Reactivity of FeVO4 towards MgMoO4in the solid-state

Article abstract of DOI:10.1023/B:JTAN.0000033185.46340.5f

A compound of the formula Fe₃Mg₂Mo₂V ₃O₂₀ has been obtained as a result of a reaction between FeVO₄ and MgMoO₄ taken at a molar ratio of 3:2. This compound crystallises in the monoclinic system and its unit cell parameters are the following: a=0.6971 nm, b=0.9055 nm, c=1.2542 nm, γ=102.00°, Z=2.

Full text of DOI:10.1023/B:JTAN.0000033185.46340.5f

Journal of Thermal Analysis and Calorimetry, Vol. 77 (2004) 33–40
REACTIVITY OF FeVO TOWARDS MgMoO IN THE
4
4
SOLID-STATE
*
M. Bosacka , Maria Kurzawa and P. Jakubus
Technical University of Szczecin, Institute of Chemistry and Environment Protection, Department
of Inorganic Chemistry, al. Piastów 42, 71-065 Szczecin, Poland
Abstract
A compound of the formula Fe
and MgMoO taken at a molar ratio of 3:2. This compound crystallises in the monoclinic system and its
unit cell parameters are the following: a=0.6971 nm, b=0.9055 nm, c=1.2542 nm, γ=102.00°, Z=2.
3 2 2 3 4
Mg Mo V O20 has been obtained as a result of a reaction between FeVO
4
4 4
Keywords: DTA, FeVO , IR, MgMoO , molybdatovanadate, XRD
Introduction
Mixed-metal oxides play an important role in many areas of physics, chemistry and
materials science [1–6]. The combination two or more metals in an oxide matrix can
produce materials with novel interesting physicochemical properties. That can lead
to using these new materials in technological applications.
It is known from literature reports that molybdates(VI) and vanadates(V) of di-
valent and trivalent metals which contain isolated VO or MoO tetrahedra have
4 4
promising catalytic properties in the process of oxidative dehydrogenation of light
alkanes [3, 7–9].
Unlike the well-known properties of molybdates(VI) and vanadates(V) of divalent
and trivalent metals in the structure of which isolated tetrahedra Mo/V–O occur, only
4
few molybdato(VI)vanadates(V) are known at all. Namely, in the recent decade a family
of molybdato(VI)vanadates(V) of divalent metals with the formula M VMoO , where
M=Mn, Mg, Co, Zn [8–11], has been obtained and thoroughly investigated. Moreover, in
2.5
8
the system Fe O –V O –MoO there exist two compounds, FeVMoO and Fe V Mo O
20
2
3
2
5
3
7
4
2
3
[14–17], and a substitutional solid solution of MoO in Fe V O with a limited solubility
3 2 4 13
[18–20], whereas in analogous systems containing chromium or aluminum instead of
iron the compounds CrVMoO and AlVMoO are formed [21, 22]. The structure and
7
7
properties of FeVMoO and Fe V Mo O are known and have been described in many
7
4
2
3
20
articles [23–25]. It is also known that FeVMoO forms solid solutions with CrVMoO
7
7
and AlVMoO [26, 27]. Molybdatovanadates of Fe Cr V Mo O type, where x is from
3
7
4–x
x
2
20
1
to 2, are also known [28].
*
Author for correspondence: E-mail: bossm@ps.pl
1
388–6150/2004/ $ 20.00
2004 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht
©

