Ind. Eng. Chem. Res. 1992,31,1807-1810 Abraham, R. J.; Griffiths, L.; Loftus, P. Approaches to Charge Calculatiom in Molecular Mechanics. J. Comput. Chem. 1982,3, 407. Affens, W. A.; Johnson, J. E.; Carhart, H. W. Effect of Chemical Structure on Spontaneous Ignition of Hydrocarbons. J. Chem. Eng. Data 1961, 6,613. American Society for Testing and Materiala. Standard Test Method for Autoignition Temperature of Liquid Petroleum Products; ANSI/ASTM D 21554% American Society for Testing and Materials: Philadelphia, 1976; p 177. Anker, L. A.; Jurs, P. C.; Edwards, P. A. Quantitative StructureRetention Relationship Studies of Odor-Active Compounds with Oxygen-Containing Functional Groups. Anal. Chem. 1990, 62, 2676. Balaban, A. T. Highly Discriminating Distance-Based Topological Index. Chem. Phys. Lett. 1982,89, 399. Burkert, U.; AUinger, N. L. Molecular Mechanics; ACS Monograph 177; American Chemical Society: Washington, DC, 1982. Ciarlet, P. G. Introduction to Numerical Linear Algebra and Optimisation; Cambridge University Press: Cambridge, U.K., 1989; p 11. Design Institute for Physical Property Data (DIPPR). Physical and Thermodynamic Properties of Pure Chemicals: Data Compilation; Daubert, T. E., Danner, R. P., Eda.;Hemisphere Publishing: New York, 1989; Vols. 1-4. Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. AMI: A New General Purpose Quantum Mechanical Molecular Model. J. Am. Chem. SOC. 1985,107,3902. Dunnivant, F. M.; Elzerman, A. W.; Jurs, P. C.; Hasan, M. N. Quantitative Structure-Property Relationships (QSPRS). Prediction of Aqueous Solubilities and Henry's Law Constants for Polychlorinated Biphenyls (PCBs). Submitted for publication, 1992. Egolf, D. S.; Brockett, E. B.; Jurs, P. C. Simulation of Carbon-13 Nuclear Magnetic Resonance Spectra of Methyl-Substituted Norbornan-2-ols. Anal. Chem. 1988,60, 2700. ESDU. Fire Hazard Properties: Flash Points, Flammability Limits and Autoignition Temperatures; ESDU Engineering Sciences Data; Item No. 82030; ESDU International: London, 1982 (amendments 1986); Vol. 8, p 6. Frank, C. E.; Blackham, A. U. Spontaneous Ignition of Organic Compounds. Znd. Eng. Chem. 1952,44 (4), 862. Frank, C. E.; Blackham, A. U. Reaction Processes Leading to the Spontaneous Ignition of Hydrocarbons. Znd. Eng. Chem. 1954, 46 (I),212. Furnival, G. M.; Wilson, R. W. Regressions by Leaps and Bounds. Technometrics 1974, 16, 499. Goldstein, H. Classical Mechanics. Addison-Wesley: Reading, MA, 1950; pp 144-156. Hilado, C. J. How to Predict if Materials Will Burn. Chem. Eng. 1970, 77 (26), 174.
