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Effects of various alkali–acid additives on the activity of a manganese oxide in the catalytic combustion of ketones
Applied Catalysis B: Environmental 33 (2001) 1–8
Effects of various alkali–acid additives on the activity of a manganese oxide in the catalytic combustion of ketones L.M. Gandía∗ , A. Gil, S.A. Korili1 Departamento de Química Aplicada, Universidad Pública de Navarra, Campus de Arrosadía s/n, 31006 Pamplona, Spain Received 28 May 2000; received in revised form 22 December 2000; accepted 28 January 2001
Abstract The catalytic combustion of acetone and methyl-ethyl-ketone (MEK) has been studied over a manganese oxide, Mn2 O3 . The reactant conversion has been followed as a function of the reaction temperature and it has been observed that lower temperatures are required for the combustion of acetone than for MEK. The performance of Mn2 O3 in the combustion reactions when alkali and acid ions are added to the oxide has also been investigated. A significant improvement of the catalyst performance is observed when Cs+ and Na+ are used as additives. The data of the ignition curves have been fitted to a simplified model where both ketone combustion reactions are assumed to have power law rate equations, which are first-order with respect to the corresponding organic molecule and do not depend on the oxygen partial pressure. Differences in the apparent activation energy values estimated with this model for acetone and MEK combustion are significant only when acid ions additives are present. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Manganese oxide; Acetone catalytic combustion; Methyl-ethyl-ketone catalytic combustion; Alkali–acid additives
1. Introduction Catalytic oxidation is a well-established technology for volatile organic compound (VOC) destruction and odour elimination. When compared to the thermal regenerative oxidation, the catalytic oxidation with recuperative heat recovery may be economically advantageous for flow rates under 850 m3 STP min−1 and VOC concentrations ranging from 50 to 10 000 ppmv [1,2]. Because of the large gas volumes that must be treated and the VOC concentrations, catalysts that are very active and selective towards complete oxidation reactions are required. The performance of catalytic combustion processes critically depends on the type of ∗
Corresponding author. E-mail address: [email protected] (L.M. Gand´ıa). 1 On leave from Aristotle University of Thessaloniki, Greece.
catalyst, its physical form and its ability to work over a wide range of operating temperatures, and on the VOC molecule nature [3,4]. Supported noble metals, typically Pt, Pd or Rh, and transition metal oxides as CuO, MnOx , Co3 O4 , Cr2 O3 , and NiO as well as mixed oxides are active VOC combustion catalysts [3–10]. Several researchers [11–19] have studied the effects of alkali and acid additives on the activity and selectivity of catalysts that are usually used in combustion reactions. However, the way in which the various additives act on the catalyst activity and selectivity is not clear at present. Ishikawa et al. [13] claimed a positive effect of the acidic properties of sulfate-doped supports on the activity of platinum in the low-temperature catalytic combustion of propane. According to the authors, a possible role of the acidic support is to prevent the oxidation of the supported platinum. Hua and Gao [17] studied the catalytic com-
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bustion of n-pentane on Pt supported on superacids and correlated the catalyst activity with the metal dispersion and the acid strength of the support. They proposed that the formation of alkane carbocations on the surface sulfates during reaction may account for the enhancement of the combustion activity by the strong acidity of the supports. Kang and Wan [14] investigated the effects of acid and base additives on the activity of chromium and cobalt oxides supported on ␥-alumina in CO and ethane oxidation. Although the conversions of the two reactions were influenced in different ways by the additives, it seems that the addition of a base additive to the oxide catalysts in general favours the complete oxidation, while the acid additives decrease it. These results may be explained by the electron ability character, donor or acceptor, of the catalyst [20]. All the cited papers indicate an important influence of the additives on the catalyst properties and behavior in combustion reactions. The aim of the present work, was to study the performance of a manganese oxide in the catalytic combustion of acetone and MEK, and to investigate the possible effects of alkali and acid additives on catalyst activity. To our best knowledge, this is the first report dealing with the effect of alkali–acid additives on the catalytic performance of a manganese oxide in the combustion of VOC, and especially of ketones.
2. Experimental The manganese oxide was prepared by the citrate method [21]. The starting materials, Mn(NO3 )2 ·4H2 O (Merck, PA) and citric acid (Panreac, PA), were dissolved in water at an equivalent ratio of 1:1. The resulting solution was slowly evaporated in a rotavapor at 333 K to form a dense gel, which was further dehydrated in a vacuum oven at 343 K. The precursor formed this way was heated in air (5 K min−1 ) in a furnace up to 823 K, where it was calcined for 4 h, forming the final oxide material. The acid modified samples, were prepared by treating 3 g of the manganese oxide with 50 cm3 of 0.01N H2 SO4 (Merck, 98%) or citric acid (Panreac, PA). In the case of the alkali modified catalysts, the quantity of Na or Cs necessary to form half a monolayer on the surface of the manganese oxide was estimated on
the basis of the BET specific surface area of the initial oxide (12.5 m2 g−1 ). Taking as a basis for the calculations the bigger ionic radius, solutions of NaNO3 (Panreac, PA) and CsNO3 (Aldrich, 99%) were prepared and mixed with the manganese oxide in suitable ion proportions. The slurries resulting from the mixing of the oxide with the acid or salt solutions were agitated at room temperature for 3 h, then evaporated slowly at 333 K under reduced pressure in a rotavapor. The solids were dried for 12 h at 383 K and finally calcined for 4 h at 773 K. For a better comparison of the catalysts, the manganese oxide was also treated with 50 cm3 of deionised water, then evaporated, dried and calcined in the same way as the modified samples. The catalysts are designated hereafter as Mn–X, where X refers to the additive used, that is S for sulfuric acid, Cit for citric acid, Na for sodium nitrate and Cs for cesium nitrate. No suffix was used in the case of the pure unmodified oxide (Mn). The phases present in the oxide prepared were identified by X-ray diffraction (XRD) analysis on a Siemens D-500 powder diffractometer, using nickel-filtered Cu K␣ (λ = 1.5405 Å) radiation. The BET specific surface areas were measured by Kr (Air Liquide, 99.995%) adsorption at 77 K using a static automatic volumetric apparatus (Micromeritics ASAP 2010 adsorption analyser). Samples were previously degassed (0.1 Pa) for 3 h at 473 K. Catalyst acidity was determined by adsorption of NH3 (Air Liquide, >99.995%), using the same static automatic apparatus mentioned before. Samples were pretreated by heating under vacuum at a rate of 5 K min−1 up to 573 K, where they were maintained for 3 h. The ammonia volume adsorbed at 448 K and an equilibrium pressure of 20 kPa [22] was considered representative of the sample acidity. Chemical analyses of the catalysts in order to determine the real content of additive present were carried out by Activation Laboratories, Ancaster, Ont., Canada, using instrumental neutron activation analysis (INAA) and infrared analysis (IR). Ketone combustion reactions were carried out in a tubular (8 mm i.d.) fixed-bed Pyrex glass reactor at atmospheric pressure. The catalyst (100–200 m particle size fraction) was diluted with inert solids (100–200 m Pyrex glass beads) in a volume ratio of 1:4 approximately, forming a catalytic bed
