Ind. Eng. Chem. Res. 2011, 50, 41–51
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Kinetics, Selectivity, and Deactivation in the Aldol Condensation of Acetaldehyde on Anatase Titanium Dioxide James E. Rekoske† and Mark A. Barteau* Center for Catalytic Science and Technology, Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716
Metal oxide catalysts are active for aldol condensation reactions of aldehydes and ketones but typically exhibit rapid deactivation. In this study, aldol condensation with dehydration of acetaldehyde to produce crotonaldehyde was catalyzed by oxidized TiO2 anatase. Particular emphasis was placed on determination of product selectivities and reaction kinetics as a function of both conversion and the extent of catalyst deactivation. Condensation of acetaldehyde is rapid and selective on anatase TiO2, with turnover frequencies exceeding 0.03 s-1 and crotonaldehyde selectivities approaching 100%. The main side reactions generating volatile products are: (a) hydrogenation of the reactant and product aldehydes, (b) a secondary cross-esterification between crotonaldehyde and acetaldehyde to form ethyl crotonate, and (c) secondary condensations involving crotonaldehyde. Catalyst deactivation was observed to affect both the rate of reaction and the product selectivities, particularly at low values of time-on-stream. Secondary condensations that deposit nonvolatile organic species on the catalyst surface are responsible for the initial deactivation of the catalyst. The catalyst mass increase during the course of reaction was observed to be directly proportional to the rate of deactivation. Selectivity patterns were impacted by deactivation in a manner that could not be explained solely by the changing conversion levels associated with the deactivation process. We conclude that deactivation during aldol condensation on the anatase polymorph of titania alters both activity and selectivity of the active sites. As a result, care must be taken to account for catalyst deactivation when comparing both catalytic activities and selectivities. Introduction Most heterogeneous catalysts show significant loss in activity with advancing time-on-stream, resulting in a fixed useful life for the catalyst. Deactivation may have many causes, including simple poisoning of active sites, pore blocking, loss of active surface area, and other sintering mechanisms.1 Regardless of the mechanism, catalyst deactivation can significantly impact both the physical and economic feasibility of potential catalytic processes. While it has long been understood that reaction kinetics and selectivity must be compared between catalyst systems at identical levels of conversion,2 it is not common to restrict comparisons to an equivalent extent of catalyst deactivation. Although catalyst deactivation can have a significant impact on the active site selectivity, most authors do not specifically account for this phenomenon when comparing the performance of different catalyst formulations. Often integral results are used or, in some cases, comparisons are made at specific time-onstream.3 These methods implicitly assume that the nature of the active sites, and thus the reaction selectivity, are not directly affected by deactivation or they are affected in a manner which is correlated with time-on-stream. Aldol condensation is an important reaction for the manufacture of several significant commodity and specialty chemicals,4 especially 2-ethylhexanal, isophorone, mesityl oxide, and crotonaldehyde. This chemistry can be catalyzed both as a homogeneous reaction in the liquid phase by strong bases5 and, in a heterogeneous gas-solid version, by zeolites and metal oxides.6-28 Since the heterogeneously catalyzed aldol condensation reaction is typically accompanied by significant catalyst * To whom correspondence should be addressed. Tel.: 1-302-8314007. Fax: 1-302-831-8620. E-mail:
[email protected]. † Current address: UOP LLC, A Honeywell Company, 25 East Algonquin Road, Des Plaines, IL 60017.
