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Synthesis and modification of zeolite NaA adsorbents for separation of hydrogen and methane.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2009; 4: 666–671
Published online 7 July 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.315
Special Theme Research Article
Synthesis and modification of zeolite NaA adsorbents for
separation of hydrogen and methane
Yanna Liu, Jingyang Xu, Lijun Jin, Yunming Fang and Haoquan Hu*
State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, 129
Street, Dalian 116012, P. R. China
Received 31 October 2008; Revised 2 March 2009; Accepted 3 March 2009
ABSTRACT: To improve the adsorption capacity of zeolite A, two kinds of zeolite NaA with submicron and hierarchical
structure were prepared. The XRD patterns indicated that both synthesized products were pure zeolite with LTA-type
framework. The SEM images and laser particle size analysis showed that the particle size of submicron zeolite NaA was
about 240 nm. The TEM and N2 adsorption/desorption isotherms proved the existence of mesopores in the hierarchical
zeolite NaA. The adsorption capacities of single component CH4 or H2 on zeolite SrA adsorbents, obtained by Sr2+
ion-exchange of submicron and hierarchical zeolite NaA, and commercial zeolite 5A adsorbent were measured by the
static volume method at 25 ◦ C and pressures up to 1 MPa. The results show that both prepared zeolite SrA adsorbents
have higher adsorption capacities of CH4 and ideal separation factors of CH4 /H2 than commercial zeolite 5A, and the
submicron zeolite SrA has the largest adsorption capacity of CH4 and ideal separation factor of CH4 /H2 .  2009 Curtin
University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: zeolite SrA; adsorption; modification; hydrogen; methane
INTRODUCTION
Coke oven gas, containing about 54–59% H2 and
24–28% CH4 , is a potential source of H2 . With the
increasing demand of high pure H2 and CH4 for
petrochemical processes and environmental protection
against greenhouse effect of CH4 , more and more
pressure swing adsorption (PSA) processes to recover
H2 and CH4 from coke oven gas have been studied.[1 – 4]
Zeolite 5A, as the most widely used adsorbent in
the PSA process purifying H2 from coke oven gas,
is not faultless in the micropore diffusion in zeolite
crystals. High diffusion limitation prevents CH4 from
diffusing into the intracrystalline channels and results
in decrease of adsorption capacity of CH4 . To deal with
the diffusional limitation imposed by zeolitic structure,
two potential solutions have been explored: one is to
decrease the crystal size and the other is to introduce
mesopores into the zeolite.
The small crystal zeolite exhibits better performances
than the large one in reducing the intracrystalline diffusion path and increasing the external surface area.
Some strategies have been exploited to decrease the
*Correspondence to: Haoquan Hu, State Key Laboratory of Fine
Chemicals, Institute of Coal Chemical Engineering, School of
Chemical Engineering, Dalian University of Technology, 129 Street,
Dalian 116012, P. R. China. E-mail: hhu@chem.dlut.edu.cn
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
crystal size by adjusting synthesis gel composition,
changing crystallization temperature or time,[5 – 10] confined space synthesis[11 – 14] and addition of surfactants
or solvents.[15 – 17] However, zeolites with too small particle size result in decrease in the micropore volume due
to less perfect crystallization.[18,19] Therefore, the particle size of a zeolite adsorbent for gas separation has to
be controlled.
Compared with micropores of the zeolite (<2 nm),
mesopores (2–50 nm) permit faster migration of guest
molecules in the host frameworks, but mesoporous
molecular sieves do not possess good hydrothermal stability because of noncrystallization of the pore wall.
