Nb and Zr modified MWW zeolites-characterisation and catalytic activity

Article abstract of DOI:10.1039/c5ra00411j

Hydrogen forms of MCM-22 and delaminated MCM-56 zeolites were modified with niobium and zirconium species. The structure/texture and surface properties, as well as the catalytic activity for dibutyl sulfide oxidation, of the two types of modified zeolites were compared. The presence of Br?nsted (BAS) and Lewis (LAS) acid sites was detected after pyridine adsorption in all the prepared materials. The highest number of BAS and LAS was observed for HMCM-22. The number of BAS significantly decreased after metal modification because of the interaction between the metal sources and the BAS. External silanol groups of zeolites also participate in metal anchoring. The catalysts obtained were tested for liquid phase dibutyl sulfide oxidation with hydrogen peroxide. The texture of the zeolites and the nature of the metal modifier have a crucial effect on the oxidation of dibutyl sulfide. MCM-56 zeolites are more active than their counterparts of MCM-22 type. Application of niobium containing zeolites significantly increases the reaction rate, leading to 95% conversion after 3 h. The remarkable role of the Nb species is due to its interaction with H₂O₂ towards peroxo/superoxo species. Modification of both zeolites with zirconium does not increase their activity because of the strong interaction of zirconium species with sulfur compounds. This journal is

Full text of DOI:10.1039/c5ra00411j

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Nb and Zr modified MWW zeolites –
characterisation and catalytic activity
Cite this: RSC Adv., 2015, 5, 22326
Anna Wojtaszek-Gurdak* and Maria Ziolek
Hydrogen forms of MCM-22 and delaminated MCM-56 zeolites were modified with niobium and zirconium
species. The structure/texture and surface properties, as well as the catalytic activity for dibutyl sulfide
oxidation, of the two types of modified zeolites were compared. The presence of Brønsted (BAS) and
Lewis (LAS) acid sites was detected after pyridine adsorption in all the prepared materials. The highest
number of BAS and LAS was observed for HMCM-22. The number of BAS significantly decreased after
metal modification because of the interaction between the metal sources and the BAS. External silanol
groups of zeolites also participate in metal anchoring. The catalysts obtained were tested for liquid phase
dibutyl sulfide oxidation with hydrogen peroxide. The texture of the zeolites and the nature of the metal
modifier have a crucial effect on the oxidation of dibutyl sulfide. MCM-56 zeolites are more active than
their counterparts of MCM-22 type. Application of niobium containing zeolites significantly increases the
reaction rate, leading to 95% conversion after 3 h. The remarkable role of the Nb species is due to its
Received 8th January 2015
Accepted 12th February 2015
2 2
interaction with H O towards peroxo/superoxo species. Modification of both zeolites with zirconium
DOI: 10.1039/c5ra00411j
does not increase their activity because of the strong interaction of zirconium species with sulfur
compounds.
www.rsc.org/advances
MCM-22 and delaminated MCM-56 are members of the
MWW zeolites family. The differences in the structure of these
Introduction
Zeolites represent an important group of inorganic crystalline zeolites signicantly affect their properties. Their unique crystal
microporous materials used in numerous large-scale chemical structure is formed by two independent 10-ring (10 MR) pore
3,21
technologies as heterogeneous catalysts thanks to their adjust- systems. The rst one is sinusoidal and is localized inside the
able acidity, high-temperature stability, and shape-selectivity zeolite layers. The second one is formed upon fusion of the
1
properties. Conventional zeolites are characterized by unin- adjacent layers. Moreover, there are 12-ring cups or pockets on
21,22
terrupted 3D framework structures. The new group of zeolites of the surface layers.
When the zeolite 3-D structure is formed,
MWW topology exhibits a layered 2D character. The most two pockets form a supercage (ca. 1.8 nm ꢀ 0.7 nm) inside the
2–6
7
known zeolites in this group are MCM-22
and MCM-56.
