Tantalum-containing catalysts for oxydehydrogenation of hydrocarbons and alcohols

Article abstract of DOI:10.1134/S003602361107014X

The first methods were developed for introducing tantalum(V) into Mg-Al hydrotalcites, which are precursors of oxide catalysts for oxydehydrogenation of hydrocarbons and alcohols. Samples of oxide tantalum(V)-containing catalysts were synthesized. Their catalytic properties were studied in the oxydehydrogenation of ethane to ethylene and ethylbenzene to styrene, oxydehydrocyclization of octane to ethylbenzene and styrene, and oxydehydrogenation of sec-butanol to ketone (octan-2-one). The transformation of ethane to ethylene over the tantalum-containing catalyst occurs with a high selectivity (92-97%) at relatively low temperatures (500°C), and the catalyst is quite efficient in conversion of ethylbenzene to styrene, dehydrocyclization of n-octane to ethylbenzene and styrene, and oxydehydrogenation of sec-butanol to octan-2-one. Comparison with a niobium-containing catalyst showed that it ensures higher yields and selectivities in similar reactions than its tantalum-containing analogue does. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S003602361107014X

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2011, Vol. 56, No. 7, pp. 1012–1016. © 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 Neorganiꢀ
cheskoi Khimii, 2011, Vol. 56, No. 7, pp. 1073–1077.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
TantalumꢀContaining 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
b Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Leninskii pr. 47, Moscow, 119991 Russia
eꢀmail: danilov@igic.ras.ru
Received November 25, 2010
Abstract—The first methods were developed for introducing tantalum(V) into Mg–Al hydrotalcites, which
are precursors of oxide catalysts for oxydehydrogenation of hydrocarbons and alcohols. Samples of oxide tanꢀ
talum(V)ꢀcontaining catalysts were synthesized. Their catalytic properties were studied in the oxydehydrogeꢀ
nation of ethane to ethylene and ethylbenzene to styrene, oxydehydrocyclization of octane to ethylbenzene
and styrene, and oxydehydrogenation of secꢀbutanol to ketone (octanꢀ2ꢀone). The transformation of ethane
to ethylene over the tantalumꢀcontaining catalyst occurs with a high selectivity (92–97%) at relatively low
temperatures (500°C), and the catalyst is quite efficient in conversion of ethylbenzene to styrene, dehydroꢀ
cyclization of ꢀoctane to ethylbenzene and styrene, and oxydehydrogenation of secꢀbutanol to octanꢀ2ꢀone.
n
Comparison with a niobiumꢀcontaining catalyst showed that it ensures higher yields and selectivities in simꢀ
ilar reactions than its tantalumꢀcontaining analogue does.
DOI: 10.1134/S003602361107014X
This work continues our studies of the effect of the
To a solution of 0.3 mol/L aluminum nitrate and
composition of hydrotalciteꢀlike hydroxo salts, which 0.6 mol/L magnesium nitrate, a solution of 2 mol/L
potassium hydroxide and 1 mol/L potassium carbonꢀ
ate was slowly added dropwise (one drop every 3 s) at
60 С while continuously stirring, with pH being
changed from 1 to 10. The forming precipitate of a
magnesium–aluminum double hydrotalciteꢀlike
hydroxo salt (figure) was washed with water to remove
potassium ions on a Nutsch filter until a sodium tetꢀ
raphenylborate test was negative.
are used as catalyst precursors, on the catalytic propꢀ
erties of the oxide catalysts that are produced from
these precursors and are active in oxydehydrogenation
of organic compounds. We previously synthesized
hydroxo salts of various compositions (Mg–Al, Mg–
Ni–Al, Mg–Ni–Co–Al, Mg–Al–Cr, and Mg–Al–
Fe) with the hydrotalcite structure [1–6] that contain
nitrate, carbonate, decavanadate, paramolybdate,
metatungstate, and pentaoxoniobate ions in anion
interlayers. Oxide catalysts prepared from such
hydroxo salts had high selectivities and ensured high
yields of desired products in oxydehydrogenation of
organics.
