A Lamellar MWW Zeolite With Silicon and Niobium Oxide Pillars: A Catalyst for the Oxidation of Volatile Organic Compounds

Article abstract of DOI:10.1002/chem.202000862

In this work, an MWW-type zeolite with pillars containing silicon and niobium oxide was synthesized to obtain a hierarchical zeolite. The effect of niobium insertion in the pillaring process was determined by combining a controllable acidity and accessibility in the final material. All pillared materials had niobium occupying framework positions in pillars and extra-framework positions. The pillared material, Pil-Nb-4.5 with 4.5 wt percent niobium, did not compromise the mesoporosity formed by pillaring, while the increase of niobium in the structure gradually decreased the mesoporosity and ordering of lamellar stacking. The morphology of the pillared zeolites and the niobium content were found to directly affect the catalytic activity. Specifically, we report on the activity of the MWW-type zeolites with niobium catalyzing the gas-phase oxidation of volatile organic compounds (VOCs), which is an important reaction for clean environmental. All produced MWW-type zeolites with niobium were catalytically active, even at low temperatures and low niobium loading, and provided excellent conversion efficiencies.

Full text of DOI:10.1002/chem.202000862

Accepted Article
Accepted Article
Title: Lamellar MWW zeolite with silicon and niobium oxide pillars - A
catalyst for the oxidation of volatile organic compounds
Authors: Anderson Joel Schwanke, Rosana Balzer, Christian Wittee
Lopes, Débora Motta Meira, Urbano Díaz, Avelino Corma,
and Sibele Pergher
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202000862
Link to VoR: https://doi.org/10.1002/chem.202000862
01/2020

10.1002/chem.202000862
Chemistry - A European Journal
Lamellar MWW zeolite with silicon and niobium oxide pillars -
A catalyst for the oxidation of volatile organic compounds
Anderson Joel Schwankea*, Rosana Balzerb, Christian Wittee Lopesa, Débora Motta
Meirac, Urbano Díazd, Avelino Cormad, Sibele Perghere
a Universidade Federal do Rio Grande do Sul, 91501-970, Porto Alegre, RS, Brasil.
*Corresponding author: anderson-js@live.com
b Universidade Federal do Paraná - UFPR, 85950-000, Palotina, PR, Brasil.
c CLS@APS sector 20, Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass
Avenue, Argonne, IL 60439, USA. Canadian Light Source Inc., 44 Innovation Boulevard,
Saskatoon, Saskatchewan S7N 2V3, Canada
d Instituto de Tecnología Química, Universitat Politècnica de València-Consejo Superior de
Investigaciones Científicas, Avenida de los Naranjos s/n, E-46022 Valencia, Spain.
e Universidade Federal do Rio Grande do Norte, 59078-970, Natal, RN, Brasil.
ABSTRACT
In this work, an MWW-type zeolite with pillars containing silicon and niobium oxide was
synthesized to obtain a hierarchical zeolite. The effect of niobium insertion in the pillaring
process was determined by combining a controllable acidity and accessibility in the final
material. All pillared materials had niobium occupying framework positions in pillars and
extra-framework positions. The pillared material, Pil-Nb-4.5 with 4.5 wt% niobium, did
not compromise the mesoporosity formed by pillaring, while the increase of niobium in
the structure gradually decreased the mesoporosity and ordering of lamellar stacking. The
morphology of the pillared zeolites and the niobium content were found to directly affect
the catalytic activity. Specifically, we report on the activity of the MWW-type zeolites
with niobium catalyzing the gas-phase oxidation of volatile organic compounds (VOCs),
which is an important reaction for clean environmental. All produced MWW-type zeolites
with niobium were catalytically active, even at low temperatures and low niobium
loading, and provided excellent conversion efficiencies.
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1. INTRODUCTION
Catalysis is directly related to the development of materials with catalytically
active sites that selectively drive target transformations with high performance under soft
and environmentally friendly conditions.[1] Niobium (Nb)-based structures have been
extensively employed in catalysis and electrocatalysis applications.[2] There is a crescent
attempt related to the use of Nb as a promoter, catalytically active phase or support in the
process of catalyst preparation.[3] An understanding of the energetics, spectroscopic
properties and molecular structures of supported niobium catalysts has permitted the
prediction of the optimal composition and structure of catalysts for specific catalytic
processes.[4] However, for practical purposes, the identification and optimization of active
sites are challenging because chemical reactions often occur at poorly defined solid–gas
or solid–liquid interfaces.[5] The synthesis strategies to obtain niobium-containing
mesoporous silica and other molecular sieves have systematically increased over time as
well as the understanding of their physicochemical properties.[3] Hierarchical zeolites
have received substantial attention in recent years. This strategy emerged from the
necessity to combine accessibility and catalytic activity that is higher than that in typical
ordered mesoporous materials, i.e., M41S and SBA-15.[6]
In the category of hierarchical zeolites, pillared zeolites belong to a “bottom-up”
approach, where the procedure involves separating and establishing pillars between
MWW zeolitic lamellae.[7] Moreover, pillared zeolites offer adjustable properties as high
