The evolution of the M1 local structure during preparation of VMoNbTeO catalysts for ethane oxidative dehydrogenation to ethylene

Article abstract of DOI:10.1039/c8ra06424e

The so-called M1 phase (the common formula (TeO)_x(Mo, V, Nb)₅O₁₄) is a very promising catalyst for ethane oxidative dehydrogenation (ODE). It shows 90% selectivity to ethylene at 78% ethane conversion (400 °C, contact time-5.5 s). The active crystal structure is formed under certain synthetic conditions in VMoNbTe mixed oxides. This paper is devoted to the analysis of how the local and average structure of the M1 phase is developed during the synthesis and what happens at particular synthetic steps. The analysis of the local structure was performed using the EXAFS and pair distribution function (PDF) methods. The EXAFS analysis of the initial VMoTe water solution and VMoNbTe slurry showed that Anderson-type heteropoly anions are formed in the solution and are preserved after fast spray-drying of the slurry. Nb cations do not enter the structure of the polyanions, but form an extended hydrated oxide matrix, where distorted NbO₆ and NbO₇ polyhedrons are connected to each other. The hydrated oxide matrix with captured polyanions provides the compositional homogeneity of the precursor. The distances in the second coordination shell are redistributed after thermal treatment at 310 °C. After being heated at T > 350°, the local structure of the M1 phase is organized and pentagonal domains are formed. These domains consist of a NbO₇ pentagonal bipyramid and five MeO₆ adjacent octahedra (Me = Mo, V). In the first stages, the building blocks are stacked along the [001] direction. The crystallization process results in the connection of the pentagonal domains to the extended polygonal grid. The formation of the regular grid with TeO_x containing channels is accompanied by the increase in ethane conversion and ethylene selectivity of the catalysts.

Full text of DOI:10.1039/c8ra06424e

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The evolution of the M1 local structure during
preparation of VMoNbTeO catalysts for ethane
oxidative dehydrogenation to ethylene†
Cite this: RSC Adv., 2018, 8, 35903
a
a
a
a
T. Yu Kardash, * E. V. Lazareva, D. A. Svintsitskiy, A. V. Ishchenko,
a
b
V. M. Bondareva and R. B. Neder
The so-called M1 phase (the common formula (TeO)
x
5
(Mo, V, Nb) O
14) is a very promising catalyst for ethane
ꢀ
oxidative dehydrogenation (ODE). It shows 90% selectivity to ethylene at 78% ethane conversion (400 C,
contact time – 5.5 s). The active crystal structure is formed under certain synthetic conditions in VMoNbTe
mixed oxides. This paper is devoted to the analysis of how the local and average structure of the M1 phase is
developed during the synthesis and what happens at particular synthetic steps. The analysis of the local
structure was performed using the EXAFS and pair distribution function (PDF) methods. The EXAFS
analysis of the initial VMoTe water solution and VMoNbTe slurry showed that Anderson-type heteropoly
anions are formed in the solution and are preserved after fast spray-drying of the slurry. Nb cations do
not enter the structure of the polyanions, but form an extended hydrated oxide matrix, where distorted
6 7
NbO and NbO polyhedrons are connected to each other. The hydrated oxide matrix with captured
polyanions provides the compositional homogeneity of the precursor. The distances in the second
ꢀ
ꢀ
coordination shell are redistributed after thermal treatment at 310 C. After being heated at T > 350 , the
local structure of the M1 phase is organized and pentagonal domains are formed. These domains consist
of a NbO
7
pentagonal bipyramid and five MeO
6
adjacent octahedra (Me ¼ Mo, V). In the first stages, the
Received 30th July 2018
Accepted 11th October 2018
building blocks are stacked along the [001] direction. The crystallization process results in the
connection of the pentagonal domains to the extended polygonal grid. The formation of the regular grid
DOI: 10.1039/c8ra06424e
with TeO
x
containing channels is accompanied by the increase in ethane conversion and ethylene
rsc.li/rsc-advances
selectivity of the catalysts.
VMoNbTe oxide catalysts show unsurpassed catalytic charac-
teristics in the ethane oxidative dehydrogenation. According
1
. Introduction
3–6
Industrial processing of associated petroleum gases and their to ref. 1 and 7, the ODE process based on the VMoNbTe oxide
components – methane, ethane and propane is in the focus of catalysts show energy and cost savings in comparison to the
both fundamental and applied research. The growth of shale pyrolysis process.
gas production and the need to decrease CO
2
emissions are the
The catalytic activity and selectivity of such catalysts are
1,2
8–12
main reasons for this. Oxidative dehydrogenation of ethane attributed to the so-called M1 phase.
The structure of the M1
13–15
(
ODE) is one of the possible processes for effective utilization of phase is studied virtually at the atomic level.
The M1 phase
is a layered compound with a common formula (TeO)0.23(Mo, V,
14 (see Fig. 1). Distorted octahedrons of MoO - and VO
existing pyrolysis processes for the production of ethylene, types and NbO pentagonal bipyramids are connected to form
a natural gas feedstock.
ODE has the potential to become an alternative to the Nb)
5
O
6
6
-
7
which is a commodity chemical widely used in industry. Pyrol- extended grids. The grids are stacked along the [001] direction
ꢀ
ysis is a high temperature (800 C) process that thermally cracks forming hexa- and heptagonal channels both of these are
x
a feedstock of ethane or liquid fuels. ODE transforms ethane to occupied by TeO species depending on the Te content in
ꢀ
ethylene catalytically, under mild conditions (300–400 C). a sample.
The implementation of the VMoNbTeO catalysts in an
industry has several challenges. One of them is a complex
a
Boreskov Institute of Catalysis SB RAS, Pr. Ak. Lavrentieva, 5, 630090, Novosibirsk,
synthetic procedure, deviations from which result in signicant
Russia. E-mail: kardash@catalysis.ru
b
16,17
changes of phase composition and structural disorder.
To
Friedrich-Alexander University of Erlangen-Nuremberg, Staudtstr. 3., 91058 Erlangen,
obtain the M1 phase, all the components should interact already
at the step where a wet precursor is synthesized, which is reached
Germany
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra06424e
18–21
in a narrow range of pH values.
