Phase ratios in the M2O-V2O5-SO 3 (M = Rb, Cs) systems and the properties of the compounds formed in these systems

Article abstract of DOI:10.1134/S0036023607030205

Powder X-ray diffraction and microscopy have been used to study phase ratios of the M₂O-V₂O₅-SO₃ (M = Rb, Cs) systems, which model the active component of rubidium-vanadium and cesium-vanadium catalysts for sulf

Full text of DOI:10.1134/S0036023607030205

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 3, pp. 419–423. © Pleiades Publishing, Inc., 2007.
Original Russian Text © V.N. Krasil’nikov, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 3, pp. 471–475.
PHYSICOCHEMICAL ANALYSIS
OF INORGANIC SYSTEMS
Phase Ratios in the M2O–V2O5–SO3 (M = Rb, Cs) Systems
and the Properties of the Compounds Formed in These Systems
V. N. Krasil’nikov
Institute of Solid-State Chemistry, Ural Division, Russian Academy of Sciences,
ul. Pervomaiskaya 91, Yekaterinburg, 620219 Russia
Received January 26, 2006
Abstract—Powder X-ray diffraction and microscopy have been used to study phase ratios of the M2O–V2O5–SO3
(M = Rb, Cs) systems, which model the active component of rubidium–vanadium and cesium–vanadium cata-
lysts for sulfuric acid production at high sulfur dioxide conversions. We have stated that each system forms four
compounds: M3VO2(SO4)2, MVO2SO4, M4V2O3(SO4)4, and MVO(SO4)2. The thermal properties of these com-
pounds and their interaction with water vapor saturated at room temperature have been studied. The unit cell
parameters have been determined for the compounds MVO2SO4 (M = K, Rb), MVO(SO4)2, and
M[VO2(SO4)(H2O)2] · H2O (M = Rb, Tl). The reciprocal transformations of the components and phases of the
M2O–V2O5–SO3 systems match the Lux–Flood ideas of the acid–base properties of oxide compounds.
DOI: 10.1134/S0036023607030205
Systems M2O–V2O5–SO3, where M is an alkali vanadium reduction upon heating, and moisture sensi-
tivity at high sulfur trioxide and alkali oxide concentra-
tions [5]. In view of this, we synthesized most samples
metal other than lithium, model the composition of the
active component of vanadium catalysts for sulfuric
acid production at high sulfur dioxide conversions [1, by solid-phase annealing under the conditions that
2]. The Na2O–V2O5–SO3 and K2O–V2O5–SO3 systems
excluded the possibility of alloying the reagents or their
contact with water vapor. The starting reagents used
have been already studied, and the relevant phase dia-
grams have been designed [3–5]. In the K2O–V2O5–SO3 were V2O5, M2CO3, M2SO4 (high purity grade), and
system (which is of applied significance), five com-
pounds (potassium oxosulfovanadates) were synthe-
sized by solid-state annealing; their compositions are
K3VO2(SO4)2, KVO2SO4, K4V2O3(SO4)4, K3VO(SO4)3,
and KVO(SO4)2 [3, 5]. Single crystals of one more com-
pound, K8V2O3(SO4)6, were pulled from the melt of the
K2S2O7–K2SO4–V2O5 system and were structurally
studied [6]. In the Na2O–V2O5–SO3 system, only two
compounds were found: Na3VO2(SO4)2 and
NaVO(SO4)2 [4]. According to works [5, 7–9], similar
compounds are also formed in the systems with rubid-
ium, cesium, or thallium oxides. Ammonium oxosulfa-
tovanadates(V) were synthesized: (NH4)4V2O3(SO4)4,
(NH4)8V2O3(SO4)6 [10], and NH4VO(SO4)2 [5, 11]. In
view of the ever increasing interest in rubidium- and
cesium-based catalysts, which are more efficient pro-
moters [2], we have studied phase ratios in the
M2O−V2O5–SO3 systems, where M = Rb or Cs.