To improve the stability and solve the diffusion limitation problem with crystalline microporous, infusion of mesopores into zeolite has been explored.[20]
Jacobsen et al .[21] reported that a single crystal of
ZSM-5 zeolite with a mesopore system was synthesized by the template of BP-2000. Mesoporous
type A, Y and ZSM-5 zeolites have been synthesized by Tao et al . using carbon aerogel with threedimensional mesopores as template.[22 – 24] To simplify
synthesis route, Ryoo et al .[25 – 27] reported a direct synthesis method to prepare zeolites with tunable mesoporous structure using the amphiphilic organosilanes
as a mesopore-directing agent. They, thus, synthesized
hierarchical pore zeolites such as MFI, AlPO-5 and
AlPO-11 zeolites. In our previous works, we have
Asia-Pacific Journal of Chemical Engineering
SYNTHESIS AND MODIFICATION OF ZEOLITE NaA ADSORBENTS
reported to synthesize an ordered mesoporous aluminosilicate with completely crystalline zeolite wall structure through recrystallization of SBA-15 using in situ
formed CMK-5 as the hard template,[28] zeolite with
tunable intracrystal mesoporosity with carbon aerogel as
a secondary template,[29] and mesoporous aggregate of
zeolite nanocrystals by self-assembly of in situ formed
zeolite nanocrystals through careful control of the crystallization without a secondary template.[30]
To improve the adsorption capacity of zeolite A for
separation of CH4 and H2 , in this paper, submicron
zeolite SrA and hierarchical zeolite SrA sorbents were
synthesized by addition of a water-soluble polymer,
poly(ethylene glycol) with average molecular weight
of 1000 (PEG-1000) and an organosilane surfactant [3(trimethoxysilyl) propyl] octadecyldimethylammonium
chloride (TPOAC) to the conventional alkaline synthesis system, and subsequent ion-exchange of Na+ with
Sr2+ . The adsorption capacities for CH4 and H2 and the
ideal separation factor of CH4 /H2 on SrA adsorbents
were measured by the static volume method, and then
compared with those on commercial zeolite 5A.
EXPERIMENTAL
Synthesis of submicron zeolite NaA
Submicron zeolite NaA was synthesized by utilizing synthesis mixtures with a molar composition:
4.5Na2 O/2SiO2 /Al2 O3 /170H2 O/31.5PEG-1000/0.75C6
H8 O7 (citric acid). Solution of PEG-1000 dissolved in
hot distilled water was dropped into the solution including NaOH and aluminium isopropoxide under the rapid
stirring. Then, silicate gel (25 wt% SiO2 ) and citric acid
were added to the obtained gel with stirring for 3 h at
0 ◦ C. Finally, the gels were transferred into teflon-lined
autoclave and crystallized at 90 ◦ C for 6 h. The products were recovered by filtration, washed with distilled
water till the pH was lower than 9, and were dried at
110 ◦ C overnight.
Synthesis of hierarchical zeolite NaA
The molar composition of the final gel was 1Al2 O3 /
3.3Na2 O/2SiO2 /128H2 O/0.08TPOAC. The organosilane surfactant was 42% TPOAC in methanol solution. TPOAC was added to a mixture of silica sol
(25 wt% SiO2 ), NaOH, sodium aluminate (Na2 O 59
wt%, Al2 O3 41 wt%) and distilled water with vigorous
stirring for 2 h. The final gel was removed to teflonlined autoclave and crystallized at 95 ◦ C for 5 h. After
crystallization, the products were collected by filtration,
washed with distilled water till the pH lower than 9,
and then were dried in air at 110 ◦ C overnight. Finally,
the products were calcined in air at 530 ◦ C for 5 h to
remove the organosilane surfactant.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Ion-exchange
Zeolite SrA was prepared by ion-exchange of Na+ with
Sr2+ . Typically, 2 g of zeolite NaA were dispersed in
50 ml strontium nitrate solution (0.2 M). The solution
pH was adjusted to 12 with strontium hydroxide. The
resultant slurry was stirred at 90 ◦ C for 24 h. The
products were washed with distilled water several times,
and then were dried in air at 110 ◦ C overnight before
calcined in air at 400 ◦ C for 4 h.
