Methyl ester production from palm fatty acid distillate using sulfonated glucose-derived acid catalyst

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Methyl ester production from palm fatty acid distillate using sulfonated glucose-derived acid catalyst

Ibrahim M. Lokman

^a^,^b

, Umer Rashid

^c^,^*

, Yun Hin Tau ﬁ q-Yap

^a^,^b^,^d^,^**

, Robiah Yunus

^c^,^e

aCatalysis Science and Technology Research Centre, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

bDepartment of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

cInstitute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

dCurtin Sarawak Research Institute, Curtin University, 98009 Miri, Sarawak, Malaysia

eDepartment of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

a r t i c l e i n f o

Article history:

Received 27 October 2014 Accepted 16 March 2015 Available online

Keywords:

Sulfonated glucose-derived acid catalyst PFAD

Esteriﬁcation Surface area Biodiesel production

a b s t r a c t

A highly potential heterogeneous solid acid catalyst derived from a carbohydrate precursor was successfully developed and applied for biodiesel production from palm fatty acid distillate (PFAD). The catalyst was synthesized by sulfonating the incomplete carbonized D-glucose using concentrated sulfuric acid to produce a sulfonated glucose-derived acid catalyst. The catalyst underwent a detailed characterization analysis in terms of its functional groups of active sites, morphological structure, thermal stability, surface area and density of acid sites. For the catalytic activity test, the sulfonated glucose- derived acid catalyst was used to esterify PFAD which contained around 85 wt.% free fatty acids (FFA).

Furthermore, it demonstrated a 95.4% conversion of FFA to fatty acid methyl esters (FAMEs) with 92.3% of FAME yield under the following optimum condition: catalyst loading of 2.5 wt.%, methanol-to-PFAD molar ratio of 10:1, reaction temperature of 75C and the reaction time was 2 h. It can be deduced from the results that a sulfonated glucose-derived acid catalyst has a high potential to esterify high FFA feedstocks, especially PFAD, to produce low cost biodiesel.

1. Introduction

The dramatic depletion of petroleum sources and the drastic increase of its price have force the demand for the supply of biodiesel. Basically, biodiesel or scientiﬁcally known as fatty acid methyl ester (FAME) can be derived from vegetable oil and animal fat, which contain triglycerides and fatty acids. The conversion of starting materials to FAME is possible in the presence of alcohol.

The processes are also known as esteriﬁcation and trans- esteriﬁcation reactions. Both reaction processes are the common reversible reactions to produce biodiesel from free fatty acids (FFA) and triglycerides (TGs), respectively. The most common starting materials to synthesize biodiesel are vegetable oils including soybean oil[1], cottonseed oil[2], Jatropha oil[3], palm oil [4], and

sunflower oil [5,6]. Recently, the increase demand for biodiesel production has led to the shortages of feedstock and related chemicals, as well as an increase in the price of the biodiesel production cost. As a result, the biodiesel price has become more expensive than petrol-diesel. Therefore, alternative non-edible and cheaper feedstocks have been searched for and studied. One of the possible feedstocks that have been focused on is palm fatty acid distillate (PFAD). In Malaysia, palm oil industries produce almost 16 million tonnes of crude palm oil per annum. Meanwhile, at least 700,000 metric tonnes of PFAD has been produced every year as a by-product in the crude palm oil's refinery process. PFAD has been used in the animal feed industry, cosmetic industry and soap industry[7,8]. PFAD consists of a high free fatty acid (FFA) content, as high as 85 wt.%, and contains almost 10 wt.% of triglycerides and small amounts of squalene, sterols and vitamin E. As compared to refined palm oil which consists of almost 100 wt.% of triglycerides [9].

The transesteriﬁcation of PFAD is impossible in the presence of a base catalyst. The base catalyst will react with the acidic FFA to initiate the neutralization and saponiﬁcation process; hence, it compensates the reaction and deactivates the catalyst. To overcome

*Corresponding author. Tel.:þ60 3 89467393; fax:þ60 3 89467006.

