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

4 Results and discussion
4.3.1.1 Ketone approach
A summary of the stability of the final 8YSZ sols obtained while simultaneously varying
the type of carboxylic acid and the solid content is depicted in Table 20. A sol is
considered stable when a sol is clear for several months at room temperature. It is clear
that the higher the aliphatic chain of the carboxylic acid, the higher the sol stability. In
fact, when nanonic acid was used, the obtained sols were stable in the whole range of
solid content. Since the carboxylic acid acts as hydrolysis/polymerization catalyst, this
behaviour can be explained considering that the lower acid strength of long-chain acid
reduces the reaction (growth) rate, decreasing in turn the formation of bigger particles
and agglomerates. Bigger particles and agglomerates can destabilize the sol due to
sedimentation. These bigger particles are avoided using long-chain acids. Moreover,
another effect to be taken into account is when the acid chain length is increased, the
8YSZ nano-particles are somehow better protected or capped, 136 reducing the possibility
of agglomeration (Figure 41-A and B).
Table 20 Summary of the screened synthesis compositions showing influence of solid content wt% and the
type of carboxylic acid on the final sol stability. Shadowed cells represent unstable sols while white ones
represent stable clear sols (stable for several months). Fixed molar ratios of the sols include Ac/Zrpropoxide
~1, water/Zr-propoxide ~4 and acid/Ac ~1. § Maximal 8YSZ wt% content was limited by the
carboxylic acid solubility, being the maximum values: 20% (acetic), 19% (propionic), 18% (caproic) and
17% (nanonic). Unstable sols start precipitating after a few days or weeks.
Solid Content,
8YSZ wt%
2.5
5.0
10
15
Max.
§
71
Carboxylic Acid
Acetic Propanoic Caproic Nanonic
Unstable Sols
Stable Sols
On the other hand, the sols with a higher solid content show a larger average particle size
(Figure 41-C) and are more stabile. The sols have been analysed with the dynamic light
scattering technique immediate after the synthesis was finalised. This effect is attributed
to the higher concentration of protecting/complexing agents (acetyl acetone and the
corresponding carboxylic acid), although the concentration of particles is higher and the
collision probability too. 5wt% structure directing agent added to stable sols showed a
similar particles size distribution with an average particle diameter of 4.5 nm and a
standard deviation (δ) of 1.9 nm (Figure 41-D). Ketone sols with 0, 50, 75, 100 mol %