Geoscience Reference
In-Depth Information
The sorption uptakes are found to decrease with the increase in experimental tem-
perature and this decrease is consistent with the observed increase in the crystallite size.
As the molecular size of the probe molecules increases, a decrease in sorption
uptake with an increase in crystallite size becomes appreciable on account of their
different packing in the zeolite voids; Al-omega shows higher sorption uptake than
Ga-omega zeolite. These omega zeolites are stable up to a temperature of 1000
C.
The AlPO
4
molecular sieves show excellent physical properties. Many are ther-
mally stable and resist loss of structure at 1000
C. Those studied for hydrothermal
stability, including AlO
4
-5, 11, and 17, show no structure loss when treated with
16% steam at 600
C. The adsorption properties of selected AlPO
4
molecular sieves
are listed in
Table 6.20
. The different structure types are arranged in order of
decreasing pore size, which is controlled by the ring size. The ring size refers to
the number of tetrahedral atoms (Al or P). For the 12-T ring structures, AlPO
4
-5
and AlPO
4
-31, adsorption of neopentane has been observed. The 10 or puckered
Table 6.20 Typical Adsorption Pore Volumes for Selected AlPO
4
-Based Structure Types
Adsorption Capacity (cc/100 g)
a
Adsorbate
5
H
2
OO
2
n
-
Butane
n
-
Hexane
Isobutane Cyclohexane Neopentane
Kinetic diameter
(nm)
5
0
0.265 0.346 0.43
0.43
0.50
0.60
0.62
P/Po
5
0.9
0.9
0.4
0.3
0.3
0.5
0.6
Temperature
(C)
5
22
2
183 22
22
22
22
22
Structure Ring
size
b
5
12
26
16
13
12
36
12
31
21
14
13
46
12
35
30
28
26
11
10
18
13
11
11
2
31
10 or
12
21
13
11
9
11
41
10 or
12
21
12
10
8
11
34
8
33
27
23
2
35
8
31
17
1
39
8
23
9
1
44
8
35
25
10
47
8
35
27
25
16
6
28
4
1
20
6
32
2
1
a
Determined by standard McBain
Baker gravimetric techniques after calcination at 500
600
C in air. Samples
activated at 350
C, 0.01 torr, prior to measurement.
b
Number of tetrahedral atoms (Al, P, and Me) in ring controlling the pore size. When structure is not known, it is
estimated from adsorption measurements.
