1622
Ind. Eng. Chem. Res. 1993,32, 1622-1630
Complete Oxidation of Ethanol, Acetaldehyde, and Ethanol/Methanol Mixtures over Copper Oxide and Copper-Chromium Oxide Catalysts Hariharan Rajesht and Umit S . Ozkan' Department of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210
Complete oxidation of ethanol, acetaldehyde, and a methanol-ethanol mixture over supported copper oxide, chromium oxide, and copper oxide/chromium oxide catalysts was investigated using a gradientless external recycle reactor. The catalysts were characterized by BET surface area measurement, X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray analysis. The effect of catalyst type and composition, reaction temperature, and feed composition has been examined.
Introduction With the Clean Air Act amendments of 1990 requiring increased use of oxygenated compounds such as alcohols and ethers in motor fuels, the problem of effectively controlling the emissions caused by burning these substances has become a more pressing issue. Alcohols, both ethanol and methanol, are currently in use, mostly as blends in gasoline, in some of the air quality nonattainment areas of the United States. Gasoline-powered vehicles typically emit unburned fuel, CO, and a mixture of C2-C4 hydrocarbons along with various cyclic compounds (Goodrich,1982). The emissions from such vehicles are generally controlled by catalysts containing a precious metal such as Pt, Pd, Rh, or Ag. Alcohols, on the other hand, are considered as "clean burning" fuels. Emissions of complex hydrocarbons, olefins, benzene, S02, and soot are almost eliminated. The above advantages are, however, offset by heavier discharges of carcinogenic aldehydes like formaldehyde and acetaldehyde which form photochemically active reactant radicals and toxic peroxyacetyl nitrates. Suitable catalytic materials thus need to be developed to handle these pollutants. A considerable amount of work has been done to investigate catalytic control of emissions from gasolinepowered vehicles. Since CO emission is one of the chief causes of environmental pollution, much of the research has been on CO oxidation. One of the earliest studies on catalytic oxidation of hydrocarbons from vehicle exhaust, was conducted at the U.S. Bureau of Mines (Stein et al., 1960). A number of catalysts were tested for activity, and it was found that oxides of Co, Ni, Mn, Cr, and Fe were the most active for the automobile catalyst muffler. Oxidation of CO and various hydrocarbons in the 200-400 "C temperature range was examined at Ford Motor Company (Yao and Kummer, 1973,1977). Oxides of Cu, Cr, Co, and Ni, both supported and unsupported, were tested. The Cu-Cr catalyst was reported to have the highest activity for CO oxidation followed by copper and then cobalt. In another study done at Ford (Goodsel, 1973), an unsupported cobalt oxide catalyst was used for CO and hydrocarbon oxidation. Oxidation of CO over Cu, Cr, and Cu-Cr mixed metal catalysts, supported on y-alumina, has been studied by Severino et al. (1983,1986). Supported copper catalysts were more active than copper-chromite catalysts when metal concentrations were smaller than 12%. A t larger metal concentrations supported copper chromites were more active. The authors have postulated
* To whom correspondence should be addressed.
+ Present address: United Catalysts Inc., Louisville,KY 40232.