H2SO4 (pure for analysis grade). Rubidium and cesium
disulfates (M2S2O7) were prepared by thermolysis of
hydrogensulfates (MHSO4) [12], which were synthe-
sized from neutral sulfates and sulfuric acid. In the
course of our experiments, we worked out original
methods for the synthesis of rubidium and cesium van-
adates and the three-component compounds of
M2O−V2O5–SO3 systems. Phase control was accom-
plished on a DRON-2 diffractometer using CuKα radi-
ation and a POLAM C-112 polarized-light microscope
in transmitted light using an optical immersion probe.
To estimate unit cell parameters. X-ray diffraction pat-
terns were recorded on a STADI-P automated diffracto-
meter equipped with a mini-PSD detector, using CuKα1
radiation, in the transmission geometry with scan steps
of ∆2θ = 0.02° in the 2θ range from 2° to 120°. Poly-
crystalline silicon with a = 5.43075(5) Å was used as
internal and external standards. X-ray diffraction data
were processed by full-profile Rietveld analysis using
FULLPROF software. Thermal analysis was performed
on a Q-1500D derivatograph in the heating mode at
10 K/min; the melting and onset decomposition tem-
peratures of individual compounds were evaluated
EXPERIMENTAL
Investigations of M2O–V2O5–SO3 systems, where M
is an alkali metal, meet several difficulties because of
their tendency toward glass formation, spontaneous using visual polythermal analysis [5].
419

420
KRASIL’NIKOV
V O
V O
2
5
2
5
Cs V O
6 16
Rb V O
2
2
6
16
Cs V O
4 11
2
Rb V O
3
5
14
CsVO
RbVO
3
3
Cs V O
Rb V O
5
3 10
5
3 10
V O (SO )
4 2
Cs V O
2 7
2
3
V O (SO )
Rb V O
2 7
4
2
3
4 2
4
2
2
Cs VO
Rb VO
3
4
3
4
4
3
4
3
1
1
Rb O
Rb SO Rb S O
SO
Cs O
Cs SO Cs S O
SO
3
2
2
4
2 2
7
3
2
2
4
2 2
7
Fig. 1. Phase diagram for the Rb O–V O –SO system.
Fig. 2. Phase diagram for the Cs O–V O –SO system.
2 2 5 3
2
2
5
3
Phase notation: (1) Rb VO (SO ) , (2) RbVO SO ,
Phase notation: (1) Cs VO (SO ) , (2) CsVO SO ,
3
2
4 2
2
4
3 2 4 2 2 4
(3) Rb V O (SO ) , and (4) RbVO(SO ) .
(3) Cs V O (SO ) , and (4) CsVO(SO ) .
4 2 3 4 4 4 2
4
2
3
4 4
4 2
RESULTS AND DISCUSSION
equilibrated in their subsolidus portions. Phase equilib-
ria in the M2S2O7 – V2O5–V2O3(SO4)2 partial systems
are determined by the thermostability of MVO(SO4)2
and V2O3(SO4)2; that is, under ordinary temperatures
and pressures, equilibrium is in fact confined to
~375°ë: at higher temperatures, these phases decom-
pose with sulfur trioxide evolution to the gas phase. The
existence of the MVO(SO4)2–SO3 quasi-binary section
was confirmed by the course of the process with the fol-
lowing sequence of transformation: (I) MVO3 + SO3
Figures 1 and 2 display the phase diagrams for the
systems studied. The difference between these dia-
grams is mainly due to the phase composition of M2O–
V2O5 binary systems, in which the following air-stable
vanadates were identified: M3VO4, M4V2O7, M5V3O10,
MVO3, Rb3V5O14, Cs2V4O11, and M2V6O16 [5, 13].
Vanadium(V) oxide and sulfur trioxide form the only
compound, V2O3(SO4)2, which is stable upon heating to
~400°ë [3, 5]. In the M2O–SO3 systems, there are
M2SO4, M2S2O7, and M2S3O10 compounds; the last are
extremely moisture sensitive and have a very low ther-
mostability [5, 14]. For this reason M2S3O10 compounds
are not indicated on Figs. 1 and 2, but the possibility of
their formation was taken into account in the design of
the phase diagrams.
