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792
Mattisson, M. F.; Legendre, K. A. Determination of the Carboxyl Content of Oxidized Starches. Anal. Chem. 1962,24,1942-1944. Mima, S.; Miya, M.; Iwamoto, R; Yoehikawa, S.Highly Deacetylated Chitosan and its Properties. J . Appl. Polym. Sci. 1983, 28, 1909-1917. Otey, F. H.; Westhoff, R. P.; Doane, W. M. Starch-Based Blown Films. Znd. Eng. Chem. Prod. Res. Deu. 1980,19, 592-595. Puchtler, H.; Meloan, S. N. On Schiff's Baser, y d Aldehyde-Fuchsin: A Review from H. Schiff to R. D. Lillie. Htstochemistry 1981,72, 321-332. Samuelson, 0.;Soderholm, I. Determination of Carbonyl Groups in Cellulose by the Hydrazine Method. Suen. Papperstidn. 1963,66, 833-838. Stepto, R. F. T.; Tomka, I. Injection Moulding of Natural Hydrophilic Polymers in the Presence of Water. Chimia 1987,41 (3), 76-81. Yoshihara, K.; Hosokawa, J.; Nishiyama, M.; Kubo, T. Isolation and Identification of a Chitoean Degrading Bacterium belonging to the Genus Pseudomonas and the Chitosanase Production by the Isolate. Agric. Biol. Chem. 1990, 54 (12), 3341-3343.
Literature Cited Conant, J. B.; Bartlett, P. D. A Quantitative Study of Semicarbazone Formation. J . Am. Chem. SOC.1932,54,2881-2899. Fukui, Y . Microfibrillated Cellulose. Ann. High Perform. Pap. SOC., Japan 1985,24, 5-12. Hirano, S. A facile method for the preparation of novel membranes from N-acyl- and N-arylidene-chitosan gels. Agric. Biol. Chem. 1978,42, 1939-1940. Hirano, S.; Matsuda, N.; Miura, 0.; Tanaka, T. N-methylenechitosan gels, and some of their properties as media for gel chromatography. Carbohydr. Res. 1979, 71,344-348. Hosokawa, J. Outline of Biodegradable Plastics and their Application for Packaging. Food Packag. 1990, 1990-2, 38-42. Hosokawa, J.; Kubo, T. Color Reversion of Ozone-bleached Kraft Pulp VII. Mokuzai Cakkaishi 1987,33, 660-666. Hosokawa, J.; Nishiyama, M.; Yoshihara, K.; Kubo, T. Biodegradable Film Derived from Chitosan and Homogenized Cellulose. Znd. Eng. Chem. Res. 1990,29, 800-805. Kunioka, M.; Kawaguchi, Y.; Doi, Y. Production of biodegradable copolyestera of 3-hydroxybutyrate and 4-hydroxybutyrate by Alcaligenes eutrophus. Appl. Microbiol. Biotechnol. 1989, 30, 569-573.
Received for review July 3, 1990 Accepted November 13, 1990
Reaction Pathways in Lubricant Degradation. 1. Analytical Characterization of n -Hexadecane Autoxidation Products Steven Blaine and Phillip E. Savage* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136
A new protocol is described for the quantitative analysis of mixtures of hydroperoxides, carboxylic acids, aldehydes, ketones, esters, and alcohols. Such complex mixtures arise during the autoxidation of lubricating oils and other hydrocarbons. The analytical protocol uses a novel combination of iodometric and potentiometric acid-base titrations and 'H and 13C NMR spectroscopies. The techniques described were developed so that they would be suitable for the analysis of hydrocarbon samples that have undergone extensive oxidative degradation. The information obtained from these complementary analytical methods results in a set of six simultaneous algebraic equations that contains the six functional group concentrations as unknowns. Solution of these equations then provides the desired concentrations. The average difference between experimental and actual concentrations was less than 0.1 M for all compounds except aldehydes. The average difference for aldehydes was 0.5 M. This analytical scheme is a useful tool for resolving the reaction pathways responsible for hydrocarbon autoxidation.
