Heterogene0us Catalytic Oxidation of Hydr0carb0 nS C. F. CULLIS
i
C-H bond strength N 100 kcal.) tend to be preserved intact. Typical products of the heterogeneous catalytic oxidation of hydrocarbons therefore include butadiene (from butenes and n-butane), maleic anhydride (from benzene and C, hydrocarbons), phthalic anhydride (from o-xylene and napthalene), acrolein-in which the aldehydic D H bond is probably stronger than a normal carbonylic C-H bond-(from propylene) and ethylene oxide (from ethylene). Thus, only relatively stable compounds are normally formed as high-yield products and many otherwise desirable conversions (e.g. phenol from benzene) are impracticable, probably simply because the required product is more easily oxidized than the initial reactant. Operation of redox cycle. Finally, there is at least a formal analogy between the mechanism of homogeneous catalyzed and heterogeneous catalyzed oxidations. Thus, in certain liquid-phase oxidations catalyzed by
-
'rn
r u
I
I
I -
Oxidation over a heterogeneous catalyst is the best way to convert a hydrocarbon or group of hydrocarbons to a particular product. A catalyst for the partial oxidation of hydrocarbons is more effectively developed by studying the reaction mechanisms than by trial and error
transition metal ions, there is evidence for some mrt of redox cycle in which the metal ions in their high valency state react with the hydrocarbon and are then reoxidized by molecular oxygen. I n the hydrogen bromidecatalyzed gaseous oxidation of hydrocarbons, too, the attacking species are bromine atoms which are regenerated by the reaction of oxygen with the resulting hydrogen bromide: RH Br+ R HBr
+ HBr + 0,
+
+ Br + HOa
Similarly, in some heterogeneous oxidations catalyzed by metal oxides, it is believed that there is a formally similar redox cycle in which the hydrocarbon reacts with the oxide ions in the catalyst lattice which are then replaced by the reduced catalyst reacting with gaseous oxygen:
+e
R H + RH'm.) RH+(&) '/a
+ Oz-(ads)
-t
Os
VOL 59
+2e
-+
products
+e
O'-(.ae)
NO. 1 2 D E C E M B E R 1 9 6 7
19
'ABLE 1. REACTIVITY OFOLEFINS FOR OXlDl 'ION OVER BISMUTH MOLYBDATE AT 460' # (FROM REF. 1)
lw=l
s
f!!p?=
JiaJbutclE , ~ , rf-2-Butene .Methyl4 -propene .Methyl-Z-butene
10
Butene .Ptntene -Methyl-f-pentene .Pcntenc (cir and
...
I t )
.Ethyl-1-hwene .Methyl-1-butene
;o-Butene
.Methyl-t -pentene .MethylQ-pentene
0.01
0.1 Oxidation
1 .
10
Figure I . Cornpariron of the rtactiviQ of olejsns for oxidation o w bismufh molybdafc of 460' C. to thof of allyl hydrogen obrtmction by methyl radicals in isoocfonr solution of 65' C. Trimglm arc for intmul OI&LS &om ref. 7)
Thus, for example, anthraquinone may he formed by passing anthracene over a vanadium pentoxide catalyst in the absence of any oxygen gas and the work of Roiter (47) and other Russian workers has shown that '*O in the catalyst lattice can appear in the gaseous oxidation products. It therefore appears that what the catalyst does essentially-whether homogeneous or heterogeneous-is to provide a new and different species to react with the hydrocarbon. One can perhaps press this last analogy a little further, for there is also sane similarity between the intermediates formed between unsaturated hydrocarbons and transition metal ion catalysts in the two types of system. Thus the bonding of olefins to the surface (20,43)during reactions over heterogeneous catalysts can be classified in terms of =-complexes similar to those formed in homogeneous solutions (e.g. CnHrPdCl,) (50).
