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Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 624-628
Polymerization of Butadiene with Cobalt Acetylacetonate-Aluminum Alkyl Catalyst K. K. Li and C. C. Hsu' Depatiment of Chemical Engineering, Queen's University, Kingston, Ontario, Canada K7L 3N6
The results of the polymerization of butadiene using CO(AC~C)~-R,AICI~_, catalyst are presented here to show the effects of chlorine substitutions in place of alkyl on aluminum and the use of mixed solvent on the conversion, microstructure,and molecular weights. The results led to the following conclusions. Et2AICI is the most effective cocatalyst for Co(Acac),. The presence of a small amount of cyclohexane in the solvent greatly enhances the catalyst activity in the system with (~-BU)~AI. The alkyls without chlorine substitution produce polybutadiene with about 50% vinyl content, and the addition of cyclohexane results in a slight decrease in vinyl formation.
Polymerization of butadiene with Ziegler-Natta catalysts can produce polymers exclusively of trans-1,4, cis-1,4, or vinyl structure. Because of their structural difference, each polymer has its own distinct physical properties. The cis-1,4 polymer is a rubber-like elastomer, whereas polymers of syndiotactic 1,2 structure are highly crystalline and nonelastic. Thus,by careful choice of the catalyst we might be able to prepare polybutadiene (PB) of desired properties for any particular end use. The mechanism of such sterospecific polymerizations is, however, not well understood. While few empirical generalizations may be considered, no definite conclusion can be drawn. The polymers of 1,4 structure tend to be produced from the catalysts with ionic characteristics, normally containing highly negative groups, particularly halogen atoms, whereas polymers of high 1,2 content are often obtained from catalysts which are prepared from the transition metal alkoxides, acetylacetonates, or those containing a high alky1:halogen ratio. However, this generalization is not entirely true since groups attached to the metal may interchange freely among themselves and the structure of the polymer will then depend on operating conditions. For example, cobalt chloride and aluminum trialkyl will give either cis-1,4, or 1,2 polymer depending on the stoichiometry of the catalyst components and the conditions under which the catalyst is prepared (Susa, 1963). Extensive literature may be found for the synthesis of high trans- or cis-PB, but a relatively small number of papers have been devoted to the preparation of 1,2-PB. The synthesis of syndiotactic 1,2-PB was first reported by Natta and his associates (1956,1968). Later, Susa (1963) revealed new catalysts, basically a combination of (C2H5)3A1(Et3A1) with CoS04, C O ~ ( P O cobalt ~ ) ~ , stearateto prodipyridine, or cobalt acetylacetonate (Co(A~ac)~) duce syndiotactic 1,2-PB over 98%. Similar results were also reported by Marconi (1967). Recently, Japan Synthetic Rubber Co. (1967) has developed successfully a commercial process of producing a nearly 100% 1,2 structure. The new polymer has a low crystallinity and is used as a thermoplastic, which can be processed with conventional molding machines. For the use as high-impact plastic, or as a possible substitute for SBR, PB of 1,2 content within the range of 10 to 25% is desirable. Polymers of equimolar distribution of cis-1,4 and 1,2 have been successfully synthesized by different research teams, notably Dawans and Teyssie (1968),Teyssie et al. (1968), Takeuchi et al. (1969), and Furukawa et al. (1971). Furukawa and his coworkers used Co(A~ac)~-Et&as catalyst 0196-4321/81/1220-0624$01.25/0
to make an equimolar PB of cis-1,4 and 1,2 polymers. They found the distribution of these two configurations being completely random, contrary to the alternating coordination mechanism suggested by Dawans et al. (1968). The formation of cis-1,4 units seems to be controlled by the addition of water to the catalyst. With the exception of using the chlorinated hydrocarbons as solvent which produces predominantly cis-1,4 polymers, the equal distribution of cis-1,4 and 1,2 vinyl is not affected by the variation of the composition of the catalyst and of additives regardless of the additives whether they are polar or electron donor. In these studies it is not clear whether the C ~ ( A c a cis) ~ the main contributor to the formation of 1,2-PB or if it is due to a combined effect of Co(Acac)3and Et&. In this work we will examine how the substitution of chlorine in place of alkyl groups on aluminum will alter the characteristics of the catalyst. We will aslo investigate the influence of the addition of cyclohexane to the solvent and whether the 1,2 insertion will be affected by replacing the ethyl groups with more sterically hindered isobutyl groups.
