Ind. Eng. Chem. Res. 2005, 44, 6403-6405
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Effect of Teflon Powder Deposition on the Catalyst in a Four-Phase Trickle-Bed Reactor Hiroshi Yamada,* Kenji Saito, Shigeo Goto, and Tomohiko Tagawa Department of Chemical Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan
To increase the conversion of hydrogenation of carbobenzoxy phenylalanine in the gas-liquidliquid-solid four-phase trickle-bed reactor, Teflon powder was deposited on the surface of the catalyst. Teflon powder was expected to break the water film on the catalyst, Pd/Al2O3, surface since it has hydrophobic property. Then organic solution, in which the reactant was dissolved, could easily contact the catalyst surface; thus, mass transfer was promoted. Three kinds of Teflon powder were tested and Fussoseparate increased the conversion. A rotary evaporator was used to control the amount of the Teflon powder deposition on the catalyst surface. Conversion was increased with an increase of the amount of Teflon powder. But excessive Teflon powder decreased the catalyst activity because it covered the active site of the catalyst. The highest activity was achieved by 0.064 wt % of Teflon powder. Introduction Design of multiphase reactors are very complicated because steps of interphase mass transfers as well as reactions are involved. There are some reactions carried out in the gas-liquid-liquid-solid four-phase system. Kinetics of selective hydrogenation of unsaturated aldehyde,1 partial hydrogenation of benzene,2 hydrogenation of nitrobenzene,3 and hydrogenation of carbobenzoxy phenylalanine4 are studied in the stirred tank batch reactor. In industry viewpoints, continuous operation is very important but there are a few publications on the flow-type reactor.5,6 In this paper enhancement of the mass transfer in the four-phase trickle-bed reactor was studied. Carbobenzoxy phenylalanine was dissolved in the organic phase and hydrogenated with gaseous hydrogen to phenylalanine by using the solid catalyst. Since the phenylalanine could not be dissolved in the organic phase, a water phase was needed to dissolve the product.4 Four-phase gas-organic-aqueous-solid existed in the reactor. Previously, Yamada has carried out the hydrogenation of carbobenzoxy phenylalanine in the trickle-bed reactor.6 Water and organic solution were introduced into the reactor simultaneously. Pd/C and Pd/Al2O3 were used as solid catalysts. For Pd/C, the conversion decreased with time. On the other hand, Pd/Al2O3 showed stable activity. However, the conversion on Pd/Al2O3 was much lower than the initial conversion on Pd/C. The different behaviors on Pd/C and Pd/ Al2O3 were explained by the different surface properties. Since carbon support has hydrophobic property, the Pd/C pellet was mainly covered with the organic solution in the tricklebed reactor. It was difficult for water to contact the catalyst. The product was deposited on the surface of the catalyst. Product covered the active site of the catalyst. On the other hand, since Pd/Al2O3 is hydrophilic, the catalyst was mainly covered with water. Water prevented the catalyst from making contact with the reactant in the organic solution. * To whom correspondence should be addressed. Tel: (+81)-52-789-4529. Fax: (+81)-52-789-3387. E-mail: yamada@ nuce.nagoya-u.ac.jp.
In this paper, increasing the effective contact area of organic solution on the Pd/Al2O3 catalyst surface was studied. Teflon powder has been frequently used to increase the gas-solid contact area in the gas-aqueous-solid three-phase reactor, such as oxidation of water-soluble reactant7 and fuel cell.8 We used Teflon powder to increase the contact area of solid-organic solution. Teflon powder is expected to break the water film on the catalyst surface since it has hydrophobic property. Then organic solution can easily contact the catalyst surface. Experimental Section A ball type 0.5 wt % Pd/Al2O3 (Nippon Engelhard Co. Ltd. Present name is N. E. Chemcat Co. Ltd.) was used as a catalyst. Palladium was supported only on the external surface of the alumina ball. The size of the catalyst was 4 mm in diameter. Three kinds of commercial Teflon powder spray were used, which were Fussoseparate (Toyo Kagaku Shokai Co. Ltd. Abbreviated as TF1), TFErub (Fine Chemical Japan Co. Ltd. Abbreviated as TF2), and Powerfrip 2 (Toyo Kagaku Shokai Co. Ltd. Abbreviated as TF3). Teflon powder was directly sprayed on the catalyst for 10 s (direct spray method) in the preliminary experiments. A rotary evaporator was also used instead of the direct spray method. Teflon powder was sprayed into a flask and then benzene and the catalyst were put into the flask. Benzene was evaporated under reducing pressure. Teflon powder was deposited on the catalyst more homogeneously than that using the direct spray method. Two kinds of reactors were used. One was the stirredtank basket-type batch reactor to determine the kinetics. Two baskets, which were packed with catalysts, were attached to the stirring shaft. The total weight of catalyst was 1.4 g; 80 cm3 of 1-octanol, in which reactant was dissolved, and the 100 cm3 of distilled water were used. Hydrogen was continuously bubbled into the reactor. The other reactor was the trickle-bed reactor (2.0 cm in the diameter) to investigate the stable continuous operation. First, 30.0 g (12.6 cm in height for Pd/Al2O3) of catalyst was packed in the reactor. Glass beads were also packed over the catalyst bed to preheat the fluids
10.1021/ie049222m CCC: $30.25 © 2005 American Chemical Society Published on Web 03/22/2005
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Figure 2. Effect of amount of Teflon powder on conversion.
