Article pubs.acs.org/EF
Efficient Antioxidant Formulations for Use in Biodiesel Maria L. Medeiros,† Angela M. M. T. Cordeiro,‡ Neide Queiroz,† Luiz E. B. Soledade,§ Antonia L. Souza,*,† and Antonio G. Souza† †
Universidade Federal da Paraíba, CCEN, Departamento de Química-Laboratório de Combustíveis e Materiais (LACOM), CEP 58051-970 João Pessoa, PB, Brazil ‡ Instituto Federal de Educaçaõ , Ciência e Tecnologia do Amazonas, Campus Manaus/Zona Leste, CEP 69020-120 Manaus, AM, Brazil § Universidade Federal do Maranhão, Campus de Pinheiro, CEP 65200-000 Pinheiro, MA, Brazil ABSTRACT: Biodiesel has a lipid origin; thus, antioxidants commonly used in oils and fats have been adopted for the improvement of the oxidative stability of biodiesel. However, these antioxidants have not shown the same efficiency when they are applied to biodiesel. This fact points out the need to investigate additives that warrant the oxidative stability of biodiesel, especially during long storage periods. In this paper, antioxidant formulations were developed for application in biodiesel, from blends including the synthetic antioxidants butylated hydroxytoluene (BHT), tert-butylhydroquinone (TBHQ), the chelating agent citric acid, and organic rosemary extracts. The antioxidants were applied at different concentrations and combinations and were evaluated by the Rancimat method, EN 14112, and by the pressurized PetroOXY method. The results showed that for ethanol soybean biodiesel, the antioxidant compositions formed by TBHQ (2000 mg kg−1) and citric acid (500 mg kg−1) and by the chloroform rosemary extract (2000 mg kg−1) and citric acid (500 mg kg−1) were the most effective in delaying the oxidative process, in a manner independent of the evaluation method. For the methanol cottonseed oil biodiesel, the best result was obtained using the combination of 1500 mg of TBHQ with 1500 mg of ethanol rosemary extract. However, the formulations using low concentrations of rosemary ethanol extract and citric acid induction periods of >6 h were obtained, reaching the limit specified by European Standard EN 14214.
1. INTRODUCTION Biodiesel is a biofuel that originates from renewable sources (oils and fats). It displays fuel properties similar to those of petrodiesel. Thus, biodiesel can be utilized in diesel cycle engines, without the need for adaptation. It is a very attractive fuel, from an environmental perspective, as besides being biodegradable, it exhibits very low sulfur and aromatic hydrocarbon contents and displays a toxicity lower than that of the diesel obtained from fossil fuels.1−4 In Brazil, the most used vegetable oils in the production of biodiesel are soybean and cottonseed oils as they are produced from oily plants, the culture of which is consolidated in the country. Both vegetable oils display high percentages of unsaturated fatty acids, so they present a low oxidative stability, a property that is also inherent to the biodiesel forthcoming from these oils.5−7 The low oxidative stability of biodiesel has been one of most critical drawbacks to its consolidation into the world energy matrix. It is responsible for oxidative processes that degrade biodiesel, harming drastically its fuel properties, especially during long storage periods.6−10 Biodiesel oxidation is directly related to the presence of allylic and bis-allylic methylene groups in the fatty acid ester chains. The radical removal of a hydrogen atom from these groups is energetically favorable, as it leads to the formation of allylic and bis-allylic radicals. Although these are resonance-stabilized radicals, they are sufficiently reactive to trigger chain radical processes, which, if not reverted, rapidly degrade biodiesel.10−12 © XXXX American Chemical Society
