Methanol oxidation on VSiBEA zeolites: Influence of v content on the catalytic properties

Article abstract of DOI:10.1016/j.jcat.2011.04.013

This study deals with the influence of V content on the catalytic properties of V_xSiBEA zeolites in the oxidation of methanol. The samples are prepared following a postsynthesis method reported earlier (S. Dzwigaj, M.J. Peltre, P. Massiani, A. Davidson, M. Che, T. Sen, S. Sivasanker, Chem. Commun. (1998) 87). The incorporation of isolated mononuclear V(V) into the framework of SiBEA is evidenced by XRD, FTIR, diffuse reflectance UV-vis and NMR. It is found that, for low V content, V(V) ions are in pseudo-tetrahedral coordination only, either in nonhydroxylated (SiO)₃VO or hydroxylated (SiO)₂(OH)VO species in framework position. For higher V content, additional species appear in extraframework position with vanadium in octahedral coordination. FTIR investigations of pyridine adsorption show that strong Bronsted and Lewis acidic centres are present in SiBEA leading to dimethyl ether only, in methanol oxidation. Upon incorporation of vanadium into the BEA framework, Lewis acidic (V⁵⁺) and basic (O^2-) centres are generated with simultaneous appearance of partial oxidation products mainly, whose total concentration increases with vanadium content. These results suggest that those centres are responsible for the oxidation activity of V_xSiBEA zeolites. The selectivity toward formaldehyde increases with the amount of vanadium present as pseudo-tetrahedral hydroxylated (SiO) ₂(HO)VO species. This selectivity is suggested to be related to the moderate nucleophilicity of the basic vanadyl oxygen (VO) of (SiO) ₂(HO)VO species.

Full text of DOI:10.1016/j.jcat.2011.04.013

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Journal of Catalysis 281 (2011) 169–176
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Methanol oxidation on VSiBEA zeolites: Influence of V content
on the catalytic properties
a
Maciej Trejda a, , Maria Ziolek , Yannick Millot b,c, Karolina Chalupka b,c, Michel Che b,c,d
,
⇑
Stanislaw Dzwigaj b,c,
⇑
a Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
b Laboratoire de Réactivité de Surface, UPMC Univ Paris 6, 4 Place Jussieu, 75252 Paris Cedex 05, France
c Laboratoire de Réactivité de Surface, CNRS, UMR 7197, 4 Place Jussieu, 75252 Paris Cedex 05, France
d Institut Universitaire de France, 103 Bd. Saint-Michel, 75005 Paris, France
a r t i c l e i n f o
a b s t r a c t
Article history:
This study deals with the influence of V content on the catalytic properties of VxSiBEA zeolites in the
oxidation of methanol. The samples are prepared following a postsynthesis method reported earlier
(S. Dzwigaj, M.J. Peltre, P. Massiani, A. Davidson, M. Che, T. Sen, S. Sivasanker, Chem. Commun. (1998)
87). The incorporation of isolated mononuclear V(V) into the framework of SiBEA is evidenced by XRD,
FTIR, diffuse reflectance UV–vis and NMR. It is found that, for low V content, V(V) ions are in pseudo-tet-
rahedral coordination only, either in nonhydroxylated (SiO)3V@O or hydroxylated (SiO)2(OH)V@O species
in framework position. For higher V content, additional species appear in extraframework position with
vanadium in octahedral coordination. FTIR investigations of pyridine adsorption show that strong
Brønsted and Lewis acidic centres are present in SiBEA leading to dimethyl ether only, in methanol oxi-
dation.
Received 7 December 2010
Revised 16 April 2011
Accepted 21 April 2011
Available online 23 May 2011
Keywords:
BEA
Zeolite
Vanadium
Methanol
Oxidation
Upon incorporation of vanadium into the BEA framework, Lewis acidic (V5+) and basic (O2ꢀ) centres are
generated with simultaneous appearance of partial oxidation products mainly, whose total concentration
increases with vanadium content. These results suggest that those centres are responsible for the oxida-
tion activity of VxSiBEA zeolites. The selectivity toward formaldehyde increases with the amount of
vanadium present as pseudo-tetrahedral hydroxylated (SiO)2(HO)V@O species. This selectivity is sug-
gested to be related to the moderate nucleophilicity of the basic vanadyl oxygen (V@O) of (SiO)2(HO)V@O
species.
Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction
aqueous NH4VO3 solution as precursor at low concentration and
pH = 2.5.