Introduction Oxidation is the primary means of lubricant degradation (Fenske et al., 1941; Korcek et al., 1986: Gunsel et al., 1988; Naidu et al., 1984,1986),and this realization has motivated much research into the chemistry of lubricant oxidation. Lubricating oils are multicomponent mixtures, however, and their complexity frustrates resolution of the controlling reaction fundamentals in experimental studies. Therefore, n-paraffins, which are simpler yet chemically relevant compounds, are frequently used as model reactants for oxidation studies. Previous studies of paraffin oxidation at mild conditions have led to the resolution of the reaction pathways, kinetics, and mechanisms for the initial stages of oxidation (e.g., Garcia-Ochoa et al., 1989; Lee et al., 1987; Brown and Fish, 1969; Van Sickle, 1972; Van Sickle et al., 1973; Suresh et al., 1988; Mill et al., 1972; Korcek et al., 1972; Jensen et al., 1979, 1981; Reddy et al., 1988; Hazlett et al., 1977). Under more severe reaction conditions, however, hydrocarbon oxidation is considerably more complex than it is in its initial stages because a larger number of reaction products form and new reaction pathways appear. One of these new pathways involves reactions that can produce 0888-5885/91/2630-0792$02.50/0
high molecular weight material (Naidu et al., 1986). These high molecular weight oxidation products lead to a deleterious increase in lubricant viscosity and eventually to the formation of insoluble material that can deposit on the lubricated surfaces. Because the later stages of oxidation are more complex than the initial stages, the controlling reaction pathways are more difficult to resolve. Nevertheless, Naidu et al. (1986) were able to develop a model for lubricant oxidation that accounts for the formation of high molecular weight and deposited products. This model lumped together all oxidation products having similar molecular weights and used first-order kinetics to account for transformations between the three different molecular weight lumps. Although the model adequately described data from thin film oil oxidation tests, more advanced models could be developed by incorporating chemical, rather than solely physical, information. Including chemical information, of course, requires a means of obtaining such information (i.e., methods of analysis for the chemical constituents in the complex mixtures of oxidation products). Identifying and quantifying the amount of each individual product from paraffin oxidation at severe conditions would be an in@ 1991 American Chemical
Society
Ind. Eng. Chem. Res., Vol. 30,No. 4, 1991 793 0
0
II
R-C-R
Ketone
1 I
R - C - OH
Carboxylic Acid
R-OH
Alcohol
R-OOH
Hydroperoxide
0 II
R-C-H
Aldehyde
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Ester
(R, R denote hydrocarbon groups)
Figure 1. Oxygen-containing functional groups present in hydrocarbon autoxidation products.
tractable analytical problem. Grouping compounds together by functional group, however, retains the key chemical features of each product and at the same time greatly reduces the scope of the analytical problem. The foregoing consideration motivated the work reported herein, which consisted of developing a scheme for quantitatively analyzing paraffin oxidation products according to their functional groups. This represents the first step in our efforts to resolve the reaction pathways for lubricant oxidation under severe conditions. Although portions of the analytical problem have been previously addressed (Brown et al., 1967; Boss et al., 1973), the literature did not provide a unified protocol for the functional group analysis of paraffin oxidation products obtained at severe conditions.
Development of Analytical Scheme Figure 1shows the types of oxygen-containing functional groups formed during liquid-phase hydrocarbon oxidation reactions. These functional groups include hydroperoxides, carboxylic acids, ketones, esters, alcohols, and aldehydes (Jensen et al., 1979, 1981; Garcia-Ochoa et al., 1989; Hombek et al., 1989; Hamilton et al., 1980; Boss and Hazlett, 1969: Hazlett et al., 1977; Brown and Fish, 1969; Reddy et al., 1988; Mill et al., 1972). Thus, our goal was to develop an analytical protocol that is capable of quantifying the concentrations of these functional groups in a complex mixture. We examined a large number of different techniques and approaches for functional group analysis before adopting the protocol reported in this paper. Techniques that we explored but eventually decided to exclude from our analytical scheme included enrichment of oxygenated products by sample elution over silica gel (Jensen et al., 1979), wet chemical methods for the determination of aldehydes (Oles and Siggia, 1974) and esters (Boss et al., 1973), and infrared spectroscopy for the quantitative determination of carbonyl-containing compounds. We found oxygenated product enrichment unnecessary because the samples of severely oxidized nhexadecane that we desired to analyze had high concentrations of these products. The wet chemical methods were discarded because they were unreliable for analyses in complex mixtures. Infrared spectroscopy, however, proved to be a reliable and useful tool, but we found that employing this technique was not necessary to meet our objectives. The sections that follow discuss in detail the methods that we did adopt for our analytical protocol. Titrimetric Methods. We initially selected the iodometric titration procedure (Hercules method I) of Mair and Graupner (1964) to quantify the hydroperoxide concentration. This widely used method reduces hydroperoxides to alcohols with iodide ion, and the reaction quantitatively liberates Is-. The concentration of Is- can then be determined by titrating with a standard thiosulfate solution, and the titration end point is detected colorimetrically. This method worked well for mildly oxidized
