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Influence of calcination conditions on the phase composition of vanadium-phosphorus oxide catalysts
203
Applied Catalysis, 9 (1984) 203-211 Elsevier Science Publishers B.V., Amsterdam
INFLUENCE
OF CALCINATION
- Printed in The Netherlands
CONDITIONS
ON THE PHASE COMPOSITION
OF VANADIUM-PHOSPHORUS
OXIDE CATALYSTS
B.K. HODNETT
and B. DELMON
Groupe de Physico-Chimie
Miner-ale et de Catalyse,
Place Croix du Sud, 1, 1348 Louvain-la-Neuve,
(Received
23 March
1983, accepted
Universite
Catholique
de Louvain,
Belgium.
5 July 1983)
ABSTRACT The influence of calcination time and temperature on the formation of phases related to VP0 catalysts with P/V ratios in the range 0.90 to 1.10 has been studied. In the preparation, lactic acid was used for the reduction of V705 and the reduced vanadium complex thus formed was reacted withotl3P04. The evolution of phases was followed by XRD. For calcinations at 773 K, low P/V ratios favour the formation of BVPO~; high ratios (>l) favour the formation of a reduced phase labelled B* (d = 4.67, 4.07, 3.14 and 2.59 A). For all P/V ratios studied, this phase was not stable in air at 773 K but transformed slowly into 6VP05. The B*/BVPO5 ratio did not influence the surface area. The B* phase was not observed after calcination above 873 K. For calcination of catalysts with a P/V ratio of 1.10, carried out at 923 K, a reduced phase with principal d spacings at 3.90, 3.13 and 2.99 fi and variously labelled B, B and (VO)2P2O7 in the literature, was observed.
INTRODUCTION Catalytic anhydride phases
activity
for the selective
over VP0 catalysts
has been associated
in which these catalysts
studies
have been published
are reported
interrelation
between
of two or more phases
another,
individual
whereby
the influence
influence
phases, whether
Industrial
have P/V ratios greater
usually
of phosphorus selectivity individual
was chosen
can stabilize
for maleic
anhydride
oxides of vanadium
Such phases might not easily catalysts
vanadium
with stoichiometric
0166-9834/84/$03.00
occur
conditions
with P/V ratios
catalysts
be related
VP0 catalysts has been
one transforms
into
[7].
on the phase composition
than 1.10 [3,4,9-111.
in the +4 oxidation
formation
and
in the range 0.90 to 1.10 is reported.
since it is already
or phosphorus
few detailed
the formation
To date, little attempt
these transformations of calcination
to maleic
with some of the large number of
of a series of VP0 catalysts
this range of compositions
and butenes
[5,7,8-J. Very often complex
[4,5,7,81.
the origin of the individual
nor the sequence
In this article
phases
of n-butane
to form [l-6]. However,
on the factors which
are a mixture
made to determine
oxidation
However,
known that a slight excess state and increase
[I]. On the other hand, formation might
be favoured
to well characterised
compositions.
0 1984 Elsevier Science Publishers B.V.
outside
of
this range.
phases of VP0
204 EXPERIMENTAL Full details already
of the method of preparation
been presented
Cl]. Briefly,
in 85% lactic acid for 16 hours, followed 85% o-H3P04 further
in water to give the desired
refluxed
obtained
by addition
of V205 by refluxing
of the requisite
P/V ratio. The resulting
was calcined
amount of
solution
was
lactic acid was evaporated
in air at 773 or 923 K. Hereafter,
notation will be used to identify catalysts.
PV 773 will signify
used for this study have
reduction
for 4 hours, after which time excess
off and the solid precursor a shorthand
of catalysts
this involved
Thus, for example,
that the P/V ratio was 1.10 and the calcination
1.10
temperature
was 773 K. The procedure measured oxidation
X-ray diffraction described
states of vanadium
previously remained
whereby
was as previously
filtering
was imposed
calcined
the 2M H2S04 solutions
double titration To determine quantities
the influence
of catalysts,
areas were average
as that used
in order to remove carbon deposits
for less than 10 hours. This consisted
in which the catalysts
of calcination
sufficient
oven at chosen
were taken as proportional no internal
reproducibility
for measuring
were dissolved,
which
of
before
with KMn04.
from the calcination
operation,
and B.E.T. surface
(AV) rested on the same principle
[l]. A modification
on these samples
patterns
[I]. The procedure
time on phase composition,
to allow XRD and AV measurements, intervals.
The amounts
standard was used because
were withdrawn
of each phase present
to the height of its characteristic
was observed
small
XRD peak. In
no significant
increase
normal
in
when CuO was used for this purpose.