The oxidative process of biodiesel may take place by means of photo-oxidation and by autoxidation. Factors such as light, heat, transition metals, water, oxygen, and traces of peroxide influence this process.10 Autoxidation is the most investigated oxidative process, both for biodiesel and for oils and fats. It comprises three steps: initiation, propagation, and termination. In the steps of initiation and propagation, in which antioxidants may interfere, free radicals and hydroperoxides, primary oxidation products, are formed. In the termination step are formed aldehydes, carboxylic acids, hydrocarbons, ketones, and polymers, the so-called secondary oxidation products. At this point, the process is completely irreversible.10,12 One of the most efficient methods for delaying the oxidation of biodiesel is the use of antioxidant additives. In commercial biodiesel are mainly utilized synthetic additives butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and tert-butylhydroquinone (TBHQ).3 Studies show that the combination of antioxidants can be a good strategy for improving the oxidative stability of lipid materials and biodiesel, as the blend can lead to a synergism, stemming from the properties of each individual antioxidant.13 The search for efficient antioxidant combinations includes synthetic and natural antioxidants, now that the phenolic Received: October 6, 2013 Revised: December 23, 2013
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2.4. Preparation of the Antioxidant Formulations. The synthetic antioxidants BHT and TBHQ and the natural antioxidants citric acid and rosmarinic acid were supplied by Sigma-Aldrich. They were applied, as well as the rosemary extracts, to the soybean and cottonseed biodiesels, using the formulations listed in Tables 1 and 2,
compounds present in plants have been undoubtedly characterized as excellent antioxidants.14 Rosmarinus off icinalis L., common rosemary, is a fragrant plant that originated from the Mediterranean region but adapted to grow in several parts of the world. It is widely known as a source of natural antioxidants, besides displaying antibacterial activity. Its therapeutic properties are chiefly ascribed to the presence of its phenolic compounds, carnosic acid and rosmarinic acid.15−17 Several studies report that the mechanism of antioxidant action of rosemary is based on the scavenging of free radicals,17 which makes rosemary extract very promising for application in biodiesel, now that the initiation step of the biodiesel oxidation process has been shown to involve the formation of free radicals. On the basis of the similarity of the profile of the fatty soybean biodiesel and cottonseed biodiesel, this work investigated the antioxidant effect of substances TBHQ, BHT, rosmarinic acid (RA), citric acid (CA), and organic rosemary extract in these biodiesels. The antioxidants were used both individually and together. The antioxidant action was evaluated by the accelerated Rancimat and PetroOxy methods in the soybean biodiesel and Rancimat method only in the cottonseed biodiesel, whereas the National Agency of Petroleum, Natural Gas and Biofuels (ANP), takes into account only the values of oxidative stability obtained by the Rancimat method.
Table 1. Antioxidant Formulations Applied to the Soybean Biodiesel and Their Respective Codes sample −1
