Ind. Eng. Chem. Res. 2002, 41, 3341-3344
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Catalytic Hydration of Propylene with MoO3/Al2O3 in Supercritical Water Kengo Tomita,† Seiichiro Koda,† and Yoshito Oshima*,‡ Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Environmental Science Center, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
We carried out a catalytic hydration of propylene with MoO3/Al2O3 in sub- and supercritical water. MoO3/Al2O3 catalyst promoted hydration of propylene to form 2-propanol as a sole product and exhibited stronger activity than Al2O3, which is known to function as an acid catalyst. It was also experimentally found that the reaction temperature near the critical temperature of water gave a maximum conversion of propylene. The rate of this catalytic hydration could be expressed as a function of both the reaction temperature and the ion product of water, implying that the catalytic activity of the acid sites in MoO3/Al2O3 is strongly affected by the concentration of H+ in the bulk phase. Introduction Supercritical fluids have recently received attention as a novel chemical synthetic reaction field. Supercritical water behaves as a nonpolar solvent, having complete miscibility with most organics and gases. Additionally, its physicochemical properties can be changed continuously with temperature and pressure.1 Because of these characteristics, supercritical water is expected as an ideal alternative to most organic solvents. Antal et al. demonstrated through the study of biomass pyrolysis that supercritical water catalyzes the dehydration reaction in superheated and supercritical water.2 Enhancement of the reaction rate by the acidic character of supercritical water has also been reported in Beckmann and pinacol rearrangements,3 hydrolysis reactions,4 and Diels-Alder reactions.5 However, chemical synthesis with a heterogeneous catalyst in supercritical water has been investigated by very few researchers.6,7 In this study, catalytic hydration of propylene in water at high temperature and pressure with MoO3 supported on Al2O3 (MoO3/Al2O3) was examined. Hydration of alkenes is one of the important reactions in synthetic chemistry, and such acid catalysts as sulfuric acid, supported phosphoric acid, and acidic cation are used in conventional processes. However, the low pH value of the reaction effluent requires the treatment of the wastewater and sometimes causes the reactor corrosion. The objective of our study is to investigate how the physicochemical properties of water affect the overall rate of the heterogeneous catalysis in supercritical water, to provide a novel reaction medium for * To whom correspondence should be addressed. Phone: +81-3-5841-3027. Fax: +81-3-3813-7294. E-mail: oshima@ esc.u-tokyo.ac.jp. † Department of Chemical System Engineering, School of Engineering, The University of Tokyo. ‡ Environmental Science Center, The University of Tokyo.
hydration, and thus to propose an alternative method to the conventional acid-catalyzed reaction. In this paper, we mainly focus on the kinetic analysis of the propylene conversion to understand how the global reaction rate depends on the reaction conditions. Experimental Section The experiments were performed using a tubular flow reactor. The propylene solution was prepared by dissolving gaseous propylene into water in a saturator overnight, where the pressure of propylene was maintained at 0.8 MPa at room temperature. The propylene solution was pumped using a high-pressure highperformance liquid chromatographic pump (Tosoh CCPPD) up to the reaction pressure, preheated while flowing in a preheat line (Hastelloy C276 tubing of 0.108 cm i.d. and 3 m length), and fed into the catalyst bed (SUS316 tubing of 0.4 cm i.d. and 25 cm length). The preheat line and the reactor were immersed in an electrically heated fluidized sand bath. The temperature of the fluid was monitored directly using a thermocouple located inside of the reaction tube at the entrance. The fluid emitted from the reactor was promptly cooled by external water cooling, depressurized using a back-pressure regulating valve, and separated into gaseous and liquid products in a gas-liquid separator. The liquid and gaseous samples were quantitatively analyzed by gas chromatography with flame ionization and thermal conductivity detectors, respectively. The catalyst used in this study was MoO3/Al2O3 (1 wt % on alumina), whose average particle size was 1 mm and surface area 88 m2/g. The loaded amount of catalyst in the reactor was 2.0 g. The catalytic activity of Al2O3 with the same particle size and surface area as the MoO3/Al2O3 catalyst was also examined. The experiments were conducted at temperatures between 100 and 420 °C and pressures between 21.6 and 31.4 MPa. The propylene concentration at the reactor entrance ranged from 3.6 × 10-3 to 3 × 10-2 mol/dm3.
