Ind. Eng. Chem. Res. 2003, 42, 2639-2643
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Pressure-Tuning the Effective Diffusivity of Near-critical Reaction Mixtures in Mesoporous Catalysts Venu Arunajatesan,† Kimberly A. Wilson,‡ and Bala Subramaniam* Department of Chemical and Petroleum Engineering, University of Kansas, Lawrence, Kansas 66045
The pressure-tunability of the diffusivity of a supercritical medium in a mesoporous catalyst, by approximately 2 orders of magnitude with moderate changes in pressure, is demonstrated. Effective diffusivities in porous Pt/γ-Al2O3 pellets at supercritical conditions were estimated by performing the geometric isomerization of 1-hexene (Tc ) 231 °C, Pc ) 31.7 bar) at supercritical conditions that encompass both the kinetic-controlled and the pore-diffusion-controlled regimes. Operation at supercritical conditions thwarts catalyst deactivation by coking and ensures steady catalyst activity. Effective rate constants (ηk) are calculated from steady-state 1-hexene conversion data obtained in a fixed-bed reactor at various supercritical pressures (37-73 bar) in the 235-310 °C range. The intrinsic kinetics parameters and the effective diffusivities are estimated from the rate constants (ηk) using the conventional theory of diffusion and reaction in catalyst pellets. The intrinsic activation energy was determined to be 184 ( 4.4 kJ/mol. The pore diffusivity of the hexene molecules was found to vary by nearly 2 orders of magnitude from 6.3 (10-6) cm2 s-1 at Fr (reduced density) ) 2.7 (235 °C, 72.8 bar) to 1.4 (10-4) cm2 s-1 at Fr ) 0.8 (311 °C, 36.7 bar), with relatively moderate changes in pressure and temperature of the nearcritical reaction mixture. Introduction Supercritical fluids (SCFs) are substances above their critical pressure and temperature and possess physical properties intermediate to those of gases and liquids. Along near-critical isotherms (1.05-1.2 Tc, for example), the physical (density, dielectric constant, etc.) and transport (diffusivity, viscosity, etc.) properties of SCFs may be sensitively pressure-tuned from gaslike to liquidlike values with relatively moderate changes in pressure around the critical pressure (Pc). In fact, unique combinations of fluid properties (liquidlike densities and gaslike diffusivities) may be obtained at nearcritical conditions. These unique properties have been exploited in heterogeneous catalysis in a variety of ways such as these: (a) the in situ extraction of heavy hydrocarbons (i.e., coke precursors) from the catalyst surface and their transport out of the pores before they are transformed to consolidated coke, thereby extending catalyst lifetime; (b) complete miscibility of reactants such as hydrogen in the reaction mixture and enhanced pore transport of these reactants to the catalyst surface, thereby promoting desired reaction pathways; (c) enhanced desorption of primary products preventing secondary reactions that adversely affect product selectivity; and (d) control of temperature rise in exothermic reactions, thereby preventing “reactor runaway” conditions. Examples of such applications are summarized elsewhere.1-3 Although the unique transport properties of nearcritical reaction media have been widely cited as an advantage for performing heterogeneous catalysis with * To whom correspondence should be addressed: Phone: 785-864-2903. Fax: 785-864-4967. E-mail: bsubramaniam@ ku.edu. † Currently with Degussa Corp., 5150 Gilbertsville Hwy, Calvert City, KY. ‡ Currently with Infineon Technologies, Ko¨nigsbruckerstr. 180, Dresden, Germany.
porous catalysts, no experimental demonstration, either direct or indirect, of the sensitive pressure-tunability of the effective diffusivity of near-critical media has been reported. The measurement of effective diffusivity, De, in a porous catalyst, especially at reaction conditions, poses a major challenge. A variety of techniques have been used to measure the pore diffusivities in a catalyst pellet.4 However, very few measurements of De at reaction conditions5-7 have been reported, and fewer report De at elevated pressures and temperatures.8 Professor Octave Levenspiel is clearly a pioneer in the chemical reaction engineering field. He among others has taught us how to effectively use theory for measuring intrinsic kinetics and transport parameters in porous catalyst pellets.9 We follow a similar approach here to demonstrate the pressure-tunability of the effective diffusivity of a supercritical medium in a mesoporous catalystsby about 2 orders of magnitude with moderate changes in pressure. Conventional theory of diffusion and reaction in porous pellets is employed to interpret the experimental measurements and to develop a better fundamental understanding of the physicochemical processes underlying simultaneous reaction and transport in porous catalysts exposed to near-critical media. The geometric isomerization of 1-hexene over a Pt/γAl2O3 catalyst is used as the model reaction. For this reaction, we previously demonstrated how to exploit near-critical media to stabilize catalyst activity10 and reported intrinsic parameters based on steady conversion data obtained at supercritical conditions.11 The temperature at which the geometric isomerization occurs (200-300 °C) is in the vicinity of the critical temperature of 1-hexene (Tc ) 231 °C, Pc ) 31.7 bar). Further, the critical properties of the products (2-hexene and 3-hexene) are nearly identical to those of the feed such that the critical properties of the reaction mixture remain virtually constant with conversion. Hence, this
10.1021/ie020835d CCC: $25.00 © 2003 American Chemical Society Published on Web 03/19/2003
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Ind. Eng. Chem. Res., Vol. 42, No. 12, 2003
Figure 1. Schematic of experimental setup.
reaction system is ideal for investigating the pressuretuning effects on the effective diffusivities in mesoporous catalysts. Clark12 reported the effective diffusivity (De) of hexene molecules in a Pt/γ-Al2O3 catalyst to be 9.8 × 10-6 cm2 s-1 at 69 bar and 251 °C. We have extended this work to demonstrate, for the first time, the sensitive changes in De at reaction conditions close to the critical point of the reaction mixture. The procedure adopted in this work to estimate De is as follows. Effective rate constants (ηk) for 1-hexene isomerization are estimated from experimental measurements of steady 1-hexene conversion data obtained with cylindrical Pt/γ-Al2O3 catalyst pellets under isothermal, diffusion-limited conditions, over a wide range of pressures and temperatures in the near-critical region. When the intrinsic kinetics parameters (k and E obtained with crushed catalyst pellets) are known, the effectiveness factor (η), the Thiele modulus (φ), and the De are then estimated, employing the conventional theory of diffusion and reaction in cylindrical pellets. Experimental Section Figure 1 shows a schematic of the experimental setup. The experiments were carried out in a fixed-bed reactor (i.d. ) 19 mm, o.d. ) 25 mm) that was passivated using Silicosteel (Supelco Co.) to virtually eliminate the reactivity of the stainless steel. The 1-hexene (>98%, Aldrich) was placed in an amber bottle with a helium headspace to minimize its exposure to the ambient oxygen and light, which promote the formation of peroxides. The peroxides are known to catalyze the formation of hexene oligomers, which are detrimental to catalyst activity.10 The peroxide content in the 1-hexene feed was reduced to
