Hydrocarbon Oxidation Reactions by High Pressure Differential Thermal Analysis Walter R. May' and lewis Bsharah Corporate Research Laboratories, Petrolite Corp., St. Louis, Mo. 63119 The oxidation of n-hexane, cyclohexane, benzene, toluene, xylene, and ASTM isooctane under oxygen at 500 psi was evaluated by a high pressure differential thermal analysis (DTA) technique. The work on isooctane was extended to define the effect of oxygen concentration and pressure on the oxidation reaction. The oxidation of isoctane was observed in the presence of a bisphenol, an amine, and a phosphite antioxidant. The effects of nine metals and their interactions with the antioxidants on the oxidation of isooctane were included. The high pressure DTA technique i s a useful tool for evaluating the oxidation characteristics of low-boiling hydrocarbons.
M o s t of the studies on the oxidation of hydrocarbons involve measuring the oxygen uptake of the material being oxidized. Hopkins (1967) demonstrated the usefulness of differential thermal analysis (DTA) for evaluating the oxidation characteristics of n-hexatriacontane, n-tetracosane, 11-tricosene, and 17-pentatriacontene. This technique is rapid, accurate, and simple to use. However, he was limited to high-boiling compounds because of his restriction to ambient pressure. We have made several studies of the oxidation of rubber by a high-pressure DTA technique (May and Bsharah, 1969, 1970; May et al., 1968). The application of this method to the study of the oxidation of low-boiling hydrocarbons was obvious. We have obtained DTA thermograms of n-hexane, cyclohexane, benzene, toluene, xylene, and isooctane under oxygen at varying pressures. The study of isooctane was extended to include evaluation of the effects of a bisphenol, an amine. and a phosphite antioxidant on the oxidation characteristics of the material. The effects of nine metals and their interactions with the antioxidants on the oxidation of the isooctane were included in the study. Experimental
hlaterials. The isooctane sample was ASTM grade 2,2,4trimethylpentane obtained from the Phillips Petroleum Co. The other hydrocarbons were reagent grade materials obtained from the Fisher Scientific Co. The antioxidants are commercially available materials, of the quality used in the oil and rubber industry. The 2,2'-methylenebis(6-tert-butylcresol)was obtained from the American Cyanamid Co.; the n-cyclohexyl-n'-phenyl-p-phenylenediamine and trisnonylphenyl phosphite came from the Uniroyal Co. The calcium. cerium. cobalt, copper, lead. manganese, and zinc naphthenates were obtained from the Shepherd Chemical Co. The ferric and vanadyl naphthenates were obtained from K and K Laboratories, Inc. All materials were used without further purification. Preparation of Samples. The samples for DTA analysis were prepared by admixing appropriate quantities of antioxidant and metal with the various hydrocarbons. Sufficient metal naphthenate was added so that the metal content in each sample was 0 . 1 5 by weight. The antioxidants were added on a 1% basis.
' To whom correspondence should be addressed. 66
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
Equipment. The DTA evaluations were carried out with a Du Pont Model 900 differential thermal analyzer in connection with a high pressure cell, designed and constructed by E. I. du Pont de Nemours and Co., Inc. DTA Curves. The normal procedure for running the Du Pont Model 900 differential thermal analyzer was followed, except for the special requirements of the pressurized cell. Micro-sized samples ( 5 to 10 mg) and a heating rate of 10°C per minute were used for the DTA evaluations. All evaluations were conducted under oxygen at 500 psi, except for isooctane, which was studied under several pressures from ambient to 600 psi. Thermograms were examined for temperatures a t the start of oxidation, beginning of the oxidation peak, and maximum of the peak. Start of oxidation is the point of exothermic deflection from the base line. The beginning of the oxidation peak is the point a t which a strong exothermic deflection occurs, usually several degrees above the start of oxidation. Results
The thermograms obtained from the hydrocarbons studied in this work are presented in Figures 1 and 2. All except benzene had a strong exotherm between 200" and 25OOC. The peak shape varied from very sharp for hexane, cyclohexane, and xylene to rather broad for toluene and isooctane. The benzene did not exhibit an exotherm below 225" C, indicating that it is inert to oxidation under the conditions of the test up to that point. The endotherm which peaked a t 250" C denoted that evaporation had occurred. Figure 3 illustrates a series of thermograms for isooctane in which the oxygen pressure was varied between ambient and 600 psi. In the ambient pressure case, a 200 cc per minute flow rate of oxygen was maintained through the DTA cell. As can be seen, only an evaporation endotherm was observed. At 150 psi, a curve was found which resembled the thermogram for benzene, indicating no oxidation up to the temperature a t which evaporation occurred. At 300 psi, the familar oxidation exotherm was present, and with increasing pressure, it became larger and more distinct. In Figure 4 is found a series of curves for isooctane, in which the oxygen content was varied by diluting with nitrogen. In all cases, a total pressure of 300 psi was
t Figure 1. Thermograms for nhexane, ASTM isooctane, and toluene under oxygen at 500 psi
