Weakly branched silica films formed by the two-step sol-gel process allow for the formation of high selectivity membranes for gas separation. 29Si NMR and gas permeation showed that reduced crosslinking leads to He/CH4 selectivity improvement from 300 to 1000. Applied in membrane reactor for cyclohexane conversion to benzene, conversions were achieved at 14 fold higher than a conventional reactor at 250°C. Hydrothermal stability studies showed that carbon templating of silica is required for hydrothermally stable membranes. From our work it was shown that with correct application of chemistry, practical membrane systems can be built to suit gas separation (e. g. hydrogen fuel) and reactor systems

Diniz da Costa, J. C.

Duke, M. C.

Duke, Mikel

da Costa, João C. Diniz

Name not available

Molecular Sieve Silica (MSS) Membranes for Gas Separation and Reaction Processes

University of Queensland eSpace

1 
MOLECULAR SIEVE SILICA (MSS) MEMBRANES FOR GAS 
SEPARATION AND REACTION PROCESSES  
M. C. DUKE, J. C. DINIZ DA COSTA 
ARC Centre for Functional Nanomaterials, School of Engineering 
 The University of Queensland, Brisbane Qld 4072, Australia,  
Weakly branched silica films formed by the two-step sol-gel process allow for the formation of high 
selectivity membranes for gas separation. 29Si NMR and gas permeation showed that reduced 
crosslinking leads to He/CH4 selectivity improvement from 300 to 1000. Applied in membrane 
reactor for cyclohexane conversion to benzene, conversions were achieved at 14 fold higher than a 
conventional reactor at 250°C. Hydrothermal stability studies showed that carbon templating of 
silica is required for hydrothermally stable membranes. From our work it was shown that with 
correct application of chemistry, practical membrane systems can be built to suit gas separation (eg 
hydrogen fuel) and reactor systems. 
1 Introduction 
The development of silica derived membranes in the last decade has led to major 
applications in gas separation at high temperature (up to 500°C). Potential applications 
are also envisaged in membrane reactors to shift the reaction equilibrium to high 
conversions. MSS membranes have been synthesised by sol-gel processes including a 
single-step [1-3] and two-step [4, 5] catalysed hydrolysis. The top layer of a MSS 
membrane provides the molecular sieve properties required to allow a molecule with a 
small kinetic diameter (dk) to diffuse whilst hindering the passage of the larger molecule. 
Controlling the MSS film pore size is fundamental to obtain high separation factors (i.e. 
permselectivities). Weakly branched fractal sol gel processes has shown promising results 
in MSS membrane technology [5, 6]. These tend to have a high contribution of silanol 
groups (Q2 and Q3 groups) which allow for a fine pore size control. However, industrial 
processes for gas separation generally contain a small concentration of water. MSS 
membranes are hydrophilic and unstable to water reaction with the silanol groups. To 
address this issue several groups have looked at surface functionalisation [7-10]. This 
paper addresses MSS film pore size formation and its functionalisation for gas separation 
and membrane reactor applications. 
2 Experimental 
MSS layers were synthesised employing a two-step catalysed hydrolysis process using 
tetraethylorthosilicate (TEOS), absolute ethanol (EtOH), 1M nitric acid (HNO3) and 
distilled water (H2O) as described elsewhere [5, 9, 10].  The mixtures ware refluxed under 
stirring at 60°C for 180 minutes in a water bath.  The final MSS membranes consisted of 
4 × silica layers, 2 × γ-alumina layers supported on an α-alumina substrate. The 
membranes were tested using a dynamic permeation apparatus to check the membrane 
quality for H2 permeation for temperatures between 150 to 250°C.  The packed bed 
membrane reactor was assembled with the membrane selective layer and Pt/Al2O3 catalyst 
2 
facing the feed stream. CP/MAS 29Si NMR was performed using a Bruker MSL 300 
spectrometer. Spectra were de-convoluted using a Gauss-Lorentz fit with PeakFitTM 
(Jandel Scientific) software. 
3 Results and Discussion 
The two-step samples consistently showed high contribution of silanol groups (Q2 and Q3) 
as depicted in Figure 1a. According to the fractal theory, high concentration of silanols in 
the sol-gel process leads to the formation of weakly branched dense films with smaller 
pore size [11]. The gas permselectivities of single-step and two-step membranes are very 
distinct as depicted in Figure 1b.  The translation of the fractal concept into thin film 
technology resulted in the production of high quality membranes.  Using gases of 
different kinetic diameters as molecular probes, permselectivity ratios close to 1000 for 
the separation of He and CH4 were achieved.  In agreement with the 29Si NMR findings, 
the permselectivity results of the two-step membranes showed superior pore size 
tailorability in the region of 3Å as compared to single-step larger pore size of 3.5 Å. 
 
