Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 455-460
455
Oxidation of Polyalkylaromatic Hydrocarbons. 12. Technological Aspects of p-Xylene Oxidation to Terephthalic Acid in Water' Milan Hronec' and Jin Ilavskji Faculty of Chemical Technology, Slovak Technical University, 8 12 37 Bratislava. Czechoslovakia
A new method of terephthalic acid preparation (about 90 mol % yields) by oxidation of a mixture of p-xylene and p-toluic acid In water as solvent is described. The reaction is catalyzed by cobalt or manganese salts, at temperatures above 140 OC,preferably about 180-190 O C . p-Xylene to be oxidized necessitates the presence of p-toluic acid and concentration of catalyst above the critical point. The results of this study suggest that the oxidation of a mixture of p-xylene and p-toluic acid in the presence of water is governed by a free-radical mechanism involving an initial abstraction of hydrogen on alkyl aromatic substrates by radical species. This mechanism was indicated by the low p value of -0.57, correlated with'6 at 160 O C , obtained from the relative reactivity of substituted toluenes and preferential reactlvity of isopropyl vs. methyl group. Substituted benzoic acids obey a linear relation with a p value of -1.05.
Introduction Oxidation of p-xylene in the liquid phase forms the basis of many important commercial processes of terephthalic acid production. The reaction is catalyzed by transient metal salts or complexes, preferably by cobalt and manganese, and proceeds in solutions of aliphatic acids, usually in acetic acid. The main intermediate of p-xylene oxidation, p-toluic acid, oxidizes to the terephthalic acid only very slowly. Using metal catalysts aIone, p-toluic acid is formed as the main product. To enhance the oxidation of p-toluic acid, new methods using co-catalysts or promotors of metal catalysts, i.e., bromine compounds (Towle and Baldwin, 1964), methyl ethyl ketone (Brill, 1960), acetaldehyde (Thompson and Neely, 1966), paraldehyde (Nakaoka et al., 1973), and eventually zirconium salts (Ichikawa and Soma, 1973) were developed. These are used in commercial processes, where in all cases acetic acid is utilized as the solvent. Recently, several patents concerning the oxidation of p-xylene in other solvents (Stark and Marsh, 1978; Hanotier, 1978),eventually in the absence of a solvent (Yamaji et al., 1978) have appeared. As an extension of our study on the catalyzed oxidation of polyalkylaromatic hydrocarbons we investigated the kinetics and technological aspects of p-xylene oxidation in water. Experimental Section Materials. Hydrocarbons, alcohols, ketones, acids, and esters were purified by distillation or crystallization. p Toluic acid was prepared by cobalt-catalyzed oxidation of p-xylene a t 130 "C with oxygen. For kinetic studies, p toluic acid was prepared by hydrolysis of p-methyl toluate (99.8% purity) and twice recrystallized from a methanol-water solution. Cobalt(II1) acetate was prepared by passing ozone through an acetic acid solution of cobalt(I1) acetate tetrahydrate. The solution was evaporated at 30-40 "C on a rotary evaporator at a pressure of about 3-4 torr. The solid obtained was dried in a desiccator over KOH and analyzed for both Co(I1) and Co(II1). Metal catalysts were obtained commercially, except for acetylacetonates, which were prepared by the known methods (Charles and Pawlikowski, 1958). Procedures. Oxidation reactions were carried out in a 250-mL stainless steel autoclave fitted with a magnetic 'Part 11: Hronec, M. Collect. Czech. Chem. Commun. 1980, 45, 1955. 0196-4321/82/ 1221-0455$01.25Io
impeller system operating at 2200 rpm, having the air inlet at the bottom and outlet through a condenser equipped with a phase separator, pressure and temperature regulator, electric heating mantle, air or water cooler, and outlet for products probe. The temperature of reactants charged in the reactor was increased gradually under applied air pressure and introduction of air and stirring, which usually takes 8-10 min. At the above-mentioned revolutions of the impeller the reaction was not influenced by transport phenomena. Analysis. Outlet gases were continually monitored for oxygen and carbon oxides using Permolyt and Infralyt instruments (Junkalor Dessau, GDR). The reaction products as free acids and also after conversion of carboxylic acids into the corresponding methyl esters with diazomethane were analyzed with a Hewlett-Packard 5830 gas chromatograph by doping with standard amylbenzoate and confirmed by Varian MAT 111 GNOM mass spectrometry. The solid products were mostly analyzed for the acid value and content of terephthalic acid. The weight percent of crude terephthalic acid contained in the reaction product was calculated as a solid undissolved in acetone after drying at 100 "C. Yields quoted in this study are based on the weight of the dried terephthalic acid obtained in this manner and are expressed as grams of terephthalic acid formed from the charged mixture of p-xylene and p-toluic acid, or calculated on 100 g of p-xylene only (theor. 