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of about 1 cm of depth. 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. An air stream saturated with the corresponding ketone (Panreac, PA) was created using a saturator, equipped with temperature and pressure control, and then diluted with pure air (SEO, 99.999%), resulting in a 600 ppmv ketone concentration in the reactor feed. Prior to the reaction, the catalysts were treated under 100 cm3 min−1 of air for 1 h at 573 K. The light-off curves for ketone combustion were obtained at a controlled heating rate of 2.5 K min−1 and at identical SBET /Qin ratios of 2 min cm −3 . Space velocities (GHSV), cal1.575 mox ketone culated at standard temperature and pressure and based on the total volume of the catalytic bed, were about 34 000 h−1 . The operating conditions of the system have been carefully selected in order to minimise heat and mass transfer phenomena. Nevertheless, the possible effect of external limitations has been evaluated by the application of standard literature criteria [23,24]. It was found this way that mass and heat transfer effects do not prevail in most cases, especially at low and medium conversions. Some individual experiments that were carried out at varying gas linear velocities and constant SBET /Qin ratio, confirmed the absence of external mass transfer effects. On-line analysis of the product stream was performed on a Hewlett-Packard 6890 gas chromatograph equipped with two column systems, a 6 ft HayeSep Q connected to a TCD for CO2 determination, and an HP-INNOWax 30 m ×0.32 mm i.d. column connected to an FID for ketone analyses.
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3. Results and discussion 3.1. Physico-chemical properties The XRD analyses of the manganese oxide and the modified oxides indicated the presence of well-crystallised ␣-Mn2 O3 in all cases. No other bulk crystalline phases containing the additives were detected at the modified oxides. The BET specific surface areas and the additive content of the samples are summarised in Table 1. A slight increase of the oxide specific surface area when the citric acid is used as modifier can be explained by a partial dissolution of the manganese oxide during the modification procedure, which was very similar to the citrate method used for the synthesis of the parent oxide. On the other hand, the incorporation of sodium and cesium caused a loss of specific surface area, probably related to a cement effect [25] which sticks the oxide and the alkali metal salt together in a sort of conglomerate. The acidity of the modified oxides was, as expected, higher than that of the parent manganese oxide in the case of the acid modified samples, and lower in the case of the alkali modified ones. As it can be seen from the results of Table 1, no significant differences were observed between the acidities of the two acid modified samples or the acidities of the two alkali modified ones. 3.2. Reaction studies In the combustion reactions of organic compounds, when the reactant conversion is followed as a function of the reaction temperature, a characteristic S-shaped
Table 1 Physico-chemical and catalytic properties of the prepared oxides. Sample
SBET (m2 g−1 )
V (cm3 STP g−1 )a
Additive (wt.%)
Ea (kJ mol−1 )b
Mn Mn–S Mn–Cit Mn–Na Mn–Cs
12.5 11.6 14.6 8.7 6.3
1.32 1.65 1.51 0.87 0.83
– 0.25 (S) – 0.41 (Na) 1.03 (Cs)
118 217 194 133 119
a
± ± ± ± ±
6 24 6 6 6
T50 (K)b
Ea (kJ mol−1 )c
540 600 542 508 495
125 162 164 125 116
Ammonia adsorbed at 448 K and 20 kPa. Apparent activation energy with 95% confidence interval; T50 for the combustion of acetone. c Apparent activation energy with 95% confidence interval; T 50 for the combustion of MEK. d Points used in model fitting lie in the conversion range 0–90%. b
± ± ± ± ±
6 4 7d 7d 8d
T50 (K)c 558 601 560 526 509
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curve is obtained, called light-off curve or ignition curve. The exact shape of the curve depends on several factors such as the reaction kinetics, the mass transport, and the reactor operating conditions [26]. By fitting the data of this curve to a suitable model describing the reaction kinetics and the reactor behavior, the best estimations for the kinetic parameters can be obtained. A series of assumptions has been made in order to develop a reasonable but simple model that can describe satisfactorily the experimental data. Both ketone combustion reactions are assumed to have power law rate equations, which are first-order with respect to the corresponding organic molecule and do not depend on the oxygen partial pressure, since oxygen is in a very large excess and its partial pressure remains practically unchanged during the reaction. Ea −ri = kPi = A exp − (1) Pi RT where −ri is the rate of reaction of ketone i, Pi the partial pressure of the ketone in the gas stream, and k the rate constant, which obeys an Arrhenius relation, A is the pre-exponential factor and Ea the apparent activation energy. The assumption of first-order kinetics has been checked by performing a series of catalytic tests at varying acetone concentrations (300, 600 and 1200 ppmv) in the reactor feed, while keeping the SBET /Qin ratio constant. The acetone conversion, as given by the light-off curves, is almost independent of the feed concentration, this being in accordance with a first-order behavior of the complete oxidation reaction with respect to acetone. The packed-bed laboratory reactor, having a bed depth to particle diameter ratio of 50–100, can be considered as a plug-flow one [23]. Adsorption phenomena and general mass transport limitations, as well as energy transport phenomena such as radial or axial temperature gradients, are considered of no great influence for the development of the simplified model. Starting from a mass balance for the ketone component i, and incorporating the reaction kinetics and the simplifying assumptions, the following equation is obtained: Vb RT0 Ea ln(1 − Xi ) = − A exp − (2) (Qin )T0 RT
where Xi is the reactant conversion, Vb the volume of the reactor bed, (Qin )T0 the volumetric flow of the feed measured at a reference temperature T0 , and R is the gas constant. Eq. (2) can be used as is to give the best estimations of the kinetic parameters by non-linear regression. Other possibilities could be to rearrange the Eq. (2) in order to express directly the conversion Xi as a function of gas temperature T, or even to take the logarithms of both equation sides in order to linearize it. All these model possibilities have been checked in the present work. Since it is the rate constant k that obeys the Arrhenius law, and we are interested in values of k rather than ln k or other functions of k, the most appropriate deviations whose sum of squares must be minimised should be the differences between the experimental values of the rate constant and the model predicted ones. Therefore, the most appropriate equation for estimating the kinetic constants seems to be Eq. (2), as it is also suggested by other researchers [27]. The best estimations of the apparent activation energy Ea found this way, are presented in Table 1, along with their 95% confidence intervals. The activity of the catalysts in the oxidation of the organic compounds can be represented qualitatively by the parameter T50 , which is defined as the temperature at which the conversion of the organic molecule reaches 50%. The values of T50 , derived from the model used are included in Table 1 and, as it can been seen, are in good agreement with the values that can be estimated directly from the experimental results. 3.3. Influence of the ketone nature on the light-off curves The light-off curves for acetone and MEK combustion over the pure manganese oxide, Mn, are shown in Fig. 1. CO2 and H2 O were the only reaction products found in all cases. As it can be seen, although the catalyst behavior is rather similar in the two cases, the acetone combustion occurs at lower temperatures than that of MEK, a fact that can be explained by the differences in the chemical structure of the particular molecules and the way that they interact with the catalyst surface. In the case of hydrocarbons, which have been relatively extensively studied, it is generally accepted that the activation of the respective weakest C–H
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Fig. 1. Light-off curves for acetone (䊉) and MEK (䊐) oxidation over manganese oxide (Mn).
