Vegetable Oil Transesterification in Supercritical Conditions Using Co

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Vegetable Oil Transesterification in Supercritical Conditions Using Co-solvent Carbon Dioxide over Solid Catalysts: A Screening Study B. Saez,† A. Santana,*,† E. Ramírez,‡ J. Maçaira,§ C. Ledesma,∥ J. Llorca,∥ and M. A. Larrayoz† †

Chemical Process Department, School of Industrial Engineering of Barcelona (ETSEIB), Polytechnic University of Catalonia, 08028 Barcelona, Spain ‡ Department of Chemical Engineering, University of Barcelona, 08028 Barcelona, Spain § Laboratory of Environmental Process Engineering and Energy (LEPAE), Department of Chemical Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal ∥ Institute of Energy Technologies, School of Industrial Engineering of Barcelona (ETSEIB), Polytechnic University of Catalonia, 08028 Barcelona, Spain ABSTRACT: The transesterification reaction employing supercritical methanol and carbon dioxide used as a co-solvent in the presence of several heterogeneous solid acid catalysts was investigated. The solid acid catalysts were prepared by impregnation methods, with appropriate precursors over magnesium aluminum silicate (cordierite). The catalysts tested were CeO2, WO3, ZnO, ZrO2, ZrO2−SO42−, mixed oxides (50−50%, w/w) WO3− ZrO2, CeO2−ZrO2, ZnO−La2O3, and Al2O3. Reaction tests were conducted at 200 °C and 20 MPa under the condition of 25:1 methanol/oil ratio at a space velocity of 4 min with a fixedbed continuous flow reactor containing ca. 5 g of catalyst. The best catalytic performance was obtained over ZrO2−SO42− with a yield toward fatty acid methyl esters (FAMEs) of 98%. This value is better to that obtained over the commercial catalyst Nafion SAC-13 (94%). The direct correlation between the conversion and catalyst total acidity was non-existent, but a positive effect of strong acid sites is evidenced.

1. INTRODUCTION Biodiesel has many advantages, such as renewable fuel, nontoxicity, biodegrabability, and burns with much lower emissions than diesel fuels.1−3 Conventionally, the biodiesel production is performed by transesterification, which is the reaction of a triglyceride with an alcohol. The product of the reaction is a mixture of methyl ester (biodiesel) and glycerol. Ethanol and methanol are the most ordinarily used alcohols, notably methanol because of its chemical and physical advantages and its low cost. Figure 1 can represent the overall transesterification reaction.

acid, it will react with the alkali catalyst in biodiesel production to form soap instead of biodiesel. To overcome those drawbacks, a large number of production alternatives have been developed. One of these options is the use of heterogeneous (solid) catalysts.5 Heterogeneous catalysts are easier to separate from the product former, avoiding the water-washing and neutralization process, which makes the process more economical. Solid base catalysts provide a faster rate reaction under mild conditions when compared to solid acid catalysts, but commonly, they are suitable for deriving biodiesel only from refined oils free fatty acid (FFA) content less than 0.5%, w/w, e.g., the first commercialized solid catalyst (Al2O3/ZnAl2O4/ ZnO), developed for the transesterification reaction by the Institute Français du Petrole.6 The use of enzymes and acid catalysts has been proposed to solve this problem. Those catalysts can promote simultaneous transesterification of triglycerides and esterification of FFAs. Furuta et al.7 evaluated tungstated zirconia alumina, sulfated zirconia alumina, and sulfated tin oxide in the transesterification of soybean oil with methanol at 200−300 °C using a packed-bed reactor. Tungstated zirconia was the most active catalyst (90% of conversion) in transesterification, followed by sulfated zirconia alumina and sulfated tin oxide. TiO2−ZrO2 [11% (w/w) Ti] and Al2O3−ZrO2 catalysts also showed similar promising performances in soybean oil transesterification with methanol in a flow reactor at 200 °C.8 A good conversion yield was also

Figure 1. Transesterification reaction scheme.

