Ind. Eng. Chem. Res. 1990,29, 1136-1142
1136
note that hydrazine, a recognized high-temperature oxygen scavenger, is produced by oxidation of its parent compound and not by hydrolysis. The principal reason for developing carbohydrazide as a scavenger was to reduce the risk of handling hydrazine in industrial environments. That objective has been realized. The increased reactivity and passivation, however, show that carbohydrazide is not just a safe form of hydrazine. These properties do not result from in situ release of hydrazine but are characteristic of carbohydrazide itself. The observed accumulation of hydrazine is apparently not material to the overall result. Registry No. I, 497-18-7; 11, 1617-13-6; Cu, 7440-50-8; 02, 7782-44-7; water, 7732-18-5; hydrazine, 302-01-2.
Literature Cited Attanasi, 0.;Serra-Zanetti, F. Effect of metal ions in organic synthesis; VII. Conversion of acylhydrazines and N-acyl-N'-tosylhydrazines to carboxylic acids and esters in the presence of copper(I1) chloride. Synthesis 1980, 314-315. Banerjea, D.; Singh, I. P. Hydrazine complexes of some divalent metal ions in aqueous solutions. 2. Anorg. Allg. Chem. 1967,349, 213-219. Benson, S. W. The induction period in chain reactions. J . Chem. Phys. 1952,20, 1605-1612. Borsche, W.; Muller, W.; Bodenstein, C. A. Relation between quinonehydrazones and p-hydroxyazo compounds. VII. Aliphatic aromatic dihydroxy4,4'-disazo compounds Justus Liebigs Ann. Chem. 1929,475,120-131. Campbell, M. J. M.; Grzeskowiak, R. An epr study of some complexes formed by interaction between copper(I1) oxyacid salts and carbohydrazide. Inorg. Nucl. Chem. Lett. 1976, 12, 545-549. Campi, E.; Ostacoli, G.; Vanni, A.; Casorati, E. Complexes of carbohydrazide with metallic ions in aqueous solution. Ric. Sci., Parte 2: Sez. A 1964, 6, 341-356. Cosper, D. R. (Nalco Chemical Co.) Unpublished data on autoxidation rates of hydrazides of aliphatic acids, 1983. Feher, F.; Linke, K. H. Concerning the oxidation of urea, semicarbazide and carbohydrazide with sodium hypochlorite. J. Prakt. Chem. 1966, 32, 190-197. Freier, R. K. Corrosion protection of steam power boilers with hydrogen peroxide. Energie 1977,29, 294-296.
Keim, G. L.; Henry, R. A.; Smith, G. B. L. The oxidation of di- and triaminoguanidine with potassium iodate. J . Am. Chem. SOC. 1950, 72, 4944-4946. Krivis, A. F.; Gazda, E. S.; Supp, G. R.; Kippur, P. Coulometric determination of carboxylic acid hydrazides. Anal. Chem. 1963, 35, 1955-1957. Meites, L. Iodometric determination of copper. Anal. Chem. 1952, 24, 1618-1620. Mill, T.; Hendry, D. G. Kinetics and mechanisms of free radical oxidation of alkanes and olefins in the liquid phase. In Comprehensive Chemical Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1980; Vol. 16 (Liquid-phase Oxidation). Mushran, S. P.; Agrawal, M. C. Mechanistic studies on the oxidation of ascorbic acid. J . Sci. Ind. Res. 1977, 36 (6), 274-283. Rynasiewicz, J. Hydrogen peroxide determination in the presence of chromate. Anal. Chem. 1954,26, 355-358. Sawyer, D. T.; Nanni, E. J. Redox chemistry of dioxygen species and their chemical reactivity. In Oxygen and Oxy-radicals in Chemistry and Biology, Rodgers, M. A. J., Powers, E. L., Eds.; Academic: New York, 1981. Sellers, R. M.; Simic, M. G. Pulse radiolysis study of the reactions of some reduced metal ions with molecular oxygen in aqueous solution. J . Am. Chem. SOC.1976, 98, 6145-6150. Slovinsky, M. Boiler additives for oxygen scavenging. .- - U S . Patent 4,269,717, 1981. Smith. P. A. S. The Chemistrv of ODen-Chain Organic Nitrogen Compounds;W. A. Benjamin, Inc.; New York, 1966: Vol. 11, p i 8 5 and references cited therein. Stoll6, R. Hydrazidicarboxylic hydrazide. Ber. Dtsch. Chem. Ges. 1910, 43, 2464-2470. Tsuji, J.; Nagashima, T.; Qui, N. T.; Takayanagi, H. Facile oxidative conversion of hydrazides of carboxylic acids to corresponding acids, esters and amides using copper compounds. Tetrahedron 1980,36, 1311-1315. Watt, G. W.; Chrisp, J. D. A spectrophotometric method for the determination of hydrazine. Anal. Chem. 1952,24, 2006-2008. Yokota, K.; Yamazaki, I. Analysis and computer simulation of aerobic oxidation of reduced nicotinamide adenine dinucleotide catalyzed by horseradish peroxidase. Biochemistry 1977, 16, 1913-1920.