zeolite crystal. Such a space can be sufficient to host even
MCM-22 exhibits a structure similar to that found earlier for bulky organic molecules. The disordering of zeolite layers, as in
8
9
10
ERB-1, SSZ-25 or PSH-3. The layered zeolites are so-called MCM-56 prepared by a one-pot synthesis procedure, can also
two-dimensional (2D) zeolites. The characteristic feature of enhance the accessibility to 12-ring pockets. The main differ-
these zeolites is that the layers could be exfoliated before their ence between MCM-22 and MCM-56 zeolites is the connection
fusion into a 3D framework. The 2D structure is associated with of layers that make up their structure. MCM-22 is formed of
ordered zeolite frameworks formed through lamellar precur- layers that are stacked in an ordered manner while MCM-56 is
11
sors. The preparation method can based on the swelling of made of very thin MWW monolayers and small crystals that are
21
MWW layered zeolite precursor followed by a number of disordered. MCM-56 shares the same primary structure with
different possible treatments. Depending on the way applied for MCM-22; however, it has a relatively low amount of supercages,
transformation of swollen precursor one can obtain different but more 12-MR surface cups and acid sites exposed to the
1
2,13
14
zeolites like ITQ-2
papers
and MCM-36. There are elegant review crystal exterior, and an easier accessibility to large organic
1
5–20
23
describing the history, methods of synthesis and molecules. Both zeolites MCM-22 and delaminated MCM-56
properties of MWW zeolites. The interest in such zeolites is still can be synthesized in a similar way, with the use of the same
growing.
structure directing agent, which is hexamethyleneimine. The
differences in synthesis are the Si/Al ratio and the crystallization
time. MCM-56 contains more Al in comparison to MCM-22
zeolite. The nal product can be obtained aer two days of
Adam Mickiewicz University in Pozna n´ , Faculty of Chemistry, Umultowska 89b, 61-614
Pozna n´ , Poland. E-mail: anna.gurdak@amu.edu.pl; Tel: +48 618291305
21,24
synthesis for MCM-56 (as opposed to ten days for MCM-22).
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Approximately 50% of BAS in MCM-56 can interact with a periodic table. Some of them, like stability or strong metal
25
large base pivalonitrile compared to <30% for MCM-22. This support interaction (SMSI) and strong metal oxide support
behavior of MCM-56 seems to be very important for the catalytic interaction (SMOSI), are very important for a good quality
38–40
transformation of bulky molecules.
catalyst.
The characteristic features of niobium compounds
MWW zeolites are important in material chemistry because are not only to act as sources of Lewis and Brønsted acidity
38
of the possibility of diverse structural, textural and composi- but also the promoter effect and the support effect. Niobium
tional modications. The use of intercalated inorganic and/or oxides remarkably enhance catalytic activity and prolong cata-
organic guests allows the design and formation of new cata- lyst life when they are added in small amounts to known cata-
26
lyst with unique properties. Modication with metals gives lysts. Niobium compounds have recently been widely studied
an opportunity to obtain bifunctional (acid-redox) catalysts. Wu as catalysts for various reactions. In the last few years they have
et al. have for the rst time synthesized the crystalline MWW been tested e.g. in the selective oxidation of hydrocarbons,
precursor containing the individual zeolitic sheets simulta- oxidative dehydrogenation of propane, oxidation of alcohols
41
neously with both tetrahedrally coordinated boron and tita- and oxidation of organic suldes. In the latter reaction,
27
nium. Ruan et al. speculated that Ti centres should occupy the niobium species can play the role of an agent promoting the
pillars sites, which have a higher steric accessibility than interaction of hydrogen peroxide with acidic hydroxyls towards
28
33
network position in conventional 3D zeolites. Different layered active hydrosuperoxo species transformed to OH radicals.
precursors were prepared containing tetrahedrally coordinated Therefore, MWW types of zeolites containing Brønsted acidity
metals incorporated into the framework of individual MWW were chosen as supports for niobium and zirconium species.
29
30
31
42
layers, such as gallium, vanadium, and lanthanum. Recent
In our previous paper we compared the textural and cata-
reports in the literature indicate the new possibility of gener- lytic properties of MCM-22 and delaminated MCM-56 zeolites
ating catalytic properties of MWW zeolites by stabilization as an modied with molybdenum. This work is to some extent a
interlayer expanded structure MWW-IEZ and modication with continuation of the studies on MWW zeolites modied with
cerium. The obtained catalysts present high activity for the other transition metals that are located in the same row of the
32
oxidation of CO to CO
2
at room temperature.
periodic table (Nb and Zr). One can expect generation of
In this work, we have prepared hydrogen forms of MCM-22 different properties via modication by niobium and zirconium
and delaminated MCM-56 zeolites to be later modied with because of the various electronic states of both metals leading
niobium or zirconium. The objective of our study was to to various cationic charges and different interactions with
comparatively examine MCM-22 and MCM-56 zeolites modied hydrogen peroxide and sulfur containing compounds.
