Surface Heterogenity of Bismuth-Molybdate Catalyst in Oxidative Dehydrogenation of Olefin* by Toru Watanabe** and Etsuro Echigoya** Summary: The oxidative dehydrogenation of C4, C5 olefins over bismuth molybdate catalyst has been investigated by a flow method and a pulse technique. The isomers of pentene were found to be less reactive than those of corresponding butenes in the case of using the flow method, but the reverse result was obtained by means of a pulse technique. The activation energy of the reaction obtained by the former was much larger than that of the latter. The value of activation energy isoprene added to the pulse of 1-butene. It was found that the reaction was strongly inhibited by isoprene added. The heat of adsorption of olefins and dienes was also measured by the gaschromatographic method. From these results, it was suggested that the active sites of bismuth molybdate catalyst were ununiformity. The pulse reaction proceeded on the strong active sites which were inhibited by the products in the flow reaction, and the mild active sites which had a similar activity were only effective for the flow reaction. 1 Introduction The novel investigation by means of the differential isotopic method (DIM) has been demonstrated by Zimin and Yanovskii1) to confirm the surface heterogenity of catalyst. They applied this technique to the bismuth molybdate catalyst in order to study the surface uniformity. That is to say, after successive adsorption of two kinds of butenes i. e., n-butene and 14C labeled one on the bismuth molybdate catalyst, the reactor temperature was gradually raised, then the desorption products were introduced into the analytical equipment. The specific radioactivity of the desorption products did not change during gradual desorption over the tem- tions. It was pointed out by Adams5) and Morita6) that the inhibitional effect of isoprene produced was responsible for the low activity of isopentenes. In this work the oxidative dehydrogenation of C4, C5 olefins over bismuth molybdate catalyst was studied and the reactivities of various olefins, the rates of reaction and the activation energies obtained by the flow method were compared with those by the pulse technique. The nature of the active sites of bismuth molybdate catalyst was discussed from the viewpoints of the effect of dienes addition on the activation energy and the heats of adsorption of butenes and diene for this catalyst, measured by the gaschromatographic method. they pointed out the uniformity of the portions 2 Experimental of the surface participation in the oxidation reaction. The studies on the catalytic oxidative dehydrogenation of C4 and C5 olefins over bismuth molybdate catalyst were reported in our previous n-butenes to butadiene was found to proceed quite selectively. However, the activity of isopentenes to isoprene by oxidative dehydrogenation was fairly low under the same condi- * Received December 14, 1971. ** Department of Chemical Engineering, Tokyo Institute of Technology (Ookayama, Meguro-ku, Tokyo, Japan) 2.1 Raw Material Pure grade (99%) 1-butene, cis-2-butene, trans-2-butene and 3-methyl-1-butene (imported by Takachiho Trading Co.) were used directly from cylinders. Reagent grade 1-pentene and isoprene were obtained from Tokyo Kasei Co., and butadiene was provided by Japan Synthetic Rubber Co. and was pure grade raw material for polymerization. 2.2 Catalyst Bismuth molybdate catalysts used in this report were prepared by the co-precipitation
Surface Heterogenity of Bismuth-Molybdate Catalyst in Oxidative Dehydrogenation of Olefin method from an ammonium molybdate solution by mixing with acidified bismuth nitrate and 4N-ammonium hydroxide so as to give a final ph of 6.0. The precipitate obtained was washed ratio was found to be about 1.0 by chemical analysis. 2.3 Experimental Procedure 2.3.1 Flow Method ~ The reactor was a 13mm ID pylex glass tube containing a concentric thermowell, and was charged with 1.0g of the catalyst, diluting to 10ml with quartz chips of the same size. The activities of the catalyst were tested at constant flow rates of 1-butene, oxygen and nitrogen The inhibitional effects of diene were investigated by the reaction using mixture of 1- butene (0.1atm), oxygen (0.1atm) and isoprene those of corresponding butenes for both the oxidative dehydrogenation and the isomeriza- constant by changing the content of nitrogen in the mixture. 