DEVELOPMENT OF MESOPOROUS MEMBRANE MATERIALS FOR CO2 SEPARATION Page: 3 of 8
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enhancing the permeation of CO2. The idea is to have adsorption sites on the pore wall for the
CO2 to facilitate its surface diffusion along the pore wall. Higher specific surface area would
increase the adsorption of CO2 and enhance the selectivity. The new mesoporous molecular
sieves developed by researchers at Mobil in the early 90s offer an opportunity for improvement.
The mesoporous molecular sieves have larger pore size than zeolites and the pore size can be
varied by the size of the organic template.
The selectivity for CO2 comes from the dopants that prefer to bond with CO2. Horiuchi et
al.1 showed that basic metal oxides in alumina could enhance the selectivity of CO2. The addition
of alkali metal oxides such as Cs20 and alkaline-earth oxides such as BaO were shown to
increase the retention time for CO2 than pure alumina. On the other hand, higher surface area is
expected to improve the adsorption of CO2 as well. Rossignol and Kappenstein2 have recently
shown that the addition of Ba to alumina can increase the surface area at elevated temperatures.
Combining the requirements of high surface area and selective adsorption of CO2, we chose Ba
as the dopant in the present study.
Our preliminary objective is to synthesize high surface-area alumina powders which can
selectively adsorb CO2 over N2. Alumina was synthesized by two methods. One is the
precipitation method using Al(NO3)3. It has been shown recently by Chuah et al.3 that holding
the precipitated alumina in the mother liquids at higher temperatures (digestion) can increase the
surface area. Another method is the templating approach using tartaric acid.4
Preliminary results show that the templating method produced alumina with higher
surface area after 500 C heat treatment for 4 hours. Mesoporous alumina has surface area -380
m2/g compared to precipitated alumina with 240 m2/g. At 500 C heat treatment, it is found that
the addition of Ba lowers the surface area of alumina. This is reasonable since the effect of Ba on
the surface area is expected to occur near the y-a phase transition temperature (~1100 C) rather
than at 500 C. It is generally believed that stabilizing dopants can delay the y-a phase transition
temperature thereby avoiding the destruction of the high surface-area structure of the y phase.
However, there is an interesting difference between the Ba-doped mesoporous alumina and Ba-
doped precipitated alumina. For the precipitated alumina, Ba doping results in larger pore size
and pore volume. On the other hand, the addition of Ba to mesoporous alumina results in smaller
pore size and pore volume.
It was found that mesoporous alumina has larger specific surface area and better
selectivity of CO2 than precipitated alumina. Ba improves the affinity of mesoporous alumina
with CO2 Phase plays an important role in selective adsorption of CO2. It is speculated that
mesoporous alumina is more reactive creating the xBaO-Al203 phase that may be more affinitive
to CO2 than N2. On the other hand, the barium alumnate phase (Ba3A206) in the mesoporous
sample does not help the adsorption of CO2. The preliminary results indicate that in addition to
surface area, pore size and pore volume are also important characteristics of porous materials.
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Shih, Wei-Heng; Zhao, Qiang & Wang, Nanlin. DEVELOPMENT OF MESOPOROUS MEMBRANE MATERIALS FOR CO2 SEPARATION, report, May 1, 2002; Pittsburgh, Pennsylvania. (digital.library.unt.edu/ark:/67531/metadc740146/m1/3/: accessed March 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.