Kinetics of Methyl Ethyl Ketone Combustion in Air at Low

Ind. Eng. Chem. Res. 2007, 46, 9037-9044

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Kinetics of Methyl Ethyl Ketone Combustion in Air at Low Concentrations over a Commercial Pt/Al2O3 Catalyst Gurutze Arzamendi,† Roberto Ferrero,† A Ä ngel R. Pierna,‡ and Luis M. Gandıá*,† Departamento de Quı´mica Aplicada, Edificio de los Acebos, UniVersidad Pu´ blica de NaVarra, Campus de Arrosadıá s/n, E-31006 Pamplona, Spain, and Departamento de Ingenierıá Quı´mica y del Medio Ambiente, Escuela UniVersitaria Polite´ cnica, UniVersidad del Paı´s Vasco, P.O. Box 1379, 20080 San Sebastia´ n, Spain

An investigation on the combustion in air of a common volatile organic compound, methyl ethyl ketone (MEK), over a commercial 0.5% Pt/Al2O3 catalyst has been performed. A tubular fixed-bed reactor operating both in the integral (ignition curves) and differential (kinetic study) modes was used at temperatures between 403 and 485 K, MEK partial pressures ranging from 12 to 191 Pa, and space times between 3160 and 25170 kgcat.‚min‚kmolMEK-1. Water and CO2 were the only reaction products. The apparent reaction orders for MEK given by an empirical power-law kinetic model decreased with reaction temperature from 0.75 at 478 K to 0.44 at 403 K. Several kinetic equations derived from mechanistic considerations have been investigated to account for this fact. The Mars-van Krevelen rate equation described well the kinetics of MEK combustion. However, it has been found that other rate equations derived from the Langmuir-Hinshelwood-HougenWatson formalism fit the kinetic data as well as or even better than the Mars-van Krevelen expression. 1. Introduction Catalytic combustion is a flameless technology in which a fuel is oxidized, most frequently with oxygen from air, in the presence of a suitable catalyst. Interest in catalytic combustion is mainly supported by differences with respect to conventional flame combustion: (i) it can be conducted at much lower temperatures, (ii) emissions of nitrogen oxides can be drastically reduced, (iii) ignition sources are not required, and (iv) it is safer and can proceed at any fuel/air ratio.1-4 Catalytic combustion is a cleaner alternative to flame combustion. This is contributing to the strengthening of this technology by improving its performance, mitigating its environmental impacts, and expanding the field of applications. In this regard, besides the well-known use of catalytic combustion in the converters to control gaseous emissions from motor vehicle exhausts,5 it has two additional major applications: on the one hand, the combustion of lean methane (natural gas)/air mixtures with feeding of the hot exhaust gases to a gas turbine for electricity generation;3,6,7 on the other hand, the destruction by complete oxidation of hazardous gaseous pollutants known as volatile organic compounds (VOCs).3,4,8 These pollutants are usually present in air streams at low concentrations (below 1%), so a flame cannot be sustained; as a result, additional fuel should be employed in the case of using conventional combustion for this application, thus increasing the operating costs and environmental impact. In contrast, catalytic combustion takes advantage of this fact since it can operate at very low fuel concentrations with high selectivities for the complete oxidation products CO2 and H2O. Combustion catalysts typically consist of noble metals, mainly platinum and palladium, supported on ceramic monolithic structures, although in recent years important progress has been made in the field of combustion catalysts based on transition metal mixed oxides.4,9 In catalytic combustion studies it is * To whom correspondence should be addressed. Tel.: +34 948 169 605. E-mail: [email protected]. † Universidad Pu´blica de Navarra. ‡ Universidad del Paı´s Vasco.