2.3.2 Pulse Method The reactor was a pylex glass U-tube (4mm ID), and was immersed in a fluidized thermal diluted with quartz chips were packed. Feed pulse of the reaction mixture was injected into a stream with a micro-syringe or a doser. The flow rate of the carrier gas, helium containing in order to obtain the rates of oxidative dehydrogenation of olefins. The same temperature ranges used in a flow method were applied to a pulse reaction technique. bed, then the inhibition caused by the produced isoprene was evaluated by addition of isoprene. The reactant was diluted with nitrogen, if necessary. 2.3.3 Heat of Adsorption The heats of adsorption of C4 olefins and diene Volume 14, No. 1, May 1972 over bismuth molybdate catalyst were measured by the usual gaschromatographic technique. All the experiments were carried out in helium carrier (50ml/min), with 25g of the catalyst and amounts of reactants were injected into the catalyst bed from micro-syringe, and then the heat of adsorption was calculated on the basis of retention-time lag of adsorbed reactant gas. 2.4 Analysis A gas chromatograph was used for the analysis of the reactants and products. For the analysis of olefins, dienes and CO2 40wt% propylene and Molecular Sieve 5A (1m) was used to separate N2, O2 and CO. 3 Results and Discussion The reactivities of olefins were measured under the same experimental conditions by means of the flow method. The reactivity of 1-butene or t-2-butene was compared with 1-pentene or t-2-pentene, respectively, at the temperature of of pentenes are substantially less reactive than On the other hand, 1-pentene was more reactive than 1-butene when the pulse technique was used as shown in Table 1-b). The above results can be explained reasonably Table 1 Reactivity of C4, C5 Olefins
Watanabe and Echigoya: Surface Heterogenity of Bismuth. as follows, considering other results concerning the inhibitional effect of dienes. (1) In the case of using the flow method, the reaction was inhibited considerably by isoprene produced, but not by the addition of butadiene. (2) In the case of using the usual pulse technique, the reaction was not inhibited by diene produced, because of the non-stationary reaction proceeding over the clean surface of the catalyst. 3.2 The Rate of Reaction 3.2.1 Reaction of 1-Butene A combination of parallel and consecutive reaction system was given in the following processes. All of the oxidative dehydrogenation and isomerization of n-butenes in this system was assumed to be the first order with respect to n- butene and independent of oxygen. The activation energy for the oxidative dehydrogenation reaction of 1-butene was found to be 40kcal/mol. On the other hand, the rate of oxidative dehydrogenation using the pulse technique could be expressed in terms of a first order equation with respect to 1-butene. The activation energy obtained was 11kcal/mol, which was much less than that obtained using the flow method. This difference in the value of activation energy between both methods can be explained by assuming that the catalyst had undergone inhibition by the products that were more strongly adsorbed than butadiene in the flow method. 3.2.2 Reaction of 3-Methyl-1-Butene The rate of oxidative dehydrogenation of 3-methyl-1-butene which was most easily converted to isoprere among isoamylenes, was studied. The following equation was proposed by Adams5) on the assumption that dienes operated by competing with oxygen and displacing it from part of the surface of the bismuth molybdate catalyst the following rate equation was presented. where K, and K' are equilibrium constants for adsorption of olefin and dienes respectively. The effect of particial pressure of oxygen on the conversion of olefin was little within the range of experimental conditions examined. Therefore, Eq. (2) can be simplified as Eq. (2'), because the term for oxygen in Eq. (2) is a constant, At first, K and k were determined at a low conversion of 3-methyl-1-butene in the partial Experiments were carried out under the constant partial pressure (0.1atm) of 3-methyl-1- butene by changing the amount of catalysts. The reaction was proceeded with high selectivity under the experimental conditions. As shown in Fig. 1, a good linear relationship was obtained between (1/r-1/k) P and P'. The full line in Fig. 2, which was calculated on the where, Ko2 and K'; equilibrium constants for adsorption of oxygen and diene, respectively. P, Po2 and P'; partial pressure of olefin, oxygen and diene, respectively. In a previous paper2), we proposed that the reaction mechanism could be considered to be the dual function catalysis type. Consequently, Fig. 1 Verification of Rate Equation for 3-Methyl- 1-Butene