Powder X-ray diffraction and microscopic examina-
tion show the following compounds in the M2O–V2O5–
SO3 (M = Rb, Cs) systems: M3VO2(SO4)2, MVO2SO4,
M4V2O3(SO4)4, and MVO(SO4)2. Compounds similar
to K3VO(SO4)3 [3, 5] or K8V2O3(SO4)6 [6] have not
been identified. The existence of rubidium and cesium
sulfovanadates of general composition mV2O5
· nM2SO4 have not been confirmed (these compounds
are thought to act as the active component of vanadium
catalysts for sulfuric acid production [15]). The prod-
ucts of the reaction between the components of M2SO4–
V2O5 systems at 300–350°ë are mixtures of solid
phases that enter the composition of the M2SO4–
M3VO2(SO4)2–M2V6O16, M3VO2(SO4)2–MVO2SO4–
M2V6O16, and MVO2SO4–M2V6O16–V2O5 partial ter-
nary systems. Rubidium and cesium vanadates are in
equilibrium only with M2SO4, and only M2V6O16 com-
pounds form binary joins with MVO2SO4 compounds.
The M2O–V2O5–M2S2O7 partial ternary systems are
(II) MVO2SO4 + SO3
(III) MVO(SO4)2 + SO3.
All vanadates experience similar transformations in
contact with sulfur trioxide; according to the M2O–
V2O5–SO3 phase diagrams (Figs. 1, 2), phase equilib-
rium is achieved by annealing samples with a fixed
composition. For this reason, it seems unlikely that
alkali vanadates exist in the active component of vana-
dium catalysts under the conditions of catalytic sulfur
dioxide oxidation. Reactions (I)–(III) can be consid-
ered as model reactions for studying the saturation of
the active component of vanadium catalysts with sulfur
trioxide in the absence of the reducing gas (sulfur diox-
ide).
The M3VO2(SO4)2 synthesis was the solid-phase
annealing of M2S2O7 + 2M2SO4 + V2O5 blends in the
temperature ranges of 200–325°C (for the rubidium
compound) and 200–300°C (for the cesium com-
pound). The alternative process was the reaction of
M2S2O7 and MVO3 at the same temperatures. Powdered
M3VO2(SO4)2 compounds are almost colorless. Color-
less grains of irregular shapes with moderate cleavage
were observed with a microscope. The refractive indices
were the following: for the rubidium compound, Ng =
1.545 0.002 and Np = 1.505 + 0.001; for the cesium
compound, Ng = 1.565 0.002 and Np = 1.525 0.001.
DTA and visual polythermal analysis show that the
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PHASE RATIOS IN THE M2O–V2O5–SO3 (M = Rb, Cs) SYSTEMS
421
M3VO2(SO4)2 compounds melt incongruently to yield in work [17]: for the rubidium compound, a =
4.9732(1) Å, b = 8.7896(1) Å, c = 16.6954(3) Å; and
for the thallium compound, a = 4.9606(1) Å, b =
8.7341(1) Å, c = 16.8504(2) Å; Z = 4, space group
P212121. In the atmosphere of water vapor saturated at
room temperature, rubidium and cesium oxosulfovana-
dates(V) are hydrolyzed:
M2SO4 crystals over the temperature ranges of 340–
585°C (the rubidium compound) and 310–560°C (the
cesium compound). When the compounds are heated above
the liquidus points, they eliminate sulfur trioxide.
