2328
Ind. Eng. Chem. Res. 1992,31, 2328-2331
Effect of Water on the Oxidation of 2-Butanone on Vanadium Oxide Ismat Jahan and Harold
H.Kung*
Ipatieff Laboratory and Department of Chemical Engineering, Northwestern University, Evamton, Illinois 60208
The oxidation of 2-butanone was studied over V205and VzOsmodified by Cs and K a t 250 "C with various partial pressures of water in the feed. Acetic acid and acetaldehyde were the most abundant products. Other major products were 2,3-butanedione and carbon oxides. The addition of water to the feed increased the activities of the catalysts as well as the selectivities to acetic acid, acetaldehyde, and 2,3-butanedione. Modification of V205with alkali metal ions increased the activity. The increase in activity due to water and alkali metal ion modification correlated well with the decrease in weight loss in thermogravimetric analysis (TGA) measurements in oxygen and with the surface carbon/vanadium ratios as determined by X-ray photoelectron spectroscopy (XPS). Thus the main effect of water and alkali metal ion modification was to reduce the formation of carbonaceous deposits on the catalysts.
Introduction The selective oxidation of butene to acetic acid and acetaldehyde can be carried out over catalysts based on V-Mo oxide (Seiyamaet al., 1977; Takita et al., 1977). The reaction involves first the hydration of butene to 2-butanol followed by oxidation to 2-butanone, which is then oxidized to acetic acid and acetaldehyde. Water has been shown to greatly increase the reaction rate, presumably by facilitating the formation of 2-butanol (Seiyama et al., 1977). Little is known of the step in which the C-C bond is cleaved and acetic acid and acetaldehyde are formed. Conceivably water could also be involved in the formation of the OH group of CH,COOH. In addition to the formation of acetaldehyde and acetic acid, the oxidation of 2-butanone over VPO (Ai,1984a)and VMoO (Takita et al., 1977) also resulted in the formation of 2,3-butanedionein a parallel reaction and other products including carbon oxides. Both acetic acid and 2,3-butanedione are useful i n d u s t d products. Since the formation of 2,3-butanedione probably does not involve water, whereas that of acetic acid might, it will be interesting to investigate how water affects this reaction. In general, in the oxidation of 2-butanone,the selectivity for the scission reaction and for the 2,3-butanedione formation correlated with the acidity and basicity of the oxides when water is present in the feed. Thus acidic oxides such as MOO, and V2O5 produce acetic acid and acetaldehyde with high selectivities,whereas over weakly basic or amphoteric oxides (NiO and Co3O,), the formation of 2,3-butanedione predominates (Takita et al., 1987). Over a VPO catalyst, increasing the partial pressure of water increased the conversion and the yields of acetaldehyde and acetic acid and decreased the yield of 2,3butanedione (Ai, 1984a). The effect on selectivity was attributed to increased Brernsted acidity, but the effect on activity was not explained. A peroxide-like intermediate was proposed for both the selective scission reaction and the formation of 2,3-butanedione. The objective of this investigation was to attempt to explain the effect of water on this reaction over V20s and alkali metal ion-modifiedV2OP We report here the results of reaction studies and TGA and XPS analyses of the catalysts. Experimental Section Catalyst Preparation. V205 (Aldrich, 99.6+ % ) was used as the catalyst without further purification. The alkali metal cation-modified V20, catalysts were prepared by first dissolving appropriate amounts of CsN03 (Aldrich,