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Hilado, C. J.; Clark, S. W. Autoignition Temperatures of Organic Chemicals. Chem. Eng. 1972, 79 (19), 75. Jure, P. C.; Chou, J. T,; Yuan, M. In Computer-Assisted Drug Design; Olsen, E. C., Christoffersen, R. E., Eds.; ACS Symposium Series 112; American Chemical Society: Washington, DC, 1979; p 103. Jurs, P. C.; Sutton, G. P.; Ranc, M. L. Carbon-13 NMR Spectral Simulation. Anal. Chem. 1989,61,1115A. Kier, L. B. In QSAR: Quantitative Structure-Activity Relationships in Drug Design; FauchBre, J. L., Alan R. Lisa: New York, 1989; p 105. Kier, L. B.; Hall, L. H. Molecular Connectivity in Structure-Activity Analysis; Wiley: New York, 1986. Kirsch, L. J.; Quinn, C. P. Progress Towards a Comprehensive Model of Hydrocarbon Autoignition. J. Chim. Phys. 1985,82 (5), 459. Morley, C. A Fundamentally Based Correlation Between Alkane Structure and Octane Number. Combust. Sci. Technol. 1987,55, 115. Morrison, R. T.; Boyd, R. N. Organic Chemistry, 4th ed.; Allyn and Bacon: Boston, 1983; Chapter 3. National Fire Protection Association. Fire Protection Guide on Hazardous Materials, loth ed.; National Fire Protection Association: Quincy, MA, 1991; p 325M-5. Neter, J.; Wasserman, W.; Kuntner, M. H. Applied Linear Stotistical Models; Irwin: Homewood, IL, 1985. Rousseeuw, P. J.; Leroy, A. M. Robust Regression and Outlier Detection; Wiley: New York, 1987. Snee, R. D. Validation of Regression Models-Methods and Examples. Technometrics 1977, 19, 415. Stanton, D. T.; Juts, P. C. Development and Use of Charged Partial Surface Area Structural Descriptors in Computer-Assisted Quantitative Structure-Property Relationship Studies. Anal. Chem. 1990,62, 2323. Stanton, D. T.; Jurs, P. C. Computer-Assisted Study of the Relationship Between Molecular Structure and Surface Tension of Organic Compounds. Submitted for publication, 1992. Stanton, D. T.; Jurs, P. C.; Hicks, M. G. Computer-Assisted Prediction of Normal Boiling Points of Furans, Tetrahydrofurans, and Thiophenes. J. Chem. Znf. Comput. Sci. 1991,31,303. Stuper, A. J.; Brugger, W. E.; Jurs, P. C. Computer-Assisted Studies of Chemical Structure and Biological Function; Wiley-Interscience: New York, 1979; p 83. Swarta, D. E.; Orchin, M. Spontaneous Ignition Temperature of Hydrocarbons. Ind. Eng. Chem. 1957,49 (3), 432. Weiner, H. Structural Determination of Paraffin Boiling Points. J. Am. Chem. SOC.1947,69, 17.
Received for review October 21, 1991 Revised manuscript received April 7, 1992 Accepted April 27, 1992
Oxidation Products of Methanol in Chlorine Dioxide Production M. Fazlul Hoq,t Bhart Indu,t William R. Ernst,*!+and Leslie T. Gelbaumt School of Chemical Engineering and Research Center for Biotechnology, Georgia Institute of Technology, Atlanta, Georgia 30332
This study provides direct evidence that formaldehyde is formed in the methanol-chlorate process of generating chlorine dioxide. A previous study showed that formic acid is a product of this reaction in a commercial chlorine dioxide generator. Formaldehyde has not been observed a t highly reactive conditions of a commercial generator; however, it was observed in a laboratory reaction under mild oxidative conditions by proton NMR spectroscopy. Formaldehyde is believed to be a short-lived intermediate which reacts rapidly after forming. Carbon dioxide was also detected in a laboratory reaction a t conditions within the range of commercial interest by carbon-13 NMR spectroscopy. Introduction Chlorine dioxide is becoming increasingly important in pulp bleaching and water purification. It is manufactured
* To whom all correspondence should be addressed. t School of
Chemical Engineering.