bond is the rate determining step for their complete oxidation [20,28,29]. The C–H bond dissociation enthalpy is usually taken as a measure of the bond strength. Accordingly reasonable correlations have been found showing that the lower the C–H bond dissociation enthalpy the easier the complete oxidation of the corresponding organic compound [28,29]. Regarding to the two ketones studied in this work, the dissociation enthalpies of the weakest C–H bonds are 411.3 kJ mol−1 for acetone (H–CH2 COCH3 ) and 386.2 kJ mol−1 for MEK (H–CH(CH3 )COCH3 ) [30]. From the difference between these values one can expect also differences between the temperatures necessary for complete oxidation of the two compounds, but in favour of MEK instead of acetone, contrary to our experimental results. Studies on the oxidation of acetone over several oxides have shown that acetone is oxidised to CO2 through the oxidative cleavage of a C–C bond giving rise to adsorbed acetate (carboxylate) fragments [9,31,32]. Finocchio et al. [9] proposed a Mars–van Krevelen type reaction mechanism, with the combustion being produced at the expense of nucleophilic lattice oxygen species. Busca et al. [28] have reported that carbonyl compounds carrying hydrogen atoms at their ␣-positions can undergo base- or acid-catalysed enolization at the surface of metal oxide based catalysts. The enolic species produced this way are strongly bound on the catalyst surface, and can play a major role conditioning the initial interaction with the catalyst and the complete oxidation of the carbonyl
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reactants [28,32]. Acetone and MEK, both carbonyl compounds with hydrogen at ␣-position, may lead to different enolic species due to their different structure. Moreover, the fact that the dissociation enthalpy of the respective weakest C–H bond is significantly lower for MEK than for acetone, may be an indication of a higher thermodynamic stability, and perhaps a lower reactivity, of the enolic species resulting from the adsorption of MEK. This is in accordance with the light-off curves obtained in our work. Another reason for the complete oxidation being harder in the case of MEK than in that of acetone, is that the MEK molecule has one more methyl group than the acetone one. This extra methyl group can allow the formation of additional adsorbed intermediate compounds (such as acrolein and acrylate [28]) in the pathway for the ketone combustion. In this regard, Baldi and co-workers [31,33] have found that over Mn3 O4 acrolein is more difficult to oxidise than acetone. However, whether the whole combustion process may be controlled by an enolic C=C double bond dissociation or by a late step such as the combustion of a carboxylate-type intermediate species, is still an open question. It should also be noted that no apparent activation energy differences are observed for acetone and MEK combustion on Mn. As shown in Table 1, the values estimated for the two processes were, respectively, 118 and 125 kJ mol. Although these values correspond to the whole combustion process, it is very interesting that, when multiple bonds are involved, the activity changes only slightly in an homologous series of compounds [34], something that may suggest that enolic species could play an important role in the ketone combustion. 3.4. Effect of additives on the light-off curves The ignition curves for acetone and MEK combustion over the modified manganese oxides are shown in Figs. 2 and 3, respectively. Interesting catalytic activity differences can be observed among the oxides for both ketones combustion. First, acetone combustion over all oxides occurs at temperatures lower than those of MEK combustion, as it is also observed for the non modified manganese oxide, Mn. As judged from the light-off curves and the corresponding T50 values that are presented in Table 1, the addition of sodium and cesium results to a considerable improvement of
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Fig. 2. Light-off curves for acetone oxidation over modified manganese oxides: Mn (䊉); Mn–Cit (䊊); Mn–S (䊏); Mn–Na (䊐); Mn–Cs (䉬).
the catalyst performance, while sulfate has a negative effect, always in comparison to the performance of Mn. No significant change of the catalytic behavior has been observed when the citric acid is used as additive. An apparent activation energy trend exists as well, with the alkali-modified catalysts having activation energies comparable to the one of the parent material, and lower than the ones of the acid-modified samples, in both combustion reactions. The characterisation results indicated that the bulk crystalline phase present in all catalysts was ␣-Mn2 O3 , the specific surface areas were modified by the pres-
Fig. 3. Light-off curves for MEK oxidation over modified manganese oxides: Mn (䊉); Mn–Cit (䊊); Mn–S (䊏); Mn–Na (䊐); Mn–Cs (䉬).
ence of additives and possible variations in the basic and acid surface properties could also been produced. The number of active sites of manganese oxide present in the reactor can markedly affect the catalytic combustion of ketones. If the additives do not modify the surface nature of the oxides or block the active sites, and taking into account that the same surface area of oxides is loaded each time in the reactor in our experiments, then a similar number of active sites is present in all catalytic runs. Although the nature of the active sites for catalytic combustion is not well established, their number is in general related to the specific surface area. As it can be seen from Figs. 2 and 3, some of the modified manganese oxides are more active than Mn, while others are less (see also T50 in Table 1). Therefore, there must be other factors that explain the observed catalytic behavior of the modified manganese oxides. One possible way in which the alkali and acid additives influence the oxide catalyst behavior, is through the enolic species that can be formed from the ketones on the oxide surface [31,32]. These species can react as well with the gas-phase molecules of the carbonyl compounds, to give aldol condensation products that can remain adsorbed on the catalyst surface and cannot be decreased before complete burning. An important effect of the acid–base properties of the catalysts on these processes could be expected due to the fact that the strongly bound enolic species and the aldol condensation are favored by both acid and base sites [28,35,36]. However, in the present work, the ketones combustion takes place at lower temperatures when alkali additives are used and only in the presence of sulfate, the combustion temperatures increase. The effect of the alkali and acid additives in the catalytic performance is often interpreted in terms of the changes produced by the additives on the electron donor or acceptor character of the catalysts [36,37]. The alkali additives normally act by increasing the electron donor ability of the manganese oxide, while acid additives increase the electron acceptor ability [20,37]. Taking into account, the reactivity of the carbonyl group, it would be expected that an increase of the electron density of the catalyst surface can enhance the electrophilic adsorption of ketone molecules, thus shifting the combustion temperatures towards lower values. The contrary effect would be expected in the case of a decrease of the electron donor ability.
