OX IDATION (0F STY R E NE- B UTAD IE NE R UBBE R CATALYZED 13Y METALLIC IONS WALTER
R .
M A Y
A N D
LEWIS
B S H A R A H
Physical-Analytical Research and Services Department, Petrolite Corp., St. Louis, Mo. 63119
The catalytic effects of nine metals on the oxidation of rubber without a n antioxidant and in the presence of a phosphite, a n amine, and a phenolic antioxidant wer'e evaluated by differential thermal analysis. The catalytic activity of the metlals generally followed their oxidation potential and the expected order of activity. Some interaction between the metal and antioxidant was observed with all three types of antioxidants. The antioxidants, for the most part, retained their traditional roles as free radical traps and peroxide scavengers. Some mild metlal synergism effects were observed, but none was in the extreme, in either the positive or negative direction.
THEcatalytic
effect of metals on oxidation has been studied in several elastomer systems. Kuz'minskfi (Kuz'minskfi et al., 195'3) investigated the effects of copper, iron, and manganese on the oxidation of styrene-butadiene and natural rubber. Lee et al. (1966) investigated the effects of 10 metal ions on oxygen adsorption in nine elastomers. They extended their work to include the effect of a metal deactivator on metal catalysis of polybutadiene oxidation. Raevskfi et al. (1966) investigated the effects of several metal deactivators on the copper- and ironcatalyzed oxidation of styrene-butadiene rubber. The object of this work was to investigate the catalytic effects of several metals on the oxidation of styrenebutadiene rubber, evaluate the role of several antioxidants in deactivating the metals, and study synergism effects between the metals. 'The oxidations were evaluated by a high-pressure differential thermal analysis (DTA) method (May et al., 1968; May and Bsharah, 1969). Experimental
Materials. The rubbler samples were prepared from cold Type 1500 styrene-butadiene latex obtained from the Texas-U. S. Chemical Co. The antioxidants were commercially available samples and of the quality used in the rubber industry. The 2,2'-methylene bis(6-tert-butylcresol) was obtained from the American Cyanamide Co.; the N-cyclohexyl-N'-phenyl-p-phenylenediamine and alkylated aryl phosphite 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 & K Laboratories, Inc. All materials were used without further purification. Preparation of Rubber Samples. A large rubber sample was prepared from the latex emulsion by the salt-acid coagulation method. The sample was dried a t 60°C in a vacuum oven and stored under nitrogen. The small samples for analysis were prepared by admixing appropriate amounts of the metallic salts and antioxidants
with portions of the rubber sample dissolved in carbon disulfide. The solution was then placed in a weighing dish, dried under vacuum, and stored under nitrogen. The total metal content in each sample was 0.1% by weight. I n cases in which two metals were used, 0.05% of each metal was added. The antioxidants were added on a 1-phr basis. Equipment. The DTA evaluations were carried out with a D u Pont Model 900 differential thermal analyzer in conjunction with a high pressure cell, designed and constructed by Du Pont. DTA Curves. Macro-size samples (10 to 20 mg) were used for the DTA evaluations. A 10" per minute heating rate was used on each sample. The normal procedure for running the D u Pont Model 900 differential thermal analyzer was followed, except for the special requirements of the pressurized cell. All evaluations were conducted under 300-psi oxygen. Our earlier work with the DTA method indicates that the peak position of the DTA curve is the most reliable indicator of the oxidation characteristics of the rubber sample (May and Bsharah, 1969). Therefore, we have reported only the position of the oxidation peak below. DTA curves for rubber under 300-psi oxygen containing all the antioxidants used in this work were published earlier (May et al., 1968). The DTA curves for samples containing various metals were identical to those without metals, except that they were positioned a t different temperatures. Results
The DTA results on the rubber samples containing the nine metals in all combinations with the antioxidants are given in Table I. Although new rubber and antioxidant samples were used as well as a slightly different technique, the DTA data on the samples without metals compared well with the earlier work (May et al., 1968). I n all cases, the DTA peaks were biased approximately 15"C below the values published earlier. However, they all fell in Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1, M a r c h 1970