* Research Center for Biotechnology. 0888-5885/92/2631-1807$03.00/0
by reducing sodium chlorate with methanol in sulfuric acid solution (Masschelein, 1979; Norell, 1988). Although this process is important commercially, no studies of the reaction mechanism have been reported. This work focuses on identifying carbon containing intermediates and byproducts of this process by NMR spectroscopy. A likely first step of the process involves oxidation of 0 1992 American Chemical Society
1808 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992
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methanol to formaldehyde HC103 + CH30H HCHO + HzO + HCIOz (1) followed by analogous steps oxidizing formaldhyde to formic acid and formic acid to carbon dioxide. Formic acid has recently been found in product streams of several commercial chlorine dioxide plants, by a proton NMR technique (Hoq et al., 1991a). The technique did not d e w any formaldehyde in these streams and is not applicable in detecting carbon dioxide. Even though formaldehyde was not detected, we suspect that it forms by eq 1and rapidly reacts in the highly oxidizing environment of a commercial reactor. Likewise, we suspect that carbon dioxide forms at the conditions of a commercial reactor. In related studies, we have shown that the rate of oxidation of formaldehyde is much greater than that of formic acid (Hoq et al., 1991b; Indu et al., 1991). We suspect that formaldehyde may occur at low concentrations in a commercial process, undetectable by NMR spectroscopy. The purpose of this study is to determine whether formaldehyde and carbon dioxide are formed in the methanol-chlorate process. The aim is to conduct experiments at conditions which allow formaldehyde and carbon dioxide to accumulate sufficiently in solution so they can be detected by proton and carbon-13 NMR spectroscopies, respectively.
10
1
I
0
500
I
1000
I
1500
Time, sec Figure 1. Concentration-time profiles for chloroue acid and chlorine dioxide during a batch reaction experiment at 25 "C. Solution concentrations include 0.23 M methanol, 0.80 M sodium chlorate, and 3.3 M sulfuric acid.
lapping signals at each wavelength (Sjostrom and Tormund, 1978).
Experimental Section
Results and Discussion a. Formaldehyde. Figure 1shows concentration-time
Fisher certified materials were used: formaldehyde solution (37 wt % containing 10-15 vol % methanol); hexamethylenetetramine (crystalline, 99 wt %); formic acid solution (88 wt %); sodium chlorate (99.9 wt %); and sulfuric acid (37 N). A 0.25 M 13Clabeled sodium carbonate solution was prepared from solid Na213C03(99.1 at. % 13C;Isotec, Inc). A solution of 13C02in sulfuric acid was generated by injecting 0.5 mL of the labeled sodium carbonate solution into a 4.5-mL solution of 2.8 M sulfuric acid. Solutions of 0.05 M 13C labeled methanol were prepared by diluting neat methanol (99.1 at. % 13C;Isotech). Proton NMR spectra were obtained using a broad band switchable probe in a Varian XL-400 Fourier transform spectrometer operating at 399.915 MHz. To ensure good signal to noise ratio and sufficient relaxation time, multiple scans (64 for most) were obtained using a pulse angle of 25' and a 4-5 pulse cycle. The large water signal was suppreased by selective saturation during a 2-5 delay time prior to acquisition. Carbon-13 spectra were obtained using a 10-mm broad band probe tuned to 100.570 MHz. To ensure semiquantitative results, a 40" pulse angle was used with a 1.0-s relaxation delay giving a 1.8s pulse cycle. WALTZ decoupling was used only during the acquisition