156.5 g). Because a mixture of p-xylene and p-toluic acid was oxidized, higher values of terephthalic acid were caused by p-toluic acid oxidation. Kinetic Measurements. The oxidation rates were measured at 160 OC and 1.25 MPa of total pressure by following the oxygen consumption. All reaction components were put into a 50-mL stainless steel vessel connected with a flexible metal capillary to an oxygen supply system and recording instruments (Hronec and Ilavsky, 1980). The solid catalyst was separately weighed on a Teflon foil placed inside the reactor and after heating of the vessel in a thermostated oil bath (5-7 min) the catalyst was mixed by shaking the vessel using a vibrator. The initial and maximal absorption rates of oxygen were calculated from the plot of absorbed oxygen vs. time, measured under vigorous agitation. Competitive Rate Study. Competitive rates of oxidation of various hydrocarbons in the presence of p-toluic acid were studied under conditions used for kinetic measurements. All reactions were run twice (with 5 1 0 % 0 1982 American Chemical Society
456
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982
-
0
1
N
I
1
2
1
L
I
I
0
mmol
3 of metal
4
Table I. Effect of Solvents on the Oxidation of p-Xylene andp-Toluic Acid at 185 "C and 2.0 MPa with Air 30 dm3 h - ' react. time, v o l % C O , / gTA/ solvent min minC 1OOgPX no solvent" 140 3.3135 79.8 water" 335 2.91125 198.9 acetic acid" 250 5.7118 95.9 1-butanol" 315 6.5175 68.5 dioxane "
benzoic acid
dioxane amyl acetateb lauryl alcohol benzyl alcoholb cyclohexanoneb
70 80 240 345 150 150 155 70 13OC 105c 225
2.9120 2.5115 1.7110
3.1125 2.9140 1.1165 1.7115
1.9125
0
Io
1
1
I
20
LO
60
80 9 HI0
Figure 1. Dependence of TA formation on Mn(I1) ( 0 )and Co(I1) (0) concentration. Conditions: 185 "C, 2.0 MPa, 75 dm3 h-*air, 30 g PX, 50 g PTA, 17 g HzO.
ethylene glycol" no solvent b waterb acetic acid
- 10
I
-1 1 -2 60.3 -2 -1 -1 5.3 0 0 2.8
Figure 2. Effect of water on TA (0,Mn; 0,Co) and carbon dioxide (A,Mn; A, Co) formation. Conditions as in Figure 1;catalysts concentration 2.16 mmol.
I
loot
1.5
2,o
2,s MPa
Figure 3. Effect of pressure on Mn(I1)-catalyzedoxidation. Conditions as in Figure 1; 2.85 mmol catalyst.
" 30 g PX, 50 g PTA, 1 7 g solvent, 1.6 mmol cobalt alkanoate + 0.69 mmol Mn(OAc),.4H20. 7 5 g PX, 5 g ITA, 17 g solvent, 2.85 mmol Mn(OAc),.4H20. Time of maximum vol 3'% CO, formation. conversion) and the mixture was analyzed twice, before and after the reaction, for the change of starting substrates by gas chromatography using an internal standard method. All reactivities were related to toluene using the expression ka _ -- log ([Alf/[Ali) kb 1% ([Blf/[Bli) where A and B refer to concentrations of the two substrates before and after the reaction. Results and Discussion In the presence of p-toluic acid, p-xylene is oxidized by air to terephthalic acid using cobalt or manganese catalysts. The reaction proceeds in the absence of promotors or cocatalysts such as acetaldehyde, paraldehyde, methyl ethyl ketone, or bromine compounds and without acetic acid as solvent. As shown in Figure 1, oxidation is unusually dependent on the catalyst concentration. Manganese produces higher yields of terephthalic acid than a cobalt catalyst. The high melting temperatures of the formed p-toluic acid (179 "C) and the desired terephthalic acid (>320"C) rerequire operation at reaction temperatures above 180 "C or use of a solvent, which has many technological advantages. In Table I is given the effect of some solvents on the terephthalic acid formation and the maximal content of COPin the off-gas at two different p-xylene to p-toluic acid ratios, expressing the selectivity of reaction. In com-
4 01
I
I
I
I
160
180
200 O C
Figure 4. Effect of temperature on TA production catalyzed by: 0.81 mmol Mn(I1)acetate, 0 ; 1.5 mmol Co(I1) alkanoate, 0;mixed catalyst 0.69 + 1.5 mmol Mn(I1) + Co(II), A;conditions as in Figure 1 and 30 dm9 h-' air flow.
