Application of Catalytic Ozone Chemistry for Improving Biodiesel

Biomacromolecules 2005, 6, 1334-1344

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Application of Catalytic Ozone Chemistry for Improving Biodiesel Product Performance Tylisha M. Baber, Daniel Graiver, Carl T. Lira, and Ramani Narayan* Department of Chemical Engineering & Materials Science, Michigan State University, East Lansing, Michigan 48824-1226, and BioPlastic Polymers & Composites, Okemos, Michigan 48864 Received September 23, 2004; Revised Manuscript Received January 8, 2005

Ozonolysis of methyl soyate (biodiesel) was conducted in the presence of methanol, dichloromethane (solvent), and triethylamine (catalyst) at -75 °C. Structural analysis, including FTIR, GC, and GC-MS, showed that the total amount of double bonds in the mixture was reduced by more than 90% after 2 h of ozonolysis. All of the esters predicted by this novel application of ozone reaction chemistry were successfully produced. Other major components were identified by GC-MS. Thermogravimetric analysis showed a dramatic decrease in the onset volatilization temperature from 135 to 73 °C, making ozonated biodiesel fuel comparable to diesel fuel (76 °C). Differential scanning calorimetric studies showed that the cooling curves for both methyl soyate and ozonated methyl soyate displayed two exothermic regions. The onset freezing temperature of ozonated methyl soyate in the “colder” region was significantly reduced from -63 to -86 °C. Furthermore, the degree of crystallinity in the “hotter” region was also reduced. Introduction Renewable diesel fuels derived from monoalkyl esters of long chain fatty acids, also known as biodiesel, are gaining ever-increasing attention as alternatives to petroleum-based fuels. Biodiesel is similar to petroleum-based middle distillate diesel fuels; the latter is the distillate fraction over 150400 °C having an average carbon number from C13 to about C21 and a flash point around 200 °C. For comparison, biodiesel derived from soybean oil (methyl soyate) is composed of slightly higher molecular weight esters and higher viscosity but similar boiling point and flash point. Further comparison of some of the key properties between these fuels is shown in Table 1.1, 2 The transesterification of biodiesel from a variety of vegetable oils has been thoroughly investigated, and many improvements have been implemented in this production process, where the triglyceride structure is broken and glycerol is replaced with simple alcohols, usually methanol.3-7 Unlike other alternative fuels, biodiesel is of particular interest because it is manufactured using existing industrial equipment and can be used in unmodified diesel engines with current fueling infrastructure.8 Furthermore, biodiesel is biodegradable, renewable, nontoxic, a reliable source of fuel, and environmentally friendly. Biofuel energy releases carbon dioxide into the atmosphere, but it is only returning to the atmosphere as much as was removed through photosynthesis during the plant’s lifetime. On the contrary, burning fossil fuel returns carbon dioxide to the atmosphere where it has been locked away in the Earth’s crust for millions of years. Thus, burning fossil fuel can cause a potential threat to global warming from the accumulation of heat-trapping carbon dioxide released after millions years of captivity. * Corresponding author: Phone: (517) 432-0775. Fax: (517) 913-6009. E-mail: [email protected].

Table 1. Properties of Petroleum-Based and Vegetable-Based Diesel Fuels kinematic viscosity (mm2/s)

fuel type

heat content (KJ/kg)

density (kg/m3)

27 °C

75 °C

cetane no.

diesel fuel biodiesel fuela

43,350 39,760

815 872

4.3 11

1.5 4.3

47 55

a

Neat soybean oil, methyl ester.

More than 1.5 million tons of sulfur dioxide is produced in the United States each year by the burning of fossil fuels in vehicle engines.9 Thus, in 2001, the EPA finalized a rule that will require sulfur levels in diesel fuel to be reduced from 500 to 15 ppm, a 97% reduction, by 2006. Unfortunately, refinery processes used to decrease the amount of sulfur in diesel also act to reduce its lubricity. In many cases, use of a fuel with poor lubricity can increase wear and cause engine failure. However, biodiesel can address this ultralow sulfur issue because it contains no sulfur in its structural features, already meeting the 2006 standard and environmental pollution due to sulfuric acid emission is not a serious issue. Biodiesel is also one of the most thoroughly tested alternative fuels on the market. In 2000, it became the only alternative fuel in the country to have successfully completed the rigorous EPA required Tier I and Tier II health effects testing under the Clean Air Act Section 211 (b).10 This evaluation includes the most stringent emission testing protocols ever required for certification of fuels and fuel additives in the United States. With all these benefits, one important issue that needs to be addressed is the thermal/oxidative stability of biodiesel as it contributes to long-term storage and engine fueling systematic problems. In fact, the Engine Manufacturers Association (EMA) has recently11 issued a warning against

10.1021/bm049397f CCC: $30.25 © 2005 American Chemical Society Published on Web 04/13/2005

