M. JIANG et al.: CHARACTERIZATION OF THE STRUCTURE AND PERFORMANCE OF CE
3+
...
423–428
CHARACTERIZATION OF THE STRUCTURE AND
PERFORMANCE OF Ce
3+
EXCHANGED LIX MOLECULAR
SIEVES
KARAKTERIZACIJA STRUKTURE IN LASTNOSTI Ce
3+
IZMENJALNIH LIX MOLEKULARNIH SIT
Mingming Jiang
1
, Mengfu Zhu
1
, Cheng Deng
1
, Jun Ma
1
, Qiaofeng Duan
2
, Meisheng Shi
1
,
Wanyu Gao
1
1
Institute of Medical Equipment, Academy of Military Medical Sciences, Tianjin 300161, China
2
Tianjin Medical Devices Quality Supervision and Testing Center, Tianjin 300384, China
zmf323@163.com
Prejem rokopisa – received: 2017-07-04; sprejem za objavo – accepted for publication: 2018-01-24
doi:10.17222/mit.2017.103
In order to prepare CeLiX zeolite molecular sieves with different Ce
3+
exchange degrees, LiX zeolite molecular sieves were
modified by Ce
3+
via a cation-exchange method. These sieves were characterized with the FT-IR, XRD, SEM, BET and XRF
techniques. The adsorption isotherms of N2 and O2 were measured with a gas-adsorption analyzer at 25 °C. The results show
that the Ce
3+
can replace the extra-framework lithium element. The introduction of Ce
3+
did not change the original crystal
structure, and the small spherical powder structure remained unchanged after the modification. The distribution of particle size
was uniform with a particle diameter near 4 μm. The exchange percentage of the Ce
3+
increased with the exchange times. After
three rounds of exchange, the exchange percentage reached 61.2 %. The pore size distribution of the LiX zeolite was 2–6 nm
before modification, while this value changed to a 7-nm medium-size pore after modification. The introduction of Ce
3+
can
significantly improve the adsorptive selectivity of the LiX zeolite for O2/N2 separation. The adsorptive selectivity of the CeLiX
was greater than 1 after three rounds of exchange, which indicated the product was oxygen-adsorbent molecular sieves.
Keywords: LiX molecular sieve, cation exchange, adsorptive selectivity, O
2
/N
2
separation
Avtorji prispevka so pripravljali CeLiX zeolitna molekularna sita z razli~nimi stopnjami izmenjave Ce
3+
kationov. LiX zeolitna
molekularna sita so modificirali s pomo~jo kationske izmenjevalne metode. Izdelana sita so nato okarakterizirali s tehnikami
FT-IR, XRD, SEM, BET in XRF. Adsorpcijske izoterme N2 in O2 so merili s plinskim adsorpcijskim analizatorjem pri 25 °C.
Rezultati analiz so pokazali, da lahko Ce
3+
zamenja zunanje ogrodje elementa litija (Li). Uvajanje Ce
3+
kationov ni spremenilo
originalne kristalne strukture in tudi struktura drobnih krogli~nih delcev je ostala po modifikaciji nespremenjena. Velikostna
porazdelitev delcev je bila enovita s premerom delcev blizu 4 μm. Procentualni dele` izmenjave kationov Ce
3+
je nara{~al s
~asom izmenjave. Po treh krogih izmenjave je procentualni dele` izmenjave dosegel 61,2 %. Pred modifikacijo LiX zeolita je
bila velikost por 2–6 nm, po modifikaciji pa se je spremenila na povpre~no 7 nm. Uvajanje Ce
3+
kationov lahko pomembno
izbolj{a adsorptivno selektivnost LiX zeolita za lo~itev O2 od N2. Adsorptivna selectivnost CeLiX je bila ve~ja kot 1 po treh
krogih izmenjave, kar ka`e na to, da je produkt primeren kot kisikovo adsorpcijsko molekularno sito.
Klju~ne besede: LiX molekularno sito, kationska izmenjava, adsorptivna selektivnost, O2/N2 separacija
1 INTRODUCTION
Molecular sieves have wide industrial applications.
