Active Acid Catalyst of Sulphated Zinc Oxide for Transesterification of Soybean Oil with Methanol to Biodiesel

I. Istadi; Didi D. Anggoro; Luqman Buchori; Dyah A. Rahmawati; Dinnia Intaningrum

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Procedia Environmental Sciences 23 (2015) 385 - 393

International Conference on Tropical and Coastal Region Eco-Development 2014(ICTCRED

ACTIVE ACID CATALYST OF SULPHATED ZINC OXIDE FOR TRANSESTERIFICATION OF SOYBEAN OIL WITH METHANOL TO BIODIESEL

I. Istadi *, Didi D. Anggoro, Luqman Buchori, Dyah A. Rahmawati, Dinnia Intaningrum

Laboratory of Chemical and Process Engineering, Department of Chemical Engineering, Diponegoro University, Jln. Prof. Soedarto, Kampus Undip Tembalang, Semarang, 50275 Indonesia

Corresponding author : istadi@che.undip.ac.id (I. Istadi)

Abstract: Active solid acid catalysts of sulphated zinc oxide (SO42"-ZnO and SO/'/ZnO) were prepared and characterized. The solid acid catalysts were investigated for their performance on transesterification of soybean oil with methanol to produce biodiesel. The SO42--ZnO and SO42-/ZnO catalysts were prepared by coprecipitation and impregnation respectively to compare their performance for biodiesel production. The catalysts were characterized by X-ray Diffraction (XRD) and Fourier Transform Infra Red (FT-IR), while the fatty acid methyl ester (FAME) component in biodiesel product was identified by Gas Chromatography - Mass Spectrometer (GC-MS) and FT-IR. The SO42--ZnO catalyst, prepared by coprecipitation, showed better performance for the transesterification process than the SO42-/ZnO catalyst, prepared by wet impregnation. The trend of performance of both catalyst was due to effect of sulfonate incorporation into the zinc oxide structure to form active acid sites. From the catalyst testing on transesterification of soybean oil with methanol at mild conditions (temperature of 65 oC, methanol to oil mole ratio of 6, and 4 wt% of catalyst loading), the SO42--ZnO catalyst exhibited promising FAME yield of 80.19% at 4 h reaction time. Therefore, the SO42--ZnO catalyst showed potential as a catalyst for transesterification of soybean oil to produce biodiesel.

Key words: sulphated zinc oxide; SO42--ZnO; soybean oil; transesterification; sulfonation

Peer-review under responsibility of scientific committee of the ICTCRED 2014

1. Introduction

Current worldwide energy policies have been focused on development of sustainable renewable energy in order to reduce greenhouse gases due to fossil fuels usage. Related to this trend, biodiesel is an alternative fuel consisting of fatty acid methyl esters (FAME) produced by transesterification of triglycerides with methanol. The

Peer-review under responsibility of scientific committee of the ICTCRED 2014 doi:10.1016/j.proenv.2015.01.055

biodiesel is environmentally benign fuel due to biodegradable, nontoxic, low hazardeous emission, and renewable. The transesterification reaction could be catalyzed by homogeneous catalysts (alkalies and acids) or heterogeneous catalysts (solid acid or base). Until now, homogeneous base catalysts (KOH or NaOH) were well known used in industry, since this process has fast reaction rate at mild reaction conditions. Unfortunately, their utilization in vegetable oils transesterification has big problems due to forming soaps as undesirable byproducts, in which large amounts of wastewater was resulted, corrosiveness, and needs longer time for separation process.

Therefore, the use of heterogeneous catalysts allows a more environmentally benign process to be used for biodiesel production. The use of heterogeneous catalysts could enhance efficiency of reactor design, enable continuous process, and improve the economics of biodiesel production. The use of heterogeneous catalysts do not produce soaps through free fatty acid neutralization, which simplifies the separation and purification processes. Due to the potential interest of heterogeneous catalyst usage, researches on the transesterification reaction using the solid catalysts for biodiesel production has increased over the past decade. Heterogeneous catalysts could overcome the mentioned problems of homogeneous catalysts usage.

Previous researchers, including base and acid solid, have investigated a variety of solid catalysts for biodiesel production. The solid base catalyst includes CaO-based oxides [1-4], zinc oxide-based catalysts [5-10], and zeolites and modified zeolites [11-17]. Heterogeneous solid acid catalysts have been used as an alternative to basic and enzyme catalysts, since they can be used in both transesterification and esterification reactions [18]. However, acid catalysts required high alcohol-to-oil molar ratios and catalyst concentrations to achieve satisfactory transesterification conversions. The solid acid catalysts included sulphated zirconia [18-25].

