Ind. Eng. Chem. Res. 2004, 43, 2345-2348
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Enhancement of the Catalytic Activity by an Ion Product of Suband Supercritical Water in the Catalytic Hydration of Propylene with Metal Oxide Kengo Tomita† 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 investigated the detailed mechanism of the relationship between the H+ concentration in the bulk phase and the activity of protonic acid sites on the catalytic surface. The theory clearly shows that the acidity of the catalytic surface is strongly affected by the change of the ion product in the bulk phase water. This phenomenon was apparent with other kinds of metal oxide catalysts and is largely caused by the change in the surface charge of the catalyst. In fact, through the kinetic analysis of the catalytic hydration of propylene with a TiO2 catalyst in sub- and supercritical water, the reaction rate of the hydration could be expressed as a function of both the reaction temperature and ion product of water. Introduction Supercritical water offers numerous advantages as a benign catalyst and solvent. Properties of supercritical water are different from those of ordinary liquids and gases and are tunable simply by changing the pressure and temperature.1 Also, at supercritical conditions, organic compounds have enhanced solubility in water2 and the hydrogen-bonding network of water is minimized.3 Therefore, the water under supercritical conditions is expected as an ideal alternative to most organic solvents. In particular, near-critical water is considered to be a strong source of hydronium cations, which in itself can act as a catalyst in reactions that require additional acid, such as acid-catalyzed reaction. Various researches have reported such enhancement of the reaction rate due to the acidic character of sub- and supercritical water, which includes Diels-Alder reactions,4 aldol condensations,5 hydrolysis reactions,6 Beckmann and Pinacol rearrangement,7 and alkylation.8,9 The heterogeneous catalysts are considered to increase the reaction rate and control the selectivity. Especially, for acid-catalyzed reactions, solid acid catalysts can significantly increase the H+ concentration in a system. In addition, supercritical water is suitable for heterogeneous reactions because mass-transfer restrictions, which limit heterogeneous reactions in general, can be avoided because of its characteristics such as high density, high diffusion coefficient, and low viscosity. Therefore, the combination of catalyst and water in sub- and supercritical water is considered as a promising technique that has high reactivity and high selectivity. In fact, some researchers have investigated the synthesis reaction with heterogeneous catalysts in sub- and supercritical water.10-13 Our previous study reported14 that catalytic hydration of propylene in sub* To whom correspondence should be addressed. Tel.: +813-5841-3027. Fax: +81-3-5841-8659. E-mail: oshima@esc. u-tokyo.ac.jp. † Department of Chemical System Engineering, School of Engineering. ‡ Environmental Science Center.
and supercritical water is promoted by a MoO3/Al2O3 catalyst, which is known to function as an acid catalyst, and that the reaction rate of propylene hydration can be expressed as a function of both the reaction temperature and ion product of water because the number and/ or activity of the protonic acid sites on the catalytic surface is related to the H+ concentration in the bulk phase. In this paper, we tried to investigate the detailed mechanism of the relationship between the H+ concentration in the bulk phase and the activity of the protonic acid sites on the catalytic surface using a theoretical analysis. Then, we conducted catalytic hydration of propylene with a TiO2 catalyst, which is one of the solid acid catalysts, to prove that the theoretical concept is reasonable. Hydration of propylene is an acid-catalyzed reaction and is expressed by the following equation:
CH2 ) CHCH3 + H2O f CH3CH(OH)CH3
(1)
Experimental Section All experiments were performed using a tubular flow reactor that can be operated isothermally and isobarically. The propylene solution was prepared by dissolving gaseous propylene into degassed water in a saturator overnight, where the pressure of propylene was maintained at 0.8 MPa at room temperature. The propylene solution was pumped up to the reaction pressure, preheated while flowing in a preheat line, and fed to the reactor, which contained a catalytic bed. The fluid emitted from the reactor was promptly cooled, depressurized, then sampled, and quantitatively analyzed by gas chromatography with flame ionization and thermal conductivity detectors. Detailed descriptions of the reactor system and experimental methods have been published previously.14 The catalyst used in this study was TiO2, whose average particle size was 0.75 mm and surface area was 15 m2/g. The amount of catalyst loaded in the reactor was 2.0 g. The experiments were conducted at temperatures between 100 and 470 °C and pressures between 21 and
10.1021/ie030805q CCC: $27.50 © 2004 American Chemical Society Published on Web 04/20/2004