**Corresponding author. Catalysis Science and Technology Research Centre, Fac- ulty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia.

Tel.:þ60 3 89466809; fax:þ60 3 89466758.

E-mail addresses: umer.rashid@yahoo.com (U. Rashid), tauﬁq@upm.edu.my (Y.H. Tauﬁq-Yap).

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this issue, an acid catalyst, e.g., homogeneous sulphuric acid has been used to esterify the PFAD [10]. However, theﬁnal product required several steps of a puriﬁcation process to recover the product from the acid catalyst. Due to above mentioned reasons;

the heterogeneous solid acid catalyst has been introduced. The heterogeneous solid acid catalyst can eliminate the corrosion, separation, emulsiﬁcation and saponiﬁcation problems[11].

Nowadays, there are a lot of heterogeneous solid acid catalysts that have been introduced for biodiesel applications. The most common heterogeneous solid acid catalyst is sulfonated-carbon nanotubes[2], niobium-MCM-41 catalysts[5], ferric alginate[12], ferric hydrogen sulfate[13] and carbohydrate-derived solid acid catalyst[14]. An ideal and effective heterogeneous solid acid catalyst must have numerous active sites with high acidity properties and quite good stability with very little leaching and deactivation of the catalyst, which is applicable for repetitive use [14,15]. In a recent research, it was reported that a sugar catalyst also known as a carbohydrate-derived solid acid catalyst has ideal properties as an excellent heterogeneous solid acid catalyst[16]. It has a potential to esterify high FFA feedstock due to the high acidity properties and the high stability of a polycyclic carbon structure[17,18]. Theoret- ically, both esterification and transesterification of high FFA feedstock using an acid catalyst requires a high temperature and longer reaction time due to the slow reaction rate process. In order to overcome the problems, a two-step process was introduced. Firstly, pre-treatment of the feedstock via an acid catalyst to reduce the amount of FFA followed by the second-step which is to transesterify the remaining triglycerides using a base catalyst[19]. On the other hand, an autoclave reactor has been used as the alternative method to skip the two-step method. However, both methods are difficult, time consuming and dissipate energy.

Previously, the carbohydrate-derived solid acid catalyst had been synthesized by Nakajima and Hara[20]and Okamura et al.

[21]. However, the effect of the activation/sulfonation step on the properties of the carbon structure and its application to esterify PFAD were not reported. In this study, we synthesized a sulfonated glucose-derived solid acid catalyst and all the catalyst were char- acterized by infrared spectroscopy (IR), thermal gravimetric analysis (TGA), x-ray diffractometer (XRD), surface area analysis, CHNOS elemental analysis, ammonia-temperature programmed desorption (TPD-NH3) and scanning electron microscopy (SEM) with energy dispersive x-ray (EDX) analysis. The effect of the sulfonation time on the esteriﬁcation of PFAD based on physical and chemical properties were investigated. The main objective was also to improve the reaction step by shortening the reaction time and lowering the reaction temperature without pre-treatment of the feedstock. The reaction was performed under a conventional reﬂux system and the optimum variables were determined for the optimization purpose.

2. Materials and methods

2.1. Chemicals and materials

The PFAD was supplied by Jomalina R&D, Sime Darby Sdn. Bhd., Malaysia. Commercialized D-(þ)-glucose was purchased from Sigma, Aldrich. Concentrated H2SO4and KOH were obtained from J.T. Baker. Meanwhile, the solvent, such as methanol, ethanol and n- hexane were supplied by Merck chemical company. The standard methyl esters for GC analysis, such as methyl heptadecanoate, methyl oleate, methyl linoleate, methyl palmitate, methyl myristate and methyl stearate were purchased from Fluka, USA. All of the chemicals used in this work were analytical grade and had no need for any further puriﬁcation.