RESULTS Figure
1 shows the X-ray diffractograms
for the times indicated. amorphous
of the catalyst
and peaks characteristic
of the presence
to appear after ca. 2 hours. The first clearly that with principal labelled
d spacings
of BVP05 [12], with principal
In
two phases,
of the calcination
time. Measured
which AV was closest
calcination
peaks characteristic
after calcination
for 38 hours.
peaks associated
AV values are also presented
concentration
with each of these
coincided
in this figure.
with the calcination
seemed to be at the expense
had
time at
above +4 that BVPO~ began
of the 8* phase.
times in excess of ca. 20 hours, an additional
and there was evidence
as a function
of the B* phase after calcination
to +4. It was only when AV increased
Its appearance
of the presence
at 3.48, 3.40 and 3.07 A, began to appear.
of the principal
for ca. 7 hours. This maximum
to appear.
phases only began
phase to emerge was
i.e. d = 4.07 for B* and d = 3.40 for BVP05 is plotted
These data show a maximum proceeded
of various
identifiable
time increased,
d spacings
phase present
Figure 2 the evolution
(t = 0) was completely
at 4.67, 4.07, 3.14 and 2.59 A. This phase will be
B* [I]. As calcination
This was the predominant
1.03 PV 773 calcined
It can be noted that the precursor
For
peak began to appear
that the peak at d = 4.07 A began to broaden and split.
205
TIME/hr
FIGURE
1
Evolution
of the X-ray diffractogram
of 1.03 PV 773 for the calcination
times indicated.
FIGURE 2
Influence
PV 773. B* (@I;
of calcination
BVPO5 (0);
The phases present
time on the phase composition
after different
calcination
PV 773 and 1.10 PV 773. Figure 3 presents for the catalyst The behaviour
of this catalyst
during
PV 773 in that only trace amounts
of phases
of the maximum
20 and 30 hours.
phosphorus
accelerate
after
that mechanical
in Figure 4. Strong stabil-
is manifest
in this figure by the
in the B* phase to calcination
of the overall grinding
times between
to appear after ca. 14 hours,
midway
composition through
after 38 hours.
calcination
did not
the rate of BVP05 formation.
The influence presented
to 1.03
and these only at
of @VP05 was almost complete
sVPO5 only started
but was still only a small fraction It was established
was obtained.
of V205 were observed.
composition
In addition,
precursor
was in sharp contrast
in 1.10 PV 773 is presented
ization of the B* phase by excess shifting
calcination
for 0.90
of the B* phase and @VP05
of the B* phase were observed
times. Also the formation
4 hours. No peaks characteristic The evolution
times were also studied
the evolution
0.90 PV 773. Once again, an amorphous
short calcination
and AV of 1.03
AV (@I.
of calcination
in Figure
time on the B.E.T. surface
5. No significant
change was observed
area of 1.03 PV 773 is in this parameter
for
206
O_
0
20 TME/t
, 20
0
3
TIME/ hr
FIGURE 3 B* (0); FIGURE 4 B* (0);
Influence of calcination
time on the phase composition
of 0.90 PV 773.
time on the phase composition
of 1.10 PV 773.
BVP05 (0). Influence of calcination RVPO5 (0).
FIGURE 5
Influence
surface area (0)
times between the s*/eVP05
of calcination
time on the B*/BVPO~
ratio (a)
and specific
of 1.03 PV 773.
16 and 95 hours. During this time large changes were observed
in
ratio.
Figure 6 summarizes
the influence
of calcination
ratio and the B.E.T. surface area of a catalyst Calcination
time was 16 hours. The ~*/sVP05
calcination
temperature
was increased
area passed through a maximum
centered
temperature
on the B*/6V.P05
with P/V ratio equal to unity.
ratio decreased
monotonically
as the
and was close to zero at 873 K. The surface around 723 K.
207
CALCINATION
FIGURE 6
Influence
specific
surface
of calcination
area (0)
FIGURE 7
Influence
(VO)2P*07
(e);
Figure 7 presents
i.e, aVP05 at (3.90),
(d
temperature
(0);
the evolution
of various
= 3.57) [13], BVP05
times,
and
of 1.10 PV 923.
phases for the catalyst
3 phases
The number
of this or any other catalyst
continued
in round brackets
7. The concentrations
d = 3.90 passed
but BVP05 formation
of the other two phases.
began to emerge
1.10 PV 923. simultaneously,
(d = 3.40) and a phase with principal
XRD peaks used to plot Figure
aVP05 and the phase with principal
expense
(0)
BVP05 (0).
3.13 and 2.99 51 [2,3,7,8,16].
calcination
ratio
time on the phase composition
which was still amorphous,
the characteristic
on the B*/BVPO~
/hr.
of 1.00 PV 773.
of calcination
aVPO5
From a precursor
TIME
TEW/K.
through
maxima
monotonically,
refers to of both
at intermediate
apparently
The B* phase was not observed
prepared
d spacings
during
at the
calcination
at 923 K.
DISCUSSION A review of the recent
literature
associated
they can form in a large number of phases. important include
in determining
Some factors
the P/V ratio [1,5,7,8,14]
[7]. However,
some confusion
of the characteristic
form. Some of the reported
and the calcination
and lack of coherence XRD patterns
phases are listed
indicates
that
have been established
the nature of the phase or mixed
and assigning
spacings
with VP0 catalysts
phases obtained.
temperature
as
These
and atmosphere
have arisen as to the labelling
of the various
in Table
phases which can
1, with the principal
d
of each.
