Synthesis and physicochemical study of hexaamminechromium(III) hexamolybdogallate

Article abstract of DOI:10.1134/S0036023611110180

Hexaamminechromium(III) hexamolybdogallate of composition [Cr(NH ₃)₆][GaMo₆O₁₈(OH)₆] ? 5H₂O (I) was synthesized and studied by mass spectrometry, IR spectroscopy, thermogravimetry, and X-ray diffraction. Crystals I are monoclinic: a = 11.77 ?, b = 10.97 ?, c = 15.49 ?, β = 115.11°, V = 1811.29 ?³, ρ_calc = 1.17 g/cm³, Z = 1. The compound obtained was used as a catalyst for soft oxidation of natural gas at 633 K.

Full text of DOI:10.1134/S0036023611110180

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2011, Vol. 56, No. 11, pp. 1698–1701. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © A.V. Oreshkina, G.Z. Kaziev, N.N. Lobanov, P.A. Shipilova, 2011, published in Zhurnal Neorganicheskoi Khimii, 2011, Vol. 56, No. 11, pp. 1781–1784.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Synthesis and Physicochemical Study
of Hexaamminechromium(III) Hexamolybdogallate
a
a
b
a
A. V. Oreshkina , G. Z. Kaziev , N. N. Lobanov , and P. A. Shipilova
a
Moscow State Pedagogical University, ul. Malaya Pirogovskaya 1, Moscow, 119991 Russia
Peoples’ Friendship University of Russia, ul. MiklukhoꢀMaklaya 6, Moscow, 117198 Russia
Received June 30, 2010
b
Abstract—Hexaamminechromium(III) hexamolybdogallate of composition [Cr(NH ) ][GaMo O (OH) ]
⋅
3
6
6
18
6
5
H O (I) was synthesized and studied by mass spectrometry, IR spectroscopy, thermogravimetry, and Xꢀray
2
3
diffraction. Crystals
I
are monoclinic:
a
= 11.77
Å,
b
= 10.97
Å,
c
= 15.49
Å,
β
= 115.11
°,
V
= 1811.29 Å ,
ρ
=
calc
3
1
.17 g/cm , Z = 1. The compound obtained was used as a catalyst for soft oxidation of natural gas at 633 K.
DOI: 10.1134/S0036023611110180
Heteropoly compounds (HPCs) constitute a mixture obtained was heated on a water bath for 3 h
unique class of coordination compounds with redox and then cooled in a desiccator. White crystals of ammoꢀ
and acid–base properties. HPCs consist of metal– nium hexamolybdogallate precipitated in one day.
oxygen octahedra sharing their corners or edges to
The qualitative and quantitative compositions of
form a strong framework of a heteropoly anion (HPA) the compound were determined by mass spectrometry.
with one (or several) hetero atom in the center. These
compounds are used as heterogeneous, acid, and oxiꢀ
dation catalysts [1–4]. Inasmuch as HPCs are easily
reduced with formation of strong colored products,
they are used as inorganic photochromic materials [5]
and in production of dyes and colored lacquers. Hexꢀ
amolybdogallates were prepared for the first time and
their thermal properties were studied by IvanovꢀEmin
and Rabovik [6]; then, Xꢀray crystallographic studies
of HPCs were performed, and the unit cell parameters
were determined by Filatenko et al. [7].
Previously, we prepared and studied tetraammiꢀ
necopper(II) [8] and tetraamminezinc(II) [9] hydroꢀ
gen hexamolybdogallates.
This work is devoted to the synthesis and physicoꢀ program package. The phases were identified with refꢀ
chemical characterization of hexaamminechroꢀ erence to the JCPDS Database (release 2001).
mium(III) hexamolybdogallate and its catalytic activꢀ
ity in soft oxidation of natural gas at 573 K.
For [Cr(NH ) ][GaMo O (OH) ] · 5H O anal.
calcd. (wt %): Cr, 4.00; N, 6.53; Ga, 5.40; Mo, 45.11;
3
6
6
18
6
2
O, 30.10; H O, 8.98.
2
Found (wt %): Cr, 4.06; N, 6.56; Ga, 5.44; Mo,
4
5.01; O, 30.01; H O, 9.03.
2
The compound was characterized by Xꢀray powder
diffraction, IR spectroscopy, and thermogravimetry.
Xꢀray powder diffraction was neasured on an XRDꢀ
6
000 diffractometer (CuK radiation, Ni filter) relative
α
to a silicon external standard. Samples were prelimiꢀ
nary ground in an agate mortar. Xꢀray powder diffracꢀ
tion patterns were first treated using WinXpow softꢀ
ware to refine the reflection positions; then, the patꢀ
terns were indexed with the use of the Powderꢀ2
IR spectrum was recorded as KBr pellets on a
PerkinElmer spectrophotometer in the range of 200–
–
1
4
000 cm .
Thermogravimetric study was performed using a
EXPERIMENTAL
Qꢀ1500 PaulikꢀPaulikꢀErdey derivatograph in a temꢀ
Compound
I
was prepared by reaction between perature range of 20–1000
С; the heating rate was
°
ammonium hexamolybdogallate and chromium(III) 10 K/min, and a sample weight was 100 mg. Calcined
acetate (1 : 4) at heating on a water bath under continꢀ alumina was used as a reference.
uous stirring. Green crystals precipitated after keeping
the reaction mixture for several days over alkali in a
desiccator. The crystals were filtered out and washed
with distilled water.
Catalytic properties of the compound synthesized
were tested for soft oxidation of natural gas containing
9
3% methane. The oxidation of methane with air oxyꢀ
gen was carried out in a flowꢀthrough reactor. The
Ammonium hexamolybdogallate required for the reactor was a quartz tube 270 mm in length and 12 mm
synthesis was prepared as described in [1] as follows. in diameter. The reaction was carried out at 573 K for
To a hot solution of ammonium paramolybdate acidiꢀ 1 h. The reaction products were condensed at the outꢀ
fied to pH 3, a gallium nitrate solution was added. The let of the reactor in a collection flask cooled with a
1698