They are common catalytic materials, adsorptive separa-
tion materials and ion-exchange materials. Microporous
molecular sieves play an increasingly prominent role in
petroleum chemicals, fine chemicals and daily-use
chemical industries.
1–4
The extra-frame cation in mole-
cular sieves can be easily exchanged. The extra-frame
cation balances the negative charge of the molecular
sieve frame and is usually located in the pore or cage of
the molecular sieve. The quantity and the location of the
extra-frame cation in molecular sieves have a strong
impact on the performance of the molecular sieve, such
as the catalytic and adsorptive performance. One or
several cations can modify the zeolite molecular sieve
with outstanding performance via the ion-exchange
method.
Zeolite molecular sieves can be X-type, Y-type or
A-type. The X-type zeolite molecular sieve is the most
extensively studied.
5,6
The ion-exchange can adjust the
surface charge property and the adsorptive performance
of the molecular sieve.
7–9
For example, in most cases, the
specific surface area of the zeolite molecular sieve
slightly decreases after the ion-exchange of the extra-
frame cations. The cations to be exchanged have diffe-
rences in size, quantity and location in the frame struc-
ture. The ion exchange can affect the specific surface
area and pore size of the zeolite molecular sieve. This
can further affect its adsorptive performance to achieve
adsorptive separation.
10
For example, the exchange of
cations with a high valence state to low valance state
reduces the amount of cations leaving a cavity and
increasing the pore size of the zeolite. The exchange of a
large-radius cation with a small-radius cation can block
the cavity of the zeolite molecular sieve and reduce the
Materiali in tehnologije / Materials and technology 52 (2018) 4, 423–428 423
UDK 543:66.017:66.067.123.8 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(4)423(2018)

effective pore size and change the volume of the crystal
cavity and specific surface area.
11
Chao et al.
12
adopted Li
+
/alkaline earth ions to mo-
dify the X-type zeolite molecular sieve by mixed-cation
exchange. After modification, the thermal stability and
adsorptive selectivity of the zeolite molecular sieve was
improved. The selectivity coefficient of the X-type
zeolite molecular sieve for N
2
/O
2
was 3.7, while that of
the LiX zeolite molecular sieve after 97 % lithium ex-
change was 10.2. Yang et al.
13
used Ag
+
to exchange the
LiX molecular sieve and obtained AgX molecular sieves
composed of 80 % Li
+
and 20 % Ag
+
. This zeolite mole-
cular sieve showed a significant increase in the N
2
adsorptive volume, but showed a small change in the O
2
adsorptive volume. The Ag
+
combined into zeolite mole-
cular sieves could improve the adsorptive volume and
selectivity coefficient for N
2
/O
2
separation. Weston et
al.
14–16
found that the N
2
adsorptive volume of a Li-LSX
molecular sieve rapidly increased only when the Li
+
ex-
change level was greater than 75 %.
The LiX zeolite molecular sieve is mainly used for
N
2
absorption as an outstanding adsorbent for N
2
/O
2
separation. It is widely used in air separation for oxygen
and nitrogen production. Andrieux
17
developed an ad-
sorbent with a high oxygen selectivity. Loading the
transition element complex on the zeolite substrate
results in a high specific surface area, and the transition
element in the complex can adsorb the oxygen in the gas
mixture and function as an oxygen adsorbent. A study by
Jasra et al.
18
showed that a NaX zeolite molecular sieve
had better oxygen adsorption performance after ex-
changing alkaline metal ions with trivalent cations.
Due to the high abundance, low cost and stable
valance state of Ce
3+
,
19
Ce
3+
was used to modify the LiX
zeolite molecular sieve and prepare a CeLiX zeolite
molecular sieve via a cation-exchange method in this
study. The structure and adsorptive property on N
2
and
O
2
were also characterized in detail.
2 EXPERIMENTAL PART
2.1 Materials and Instruments
The LiX zeolite molecular sieve (n(Si):n(Al)=1.4,
powder, Shanghai Hengye Chemical Ltd.), cerium chlo-
ride (analytical grade, Tianjin Guangfu fine-chemical
institute), cerium nitrate (analytical grade, Tianjin Che-
mical No.3 factory).
