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Production of methyl esters from waste cooking oil using a heterogeneous biomass-based catalyst
Renewable Energy 114 (2017) 638e643
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Production of methyl esters from waste cooking oil using a heterogeneous biomass-based catalyst Mohammed Abdillah Ahmad Farid a, Mohd Ali Hassan a, b, *, Yun Hin Taufiq-Yap c, d, Mohd Lokman Ibrahim d, e, Mohd Ridzuan Othman a, b, Ahmad Amiruddin Mohd Ali a, f, Yoshihito Shirai f a
Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia Department of Food and Process Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia c Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia d Catalysis Science and Technology Research Centre (PutraCAT), Faculty of Science, University Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia e UiTM Foundation Centre, Universiti Technologi Mara, Campus Dengkil, 43800, Dengkil, Selangor, Malaysia f Graduate School of Life Sciences and System Engineering, Kyushu Institute of Technology, 808-0196, Hibikino 2-4, Wakamatsu-ku, Kitakyushu-shi, Fukuoka, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2016 Received in revised form 23 May 2017 Accepted 10 July 2017 Available online 17 July 2017
Fatty acid methyl esters (FAME) production from waste cooking oil was successfully carried out using a newly developed heterogeneous biomass-based catalyst. Activated carbon produced from oil palm biomass was calcined with potassium phosphate tri-basics (K3PO4) in order to synthesize a high catalytic heterogeneous catalyst. As it is characterized with substantial surface area of 680 m2/g and basicity amount of 11.21 mmol/g, 98% of FAME yield was achieved under optimum reaction parameters of 5 wt% catalyst loading, 12:1 methanol to oil molar ratio at 60 C for 4 h. The catalyst was shown to be reusable, with more than 76% FAME yield after 5 consecutive cycles. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Methyl esters Heterogeneous catalyst Oil palm biomass Activated carbon Waste cooking oil
1. Introduction Besides global warming, population growth and global industrialization have caused increased energy demand and inevitable depletion of fossil fuels [1]. Increasing concern on environmental conservation and energy security issue have encouraged research efforts for alternative fuels, such as biodiesel [2]. Biodiesel is a renewable, green and clean-burning fuel that consists of long-chain methyl esters, which are normally produced from agricultural oils [3]. The lack of policies and inefficient waste management of waste cooking oil (WCO) have led to its indiscriminate disposal [4]. Since
* Corresponding author. Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia. E-mail address: [email protected] (M.A. Hassan). http://dx.doi.org/10.1016/j.renene.2017.07.064 0960-1481/© 2017 Elsevier Ltd. All rights reserved.
it was abundant and cheap, some unregulated industries have made huge profits out of recycling this waste. Therefore, the exploitation of waste cooking oil (WCO) in biodiesel production is a potential alternative that is beneficial towards the environment [5]. Recently, heterogeneous catalyst has received attention for biodiesel production. Apart from its advantages on reusability and ease of separation, it could be prepared from cheap biomass [6]. It has been reported that oil palm empty fruit bunch (OPEFB) can be used as a catalyst for biodiesel production, after being functionalized with acids or bases [6,7]. In a previous report, biochar supported calcium oxide (CaO) was used in transesterification of Mesua ferrea seed oil resulting > 90% of fatty acid methyl esters (FAME) yield produced under the optimal conditions [8]. In other study, activated carbon supported heteropoly acid catalyst was used in transesterification of crude Jatropa oil in an ultrasoundassisted reactor system. The catalyst with 20% catalyst loading resulted in 87.3% of FAME yield in 40 min [7]. Recently, transesterification of waste cooking oil was found to be efficiently
M.A. Ahmad Farid et al. / Renewable Energy 114 (2017) 638e643
catalysed by potassium phosphate tri-basics (K3PO4), producing 97.3% of FAME yield [9]. Previously, K3PO4 catalyst is generally used as an anti-microbial agent for poultry processing and food additives [4,5]. Therefore, since it is safe and basic in characteristic, it should be explored for heterogeneous based transesterification reaction. The aim of the present work was to synthesize and characterize a biomass-based catalyst for FAME production using WCO as feedstock. The catalyst was characterized with scanning electron microscope (SEM), energy-dispersive X-ray (EDX) spectroscopy, BrunauereEmmetteTeller (BET), Barrett-Joyner-Halenda (BJH), Fourier transform infrared (FT-IR), X-ray diffraction (XRD) and temperature-programmed desorption of carbon dioxide (TPD-CO2). The influence of methanol to oil molar ratio, catalyst loading and reaction temperature on FAME production was studied. The catalyst reusability and leaching properties were also being carried out for several consecutive reaction cycles. 