customary to evaluate the performance of a catalyst and compare the capability of different samples by means of the so-called ignition or light-off curves.10,11 These are conversion-reaction temperature graphs obtained by measuring the fuel or VOC conversion as the reaction temperature is increased, although the experiment can be conducted at decreasing temperatures starting from complete conversion. On increasing the temperature, the reaction initially is usually in the kinetic regime until the process known as light-off occurs, resulting in a sudden increase of the reaction rate and organic compounds conversion. At even higher temperatures the reaction rate increase becomes slow due to the depletion of the reactant concentration caused by the fast chemical kinetics and the reaction may eventually enter the mass-transfer controlled regime. The general pattern of the ignition or light-off curves is sigmoid or S-shaped, as for any irreversible chemical reaction. Nevertheless, masstransfer effects have a strong influence on the shape of the lightoff curves, as shown by Duprat.11 In the presence of masstransfer limitations the combustion rate slows down so that complete oxidation is achieved at temperatures much higher than those in the absence of transport effects and the ignition curves move away from S-shaped at relatively high conversions. Lightoff curves provide useful information such as the minimum space time (catalyst amount) required to achieve complete combustion of a given feed stream at a convenient reaction temperature. However, reliable catalytic combustor design demands knowledge of the reaction kinetics, especially when VOCs mixtures have to be treated since it is habitual that strong mutual inhibition or “mixture” effects between the several components give rise to important changes in the temperature required to achieve complete oxidation.12 Nevertheless, there is a clear lack of knowledge about the reaction kinetics and mechanisms in the field of catalytic combustion of VOCs. In spite of being a very active research area, most of the published work is about catalyst formulation and performance evaluation by means of the ignition curves of some selected VOCs. Methyl ethyl ketone (2-butanone, MEK) is a widely used chemical due to its outstanding solvent properties and low price in its boiling range.13 It is used as a solvent in a variety of coatings involving resins, cellulose acetate, and cellulose nitrate.

10.1021/ie071156b CCC: $37.00 © 2007 American Chemical Society Published on Web 11/22/2007

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Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007

It is a dewaxing agent in the refining of lubricant oils and is a denaturing agent for alcohols, and it is used in vegetable-oil extraction processes. It is also present in many household products as lacquer, adhesives, varnishes, paint removers, and cleaning products. Therefore, there is an interest in controlling the presence of MEK in ambient air and gaseous emissions.14 There are few reports on the catalytic combustion of MEK. Among the catalysts considered in these studies are Pt, Ni, and Cr alloy,15 platinum-based membranes,16 bulk and supported manganese oxides,17,18 platinum on alumina-pillared clays,19 Fe2O3-based membranes,20 transition-metal-doped ZrO2,21 and Pd/alumina.22 As the kinetics of this reaction is concerned, it has been studied only by Lou and Chen over a commercial foam-type catalyst,15 Picasso Escobar et al. over Fe2O3,23 and Choudhary and Deshmukh over Cr-doped zirconia.24 The aim of this work is to study the combustion of MEK in diluted air streams over a commercially available Pt/Al2O3 catalyst. Emphasis has been put on deriving kinetic expressions describing the rate of this reaction. To the best of our knowledge, this is the first report on the kinetics of the complete oxidation of this ketone on an alumina-supported platinum catalyst.

catalytic bed is allowed to stabilize for about 1 h and then the temperature is decreased by switching on a programmed ramp with the temperature controller to measure the ignition curves. The ramp was stopped when the reactor temperature reached the values at which the analyses of the products stream wanted to be performed. The chromatographic analyses required about 9 min to confirm that the only reaction products were those of the complete oxidation of MEK; after that time the ramp was run again to allow further decrease of the reactor temperature until completion of the ignition curve. In the case of the kinetic experiments, the temperature was allowed to decrease from 673 K to the selected value between 403 and 485 K at a rate of about 10 K‚min-1. After the reaction temperature was reached, the evolution of MEK conversion with time-on-stream was measured at constant temperature and space time. It was found that conversion slowly decreased until a constant value that was typically reached after 5-7 h on-stream. The stationary values obtained at various temperatures and MEK concentrations were very reproducible and they were used in the kinetic study. 2.3. Data Analysis. Taking into account that the kinetic experiments were conducted in the differential regime, the reaction rate for MEK combustion (-RMEK) was calculated according to

