Synthesis of ferrisilicate with the MCM-22 structure
Peng Wu, Hong Lin, Takayuki Komatsu and Tatsuaki Yashima*
Department of Chemistry, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo, 152, Japan
A new ferrisilicate molecular sieve with the MCM-22
structure is synthesized in Al-free form and exhibits low
activity as a solid-acid catalyst and significant activity for
the selective catalytic reduction of NO with NH3.
was transformed into the H form by ion exchange with 0.1 m
NH4NO3 followed by calcination in air at 723 K for 2 h. A
reference sample of the Al analogue, Al-MCM-22, was
synthesized according to ref. 12. These two samples were
characterized with XRD, IR, N2 adsorption and catalytic
studies.
A new zeolite MCM-22 patented recently1 has been proposed to
contain two independent pore systems, both of which are
accessed through rings composed of ten tetrahedral (T) atoms.2
One of these pore systems consists of large supercages of
12-membered rings. This unique pore structure of the alumino-
silicate MCM-22 means it is an interesting catalyst for a wide
variety of acid-catalysed reactions.3–5
Fe-MCM-22 was nearly white, indicating the absence of iron
oxides within the crystals. The XRD pattern of Fe-MCM-22
was very similar to that of its aluminosilicate analogues
reported previously12 except that the relative peak intensities
were somewhat different (Fig. 1). Its framework IR spectrum
was also similar to that of Al-MCM-22. Upon substitution of Fe
for Al, some vibration bands were shifted to lower wave-
numbers which can be taken as evidence that Fe ions are present
in the tetrahedral framework sites since Si–O–Fe vibrations are
weaker than those of Si–O–Al. The internal asymmetric
vibration has been reported to show a larger shift than the
external one upon the substitution of Fe for Al in beta and
MOR.9,10 However, the extent of the shift of these two
vibrations was reversed for MCM-22 (Table 1), which may be
due to the unique structure of the MCM-22 zeolite. From N2
adsorption experiments, Fe-MCM-22 had a comparable micro-
pore volume and specific surface area to Al-MCM-22. These
data were also similar to those reported for MCM-22 alumino-
silicates4 and is a clear indication that Fe-MCM-22 has a high
degree of crystallinity.
Isomorphous substitution of other elements for Si or Al into
the silica based zeolite framework to prepare the corresponding
metallosilicates is an effective way to modify the activity and
selectivity of zeolite catalysts. Isomorphous substitution of Ga,
Fe, As, Ti, V, Sn, etc. into the framework has already been
reported. Among these metallosilicates, ferrisilicates have been
used as both solid-acid catalysts and oxidation catalysts. The
acid strength of MFI type (ZSM-5 type) ferrisilicate is moderate
and weaker than that of the Ga and Al analogues.6 More
importantly, we have reported that MFI type ferrisilicate is a
more selective catalyst than Fe3+-exchanged ZSM-5 and iron
oxide supported on silicalite in the oxidative dehydrogenation
of alkanes,7 and that it is also an active catalyst for the selective
catalytic reduction (SCR) of NO with NH3 in the presence of
O2.8 Ferrisilicates have been synthesized for MFI,6 beta,9 MOR
(mordenite),10 FER (ferrierite), etc.,11 but, as yet, not for the
MCM-22 topology. Here, we report for the first time, the
synthesis of iron-substituted MCM-22 zeolite and its phys-
icochemical and catalytic properties.
If trivalent Fe3+ ions are incorporated into the silicate
framework, Si(OH)Fe hydroxy groups must be generated when
Fe-MCM-22 was exchanged into the H form. Fig. 2 shows IR
spectra of both H-form Fe-MCM-22 and H-form Al-MCM-22
in the hydroxy region. For Al-MCM-22 [Fig. 2(a)], the bands at
3748 cm21 (external silanols), 3730 cm21 (internal silanols),
3663 cm21 (associated with extraframework Al species) and
3621 cm21 [the structural Si(OH)Al hydroxy groups] were
MCM-22 type ferrisilicate (Fe-MCM-22) was synthesized
using fumed silicon dioxide (Cab-o-sil M7D) as the source of
silicon to obtain an Al-free material (Si/Al
> 10000).
Hexamethyleneimine (HM) and Fe(NO3)3·9H2O were used as
the template and iron source, respectively. In a typical synthesis,
silicon dioxide and sodium hydroxide were dissolved in an
aqueous solution of HM with stirring for 2 h. The resultant
mixture was then added dropwise into an aqueous solution of
(Fe(NO3)3. The mixture was stirred for another 2 h to obtain a
clear gel. The pH of the final gel was ca. 12 and its molar
composition was SiO2 :0.03 Fe2O3 :0.15 Na2O:45 H2O. The
gel was then transferred into a Teflon autoclave and heated at
408 K for 11 days under stirring (60 rpm). The resultant solid
product was filtered and washed thoroughly with deionized
water, dried at 403 K and calcined in air at 753 K for 5 h to
remove the template. The Na form of the zeolite thus obtained
5
15
25
10
20
30
2θ / °
Fig. 1 X-Ray diffraction pattern of Fe-MCM-22
Table 1 Physicochemical and catalytic properties of Fe-MCM-22 and Al-MCM-22
N2 adsorption
Catalytic activity/mol%
n
asym/cm21
Si/M ratio
in gel
Micropore/
ml g21
Surface area/
m2 g21
o-Xylenea
Tolueneb
External
Internal
isomerization
methylation
Fe-MCM-22
Al-MCM-22
15
15
0.197
0.203
544
551
1229
1244
1089
1092
6.1
67.5
23.9
81.6
a o-Xylene isomerization: cat., 0.4 g; W/F = 7.2 g h mol21; T = 573 K; He, 20 ml min21; time on stream (TOS) = 1 h. b Toluene methylation: cat., 0.4
g; W/F = 6.8 g h mol21; T 573 K; MeOH–toluene molar ratio = 2:1; He, 20 ml min21; TOS = 1 h.
Chem. Commun., 1997
663
Wu, Peng
