Sub- and supercritical esterification of palm fatty acid distillate with carbohydrate-derived solid acid catalyst

Sub- and supercritical esterification of palm fatty acid distillate with carbohydrate-derived solid acid catalyst

Accepted Manuscript Sub- and supercritical esterification of palm fatty acid distillate with carbohydrate-derived solid acid catalyst Ibrahim M. Lokma...

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Accepted Manuscript Sub- and supercritical esterification of palm fatty acid distillate with carbohydrate-derived solid acid catalyst Ibrahim M. Lokman, Motonobu Goto, Umer Rashid, Yun Hin Taufiq-Yap PII: DOI: Reference:

S1385-8947(15)01184-5 http://dx.doi.org/10.1016/j.cej.2015.08.102 CEJ 14094

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

4 June 2015 12 August 2015 23 August 2015

Please cite this article as: I.M. Lokman, M. Goto, U. Rashid, Y.H. Taufiq-Yap, Sub- and supercritical esterification of palm fatty acid distillate with carbohydrate-derived solid acid catalyst, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej.2015.08.102

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Sub- and supercritical esterification of palm fatty acid distillate with carbohydrate-derived solid acid catalyst Ibrahim M. Lokmana, b, c, Motonobu Gotod, Umer Rashide, f, Yun Hin Taufiq-Yapa,b,* a

Catalysis Science and Technology Research Centre, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia. b Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia. c School of Chemistry, Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia. d Department of Chemical Engineering, Nagoya University, Furo-cho, Chikusa-ku, 464-8603. Nagoya, Japan. e Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia. f Chemistry Department, College of Science, King Saud University, Riyadh, a14S, Saudi Arabia.

*Corresponding author:

Prof. Dr. Yun Hin Taufiq-Yap Catalysis Science and Technology Research Centre Faculty of Science, Universiti Putra Malaysia 43400 UPM Serdang, Selangor, Malaysia. Tel: +603-89466809, Fax: +603-89466758 Email: [email protected]

ABSTRACT The esterification of palm fatty acid distillate (PFAD) in supercritical methanol was investigated by using carbohydrate-derived solid acid catalyst. The catalysts were prepared by sulfonation of incomplete carbonized glucose and starch, which had been coded as sulfonated-ICG and sulfonated-ICS, respectively. The contents of fatty acid methyl ester (FAME) and its yield were determined by using gas chromatography techniques. The effects of sub- and supercritical operating conditions such as methanol/PFAD molar ratio, catalyst amount, reaction temperature and reaction time were analyzed to determine their optimum operating conditions. At optimum reaction temperature of 290C, methanol/PFAD molar ratio of 6/1, catalyst amount of 1 wt.% and 5 min reaction time, the esterification of PFAD in supercritical methanol with the presence of sulfonated-ICS and -ICG catalysts resulted 97.3% and 95.4% of FAME; both catalysts yield significantly higher percentages compared to uncatalyzed reaction. Alongside of its potential in enhancing the efficiency of production process, the utilization of carbohydrate-derived solid acid catalyst in supercritical methanol method had also resulting fast reaction and energy saving.

Keywords: Esterification; Supercritical methanol; Palm fatty acid distillate; Carbohydrate-derived solid acid catalyst; Fatty acid methyl ester.