2. Materials and methods 2.1. Materials Samples of OPEFB were collected from FELDA Serting Hilir Palm Oil Mill in Negeri Sembilan, Malaysia. For oil feedstock, WCO was collected from a nearby residential area located around Taman Sri Serdang, Seri Kembangan Selangor, Malaysia. KOH (95.5%) and analytical grade n-hexane (99.9%) were supplied by Merck, USA. Methanol (95%) was purchased from Friendemann Shmidt Chemical, Australia. K3PO4 (98%) was purchased from Sigma-Aldrich, USA. N2 (99%) was purchased from Malaysian Oxygen Berhad (MOX). Supelco FAME mix standard was purchased from SigmaAldrich, Germany. Methyl heptadecanoate (99%) was purchased from Sigma-Aldrich, Germany. 2.2. Catalyst preparation The collected press-shredded OPEFB was ground using cyclone grinder (Sima, Malaysia) to produce samples of 29 ± 1 mm in size. The sample was then washed several times with distilled water to remove dirt and residues, followed by drying at 105 C for 16 h. Activated carbon was produced by a two-step process, beginning with carbonization in a furnace (Densply Creamco, USA) under continuous flow of N2 at 700 C for 2 h. The biochar was ground using Waring blender (Hung Chuan Machinery, Taiwan) and sieved to achieve biochar size of 250 mm, followed with KOH impregnation for 2 h at 1:0.5 biochar to KOH weight ratio. The mixture was then dried at 105 C for 16 h and activated at 700 C for 2 h under continuous flow of N2. The OPEFB-derived activated carbon obtained was neutralized with 0.1 M HCl, followed with hot water repeatedly until the pH of the washing solution reached 6 to 7, and dried at 105 C for 16 h. Biomass-based catalyst was prepared via wet-impregnation and calcination process. At 1:1 OPEFB-derived activated carbon to K3PO4 weight ratio, both were mixed in 100 mL of distilled water, which was continuously stirred at 80 rpm for 2 h. The impregnation mixture was then dried at 105 C for 16 h and subsequently calcined for 3 h under continuous flow of N2 at 500 C. The catalyst produced was stored in the desiccator at 25 C. The produced potassium phosphate tri-basics supported activated carbon catalyst (K3PO4/ AC) was later used in the biodiesel reaction from WCO. 2.3. Catalyst characterization Catalyst surface morphology and composition were examined using scanning electron microscopy (SEM) (JEOL, JSM- 6290LV instrument) equipped with energy-dispersive X-ray (EDX)
639
(Shimadzu, EDX-720). In order to evaluate the surface area, pore volume and pore size, surface area measurement analyser (Micromeritics ASAP 2000, USA) was used according to BrunauerEmmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) method. Catalyst crystallinity and functional groups were examined via Xray diffraction (XRD) using a Shimadzu Diffractometer Model XRD 6000 and Fourier transform infrared (FT-IR) spectroscopy, respectively. Temperature-programmed desorption of carbon dioxide (TPD-CO2) was performed to determine basicity of the developed catalyst using AutoChem II 2920 chemisorption analyser. 2.4. Methyl esters production At first, the WCO was filtered through vacuum filtration to remove the food impurities and dried at 105 C for 1 h to remove the residual moisture. The reaction was performed by using a 1 L three-neck round-bottom flask equipped with magnetic stirrer bar and reflux condenser on a digital heating mantle (Misung Scientific Co., Korea). In the presence of 50 g of pre-treated WCO, the methanol and K3PO4/AC were added into the flask. After the reaction was completed, the product mixture was separated via centrifugation at 10,000 rpm for 10 min. The effect of the reaction variables such as methanol to oil molar ratio (3:1e15:1), catalyst loading (1e6 wt%), and reaction temperature (30e70 C) was investigated by sampling at an hourly interval during the 6 h reaction time. The quantitative analysis of the produced FAME was analysed using gas chromatography (Shimadzu GC-14C) equipped with flame ionization detector (GC-FID) and polar RTX65 capillary column (30 m 0.5 mm 0.25 mm). For sample preparation, 100 mL of sample was added with 100 ppm of methyl heptadecanoate (internal standard) and dissolved in n-hexane (solvent) at the desired dilution factor. The injector and detector ports were set to 230 C and 270 C, respectively. One mL of sample was injected into the oven at 140 C and heated up to 250 C at 5 C/min. The FAME yield was determined by following Eq. (1).
Total mole of methyl esters 100% FAME yieldð%Þ ¼ Weight of WCO 3 Molecular Weight of WCO
(1)
2.5. Catalyst reusability and leaching The reusability of the catalyst was studied by conducting 4 successive reaction cycles. The spent catalyst was recovered from the reaction mixture and re-calcined for 3 h under continuous flow of N2. The leaching of potassium (K) and phosphorus (P) was determined by inductively coupled plasma-atomic emission spectrometric (ICPeAES) using Perkin Elmer Emission Spectrometer Model Plasma 1000. 3. Result and discussions 3.1. Catalyst characterization The reason behind carbonization is to increase the carbon content and generate porosity, while activation helps in pores enlargement. In this work, potassium hydroxide (KOH) is used as activating agent in order to enhance surface area and pore structure of the mesoporous carbon [10]. After activation is completed, neutralization is applied in order to remove the excess KOH from the surface of OPEFB-derived activated carbon and to allow immobilization by K3PO4 catalyst during the calcination process.
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A
B
C
D
Fig. 1. Scanning electron micrographs of (A) K3PO4/AC (view of capillary pores); (B) K3PO4/AC (view of outer surface); (C) OPEFB-derived activated carbon (view of capillary pores); and (D) OPEFB-derived activated carbon (view of outer surface).