Received for reuiew July 25, 1989 Accepted January 31, 1990
Methanol Oxidation over Nonprecious Transition Metal Oxide Catalysts Umit S. Ozkan,* Richard F. Kueller,+ and Edgar Moctezuma Department of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210
Methanol oxidation over nonprecious transition metal oxide catalysts was studied in a gradientless external recycle reactor. The catalysts (oxides of Cr, Mn, Fe, Co, Ni, Cu), which were prepared by using the incipient wetness technique, were supported on 1/8-in. -y-Al,O, tablets. The catalysts were characterized by using BET surface area measurement, X-ray diffraction, laser Raman spectroscopy, scanning electron microscopy, and energy dispersive X-ray analysis techniques. All the catalysts exhibited similar activities for methanol conversion, but the Cu catalyst was found to be considerably more selective to COz. The order of the reaction was 1.2 with respect to methanol concentration and appeared to range between 0.5 and 0 with respect to oxygen concentration. As the use of alcohol-fueled vehicles grows, effective catalytic control of emissions from these vehicles is becoming increasingly important. Gasoline-powered vehicles typically emit unburned fuel, CO, and a mixture of C2-C, hydrocarbons along with various cyclic compounds (Goodrich, 1982). The catalysts used to control these emissions generally contain a precious metal (Pt, Pd, Rh, Ag) supported on a ceramic monolith. The emissions from pure-alcohol-fueled vehicles contain the unburned fuel and CO, but the number of partially oxidized fuel derivatives
* To whom correspondence should be addressed. t Present address: Dow Chemical, Midland,
MI 48674.
0888-5885/90/2629-1136$02.50/0
are much smaller compared to gasoline combustion. Since complex hydrocarbons, sulfur, and lead are absent from the exhaust, nonprecious transition metal oxide catalysts can offer an effective and inexpensive alternative. Compared to the considerable work done on gasoline engine emission control (Stein et al., 1960; Yao and Kummer, 1973, 1977; Severino and Laine, 1983; Saverino et al., 19861, the number of studies on exhaust emission control from alcohol-fueled vehicles is much fewer. Most of the earlier work has been on either ethanol oxidation (Yao, 1984; McCabe and Mitchell, 1983, 1984; Gonzales and Nagai, 1985) or on aldehyde oxidation (McCabe and McCready, 1984; McCabe and Mitchell, 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1137 1983; Foster and Masel, 1986; Lapinski et al., 1987). Some of the more recent studies have focused on methanol oxidation and raised several interesting questions about the reaction scheme, catalyst-support interactions, and the choice of metal (Plummer et al., 1987; McCabe and Mitchell, 1986, 1987). This research examines the catalytic oxidation of methanol over a series of first-row transition metal oxide catalysts (Cr, Mn, Fe, Co, Ni, Cu). The metal oxides were prepared by the wet impregnation method and were all supported on y-alumina. The oxidation experiments were performed by using a fixed-bed integral reactor. The reactor was equipped with an external recycle loop. When the recycle ratio is in excess of 25, CSTR behavior is simulated; heat- and mass-transfer coefficients are large, and local heat- and mass-transfer effects do not disguise the observed kinetics (Serrano and Carberry, 1985). Isothermal reaction data were collected at CSTR conditions using a recycle ratio of 50. The effect of temperature on catalytic activity and selectivity was examined by varying the reaction temperature in the range 30-300 "C. The effects of metal loading and methanol concentration were also investigated. The catalysts were characterized by nitrogen BET surface area measurement, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and