www.elsevier.com/locate/micromeso
Microporous and Mesoporous Materials 69 (2004) 209–215
First stoichiometric large-pore chromium(III) silicate catalyst
Paula Brand~ao a, Anabela Valente a, Artur Ferreira b, V�ıtor S. Amaral c, Jo~ao Rocha a,*
a Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugalb ESTGA, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
c Department of Physics, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
Received 21 November 2003; received in revised form 16 February 2004; accepted 17 February 2004
Abstract
The first example of a large-pore framework chromium-silicate containing stoichiometric amounts of hexa-coordinated chro-
mium(III) has been reported. Samples were characterised by powder X-ray diffraction (XRD), scanning electron microscopy (SEM),
diffuse reflectance spectroscopy ultraviolet–visible (DR UV–VIS) spectroscopy, thermogravimetry (TG), adsorption of different
probe molecules and model catalytic tests; AV-15 is an active catalyst for the liquid phase epoxidation of cyclohexene to cyclohexene
oxide, the oxidation of cyclohexane to cyclohexanol and cyclohexanone, and the oxidative dehydrogenation of cyclohexanol to
cyclohexanone, under mild reaction conditions (using H2O2, at 50 �C). The catalyst is recyclable without significant loss of catalytic
activity, but leaching tests indicate that catalysis is mainly due to some leached chromium species. In gas phase, AV-15 exhibits
mainly oxydehydrogenation activity in the conversion of ethanol, 2-propanol and cyclohexanol, using air, yielding the corre-
sponding carbonyl product.
� 2004 Elsevier Inc. All rights reserved.
Keywords: Chromium silicate; AV-15; Large-pore; Hydrothermal synthesis; Oxidation reactions
1. Introduction
Heterogeneous catalytic oxidation, both in the va-
pour and liquid phase, is an important technological
area in the field of processes for the production of bulk
organic chemicals, the production of fine chemicals andfor pollution abatement [1]. A main advantage in he-
terogeneous catalysts is the easy and economical sepa-
ration of products from the catalyst. A large number of
studies have been devoted to the development of new
materials for catalytic oxidation, principally transition
metal substituted zeolites, zeotypes and mesoporous
materials. A primary difficulty relates to the stability of
these materials in the reaction environment. Some of themajor commodity chemicals, such as formaldehyde and
ethylene oxide, are produced by heterogeneous oxida-
tion in the gas phase at temperatures higher than 600 K
[1]. Hence, high thermal stability is a constraint to
materials developed for such applications. In liquid
phase, the stability of the catalyst to metal leaching
*Corresponding author. Tel.: +351-234-370730; fax: +351-234-
370084.
E-mail address: [email protected] (J. Rocha).
1387-1811/$ - see front matter � 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2004.02.011
during operation in the presence of oxidizing agents is
crucial, but often questionable [2].
It is well known that chromium supported on various
inorganic oxides, such as c-Al2O3, SiO2, ZrO2, and
molecular sieves, are interesting catalysts for a wide
range of redox and polymerisation reactions [3,4]. At-tempts have been made to prepare stable chromium-
based heterogeneous catalysts by incorporating small
amounts of metal in the framework of molecular sieves
[5–11]. For most of these materials framework insertion
requires chromium to be tetra-coordinated, but instead
they contain hexa-coordinated CrIII, most likely present
as extra-framework metal species [3,7].
As a result of a comprehensive effort for prepar-ing novel mixed tetrahedral–octahedral microporous
framework silicates [12], we wish to report the synthesis
and characterization of a large-pore chromium-silicate
containing stoichiometric amounts of hexa-coordinated
CrIII in the framework (AV-15, Aveiro microporous
solid number 15). The catalytic properties of AV-15 are
studied for the liquid phase oxidation of cyclohexene,
cyclohexane and cyclohexanol, under mild conditionsand gas phase conversion of ethanol, 2-propanol and
cyclohexanol using air.
