The Oxidation Activity and Acid-base Properties of. V2O5-K2SO4-H2SO4 Catalysts* by Mamoru Ai**

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1 The Oxidation Activity and Acid-base Properties of V2O5-K2SO4-H2SO4 Catalysts* by Mamoru Ai** Summary: The vapor-phase oxidation of 1-butene, butadiene, and acetic acid were carried out in the presence of excess air over two series of V2O5-K2SO4-H2SO4 catalysts, V2O5-K2SO4- H2SO4 (2-x-1, mole ratio) and V2O5-K2SO4-H2SO4 (2-1-y) where x and y were varied of the catalysts was investigated. The acidity and basicity of V2O5-K2SO4-H2SO4 catalysts were obtained from the values of dehydration activity for isopropyl alcohol (IPA), rp, and the ratio of (dehydrogenation activity for IPA)/(dehydration activity for IPA), ra/rp, which were measured in the presence of excess air. It has been found that the acid-base properties of the catalysts are changed largely by the contents of K2SO4 and H2SO4, and that oxidation activity and selectivity can be interpreted in terms of the acid-base properties of the catalysts. It can be concluded that the acid-base conception in which the catalytic activities are governed by the acidbase properties between the catalyst and reactant is still effective in explaining the function of the K2SO4 and H2SO4 added to V2O5. 1 Introduction The V2O5-K2SO4-H2SO4 (V2O5-K2O-SO3) catalyst system has been typically used for a long time in synthesis of sulfuric acid from sulfur dioxide and, moreover, this catalyst system is effective for selective oxidation of 0-xylene and naphthalene to form phthalic anhydride. Many system, and this modification, in turn, induces a determine the role of K2SO4 in this system in connection with the structural, electronic, and physical properties of the catalysts, but no definitive explanation has yet been made. It is important to note the evidence reported by Kakinoki et al.5),8),9) that the presence of H2SO4 in the V2O5-K2SO4 system is indispensable for oxidation of hydrocarbons. In previous works, we have pointed out that the activities and selectivities in mild oxidation reactions are governed by the acid-base properties of the catalyst and the reactant, and that this conception is effective for explaining the results of oxidation over many MoO3 and V2O5-based catalysts such as MoO3-P2O510), MoO3-Bi2O3- P2O511),12), MoO3-TiO213), MoO3-SnO213),14), MoO3-Fe2O313), MoO3-V2O515), V2O5-P2O516), * Received December 10, ;** Research Laboratory of Resources Utilization, Tokyo of Technology (Ookayama, Meguroku, Tokyo 152) V2O5-TiO213), V2O5-SnO213),17), and V2O5- Fe2O313). We have also found from our earlier studies18) that the addition of K2SO4 to V2O5 decreases the acidity of the catalyst remarkably, while that of H2SO4 to V2O5 increases the acidic nature. This fact leads us to expect that the addition of K2SO4 as well as H2SO4 to V2O5 modifies the acid-base properties of the catalyst change in the catalytic behavior. In order to clarify the functions of K2SO4 and H2SO4 in this catalyst system, we have attempted in the present work to ascertain how the addition of K2SO4 to V2O5-H2SO4 and that of H2SO4 to V2O5-K2SO4 modify their acid-base properties as well as their oxidation activity and selectivity, and to see whether or not the catalytic behavior can be interpreted in connection with the acidbase properties between the catalyst and the reactant. 2 Experimental 2.1 Catalysts The catalysts used in this study were two series of V2O5-K2SO4-H2SO4 catalysts. In Series A, V2O5-K2SO4-H2SO4 (2-x-1, mole ratio); the the H2SO4/V2O5 ratio constant at 1/2. In Series B, V2O5-K2SO4-H2SO4 (2-1-y, mole ratio); the