L.M. Gand´ıa et al. / Applied Catalysis B: Environmental 33 (2001) 1–8
A positive effect of the alkali doping has been recently observed by several investigators in various reaction systems [19,38,39]. In these works, the effect of the additives has been related with the basic and acid surface properties of the catalysts and with the possible creation of new reaction pathways. Another proposed explanation of the positive action of the additives is that they help the removal of carbonaceous deposits from the surface or prevent the undesired adsorption of side products on the active sites. In our case, the presence of alkali (sodium or cesium) additives at the manganese oxide could favour the combustion of the adsorbed species (enolic species and aldol condensation products) on the catalyst surface, thus reducing the combustion temperature. 3.5. Stability tests A series of long catalytic runs was performed with the Mn, Mn–Cs and Mn–Cit oxides, with the reactor operating at constant combustion temperature, in order to investigate the effect of the additives presence on the maintenance of the catalytic performance. Reaction temperatures were selected as to achieve in each case a relatively high initial ketone conversion of about 80%. The evolution with time-on-stream of the MEK conversion for Mn and Mn–Cit at 573 K, and Mn–Cs at 538 K, is presented in Fig. 4. As it can be seen the three catalysts exhibit a very similar behavior, particularly Mn and Mn–Cit, in general accordance with the evolution shown by their respective light-off
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curves (see Fig. 3). In all samples, after a relatively rapid increase from about 80 to 90% during the first hour of reaction, the conversion then decreases almost linearly with time, reaching again a value close to the initial one after about 48 h on-stream. In the case of the acetone combustion, the evolution of the catalyst performance with time-on stream is similar to that of the combustion of MEK. There is a clear lack of stability data in the literature concerning the catalytic combustion of volatile organic compounds. The presence of a stabilisation period of about 16 h, at the end of which the conversion reached an almost constant value, has been reported for the catalytic combustion of ethyl acetate, n-hexane and benzene on ␥-MnO2 [40]. It was concluded that the stabilisation period was a transient one, that consisted in the time needed to get a constant coverage of the catalyst surface by the reactants and products, specially water, at the very low concentrations that exist in the gas stream. The activity decrease taking place during this period was attributed to a competition of the reactants and water for specific sites. In our case, it can be said that the conversion attains an almost constant value only during a 16 h period of time from the first two hours on-stream. Taking into account the chemical properties of MEK and the important role that strongly bound enolic species may play in this case, a possible explanation for the observed activity decay would be the deactivation by fouling due to surface accumulation of heavy by-products. In fact, surface enolic species coming from MEK adsorption, can react as well with gas-phase MEK molecules to give aldol condensation products, in a similar way as it has been reported for acetone [35]. These products could remain adsorbed on the catalyst surface, leading eventually to deactivation. Similar arguments have been also pointed out for acetone combustion on supported Mn2 O3 [41] and the previously mentioned ethyl acetate combustion [40,42].
4. Conclusions
Fig. 4. Evolution of MEK conversions with time-on-stream for Mn (䊉) at 573 K, Mn–Cit (䊊) at 573 K, and Mn–Cs (䉬) at 538 K.
In conclusion, lower temperatures are required for acetone than MEK combustion on Mn2 O3 . The performance of Mn2 O3 in the combustion reactions of acetone and MEK was observed to improve (lower
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combustion temperatures were required) when sodium and cesium ions were used, and to decrease due to the addition of sulfate ions. No influence was observed due to the addition of citric acid. In all cases, the respective ignition curves result reasonably well described by a simplified model where both ketone combustion reactions are assumed to have power law rate equations, which are first-order with respect to the ketone and do not depend on the O2 partial pressure. No significant differences between the apparent activation energies estimated with this model were found for the combustion of the ketones on pure and alkali modified manganese oxides. On the contrary, the combustion reaction processes took place with noticeably higher apparent activation energies over the sulfuric acid modified manganese oxide.
