Microwave fabrication of MFI zeolite crystals with a fibrous morphology and their applications

Article abstract of DOI:10.1002/anie.200461403

Metal incorporation and synthesis of MFI zeolites with microwave irradiation result in crystal stacking in one direction to form a fibrous morphology (see images of the Ti-MFI zeolite). These materials display several interesting physicochemical propertie

Full text of DOI:10.1002/anie.200461403

Angewandte
Chemie
Communications
Metal-containing zeolites synthesized under microwave conditions
stack in one direction to form a fibrous morphology. The resulting
materials show enhanced shape-selective adsorption of xylene isomers
and have potential applications in microreactors and separation
technology. For more information see the Communication by J.-S.
Chang, S.-E. Park et al. on the following pages.
556
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DOI: 10.1002/anie.200461403
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Angewandte
Chemie
Zeolite Synthesis
method, we prepared fibrous MFI zeolites that exhibited
useful physicochemical properties due to their self-stacking.
Pure and metal-incorporated MFI crystals were synthe-
sized by microwave-heating precursor solutions at 438 K.
These samples will be denoted as M-MFI-MW, where M
stands for the incorporated metal (Ti, Fe, Zr, and Sn) and MW
for the microwave conditions; metal-free MFI from a micro-
wave reaction will be denoted as Si-MFI-MW.
The powder X-ray diffraction (XRD) patterns of Ti-MFI-
MW samples show the characteristic peaks for the MFI
structure without any impurities. The IR spectra of the
calcined Ti-MFI-MW samples exhibit an absorption band at
960 cmÀ1 for the stretching mode of a [SiO4] unit bonded to a
Ti4+ ion (O3SiOTi). UV/Vis diffuse reflectance spectra,
collected in vacuo after evacuation (ꢀ 10À5 torr) at 627 K,
revealed a strong peak around 210–240 nm, assignable to the
ligand-to-metal charge-transfer absorption band from the
oxygen to tetrahedral titanium(iv) atom in the zeolite frame-
work.[14] The Ti-MFI-CH from the hydrothermal syntheses
also showed practically the same XRD and IR data, indicating
that the difference in the synthesis methods did not influence
the identity of the final products.
However, we observed that the microwave synthesis
induces a dramatic change in the morphology depending on
the composition. Si-MFI-MW and Ti-MFI-CH show the
characteristic pucklike crystals of submicrometer sizes with
well-developed large (010) faces (Figure 1a and b). The
aggregated morphology of individual particles appears to be
attributed to superfast crystallization (Figure 1b).[9] The
microwave reactions of incorporated metal systems produced
similar primary crystals, but this time the crystals are all
Microwave Fabrication of MFI Zeolite Crystals
with a Fibrous Morphology and Their
Applications**
Young Kyu Hwang, Jong-San Chang,* Sang-Eon Park,*
Dae Sung Kim, Young-Uk Kwon, Sung Hwa Jhung,
Jin-Soo Hwang, and Min Seok Park
Zeolite molecular sieves are good candidates for designed
microscopically and nanoscopically engineered materials due
to their well-defined pore structures.[1,2] Among them, silica-
lites, zeolites having the MFI framework topology, have been
studied extensively with regards to the selective separation of
hydrocarbons[3] and the development of electrode materials,[4]
membranes,[5] and catalysts.[6] Because many emerging appli-
cations of porous materials require precise control of crystal
size, shape, and orientation,[7,8] the development of new
synthesis strategies is increasingly important. For example,
the properties of spherical mesoporous silica particles with a
narrow particle size distribution as a material for column
chromatography are superior to those of curved hexagonal
rods with the same structure.[7]
Recently, it has been reported that the microwave syn-
thesis method could provide an efficient way to control
particle size distribution,[9] phase selectivity,[10] and macro-
scopic morphology[11] in the synthesis of porous materials as
well as result in fast crystallization.[12] However, to our
knowledge, there are few reports of microwave techniques
being utilized to control the morphology of porous materi-
als.[11,13] Here, we report on new applications of the micro-
wave method for the fabrication of MFI-type zeolite crystals
incorporating Ti and other metal species. By using the
[*] Dr. Y. K. Hwang, Dr. J.-S. Chang, Dr. D. S. Kim, Dr. S. H. Jhung,
Dr. J.-S. Hwang, Dr. M. S. Park
Research Center for Nanocatalysis
Korea Research Institute of Chemical Technology (KRICT)
P.O. Box 107, Yusung, Taejon 305-600 (Korea)
Fax: (+82)42-860-7676
E-mail: jschang@krict.re.kr
Prof. S.-E. Park
Department of Chemistry, Inha University
Incheon 402-751 (Korea)
Fax: (+82)32-874-7674
E-mail: separk@inha.ac.kr
Prof. Y.-U. Kwon
Department of Chemistry and BK-21
School of Molecular Science, Sungkyunkwan University
Suwon 440-746 (Korea)
