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R. F. WATERS, J. B. PERI, G. S. JOHN,' and H. S. SEELIG Research and Development Department, Standard Oil Co. (Indiana), Whiting, Ind.
High Temperature Changes in
Alumina Structure and Activity of PlatinumAlumina Catalysts At high temperatures, reforming catalysts prepared from gammaalumina retain surface area, resist transformation of the alumina, and retain reforming activity better than those prepared from the eta form PLAmruM-on-alumina reforming catalysts become fouled with carbonaceous deposits during use. These can be removed, and the catalytic activity can be completely restored by a proper regeneration. However, if overheating occurs surface area is destroyed, and the active gamma or eta alumina is transformed into catalytically inactive alpha or theta ( 6 ) . These changes are irreversible and result in permanent deactivation.
Literature Background Subject Ref. Nomenclature and x-ray patterns of (6) aluminas Calculation of surface areas from (1) primary-particle size in SiOz-AlzOa gels Importance of self-steaming in (7) high-temperature aging of SiOzAh08 gels Transformation of y- to a-AleOa (4) below 500' C. at water vapor partial pressure of 2000 p.s.i. Theory of diffusion in solids (6)
I n the work described here, effects of temperature and water vapor on surface area and transformation were measured, as well as the effect these changes have on catalytic activity. The results showed that reforming catalysts prepared from gamma-alumina are better able to maintain activity in a regenerative reforming process. The investigations were supplemented by electron microscopestudies.
silica glass tubes that extended about 12 inches beyond the furnace at both ends. Air passed over the catalyst was either dry or saturated with water vapor at 25" C. Total flow was metered and then divided into seven equal streams so that the flow rate through each tube was 2 liters per hour. A catalyst sample of about 3 cc. was placed in the cool outlet portion of each tube and, at the desired time, was pushed into the heated central portion. Because the catalysts were tested in pellet form, sample boats were unnecessary. The test was terminated by pushing the sample on through the tube to the cool inlet portion. Heating rate of the catalyst was several hundred degrees per minute, except in one experiment where the temperature was raised only 1' C. per minute from room temperature to 927' C. This procedure minimized self-steaming by sweeping out most of the liberated water at low temperatures. After heat treatment, surface areas were measured by the conventional Brunauer-EmmettTeller method. Alumina type was determined by conventional x-ray diffraction techniques (6).
For the electron micrographs, the catalyst was ground by mortar and pestle, with a small amount of oleic acid as a dispersant and suspended in iso-octane (2,2,4-trimethylpentane). The suspension was deposited on a 200-mesh screen covered with a graphite film, dried, and then washed with pentane. For each sample, a t least 20 fields were photographed at a magnification of 17,000 diameters with an RCA EMU-2D electron microscope. Catalyst activities were measured in a reforming test in which low-octane naphtha is converted to high-octane gasoline ( 3 ) . The ability of the catalyst to accomplish dehydrocyclization is measured. Activity is proportional to dehydrocyclization ability and, for heattreated catalysts, is relative to comparable fresh catalyst activity. Changes in Surface Area, Alumina Type a n d Structure
Water catalyzes the effects of temperature on surface area and alumina type (Figures 1 and 2), but it has less effect on the gamma catalyst. Although selfsteaming is important with silica-alumina IO0
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Experimental
The gamma-based ultraforming catalysts had an area of 220 square meters per gram and a pore volume of 0.41 cc. per gram; the eta-based experimental catalyst area was 225 square meters per gram, and pore volume was 0.44 cc. per gram. Catalysts were heated in air in a horizontal tubular furnace at a series of temperatures for various lengths of time. In the furnace was a stainless steel block with two thermowells and seven longitudinal holes containing 15-mm. 96% 1 Present address, Notre Dame University, South Bend, Ind.
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Figure 1. When catalysts are heated in wet or dry air, area for both drops rapidly at first and then approaches limiting values Open symbols, gomma catalyst Solid symbols, eta catalyst
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Figure 2. In wet or dry air, etaalumina goes rapidly to theta-alumina; transformation of gamma- to alpha-alumina is slower Open symbols, alpha- from gamma-alumina Solid symbols, theta- from eta-alumina
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catalyst ( 7 ) , the slow heating experiment showed that the effect on alumina catalyst is moderate. Aluminas consist of aggregates of small primary particles, similar to those found in silica-alumina catalyst ( I ) , ranging in diameter from about 25 to over 200 A. The size and manner of aggregation of these particles determines surface properties (2). Primary particles of gamma catalyst are randomly packed; those of eta catalyst are frequently packed in an orderly fashion that results in the formation of rectangular or hexagonal platelets. These platelets often appear to be stacked in laminar aggregates. Heating in either dry or wet air causes growth of the primary particles. For eta catalyst, the ordered structure of the aggregates observed in the original catalyst is preserved. However, in both catalysts, the primary particles composing the aggregates become more irregular in shape.