Improving Biodiesel Product Performance

using over 5 vol % biodiesel in petroleum-based fuel because the poor oxidative stability of biodiesel can cause a variety of engine performance problems, including filter plugging, injector coking, piston ring sticking and breaking, and severe degradation of engine lubrication. The Fuel Injection Equipment manufacturers have also expressed a concern that blends containing more than 5 vol % biodiesel can cause reduced product service life and injection equipment failure.12 As for storage issues, the EMA statement concluded that the poor oxidation stability of biodiesel fuel could result in long-term storage problems due to deposit formation in tanks, fuel systems and filters. The thermal/oxidative stability problem has been known for many years and is inherent in the structure of the biodiesel mixture. It has been established that the thermal/oxidative stability of fatty esters is directly related to the number of double bonds present in the hydrocarbon tail.13 Oxidative degradation is known to initiate from the instability of the carbon-carbon double bonds within in the mixture where peroxides are formed, which further react to degrade the structure. The oxidative reaction occurs when the fuel is in contact with air (oxygen), and it is further affected by temperature, certain metals, and ultraviolet light.14,15 The oxidative instability of biodiesel is of particular concern because the products of these reactions are particulate solids, also termed “filterable sediments”, which tend to deposit on hot surfaces and eventually cause severe damage to the engine and the fuel system. The rate of oxygen reactivity is much higher in fatty esters containing multiple double bonds than those containing single or no double bonds. In fact, the relative autoxidation rate of methyl stearate, methyl oleate, methyl linoleate, and methyl linolenate was found to be 1, 11, 114, and 170, respectively, at comparable temperatures.16 It was further concluded that the rate of oxidation of nonconjugated polyunsaturated fatty esters is the fastest because of the high activation energy of methylene groups between the two double bonds.17 However, irrespective of oxidation rates, the reaction mechanism in every unsaturated fatty acid involves the formation and decomposition of hydroperoxides, leading to the formation of cross-linking and chain scission reactions. Although saturated fatty esters are also susceptible to degradation by oxidation, the autoxidation is much slower and these compounds are essentially inert at temperatures below 100 °C. The objective of this study is to chemically modify biodiesel by eliminating the double bonds to produce a mixture of methyl and dimethyl esters for enhancing its thermal/oxidative stability. Clearly, simple hydrogenation will not work as this will lead to long chain saturated fatty acids, which have relatively high melting point and thus will cause solidification and will adversely affect the lowtemperature flow properties of the fuel. Instead, a newly developed catalytic ozonolysis reaction was used, in which ozone should cleave the unsaturated fatty acids at the double bonds and the newly terminal carbons should react with methanol to yield methyl and dimethyl esters. It was previously reported18,19 that, in the presence of sodium methoxide, ozonides of pure methyl oleate were converted

Biomacromolecules, Vol. 6, No. 3, 2005 1335

directly to methyl nonanoate and dimethyl azelate, in a single step, instead of the usual formation of aldehyde and carboxylic groups. Such reactions were further demonstrated with unsaturated ethers, esters, and amides. Using this same approach, it is hypothesized that the cleavage of double bonds in a mixture of unsaturated fatty esters would form fragmental and terminal methyl and dimethyl esters in a single step. Thus, in this novel and unique application of ozone reaction chemistry, the predicted esters should improve the thermal/oxidative stability of biodiesel without adversely impacting its low-temperature flow properties. The reactions of ozone with alkenes are the best-known of all reactions involving ozone.20 Ozone is known to react with double bonds in olefins to form ozonide intermediates, which in the absence of other reactants spontaneously decompose to yield a mixture of aldehydes and carboxylic acids via the classical Criegee mechanism.21 Ozone is a wellknown and very powerful oxidative agent that is particularly suitable to efficiently attack double bonds. At the end of the oxidative reaction, any unreacted ozone dissociates back to oxygen, leaving no byproducts that must be neutralized or removed by most other oxidative procedures. Since the early ozonolysis experiments, it was realized that no other oxidative agent is capable of reacting with double bonds as fast and with great efficiency as ozone.22 The production and use of ozone is relatively simple and consists of passing oxygen or air near high electrical potential electrodes and then directly into the reaction mixture. This silent electric discharge technology is the most economical and convenient method for the production of large quantities of ozone.23 Unlike most other oxidative processes, the use of ozone does not require special packaging, storage, complicated handling procedures or expensive transportation protocols because ozone is produced and used on-site. Ozonolysis has also been successfully used on an industrial scale for chemical synthesis. The first ozonolysis process commercialized was the production of nonanoic and azelaic acids from oleic acid.24 Experimental Section Chemicals. Soybean oil, methyl esters (methyl soyate, [67784-80-9]), and premium No. 2 diesel fuel [68334-305] were graciously provided by Zeeland Farm Soya (Zeeland, MI) and Michigan State University (East Lansing, MI), respectively. Analytical grade methanol [67-56-1] was purchased from Mallinckrodt Baker, Inc (Phillipsburg, NJ). Analytical grade triethylamine [121-44-8] and petroleum ether [8032-32-4] were supplied by Spectrum Quality Products, Inc. (Gardena, CA). Calcium sulfate [7778-189], purified grade and anhydrous, and hexane [110-54-3], UV high purity, were purchased from Columbus Chemical Industries, Inc. (Columbus, WI) and Burdick & Jackson (Muskegon, MI), respectively. Both hydrochloric acid [764701-0], 36.5-38 wt %, and analytical grade dichloromethane [75-09-2] were purchased from EMD Chemicals, Inc. (Gibbstown, NJ). The nine fatty acid methyl ester (FAME) standards used in this study, methyl propionate (propionic acid, methyl ester, [554-12-1]), methyl hexanoate (hex-