Some of the phases
listed,
well determined
structures.
to be identical
to aVPO5
such as BVP05 [12], VP05.2H20
Of the others,
C133. (VO)2P207
[15] and aVP05 have
the phase labelled
X [4] would
appear
C81, the B phase C2,31 and the 8 phase
[73 also appear very similar. The main difference appears to be in the amounts +4 which they contain. Originally, the B phase was reported to be a
of v+5 and V
208 mixture
of these two oxidation
states. Trifiro.
et al. [I61 used 6 phase as a
label for the V+4 part of the B phase and claimed a P:V ratio between structure
is assumed
to be an oxidised
In this work, marked differences compositions
which depended
prepare catalysts
which exhibited
ratios <0.09 or >l.lO.
were observed
monophasic
crystalline
phases had to be accommodated;
appear
in any of.these
strongly
was facile stabilized
phase of vanadium be formed.
phases).
by excess
by Bordes
of a phase
in the evolution
of the phase
It was considerably
XRD patterns
whose X-ray diffractograms
easier to
if they possessed
in addition,
phosphorus
observed
i.e., close to unity, depended
time as well as the P/V ratio (Figures B' could be detected
might
(P/V = 0.90),
state of vanadium
in 1.10 PV 773 that a reduced
the final phase composition
since non-
non-stoichiometry
the +4 oxidation
P/V
were composed
were observed,
In the lower end of the range studied
(Figure 3) whereas
P/V ratios,
phase labelled
the XRD pattern
(ox in Figure 4) had almost fully evolved
However,
intermediate
favoured
(This does not mean that these compounds
of the materials
with
form of the B phase [17].
upon their P/V ratios.
exclusively
oxidation
structure
[5,83. A recent patent has described
B', which
it is non-stoichiometric
they assign a tripolyphosphate
to it [7] rather than the pyrophosphate
and Courtine labelled
1 and 1.1 [7]. In addition,
before
6VPO5 began to
for catalysts strongly
was so
crystalline
with
upon calcination
1 and 2). In addition, quantities of the
at extended
calcination
times for these P/V
ratios. The P/V ratio at which a mixture values by increasing
at P/V = 1.10 from calcination composition
reported
The work presented
d spacing
E.S.R. study of VP0 catalysts Nakamura
prepared
to higher
was observed
differences
in phase
must be taken of this factor. light on the structure
at 3.90 8, variously
labelled
phase observed
using lactic acid as reducing
et al. [18] reported
ions in the +4 oxidation actions.
cognizance
here does not shed any additional
prepared
can be shifted
This phenomenon
at 923 K. Thus, in comparing
except that it was the V+4 carrying
at 923 K of catalysts
agents,
is observed
temperature.
in the literature,
of the phase, with principal (VO)2P207,
of phases
the calcination
using hydroxy some aggregation
B, B and
after calcination agent.
carboxylic
From their
acid reducing
or clustering
state which gave rise to highly efficient
of vanadium
exchange
inter-
Catalysts prepared using oxalic acid as reducing agent were considered +4 homogeneously dispersed. The catalysts used in that study were V
to possess
largely amorphous
and calcined
(2 hours) than used here. be reversed
at lower temperatures
It is not clear whether
by more severe calcination
A definitive
structure
of the data presented may be closely
this clustering
cannot be assigned
to the phase labelled
here, but is also derived
calcination
from an amorphous
(>20 hours) at 773 K. However,
after treatments
behaviour
times could
conditions.
related to the B' phase [17], small amounts
after prolonged been detected
(673 K) and for shorter
at temperatures
B* on the basis precursor.
It
of which were detected the B' phase has also
below 753 K, i.e. during catalytic
209
TABLE 1 Principal
XRD d spacings
for VP0 catalysts.
Phase
Principal
XRD
d
w&j
3.48, 3.40, 3.07
Ref.
Description
spacings/a
Isostructural
cam
with
V in +5 oxid-
BVOS04.
ation state. Features corner
sharing
VO6
octahedra. Isostructural
3.57, 3.07, 3.01
ctVPO5
state.
Features
sharing Phase X
3.57, 3.07, 3.00
Patent
a"VP05
3.10, 3.00, 1.96
Similar
La.131
with
V in +5 oxid.
aVOS04.
corner
VO6 octahedra.
c41 structure
to
cal
ctVP05 with elongation along a-axis of unit cell due to trapped water.
NO),P*07
V in +4 oxid. state.
3.87, 3.14, 2.99
Features
cal
edge sharing
V06 octahedra. B phase
Not isolated
3.90, 3.13, 2.98
in its pure
form. Recognized
phase of a complex
8 phase
lyst. Patent. +4 containing V
3.87, 3.14, 2.98
c31
as active cata-
part of
C7,161
B phase. Tripolyphosphate structure
assigned
on basis
of infra red spectra. [I, this
4.67, 4.07, 3.14, 2.59
B* phase
work] VP05.2H20
7.50, 3.75, 3.14
V in +5 oxid. state
B' phase
4.58, 4.02, 3.66, 3.12
Reported
to be the oxidi-
dized equivalent
Cl51 Cl71
of the
6 or B phase.
testing
or calcination
has vanadium
in air (16 hours) at 723 K, which suggests
in the +4 oxidation
state.
In fact the s*/BVP05
723 K for Eigure 6, may be an overestimate at d=4.02 A, poorly resolved
that this phase
ratio, measured
since a contribttion
at
from the B' peak
from the B* peak at d = 4.07 A, is included.
