Inorganic Microporous Membranes for Gas Separation in Fossil Fuel ...

4 Results and discussion
be enhanced by a shorter path length. The path length can be reduced by grinding the
as-made crystals into smaller particles. Ball milling was used as the grinding method.
The crystal size obtained by optical microscopy reduces two orders of magnitude to
1-10 µm by ball milling for 0 to 26 hours (Figure 33).
Crystal size [µm]
120
100
80
60
40
20
0
0 5 10 15 20 25 30
Ball mill in hours
Figure 33 Crystal size of large synthesised DOH as function of ball milling time.
~1 µm DOH crystal can be observed after ball milling for 72 hours (Figure 34-A and B).
Decreasing the crystal size by ball milling is coupled with a significant crystallinity
decrease during the 72 hours of ball milling (Table 18). Attempts to hydrothermally
recrystallise the ball milled products were unsuccessful. Treating the ball milled DOH
products with aqueous NH4F did not separate the crystalline and amorphous phases.
Calcination of the 72 hours ball milled sample at 900ºC for 5 hours did not result in
further crystallinity loss. The ~35% crystalline DOH product did not contain any carbon.
Once again, the peak intensity at the low diffraction angle increased after calcination
indicating the presence of empty cages (XRD pattern not shown). The few DOH crystals
left are SDA free. This indicates that the template removal is strongly dependent on the
crystal size. The quantity of expelling of the combusted SDA is dependent on the path
length of the transported species should travel.
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