Stoichiometrically, it requires 3 mol of alcohol/1 mol of triglyceride; the reaction (triglyceride and alcohol) are reversible reactions; and the excess of alcohol must be added to improve the reaction toward the desired product. The transesterification reaction can be carried out under alkaline, acid, or enzyme catalysts. The most widely used industrial process for transesterification is based on KOH and NaOH.4 However, if the raw material (fat and oil) contains free fatty © 2014 American Chemical Society

Received: March 26, 2014 Revised: August 11, 2014 Published: August 18, 2014 6006

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obtained by Lacome et al.,9 with zirconia supported on alumina at 200 °C [90% of the fatty acid methyl ester (FAME) yield at 7 h]. Jitputti et al.10 found in the transesterification of crude palm kernel oil and crude coconut oil with supercritical (SC) methanol at 200 °C the following catalyst activity for acid catalysts: sulfated zirconia (90% FAME yield at 4 h) ≥ sulfated tin oxide > ZnO > ZrO2. The higher FFA and water concentration in the coconut oil with respect to the crude palm kernel oil reduces the activity of all of the catalysts, showing the sensitivity of catalysts to impurities. Chen et al.11 discovered that, by introducing 2% (w/w) catalyst at an alcohol/oil molar ratio of 12:1 in the transesterification of cotton seed oil to methyl esters, the methyl ester yields obtained after 8 h of reaction time in the presence of TiO2− SO42− and ZrO2−SO42− were 90 and 85%, respectively. Zinc oxide was reported as a catalyst in a rapeseed oil transesterification with methanol carried out at 170−230 °C by Stern et al.12 in a batch reactor. The ZnO catalyst gave good results (FAME yield of >90% at 6 h). ZnO supported on alumina by impregnation and calcined at 500 °C showed a fairly similar performance to commercial zinc oxide. Zirconiasupported tungsten oxides [2.5−25% (w/w) WO3 loading] calcined at 500 °C were tested as solid acid catalysts for esterification of palmitic acid with methanol.13 It was found that the maximum conversion of 98% was obtained in 6 h of reaction time and the catalyst acidity decreased by increasing the amount of WO3 because of the excess coverage of WO3 species on ZrO2. The high catalyst activity was attributed to the formation of the tetragonal phase of ZrO2. Park et al.14 stated that the optimum reaction conditions for tungsten oxide zirconia transesterification with a FFA conversion of 96% were 20% (w/w) WO3−ZrO2 at 150 °C, 0.4 g/mL (oil), 9:1 (alcohol/oil molar ratio), and 2 h of reaction time. Even though aforementioned promised results were obtained by the use of heterogeneous solid catalysts in transesterification, three-phase formation along with oil and alcohol in the reactor arise from its introduction. That leads to mass-transfer limitation that results in low rates of reaction.15 Another method is the use of alcohol in the SC state in the absence of a catalyst. The pioneer work16 found that transesterification was carried out in SC methanol without any catalyst because of the high dissociation constant of SC alcohol. Oils and fats consisting of triglycerides and FFAs were converted to their methyl esters through transesterification and methyl esterification reactions. In comparison to conventional biodiesel production, it has been reported that SC transesterification is superior because it can solve several problems, such as a low rate of reaction because of mass-transfer resistance, long time treatments, and complicated separations.17−22 Under SC conditions, conversion increases 50−95% in a very short time (4−10 min).13−15 However, the main drawbacks of biodiesel synthesis using SC alcohols are the large alcohol/oil ratios (42:1) needed and the high cost of the apparatus because of the high temperature and pressure (250− 400 °C and 35−60 MPa).23−25 Hence, researchers have sought to reduce the reaction conditions. The use of a co-solvent and catalyst in combination with SC conditions has been proposed to allow for mild conditions. Co-solvents, such as hexane or propane with a small amount of catalyst (calcium oxide) added into the reaction mixture, can decrease the operating temperature, pressure, and amount of alcohol.26−33 The use of a co-solvent, such as the use

