OXIDATION OF ISOBUTENE OVER SELEN IU M DIOXIDE-MODI FI ED COPPER OXIDE CATALYSTS R . S. M A N N A N D K . C . Y A O Department of Chemical Engineering, University of Ottawa, Ottawa, Canada The vapor-phase catalytic air oxidation of isobutene to methacrolein over a supported copper oxide catalyst in the presence of selenium dioxide was investigated in a flow system to establish the conditions for maximum conversion, selectivity, and optimal amount of SeOz that can b e used for promotional effect. By using gas chromatographic technique accurate quantitative analyses of the products were obtained. The data for product distribution as a function of the operating conditions are presented. HE selective air oxidation of isobutene to methacrolein is of T c o n s i d e r a b l e importance. Though several patents (Barclay et al., 1961 ; Dowden and Caldwell, 1961 ; Farbenfabriken, 1960; Montecatini, 1960; Hadley, 1953, 1955, 1957) during the last 15 years have described the use of various metal oxides in the partial oxidation of isobutene to methacrolein, the kinetic studies have been reported only recently (Mann and Rouleau, 1964; Shapovalova et al., 1966). The air oxidation of propylene to acrolein over CuSeOa supported on activated alumina and silica gel has been studied recently (Kominami et al., 1962; Kruzhalov et al., 1962). They proposed that selenium assisted the rupture of the chain reaction for the decomposition of the allyl hydroperoxide radical or acted as a promoter for the decomposition of the allyl hydroperoxide radical or for the active centers of the catalyst during reaction. Kominami et al. found that feeding of selenium prevented decrease in the yield and enabled synthesis of acrolein in high concentrations for a long time. T h e effect of several additives upon the catalytic activity and selectivity of metals and semiconductors has been investigated for propylene and ethylene oxidation (Margolis et al., 1962). T h e additives having electronegative ( E ) value greater than that of
the catalyst decreased the activity of the catalyst and increased the work function and the selectivity of catalyst for the oxidation of ethylene and propylene. Also, in many patents (Hadley, 1953, 1955, 1957), it is claimed that the addition of selenium to oxidation catalyst increased the life of the catalyst and its selectivity in the oxidation of olefins to unsaturated aldehydes. However, the literature does not mention the selectivity and optimum amounts of selenium dioxide for different operating conditions in the oxidation of isobutene to methacrolein. This paper reports the effects of various amounts of selenium dioxide under different operating conditions on the conversion of isobutene to methacrolein. Experimental
T h e selective air oxidation of isobutene to methacrolein in a constant and continuous supply of selenium dioxide was investigated in an isothermal integral flow reactor. T h e equipment was constructed of 316 stainless steel and was similar to that used by Mann and Rouleau (1964), with slight modifications and addition of a selenium dioxide vaporizer. T h e apparatus used to study is shown schematically in Figure 1. The reactants (isobutene and air) were well mixed before they entered the preheating section. Two streams of air
t L
L
VP
Figure 1 . 81 Bz CiCz DlDz
Ice trap liquid nitrogen trap Temperature controllers Drying tubes
FIF~ VP GP
GM
Flow diagram for oxidation of isobutene
Furnaces Vapor fractometer Gas partitianer W e t gas meter
M1MzM3Ma R RlRzR3 .PiPzP3 PIPzP~
S
Manometers Reactor Rotameters Pressure gages
VOL. 6
TI T2 V
NO. 4
Sampling value Air tank lsobutene tank Selenium dioxide vaporizer
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1967
263
were used, one through the selenium dioxide vaporizer (Figure 2) to carry its vapor and the other bypassing it. Selenium dioxide vaporizer was made of 304 stainless steel tubing, 6 inches long and 1 inch in 0.d. with a Swagelok connection on the top. The vaporizer was heated in an electric furnace, the temperature of which was well controlled to within 1 0 . 5 ' C. The flow rate of the inlet air was kept constant and different amounts of selenium dioxide vapor carried by air were obtained by adjusting the temperature of the vaporizer. Thus selenium and its compounds (oxides) were uniformly distributed over the thin layer of catalyst bed. Since the catalyst used was in very small amounts, the selenium retained by the catalyst could not be determined very accurately. However, most of it was recovered from the product stream by condensation. An air condenser was installed in the exit line of the reactor to remove the condensed selenium, making sure that selenium did not pollute the air. Analysis of Products. T h e main features of the analytical procedure for analyzing the reaction products were described earlier (Mann and Rouleau, 1964). Catalyst. The pumice-supported copper oxide catalyst containing 16% by weight of copper was prepared by impregnating 20- to 40-mesh crushed pumice stone, supplied by the Fisher Scientific Co., with a copper nitrate solution and drying it a t 105' C. for 6 hours. T h e dried catalyst was subsequently calcined a t 500' C. for 6 hours and placed in the reactor. The catalyst was activated by passing air over it for 12 hours before any experimental run was made. Results and Discussion