laser Raman spectroscopy techniques. Experimental Section Catalyst Preparation. The catalysts used in this study were high surface area first-row transition metal oxides (Cr, Mn, Fe, Co, Ni, Cu) supported on l/ejn. tablets of y-alumina (Harshaw, A1-3438 T). Metal nitrates were used to supply the metal. Each catalyst was prepared by a two-step wet impregnation of the support. A volume of double-distilled water equal to twice the pore volume of the alumina support (0.5 cm3/g) was used to dissolve the metal nitrate salt at each step. The metal nitrate solution and the support were contacted at 100 "C for 2 h during the first step and 4 h during the second step. The mixture was dried at 125 "C at each step. The catalysts were all calcined at 500 "C for 9 h under an oxygen atmosphere. The metal loading for all the catalysts in this study was held at 8 mol of metal per 100 mol of y-A1203 support. Two additional copper catalysts at mole ratios of 0.04 and 0.10 were also examined. Catalyst Characterization. BET Surface Area. The surface areas of the catalyst samples were measured by using a Micromeritics 2100E Accusorb instrument. Nitrogen was used as the adsorbate. X-ray Diffraction. X-ray diffraction patterns were obtained by using a Scintag PAD V diffractometer. The incident beam was copper K, radiation (A = 1.543 A), and a nickel filter was used. A Data General computer system was used to collect and process the output data. Scanning Electron Microscopy. Electron micrographs were taken on the Hitachi 53-510 scanning electron microscope. The accelerating voltage was 25 kV, and the filament current was 90 A. Magnification ranged from 30X to 6000X. Samples were prepared by splitting a pellet and mounting it on a lf2-in. carbon disk with low resistance cement so that the split cross section was exposed. The samples were gold coated to prevent specimen charging. Energy Dispersive X-ray Analysis. A JEOL (JXA35) SEM equipped with an EDAX system was used to examine the samples. Characteristic X-ray lines of copper and aluminum were used for elemental dot maps.
Heal Exchanger
uu u O2
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N2
Figure 1. Feed and reactor system.
Laser Raman Spectroscopy. Raman spectra were obtained by using a SPEX 1403 Ramalog 9-1 spectrometer. The excitation source was the 514.5-nm line of a 5-W Ar ion laser (Spectra Physics, Model 2016). Laser power was 150 mW. A personal computer was used for data collection. The scan rate was 1 cm-l/s, integration time was 1 s, and generally 16 scans between 200 and 1200 cm-' were collected for each sample. Reactor Configuration. Feed and Reactor System. Figure 1 shows the feed and reactor system used in the oxidation experiments. Compressed gas cylinders supplied the gas feed and were regulated by four mass flow controllers (Tylan, Model FC-260). The nitrogen (high purity grade, Union Carbide), oxygen (extra dry grade, Linde), COz (high purity grade, Linde), and dimethyl ether (1.1% (CH3)20in N2, Matheson) tanks were all maintained at a delivery pressure of 20 psig. The gases were combined in a manifold. Liquid methanol was introduced into the feed system by a Sage syringe pump (Model 341B, Fisher Scientific). Hamilton gas-tight syringes fitted with Leur Lock tips were used to deliver the methanol. The reactor system utilized a fixed-bed integral reactor equipped with an external recycle loop. The reactor measured 25.4-mm 0.d. X 23.6-mm i.d. and 500 mm in length. It was constructed from 316 stainless steel. Five Chromel-Alumel (type K) thermocouples were placed in the reactor (Omega, SCASS-0626024)to measure axial and radial temperature gradients. The length of the catalyst bed, which was placed in the middle of the reactor, was 120 mm. The catalyst was diluted with inert solids