2 Ai: The Oxidation Activity and Acid-base Properties of V2O5-K2SO4-H2SO4 Catalysts K2SO4/V2O5 ratio constant at 1/2. The catalysts were prepared as follows: the required quantities of NH4VO3 and H2SO4 were dissolved in hot water by using oxalic acid. A solution of K2SO4 was 20 mesh pumice was mixed with it; then the mixture was evaporated with vigorous stirring. The catalysts were calcined in a flow of air at 2.2 Acidity and Basicity Measurements Since the surface areas of V2O5-K2SO4-H2SO4 catalysts are very small (about 0.7m2/g), it is not easy to measure accurately the amount of NH3 or CO2 irreversibly adsorbed. However, from a comparison of the catalytic activities for isopropyl alcohol (IPA) with the acidity-basicity data directly measured on many kinds of mixed metal-oxide catalysts such as MoO3-Bi2O3-P2O512), MoO3 TiO213), MoO3 SnO213),14) MoO3-Fe2O313) V2O5-TiO213), V2O5-SnO213),17), V2O5-Fe2O313), WO3-P2O519), WO3-P2O5-XnOm19), and TiO2-Xn 0m20) (XnOm: various kinds of metal oxides), it has been found that the activity for dehydration of IPA to propylene (rp) represents the acidity of the catalyst and that the value of ra/rp ratio, where ra is the activity for dehydrogenation of IPA to acetone, is also valid enough as an index of basicity. That is, (dehydrogenation rate) rate) (dehydration These results prove that both acidic and basic sites take part almost equally in the dehydrogenation of IPA: This reaction may proceed by a concerted mechanism, for example12): Therefore, we used in this study the values of rp and ra/rp as measures of acidity and basicity of the catalysts, respectively. 2.3 Catalytic Activity Measurements The vapor-phase oxidation of 1-butene, 1,3- butadiene, and acetic acid and dehydration and dehydrogenation of IPA were carried out in an ordinary continuous-flow-type reaction system. The concentration of butene, butadiene, acetic acid, and IPA were about 0.67, 0.67, 1.5, 1.65 Volume 18, No. 1, May 1976 Fig. 1 Effect of K2SO4 Content on the Surface Area mol% in air, respectively. The total flow rate 3 Results and Discussion 3.1 Effect of K2SO4 Added to V2O5-H2SO4 Catalyst System In this section, measurements were performed over a series of V2O5-K2SO4-H2SO4 (2-x-1) catalysts with different content (x) of K2SO Surface Areas The surface areas of V2O5-K2SO4-H2SO4 (2-x-1) catalysts were checked by the BET shows the dependency of the surface area on K2SO4 content (x). It is found that the surface area decreases with the addition of a small amount of K2SO4 (x<0.4) to V2O5-H2SO4 (2-1), and with a further increase in the K2SO4 content, it remains constant at about 0.7m2/g Acidity and Basicity The rates of dehydration and dehydrogenation of IPA; rp and ra (mol/hr.g-cat), which are known to be almost zero-order with respect to the IPA concentration10),15),21),22), were obtained from the data at low conversion of IPA. The values of rp/s and ra/rp are plotted as a function of K2SO4 content in Fig. 2. By increasing the content of K2SO4, the value of rp/s, namely, acidity, sharply decreases at x<0.2; while the value of ra/rp namely, basicity, slightly increases with K2SO4 content at x<0.2, but it is very low over the whole range of x.