with Nb and Zr species. The structure/texture and surface
properties as well as the catalytic activity for dibutyl sulde
oxidation with hydrogen peroxide were compared for the two
types of zeolites modied with different metals (Nb and Zr). The
Experimental
Zeolite preparation
choice of niobium and zirconium as modiers of zeolites was The MCM-22 and MCM-56 zeolites were synthesized using the
dictated by the presence of both elements as neighbours in the same reagents: SiO (Ultrasil 3VN – Degussa), sodium aluminate
2
same row of the periodic table but the different oxidation states (53% of Al O and 42.5% of Na O, Riedel-de Haen), distilled
2
3
2
of both metals are expected to generate different surface prop- water, sodium hydroxide (Chempur), and hexamethyleneimine
erties of the nally obtained catalysts based on both mentioned (HMI – Aldrich) as the template.
zeolites. One can expect different interactions of the Zr and Nb
species with the reactants in the oxidation of dibutyl sulde. At dure described in ref. 3. The synthesis gel had the following
least one of the reactants has to be chemisorbed on the catalyst composition: SiO /Al ¼ 0.18, H O/SiO
¼ 54, OH/SiO
surface. Considering the reaction pathways and based on recent 44.9, Na/SiO
¼ 0.18, HMI/SiO
The MCM-22 zeolite was prepared according to the proce-
2
2
O
3
2
2
2
¼
2
2
¼ 0.35. The synthesis mixture
33,34
papers
it can be expected favorable chemisorption of H O2 was stirred for 30 min at RT and then loaded into a Teon-lined
2
3
on niobium species playing the role of active sites (the forma- Parr reactor (300 cm ). Hydrothermal synthesis was carried out
tion of peroxo species) or interaction with Brønsted acid sites at 443 K for 40 h with continuous stirring. The product, MCM-22
towards the formation of active intermediates (hydrosuperoxo precursor (MCM-22P), was recovered by ltration, washed with
species). In turn, zirconium species are expected to interact distilled water, and dried at 383 K.
35
preferentially with dibutyl sulde. The use of these two
different modiers (Zr and Nb) allows us to draw a picture of the proposed by Fung et al. with the synthesis gel composition:
most favorable reaction pathway (via chemisorption of dibutyl SiO /Al ¼ 0.21, H O/SiO ¼ 20, Na/SiO ¼ 0.21,
¼ 23, OH/SiO
sulde or H O ).
HMI/SiO2 ¼ 0.35. The resulting synthesis gel was stirred for
Zirconium containing zeolites are characterized by good 30 min at RT. Hydrothermal synthesis was carried out in a
The MCM-56 was synthesized according to the method
7
2
O
2 3
2
2
2
2
2
2
36
stability and easy regeneration through calcination. They have Teon-lined Parr reactor at 416 K for 48 h upon continuous
widespread application in a wide range of reactions, especially stirring. The resulting catalyst (MCM-56) was ltered off, washed
in reactions requiring moderate acidity and oxidizing capabil- with distilled water and dried at 383 K.
ities like hydrogenation, hydroxylation, oxidation or epoxida-
To obtain the hydrogen forms of the MCM-22 and MCM-56
tion. Niobium compounds exhibit special properties not zeolites, the catalysts were modied by a twice-repeated ion
shown by the compounds of neighbouring elements in the exchange procedure at 323 K for 3 h using 0.5 M NH NO
37
4
3
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solution. Aer ion exchange, the catalysts were dried at 373 K treatments. The reported spectra are the results of this
ꢁ2
for 12 h and calcined at 823 K for 6 h.
subtraction and are normalized to 10 mg cm .
Modication of zeolites with niobium and zirconium
Cyclisation and dehydration of 2,5-hexanedione (2,5-HDN)
Nb and Zr containing catalysts were prepared by an impregna-
tion method using an aqueous solution of ammonium nioba-
te(V) oxalate hydrate (Sigma-Aldrich) for Nb/HMCM-22 and Nb/
HMCM-56 (5 wt% of Nb) and zirconium(IV) oxynitrate hydrate
The catalysts were tested for 2,5-hexanedione (2,5-HDN) dehy-
dration and cyclisation as the test reactions. A tubular, down-

ow reactor (Ø ¼ 8 mm; length l ¼ 80 mm) was used for the
2,5-HDN dehydration and cyclisation reaction that was carried
(
Sigma-Aldrich) for Zr/HMCM-22 and Zr/HMCM-56 (5 wt% of
out at atmospheric pressure, using nitrogen as the carrier gas.
