石 油 学 会 誌 Sekiyu Gakkaishi, 32, (2), 87-91 (1989) 87

Alpha-methylstyrene from Dehydrogenation of Cumene over Fe2O3-Cr2O3-K2CO3 Catalyst

Hiroshi MIURA*, Satoshi TAKAHASHI, Yohji MIZUSHIMA, Kazuo SUGIYAMA, and Tsuneo MATSUDA

Department of Applied Chemistry, Saitama University, Shimo-okubo, Urawa-shi 338

(Received July 29, 1988)

The rate of dehydrogenation of cumene over Fe2O3-Cr2O3-K2CO3 catalyst to derive α-methylstyrene has been studied. The reaction rate has been interpreted on the basis of a reaction model taking account

of competitive adsorption of cumene, α-methylstyrene, styrene and CO2. The kinetic parameters were compared with those of dehydrogenation of ethylbenzene. The dehydrogenation of cumene proceeded more rapidly than that of ethylbenzene because the effect of product retardation was less serious.

operated at ambient pressure. Kinetic parameters 1. Introduction were obtained by means of differential reactor While detailed studies have been made on the technique, keeping the cumene conversion less dehydrogenation of ethylbenzene1)-5), there have than 10% (in most cases less than 5%). Such been only few papers on the dehydrogenation of experiments were carried out at a temperature of alkylbenzenes other than ethylbenzene.5),7),8) 535℃, with catalyst weighing 0.5g, molar ratio The decomposition of cumene has frequently of cumene/H2O=1/12 and contact time of 10- been tried in studying the acid-base properties of 30g・h/mol. The rate equation thus obtained was oxide catalysts, since cumene is decomposed to verified along with the data obtained using the benzene and propylene over Bronsted acid catalysts9), integral reactor, in which case cumene conversion whereas it is dehydrogenated over base catalysts. The had changed from 5% to 80%. Such experiments product of dehydrogenation, α-methylstyrene (α- were carried out at temperatures of 535, 620 and MS), is recently being used as a raw material for such 660℃, with catalyst weighing 0.5-2g, and specialty plastics as heat resistant ABS resin and contact time of 10-100g・h/mol. modified alkyd or polyester resins. This reaction, 3. Results and Discussion however, has not been studied on kinetics using industrial catalysts. We examined this reaction Carra and Forni10) reported that the rate of over an industrial Fe2O3-Cr2O3-K2CO3 catalyst, for ethylbenzene dehydrogenation over Fe2O3-K2CO3- kinetic analysis, comparing the kinetic parameters Cr2O3 (Shell 105) was interpreted on the basis of a with those of dehydrogenation of ethylbenzene. kinetic model taking into account of the competitive adsorption of ethlbenzene and styrene. We also 2. Experimental suppose a similar model for cumene dehydrogenation; Catalyst: A commercial Fe2O3-K2CO3-Cr2O3 the Langmuir-Hinshelwood type unimolecular catalyst, Nissan Girdler G-64A (1/8" extrude), was reaction of adsorbed cumene, including competitive used. The extrude was ground and granules in the adsorption of reactant and products. The rate range of 20 and 60 mesh were submitted for equation is expressed as eq. (1). experiments. Since this catalyst is reduced (Fe2O3→Fe3O4) during the reaction, accompanied with a gradual change in activity, it was prereduced in a flow of H2/H2O (1:5 molar ratio) in the reactor at 600℃ for 5h. After this pretreatment, (1) there was no change in the catalytic activity except the first 1 hour on stream. When cumene conversion is low, the yield of Kinetic Measurements: The reaction was carried by-products (styrene, toluene, benzene, CO2, CH4, out using a conventional flow reactor equipped CO, C2H4) is very small because selectivity to α- with a stainless steel reactor tube (10mm diameter) methylstyrene is higher than 95%, and their

* To whom correspondence should be addressed. adsorption terms may be neglected (eq. 2).

石 油 学 会 誌 Sekiyu Gakkaishi, Vol. 32, No. 2, 1989 88

cate the mol percentage of cumene in the feed mix- (2) ture, eq. (3) becomes eq. (7), on the assumption that the amount of α-MS formed by the reaction is To simplify this equation further, the effect of much smaller than that added in the feed. The cumene pressure on the reaction rate was examined, equation is rearranged (eq. 8), which indicates that changing H2O/cumene ratio in the feed. It was plotting k/r against 1/M, a straight line should be found that the rate was independent of the obtained, both the slope and the intercept giving cumene pressure in the range of 40-120Torr. Equa- zα value. tion (2), therefore, is further simplified (eq. 3). r=kM/[M+zα(1-M)] (7) k/r=(1-zα)+zα/M (8) First calculating the approximate value of k using eq. (6), a temporary zα value was obtained by eq. (8). This temporary value was used for eq. (5) (3) to get more accurate value of k, which was again used in the calculation of zα from eq. (8). The