The effect of various variables-weight ratio of selenium in the feed to the copper in the catalyst, 2, oxygen-isobutene ratio in the feed, R , and operating temperature, T-on the conversion, X,of isobutene, and the rates of formation, r, of various products, carbon dioxide, water, and methacrolein and the selectivity, S,of the catalyst for methacrolein production were investigated. The weight of the catalyst (0.6547 gram) and the weight of isobutene (0.3898 gram mole per hour) charged into the reactor were maintained constant. Different feed compositions (airisobutene ratios) were obtained by adjusting the air flow rate. While conversion is referred to as the moles of isobutene consumed (reacted) per hour to the moles of isobutene fed per hour, the rate of formation is referred as the moles of various
products formed per hour per gram of catalyst. The ratio of moles of methacrolein produced per hour to the moles of isobutene reacted has been defined as selectivity, S. The yields (a) are based upon the moles of methacrolein produced in proportion to the total products formed. Table I gives some typical results obtained from various runs, where feed composition, temperature, and ratio of selenium dioxide-copper in the catalyst were varied. The effect of different amounts of selenium dioxide in the feed on the conversion, rate of formation and selectivity for methacrolein for a W / F (reciprocal of space velocity) = 1.68, and oxygen-isobutene ratio, I? = 0.7168 a t 425' C. is shown in Figure 3. The conversion of isobutene increased rapidly with increased amounts of selenium dioxide up to Z = 0.0067, and then started decreasing with further amounts of selenium dioxide. T h e rate of carbon dioxide formation slowly decreased in the beginning, and decreased rapidly with increasing amounts of selenium dioxide up to Z = 0.020, beyond which there was no substantial decrease with further increased amounts of selenium dioxide. The rate of water formation increased slowly with increasing amounts of selenium dioxide up to 2 = 0.0067 and then decreased. The rate of methacrolein formation increased steadily with increased amounts of selenium dioxide up to 2 = 0.020, then decreased slightly. The selectivity for methacrolein, S, first increased rapidly with increased amounts of selenium dioxide and then decreased. T h e optimum amount of selenium dioxide giving the highest selectivity was 2 = 0.031 or 0.7y0 by weight of the pumice-supported catalyst. Figure 4 shows the effect of various oxygen-isobutene ratios in the feed, R, on the conversion, rate of formation, and selectivity for 2 = 0.020 a t 425' C. While the conversion of isobutene and rate of methacrolein formation increased steadily with feed ratios, the rates of carbon dioxide and water formation increased rapidly. T h e selectivity for methacrolein decreased with increased oxygen-isobutene ratios in the feed. Figure 5 shows the effect of temperature on the oxidation reaction a t a W / F ratio of 1.68, and oxygen-isobutene ratio of
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264
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l&EC P R O D U C T RESEARCH A N D DEVELOPMENT
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0.03
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GM. SELENIUM / GM. COPPER
Figure 3.
Effect of selenium dioxide on conversion Rate of formation and seectlvity
0,9910, in the presence of selenium dioxide (Z = 0.020). With increasing temperature from 350' to 425' C., the conversion increases, but the selectivity remains nearly constant. I n the oxidation of isobutene over copper oxide catalyst, the presence of selenium dioxide in smaller amounts reduces the formation of carbon dioxide, thereby increasing the selectivity of the catalyst for methacrolein production. This is in agreement with the findings of Margolis et al. (1962) and of Hauffe (1955). Enikeev et al. (1960) consider selenium dioxide as an acceptor impurity (ap-type impurity) in the copper oxide catalyst. T h e presence of selenium dioxide increases the hole concentration (decreases the free electron concentration) of the valence (condiction) band of the catalyst, thereby reducing the surface concentration of adsorbed oxygen.
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265
Conclusions
X
The catalytic air oxidation of isobutene over a pumice-supported copper oxide catalyst in the presence of varying amounts of selenium dioxide was investigated between 350’ and 425’ C., a TV/F ratio of 1.68, and oxygen ratio of 0.7 to 1.7 to establish the conditions for maximum conversion and yield. The highest selectivity of 50.17Y0 was obtained a t 30.8470 conversion a t 400’ C . in the presence of selenium dioxide (2= 0.020) and oxygen-isobutene ratio of 0.7168. However, a t a sacrifice in yield (33.2%) higher conversions (45%) were obtained a t 375’ C. for an oxygen-isobutene ratio of 1.6644 in the presence of selenium dioxide (2= 0.0067).
