Gas Transport Mechanisms through New Templated Silica
Membranes Prepared by Sol-Gel Method
M. Pakizeh, M. R. Omidkhah*, A. Zarringhalam and M. Shariati Niasar
Department of Chemical Engineering, Tarbiat Modares University,
P.O.Box:14115-143, Tehran, Iran
Abstract
Different templated silica membranes were prepared by sol-gel polymerization route using
cationic and non-ionic surfactants as template agent. -alumina support and prepared silica
membranes were used in single-gas permeation and mixed-gas separation experiments. Gas
transport mechanisms through porous membranes as well as influences of main parameters in
each mechanism have been reviewed in details. The characteristics of gas transport through
the support as macroporous membrane and templated silica membranes were investigated
with several gas diffusion models in the range of 200-400oC and at pressure differences
ranging from 0.2 to 1 bar. The experimental gas permeation data of alumina support were
analyzed using the viscous flow model and results clearly showed good agreement between
the model analysis and the experimental data. The prepared templated asymmetric silica
membranes represented different behavior each other due to their different characteristics.
The experimental data of silica membrane which templated by polyoxyethylene(4) lauryl
ether (Brij30) showed that the permeation followed the Knudsen diffusion model in which the
permeance decreased with increasing temperature. The experimental gas permeation data of
cetylpyridinium bromide (CPB) templated silica membrane as well as non-templated silica
membrane as microporous membranes revealed that they followed the configurational
diffusion model in which the permeance increased with temperature. Separation factors
obtained from mixed-gas separation experiments with 50/50 (vol%) gas mixtures were
slightly lower than the permselectivities.
1. Introduction
Supported inorganic membranes have attracted considerable attentions due to their
outstanding thermal and chemical stability over the past half century [1]. Among the
inorganic membranes, supported silica membranes have been frequently utilized in gas
separation processes by many researchers due to their synthesis convenience and low cost [2].
By far, the sol-gel method due to introducing desirable characteristics such as: high surface
area, high pore volume and bimodal pore size distribution has been reported as the best
method for silica membrane synthesis [3,4]. Recently, the sol - gel method has been combined
with template technology to improve the silica membranes performances. Template
technology uses template agents which can be ionic or nonionic surfactants. Kresge et al.
from Mobil Oil Corporation have used this technology and produced the class of mesoporous
materials known as MCM – 41with hexagonal arrays structure for first time [5]. McCool et
al.
produced
supported
silica
membranes
which
templated
by
cetyltrimethylammonium bromide (CTAB) in hydrothermal and dip-coating methods.
They have reported that dip-coating membranes have a flux three times greater than
*
Omidkhah@modares.ac.ir
hydrothermally prepared also the permeation characteristics of both membranes were
governed by Knudsen diffusion model [6]. Zhong and coworkers have used
dodcyltrimethylammonium chloride (DTAC) as template agent and have obtained
mesoporous silica membrane. They have carried out the gas permeation experiments for
C2H4, N2, C2H6 and CO2 gases which have been controlled by Knudsen diffusion model [7].
Doohwan and Oyama have prepared a novel silica membrane namely Nanosil which has
been contained a deposited silica layer on Vycor glass as support. They have presented gas
permeation experiments of He, Ne, H2, CO2, CO and CH4 both on Vycor glass and the
Nanosil membranes and have found that the gas permeation mechanism on the Vycor glass
has followed a combination of surface diffusion and Knudsen permeation models, while the
permeation of the Nanosil has been controlled by a statistical solid state diffusion model. This
model could justify unexpected permeation of He, H2 and Ne gas (He>H2>Ne) through
membrane whereas molecular size of H2 is greater than Ne molecule [8]. So et al. have
prepared silica membrane in situ sol-gel and dip-coating methods with two step pore
modification. With pore modification by Pd precursor, H2 selectivity was increased while
Knudsen diffusion was replaced by surface diffusion model [9]. Thomas and coworkers have
investigated the involving gas permeation mechanisms in multilayer membrane. They have
proposed the configurational diffusion model and found good agreement between this model
and experimental data for He and H2 gases [10].
2. Mass transfer in inorganic porous membranes
Mass transfer in porous media has been frequently studied by many researchers [11, 12].
Although, based on pore size and mean free path of gas molecules, several mechanisms have
been presented, but in many cases these mechanisms due to existence of a large range of pore
sizes cannot be used alone. Adsorptions and diffusion are two main phenomena that have
important influences on overall gas transport in some cases. The interaction between gas
molecules and membrane surface is considered by adsorption while diffusion describes the
gas molecules transport through the membrane. Recently, the activated configurational
diffusion model has been proposed as the most advanced model for the analyzing the
permeance results of microporous membranes.
2.1. Viscous flow model
When the number of intermolecular collisions is much greater than collisions between gas
molecules and membrane surface which can be happened in the case of larger pores, the flow
in a capillary can be expressed by Darcy's law [13]. The molar flux in narrow capillaries with
laminar flow, can be described by a Hagen- Poiseuille type law:
 rp2 P dp
Jv 
(1)
8 RT dz
Eq. (1) is modified for real systems by introducing  (porosity) and  (tortuosity):
Jv 
2
  rp P dp
.
 8 RT dz
(2)
By integration of Eq. (2) across the membrane thickness (L) gives the permeation:
2
 Jv
 rp Pm
Qv 

