Study on Autoxidation Kinetics of Fats by ... - ACS Publications

Differential scanning calorimetry was used to follow the oxidation course of the saturated fatty acids lauric, myristic, palmitic, and stearic and the...
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Ind. Eng. Chem. Res. 2000, 39, 7-12

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Study on Autoxidation Kinetics of Fats by Differential Scanning Calorimetry. 1. Saturated C12-C18 Fatty Acids and Their Esters Grzegorz Litwinienko,*,† Andrzej Daniluk,† and Teresa Kasprzycka-Guttman†,‡ Department of Chemistry, Warsaw University, 02-093 Warsaw, Pasteura 1, Poland, and Drug Institute, 00-725 Warsaw, Chelmska 30/34, Poland

Differential scanning calorimetry was used to follow the oxidation course of the saturated fatty acids lauric, myristic, palmitic, and stearic and their ethyl esters and, additionally, glycerol tripalmitate and tristearate. Assessment of the oxidative stability of these fat components was performed at 90-300 °C without any solvents and in the absence of free-radical initiators. The calculated overall activation energies were in the range 106.0-123.3 kJ/mol. Obtained overall rates constants of autoxidation (k) were not dependent on the length of the carbon chain, and k’s for acids were greater than those for esters and triglycerides. The applicability of thermoanalytical methods to determine the kinetic parameters of lipid oxidation was verified by comparison with values of activation energies and rate constants of oxidation determined by GC. Introduction Natural saturated fatty C12-C18 acids and their esters play important structural and metabolic roles in plant and animal fats, and this group of fats is of great importance by reason of the fact that it is a source of the best quality edible solid fats. The mechanisms of autoxidation and decarboxylation processes were intensively studied for oxygen-containing compounds alcohols, saturated and unsaturated monoand dicarboxylic acids, and their esters and were summarized by Denisov in monographs,1,2 but the majority of the described experiments were performed for catalyzed oxidation of saturated fatty acids and their esters with solvent and processes were initiated by freeradical initiators. From the industrial point of view, the determinations of the oxidative stability of these compounds in bulk (storage, transport, and application) are more important than the determinations of reactivity and absolute rate constants of initiated autoxidation steps in various solvents. For example, for the system cumene and stearic acid, oxidation of the solvent is the reason for initiated oxidative decarboxylation of stearic acid while it does not take place in biphenyl.1 The technical application of saturated acids and their esters is wide: they are a major part of vegetable butters, edible and confectionery fats, pharmacy oils, polishes, and high class soaps. Animal fats are also used as food, perfume and soap components, lubricants, and waterproofing agents. Moreover, some oils such as palm oil (40% of palmitic acid) are used in the tin plate industry as a protective coating against atmospheric * To whom correspondence should be addressed. E-mail: [email protected]. Fax: (48)(22) 822-59-96. Phone: (48)(22) 822-02-11 ext. 335. † Warsaw University. ‡ Drug Institute.

oxidation of the hot metal.3 Carboxylic acid derivatives such as wool fat and various types of stearins are corrosion inhibitors and lubricant components.4 Various physical and chemical processes in chemical, cosmetic, detergent, pharmaceutical, and rubber industries and extreme conditions of use (for example, lubricant action at high temperature or frying processes) are primary reasons of autoxidation and deterioration of edible and nonedible fatty materials. However, great attention is only paid to low-temperature oxidation of fats because of biochemical consequences of lipid deterioration. In contrast, a good example of pure and applied research on autoxidation is a large number of publications about autoxidation of saturated hydrocarbons because of their great significance in petroleum and lubricant industries.5-14 At the initial stages of hydrocarbon oxidation the rate of the process (rRH) can be described by a classical rate equation for autoxidation:

rRH ) kp(Ri/2kt)1/2[RH]

(1)

where Ri is the rate of production of free radicals, kp and kt are the rate constants of propagation and termination, and [RH] is the concentration of hydrocarbon. During the very initial stages of oxidation, the reaction may occur by direct attack of oxygen at C-H bonds within the hydrocarbon,6 [RH] is assumed to be constant, and the rate of initiation Ri can be taken to be effectively constant. Therefore, kp(Ri/2kt)1/2 ) k ) const, and k is a global first-order reaction rate constant.9,10 With the progress of oxidation, the hydroperoxides decompose to ketones, alcohols, and fatty acids as follows:

