.6
-
0) v1
Ln
.-
U
..5 z -6 Static
0 c
.-E
U
1.4
5 m
-
5-
0 .-c m
1.3
1 5
10
15
, 100 Parts Adiprene LlOO 12.5 Parts MOCA
rr
0 l1 100
System
60
70
80 90 Processing Temperature,
100
1 3
OC
20
Wt. Percent Adiprene L315
Figure 31. Effect of composition of Adiprene L315-Adiprene L100 solution on compression-deflection characteristics
Figure 32. Effect of processing temperature on compressiondeflection characteristics
physical characteristics can be obtained predictably. I t is now feasible to engineer shock-mitigating systems with polymeric materials that are comparable in sophistication t o mechanical systems.
to Herbert F. Minter and Richard G. Black, Westinghouse R&D Center, for reviewing the manuscript.
Acknowledgment
The authors thank George E. Rudd, Westinghouse R&D Center, who performed much of the mechanical testing and contributed t o the design of the upper region liner pads. Appreciation is also expressed t o William Volz and Donald Wold, Westinghouse Missile Launching and Handling Department, who contributed to the mechanical analysis of the launcher requirements. We are grateful
Literature Cited
Mendelsohn, M . A., Black, R . G., Runk, R. H.. Minter, H.F., J . Appl. Polym. Sei., 9, 2716 (1963). Mendelsohn, M. A., Black, R. G., Runk, R. H.. Minter, H. F., J . Appl. Polym. Sci., 10, 443 (1966). Mendelsohn, M. A . , Runk, R . H., Connors, H. J., Rosenblatt, G. B., Division of Organic Coatings and Plastics Chemistry, 160th Meeting, ACS, Chicago. Ill.. September 1970. RECEIVED for review August 1 2 , 1950 ACCEPTED October 29. 1950
CATALYST SECTION
Catalytic Oxidation of Olefins over HalogenModified Copper Oxide Catalysts R. S. Mann' and K. C. Yao Department of Chemical Engineering, Uniuersit? of Ottarra. Otiaica 2, Canada T h e field of heterogeneous vapor phase catalytic oxidation of hydrocarbons to useful oxygenated products has advanced rapidly in recent years. Though the processes for producing acrolein and methacrolein by vapor phase oxidation of unsaturated hydrocarbons in the presence I
T o whom correspondence should be addressed.
of solid catalysts, such as copper oxide and bismuth molybdate, are well known, these processes have not proved entirely satisfactory, primarily because of' difficulties in avoiding complete oxidation to carbon oxides. During the last 16 years, several patents (Besozzi et al., 1967: Brill et al., 1966; Cheney and Breier. 1939: Cheney and Stephen, 1957; Hadley, 1953, 1957) have Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
25
Based on the concept that t w o types of active centers exist on the copper oxide catalyst on which oxygen and olefin are selectively chemisorbed, vapor phase catalytic oxidation of propene, isobutene, and its primary oxidative product, methacrolein, was investigated over modified copper oxide catalysts in a tubular flow reactor. The skeleton catalyst (copper oxide) was modified with additives, such as bromine, chlorine, and iodine, all having electronegativities higher than that of copper. All these modified catalysts show marked increase in selectivity for methacrolein production as compared t o the skeleton catalyst. The results are explained on the basis of the partial oxidation of isobutene to methacrolein being a p-type reaction, and its further oxidation to carbon dioxide, an n-type reaction under optimum conditions. The promotional effects of different modifiers are discussed. The active phase of the catalyst for aldehydes formation has been identified as cuprous oxide.