3
4
4 4
BOSACKA et al.: FeVO TOWARDS MgMoO
The solid solution formed in the system Fe O –V O –MoO results from an incor-
2
3
2
5
3
6+
5+
poration of Mo ions into the sites of V ions in the crystal lattice of Fe V O . In the
2
4
13
above mentioned system this phase crystallizes along the line Fe V O –FeVMoO
7
2
4
13
[17, 19, 20], however the literature information on how the compensation of excessive
charge introduced into the Fe V O lattice by the Mo ions occurs – is contradictory.
From one side, it has been proven by Mössbauer spectroscopy that the excessive positive
6+
2
4
3
3+
charge is compensated by a reduction of an amount of Fe ions, equivalent with respect
6
+
2+
to Mo , to Fe ions [20]. Thus the substitution mechanism of Mo for V is described
6+
5+
3+
2+
x
by the formula Fe2 Fe V Mo O , where 0<x≤0.6 [20]. On the other hand, Wang et
al. have reported that the overall charge is balanced by introduced oxygen atoms accom-
5+
panying the substitution of Mo for V . These authors have presented the substitution
–x
4–x
x
13
6+
6+
5+
mechanism of Mo for V by the formula Fe V Mo O indicating that the substi-
13+x/2
2
6+
4–x
x
5+
tution causes no reduction of either V or Mo ions, however they do not comment at
all on the oxidation state of iron in the formed phase [29].
Recently, new solid solution crystals of (Mg1–xFe )(Mo V )O (0.13≤x≤0.47)
x
2–x
x
7
have been obtained which are formed in the quaternary MgO–Fe O –V O –MoO
3
2
3
2
5
system on the FeVMoO –MgMo O tie line [30].
7
2
7
The aim of the work was investigating whether on another tie line, namely
FeVO –MgMoO , of the quaternary system Fe O –MgO–V O –MoO , any other
hitherto unknown molybdato(VI)vanadates(V) with mixed cation are formed.
4
4
2
3
2
5
3
Experimental
The initial reagents used for preparation of phases needed in the syntheses were: Fe O
2
3
(
a.p., POCh), V O (a.p., Riedel–de Haën), 3MgCO ⋅Mg(OH) ⋅H O (a.p., POCh) and
2
5
3
2
2
MoO (a.p., POCh). From these reagents the phases FeVO , MgMoO , Mg V O and
Fe (MoO ) were obtained as a result of heating the stoichiometric mixtures of suitable
3
4
4
3
2
8
2
4 3
compounds in the following conditions:
•
•
•
•
FeVO4:
550°C(24 h)+600°C(24 h)+600°C(24 h)
550°C(24 h)+650°C(24 h)+700°C(24 h)
550°C(24 h)+650°C(24 h)+700°C(24 h)
650°C(24 h)+700°C(24 h)+700°C(24 h)
MgMoO4:
Mg V O :
Fe (MoO ) :
3
2
8
2
4 3
Reacting substances, i.e. FeVO and MgMoO , weighed in appropriate portions
4
4
were always homogenised by grinding and shaped into pellets. The pellets were
placed in porcelain crucibles and heated in the atmosphere of air. On each heating
stage the samples were gradually cooled to ambient temperature, ground and sub-
jected to DTA investigations and their contents determination by XRD method. After
shaping them into pellets again they were heated further. This procedure was re-
peated until samples in equilibrium state were received.
The DTA investigations were performed by means of a derivatograph (MOM,
Hungary). Samples of 1000 mg by mass were placed in quartz crucibles. All mea-
surements were conducted in the atmosphere of air, in the temperature range
–
1
2
0–1000°C, at a heating rate of 10° min .
J. Therm. Anal. Cal., 77, 2004

BOSACKA et al.: FeVO
4
TOWARDS MgMoO
4
35
Phases occurring in the samples were identified on the base of an X-ray phase
analysis performed with the use of the diffractometer DRON-3 (Bourevestnik, Sankt
Petersburg, Russia). The source of radiation was a cobalt tube (CoK ) with an iron
α
filter, angular range was 12–52° 2θ with step size 0.02° 2θ and counting time per
step 10 s. The identification of phases was conducted by comparison with the XRD
characteristics contained in the PDF cards [31].
Unit cell parameters were determined by applying the program Powder [32, 33].
For indexing – 30 diffraction lines were selected the positions of which were read
from a diffraction pattern recorded for a sample with internal standard KCl (space
group Fm3m; a=b=c=0.629 nm).
The density was determined by the method described in the work [34].
–
1
IR spectra were recorded in the wave-number range 1100–250 cm by means of
the spectrometer Specord M80 (Carl Zeiss, Jena, Germany). The technique of press-
ing pellets with KBr at a mass ratio of 1:300 was applied.
Results and discussion
In order to investigate the solid-state reaction occurring between FeVO and MgMoO ,
4
4
five mixtures were prepared from separately obtained FeVO and MgMoO . Table 1
4
4
presents the composition of initial mixtures, their heating conditions and XRD analysis
results after the final heating stage. The research results compiled in Table 1 prove that
FeVO and MgMoO are not inert towards each other, but react in the solid-state. In the
4
4
diffraction patterns of samples containing in their initial mixtures up to 60 mol%
FeVO a set of diffraction lines characteristic for MgMoO was recorded together with
4
4
another set of lines that could not possibly be ascribed to any phases known to be
formed in the quaternary system Fe O –MgO–V O –MoO . This latter set of lines was
denoted as LX. Figure 1 presents the powder diffraction pattern of the sample contain-
2
3
2
5
3
Fig. 1 Powder diffraction patterns of a – MgMoO
4
4
+FeVO mixture and of
b – Fe
lines in curve b all belong to Fe
3
Mg
2
Mo
2
V
3
O
20, where ¯ – MgMoO
4
3
, — – FeVO
4
and the non-marked
3
Mg Mo
2
2
V
O
20
J. Therm. Anal. Cal., 77, 2004

