Effect of the composition of (Mo, Nb, V, Ti)/Γ-Al2O3 surface oxide structures on the oxidative dehydrogenation of ethane to ethylene

Article abstract of DOI:10.1134/S1070427216010055

Molecular layering method was used to synthesize on the surface of γ-Al₂O₃ vanadium-titanium oxide structures of varied composition and structure from the gas phase and niobium- and molybdenum-containing additives from the liquid pha

Full text of DOI:10.1134/S1070427216010055

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2016, Vol. 89, No. 1, pp. 34−39. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © S.V. Mikhailovskii, N.A. Zhilyaeva, A.A. Obletsova, M.M. Ermilova, N.V. Orekhova, A.A. Malygin, A.B. Yaroslavtsev, 2016, published in
Zhurnal Prikladnoi Khimii, 2016, Vol. 89, No. 1, pp. 37−43.
VARIOUS TECHNOLOGCAL
PROCESSES
Effect of the Composition of (Mo, Nb, V, Ti)/γ-Al2O3 Surface
Oxide Structures on the Oxidative Dehydrogenation
of Ethane to Ethylene
S. V. Mikhailovskiia, N. A. Zhilyaevab, A. A. Obletsovaa, M. M. Ermilovab,
N. V. Orekhovab, A. A. Malygina, and A. B. Yaroslavtsevb
a St. Petersburg State Technological Institute (Technical University), Moskovskii pr. 26, St. Petersburg, 190013 Russia
b Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninskii pr. 29, Moscow, 119991 Russia
e-mail: Mikhailovsky83@mail.ru
Received December 31, 2015
Abstract—Molecular layering method was used to synthesize on the surface of γ-Al2O3 vanadium-titanium oxide
structures of varied composition and structure from the gas phase and niobium- and molybdenum-containing ad-
ditives from the liquid phase. The resulting composites and the starting aluminum oxide were studied by X-ray
diffraction analysis and chemical-analytical and adsorption methods. The catalytic properties of the synthesized
products were examined in the oxidative dehydrogenation of ethane. It was shown that the activity of the samples
is affected not only by the composition and structure of surface structures, but also by the relative amounts of
components in mechanical mixing of vanadium-titanium oxide and niobium oxide supported systems.
DOI: 10.1134/S1070427216010055
One important problem in both fundamental and
practical regards is the selective conversion of a required
component in a mixture of hydrocarbons to the required
target product.As an example of a component of this kind
can serve ethane present in considerable concentrations
(from 5–10 to 20–30%) in some natural gas fields and
gases formed in pyrolysis of oil raw materials. Ethane is
hardly used and is burnt together with methane. At the
same time, its dehydrogenation will make it possible
to obtain inexpensive ethylene and products of its
polymerization, the world’s demand for which is steadily
increasing to become 150 million tons per year according
to prognoses for 2015 [1].
also insufficient and further studies were concerned with
mixed oxide systems (solid solutions) containing approxi-
mately 1 to 50% impurity element oxides (in particular,
the mixtures containing no V, Mo, and Ni at all), including
double [3, 6–8], ternary [9], and quaternary oxide systems
[9–12]. In addition to mixing of V, Mo, and Ni oxides, it
was found effective to add Nb to the mixtures, although
niobium oxide by itself does not exhibit a high activity
in the reaction of oxidative dehydrogenation of ethylene.
The most extensively studied among mixed oxide systems
of four elements is Mo–V–Te–Nb–O. It is assumed that
the role played by vanadium oxide in this system consists
in the activation of ethane, with molybdenum oxide en-
hancing the catalytic activity of vanadium atoms. The role
of tellurium and niobium is related to raising the selectiv-
ity of the vanadium–molybdenum system with respect to
ethylene. The Mo–V–Te–Nb–O system is distinguished
by activity below 400°C, which makes it possible to re-
duce the energy expenditure and minimizes the number
of side reactions [12, 13]. However, the development of
catalytic systems of this kind with optimal composition
is still a complicated and long empirical process.
The dehydrogenation of lower paraffins (C2–C4) poses
an important problem associated with the poor reactivity
of these hydrocarbons, reversibility of the nonoxidative
dehydrogenation, and high process temperatures [2].
Using the oxidative variant of dehydrogenation makes it
possible to substantially lower the process temperature.
