Page 269 - Principles of Catalyst Development

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APPENDIX 6
for a 2-hr period. Repeat this procedure three times using just enough of
the Hel solution to completely cover the gel each time.
The gel is washed free of chloride ions and then dried for 4 hr at about
150°C. The product should be transparent at this stage. If any opacity is
observed, the drying period should be prolonged. The dried solid is then
calcined in air at 550°C.
The calcined product has extremely small pores of about 1 nm, charac-
teristic of dessicant gels. The pore size can be controlled over a wide range
by changing the pH of the treating solution. For example, using 1 % NH 4Cl
instead of 1 N HCI gives a much lower surface area and density, and the
pore size will be approximately three times as large.

1.3. Chromic Oxide Gel (Alkane Dehydrogenation)
Add 2500 cm' of 0.1 N N H4 solution at 6 cm' per minute to 5000 cm]
of 0.1 N Cr(NO])] 9H:O at 20°C with stirring. The original precipitate
disappears after about 2 hr, but a permanent precipitate forms later. At this
point, add the rest of the alkali rapidly. Dark green hydroxide separates as
a gelatinous precipitate. Allow the precipitate to settle and decant the
supernatant liquid as completely as possible.
Wash the gel with 5% ammonium carbonate solution, adding sufficient
solution to bring the level up to the original mark. Stir the suspension for
30 min, allow the precipitate to settle, and decant again. Repeat five times
or until the precipitate begins to peptize.
Filter with a Buchner funnel and dry at 100-105°C for 24 hr. Before
use, the catalyst is heated in hydrogen while raising the temperature slowly
to about 380°C.

2. DUAL OXIDES

2.1. Silica-Alumina Gel (Support and Cracking Catalyst)

Prepare two solutions. Solution A is 465 em' of water glass in 900 cm 3
3
of H 20. Solution B is 67 cm' of 4 N HCl plus 140 cm of 0.5 N AI2 (S04h
solution in 400 em' of H 20. Cool both solutions to about 5°C. Add solution
A to solution B rapidly with vigorous stirring. Pour into a tray as soon as
mixing is complete. The gel time is about 35 s. In about 1 hr, cut the gel
into cubes approximately 2 em on a side. Allow the gel to age for 48 hr and
transfer to a Buchner funnel. Base exchange with a 2% solution of
AI2 (S04)3 18H2 0 three times for 2-hr periods and then once overnight. Wash
with distilled water until free of sulfate ions. Dry for 4 hr at 140 to 180°C.