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anoic acid, methyl ester, [106-70-7]), methyl nonanoate (nonanoic acid, methyl ester, [1731-84-6]), methyl myristate (tetradecanoic acid, methyl ester, [124-10-7]), methyl palmitate (hexadecanoic acid, methyl ester, [112-39-0]), methyl stearate (octadecanoic acid, methyl ester, [112-618]), methyl oleate (cis-9-octadecenoic acid, methyl ester, [112-62-9]), methyl linoleate (cis,cis-9,12-octadecadienoic acid, methyl ester, [112-63-0]), and methyl linolenate (cis,cis,cis-9,12,15-octadecatrienoic acid, methyl ester, [30100-8]), were all chromatographically >99% pure and purchased from Nu-Chek Prep, Inc. (Elysian, MN). Two additional reference standards, dimethyl azelate (nonanedioic acid, dimethyl ester, [1732-10-1]) and dimethyl malonate (propanedioic acid, dimethyl ester, [108-59-8]), were purchased from TCI America (Portland, OR), with each having a 98% purity. Unless noted, all chemicals were used as received. Ozonolysis of Methyl Soyate. A solution of methyl soyate (15 g), methanol (60 mL), triethylamine (3 g), and dichloromethane (120 mL) was added to a reaction vessel equipped with a glass tube that was tipped with a fritted glass disk. The theoretical molar ratio of methyl soyate (based on 1.5 double bonds per mol of methyl soyate) to methanol is 1:3; thus, excess methanol was used experimentally at a methyl soyate/methanol molar ratio of about 1:28 to ensure that the required amount of the methanol is reacted. The theoretical molar ratio is based on the mechanism of Marshall et. al.,19 which suggests that for every double bond cleaved 2 mol of methanol reacts to produce dimethyl and methyl esters. Triethylamine and methanol were premixed at a molar concentration of 0.5 M. The solution was cooled to about -75 °C by immersing the reaction vessel into a dewar surrounded by a dry ice/propanol bath. Ozone was produced by passing oxygen through a Praxair Trailigaz OZOBLOC Model OZC1001 ozone generator (Cincinnati, OH). The concentration of ozone in the feedgas was maintained within the range of 6-10 wt %. The manufacture’s calibration data for ozone concentration under different conditions was used as the basis for the concentration of ozone in the feedgas stream. The pressure of the generator was operated at about 83 KPa. The exit port of the ozone generator was connected with Tygon tubing to the inlet of the glass tube, and the gaseous oxygen/ ozone mixture was delivered to the reaction solution through the fritted disk at a flow rate of 165 cm3/s. The exhaust outlet was connected to a potassium iodide aqueous solution trap, in which excess ozone was rapidly decomposed. The reaction is conducted within a fume hood and ozone is always contained within the reaction system. A schematic of the process flow design is depicted in Figure 1. After the specified reaction time, the generator was shutdown and the reaction products were flushed for 10 min with oxygen to remove excess ozone. The solution was allowed to warm to room temperature, and dichloromethane was evaporated under a vacuum at 40 °C. The solution was transferred to a separatory funnel and the base was neutralized with a 2 M hydrochloric acid aqueous solution, which resulted in the separation of two phases. To distinctively separate the two phases, they were washed with an equal volume of petroleum ether. The aqueous layer was discarded,

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Figure 1. Experimental process flow diagram for ozonolysis.

and petroleum ether and residual methanol were removed from the organic layer under vacuum at about 60 °C. The resultant product was dried over anhydrous calcium sulfate and filtered. Ozonolysis was also conducted without the addition of dichloromethane to study solvent effects on the ozonolysis of methyl soyate. Under these conditions, triethylamine was used as the solvent. The separation process was the same, except that the first evaporation step was omitted. FTIR Analysis. A Perkin-Elmer model Spectrum 1000 (Boston, MA) was used to study the degradation of double bonds with respect to reaction time. FTIR spectra (resolution, 2 cm-1; scan, 64; range, 4000-500 cm-1) were performed after spreading a thin film of sample between two sodium chloride pellets. A background spectrum was also collected under identical conditions GC Analysis. Analysis of all standards and samples were performed with a Hewlett-Packard instrument (Palo Alto, CA) model HP-5890 Series II equipped with an Alltech Associates (Deerfield, IL) Heliflex AT-1 capillary nonpolar column (15-m length, 0.53-mm i.d., 1.5-µm thickness) and a flame-ionization detector. Helium and nitrogen were the carrier and auxiliary makeup gases, respectively, to obtain a total flow rate of 25 mL/min. The column temperature programming conditions were as follows: temperature was initially set at 40 °C for 2 min; rate 1 was 10 °C/min from 40 to 180 °C, held isothermally for 8 min; rate 2 was 5 °C/ min from 180 to 240 °C, held isothermally for 5 min; rate 3 was 30 °C/min from 240 °C to a final temperature of 300 °C, held isothermally for 5 min. The flow rate of hydrogen was 35 mL/min and the flow rate of air was about 400 mL/ min. Temperatures of the injector and detector were 280 and 320 °C, respectively. Splitless injection was used at a sample size of 1.0 µL. A total of eleven stock solutions, were prepared at a concentration of about 1.0 g/mL in hexane and were used to prepare one standard solution. An internal standard was used to normalize the amount of each of the desired components. GC-MS Analysis. A JEOL AX-505H double-focusing mass spectrometer (JEOL, USA, Peabody, MA) coupled to a Hewlett-Packard 5890J gas chromatograph via a heated interface was used to determine the absolute molecular weight of selected samples. A RT2560 capillary polar column (100-m length, 0.25-mm i.d., 0.20-µm film thickness) from Restek Corporation (Bellefonte, PA) was used for the separation of mono- and dicarboxylic acid methyl esters. A ramp rate of 10 °C/min from a starting temperature of 100-


