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Replacement of liquid homogeneous catalysts with solid heterogeneous catalysts would greatly solve problems associated with expensive separation protocols involved with the former, yielding a cleaner product and greatly decreasing the cost of synthesis. As part of ongoing efforts to investigate heterogeneous 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 (a synthetic triglyceride in the liquid phase) with methanol on a number of solid acid catalysts. Liquid-phase reaction was performed at 60oC for 8 hours using a stirred batch reactor. The best catalyst performance (sulfated zirconia, SZ) was choosing for further study in different molarities of acid solution in catalyst preparation. Analysis of the samples was done using HP-LC for quantitify conversion of triacetin to methyl ester. IR-pyridine was used to characterize the 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 the suitable characteristic solid acid catalyst for biodiesel production.
Keywords: Transesterification; Biodiesel; Triacetin; Acidity
During this time, industry grew to meet the needs of an ever-growing population; the most efficient means of transporting industrial goods and services is through the use of fuel transportation whether by truck, rail, or ship. However, these vehicles are still using fuel derived from hydrocarbon such as petrol and diesel. The consumption of transportation fuel is increasing continuously, while the reserves of crude oil are declining. The extra supplement of fuel is needed due to the predicted shortfall in the world crude oil production . The exhausted gases from these vehicles emit the green house gases such as carbon monoxide (CO), sulphur (S) and nitrogen oxide (NOx). These pollutant gases can cause the hazard to human health. One way to avoid these problems is to have alternative fuels that renewable and produce less or no pollutant. Transesterification (Figure 1) 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.
Triacetin Methanol Glycerol Acetic acid methyl ester
Figure 1. Transesterification reactions of triacetin with methanol
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 . 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. Figure 2 shows the advantages and disadvantages of heterogeneous acid catalyst;
• Reusable and do not form soaps
• More tolerant to water and free fatty acids
in the feedstock
• Improve product yield and purity
• Simpler purification process for glycerol
• Ease of separating biodiesel product
• Require elevated temperatures and
pressures to work well.
• Those on solid support tend to show less
activity than the active species in solution
• There is the possibility of leaching which
might contaminate the biodiesel.
Figure 2. The advantages and disadvantages of heterogeneous acid catalyst
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 (prone to donate a proton) as well as Lewis (prone to accept an electron pair) 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.
According to Sugeng et al, 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.
More research, especially of a fundamental nature using model compound, is required to in order to better delineate esterification and transesterification reactions on solid acid catalysts . The chemistry and structural simplicity of triacetin simplifies the identification and quantification of reaction products, while keeping the same chemical functionality shared by every TG . 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 .
In this work, triacetin (the acetic acid triester of glycerol) 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. Therefore, this study should be considered an initial exploration of TG transesterification by solid acid catalyst. The conversion result using these solid catalysts was compared with their acidity in an effort to find the suitable solid acid catalyst and its role in biodiesel synthesis.
Catalyst preparation and testing
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 (SO4/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 kneaded 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 (SO4/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 SO4/SnO2 (STO).
The best catalyst performance (sulfated zirconia, SZ) was choosing 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.
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 CuK„ƒ 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.
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 (Lopez et al). 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.
Method of analysis
High Performance Liquid Chromatography (HPLC) was used to evaluate the conversion efficiencies 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.
RESULT AND DISCUSSION
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
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, among acid catalysts the order of acidity was: ZSM-5› SZA › WZA › WZ › SZ
After calcinations of pure zirconia at 600oC and above, zirconia transform 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 suppresses the formation of the monoclinic phase of zirconia. Figure 3 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 ay 31.4 o.
Figure 3. XRD pattern of SO42- /ZrO2 immersed with 0.5M to 5.0M of H2SO4
The triacetin conversion after transesterification varied significantly among the catalysts tested. Figure 4 shows the conversion of triacetin after the screening process in transesterification reaction at 60OC. Among acid catalysts the order of reactivity was: H2SO4 ›SZ ›ZSM-5 › SZA ›WZA ›STO › ZrO2 ›WZ
Figure 4. 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.
Role of acidity in biodiesel production
For study in different preparation of sulfated zirconia were using different molarities of sulfuric acid. Based on this result, Figure 5 shows the total conversion of triacetin to acetic acid methyl ester in the function of sulfate ion loading of zirconia. The conversion of acetic acid methyl ester increases linearly with the increase of sulfate ion. These results reveal that the acidity of catalysts role play the activity of catalysts towards the transesterification of triacetin directly. 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 sulfer atom for every two zirconium atom. The Bronsted acididty also maximizes at this level. The concentration of Bronsted acid site increases as the sulfate concentration approaches monolayer coverage.
Figure 5. (a)Methyl esters calibration curve, method of quantification by areas (b) conversion of acetic acid methyl ester
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 per specific surface area of catalysis increases with the acidity concentration.