a mechanism of electron transfer between copper and chromium with copper being the more active species. Complete oxidation research for alcohols has largely been done by the automobile companies Ford and General Motors. Both precious metal (Pt, Pd, Ag, and Rh) and base metal oxides have been studied previously for oxidation of ethanol (Yao, 1984; McCabe and Mitchell, 1983, 1984; Gonzalez and Nagai, 1985; Ismagilov et al., 1979,1983). In general, Pt and Pd catalysts were found to be far superior compared to the base metal oxides because they exhibited higher activity for ethanol conversion to C02 and less acetaldehyde formation. Research on complete methanol oxidation has been reported by Plummer et al. (1987) and McCabe and Mitchell (1986, 1987). Catalysts containing highly dispersed Pt, Pd, Ag, Rh, and Cu-Cr supported on y-alumina beads have been studied. Formaldehyde, dimethyl ether, and methyl formate were formed as intermediate products in the oxidation reaction. Some work has also been reported on formaldehyde and acetaldehyde oxidation (McCabe and McReady, 1984; McCabe and Mitchell, 1983,1984,1988; Foster and Masel, 1986; Lapinskii et al., 1987). In a previous publication (Ozkan et al., 1990), catalytic oxidation of methanol over a series of first row transition metal oxide catalysts using a gradientless external recycle reactor was reported. All catalysts that were studied exhibited similar activities for methanol conversion, but the supported copper catalyst was found to be considerably more selective to COa. In this paper, we report the complete oxidation of ethanol, acetaldehyde, and a methanol-ethanol mixture over supported catalysts containing oxides of copper, oxides of chromium, and mixed oxides of copper and chromium. All catalysts were prepared by a wet impregnation technique and were supported on y-alumina. Oxidation experiments were performed by using a fixed bed integral reactor 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 external heat- and mass-transfer effects do not disguise the observed kinetics (Serrano and Carberry, 1985). Using a recycle ratio of 50, isothermal reaction data were collected at CSTR conditions. The effect of temperature on catalytic activity and selectivity was examined by varying the reaction temperature in a 50300 OC temperature range. The effects of metal loading and metal composition were investigated for the ethanol oxidation experiments. The catalysts were characterized by nitrogen BET surface area measurements and X-ray diffraction, scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDXA) techniques.
0888-5885/93~2632-1622$Q4.00/00 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 1623
Experimental Section Catalyst Preparation and Characterization. The catalysts used in this study were high surface area oxides of copper, oxides of chromium, and mixed oxides of copper and chromium. All catalysts were supported on 1/8-in. y-alumina pellets (Engelhard Corporation, A1-3438T). Metal nitrates were used as precursors to supply the metal. A two-step wet impregnation of the y-alumina support was the method used to prepare these catalysts. 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 at each step. The metal nitrate solution and the support pellets were contacted in a rotary evaporator (Buchi, Model RE-121) a t 80 "C for 2 h during the first step and 4 h during the second step. The mixture was dried under vacuum after each step. The pellets were then calcinedat 500 OC for 9 h under an oxygen atmosphere. This two-step procedure ensured that the active catalytic species was well dispersed throughout the support, and not on the exterior surface alone. Three different copper catalysts a t mole ratios (moles of metal per mole of y-Al& support) of 0.04, 0.08, and 0.10 were prepared by this technique. Copper-chromium mixed metal oxide catalysts were examined at a total metal loading of 0.08 mol of metal/mol of support. In this case, however, the metal composition was varied and catalysts with Cu:Cr ratios of 1:1, 2:1, and 1:2 were prepared. In addition to the above catalysts, a pure Cr catalyst a t a mole ratio of 0.08 was also studied for purposes of comparison. The catalysts were characterized by nitrogen BET surface