Crystalline microporous zeolites containing transition metal
ions well dispersed in the structure are important catalytic materi-
als in reactions, such as selective oxidation, hydroxylation and
selective catalytic reduction of NOx [1–3]. Their catalytic perfor-
mance strongly depends on the nature, local environment and, in
particular, content of transition metal ions. We recently reported
[4–9] that it is possible to control the content of vanadium in
BEA zeolite by using a two-step postsynthesis method that con-
sists, in the first step, in the creation of vacant T-atom sites with
associated silanol groups by dealumination of BEA zeolite with
nitric acid to form SiBEA zeolite and, in the second step, in the
incorporation of vanadium into the vacant T-atom sites using
Oxidative dehydrogenation of methanol leads mainly to forma-
tion of formaldehyde, a desirable intermediate in synthesis of dif-
ferent organic compounds. The first commercial production began
in the end of the 19th century in Germany using Cu catalyst, which
was later replaced by Ag catalyst [10]. The process was carried out
at high methanol concentration of above 40% at a temperature
from the range 823–1023 K. In the middle of the 20th century, a
procedure was developed in which a lower methanol concentra-
tion as well as lower temperature could be applied. This effect
was achieved with the iron molybdate catalyst. Industrial iron
molybdate catalysts applied for methanol oxidation have been
described earlier [11]. Nowadays, both catalytic systems leading
to formaldehyde are used. Formaldehyde and other products are
formed following mechanisms that depend on the catalyst type.
Methanol oxidation can be used as a test reaction to identify the
types of active site present on catalyst surfaces [12–21]. The reac-
tion network related to methanol oxidation [14] involves two main
⇑
Corresponding authors. Address: Laboratoire de Réactivité de Surface, UPMC
Univ Paris 6, 4 Place Jussieu, 75252 Paris Cedex 05, France (S. Dzwigaj).
E-mail addresses: tmaciej@amu.edu.pl (M. Trejda), stanislaw.dzwigaj@upmc.fr
(S. Dzwigaj).
0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.04.013

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M. Trejda et al. / Journal of Catalysis 281 (2011) 169–176
+ O
+ O
+ O
centrifugation, washed with distilled water and finally dried in
air overnight at 353 K.
CH3OH
HCHO
(HCOOH)
CO2 i) oxidation
- H2O
- H2O
The SiBEA solid was then contacted with an aqueous solution of
ammonium metavanadate (NH4VO3) in excess (2 g zeolite in 20 ml
of solution) and with a concentration varying from 0.25 ꢁ 10ꢀ2 to
9 ꢁ 10ꢀ2 mol Lꢀ1. Because of its low concentration at pH = 2.5,
the aqueous solution is expected to mainly contain monomeric
VOþ2 ions [22]. The suspension was left for 3 days at room temper-
ature without any stirring. The solids obtained were recovered by
centrifugation and dried at 353 K overnight. The samples were
labelled VxSiBEA with x = 0.3, 0.7, 2.05 and 4.7 V wt.%.
+ CH3OH
+ 2 CH3OH - 2 CH3OH + CH3OH
(CH3O)2CH2
HCOOCH3
+ H2O
CO
CH3OCH3
ii) dehydration
+ H2O
+ H2O
+ H2O
Fig. 1. Reaction network for methanol oxidation leading to various products (see
text), adapted from Ref. [9].
pathways: (i) oxidation requiring oxygen (indicated by O in Fig. 1)
either from the gas phase (dioxygen) or from the catalyst (mono-
nuclear oxygen) and (ii) dehydration that does not require oxygen.
With the exception of dimethyl ether (CH3OCH3) directly pro-
duced by bimolecular dehydration of methanol, all other carbon-
containing products (indicated in bold in Fig. 1) require at least
one oxidation step.
Tatibouet has reviewed the use of methanol oxidation to probe
the acido-basic character of the catalytically active sites [14].
While dimethyl ether selectivity is generally related to the acidic
character of the catalyst employed and to its dehydration ability,
the other products require catalyst with increasing basicity or
nucleophilic character in the order:
2.2. Techniques
Powder X-ray diffractograms (XRD) were recorded with a Sie-
mens D5000 apparatus using the Cu Ka radiation (k = 154.05 pm).
