Microporous and Mesoporous Materials 118 (2009) 361–372
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Microporous and Mesoporous Materials
journal homepage: www.elsevier.com/locate/micromeso
The effect of Na2SO4 salt on the synthesis of ZSM-5 by template free
crystallization method
Na Young Kang a, Bu Sup Song a, Chul Wee Lee a, Won Choon Choi a,
Kyung Byung Yoon b,*, Yong-Ki Park a,*
a
b
Division of Advanced Chemical Technology, Korea Research Institute of Chemical Technology (KRICT), Jang-dong 100, Daejeon 305-600, Republic of Korea
Department of Chemistry, Sogang University, Seoul 121-742, Republic of Korea
a r t i c l e
i n f o
Article history:
Received 20 May 2008
Received in revised form 3 September 2008
Accepted 6 September 2008
Available online 23 September 2008
Keywords:
Effect of Na2SO4
ZSM-5
Sodium silicate
Template-free
Oxyanion
a b s t r a c t
ZSM-5 was synthesized by template-free method using sodium silicate as a silica source and the content
of Na2O in mother liquid was controlled by H2SO4. To see the effect of Na2SO4 generated by added H2SO4,
hydrothermal crystallization was carried out with two types of mother liquids having low and high
Na2SO4 contents at 170 °C. The physical and chemical properties of ZSM-5s prepared in two different conditions were compared by XRD, EDS, FTIR, micropore analyzer and particle size analyzer. High crystalline,
isometrical shaped and uniform 1–2 lm sized ZSM-5s with relative crystallinity around 100% could be
obtained successfully from the mother liquids having the molar range of (11.0–21.0) Na2O:(21.4–11.4)
Na2SO4:100.0 SiO2:2.0 Al2O3:4000.0 H2O. Through the crystallization in the absence and presence of extra
Na2SO4, it was found that the condensation rate of aluminosilicate was increased by the Na2SO4 and
resulted in the formation of amorphous phase instead of crystalline ZSM-5. The formation of crystalline
HZSM-5 was prohibited critically if the total amount of Na2SO4 in mother liquid was higher than 24.4
mole based on 100.0 mole of SiO2. The enhanced condensation reaction and the preferential formation
of agglomerated amorphous aluminosilicates in the presence of Na2SO4 could be explained by the oxyanion effect of SO2
4 generated by Na2SO4.
Crown Copyright Ó 2008 Published by Elsevier Inc. All rights reserved.
1. Introduction
Because the ZSM-5 has extremely high thermal and acid stability, shape selectivity and activity in catalytic reactions, which are
very critical for industrial application, it has been used widely in
many petrochemical catalytic processes such as cracking, isomerization, aromatization and alkylation process [1]. Since the Mobil
Oil Corporation, in 1972, issued a first patent on the synthesis of
a pentasil aluminosilicate zeolite and termed ZSM-5 by using
TPA+ ion (tetrapropylammonium ion) as templates [2], there has
been a lot of approaches to synthesize it in more mild conditions
and from the cheaper raw materials.
Generally, high crystalline ZSM-5 could be synthesized easily
via a hydrothermal method from a mother liquid containing silica
sol, sodium aluminate, and sodium hydroxide together with organic template cations such as TPA+ and TEA+. These raw materials are
quite efficient precursors to obtain high crystalline ZSM-5 but it
has some drawbacks in economics and environment due to relatively high raw material cost and toxicity. Therefore, there have
* Corresponding authors. Tel.: +82 42 860 7672; fax: +82 42 860 7388 (Y.-K.
Park).
E-mail addresses: [email protected] (K.B. Yoon), [email protected] (Y.-K.
Park).
been a lot of efforts to synthesize ZSM-5 with or without seed from
cheaper silica sources such as sodium silicate and silicic acid in the
absence of template [3–14].
For the template-free synthesis of ZSM-5, the synthetic conditions such as types of raw material, molar composition, mixing
procedure, aging time and crystallization temperature should be
controlled precisely because the working windows for each parameter are much narrower than those in templated systems. The
selection of silica source is a most important one and only after
its decision the other parameters such as Al2O3/SiO2 ratio, Na2O/
SiO2 ratio, mixing procedure and time of reactants, and the aging
time are optimized. The silica source should be chosen based on
two guidelines of material cost and quality of obtained ZSM-5.
Among the several types of silica sources such as silica sol, amorphous silica and water glass, the water glass could be considered
as one of attractive silica sources because high crystallinity of
ZSM-5 is guaranteed without the loss of product yield from this
most cheap and easy market accessible water glass.
One difficulty encountered in water glass system can be ascribed to its high content of alkali ingredients more than that required for ZSM-5 synthesis. That is, the minimum Na2O/SiO2
mole ratio of commercially accessible sodium silicate is 0.33 as
shown in Eq. (1) but the Na2O/SiO2 molar ratio of mother liquid required for the synthesis of high crystalline pure ZSM-5 is in the
1387-1811/$ - see front matter Crown Copyright Ó 2008 Published by Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2008.09.016
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N.Y. Kang et al. / Microporous and Mesoporous Materials 118 (2009) 361–372
range of 0.1–0.25. To remove the excess Na2O, it should be neutralized with acids such as sulfuric acid, nitric acid and hydrochloric
acid. Among the various types of acids, sulfuric acid is a preferred
one due to its low corrosiveness and price. During the neutralization process, considerable amount of Na2SO4, which corresponds
to 1/3 – 2/3 of Na2O included in water glass, is generated as shown
in Eq. (2), That is, the formation of Na2SO4 salt is unavoidable if
water glass is used as a silica source.
2. Experimental
2.1. Synthesis of ZSM-5 zeolite
For the synthesis of ZSM-5, sodium silicate (30.0 wt.% SiO2), aluminum sulfate (8.0 wt.% Al2O3) were used as a silica and alumina
source. Finally, to adjust Na2O/SiO2 ratio, 25 wt.% H2SO4 was added
and Na2SO4 was formed during this procedure according to Eq. (2).