area measurements and X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray analysis techniques. The details of the characterization procedures have been described earlier (Ozkan et al., 1990). Oxidation Reactor System. The feed and reactor system used in the oxidation experiments was described previously (Ozkan et al., 1990). The gas feed system consisted of compressed gas cylinders for each material. The followinggases were used as reactants: nitrogen (highpurity grade, Union Carbide), oxygen (extra-dry grade, Linde), 500ppm acetaldehyde in nitrogen (Scott Specialty Gases) and compressed air (high purity, oil-free grade, Linde). In addition to these, C02 (high-purity grade, Linde) and CO (high-purity grade, Linde) were used for product identification and response factor determination. All gas cylinders were maintained a t a delivery pressure of 20 psig, and the flow rate of the feed gas was regulated by four mass flow controllers (MFCs, Tylan Model FC 260). Liquid ethanol or the methanol-ethanol mixture was introduced into the feed system by a Sage syringe pump (Model 341B, Fischer Scientific). The reactor system used in the experiments consisted of a tubular fixed bed integral reactor equipped with an external recycle loop. The reactor was constructed from 316 stainless steel, and measured 25.4-mm 0.d. X 23.6-mm i.d. and 500 mm in length. In order to measure the axial and radial temperature gradients, five Chromel-Alumel, type K thermocouples were placed inside the reactor a t different positions. 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 (3-mm Pyrex glass beads) in a volume ratio of 1:4. 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 (3/8-in.-o.d. tube, 3/4-in.i.d. shell, 1 m in length), a double-diaphragm air pump (Thomas, Model 2107CA14), 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, 5890A). The GC was equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Two six-port valves were used for sample injection, and a four-port valve was used as a column isolation valve. Porapak Q, Hayes Sep Q, 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. Ethanol Oxidation. Oxidation experiments using a feed stream consisting of ethanol, oxygen, and nitrogen were performed using different catalysts. The stainless steel reactor, filled with diluent Pyrex glass beads, was also tested for catalytic activity. Ethanol oxidation over the bare y-alumina support was examined to determine the nature of products that were formed. Two series of supported catalysts were used in these experiments, namely, copper oxide catalysts and copperchromium mixed oxide catalysts. For the copper series, three different metal loadings (0.04,0.08, andO.10 mole ratio) were used. For the copperchromium series the total metal content of the catalyst was held constant (0.08 mol of metal/mol of support),while the metal composition was changed. Three catalysts (2Cu: lCr, 1Cu:2Cr and 1Cu:lCr) were tested in the oxidation experiments. A pure chromium catalyst (0.08 mole ratio) was also studied to make a comparison between copper, copper-chromium, and chromium catalysts. The concentration of ethanol, oxygen, and nitrogen and the recycle ratio were kept constant for these activity and selectivity experiments, while the temperature was varied. The reaction parameters used were as follows: temperature, 50-300 OC; pressure, 8 psig; feed composition, 0.35 % C~HBOH, 3.83% 0 2 , 95.82% N2; recycle ratio, 50; total catalyst surface area, 3000 m2. Acetaldehyde Oxidation. To gain further insight into the reaction pathway, the oxidation of acetaldehyde (the principal partial oxidation intermediate of ethanol oxidation) was studied. Acetaldehyde, supplied as a gas mixture with nitrogen (500 ppm) was used in these experiments. The feed stream consisted of 0.047 % CH3CHO, 2.416% 0 2 , and 97.536% N2. Three different catalysts, copper (0.08 mole ratio), copper-chromium (1mol of Cu:l mol of Cr), and chromium (0.08 mole ratio), were examined for catalytic activity in a 100-300 "C temperature range. The recycle ratio used in these experiments was 50, and the total catalyst surface area used in the reactor was 100 m2. Oxidation of Methanol-Ethanol Mixtures. Very little information is available on the systematic study of catalytic oxidation of volatile organic compound (VOC) mixtures. Reported work usually tends to be wellcharacterized research on pure component oxidation. Catalytic oxidation of a methanol-ethanol mixture was therefore studied in order to understand the behavior of the individual alcohols in mixtures. The methanol-ethanol mixture was prepared by mixing equal volumes of methanol