Among the heterogeneous base catalysts, CaO was the most studied and the great potential for biodiesel production due to its low price and desired activity. The physical-chemical characteristics of CaO-based oxides showed improvement of surface area and thermal stability. This is probably due to the great enhancement of oil diffusion into catalyst surface which has been changed to be hydrophobic. In terms of solid acid catalysts, sulphated zirconia showed good catalyst for transesterification of vegetable oils [18, 21-23, 25] due to its strong acid properties and high activity and selectivity for the esterification of fatty acids with a variety of alcohols. The calcium oxide- and sulphated zirconia - based catalysts showed the potential of being used as a recyclable catalyst after simple regeneration without any significant loss in activity. In another researches, the transition metal oxide of zinc oxide was reported as a cheap, stable, reusable, commercially available, environmentally benign catalyst, and used in many catalytic reactions [2, 5]. It was also widely used as catalyst support. The ZnO-based metal oxides also showed good basic solid catalyst when modified with alkaline or alkaline earth metals for the transesterification of vegetable oils [26-28]. The modified ZnO was reported as a good catalyst for the transesterification of rapeseed oil owing to its high activity and minimum weight loss in supercritical methanol [28]. However, only a few previous researchers focused on development of potential sulphated-ZnO as a solid acid catalyst. Therefore, sulphate modified-ZnO, either prepared by coprecipitation or impregnation method, needs to be more studied for optimizing biodiesel production by transesterification process of vegetable oil. The incorporation of sulphate ions are expected showing strong influence on the phase modification of zinc oxide from thermodynamically more stable monoclinic to the metastable tetragonal or hexagonal phase. The sulfation is also expected leading to acidity increasing which may be due to presence of SO42- ions on the surface of ZnO and also due to the formation of tetragonal or hexagonal phase as well as the increase in Lewis acidity of zinc oxide due to calcinations.

The aim of this paper is to obtain active catalysts based on sulphated zinc oxide which is prepared by coprecipitation and/or impregnation for the transesterification of soybean oil with methanol at mild reaction conditions. For this purpose, a series of catalysts were prepared, characterized, and compared, i.e. SO42--ZnO, SO42-/ZnO, commercial ZnO, and synthesized ZnO. These catalysts were characterized by XRD and FT-IR, while the biodiesel product was identified by GC-MS and FT-IR.

2. Materials and Methods 2.1. Materials

Soybean oil was purchased from the local market (Happy-Indofood) to be used as raw material of triglyceride, while methanol (Merck, 99.9%) were used as reactant of transesterification. In terms of catalyst preparation, Zn(NO3)2.4H2O (Merck, 98%) and ZnSO4.7H2O (Merck, 99.5%) were used as precursor of zinc oxide, while H2SO4 (Merck, 95-97%) and NH4OH (Merck, 25%) were used as catalyst pretreatment or sulphation.

2.2. Catalysts Preparation

For synthesis of SO42-/ZnO, ZnO was prepared by calcining ZnSO4.7H2O at 727 oC for 5 h in a muffle furnace (Ney Vulcan 3-550). Thus, the resulted ZnO powder was immersed while stirred in 2 M solution of H2SO4 for 6 h at room temperature (28 oC). The mixed wet solid was dried in an oven (Memmert) at 110 oC for overnight. The resulted solid was calcined at 300 oC for 3 h in the muffle furnace.

For synthesis of SO42--ZnO, 2 M solution of Zn(NO3)2 was prepared by dissolving Zn(NO3)2.6H2O in distilled water. After preparation of zinc nitrate solution, ammonium hydroxide was drop wise added to it from burette. The reaction mixture was continuously stirred in a magnetic stirrer at room temperature for 30 minute till complete precipitation (pH 10-12). The precipitate was washed with distilled water and then dried in the oven at 100 oC for overnight. The dried precipitate was crushed, milled, and then sieved to get uniform sized particles. After sieving, the dried precipitate (Zn(OH)2) was immersed while stirred in 1 N H2SO4 solution for 2 h. The mixed solid was then dried in the oven at 100 oC for overnight. The resulted solid was calcined in the box furnace at 500 oC for 3 h. The calcined catalyst was crushed, milled, and then sieved to get uniform sized particles. Meanwhile, commercial ZnO (MERCK, 99%) and the synthesized ZnO from ZnSO4.7H2O (synthesized ZnO) was used for comparison.