experimental data are shown in Fig. 2. The zα 3.1 Effect of α-Methylstyrene value obtained from the slope was in agreement Equation (3) shows that the rate is controlled by with that from the intercept, and the value was the value of zα, which indicates the relative decided to be 4.28. strength of α-MS adsorption with respect to 3.2 Effect of the By-Products cumene. The effect of adding α-MS to the feed When cumene conversion is high, considerable mixture was examined. Figure 1 shows that the amounts of by-products are formed and the effect reaction rate decreased significantly by the addi- of their adsorption should be included in the rate tion of α-MS. Then, determination of the zα value equation. We found the following by-products: following the method of Carra and Forni10) was CO, CH4, CO2, C2H4, benzene, toluene, styrene. tried. Because the formation of CO and C2H4 was very Let X indicate the conversion of cumene to α- small, the effect of their adsorption seems negligible. MS, eq. (3) becomes eq. (4) and k is obtained from The adsorption of methane seems very weak at eq. (5). When X is sufficiently small, eq. (5) is such high temperatures. When benzene or toluene simplified as eq. (6), using the approximation, was added to the feed, the rate of reaction changed -ln(1-X)≒X. only slightly. Thus, only the z values of styrene r=dX/dt=k(1-X)/[(1-X)+zαX] (4) (zst) and CO2 (zco2) were obtained, adding them to k=1/t[X(1-X)+zαln(1/1-X)] (5) the feed mixture. Analysis was made in the same

≒X/t (6) manner as the case of zα, and the results are When α-MS is added to the feed, and let M indi- shown in Table 1. Finally, eq. (9) was obtained as

Fig. 2 Relation of k/r to 1/M in Eq. 8 for Dehydrogena- Fig. 1 Effect of α-MS or Styrene Added to the tion of Cumene Feed Mixture on the Rate of Cumene Dehy- Relative adsorption coefficients, zα and zst, drogenation were calculated from the slope and intercept of

◎ no addition, ○ α-MS addition, ● styrene the straight lines. ○ α-MS addition, ○ styrene addition. Reaction temperature: 535℃ addition.

石 油 学 会 誌 Sekiyu Gakkaishi, Vol. 32, No. 2, 1989 89

Table 1 Kinetic Parameters for Dehydrogenation of Cumene and Ethylbenzene

Fig. 4 Relationship of Conversion and Reaction Temperature: Comparison of Dehydrogenation of Cumene with That of Ethylbenzene

□: cumene dehydrogenation, ○: ethylbenzene

dehydrogenation.

750℃ for 5hr. In the absence of the trap, however, the evolution of CO2 was very slow at temperatures of 600-650℃ and only 20% of K2CO3 was decomposed after 1hr at 650℃. The uptake of CO2 on the catalyst in which 98% of K2CO3 was decomposed was also examined, and it was found that the absorption process was slow; Fig. 3 Correspondence of Experimental Results with Calculated Values Using Eq. 9 only 30% of K2CO3 was restored after 3hr in contact with CO2 at 650℃. Thus, it was found Solid lines: calculated, using eq. 9

Plots: experimental results. □: 660℃, that the evolution-absorption of CO2 was a very

△: 620℃, ○: 535℃. slow process compared with the rapid chang in the catalytic activity following the concentration of the rate equation of cumene dehydrogenation over CO2 in the feed mixture. In such brief experiments, Fe2O3-K2CO3-Cr2O3 catalyst. the effect of CO2 seemed to be ascribed mainly to r=kPcu/(Pcu+zαPα+zstPst+zco2Pco2) (9) its adsorption, as supposed in eq. (9). zα=4.28, zst=1.35, zco2=2.39. 3.3 Comparison with Dehydrogenation of Eth- Equation (9) was compared with experimental ylbenzene results using an integral reactor, in the conversion Reaction rates in dehydrogenation of cumene range of 5-80%. As shown in Fig. 3, a good and ethylbenzene were compared at the same reac- agreement between the experimental results and tion conditions. Figure 4 shows that dehydrogena- the rate equation was found. tion of cumene proceeded more rapidly than that In eq. (9) we supposed that the effect of CO2 was of ethylbenzene. The former is thermodynamically limited to competitive adsorption with the reactant, more favorable than the latter11), but in this case, cumene. Hirano2) reported, however, that the conversion was kept low and the difference seemed presence of CO2 in the gas phase resulted in the to be due to kinetic reasons. An attempt was made decomposition of KFeO2, which was regared as the to obtain the rate equation for ethylbenzene active phase in the catalyst system. We tried to dehydrogenation, adding styrene to the feed estimate the extent of K2CO3 decomposition, mixture to determine z'(=bst/beb). As shown in which is contained in the fresh catalyst, at high Table 1, z'st for ethylbenzene dehydrogenation was temperature by measuring the amount of CO2 larger than zα for cumene, suggesting that the evolved. The catalyst in the amount of 1.0g was product retardation was more serious for ethyl- packed in a quartz U-tube, and the temperature benzene than for cumene. The rate constant, keb, elevated gradually while 50Torr of air was at 620℃ was smaller than kcu, whereas the recirculated. When the recirculating gas was activation energy Eeb was larger than Ecu. passed through a liquid nitrogen trap, more than Such differences in kinetic parameters are 98% of K2CO3 was decomposed by heating at understood by the relative stability of the reaction

石 油 学 会 誌 Sekiyu Gakkaishi, Vol. 32, No. 2, 1989 90

relative adsorption coefficient of product, zα, is smaller than z'st and, accordingly, effect of product retardation is less serious for cumene. 3. The relative strength of adsorption was decided to be as follows: α-MS>CO2>styrene> cumene>ethylbenzene.