1624 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 Table I. Results of BET Surface Area Analysis catalyst mole ratio surface area (m2/g) -pAl~Oasupport 1.0 175 copper catalysts 0.04 185 cu cu 0.08 195 cu 0.10 170 copper chromium catalysts 0.08 205 1Cu:lCr 2Cr:lCu 0.08 195 0.08 190 2Cu:lCr chromium catalyst Cr 0.08 200
and ethanol and then stirring the mixture to achieve homogeneity. The reaction parameters used in these experiments were as follows: temperature, 50-300 "C; pressure, 8 psig; feed composition, 0.172% C~HBOH, 0.248% CHsOH, 5.38% 02, and 94.20% Nz; total catalyst surface area, 3000 m2; recycle ratio, 50. Oxidation experiments consisted of a blank run (reactor filled with Pyrex glass beads) and catalytic runs with three different catalysts, namely, Culy-alumina (0.08 mole ratio), CuCrly-alumina (lCu:lCr), and Crly-alumina (0.08 mole ratio). Initial Rate Experiments. The last set of experiments examined the effectof ethanol concentration on conversion and selectivity over a 0.08 mole ratio copper catalyst under initial rate conditions. For these experiments, the temperature was kept constant (130 "C) to give an ethanol conversion less than 10% ,while the ethanol concentration was varied at three different oxygen concentrations. Catalyst surface area used in the reactor was 3000 m2. The percent conversion of ethanol (or acetaldehyde) is defined as (moles of ethanol (or acetaldehyde) consumed per moles of ethanol (or acetaldehyde) in feed) 100%.The yield of product A is defined as (moles of A produced per moles of reactant (ethanol or acetaldehyde) in feed) (1/ y)(lOO%),where y is the ratio of the number of carbon atoms in the reactant to the number of carbon atoms in the product. For the methanol-ethanolmixture oxidation runs, in addition to determining the individual alcohol conversions, an overall conversion was also calculated. Since both alcohols oxidize to give some common products (e.g., COz), all product yields for the mixture oxidation experiments are expressed as a percentage of carbon in the feed that is converted to the specific product. For all oxidation experiments, the carbon balances were 95 % or higher. Results Catalyst Characterization. The BET surface areas of the catalysts used in this study are listed in Table I. The surface areas of the supported catalysts were all found to be close to the surface area of the support material (175 m2/g). The d spacings and relative intensities obtained from the X-ray diffraction patterns permitted identification of the oxide phase present on the alumina. In the case of copper catalysts, the oxide phase of copper present on the alumina support corresponded to CuO, as identified by comparison to the JCPDS files. The Cu-Cr mixed metal catalysts and the Cr catalyst however did not show any identifiable peaks over the diffraction pattern of the alumina, suggesting that the metal ions were highly dispersed in these catalysts. All catalysts were examined under a scanning electron microscope. No new phases or phases different from the alumina were observed for the catalysts. The Cu and Cu-Cr catalysts were characterized using EDXA. Elemental dot maps and SEM images
revealed a high degree of dispersion of the metals throughout the pellet. Oxidation Experiments. Ethanol Oxidation. Blank Reactor Runs: The stainless steel reactor was tested for catalytic activity by performing blank reactor runs. Acetaldehyde, COz, and CO were the major carboncontaining products of ethanol oxidation. Below a temperature of 250 "C, less than 10% ethanol conversion was observed. It was necessary to raise the temperature above 400 "C, to reach conversion levels of 90% or above. The maximum yield of acetaldehyde obtained in blank reactor runs was about 40% at 350 "C, after which acetaldehyde formation dropped rapidly. Ethanol Oxidation