Capping with multivalent surfactants for zeolite nanocrystal synthesis

Article abstract of DOI:10.1002/anie.201303088

Multiammonium surfactants exhibited a remarkable capping effect for zeolite synthesis in the forms of nanoparticles, nanorods, and nanosponges in cases where common monovalent surfactants failed. A nanorod-shaped mordenite zeolite synthesized in this manner showed significantly enhanced catalytic lifetimes in acid-catalyzed cumene synthesis reactions.

Full text of DOI:10.1002/anie.201303088

.
Angewandte
Communications
DOI: 10.1002/anie.201303088
Nanoparticles
Capping with Multivalent Surfactants for Zeolite Nanocrystal
Synthesis**
Changbum Jo, Jinhwan Jung, Hye Sun Shin, Jaeheon Kim, and Ryong Ryoo*
Nanoparticles have attracted a great deal of attention because
they have a large accessible surface area and a remarkable
difference in physicochemical properties when compared to
as to justify a general application to the synthesis of other
zeolites. We believe that the lack of effectiveness of zeolite
capping was due to the presence of other cationic species,
[
1]
+
bulk solids. A versatile route to downsizing crystals to the
such as Na and zeolite structure-directing ammonium ions
[2]
+
nanoscale is to use an organic surfactant as a capping agent.
that could compete with CTA ions during the hydrothermal
The surfactant prevents further crystal growth by covering
nanoparticle surfaces by electrostatic attraction, coordination
bonding, hydrogen bonding, or even using weak van der
Waals interaction to the surface atoms. The surfactant-
capping route is generally applied to the synthesis of various
kinds of nanostructures, including nanoparticles and nano-
synthesis of zeolites.
Synthesis of nanocrystalline zeolites using multivalent
surfactants (MSs) has been reported recently. For example,
Ryoo and co-workers synthesized MFI, MRE, beta, and
MTW zeolites in the form of nanosheets or nanosponges,
+
+
using C H N (Me) C H N (Me) C H and other cationic
2
2
45
2
6
12
2
6
13
[3]
[11]
rods composed of metals, metal oxides, and chalcogenides.
Zeolites are a family of microporous crystalline alumi-
surfactants containing three or more ammonium ions. The
role of the surfactants was to direct a mesostructure by
forming a micelle while micropores were generated by
individual surfactant head groups. Hensen and co-workers
synthesized a mesoporous CHA zeolite by incorporating
[4]
nosilicates consisting of more than 200 types of frameworks.
Currently, quite a few zeolites function as important ion-
[
5]
exchangers, molecular sieves, and catalysts. Some zeolites
are found as large mineral rocks in nature, but most zeolites
used in the chemical industry are synthesized in the form of
micrometer-sized crystallites. Despite the micrometer-scale
+
+
C H N (Me) C H N (Me) C H as a mesopore-generating
2
2
45
2
4
8
2
4
9
agent into a synthesis composition containing N,N,N-tri-
methyl-1-adamantanammonium hydroxide for the micropore
[
12]
size, the synthesized zeolite particles still contain more than
generation. The mesopore generation was attributed to the
effect of growth interruption of zeolite crystals. However,
such an effect was not so far confirmed for other zeolites or
other surfactants.
9
1
0
micropores. This characteristic can slow the rate of
molecular diffusion through the microporous framework,
[
6]
often leading to limited catalytic performance. To resolve
the diffusive limitation, there have been numerous
approaches focusing on the synthesis of nanocrystalline
Herein, we show that various zeolite nanocrystals could
readily be synthesized, such as MOR, FAU(X), CHA, and
MFI types, when cationic MSs that contain two or more
ammonium head groups were added to the hydrothermal
synthesis compositions. The zeolite nanocrystals became
agglomerated so that they possessed intercrystalline meso-
pores. We characterized the zeolite samples using scanning
electron microscopy (SEM), transmission electron microsco-
py (TEM), X-ray diffraction (XRD), and N2 adsorption–
desorption isotherms. Our results indicated that the MSs
could protect zeolite nanocrystal surfaces from further growth
more effectively than CTABr, which is probably due to strong
electrostatic interactions. Because of their nanocrystal size,
the MOR zeolites obtained by MS-capping exhibited high
catalytic performance as solid acid catalysts for cumene
synthesis reaction.
[7]
zeolites. However, most approaches were limited to partic-
[
8]
ular types of zeolites under special synthesis conditions or
were dependent on specially synthesized zeolite structure-
[
9]
directing agents (SDA). Cetyltrimethylammonium bromide
(
CTABr) is well known for its mesopore-directing effect in
[10a]
the synthesis of mesoporous silicas.
However, CTABr fails
to function as a mesopore-directing agent when the surfactant
was added to a zeolite synthesis composition in an attempt to
synthesize mesoporous material with a crystalline zeolitic
[10b,c]
framework.
There are a few reports on the synthesis of
LTA (Linde type A) zeolite nanocrystals in the presence of
CTABr by a low crystallization and growth rate techni-
[
10d–f]
que.
However, the surfactant capping was not so effective
+
+
[
*] C. Jo, J. Jung, H. S. Shin, J. Kim, Prof. R. Ryoo
Center for Nanomaterials and Chemical Reactions
Institute for Basic Science(IBS)
Daejeon 305-701 (Republic of Korea)
and
We
tested
C H N (Me) C H N (Me) C H -
18 37 2 6 12 2 6 12
+
N (Me) C H (“C -N -C ” for brevity) as a surfactant for
2
18 37
18
3
18
the capping of MOR zeolite nanocrystals. The surfactant was
added to a conventional synthesis composition for a MOR
zeolite. The synthesis temperature was 423 K (see the
Experimental Section). The zeolite sample synthesized with
Department of Chemistry, KAIST
Daejeon 305-701 (Republic of Korea)
E-mail: rryoo@kaist.ac.kr
C -N -C is denoted by MOR-nrod. On the other hand,
18
3
18
Homepage: http://rryoo.kaist.ac.kr
a control sample synthesized without using C -N -C is
18 3 18
[
**] This work was supported by the Institute for Basic Science in Korea.
denoted by MOR-bulk. The XRD pattern of MOR-nrod was
consistent with the structure of the MOR-type zeolite
(Supporting Information, Figure S1). As compared to
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303088.
1
0014
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Angew. Chem. Int. Ed. 2013, 52, 10014 –10017