[**] This work was supported by the Korean Ministry of Science and
Technology through the National Science Programs for Key Nano-
technology and Institutional Research Program. The authors thank
especially Prof. J. M. Kim for helpful collaboration, Dr. C. H. Xue and
T. H. Jin for experimental assistance, and Dr. J. S. Yoo and P. M.
Forster for useful discussions.
Figure 1. SEM images of a) Si-MFI-MW, b) Ti-MFI-CH (Si/Ti=70),
c) Ti-MFI-MW (Si/Ti=120), d) Ti-MFI-MW (Si/Ti=70), and e) Sn-MFI-
MW, and f) TEM image of Sn-MFI-MW. Inset is a large-scale image of
(c).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
stacked on top of one another along their [010] direction to
case. Further work is needed to fully understand the role of
the incorporated metals in the stacking of crystals under
microwave conditions.
form a wormlike or fibrous morphology (Figure 1c–f). This
stacking is sufficiently robust that it is not destroyed by
sonication for 1 h or more, indicating that this morphology is
not a result of simple aggregation but of strong chemical
bonds between the crystals. This fibrous morphology persists
as long as there is incorporated Ti and its concentration is
kept in the range of Si/Ti = 70–230. When the concentration
of Ti is increased (Si/Ti ꢀ 50), the product is composed of
isolated ellipsoidal crystals (not shown). This is probably
because the high concentration of Ti interferes with the
mechanism of crystal growth, and the lack of flat surfaces
does not allow crystal stacking. The crystals with other
incorporated metals (Fe, Zr, Sn) also show the fibrous
morphology, and we show the images of Sn-MFI-MW in
Figure 1e and f.
We also have monitored the reaction progress in the Ti-
MFI-MW system by XRD and scanning electron microscopy
(SEM). The XRD patterns show that the crystals are starting
to grow out of embryos after about 10 min, grow rapidly
during the next 30 min, and reach the full crystallization at
about 60 min. The SEM images show that the stacking of
crystals begins at 20 min and culminates after 60 min. These
observations suggest that the fibrous morphology is achieved
through interactions between already present crystals (see the
Supporting Information).
The concentration of the reagent also influences the
stacking behavior of crystals. When the concentration is too
low (H2O/SiO2 = 50 and 100), the crystals become larger than
those mentioned above (H2O/SiO2 = 22) and the stacking
occurs to a much lesser extent. Probably, the large crystal sizes
and the dilute conditions provide fewer chances for the
crystals to meet with the correct relative orientation for
stacking. When the concentration is too high (H2O/SiO2 =
10), the product is composed of granules of much smaller sizes
because the reaction occurs too quickly for the crystals to
attain the pucklike morphology, and the lack of large and flat
surfaces prohibits the crystal stacking.
The fibrous morphology of Ti-MFI-MW crystals leads to
several unique properties that may be advantageous for
applications. To show this, we chose a fibrous Ti-MFI-MW
synthesized from a Si/Ti = 70 solution and compared its
properties with those of nonfibrous MFI zeolites such as Si-
MFI-MW, Ti-MFI-CH, and Ti-MFI-MW (hereafter, NF-Ti-
MFI-MW) synthesized from a solution with H2O/SiO2 = 100.
The packing density of fibrous Ti-MFI-MW powder is 1.9
times lower than that of the Si-MFI-MW and Ti-MFI-CH
powders. However, the BET surface areas and micropore
volumes from N2-adsorption experiments (Table 1) show that
Although it is not clear how the
incorporated metals induce the
Table 1: Physicochemical and catalytic properties of Ti-MFI zeolites according to the morphology and
preparation method.