Nomenclature

bx: adsorption coefficient of compound x zx: relative adsorption coefficient for cumene dehydro-

genation, bx/bcu z'x: relative adsorption coefficient for ethylbenzene dehydrogenation, bx/beb Px: partial pressure of compound x r: reaction rate, mol/g・min Fig. 5 Effect of Styrene Addited to the Feed k: reaction rate constant, mol/g・min Mixture of Ethylbenzene Dehydrogenation X: conversion of cumene

Reaction temperature; 575℃ t=W/F: contact time, g・min/mol M: mol percent of cumene in the feed mixture Ecu,Eeb: activation energy of dehydrogenation intermediates. From the z values of Table 1, the cu: cumene relative strength of adsorption of the reactants and α=α-MS: α-methylstyrene products are calculated as follows; eb: ethylbenzene bcu/beb=15.6, bα/bst=3.2. st: styrene It is clear that the effect of increase in the References adsorption strength of reactants (bcu>beb) is more significant than the increase in the effect of 1) Hirano, T., Shokubai (Catalyst), 29, 641 (1987). product retardation (bα>bst), resulting in smaller 2) Hirano, T., Applied Catal., 26, 65 (1986). value of zα than that of z'st. 3) Hirano, T., Applied Catal., 26, 81 (1986). 4) Hirano, T., Applied Catal., 28, 119 (1986). From the values of relative adsorption coefficients, 5) Miyata, K., Ozawa, S., Yamazawa, Y., Ogino, Y., Sekiyu the following order of adsorption strength was Gakkaishi, 31, 210 (1988). found: 6) Lee, E. H., Catal. Rev. Sci. Eng., 8, 285 (1973). α-MS>CO2>styrene>cumene>ethylbenzene. 7) Forni, L., Valerio, A., Ind. Eng. Chem. Process Des. Develop., 10, 552 (1971). 4. Conclusions 8) Tanaka, K., Fujiwara, H., Akaho, M., Sekiyu Gakkaishi, 29, 384 (1986); ibid, 29, 391 (1986). 1. The rate of dehydrogenation of cumene was 9) Tanabe, K., "Solid Acids and Bases" p. 127 (1970), interpreted on the basis of a Langmuir-Hinshelwood Kodansha, Tokyo. type kinetic model including the competitive 10) Carra, S., Forni, L., Ind. Eng. Chem. Process Des. Develop., 4, 281 (1965). adsorption of cumene, α-MS, styrene and CO2. 11) Ohta, N. ed., "Shokubai Kogaku Koza" vol. 7, p. 202 2. Dehydrogenation of cumene proceeds more (1969), Chijin Shokan, Tokyo. easily than that of ethylbenzene, because the

石 油 学 会 誌 Sekiyu Gakkaishi, Vol. 32, No. 2, 1989 91

要 旨

Fe2O3-Cr2O3-K2CO3触 媒 に よ る ク メ ン の α-メ チ ル ス チ レ ン へ の 脱 水 素 反 応

三 浦 弘, 高 橋 聡, 水 嶋 洋 三, 杉 山 和 夫, 松 田 常 雄

埼 玉 大 学 工 学 部 応 用 化 学 科, 338浦 和 市 下 大 久保255

Fe2O3-Cr2O3-K2CO3触 媒 を 用 い た ク メ ン の α-メ チ ル ス チ CO2を 加 え た と き の 反 応 速 度 の 変 化 か ら, 相 対 吸 着 係 数zα, レ ン へ の 脱 水 素 反 応 の 速 度 論 的 研 究 を行 っ た。 反 応 速 度 は, ク zst, zCO2を測 定 し, お の お の4.28, 1.35, 2.39で あ る こ とが メ ン, α-メ チ ル ス チ レ ン, ス チ レ ン, CO2の 競 争 吸 着 を 仮 定 わ か っ た。 エ チ ルベ ンゼ ンの 脱 水 素 に比 べ て クメ ンの脱 水 素 は し た Langmuir-Hinshelwood 型 の 速 度 式(9)で 整 理 で き た。 さ 容 易 に進 行 す る。 相 対 吸 着 係 数 の 比 較 か ら, エ チ ル ベ ンゼ ンの ら に, 原 料 中 に α-メ チ ル ス チ レ ン, ス チ レ ン, 脱 水 素 の 場 合 の 方 が 生 成 物 阻 害 の 影響 が 強 く現 れ る た め で あ る r=kPcu/(Pcu+zαPα+zstPst+zCO2PCO2) (9) こ とが わ か った。

Keywords Cumene, Dehydrogenation, Iron oxide catalyst, Kinetics, 2-Propenylbenzene

石 油 学 会 誌 Sekiyu Gakkaishi, Vol. 32, No. 2, 1989