time to suppress any nuclear Overhauser effect. In proton NMR experiments, cyclohexane in carbon tetrachloride was used as a reference standard. A 10% (v/v) solution of cyclohexane in carbon tetrachloride was placed inside a capillary tube inserted co-axially into the NMR tube. The cyclohexane signal was set at 1.38 ppm with reference to TMS. In '3c experiments, NMR signals were referred to methanol at 49 ppm. Results of a batch reaction experiment shown in Figure 1were obtained at 25 "C by combining 0.23 M methanol, 0.80 M sodium chlorate, and 3.3 M sulfuric acid in a 250-mL stirred flask (200-mL solution) and monitoring UV signals at 250, 322, and 357 nm, which correspond respectively to absorbances for chlorous acid (c = 140 cm-' M-l), chlorine (c = 75 cm-' M-l), and chlorine dioxide (E = 1171 cm-' M-l). Concentrations of these species were found by a matrix procedure which corrected for over-
profiles of chlorous acid and chlorine dioxide monitored during a batch reaction experiment in which we combined methanol, sodium chlorate, and sulfuric acid in a stirred flask. In Figure 1,chlorous acid forms rapidly prior to the formation of chlorine dioxide, approaching a nearsteadystate value (ca.0.0002 M) with increasing time. The chlorine dioxide rate increases with time, indicated by the change of slope of the chlorine dioxide concentration-time profile, starting at zero and approaching a constant value. These observations provide support for our proposed first step in the chlorine dioxide forming process, eq 1,lending indirect evidence that formaldehyde is the initial oxidation product of methanol in this process. Figure 2 shows a proton NMR spectrum of a reaction mixture containing 1.2 M methanol, 0.15 M sodium chlorate, and 4.5 M sulfuric acid, taken 18 min after the reactants were mixed and reacted at -5 "C. The signal at 8.1 ppm was assigned as formic acid by comparison with a spectrum of formic acid in 4.5 M sulfuric acid. Signals in the range 4.4-4.6 ppm were assigned to derivatives of formaldhyde by comparison with the spectra of both commercial formaldehyde and formaldehyde generated from hexamethylenetetramine, both in 4.5 M sulfuric acid. The signal at 4.5 ppm most likely corresponds to dihydroxymethane. The other signals within this range correspond to oligomeric hydrates of formaldehyde (Gorrie et al., 1973). None of the spectra contained a signal between 9 and 10 ppm, which is the range aldehydic protons would appear (Silverstein and Bassler, 1967). This is not an unexpected result since, in aqueous solution, formaldehyde undergoes nearly complete equilibrium conversion (K = ca. 2000) to dihydroxymethane (Lowry and Richardson, 1987; Bell and Evans, 1966). Any carbonic acid, formed from carbon dioxide, would not be observed since its signal would merge with the large water signal in the proton spectrum. The temperature and reactant concentrations of the NMR experiment are quite different from those found in a commercial chlorine dioxide generator, where we found no formaldehyde. Commercial generators typically operate at greater than 50 "C, with high clorate to methanol ratio.
Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 1809
Water
CYclohexane
Methanol
4.8
4.6
4.4
4 . 2 PPM
Formic Acid
&
?-
7
6
5
u 1111, I I I I
4
I I / I ,I I I I
2
I1 I I , ,
,
I ,
1 PPM
0
Figure 2. NMR scan of methanol-chlorate-sulfuric acid solution after reacting for 18 min at -5 "C. Initial concentrations include 1.22 M methanol, 0.15 M sodium chlorate, and 4.5 M sulfuric acid. Signals are referred to cyclohexane at 1.38 ppm.
