Vanadyl hydrogenphosphate sesquihydrate as a precursor for preparation of (VO)2P2O7 and cobalt-incorporated catalysts

Article abstract of DOI:10.1016/S1381-1169(00)00145-X

Vanadyl hydrogenphosphate sesquihydrate (VOHPO₄ · 1.5H₂O) was prepared by reduction of VOPO₄ · 2H₂O suspended in refluxed 1-butanol and activated at 480°C over 10 h on-stream to give (VO)₂P₂O₇, which revealed high specific activity per unit surface area in the vapor-phase oxidation of n-butane. Cobalt-modified (VO)₂P₂O₇ catalysts were prepared in the present work by intercalating sesquihydrate precursor with cobaltous acetate employing 1-butanol as solvent. The catalysts prepared from cobalt-intercalated precursor showed high specific activity, but both selectivities to maleic anhydride and surface area decreased with increasing cobalt-contents. Presumably, intercalated cobalt species retarded topotactic dehydration of layered precursor to afford crystalline active species (VO)₂P₂O₇, leading to an increase in unfavorable combustion activity due to pentavalent β-VOPO₄ species. The activity of catalysts prepared for reference by kneading vanadyl hydrogenphosphate hydrates with an additive Co₂P₂O₇ was interpreted as the variation in surface area, and cobalt has none of positive effects in these catalysts. (C) 2000 Elsevier Science B.V.

Full text of DOI:10.1016/S1381-1169(00)00145-X

Journal of Molecular Catalysis A: Chemical 158 Ž2000. 559–565
www.elsevier.comrlocatermolcata
Vanadyl hydrogenphosphate sesquihydrate as a precursor for
ž /
preparation of VO P O and cobalt-incorporated catalysts
2
2
7
Tomohiro Ishimura, Shigeru Sugiyama, Hiromu Hayashi)
Department of Chemical Science and Technology, Faculty of Engineering, UniÕersity of Tokushima, Minamijosanjima,
Tokushima 770-8506 Japan
Received 16 August 1999; accepted 14 January 2000
Abstract
Vanadyl hydrogenphosphate sesquihydrate ŽVOHPO P1.5H O. was prepared by reduction of VOPO P2H O suspended
4
2
4
2
in refluxed 1-butanol and activated at 4808C over 10 h on-stream to give ŽVO. P O , which revealed high specific activity
2
2
7
per unit surface area in the vapor-phase oxidation of n-butane. Cobalt-modified ŽVO. P O catalysts were prepared in the
2
2
7
present work by intercalating sesquihydrate precursor with cobaltous acetate employing 1-butanol as solvent. The catalysts
prepared from cobalt-intercalated precursor showed high specific activity, but both selectivities to maleic anhydride and
surface area decreased with increasing cobalt-contents. Presumably, intercalated cobalt species retarded topotactic dehydra-
tion of layered precursor to afford crystalline active species ŽVO. P O , leading to an increase in unfavorable combustion
2
2
7
activity due to pentavalent b-VOPO4 species. The activity of catalysts prepared for reference by kneading vanadyl
hydrogenphosphate hydrates with an additive Co P O was interpreted as the variation in surface area, and cobalt has none
2
2
7
of positive effects in these catalysts. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Cobalt-intercalated vanadyl hydrogenphosphate sesquihydrate; ŽVO. P O catalyst; Butane oxidation to maleic anhydride
2
2
7
1
. Introduction
hemihydrate precursor ns0.5
Ž . w3x with te-
travalent vanadium, of which structure was de-
termined by the X-ray single crystal analysis
w4–6x, are usually obtained by reduction of
Crystalline vanadyl pyrophosphate
ŽVO.2
-
P O , which is an active species w1,2x for selec-
2
7
tive oxidation of n-butane to maleic anhydride,
are prepared by the topotactic dehydration of
vanadyl hydrogenphosphate hydrates VOHPO4 P
vanadium pentoxide in concentrated HCl,
andror alcohols followed by the reaction with
85% H PO . Reduction of pentavalent vana-
3
4
nH O
Ž
ns0.5 and 1.5
.
at 400–5008C. The
dium species in rather strongly reductive alco-
hols such as 2-butyl w5x, i-butyl and benzyl
alcohol w7x also gives hemihydrate.
2
The sesquihydrate precursor
obtained by reducing VOPO P2H O in the less
ns1.5 was
Ž .
)
Corresponding author. Tel.: q81-88-6567430; fax: q81-88-
4
2
6
55-7025.
E-mail address: hayashi@chem.tokushima-u.ac.jp
ŽH. Hayashi..
reductive 1-ols such as 1-butyl alcohol w8x. The
layer structure of sesquihydrate with wide spac-
1
381-1169r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S1381-1169Ž00.00145-X