RCH(OOH)R1 f R(CdO)R1; k2

10.1021/ie9905512 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/04/1999

(2)

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Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000

RCH(OOH)CH2R1 f RCOOH + R1COOH; k3 (3) ROOH f RO• + •OH; k4

(4)

RO• + RH f ROH + R•; k5

(5)

Because k4 , k5, the formation of alcohols is determined by a decomposition step described by first-order kinetic equation dC/dt ) k4 [ROOH],14 and the overall decomposition reaction of hydroperoxides is assumed as first order. The total rate of autoxidation is determined by the rate of hydroperoxides formation (during the initial stage, eq 1), and in the early stages, when the hydrocarbon conversion and the hydroperoxides concentration are low, the decomposition of primary products is expected to be first order:

rdecomp ) (k2 + k3 + k4)[ROOH]

(6)

Differential scanning calorimetry (DSC) have found application in the investigation of both physical phenomena (i.e., changes in crystallographic properties, melting, sublimation, and adsorption) and chemical phenomena (dehydratation, decomposition, oxidation, and reduction), and review works of Dollimore15-17 give a wide insight into recent literature in these fields. Stability of petroleum oils under oxidative conditions characterized by DSC was described in a review concerning thermal analysis of petroleum products.18 Because autoxidation of fats, fatty acids, and lipids is attended by heat transport, DSC is a valuable method to follow the course of isothermal and nonisothermal oxidation. In our previous papers19,20 we reported several examples of papers in this field. The thermal oxidative decomposition of fats and petroleum products is a complex process, and the value of heat evolved is the net sum of concurrent processes at a particular temperature. Assuming an overall first-order process (eqs 1 and 6), the heat evolved (H) at time t is proportional to the amount of reacted substrate and the following kinetic equation is obtained:

(

)

HT - H dR 1 dH ) ) Ze-E/RT ) Ze-E/RT(1 - R) HT dt HT dt (7) where HT is the total heat evolved, E is the activation energy, R is the gas constant, Z is the Arrhenius preexponential factor, T is the absolute temperature, and R ) H/HT is the degree of conversion. This equation is well-known in chemical kinetics as the first-order rate equation given in form dR/dt ) k(1 - R) where k is the Arrhenius rate constant:

k ) Z exp(-E/RT) method21

(8)

is a nonisothermal The Ozawa-Flynn-Wall method where the temperature is increased with programmed linear heating rate β ) dT/dt. The extrapolated start of the oxidation process and the maximum of heat flow are the points of constant degree of conversion. Therefore, in the Ozawa-Flynn-Wall method by introduction of β into eq 7 and combination with eq 8, the temperature of the start or temperature of the maximum of heat flow obtained from the several DSC scans (each run with different β’s from 1 to 20 K/min) can be used for the determination of kinetic

parameters from equation

log β ) aT-1 + b

(9)

When log β versus 1/T is plotted, the straight line with the slope a ) -0.4567E/R and reciprocal b ) -2.315 + log(ZE/R) should be obtained. Therefore, the activation energy E is calculated from

E ) -2.19R

d log β dT-1

(10)

Values of E and preexponential factor Z (calculated from reciprocal b) can be used to calculate the rate constant of reaction given by the Arrhenius equation (eq 8). The aim of this work was to study the oxidation of the saturated fatty acids lauric, myristic, palmitic, and stearic and their derivatives to have well-defined kinetic parameters of oxidation. A comparison of these parameters for free fatty acids, ethyl esters, and triglycerides allows one to make assessment of the role of the carboxylic group (free or esterified) in the oxidation process. Additionally, it is possible to study the influence of the carbon chain length on the oxidation parameters E, Z, and k. Similar investigations (without initiators and without solvent) were conducted only for hydrocarbons. First publications1,5 suggested that oxidative reactivity increased with an increase in the length of the carbon chain. Recent works22,23 did not confirm this dependency, and the gross reactivities of the n-paraffins determined from the amount of the monofunctional derivatives were similar. Authors explained this discordance by probable formation of difunctional products of oxidation (not monitored in these experiments). Thermal analysis is a good tool for verification of these experiments. The progress of the oxidation process can be continuously monitored, and it is possible to investigate the oxidative behavior of nonvolatile organic compounds to have a well-defined reference data of their reactivity. Experimental Procedures Materials. Fatty acids lauric, palmitic, and stearic and ethyl esters laurate, myristate, palmitate, and stearate (all 99%) were purchased from Sigma-Aldrich. The purity of glycerol tripalmitate (BDH) and tristearate (Fluka AG) was 98%. The acids and esters were used without further purification. All investigated compounds were stored under nitrogen at 0 °C. Methods. All calorimetric measurements were carried out using a DSC apparatus: Du Pont model 910 differential scanning calorimeter with a Du Pont 9900 thermal analyzer and a normal pressure cell. The apparatus was calibrated with a high-purity indium standard. The study was carried out in an oxygen atmosphere. Experiments were performed under an oxygen flow of about 6 dm3/h. Samples (5 mg) of compounds were heated from 90 to 300 °C in an open aluminum pan with linear heating rate β (2-20 K/min). The extrapolated onset temperatures (Te) and temperatures of maximum heat flow (Tp1 and Tp2) are defined in Figure 1a, and they were determined from each DSC scan by using program GENERAL V4.01 (TA Instruments). Typical DSC curves of oxidation of ethyl palmitate and palmitic acid obtained for various β are presented in Figure 1b,c. Te, Tp1, and Tp2 determined for investigated compounds are listed in Table 1. All

Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 9 Table 1. Values of Extrapolated Onset Temperatures Te and Temperatures of First Peak, Tp1, and Second Peak, Tp2, Obtained for Oxidation at Different Heating Rates β β/ β/ (K/min) Te /°C Tp1 /°C Tp2 /°C (K/min) Te /°C Tp1 /°C Tp2 /°C 5 10 15 20

Lauric Acid 183.3 202.1 193.5 215.4 199.2 229.0 203.9 233.5

227.8 237.2 244.0 247.2

3 5 7 10

Ethyl Laurate 183.8 202.1 188.2 215.8 195.6 229.0 204.8 233.5

2 5 10 15 20 25

Palmitic Acid 158.1 174.2 169.0 189.0 178.0 200.4 182.0 186.1 206.7 190.8

192.3 204.1 219.9 224.1 228.1 231.7

1 3 5 7 10

Ethyl Myristate 176.7 191.6 190.7 208.9 196.8 218.7 200.0 222.8 211.5 231.4

209.4 234.3 244.5 256.1 274.5

1 3 5 7 10

Ethyl Palmitate 170.6 187.9 185.9 204.6 190.9 210.4 197.1 213.7 201.4 219.8

199.8 216.5 220.6 224.4 234.5

2 4 5 8 10 12 15 17 20

Stearic Acid 162.9 190.0 173.7 196.7 178.0 203.0 182.5 208.9 183.6 208.6 188.3 213.7 192.6 218.9 193.0 219.5 196.0 221.4

215.0 233.0 237.0 253.5 257.3 262.3 268.1 268.6 271.1

Glycerol Tripalmitate 1 161.3 197.4 2 170.1 214.6 5 179.5 235.5 10 195.9 244.1 15 200.9 257.7 17 202.2 260.5 20 206.2 262.1 2 5 10 12 15 17 20

5 7 10 12

Ethyl Stearate 177.2 192.8 187.7 212.7 197.9 223.6 204.3 228.2 206.6 230.3 208.2 232.6 216.6 238.4

208.3 227.8 245.5 251.9 266.3

Glycerol Tristearate 163.1 248.4 165.1 259.0 171.5 266.2 173.3 275.2

Hewlett-Packard 6890 gas chromatograph (column HP5, 30 m × 0.32 mm i.d.; film thickness, 0.25 µm; carrier gas, argon; FID, 300 °C; split, 100:1; oven program, 1 min at 50 °C and then increased at 10 °C/min to 200 °C). A weighed sample (about 10 mg) of lauric acid was heated at 154 °C under oxygen flow in a DSC cell for 5 min. After the oxidation was stopped, the sample was diluted with acetone to 5 mL and the solution was analyzed by GC. The next samples of the acid were oxidized at the same conditions for 11, 24, and 30 min and analyzed. The retention time for lauric acid was 14.74 min. Logarithms of concentrations (Cacid) of unreacted acid (in milligrams of acid per milligrams of reaction mixture) versus reaction time (t) gave linear dependency Figure 1. (a) DSC curve of stearic acid oxidation with a defined temperature of the extrapolated start of oxidation (Te) and temperatures of maximum heat flow (Tp1 and Tp2). Heating rate β ) 2 K/min. (b) Typical DSC scans of ethyl palmitate oxidation. Numbers denote heating rates in K/min. (c) DSC plots of palmitic acid oxidation for different heating rates.