parison with acetic acid, which is most frequently used as solvent in oxidation reactions, oxidation in water as solvent leads to the formation of greater yields of terephthalic acid. Concentration of water in the reaction system influences the yield and selectivity of the oxidation. Carbon dioxide as a byproduct is produced at a different rate during the oxidation. As seen from Figure 2, water decreases the maximal content of carbon dioxide in the off-gas and up to concentrations of 15-20 wt % in the case of cobalt catalyst water also increases the yield of terephthalic acid and has a small influence on the manganese catalyst. Higher concentrations of water decrease the terephthalic acid yield. The oxidation is also profoundly affected by oxygen pressure and temperature of the system, as indicated in Figures 3 and 4. The oxidation of p-xylene in
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No.
0,75 "E 0
Table 11. Effect of Acids o n Mn(1I)" and C O ( I I ) ~ Acetate Catalyzed Oxidation of p-Xylene in Water
8%
~~~
react. time, v o l % C O , / g T A / min minC 1OOgPX
acid n N
0
0
10
3, 1982 457
20
30
40
min
Figure. 5. Effect of PTA on reaction rate. Conditions: 160 "C, 1.25 MPa Oz, 56.6 mmol PX, 111.1 mmol HzO,0.31 mmol Co(II1) acetate, p-toluic acid; 0, 0; 0, 1.1 mmol; A, 3.7 mmol. 60 I
acetic" monochloroacetic a mucochloric" benzoic a p-toluic" terephthalic " 3,5-dimethylbenzoica phenylacetic " 3,6-endomethylene tetrahydrophthalic" aceticb acetic bve benzoic p-toluicb
190 55 370 85 345 65 320 145 105
4.5135 3.1145 12.9115 3.2145 3.1125 0.8125 3.01170 4.7185 3.3110
185 304 155 375
2.4160 2.4128 2.9195 2.51130
4.3f 0 0 0 60.3 14.4d 24.7 1.9 0 14.1 2 8.3 52.6
a 70 g PX, 10 g acid, 17 g H,O,2.85 mmol Mn(OAc),. 4H,O, 185 'C, 2.0 MPa, air flow 30 d m 3 h-'. 50 g PX, 20 g acid, 20 g H,O, 2.16 mmol Co(OAc),.BH,O, 185 'C, 2.0 MPa, air flow 30 d m 3 h-l. Time of maximum vol % CO, formation. Equal as initial amount. e H,O exchange with 20 g AcOH. f 50.6%PX conversion and 42.3 mol % p-toluic acid.
4
Table 111. Effect of Cobalt and Manganese Salts o n the Catalyzed Oxidation of p-Xylene and p-Toluic Acid in Water"
2otI
metal salt PX PX+ PTA
Figure 6. Amount of TA formed at different PX to PTA ratios. Conditions: 185 "C, 2.0 MPa, air flow 75 dm3 h-l, PX + PTA = 80 g, 2.1 mmol Co(I1)alkanoate 0 , 1 7 g HzO;0 , 5 0 g HzO;0 , 2 . 7 mmol Mn(I1) acetate and 17 g HzO.
