As part of ongoing efforts to investigate the influence of acidity of solid catalyst for biodiesel synthesis, a fundamental conversion of triglyceride and characterization of the catalyst in term of acidity was carried out for the transesterification of triacetin with methanol on a number of solid acid catalysts. Liquid-phase reaction was performed at 60oC for 8 hours using a stirred batch reactor. To compare the catalytic activity of the same catalyst with different acidity, the sulfated zirconia, were further modified with different molarities of acid solution. X-Ray diffraction and IR-pyridine were used to characterize the structure, acid sites concentration and strength of the solid catalyst. The conversion result using these solid catalysts was compared with their acidity in an effort to find influence of solid acid catalyst for biodiesel production. The conversion of triacetin was improved as the amount of acid site increased. The amount of acid sites of sulfated zirconia affected significantly the catalytic activity in the transesterification of triacetin.
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Keywords: Transesterification; Biodiesel; Triacetin; Acidity
Biodiesel is an alternative fuel for diesel engines that is gaining attention and would continue to grow at a fast pace after the price of crude oil increases due to the predicted shortfall in the world crude oil production and with increasing concern over global warming [1,2]. Its primary advantages are that it is one of the most renewable fuels currently available and it is biodegradable and non-toxic. It can also be used directly in most diesel engines without requiring extensive engine modifications.
Transesterification of triglycerides (TGs) with methanol is the central reaction in the synthesis of biodiesel from oils and fats produce chemical compounds known as fatty acid methyl ester (biodiesel). Current reaction production commercially, carried out using homogeneous base catalysts, which are corrosive and non-reusable but also produce neutralization waste, leading to increased costs and environmental concerns. Although homogeneous alkali catalyst (e.g., NaOH) are commonly used to produce biodiesel by transesterification of triglyceride and methanol, solid acid catalyst are attractive because they are easy to separate and recover from the product mixture, and also show significant activity in the presence of fatty acid impurities, which are common in low-cost feedstock .
The ability of solid acid to act as a catalyst is consequences of the presence of different types of acid sites which are provide a surface for the chemical reaction to take place on. In general, solid acid may exhibit Bronsted as well as Lewis acid sites. In order for the reaction to occur one or more of the reactants must diffuse to the catalyst surface and adsorb into it. After reaction, the products must desorb from the surface and diffuse away from the solid surface. A good catalyst need to adsorb the reactant molecules strongly enough for them to react, but not so strongly that the product molecules stick more or less permanently to the surface. Solid acid have found widespread application in the catalysis field and belong to the group of new catalytic material which are greatly develop in recent years [2,3,4,5,12]. However research dealing with the use of solid acid catalysts for biodiesel synthesis has been limited due to low expectations about reaction rates and probable undesired side reactions. On the other hand, the acidity strength of the solid acid is not uniform and strong acidic sites are not durable makes some difficulties in practical application of the solid acid. Solid acid catalysts, such as SO42-/SnO2  and zeolites  used for triglyceride transesterification have shown high catalytic activities. The esterification of free fatty acids (FFA) by solid acid catalysis also has been explored . Sulfated zirconia (SO42-/ZrO2), sulphated tin oxide (SO42-/SnO2) and sulfated titanium oxide (SO42- /TiO2) were the representative catalysts that showed good catalytic activities . There are too many applications of such catalysts in the reactions such as alkylation, isomerization and esterification and so on, but there is no specific study on the effect of amount of acidity on the biodiesel production. Based on these points, it is necessary to do some study focused on the influence of acidity of solid acid in the transesterification of triacetin with methanol.
