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

2 Theoretical background
2.3.3.2 Microporous transition metal oxide membranes
In contrast to sol-gel derived non-transition silica membranes, the formation of
microporous transition titania or (yttria stabilized) zirconia is difficult due to the fast and
hard to control hydrolysis and condensation reactions of Ti or Zr-precursors. 81 The fast
reactivity can be a result of the larger charge density of Zr or Ti compared with Si 76 or
by the heterogeneousity of the commercially purchased alkoxide precursors. 80,81 Several
groups have reported the synthesis of microporous YSZ (yttria doped zirconia) 78,81,83,104-
111 or titania 78,82,83,100-102,104,112,113 materials, however they were not successful in
producing membranes with a significantly larger permselectivity than the Knudsen factor
for the gasses H2, N2, CO2 and CH4 as a result of the pore sizes of ~0.9 nm and larger. 114
2.3.4 Selection of Sol-Gel derived membrane material
Microporous sol-gel derived silica membranes are found nowadays industrial in
pervaporation applications. Lab scale sol-gel derived membranes offer good gas
separation factors which is mainly based on the molecular sieving compared with
sorption properties. Good prospects are therefore envisaged for applications such as
dehydrogenation of hydrocarbons, coal gasification, the postcombustion concept and the
water-gas shift process in the precombustion concept.
Modified sol-gel derived non-transition metal oxide silica membranes have not found
their way into the large scale gas separation. An alternative method to obtain
hydrothermal stable microporous membranes is to develop non-silica sol-gel derived
materials with the following requirements:
o Separative to H2 from CO2 or CO2 from N2 by means of molecular sieving
o Defect free microporous layers
o Comparable permeabilities to silica membranes
o Stable in hydrothermal conditions
2.3.4.1 Microporous transition metal oxide membranes
Sol-gel derived microporous crystalline titania membranes are commercially today
available with a pore size of 0.9 nm. 101 Sol-gel derived microporous crystalline (yttria
doped) zirconia material is prepared at lab scale. Both, titania and zirconia, sol-gel
derived transition metal oxide materials are being increasingly studied due to a more
controllable sol-gel synthesis using precursor modifiers. 78-80,82,83,110,115
Crystalline materials are desirable due to their expected thermal and chemical stability.
However, state of the art silica layers are amorphous and contain pore sizes in the range
of 0.5 nm, whereas the titania and zirconia materials tend to crystallise at lower
temperatures and might have larger particles than their silica counterparts, resulting in
larger pores. It is known that a mixture of titania and zirconia has a higher crystallisation
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