73
case except with the amine, where it was a weak promoter. Manganese ranged from the most active in the amine case to no effect in the phenol case. The data on the metal-metal combinations are given in Table 111. These samples were all evaluated in the absence of an antioxidant. All possible combinations were run for a total of 36 samples. Table IV was constructed to help sort out the metalmetal interactions. The symbols indicate how data for metal-metal combinations compare with data for the metals alone listed in the column on the left-for example, the combination lead-cobalt promoted oxidation over lead alone, but was the same as cobalt alone. The unique catalytic properties of cobalt are illustrated by the fact that every combination containing cobalt was promoted, compared with other metals alone. Calcium, by contrast, did not promote oxidation in any case. Positive and negative synergism should show up when the metal-metal combinations are either better or poorer than either of the original metals. The situations where this arose are:
Table 1. Data from DTA Thermograms of Styrene-Butadiene Rubber under 300-PSI Oxygen
(Antioxidant-metal combinations)
D T A Oxidation Peak, C Antioxidant No antioxidant Amine Phenol Phosphite
Metal Blank Calcium Cerium Cobalt Copper Iron Lead Manganese Vanadyl Zinc
140 138 109 85 118 119 147 120 130 143
220 215 165 195 190 180 215 153 225 197
190 180 180 147 160 171 183 190 168 191
158 160 123 94 134 150 172 127 155 158
the same order and a valid comparison can be made of the data. The bias was probably caused by the fact that, in this work, the samples were dissolved in CSZ to facilitate addition of the antioxidant and metal, whereas in the earlier work this step was not used. The position of the peak maximum is reproducible and seldom varies more than &lot o 2"C. The reliability of this measurement as an indicator of antioxidant activity has been demonstrated in a correlation of DTA data with physical test measurements on compounded and cured rubber containing the same antioxidants (May and Bsharah, 1969). The order of effectiveness of the metals as catalysts changed with the antioxidant. The metals were divided into four groups for each antioxidant, ranging from very effective to no effect (Table 11). Zinc, lead, vanadyl, and calcium were in Group I11 or IV in all cases, except for vanadyl in the phenol case, indicating that these metals have little or no effect on the oxidation of rubber. The iron, cerium, and copper all appeared to some degree as activators. Cobalt was a strong activator in every
Positiue Synergism
Negatioe Synergism
Cobalt-vanadyl Cobalt-zinc Copper-lead Copper-vanadyl Lead-zinc Vanadyl-zinc
Copper-cerium Cerium-iron Cerium-manganese Copper-iron Copper-manganese Iron-lead
Copper and lead appeared in both positive and negative synergism situations, and the lead-zinc combination showed some synergism. This point is significant to the rubber manufacturer, since compounds containing zinc and lead are often used in compounding rubber. Cerium and iron appeared several times in the negative column. This is rather surprising, since both metals are usually considered oxidation promoters. Discussion
Degradation of rubber proceeds by a free radical chain reaction mechanism. The free radicals are usually formed
Table II. Ranking of Metals in Order of Effectiveness in Promoting Oxidation
Antioxidant Gmup
I. Very effective 11. Effective 111. Small effect IV. No effect
None
Amine
Phenol
Phosphite
Co Fe, Mn, Ce, Cu VO Blank, Ca, Zn, P b
Mn Ce, Fe Cu, Zn, Co Blank, Ca, Pb, VO
co Fe, VO, Cu Ce, Ca, P b Blank, Mn, Zn
Co Mn, Ce Fe, Cu Blank, Ca, Zn, VO, P b
Table 111. Data from DTA Thermograms of Styrene-Butadiene Rubber under 300-PSI Oxygen
(Metal-metal combinations)
Cerium
Cobalt
Copper
74
Manganese
Vanadyl
zinc
133 128 121 132
133 110 98 110 132 139 127
154 108 75 116 132 139 132 137
O
First Metal Calcium Cerium Cobalt Copper Iron Lead Manganese Van ad y1
Second Metal Imn Lead DTA Oxidation Peak, C
113
65 97
117 110 90
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1 , March 1970
142 110 80 117
145 108 76 110 133
118
126
Table I V . Relative Synergistic Effects of Various Metal Combinations Compared with Metal Alone
Original Metal
Added Metal"
-
Calcium
Calcium Cerium Cobalt Copper Iron Lead Manganese Vanadyl Zinc a
Cerium
Cobalt
Copper
Iron
Lead
Manganese
Vanadyl
Zinc
+
+
+
0 0 0
+
+ 0 + + +
0 0
+
-
+ + + + + +
0 -
-
-
+ -
+
+
+
0 0
+
-
-
+
+ 0 -
+ 0 +
0
0 -
-
+
+
+
+ promotes, 0 m effect, ~- retards.
