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

4 Results and discussion
successful reproduced seeded DOH synthesis still includes significantly amount of SDA.
This could have resulted in almost completely templated DOH. The chemical analysis
confirm this expectation with a 5.5 wt% (0.22 mol%) carbon content. This amount,
related to the theoretically determined SiO2/ADA ratio of 34 in DOH, results in a ~75%
cage occupancy. To summarise, it was not possible to synthesize DOH without the
addition of (small amounts of) SDA by using secondary growth of seeds.
Post treatments
Several post treatments are compared in order to remove the organic template from the
largest DOH [5 12 6 8 ] cages, namely 1) conventional calcinations, 2) conventional
calcinations of DOH synthesised with traces of titanium to assist the burnout by catalytic
oxidation, 3) reducing the crystal size to shorten the path length of the oxygen and
combusted gasses by means of mechanical ball milling and 4) calcination in combination
with pressurised air.
1) Conventional calcinations
The structure directing agent (SDA) occluded in DOH could not be removed completely
by means of conventional calcination in air under ambient pressure and at temperatures
between 700 and 900ºC. This is in line with other published results. 66,71-73 If all large
cages are occupied by an ADA molecule the organic compounds amount to a total of
6.9 wt.%. A complete SDA removal is observed at about 1400ºC, which is coupled with
DOH structure breakdown (Figure 30). The XRD patterns of the ‘as-made’ and the
calcined sample at 900ºC for 5 hours clearly show a broad diffraction around 22° 2Θ
indicating amorphous silica (Figure 31). This typical crystallinity loss after calcination is
coupled with an increase in the intensity for the low angle and is typical of emptying
zeolite cages. Several samples suffer significant crystallinity loss after calcination.
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