The MVO2SO4 (M = K, Rb, Cs) compounds were
synthesized by solid-phase annealing of M2S2O7 +
V2O5 blends at 300–420°C. Powdered RbVO2SO4 is
orange-red; the crystals have short prismatic or platy
habit, which is also characteristic of KVO2SO4 crystals
[3, 5, 7]. Powdered CsVO2SO4 has an intense yellow
color; the crystal habit is platy. The refractive indices
are the following: for RbVO2SO4, Ng = 2.016 0.010
(crimson-red) and Np = 1.724 0.003 (colorless); for
CsVO2SO4, Ng = 1.870 0.005 (light yellow) and Np =
1.650 0.002 (colorless). Processing the X-ray patterns
for MVO2SO4 (M = K, Rb) on the assumption of the
orthorhombic system gave the following results: for the
potassium compound, a = 11.1004(2) Å, b = 8.2626(2) Å,
c = 5.4772(1) Å; for the rubidium compound, a =
10.8203(2) Å, b = 8.9051(2) Å, c = 5.5726(1) Å; Z = 4,
space group P212121. The unit cell parameters for the
CsVO2SO4 structure are found in work [16]. The melt-
ing point of MVO2SO4 is 435°C for the rubidium com-
pound and 428°C for the cesium compound. When the
compounds are heated above the indicated tempera-
tures, they eliminate sulfur trioxide to the gas phase.
Rb3VO2(SO4)2 + 3H2O
(1)
= Rb[VO2(SO4)(H2O)2] ⋅ H2O + Rb2SO4,
Rb4V2O3(SO4)4 + 7H2O
(2)
= 2Rb[VO2(SO4)(H2O)2] ⋅ H2O + 2RbHSO4,
RbVO(SO4)2 + 4H2O
(3)
= Rb[VO2(SO4)(H2O)2] ⋅ H2O + H2SO4.
The hydrolysis products of cesium oxosulfovana-
dates(V) are different in that they contain
Cs[VO2(SO4)(H2O)2] instead of Rb[VO2(SO4)(H2O)2] ·
H2O. H2SO4 is evolved during MVO(SO4)2 hydration,
which increases the water sensitivity of the samples and
enhances the decomposition of the compounds.
Yellow Rb[VO2(SO4)(H2O)2] · H2O crystals are
shaped as flattened prisms with perfect cleavage along
pinacoid (001) and moderate cleavage along (100). The
extinction of the crystals lying on their pinacoidal faces
is vertical, while for the other orientations the extinc-
tion is usually oblique; the crystals are monoclinic. Ple-
ochroism is well defined: Ng = 1.615 0.002 (yellow),
Nm = 1.597 0.002 (light yellow), and Np = 1.558
0.001 (colorless). The unit cell parameters for the crys-
tal structure of M[VO2(SO4)(H2O)2] · H2O (M = Rb, Tl)
compounds as determined from the crystal data for
K[VO2(SO4)(H2O)2] · H2O [18], are the following: for
M = Rb, a = 6.2601(7) Å, b = 9.99736(8) Å, c =
6.69303(5) Å, β = 107.83(1)°; for M = Tl, a =
6.25817(5) Å, b = 9.96217(9) Å, c = 6.70379(6) Å, β =
107.83(1)°, Z = 2, space group P21. The orange-yellow
Cs[VO2(SO4)(H2O)2] crystals are fibrous. The fibers
observed with a microscope are usually bent and bun-
dled; their thicknesses do not exceed 0.5 µm. The
refractive indices of the fibers are Ng = 1.712 and Np =
1.598.
The M4V2O3(SO4)4 synthesis was performed by
means of solid-phase annealing of 2M2S2O7 + V2O5
blends at 250–380°ë. Powdered Rb4V2O3(SO4)4 is
orange; microscopic examination shows crystals
shaped as regular rhombic prisms and their fragments,
which have vertical extinction in all sections. For
Cs4V2O3(SO4)4, irregular-shaped grains are typical.
The refractive indices are the following: for
Rb4V2O3(SO4)4, Ng = 1.762 0.005 (orange) and Np =
1.569 0.002 (colorless); for Cs4V2O3(SO4)4, Ng =
1.781 0.005 (light yellow) and Np = 1.584 0.002
(colorless). The M4V2O3(SO4)4 compound melt with-
out decomposition at 416°C for the rubidium com-
pound and 412°C for the cesium compound. At temper-
atures ~20°ë above the melting points, sulfur trioxide
evolution from M4V2O3(SO4)4 melts starts.