Various metal oxides were studied as catalysts for this
process, with oxygen compounds of V, Mo, and Ni found
to be the most active [2–6]. However, their activity was
34

EFFECT OF THE COMPOSITION OF (Mo, Nb, V, Ti)/γ-Al2O3 SURFACE OXIDE STRUCTURES
35
When studying single oxides in the oxidative de-
hydrogenation of ethane, researchers mostly consider
supported systems, whereas mixed oxide catalysts are
mostly of bulk type. One of advantages of supported,
i.e., heterogeneous solid-phase systems of the core (sup-
port)–shell (catalytic additive), is the possibility of using
matrices with large specific surface area. This can strongly
improve the catalytic properties of structured supported
systems, compared with bulk catalysts, because of the
increasing amount of accessible active centers per unit
mass of a sample. In addition, a complex multilayer shell
constituted by layers of varied composition and structure
on the surface of the core-support, which can also affect
the type of surface centers and their activity in the cata-
lytic reaction.
Two-component vanadium-titanium oxide structures
of varied composition and structure (sets I and II) were
formed on the surface of γ-Al2O3 were formed by the
molecular layering method via alternating treatments of
the support in prescribed order with vapors of VOCl3,
TiCl4 [special-purity grade, 12-3 brand, TU (Technical
Specification) 6-09-2118–77] and water. The synthesis
was performed in a flow-through installation with a
vertical glass reactor under the previously described
conditions [14]. The chemical composition and the order
and number of treatments (ML cycles) of the solid-phase
matrix with appropriate reagents are listed in the table.
Molybdenum and niobium oxide structures (1Mo and
1Nb samples in the table, respectively) were synthesized
on the surface of γ-Al2O3 with the chlorides MoCl5 and
NbCl5 (Sigma Aldrich, 99.9%). Because these solid
chlorides have at room temperature a vapor pressure that
is substantially lower than that of liquid VOCl3 and TiCl4,
a previously suggested modernized procedure was used,
in which porous silica is modified with molybdenum(V)
oxide structures via its treatment at room temperature with
a solution of MoCl5 in a dried solvent, e.g., CCl4. Thus,
a liquid dried medium, a carrier, rather than a gaseous
medium, was used to deliver chloride molecules into the
reaction chamber with a solid-phase matrix.
The efficiency of this approach was demonstrated in
[14] for the example of a system constituted by γ-Al2O3
(core) with surface oxide structures (shell) of the nVOx/
mTiOy type, produced by the molecular layering (ML)
method. Here, n and m are the numbers of cycles of the
ML of the corresponding element oxide structures: n =
1–3, m = 1–3. A study of these systems in the reaction of
oxidative dehydrogenation of methanol to formaldehyde
demonstrated that presence of titanium oxide centers,
together with oxide structures of vanadium, improves the
selectivity of the process with respect to formaldehyde.
The XPS method demonstrated that a part of core centers
remain accessible to regents at any values of n and m.
In the present study, we synthesized niobium- and
molybdenum-containing aluminum oxide samples in
a vertical flow-through glass reactor (the scheme is
shown in Fig. 1), with acetonitrile of chemically pure
grade serving as the organic solvent. An aluminum
oxide powder was placed in reactor 1 on grid 2 having
the form of a glass filter (Duran, porosity 0), covered
with cap 3, and calcined at 500°C in a flow of N2 dried
to a dew point not worse than –55°C. The temperature
was monitored and controlled with an ARCOM D44
temperature indicator controller 4 with a chromel-
alumel thermocouple, connected to resistive furnace 5.
Further, γ-Al2O3 was cooled to room temperature and,
with nitrogen delivered from below, first the solvent
and then a weighed portion of a metal chloride (NbCl5
or MoCl5) were introduced into the reaction zone in
amounts preliminarily calculated on the basis of the
possible concentration of an element in the monolayer
(0.4–0.6 mmol g–1 of a sample). In the process, the gas
flow precluded penetration of liquid phase 6 to below the
partition and simultaneously promoted an active agitation
of the liquid and solid phases. Upon addition of MoCl5 to
acetonitrile, the solution was colored red-brown, with the
In various studies, the oxidative dehydrogenation of
ethane to ethylene was performed with the role of sup-
ports of simple and complex oxide systems played by
matrices of varied chemical nature, such as Al2O3, SiO2,
zeolites, and TiO2 [3–6, 8], with γ-Al2O3 being the most
widely used.