210 The evolution
of the B* phase during calcination
ures 2 and 4) suggests
that vanadium
phase and that it was the precursor
of 1.03 and 1.10 PV 773 (Fig-
is in the +4
oxidation
773 K. The fact that the 6, 6 or (VO),P,O7
was composed
of clusters
of vanadium
phase or that some phase segregation ation, which could be reversed the features been reported
ent with V40g; catalysts formation
and R* content
containing
from n-butane
[I]; the fact that mechanical indicates
XRD pattern
other factors would
was observed
[I]; X.P.S. studies
grinding
indicated
with widely
midway
that no passivating
had built up which might make further
domains
A similar
a* phase were highly selective
and a correlation
through
barrier
oxidation
study did not reveal the presence
calcin-
of B* does have some of
in 1.03 PV 773 was 4.0 and not 4.5 as would
after calcination
had no effect
by a phosphorus
in maleic
in surface
~*/6VP05
of phosphorus
ratios
procedure
rich layers
an analytical
of any phosphorus
an-
selectivity
no change
the calcination
impossible;
concent-
be consist-
between
differing
has
suggest
the AV value at which a maximum
i.e. P/V ratio, for catalysts
composition,
microscopy
[7]. However
had not occurred;
calcin-
in the lower temperature
at 923 K. The XRD pattern
for other VP0 catalysts
ration of B* was observed
hydride
had occurred
out at
during
that the amorphous
ions surrounded
of V40g [19], but it is not identical.
that phase segregation
was carried
type phase was observed
ation at 923 K and B* at 773 K would at first sight suggest precursor
state also in this
of BVPO~ when calcination
electron
or vanadium
rich
[I].
The exact mechanism of vanadium,
cannot yet be settled. phosphorus
whereby
presumably
excess
by inhibiting On balance,
is homogeneously
phosphorus migration
the evidence
dispersed
throughout
stabilizes of oxygen
the +4 oxidation
through
state
the lattice,
to date would suggest
that excess
the bulk for materials
calcined
at 773 K or above. As stated above a correlation maleic
anhydride
formation
this is not the exclusive samples
prereduced
selectivities
was found between
from n-butane
B* content
[il. However
active phase as catalysts
in hydrogen
and selectivity
other results
containing
such that they become amorphous
suggest
in
that
the B phase and also show good
[20].
CONCLUSIONS
In summary it may be stated that excess phosphorus oxidation
of vanadium
the related
phases,
which amorphous
either
in the B* phase when calcined
8, B or (VO)2P207,
by calcination
exhibiting
monophasic
in air at 773 K, provided
ly removed from unity; catalysts calcination
during calcination
at 773 K or in one of
occur depends
XRD patterns
could readily
the P/V ratio was sufficient-
with P/V ratios close to unity required
Higher calcination
temperatures
the +4
at 923 K. The rate at
times at 773 K to reach steady state compositions
tures of phases.
stabilizes
phases -t reduced phases + BVPO~ transformations
upon the P/V ratio. Catalysts be obtained
strongly
favoured
extended
and exhibited
formation
mix-
of @VP05 and
211 resulted
in lower surface
phases are finally
formed
areas.
The role of reducing
after calcination
agent
in determining
which
is not yet clear.
ACKNOWLEDGEMENTS One of us (B.K.H.) wishes Scientifique
(Belgium)
to thank the Service
de Progranation
de la Politique
for a fellowship.
REFERENCES B.K. Hodnett, PH Permanne and B. Delmon, Appl. Catal., 6 (1983) 231. R.A. Mount and H. Rafelson, U.S. Patent No. 8330,354 (1975). R.A. Schneider, U.S. Patent No. 3,864,280 (1975). I.C.I. Belgian Patent No. 867,189 (1978). E. Bordes and P. Courtine, 3. Catal., 57 (1979) 236. L. Morselli, F. Trifiro and L. Urban, J. Catal., 75 (1982) 112. G. Poli, I. Resta, 0. Ruggeri and F. Trifiro, Appl. Catal...,1 (1981) 395. E. Bordes, Thesis, Universite de Technologie de Compiegne (1979). E.M. Boghosian, U.S. Patent No. 3,862,146 (1975) R.O. Kerr, U.S. Patent No. 4,056,487 (1966). B. Weinstein, A.. Jurewicz and L.B. Young, U.S. Patent No. 3,931,046 (1976). R. Gopal and C. Calvo, J. Solid State Chem., 5 (1972) 432. B. Jordan and C. Calvo, Can. J. Chem., 51 (1973) 2621. B.K. Hodnett and B. Delmon, Submitted for publication J. Catal., G. Ladwig, Z. Anorg. Alla. Chem., 338 (1965) 266. G. Poli, 0. Ruggeri and F. Trifiro, Ninth Intl. Symp. Reactivity of Solids, Cracow (1980) 512. G.J. Hutchings and R. Higgins, U.S. Patent No. 4,209,423 (1980). M. Nakamura, K. Kawai and Y. Fujiwara, J. Catal., 34 (1974) 345. A.S.T.M. Powder Diffraction File card 23-720 (V4Og). B.K. Hodnett and 8. Delmon, submitted Ind. Eng. Chem., Fund.