The catalyst powder was pressed, crushed and sieved to the
fraction of 0.5 < Ø < 1.0 mm. A total of 0.05 g of this fraction was
put into the reactor. The catalyst was rst activated for 2 h at
Zr). HMCM-22 or HMCM-56 was placed in a ask connected to a
rotary vacuum evaporator (BUCHI Rotavopar RII) and evapo-
rated for 1 h. Next, a solution of niobium or zirconium source
was added and the wet material was rotated (100 rpm) at 353 K
for 10 h. At 353 K the evaporation started. Aer evaporation,
the material was dried in an oven at 383 K for 8 h (heating rate
ꢁ1
6
23 K (heating rate 15 K min ) under nitrogen ow
3
ꢁ1
3
(40 cm min ). Subsequently, a 0.5 cm of HDN (Fluka, GC
grade) was passed continuously into the catalyst at 623 K. The
substrate was delivered with a pump system (KD Scientic) and
vaporised before being passed through the catalyst with the ow
ꢁ
1
5
5
K min ) and then calcined at 773 K for 4 h (heating rate
ꢁ
1
K min ).
3
ꢁ1
of nitrogen carrier gas (40 cm min ). The reaction products
were collected for 30 min downstream of the reactor in a cold
trap (liquid nitrogen and 2-propanol) and analyzed by gas
chromatography (SRI 310 C, MXT-1 column 30 m, temperature
of column 373 K) with a TCD detector. Helium was applied as a
carrier gas.
Characterization methods
The materials prepared were characterized using XRD, N
adsorption/desorption, UV-vis, and XPS.
2
XRD measurements were carried out on a Bruker AXS D8
Advance diffractometer with Cu Ka radiation (l ¼ 0.154 nm)
ꢂ
ꢂ
with a step size of 0.05 in the wide-angle range (6–60 ).
N
adsorption/desorption isotherms were obtained in a Dibutyl sulde oxidation with H O
2
2 2
Quantachrome Instruments Autosorb iQ2 at 77 K. Samples were
pretreated in situ under vacuum at 573 K. The surface area was
calculated by the BET method. The volume of mesopores was
determined from BJH, whereas the micropore surface area and
micropore volume were determined by the t-plot method.
X-ray photoelectron spectroscopy (XPS) measurements
were performed with a Specs photoelectron spectrometer
equipped with a monochromatic microfocused Al Ka X-ray
source (1486.7 eV).
UV-vis spectra were recorded on a Varian-Cary 300 Scan UV-
visible spectrophotometer with an integrated sphere CA-30I.
The catalysts, dehydrated at 673 K for 2 h, in the form of
powders were placed into a cell equipped with a quartz window.
The Kubelka–Munk function (F(R)) was used to convert the
reectance measurements into equivalent absorption spectra
using the reectance of SPECTRALON as a reference.
The catalytic reaction between dibutyl sulde and hydrogen
peroxide was carried out in a glass ask equipped with a water
jacket (connected to the cryostat – Labortechnik Medingen
K21), magnetic stirrer, a reux condenser and a membrane for
sampling. Before the reaction, 0.04 g of calcined catalyst was
3
placed into the ask, where methanol (10 cm ) was added. Aer
stirring for 1 h, 96% dibutyl sulde (2 mmol – Aldrich) was
added, followed by the dropwise addition of 35% hydrogen
peroxide (2 mmol – Aldrich). The oxidation was conducted by
stirring at 303 K for 8 h. The reaction mixture was analysed each
1
h using a gas chromatograph (Varian CP 3800) equipped with
a 60 m VF-5ms capillary column, working in the temperature
ꢁ1
range of 333–523 K (temperature ramp 10 K min ), and a FID
detector.
Results and discussion
Acidity measurements
Texture/structure characterization
Infrared spectra were recorded with a Bruker Vector 22 FTIR Fig. 1 presents the X-ray patterns of the HMCM-22 and HMCM-
spectrometer using an in situ cell. The samples were pressed 56 materials. All synthesized zeolites show well-dened crystal
ꢁ
2
under low pressure into thin wafers of ca. 10 mg cm and structures. The appearance of XRD peaks characteristic of the
placed inside the vacuum cell. Before pyridine adsorption, the materials with MWW structures indicates the successful
catalysts were outgassed at 673 K for 2 h and pyridine was then synthesis of the MCM-22 and MCM-56 samples. Impregnation
admitted at 473 K. Aer saturation with pyridine, the samples of both layered and delaminated MWW zeolites with Nb or Zr
were degassed at 473 K for 5 min. The catalysts were outgassed does not change the zeolites' structure (XRD patterns not shown
at different temperatures aer pyridine adsorption. The spec- here). No XRD reections from crystalline metal oxide phases
trum without any sample (“background spectrum”) was sub- were observed in the XRD patterns. The absence of the peaks
tracted from all recorded spectra. The spectra of the activated from metal oxides can be caused by good dispersion of metal
samples (aer outgassing at 673 K) were subtracted from those oxide, or if the Nb and Zr are present not only in the form of
recorded aer the adsorption of pyridine followed by various crystalline oxides in the samples prepared.