99.99%) or KNOB(Aldrich, 99.9%) in 100 mL of doubly distilled water. One hundred grams of Vz05 was added to this solution, which was then evaporated with constant stirring to a paste at approximately 70 "C. The paste was calcined at 540 "C for 4 h. The catalysta 3CsVO and lOKVO contained 3 g of Cs and 10 g of K, respectively, in 100 g of V2OP The amounta of Cs and K were confirmed by atomic absorption spectrometry. The BET surface areas of the catalysts were V205, 3.1; 3CsV0, 1.0; and 10KV0, 1.0 m2/g. Reaction Studies. Reactions were run at 250 "C in a conventional flow system with a quartz U-tube microreactor of 15 mL containing a fixed catalyst bed. In the absence of a catalyst, there was negligible reaction. The standard feed was 3 vol % 2-butanone (Aldrich, 99.9%), 6 vol % oxygen (Bennett, high purity), and the balance He (Bennett, high purity) and HzO. The partial pressure of water was varied from 0 to 24 vol % . All produds were analyzed by on-line gas chromatography using thermal conductivity and flame ionization detectors in series and two columns in parallel: a 6-ftPorapak T column operated between 70 and 190 O C to separate O2 + CO, COz,HzO + acetaldehyde, 2-butanone, 2,3-butanedione,and acetic acid, and a 12-ft molecular sieve 13X column at room temperature to separate O2 and CO. The reaction runs were Canried out for more than 8 h before steady-statedata were collected. Catalyst Characterization. XPS analyses were performed using a VG Scientific spectrometer system with an Al Ka (1486 eV) monochromaticX-ray source. All samples were supported on indium foils (Aldrich, 99.99%). The sensitivity factors, as provided by the manufacturer, were 0.2 for C Is, 1.2 for V 2p3/2, and 1.0 for Cs 3d peaks. Thermogravimetric analyses (TGA) were performed using a TG2 Dupont 9900 instrument. Approximately 100 mg of catalyst was used in each analysis. The used samplea were those that had been used for 26 h each in the oxidation reaction. Hence they had been exposed to the same quantities of 2-butanone. After 26 h of reaction, the 2butanone, H20, and O2 flows were turned off. About 40 mL/min He was passed over the catalysts at the reaction temperature for 1 h to remove weakly adsorbed species. TGA was carried out in 02.During a TGA experiment, the catalyst was heated from room temperature to 150 OC at 10 "C/min. The catalyst was allowed to equilibrate at this temperature for 30 min before the temperature was again increased to 300 "C at 10 "C/min. X-ray diffraction was performed using a Rigaku diffradometer (Denke Ltd.) with Cu Ka radiation. The X-ray patterns of the 3CsVO and lOKVO samples were the same as that of Vz05. No
0888-5885/92/2631-2328$03.00/00 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 10, 1992 2829
C 0 N
50
B
401
I
X
v
E
-- I
I
301
A
R
0
N
10
% i -
,
-I--_--.--L--
1
d
'
40
j
,
1
0
,
,
,
3
2
,
I a
6
4
W/F x 100 (g catalyst-min/mL) Figure 3. Effect of modifying cation on selectivity for acetic acid and acetaldehyde. T = 250 "C;feed composition = 3% 2-butanone, 6% 02,15% HzO, and balance He; = 10KV0, A = ~CSVO,+ VZO5.
c I
0
vN E S I
*'
8
A
A
A
l
+
401
I '
m
A
+
A
0 1 %
2ol
L
1
A
I
~
+
+ I
kd
oL-----l o
1
!2
3
2
4
8
5
W/F x 100 (g catalyst-min/mL)
v)
Figure 4. Effect of modifying cation on selectivity for 2,3-butanedione. T = 250 "C; feed composition = 3% 2-butanone, 6% 02,15% HzO, and balance He; = 10KV0, A 3CsV0, + = VzOp 400
120
T
E
1 19.8
w
M -900
119.6
R
E I
0
119.4
T
119.2
H
R
E
f
116.0
0
E
26
so
76
100
TIME ( m i d Figure 5. TGA of V,05 wed without water in the feed. O2flow = 15 mL/min.