2.3. Catalysts Characterization

The X-ray diffraction (XRD) patterns of the catalysts were recorded on an X-ray diffractometer (PANalytical) operating under 40 KV and 40 mA with the scan rate of 2°/min. The diffractometer employing Cu-Ka radiation to generate diffraction patterns from powder crystalline samples at ambient temperature. The Cu-Ka radiation was generated at scanning angle (29) from 5o to 90o. The Fourier Transform Infra Red (FTIR) spectrum of the catalysts were recorded on a SHIMADZU instrument using the KBr pellet technique and Attenuated Total Reflectance (ATR) method. The infra red scanning was done at 298 K and wave range of 4000-400 cm-1 with 2 cm-1 resolution.

2.4. Catalysts Testing for Transesterification

The sulphated zinc oxide (SO42--ZnO and SO42-/ZnO) were tested for transesterification of vegetable oil to biodiesel using methanol. The sulphated ZnO catalysts were also compared to the commercial ZnO as well as ZnO synthesized from zinc sulphate. The transesterification reaction was carried out as follows. A 250 ml mixed solution of methanol and soybean oil with methanol to soybean oil molar ratio of 6:1, and 4 wt% solid acid catalyst were fed into a 500 ml flask as a batch reactor equipped with thermocouple and magnetic stirrer. The reactants were stirred at 300 rpm and the heating was started and kept at temperature of 65 oC. The good stirring was purposed to maintain uniform temperature and suspension. After running the transesterification reaction for a desired duration, the heater was switched off and the reactor was allowed to cool down naturally. After cooling, the product was discharged out and filtered using filter paper to separate the solid catalyst from the mixture of biodiesel-glycerol. The resulted FAME biodiesel was characterized and identified using GC-MS (SHIMADZU) equipped with Agilent DB-1 column and helium as a gas carrier. Additionally, in order to confirm the identified fatty acid methyl ester by GC-MS, the biodiesel product was also characterized by FT-IR spectroscopy.

3. Results and Discussion

3.1. Catalyst Characterization

Following reaction is supposed to take place in the preparation of main ZnO catalyst:

In step 1 (Equation 1), zinc nitrate gave precipitated zinc hydroxide and ammonium nitrate. After washing, drying, and calcination (step 2) of dried precipitate of zinc hydroxide, a solid zinc oxide was obtained. Therefore, the zinc oxide was wet-impregnated with aqueous sulfuric acid solution which resulted in sulphated zinc oxide (SO42--ZnO). In this research, four catalysts were compared, i.e. SO42-/ZnO, SO42--ZnO, synthesized ZnO, and commercial ZnO.

Zn(NO3)2 (aq) + 2NH4OH (aq) ^ Zn(OHh w + 2NH4NO3 (a<D Zn(OH)2 (s) ^ ZnO (s) + H2O (g)

(1) (2)

Structure analysis of compounds in the studied catalysts was investigated by FT-IR spectroscopy as shown in Figure 1. A broad absorption peak in the range of 3500-3700 cm-1 is present in all figures which can be attributed to the characteristic absorption of hydroxyl groups (O-H) by OH stretching vibration [6, 27]. This broad peak of OH represents a large number of water was adsorbed in the catalyst surface. The commercial and the synthesized ZnO showed similar FT-IR peaks trend. This fact suggests that the preparation of synthesized zinc oxide from zinc nitrate was well prepared. Peaks at 1630 cm-1 is due to bending of water molecules [27]. The peaks in the range of 4501000 cm-1 is due to the presence of Zn-O bond [29, 30] for four catalysts. Sharper peaks evidence in the commercial ZnO than the synthesized ZnO indicates that the commercial ZnO is better than by coprecipitation of zinc sulphate. Metal-oxygen bonding in metal oxide catalyst is important as active component of the catalyst. Modification of the catalyst by SO42- promoter was expected to change the catalyst structure. The modification was able to generate acid sites in the catalyst (Figure 1). From Figures 1(iii) and 1(iv), peaks at 1230-1370 cm-1 was assigned to S=O bonding in SO42- [10, 22], while two bands at 1041 and 1182 cm-1 in the solid acid was assigned to the SO2 asymmetric and symmetric stretching modes, respectively [22, 31, 32]. It indicates that the covalently linked sulfonic acid group was formed on the catalyst surface. However, the evidence of N-H peak at coprecipitated ZnO (Figure 1(i)) shows that the catalyst after coprecipitation is not completely washed by water to remove unreacted ammonia. Regarding the role of acid sites of the catalyst, the strong Bronsted acid sites is evidence at 3650 cm-1 due to O-H stretching on the sites. This fact is associated to the simultaneous absence of disulfate species and evidence of molecular water [22]. The band at 1620 cm-1 is attributed to the C=O stretching mode of the -COOH groups [31]. Therefore, the following functional groups, -COOH, -SO3H and -OH, were found in the solid acid catalyst.