Experimental Work The toluene used as solvent in this study was Fisher analytical grade with a purity of better than 99%. The rubber grade l,&butadiene of 99% purity and the cyclohexane of 99.99% purity were obtained from Phillips Petroleum Co. The water content in those hydrocarbons was determined by Karl Fischer titration. The cobalt acetylacetonate supplied by Ventron Corp. and aluminum alkyls supplied by Ether Corp. were used as received. A simple bottle-polymerization technique was used throughout this investigation. The detailed description of the reactor and the experimental procedure are given elsewhere (Li, 1976). All polymerization experiments were carried out at 25 "C in toluene under nitrogen atmosphere. At the end of 2 h polymerization time the reaction mixture was poured into methanol to stop the reaction and at the same time to precipitate out the PB. After filtration the polymer product was dried under vacuum and the dry polymer weighed to determine the yield. The catalysts employed are the combinations of cobalt acetylacetonate with different aluminum alkyls to study the effects of chlorine content in alkyls and of different alkyl groups on polymerization: those being studied include EtAlCl,, Et2A1C1,Et3A1, and ( ~ - B U ) ~The A ~ .order of addition of catalyst components is usually important 0 1981 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20,No. 4, 1981 825
Table I. Polymerization Results with Co(Acac),-Et,AlCl as Catalysts at 25 "C ([MI, = 1.38 mol/L and [H,O]= 9.3 mmol/L; Time = 2 h ) expt. [All/ % % cis- M , X M,; no. [Co] %conv. 1,2 1,4 10[All = 18.8 mmol/L 43 68.5 2.0 97 9.7 2.0 AD-1 3 93 38.5 1.0 96 12.0 4.6 4 140 17.6 2.0 93 17.6 8.3 6 280 5.7 7
140
[All = 28.2 mmol/L 46.6 1.0 95
23.5
tI
9.90
in the polymerization using Ziegler-Natta type catalysts. Different procedures have been tried for the preparation of catalyst in order to obtain the best results. Two types of additives were employed to study their effects on polymerization; the electron donors and saturated cyclic hydrocarbon solvents such as cyclohexane. The molecular weight averages were determined using GPC, a standard Waters unit, Model 200. Four columns ranging from 1 X lo3 A to 3 X lo6 A were used. The calibration of the GPC was based on the principle that the polymer species of different sizes were separated in the column according to their hydrodynamic volumes, [TIM, where [ T ] is the intrinsic viscosity and M is the molecular weight of polymer. Thus, a universal calibration curve can be established by determining the elution volumes of the narrow fractions of the standard polystyrene samples with known molecular weights and intrinsic viscosities. The universal plot of log [VIMvs. elution volume obtained in this study shows a good straight line within the range of interest. The use of the universal calibration curve for the determination of the molecular weight of PB requires the intrinsic viscosities of the narrow fractions of PB which unfortunately are not readily available. Therefore, we used four standard PB samples with certified M, and M, by Phillips Petroleum Co. to estimate the best values of the parameters K and a in the Mark-Houwink equation for viscosity, [v] = KM".The four standard samples cover the molecular weight range from 1.8 X lo4to 2.6 X lo5. The M-H equation was then used in subsequent determinations of the molecular weight of unknown PB samples. The molecular weights determined from this method had an average error of about 5% with possible maximum error of 12%. The composition of microstructure of PB was determined by infrared spectrometry. In this work, a Beckman Model IR8 spectrophotometer equiped with a KC1 cell of path length 1.0 mm was used. According to Silas et al. (19591, three absorption peaks were chosen for calculation: 10.3,11.0, and the combined one of 12.0 to 16.0. In the case of the broad peak between 12.0 and 16.0, the area bounded by the two wavelengths was taken. This method is based on the assumption that PB can be regarded as a simple three-component mixture of trans-1,4, cis-1,4, and 1,2 vinyl. The analysis then involves solving a set of three simultaneous equations of absorbance at a particular wavelength. The four standard samples were again used for the calibration in obtaining the molar absorptitivies. Results and Discussion No polymer was obtained with the catalyst system of Co(Acac),-EtAIClz for a 2-h polymerization time regardless of how the catalyst was prepared. It was noticed that when the two catalyst components were mixed the temperature of the mixture rose quickly, which seems to suggest a rapid
031 0
i
\
' '
01
I 02
I 03
04
05
Cco], m m o l e / l
Figure 1. Plot of log (1 - X) vs. cobalt concentration for Co(Acac)B-EhAlCl catalyst system: [All = 18.8mmol/L; [H,O] = 9.3 mmol/L; [MIo = 1.38 mol/L. Temperature = 25 "C and reaction time = 2 h.
