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

2 Theoretical background
Figure 11 Schematic drawing of the colloidal and polymeric sol-gel technique 77
The particle size of polymeric sols may be in the range of 2-20 nm whereas colloidal sols
have particle sizes up to 1000 nm in diameter. Polymeric sols are prepared by adding
small amounts of water to keep the hydrolysis reaction slow, equation (6). The water can
be added in a controlled method by:
i) slowly adding a high ratio alcohol/water solution,
ii) in situ production of H2O by an esterification reaction,
iii) dissolving an alkaline base or
iv) adding a hydrated salt into the alkoxide solution in alcohol.
The catalyst can be applied to enhance the condensation reactions. Precursor modification
or stabilisation is another method used to modify the precursors in order to reduce the
hydrolysation rate. Ketones, amines and carboxylic acids can stabilise the precursor and
thereby decrease the hydrolysis reaction. Common precursor modifiers are acetyl acetone
78,79 and diethanolamine 78-83 for titania and zirconia polymeric sols.
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