5
60
T. Ishimura et al.rJournal of Molecular Catalysis A: Chemical 158 (2000) 559–565
ing capable of intercalating additives appears of
benefit for preparing improved catalysts. Activi-
ties of vanadyl pyrophosphate and cobalt-incor-
porated catalysts prepared by the sesquihydrate
precursor are described in the present paper.
Prior to the comparative reaction studies, the
tetravalent sesquihydrate precursor was acti-
vated by dehydrating under butane-oxidation
conditions at least 10 h on-stream at 4808C to
afford vanadyl pyrophosphate
ŽVO.2 P O with
2 7
high surface area. The vapor-phase catalytic
oxidation of n-butane was carried out by a
conventional fixed-bed flow apparatus at 300–
2
. Experimental
4
808C with a space velocity SV of 2040
Ž .
Vanadium phosphate dihydrate VOPO P
mlrg-catrh for a feed gas composition of 2%
n-butane and 20% oxygen with helium balance.
4
2
H O 10.0 g, 50.5 mmol was added to 150 ml
Ž .
2
of 1-butanol and refluxed at 808C for 24 h.
After being cooled to room temperature, the
resultant precipitates were filtered, washed with
acetone, and dried overnight in a desiccator to
3. Results and discussion
afford 9.3 g
Ž
yield 97%
.
of white-blue powder
3.1. Reduction of VOPO P2H O in alcohols to
4
2
of VOHPO P1.5H O. Powder X-ray diffraction
VOHPO PnH O
4
2
4
2
Ž
XRD patterns were recorded with a diffrac-
.
tometer RINT 2000 of Rigaku, Tokyo, using
CuKa radiation at 40 kV and 100 mA. Chemi-
cal composition of catalysts and the precursors
was analyzed by an inductively coupled plasma
apparatus ICPS-5000 of Shimadzu, Kyoto.
Thermogravimetric and differential thermal
Vanadyl hydrogenphosphate hydrates
VOHPO PnH O are usually prepared by reduc-
4
2
tion of pentavalent vanadium species. Fig. 1
shows a pair of TG-DTA curves of a hemihy-
drate ns0.5 prepared by reduction of VOPO4
P2H O in 2-butanol, signifying a single stage
a
Ž .
Ž
.
2
analysis
Ž
TG-DTA
.
was measured by a thermal
dehydration of VOHPO P0.5H O to
Ž
VO.2
-
4
2
analyzer, Model TAS-100 of Rigaku, Tokyo.
Surface area was determined by nitrogen ad-
sorption method with a volumetric apparatus
P-700 of Shibata, Tokyo.
P O at 4508C. Powder XRD patterns of the
2
7
hemihydrate and pyrophosphate prepared there-
from are shown in Fig. 1 b1 and b2 , respec-
tively, and agreed well with literature w2,5,9x.
Ž
.
Ž
.
Fig. 1. TG-DTA Ža. and powder XRD patterns Žb. for VOHPO P0.5H O. XRD patterns: Žb1., authentic VOHPO P0.5H O; Žb2.
4
2
4
2
ŽVO. P O obtained at 7008C under nitrogen.
2
2
7

