5
Sodalites as ultramicroporous frameworks for hydrogen separation at elevated temperatures: thermal stability, template removal, and hydrogen accessibility Zhenkun Zheng Vadim V. Guliants Scott Misture Published online: 27 April 2008 Ó 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 Á Thermal stability Á Temperature-programmed desorption Á 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 H 2 , He and H 2 O[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/N 2 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 H 2 permeance of 1.14 9 10 -7 mol m -2 s -1 Pa -1 and H 2 /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/N 2 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 [57]. 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 m 2 /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 H 2 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 Á 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

Sodalites as ultramicroporous frameworks for hydrogen separation at elevated temperatures: thermal stability, template removal, and hydrogen accessibility

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Page 1: Sodalites as ultramicroporous frameworks for hydrogen separation at elevated temperatures: thermal stability, template removal, and hydrogen accessibility

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

Page 2: Sodalites as ultramicroporous frameworks for hydrogen separation at elevated temperatures: thermal stability, template removal, and hydrogen accessibility

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

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Page 3: Sodalites as ultramicroporous frameworks for hydrogen separation at elevated temperatures: thermal stability, template removal, and hydrogen accessibility

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

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Page 4: Sodalites as ultramicroporous frameworks for hydrogen separation at elevated temperatures: thermal stability, template removal, and hydrogen accessibility

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

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Page 5: Sodalites as ultramicroporous frameworks for hydrogen separation at elevated temperatures: thermal stability, template removal, and hydrogen accessibility

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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