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[12] H. Noller, H. Vinek, J. Mol. Catal. 51 (1989) 285. [13] A. Ishikawa, S. Komai, A. Satsuma, T. Hattori, Y. Murakami, Appl. Catal. A 110 (1994) 61. [14] Y.-M. Kang, B.-Z. Wan, Appl. Catal. A 114 (1994) 35. [15] G.A. El-Shobaky, A.M. Ghozza, G.A. Fagal, M. Shouman, Mater. Lett. 19 (1994) 79. [16] A.S.C. Brown, J.S.J. Hargreaves, B. Rijniersce, Catal. Today 45 (1998) 47. [17] W. Hua, Z. Gao, Appl. Catal. B 17 (1998) 37. [18] W.-H. Cheng, Ch.-Y. Shiau, T.H. Liu, H.L. Tung, J.-F. Lu, Ch.Ch. Hsu, Appl. Catal. A 170 (1998) 215. [19] J. Haber, T. Machej, M. Derewinski, R. Janik, J. Krysciak, H. Sadowska, J. Janas, Catal. Today 54 (1999) 47. [20] V.D. Sokolovskii, Catal. Rev.-Sci. Eng. 32 (1990) 1. [21] P. Courty, H. Ajot, C. Marcilly, B. Delmon, Powder Technol. 7 (1973) 21. [22] ASTM D 4824-94, Standard test method for determination of catalyst acidity by ammonia chemisorption, Annual Book of ASTM Standards, Vol. 5.04, 1999, p. 328. [23] D.E. Mears, Ind. Eng. Chem. Process Des. Dev. 10 (1971) 541. [24] G.F. Froment, K.B. Bischoff, Chemical Reactor Analysis and Design, 2nd Edition, Wiley, New York, 1990. [25] V. Perrichon, M.C. Durupty, Appl. Catal. 42 (1988) 217. [26] L. van de Beld, M.P.G. Bijl, A. Reinders, B. van der Werf, K.R. Westerterp, Chem. Eng. Sci. 49 (1994) 4361. [27] N.H. Chen, R. Aris, AIChE J. 38 (1992) 626. [28] G. Busca, E. Finocchio, G. Ramis, G. Ricchiardi, Catal. Today 32 (1996) 133. [29] A. O’Malley, B.K. Hodnett, Catal. Today 54 (1999) 31. [30] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 75th Edition, CRC Press, Boca Raton, FL, 1994. [31] M. Baldi, F. Milella, G. Ramis, V. Sanchez Escribano, G. Busca, Appl. Catal. A 166 (1998) 75. [32] G. Busca, E. Finocchio, V. Lorenzelli, G. Ramis, M. Baldi, Catal. Today 49 (1999) 453. [33] M. Baldi, E. Finocchio, F. Milella, G. Busca, Appl. Catal. B 16 (1998) 43. [34] G.K. Boreskov, in: J.R. Anderson, M. Boudart (Eds.), Catalysis Science and Technology, Vol. 3, Springer, Berlin, 1982, p. 120. [35] G. Busca, V. Lorenzelli, J. Chem. Soc., Faraday Trans. 1 (78) (1982) 2911. [36] K. Tanabe, M. Misono, Y. Ono, H. Hattori, New Solid Acids and Bases: Their Catalytic Properties, Elsevier, Amsterdam, 1989. [37] W.-D. Mross, Catal. Rev.-Sci. Eng. 25 (1983) 591. [38] Ch.-H. Lee, Y.W. Chen, Appl. Catal. B 17 (1998) 279. [39] C.A. Querini, L.M. Cornaglia, M.A. Ulla, E.E. Miró, Appl. Catal. B 20 (1999) 165. [40] C. Lahousse, A. Bernier, P. Grange, B. Delmon, P. Papaefthimiou, T. Ioannides, X. Verykios, J. Catal. 178 (1998) 214. [41] M. Paulis, L.M. Gandía, A. Gil, J. Sambeth, J.A. Odriozola, M. Montes, Appl. Catal. B 26 (2000) 37. [42] P. Papaefthimiou, T. Ioannides, X.E. Verykios, Appl. Catal. B 13 (1997) 175.
Effects of various alkali–acid additives on the activity of a manganese oxide in the catalytic combustion of ketones L.M. Gandía∗ , A. Gil, S.A. Korili1 Departamento de Química Aplicada, Universidad Pública de Navarra, Campus de Arrosadía s/n, 31006 Pamplona, Spain Received 28 May 2000; received in revised form 22 December 2000; accepted 28 January 2001
Abstract The catalytic combustion of acetone and methyl-ethyl-ketone (MEK) has been studied over a manganese oxide, Mn2 O3 . The reactant conversion has been followed as a function of the reaction temperature and it has been observed that lower temperatures are required for the combustion of acetone than for MEK. The performance of Mn2 O3 in the combustion reactions when alkali and acid ions are added to the oxide has also been investigated. A significant improvement of the catalyst performance is observed when Cs+ and Na+ are used as additives. The data of the ignition curves have been fitted to a simplified model where both ketone combustion reactions are assumed to have power law rate equations, which are first-order with respect to the corresponding organic molecule and do not depend on the oxygen partial pressure. Differences in the apparent activation energy values estimated with this model for acetone and MEK combustion are significant only when acid ions additives are present. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Manganese oxide; Acetone catalytic combustion; Methyl-ethyl-ketone catalytic combustion; Alkali–acid additives
1. Introduction Catalytic oxidation is a well-established technology for volatile organic compound (VOC) destruction and odour elimination. When compared to the thermal regenerative oxidation, the catalytic oxidation with recuperative heat recovery may be economically advantageous for flow rates under 850 m3 STP min−1 and VOC concentrations ranging from 50 to 10 000 ppmv [1,2]. Because of the large gas volumes that must be treated and the VOC concentrations, catalysts that are very active and selective towards complete oxidation reactions are required. The performance of catalytic combustion processes critically depends on the type of ∗
Corresponding author. E-mail address: [email protected] (L.M. Gand´ıa). 1 On leave from Aristotle University of Thessaloniki, Greece.
catalyst, its physical form and its ability to work over a wide range of operating temperatures, and on the VOC molecule nature [3,4]. Supported noble metals, typically Pt, Pd or Rh, and transition metal oxides as CuO, MnOx , Co3 O4 , Cr2 O3 , and NiO as well as mixed oxides are active VOC combustion catalysts [3–10]. Several researchers [11–19] have studied the effects of alkali and acid additives on the activity and selectivity of catalysts that are usually used in combustion reactions. However, the way in which the various additives act on the catalyst activity and selectivity is not clear at present. Ishikawa et al. [13] claimed a positive effect of the acidic properties of sulfate-doped supports on the activity of platinum in the low-temperature catalytic combustion of propane. According to the authors, a possible role of the acidic support is to prevent the oxidation of the supported platinum. Hua and Gao [17] studied the catalytic com-
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bustion of n-pentane on Pt supported on superacids and correlated the catalyst activity with the metal dispersion and the acid strength of the support. They proposed that the formation of alkane carbocations on the surface sulfates during reaction may account for the enhancement of the combustion activity by the strong acidity of the supports. Kang and Wan [14] investigated the effects of acid and base additives on the activity of chromium and cobalt oxides supported on ␥-alumina in CO and ethane oxidation. Although the conversions of the two reactions were influenced in different ways by the additives, it seems that the addition of a base additive to the oxide catalysts in general favours the complete oxidation, while the acid additives decrease it. These results may be explained by the electron ability character, donor or acceptor, of the catalyst [20]. All the cited papers indicate an important influence of the additives on the catalyst properties and behavior in combustion reactions. The aim of the present work, was to study the performance of a manganese oxide in the catalytic combustion of acetone and MEK, and to investigate the possible effects of alkali and acid additives on catalyst activity. To our best knowledge, this is the first report dealing with the effect of alkali–acid additives on the catalytic performance of a manganese oxide in the combustion of VOC, and especially of ketones.