in a volume ratio of 1to 4. The total surface area in the reactor was held at 3000 m2 for each catalyst. The reactor was placed in a fluidized sand bath (Techne, FB-08) for uniform heating. The temperature was controlled by a Eurotherm type K temperature controller. All of the reactor inlet and outlet lines were heated to prevent condensation and to preheat the feed. An important feature of the reactor system was the external recycle loop. This allowed the reactor to function as either a fixed-bed integral reactor or a gradientless differential reactor. The recycle loop consisted of a 316 stainless steel heat exchanger (3/s-in. 0.d. tube, 3/4-in.i.d. shell, 1 m in length), a double diaphragm air pump (Thomas, Model 2107CA14, with Nordel diaphragms), and a flowmeter (Cole Parmer, Model 5-3217-32). The reactor feed and product gases were analyzed by an on-line gas chromatograph, GC (Hewlett-Packard, 5890 A). Product identification was also confirmed by GC-mass spectrometry (Finnigan, Model 4021). The GC was equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Two six-portvalves were used for sample injection, and a four-port valve was used
1138 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 Table I. BET Surface Areas and Weight Percent Loadings of the Catalysts surface area, m2/g wt% catalysts (mole ratio) 190 ?'-A1203 Cr/yAlZO3(0.08) 200 7.3 Mn/y-Alz03(0.08) 185 11.0 Fe/y-Alz03(0.08) 190 11.1 Co/~-AlzO,(0.08) 220 15.9 Ni/r-A1203(0.08) 190 5.5 Cu/~-AlzO3(0.04) 200 3.0 Cu/~-AlzO3(0.08) 210 5.9 Cu/~-AlzO3(0.10) 175 7.2
as a column isolation valve. Hayes Sep Q, Hayes Sep T, and molecular sieve 5A columns were used for separating the species found in the product and feed streams. Two Hewlett-Packard integrators (Models 3390 A and 3396 A) were used to process the signals from the detectors. Oxidation Experiments. Methanol oxidation was studied over a series of transition metal oxides. The first series of experiments examined catalyst activity and selectivity over different catalysts which included supported oxides of Cr, Mn, Fe, Co, Ni, and Cu with an oxide to alumina mole ratio of 0.08 and the alumina support itself. The second set examined the effect of metal loading on the catalyst activity and selectivity for the copper catalyst. The last set of experiments examined the effect of methanol concentration on conversion and selectivity over the copper catalyst under initial rate conditions. The concentration of methanol, oxygen, and nitrogen and the recycle rate were held constant for the first and second set of experiments, while the temperature was varied. For the experiments performed to assess the effect of methanol concentration, the temperature was kept constant at a level to give methanol conversions less than 10%. The methanol concentration was varied at two different oxygen concentrations. The reaction parameters used in this study were as follows: temperature, 25-300 "C; pressure, 8 psig; recycle ratio, 50; feed composition, 0.32% CH,OH, 3.63% 02,96.05% N,; molar flow rate of fresh feed, 1.25 mol/h; total catalyst surface area, 3000 m2. The reaction parameters for the experiments conducted to assess the effect of methanol concentration under initial rate conditions were as follows: temperature, 109 OC; pressure, 6 psig; recycle ratio, 60; feed composition, 0.18-0.74% CHSOH, 24.9% and 49.8% 02,49.&74.8% N,; molar flow rate of fresh feed, 1.07 mol/h. The percent methanol conversion is defined as (moles of methanol consumed/moles of methanol in feed)100%. The yield of product A is defined as (moles of A produced/moles of methanol in feed)(l/y)100%, where y is the ratio of the number of carbon atoms in the reactant to the number of carbon atoms in the product. For all oxidation reactions, the carbon balances were 96% or higher.
