Membrane Gas Separation

8.4
Conclusions 
Physical and gas transport properties of HBPI – silica hybrid membranes prepared by the 
sol – gel method with two kinds of alkoxysilanes, TMOS and MTMS, were investigated. 
From the comparison of FTIR spectra for the TMOS and MTMS systems, the red shift 
of the Si – O – Si peaks is observed for the hybrid membranes prepared with MTMS, sug-
gesting the formation of a looser Si – O – Si network caused by the methyl groups in MTMS, 
which induce large enhancements of mean size and total amount of free volume elements 
compared to those of the TMOS system. Thermal stabilities of the hybrid membranes are 
essentially improved by hybridization with silica. The T
g
and
T
d
5
of the hybrid membranes 
increase with increasing silica content, resulting from the formation of three - dimensional 
Si – O – Si network. However, the T
g
values slightly decrease at low silica content region 
for MTMS and TMOS/MTMS combined systems due to the insuffi cient and loose forma-
tion of three - dimensional Si – O – Si network and the enhanced free volume holes. The CTE 
of the TMOS system greatly decreases with increasing silica content, and in contrast, the 
CTE of the MTMS system increases with increasing silica content. CO 
2
, O 
2
, N 
2
, and CH 
4
permeability coeffi cients of the hybrid membranes increase with increasing silica content 
because of additional formation of free volume elements. In particular, MTMS and 
TMOS/MTMS combined systems show enormous improvements of gas diffusivity 
because of large enhancements of mean size and total amount of free volume elements 
attributed to loose Si – O – Si networks. The TMOS/MTMS combined systems also exhibit 
simultaneous large enhancements of CO 
2
permeability and CO 
2
/CH 
4
separation ability. 
This fact indicates the mean size and distribution of free volume elements in the TMOS/
MTMS combined systems are successfully controlled to achieve the unique selective CO 
2
/
CH 
4
separation ability, and are expected to apply to high - performance gas separation 
membranes.