2. Experimental The manganese oxide was prepared by the citrate method [21]. The starting materials, Mn(NO3 )2 ·4H2 O (Merck, PA) and citric acid (Panreac, PA), were dissolved in water at an equivalent ratio of 1:1. The resulting solution was slowly evaporated in a rotavapor at 333 K to form a dense gel, which was further dehydrated in a vacuum oven at 343 K. The precursor formed this way was heated in air (5 K min−1 ) in a furnace up to 823 K, where it was calcined for 4 h, forming the final oxide material. The acid modified samples, were prepared by treating 3 g of the manganese oxide with 50 cm3 of 0.01N H2 SO4 (Merck, 98%) or citric acid (Panreac, PA). In the case of the alkali modified catalysts, the quantity of Na or Cs necessary to form half a monolayer on the surface of the manganese oxide was estimated on
the basis of the BET specific surface area of the initial oxide (12.5 m2 g−1 ). Taking as a basis for the calculations the bigger ionic radius, solutions of NaNO3 (Panreac, PA) and CsNO3 (Aldrich, 99%) were prepared and mixed with the manganese oxide in suitable ion proportions. The slurries resulting from the mixing of the oxide with the acid or salt solutions were agitated at room temperature for 3 h, then evaporated slowly at 333 K under reduced pressure in a rotavapor. The solids were dried for 12 h at 383 K and finally calcined for 4 h at 773 K. For a better comparison of the catalysts, the manganese oxide was also treated with 50 cm3 of deionised water, then evaporated, dried and calcined in the same way as the modified samples. The catalysts are designated hereafter as Mn–X, where X refers to the additive used, that is S for sulfuric acid, Cit for citric acid, Na for sodium nitrate and Cs for cesium nitrate. No suffix was used in the case of the pure unmodified oxide (Mn). The phases present in the oxide prepared were identified by X-ray diffraction (XRD) analysis on a Siemens D-500 powder diffractometer, using nickel-filtered Cu K␣ (λ = 1.5405 Å) radiation. The BET specific surface areas were measured by Kr (Air Liquide, 99.995%) adsorption at 77 K using a static automatic volumetric apparatus (Micromeritics ASAP 2010 adsorption analyser). Samples were previously degassed (0.1 Pa) for 3 h at 473 K. Catalyst acidity was determined by adsorption of NH3 (Air Liquide, >99.995%), using the same static automatic apparatus mentioned before. Samples were pretreated by heating under vacuum at a rate of 5 K min−1 up to 573 K, where they were maintained for 3 h. The ammonia volume adsorbed at 448 K and an equilibrium pressure of 20 kPa [22] was considered representative of the sample acidity. Chemical analyses of the catalysts in order to determine the real content of additive present were carried out by Activation Laboratories, Ancaster, Ont., Canada, using instrumental neutron activation analysis (INAA) and infrared analysis (IR). Ketone combustion reactions were carried out in a tubular (8 mm i.d.) fixed-bed Pyrex glass reactor at atmospheric pressure. The catalyst (100–200 m particle size fraction) was diluted with inert solids (100–200 m Pyrex glass beads) in a volume ratio of 1:4 approximately, forming a catalytic bed
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of about 1 cm of depth. 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. An air stream saturated with the corresponding ketone (Panreac, PA) was created using a saturator, equipped with temperature and pressure control, and then diluted with pure air (SEO, 99.999%), resulting in a 600 ppmv ketone concentration in the reactor feed. Prior to the reaction, the catalysts were treated under 100 cm3 min−1 of air for 1 h at 573 K. The light-off curves for ketone combustion were obtained at a controlled heating rate of 2.5 K min−1 and at identical SBET /Qin ratios of 2 min cm −3 . Space velocities (GHSV), cal1.575 mox ketone culated at standard temperature and pressure and based on the total volume of the catalytic bed, were about 34 000 h−1 . The operating conditions of the system have been carefully selected in order to minimise heat and mass transfer phenomena. Nevertheless, the possible effect of external limitations has been evaluated by the application of standard literature criteria [23,24]. It was found this way that mass and heat transfer effects do not prevail in most cases, especially at low and medium conversions. Some individual experiments that were carried out at varying gas linear velocities and constant SBET /Qin ratio, confirmed the absence of external mass transfer effects. On-line analysis of the product stream was performed on a Hewlett-Packard 6890 gas chromatograph equipped with two column systems, a 6 ft HayeSep Q connected to a TCD for CO2 determination, and an HP-INNOWax 30 m ×0.32 mm i.d. column connected to an FID for ketone analyses.
3
3. Results and discussion 3.1. Physico-chemical properties The XRD analyses of the manganese oxide and the modified oxides indicated the presence of well-crystallised ␣-Mn2 O3 in all cases. No other bulk crystalline phases containing the additives were detected at the modified oxides. The BET specific surface areas and the additive content of the samples are summarised in Table 1. A slight increase of the oxide specific surface area when the citric acid is used as modifier can be explained by a partial dissolution of the manganese oxide during the modification procedure, which was very similar to the citrate method used for the synthesis of the parent oxide. On the other hand, the incorporation of sodium and cesium caused a loss of specific surface area, probably related to a cement effect [25] which sticks the oxide and the alkali metal salt together in a sort of conglomerate. The acidity of the modified oxides was, as expected, higher than that of the parent manganese oxide in the case of the acid modified samples, and lower in the case of the alkali modified ones. As it can be seen from the results of Table 1, no significant differences were observed between the acidities of the two acid modified samples or the acidities of the two alkali modified ones. 3.2. Reaction studies In the combustion reactions of organic compounds, when the reactant conversion is followed as a function of the reaction temperature, a characteristic S-shaped
Table 1 Physico-chemical and catalytic properties of the prepared oxides. Sample
SBET (m2 g−1 )
V (cm3 STP g−1 )a
Additive (wt.%)
Ea (kJ mol−1 )b
Mn Mn–S Mn–Cit Mn–Na Mn–Cs
12.5 11.6 14.6 8.7 6.3
1.32 1.65 1.51 0.87 0.83
– 0.25 (S) – 0.41 (Na) 1.03 (Cs)
118 217 194 133 119
a
± ± ± ± ±
6 24 6 6 6
T50 (K)b
Ea (kJ mol−1 )c
540 600 542 508 495
125 162 164 125 116
Ammonia adsorbed at 448 K and 20 kPa. Apparent activation energy with 95% confidence interval; T50 for the combustion of acetone. c Apparent activation energy with 95% confidence interval; T 50 for the combustion of MEK. d Points used in model fitting lie in the conversion range 0–90%. b
± ± ± ± ±
6 4 7d 7d 8d
T50 (K)c 558 601 560 526 509