the absence of p-toluic acid is extremely slow and always stops by itself without terephthalic acid formation (Figures 5 and 6). Similarly, at conditions in Figure 6, practically no oxidation of p-toluic acid is observed when p-xylene is not present. The oxidation of a mixture of these two reagents in a broad range of ratios leads to almost equal yields of terephthalic acid, in dependence on both the amount of water in the system and type of catalyst. Differences, however, are in the reaction times. The substitution of p-toluic acid by another aromatic or aliphatic acid, is in each of the cases studied less effective (Table 11). Using monochloroacetic acid, benzoic, terephthalic, or 3,5-endomethylene tetrahydrophthalic acid, the reaction after a short time stops rapidly. In the case of mucochloric acid the oxidation runs a long time but instead of terephthalic acid formation a considerable decarboxylation proceeds, indicated by a high COPcontent in the off-gas. Moreover, the corrosivity of the system is extremely high. In acetic acid as solvent the oxidation of p-xylene stops at the p-toluic acid stage. To obtain the desired terephthalic acid in this solvent, higher amounts of a catalyst than catalytical ones are required (Chester et al., 1977). The previous Figures 2 and 6 show that higher yields of terephthalic acid are obtained with a manganese catalyst. The catalytic effect of cobalt and manganese is strongly dependent also on the ligands of metals (Table 111). The unusual dependence of terephthalic acid yield upon the concentration of a metal catalyst, i.e., strong increase of terephthalic acid yield in a very narrow range of con-
CO(SO,) * 7 H ,O CO(OAc),.4H,O Co(AcAc), Co(NCS),Py, Co(OAc), (79.2 % Coul) CoCrO, Co ( N 0,),*6H,O cobalt alkanoate (10.5 % Co) CoCI,*6H,O cobalt carbonate cobalt phthalocyanine cobalt hydroxide MnO MnO, Mn( 0Ac ),a4 H,O MnCl ,.4H,O MnSO, MnCO, Mn( AcAc),
amt, g
react. time, gTA/ min 1OOgPX
1.5 0.52 1.5 1.5 1.5 1.5 1.5 1.3
121 200 180 110 280 280 125 150
5.0 93.7 7.0 1.3 115.8 18.2 15.9 108.6
1.5 1.5 0.49 1.5 1.5 1.5 0.8 1.5 1.5 1.5 0.8
125 150 123 160 120 130 400 85 125 135 405
3.3 54.0 1.5 62.6 13.1 14.7 213.4 10.1 8.1 13.0 208.1
" 30 g PX, 50 g PTA, 17 g H,O, 185 'C, 2.0 MPa, air flow 30 d m 3 h - l .
loot
v mmol
Mn
Figure 7. Dependence of TA formation on Mn(I1) acetate concentrations at 0,165 "C; 0 , 185 "C; and 0 , 205 "C. Conditions as in Figure 1.
458
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982
Table IV. Reactivities of Aromatics toward Co(111), Mn(111), and Some Radicals aromatic hydrocarbon
Co( 111), 160 O c a
Mn(III), 160 'Ca
Co( 111) 1 0 5 'C6
ROO., 90 "Cc,e
t-BuO *, 110-160 "Cf,e
c1. 40 oCA,e
toluene ethylbenzene cumene
1 4.7 5.3
1 4.9 5.6
1 1.2 0.1
1
7.8 13.3
1 4.02 6.41
2.5 5.5
1
a Our own experiments, 1.25 MPa, 0.28 mmol Mn(II1) acetate, 111.1mmol H,O, 14.7 mmolp-toluic acid, and 18.8 mmol each hydrocarbon. I n acetic acid (Onopchenko et al., 1973). Russell (1956). Russel et al. (1963). e Per active hydrogen. f Brooks (1957).
I1I 0
1
2
3 mmol
4 Mn
Figure 8. Dependence of TA formation on Mn(I1) acetate concentration using: 0 , O g HzO; 0,19g HzO;and 0 , 6 0 g HzO as solvent. Conditions as in Figure 1.
centrations, was studied at various temperatures (Figure 7), various amounts of water as solvent (Figure 8) and various ratios of reagents of p-xylene and p-toluic acid (Figure 9 and 10). In the last case, at higher p-xylene to p-toluic acid ratios, the oxidation can be realized only in a narrow range between the minimal and maximal value of critical concentration for both cobalt and manganese catalysts. In the absence of water, in the concentration range of 1.5-2.5 m o l of Mn(II) acetate, the reaction starts, but after 15-30 min of oxygen absorption it immediately stops. The above described method for the preparation of terephthalic acid is based on the oxidation of a mixture of p-xylene with p-toluic acid in water as solvent catalyzed by cobalt or manganese salts in the absence of aldehyde, ketone, halogenide or other activators. High yields of terephthalic acid (>90mol %) are obtained with catalytic concentrations of cobalt or manganese salts, while the latter are more effective. These findings differ from the known catalytic systems of oxidation of alkyl aromatics in acetic acid solvent (Sheldon and Kochi, 1976), where (a) large amounts of catalyst in a higher valency state are required to effect the selective oxidation; (b) with Co(1I) ions alone, reaction does not proceed readily; (c) cobalt catalysts are more effective than manganese ones. In acetic acid Co(II1) functions primarily via electron transfer and Mn(II1) is effective in both electron transfer and a freeradical pathway, depending on experimental conditions and the reactivity of the substrates. The electron transfer mechanism proposed mostly on the basis of the relative reactivity of substituted toluenes toward cobaltic acetate involves the reversible interaction of Co(II1) with the aromatic hydrocarbon leading to the formation of the corresponding radical cation RCHS
Co(II1)
[RCHJ+*-* RCHy
+ H+
Methyl groups are preferentially attacked and most striking is the inertness of the tertiary isopropyl hydrogen (Onopchenko et al., 1972).
