Catalytic Reforming of Ethanol to Acetaldehyde for Lean-NO x

Catalytic Reforming of Ethanol to Acetaldehyde for Lean-NOx Emission Control ... from liquid hydrocarbon for controlling diesel engine NOx emissions i...
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Ind. Eng. Chem. Res. 2004, 43, 6343-6348

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Catalytic Reforming of Ethanol to Acetaldehyde for Lean-NOx Emission Control Jong-Hwan Lee,* Steven J. Schmieg, and Se H. Oh Chemical and Environmental Sciences Laboratory, General Motors Research and Development Center, Warren, Michigan 48090

The on-board production of a reducing gas mixture from liquid hydrocarbon for controlling diesel engine NOx emissions is being considered as an alternative to the other technologies that require supplemental reducing agents such as urea. Among the catalysts evaluated for the formation of acetaldehyde by partial oxidation of ethanol under the reaction conditions that are suitable for the on-board side-stream fuel reforming device, silver, molybdenum, and vanadium catalysts have been identified to be the most promising for this application. Mo/Al2O3 catalysts exhibit the highest acetaldehyde yield over a wide range of reaction temperatures. V/TiO2 is effective at lower temperatures, while Ag/Al2O3 is effective at higher temperatures, although the reaction temperature window is narrow for both catalysts. On both Mo/Al2O3 and Ag/Al2O3, the reaction rate for acetaldehyde formation was found to be more than 100 times higher than that for CO2 formation, which indicates the high selectivity of these catalysts. 1. Introduction Diesel engines can achieve substantially higher fuel efficiency than stoichiometric gasoline engines. However, because these engines operate with excess oxygen, it becomes very difficult to reduce nitrogen oxides (NOx) in the exhaust. The lack of effective and affordable methods for meeting the future NOx emission standards has delayed the introduction of these fuel-efficient engines to the U.S. market. One of the technologies considered to date is the use of catalysts that can selectively reduce NOx in the presence of excess oxygen using engine-out hydrocarbons. A new class of materials for selective catalytic reduction (SCR) of the NOx has been developed to promote the reaction between the reductants and NOx over the competing combustion reaction. However, despite the extensive efforts made to utilize the hydrocarbons in the exhaust as the reductant (HC-SCR), many serious problems such as poor activity, narrow operating temperature window, and insufficient durability still hinder the commercial application of this technology.1 Therefore, the injection of appropriate reductants into the exhaust to improve stoichiometry and kinetics has been considered for the control of NOx emissions from diesel engines. In fact, the SCR of NOx using ammonia (NH3) as the reductant (NH3-SCR) has been proven effective and used commercially for the removal of NOx emissions from stationary sources since the 1970s. Because of the safety and regulatory problems associated with storage, handling, and transportation of NH3, the use of an aqueous urea solution has been explored as a storage compound of NH3. For example, the SCR of NOx using urea as the reductant (urea-SCR) for heavy duty diesel applications has attracted a lot of attention in the past several years. However, because of several major issues, most notably the lack of urea distribution infrastruc* To whom correspondence should be addressed. Tel.: (586) 986-2099. Fax: (586) 986-8697. E-mail: [email protected].

ture, it is unlikely that this technology will be widely implemented for light duty diesel applications in the U.S.1 Although typical hydrocarbons present in engine exhaust are generally less active and selective reductants compared to ammonia, the NOx reduction efficiency of HC-SCR catalysts can be greatly improved if an appropriate hydrocarbon species is used. For example, acetaldehyde (CH3CHO) was found to be very effective for reducing NOx over alumina, BaY and NaY.2,3 Thus, it would be desirable if acetaldehyde can be produced on-board for use with other NOx emission control technology including a HC-SCR catalyst. In fact, the concept of on-board production and use of appropriate reductants from engine fuel for NOx emission control has been around for many years.4 Among a number of methods to prepare acetaldehyde, it should be prepared from a liquid hydrocarbon source, which can be safely and conveniently handled and carried on a vehicle. Thus, it appears to be most appropriate to consider the partial oxidation of ethanol with ambient air to produce acetaldehyde (C2H5OH + 1/ O f CH CHO + H O) on-board for vehicle applica2 2 3 2 tions. The SCR of NOx with acetaldehyde produced from ethanol on-board offers a number of advantages over the urea-SCR technology. Ethanol has less serious issues associated with distribution infrastructure, inuse compliance, freezing temperature, unreacted NH3 slip, etc. For example, ethanol can be more conveniently distributed as a fuel additive. In fact, ethanol is being considered as a replacement for methyl tert-butyl ether as an oxygenated fuel additive to lower emissions.5 Because only a small amount of acetaldehyde is required for NOx reduction, no more than 2% of ethanol needs to be mixed with engine fuel (e.g., E-diesel). This translates into less than 2% of fuel economy penalty excluding the electrical requirements such as dosing control and catalyst heating. Upon refueling using existing devices at gas stations, ethanol can be conveniently distilled off by an on-board distillation device and stored in a small storage tank for acetaldehyde production.