2.2. Catalyst preparation

The preparation of the glucose-derived solid acid catalyst was adopted and modiﬁed as proposed by Zong et al.[17]and Lou et al.

[18]. Briefly, 15 g of D-Glucose powder was heated at 400C for 12 h under N2flow to produce a black solid of incomplete carbonized glucose (ICG). The produced material was then milled to powder form. 4 g of ICG powder was mixed with 100 mL of concentrated H2SO4and heated at 150C for 5, 8, 10, 12 and 15 h in N2flow to introduce the sulfonic group (eSO3H) on an aromatic carbon structure, designated as ICG_(x)-SO₃H where‘x’was the sulfonation time. Then, the mixture was diluted with distilled water and the black precipitate was collected. The black precipitate was washed with hot distilled water (>80C) to remove any excess sulfate ions and impurities. The resulting sulfonated-glucose derived solid acid (ICG-SO3H) catalyst was then dried in an oven at 80C for about 24 h to remove excess moisture.

2.3. Catalyst characterization

The crystallinity of the catalyst was determined by X-ray diffraction (XRD). The analysis was conducted using XRD (Shi- madzu, XRD 6000) with the scan range of theta (q^{) from 2}^to60^at a scanning rate of 4 degree min¹. A morphological study of the catalysts has been performed by using scanning electron microscopy (SEM) (JEOL, JSM-6400). An elemental composition of the catalyst was performed by Shimadzu X-ray Fluorescence (XRF) (Rayny EDX-720 spectrometer) and also by using the CHNS elemental analysis (Leco, TruSpec^® CHN). Fourier transform infrared (FTIR) from Perkin Elmer (1725 X) was used for the qual- itatively determination of the functional group's presence in the structure of the catalyst after the activation with H2SO4. The ammonia-temperature programmed desorption (TPD-NH3) (Thermo Finnigan, TPDRO 1100 series) was used to determine the density of the acid sites of the catalyst. The thermal stability of the catalyst was analyzed by using the thermogravimetric analysis (TGA) equipped with differential thermogravimetry (DTG) using Mettler Toledo 990. Meanwhile, The BET surface area measurement was carried out by using a Sorptomatic 1990 series (Thermo Fin- nigan) instrument. The samples were degassed and heated over- night at 150C prior to the surface area analysis.

2.4. Catalytic activity of the catalyst

The esterification reaction of PFAD was carried out by using the conventional reflux technique. A 150 mL two-necked round bottom flask was used and immerged in an oil bath. The condenser was coupled with the round bottomflask to recondense the evaporated methanol. In a typical reaction, 5 g of PFAD was mixed with the calculated amount of methanol and catalyst. The suspension was then refluxed at the desired temperature with a stirring speed of 600 rpm. At the end of the reaction process, the catalyst was separated from the suspension by centrifugation at 4000 g within 15 min. The methanol was then separated and recovered from the product. The separated product (PFAD methyl ester) was collected for further analysis.

2.5. PFAD methyl ester analysis

The fatty acid conversion was calculated based on the difference of the acid value of the feedstock and acid value of the product according to the AOCS 5a-40 standard method[6]. The FFA conversion was calculated according to Eq.(1), where,AVfand AVp stand for the acid value of the feedstock and the product, respectively.

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FFA conversion;%¼AVfAVp

AVf 100% (1)

Meanwhile, the analysis of the product yield was carried out by gas chromatography (GC) equipped with an FID detector. A highly polar capillary column BPX 70, SGE Company (length: 30 m, ID:

0.25 mm and capillary: 0.25mm) was used for the separation of the FAME compounds. Then-hexane was used as the solvent for the dilution of the product samples and FAME Standard. 500 ppm of each standard methyl oleate, methyl palmitate, methyl linoleate, methyl myristate and methyl stearate were prepared as the reference standard. Meanwhile, methyl heptadecanoate was used as the internal standard and prepared together with the sample product.