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Fig. 1 XRD patterns of HMCM-22 and HMCM-56.
Fig. 2 UV-vis spectra of Nb/HMCM-22, Zr/HMCM-22, Nb/HMCM-56,
and Zr/HMCM-56.
The materials prepared show surface areas and pore volumes
(Table 1) typical of HMCM-22. In the case of the HMCM-56
zeolites some degradation or pore blockage seem to occur if
compared with the typical MCM-56 materials reported in the
literature. Impregnation of MWW zeolites by metals (Nb or Zr)
leads to changes in the textural parameters. In all cases, the BET
surface areas of HMCM-22 and delaminated HMCM-56
decrease aer metal introduction. This fact is caused by a
decrease in the external surface area, which could be a conse-
quence of the Nb and Zr being located partially outside the
zeolite pores.
collated in Table 2. The XPS study indicated binding energies
(BE) typical of niobium +5 and zirconium +4. The binding
energies of Nb 3d and Zr 3d located in the zeolites are shied to
higher values than in the case of bulk niobium and zirconium
oxides. These results are noted for all catalysts, both MCM-22
and delaminated MCM-56. Higher values of BE can suggest
different surroundings of niobium and zirconium in zeolites
than those in bulk oxides.
Acidity measurements
The state of metals
The surface properties of the prepared zeolites were investi-
gated by infrared spectroscopy combined with the adsorption
of pyridine. According to the literature, when pyridine is
To examine the state of niobium and zirconium in the zeolites
synthesized in detail, XPS and UV-vis spectroscopy were used.
The UV-vis spectra of niobium containing zeolites (Fig. 2)
are characterized by bands at ca. 220 and 250 nm, which
are typical of tetrahedrally coordinated niobium(V) species
43,44
Table 2 Binding energies from XPS study
in different surroundings.
niobium species gives a characteristic adsorption band at ca.
30 nm. This band is not observed in the spectra of both the
The octahedrally coordinated
BE [eV]
3
5+
4+
Nb/HMCM-22 and the delaminated Nb/HMCM-56 zeolites.
Nb
Zr
The UV-vis spectra of the zirconium modied zeolites
Fig. 2), i.e. Zr/HMCM-22 and delaminated Zr/HMCM-56, show
Catalyst
3d5/2
3d3/2
3d5/2
3d3/2
(
one band at 207 nm. According to the literature, this band can Nb O5
207.10
—
208.60
—
—
—
—
—
211.40
—
—
—
—
—
184.07
182.50
—
—
—
—
186.40
—
2
be assigned to the ligand to metal charge transfer (LMCT) from HMCM-22
2
ꢁ
4+
45,46
Nb/HMCM-22
Zr/HMCM-22
O
to an isolated Zr ion in a tetrahedral coordination.
For further investigation of the niobium and zirconium
states and localization in the prepared materials, XPS spec-
ZrO
HMCM-56
2
—
—
troscopy was applied. The results obtained for all samples, as Nb/HMCM-56
well as for niobium(V) and zirconium(IV) bulk oxides, are Zr/HMCM-56
207.60
—
210.30
—
—
182.78
—
185.17
Table 1 BET surface area and pore volume of zeolites obtained
2
ꢁ1
2
ꢁ1
2
ꢁ1
3
ꢁ1
3
ꢁ1
Catalyst
S
total (m g
)
S
micro (m g
)
S
exter (m g
)
V
total (cm g
)
V
micro (cm g
)
HMCM-22
461
423
419
212
195
183
341
336
317
119
119
122
120
87
101
92
76
60
0.38
0.27
0.39
0.84
1.14
1.22
0.18
Nb/HMCM-22
Zr/HMCM-22
HMCM-56
Nb/HMCM-56
Zr/HMCM-56
0.18
0.17
0.06
0.06
0.06
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ꢁ
1
In consequence, the calculated numbers of BAS (1544 cm
)
ꢁ1
and LAS (1453 cm ) are higher for HMCM-22 and its metal
modied materials (Table 3). The introduction of niobium and
zirconium decreases the number of BAS and LAS. The drop in
the LAS and BAS numbers (aer evacuation at 573 K) oscillates
between 22 and 35% independently of the zeolite type. The
exception is Nb/HMCM-56 in which only a 6% drop in the LAS
(aer evacuation at 573 K) is observed aer Nb loading. It
suggests the higher strength of the LAS in this material. A
similar impact of metal (Mo) introduction on HMCM-22 acidity
50
had been observed before by Huang et al., and also in our
42
previous study with molybdenum.