The detail mole composition of mother liquids is described in
Table 1. ZSM-5 was synthesized according to the overall experimental procedure described in Fig. 1. The composition of the
mother liquid was changed in the molar range of 30–80 SiO2/
Al2O3, 0.10–0.22 Na2O/SiO2 and 40 H2O/SiO2. After mixing silica
source and alumina source vigorously for 30 min, it was introduced
into a 100 ml autoclave lined with Teflon and then crystallized
hydrothermally in a rotating oven maintained at 170 °C and 200
rpm for 6–24 h. After completion of the crystallization, the synthesized product was cooled to room temperature, rinsed with distilled
water, and then dried at 120 °C for 12 h. From the solid obtained
after drying, solid product yield was calculated by using Eq. (3)
Sodium silicate : 2NaOH þ 3SiO2
! 3Na2=3 SiO2 1=3 þ H2 OðNa2 O=SiO2 ¼ 0:33Þ
ð1Þ
Mother liquid : 3Na2=3 SiO2 1=3 þ H2 SO4
! Na2 SO4 þ 3SiO2 þ H2 O
ð2Þ
This study was motivated after finding the fact that the crystallization time was much shorter in the case of ZSM-5 synthesis with
sodium silicate than that with silica sol even though the synthetic
procedure and molar composition of mother liquid were same.
That is, when the ZSM-5 was synthesized from the mother liquid
having the same molar composition (18 Na2O:100 SiO2:2
Al2O3:4000 H2O), high crystalline one was obtained within a day
from the sodium silicate while more than two days from the silica
sol. The main difference is the presence of Na2SO4 in the mother
liquid.
So, we were curious about the role of Na2SO4 salt which is generated inevitably during the hydrothermal crystallization of ZSM5. Even though the presence of Na2SO4 in mother liquid brings so
big changes in crystallization rate and morphology, there have
been no studies to see the effect of Na2SO4 systematically in the
template-free synthesis of ZSM-5 using sodium silicate as a silica
source.
However, to see the effect of Na2SO4 independently is not easy
because the content of Na2SO4 and Na2O change simultaneously
by the adding of sulfuric acid. Therefore, to see the effect of
Na2SO4 more clearly, two different synthetic compositions which
depend sensitively on the content of Na2SO4 were chosen and then
extra Na2SO4 was added; one is in low edge of Na2O/SiO2 and the
other is in high edge of Na2O/SiO2 in which ZSM-5 phase could be
obtained. That is, firstly, the crystallization composition windows
depending on the ratio of Al2O3/SiO2, Na2O/SiO2 and H2O/SiO2
were determined by controlling molar compositions of water, aluminum sulfate and sulfuric acid. Secondly, based on these crystallization windows, two types of mother liquid having different
Na2O/SiO2 ratio were prepared after fixing the Al2O3/SiO2 and
H2O/SiO2 ratio and the Na2SO4/SiO2 was changed by adding extra
Na2SO4.
Solid product yield ðwt:%Þ
¼
Weight of solid obtained after crystallization
Weight of solid in mother liquid
ð3Þ
To change the prepared zeolites into H-form, they were ionexchanged with 500 ml of 0.5N NH4NO3 at 60 °C for 6 h and it
was repeated two times. Finally, they were calcined at 550 °C
and air atmosphere for 6 h.
2.2. Characterization
Synthesized ZSM-5 zeolite products were characterized by X-ray
diffraction (XRD; Rigaku, model D/Max-2200V) using Cu Ka radiation operated with a 2h scanning speed of 4° min1. The relative
crystallinity was determined from the peak area between 2h = 22–
25° using peak area of highly crystalline ZSM-5 (Zeolyst, CBV
5524G) as a reference. The degree of the crystallinity was calculated
from the following Eq. (4), as previously done by Ping Wang [15].
Relative crystallinity ð%Þ ¼
Peak area of product
Peak area of reference sample
ð4Þ
Crystal morphology and size were identified by SEM (Akasi Alpha-25A) equipped with image analyzer (Oyang system Co.). The
pore size and surface area were measured by micropore analyzer
(Micromeritics Co. ASAP2010) using Argon adsorption at 77K. Elemental analysis was conducted by EDS to confirm SiO2/Al2O3 molar
ratio of the obtained solid products. To monitor the effect of Na2SO4
Table 1
Crystallization field of zeolite depending on Na2O/SiO2, Na2SO4/SiO2 and Al2O3/SiO2 ratio
x
y
z (SiO2/Al2O3 ratio)
2.50 (40)
2.00 (50)
1.25 (80)
Molar composition = x Na2O:y Na2SO4:100 SiO2:z Al2O3:4000 H2O
22.0
10.4
Mordenite
3.30 (30)
Mordenite
Mordenite
Amorphous
21.0
20.0
19.0
18.0
17.0
16.0
14.0
13.0
12.0
11.0
10.0
Mordenite
Mordenite
Mordenite
Mordenite
Mordenite + ZSM-5
Mordenite + ZSM-5
ZSM-5
ZSM-5
ZSM-5
ZSM-5
ZSM-5
Mordenite
Mordenite
Mordenite + ZSM-5
ZSM-5
ZSM-5
ZSM-5
ZSM-5
ZSM-5
ZSM-5
ZSM-5
Amorphous
ZSM-5
ZSM-5
ZSM-5
ZSM-5
ZSM-5
ZSM-5
ZSM-5
ZSM-5
ZSM-5
Amorphous
Amorphous
11.4
12.4
13.4
14.4
15.4
16.4
18.4
19.4
20.4
21.4
22.4
Mordenite
Mordenite
Mordenite
Mordenite
Mordenite
Mordenite
Mordenite + ZSM-5
ZSM-5
ZSM-5
ZSM-5
ZSM-5
N.Y. Kang et al. / Microporous and Mesoporous Materials 118 (2009) 361–372
(1) Sodium silicate+ H 2 O
(2) Aluminum sulfate
+ H 2SO4 + H 2O + (Salt)
Mix ing: (1) + (2)
Hydr other mal Cr ystallization
o
at 170 C, 200 r pm for 6~ 24h
Filtr ation and Washing
o
Dr ying at 120 C for 12 hr s
Ion Ex change with NH 4 NO3
o
Calcination at 550 C for 6 hr s
Fig. 1. Synthetic procedure of ZSM-5 from sodium silicate without templating
agent.
on nucleation and crystal growth of zeolite, the particle size change
and Si–O bond formation rate of mother liquid were monitored by
Particle size analyzer (ELS-Z2, Otsuka) and FTIR in ATR mode (Avartar 360, Nicolet), respectively, depending on aging time.