Results Catalyst Characterization. BET Surface Area. The BET surface areas and weight compositions of the catalysts are listed in Table I. The surface areas of the supported catalysts were found to be fairly close to the surface area of the support material. X-ray Diffraction. The d spacings and relative intensities obtained from the X-ray diffraction patterns permitted identification of the oxide phase present on the alumina. The XRD patterns showed that the oxide phases of manganese, iron, cobalt, nickel, and copper present on the alumina support corresponded to Mn2O3, Fe2O3, Co304, NiO, and CuO, respectively, as identified by a comparison to the JCPDS (Joint Committee on Powder Diffraction
Standards) files, 31-825,33-664A,9-418,22-1189, and 31825. The Cr catalyst did not show any identifiable peaks superimposed over the diffraction pattern of alumina. Laser Raman Spectroscopy. Despite the poor Raman scattering characteristics of these catalysts, Raman spectra were obtained for catalysts containing Cr, Fe, and Co. Along with the spectra of the catalysts the spectra of the pure metal oxide standards were also obtained. The chromium catalyst exhibited Raman bands at 874 and 996 cm-'. These do not correspond to the bands from the Cr203standard, which has strong Raman bands at 300,339, 390, 540, and 598 cm-'. The chromium phase on the catalyst clearly is not Cr203. Comparison of the bands observed from the Cr catalyst with those obtained from CrO, standard, however, shows close agreement. The Raman spectrum for the Fe catalyst exhibited bands at 216, 278, and 396 cm-', which agreed closely with those observed in FezO, standard. The Co catalyst had Raman bands at 464,808, and 671 cm-'. The same band positions were obtained when the Raman spectrum of Co30, was taken. This reconfirmed the X-ray diffraction results, indicating that the bulk oxide phases present on the Feand Co-supported catalysts were Fez03 and Co304,respectively. Scanning Electron Microscopy. All of the catalysts were examined by using a scanning electron microscope (Hitachi S-510). Additional SEM images were taken of the copper catalyst. The cross section, the outside surface, and the crushed powder of the pellet were examined. The alumina structure appeared flat and platelike for both the pellet and the powder. With the exception of the copper catalyst, the other catalysts did not show any new or different phases from the alumina at magnifications ranging from 30X to 1500X. Upon examination of the micrographs of the Cu catalyst at low magnification (Figure 2), the flat platelike structures were again visible, but in the "pits" another structure was detected. At higher magnification, these spongelike structures were clearly seen. Energy Dispersive X-ray Analysis. The copper catalyst was also examined in an SEM equipped with an EDAX unit (JEOL JXA-35) and elemental dot maps, and SEM images were obtained. Dot maps were taken a t low and high magnification. Low-magnification results showed that Cu and A1 were both well dispersed throughout the pellet. The Cu dot map was slightly brighter at the edge of the pellet, indicating a slightly higher concentration of Cu atoms there. Cu and Al dot maps were also taken after increasing the magnification and isolating the spongelike structures (Figure 3). The Cu now appeared conlderably more concentrated in the pit, while the A1 was concentrated outside the pit. Examination of the SEM image showed that Cu concentration corresponded to the sponges, whereas the flat plates corresponded to Al. Dot maps of other catalysts were also taken. They showed that metal was well dispersed throughout the catalyst. Separate phases were not detected with either the SEM imaging or the elemental dot mapping. Reaction Studies. Blank Reactor Runs. The stainless steel reactor was tested for catalytic activity by performing blank reactor runs. Methanol oxidation over the inert packing material (Pyrex beads) resulted in no reaction up to 325 "C. A 90% conversion of methanol was obtained a t 475 "C. Oxidation Studies over First-Row Transition Metal Oxide Catalysts. The selectivity and yield of methanol oxidation were examined over six supported metal oxide catalysts and a bare alumina support. Methanol conver-
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1139
Figure 2. SEM micrographs of CuO/yAl2O3.