10.1021/ie049680v CCC: $27.50 © 2004 American Chemical Society Published on Web 09/03/2004

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The purpose of this study is to evaluate the feasibility of side-stream on-board catalytic reforming of ethanol to produce acetaldehyde for use with an HC-SCR system for the reduction of NOx emissions from diesel engines. A series of catalysts have been formulated and evaluated for their effectiveness for converting ethanol to acetaldehyde under the reaction conditions that are suitable for an on-board fuel reforming device. Aluminasupported silver and molybdenum catalysts have been identified to be the most promising and investigated further to provide a guideline for the development of an effective catalytic ethanol reforming device. 2. Experimental Section 2.1. Catalyst Preparation and Characterization. Ag/Al2O3 containing 2 wt % Ag was prepared on γ-Al2O3 powder (Sasol, Catalox SBa-200; surface area ) 205 m2/ g) by the incipient-wetness method using AgNO3 (JT Baker) as a precursor. The sample was dried at 110 °C overnight and calcined at 500 °C for 4 h. Mo/Al2O3 catalysts were prepared on γ-Al2O3 powder by the same incipient-wetness method using (NH4)6Mo7O24‚4H2O (Alfa Aesar). Mo catalysts containing 1-20 wt % Mo were prepared by dissolving appropriate amounts of Mo precursor in the same amount of H2O, which was then used to wet the alumina powder. The samples were dried at 110 °C overnight and calcined at 450 °C for 4 h. Cu catalysts were prepared on Y zeolite and γ-Al2O3 powder. CuY containing 6.5 wt % Cu was prepared from NaY (Zeolyst, CBV-100, 10 wt % Na, Si/Al ) 2.55) by the ion-exchange method using Cu(CH3CO2)2 (JT Baker) as a precursor at room temperature for 24 h, followed by drying at 110 °C overnight and calcination at 400 °C for 4 h. Cu/Al2O3 catalysts containing 1-5 wt % Cu were prepared on alumina powder by the same incipient-wetness method using Cu(NO3)2‚3H2O (Fisher) as a precursor. The samples were then dried at 110 °C overnight and calcined at 450 °C for 4 h. WO3/Al2O3 catalysts containing 5 and 10 wt % W were prepared by mixing and grinding powders of WO3 (Alfa Aesar) and γ-Al2O3 in an agate mortar, followed by calcination at 500 °C for 4 h. V2O5/TiO2 containing 5 wt % V was prepared on TiO2 powder (Hombikat UV100; surface area ) 309 m2/g) by the wet impregnation method using NH4VO3 (EM) as a precursor. The resulting yellow samples were then dried at 110 °C overnight and calcined at 450 °C for 4 h. V2O5/ Al2O3 containing 10 wt % V was prepared on γ-Al2O3 powder in the same manner. All of the catalysts prepared on powder were pelletized to 60-80 mesh sizes to minimize the pressure drop across the catalyst bed. The concentrations of various metals reported in this paper were obtained by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Perkin-Elmer Optima 300 instrument in the Chemical Analysis group at the R&D Center. 2.2. Catalyst Evaluation. The partial oxidation of ethanol to acetaldehyde was carried out using a packedbed flow reactor system. A total of 0.23 g of catalyst was loaded in a 3/8-in.-o.d. quartz tube with a quartz frit, which was then placed in an electric furnace. The reaction temperature was measured by a thermocouple located slightly above the catalyst bed in the inlet. Unless specified otherwise, the activity was measured using a feed gas containing 19% O2, 2.5% H2O, 1000