of carbon dioxide (CO2) in the SC transesterification, has been investigated and well-documented in the literature.23−25,34−36 In previous works,23−25 biodiesel was produced by the transesterification of triglycerides with compressed methanol or ethanol in the presence of Nafion SAC-13 as a heterogeneous catalyst and with SC CO2 as the co-solvent. All experiments were performed continuously in a fixed-bed reactor, with a mass flow rate of 6−24 mL/min at 150−200 °C and 20−25 MPa. The molar ratio of oil/alcohol and catalyst amount were kept constant (25 and 9 g, respectively). The obtained results showed that the observed reaction rate was 20 times faster than that of conventional biodiesel production processes. However, the amount of free glycerol in the biodiesel phase was found above the specification limits, which, in combination with the low value of the maximum operating temperature of Nafion SAC-13 (210 °C) and its high cost, makes the biodiesel production scale-up not feasible. In this work, several solid catalysts, such as ZrO2, ZrO2− SO42−, CeO2, WO3, ZnO, mixed oxides (50−50%, w/w) WO3−ZrO2, CeO2−ZrO2, ZnO−La2O3, and Al2O3, were prepared by impregnation of appropriate precursors over magnesium aluminum silicate (cordierite) and applied for continuous transesterification of triglycerides using SC fluids (methanol and carbon dioxide). The aim is to produce biodiesel using different types of cheaper heterogeneous solid catalysts, which would allow work at lower temperatures and observation of how their nature influences conversion and product distribution. Reaction tests were conducted under operating conditions employed in previous transesterification works23−25 over Nafion SAC-13 for the sake of comparison.

2. EXPERIMENTAL SECTION 2.1. Material and Catalyst Preparation. In this work, biodiesel was synthesized from vegetable-sunflower-based oil (S5007) purchased from Sigma-Aldrich. The mixture of methanol/CO2 was provided by Abello Linde S.A. The solvents, standards, and reagents used in the derivatization step required for the chromatographic analysis were supplied by Sigma-Aldrich. Aqueous solutions of H2SO4 and nitrate or chloride salts (CeNO3·6H2O, WCl6, ZrOCl2·8H2O, Zn(NO3)2·6H2O, and La(NO3)3·6H2O) used as catalysts precursors were provided by Fluka, and boehmite suspension [γ-AlO(OH)] was provided from Sasol. A series of solid catalysts CeO2, ZrO2, ZrO2−SO42−, ZnO, WO3, mixed oxides (50−50%, w/w) CeO2−ZrO2, WO3−ZrO2, CeO2− ZrO2, ZnO−La2O3 (3:1 ratio), and Al2O3 were prepared by impregnation of appropriate precursors over magnesium aluminum silicate (cordierite, Mg2Al4Si5O18). The resulting samples were dried and calcined in air for 2−5 h to obtain the corresponding active oxides. Aluminum-based catalyst was calcined at 450 °C; zirconia-, ceria- and ZnO-based catalysts were calcined at 500 °C; and WO3-related catalysts were calcined at 750 °C. This procedure was repeated several times to obtain the desired active phase weight gain [ca. 20% (w/w) with respect to the cordierite support] for the sake of comparison to Nafion SAC-13. Initially, silica (SiO2), alumina (Al2O3), and cordierite were chosen as possible catalyst supports, but after stirring the particles into the reactor, physical breakage and attrition were detected for silica and alumina. Under process conditions (200 °C and 25 MPa) in the presence of oil and alcohol, cordierite presented no appreciable reactivity (FAME content of Al2O3 > ZrO2 > WO3−ZrO2 > ZnO > CeO2−ZrO2 > ZnO−La2O3 > Nafion SAC-13 > WO3 ≫ CeO2. It should be recalled that, for all samples, ca. 5 g of catalyst was used, whereas in the case of SAC-13, the amount tested was 9 g. To obtain insight into the catalytic activity tendency, Table 2 shows the total density of acid sites and their distribution determined by NH3-TPD. Two NH3 desorption peaks were observed in each case, corresponding to weak and strong acid sites. Clearly, the acidity of Al2O3 was much higher than that of the other catalysts prepared. As expected, after the introduction of sulfate anions, new Brønsted acid sites were formed on the catalyst surface of ZrO2. The total acidity of the catalysts varied as follows: Al2O3 ≫ ZrO2−SO42− > CeO2 > ZrO2 > WO3−ZrO2 > CeO2−ZrO2 > WO3 ≫ ZnO > ZnO−La2O3. It is inferred from Tables 1 and 2 that there is no direct relationship between total acidity and catalytic performance. For instance, tungsten−zirconia mixed oxide showed a FAME yield between those of ZrO2 and WO3, in line with the total acid site distribution, but ceria−zirconia mixed oxide did not. On the other hand, ZnO−La2O3 did not exhibit higher activity than that of the ZnO catalyst, in contrast to that reported by Yan et al.40 for the transesterification of unrefined and waste oils using the same catalyst, where the strong interaction between Zn and La species was claimed to enhance the catalytic activity. Sulfated zirconia was the most active catalyst. Catalytic activity over sulfated zirconia, which is regarded as a strong acid catalyst because of the high acidic strength of sulfate anions on the surface of zirconia, was dramatically improved in comparison to that over ZrO2. In contrast, although Al2O3 showed much higher total acidity than sulfated zirconia, its activity toward FAMEs was substantially lower (86.1 versus 97.9% for the same amount of catalyst). The experimental data reported show that there is no direct correlation between the FAME yield and total acidity. However, it is observed that those catalysts with a higher relative number of strong acid sites (ZrO2−SO42−, Al2O3, WO3−ZrO2, and ZrO2) are exactly those that are more active toward FAME formation, suggesting that the presence of strong acid sites markedly favors the transesterification reaction. Even if acidity is an important factor to consider in catalyst design, adsorption and desorption of reactants, products, and/