.Methyl-1-buienc Methyl-1 -pentcnc -Ethyl-l-pentene
1.9 1.4
Diffusion (56). If the rates of diffusion are lower than or even comparable with the rates of other processes, true chemical kinetics will not be observed. Certainly, for very fast reactions (e.g., with very high activity catalysts), only the external surface of the catalyst will be accessible to the reactants and the overall rate of chemical change may become limited by the rate of mass transfer to the surface. O n the other hand, circumstances may arise with porous catalysts where the rate of reaction is small compared with the rate of external diffusion but is nevertheless affected by the rate of diffusion within the catalyst pores. The effect of slow pore diffusion on selectivity is quite complex. Most oxidations involving the formation of some required intermediate product proceed by some scheme which can be designated:
Physical Focton in Heterogeneous Catalytic Oxidation
Although there are a number of obvious similarities between homogeneous and heterogeneous oxidation, there are certain physical complications, found only in heterogeneous systems, which are a direct result of the fact that reaction takes place at a gas-solid interface. Thus, in any complete heterogeneous reaction, apart from chemical reaction taking place between the adsorbed species, there is the necessity of the reactants and products diffusing to and from and being adsorbed on or desorbed from the surface. 20
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Now the concentration of A within the pores will be markedly decreased for a rapid reaction taking place on a very porous catalyst. If the two competing reactions: /ethylene
fBe.g., ethylene
oxide
A
\iC
\COP
+ H20
are of the same kinetic order, then the catalyst pore structure will not affect the relative proportions of the two processes. But if, for example, A -c B were firstorder with respect to A and A + C were second-order with respect to A, then the second reaction would be slowed down more than the first, when the concentration of A is decreased and vice versa. Using a catalyst with small pores is then equivalent to lowering the total partial pressure of A so that, if a particular catalytic process gives higher yields of a desired product at low pressures, then small-pore catalysts should further improve the yields, and vice versa. What is perhaps however more important in the heterogeneous catalytic oxidation of hydrocarbons is the occurrence of unwanted consecutive reactions : A +- B + D. This tends to occur in almost every industrially important reaction since the required product is invariably thermodynamically unstable in the presence of oxygen. With a porous catalyst, where A diffuses into the pores and reacts to form B, the product molecules must make many collisions with the pore walls before they escape into the gas-phase. Thus, while they are trying to find their way out, they stand a high probability of being converted to D. T o minimize this “overoxidation,” catalysts should therefore have as little as possible internal pore structure. I n the heterogeneous catalytic oxidation of hydrocarbons, unwanted consecutive reactions are probably more undesirable than unwanted concurrent reactions, so that the best oxidation catalysts have only large diameter pores and are generally of fairly low surface area. Sorption. The adsorption and desorption equilibria which reactants and products tend to reach at different parts of the catalyst are governed by rates which are complex functions of the concentration and nature of the species competing for sites and of the precise structure of the solid catalyst. Certainly for a number of reactions, the overall rate may be governed by the rates of adsorption of reactants or of desorption of products. This is one reason why kinetic measurements on heterogeneous reactions are much less significant than those on homogeneous reactions. The order of reaction and the activation energy often vary considerably with the
F. Cullis is Professor of Physical Chemistry at the City University, London, England. T h i s paper was presented at the Fifth Oxidation Conference at Stanford Research Institute, August 7 I , 1966. AUTHOR C.