Characterization
Powder X-ray diffraction (XRD) patterns of commercial, submicron and hierarchical zeolite NaA adsorbents
were recorded on a Rigaku DMAX-2400 diffractor
with CuKα radiation. The morphology of the crystals was investigated using scanning electron microscope (SEM) on a KYKY2800B electron microscope
and their particle size distribution was measured by
Zetasizer 1000. The transmission electron microscopy
(TEM) micrographs were acquired with a Philips Tecnai
G2 20 microscope. The nitrogen adsorption/desorption
isotherms were obtained by using a Micromeritics
ASAP2020 adsorption unit at −196 ◦ C.
Adsorption measurement
The adsorption measurements of CH4 and H2 were carried out using a static volumetric apparatus shown in
Fig. 1. The apparatus basically consists of adsorption
cell, reference cell, pressure transducer, vacuum pump,
valves and connection tubes. The volume of the reference cell was 64.5 ml and the volume of the adsorption cell besides the adsorbents was measured by the
expansion of helium gas. Detailedly, pure gas was first
introduced into the reference cell, and the pressure was
measured after the cell was stabilized. Then the valve
between the reference and adsorption cells was opened
to allow the gas to contact the adsorbent. Finally, the
pressure in cells was measured after equilibrium was
achieved, and the number of mole adsorbed by the
adsorbent was calculated. During the measurement, the
reference and adsorption cells were immerged in a water
bath maintained at 25 ◦ C using thermostat. The adsorption equilibrium state was determined when the pressure
of the cells were constant. Commercial zeolite 5A from
Anshan Iron and Steel Group Corp., prepared submicron zeolite SrA and hierarchical zeolite SrA were used
as adsorbents. The commercial zeolite 5A was ground
and the prepared zeolite SrA samples, as the sample
adsorbents, were shaped and sieved to 20–40 mesh and
calcined at 400 ◦ C for 3 h, then cooled down to the
room temperature in a vacuum drying chamber before
the measurement. The chosen adsorption temperature
Asia-Pac. J. Chem. Eng. 2009; 4: 666–671
DOI: 10.1002/apj
667
Y. LIU ET AL.
V1
Asia-Pacific Journal of Chemical Engineering
samples are pure zeolites with Linde Type A (LTA)type framework. Compared with hierarchical zeolite,
submicron zeolite NaA has high intensity of the diffraction peak, indicating of high crystallization. Figure 2(b)
shows the XRD patterns of hierarchical zeolite NaA at
the small angle, where a peak at about 0.7◦ of diffraction angles 2θ , suggesting the existence of ordered
mesopores in the prepared hierarchical zeolite NaA in
addition to micropores. After Sr2+ ion-exchange of submicron and hierarchical zeolite NaA, the frameworks of
the zeolite keep unchanging except for the relative weak
intensity and broad diffraction peaks.
2
V3
M1
V2
V7
V8
V5 M2
V6
3
1
A.C
R.C
4
5
N2 adsorption/desorption
Figure 1. Static volumetric adsorption apparatus (V1–V6:
on–off valve; V7, V8: needle valve; M1: pressure gauge; M2:
pressure transducer; R.C: reference cell; A.C: adsorption
cell; 1: adsorption gas; 2: vacuum pump; 3: filter; 4, 5:
thermostat). This figure is available in colour online at
www.apjChemEng.com.
The nitrogen adsorption/desorption isotherms on hierarchical zeolite NaA at −196 ◦ C are presented in Fig. 3.
Obviously, the nitrogen adsorption isotherms belong
to the type-IV isotherm, which is the characteristic
was 25 ◦ C and the pressure in the range of 0–1.0 MPa.
The ideal separation factor was defined as follows:
Volume adsorbed (cm3/g, STP)
150
ni
αij =
nj
where ni and nj are the amounts of component i and j
adsorbed in equilibrium, respectively.