Calcination is applied to develop a catalyst capable of immobilizing a homogeneous catalyst on the surface of any support material [11]. Fig. 1 shows the morphological properties of OPEFB-derived activated carbon and K3PO4/AC. The micrographs have demonstrated no distinct changes between both samples. Therefore, it can be suggested that the calcination of K3PO4 has given no effect towards the support structure. Moreover, the porosity observed on the samples indirectly allowed more K3PO4 particles to be distributed and immobilized on the outer surface and inside the capillary pores, leading to the increase of catalytic sites [7,12]. Basically, high surface area provides more reaction sites which lead to faster reaction, as more reactants can reach the accessible active sites at one time [13]. Based on the results in Table 1, surface area, pore volume and average pore radius of activated carbon were 694 m2 g1, 0.343 cm3 g1 and 1.90 nm, respectively. After calcination, the surface area, pore volume and pore diameter decreased due to the successive penetration by K3PO4 inside the capillary pores that caused the reduction of activated carbon’s accessible area. This finding is in agreement with the previous reported literature [12]. The EDX spectroscopy was used to determine the surface elemental composition of the catalyst. As shown in Table 1, compositions of K and P on the surface of K3PO4/AC increased substantially after the calcination with K3PO4. It signifies that the porous structure has eventually enhanced the adsorption and accessibility of the K3PO4 particles onto the surface of activated carbon [14e16]. For a base-type catalyst, TPD-CO2 analysis was performed to investigate the basicity amount. The volume of desorbed gas particles was measured and given as mmol of CO2 per g of sample (mmol/g). Calcination of catalyst on the surface of OPEFB-
derived activated carbon, which acted as the support, has synthesized the base active sites, which contributed to the catalytic activity. Table 1 shows that the resulted basicity values increased after the calcination of K3PO4. XRD patterns obtained were illustrated in Fig. 2. According to the OPEFB-derived activated carbon spectrum, no peak was observed except at 25.1. The broad peak was possibly due to the turbostratic structure of disordered microcrystalline layer that the mimicked microcrystalline structure created due to heteroatoms (lattice sites) of hydrogen or oxygen [17]. It also can be deduced that the OPEFB-derived activated carbon has an amorphous structure. In addition, the pattern of peak origin from K3PO4 was visualized on the K3PO4/AC at the diffraction angle of 2q ¼ 31.56 . FTIR spectroscopy (Perkin Elmer Spectrum 100) was conducted to detect the presence of surface functional groups that contributed to the catalytic activity of a catalyst. Theoretically, multiple different functional groups present on a subject material may act together to contribute a cooperative catalysis capability. The FTIR spectrum displayed in Fig. 2 was analysed at 4 cm1 resolution from 600 cm1 to 4000 cm1 wavelength. According to the result, the OPEFB-derived activated carbon appeared to have a broad band of CeO stretching vibration of primary alcohol band at 1032 cm1, which are attributed to the presence of lignin. Less functional groups observed on the surface of OPEFB-derived activated carbon was due to the high temperature subjected during carbonization and neutralization procedure that caused degradation of hemicellulose, cellulose and lignin and removal of the impregnated KOH from the surface of OPEFBderived activated carbon, respectively [18]. However, during the
Table 1 Surface area, pore volume, average pore radius, basicity (TPD-CO2), and surface elemental distribution of OPEFB-derived activated carbon and K3PO4/AC. Sample
OPEFB-derived activated carbon K3PO4/AC
Surface area (m2/g)
Pore volume (cm3/g)
Average pore radius (nm)
TPD-CO2 (mmol/g)
Surface elemental distribution C
O
P
K
N
H
S
694 680
0.343 0.282
5.27 5.09
0.64 11.21
75.43 64.65
20.96 14.28
0.00 3.64
0.19 15.13
1.47 0.91
1.81 1.12
0.14 0.10
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Fig. 2. (A) XRD diffractograms of OPEFB-activated carbon, K3PO4, and K3PO4/AC; (B) FTIR spectra of OPEFB-derived activated carbon and K3PO4/AC.
calcination with K3PO4, the surface of OPEFB-derived activated carbon revealed the existence of three newly developed functional groups which are attributed to OeH stretching vibration of primary alcohol between 3416e3173 cm1, OeH stretching vibration of primary alcohol at 1650 cm1, CeO stretching vibration of primary alcohol at 1015 cm1, and CeH bending vibration at 772 cm1. The functional groups revealed after calcination represented the origin K3PO4 functional groups, which are in good agreement with previous work [19]. Since the catalyst has basic property, the presence of hydroxyl functional groups also indicated the existence of moisture that was adsorbed onto the surface of K3PO4/AC catalyst. 3.2. Methyl esters production Optimum reaction conditions of WCO using K3PO4/AC were investigated by assessing the effect of catalyst loading, alcohol to oil molar ratio and reaction time. Sampling was conducted at an hourly interval during the 6 h reaction time. 3.2.1. Effect of catalyst loading As shown in Fig. 3, the optimal catalyst loading used in production process was determined, ranging from 1 to 6 wt% using K3PO4/AC under fixed reaction conditions of 12:1 methanol to oil molar ratio at 60 C for 6 h. The results showed that FAME conversion yield increased as the catalyst loading was increased. It can be seen that, the reaction using 5 wt% of catalyst loading reached the highest FAME yield of 98% at 4 h reaction time. It can be reasoned that, the high amount of catalyst loading increased the amount of active sites, where reaction can occur faster. This is in line with improved reaction kinetics at higher catalyst loadings as previously reported [20]. Catalyst loading of 6 wt% revealed neither
Fig. 3. Effect of catalyst loading on biodiesel production from waste cooking oil. *The experiment was conducted under fixed conditions of 12:1 methanol to oil molar ratio at 60 C for 6 h.
