I n d . Eng. Chem. Res. 1989,28, 1587-1589 Contaminated with TCE and PCE: Effect of Lime Softening. Annual Conference,Amer. Water Works Assoc., Los Angeles, CA, June 18-22,1989. Staehelin, J.; HoignB, J. Decomposition of Ozone in Water: Rate of Initiation by Hydroxide Ion and Hydrogen Peroxide. Enuiron. Sci. Technol. 1982, 16, 676.
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Staehelin, J.; HoignB, J. Decomposition of Ozone in the Presence of Organic Solutes Acting as Promoters and Inhibitors of Radical Chain Reactions. Enuiron. Sci. Technol. 1985, 29,1206.
Received for review April 5, 1989 Accepted June 28, 1989
Catalytic Hydroxylation of Benzene on Telluric Acid Dispersed on Silica Tatsuo Fukushi, Wataru Ueda,* Yutaka Morikawa, Yoshihiko Moro-oka, and Tuneo Ikawat Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 227 Japan
Hydroxylation of benzene by O2 at 748-834 K over carrier-supported telluric acid catalysts has been investigated. It has been found that silica-supported telluric acid in the presence of water is catalytically active for the formation of phenol. Catalysts supported on other carriers and unsupported solid telluric acid showed no ability to hydroxylate benzene to phenol. Finely dispersed telluric acid is suggested as the active site; its dispersion is thought to be especially stabilized on silica gel by the presence of water vapor. Recently, catalytic hydroxylations of alkanes and aromatic compounds have been attempted extensively in liquid-phase or gas-phase systems. The reactions are of interest in connection with the C-H bond activation and the modeling of enzymatic catalysis and are of great importance in recent industrial trends toward the utilization of cheaper feedstocks and the development of highly efficient synthetic processes. It is well-known that aromatic compounds are oxidized to phenols in the liquid phase in many metallic systems, such as Fenton’s reagent (Fe2+-H202)(Walling, 1975), the Udenfriend system (Fe2+-EDTA-ascorbic acid-02 or H202)(Udenfriend et al., 1954), and Hamilton’s reagent (Fe3+-catechol-H202) (Hamilton and Friedman, 1963). The former two cases involve the hydroxyl radical as the hydroxylating agent, while the latter is thought to involve a catechol-metal ion complex as the electrophilichydroxylatingspecies. Among the various possible hydroxylations, catalytic hydroxylation of benzene to phenol with an oxidant, especially 02,has been a long-standing objective. Sasaki et al. reported that benzene was hydroxylated by air under ambient conditions in the presence of cuprous chloride (Kinoshita et al., 1983). Recently, Takehira et al. succeeded in oxidizing benzene with the extended cuprous chloride-dioxygen catalytic system (Orita et al., 1987). Other effective systems are synthetic metalloporphyrins with various oxidants, (Groves and Nemo, 1983; Guengerich and Macdonald, 1984) and dioxygen-macrocyclic polyamine-metal complexes (Kimura and Machida, 1984), which are of interest because of their resemblance to enzymatic hydroxylations. In contrast to the well-known success of liquid-phase hydroxylations, little has been reported on the gas-phase oxidation of aromatic compounds by heterogeneous catalytic systems. Although publications have appeared on the one-step catalytic oxidation of benzene to phenol in heterogeneous systems such as Cu-P-O catalyst with dioxygen (Ioffe and Levin, 1961) or with a silica-supportedvanadium oxide catalyst that uses N20 as an oxidant (Iwamoto et al., Present address: Department of Industrial Chemistry, Faculty of Engineering, Kantogakuin University, 4834 Mutsuura, Kanazawa-ku, Yokohama, 236 Japan. 0888-5885/89/2628-1587~0~.50 f0
1983), little success has been achieved in obtaining phenol in substantial amounts. In the course of our studies on the catalytic properties of telluric acid dispersed on various supports, we have found that highly dispersed telluric acid on silica gel is a new type of hydroxylation catalyst for aromatic compounds; benzene is catalytically oxidized to phenol, with O2 as the oxidant, in the presence of water. This report describes the catalytic performance of supported telluric acid and the role of water in the catalysis.