deactivation, the economics of this process could be substantially improved if the problems associated with catalyst deactivation could be overcome. In the present work, we investigate the time-on-stream dependence of the aldol condensation of acetaldehyde catalyzed by the anatase polymorph of titanium dioxide. We have observed that both the activity and reaction selectivity are affected by the chronological reaction time. Perhaps more important is the observation that the product selectivities were dependent on time-on-stream in a manner which could not be accounted for solely by the changing conversion level associated with catalyst deactivation. These results suggest a complex relationship between catalyst selectivity and deactivation in metal-oxidecatalyzed aldol condensation reactions. Experimental Section All experiments were conducted in a novel flow-through microbalance reactor system employing a commercial inertial mass measurement device as the reactor.29-31 The mass measurement device (TEOM 1500, Rupprecht and Patashnick Co, Inc., Albany, NY) utilizes inertial principles to provide microgram resolution of catalyst mass changes and rapid temporal response in a down-flow microreactor. All flows within the upstream gas handling manifold were controlled by electronic mass flow controllers (Tylan FC-260). The temperatures of the preheat and catalyst zones were controlled by a two-zone electric furnace. Reactant purity and product identification were verified by two main methods: gas chromatography (GC) and quadrupole mass spectrometry (QMS). The GC section consisted of a Hewlett-Packard 5890 Series II Plus gas chromatographic mainframe designed to allow simultaneous analysis for C1-C10 hydrocarbons and oxygenates, permanent gases, and water. Analyses for hydrocarbons and oxygenates were carried out on a 100 m, 0.25 mm o.d. HP-1
10.1021/ie100394v 2011 American Chemical Society Published on Web 07/28/2010
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capillary column using a flame ionization detector, while permanent gases and water were completely separated using a thermal conductivity detector and a sequenced flow pattern through two 1/8 in. packed columns (an 8 ft HaySep Q and a 10 ft 13X molecular sieve). Acetaldehyde was used as received from Aldrich (99.5%) and fed to the reactor by saturation in a stream of helium carrier (Matheson, 99.999%). The temperature of the saturator was maintained by immersion in a MGW/Lauda RM6 thermostatted bath containing an approximately 50/50 mixture of 2-propanol and water. The partial pressure in the feed was measured by GC analysis and compared to quantitative standards (Scott Specialty Gas). Anatase TiO2 obtained from Aldrich (99.9+%, BET surface area 10 m2/g), used throughout this investigation, was pretreated by heating to 673 K in a flow of 50 cm3 min-1 dry air (Matheson, Extra Dry) and oxidized for 2 h. After oxidation, the reactor temperature was changed to the temperature of interest and, once the temperature was stable, the flow was switched to helium and the reactor was purged for 20 min. After this purge period, the reactor was isolated from the flow stream and flow of the carrier through the acetaldehyde saturator was begun, analyses to determine the composition of the reactor feed were performed and the reaction was commenced. Conversions ranging from 0.3 to 26% were investigated by changing typical reactor variables such as catalyst mass, gas flow rate, and temperature. Temperatures greater than 548 K could not be readily investigated, as sufficiently low conversions could not be obtained by altering other experimental variables without inducing experimental difficulties. Initial experiments indicated a significant deactivation during reaction; the activity and selectivity as a function of time-onstream were therefore determined. Since analysis time was typically 50 min, several reaction/oxidation cycles were necessary to map out the time-on-stream behavior, with the data collected in a random order with respect to time-on-stream. Before each run, the catalyst was returned to its original activity state by repeating the original oxidation step at 673 K for 2 h. As previously mentioned, the mass of the catalyst bed was continually monitored and recorded during the entire portion of each reaction/oxidation cycle. Results Aldol condensation is a facile and selective base-catalyzed reaction on the oxidized surface of anatase TiO2, occurring with reasonable rates and high selectivities at temperatures as low as 373 K. However, significant deactivation of the catalyst does occur during the first 10 min on stream, decreasing the reaction rate by as much as a factor of 10. Figure 1 shows the time-onstream dependence of the acetaldehyde consumption rate for three different temperatures from 423 to 523 K. Rates of reaction are expressed as turnover frequencies (inverse seconds), normalized based on the adsorption capacity of acetaldehyde on the anatase polymorph of TiO2 (2.2 molecules nm-2).31 For all three examples shown, the rate declined quite rapidly for the first 6-10 min and then remained approximately constant with increasing time-on-stream. The absence of chromatographic effects was verified by monitoring the temporal response of the reactor effluent to a step change in the inlet gas. Once feed flow is cut in, it takes significantly less than 1 min for the reactor effluent to reach the acetaldehyde concentration of the feed at low temperature. The same period of time ((15 s) was required for equilibration when the experiment was repeated with crotonaldehyde, the main reaction product. In addition, any