over the y-Al203Support: Ethanol conversion increased with increasing temperature, but the nature of products was different. Instead of oxidation products, ethylene and diethyl ether (DEE), which are dehydration products, were detected. Yield to DEE increased at first, reached a maximum, and then decreased. Ethylene was detected at all temperatures higher than 150 "C, while C02 formation took place only at a temperature higher than 225 "C. Oxidation Studies over Culy-AlzOs Catalysts: The effect of metal loading on ethanol oxidation was studied over three different copper catalysts (0.04,0.08, and 0.10 mole ratio). The variation of conversion and product yield as a function of catalyst bed temperature is shown in Figure 1. For all three catalysts, ethanol conversion increased with increasing temperature. At any given temperature, ethanol conversion was greater for the catalyst with the higher metal loading. Significant DEE formation was detected only at the lower metal loading of 0.04 mole ratio. The yield to acetaldehyde over all three catalysts was very similar, differing only in the temperature required for maximum acetaldehyde yield. For the 0.04 mole ratio Cu catalyst, C02 formation took place only at a temperature greater than 150 "C while for the other two catalysts C02 was detected at temperatures as low as 100 "C. At any given temperature, the 0.1 mole ratio catalyst gave the highest yield to COz. Oxidation Studies over Cu-Cr/y-Al& Catalysts: Ethanol oxidation over three supported Cu-Cr mixed metal oxide catalysts was examined for catalytic activity and selectivity. Figure 2 presents the variation of percent conversion and yield with temperature in ethanol oxidation over lCu:lCr, 2Cu:lCr, and 1Cu:2Cr catalysts. As with the copper catalysts, as the temperature increased the ethanol conversion rose sharply over all three catalysts. The other common feature was the yield to acetaldehyde which went through a maximum with increasing temperature. At any given temperature, the ethanol conversion was highest for the catalyst which had 2 mol of Cu/mol of Cr (2Cu:lCr). When the maximum acetaldehyde yield for the three catalysts was compared, again the 2Cu:lCr catalyst was better since it gave the lowest value. This catalyst also promoted ethanol oxidation to COz at a temperature lower than that required for the other two catalysts in this series. Interestingly, traces of CO were observed only for the catalyst containing 2 mol of Cr/mol of cu. Comparison of Catalysts for Ethanol Oxidation: To make a comparison between copper, copper-chromium, and chromium catalysts, oxidation experiments were performed using a 0.08 mole ratio Cr catalyst. The variation of conversion and yield for the pure Cr catalyst is shown in Figure 3. In addition to acetaldehyde and C02, CO formation was significant beyond a temperature of 200 "C. Maximum yield to acetaldehyde was about
Ind. Eng. Chem. Res., Vol. 32, No. 8,1993 1626
.vl 5 '
.C(
-
20
"
so
100
200
150
250
300
50
100
Temperature ( O C)
150
200
250
Temperature ( O C)
(a)
(a) 100 I
50
100
200
IS0
250
i
300
50
Temperature ( O C) (b)
20
300
100
150
200
250
300
Temperature ( O C) (b)
-
50
100
150
200
250
Temperature ( O C) (c) Figure 1. Ethanol oxidation: variation of conversionand yield with temperature. (a) 0.04 Cu; (b)0.08 Cu; (c) 0.10 Cu.
30%. The temperature requiredto oxidize 50% and 90% of the ethanol for five catalysts at the same metal loading (0.08 mole ratio) is shown in Figure 4. The temperature
50
100
150
200
250
Temperature ( O C) (C)
Figure2. Ethanol oxidation: variation of conversionand yield with temperature. (a) 1Cu:lCr; (b) 2Cr:lCu; (c) 2Cu:lCr. for 50%conversion ranged between 185 and 220 "Cfor all
the catalysts, with the Cr catalyst requiring the lowest
1626 Ind. Eng. Chem. Res., Vol. 32,No. 8, 1993
1w Ethanol 01
/
0,08&
"^
u-
1w
50
IS0 ZW Temperature (" C)
Ethanol Oxidation
3w
250
Figure 3. Ethanol oxidation over a 0.08 mole ratio Cr catalyst.
cu
2C" 1Cr
IC" IC,
cr
1CU 2C'
Catalyst ppe
Ethanol Oxidation
250
I
Figure 5. Product yield from ethanol oxidation at 200 OC.
i
cu
xu:1cr
IC": ICr
1cu:zcr
IW 1
I Acetaldehyde Oxidation
Cr
Catalyst Typt
Figure 4. Comparison of the temperature required for 50% and 90% ethanol conversion.