In this work, triacetin was used as a model compound for TGs to provide fundamental insight into TG transesterification catalyzed by solid acid catalysts. Triacetin, having the same chemical functionality of any triglyceride molecule, shares the same reactivity principles of triglycerides. But triacetin differs from common TGs in terms of size. Transesterification of triacetin with methanol proceed via three consecutive and reversible reactions where the triacetin ligands combine with the methanol to produce an acetic acid methyl ester and a glycerol as its by-product. The stoichiometric reaction requires 1 mole of triacetin and 3 moles of methanol to produce 3 moles of fatty acid methyl ester and 1 mole glycerol. Methanol and to lesser extent alcohol are the most frequently used alcohol in biodiesel synthesis because of their availability and low cost . Therefore, this study should be considered an initial exploration of TG transesterification by solid acid catalyst. The catalytic activities of solid acid catalysts having different acidities were investigated in the transesterification of triacetin and the influences of acidic properties of the solid acid were discussed in the reaction relating to the conversion of triacetin.
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2.1 Catalysts preparation
The solid acid catalysts studies were zirconia, sulfated zirconia, sulfated zirconia-alumina, tungsten zirconia, tungsten zirconia-alumina, sulfated tin oxide and zeolite ZSM-5. All these catalysts have high concentration of acid sites and showed great promise for biodiesel forming reaction. Sulfated zirconia (SO42-/ZrO2) was prepared by kneading a mixture of hydrated zirconia (amorphous), hydrated alumina, and ammonium sulfated together with deionized water. By calcining in air at 675 OC for 1.5 h, sulfated zirconia-alumina (SZA) was obtained . Tungstated zirconia (WO3/ZrO2) was prepared following the literature . A mixture of hydrated zirconia powder, of hydrated alumina, and aqueous ammonium metatungstate solution (50 wt% WO3), and deionized water were put into a kneader with stirring vanes. The mixture was mixed for 25 min in the kneader. By drying and calcined at 800 OC for 1 h, tungstated zirconia-alumina (WZA) was obtained. Sulfated tin oxide (SO42-/SnO2) was prepared by immersed tin oxide powder in a glass flask with 3 mol-1 ? H2SO4, stirred for 1 hour, filtered by suction, dried at 100 OC for 2 hours, and finally calcined in air at 500 OC for 3 hours to obtain SO42-/SnO2 (STO).
The best catalyst performance sulfated zirconia (SZ), was selected for the further study in modification using different molarities of H2SO4. Zirconium hydroxide Zr(OH)4 was immersed in a 0.5 M, 1.0M, 1.5M, 2.0M, 2.5M, 3.0M, 3.5M, 4.0M and 5.0M H2SO4 solution for 30 min, then filtered and dried at 110 OC for 24 h yielding the sulfated salt (SO42-/ZrO2). Finally, it was calcined at 600 OC for 2 h. As a reference, plain Zr(OH)4 was calcined at 600 OC for 2 hours. Zirconium hyroxide (99%), tin oxide (99.9 %) metal basis, aluminium hydroxide and triacetin (99%) were purchased from Aldrich. Ammonium metatungstate hydrate was purchased from Fluka and zeolite ZSM-5 (30) was purchased from Zeolyst.
2.2 Catalyst characterization
The structures of sulfated zirconia were determined by x-Ray Diffraction techniques. This method was based on the fact that every crystalline material has its own characteristic diffractogram. Approximately 1 g of sample was dried overnight in air at 110°C. XRD patterns were acquired on a Siemens D5000 goniometer using CuKα radiation in the range of 2θ from 2° to 70° at a scanning speed of 3° per minute. The XRD patterns were taken within this range since most of the intense peaks were observed.
IR-pyridine was determined by Shimadzu 3000 FTIR spectrometer. The self-supporting wafer of 13 mm diameter was made by pressing about 10 mg of fine sample powder under five tons pressure for 15 seconds. The thin wafer was placed in a ring type sample holder and transferred into the IR cell. The IR cell containing the sample wafer was attached to the vacuum line and dehydrated at 400 OC under vacuum at 10-8 mbar. The IR spectra of these samples were recorded in the range wave number 1700-1400 cm-1.