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1. Introduction Combustion of fossil fuel emits carbon monoxide, hydrocarbon, sulfur dioxide, and hazardous particles caused a critical damage to the environment [1-2]. A previous study showed substitutes for energy sources are crucial to replace the nonrenewable fossil fuel and one of the best alternative fuels reported is the biodiesel [3]. Biodiesel is derived from renewable feedstock such as vegetable oil and animal fats [4-6] and produced by transesterification of triglycerides (e.g. palm oil, jatropha oil, cotton seed oil, soybean oil and corn oil) in short chain of alcohol (e.g. methanol and ethanol) with the presence of catalyst [7-8]. However, those reported feedstocks are expensive and not compatible for low-cost biodiesel production. Recent studies revealed palm fatty acid distillate (PFAD) became the promising candidate as the biodiesel feedstock, which is a by-product from refinery of crude palm oil consisting more than 85 wt.% of free fatty acid (FFA). A number of research papers had been reported focusing on the utilization of PFAD for biodiesel production via esterification and transesterification processes [9-12]. Normally, the common conventional reflux by homogenous acid catalyst such as H2SO4 and HCl were used to carry out the reaction process [13-14]. The product purification is quite challenging due to the solubility problem of the catalyst during the treatment step. Therefore, the heterogeneous solid acid catalysts were introduced based on their solid physical properties with the advantages of easy separation and high reusability. However, it is well known that the acid-catalyzed reactions are time consuming and require lots of catalyst to complete the reaction [15]. Various heterogeneous solid acid catalysts had been introduced, such as sulfonated-carbon nanotubes [16], niobiumMCM-41 catalysts [17], ferric alginate [12], and ferric hydrogen sulphate [18]. Basically, the important factors that contribute to define the efficiency of the solid

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catalyst are the stability, numerous active sites, recyclability, hydrophobicity and lowcost [11,19]. In the last decade, novel carbohydrate-derived solid acid catalyst was developed by several groups of researcher [20-24]; it was reported to have ideal properties similar to solid acid catalyst such as high stability, reusability and numerous active sites. Activation by sulfonation of incomplete carbonized carbohydrate produces a rigid carbon structure consisting of poly-aromatic carbon rings [23] and introduces numerous sulfonic (SO3H) groups on the carbon surface, which plays an important role during the reaction process. Over the last decade, Saka and Kusdiana research groups had proposed an alternative method to produce biodiesel via non-catalytic transesterification of vegetable oils in supercritical methanol [25-26]. The research studies managed to produce high yield of biodiesel originated from rapeseed oil by using supercritical methanol. Based on the theory, the critical temperature to excite the methanol molecules into supercritical state is 239C with 8.08 MPa pressure with the elimination between gas and liquid boundaries. Another study found out that the maximum conversion of rapeseed oil to biodiesel was achieved due to high-energy transfer rate of homogenous catalyst, methanol and feedstock [27]. As reported by Ghoreishi and Moein [28], at 271C, 33.8:1 methanol-to-oil molar ratio, 20.4 min reaction time at 23.1 MPa pressure, the reported FAME yield derived from vegetable oil was 95.3%. Another study by Shin et al. [29] produced biodiesel from waste lard by supercritical methanol method at temperature of 335C, 45:1 of methanol-to-oil molar ratio, and pressure of 20MPa within 15 min of the reaction. A research report by Yujaroen et al. [30] study the esterification reaction of palm fatty acid to biodiesel by non-catalytic supercritical methanol, 95% of production yield was achieved at 300C using 1:6 of feedstock-to-methanol molar ratio within 30 min reaction time.

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However, the contra point of this process was that the non-catalytic supercritical methanol requires high energy while reaction time was not improved significantly. The aim of this work was to improve and enhance the efficiency of the supercritical methanol reaction by introducing a stable heterogeneous carbohydratederived solid acid catalyst into the reaction system. To date, no information has been reported on the use of particular catalyst for supercritical esterification of PFAD. The effects of operating conditions include methanol-to-PFAD molar ratio, amount of catalyst, reaction temperature and reaction time were intensively studied.

2. Materials and methods 2.1.Materials PFAD was kindly provided by Jomalina R&D, SIME DARBY Co., Malaysia. The PFAD sample (average molecular weight was 269.4 g mol-1) consists of 95 wt.% FFA content (1.9 wt.% myristic acid, 45.7 wt.% palmitic acid, 4.3 wt.% stearic acid, 40.2 wt.% oleic acid and 7.9 wt.% linoleic acid). The commercial D-glucose, starch and concentrated H2SO4 were purchased from MERCK chemical company. The nhexane was obtained from J.T. Baker chemical company. A mixture of standard FAMEs consist of methyl oleate, methyl palmitate, methyl myristate, methyl linoleate and methyl stearate was purchased from Supelco, SIGMA. Methyl heptadecanoate was used as the internal standard for analysis of FAME content.