RESULTS AND DISCUSSION
XRD patterns
Adsorption
Desorption
100
50
0
0.0
0.4
0.6
0.8
1.0
N2 adsorption/desorption isotherms of hierarchical mesoporous zeolite NaA at −196 ◦ C. This figure is
available in colour online at www.apjChemEng.com.
Figure 3.
hierarchical zeolite NaA
(a)
commerical zeolite 5A
20
30
40
50
2θ (degree)
hierarchical zeolite NaA
Intensity (a.u.)
(b)
submicron zeolite NaA
10
0.2
Relative pressure (P/P0)
The XRD patterns of commercial, submicron and hierarchical zeolite NaA are shown in Fig. 2(a). The existence of characteristic peaks at the 2θ of 7.2, 10.2,
24.0, 30.8 and 34.2◦ suggests that both of the prepared
Intensity (a.u.)
668
1
3
2
2θ (degree)
4
Figure 2. Wide-angle XRD patterns of commercial zeolite 5A, submicron and hierarchical
zeolite NaA (a) and small-angle XRD patterns of hierarchical zeolite NaA (b).
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2009; 4: 666–671
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
SYNTHESIS AND MODIFICATION OF ZEOLITE NaA ADSORBENTS
of materials with mesoporosity. An explicit adsorption hysteresis can be seen, demonstrating that prepared
hierarchical zeolite has predominant mesopores. A distinct increase in the adsorption quantity in the region
0.4 < P /P0 < 0.9 can be interpreted as capillary condensation in the mesopores.
Adsorption capacity and ideal separation
factor of CH4 and H2 on zeolite samples
Separation of gas mixture by PSA can be based on
either different adsorption affinities of the components
(equilibrium separation), or different diffusion rates in
the adsorbent (kinetic separation). For the separation
of CH4 and H2 , zeolite 5A has been studied[31] taking
advantage of the equilibrium selectivity to CH4 . It is
still necessary to increase the adsorption capacity of the
adsorbent.
The static amounts of CH4 and H2 adsorbed on commercial 5A, submicron SrA and hierarchical SrA are
presented in Fig. 6. It is found that the adsorption capacities of CH4 and H2 are in the order of submicron
SrA > hierarchical SrA > commercial 5A. The amount
of CH4 adsorbed on every adsorbent is always higher
than that of H2 , due to the higher polarizability of
CH4 molecule. Of the five types of physical adsorption defined by Brunauer et al .,[32] the experimental
isotherms of CH4 closely resemble the type-I Langmuir
isotherm, while those of H2 show linearity. The type-I
represents the single molecular surface adsorption and
may apply to microporous adsorbents with small pore
sizes.
The ideal separation factors of CH4 /H2 adsorbed on
the adsorbents were given in Table 1. It is found that
all three adsorbents have the largest ideal separation
factors at 0.4 MPa and decrease with increasing pressure. It is also shown that the ideal separation factors
of prepared adsorbents are always higher than that of
commercial adsorbent and the submicron zeolite SrA
has higher ideal separation factor than the hierarchical zeolite SrA at the same pressure and temperature.
Comparing with ordinary zeolite crystals in micrometer
size, nanosized and submicron zeolites exhibit properties such as large external surface area and reduced
diffusion limitation. For adsorption, larger external surface area means more adsorption sites, resulting in
an increase in amount adsorbed. Short and uniform
pore paths avoid unacceptably slow diffusion of adsorbate molecules to and from the active sites of zeolite
SEM and particle size analysis of submicron
zeolite NaA
The particle size of submicron zeolite NaA was
analyzed by SEM and laser particle size analysis, and
the results are presented in Fig. 4. In Fig. 4(a), the submicron zeolite NaA is cubic and the particle size of the
crystal is about 0.40 µm, much smaller than conventional zeolite NaA with 2–3 µm, which is in agreement
with the results of particle distribution measured by
laser particle size analysis (Fig. 4(b)). In Fig. 4(b), it
is found that the particle size of submicron zeolite NaA
concentrates between 200 and 320 nm and the average
particle size is about 240 nm. The results suggest that
the PEG added into the synthesis system can effectively
limit the crystals to grow up and obtain zeolite crystals
with small particle.