Experimental Section Supported telluric acid catalysts were prepared by impregnating silica gel (Fugi-Davidson ID gel) or other supports with an aqueous solution of telluric acid (HeTe06). Telluric acid contents in the supported catalysts were calculated on the basis of telluric acid concentrations in the preparation solutions. K2Te04/Si02was prepared from an aqueous solution of K2TeO4by the same procedure. H2Te04/Si02was obtained by immersing the supported potassium tellurate in 0.1 N HC1 solution for 1 2 h at room temperature, followed by filtering and drying at 383 K. Since some of the tellurium on the silica dissolved in the HC1 solution during immersion, the telluric acid content was determined by X-ray fluorescence analysis. All catalysts were pretreated in a nitrogen/oxygen stream containing water (10%) for 2 h at reaction temperature before reaction. Catalytic oxidation of benzene was carried out in a continuous flow reactor at atmospheric pressure. The quartz reactor, having an inner diameter of 13 mm and a heated length of 15 cm, has shown no catalytic activity under the reaction conditions applied. The reactants entered at the top and exited at the bottom of the reactor. The catalyst powder (1 g) diluted with 5 g of quartz granules was placed in the middle of the heated zone. The bottom of the reactor was filled with quartz granules to decrease the volume of the postcatalyst zone. Benzene was introduced through a benzene bubbler connected via a manifold system with N2 and O2 streams. Water was introduced from a syringe pump through a vaporizer at about 373 K. Phenol was identified by mass spectrometry (mle 0 1989 American Chemical Society
1588 Ind. Eng. Chem. Res., Vol. 28, No. 11, 1989 Table I. Catalytic Activity of Supported Telluric Acid for the Oxidation of Benzene' rate of formation, mmol-min-'. (mol of Te)-l carbon content, reaction catalyst wt % temp, K phenol oxidesb 0.0 0.1 100 148 0
I
2
3
4
5
6
7 8 5 Time on stream l h l
1 0 1 1
12
I3
1 4 1 5
Figure 1. Catalytic oxidation of benzene over 5 wt % silica-supported telluric acid in the presence or absence of water. The reaction temperature was 748 K. For reaction conditions, see footnote a , Table I. (0)Phenol, (A)carbon dioxide, ( 0 ) carbon monoxide.
= 94). The reactants and products were analyzed by gas chromatography. The catalysts maintained stable activity after an initial deactivation lasting a few hours (Figure 1).
Results and Discussion Table I summarizes the activities of telluric compounds supported on various carriers. In all cases, the oxidized products of benzene were phenol, carbon oxides, and traces of benzoquinone and maleic acid. Substantial amounts of carbon oxides were formed in all cases, except for silicasupported telluric acid. Since the percentage conversion of benzene was very low (less than 1%) over every catalyst, the formation rates of phenol and carbon oxides are listed in the table. Table I clearly shows that phenol is a major oxidized product in the oxidation of benzene only with silica-supported telluric acid. Other supports were extremely inefficient for the formation of phenol, even though the catalytic activity of the telluric acid supported on them was relatively high. Nb205-and Zr02-supported catalysts were inactive for such oxidation. As SiOz was the most effective support for the formation of phenol among the various supports used here, the effect on the reaction of telluric acid content was also investigated (Table I). Silica gel itself did not show any activity under the described reaction conditions. The rate of phenol formation per mole of telluric acid decreased markedly with increasing content of telluric acid. Interestingly, unsupported telluric acid showed no ability to catalyze the phenol formation. Evidently, the high dispersion of telluric acid on the silica gel, in the low loading region, is conducive to the desired reaction. Since unsupported telluric acid is inactive and it is reasonable to assume that aggregated telluric compounds exist on the support predominantly in the higher loading region, it can be concluded that deposition of telluric acid in the aggregated state does not lead to the formation of active sites. If ammonia treatment increases the number of Si-OH sites on the surface of silica gel, it might be expected to facilitate the impregnation of telluric acid in a highly dispersed state, even in the case of higher loading, and to permit attainment of high activity. Actually, when silica gel was treated with ammonia water before the impregnation, the resulting 5 wt % catalyst showed about the same activity for phenol formation as the untreated 0.5 wt % catalyst (Table I), though with increased carbon oxide formation. The apparent activation energy for the formation of phenol under the experimental conditions was 160 kJ-mol-' for 5 wt % telluric acid on silica gel. The rate of phenol formation was first order in benzene pressure and independent of O2 pressure. The presence of water vapor is essential to the formation of phenol in the vapor-phase oxidation of benzene on silica-supported telluric acid. Figure 1 demonstrates the
0.5 5 5 5 5 10 5 5 5 5
5 5 5 5 2.7
148 748 798 819 834 748 748 148 748 748 148 748 148 748 748
4.0 1.9 6.3 10.4 12.8 0.8 5.9 0.2 0.2 0.0 0.0 0.0 g g 0.4
5.1 5.0 14.0 59.8 149 1.8 14.2 21.1 0.3 1.7 10.7 9.1 g g 0.3
"The reaction conditions were as follows: catalyst weight 1.0 g; total flow rate of gases, 75 mlemin-'; partial pressure of Nz, 0.67, 02,0.11, benzene, 0.11, HzO, 0.11 atm. *Benzene basis. Unsupported solid telluric acid was used as a catalyst. dSilica gel was treated with ammonia water (pH 10) before impregnation. e Reaction was carried out without oxygen. 'Different reaction conditions were employed: catalyst weight, 1.0 g; total flow rate, 64 ml-min-'; partial pressure of Nz, 0.63, Oz, 0.16, benzene, 0.02, HzO, 0.19 atm. gNo reaction.