decomposition of the catalysts. The experimental results from the catalyst system of C~(Acac)~-Et,AlClare given in Table I. The aluminum diethyl chloride must be added to the monomer solution before the addition of cobalt acetylacetonate; otherwise polymerization will not be initiated. A plot of log (1.0 - X ) , where X is the fractional conversion, vs. cobalt concentration shows a straight line (Figure 1). This indicates a first-order dependence of the polymerization rate on cobalt concentration. This is found for most Ziegler-Natta polymerizations (Gippin, 1962; Cooper et al., 1963; Loo and Hsu, 1974; Hsu and Ng, 1976). What is interesting is that the line does not go through 1 at the ordinate at zero concentration of cobalt acetylacetonate; instead, at 1.0 - X equal to 1,the cobalt concentration has a value of 0.05 mmol/L. This concentration can be considered as minimum concentration of cobalt required for initiation. It is quite obvious that a certain amount of cobalt catalyst is reacted or decomposed forming inactive compounds. As compared with the polymerization intiated by the CoClz-pyridine catalyst studied by Hsu and Ng (1976), there is no appreciable difference in microstructure. In both cases the butadiene polymers are consistently over 90% cis configuration. The molecular weight, however, is one order less than those obtained with cobalt chloride catalyst. This is partly due to the lower catalyst activity of Co(Acac)3-EtA1Clz as compared to CoClz-pyridine. Almost ten times more cobalt catalyst must be used than the amount of CoC12-pyridine in order to obtain a comparable polymerization rate. As expected, the molecular weight decreases as cobalt concentration increases, but the decrease in number average molecular weight is faster than the corresponding weight average at constant temperature and water contents. Thus, as cobalt concentration increases, the dispersion of the product distribution is also increased. This could be attributed to the first-order deactivation of catalyst (Loo and Hsu, 1974). For the system of C~(Acac)~-Et~Al, aluminum alkyl again has to be added before the addition of the cobalt acetylacetonate as in the previous case; the reaction will not be initiated otherwise. In general, the activity of this catalyst system is low; it produces only about 5 to 15% conversion during a 2-h polymerization time under the conditions studied. The results are summarized in Table 11. It can be seen that the concentration of cobalt acetylacetonate has no apparent effect on the monomer conversion. A mild temperature increase was observed when Et,A1 was added to the reaction mixture. This increase
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Table 11. Polymerization Results with Ca(Acac),-Et,Al as Catalyst at 25 "C ([MI,= 1.38 mol/L; Time= 2 h ) catalvst concn. expt. no.