stream of 15 d / m h of Op The three uaed V205 eamplea were used in reaction runs with 0,16, and 25% water, and the two used Cs-modified V2O6 were used with 0 and 16% water in the feed. Figure 5 show the TGA profile of the V2O5 used without water. It showed a weight l w when the sample was first heated due to l a of adeorbed water. Then there was another weight loes when the temperature was raised from 150 to 300 OC followed by a weight gain. The weight loss was attributed to the removal of surface
2330 Ind. Eng. Chem. Res., Vol. 31, No. 10, 1992 Table I. Conversion and Product Distribution on VzOs, 3CsV0, and lOKVO (Oxygen/2-Butanone = 2/1, Temperature = 260 "C) selectivities,"**% water, % W/F,Cg-minlml conv, % CH3CHO CH3COOH co COZ CH3COCOCH3 V206Catalyst 0 3.51200 9 f 1.5 35.0 34.6 6.6 11.3 12.5 28.5 36.0 8.7 16.7 10.0 3.5/100 18 f 1 26.0 42.0 8.0 14.2 9.5 3.5150 40 f 1 15.0 36.0 16.0 32.6 0.5 6.0150 71 f 1 27.0 43.0 1.2 4.5 23.9 15 2.0/100 10 f 1.5 28.0 47.0 2.3 7.1 15.1 3.51100 39 f 1 73 f 2 6.8 65.0 5.7 12.3 9.5 5.81100 4.5150 look2 4.2 73.9 6.9 15.1 0.0 0
15 0
15
61100 61100
30 f 1 75 f 2
3CsVO Catalyst 29.9 12.4
32.3 41.4
4.5 4.0
12.9 15.4
20.4 26.7
1/100 1/100
4 f 0.2 26 f 1
lOKVO Catalyst 32.7 8.6
43.3 58.9
3.1 2.3
10.8 15.3
10.0 14.1
"Small amounts of unidentified products were not included. bError in selectivities are f1.5%. 'Weight of catalystltotal flow rate. Table 11. TGA and XPS Data of Used and Fresh Catalysts" reaction condition catalyst water, % conv, % Vz05(used) 0 18 15 33 25 40 3CsVO (used) 0 30 15 60 V205g(fresh)
TGAb wt loss, % 0.78 f 0.05 0.31 f 0.05 0.15 f 0.05 0.28 f 0.05 trace
A B
XPS data In/Vc*d 1.0 2.2 0.1 2.3 0.6
CS/V'
0.2 0.3
5.4 15 9.5
C 3CsVOh (fresh)
0.9 2.0 1.2
f f
A B
C/V" 3.7 2.2 1.3 1.7 1.2
0.4 0.4
1.0 0.8
Catalysta used for 26 h of reaction. Weight loss during heating from 150 to 300 "C in 15 mL/min 02.Indium was used to support the Catalysts for XPS. Area ratios, *20%. e Mole ratios, *20%. 'In not looked for. Sample A, no pretreatment; sample B, heated in air at 520 OC for 2 min; sample C, heated in air at 550 OC for 45 min. Sample A, no pretreatment; sample B, heated in air at 550 OC for 30 min.
carbonaceous species by reaction with O2and the weight gain to reoxidation of the catalyst. The TGA profiles of other catalysts showed similar features but of different magnitudes. Figure 6 shows that for 3CsVO used in 15% water in which the weight loss when the sample was heated between 150 and 300 OC was much smaller, whereas the subsequent weight gain was much larger than the profile in Figure 5. Table I1 summarizes the weight losses of the catalysts due to combustion of surface carbonaceous species. On both the V206 and 3CsVO catalysts, an increase in the partial pressure of water in the feed stream reduced the weight loss in the TGA, Le., the amount of surface carbonaceous species. A comparison of the data for 3CsVO and V206showed that the Cs-containing sample had fewer surface carbonaceous species. When the same set of ueed catalyets was examined by XPS,their surface C/V ratios were found to decreaee with increasing partial pressure of water in the feed and were lower for the Cs-modified catalyst (Table 11). These trends were consistent with the conclusion from the TGA experiments that there was leas carbon deposit on the surface of Cs-modified V205and on catalysts used with water in the feed. The XPS data of freah catalysta are also shown in Table 11. These catalysts had subatantially lees coke than the used ones. The data for the fresh samples after different pretreatments showed that the increase in the C/V ratios after use was much larger than the variation due to handling of the sample.
