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

Summary
CO2 capture and storage might have an important role in stabilising the global concentration
of CO2. Power plants are primary candidates for CO2 capture and storage because they have
great potential (up to 45% of CO2 emission reduction in the year 2050 compared to 2005).
Inorganic membranes (Zeolites or TiO2-ZrO2) are candidates for separating H2 from CO2
(precombustion) or CO2 from the flue gas (mainly N2) at the End-of-Pipe (postcombustion) in
fossil fuel power plants.
All-silica Dodecasil 1H (DOH) zeolite type was selected for the hydrothermal stability
and possible ability to separate H2 from other gasses under precombustion concept conditions.
The DOH crystal size can be synthesised to thin hexagonal plates with sizes in the order of 10
µm. This crystal size is too large for membrane formation or to act as seeds for the layer
formation, by means of secondary growth of DOH nuclei.
The removal of the complete structure directing agent (SDA) content from the pores in the
DOH structure could not be obtained through calcination in air for extended periods at
elevated temperatures. Quasi SDA-free DOH, with a high crystallinity, was obtained after
calcination at 900ºC for 5 hours when the atmospheric pressure was increased twice to 50
MPa for 30 min.
The prepared all-silica DOH, that is quasi SDA-free, might present hydrothermal stable
microporous material with pores that are inaccessible for CO2 and accessible to H2.
Polymeric Y2O3 or TiO2 mixed ZrO2 sols are synthesised for the preparation of
ultramicroporous powders and thin films on γ-Al2O3 intermediate layers supported by α-
Al2O3 disks as potential gas separation membranes. Two routes have been selected being the
Ketone- and the Amine-approach based on the precursor modifiers.
8 mol% yttria stabilised zirconia (8YSZ) calcined at 450ºC is microporous with a BET
specific surface area of ~50 m 2 /g. 8YSZ layers might form microporous layers with low
permeability. 30-50 nm thin cubic 8YSZ films, prepared by the Ketone-approach, show He
and N2 transport by Knudsen diffusion due to defects or to the too large pores in the final
membrane layer.
As expected from the linear polymeric Amine-Sols, the amorphous binary TiO2-ZrO2
materials are microporous between 400 and 500ºC. The highest BET specific surface area of
~200 m 2 /g with an estimated pore size of ~1.0 nm (gas physisorption) is obtained for the
Ti0.5Zr0.5O2 calcined at 500ºC using the Amine-approach. The crystallisation temperature of
orthorhombic Ti0.5Zr0.5O2 is between 550 and 600ºC which is ~250ºC higher than that of
single oxides.
20-60 nm thin, homogeneous films can be prepared from TiO2, ZrO2 and binary oxides on γ-
Al2O3 membranes using calcination temperatures in the range of 400 to 600ºC. H2/CO2
permselectivity higher than the Knudsen factor is observed for Ti0.5Zr0.5O2 films calcined at
500 and 600ºC. These films can contain a pore size distribution of ~0.3 to ~0.5 nm in
diameter which is comparable to state of the art SiO2 membranes. The maximal He permeance
of 1·10 -7 mol/m 2 sPa and a maximal permselectivity of He/N2 =14 or H2/CO2=6 values are
lower than state of the art SiO2 membranes but higher than TiO2-ZrO2 membranes published
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