Figure 1. Time-on-stream dependence of the rate of acetaldehyde conversion on oxidized anatase TiO2 at temperatures ranging from 423 to 523 K and an acetaldehyde partial pressure of 151.5 Torr. The rate of reaction is expressed as a turnover frequency normalized based on the adsorption capacity of acetaldehyde on oxidized anatase TiO2 (2.2 molecules per nm2).31
chromatographic effect would be expected to decrease with increasing temperature,32 which was not observed. Mass balance closures (including mass deposited on the catalyst, discussed below) at all time-on-stream values presented in Figure 1 ranged from 97.5 to 102%. The origin of the activity decay in Figure 1 is evident from the catalyst mass data as a function of time-on-stream. After the gaseous sample was obtained for analysis, the feed of acetaldehyde was terminated and the catalyst was purged with dry helium for a period of 1 h. The difference between the mass of the catalyst before reaction and after the reaction and purge sequence is considered to result from “irreversibly adsorbed” species. Figure 2 shows the mass of anatase TiO2 during the reaction process at 423 K and 151.5 Torr acetaldehyde partial pressure for a 10 min time-on-stream run. The irreversibly adsorbed mass was determined to be 469 µg per 100 mg of TiO2 after 10 min of exposure to acetaldehyde at the conditions described above. An identical procedure was used to determine the mass of irreversibly adsorbed species after each run. The values for the mass of irreversibly adsorbed species obtained for runs at the same three temperatures as in Figure 1 are plotted versus the relative reaction rate in Figure 3. The relative rate is normalized to the observed rate of reaction at 1 min time-onstream. Differences in the amount of mass deposited on the catalyst during this first minute of time-on-stream account for the offset of the curves at different temperature in Figure 3. A very strong negative correlation between the observed rate of acetaldehyde consumption and the mass of irreversibly adsorbed species is apparent. This result indicates that the activity decay in Figure 1 is caused by a deactivation mechanism involving considerable mass deposition on the catalyst. In fact, Figure 2 shows that the amount of mass deposited after only 10 min on stream is several times that of the acetaldehyde adsorption capacity of the catalyst. For adsorption at room temperature, the acetaldehyde uptake is 2.2 molecules nm-2.31 The postre-
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Figure 2. Mass increase of oxidized anatase TiO2 observed during reaction of acetaldehyde at 423 K and an acetaldehyde partial pressure of 151.5 Torr. Time of exposure is 10 min. After a 60 min He purge, the amount of irreversibly adsorbed mass was determined to be 469 µg on a 100 mg of anatase TiO2 sample. Table 1. Rate of Acetaldehyde Conversion on Anatase TiO2 as a Function of Partial Pressure and Time-on-Stream at 423 Ka rate of acetaldehyde consumption (s-1) time on CH3CHO stream (min) P ) 75 Torr P ) 151.5 Torr P ) 304 Torr order 1 5 10 20 30 40 50 60 90 120 a
Figure 3. Correlation between the observed amount of irreversibly adsorbed mass on the anatase TiO2 catalyst and the relative rate of acetaldehyde conversion at 423, 473, and 523 K. All data were recorded at an acetaldehyde partial pressure of 151.5 Torr. The relative rate is defined as the ratio of the reaction rate at a specific time-on-stream value to the initial rate of reaction determined at 1 min time-on-stream.
action purge in Figure 2 removed an amount of mass nearly equal to 2.2 molecules of acetaldehyde per nm2, leaving 3 times
8.25 × 10-3 1.90 × 10-3 8.78 × 10-4 7.65 × 10-4 7.28 × 10-4 6.74 × 10-4 7.06 × 10-4 7.63 × 10-4 7.58 × 10-4 7.85 × 10-4
13.8 × 10-3 3.36 × 10-3 1.64 × 10-3 1.37 × 10-3 1.48 × 10-3 1.43 × 10-3 1.46 × 10-3 1.64 × 10-3 1.42 × 10-3 1.42 × 10-3
22.3 × 10-3 5.50 × 10-3 2.87 × 10-3 2.53 × 10-3 2.72 × 10-3 2.74 × 10-3 2.80 × 10-3 2.81 × 10-3 2.54 × 10-3 2.61 × 10-3
0.71 0.76 0.85 0.85 0.94 1.00 0.98 0.93 0.86 0.86
Resulting order with respect to acetaldehyde is also indicated.
that amount of mass irreversibly adsorbed on the catalyst. Given the high coverage of the catalyst with reversibly and irreversibly adsorbed species under the conditions of these experiments, the occurrence of polycondensation reactions of acetaldehyde is not surprising. The consumption of acetaldehyde increased with increasing partial pressure of acetaldehyde regardless of time-on-stream or temperature, as shown in Tables 1 and 2. Determination of a reaction order is complicated because of catalyst deactivation; however, values in Tables 1 and 2 suggest that the activity of the catalyst is approximately proportional to the partial pressure of acetaldehyde at all reaction times and temperatures. The deviation from first-order kinetics at short time-on-stream (where the majority of the deactivation occurs) suggests that the deactivation rate is independent of acetaldehyde partial pressure. In agreement with this conclusion, the mass of irreversibly adsorbed species retained on the catalyst was found to be
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Table 2. Rate of Acetaldehyde Conversion on Anatase TiO2 as a Function of Partial Pressure and Time-on-Stream at 523 Ka rate of acetaldehyde consumption (s-1) time on CH3CHO stream (min) P ) 75 Torr P ) 151.5 Torr P ) 304 Torr order 1 5 10 20 30 40 50 60 90 120 a
13.8 × 10-3 6.01 × 10-3 5.62 × 10-4 5.22 × 10-4 4.82 × 10-4 4.88 × 10-4 5.16 × 10-4 4.98 × 10-4 5.29 × 10-4 5.68 × 10-4
22.8 × 10-3 10.8 × 10-3 10.2 × 10-3 10.6 × 10-3 9.89 × 10-3 10.1 × 10-3 10.5 × 10-3 10.3 × 10-3 9.84 × 10-3 10.3 × 10-3
39.1 × 10-3 19.5 × 10-3 18.9 × 10-3 20.9 × 10-3 20.5 × 10-3 20.4 × 10-3 22.1 × 10-3 21.5 × 10-3 18.9 × 10-3 17.9 × 10-3
0.74 0.84 0.86 0.99 1.03 1.02 1.04 1.04 0.91 0.82
Resulting order with respect to acetaldehyde is also indicated.