described the use of selenium, sulfur, halogen, and their compounds in conjunction with a skeleton catalyst to obtain an improved catalyst for selective oxidation. However, neither any published information nor a generalized theory indicating the role of such modifier and its optimum amount in enhancing the selectivity of the skeleton catalyst is available. Margolis et al. (1962) studied the effect of several additives on the copper oxide catalyst for propene oxidation. They correlated the change in work function, as induced by additives, the activation energies for acrolein and carbon dioxide formation, and the selectivity of the reaction. They found that in the presence of additives having electronegativities higher than the copper oxide catalyst (such as S and Cl), the work function of the catalyst and the selectivity for acrolein increased. However, they did not consider the effect of quantity of modifiers on the variation in product distribution and the selectivity of the catalyst. The stability of the modified catalyst was not mentioned, though in general the additives are unstable a t reaction temperature and are continuously lost from the catalyst surface. The catalytic oxidation of isobutene (2-methylpropene) over selenium dioxide- and sulfur dioxide-modified copper oxide catalyst has been investigated by Mann and Yao (1967, 1969). They found that in the presence of very small amounts of selenium dioxide or sulfur dioxide, the selectivity of the catalyst was much improved. It has been suggested that by controlling the distribution of the density of positive holes and free electrons on the catalyst surface, with a controlled valency of the skeleton catalyst, a high selectivity in the partial oxidation would be attained. The purpose of the present work was to extend the work further and to include other olefins and other electronegative gases, such as chlorine, bromine, and iodine, to a better understanding of the promotional effect of these modifiers. Experimental
Apparatus and Procedure. The catalytic air oxidation of olefins was investigated in an isothermal integral flow reactor. The equipment was made of 316 stainless steel and was basically similar to the one used earlier (Mann and Yao, 1969). The flow rates of air and olefin were measured by calibrated rotameters and they were well mixed before they entered the preheating section. The reactor was made of 6-inch-long and -inch-0.d. stainless steel tubing. A stainless steel porous plate which served 26 Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
as the support of the catalyst bed was welded a t the bottom of the reactor. The reactants with the added modifier passed upward through the catalyst bed. The reactor was immersed and heated in a fluidized bed furnace. The hot effluents from the reactor were directly introduced into the Perkin-Elmer Vapor Fractometer for determining aldehydes and water. The exit gases from the high temperature sampling valve of the Fractometer were led to the condenser and traps to remove water and aldehydes and then injected into the Fisher Gas Partitioner for determining carbon oxides, oxygen, nitrogen, and olefins. The modifiers were introduced continuously into the system via a feed preparation unit. Where bromine was used for doping, compressed methyl bromide gas obtained from the Matheson Co., with a purity of 99.5'c minimum, was used as the source. The air-bromide mixture was prepared in a static blending system and the amount of bromine in the mixture was controlled by the partial pressure of methyl bromide. The mixture (41.43 ml/minute STP) was admitted to the preheater through a calibrated rotameter and further mixed with the air and olefin streams from the feed preparation unit. When chlorine and iodine were used as the modifiers, tetrachloroethylene (minimum purity 99.9%) and propyl iodide diluted with research grade n-heptane were used as the sources. The mixtures were introduced continuously and precisely by a motorized syringe pump (Harvard Instrument Co.) into a Hamilton hot inlet, and were carried away by the preheated air stream. Where methacrolein oxidation was investigated in the presence and absence of modifiers, reagent grade methacrolein was introduced into the preheater through another syringe pump and vaporizer, and carried to the reactor by the preheated air stream. During the runs, though the flow rate of the olefin charged into the reactor was maintained constant, different feed ratios (oxygen-olefin) R, were obtained by adjusting the air flow rate, while the reciprocal of space velocity was varied by changing the amount of catalyst, and the weight ratios of the modifier to olefin in the feed, 2, were controlled by the dilution ratios or speed of syringe pump and size of the syringe. Analysis. The reaction products and the unreacted reactants were analyzed with two gas chromatographs. The column used for the separation of aldehydes and water was 4 meters long and consisted of 10% by weight of Carbowax 1500 coated on Teflon powder. The rest of the components were analyzed by a Fisher Gas Partitioner