2. Experimental Section 2.1. Experimental Setup. A tubular (8 mm i.d.) fixed-bed Pyrex glass reactor operated at atmospheric pressure was used to perform the MEK combustion experiments. A commercial 0.5% Pt on alumina catalyst (PRO-CAT-S30, Johnson Matthey, 3 mm pellets type 73, specific surface area of 95 m2‚g-1) was considered in this study. A thermocouple placed inside the reactor, in the center of the catalyst bed, monitored the reaction temperature. Mass flow controllers (Bronkhorst) monitored and controlled the flow of gases used to obtain the feed mixture and to pretreat the catalyst. The reactor feed consisted of an air stream saturated with MEK (Panreac, PA) that was created using a saturator equipped with temperature and pressure control and then diluted with synthetic air (Praxair 99.999%, 21% O2) to reach the selected inlet partial pressure and space time. Prior to each experiment, the catalyst was treated under 100 cm3 STP‚min-1 of synthetic air for 1 h at 673 K. On-line analysis of the product stream was performed on a Hewlett-Packard 6890 gas chromatograph, equipped with a 6 ft HayeSep Q column connected to a TCD for CO2 and water determination, and an HP-INNOWax 30 m × 0.32 mm i.d. column connected to an FID for MEK analyses. Water and CO2 were the only reaction products found. In general, the MEK conversion results were reproducible within 2% and mass balance closures were better than (1%. 2.2. Experimental Conditions and Procedures. When the kinetic experiments were performed, the reactor was operated in the differential mode (conversion below 15%) at temperatures between 403 and 478 K and MEK partial pressures in the feed stream ranging from 12 to 191 Pa (118-1885 ppm). Total volumetric gas flow rates ranged from 187 to 950 cm3 STP‚min-1 and the space times varied from 3160 to 25170 kgcat.‚min‚kmolMEK-1. Catalytic runs were carried out also to evaluate the light-off performance. To this end, once the pretreatment of the catalyst at 673 K for 1 h under 100 cm3 STP‚min-1 of synthetic air has finished, the MEK-air mixture is fed into the reactor using a 6-way valve while keeping the reactor temperature at 673 K. The conversion of MEK at this temperature was always 100% in the range of space velocities examined in this work. The

(-RMEK) )

XMEK

(1)

(W/FMEK,0)

where XMEK is the measured MEK conversion at the reactor outlet and W/FMEK,0 the space time referring to the amount of catalyst loaded into the reactor and the inlet MEK molar flow rate. In the present work, the reaction rate data have been fitted to the selected kinetic models by means of nonlinear regression analysis using the Nelder and Mead25 algorithm of direct search and a modified Levenberg-Marquardt method26 furnished by the DBCPOL and DRNLIN subroutines, respectively, in the IMSL library. These algorithms allow minimizing the objective function for the normalized residual sum of squares (NRSS), N

NRSS )

∑

n)1

(

)

(-Re,n) - (-Rn) (-Rn)

2

(2)

where (-Re,n) is the estimated reaction rate, (-Rn) the nth value of the experimentally measured reaction rate, and N the total number of experiments. The criterion adopted was that convergence was achieved when the NRSS between two consecutive iterations was less than 10-4. To improve the stability of the nonlinear least-squares algorithm, the kinetic (k) and equilibrium (K) constants were reparametrized as follows,27,28

(-ER (T1 - T′1 )) -∆H 1 1 K(T) ) K′‚exp( R (T T′)) k(T) ) k′‚exp

(3) (4)

where T′ is an arbitrary reference temperature; it was taken as 473.15 K, that is, within the range of temperatures considered in the kinetic study; k′ and K′ are the values of the constants at T′; E and ∆H are the activation energy and the enthalpy of adsorption, respectively; and R is the universal gas constant. As the discrimination between rival kinetic models is concerned,

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Figure 1. Light-off curves for MEK combustion in air at 101.3 kPa (PMEK,0 ) 47.3 Pa) over a commercial 0.5% Pt/alumina catalyst at a space time of 25170 kgcat.‚min‚kmolMEK-1. Reactions were performed with catalyst particles of mean size: 3 × 3 mm pellets (0), 3 × 1.5 mm pellets (2), 1.5 mm (4), 1 mm (b), 0.75 mm (O), 0.45 mm (]), 0.25 mm (3), and 0.15 mm (1).

Figure 2. Light-off curves for MEK combustion in air at 101.3 kPa (PMEK,0 ) 47.3 Pa) over a commercial 0.5% Pt/alumina catalyst at a space time of 25170 kgcat.‚min‚kmolMEK-1. Reactions were performed with the following amounts of catalyst and gas flow rates: 0.100 g and 187.5 cm3 STP‚min-1 (b), 0.200 g and 375 cm3 STP‚min-1 (4), and 0.400 g and 750 cm3 STP‚min-1 (2).

