INFRARED AND GRAVIMETRIC STUDYOF SURFACE HYDRATION OF 7-ALUMINA
211
Infrared and Gravimetric Study of the Surface Hydration of -,-Alumina
by J. B. Peri Research and Development Department, American Oil Company, Whiting,Indiana
(Received July 80, 1084)
Infrared and gravimetric studies have yielded new information on the surface hydration of y-alumina. Dry y-alumina was found to "chemisorb" one molecule of water per 11-16 of surface a t lOO", depending on rehydration procedure. Desorption of chemisorbed water on subsequent heating under vacuum was studied at temperatures between 100 and 900". Characteristic dehydration isotherms showed that a rapid initial weight loss was followed by a slow continuing loss. Because of these characteristics, the extent of surface hydration is governed primarily by the drying temperature. In addition to the three major isolated hydroxyl bands previously noted in infrared spectra of dry alumina (3800, 3744, and 3700 cm.-I), two additional bands are often observed a t 3733 and 3780 cm.-'. Changes in the bands were studied as a function of temperature. Rehydration of the surface between 400 and 800" was also studied by direct spectroscopic observation of the hot alumina. Results show that hydroxyl groups, although apparently mobile, persist as such on the surface at high temperatures on distinct types of sites rather than in a completely random state. Readsorption of water cannot alone explain the difficulty in removing residual hydroxyl groups a t high temperatures. Free energy of activation for desorption of water apparently increases continuously as the surface concentration of hydroxyl groups decreases. Although "strain" sites are undoubtedly created by dehydration, characterization of such sites as strained A1-0-A1 linkages seems inadequate.
Introduction Strongly dried aluminas chemisorb at least a monolayer of water when exposed to moisture at room temperature, Although several crystallographic forms of high-area alumina exist, available information suggests that their surface hydration is similar. The forms most often investigated are usually characterized as either y- or 7-alumina, but mixtures of these two rather similar forms are probably common. The hydrated surfac: of y-alumina retains 13 molecules of water per 100 A.2of surface after evacuation at 25" for 100 hr.,' and even after dzying at 120", it may still retain 8.25 molecules per 100 A.2.2 Although much of this water is held as hydroxyl groups on the surface, infrared studies show that some of the water strongly adsorbed a t 25 " remains molecular. When wet y-alumina is heated above loo", some of the adsorbed water is desorbed while some reacts to form hydroxyl groups. At higher temperatures these, as well as pre-existing hydroxyl groups, gradually condense to eliminate water. Surface hydroxyl groups are
not completely eliminated, however, even by drying under vacuum at 800-1000°.3 Important catalytic properties of y-alumina are associated with "acid" sites created on the surface during removal of hydroxyl groups at temperatures above 400". Hydroxyl groups of at least three chemically distinct types remain on the surface after the alumina has been dried under vacuum above 650°,3 but none of these types appears catalytically active, per se, for the isomerization of l - b ~ t e n e . ~Instead, the active sites appear to involve abnormal exposure of aluminum ions4or creation of strained AI-0-AI linkages5 on the surface
(1) J. J. Kipling and D. B. Peakall, J . Chem. SOC.,834 (1957). ( 2 ) J. H. de Boer, J. M. H. Fortuin, B. C. Lippens, and W. H. Meijs, J . Catalysis, 2 , 1 (1963). (3) J. B. Peri and R. B. Hannan, J . Phys. Chem., 64, 1526 (1960). (4) J. B. Peri, Actes Congr. Intern. de Catalyse, P, Paris, 1960, 1, 1333 (1961). (5) E. B. Cornelius, T. H. Milliken, G. A. Mills, and A. G. Oblad, J . Phys. Chem., 59, 809 (1955).
Volume 69, Number I
January 1966
when hydroxyl groups are removed. Some degree of strain is known t o characterize crystal surfaces.' Although much progress has been made toward explaining the surface hydration of aluminas, present knowledge remains meager. Because the crucial importance of surface hydration has often been overlooked, much of the literature on catalytic properties of aluminas is of questionable significance. Further understanding of the details of surface hydration is clearly needed. The present work was undertaken to obtainadditional information on the surface hydration of y-alumina by infrared and gravimetric techniques. Alumina aerogel plates were studied.
Experimental Muleriuls. Because minor differences in procedure may lead to unsatisfactory plates or may otherwise influence the properties of the alumina, the preparative procedure is described in detail. Unless otherwise noted, reagent grade chemicals were used. Alumina sol was typically prepared from 80 g. of aluminum metal pellets (0.3 to 0.6 em., 99.99 +%pure), 60 ml. of glacial acetic acid, 3 1. of distilled water, and 0.1 g. of red mercuric oxide. These materials were combined in a three-necked Pyrex flask equipped with a reflux condenser, a stirrer, and a thermocouple. The flask was warmed with a heating mantle to initiate the reaction. Then the mantle was dropped, and the reaction was allowed to proceed at 65-70" for 24 to 48 hr. The sol was decanted from any unreacted aluminum or coarse precipitate and, if it showed more than a slight milky opalescence, was centrifuged for 15 min. at 30,000 g in an angle-head centrifuge. The clear sol (-5% AlzOa)was floated as a 0.3- t o 0.6-cm. layer on clean mercury or carbon tetrachloride in a 20.3-cm. crystallization dish. Open weighing bottles of 10% aqueous ammonia solution were suspended inside the dish, which was covered with a large watch glass. Gelation normally required about 2 hr., but the time varied with the thickness of the sol layer and the amount and concentration of the ammonia solution. (Excessive exposure to ammonia gave opaque plates, whereas correct exposure gave clear, firm plates.) The plates were broken into suitable pieces and carefully transferred to a crystallization dish containing methanol. (The plates never occupied more than 20% of the volume of methanol in the dish.) The dish was gently swirled once or twice a day, and the methanol was replaced at 2- to 4 d a y Intervals; it was changed at least four times over a 2-week period.
After alcohol exchange, the plates were packed loosely in a glass autoclave liner (3.2 cni. i.d. and 26.7 cm. in length) filled with pure methanol. The liner was placed in a stainless steel autoclave, and methanol was added to fill the autoclave completely, which WRS then sealed and heated slowly to 260" while the methanol was bled off slowly, as necessary, to keep the pressure below 1500 p.s.i. Between 260 and 280" bleeding was continued for about 2 hr. to maintain the pressure as long as possible in the range from 1200 to 1500 p.s.i. Ultimately, the pressure was slowly lowered, arid the autoclave was evacuated for about 0.5 hr. while still hot (-300'). The clear aerogel was removed from the autoclave and calcined in air or oxygen at 600" before use. X-Ray analysis showed that the original sol, which had been dried in air at 130", was mainly amorphous with some microcrystalline Boehmite; the calcined aerogel was ?-alumina. For electron micrographs, the calcined aerogel was ground in ethanol, and a thin film of the suspension was dried onto a carbon supporting film. Figure 1 illustrates the typical rod- or lath-like appearance of the alumina particles. Some samples also contained sheee like structures. Spectroscopic analysis of a typical plate showed