2 = g. of selenium in feed/g. of copper in catalyst
Ac knowledgment
The authors are indebted to the Selenium-Tellurium Development Association, Kew York, for financial support of the project and a fellowship to one of the authors (K. C. Y . ) .
Nomenclature
W = weight of catalyst, g. F = moles of isobutene per hr. = moles of oxygen/moles of isobutene in feed S = selectivity, moles of methacrolein formed/mole of isobutene reacted r = rate of formation, moles of product formed per g. of catalyst per hr. T = temperature, O C. @ = yield, moles of methacrolein formedlmole of total product
= conversion, moles of isobutene reacted per mole iso-
butene fed
literature Cited
Barclay, J. L., Bethell, J. R., Bream, J. B., Hadtey, D. J., Jenkins, R. H., Stewart, D. G., Wood, B. (to Distillers Co.), Brit. Patent 864,466 (April 6, 1961). Barclay, J. L., Hadley, D. J., Stewart, D. G. (to Distillers Co.), Brit. Patent 873,712 (July 26, 1961). Dowden, D. A . Caldwell, A. M. U. (to Imperial Chemical Industries), Brit. Patent 828,812 (Feb. 24, 1961). Enikeev, E. Kh., Isaev, 0. V., Margolis, L. Ya., Ktnet. Katal. 1, 431 (1960). Farbenfabriken Bayer A.-G., Brit. Patent 839,808 (June 29, 1960). Hadley, D. J. (to Distillers Co.), Brit. Patent 694,353 (July 22, 1953); 727,318 (March 30, 1955); U. S. Patent 2,716,665 (Aug. 30, 1955); 2,810,763 (Oct. 22, 1957). Hauffe, K., AdLan. Catalysts 7, 213 (1955). Kominami, N., Kogyo Kagaku Zasshi 65, 1514-33 (1962). Kruzhalov, B. D., Shestukhin, E. S., Garnish, A. M., Ktnet. Katal. 3, 247-51 (1962). Mann, R. S., Rouleau, D., IND.ENG.CHEM.PROD.RESEARCH 3, 94 (1964). DEVELOP. Margolis, L. Ya., Enikeev, E. Kh., Isaev, 0. V., Krylova, A. V., Kushnerov, M. Ya., Ktnet. Katal. 3, 181 (1962). Montecatini, Brit. Patent 847,564 (Sept. 7, 1960). Shapovalova, L. P., Gorokhovastiskir, Ya. B., Rubanik, M. Ya., Kinet. Katal. 5 , 330 (1966). RECEIVED for review April 27, 1967 ACCEPTEDOctober 2, 1967 Material supplementary to this article has been deposited as Document No. 9680 with the AD1 Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington, D. C. A copy may be secured by citing the document number and by remitting $1.25 for photoprints or $1.25 for 35-mm. microfilm. Advance payment is required. Make checks or money orders payable to Chief, Photoduplication Service, Library of Congress.
OXIDATION OF LEVOPIMARIC ACID, PINE GUM, AND VARIOUS ROSINS WITH SINGLET OXYGEN WALTER H . SCHULLER AND RAY W.
LAWRENCE
AVaL’alStores Laboratory, Southern Utilization Research and Development DiLiision, U. S. Department of Agriculture, Olustee, Fla.
acid (I) is now a readily obtainable raw ma(Summers el ul., 1963), available from pine gum. The photosensitized oxidation of I has been shown to give levopimaric acid transannular peroxide (Moore and Lawrence, 1958; Schuller et al., 1964) (11) (Procedure a), a reactive multifunctional, potentially useful intermediate. EVOPIMARIC
L terial
+
a)AIR LIGHT+ SENSlTlZlNB DYE
I t has been claimed recently that “singlet oxygen (Procedure b) displays a chemistry identical to that of the intermediate in photosensitized oxidation” (Foote et al., 1965). We have found that singlet oxygen, generated by the reaction of sodium hypochlorite and hydrogen peroxide, does give 11. Similarly, the reaction of pine gum and gum rosin with singlet oxygen also gives mixtures of peroxides similar to those prepared by photosensitized oxidation (Schuller et a/., 1964). Discussion of Results
COOH
I 266
I
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
The sensitivity of levopimaric acid transannular peroxide (11) to base (Moore and Lawrence, 1959) led to a n attempt to