conditions. When adsorption is rate-controlling and the active sites are only sparsely occupied, the reaction will tend to be first-order with respect to the reactant and the activation energy will be that for this adsorption process. When chemical reaction within the adsorbed layer is rate-determining, the active sites will become saturated with the reactant. Under these conditions, the order with respect to the reactant will tend to zero and the measured activation energy will be the true value for the surface reaction. In industrially important oxidation reactions, where oxidation must be stopped at an intermediate stage, it is important that desorption of the product from the surface-like diffusion out of the catalyst pores-should be fast. I n general, hydrocarbons are less strongly adsorbed than their more polar oxygen-containing partial oxidation products. However, in many systems, the addition of water vapor has a beneficial effect on selectivity and may well act by product displacemente.g., yields of acrolein from propylene obtained over a Bi/P/Mo catalyst are considerably increased by the addition of steam to the reacting gases (54). T h e complications resulting from the occurrence of diffusion and sorption processes emphasize the importance of knowing as much as possible about such factors as specific surface area and pore size distribution. In most investigations, little or no information is given about such properties of the catalyst but, with the development of improved techniques, the measurement of such quantities can now be made with increasing ease (Table 11). The formation and maintenance of the catalyst in the right physical form can often be facilitated by depositing it on a suitable support or carrier. The main function of this supporting solid is often simply to give the catalyst mechanical strength but it sometimes also alters its texture and, hence, such properties as the area of the crystallites, the porosity, and the defect structure. Since most oxidation processes are highly exothermic, another important physical consideration is the ease of removal of the heat of reaction, since, if there is a runaway increase of temperature, only complete combustion of the hydrocarbon will take place. To facilitate the maintenance of as nearly as possible isothermal conditions, long narrow catalyst beds are frequently used and the catalyst may also be diluted with an inert solid, perhaps using the greatest dilution at the inlet end where the partial pressures of the reactants (and hence the fluid has heat evolution) are highest (35). VOL. 5 9
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The catalyst must not only be in a suitable physical form to permit diffusion of reactants and products, but also chemically correct for the desired conversion
Alternatively, the use of idized beds of catalyst material has the advantages of more efficient temperature control and of increased available surface:volume ratio of the catalyst. Both these advantages arise from the fact that the reactor contains an intimate mixture of homogeneous reactants and finely divided solid catalyst. Although the technology of fluidized beds has been developed only recently (350), their use would seem to possess many desirable attributes for the study of oxidation reactions. Chemical Factors in Heterogeneous Cutalytic Oxidation
I n addition, however, to getting the catalyst into the right physical form to minimize such unwanted effects as slow diffusion into pores and overheating, it is, of course, necessary to have a catalyst which is, as nearly as possible, chemically correct for bringing about the desired conversion as selectively as possible. A variety of solids can act as heterogeneous catalysts for the partial oxidation of hydrocarbons, but the commonest catalysts are transition metal oxides (or transition metals which under operating conditions become covered with transient layers of the correspondingoxide). These metal oxides are semiconductors and it has been possible to correlate quite successfully their catalytic
TABLE il. HETEROGENEOUS CATALYTIC OXIDATION OF HYDROCARBONS. SOME IMPORTANT PROPERTIES OF CATALYSTS M&%JWU7Zlftl
‘omity
0
article sire and shi
‘hase identification
‘alency of transitio metal ions laturc of catalystadsorbaka involvi in reaction
INDUSTRIAL AND E N G I N E E R I N G C H E M I S l r r
Physical adsorption.of ga such an A, NI,or Kr-wzc w their b.0. (58) Analysis of shape of adsorption or desarption isotherm in region of capillary condensation (suitable for small pores, 500 A.) (40) Electmn microscopy (2). X-ray diffraction (23.34) Optical microscopy (24). X-ray diffraction (76,23, 34). “Selected ama” electron diffraction (75). Differential thermal analysis ( 4 ) Infrared absorption spectroscopy (77). Magnetic susceptibility (39, 47). Electron spin mnancc (72,27,30,32,37) Heat of adsorption (78). Isotope exchange (6, 57). Infrared spcevoscopy (5, 73, 77,53). Electrical conductivity (79, 25 57). Electron spin resonalic (36, 39A, 42, 45,46,52).