ppm ethanol, and balance N2 because ambient air containing oxygen and some moisture will likely be used for on-board ethanol reforming. Pure water and ethanol were injected using separate sets of a syringe pump and a wick. The injected liquids were then evaporated and heated along with the gas lines to over 120 °C. The total gas flow rate was 272 mL/min, and the gas hourly space velocity was estimated to be 52 000 h-1. The concentrations of ethanol, acetaldehyde, and other reaction products were continuously measured with a Nicolet Nexus 670 Fourier transform infrared (FT-IR) spectrometer with a 2-m gas cell heated at 165 °C and pressurized to 940 Torr. The catalysts were evaluated for their activity toward ethanol conversion as well as their selectivity toward acetaldehyde production. The reaction temperature was increased stepwise by 25 °C and maintained for 1 h at each temperature or until the steady-state conversion was achieved. The ethanol conversion was calculated based on the difference in the ethanol concentration before and during the reaction.

% C2H6O conversion ) [ethanol(in) ethanol(out)]/ethanol(in) × 100 The selectivity to acetaldehyde is defined as the amount of acetaldehyde produced divided by the difference in the ethanol concentration before and during the reaction.

% selectivity to C2H4O ) acetaldehyde(out)/ [ethanol(in) - ethanol(out)] × 100 In this study, the acetaldehyde yield is defined as the amount of acetaldehyde produced divided by the amount of ethanol in the feed gas mixture.

% acetaldehyde yield ) acetaldehyde(out)/ ethanol(in) × 100 Mass balances were checked by comparing the amount of carbon associated with ethanol and the reaction products such as acetaldehyde, acetic acid, ethylene, carbon monoxide, and carbon dioxide. Reasonably good carbon balance (∼95%) was obtained for all of the experiments, indicating that other unidentified species were present only in negligible amounts. 3. Results and Discussion 3.1. Effects of Catalyst Composition. Among various catalysts prepared and tested for the partial oxidation of ethanol to acetaldehyde, those materials that showed high ethanol conversion activities are shown in Figure 1a,b. The supports alone showed no activity toward the acetaldehyde formation and thus are not included in the figures. First, a Ag/Al2O3 catalyst containing 2 wt % Ag was prepared and tested in view of the earlier literature report that Ag/Al2O3 is active for acetaldehyde production as a result of the partial oxidation of ethanol.6 Furthermore, the commercial process for acetaldehyde production from ethanol employs a Ag catalyst.7 The Ag catalyst was effective in converting ethanol to acetaldehyde at higher temperatures among the catalysts tested. Copper is also well-known and has been used to produce acetaldehyde from ethanol commercially.7 Thus, 3% Cu/Al2O3 and 6.5% CuY were prepared and tested

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Figure 1. Catalysts evaluated for the partial oxidation of ethanol to acetaldehyde: (a) ethanol conversion; (b) acetaldehyde yield. Reaction conditions: 19% O2, 2.5% H2O, 0.12% C2H5OH; SV ) 52 000 h-1.

under our reaction conditions. As shown in Figure 1a, both Cu catalysts were reasonably active in converting ethanol. However, they produced mostly combustion products such as CO and CO2, thus yielding very little acetaldehyde as shown in Figure 1b. Especially, 6.5% CuY produced a significant amount of ethylene with almost no acetaldehyde. The observed poor selectivity over these Cu catalysts can be attributed to the formation of copper oxides under the oxidizing reaction conditions. V2O5/TiO2 has been reported to be effective for the partial oxidation of ethanol to acetaldehyde in the literature.8 As shown in Figure 1a, V2O5/TiO2 containing 5 wt % V was highly effective in ethanol conversion with the light-off temperature in the range of 100-125 °C. It was also very effective in producing acetaldehyde selectively: ∼60% acetaldehyde yield at only 125 °C. Because tungsten oxide is known to oxidize methanol to formaldehyde,9 the physical mixtures of WO3 and Al2O3 powders containing 5 and 10 wt % W were tested. However, very little acetaldehyde was produced despite high ethanol conversion. Although the dispersion of WO3 in these physical mixtures might have been relatively low, the observed activity was too low to be explained solely by poor dispersion of WO3.

Figure 2. Effects of molybdenum weight loading: (a) ethanol conversion; (b) acetaldehyde yield. Reaction conditions: 19% O2, 2.5% H2O, 0.12% C2H5OH; SV ) 52 000 h-1.