2346 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004
31 MPa. The propylene concentration at the reactor entrance ranged from 3.6 × 10-3 to 3 × 10-2 mol/dm3.
KD )
Results and Discussion Acidity of the Catalytic Surface in Water. According to our previous research,14 the profiles of propylene conversion with MoO3/Al2O3 could be explained by the effects of both the reaction temperature and ion product of water in the bulk phase. Also, the reaction rate of propylene was expressed, using LangmuirHinshelwood kinetics, by the equation
[C3H6] r ) kMoKw0.40 [H2O]0
(2)
where kMo is the rate constant and Kw is the ion product of water ()[H+][OH-]).15 Also, in case of the Al2O3 catalyst, the reaction rate could also be expressed using the Kw value raised to the 0.40th power. This is in consideration that the number and/or activity of the protonic acid sites on the catalytic surface is related to the H+ concentration in the bulk phase. A number of previous researchers reported that the development of acidity on the catalytic surface is caused by the dissociation of water molecules.16-18 Also, the strength of the acidity is represented by the equation
pKa ) -log Ka ) -log
(
-
+
)
[MOH ][HS ] [H2OS]
(3)
where M are activated sites, [HS+] and [H2OS] are concentrations of the hydrogen ion and surface-bound water on the catalytic surface, respectively. Tanaka et al.18 reported that the acidity increases with the electronegativity of the central metal ion and that the relationship between pKa values of eq 3 and the parameter Xi is linear. When an insoluble oxide is suspended in water, the surface becomes hydrated. The hydroxyl groups or the water molecules coordinated to the metal ion of the hydrated surface may ionize as an acid or as a base depending on the proton activity of the surrounding fluid phase. Tanaka et al. reported that the acidic nature of the hydrated surface is characterized by its point of zero charge (pzc), which is the pH of the equilibrium solution at which the surface is uncharged,19,20 and that the relationship between pzc and the parameter Xi of the corresponding metal ion is linear. Consequently, it is found that the relationship between the acidity on the catalysts and pzc is linear, and from the results of Tanaka’s study, the relationship is expressed by the following equation:
pKa ) 1.8pzc - 11.2
(4)
Be´rube´ and De Bruyn21 have derived a thermodynamic relationship for the variation of pzc with temperature. Their expression is given by
∆H C 1 4.6R pKD - pzc ) + 2 T T
[
the dissociation constant of water given by the equation
]
(5)
where ∆H represents the standard enthalpy difference for the transfer of H+ and OH- ions from the bulk solution to the surface, C is a constant value, and KD is
Kw [H+][OH-] ) [H2O] [H2O]
(6)
and rearrangement of eq 5 gives the relationship among pzc, pKD, and T can be given as eq 7, where D is a
1 D pzc ) pKD 2 T
(7)
constant value. From eqs 4 and 7, the value of pKa is
D 1 - 11.2 pKa ) 1.8 pKD 2 T
(
)
(8)
and eq 9 is obtained from eqs 3, 6, and 8. The
[MOH-][HS+] [H2OS]
)
( ) Kw
[H2O]
0.9
× 10-1.8D/T-11.2
(9)
concentration of MOH- is equal to the concentration of HS+ because one molecule of water dissociates to one MOH- and one HS+. Therefore, eq 9 is rearranged as eq 10. [H2OS], which is the concentration of adsorbed
[HS+] ) (Kw)0.45[H2OS]0.5[H2O]-0.45 × 10-0.9D/T-5.6 (10) water on the catalytic surface, can be expressed by the concentration of water in the bulk phase and temperature; therefore, this concentration is independent of Kw. Finally, it can be concluded that the concentration of the hydrogen ion on the catalytic surface is proportional to Kw0.45. It is well-known that the rate of catalytic hydration is found to be of first order in the propylene concentration and protonic acid (HS+) concentration on the catalytic surface.22,23 Therefore, the rate of catalytic hydration becomes proportional to Kw, which is the ion product of the bulk phase water raised to the 0.45th power. The result agrees with our experimental observation that the rate is proportional to Kw raised to the 0.4th power. On the basis of the results, it can be concluded that a change of the ion product of the water in the bulk phase strongly affected the concentration of the hydrogen ion on the catalytic surface primarily by a change of the surface charge of the catalyst. Propylene Hydration with a TiO2 Catalyst and Kinetics. According to the above considerations, regardless of the kinds of catalysts, the reaction rate is in proportion to the Kw value raised to the 0.45th power. Then, we conducted the catalytic hydration of propylene with a TiO2 catalyst, which is one of the solid acid catalysts. The temperature dependence of propylene conversion with a TiO2 catalyst is shown in Figure 1. The reaction pressure was kept constant at 25 MPa. The products were 2-propanol, which is formed through the hydration of propylene, and acetone, which is a product of the dehydrogenation of 2-propanol24 at any temperature. However, the yield of acetone was extremely small, and the selectivity of 2-propanol was more than 90% for any condition. The carbon balance was always close to unity. For comparison, the profile of propylene conversion without any catalyst is also shown in Figure 1. In the
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2347
Figure 1. Temperature effect on the propylene conversion with a TiO2 catalyst and without a catalyst at 25 MPa (9, TiO2; b, no catalyst).