Figure 4. Time-on-stream dependence of the amount of irreversibly adsorbed mass on oxidized anatase TiO2 during reaction of acetaldehyde at partial pressures ranging from 75 to 304 Torr.
independent of the acetaldehyde partial pressure at all times on stream, as shown in Figure 4. The product slate for acetaldehyde conversion on oxidized anatase TiO2 was weakly dependent on time-on-stream at low temperature, with crotonaldehyde selectivities (determined on a carbon-content basis of volatile products) ranging from 86 to
100%, as indicated in Table 3 and Figure 5a. In contrast, the selectivity to crotonaldehyde was essentially independent of time-on-stream at 523 K, remaining constant at 93% as shown in Table 4 and Figure 6a. Comparing Tables 3 and 4, we note that both the lowest and highest crotonaldehyde selectivities were obtained at the lowest temperature investigated. Initially, at short times on stream, the anatase catalyst is less selective to crotonaldehyde at low temperature than at high temperature; conversely, at longer times on stream (>10 min) the catalyst is more selective to crotonaldehyde for lower temperature operation. It should be noted, however, that the large amounts of acetaldehyde that react to irreversibly adsorbed species mainly in the first 10 min on stream are not considered in these selectivity calculations, thus the short time-on-stream selectivities do not give a complete picture of the fate of the acetaldehyde reacted. At all temperatures and reaction times, the cis/trans ratio of crotonaldehyde in the product stream was essentially equilibrated. Minor products included light hydrocarbons (C1-C2), hydrogenation products (ethanol and butyraldehyde), a cross-esterification product (ethyl crotonate), secondary aldol condensation products (e.g., 2,4-hexadienal), methyl vinyl ketone (MVK), and benzene, as shown in Figures 5b and 6b. At short times on stream, trace amounts (e0.1% selectivity) of carbon oxides were observed at 548 K only. Selectivity to crotonaldehyde and minor products was found to be essentially independent of acetaldehyde partial pressure within the range (75-304 Torr) investigated. In order to examine the rank of the different observed products, a series of experiments at each temperature was performed in which the conversions at 1 and 10 min time-onstream were varied by alteration of the space velocity and carefully recorded. The results obtained at 423 K and 151.5 Torr are shown in Figure 7. The only primary products of acetaldehyde reaction at 423 K appear to be the initial aldol condensation products (crotonaldehyde and/or MVK, discussed below), with hydrogenation to ethanol appearing as a primary route at short times on stream. The same conclusion was reached within the entire temperature range investigated (373-523 K). Deactivation again complicates the estimation of an apparent activation energy. In principle, an Arrhenius plot can be constructed for every different time-on-stream; however, this is likely to have the most meaning for initial and stable reaction rates. Such an Arrhenius plot is shown in Figure 8 with apparent activation energies determined for time-on-stream values of 1 min (initial) and 20 min (stable) of 9 and 37 kJ mol-1, respectively. While these values are quite low, they compare favorably to values determined for other solid-catalyzed aldol condensation reactions.8,33 These results are also consistent with acetaldehyde temperature programmed desorption results on
Table 3. Product Selectivities from Acetaldehyde Conversion on Oxidized Anatase TiO2 at 423 K and 151.5 Torra selectivity (moles of acetaldehyde reacted to product per 100 mol converted) TOS(min)
conversion (%)
mass balance (%)
ethanol
trans-crotonaldehyde
cis-crotonaldehyde
ethyl crotonate
2,4-hexadienal
benzene
1 5 10 20 30 40 50 60 90 120
3.14 0.73 0.37 0.31 0.33 0.30 0.33 0.37 0.30 0.30
98.7 99.7 101.3 99.7 99.4 99.9 99.5 101.0 98.5 99.9
0.36
81.09 87.20 90.02 90.23 90.56 90.99 90.54 89.96 90.11 89.91
5.03 5.13 9.98 9.77 9.44 9.01 9.46 10.04 9.89 10.09
9.08 5.12
2.73 1.06
0.46
a
Small levels of methyl vinyl ketone, MVK, () 0.30%) were observed at short times on stream. The balance of the products at short times on stream is light (C1-C2) hydrocarbons.