Figure 3. (a) SEM image of the catalytic coating in sample ZrO2. (b) FIB cut of the catalytic coating. (c) SEM image corresponding to the FIB cut. The catalytic coating has a thickness of about 100−200 nm. (d) Detail of the catalytic coating, which shows high porosity.

where particles of about 50−100 nm in size exhibit a homogeneous aspect and are well-distributed. To obtain a direct measurement of the catalytic coating thicknesses, the samples were cut with an ion beam. Prior to this, a strip of platinum was deposited in the area of interest to obtain a clean cut (Figure 3b). Figure 3c corresponds to a SEM profile image after FIB, showing the platinum strip layer, the catalytic layer, and the cordierite substrate in the ZrO2 sample (confirmed by EDX). The catalytic layer is continuous and exhibits a thickness of about 100−200 nm. Taking into account the dimension of the catalytic layer and its high porosity (Figure 3d), no strong mass-transfer limitations are expected. Similar results were obtained over all of the samples tested. Metal leaching is a problem in many metal-loaded catalysts. In this work, the catalyst used at the transesterification reaction was kept at room temperature for 1 week, and after that, it was reused at a new transesterification reaction using the same operation conditions but without alteration of the yield. The results obtained in this study were compared to the results reported by other researchers for vegetable oil transesterification with methanol over ZrO2-based catalysts. The result over Nafion SAC-13 was included too (see Table 3). The yield obtained in this study over WO3−ZrO2, defined as the ratio of 6009

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Table 3. Summary of ZrO2-Based Catalysts Used for Biodiesel Production in SC Methanol reaction conditions type of oil

catalyst

temperature (°C)

pressure (MPa)

time (min)

methanol/oil ratio

yield (%)

palm kernel palm kernel cotton seed waste acid synthetic synthetic synthetic synthetic oil

ZrO2 ZrO2−SO42− ZrO2−SO42− WO3−ZrO2 Nafion SAC-13 ZrO2−SO42− ZrO2 WO3−ZrO2

200 200 230 150 200 200 200 200

5 5 1

60 60 480 120 4 4 4 4

6:1 6:1 12:1 9:1 25:1 25:1 25:1 25:1

64.5 90.3 85.0 96.0 94.0 98.0 94.0 85.8

20 20 20 20

4. CONCLUSION Biodiesel production from synthetic oil via the SC method with CO2 as a co-solvent and a solid acid catalyst is feasible and superior to the conventional transesterification method. The advantage of this method is that it requires mild operation compared to those in the conventional SC method. The reaction rate compared to conventional methods is significantly higher, and less energy is required for the process. The high reaction rates and milder reaction conditions using this environmentally friendly method cause it to be feasible for use in industry. When a wide range of solid catalysts are tested with different acidities, it has been demonstrated that there is no direct relationship between total acidity and yield of FAMEs, whereas strong acid sites are beneficial for the reaction. ZrO2-based samples were found to be optimum catalysts for transesterification of synthetic oil. An outstanding FAME yield around 98% after 4 min at 200 °C and 20 MPa was obtained over the ZrO2−SO42− catalyst. AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the Spanish Ministry of Science, Technology, and Innovation (Grant ENE200914502 and ENE2012-36368). J. Llorca is thankful to the Catalan Institution for Research Advanced Studies (ICREA) Academia program.

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the weight of methyl esters to the weight of vegetable oil used, was relatively lower than those reported in the literature, although the reaction time was significantly lower (4 versus 120 min). In contrast, ZrO2-based catalysts showed promising results to be used as heterogeneous catalysts for the SC transesterification because they give high methyl ester yields with a reaction time of only 4 min.

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