Then, 1mL of the sample solution was injected into the GC injector port. The GC injector port was set at 230 C and the detector temperature was 270C. However, the GC oven was programed with the increasing starting temperature from 100C up to 250C with 10C/min of the temperature rate. The formula to calculate the product yield was as depicted in Eq.(2).

FAME yield;%¼ weight of FAME produced

weight of theoritical FAME100% (2)

2.6. Catalyst reusability and leaching test

The catalyst reusability was studied in order to study the deactivation and the ability of the catalyst to be recycled without reactivation. 2.5 wt. % of the ICG(15)-SO3H catalyst was used for this reaction. The reaction was carried out at the 75C reaction temperature with a 10:1 methanol-to-PFAD molar ratio within a 2 h period. The recovered catalyst from the product mixture at each cycle was then washed with methanol and followed by hexane to remove polar and non-polar compounds from the surface of the catalyst. The product from each reaction was analyzed to calculate the conversion of FFA to PFAD methyl esters according to Eq.(1)and Eq.(2). Meanwhile, about 0.005 g of the catalyst was collected from every reaction cycle for Sulfur determination by using a CHNOS instrument to determine the amount of leached active site (eSO3H) from the catalyst.

2.7. Statistical analysis

The catalytic activity evaluation of all the catalyst samples were statistically analyzed individually in triplicate as mean±SD using SPSS software (IBM, PASW 130 Statistics 19, USA).

3. Results and discussion

3.1. Characterization of the catalyst

Fig. 1depicts the IR spectra of the unsulfonated ICG catalyst and the sulfonated ICG(15)-SO3H catalyst. The bands at 1591.45 cm¹ and 1588.87 cm¹ were attributed to the aromatic ring C]C stretching mode presence in the polyaromatic carbon sketch.

Meanwhile, the bands at 1692.31 and 1698.45 cm¹ for both samples were assigned to the C]O stretching mode of theeCOOH group, respectively. The presence of sulfonic groups was conﬁrmed by clear and strong vibration bands at 1030.93 cm¹ for eSO₂ symmetric stretching and 1163.85 cm¹ for eSO2 asymmetric stretching as shown inFig. 1(b), proving the presence of theeSO3H group covalently bonded to the polyaromatic carbon structure [22,23]. Although these peaks were also present inFig. 1(a), they had a weak intensity. This was due to the efﬁcient IR adsorption

ability of the carbon frameworks that were rearranged from the high temperature thermal treatment, which were the same as reported by Nakajima and Hara[20].

The SEM images, Fig. 2 showed the irregular and aggregate particles of the unsulfonated ICG and sulfonated ICG₍₁₅₎-SO₃H catalyst at the 100mm scale. The diameter size of the ICG catalyst ranged from 10 to 50 mm; meanwhile, it was 5e20 m^{m for the} ICG(15)-SO3H catalyst. The SEM images revealed that the size of the particles of the catalyst decreased due to the acid treatment during the sulfonation process.

Fig. 3depicted the XRD patterns of the ICG and ICG(15)-SO3H catalysts. Both samples have one broad and one weak peak at 2q¼10e30and 2q¼40e50which were assigned to the C(002) and C(101), respectively. Such peaks theoretically were assigned for the amorphous carbon which is composed of the oriented random fashion of aromatic carbon sheets [17]. This indicated that the samples were comprised of a high content of non-graphitic carbon structure[20,24,25]. The glucose was heated at 400C in an inert media and the CeOeC bonding of the structure started to break the cleavage of the bonding. Due to this reason, the amorphous structure and the formation of polycylic carbon sheets became clearer.

This may be important in the catalyst's activity during the esteri- ﬁcation process as reported by Zong et al.[17].