TEM of hierarchical zeolite NaA
TEM images of hierarchical zeolite NaA are shown in
Fig. 5. Figure 5(a) illustrates that hierarchical zeolite
NaA is in the shape of cubic with a particle size
of about 1 µm, and the crystals aggregate to some
extent. The wormhole-like mesopores can be seen
from the low-resolution images shown in Fig. 5(b)
and (c). The high-resolution images (Fig. 5(d) and (e))
exhibit a completely crystallized zeolite NaA crystal
permeated by randomly oriented small mesopores of
nearly the same size. The clear lattice image, with a
lattice spacing of 1.22 nm for the (200) plane, indicates
the high crystallinity of the as-synthesized hierarchical
zeolite NaA.
(a)
% In class
60
(b)
40
20
0
30
50
100
500
1000
Diameter (nm)
Figure 4. SEM images (a) and particle size distributions (b) of submicron zeolite NaA.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2009; 4: 666–671
DOI: 10.1002/apj
669
Y. LIU ET AL.
Asia-Pacific Journal of Chemical Engineering
(b)
(a)
(c)
(d)
(e)
1.22 nm (200)
Figure 5. TEM images of hierarchical zeolite NaA.
0.4
Amount adsorbed (mmol.g-1)
Amount adsorbed (mmol.g-1)
670
H2
0.3
0.2
Submicron SrA
Hierarchical SrA
Commercial 5A
0.1
0.0
0.0
0.2
0.4
0.6
0.8
Pressure (MPa)
1.0
1.2
1.6
CH4
1.2
0.8
0.4
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Pressure (MPa)
Adsorption isotherms of CH4 and H2 on submicron SrA, hierarchical SrA
and commercial 5A adsorbents at 25 ◦ C. This figure is available in colour online at
www.apjChemEng.com.
Figure 6.
intracrystalline. With hierarchical materials by infusing
mesopores into the microporous materials, the problem
of diffusion limitation in micropores and mass transfer
could be solved.
Table 1. Ideal separation factor of CH4 /H2 on different
adsorbents.
Ideal separation factor of CH4 /H2
Pressure/MPa
CONCLUSIONS
In this work, a submicron and a hierarchical zeolite
SrA were synthesized with PEG-1000 and organosilane,
respectively, as surfactants to obtain NaA followed by
ion-exchange of Na+ with Sr2+ , and used for adsorption of CH4 and H2 . The particle size of submicron
zeolite NaA was proved to be about 240 nm and the
hierarchical zeolite NaA was testified to possess ordered
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
0.4
0.6
0.8
1.0
Commercial
5A
Hierarchical
zeolite
SrA
Submicron
zeolite
SrA
8.00
7.05
5.94
5.30
7.99
7.40
6.80
6.10
9.14
8.03
6.90
6.17
mesopores in addition of micropores. At the same pressure and temperature, the amounts of CH4 adsorbed are
Asia-Pac. J. Chem. Eng. 2009; 4: 666–671
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
SYNTHESIS AND MODIFICATION OF ZEOLITE NaA ADSORBENTS
always higher than that of H2 , due to its higher polarizability. The isotherms of CH4 belong to the type-I
Langmuir isotherm, while the isotherms of H2 show linearity. The submicron zeolite SrA adsorbent shows the
highest adsorption capacity of CH4 and the ideal separation factor of CH4 /H2 , compared with the hierarchical
zeolite SrA and commercial 5A. The smaller particle
size of zeolite SrA and the presence of mesopores in
zeolite SrA crystals offer the possibility of increase in
the amount of gas adsorbed with large molecule.
Acknowledgements
This research was performed with the support of the
National Basic Research Program (973 program) of
China (No. 2005CB221202).
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DOI: 10.1002/apj
671
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