temperature. For 90% conversion of ethanol, the temperature ranged from 225 to 240 O C with the pure Cu catalyst requiring the lowest temperature. The product yield over different catalysts at 200 "C is shown in Figure 5. For all the catalysts with the exception of Cr, acetaldehyde and COz were the only major carboncontaining products. The acetaldehydeyield was greater than the COz yield for the lCu:lCr, the lCu:2Cr, and the pure Cr catalysts. However, for the 2Cu:lCr and the pure Cu catalysts, the COz yields were higher than the acetaldehyde yield at 200 "C. The lowest acetaldehyde yield (approximately 10%)was for the pure Cu catalyst. Acetaldehyde Oxidation. Since acetaldehyde is the principal partial oxidation intermediate of ethanol oxidation, some experiments were conducted using acetaldehydeas the feed. The resultsof acetaldehydeoxidation over three catalysts, namely, Cu, Cu-Cr (1mol of Cu:l mol of Cr), and Cr, all supported on alumina are shown in Figure 6. COz was the only product of acetaldehyde oxidation over all three catalysts. For all three catalysts theconversionlevelwaslessthan20% uptoatemperature of 200 O C . Below 200 OC, the Cu and the Cu-Cr catalysts, exhibited almost similar activity for acetaldehyde oxidation. The CrlyAlzOs catalyst was more active than the other two catalysts studied, below 200 "C. Between 200
"
50
ion
1SO
200
250
Temperature (' C)
Figure 6. Variation of acetaldehyde conversion with temperature over Cu, Cu-Cr, and Cr catalysta.
and 300 "C, however, the catalysts exhibited a totally different behavior. The 0.08 mole ratio Cu catalyst was the most active for acetaldehyde oxidation. This was followed by the Cu-Cr mixed metal catalyst, while the Cr catalyst had the lowest activity. A t a temperature of 250 O C , the copper catalyst gave 80% conversion, the Cu-Cr catalyst gave 72% conversion, while the Cr catalyst gave only 55% conversion. Oxidation of a Methanol-Ethanol Mixture. Blank Reactor Runs: The methanol-ethanol mixture was oxidized in the stainless steel reactor filled with Pyrex glass beads. Formaldehyde, acetaldehyde, CO, and COz were the carbon-containing products of oxidation that were detected and separated. An overall feed conversion of 76% was obtained at a temperature of 390 "C. Oxidation Studies ouer Cu, Cu-Cr, and Cr Catalysts: Oxidation of a methanol-ethanol mixture over three different catalysts (Cu, 1Cu-ICr, and Cr) was studied in the temperature range between 50 and 300 O C . The metal loading for all three catalysts was held at 0.08 mol of metal/ mol of support. The results of the alcohol mixture
Ind. Eng. Chem. Res., Vol. 32, No. 8,1993 1627 275 I mcCmvTemp
Methanol-Ethanol Mixture Oxidation
Elhanol conv.
50
2.0
100
3ca C"
Ethanol conv. Acet. fonnation
e,
.$ v
9 m 0.5
0.0
100
2w
150
250
3w
250
300
Temperature (" C)
(b)
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Ethanolconv. Acet formation COZformation
0
1.0
2
v
22
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...
50
100
150
m
C*
Catalyst Type
-2
50
Cu~Cr
Figure 8. Comparison of the temperature required for 50% and i
A
-8
250
Temperature ("C) (a)
,
E $ 1.5
200
150
Temperature ("C)
(d Figure 7. Methanol-ethanol mixture oxidation over (a) 0.08 Cu, (b) lCu:lCr,and (c) 0.08 Cr.
oxidation over a supported copper catalyst are shown in Figure 7a. Since the feed contains unequal amounts of methanol and ethanol, the rate of conversion of reactants
90% overall carbon couveraion and 50% COEyield.