The thermal stability of the unsulfonated and sulfonated ICG catalysts were determined by TGA as presented in Fig. 4. Both samples had weight loss occurring at around 100C due to physi- cally absorbing water on the catalysts [17]. A steady loss was observed for the unsulfonated ICG catalyst starting from 500 to 700 C which could be attributed to the decomposition of the carbon structure. However, the ICG(15)-SO3H showed three stages of weight loss which were completely different with the unsulfonated ICG catalyst. The weight loss was observed at around 201C from 100 to 300C, which was assigned to the decomposition ofeSO3H groups similar to the one observed by Shu et al. [2]. Further decomposition of the carbon structure at 586 C was observed.

These results revealed that the prepared ICG(15)-SO3H catalyst were stable up to 300C before the sulfonic group started to decompose;

this was the same as the thermal stability of the sugar catalyst reported by Zong et al.[17].

The density and distribution of the acid sites of the catalyst were determined by NH₃-TPD. Fig. 5 (a) and (b) show the ammonia desorption curves for both the unsulfonated ICG and ICG-SO3H, respectively. The unsulfonated ICG sample had only one broad Fig. 1.IR spectra of incomplete carbonized glucose before sulfonation (a) ICG and after sulfonation (b) ICG(15)-SO3H.

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desorption peak maximized at 550C from 420 to 850C, which was assigned to the strong acid sites with the density of the acid site up to 0.12±0.05 mmol/g as tabulated inTable 1. However, for sulfonated ICG(15)-SO3H, one extra desorption peak was observed maximized at 200C from 150 to 350C beside the peak at 600C ranging from 410 to 900C. This peak was due to the interaction of eNH3with incompletely formed carbon sheets and also the interaction with the eSO₃H [26]. Both of the distinct ammonia desorption's peaks corresponded to the presence of a weak and strong Brønsted acid site[27], which was important to ensure that the catalyst was chemically and thermally stable up to 300C before theeSO3H groups started to decompose as discussed in the TGA part. The same TPD proﬁle of the sulfonated carbon catalyst was reported by Dawodu et al. They reported the production of biodiesel from non-edible seed oil with 15% FFA using a sulfonated glucose catalyst[28].

3.2. Effect of different sulfonation time on texture properties and catalytic activity

Table 1showed the texture properties of the sulfonated ICG- SO3H catalyst prepared at different sulfonation times (5, 8, 10, 12 and 15 h). The catalytic activity was analyzed based on the PFAD esterification process. The unsulfonated ICG catalyst showed lower activity and only esterified about 5.6% of FFA. This was due to lower total acid site density and lower BET surface area as compared to the sulfonated ICG-SO₃H catalyst. The results revealed that a longer sulfonation time increased the total acid site density, BET surface area and S content of the catalyst, which suggested that both oxidation and activation occurred during the sulfonation process [26]. ICG₍₁₅₎-SO₃H depicted a larger surface area of 10.67 m²/g, with 4.23 mmol/g total acid site density and 4.89% S content as compared to the other prepared ICG-SO3H at a lower sulfonation time. The prepared ICG₍₁₅₎-SO₃H in this condition also exhibited a larger surface area and acid site density than other reported sulfonated sugar catalysts, which averaged a surface area less than 5 m²/g and acid density less than 4 mmol/g [17,18,21,28]. All S atoms in the sulfonated catalyst have previously been shown to be present aseSO3H groups[17]. The larger surface area provided much more space for anchoring theeSO₃H groups' molecules and theoretically enhanced the catalytic activity of the catalyst. The esterification process of PFAD with the ICG(15)-SO3H catalyst showed higher catalytic activity with 81.5% of FFA conversion at 2 wt.% of the catalyst, 10:1 methanol-to-PFAD molar ratio, 70C and 2 h reaction time (non-optimized condition).

3.3. PFAD esteriﬁcation process 3.3.1. Effect of methanol amount

The amount of methanol to acid oil is the important variable to ensure the completion of FFA conversion to FAMEs. A series of reactions was carried out with different amounts of methanol (1e18 methanol molar ratio) while other reaction conditions were kept constant using 1 wt.% of the ICG(15)-SO3H catalyst. The obtained results are depicted inFig. 6(a). It was observed that the FFA conversion was increased with an increase in methanol-to-PFAD molar ratio from 1:1 until 10:1 equivalent to 77.8% of the FFA conversion.