&mol/L ' % % %cis- M , x [Co] [All [H,O] conv. 1 , 2 1,4
AT-21 -8 -20 -13 -35 -34 -32 -33 -4 -1 -18 -12
0.2 0.2 0.2 0.4 0.2 0.2 0.2 0.2 0.07 0.12 0.33 0.41
12.9 17.3 25.9 34.5 17.3 17.3 17.3 17.3 17.3 17.3 17.3 17.3
9.3 9.3 9.3 9.3 6.8 7.6 11.6 12.8 9.3 9.3 9.3 9.3
trace 61.1 34 0.7 27 trace 7.4 52 11.0 4 4 8.0 33 5.0 46 7.1 32 11.7 35 5.5 46 9.5 51
M,X
59 70
33.0
8.0
47 50 60 48 57 55 52 46
36.3 18.1 6.7 5.2 14.9 41.5 14.2 8.54
2.8 4.5 2.1 1.0 2.6 11.5 3.4 2.0
in temperature is believed to be a result of partial decomposition of EbA1, which produces compounds not capable of initiating polymerization reaction. Consequently, only a small fraction of catalyst turns into active complexes. It can be seen from the results in Table I1 that the concentration of cobalt has little influence on conversion. The most noticeable difference of this catalyst system vs. the Co(Acac),-Et2A1C1 is the high 1,2 content in the polymer products, which seems to agree with the general feature of the Ziegler catalyst that stronger ionic characteristics favor the 1,4 polymerization (Marconi, 1967; Natta and Porri, 1968). Table I1 also shows that there is a slight increase in cis-1,4 polymerization at high triethylaluminum concentrations. The exact relationship is difficult to obtain because too little or too much of triethylaluminum will produce no polymer. The addition of a compound such as diethyl sulfite has an adverse effect on polymerization rate, but in its presence, the polymerization tends to be in favor of 1,2 coordination. Adding triethylphosphine almost doubles the vinyl content as shown in Table 111. This is contrary to the results obtained by Wang and Liao (1964),who studied the CoC12.4pyridine-Et2AlCl system and reported no change in microstructure when diethyl sulfite or triethylphosphine was added to the reaction mixture. The effect on the polymerization rate was more complicated. The rate shows a sharp increase up to about 3 mmol/L; the rate starts to drop with further increase of concentration. The partial replacement of the toluene solvent with cyclohexane does not have a significant effect on the yield, but it does change the microstructure as shown in Figure 2, the effect ranging from 35 to 55% increase in vinyl content with cyclohexane increasing from zero to about 18%. The cis content decreases accordingly, whereas the trans-1,4 content remains fairly constant. Water has some effect on the conversion within the range studied, but
20
0
2
4
6
8
IO
12
I4
16
V O L U M E OF C Y C L O H E X A N E A D D E D . m i
Figure 2. Effect of cyclohexane on microstructure for Co(Acac)SEt& system: [All = 17.3 mmol/L; [Co] = 0.2 mmol/L; [HzO]= 9.3 mmol/L; [MI, = 1.38 mol/L. Temperature = 25 OC, reaction time = 2 h, and total solvent = 85 mL.
0.i
02
0
0.4
tcol.
0.6
08
mmole/i
Figure 3. Plot of log (1 - X) vs. cobalt concentration for Co(Aca~)~-(i-Bu)A system: [All = 18.6 mmol/L; [H20]= 9.3 mmol/L; [MI, = 1.38 mol/L. Temperature = 25 "C and reaction time = 2 h.