Applied Catalysis, 9 (1984) 203-211 Elsevier Science Publishers B.V., Amsterdam
INFLUENCE
OF CALCINATION
- Printed in The Netherlands
CONDITIONS
ON THE PHASE COMPOSITION
OF VANADIUM-PHOSPHORUS
OXIDE CATALYSTS
B.K. HODNETT
and B. DELMON
Groupe de Physico-Chimie
Miner-ale et de Catalyse,
Place Croix du Sud, 1, 1348 Louvain-la-Neuve,
(Received
23 March
1983, accepted
Universite
Catholique
de Louvain,
Belgium.
5 July 1983)
ABSTRACT The influence of calcination time and temperature on the formation of phases related to VP0 catalysts with P/V ratios in the range 0.90 to 1.10 has been studied. In the preparation, lactic acid was used for the reduction of V705 and the reduced vanadium complex thus formed was reacted withotl3P04. The evolution of phases was followed by XRD. For calcinations at 773 K, low P/V ratios favour the formation of BVPO~; high ratios (>l) favour the formation of a reduced phase labelled B* (d = 4.67, 4.07, 3.14 and 2.59 A). For all P/V ratios studied, this phase was not stable in air at 773 K but transformed slowly into 6VP05. The B*/BVPO5 ratio did not influence the surface area. The B* phase was not observed after calcination above 873 K. For calcination of catalysts with a P/V ratio of 1.10, carried out at 923 K, a reduced phase with principal d spacings at 3.90, 3.13 and 2.99 fi and variously labelled B, B and (VO)2P2O7 in the literature, was observed.
INTRODUCTION Catalytic anhydride phases
activity
for the selective
over VP0 catalysts
has been associated
in which these catalysts
studies
have been published
are reported
interrelation
between
of two or more phases
another,
individual
whereby
the influence
influence
phases, whether
Industrial
have P/V ratios greater
usually
of phosphorus selectivity individual
was chosen
can stabilize
for maleic
anhydride
oxides of vanadium
Such phases might not easily catalysts
vanadium
with stoichiometric
0166-9834/84/$03.00
occur
conditions
with P/V ratios
catalysts
be related
VP0 catalysts has been
one transforms
into
[7].
on the phase composition
than 1.10 [3,4,9-111.
in the +4 oxidation
formation
and
in the range 0.90 to 1.10 is reported.
since it is already
or phosphorus
few detailed
the formation
To date, little attempt
these transformations of calcination
to maleic
with some of the large number of
of a series of VP0 catalysts
this range of compositions
and butenes
[5,7,8-J. Very often complex
[4,5,7,81.
the origin of the individual
nor the sequence
In this article
phases
of n-butane
to form [l-6]. However,
on the factors which
are a mixture
made to determine
oxidation
However,
known that a slight excess state and increase
[I]. On the other hand, formation might
be favoured
to well characterised
compositions.
0 1984 Elsevier Science Publishers B.V.
outside
of
this range.
phases of VP0
204 EXPERIMENTAL Full details already
of the method of preparation
been presented
Cl]. Briefly,
in 85% lactic acid for 16 hours, followed 85% o-H3P04 further
in water to give the desired
refluxed
obtained
by addition
of V205 by refluxing
of the requisite
P/V ratio. The resulting
was calcined
amount of
solution
was
lactic acid was evaporated
in air at 773 or 923 K. Hereafter,
notation will be used to identify catalysts.
PV 773 will signify
used for this study have
reduction
for 4 hours, after which time excess
off and the solid precursor a shorthand
of catalysts
this involved
Thus, for example,
that the P/V ratio was 1.10 and the calcination
1.10
temperature
was 773 K. The procedure measured oxidation
X-ray diffraction described
states of vanadium
previously remained
whereby
was as previously
filtering
was imposed
calcined
the 2M H2S04 solutions
double titration To determine quantities
the influence
of catalysts,
areas were average
as that used
in order to remove carbon deposits
for less than 10 hours. This consisted
in which the catalysts
of calcination
sufficient
oven at chosen
were taken as proportional no internal
reproducibility
for measuring
were dissolved,
which
of
before
with KMn04.
from the calcination
operation,
and B.E.T. surface
(AV) rested on the same principle
[l]. A modification
on these samples
patterns
[I]. The procedure
time on phase composition,
to allow XRD and AV measurements, intervals.
The amounts
standard was used because
were withdrawn
of each phase present
to the height of its characteristic
was observed
small
XRD peak. In
no significant
increase
normal
in
when CuO was used for this purpose.