2.2. Synthesis and characterization of the catalysts 12 g of each both D-glucose and starch were calcined at 400°C under N2 flow for 12 h to produce incomplete carbonized glucose (ICG) and starch (ICS). These incomplete carbonized carbons were then milled to powder and activated with the SO3H group through sulfonation process. The sulfonation was carried out by thermal

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treatment of incomplete carbonized carbon with concentrated H2SO4 (1 g 20 mL-1) for 12 h at 160°C in inert environment. The suspension was then diluted with 1 L of distilled water and filtered. The black powder was vigorously washed with hot distilled water until pH of filtrates showing pH 6 or equivalent to the pH of distilled water. The purification was repeated to ensure the excess sulfate ions are completely removed from the catalyst. The carbohydrate-derived solid acid catalyst was dried at 80°C for further analysis. Characteristic evaluations of the prepared catalysts were completed by infrared spectroscopy (IR), ammonia-temperature programmed desorption (NH3-TPD), Raman spectroscopy and BET analysis.

2.3.Esterification of PFAD by supercritical methanol A batch type supercritical methanol reactor was used throughout the experiment. Basically, the supercritical methanol reactor system consists of one electrical furnace adjacent to the stainless steel reactor (volume: 8.8 mL, maximum pressure: 50 MPa).It was equipped with the temperature controller as illustrated in Fig. 1. The reactor was firstly charged with a mixture of specified amount of catalyst (1-5 wt.%) and methanol-to-PFAD molar ratio of (1:1 to 15:1) before heated in the range of 190 – 290°C. After the completion of the reaction, the sample vessel was removed from the furnace and soaked into cool water to stop the reaction. The product mixture was discharge from the vessel and centrifuged at 4000 rpm for 15 minutes to separate the solid catalyst. The product was allowed to settle to three layers – the bottom layer was by-product water, followed by FAME and seen as the top layer was unreacted methanol, which was removed by evaporation. The FAME oil, which had been produced as the middle layer, was collected for biodiesel analysis.

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There are several approaches to estimate the critical properties of reaction mixtures obtained in different reaction conditions. Olivares-Carrillo et al., studied the common formulae that had been used to calculate the critical temperature and pressure of the pure component present during the synthesis of biodiesel using supercritical methanol [31]. In this work, the critical temperature and pressure of the starting materials and mixture were calculated according to the Lydersens’s method of group contributions [32] with the application of Lorentz-Berthelot-type mixing rules as shown in following equations (Eq. 1-8) [33]. The terms

and

were

estimated by the combination of these equations below:

(1) (2) (3) (4)

(5)

(6)

(7) (8)

where i and j stand for PFAD and methanol, respectively. x is the mole fraction of PFAD or methanol, Tc is the critical temperature of PFAD or methanol, Vc is the molar volume of PFAD or methanol, zc is the compressibility factor of PFAD or methanol, Tcm is the critical temperature of mixture, Vcm is the molar volume of

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mixture, zcm is the compressibility factor of mixture, and Pcm is the critical pressure of mixture.

2.4. Reusability of catalysts Catalyst’s reusability and deactivation analysis were carried out at 290C, 6/1 of methanol/PFAD molar ratio, 3 wt.% of catalyst and 10 min reaction time. As the completion of the reaction, the catalyst was separated and collected by centrifugation for subsequent reaction cycle. The biodiesel product in each cycle was collected for FAME yield determination.