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Figure 5. Time-on-stream dependence of the (a) major and (b) minor product carbon-basis selectivities observed during the reaction of acetaldehyde on oxidized anatase TiO2 at 423 K and 151.5 Torr of acetaldehyde. Table 4. Product Selectivities from Acetaldehyde Conversion on Oxidized Anatase TiO2 at 523 K and 151.5 Torra selectivity (moles of acetaldehyde reacted to product per 100 mol converted) TOS (min)
conversion (%)
mass balance (%)
methyl vinyl ketone
1 5 10 20 30 40 50 60 90 120
5.20 2.78 2.32 2.45 2.31 2.30 2.41 2.34 1.97 1.70
99.8 100.4 100.2 101.8 100.8 100.4 99.3 99.3 101.4 99.3
1.15 1.19 1.24 1.32 1.30 1.31 1.32 1.33 1.24 1.33
trans-crotonaldehyde
cis-crotonaldehyde
ethyl crotonate
2,4-hexadienal
benzene
83.58 81.56 82.65 82.32 83.56 83.42 83.62 83.90 84.27 83.90
10.36 10.21 10.09 10.28 10.31 10.25 10.23 10.30 10.39 10.30
0.32 1.89 1.49 1.46 1.41 1.45 1.42 1.43 1.43 1.46
2.47 2.57 1.76 1.62 1.60 1.58 1.63 1.58 1.60 1.74
0.35 1.08 1.66 1.46 1.32 1.27 1.06 0.54 0.25 0.78
a Small levels of ethanol (0.79%) and butyraldehyde (0.43%) were observed at short times on stream. The balance of the products at short times on stream is light (C1-C2) hydrocarbons.
polycrystalline anatase and rutile TiO234 as well as the TiO2(001) single-crystal surface35 showing the aldol condensation products as desorption-limited features at 400-410 K. Discussion Activity, Selectivity, and Kinetics Results. The conversion of acetaldehyde into crotonaldehyde proceeds rapidly on anatase TiO2 powder, with carbon-based selectivities easily exceeding 90% and turnover frequencies reaching nearly 0.04 s-1. Though several earlier investigations have been reported for the cross-condensation of carbonyl compounds on metal oxide catalysts,12-18,23,36 relatively few studies have examined the simple coupling reactions of light aldehydes and ketones25-27,37-39 and most of these have utilized transient techniques and should be considered only semiquantitative in nature due to the occurrence of deactivation. Hugueny et al.40 observed the condensation of acetaldehyde to crotonaldehyde and water on silica-alumina catalysts below 473 K; above this temperature, product
degradation through cracking occurred extensively, as light hydrocarbons became the predominant product. Young and Sheppard41 reported the formation of crotonaldehyde adsorbed on silica with infrared spectroscopy by heating a layer of adsorbed acetaldehyde to 393 K. Robinson and Ross33 observed the formation of crotonaldehyde upon adsorption of acetaldehyde on γ-alumina and aluminum-impregnated silica gel at 294 K. The catalytic activity of the aluminumcontaining silica gels at 463 K was found to decrease with increasing pretreatment temperature, with gels treated at 1273 K in dry N2 being completely inactive for the condensation reaction. More recently, Chavez Diaz et al.42 studied the adsorption and reaction of acetaldehyde on the proton form of MFI zeolite using Fourier transform infrared spectroscopy. Even at very low partial pressure (2 Torr), substantial conversion of the adsorbed acetaldehyde into crotonaldehyde was observed at 313 K. The authors also noted the disappearance of the surface OH band at 3600 cm-1 upon the adsorption of acetaldehyde,
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Figure 6. Time-on-stream dependence of the (a) major and (b) minor product carbon-basis selectivities observed during the reaction of acetaldehyde on oxidized anatase TiO2 at 523 K and 151.5 Torr of acetaldehyde.
Figure 7. Observed product carbon-basis selectivities at very low conversion of acetaldehyde at 423 K, 151.5 Torr, and 10 min time-on-stream. Conversion was adjusted by varying the space velocity.
Figure 8. Arrhenius plot showing the temperature dependence of the rate of acetaldehyde conversion on oxidized anatase TiO2 at 1 and 20 min timeon-stream and a partial pressure of acetaldehyde of 151.5 Torr.
suggesting proton transfer as the primary means of acetaldehyde interaction with H-MFI. These earlier investigations were all conducted on solid acid catalysts; as such, it is reasonable to assume the aldol condensation reaction follows an acid-catalyzed
mechanism on these materials. Further, all of these studies noted significant deactivation of the catalysts, presumably due to strong interactions between polycondensation products and the active sites.