2.3. Cracking activity test
To evaluate catalytic activity of ZSM-5s synthesized in content
of Na2SO4, naphtha cracking reaction, one of representative reactions to evaluate catalytic activity of ZSM-5, was carried out in a
fixed-bed flow reactor made of Inconel under atmospheric pressure. 0.5 g of catalyst was put into the reactor and then pretreated
in helium flow at 500 °C for 1 h before reaction. The cracking reaction was done in the following condition; naphtha/steam(wt/
wt) = 2, WHSV = 2 h1, temperature = 675 °C. The gas and liquid
products were separated by cooling to 2 °C and then analyzed
by an GC (HP 6890) equipped with NP-1(Alltech) and ATTMPETRO(Alltech) columns, respectively.
3. Results and discussion
3.1. Crystallization field of zeolite depending on Na2O content and
SiO2/Al2O3 ratios
A series of hydrothermal synthesis of zeolite was carried out by
using sodium silicate as a silica source at 170 °C for 24 h while the
363
molar composition of Na2O and Na2SO4 was changed by adding
H2SO4 at four different SiO2/Al2O3 ratios of 30, 40, 50 and 80 which
correspond to the Al2O3 mole fraction of 3.3, 2.5, 2.0 and 1.25 (Table 1). Initially, a mother liquid containing 32.4 mole of Na2O based
on 100 SiO2 was prepared and then its content was reduced by
neutralizing with sulfuric acid. During this step, stoichiometric
amount of Na2SO4 was formed according to Eq. (2). The content
of Na2SO4 was decided by the amount of added sulfuric acid and
the sum of Na2O and Na2SO4 was maintained to a constant value
of 32.4.
Two interesting things were found in this experiment. Firstly,
the Na2O composition window for ZSM-5 phase was widened as
the molar ratio of SiO2/Al2O3 increased. That is, ZSM-5 could be
synthesized in the range of 10–13 mole of Na2O at SiO2/Al2O3 molar ratio of 30 but it was widened to 12–21 mole at SiO2/Al2O3 molar ratio of 80. Secondly, Mordenite phase was obtained if the
content of Na2O is too high while an amorphous phase was obtained if its content was too low. As can be seen in Table 1, quite
predictable crystallization field was obtained depending on the
contents of Na2O and Na2SO4 and SiO2/Al2O3 ratio even though
the absolute values are different from those reported by Ping Wang
[15].
The trend in crystallization field could be explained as the stability of aluminosilicate depending on the alkalinity of mother liquid.
If the aluminosilicate is exposed at high alkaline condition, it becomes more soluble with decrease of its condensation rate. Thereby, it will lead to the transformation of comparatively less
amount of aluminosilicate to the crystalline solid product. As reported by Jacobs, this may be the reason for the formation of Mordenite which is more metastable than ZSM-5 with decreased solid
product yield [16]. Oppositely, if the aluminosilicate is exposed to
a low alkaline condition, it becomes less soluble and condensation
will occurs more rapidly. The increased condensation rate will lead
to the formation of amorphous phase with high solid product yield
because the aluminosilicate does not have enough time to form a
crystalline phase such as ZSM-5 and Mordenite. This trend could
be seen clearly from the XRD patterns of solid products obtained
at different Na2O contents (Fig. 2). The XRD patterns were obtained
after hydrothermal crystallization at 170 °C for 24 h with mother
liquids having molar composition of (10.0–20.0) Na2O:(22.4–12.4)
Na2SO4:100 SiO2:2 Al2O3:4000 H2O. In the range of 11.0–18.0 mole
of Na2O, the XRD patterns of high crystalline ZSM-5 were obtained.
However, out of this range, two different types of solid products
were obtained. At 20 mole of Na2O (high alkaline condition), mixed
XRD patterns of ZSM-5 and Mordenite were obtained while at 10
mole of Na2O (low alkaline condition), an amorphous one was obtained (Fig. 2a and f). Another evidence for the dependence of
Na2O could be seen from the changes of crystallinity and solid product yield (Table 2). As same as in the XRD patterns, almost 100% relative crystallinity of ZSM-5s with was obtained in the range of 12.0
- 18.0 mole of Na2O but they decreased rapidly out of this range.
That is, below 12 mole of Na2O, poor crystalline ZSM-5s were
obtained.
The solid product yield calculated by Eq. (3) was also monitored
depending on the content of Na2O (Table 2). It depends linearly on
the content of Na2O. The higher the content of Na2O, the less solid
product yield was obtained. This means that the solubility of aluminosilicate strongly depends on the content of Na2O in the
mother liquid. Therefore, to obtain high solid product yield, it is required to maintain the content of Na2O as low as possible.
Fig. 3 shows the SEM images of solid products synthesized
from the same mother liquid described in Table 2. In the range of
(10.0–20.0) Na2O and (22.4–12.4) Na2SO4, 1–2 lm uniform-sized,
isometrical-shaped and high crystalline ZSM-5s were obtained
(Fig. 3b–e). However, out of this range, irregular-shaped and
0.2–0.3 lm sized fine spheroidal particles were obtained. From
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N.Y. Kang et al. / Microporous and Mesoporous Materials 118 (2009) 361–372
ing to Eq. (2) in introduction section. If sodium silicate having
Na2O/SiO2 molar ratio of 0.33 is used as a silica source and SiO2/
Al2O3 ratio is fixed to 50, then the molar compositions of Na2O
and Na2SO4 in mother liquid are decided by the added sulfuric acid
and the sum of Na2O and Na2SO4 becomes 32.4 mole according to
Eq. (5).