values are averages of at least three determinations. The apparent activation energies of oxidation and Z factors were calculated by the Ozawa-Flynn-Wall method21 from eqs 9 and 10. A full description of the calculation procedure was shown in previous papers.19,20 To supply additional evidence of reality of obtained activation energies, the measurements of the reaction progress were carried out. The measurements of loss of fatty acids during oxidation were performed using a

ln(Cacid) ) -kt + const

(11)

where slope k is the rate constant of the first-order reaction of oxidation.25 The same experiments and calculation were carried out for stearic acid (at temperatures 155, 166, and 180 °C and GC retention time 18.78 min). Obtained rate constants were used for calculation of activation energies of oxidation from the transformed form of eq 8. Results and Discussion Temperatures Te, Tp1, and Tp2 determined for oxidation of each studied compound are presented in Table 1. An increase of the heating rate (β) results in a shift

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Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000

Table 2. Parameters a and b of Equation 9 with Standard Deviations (σa and σb), Square Regression Coefficients (R2), and Calculated Kinetic Parameters: Activation Energy Ea, Preexponential Factor Z, and Rate Constant of Oxidation k at 160 °C Ea/(kJ/mol)

Z/s-1

k (160 °C)/s-1

Lauric Acid 0.9996 0.9866 0.9972

116.7 ( 1.6 80.2 ( 6.6 143.6 ( 5.4

1.35 × 1011 3.41 × 106 5.62 ×1012

1.1 × 10-3 7.2 × 10-4 2.7 × 10-5

Ethyl Laurate 0.9483 0.9867

118.7 ( 19.6 80.2 ( 6.6

1.45 × 1011 3.41 × 106

7.0 × 10-4 7.2 × 10-4

119.0 ( 12.0 107.6 ( 3.1 75.9 ( 6.6

8.94 × 1010 1.43 × 109 1.89 × 105

3.9 × 10-4 1.5 × 10-4 1.3 × 10-4

125.3 ( 3.6 115.8 ( 10.8 114.9 ( 6.0

4.16 × 1012 7.82 × 1010 1.85 × 1010

3.2 × 10-3 8.4 × 10-4 2.5 × 10-4

Ethyl Palmitate 0.03 0.9960 0.04 0.9929 0.08 0.9756

124.5 ( 4.5 131.3 ( 6.4 131.1 ( 12.0

5.91 × 1011 9.09 × 1011 4.46 × 1011

5.6 × 10-4 1.3 × 10-4 6.8 × 10-5

13.70 10.59

Glycerol Tripalmitate 0.05 0.9888 0.04 0.9915

108.0 ( 4.3 91.0 ( 3.2

1.33 × 1010 1.22 × 107

1.2 × 10-3 1.3 × 10-4

0.26 0.36 0.19

14.81 15.36 9.58

0.04 0.05 0.04

Stearic Acid 0.9881 0.9821 0.9883

115.4 ( 4.8 126.7 ( 6.5 82.6 ( 3.4

1.60 × 1011 5.17 × 1011 1.32 × 106

1.9 × 10-3 2.7 × 10-4 1.4 × 10-4

-5.82 -5.44 -4.46

0.41 0.23 0.12

13.29 11.96 9.58

0.06 0.04 0.02

Ethyl Stearate 0.9757 0.9909 0.9979

106.0 ( 7.5 99.1 ( 4.2 81.2 ( 2.1

5.26 × 109 2.63 × 108 1.34 × 106

8.6 × 10-4 2.9 × 10-4 2.1 × 10-4

-6.47

0.97

15.65

Glycerol Tristearate 0.04 0.9569

117.8 ( 17.7

1.09 × 1012

6.7 × 10-03

-4.22

0.41

9.79

76.9 ( 7.5

2.29 × 106

1.2 × 10-03

R2

a

σa

b

σb

start 1st peak 2nd peak

-6.41 -4.40 -7.89

0.09 0.36 0.30

14.74 9.98 16.45

0.01 0.04 0.02

start 1st peak

-6.52 -4.40

1.08 0.36

14.78 9.98

0.05 0.04

start 1st peak 2nd peak

-6.54 -5.91 -4.17

0.66 0.17 0.36

14.57 12.73 8.70

Ethyl Myristate 0.08 0.9705 0.02 0.9974 0.07 0.9779

start 1st peak 2nd peak

-6.88 -6.36 -6.31

0.20 0.60 0.33

16.26 14.50 13.87

0.03 0.07 0.05

start 1st peak 2nd peak

-6.84 -7.21 -7.23

0.25 0.35 0.66

15.41 15.62 15.31

start 1st peak

-5.93 -4.99

0.24 0.18