3 Ai: The Oxidation Activity and Acid-base Fig. 2 Values of rp/s and ra/rp as a Function K2SO4 Content (x) of as a Func- Fig. 4 Selectivity to Maleic Anhydride tion of the K2SO4 Content (x) Fig. 3 Oxidation Activities for Butadiene and Acetic Acid as a Function of K2SO4 Content (x) It may be natural that, because of the high content of H2SO4, the addition of K2SO4 cannot decrease the acidity so markedly as in the case where K2SO4 is added to pure V2O5, and that V2O5-K2SO4-H2SO4 (2-x-1) catalysts are highly acidic and scarcely basis Oxidation Activity for Butadiene As a measure of oxidation activity for basic reactants, the initial rate of overall consumption convenience in the experimental procedures. K2SO4 content in Fig. 4. function of K2SO4 (x) in Fig. 3. The rb/s value decreases with K2SO4 content (x) in the same fashion as the rp/s value shown in Fig. 2. This correlation leads us to consider that oxidation of a basic reactant is governed by the activation of the reactant on the acidic sites of the catalyst, which may associate with the acid-base type affinity between the catalyst and the reactant Oxidation Activity for Acetic Acid Attention is now given to the oxidation activity for acidic reactants. As a typical reactant among these compounds, acetic acid was chosen. The initial rate of oxidation of acetic acid to value of ra/s is also shown in Fig. 3. The change of K2SO4 content causes only a slight change in the ra/s value, but a maximum of the ra/s curve is similar to that of the ra/rp curve shown in Fig. 2. This fact suggests that the oxidation reactions is governed by activation of the acidic reactant on the basic sites of the catalyst Selectivity in Olefin Oxidation Since V2O5-K2SO4-H2SO4 catalysts are acidic, as has been mentioned above, the selectivity in oxidation of both butene and butadiene to maleic anhydride in the presence of excess air (0.67mol% for every catalyst and plotted as a function of the The selectivity in oxidation of butadiene decreases steadily with increase in the K2SO4 con-

4 Properties of V2O5-K2SO4-H2SO4 Catalysts tent, namely, with a decrease in the acidity of the catalyst. This indicates that acidic nature is indispensable for oxidation of butadiene to maleic anhydride, which can be understood from the On the other hand, the selectivity of maleic anhydride in the oxidation of 1-butene is very low in the case of V2O5-H2SO4 (2-1) catalyst, but it increases with increasing K2SO4 content and shows a broad maximum at x=0.3, although it is always lower than the selectivity in the oxidation of butadiene. The results can be understood from the evidence16) that, when butene is oxidized over V2O5- based catalysts not containing P2O5, the C-C fission of butene takes place in preference to the allylic C-H fission in the first step of butene oxidation, and the selectivity to maleic anhydride is greatly controlled by the C-C fission which is assumed to be related to the acid strength of the catalyst, and that the addition of P2O5 to V2O5-based catalysts plays a role in lowering the acid strength and, consequently, in suppressing the C-C fission of butene. According to these considerations, it may be supposed that the addition of K2SO4, as that of P2O5, plays a role in lowering the acid strength and in suppressing the C-C fission of butene. 3.2 Effect of H2SO4 Added to V2O5-K2SO4 Catalyst System Another series of experiments was performed using V2O5-K2SO4-H2SO4 (2-1-y) catalysts with different H2SO4 contents (y) Surface Areas The surface areas of V2O5-K2SO4-H2SO4 (2-1-y) catalysts are shown as a function of H2SO4 content (y) in Fig. 5 (broken line). The surface area is very small and remains constant at about small amount of such acidic substance as H2SO Acidity and Basicity The values of rp/s and ra/rp are plotted as a function of H2SO4 content in Fig. 5. The value of rp/s, namely, acidity, is low in the low range of H2SO4 content (y<0.5), but with