EFFECT of the INTRAWALL MICROPOROSITY on the DIFFUSION
CHARACTERIZATION of BI-POROUS SBA-15 MATERIALS
VINH THANG HOANG a,QINGLIN HUANG b, MLADEN EIC b*,
TRONG ON DO a, and SERGE KALIAGUINE a
a
Department of Chemical Engineering, Laval University, Ste-Foy,
Québec, Canada, G1K 7P4
b
Department of Chemical Engineering, University of New
Brunswick, P.O Box 4400, Fredericton, N.B., Canada, E3B 5A3
Abstract
In our recent study [1], a series of synthesized SBA-15 materials were
characterized by nitrogen adsorption/desorption isotherms at 77K, and four of
them (MMS-1-RT, MMS-1-60, MMS-1-80 and MMS-5-80) were investigated
by the ZLC technique involving n-heptane diffusion at a low concentration
level. SBA-15 materials proved to have a broad pore size distribution within the
micropore/small-mesopore range in the walls of their main mesoporous
channels. Desorption/diffusion of n-heptane took place inside the intrawall
pores and depended on the relative content of micropores. In this paper, the four
selected samples were further investigated with regard to diffusion of two other
probe molecules having different kinetic diameter, i.e., cumene and mesitylene.
The results show that the diffusivities and activation energies of the three
different probe molecules in MMS-1-RT, MMS-1-60 and MMS-1-80 samples
are of the same order, while they show an opposite trend relative to their size in
MMS-5-80 sample (the lowest micropore content). The transport of these probe
molecules is controlled by a combination of micro- and meso-pore diffusion
resistances in the intrawall pores. The mesopores in main channels played more
and more important role in diffusion processes where the relative micropore
volume was decreased, and the critical size of probe molecules increased. The
diffusivities in SBA-15 samples were significantly higher than in ZSM-5
zeolites for cumene and mesitylene. The activation energies, in contrast, were
found to be much smaller than in corresponding MFI- zeolites.
Keywords: Diffusion, micro/mesoporosity, SBA-15, MFI-zeolites, n-heptane,
cumene, mesitylene, ZLC technique
1. Introduction
In a previous paper [1], we reported the formation and structural
characterization of a series of bi-porous SBA-15 materials. These materials
were found to have not only an array of hexagonally ordered primary channels
but also a certain amount of smaller pores with a broad pore size distribution in
the micropore and small mesopore range within the mesoporous walls. The
micropores and small mesopores constitute the intrawall porous structure of the
SBA-15 materials. The microporosity could be controlled systematically by the
synthesis conditions. These results are similar to those reported by several
authors [2-9], who indicated the existence of micropores within the pore walls
of mesopores. Furthermore, the diffusion of n-heptane in four selected SBA-15
samples (MMS-1-RT, MMS-1-60, MMS-1-80 and MMS-5-80), having
different micropore volumes, has been studied by the Zero Length Column
(ZLC) technique. Intrawall pores were found to play a major role in controlling
diffusion at a low concentration level. The overall diffusion process of nheptane was controlled by a combination of micro- and mesopore-diffusion
resistances in the mesoporous walls and depended on the relative content of
micropores. In the samples that have relatively high content of micropores, nheptane diffusion process is similar to that of typical microporous adsorbents.
As the micropore content is decreased, diffusion becomes more and more
controlled by secondary mesopores of the intra-wall pore structure. Furthermore
primary mesopores, comprising the main channel structure could also play a
role in facilitating transport through the biporous structure of SBA-15 materials
with low micropore content.
Studies of the simultaneous influence of micropores and mesopores on the
mechanisms as well as transport properties of sorbate molecules in the SBA-15
mesoporous materials is an essential first step in developing novel materials for
future applications [10-13]. The overall diffusion process in porous materials is
often governed by contributions from different transport steps. These can
include slow transport through the micropores, and relatively fast diffusion in
mesoporous channels. In this regard, using probe molecules with different
kinetic diameters in the diffusion study is a promising strategy.
In continuation of our previous study introduced above, the diffusion of cumene
and mesitylene in the four selected bi-porous SBA-15 samples was investigated
using the ZLC method. The diffusivity data obtained for bi-porous SBA-15
materials are compared and interpreted. The specific effect of microporosity on
the diffusion of probe molecules was investigated in some detail.