I
.
properties with their semiconductivity. Semiconductors have, according to the band theory, a filled band of bound electrons at a temperature of absolute zero, separated by an energy gap from a conduction band which is empty at absolute zero but which is able to accdmmodate electrons of suitable energies at other temperatures. With pure transition metal oxides, the energy gap between the filled and conduction bands is too great for an appreciable number of electrons to be excited into the conduction band at normal temperatures (Figure 2). However, imperfections in the crystal lattice, resulting from the nonstoichiometry of the oxide or the presence of foreign ions, introduce discrete impurity levels into which electrons can be excited more easily than into the conduction band proper. With n-type semiconductors, the energy of electrons in impurity levels is quite close to that of the conduction band, and excitation from these levels into the conduction band leads to electronic (n-type) conduction (Figure 2). On the other hand, with!-type semiconductors, the energies of the impurity bands are quite close to that of the filled band and oxidation into these levels from the filled band leaves vacancies which lead to positive hole @-type) conduction. When, for example, oxygen is adsorbed on an n-type oxide, electrons are transferred from electron-donor sites near the surface to give species such as O%-,0-, and 0". A negative charge is thus built up on the surface which is compensated for by a positive charge, which extends some distance below the surface due, to the deficiency of electrondonor sites. This means that adsorption of oxygen, as it proceeds, requires more and more energy for the required electron transfer. On the other hand, with a p-type oxide, electrons are transferred from the filled band of the semiconductor where they are readily available, and so adsorption of oxygen as negative ions will continue until much higher surface coverages are obtained. Now in accordance with this reasoning, it is found that in generalp-type oxides are much more activ oxidation catalysts than are n-type oxides (Table 111) However, as in homogeneous oxidation, activity and selectivity do not run parallel. Thus, just as the m a t highly reactive free radicals tend to be unselective in their attack, sop-type oxides show little or no selectivity; similarly low activity n-type oxides are quite selective. Thus, neither !-type nor n-type metal oxides on their own are good catalysts for the oxidation ofhydrocarbons. Nor does it seem possible to achieve high selectivity and activity by combining the two types of oxide. Nevertheless, the best results are obtained by using mixture^ of oxides containing at least two different metals Thus, subgroup A oxides (In, T1, Sn, Pb, Bi, Se, Te), although of low activity when used alone, frequently give catalysts of moderately good activity and selectivity
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Figwe 2. Types of ~ c m i c o n d w t ~ ~
Oxide
-
VOL 59
NO. 1 2
DECEMBER 1 9 6 7
23
i
F i p t 3 . The unriations of produd formation and of surfme arso with catalyst age (temperatwe 3ooo C.; petll-2-mr; oaygen:ol& = 1.25; cotllmf time, 33.6sec.); 0,pmt-2-c~;0 acetaldehyde; propionaIdehyde; 0 acetone; 0 trans-2,3-epo?ypmroM; @ efhylcna; v methanol; *pmtanone-2pluspcntnnone-3; v spea& surface arc4 (after degassing at 350' C.) (from rcf. 16)
F i p e 4. Bani mechanism fathe hetwogenemu cataIytic oxidation of hydrocarbons
24
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
F i p e 5. Possible mechanism fa the catalytic oxidation of popylana fo acrolein