significant increase in FAME yield nor faster reaction completion, as the reaction reached equilibrium at 5 wt% catalyst loading for 4 h reaction period. 3.2.2. Effect of methanol to oil molar ratio Generally, transesterification of 1 mol of triglyceride requires 3 mol of methanol. However, in order to investigate the optimal amount of methanol required for transesterification of the WCO, different methanol to oil molar ratios (3:1 to 15:1) were investigated using the K3PO4/AC, under fixed reaction conditions of 5 wt% catalyst loading at 60 C for 6 h. As illustrated in Fig. 4, FAME yield using 12:1 and 15:1 methanol to oil molar ratio has reached equilibrium at early 4 h reaction time. No further increment in FAME yield was observed except for transesterification using lower methanol to waste cooking oil molar ratio than 12:1, which continued to increase toward the end of the reaction. Transesterification continued within the reactant, as they existed in the form of small droplets within the reaction mixture [21]. Therefore, it can be suggested that the higher amount of methanol in the mixture enhanced the reaction by increasing the accessibility and provided additional medium for the reaction to take place, thereby leading to a shorter conversion period. 3.2.3. Effect of reaction temperature The effect of temperature on FAME yield was studied at 5 different reaction temperatures of 30 C, 40 C, 50 C, 60 C, and 70 C using the K3PO4/AC under fixed conditions of 5 wt% catalyst loading and 12:1 methanol to oil molar ratio for 6 h. As shown in Fig. 5, it can be seen that FAME yield increased with the increase of temperature. The more heat supplied to the reaction mixture
Fig. 4. Effect of methanol to oil molar ratio on FAME production from waste cooking oil. *The experiment was conducted under fixed conditions of 5 wt% catalyst loading at 60 C for 6 h.
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M.A. Ahmad Farid et al. / Renewable Energy 114 (2017) 638e643 Table 2 FAME yield, potassium (K) and phosphorus (P) content for 5 consecutive cycles. Run
EN 1 14214
FAME yield (%) 96.5 K content (ppm) 5.0 P content (ppm) 10
2
3
4
5
98 ± 1.4 90.4 ± 2.2 83.2 ± 1.9 80.8 ± 3.1 76.6 ± 2.4 5 ± 2.3 5 ± 1.2 5 ± 0.8 5 ± 0.3 4 ± 2.0 <1 <1 <1 <1 <1
*The experiment was conducted under optimum conditions of 12:1 methanol to oil molar ratio, 5 wt% catalyst loading and 60 C reaction temperature for 4 h.
Fig. 5. Effect of reaction temperature on FAME production from waste cooking oil. *The experiment was conducted under fixed conditions of 12:1 methanol to oil molar ratio and 5 wt% catalyst loading for 6 h.
increased the kinetic energy which moves the molecules faster in motion, and therefore made the reaction completed more rapidly [22]. The optimal temperature recorded was at 60 C as the FAME yield reached equilibrium at the shortest reaction time. No substantial increase of FAME yield was observed after 4 h of reaction period as it reached equilibrium. However, for the reactions conducted below 60 C, their reaction yields continued to increase until 6 h of reaction period. At 70 C reaction temperature, there was formation of bubbles, which turned the reaction mixture into a 3phase interface (oil-methanol-catalyst) [23]. 3.3. Reusability and leaching properties Catalyst stability and reusability are crucial from an economic point of view [13]. Therefore, in order to investigate the catalyst reusability, consecutive reaction cycles were conducted using the same catalyst and operating parameters. In each reaction cycle, the spent catalyst was recovered through centrifugation. Since the presence of polar and non-polar impurities on the surface of the catalyst will act as a barrier from exposing active sites towards reactants, thus the recovered catalyst was re-calcined in order to restore its catalytic activity [7,24]. The K3PO4/AC was used repeatedly for 5 reaction cycles under the condition of 12:1 methanol to oil ratio and 5 wt% of catalyst loading at 60 C for 4 h. The relationship between the FAME yield and number of reaction cycle is shown in Fig. 6. For cycles 1 to 5, a slight reduction in FAME yield
was recorded from 98 ± 1.4% to 76.6 ± 2.4%. Loss of catalytic activity is caused by leaching of the catalyst’s active species prompting accumulation of these inorganic contaminants in the crude biodiesel [25]. Therefore, elemental analysis was carried out on liquid sample of the crude biodiesel produced from every reaction cycle. The result obtained was compared against the European Biodiesel Standard (EN 14214), as shown in Table 2. It was found that the amount of K that leached into the crude biodiesel was more substantial than P. Therefore, since K is the main active species of K3PO4/AC catalyst, the leaching has resulted in the gradual reduction of FAME yields from one reaction cycle to another. 4. Conclusions A biomass-derived activated carbon catalyst was successfully developed with the ratio of 1:1 activated carbon to K3PO4 weight ratio and calcined at 500 C. For transesterification of the WCO, under the optimum conditions of 5 wt% catalyst loading, 12:1 methanol to oil molar ratio at 60 C for 4 h, the highest FAME yield of 98% was obtained. The FAME conversion is a direct consequence of the high catalyst surface area and catalytic strength. The newly developed catalyst has demonstrated retention of catalytic activity for 5 reaction cycles, with higher than 76% of FAME yield. The slight reduction was due to the leaching of K and P. It is proposed that the newly developed biomass-derived carbon supported K3PO4 catalyst has potential in methyl esters production from WCO. Acknowledgments The authors would like to acknowledge the financial support provided by JICA-JST SATREPS project, Ministry of Higher Education, Malaysia and Universiti Putra Malaysia Graduate Research Fund (GRF). References
Fig. 6. Reusability of the catalyst for 5 consecutive reaction cycles in FAME production from waste cooking oil. *The experiment was conducted under optimum conditions of 12:1 methanol to oil molar ratio, 5 wt% catalyst loading, 60 C reaction temperature for 4 h.