stacking of crystals under the micro-
wave conditions, it appears to be
related with the magnitude of the
Catalyst[a]
SBET
Pore volume Hydrophob. Reaction Styrene epoxidation[c]
[m2 gÀ1
]
[mLgÀ1
]
index[b]
time [h] Conv. [%][d] Sel. [%][e]
À
local dipole moment of the M O
bonds. Electrically insulating mate-
rials absorb microwave energy
through the oscillation of dipoles,
and the magnitude of absorption
increases with an increase in the
dipole moment. The magnitude of
Ti-MFI-MW (fibrous)
423
0.21
8.2
4
8
4
8
4
8
55.6
70.5
39.5
48.7
39.5
48.0
63.1
65.2
54.5
51.9
52.5
44.2
NF-Ti-MFI-MW (nonfibrous) 432
Ti-MFI-CH (nonfibrous) 453
0.22
6.3
0.25
6.0
the dipole moment is determined [a] Ti-MFI-MW (fibrous): prepared by microwave irradiation for 60 min with a H2O/SiO2 ratio of 22; NF-
Ti-MFI-MW (nonfibrous): prepared by microwave irradiation for 60 min with a H2O/SiO2 ratio of 100; Ti-
MFI-CH (nonfibrous): prepared by the conventional hydrothermal method at 438 K for 24 h in a Si/Ti
ratio of 70. [b] The hydrophobicity index was calculated from the amount of adsorbed water and toluene.
[c] Reaction conditions: 343 K, 5 mL styrene, 1.3 mL H2O2 (30%), 18 mL acetone (solvent), 0.35 g
catalyst. [d] (Styrene conversion/maximum expected conversion)ꢀ100. [e] Other products: benz-
aldehyde and phenylacetaldehyde.
mainly by the difference in the
electronegativities (Dc) of the two
À
bonded atoms. Therefore, the Ti O
bond (Dc = 2.18 according to the
Allred–Rochow scheme)[15] is
a
better microwave absorber than
À
the Si O bond (Dc = 1.76). The
À
Ti O bonds on the surfaces are strongly activated by micro-
wave absorption and can undergo condensation reactions to
form Ti-O-Ti and/or Ti-O-Si bonds between crystals. The
same explanation applies to the Fe-MFI-MW and Zr-MFI-
all three Ti-MFIs (Ti-MFI-MW, Ti-MFI-CH, and NF-Ti-MFI-
MW) have comparable pore characteristics, suggesting that
the crystal stacking hardly affects the intracrystalline pore
structure and that the reduced density of Ti-MFI-MW is a
result of the fibrous morphology. The hydrophobicity index
data from the adsorption mass ratio of toluene and water
show that the hydrophobicity of fibrous Ti-MFI-MW is 36%
higher than that of Ti-MFI-CH (Supporting Information). It
can be rationalized in terms of the consumption of terminal
hydroxyl groups by stacking crystals in Ti-MFI-MW. Accord-
ing to IR spectra of Ti-MFI-MW dehydrated at 673 K under
vacuum, the intensity of surface hydroxyl band (ca.
3740 cmÀ1) of the Ti-MFI-MW is much smaller than those of
À
À
MW zeolites because of the large Dc values for Fe O and Zr
O bonds. In the case of Sn-MFI-MW, the Dc value of the Sn
O bond (1.78) is rather small, close to that of Si O bond.
However, because Sn is a large atom, the valence electron
À
À
À
density of the Sn O bond is shifted to the O side, making this
bond more polar than would be estimated by the Dx value
alone. In other words, the homopolar contribution to the
dipole moment[16] and the above explanation of microwave
absorption by polar bonds can be applied to the Sn-MFI-MW
558
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Angew. Chem. Int. Ed. 2005, 44, 556 –560

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Chemie
Ti-MFI-CH and even NF-Ti-MFI-MW. (Supporting Informa-
tion). The enhanced surface hydrophobicity affects the
catalytic properties of Ti-MFI-MW. As summarized in
Table 1, Ti-MFI-MW is more reactive and selective than
nonfibrous Ti-MFI-CH and NF-Ti-MFI-MW for the epox-
idation of styrene with H2O2 to produce styrene oxide.