Chemical composition,
1
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The goal of the present work is to study the development of
the local structure of the M1 phase in the VMoNbTe oxide
samples with cationic composition V0.3Mo Nb0.12Te0.23
,
1
3
4
typical for highly active catalysts. We started with the
Te0.23 solution and analyzed the local structure
V
0.3Mo
1
changes during all the synthetic steps using EXAFS and pair
distribution function (PDF) methods.
Both methods provide information about the local struc-
ture for highly disordered materials, amorphous structures,
and liquids. PDF is a powerful tool to study and describe the
structure of both nanoparticles and highly disordered mate-
rials. PDF curve is a measure of the probability of nding pairs
of atoms in the material separated by a distance r. As PDF data
Fig. 1 Schematic representation of the M1 phase in the (001) plane (a)
and the stacking of the grids along the [001] direction (b). Mo atoms are
shown in blue, Nb – green, V – black, Te – brown, O – red. Atomic
coordinates taken from the ref. 14.
drying and the conditions of heat treatment also have a strong is obtained from a diffraction experiment, it provides all atom-
3
5
inuence on the phase composition of the samples. M1 oxide pair correlations in the material. EXAFS data provides
2
2–25
phase is prepared either by hydrothermal synthesis
slurry” method, which implies mixing of aqueous solutions of
initial reagents followed by solvent removal and thermal treat- study the structural evolution from the mixed V0.3Mo Te
or by the chemically resolved information on the local structure.
“
We used EXAFS spectroscopy for Mo–K and Nb–K edges to
1
0.23
26,27
ment in combined mode.
The essential synthetic steps and solution to V0.3Mo Nb0.12Te0.23 slurry, amorphous spray-dried
1
required conditions are mostly known from empirical ndings, sample and highly disordered oxide precursor formed aer
whereas the structural transformations that occur at each step calcination. The joint PDF analysis and EXAFS spectroscopy
are still ill-understood. Controlled synthesis of M1 phase with were applied to model the atomic structure of a highly disor-
predictable surface properties requires a deeper investigation of dered precursor of the M1 phase. X-ray photoelectron spec-
the reactions proceeding in the initial solutions and during troscopy (XPS) was applied to address the changes in the
crystallization.
oxidation state of the components (Mo, V, Nb and Te) during
The detailed analysis of chemical transformations that occur the thermal treatment of the dried precursor.
in the mixed solutions of the starting components was performed
28–30
by Raman and NMR spectroscopy methods.
It was shown that
in the initial water mixed solution, that contains Mo, V and Te,
different heteropoly anions (HPA) are formed and their compo-
sition, structure, a degree of condensation and protonation
2
. Experimental
2
.1. Sample preparation
For preparation of solutions, the following starting materials
According to ref. 21 and 31 the optimal pH to obtain the best were used: ammonium paramolybdate (PMA) (NH ) -
28
changes with component concentrations, temperature and pH.
4
6
catalysts should be z3. At this pH values Anderson-type heter- Mo O $4H O, ammonium metavanadate (MVA) NH VO and
7
24
2
4
3
6ꢁ
5ꢁ
27
opoly anion [TeMo O ] and polyoxoanion [TeV Mo O ] as telluric acid (TA) H TeO . All the compounds were the best
the major species were detected. This pH value is reached when available reagent grade and were used without further
6
24
5
3
6
6
28
Nb oxalate solution is added to the VMoTe solution. It results in purication.
the formation of a VMoNbTe slurry. Although in ref. 29 and 30 it
For preparation of the solution with a Mo/V/Te molar ratio
was detected that the structure of anionic species does not (S1 sample) corresponding to the optimum catalyst composi-
change aer spray-drying of the slurry, the particular role of Nb at tion V0.3Mo Nb0.12Te0.23, PMA (1.77 g), MVA (0.351 g), and TA
1
ꢀ
this synthesis stage is yet unclear.
(0.528 g) were dissolved in distilled water (10 mL) at 80 C with
ꢀ
The development of the M1 crystal structure during the stirring, followed by cooling to ca. 40 C. The pH of the resulting
thermal treatment of VMoNbTe dried precursor was studied by solution was z6.
19,29,32
several groups.
The successive synthetic procedure includes
For obtaining a V0.3Mo Nb0.12Te0.23Nb
gel (G1 sample),
1
0.12
ꢁ1
ꢀ
2ꢁ
short activation in air ow at 310 C with a subsequent heating in a Nb oxalate solution (2.5 mL, 46.5 mg mL Nb; C O4 /Nb ¼
2
ꢀ
He ow up to 550–600 C. The oxides precursors are semi- 3 : 1) was added to a V0.3Mo Nb0.12Te0.23 solution.
1
ꢀ
crystalline up to 550 C when M1 crystallization stars. Such
The gel with a bulk composition V0.3Mo Te0.23Nb0.12 was
1
crystallization is a non-equilibrium process. Minor deviations dried using a spray-drying procedure (sample D1) or under
from the optimal synthetic procedure might result in the ambient conditions at room temperature (sample D2). A lab
formation of additional phases such as M2, Mo O , and MoO . spray-dryer (Buchi-290) was used; the inlet temperature was
5
14
3
ꢀ
ꢀ
The M2 phase usually co-exists with the M1 phase in the active 220 C and the outlet temperature was 180 C.
13
catalysts. Its structure is closely related with a hexagonal bronze.
Mo
mixed oxides and has a similar structural arrangement to M1.
The solid resulting aer spray-drying was divided into
14-type of the structure is usually formed in the VMoNb several parts. One sample was calcined in air at 310 C for 1
hour (H310 sample). Other samples were heated in two steps:
14 and M1 in the arrangement rst in air at 310 C for 1 hour and then in He ow at 350 C
of the polygonal grids is the absence of big heptagonal channels (H350), 400 (H400), 450 (H450) and 550 C (H550). Heating in
ꢀ
5
O
33
ꢀ
ꢀ
The only difference between Mo
5
O
ꢀ
22
in the Mo O structure.
He ow was conducted for 2 h for every sample.
5
14
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.2 Characterization techniques
RSC Advances
ꢀ
2
at atmospheric pressure and temperature of 400 C using feed
consisting of a mixture C H : O : N ¼ 10 : 10 : 80 (% vol).