210 P. Brand~ao et al. / Microporous and Mesoporous Materials 69 (2004) 209–215
2. Experimental
2.1. Synthesis
AV-15 was synthesised in teflon-lined autoclaves understatic hydrothermal conditions. Typically, an alkaline
solution was prepared by mixing 6.11 g sodium silicate
solution (Na2O 8 wt.%, SiO2 27 wt.%, Merck), 9.26 g
H2O and 0.90 g NaOH (Merck). A second solution was
prepared by mixing 6.22 g H2O with 1.20 g Cr2(SO4)3 Æ15H2O (Merck). These two solutions were combined
and stirred thoroughly. The gel, with a composition
10.6Na2O:15.1SiO2:Cr2O3:475.1H2O, was autoclavedfor seven days at 230 �C. The green crystalline pow-
der was filtered off, washed and dried at 60 �C over-
night. The synthesis of AV-15 is easily reproducible.
2.2. Characterisation
Powder XRD data were collected on a X’Pert MPD
Philips difractometer (CuKa X-radiation) with a curvedgraphite monochromator, a automatic divergence slit
(irradiated length 20.00 mm), a progressive receiving slit
(slit’s height 0.05 mm) and a flat plate sample holder, in
a Bragg–Brentano para-focusing optics configuration.
Intensity data were collected by the step counting
method (step 0.02�, time 38 s) in the range 2h 3–32�. The
in situ work was carried out using an Anton Parr high-
temperature chamber and a 10 �C/min heating rate.SEM images were recorded on a Hitachi S-4100 Field
Emission Gun tungsten filament working with a voltage
of 25,000 V. Chemical composition was determined by
energy dispersive analysis of X-rays (EDAX) on a SEM
instrument. DR UV–VIS was performed on a Jasco
V-560 PC spectrometer using BaSO4 as the reference
material. Magnetic susceptibility measurements were
performed on a SQUID (superconducting quantuminterference device) magnetometer, model MPMS5
from Quantum Design. The measurements were taken
under an applied magnetic field 100 Oe on heating
from 5 to 300 K. The sample was previously cooled with
the magnetic field applied from room temperature to
5 K. TGA and DSC curves were measured with TGA-
50 and DSC Shimadzu analysers. The samples were
heated under inert atmosphere and air with a rate of5 �C/min. Nitrogen adsorption measurements at 77 K
were performed using a Micromeritics ASAP 2010
V1.01 B automatic instrument. Pore size distribution
was determined using the density-functional theory
(DFT) Plus Software for data files generated from the
ASAP instrument. Adsorption of benzene and m-xylene
was measured at 298 K, using a gravimetric ad-
sorption apparatus equipped with a CI electronic MK2-M5 microbalance and an Edwards Barocel pressure
sensor. Before analysis, the solid was degassed at
523 K.
2.3. Catalytic tests
The catalytic performance of AV-15 was evaluated
for the liquid phase and gas phase reactions of organic
compounds. The liquid phase oxidations of cyclohexene,cyclohexane and cyclohexanol were carried out in a
micro-reaction vessel equipped with a magnetic stirrer,
using H2O2 (30% aq.) as oxidant, at 50 �C. In a typical
run, the reactor was loaded with 50 mg catalyst, 1.2
mmol substrate, 2.6 mmol H2O2, and 1 cm3 acetonitrile,
used as solvent. Blank reactions under the same condi-
tions without a catalyst were also performed. Samples
were withdrawn periodically and analyzed using a gaschromatograph (Varian 3800) equipped with a semi-
capillary CP WAX 52CB column (30 m · 0.53 mm) and
a flame ionization detector.
The gas phase conversion of ethanol, 2-propanol and
cyclohexanol was carried out in a fixed bed continuous
flow reactor (reaction tube of 7 mm inner diameter).
Prior to the reaction the catalyst (50 mg) was placed in
the reactor bed and heated in situ at 300 �C in flowingN2 (20 cm3/min) for 30 min. Afterwards 8 mmol/h
alcohol was introduced using a syringe pump and fed to
the reactor in a stream of air (20 cm3/min). For the
reactions carried out in the absence of oxygen only the
alcohol was introduced after dehydrating the catalyst.