hexadecane samples, which were colorless, but the yellow to brown color of the more heavily oxidized samples prohibited visual detection of the colorimetric end point. Therefore, we finally selected the amperometric titration procedure of Habeeb et al. (1987),which is identical with the Mair and Graupner (1964) hydroperoxide analysis except for the method of end point detection, for the analysis of all samples. The hydroperoxide determinations were performed by using a Metrohm 670 titroprocessorand a Metrohm E585 polarizer. The apparatus for this titration includes a double platinum electrode connected to a polarizer that maintains a potential difference across the electrode. The titroprocessor controls the potential difference such that a constant current is maintained throughout the titration. The titration end point is detected as the titrant volume at which the first derivative of the potential difference with respect to the titrant volume is a maximum. We also selected a titrimetric method to quantify the concentration of carboxylic acids. We performed these titrations in a specially designed cell consisting of a 150-mL beaker with two side arms. One side arm held a combination pH electrode (Fisher Scientific) and maintained it securely off the cell bottom to permit stirring of the solution with a magnetic stir bar. The other side arm was a gas inlet port through which argon flowed during the titration. Blanketing the headspace in the titration cell with argon removed carbon dioxide from the cell and thereby precluded the formation of bicarbonates. A Fisher Scientific Accumet pH meter 910 displayed the solution pH, which was recorded as a function of titrant volume. The inflection point on the pH versus volume titrant curve provided the titration end point. The procedure for carboxylic acid determination involved dissolving an aliquot of oxidate in 70 mL of a toluene/isopropanol/water(0.500/0.495/0.005 v/v) solvent. Adding excess base (potassium hydroxide) to this mixture neutralized the carboxylic acids present and provided an alkaline pH. Back-titrating with a standard hydrochloric acid solution and determining the difference between the volume of acid required to titrate the sample and that required to titrate a blank sample permitted calculation of the quantity of carboxylic acids in the sample. This titration procedure is identical with that used by Jensen et al. (1979) for carboxylic acid determination with the exception of a change in solvent. The toluene/water (0.53/0.47 v/v) solvent used by Jensen et al. (1979) gave erratic pH readings during our titrations of oxidized hexadecane. This was likely due to incomplete dissolution of our severely oxidized hexadecane samples in the toluene/water solvent, which was used by Jensen et al. (1979) for the analysis of only mildly oxidized hexadecane samples. We selected a toluene/isopropanol/water(0.500/0.495/ 0.005 v/v) solvent, the solvent used in the ASTM test D 664, as an alternative, and the hydrochloric acid and potassium hydroxide reagents were prepared in isopropyl alcohol instead of in water. With these changes in solvent, the procedure of Jensen et al. (1979) gave pH readings that were stable and led to reproducible results. Spectroscopic Methods. I3C and 'H nuclear magnetic resonance (NMR) are the spectroscopic methods used for the analysis. NMR spectroscopy exploits the different chemical environments that exist for carbon atoms within different types of organic compounds. For example, the carbonyl carbons in ketones and aldehydes appear at chemical shifts of 185-220 ppm relative to tetramethylsilane (TMS), and carbonyl carbon atoms in carboxylic
794 Ind. Eng. Chem. Res., Vol. 30, No. 4, 1991
acids and esters exhibit chemical shifts of 160-185 ppm (Cooper, 1980). Carbonyl carbon atoms in the latter compounds, which are also singly bonded to another oxygen atom, appear upfield of the carbonyl carbons in the former cQmpounds, which are attached to only hydrocarbon moieties. A thud region of interest in the 13CNMR spectrum appears at 45-83 ppm relative to TMS (Cooper, 1980), where carbon atoms singly bonded to an oxygen atom (C-0)resonate. The types of compounds giving rise to signals in this region include alcohols, esters, and hydroperoxides. We prepared the samples for 13CNMR analysis by first dissolving them in dibromomethane-d2. A small amount of benzene, at known concentration, was then added to each sample to provide an internal standard for determining the concentrations of oxygen-containing functional groups in the sample. Benzene is a good internal standard because none of the hexadecane autoxidation products appear in the aromatic region of the 13C NMR spectrum (110-133 ppm) and because all six carbon atoms in benzene are equivalent and they give a single peak in the 13CNMR spectrum. 13C NMR analyses were performed on a Bruker 360MHz instrument using a broad-band probe set to a radio frequency of 90.6 MHz. We eliminated the nuclear Overhauser effect (NOE) and obtained quantitative 13CNMR spectra by using inverse-gated decoupling,adding the spin relaxation agent chromium(II1) acetylacetonate (Cr(C,H702)3)to the sample at approximately 0.05-0.1 M concentration and allowing a 3-s delay time between pulses. In order to achieve an adequate signal to noise ratio, we collected at least 1000 scans. We integrated all of the peaks in the regions of interest in the spectrum and determined the concentrations of oxygenated products by comparing the area of the internal standard peak to the areas of the other peaks in the spectrum. The total concentrations of compounds within each of the three spectral regions were calculated from the sum of the integrated peak areas in each region by using eq 1-3, where Ci = concentration of species i, Ai = inte-
The preparation of the 'H NMR samples was identical with that of the 13C NMR samples except that the spin relaxation agent was omitted. 'H NMR spectra were obtained by using the same Bruker 360-MHz instrument as described for our 13C NMR analyses, using a radio frequency of 360.1 MHz. An adequate signal to noise ratio was provided with 50 scans. We determined the concentration of aldehyde present in the oxidation products by comparing the integrated area of peaks in the region of 9-10 ppm to the integrated area of the internal standard peak. The aldehyde concentration was calculated by using eq 4.