0
1
2
3
4 mmol
5 Mn
Figure 9. Effect of PX to PTA ratio on critical concentration of Mn(I1) acetate catalyst. Conditions: 190 "C, 2.0 MPa, 17 g H20; 0 , 30 g PX, 50 g PTA, 75 dm3 h-' air; A, 70 g PX, 10 g PTA, 30 dm3 h-l air.
50
0
1
mmol of metal
Figure 10. Effect of metal catalysts on their critical concentration. Conditions: 180 "C, 2.0 MPa, 75 g PX, 5 g PTA, 17 g H20, air flow 60 dm3 h-'; A, Co(I1) alkanoate; 0,Mn(I1) acetate: 0 , Mn(I1) acetate in the absence of water.
In our work, the results suggest that the oxidation of p-xylene and p-toluic acid mixture in the presence of water is governed by a different mechanism. The relative reactivity of substituted toluenes toward Co(II1) follows a linear Hammett n+ - p relationship (Figure 11) with a p value of -0.57. The values for xylenes and mesitylene used in the plot have been divided by the statistical factors 2 and 3. This value is very low in comparison with a p value of -2.66 obtained at 90 "C in a Co(II1)-acetic acid system indicating an electron-transfer pathway (Morimoto and Ogata, 1967); however, it is very close to p values of -0.6 and -0.68, which are known for hydrogen abstraction by peroxy radicals (Russell, 1956; Kennedy and Ingold, 1966). The reactivity sequence, summarized in Table IV,observed by us under conditions of commercial processes for toluene, ethylbenzene, and cumene, is different from that reported with Co(II1) in acetic acid solvent (Sakota et al., 1969; Onopchenko et al., 1972). The reactivity sequence is almost the same in Co(II1) and Mn(II1) systems and is in
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982 459 Table V. Oxidation Products of p-Cymene (22.4 mmol) Oxidation in the Presence ofp-Toluic Acid (0.7 mmol) and Water (55.5 mmol) at 1 1 5 "C and 1.25 MPa with Oxygen. Catalyst: 0.39 m o l Co(II1) Acetate products wt % react. time, min
-p -methylacetophenone
p-toluic acida
p-isopropylbenzoic acid
terephthalic acid
reactivity, i-Pr:Me
14 50
14.1 23.9
3.4 6.8
4.5 9.3
0 1.2
3.8:l 3.6:l
Initial concentration 3.2 wt %.
- 0.2
0
42 f*
0,4
Figure 11. Relation between the relative reactivities of substituted toluenes toward Co(II1) acetate and the Hammett substitution constants 8. Conditions: 160 OC, 1.25 MPa; 0.28 mmol Co(III), 14.7 mmol PTA, 111.1 mmol H20, 14.7 mmol each hydrocarbon.