Reaction (3) is convenient for the synthesis of
M[VO2(SO4)(H2O)2] · H2O: sulfuric acid can easily be
removed by washing with ethanol or acetone on a vac-
uum filter. On the other hand, M[VO2(SO4)(H2O)2] ·
H2O (M = K, Rb) compounds are efficient precursors in
KVO2SO4 and RbVO2SO4 synthesis, which is based on
the following reactions:
To prepare the MVO(SO4)2 (M = Rb, Cs, Tl) com-
pounds, we heated MVO3 + 2H2SO4 blends at 150–
320°ë. MVO(SO4)2 powders are orange-yellow; micro-
scopic examination shows needlelike and prismatic
crystals with well-defined pleochroism and vertical
extinction in all sections. The refractive indices are as
follows: for RbVO(SO4)2, Ng > 1.785 (orange-yellow),
Nm = 1.722 0.005 (yellow), and Np = 1.599 0.002
(colorless); for CsVO(SO4)2, Ng > 1.785 (yellow), Nm =
1.736 0.005 (light yellow) and Np = 1.623 0.002
(colorless). The unit cell parameters for the MVO(SO4)2
(M = Rb, Tl) compounds were determined from data
obtained for their potassium and ammonium analogues
MVO3 + 2H2SO4 = MVO(SO4)2 + 2H2O↑,
(4)
MVO(SO4)2 + 4H2O
(5)
= M[VO2(SO4)(H2O)2] ⋅ H2O + H2SO4,
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 3 2007

422
KRASIL’NIKOV
Cs+
Compounds formed in M2O–V2O2–SO3 systems
M2S2O7 + MVO3 = M3VO2(SO4)2
(S2O72– + O2– = 2SO24–,V2O–3 = VO2+ + O2–),
Compound
Na+
K+
Rb+
(9)
M3VO2(SO4)2
MVO2SO4
+
–
–
–
+
+
+
+
+
+
+
+
+
–
+
+
+
–
M4V2O3(SO4)4
M3VO(SO4)3
MVO(SO4)2
MVO(SO4)2 + MVO3= 2MVO2SO4
(VO3+ + O2– = VO+2 ,VO–3 = VO2+ + O2– ),
(10)
(11)
+
+
M[VO2(SO4)(H2O)2] · H2O = MVO2SO4 + 3H2O↑. (6)
V2O5 + SO3 = V2O3(SO4)2 (V2O5
= VO3+ + VO2+ + 2O2–, 2SO3 + 2O2– = 2SO24– ).
Exposure of MVO(SO4)2 to water vapor saturated at
room temperature can yield all known compounds of
the M[VO2(SO4)(H2O)2] · H2O (R = K, Rb, Tl, NH4)
family [5].
Reactions (7)–(10) are only schematic; they do not
fully reflect the chemistry of real multistage processes
whose final stages they are.
Rb[VO2(SO4)(H2O)2] · H2O is dehydrated at 115°C
to yield a red-brown glassy phase, whose crystalliza-
tion appears in the DTA curve as an exotherm at 328°C.
The endotherm at 435°C is due to the melting of crys-
talline RbVO2SO4. Weight loss observed above 435°C
is due to the evolution of sulfur trioxide from the melt.
The anomalies observed in the TG and DTA curves at
T ≈ 100°C indicate the nonequivalence of water mole-
cules in the crystal structure of Rb[VO2(SO4)(H2O)2] ·
H2O, in correlation with IR spectra [19] and X-ray
structure data for K[VO2(SO4)(H2O)2] · H2O [18]. The
Cs[VO2(SO4)(H2O)2] dehydration occurs in a tempera-
ture range of about ~100–400°C; the anhydrous prod-
uct is a red-brown glass, whose crystallization into a
yellow CsVO2SO4 phase is made possible by long-term
annealing at 400°C.