2
6
2
2
X-ray diffraction was performed using Bruker D8 Advance
diffractometer and CuKa-radiation. The diffraction intensities
were measured using a LynxEye position sensitive detector. XRD
Ethane conversion and selectivity to the reaction products
were calculated as in ref. 31 Additional details on catalytic
experiment are in ESI.†
ꢀ
ꢀ
patterns were collected in the 2q range of 5–70 , with a 0.02
step size and a 3 s collection time. Phase analysis was performed
using the ICDD PDF-2 database. The Rietveld renement was
performed using a Topas v.4.3 (Bruker) soware.
3
. Results and discussion
XPS measurements were performed on a ES-300 photoelec-
tron spectrometer (KRATOS Analytical) using an MgKa (hn ¼
3.1 XRD analysis of structural transformations
1253.6 eV) X-ray source (the power of 180 W). Spectrometer Table 1 shows the bulk chemical composition of VMoNbTeO
calibration was performed using values of BE(Au4f7/2) ¼ 84.0 eV samples aer spray-drying and heating in He at different
and BE(Cu2p3/2) ¼ 932.7 eV for Ar-sputtered gold and copper temperatures.
foils, respectively. Before measurements all samples were
We used XRD data for a preliminary characterization of the
pressed into a pellet followed by the xation using a conducting solid intermediates obtained by spray-drying, heating in He
carbon tape. No charging effect was observed during spectrum ow and the nal crystalline sample.
acquisition. Processing of spectral data (curve tting, area
Fig. 2 displays the XRD patterns of the VMoNbTeO samples
calculation, difference spectra, etc.) was performed using the aer spray-drying (D1 sample), short-term heating in air at
ꢀ
homemade XPS-Calc program, which has been tested on 310 C (H310) and heated in He ow (H350, H450 and H550).
36–38
a number of systems.
The procedure of curve tting was The diffraction patterns for the H400 and H500 samples were
performed using an approximation based on a combination of similar to the XRD pattern of the H450 sample. According to the
the Gaussian and Lorentzian functions with the subtraction of XRD data, D1 sample is an amorphous solid. The following
ꢀ
Shirley-type background.
heating in He below 550 C results in a formation of disordered
Mo–K and Nb K X-ray absorption spectra were recorded on solids (H310, H350, H450 and H500). The diffraction pattern of
an EXAFS station at the Siberian Synchrotron Radiation Center these samples is characterized by the presence of two peaks at
ꢀ
ꢀ
ꢀ
(
Novosibirsk) using the standard transmission procedure. The 2q ¼ 22.20 and 45.2 and a broad diffuse scattering at 2q z 27 .
spectra were recorded at electron energy of 2 GeV in the VEPP-3 The crystallization of the bulk M1 phase occurs aer heating in
ring and current of 80 mA. The spectrometer had a Si(111) He at 550 C (H550). Curve 4 shows the Rietveld t of the
ꢀ
double monochromator and proportional chambers as detec- experimental pattern. The structural parameters of the tting
tors. Data processing and analysis were performed using Athena model were taken from the ref. 14 The full-prole Rietveld
39
and Artemis soware. For all samples, the oscillating part of renement of the diffraction data conrmed that the sample is
2
the EXAFS spectrum (c(k)) was analyzed in the form of k c(k) in a single phase. Table S1 (ESI†) shows the tting results and
ꢁ
1
˚
the wave number range k ¼ 2.5–12.5 A
.
structural parameters of the crystalline phase.
ꢀ
ꢀ
X-ray PDF data were collected at beam line 11-ID-B, APS,
The peaks at 2q ¼ 22.20 and 45.2 , observed for the H350
˚
Argonne at an energy of 58.2636 keV, i.e. l ¼ 0.2128 A. Data and H450 samples, aer crystallization become (001) and (002)
collection was at room temperature as well. The samples were reections of the M1 structure. It indicates that at the rst stage
stored in 0.18 cm polyimide capillaries (Cole-Parmer, Illinois). of crystallization the long-range order appears in the [001]
40
Data treatment was performed with PDFgetX3. Building and direction, which corresponds to the interlayer distance of 4.0 A˚
renement of PDF for nanoparticles was done using DISCUS (see Fig. 1b). So, we will identify these samples as “semi-
41
program. To visualize structural models, VESTA 3 soware was crystalline materials” to indicate the presence of the ordering
42
used.
in the [001] direction and disorder in other directions. The
TEM data was obtained using JEM-2010 (accelerating voltage crystallization process is connected with ordering in the (001)
˚
of 200 kV, resolution of 1.4 A) and JEM-2200FS (eld electron plane when hk0 reections appear on the XRD pattern.
emission cathode, accelerating voltage of 200 kV, resolution of
˚
0
.8 A) transmission electron microscopes. Particles were
deposited by dispersing the sample suspension in ethanol on Table 1 The chemical composition and specific surface area of
VMoNbTeO samples after heating in He at different temperatures
a copper substrate using an ultrasonic dispergator. The local
analysis of the elemental composition of the samples was per-
formed with the use of a Phoenix EDAX spectrometer with
a Si(Li) detector and an energy resolution of about 130 eV.
Chemical composition
ꢀ
2
ꢁ1
Sample
Tcal, C
SBET, m g
V/Mo
Nb/Mo
Te/Mo
D1
220
350
400
450
500
550
—
0.30
0.30
0.30
0.30
0.30
0.30
0.11
0.11
0.11
0.11
0.11
0.11
0.23
0.23
0.20
0.21
0.18
0.17
2
.3 Catalytic testing
H350
H400
H450
H500
H550
4.5
5.0
6.7
7.3
5.4
The ethane oxidative dehydrogenation (ODE) was carried out
in a xed-bed glass tubular reactor with on-line chromato-
graphic analysis of the reaction mixture components. Catalyst
fraction 0.25–0.50 mm was used. Experiments were performed
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The BE(Nb3d5/2) value in case of Nb
2
O
5
is varied in the range
45,46
5+
2
07.2–207.5 eV,
while corresponding value for Nb in the
Mo O - or M1-like structures decreases down to 206.8–
5
14
33,47,48
206.9 eV.
Observed Nb3d5/2 shi during high-temperature
heating was probably related to the change of local coordination
environment accompanied by the insertion of niobium in M1-
like structure of VMoNbTe mixed oxide. Note that such
ꢀ
a process occurred only at T > 400 C.