start 1st peak 2nd peak

-6.34 -6.96 -4.54

start 1st peak 2nd peak start 1st peak 2nd peak

Palmitic Acid 0.9966 0.9828 0.9893

0.03

of the temperature ranges of the observed processes, and this phenomenon is the basis of the DSC nonisothermal methods for calculation of the kinetic parameters. In all cases the exothermal effect of oxidation begins above 160 °C (Figure 1 and Table 1). There are usually two peaks on the obtained DSC curves, indicating that at least two main processes take place during oxidation. The onset point and first maximum are connected with autoxidation (initiation and formation of primary products of autoxidation; see Introduction), while the second maximum is the further oxidation of formed alcohols, ketones, and esters. In our previous experiments with linolenic acid thermoxidation, addition of antioxidants retarded the start of oxidation and (insignificantly) the first peak, but the influence of antioxidants on the temperature of the second peak was not observed;19,26 thus, the monitoring of Te allows one to calculate the overall kinetic parameters of autoxidation. The parameters of eq 9 and kinetic parameters calculated for individual stages of oxidation (start, first, and second peaks) are listed in Table 2. Errors of activation energies were calculated from the standard deviations of eq 9. In the case of ethyl laurate and ethyl palmitate, the exothermic peaks were overlapped and Tp2 was difficult to determine; thus, the kinetic parameters of its oxidation were calculated only for Te and Tp1. Measurements of the rate constant of oxidation of stearic acid done by the conventional method (described in the Methods subsection) confirmed the first-order dependency. The typical plot of log(Cacid) vs time of oxidation for stearic acid is presented in Figure 2. The

0.9815

Figure 2. Plot of ln(Cacid) vs time of oxidation at 180 °C for stearic acid.

GC measurements gave the following values:

k(180 °C) ) (1.01 ( 0.03) × 10-3 s-1 k(166 °C) ) (0.37 ( 0.07) × 10-3 s-1 k(155 °C) ) (0.16 ( 0.02) × 10-3 s-1 Calculated E denotes 120.1 ( 14.9 kJ/mol. Similarly, rate constants of lauric acid oxidation were as follows:

Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 11

k(183 °C) ) (1.01 ( 0.20) × 10-3 s-1 k(171 °C) ) (0.61 ( 0.05) × 10-3 s-1 k(154 °C) ) (0.14 ( 0.04) × 10-3 s-1 The activation energy calculated from these rate constants is 112.6 ( 13.5 kJ/mol. Both values of E are in good agreement with values presented in Table 2. This comparison shows the accuracy of kinetic parameters obtained by the Ozawa-Flynn-Wall method. It follows from the tabulated results that the values of E do not vary substantially and the differences can be demonstrated within the experimental error of 3-16%. Presented activation energies are similar to value 108.8 kJ/mol for n-octane oxidation obtained by Van Sickle.7 Korcek et al.27 reported values Ep - 1/2Et ) 66 ( 2 kJ/mol (Ep and Et are the activation energies of propagation and termination, respectively) for autoxidation of n-hexadecane, n-tridecane, and cyclohexane in the presence of tert-butyl hydroperoxide at 4070 °C initiated by azo(bisisobutyronitrile) (AIBN). The overall activation energy of autoxidation is given by equation27