or rate of formation of products is plotted on the y-axis (pmol/(m2-h)). Acetaldehyde and COz were the only carbon-containing products of oxidation. At any temperature higher than 200 "C the rate of methanol conversion was greater than the rate of ethanol conversion. Formaldehyde and CO were not observed in the temperature range under investigation. Over the Cu-Cr catalyst (1mol of Cu:l mol of Cr), the oxidation behavior was quite similar to that observed on the Cu catalyst (Figure 7b). Acetaldehyde formation, however, was quite higher, reaching a maximum value of 13.5% at 200 "C. No product formation was observed below a temperature of 100 "C. Formaldehyde and CO were not observed in the reaction products. Examining the mixture oxidation over Crly-AlzOS catalyst (Figure 7c), again, a similar behavior was observed. For this catalyst however, in addition to acetaldehyde and COz as the carbon-containing products, CO was produced at temperatures greater than 150 "C. Rate of acetaldehyde formation went through a broad maximum with a maximum yield of approximately 14% at 160 "C. Formaldehyde wm not detected in the reaction producta. Catalyst Comparison: Figure 8shows the temperature required for 50% and 90% overall carbon conversion and the temperature required for 50% of the total carbon entering the system to get converted to COz. Three catalysts, namely, Cu, Cr, and Cu-Cr (1mol of Cu/mol of Cr), have been compared. The Cr catalyst required the lowest temperature for 50% conversion (171 "C) while the other two catalysts required about 200 "C. The temperature required for 90% conversion for all three catalysts ranged between 225 and 240 OC, with no major differences existing between the Cu and the Cu-Cr catalysts. The pure Cr catalyst however required a higher temperature. For 50% carbon conversion to CO2, the temperature required increased as one went from pure Cu to pure Cr. The conversion of total carbon to products in methanolethanol mixture oxidation a t 200 OC is shown in Figure 9. For all three catalysts studied, acetaldehydeand CO2 were the major carbon-containing products. No formaldehyde was produced for any of the three catalysts studied, under reaction conditions. Oxidation ofthealcohol mixture over the Cr catalyst also produced CO as a partial oxidation intermediate. Acetaldehyde formation was the lowest for
1628 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993
squares analysis showed the order of the reaction was 0.2 with respect to ethanol concentration.
Methanol-Ethanol mixtun oxidation
50
40
30
20
IO
" Y
cu-Cr
C"
Cr
CaralystTypc
'
Figures. Conversionoftotalcarhntapducta inmethanol-ethanol mixture oxidation at 200 O C .
'"
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-B
2
I
I
Initial Rate Experiment
-
Oxygen conc. = 30 40
Temperalum = 130 'C
16.6 -
2,-. I
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'
'
'
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.
Discussion The BET surface areas for all the catalysts (except the 0.10 mole ratio catalyst) was higher than the surface area of the 7-alumina support. The SEM micrographs and EDXA dot maps showed the physical appearance of the catalyst surface and the location of the different metals. X-ray diffraction permitted identification of the oxide phase for the Cu catalysts. The oxidation experimentsin this work were performed with a recycle ratio of 50 to simulate CSTR behavior in the reactor. Using the interparticle heat- and masstransfer limitation criteria outlined by Carberry (1976, 1987), it was concluded that external heat- and masstransfer effects were negligible. The magnitudes of the intraparticle concentration and temperature gradients were examined using the following criteria formulated previously by Carberry (1976, 1987):
"
2.0
'
'
.
'
I
2.5
-In Cethanol [mollm'l Figure 10. Effect of ethanol concentration on the initial reaction rate over 0.08 Cul-pAkO~.