Among the catalysts shown in Figure 1, molybdenum was identified to be the most active and selective in producing acetaldehyde from ethanol over a wide range of temperatures under our reaction conditions. Thus, more samples containing various amounts of Mo (1-20 wt %) were prepared on γ-Al2O3 powder. As shown in Figure 2a, the light-off temperature for ethanol conversion was lowered with increasing Mo loading. The percent acetaldehyde yield, shown in Figure 2b, was also improved with the higher Mo loadings. Over 70% acetaldehyde yield was observed with Mo/Al2O3 catalysts containing 7-20% Mo at 200-300 °C. In particular, an acetaldehyde yield as high as ∼90% was obtained with 10 wt % Mo catalyst at 225 °C. For the same 5 wt % loading shown in Figure 3, the V/TiO2 catalyst exhibited significantly higher activity at lower temperature compared to Mo/Al2O3; however, the temperature window for acetaldehyde formation was narrower over V/TiO2. Also, Mo/Al2O3 was more selective over a wide range of temperatures: acetaldehyde was the only major product at all temperatures above the light-off temperature. On the other hand, the major product over V/TiO2 changed from acetaldehyde to acetic acid and combustion products (e.g., CO and CO2) with increasing reaction temperatures. For the same 10 wt % loading on alumina, the Mo/ Al2O3 catalyst was more effective in converting ethanol

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Figure 3. Comparison between V/TiO2 and Mo/Al2O3: (a) ethanol conversion and acetaldehyde yield; (b) product distribution. Reaction conditions: 19% O2, 2.5% H2O, 0.12% C2H5OH; SV ) 52 000 h-1. Solid line: 5 wt % Mo/Al2O3. Broken line: 5 wt % V/TiO2.

to acetaldehyde at lower temperatures compared to V/Al2O3 as shown in Figure 1: 89% vs 71% at 225 °C. However, they both exhibited similar selectivity toward acetaldehyde and similar product distribution patterns, although more CO and CO2 were produced over V/Al2O3. Because V/Al2O3, unlike V/TiO2, did not perform better than Mo/Al2O3, it was suspected that there is a beneficial effect from the TiO2 support for the partial oxidation of ethanol. Indeed, it was reported that the reaction rates followed the order Mo/TiO2 > Mo/Al2O3 > Mo/SiO2, although the activation energy did not vary with support. The differences in rates were attributed to different electronic energy states in the metal oxide-support interface.10 This suggests that Mo catalysts can be further optimized by using a different support such as TiO2. 3.2. Effects of Reaction Conditions. 3.2.1. Effects of Partial Pressures of H2O and O2. The partial pressures of oxygen and water vapor are known to influence the activity and product selectivity in the partial oxidation of an alcohol. Hence, it is expected that the performance of the on-board ethanol reforming device for NOx emission control will be influenced by the different humidity levels in different climates. Therefore, the effects of different partial pressures of O2 and H2O in the reaction feed were investigated using