After 30 min, the effluent gas was periodically injected
into the GC by a six-port VICI gas-sampling valve.
3. Results and discussion
3.1. Characterization
The powder XRD pattern of AV-15 is shown in Fig.
1 and the d-spacing and intensities of the main reflec-
tions are given in Table 1. The several peaks were in-dexed to different reflections with DICVOL [13] using
the first 20 lines. The reflections were poorly resolved
making it difficult to obtain a precise solution for the
unit cell. Our best result (M(20)¼ 5) was achieved by
indexing AV-15 on a triclinic cell with a ¼ 15:008 �A,
b ¼ 13:738 �A, c ¼ 11:896 �A, a ¼ 59:78�, b ¼ 110:84�,c ¼ 120:73� (volume 1806 �A3). In situ powder XRD
pattern remains unchanged until 500 �C in vacuum,whereas in air some loss of crystallinity is observed.
SEM images reveals that AV-15 crystals are narrow
plate-like particles with ca. 10–20 lm in length and ca. 2
lm width (Fig. 2). EDAX (energy dispersive absorption
of X-rays) yields Si/Cr and Na/Cr molar ratios of ca. 6
and 3, respectively. The DR UV–VIS spectrum shows
three bands centred at 290, 445 and 640 nm, typical of
trivalent chromium in octahedral coordination (Fig. 3).These results explain the green colour of the as-syn-
thesised sample, similar to most common minerals
containing octahedral CrIII, such as chromite [3]. Mag-
6 9 12 15 18 21 24 27 30 33 36 39 42 452θ /º
I rel
Fig. 1. Powder XRD pattern of chromium-silicate AV-15. The tick marks depict Bragg reflections.
Table 1
Powder XRD data of AV-15
d/�A I=I0
12.641 100
10.595 1
7.135 26
6.828 6
6.361 3
6.301 2
5.903 1
5.746 1
5.352 1
5.242 2
4.961 1
4.647 1
4.485 1
4.289 2
4.261 2
4.228 3
3.790 1
3.680 2
3.490 3
3.450 3
3.388 7
3.205 2
3.176 4
3.152 2
3.070 2
3.041 2
Fig. 2. SEM image of AV-15 crystals.
250 350 450 550 650 750
Wavelength (nm)
% R
(a.u
.)
640445
290
Fig. 3. Diffuse reflectance UV–VIS spectrum of AV-15.
P. Brand~ao et al. / Microporous and Mesoporous Materials 69 (2004) 209–215 211
netic susceptibility measurements (SQUID) show that
AV-15 is paramagnetic with a pseudo-Curie temperature
of 0.71 K and magnetic moment of 3.69 lB, indicating
CrIII is present. These results indicate that chromium
in AV-15 is present as Cr3þ in octahedral coordination
and a possible formula for dehydrated AV-15 is
Na3CrSi6O15. TGA analysis under inert atmosphere
shows a weight loss in the range 50–400 �C corre-sponding to the loss of ca. 4 mol H2O per formula unit
(Fig. 4). The DSC curve obtained under similar condi-
tions exhibits two endothermic peaks at ca. 80 and 130
�C ascribed to the loss of water located on the external
surface and in the pores of AV-15.
The N2 adsorption–desorption isotherm of AV-15 at
77 K is shown in Fig. 5. The significant adsorption at
very low relative pressures is supposedly due to micro-
pore filling. Since the isotherm increases further with
85
90
95
100
0 100 200 300 400 500Temperature (ºC)
Wei
ght (
%)
-1.5
-0.5
0.5
1.5
2.5
DSC
(mW
)
Fig. 4. TGA and DSC curves of AV-15 recorded in inert atmosphere.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.0 0.2 0.4 0.6 0.8 1.0
p/p0
mm
ol/g
0 2 4 6 8 10pore width (nm)
sorp
tion
(a.u
.)
