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Sodalites as ultramicroporous frameworks for hydrogenseparation at elevated temperatures: thermal stability,template removal, and hydrogen accessibility
Zhenkun Zheng Æ Vadim V. Guliants ÆScott Misture
Published online: 27 April 2008
� Springer Science+Business Media, LLC 2008
Abstract Sodalite (SOD) is a highly promising porous
structure for hydrogen separation from larger gas molecules
due to the presence of small (*2.8 A) six-membered ring
openings of the sodalite cages. Thermal stability, template
removal, and the release of encapsulated hydrogen were
studied for low-silica (Si/Al = 1), high-silica (Si/Al = 5)
and pure-silica (Si/Al = ?) sodalites. The release of
encapsulated hydrogen from sodalite cages was observed at
380, 550, and 480 �C for low-silica, high-silica and pure-
silica sodalites, respectively, suggesting the operating tem-
peratures for hydrogen separation employing these sodalite
structures.
Keywords Sodalite � Thermal stability �Temperature-programmed desorption � Hydrogen
1 Introduction
Ultramicroporous sodalite (SOD) is a highly promising ul-
tramicroporous structure for the membrane separation of
small gas molecules, such as hydrogen, due to the presence of
6-membered rings (2.8 A opening) accessible only to H2, He
and H2O [1]. The sodalite structure may be prepared as
the low-silica (Si/Al = 1.0), high-silica (Si/Al = 5) and
pure-silica (Si/Al = ?) frameworks [1]. To date, only few
studies were reported on low-silica sodalite membranes,
whereas the high-silica and pure-silica sodalite membranes
have not been investigated. For instance, Julbe et al. [2]
reported a relatively low He/N2 permselectivity of 6.2 at
115 �C for low-silica sodalite membranes obtained by
microwave-assisted hydrothermal synthesis, which likely
contained intercrystalline micropore defects. Using the same
synthesis method, Xu et al. [3] reported low-silica sodalite
membranes, which displayed relatively high H2 permeance
of 1.14 9 10-7 mol m-2 s-1 Pa-1 and H2/n-butane perm-
selectivity of[1,000 at room temperature based on single-
gas permeances. Lee et al. [4] fabricated low-silica sodalite
membranes by vacuum seeding and secondary growth
methods and reported the He/N2 permselectivity of 2.25 at
90 �C at transmembrane pressure of *2 bars.
Recent molecular modeling studies of hydrogen diffu-
sion through pure- and low-silica sodalite structures
confirmed their promise for hydrogen separation applica-
tions [5–7]. Van den Berg et al. [6, 7] reported the
hydrogen diffusion coefficient through pure-silica sodalite
in the range of 1.6 9 10-10-1.8 9 10-9 m2/s at 700–
1200 K. As compared to pure-silica sodalite, low-silica
sodalite was predicted to have a higher hydrogen diffusion
coefficient at low temperatures due to a larger pore window
opening while they became comparable at high tempera-
tures [5]. However, despite the promise of low-, high- and
pure-silica sodalites for H2 separation, no information is
currently available about (1) the release of organic template
decomposition products from the high-silica and pure-
silica sodalite structures; (2) the thermal stability of the
sodalite structures; and (3) the operating temperatures for
hydrogen permeation through the sodalite structures, which
is critical for the development of highly selective and
permeable sodalite membranes for hydrogen separation.
Z. Zheng � V. V. Guliants (&)
Department of Chemical and Materials Engineering,
University of Cincinnati, Cincinnati, OH 45221-0012, USA
e-mail: [email protected]
Z. Zheng
e-mail: [email protected]
S. Misture
New York State College of Ceramics at Alfred University,
2 Pine St., Alfred, NY 14802, USA
123
J Porous Mater (2009) 16:343–347
DOI 10.1007/s10934-008-9206-y
In the present study, we investigated the template
removal, thermal stability, and hydrogen accessibility of
low-silica, high-silica and pure-silica sodalites in order to
evaluate the feasibility of sodalites for H2 separation.