2.5.Biodiesel analysis The FAME yield was analyzed using gas chromatography (GC-2014, Shimadzu, Japan) equipped with flame ionization detector (FID). The GC was equipped with the HP-Innowax capillary column (30 m length, 0.25 mm I.D., 0.25 μm film) (USA) for separation of FAME species. The oven temperature was programmed with two steps of temperature increment. Initially, the temperature was set at 60C and held for 5 min, increased to 200C at 10C min-1. Again, the oven temperature was increased up to 240C at 5C min-1 and held for 5 min. The injector port and detector port was set at 250C. Hydrogen was used as the makeup and helium gas was utilized as the carrier gas. The sample was dissolved in n-hexane, 1 μL of sample was injected into the system for FAME analysis. The FAME content was determined according to the European regulated procedure EN14103 and calculated using the following equation;

Where, WFAMEs is weight of FAME product and Wraw material is weight for raw material.

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3. Results and discussion 3.1.Catalyst Characterization After the acid activation, illustrations of both infrared spectra of sulfonatedICG (Fig. 2 (a)) and -ICS catalysts (Fig. 2 (b)) can be observed. Two vibration peaks were identified at 1034 cm-1which attributed to the -SO2- symmetric stretching and 1181 cm-1 that attributed to the-SO2- asymmetric stretching vibrations; demonstrating the presence of -SO3H group [34]. A peak around 1716 cm-1 is assigned for C=O for COOH group. The adsorption peak at 1600 cm-1 coupled with weak absorption peak at 1382 cm-1 could be assigned for C=C stretching mode in poly aromatic rings [19]. The IR spectra suggested that both sulfonated-ICG and ICS were successfully functionalized with –SO3H groups. Guo et al. [35] synthesized a sulfonated-carbon solid acid catalyst from lignin, reported the similar infrared profile for the presence of -SO3H group and C=C aromatic carbon rings on the carbon structure. The Raman spectra of all the carbon catalysts are presented in Fig. 3. Both catalysts displayed two distinct signals corresponding to the defects induced in the D band (1345 cm-1, A1g D breathing mode) and sp2 carbon lattice G band (1570 cm-1, E2g G breathing mode, which indicated the presence of polycyclic carbon rings [36-37]. Usually, the ratio of the relative intensity of I(D)/I(G) band normally used to evaluate the degree of structural order of defect and graphities. Theoretically, the degree of sample defect and graphitization can be attributed to the structural changes during the thermal treatment process [37-38]. In this study, the I(D)/I(G) values found for sulfonated-ICS (0.76) and -ICG (0.75) were slightly higher compared to the ICG (0.68) and ICS (0.67). These were due to the introduction of -SO3H group on the incomplete carbonized sp2, which increased the defects of the carbon network [37].

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The breaking of the C-C bond enhanced the potential of non-graphitic carbon to form a stable covalent bond with –SO3H group [39]. The catalyst acid sites distribution and density were determined by NH 3-TPD as shown in Fig. 4. The incomplete carbonized carbon of ICS and ICG were showed in Fig. 4(a) and (b), which had one broad peak at temperature range from 450 to 850°C and maximized at 550°C with total acid site density of 5.57 ± 0.31 mmol g1

and 0.12 ± 0.05 mmol g -1 (Table 1). After the activation of the catalyst, a new peak

was observed in Fig. 4(c) and (d) and appeared at lower temperature in the range of 100 to 350°C for both sulfonated-ICS and -ICG samples, which comprised with 6.97 ± 0.56 and 4.23 ± 0.11 mmol g -1 of –SO3H group densities (Table 1). Both peaks at lower and higher temperature normally corresponded to the existence of Lewis acid sites and strong Brønsted acid sites, respectively. It was observed, due to the interactions of -NH3 gaseous with -SO3H and –COOH groups on the carbon sheets [40]. BET surface area analysis of the carbon materials were analyzed by physically adsorption of the N2 gas on the surface of the catalyst – all surface area, average pore volume and pore size were tabulated in Table 1. It was demonstrated that, after the activation process with -SO3H groups, the surface area for both sulfonated-ICS and ICG catalysts were slightly increased from, 10.54 and 3.65 m2g-1 to 12.40 and 10.67 m2g-1, correspondingly. This might be due to the defect on the carbon sheets by extensive thermal treatment, which resulted in smaller particle of carbon catalyst. However, the average pore volume and pore size were slightly decreased as a result from incorporated–SO3H groups that had occupied on and inside the pore of the catalysts [41].