when used in conjunction with a transition metal oxide (e.g., Bi/Mo, Sn/V). The way in which such second components exert their effect varies from system to system. Small quantities of foreign ions introduced into a metal oxide lattice produce lattice defects and hence alter the semiconductivity. However, many oxidation catalysts contain comparable quantities of two different metals and in such cases compound formation or solid solution may occur, so that the lattice is entirely different in structure and dimensions from that of either single compound. I t has been suggested (70) that effective additives change the metaloxygen bond strength in such a way that both rapid interaction of lattice oxygen with the hydrocarbon and rapid reoxidation of the metal ions by molecular oxygen can occur. One interesting postulate is that catalysts which cause nonselective oxidation are those which chemisorb oxygen as single atom species : the adsorbed oxygen atoms would be highly reactive and hence nonselective. For the dissociative chemisorption of oxygen, however, it is probably necessary for the active metal ions to be close together. The increase in selectivity which accompanies the addition to an active $-type oxide of some second component may simply be due to the increased separation by diluting of the active centers. In practice, of course, adsorption takes place not only of oxygen, but also of the hydrocarbon as a result of a-complex formation. Both processes are likely to be favored by low-valency metal oxides, the former because more complete transfer of electrons to the oxygen is required and the latter because of the required feedback of energetically suitable &orbitals of the ion to the vacant antibonding orbitals of the unsaturated hydrocarbon. It has been postulated that an additive which gives improved selectivity does so because it produces a lattice of a different type-or with different unit cell dimensions-in which the active metal ion more readily assumes its low valency state. One further important consideration regarding the chemical nature of the catalyst is that its physicochemical properties should not change as it is used. In several cases, the valency states of a metal oxide are changed when it is used as a catalyst for hydrocarbon oxidation. Reduction to a lower oxide-producing a change in the equilibrium unit cell size-is sometimes particularly rapid when fuel-rich mixtures'are used and has been wellestablished with VZOScatalysts (26, 27, 3 7 , 48), where the pentavalent oxide is reduced to Vz04.a4 and Vz04. Once the lower oxide is formed, the catalyst consists of a mixture of oxides containing different cations of the same metal and as a result the activity and selectivity often alter. For example, during the oxidation of pentene-2 over vanadium pentoxide ( 9 ) , the unreduced oxide gives acetaldehyde, propionaldehyde, acetone, and 2,3-epoxypentane as a the sole oxygenated products. Once the lower oxide separates, however, the selectivity decreases markedly and in addition CZ, Cg, and Cd
epoxides, methanol, crotonaldehyde, methyl ethyl ketone, diethyl ketone, and methyl n-propyl ketone are all formed (Figure 3).
General Mechanism for Heterogeneous Catalytic Oxidation of Hydrocarbons
There are a number of physical and chemical requirements of an effective catalyst for the oxidation of hydrocarbons and it is perhaps not surprising that such catalysts are still discovered largely on a trial-and-error basis. Nevertheless, it is now clear that the basic chemical mechanism on all catalysts can, under conditions where mass-transfer processes are not rate controlling, be represented by quite a simple scheme (Figure 4) involving the adsorption of oxygen as anions and/or of hydrocarbons as cations. I n order that a surface should have a catalytic effect on such a mechanism, it is necessary that mobile electrons should be available, electron transfer processes between the lattice and the adsorbed species should be energetically possible between the lattice and the adsorbed species, and distortion of the activated complex into products should be feasible. The numerous possible reaction paths in step 4 could clearly lead to a variety of products. It would be