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J. J. FRIPIAT, A. L ~ O N A RAND D , J. B. UYTTERHOEVEN
and hydrogen is not readily adsorbed on the copper silica-alumina sample. Alloys may be more common on reduced supported metal oxide catalysts than has been realized. This idea was expressed recently by Endter, who reported that platinum-aluminum alloys were formed during used the preparation Of a in the synthesis of hydrogen cyanide.28
Acknowledgment. The authors wish to express their appreciation to Dr. W. K. Hall of Mellon Institute, Pittsburgh, Pa., for the unsupported nickel-copper alloy samples and to Miss Winifred Roensch for the X-ray diffraction analyses. (28) F. Endter, presented at the 147th National Meeting of the American Chemical Society, Philadelphia, Pa., April 5-10, 1964.
Structure and Properties of Amorphous Silicoaluminas. 11. Lewis and Brqhsted Acid Sites
by J. J. Fripiat,l A. Lhnard, and J. B. Uytterhoeven Laboratoire de Physico-Chink Mindrale, Agronomic Institute of the University of Louvain, HherlCLouvain, Belgium (Received February 8,1966)
The schematic structures derived previously from coordination number measurements and infrared spectroscopy suggested different possible origins for Br@nstedand Lewis acid site$ in silicoaluminas with compositions ranging from pure silica to pure alumina. To check these hypotheses, cation-exchange capacities and ammonia adsorption isotherms were measured, and infrared spectra of adsorbed species were recorded. The comparison between structural and adsorption data shows the existence of two probable Lewis acid types located either on silicon atoms in high alumina samples or on aluminum atoms in low alumina samples. The critical composition is somewhat lower than 40% in A l 2 0 3 . Brgnsted acid sites originate from isomorphic substitution of silicon by aluminum in the tetrahedral network.
Introduction Acid properties of silicoalumina surfaces are due to Brnsted or Lewis sites according to the hydration state and to the surface structure, which reflects to some extent the organization in the bulk. Therefore, it appears interesting to relate the schematic structures obtained previously2 from X-ray fluorescence spectroscopy and from infrared to acid properties. The coordination numbers of aluminum and silicon were obtained from the comparison of the angular positions of the A1 KCYand Si KCYlines with the angular positions of the corresponding lines in materials of The Journul of Physicd Chemistry
known structures where aluminum or silicon have the coordination numbers 0, 4, or 6, respectively. For aluminum the results were expressed in terms of relative contents in fourfold coordinated atoms (%AI"). This was justified by the distribution of aluminum atoms into two main types of sites where they are either tetrahedrally or octahedrally coordinated with respect to oxygen atoms. Infrared spectroscopy in the Si-0 (1) The University of Louvain and M.R.A.C. Tervuren. (2) A. Leonard, S. Suzuki, J. J. Fripiat, and C. Chem., 68, 2608 (1964).
De Kimpe, J . Phy8.
LEWISAND BR~~NSTED ACIDSITESIN SILICOALUMINAS
E
0
I
I 0-9-0
0-Si-0
0
0
I 0
I OH
0
I 0 I
I 0
I I 0-SI -04-0 -Si -0 I
I 0
0
o-si-0 I
0
C
0
D
0-si-0
I I &Si- 0-Si -0 I I
I 0-Si -0
dm. Ala
S?* . AIm
0
F
9
0
I I OTi-O-S,i -0
I 0
0
0
G
0
I
os,i-o 0
0
\&/
' 0 0-S'i-0
d
&/O
o"0 I
0-44 0
Figure 1. Schematic structures.
stretching region was used essentially to check these data. The schematic structures proposed in the first paper* are represented in Figure lA, C, and D. As far as aluminum is concerned, three different oxygen environments were actually taken into consideration, corresponding, respectively, to aluminum tetrahedra sharing corners with silicon tetrahedra (A), to aluminum octahedra (C), or to aluminum tetrahedra sharing edges (D). The distribution of these different types depends on the aluminum content and the pretreatment temperature. Hydrated silicoaluminas rich in alumina (>SO% Alz03) contain essentially aluminum octahedra (C), which, by dehydration, are converted
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into aluminum tetrahedra sharing edges (D). Samples poor in alumina (
