1424
INDUSTRIAL AND ENGINEERING CHEMISTRY
Craze resistance and gloss and color retention are probably the three basic properties that any enamel must possess a t the required temperature level. For any particular application, a system must pass many specialized tests. No single term or expression was found that would give a clear picture of the changes occurring in an enamel film on prolonged heating. Considerable time was spent attempting t o show that copolymerization was occurring b y a transesterification reaction between the excess hydroxyl groups of the alkyd resin and the ethoxy groups of the siloxane. Although no direct chemical proof of the reaction was devised, it is believed that the evidence pointing to copolymerization is convincing. The three results cited to show copolymerization are all consistent with the hypothesis. An added factor is the difference in properties between known alkyd and silicone mixtures and those thought to be copolymers. The poor gloss retention of the mixtures is outstanding. The mixtures also discolor excessively considering their composition. The increased craze life of mixtures of an alkyd and phenylethoxypolysiloxane over a copolymer of the same composition is quite possibly peculiar to this system where the silicone is brittle. However, in this instance the difference between mixtures and copolymers shows up in craze life. The copolymers, being chemically united, tend to become hard and brittle, and tend to crack under the heating and cooling stresses. The mixtures, on the other hand, have stable silicone polymers imbedded in an alkyd matrix which decomposes and ruins the film integrity. The resulting film is not coherent enough to craze as easily as the copolymer film. One of the conclusions reached during this project was that the alkyd portion of the copolymer contributed most to degradation of properties and presented the best opportunity for improvement. A study of heat-resistant alkyd resins has subsequently been made, and the alkyds made with 2-ethylhexanoic acid, trimethylolethane, and phthalic anhydride have much better color and gloss retention than those made with lauric acid, glycerol, and phthalic anhydride. An alkyd-silicone resin containing 50% silicone was recently made along these lines, and it has better color retention than a commercial alkyd-silicone resin having 75% silicone. However, the gloss retention was not as good as the commercial resin. Results indicate that there is still room for substantial improvement in the alkyd portion of the alkyd-dicone copolymers.
Vol. 47, No. 7
ACKNOWLEDGMENT
The authors wish to express their appreciation to the Lilly Varnish Co. which sponsored the research and furnished many of the materials used; to Linde Air Products Co. for furnishing many of the organosilicon compounds and many helpful suggestions; and to Dow-Corning Co., General Electric Co., the Plaskon Division of Libbey-Owens-Ford, and Armour and Co. which obligingly furnished materials for this research. LITERATURE CITED
(1) Bowman, A., and Evans, F. hI., Brit. Patent 583,754 (Nov. 28, 1945). (2)
British Thompson-Houston Co., Ltd., Ibid., 650,241 (Feb. 21, 1951).
(3) Doyle, C. D., and Nelson, A . C., U. S. Patent 2,587,295 (Feb. 26, 1952). (4) Flory, P. S., J . Am. Chem. Soc., 63,3083 (1941). (5) Goodwin, J. T., and Hunter, M. J., U. S. Patent 2,584,341 (Feb. 5, 1952).
(6) Ibid., 2,584,342. (7) Ibid., 2,584,343. ( 8 ) Ibid., 2,584,344. (9) Ibid., 2,589,243 (March 18, 1952).
(10) Hedlund, R. C., Ofic. Digest Federation P a i n t & V a r n i s h Production Clubs, 26, 352, 356-7 (1954). (11) Hedlund, R. C., Org. F i n i s h i n g , 15, 16-18 (1954). (12) Hunter, M. J., and Rauner, L. A., U. S. Patent 2,584,351 (Feb. 5, 1952). (13) Ibid., 2,628,215(Feb. 10, 1953). (14) Kress, B. H., and Hoppens, H. A., Ofic.Digest Federation Paint & V a r n i s h Production Clubs, No. 333, 689-99 (1952). (15) Lawson, W. F., U. S. Patent 2,048,799(July 28, 1936). (16) (17) (18) (19) (20) (21)
MeGregor, R. R., “Silicones and Their Uses,” pp. 112-16, 284, hIcGraw-Hill, New York, 1954. Millar, R. L., U. S. Patent 2,663,694(Dee. 22, 1953). Patterson, J. R . , IND.ENG.CHEM.,39, 1376 (1947). Patterson, J. R., Ofic. Digmt Federation P a i n t & V a r n i s h Production Clubs, No. 266, 144 (1947). Patterson, J. R., Org. F i n i s h i n g , 8, No.4 , 32-7 (1947). Pattison, E. S., P a i n t V a r n i s h Production, 42, No. 10,23-5, 81 (1952).