2. Materials and Diffusion Measurements
Four hexagonal SBA-15 mesoporous silica samples (MMS-1-RT, MMS-1-60,
MMS-1-80 and MMS-5-80) having different microporosities described in the
previous paper [1] were used. The physico-chemical properties of these
materials are summarized in Table 1. n-Heptane, cumene and mesitylene were
used as probe molecules. n-Heptane has a kinetic diameter of 0.43 nm about
twice smaller than mesitylene (0.87 nm), while, cumene or isopropyl benzene
has a kinetic diameter of 0.67 nm [14].
Diffusion measurements were carried out using the standard ZLC method.
Details of the method are given elsewhere [1,15]. Temperatures investigated in
this study were in the range of 30 to 70oC. Sorbate gas-phase concentrations
were adjusted and maintained within the linear region of the adsorption
isotherm as required by the ZLC theory. Separate blank experiments were
carried out to determine a cut-off point for the analysis of ZLC curves and then
desorption data were fitted with the ZLC model [1].
3. Results and discussion
The effect of purge gas flow rate on ZLC desorption curves for mesitylene
diffusion measurements in MMS-1-60 sample of a bi-porous micro/mesoporous
structure is displayed in Figure 1.
1
1
(b)
(a)
100 ml/min
C/Co
C/Co
50 ml/min
200 ml/min
0.1
150 ml/min
0.1
0
100
200
300
Ft (ml)
400
500
600
0
100
200
300
400
Time (sec)
Figure 1. Effect of flow rate on experimental (symbols) and theoretical (solid lines)
ZLC curves of mesitylene over MMS-1-60 sample at 50oC: F (ml/min) = 50 (); 100
(); 150 (); and 200 ()
Table 1. Textural properties of mesoporous SBA-15 silica*
Sample
MMS-1RT
1
RT
680
362
0.447
MMS-160
1
60
750
415
0.579
MMS-180
1
80
860
578
0.854
Heating time (day)
Synthesis temperature (oC)
SBET (m2/g)
Smeso (m2/g)
Total pore volume, Vt
(cm3/g)
Total mesopore volume, Vmeso
0.324
0.459
0.747
(cm3/g)
Primary mesopore diameter
4.6
5.5
7.1
(nm)
Micropore volume (cm3/g)
0.121
0.119
0.105
Total intrawall pore volume
0.187
0.220
0.332
(cm3/g)
% Micro-porosity (percentage
of micropore volume in the
64.7
54.1
31.6
total intrawall volume)
* Data taken from Vinh Thang et al. [1]
MMS5-80
5
80
920
733
1.106
1.053
7.8
0.059
0.318
18.6
Figure 1a shows ZLC desorption
Table 2. Diffusivity data of mesitylene in
curves at 50oC and different flow
MMS-1-60 sample at 50oC at various flow
rates. In this diagram normalized
rates
sorbate effluent concentrations
Flow rate Temperature
are plotted against the total purge
L
Deff/R2 (s-1)
(ml/min)
(oC)
volume, Ft. At the short time
50
4.2
0.29810-3
asymptotes (Ft < 100ml) the
100
5.3
0.30010-3
50
ZLC desorptions curves are
150
8.0
0.30110-3
similar, indicating that the
200
10.0
0.30010-3
transport is approaching the
equilibrium control limit. However, at the long time region (Ft > 200ml) the
curves clearly diverge, thus confirming all of them being in kinetically
(diffusion) controlled regime regardless of the purge flow rate [16]. The same
results indicating diffusion control regime were obtained for the other two
sorbates investigated in this study, i.e., n-heptane and cumene. The values of
effective diffusional time constant (Deff/R2) derived from the model fitting with
experimental ZLC curves at different purge flow rates as shown in the Figure 1b
are listed in Table 2. The fitted Deff/R2 values were found to be practically the
same for all flow rates in conformation with the summary curves shown in
Figure 1a. This analysis also generally confirms the diffusion-controlled
mechanism in this mesoporous solid that possesses distinctive micropores in the
walls of its principal mesoporous structure.