virtually no influence on microstructure. The polymerization results using Co(A~ac),-(i-Bu)~Al as catalyst are given in Table IV. For this system, the catalyst components must be mixed before being introduced to the monomer solution. This is contrary to most of the catalyst systems of similar nature, where the aluminum alkyl is usually added first to act as a scavenger to suppress the undesirable influence from impurities in the system. Figure 3 is a plot similar to Figure 1,but the line meets at log (1.0 - X)= 1.0 at zero concentration of cobalt compound; thus all the cobalt acetylacetonate added to the system can be assumed to form stable and active complexes. However, the yield is still low even though it is slightly better than that with Et3A1 as catalyst. The polymer obtained contains about 40% 1,2-vinyl and 50% cis-l,4. A slight increase in water concentration, for example from 9.3 mmol/L to 11.6 mmol/L, increases the conversion from about 10% to 35% as shown in the series B of Table IV. It is believed that (i-Bu),Al reacts first with water forming aluminum alkoxides. These are the alkoxides which are responsible for the formation of active complexes with cobalt catalyst. To study quantitatively the effect of water on the polymerization, an independent experi-
Table 111. Effects of Additives o n Polymerization with Co(Acac),-Et,& as Catalyst at 25 "C ([Co] = 0.2 mmol/L; [All = 17.3 mmol/L; [Mol = 1.38 mmol/L; and [H,0] = 9 . 3 mmol/L; Time = 2 h ) expt. no. additive % conv. % 1,2 % cis-1,4 Mw X M, X AT-8 -9 -14 -22 -23 - 24 -25 - 26 -27 -28 -29 -30
Et$, 1 0 mmol/L Ph,P, 0 . 2 mmol/L cyclohexane, 5 mL cyclohexane, 3 mL cyclohexane, 7 mL cyclohexane, 10 mL cyclohexane, 1 5 mL cyclohexane, 15 mL cyclohexane, 1 2 mL cyclohexane, 11 mL cyclohexane, 9 mL
16.1 3.2 2.9 5.9 6.4 8.9 9.8 3.8 4.5 8.5 7.5 5.1
34 45 72 45 44 48
52 57 55 51 53 34
59 48 27 54 53 47 44 41 42 44 45 63
33.0 9.0 5.6 14.4 17.3 19.3 24.2 10.6 22.6 24.1 8.2 15.4
8.0 1.9 1.0 2.2 3.9 3.9 4.7 1.7 3.7 2.3 1.8 2.0
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 627
Table IV. Effects of Water and Cyclohexane on Polymerization with Co(Acac),-(i-Bu),Al Catalyst at 25 "C ([MI, = 1.38 mol/L; [Co] = 0.44 mmol/L; [All= 18.6 mmol/L; Total Volume = 8 5 mL; Time = 2 h ) expt. no. B-17 -18 -19 - 20 - 21 -22
BC-14 -15
-20 -13 -12 -3 1 -11 -34 -10 -3 2 -1
-9 -3 5 -3 -5 -21 -7
[H,OI,
mmol/L cyclohexane, mL 9.3 10.5 10.9 11.6 12.8 12.8 9.3 9.3 9.3 9.3 9.3 9.3 9.3 9.3 9.3 9.3 9.3 9.3 9.3 9.3 9.3 9.3 9.3
1.0 1.0 1.5 2.0 3.0 3.0 4.0 4.0 5.0 5.0 10.0 15.0 15.0 20.0 20.0 20.0 25.0
% conv.
% 1,2
% cis
9.8 28.8 28.7 35.3 20.8 18.5 16.7 11.0 32.0 33.5 31.7 34.0 38.1 35.9 25.0 31.6 20.6 13.8 14.8 18.5 17.1 12.1 14.5
58 43 45 42 31 45 39 31 40 38 22 44 27 38 47 42 43 36 47 42 20 45 51
38 55 50 55 66 53 59 68 58 59 76 49 70 55 49 53 52 62 51 55 77 53 46
M, x
M,
X
4.1 12.2 16.3 12.4
1.7 4.2 5.6 3.7
26.7 3.6
8.0 1.7
6.7 3.4 8.0 15.7 3.5 7.0 18.4 24.9 2.8 8.3 12.1 16.3 7.0 18.2 33.4
2.5 1.1 1.5 2.2 1.1 2.1 3.2 5.2 1.2 1.6 2.5 4.4 0.6 2.7 7.1
40 0
[All/t%Ol
Figure 4. Plot of initial polymerization vs. [Al]/[H,O] for Co(Acac),-(i-Bu),Al system (20 "C); [H,O] = 15.4 mmol/L; [MI = 1.4 mol/L.