115
1 T E M
w E
-300
'14
I
G
H T
- 200
113
u
R E
rn 9
E R A T
- 100
112
d
e Q
111 0
50
100
150
200
TIME ( m i d Figure 6. TGA of 3CsVO used with 15% water in the feed. O2flow = 15 mL/min.
Discussion The results reported here indicate that the steady-state activities of V206and K- or Cs-modified V2O6increased with increasing partial preasure of water in the feed in the oxidation of 2-butanone. A similar effect has been observed with the VPO catalyst for the same reaction (Ai, 1984a). The addition of steam in the oxidation of hydrocarbons can either increase or decrease the activity, depending on the hydrocarbon and the catalyst (Ai, 1984a,b; Arnold and Sundaresan, 1988; Liu et al., 1982).
Ind. Eng. Chem. Res., Vol. 31, No. 10, 1992 2331
For example, Takita et al. (1977) reported that the rate of butene oxidation over a VMoO catalyst was increased by the addition of steam in the feed, presumably because water generates Bransted acid sites which catalyze the hydration of butene to sec-butyl alcohol. On the other hand, water blocks the active sites and decreases the activity for the oxidation of butane over a VPO catalyst (Arnold and Sundaresan, 1988). In this study, the change in the activity of the catalyst can be correlated with the amount of surface carbonaceous species, as indicated by the weight loss per unit weight of catalyst in the TGA experiments and supported by the surface C/V ratios (Table 11). The higher the activity, the less was the weight loss and the smaller was the C/V ratio for both the V205and 3CsVO catalysts. Thus water enhances the activity of the catalyst by reducing the formation of surface carbonaceous species. Modifying V205 with an alkali metal ion such as Cs reduces the quantity of carbonaceous deposits on the catalyst surface, as shown by the data in Table 11. Correspondingly it also increases the activity of the catalyst when compared on a basis of per unit surface area, since the data in Table I show that 3CsVo was as active as V205 on a weight basis but its surface area was only 1 / 3 that of the latter. Similarly,the addition of K also increased the activity of the catalyst on a surface area basis. This effect of the alkali metal ion is similar to that observed in steam reforming of hydrocarbons, where the addition of alkali metals results in reduced amounts of coke on the catalyst (Hadden et al., 1991). To determine whether water could react with the surface carbonaceous deposits, the following experiment was conducted. One gram of V205 that had been used in the oxidation of 2-butanone with no water in the feed was treated with a stream of 75 mL/min He containing 25 vol % water at 250 OC for 5 h, and the exit stream was passed through an ice trap. According to the TGA and XPS results (Table 11),this catalyst contained a large amount of coke. The liquid collected in the ice trap was analyzed by gas chromatography to contain 2-butanone, 2,3-butanedione, acetic acid, and some unidentified products. TGA performed on this used catalyst before the water treatment showed a loss of 0.78 wt % due to combustion of carbonaceous deposit (Table 11). After the water treatment, the loss was 0.41 w t %. Thus about 50% of the carbonaceous deposit was removed by reaction with water to produce products, the majority of which were those of the oxidation of 2-butanone. It was observed here that the addition of K or Cs increased the activity of VZOp Takita et al. (1980) have reported similar results upon addition of 2 atom % Na20 to V2OP These observations suggest that the enhancement of activity by water is not due to the generation of additional Bransted sites, as was proposed by Miyata et al. (1989). Instead, one might suggest that Bransted acidity may be responsible for polymerization of surface intermediates that poison the catalyst. Thus the effect of water can be explained by the following scenario. Increasing water partial pressure in the feed results in additional Brernsted acidity on the catalyst surface. The increase in Bransted acidity could lead to increased carbonaceous deposits as suggested above. The relative rates of the two reactions, i.e., removal of the carbonaceous deposits by reaction with water and formation of those species due to increased surface Bransted acidity, determine the effect of water on the activity. The observed increase in activity with increased water partial pressure then signifies that the reaction of water with