We explored conditions which more closely represent those in a commercial generator but were unable to detect formaldehyde. At conditions of our experiment, low temperature and high methanol to chlorate ratio, further reaction of formaldehyde is less favored. b. Carbon Dioxide. We measured the carbon-13 spectrum of a solution of 13Clabeled sodium carbonate in 2.8 M sulfuric acid, observing a chemical shift of 124 ppm for carbon dioxide which evolved from solution. This signal was referenced to a methanol signal at 49 ppm. We measured the carbon-13 NMR spectra of methanol-sodium chlorate-sulfuric acid solutions, reacted for various lengths of time. Although low solubility of carbon dioxide in the strongly acidic solutions made it impossible to quantitatively monitor the formation of carbon dioxide or to measure concentration-time trends, we were able to verify qualitatively that carbon dioxide formed by observing that the signal at 124 ppm was present. In one typical experiment, we observed the 124 ppm signal for a solution of 2.5 M sulfuric acid, 2 M sodium chlorate, and 0.05 M 13Clabeled methanol after it had been heated at 75 "C for 15 min. These reactant concentrations and this temperature fall within the range of commercial operating conditions. At ambient conditions, we also detected carbon dioxide; however, longer reaction times and higher acid concentrations were required in order to produce detectable concentrations. For example, we observed carbon dioxide in a solution of 4.5 M sulfuric acid, 0.15 M sodium chlorate, and 0.25 M '3c labeled methanol only after it had reacted for 7 days at ambient conditions in a sealed NMR tube. By adding chlorine to the reaction solution, we were able to reduce the reaction time required to produce detectable concentrations of carbon dioxide. There are several possible reactions which might produce carbon dioxide in a chlorine dioxide reactor. 2H+ + 2C103- + HCOOH 2C1O2 + 2Hz0 + COz (2)
-
We showed earlier (Hoq et al., 1991b) that when chlorine is present, carbon dioxide can form by the reaction Clz
+ HCOOH
-
2H+ + 2C1-
+ COZ
(3)
Equation 2 is favored at high acid concentrations typically found in a commercial generator. Equation 3 becomes more important at low acid concentrations (Hoq et al., 1991b) . Conclusions We have demonstrated that formaldehyde and carbon dioxide form when chlorine dioxide is generated by the reaction of methanol and chlorate ions. Formaldehyde is not detectable in commercial chlorine dioxide process solutions because the highly reactive environment rapidly oxidizes any formaldehyde that forms and prevents it from accumulating to levels detectable by NMR spectroscopy. Formaldehyde is detectable in a laboratory reaction solution at mild oxidative conditions, including ambient temperature and low chlorate to methanol ratio. Carbon dioxide was detected at conditions in the range of commercial operation but not under the mild conditions used to detect formaldehyde. Adding a low concentration of chlorine to the reaction solution increased the production of carbon dioxide at ambient temperature. Acknowledgment We gratefully acknowledge financial support from Eka Nobel, Inc. We thank Mr. John Winters and Mr. Joel Tenney for helpful discussions regarding commercial chlorine dioxide processes. Registry No. CH,OH, 67-56-1; C103-, 14866-68-3; CIOz, 10049-04-4.
I n d . Eng. Chem. Res. 1992,31,1810-1813
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Literature Cited Bell, R. P.; Evans, P. G. Kinetics of the Dehydration of Methylene (London),Ser A. 1966, Glycol in Aqueous Solution. h o c . R.SOC. 291, 297. Gorrie, T. M.; Raman, S. K.; Rouette, H. K.; Zollinger, H. NMRInvestigation of the Formaldhyde Addition and Oligomerisation Equilibria in the System Formaldehyde/ Water/Acetic Acid/ Hydrochloric Acid. Helv. Chim. Acta 1973,56,(Fasc. 1 (a)),175. Hoq, M. F.; Ernst, W. R.;Gelbaum, L. T.NMR Procedure for Determining Methanol and Formic Acid in Chlorine Dioxide Plant Solutions. TAPPI J. 1991a, 74,217. Hoq, M. F.; Indu, B.; Emst, W. R.;Neumann, H. M. Kinetics of the Reaction of Chlorine with Formic Acid in Aqueous Sulfuric Acid. J. Phys. Chem. 1991b,95,681. Indu, B.;Hoq, M. F.; Ernst, W. R.; Neumann, H. M. Kinetics of the Reaction of Chlorine with Formaldehyde in Aqueous Sulfuric
Acid. Znd. Eng. Chem. Res. 1991,30,1077. Lowry, T. H.; Richardson, K. S. Reactions of Carbonyl Compounds. Mechaniem and Theory in Organic Chemistry; Harper and Row: New York, 1987;pp 661-664. Masechelein, W. J. Industrial Synthesis. Chlorine Dioxide; Ann Arbor Science: Ann Arbor, MI, 1979; p 120. Norell, M. US. Patent No. 4,770,868,1988. Silverstein, R. M.; Baesler, G. C. Nuclear Magnetic Resonance Spectroscopy. Spectrometric Identification of Organic Compounds. Wiley: New York, 1967;p 118. Sjostrom, L.; Tormund, D. Determination of Inorganic Chlorine Compounds and Total Chlorine in Spent Bleaching Liquors.