by heat, light, oxygen, and other initiators which then react with either oxygen or rubber to produce more free radicals. I n our case, the metal catalysts fall in the "other initiators" category. Chalk and Smith (1957) stated the electron transfer mechanism for metal catalysts as follows:
RH + R * + H * R * + 0,- ROO. ROO. + RH ROOH + Re ROOH + M " - + ROO. + H- + M ( " - ' ) ROOH + M("')-+ RO- + O H + M"' RO. + RH ROH + Re 2 R 0 0 + inactive products +
-+
Antioxidants interrupt the process by terminating the free radicals and decomposing the peroxides into harmless by-products. Antioxidants are expected to deactivate the metal catalysts by either stabilizing their oxidation state, so they do not participate in the redox reaction (Jones, 1964), or neutralizing the peroxides and free radicals produced by the metals. Deactivation of metals by antioxidants is a difficult area to assess. Antioxidants generally contain bulky substituent groups around the electron donor atom (such as the phenol and annine compounds used in this work), making complex for mation difficult for steric reasons. Recently May et al. (1967) established that phenolic antioxidants do not form complexes with metals within the error of the experimental method. However, phenylenediamines, which are the basic structure of many amine antioxidants, can form complexes with cobalt and nickel (Duff, 1968a,b; Markis et a1 , 1967). Raevsk; et al. (1966) reported that compounds such as o-hydroxyquinoline, y-hydroxyphenyl-2-naphthylamine, and p-hydroxydiphenylamine form inactive complexes with copper and iron which increase the stability of styrene-butadiene rubber. Phosphite complexes with palladium (Malatesta and Angoletta, 1957), nickel (Leto and Leto, 1961; Vinal and Reynolds, 1964), cobalt (Vol'pin and Kolomnikov, 19661, and iron (Kruse and Atalia, 1968) are known. Some of' these complexes include ligands derived from triphenyl phosphite. Although none of these phosphites are antioxidants, the structures cited are sufficiently similar to trisnonyl phenyl phosphite to assume that it should be capable of forming a complex
with metals. The sensitivity of metals to deactivation by antioxidants is further emphasized by the work of Jones et al. (1959), who found that replacement of only one solvent molecule by a ligand deactivated the metal as a catalyst. The fact that the antioxidants used in this work did not all affect the metals in the same way is shown in Table 11. The order of catalytic activity of the metals in the presence of the phenolic antioxidant was different from the set without an antioxidant, indicating that the phenol has some interaction with the metals. On the basis of comparison with the blank system, the amine appeared to have the most interaction with the metal. T h e amine deactivated the cobalt more than the other metals. T h e effectiveness of the antioxidants in retarding oxidation followed the expected order: amine > phenol > phosphite (Barnhart and Newby, 1959). This indicates that although metal deactivation may be a part of the mechanism, free radical trapping and peroxide decomposition are more important. The catalytic activity of the metals appears to be related to their oxidation potential. The order of catalytic activity of the metals is Co > Fe = VO > Ce = Cu > M n = P b > Zn > Ca = blank. The only metal in the group which does not easily reduce (Ca) had no catalytic effect. The other metals followed roughly the oxidation potentials cited as follows (Lange, 1961). Volts
Co + e = Co-' Ce+ + e = C e - ' Mn+'+e=Mn'' Fe-' + e = Fe-2 V O L L 211'+e = V + ' + H , O Cu?+ e = Cub Pb-' + 2e = Pb Zn-' + Xe - Zn
1.82 1.61 1.51 0.77 0.4 0.170 -0.126 -0.76
This indicates that the electromotive force associated with reduction is related to the catalytic acitivity of the metals. I n evaluating the data on the metal-metal combinations, three possible interactions may be taking place: positive and negative synergism and simple addition. The combinations which demonstrated synergism effects are listed above. None of the combinations showed extreme synergism in either direction. Copper appeared as both a positive and a negative synergist. This may be related to its position in about the center of the electromotive force table. Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 1 , M a r c h 1970
75
Conclusions
The catalytic, metal synergism, and metal-antioxidant interaction effects on the oxidation of styrene-butadiene rubber were adequately demonstrated by the high pressure DTA method of evaluating rubber samples for oxidation characteristics. The electron transfer mechanism of catalytic activity of metals in the oxidation of hydrocarbons as stated by Chalk and Smith (1957) was corroborated. The three antioxidants studied interacted with the metals to some extent as deactivators but, for the most part, retained their traditional role of free radical traps and peroxide scavengers. The phenolic antioxidant had some effect on the catalytic activity of the metals, indicating that the antioxidants interact with metals through mechanisms other than metal deactivation. Some mild metal synergism effects were observed, but none was in the extreme, in either the positive or negative direction. literature Cited
Barnhart, R. S., Newby, T. H., “Introduction to Rubber Technology,” M. Morton, ed., p. 130, Reinhold, New York, 1959. Chalk, A. J., Smith, J. F., Trans. Faraday Soc. 53, 1214, 1235 (1957). Duff, E. D., J . Chem. Soc. 1968a, 836. Duff, E. D., J . Inorg. Nucl. Chem. 30, 861, 1257 (1968b). Jones, M., “Elementary Coordination Chemistry,” p. 226, Prentice-Hall, Englewood Cliffs, N.J., 1964.