The Mo3d spectrum of the D1 sample was characterized by
the BE(Mo3d5/2) value equal to 232.7 eV and integral halfwidth
6
+
of 1.7 eV (Fig. 3b). It corresponds to Mo state in the MoO3
49,50
ꢀ
structure.
Heating in helium at 400 and 550 C (H400 and
H450 samples) resulted in the signicant spectral broadening
of Mo3d line (FWHM ¼ 1.9–2.1 eV), indicating the contribution
from two components. Corresponding Mo3d spectra were
curve-tted into two doublets with BE(Mo3d5/2) values of 232.3
and 233.0 eV. It was reliably shown that the VMoNbTe oxide
5
+
6+
with M1 structure contains the combination of Mo and Mo
1
3,47
species.
Thus, components with BE(Mo3d ) values of 232.3
5
/2
6+
5
+
and 233.0 eV were attributed to Mo and Mo states, respec-
tively. The surface Mo ratio for samples heated at 400 and
5
+
ꢀ
5
50 C was close to ꢂ52 and ꢂ57%, respectively. The shi of
6
+
Mo3d5/2 peak from 232.7 to 233.0 eV in case of Mo state can be
related to the change of molybdenum coordination environ-
ment during the formation of M1 structure.
An increased FWHM value was the reason for curve-tting of
V2p into two components with BE(V2p3/2) near ꢂ516.2 and
4
+
1
Fig. 2 XRD patterns for the V0.3Mo Nb0.12Te0.23 samples after spray-
ꢂ517.2 eV (see Fig. 3c). Such components were attributed to V
dying and thermal treatment. 1 – D1 sample (spray-dried precursor), 2
H350, 3 – H450, 4 – H550. The red curve shows the Rietveld fit for
the crystalline M1 phase.
5+
51,52
4+
and V states.
In case of the D1 sample the small V
–
contribution appeared immediately during spectra acquisition
5
+
(
see Fig. S1, ESI†), while the predominant presence of V state
was observed on its initial surface. The H400 and H550 samples
were characterized by an unchangeable surface state during XPS
3
.2 XPS study
4
+
Fig. 3 presents curve-tted Nb3d spectra for different VMoNb-
TeO samples: from the spray-dried precursor (D1 sample) up to
oxide heated in He at 550 C (H550). Table 2 summarizes the
measurements. The V ratio at the surface of such samples was
4
+
5+
close to ꢂ58%. The combination of V and V states is
ꢀ
a characteristic feature of VMoNbTe oxides with M1 struc-
XPS data for all studied samples. A surface chemical composi-
tion of studied samples during all synthetic steps is shown in
ESI, Table S2.† For the D1 sample, the element atomic ratios
were different from the bulk. In the course of heating, the
surface V/Mo and Nb/Mo rations were approaching the bulk
values. Te/Mo surface ration was increasing during heating in
He, which shows the surface enrichment by Te in the crystalline
H550 sample.
The value of FWHM (full width at half maximum) for all
experimental Nb3d lines was close ꢂ1.7 eV, indicating the
presence of only single niobium state. The D1 sample was
characterized by the Nb3d5/2 peak position near 207.1 eV.
13,53
ꢀ
ture.
So, it can be suggested that the heating up to 400 C is
enough for insertion of vanadium into nal mixed oxide.
For all solid samples the maximum of Te3d5/2 peak was
4
+
located near 576.5 eV (see Fig. S2, ESI†) corresponded to Te
50
species. So, the main tellurium state on the surface of
4
+
VMoNbTe oxides is an oxidized form of Te .
3.3 EXAFS study
EXAFS analysis of Mo and Nb local structures was performed
because of their higher scattering power in comparison to V and
Te. Furthermore, Mo is an element that determines the struc-
ture, whereas Nb role is still debatable.
ꢀ
Heating in helium at T # 400 C didn't result in any evident
spectral changes for Nb3d line (for example, broadening or
shi). The shi of Nb3d5/2 peak ꢂ0.3 eV towards smaller
binding energies (BE) was observed for the H450, H500 and
H550 samples (see Table 1). The Nb3d spectra obtained for the
H450, 500 and 550 samples were similar to each other. The
BE(Nb3d5/2) value close to ꢂ207 eV corresponds to oxidized
In order to interpret the spectra of the initial solution, an
EXAFS spectrum for a water solution of the (NH ) [TeMo O ]
salt was measured (see Fig. 4, curve 1). Curves 2–4 in the Fig. 4
show the pseudo-radial distribution functions (RDF) obtained
via Fourier transformation of the Mo–K EXAFS spectra for the S1
4
6
7 24
sample (V0.3Mo Te
solution), and dried powders D1 and D2
0.23
1
5+
43,44
niobium in form of Nb .
So, the niobium state on the
correspondently. RDF curve for the G1 sample was similar to the
RDF of the D1 sample and is not shown. Curves 5–7 in the Fig. 4
5
+
surface of all studied samples can be considered as Nb .
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Fig. 3 Photoelectron Nb3d, Mo3d and V2p spectra: 1 – D1 sample; 2 – H400 sample; 3 – H550 sample. The FWHM values (full width at half
maximum) are presented for experimental spectral lines.
show RDFs for the H350, H450 and H550 samples. The RDFs are interpretation of EXAFS data on the determination of mean
not phase-shi corrected and the peaks are shied by approx- interatomic distances within the atomic groups and compar-
˚
imately z0.4 A with respect to the crystallographic distances.
ison of different samples.