Fig. 4 presents the FTIR spectra in the hydroxyl region for
HMCM-22 as an example of changes in this region aer pyri-
Fig. 3 FTIR spectra after pyridine adsorption at 473 K on: (a) HMCM-
51
dine adsorption. According to the literature, the band at
5
6; (b) Zr/HMCM-56; (c) Nb/HMCM-56; (d) HMCM-22; (e) Zr/HMCM-
2; and (f) Nb/HMCM-22. Spectra are normalized to 10 mg of zeolite.
ꢁ
1
3
620 cm corresponds to bridged hydroxyl groups (Al–OH–Si),
2
ꢁ
1
whereas the band at 3670 cm is oen ascribed to hydroxyl
groups linked to extra-framework aluminium species (gener-
ꢁ
1
ated upon calcination); and the bands at ꢃ3730 and 3748 cm
are assigned to internal and external silanol groups, respec-
coordinated to Lewis acid sites (LAS), characteristic bands at
ꢁ1
ꢁ1
47
ꢃ1450 cm and ꢃ1610 cm appear. The intensity of the rst
one is related to the number of LAS, whereas the position of
the second band characterizes the strength of the LAS.
Adsorption of pyridine on Brønsted acid sites (BAS) gives a band
ꢁ
1
tively. Two IR bands dominate in all spectra, 3621 cm (BAS)
ꢁ
1
and 3748 cm
(originated from external silanol groups).
ꢁ1
Moreover, the 3728 cm band coming from vibrations in the
ꢁ1
ꢁ
1
ꢁ1
internal silanol groups as well as the bands at 3671 cm
at ꢃ1550 cm and two other ones in the 1620–1640 cm range
48
assigned to hydroxyls linked to the extra-framework aluminium
species are present in the spectra presented in Fig. 4. Intro-
duction of metals (Nb or Zr) reduces the intensity of the
3620 cm and 3748 cm bands in the spectra of Nb/HMCM-22
or Zr/HMCM-22. These results indicate the interaction of metals
species with BAS and external silanol groups. Interestingly,
internal silanol groups and hydroxyls linked to extra-framework
aluminium species are not involved in the modication of
HMCM-22 with metal species.
For the evaluation of the acid/base properties of the
synthesized zeolites in reaction conditions, dehydration and
cyclisation of 2,5-hexanedione was carried out. The main
products of this reaction are: 2,5-dimethylfuran (DMF) formed
in the presence of acidic centers and 3-methyl-2-cyclopentenone
coming from the vibration of pyridinium cations. Moreover,
pyridine can also interact with hydroxyls on the zeolite surface
giving adsorption bands at 1596 cm and 1445 cm assigned
to hydrogen bonded species. The IR band at ca. 1490 cm is
assigned to both pyridine coordinated to LAS and pyridinium
cations.
ꢁ
1
ꢁ1
ꢁ
1
ꢁ1
ꢁ
1
The spectra (normalized to 10 mg of catalyst) aer pyridine
adsorption at 473 K and outgassing at 573 K for 0.5 h are pre-
sented in Fig. 3. All prepared catalysts show acidic character.
The characteristic bands from pyridine adsorbed on Lewis
ꢁ
1
ꢁ1
(
1610, 1453 cm ) and Brønsted (1544 and 1635 cm ) acidic
sites are well visible. The main difference between the layered
and the delaminated materials is the intensity of these bands.
All HMCM-22 and metal-modied zeolites exhibit more intense
bands than the delaminated HMCM-56 and metal modied
materials.
52,53
(MCP) formed in the presence of basic centers.
The ratio of
Table 3 BAS and LAS number in MCM-22 and MCM-56 zeolites,
2
ꢁ1
calculated using absorption coefficients: LAS 3(L) ¼ 0.165 cm mmol
2
ꢁ1
and BAS 3(B) ¼ 0.044 cm mmol (ref. 49)
Number of Lewis acid
sites occupied by
Number of Brønsted acid
sites occupied by pyridine
ꢁ1
ꢁ1
pyridine [mmol g
aer evacuation at
]
[mmol g ] aer
evacuation at
Catalyst
473 K 523 K 573 K
473 K
523 K
573 K
HMCM-22
1914
1538
1748
837
869
748
1651
1164
1380
679
651
565
1455
1105
1125
538
516
413
12 961 12 080 10 895
Nb/HMCM-22
Zr/HMCM-22
HMCM-56
Nb/HMCM-56
Zr/HMCM-56
9216
9750
3400
3009
3141
9644
8742
2722
2452
2330
8663
7786
2286
1725
1699
Fig. 4 FTIR spectra after outgassing at 673 K for 2 h of: (a) HMCM-22;
(b) Nb/HMCM-22; and (c) Zr/HMCM-22. Spectra are normalized to
1
0 mg of zeolite.