: Mordenite
(a)
(b)
xNa2 O þ yNa2 SO4 ¼ 32:4 and yNa2 SO4 ¼ wH2 SO4
Where wH2 SO4 = H2SO4 mole, xNa2 O = Na2O mole, yNa2 SO4 = Na2SO4
mole)
If sulfuric acid is used as a neutralizing agent, then it is impossible to change independently the content of Na2SO4 in mother liquid without changing that of Na2O. Therefore, to see the effect of
Na2SO4, the content of Na2SO4 was changed by Na2SO4 itself instead of the sulfuric acid. That is, two mother liquids having 12
and 18 mole of Na2O, which correspond to the regions of phase
transformation ‘‘from ZSM-5 to Mordenite” and ‘‘from ZSM-5 to
amorphous phase,” were prepared and then desired amount of extra Na2SO4 was added to these mother liquids.
(c)
Intensity (cps)
ð5Þ
(d)
(e)
(f)
5
10
15
20
25
30
35
40
45
50
2Theta (deg.)
Fig. 2. XRD patterns of solid products synthesized from the mother liquids
described in Table 2 (a) x = 20.0, y = 12.4; (b) x = 18.0 y = 14.4; (c) x = 16.0,
y = 16.4; (d) x = 14.0, y = 18.4; (e) x = 12.0, y = 20.4; (f) x = 10.0, y = 22.4, where
the molar composition = x Na2O:y Na2SO4:100 SiO2:2 Al2O3:4000 H2O.
Table 2
Relative crystallinity and yield of ZSM-5 depending on Na2O/SiO2, Na2SO4/SiO2 (Silica
source = sodium silicate, Al2O3/SiO2 = 2)
x
y
Relative Crystallinity (%)
Solid Yield (wt%)
Molar composition = x Na2O:y Na2SO4:100 SiO2:2 Al2O3:4000 H2O
22.0
10.4
Mordenite
42.5
21.0
20.0
19.0
18.0
17.0
16.0
14.0
13.0
12.0
11.0
10.0
11.4
12.4
13.4
14.4
15.4
16.4
18.4
19.4
20.4
21.4
22.4
Mordenite
Mordenite
Mordenite
100.2
106.2
96.6
109.6
93.9
93.1
74.5
21.3
45.4
49.6
46.9
58.4
60.3
66.2
69.78
64.9
68.1
71.4
78.1
the results of relative crystallinity, solid product yield, XRD and
SEM, it was confirmed that cubical-shaped and uniform sized high
crystalline ZSM-5 could be synthesized successfully even from sodium silicate without templating agent if the content of Na2O is
controlled properly in a certain range.
3.2. Effect of Na2SO4 salt
As can be seen in the column 1 and 2 of Table 2, if sodium silicate is used as a silica source, the formation of Na2SO4 is unavoidable because sulfuric acid has to be added to neutralize excess
Na2O. This means that the contents of Na2SO4 and Na2O in mother
liquid are decided automatically by the added sulfuric acid accord-
3.2.1. Effect of Na2SO4 at 0.18 Na2O/SiO2 molar ratio
To see the effect of Na2SO4 near the region of phase transformation from ZSM-5 to Mordenite, 0–14 mole of extra Na2SO4 were
added additionally to the mother liquid having the molar composition of 18.0 Na2O:14.4 Na2SO4:100.0 SiO2:2.0 Al2O3:4000.0 H2O
and then synthesized at 170 °C for 24 h. The details of mother liquid composition and the results of relative crystallinity and solid
product yield are described in Table 3a. If the content of Na2SO4
was less than 8 mole, which corresponds to the total Na2SO4 content of 22.4 mole, both of relative crystallinity and solid product
yield were not influenced so much. However, further addition of
Na2SO4, higher than 10 mole, led to the rapid decrease of relative
crystallinity together with some increase of solid product yield.
This means that the condensation of aluminosilicate is more favorable in the presence of Na2SO4 even though it is not helpful for the
formation of crystalline phases such as ZSM-5 and Mordenite.
From this result it can be induced that the Na2SO4 plays an important role to enhance dehydration of aluminosilicate and leads to
the formation of more polymeric aluminosilicate.
Moreover, the crystal phase stability and morphology change
depending on the content of Na2SO4 were also monitored by XRD
patterns and SEM images. As can be seen in Fig. 4, no other crystal
phases except ZSM-5 were observed even though the content of
Na2SO4 was higher than 10 mole. The role of Na2SO4 is much different from that of Na2O which plays an important role as a structure
directing agent. That is, different from the phase from ZSM-5 to
Mordenite in excess Na2O condition, no phase transformation
was occurred by the addition of Na2SO4 even its content is higher
than 10 mole. Therefore, it could be suggested that the Na2SO4 just
interrupts the formation of crystalline ZSM-5 by the enhanced condensation reaction of aluminosilicate. It is a good comparison with
the Na2O which plays an important role as a structure directing
agent to provide a strong driving force for the formation of certain
crystal phases.
The morphology changes were also observed on the products
prepared in excess Na2SO4 condition. As can be seen in the SEM
images of Fig. 5, the addition of Na2SO4 leads to the formation of
more tightly aggregated particles together with reduced particle
size. If the content of Na2SO4 is higher than 10 mole (Fig. 5d), the
particles aggregated with crystalline ZSM-5 and amorphous phase
could be seen clearly. Therefore, to obtain high crystalline ZSM-5, it
is recommended to reduce the content of Na2SO4 in the mother liquid as low as it can be.
To see how much the physical properties are influenced by the
Na2SO4, micropore and EDS analysis were carried out for the three
solid products prepared from mother liquids having 0, 8 and
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N.Y. Kang et al. / Microporous and Mesoporous Materials 118 (2009) 361–372
Fig. 3. SEM images of solid products synthesized from the mother liquids described in Table 2 (a) x = 20.0, y = 12.4; (b) x = 18.0 y = 14.4; (c) x = 16.0, y = 16.4; (d) x = 14.0,
y = 18.4; (e) x = 12.0, y = 20.4; (f) x = 10.0, y = 22.4, where the molar composition = x Na2O:y Na2SO4:100 SiO2:2 Al2O3:4000 H2O.