the statistical parameters considered have been the model selection criterion (MSC) and the coefficient of determination (DC).23 3. Results and Discussion 3.1. Ignition Curves. The commercial 0.5% Pt/alumina catalyst (PRO-CAT-S30, Johnson Matthey Chemicals) is delivered in the form of 3 × 3 mm pellets. Possible effects associated with mass-transfer limitations within the porous pellets were first examined by conducting several light-off experiments with catalyst particles of different size. To this end the pellets were crushed in an agate mortar and sieved in different size fractions from the starting 3 mm to 0.15 mm. The ignition curves for MEK combustion obtained in this series of experiments are depicted in Figure 1. Reactions were carried out with a gas flow rate of 750 cm3 STP‚min-1 and space time of 25170 kgcat.‚min‚kmolMEK-1. Possible effects related to textural changes due to the grinding of the pellets can be discarded. In fact, the specific surface area of the several catalyst fractions was evaluated according to the BET equation from the nitrogen adsorption-desorption data at 77 K measured by the static method in an automatic volumetric Micromeritics ASAP 2010 adsorption analyzer. The specific surfaces areas varied very slightly from 95 m2‚g-1 for the original pellets to 90 m2‚g-1 for the fraction with the smallest mean particle size (0.15 mm). As can be seen from Figure 1, once ignition was reached, MEK combustion suffers from severe pore diffusion limitations when conducted with the 3 × 3 mm pellets due to fast oxidation kinetics at temperatures above about 550 K. The light-off or ignition temperature, T50, is usually defined as the temperature at which the VOC conversion achieved is 50%. This temperature decreases from 530 K with the entire pellets to 450 K with particles of 0.15 mm. Nevertheless, there are no significant differences between the results obtained with catalyst particles of mean diameter below 0.45 mm. As a result, the rest of this study was performed with particles of 0.25 mm mean diameter to avoid intraparticle mass-transfer effects. As the possible presence of external mass-transfer effects is concerned, that is, limitations to the transport from the bulk gas phase to the outer surface of the catalyst particles, some

Figure 3. Light-off curves for MEK combustion in air at 101.3 kPa (PMEK,0 ) 47.3 Pa) over a commercial 0.5% Pt/alumina catalyst at constant gas flow rate of 750 cm3 STP‚min-1. Reactions were performed with the following amounts of catalyst: 0.049 g (O), 0.100 g (b), 0.200 g (4), and 0.400 g (2).

experiments were carried out at the same space time (W/FMEK,0 ) 25170 kgcat.‚min‚kmolMEK-1) but at varying gas linear velocities by increasing proportionally the catalyst amount (W) and the total volumetric gas flow rate (Q0). The ignition curves for MEK combustion obtained in these catalytic runs are depicted in Figure 2. It can be seen that there is no effect on the gas velocity within the range of experimental conditions considered in this work, so one can reasonably assume that the results are free from external mass-transfer effects. Finally, the effect of space time in the ignition curves for MEK combustion over the 0.5% Pt/alumina catalyst (0.25 mm particles) has been studied by varying the mass of catalyst loaded into the reactor at constant MEK partial pressure in the feed (47.3 Pa, 467 ppm) and volumetric gas flow rate (750 cm3 STP‚min-1). In this series of experiments the mass of catalyst was varied between 0.049 and 0.400 g, thus resulting in space time values in the 3162-25170 kgcat.‚min‚kmolMEK-1 range. As shown in Figure 3, the amount of catalyst (or space time) has a noticeable effect on the ignition curves and consequently in