RESULTS Figure
1 shows the X-ray diffractograms
for the times indicated. amorphous
of the catalyst
and peaks characteristic
of the presence
to appear after ca. 2 hours. The first clearly that with principal labelled
d spacings
of BVP05 [12], with principal
In
two phases,
of the calcination
time. Measured
which AV was closest
calcination
peaks characteristic
after calcination
for 38 hours.
peaks associated
AV values are also presented
concentration
with each of these
coincided
in this figure.
with the calcination
seemed to be at the expense
had
time at
above +4 that BVPO~ began
of the 8* phase.
times in excess of ca. 20 hours, an additional
and there was evidence
as a function
of the B* phase after calcination
to +4. It was only when AV increased
Its appearance
of the presence
at 3.48, 3.40 and 3.07 A, began to appear.
of the principal
for ca. 7 hours. This maximum
to appear.
phases only began
phase to emerge was
i.e. d = 4.07 for B* and d = 3.40 for BVP05 is plotted
These data show a maximum proceeded
of various
identifiable
time increased,
d spacings
phase present
Figure 2 the evolution
(t = 0) was completely
at 4.67, 4.07, 3.14 and 2.59 A. This phase will be
B* [I]. As calcination
This was the predominant
1.03 PV 773 calcined
It can be noted that the precursor
For
peak began to appear
that the peak at d = 4.07 A began to broaden and split.
205
TIME/hr
FIGURE
1
Evolution
of the X-ray diffractogram
of 1.03 PV 773 for the calcination
times indicated.
FIGURE 2
Influence
PV 773. B* (@I;
of calcination
BVPO5 (0);
The phases present
time on the phase composition
after different
calcination
PV 773 and 1.10 PV 773. Figure 3 presents for the catalyst The behaviour
of this catalyst
during
PV 773 in that only trace amounts
of phases
of the maximum
20 and 30 hours.
phosphorus
accelerate
after
that mechanical
in Figure 4. Strong stabil-
is manifest
in this figure by the
in the B* phase to calcination
of the overall grinding
times between
to appear after ca. 14 hours,
midway
composition through
after 38 hours.
calcination
did not
the rate of BVP05 formation.
The influence presented
to 1.03
and these only at
of @VP05 was almost complete
sVPO5 only started
but was still only a small fraction It was established
was obtained.
of V205 were observed.
composition
In addition,
precursor
was in sharp contrast
in 1.10 PV 773 is presented
ization of the B* phase by excess shifting
calcination
for 0.90
of the B* phase and @VP05
of the B* phase were observed
times. Also the formation
4 hours. No peaks characteristic The evolution
times were also studied
the evolution
0.90 PV 773. Once again, an amorphous
short calcination
and AV of 1.03
AV (@I.
of calcination
in Figure
time on the B.E.T. surface
5. No significant
change was observed
area of 1.03 PV 773 is in this parameter
for
206
O_
0
20 TME/t
, 20
0
3
TIME/ hr
FIGURE 3 B* (0); FIGURE 4 B* (0);
Influence of calcination
time on the phase composition
of 0.90 PV 773.
time on the phase composition
of 1.10 PV 773.
BVP05 (0). Influence of calcination RVPO5 (0).
FIGURE 5
Influence
surface area (0)
times between the s*/eVP05
of calcination
time on the B*/BVPO~
ratio (a)
and specific
of 1.03 PV 773.
16 and 95 hours. During this time large changes were observed
in
ratio.
Figure 6 summarizes
the influence
of calcination
ratio and the B.E.T. surface area of a catalyst Calcination
time was 16 hours. The ~*/sVP05
calcination
temperature
was increased
area passed through a maximum
centered
temperature
on the B*/6V.P05
with P/V ratio equal to unity.
ratio decreased
monotonically
as the
and was close to zero at 873 K. The surface around 723 K.
207
CALCINATION
FIGURE 6
Influence
specific
surface
of calcination
area (0)
FIGURE 7
Influence
(VO)2P*07
(e);
Figure 7 presents
i.e, aVP05 at (3.90),
(d
temperature
(0);
the evolution
of various
= 3.57) [13], BVP05
times,
and
of 1.10 PV 923.
phases for the catalyst
3 phases
The number
of this or any other catalyst
continued
in round brackets
7. The concentrations
d = 3.90 passed
but BVP05 formation
of the other two phases.
began to emerge
1.10 PV 923. simultaneously,
(d = 3.40) and a phase with principal
XRD peaks used to plot Figure
aVP05 and the phase with principal
expense
(0)
BVP05 (0).
3.13 and 2.99 51 [2,3,7,8,16].
calcination
ratio
time on the phase composition
which was still amorphous,
the characteristic
on the B*/BVPO~
/hr.
of 1.00 PV 773.
of calcination
aVPO5
From a precursor
TIME
TEW/K.
through
maxima
monotonically,
refers to of both
at intermediate
apparently
The B* phase was not observed
prepared
d spacings
during
at the
calcination
at 923 K.
DISCUSSION A review of the recent
literature
associated
they can form in a large number of phases. important include
in determining
Some factors
the P/V ratio [1,5,7,8,14]
[7]. However,
some confusion
of the characteristic
form. Some of the reported
and the calcination
and lack of coherence XRD patterns
phases are listed
indicates
that
have been established
the nature of the phase or mixed
and assigning
spacings
with VP0 catalysts
phases obtained.
temperature
as
These
and atmosphere
have arisen as to the labelling
of the various
in Table
phases which can
1, with the principal
d
of each.