Chydroperoxide + Calcohol + Cester =
T1 and T2 are the concentrations determined by the titrimetric methods for hydroperoxides and carboxylic acids, respectively, and the NMRi terms are as defined previously.
(3) grated area of peak i, or total integrated area of peaks within spectral region, VNMR = volume of NMR sample (oxidate + solvent + benzene), Vo = volume of oxidate added to NMR sample tube, and NMRi = concentration sum for equation i obtained from NMR analysis. 'H NMR was the second spectroscopic technique we employed, and it exploits the different chemical environments that exist for hydrogen atoms within different organic compounds. Of particular interest in our analytical protocol is the resonance of the aldehydic hydrogen at approximately 8-10 ppm relative to TMS. No other protons likely to be present in the reaction products appear in this region of the spectrum, so 'H NMR analysis allows the determination of aldehydes separately from that of other carbonyl-containing compounds (Gordon and Ford, 1972).
Calculating Functional Group Concentrations. Information obtained from the titrimetric and spectroscopic analyses must be combined to calculate individual functional group concentrations. Of the six functional group concentrations we sought to determine, two (hydroperoxides and carboxylic acids) were determined independently by using the titrimetric methods, and one (aldehydes) was determined independently by using the 'H NMR analysis. The remaining three functional group concentrations can be determined via algebraic manipulations of eqs 1-3, which were derived from the 13CNMR analysis. The resulting set of equations that can be used to calculate the concentrations of the six different oxygen-containing functional groups is listed below. Chydroperoxide Ccarboxylicacid
Caldehyde
=
Tl
(5)
T2
(6)
= NMR4
(7)
Ckebne= NMR3 - NMR4 Calcohol= NMRl - NMR2 - Ti Cesbr= NMR2 - T2
(8)
+ T2
(9) (10)
Assessment of Analytical Methods To assess the reliability of our analytical protocol, we prepared 35 model mixtures containing known concentrations of different oxygen-containing compounds in hexadecane, and we analyzed these mixtures using the titrimetric and spectroscopic methods discussed above. The model mixtures contained heptaldehyde (95% ), 2nonanone (99+%), ethyl caprate (99+%), octanoic acid (99.5+%) or butyric acid (99+%), 3-octanol (99%),vinyl 2-ethylhexanoate (99%), e-caprolactone (99+% 1, y-butyrolactone (99+%), and tert-butyl hydroperoxide (90% ), where the number in parentheses is the nominal purity. All of these compounds were obtained from Aldrich Chemical Co. and used as received. The model mixtures contained hydroperoxide concentrations ranging from 0.015-1.18 M, carboxylic acid concentrations ranging from 0.039-8.65 M, ester concentrations (which included vinyl esters and lactones) ranging from 0.032-1.47 M, alcohol concentrations ranging from 0.056-1.08 M, ketone concentrations ranging from 0.013-2.04 M, and aldehyde concentrations ranging from 0.099-1.83 M. The model mixtures were stored in a freezer at -20 "C and were thawed prior to analysis.
Ind. Eng. Chem Res., Vol. 30, No. 4, 1991 796
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8
/
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ACTUAL CONCENTRATION (M)
Figure 3. Comparison of actual and experimental concentrations of alcohols + hydroperoxides + esters in the model mixtures, as determined by 'BC NMR analysis.