t a 1 I-
s300 PSI
i
+ Figure 3. Effect of varying oxygen pressure on oxidation peak of ASTM isoctane
. \
3 TEMPERATURE, “C.
TEMPERATURE, *C
t Figure 2. Thermograms for cyclohexane, xylene, and benzene under oxygen at 500 psi
Q
I
E BENZENE
0
50
100
150
200
,250
300
Figure 4. Effect of varying oxygen content at constant pressure on 1 oxidation peak of ASTM isooctane
So
TEMPERATURE, “C.
maintained. The oxygen content was varied between 0 and 100cC in 25% increments. Under pure nitrogen, there was no peak, although after about 150°C there was a drift in the exothermic direction which could have been caused by oxygen absorbed in the material. A distinct exothermic oxidation peak appeared and became more pronounced with increasing oxygen content. This peak shifted to lower temperatures with increasing 0 2 content. The peak for oxidation in 75% oxygen was a few degrees below the one in which pure oxygen was used. We attribute this to experimental error. The thermograms for the isooctane containing various antioxidants and metals were similar to the original isooctane, except that they were displaced either to lower temperatures, indicating promoted oxidation, or to higher temperatures, indicating retarded oxidation. Therefore, rather than reproduce all the thermograms, the pertinent data on these evaluations are given in Table I. All these evaluations were carried out under oxygen a t 300 psi. Our earlier work using high pressure DTA to evaluate the oxidation of rubber indicated that the peak maximum is the most reliable indicator of the oxidation charac-
I 300
200 25.0 T EM P EMTURE “C
loo
teristics of the sample (May and Bsharah, 1969). Therefore, we used the peak maximum to compare the oxidation of the isooctane with the various antioxidantmetal combinations. In Table 11, the metals are ranked according to their effectiveness in promoting oxidation. Iron and manganese Table I. Data for DTA Thermograms of ASTM Isooctane with Antioxidant-Metal Combinations DTA peak maximum, ’ C No Metal
Blank Calcium Cerium Cobalt Copper
Iron Manganese Lead Vanadyl Zinc
antioxidant
Amine
Phenol
Phosphite
225 252 210 182 192 200 201 210 210 210
278 277 252 266 295 223 215 232 260 275
253 262 223 263 226 215 211 262 257 272
259 258 248 272 300 214 198 267 257 263 ~~~~
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
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Table It. Ranking of Metals in Order of Effectiveness in Promoting Oxidation Antioxidant Group
Very effective Effective No effect Retarding
Amine
None
Co, Cu, Fe, Mn Ce, Pb, VO, Zn Blank, Ca
Phenol
Fe, Mn, Pb
Mn,Fe
Ce, Co, VO
Ce, Cu Blank, VO Ca, Zn, Co, Pb
Blank, Ca, Zn cu
are strong promoters in every case. Copper and cobalt, considered strong promoters, ranged from effective to retarding in the presence of various antioxidants. Calcium did not promote oxidation in any case; it was either inert or exhibited an inhibiting effect. Cerium, lead, vanadyl, and zinc ranged between the effective and the retarding categories, and in the case of the amine, lead was found in the very effective category. Discussion
The accepted pathway for the oxidation of hydrocarbons is by the radical chain mechanism proposed by Bolland (1949) and expanded by Bateman (1954). By this mechanism, an initiator caused formation of a free radical which reacts with oxygen to form a peroxide, leading to other free radicals and the oxidation products. The reaction of oxygen with the free radical is extremely rapid and probably accounts for the peak shifts toward lower temperatures observed in Figures 3 and 4 as oxygen concentration is increased. The oxidation peaks observed for the low-boiling hydrocarbons under oxygen a t 300 psi were about 100"C lower than those observed by Hopkins (1967) for the high-boiling hydrocarbons under oxygen a t atmospheric pressure. The evidence discussed above indicates that this was a t least partly caused by the increased concentration of oxygen in the system used for this work. An increasing difficulty in oxidizing the hydrocarbon