10 wt % Mo/Al2O3 and 2 wt % Ag/Al2O3, which have been identified as highly effective for the partial oxidation of ethanol to acetaldehyde. First, when 0 and 2.5 vol % H2O conditions were compared over Mo/Al2O3 at 200-275 °C, some changes in the product distribution were observed as the combustion and dehydration reactions were promoted in the absence of water vapor. More combustion reaction products like CO and CO2 and a dehydration reaction product like C2H4 were produced at the expense of the partial oxidation products such as acetaldehyde and acetic acid. Nonetheless, still over 90% ethanol conversion and ∼70% acetaldehyde yield were obtained in the absence of water vapor. The effect of water vapor on the product distribution is particularly pronounced in Ag/Al2O3. When 2.5% H2O was included in the feed, ethylene formation was suppressed (40-50 ppm f 12-24 ppm) while acetic acid formation was promoted (0 ppm f 72-116 ppm). However, there was hardly any effect of water vapor on acetaldehyde formation. In fact, a slightly higher acetaldehyde yield was obtained without H2O in the feed. In the absence of H2O, the light-off temperature was slightly lowered (∼20 °C) but the selectivity to acetaldehyde was unaffected. The effect of the oxygen concentration was studied over Ag/Al2O3 only using 10% O2 to test the possibility of using oxygen in the engine exhaust. Practically the same ethanol conversion and acetaldehyde yield were obtained in both cases without any significant difference in product distribution. 3.2.2. Effects of the Partial Pressure of C2H5OH. Once acetaldehyde is produced from ethanol using air as the oxidant in a side-stream device, it can be mixed with the engine exhaust stream and used to reduce NOx over a downstream lean-NOx catalyst. The amount of acetaldehyde required for this NOx reduction reaction depends on the activity of the lean-NOx catalyst. For example, it was estimated that only 1 mol of acetaldehyde is required to reduce 2 mol of NO2 to 1 mol of N2 over alumina at 475 °C.2 On the other hand, it was estimated that 1.8 mol of acetaldehyde is required to reduce 1 mol of NO over NaY.3 Therefore, if the engineout NOx is 250 ppm and NaY is used downstream, 450 ppm of acetaldehyde would be required to reduce NOx. This translates into the production of 1.4% acetaldehyde required in the side-stream reactor if acetaldehyde is produced in 1 L/s of air flow and mixed with 30 L/s of engine exhaust flow. Thus, at 89% acetaldehyde yield over 10% Mo/Al2O3, ∼1.6% ethanol should be mixed with the air flow in the side-stream reactor. That is, the molar ratio of ethanol/H2O/O2 should be 1.6:2.5:19 in the device, instead of 0.1:2.5:19 used for screening catalysts in this study. The effect of relatively high partial pressures of ethanol was investigated by using a feed gas containing less O2 and H2O. Increasing the ethanol concentration to maintain the ratio was not possible because of the difficulty associated with measuring the large amount (>0.3%) of acetaldehyde with FT-IR spectroscopy. Instead, 1.1% O2 and 0.15% H2O were mixed with 0.12% ethanol for reactor experiments. Similar levels of selectivity to acetaldehyde were maintained at all temperatures over Mo/Al2O3. However, because ethanol conversion was suppressed at 200 °C (91% f 79%), a lower acetaldehyde yield was obtained as a result. Interestingly, more C2H4 and less

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Figure 4. Product concentrations across the catalyst bed: (a) 10 wt % Mo/Al2O3 at 200 °C; (b) 2 wt % Ag/Al2O3 at 275 °C. Reaction conditions: 1.1% O2, 0.15% H2O, 0.12% C2H5OH.

Figure 5. Product yields at different levels of ethanol conversion. (a) 10 wt % Mo/Al2O3 at 200 °C; (b) 2 wt % Ag/Al2O3 at 275 °C. Reaction conditions: 1.1% O2, 0.15% H2O, 0.12% C2H5OH.

acetic acid were produced at all temperatures. On the other hand, the light-off temperature for ethanol conversion over Ag/Al2O3 was lowered by 50-75 °C. The acetaldehyde yield was also improved to 78% at 300 °C. This improved activity can be attributed to the more favorable feed gas ratio for Ag/Al2O3, because the commercial processes using Ag and Cu catalysts operate under ethanol-rich conditions.7 Nonetheless, Ag/Al2O3 still exhibited lower overall acetaldehyde yields and a narrow temperature window compared to Mo/Al2O3. 3.3. Reaction Pathways and Kinetics. A rational design and optimization of a catalytic reformer requires understanding of the reaction mechanism and kinetics of the promising reformer catalysts identified in this study. Therefore, additional experiments were carried out with 10 wt % Mo/Al2O3 and 2 wt % Ag/Al2O3 catalysts to examine how the product yields are affected by the ethanol conversion. Such relationships provide insight into the reaction pathway and kinetics.11 In these experiments, the space velocity was varied by changing the amount of catalyst in the reactor (from 0.013 to 0.23 g) while using the reaction feed containing 0.1% ethanol, 1.1% O2, and 0.15% H2O at a fixed temperature, and the ethanol conversion and product distribution were monitored. The concentrations of ethanol and major products across the catalyst beds are shown in Figure 4a,b. The acetaldehyde formation and ethanol consumption in-

creased gradually across the Mo catalyst at 200 °C. On the contrary, most of the acetaldehyde production and ethanol conversion occurred over a very small amount of Ag/Al2O3 at 275 °C. Ethanol conversion then increased steadily over the remaining Ag catalyst but produced mostly acetic acid and CO2. Interestingly, the acetaldehyde produced was not consumed with increasing catalyst amount. When product yields were plotted as a function of the ethanol conversion as shown in Figure 5, linear relationships were obtained at lower ethanol conversions, which indicates irreversible, first-order, parallel reactions for acetaldehyde formation (reaction 1) and ethanol combustion (reaction 2). From the slopes of the lines k1