surface carbonaceous deposits or their precursors is faster than their rate of formation. Relative to the case of no water in the feed, the presence of water increased the selectivityto scission products and 2,3-butanedione over V205and decreased the selectivity to COXwhen compared under comparable conversions (Table I). This could be because water competes with the partial oxidation products for adsorption sites and thereby reduces combustion due to secondary reactions. The effect of secondary reactions can be significant, since the selectivities for acetic acid and COXincreased at the expense of acetaldehyde and 2,3-butanedione on increasing conversion. Conclusion It is shown in this study that, in the oxidation of 2-butanone over V205 and Cs- and K-modified V205 catalysts, water reacts with carbonaceous deposits on the catalyst surface and their precursors and renders the catalyst more active. Unlike the effect on activity,the presence of steam shows only minor effects on the selectivity. Thus Bransted acid sites are not responsible for selective C-C bond scission. Acknowledgment This work was supported by the Division of Chemical Sciences,Basic Energy Sciences,Department of Energy. The TGA experiments were performed with the help of D. Tomczak. Registry No. Vz05,1314-62-1; Cs, 7440-46-2; K, 7440-09-7; CH&(O)CH&H3,78-93-3; CH3C02H, 64-19-7; CH,CH(O), 7507-0; CH&(O)C(O)CH,, 431-03-8; HzO, 7732-18-5.
Literature Cited Ai, M. Oxidation of Methyl Ethyl Ketone to Diacetyl on VzOs-PzOs Catalysts. J. Catal. 1984a, 89,413. Ai, M. Catalytic Activity of Cesium Salt of 12-Molybdophosphoric Acid Containing a Vanadium Promoter in Selective Oxidation. J. Catal. 198413,85,324. Arnold, E. W.; Sundaresan, S. Effect of Water Vapor on the Activity and Selectivity Characteristics of a Vanadium Phosphate Catalyst towards Butane Oxidation. Appl. Catal. 1988,41, 225. Hadden, R. H.; Howe, J. C.; Waugh, K. C. Hydrocarbon Steam Reforming Catalysts-Alkali Induced Resistance to Carbon Formation. Catalyst Deactivation 1991; Bartholomew, C. H., Butt, J. B., Eds.; Elsevier: Amsterdam, 1991; p 171. Liu, R. S.; Iwamoto, M.; Lunsford, J. H. Oxidation of Methane over Supported MOO,Catalyst. J. Chem. SOC., Chem. Commun. 1982, 78. Miyata, H.; Kohno, M.; Ono, T.; Ohno, T.; Hatayama, F. Structure of Vanadium Oxides on ZrO, and the Oxidation of Butenes. J. Chem. SOC.,Faraday Trans. 1 1989,85,3663. Seiyama, T.; Nita, K.; Maehara, T.; Yamazoe, N.; Takita, Y. Oxyhydrative Scission of Olefins. I. Oxidation of Lower Olefins. J. Catal. 1977,49, 164. Takita, Y.; Nita, K.; Maehara, T.; Yamazoe, N.; Seiyama, T. Oxyhydrative Scission of Olefins 11. J. Catal. 1977, 50, 364. Takita, Y.; Inagawa, K.; Yamazoe, N.; Seiyama, T. Oxidation of Methylethylketone over Metal Oxide Catalysts. Oxid. Commun. 1980, 1, 135. Takita, Y.; Inokuchi, K.; Kobayashi, 0.;Hori, F.; Yamazoe, N.; Seiyama, T. Oxidation of Ketones over Metal Oxide catalysts. I. Catalvtic Svnthesis of Biacetvl from Methvl Ethvl Ketone. J. Catai 1984 90,232. Takita, Y.; Hori, F.; Yamazoe, N.; Seiyama, T. Oxidation of Ketones over Metal Oxide Catalysts. 11. Biacetyl Synthesis over Co3O4Lanthanide Oxide Catalysts. Bull. Chem. SOC. Jpn. 1987, 60, 2757.
Received for review October 7, 1991 Revised manuscript received March 20, 1992 Accepted July 21, 1992