accord with free radical mechanism. This mechanism is acceptable for explaining the unusual catalytic activity of manganese in comparison with cobalt and for the dependence of oxidation on catalyst concentration. Probably the metal catalyst present in the reaction system in the form of metal toluates is only effective in the early stages of the reaction and a relatively low critical catalyst concentration is needed to produce stationary concentrations of radicals derived from the corresponding hydroperoxides and peracids. Higher catalyst concentrations affect the oxidation reaction only in the case when the initial amount of p-toluic acid is low (Figures 9 and 10). The reaction is inhibited probably as a consequence of competitive oxidation of carboxylic acid ligands, manifested by a relative increase of carbon dioxide in the off-gas. It is the same, whether the catalyst is introduced to the reaction system in a lower or higher valency state. Rates of oxidation and yields of terephthalic acid are equal. These results cannot be rationalized on the basis of an electron-transfer mechanism, where a reversible step is assumed. Additional evidence for a radical attack is the finding that during the oxidation of a p-cymene and p-toluic acid mixture in water, tertiary hydrogen of the isopropyl group is preferentially abstracted (Table V). Under such conditions, p-methylacetophenone is the major product in the cobalt system and the reactivity of the isopropyl group vs. methyl is 3.7:l. Selectivities are analogous to those obtained in noncatalytic free radical oxidation, 3.5:l (Serif et al., 1953);3.21 (Russell,1956) and with Co(1II)-catalyzed oxidation of p-cymene in the presence of chloride ions, 3.2:l (Onopchenko et al., 1972). The investigated reaction system consists of a water and a hydrocarbon phase, each containing p-toluic acid and catalyst in various ratios and concentrations. The same apparent activation energy, 93.2 kJ mol-', in the maximal reaction rate region under different molar ratios of pxylene to p-toluic acid and water 1:0.26:2 and 1:0.53:1.31 over the temperature range 140-170 "C,indicate that the oxidation is not influenced by mass transfer. Probably, the reaction proceeds predominantly in the hydrocarbon phase. The rate measurements of p-xylene oxidation with
- 0.2
0
0.2
6
0.4
Figure. 12. Effect of substituted benzoic acids on p-xylene oxidation in water. Conditions: 160 OC, 1.25MPa; 0.39 mmol Co(II1) acetate; 56.6 mmol PX, 111.1 mmol H20,14.7 mmol each aromatic acid.
oxygen at 160 "C in the presence of water shows that the initial rate of oxygen uptake increases linearly with initial concentration of p-toluic acid in the reaction system up to concentrations of 10 wt % calculated on p-xylene. For the different meta- and para-substituted benzoic acids a linear relation between the initial rate of p-xylene oxidation by Co(II1) acetate and the Hammett substitution constants u was obtained (Figure 12). The negative value of -1.05 calculated for. the p constant shows that electron-withdrawing substituwts retard the reaction. Such dependence and also a low p value was found for the oxidation of substituted toluenes (Figure 11). A study of thewproductsof oxidation showed that pmethylbenzaldehyde and 4-carboxybenzaldehyde are the main intermediate products (3-7 mol % ) which play an important role in the radical production at high temperature metal catalyzed oxidation of p-xylene and p-toluic acid mixtures in water. Further kinetic data are necessary to elucidate detailed mechanisms under technological conditions.
Conclusions The results obtained in this study demonstrate the unique and unexpected reactivity of the investigated catalytic system producing yields of terephthalic acid higher than 90 mol %. Differences in selectivity toward the attack at methyl vs. isopropyl groups and relative reactivity of substituted toluenes toward Co(II1) are more compatible with a free-radical mechanism involving initial abstraction of hydrogen on alkyl aromatic substrates by radical species.
Literature Cited Brlll, W. F. Ind. Eng. Chem. 1960, 52, 837. Brooks, J. H. T. Trans. Faraday SOC. 1957, 53,327. Charles, R. G.; Pawlkowski. M. A. J . W y s . Chem. 1958, 6 2 , 440. Chester, A. W.; Scott E. J. Y.; Landls, P. S. J . Catal. 1977, 4 6 , 308. Hanotler, J. D. V. German Offen 2 745 918, Apr 27, 1978. Hronec, M.; IIavskR J. Czech. Patent 201 811, July 10, 1980. Ichlkawa, Y.; Soma, K. Japanese Patent 73 96 545, Dec IO, 1973. Kennedy, R. B.; Ingold. K. U. Can. J . Chem. 1966, 4 4 , 2381. Morimoto, T.; Ogata, Y. J . Chem. Soc., B 1967, 1353. Nakaoka, K.; Miyama. Y.; Matsuhlsa, S.; Wakamatsu, S. Ind. Eng. Chem. Rod. Res. D e v . 1973, 12, 150. Onopchenko, A.; Schultz, J.G.D.; Seekircher, R. J . Org. Chem. 1972, 37, 1414. Russell, G. A. J . Am. Chem. SOC. 1956, 78, 1047.