P 8 RTL
where
Pm  ( P1  P2 ) / 2
(3)
Thus, in this Flow, the permeation is proportional to the mean pressure and squared hydraulic
radius (r2). Also the permeation decrease with temperature for gases because the viscosity of
gases increases with temperature.
2.2 Knudsen diffusion model
When the number of intermolecular collisions is much less than molecule-wall collisions,
Knudsen diffusion occurs. In this regime, diffusion occurs in the result of consecutively
collisions of gas molecule with pore wall. The Knudsen diffusivity is obtained from the
geometric parameters related to the membrane and gas kinetic velocity:
D= (geometric parameters).(gas kinetic velocity)
1/ 2
 d p  8RT 

(4)
Dkn  

 3  M 
Where dp is the pore diameter, R the gas constant and M is molecular weight of diffusing gas
[8,13].
By substituting Eq. (4) in macroscopic description of transport process (Fick's first law; J= D(c)c), the expression for the Knudsen flow in a porous membrane is obtained:
0.5
 2 rp  8  dp
J kn 
.
(5)


3   RTM  dz
After integration of Eq. (5) over the membrane thickness L the permeation is found to be:
1/ 2
 J kn 2rp  8 
Qkn 



P
3L  RTM 
(6)
As can be seen in Eq.(6), in this flow regime, the permeation of a single gas is proportional to
the average pore radius (rp) and independent of mean pressure. Also, every diffusing gas
should have the same value of Q (MT)1/2 = (dp/L) (8/9R)1/2 which is independent of
temperature if gas transport through the membrane governed only by Knudsen diffusion.
2.3. Surface diffusion model
In this model, gas molecules adsorb on the surface of membrane at first and in subsequence
followed by diffusion to the other side of membrane where they desorbs to the gaseous bulk.
Based on different assumption about the situation of adsorbed molecules, several adsorption
models can be found in the literature [14]. Among them BET and Langmuir adsorptions
models have been frequently used by researchers. In gas separation process using porous
membranes, gas adsorption usually occurs in the state of monolayer. Monolayer adsorption
has been well described by Langmuir adsorption model [15].

q
bp

q s 1  bP
(7)
where  is the fractional occupancy of adsorption sites, q the amount of adsorbed gas
molecules per unit mass of adsorbent (molg-1), qs the saturation amount of adsorbed
molecules, P the pressure (pa) and b is an equilibrium adsorption constant (Pa-1). Equilibrium
adsorption constant (b) has the following dependency on temperature:
H a
(8)
b  b0 exp(
)
RT
Where Ha is the heat of adsorption. At high temperature and low pressure (bp<<1) Eq. (7) is
reduced to Henry's equation
q  q s bp  KP  K 0 exp(
H a
)P
RT
(9)
where K is Henry's constant (mol-1g-1pa-1). In this regime, the amount of adsorbed molecules
varies linearly with pressure. Concentration of gas molecules at membrane surface is product
of membrane density () and the amount of adsorbed molecules:
H a
(10)
c  q  K 0 exp(
)P
RT
By introducing the Eq. (10) in Fick's first law:
J s   D0 (q) K 0 exp(
H a
 E dp
) exp (
)
RT
RT dz
(11)
By integration Eq. (11) across the membrane, permeation is obtained for surface diffusion
model:
D0 (q) K 0
 H a  E 
(12)
Q s  Q0 exp 
where
Q0 

