NOTES
3345 the Lever Brothers Co. for permission to publish this paper. (7) For example, by combining eq 25 and 29 in H. M. Princen in “Surface and Colloid Science,” Vol. I, E. Matijevic and F. Eirioh, Ed., Interscience Publishers, New York, N. Y., in press.
Isobutane Chemisorption on Synthetic Faujasites
by P. Donald Hopkins and R. L. Stoffer Research and Development Department, American Oil Company, Whiting, Indiana (Received A p r i l 17, 1968)
Figure 3. Measured intensity profile for 0.2% NaCL2S04.f 0.4 M NaCl.
theory, according to which the radius of curvature of a cylindrical interface in a gravitational field is given by7 P =
dY d sin cp
=
-[2dg(l
- sin
~ p ) / y ~ ] - ’ (11) /~
so that
Substituting for dy/dq in eq 7 yields eq 10, which can be used to obtain lo from the measured intensity a t large a. This is preferable to measuring Io directly in the transmitted beam at a = 0. Experimentdly, the intensity was measured with a photomultiplier tube (RCA 6199) which could be moved a t constant speed through a circular arc behind the cell. The signal was recorded, and a typical curve is shown in Figure 3. It indicates that geometrical optics is insufficient to describe the system quantitatively. The fringes a t large angles are definitely due to diffraction, resulting from the small size of the transition region and the large curvature of the bounding surfaces. This severely complicates the problem and, although we feel that the curve contains, in principle, the necessary information for a complete description of the transition region, we have not yet succeeded in analyzing these diffraction effects satisfactorily. It is also possible that the intensity profile is not sufficiently sensitive to the detailed shape of the transition region. Alternative techniques are being investigated. Although these initial attempts a t studying the transition region have not been successful, we stress again the importance of this region as a potential source of valuable information on interaction forces in thin liquid films. Acknowledgment. The author expresses his thanks to
In recent years zeolites, particularly synthetic faujasites, have been introduced as catalysts for cracking and other hydrocarbon conversion reactions. Faujasites both by themselves and dispersed in matrices of clay or amorphous silica-alumina have demonstrated marked increases in cracking and isomerization activity over amorphous silica-alumina catalysts; e.g., increases in activity for cracking of hexane by factors of up to lo4 have been observed.’ The reasons for these increases are not well understood. The structure of synthetic faujasite has been determined;2 the large spherical cavities in faujasite (entrance diameter about 8 & are easily accessible to a large variety of hydrocarbon molecules. Cracking is believed to occur primarily within the large cavity system and it has been suggested that the stereoregularity of the faujasite results in a marked increase in the number of catalytically active sites compared to silicaa l ~ m i n a . ~On the other hand, one active site in every lo4 cavities has been claimed to be sufficient t o explain cumene cracking rates over synthetic hydrogen Y (HY) fauja~ite.~ MacIver, et aL,5 found two types of chemisorbed isobutane on silica-alumina, reversibly adsorbed isobutane which underwent exchange with gas phase isobutane, and irreversibly adsorbed isobutane which could be removed only by burning in oxygen. Larson and Hall6 further separated reversibly adsorbed isobutane into two types based on rate of exchange. Reversible adsorption reached an equilibrium, but irreversible (1) J. N. Miale, N. Y. Chen, and P. B. Weisz, J. Catal., 6, 278 (1966). (2) L. Broussard and D. P. Shoemaker, J . Amer. Chem. Soc., 82, 1041 (1960). (3) J. A. Rabo, P. E. Pickert, D. N. Stamires, and J. E. Boyle, in
“Actes du Deuxieme Congres International de Catalyse,” Editions Technip, Paris, 1961, p 2055. (4) J. T. Richardson, J . Catal., 9, 182 (1967). (5) D. 8. MacIver, P. H. Emmett, and H. 8. Frank, J. Phys. Chem., 62, 935 (1958).
(6) J. G. Larson and W. K. Hall, J. Amer. Chem. Soc., 85, 3570 (1963).
Volume 78, Number 0 September 196’8
NOTES
3346 adsorption was a linear function of time. The amounts adsorbed were very small (