containing two columns in series. The first column was packed with 6-foot-long, 3 0 7 HMPA (hexamethyl phosphoramide) coated on Celite diatomaceous silica, followed by 1-foot-long 30% DEHS (di-2-ethylhexylsebacate) coated on Celite diatomaceous silica. The second column was packed with 7-foot-long molecular sieve 5A, followed by 5-foot-long uncoated Celite diatomaceous silica. Nitrogen gas was used as the key component because of its chemical inertness. Catalysts. The supported copper oxide catalyst was prepared as reported earlier (Mann and Yao, 1967), except that the chlorine- and iodine-modified copper oxide catalysts were supported on inactivated alumina with a surface area less than 1 m' per gram, supplied by the Norton Co. Inactivated alumina was used instead of pumice stone to remove the heat of reaction more easily. Results and Discussion
Experimental data were collected by means of a quasiisothermal fixed-bed reactor under atmospheric pressure. The steady state, usually reached within 5 hours of operation, was realized not only from the operating conditions but also from the products analyses. The effect of various variables on the conversion of olefins, X (moles of olefin
reacted per mole of olefin fed), the yields of various products, Y (moles of various products formed per mole of olefin fed), and the selectivity, S, of the catalyst for acrolein or methacrolein formation (moles of acrolein or methacrolein formed per mole of olefin reacted) was investigated (Table I). The effect of different amounts of bromine (calculated from methyl bromide) in the feed on the catalytic oxidation = 0.819, W F = 1.61 at of isobutene for feed ratio 400°C. is shown in Figure 1. The conversion of isobutene was only slightly affected. but the yields of carbon dioxide and water decreased rapidly with increased amount of bromine in the feed. On the other hand. the yield of methacrolein and selectivity for methacrolein production increased steadily with increased amounts of bromine. Figure 2 shows the effect of the feed ratio on the conversion of isobutene and selectivity for methacrolein in the presence and absence of bromine at W F = 1.61 and 4OOOC. In the presence of bromine (2 = 0.0007), though the conversion was slightly affected. the selectivity was almost double that of the unmodified catalyst under the same operating conditions. While in the absence of bromine, the selectivity decreased with feed ratio. it remained nearly constant in the presence of bromine in the feed ratio range, R = 0.5 to 1.0. ~~~~
~~
Table I. Effect of Variables on Conversion Yields and Selectivity in Olefin Oxidation
Run no
T, ' C
Feed ratio,
201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 501 502 503 504 505
400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 375 375 375 375 375 375 375 327 327 327 327 32i 400 400 400 400 400
0.5819 0.5819 0.5819 0.5819 0.6591 0.6591 0.6591 0.6591 0.8185 0.8185 0.8185 0.8185 0.8185 0.9299 0.9299 0.9299 0.9299 0.9543 0.9543 0.9543 0.9543 0.9543 0.9543 0.6332 0.6332 0.6332 0.6332 0.6332 0.6332 0.6332 7.6976 7.6976 7.6976 7.6976 7.6976 0.9543 0.9543 0.9543 0.9543 0.9543
Temp,
R
Modifier
CH %Br
C?H?Cl,
CZH?CL
CZHJCl.
CIH-I
Weight ratio of modifier to hydrocarbon
0 0.00009 0.00023 0.00070 0 0.00009 0.00023 0.00070 0 0.00009 0.00023 0.00042 0.00070 0 0.00009 0.00023 0.00070 0 0.00050 0.00100 0.00250 0.00500 0.01020 0 0.00030 0.00075 0 .OO 150 0.00300 0.00730 0.01480 0 0.00120 0.00270 0.00540 0.01080 0.00030 0.00050 0.00070 0.00140 0.00300
Feed, mole/hr
C lHh
...
... ...
... ... ...
... ... ...
...
...
...
...
...
...
... ... ... ... ... . t .
...
...
0.1047 0.1047 0.1047 0.1047 0.1047 0.1047 0.1047
... ... ... ...
C,H.
0.3626 0.3626 0.3626 0.3626 0.3626 0.3626 0.3626 0.3626 0.3626 0.3626 0.3626 0.3626 0.3626 0.3626 0.3626 0.3626 0.3626 0.1138 0.1138 0.1138 0.1138 0.1138 0.1138
..,
... ... ... ... ... ... ...
... ...
...
... ...
...
0.1138 0.1138 0.1138 0.1138 0.1138
... ...
... ...
C4Hf.O
O?
N.
... ... ... ... ...