surface area and sorption capacities, mesoporous dimensions obtained from the control
of the length of pillars and tuneable hydrophobicity/hydrophilicity.[8] Consequently, the
first pillared zeolite structure was called MCM-36,[9] and was obtained from the
expansion of a lamellar zeolitic precursor with MWW topology known as MCM-22P.[10]
The lamellar zeolitic precursor was intercalated with surfactants followed by further
addition of a pillaring agent-based silicon alkoxide, which after calcination, formed rigid
SiO2 pillars between the lamellae. MCM-36 presents remarkable performance as catalyst
for vacuum gas oil cracking and alkylation and support for enzymes.[9, 11] Subsequently,
the stabilization of SiO2 pillars was extended to other lamellar zeolites including Ti-
MCM-36 titanosilicates, ITQ-36 (FER), pillared MCM-39 (NSI), pillared MFI
nanosheets, pillared ilerite (RWR), pillared IPC-1P and Ti-IPC-1PISi (UTL).[12]
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10.1002/chem.202000862
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In contrast, SiO2 pillars are also considered as inert space fillers and associated
with partial blockage of active sites present between the external surfaces of lamellae.[13]
A way to compensate this blockage involves inserting active species in the pillars, which
could be combined with the accessibility of the formed galleries. However, studies
focused on lamellar zeolites which the pillars play a role as active species are scarce and
still a real opportunity for advanced studies on pillared zeolites.[14] MCM-36 with
alumina-magnesia as mixed pillars was the first example of this approach and had pore
diameters of 2-4 nm, and this concept was extended to other mixed pillars with BaO–
Al2O3, Al2O3–SiO2, MgO–Al2O3–SiO2, and BaO–Al2O3–SiO2.[15] A pillared MWW
zeolite with Si/Ti as mixed pillars was reported as catalysts for epoxidation of
cyclooctene.[16] Recently, a pillared MWW with niobiosilicate was reported, and the
materials were active for cyclohexene oxidation and methylene blue discoloration with
hydrogen peroxide.[17] Other successful examples of zeolite with pillars as active species
include TS-1 (MFI) with Si/Ti pillars applied for epoxidation of cyclooctene, norbornene
and α-pinene,[18] and TiIPC-1PITi (UTL) with Si/Ti pillars applied for epoxidation of
cyclooctene, norbornene, and linalool.[12g]
In fact, niobium oxide proved to be a versatile catalyst for several reactions, e.g.,
oxidation, esterification, dehydration, dehydrogenation, hydrogenation, photocatalysis
and other applications, such as enzyme immobilization and chemical sensing.[4a, 5, 19]
Additionally, the interest to prepare niobium-containing microporous and mesoporous
materials, such as MFI, FAU, BEA, AlPOs, SBA-3, MCM-41, SBA-15, KIT-5, and a
layered niobium oxide, is still growing.[20] Here, we prepared a lamellar MWW-type
zeolite with silicon and niobium oxide as pillars where the catalytic performance can be
optimized by careful control of physicochemical properties.
We focused on the catalytic gas-phase oxidation of toxic volatile organic
compounds (VOCs), which represents an important transformation for environmental
remediation and where enhanced catalytic performance could provide improved
degradation efficiencies with decreased energy costs.[21] Catalytic oxidation has been
recognized as the most effective approach, mainly due to its high degradation efficiency,
low energy cost, and the potential for the removal VOCs in low concentrations.[22] We
demonstrate that the physicochemical properties of MWW-type zeolite with pillars of
silicon and niobium oxide have a strong impact on the gas-phase oxidation activity of
VOCs.
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2. EXPERIMENTAL
2.1 Synthesis of MCM-22 and its precursor: In a plastic beaker containing
9.0 mol of distilled water, it was added 18.5 mmol of sodium hydroxide (NaOH, Sigma
Aldrich) kept in stirring during 10 min. Then, it was added 9.0 mmol of sodium
aluminate (NaAlO3, Riedel-de-Haën) under continuous stirring until its complete
dissolution. Then, 0.1 mol of the organic structure directing agent, hexamethyleneimine
(HMI, Sigma Aldrich), was added and followed by the addition of 0.2 mol of silica
(Aerosil 200, Degussa). After 2 h of continuous stirring, the mixture was placed in
autoclaves containing PFTE recipes and kept in an oven at 135 °C with a system of
rotation of the autoclaves during 7 days. Afterward, the material was filtered with
distilled water, dried and stored in a plastic flask. The precursor (P) was calcined for 12
h at 580 °C to eliminate the HMI and to obtain the MCM-22
2.2
h in stirring at room temperature, 25 °C. The mixture was done with 4.8 g of distilled
water, 1.2 of MCM-22 precursor and 12.0 of 29 wt%
Swelling precursor: The material was obtained with a mixture kept for 18
g
g
a
hexadecyltrimethylammonium bromide (C16TABr, 98%, Sigma Aldrich) solution
previously exchanged by OH- ions with an ionic exchange resing (Dowex). The material
was washed several times until pH = 9 and labeled as Swollen(P).
2.3
Pillaring: The procedure was done similar to the literature.[12g, 18] It was
used N2 atmosphere and reflux condenser system for 24 h at 80 °C. The Swollen(P)
material was added with tetraethyl orthosilicate (TEOS, Sigma Aldrich) with a 1:5 wt/wt
ratio and niobium ethoxide (99.9%, Sigma Aldrich) with Si/Nb molar ratios = 50, 25,
15, 10. The resulting materials were hydrolyzed with water for 12 h at 40 °C and dried.
