Geoscience Reference
In-Depth Information
space occupying (filling) species would, in principle, allow a high-temperature
synthesis of open, porous structures
[11]
.
Temperature can obviously affect the rate of nucleation and crystal growth. The
rate of nucleation and the linear rate of crystal growth are shown as follows
[28]
:
Þ
2
1
dN
=
dt
A
½
exp
ð
Et
5
r
kt
5
The coefficients K, A, and E in
Table 6.14
and K in
Table 6.15
all increase with
temperature, indicating that linear rates of crystal growth (coefficients K) and rates
of nucleation (coefficients A and E) both increase with rising temperatures.
When the temperature is elevated, the induction time of s-shaped crystallization
curves is shortened. It is also observed that the temperature strongly affects the
induction time. Zhdanov and Samulevich
[80]
have developed a method to analyze
nucleation and crystal growth:
dln
ð
1
=θÞ
En
R
Þ
5
d
ð
1
=
T
where
is the induction time, i.e., a point on the crystallization curve where the
conversion into the crystallization phase just begins. The apparent activation energy
for the nucleation of phillipsite was given as 13.5 and 14.3 kcal/mole with and
without stirring respectively, while for the Al-free end member of the ZSM-5 series
(silicalite l), this energy was given as 9.1 kcal/mole
[90,91]
. The En value for Na-
Y was obtained as 17 kcal/mole
[91]
.
θ
6.5.5 Alkalinity (pH)
Alkalinity of the media plays a vital role in crystal growth, materials synthesis/
preparation, and processing on the whole. It influences the supersaturation, kinetics,
Table 6.14 K, A, and E, for Zeolite Na-A as Function of Alkalinity and Temperature
[89]
K
(
μ
mh
2
1
)
3
10
6
T
(k)
H
2
O/Na
2
O
A
E
343
20
0.050
20.0
0.102
343
30
0.027
13.2
0.033
343
40
0.017
9.0
0.115
333
20
0.03
1.0
0.062
343
20
0.05
20.0
0.102
353
20
0.06
200.0
0.132
Table 6.15 Linear Rates K
5
0.5
Δ
l/
Δ
t for Growth of Na-X
[80]
T (k)
343
353
363
373
K (
μ
mh
2
1
)
0.0175
0.0375
0.0625
0.1071