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curve is obtained, called light-off curve or ignition curve. The exact shape of the curve depends on several factors such as the reaction kinetics, the mass transport, and the reactor operating conditions [26]. By fitting the data of this curve to a suitable model describing the reaction kinetics and the reactor behavior, the best estimations for the kinetic parameters can be obtained. A series of assumptions has been made in order to develop a reasonable but simple model that can describe satisfactorily the experimental data. Both ketone combustion reactions are assumed to have power law rate equations, which are first-order with respect to the corresponding organic molecule and do not depend on the oxygen partial pressure, since oxygen is in a very large excess and its partial pressure remains practically unchanged during the reaction. Ea −ri = kPi = A exp − (1) Pi RT where −ri is the rate of reaction of ketone i, Pi the partial pressure of the ketone in the gas stream, and k the rate constant, which obeys an Arrhenius relation, A is the pre-exponential factor and Ea the apparent activation energy. The assumption of first-order kinetics has been checked by performing a series of catalytic tests at varying acetone concentrations (300, 600 and 1200 ppmv) in the reactor feed, while keeping the SBET /Qin ratio constant. The acetone conversion, as given by the light-off curves, is almost independent of the feed concentration, this being in accordance with a first-order behavior of the complete oxidation reaction with respect to acetone. The packed-bed laboratory reactor, having a bed depth to particle diameter ratio of 50–100, can be considered as a plug-flow one [23]. Adsorption phenomena and general mass transport limitations, as well as energy transport phenomena such as radial or axial temperature gradients, are considered of no great influence for the development of the simplified model. Starting from a mass balance for the ketone component i, and incorporating the reaction kinetics and the simplifying assumptions, the following equation is obtained: Vb RT0 Ea ln(1 − Xi ) = − A exp − (2) (Qin )T0 RT
where Xi is the reactant conversion, Vb the volume of the reactor bed, (Qin )T0 the volumetric flow of the feed measured at a reference temperature T0 , and R is the gas constant. Eq. (2) can be used as is to give the best estimations of the kinetic parameters by non-linear regression. Other possibilities could be to rearrange the Eq. (2) in order to express directly the conversion Xi as a function of gas temperature T, or even to take the logarithms of both equation sides in order to linearize it. All these model possibilities have been checked in the present work. Since it is the rate constant k that obeys the Arrhenius law, and we are interested in values of k rather than ln k or other functions of k, the most appropriate deviations whose sum of squares must be minimised should be the differences between the experimental values of the rate constant and the model predicted ones. Therefore, the most appropriate equation for estimating the kinetic constants seems to be Eq. (2), as it is also suggested by other researchers [27]. The best estimations of the apparent activation energy Ea found this way, are presented in Table 1, along with their 95% confidence intervals. The activity of the catalysts in the oxidation of the organic compounds can be represented qualitatively by the parameter T50 , which is defined as the temperature at which the conversion of the organic molecule reaches 50%. The values of T50 , derived from the model used are included in Table 1 and, as it can been seen, are in good agreement with the values that can be estimated directly from the experimental results. 3.3. Influence of the ketone nature on the light-off curves The light-off curves for acetone and MEK combustion over the pure manganese oxide, Mn, are shown in Fig. 1. CO2 and H2 O were the only reaction products found in all cases. As it can be seen, although the catalyst behavior is rather similar in the two cases, the acetone combustion occurs at lower temperatures than that of MEK, a fact that can be explained by the differences in the chemical structure of the particular molecules and the way that they interact with the catalyst surface. In the case of hydrocarbons, which have been relatively extensively studied, it is generally accepted that the activation of the respective weakest C–H
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Fig. 1. Light-off curves for acetone (䊉) and MEK (䊐) oxidation over manganese oxide (Mn).
bond is the rate determining step for their complete oxidation [20,28,29]. The C–H bond dissociation enthalpy is usually taken as a measure of the bond strength. Accordingly reasonable correlations have been found showing that the lower the C–H bond dissociation enthalpy the easier the complete oxidation of the corresponding organic compound [28,29]. Regarding to the two ketones studied in this work, the dissociation enthalpies of the weakest C–H bonds are 411.3 kJ mol−1 for acetone (H–CH2 COCH3 ) and 386.2 kJ mol−1 for MEK (H–CH(CH3 )COCH3 ) [30]. From the difference between these values one can expect also differences between the temperatures necessary for complete oxidation of the two compounds, but in favour of MEK instead of acetone, contrary to our experimental results. Studies on the oxidation of acetone over several oxides have shown that acetone is oxidised to CO2 through the oxidative cleavage of a C–C bond giving rise to adsorbed acetate (carboxylate) fragments [9,31,32]. Finocchio et al. [9] proposed a Mars–van Krevelen type reaction mechanism, with the combustion being produced at the expense of nucleophilic lattice oxygen species. Busca et al. [28] have reported that carbonyl compounds carrying hydrogen atoms at their ␣-positions can undergo base- or acid-catalysed enolization at the surface of metal oxide based catalysts. The enolic species produced this way are strongly bound on the catalyst surface, and can play a major role conditioning the initial interaction with the catalyst and the complete oxidation of the carbonyl
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reactants [28,32]. Acetone and MEK, both carbonyl compounds with hydrogen at ␣-position, may lead to different enolic species due to their different structure. Moreover, the fact that the dissociation enthalpy of the respective weakest C–H bond is significantly lower for MEK than for acetone, may be an indication of a higher thermodynamic stability, and perhaps a lower reactivity, of the enolic species resulting from the adsorption of MEK. This is in accordance with the light-off curves obtained in our work. Another reason for the complete oxidation being harder in the case of MEK than in that of acetone, is that the MEK molecule has one more methyl group than the acetone one. This extra methyl group can allow the formation of additional adsorbed intermediate compounds (such as acrolein and acrylate [28]) in the pathway for the ketone combustion. In this regard, Baldi and co-workers [31,33] have found that over Mn3 O4 acrolein is more difficult to oxidise than acetone. However, whether the whole combustion process may be controlled by an enolic C=C double bond dissociation or by a late step such as the combustion of a carboxylate-type intermediate species, is still an open question. It should also be noted that no apparent activation energy differences are observed for acetone and MEK combustion on Mn. As shown in Table 1, the values estimated for the two processes were, respectively, 118 and 125 kJ mol. Although these values correspond to the whole combustion process, it is very interesting that, when multiple bonds are involved, the activity changes only slightly in an homologous series of compounds [34], something that may suggest that enolic species could play an important role in the ketone combustion. 3.4. Effect of additives on the light-off curves The ignition curves for acetone and MEK combustion over the modified manganese oxides are shown in Figs. 2 and 3, respectively. Interesting catalytic activity differences can be observed among the oxides for both ketones combustion. First, acetone combustion over all oxides occurs at temperatures lower than those of MEK combustion, as it is also observed for the non modified manganese oxide, Mn. As judged from the light-off curves and the corresponding T50 values that are presented in Table 1, the addition of sodium and cesium results to a considerable improvement of
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Fig. 2. Light-off curves for acetone oxidation over modified manganese oxides: Mn (䊉); Mn–Cit (䊊); Mn–S (䊏); Mn–Na (䊐); Mn–Cs (䉬).