'9c NMR Analysis. The '3c NMR analysis yields the concentrations of the groups of oxidation products noted in eqs 1-3. Figure 2 displays a representative 13C NMR spectrum of one of the model mixtures, and it identifies the functional groups that appear in the three different regions. We determined these concentration s u m s for the model mixtures and obtained a mean error (i.e., the mean of the absolute values of the differences between the calculatsd and actual concentrations) of 0.10 M for alcohols + hydroperoxides + esters (NMR,), a mean error of 0.066 M for carboxylic acids + esters ,"1( and a mean error of 0.54 M for ketones + aldehydes (NMR3). Figure 3 displays a plot of the experimentally determined concentrations versus the actual concentrations in the model mixtures for NMR,. A line with slope equal to unity is included in this plot, and if the 13C NMR analyses were perfectly accurate, all of the data would fall on this line. Thus, a measure of the error for each concentration measurement is the vertical distance between the data point and the line of the slope equal to 1. The mean error was calculated as the average of the individual errors over all samples analyzed. Figure 3 shows that the 13C NMR analyses for alcohols + hydroperoxides + esters were quite
good. Consequently, the resulting mean error of 0.10 M can be taken as indicative of a reliable quantitative analysis. The error for the NMR,determination was much higher than the errors for NMRl and N W , and this prompted an investigation into the source of the error. Since each of the model mixtures contained only one aldehyde and one ketone, it was possible to distinguish the individual aldehyde and ketone peaks in the 13C NMR spectrum, thereby making it possible to quantify each of these functional groups individually. The mean error for the determination of the ketone concentration was found to be only 0.085 M, whereas that for the aldehyde concentration was 0.46 M. This indicated that the NMR8 error was primarily associated with the determination of aldehyde. We next analyzed a set of mixtures containing only one aldehyde and one ketone dissolved in hexadecane. These analyses proved to be quantitative for both ketones and aldehydes, as the mean error for the ketone was 0.023 M and the mean error for the aldehyde was 0.074 M. The dramatic improvement in the aldehyde analysis for a simple binary mixture suggested that one or more of the components in the more complex model mixtures must be interfering with the analysis. A rapid chemical reaction involving the aldehyde and one or more of the other species present in the model mixtures would be one possible explanation. Aldehydes. The 'H NMR analysis yields the concentration of aldehydes present in the oxidation products. Figure 4 displays a representative 'H NMR spectrum of one of the model mixtures, and it identifies the peak due to the aldehydic hydrogen. We determined the aldehyde concentration for the model mixtures and obtained a mean error of 0.51 M. This error is quite high, and it is comparable to the error of 0.46 M obtained from the determination of aldehydes using '9c NMR spectroscopy. The analysis of simple mixtures, however, which contained only one aldehyde and one ketone dissolved in hexadecane, gave a significantly lower mean error (0.11 M) for the aldehyde
796 Ind. Eng. Chem. Res., Vol. 30, No. 4,1991
Ber
ne
Figure 4. 'H NMR spectrum of a model mixture.
determination. This result was consistent with the I3C NMR analysis for aldehydes in a binary mixture, and it confirms the notion that some component present in the more complex model mixtures interfered with the analysis for aldehydes. Although it appears that some species present in the model mixtures interacted with the aldehyde in such a way as to prevent accurate aldehyde determination, a key finding of these analyses is that both 'H and 13C NMR spectroscopies gave essentially the same results for the aldehyde analysis. This is an important result because it implies that combining the 'H and 13C NMR measurements for aldehydes + ketones (NMR3) and aldehydes (NMR,) in eq 8 will yield an accurate determination of the ketone concentration. Ketones. The ketone concentration was calculated by combining results obtained from 'H and 13C NMR methods according to eq 8. The results obtained were quite good, as the mean difference between the true and the calculated ketone concentrations in the model mixtures was only 0.054 M. Hydroperoxides. The hydroperoxide analysis was essentially quantitative for all of the model mixtures. The mean residual error was only 0.022 M, but the experimentally determined hydroperoxide concentrations were often lower than the actual concentrations. This difference can be attributed to the instability of alkyl hydroperoxides, which decompose even at ambient temperatures (Hiatt, 1980). Thus, it is possible that some decomposition occurred while the model mixtures were standing a t room temperature. To check this hypothesis, we analyzed several mixtures as rapidly as possible after thawing (e.g., 10 min) and obtained a smaller residual error than we did for samples that stood at room temperature for a longer time (e.g., 60 min) before analysis. The experimentally determined concentrations for these mixtures were still lower than the actual concentrations, but minimizing the roomtemperature standing time apparently improved the accuracy of the hydroperoxide determination. It is inter-
esting to note that only three of the experimentally determined hydroperoxide concentrations were higher than the actual concentrations, and these were for the three mixtures with the highest actual hydroperoxide concentrations (=1 M). Carboxylic Acids. Analysis of a single carboxylic acid in hexadecane gave a small residual error (