RECEIVED for review October 1 1 , 1954. ACCEPTED February 16, 1955. Division of Industrial and Engineering Chemistry, 126th Meeting, AC8, New York, September 1954.
Vanadium Oxides as Oxidation Catalysts G. L. SIMARD1, J. F. STEGER2, R. J. ARNOTT?, AND L. A. SIEGEL Research Division, .4merican Cyanamid C o . , Stamford, Conn.
T
HE selective catalytic oxidation of hydrocarbons-for example, of naphthalene or o-xylene to phthalic anhydrideis a process of considerable commercial importance. Approsiinately 300,000,000 pounds of phthalic anhydride were produced in the United States in 1953 and 315,000,000 pounds have been projected for 1954. As normally operated, the process for the catalytic oxidation of naphthalene consists of passing a stream of vaporized hydrocarbon a t about 1 mole % concentration in air through a catalyst bed a t temperatures between 400’ and 500’ C. for contact times of a few tenths of a second. Typical yields are 70 to 90 pounds of phthalic anhydride per hundred pounds of naphthalene. Other products of reaction are carbon oxides, 1 2
a
Present address, Schlumberger Well Surveying Corp., Ridgefield, Conn. Present address, Morton Salt Co., Chicago, 111. Present address, The Texas Co., Houston, Tex.
water, and minor quantities of quinones and maleic anhydride. I n fixed bed operation, vanadium oxide-type catalysts supported on silicon carbide are frequently used. As an approach to the development of oxidation catalysts, a fundamental study of unpromoted vanadium oxides was undertaken. o-Xylene was chosen as the hydrocarbon for oxidation because i t permitted the use of a simple feed system. For the most part, the measurements were of bulk properties-i.e., properties of more regular structures than would be expected on the catalyst surface. A qualitative picture of the surface may nevertheless b e obtained since the arrangement of the surface atoms, although subject to considerable disorder, will tend toward the stable structures of the bulk oxides. The studies have been of a survey nature, but the results have helped clarify the catalytic behavior of vanadium oxides. The vanadium oxides formed in the oxidation of o-xylene are
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1955
/
I
I
I
1425
a
DEPTH
Figure 1. Reduction, exotherm temperature, and oxide composition of catalyst after normal operation with 1.1 mole 70 o-xylene in air
considered first. The properties of those oxides which are believed to determine the catalytic mechanism are then described. COMPOSITION O F A VANADIUM OXIDE CATALYST DURING OXIDATION O F o-XYLENE TO PHTHALIC ANHYDRIDE
From the color and chemical analyses of used catalysts initially charged as vanadium pentoxide, it definitely appeared that reduction occurred during reaction. Indications of this have been published ( 2 4 , 17). X-ray and chemical measurements were undertaken to determine the actual oxide phases present. At a predetermined time during the oxidation of o-xylene, the coniposition of the catalyst was preserved b y rapidly displacing the hydrocarbon and air mixture with prepurified grade nitrogen and lowering the temperature. The catalyst was then removed and subjected t o examination. Composition was correlated with bed depth and catalyst temperatures. The catalyst, as charged, was a 5y0coating of vanadium pentoxide on 6- to 8-mesh silicon carbide granules. A steel tube converter, 23 inches in length and 0.66 inch in internal diameter, was used. The converter was immersed in a molten salt bath with one end connected to a liquid displacement feeder containing the o-xylene and the other end to a water reflux system for collecting phthalic anhydride. A preheater vaporized and raised the temperature of the o-xylene to the salt bath temperature before i t entered the catalyst bed. Heat liberated b y the reaction raises the catalyst temperature above that of its immediate surroundings. This temperature difference, expressed as a function of the catalyst bed depth, is called the exotherm. The bath temperature was controlled to 1 2 ’ C. Temperatures were measured b y an axially positioned movable thermocouple within the converter tube. The phthalic anhydride produced was determined b y titration. The following conditions of operation were studied: 1. Period of normal operation with 1.1 mole % o-xylene 2. Initial 3 hours of operation (break-in period) with 1.1mole yo o-xylene 3 . Overloading with 3.3 mole yoo-xylene after normal operation with 1.1 mole yo o-xylene
Conditions prior t o quenching are shown in Table I. The data are shown in Figures 1, 2, and 3. I n the upper half of the figures, the percentage reduction of the catalyst determined
,
Figure 2. Reduction, exotherm temperature, and oxide composition of catalyst after 3-hour break-in with 1.1 mole 70o-xylene in air