3
6
4 4
BOSACKA et al.: FeVO TOWARDS MgMoO
Table 1 Initial mixtures composition, their heating conditions and XRD analysis results of the
samples after the last heating stage
Composition of initial
mixtures/mol%
No.
1
Heating conditions/°C
00
650
Phases at equilibrium
6
MgMoO
4
+FeVO
4
4
4
4
*
6
5
4
4
0MgMoO
4
+40FeVO
4
MgMoO +LX
4
*
LX +MgMoO4
7
6
00
00
*
MgMoO
4
+FeVO
+LX (tr.)
*
0MgMoO
4
+50FeVO
4
650
MgMoO +LX
4
2
3
*
LX +MgMoO4
7
6
00
00
*
MgMoO
4
+FeVO
+LX (tr.)
*
2.8MgMoO
4
+57.2FeVO
4
650
MgMoO +LX
4
*
LX +MgMoO4
7
6
00
00
*
MgMoO
4
+FeVO
+LX (tr.)
*
0MgMoO
4
+60FeVO
4
650
MgMoO4(tr.)+FeVO4(tr.)+LX
4
5
*
7
6
00
00
LX
*
MgMoO
4
+FeVO
4
+LX (tr.)
*
LX +FeVO
33.3MgMoO
4
+66.7FeVO
4
650
00
4
*
LX +FeVO
7
4
*
LX – new compound; (tr.) – traces
ing 40 mol% MgMoO and 60 mol% FeVO after the final heating stage (curve b) and
4
4
in their initial mixture (curve a). The diffraction pattern of a sample containing
0 mol% FeVO in its initial mixture contained only the LX set of lines. On the
diffractogram of the last listed sample beside the LX set some lines characteristic for
6
4
FeVO were registered.
4
The obtained results indicate that the solid-state reaction between iron(III)
orthovanadate(V) and magnesium molybdate(VI) leads to a formation of a new com-
pound, which can be expressed by the equation:
2
MgMoO4 (s)+3FeVO4 (s) = Fe Mg Mo V O
3
(1)
2
2
3
20 (s)
The compound Fe Mg Mo V O was also obtained as a product of a solid-state
3
2
2
3
20
reaction occurring among Fe (MoO ) , Mg V O and FeVO mixed at a molar ratio of
8
2
4 3
3
2
4
J. Therm. Anal. Cal., 77, 2004

BOSACKA et al.: FeVO
4
TOWARDS MgMoO
4
37
2
:2:5 or among 3MgCO ⋅Mg(OH) ⋅H O, MoO , Fe O and V O heated in the same
3
2
2
3
2
3
2
5
conditions as the sample No. 4. The reactions taking place can be written in a form:
2
Mg V O +2Fe (MoO ) +5FeVO4 (s) = 3Fe Mg Mo V O
3
(2)
3
2
8 (s)
2
4 3 (s)
3
2
2
20 (s)
[
3MgCO ⋅Mg(OH) ⋅H O] +3Fe O +3V O
=
3
2
2
(s)
2
3 (s)
2
5 (s)
=
2Fe Mg Mo V O
2
+3CO ↑+2H O↑
(3)
3
2
3
20 (s)
2
2
The products of the reactions (1), (2) and (3) were subjected to chemical analysis
and their densities were determined. The chemical analysis showed that the mean content
of metallic elements in the investigated samples amounted to 18.3 mass% Fe, 5.8 mass%
Mg, 21.5 mass% Mo, 17.8 mass% V, which can be compared to the values calculated on
the base of the assumed formula: 19.02 mass% Fe, 5.52 mass% Mg, 17.35 mass% V,
–3
21.78 mass% Mo. The density of Fe
pound Fe Mg Mo V O is honey yellow. Its melting temperature read from the DTA
Mg Mo V O20 is equal to 3.79 g cm . The com-
3 2 2 3
3
2
2
3
20
curve (Fig. 2) as the beginning of endothermic effect amounts to 850±5°C. The XRD
analysis results of Mg Fe Mo V O molten at 890°C and next rapidly quenched to am-
bient temperature proved that this compound melts congruently.
2
3
2
3
20
3 2 2 3 20
Fig. 2 DTA curve of Fe Mg Mo V O
Results of indexing the powder diffraction pattern of Fe Mg Mo V O are pre-
20
3
2
2
3
sented in Table 2. The compound Fe Mg Mo V O crystallises in the monoclinic sys-
3
2
2
3
20
tem, a primitive unit cell with the following parameters: a=0.6971 nm, b=0.9055 nm,
3
c=1.2542 nm, β=102.00°. The unit cell volume is V=774.4 nm . The number of
stoichiometric units per unit cell Z=2 and the density amounts to dcalc.=3.78 g cm .
–3
Figure 3 presents IR spectra of the compound Mg Fe Mo V O (curve b) and of
2
3
2
3
20
the physical mixture of magnesium molybdate(VI) with iron(III) orthovanadate(V) at a
molar ratio of 2:3 (curve a) – for comparison. The oscillation spectrum of
Mg Fe Mo V O is definitely less complex than the spectrum of the components mix-
2
3
2
3
20
ture from which Mg Fe Mo V O was obtained. In the diagram of Mg Fe Mo V O
20
2
3
2
3
20
2
3
2
3
four regions of absorption bands can be distinctly distinguished. Namely, the first one
–1
exhibiting two maxima at 975 and 928 cm can be ascribed with great probability to
J. Therm. Anal. Cal., 77, 2004