Fig. 5. Nitrogen adsorption (þ)–desorption (�) isotherm at 77 K,
benzene adsorption (�) at 298 K and pore size distribution of AV-15.
212 P. Brand~ao et al. / Microporous and Mesoporous Materials 69 (2004) 209–215
increasing p=p0, the presence of some mesoporosity is
evident, probably corresponding to inter-particle voids.
A hysteresis loop with a very steep region of the
desorption branch which leads to a lower closure point
of the loop at p=p0 � 0:45 is characteristic of aggregates
of plate-like particles leading to slit shaped pores. These
results are in accord with the AV-15 morphology re-
vealed by SEM. The apparent micropore volume, esti-mated from either the t-plot or DR-plot methods, is 0.06
cm3/g. The Langmuir specific surface area is 183 m2 g�1,
of which less than 10% corresponds to external surface
area (16 m2 g�1 calculated from the t-plot). When so-
dium ions are partially substituted by the more bulky
potassium ions the specific surface area decreases ca.
50%. The maximum of the pore size distribution curve
corresponds to a pore width of 8 �A (Fig. 5). The AV-15
sorption capacity for benzene (kinetic diameter 6.8 �A) is
ca. 1 mmol g�1solid (taken at p=p0 � 0:4, Fig. 5), while for
larger adsorbate molecules such as m-xylene (7.4 �A) it is
much smaller (ca. 0.4 mmol g�1solid).
3.2. Catalytic tests
3.2.1. Liquid phase
Several chromium catalysts are known to effectively
oxidize organic compounds with hydrogen peroxide or
tert-butyl hydroperoxide [14–16]. The catalytic perfor-
mance of AV-15 was evaluated for the liquid phase
oxidation of cyclohexene, cyclohexane and cyclohexa-nol, using dilute H2O2 as oxidant, at 50 �C and the
catalytic results are shown in Table 2. Control experi-
ments without a catalyst gave negligible substrate con-
sumption at 24 h.
The oxidation of cyclohexene in the presence of AV-
15 proceeds to 40% conversion within 7 h. Cyclohexene
oxide is the main product, but undergoes acid-catalysed
epoxide ring opening and consecutive oxidation reac-tions, decreasing selectivity.
The catalytic oxidation of cyclohexane yields cyclo-
hexanone and cyclohexanol as the only products in a
molar ratio of 1.7. The turnover frequency of cyclo-
hexane conversion in the presence of AV-15 is 0.46
mmol g�1cat h
�1. The activity of AV-15 may be compared
to that of CrIII incorporated into the framework of
zeolite-b, in which case cyclohexane conversion usingH2O2 as oxidant, at 85 �C, was reported to be 7.8% after
6 h [17]: with AV-15 ca. 11% conversion is achieved. The
oxidation of cyclohexane in the presence of chromium
silicalite-1 (CrS-1, bearing a MFI structure) using a
stronger oxidizing agent than H2O2, i.e., tert-butyl
hydroperoxide (TBHP), at 100 �C, gives equimolar
amounts of cyclohexanone and cyclohexanol at 8.5%
cyclohexane conversion, achieved after 12 h [18].Cyclohexanol conversion in the presence of AV-15
produces cyclohexanone with 100% selectivity at 29%
conversion (Table 2). Similar results have been reported
by Parentis et al. [19] for the oxidation of cyclohexanol
by TBHP in the presence of silica supported CrIII, at 70
�C, though the reaction rate in the latter case is faster
(3.5 mmol g�1cat h
�1, TOF estimated at 7 h reaction) than
that observed for AV-15 (0.8 mmol g�1cat h
�1). The cata-lytic results with AV-15 show that cyclohexanol is more
easily converted than cyclohexane and in the latter
reaction cyclohexanol is subsequently converted to
cyclohexanone.