10.1021/ie020012o CCC: $22.00 © 2002 American Chemical Society Published on Web 06/18/2002
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in supercritical water, whereas Figure 2 suggests that, in supercritical water, hydration of olefin, which is the reverse reaction of dehydration of alcohol, is not so much acid-catalyzed as dehydration under the condition adopted in this study and that the heterogeneous catalysts play a substantial role for hydration. It is also suggested from Figure 2 that Al2O3 also has catalytic activity, though the conversion was smaller than that in the case of MoO3/Al2O3. The higher catalytic activity of MoO3/Al2O3 is probably ascribed to the Brønsted acid sites of MoO3 and/or the formation of new acid sites through the interaction between MoO3 and Al2O3.8 Because the product was exclusively 2-propanol at any temperature and pressure, such a hydration of propylene as shown in the following equation is considered to be promoted by the catalysts.
CH2dCHCH3 + H2O f CH3CH(OH)CH3 Figure 1. Pressure effect on propylene conversion with MoO3/ Al2O3 at 348 °C (9) and 385 °C (b).
Incidentally, we examined the same reaction with a smaller particle size ranging from 0.36 to 0.50 mm to see if the reaction rate would be dependent on the particle size of the catalyst. Because the results for the small size were identical with the case of a 1 mm particle size, the effect of the particle size on the reaction rate is negligibly small under such reaction conditions. When one of the mechanisms of the gas-phase catalytic reactions is consulted, the kinetic description of the propylene hydration rate is obtained by applying Langmuir-Hinshelwood (LH) kinetics where two reactants are competing with each other for the adsorption sites as eq 1,9-11 where KP and KH are the adsorption
r)
kKPKH[C3H6][H2O]0 (1 + KP[C3H6] + KH[H2O]0)2
(1)
equilibrium constants of propylene and water, respectively, k is the specific rate constant on the surface, and the reaction is assumed to be first order on the adsorbed propylene and water concentration individually. Because of the high concentration of water, we assume that Figure 2. Temperature effect on propylene conversion with three different catalysts at 25.5 MPa (catalysts: 9, MoO3/Al2O3; b, Al2O3; 2, no catalyst).
KH[H2O]0 . 1 + KP[C3H6] The simplified LH rate equation is
Results and Discussion Figure 1 shows the dependence of propylene conversion on the reaction pressure using the MoO3/Al2O3 catalyst at 348 and 385 °C. The conversions of propylene monotonically increased with increasing pressure at both temperatures. However, no reaction took place below 24 MPa at 385 °C. The conversion at 385 °C was always lower than that at 348 °C. The dependence of propylene conversion on the reaction temperature with the MoO3/Al2O3 catalyst is shown in Figure 2. The reaction pressure was kept constant at 25.5 MPa. Concerning the effect of temperature, the conversion of propylene was zero below 200 °C, started to increase with an increase of temperature, reached a maximum (10%) at around the critical temperature of water, and suddenly dropped over the critical temperature. For comparison, the profiles of propylene conversion with the Al2O3 catalyst and without any catalyst are also shown in Figure 2. In the absence of catalyst, the propylene conversion was extremely small at any temperature. According to the previous literature,2 dehydration of alcohol is homogeneously acid-catalyzed
r ) kLH
[C3H6] [H2O]0
(2)
where
( )
kLH ) k
KP KH
(3)
A first-order plot for the propylene concentration was made in Figure 3. The linear relationship in Figure 3 suggests that the apparent reaction order of propylene can be regarded as unity. Therefore, the variance of the propylene conversion in Figures 1 and 2 is not due to propylene concentration effects but to an effect on the rate constant. When eq 2 was applied to the experimental data in Figure 2, the overall rate constant, kLH, at each temperature was calculated, using the density of water from steam table.1 The Arrhenius plot of the rate constant is shown in Figure 4. It is obvious from Figure 4 that the dependence of the overall rate constant on temperature
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Figure 3. First-order plot for the propylene concentration in the hydration with MoO3/Al2O3 at 385 °C and 25.5 MPa.
