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Effects of Lignins on Antioxidant Biodiesel Production in Supercritical Methanol Shimin Kang, Xianglan Li, Biao Li, Juan Fan, and Jie Chang* Pulp and Paper Engineering State Key Laboratory, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, People’s Republic of China
’ INTRODUCTION Biodiesel is gradually gaining acceptance in the market as an environmentally friendly alternative diesel fuel.1 However, poor oxidative stability of biodiesel is a significant problem associated with the commercial application. Oxidation instability can lead to the formation of undesirable oxidation products, such as aldehydes, alcohols, shorter chain carboxylic acids, insolubles, gum, and sediment in the biodiesel.2 The most promising approach for improving oxidative stability of biodiesel is treatment with antioxidants because it facilitates the use of existing storage tanks and fuel-handling systems without requiring upgrades or redesign.3 However, adding conventional antioxidants means increased cost, because the commercial antioxidants are costly chemicals. Lignins are largely byproducts of the paper industry and cellulosic ethanol plants, which were traditionally viewed as waste materials or low-value products.4 Lignins are polyphenolic compounds, containing three phenylpropane units (coniferyl, sinapyl, and p-coumaryl alcohol), as shown in Figure 1;5 the three units are linked by β-ether or C C bond. In supercritical alcohols, lignin can be decomposed to lower molecular phenolics with or without catalysts.6 9 It was indicated that supercritical methanol treatment is very effective to depolymerize model compounds of lignin into the lower molecular products mainly by the cleavage of the dominant β-ether structure.8,9 Lignin possesses antioxidant properties because of its phenolic structure.10,11 However, in most oil substances, poor solubility and mobility of lignin limit its antioxidant activity and applications. Phenolic compounds can be used as an antioxidant because of their phenolic rings and hydroxyl substituents. The antioxidant power is improved greatly when lignin depolymerized to phenolic products, and the lignin-liquefied phenolic products have great potential to be used as a liquid antioxidant.12 When vegetable oils (mainly triglycerides) and lignins are put in supercritical methanol, biodiesel [fatty acid methyl esters (FAMEs)] can be synthesized through transesterification. The overall transesterification is a three-step reaction, with diglyceride and monoglyceride as the intermediates.13 Simultaneously, lignins can be liquefied to phenolic compounds, which can act as antioxidants in biodiesel. Scheme 1 shows the overall transesterification and lignin degradation reaction in supercritical methanol in the experiments. The object of the work was to investigate and analyze the oxidative stability of biodiesel produced in supercritical methanol with various lignin additions. ’ EXPERIMENTS AND RESULTS Magnesium lignosulfonate (ML), commercial alkaline lignin (CAL), dealkaline lignin (DL), and black liquor alkaline lignin r 2011 American Chemical Society
(BLAL) were used in the experiments (for details of the sources of lignins, fatty acid profile of the rapeseed oil, instruments, analysis methods, and operating conditions, see the Supporting Information). The pH values of various lignin solutions (0.01 g/mL) were shown in Table 1. The autoclave was loaded by 100 mL of methanol and 57 mL of rapeseed oil with or without 0.1 g of the designed kinds of lignin; the reaction temperature was 300 °C. The obtained products formed a two-layer solution by gravimetric precipitation. When lignin was added to the reaction, the top layer was filtered by a filter fabric and the residue content settled on the filter fabric was negligible, which meant that lignin was almost completely converted. Methanol in the filtered top layer was removed using a vacuum evaporator. The remaining top layer was biodiesel. The compositions of the biodiesel samples were analyzed by gas chromatography mass spectrometry (GC MS). With or without lignin addtion in the reaction, diglycerides and triglycerides were not detected in all of the samples. The types of FAMEs were almost the same, and FAMEs were the main constituents. Monoglycerides [such as 9-octadecenoic acid (Z)-2,3-dihydroxypropyl ester] and some other byproducts were detected. To ascertain the production of phenolics, the biodiesel produced with BLAL addition was first extracted by 5% NaHCO3 solution and then the remaining biodiesel was extracted by 5% NaOH solution. The NaOH solution was extracted by ethyl acetate after acidification, and then the ethyl acetate phase was analyzed by GC MS. As shown in Figure 2, there were also phenolics in the products when lignin was added in the reaction. Table 2 shows the relative peak area in the GC MS analysis. When CAL, DL, BLAL, and ML were added, the total monoglyceride content greatly decreased, while the total FAME content increased. This means that the four lignins promoted transesterification reactions. Moreover, BLAL had the optimal effects. The improvement of transesterification reactions indicates that all of these lignins can be used as catalysts under supercritical methanol reaction conditions. CAL and BLAL can be used as catalysts because they are alkaline, as shown in Table 1. Alkali is a popular catalyst in transesterification; it has been indicated that FAME yield was greatly improved even when a little CaO or NaOH was added.14 While DL and ML are acidic (as shown in Table 1), phenolic compounds have weak acidity under common conditions. Acid catalysis is also an important method for biodiesel production because acid can simultaneously perform transesterifaction of triglyceride and alkylesterification of the free fatty Received: March 19, 2011 Revised: May 31, 2011 Published: May 31, 2011 2746
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Figure 1. Phenylpropane unit of lignin.
