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

2 Theoretical background
the material’s adsorption capacity for various gas molecules. Kuznicki et al. 55 claimed
even that ETS-4 can be tuned to penetrate or absorb oxygen and to exclude nitrogen into
the crystal cages, resulting in an oxygen-selective adsorbent.
Known 8-ring zeolite type membranes are Zeolite Type A (LTA, Linde Type A, Figure
7) and deca-dodecasil 3R. The Si/Al ratio for zeolite (Type) A is 1-3.7. Recently,
hydrophobic all-silica zeolite A was prepared by using a supramolecular organic
structure-directing agent. 56 Guan et al. 57 prepared aluminosilicate LTA zeolite
membranes with rather low permselectivity of 7.5 for H2/N2. Hedlund et al. 58,59 prepared
ultra thin layers of 200 nm or less of LTA.
The low H2/CO2 permselectivity of LTA membranes can be explained by the possibility
that both H2 and CO2 can enter the pores and by the fact that intercrystalline pores may
be present as a result of poor poly-crystallisation. This drawback is less critical for
pervaporation applications. Nowadays, the Japanese company NGK prepares
commercially available LTA and DDR membranes.
2.2.4 Selection of zeolite types
A B C
Figure 7 Zeolite frameworks of MFI (A), LTA (B) and FAU (C).
Lab scale zeolite membranes offer good gas separation factors predominantly determined
by the sorption properties and less by molecular sieving. This is mainly due to the
presence of intercrystalline pores (larger than 2 nm) as a result of poor polycrystallisation.
Nevertheless, zeolite membranes are promising candidates for CO2
capture.
In this study, zeolite types are selected for the pre and postcombustion concept. Sorption
properties of H2, CO2 and N2 are less pronounced at elevated temperatures. Therefore the
zeolite types are selected primarily as molecular sieves. These requirements exclude
zeolite types with 10 or more rings (Figure 8).
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