°
For anion exchange with partial replacement of
carbonate ions by decavanadate ions (V10O28)6–, paraꢀ
molybdate ions (Mo7O24)6–, or metatungstate ions
The purpose of this work was to develop a method
for introducing tantalum(V) into these catalysts and
study its effect on their efficiency in oxydehydrogenaꢀ
tion of organic compounds.
EXPERIMENTAL
60
50
40
30
20
10
, deg
Precursors were synthesized by reacting a solution
of metal nitrates with a solution of potassium hydroxꢀ
ide and potassium carbonate with subsequent anion
exchange of nitrate and carbonate ions for the correꢀ
sponding polyoxometalates according to the following
procedure.
2
θ
Diffractogram of magnesium aluminum double hydrotalꢀ
citeꢀlike hydroxocarbonate pentatantalate
AlMg (OH) ][(CO ) (Ta O H O].
[
)
· n
1.77
5.54
3 0.465
5
16 0.01 2
1012

TANTALUMꢀCONTAINING CATALYSTS
1013
(H2W12O40)6–, the paste obtained after precipitation
Noteworthy, the method developed for introducing
and washing was diluted with water until the solidꢀtoꢀliqꢀ polyoxotantalates into the composition of hydrotalꢀ
uid ratio was 1 : 2, and was then added with a necessary citeꢀlike hydroxo salts by anion exchange is the first
amount of either 0.15 M potassium decavanadate, or one.
0.15 mol/L ammonium paramolybdate, or 0.5 mol/L
sodium tungstate, depending on a required final comꢀ
position. The pulp was stirred for 10 min, after which
0.2 M nitric acid was added dropwise to it until pH
become 4.5 in the case of exchange of carbonate ion
for decavanadate or paramolybdate ions [7, 8] and
until pH become 5.0 in the case of exchange of carꢀ
bonate ion for metatungstate ion [9]. After keeping the
mixture at a necessary pH value for 10 min in the case
of exchange for decavanadate or paramolybdate ions
and for 30 min in the case of exchange for metatungꢀ
state ion, the precipitate was filtered off and washed
with water to remove potassium, ammonium, and
sodium ions.
The phase and chemical compositions of the synꢀ
thesized hydroxo salts were determined by chemical
analysis and Xꢀray powder diffraction (DRONꢀ2.0
diffractometer, Cu
K radiation).
α
The potassium content was found gravimetrically
as tetraphenylborate; aluminum, volumetrically by
back titration of excess EDTA with zinc nitrate in the
presence of Xylenol Orange; and magnesium, chelatoꢀ
metrically with Eriochrome Black in an ammonia
buffer solution while masking aluminum by triethanoꢀ
lamine hydrochloride. Vanadium was determined volꢀ
umetrically by titration of ferrous ammonium sulfate
in the presence of phenylanthranilic acid as an indicaꢀ
tor; molybdenum, gravimetrically with
α
ꢀbenzoin
oxime; and tungsten and tantalum, as WO3 and Ta2O5
after acid hydrolysis of samples.
To introduce tantalum into the composition of the
magnesium–aluminum double hydrotalciteꢀlike
hydroxo salt, a method was developed for synthesizing
potassium polyoxotantalate and introducing it into the
hydroxo salt by anion exchange. For this purpose, tanꢀ
talum oxide Ta2O5 was alloyed with potassium carbonꢀ
ate in platinum dishes in different molar ratios under
various temperature conditions.
RESULTS AND DISCUSSION
Polyoxometalate (decavanadate, paramolybdate,
or metatungstate) ions replace carbonate ion in the
interlayer space of the magnesium aluminum double
hydrotalciteꢀlike hydroxo salt at pH 4.5–5.0, whereas
pentatantalate does it at pH 13. For this reason, when
preparing precursors of oxydehydrogenation catalysts
containing tantalum as pentatantalate and other polyꢀ
oxometalates, we had to use a mechanical mixture of
two isomorphous phases, one of which was magneꢀ
sium–aluminum hydrotalciteꢀlike hydrocarbonate
pentatantalate, and the other was aluminum–magneꢀ
sium hydroxocarbonate containing either decavanaꢀ
date, or paramolybdate, or metatungstate ion sepaꢀ
rately or together in various combinations.