the Cu catalyst, and this catalyst also gave the highest carbon conversion to COZat 200 "C. InitialRate Experiments. Arateequationforethanol oxidation at three different oxygen concentrations was determined. Oxidation experimentswere performed over the Cu catalyst (0.08 mole ratio) by keeping the oxygen concentration constant at an excess level and varying the ethanol concentration. The kinetic data were collected at low conversion levels to simulate the initialrateconditions. Thereactionratewasobtainedfromasimplemassbalance for a CSTR YCethX -rate = -
e
where Y is the volumetric feed flow rate, 0 is the surface area of the catalyst in the reactor, Csth is the ethanol concentration, and X is the overall ethanol conversion. Data were collected at four different ethanol concentrations and at three different excess oxygen concentrations. All of the data were taken at 130 OC. Figure 10 shows the dependence of the rate on ethanol concentration at an oxygen concentration of 30%. The results of a least-
ACi = 0 if X/((1- X)a2De,Tt < 1.0 AT-
= (-AIf)D&JA
(2)
(3)
where ACi = intraparticle concentration gradient, X = conversion, a = pellet surface area to volume ratio, Der = effective diffusivity, ATm- = maximum intraparticle temperature gradient, C. = surface concentration, A = thermal conductivity of the pellet, AH = heat of reaction, and T = residence time. Using the above criteria, the maximum temperature gradients in the pellet were found to be less than 1 'C under the reaction conditions used in this study. The concentration gradientswithin the pellets were also found to be negligible for conversion levels less than 75 %. The intraparticle concentration gradients became somewhat significant only at conversion levels 80% and above. The details of these calculations have been presented elsewhere (Kueller, 1989; Rajesh, 1991). Using this criteria, it was made certain that the data obtained represented the intrinsic kinetics of the system. Reaction products were more difficult to quantify at low concentrations (lower temperatures) due to imperfect detector responses, and therefore carbon balances were generally slightly off at lower conversion. Carbon balance closure was generally obtained to within 5 % for all experiments. Due to the inherent inaccuracies involved in auantifvine water.. oxveen and hvdroeen balances were lesi reliable. All the catalysts used exhibited large adsorption cauacities for the reactants, and the amount adsorbed was found to be sensitive to temperature. A t lowtemperature the adsorption-desorption equilibrium was approached slowly (4-5 h) upon exposure to the feed stream. Steadystate conditions were reached more quickly at higher temperatures. Desorption of the alcohol and aldehyde species was also observed as temperature was increased over the catalyst surface. Blank reactor experiments indicated that the contribution from the homogeneous reaction or from the activity of the reactor walls was not significant until very high temperatures (350 "C) were reached. On the basis of the products formed during the reaction experiments, the oxidation of ethanol over the various catalysts tested involves one or more of the following reactions: _I
- -
I
C,H,OH
+ 30,
-
2C0,
+ 3H,O
(4)
---
+ (1/2)0, C,H,OH + 20,
C,H,OH
C,H,OH
Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 1629 CH,CHO
+ H,O
2CO + 3H,O
+ H,O (C,H,),O + H,O C,H,
(5) (6) (7)
2C,H,OH (8) Ethanol oxidation over the yAlzO3 support resulted in dehydration products, namely ethylene and diethyl ether (DEE), through reactions 7 and 8. Both these products are formed by base reactions (dehydration) and are promoted by catalysts having acidic properties. Alumina contains Lewis acid sites (AP+cations) which are involved in reactions of alcohols to ethers. Alumina also contains Brcansted acid sites which promote dissociation of the alcohol hydroxyl group during olefin formation. Hence, the formation of ethylene and DEE on the alumina support can be explained by the presence of acid sites present on the surface. Ethanol oxidation over the supported metal oxide catalysts resulted in partial and total oxidation products (acetaldehyde, COz, CO, and water) through reactions 4, 5, and 6 indicating that the metal oxides were responsible for the oxidation reactions. Both acetaldehyde and COZ were produced simultaneously over all the catalysts in the temperature range under investigation. It is therefore possible that some of the ethanol in the feed is oxidized in sequential fashion to acetaldehyde and then to COz, illustrated by the following series mechanism: C,H,OH
-
CH,CHO
- CO
CO,
(9) However, direct oxidation of ethanol to COZ(i.e., without the formation of acetaldehyde) may also occur according to the reaction
-
C,H50H 2C0, (10) Examining ethanol oxidation over the Cu catalysts, it was found that, at low metal loading (0.04 mole ratio), there was some DEE being formed. This could be due to the fact that, at lower metal concentrations, all of the alumina acid sites were not completely blocked. At higher metal loading (0.08 and 0.10 mole ratio), the amount of DEE,that was formed was very low (