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Production of methyl esters from waste cooking oil using a heterogeneous biomass-based catalyst Mohammed Abdillah Ahmad Farid a, Mohd Ali Hassan a, b, *, Yun Hin Taufiq-Yap c, d, Mohd Lokman Ibrahim d, e, Mohd Ridzuan Othman a, b, Ahmad Amiruddin Mohd Ali a, f, Yoshihito Shirai f a
Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia Department of Food and Process Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia c Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia d Catalysis Science and Technology Research Centre (PutraCAT), Faculty of Science, University Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia e UiTM Foundation Centre, Universiti Technologi Mara, Campus Dengkil, 43800, Dengkil, Selangor, Malaysia f Graduate School of Life Sciences and System Engineering, Kyushu Institute of Technology, 808-0196, Hibikino 2-4, Wakamatsu-ku, Kitakyushu-shi, Fukuoka, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2016 Received in revised form 23 May 2017 Accepted 10 July 2017 Available online 17 July 2017
Fatty acid methyl esters (FAME) production from waste cooking oil was successfully carried out using a newly developed heterogeneous biomass-based catalyst. Activated carbon produced from oil palm biomass was calcined with potassium phosphate tri-basics (K3PO4) in order to synthesize a high catalytic heterogeneous catalyst. As it is characterized with substantial surface area of 680 m2/g and basicity amount of 11.21 mmol/g, 98% of FAME yield was achieved under optimum reaction parameters of 5 wt% catalyst loading, 12:1 methanol to oil molar ratio at 60 C for 4 h. The catalyst was shown to be reusable, with more than 76% FAME yield after 5 consecutive cycles. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Methyl esters Heterogeneous catalyst Oil palm biomass Activated carbon Waste cooking oil
1. Introduction Besides global warming, population growth and global industrialization have caused increased energy demand and inevitable depletion of fossil fuels [1]. Increasing concern on environmental conservation and energy security issue have encouraged research efforts for alternative fuels, such as biodiesel [2]. Biodiesel is a renewable, green and clean-burning fuel that consists of long-chain methyl esters, which are normally produced from agricultural oils [3]. The lack of policies and inefficient waste management of waste cooking oil (WCO) have led to its indiscriminate disposal [4]. Since
* Corresponding author. Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia. E-mail address: [email protected] (M.A. Hassan). http://dx.doi.org/10.1016/j.renene.2017.07.064 0960-1481/© 2017 Elsevier Ltd. All rights reserved.
it was abundant and cheap, some unregulated industries have made huge profits out of recycling this waste. Therefore, the exploitation of waste cooking oil (WCO) in biodiesel production is a potential alternative that is beneficial towards the environment [5]. Recently, heterogeneous catalyst has received attention for biodiesel production. Apart from its advantages on reusability and ease of separation, it could be prepared from cheap biomass [6]. It has been reported that oil palm empty fruit bunch (OPEFB) can be used as a catalyst for biodiesel production, after being functionalized with acids or bases [6,7]. In a previous report, biochar supported calcium oxide (CaO) was used in transesterification of Mesua ferrea seed oil resulting > 90% of fatty acid methyl esters (FAME) yield produced under the optimal conditions [8]. In other study, activated carbon supported heteropoly acid catalyst was used in transesterification of crude Jatropa oil in an ultrasoundassisted reactor system. The catalyst with 20% catalyst loading resulted in 87.3% of FAME yield in 40 min [7]. Recently, transesterification of waste cooking oil was found to be efficiently
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catalysed by potassium phosphate tri-basics (K3PO4), producing 97.3% of FAME yield [9]. Previously, K3PO4 catalyst is generally used as an anti-microbial agent for poultry processing and food additives [4,5]. Therefore, since it is safe and basic in characteristic, it should be explored for heterogeneous based transesterification reaction. The aim of the present work was to synthesize and characterize a biomass-based catalyst for FAME production using WCO as feedstock. The catalyst was characterized with scanning electron microscope (SEM), energy-dispersive X-ray (EDX) spectroscopy, BrunauereEmmetteTeller (BET), Barrett-Joyner-Halenda (BJH), Fourier transform infrared (FT-IR), X-ray diffraction (XRD) and temperature-programmed desorption of carbon dioxide (TPD-CO2). The influence of methanol to oil molar ratio, catalyst loading and reaction temperature on FAME production was studied. The catalyst reusability and leaching properties were also being carried out for several consecutive reaction cycles. 2. Materials and methods 2.1. Materials Samples of OPEFB were collected from FELDA Serting Hilir Palm Oil Mill in Negeri Sembilan, Malaysia. For oil feedstock, WCO was collected from a nearby residential area located around Taman Sri Serdang, Seri Kembangan Selangor, Malaysia. KOH (95.5%) and analytical grade n-hexane (99.9%) were supplied by Merck, USA. Methanol (95%) was purchased from Friendemann Shmidt Chemical, Australia. K3PO4 (98%) was purchased from Sigma-Aldrich, USA. N2 (99%) was purchased from Malaysian Oxygen Berhad (MOX). Supelco FAME mix standard was purchased from SigmaAldrich, Germany. Methyl heptadecanoate (99%) was purchased from Sigma-Aldrich, Germany. 