chemically versus depth in the catalyst bed is shown as a full line. The percentage reduction is expressed as moles of oxygen lost from S’ZO~, reduction at 1370 corresponding to an over-all vanadium t o oxygen ratio for V204.34, and t h a t at 20% for VzO4. The broken curve shows the exotherm temperatures of the catalyst read on the right hand ordinate. I n the lower half of the figure are the composition data as determined b y quantitative measurement of x-ray diffraction patterns of the catalyst coating. At the point marked “ E J ’ the catalyst surface was examined by electron diffraction. Normal Operation with 1.1 Mole % o-Xylene. There was general correspondence between the exotherm and the degree of catalyst reduction as shown in Figure 1. The crystalline phases identified were the commonly known oxides, VZOSand V204, and the less familiar oxide Vz04.34 (also called V12028). . Electron diffraction data obtained at the position of the exotherm maximum indicated t h a t this oxide overlaid the VzO4 and therefore contracted the gaseous reactants. Although well known crystallographically, the only published reference to the occurrence of VZOW in a reaction is a suggestion that it may form in the reduction of VpOa with sulfur dioxide ( 9 ) . There mas an obvious dependence of relative ratio of oxides on catalyst bed depth and accordingly on the concentration of oxidizable hydrocarbon. This was also shown b y comparison of these data with data obtained at 0.5 mole % o-xylene concentration. At 0.5 mole %, the oxidation level of the catalyst was roughly twice that a t 1.1 mole yo. Concerning the relative positions of V Z Oand ~ VzO4.34, the more reduced oxide could plausibly b e the underlying layer. The
Table I.
Operating Conditions
Catalyst: 5 % VrOs coating on 6- t o 8-mesh S i c Feed: o-Xylene (93% purity) in air. Contact time, 0.26 sec.; bath temp., 460’ C, Operating Time, Conversion to Total Hours Continuous Total MoleAcids, % Operation Normal 1.1% feed 41 5 58 Initial 3-hour break-in with 1.1% feed 3 3 57 Overloading with 3.3% o-xylene 45 5
INDUSTRIAL AND ENGINEERING CHEMISTRY
1426
catalyst coating may be thought of as a loose agglomeration of vanadium oxide particles. The feed can penetrate this porous arrangement but will be relatively stagnant in the interparticle voids. There will therefore be a longer contact time and higher local temperatures in the voids arising from pobrer heat dissipation. This would result in more severe reducing conditions than those existing a t the catalyst surface. This condition would persist until the underlying particles were inactivated by overreduction. This change is probably one of several occurring during the break-in period. Concerning the presence or absence of Vz06, a superficial layer of about 50 A. or less over V ~ 0 4 . 8 ~ or an amount less than 5% could have escaped detection by electron diffraction.
301
Vol. 47, No, 7
because the increased amount of o-xylene oxidizing produced heat faster than the heat could be removed. As the catalyst a t any point became inactivated, the zone of maximum temperature moved down the catalyst tube and finally passed out of the exit end. At the exit portion of the tube VPOSwas present and essentially only VzO4 elsewhere. V Z O was ~ probably a result of the high exotherm a t the exit. The small quantity of VzO4.34 found could have been residue core in the individual particles which were formerly V204.a4in the catalyst coating, the time of reduction having been too short for complete conversion t o V Z O ~ . Since catalytic oxidation to phthalic anhydride completely stops under the conditions represented by the quenched catalyst, neither T i 2 0 4 nor V203 can be considered catalytically active. The general conclusions from these data are:
1. Vanadium in an unpromoted vanadium oxide catalyst during catalytic oxidation is in V + 4and V+6 states. 2. I n bulk, these states are associated with oxygen in three crystalline forms-VZ04, vZo4.34, and Vz06. 3. The relative proportions of these oxides are dependent on the hydrocarbon concentration in the feed air; the changes with time of operation observed were in a direction t o establish a characteristic composition. 4. VZO4and VzOs are not catalytically active for the production of phthalic anhydride or intermediates. Further consideration of the active catalyst can therefore be limited to Vz06, V&a.a4, and t o intermediate structures which may exist on the surface. INTERACTION OF 0-XY LENE WITH VANADIUM PENTOXIDE
z 0 t
8 504 8 "204.34
l o
c
"
, ,'/ 3
m
I
I
8
12
I
I
16 INCHES
DEPTH
Figure 3. Reduction and oxide composition of catalyst after overloading with 3.3 mole q~ o-xylene in air.