150 °C, follow by another ramp of 3 °C/min to a final temperature of 240 °C were the column temperature programming conditions. An ionization voltage of 70 eV over the mass range of 45-750 amu was used to fragment the components. The carrier gas was helium and the injection volume was 1.0 µL. Other operating conditions were splitless injector temperature of 250 °C, interface temperature of 240 °C, and ion source temperature of 200 °C. A NIST/EPA/ NIH Mass Spectral Library, version 2.0, was used for compound identification. Also, a ShraderTSS Pro, version 3.0, operating system (Shrader Analytical and Consulting Labs, Detroit, MI & JEOL Ltd, Peabody, MA) was used for data collection and analysis. TGA Analysis. TGA analysis was carried out with a TA Instruments (New Castle, DE) model TGA 2950 thermobalance. The instrument was operated in the dynamic mode with a heating rate of 5 °C/min, where 40 and 350 °C were the initial and final temperatures, respectively. On average, 4 mg was the initial mass of each sample analyzed. A reactive atmosphere, nitrogen/air gas mixture, surrounding the sample was used to study the effect of air oxidation on volatility and to simulate the environmental conditions of a diesel engine combustion chamber. DSC Analysis. DSC analysis was carried out with a TA Instruments (New Castle, DE) model DSC Q1000, equipped with a 50-position auto-sampler, PC-based controller and a refrigerated cooling system (RCS) for subambient temperature scans. The TA Instruments DSC Q series are specifically designed to take into consideration extraneous heat flow effects using TZERO technology (TA Instruments, New Castle, DE). This advanced technology accounts for the cell resistances and capacitances, as well as pan effects.25 The DSC chamber was continuously purged with low-pressure nitrogen gas (99.9999% purity) at a flow rate of 50 mL/ min, to create a reproducible and dry atmosphere. On average, 10.0 mg samples were weighted to the nearest 0.1 mg with a microbalance and sealed in aluminum pans. An empty, sealed aluminum pan was used as a reference. For cooling scans, samples were first held isothermally at 40 °C for 10 min to establish thermal equilibrium. Previous studies have shown that calorimetric properties vary significantly with temperature scanning rate.26,27 It was concluded that, to obtain thermal event temperatures close to the thermodynamic value and high accuracy measurements, slow scanning rates should be used. A scanning rate of 5 °C/min is a convenient and reliable rate to use because it gives a good combination of resolution and flat baselines.26 Therefore, after equilibration, the samples were cooled to -90 °C at a scanning rate of 5 °C/min and held isothermally for 10 min and then heated back to 40 °C at the same rate. Each sample was scanned three consecutive times to eliminate thermal history. The third scan of every sample was taken as the experimental data. Heat flow vs temperature thermograms were analyzed to determine the crystallization properties of the samples. The DSC instrument was calibrated with the high purity metal indium (melting point 156.51 °C, ∆Hfusion: 28.71 J/g).


Results and Discussion Soy-based biodiesel, or methyl soyate, is a mixture of fatty acid methyl esters. Its exact composition depends on the type of soy, the maturity of the bean, geographical location, climate, the extraction process that was used to separate and isolate the oil, and the transesterification process of the triglycerides with methanol. The composition of methyl soyate used in this study was methyl palmitate (C16:0, 11 wt %), methyl stearate (C18:0, 5 wt %), methyl oleate (C18:1, 25 wt %), methyl linoleate (C18:2, 51 wt %), and methyl linolenate (C18:3, 7 wt %). As previously mentioned, many of the problems associated with use and storage of the biodiesel are related to the unsaturated fatty acids. Simple hydrogenation removes the double bonds yielding long-chain saturated fatty acid esters. This chemical modification process is not suitable because the melting points and the low-temperature flow properties of these esters are too high, yielding a fuel that will solidify around room temperature. Instead, catalytic ozone chemistry in the presence of methanol was used to cleave the double bonds and react the ozonide intermediates with methanol, in a single step. The oleic, linoleic, and linolenic fatty esters should react with ozone, and the resulting product mixture is expected to consist of shorter molecular weight, saturated methyl and dimethyl esters then the original methyl soyate mixture, which are illustrated in Figure 2. For example, the cleavage of methyl oleate under these conditions is expected to yield methyl nonanoate and dimethyl azelate. Similarly, cleavage of methyl linoleate is expected to yield dimethyl azelate and two low molecular weight fragments (methyl hexanoate and dimethyl malonate), whereas methyl linolenate would yield dimethyl azelate, methyl propionate, and dimethyl malonate. Since methyl stearate and methyl palmitate contain no unsaturation, they should not be affected by the ozonolysis process and will remain in the product mixture. Structural Analysis. FTIR Analysis. Direct structural evidence to the nature of the catalytic ozonolysis can be observed from the FTIR spectra. As expected, the double bonds in the fatty esters were readily attacked and cleaved by the ozone as indicated by the disappearance of the absorbance peak at 3005 cm-1, shown in Figure 3a. Furthermore, under the reaction conditions that were employed, the ozonide intermediate reacted with methanol to yield methyl esters instead of spontaneously decomposing to a mixture of aldehydes and carboxylic acids. This direct conversion to methyl ester is evident by the changes in the carbonyl region of the spectrum and the lack of adsorption peaks due to hydroxyl groups. Thus, the adsorption peak related to the CdO stretching mode at 1745 cm-1 in methyl soyate becomes more intense with an additional shoulder at 1710 cm-1 after an exposure time of 120 min to ozone, illustrated in Figure 3b. No other peaks are observed in this region that should be apparent if a carboxylic acid or its salts would have been formed. The extent of the ozonolysis reaction as a function of time was determined from the FTIR spectra by calculating the ratio of the areas of the CdC peak to the -CH2-, -CH3 stretching peak, and normalizing this ratio to 1.0 for the starting methyl soyate sample, shown in Figure 4a. Although