Scheme 1. One-Step Production of Phenolics Containing Biodiesel in Supercritical Methanol Figure 3. Relationship between oxidation times and acid values of biodiesel produced with or without lignin addition.
Table 1. pH Values of Lignin Solution lignin
BLAL
CAL
ML
DL
pH
9.85
9.03
5.91
3.75
Figure 4. Relationship between oxidation times and POVs of biodiesel produced with or without lignin addition. Figure 2. Phenolics in the ethyl acetate phase extracted from biodiesel produced with BLAL addition by GC MS analysis.
Table 2. Percentage of the Relative Peak Area percentage of the relative peak area (%) addition of lignins
total FAMEs
total monoglycerides
others
none
75.7
20.0
4.3
0.1 g of CAL
87.7
8.7
3.6
0.1 g of DL
86.8
9.0
4.2
0.1 g of BLAL
88.7
8.2
3.1
0.1 g of ML
86.6
9.6
3.8
acids to produce the alkyl esters; for example, sulfuric acid is commonly used as a catalyst.13 In the oxidation stability experiments, 25 g of biodiesel sample was put into a 3.5 cm diameter tube, which was put in a heating oil
bath pot set at 110 °C. Oxygen was bubbled through the sample at a rate of 130 mL/min. In regular intervals, the peroxide value (POV) and acid value were determined, as shown in Figures 3 and 4. The initial acid value (IAV) of the biodiesel was various. The IAV of biodiesel with DL added was greatly higher than that of all of the others, while BLAL-added biodiesel had the lowest IAV. One possible reason for the high IAV of DL-added biodiesel was the high inorganic acid content in the DL (as shown in Table 1); the inorganic acid was not absolutely evaporated during methanol evaporation but at last remained in the biodiesel. However, the specific mechanism of the various IAVs with different lignin additions needs further studies. The acid value and POV change trends with oxidation time with lignin additions more gentle than that without lignin additions. This meant that the antioxidant ability of biodiesel has been improved with all kinds of lignin additions. There were mainly two reasons: (1) Phenolic compounds and biodiesel were produced and mixed in supercritical methanol. Phenolics acted as chain breakers to avoid or decrease biodiesel oxidation in the oxidation experiments. (2) As shown in Table 2, much more monoglycerides remained in 2747
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Table 3. Specific Properties of Biodiesels Prepared with or without BLAL Addition in the Reaction addition of lignin
total glycerol
viscosity at
freezing
cloud
content (wt %) 40 °C (mm2/s) point (°C) point (°C)
none 0.1 g of BLAL
1.05 0.60
4.87 4.67
22 22
4 5
the biodiesel without lignin additions, and the monoglycerides may act as pro-oxidants; it was reported that monoglycerides had pro-oxidant effects in soybean oil.15 Besides, the β hydrogen in the monoglycerides is readily susceptible to elimination, which is conductive to the formation of unsaturated compounds and free fatty acids. Therefore, the decreased monoglyceride content with lignin additions in the reaction can also result in high thermal oxidation stability. Considering the FAME yields and antioxidant effects of the biodiesel with or without lignin additions, as shown in Table 2 and Figures 3 and 4, all of the four lignins have shown the catalytic effects and the biodiesels produced with lignin additions showed better antioxidant abilities, while BLAL showed the best improvement of these properties. Specific properties of biodiesels prepared with or without BLAL addition were compared. As shown in Table 3, although the total glycerol content were higher than that of the American Society for Testing and Materials (ASTM) D6751 standard (e0.250 wt %), owing to the high total monoglyceride content in the biodiesel, it was greatly decreased when BLAL was added in the reaction. The cloud point was also greatly decreased when BLAL was added in the reaction. This was probably also because of the lower content of monoglycerides; it has been reported that monoglycerides significantly raise the cloud point of biodiesel.16 While the viscosity of both of the biodiesels at 40 °C was conforming to the ASTM D6751 standard (1.9 6.0 mm2/s), the freezing points were the same whether BLAL was added in the reaction or not. The lowered cloud point and improved antioxidant properties indicate that the addition of BLAL is beneficial to biodiesel production. Considering that BLAL is abundantly used as a waste material and the biodiesel which possesses antioxidant abilities will gain added acceptance in the market, it is promising to produce biodiesel by one step in supercritical methanol with BLAL addition.
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’ ASSOCIATED CONTENT
bS
Supporting Information. Details of the sources of lignins, instruments, analysis methods, fatty acid profile of the rapeseed oil, and reaction operating conditions. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledge the financial support from the National Basic Research Program of China (973 Program) (2010CB732205). ’ REFERENCES (1) Janaun, J.; Ellis, N. Renewable Sustainable Energy Rev. 2010, 14, 1312–1320. 2748
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