Figure 3. SEM micrograph and dot maps of CuO/y-A1203 cross section: (a, left) Cu dot map, (b, middle) SEM image, (c, right) A1 dot map.
sion and yields to specific products were recorded a t various temperatures. The six catalysts were also compared for oxidation activity and selectivity to carbon dioxide. The ratio of moles of metal to moles of alumina was 0.08 for each catalyst studied. Figure 4 presents the variation of percent conversion and yields with temperature in methanol oxidation over Cr03/ ?-A12039 Mn203/ ?-A12039 Fe203/Y'A1203, c0304/YA1203,NiO/y-Al2O3,and CuO/y-A1203catalysts. An examination of this figure shows that the first five catalysts exhibited a similar behavior in the oxidation of methanol (Figure 4a-e). As the temperature increased, the methanol conversion rose sharply over all five catalysts. The other common feature observed over these catalysts was the large yields of dimethyl ether (DME), which went through maxima with increasing temperature. A t low temperatures (below 200 "C), dimethyl ether was the only major reaction product. Especially Fe203and NiO catalysts showed very high yields.of dimethyl ether even a t moderately high temperatures. It was also observed that, once the dimethyl ether yield began to decline, the carbon dioxide yield began to increase sharply. A t higher temperatures, the CO yield went through a shallow maximum. In addition to dimethyl ether, C02,and CO, trace quantities of formaldehyde were observed over NiO and Fe203catalysts. Over Cr03, methyl formate was found in tracequantities a t lower temperatures. CuO/yA1203,the last catalyst in the series of the
first-row transition metal oxide catalysts studied, however, exhibited a distinctly different behavior (Figure 4f). As with the other catalysts, methanol conversion increased rapidly with increasing temperature. But unlike the previous catalysts, C02 was the only reaction product detected. Neither DME nor CO formed. C02 formation occurred a t temperatures approximately 50 "C lower than any of the other catalysts studied. The results from methanol oxidation over pure 7-A1203 are shown in Figure 5a. As with all of the catalysts, the' overall methanol conversion showed a similar increase with increasing temperature. Up to a temperature of 225 "C, DME was the only reaction product observed. At temperatures above 250 "C, DME was still the major reaction byproduct. CO and C02slowly started to form above 250 "C. Interestingly, C02,as opposed to CO, was the minor product, accounting for less than 5% of the methanol yield even a t 275 "C. The temperature required to achieve methanol conversion levels of 90% and 50% for each catalyst is shown in Figure 6. For 90% conversion, the temperature ranged between 200 and 260 "C, with Cu and Cr requiring the lowest and Co and Fe the highest temperatures. No large differences existed among the catalysts. By examination of the temperature required to convert 50% of the methanol, again no major -differences are seen between the catalysts. The temperature ranged between 130 and 160
1140 Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990
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"C. This reflects that the overall methanol conversion is quite similar for all of the catalysts. When the temperatures required to yield 50% C 0 2 were compared (also shown in Figure 6), the differences between the catalysts began to appear more clearly. The temperatures now ranged from 150 to 240 "C. Copper catalyst required only 155 "C to yield 50% C02, whereas the other catalysts needed well over 200 "C. When the CO, yields at 150 and 200 "C were plotted for all of the catalysts, the differences were much more pronounced (Figure 7). At 150 O C , with the exception of Cu, all of the catalysts had a negligible
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COzyield. The COzyield over the Cu catalyst, on the other hand, was approximately 45% and corresponded to near 100% selectivity. At 200 "C, the copper catalyst had a C 0 2 yield close to 90%, whereas the other five catalysts had CO, yields ranging from 6% to 40% only. Since COz is the desired product in this study, the copper catalyst was clearly superior to all of the other catalysts studied. Further work was undertaken on the copper catalyst. Effect of Recycle. To investigate the effect of the presence of the recycle loop on the product distribution and activity, oxidation experiments were performed at 150 and 200 O C , bypassing the recycle loop and using a total flow rate of 935 cm3 (STP)/min. The removal of the recycle loop did not have a major effect on the product distribution although the overall conversion decreased due to lower residence times. The only major product observed over the copper catalyst was again carbon dioxide. Other catalysts gave dimethyl ether as the major reaction products with smaller quantities of carbon dioxide and carbon monoxide. Formaldehyde was also detected in trace quantities. Effect of Metal Loading. The effect of metal loading on methanol oxidation was studied over copper catalysts. The methanol conversion increased with increasing metal loading. At any given temperature, the methanol oxidation