The composition and structure were characterized by
NICOLET 6700 Fourier transform infrared (FTIR) spec-
trometer from Thermo Fisher Scientific (US), MiniFlex
600 X-ray diffraction (XRF) instrument from Rigaku
(Japan), 1530 VP scanning electron microscope from
LEO (Germany), ASAP2020 specific surface area and
porosity analyzer from Micrometritics (US), SRS3400
X-ray fluorescent spectrometer from Bruke (Germany)
and 3H-2000PH gas adsorption analyzer from BSD
(China).
2.2 Modification of molecular sieve
The LiX zeolite molecular sieve was modified by
cation exchange in an aqueous solution. A pre-treated
LiX molecular sieve (calcined at 300 °C for 2 h) was
continuously mixed with cerium chloride solution under
certain conditions via an isothermal magnetic mixer
(Table 1). The middle product was then filtered, washed,
dried and calcined to produce a product after one
exchange. This experimental procedure was repeated
three times. LiX zeolite molecular sieves with different
Ce
3+
exchange degrees were eventually obtained and
denoted as CeLiXn (n represents the times of the ex-
change).
Table 1: Ion-exchange reaction conditions
Mole-
cular
sieve
Raw
material
Dosage
(g)
Solution
concen-
tration
(mol·L
–1
)
Solution
volume
(mL)
Reaction
tempe-
rature
(°C)
Reac-
tion
time
(h)
CeLiX1 LiX 12 0.1 200 80 3
CeLiX2 CeLiX1 8 0.1 200 80 3
CeLiX3 CeLiX2 4 0.1 200 80 3
3 RESULTS AND DISCUSSION
3.1 Structure of Molecular Sieve Frame
3.1.1 FTIR analysis
Figure 1 shows the adsorption peak at 3500 cm
–1
,in-
dicating the stretching vibration of the –OH group from
free water molecules adsorbed on the surface of a mole-
cular sieve. The adsorption peak at 3500 cm
–1
is attri-
buted to the crystal water inside the zeolite molecular
sieve. The 1000 cm
–1
and 725 cm
–1
adsorption peaks are
attributed to the asymmetrical and symmetrical stretch-
ing vibrations of the tetrahedron inside the zeolite
molecular sieve, respectively. These are characteristic of
the X-type zeolite.
20
These results show that introducing
the cerium ion did not damage the original structure, and
the crystal structure remained unchanged.
M. JIANG et al.: CHARACTERIZATION OF THE STRUCTURE AND PERFORMANCE OF CE
3+
...
424 Materiali in tehnologije / Materials and technology 52 (2018) 4, 423–428
Figure 1: Infrared spectra of zeolite molecular sieve (a for LiX, b for
CeLiX1, c for CeLiX2, d for CeLiX3)

The location of the adsorption peaks in the FTIR
remains the same before and after the Ce
3+
modification.
The only change was the intensity of some adsorption
peaks. The characteristic peaks of the molecular sieves
modified by different reaction times have the same loca-
tion; only the peak strength is inversely proportional to
the exchange times. The decrease in peak strength is
possibly due to the combination of anions in the cerium
salt with Si-OH or Al-OH groups on the surface of the
LiX zeolite molecular sieves. This might also be due to
the effect of Ce
3+
itself on the molecular sieves.
10,21
3.1.2 XRD analysis
Figure 2 shows the XRD spectra of CeLiX zeolite
molecular sieve. Figure 2 compares the LiX zeolite
molecular sieve before and after the Ce
3+
exchange. The
diffraction peak of the LiX molecular sieves has a small
change on the diffraction angle. Its strength decreases,
indicating that the ion exchange has a small impact on
the basic structure of zeolite molecular sieves. However,
introducing Ce
3+
decreases the strength of the diffraction
peak of the LiX zeolite molecular sieve. This might be
due to two reasons. First, cerium chloride is a strong-
acid and weak-base salt. The hydrolysis of cerium
chloride makes the solution acidic and destroys the
silicon aluminum frame of the zeolite molecular sieve.