The mechanical mixture was prepared by mixing
the two phases with addition of water at the solidꢀtoꢀ
liquid ratio 1 : 2 for 2 h. The mixture was quite uniꢀ
form, which was suggested by sedimentation features
and also by the results of chemical analysis of pulp
samples taken in the upper and lower parts of the sedꢀ
imentation cylinder (V : Ta, Mo : Ta, and W : Ta ratios
in these samples were equal).
The alloy formed at a К2СО3 : Та2О5 molar ratio of
32 : 1 or lower while keeping the reactants at 900°С for
30 min and then at 1050°С for 30 min, did not comꢀ
pletely dissolve in boiling water and contained 25–
35% insoluble residue. The alloy produced at a К2СО3 :
Та2О5 molar ratio of 50 : 1 while alloying at 900°С for
30 min and then at 1100°С for 60 min completely disꢀ
solved in hot (70°С) water. Evaporation and cooling of
the resulting solution caused precipitation of crystals,
which were washed with water and alcohol to remove
the mother liquor and dried in a desiccator over
CaCl2/CaO. Chemical analysis of the crystals deterꢀ
mined that the К2О : Та2О5 molar ratio was 1.4 : 1.0 and
corresponded to the previously described [10] pentaꢀ
tantalate К7Та5О16 · aq.
For anion exchange of carbonate ion of the Mg–Al
hydroxo salt for polyoxotantalate ion [Ta5O16]7–, a
solution provided by dissolving a K2CO3–Ta2O5 alloy
in water was used (the pH of the solution is 13.0).
To study the effect of tantalum(V) on the catalytic
properties of the oxide catalysts, we synthesized mixꢀ
tures of isomorphous hydrotalciteꢀlike hydroxo salts
(Tables 1, 2). The samples synthesized (Table 1, samꢀ
ples 1–5) were subjected to heat treatment, which
consisted in drying the precipitate at 100–120°С until
it acquired 40% water content, pelletizing, annealing
in a muffle furnace in an air flow with continuously
Pentaoxotantalate [Та5О16]7– is known [11] to exist
in solution over a wide alkali concentration range
(0.01–3.00 M KOH), which corresponds to pH 12 and
higher. Anion exchange was performed at pH 12, 13,
and 14 for 6 h at a solidꢀtoꢀliquid ratio of 1 : 4. It was
found that all of the polyoxotantalate passes to the
solid phase at pH 13 and 14. However, because a
noticeable (>10%) amount of aluminum passes to the
solution at pH 14, the exchange was finally carried out
at pH 13. In this case, about 5% aluminum passed to
increasing temperature at a rate of 100 deg/h to 500°С
,
and keeping at this temperature for 4–5 h.
The catalytic properties of the materials were invesꢀ
the solution and the hydrotalcite structure was conꢀ tigated in a quartz flow reactor with 1–2 mL of cataꢀ
served (hexagonal lattice,
(figure).
a
= 3.0 Å, and
c
= 7.6 Å) lyst. The oxidizer was atmospheric oxygen. To mainꢀ
tain isothermicity, the catalyst was mixed with the
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 7 2011

1014
KRASNOBAEVA et al.
Table 1. Compositions of samples of isomorphous hydrotalciteꢀlike hydroxo salts—precursors of oxide catalysts for oxyꢀ
dehydrogenation of organic compounds
Sample no.
Composition of mixture of hydroxo salts
[AlMg2.2(OH)6.4][(CO3)0.40(Ta5O16)0.03 · nH2O]
[AlMg1.77(OH)5.54][(CO3)0.392(Ta5O16)0.005(Mo7O24)0.03
[AlMg2(OH)6][(CO3)0.37(Ta5O16)0.02(V10O28)0.02 H2O]
[AlMg1.76(OH)5.52][(CO3)0.414(Ta5O16)0.004(V10O28)0.004(Mo7O24)0.02
[AlMg1.76(OH)5.52][(CO3)0.408(Ta5O16)0.004(V10O28)0.004(Mo7O24)0.02 · (H2W12O40)0.002
1
2
3
4
5
· nH2O]
·
n
· nH2O]
· nH2O]
Table 2. Compositions of the initial isomorphous hydrotalciteꢀlike hydroxo salts from which precursors of oxide catalysts for oxydeꢀ
hydrogenation of organic compounds were obtained by mixing in the ratio 1 : 1
Samꢀ
ple no.