A recycling test was carried out for AV-15 in the
oxidation of cyclohexanol. Before reuse the solid was
separated from the reaction solution by centrifugation,
washed with acetonitrile and acetone and dried at 60 �C.The reaction rate and selectivity remained practically the
same (Table 2). No major morphological and structural
changes were observed after catalysis, as ascertained by
Table 2
Liquid phase oxidation of organic compounds in the presence of AV-15
Substrate TOFa (mmol g�1cat h
�1) Conversionb(%) Product Selectivityc(%)
Cyclohexene 1.35 51 Cyclohexene oxide 41
2-Cyclohexen-1-one 31
1,2-Cyclohexanediol 4
2-Hydroxy-1-cyclohexanone 14
Cyclohexane 0.46 24 Cyclohexanol 37
Cyclohexanone 63
Cyclohexanol
1st run 0.82 29 Cyclohexanone 100
2nd run 0.83 32 Cyclohexanone 100
a Turnover frequency calculated for 7 h reaction.b Substrate conversion after 24 h.c Products identified by GC–MS. For cyclohexene oxidation minor amounts of other products such as adipic aldehyde and 2-cyclohexen-1-ol are
formed.
Table 3
Gas phase reactions of alcohols in the presence of AV-15 at 300 �C
Alcohol Experimental
conditions
Conversiona
(%)
Selectivityb (%)
ENE ONE
Ethanol AV-15/air 27 9c 85
AV-15/N2 12 100 –
No catal./air 4 100 –
2-Propanol AV-15/air 45 20 80
AV-15/N2 50 86 14
No catal./air 16 69 31
Cyclohexanol AV-15/air 26 33 67
AV-15/N2 10 100 –
No catal./air 7 66 34
a Based on conversion at 260 min on-stream.b Selectivity to the corresponding olefin (ENE) and carbonyl prod-
uct (ONE).c Remaining products are ethyl acetate (5% selectivity) and acetic
acid (1% selectivity).
P. Brand~ao et al. / Microporous and Mesoporous Materials 69 (2004) 209–215 213
SEM and powder XRD. However, the Si/Cr molar ratio
of the recycled catalyst increased approximately 10%,
indicating some loss of chromium ions. To test for
leaching the reaction medium containing fresh AV-15
was filtered at the reaction temperature and the filtrate
was allowed to react further. The hot filtrate reacted at a
similar rate to that observed in the presence of AV-15.
The same is observed when a similar experiment iscarried out for the solid recovered from the second
reaction cycle, as well as for AV-15 washed with
ammonium acetate to remove possible non-framework
chromium species. These results indicate that, under the
applied experimental conditions, AV-15 is subject to
leaching and the soluble chromium species are capable
of catalysing the reaction. Chromium(III) can be oxi-
dized to CrVI in solution, which is an extremely activecatalyst and concentrations as low as 1–2 ppm CrVI can
catalyse the reaction efficiently [2]. When tert-butyl
hydroperoxide was used as oxidant no reaction oc-
curred, probably because no leaching occurs. The
chromium species present in solution may be removed
from the framework of AV-15 or from impurities of
unreacted gel.
3.2.2. Gas phase
The relatively high thermal stability of AV-15 makes
it an interesting candidate for gas phase catalytic oxi-
dation reactions. The oxidation of alcohols to carbonyl
compounds is a key reaction in organic synthesis [20].
Redox and nonredox CrIII sites in chromium catalysts
have been proposed as active for dehydrogenation
reactions [21, and references therein]. Herein, the cata-lytic performance of AV-15 is evaluated for the gas
phase conversion of alcohols, namely ethanol, 2-pro-
panol or cyclohexanol using air, at atmospheric pres-
sure, in a fixed bed continuous flow reactor, at 300 �C.
In these experiments the main product is the corre-
sponding carbonyl compound, which results from the
(direct and/or oxidative) dehydrogenation of the alco-
hol.