Representative experimental and theoretical ZLC curves for mesitylene and
cumene in four SBA-15 samples at different temperatures are presented in
Figures 2. A good agreement between experimental results and theoretical
fittings is observed for these two sorbates as well. Summary of effective
diffusional time constants (Deff/R2) extracted from the model fittings of the
experimental ZLC curves together with corresponding parameter L values for
all sorbates are summarized in Table 3. The table also includes the values of
effective diffusivity and activation energy, which were obtained from particle
size analysis using SEM micrographs [1], and Arrhenius-type relationship,
respectively.
As can be seen from Table 3, the effective diffusivity values of molecules in
the four SBA-15 samples were found to be in the order of mesitylene < cumene
< n-heptane, which, as expected, shows the opposite trend with respect to their
critical molecular sizes, thus indicating the important role of this geometrical
factor regarding diffusion rate. Furthermore, the effective diffusivities for all
sorbates increase with the decrease of the relative content of micropore volume
in these samples, e.g., the diffusion rate of each sorbate is in the following
order: MMS-5-80 > MMS-1-80 > MMS-1-60 > MMS-1-RT.
In comparison to MFI-zeolites, i.e., all silica zeolite silicalite, diffusion of
cumene and mesitylene in SBA-15 samples is generally much faster. For
example, values of the effective diffusivity for mesitylene in SBA-15 samples
are about six orders of magnitude higher than in ZSM-5 zeolite measured from
the liquid phase using volumetric method as reported by Choudhary et al.
[14,17]. For cumene, the values of the effective diffusivities are about four
orders of magnitude higher than those reported by Choudhary et al. [18] also
measured from the liquid phase.
It is very unlikely that relatively large organic molecules, in particular
mesitylene, can penetrate the internal structure of silicalite (micropores of the
main channels are only about 0.6 nm in diameter). It is plausible that diffusion
in this case occurs at the external surface of silicalite crystals. On the other hand
it is obvious that the mesoporous structure of SBA-15 samples has a very strong
effect on diffusion. The introduction of main and small mesopores is equivalent
to an increase of external surface area and makes probe molecules more easily
accessible to micropores. However, diffusivity of relatively small n-heptane
molecule in SBA-15 samples is roughly equal to the diffusivity in silicalite
(values extrapolated from the ZLC data presented in Kärger and Ruthven [19]).
1
1
(a)
30oC
o
(a)
0.1
C/Co
C/Co
30 C
70oC
70oC
0.1
50oC
o
50 C
0.01
0.01
0
100
200
300
400
0
50
100
Time (sec)
150
200
Time (sec)
1
1
(b)
(b)
o
o
0.1
30 C
C/Co
C/Co
30 C
70oC
0.1
70oC
o
o
50 C
50 C
0.01
0.01
0
100
200
300
400
0
50
100
Time (sec)
150
1
1
(c)
(c)
o
o
30 C
C/Co
C/Co
30 C
0.1
o
70 C
0.1
o
70 C
50oC
o
50 C
0.01
0.01
0
100
200
300
400
0
50
100
Time (sec)
150
200
Time (sec)
1
1
(d)
o
30 C
0.1
(d)
C/Co
C/Co
200
Time (sec)
30oC
0.1
o
o
70 C
70 C
o
50 C
o
50 C
0.01
0.01
0
100
200
300
400
0
50
100
Time (sec)
Time (sec)
Mesitylene
Cumene
150
200
Figure 2. Effect of temperature on experimental (symbols) and theoretical (solid lines)
ZLC curves for mesitylene and cumene, lnc/co versus t, at flow rate of 200ml/min: (a)
MMS-1-RT; (b) MMS-1-60; (c) MMS-1-80; and (d) MMS-5-80
Table 3. Diffusivity data of n-heptane*, cumene and mesitylene in MMS-x-y
samples
T (oC)
L