2 Experimental
Low-silica sodalite was synthesized in the absence of
organic structure-directing agents (SDAs), whereas high-
silica and pure-silica sodalites were obtained in the pres-
ence of tetramethylammonium hydroxide (TMAOH) and
ethylene glycol (EG), respectively, acting as SDAs. The
details of preparation methods were described elsewhere
[8–10]. Their phase composition and morphology were
investigated by X-ray diffraction (XRD) (Siemens, CuKa)
and Scanning Electron Microscopy (SEM) (Cambridge S-
90). The thermal stability and thermal expansion of the
sodalites in air were studied using high temperature X-ray
diffraction (HTXRD) [11]. The template removal was
studied by thermo-gravimetric analysis (TGA) and differ-
ential scanning calorimetry (DSC). Temperature-
programmed desorption (TPD) of hydrogen encapsulated
in sodalites was performed in a tubular furnace equipped
with an on-line mass spectrometer (SRI QMS200amu
system).
3 Results and discussion
Figure 1 shows the XRD patterns of low-silica (Si/
Al = 1.0), high-silica (Si/Al = 5) and pure-silica (Si/
Al = ?) sodalites. No crystalline or amorphous impurities
were observed. The crystalline domain size and unit cell
parameters of sodalites were calculated using Scherrer’s
formula and are shown in Table 1. The domain sizes of
sodalites are in the range of 20–50 nm. The unit cell
parameters of all sodalites are very close to one another and
consistent with the reported value [12]. The small variation
of unit cell parameters of sodalites is probably due to the
difference of the framework Si/Al ratios. Figure 2 shows
SEM micrographs of (a) low-silica, (b) high-silica, and (c)
pure-silica sodalite powders. The crystal size of the low-
silica sodalite is less than 100 nm and the size of high-
silica sodalite crystals is very uniform and around 30 nm.
Pure-silica sodalite crystals were much larger (*30 lm),
cubic in shape exhibiting dodecahedral truncation.
The behavior of the three sodalites is strikingly different
during heating as shown by comparison of Figs. 3–5. For the
low-silica sodalite (Fig. 3) the as-synthesized sodalite is
stable to *250 �C, at which point additional diffraction
peaks are noted. The intermediate phase assemblage
remains until *500 �C when the XRD pattern of pure
sodalite is recovered. Additional heating results in decom-
position at *900 �C to form nepheline, NaAlSiO4 [13]. In
contrast, high-silica sodalite (Fig. 4) begins to transform to
an amorphous phase at *600 �C and then becomes fully
amorphous by 900 �C. As shown in Fig. 5, the pure-silica
sodalite phase is stable up to *1000 �C. A significant
change in the crystallite size or microstrain is noted by the
change in peak width beginning at *300 �C. The thermal
expansion coefficients of pure-silica sodalite calculated
from the XRD patterns showed that sodalite framework
slightly expanded up to 400 �C and slightly contracted
above this temperature during calcination (Fig. 6). This
change in the sodalite unit cell volume with temperature
may be explained by the partial release of gaseous decom-
position products of the organic template occluded inside
the sodalite cages during synthesis. However, sodalite
structure always shows negative thermal expansion coeffi-
cient after the removal of organic template indicating the
importance of selecting appropriate support materials to
avoid the mismatch of thermal expansion coefficients
between the sodalite layer and membrane support.
Figure 7 shows the removal of water from low-silica
sodalite phase and organic template occluded in the soda-
lite cages of high-silica and pure-silica sodalite phases. The
weight loss observed by TGA for the low-silica sodalite
0 5 10 15 20 25 30 35 40 45 50
2 Theta / Degree
Inte
nsity
/ a.
u.
a
b
c(110)
(200)
(211)
(220)
(310)(222) (330)(400)
(332)
Al holder
Fig. 1 XRD patterns of (a) low-silica (Si/Al = 1), (b) high-silica (Si/
Al = 5), and (c) pure-silica (Si/Al = ?) sodalites
Table 1 Crystalline domain size and unit cell parameters of sodalites
Domain
size (nm)
Unit cell
parameter (A)
(This work)
Unit cell
parameter (A)
(Prior work)
Low-silica SOD 34 8.8890 8.8700 [12]
High-silica SOD 21 8.9237 N/A
Pure-silica SOD 48 8.8414 N/A
344 J Porous Mater (2009) 16:343–347
123
Fig. 2 SEM images of (a) low-
silica (Si/Al = 1), (b) high-
silica (Si/Al = 5), and (c) pure-
silica (Si/Al = ?) sodalites
Fig. 3 3-D perspective view of the in-situ diffraction patterns (Co
radiation) of low-silica sodalite from room temperature to 1,000 �C
measured isothermally in steps of 100 �C)
Fig. 4 In-situ diffraction patterns (Co radiation) of high-silica
sodalite from room temperature to 1,000 �C measured isothermally
in steps of 100 �C Fig. 6 Thermal expansion coefficient of pure-silica sodalite
Fig. 5 In-situ diffraction patterns (Co radiation) of pure-silica
sodalite at from room temperature to 1,000 �C measured isothermally
in steps of 100 �C
J Porous Mater (2009) 16:343–347 345
123
was *10% from 50 to 1200 �C due to the removal of
water since no organic template was used in hydrothermal
synthesis. The total weight loss was *40% in the 50–
1200 �C range for high-silica sodalite and *9% in the
480–760 �C range corresponding to the removal of *53%
of occluded TMA+ assuming that each sodalite cage con-
tained one TMA+ and a Si/Al ratio of 5 [14].