Figure 1. Effect of Teflon powder on conversion.
and to calm the flow patterns. Then, 1.7 × 10-8 m3/s of 1-octanol, in which the reactant was dissolved, and 5.0 × 10-8 m3/s of the distilled water were flowed into the reactor continuously. The reactor was filled with the distilled water to prewet the catalyst before start-up. After the temperature of the reactor reached the reaction temperature, the water was drained. The experiment continued for 2 days; the reactor was filled with distilled water after the first day of the experiment. The initial concentration of the reactant was 100 mol/ m3 and the reaction temperature was 323 K. The hydrogen flow rate was fixed at 1.7 × 10-6 m3/s in both reactors. Organic samples were injected into a liquid chromatograph with a 14 cm column of TOSO TSKgel ODS80TM at 313 K and a UV absorbance detector of 254 nm. The carrier liquid was composed of 0.055 vol % trifluoroacetic acid, 45 vol % acetonitrile, and 55 vol % water. The reactant, carbobenzoxy phenylalanine, and the product, toluene, could be detected in the organic samples. The objective product, phenylalanine, was not detected because it was dissolved in water. Results and Discussion Effect of Teflon powder was examined in the tricklebed reactor. Teflon powder was expected to break the water film on the catalyst surface since it has hydrophobic property. Then organic solution could easily contact the catalyst surface. Increasing of mass transfer from organic solution to catalyst increased conversion of carbobenzoxy phenylalanine. Teflon was deposited on the catalyst surface by the direct spray method as a preliminary experiment. Figure 1 shows the conversion profiles of Teflon-deposited catalysts. The break line, Xa ) 0.44, was the conversion profile of the catalyst without Teflon (non-Teflon).6 Three kinds of Teflon powder were examined. TF2-deposited catalyst had lower activity than non-Teflon catalyst. Teflon powder covered the active site, and the catalyst lost activity. TF3-deposited catalyst had lower activity than non-Teflon catalyst on the first day. But it increased to the same activity as non-Teflon catalyst on the second day. Teflon powder, which once covered the active site, might be peeled off from the catalyst surface. TF1-deposited catalyst had lower activity than non-Teflon catalyst for the initial
4 h. But the conversion became higher than non-Teflon catalyst after 4 h. A part of Teflon was peeled off from the catalyst surface, and the conversion was increased to 0.54. The activity was then stabilized. These results suggested that the amount of Teflon powder controlled the catalyst activity. The active site under the Teflon powder did not work. But the active site near the Teflon powder worked more efficiently than the other part since it can easily be contacted with the organic reactant. The excess of TF1 was peeled off and an optimal amount of Teflon powder remained on the surface of the catalyst. The amount of Teflon powder, TF1, was varied. After this experiment a rotary evaporator method was used to deposit TF1 on the catalyst. The rotary evaporator method could control the amount of Teflon powder more precisely than the direct spray method. Figure 2 shows the experimental results. The conversion was stable and higher than that of the non-Teflon catalyst. The 0.064 wt % catalyst had the highest activity, Xa ) 0.56. From the kinetic equation of this reaction,4 reaction rate enhancement was estimated. The reaction rate became 1.4 times larger with the Teflon deposition. Both the 0.10 wt % catalyst and the 0.020 wt % catalyst showed smaller conversions than the 0.064 wt % catalyst. Teflon powder covered too much of the active site on the 0.10 wt % catalyst. And the hydrophobic property of 0.020 wt % catalyst was not enough. The reaction rate was measured by using the stirredtank batch reactor. 0.064 wt % TF1-deposited catalyst and non-Teflon catalyst were used. The rotating speed of the stirrer and the flow rate of hydrogen were adjusted to minimize the resistances of interphase mass transfer. Both water and organic solution can contact the catalyst surface whether the catalyst surface property is hydrophilic or hydrophobic in the stirred-tank batch reactor. Figure 3 shows the reaction profiles under the conditions where the resistance of the mass transfer was negligible. The initial reaction rate of TF1-deposited catalyst was lower than that of non-Teflon catalyst by 20%. This decrease of initial reaction rate means 20% of the active site was covered with Teflon powder. The remaining 80% of active site increased the catalyst activity in the trickle-bed reactor (Figure 2). Active sites that were not covered with Teflon powder increased their activity of 1.75 times on average in the tricklebed reactor. Figure 4 shows the surface properties of non-Teflon catalyst and 0.064 wt % Teflon-deposited catalyst. A
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deposited catalyst was used. Thus, increase of contact between organic solution and catalyst enhanced the mass transfer and increased catalyst activity shown in Figure 2. Conclusion
Figure 3. Reaction profiles in batch reactor.
Improvement of mass transfer in the gas-liquidliquid-solid four-phase trickle-bed reactor was studied by using Teflon deposition on the catalyst. Hydrogenation of carbobenzoxy phenylalanine was carried out in the trickle-bed reactor. Teflon powder broke the water film on the surface of Pd/Al2O3 and organic solution contacted the catalyst instead of water. Conversion was increased with increase of the amount of Teflon powder. But excessive Teflon powder decreased catalyst activity because it covered too much of the active site of the catalyst. The highest activity was achieved by 0.064 wt % Teflon powder. Teflon powder covered 20% of the active site but it increased conversion from 0.44 to 0.56. Nomenclature Xa ) conversion of carbobenzoxy phenylalanine r ) initial reaction rate constant in stirred-tank batch reactor [mol/s kg-cat]
Literature Cited
Figure 4. Hydrophobic property of catalyst surface.
water drop was put on the catalyst surface. The water drop was spread on the surface of the non-Teflon catalyst. But the water drop had a higher face contact angle on the surface of TF1-deposited catalyst than that of non-Teflon catalyst. Teflon on the catalyst surface repelled the water and water did not cover all the surface of the catalyst. Organic solution in which reactant was dissolved could contact the catalyst surface instead of water in the trickle-bed reactor when Teflon-
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Received for review August 25, 2004 Revised manuscript received February 15, 2005 Accepted February 18, 2005 IE049222M
