Niobium-containing catalysts for oxydehydrogenation of hydrocarbons and alcohols

Article abstract of DOI:10.1134/S0036023611020148

The first methods are developed for introducing niobium(V) into Mg-Al hydrotalcites used as precursors of oxide catalysts for oxydehydrogenation (OD) of alkanes and alcohols. Samples of niobium(V)-containing oxide catalysts are synthesized. Their catalytic properties are studied in oxydehydrogenation of ethane and ethylbenzene to styrene, oxidation dehydrocyclization of octane into ethylbenzene and styrene, and oxydehydrogenation of sec-butanol to ketone (octane-(2)-one). It is ascertained that ethane transformation into ethylene is highly a selective highly process (92-97%) at low temperatures (450-500°C) in the presence of a niobium-containing catalyst; the catalyst is appreciably efficient in ethylbenzene transformation to styrene and dehydrocyclization of n-octane to ethylbenzene and styrene, and in oxydehydrogenation of secbutanol to octane-(2)-one. All the catalysts studied operate stably in OD reactions; no decrease in their activity or selectivity was detected after 50 h operation.

Full text of DOI:10.1134/S0036023611020148

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2011, Vol. 56, No. 2, pp. 168–172. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © O.N. Krasnobaeva, I.P. Belomestnykh, T.A. Nosova, T.A. Elizarova, G.V. Isagulyants, S.P. Kolesnikov, V.P. Danilov, 2011, published in Zhurnal Neorganꢀ
icheskoi Khimii, 2011, Vol. 56, No. 2, pp. 204–208.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
NiobiumꢀContaining Catalysts for Oxydehydrogenation
of Hydrocarbons and Alcohols
O. N. Krasnobaevaa, I. P. Belomestnykhb, T. A. Nosovaa, T. A. Elizarovaa,
G. V. Isagulyantsb, S. P. Kolesnikovb, and V. P. Danilova
a Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
eꢀmail: danilov@igic.ras.ru
b Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Leninskii pr. 47, Moscow, 119991 Russia
Received November 18, 2009
Abstract—The first methods are developed for introducing niobium(V) into Mg–Al hydrotalcites used as
precursors of oxide catalysts for oxydehydrogenation (OD) of alkanes and alcohols. Samples of niobium(V)ꢀ
containing oxide catalysts are synthesized. Their catalytic properties are studied in oxydehydrogenation of
ethane and ethylbenzene to styrene, oxidation dehydrocyclization of octane into ethylbenzene and styrene,
and oxydehydrogenation of secꢀbutanol to ketone (octaneꢀ(2)ꢀone). It is ascertained that ethane transformaꢀ
tion into ethylene is highly a selective highly process (92–97%) at low temperatures (450–500
ence of a niobiumꢀcontaining catalyst; the catalyst is appreciably efficient in ethylbenzene transformation to
styrene and dehydrocyclization of ꢀoctane to ethylbenzene and styrene, and in oxydehydrogenation of secꢀ
°C) in the presꢀ
n
butanol to octaneꢀ(2)ꢀone. All the catalysts studied operate stably in OD reactions; no decrease in their activꢀ
ity or selectivity was detected after 50 h operation.
DOI: 10.1134/S0036023611020148
This work continues studies on the effect of the aluminum nitrate (0.3 mol/L) and magnesium nitrate
composition of hydrotalciteꢀlike hydroxo salts used as (0.6 mol/L) at 60°C under continuous stirring until
catalyst precursors on the catalytic properties of oxide pH changed from 1 to 10. The resultant precipitate,
catalysts for oxydehydrogenation (OD) of organic which was a hydroxytalciteꢀlike double hydroxo salt of
compounds.
magnesium and aluminum (figure, curve 1), was
washed with water in a Nutch filter to remove potasꢀ
sium ions until the negative test to sodium tetrapheꢀ
nylborate was obtained.
Earlier [1–5], we have synthesized hydroxo salts
having hydrotalcite structure with various composiꢀ
tions (Mg–Al, Mg–Ni–Al, Mg–Ni–Co–Al, Mg–
Al–Cr, Mg–Al–Fe), containing nitrate, carbonate,
decavanadate, paramolybdate, and metatungstate ions
in their anionic interlayers. Oxide catalysts prepared
from them were highly selective and provided high
yields of target products in the oxydehydrogenation of
organic compounds.