Table 3
Relative crystallinity and solid yield of ZSM-5 depending on the added amount of Na2SO4 (Silica source = sodium silicate)
x (Na2O) (mol)
y (Initial Na2SO4) (mol)
z (Added Na2SO4) (mol)
Molar composition = 18 Na2O:(x + 14.4) Na2SO4:100 SiO2:2 Al2O3:4000 H2O
(a)
18 (Na2O/SiO2 = 0.18)
14.4
4
6
8
10
12
14
20.4
2
(b)
12 (Na2O/SiO2 = 0.12)
4
6
[y + z ] (Total Na2SO4) (mol)
Relative Crystallinity (%)
Solid yield (wt%)
18.4
20.4
22.4
24.4
26.4
28.4
22.4
24.4
26.4
106.8
98.7
103.5
78.2
75.1
50.5
113.2
76.7
11.7
58.0
53.1
55.8
59.3
62.3
67.0
74.3
76.4
81.1
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N.Y. Kang et al. / Microporous and Mesoporous Materials 118 (2009) 361–372
different from that in the region of phase transformation from
ZSM-5 to Mordenite. That is, the relative crystallinity decreased
more rapidly as the content of Na2SO4 increased. It decreased to
76.7% and 11.7%, respectively, even by the addition of 4 and 6 mole
of extra Na2SO4. As can be seen in Figs. 6 and 7, coincided dependence of Na2SO4 was also observed in SEM images and XRD patterns. When 6 mole of Na2SO4 was added to the mother liquid,
the solid product agglomerated only with amorphous fine particles
was obtained instead of crystalline ZSM-5.
The reason for the more sensitive dependence of Na2SO4 could
be explained as follows. The high alkaline mother liquid in the region of phase transformation from ZSM-5 to Mordenite contains
only 14.4 mole of Na2SO4 and the relative crystallinity began to decrease rapidly after addition of 10 mole of extra Na2SO4, which corresponded to the total amount of 24.4 mole of Na2SO4. Different
from the high alkaline mother liquid, the low alkaline mother liquid in the region of phase transformation from ZSM-5 to amorphous contains already 20.4 mole of Na2SO4 and the relative
crystallinity began to decrease rapidly even after addition of
4 mole of extra Na2SO4, which corresponded to the total amount
of 24.4 mole of Na2SO4. When the relative crystallinities, XRD patterns and micropore analysis are compared based on the total
amount of Na2SO4, quite well coincided results are obtained. This
means that if the total amount of Na2SO4 in mother liquid is higher
than 24.4 mole based on 100 mole of SiO2 then the formation of
ZSM-5 is severely interrupted and amorphous aluminosilicate is
obtained.
(a)
Intensity (cps)
(b)
(c)
(d)
(e)
(f)
5
10
15
20
25
30
35
40
45
50
2Theta (deg.)
Fig. 4. XRD patterns of solid products synthesized from the mother liquids
described in Table 3a; (a) x = 4, (b) x = 6, (c) x = 8, (d) x = 10, (e) x = 12, (f) x = 14,
where the molar composition = 18.0 Na2O:(x + 14.4) Na2SO4:100 SiO2:2 Al2O3:4000
H2O.
14 mole of extra Na2SO4. As shown in Table 4, the ZSM-5s prepared
with 0 and 8 mole of Na2SO4 revealed high BET surface area (about
430 m2/g) with high crystallinity but the ZSM-5 prepared with
14 mole of Na2SO4 revealed very low BET surface area (164 m2/g)
with poor crystallinity. The pore volume also showed same dependence on Na2SO4 as that of BET surface area. If 14 mole of extra
Na2SO4 was added to the mother liquid, the pore volume of solid
product decreased rapidly from 0.16 to 0.60 cm3/g. Different from
the pore volume, however, all the solid products revealed constant
pore diameter independent of the amount of Na2SO4. This provides
further evidence that the solid product is a physical mixture of
crystalline ZSM-5 and amorphous phase.
Chemical composition change was also monitored. Different
from the relative crystallinity and BET surface area, it didn’t show
any dependence on the content of Na2SO4. All of the solid products
revealed constant SiO2/Al2O3 ratio of around 25 regardless of the
added Na2SO4. This means that the condensation of silica and alumina occurred equally proportional to the mole composition of
mother liquids independent of the Na2SO4 content.
3.2.2. Effect of Na2SO4 at 0.12 Na2O/SiO2 molar ratio
To see the effect of Na2SO4 near the region of phase transformation from ZSM-5 to amorphous, that is, in the region of low Na2O
content, 0–6 mole of Na2SO4 were added additionally to the
mother liquid having the molar composition of 12.0 Na2O:20.4
Na2SO4:100.0 SiO2:2.0 Al2O3:4000.0 H2O and then synthesized at
170 °C for 24 h. The details of mother liquid composition and the
results of relative crystallinity and solid product yield are described in Table 3b. Comparing Table 3a and b, it could be seen that
the dependence of relative crystallinity on the Na2SO4 content is
3.3. Effect of Na2SO4 on crystallization time
As can be seen in the results of previous section, the crystallinity of ZSM-5 is strongly influenced by the Na2SO4 in mother liquids.
Therefore, the crystallinity and morphology of solid products were
examined depending on crystallization time in the absence and
presence of extra Na2SO4 (Table 5, Figs. 8 and 9). For this experiment, two types of mother liquids having low Na2SO4 (18.0
Na2O:14.4 Na2SO4:100.0 SiO2:2.0 Al2O3:4000.0 H2O) and high
Na2SO4 (18.0 Na2O:28.4 Na2SO4:100.0 SiO2:2.0 Al2O3:4000.0 H2O)
were prepared and then crystallization was carried out at 170 °C
for 6–24 h.