Three-Hour Break-in with 1.1 Mole % *Xylene. The data, Figure 2, are for a reaction quenched after only 3 hours of operation. This is a period of considerable instability of the exotherm and of product yield. V204.34 was again found overlying VzOl. The catalyst was not as reduced as after prolonged use, but there was a definite trend in the entry region t o the values observed after longer operation. These data suggest that a steady state catalyst composition is eventually attained. Overloading with 3.3 Mole % +Xylene after Operation with 1.1% o-Xylene. The effect of drastically increasing the hydrocarbon concentration during operation is of interest. Under such conditions, catalytic oxidation stops and is not re-established until the catalyst is reoxidized by long heating in air. A reaction which had been operated normally for a period of 45 hours in 1.1%o-xylene was suddenly overloaded with a 3.3% feed. This still includes twice the amount of oxygen necessary t o give phthalic anhydride and water or 0.6 times the amount to give carbon dioxide and water. At first, the temperature of the exotherm increased without changing position. Then the maximum progressively moved down the tube while the temperature continued t o increase. I n a total of 3 t o 4 minutes, the exotherm had passed to the exit end of the converter and there a temperature of 600" C. was recorded. The catalyst was immediately quenched. From Figure 3, inactivation of the catalyst is definitely associated with overreduction. The temperature increased rapidly
The reduction of a catalyst charged as vanadium pentoxide during reaction suggested t h a t oxygen ions of the catalyst might interact directly with the hydrocarbon. This would be of particular interest if the interaction formed phthalic anhydride. A study was therefore made of the products obtained on passing o-xylene in the absence of gaseous oxygen over the catalyst. The 23-inch converter system already described was used in these experiments. The feed was 1 mole yoo-xylene in prepurified grade nitrogen (less than 0.002% H P or OS). The total amount of hydrocarbon fed was about that oxidizable to phthalic anhydride by the oxygen available in reducing V ~ O Sto VzO4.34. The duration of the run was 2 to 5 minutes after which the hydrocarbon was shut off and the catalyst quenched. Other experi, mental conditions and the results are given in Table 11. The acids produced were those normally ,observed in catalytic oxidation-phthalic and maleic. The COZ/CO ratio was of the order usually found in the break-in period of a catalyst. The conversion calculated from products was 30% of the o-xylene fed.
Table 11. o-Xylene on Vanadium Pentoxide Catalyst: VzOs on 6- to 8-mesh S i c Feed:
Oxide Surface Area (Total), Sq. M.
60-125
1% +Xylene (90-93% purity) in nitrogen. Contact time, 0.1-0.2 sec.; temp., 463' C.
Reduction Chemical, % X-ray 12-13 VOOs VnOm (trace VzO4)
+
Products 847 phthalic anhydride 16% maleic anhydride COe/CO 5.3
-
The proper active surface and a source of oxygen evidently existed for the formation of phthalic anhydride. Except when the hydrocarbon first contacted the surface, the formation of phthalic could not have occurred on a simple V P O structure. ~ Phthalic may have formed on one o r all of the variety of intermediate defect structures between Vz06 and VPOM occurring during reduction. The supply of oxygen to the hydrocarbon a t the oxide surface was necessarily through the diffusion of vana-
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1955
dium or oxygen ions in the solid oxide and occurred rapidly. The role of gaseous oxygen in a conventional catalytic oxidation could therefore be a secondary one, with the reaction proceeding through the following steps: 1. The chemisorption of hydrocarbon on the catalyst surface 2. T h e reaction of the chemisorbed hydrocarbon with oxygen *on8 of the catalyst 3. Desorption of intermediate or final products 4. Replenishment of oxygen to the catalyst from the feed air
0.f
0 ,
,
X c o
,
m:
xo
,
x;
-0.2
300
200
IO0
400
1427
Vanadium pentoxide preparations t h a t were insufficiently was added desorbed oxidized or t o which a small quantity of oxygen on heating t o 400" C. and adsorbed oxygen on cooling to room temperature. I n the neighborhood of 400" C., irreversible adsorption took place according to a square root of time law. Figure 46 shows the oxygen take-up of a sample prepared by adding Vz04t o molten VzOj under nitrogen in the mole ratio of 1 :27. The amount of impurity V +4 in these experiments was too small t o permit its detection either as a constituent of a separate crystalline phase or as a solid solution with vanadium pentoxide. A conclusion which can be drawn is t h a t the lattice defects caused by the introduction of V+* ions, while causing no obvious change of the vanadium pentoxide structure, markedly changed the response of oxide t o gaseous oxygen. This was also indicated in the studies of oxygen exchange with vanadium pentoxide b y Farkas and coworkers (6) who state:
.