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More recently, the adsorption and reaction of acetaldehyde on oxidized single crystal26,35 and polycrystalline powders26,34,43,44 of TiO2 have been studied by temperature programmed techniques. We previously reported that the adsorption and reaction of acetaldehyde in temperature programmed desorption (TPD) experiments on anatase TiO2 powder gave a selectivity to condensation products of nearly 94%, with a peak temperature of 430-450 K.34 This is remarkably similar to the selectivities for condensation products (cis/trans-crotonaldehyde and methyl vinyl ketone) reported in Table 3, which total between 86 and 100%. Further, condensation of acetaldehyde in the flow reactor studies was observed within the same temperature interval as the desorption of condensation products from anatase TiO2 powders.34 Idriss et al. have also reported results for TPD of acetaldehyde from the {011}- and {114}-faceted surfaces of a TiO2(001) single crystal and polycrystalline anatase powder.26 Desorption of crotonaldehyde, crotyl alcohol, butenes, ethanol, and water was observed in TPD of adsorbed acetaldehyde on both surfaces, though some differences in the quantity of products and desorption temperatures were observed. While acetaldehyde hydrogenation to ethanol was the major reaction on the {011}faceted surface, insignificant quantities of ethanol were observed on the {114}-faceted surface. Further, desorption of condensation products occurred in two channels (450 and 560 K) on the {011}-faceted surface, while only one feature was observed on the {114}-faceted surface (400 K). As was observed for the thermal desorption results,26 the product slate, selectivities, and kinetic features observed from flow reactor studies of acetaldehyde condensation on anatase TiO2 powders are more similar to results obtained on the {114}-faceted surface than those on the {011}-faceted surface; Idriss et al. reported nearly 75% selectivity for crotonaldehyde and crotyl alcohol on the anatase TiO2 powder, and nearly 60% on the {114}-faceted TiO2(001) surface. Only insignificant amounts of hydrogenation products were observed in the present flow reactor studies on anatase powders, mostly at short times on stream, again consistent with results from Idriss et al. on both the {114}-faceted TiO2(001) surface and the anatase TiO2 powder TPD studies.26 Luo and Falconer studied the adsorption and reaction of acetaldehyde on both mixed-phase (25% anatase, 75% rutile)43 and pure anatase44 TiO2 under TPD and temperature programmed hydrogenation (TPH) conditions. On anatase, the authors reported TPH spectra for acetaldehyde conversion that were very similar to the TPD spectra reported by Idriss et al.,26 which used the same polycrystalline TiO2 sample. Small amounts of acetone and carbon oxides were also observed in the TPH experiments, consistent with the previous TPD studies on single crystal and polycrystalline powder surfaces.26,34,35 However, in contrast to the previous studies, Luo and Falconer observed the formation of benzene, 2,4-hexadienal, and other high molecular weight products in TPH experiments performed on mixed-phase polycrystalline TiO2.43,44 Benzene, 2,4-hexadienal, and other high molecular weight compounds (e.g., ethyl crotonate, methyl vinyl ketone) were also observed during the steady-state conversion of acetaldehyde on anatase TiO2 in the present flow reactor studies. Interestingly, the formation of these higher molecular weight compounds is correlated with the retention of carbonaceous residue on the surface of the catalyst during experiments. In their TPH work on anatase TiO2, for example, Luo and Falconer44 reported that only about 4-5% of the original acetaldehyde remained on the catalyst surface after completion of the experiment as measured by temperature programmed
47
oxidation of the residue. This is consistent with the our previous results,34 which showed that only 5% of the adsorbed acetaldehyde remained on the anatase TiO2 surface after TPD using an inertial mass measurement technique. In these temperature programmed experiments on anatase TiO2, no high molecular weight products were observed.26,34,44 In contrast, Luo and Falconer reported that 42% and 67% of the originally adsorbed acetaldehyde remained on the mixed-phase TiO2 surface following TPH and TPD experiments, respectively, while observing high molecular weight products in these same experiments.43 Finally, in the present flow reactor studies, small quantities of these higher molecular weight products were observed, particularly at higher temperatures, along with clear evidence that catalyst surface builds and retains carbonaceous species (Figure 2). The reasons for this observation are currently unclear but may be indicative of modification of the surface active sites due to the deposition of carbonaceous species on the catalyst. We note, for example, that formation of the higher molecular weight products in the temperature programmed experiments of Luo and Falconer occurred at high temperature (above 550 K), at which point substantial amounts of carbonaceous species are likely to have been formed based on the results of the current study. Rasko´ and Kiss studied the adsorption and reaction of acetaldehyde on the surface of TiO2, CeO2 and Al2O3 metal oxides by Fourier transform infrared (FTIR) spectroscopy45 on both fully oxidized and H2-treated materials. Upon adsorption of acetaldehyde on the surfaces of the metal oxides at 300 K, spectral features associated with adsorbed crotonaldehyde, ethoxy, and acetate species were observed in addition to various bands from acetaldehyde interaction with the surface for all samples. When the adsorbed layers