Infrared spectra were registered with a Bruker Vector 22 FTIR
spectrometer. Samples were pressed at ꢂ0.2 tons cmꢀ2 into thin
wafers of ca. 10 mg cmꢀ2 and placed inside the IR cell. Catalysts
were outgassed at 673 K for 3 h and then contacted with pyridine
(PY) at 423 K for 0.5 h. After saturation with PY, the samples were
outgassed at 423, 473, 523 and 573 K for 0.5 h at each temperature.
Spectra were recorded at room temperature in the range
4000–400 cmꢀ1. The spectrum of the IR cell without any sample
(‘‘background spectrum’’) was subtracted from all recorded spec-
tra. The IR spectra of the activated samples (after outgassing at
673 K) were subtracted from those recorded after adsorption of PY.
29Si NMR spectra of samples, transferred at ambient atmo-
sphere into 7 mm zirconia rotors, were recorded with a Bruker
Avance spectrometer at 99.4 MHz, some in cross-polarization
(CP) mode (29Si CP MAS NMR). The chemical shifts of silicon were
measured with reference to tetramethylsilane (TMS). 29Si MAS
CH2O 6 ðCH3OÞ2CH2 < HCOOCH3 < CO; CO2
On the basis of the product favoured, three main types of character
can be distinguished: highly acidic (dimethyl ether), highly basic
(carbon oxides) and bifunctional acid–base (mild oxidation prod-
ucts). From this picture, selectivity appears to be the key parameter
to evaluate the acid–base properties of the catalyst.
Practically, the products observed depend on the catalyst nat-
ure, reaction temperature and contact time (both influencing con-
version) as well as reactant partial pressure. For instance, an
increase in conversion favours consecutive reactions, leading at
high conversion predominantly to the formation of CO2, the most
thermodynamically favoured product.
NMR spectra were obtained at 5 kHz spinning speed, 2.5 ls excita-
tion pulse and 10 s recycle delay. 3-(Trimethylsilyl)-1-propanosul-
phonic sodium salt was used for setting the Hartmann–Hahn
condition. The proton
recycle delay were 6.8
p
/2 pulse duration, the contact time and
ls, 5 ms and 5 s, respectively.
1H MAS NMR spectra were recorded at 500 MHz with a 90°
Vanadium-based species exhibit significant activity in methanol
oxidation [12,14,16]. Moreover, the environment of active sites
strongly affects activity and selectivity [14,15]. The high selectivity
toward formaldehyde is observed when the nucleophilicity of oxy-
gen present in the neighbourhood of vanadium is moderate [15].
The increase in nucleophilicity in the surrounding of vanadium
species leads to the stronger chemisorption of formaldehyde and
the formation of methylal, which can be transformed into methyl
formate by further reaction with CH3OH. Further increase in nucle-
ophilicity causes the total oxidation of methanol.
This paper deals with the influence of vanadium content on the
catalytic properties of VxSiBEA zeolites in methanol oxidation as
evidenced by XRD, FTIR, 29Si MAS NMR, 1H–29Si CP MAS NMR, 1H
MAS NMR, DR UV–vis, 51V MAS NMR and catalysis data. To the best
of our knowledge, this is the first report on the oxidation of meth-
anol catalysed by vanadium incorporated into the framework of
BEA zeolite by the two-step postsynthesis method described above
and reported earlier [4].
pulse duration of 3 ls and a recycle delay of 5 s. To record only
the proton signal of the sample, the equipment for rotation
(12 kHz) was carefully cleaned with ethanol and dried in air at
room temperature. The proton signals from probe and rotor were
subtracted from the total free induction decay.
DR UV–vis spectra were recorded at ambient atmosphere on a
Cary 5000 Varian spectrometer equipped with a double integrator
with polytetrafluoroethylene as reference.
51V NMR spectra were recorded with a Bruker Avance 500 spec-
trometer at 131.6 MHz and with a 2.5-mm zirconia rotor spinning
at 35 kHz. The spectra were acquired with 0.5-s recycle delay and
pulse duration of 3.5 ls. Chemical shifts of vanadium were mea-
sured with reference to NH4VO3 (d = ꢀ570 ppm).
2.3. Methanol oxidation
The reaction was performed in a fixed-bed flow reactor on
0.04 g of pressed catalyst (particles with 0.5–1 mm diameter).
The sample was activated in flowing He (40 cm3 minꢀ1) at 673 K
for 2 h and then cooled to 523 K, the reaction temperature. A gas
mixture of MeOH and O2 (MeOH/O2 molar ratio = 2), diluted with
2. Experimental
2.1. Materials
He as carrier gas, was used with a total flow rate of 40 cm3 minꢀ1
.