Some of the phases
listed,
well determined
structures.
to be identical
to aVPO5
such as BVP05 [12], VP05.2H20
Of the others,
C133. (VO)2P207
[15] and aVP05 have
the phase labelled
X [4] would
appear
C81, the B phase C2,31 and the 8 phase
[73 also appear very similar. The main difference appears to be in the amounts +4 which they contain. Originally, the B phase was reported to be a
of v+5 and V
208 mixture
of these two oxidation
states. Trifiro.
et al. [I61 used 6 phase as a
label for the V+4 part of the B phase and claimed a P:V ratio between structure
is assumed
to be an oxidised
In this work, marked differences compositions
which depended
prepare catalysts
which exhibited
ratios <0.09 or >l.lO.
were observed
monophasic
crystalline
phases had to be accommodated;
appear
in any of.these
strongly
was facile stabilized
phase of vanadium be formed.
phases).
by excess
by Bordes
of a phase
in the evolution
of the phase
It was considerably
XRD patterns
whose X-ray diffractograms
easier to
if they possessed
in addition,
phosphorus
observed
i.e., close to unity, depended
time as well as the P/V ratio (Figures B' could be detected
might
(P/V = 0.90),
state of vanadium
in 1.10 PV 773 that a reduced
the final phase composition
since non-
non-stoichiometry
the +4 oxidation
P/V
were composed
were observed,
In the lower end of the range studied
(Figure 3) whereas
P/V ratios,
phase labelled
the XRD pattern
(ox in Figure 4) had almost fully evolved
However,
intermediate
favoured
(This does not mean that these compounds
of the materials
with
form of the B phase [17].
upon their P/V ratios.
exclusively
oxidation
structure
[5,83. A recent patent has described
B', which
it is non-stoichiometric
they assign a tripolyphosphate
to it [7] rather than the pyrophosphate
and Courtine labelled
1 and 1.1 [7]. In addition,
before
6VPO5 began to
for catalysts strongly
was so
crystalline
with
upon calcination
1 and 2). In addition, quantities of the
at extended
calcination
times for these P/V
ratios. The P/V ratio at which a mixture values by increasing
at P/V = 1.10 from calcination composition
reported
The work presented
d spacing
E.S.R. study of VP0 catalysts Nakamura
prepared
to higher
was observed
differences
in phase
must be taken of this factor. light on the structure
at 3.90 8, variously
labelled
phase observed
using lactic acid as reducing
et al. [18] reported
ions in the +4 oxidation actions.
cognizance
here does not shed any additional
prepared
can be shifted
This phenomenon
at 923 K. Thus, in comparing
except that it was the V+4 carrying
at 923 K of catalysts
agents,
is observed
temperature.
in the literature,
of the phase, with principal (VO)2P207,
of phases
the calcination
using hydroxy some aggregation
B, B and
after calcination agent.
carboxylic
From their
acid reducing
or clustering
state which gave rise to highly efficient
of vanadium
exchange
inter-
Catalysts prepared using oxalic acid as reducing agent were considered +4 homogeneously dispersed. The catalysts used in that study were V
to possess
largely amorphous
and calcined
(2 hours) than used here. be reversed
at lower temperatures
It is not clear whether
by more severe calcination
A definitive
structure
of the data presented may be closely
this clustering
cannot be assigned
to the phase labelled
here, but is also derived
calcination
from an amorphous
(>20 hours) at 773 K. However,
after treatments
behaviour
times could
conditions.
related to the B' phase [17], small amounts
after prolonged been detected
(673 K) and for shorter
at temperatures
B* on the basis precursor.
It
of which were detected the B' phase has also
below 753 K, i.e. during catalytic
209
TABLE 1 Principal
XRD d spacings
for VP0 catalysts.
Phase
Principal
XRD
d
w&j
3.48, 3.40, 3.07
Ref.
Description
spacings/a
Isostructural
cam
with
V in +5 oxid-
BVOS04.
ation state. Features corner
sharing
VO6
octahedra. Isostructural
3.57, 3.07, 3.01
ctVPO5
state.
Features
sharing Phase X
3.57, 3.07, 3.00
Patent
a"VP05
3.10, 3.00, 1.96
Similar
La.131
with
V in +5 oxid.
aVOS04.
corner
VO6 octahedra.
c41 structure
to
cal
ctVP05 with elongation along a-axis of unit cell due to trapped water.
NO),P*07
V in +4 oxid. state.
3.87, 3.14, 2.99
Features
cal
edge sharing
V06 octahedra. B phase
Not isolated
3.90, 3.13, 2.98
in its pure
form. Recognized
phase of a complex
8 phase
lyst. Patent. +4 containing V
3.87, 3.14, 2.98
c31
as active cata-
part of
C7,161
B phase. Tripolyphosphate structure
assigned
on basis
of infra red spectra. [I, this
4.67, 4.07, 3.14, 2.59
B* phase
work] VP05.2H20
7.50, 3.75, 3.14
V in +5 oxid. state
B' phase
4.58, 4.02, 3.66, 3.12
Reported
to be the oxidi-
dized equivalent
Cl51 Cl71
of the
6 or B phase.
testing
or calcination
has vanadium
in air (16 hours) at 723 K, which suggests
in the +4 oxidation
state.
In fact the s*/BVP05
723 K for Eigure 6, may be an overestimate at d=4.02 A, poorly resolved
that this phase
ratio, measured
since a contribttion
at
from the B' peak
from the B* peak at d = 4.07 A, is included.