2.2. Catalyst preparation The collected press-shredded OPEFB was ground using cyclone grinder (Sima, Malaysia) to produce samples of 29 ± 1 mm in size. The sample was then washed several times with distilled water to remove dirt and residues, followed by drying at 105 C for 16 h. Activated carbon was produced by a two-step process, beginning with carbonization in a furnace (Densply Creamco, USA) under continuous flow of N2 at 700 C for 2 h. The biochar was ground using Waring blender (Hung Chuan Machinery, Taiwan) and sieved to achieve biochar size of 250 mm, followed with KOH impregnation for 2 h at 1:0.5 biochar to KOH weight ratio. The mixture was then dried at 105 C for 16 h and activated at 700 C for 2 h under continuous flow of N2. The OPEFB-derived activated carbon obtained was neutralized with 0.1 M HCl, followed with hot water repeatedly until the pH of the washing solution reached 6 to 7, and dried at 105 C for 16 h. Biomass-based catalyst was prepared via wet-impregnation and calcination process. At 1:1 OPEFB-derived activated carbon to K3PO4 weight ratio, both were mixed in 100 mL of distilled water, which was continuously stirred at 80 rpm for 2 h. The impregnation mixture was then dried at 105 C for 16 h and subsequently calcined for 3 h under continuous flow of N2 at 500 C. The catalyst produced was stored in the desiccator at 25 C. The produced potassium phosphate tri-basics supported activated carbon catalyst (K3PO4/ AC) was later used in the biodiesel reaction from WCO. 2.3. Catalyst characterization Catalyst surface morphology and composition were examined using scanning electron microscopy (SEM) (JEOL, JSM- 6290LV instrument) equipped with energy-dispersive X-ray (EDX)
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(Shimadzu, EDX-720). In order to evaluate the surface area, pore volume and pore size, surface area measurement analyser (Micromeritics ASAP 2000, USA) was used according to BrunauerEmmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) method. Catalyst crystallinity and functional groups were examined via Xray diffraction (XRD) using a Shimadzu Diffractometer Model XRD 6000 and Fourier transform infrared (FT-IR) spectroscopy, respectively. Temperature-programmed desorption of carbon dioxide (TPD-CO2) was performed to determine basicity of the developed catalyst using AutoChem II 2920 chemisorption analyser. 2.4. Methyl esters production At first, the WCO was filtered through vacuum filtration to remove the food impurities and dried at 105 C for 1 h to remove the residual moisture. The reaction was performed by using a 1 L three-neck round-bottom flask equipped with magnetic stirrer bar and reflux condenser on a digital heating mantle (Misung Scientific Co., Korea). In the presence of 50 g of pre-treated WCO, the methanol and K3PO4/AC were added into the flask. After the reaction was completed, the product mixture was separated via centrifugation at 10,000 rpm for 10 min. The effect of the reaction variables such as methanol to oil molar ratio (3:1e15:1), catalyst loading (1e6 wt%), and reaction temperature (30e70 C) was investigated by sampling at an hourly interval during the 6 h reaction time. The quantitative analysis of the produced FAME was analysed using gas chromatography (Shimadzu GC-14C) equipped with flame ionization detector (GC-FID) and polar RTX65 capillary column (30 m 0.5 mm 0.25 mm). For sample preparation, 100 mL of sample was added with 100 ppm of methyl heptadecanoate (internal standard) and dissolved in n-hexane (solvent) at the desired dilution factor. The injector and detector ports were set to 230 C and 270 C, respectively. One mL of sample was injected into the oven at 140 C and heated up to 250 C at 5 C/min. The FAME yield was determined by following Eq. (1).
Total mole of methyl esters 100% FAME yieldð%Þ ¼ Weight of WCO 3 Molecular Weight of WCO
(1)
2.5. Catalyst reusability and leaching The reusability of the catalyst was studied by conducting 4 successive reaction cycles. The spent catalyst was recovered from the reaction mixture and re-calcined for 3 h under continuous flow of N2. The leaching of potassium (K) and phosphorus (P) was determined by inductively coupled plasma-atomic emission spectrometric (ICPeAES) using Perkin Elmer Emission Spectrometer Model Plasma 1000. 3. Result and discussions 3.1. Catalyst characterization The reason behind carbonization is to increase the carbon content and generate porosity, while activation helps in pores enlargement. In this work, potassium hydroxide (KOH) is used as activating agent in order to enhance surface area and pore structure of the mesoporous carbon [10]. After activation is completed, neutralization is applied in order to remove the excess KOH from the surface of OPEFB-derived activated carbon and to allow immobilization by K3PO4 catalyst during the calcination process.
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A
B
C
D
Fig. 1. Scanning electron micrographs of (A) K3PO4/AC (view of capillary pores); (B) K3PO4/AC (view of outer surface); (C) OPEFB-derived activated carbon (view of capillary pores); and (D) OPEFB-derived activated carbon (view of outer surface).