The conversion of ethanol produces mainly acetal-
dehyde with 85% selectivity at 27% conversion, after 260
min on-stream (Table 3). Some by-products such as
ethylene, ethyl acetate and acetic acid are formed. The
conversion of cyclohexanol over AV-15 gives cyclohex-
anone in 67% selectivity at 26% conversion, and the onlyby-product formed is cyclohexene. The oxydehydro-
genation activity (OD) of AV-15 for these reactions was
demonstrated by using nitrogen instead of air, under
identical experimental conditions, which resulted exclu-
sively in the dehydration of the alcohol to the corre-
sponding olefin. AV-15 exhibits OD activity for the
conversion of 2-propanol, giving acetone with 80%
selectivity at 45% conversion (TOS (time-on-stream)¼260 min). Without oxygen, the acetone yield is lower
(7% compared to 36% with air), whereas the propene
yield is higher (43% compared to 9% with air).
0
20
40
60
80
100
0 100 200 300
TOS (min)0 100 200 300
TOS (min)0 100 200 300
TOS (min)
Con
v., S
elec
t. (%
)
0
20
40
60
80
100
Con
v., S
elec
t. (%
)
0
20
40
60
80
100
Con
v., S
elec
t. (%
)
A B C
Fig. 6. Catalytic performance of AV-15 in the conversion (�) of ethanol (A), 2-propanol (B) or cyclohexanol (C), and selectivity to the corresponding
carbonyl (�) or olefin (þ) compound as a function of time-on-stream (TOS).
214 P. Brand~ao et al. / Microporous and Mesoporous Materials 69 (2004) 209–215
The observed OD activity may be due to the presenceof basic sites in AV-15. It has been found that basic sites
of molecular sieves play a very important role in the
dehydrogenation of alcohols [22–24]. On the other hand,
the dehydration activity, observed especially for the
conversion of 2-propanol to propene under N2 stream, a
typical acid-catalysed reaction [23], indicates that AV-15
possesses some surface acidity. Products resulting from
consecutive reactions (such as isomerization, dispro-portionation) of, for example, cyclohexene were not
detected, suggesting that the acid sites in AV-15 are
weak [25].
The conversion profiles shown in Fig. 6 indicate that
AV-15 slowly deactivates with TOS and simultaneously
the dehydrogenation activity tends to decrease whereas
the dehydration activity increases. The catalyst deacti-
vation may be partly due to the formation of coke. Infact, the originally green catalyst powder turned to a
brownish colour after the reaction, probably due to
accumulation of undesorbed dimeric and heavier olig-
omers. The DSC analysis carried out on the used sample
under air atmosphere, shows a broad exothermic peak
centred at 408 �C that does not appear for the as-syn-
thesized sample. Hence, this band may be assigned to
the decomposition of organic matter present in the solid,indicating that during gas-phase experiments the prod-
ucts desorption is hindered. Under the applied reaction
conditions AV-15 lost 36% crystallinity (estimated by
normalizing the sum of the areas of the powder XRD
peaks to the corresponding areas of the as-synthesised
AV-15 sample), which may also account for the ob-
served catalyst deactivation.
4. Conclusions
We report the synthesis and characterisation ofAV-15,
which contains stoichiometric hexa-coordinated chro-
mium(III). This material is, to the best of our knowl-
edge, the first example of a large-pore chromium silicate.
AV-15 is an active catalyst for the liquid phase epoxida-tion of cyclohexene to cyclohexene oxide, the oxidation
of cyclohexane to cyclohexanol and cyclohexanone, and
the oxidative dehydrogenation of cyclohexanol to cyclo-hexanone, undermild reaction conditions (usingH2O2, at
50 �C). The catalyst could be recycled without significant
loss of catalytic activity, but leaching tests confirmed that
catalysis is mainly due to some chromium species leached
from AV-15 during the oxidative transformations. The
catalytic performance of AV-15 was further evaluated for
the gas phase conversion of primary and secondary
alcohols using air (atmospheric pressure), at 300 �C. AV-15 exhibits mainly oxydehydrogenation activity in the
conversion of ethanol, 2-propanol and cyclohexanol,
yieldingmainly the corresponding carbonyl product. AV-
15 also exhibits some dehydration activity responsible for
the formation of the corresponding olefin as a by-prod-
uct. The olefins did not undergo any consecutive reac-
tions, suggesting that the acid sites in AV-15 are weak.