RT
L


Qo (molm-2s-1pa-1) and Ha-E (KJmol-1) are the model parameters for surface diffusion
mechanism.
2.4. Configurational diffusion model
The configurational diffusion mechanism has been applied only for microporous materials. In
this model is assumed that gas molecules can not escape completely from the potential field
of the solid [17]. Several researchers have nominated this model as the gas translational (GT)
diffusion model in literature [17,18]. Two main concepts are considered by this mechanism;
the first is related to the Knudsen model and the later is related to the moving of gas
molecules between sorption sites in a translational mode by overcoming the barriers formed
by small channels which connect adjacent sorption sites:
 Ec
8RT
(13)
Dc   g d p
exp(
)
M i
RT
With
Jc  
1 
D CP
RT 
(14)
By integration of Eq (14) across membrane thickness L, Permeation is obtained:
Qc  
JC g d p 

.
P
L 
 Ec
8
exp(
)
MRT
RT
(15)
The temperature dependency of diffusivity of this model differs completely from that of the
Knudsen model. The parameter g is the probability that a gas molecule can overcome the
energy barrier Ec. Also the other models such as vibrational gas diffusion and statistical
solid state diffusion model have been used by researchers [8].
3. Experimental procedures
3.1 Membrane preparation
In order to minimization of polycondensation rates of silica species, the standard sol was
prepared in two acid-catalyzed reaction steps. With these procedures, the formation of highlybranched polymeric clusters was avoided. In the first step, 40 ml TEOS was added to a
mixture of 38.7 ml EtOH , 4 ml DI-water, followed by the adding nitric acid (0.012 ml)
while stirring the mixture (pH=4.6). The sol was refluxed for 1.5 h at 70 oC. In the second
step, additional water and acid were added to the prepared sols so that the final molar ratios of
sols were reached to the values of Table1 (pH=1.8).
Table 1- Molar ratio of templates and precursors used in sols preparation
Molar
ratios
Sample
TEOS
H2O
ETOH
HNO3
Template
Si(TF)
1
6
3.8
0.086
0.0
Si(CPB)
1
6
3.8
0.086
0.14
Si (Brij)
1
6
3.8
0.086
0.14
The sol samples were then cooled to the room temperature, moreover the corresponding
amount of surfactants powder were added and stirred at 250 rpm for 5 min. The obtained sols
were referred as Si(CPB), Si(Brij) and Si(TF) considering their used template. As can be seen
in Table 1, all the templates concentrations are above the Critical Micellar Concentration
(CMC) [25]. The dipping solutions were aged for 3 days to allow the polymeric silica
particles to organize and form around the template micelles. The home made polished alumina tube with the average pore size of 80nm and porosity of 30%, as measured by Pascal
440 series Hg porosimetry, was used as support. The inner and outer diameters of this support
tube were: 10 and 12 mm and its length was 5cm. It was first cleaned in acetone by ultrasonic
regenerator (Ultrawave) for 20 min and then dried in a laminar flow cabinet. The silica film
on outer surface of the support may be damaged during the assembling to membrane holder,
therefore, it was coated on inner surface of the support. The cleaned supports were dipped in
the aged polymeric sols for 2 min. Dipping and withdrawal rates were approximately 2cms-1.
The as-coated membranes tubes were then dried in a laminar flow cabinet for 2 h. Sol-gel
transition was occurred immediately after withdrawal of the supports from the dipping sols,
due to fast evaporation of solvent. The as-prepared Membranes were then calcined in air at
600oC for 4 h with the heating and cooling rates of 0.5◦Cmin-1. The dipping sols first were
converted into wet gels after several weeks, then into clear films in laminar flow cabinet. Each
clear film was ground and then calcined under the same conditions as membrane synthesis
step.
3.2. Unsupported membrane characterization
The surface area of the unsupported membranes was calculated by Brunaver-Emmett-Teller
(BET) method using N2 adsorption-desorption isotherms at relative pressure( P/P0) of 0.01 to
0.995 at 77 K ( Micromeritics ASAP 2000). Pore volume was obtained considering the
amount of N2 adsorbed. Average pore diameter of unsupported membranes were obtained
using Barrett- Joyner- Halenda (BJH) method.
3.3. Gas permeation
The as-prepared silica membranes were encapsulated in a custom-made stainless steel
membrane holder by placing the two end sides of the tubular membranes into two aluminum
annulars and sealed completely with high temperature resistant epoxy resin. A schematic
representation of the membrane holder has been given in Figure 1.
Sweep Gas Inlet
Heater
Stainless steel
module
Retentate
Stream
Feed Inlet
Annular aluminium
Membrane
Heater
Permeate Stream
Figure 1: A schematic representation of the membrane holder
The permeation properties of the tubular support and silica membranes were measured over
the temperature range of 200-400 oC and pressure difference range of 0.2-1bar using the
experimental set-up depicted in Figure2. Prior to the gas permeation experiments, ambient
moisture was removed with dry argon flowing at 100°C for 4 h through the membranes.
Single component permeation of a series of gases, considering the kinetic diameter, i.e. H2
(2.89Å), CO2 (3.3Å), CH4 (3.8Å) was measured by flowing a stream of each gas through the
membrane.
Vent