effect of the water feed on the reaction. The rate of carbon oxide formation increased markedly when the water feed was stopped after the reaction had achieved steady state. The rate of phenol formation, on the other hand, immediately decreased steeply but was then restored. When the introduction of water was resumed, the initial rates were almost reestablished. The initial steep decrease in the rate of phenol formation and the remarkable increase in the rate of carbon oxide formation after the water feed was discontinued strongly indicate the influence of water on the course of the oxidation. Water may have two possible roles: one is that of a primary oxidizing agent and the other is that of maintaining catalytic site activity. Table I shows that no catalytic oxidation of benzene occurs without oxygen. Through tracer experiments, it was revealed that 4% phenolJ80 was formed in the presence of water containing 20 mol % H2180. Evidently, the primary oxidizing agent is not water but dioxygen. Therefore, it can be presumed that the water interacts with the active sites to maintain their activity for phenol formation and to suppress more extensive oxidation to carbon oxides. Although the importance of the support and water in the oxidation described here is adequately demonstrated, their effects are not fully explicable, partly because we have devoted little effort to characterizing the catalyst. Nevertheless, our results may be rationalized by assuming that the active phase for oxidation of benzene to phenol is telluric acid. That unsupported telluric acid is not an effective catalyst may be due to the instability of solid H6Te06at elevated temperatures even in an atmosphere of water and oxygen. The acid changes easily into Te02 through the dehydration and reduction. X-ray diffraction analysis showed that telluric acid had in fact been converted to TeOz under the reaction conditions. It seems that telluric acid is much more stable when it is maintained in an immobilized, highly dispersed state in a water vapor atmosphere. This situation can only be attained on a silica gel support. TeOz was indeed detected by XRD in the 10
Ind. Eng. C h e m . Res. 1989,28, 1589-1596 wt % telluric acid/Si02 system, which was less effective
than highly dispersed telluric acid/SiOz. Furthermore, Te02was also detected by XRD even in the lightly loaded catalyst when water feed was omitted. To confirm that telluric acid is the active catalyst for phenol formation, additional experiments were carried out using K2Te04/Si02and its acid form, H2Te04/SiOz. The results are shown in Table I. As expected, the K2Te04/ Si02 catalyst was almost inactive for benzene oxidation, whereas its acid form was active. It has been reported by Iwamoto et al. (1983) that group 5 and 6 metal oxides supported on silica gel, especially vanadium oxide, are active for the formation of phenol, with N20 as an oxidant, while only a trace amount of phenol can be formed when O2is used instead of N20. In fact, from our separate experiments with group 5 and 6 metal oxide catalysts supported on silica gel, selectivity for phenol production in the oxidation of benzene by O2 could not exceed 2% in most cases. Bi203/Si02and Se02/Si02were practically inactive. By contrast, the selectivity of telluric acid supported on silica gel exceeded 40%. At the present stage, the one-pass yield of phenol is less than 1% Nevertheless, a much higher yield can be expected by further optimization of the reaction conditions, the catalyst design based on the telluric acid catalyst, and reactor design. For example, by use of a recirculating flow reactor, 7% yield of phenol at 16% conversion of benzene was attained over 5 wt 90 silica-supported telluric acid catalyst at 733 K (conditions;catalyst weight, 5 g; total flow rate, 56 mlsmin-'; partial pressure, N2, 0.2, 02,0.17, benzene, 0.03, H20,0.6 atm; flow rate of recirculating gas, 500 mL-min-9. Further details will be reported elsewhere.
.
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Registry No. Benzene, 71-43-2;telluric acid, 7803-68-1;water, 7732-18-5; phenol, 108-95-2.