Table 5 and Fig. 8 show the dependences of relative crystallinity, solid product yield and XRD patterns on the crystallization
time. The crystallization rate at 28.4 mole of Na2SO4 was much faster than that at 14.4 mole of Na2SO4. Especially, the induction period of crystallization in high Na2SO4 is much shorter than that in
low Na2SO4. That is, it took only 18 h to obtain high crystalline
ZSM-5 having relative crystallinity of 99.7% in high Na2SO4 while
it took 24 h to obtain high crystalline one in low Na2SO4. As shown
in Fig. 9a and b, they also showed some differences in morphology.
The more agglomerated solid product was obtained at high content
of Na2SO4. The extent of agglomeration could be estimated easily
from the SEM images at 12 h. The agglomerated particles were
formed from the beginning of crystallization and they maintained
their form continuously over the whole crystallization period.
The condensation behavior could be inferred from the pattern of
solid product yield depending on crystallization time. Both of the
products revealed high solid product yield even at the beginning
of crystallization time before the crystalline ZSM-5 was formed.
Then their yields decreased rapidly when the crystalline ZSM-5 began to be formed. This means that amorphous precipitates are
formed initially through the condensation reaction and then transformed to the crystalline ZSM-5 after having induction period for
nucleation. According to Kalipcila [14], two types of crystallization
routes have been proposed; one is a direct transformation of amorphous precipitates into a crystalline zeolite and the other is a mediated crystallization. In the latter case, the crystallization proceeds
N.Y. Kang et al. / Microporous and Mesoporous Materials 118 (2009) 361–372
367
Fig. 5. SEM images of solid products synthesized from the mother liquids described in Table 3a; (a) x = 4, (b) x = 6, (c) x = 8, (d) x = 10, (e) x = 12, (f) x = 14, where the molar
composition = 18.0 Na2O:(x + 14.4) Na2SO4:100 SiO2:2 Al2O3:4000 H2O.
Table 4
The physical and chemical properties of ZSM-5 synthesized with different amount
Na2SO4
Sample
x (Added
Na2SO4,
mol)
Relative
crystallinity
(%)
SiO2/Al2O3
ratio by
EDS
BET
(m2/
g)
Horvath–Kawazoe
Pore
diameter
(Å)
Pore
volume
(cm3/g)
Molar composition = 18 Na2O:(x + 14.4) Na2SO4:100 SiO2:2 Al2O3:4000 H2O
(a)
0
97.0
25.0
437.0 6.1
0.163
(b)
8.0
103.5
25.6
431.8 6.0
0.161
(c)
14.0
50.5
25.6
163.9 6.3
0.060
through two reaction steps; firstly amorphous precipitates dissolved into soluble species and then nucleation and crystal growth
proceed separately from these soluble species. As can be seen in
Fig. 9, only homogeneous species were found without any mixing
of amorphous and crystalline phases even though their morphologies changed depending on the crystallization time. From this result, it could be suggested that the crystallization proceeds
through direct transformation of amorphous precipitates into a
crystalline zeolite in template-free synthesis of ZSM-5 from sodium silicate. That is, the observed agglomerated precipitates are
true chemical or structural precursors to the final solid products
as suggested by Cundy [17].
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N.Y. Kang et al. / Microporous and Mesoporous Materials 118 (2009) 361–372
Intensity (cps)
(a)
(b)
(c)
5
9
13
17
21
25
29
33
37
41
45
49
2Theta (deg.)
Fig. 6. XRD patterns of solid products synthesized from the mother liquids
described in Table 3b; (a) x = 2, (b) x = 4, (c) x = 6, where the molar composition =
12.0 Na2O:(x + 20.4) Na2SO4:100 SiO2:2 Al2O3:4000 H2O.
The reduced crystallization time in the presence of high Na2SO4
could be explained by the effect of oxyanions provided by Na2SO4
salt as suggested by Kumar et al. [18]. They evaluated the effect of
3
oxyanions such as ClO4 , PO3
4 and AsO4 in the synthesis of MFItype microporous materials. According to their results, the crystallization time decreased linearly until an optimum concentration of
oxyanions reached, after which constant crystallization time remained. Luo and co-workers also observed decreased crystallization time in hydrothermal synthesis of EMT zeolite by the
addition of sodium phosphate (Na3PO4 12H2O). When 0.2 mole
of Na3PO4 12H2O was added to the mother liquid having the molar composition of (2.1 Na2O:10.0 SiO2:1.0 Al2O3:0.3 (18-crown-6):
140.0 H2O), the crystallization time decreased from 25 days to 6
days. This unusual result is ascribed to the phosphate acting as
an emulsifying and water-structure-breaking agent [19].
Another thing to be concerned is that the relative crystallinity of
solid product decreased rapidly from 99.8% to 52.4% at high concentration of Na2SO4 as the crystallization time increased from
18 to 24 h. The similar phenomenon was also observed in template-free synthesis of ZSM-5 with silicic acid and colloidal silica
by Nastro et al. [20]. They explained that this decrease is caused
by the decomposition of ZSM-5 because its stability range is narrower in template- free system relative to the template containing
system. Shiralkar and Clearfield reported that eventually they
transformed to more stable phases like Quartz and Mordenite type
zeolites in the extend times of crystallization [7].
3.4. Effect of Na2SO4 on condensation rate
Even though the detail data is not included in this paper, an
interesting thing was found during the aging time of mother liquid
before autoclaving for hydrothermal synthesis. The mother liquids
having more Na2SO4 gelated more rapidly than that of less one. If
the mother liquid contained 28.4 mole of Na2SO4, then it became
a hard gel and could not be stirred within an hour even at room
temperature. This phenomenon motivated to monitor the particle
size change of mother liquids depending on aging time of mother
liquid. Because the concentration of original mother liquid is too
high to measure particle size by light scattering method, it was di-
Fig. 7. SEM image of solid products synthesized from the mother liquids described
in Table 3b; (a) x = 2, (b) x = 4, (c) x = 6, where the molar composition = 12.0
Na2O:(x + 20.4) Na2SO4:100 SiO2:2 Al2O3:4000 H2O.
luted 4 times with water; that is, the content of water increased
from 4000 mole to 16,000 mole.