This work is aimed at developing a method for
introducing niobium(V) to the catalysts and studying
its effect on the properties thereof in OD reactions of
a number of organic compounds.
In order to carry out the anion exchange with parꢀ
tial replacement of carbonate ions by decavanadate
(V10O28)6–, paramolybdate (Mo7O24)6–, and metatungꢀ
state (H2W12O40)6– ions, the paste obtained after preꢀ
cipitating and washing was diluted with water to the
ratio S : L = 1 : 2, then a solution of potassium decaꢀ
vanadate (0.15 M), or ammonium paramolybdate
(0.15 M), or sodium tungstate (0.5 M) was added in an
amount depending on the final composition required.
The pulp was stirred for 10 min, then nitric acid (0.2 M)
was dropped to it until pH reached 4.5 for the
exchange of carbonate ion for decavanadate and paraꢀ
molybdate ions [6, 7] and until pH reached 5.0 for the
exchange of carbonate ion for metatungstate ion [8]. The
mixture was kept at the required pH value for 10 min for
the exchange for decavanadate and paramolybdate
ions and for 30 min for the exchange for metatungstate
ion; the precipitate was filtered and washed with water
to remove potassium, ammonium, and sodium ions.
EXPERIMENTAL
Precursors were synthesized by reacting solutions
of metal nitrates with potassium hydroxide and potasꢀ
sium carbonate followed by anion exchange of nitrate
and carbonate ions for the corresponding polyoxometꢀ
alates according to the following procedure.
A solution containing potassium hydroxide and
potassium carbonate (2 M KOH + 1 M К2СО3) was
For introducing niobium into hydrotalciteꢀlike
slowly dropped (1 drop/3 s) to a solution containing magnesium aluminum hydroxo salt, procedures were
168

NIOBIUMꢀCONTAINING CATALYSTS
169
1
2
60
50
40
30
20
10
, deg
2
θ
Diffraction patterns for (
hydrotalciteꢀlike magnesium aluminum hydroxocarbonate hexaniobate [AlMg (OH) ][(CO )
1) hydrotalciteꢀlike magnesium aluminum hydroxo nitrate [AlMg (OH) ][(NO ) · nH O] and (2)
2 6 3 2
(Nb O
)
19 0.017
].
2
6
3 0.432
6
developed for synthesizing potassium polyoxoniobate
To ascertain the phase and chemical composition
and introducing it into the hydroxo salt by anionic of the hydroxo salts synthesized, we used chemical
exchange. For this purpose, we carried out experiꢀ analysis and Xꢀray powder diffraction (DRONꢀ2.0,
ments on alloying niobium oxide Nb2O5 with potasꢀ Cu
K radiation).
α
sium carbonate at different molar ratios K2CO3 :
Nb2O5 (7 : 1, 10 : 1, 20 : 1).
In chemical analysis, potassium was determined
gravimetrically in the form of tetraphenylborate; aluꢀ
minum was determined volumetrically by back titraꢀ
tion of excess Trilon B with zinc nitrate with Xylenol
Orange; and magnesium, complexonometrically with
Eriochrome Black in an ammonia buffer solution
masking ammonia with triethanolamine chloride.
Vanadium was determined volumetrically by titration
of Mohr’s salt in the presence of phenylanthranylic
acid as an indicator; molybdenum, gravimetrically
The alloying was carried out in platinum bowls first
at 900
°
С
(for 30 min), then at 1050 С (for 40 min).
°
The resultant glassy cake was dissolved in a minimal
amount of boiling water, cooled, and then thickened
under vacuum over concentrated sulfuric acid until
crystal were formed; the crystals were washed with
water to remove the mother liquor and dried over
СаCl2/CaO in an exiccator.
with ꢀbenzoin oxime; tungsten and niobium, graviꢀ
metrically after performing acidic hydrolysis of the
samples (gravimetrical forms were WO3 and Nb2O5).
α
Analysis of the crystals in all cases showed the molar
ratio K2O : Nb2O5 = 4 : 3, which corresponds to potassium
hexaniobate (“4 : 3 salt”) of composition K8[Nb6O19] · aq
that was earlier described in literature [9].