mental study was conducted by modifying the reactor setup to allow measurements of conversion vs. time. From the conversion data we calculated polymerization rates as a function of time. Since the rate of catalyst decay (deactivation) depends on the initial composition of catalysts, only initial polymerization rates were studied for the water effect. Figure 4 is a plot of the initial polymerization rate per mole of cobalt vs. the aluminum to water ratio over aluminum concentrations from 11.3 to 25.8 mmol/L, keeping cobalt concentration fixed at a concentration of 1.26 mmol/L. The effect of the [Al]/[HzO] ratio is evident. The initial polymerization rate is essentially zero when [Al]/[H,O] drops below 0.8. The maximum rate occurs a t ratio of about 1.0. The rate decreases sharply beyond the ratio of 1.0 and levels off at about 1.4. Thus the most active complexes are formed by a one-to-one reaction between HzO and (i-B&Al. A small variation of water concentration does not have any appreciable effect on the microstructure. The molecular weight, on the other hand, varies in parallel with the conversion. Both the number and weight averages
IO
0 VOLUME
OF
20
30
C Y C L O H E X A N E ADDED, m l
Figure 5. Effect of cyclohexane on monomer conversion for Co(Acac)&Bu)& system: [All = 18.6 mmol/L; [Co] = 0.44mmol/L; [H20! = 9.3 mmol/L; [MI,= 1.38 mol/L. Temperature = 25 O C , reaction time = 2 h, and total solvent = 85 mL.
reach a maximum at about the same water content which gives a maximum conversion. Thus, within the range of water concentration studied, water merely changes the propagation rate but has no effect on the chain transfer reaction, which is generally true for other similar systems (Hsu and Ng, 1976). The most striking result obtained in this system is from those experiments where the cyclohexane was added to the toluene as mixed solvent. As shown in Figure 5 there is a sharp increase in product yield when a small amount of cyclohexane was added, followed by a rapid decrease when cyclohexane exceeds about 5 mL of a total solvent of 85 mL. The solvating effect certainly is not sufficient to explain this phenomenon. If it were true, the water should have a much stronger effect. The effect of water, as shown previously, is much less drastic than that of cyclohexane, however. Normally saturated cyclic compounds are poor solvents for Zieger-Natta catalysts. It seem then that the presence of a small amount of cyclohexane has a strong local effect facilitating the coordination reaction, but as the quantity increases, the overall effect as a solvent which
628
Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 628-636
is usually poor for that kind of catalyst becomes evident. The concentration of (i-Bu),Al also has a significant effect of conversion as shown in Table IV; the polymerization rate increases as the concentration of ( ~ - B U ) in~A~ creases and reaches a maximum. (i-Bu),Al has no effect on the microstructure of the polymer. Conclusions Among the aluminum alkyls studied, Et&C!l is the most effective cocatalyst for C ~ ( A c a c ) However, ~. this system is still much less efficient as compared to CoClz systems. There is no obvious correlation of the number of chlorine substitutes to the reactivity of catalyst. The chlorine substitute improves the stereospecificity of the catalyst. Both Et3A1and (i-Bu),Al give about 30 to 40 5% 1,2-vinylpolymerization whereas alkyls containing chlorine substitutes give high cis configuration. The chlorine substitution increases the Coulomb attraction between the complexes and the monomer molecules, which greatly facilitates the cis-1,4 insertion. Thus,the aluminum catalyst with chlorine substitution in place of alkyl gives a higher overall polymerization rate. The addition of a small amount of cyclohexane to the C~(Acac)~-(i-Bu)~Al system strongly promotes the reactivity of the catalyst if the amount is kept low. No satisfactory explanation can be given, however, at this time; a detailed mechanistic study of this particular polymerization reaction is required in order to fully understand the