Figure 5. Relationship between the rate constant, kLH, and the ion product of water at 348 °C (9) and 385 °C (b).
Figure 4. Arrhenius plot with a rate constant, kLH, for the hydration of propylene at 25.5 MPa.
Figure 6. Arrhenius plot with a rate constant divided by Kw0.40, k′, for the hydration of propylene at 25.5 MPa.
is not explained by a simple Arrhenius expression. We therefore considered that the rate constant is also a function of the ion product of water, taking into account the finding that the profile of propylene conversion in Figures 1 and 2 resembles the pressure and temperature dependences of the ion product of water.1,12 We attempted to incorporate the ion product effect into eq 2, as described in eq 4. In eq 4, Kw is the ion product of c
r ) k′Kw
[C3H6] [H2O]0
r ) k′Kw0.40 (4)
water ()[H+][OH-])1,12 and c is the exponent to be determined. With the arrangement of eq 4, the relationship among the propylene conversion, Kw, and c is given in eq 5.
ln
[
]
-ln(1 - X) [H2O]0 ) ln k′ + c ln Kw W/F
is 0.40 ((0.02) at 348 °C and 0.40 ((0.004) at 385 °C. The numbers in the parentheses represent the 95% confidence interval. Because the same value of c was obtained at two different temperatures, we conclude that the overall reaction rate can be described as follows:
(5)
The experimental value for the left-hand side in eq 5 was plotted against ln Kw in Figure 5. From the slope at each temperature, the exponent c with respect to Kw
[C3H6] [H2O]0
(6)
The Arrhenius plot of k′ is shown in Figure 6. The good linear relationship in Figure 6 suggests that the reaction rate of the catalytic hydration in supercritical water is determined by both the temperature and ion product of water. The value of the activation energy is approximately 104 ((5.8) kJ/mol. The numbers in the parentheses represent the 95% confidence interval. We also examined a different model in which the reaction between adsorbed water and bulk phase propylene (the Eley-Rideal model) is considered. The propylene hydration rate can be described as follows:
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r)
kerKH[C3H6][H2O]0 1 + KH[H2O]0
(7)
The result of the regression analysis based on the EleyRideal model also suggests that this reaction rate can be organized by exponential dependence in Kw, though the apparent dependence of the overall reaction rate on Kw is slightly different (order ) 0.34). According to the previous researchers,9,10 the rate of catalytic hydration is proportional to the concentrations of propylene and protonic acid on the surface of the catalyst, though no information on the effect of the H+ concentration in the bulk phase on the reaction rate is available. The present finding that the reaction rate of heterogeneous catalysis in supercritical water depends on the ion product of water implies that the H+ concentration in the bulk phase is related to the number and/or activity of protonic acid sites on the catalyst surface, though the detailed mechanism is left to future studies. Concluding Remarks We carried out a catalytic hydration of propylene with MoO3/Al2O3 in sub- and supercritical water. The following conclusions can be drawn from the results. (1) MoO3/Al2O3 catalyst promoted hydration of propylene to form 2-propanol as an exclusive product and exhibited stronger activity than Al2O3, which is known to function as an acid catalyst. (2) The conversions of propylene monotonically