Molybdate Catalyst in Oxidative Dehydrogenation of Olefin Fig. 2 Variation of Isoprene Yield from 3-Methyl- 1-Butene with Contact Time (W/F) Fig. 3 Inhibitional Effect of Isoprene (Flow Method) basis of using Eq. (2'), agrees with the experimental data satisfactorily. From the temperature-dependency of k, K and K', the activation energy of the reaction and the heat of adsorption of 3-methyl-1-Butene and isoprene over bismuth molybdate catalyst were estimated to be 42kcal/mol, 12kcal/mol and 15kcal/mol, respectively. The activationenergy of oxidative dehydrogenation of 3-methyl- 1-butene estimated by the pulse technique was found to be 8.3kcal/mol, and this value was also much less than that for the flow method. The surface of the bismuth molybdate catalyst is considered to be heterogeneous from the large difference between the values of activation-energy obtained by means of the flow method and the pulse technique. The value of frequency factor calculated on the basis of the data obtained by the flow method was about 105 times as large as that of the pulse technique. The results obtained indicate that there exists a distribution of the kind of active centers on the catalyst surface. 3.3 Inhibitional Effect of the Pressure of Dienes 3.3.1 Flow Method The rate-equation for the oxidative dehydrogenation of 1-butene was expressed as follows when a constant volume of isoprene is fed with reactants. r=kkp/(1+k'p') (3) Eq. (3) is transformed to Eq. (3') 1/r=1/kKP+K'P'/kKP (3') Fig. 4 Inhibitional Effect of Isoprene on Butadiene Yield by Pulse Technique As shown in Fig. 3, a linear relationship was obtained between 1/r and P'. The value of slope (K'/kKP) was found to be independent of the pressure of isoprene added, hence it is considered that the active sites of the catalyst contributed the reaction are uniformity. Fig. 3 gives the activation-energy of the reaction of 38kcal/mol and the heat of adsorption of iso- values agree with that described in the preceding section. 3.3.2 Pulse Method When various amounts of butadiene were added to 1-butene pulse at the temperature served. However, when isoprene was fed with 1-butene, inhibition effect was observed as shown in Fig. 4 and 5. From these results, the acti- Volume 14, No. 1, May 1972
Surface Heterogenity of Bismuth-Molybdate Catalyst in Oxidative Dehydrogenation of Olefin Fig. 5 Inhibitional Effect of Isoprene (Pulse Technique) on Isomerization Reaction Fig. 7 Inhibitional Effect of Isoprene (Pulse Technique) on Activation Energy Fig. 6 Inhibitional Effect of Isoprene (Pulse Technique) on Arrhenius Relationship vation-energy of the oxidative dehydrogenation was calculated by using the Arrhenius relationship (Fig. 6) on the assumption of the first order kinetics with respect to 1-butene. As can be seen in Fig. 6, the strong effect of inhibition was observed in the low temperature range. Fig. 7 shows that the value of activationenergy of the reaction increased with the amount of isoprene added in the reactant. Therefore, the strongly active sites of the catalyst surface are poisoned by the isoprene adsorbed. Con sequently, the reaction seems to proceed on the milder active sites. 3.4 Heat of Adsorption The heat of adsorption of n-butene and butadiene over bismuth molybdate catalyst was measured by the usual gaschromatographic technique. Fig. 8 gives the relationship between the heat of adsorption and the size of pulse. If the dependency of the size of pulse can be substituted for that of the coverage, the Fig. 8 The Influence of Pulse Size on Heat of Adsorption over Bi2O3-MoO3 Catalyst by Means of Pulse Technique variation of heat of adsorption on the coverage is especially large in the low coverage range. The initial heat of adsorption of 1-butene ob- It was considered that the severe decrease in the heat of adsorption with increasing degree of surface coverage is attributed to the ununiformity of the active sites. Literature 1) Zimin, R. A., Yanovskii, M. I., Kinetika i Kataliz, 8, (4), 936 (1967). 2) Watanabe, T., Echigoya, E., Kogyo Kagaku Zasshi, 74, (1), 40 (1971). 3) Watanabe, T., Kuwajima, H., Echigoya, E., ibid., 74, (1), 44 (1971). 4) Watanabe, T., Kawakami, T., Echigoya, E., ibid., 74, (11), 2281 (1971). 5) Adams, C. R., 3rd. Intern. Cong. on Catalysis, Amsterdam (1964). 6) Nishikawa, E., Morita, Y. et al., Kogyo Kagaku Zasshi 73, (7), 1674 (1970).