6
ꢁ
For an interpretation of EXAFS spectra in the mixed Mo-
Fig. 5b shows the structure of the [TeMo
7
O
24
]
anion. The
based compounds, the severe distortion of MoO octahedrons, RDFs for the (NH ) [TeMo O ] salt and the S1 sample (see
6
4 6
7 24
where the central atom is shied from the center towards one of Fig. 4, curve 2) show three characteristic peaks of the Anderson
the edges, should be considered. Due to the distortion, the Mo– anion: the rst and the second peak could be attributed to Mo–
O lengths in the rst coordination sphere vary from 1.8 to 2.4 A˚ O distances in the distorted polyhedron, the third peak corre-
˚
(
see Fig. 5a) which causes a wide variation of metal–metal sponds to the mean Mo–Me (Me ¼ V, Mo) distance of 3.3 A. This
54
distances in the second coordination shell. A similar distor- distance corresponds to the distance between adjacent octahe-
5
+
5+
tion is typical for Nb and V oxides. Therefore, the inclusion drons as shown in the Fig. 5b. According to ref. 21 vanadium
of all the paths in the modeling of EXAFS spectra is impossible, could substitute Mo in the Anderson heteropoly anion, there-
because it would produce an excessive number of correlated fore, vanadium could also contribute to the second shell
parameters. Because of this, we performed only qualitative distances. No additional peaks are present on the RDF curves,
Table 2 XPS data for studied VMoNbTe oxides on different preparation steps
Nb3d
Mo3d
V2p
5+
5+
6+
4+
5+
Nb
Mo
Mo
V
V
Sample
BE, eV
FWHM, eV
BE, eV
%
BE, eV
%
FWHM, eV
BE, eV
%
BE, eV
%
FWHM, eV
a
a
D1
207.1
207.05
207.1
207.1
206.95
206.9
206.8
1.7
1.9
—
—
33
53
52
53
55
57
232.6
232.8
233.0
233.0
233.0
233.0
233.0
100
67
47
48
47
45
43
1.7
1.8
1.7
1.75
1.7
1.6
1.7
516.2
516.3
516.2
516.2
516.2
516.3
516.2
13
44
56
58
55
57
58
517.3
517.3
517.1
517.1
517.1
517.1
517.1
87
56
44
42
45
43
42
1.7
H310
H350
H400
H450
H500
H550
232.1
232.3
232.3
232.3
232.3
232.4
1.85
1.75
1.7
1.7
1.7
1.75
1.75
1.75
1.75
1.75
1.6
a
The partial reduction of vanadium was observed during spectra acquisition.
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˚
distance that corresponds to the interlayer distance of 4.0 A
ꢀ
appears for samples, heated at temperature $450 C.
The RDF curve for Mo–K edge of the H550 sample (see Fig. 4,
curve 7) can be described by six groups of interatomic distances:
three groups of metal–oxygen distances and three groups of
metal–metal distances. The typical distances of the M1 phase
5
within the {(Nb)Me } domain are shown in Fig. 5c. The Mo–O
distances correspond to the bond lengths in the distorted MoO
6
˚
octahedra: short 1.7 and 2.3 A in the [001] direction and average
˚
13
˚
2
.0 A in the (001) plane. The short distance 1.7 A is clearly
resolved only for the H550 sample, indicating the structural
ordering. For the H350 and H450 samples, distances at 1.7 and
˚
2
.0 A are not resolved. Mo–Me (Me ¼ Mo, V, Nb) distances
indicate different types of polyhedral connection in the M1
˚
structure. The short distance 3.3 A corresponds to the mean
distance between the pentagonal bipyramid and the edge-
˚
sharing octahedrons, the longer 3.8 A – between corner-
Fig. 4 Pseudo-radial distribution functions (RDF) obtained via Fourier
transformation of Mo–K edge EXAFS spectra. 1 – The water solution of
sharing octahedrons in the (001) plane. The 4.0 A peak corre-
˚
sponds to the distance between polygonal grids along the [001]
(
4
NH )
6 6
TeMo O
24; 2 – S1; 3 – D1; 4 – D2; 5 – H350, 6 – H450 and 7 –
13
direction (see Fig. 1b).
H550.
Thus, for the H350 and H450 samples, we observe a recon-
struction of the metal–metal coordination shell. There appears
a peak, which corresponds to the distance between the
pentagonal bipyramid and the adjacent octahedrons. The
crystallization of the M1 phase results in an increase of the
intensities of other Me–Me peaks indicating the appearance of
the long-range ordering in the polygonal grid.
The RDFs for the Nb–K edge of the sample V0.3Mo Te0.23-
1
Nb0.12 at various synthetic steps are shown in Fig. 6. The Nb
source for synthesis is Nb-oxalate solution, which is shown by
the RDF curve is in Fig. 6 (curve 1). The RDF of an Nb-oxalate
solution is characterized by the single Nb–O peak, whereas
peaks in the second coordination sphere are not pronounced.
The general formula of a niobium oxalate solution could be
described as NH [NbO(C O ) (H O) ]$nH O. The characteristic
4
2
4 2
2
2
2
structural unite of Nb-oxalate has a pentagonal bipyramidal
55
structure with a short Nb]O double bond and bidentate
oxalate-ligands.
The addition of Nb-oxalate solution to the S1 solution causes
the formation of the orange-colored gel and signicant changes
Fig. 5 The structural arrangement and typical interatomic distances in the Nb local structure (G1 sample, Fig. 6, curve 2). For the G1
6
for (a) the MoO octahedron, (b) Anderson-type HPA and (c) the
pentagonal domain in the M1 structure.
sample, the Nb–O peak is shied to the longer distances indi-
cating the Nb–O bond length typical for Nb oxides, which are
55,56
longer than those of the oxalate complex.
formation of an amorphous hydrous niobium oxide Nb
nH O, which contains distorted NbO , NbO and NbO
polyhedra.
It indicates the
indicating that anions are not included in the ordered
structures.
2
O
5
-
$
2
6
7
8
The local structure of the Anderson anion is preserved
during spray-drying in the D1 sample (see Fig. 4, curve 3).
However, for the D2 sample, we observe a decrease in the
intensity of the Mo–Me peak (see Fig. 4, curve 4). Thus, the
prolonged aging of the wet precursor (G1 sample) in air under
ambient conditions caused the destruction of the Anderson
anion.
55
The characteristic feature of the G1 sample is the appearance
of the pronounced peak that corresponds to the Nb–Nb
distance. The tting of the spectra showed that its peak corre-
˚
sponds to the distance of 3.4 A, typical for Nb–Nb octahe-
56
drons, connected by common edges.
The integration of Nb cations into the structure of the
Anderson anion by the substitution of Mo cations in the HPA
structure (see Fig. 5b) would result in an appearance of the
similar Nb–Me distances as on the Mo-RDF of the G1 and D1
The subsequent heating in He ow at higher temperature
results in the appearance of new Mo–Me peaks corresponding
to the interatomic distances (see Fig. 4, curves 6 and 7) that are
23
characteristic of the local structure of the M1 phase. The
˚
samples, namely 3.30 A. However, Nb–Nb distances are longer
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structure, we would observe the decrease in Me–Me intensities
for the both Nb–K and Mo–K RDFs.