T. Ishimura et al.rJournal of Molecular Catalysis A: Chemical 158 (2000) 559–565
561
Sesquihydrate
Ž
ns1.5
2
.
was obtained by re-
a highly active catalyst on the weight-basis
often resulted in the effect of an increased sur-
face without appreciable improvements in the
intrinsic activity based on the unit area. Hutch-
ings and Higgins w11x proposed a simplified
kinetic model for n-butane oxidation involving
a parallel-consecutive reaction scheme of pseudo
first-order reactions:
ducing VOPO P2H O at 808C in 1-butyl alco-
hol. TG-DTA curves of seven sesquihydrate
samples superimposed on each other as given in
4
Fig. 2
and stepwise dehydration through hemihydrate
toward
VO.2 P O level. The first stage of an
a , showing the reproducible preparations
Ž .
Ž
2
7
endothermic decrease in weight at around 1508C
would be 1 mol of dehydration of sesquihydrate
to hemihydrate. The observed XRD pattern
given in Fig. 2
calcining sesquihydrate at 2008C was identified
as less crystalline
b2 for the product obtained by
Ž .
Ž
VO.2 H P O w6,10x, which
4
2
9
is known w4–6x to be another formulation of,
and the same material as VOHPO P0.5H O
4
2
Thus, specific activity
qately eliminate the effect of surface area.
Ž
k qk2.rsa could ade-
1
given in Fig. 1
Ž
b1
.
. Further dehydration of in-
termediate hemihydrate to vanadyl pyrophos-
phate was observed at lower temperature of
Fig. 3 compares conversion of n-butane a
Ž .
and selectivity to maleic anhydride
VO.2 P O catalysts prepared from precursors
b of
Ž .
3
20–3508C
Ž
Fig. 2
4508C; Fig. 1
Ž
a,b2.. than does the authen-
a,b1..
Ž
tic hemihydrate
Ž
Ž
.
2
7
of vanadyl hydrogenphosphate hemihydrate
s0.5; triangles and sesquihydrate ns1.5;
circles at a reaction temperature of 4208C,
where the Hutchings’ plots are given in
Ž
n
.
Ž
.
3
.2. ActiÕity of (VO) P O catalysts
2 2 7
c .
Ž .
Catalysts from hemihydrate precursor obtained
A wide variety of procedure for preparing
by reduction of V O in benzyl alcohol w12x
2
5
2
V–P–O catalysts makes it difficult to compare
the catalyst performance on the same basis. The
activity is usually related to the surface area and
have a high suface area of 35–40 m rg and
often show high activity, but the activity
k2 in the present work lies on a same line of
Ž
k q
1
.
Fig. 2. TG-DTA Ža. and powder XRD patterns Žb. for seven samples of VOHPO P1.5H O. XRD patterns: Žb1. authentic VOHPO P1.5H O;
4
2
4
2
Žb2. ŽVO. H P O obtained by calcining sesquihydrate at 2008C under nitrogen; Žb3. ŽVO. P O obtained at 7008C.
2
4
2
9
2
2
7

5
62
T. Ishimura et al.rJournal of Molecular Catalysis A: Chemical 158 (2000) 559–565
Fig. 3. Effect of surface area on conversion of n-butane Ža., selectivity to maleic anhydride Žb. and Hutchings’ plot w11x Žc. in the
vapor-phase oxidation of n-butane over ŽVO. P O catalysts. Slope in Žc. indicates specific activity. Catalyst precursors: triangles,
2
2
7
VOHPO P0.5H O; circles, VOHPO P1.5H O. Reaction conditions: temperature, 4208C; feed composition, C H :O :Hes1:10:40, flow
4
2
4
2
4
10
2
rate, 51 mlrmin; space velocity, 2040 mlrg-catrh.
hemihydrate
Ž
Fig. 3
Ž
c
.
, triangles
.
. A catalyst
similar to the hemihydrate was proposed for
orthorhombic unit cell with lattice constants
obtained by the stepwise dehydration of the
sesquihydrate precursor through intermediate as
˚
as7.43, bs9.62 and cs7.97 A. Enlarged
Ž
b2
of 9.6 m rg, but the activity lies on a same line
of sesquihydrate Fig. 3 , circles . The slope
of Hutchings’ plot indicates the specific activity
and thus Fig. 3 shows higher specific activity
.
showed a low surface area
layer interstices of 7.97 A in sesquihydrate
˚
given in Fig. 2
2
˚
compared with that of 5.70 A in hemihydrate
Ž
Ž
c
.
.
appears quite reasonable to hold another
molecule of water in agreement with TG-DTA
Ž
c
.
in Fig. 2
However, an extended diffraction measure-
ment toward lower angles 2u s3–608 in this
work revealed a strong new reflection at around
.58 as shown in Fig. 2 b1 , suggest-
a .
Ž .
of catalyst prepared from the sesquihydrate pre-
cursor. Conversion of n-butane increased in
parallel with increasing surface area for both
catalysts as in Fig. 3
while to note that the selectivity to maleic anhy-
dride on
VO.2 P2O7 catalysts prepared from
sesquihydrate decreased with increasing surface
area circles while unchanged over a wide range
of surface area in the case of hemihydrate pre-
cursor triangles as compared in Fig. 3
Ž
.
a , but it appears worth-
Ž .
5
Ž
ds16 A˚
.
Ž
.
ing the structure based on the above mentioned
Ž
indexing w8x leaves further improvements. We
could not yet fully figure out at present, but the
Ž
.
structure of VOHPO P1.5H O appears quite
4
2
reminiscent of a-Zr
structure was extensively investigated w13,14x to
ŽHPO4.2 PH O, of which
2
Ž
.
Ž
b
.
.
be a layer structure with a lattice constant of
˚
˚
3
.3. Structure of Õanadyl hydrogenphosphate
cs15.414 A and the layer separation of 7.6 A.
The wide layer spacing of sesquihydrate capable
of intercalation would be of benefit for provid-
ing a general procedure in the preparation of
hydrates
Vanadyl monohydrogenphosphate hemihy-
drate VOHPO P0.5H O crystallized from aque-
modified
tives.
ŽVO.2 P O catalyst incorporated addi-
4
2
2 7
ous media is orthorhombic with as7.420 1 ,
Ž .
bs9.609 2 , cs5.6931 7
Ž . Ž .
A˚ , Zs4 and space
group Pmmn w4–6x. Powder XRD pattern of
sesquihydrate VOHPO P 1.5H O was tenta-
3.4. Cobalt-incorporated (VO) P O catalysts
2 2 7
4
2
tively indexed in the previous communication
w8x based on the powder XRD data taken in the
range of 2u s10–508, and a layer structure
A number of promoters, e.g., Co, Ni, Fe, Mo
and Nb, have been shown to be effective for
improving vanadium phosphorus oxide catalysts