To eliminate the organic content, calcination was done using a vertical tubular reactor
with a ramp of 0.05 °C s-1 up to 550 °C (N2 flowing) kept at the same temperature for
an additional 8 h (O2 flowing). The Nb-containing pillared MWW-type materials were
labeled as Pil-Nb-x, where x is the Nb percentage (%) determined by ICP. Niobium
oxide (99.9%, Alfa Aesar) was used as a reference.
2.4
Oxidation of VOCs: The conditions were: 30 mg catalyst; benzene (1.3 g
m−3), toluene (0.7 g m−3), and ο-xylene (0.6 g m−3) added to air; gas flow rate of 20 cm3
min−1; residence time of 0.3 s; space velocity of 12000 mL.g-1 h−1; and temperature range
of between 50 and 300 °C. It was used quartz reactor under atmospheric pressure. The
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data were collected after 2 h on-stream at room temperature and the products were
determined with a GC-MS. The subtract and product mixtures were analyzed using two
online gas chromatographs. The conversion of BTX follows:
Conv. (%): 푋푖푛푝푢푡−푋표푢푡푝푢푡 . 100
푋푖푛푝푢푡
Where X represents the benzene, toluene or o-xylene.
2.5
Characterization: The elemental analyses of silicon, aluminium and
niobium were recorded with a 715-ES (Varian, ICP). The morphology of samples was
analysed on an Ultra 55 at 2 kV (ZEISS, FESEM). TEM images were collected JEM
2100F (JEOL, 200 kV) using high-angle annular dark-field (HAADF) and bright field
(BF) and modes. The crystalline structure of solids was measured on diffractometer
Cubix (PANalytical) using copper K-alpha radiation. The textural properties were
collected with a N2 and Ar sortometer ASAP 2010 and ASAP 2020 (Micromeritics),
respectively, outgassing the samples at 300 °C for 12 h before the measurements. The
Brunnauer, Emmet and Teller method was used to calculate the specific surface area.
Infrared spectroscopy with adsorption-desorption of pyridine as probe molecule was
performed in a Nicolet 710 (Thermo Fisher Scientific) and the absorption coefficients
were calculated according to Emeis method.[23] Diffuse reflectance UV-Vis
spectroscopy was collected on a Cary 5000 (1.12, Varian). The experiments of X-ray
photoelectron spectroscopy (XPS) were carried out in an Omicron-SPHERA station. X-
ray Absorption Spectroscopy experiments were carried out at Nb K-edge (18986 eV) at
the 20 BM beamline of Argonne National Laboratory (Lemont, IL, USA). The samples
were pelletized (pellets of an optimized thickness) and the spectra were collected in
transmission mode employing ionization chambers. Energy calibration was checked by
Nb foil placed between I1 and I2 ionization chambers. Reference compounds were also
measured in order to have a proper comparison with the measured samples. In order to
ensure spectral reproducibility and a good signal-to-noise ratio, three scans were
acquired at each measurement step. Data treatment has been performed using the
IFEFFIT-based package [24]
.
3. RESULTS AND DISCUSSION
The diffractograms of all solids obtained are shown in Figure 1. For the precursor
MCM-22(P), the diffraction peaks located at 2θ = 3.3° and 6.6° are related to the (001)
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and (002) reflections of the MWW zeolitic lamellar stacks, where the individual lamella
have a 2.5 nm thickness.[25] The d001 spacing of the MCM-22(P) with 2.6 nm corresponds
to the sum of the thickness of a MWW lamellae and the 0.1 nm thickness of the silanol
groups located on the upper and lower surface of each lamella. After calcination, silanol
groups between the lamellae are condensed, and HMI is removed for the formation of the
tridimensional MCM-22 zeolite.
The swollen precursor shows an increase in the d001 spacing, where the peak
position shifted to 2θ = 2.2° (4.0 nm) due to surfactant molecules accommodated between
the MWW lamellae. This finding is confirmed by the widening of (101) and (102) peaks
due to loss of the ordering of the stacked lamellae along the c axis.
Figure 1. Diffractograms of MCM-22(P), Swollen(P), MCM-22, Pil-Nb-4.5, Pil-Nb-
10.5 and Pil-Nb-14.8.
The introduction of niobium and silicon as oxide pillars maintains the separation
of the individual MWW lamellae, which is observed by the union of the (101) and (102)
diffraction peaks to form a broad peak due to the loss of vertical arrangement of the
lamellae along the c axis in all pillared materials. For the Pil–Nb-4.5 material, it is
observed that the (00l) diffraction peaks are maintained after calcination with a widening
of the (00l) peaks, an increase in the d001 to 4.0 nm. The XRD patterns of the Pil-Nb-10.5
and Pil-Nb-14.8 materials show a decrease in the d001 spacing to 3.6 nm. Moreover, a
gradual decrease in the order of the lamellar stacking with an increase in the niobium
content observed by the widening of the (00l) diffraction band is attributed to the increase
in the geometry distortions in pillars. Also, it is observed that the pillars do not collapse
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for the Pil-Nb-14.8 material since the broad peak in the region between 2θ = 8 – 10° is
maintained as well as the broad (002) peak at 2θ = 5.1°, which is located at a lower angle
than the MCM-22(P) (002) peak at 2θ = 6.6°.