The reaction products were analysed by gas chromatography (GC
8000 Top) equipped with flame ionization (FID) and thermal con-
ductivity (TCD) detectors. Reactants and products were separated
on a 60 m DB-1 column filled with dimethylpolysiloxane kept at
313 K.
A
tetraethylammonium BEA zeolite (TEABEA) (Si/Al = 11)
provided by RIPP (China) was treated with a 13 N HNO3 solution
(4 h, 353 K) under stirring to obtain dealuminated BEA
(Si/Al = 1000) noted thereafter SiBEA, which was separated by
a

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M. Trejda et al. / Journal of Catalysis 281 (2011) 169–176
171
The structure of the (SiO)2(HO)V@O species with the wavenum-
bers of its associated vibrators can now be refined (Fig. 3), on the
basis of the above results (SiO–H at 3620 and 3645 cmꢀ1) and ear-
3. Results and discussion
3.1. Incorporation of vanadium into the framework of dealuminated
BEA
lier data obtained by photoluminescence (V@O at 1018 cmꢀ1
,
referred to as species
a with structure A) [5], theoretical calcula-
tions [23] and IR and photoacoustic spectroscopies (SiAOV at 980
3.1.1. X-ray diffraction
and SiAOH at 950 cmꢀ1) [6].
Incorporating vanadium into SiBEA leads to an increase of the
d302 spacing (results not shown) from 3.912 (SiBEA) to 3.941 Å
(V2.05SiBEA), indicating some expansion of the BEA structure and
confirming incorporation of vanadium into the framework, in line
with earlier conclusions for VSiBEA zeolites [4–7].
The absence of diffraction lines due to extraframework com-
pounds in VxSiBEA indicates a good dispersion of vanadium. It is
important to note that the surface area (655 m2 gꢀ1) and the micro-
pore volume (0.24 cm3 gꢀ1) of SiBEA do not change after incorpora-
tion of vanadium, suggesting that the latter does not sensibly affect
the zeolite porosity.
The acidic sites in SiBEA and VxSiBEA were probed by means of
pyridine adsorption (see Section 2), and Fig. 4 gives the corre-
sponding IR spectra.
For SiBEA, two bands typical of pyridinium cations are observed
at 1545 and 1638 cmꢀ1, indicating the presence of Brønsted acidic
centres (most likely related to the acidic proton of Al–O(H)–Si
groups, present as traces after the dealumination step). The bands
at 1454 and 1622 cmꢀ1 correspond to pyridine interacting with
strong Lewis acidic centres (Al3+
) and those at 1445 and
1596 cmꢀ1 (Fig. 4) to pyridine interacting with weak Lewis acidic
centres (hydroxyls) and/or pyridine physisorbed, in line with earlier
data for BEA [6].
3.1.2. FT-IR
The introduction of vanadium leading to VxSiBEA generates
bands at 1449 and 1608 cmꢀ1 corresponding to pyridine adsorbed
on Lewis acidic centres (V5+). The bands at 1545 and 1638 cmꢀ1
observed for both VxSiBEA and SiBEA indicate a similar strength
of Brønsted acidic centres. In contrast, for VxSiBEA, new Lewis
acidic centres exhibit a lower strength than that of centres present
on SiBEA, as shown by the position of the bands (1608 vs.
1622 cmꢀ1). These bands disappear upon outgassing at increasing
temperature (Table 1) as discussed below. As reported earlier for
BEA zeolite [6], the Brønsted acidic centres evidenced in VxSiBEA
are related to the acidic proton of OH group of framework hydrox-
ylated (SiO)2(HO)V@O species as deduced from their disappear-
ance upon adsorption of pyridine (results not shown). However,
as reported earlier for BEA and sodalite systems, only a small part
of all framework tetrahedral V(V) ions appears as such species [23].
The number of acidic centres calculated from the amount of
pyridine outgassed at 473 K for 0.5 h is given in Table 1. One can
notice that the number of Lewis acidic centres increases with the
vanadium content (0.3, 0.7 and 2.05 wt.%) with some saturation
for 4.7 wt.% (V4.7SiBEA), suggesting that not all vanadium ions are
accessible for pyridine, as in the case of vanadium oxide clusters.