When the molar ratio increased from 10:1 to 18:1, there was no significant increment observed. Due to the reversible esterification reaction process, an excess amount of methanol was needed to shift the equilibrium to the right hand side. However, too much methanol showed insignificant changes. This result could possibly be attribute to the high water content produced, which reacted with the FAMEs thus, redirecting the reaction backwards[29e32]. On the other hand, the 10:1 methanol-to-PFAD molar ratio was chosen Fig. 2.SEM images of incomplete carbonized glucose before sulfonation (a) ICG and

after sulfonation (b) ICG(15)-SO3H.

Fig. 3.XRD patterns of incomplete carbonized glucose unsulfonated (a) ICG and sulfonated (b) ICG(15)-SO3H.

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for further study based on the equilibrium conversion and low- production cost. Zhang and Jiang (2008) used a 24:1 methanol- to-oil molar ratio with 2 wt.% sulphuric acid, 80 min reaction time and 60C reaction temperature to esterifyZanthoxylum bungeanumseed oil from 45.51 mg KOH/g to 1.16 mg KOH/g by only one-step acid-catalyzed esteriﬁcation[33].

3.3.2. Effect of amount of ICG(15)-SO3H catalyst

Fig. 6(b) depicted the effect of the catalyst amount on the FFA conversion. The dosage of the ICG₍₁₅₎-SO₃H catalyst ranged from 0.5 to 3.5 wt.%. Other fixed conditions were the methanol-to-PFAD molar of 10:1, reaction time of 2 h and reaction temperature of 70C. As observed, the FFA conversion increased with an increase in the catalyst amount. From 0.5 to 2.5 wt.% of the catalyst, the FFA conversion was increased from 35.7 to 86.1%, which was about a 50.4% increment. However, further addition of the catalyst showed no significant increment of FFA conversion. This was due to the rate of the mass transfer or contact rate among the catalyst, methanol and the feedstock reaching the optimum condition (equilibrium point)[30]. Thus, it is clear that the addition of the catalyst more than 2.5 wt.% does not have much influence to reduce the FFA content. The reaction was further studied with 2.5 wt.% of the ICG₍₁₅₎-SO₃H catalyst for further optimization analysis. Guo et al.

[34]have used an acidiﬁed soybean soapstock for biodiesel production using the lignin-derived carbonaceous solid acid catalyst.

They reported that 7 wt.% of the catalyst was sufﬁcient to increase the FFA conversion up to 96% at a 70C reaction temperature and 9:1 methanol-to-oil molar ratio within a 5 h reaction time.

3.3.3. Effect of reaction temperature

Due to the endothermic reaction of the esteriﬁcation process, the activation energy was required to activate the protonation of FFA by a solid acid catalyst[27,28]. To study the inﬂuence of the reaction temperature on the FFA conversion, a number of Fig. 4.TGA analysis of incomplete carbonized glucose before sulfonation (a) ICG and after sulfonation (b) ICG(15)-SO3H.

Fig. 5.TPD-NH3of incomplete carbonized glucose (a) ICG and (b) ICG(15)-SO3H.

Table 1

Effect of different sulfonation time on acid sites density, S content, surface area and FFA conversion.

Catalyst Pyrolysis time^a(h) Sulfonation time^b(h) Acid site density^c(mmol/g) S content^d(%) BET, (m²/g) FFA conversion^e(%)

Unsulfonated-ICG 12 e 0.12±0.05 0 3.65±0.23 5.6±0.35

ICG(5)-SO3H 12 5 0.18±0.08 1.61±0.15 4.03±0.20 12.4±0.75

ICG(8)-SO3H 12 8 0.21±0.10 2.08±0.12 6.62±0.60 56.9±0.80

ICG(10)-SO3H 12 10 2.48±0.15 2.50±0.12 8.46±0.70 64.2±0.70

ICG(12)-SO3H 12 12 3.42±0.14 3.36±0.11 9.88±0.50 72.4±0.90

ICG(15)-SO3H 12 15 4.23±0.11 4.89±0.10 10.67±0.90 81.5±0.60

Ref.