3
8
4 4
BOSACKA et al.: FeVO TOWARDS MgMoO
3 2 2 3 20
Table 2 Results of indexing the powder diffraction pattern of Fe Mg Mo V O
No.
dexp/nm
dcal/nm
h k l
0
I/I /%
1
2
0.6565
0.4778
0.6571
0.4778
1 1 0
1 1 1
14.0
2.5
0
0
.4605
.4614
0 2 1
1 2 0
3
0.4605
3.2
4
5
6
7
8
9
0.3766
0.3411
0.3282
0.3243
0.3093
0.2733
0.2672
0.2641
0.2629
0.3738
0.3413
0.3281
0.3243
0.3092
0.2734
0.2672
0.2641
0.2630
2 0 1
2 1 1
2 2 0
1 0 2
1 3 1
2 3 0
10.9
7.0
100.0
17.6
13.9
11.1
6.7
1
1
1
0
1
2
3 –3 0
1 –3 2
2 –2 2
6.0
9.3
0
0
.2389
.2390
2 2 2
3 –4 0
1
3
0.2388
2.8
0
0
.2263
.2261
4 –1 0
3 –4 1
1
1
1
4
5
6
0.2264
0.2252
0.2190
4.7
4.3
0.2253
3 0 2
0
0
.2191
.2187
1 1 3
3 3 0
16.0
4.8
1
1
7
8
0.2116
0.2076
0.2115
0.2075
3 –5 0
1 2 3
2 3 2 3 20
Fig. 3 IR spectra of a – initial mixture and b – Mg Fe Mo V O
J. Therm. Anal. Cal., 77, 2004

BOSACKA et al.: FeVO
4
TOWARDS MgMoO
4
39
stretching vibrations of M-O bonds in the tetrahedra VO and MoO [35, 36]. The sec-
4
4
–1
ond small band with its maximum at 650 cm can be associated with stretching vibra-
tions of bridging bonds M–O–M [37, 38] and the next band with a broad maximum at
–1
490 cm is certainly due to stretching vibrations of the bonds Fe–O and Mg–O in the
polyhedra FeO , MgO and MgO [38]. The last region of absorption bands lying at
6
6
5
–1
400–250 cm possesses several maxima caused probably by bending vibrations of
M–O bonds in the tetrahedra VO or MoO [35, 38].
4
4
Conclusions
The presented research results allow the following conclusions to be drawn:
•
In the solid-state, FeVO and MgMoO are not inert towards each other, but
4 4
enter into reaction at a molar ratio of 3:2 with a formation of a hitherto not de-
scribed in literature phase Mg Fe Mo V O . This compound can also be ob-
tained according to Eqs (2) and (3).
2
3
2
3
20
•
•
The compound Mg Fe Mo V O melts congruently at 850±5°C.
2
3
2
3
20
The compound Mg Fe Mo V O crystallises in the monoclinic system and
its unit cell parameters are the following: a=0.6971 nm, b=0.9055 nm,
c=1.2542 nm, β=102.00°.
2
3
2
3
20
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