RESULTS AND DISCUSSION
To carry out the anion exchange of the carbonate
ion of the Mg–Al hydroxo salt for polyoxoniobate ion
To prepare the precursors of the OD catalysts,
which contain niobium in the form of hexaniobate and
other polyoxometalates (decavanadate, paramolybꢀ
date, and metatungstate ions), in view of the fact that
the latter replace carbonate ion in the interlayer space
of hydrotalciteꢀlike Mg–Al hydroxo salt at pH 4.5–
5.0 (this value being pH 13 for hexaniobate), we had to
use a mixture of two isomorphous phases: hydrotalꢀ
citeꢀlike aluminum magnesium hydroxocarbonate
hexaniobate and aluminum magnesium hydroxocarꢀ
bonate containing either decavanadate, or paramolybꢀ
date, or metatungstate ions, individually or together in
various combinations.
(
[Nb6O19]8–), we used the solution obtained by disꢀ
solving the alloy of K2CO3 with Nb2O5 having the 10 : 1
molar ratio (pH 13.0) and Mg–Al hydroxo salt.
The anion exchange was carried out at three pH
values (11, 12 and 13) by acidifying the solution until
the required pH value was attained for 6 h at the ratio
S : L = 1 : 4. The maximal amount of polyoxoniobate
was found to pass into the solution at pH 13, while
only a small amount of aluminum (5–7%) passes from
the precipitate into the solution and hydrotalcite
structure is retained (hexagonal crystal system;
3.02 Å, = 7.7 Å) (figure, curve ). Judging from the
published data, we have pioneered the synthesis of
а =
с
2
The aboveꢀmentioned mixture was prepared by
hydrotalciteꢀlike hydroxo salts containing polyoxoꢀ stirring for 2 h (to attain homogeneity) of the two starting
niobate ions. hydroxo salts in aqueous medium in the ratio S : L = 1 : 2.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 2 2011

170
KRASNOBAEVA et al.
Table 1. Composition of the samples of niobiumꢀcontaining isomorphous hydrotalciteꢀlike salts used as precursors for the oxyꢀ
dehydrogenation of organic compounds
Sample no.
Hydroxo salt mixture
[AlMg2(OH)6][(CO3)0.42(Nb6O19)0.02 · nH2O]
1
2
3
4
5
[AlMg1.5(OH)5][(CO3)0.424(Nb6O19)0.004(Mo7O24)0.02
[AlMg1.8(OH)5.6][(CO3)0.33(Nb6O19)0.02(V10O28)0.03
[AlMg1.5(OH)5][(CO3)0.379(Nb6O19)0.004(V10O28)0.005(Mo7O24)0.03
[AlMg1.6(OH)5.2[(CO3)0.407(Nb6O19)0.003(Mo7O24)0.02(V10O28)0.005(H2W12O40)0.002
·
n
H2O]
·
nH2O]
· nH2O]
· nH2O]
Table 2. Composition of the initial isomorphous hydrotalciteꢀlike hydroxo salts used for preparing precursors for the oxydehyꢀ
drogenation of organic compounds (Table 1) by mixing in the 1 : 1 weight ratio
Samꢀ
ple no.
Niobiumꢀcontaining hydroxo salt
[AlMg2(OH)6][(CO3)0.34 · (Nb6O19)0.04
Niobiumꢀfree hydroxo salt
1
2
3
4
5
·
n
H2O]
H2O] [AlMg1.5(OH)5][(CO3)0.38(Mo7O24)0.04
H2O] [AlMg1.8(OH)5.6][(CO3)0.32(V10O28)0.06
[AlMg2(OH)6][(CO3)0.5] · nH2O]
[AlMg1.5(OH)5][(CO3)0.468 · (Nb6O19)0.008
[AlMg1.8(OH)5.6][(CO3)0.34 · (Nb6O19)0.04
[AlMg1.5(OH)5][(CO3)0.468 · (Nb6O19)0.008
·
n
·
·
n
H2O]
H2O]
·
n
n
·
n
H2O] [AlMg1.5(OH)5][(CO3)0.29(V10O28)0.01(Mo7O24)0.06
H2O] [AlMg1.6(OH)5.2[(CO3)0.338(V10O28)0.01(Mo7O24)0.04
(H2W12O40)0.004] · H2O]
· nH2O]
[AlMg1.6(OH)5.2][(CO3)0.476 · (Nb6O19)0.006
·
n
·
n
The resultant mixture was sufficiently homogeꢀ
neous, which was attested by the character of sediꢀ moles of reagent passed over the catalyst)
mentation, and by the data of chemical analysis of the
pulp sampled from the top and bottom parts of the
sedimentation cylinder (V : Nb, Mo : Nb, and W : Nb
ratios were equal in these samples).
