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

4 Results and discussion
hours using ethylenediamine as a base. Lower aging temperatures resulted in larger
crystals (Figure 29). The crystal size is estimated from several crystals observed with the
optical micrograph. This indicates that the nucleation of primary building units occurs at
moderate temperatures (50-80ºC), which is typical for zeolite formation. At higher
temperatures (200ºC), DOH crystal growth is stimulated. These results suggest that the
DOH nucleation and growth can be decoupled.
The crystal size in the range of 10 µm can be too large to act as nuclei for the formation
of DOH thin layers. It is believed that zeolite membrane layers should have a desired
thickness of less than 20 µm. 133
Crystal size [µm]
160
120
80
40
0
20 40 60 80
Aging temperature [ºC]
Figure 29 Crystal size as function of the aging temperature using the gel composition of
100SiO2:11ADA:267EN:7405H2O. Two samples at each aging temperature of 25, 50 and 80ºC were
prepared. The crystallisation was performed by rotating the autoclave at 200˚C for 4 days. The crystal
sizes were determined with an optical microscope.
4.2.2.2 Preparation of quasi template free DOH
Seeding DOH synthesis
The addition of DOH seeds enhances the DOH crystallisation. However, the
reproducibility of a synthesis with seeds in combination with low concentration of SDA
was proven to be difficult. Nevertheless, low yield of pure DOH was obtained using a
low amount of seeds (0.28-0.60 wt% compared to the total gel mass) in combination with
Si/ADA = 100 molar ratio instead of 9. These results confirm the possibility of DOH
synthesis by secondary growth as published by Grebner et al. 71-73 However, the
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