were heated, acetaldehyde, crotonaldehyde, water, and benzene were all observed in various desorption channels from 400 to 673 K with oxidized TiO2; no CO or CO2 was detected up to 673 K. Both water and crotonaldehyde exhibited the same peak temperature on the oxidized TiO2 surface (473 K), while benzene desorption peaked at higher temperature (573 K). The condensation of acetaldehyde on pure and transitionmetal impregnated CeO2 catalysts has been investigated by Idriss et al.27 Crotonaldehyde and crotyl alcohol (from hydrogenation of the CdO group in crotonaldehyde) were observed as major products in the temperature programmed desorption (TPD) of acetaldehyde adsorbed on oxidized pure CeO2 powders, with combined selectivities approaching 50% on a carbon basis. Crotonaldehyde desorbed in a single feature near 390 K, while crotyl alcohol was formed in two reaction channels at 390 and 570 K. No butyraldehyde (from hydrogenation of the CdC in crotonaldehyde) was observed in the desorbing products from pure CeO2. The adsorption of acetaldehyde on CeO2 was also investigated with infrared spectroscopy;27 the presence of acetates and ethoxides was detected upon room temperature adsorption. Heating the adsorbed layer to 390 K resulted in nearcomplete removal of ethoxide species, an increase in the intensity from acetates, and appearance of new bands attributable to adsorbed crotonaldehyde. Madhavaram and Idriss have examined the chemistry of uranium oxide surfaces in some detail, including the reaction of acetaldehyde46 using temperature programmed desorption techniques. The three major oxide surfaces of uranium (β-UO3, R-U3O8, and UO2) all showed different characteristic reaction channels for acetaldehyde desorption and reaction. Aldolization is favored over R-U3O8 with a maximum desorption rate of crotonaldehyde at 350 K and an observed carbon selectivity of
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68%. Reduction to ethanol (580 K, 33% selectivity) and reductive coupling to C4 hydrocarbons (540-550 K, 37% selectivity) are favored over the UO2 surface, and only small amounts of aldol products (340-350 K, 15% selectivity) were observed. Reaction of acetaldehyde on the β-UO3 surface resulted in the formation of the coupling product furan (400-420 K, 48% selectivity) along with aldolization to crotonaldehyde (400 K, 33% selectivity) at higher exposures. All of these previous results are consistent with the behavior observed for acetaldehyde condensation on anatase TiO2 between 423 and 548 K. Further, the similarities for the condensation of acetaldehyde on a diverse group of catalytic materials (silica-alumina to CeO2) are striking: condensation occurs at low temperature and evolution of the product is typically desorption-limited. Such similarities in thermal requirements may be taken as an indication of kinetic and/or mechanistic similarities in the condensation processes on these materials. In addition, we note that the selectivities reported in this paper are among the highest reported in the open literature for the condensation of acetaldehyde. High selectivity has been previously reported for other aldol condensation reactions47-50 in all cases, and a significant decay of activity was observed with time-on-stream. Hattori and co-workers47,48 have observed conversions ranging from 10 to 20% with selectivities greater than 99% for the aldol condensation of acetone on variously treated surfaces of MgO and CaO. In addition, Ai49 has reported the formation of methyl vinyl ketone through the selective crosscondensation of acetone and formaldehyde. Finally, the selective condensation of butyraldehyde to 2-ethylhexanal has also been reported.50 We note that the selectivities and conversions reported in this work are similar to those mentioned above, adding anatase TiO2 to the list of selective heterogeneous catalysts for aldol condensations. Kinetics Results and Comparison to Temperature Programmed Experiments. As noted above, the apparent activation energies determined from the Arrhenius plot in Figure 8 for the conversion of acetaldehyde on the oxidized surface of TiO2, while low, are consistent with previous results reported in the literature8,12,13,16 as well as those extracted from TPD experiments.26,34 Malinowski and Basinski8 reported an apparent activation energy of 10-12 kcal mol-1 for the cross-condensation of acetaldehyde with formaldehyde on a NaOH/SiO2 catalyst, while Ai12,13 has reported values ranging from 3.5 to 15 kcal mol-1 for the same reaction, depending on the catalyst. Negative apparent activation energy values have also been reported for this reaction catalyzed by metal oxide-loaded zeolites.16 These values illustrate the large range of apparent activation energy values reported in the literature, making quantitative comparison difficult. An additional complication to quantitative comparison is the observed time-on-stream dependence of the apparent activation energy, as illustrated in Figure 8. Even with these complications, the activation energies determined in the present investigation are consistent with a low activation energy process, as has been previously reported. We also note that the observed near first-order kinetics for aldol condensation, the low apparent activation energy for the reaction, and the temperature-programmed desorption reaction results can all be consistently interpreted using simple Langmuir-Hinshelwood kinetics. Previous results26,34,35 for the temperature-programmed reaction/desorption of acetaldehyde on anatase and rutile TiO2 powders have shown that desorption of the condensation product occurs within the same temperature window as the steady-state condensation reaction reported here.