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the light-off temperatures. The value of T50 for MEK combustion decreases from 519 to 450 K with an eightfold increase of the space time, from 3162 to 25170 kgcat.‚min‚kmolMEK-1. Obviously, the reactor performance improves at increasing space times, thus resulting in a higher MEK conversion at a given reaction temperature. Therefore, the ignition curves are shifted to lower temperatures as the space time increases. In the field of the complete oxidation of VOCs, it is generally assumed that relatively important variations of the space time or space velocity give rise to only small changes in the temperature at which a given VOC conversion is achieved due to the exponential dependence on the reaction temperature of the rate constants. Nevertheless, it seems that in this respect the VOC concentration and catalyst nature play an important role. In fact, Pina et al.16 did not find a significant effect of the weight-hourly space velocity (WHSV) in the ignition curves for the combustion of MEK (1750-3100 ppm in air) over Pt/ γ-Al2O3-based catalytic membranes. The value of T50 was found to increase only from 413 to 423 K when increasing the space velocity from 1674 to 2658 h-1. In our case, the WHSV values were in the range 145-1183 h-1, and a significant effect of this variable has been found (Figure 3). In previous studies, T50 values in the 483-553 K range were found for the combustion of 600 ppm MEK in air over Pt catalysts supported (2.3 wt. % Pt) on several pillared clays at WHSV of 432 h-1,19 whereas values between 508 and 603 K were measured for a series of unsupported manganese oxides at WHSV in the 5101200 h-1 range.17 Similar results were found by Picasso et al.22 for the complete oxidation of MEK (1550 ppm in air) over Fe2O3-based catalysts, although at WHSV of only 80-170 h-1. Irusta et al.,29 on the other hand, obtained T50 values between 508 and 523 K when treating 1600 ppm of MEK in air over a series of Mn and Co perovskites at WHSV of 178 h-1. From the above revision it can be concluded that the performance of the 0.5% Pt/alumina catalyst (PRO-CAT-S30) is close to that of Pt/γ-Al2O3-based catalytic membranes but superior to that of Mn and Fe oxides and Pt on pillared clays. 3.2. Kinetic Study. The evolution of MEK conversion with time-on-stream for a representative set of combustion reactions conducted at constant space time of 6327 kgcat.‚min‚kmolMEK-1 with a MEK partial pressure in the feed stream of 47.3 Pa and several temperatures between 403 and 463 K is shown in Figure 4. In each case the initial MEK conversion is almost co-incident with that obtained in the light-off experiments carried out at decreasing reaction temperatures. However, a noticeable decrease of MEK conversion with time-on-stream can be appreciated in Figure 4 for the experiments carried out at constant temperature, although a pseudo steady state is reached after about 400-450 min. This behavior was not due to irreversible deactivation phenomena since the initial conversion is restored simply by pretreating again the catalyst under 100 cm3 STP‚min-1 of synthetic air for 1 h at 673 K. Moreover, the evolution of the MEK conversion turned out extremely reproducible. A similar behavior has been reported by Lahousse et al.30 for the combustion of benzene and ethyl acetate over MnO2 and Pt/TiO2 catalysts. As discussed by these authors, the VOCs conversion decreases because a transition period is required to get a steady coverage of the catalyst surface by the reactants and the products. The relatively long duration of this transient period can be related to the very low concentration of the organic compound in the gaseous phase. Insofar as the stable MEK conversion reached after the transient period entered the differential regime, the result was used to calculate the reaction rate according to eq 1. A set of

Figure 4. Evolution of MEK conversion with time-on-stream for a series of combustion reactions conducted at 101.3 kPa (PMEK,0 ) 47.3 Pa) and 6327 kgcat.‚min‚kmolMEK-1 over a commercial 0.5% Pt/alumina catalyst. Reaction temperatures were 403 K (2), 418 K (4), 433 K (b), 448 K (1), and 463 K (3).

Figure 5. Rate of MEK combustion in air at low concentrations over a commercial 0.5% Pt/alumina catalyst. Reaction temperatures were 403 K (2), 418 K (4), 433 K (b), 448 K (1), 463 K (3), and 478 K (]). Lines correspond to the fitting of the kinetic data to the Mars-van Krevelen rate equation (s), the LHHW-I-MEK expression (- - -), and the LHHW-IIMEK rate equation (---).

kinetic data has been obtained in this way varying the space time, the MEK partial pressure in the feed stream, and the reaction temperature. The results are shown in Figure 5 as a semilog plot. In what follows the analysis of the kinetic data will be presented considering both empirical and mechanistic approaches to derive reaction rate expressions describing the experimental results. 3.2.1. Power-Law Rate Model. This is the simplest approach to describe the dependence of reaction rate on temperature and partial pressures,

(-RMEK) ) kPL‚POβ 2‚PRMEK

(5)

where kPL is the kinetic constant of the power-law model and PO2 and PMEK are the partial pressures of oxygen and MEK, respectively. In the experimental conditions relevant to this work the pressure drop in the catalytic bed is of the order of 20.3 kPa; this gives an absolute pressure at the reactor entry of about 121.6 kPa and an oxygen partial pressure of 25541 Pa. This

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Figure 6. Power-law rate model plot for the combustion of MEK in air over a commercial 0.5% Pt/alumina catalyst. Reaction temperatures were 403 K (2), 418 K (4), 433 K (b), 448 K (1), 463 K (3), 478 K (]), and 485 K (×).

value represents a considerable excess with respect to the stoichiometric amount of oxygen necessary for the complete oxidation of MEK: 5.5 × 191 ) 1050 Pa for the highest concentration of MEK in the feed stream considered. Moreover, as the reactor was operated in the differential regime (XMEK < 0.15), the oxygen consumption is low. As a result, the oxygen partial pressure through the catalytic bed has been considered constant, so it can be incorporated into the apparent kinetic constant k′PL,

(-RMEK) ) k′PL‚PRMEK

(6)