expected, however, that on surfaces the physicochemical properties of which favor given reactions in this scheme (producing for example high concentrations of adsorbed R H + and 0-2), high selectivity will be attained. Changes in selectivity during oxidation can also be explained on this general mechanism, since the relative availability of the various species involved in step 4 will change if steps 3a or 3b become rate-determining. Thus, for example, if step 3a is slower than steps 3b and 4, as is frequently the case, the catalyst will become reduced and its selectivity will change. The ideal heterogeneous oxidation catalyst is therefore one which causes a given reaction in step 4 to proceed to the exclusion of all others and also transfers electrons rapidly enough to regenerate the starting materials for step 4. Moreover these electron-transfer steps should have rates such that the phase composition of the catalyst remains unaltered. Now, how is it known, in a given reaction, what species are involved in step 4? It has already been pointed out that purely kinetic measurements are difficult to interpret but in some systems the nature of the reacting species can be determined by the use of isotopic tracer techniques. This can be illustrated by discussion of the oxidation of propylene to acrolein. This reaction can be carried out over a variety of catalysts (Table IV) but it is only quite recently that a start has been made in elucidating the mechanism. I n principle, two types of mechanism are possible (Figure 5) : (A) Propylene may be adsorbed by opening of the a-bond to form a a-complex followed by reaction of adsorbed oxygen either with the methylene group or with the methyl group. (B) The olefin may be dissociatively chemisorbed forming a VOL. 5 9
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DECEMBER 1 9 6 7
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symmetrical allyl group or it may be chemisorbed with simultaneous addition of a hydrogen atom from the surface forming a symmetrical isopropyl group. I n either case of (B), both terminal carbon atoms will have an equal chance of forming a C=O bond. (A) and (B) can be distinguished if one of the terminal positions is labeled with isotopic carbon or deuterium. If 1- or 3-labeled propylenes are oxidized then, according to (B), 50% of the labeled isotope should appear in the CHI and CHO groups. This has been shown to be the case "C-, and over both C u and Bi/Mo catalysts using W-, D-labeled propylenes (3, 38,4?,49,55). I n the case of (A), it is unWrely that the amounts of attack at the two and carbon atoms would be exactly equal. It was also shown that a t the reaction temperature the recovered propylene had scarcely isomerized at all, proving that the isomerization taking place during the formation of acrolein was not caused by the prior isomerization of propylene on the catalyst but only occurred during the oxidation of propylene through a symmetrical intermediate. I n order to determine whether this intermediate was allyl or isopropyl, further experiments were carried out with a catalyst surface which had been treated with deuterium oxide. Under these conditions, the route via allyl (involving no transfer of hydrogen from the surface to the adsorbed hydrocarbon) should give no deuteroacrolein, whereas that via isopropyl should give 50% monodeuteroacrolein. Scarcely none
r
TABLE IV.
I