This result suggests that the rate of formation of volatile aldol condensation products may be desorption-limited. Recently, Singh et al.51 performed low-temperature FTIR experiments of the adsorption and reaction of acetaldehyde on mixed-phase (25% anatase, 75% rutile) TiO2 powder using very precise sample heating. Upon adsorption of acetaldehyde at 223 K and after heating up to 250 K, only features attributable to adsorbed acetaldehyde were observed on the catalyst surface. However, after heating in the range of 250-257 K, substantial changes in the structure of the adsorbates were apparent. These changes were consistent with the formation of crotonaldehyde and crotyl alcohol by aldol condensation of acetaldehyde; spectroscopic features associated with both species were observed.51 This mixed-phase TiO2 sample was the same source as was used by Luo and Falconer,43,44 who reported a maximum desorption rate of crotonaldehyde at 400 K in TPH and TPD experiments. Taken together, these two reports would seem to confirm that the generation of aldol products from acetaldehyde is desorption-limited. Desorption limitation of product formation may contribute to the appearance of larger products through secondary condensations by keeping the surface concentration of the initial condensation products high, as noted above. Indeed, we have previously demonstrated strong similarities between the modes of adsorption of acetaldehyde and crotonaldehyde on the surfaces of anatase TiO231 as have others more recently.45,51 Active Site Deactivation and Dependence of Rate and Selectivity. Ji et al. have studied the condensation of acetaldehyde on a variety of silica-supported catalysts.52 When alkali metals (Li, Na, K, and Cs) were supported on silica, high selectivity (90%) to aldol products (crotonaldehyde and crotyl alcohol) at moderate to low (>20%) conversion was obtained between 523 and 648 K; minor amounts of ethyl ether and ethyl acetate were also observed. The authors reported that it was not possible to obtain higher conversion on these catalysts without a drop in selectivity.52 When molybdenum and tungsten oxides were supported on silica, high selectivities (90%) to aldol products were again obtained at low conversions (10-15%). Finally, the authors also studied zirconia and SO4--promoted zirconia supported on silica. Unlike the other silica-supported catalysts studied, zirconia-on-silica catalysts were unstable, with conversion declining from about 35% to near 10% over 300 min on stream. During this decline in conversion, the selectivity to aldol products increased from 80% to near 100%. The dependence of the both the rate of reaction and the nature and amount of adsorbed products observed by Ji et al.52 is a clear indication that the state of the catalyst is altered during the course of the reaction. Indeed, we report here a similar strong and rapid deactivation of anatase TiO2 during aldol condensation of acetaldehyde. The rate of acetaldehyde conversion on anatase declined by more than a factor of 3 during the first 10 min of reaction as illustrated in Figure 1. The correlation between activity loss for volatile product formation and increased catalyst mass, shown in Figure 3, suggests that the deposition of strongly bound adsorbates is directly responsible for the loss in catalytic activity. A complementary study of the capacity and resulting surface intermediates formed by the adsorption of acetaldehyde and crotonaldehyde on TiO2 provides a partial description of the mechanism for the formation of these nonvolatile species.31 The adsorption of acetaldehyde and crotonaldehyde was found to be competitive, occurring on similar sites with comparable strengths and resulting in similar adsorbed species. Considering these adsorptive similarities along with the capability of both aldehydes to participate in aldol condensation reactions, it is
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logical to presume the nonvolatile species responsible for loss in catalytic activity result from the secondary condensation reactions. While not directly probed in this investigation, several additional observations regarding the nature of the species deposited during the deactivation process which support this conclusion can be made by examining the associated kinetic features. The rate of volatile product formation was observed to increase with increasing partial pressure of acetaldehyde in a fashion which was near, but slightly less than, proportional. However, the rate of mass deposition on anatase was shown to be essentially independent of the partial pressure of acetaldehyde in the gas-phase, as shown in Figure 4. The combination of zero-order mass deposition and first-order reaction to volatile products may explain the intermediate reaction orders observed in the initial portion of the reaction (Tables 1 and 2.) Zeroorder dependence of mass deposition on partial pressure was observed throughout the temperature and pressure ranges investigated. The situation most likely to provide zero-order kinetics is one in which the surface species responsible for kinetic control of carbon deposition are adsorbed on the catalyst surface to capacity. Regardless of the controlling phenomenon, the kinetic dependences of the formation of volatile and nonvolatile species on acetaldehyde partial pressure are different. While this difference does not preclude the mechanisms of these two processes from being similar, it does provide one indication that the controlling steps are either different or involve different chemical species. The deactivation kinetics may be interpreted as resulting from a rate-limiting condensation reaction of two abundant surface intermediates, resulting in low-order kinetics with respect to acetaldehyde partial pressure. Similar to this observed difference in reaction order with respect to acetaldehyde, the initial (1 min time-on-stream) rate of irreversible adsorbate deposition was found to have a slightly negatiVe apparent activation energy on both anatase (-8 kJ mol-1) and rutile (