increased with increasing pressure. However, no reaction took place below 24 MPa at 385 °C, and the conversion at 385 °C was always lower than that at 348 °C. The reaction temperature near the critical temperature of water gives a maximum conversion of propylene. (3) This anomalous dependence of the overall rate constant on temperature cannot be explained by the simple Arrhenius expression but can be expressed as a function of both the reaction temperature and ion product of water because the H+ concentration in the bulk phase is related to the number and/or activity of protonic acid sites on the catalyst surface. Acknowledgment This work was partly supported by a grant from the “Research for the Future” program by the Japan Society for the Promotion of Science (96P00401). We also
gratefully acknowledge Sud-Chemie Nissan Catalysts Inc. for providing us with the catalysts. Nomenclature r ) reaction rate of propylene [mol‚(kg of catalyst)-1‚s-1] k ) rate constant [mol‚(kg of catalyst)-1‚s-1] KP ) adsorption equilibrium constant of propylene [m3/mol] KH ) adsorption equilibrium constant of water [m3/mol] kLH ) apparent first-order rate constant according to the LH mechanism [mol‚(kg of catalyst)-1‚s-1] Kw ) ion product of water [(mol/dm3)2] c ) exponent with respect to Kw k′ ) kLH/Kwc X ) conversion of propylene W ) amount of catalyst [g] F ) volume flow rate [dm3/min] ker ) rate constant in the Eley-Rideal model [mol‚(kg of catalyst)-1‚s-1]
Literature Cited (1) Haar, L.; Gallagher, J. S.; Kell, G. S. NBS/NRC Steam Tables; Hemisphere: Washington, DC, 1984. (2) Ramayya, S.; Brittain, A.; Dealmeida, C.; Mok, W.; Antal, M. J. Acid-catalyzed dehydration of alcohols in supercritical water. Fuel 1987, 66, 1364. (3) Ikushima, Y.; Hatakeda, K.; Sato, O.; Yokoyama, T.; Arai, M. Acceleration of synthetic organic reactions using supercritical water: Noncatalytic Beckmann and pinacol rearrangements. J. Am. Chem. Soc. 2000, 122, 1908. (4) Taylor, J. D.; Steinfeld, J. I.; Tester, J. W. Experimental measurement of the rate of methyl tert-butyl ether hydrolysis in sub- and supercritical water. Ind. Eng. Chem. Res. 2001, 40, 67. (5) Korzenski, M. B.; Kolis, J. W. Diels-Alder reactions using supercritical water as an aqueous solvent medium. Tetrahedron Lett. 1997, 38, 5611. (6) Watanabe, M.; Inomata, H.; Smith, R. L.; Arai, K. Catalytic decarboxylation of acetic acid with zirconia catalyst in supercritical water. Appl. Catal. 2001, 219, 149. (7) Bro¨ll, D.; Kramer, A.; Vogel, H.; Lappas, I.; Fuess, H. Heterogeneously catalyzed partial oxidation in supercritical water. Chem. Eng. Technol. 2001, 24, 142. (8) Yamadaya, S.; Kabe, T.; Oba, M.; Miki, Y. Surface acidity of molybdena-alumina catalysts. Nihon Kagaku Kaishi 1976, 2, 227. (9) Sonnemans, M. H. W. Hydration of propene over acidic zeolites. Appl. Catal. 1993, 94, 215. (10) Sonnemans, M. H. W. Hydration and etherification of propene over H-ZSM-5. 1. A kinetic study. Ind. Eng. Chem. Res. 1993, 32, 2506. (11) Williams, B. P.; Young, N. C.; West, J.; Rhodes, C.; Hutchings, G. J. Carbonyl sulphide hydrolysis using alumina catalysts. Catal. Today 1999, 49, 99. (12) Marshall, W. L.; Frank, E. U. Ion product of water substance, 0-1000 °C, 1-10,000 bar. New international formulation and its background. J. Phys. Chem. Ref. Data 1981, 94, 295.
Received for review January 4, 2002 Revised manuscript received May 3, 2002 Accepted May 3, 2002 IE020012O