Kresnawahjuesa et al. reported the weight loss of *8% in
the 480-760 �C range for high-silica sodalite [14]. The
total weight loss for pure-silica sodalite was *13% at 50–
1200 �C corresponding to the removal of *86% of
occluded EG assuming that each pure-silica sodalite cage
contained one EG molecule (Si12O24 � 2C2H4(OH)2) [8].
The hydrogen accessibility in these sodalite phases was
investigated in temperature-programmed desorption (TPD)
of hydrogen encapsulated in sodalite cages. 500 mg of
as-synthesized sodalite was first heated under argon flow
(30 mL/min) up to 300, 650 and 800 �C at a ramp rate of
5 �C/min for low-silica, high-silica and pure-silica soda-
lites, respectively, in order to dehydrate these phases and
remove the organic template. After 1 h at these high tem-
perature conditions, the gas flow was switched to hydrogen
(30 mL/min) and the sodalite phases were cooled down to
room temperature at a ramp rate of 0.5 �C/min to encap-
sulate hydrogen in sodalite cages. The reactor was purged
with argon flow of 30 mL/min for 30 min and the gas flow
was switched to argon (2.5 mL/min) for another 30 min.
Encapsulated hydrogen was released by heating these
phases at 5 �C/min under the argon flow of 2.5 mL/min
using an on-line mass spectrometry. As shown in Fig. 8,
the encapsulated hydrogen was released from the sodalite
cages at 380, 550 and 480 �C for low-silica, high-silica and
pure-silica sodalites, respectively, suggesting the operating
temperatures for H2 separation employing low-silica, high-
silica and pure-silica sodalite membranes. The observed
slope for pure-silica sodalite over 650 �C (Fig. 8) is
probably due to the contraction and distortion of pure-silica
sodalite structure. These three sodalite structures exhibited
different hydrogen accessibility due to differences in their
framework Si/Al ratios and thermal stability. Among three
sodalite structures, high-silica sodalite was the least
ordered. According to high temperature XRD data (Fig. 4),
this sodalite structure exhibited an almost complete loss of
crystallinity between 500 and 700 �C, which may also
explain its low hydrogen accessibility. On the other hand,
thermally stable low-silica and pure-silica sodalites dis-
played hydrogen accessibility behavior consistent with
their cubic cell parameter (Table 1).Fig. 7 TGA curves of (a) low-silica, (b) high-silica and (c) pure-
silica sodalites (flowing gas: 50 mL/min argon, heating rate: 5 �C/
min)
0 100 200 300 400 500 600 700 800
Temperature / oC
a
b
c
Mas
s Sp
ectr
omet
ry I
nten
sity
/ a.u
.
Fig. 8 Temperature-programmed desorption (TPD) of encapsulated
hydrogen for (a) low-silica; (b) high-silica; (c) pure-silica sodalites
346 J Porous Mater (2009) 16:343–347
123
4 Conclusions
We reported for the first time thermal stability, template
removal, and hydrogen accessibility of sodalite phases
possessing different framework Si/Al ratios. The observed
negative thermal expansion coefficients for sodalite struc-
tures suggest that membrane support should also possess
negative thermal expansion coefficient to avoid the thermal
stress between sodalite layer and membrane support. The
obtained results indicated that it is possible to apply low-
silica, high-silica and pure-silica sodalite membranes for
hydrogen separation at 380–500 �C, 550–600 �C, and 480–
1000 �C, respectively, based on the molecular size effect.
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