10
8.0
n-Heptane
20
8.0
30
8.0
30
6.8
MMS-1-RT
50
Cumene
7.3
(12 m)
70
7.9
30
7.3
50
Mesitylene
7.8
70
8.5
10
11.5
n-Heptane
20
11.5
30
12.3
30
8.0
MMS-1-60
50
9.0
Cumene
(12 m)
70
10.2
30
7.0
50
7.5
Mesitylene
70
9.8
10
12.0
n-Heptane
20
13.0
30
14.5
30
9.4
MMS-1-80
50
10.7
Cumene
(15 m)
70
11.7
30
8.0
50
9.0
Mesitylene
70
11.2
10
12.0
20
n-Heptane
16.0
30
20.0
30
12.0
MMS-5-80
50
12.6
Cumene
(15 m)
70
15.0
30
10.0
50
12.0
Mesitylene
70
14.0
* Data taken from Vinh Thang et al. [1]
Sample
Sorbate
Deff/R2 (s-1.103)
0.50610-3
0.69210-3
0.94410-3
0.34210-3
0.52010-3
0.75910-3
0.20210-3
0.30310-3
0.41810-3
0.62510-3
0.83710-3
1.10210-3
0.39610-3
0.57010-3
0.78810-3
0.24010-3
0.33910-3
0.45610-3
0.96110-3
1.15010-3
1.37510-3
0.51910-3
0.69910-3
0.91310-3
0.30310-3
0.40010-3
0.51510-3
1.38810-3
1.52610-3
1.65910-3
0.70710-3
0.85210-3
1.00210-3
0.38610-3
0.48110-3
0.58310-3
Deff (m2.s-1)
1.82210-14
2.49110-14
3.39810-14
1.23110-14
1.87210-14
2.73210-14
0.72710-14
1.09110-14
1.50510-14
2.25010-14
3.01310-14
3.96710-14
1.42510-14
2.05210-14
2.82710-14
0.86410-14
1.22010-14
1.64210-14
5.40610-14
6.46910-14
7.73410-14
2.91910-14
3.93210-14
5.13510-14
1.70410-14
2.25010-14
2.89710-14
7.80810-14
8.58410-14
9.33210-14
3.97710-14
4.79210-14
5.63610-14
2.17110-14
2.70510-14
3.27910-14
E (kJ/mol)
22.2
17.2
15.8
20.2
14.8
13.9
12.8
12.2
11.4
6.5
7.5
8.9
2
-1
14
Deff (m .sec x10 )
The variation of the effective diffusivity for n-heptane, cumene and
mesitylene with regard to the primary mesopore size of SBA-15 materials at 30
o
C is plotted in Figure 3. The figure displays similar trends for all sorbates.
Apparently, the effective diffusivity increases only marginally as mesopore size
increases from 4.6 to 5.5 nm,
while it increases more
12
significantly as mesopore size
MMS-5-80
10
increases from 5.5 to 7.8 nm.
MMS-1-80
This observation indicates that
8
primary mesopores do not
6
MMS-1-60
influence the diffusion process
MMS-1-RT
in MMS-1-60 and MMS-1-RT
4
samples. However, it further
2
points out that those mesopores
may play a significant role in
0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
facilitating transport through
Mesopore diameter (nm)
the biporous structure of SBA15
materials
with
low
Figure 3. Dependence of the effective
micropore content such as
diffusivity (Deff) on the mesopore diameter at
MMS-1-80 and MMS-5-80.
30oC for: () n-heptane; () cumene; and
Knudsen diffusion values of the
() mesitylene
probe molecules investigated in
this study in pores of 4-5 nm in diameter are 6-7 orders of magnitude larger than
effective diffusivities determined experimentally here. It is therefore anticipated
that sorbate molecules effectively slide along the surface rather than execute
long trajectories with significant radial displacement in the primary mesopores.
The activation energy of desorption of mesitylene and cumene in SBA-15
samples having different micropore volume is in the following order: MMS-1RT > MMS-1-60 > MMS-1-80 > MMS-5-80. The trend is the same as that of nheptane and has been explained in detail in the previous paper [1]. Satterfield et
al. [20] reported activation energy of 53.1-66.5 kJ/mol for cumene diffusion in
H-mordenite and Choudhary et al. [18] obtained values of 27.5-50.2 kJ/mol for
the same species diffusion in ZSM-5. Our activation energy values for diffusion
of cumene in SBA-15 samples are much lower in comparison with the above
literature values. This conforms to a general pattern of less activated process
involving diffusion in mesoporous adsorbents.