0.2110 0.2110 0.2110 0.2110 0.2390 0.2390 0.2390 0.2390 0.2968 0.2968 0.2968 0.2968 0.2968 0.3372 0.3372 0.3372 0.3372 0.1086 0.1086 0.1086 0.1086 0.1086 0.0663 0.0663 0.0663 0.0663 0.0663 0.0663 0.0663 0.0663 0.0663 0.0663 0.0663 0.0663 0.0663 0.1086 0.1086 0.1086 0.1086 0.1086
0.7963 0.7963 0.7963 0.7963 0.9021 0.9021 0.9021 0.9021 1.1200 1.1200 1.1200 1.1200 1.1200 1.2724 1.2724 1.2724 1.2724 0.4100 0.4100 0.4100 0.4100 0.4100 0.4100 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.4100 0.4100 0.4100 0.4100 0.4100
... ... ... ...
... ... ...
... ... ... ... ... ...
...
... ...
...
. I .
... ...
... ... ... ... ...
0.0086 0.0086 0.0086 0.0086 0.0086
... .
I
.
...
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
27
Table I. (Continued) Yo
Yield mole/hr/mole/hr
Run no
YC0
yC H 0
yC H 0
YH 0
201 202 203 204 205 206 207 208 209 210 211 212 213 214 '15 216 217 401 102 403 404 405 106 407 408 409 410 411 412 413 414 415 416 417 418 501 502 503 504 505
0.3359 0.3052 0.2399 0.0965 0.3673 0.339 0.2675 0.0984 0.4299 0.3747 0.3229 0.2418 0.0932 0.4597 0.4007 0.3488 0.0951 0.5878 0.4639 0.4349 0.3418 0.2838 0.2513 0.4116 0.3228 0.2788 0.2578 0.2406 0.2798 0.4403 2.8837 2.2441 1.8139 1.383 1.2790 0.46,57 0.4103 0.4332 0.5738 0.6423
0.0590 0 0777 0.0874 0.1232 0.0703 0.0827 0.0959 0.1323 0.0882 0.1020 0.1155 0.1431 0.1547 0.0990 0.1144 0.1348 0.1778 0.0500 0.2293 0.2785 0.3040 0.2732 0.2170
... ... ...
... ... ... ... ...
0.0735 0.1766 0.2043 0.2082 0.1776 0.1088 0.0171
0.3604 0.4161 0.3521 0.2027 0.4608 0.4594 0.3998 0.2206 0.5959 0.5554 0.5355 0.4440 0.2688 0.6387 0.6105 0.6621 0.3058 0.6115 0.6520 0.6432 0.6019 0.5369 0.4683 0.4832 0.4976 0.4823 0.4660 0.4183 0.3868 0.0415 1.7209 1.4883 1.4069 1.2906 1.2325 0.6054 0.6370 0.5957 0.5325 0.5475
...
... ... ...
...
... ... 0.2434 0.3189 0.2882 0.1344 0.0307
...
... ... ... ... ... ... ... ... ... ... ... ... ...
...
... ...
...
... ...
...
...
... ... ... ... ... ... ... ...