The number of Brønsted acidic centres increases with vanadium
content to reach a maximum for 2.05 wt.%, before decreasing for
As shown earlier [4,6], the FT-IR spectrum of TEABEA zeolite cal-
cined at 823 K for 15 h exhibits five IR bands due to the OH stretch-
ing modes of AlOH groups at 3781 and 3665 cmꢀ1, SiAO(H)AAl
groups at 3609 cmꢀ1, isolated SiOH groups at 3740 cmꢀ1 and
H-bonded SiOH groups at 3520 cmꢀ1
.
The treatment of TEABEA zeolite by aqueous HNO3 solution
leads to the removal of framework Al atoms, as evidenced by the
disappearance of bands at 3781 and 3665 (AlOH groups) and
3609 cmꢀ1 (SiAO(H)AAl groups), as shown earlier for VSiBEA zeo-
lite [4–6]. The appearance of narrow bands at 3736 and 3710 cmꢀ1
related to isolated internal and terminal silanol groups and of a
broad band at 3520 cmꢀ1 due to H-bonded SiOH groups in SiBEA
reveals the presence of vacant T-atom sites associated with silanol
groups, in line with earlier data for VSiBEA zeolite [4–6].
The impregnation of SiBEA by aqueous NH4VO3 solution re-
duces the intensity of the latter bands, particularly that at
3520 cmꢀ1 due to H-bonded SiOH groups (Fig. 2), suggesting that
silanol groups react with the vanadium precursor to form stable
nonhydroxylated framework pseudo-tetrahedral (SiO)3V@O spe-
cies, in line with earlier work on BEA zeolite [5,23]. Simulta-
neously, two FTIR bands at 3645 and 3620 cmꢀ1 appear which
are assigned to the hydroxyl vibration of (SiO)2(HO)V@O species
located at two different crystallographic sites, in line with earlier
data for VSiBEA zeolite [7,23]. Those V species are in much lower
amount than nonhydroxylated (SiO)3V@O species, confirming ear-
lier data on VSiBEA zeolite [5,6,23].
V4.7SiBEA.
Table 1 also gives the total number of acidic centres determined
from the amount of pyridine remaining adsorbed after outgassing
the samples at 473, 523 and 573 K, making it possible to estimate
the acid strength. There is a large difference in the acid strength
between SiBEA and VxSiBEA. For SiBEA, outgassing at 573 K leads
to a decrease of ca. 20% of acidic centres compared to the situation
after outgassing at 473 K, whereas for V0.3SiBEA, this decrease is ca.
85%. Thus, the acid strength of SiBEA appears to decrease after a
0.1
3645
3736
3620
3520
SiBEA
V0.7SiBEA
950
Si
O
H
O
V2.05SiBEA
V4.7SiBEA
3645, 3620
H
1018
O
O
V
O
980
4000
3800
3600
3400
3200
3000
Si
Si
980
Wavenumber (cm-1)
Fig. 2. FTIR spectra recorded at room temperature of SiBEA, V0.7SiBEA, V2.05SiBEA
and V4.7SiBEA outgassed at 673 K for 3 h (10ꢀ3 Pa) in the vibrational range of OH
groups.
Fig. 3. Schematic representation of the hydroxylated (SiO)2(HO)V@O species
hydrogen bonded to a neighbouring SiOH group, with the wavenumbers (cmꢀ1
of the associated vibrators.
)

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M. Trejda et al. / Journal of Catalysis 281 (2011) 169–176
1449
-111.4
-114.5
1608
1622
0.1
1545
1445
1596
-103.0
-103.8
1490
1638
V
2.05SiBEA
1576
-111.0
-113.6
V
4.7SiBEA
2.05SiBEA
V
-102.0
-102.0
V0.7SiBEA
V0.7SiBEA
-111.2
-112.3
-114.5
V0.3SiBEA
SiBEA
1454
-103.8
-102.5
V0.3SiBEA
1700
1650
1600
1550
1500
1450
1400
-111.1
-114.2
Wavenumber (cm-1)
Fig. 4. FTIR spectra recorded at room temperature after adsorption of pyridine at
423 K followed by desorption at 473 K for 0.5 h of SiBEA, V0.3SiBEA, V0.7SiBEA,
SiBEA
V
2.05SiBEA and V4.7SiBEA.