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Figure 2. SEM images of MOR zeolite samples synthesized with multi-
Figure 1. a) SEM image and b) TEM image of MOR nanorods, which
valent capping surfactants: a) MOR nanosheets synthesized with
a divalent surfactant; b) MOR nanocrystals synthesized with a prolinol
derivative trivalent surfactant.
were synthesized with a trivalent surfactant, C -N -C . For compar-
1
8
3
18
ison, SEM image of MOR-bulk is shown in the inset of (a).
+
+
MOR-bulk, MOR-nrod exhibited XRD line broadening. The
estimation of crystal size using the Scherrer equation from
structural formula of C H N (Me) C H N (Me) C H
18 37 2 6 12 2 18 37
was used as the capping agent (Figure 2a). When a prolinol
+
(
200) reflection gave 17 nm for the MOR-nrod zeolite. The
derivative,
trivalent
surfactant
C H N (Me) C H -
18 37 2 6 12
+
+
SEM image for MOR-nrod (Figure 1a) exhibited agglomer-
ation of nanorods with a flat cross section. Typical dimension
of the nanorods ranged from 10 to 15 nm in width, 4 to 6 nm in
thickness, and 100 to 300 nm in length, as judged from TEM
imaging (Figure 1b; Supporting Information, Figure S1). The
longest side of the nanorods is parallel to the c-axis of a MOR
structure (Supporting Information, Figure S1). On the other
hand, the MOR-bulk sample exhibited micrometer-sized
crystal morphologies with 1–2 mm particle diameters (inset
of Figure 1a). The t-plot method was used to determine the
external crystal surface area of these zeolite samples. The
external surface area for MOR-nrod obtained in this manner
N (Me) CH C H CH N (Me) (C H CH OH) (see the Sup-
2 2 6 4 2 4 7 2
porting Information, Figure S3 for the structure) was used,
the morphology of the MOR zeolite was observed to be an
aggregation of nanocrystals (MOR-ncrl in Figure 2b). The
diameter of the individual MOR nanocrystals ranged from 10
to 30 nm, as determined by high-resolution TEM (Supporting
Information, Figure S3). On the other hand, when the
+
monovalent cationic surfactant C H N (Me) C H , or
1
8
37
2
18 37
+
CTA , was tested as a capping agent, the resultant product
was very similar to MOR-bulk. From this result, we believe
that the multiple charges are important for the capping effect
of a cationic surfactant during zeolite synthesis. Certainly, the
exclusive use of these surfactants does not make them MOR
2
À1
2
À1
was 220 m g , whereas MOR-bulk was only 5 m g (Sup-
porting Information, Figure S1). The difference in external
surface areas was in good agreement with their particle size
difference. Elemental analysis (EA) revealed that the freshly
synthesized MOR-nrod sample (before calcination) con-
tained organic components corresponding to 15% (w/w) of
its total mass. Before the EA, the MOR-nrod sample was
washed with ethanol and water to remove any organic
components that might be weakly physisorbed. Therefore,
the 15% (w/w) loss could be assigned to the C -N -C
[
11a,c]
zeolite structure-directing agents.
The surfactant-capping route was further tested for
FAU(X), MFI, and CHA zeolites using C -N -C . All the
resultant zeolites gave SEM images showing agglomerates of
nanocrystals (Figure 3). On the other hand, when these
zeolites were synthesized without C -N -C , all the products
1
8
3
18
1
8
3
18
were micrometer-sized crystals with smooth surfaces (Sup-
porting Information, Figure S5). The crystal sizes were 20–
1
8
3
18
surfactant. Since the C -N -C molecule was too bulky to
1
8
3
18
enter the micropore aperture of MOR zeolite, the amount of
surfactant was believed to remain on the external surfaces
through electrostatic interactions between the ammonium
heads of the surfactants and the zeolite framework. The
surfactant amount, based on the monolayer assumption, was
consistent with the measured 15% loss (see the Supporting
Information, Section S2 for a detailed calculation). There-
fore, the generation of the nanocrystals could be attributed to
the capping effect of C -N -C . The surfactant capping was
1
8
3
18
confirmed to work effectively until the concentration of the
reaction gel was increased twice by decreasing the amount of
water to a half as compared to the gel composition described
in the Experimental Section. Below this water content,
however, the nanocrystalline MOR zeolite product contained