Second, multiple exchanges damage the zeolite mole-
cular sieve. Nonetheless, the XRD spectra of the CeLiX
zeolite molecular sieve have a characteristic peak of the
LiX zeolite molecular sieve. This indicates that despite
some reduction in the integrity of the crystal, introducing
Ce
3+
does not change the original crystal structure of the
zeolite, which both have a cubic pattern.
3.1.3 SEM analysis
We used the SEM technique to characterize the mor-
phology before and after modification (Figure 3).
Compared to the unmodified LiX zeolite molecular
sieve, the powder surface after modification is coarser
and shows signs of corrosion. Nevertheless, both have a
small spherical powder structure with a uniform particle
size distribution and a diameter of about 4 μm. The
powder particle of the CeLiX zeolite molecular sieve has
an octahedral structure, and its lattice structure does not
collapse.
3.2 Specific Surface Area and Pore Diameter Analysis
3.2.1 Specific surface area of molecular sieve
The BET technique was used to measure the N
2
adsorption data at –196 °C for CeLiX zeolite molecular
sieves that were treated for different exchange times. The
specific surface area can be calculated using the BET
equation (Table 2). The CeLiX zeolite molecular sieve
has a slightly smaller specific surface area than that of
the LiX zeolite molecular sieve perhaps because of the
smaller radius of Li
+
(0.068 nm) relative to Ce
3+
(0.103
nm). The charge density of Li
+
is also lower than Ce
3+
.
This makes it difficult for Ce
3+
to find a stable location in
the pores of the zeolite molecular sieve. Therefore, the
content of Ce
3+
in the CeLiX zeolite molecular sieve is
not high. The change in the specific surface area of the
CeLiX zeolite molecular sieve is small. In addition, the
XRF analysis shows that the Ce
3+
exchange percentage
increases with exchange times. After three rounds of
exchange, the exchange percentage of cerium ion reaches
61.2 %.
Table 2: Ce
3+
exchange percentage and specific surface area of CeLiX
zeolite molecular sieves
Molecular Sieve
Specific surface
area /(m
2
g
–1
)
Ce
3+
exchange
percentage/%
LiX 56.31 0
CeLiX1 50.74 26.4
CeLiX2 51.02 46.5
CeLiX3 48.67 61.2
M. JIANG et al.: CHARACTERIZATION OF THE STRUCTURE AND PERFORMANCE OF CE
3+
...
Materiali in tehnologije / Materials and technology 52 (2018) 4, 423–428 425
Figure 3: SEM image of zeolite molecular sieve: a) for LiX, b) for
CeLiX1, c) for CeLiX2, d) for CeLiX3, 6 K magnification)
Figure 2: XRD spectra of zeolite molecular sieve: a) for LiX, b) for
CeLiX1, c) for CeLiX2, d) for CeLiX3)

3.2.2 Pore size of zeolite molecular sieve
Figure 4 shows the adsorption-desorption curves of
the LiX and CeLiX zeolite molecular sieves measured at
the temperature of liquid nitrogen. The shape of the
curve is related to the pore structure. The IUPAC
classification shows four types (I, II, IV, VI) of curves
that fit the porous materials. The adsorption-desorption
curves of LiX and CeLiX in Figure 4 match type IV.
Furthermore, the zeolite molecular sieve showed a
single-layer adsorption with more obvious capillary
condensation. This shows a significant increase in the
large pores after modification.
If the desorption and adsorption processes are not
completely reversible, then the discrepancy between the
adsorption and desorption curves can be observed. This
is called hysteresis, and the adsorption and desorption
curves form a hysteresis loop. IUPAC classifies
hysteresis loops into four types (H1, H2, H3, and H4). In
Figure 4, the hysteresis loops are formed by the
adsorption-desorption curve for LiX and CeLiX zeolite
molecular sieves. These are H3 and H1 types,
respectively. The H3 type is mostly attributed to the
narrow slit pore channel, and the H1 type is mostly
attributed to the pore channel with a uniform size and
regular shape. The common pore structures result in a
H1-type hysteresis loop. These are individual and
cylindrically narrow pores and long channels with a
uniform pore size. The packing of uniform spherical
particles forms the cavity. This result agrees with the
SEM image, indicating that the prepared CeLiX zeolite
molecular sieves are spherical particles with uniform
size.