SB + REE (50 mg kg ) SB + REE (100 mg kg−1) SB + REE (200 mg kg−1) SB + CA (50 mg kg−1) SB + CA (100 mg kg−1) SB + CA (200 mg kg−1) SB + REE (50 mg kg−1) + CA (25 mg kg−1) SB + REE (100 mg kg−1) + CA (25 mg kg−1) SB + REE (200 mg kg−1) + CA (25 mg kg−1) SB + TBHQ (50 mg kg−1) SB + TBHQ (100 mg kg−1) SB + TBHQ (200 mg kg−1) SB + TBHQ (50 mg kg−1) + CA (25 mg kg−1) SB + TBHQ (100 mg kg−1) + CA (25 mg kg−1) SB + TBHQ (200 mg kg−1) + CA (25 mg kg−1) SB + REE (2000 mg kg−1) SB + RHE (2000 mg kg−1) SB + RCE (2000 mg kg−1) SB + RACE (2000 mg kg−1) SB + REE (2000 mg kg−1) + CA (500 mg kg−1) SB + RHE (2000 mg kg−1) + CA (500 mg kg−1) SB + RCE (2000 mg kg−1) + CA (500 mg kg−1) SB + RACE (2000 mg kg−1) + CA (500 mg kg−1) SB + TBHQ (2000 mg kg−1) SB + TBHQ (2000 mg kg−1) + CA (500 mg kg−1)
2. MATERIALS AND METHODS 2.1. Biodiesel Syntheses. Soybean biodiesel (SB) was synthesized in the laboratory by alkaline transesterification of the triacyl glycerides present in refined soybean oil, using KOH as a catalyst and the ethanol route. In the synthesis were utilized 1000 g of oil, 350 g of ethanol, and 15 g of KOH. After the synthesis, the biodiesel was washed with an aqueous hydrochloric acid solution and then dried, in vacuum, at 90 °C for 30 min.18The cottonseed methanol biodiesel (CB) utilized was obtained from Usina de Caetés, Pernambuco state, Brazil, in the absence of antioxidant additives. Initially, the work would investigate only ethyl esters, the ethanol route, as in Brazil, commercial biodiesel is not synthesized by this route; methanol biodiesel from cotton oil was also studied. The physicochemical properties of the two different biodiesel samples were determined according to European Standard EN 14214.19 2.2. Determination of the Fatty Acid Ester Profiles. The identification of fatty acid esters in the samples was performed with a gas chromatograph−mass spectrometer (GC-MS) from Shimadzu (model GCMS-QP2010). The capillary column was a Durabond DB-23 column (30 m × 0.25 mm and 0.25 μm film thickness) in a temperature range of 40−250 °C. A total of 1 μL of biodiesel was injected using the split mode, and the initial column temperature was 90 °C, with heating rates of 10 °C/min up to 200 °C, 3 °C/min up to 210 °C, and 20 °C/min up to 230 °C.20 2.3. Preparation of the Rosemary Extracts. The rosemary leaves utilized for the preparation of the extracts were acquired from a ́ state, Brazil. After being commercial supplier in João Pessoa, Paraiba made hygenic, the leaves were dried in an air circulation stove at 60 °C for 24 h. Next the leaves were powdered in a processor, weighed, and subjected to the extraction with hexane and with ethanol, in individual containers, for 15 days. The extracts were filtered, and the solvents were removed with a rotary vacuum evaporator. The extracts were transferred to amber glass containers and exposed to natural drying for the elimination of the remaining solvents. The solid residues of the vegetable materials, after the filtration of the hexane extracts, were reutilized to obtain the chloroform and ethyl acetate extracts, using a similar procedure. The solvent-free rosemary extracts using hexane (RHE), chloroform (RCE), ethyl acetate (RACE), and ethanol (REE) were stored in amber glass containers in the refrigerator until they were used.
code SB50REE SB100REE SB200REE SB50CA SB100CA SB200CA SB50REE25CA SB100REE25CA SB200REE25CA SB50TBHQ SB100TBHQ SB200TBHQ SB50TBHQ25CA SB100TBHQ25CA SB200TBHQ25CA SB20000REE SB2000RHE SB2000RCE SB2000REAE SB2000REE500CA SB2000RHE500CA SB2000RCE500CA SB2000RACEE500CA SB2000TBHQ SB2000TBHQ500CA
Table 2. Antioxidant Formulations of Antioxidants Applied to Cottonseed Oil Biodiesel and Their Respective Codes sample
code −1