Ind. Eng. Chem. Prod. Res. Dev. 1082, 21, 460-461
460
Russell, 0. A,; Ito, A.; Hendry. D. G. J . Am. Chem. SOC. 1963, 8 5 , 2976. Sakota, K.; Kamiya, Y.; Ohta, N. Can. J . Chem. lS69, 4 7 , 387. Serif, G.: Hunt. G.; Bourns, A. Can. J . Chem. 1953, 31, 1229. Sheldon, R.; Kochi, J. K. Adv. Cats/. 1976, 2 5 , 272. Stark, L.: Marsh, D. R. US. Patent 4081 463. Mar 28, 1978. Thompson, B.; Neeiy, S. D. U S . Patent 3240803, Mar 15, 1966. Towle, P. H.:Baldwln, R. H. Hydrocarbon Process. 1984, 4 3 , 149.
Yamajl, T.; Yoshisato, E.; Hiramatsu, T.; Hirose, 1. Japanese Patent 78 112 830, Oct 2, 1978.
Received for review April 5, 1981 Revised manuscript received November 5, 1981 Accepted January 12, 1982
Determination of the Thermal Decomposition Kinetics of Polyurethane Foam by Guggenheim’s Method J. Richard Ward‘ and Leon J. Decker U S . Army Ballistic Research Laboratory, Aberdeen Proving Ground, Maryland 21005
The kinetics of the thermal degradation of a rigid polyurethane foam used to reduce gun wear was determined to illustrate how Guggenheim’s method could be applied to polymer decomposition. The polyurethane foam decom osed in two distinct steps. The first-order activation parameters for each rate coefficient are k , = 2.0 X 10‘ s exp(-134 kJlmollRT) and k , = 1.2 X 101os-’ exp(-154 kJlmollRT).
8 -’
Introduction The thermal decomposition rate of polyurethanes has long been of interest to polymer chemists in their endeavor to reduce the flammability of these widely used materials. A recent thesis (Ramakrishnan, 1975) reviews this history. Thermal decomposition of polyurethanes is also of interest in interior ballistics. A high-density polyurethane foam glued to the inside wall of a cartridge case reduces gun barrel we= (Dickinson and McLennon, 1968; Joseph, 1958). A common technique to measure the kinetics of the thermal decomposition is to monitor mass loss vs. time at constant temperature (“isothermal”), or mass loss vs. temperature at a constant heating rate (“dynamic”). The dynamic technique is experimentally convenient, since activation parameters can be determined in a single experiment. The isothermal technique, by contrast, requires rate coefficient determinations at several temperatures; the time for a reaction to go to completion is much longer than in a dynamic run; and the rate coefficient is sensitive to the value of mass selected as the end of the reaction. For polymers this choice can be arbitrary, since polymers typically decompose by consecutive reactions. The experimental simplicity of the dynamic method ptompted many investigators to apply this technique to polymer decomposition kinetics (Flynn, 1969). A number of investigators have questioned the validity of the dynamic method, since rate coefficients determined isothermally did not agree with those determined with the dynamic method. MacCallum and Tanner (1970a,b) go so far as to question the validity of the equations used in the dynamic method. In this report, the decomposition of a polyurethane is measured using Guggenheim’s method to evaluate the fit-order rate coefficients. Guggenheim‘stechnique seems particularly suited, since most polymers decompose by first-order kinetics (Rumao and Frisch, 1972; Dyer and
Table I. Composition of Polyurethane Foam resin prepolymer ingredients
parts by wt
polyethylene glycol 200 polypropylene glycol 1200 castor oil 2,4-toluene diisocyanate cat. mixture ingredients
10.5 6.5 36.5 46.5
polypropylene glycol glycerine polyethylene glycol ferric acetylacetonate nigrosine black dibutyltin dilaurate a Remainder of foam is resin.
parts by wta 10.0 7.5 3.75 0.15 0.25 0.30
Table 11. Mans vs. Time a t 533 K 15.18 14.94 14.72 14.32 13.94 13.52 13.12
12.76 12.40 12.02 11.70 10.96 10.48 a
~equals t
12.40 12.22 12.02 11.90 11.42 11.14 10.96 10.74 10.58 10.40 10.26 9.88 9.68
1.0 1.5 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 13.0 15.0
7 min.
Hammond, 1964)under an inert atmosphere, but the exact mass loss corresponding to the end of the reaction is difficult to discern for consecutive reactions. Experimental Section Samples of polyurethane foam were cut from a piece of foam taken from a 105-mm tank-gun cartridge (Joseph,
This article not subject to U S . Copyright. Published 1982 by the American Chemical Society