The existence of micropores and small mesopores has been verified in our
previous communication [1]. Imperor-Clerc et al. [8] have reported the
existence of micropores corona surrounding the mesopores of SBA-15 materials
(pore diameter < 2 nm). Ryoo et al. [4] have proposed a micropore-mesopore
network in SBA-15 silica by TEM imaging of the inverse platinum replica.
Recently, Galarneau et al. [9] have clearly suggested the presence of two kinds
Intrawall pores
Mesopore channel
Micropores
Porous wall
between two
straight
mesopore
channels
Small mesopores
Mesopore channel
Figure 4. Schematic structural model of SBA-15 material
of complementary porosity, an ultramicroporosity (pore size < 1 nm), and a
secondary porosity with a very broad pore size distribution between 0.15 nm
and 3-5 nm (depending on synthesis temperature). They also suggested that
structural mesopores are connected through micropores. Based on our study and
literature results, a schematic diagram of SBA-15 materials is shown in Figure
4. These materials have an array of hexagonally ordered primary mesopore
channels and a certain amount of intrawall pores within the mesoporous walls.
The intrawall pores include micropores ranging from ultra- to super-micropores
and small mesopores, which may open at both ends, i.e., interconnecting with
main mesopore channels, or closed at one end. The structure of SBA-15
materials changes with different synthesis conditions. When the synthesis
temperature and length of heating time are increased, the mesopore size
increases, the wall thickness decreases, and the collapse of the
ultramicroporosity follows the formation of secondary porosity, spanning from
supermicropores to small mesopores which represent bridges between adjacent
mesopores.
Generally, if entering a pore is an activated process for a sorbate, the energy
barrier is determined by the size of the sorbate molecule and the pore structure.
The order of kinetic diameters of sorbates is n-heptane < cumene < mesitylene.
It is thus expected that the activation energy of n-heptane is the smallest and
that of mesitylene the largest. However, it is interesting to observe that the
activation energy in MMS-1-RT, MMS-1-60 and MMS-1-80 is in the opposite
order, e.g., n-heptane > cumene > mesitylene, while that in MMS-5-80 is as
expected n-heptane < cumene < mesitylene. These results logically suggest that
the overall diffusion process in the micro-mesoporous SBA-15 materials is
controlled by the micropores/small mesopores (intrawall pores) in the walls of
the main channels.
By using Horvath-Kawazoe model to analyze nitrogen desorption/adsorption
isotherm data of MMS-1-RT, MMS-1-60 and MMS-1-80 samples, we estimated
the pore size of microporous channels. When the synthesis temperature
increases from room temperature to 80oC, the micropore sizes span from about
0.4 to 0.7 nm, close to the kinetic diameter of n-heptane and cumene,
respectively. This provides evidence that n-heptane, and to a lesser extent
cumene, can penetrate the microporous channels within the mesoporous wall of
these materials. On the other hand, mesitylene having a kinetic diameter of 0.87
nm is very likely excluded from these micropores. Therefore, the adsorbed
amount of sorbates in micropores is in the decreasing sequence of n-heptane,
cumene and mesitylene. This in turn allows to conclude that the diffusion of
smaller molecules, such as n-heptane, is more strongly, if not entirely,
controlled by the diffusion resistance in micropores, while the transport of the
larger molecules, i.e., cumene and mesitylene, is more controlled by mesopore
resistances in intrawall pores, which can explain the trend of activation energy
in the three SBA-15 samples.
In the case of MMS-5-80 sample, which was synthesized at 80oC for 5 days,
all of its ultra-micropores collapsed and only supermicropores and small
mesopores having pore size > 1 nm remained in the mesoporous walls. In this
case, all molecules can be transported through the intrawall pores and the
diffusion involving this material is predominantly controlled by resistance in the
secondary and possibly in primary mesopores. This has been confirmed by the
lowest values of activation energies for this sample. Moreover the activation
energy in MMS-5-80 was in the expected increasing order for n-heptane,
cumene and mesitylene, although their differences were not significant.
4. Conclusion
In this work, SBA-15 mesoporous materials were further characterized with
probe molecules, i.e., cumene and mesitylene using the ZLC technique.