of the acrolein was in fact deuterated, and thii observation provides strong evidence in favor of the allyl route. Other tracer experiments show that most of the carbon dioxide formed in the reaction comes from further oxidation of acrolein, rather than by a concurrent mode of oxidation of. propylene. This was proved by using either "C-labeled propylene or acrolein (or other intermediates) and measuring the specific activities of the acrolein and carbon dioxide formed as a function of time (22, 28, 29). The findings illustrate the general point that, in the heterogeneous catalytic oxidation of hydrocarbons, unwanted consecutive reactions are often more serious than unwanted concurrent reactions. Finally, it would be of interest to be able to relate the different products formed in a given oxidation system to the reactions of specific adsorbed species. An attempt to do this has been made in some recent work on the oxidation of various pentenes over vanadium pentoxide (8). Thus, for example, the main oxgenated products formed during the catalytic oxidation of 2-methyl-but-2-eue over a new catalyst are acetaldehyde, acetone, ethylene, and propylene, together with smaller amounts of 3-methylbutan-2-one and 2,3-epoxymethylbutane. It seems reasonable to suggest that these products arise by the interaction of two forms of surface cation with 0" to give two different surface-bonded anions:
HETEROGENEOUS CATALYTIC OXIDATION OF HYDROCARBONS. OXIDATION OF PROPYLENE TO ACROLEIN
CulO on S i c Cu:O on Si@ CuO on SiO# CuO on Al9Ut Bi/P/Mo oxides (9:l:lZ) Bi/W oxides. (1:1) Bi/Mo/As oxides (5: 18:l) SnlSb oxides (32:l) Sn/Sb oxides (1:2) SnfSb oxides (3:l) Sb/W oxides
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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The major fate of these anions is apparently to undergo fission of the central C-C bond to give carbonyl compounds and olefins: 0(CH3)zC-CHCHa
(CH3)zC0 4- CHz = CHZ
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(CH3)2C-CHCH3
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The amounts of acetone and acetaldehyde formed (which vary with the experimental conditions) presumably indicate the relative extents to which the two respective precursor cations are formed initially and their propensity to react with O-2. If the adsorbed anions lose an electron to the surface before the C-C bond breaks, the resulting adsorbed alkoxy radicals may rearrange to give 2,3-epoxy-2-methylbutaneand 3-methylbutan2-one : 00‘ (CH3)2i-CHCH3
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(CH3)2CH--COCH3 The reactions of 0-2with the adsorbed olefin are thus capable of explaining the formation of the six major products. O n an old (reduced) catalyst, however, the selectivity decreases and a whole range of further products is formed. This change in selectivity is probably due to the fact that the rate-determining step is the replacement of 0-2ions in the lattice by the reaction: l/202(g) 2 e -t 0-2 The slowness of this reaction results in the adsorbed hydrocarbon having to react in many cases with the intermediate oxygen ions 0- and 0 2 - and indeed with molecular oxygen itself. These extra interactions of course give rise to a very much greater range of products than do those involving only O+. Thus, as the catalyst gets older, there is an increasing tendency for the products
+
to resemble those formed during the homogeneous gaseous oxidation of 2-methyl-but-2-ene. This study suggests quite clearly, however, that for highly selective catalytic oxidation of hydrocarbons, the structure of the catalyst should ensure the presence of a single anionic oxygen species at the surface and the structure of the hydrocarbon should be such that its initial reaction at the surface produces a single species. Finally, of course, the further oxidation of the desired product-both while it is still adsorbed on the surface and after its return to the gas phase-must be slow. REFERENCES (1) Adams, C. R., Proc. 3rd Intern. Congr. Catalysis, Amsterdam, p. 240 (1965). (2) Adams, C. R., Benesi, H. A., Meisenheimer, J. L., J . Catalysis 1, 336 (1962). (3) Adams, C. R., Jennings, T. J., Ibid., 2, 63 (1963). (4) Bhattacharyya, S. K., Ramachandran, V. S., Ghosh, J. C., Aduan. Catalysis 9, 114 (1957). (5) Blyholder, G., J . Catalysis 2, 138 (1963). (6) Boreskov, G . K., Advan. Catalysis 15, 285 (1964). (7) Buckley, R. P., Szwarc, M., Proc. Roy. Soc. (London) Ser. A240, 396 (1957). ( 8 ) Butt, N. S.,Fish, A,, J. Catalysis5,205,494 (1966). 