T
PG
F
F
Membrane
F
Heater
GC
F
T
Needle valve Reducer 3-way valve
F
CO2
H2
CH4
Mass flow
controller
Ar
DC 24 V
Gas cylinder
Pressure
Gauge
Temperature
Indicator
GC
Gas
Chromatography
DC
24 V
PC
Power
supply
Figure2: Experimental set-up for single-gas and mixed-gas permeation.
For mixed-gas permeation measurement, a per-mixed gas with known composition was used
and the composition of both effluent streams were analyzed using an online Philips PU4410
Series Gas Chromatograph equipped with a stainless steel packed column (Hayesep N,
80/110, 2M) and thermal conductivity detector. The flow rate and the pressure of the tube side
stream as well as the temperature of the membrane were varied. Pure argon with constant
flow rate of 25cc/min was used as sweep gas, flowing in the shell side of the permeation cell
at ambient pressure. Gas flow rate exiting from the shell side was measured by a calibrated
soap-film flowmeter. The experiments were conducted under steady-state conditions (no
change in compositions and flow rate with time). The gas flux was then calculated using the
outlet gas flow rate and the concentration of the permeated gas outcoming from the shell side.
The permeance, Q, is considered as ratio of gas flux, J (mol/m2 s), and the pressure difference
(Q (mol/m2 s Pa) = -J/P). Permselectivity is defined as ratio of permeances of individual
pure gases and the separation factor (i/j) is defined by following expression:
retentate
y ipermeate y j
 i / j  permeate . retentate
(i and j are the components of feed)
(16)
yj
yi
y AB is the mole fraction of component A in stream B.
Infinity separation factor is obtained when y jpermeate  0.0 which can be obtained for dense
membranes
4. Results and discussion
Four types of membranes were studied in this work. One was a -alumina support, and the
second was Si(TF), a supported type membrane consisting of a thin film of non-templated
silica deposited on support. The two later membranes were Si(CPB) and Si(Brij), supported
silica membrane which were templated by CPB and Brij surfactants, respectively. The
average pore size, pore volume and the surface area of unsupported membranes were
calculated by BJH and BET methods using N2 adsorption/desorption data and the results have
been illustrated in Table 2.
Table 2- Textural characteristics of different samples of unsupported silica membranes
Surface
Average
Pore
area
pore size
volume
(m2/g)
(nm)
(cm3/g)
Si(TF)
121
0.987
0.0238
Si(CPB)
1160
1.51
0.426
Si(Brij)
482
2.86
0.372
Sample
Permeance × 10-5 (mol/m2 s Pa)
Such high surface areas of the Si(CPB), and Si(Brij) membranes compared to the Si(TF)
membrane are due to the application of the templates. Average pore sizes of the Si(TF) and
Si(CPB) samples categorize them in the microporous regime. In the Si(TF) sample, due to
the absence of self assembled organic component, polycondensation of inorganic silicate or
agglomeration of particles couldn’t occur around the specific structure of template.
Formation of these particles without a tailoring template, accompanied with the removing of
small ligand (ethyl group) which was covalently bound to silica network, lead to lower pore
size, pore volume and surface area compared to other templated silica membranes. Average
pore sizes of the Si(Brij) sample was in mesoporous regime. Differences between average
pore sizes of these samples depend on the nature of the template and interaction between the
silica particles and micelles. The adsorption of surfactants, e.g. CTAB and CPB, on silica
surface at different micelle concentrations has been also studied by Atkin and coworkers
[19,20].
Since the pore size of alumina support is much greater than the mean free paths of the
diffusing gases (H2, CO2 and CH4), transport through the support membrane has generally
been described by the viscous flow model. A property of gas transport by the viscous flow
model is that the permeance increases linearly with increasing mean pressure at constant
temperature . Also at constant pressure difference, the permeance decreases with increasing
temperature. Gas permeance data on alumina support was studied and the results are shown in
Figures 3 and 4. In spite of the molecular size of CH4 (3.8Å) being greater than CO2 (3.3Å),
permeation of CH4 was greater than CO2 through the support.
1.5
H2
1.3
1.1
CH4
0.9
CO2
0.7
0.5
1
1.2
1.4
1.6
Mean pressure(bar)
Figure 3: Gas permeation data through alumina support versus mean pressure at 400°C.
Permeance × 10-5 (mol/m2 s Pa)
1.6
H2
1.2
CH4
0.8
CO2
0.4
170
220
270
320
370
420
Temperature(°C)
Figure4: Temperature dependence of the permeance for the support at P=0.6 bar, Pm = 1.3.
This implies that the support controls the permeation considering the viscosity and not the
molecular size, again was in agreement with viscous flow model. The permeance data was
fitted to the viscous flow model (Eq.(3)) by the least squares method and the best-fit
parameters are shown in Table 3.
Table 3- Determined the viscous flux parameter for support in different single gas
permeations
Regression
Gas
ε/τ
H2
0.209
0.998
CH4
0.208
0.997
CO2
0.21
0.998
coefficient
In the each case of next section, the membrane contains a silica thin layer deposited on alumina as support, therefore the overall resistance for gas permeation is a summation of
resistance on the support and the silica layer:
1
1
1
(17)


