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

2 Theoretical background
Dry or wet gel conversion method
This method is based on deposition of amorphous alumino-silicate gel on the support.
The crystallisation is initiated using the vapour of amines such as triethylamine,
ethylenediamine and water. The process is called ‘steam-assisted crystallisation’ if the
dry gel contains structure directing agents (SDA). Vapour Phase Transport is the
terminology used when the gel composition is SDA-free. However, many zeolite layers
present cracks, which can be explained by the large volume shrinkage from the gel state
to the zeolite layer.
Secondary growth
Seeding the membrane initially can be beneficial due to the decoupling of the nucleation
and the growth processes to obtain a controlled microstructure (Figure 6). Generally, the
seeds are synthesised at lower temperatures than the classical recipes in order to form
smaller crystals. 43 The seed size can vary between 50 to 2000 nm. Depending on the
temperature, induction period and gel composition, the seed synthesis time can be very
long (up to several months). 43 Commercial zeolite crystals can be grinded to acquire
small seeds 200 nm in size.
Figure 6 Schematic drawing of secondary grown zeolite membrane layer
There are several methods for attaching seeds to the surface of the support. By (i)
changing the pH of the solution for matching the zeta potentials of alumina support and
zeolitic seeds. Seeds (ii) can be rubbed into the support with help of cationic polymers.
As an alternative to seed rubbing, colloidal zeolite particles, which achieve a greater
control of the membrane microstructure, can be deposited on the supports. The
attachment can be improved by heating the seeded supports to 150-200ºC in order to
condensate the hydroxyl groups.
The use of sols with nanosized primary building units followed by a secondary growth
process were firstly studied by Tsapatsis et al. 46 The substrate is deposited (e.g. by dip
coating) with nanosized zeolitic nuclei and these zeolite sols are connected
hydrothermally. This process can result in a zeolite membrane with a thickness of about
200 nm.
Another (iii) recent zeolite seeds preparation method is the use of laser ablation. The
depositing of zeolite seeds is prepared by using pulsed laser deposition and followed by
hydrothermal growth of the layer. 38
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