NAMS 2002 Workshop - ICOM 2008

Inorganic Membranes III – 1 – Keynote
Friday July 18, 9:30 AM-10:15 AM, O’ahu/Waialua
Silica Network Engineering For Highly Permeable Hydrogen Separation
Membranes
T. Tsuru (Speaker), Hiroshima University, Hiroshima, Japan - tsuru@hiroshima-u.ac.jp
K. Yada, Hiroshima University, Hiroshima, Japan
M. Kanezashi, Hiroshima University, Hiroshima, Japan
Inorganic membranes are promising for possible application to high temperature
separation systems and membrane reactor systems [1-3]. Metal membranes,
which shows 100% selectivity to hydrogen and high permeances at
temperatures, have several disadvantages such as expensive cost, degradation
with hydrocarbon and acid gases, and hydrogen brittleness at low temperatures.
On the other hand, amorphous silica, which can be derived from the sol-gel
processing or CVD (Chemical vapor deposition), is a microporous material,
consisting of silica network which allows the permeation of small molecules such
as helium and hydrogen. In this paper, recent progress in the control and design
of silica network by sol-gel processing will be discussed to develop highly
permeable hydrogen separation membranes.
The sol-gel process is divided into two main routes: the polymeric sol-gel route
and the colloidal sol-gel route1). In the colloidal sol route where the hydrolysis
and condensation reaction of alkoxide (tetraethoxysilane (TEOS) for SiO2
membranes) is fast, the rapid condensation reaction causes particulate growth
and/or the formation of precipitates. In the polymeric sol route, the hydrolysis
reaction is slower, resulting in a partially hydrolyzed alkoxide and the formation of
a linear inorganic polymer. Pore sizes can be controlled by the void spaces
among the packed colloidal particles (i. e. interparticle pore) in the colloidal sol
route and by the size of the gel network in the polymeric gel route, respectively.
By controlling the preparation condition of silica sols (pH, temperature,
concentration, aging time etc.), pore sizes of SiO2 membranes were found to be
precisely tuned in the subnanometer range. SiO2 membranes showing highly
hydrogen selectivity over nitrogen [3], as well as showing a large H2 permeation
rate with low H2/N2 but high H2/SF6 separation factors, were successfully
prepared [1].
Another strategy to control silica network is the utilization of structured alkoxides,
such as (EtO)3-Si-(CH2)n-Si-(OEt)3 (n=1-6). Since the silicone atoms are more
distant with each other than the case of TEOS, due to the existence of -C2H4-,
the silica network can be expected to be formed more loosely than the case of
TEOS. Silica membranes prepared from bis(triethoxysilil)ethane (BTESE, n=2)