Although the fibrous structure of Ti-MFI-MW may limit the
diffusion of reactants and products, this drawback is more
than offset by the positive effect of the increased hydro-
phobicity for this reaction.[17]
xylene in the low-temperature range 323–373 K (shaded area
of Figure 2a), implying that this material can be applied in
producing highly pure p-xylene from a xylene mixture when
combined with temperature swing desorption. The preferen-
tial adsorption of p-xylene (p-xylene/o-xylene = 9.9) in the
fibrous Ti-MFI-MW has also been confirmed in competitive
liquid-phase adsorption of a p-xylene/o-xylene mixture at
293 K (see the Supporting Information).
The MFI structure has three different sites for adsorption:
straight and sinusoidal channels along the [010] and [100]
directions, respectively, and their intersections. The diffusivity
in the straight channel is reportedly much higher than in the
sinusoidal channel.[18] Because of the crystal stacking along
the [010] direction, the straight channels of Ti-MFI-MW are
longer and their openings are much fewer than those of the
nonfibrous Ti-MFI-CH crystals. Therefore, the difference
between the diffusivities of p- and o-xylene would be
amplified, resulting in the enhanced adsorption selectivity
of Ti-MFI-MW. Further work is needed to fully understand
the present adsorption behaviors.
Even more importantly, the fibrous Ti-MFI-MW zeolite
presents significantly enhanced shape selectivity in adsorp-
tion. Figure 2 shows the temperature-programmed desorption
In summary, we have demonstrated that the coupled
effect of metal incorporation at a suitable concentration level
and microwave irradiation during the synthesis of MFI
zeolites produces fibrous MFI crystals. This effect appears
to be related to the dipole moment strength of the surface
À
M O bonds. The fibrous Ti-MFI-MW zeolites show several
interesting properties such as lower packing density, higher
hydrophobicity, enhanced catalytic properties, and higher
shape-selectivity in adsorption as compared to nonfibrous
MFI zeolites.
Experimental Section
Ti-MFI-MW and Fe-MFI-MW were synthesized at 438 K for 60 min
under microwave (MW) irradiation at 600 W. Tetraethyl orthosilicate
(TEOS, Aldrich, 98%), tetrapropyl orthotitanate (TPOT, Aldrich,
96%) and tetrapropylammonium hydroxide (TPAOH, Aldrich,
25%) were used. The molar composition of the reaction mixture
was 1TEOS:0.2TPAOH:0–0.05TPOT or 0–0.05FeCl3:1isopropyl
alcohol:22.2H2O. The precursor gel of 30–60 g was loaded in a 100-
mL Teflon autoclave, which was sealed and placed in a microwave
oven (Mars-5, CEM). Ti-MFI-CH was prepared under conventional
hydrothermal conditions at 438 K for 24 h with the same precursor-
gel composition as Ti-MFI-MW. Sn- and Zr-MFI zeolites were
synthesized with the same precursor compositions as Ti-MFI-MW at
438 K except for the modification of metal alkoxides using acetyl-
acetone as a chelating agent to reduce the rate of alkoxide hydrolysis.
Further details are given in Supporting Information.
Vapor-phase adsorption and TPD measurements of xylene
isomers were conducted in a plug-flow glass reactor with an internal
diameter of 6 mm at 323 K. The xylene mixture that passed through
the reactor was analyzed using an on-line gas chromatograph (Donam
model DS-6200) that was equipped with a capillary column (Bentone-
34) and a flame ionization detector. TPD profiles of the adsorbed
xylenes were obtained by temperature-programmed heating (ramp
rate: 5 KminÀ1) from 323 K to 573 K after sufficient desorption of
physisorbed xylenes at 323 K. Detailed methods for the adsorption
and TPD analysis of xylene isomers are described in the Supporting
Information.
Figure 2. Xylene TPD profiles of a) fibrous Ti-MFI-MW and b) nonfi-
brous Ti-MFI-CH after vapor-phase competitive adsorption of an equi-
&
*
molar mixture of p-xylene ( ) and o-xylene ( ) at 323 K. Adsorbent
weight: 0. 25 g; carrier gas: nitrogen (flow rate=30 mLminÀ1); ramp
rate=5 KminÀ1; mL xylenes per gram of adsorbent=0.168 for plot a
and 0.175 for plot b.
(TPD) profiles of xylene isomers after vapor-phase adsorp-
tion of an equimolar mixture of p-xylene and o-xylene over
Ti-MFI-MW and Ti-MFI-CH at 323 K.[3,8] The xylene TPD
patterns are similar over the two adsorbents, that is, two peaks
for p-xylene and one peak for o-xylene, while the fibrous Ti-
MFI-MW reveals a remarkably enhanced adsorption selec-
tivity to p-xylene (p-xylene/o-xylene = 10.7), which is 6.7
times higher than that of the nonfibrous Ti-MFI-CH. More-
over, it is notable that Ti-MFI-MW selectively desorbs only p-
Received: July 23, 2004
Published online: November 11, 2004
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Keywords: crystal growth · microporous materials · zeolites
.
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