Qmembrane Qsup port Qsilica
By subtracting the resistance of support from the measured resistance of supported silica
membrane, the permeance of silica layer was calculated. Generally, mesoporous membranes
with pore sizes of 2- 20nm exhibit Knudsen diffusion as dominating permeation mechanism.
Since the ratio of the mean free path of diffusing gas molecules (H2, CH4 and CO2) to the pore
size of Si(Brij) (2.86nm) are greater than 10, therefore gas transport through membrane can be
governed by Knudsen diffusion mechanism. As described before, in this regime, permeance
shows an inverse square root dependence on temperature and molecular weight of diffusing
gas molecule. In the other words the value of Q(MT)1/2 which is equal to (dp/L)(8/9R)1/2
for each diffusing gas must be relatively constant and independent of temperature (Eq(6)).
Using this approach, the gas permeance data were analyzed and the results are shown in
Figures 5 and 6.
Permeance × 10-6 (mol/m2 s Pa)
4.8
4.2
3.6
3
1
1.2
1.4
1.6
Mean pressure(bar)
2
Permeance × 10-6 (mol/m2 s Pa)
Permeance × 10-6 (mol/m2 s Pa)
Figure 5: Hydrogen permeance versus mean pressure for the Si(Brij) membrane (T = 400°C).
1.7
CH4
1.4
1.1
CO2
0.8
1
1.2
1.4
1.6
4.8
H2
3.8
2.8
CH4
1.8
CO2
0.8
150
250
Mean pressure(bar)
Figure6: CO2 and CH4 permeances versus mean-
350
450
Temperature(°C)
Figure7: Permeances of H2, CH4 and
CO2 versus pressure for the Si(Brij) membrane (T = 400°C).
temperature for the
Si(Brij) membrane (P= 0.6 bar).
Figure7 illustrates that permeance of gases decrease with increasing temperature similar to
support. Generally, CO2 permeation through microporous or mesoporous membranes is
governed by surface diffusion model at lower temperatures. However we conducted the gas
permeation in both cases of single-gas and mixed-gas permeation at higher temperatures (473673K) and no contribution of surface diffusion mechanism was observed. Nearly constant
values of Q(MT)1/2 for H2 and CH4 over the applied temperature range imply that the
permeances of these gases through Si(Brij) membrane is controlled by Knudsen diffusion
model(Figure 8).
1.5
1.2
0.9
0.6
H2
0.3
CH4
CO2
0
150
4.5
4.2
2.4
3.9
2.2
3.6
2
3.3
Si(TF)
Si(CPB)
1.8
250
350
450
Permeance × 10 -6 (mol/m 2 s Pa)
Permeance × 10 -6 (mol/m 2 s Pa)
Q(MT)1/2× 10-4 (mol (g°C)1/2/m2 s Pa)
2.6
3
1
Temperature(°C)
1.2
1.4
1.6
Mean pressure(bar)
Figure8. Q (MT)1/2 versus temperature for the Si(Brij)
Figure9. Hydrogen permeance
versus mean pressure membrane (P= 0.6 bar).
for Si(TF) and
Si(CPB) membranes at 400°C.
In contrast to alumina support there is not an increase of gas permeances with increasing the
pressure across the Si(TF) and Si(CPB) membranes as can be seen in Figures 9 and 10. The
permeances through alumina support as macroporous membrane and Si(Brij) as mesoporous
membrane decrease with increasing temperature over the entire range as expected from the
viscous flow and Knudsen diffusion models while the permeance through the Si(TF) and
Si(CPB) membranes shows an increase
with temperature .
0.15
0.016
0.145
0.012
0.14
0.008
0.135
0.004
0
0.13
1
1.2
1.4
Mean Pressure(bar)
1.6
Permeance × 10 -6 (mol/m 2 s Pa)
Permeance × 10 -6 (mol/m 2 s Pa)
0.02
Figure10. CO2 and CH4 permeances versus
mean pressure at 400°C for Si(TF) and
Si(CPB) membranes.
The results imply that the gas diffusion
mechanism
for
Si(TF)
and
Si(CPB)
membranes follows an activated diffusion
mechanism.
Increasing
of
the
gas
permeances through both the Si(TF) and
Si(CPB) microporous membranes with
increasing temperatures have been presented by Figure 11 and 12.
The permeation
characteristics of the hydrogen molecules on the silica layer of both Si(TF) and Si(CPB)
membranes were studied by comparing it with the gas permeability on the asymmetric silica
membranes using Eq.(17) and the results have been shown in Figure 13. The calculations for
H2 diffusion, revealed that the resistances of silica layers against the mass transfer were 88%
and 74% of overall resistance for the Si(TF) and Si(CPB) membranes, respectively
Temperature contributes in exponential terms as well as in gas kinetic velocity. Eq.(15) can be
rewritten as:
ln
 T Q   ln  LA
c