Coke formation and partial loss of crystallinity accountsfor some catalyst deactivation at 300 �C.
Acknowledgements
The authors thank FCT, PRAXIS XXI, POCTI and
FEDER (Portugal) for financial support and the
Materials Institute of Porto (IFIMUP) for performingthe SQUID magnetic measurements.
References
[1] B.K. Hodnett, Heterogeneous Catalytic Oxidation. Fundamental
and Technological Aspects of Selective and Total Oxidation of
Organic Compounds, John Wiley & Sons Ltd, UK, 2000.
[2] I.W.C.E. Arends, R.A. Sheldon, Appl. Catal. A: Gen. 212 (2001)
175.
[3] B.M. Weckhuysen, I.E. Wachs, R.A. Schoonheydt, Chem. Rev.
96 (1996) 327.
[4] X. Zhang, Y. Yue, Z. Gao, Catal. Lett. 83 (2002) 19.
[5] N. Van der Puil, Widyawati, J.C. Jansen, H. Van Bekkum, Stud.
Surf. Sci. Catal. 84 (1994) 211.
[6] N. Rajic, D. Stojakovic, S. Hocevar, V. Kaucic, Zeolites 13 (1993)
384.
[7] B.M. Weckhuysen, R.A. Schoonheydt, Stud. Surf. Sci. Catal. 84
(1994) 965.
P. Brand~ao et al. / Microporous and Mesoporous Materials 69 (2004) 209–215 215
[8] M.R. Klotz, US Patent 4,299,808 (1981); US Patent 4,363,718
(1982); US Patent 4,405,502 (1983); US Patent 4,431,748 (1984).
[9] E.M. Flanigen, B.M.T. Lock, L. Patton, S.T. Wilson, US Patent
4,759,919 (1988).
[10] Z. Zhu, Z. Chang, L. Kevan, J. Phys. Chem. B 103 (1999) 2680.
[11] P. Brand~ao, A. Philippou, A. Valente, J. Rocha, M.W. Anderson,
Phys. Chem. Chem. Phys. 3 (2001) 1773.
[12] J. Rocha, M.W. Anderson, Eur. J. Inorg Chem. (2000) 801.
[13] A. Boultif, D.J. Lou€er, Appl. Crystallogr. 21 (1991) 987.
[14] G. Suss-Fink, G.B. Shul’pin, Petrol. Chem. 42 (2002) 15.
[15] J. Muzart, Chem. Rev. 92 (1992) 113.
[16] R.S. Cruz, J.M.S. Silva, U. Arnold, U. Schuchardt, J. Mol. Catal.
A: Chem. 171 (2001) 251.
[17] S. Yuvaraj, M. Palanichamy, V. Krishnasamy, Chem. Commun.
(1996) 2707.
[18] A.P. Singh, T. Selvam, J. Mol. Catal. A: Chem. 113 (1996) 489.
[19] M.L. Parentis, N.A. Bonini, E.E. Gonzo, React. Kinet. Catal.
Lett. 76 (2002) 243.
[20] R.A. Sheldon, J.K. Kochi, Metal Catalysed Oxidations of Organic
Compounds, Academic Press, New York, 1981.
[21] R.L. Puurunen, S.M.K. Airaksinen, A.O.L. Krause, J. Catal. 213
(2003) 281.
[22] Y. Matsumura, K. Hashimoto, S. Yoshida, J. Catal. 122 (1990)
352.
[23] P.E. Hathaway, M.E. Davis, J. Catal. 116 (1989) 263.
[24] P.E. Hathaway, M.E. Davis, J. Catal. 116 (1989) 279.
[25] J. Datka, B. Gil, O. Vogt, J. Rakoczy, in: I. Kiricsi, G. P�al-
Borb�ely, J.B. Nagy, H.G. Karge (Eds.), Porous Materials in
Environmentally Friendly Processes, vol. 125, Elsevier, Amster-
dam, 1999, p. 409.
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