Calcination is applied to develop a catalyst capable of immobilizing a homogeneous catalyst on the surface of any support material [11]. Fig. 1 shows the morphological properties of OPEFB-derived activated carbon and K3PO4/AC. The micrographs have demonstrated no distinct changes between both samples. Therefore, it can be suggested that the calcination of K3PO4 has given no effect towards the support structure. Moreover, the porosity observed on the samples indirectly allowed more K3PO4 particles to be distributed and immobilized on the outer surface and inside the capillary pores, leading to the increase of catalytic sites [7,12]. Basically, high surface area provides more reaction sites which lead to faster reaction, as more reactants can reach the accessible active sites at one time [13]. Based on the results in Table 1, surface area, pore volume and average pore radius of activated carbon were 694 m2 g1, 0.343 cm3 g1 and 1.90 nm, respectively. After calcination, the surface area, pore volume and pore diameter decreased due to the successive penetration by K3PO4 inside the capillary pores that caused the reduction of activated carbon’s accessible area. This finding is in agreement with the previous reported literature [12]. The EDX spectroscopy was used to determine the surface elemental composition of the catalyst. As shown in Table 1, compositions of K and P on the surface of K3PO4/AC increased substantially after the calcination with K3PO4. It signifies that the porous structure has eventually enhanced the adsorption and accessibility of the K3PO4 particles onto the surface of activated carbon [14e16]. For a base-type catalyst, TPD-CO2 analysis was performed to investigate the basicity amount. The volume of desorbed gas particles was measured and given as mmol of CO2 per g of sample (mmol/g). Calcination of catalyst on the surface of OPEFB-
derived activated carbon, which acted as the support, has synthesized the base active sites, which contributed to the catalytic activity. Table 1 shows that the resulted basicity values increased after the calcination of K3PO4. XRD patterns obtained were illustrated in Fig. 2. According to the OPEFB-derived activated carbon spectrum, no peak was observed except at 25.1. The broad peak was possibly due to the turbostratic structure of disordered microcrystalline layer that the mimicked microcrystalline structure created due to heteroatoms (lattice sites) of hydrogen or oxygen [17]. It also can be deduced that the OPEFB-derived activated carbon has an amorphous structure. In addition, the pattern of peak origin from K3PO4 was visualized on the K3PO4/AC at the diffraction angle of 2q ¼ 31.56 . FTIR spectroscopy (Perkin Elmer Spectrum 100) was conducted to detect the presence of surface functional groups that contributed to the catalytic activity of a catalyst. Theoretically, multiple different functional groups present on a subject material may act together to contribute a cooperative catalysis capability. The FTIR spectrum displayed in Fig. 2 was analysed at 4 cm1 resolution from 600 cm1 to 4000 cm1 wavelength. According to the result, the OPEFB-derived activated carbon appeared to have a broad band of CeO stretching vibration of primary alcohol band at 1032 cm1, which are attributed to the presence of lignin. Less functional groups observed on the surface of OPEFB-derived activated carbon was due to the high temperature subjected during carbonization and neutralization procedure that caused degradation of hemicellulose, cellulose and lignin and removal of the impregnated KOH from the surface of OPEFBderived activated carbon, respectively [18]. However, during the
Table 1 Surface area, pore volume, average pore radius, basicity (TPD-CO2), and surface elemental distribution of OPEFB-derived activated carbon and K3PO4/AC. Sample
OPEFB-derived activated carbon K3PO4/AC
Surface area (m2/g)
Pore volume (cm3/g)
Average pore radius (nm)
TPD-CO2 (mmol/g)
Surface elemental distribution C
O
P
K
N
H
S
694 680
0.343 0.282
5.27 5.09
0.64 11.21
75.43 64.65
20.96 14.28
0.00 3.64
0.19 15.13
1.47 0.91
1.81 1.12
0.14 0.10
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Fig. 2. (A) XRD diffractograms of OPEFB-activated carbon, K3PO4, and K3PO4/AC; (B) FTIR spectra of OPEFB-derived activated carbon and K3PO4/AC.
calcination with K3PO4, the surface of OPEFB-derived activated carbon revealed the existence of three newly developed functional groups which are attributed to OeH stretching vibration of primary alcohol between 3416e3173 cm1, OeH stretching vibration of primary alcohol at 1650 cm1, CeO stretching vibration of primary alcohol at 1015 cm1, and CeH bending vibration at 772 cm1. The functional groups revealed after calcination represented the origin K3PO4 functional groups, which are in good agreement with previous work [19]. Since the catalyst has basic property, the presence of hydroxyl functional groups also indicated the existence of moisture that was adsorbed onto the surface of K3PO4/AC catalyst. 3.2. Methyl esters production Optimum reaction conditions of WCO using K3PO4/AC were investigated by assessing the effect of catalyst loading, alcohol to oil molar ratio and reaction time. Sampling was conducted at an hourly interval during the 6 h reaction time. 3.2.1. Effect of catalyst loading As shown in Fig. 3, the optimal catalyst loading used in production process was determined, ranging from 1 to 6 wt% using K3PO4/AC under fixed reaction conditions of 12:1 methanol to oil molar ratio at 60 C for 6 h. The results showed that FAME conversion yield increased as the catalyst loading was increased. It can be seen that, the reaction using 5 wt% of catalyst loading reached the highest FAME yield of 98% at 4 h reaction time. It can be reasoned that, the high amount of catalyst loading increased the amount of active sites, where reaction can occur faster. This is in line with improved reaction kinetics at higher catalyst loadings as previously reported [20]. Catalyst loading of 6 wt% revealed neither
Fig. 3. Effect of catalyst loading on biodiesel production from waste cooking oil. *The experiment was conducted under fixed conditions of 12:1 methanol to oil molar ratio at 60 C for 6 h.