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Figure 2. Ozonolysis products from the unsaturated fatty ester components in methyl soyate.

Figure 3. FTIR spectra of (a) methyl soyate in the CdC, -CH2-, and -CH3 regions at the six reaction time intervals and (b) methyl soyate in the CdO region at an ozone exposure time of 0 and 120 min.

it was recognized that the peak area determined by simple baseline correction is not completely accurate, this method was sufficient to determine the progress of the reaction over time, which is depicted in Figure 4b. It is evident from the data that over 90% of the double bonds were cleaved after 120 min, when dichloromethane was used as a solvent. Surprisingly, no reaction was observed when dichloromethane was not added and triethylamine served as both a catalyst and a solvent. It has been shown28 that when a methanol/methyl soyate solution is cooled to -1.6 °C, two separate liquid layers are observed, due to the poor solubility of methyl soyate in methanol at low temperature. Thus, at the low temperature (-75 °C) of the ozonolysis reaction, a separate liquid phase of methyl soyate most likely remained at the bottom of the reaction flask unaffected by ozone. GC Analysis. There are several types of quantitative methods commonly used in GC analysis. For the purposes of this study, an internal standard normalization technique was used to quantify the compounds of interest. A standard solution was prepared containing known amounts of the internal standard and each methyl and dimethyl ester of interest. Then, the same amount of internal standard was added to samples containing the desired esters in unknown amounts. Each component was identified by the comparison

Figure 4. (a) Sample of a FTIR spectrum used to calculate the relative amount of residual double bonds as a function of reaction time. (b) Degradation of double bonds as a function of reaction time.

of their retention times between the standard solution and the samples, which were within 0.001-0.004 min of agreement. Peak identification and the corresponding component are also listed in Table 2. Figure 5 shows the GC chromatogram for the standard solution. Unfortunately, methyl propionate is highly volatile and was lost during the workup procedure with the solvent and is not observed as a separate distinct peak in both the standard solution and the samples. Furthermore, methyl linoleate and methyl linolenate are too similar in structure



Figure 5. Gas chromatogram of the standard solution. Table 2. GC and GC-MS Peak Identification (ID) and the Corresponding Component for All Components Present the Product Mixture esters predicted by ozonolysis

other major components in product mixture

peak ID

component

peak IDb

component

DM MH MN DA STDa MP ML MLn MO MS

dimethyl malonate methyl hexanoate methyl nonanoate dimethyl azelate methyl myristate methyl palmitate methyl linoleate methyl linolenate methly oleate methyl stearate

A B C D E F G

1,1-dimethoxyhexane methyl octanoate 1,1-dimethoxynonane nonanal methyl heptadecanoate 9-(oxo)-methyl nonanoate methyl isocanoate

a Internal standard used for quantification. b Peaks identified by GCMS only.

to be separated into two distinct peaks in our column and thus eluted off the column as a single peak (ML, MLn) with methyl oleate tailing them (MO), producing a doublet. The GC chromatograms of nonozonated methyl soyate (MSy) and a sample of ozonated methyl soyate (OMSy) at a reaction time of 100 min are illustrated in Figure 6, parts a and b, respectively. It is clearly evident that the peaks associated with the unsaturated components of MSy (methyl oleate, methyl linoleate, and methyl linolenate) have greatly

reduced in size, implying the reaction of these components by ozone. The comparison of the retention times between the standard and samples suggests that all esters theoretically expected to be present after ozonolysis are experimentally present in the sample, with the exception of methyl propionate. Methyl propionate may have been eluted off with the solvent peak or perhaps lost during the evaporation steps of the separation process. Also, other major unexpected peaks, which are not labeled, are observed in Figure 6b. This observation implies that other fragments are also produced during the ozonolysis of methyl soyate. The esters of interest were quantified by normalization, similar to the FTIR quantitative method. The peak area of each of the expected esters was divided by the peak area of the internal standard, taking this ratio to be 1.0 for the internal standard. The progress of the ozonolysis reaction can clearly be observed by a plot of these normalized concentrations as a function of time, shown in Figure 7. As expected, no change is observed in the concentration of the saturated fatty esters (methyl stearate and methyl palmitate), whereas the concentrations of dimethyl azelate, methyl hexanoate methyl nonanoate, and dimethyl malonate were continuously increasing with reaction time. Moreover, the concentrations of the unsaturated fatty esters continuously decreased with reaction time. It must also be noted that after 2 h of ozonolysis the normalized concentration of the total unsaturated fatty esters was reduced from about 3.8 to less than 0.32, a more than 90% reduction. GC-MS Analysis. The expected products were identified based on molecular weights using GC-MS. Figure 8 shows chromatograms of MSy and the same OMSy sample that was previously analyzed by GC alone. Since a 100-m column was used, all components in MSy separated into distinct peaks. Also a polar column was used, generating different retentions than in the GC analysis, where a nonpolar column was used. For each scan, a mass spectrum was generated and each peak was identified by molecular weight. Figure 9 shows the experimental mass spectrum (top) for the desired

Figure 6. Gas chromatograms of (a) MSy and (b) OMSy at a reaction time of 100 min.