Regarding the niobium speciation, it is well known that the sensitivity of XRD
requires greater than 5% of crystalline phases to be present, and part of the Nb might be
in the oxide form occupying extra-framework positions.[20b] Another possibility lays in
the formation of an amorphous niobium oxide phase, which is not detected by XRD. In
our case, no crystalline phases of Nb oxide are present even in pillared samples with a
high Nb content, i.e., 14.8% (Pil-Nb-14.8). This was confirmed by the absence of
crystalline Nb oxide XRD diffraction peaks at 2θ = 22.6, 28.3 and 36.6°, which are shown
as a reference in Figure 1. The Nb local environment will be addressed further.
Table 1 shows the chemical composition of the samples. The Swollen(P) had a
Si/Al molar ratio = 18, which is less than the MCM-22 ratio of Si/Al = 22. This decrease
is due to the partial desilication of the MCM-22(P) during the swelling procedure with an
alkaline media. On the other hand, the pillared samples Pil-Nb-4.5, Pil-Nb-10.5 and Pil-
Nb-14.8 and have an increase in the Si/Al ratio of 28, 36 and 32, respectively, confirming
that materials contain more silicon due to the pillars.
Table 1. Composition and textural properties of MCM-22, Pil-Nb-4.5, Pil-Nb-10.5, Pil-
Nb-14.8 and Pil-Nb-24 materials.
Sample
Si/Nb
Si/Al
SBET
Sext
VTP
Vmicro
Vmeso
2 -1
2 -1
3 -1
3 -1
3 -1
(ICP)
(ICP)
(m g
)
(m g
)
(cm g
)
(cm g
)
(cm g )
MCM-22
Pil-Nb-4.5
Pil-Nb-10.5
Pil-Nb-14.8
--
22
450
120
727
373
331
0.64
0.90
0.46
0.42
0.15
0.09
60
35
19
28
36
32
833
711
614
0.03
0.15
0.14
0.33
0.09
0.08
The N2 isotherms of the pillared samples are shown in Figure 2a. The Pil-Nb-4.5
material shows steep adsorption between 0.12 and 0.4 of the p/p0 region that was created
by pillaring and is due to capillary condensation occurring in the mesopores. For the other
pillared materials, the N2 adsorption at low pressures is mainly due to the presence of
micropores, while the MCM-22 zeolite possesses a type I isotherm typical of materials
with micropores.[26]
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The pore size distribution of the MWW-type materials presented in Figure 2b
shows pore widths (Wp) ranging between 2 - 4 nm (small mesopores) in the case of Pil-
Nb-4.5. The decreasing Wp to the range of supermicropores was observed for the pillared
samples with high Nb content, Pil-Nb-4.5 and Pil-Nb-10.5. In the specific case of Pil-Nb-
14.8, the MWW lamellae did not collapse when their Wp profiles are compared with those
of the MCM-22 zeolite. These results are consistent with the XRD results that were
previously discussed.
Figure 2. (a) N2 adsorption isotherms and (b) pore widths of MCM-22, Pil-Nb-4.5, Pil-
Nb-10.5 and Pil-Nb-14.8 materials and (c) micropore size distribution of Pil-Nb-4.5 in
using Horwath-Kawazoe method from the Ar adsorption isotherm.
The textural properties of the pillared MWW-type materials in Table 1 show an
increase in the specific surface when the Nb content decreases. The Pil-Nb-4.5 sample
shows a specific surface of 833 m² g-1 (383 m² g-1 more than MCM-22) and an increase
in pore volume related to Vmeso created by pillaring. Besides, the values of Sext for the Pil-
Nb-4.5 sample with 727 m² g-1 are six times higher than that of the MCM-22 sample and
confirms that pillars separate the MWW lamellae. For the Pil-Nb-10.5, Pil-Nb-14.8
materials, the Sext values are three times higher than that of MCM-22. On the other hand,
Vmicro of Pil-Nb-10.5 and Pil-Nb-14.8 are similar to that of the MCM-22 zeolite and may
indicate that the interlamellar mesoporosity was not completely maintained when
concentrations of Nb is higher than 4.5%. The low Vmicro of Pil-Nb-4.5 (0.03 cm3 g-1)
reveals that the portion of micropores formed in the condensation of silanol groups
between MWW lamellae (the case of MCM-22) was replaced for interlamellar mesopores
formed by pillaring. Furthermore, the silica could partially fill the ten-membered ring
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sinusoidal micropore channels in individual MWW lamellae, which also contributes to
the low micropore volume. To obtain more information about the micropores in Pil-Nb-
4.5, Ar adsorption isotherm was performed and the microporous size distribution (using
Horwath-Kawazoe method) is shown in Figure 2c. It was observed the maximal
distribution centred at 0.5 nm corresponds to 10-membered ring sinusoidal micropore
channels in the MWW lamella, which confirms the presence of micropores. Moreover,
the Vmicro estimated was 0.03 cm³ g-1, which is similar to the Vmicro, obtained from the N2
adsorption isotherm.
Figure 3 shows the SEM of the synthesized materials. A coin-like morphology is
shown for the MCM-22 sample (Figure 3a), where 1.4 µm hexagonal crystals are
observed. The morphology and crystal habits are maintained for Pil-Nb-4.5, Pil-Nb-10.5,
and Pil-Nb-14.8 (Figure 3 b, c and d, respectively) with aggregation in the case of Pil-
Nb-14.8, which is associated with the pillaring process [27]. In addition, no bulk phases of
niobium oxide are observed.
Figure 3. The SEM images of (a) MCM-22, (b) Pil-Nb-4.5, (c) Pil-Nb-10.5 and (d) Pil-
Nb-14.8.