CB + REE (1000 mg kg ) CB + REE (1500 mg kg−1) CB + REE (2000 mg kg−1) CB + REE (2500 mg kg−1) CB + REE (3000 mg kg−1) CB + REE (3500 mg kg−1) CB + REE (4000 mg kg−1) CB + RCE (1000 mg kg−1) CB + RCE (1500 mg kg−1) CB + RCE (2000 mg kg−1) CB + RCE (2500 mg kg−1) CB + RCE (3000 mg kg−1) CB + RCE (3500 mg kg−1) CB + RCE (4000 mg kg−1) CB + TBHQ (500 mg kg−1) CB + TBHQ (1000 mg kg−1) CB + TBHQ (2000 mg kg−1) CB + TBHQ (2500 mg kg−1) CB + TBHQ (3000 mg kg−1) CB + BHT (2000 mg kg−1) CB + BHT (2500 mg kg−1) CB + BHT (3000 mg kg−1) CB + REE (1500 mg kg−1) + TBHQ (1500 mg kg−1) CB + RA (1000 mg kg−1) B
CB1000REE CB1500REE CB2000REE CB2500REE CB3000REE CB3500REE CB4000REE CB1000RCE CB1500RCE CB2000RCE CB2500RCE CB3000RCE CB3500RCE CB4000RCE CB500TBHQ CB1000TBHQ CB2000TBHQ CB2500TBHQ CB3000TBHQ CB2000BHT CB2500BHT CB3000BHT CB1500REE1500TBHQ CB1000RA
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respectively. The formulations using extracts were applied at high concentrations, taking into account the fact that the extracts are not pure substances and are constituted by substances of varied chemical nature and not only by phenolic compounds. 2.5. Oxidative Stability. The Rancimat method was used to estimate the effect of the antioxidants. The tests were conducted in Metrohm Rancimat 873 equipment, at 110 °C, according to the ISO EN 14112 standard.21 The induction period (IP) was obtained by the conductivity curve onset. The PetroOxy method was also used to assess oxidative stability. The PetroOxy analyses were performed with 413 Petrotest equipment, according the methodology reported elsewhere: a 5 mL biodiesel sample was placed in a hermetically sealed chamber.22 After the sample had been introduced, the chamber was purged with oxygen for the complete removal of air and then oxygen was injected with the system at room temperature until a pressure of 700 kPa was reached. At this point, the temperature was increased to 110 °C, which provoked the increase in pressure to a value that depends on the composition of the sample being analyzed. The induction period is the measurement of the time interval between the beginning of the test and the decrease of 10% in the maximal pressure achieved.
Table 5. Induction Periods (IPs in hours) and Protection Factors (percent)25 of Soybean Biodiesel with and without Antioxidants Using the Rancimat and PetroOxy Methods
3. RESULTS AND DISCUSSION 3.1. Fatty Profile of Biodiesel Samples. The fatty acid profile of the samples (Table 3) showed that the biodiesels Table 3. Fatty Acid Ester Composition of Biodiesels % fatty acid ester
soybean oil
cottonseed oil
C16:0 (palmitate) C18:0 (stearate) C18:1 (oleate) C18:2 (linoeate) C18:3 (linolenate) saturated unsaturated
11.0 4.0 25.6 53.3 6.1 15.0 85.0
26.1 3.8 16.1 54.0 − 29.9 70.1
a
sample
IP Rancimat
protection factor
IP PetroOXY
protection factor
SB SB50REE SB100REE SB200REE SB50CA SB100CA SB200CA SB50REE25CA SB100REE25CA SB200REE25CA SB50TBHQ SB100TBHQ SB200TBHQ SB50TBHQ25CA SB100TBHQ25CA SB200TBHQ25CA SB2000REE SB2000RHE SB2000RCE SB2000RACE SB2000REE500CA SB2000RHE500CA SB2000RCE500CA SB2000RACE500CA SB2000TBHQ SB2000TBHQ500CA
3.8 5.1 5.6 5.8 3.7 3.1 3.3 5.8 5.9 6.6 3.6 3.8 3.9 3.0 4.1 5.1 11.0 7.4 11.6 9.8 10.3 9.6 12.9 9.5 12.0 22.5
a 34.2 47.4 52.6 a a a 52.6 55.3 73.7 a a a a a 34.4 189.5 94.7 205.3 157.9 171.1 152.6 239.5 150.0 215.8 492.1
1.6 2.0 2.0 2.3 1.7 1.6 1.6 2.0 2.2 2.2 1.5 1.7 1.4 1.7 1.9 1.8 3.3 2.6 3.5 3.3 4.0 3.5 3.4 2.5 3.2 5.9
a 25.0 25.0 43.8 a a a 25.0 37.5 37.5 a a a a 18.8 12.5 106.3 62.5 118.8 106.3 150.0 118.8 112.5 56.3 100.0 268.8