T. Ishimura et al.rJournal of Molecular Catalysis A: Chemical 158 (2000) 559–565
563
w11,15x. However, it appears not yet clear where
is the effect coming from. It is rather common
that the preparation procedure strongly affects
the activity of modified catalysts with additives.
Attempts were made in the present work to
compare cobalt-incorporated
ŽVO.2 P2O7 cata-
lysts prepared by kneading and intercalation
methods. Coprecipitation and impregnation w16x,
and doping w17x were employed for preparing
cobalt-modified catalysts, but intercalation has
not yet been reported.
Fig. 5. Lattice constants of cobalt-intercalated VOHPO P1.5H O.
4
2
3
.4.1. Intercalation of cobalt acetate into the
longed refluxing for 40–80 h. The precipitates
attempted to activate at 4808C, as usual, showed
low activity and the major component detected
layer spacing of VOHPO PnH O
4
2
Intercalation of additives in the interstices of
catalyst precursor with a layer structure such as
VOHPO PnH O is of interest, if possible, to
assure the atomic dispersion of additives to
afford well-defined catalysts. However, hemihy-
by XRD was V O .
2
5
4
2
For topotactic dehydration w4–6x of the pre-
cursor hydrates to afford active species
Ž
VO.2 P O , the layer structure should be re-
drate
aqueous solution of Co
for 80 h. Attempted intercalation of cobalt into
the wide layer spacing of sesquihydrate ns
.5 in aqueous phase also resulted in failure to
Ž
ns0.5
.
precursor was intact with an
2 7
Ž
OAc.2 refluxed at 808C
tained in the intercalation of additives. To avoid
decomposition as observed in aqueous phase,
another attempt has been made in the present
work to intercalate cobalt species in 1-butanol,
which was employed as reducing solvent in
preparing sesquihydrate. To a solution of 100
Ž
1
.
decompose the structure of vanadyl hydrogen-
phosphate hydrate, which dissolved in aqueous
cobaltous acetate within 10–20 h under reflux at
ml of 1-butanol containing 1.006 g
Ž
4.0 mmol
40.0
.
of Co OAc.2 P4H O was added 7.597 g
Ž
Ž
8
08C to give dark blue solution followed by the
2
precipitation of deep green powder during pro-
Fig. 4. Powder XRD patterns of cobalt-intercalated VOHPO4P
Fig. 6. Powder XRD patterns of ŽVO. P O catalysts prepared
2 2 7
1
0
.5H O. CorV Žatomic ratio.: Ža., 0.00; Žb., 0.05; Žc., 0.10; Žd.,
from cobalt-intercalated VOHPO P1.5H O. CorV Žatomic ratio.:
2
4
2
.28; Že., 0.90. B: Co ŽOAc. ŽOH..
Ža., 0.00; Žb., 0.05; Žc., 0.10; Žd., 0.18. ': b-VOPO .
4
3
5