Two types of diluted mother liquids having low (14.4 mole) and
high (28.4 mole) contents of Na2SO4 were prepared and then their
particle size was monitored at room temperature. As shown in
Fig. 10a and b, from the series of data, differences in particle size
and its growing rate could be seen clear depending on the quantity
of Na2SO4 in the mother liquids. In the case of the mother liquid
having low Na2SO4, initially 7–8 nm sized fine particles were
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N.Y. Kang et al. / Microporous and Mesoporous Materials 118 (2009) 361–372
sumed that the Na2SO4 induces high nucleation then more fine
particles have to be formed on the mother liquid with high Na2SO4
at the initial stage of crystallization. However, completely opposite
result was obtained. That is, bigger particles, even the fraction is
not so high, were obtained from the mother liquid with high
Na2SO4. It is thought that the bigger particles are formed through
the agglomeration of primary particles and it is mainly caused by
the Na2SO4. In the SEM images of previous section, it was also discussed that the agglomeration of crystal was more pronounced in
the presence of Na2SO4. Because the agglomeration of primary particles is closely related with condensation reaction, it could be suggested clearly that the addition of salt leads to the increase of
condensation reaction.
To see how much the condensation rate of silica is influenced by
the Na2SO4, three different types of mother liquids having 14.4,
21.4 and 28.4 mole of Na2SO4 based on 18.0 Na2O:100.0 SiO2:2.0
Al2O3:4000.0 H2O were prepared, respectively and then the Si–O
stretching bands were monitored by a FTIR in ATR mode. As can
be seen in Fig. 11, the IR band intensities at 1050 and 1220 cm1
showed different time dependence depending on the concentration of Na2SO4. According to Kulkarni et al. and Chao et al.
[21,22], these bands could be assigned to the symmetric and asymmetric stretching vibration of Si–O bonds, respectively, located at
tetrahedral site of silica (the T–O linkage). As the concentration
of Na2SO4 increased from 14.4 mole to 28.4 mole, the increasing
rate of these two bands depending on time became more pronounced. However, the dependency of intensity change on the content of Na2SO4 was different. In the case of 14.4 mole to 21.4 mole
of Na2SO4 where high crystalline ZSM-5 could be obtained, the increase of intensities were not so high even after 20 h of aging
(Fig. 11a, b). However, in the presence of 28.4 mole of Na2SO4
where poor crystalline ZSM-5 was obtained, the high increase of
intensity was observed (Fig. 11c). This means that there exists a
Table 5
Effect of Na2SO4 on crystallization time of ZSM-5
Time
(hrs)
x = 14.40 (low Na2SO4)
x = 28.40 (high Na2SO4)
Relative
crystallinity (%)
Relative
crystallinity (%)
Solid yield
(wt%)
Solid yield
(wt%)
Molar composition = 18 Na2O:x Na2SO4:100 SiO2:2 Al2O3:4000 H2O
6
Amorphous
62.7
Amorphous
12
Amorphous
61.0
Amorphous
18
19.88
59.8
99.77
24
93.11
54.4
52.35
72.1
75.2
58.1
64.1
formed. Once formed, they grew very slowly and reached to 10 nm
even after 7.5 h (Fig. 10a). Different from the low Na2SO4, the
growing rate of particle in high Na2SO4 was quite high. Even at
0.5 h, a shoulder peak corresponding to 20 nm in size was detected
together with main peak at 7–8 nm. The main peak disappeared
completely after 2.5 h and the shoulder peak at 20 nm peak became a dominant one. The particles grew continuously and
reached to about 50 nm in size at 7.5 h (Fig. 10b). The growing rate
of particle in high Na2SO4 content was 5 times higher than that in
low Na2SO4. Generally, the growing rate of particle in colloidal
solution is controlled by two factors; nucleation rate and condensation rate, which have the same meaning as initiation and propagation in polymerization reaction. Usually, the nucleation step
involves in early stage of crystallization and more fine particles
are obtained if its rate is high. Different from the nucleation, the
condensation reaction involves over the whole process of crystallization and much more related with particle size growing. Based on
these concepts, the growing of particle size depending on the content of Na2SO4 could be explained. At 0.5 h, corresponding to the
initial stage of crystallization, fine particles with narrow size distribution were obtained regardless of content of Na2SO4. If it is as-
a
b
Time = 6 hrs
Time = 12 hrs
Time = 12 hrs
Time = 18 hrs
Time = 18 hrs
Time = 24 hrs
Time = 24 hrs
Intensity (cps)
Time = 6 hrs
5
10
15
20
25
30
35
2 Theta (deg.)
40
45
50 5
10
15
20
25
30
35
40
45
50
2 Theta (deg.)
Fig. 8. XRD patterns of solid products depending on crystallization time; the mother liquids have molar composition of (a) 18.0 Na2O:14.4 Na2SO4:100 SiO2:2 Al2O3:4000
H2O, and (b) 18.0 Na2O:28.4 Na2SO4:100 SiO2:2 Al2O3:4000 H2O.
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N.Y. Kang et al. / Microporous and Mesoporous Materials 118 (2009) 361–372
Fig. 9. SEM images of solid products depending on crystallization time; the mother liquids have molar composition of (a) 18.0 Na2O:14.4 Na2SO4:100 SiO2:2 Al2O3:4000 H2O,
and (b) 18.0 Na2O:28.4 Na2SO4:100 SiO2:2 Al2O3:4000 H2O.
critical concentration of Na2SO4 in which the condensation rate of
silica speeds up rapidly. As shown in crystallization field diagram
of Table 1, it is thought the 24.4 mole of Na2SO4 is the critical concentration under which high crystalline ZSM-5 could be obtained.
These results also well coincide with that of the particle size
change where the aggregation of particle is more pronounced in
the presence of 28.4 mole Na2SO4 (Fig. 10b).
In the view point of chemistry, however, still it is not clear why
condensation rate of alluminosilicate increased by the Na2SO4.