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1141 20.0
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was greater for the catalyst with the higher metal loading. The only reaction product for the Cu catalysts with both 10% and 8% metal loading was C02. When the methanol conversion was examined for the Cu catalyst with 4 % metal loading (Figure 5b), DME was detected. Instead of C02being the only reaction product a t low temperatures, as with the other two Cu catalysts, DME was now the major product, and there was no CO being produced. Although the quantity of DME formed was small compared to the quantity obtained over other metal catalysts, it was still present in appreciable amounts. No C02formation took place below 150 "C over this catalyst, although the other copper catalysts gave appreciable C 0 2 yields a t temperatures as low as 100 "C. Effect of Methanol Concentration. To examine the effect of methanol concentration on activity and product distribution, oxidation experiments were performed over the Cu catalyst with the mole ratio of 0.08 by keeping the oxygen concentration constant a t an excess level and varying the methanol concentration. The kinetic data were collected at low conversion levels to simulate the initial rate conditions. The reaction rate was obtained from a simple mass balance for a CSTR: -rate =
ucmethx 0
where u is the volumetric feed flow rate, 0 is the surface area of the catalyst used, Cmeth is the methanol concentration, and X is the overall methanol conversion. Data were collected a t five different methanol concentrations and at two different excess oxygen concentrations. All of the data were taken a t 109 "C. Figure 8 shows the dependency of the rate on methanol concentration a t the low value of excess oxygen. The results of a least-squares analysis showed the order of the reaction was 1.2 with respect to methanol concentration and appeared to range between 0.5 and 0.0 with respect to oxygen concentration.
Discussion The BET surface areas for all of the catalysts generally ranged f10% of surface area of the base alumina support. The phase present for each catalyst was identified either by X-ray diffraction, by laser Raman spectroscopy, or by both. The SEM micrographs and EDAX dot maps showed the physical appearance and location of the CuO phase present
on the alumina. The SEM images for the Cu catalyst clearly depicted a phase separate from A1203.While metal oxide phases were detected by XRD and LRS for catalysts other than the Cu catalysts, no separate phases were visible in the SEM. The oxidation experiments were performed with a recycle ratio of 50 to simulate CSTR behavior in the reactor. Using the heat- and mass-transfer limitation criteria outlined by Carberry (1976,1987), it Was concluded that inter- and intraphase heat- and mass-transfer effects were negligible. In this way it was made certain that the data obtained represented the true kinetics and not heat- and mass-transfer effects. In all experiments, the carbon balance closures were higher than 96%. The oxidation results for all of the metal oxide catalysts (Cr, Mn, Fe, Co, Ni, Cu) showed very similar activities for methanol oxidation. Methanol oxidation increased rapidly with increasing temperature. The temperatures required for 90% methanol oxidation ranged between 200 and 260 "C for the catalyst series. When the temperature needed to achieve a 50% methanol conversion was compared for all of the catalysts, a 30 "C temperature range was observed (130-160 "C). The temperature ranges for 90% and 50% conversion of methanol over all of the catalysts are relatively small considering the diverse nature of the catalysts. When the temperature levels required for a 50% C 0 2 yield were compared, however, differences between the catalysts became more evident. For the Cr, Mn, Fe, Co, and Ni catalysts, the temperature changed in a 60 "C range (200-260 "C). The temperature required for 50% C02 yield over the copper catalyst, on the other hand, was 155 "C, 45 "C lower than that required over any other catalyst. When the C02yield was compared for all of the catalysts, differences between the catalysts became even more pronounced. The copper catalyst was the only one to form significant amounts of C02. The 45% C02yield at 150 "C corresponded to near 100% selectivity. All of the other catalysts principally formed dimethyl ether a t this temperature. In these