Literature Cited Groves, J. T.; Nemo, T . E. Aliphatic Hydroxylation Catalyzed by Iron Porphyrin Complexes. J . Am. Chem. SOC.1983, 105,
6243-6248. Guengerich, F. P.; Macdonald, T . L. Chemical Mechanisms of Catalysis by Cytochromes P-450 A Unified View. Acc. Chem. Res.
1984,17,9-16. Hamilton, G. A.; Friedman, J. P. A Hydroxylation of Anisole by Hydrogen Peroxide Requiring Catalytic Amounts of Ferric Ion And Catechol. J. Am. Chem. SOC.1963,85,1008-1009. Ioffe, I. I.; Levin, Ya. S. Direct Oxidation of Benzene to Phenol. Sb. Statei, Nauchno-Issled. Inst. Org. Poluprod. Krasitelei 1961, No. 2,88-117; Chem. Abstr. 1962,57,1177. Iwamoto, M.; Hirata, J.; Matsukami, K.; Kagawa, S. Catalytic Oxidation by Oxide Radical Ions. 1. One-Step Hydroxylation of Benzene to Phenol over Group 5 and 6 Oxides Supported on Silica Gel. J . Phys. Chem. 1983,87,903-905. Kimura, E.;Machida, R. A Mono-oxygenase Model for Selective Aromatic Hydroxylation with Nickel(I1)-MacrocyclicPolyamines. J. Chem. Soc., Chem. Commun. 1984,499-500. Kinoshita, T.;Harada, J.; Ito, S.; Sasaki, K. Aerial Oxidation of Benzene in the Presence of Electrochemically Generated Cu+ Ions. Angew. Chem., Int. Ed. Engl. 1983,22,502. Orita, H.; Hayakawa, T.; Shimizu, M.; Takerhira, K. Catalytic Hydroxylation of Benzene by the Copper-Ascorbic Acid-02 System. J. Mol. Catal. 1987,42,99-103. Udenfriend, S.; Clark, C. T.;Axelrod, J.; Brodie, B. B. Ascorbic Acid in Aromatic Hydroxylation I. A Model System for Aromatic Hydroxylation. J. Biol. Chem. 1954,208, 731-739. Walling, C. Fenton's Reagent Revisited. Acc. Chem. Res. 1975,8,
125-131. Received for review December 29, 1988 Accepted July 5, 1989
Testing Catalysts for Production Performance and Runaway Limits Imre J. Berty, Jozsef M. Berty,* Paul T. Brinkerhoff, and Tibor Chovan Berty Reaction Engineers, Ltd., 100 Lincoln Street, Akron, Ohio 44308-1708
The results of laboratory catalyst tests, conducted in recycle reactors under fixed conditions of feed rate and composition and otherwise at average production conditions, permit the evaluation of catalyst performance for production reactors. These tests are performed in short steady-state runs at stepwise increasing temperatures until a specified product concentration is reached. From these results, in addition to performance evaluation, the thermal stability criteria of the reaction can also be calculated. This information is needed to maximize production within the thermal runaway limit. Since the thermal runaway limit, estimated from the catalyst test, does not contain assumptions on kinetics, the experimentally evaluated runaway limit can be used as a benchmark to help discriminate between kinetic models that were developed from other data sets. The evaluation of the performance as well as the thermal runaway limit is shown on actual experimental measurements made for the production of ethylene oxide by oxidation of ethylene. Mathematical derivations are kept simple and explanations detailed so that the method could be used without much difficulty under practical conditions.
Catalyst Tests in Recycle Reactors Evaluation of catalysts for production units requires answers to the following questions of economic importance: (1)What is the product concentration in the reactor effluent? (2) What is the production rate per unit reactor volume filled with catalyst? (3) What is the selectivity of the process? The answer to the first question is important to estimate the product separation cost and the cost of recycling any unconverted readants. The second answer helps to clarify the investment cost in the synthesis loop. The third 0888-5885/89/2628-1589$01.50/0
question addresses the raw material cost and the byproduct disposal charges. All these are considered in relation to the cost of the catalyst for its economical lifetime. The three questions mentioned above are interrelated; therefore, it is practical to introduce some standardization and simplification. For quality control tests and also for experiments to optimize a related, particular group of catalysts, one useful simplification is to require that in the tests a standard product concentration should be reached with the feed rate and feed concentration held at some standard level. This can be done by changing the temperature to reach the desired level of product concentra0 1989 American Chemical Society