TEM micrographs in the bright-field mode for the Pil-Nb-4.5 sample are shown
in Figure 4a and the d100 spacing of 1.26 nm was determined by the intensity profile (see
inset). This value is similar to the d100 spacing obtained by XRD (Figure 1) for the Pil-
Nb-4.5 sample at 2θ = 7.29° (1.21 nm), indicating that the MWW zeolitic structure is
preserved. Figure 4b shows six elongated individual MWW lamellae separated by
approximately 2 nm each one, which is nearly the size of the gallery of 1.91 nm obtained
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by the XRD pattern of the Pil-Nb-4.5 sample (d001 spacing subtracted by the thickness of
individual MWW lamellae, 2.5 nm).
Figure 4. TEM image in BF mode of Pil-Nb-4.5 material along to the (a) a plane and
(b) c plane.
The HAADF-STEM shown in Figure 5 examines the presence of Nb with single-
point elemental analysis between individual MWW lamellae (region of pillars). Silicon
(40.56%) and oxygen (54.25%) are the major elements, and niobium (4.36%) is due to
the pillars. No regions with a high density or bulk phases are observed in Figures 4 and
5, confirming the good distribution of niobium and supporting his presence within the
pillars.
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Figure 5. HAADF-STEM point elemental mapping between two MWW lamellae in the
Pil-Nb-4.5 sample.
To examine the state of Nb and the possibility of its extra-framework position, DR
UV-Vis analysis of pillared samples was conducted and the results are shown in Figure
6a. All samples contain major peaks between 210 - 250 nm that are typical of the niobium
tetrahedral coordinated species present in the framework.[20a],[20e],[28] The DR UV-Vis
spectrum for niobium oxide (Figure 6b) with octahedral geometry shows a characteristic
band at approximately 330 nm, which is not observed in any pillared sample.
Figure 6. DR UV-Vis spectra of (a) Pil-Nb-4.5, Pil-Nb-10.5, Pil-Nb-14.8 materials and
(b) niobium oxide.
Niobium species in Pil-Nb-4.5, Pil-Nb-10.5, Pil-Nb-14.8 and niobium oxide were
also investigated by XPS and shown in Figure 7. The Nb3d doublet was divided into 5/2
(green lines) and 3/2 (blue lines) components and the binding energies (BE) are
characteristic of Nb5+ species. The BE of niobium oxide appears at 207 and 210 eV and
these signals are present in Pil-Nb-4.5, Pil-Nb-10.5 and Pil-Nb-14.8 samples and are
related to the extra-framework niobium oxide (Nb2O5). Moreover, the signals at higher
BE confirms niobium with distinct surroundings (NbOSi), which is also reported by other
authors.[17, 29] These results may indicate that an amorphous phase of niobium is also
present, which supports the absence of characteristic peaks of crystalline niobium oxide
that were not detected by XRD in the pillared materials as shown in Figure 1. However,
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DR UV Vis spectra of pillared samples do not feature bands of niobium with octahedral
geometry, which suggests that Nb2O5 may be present in small proportion.
On the other hand, Nb3d5/2 and Nb3d3/2 components are observed with
approximately the same BE (203 and 205 eV) and can be attributed to the presence of
other niobium species, however, these BE display very low intensities when compared to
the intensity of the BE above 205 eV, and may indicate the content is lower than the
detection limit. According to the literature, BE of 203.9 eV can be assigned to NbO (Nb2+)
and the BE at 205.9 eV can be assigned to NbO2 (Nb4+) and the surface of those species
could be are oxidized predominating the presence of Nb2O5.[30] The XPS analysis of the
MCM-22 sample was not performed because it does not have any BE in this region, as
previously reported.[29, 31]
Figure 7. XPS spectra of Pil-Nb-4.5, Pil-Nb-10.5, Pil-Nb-14.8 and niobium oxide.
It has been reported that the complete incorporation of niobium into silica-based
materials may depend on the type of Nb source (oxalated, ammoniacal, chloride), pH and
silica source.[3] To a deeper understanding of the niobium incorporation in the pillared
material, X-ray absorption spectroscopy (XAS) analysis was performed on the sample
with high niobium content, Pil-Nb-14.8. XAS has become a powerful technique in all the
fields of materials science and, particularly, in catalysis. The reason relies on its atomic
selectivity which enables the characterization of a specific element within complex
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matrices composed by several chemical elements. XAS is highly sensitive to low element
concentration as well, being able to measure the absorption edge of elements containing
considerable small amounts (hundreds of ppm) respect to the total mass of the sample. In
this work, XAS has been employed to obtain electronic and structural information of the
Nb centers.
Figure 8 displays the XAS data of Pil-Nb-14.8 and respective standards. The
XANES spectra Figure 8a shows that Nb centers are mainly present as Nb5+ in sample
Pil-Nb-14.8 once the absorption edge is positioned at the same energy as for the Nb(V)-
based compounds, which corroborates with XPS results. As previously reported, a change
occurs in the Nb coordination state in supported niobia, from tetrahedral to octahedral,
upon adsorption of water molecules [32], while for dry Nb2O5 the weak pre-edge feature
is related to distorted octahedral metal sites. In Pil-Nb-14.8, the coordination of water
molecules could convert the Nb sites from tetrahedral to octahedral geometry, resulting
in low pre-edge intensity. Notwithstanding, the local environment of Nb in our sample is
in agreement with that ascribed for Nb tetrahedrally coordinated.[20a, 33]
Figure 8. Normalized XANES spectra at Nb K-edge (a) and respective k2-weighted |FT|
spectra (b) of Pil-Nb-14.8.