Figure 3 shows the effect on the yield of methacrolein and carbon dioxide. The yield of methacrolein increased linearly with increasing feed ratio in the presence and absence of bromine. Though the yield of carbon dioxide increased with the feed ratios in the absence of bromine, it was almost unaffected by feed ratios for 2 = 0.0007. These linear relationships among the conversion, yields, and feed ratio suggest this reaction to be approximately first-order with respect to oxygen, in agreement with the findings of Isaev and Margolis (1960). The mechanism is not much affected by the presence of the moderators, and the increase in selectivity for methacrolein production was mainly due to the suppression of the undesired further oxidations of aldehydes to carbon dioxide. The effect of different amounts of chlorine (calculated from tetrachloroethylene) in the feed on the catalytic oxidation of isobutene over inactivated alumina-supported copper oxide catalyst for R = 0.955 a t 400" C and W /F = 5.2'7 is shown in Figure 4. The conversion and the yield of methacrolein first increased rapidly with increased amounts of chlorine up to 2 = 0.0010, and then decreased with further amounts of chlorine. The yield of carbon dioxide decreased drastically initially, and then slightly with the increasing amount of chlorine. The yield of water slightly decreased initially and then much more rapidly with increasing amounts of chlorine. The selectivity for 28
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
O h
Conversion
14.83 18.03 16.82 15.63 18.91 20.51 18.36 17.04 23.44 23.82 22.33 22.09 20.04 26.58 25.70 26.19 22.11 20.12 34.09 37.69 57.87 34.18 28.38 21.01 28.36 29.70 29.41 25.78 20.15 16.33 72.09 54.65 39.53 31.39 29.06 34.44 41.65 37.35 27.94 21.35
Selectivity
for acrolein or methacrolein
40.28 43.11 51.96 78.83 39.21 40.32 52.25 77.66 37.64 42 82 51.72 64.79 77.16 37.24 44.52 51.47 80.42 24.89 67.26 73.89 80.27 79.94 76.47 35.00 62.28 68.81 70.77 68.88 54.02 10.32
. . ... ...
...
... 70.66 76.58 77.17 48.11 14.40
methacrolein formation increased very rapidly and then slightly decreased with the addition of chlorine modifier. The optimum amount of chlorine giving highest yield and selectivity for methacrolein was about Z = 0.0015. The effect of iodine (from propyl iodide) in the feed on conversion, yields, and selectivity for methacrolein under the same operating conditions as used in chlorinemodified catalyst is shown in Figure 5 . The trend in the conversion of isobutene and selectivity for methacrolein was similar to the one when chlorine was used as a modifier, but was more sensitive to the presence of further amounts of iodine. The yield and selectivity for methacrolein decreased very rapidly, when iodine feed exceeded its optimum amount (2 = 0.0005). On the other hand, the effect of increasing iodine in the feed on the yield of carbon dioxide was different from those obtained with chlorine, bromine, sulfur, and selenium doping. Although initially it decreased first with increasing amount of iodine, it increased when the added iodine in the feed exceeded its optimum amount. At this stage, a drastic decrease in the selectivity for methacrolein was reflected. Similar results were observed when methyl iodide was used as the source of iodine. Either on reducing the amount of iodine in the feed or increasing the isobutene charge to maintain the optimum iodine-isobutene ratio, the yield of carbon dioxide again decreased and high conversion
0
Yield, C 4 H s O
0
Yield, COY
c
I
I
I
I
I
L
I
I
I
I
05
06
07
I 08
FEED RATIO Figure 1. Effect of bromine in feed on conversion yield and selectivity for isobutene oxidation
0
I
I
09
1.0
(A)
Figure 3. Effect of feed ratio on yields of methacrolein and carbon dioxide over bromine-modified catalyst
Converslon, C q H 8
A Selectivity, C ~ H ~ I Z =0.0007
-
-A-A
A
-
I
I
I
I
I
05
06
07
08
09
FEED
RATIO
I IO
(E)
Figure 2. Effect of feed ratio on conversion a n d selectivity for isobutene oxidation in presence and absence of bromine
0
2
4
6
6
IO
2 , WEIGHT RATIO ( C I / C 4 H 8 ) X 1 0 3
Figure 4. Effect of chlorine in feed on conversion yield and selectivity for isobutene oxidation
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
29
I I I
0 Conversion
/
0 Y i e l d .. C n. H -c -O A Selectivitv CAH,I: 0 -
\
I
5
I
, C4 H e
I
10
I 15
I
20
I
25
I I
30
2 , WEIGHT RATIO ( I / C 4 H s ) X IO4 Figure 5 . Effect of iodine in feed on conversion yield and selectivity for isobutene oxidation
and selectivity were obtained. This indicated that the modifier was more closely related to the olefin charged than to other variables for an optimum production of unsaturated aldehydes. The catalytic oxidation of propene over the same chlorine-modified catalyst as used in isobutene oxidation was investigated for R = 0.633 and W F = 6.38 a t 375. C. The results as shown in Figure 6 were similar to those obtained in isobutene oxidation in the presence of iodine.