The Nb-RDF for the H350 sample shows broadening of all
the peaks (see Fig. 6, curve 5). It is an indication of the strong
structural disorder of the Nb local structure that takes place
aer the sample is heated in He ow.
The Nb-RDF for the H450 sample (see Fig. 6, curve 6), is
similar to the RDF curve as H550 sample. It shows that the local
Nb structure in the semi-crystalline sample is similar to those of
˚
M1 crystal structure. The absence of the 1.8 A peak and lower
intensities of the RDF peaks corresponds to a disordering of the
structure. However, the main characteristic features of the M1
˚
phase is the 3.3 A distance, presented on the RDF curve of the
semi-crystalline sample. It is known from the previous HAADF-
STEM studies, that Nb occupies pentagonal columns of the M1
57
structure. The similar Nb–K EXAFS spectra were obtained for
23
a phase-pure M1 structure by Sanz et al. Our data support
Fig. 6 Pseudo-radial distribution functions (RDF) obtained via Fourier this nding.
transform of Nb–K edge EXAFS spectra. 1 – Nb-oxalate solution; 2 –
G1 gel; 3 – D1 sample; 4 – D2 sample; 5 – H350; 6 – H450; 7 – H550.
The data obtained by EXAFS show that the {(Nb)Me } (Me ¼
5
Mo, V) domain is already formed in the semi-crystalline samples
despite the long-range ordering along the [001] direction only.
than Mo–Mo distances in the D1 and G1 samples. So, it could be
concluded that Nb cations do not enter the structure of the HPA. 3.4 X-ray PDF study
The tting results and used parameters are shown in the ESI
As the detailed modeling of the EXAFS data is complicated due
to the presence of a number of different distances even in
a crystalline sample, we performed a PDF analysis and structure
simulation for a semi-crystalline sample (H450). The PDF
advantage to EXAFS is the possibility of direct measuring of the
interatomic distances.
X-ray PDFs for the H450 and H550 samples are shown in
Fig. 7a and b. The PDF data shows that the local structures of
the samples with crystalline M1 phase and semi-crystalline
precursor are similar. The rst three peaks at 1.8, 2.0 and 2.3
A˚ correspond to different Me–O (Me ¼ V, Nb, Mo) interatomic
(
see Fig. S3 and Table S3†).
The difference between Nb local structure and Mo local
structure within the VMoNbTeO gel is also revealed as
a different behavior upon its drying. The Nb local environment
does not change either during spray-drying or long aging in the
ambient air (D1 and D2 samples, Fig. 6, curves 3 and 4).
However, the long aging causes the signicant decrease of
intensity of the Mo–Mo (or Mo–V) peak on the Mo-RDF, which is
connected with a destruction of the HPA (D1 and D2 samples,
Fig. 5, curves 3 and 4). If Nb cations were included in the HPA
ꢀ ꢀ
1
Fig. 7 (a) X-ray PDF data for the sample with composition V0.3Mo Nb0.12Te0.23 after heating in he at 450 C and 550 C. (b) Experimental PDFs in
a wide range of distances and calculated PDF for a pentagonal domain from Fig. 5c.
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distances in the highly distorted MeO
samples. The PDF maxima near 3.3, 3.7 and 4.0 A match three structural feature of semi-crystalline oxide phases in the
different types of Me–Me distances in the M1 structure. The VMoTe and VMoNb oxide systems.
6
octahedrons for both
The similar pentagonal domains were found as a main
˚
5
8,59
However, despite the
appearance of such distances is in good agreement with the structural similarity of the semi-crystalline phases in these
Mo–K and Nb–K edge EXAFS data (see Fig. 4, curve 7 and Fig. 7, systems, the phase composition aer crystallization is
curve 6).
different. In the V0.3Mo
1
Te0.23 oxide system, M2 phase and
5
9
The difference in the long-range order between the samples (Mo, V)
heated at 450 and 550 C is displayed in the PDF region above 7– semi-crystalline
5
O
14 phases are formed, while the calcination of the
ꢀ
V
0.3Mo
1
Nb0.37 precursor results in the
A˚ . For the semi-crystalline sample, we observe the fall of peak appearance of the (Mo, V, Nb)
O14 mixed oxide.
33
8
5
˚
intensities above 8 A distance range. The crystalline sample
shows evident G(r) maxima beyond the distance range of 20 A˚ 3.5 TEM study
shown in Fig. 7. The semi-crystalline sample has no intensive
In order to conrm PDF and EXAFS structural data, the TEM
analysis was performed. We started from the H310 sample, as
D1 sample was very unstable under electron beam exposure.
Fig. 9 shows the TEM image of the H310 sample. The images
show big porous spongy particles in the sample. The EDX
˚
peaks at the distances >20 A.
The observed features of the local structure are similar to
those previously reported by our group in ref. 58, where the
development of the Mo O14-like local structure was studied in
5
the three-component VMoNb system. To simulate the structure
of a semi-crystalline VMoNb oxide, a model based on the
pentagonal domain was developed.
A similar procedure was applied in current work to simulate
the structure of a VMoNbTe oxide of the M1 phase. The unit cell
of the M1 structure was expanded along a, b, and c directions to
build a sufficiently large crystal. During the second step, an
ellipsoid particle was cut from the big crystal. The procedure is
illustrated in Fig. 8. The pentagonal bipyramid was placed in
the center of the ellipsoid. The long axis of the ellipsoid corre-
sponds to the [001] direction in the M1 crystal.
1
analysis shows the composition Mo V0.29Te0.23Nb0.17, which
corresponds well to the bulk chemical composition. The parti-
cles are amorphous, which corresponds well to the EXAFS and
XRD data.
Fig. 10 shows TEM images for the H450 sample. The sample
is characterized by the presence of the disoriented crystallites
within the amorphous matrix. The diffraction pattern taken
from the area is marked in the Fig. 10b shows only 4.0 A˚
reections. In the Fourier ltered image, it can be seen that
there is no pronounced boundary between the amorphous
structure of the matrix and the crystallites, i.e. crystallization
occurs from the amorphous phase. Some structural features of
the M1 phase can be recognized in the Fourier ltered image.