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Figure 7. Normalized concentration versus reaction time for (a) the expected esters of OMSy and (b) the five common methyl esters of MSy. Table 3. Onset Volatilization Temperature at the Six Reaction Time Intervals reaction time (min)

onset volatilization temperature (°C)

0 40 60 80 100 120 diesel

135.3 65.5 71.6 73.1 71.7 72.9 75.8

products, which is compared against the mass spectrum (bottom) generated by the NIST/EPA/NIH Mass Spectral library. The statistical probability of the match between the experimental and theoretical mass spectrums for all of the desired ester was above 80%. Identification of other major peaks include 1,1-dimethoxyhexane, methyl octanoate, 1,1dimethoxynonane, nonanal, methyl heptadecanoate, 9-oxomethyl nonanoate, and methyl isocanoate, which are listed in Table 2. Ozone is known to react with methanol and other oxygen containing compounds. The ozone reaction with methanol may proceed through the formation of the following transition states: (1) attack on the C-H bond, (2) attack on the O-H bond, and (3) attack on the oxygen atom.20 Thus, due to the similar structure of the methylated terminal end of the methyl esters to that of methanol, ozone could have possibly reacted with this functional group and with the double bonds simultaneously. Furthermore, the expected esters formed could have been further attacked by ozone and methanol at these methylated sites. The peaks labeled with an asterisk were not recognized nor did the library provide a satisfactory match (statistical probability >70%) for these components.

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Structural analysis, including FTIR, GC, and GC-MS, support that in the presence of dichloromethane the double bonds within the mixture diminished as reaction time increased. Both GC and GC-MS analytical techniques support the original hypothesis that methyl oleate, methyl linoleate, and methyl linolenate are attacked and cleaved by ozone by identifying the expected products of methyl hexanoate, methyl nonanoate, dimethyl malonate, and dimethyl azelate. Moreover, the amounts of methyl stearate and methyl palmitate remained unchanged, which implies that ozone did not react with these components during the reaction time intervals analyzed. In addition to the formation of these esters, both GC and GC-MS analytical methods showed that other fragments were in the product mixture. The presence of these other components should not have an adverse effect on the oxidative stability of the modified alternative fuel because they do not contain unsaturation in their structural features. Thermal Analysis. TGA Analysis. Fuel volatilization is one of the most fundamentally important qualities of fuel in engines because it has a major affect on the vapor-air ratio in the cylinder and, therefore, influences the ignition quality of the fuel. The TGA profiles of MSy and OMSy at reaction times of 40 and 120 min, and No. 2 premium diesel fuel are illustrated in Figure 10. It is clearly evident in the profile that, as the reaction time increased, the volatility characteristics of the resulting ester mixture approached that of diesel fuel. Since the boiling points of fatty acid methyl esters increase with the length of the hydrocarbon tail (or with molecular weight), it is not surprising that volatility increased as the heavier, long chain, unsaturated components are fragmented to shorter, lighter, saturated components, shifting its TGA profile toward diesel fuel. Table 3 shows the onset volatilization temperature at each reaction time interval. The onset volatilization temperature was inferred from the point of intersection of lines drawn tangent to the initial baseline and the rate of weight loss curve on the derivative TGA plot, shown in Figure 10. There was a dramatic decrease in the onset volatilization temperature from MSy (135 °C) to OMSy (an average of 73 °C), making OMSy a more volatile fuel and comparable to diesel fuel (76 °C). This reduction occurred because of the significant difference in composition between the original and the chemically modified fuels, due to the breaking of the heavier longchain unsaturated components into smaller and lighter fractions. DSC Analysis. Thermograms of MSy and OMSy at various reaction times are compared in Figure 11a. The cooling curves for both MSy and OMSy display two major exothermic regions; Region 1 between +10 and -10 °C and region 2 less than -65 °C. This behavior is consistent with mixtures of fatty compounds, such as mixtures of unsaturated and saturated methyl esters.29 The length of the hydrocarbon chains and their degree of unsaturation strongly influence the phase transitions of fatty mixtures. Components with short-chain hydrocarbons will crystallize at a lower temperature than components with long-chain hydrocarbons because the intermolecular forces are weaker between the shorter fatty tails. Intermolecular forces among neighboring hydrocarbon tails increase as the tails get longer, enhancing the tendency to crystallize. In terms of saturation, saturated components



Figure 8. Chromatograms of (a) MSy and (b) OMSy at a reaction time of 100 min.

are flexible since every carbon-carbon bond is free to rotate, forming closely packed and well-ordered crystals. Unsaturation present in methyl esters has a pronounced lowering

effect on freezing points and polyunsaturation more so than monounsaturation.30 Because rotation around the double bond is hindered, the presence of cis double bonds produces a

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Figure 9. Experimental (top) and theoretical (bottom) mass spectra of (a) hexanoic acid (methyl hexanoate), (b) nonanoic acid, methyl ester (methyl nonanoate), and (c) nonanedioic acid, dimethyl ester (dimethyl azelate). R.T. is the retention time.