t r\-
The yield of carbon dioxide decreased first, and then increased with increasing amount of chlorine in the feed. In the case of propene oxidation, the optimum weight ratio of chlorine-propene was about 2 = 0.00075, which is comparatively less than that required for isobutene oxidation (2= 0.0015). The oxidation of methacrolein, the primary oxidative product in isobutene oxidation, was investigated in order to find whether carbon dioxide is formed from the further oxidation of methacrolein or the parallel oxidation of isobutene, and to obtain some insight concerning the promotional effect of the modifiers. The catalyst was the same as used in the isobutene oxidation, in which chlorine was used as a modifier. In the absence of the modifier, methacrolein oxidation was sensitive to temperature and oxygen-methacrolein feed ratio. The reaction was almost complete at temperatures higher than 350" C. On the other hand, in the absence of the catalyst, the conversion of isobutene was negligible even a t 400°C when chlorine modifier was added. This suggests that a t temperatures higher than 350" C, carbon dioxide originates mostly from the further oxidation of aldehydes, in agreement with the observations of Voge et al. (1963) and Isaev et al. (1958). Figure 7 shows the effect of different amounts of chlorine in the feed on the catalytic oxidation of methacrolein at 327OC. The conversion of methacrolein declined with increased amounts of chlorine. The results are in agreement with the earlier findings in isobutene oxidation over chlorine-modified catalyst (Figure 4) and indicate that the promotional effect of modifiers was mainly due t o the successful suppression of the undesired further oxidation. The result of different modifiers used in conjunction with copper oxide catalysts in the oxidation of isobutene was compared with their electronegativities (Little and Jones, 1960) and are given in Table 11. The optimum range of modifier-isobutene weight ratio for methacrolein formation is about 0.0005 to 0.0015. A continuous supply of modifiers in the feed is essential for maintaining a high selectivity for methacrolein formation.
0 Conversion, C 3 H6
70
Y i e l d , COP Yield , H 2 0 Yield, C 3 H 4 0 A Selectivity ,C3H40 0
Figure 6. Effect of chlorine in feed on conversion yield and selectivity for propene oxidation
0
I
I
I
I
I
I
2
4
6
0
10
z, 30
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
WEIGHT RATIO ( CIIC~H,)
1
12
x lo3
14
CunO (non stoi.) I
Temperature Catalyst Feed Air Methacrolein
60
327% 0 6 gm 03162 g m mol /hr 0 0086 gm.mal /hr
v,
a
40
z
0 0
'
30
0
2I IO
0
2
4
8
6
IO
12
2 , WEIGHT RATIO ( C I / C 4 H 6 0 ) X lo' Figure 7. Effect of chlorine in feed on conversion of methacrolein
The color of the fresh catalyst was dull black and the modified catalyst discharged from the reactor was reddish orange. The active catalyst for selective oxidation was identified as cuprous oxide by x-ray diffraction studies (Table 111). As mentioned by Mann and Yao (1968), when olefin (a donor-type gas) is adsorbed on the cuprous oxide catalyst (a p-type semiconductor), because of the lack of sufficient positive holes for charged adsorption of olefin, the surface concentration of charged olefin is always low. On the other hand, when oxygen (an acceptor-type gas) is adsorbed on the cuprous oxide surface, the supply of electrons from the valence band of the catalyst is always large. This in turn results in much higher surface concentration of charged oxygen than charged olefin. Also, from the stoichiometric relation, it is understood that a higher surface concentration of oxygen ions would be more favorable for complete oxidation. Hence, the selectivity of the catalyst for olefin oxidation in the absence of the modifier is always limited. Trace amounts of modifiers such as selenium, sulfur, chlorine, bromine, and iodine having electronegativities higher than that of copper would decrease the supply of electrons and increase the concentration of positive holes on the catalyst surface, which in turn increases the selectivity of the catalyst for partial oxidation. Modifiers like bromine and chlorine are good electron acceptors (Smoluchowski, 1953). The catalytic oxidation of olefin can also be realized as taking place by a redox cycle of the copper oxide catalyst Activated
CUO, C u r 0 . I cuo in air
CuO-
Olefin
Cu? 0 (stoi.)