2.3 Transesterification of triacetin with methanol
In all cases, the reaction mixture media contained anhydrous methanol and triacetin in a 6:1 (methanol:triacetin) molar ratio (3x stoichiometry for complete conversion), which in the proportion commonly used with homogeneous catalysts. Prior to a reaction, a sample of reactant mixture was analyzed in order to obtain initial concentrations. Catalysts loading was 2 wt. % (wt. cat/wt. reaction mixture) for all the studied catalysts . Reaction were carried out in a 500 ml three-neck round bottom flask on a Heildoph (model MR 3003) hot plate with constant temperature bath, Haake (model F6/B6) at 60 OC and stirred at 500 rpm for 8 h.
2.4 Method of analysis
High Performance Liquid Chromatography (HPLC) was used to evaluate the conversion of triglyceride into biodiesel. Analyses were performed using Hewlett packard series 1100 with a refractive index detector. The size exclusion column was a Princeton SHERE CYANO 60A column from Princeton Chromatography Inc., with a length of 250 mm and ID of 4.6 mm. The mobile phase used was THF (HPLC grade). The column temperature was 25°C and the mobile phase flow rate was held at 0.35 mL/min. A recorder model Water 740 registered the detector signal and performed the integration of the peaks. The run time was 20 min. approximately, 40 µL samples was injected manually in an injector loop of 20 µL.
3.0 Result and discussion
3.1 Catalyst characterization
3.1.1 Acidity measurement
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The type of acid that were present in the catalyst were further characterized using pyridine adsorption method. The reaction on the Bronsted acid site result in the formation of a pyridinium ion while the pyridine molecule is coordinative bound to Lewis acid sites. Both the pyrinidium ion and coordinatavely bound pyridine show characteristic IR absorption bands. In order to elucidate the role of the surface acidity in solid acid catalyst, Py-FTIR was used to characterize the surface acidity of catalyst. For comparison, the relative amount of Bronsted and Lewis acid sites estimated from the integrated area of the band at 1540 cm-1 and 1450 cm-1 at 200 oC is given in Table 1.
Table 1. Concentration of Bronsted and Lewis acidity of the sample
Solid acid catalyst
(at 1545 cm-1)
(at 1450 cm-1)
The amount of acidity is calculated with this equation :
Amount of Bronsted acid site =
(Integrated of Bronsted Band Area * 0.7857) Equation 1
(0.03 * 5.92)
Amount of Lewis acid site =
(Integrated of Lewis Band Area * 0.7857) Equation 2
(3.8 * 5.92)
Based on this result, the order of acidity was: ZSM-5› SZA › WZA › WZ › SZ.
Figure 1 shows IR-pyridine peak area of Sulfated Zirconia with different molar of H2SO4 during catalyst preparation. From this result, it can be suggested that acidity of sulfated zirconia was increased when the concentration of sulfuric acid added until 2.0M. Addition of more concentration of sulfuric acid cause decreases of catalyst acidity. According to Triwahyono et al. (2002), number and strength of acid sites play important role in the chemical reaction and showed that acidity properties of catalyst greatly affected by the amount of sulfated ion loading . For a limited range of sulfated loading investigated in their study of sulfated zirconia, it appears that that conversion per specific surface area of catalysts increase with the sulfated ion loaded. This means that if the acidity of catalyst is low, the conversion rates of reaction will be low due to the low concentration of acid sites, and when the acidity is too high, deactivation by coking maybe occur or undesirable by-product might be formed requiring additional and expensive separation procedures.
Figure 1. IR-pyridine peak area of Sulfated Zirconia with different molar of H2SO4 during catalyst preparation
It has been identified that sulfation of zirconia causes the change in the surface area and the crystalline structure of zirconia and the additional sulfate ion promotes the acidity and activity . Furthermore, it is clear that the sulfate content of the catalyst depends on the quantity of sulfuric acid used for impregnation. The increase in the amount of acid concentration decreases the intensity of acid sites.