The k2-weighted |FT| spectra of the sample and Nb standards are represented in
Figure 8b. As already commented for the XANES, the EXAFS spectra of the sample and
Nb2O5 are rather similar, with the presence of a split first-shell related to the Nb-O
contribution. However, the position of this peak in the spectrum of Nb2O5 is shifted to
longer R-values than that of the sample, which reinforces that Nb local order is different.
Also, by looking at the higher shells one can notice that they are not superimposed,
pointing out to a different order at higher distances. The similarities observed here could
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10.1002/chem.202000862
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be ascribed to a certain hydration degree of the sample during XAS measurements or even
to the presence of Nb2O5 domains in the silica-based pillars.
The surface properties of MWW materials were characterized by FTIR
spectroscopy with adsorption-desorption of pyridine as a probe molecule. Figure 9a
shows the FTIR spectra in the hydroxyl region and the vibration bands centered at 3730
and 3748 cm-1, which are assigned to the internal and external silanol groups, respectively
[34]. Moreover, the vibration bands at 3620 cm-1 and 3670 cm-1 correspond to surface
bridged hydroxyl groups associated with framework Al–OH–Si and extra-framework
aluminum species generated upon calcination, respectively [35]
.
Figure 9. FTIR after outgassing at 400 °C for 2 h and spectra after pyridine adsorption at
150 °C for samples MCM-22, Pil-Nb-4.5 and Pil-Nb-10.5 with (a) hydroxyl group
regions and BAS and (b) LAS sites. Spectra were normalized to 11.3 mg sample.
The intensity of the band at 3748 cm-1 for the Pil-Nb-4.5 and Pil-Nb-10.5 samples
is higher than that of the MCM-22 zeolite, which is associated with silanols from the
pillar walls between MWW lamellae. Moreover, a smaller and wider band is observed for
the sample Pil-Nb-10.5 and suggests that the increase of Nb content reduces the silanol
groups, and consequently increases the hydrophobicity, which is beneficial for reactions
where apolar reactants (as BTX) are involved. Hence, the increase of Nb content could
stress the pillars between lamellae and decrease the lamellar stacking along the c axis,
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10.1002/chem.202000862
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which is also reported when Zr was inserted in pillared clay material.[36] Also which
decrease after pillaring since Si-O-Si bonds were formed between silanol groups of the
MWW lamellae and pillars upon calcination. Finally, the band at 3620 cm-1 is maintained
for pillared materials and indicates that the framework of Al–OH–Si species were
preserved.
Table 2. The number of BAS and LAS (µmol Py g-1) of Pil-Nb-4.5, Pil-Nb-10.5 and
MCM-22.
150 °C
250 °C
Total BAS LAS
350 °C
Total BAS LAS Total
BAS
LAS
23
Sample
Pil-Nb-4.5
Pil-Nb-10.5
MCM-22
51
45
54
74
71
72
45
21
48
11
6
15
56
27
63
39
15
36
11
6
15
50
21
51
26
18
The MWW-type pillared materials show acid sites with different natures and
strengths with pyridine as the adsorbed molecule in Figure 9b and values in Table 2.
Specifically, vibration bands at 1620 – 1640 cm-1 are associated with the vibration of
pirydinium cations. The bands at 1610 cm-1 (1453 cm-1) and 1544 cm-1 (1635 cm-1) are
characteristics of pyridine adsorbed on Lewis (LAS) and Brønsted (BAS) acid sites,
respectively, and the band at 1490 cm-1 is characteristic of pyridine adsorbed on both LAS
and BAS acid sites. Furthermore, the vibration band observed at ~1447 cm-1 is assigned
to hydrogen-bonded species that result from the interaction of hydroxyl groups on the
zeolite surface [34]. The band at 1610 cm-1 for the MCM-22, Pil-Nb-4.5 and Pil-Nb-10.5
becomes prominent and suggests the increase of LAS with the increase of Nb content.
Hence, the signal at ~1447 cm-1 is more intense for Pil-Nb-4.5 and Pil-Nb-10.5 samples,
probably by the interaction of pyridine with hydroxyl groups formed after pillaring.
The intensity of the band at 1544 cm-1 decreases after pillaring, which indicates
that the number of BAS was “diluted” by the insertion of silica pillars and increases with
niobium content. On the other hand, LAS estimated by the signal at 1454 cm-1 increases
with the increase of Nb content.
A schematic representation of the synthesized MWW-type materials from the
MCM-22(P) sample to pillared materials with silicon and niobium oxide is shown in
Figure 10. The swelling procedure is able to intercalate surfactant molecules between
individual MWW lamellae, and an increase in the d001 spacing from 2.6 to 4.0 nm is
obtained. The Swollen(P) material and a pillaring solution containing silicon and niobium
alkoxides with different molar ratios are mixed. The surfactant molecules were removed
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by calcination with the simultaneous formation of rigid pillars of silicon and niobium
oxide between MWW lamellae. Niobium occupying both framework (green dots) and
extra-framework (blue ellipsoids) positions were present in pillared materials, according
to the XPS and XAS analyses. The increase of Nb content (higher than Nb 4.5 wt%)
decreases the d001 spacing, the order of lamellar stacking and the interlamellar
mesoporosity.