sodalite zeolite as an impurity phase.
The nanomorphologies of MOR zeolites could be con-
trolled by the use of different capping surfactants. For
example, MOR zeolite was synthesized in the form of
nanosheet (MOR-nsht) when a divalent surfactant with the
Figure 3. SEM images of a) FAU, b) MFI, and c) CHA zeolite nano-
particles, which were synthesized with C -N -C as a capping agent.
1
8
3
18
40 nm in the case of the nanocrystalline FAU(X) zeolites (see
the Supporting Information, Figure S4 for TEM). The
2
À1
external surface area was 118 m g , and this was much
2
À1
higher than 2 m g of the bulk counterpart. Similarly, crystal-
capping effect of the trivalent surfactant was confirmed in
MFI zeolite synthesis. The MFI zeolite synthesized with the
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surfactant has the form of agglomerates of 10–20 nm thick Experimental Section
nanosheets (Supporting Information, Figure S4). In the case
of CHA zeolite, nanocrystalline (20–30 nm) CHA zeolite
possessing intercrystalline mesopores was also obtained by
the addition of C -N -C surfactant (Figure 3). This zeolite
possessed intercrytalline mesopores amounting to 0.2 cm g .
However, the mesoporosity decreased gradually when the
zeolite product was kept in the synthesis reaction mixture
over a long period of several days after the initial nanocrystal
formation. It thus seemed that the crystal capping effect
decreased owing to Ostwald ripening.
The decrease in particle size to a nanometer dimension
can be a remarkable benefit where zeolite catalysts suffer
from diffusion limitations. For example, the MOR-nrod with
a Si/Al ratio of 8.5 exhibited much higher activity and longer
catalytic lifetime in the cumene synthesis reaction than
showed by the MOR-bulk zeolite (Si/Al = 7). Cumene is
a precursor for the synthesis of other industrially important
chemicals, such as phenol and acetone. The conversion rates
of the reactants were plotted (Supporting Information, Fig-
ure S6) as a function of time on stream to determine the
lifetime of catalysts. MOR-nrod exhibited an initial activity of
Detailed procedures for the synthesis of multivalent surfactants
presented in this paper are described in the Supporting Information.
MOR nanorods with C -N -C as a capping agent were synthesized
18
3
18
as follows: Aluminum sulfate octadecahydrate (ꢀ 98%, Sigma–
1
8
3
18
Aldrich) and C -N -C was completely dissolved in distilled water.
3
À1
18
3
18
Into the clear solution, sodium silicate (Si/Na = 1.75, 29 wt% SiO2,
Shinheung Silicate Co., Ltd.) was added at once and the mixture was
vigorously shaken by hand for 10 min. Then, sulfuric acid (47%,
Wako) was dropped into the gel mixture. The resultant gel had
a molar composition of 30:1.5:1.3:8.57:1200:4.5 SiO /Al O /C -N -
2
2
3
18
3
C /Na O/H O/H SO . After continuous mixing using a magnetic
1
8
2
2
2
4
stirrer for 12 h at 298 K, the resultant gels were transferred into
a Teflon-lined stainless steel autoclave. The autoclave tumbled at
60 rpm in an oven heated at 423 K. The sample was collected from
a homogeneous gel solution at 8 days. All of the samples were dried at
373 K and calcined in air at 853 K. The production yield of MOR-
nrod was more than 85%. The synthesis method of MOR zeolites
+
synthesized by using C -N -C , CTA , C -N -C , and prolinol-
18
1
18
18
2
18
containing surfactant as capping agents is the same as that of MOR
nanocrystal synthesized by using C -N -C . The synthesis procedures
1
8
3
18
of other zeolite nanocrystals are described in the Supporting
Information.
Received: April 12, 2013
Revised: June 7, 2013
8
5% and maintained this value even after 20 h. In contrast,
MOR-bulk showed very low initial activity (20%) and rapidly
deactivated. This high catalytic performance of MOR-nrod
can be attributed to reactions occurring at the external
Published online: August 1, 2013
Keywords: capping agents · multivalent surfactants ·
.
[
11e]
nanoparticles · zeolites
surfaces,
mesoporous zeolite.
for the successful production of highly crystalline MOR
or the short diffusion path lengths in the
[
13]
So far, no direct synthetic methods
[14]
nanocrystals less than 100 nm in size have been reported.
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