was noted in passing from a straight-chain compound to branched-chain to aromatic systems. The n-hexane and cyclohexane had peaks a t about 2OO0C, followed by isooctane a t 225" C, xylene a t 235" C, toluene a t 240" C, and no peak for benzene. Hopkins (1967) also noted the inertness of aromatics toward oxidation, comparing pyrene with hexat riacontane. The three antioxidants chosen for this study consisted of a phenolic type, which functions as a free radical trap, a phosphite type. which functions as a peroxide scavenger, and an amine, which can function in both capacities (Barnhart and Newby, 1959). The general mechanism for antioxidant reactions follows:
R. + A H + R H + A * ROO. + AH + ROOH + A * ROO. + A * + ROOA RO. + AH + ROH + A . ROOH + AH -+ harmless fragments As shown in Table I, the amine was the most effective antioxidant, followed by the phosphite and the phenol. The phosphite had a slight edge on the phenol in this application, which is the opposite of observations in rubber systems (May et al., 1968). This implies that interruption of the oxidation reaction by elimination of the peroxide is more important than reaction with the hydrocarbon free radical. 68 Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
Phosphite
Fe, Mn Ce Blank, Ca. VO Co, Cu, Pb, Zn
In earlier work from this laboratory (May and Bsharah, 1970), the effects of various antioxidant-metal combinations on the oxidation of SBR rubber were studied. We have extended this work to the oxidation of isooctane. A metallic ion is expected to promote or retard oxidation by the one-electron transfer mechanism:
M"' + R H + R . + M ' + HM"' + 02 Z (M01)"(MO1)"- + R H + M"- + R . + HOz. ROOH+M"-'+ RO + OH- + M" -
i
The order of catalytic activity of the metals in the absence of an antioxidant in rubber was somewhat different from the order of the same metals observed in the isoctane system. This is not surprising, since the order of the catalytic activity of several metals in sterates (George et al., 1946a,b, 1942), 2,5-dimethylhexane (Wibaut and Strang, 1951), cyclohexene (Banks et al., 1954), and n-heptane (Prizkow and Miller, 1946) was different for each system. However, in all these systems, cobalt, iron, copper, and manganese are effective oxidation catalysts. The catalytic activity of the metals, in the absence of an antioxidant, roughly follows their oxidation potentials. This substantiates the one-electron transfer mechanism cited above. Although there are some deviations in order of catalytic activity vs. the standard oxidation potentials, this is expected, since the potentials are for a water system, whereas the catalysis of the oxidation is in a purely hydrocarbon system. If the antioxidant is to deactivate the metal catalysts so that they do not promote the oxidation reaction, it must stabilize the oxidation state of the metal (Jones, 1964). The antioxidants used are relatively weak metal deactivators, but it was hoped that some interactions could be observed between them and the metal catalysts. There is evidence that it is unlikely that phenolic-type antioxidants can form complexes with metals (May et al., 1967), whereas amines and phosphites do form metal complexes (Duff, 1968a,b; Kruse and Atalia, 1968; Marks et al., 1967; Vol'pin and Kolomnikov, 1966). Iron and manganese were effective catalysts in all cases, indicating very little interaction of the antioxidants with these metals as deactivators. The catalytic properties of calcium, cobalt, lead, the vanadyl ion, and zinc were fairly effectively cancelled by the antioxidant. Calcium appeared as an antioxidant in the case in which a regular antioxidant was not used. Cerium catalyzed oxidation even in the presence of the antioxidants. This indicates that metal complex formation may be occurring in a t least some of the cases studied, for cerium normally coordinates only to oxygen and nitrogen donor atoms present in chelating agents (Jones, 1964). None of the antioxidants used in this work would be very likely to serve as a chelating agent.