Figure 10. Schematic representation of the synthesized materials: MCM-22(P),
Swollen(P), Pil-Nb-4.5 and Pil-Nb-14.8.
Figure 11 shows the conversion percentage for the oxidation of benzene, toluene,
and o-xylene as a function of reaction temperature for the Pil-Nb-4.5, Pil-Nb-10.5, Pil-
Nb-14.8 and pure MCM-22 (as the catalysts for the control experiment). The inset in
Figure 11 (image a) shows a scheme for the BTX oxidation reaction. We were interested
in investigating how the catalytic performances of Pil-Nb-4.5, Pil-Nb-10.5 and Pil-Nb-
14.8 towards BTX oxidation are dependent upon their physicochemical properties. No
significant conversion is detected for pure MCM-22. To investigate the influence of the
surface area and accessibility, the MWW zeolite containing only silica pillars was tested
and shown similar conversions than pure MCM-22. The MWW pillared materials
containing niobium show good catalytic activities at relatively low temperatures (the
BTX oxidation begins just below at 50 °C) and low metal loadings (4.5 wt%), showing
that the appropriate combination of niobium can provide higher activities related to their
individual counterparts. The BTX conversion increases with temperature for all catalysts,
as expected, and only H2O and CO2 are detected as reaction products.
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Figure 11. Catalytic oxidation of (a) benzene, (b) toluene, and (c) ο-xylene as a function
of temperature, catalyzed by Pil-Nb-4.5, Pil-Nb-10.5, Pil-Nb-14.8 and pure MCM-22.
Figure 11 shows that the BTX conversion increases as the niobium content
increases. Specifically, there are 86 and 83 and 78% conversion for benzene; 66, 60 and
56% conversion for toluene; and 34, 24 and 13% conversion for o-xylene at 300 °C when
the catalysts Pil-Nb-14.8, Pil-Nb-10.5 and Pil-Nb-4.5 are compared, respectively.
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In addition, the oxidation of organic compounds promoted by metal catalysts is
recognized as a catalyst structure-sensitive reaction.[37] Interestingly, the BTX conversion
increases as the Vmicro of the pillared materials remains close to the value of pure MCM-
22. For example, at 300 °C, the conversion efficiency for benzene is 86% of that for Pil-
Nb-14.8 (Vmicro of 0.14 cm3 g-1) but only 72% of that for Pil-Nb-4.5 with a Vmicro of 0.03
cm3 g-1.
The pillaring procedure increases the external surface for the pillared materials,
which allows the rapid diffusion of reactants and products into the active sites, confirming
that the nature and structure of the pillared materials can influence their catalytic
performance. Therefore, it is plausible that the diffusion of gas molecules does not prevent
access to the gallery region between the lamellae, and the increase in Nb content between
MWW lamellae could improve the catalytic activity for BTX oxidation.
Considering the catalyst with high Nb content (Pil-Nb-14.8%), the increased
activities can be explained by its strong niobium-support interactions and oxygen vacancy
concentrations. In oxidation reactions, the catalyst is subjected to cycles of oxidation and
reduction, in which both processes are affected by the oxygen mobility of the catalyst.[37]
The XPS results show the predominant presence of Nb5+ and a minority presence of Nb2+
and Nb4+.
Therefore, the mechanism for describing hydrocarbon oxidation is a redox
mechanism involving the reduction of the catalyst by adsorbed hydrocarbon (benzene,
toluene or xylene) with the participation of the oxygen from the lattice oxide and the
formation of CO2 that is desorbed together with H2O molecules. In this case, the reduced
catalyst surface (Nb4+ or Nb2+) is re-oxidized (Nb5+) by the molecular oxygen in the gas-
phase and this mechanism occurs via Mars-van Krevelen.[37-38] Thus, the mechanism
involves the supply of oxygen by the oxide followed by the introduction of oxygen
species from the oxide into the structure of the hydrocarbon molecules and the reoxidation
of the reduced solid by the oxygen-containing gas phase. In our case, the Pil-Nb-14.8
catalyst created the most active catalyst for oxidation. We believe that our results
represent an important improvement in terms of performance relative to conventional
systems. It is interesting to note that a slight decrease in the surface area is detected when
the Nb content increases but this does not affect the catalytic oxidation of the BTX.
For instance, the conversion percentages are also superior to other catalysts
reported in the literature, as presented in Table 3, which compares the Pil-Nb-4.5 sample
fabricated here to conventional catalysts and noble metals, such as Au and Pd, supported
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in mesoporous materials under the same experimental conditions. A comparison is also
made with other lamellar materials such as layered double hydroxides (LDHs) and
pillared interlayered clay (PILC) previously reported as catalysts for VOCs oxidation.
Table 3. Oxidation of BTX over Pil-Nb-4.5 material and other catalysts previously
reported in the literature.
