Geoscience Reference
In-Depth Information
It was well known that the largest ring in silica zeolites consisted of 12 tetrahe-
dral (12 T-atoms), and this concept remained for over 180 years, since the first
discovery of natural zeolite with 12-T atom rings, e.g., gmelinite, took place during
1807
[18,19]
. Davis et al.
[131]
have synthesized the first molecular sieve with rings
that possess greater than 12 T-atoms. VPI-5 is a family of aluminophosphate-based
molecular sieves with the same 3D topology. The extra-large pores of VPI-5 contain
unidimensional channels circumscribed by rings that have 18-membered rings and
possess free diameters of approximately 12
13
˚
[131]
. It was only during 1996
that Freyhardt et al.
[132]
synthesized the first aluminosilicate zeolite with its ellipti-
cal 14-tetrahedral-atom pore opening. For the synthesis of VPI-5, pseudoboehmite,
85 wt% H
3
PO
4
, tetrabutylammonium hydroxide (TBA), and n-dipropylamine (DPA)
were used. Schmidt et al.
[133]
have studied the VPI-5 synthesis in detail. Here, we
describe only the synthesis of VPI-5 to show the peculiarities involved in the synthe-
sis of aluminophosphate zeolites in general. In this regard, the experimental results
of Schmidt et al.
[133]
are very interesting for a reader. The reaction temperature
was varied from 100
C to 200
C using two different gel compositions in order to
find the optimum conditions for VPI-5 crystallization.
Figure 6.29
shows the
1.O Pr
2
NH: 1.OAl
2
O
3
: 1.OP
2
O
5
:4OH
2
O
90°C/24 h
90°C/24 h
I
II
III
IV
MV, DPADHP,
*
MV, DPADHP,
*
150°C/336 h
200°C/24 h
150°C/336 h
200°C/24 h
MV
200°C/24 h
*
11
11, 41, 31, C,
11,C,T
11
MV
= metavariscite
DPADHP = di-
n
-propylammonium di-hydrogenphosphate
C
= AIPO
4
(C)
T
= AIPO
4
(T)
*
= unreacted AL (OH)
3
XH O
11
= AIPO
4
- 11
41
= AIPO
4
- 41
31
= AIPO
4
- 31
Figure 6.28 Effect of various hydrothermal treatments on the amine containing AlPO
4
gel
[131]
.