Protection effect of 400% in relation to that of CB. In spite of the satisfactory results with the antioxidant TBHQ, its efficacy was much lower compared with recently published values,29 in which the addition of 300 mg of TBHQ kg−1 to cottonseed biodiesel, synthesized in the laboratory, was sufficient to meet the oxidative stability parameters from the standard EN 14214, adopted by the Brazilian National Agency of Petroleum, Natural Gas and Biofuels (ANP). In this work, in cotton biodiesel, higher concentrations of TBHQ were investigated, taking into account the fact that the concentration of 500 mg kg−1 was not sufficient to correct the oxidative stability of biodiesel. Another reason was to investigate if higher concentrations of the antioxidant would provoke pro-oxidant effects in the biodiesel. Such a difference in the TBHQ antioxidant activity can be ascribed to factors that interfere in the antioxidant behavior in the biodiesel. These factors include the biodiesel synthesis process, the quality of the raw material oil, which is associated with many variables, and the antioxidant purity in the moment E
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Figure 2. Influence of the antioxidant concentration on the oxidative stability of commercial cottonseed biodiesel, evaluated by the induction period (Rancimat) and protection factor: (a) rosemary ethanol extract, (b) rosemary chloroform extract, (c) TBHQ, and (d) BHT.
among the phenolic constituents of rosemary extract and TBHQ. Both of them have been cited as antioxidants with a powerful protecting effect in vegetable oils, besides being thermally stable, thus justifying the induction periods obtained by the Rancimat method.32
of its application, without forgetting operating errors during the determination of the induction period. In relation to the antioxidants RCE and BHT (panels b and c of Figure 2, respectively), the effect was not satisfactory. Even at a concentration of 3000 mg kg−1, the formulations with these antioxidants did not yield an induction period of 6 h. The poor results with the synthetic antioxidant BHT were not surprising, as according to previously reported data30 this antioxidant undergoes thermal degradation at a temperature lower than that utilized in EN 14112. In relation to the result for extract RCE, the result was unexpected, once it displayed an excellent antioxidant effect when it was applied to soybean biodiesel (Table 5). The factors discussed with respect to the TBHQ behavior may also be taken into account for RCE, besides the fact that the extract was applied to different biodiesels. The two formulations, utilizing rosmarinic acid and the combination of TBHQ and REE, displayed quite promising results. Rosmarinic acid, for example, was shown to be an efficient antioxidant to be used in cotton biodiesel. Only 1000 mg kg−1 was needed to correct the biodiesel stability to surpass the minimal value required by standard EN 14214. Such a powerful effect of rosmarinic acid is probably ascribed to its structure containing a larger number of phenolic hydroxyls than TBHQ and BHT.31 Nevertheless, the usage of rosmarinic acid as an antioxidant in biodiesel is economically unfeasible. The combination of TBHQ and REE presented an induction period of 22.8 h, with a protection factor of 1040%. This excellent antioxidant effect is attributed to a synergetic effect
4. CONCLUSIONS The antioxidant formulations evaluated in this work produced quite promising results in the biodiesels, especially those based on the combination of antioxidants. In soybean biodiesel, for example, the rosemary extracts corrected the oxidative stability to meet the parameters established by standard EN 14112, even at a concentration of 200 mg of rosemary ethanol extract kg−1 combined with 25 mg of citric acid kg−1. In the formulations applied to cottonseed biodiesel, the chloroform rosemary extract and BHT were shown to have low efficiencies and did not yield induction periods to meet the standards established by standard EN 14214, even using the highest