the catalyst performance, while sulfate has a negative effect, always in comparison to the performance of Mn. No significant change of the catalytic behavior has been observed when the citric acid is used as additive. An apparent activation energy trend exists as well, with the alkali-modified catalysts having activation energies comparable to the one of the parent material, and lower than the ones of the acid-modified samples, in both combustion reactions. The characterisation results indicated that the bulk crystalline phase present in all catalysts was ␣-Mn2 O3 , the specific surface areas were modified by the pres-
Fig. 3. Light-off curves for MEK oxidation over modified manganese oxides: Mn (䊉); Mn–Cit (䊊); Mn–S (䊏); Mn–Na (䊐); Mn–Cs (䉬).
ence of additives and possible variations in the basic and acid surface properties could also been produced. The number of active sites of manganese oxide present in the reactor can markedly affect the catalytic combustion of ketones. If the additives do not modify the surface nature of the oxides or block the active sites, and taking into account that the same surface area of oxides is loaded each time in the reactor in our experiments, then a similar number of active sites is present in all catalytic runs. Although the nature of the active sites for catalytic combustion is not well established, their number is in general related to the specific surface area. As it can be seen from Figs. 2 and 3, some of the modified manganese oxides are more active than Mn, while others are less (see also T50 in Table 1). Therefore, there must be other factors that explain the observed catalytic behavior of the modified manganese oxides. One possible way in which the alkali and acid additives influence the oxide catalyst behavior, is through the enolic species that can be formed from the ketones on the oxide surface [31,32]. These species can react as well with the gas-phase molecules of the carbonyl compounds, to give aldol condensation products that can remain adsorbed on the catalyst surface and cannot be decreased before complete burning. An important effect of the acid–base properties of the catalysts on these processes could be expected due to the fact that the strongly bound enolic species and the aldol condensation are favored by both acid and base sites [28,35,36]. However, in the present work, the ketones combustion takes place at lower temperatures when alkali additives are used and only in the presence of sulfate, the combustion temperatures increase. The effect of the alkali and acid additives in the catalytic performance is often interpreted in terms of the changes produced by the additives on the electron donor or acceptor character of the catalysts [36,37]. The alkali additives normally act by increasing the electron donor ability of the manganese oxide, while acid additives increase the electron acceptor ability [20,37]. Taking into account, the reactivity of the carbonyl group, it would be expected that an increase of the electron density of the catalyst surface can enhance the electrophilic adsorption of ketone molecules, thus shifting the combustion temperatures towards lower values. The contrary effect would be expected in the case of a decrease of the electron donor ability.
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A positive effect of the alkali doping has been recently observed by several investigators in various reaction systems [19,38,39]. In these works, the effect of the additives has been related with the basic and acid surface properties of the catalysts and with the possible creation of new reaction pathways. Another proposed explanation of the positive action of the additives is that they help the removal of carbonaceous deposits from the surface or prevent the undesired adsorption of side products on the active sites. In our case, the presence of alkali (sodium or cesium) additives at the manganese oxide could favour the combustion of the adsorbed species (enolic species and aldol condensation products) on the catalyst surface, thus reducing the combustion temperature. 3.5. Stability tests A series of long catalytic runs was performed with the Mn, Mn–Cs and Mn–Cit oxides, with the reactor operating at constant combustion temperature, in order to investigate the effect of the additives presence on the maintenance of the catalytic performance. Reaction temperatures were selected as to achieve in each case a relatively high initial ketone conversion of about 80%. The evolution with time-on-stream of the MEK conversion for Mn and Mn–Cit at 573 K, and Mn–Cs at 538 K, is presented in Fig. 4. As it can be seen the three catalysts exhibit a very similar behavior, particularly Mn and Mn–Cit, in general accordance with the evolution shown by their respective light-off
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curves (see Fig. 3). In all samples, after a relatively rapid increase from about 80 to 90% during the first hour of reaction, the conversion then decreases almost linearly with time, reaching again a value close to the initial one after about 48 h on-stream. In the case of the acetone combustion, the evolution of the catalyst performance with time-on stream is similar to that of the combustion of MEK. There is a clear lack of stability data in the literature concerning the catalytic combustion of volatile organic compounds. The presence of a stabilisation period of about 16 h, at the end of which the conversion reached an almost constant value, has been reported for the catalytic combustion of ethyl acetate, n-hexane and benzene on ␥-MnO2 [40]. It was concluded that the stabilisation period was a transient one, that consisted in the time needed to get a constant coverage of the catalyst surface by the reactants and products, specially water, at the very low concentrations that exist in the gas stream. The activity decrease taking place during this period was attributed to a competition of the reactants and water for specific sites. In our case, it can be said that the conversion attains an almost constant value only during a 16 h period of time from the first two hours on-stream. Taking into account the chemical properties of MEK and the important role that strongly bound enolic species may play in this case, a possible explanation for the observed activity decay would be the deactivation by fouling due to surface accumulation of heavy by-products. In fact, surface enolic species coming from MEK adsorption, can react as well with gas-phase MEK molecules to give aldol condensation products, in a similar way as it has been reported for acetone [35]. These products could remain adsorbed on the catalyst surface, leading eventually to deactivation. Similar arguments have been also pointed out for acetone combustion on supported Mn2 O3 [41] and the previously mentioned ethyl acetate combustion [40,42].
4. Conclusions
Fig. 4. Evolution of MEK conversions with time-on-stream for Mn (䊉) at 573 K, Mn–Cit (䊊) at 573 K, and Mn–Cs (䉬) at 538 K.
In conclusion, lower temperatures are required for acetone than MEK combustion on Mn2 O3 . The performance of Mn2 O3 in the combustion reactions of acetone and MEK was observed to improve (lower
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combustion temperatures were required) when sodium and cesium ions were used, and to decrease due to the addition of sulfate ions. No influence was observed due to the addition of citric acid. In all cases, the respective ignition curves result reasonably well described by a simplified model where both ketone combustion reactions are assumed to have power law rate equations, which are first-order with respect to the ketone and do not depend on the O2 partial pressure. No significant differences between the apparent activation energies estimated with this model were found for the combustion of the ketones on pure and alkali modified manganese oxides. On the contrary, the combustion reaction processes took place with noticeably higher apparent activation energies over the sulfuric acid modified manganese oxide.
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