8  E c  1 

 
MR  R  T 
A
g d p 
(18)

A linearization was used by plotting the values of ln( T Qc ) versus 1/T and the best-fit
parameters was calculated which have been indicated in Tables 4 and 5.
Table 4-Values of configurational model parameters for Si(TF) membrane
Gas
A (m)
Regression
ΔEc (J/mol)
coefficient
H2
1.151×10-8
5403
0.9997
CO2
0.943×10-8
11051
0.9988
CH4
4.57×10-10
9833
0.9898
Moreover, the activation energies of CO2 are higher than those of CH4 and H2 in both Si(TF)
and Si(CPB) membranes. The activation energy of CH4 is 9.833 kJ/mol and 6.076 kJ/mol for
Si(TF) and Si(CPB) membranes respectively. de Vos et al.[22] have reported activation
energy of CH4 around 10 kJ/mol for Si(400) that is vary close to that of the Si(TF) in present
study also it is in agreement with Tsai et al.[21] findings on single-layer silica membrane
(9.64kJ/mol).
Table 5- Values of configurational model parameters for Si(CPB) membrane
Gas
A (m)
ΔEc
Regression
(J/mol)
coefficient
H2
1.099 ×10-8
3753
0.9976
CO2
1.032 ×10-8
6076
0.9985
CH4
4.587×10-10
9605
0.9878
The activation energy of H2 was the lowest in both cases, indicating that hydrogen molecules
overcome rapidly the barrier energy of the membranes and move easily between sorption sites
in comparison with CH4 and CO2 molecules. The activation energy of H2 for both Si(TF) and
Si(CPB) membranes are greater than the reported value (2968 J/mol) by Thomas et al. [10]
for their composite membrane. While the A parameter of the Si(TF) and Si(CPB) are very
close to that of their composite silica membrane for H2 (1.09×10-8 m).
Table 6- Permselectivity (Pi/j) and separation factor (i/j) for Si(Brij), Si(TF)
and Si(CPB)membranes at T = 400°C and Pm=1.3 bar
Pi/j
α i/j
H2/CH4 H2/CO2
H2/CH4 H2/CO2
Si(Brij)
2.94
5
2.88
4.79
Si(TF)
149
15.5
131
12.9
Si(CPB)
135
35
123
28.4
Membrane
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