Conversion, % = (moles of converted compound /
100
Selectivity, % = (moles of target product/moles of
converted compound) 100
Yield, % = (moles of target product/moles of
reagent passed over the catalyst) 100
×
;
×
;
×
.
To elucidate the effect of Nb(V) on the catalytic
properties of oxide catalysts, we synthesized a set of
mixtures of hydrotalciteꢀlike isomorphous hydroxo
salts (Tables 1, 2).
The catalysts that were synthesized from samples
1–5 (Table 1) were used to study oxydehydrogenation
of ethane (Table 3) . The numbering of the catalysts
corresponds to the numbering of their precursors
(Table 1). The С2Н6/О2 ratio (mol/mol) was varied
from 1/0.3 to 1/0.5.
The samples synthesized (Table 1) were subjected
to heat treatment, which consisted in drying the preꢀ
The analysis of the results of catalytic studies shows
that, over niobiumꢀcontaining catalysts, the converꢀ
sion of ethane into ethylene occurs with high selectivity
cipitate at 100–120
40%, pelletizing, and igniting in a muffle furnace in an
air flow at 100 K/h to 500 ; then, the samples were
°
С until the moisture content was
°
С
(92–97%) at low temperatures (450–500 С) (Table 3).
°
kept at this temperature for 4–5 h. Catalytic properties
of the resultant materials were studied in a flowꢀ
through quartz reactor charged with 1–2 mL of the
catalyst. The oxidizer used was atmospheric oxygen.
To maintain isothermicity, the catalyst was mixed with
an equal volume of crushed quartz. The reaction temꢀ
perature, the alkane flow rate, and the alkane/oxygen
ratio in the initial mixture were varied widely. Reaction
products were analyzed by liquid chromatography
using a column packed with PoropackꢀQ. The contact
gas was analyzed for the unreacted starting compound
Rising temperature increases conversion from 19 to
58% with a simultaneous decrease in selectivity to 72–
74% (Table 3). Moreover, making the catalyst compoꢀ
sition more complex, from Nb–Al–Mg–O via Nb–
Mo–Al–Mg–O and Nb–V–Al–Mg–O to Nb–Mo–
V–Al–Mg–O and Nb–Mo–V–W–Al–Mg–O,
results in an enhancement of catalytic characteristics
and a rise in conversion from 12 to 20% and in selecꢀ
tivity from 92 to 97% (Tables 1, 3), the process occurꢀ
ring at low temperatures (450–500
°
С).
(ethane, nꢀoctane, ethylbenzene, and butyl alcohol),
It should be emphasized that unlike over Nbꢀconꢀ
dehydration product (ethylene, styrene, ethylbenꢀ taining catalysts, the oxydehydrogenation of ethane
zene, and octaneꢀ(2)ꢀone), СО2, and methane. The over catalysts of other compositions (Ni–V–Mo–W–
results of the experiments were used to calculate conꢀ Al–Mg–O, Fe–V–Mo–W–Al–Mg–O) at low temꢀ
version of the starting compound, selectivity of the peratures does not virtually occur (conversion is 7–7.5%,
reaction, and yield of the target products:
selectivity is 40–45%, yield is 2.6–3.3%; Table 4).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 2 2011

NIOBIUMꢀCONTAINING CATALYSTS
171
Table 3. Results of the oxydehydrogenation of ethane over niobiumꢀcontaining catalysts of various compositions
Sample
no.