, . in most instances, the exchange reaction was controlled by the surface reaction and the diffusion of the oxygen in the bulk of Vz05 wa5 fast com ared with the surface reaction. It is believed that this high & f u s i o n rate was made possible by lattice imperfections caused by the presence of foreign ions or other lattice defects.
X- T INCREASING
8 9
0 - T DECREASING
1
14
An activation energy of 45 kcal. per mole was obtained for oxygen exchange on vanadium pentoxide, and this was interpreted as indicative of either the dissociation of oxygen on the surface of the oxide or the loosening of V-0 bonds.
0.6 0.4
02 -0.2 O I
X X
x-K
i
x-,x
ii./
-0 4 c
I
I
I
zoo
loo
TEMPERATURE IN
1
'c.
I 300
I
I 400
Figure 4. Adsorption and desorption of oxygen by vz05 at oxygen pressure of 170 mm. (a) Fully oxidized
VaOs; ( b ) Vi05 containing 3.7 mole % Vi01
Recent experiments with o-xylene using normal conditions and conversions below 20% have shown o-tolualdehyde to be an isolatable intermediate. Accordingly, the transformation of o-xylene t o phthalic anhydride does not necessarily occur only in one stage of adsorption. The question of whether an intermediate desorbs and then oxidizes further either in the gas phase or on readsorption is important t o the mechanism. The described experiments in the absence of gaseous oxygen suggest t h a t ,the gas phase need not be considered. There may, however, be successive desorptions and readsorptions of intermediates in the course of reaching phthalic anhydride, which is a product relatively stable over the catalyst. "Overoxidation" products such as maleic acid and carbon dioxide are evidence of variations in the activity of adsorption sites on the catalyst or in the mode of breakdown of the hydrocarbon. INTERACTION OF OXYGEN WITH VANADIUM OXIDES
The interaction of gaseous oxygen with V Z O ~and . ~ ~Vz06was studied b y measurements of oxygen uptake. I n VZO~,the adsorption and desorption of oxygen was followed on varying t h e temperature a t a fixed pressure. Vz04.84 was studied b y following the uptake of oxygen as a function of time a t fixed temperatures and a t the partial pressure of oxygen in air. As shown in Figure 4 4 V Z O ~prepared , from a fusion through which oxygen had bubbled, showed no measurable adsorption or desorption of oxygen with the experimental sensitivity of h O . 1 ml. of oxygen per gram of vanadium pentoxide, a t standard temperature and pressure on cyclic heating and cooling between 25" and 400" C. at 170 mm. oxygen pressure.
Figure 5.
Uptake of oxygen by temperatures
VzO4.84
at various
I n V Z O ~ the . ~ ~uptake , of oxygen in the temperature range 404" to 497" C. followed a square root of time or parabolic rate law, Figure 5 . This behavior is expected where diffusion processes are involved and is commonly observed in the oxidation of metals. A theoretical derivation for this type of process can be made on the basis of an ionic mechanism ( 4 ) . This law also described the oxidation of VzOa and V204. An activation energy of 30 kcal. per mole was calculated for t h e oxidation of V 2 0 4 . 3 4 , 30 for VzO4, and 17 for VZO~. These values are consistent with those for t h e oxidation of metals, the value for vanadium being 31 kcal. per mole (11). The uptake of oxygen by V 2 0 4 . 3 4 was independent of oxygen pressure above pressures of 10 to 20 mm. Although chernisorption takes place, i t is therefore not a rate determining step in the oxidation of V204.34 above 20 mm. The partial pressure of oxygen used in the catalytic reaction is 150 mm. The primary implication of these data is t h a t the uptake of
,'
INDUSTRIAL AND ENGINEERING CHEMISTRY
1428
Figure 6.