3.2. Esterification of PFAD in supercritical methanol

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3.2.1. Effect of catalyst amount The effect of catalyst amount on the esterification of PFAD in supercritical methanol was studied (0 to 5 wt.%). A series of reactions were carried out at 290°C, for 30 min with 6/1 of methanol/PFAD molar ratio, as shown in Fig 5. Un-catalyzed reactions (0 wt. % catalyst) gave only 78.4% FAME yield. However, the FAME yields increased up to 95% (sulfonated-ICS) and 99% (sulfonated–ICG), with an addition of 1 wt.% catalyst. It strongly supported the hypothesis of ‘an addition of catalyst increased the rate of reaction process’ [24]. The high FAME yield can be explained through the role of catalyst, which provides the alternative way with low activation energy, thus will increase the reaction rate and conversion [42]. On the contrary, addition of excess catalyst from 2 to 5 wt.% showed insignificant drop of FAME yield, which can be clarified by two following reasons; (1) the hydrophilic group of carbohydrate-derived solid acid catalyst has tendency to entrap water molecule on the surface of the catalyst. The existence of water molecule inhibits the esterification reaction of PFAD [30]; (2) due to the agglomeration of the catalyst inside the system, it can be observed after the reaction process, that the catalyst was agglomerate and viscous. This phenomenon will lead to the mixing and diffusion problem [43]. Fig. 5 also showed the increment of the pressure from P = 7 MPa to 25 MPa with addition of 1 wt.% catalyst. The system pressure was found to be increased up to P = 40 MPa with addition of 5 wt.% catalyst. It was observed that the presence of catalyst in supercritical system increases the synergistic effect resulted rapid diffusion and high miscibility of methanol and PFAD molecules, at the same time increases the pressure of the system [44]. On the other hand, high pressure could cause negative impact and major damage to the reactor. Our present work has found that the optimum catalyst amount for esterification of PFAD in supercritical methanol

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was 1 wt.% and it was used for further optimization analysis. Thus, it revealed that, the system successfully reduces the amount of catalyst needed for the conventional biodiesel production process. As reported by Asri et al. [43], the transesterification of refine palm oil in sub- and supercritical methanol with γ-alumina supported heterogeneous base catalyst showed the presence of 3 wt.% of catalyst at 290°C reaction temperature significantly reduced the reaction times and required catalyst amount [43].

3.2.2. Effect of methanol-to-PFAD molar ratio The effect of the methanol/PFAD molar ratio on the esterification reaction of PFAD was studied in the range of 1 – 15 as shown in Fig. 6. It was observed that the yield of FAME increased as the methanol/PFAD molar ratios increase from 1:1 to 6:1, from 43.5% and 55.2% up to 88.3% and 96.9%for sulfonated-ICG and -ICS, respectively. But, an addition of methanol from 9:1 to 15:1 showed no increment in conversion. This result may be attributed to the higher water molecules contained in the system since the methanol use in this work was commercialized grade. By referring to the trend of the results, it was obvious that the molar ratio of methanol to oil ratio play an important role to effect the FAME yield. According to stoichiometric equation for esterification of PFAD, 1 mol of methanol is required to react with 1 mol of PFAD to produce 1 mol of FAME and 1 mol of water. Due to the reversible reaction, an excess of methanol is required to shift the reaction forward to the end product [43,45]. The larger amount of methanol provides high potential for the reaction between molecules of PFAD and methanol. Furthermore, the critical temperature of the mixture decreased when the amount of methanol increased because of methanol’s low critical temperature [30]. However, an excess of methanol was not favorable because it will increase the production cost. Thus, 6:1 of methanol/PFAD

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molar ratio was chosen as the optimum ratio and used for further optimization reaction. Yujaroen et al. studied the effect of methanol-to-PFAD molar ratio on the conversion of PFAD in supercritical methanol. They reported the same molar ratio of methanol to PFAD of 6:1 was the most suitable molar ratio which resulted 95% of FAME yield at 300°C reaction temperature [30].