-80
-90
-1
-110
-120
-130
δSi (ppm)
Table 1
Fig. 5. 29Si MAS NMR spectra recorded at room temperature of SiBEA, V0.3SiBEA,
Number of acidic centres of VxSiBEA zeolites determined from the amount of pyridine
remaining adsorbed after outgassing the samples at 473 K (number of Lewis or
Brønsted acidic centres), and 473, 523 and 573 K (total number of acidic centres).
V
0.7SiBEA and V2.05SiBEA as prepared in 7 mm (external diameter) zirconia rotors.
Sample
Lewis
acidic
centres
Brønsted
acidic
Total number of acidic centres
(ꢁ1017
)
-111.0
centres
Pyridine Desorption Temperature
(ꢁ1017
)
(ꢁ1017
)
473 K
523 K
573 K
-114.5
-102.0
SiBEA
17.8
15.0
71.7
7.4
2.1
3.8
6.8
5.4
25.2
17.1
75.5
167.0
182.3
22.4
7.0
62.2
115.6
100.3
20.1
2.8
30
57.9
34.6
V
V
V
V
0.3SiBEA
0.7SiBEA
2.05SiBEA 160.2
4.7SiBEA 176.9
V2.05SiBEA
-102.0
-103.8
-110.9
-113.6
-91.8
-102.0
0.7SiBEA
V
-103.8
small amount of vanadium is incorporated (compare spectra a and
b of Fig. 4). These results evidence the role of the surroundings of
acidic centres on their strength.
-111.0
-114.5
-92.0
-102.5
V0.3SiBEA
-111.1
-114.2
3.1.3. 29Si MAS NMR and 1HA29Si CP MAS NMR
The 29Si MAS NMR spectrum of SiBEA (Fig. 5) shows three peaks
at ꢀ102.5, ꢀ111.1 and ꢀ114.2 ppm. The latter two peaks are due to
framework Si atoms in a Si(OSi)4 environment, located at different
crystallographic sites, in line with earlier data for BEA system [8].
The broad peak at ꢀ102.5 ppm corresponds to Si atoms in a Si
(OH)(OSi)3 environment, as reported earlier for BEA zeolite [24],
in line with the removal of nearly all Al atoms upon dealumination,
leading to a ratio Si/Al = 1000.
-92.2
SiBEA
-80
-90
-100
-110
-120
-130
δSi (ppm)
Fig. 6. 1HA29Si CP MAS NMR spectra recorded at room temperature of SiBEA,
0.3SiBEA, V0.7SiBEA and V2.05SiBEA as prepared in 7 mm (external diameter)
V
This is confirmed by the increased intensity of the ꢀ102.5 ppm
peak in the NMR spectrum in CP mode (Fig. 6), which enhances the
signal of 29Si nuclei close to protons, as in the case of the Si(OH)
(OSi)3 species. Moreover, a small amount of Si atoms in a Si
(OH)2(OSi)2 environment is evidenced by the very weak peak at
zirconia rotors.
corresponding to Si atoms in the Si(OH)(OSi)3 environment is smal-
ler for V2.05SiBEA than that observed at ꢀ102.5 ppm for SiBEA
(Fig. 6). Moreover, the doublet observed at around ꢀ(102.0–
103.8) ppm for V0.3SiBEA and V0.7SiBEA suggests two types of sur-
roundings of Si atoms in Si(OH)(OSi)3 species.
about ꢀ92.2 ppm. The peak at ꢀ111.1, with
a shoulder at
ꢀ114.2 ppm, appears to be due to framework Si atoms in a Si(OSi)4
environment, located at different crystallographic sites, in line
with earlier data for BEA zeolite [8].
After incorporation of increasing amount of V into SiBEA, the
intensity of the peak at around ꢀ(102.0–103.8) ppm is reduced
(Fig. 5), confirming the reaction between NH4VO3 and SiOH groups.
It is important to note that up to four H-bonded SiOH groups may be
consumed by each V ion incorporated into a vacant T-atom site. This
is confirmed by the CP spectra of V0.3SiBEA, V0.7SiBEA and V2.05SiBEA
(Fig. 6), which show that the peak at about ꢀ(102.0–103.8) ppm
3.1.4. 1H MAS NMR
In the 1H MAS NMR spectrum of SiBEA (Fig. 7), two main peaks
are observed at 1.3 and 5.4 ppm due to isolated SiOH protons and
H-bonded SiOH groups present at vacant T-atom sites, respec-
tively, as shown earlier for different zeolites [25–29].
In addition, the small peak at 3.2–2.8 ppm is probably due to
protons of H-bonded SiOH groups located in a second type of