SYNTHESIS AND PHYSICOCHEMICAL STUDY
1699
I, %
I
100
90
80
70
60
50
40
30
20
10
1
616
1655
380
3
456
1400
9
44 892 652
3
500
2000 1500 1000
500 100
ν, cm
–1
0
5
15
25
35
45
55
65
, deg
2θ
Fig. 1. Xꢀray line pattern of compound
I.
Fig. 2. IR spectrum of compound I.
Yield, %
2.02
Δ
m
, %
10
1
3
T, °C
2.00
TG
1.98
1.96
1.94
9
7
5
3
1
00
780
00 TGA
00
1.92
2
2
0
0
1
.90
.88
3
80
00
1
3
T
00
1
20
1.86
1.84
0
30
Time, min
60
6
7.4
69.9
Selectivity, %
87.4
1
2
3
–(NH ) [GaMo O (OH) ] · 7H O
4 3 6 18 6 2
–[Co(NH ) ] · H[GaMo O (OH) ] · 4H O
3
6
6
18
–[Cr(NH ) ][GaMo O (OH) ] · 5H O
6
2
3
6
6
18
6
2
Fig. 3. Thermal curves for compound
I.
Fig. 4. Catalytic properties of HPCs.
mixture of ice, water, and sodium chloride. Then, the remove the crystal water (this treatment did not
gas mixture was passed through a PdCl2 solution (to change the HPA structure).
determine carbon(II) oxide) and through a sodium
hydroxide solution (0.01 N, 100 mL). Formaldehyde
RESULTS AND DISCUSSION
was determined by spectrophotometry on an SFꢀ4
spectrophotometer, and carbon(IV) oxide was deterꢀ
mined by titrating alkali solutions with hydrochloric
acid before and after the reaction.
The catalysts used were silica gelꢀsupported 6ꢀhetꢀ Table 1). The comparison of the Xꢀray diffraction data
eropoly compounds with Perloffꢀtype structures: with the PCPDFWIN Database has shown that the
The individuality and purity of the compound synꢀ
thesized were confirmed, and its unit cell parameters
were determined by Xꢀray powder diffraction (Fig. 1,
[
Сr(NH ) ][GaMo O (OH) ]
⋅
5H О, [Со(NH ) ]
⋅
compound is individual, contains no possible admixꢀ
and tures, and belongs to the monoclinic system with the
following unit cell parameters: = 11.77 = 10.97
= 15.49 = 115.11 = 1811.29 Å . The pycnomꢀ
to 633 K. Inasmuch as HPCs are hydrated to a rather eter density determined by Syromyatnikov’s method
3
6
6
18
6
2
3 6
Н[GaMo O (OH) ]
⋅
4H O,
6
18
6
2
(NH ) [GaMo O (OH) ]
⋅
7H O
.
a
Å,
b
Å,
4
3
6
18
6
2
3
These catalysts are used at the temperatures of up
c
Å,
β
°, V
3
high degree, they were preliminary heated at 400°C to [10] was
ρ
= 1.17 g/cm (Z = 1).
calc
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 11 2011