0
10
20
30
40
50
60
70
Q4 Q3 Q2
Functional Groups
Co
n
ce
n
tr
at
io
n
 
(%
)
two-step
single-step
 
Figure 1a. Concentration of MSS functional groups from 
CP/MAS 29Si NMR results. 
 
Kinetic Diameter (dk)
Pe
rm
s
e
le
c
tiv
ity
2.55 2.7 2.85 3 3.15 3.3 3.45 3.6 3.75 3.9
1
2
3
5
7
10
20
30
50
70
100
200
300
500
700
1000
He/CH4 H2/CH4 CO2/CH4 CH4/CH4
Single-Step
Tw o-Step
 
Figure 1b. Permselectivity versus molecular 
kinetic diameter. 
 
Figure 2a compares the cyclohexane conversion to benzene in the packed bed 
membrane reactor (MRx) and conventional packed bed reactor (Rx) using membranes 
with H2 permeance of 1×10-7 mol.m-2.s-1.Pa-1. The conversion over Pt/Al2O3 catalyst on 
the Rx was small and close to equilibrium value. On the other hand, the MRx yielded an 
14 fold conversion higher than the Rx at 250°C. These results illustrate the capabilities of 
the MRx to shift the reaction yield successfully beyond the equilibrium value, by simply 
removing the H2 product from the reactor side.  
Figure 2b shows how silica membranes are affected by process steam and the 
improvement offered by carbon functionalisation. Conventional MSS membranes 
selectivity is reduced by half after exposure to steam. This is largely attributed to the 
silanol groups which react with water causing pore opening [9, 10]. By the same token, 
functionalised carbon templated MSS (CTMSS) membrane showed improved selectivity 
after exposure to steam, clearly indicating that functional groups can maintain and 
improve selectivities in membrane technology. Functionalised MSS membranes can 
3 
therefore find applications for reformate clean up in fuel cell technology and membrane 
reactors such as the dehydrogenation reactions and the water gas shift reaction. 
Temperature (oC)
Cy
c
lo
he
x
a
n
e
 
Co
n
v
e
rs
io
n
 
(%
)
200 205 210 215 220 225 230 235 240 245 250
0.5
0.7
1
2
3
4
5
7
10
20
Membrane Reactor (MRx)
Equilibrium
Conventional Reactor (Rx)
 
Figure 2a – Conversion of cyclohexane to benzene. 
 
0
5
10
15
20
25
30
35
Initial (dry) During steam After steam (dry)
MSS
CTMSS
 
Figure 2b – H2/CO selectivities for membranes with 34 
mol% steam exposure at 200°C 
4 Conclusions 
Weakly branched silica formed by the two-step sol-gel process was highly selective to 
gases, which was attributed to a high silanol concentration. Combining the membrane 
selectivity with reaction in a packed bed membrane reactor, the conversion could be 
increased over standard packed bed reactor equilibrium. In systems that contain steam, 
carbon functionalisation of silica surfaces was required for stable operation, as water 
reacted with silanol groups resulting in pore widening.  
Acknowledgments 
The authors acknowledge the support from the Australian Research Council (ARC). 
References 
1. R.S.A. de Lange, J.H.A. Hekkink, K. Keizer and A.J. Burggraaf, Microp. Mat., 4, 169-186 (1995). 
2. B.N. Nair, T. Yamaguchi, T. Okubo, H. Suematsu, K. Keizer and S.-I. Nakao, J. Membr. Sci., 135, 237-243 
(1997). 
3. R.M. de Vos and H. Verweij, J. Membr. Sci., 143, 37-51 (1998). 
4. C.Y. Tsai, S.-Y. Tam, Y. Lu and C.J. Brinker, J. Membr. Sci., 169, 255-268 (2000). 
5. J.C. Diniz da Costa, G.Q. Lu, V. Rudolph and Y.S. Lin, J. Membr. Sci., 198, 9-21 (2002).  
6. R. Schafer, M. Noack, P. Kolsch, S. Thomas, A. Seidel-Morgenstern and J. Caro, Sep. Purif. Technol., 25, 
3-9 (2001). 
7. R.M. de Vos, W.F. Maier and H. Verweij, J. Membr. Sci., 158, 277-288 (1999). 
8. J. Campaniello, C.W.R. Engelen, W.G. Haije, P.P.A.C. Pex and J.F. Vente, Chem. Comm., 834-835 
(2004). 
9. M.C. Duke, J.C. Diniz da Costa, G.Q. (Max) Lu, M. Petch and P. Gray, J. Membr. Sci., 241, 325-333 
(2004). 
10. S. Giessler, L. Jordan, J.C. Diniz da Costa and G.Q.M. Lu, Sep. Purif. Technol., 32, 255-264 (2003). 
11. C.J. Brinker and G.W. Scherer, Sol-Gel Science, Boston: Academic Press, 1990. 


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Molecular Sieve Silica (MSS) Membranes for Gas Separation and Reaction Processes

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