5
64
T. Ishimura et al.rJournal of Molecular Catalysis A: Chemical 158 (2000) 559–565
crystalline sesquihydrates with increasing inter-
calated cobalt. In case of higher concentrations
of cobaltous acetate in 1-butanol, the resultant
cobalt-intercalated sesquihydrate was contami-
nated as in Fig. 4 e with Co3
Ž . ŽOAc. ŽOH. w18x,
5
which is very soluble in water and could be
removed with great ease by washing quickly
with a small amount of water. Slightly enlarged
lattice constants of
due to intercalation of cobalt species without
change in those of 220 reflections ab-plane
are shown in Fig. 5, signifying an easy access of
cobalt species to the wide layer spacing along
c-axis without decomposing the structure. Thus,
the upper-limit for sesquihydrate precursor ca-
pable of intercalating cobalt species was at
around CorVs0.28.
001 reflections c-axis
Ž . Ž .
Fig. 7. A decrease in surface area of ŽVO. P O catalysts pre-
Ž
.
Ž
.
2
2
7
pared from cobalt-intercalated VOHPO P1.5H O with increasing
4
2
cobalt-contents.
mmol of VOHPO P1.5H O and the magneti-
.
4 2
cally stirred suspension was heated at 808C
under reflux. The wine-red color quickly disap-
peared within 10 min, indicating the effective
uptake of cobalt species into the precursor hy-
drate without decomposition of the skeletal lat-
tice. After 1 h of refluxing, the precipitates were
filtered, washed with acetone, and dried
overnight in a desiccator to afford 8.279 g of
grayish white-blue powder. The filtrate was col-
orless and atomic ratio of CorV in the solid
mass analyzed by ICP was found to be 0.104,
showing that every cobalt species in the ingredi-
ent was entrapped into the precursor hydrate.
Powder XRD patterns given in Fig. 4 show less
3.4.2. Actitity of cobalt-incorporated V–P–O
catalysts
The cobalt-intercalated precursors were acti-
vated as usual at 4808C under butane-oxidation
conditions at least over 10 h on-stream to afford
active
without appreciable change in the XRD patterns
at low contents of cobalt Fig. 6
b,c... However,
pentavalent b-VOPO phase was detected along
ŽVO.2 P O phase as evidenced in Fig. 6
2 7
Ž
Ž
4
with d for
ŽVO.2 P2O7 as shown in Fig. 6Ž .
Fig. 8. Activity Ža. ŽHutchings’ plot w11x. and selectivity patterns Žb. of catalysts with and without incorporated cobalt in the vapor-phase
oxidation of n-butane. Slope in Ža. indicates specific activity. Catalyst precursors: triangles, VOHPO P0.5H O; circles, VOHPO P1.5H O.
4
2
4
2
White symbols, without Co; Half-black symbols, kneaded with Co P O ; Center-black circles, cobalt-intercalated VOHPO P1.5H O Žwith
2
2
7
4
2
CorVs0.05, 0.10 and 0.18.. Reaction conditions: same as in Fig. 3.

T. Ishimura et al.rJournal of Molecular Catalysis A: Chemical 158 (2000) 559–565
565
catalyst with a high cobalt-content. Surface area
of catalysts decreased with increasing cobalt-
contents as shown in Fig. 7.
drate could be intercalated with cobaltous ac-
etate to afford modified
VO.2 P O with high
activity, but both selectivity to maleic anhydride
and surface area decreased with increasing
cobalt-contents. Presumably, intercalated addi-
tives retarded topotactic dehydration of layered
precursor to afford the crystalline active species
Ž
2
7
Fig. 8 compares activity
meleic anhydride in the vapor-phase cat-
a and selectivity to
Ž .
b
Ž .
alytic oxidatioin of n-butane over cobalt-incor-
porated V–P–O catalysts prepared by the inter-
calation
method
Ž
center-black circles
half-black symbols on Hutchings’ plot.
white sym-
given in Fig. 3 are also plotted for refer-
ence. Sesquihydrate precursor gives active
VO.2 P2O7 catalysts as demonstrated in Fig.
, and catalysts from cobalt-intercalated pre-
cursor were much more active as in Fig. 8
.
and kneading
ŽVO.2 P O , leading to an increase in the unfa-
vorable combustion activity due to pentavalent
2 7
Ž
.
Results of catalysts without cobalt
Ž
b-VOPO species. Cobalt has none of the posi-
4
bols
.
tive effects in catalysts prepared by simple
kneading procedure.
Ž
3 a
Ž .
a ,
Ž .
Acknowledgements
scattering beyond the sesquihydrate line, but
selectivity to maleic anhydride decreased with
We would like to appreciate Dr. Ikuya Mat-
suura for his stimulating suggestions for the
present project.
increasing cobalt-content as shown in Fig. 8
center-black circles
Cobalt-incorporated catalysts were prepared
b
Ž .
Ž
.
.
by another method for reference, where the
precursor hydrate VOHPO PnH O and an addi-
4
2
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2
2
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2
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vated at 4808C over 10 h on-stream to give
VO.2 P O , which revealed high specific activ-
ity in the vapor-phase oxidation of n-butane.
Ž
2
7
Employing 1-butanol as solvent, the sesquihy-