Space
Metal
Temp. Conv. Velocity
Catalysts
loading
(%)
Substrate
Reference
(°C)
(%)
(mL.g-
1.h-1)
Pil-Nb-4.5
4.5
4.5
5.0
5.6
2.54
5.0
5.6
Benzene
Toluene
Benzene
Benzene
Benzene
Toluene
Toluene
Toluene
300
300
300
300
270
300
300
255
78
56
61
68
50
44
49
50
12000
12000
12000
12000
30000
12000
12000
40000
This work
Pil-Nb-4.5
This work
[39]
Au/MCM-41
Au-Pd/MCM-41
Cu/MnOx–CeO2
Au/MCM-41
Au-Pd/MCM-41
AuCo-10
[39]
[40]
[39]
[39]
[41]
8.5(Co
)
1.21 (Au)
[42]
[43]
[43]
[44]
[44]
[45]
Au/TiO2
1.3
0.5/2.5
3.0
Toluene
Benzene
Benzene
Benzene
Toluene
Benzene
375
250
290
260
250
300
50
50
10
99
99
60
35000
60000
60000
60000
60000
20000
Cu0.5Co2.5Al LDH
Cu3Al-MMO LDH
CoAlONH4 LDH
CoAlONH4 LDH
MnCe(6:1)/Al-PILC
*
*
6:1
LDH: Layered Double Hydroxide; * Hierarchical CoII2.8 CoIII1 LDH; PILC: Al-Pillared
Interlayered Clay.
The remarkable catalytic activities observed for the Pil-Nb-4.5, Pil-Nb-10.5 and
Pil-Nb-14.8 samples are related to four main factors. (1) The unusual surface morphology
allows the production of materials with a high surface area by the separation of the MWW
lamellae by pillars that increase in the external surface; this is reflected by the excellent
performance for various catalytic processes related to the MWW-type zeolite
counterparts. (2) The difference in the electronic structure is important because it allows
different redox ratios on the surface, which are catalytically active sites for oxidation
reactions. (3) The uniform distribution of niobium without significant agglomeration is
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10.1002/chem.202000862
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important; a uniform distribution is often crucial for catalytic activities and the presence
of Nb2+, Nb4+ and Nb5+ on the surface of the catalyst provides very catalytically active
sites for the oxidation reaction. (4) The presence of Lewis acid sites provided by the
introduction of niobium, which is characterized by FTIR spectroscopy with pyridine as
the probe molecule (Figure 9).
We believe that the niobium metal centers act as Lewis acid sites on the surface
of the catalyst, which should be an additional stabilizing effect of the adsorbed species.
The presence of Lewis acid sites present in the catalyst may allow the substrate molecules
to interact with the acid sites of the catalyst and electrostatically activate the methyl group
in order to the oxidation reaction occurs. The remarkable catalyst activity, stability and
low Nb loading described herein along with the application of green chemistry principles
enables the design of safer materials.
We also investigated the catalytic stability of the Pil-Nb-4.5, Pil-Nb-10.5 and Pil-
Nb-14.8 materials, increasing the time of the reaction for the benzene oxidation that
showed the highest activity (Figure 12). These experiments confirmed that no significant
loss of catalytic activity is detected even after 36 h, showing that these materials may
represent good candidates for gas-phase catalytic applications.
Figure 12. Benzene conversion percentages as a function of time with Pil-Nb-4.5, Pil-Nb-10.5,
Pil-Nb-14.8 and MCM-22 zeolite.
Regarding the operational conditions of the reaction (gas flow rate, type of reactor,
the composition of the reaction mixture) and the type of structure of the hydrocarbon,
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they can reflect directly on the oxidation BTX. Moreover, most reported catalysts are
combined with other very active phases, such as noble metals or metal oxides (Au, Pd,
Pt, Co, Mn).[46] In our work, a hierarchical MWW zeolite with a lamellar structure
containing pillars of silicon (an inert phase) and niobium oxide was studied to focus on
the contribution of the niobium, which presents a very high activity. It is interesting to
note that the conversion of benzene and toluene by Pil-Nb-4.5 was higher than that of Au-
based catalysts. These results indicate that MWW-type zeolites containing pillars with
silicon and niobium oxide can be considered promising catalysts for BTX oxidation.
4. CONCLUSION
It was possible to obtain hierarchical MWW zeolites with combined micro and
mesoporous architectures where niobium participates as active species in pillars. The
niobium content reveals a profound impact on the physical and chemical properties,
resulting in a gradual decrease to the basal spacing and, consequently, the interlamellar
mesoporosity at concentrations high than 4.5 % Nb. However, even with the highest
concentration of 14.8% niobium, the robustness of pillars still preventing the structural
interlamellar collapse. The set of spectrometric analyses reveals different niobium
environments, a result of the difference of the electronic structure which allows different
redox ratios on the surface, which played a key role in catalytic performance. The higher
catalytic activity occurred with pillared material with the high niobium content (Pil-Nb-
14.8), with conversions of benzene, toluene, and o-xylene of 86, 66 and 34% respectively.
This indicates that the diffusion of reagents and products to the active sites located on the
pillars was not limited even with the reduction of interlamellar mesoporosity. All pillared
materials exhibited high stability with no significant loss of activity occurred even after
36 h of reaction. Therefore, the present results may have important implications for the
design and synthesis niobium-containing MWW-type materials with pillars as active
species and excellent performance for catalytic oxidation of VOCs, and possibly many
other industrially relevant oxidation reactions.
ACKNOWLEDGMENTS
A.J.S. thanks the Cordenação de Aperfeiçoamento de Pessoal de Nível Superior –
Brasil (CAPES) – Finance Code 001, the PDSE program (process number
99999.004779/2014-02) and the prof. Claudio Radtke from the PPGQ-UFRGS for the
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10.1002/chem.202000862
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XPS analyses. C.W.L. is grateful to INOMAT/CAPES for a postdoctoral fellowship. U.D.
thanks the funding by the Spanish Government (MAT2017-82288-C2-1-P).
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