C2H5OH + 1/2O2 98 CH3CHO + H2O k2

C2H5OH + 3O2 98 2CO2 + 3H2O

(1) (2)

in Figure 5a,b, considering the stoichiometry of these two reactions, k1/k2 was estimated to be around 150 over Mo/Al2O3 at 200 °C and around 110 over Ag/Al2O3 at 275 °C at lower ethanol conversions. This indicates the much greater selectivity of these catalysts for the partial oxidation of ethanol to acetaldehyde against the total oxidation to CO2.

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On the other hand, at higher ethanol conversions, the CO2 yield deviated from this linear relationship over both catalysts. The ethanol partial oxidation reaction may be expanded as a consecutive reaction in which acetaldehyde is an intermediate: C2H5OH f CH3CHO f CO2. Considering the scale and stoichiometry of the reaction, the observed increase in the CO2 yield can be attributed to the further oxidation of acetaldehyde. Nonetheless, given the linear relationship between the acetaldehyde yield and ethanol conversion up to 80% ethanol conversion, the further reaction of acetaldehyde to CO2 is expected to be small even at high ethanol conversions, most likely because of its high activation energy. This is also supported by the fact that acetaldehyde was not consumed with increasing catalyst amount. Depending on the engine-out NOx level and exhaust flow rate, the amount of ethanol injected into the catalyst reformer will vary. To maintain the feed gas composition, the flow rate through the reformer catalyst will also have to change accordingly. The above observations suggest that a larger amount of Mo/Al2O3 at lower temperatures, but a smaller amount of Ag/Al2O3 at higher temperatures, would be required to ensure adequate acetaldehyde production at various flow rates. For example, to obtain 89% acetaldehyde yield over 10% Mo/Al2O3 washcoated on a monolith at 52 000 h-1 under the operation conditions that were described earlier (i.e., 1 L/s of air flow), the size of a side-stream reformer would be on the order of 200 mL. Acknowledgment We express our gratitude to Noel M. Potter for his help with timely materials characterization and Byong K. Cho for a number of exciting discussions.

Literature Cited (1) Johnson, T. V. Diesel emission control in review. SAE 200101-0184. (2) Lee, J.-H.; Yezerets, A.; Kung, M. C.; Kung, H. H. Hydrocarbon reaction pathway in selective NO reduction over a bifunctional SnO2/Al2O3 catalyst. J. Chem. Soc., Chem. Commun. 2001, 15, 1404. (3) Schmieg, S. J.; Cho, B. K.; Oh, S. H. Selective catalytic reduction of nitric oxide with acetaldehyde over NaY zeolite catalyst in lean exhaust feed. Appl. Catal. B 2004, 49, 113. (4) Nakatsuji, T.; Yasukawa, R.; Tabata, K.; Ueda, K.; Niwa, M. Catalytic reduction system of NOx in exhaust gases from diesel engines with secondary fuel injection. Appl. Catal. B 1998, 17, 333. (5) Nadim, F.; Zack, P.; Hoag, G. E.; Liu, S. United States experience with gasoline additives. Energ. Policy 2001, 29, 1. (6) McCabe, R. W.; Mitchell, P. J. Reaction of ethanol and acetaldehyde over noble metal and metal oxide catalysts. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 196. (7) Ullmann, F.; Gerhartz, W.; Yamamoto, Y. S.; Campbell, F. T.; Pfefferkorn, R.; Rounsaville, J. F. Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH Publishers: Weinheim, Germany, 1985. (8) Quaranta, N. E.; Soria, J.; Corberan, V. C.; Fierro, J. L. G. Selective oxidation of ethanol to acetaldehyde on V2O5/TiO2/SiO2 catalysts. J. Catal. 1997, 171, 1. (9) Hutchings, G. J.; Taylor, S. H. Designing oxidation catalysts. Catal. Today 1999, 49, 105. (10) Zhang, W.; Desikan, A.; Oyama, S. T. Effect of support in ethanol oxidation on molybdenum oxide. J. Phys. Chem. 1995, 99, 14468. (11) Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis and Design; John Wiley & Sons: New York, 1979.

Received for review April 20, 2004 Revised manuscript received July 22, 2004 Accepted August 4, 2004 IE049680V