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Table 4 The conversion and selectivity in dehydration and cyclisation
of 2,5-hexanedione (mass of catalyst – 50 mg; reaction temperature –
6
23 K; 2,5-HDN volume – 0.5 mL; reaction time – 0.5 h; DMF – 2,5-
dimethylfuran; MCP – 3-methyl-2-cyclopentenone)
Selectivity, %
Conversion of
Catalyst
2,5-HDN, %
MCP
DMF
MCP/DMF
Nb/HMCM-22
Zr/HMCM-22
Nb/HMCM-56
Zr/HMCM-56
88
48
74
34
2
18
4
98
82
96
77
0.02
0.21
0.04
0.29
23
Fig. 5 Conversion vs. reaction time in dibutyl sulfide oxidation with
H
2
O
2
(mass of catalyst – 40 mg; reaction temperature – 303 K;
MCP to DMF products allows the estimation of the acid-base
character of the catalyst. If acidic centers dominate on the
catalyst surface, the selectivity ratio MCP/DMF is <1. The ratio
MCP/DMF > 1 indicates that basicity is dominant. When MCP/
DMF ꢃ 1, the catalyst has both acid and base characters.
The results of the dehydration and cyclisation of
methanol – 10 mL; dibutyl sulfide – 2 mmol; H – 2 mmol).
2
O
2
catalyst, Nb/HMCM-56, led to 80% conversion of dibutyl sulde
with a very high yield of sulfoxide – Fig. 6). The high activity of
niobium containing materials can be explained by the ability of
(
2,5-hexanedione performed over niobium and zirconium cata-
34
niobium to activate the hydrogen peroxide. It has been
postulated that the electrophilicity of oxygen in H increases
in contact with an oxometal group, thus the nucleophilic
attack of sulfur is more favourable.
In spite of the rapid reaction progress for niobium contain-
ing zeolites, it can be observed that Nb/HMCM-56 is more active
than Nb/HMCM-22. This feature could be related to the
different structure of both materials (e.g. volume of mesopores).
Catalysts used in this work exhibit Brønsted (BAS) and Lewis
lysts are shown in Table 4. All tested zeolites show acidic
character, and the main product observed is DMF, which is
formed on Brønsted acid sites. It is important to point out that
for the MCM-22 type zeolites (Nb/HMCM-22 and Zr/HMCM-22)
a higher value of 2,5-HDN conversion was observed. The higher
conversion on Nb/HMCM-22 and Zr/HMCM-22 than on the
appropriate MCM-56 zeolites, should be related to the higher
number of BAS proved by pyridine adsorption. However, not
only the number of BAS inuences the activity of catalyst, but
also the nature of the metal. Zirconium containing catalysts
show almost two times lower conversion of 2,5-HDN than
niobium catalysts.
2 2
O
56
(LAS) acid sites. Both of them can participate in the oxidation of
dibutyl sulde. To consider the role of these centres, one has to
discuss the chemisorption of both reagents. Dibutyl sulde is
expected to chemisorb, if at all, on niobium or zirconium
cations. Recently, it was established that the interaction of the
acidic surface hydroxyls with hydrogen peroxide gives rise to the
33
Dibutyl sulde oxidation with H O2
2
ꢁ
formation of hydrosuperoxo species (HO2 ). These species
Fig. 5 presents dibutyl sulde conversion versus the reaction
time on niobium and zirconium catalysts. Dibutyl sulde
oxidation with hydrogen peroxide performed in the absence of a
catalyst led to sulde conversion of 78% aer 8 h. The appli-
cation of catalysts signicantly increases the reaction rate,
leading to a nal conversion of 87% (Nb/HMCM-22) and 95%
2 2
interact with the excess of H O towards hydroxyl radicals,
which are the most active species in the catalytic oxidations. The
ꢁ
role of the niobium centres is to trap superoxo (O
2
c) and per-
2ꢁ
oxo (O
2
) species on the surface. This process leads to
(Nb/HMCM-56) aer much shorter times and a very high yield
of sulfoxide. The modication of zeolites with zirconium does
not improve the activity of catalysts used. The reaction in the
presence of both Zr/HMCM-22 and delaminated Zr/HMCM-56,
proceeds very slowly, and the obtained conversions are similar
(Zr/HMCM-56) or even lower (Zr/HMCM-22) than in the case of
oxidation of dibutyl sulde without a catalyst. The low activity of
zirconium containing zeolites can be caused by the strong
interaction between zirconium species in the catalyst and sulfur
containing compounds. Similar properties of this transition
54,55
metal were observed in the papers
for the interaction
between SO2 and H S with ZrO . A totally different effect is
2
2
Fig. 6 Conversion and yield to sulfoxide and sulfone in dibutyl sulfide
oxidation with H after 10 min of the reaction (mass of catalyst –
0 mg; reaction temperature – 303 K; methanol – 10 mL; dibutyl
observed in the presence of both Nb/HMCM-22 and Nb/HMCM-
6 samples, which led to the faster reaction than in the blank
2 2
O
5
4
2 2
test (Fig. 5). Aer 10 min of the reaction time the most active sulfide – 2 mmol; H O – 2 mmol).