0 ) Butt, N. S., Fish, A., Saleeb, F. Z . , Ibid., 5, 508 (1966). (10) Callahan, J. L., Grasselli, R. K., Am. Inst. Chem. Engrs. J. 9, 755 (1963). (11) Chapple, F. H., Stone, F. S., Proc. Brit. Ceram. Soc. 1, 45 (1964). (12) Cossee, P., van Reijen, J., Proc. 2nd Intern. Congr. Catalysis, Paris, 1968, 1 6 7 0 (1961). (13) Crawford, E., Quart. Rev. (London) 14, 378 (1960). (15) De Boer, J. H., J . Catalysis 3 , 32 et seq (1964). (16) D’Eye, R . W. M., Wait, E., “X-ray Powder Photography in Inorganic Chemistry,’’ Butterworths, London, 1960. (17) Eischens, R. P., Pliskin, W. A., Advan. Catalysis 10, 1 (1958). (18) Gale, R. L., Haber, J., Stone, F. S., J. Catalysis 1, 32 (1962). (19) Garcia de la Banda, J. F., Ibid., p. 136. (20) Garnett, J. L., Sollich-Baumgarten, W. A., Advan. Catalysis 16, 95 (1966). (21) Germillion, A. F., Knox, W. R., J . Catalysis 1, 216 (1962). (22) Golovina, 0. A,, Isaev, 0. V., Sakharov, M. M., Dokl. Akad. N o d . S.S.S. R. 142, 619 (1962). (23) Guinier, A., “Small Angle Scattering of X-Rays,” Wiley, New York, 1955. (24) Hartshorne, N. H., Stuart, A., “Crystals and the Polarizing Microscope,” 3rd ed., Edward Arnold, London, 1960. (25) Hedvall, J. A., Kinetika i Kataliz. 3 , 3 (1962). (26) Ioffe, I. I., Ezhkova, Z . I., Lyubarsky, A. G., Kinelika i Kataliz3, 194 (1962). (27) Iofle, I. I., Ezhkova, 2. I., Lyubarsky, A. G., Zh. F i z . Khirn. 35, 2348 (1961). (28) Isaev, 0. V., Margolis, L. Ya., Roginsky, S. Z., Z h . Obshchei Khim. 29, 1 5 2 2 (1959). (29) Isaev, 0. V., Margolis, L. Ya., Sazanova, I. S., Dokl. Akad. Nauk. S.S.S.R. 29, 141 (1959). (30) Kehl, W. L., Poole, C. P., Jr., MacIver, D. S., J. Catalysis 1, 407 (1962). (31) Kazansky, V. B., Ezhkova, Z . I., Lyubarsky, A. G . , Voevodsky, V. V., Iofle, I. I., Kinetik~i Kataliz 2, 862 (1961). (32) Kazansky, V. B., Pecherskaya, Yu. I., Zbid., p. 454. (33) Kiselev, A. I. “The Structure and Properties of Porous Materials,” Everett and Stone, Eds., Butterworths, London, 1958, p. 195. (34) Klug, H. P., Alexander, L. E., “X-ray Diffraction Procedures,” Wiley, New York, 1954. (35) Kominami, N., Shibata, A., Minekawa, S., Kogyo Kagaku Zarshi 65, 1510 (1962). (35a) Levenspiel, O.,“ Chemical Reaction Engineering,” New York, John Wiley and Sons, Inc, 1962. (36) Lunsford, J. H., J. Phys. Chem. 3, 293 (1966). (37) MacIver, D. S., O’Reilly, D. E., J . Phys. Chem. 70, 3464 (1966). (38) McCain, C. C., Cough, G., Godin, G. W., Nature 198, 989 (1963). (39) “Magnetochemistry,” 2nd ed., Interscience, New York, p. 374, 1956. (39A) Proc. 3rd Intern. Congr. Catalysis, 586 (1964). (40) Ritter, H. L., Drake, L. C., IND. ENG.CHEM.(ANAL.ED.) 17, 782 (1945). (41) Roiter, V., Kinetiko i Katalit 1, 63 (1960). (42) Ronney, J. J., Pink, R. L., Proc. Chem. SOC.70, 142 (1961). (43) Rooney, J. J., Webb, G., J . Catalysis 3,488 (1964). (44) Sachtler, W. M. H., De Boer, N. H., Proc. 3rd Intern. Congr. Catalysis, Amsterdam, 252 (1965). (45) Sancier, K. M., J . Catalysis 3, 293 (1964). (46) Ibid., 5 , 314 (1966). (47) Selwood, P. W., Advan. Catalysis 3, 27 (1951). (48) Simard, G. L., Steger, J. F., Arnott, R. J., Stiegel, L. A,, IND. ENC.CHEM. 47, 1424 (1955). (49) Sixma, F. L. J., Rec. Trau. Chem. 82, 901 (1963). (50) Smidt, J., Chem. Znd. London, 1962, p. 54. (51) Suhrsann, R., Welder, G., J . Catalysis 1, 208 (1962). (52) Turkevich, J., Stamires, D. N., J. Am. Chem. Soc. 86, 749 (1964). (53) Terenin, A., Aduan. Catalysis 15, 227 (1966). (54) Veatch, F. Callahan J. L. Milberger E. C., Foreman, R. W., Proc. 2nd Inter. Congr. Ca)talysis Paris: 1!360,’2647 (1961f. (55) Voge, H. H., Wagner, C. D., Stevenson, D. P., J . Catalysis 2, 58 (1963). (56) Wheeler, A., Catalysis 2, 105 (1955). (57) Wolkenstein, T., Aduan. Cutalysis 12, 189 (1960). (58) Young, D. M., Crowell, A. D., “Physical Adsorption of Gases,” Butterworths, London, 1962, p. 182.
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