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

4 Results and discussion
The coupled loss of SDA and crystallinity during calcination might be explained by the
collapse of the DOH surface structure. While the inner part of the DOH crystals
maintains their crystal structure including the trapping of some of the template and the
outer part of the DOH crystals collapses. In principle, O2 with a kinetic diameter of 3.46
Å, CO of 3.70 Å and CO2 of 3.30 Å are inaccessible to the DOH structure due to the
smaller window opening compared to the kinetic diameter of these gasses. However, the
DOH structure might ‘breathe’ (elastic vibration) at elevated temperatures allowing the
transport of these gasses. This behaviour has been observed for other zeolites. 134
The remains of the burned out ADA might react with the oxygen from the silica
framework at elevated temperatures. This could explain the decrease in cage occupancy
coupled with the DOH structure loss. This might occur preferentially at the outer surface
of the DOH crystals. SDA combustion is studied by DTA/TG measurements in air and in
nitrogen. DTA/TG plots show similar behaviour for both gasses indicating that the
weight loss is independent of the atmosphere and could explain the internal combustion.
In a nut shell, conventional calcination at ambient pressure removes only a part of the
organic SDAs.
2) Catalytic oxidation by titanium incorporation in the DOH framework
Titanium was incorporated into the DOH framework in order to enhance the conventional
calcination by means of catalytic oxidation. Small amounts of titanium isopropoxide or
titanium n-propoxide were added to the silica source or to the final sol composition.
Similar attempts have been performed to incorporate iron or aluminium and have resulted
in multi phase zeolite products.
Highly crystalline Ti-DOH products are obtained after ~27 days of crystallisation at
200ºC in the presence of a small amount of titanium in the gel (molar ratio Si/Ti 10-∞,
see Table 17). Electron energy dispersive X-ray analysis and Inductively Coupled Plasma
analysis confirm the presence of titanium in the samples. Raman spectroscopy presents
bands at 960 cm -1 indicating the presence of Ti-O-Si bonds.
Table 17 Cage occupancy of as-made and after calcination for samples with different Si/Ti molar ratios
Sample # Crystal size Si/Ti Cage occupancy
As-made Calcined at 900ºC
Ti-1 Ca. 100 µm ∞ 91% 31%
Ti-2 100 76% 24%
Ti-3 Ca. 40 µm ∞ - 11%
Ti-4 100 - 4%
60