in an oven and after calcination at 500°C is shown in Figure 1A-C.

Figure 1 Preparation of (A) natural kaolin 150 mesh, (B) kaolin after oven heating, and (C) kaolin after calcination (metakaolin)

3.2.   The physical properties of activated Kaolin

Comparison of the physical properties of activated kaolin using 1M or 2M NaOH, the kaolin was separated from the NaOH solution after washing with distilled water. The dried kaolin and the calcined kaolin (metakaolin) are shown in Figure 2A-E.
        All heterogeneous catalysts were produced, namely Kb1M, Kb2M, and Ka1M. The addition of NaOH and HCl aims to dissolve silica and alumina from natural kaolin. NaOH acts as a metallizer and a base agent because the kaolin structure forms an excess negative charge on the aluminum ion in order to support cations that are needed outside the framework to neutralize its surface charge. The addition of NaOH as a mineralizer in the synthesis of kaolin catalysts due to the capacity of water as a solvent at high temperatures is often unable to dissolve substances in the crystallization process. The activation process can enlarge the pore size and open the pores of natural kaolin. On the other hand, the calcination process at the activation stage aims to ensure the formation of crystals on the kaolin surface. In general, the reaction mechanisms are illustrated in Equations (4) and (5);