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increased production of hydroxyl radicals (cOH). They are UV-vis spectra of catalysts aer the reaction reveal some
generated via electroprotic liquid phase reaction H O increase of the band at ca. 220 nm caused by the loading of
+ O2 c(aq) + H O(aq). Trapping of superoxo sulfur compounds in the pores.
+
2(aq)
2
ꢁ
ꢁ
HO2
¼ cOH
(aq)
(aq)
2
and peroxo species by niobium centres leads to a substantial
The comparison of the results presented in this paper
42
shi of the reaction equilibrium to the right enhancing the and those published for Mo containing zeolites allows us to
production of hydroxyl radicals, which interact with dibutyl present the following order of activity dependence on the nature
sulde very quickly. This process is responsible for the of the metal modier: Mo > Nb [ Zr.
substantial increase of oxidation activity at the beginning of the
The oxidation of dibutyl sulde with hydrogen peroxide
reaction when the concentration of H is the highest. If the showed that the zeolite structure (MCM-22 or MCM-56) and the
2 2
O
concentration of BAS on the catalyst surface is higher, as in the nature of the modier (Nb or Zr) have an impact on the catalytic
MCM-22 series, higher production of hydroxyl radicals is behaviour. MCM-56 modied with niobium gives very high
expected and it can lead to the easy recombination of radicals activity and selectivity for sulfoxide.
decreasing the catalytic activity. In the case of Zr containing
catalysts dibutyl sulde seems to be preferentially chemisorbed
on zirconium LAS. It is known that ZrO strongly chemisorbs
2
Conclusions
5
4,55
57
sulfur compounds,
and also dialkyl suldes. The strongly ꢄ Modication of HMCM-22 and HMCM-56 zeolites with
held dibutyl sulde is not reactive towards hydrogen peroxide niobium and zirconium does not change the structure of cata-
and Zr species are blocked. Therefore, Zr containing catalysts lysts, as conrmed by XRD study.
are not active (the activity is lower than in the blank test).
ꢄ Crystalline metal oxides are not detected on the zeolite
The discussion presented above allows us to suggest that the surfaces and it has been proved that both niobium and zirco-
effective reaction pathway in the oxidation of dibutyl sulde nium species are present in a tetrahedral coordination.
proceeds through the generation of hydroxyl radicals interact-
ing with dibutyl sulde from the liquid phase. One can nd external silanol groups on the zeolite surface. Therefore,
conrmation of the interaction of the Nb containing zeolites modication of both HMCM-22 and HMCM-56 zeolites with Nb
ꢄ Metal loading involves the interaction with BAS and
with H
2 2
O in the UV-vis spectra aer treatment of the catalysts or Zr decreases the number of BAS.
with hydrogen peroxide. The example for Nb/HMCM-22 is
shown in Fig. 7. Zeolite treated with hydrogen peroxide showed play a crucial role in the oxidation of sulde to sulfoxide (>90%
ꢄ Nb species and the texture of zeolite (MCM-22 vs. MCM-56)
an increase in the absorption intensity in the range 300–400 conversion of dibutyl sulde is reached aer 1 h of the reaction
ꢁ
nm, assigned to charge transfer from O
niobium.
2
c (superoxo species) to on Nb/HMCM-56).
33,34
ꢄ Zirconium loaded on zeolites even decreases the rate of the
The catalysts aer the reaction were studied by XPS and UV- reaction performed without the use of any catalyst due to its
vis methods. The XPS results showed the same concentration of strong interaction with sulfur compounds.
metal species before and aer oxidation of dibutyl sulde. It
suggested that leaching of the metal phase does not occur. The
Acknowledgements
We are grateful to the National Science Center in Poland for
support (project no. 2011/01/B/ST5/00847). Acknowledgements
to Andrzej Nowak and Dr Maciej Trejda for taking part in some
experimental works.
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