solvent effect. The addition of cyclohexane does affect the microstructure, but in a minor way. Literature Cited Cooper, W. D.; Evaves, D. E.; Vaughan, G. Makromol. Chem. 1963, 67, 229. Dawans, F.; Teyssie, Ph. Makromol. Chem. 1967, 709, 68. Furukawa, J.; Haga, K.; Kobayashi, E.; Iseda, Y.; Yoshimoto, T.; Sakamoto, K. folym. J . 1971, 2-3, 371. Gippin, M. Ind. Eng. Chem. prod. Res. Dev. 1962, 1 , 32. Hsu, C. C.; Ng, L. AIChE J . 1976, 22, 66. Ichikawa, M.; Lakeuchi, Y.; Kogure, A. (Japan Synthetic Rubber Co. Ltd.), U.S. Patent 3 498 963, 1967. Li, K. K. M.Sc. Thesis, Queen’s University, Klngston, Ontario, Canada, 1976. Loo, C. C.; Hsu, C. C. Can. J . Chem. Eng. 1974, 52, 374. Marconi, W. “The Stereochemistry of Macromolecules”; Ketley, A. D., Ed.; Vol. I,Marcel Dekker: New York, 1967; Chapter 5. Natta, G.; Corradini, P. J. folym. Sci. 1956, 20, 251. Natta, G.; Porri, L. “Polymer Chemistry of Synthetic Elastomers”; Kennedy, J. P.; Tornquist, E. G. M., Ed.; Part I1 Interscience; New York, 1968; Chapter 7. Silas, R. S.; Yates. J.; Thornton, V. T. Anal. Chem. 1959, 31-4 529. Susa, E. J . folym. Scl. 1963, C4, 399. Takenchi, Y.; Ichikawa, M. 22nd Annual Meeting of the Chemical Society, Japan, Reprints, 1969, p 2122. Teyssie, Ph.; Dawans, F.; Durand, J. P. J. folym. Sci., fall C 1968, 22, 221. Waii,.F. S.; Liao, Y. C. Gaofenzi Tangxun 1964, 6- 1 , 55.
Received for review November 3, 1980 Revised manuscript received April 8, 1981 Accepted May 6, 1981
This research project was supported by the Natural Sciences and Engineering Council of Canada, Grant A2413.
Catalytic Combustion of Low Heat Value Gas. 1 Substoichiometric Combustion with Supported Platinum Catalysts Ajay M. Madgavkar,’ Roger F. Vogel, and Harold E. SwlR’ Gulf Research & Development Company, Pittsburgh, Pennsylvania 15230
Experimental results of substoichiometric (oxygen deficient) catalytic combustion of a typical low heat value (LHV) flue gas are presented. Such a combustion over supported platinum catalysts resulted in significant carbon monoxide generation between air equivalence ratio (AER) values of 0.40 and 0.75. It is believed that this is mainly due to steam reforming reactions by the water produced during combustion. Stable combustion was demonstrated for compositions containing high amounts of hydrogen sulfide and no permanent deactivation of the catalysts was apparent. The production of carbon monoxide was found to be a function of several other variables. These included hydrocarbon composition of the feed, pressure of combustion reaction, feed temperature, and the catalyst characteristics which included its preparational procedure and platinum concentration. Good correlation between data obtained in this study using simulated field test conditions and an actual field test was achieved.
Introduction In the future, there will be increased sources of low heat value (LHV) flue gases from operations such as enhanced oil recovery. The present standard practice is to incinerate such gases or, where allowed, vent to the atmosphere. The cost of incineration is high and fewer areas are allowing venting. A typical LHV gas from a “Fireflood (Kaye et al., 1979) contains between 3 and 8% by volume hydrocarbons, mainly methane, with heat values varying from 40 to 100 Btu/SCF. The low magnitude of heat value is misleading since such an operation can yield 10 to 20 MM SCFD of the LHV gas, which represents roughly 0.4 to 2
billion Btu of energy per day. Besides the loss in energy, venting this gas can cause serious environmental pollution problems, accentuated by the presence of hydrogen sulfide. Some LHV gases cannot support thermal combustion. In such cases, catalysis offers a possible means to maintain stable combustion, thus making it possible to recover a portion of the thermal energy. For many operations, the gas composition can vary with time, thus changing the heat content of the LHV gas. An example of this is shown in Figure 1(Kaye et al., 1979). Complete combustion of such a gas would continuously change the exhaust gas temperature. From an operational point of view, in order to have better control over combustion temperature and/or to protect downstream equipment from temperature excursions, temperature variations can be prevented by fixing
Occidental Research Corp., Irvine, Calif. 0196-4321/81/1220-0628$01.25/0
0
1981 American Chemical Society