Conclusions
The high-pressure DTA technique appears to be a good method for screening antioxidant and metal deactivator activity in low-boiling hydrocarbons. The data obtained in this study conformed to t4e accepted mechanisms for oxidation of hydrocarbons, antioxidant behavior, and the catalysis of oxidation by metallic ions. The small advantage of the phosphite antioxidant over the phenolic antioxidant indicates that elimination of peroxide species is more important than elimination of free radicals in the oxidation of isooctane, whereas the opposite is true in the oxidation of rubber. Interaction was found between the antioxidants and the metals, indicating deactivation of the metals by the antioxidants in some cases. Acknowledgment
The assistance of Linda Y. Waring in the preparation and evaluation of the samples is gratefully acknowledged. Thanks are due Michael J. Michnick for several fruitful discussions of this work. literature Cited
Banks, G. L., Chalk. A. J., Dawson, J. E., Smith, J. F.. Nature, 174, 274 (1954). Barnhart, R. S., Newby, T. H., “Introduction to Rubber Technology,” M. Morton, Ed., p 130, Reinhold, New York, 1959. Bateman, L., Quart. Reo. (London), 8, 147 (1954). Bolland, J. L., Quart. Reo. (London), 3, 1 (1949). Duff, E. J., J . Chem. Soc., (A) 1968a, 836.
Duff, E. J., J . Inorg. Nucl Chem., 30, 861, 1257 (1968b). George, P., Rideal, E. K., Robertson, A., J . Inst. Petrol., 32, 382 (1946a). George, P., Rideal, E. K., Robertson, A., Nature, 149, 601 (1942). George, P., Rideal, E. K., Robertson, A,, Proc. Roy. SOC. London, A185, 308, 377 (194613). Hopkins, R . D., Ind. Eng. Chem. Prod. Res. Develop., 6, 247 (1967). Jones, M. M., “Elementary Coordination Chemistry.” p 226, Prentice-Hall, Englewood Cliffs, N . J., 1964. Kruse, W., Atalia, R. H., Chem. Cornrnun., 1968, 921. Marks, D. R., Phillips, D. J., Radfern, J. P., J . Chem. SOC.,(A) 1967, 1464. May, W. R., Bsharah, L., Ind. Eng. Chem. Prod. Res. Develop., 8, 185 (1969). May, W. R., Bsharah, L., Ind. Eng. Chem. Prod. Res. Develop., 9, 73 (1970). May, W. R., Bsharah, L., Merrifield, D. B., Ind. Eng. Chem. Prod. Res. Develop., 7, 57 (1968). May, W. R., Mathew, W. R., Bsharah, L., Ind. Eng. Chern. Prod. Res. Develop., 6, 185 (1967). Pritzkow, W., Miller, K. A., Ber., 89, 2321 (1946). Vol’pin, M. E., Kolomnikov, I. S., Dokl. Ahad. Nauh, SSSR, 170, 1321 (1966). Wibaut, J. P., Strang, A., Koninhl. Ned. Akad. Wetenshap. Proc., B54, 102 (1951).
RECEIVED for review March 25, 1970 ACCEPTED October 15. 1970 Division of Petroleum Chemistry, 158th Meeting, ACS, New York, September 1969.
Effect of Calcium Sulfate Additions on the Grindability of Portland Cement Clinker Kaissar M. Hannal and Soliman A. El-Hemaly Refractories and Building Materials Laboratory, National Research Center, Cairo, Egypt, U.A.R. Laboratory grinding tests were conducted in a small stainless steel vibratory ball mill to investigate the effect of various forms of anhydrous and hydrated calcium sulfate on the grindability of portland cement clinker. The specific surface of the grind products was used as a measure of the extent of grinding. Calcium sulfate dihydrate, whether natural or chemical reagent gypsum, provided a pronounced increase in surface production. However, the effectiveness of gypsum as a grinding aid diminished when it was employed in partially or completely dehydrated form and ball coating was encountered during the grinding. The effect of water additions on the fine grinding of portland cement clinker was also investigated.
C a l c i u m sulfate, generally in the form of gypsum, is universally added to portland cement clinker to control the rate of the initial reactions, so as to prevent the undesirable flash set. The final stage in the process of manufacturing portland cement involves fine grinding of the clinker with a 4 to 6% addition of gypsum. The dominant use of gypsum is due not only to its effectiveness
’ Present address. Clarkson College of Technology, Potsdam, N. Y. 13626
as a retarder, but also to its harmless effect on strength development and volume stability when not used in excess. However, very little information is available with respect to the influence of calcium sulfate on the grinding efficiency of clinker. Webb and Moody (1958) investigated the possibility of substituting anhydrite for all or part of the gypsum used for regulation of set in portland cements. On the basis of plant scale studies, they concluded that natural anhydrite could be used as a safe and effective retarder Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
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