Jones, P., Tobe, M. L., Wynne-Jones, W. F. K., Trans. Faraday Soc. 55, 91 (1959). Kruse, W.,-Atalia, R. H., Chem. Comm. 1968, 921. Kuz’minskii, A. S., Zaitseva, V. D., Lezhnev, N. N.,Dokl. Akad. Nauk SSSR 125, 1057 (1959). Lange, N. A., ed., “Handbook of Chemistry,” p. 1213, McGraw-Hill, New York, 1961. Lee, L. H., Stacy, C. L., Engel, R. G., J . Appl. Polymer Sei. 10, 1699, 1717 (1966). Leto, J. R., Leto, M. F., J . Am. Chem. Soc. 83, 2944 (1961). Malatesta, L., Angoletta, M., J . Chem. Soc. 1957, 1186. Marks, D. R., Phillips, D. J., Radfern, J. P., J . Chem. Soc. A1967, 1464. May, W. R., Bsharah, L., IND. ENG.CHEM.PROD. RES. DEVELOP.8, 185 (1969). May, W. R., Bsharah, L., Merrifield, D. B., IND.ENG. CHEM.PROD. RES. DEVELOP. 7, 57 (1968). May, W. R., Matthew, W. R., Bsharah, L., IND. ENG. CHEM.PROD. RES. DEVELOP.6, 185 (1967). Raevski’l, A. A., Kovrizhko, L. F., Shishkina, U. V., Malyugina, A. L., Smyslova, A. F., Khim. Vysokomol. Soedin. 4, 160 (1966); C A 68, 79303m. Vinal, R. S.,Reynolds, L. T., Inorg. Chem. 3, 1062 (1964). Vol’pin, M. E., Kolomnikov, I. S., Dokl. Akad. Nauk SSSR 170, 1321 (1966). RECEIVED for review May 28, 1969 ACCEPTED October 29, 1969 Division of Rubber Chemistry, ACS, Los Angeles, Calif., April 29 to May 2, 1969.
ISOBUTANE-OLEFIN ALKYLATION WITH INHIBITED ALUMINUM CHLORIDE CATALYSTS A .
K .
R O E B U C K ’
A N D
B .
1.
E V E R I N G ’
Research and Development Department, American Oil Co., Whiting Ind. 46394
Addition of certain aromatic hydrocarbons or certain metal chlorides to a solution of aluminum chloride in aluminum chloride-dialkyl ether complex gives a n alkylation catalyst that is much more active and selective than present commercial catalysts. Parasitic side reactions such as isomerization, disproportionation, hydrogen transfer, and cracking are dramatically reduced. The effect of the inhibitor seems to be connected with the reduced acidity of the inhibited catalyst.
THE alkylation
of isoparaffins with monoolefins is catalyzed both by strong Bransted acids such as sulfuric acid and hydrogen fluoride and by strong Lewis acids such as aluminum chloride. In the commercial alkylation of isobutane (2-methylpropane) with C3 and C4 olefins to produce C iand Cs isoparaffins of >90 Research octane number, sulfuric acid and hydrogen fluoride have long been preferred; aluminum chloride catalyzes side reactions I Present address, Calumet Campus, Purdue University, Hammorid, Ind. ’Present address, 337 Homeland South Way, Baltimore, Md.
76
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1, March 1970
such as isomerization, cracking, and disproportionation that produce either hydrocarbons of poorer octane number or excessive amounts of sludge (Iverson and Schmerling, 1958). The octane number of alkylate is primarily a direct function of the trimethylpentane content (Stiles, 1956). If alkylate consisted only of an equilibrium mixture of trimethylpentanes, its Research octane number would be 102.5 (American Society for Testing and Materials, 1958, 1964; Posen et al., 1945). Since the Research octane number of today’s commercial alkylate is 92 to 96, there is still considerable incentive to improve selectivity.