Lou et al.[18] e e 1.6 4.7 4.1 e

Okamura et al.[21] e e 0.74 e 2 e

Zong et al.[17] 15 15 e 4.7 4.13 e

Sample code: Incomplete Carbonized Glucose ICG(sulfonation time)eSO3H.

aThermal treatment in inert environment at 400C.

b Thermal activation with concentrated H2SO4at 150C.

c Acid sites density calculated by TPD-NH3.

d S content estimated by EDX analysis.

e Reaction condition (non-optimized): 2 wt.% catalyst, 10:1 methanol molar ratio, 2 h reaction time, 70C reaction temperature.

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experiments with temperatures ranging from 65 to 90 C were carried out using the ICG(15)-SO3H catalyst. As shown inFig. 6(c), it was found that the optimum reaction temperature was 75C. At this point, the FFA conversion was increased up to 95.4% from 75.2%

at the temperature of 65 C. However, the FFA conversion was slightly decreased when the temperature was higher than 75C. As reported by Leung and Guo[35], they produced biodiesel from neat canola oil and used frying oil. They mentioned that higher temperatures above 50C had negative impacts to the reaction. Hayyan et al.[36]had studied the reduction of the FFA content in sludge palm oil via an acid catalyst. They also reported the reduction of FFA was decreased when the reaction was at above 60 C, and it depended on the viscosity of the feedstock. At this point, at the 75C reaction temperature, the activation energy was sufﬁcient to protonate the carbonyl groups from FFA and resulted in the maximum reaction rate.

3.3.4. Effect of reaction time

In order to achieve a high FFA conversion, sufﬁcient contact time must be provided. A series of experiments were carried out to study the effect of the reaction time on the reduction of the FFA content.

Theﬁxed conditions were: 2.5 wt.% of the ICG(15)-SO3H catalyst, 10:1 methanol-to-PFAD molar ratio and 75 C reaction

temperature.Fig. 6(d) shows the FFA conversion versus different reaction times within 6 h. It was observed that the conversion of FFA increased with an increase in reaction time. The reaction reached the maximum conversion of 94.8% after 2 h. This was followed by an insignificant increment of the FFA conversion from 3 to 6 h of reaction time. Therefore, in order to save the cost and energy during the biodiesel production process, a 2 h reaction time is sufficient to esterify more than 90% of FFA to FAMEs. A study by Dehkhoda et al.[24]showed that the esterification process reduces the FFA level of high FFA feedstock by 92% using 5 wt.% of the sulfonated biochar catalyst, 18:1 methanol-to-oil molar ratio within a 3 h reaction time, and no significant increment was reported at a longer reaction time.

3.4. Yield of the PFAD methyl esters at optimum conditions The optimum conditions for esteriﬁcation of PFAD were found to be the 10:1 methanol-to-PFAD molar ratio, 75 C reaction temperature, 2.5 wt.% of the ICG(15)-SO3H catalyst and 2 h reaction time.

These operating conditions resulted in a 95.4% FFA conversion and 92.3% PFAD methyl ester yield (Calculated by Eq.(2)) as presented inFig. 8. Whereas, Fig. 7showed the GC chromatograms of the FAMEs' reference standard and the PFAD methyl esters (product).