3.2.3. Effect of reaction temperature To study the effect of reaction temperature, a series of reactions were carried out with the temperature ranging from 190 to 290C as presented in Fig. 7. It was observed that, the reaction temperature was significantly influenced the esterification of PFAD – the average percentage of FAME yield increased when the reaction temperature increased. At 190C, both reactions catalyzed gave different FAME yield (74.4 by sulfonated-ICS and 67.1% by –ICG catalysts). Meanwhile, at 290C, the FAME yield for both reactions was increased up to 97.3 and 95.4%, respectively; which may be due to the weakening of the hydrogen bonding of methanol. It will cause the reduction of methanol polarity and dielectric constant – meaning the system has a hydrophobic nature; thus, increasing the methanol solubility in oil [25-26]. It is noted that, when set temperature was 290C, the exact temperature of the reaction mixture inside the vessel was 282C. It is equivalent with the critical temperature of the methanol/PFAD mixture with the molar ratio of 6:1 is 282C (calculated by Lorentz-Berthelot-type mixing rule as shown in Eq. 1 to 8). The same theory was reported by Yujaroen et al. [30], in their work on un-catalyzed esterification reaction of PFAD in supercritical reactor. Under supercritical state of methanol/PFAD mixture, the methanol was completely homogeneous with oil, resulted in the fast reaction rate.

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3.2.4. Esterification reaction rate The PFAD esterification rate profiles of at sub- and supercritical state were observed by varying the reaction time from 5 to 30 min, as shown in Fig. 8. It was discovered that the addition of the sulfonated-ICS and ICG catalyst in the system under supercritical condition enhanced the reaction rate. At 190C, the reaction catalyzed by sulfonated-ICG required 25 min to yield 95% conversion as well as, the sulfonated-ICS required 15 min to reach 92% FAME yield. Surprisingly, both reactions catalyzed by sulfonated-ICG and ICS showed great performance at temperature 290C, resulted in more than 95% conversion of PFAD to FAME within 5 min. This can be explained by the effect of the supercritical methanol condition, which had increased the solubility of the methanol in oil [46]. Bunyakiat et al. [47] reported that the increasing of reaction temperature reaching supercritical point simultaneously increased the reaction rate. However, the critical point of the reaction mixture depends on the amount of methanol-to-oil molar ratio [47]. As calculated by using the Eq. 1 - 8, the critical point of the reaction system with 6:1 methanol-to-oil molar ratio was 282 °C (set temperature 290 °C) as discussed in section 3.2.3. Thus, the presence of the solid acid catalyst in the reaction mixture at this point drives the reaction rate to the maximum; hence shortening the reaction time. Samniang et al. [48], reported the biodiesel production from Jatropha oil and Krating oil in supercritical methanol reaction. They concluded that optimum reaction temperature and optimum reaction pressure significantly increase the reaction rate and FAME yield.

3.3.The effect of presence of catalyst

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The important of catalyst in esterification of PFAD was illustrated in Fig 9, which showed the comparison between catalyzed and un-catalyzed esterification reaction of PFAD. In addition of carbohydrate-derived solid acid catalyst in the supercritical reaction system, the FAME yield and reaction rate were increased. Based on the observation, within 5 mins of reaction at optimal operating conditions, the FAME yield reached 97% for the reaction catalyzed by sulfonated-ICS and 96% for ICG. However, only 54% FAME yield was observed after 5 min in un-catalyzed reaction; which consequently revealed that the presence of catalyst significantly increased the reaction rate by 6 times faster than the un-catalyzed reaction. Cho et al. [49], in their study on the reaction kinetics, discussed the effect of reaction temperature on the esterification of PFAD to produce biodiesel. They reported that, at temperature above 250C, the non-catalytic reaction could be completed within 180 min, producing 98% of FAME yield. Meanwhile, Yujaroen et al. [30] reported that the 95% of PFAD could be converted to biodiesel within 30 min in non-catalytic supercritical condition at 300C. In this study, we successfully shortened the reaction time from 30 min (non-catalyzed) to 5 min (catalyzed by carbohydrate-derived solid acid catalyst). These results confirmed the potential benefit of the catalyst in supercritical technology for biodiesel production for lowering the energy consumption by shortening the reaction time.