concentrations. On the other hand, the formulations employing the ethanol rosemary extract and TBHQ presented excellent results and formulation CB1500REE1500TBHQ displayed a remarkable synergetic effect, with a protecting effect of 1040% in relation to CB. Among the antioxidants used in cotton biodiesel, rosmarinic acid was the most efficient, needing only 1000 mg of this antioxidant kg−1 to increase the cottonseed biodiesel induction period above the minimal limit established by standard EN 14214; however, its high cost makes unfeasible its us as a biodiesel additive. F
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(20) Santos, N. A.; Damasceno, S. S.; Araújo, M. H. P.; Marques, C. V.; Rosenhaim, R.; Junior Fernandes, J. V.; Queiroz, N.; Santos, I. M. G.; Maia, A. S.; Souza, A. G. Caffeic acid: An Efficient Antioxidant for Soybean Biodiesel Contaminated with Metals. Energy Fuels 2011, 25, 4190−4. (21) European Committee for Standardization (CEN). EN 14112: Fat and Oil Stability (accelerated oxidation test); in German, 2003. (22) Damasceno, S. S.; Santos, N. A.; Santos, I. M. G.; Souza, A. L.; Souza, A. G.; Queiroz, N. Caffeic and ferulic acids. An investigation of the effect of antioxidnats on the stability of soybean biodiesel during storage. Fuel 2013, 107, 641−6. (23) Santos, E. M.; Piovesan, N. D.; Barros, E. G.; Moreira, M. A. Low linolenic soybeans for biodiesel: Characteristics, performance and advantages. Fuel 2013, 104, 861−4. (24) Waynick, J. A. Chracterization of Biodiesel Oxidation and Oxidation Products. Technical Literature Review; National Renewable Energy Laboratory (NREL): Golden, CO, 2005; Vol. 540, pp 390−6. (25) Suja, K. P.; Abraham, J. T.; Thamizh, S. N.; Jayalekshmy, A.; Arumughan, C. Antioxidant efficacy of sesame cake extract in vegetable oil protection. Food Chem. 2004, 84, 393−400. (26) Ramalho, E. F. S. M.; Albuquerque, A. R.; Souza, A. L.; Barros, A. K.; Maia, A. S.; Santos, I. M. G.; Souza, A. G. Use of diferente techniques in the evaluation of the oxidative stability of poultry fat biodiesel. Journal of Thermal Analysis and Calorimetry 2011, 106, 787− 91. (27) Araújo, S. V.; Luna, F. M. T.; Rola Junior, E. M.; Azevedo, D. C. S.; Cavalcante Junior, C. L. A rapid method for evaluation of the oxidation stability of castor oil fame: influence of antioxidant type and concentration. Fuel Process. Technol. 2009, 90, 1272−7. (28) Neumann, A.; Jebens, T.; Wiembicki, V. A method for determining oxidation stability of petrodiesel, biodiesel, and blended fuels. Am. Lab. (Southport, CT, U.S.) 2008, 40, 22−3. (29) Fernandes, D. M.; Serqueira, D. S.; Portela, F. M.; Assunçaõ , R. M. N.; Munoz, R. A. A.; Terrones, M. G. H. Preparation and characterization of methylic and ethylic biodiesel from cottonseed oil and effect of tert-butyl-hydroquinone on its oxidative stability. Fuel 2012, 97, 658−61. (30) Santos, N. A.; Cordeiro, A. M. T. M.; Damasceno, S. S.; Aguiar, R. T.; Rosenhaim, R.; Carvalho Filho, J. R.; Santos, I. M. G.; Maia, A. S.; Souza, A. G. Commercial antioxidants and thermal stability evaluations. Fuel 2012, 97, 638−43. (31) Leopoldini, M.; Russo, N.; Toscano, M. The molecular basis of working mehanism of natural polyphenolic antioxidant. Review of Food Chemistry 2011, 125, 288−306. (32) Rodriguez-Rojo, S.; Visentin, A.; Maestri, A.; Cocero, M. J. Assisted extraction of rosemary antioxidants with green solvents. J. Food Eng. 2012, 109, 98−103.
AUTHOR INFORMATION
Corresponding Author
*Phone and fax: +55 83 32167441. E-mail: antonia_lucia@ yahoo.com.br. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the Brazilian agencies FINEP, MCT/CGTS, CNPq, and CAPES.
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