The goal of our study was to examine the influence
exerted by the composition of new element oxide systems
(Ti, V, Mo, and Nb as elements) based on γ-Al2O3,
produced by the molecular layering system, on the process
of the oxidative dehydrogenation of ethane to ethylene.
EXPERIMENTAL
As the starting substrate served active aluminum
oxide of A-1 brand [GOST (State Standard) 8136–85].
Its grains were ground and sieved to obtain the powder
fraction with particle size of 0.2–0.4. The fraction was
washed with distilled water to remove residual amounts
of the dust fraction, dried to air-dry state at 120°C, and
then calcined in air at 500°C for no less than 4 h.
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36
MIKHAILOVSKII et al.
To substitute chlorine in surface structures, they
were treated with water vapor. The stage of vapor-phase
hydrolysis was performed in a flow of air, with the
temperature gradually raised from 190–200 to 500°C
until hydrogen chloride completely ceased to be evolved
at the reactor outlet. It can be assumed that, in this case,
the following reaction occurs on the surface by the scheme
(=Al –O–)nECl5–n + (5–n) H2O
→ (=Al –O–)nE(OH)5–n + (5–n)HCl.
(2)
Atreatment in an air flow with increasing temperature
is apparently also accompanied by oxidation of Mo(V)
to Mo(VI). Also, with the temperature raised to 500°C,
there presumably occurs partial dehydroxylation of
surface centers.
1Mo and 1Nb samples were subjected to a chemical
analysis by atomic-emission spectroscopy with inductively
coupled plasma (AES ICP) on a Shimadzu ICPE-900
spectrometer upon their preliminary dissolution in aqua
regia. The methods used to determine the content of
titanium and molybdenum in the samples by AES ICP
and photocolorimetry were described in [14].
Nitrogen
Fig. 1. Schematic of a flow-through reactor for synthesis of
Mo- and Nb-containing aluminum oxide samples. (1) Reactor,
(2) grid, (3) cover, (4) temperature indicator controller with
thermocouple, (5) resistive furnace, and (6) liquid phase
agitated together with the solid phase by a flow of nitrogen.
The specific surface area Ssp of the starting and
modified products was found from the low-temperature
adsorption of nitrogen with a Micromeritics ASAP 2020
sorption meter.
color subsequently becoming bright green. In the course
of the synthesis process, the solution and solid-phase
colors acquired a yellow tint. Upon addition, NbCl5 to
acetonitrile, the solution was colored yellow.
An X-ray phase analysis (XPA) was made on a Difrei
(Scientific Instruments ZAO) diffractometer with CuKα
radiation at Bragg angles 2θ = 18°–61°.
The samples were tested in the reaction of oxidative
dehydrogenation of ethane in the presence of atmospheric
oxygen and helium carrier-gas in a pulsed metallic mi-
croreactor in the temperature range 250–450°C under
atmospheric pressure. The molar ratio between ethane and
atmospheric oxygen was 1.3 : 1; volume of an introduced
pulse, 1 mL; catalyst mass 0.1–0.3 g; and volume of the
catalyst mixed with a quartz powder, 2 mL. The reaction
products and the starting reagents (ethane, ethylene, car-
bon dioxide) were analyzed with a Kristallyuks 4000M
gas chromatograph with heat-conductivity detector and
During the first stage of the synthesis, not only ace-
tonitrile was gradually removed in the flow of a gas, but
also hydrogen chloride is evolved at the reactor outlet,
which is presumably due to the interaction of the chlorides
with hydroxy groups on the surface of aluminum oxide.
The system with a sample was purged with a flow of gas
at room temperature until the solvent was completely
removed and a loose air-dry powder was obtained. Then
the temperature was gradually raised first to 60–80°C
and then to 190–200°C and the process was continued
until the release of HCl at the reactor outlet completely
ceased. Thus, it can be assumed that, in the interaction
with niobium and molybdenum chlorides in dried ace-
tonitrile, the following reaction occurs on the surface of
aluminum oxide by the scheme
Porapak Q column. The conversion of ethane (XC H
,
%), selectivity with respect to ethylene (SC H4, %), 2an6d
2
yield of ethylene (YC H4, mmol gram-atom–1 V) were
calculated as follows:2
ХC2H = (с(C H ) – с(C H ) ) × 100/с(C H ) ,
2 6 0
(3)
n(=Al–OH) + ECl5 → (=Al –O–)nECl5–n + nHCl, (1)
6
2
6 0
2
6 1
where c(C H ) is the initial concentration of ethane
where E is Nb or Mo.