a further increase in the H2SO4 content, it increases and goes Fig. 5 Surface Area and Values of rp/s and ra/rp as a Function of H2SO4 Content (y) Fig. 6 Oxidation Activities for Butadiene and Acetic Acid as a Function of H2SO4 Content (y) content, acidity increases but basicity decreases. However, it is worth noting that addition of a increases acidity to a significant extent, as in the case where MoO3 is combined with TiO2, SnO2, and Fe2O213),14) Oxidation Activities In Fig. 6 the values of both rb/s and ra/s are shown as a function of H2SO4 content (y). through a broad maximum at about y=2. On It was found that the trends of rb/s and ra/s curves the other hand, the value of ra/rp, namely, bisicity, are similar to those of rp/s and ra/rp (shown in sharply increases with the addition of a small Fig. 5), respectively. These results also suggest that the oxidation reactions are governed by the very low when y is greater than 1. activation of the reactants which may be ascribed It may he natural due to the acidic nature of to acid-base type affinity between the reactant H2SO4 that, with an increase in the H2SO4 and the catalyst. Volume 18, No. 1, May 1976

5 Ai: The Oxidation Activity and Acid-base Properties of V2O5-K2SO4-H2SO4 Catalysis sites contribute to the activation of acidic reactants such as acetic acid. We would like to consider that the function of the K2SO4 and H2- SO4 added to V2O5-based catalysts is to modify the acid-base properties to be proper for the intended reactions. References as a Func- Fig. 7 Selectivity to Maleic Anhydride tion of H2SO4 Content (y) 1) Kiyoura, R., Nippon Kagaku Kaishi, 61, 72 (1940); Ryusan, 2, 223 (1949). 2) Frazer, J. H., Kirkpatric, W. J., J. Am. Chem. Soc., 62, 1659 (1940). 3) Boreskov, G. K., Kasatkina, L. A., Popovski, V. V., Balovnev, Yu. A., Kinelika i Kataliz, 1, 229 (1960). 4) Tarama, H K., Teranishi, S., Yoshida, S., Tamura,., Yoshida, H., Shokubai, 3, 187 (1961) Selectivity in Olefin Oxidation In Fig. 7 the selectivity of both butene and butadiene to form maleic anhydride is shown as a function of H2SO4 content (y). The results indicate that the acidic property is indispensable for "acid-formation" reactions. 4 Conclusion It was found that acid-base properties of V2O5- K2SO4-H2SO4 catalysts are changed largely by the contents of K2SO4 and H2SO4, and the oxidation activity and selectivity can be interpreted in connection with the acid-base properties of the catalysts, as in the case of many V2O5 and MoO3-based mixed oxide catalysts. The results obtained from the V2O5-K2SO4-H2SO4 catalysts also follow our theory that, in mild oxidation reactions, oxidation activity is governed by activation of the reactants and that the acidic sites of the catalyst contribute to the activation of basic reactants such as olefins, and the basic 5) Kakinoki, H., Sahara, N., Kamata, I., Aigami, Y., ibid., 4, 113 (1962). 6) Sakamoto, K., Ishida, K., Kato, A., Seiyama, T., ibid., 7, 338 (1965). 7) Seiyama, T., Chemistry and Chemical Industry, 21, 1002 (1968). 8) Kamata, I., Tanaka, T., Kakinoki, H., Suzuki, H., Shokubai, 5, 216 (1963). 9) Oguma, I., Aigami, Y., Mizushima, F., Kakinoki, H., Suzuki, H., ibid., 6, 50 (1964). 10) Ai, M., Suzuki, S., J. Catalysis, 30, 362 (1973). 11) Ai, M., Ikawa, T., ibid., 40, 203 (1975). 12) Ai, M., Suzuki, S., Bull. Japan Petrol. Inst., 16, (2), 118 (1974). 13) Ai, M., Ikawa, T., Shokubai, 17, 10p (1975). 14) Ai, M., J. Catalysis, 40, 328 (1975). 15) Ai, M., Suzuki, S., Nippon Kagaku Kaishi, 1972, ) Ai, M., Suzuki, S., Bull. Chem. Soc. Japan, 47, 3074 (1974). 17) Ai, M., J. Catalysis, 40, 318 (1975). 18) Ai, M., Suzuki, S., Shokubai (Catalyst) Meeting, 33rd Preprint, 116 (1973). 19) Ai, M., Shokubai, 17, 87p (1975). 20) Ai, M., Shokubai (Catalyst) Meeting, 37th Preprint, 58 (1975). 21) Ai, M., Suzuki, S., Bull. Japan Petrol. Inst., 15, (1), 18 (1973). 22) Ai, M., Suzuki, S., Bull. Chem. Soc. Japan, 46, 321 (1973).