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106 CHAPTER 6
Appendix 7. For do-it-yourself enthusiasts, detailed recipes are given in
Appendix 6.
6.3. DUAL OXIDES
Dual oxides are intimate combinations of two oxides, resulting in
synergistic catalytic properties. For high dispersion and interaction, best
results are obtained through co precipitation and subsequent treatment of
the two gels. Common examples are SiOr AI 20 3 , NiO-AI 20 3 , and zeolites.
Dual oxides are not separate compounds, since they exist over a wide range
of compositions, but are more like solid solutions. This is especially true
for low calcination temperatures. With high heat treatments, solid state
reactions occur and identifiable compounds "precipitate" out.
The best known dual oxide is SiOr AI 20 3, used as a cracking catalyst
or acidic support. The preparation is very similar to that described for Si0 2 •
Two solutions are prepared, the first a 30% mixture of water glass and
water, the second a mixture of 4N HCI and 0.5N AI 2(S04)3. These are
cooled to 5°C and mixed rapidly in a 2: 1 ratio. As the pH of the acidified
water glass reaches 6, precipitation of both silica and alumina sols begin
and gelation takes about 45 sec. The combined gel is aged for two days,
treated with 2% AI 2(S04)3 solution to ion-exchange Na, and washed free
of sulfate ions. Preparation is completed with drying and calcination at
5500C. (163)
The resulting catalyst contains about 25% Al 20 3 and is amorphous,
i.e., has no distinctive x-ray diffraction pattern indicating long-range crystal-
linity. Acidic properties are described in Chapter 4.
Coprecipitated NiO-AI 20 J samples are typical of a wide variety of
dual hydrous oxide gels. They are precursors to reduced Nij AI 20 3 catalysts
used in hydrogenation and methanation. Although these catalysts are pro-
duced by other routes, the intimate interactions fostered by coprecipitation
are believed to impart improvements in activity and stability to the nickel
crystallites, 11641 although mechanical strength is lower. It may also be poss-
ible that irreducible compounds of NiO and A1 20 3 , facilitated by coprecipi-
tation, play some role in forming stable promoters.