Figure 2. Pressure effect on the propylene conversion with a TiO2 catalyst at 360 °C.
absence of catalyst, the propylene conversion was extremely small at any given temperature. The TiO2 catalyst promoted the hydration of propylene because the TiO2 catalyst has a high Brønsted acidity.25 Figure 1 shows that the reaction temperature near the critical temperature of water gave a maximum conversion of propylene as well as MoO3/Al2O3. Also, Figure 2 shows the pressure dependence of propylene conversion using a TiO2 catalyst at 360 °C. The conversion of propylene increased with increasing pressure. We have already reported that 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. The same method used in our previous study with the MoO3/Al2O3 catalyst was used to determine the reaction rate of propylene hydration with a TiO2 catalyst. At this point, we considered only the 2-propanol yield because the yield of acetone was negligible. First, the following reaction rate was assumed:
[C3H6] [C3H6] r)k ) kTiKwc [H2O]0 [H2O]0
(11)
where c is the exponent to be determined. When eq 11 was rearranged, the relationship of propylene conver-
Figure 3. Relationship between the rate constant, k [)ln(-ln(1 - X)[H2O]/(W/F)], and the ion product of water at 360 °C.
Figure 4. Arrhenius plot with a rate constant divided by Kw0.43, kTi, for the hydration of propylene at 25 MPa.
sion, Kw, and c is given as eq 12:
ln
[
]
-ln(1 - X) [H2O]0 ) ln kTi + c ln Kw W/F
(12)
where X is the conversion of propylene and W/F is the reaction time, which is defined as the value of the amounts of catalysts, W, divided by volume flow rate, F. The experimental value for the left-hand side in eq 12 was plotted against ln Kw, as shown in Figure 3. From the slope, the exponent c with respect to Kw is 0.43 ((0.053). The numbers in parentheses represent a 95% confidence interval. Thus, it can be concluded that the overall reaction rate is
[C3H6] r ) kTiKw0.43 [H2O]0
(13)
The Arrhenius plot of kTi is shown in Figure 4. The good linear relationship in Figure 4 suggests that the reaction rate of the hydration with TiO2 in sub- and supercritical water is determined by both the temperature and ion product of water, the same as the case with MoO3/Al2O3. The value of the activation energy is approximately 84.4 ((24.9) kJ/mol. The numbers in parentheses represent a 95% confidence interval.
2348 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004
On the basis of the result of kinetic analysis, because the order for the ion product is almost the same with all catalysts, TiO2, MoO3/Al2O3, and Al2O3, the validity of the above theoretical considerations can be demonstrated. Thus, the order for the ion product is approximately 0.45 for all catalysts, which is a function of the acidity on the catalytic surface. Concluding Remarks We investigated the detailed mechanism of the relationship between the H+ concentration in the bulk phase and the activity of the protonic acid sites on the catalytic surface using a theoretical analysis. Also, we carried out a catalytic hydration of propylene with TiO2 in sub- and supercritical water in order to prove that the theoretical concept is reasonable. The following conclusions can be drawn from the results: (1) The theory clearly shows that, regardless of the kind of metal oxide catalyst, the acidity of the catalytic surface is strongly affected by the change in the ion product in the bulk phase water as a consequence of the change in the surface charge of the catalyst. (2) The validity of this theoretical consideration can be demonstrated because, through the kinetic analysis, the reaction rate of the propylene hydration with TiO2 could be expressed as a function of both the reaction temperature and ion product of water and the order for the ion product in the reaction rate, as well as for the MoO3/Al2O3 and Al2O3 catalysts, was approximately 0.4. Acknowledgment This work has been conducted under the entrustment contract between New Energy and Industrial Technology Development Organization (NEDO) and Japan Chemical Innovation Institute (JCII). Also, we 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] kMo ) rate constant with the MoO3/Al2O3 catalyst [mol‚(kg of catalyst)-1‚s-1] Kw ) ion product of water [(mol/dm3)2] pKa ) strength of the acidity Xi ) electronegativity of the central metal ion HS+ ) acid on the catalytic surface H2OS ) surface-bound water on the catalytic surface pzc ) point of zero charge KD ) water dissociation constant C, D ) constant values k ) rate constant [mol‚(kg of catalyst)-1‚s-1] kTi ) rate constant with the TiO2 catalyst [mol‚(kg of catalyst)-1‚s-1] c ) exponent with respect to Kw X ) conversion of propylene W ) amount of catalyst [g] F ) volume flow rate [dm3/min]
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Received for review November 4, 2003 Revised manuscript received February 27, 2004 Accepted March 3, 2004 IE030805Q