Mechanism of Structural Changes
Initial Growth. During initial growth, the smallest, least-stable particles rapidly lose identity by diffusing into and becoming part of neighboring particles. This process decreases in rate as particle size increases at constant temperature. At higher temperatures, a larger percentage of the particles grow, and a larger reduction in area occurs. But, initial growth is not accompanied by appreciable loss of pore volume. Because both fresh catalysts have about the same average particle size, differences must be due to the type of alumina present. Eta catalyst loses area more rapidly because of an intrinsic difference in crystal lattice or because of the greater number of interparticle contacts associated with its more-ordered aggregate structure. Water does not change the mechanism of the initial growth in the gamma catalyst; results obtained with both dry and wet air fall on the same line when area is plotted against pore volume. Comparable data on eta catalyst are not 150
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available. Water probably increases the rate of surface-area decline for both catalysts by increasing mobility of ions ( 4 ) . Transformation. Alpha-alumina begins to appear in the gamma catalyst when the area is about 130 square meters per gram (Figure 3). Apparently the alumina will not begin to transform at a measurable rate until the primary particles reach a critical size. The reason for this behavior is not clear. Results for the eta catalyst indicate that it also has a critical surface area, although no points were obtained below 35Yo theta alumina. Differences in the direction and rates of the transformation of the gamma and eta catalysts may result from differences in structure. Both aluminas are believed to contain structural vacancies (6) as well as the random vacancies that exist in all real solids. The presence of such defects can assist the diffusion of ions through the lattice ( 5 ) . Differences in the observed transformations may therefore be due to differences in amount or kind of defect structure. Structural differences revealed by the electron microscope may also govern the way transformation occurs. Final Growth. Because transformation of either gamma- or eta-alumina involves a density change of only 1070, this can account for only a small decrease in surface area. Once transformation begins, surface area decreases further and faster than can be explained by a combination of continued initial growth and transformation, thus suggesting the growth of alpha- or theta-alumina particles. Final growth probably occurs by a different mechanism than does initial growth. Its rate is relatively rapid, even though overheating may have removed much of the original defect structure. Particle size should affect final growth as it does initial growth; the smallest particles will tend to lose surface energy by increasing in size. Effects of differences in temperature, alumina type, and water vapor on final growth are also shown in Figure 3. In gamma catalyst. the final-growth rate in dry air is slower than the transformation
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Catalyst Activity
Overheating the catalyst h q the expected effect on activity. Eta catalyst loses some activity before any transformation occurs (Figure 4). A given amount of theta-alumina has a less deleterious effect upon activity than does the same amount of alpha-alumina. Because theta-alumina forms at a lower temperature than alpha: the surface probably undergoes less chemical change, and there is less impairment of catalytic activity. However, higher temperatures are required to change gamma- to alphaalumina, and a reforming catalyst prepared from gamma-alumina can withstand a more severe thermal treatment before it loses any activity (Figure 4). Acknowledgment
The authors are indebted to \I7. A. Kimball for x-ray measurements and their interpretation. literature Cited (1) Adams, C. R., Voge, H. H., J. Phyys. Chem. 61, 722 (1957). (2) Ashley, K. D., Inres, W. B., IND.ENG. CHEM.44, 2857 (1953). (3) Brennan, H. M., Seelig, H. S.. Vander. Haar, R. W . , U. S. Patent 2,840,514 (June 24, 1958). ( 4 ) Cooke, P. h‘.; Haresnape, J. N., Trans. Faraday SOC.43, 395 (1947). (5) Jost, W., “Diffusion in Solids, Liquids, Gases,” Academic Press, S e w York, 1951.
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(6) Russel, A. S., Gitzen, W. H., Newsome, J. W., Ricker, R. W., Stowe, V. W., Stumpf, H. C.: Wall, J. R., Wallace,
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Paul,- “Alumina Properties,” Tech. Paper No. 10 (revised), Aluminum Co. of America, Pittsburgh, Pa., 1956. (7) Schlaffer, W. G., Morgan: C. Z., Wllson, J. N.. J . Phvs. Chern. 61. 714 (1957).
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rate at the lower temperatures of 899’ and 943’ C. Therefore, the low-ternperature points lie on separate curves, and the loss of area depends primarily on the initial growth process; final growth as defined here is probably negligible. As temperature is increased, a point is reached at which the rates of transformation and final growth become equal. Data obtained at any higher temperature therefore fall on the same curve. In eta catalyst, the final growth is more rapid than the transformation; all the points for this catalyst therefore lie on a single line. The different growth rates for the two catalysts are probably due to the structural differences. Water increases the rate of final growth even more than it increases the transformation rate. Therefore, for both catalysts, results obtained with wet air a t different temperatures all fall on the same lines in Figure 3.
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Figure 3. Transformation does not begin until surface area falls below a critical value Open symbols, dry air Solid symbols, wet air
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Figure 4. For equal quantities of high temperature alumina, gamma-based catalyst has a lower relative activity, but at equal severity of treatment, it has a higher activity
INDUSTRIAL AND ENGINEERING CHEMISTRY
RECEIVED for review March 16, 1959 ACCEPTEDMarch 3, 1960 Division of Petroleum Chemistry, 134th Meeting, ACS, Chicago, Ill., September 1958.