The X-ray Diffraction pattern of SO42-/ZnO and SO42--ZnO catalysts are shown in Figures 2(a) and 2(b) respectively. The catalysts structure were also compared to the synthesized ZnO (prepared from zinc sulphate precursor) and the commercial ZnO as shown in Figures 2(c) and 2(d), respectively. With respect to the SO42--ZnO catalyst, Figure 2(b) shows existence of two phases of monoclinic Zn3O(SO4)2 (ICSD 01-071-2475) and hexagonal ZnO (ICSD 01-076-0704), in which the main phases are shown by Zn3O(sO4)2 for 72 wt% followed by ZnO for 28 wt%. Meanwhile, 95 wt% monoclinic phase of gunningite (Zn(SO4).H2O (ICSD 01-081-0022) and 5 wt% hexagonal ZnO (ICSD 01-079-0208) was detected in SO42-/ZnO catalyst as shown in Figure 2(a). It is suggested that the SO42--ZnO catalyst has better structure and phase than the SO42-/ZnO catalyst. Meanwhile, crystallite size, crystal structure, and crystal cell volume of four catalysts is listed in Table 1.

Fig. 1. FT-IR spectra for the catalysts: synthesized ZnO (i), commercial ZnO (ii), SO42-/ZnO (iii), and SO42--ZnO

Position (26)

Fig. 2. The X-ray Diffraction pattern of the catalysts of SO42-/ZnO (a), SO42--ZnO (b), synthesized ZnO (c), and commercial ZnO (d)

Table 1. Crystallite size of the catalysts using Scherrer equation

Catalysts Crystal Structure Crystallite size (nm) Crystal cell volume (106 pm3)

SO42- - ZnO monoclinic/hexagonal 15.908 344.01/47.77

SO42-/ ZnO monoclinic 8.375 353.75

Commercial ZnO hexagonal 14.361 47.61

Synthesized ZnO monoclinic/hexagonal 8.368 709.2/48.18

According to the FT-IR and XRD analysis, it is observed that ZnO modified by sulphation was found to be an active catalyst for the transesterification of soybean oil with methanol which may be due to the creation of Bronsted acid sites as predicted in Figure 3 [5]. It was also reported that higher sulfur content corresponds to higher acidity of the catalyst and consequently higher catalytic activity [33].

Fig. 3. Predicted structure of the Bronsted and Lewis acid sites on the sulphated zinc oxide

3.2. Catalytic Activity for Transesterification of Soybean Oil to Biodiesel

The catalyst (SO/'-ZnO) was tested for transesterification of soybean oil to produce fatty acid methyl ester at mild reaction conditions (65 oC and reaction time of 4 h). In this reaction testing, methanol to soybean oil molar ratio is 6:1, while weight ratio of catalyst to oil is 4%. Yield of biodiesel (FAME) from this reaction is 80.19 % which implies the potential of the SO42--ZnO acid catalyst for the transesterification process. Meanwhile, biodiesel yield of 75.5 % was resulted when using the SO42-/ZnO catalyst at similar reaction conditions. Therefore, the SO42--ZnO catalyst showed higher biodiesel yield than the SO42-/ZnO catalyst. That's mean, incorporation of sulphate group or sulfonation to generate Bronsted acid sites in the coprecipitated SO42--ZnO is better than by impregnation. The incorporation of sulphate group in the form of monoclinic Zn3O(SO4)2 as detected in XRD results (Figure 2 and Table 1) is able to generate Bronsted acid sites and Lewis acid sites as predicted in Figure 3. Meanwhile, impregnation of sulphate on the ZnO (become SO42-/ZnO) showed weak acid sites as the FAME yield is lower than that by coprecipitation. From the catalyst characterization, the presence of -COOH, -SO3H and -OH functional groups leads to the formation of Bronsted and Lewis acid sites as predicted in Figure 3. The higher sulfur content corresponds to higher acidity of the catalyst and consequently higher catalytic activity [5].

The catalyst surface should posses some hydrophobic character for promoting the preferential adsorption of oily hydrophobic species on the catalyst surface and also to avoid deactivation of catalyst sites by strong adsorption of polar by products like glycerol or water. A strategy for controlling the hydrophobicity of metal oxide is by sulfonation as studied in this research. The enhanced catalytic performance in the transesterification reaction was explained by improved accessibility of the active sites and the hydrophobic organic groups into the zinc oxide. The Lewis acid site favor the formation of electrophilic species, which determine the rate of desorption followed by the rate of the transesterification reaction [34]. However, stronger Lewis acid did not favor product desorption easily which in turn makes the transesterification reaction slow [5, 35].