Cu20 (stoichiometric)
Diffusion of )
small amount of oxygen
C u 2 0 (non stoi.)
Excess
oxygen
CuO
Olefin and oxygen behave as reducing and oxidizing agents, respectively. Without the presence of a modifier, the probability and stability of these valency states of copper on catalyst surface are determined by operating conditions only. An increase in the severities of the oxidation process-feed ratio (Ol/olefin), reciprocal of space velocity ( W ,F ) , and reaction temperature, favors the formation of cupric oxide and decreases the specificity for the desired products. The presence of a modifier of the type used in this work favors a shift in the equilibrium toward cuprous oxide, making the catalyst more stable in its lower valency state and more selective to the formation of acrolein and methacrolein than the unmodified catalyst under the same operating conditions. Experimental results support this viewpoint. While unmodified copper oxide. a mixture of cupric and cuprous oxide. gave moderate selectivity for acrolein formation from propene oxidation, very high selectivities were obtained over copper oxide catalyst modified with proper amounts of chlorine in the feed. In the presence of an optimum amount of chlorine. the catalyst was virtually all cuprous oxide and no cupric oxide. When more than the optimum amount of chlorine was used as a modifier, selectivity for acrolein decreased, and the catalyst was found to be a mixture of cupric and cuprous oxide and other unidentified compounds which were presumably metallic copper or copper halide. This is in agreement with the earlier findings of Popova et al. (19611, Isaev et al. (1958\,Kominami et al. (1962), and Wood et al. (1969). Recently, Sachtler and deBoer (1964) and Moro-oka et al. (1967) correlated the catalytic activity with the heats of formation or the reducibilities of metal oxide catalysts in propene oxidation. Moro-oka et al. found that the larger the heats of formation AH^^.^^), the less active were the catalysts. Sachtler and deBoer reported that a high reducibility of the catalyst (corresponding to a low 9fx1.o value) increased propene conversion but decreased the selectivity for acrolein.
Table II. Modified Copper Oxide Catalyst for Oxidation of Olefin (Isobutene)
Modifier
Electronegativity
...
1.75 2.21 2.44 2.48 2.74 2.83 4.10
I S Se Br
c1 0
Optimum weight ratio (modifierolefin)
YO
YO
Conv
Selectivity
increase in selectivity
0.0005 0.0006 0.0005 0.0007 0.0015
29.05 29.00 29.00 29.00 29.00 29.34
33.80 57.00 43.58 44.00 80.00 81.34
53.84 28.93 30.17 136.70 140.70
~
O h
~~
~
Table Ill. X-Ray Diffraction Patterns of Catalysts Catalyst
1. Fresh 2. Used without modifier 3. Used with optimum chlorine modifier 4. Used with overdoped chlorine modifier
cu-0
CuO
Unknown
None Some Major
Major Major None
Trace Trace Trace
Major
Some
Some
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
31
The present work is in agreement with the findings Literature Cited of Moro-oka and Sachtler. The properly modified catalyst Besozzi. A. J., Brill, W . F., U.S. Patent 3,301,906 (January has the structure of cuprous oxide (lHbl.o = -43.0 kcal 1967). per mole). This is more selective than the unmodified Brill, W . F., Besozzi, A. J., U.S.Patent 3,274,255 (Sepcatalyst. which has the structure of cupric oxide ( - I H ~ ~ . ~ tember 1966). = -38.5 kcal per mole). The increase in lHhl.Ovalue Cheney, H. A., Breier, I. L., U. S. Patent 2,879,300 (March on modification suggests that in the presence of an 1959). Cheney, H. A., Stephen, A. H., U.S.Patent 2,807,647 optimum amount of modifier, the charged adsorption of (September 1957). oxygen becomes difficult (requires more energy). The cupHadley, D. J., Brit. Patent 694,353 (July 1953). rous oxide becomes more stable even under high severe Hadley, D. J., U.S. Patent 2,807,647 (September 1957). operating conditions. Isaev, 