3.4.Reusability and deactivation of catalyst As shown in Fig. 10, the stability of sulfonated-ICS and -ICG catalysts were determined through number of reactions that had been completed using the same catalyst without any reactivation step. A series of reaction was completed at these conditions; the pressure was controlled in the range of 20 – 30 MPa, temperature was 290C with 6/1 methanol molar ratio, and 3 wt.% of catalyst for 10 min of reaction 15

time. It was observed that both sulfonated-ICS and-ICG had a potential for recycle up to 10 times and produced around 75% of FAME yield. The result suggested that the sulfonated-ICS and -ICG catalysts had good stability in rough reaction condition. According to Mo et al. [50] and Correia et al. [51], the main factor which caused the deactivation of catalyst is the leaching of active sites in the reaction medium, which decrease the FFA and triglycerides conversion. From our observation, the major advantage of supercritical reaction was the short reaction or short contact time of the produced catalyst with the methanol and oil; hence, reducing the leaching of –SO3H active sites from the aromatic polycyclic carbon support.

4. Conclusion In the current research, the potential of the carbohydrate-derived solid acid catalyst to catalyze the esterification reaction of PFAD in supercritical methanol was determined. The catalysts were prepared by simple sulfonation of incomplete carbonized glucose and starch and the results of catalytic activity test revealed that the carbohydratederived solid acid catalyst improve the supercritical methanol esterification reaction of PFAD by 6 times faster than the un-catalyzed reaction. The reaction catalyzed by 1 wt.% of sulfonated-ICG and –ICS catalysts successfully produced more than 95% of FAME yield within 5 min at methanol/PFAD molar ratio of 6/1 and 290C reaction temperature within the range of 20 to 30 MPa of pressure system. The catalyst also showed potential to be recycled up to 10 times. To be summarized, the presence of carbohydrate-derived solid acid catalyst in the supercritical reactor system improves the esterification reaction rate of PFAD and offers fast reaction, energy saving and low-cost biodiesel production technology.

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Acknowledgement The authors thank to Department of Chemical Engineering, Nagoya University for the use of laboratory equipment. Financial support from the Ministry of Higher Education Malaysia and Universiti Teknologi MARA for one of the authors (Ibrahim M. Lokman) is acknowledged. Financial assistance from MOSTI-eScience (Vot No: 5450746; Project No: 06-01-04-SF-1780) is also gratefully acknowledged.

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Caption of Figures and Table: Fig. 1. Schematic diagram of supercritical reactor. Fig. 2. Infrared spectra of (a) sulfonated-ICG and (b) sulfonated-ICS. Fig. 3. Raman spectra of (a) ICS, (b) ICG, (c) sulfonated-ICS and (d) sulfonated-ICG. Fig. 4. NH3-TPD profiles of (a) ICS, (b) ICG, (c) sulfonated-ICS and (d) sulfonatedICG. Fig. 5. Effect of catalyst amount on FAME yield and system pressure in supercritical methanol. (Operating parameters: methanol/PFAD molar ratio of 6/1, reaction temperature of 290°C, reaction time of 30 min). Fig. 6. Effect of methanol/PFAD molar ratio on FAME yield in supercritical methanol. (Operating parameters: reaction temperature of 290°C, catalyst amount of 1 wt.%, reaction time of 30 min, and P = 20 - 30 MPa). Fig. 7. Effect of reaction temperature on FAME yield in supercritical methanol. (Operating parameters: methanol/PFAD molar ratio of 6/1, catalyst amount of 1 wt.%, reaction time of 10 min and P = 20 - 30 MPa). Fig. 8. Esterification reaction rate in sub- and supercritical methanol at 190 and 290 C with sulfonated carbon-based solid acid catalyst (a) sulfonated-ICG and (b) sulfonated-ICS. (Operating parameters: methanol/PFAD molar ratio of 6/1, catalyst amount of 1 wt.% and P = 20 - 30 MPa). Fig. 9. Comparison between catalyzed reaction and non-catalyzed reaction in supercritical methanol. (Optimum operating parameters: methanol/PFAD molar ratio of 6/1, catalyst amount of 1 wt.%, reaction temperature of 290C and P = 20 - 30 MPa). Fig. 10. Reusability potential and deactivation analysis of the catalysts. (Operating parameters: methanol/PFAD molar ratio of 6/1, catalyst amount of 3 wt.%, reaction temperature of 290 C, reaction time of 10 min and P = 20 - 30 MPa). Table 1. The textural properties of the catalyst carbon materials.

