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

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

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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

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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 Schwanke^a*, Rosana Balzer^b, Christian Wittee Lopes^a, Débora Motta

Meira^c, Urbano Díaz^d, Avelino Corma^d, Sibele Pergher^e

^aUniversidade Federal do Rio Grande do Sul, 91501-970, Porto Alegre, RS, Brasil.

*Corresponding author: anderson-js@live.com

^bUniversidade Federal do Paraná - UFPR, 85950-000, Palotina, PR, Brasil.

^cCLS@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

^dInstituto de Tecnología Química, Universitat Politècnica de València-Consejo Superior de

Investigaciones Científicas, Avenida de los Naranjos s/n, E-46022 Valencia, Spain.

^eUniversidade 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

SiO₂pillars 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 SiO₂pillars 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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In contrast, SiO₂pillars 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

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 N₂atmosphere 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^-1up to 550 °C (N₂flowing) kept at the same temperature for

an additional 8 h (O₂flowing). 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^-1h⁻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 N₂and 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 I₁and I₂ionization 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 d₀₀₁spacing 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 d₀₀₁spacing, 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 d₀₀₁to 4.0 nm. The XRD patterns of the Pil-Nb-10.5

and Pil-Nb-14.8 materials show a decrease in the d₀₀₁spacing 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

S_BET

S_ext

V_TP

V_micro

V_meso

2 -1

3 -1

(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 N₂isotherms 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/p⁰region that was created

by pillaring and is due to capillary condensation occurring in the mesopores. For the other

pillared materials, the N₂adsorption 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 (W_p) ranging between 2 - 4 nm (small mesopores) in the case of Pil-

Nb-4.5. The decreasing W_pto 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 W_pprofiles are compared with those

of the MCM-22 zeolite. These results are consistent with the XRD results that were

previously discussed.

Figure 2. (a) N₂adsorption 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^-1more than MCM-22) and an increase

in pore volume related to V_mesocreated by pillaring. Besides, the values of S_extfor the Pil-

Nb-4.5 sample with 727 m² g^-1are 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 S_extvalues are three times higher than that of MCM-22. On the other hand,

V_microof 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 V_microof Pil-Nb-4.5 (0.03 cm³g^-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 V_microestimated was 0.03 cm³ g^-1, which is similar to the V_micro, 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 d₁₀₀spacing of 1.26 nm was determined by the intensity profile (see

inset). This value is similar to the d₁₀₀spacing 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 (d₀₀₁spacing 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 Nb₂O₅may 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 Nb₂O₅the 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

Nb₂O₅are 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 Nb₂O₅is 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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be ascribed to a certain hydration degree of the sample during XAS measurements or even

to the presence of Nb₂O₅domains 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^-1and 3670 cm^-1correspond 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^-1for 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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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^-1is 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^-1are 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^-1is characteristic of pyridine adsorbed on both LAS

and BAS acid sites. Furthermore, the vibration band observed at ~1447 cm^-1is assigned

to hydrogen-bonded species that result from the interaction of hydroxyl groups on the

zeolite surface ^[34]. The band at 1610 cm^-1for 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^-1is 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^-1decreases 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^-1increases

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 d₀₀₁spacing 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 d₀₀₁spacing, 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 CO₂are 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 V_microof 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 (V_microof 0.14 cm³g^-1) but only 72% of that for Pil-Nb-4.5 with a V_microof 0.03

cm³g^-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 CO₂that 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

5.0

5.6

2.54

5.0

5.6

Benzene

Toluene

Benzene

Toluene

300

270

300

255

78

56

61

68

50

44

49

50

12000

30000

12000

40000

This work

Pil-Nb-4.5

This work

[39]

Au/MCM-41

Au-Pd/MCM-41

Cu/MnO_x–CeO₂

Au/MCM-41

Au-Pd/MCM-41

AuCo-10

8.5(Co

)

1.21 (Au)

Au/TiO₂

1.3

0.5/2.5

3.0

Toluene

Benzene

Toluene

Benzene

375

250

290

260

250

300

50

10

99

60

35000

60000

20000

Cu_0.5Co_2.5Al LDH

Cu₃Al-MMO LDH

CoAlONH₄LDH

MnCe(6:1)/Al-PILC

*

6:1

LDH: Layered Double Hydroxide; * Hierarchical CoII_2.8CoIII₁LDH; 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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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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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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A Lamellar MWW Zeolite With Silicon and Niobium Oxide Pillars: A Catalyst for the Oxidation of Volatile Organic Compounds

DOI: 10.1002/chem.202000862

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