0. V., Kushnerov, M. Ya., Margolis, L. Ya., Proc. Acad. Sei. U S S R Phys. Chem. Sect., 119, 131 (1958). Conclusions Isaev, 0. V., Margolis, L. Ya., Kinet. Catal., 1, 237 (1960). The presence of small amounts of modifiers having Kominami, N., Shilata, A . , Minekawa, S., Kogyo Kagaku electronegativities higher than that of copper which could Zasshi, 65,1510 (1962). modify the surface properties, such as by changing the Little. E. J., Jones, M. M., J . Chem. Educ., 37, 231 (1960). distribution of the two different types of active sites for Mann, R. S., Yao, K. C., Aduan. Chem. Ser., No. 76, oxygen and olefin adsorption, enhances the activity and 276 (1968). Mann, R . S., Yao, K . C., Ind. Eng. Chem. Prod. Res. selectivity of the skeleton catalyst. An increase in the Develop., 6, 263 (1967). conversion of olefin in the presence of a small amount Mann, R. S., Yao, K. C., Ind. Eng. Chem. Prod. Res. of modifier and a decrease when overdoped with the Develop., 8, 331 (1969). modifier suggest that a true catalyst modification is Margolis, L. Ya., Enikeev, E. Kh., Isaev, 0. V., Krylova, involved rather than simply catalyst poisoning or selective A. V., Kushnerov, M. Ya., Kinet. Catal., 3, 153 (1962). poisoning. Moro-oka, K., Morikawa, Y., Ozaki, A., J . Catal., 7, 23 For an optimum production of acrolein or methacrolein (1967). over modified copper oxide catalyst, the modifier is more Popova, N.I., Milman, F. A., Latyshev, V. P., Izu. Sibirclosely related to olefin feed than the other variables in sko Otd. Akad, Nauk S S S R , 7, 77 (1961). the experimental range. The optimum weight ratio of Sachtler, W. M. H., deBoer, N. H., Proceedings of 3rd International Congress on Catalysis. Vol 1. p 252, Northmodifier-isobutene in the feed is in the range of 0.0005 Holland Publishing Co., Amsterdam, 1964. to 0.0016. The optimum range for acrolein formation is Smoluchowski, R., Reu. Mod. Phys., 25, 178 (1953). less than the one required for methacrolein formation. Voge, H . H., Wagner, C. D., Stevenson, D. J., J . Catal., Under optimum conditions. the partial oxidation of 2 , 5 8 (1963). olefins to the corresponding aldehydes is a p-type reaction Wood, B. J., Wise, H., Yolles, R. S., J . Catai., 15, 355 (reaction rate accelerated with increase in positive hole (1969). concentrations on the catalyst surface) and its further RECEIVED for review April 30, 1970 oxidation to carbon oxides is an n-type reaction (reaction ACCEPTED October 20, 1970 rate accelerated with increase in concentration of free electrons on the catalyst surface) under optimum condiFinancial assistance provided by the National Research Council tions. of Canada through Grant A-1125.
Nickel on Nonstoichiometric Titanium Carbide Effect of Electronic Interaction between Metal and Support on Catalytic Activity and Selectivity Larry A. Maddox' and Howard F. Rase2 Department
of
Chemical Engineering, The University
A l t h o u g h catalyst art has definitely assigned synergistic effects to the active catalyst component and its support, very few definitive studies have been made to probe this phenomenon. Present knowledge of solid-state physics would suggest that electronic interaction between a
' Preieiit addresi. Celanese Chemical Corp., Clarkwood, Tex.
784116 -
32
To whom correspondence should be sent. Ind. Eng. Chern. Prod. Res. Develop., Vol. 10, No. 1, 1971
of Texas
at Austin, Austin, Tex. 78712
catalyst and its support is definitely possible and can be a major attribute in setting ultimate performance characteristics. To study such interaction in a meaningful way, one must select a system of potential ultility which can be precisely characterized. These criteria seem to be admirably satisfied by a nickel-titanium carbide catalyst developed in this research study. Nickel was selected as the active catalyst component because of its wide commer-