3.1.2 Physical structure
After calcinations of pure zirconia at 600oC and above, zirconia transforms into a monoclinic phase (2θ = 28.3o and 31.4o) from a tetragonal phase (2θ = 30.2o) of zirconia . However the addition of sulfate ion stabilizes the tetragonal phase of zirconia and suppresses the formation of the monoclinic phase of zirconia. Figure 2 shows the XRD patterns of the prepared samples with different concentration of sulfuric acid. The result show that the tetragonal phase is a dominant structure of zirconia and the addition of sulfate ion up to 2.0M does not considerably change the crystallinity of the catalyst. However, further additions of the sulfate ion will collapse the structure of zirconia. In the addition of 3.5M and above, a new peak associate with the monoclinic phase of zirconia was observed at 31.4 o.
Figure 2. XRD pattern of SO42- /ZrO2 immersed with 0.5M to 5.0M of H2SO4 at 2θ scale.
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3.2 Reaction chemistry
In the present work, the performance of several solid acid catalysts for the transesterification of triacetin has been carried out to study the effect of acidity on the conversion. The catalysts were characterized for their structure and acidity. The conversion of triacetin in presence of solid acid catalyst using packed bed reactor has been investigated. It was found that sulfated zirconia (SZ) gave highest conversion (61.56%) among other solid catalysts studied.
The triacetin conversion after transesterification varied significantly among the catalysts tested. Figure 3 shows the conversion of triacetin after the screening process in transesterification reaction at 60OC. Among acid catalysts the order of reactivity was: SZ ›ZSM-5 › SZA ›WZA ›STO › ZrO2 ›WZ
Figure 3. Conversion of triacetin in transesterifiacation process at 60 OC
From the result, we found that an acidity property of the catalyst is not influence the reaction conversion in the transesterification process. It might be another factor such as an interconnected system of large pores, a moderate to high concentration of strong acid sites and a hydrophobic surface .
3.3 Influence of conversion of triacetin and acidity of sulfated zirconia
In order to compare the catalytic activities on the same catalyst with various acidities, the sulfated zirconia with different acidity were employed in the reaction. Acidity of the sulfated zircornia was further modified with different molar of H2SO4 and its performance for transesterification was tested. Figure 4 represents the catalytic activities in the transesterificaction of triacetin. The conversion of triacetin increases linearly with the increase of acid amount. The result showed that sulfated zirconia impregnated with 2.0 M of H2SO4 gave highest conversion (68.5%) due to its highest acidity concentration and no reaction was observed for 4.0 M and 5.0 M of H2SO4. These results reveal that the acidity of catalysts play a role in the activity of catalysts towards the transesterification of triacetin directly. This result similarly compared to Chung et al.  where it disclose that the conversion of oleic acid was influenced mainly on acid amount of the zeolite catalysts. The excessive amount of sulfate ion generates large number of strong acid sites, which is favourable for transesterification . Zalewski et al. reported that the most active catalyst system for chemical reaction was obtained when sulfated loading approached monolayer coverage . This corresponds to a surface coverage of a one sulphur atom for every two zirconium atom. The Bronsted acidity also maximizes at this level. The concentration of Bronsted acid site increases as the sulfate concentration approaches monolayer coverage.
Figure 4. Relationship between conversion of triacetin and acid amount of sulfated zirconia to the various concentration of sulfuric acid used in the preparation of sulfated zirconia. (kenapa data acid amount tak kena exactly pada bar chart)
Sulfated zirconia gives highest conversion of triacetin to methyl ester compared to other catalyst. Results suggest that the properties of sulfated zirconia catalysts could be affected by the concentration amount of H2SO4. The properties particularly the number and the strength of acid sites play an important role in the transesterification. For a limited range of H2SO4 concentration investigated in this study of sulfated zirconia, it appears that the conversion of triacetin increases with the acidity concentration.