Structures of
V204.34
Vol. 47, No. 7
(left) and Vz06 (right)
VzOa.sa: monoclinic; a = 11.90 A., b = 3.67 A., c = 10.12 A,, p = 100°-51'; molecules per unit cell, 1 (VuOzs); space group C2/m. VZOS: orthorhombic; a = 11.48 A., b = 4.36 A., c = 3.55 A . , molecules per unit cell, 2; space group Pmn.
b
0 ~ 0 ~ 000 000
0
o o o o o(0)
Ib
V-0
LAYER
0 0
I o
0 0 0
a r
( b ) OXYGEN LAYER
000~0
o*o*o 0 0 0 \Ooo0 0 0 0 0
0
( c ) TOP
0
0
(I) Vt20
LAYER
(2)
0
LAYER
(3) V+-O
LAYER
(4) V t 4 0
LAYER
(5)
0
a (6)V+?O
of oxygen to vanadium oxides. I n view of the ability of a hydrocarbon to remove oxygen from the catalyst and form phthalic anhydride in the absence of gaseous oxygen, an ionic mechanism may also be hypothesized for supplying oxygen t o the hydrocarbon in a normally conducted catalytic reaction. It is postulated that a molecule of oxygen striking the catalyst surface first dissociates, then ionizes and becomes incorporated into the vanadium oxide structure. If the catalyst is to act as a transfer medium for oxygen, the changes in structure between the oxides v 2 0 4 . 8 4 and VsO; should occur readily to accommodate the addition or removal of oxygen ions. The relation of the two structures i s now considered and finally factors related to t h e electronic properties of the oxides which are pertinent to their action as catalysts are discussed. STRUCTURES OF VzOa AND VZO4.34 AND THEIR TRANSFORMATION
This section is a written version of a motion picture presented a t the 1951 AAAS Gordon Conference on Catalysis.
LAYER LPVrR
PLANE
Figure 7. Arrangement of vanadiums and oxygens in Vz04.34 Large open circles are oxygen, small open circles are V+6, and small solid circles are V+4. (a) Vanadium-oxygen layer i n the a-b plane; vanadiums are here either all V +E or V +4. (b) Oxygen layer i n a-b plane. (c) Top layer of vanadium and oxygens i n a-c plane illustrating the stacking of V - 0 and 0 layers to produce the VzOma structure
oxygen b y the catalysts is controlled b y V+4 and V + j ions acting as impurity centers in the otherwise orderly structural arrangements of V2O6 and V204.34, respectively. This behavior is qualitatively plausible according to present concepts on the effect of impurities on t h e properties of solids. Whether the impurity ions produce disturbances of charge only or of both charge and structure was not revealed b y the experiments. Considerations on t h e relation between Vz05 and V Z O ~structures .~~ in the following section, however, strongly suggest that local variations in structure can readily occur. T h e data thus far suggest a n ionic mechanism for the addition
(3)LESS MIGRATED
1 0 00 0 0 0 0
(4)
UNCHANGED
(5)
UNCHANGED
vao
Figure 8. Layer arrangement in a-c plane of vanadiums and oxygens after oxygen adsorption and migration of vanadiums and oxygen
The difference between VzOs-like and V~0~.~4-like structures is believed sufficiently small t h a t transformation from one t o the other may occur with only slight movements of the atoms. Energywise, this is a particularly favorable situation for the addition or removal of oxygen. The active catalyst surface is visualized as any of the series of structures intermediate between V Z O and ~ V204.a4-defect structures in a continuously changing pattern of V + 4and V+b. Figure 6 shows the structures of Vz04.a4 and V20, (1, IS).
July 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
From the crystallographic point of view, the structures of V204.34 (or Vlz0z~)and VZOSare complex. Qualitatively, Vz04.34 appears to build up in a fairly close-packed manner that is usually found in ionic crystals. Vanadium atoms are surrounded by six oxygens arranged octahedrally. The vanadium ions are present in both the V+4 and the V +6 state. V2O8 is built of deformed tetrahedra,
1429
valence requirements of the compound, and facilitates the description of the structural transformation. Now suppose layers 1 and 2 of Figure 7 c stripped off leaving a V+4-0 face. Oxygen molecules adsorb on layer 3, dissociate to oxygen atoms, and fill the positions of layer 2. The absence of Vf4 ions in layer 1 in the new system has altered the electrical balance of the structure causing a net repulsion between the V f 4 ions of layers 3 and 4. As a result of the unbalance, all the V f 4 ions and one of the oxygen ions of layer 3 migrate into the adsorbed layer, becoming a layer of oxygen ions and V r5 ions. The adsorbed layer is then a typical V-0 layer as in Figure 7a with the oxygen layer of Figure 7b behind it. The over-all arrangement is now like that in Figure 8. Structurally, the change has been to alternate V-0 layers with oxygen layers and to change the unit cell from monoclinic to orthorhombic ( a = 90"). While slightly involved in description, this has been accomplished by atomic migration through the distance of only a single atom layer. The complete transformation to the Ketelaar structure fpr Vz06is obtained by a further shift of vanadiums and oxygens in the manner shown in Figure 9. When the resulting layers are stacked on end, the VzOj structure, Figure 10, is formed.
