Geoscience Reference
In-Depth Information
(ripening) period is very crucial in the synthesis of a desired zeolite, and it differs
from aluminosilicate zeolites to aluminophosphate zeolites
[78]
. One of the impor-
tant steps occurring during the aging period is the (partial) dissolution or depo-
lymerization of the silica sol, which is promoted by the alkaline conditions for
zeolite synthesis
[40]
. Cook and Thompson
[79]
have simulated the hydrothermal
zeolite synthesis with the aging of the amorphous gel at low temperature followed
by elevated temperature crystallization by population balance methods.
The amorphous gel is formed at room temperature, homogenized, and left to age
for a period of time at room temperature. The subsequent crystallization at elevated
temperature after aging proceeds more quickly than in the nonaged case. Several
authors have noted that germ nuclei formed within the gel (or the solution) during
gel aging grow to observable sizes upon subsequent high-temperature synthesis
[40,80]
. Zhdanov and Samulevich
[80]
reported that aging zeolite Na-A gels for up
to 3 days resulted in a more rapid conversion upon heating and in smaller final
crystal sizes. They concluded that nuclei form during aging and then grow to
observable size during heating. Subotic and Bronic
[81]
reported experimental
values of percent crystallinity versus time for zeolite Na-A syntheses at 80
C, for
which some of the amorphous gel/water solutions had been aged at 25
C for as
many as 17 days prior to crystallization. Upon heating the aged solutions, those
aged for longer times were shown to crystallize faster after a shorter induction
time. These data have been plotted in
Figure 6.8
. The evolution of the average zeo-
lite crystal size with time, based on the computer simulations is shown in
Figure 6.9
. As the aging time increases, the number of nuclei increases, thereby
decreasing the final average crystal size. This result is perfectly logical, as with
more nuclei present, the reagent mass is distributed among more crystals, resulting
in smaller crystals on average.
Figure 6.10
shows the number of nuclei formed per
day by dividing values of Ns (No. of nuclei formed during aging).
1.00
AB C DE
0.80
0.60
0.40
0.20
0.00
0.00
1.00
2.00
3.00
Time (h)
4.00
5.00
6.00
Figure 6.8 Crystallization curve simulated assuming nuclei form during reaction time
[67]
.