CONCLUSIONS
In summary, an essential feature of M2O–V2O5–SO3
systems, which model the active component of vana-
dium catalysts for sulfuric acid productionat high sulfur
dioxide conversions, is the formation of vanadium(V)
complexes, namely, alkali oxosulfovanadates(V). The
table lists the compositions of alkali oxosulfovana-
dates(V) ceramically synthesized in work [5]. One can
see that potassium forms five compounds, rubidium
and cesium each form four compounds, and sodium
forms only two. X-ray structural data for CsVO2SO4
[16], Cs4V2O3(SO4)4 [20], and KVO(SO4)2 [17] fully
confirm the existence of discrete short V–O bonds and
heavily distorted SO4 groups with symmetry lower than
Td, which was first inferred from vibrations spectros-
In accordance with the Lux–Flood classification of
oxide compounds, alkali oxide in the M2O–V2O5–SO3
(M = Na, K, Rb, Cs) systems is a typical base (an O2–
donor), vanadium(V) oxide is a relatively weak acid (it
can act as a donor or acceptor of O2–), and sulfur triox- copy data in works [3, 4, 7, 8, 10] and 51V NMR data in
ide is a stronger acid than M2S2O7 (it is an O2– accep-
tor). For this reason, addition of sulfur trioxide to M2O–
V2O5 binary systems is equivalent to acidifying, which
is well illustrated by the M2O–V2O5–SO3 phase dia-
grams (Figs. 1, 2): according to these diagrams, the
increasing SO3 concentration first induces the forma-
tion of alkali-poor vanadates and neutral sulfates and,
then, oxosulfovanadates(V) and polysulfates. The
acid–base properties of the components and phases of
the M2O–V2O5–SO3 systems are illustrated by the fol-
lowing equations:
work [21]. The VO+2 group in the CsVO2SO4 structure
is heavily distorted, because one of its oxygen atoms
forms a bridging bond 1.7249 Å long with the neighbor-
ing vanadium atom [16]. In the Cs4V2O3(SO4)4 struc-
ture, the V2O3(SO4)4+ complex anion includes two iso-
4
lated V–O bonds with lengths of 1.577 and 1.583 Å and
a bridging V–O–V bond with V–O distances equal to
1.7822 and 1.775 Å; the sulfato groups are chelate coor-
dinated [20]. In the KVO(SO4)2 structure, there is a sin-
gle, very short isolated V–O bond (1.52 Å) [17]. The
sulfato groups are flattened, which explains the anoma-
lous splitting of the antisymmetrical stretching vibra-
tions ν3(SO4) in the IR spectra of MVO(SO4)2 and the shift
2M2SO4 + 4V2O5 = 2MVO2SO4
(7)
+ M2V6O16 (4V2O5 = 2VO+2 + V6O2–),
16
of this mode to the higher frequencies (cm–1): 1355 (K),
1340 (Rb), 1340 (Cs), 1320 (Tl), and 1335 (NH4) [5]. In
view of the above, it is logical to suggest that other
compounds of the MVO2SO4, M4V2O3(SO4)4, and
MVO(SO4)2 families have similar structures (table).
M2S2O7 + V2O5 = 2MVO2SO4
(8)
(S2O72– + O2– = 2SO24–, V2O5 = 2VO+2 + O2– ),
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PHASE RATIOS IN THE M2O–V2O5–SO3 (M = Rb, Cs) SYSTEMS
423
11. V. N. Krasil’nikov, Zh. Neorg. Khim. 34 (7), 1748
ACKNOWLEDGMENTS
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This work was supported by the State Program for
Support of Leading Scientific Schools (project
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(5), 905 (1952).
13. V. N. Krasil’nikov, M. P. Glazyrin, A. A. Ivakin, et al.,
Zh. Neorg. Khim. 28 (3), 739 (1983).
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