Sven. Papperstidn. 1978,4,114. Received for review October 25, 1991 Revised manuscript received March 30, 1992 Accepted April 22, 1992
Oxidation and Weight Loss Characteristics of Commercial Phosphate Esters Sundeep G. Shankwalkar* and Douglas G. Placek Process Additives Division, FMC Corporation, P.O. Box 8, Princeton, New Jersey 08543
Neutral phosphate ester compounds find wide application as plasticizers and flame-retardant additives in the polymer industry. They are also used extensively in the lubrication industry as fire-resistant functional fluids and as lubricant additives. In these industrial applications, the oxidative stability and volatility of the phosphate ester is of critical importance. This paper evaluates the oxidative stability and relative volatility of several commercially available phosphate esters. The techniques of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) have been used to evaluate these thermal properties.
Introduction Phosphate esters find use as flame retardants in polymeric and engineering plastic applications. They are used in place of flammable organic plasticizers when fire-resistance properties are desired. In the lubrication field, phosphate esters are used in high-temperature applications which warrant the use of fire-resistant fluids. Typically, they are used as hydraulic fluids in applications where molten metal, or open flames, preclude the use of hydrocarbon fluids. Phosphate esters are also used in a variety of lubricant formulations as antiwear additives. Any application at a high temperature in the presence of air could affect the thermal and oxidative stability of a phosphate ester. Under these conditions, oxidation predominates thermal decomposition (Gunsel et al., 1988), and these effects along with volatility can severely impact on the performance and useful life of the phosphate ester. Oxidative stability can be characterized at room temperature or under isothermal conditions; however, for repeatability and ease of operation, it should be measured at higher temperatures and under dynamic conditions (Koski and Saarela, 1982). This study evaluates the onset of oxidation and the weight loss characteristia of commercially available triaryl, trialkyl, and alkyl-aryl phosphate ester products. A discussion of the relationship between oxidative stability and structure of the phosphate ester is also included in this paper.
Experimental Technique All DSC measurements for determining the onset of oxidation were made using a Mettler DSC 25. In this technique (ASTM E537), the heat exchanged with the sample over a defined temperature range is measured as 0888-5885/92/2631-1810$03.00/0
the difference between the heat flow to the sample and that to the reference cell. This difference in heat flow is recorded as an exothermic or endothermic peak on the DSC scan, which relates to a physical or chemical change taking place in the material. Weight loss measurements were made using a Mettler TG 50 system. In this technique (ASTM D3850), the change in the mass of the sample is measured over a defiied temperature range. The sample weight loss can be related to volatility, or decomposition, taking place in the material. All measurements were made between 30 and 400 OC, at a heating rate of 10 OC/min with the sample enclosed in a aluminum container with a perforated (one hole) lid. Initial testing of a few phosphate ester samples in air and oxygen did not show any significant differences in their thermal profiles. For the purpose of this study, all DSC/TGA measurements were made in oxygen at 50 mL/min. Data acquisition and analysis were carried out using a Mettler TA 72 thermal analysis software system. The data presented in this paper represent the average values obtained after evaluating three to five different commercial samples, of similar composition. Sixteen different types of commercial phosphate esters were selected for testing. They are categorized in Table I. The basic molecular structure of the phosphate esters evaluated in this study are shown in Figure 1. A more detailed review of the composition of commercial phosphate ester products is covered in the work by Marino and Placek (1992).
Results and Discussion A summary of the onset of oxidation and weight loss data, as determined by DSC/TGA measurements in oxygen, is presented in Table 11. 0 1992 American Chemical Society