3.3. Characterization of Biodiesel Product using FT-IR and GC-MS

Characteristics of the soybean oil as raw material and the biodiesel product are specified in Table 2. It is shown that the biodiesel product is fulfil the requirement of SNI (Standard Nasional Indonesia) standard. Viscosity of the biodiesel product is within the range of standard, including density and acid value. Resulted biodiesel, mainly composed of fatty acid methyl esters, was analyzed using GC-MS to identify compound contents of the resulted biodiesel. To confirm the structural FAME content of the resulted biodiesel, a FT-IR spectroscopy was used to identify functional groups. The fatty acid methyl esters analysis identified by GC-MS is presented in Figure 4 and listed in Table 3. Main component of FAME product are 9,12-octadecadienoic acid methyl ester (C19H34O2), 9-octadecenoic acid methyl ester (C19H38O2), and eicosanoic acid methyl ester (C21H42O2). The IR spectra of biodiesel product are also depicted in Figure 5 to confirm the GC-MS identification. From Figure 5, detected bands at 1099, 1121, 1163, and 1238 cm-1 were assigned to stretching mode of C-O, while appeared bands at 1400, 1377, 1418, and 1462 cm-1 were respected to bending mode of C-H. Stretching mode of C-H was also detected at bands of 2853 and 2924 cm-1. With respect to C=O bonding in ester, the band at 1744 cm-1 was assigned to stretching mode of C=O. From the mentioned IR spectra, it is inferred that various groups composing fatty acid methyl esters are detected, i.e. alkana (-CH3), ester (-COO-), and alkana (C-H).

Table 2. Characteristic of soybean oil raw and biodiesel product and the comparison to standard

Parameters Soybean Oil Biodiesel Product SNI Standard

Density (gram/ml) 0.92 0.888 0.85-0.89

Kinematic viscosity (cSt) 15 5.73 2.3 - 6.0

Acid Number (%) 0.18 0.17 Max 0.8

Calorifc Value (cal/gram) 9320 9350 -

Table 3. GC-MS identification of biodiesel product from soybean oil

Peak Number Compound Formula Identified Chemical Compound Composition (%)

1 C10H16O 2,4 Decadienal 11.45

2 CioHi60 2,4 Decadienal 22.18

3 C14H3402SI2 Trimethylsilyl Ester 2.09

4 C21H4202 Hexadecanoic Acid Methyl Ester 4.28

5 C21H4202 Eicosanoic Acid Methyl Ester 9.73

6 C19H3402 9,12- Octadecadienoic Acid Methyl Ester 28.72

7 C19H3802 9- Octadecenoic Acid Methyl Ester 19.37

8 C19H3802 Octadecanoic Acid Methyl Ester 2.18

Retention Time (minute) Fig. 4. GC-MS chromatogram of biodiesel product

3900 3400 2900 2400 1900 140 0 900 4 00 Wavenumber (cm-1) Fig. 5. FTIR characterization of biodiesel product

4. Conclusions

In this study, SO42--ZnO showed potential performance as catalyst for transesterification of soybean oil to fatty acid methyl ester. The SO42--ZnO exhibited an active solid acid catalyst for the transesterification reaction which could be attributed to the presence of Bronsted/Lewis acid sites on the catalyst surface due to incorporation of sulfonate groups into the catalyst surface. From the catalyst testing for transesterification of soybean oil at mild conditions (temperature of 65 oC, methanol to oil molar ratio of 6, and 4 wt% of catalyst loading), the SO42--ZnO catalyst exhibited promising yield of fatty acid methyl esters of 80.19% at 4 h reaction time, better than that of SO42-/ZnO catalyst which only showed 75.5% yield. However, next studies should be focused on proofing presence and strength of Bronsted/Lewis acid sites as well as stability usage of the catalyst for biodiesel production.

Acknowledgement

The authors would like to express their sincere gratitudes to the Directorate General of Higher Education (DIKTI) for the financial support received under the project of Penelitian Hibah Kompetensi Year 2013 by Contract No.: 314b-5/UN7.5/PG/2013.

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Active Acid Catalyst of Sulphated Zinc Oxide for Transesterification of Soybean Oil with Methanol to Biodiesel Academic research paper on "Chemical sciences"

Abstract of research paper on Chemical sciences, author of scientific article — I. Istadi, Didi D. Anggoro, Luqman Buchori, Dyah A. Rahmawati, Dinnia Intaningrum

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