2.5
Figure 9. Arrangement of Figure 7 with displacement paths of shaded VZOpairs shown
2.0
1.5
which join in chains, these chains then forming sheets. The structure is completed b y piling up the sheets. From the vanadium-oxygen separations, it is surmised that the binding between sheets is much weaker than that within a sheet. While the vanadiums are surrounded tetrahedrally b y four oxygens, the packing of tetrahedra is such that there is essentially an octahedral arrangement. [Since the original presentation of this material, a revised structural determination for VZOShas become available (5). The slight shifts in structure do not affect the structure transformation discussed but the vanadium-oxygen coordination is altered from tetrahedral to trigonal bipyramidal.] The transformation of VzO4.84 plus adsorbed oxygen to vzo5 is illustrated b y a hypothetical example. The reverse transformation, Vz06to Vz04.34, could be shown by simply reversing the steps of the example. The structure of VzOa.34 may be described as
1.0
a
s
.5
0
-5
ID
-15
I
10 .
I
1.5
I
2.0
I
2.5
I
3.0
I
3.5
V T x lo3 Figure 11. Variation of electrical resistance of vanadium oxides with temperature
The transformation illustrated is considerably simpler when viewed with three-dimensional models and is typical of the ease by which structure or charge defects, whatever their cause, may be compensated. This ease of adjustment is believed a most important characteristic of vanadium oxide catalysts. An independent discussion of a simple transformation has been given recently (8). Figure 10. Arrangement after shift of VzO pairs; stacking of these planes produces ICetelaar's Vz06 structure
composed of vanadium-oxygen layers and oxygen layers, shown in Figures 7a and 7b, which stack in the manner of Figure 7c. The crystal structure data do not distinguish between V f 4 and V f 6 ions and the ordered arrangement of Vf4 and Vf6 ions shown in Figure 7c is arbitrary. It has been adopted since it emphasizes the presence of two valences of vanadium, satisfies the over-all
ELECTRONIC PROPERTIES OF VzOa.st A N D V2Oa
During recent years success has been reported in the correlation of electronic properties with catalytic activity (7, IO). As is common to other oxidation catalysts, vanadium is a transition element capable of more than one state of oxidation. By definition, these elements have incompletely filled subshells in their electron configurations. Elemental vanadium is characterized by having only three electrons out of a possible ten in a 3d subshell and two electrons in the 4s shell. The primary adsorption of hydrocarbon may involve a 7r-orbital of the hydrocarbon and one of the orbitals of the vanadium ion. A well known 7r-complexing of hydrocarbons with metals is silver (19). A similar
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INDUSTRIAL AND ENGINEERING CHEMISTRY
process of adsorption has recently been postulated for benzene and naphthalene on nickel (16). The coexistence in the vanadium catalyst of two valence states between which electron interchange can readily occur without undue restraints of structure is believed to be a property of many catalysts (6). A measure of the availability of electrons from the solid for electron transfer between the reactants may be obtained from electrical conductivity measurements. Electrons trapped b y defects in the lattice can be removed with considerably less energy than electrons from ions in normal positions. Published results indicate that vanadium pentoxide behaves as an excess semiconductor-semiconduction caused by interstitial metal ions (8), Figure 11 shows measured values of resistance plotted against the reciprocal of temperature. The measurements were made on powder samples, under pressure, using a d.c. Wheatstone bridge. The activation energy for the conduction is calculated from the slope of the curve to be 7.6 kcal. as compared to the reported value of 9.2 kcal. for single crystals. Also shown in Figure 11 are resistance data for VzO4.84. Above approximately 100’ C., VzO4.84 appears to be a metallic-type conductor, and effectively no activation energy is required for electron