E ) Ep + 1/2Ei - 1/2Et

(12)

and taking into account the activation energy of initiation by AIBN, Ei ) 127.3 kJ/mol,28 with the E value of initiated autoxidation of about 130 kJ/mol, which is in agreement with our results. Denisov described Ep between cumylperoxy radicals and normal saturated acids C2-C9 at 100-125 °C as 63.1-67.7 kJ/mol and for the same reaction with esters of dicarboxylic acids: dimethyl succinate, glutarate, azelate, and sebacate Ep ) 57.3-78.2 kJ/mol. Our values of Ep calculated from eq 12 by using Ei ) 142.0 kJ/mol, Et ) 24.9 kJ/mol,29 and values of E calculated for the start of the autoxidation process (Table 2) are in the range 49.5-68.5 kJ/ mol and are in good agreement with the previous values. Because there are lack of the data about absolute Ei for oxidation of normal fatty acids without initiators, in this assessment we used value Ei for diethyl sebacate.1 All initial kinetic parameters indicate that the start of the process is similar for all three kinds of investigated compounds: fatty acids, their ethyl esters, and triglycerides. The overall rate constants of palmitic acid oxidation calculated by us for the first peak processes are 3.5 × 10-3 s-1 (at 180 °C) and 8.4 × 10-4 s-1 (at 166 °C). They are in good agreement with the rate constants k2 + k3 + k4 (eq 6) for n-hexadecane oxidation given by Blaine and Savage,14 which are 2.5 × 10-3 s-1 (at 180 °C) and 6.3 × 10-4 s-1 (at 166 °C). The rate of the propagation reaction with the carboxylic group

one to state that in our experiments the oxidation on the carboxylic group (eq 13) does not play such a great role as in hydrocarbon solvents. Parameters calculated from the first peaks (decomposition of peroxides) for acids are close to the ester ones. Similarity of kinetic parameters listed in Table 2 demonstrates that the rate of autoxidation of saturated C12-C18 fatty acids is not correlated with the length of their carbon chain. Lauterbach22 described a similar observation, but in those experiments only monofunctional derivatives of oxidation were monitored. However, the increasing reactivity was expected, and authors suggested that probably more difunctional derivatives were formed (not monitored). In our DSC measurements the complete information about heat evolved by all running thermal processes and gross reactivity of the oxidized compound is straightly received. It is evident that the start of exothermal effects and kinetic parameters of oxidation do not depend on the chain length. Conclusions A nonisothermal DSC study of the oxidation of lauric, myristic, palmitic, and stearic acids and their esters with ethanol and glycerol at temperatures 150 and 300 °C was described. Presented data have respect to oxidation in the liquid phase in the absence of other solvents and free-radical initiators. The kinetic parameters E, Z, and k concern the total effect of oxidation and can be used for comparison of the oxidative stability of fats and their components. For the start of the process the range of the overall activation energy is 106.0-125.3 kJ/mol, and these data were confirmed by the GC method (for first-order kinetics). The start of the oxidation is similar for acids and both ethanol and glycerol esters; therefore, oxidation does not take place in the short alkoxyl group of the ester, but the attack of oxygen takes place at the carbon chain at the C-H bond. The autoxidation of acid is 2-5 times faster than that of esters. Aliphatic saturated fatty acids and their derivatives resemble closely the hydrocarbons, and dependence of the carbon chain length on kinetic parameters of oxidation was not observed. The process of further oxidation and decomposition of oxidation products at higher temperatures is monitored as first and second exothermic peaks in the DSC curve. Acknowledgment This work was supported by a grant from the State Committee For Scientific Research (Grant GR-976/98/ 99). Literature Cited

ROO• + R1COOH f ROOH + R1COO•

(13)

with activation energy 89.9 ( 2.0 kJ/mol is known to be 12 times greater than the propagation rate at the carbon chain:1

ROO• + R1H f ROOH + R1•

(14)

In our experiments rate constants calculated for the start of oxidation (Table 2) for acids are only 2-5 times greater than those for esters. These comparisons allow

(1) Denisov, E. T.; Mitskevich, N. I.; Agabekov, V. E. LiquidPhase Oxidation of Oxygen Containing Compounds; (a) Nauka i Technika: Minsk, Russia, 1975 (in Russian). (b) Consultants Bureau: New York, 1977 (in English). (2) Denisov, E. T. The Oxidation of Alcohols, Ketones, Ethers, Esters and Acids in Solution. In Comprehensive Chemical Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, The Netherlands, 1980; Vol. 16. (3) Brockmann, R.; Gunter, D.; Kreutzer, U.; Lindemann, M.; Plachenka, J.; Steinberner, U. Fatty Acids. Ullmann’s Encyclopedia of Industrial Chemistry; VCH Verlagsgesellschaft mbH: Weinheim, Germany, 1990; Vol. A10.

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Received for review July 26, 1999 Revised manuscript received October 18, 1999 Accepted October 19, 1999 IE9905512