2
6 0
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EFFECT OF THE COMPOSITION OF (Mo, Nb, V, Ti)/γ-Al2O3 SURFACE OXIDE STRUCTURES
37
Chemical analysis data and adsorption characteristics of catalyst samples synthesized in the study
Sample
designationa
Modifier content per 1 g of a sample, mmol g–1
Ssp, m2 g–1 (BET)
set
I
Ti
V
Mo
Nb
γ-Al2O3-500
1Ti1V
225
232
227
225
211
203
178
208
213
–
0.52
0.52
0.53
1.44
1.32
1.36
–
0.54
0.93
1.21
0.47
0.75
1.04
–
–
–
–
–
1Ti2V
1Ti3V
–
–
II
3Ti1V
–
–
3Ti2V
–
–
3Ti3V
–
–
–
–
–
1Mo
0.75
–
–
1Nb
–
–
0.55
0.19
(3Ti3V+1Nb)b
0.88
0.67
–
a In the sample designation the symbol of an element means the corresponding element oxide structure synthesized by the ML method; the
digit before the symbol, the number of ML cycles in this synthesis; and the order of symbols, the order in which a structure is fabricated.
b Mechanical mixture of samples with a content of elements per 1 g of a mixture.
(mol %), and c(C H ) is the concentration of unreacted
ethane (mol %);
size ~10 nm) were observed in the X-ray diffraction
patterns of Mo- and Nb-containing samples, as it also
previously occurred for diffraction patterns of vanadium-
titanium oxide products [14]. In this case, the intensity
of the γ-Al2O3 reflections decreases only slightly as
compared with the starting matrix. The specific surface
areas (see table) of 1Mo and 1Nb samples decrease only
slightly as compared with the starting aluminum oxide.
Apparently, this is mostly due to the decrease in the
mass fraction of the porous support in these samples.
This behavior was demonstrated in [14] for titanium,
vanadium, and vanadium-titanium oxide structures
formed by the ML method on aluminum oxide. Thus, it
can be assumed that Mo and Nb oxide centers are rather
uniformly distributed surface structures that have no
pore-covering coarse formations.
2
6 1
SC2H = сC2H /(с(C H ) – с(C H6)1),
(4)
4
4
0
6
2
2
where cC H4 is the concentration of ethylene being formed
(mol %);2
ΥC2H = ХC2H SC2H N(C H ) /matomV
,
(5)
4
6
4
2 6 0
where N(C H ) is the initial amount of ethane in the
2
6 0
volume (mmol), and matomV is the number of gram-atoms
of vanadium in a weighed portion of the catalyst (gram-
atom V).
RESULTS AND DISCUSSION
The results of a chemical analysis of the samples
synthesized in the study and their specific surface
areas are listed in the table. For all the samples, the
gain in the modifier content in the first ML cycle is
somewhat larger than that in the subsequent cycles. The
element concentrations in 1Mo and 1Nb samples are
comparable with the gain in Ti and V in a synthesis of the
corresponding oxide structures in an ML cycle.
Figure 2 shows temperature dependences of the yield
of ethylene in the oxidative dehydrogenation of ethane for
samples of sets I and II and for a mixed sample. It can be
seen that the yield per gram-atom of vanadium is lower
for sample 1Ti3V, compared with other samples of set I
(Fig. 2a). Similarly, the yield for 3Ti3V at temperatures
below 400°C is lower than that for 3Ti2V (Fig. 2b). This
may indicate that, on the one hand, the optimal content
of vanadium for the oxidative dehydrogenation of ethane
is close to the content in samples obtained in 1 or 2 ML
Strongly blurred broadened peaks of a support with
poor crystallinity (pseudoboehmite structure, crystallite
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38
MIKHAILOVSKII et al.