conduction. Even below 100’ C., where an activation energy of 15 to 20 kcal. per mole is required, VzOd.84 is a better conductor than V Z Ob~y a factor of about 100. The addition of o.570 V+4 to VzOsshows a marked lowering in resistance and an approach to V ~ O ~ . ~ ~ - tconduction. ype It is therefore evident that electrons are readily available for participation in the catalytic reaction. I n VzO4.s4, free conduction exists a t temperatures of operation. I n Vz05, an activation energy is required, but it is smaller than that of the catalytic reaction. For intermediate compositions, such as 0.5% V+4 in V 2 0 5 , the resistance is markedly lower and approaches Vz04.84 in form. This behavior is entirely analogous to that of silicon containing boron impurity. I n the latter the following interpretation has been given: The impurities have moved so close together that appreciable overlapping of their wave functions occurs. Under these conditions, the extra electrons and impurity atoms play the same role as electrons and ions in metal, and the excess electrons move as a degenerate electron gas through the irregularly distributed array of positive ions (16). CONCLUSIONS
The authors picture the active surface on an unpromoted vanadium oxide catalyst as a dynamic surface of V+4, V+5, and 0 - 2 ions in continually varied and changing structures. These may have V104.a4-like features in certain areas and VzOb-like in others, the changes of structure balancing the interchange of valence. The areas of structural change may be only of unit or partial unit cell size and readily transform from one to the other
Vol. 47,No. 7
with but slight adjustment of ion positions. Many ions are in disordered positions b u t constantly exchange with more ordered ions. An ample supply of defect centers and low energy electrons are available for the reaction. Accordingly, the factors necessary for a good catalyst are inherently provided. This picture is in accord with the fact that a vanadium oxide catalyst is a “rugged” catalyst, one of small surface area and relatively insensitive to promotor additions. Concerning the interaction mechanism between the reactants, direct combination of a vanadium-hydrocarbon chemisorption complex on the surface with oxygen ions of the catalyst is suggested, followed by desorption of a partially or totally oxidized product and replenishment of the catalyst oxygen through adsorption and dissociation of oxygen from the gas phase. ACKNOWLEDGMENT
The authors are indebted to Geraldine Clifford and M. C. Botty for carrying out some of the experimental measurements and to K. W. Saunders, J. K. Dixon, and D. J. Salley for many helpful discussions. LITERATURE CITED
(1) Aebi, F., Helv. Chim. Acta, 31, 8 (1948). (2) Boros, J., 2. P h y s i k , 126, 721 (1949). (3) Bystrom, A,, Wilhelmi, K. A,, and Brotzen, O., Acta Chem. Scand., 4, 1119 (1950). (4) Cabrera, N., and Mott, N. F., Repts. Progr. in Phys., 12, 163 (1949). ( 5 ) Cameron, W., Farkas, A., and Litz, L., J. Phys. Chem., 57, 229 (1953). (6) Cook, M. A., and Oblad, A. G., IND.ENQ.CHEM.,45, 1456 (1953). (7) Dowden, D. A., J. Chem. Soc., 1950, p. 242. (8) Fenimorc, C. P., Division of Physical and Inorganic Chemistry, ACS, Kansas City, Mo., March 30, 1954. (9) Flood, H., and Kleppa, 0. J., J. Am. Chem. Soc., 69, 998 (1947). (10) Garner, W. E., Gray, T. J., and Stone, F. S., Proc. R o y . SOC. ( L o n d o n ) , 197A, 294 (1949). (11) Gulbransen, E. A., and Andrews, K. F., J. Electrochem. SOC.,97, 396 (1950). (12) Hepmer, F. R., Trueblood, K. N., and Lucas, H. J., J . Am. Chem. Soc., 74, 1333, 1338 (1952). (13) Ketelaar, J. A. A., 2. Krist., 95, 9 (1936). (14) Senseman, C . E., and Nelson, O., IND. ENG.CHEM.,15, 521 (1923). (15) Shockley, W., “Electrons and Holes in Semiconductors,” Van Nostrand, New York, 1951. (16) Suhrmann, R., and Schulz, K., J . Colloid Sci., Suppl. 1, 50 (1954). (17) Weiss, J. M., Downs, C. R., and Burns, R. M., IND. ENG. CHEM.,15, 965 (1923). RECEIVED for review November 24, 1954. ACCBIPTED February 3, 1955. Presented in part a t AAAS Gordon Research Conferenoe on Catalysis, Colby Junior College, New London, N. H., June 28, 1951.