However, these structural fragments are disordered within the
matrix. This nding conrms the PDF observation on the
similarity of the local structure in the crystalline M1 phase and
its semi-crystalline precursor.
Fig. 11 shows TEM images of the H550 sample. According to
the XRD data, the crystallization of the M1 phase was observed
for this sample. The TEM images show the large particles with
an ordered crystalline lattice. The diffraction data shows the set
of reections that correspond to the interplanar distances in the
M1 phase.
Fig. 7b shows the calculated PDF from the ellipsoidal particle
with the length of 7 nm along the [001] direction and width of
3
nm in a perpendicular direction. The simulated PDF shows
good qualitative agreement with the experimental PDF.
According to the PDF data, although the semi-crystalline
samples do not show a long-range order in the (001) plane
ꢀ
before calcination at 550 C, the M1 local structure is already
formed. The main fragment of the local structure is a pentag-
onal bipyramid in the {(Nb)Me } domain (Me ¼ Mo, V). These
5
domains are rstly stacked on the top of each other, building
the ordering along the [001] direction. As a result, the (001)
reection appears on the XRD pattern.
Fig. 8 Nanodomain structure building from the M1 structure.
Fig. 9 TEM image of the H310 sample.
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Fig. 10 TEM of the H450 sample. (a) The microstructure of the particles (b) the red square shows the sample area, where diffraction was
measured (c) the Fourier filtered image of the sample area on the (b) image.
Table 3 The catalytic properties of VMoNbTeO catalysts in ethane
oxidative dehydrogenation
Selectivity, %
ꢀ
a
Sample
T, C
Ethane conversion , %
C
2
H
4
CO
CO
2
H350
H400
H500
H550
350
450
500
550
10.2
12.1
41.5
77.6
73.0
80.9
88.4
90.0
18.6
13.6
7.5
8.4
5.5
4.1
3.3
6.7
a
Ethane conversion at a contact time 5.5 s.
Both ethane conversion and ethylene selectivity increase
with the rise in temperature of the sample heating in He. The
crystalline M1 phase occurred aer the catalyst's heating at
ꢀ
5
50 C. The crystalline M1 phase shows the highest ethane
conversion and ethylene selectivity. H350 sample slowly
changes its catalytic characteristic with time in a stream,
therefore the initial ethane conversion and reaction products
selectivity are shown in Table 3.
Fig. 11 TEM image of the H550 sample. The red square area was used
to obtain a Fourier filtered image and diffraction data.
For all synthesized catalysts the main reaction product in
ODE is ethylene, and carbon oxides (CO, CO ) are by-products.
2
The dependence of products selectivity on ethane conversion
for the H500 and H550 samples is shown in Fig. 12. Selectivity to
3.6 Catalytic measurements
ethylene decreases, while CO and CO
ethane conversion growth. This is in agreement with literature
2
selectivities increase with
Catalytic properties of the VMoNbTeO samples aer heating at
different temperatures in the reaction of ethane oxidative
dehydrogenation are presented in Table 3.
3,8–10,31
data
and reects the triangular consecutive-parallel
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(
1). Preparation of V0.3Mo
1
Te0.23 aqueous solution from
(NH ) Mo O $4H O, NH VO and H TeO .
4 6 7 24 2 4 3 6 6
(
2). Addition of niobium oxalate solution to the V0.3Mo Te
1 0.23
solution, which resulted in the formation of the V0.3Mo1-
Nb0.12Te0.23O gel (wet precursor).
(
(
3). Spray-drying procedure giving dried precursor.
ꢀ
4). Calcination at 310 C in air, followed by heating in He
ꢀ

ow at 550 C.
The schematic representation of all synthetic steps and
structural transformations that were detected in this work are
summarized in Fig. 13.
The obtained Mo–K edge EXAFS data for the synthetic steps
1
–3 is in a good agreement with the literature ones and our
28–30
previous ndings.
The local structure that is typical for an
Anderson-type heteropoly anion was detected for the initial
Te0.23 solution. It remains unchanged in wet and dried
V
0.3Mo
1
precursors. It was shown that Nb cations do not enter the
structure of the VMoTe heteropoly anions during the second
Fig. 12 Dependence of products selectivity on ethane conversion for
ꢀ
the sample calcined at 500 (-, ,) and 550 C (:, D). Reaction
preparation step and form a spongy hydrated NbO matrix. The
x
ꢀ
temperature 400 C, C
H
2 6
: O
2
: N
2
¼ 10 : 10 : 80 (% vol).

rst coordination shell of Nb in this matrix is typical for NbO6
octahedrons, which are connected to each other by common
edges. The local structure of the Nb in the gel is rather stable
and does not change aer prolonged drying in air, while Mo
local structure changes signicantly (D2 sample, see Fig. 4). Our
data are in good agreement with in situ Raman data, which
show no changes in the HPA structure upon addition of Nb-
oxalate solution to the MoVTe solution.
scheme of ODE reaction: ethylene and carbon oxides (both CO
and CO ) are formed from ethane via parallel way and CO are
2
x
formed from ethylene via the consecutive route. However, the
increase in ethylene selectivity and ethane conversion, as shown
in Table 3, reect the catalytic properties of the samples with
the different structure.
30
10
Our ndings are in a good agreement with Konya et al., who
The chemical interaction of all components in the amor-
phous precursor occurs when the initial polyanions are
showed that crystalline M1 phase has superior catalytic prop-
erties in ODE reaction than “amorphous” Mo
3
VO
x
oxide.
ꢀ
destroyed aer the thermal treatment at 310 C.
The similar results of the fourfold increase in activity in
VMoNbTeO catalysts were reported in ref. 20 for the reaction of
propane ammoxidation to acrylonitrile. It was shown that
reaction constant and acrylonitrile selectivity are the highest
when the semi-crystalline sample is transformed into the crys-
talline M1 phase.
ꢀ
The heating in He ow at 400–450 C results in the formation
5
of the pentagonal domains {(Nb)Me } according to the PDF and
EXAFS data. Nb cations enter the centers of pentagonal bipyr-
amids, whereas Mo cations enter the adjacent octahedrons.
According to XPS data, the heating of the spray-dried
precursor (D1 sample) is accompanied by a reduction of
molybdenum and vanadium cations caused by the decompo-
sition of ammonia salts of the HPA-anion and oxalate anions.