0.44 at 403 K, and from the corresponding Arrhenius plots, the apparent activation energies increase with MEK partial pressure from 59 kJ‚mol-1 for 12 Pa to 73 kJ‚mol-1 for 191 Pa. Picasso Escobar et al.23 estimated an apparent reaction order for MEK combustion over Fe2O3 of only 0.36 in the 483-583 K temperature range, and a high apparent activation energy of 117 kJ‚mol-1, comparable with values previously reported over manganese oxides.17 Choudhary and Deshmukh,24 on the other hand, reported also a change of the apparent reaction order for MEK with reaction temperature over Cr-doped zirconia from 0.71 at 485 K to 0.99 at 523 K; the apparent activation energy was 54 kJ‚mol-1, which is more in line with the values obtained in this work over supported Pt. In spite of the reasonably good fit to the kinetic data provided by the power-law model, the variation of the kinetic parameters with temperature and composition makes it invalid, pointing to the need for more elaborated models derived from mechanistic considerations. 3.2.2. Mars-van Krevelen Rate Equation. From its origin31 the rate equation derived from the so-called Mars-van Krevelen mechanism is being routinely considered in the kinetic modeling of both partial and complete catalytic oxidation reactions of organic compounds. Basically, it is a surface redox mechanism comprising the oxidation of the hydrocarbon from the gas phase with lattice oxygen and the reoxidation of the catalyst by gasphase oxygen.4 The fact that the participation of lattice oxygen is claimed has prompted a good reception of this mechanism, especially between those which are working on selective hydrocarbons oxidation with oxide catalysts. Nevertheless, it is also frequently considered in studies on VOCs complete oxidation over oxide23,24,32 and Pt12,28,33 catalysts, even to describe mixture effects.34 In our case, the resulting rate expression assuming first-order reaction for both MEK and oxygen is

or equivalently,

ln(-RMEK) ) ln k′PL + R‚ln PMEK

(-RMEK) )

(7)

A plot of the natural logarithms of (-RMEK) and PMEK is shown in Figure 6. As can be seen, this model fits the experimental results well for temperatures below 478 K, whereas the data at 485 K clearly deviate from the power-law rate equation. The apparent reaction orders (R) for MEK decrease as the reaction temperature increases from 0.75 at 478 K to

kMvK‚kox‚PMEK‚PO2 kox‚PO2 + ν‚kMvK‚PMEK

where kMvK and kox are the kinetic constants of the MEK oxidation and catalyst reoxidation, respectively, and ν is the stoichiometric coefficient for MEK combustion (ν ) 5.5). Fitting of the kinetic data to eq 8 is shown in Figure 5 as solid lines and the estimated parameters are included in Table 1 (kinetic constants are given at 473 K). This rate equation fits the data

Table 1. Estimated Kinetic Parameters of the Rate Equations Considered for the Combustion of MEK in Air over a Commercial 0.5% Pt/Alumina Catalysta,b

a

parameter

Mars-van Krevelen

LHHW-I-MEK

normalized residual sum of squares (NRSS) model selection criterion (MSC) coefficient of determination (DC) kMvK [kmol/(kgcat.‚Pa‚s)] kox [kmol/(kgcat.‚Pa‚s)] EMvK (kJ‚mol-1) Eox (kJ‚mol-1) kI [kmol/(kgcat.‚Pa‚s)] KMEK,I (Pa-1) EI (kJ‚mol-1) -∆HMEK,I (kJ‚mol-1) k′II [kmol/(kgcat.‚Pa‚s)] EII (kJ‚mol-1) KMEK,II (Pa-1) -∆HMEK,II (kJ‚mol-1)

10-11

10-11

2.25 ×

2.24 ×

LHHW-II-MEK 2.88 × 10-12

5.216

5.218

5.410

0.9020

0.9024

0.9155

(1.2 ( 0.2) × 10-8 (2.7 ( 0.6) × 10-10 58.2 ( 6.8 83.5 ( 7.4

(8)

(5.0 ( 1.0) × 10-11 0.009 ( 0.003 83.5 ( 7.4 25.3 ( 13.1

(1.1 ( 0.2) × 10-8 62.0 ( 6.1 0.0031 ( 0.0009 17.7 ( 11.1

Values of the kinetic and equilibrium adsorption constants are given at 473 K. b Intervals are given at the 95% confidence level.