The pore size distribution is shown in Figure 5. After
exchange with Ce
3+
, the 7 nm medium-size pore was
created in the LiX zeolite molecular sieve. This shows
that the ion-exchange method cannot only decorate the
surface property of zeolite molecular sieve, but can also
adjust the structure of the medium-size pores. The
number of pores with diameters between 2 nm and 6 nm
greatly decreased after the ion-exchange. This indicates
that ion exchange can impact the original pore channel.
The result matches the adsorption isotherm and shows
the increased pore diameter.
3.3 N
2
and O
2
Adsorption Performance of Zeolite Mo-
lecular Sieve
The adsorption isotherm curve of CeLiX zeolite
molecular sieves with N
2
and O
2
is shown in Figure 6 at
25 °C and 5–110 kPa. The N
2
adsorption of LiX zeolite
molecular sieve decreases while O
2
adsorption increases
after cerium exchange. After three rounds of exchange,
the O
2
adsorption quantity of the CeLiX3 zeolite
molecular sieve is larger than the N
2
adsorption quantity.
This is related to the existing f-orbital in the Ce
3+
as seen
in reference.
22
The Ce
3+
hasa4f
l
electron structure, and
the Ce-O bond has both ionic and valence components.
This makes Ce
3+
easier to oxidize to stable CeO
2
and
increases the O
2
adsorption quantity.
The N
2
and O
2
adsorbing capacity and the N
2
/O
2
adsorptive selectivity of LiX and CeLiX zeolite
M. JIANG et al.: CHARACTERIZATION OF THE STRUCTURE AND PERFORMANCE OF CE
3+
...
426 Materiali in tehnologije / Materials and technology 52 (2018) 4, 423–428
Figure 5: Pore size distribution curves of: a) LiX and b) CeLiX Figure 4: Adsorption-desorption curves of: a) LiX and b) CeLiX

molecular sieve at 25 °C and 110 kPa are listed in Table
3. The N
2
adsorbing capacity of the CeLiX zeolite
molecular sieve gradually decreased to 5.8 cm
3
/g with
increasing exchange times. This is because the major
force between Ce
3+
and N
2
is  complexation. This is
weaker than the polarization between Li
+
and N
2
. The O
2
adsorption capacity initially increases, which indicates
that introducing Ce
3+
increases the adsorption of LiX on
the zeolite molecular sieve. This is possibly because Ce
3+
increases the active sites for O
2
adsorption on LiX zeolite
molecular sieves. This is related to the pH of the salt
solution used for the exchange.
Table 3: Nitrogen and oxygen adsorption properties of LiX and
CeLiX molecular sieve (25 °C)
Molecular
sieve
Adsorbing capacity
/(cm
3
·g
–1
) N 2
/O
2
O
2
/N
2
N
2
O
2
LiX 15.0 3.5 4.3 0.2
CeLiX1 13.2 4.4 3.0 0.3
CeLiX2 8.5 5.4 1.6 0.6
CeLiX3 5.8 8.1 0.7 1.4
4 CONCLUSIONS
The cation-exchange method from an aqueous
solution can successfully introduce Ce
3+
into the LiX
zeolite molecular sieve. The introduction of Ce
3+
reduced
the crystallinity of the LiX zeolite molecular sieve while
maintaining the same crystal structure as X-type zeolite
molecular sieves. The powders had a small spherical
structure before and after the modification. The particle
diameter was around 4 μm. Corrosion was seen on the
particle surface after modification. Modification with
Ce
3+
increased the pore diameter of the zeolite molecular
sieve and made the pore size distribution more uniform.
These results show that the modification of LiX zeolite
molecular sieves by Ce
3+
is feasible. Adding Ce
3+
can
enhance the adsorption capability of zeolite molecular
sieves for O
2
and suppress N
2
adsorption. This reverses
its adsorptive selectivity to N
2
/O
2
and makes it an oxygen
superior adsorptive agent.
Acknowledgments
This work was supported by the Twelfth Five-Year
Project of China (Grant No. CWS12J109).
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