Figure 10. TGA profile of MSy (0 min), OMS at reaction times of 40 and 120 min, and no. 2 diesel fuel.

Figure 11. (a) DSC cooling thermograms of MSy and OMSy at a scanning rate of 5 °C/min. (b) Onset crystallization temperatures for regions 1 and 2 as a function of reaction time. (c) Enthalpies of crystallization for regions 1 and 2 as a function of reaction time.

distinct bend in the hydrocarbon chain. This bend prevents the formation of closely packed, well-ordered crystals and, hence, decreases the intermolecular forces among the hydrocarbon chains. Consequently, cis unsaturated fatty esters have much lower freezing points than saturated fatty esters. Thus, for both MSy and OMSy thermograms, the higher temperature region (region 1) is the freezing transition of the high molecular weight, long-chain saturated methyl esters, and the lower temperature region (region 2) is the freezing transition of the unsaturated methyl esters.


The onset crystallization temperature, TC1, was defined as the intersection of a line tangent to the scan at the point of sharpest slope and a line tangent to a baseline segment on the “hot” side of the onset crystallization peak. Similarly, the onset freezing temperature, TC2, was defined as the point where a line tangent to the scan at the point of sharpest slope on the “hot” side of the freezing peak intersects a line across the base of the region. The MSy thermogram is comparable to that of unwinterized methyl soyate (TC1 ) 3.7 °C) of Lee et al.28 For region 2 of the OMSy thermograms, it is clearly evident that TC2 has been significantly reduced from about -63 °C (for MSy) to a minimum temperature of about -86 °C. This phenomenon occurred because of the cleavage of the double bonds, which reduced the length of the hydrocarbon tail into smaller fragments and the formation of methyl and dimethyl esters at the terminal position, as predicted. Also, the size of the peak diminished due to the reduction of the unsaturated portion of the oil. After 120 min of reaction time, there was nearly no phase transition occurring in region 2. As illustrated in Figure 11b, there was a slight increase in both TC1 and TC2, starting at a reaction time of 40 min. The heats of crystallization (∆HC) for both regions 1 and 2 were calculated by taking the area under the peak of the phase transition region. For OMSy, the heats of fusion for region 1 were relatively large compared to that of region 2. This can be explained by knowing that more released energy is required for the large, heavier, saturated components to crystallize than for the short, lighter, saturated and reduced unsaturated components. However, the exact opposite was true for MSy because there are more long-chained unsaturated fatty esters (∼85 wt %) than long-chained saturated fatty esters (∼15 wt %) present in the unmodified methyl soyate. A plot of enthalpy of crystallization versus reaction time for both regions is depicted in Figure 11c. Although the amount of the saturated components remains constant throughout ozonolysis, the degree of crystallinity decreased with reaction time. This observation may suggest that both the size and amount of crystals formed are being reduced as reaction time is increased. Cold-flow improvers, or fuel additives, significantly alter the size and tendency of wax crystal formation, producing smaller and more compact crystal.31 They interact with the waxes that separate from the fuel as it cools, making small, three-dimensional crystals rather than larger platelet crystals produced by an untreated fuel. Thus, because of the shape of the modified crystals, the wax layer on the main filter is permeable, allowing liquid fuel to pass through, whereas the large unmodified platelet crystals readily interlock, making a structure that will impede flow through the waxy layer on the main filter.31 Therefore, the other major components present in the product mixture could serve as natural fuel additives by increasing the wax tolerance of a fuel system and, thus, should not have an adverse effect on the low-temperature flow properties of biodiesel. Conclusions This study investigated the hypothesis that in the presence of methanol and a catalytic base the cleavage of double bonds in a mixture of unsaturated fatty esters would form fragmental and terminal methyl and dimethyl esters in a single

1344


conversion step. Structural analysis, including FTIR, GC, and GC-MS, confirmed that the total amount of double bonds was reduced by more than 90% after 2 h of ozonolysis. As a result of this cleavage, all of the esters predicted by ozone reaction chemistry were successfully produced, with the exception of methyl propionate, including dimethyl malonate, methyl hexanoate, and dimethyl azelate. Methyl propionate is a low molecular weight, C3 ester and was most likely lost during the evaporation steps of the separation procedure. Other major unpredicted compounds were also identified by GC-MS and were perhaps produced by the reactive nature of ozone at the methylated sites of the original methyl esters and the newly formed methylated methyl and dimethyl esters. Surprisingly, no reaction was observed when dichloromethane was not added to the reaction mixture. The poor solubility of methyl soyate in methanol at low temperature, creating a two-phase liquid mixture, may be the reason for this observation. Thermogravimetric analysis showed that the volatile characteristic of ozonated biodiesel fuel is comparable to that of diesel fuel. There was a dramatic decrease in the onset volatilization temperature from MSy (135 °C) to OMSy (73 °C), making OMSy a more volatile fuel and similar to diesel (76 °C). Differential scanning calorimertic studies showed that the cooling curves for both MSy and OMSy display two major exothermic regions: region 1 between +10 and -10 °C and region 2 around -65 °C for MSy and about -80 °C for OMSy. After ozonolysis, TC2 for OMSy was significantly reduced from about -63 °C to a minimum temperature of about -86 °C. Also, after 120 min of reaction time, nearly no phase transition occurred in region 2 as the peak in this region was flatten out. However, after a reaction time of 40 min, there was a slight increasing trend in both TC1 and TC2. Further calorimetric data showed that, even though the phase transition temperatures increased slightly as the reaction progressed, the degree of crystallinity in region 1 was reduced. Therefore, the other major components in the product mixture could serve as natural fuel additives by increasing the wax tolerance of a fuel system and, thus, should not have an adverse effect on the low-temperature flow properties of biodiesel, such as viscosity. Catalytic ozonolysis whereby methyl soyate is exposed to ozone in order to cleave the double bonds and yield methyl and dimethyl esters appears to be very promising and suitable for biodiesel applications. In this project standard, solvent-based ozone reaction chemistry conditions were employed to validate the hypothesis that ozone chemistry can be used effectively to eliminate the double bonds and form volatile methyl and dimethyl ester products. Ambient temperature and a solvent-free system can now be engineered to develop an economically viable process.