9042


satisfactorily in accordance with the relatively high value of the MSC parameter (5.22). Choudhary and Deshmukh24 found a lower value of the kMvK constant at 485 K (1.3 × 10-9 [kmol/(kgcat.‚Pa‚s)]) for the combustion of MEK over Crdoped zirconia, and Picasso Escobar et al.23 obtained significantly higher activation energies for the oxidation (96.2 kJ‚mol-1) and surface reoxidation (131.1 kJ‚mol-1) steps for the combustion of MEK over Fe2O3. Therefore, it can be concluded that the performance of the supported platinum catalysts is better than that of metal oxides for the destruction of this VOC. Vannice,35 in a recent and excellent paper, performs the first critical analysis of the Mars-van Krevelen rate expression. As pointed out by the author, the original derivation of the equation is inconsistent and incorrect; it has no physical relevance and must be considered only as a mathematical function for kinetic data fitting. This is mainly due to the fact that oxygen dissociation is not included in the reaction scheme whereas the participation of lattice oxygen and the reoxidation of the catalysts surface are claimed. Moreover, the hydrocarbon oxidation step is treated as an elementary step according to an Eley-Rideal reaction; this is very unlikely due to the high number of chemical bonds that have to be broken and formed. Vannice shows that rate equations derived from more realistic models of the Langmuir-Hinshelwood and Hougen-Watsontype can fit the kinetic data as well as or even better than the Mars-van Krevelen expression.35 This type of equation is considered in the next section. 3.2.3. Langmuir-Hinshelwood-Hougen-Watson (LHHW) Rate Equations. This classical approach to analyze the kinetics of reactions catalyzed by solids lies in proposing a reaction scheme consisting of a series of steps describing the adsorption of reactants, surface reaction, and products desorption.36 The steps are treated as elementary reactions and the one supposed to be the slowest is considered as the rate-determining step, whereas all the other steps are assumed to be quasi-equilibrated. This, together with active sites balances, allows the derivation of rate equations that can be checked for kinetic data fitting. As far as the combustion of VOCs on platinum catalysts is concerned, a number of LHHW rate expressions have been proposed whose mathematical derivation can be found in the literature. Some of the equations provided satisfactory rate data fitting and gave good description of VOCs mixture effects. To our knowledge, there is no previous report on the kinetics of MEK combustion on Pt catalyst. Our approach has been to take into consideration the mechanisms and rate equations previously proposed by other researchers for the combustion of VOCs such as benzene, styrene, acetone, methylene chloride, methanol, methyl tert-butyl ether, n-hexane, and toluene on Pt12,27,28,34 and MEK on oxides,23,24 although the number of possible rate equations is much higher. However, we have not changed the oxygen partial pressure in our experiments because the combustion of VOCs is usually conducted under very high oxygen excess, resulting in an essentially constant oxygen concentration through the reactor. As a result, we have no information as to distinguish between mechanisms differing only in the way that oxygen is added to the hydrocarbon: reaction between adsorbed VOC and adsorbed activated O2 or a single oxygen atom, adsorbed in the same or different type of active sites.35 Also for this reason, in spite of the many possible kinetic equations, we have verified that those that better describe the combustion of MEK on the commercial platinum catalyst are, from a mathematical point of view, essentially of

two types that we have named LHHW-I (eq 9) and LHHW-II (eq 10),

(-RMEK) ) (-RMEK) )

Γ1‚PMEK (1 + Γ2‚PMEK) Γ1‚PMEK (1 + Γ2‚PMEK)2

(9)

(10)

where the parameters Γ1 and Γ2 are combinations of the kinetic and equilibrium adsorption constants; the parameter Γ1 usually depends on the oxygen partial pressure, (PO2)n, where n is most frequently 1 or 1/2. As concerns the reaction schemes originating these rate equations, the main difference is the number of active sites involved in the rate-determining step: one for LHHW-I and two for LHHW-II. As an illustrative example of the LHHW-I type, it is worthwhile to consider one of the mechanisms proposed by Barresi and Baldi12 for the combustion of benzene and styrene over a commercial monolithic platinum-based catalyst. When adapted to MEK combustion, the mechanism is as follows: KMEK,I

MEK + S 798 MEK - S +O2

kI

MEK - S + O2 98 MEK′ - S 98 products + S This is the so-called sticking mechanism which includes reaction between oxygen from the gas phase and adsorbed MEK. S is an active site, KMEK,I the equilibrium constant for MEK adsorption, and kI the kinetic constant for the rate-determining step. MEK′-S represents an intermediate partially oxidized species which is rapidly converted in CO2 and H2O. Assuming that MEK adsorption has reached quasi-equilibrium conditions, the rate for MEK combustion (LHHW-I-MEK) according to this mechanism is

(-RMEK) )

kI‚KMEK,I‚PO2‚PMEK (1 + KMEK,I‚PMEK)

(11)