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Table 1 Sample

ICS ICG Sulfonated-ICS Sulfonated-ICG

Acid sites density (mmol g-1) SO3H Total

0 0 6.97 ± 0.56 4.23 ± 0.11

5.57 ± 0.31 0.12 ± 0.05 12.54 ± 0.23 6.76 ± 0.54

Surface area (m2g-1) 10.54 ± 0.35 03.65 ± 0.23 12.40 ± 0.48 10.67 ± 0.90

BET Average pore volume (cm3g-1) 3.90 ± 0.23 3.74 ± 0.32 3.60 ± 0.35 3.61 ± 0.27

Average pore size (nm) 20.3 ± 0.05 17.5 ± 0.04 16.1 ± 0.02 15.8 ± 0.06

23

Furnace

Temperature controller

re ac to r

Pressure detector AKICO reactor vessel, Pmax = 50MPa

Fig. 1.

24

Transmittance (%)

(a) (b)

1716

1382 1600 1034 1181

3500

2500 1500 Wavenumber (cm-1)

500

Fig. 2.

25

G Raman intensity (a.u.)

D (a)

(b) (c) (d) A1g 500

1000

E2g

1500 2000 Wavenumber (cm-1)

2500

Fig. 3.

26

Intensity (a.u.)

(a)

(b)

(c) (d)

50

250

450 650 Temperature (°C)

850

Fig. 4.

27

FAME yield (%)

80

40

60

30

40

20

20

10

0

0

0

1 2 3 4 Catalyst amount (wt.%)

Pressure (MPa)

50

100

Uncatalyzed Sulfonated-ICG Sulfonated-ICS Pressure

5

Fig. 5.

28

100

FAME yield (%)

80 60 Sulfonated-ICG 40

Sulfonated-ICS

20 0 1:1

3:1 6:1 9:1 12:1 Methanol/PFAD molar ratio

15:1

Fig. 6.

29

FAME yield (%)

100

80 60 Sulfonated-ICG

40

Sulfonated-ICS

20 0 190

210 230 250 270 Reaction temperature (°C)

290

Fig. 7.

30

(a)

(b) 100

90 80

190 °C 290 °C

70

60 50

FAME yield (%)

FAME yield (%)

100

90 80 70 60

190 °C 290 °C

50 5

10 15 20 25 30

Reaction time (min)

5 10 15 20 25 30 Reaction time (min)

Fig. 8.

31

100

FAME yield (%)

80 60 unCatalyzed 40

Sulfonated-ICG Sulfonated-ICS

20 0 1

5

10 15 20 Reaction time (min)

25

30

Fig. 9.

32

FAME yield (%)

100 90 80 70 60 50 40 30 20 10 0

Sulfonated-ICG Sulfonated-ICS

0

1

2

3

4 5 6 7 Catalyst cycle

8

9 10

Fig. 10.

33

Research Highlights 

The heterogeneous catalysts in supercritical methanol were introduced for biodiesel production using PFAD.



The biodiesel yields in catalyzed supercritical methanol were 97.3% and 95.4%.



The heterogeneous catalysts exhibited excellent stability and reusability in supercritical conditions.

34