Baber et al.

Acknowledgment. USDA-SBIR Grant No. 2004-3361014344 to BioPlastic Polymers & Composites. Bev Chamberlin of the MSU Mass Spectrometry Facility for conducting the GC-MS analysis. References and Notes (1) Altin, R.; Cetinkaya, S.; Yucesu, H. S. Energy ConV. Manag. 2001, 42, 529-538. (2) Peterson, C. L.; Reece, D. L.; Hammond, J.; Thompson, J.; Beck, S. M.; ASAE International Winter Meeting; Atlanta, Georgia, 1994. (3) Austrian Biofuel Institute, 1997. (4) Schwab, A. W.; Bagby, M. O.; Freedman, B. Fuel 1987, 66, 13721378. (5) Mittelbach, M.; Woergetter, M.; Pernkopf, J.; Junek, H. Energy Agric. 1983, 2, 369-384. (6) Schwab, A. W.; Dykstra, G. J.; Selke, E.; Sorenson, S. C.; Pryde, E. H. J. Am. Oil Chem. Soc. 1988, 65, 1781-1786. (7) Freedman, B.; Butterfield, R. O.; Pryde, E. H. J. Am. Oil Chem. Soc. 1986, 63, 1375-1380. (8) Clark, S. J.; Wagner, L.; Schrock, M. D.; Piennaar, P. G. J. Am. Oil Chem. Soc. 1984, 61, 1632-1638. (9) National Biodesel Board; http://www.biodiesel.org/pdf_files/ bdreport.pdf, 2001; Vol. November 2003. (10) National Biodiesel Board, http://www.biodiesel.org/pdf_files/ Benefits%20of%20Biodiesel. PDF Vol. 2003. (11) EMA Technical Standard, 2003. (12) Norman, M.; Kelly, W.; Manabe, M.; Krieger, K.; FIE Manufacturers, 2000. (13) Lundberg, W. O., Ed. Autoxidation and Antioxidants; Interscience Publishers: New York, 1961; Vol. I. (14) Turney, T. A. Oxidation Mechanism; Butterworth: London, 1965. (15) Hucknall, D. J. SelectiVe Oxidation of Hydrocarbons; Academic Press: New York, 1974. (16) Stirton, A. J.; Turer, J.; Riemenschneider, R. W. Oil Soap 1945, 22, 81-83. (17) Farmer, E. H.; Sutton, D. A. J. Am. Oil Chem. Soc. 1942, 139-148. (18) Marshall, J. A.; Garofalo, A. W. J. Org. Chem. 1993, 58, 36753680. (19) Marshall, J. A.; Garofalo, A. W.; Sedrani, R. C. Synlett 1992, 643645. (20) Rakovsky, S.; Zaikov, G. Kinetics and Mechanism of Ozone Reactions with Organic and Polymeric Compounds in Liquid Phase; Nova Science Publishers: New York, 1998. (21) Bailey, P. S., Ed. Olefinic Compounds; Academic Press: New York, 1978; Vol. 1. (22) Harris, C. D. Ber. Discuss. Chem. Ges. 1993 1903, 36. (23) Schober, B. D. Chim. Oggi 1995, 13, 21-24. (24) Goebel, C. G.; Brown, A. C.; Oehlschlaeger, H. F.; Rolfes, R. P.; U.S. Patient 2,813,113, 1957. (25) Dallas, G.; Groh, J.; Kelly, T.; Danley, R. Am. Lab. 2001, 33, 2629. (26) Lee, I.; Johnson, L. A.; Hammond, E. G. J. Am. Oil Chem. Soc. 1995, 72, 1155-1160. (27) Cebula, D. J.; Smith, K. W. J. Am. Oil Chem. Soc. 1991, 68, 591595. (28) Lee, I.; Johnson, L. A.; Hammond, E. G. J. Am. Oil Chem. Soc. 1996, 73, 631-636. (29) Dunn, R. O. J. Am. Oil Chem. Soc. 1999, 76, 109-115. (30) Walter M. Budde, J. In Fatty Acids and Their Industrial Applications; Pattison, E. S., Ed.; Marcel Dekker, Inc.: New York, 1968. (31) Owen, K.; Coley, T. In AutomotiVe Fuels Handbook; Society of Automotive Engineers, Inc.: Warrendale, 1990.

BM049397F

Application of Catalytic Ozone Chemistry for Improving Biodiesel

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