Fitting of the kinetic data to eq 11 is shown in Figure 5 and the estimated parameters are included in Table 1. In reality, fittings to eqs 11 and 8 cannot be distinguished in Figure 5 because they are mathematically equivalent. In fact, the Marsvan Krevelen rate equation (eq 8) can be written also as follows:

(-RMEK) )

kMvK‚kox‚PMEK‚PO2 kox‚PO2 + ν‚kMvK‚PMEK

)

kMvK‚PMEK ν‚kMvK 1+ ‚P kox‚PO2 MEK (12)

As a matter of fact, the statistical parameters included in Table 1, NRSS, MSC, and DC, are almost the same for both the Mars-van Krevelen rate equation and the LHHW expression given by eq 11. The identical values of the activation energy (83.5 kJ‚mol-1) for the slowest steps of the two schemes, surface reoxidation and surface reaction between adsorbed MEK and gas-phase oxygen, respectively, are also noteworthy. In the case of the LHHW mechanisms including surface reaction between two species adsorbed on the same type of sites as the rate-determining step, we have adapted to platinum the scheme proposed by Picasso Escobar et al.23 for MEK conversion on iron oxide,

Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 9043 KMEK,II

MEK + S 798 MEK - S KO

2,II

O2 + 2S 798 2O - S kII

MEK - S + O - S 98 products + 2S where S is an active site, KMEK,II and KO2,II are the equilibrium constants for the adsorption of MEK and oxygen, respectively, and kII is the kinetic constant for the rate-determining step. Assuming that MEK and O2 adsorption have reached quasiequilibrium conditions, the rate for MEK combustion according to this mechanism is

(-RMEK) )

kII‚KMEK,II‚xKO2,II‚PO2‚PMEK (1 + xKO2,II‚PO2 + KMEK,II‚PMEK)2

(13)

As suggested by Picasso Escobar et al.,23 assuming that (KO2,II‚PO2)1/2 is negligible, eq 13 can be simplified to (LHHWII-MEK)

(-RMEK) )

k′II‚PMEK (1 + KMEK,II‚PMEK)2

(14)

where k′II ) kII‚KMEK,II‚(KO2,II‚PO2)1/2. Fitting of the kinetic data to eq 14 is shown in Figure 5 and the estimated parameters are included in Table 1. As can be seen, this rate equation also fits the kinetic data well, but provides the best statistical parameters of the three kinetic expressions considered, with a reasonably high value of the MSC (5.410). The value adopted by the group k′II is comparable with that of the kinetic constant for the hydrocarbon oxidation step of the Mars-van Krevelen scheme. On the other hand, the enthalpy for MEK adsorption (17.7 kJ‚mol-1) is considerable, but slightly lower than the value given by the previous (LHHW-I-MEK) model (25.3 kJ‚mol-1). This value is very close to the one obtained by Picasso Escobar et al.23 for MEK combustion over Fe2O3 (25.7 kJ‚mol-1). In conclusion, the classical Mars-van Krevelen rate equation describes well the kinetics of MEK combustion in air at low concentrations over a commercial platinum on alumina catalyst. However, as recently pointed out by Vannice,35 this rate expression has no physical relevance and must be considered only as a mathematical function for kinetic data fitting. Other rate equations derived from mechanistic considerations according to the Langmuir-Hinshelwood-Hougen-Watson formalism fit the kinetic data as well as or even better than the Marsvan Krevelen expression. Due to the conditions of significant oxygen in excess relevant for VOCs combustion, the MEK complete oxidation has been equally well described by models considering reaction between adsorbed MEK and gas-phase oxygen or surface reaction between adsorbed MEK and oxygen atoms as the rate-determining steps. Literature Cited (1) Prasad, R.; Kennedy, L. A.; Ruckenstein, E. Catalytic Combustion. Catal. ReV.-Sci. Eng. 1984, 26, 1. (2) Pfefferle, L. D.; Pfefferle, W. C. Catalysis in Combustion. Catal. ReV.-Sci. Eng. 1987, 29, 219. (3) Hayes, R. E.; Kolaczkowski, S. T. Introduction to Catalytic Combustion; Gordon and Breach Science Publishers: Amsterdam, 1997. (4) Hodnett, B. K. Heterogeneous Catalytic Oxidation; John Wiley & Sons: Chichester, 2000.

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ReceiVed for reView August 23, 2007 ReVised manuscript receiVed September 20, 2007 Accepted September 21, 2007 IE071156B

Kinetics of Methyl Ethyl Ketone Combustion in Air at Low

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