1
700
ORESHKINA et al.
data for
Table
1. Xꢀray
6
powder
18
diffraction
H
[
Сr(NH ) ][GaMo O (OH) ]
3
⋅
5H О
Mo
Mo
6
6
2
O Ga
2
θ
, deg
d
,
Å
I
, %
h
k
l
–
1
A wellꢀdefined band at 830 cm associated with the
vibrations of the Ga–O bond does not appear because
8
.29
10.65
7.79
7.65
7.51
5.93
5.36
5.33
5.18
4.98
4.94
4.88
3.97
3.72
3.55
3.42
3.07
3.05
2.67
2.65
2.59
1.98
1.85
1.77
1.53
1.52
100
16
11
7
1
0
1
1
0
1
0
0
1
0
1
2
2
0
1
2
0
2
1
4
2
3
4
6
7
5
0
1
0
2
2
0
0
1
2
2
0
1
3
4
1
5
3
4
0
2
3
6
1
2
6
–1
11.34
11.55
11.77
14.93
16.53
16.63
17.03
17.80
17.94
18.18
22.35
23.90
25.05
25.98
29.02
29.22
33.43
33.69
34.57
45.83
49.04
51.55
60.30
60.98
–1
–1
–1
0
of vibration overlapping. Bands below 400 cm are due
to the bending vibrations of cis МоО2 groups and the
Mo–O–Mo bridging bonds. The IR spectrum also
contains intense bands at 1400 and ~1616 cm , which
are associated with the bending vibrations of coordiꢀ
nated ammonia.
Bands in the region of 3456–3651 cm–1 can be
assigned to the vibrations of water and hydroxyl
groups.
The thermogravimetric data have shown three
endotherms (Fig. 3).
ꢀ
–1
27
9
2
26
30
8
2
–2
1
5
–2
–1
–2
1
3
The first endotherm (at 120
removal of five crystal water molecules; the second
endotherm (at 380 ) is due to elimination of six
°С) corresponds to
2
10
2
°С
–2
2
ammonia molecules and three molecules of water of
crystallization and decomposition of the HPA. The
endotherm at 780°С corresponds to removal of six
molybdenum oxide molecules. The scheme of thermal
decomposition is as follows:
3
9
–2
–3
–4
–1
–4
–5
–1
1
8
1
[
Cr
(
NH
3
)
6][GaMo
Cr NH
6
O
18
(
OH 6
)
]
⋅5H
O
2
7
−5H O
⎯⎯⎯⎯⎯→
2
4
[
(
)
6][GaMo
O
18
(
OH 6
)
]
3
6
°
20 C
1
3
−
3H O, − 6NH3
2
2
→
→ 1 2Cr O ⋅1 2Ga O
2 3 2 3
°
380 C
6
3
–1
1
−6MoO3
×
6MoO3
→ 1 2Cr O ⋅1 2Ga O .
2 3 2 3
4
°
80 C
7
This HPC can be used as a catalyst for soft oxidaꢀ
tion of natural gas in a quartz flowꢀthrough reactor at
a constant temperature of 633 K.
The comparison of the IR spectra of the comꢀ
pounds synthesized with the spectra of previously
studied HPCs [11] shows their identity, which allowed
us to carry out band assignment (Fig. 2).
The use of HPCs as catalysts is possible on account
of their thermal stability and the possibility to be
obtained in different physical states, thereby with taiꢀ
lored characteristics (porosity or specific surface area).
Kozhevnikov [12] considered the mechanism of
methane oxidation with formation of intermediate
methyl radicals and their evolvution into the gas
The fundamental vibrations of terminal cisꢀMoO2
groups and bridging Mo–O–Mo groups appear in the
–
1
–1
region 200–1000 cm . A strong doublet at 892–944 cm
corresponds to the antisymmetric and symmetric
stretching vibrations of the terminal cisꢀMoO groups.
–
2
phase. The formation of СН О groups after introducꢀ
3
–1
The band at 521–656 cm (
ν
s) and the strong band at
−
–1
tion of CH3 radicals was proved by IR spectroscopy. In
6
52 cm (
ν
as) are assigned to the symmetric and antiꢀ
n+
(n – 1)+
this process,
М
from the HPA is reduced to
М
.
symmetric stretching vibrations of the Mo–O fragꢀ
ment
Then, СН
О– converts into formaldehyde [13–15].
3
Table 2. Catalytic properties of HPC/SiO2
Heteropoly compounds
Cr(NH ) ][GaMo O (OH) ] 5H O
Selectivity on CH O,
S
, %
Yield of CH O,
η, %
2
2
[
[
(
⋅
87.40
69.90
67.4
2.01
1.90
2.0
3
6
6
18
6
2
Cr(NH ) ]
⋅
H[GaMo O (OH) ]
⋅
4H O
3
6
6
18
6
2
NH ) [GaMo O (OH) ]
⋅
7H O
4
3
6
18
6
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 11 2011