Diffusivity values for cumene and mesitylene in SBA-15 samples are much
higher than those in conventional microporous zeolites, such as ZSM-5. It was
established that diffusion of the probe molecules takes place entirely through
the intrawall porous structure and is controlled by the combination of micropore
and small mesopore resistances. The primary mesopores may also play a role in
facilitating diffusion through the biporous structure of SBA-15 materials with
low micropore content. With a decrease of the relative micropore volume, the
diffusivity increases and activation energy decreases, and diffusion in small
mesopores play a more and more important role. For MMS-1-RT, MMS-1-60
and MMS-1-80 samples, with the increase of molecular sizes, less and less
probe molecules can enter the micropores yielding a more mesopore diffusion
controlled process and a lower activation energy. For MMS-5-80, the diffusion
is similar to the diffusion through a pure mesoporous material, and the trend of
activation energy is opposite to that of diffusivity and the same as that of
molecular size. These observations seem to imply that there is a large fraction
of micropores with size less than 0.7 nm in MMS-1-RT, MMS-1-60 and MMS1-80 samples, and most of micropores in MMS-5-80 sample are larger than 1
nm.
By using probe molecules of different critical size, the ZLC method proves to
be a simple and reliable technique to study the diffusion mechanism and relate it
to structure of the materials.
Acknowledgments
This project was supported by the Natural Science and Engineering Research
Council of Canada (NSERC) through the Industrial Chair in Nanoporous
Materials at Laval University.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
H. Vinh Thang, Q. Huang, M. Eić, D. Trong On, S. Kaliaguine,
(2004).Langmuir, in press.
K. Miyazawa, S. Inagaki, (2000) Chem. Commun. 2121.
M. Kruk, M. Jaroniec, C.H. Ko, R. Ryoo, (2000) Chem. Mater. 12, 1961.
R. Ryoo, C.H. Ko, M. Kruk, V. Antochshuk, M. Jaroniec, (2000) J. Phys.
Chem. B 104, 11465.
Y. Sun, Y. Han, L. Yuan, S. Ma, D. Jiang, F.S. Xiao, (2003) J. Phys.
Chem. B 107, 1853.
M. Choi, W. Heo, F. Kleitz, R. Ryoo, . (2003) Chem. Commun1340.
C.M. Yang, B. Zibrowius, W. Schmidt, F. Schuth, (2003) Chem. Mater.
15, 3739.
M. Imperor-Clerc, P. Davidson, A. Davidson, (2000) J. Am. Chem. Soc.
122, 11925.
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
A. Galarneau, H. Cambon, Di F. Renzo, R. Ryoo, M. Choi, F. Fajula,
(2003) New J. Chem. 27, 73.
D. Trong On, S. Kaliaguine, (2003).US Patent 6,669,924 B1
D. Trong On, D. Desplantier-Giscard, D. Danumah, S. Kaliaguine, (2001)
Appl. Catal. A: General 222, 299.
Y. Liu, W. Zhang, T.J. Pinnavaia, (2000) J. Am. Chem. Soc. 122, 8791.
H. Vinh Thang, A. Malekian, M. Eić, D. Trong On, S. Kaliaguine, (2002)
Proc. 3th Internat. Mesostructured Mater. Symp. and (2003) Stud. Surf.
Sci. Catal. 146, 145.
V.R. Choudhary, V.S. Nayak, T.V. Choudhary, (1997) Ing. Eng. Chem.
Res. 36, 1812.
M. Jiang, M. Eić, (2003) Adsorption 9, 225.
S. Brandani, M. Jama, D.M. Ruthven, (2000) Chem. Eng. Sci. 55, 1205.
V.R. Choudhary, A.P. Singh, (1986) Zeolites 6, 206.
V.R. Choudhary, A.S. Mamman, V.S. Nayak, (1989) Ind. Eng. Chem. Res.
28, 1241.
J. Kärger, D.M. Ruthven, (1992).Diffusion in Zeolites and Other
Microporous Materials, John Wiley and Sons, Inc.: New York
C.N. Satterfield, J.R. Katzer, W.R. Vieth, (1971) Ind. Eng. Chem.
Fundam. 10, 478.