210 The evolution
of the B* phase during calcination
ures 2 and 4) suggests
that vanadium
phase and that it was the precursor
of 1.03 and 1.10 PV 773 (Fig-
is in the +4
oxidation
773 K. The fact that the 6, 6 or (VO),P,O7
was composed
of clusters
of vanadium
phase or that some phase segregation ation, which could be reversed the features been reported
ent with V40g; catalysts formation
and R* content
containing
from n-butane
[I]; the fact that mechanical indicates
XRD pattern
other factors would
was observed
[I]; X.P.S. studies
grinding
indicated
with widely
midway
that no passivating
had built up which might make further
domains
A similar
a* phase were highly selective
and a correlation
through
barrier
oxidation
study did not reveal the presence
calcin-
of B* does have some of
in 1.03 PV 773 was 4.0 and not 4.5 as would
after calcination
had no effect
by a phosphorus
in maleic
in surface
~*/6VP05
of phosphorus
ratios
procedure
rich layers
an analytical
of any phosphorus
an-
selectivity
no change
the calcination
impossible;
concent-
be consist-
between
differing
has
suggest
the AV value at which a maximum
i.e. P/V ratio, for catalysts
composition,
microscopy
[7]. However
had not occurred;
calcin-
in the lower temperature
at 923 K. The XRD pattern
for other VP0 catalysts
ration of B* was observed
hydride
had occurred
out at
during
that the amorphous
ions surrounded
of V40g [19], but it is not identical.
that phase segregation
was carried
type phase was observed
ation at 923 K and B* at 773 K would at first sight suggest precursor
state also in this
of BVPO~ when calcination
electron
or vanadium
rich
[I].
The exact mechanism of vanadium,
cannot yet be settled. phosphorus
whereby
presumably
excess
by inhibiting On balance,
is homogeneously
phosphorus migration
the evidence
dispersed
throughout
stabilizes of oxygen
the +4 oxidation
through
state
the lattice,
to date would suggest
that excess
the bulk for materials
calcined
at 773 K or above. As stated above a correlation maleic
anhydride
formation
this is not the exclusive samples
prereduced
selectivities
was found between
from n-butane
B* content
[il. However
active phase as catalysts
in hydrogen
and selectivity
other results
containing
such that they become amorphous
suggest
in
that
the B phase and also show good
[20].
CONCLUSIONS
In summary it may be stated that excess phosphorus oxidation
of vanadium
the related
phases,
which amorphous
either
in the B* phase when calcined
8, B or (VO)2P207,
by calcination
exhibiting
monophasic
in air at 773 K, provided
ly removed from unity; catalysts calcination
during calcination
at 773 K or in one of
occur depends
XRD patterns
could readily
the P/V ratio was sufficient-
with P/V ratios close to unity required
Higher calcination
temperatures
the +4
at 923 K. The rate at
times at 773 K to reach steady state compositions
tures of phases.
stabilizes
phases -t reduced phases + BVPO~ transformations
upon the P/V ratio. Catalysts be obtained
strongly
favoured
extended
and exhibited
formation
mix-
of @VP05 and
211 resulted
in lower surface
phases are finally
formed
areas.
The role of reducing
after calcination
agent
in determining
which
is not yet clear.
ACKNOWLEDGEMENTS One of us (B.K.H.) wishes Scientifique
(Belgium)
to thank the Service
de Progranation
de la Politique
for a fellowship.
REFERENCES B.K. Hodnett, PH Permanne and B. Delmon, Appl. Catal., 6 (1983) 231. R.A. Mount and H. Rafelson, U.S. Patent No. 8330,354 (1975). R.A. Schneider, U.S. Patent No. 3,864,280 (1975). I.C.I. Belgian Patent No. 867,189 (1978). E. Bordes and P. Courtine, 3. Catal., 57 (1979) 236. L. Morselli, F. Trifiro and L. Urban, J. Catal., 75 (1982) 112. G. Poli, I. Resta, 0. Ruggeri and F. Trifiro, Appl. Catal...,1 (1981) 395. E. Bordes, Thesis, Universite de Technologie de Compiegne (1979). E.M. Boghosian, U.S. Patent No. 3,862,146 (1975) R.O. Kerr, U.S. Patent No. 4,056,487 (1966). B. Weinstein, A.. Jurewicz and L.B. Young, U.S. Patent No. 3,931,046 (1976). R. Gopal and C. Calvo, J. Solid State Chem., 5 (1972) 432. B. Jordan and C. Calvo, Can. J. Chem., 51 (1973) 2621. B.K. Hodnett and B. Delmon, Submitted for publication J. Catal., G. Ladwig, Z. Anorg. Alla. Chem., 338 (1965) 266. G. Poli, 0. Ruggeri and F. Trifiro, Ninth Intl. Symp. Reactivity of Solids, Cracow (1980) 512. G.J. Hutchings and R. Higgins, U.S. Patent No. 4,209,423 (1980). M. Nakamura, K. Kawai and Y. Fujiwara, J. Catal., 34 (1974) 345. A.S.T.M. Powder Diffraction File card 23-720 (V4Og). B.K. Hodnett and 8. Delmon, submitted Ind. Eng. Chem., Fund.