oxidation reactions, formaldehyde was observed only in trace quantities over NiO and Fe203 catalysts. There was no formaldehyde formation over the CuO catalysts. Experiments were repeated a t 150 and 200 "C, bypassing the recycle loop and using a total volumetric flow rate of 935 cm3 (STP)/min. Formaldehyde was observed only on non-copper catalysts in small quantities (less than 0.1% yield). Dimethyl ether was still the major reaction product over these catalysts. The product analysis was further verified by use of a gas chromatograph-mass spectrometer system (Finnigan, Model 4021). A comparison of these results with those obtained over alumina-supported catalysts containing group 9, 10, and 11 metals (McCabe and Mitchell, 1986) shows that CuO catalysts are quite promising for complete oxidation of methanol, giving high selectivity and activity at temperature levels comparable to those used with Pt and Pd catalysts. A direct comparison is not possible since the metal catalysts used in the two studies are not the same. To our knowledge this is the first study that focused on first-row transition metal oxides (Cr, Mn, Fe, Co, Ni, Cu) in a systematic manner, although the same catalysts have been used in ethanol oxidation previously. One of the major differences between the two sets of studies is the formation of methyl formate over Pt and Pd catalysts. Also formation of formaldehyde over Rh catalysts is reported to be more significant, whereas in our study formaldehyde was observed only on non-copper catalysts in trace quantities.
1142 Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990
When the effect of metal loading on activity was investigated over the CuO catalysts, methanol oxidation was seen to increase, as expected, with increased metal loading. The only reaction products formed over the 8% and 10% Cu catalysts were COz and H20.However, the 4 % Cu catalyst formed significant amounts of dimethyl ether at low temperatures (30% yield at 160 "C). Methanol oxidation experiments performed over the bare alumina support provided some important clues that might explain this phenomenon. Alumina was 100% selective to DME at temperatures up to 210 "C. As the CuO concentration was increased over the alumina support, a CuO phase (spongelike structures) formed and effectively blocked alumina sites responsible for DME formation. The lower metal loadings perhaps did not provide enough CuO to block all of the alumina sites. When the results of oxidation experiments performed over the metal oxide catalysts are compared with those obtained over the alumina support, it is seen that some oxidation reactions occur simultaneously with the dehydration reaction. Oxidation becomes predominant as DME formation rapidly decreases. The alumina support, however, never exhibited this predominantly oxidative mode. This suggests that the metal oxides were responsible for the oxidation reactions. Since alumina, containing both Lewis and Bransted acid sites, is known to successfully catalyze dehydration reactions, it is quite likely that these acid sites are responsible for dehydration reactions and, in our case, for dimethyl ether formation. Studies performed under the initial rates over the Cu catalyst showed the dependency of rate on methanol concentration to be approximately first order. The order of the reaction with respect to oxygen, on the other hand, varied between 0.0 and 0.5. This implies that the reaction rate is a stronger function of methanol than oxygen. This work attempted to identify some possible catalysts that would be less expensive and more readily available than the traditional precious metal oxide catalysts used to control engine emissions from methanol-fueled vehicles. From a comparative study of several transition metal oxide catalysts, all supported on y-alumina, the CuO catalyst was as active as the other catalysts and much more selective to COz. Copper was the only catalyst that did not form dimethyl ether at low temperatures and at metal loadings above -6% and was 100% selective to COP. A distinct CuO phase was seen in the SEM micrographs and identified with energy dispersive X-ray analysis. The Cu catalyst clearly stood out as the most promising one among the catalysts tested in this study for complete methanol oxidation and showed that it can be a potential candidate for catalytic emission control in methanol-fueled vehicles. Acknowledgment The financial support from The Ohio State University Office of Research and Graduate Studies is gratefully ac-
knowledged. We also thank Karen Latvala and Todd Harris for their technical assistance. Registry No. Cr03, 1333-82-0; Mn203, 1317-34-6; Fe203, 1309-37-1; Co304, 1308-06-1; NiO, 1313-99-1; CuO, 1317-38-0;
CH,OH, 67-56-1; CO2, 124-38-9.
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