Composition
of tantalumꢀcontaining hydroxo salt
Composition of tantalumꢀfree hydroxo salt
–
1
2
3
4
5
[AlMg2.2(OH)6.4][(CO3)0.40(Ta5O16)0.03 · nH2O]
[AlMg1.77(OH)5.54][(CO3)0.465(Ta5O16)0.01 · nH2O] [AlMg1.77(OH)5.54][(CO3)0.32(Mo7O24)0.06
· nH2O]
[AlMg2(OH)6][(CO3)0.36(Ta5O16)0.04 · nH2O] [AlMg2(OH)6][(CO3)0.38(V10O28)0.04 H2O]
·
n
[AlMg1.76(OH)5.52][(CO3)0.472(Ta5O16)0.008 · nH2O] [AlMg1.76(OH)5.52][(CO3)0.356(V10O28)0.008(Mo7O24)0.04
[AlMg1.76(OH)5.52][(CO3)0.472(Ta5O16)0.008 · nH2O] [AlMg1.76(OH)5.52][(CO3)0.344(V10O28)0.008
· nH2O]
(Mo7O24)0.04 (H2W12O40)0.004 · nH2O]
same volume of ground quartz. The reaction temperaꢀ
ture, hydrocarbon feed flow rate, and hydrocarbon :
oxygen ratio in the feed were varied over wide ranges.
The reaction products were analyzed by liquid chroꢀ
matography on a Porapak Q column. In the contact
gas, we determined the unreacted initial hydrocarbon
yield, % = ([produced hydrocarbon], mol/[passed
hydrocarbon], mol) 100
The catalysts synthesized from samples 1–5 (Table 1)
were use to perform oxydehydrogenation of ethane
(Table 3). The catalysts in Table 3 are numbered to
correspond to the number of its precursor in Table 1.
Analysis of the results of the catalytic studies
showed that the transformation of ethane to ethylene
occurs over the tantalumꢀcontaining catalysts at high
×
.
(ethane, nꢀoctane, ethylbenzene, and butanol), dehyꢀ
drogenation product (ethylene, styrene, ethylbenꢀ
zene, and octanꢀ2ꢀone), СО2, CO, and methane.
From the experimental results, the conversion, reacꢀ
tion selectivity, and yields of desired products were
calculated formulas as follows:
selectivity (90–96%) at low temperature (500°С
)
(Table 3). Increasing temperature leads to an increase
in conversion from 17–19% to 41–42% and to a
simultaneous decrease in selectivity from 90–96% to
72–73% (Table 3). Moreover, making the catalyst
composition more complex from Ta–Al–Mg–O
through Ta–Mo–Al–Mg–O and Ta–V–Al–Mg–O
conversion, % = ([transformed hydrocarbon],
mol/[passed hydrocarbon], mol)
selectivity, % = ([produced hydrocarbon],
mol/[transformed hydrocarbon], mol) 100
×
100;
×
;
Table 3. Oxydehydrogenation of ethane over tantalumꢀcontaining catalysts of various compositions
Samꢀ
ple no.
Temperature, C2H6 : O2 dilution ratio,
Catalyst composition
Conversion, % Yield, % Selectivity, %
°C
mol/mol
1
2
3
Ta–Al–Mg–O
500
500
500
700
500
500
700
1/0.35
1/0.40
1/0.35
1/0.50
1/0.30
1/0.30
1/0.50
13.6
16.5
17.0
57.3
17.5
19.4
57.3
12.5
15.0
15.4
41.4
16.1
18.7
42.0
91.89
91.00
90.00
72.30
91.15
96.00
73.30
Ta–Mo–Al–Mg–O
Ta–V–Al–Mg–O
Ta–V–Al–Mg–O
4
5
6
Ta–Mo–V–Al–Mg–O
Ta–Mo–V–W–Al–Mg–O
Ta–Mo–V–W–Al–Mg–O
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 7 2011

TANTALUMꢀCONTAINING CATALYSTS
1015
Table 4. Results of oxidative transformation compounds over the Ta–V–Mo–Al–Mg–O catalyst (Table 1, sample 4)
Sam
ple
no.