Therefore, more fundamental studies are required to verify the role
of Na2SO4.
3.5. Cracking activity of ZSM-5s synthesized in different Na2SO4
concentration
For the three types of ZSM-5s synthesized in three different
content of Na2SO4, naphtha cracking reaction, one of representative reactions to evaluate catalytic performance of ZSM-5, was car-
371
Weight Fraction (%)
Weight Fraction (%)
Weight Fraction (%)
Weight Fraction (%)
N.Y. Kang et al. / Microporous and Mesoporous Materials 118 (2009) 361–372
50
a
Table 6
Naphtha cracking activity of over the ZSM-5s synthesized in different concentration
of Na2SO4
b
40
Time = 0.5 hr
Time = 0.5 hr
30
20
10
0
50
40
Time = 2.5 hr
Time = 2.5 hr
30
20
10
0
50
40
Time = 5.0 hr
Time = 5.0 hr
30
20
Na2SO4 (mol)
Commerciala
ZSM-5
14.4
(low)
22.4
(medium)
28.4
(high)
SiO2/Al2O3 (mol ratio)
Relative crystallinity (%)
Product yield (wt%)
H2
CO
CO2
CH4
C2=
C2
C3=
C3
C4
C5
BTX
Others
C2= + C3=
C2=/C3=
29
100.0
50
97.0
50
103.5
50
50.5
0.1
0.8
0.6
9.1
18.7
8.1
15.9
7.8
8.2
3.5
21.1
3.0
34.6
1.2
0.1
1.0
0.5
9.7
19.4
8.9
15.6
8.8
7.5
2.6
20.8
1.4
35.0
1.2
0.1
0.9
0.5
9.3
20.2
7.4
16.7
6.3
7.1
3.1
24.2
1.0
36.9
1.2
0.1
0.6
0.2
7.2
14.9
4.9
15.3
4.0
7.1
6.0
18.0
5.0
30.2
1.0
Reaction condition: Naphtha/Steam = 2, WHSV = 2 h1, Temp.= 675 °C.
a
Commercial ZSM-5 produced by Albermarle corp. and having SiO2/Al2O3 of 29.
10
0
50
40
(Table 6). Light olefins yield (C2= + C3=) and Ethylene/Propylene ratio (C2=/C3=) are good barometers to decide catalytic properties of
ZSM-5. 35.0% and 36.9% light olefin yields were obtained over
the ZSM-5s synthesized in the presence of 14.4 mole and 22.4 mole
of Na2SO4 while 30.2% yield was obtained over the ZSM-5 synthesized in the presence of 28.4 mole. That is, regardless of the content
of Na2SO4, once ZSM-5s with high crystalline and high surface area
are obtained their cracking activities are guaranteed. This means
that the Na2SO4 in mother liquid only influences the degree of
ZSM-5 crystallization and does not influence the physical and
chemical properties of prepared ZSM-5. Through the naphtha
cracking activity test, it could be suggested that the Na2SO4 in
the mother liquid involves only in hydrothermal crystallization
process and does not affect the properties of prepared ZSM-5.
Time = 7.5 hr
Time = 7.5 hr
30
20
10
0
1
100 1
10
10
Particle size (nm)
100
Particle size (nm)
Fig. 10. Particle size distribution depending on aging time of mother liquids having
the molar composition of (a) 18.0 Na2O:14.4 Na2SO4:100 SiO2:2 Al2O3:16000 H2O,
and (b) 18.0 Na2O:28.4 Na2SO4:100 SiO2:2 Al2O3:16000 H2O.
ried out and then the results were compared with that of commercial ZSM-5 produced by Albermarle. As shown in Tables 4 and 6,
the ZSM-5s synthesized in the presence of 14.4 mole and 22.4 mole
of Na2SO4 revealed high crystallinity and surface area and is comparable to that of commercial ZSM-5 while that synthesized in the
presence of 28.4 mole of Na2SO4 revealed low crystallinity and surface area. Depending on the crystallinity and surface area of prepared ZSM-5s, quite different cracking activities were observed
60
(a) 14.4 mole
Na 2SO 4
4. Conclusion
High crystalline ZSM-5 could be synthesized successfully using
sodium silicate as a silica source by template-free method. For the
synthesis of high crystalline ZSM-5, the content of Na2O in mother
50
Reflectance (%)
(c) 28.4 mole
Na 2SO 4
(b) 21.4 mole
Na 2SO 4
0 hr
1 hr
5 hr
20 hr
40
30
20
10
1300
1200
1100
Wavenumber (cm-1)
1000 1300
1200
1100
Wavenumber (cm-1)
1000 1300
1200
1100
1000
Wavenumber (cm-1)
Fig. 11. IR spectra of mother liquid having the molar composition of (a) 18.0 Na2O:14.4 Na2SO4:100 SiO2:2 Al2O3:4000H2O, (b) 18.0 Na2O:21.4 Na2SO4:100 SiO2:2 Al2O3:4000
H2O and (c) 18.0 Na2O:28.4 Na2SO4:100 SiO2:2 Al2O3:4000H2O depending on aging time at room temperature.
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N.Y. Kang et al. / Microporous and Mesoporous Materials 118 (2009) 361–372
liquid had to be controlled precisely by sulfuric acid and stoichiometric amount Na2SO4 corresponding to the added sulfuric acid
was generated. The Na2SO4 in mother liquid influenced the properties of final product such as relative crystallinity, morphology, BET
surface area and pore volume. It was found that the Na2SO4 prohibits the formation of crystalline ZSM-5 by the enhanced condensation reaction of silicate and it becomes critical if the total
amount of Na2SO4 in mother liquid is higher than 24.4 mole based
on 100 mole of SiO2. The enhanced condensation reaction could be
explained by the oxyanion effect of SO2
4 generated by Na2SO4.
Acknowledgements
This research was supported by a grant from Carbon Dioxide
Reduction & Sequestration Research Center funded by the Ministry
of Science and Technology of Korea.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.micromeso.2008.09.016.
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