Major changes in comparison with the D1 sample were
observed for the spray-dried precursor and sample heated in He
3.7 Development of the M1 structure
Although progress in describing of structural features of the
active M1 phase in VMoNbTe oxide catalysts has already been
achieved, the detailed understanding of its formation during
the synthesis is still required. In this study, we analyze the
transformations that occur in the course of a ‘slurry’ synthetic
route with particular attention to Nb and its role in the struc-
tural development of the active phase.
ꢀ
at T > 400 C. It correlates with the EXAFS and PDF data that
show that the local structure of the M1 phase was already
ꢀ
formed aer heating at 400 C.
The oxidation state of niobium cations during all the
5
+
synthetic steps is attributed to the Nb state. However, the
position of Nb3d5/2 peak is highly sensitive to the local envi-
ronment of niobium. The shi of Nb3d line indicating the
rearrangement of the local structure was observed only aer
The ‘slurry’ method of synthesis of V0.3Mo
1
Nb0.12Te0.23 oxide
catalysts includes the following sequential steps for the prepa-
18,19
ration of the nal catalyst:
Fig. 13 The schematic diagram to represent the structural transformations that occur during the synthesis of the M1 phase in the VMoTeNbO
mixed oxide.
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heating at T > 400 C. So, the ordering of the local Nb structure oxides by hydrothermal synthesis,
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ꢀ
10,68
the slurry method for
59
occurred only aer the pentagonal domains {(Nb)Me } has been such compositions gives only nanosized M1 crystallites. The
5
ꢀ
formed. The crystallization of M1 phase caused the further shi heating of the nanosized crystallites at 550 C results in
of the Nb3d line and resolving of the Nb–O different distances destruction of the M1 phase and crystallization of M2 and (Mo,
of the RDF curve of the EXAFS spectrum.
The {(Nb)Me } domains are well known in the molybdenum
V)
5
O
14 oxides.
5
Fig. 14 is a scheme showing how the chemical composition
polyoxometalate chemistry. They are the building blocks of of the system containing V, Mo, Te, and Nb stimulates the
Kepplerate anions, that are formed in reduced molybdates formation of various crystalline products.
30
solutions under hydrothermal conditions.
Furthermore,
According to the literature data, vanadium cations enter the
Kepplerate anions have been reported as building blocks in centers of single octahedrons that connect pentagonal frag-
60,61
62
63
14
VMo,
MoVSb, and VMoW oxide catalysts with M1 or ments into the polygonal network. A similar process of vana-
Mo O -like structure. Similar pentagonal domains are formed dium interaction with Kepplerate ion was described for VMoW
5
14
64
63,69
aer the hydrothermal treatment of the Nb-oxalate solution.
Aberration-corrected STEM images of the distorted Nb oxide
conrmed the presence of the same pentagonal building blocks formed either into the M1 structure, or into the M2 structure, or
oxides with Mo
5
O14-like structure.
Thus, the VMoNb(Te) semi-crystalline oxide can be trans-
65
as in the semi-crystalline precursor of the M1 phase.
5
into the Mo O14-type structure. The nal structure of the
During the thermal treatment in helium, the M1 crystalli- polygonal grid is determined by the chemical composition and
zation is rstly accompanied by the stacking of the structural the thermal treatment conditions. The niobium cations are
70,71
fragments along the [001] direction. The next step is ordering in responsible for the stability of the pentagonal fragments,
the (001) plane via connection of pentagonal fragments by tellurium – for the presence of hexa- or heptagonal channels.
the
6
7
single octahedra.
Structural exibility explains the narrow pH range of the
According to our previous ndings, only Mo
5
O
14-type of the initial slurry, at which the most active VMoNbTeO catalysts with
21,28,31
structure with pentagonal domains is formed in the the largest amount of M1 phase are formed.
This narrow
58
V
0.3Mo
1
Nb0.37 oxide system. The addition of Te in the mixed pH interval corresponds to the isoelectric point of the Nb O -
2 5
28
oxide causes the formation of the M1 phase. However, the local $nH
2
O. The formation of the gel provides the uniform distri-
structure of the semi-crystalline VMoNb and VMoNbTe oxides is bution of all components. Quick spray drying contributes to the
5
8
20
similar. The major difference between these structures is preservation of this homogeneity in a precursor. The thermal
66
ꢀ
a presence of big hexagonal and heptagonal channels. Note treatment of the precursor in the range of 300–400 C forms
that the Te release during the thermal treatment of the M1 a nanostructured oxide with a local structure close to that in the
6
7
65
phase results in the formation of the (Mo, VNb)
5
O
14 structure.
M1 phase, crystallized at the high temperature.
The obtained data show that the local structure, surface
The role of Nb in the crystallization process could be
attributed to the stabilization of pentagonal bipyramids. composition and oxidation state of the elements for the semi-
Although the M1 phase could be obtained in VMo and VMoTe crystalline VMoNbTe sample and corresponding crystallized
Fig. 14 The schematic representation of the development of the structurally-related M1, M2 and Mo
systems via the transformations of pentagonal domains.
5
O
14-type oxides in the VMo(Nb/Te) oxide
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M1 phase are similar. However, its catalytic properties in the
ODE reaction differ signicantly. The crystalline M1 structure is
Acknowledgements
formed aer the pentagonal domains are connected within This work was supported by the Russian Science Foundation,
a polygonal grid. It is accompanied by the appearance of grant 17-73-20073.
ordered channels with TeO
x
units. The presence of such units is
an essential feature of crystalline M1 phase in the VMoNbTe
References
12
5+
oxide. Some of the TeO
in the polygonal networks. Such Te–O–V species are discussed
x
units are connected with V species
5
+
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–C
3
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x
by our previous data. It was shown that Te-free (Mo, V, Nb) O
oxide is inactive in ODE reaction.
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14
33
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M1 phase in the VMoNbTe oxide is supported by the ADF-STEM
74–76
imaging data.
It was demonstrated that the M1 phase
contains many nanosized defects and nascent phases,
including structurally disordered pentagonal domains co-
joined in pairs, triplets, and quadruplets etc, which could be
easily formed during the non-equilibrium crystallization
process.
5 A. M. Gaffney, R. Ghosh, R. Song, C. Y. Yeh and T. Langner,
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