significant increase in FAME yield nor faster reaction completion, as the reaction reached equilibrium at 5 wt% catalyst loading for 4 h reaction period. 3.2.2. Effect of methanol to oil molar ratio Generally, transesterification of 1 mol of triglyceride requires 3 mol of methanol. However, in order to investigate the optimal amount of methanol required for transesterification of the WCO, different methanol to oil molar ratios (3:1 to 15:1) were investigated using the K3PO4/AC, under fixed reaction conditions of 5 wt% catalyst loading at 60 C for 6 h. As illustrated in Fig. 4, FAME yield using 12:1 and 15:1 methanol to oil molar ratio has reached equilibrium at early 4 h reaction time. No further increment in FAME yield was observed except for transesterification using lower methanol to waste cooking oil molar ratio than 12:1, which continued to increase toward the end of the reaction. Transesterification continued within the reactant, as they existed in the form of small droplets within the reaction mixture [21]. Therefore, it can be suggested that the higher amount of methanol in the mixture enhanced the reaction by increasing the accessibility and provided additional medium for the reaction to take place, thereby leading to a shorter conversion period. 3.2.3. Effect of reaction temperature The effect of temperature on FAME yield was studied at 5 different reaction temperatures of 30 C, 40 C, 50 C, 60 C, and 70 C using the K3PO4/AC under fixed conditions of 5 wt% catalyst loading and 12:1 methanol to oil molar ratio for 6 h. As shown in Fig. 5, it can be seen that FAME yield increased with the increase of temperature. The more heat supplied to the reaction mixture
Fig. 4. Effect of methanol to oil molar ratio on FAME production from waste cooking oil. *The experiment was conducted under fixed conditions of 5 wt% catalyst loading at 60 C for 6 h.
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M.A. Ahmad Farid et al. / Renewable Energy 114 (2017) 638e643 Table 2 FAME yield, potassium (K) and phosphorus (P) content for 5 consecutive cycles. Run
EN 1 14214
FAME yield (%) 96.5 K content (ppm) 5.0 P content (ppm) 10
2
3
4
5
98 ± 1.4 90.4 ± 2.2 83.2 ± 1.9 80.8 ± 3.1 76.6 ± 2.4 5 ± 2.3 5 ± 1.2 5 ± 0.8 5 ± 0.3 4 ± 2.0 <1 <1 <1 <1 <1
*The experiment was conducted under optimum conditions of 12:1 methanol to oil molar ratio, 5 wt% catalyst loading and 60 C reaction temperature for 4 h.
Fig. 5. Effect of reaction temperature on FAME production from waste cooking oil. *The experiment was conducted under fixed conditions of 12:1 methanol to oil molar ratio and 5 wt% catalyst loading for 6 h.
increased the kinetic energy which moves the molecules faster in motion, and therefore made the reaction completed more rapidly [22]. The optimal temperature recorded was at 60 C as the FAME yield reached equilibrium at the shortest reaction time. No substantial increase of FAME yield was observed after 4 h of reaction period as it reached equilibrium. However, for the reactions conducted below 60 C, their reaction yields continued to increase until 6 h of reaction period. At 70 C reaction temperature, there was formation of bubbles, which turned the reaction mixture into a 3phase interface (oil-methanol-catalyst) [23]. 3.3. Reusability and leaching properties Catalyst stability and reusability are crucial from an economic point of view [13]. Therefore, in order to investigate the catalyst reusability, consecutive reaction cycles were conducted using the same catalyst and operating parameters. In each reaction cycle, the spent catalyst was recovered through centrifugation. Since the presence of polar and non-polar impurities on the surface of the catalyst will act as a barrier from exposing active sites towards reactants, thus the recovered catalyst was re-calcined in order to restore its catalytic activity [7,24]. The K3PO4/AC was used repeatedly for 5 reaction cycles under the condition of 12:1 methanol to oil ratio and 5 wt% of catalyst loading at 60 C for 4 h. The relationship between the FAME yield and number of reaction cycle is shown in Fig. 6. For cycles 1 to 5, a slight reduction in FAME yield
was recorded from 98 ± 1.4% to 76.6 ± 2.4%. Loss of catalytic activity is caused by leaching of the catalyst’s active species prompting accumulation of these inorganic contaminants in the crude biodiesel [25]. Therefore, elemental analysis was carried out on liquid sample of the crude biodiesel produced from every reaction cycle. The result obtained was compared against the European Biodiesel Standard (EN 14214), as shown in Table 2. It was found that the amount of K that leached into the crude biodiesel was more substantial than P. Therefore, since K is the main active species of K3PO4/AC catalyst, the leaching has resulted in the gradual reduction of FAME yields from one reaction cycle to another. 4. Conclusions A biomass-derived activated carbon catalyst was successfully developed with the ratio of 1:1 activated carbon to K3PO4 weight ratio and calcined at 500 C. For transesterification of the WCO, under the optimum conditions of 5 wt% catalyst loading, 12:1 methanol to oil molar ratio at 60 C for 4 h, the highest FAME yield of 98% was obtained. The FAME conversion is a direct consequence of the high catalyst surface area and catalytic strength. The newly developed catalyst has demonstrated retention of catalytic activity for 5 reaction cycles, with higher than 76% of FAME yield. The slight reduction was due to the leaching of K and P. It is proposed that the newly developed biomass-derived carbon supported K3PO4 catalyst has potential in methyl esters production from WCO. Acknowledgments The authors would like to acknowledge the financial support provided by JICA-JST SATREPS project, Ministry of Higher Education, Malaysia and Universiti Putra Malaysia Graduate Research Fund (GRF). References
Fig. 6. Reusability of the catalyst for 5 consecutive reaction cycles in FAME production from waste cooking oil. *The experiment was conducted under optimum conditions of 12:1 methanol to oil molar ratio, 5 wt% catalyst loading, 60 C reaction temperature for 4 h.
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