SYNTHESIS AND PHYSICOCHEMICAL STUDY
1701
According to the data obtained (Table 2, Fig. 4),
the use of the [Сr(NH ) ][GaMo O (OH) ] 5H О
catalyst at the temperature of the experiment (633 K)
gives the highest formaldehyde yield (2.01%) with high
selectivity of 87.40%, which is 0.11% higher than the
4. V. S. Sergienko and M. A. PoraiꢀKoshits, Itogi Nauki
Tekh., Ser. Kristallokhimiya 19, 79 (1985).
5. D. B. Dubovik, T. I. Tikhomirova, A. V. Ivanov, et al.,
J. Anal. Chem. 58 (9), 802 (2003).
6. B. N. IvanovꢀEmin and Ya. I. Rabovik, Zh. Neorg.
Khim. 3 (10), 2429 (1958).
7. L. A. Filatenko, B. N. IvanovꢀEmin, S. Kin’ones, et al.,
⋅
3
6
6
18
6
2
yield for the [Со(NH ) ]
⋅
Н[GaMo O (OH) ]
⋅
4H O
3
6
6
18
6
2
catalyst. When the ammonium HPC was used, the
yield was 2.0%; however, the selectivity was low
Zh. Neorg. Khim. 18 (3), 799 (1973).
8. G. Z. Kaziev, A. V. Oreshkina, S. Holguin Quinones, et
(
67.40%). The minimum yield was observed for the
[Со(NH ) ] Н[GaMo O (OH) ] 4H O catalyst.
⋅
⋅
al., Russ. J. Inorg. Chem. 51 (11), 1711 (2006).
3
6
6
18
6
2
9
. A. V. Oreshkina and G. Z. Kaziev, Aspirant Soiskatel,
No. 2, 111 (2007).
Comparing the catalytic properties of gallium HPCs
under study, we can conclude that 6ꢀHPC salts have
better properties than the simple gallium salts studied
previously (the yield was less than 1.2% at 633 K).
10. F. V. Syromyatnikov, Min. Syr’e, No. 6, 908 (1930).
1
1. K. Nakamoto, Infrared and Raman Spectra of Inorganic
and Coordination Compounds (Wiley, New York, 1986;
Mir, Moscow, 1991).
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12. I. V. Kozhevnikov, Usp. Khim. 62 (5), 511 (1993).
1
1
1
3. A. A. Davydov and O. I. Goncharov, Usp. Khim. 62 (2),
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4. I. V. Kozhevnikov and K. I. Matveev, Usp. Khim. 51
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486 (1983).
1
2
3
. E. A. Nikitina, Heteropoly Compounds (Goskhimizdat,
Moscow, 1962) [in Russian].
1
. M. A. PoraiꢀKoshits and L. O. Atovmyan, Itogi Nauki
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 11 2011