Temperature, Dilution, mol/mol
Catalyst
Nb–Al–Mg–O
Conversion, %
Yield, %
Selectivity, %
°C
C2H6/O2
1
2
500
500
700
500
450
700
450
1/0.35
1/0.35
1/0.5
12.0
17.0
57.3
17.0
19.0
58.3
19.7
10.8
15.8
41.4
15.4
17.1
42.4
19.0
92.0
92.8
72.3
92.0
94.0
74.3
97.0
Nb–Mo–Al–Mg–O
Nb–Mo–Al–Mg–O
Nb–V–Al–Mg–O
3
4
5
1/0.35
1/0.30
1/0.50
1/0.30
Nb–V–Mo–Al–Mg–O
Nb–V–Mo–Al–Mg–O
Nb–V–Mo–W–Al–Mg–O
Table 4. Results of the oxydehydrogenation of ethane over nickelꢀ and ironꢀcontaining catalysts
Sample
no.
Dilution, mol/mol
C2H6/O2
Catalyst
Temperature, °C
Conversion, %
Yield, %
Selectivity, %
1
2
Ni
–
V
–
Mo
–
W
–
Al
–
Mg
–
O
550
550
1/1
6.90
7.45
2.65
3.30
38.7
44.6
Fe–
V
–
Mo
–
W
–
Al
–
Mg
–
O
1/0.65
Apart from the oxydehydrogenation of ethane over total yield of styrene and ethylbenzene reaches 20–
niobiumꢀcontaining catalysts (Table 1, sample 4), we 25% with a total selectivity of up to 99%.
studied the following reactions:
For the oxydehydrogenation of secꢀbutanol over
the Nb–V–Mo–Al–Mg–O catalyst, optimal condiꢀ
tions were determined for producing octaneꢀ(2)ꢀone
to provide high yields (up to 67%) and selectivities
(96–97%).
oxydehydrogenation of ethylbenzene into styrene
C6H5CH2CH3 + O2
→
C6H5–CH=CH2 + H2O + CO2,
oxydehydrocyclization of octane into ethylbenzene
and styrene
С8H18
Notable is the stable operation of all catalysts studꢀ
ied in oxydehydrogenation; no decrease in activity or
selectivity was detected after 50ꢀh performance.
To summarize, efficient niobiumꢀcontaining cataꢀ
lysts have been developed for selective conversion of
alkanes, alkylaromatic alkanes, and alcohols in oxydeꢀ
hydrogenation reactions.
→
С8H16
→
C6H5 C2H5 + C6H5CH=CH2,
oxydehydrogenation of secꢀbutanol into ketone
(octaneꢀ(2)ꢀone).
The results are displayed in Table 5.
As seen in Table 5, the catalyst studied (Nb–V–
Mo–Al–Mg–O) (Table 1, sample 4) is sufficiently
efficient in conversion of ethylbenzene into styrene
ACKNOWLEDGMENTS
and dehydrocyclization of
and styrene. The yield of styrene reaches 68% with a
selectivity of 97–98% at 500
tion of ꢀoctane which has been poorly studied, the Research Program “Development of Methods for
nꢀoctane into ethylbenzene
This work was supported by the Presidium Russian
С. In the dehydrocyclizaꢀ Academy of Sciences through the Fundamental
°
n
Table 5. Results of the oxidative conversion of alkanes over the Nb
–V
–Mo
–Al–Mg–O catalyst (Table 1, sample 4)
Samꢀ
ple no.
Starting alkane
Dilution, mol/mol
Temperature, °C Conversion, %
Selectivity, %
1
2
Ethylbenzene (С6Н5СН2СН3) С6Н5C2Н5/О2/Н2О 1/1/7
500
500
70
97.5
89.0
nꢀOctane (С8Н18)
С8Н18/О2
1/2
1/2
28.1
500
22.7
90.0
3
Secꢀbutanol (С4Н9ОН)
С4Н9ОН/О2
1/1
1/1
1/1
320
340
340
45.0
55.0
70.0
97.0
96.8
96.0
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 2 2011

172
KRASNOBAEVA et al.
4. O. N. Krasnobaeva, I. P. Belomestnykh, G. V. Isagulyꢀ
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5. O. N. Krasnobaeva, I. P. Belomestnykh, G. V. Isagulyꢀ
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[Russ. J. Inorg. Chem. 54 (12), 1862 (2009)].
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