Initial compound
Dilution ratio, mol/mol Temperature, °C Conversion, % Yield, % Selectivity, %
1
2
C6H5CH2CH3 ethylbenzene C6H5C2H5/O2/H2O 1/1/7
420
54.0
48.9
90.5
nꢀOctane (C8H18)
C8H18/O2
1/2
450
450
20.1
20.7
17.1
18.4
85.0
89.0
1/2
3
2ꢀButanol (C4H9OH)
C4H9OH/O2
1/1
1/1
1/2
320
340
340
38.0
40.0
56.0
36.1
37.6
52.1
95.0
94.0
93.0
to Ta–Mo–V–Al–Mg–O and Ta–Mo–V–W–Al– for the production of the carbonyl compound (octanꢀ
Mg–O improves the catalytic characteristics; namely, 2ꢀone) to ensure quite a high yield of 52% and a selecꢀ
the conversion increases from 13.6% to 19.4%, the tivity of 93% at 340°С
.
selectivity increases from 91.89% to 95.00% (Table 3),
and the process occurs at relatively low temperature
Noteworthily, the Nbꢀcontaining catalyst the comꢀ
position of which is similar to that of the studied tanꢀ
talumꢀcontaining catalyst (Table 1, sample 4) [10] gives
better results in the reactions presented in Table 4. For
example, the styrene yield from ethylbenzene over the
niobiumꢀcontaining catalyst reaches 68%.
In the nꢀoctane dehydrocyclization, the total yield
of styrene and ethylbenzene reaches 20–25% at a total
selectivity of up to 90%.
To summarize the studies performed, we have
developed efficient tantalumꢀcontaining catalysts for
selective conversion of alkanes, alkyl aromatic hydroꢀ
carbons, and alcohols in oxydehydrogenation reacꢀ
tions.
(
500°С).
Noteworthy, niobiumꢀcontaining catalysts of simiꢀ
lar compositions when used in oxydehydrogenation of
ethane ensure similar conversion 12–19% and selecꢀ
tivity 92–97% [10] at 450–500°С. Unlike the reactions
over tantalumꢀ and niobiumꢀcontaining catalysts, the
oxydehydrogenation of ethane hardly occurs over catꢀ
alysts of other compositions, for example, Ni–V–
Mo–W–Al–Mg–O and Fe–VꢀMo–W–Al–Mg–O,
at low temperatures (500°С) (conversion: 7.0–7.5%,
selectivity: 40–45%).
Along with oxydehydrogenation of ethane over
tantalumꢀcontaining catalysts (Table 1, sample 4), we
also studied the following reactions (Table 4):
ACKNOWLEDGMENTS
oxydehydrogenation of ethylbenzene to styrene,
This work was supported by the Presidium of the
Russian Academy of Sciences (project 7P4, subproꢀ
gram “Targeted Synthesis of Inorganic Substances with
Tailored Properties and Creation of Functional Materiꢀ
als on Their Basis” of the Basic Research Program
“Development of Methods for Producing Chemical
Substances and Creation of New Materials”).
С6Н5СН2СН3 + О2
C6H5–CH=CH2 + H2O + CO2;
oxydehydrogenation of octane to ethylbenzene and
styrene,
C8H18
C6H5C2H5 + C6H5CH=CH2;
and oxydehydrogenation of secꢀbutanol to ketone
(octanꢀ2ꢀone).
The data in Table 4 demonstrate that the investigated
catalyst, namely, Ta–V–Mo–Al–Mg–O (Table 1,
sample 4), is quite efficient in the conversion of ethylꢀ
benzene to styrene and the dehydrocyclization of
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n
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n
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