(a)
is deposited on the core, aluminum oxide, because the
concentration of titanium oxide structures is insufficient
for overlapping the matrix. In this case, vanadium oxide
structures bound to aluminum oxide turn out to be les
active than those bound to titanium oxide structures.
It can be seen from a comparison of Figs. 2a and 2b
that the yield of ethylene grows with increasing number
of cycles of ML of titanium oxide structures.Apparently,
this is due to the fact that, for samples of set II, the matrix
is overlapped by titanium oxide structures to a greater
extent, and the amount of vanadium oxide structures
bound to Ti–O structures is considerably larger. Thus,
the increase in the yield may be due both to the lesser
involvement of the matrix itself in the catalytic process
and to an increase in the dehydrogenating activity of
vanadium oxide centers. Apparently, three ML cycles of
titanium oxide structures are insufficient for overlapping
the whole matrix.
(b)
These considerations agree with the results of [14],
where it was shown that the overlapping of the surface
of the γ-Al2O3 core with titanium oxide structures, which
hinders the access of reagents to its active centers, may
be one of factors enhancing the catalytic activity of sup-
ported vanadium titanium oxide systems in the reaction of
oxidative dehydrogenation of methanol to formaldehyde.
(c)
These effects are also manifested when the temperature
dependences of the selectivity with respect to ethylene
are compared for samples of sets I and II (Fig. 3).Among
samples of set I, sample 1Ti1V was found to be the most
selective in the temperature range 370–440°C. With
increasing content of vanadium and decreasing Ti/V ratio,
the selectivity decreases in the order 0.73 : 0.40 : 0.36.
On passing from sample 1Ti3V to 3Ti3V, the selectivity
becomes 1.7 times higher (370°C), but it grows within
set II with increasing vanadium content. For example, the
selectivity at 370°C is 0.33 for sample 3Ti2V and 0.61
for sample 3Ti3V.
T, °C
Fig. 2. Dependence of the ethylene yield Y on temperature T.
(a) Samples of set I: (1) 1Ti1V, (2) 1Ti2V, and (3) 1Ti3V;
(b) samples of set II: (1) 3Ti2V and (2) 3Ti3V; (c) samples
(1) (3Ti3V + 1Nb) and (2) 3Ti3V.
A study of molybdenum- and niobium-containing
samples demonstrated that the maximum conversion of
thane for these samples is about 5% in the temperature
range 370–440°C, with 1Mo sample being somewhat
more active, and 1NB sample, more selective.
cycles of vanadium oxide structures. Apparently, the
3rd cycle yields to a significant extent vanadium oxide
structures bound to lower lying V–O structures, rather
than with titanium oxide structures, which may impair
their catalytic properties. On the other hand, the fact the
yield curves are close for 1Ti1V and 1Ti2V samples in the
temperature range 400–450°C may be also indicative of a
certain positive effect of the additional overlapping of ac-
tive centers of the matrix by these structures. For samples
of set I, a considerable part of vanadium oxide structures
It was of interest to examine the influence exerted by
niobium additive on the increase in the selectivity of the
ethylene yield in the course of the oxidative dehydroge-
nation of ethane. We examined several sample mixtures
and, in particular, the oxide system (3Ti3V + 1NB) with
Ti : V : Nb atomic ratio of 1 : 0.76 : 0.21. The tempera-
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EFFECT OF THE COMPOSITION OF (Mo, Nb, V, Ti)/γ-Al2O3 SURFACE OXIDE STRUCTURES
39
S, fraction units
the related selectivity as compared with the pure 3Ti3V
sample.
(4) The results obtained in the study demonstrate
that the molecular layering method is promising for
fabrication of surface oxide heterostructures, which
can implemented by development of catalysts for the
oxidative dehydrogenation of ethane to ethylene and
optimization of their composition.
ACKNOWLEDGMENTS
The study was in part financially supported by the
Ministry of Education and Science of the Russian
Federation, Basic research program no. 5 of the Presidium
of the RussianAcademy of Sciences, and Russian Science
Foundation (agreement no. 14-13-00597).
T, °C
Fig. 3. Dependence of the selectivity S with respect to ethylene
on temperature T for samples (1) 1Ti1V, (2) 1Ti2V, (3) 1Ti3V,
(4) 3Ti2V, (5) 3Ti3V, and (6) (3Ti3V + 1Nb).
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