Fig. 6.(a). The effect of methanol-to-PFAD molar ratio on the conversion of FFA. Esterification conditions: Catalyst amount¼1 wt.% ICG(15)-SO3H, reaction temperature¼70C and reaction time¼2 h (b). The effect of ICG(15)-SO3H amount on the conversion of FFA. Esterification conditions: methanol-to-PFAD molar ratio¼10:1, reaction temperature¼70C and reaction time¼2 h (c). The effect of reaction temperature on the conversion of FFA. The reaction was done under the esterification conditions, methanol-to-PFAD molar ratio¼10:1; catalyst amount¼2.5 wt.% ICG(15)-SO3H and reaction time¼2 h (d). The effect of reaction time on the conversion of FFA. The reaction was done under esterification conditions, methanol-to-PFAD molar ratio¼10:1, catalyst amount¼2.5 wt.% ICG(15)-SO3H and reaction temperature¼75C.

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Five FAME components were identiﬁed as consisting of 45.1% of methyl palmitate, 40.3% of methyl oleate, 8.3% of methyl linoleate, 4.2% of methyl sterate and 2.1% of methyl myristate.

3.5. Deactivation and reusability analysis of the catalyst

The reusability of the prepared ICG₍₁₅₎-SO₃H catalyst was carried out at the 75 C reaction temperature, 10:1 methanol-to-PFAD molar ratio, 2.5 wt.% of the catalyst and 2 h reaction time. The catalyst was recovered, separated from the product, washed and dried before it was reused for the next reaction cycle under the same operating conditions.Fig. 8presents the reusability of the ICG(15)-SO3H which was up to 6 times and resulted in 81.5% FFA conversion after six cycles with a 73.4% FAME yield. It showed that a loss of activity was detected in each cycle. Although the sulfur content was detected in the product, the sulfur concentration was lower than the maximum limit of the ASTM D6751 and EN 14214

standards, which was less than 0.0015 ppm and 0.001 ppm, respectively.

A study has been carried out by Chin et al.[37]. They produced methyl ester from PFAD using a sulfonated sugar cane bagasse acid catalyst. At the temperature of 170C, methanol content of 20:1, 11.5 wt.% of the catalyst and 30 min reaction time, the activity of the catalyst dropped 15% after 5 reaction cycles from 80% to 65% of the methyl ester content.

4. Conclusions

The PFAD methyl ester has been successfully produced from the esterification of PFAD using a sulfonated glucose-derived solid acid catalyst with the one-step conventional reflux method. The catalyst was prepared by the incomplete carbonization of glucose at 400C followed by the sulfonation process using H2SO4at 150 C. The results showed that the polycyclic aromatic carbon sheets strongly supported the attachment of theeSO₃H group, which enhanced the acid properties of the catalyst. The ICG(15)-SO3H revealed the highest conversion of PFAD as compared with the other prepared catalysts. The optimum variables for the esterification reaction were achieved after a series of reactions within a 2 h reaction time at a 75C reaction temperature, 10:1 methanol-to-PFAD molar ratio and 2.5 wt.% of the catalyst. These optimum conditions provided a 95.4% FFA conversion with a 92.3% FAME yield. The reusability study also revealed the maximum stability of the catalyst was up to 6 cycles. In conclusion, the sulfonated ICG-SO3H catalyst has a high potential as a stable and active heterogeneous Brønsted acid catalyst for the esterification of high FFA feedstocks especially for PFAD.

Acknowledgments

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 the PUTRA grant-UPM (Vot No: 9344200) and MOSTI-eScience (Vot No: 5450746) is also gratefully acknowledged.

Fig. 8.Reusability of the catalyst and the amount of leached-sulfur estimated by CHNS elemental analysis for 6 reaction cycles. The experiment was done under optimum esteriﬁcation condition: methanol-to-PFAD molar ratio ¼ 10:1; catalyst amount¼2.5 wt.% ICG(15)-SO3H; reaction temperature¼75C; reaction time¼2 h.

Fig. 7.GC-FID chromatograms of (a) FAMEs standards and (b) PFAD methyl esters produced from esteriﬁcation of PFAD at optimized condition: methanol-to-PFAD molar ratio of 10:1, catalyst amount¼2.5 wt.% ICG(15)-SO3H; reaction temperature¼75C; reaction time¼2 h.

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