Page 116 - Principles of Catalyst Development
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104 CHAPTER 6
give more consistent and uniform results than laboratory ovens, hot plates,
and furnaces. m,160) Often these units are staged to achieve optimum results,
for example, rapid, humid drying during the constant rate period, followed
by slow control during the declining rate.(161)
Dried xerogels contain 25'}'0 -30% water, encapsulated in fine pores or
chemically bound to the oxide. In this moist state the material is sometimes
easier to form into pellets and extrudates. This is sometimes done for
convenience, provided subsequent calcination does not weaken or otherwise
harm the particles.
6.2.6. Calcination
Calcination is further heat-treatment beyond drying. Several processes
occur: (I) loss of chemically bound water or CO 2 , (2) changes in pore size
distribution, (3) active phase generation, (4) surface conditioning, and (5)
stabilization of mechanical properties.
Alumina is a good example with which to demonstrate all of these
features. (64) Figure 6.9 shows the progression through all steps from precipi-
tation to calcining. After drying, a hydrous oxide known as "boehmite" is
produced with a structure AI 20 3 nH 20. With pure boehmite n = 1, but values
up to 1.8 are found. The structure of boehmite is distinctive and the surface
hydrated as shown in Fig. 4.17. Upon calcination above 300°C, a series of
AI (OH) 3 gel
I pH :: 9, 30' C
t 2·10daVs
. boehmite'·
AI 2 03 nH 2 0 n = 1.0 crystalline
to
300 C n = 1.8 gelatinous
or
pseudo
cubic close packed usually from higher values 01 n
ABCABC
defect spinel 250 m 2 Ig
AI [ AI 5.'3 0 1 :) J 04 450 C "'/" group
low temperature
AI 2 0 3 mH 2 0
m :: 0 10 0.6
H H 900C
~ 9
AI· 0 - AI
dislorled spinel "s " group
1,000 C high temperature
anhydrous AI 2 0 3
monoclinic
, ,200 'c
hexagonal
ABABAB
figure 6.9. Preparational steps for aluminas.