A comparison of the reactivity of "nonequilibrated" and "equilibrated" V-P-O catalysts: Structural evolution, surface characterization, and reactivity in the selective oxidation of n-butane and n-pentane

Article abstract of DOI:10.1006/jcat.1996.0123

Changes occurring on thermal treatment of the precursor of vanadium/phosphorus mixed oxide, the industrial catalyst for the oxidation of n-butane, were studied. The precursor was mixed with stearic acid, used as an organic binder for pelletization of the powder. The calcination of the precursor leads to a partially oxidized compound, constituted of an amorphous V^IV-P mixed oxide and a crystalline hydrated V^V-P-O phase. The calcined compound, when left in a 1% hydrocarbon/air stream for 100 h leads to a "nonequilibrated" catalyst, and after 1000 h to the "equilibrated" catalyst. The catalytic activity of the nonequilibrated and equilibrated catalysts in n-butane and n-pentane oxidation was studied and compared; the chemical-physical features of the two catalysts were studied by means of XRD, FT-IR, chemical analysis, TGA, XPS, and TPD. Only well crystallized (VO)₂P₂O₇ was detected in the equilibrated catalyst and a homogeneous distribution of surface centers seems to be present on its surface. In the case of nonequilibrated catalyst, a poorly crystallized (VO)₂P₂O₇ is present together with an amorphous V^IV-P-O phase and γ-VOPO₄; these phases define a heterogeneous distribution of at least two kind of surface centers. This surface heterogeneity gives rise to a catalyst less selective in n-butane oxidation to maleic anhydride and less specific in the conversion of n-pentane to phthalic anhydride.

Full text of DOI:10.1006/jcat.1996.0123

JOURNAL OF CATALYSIS 160, 52–64 (1996)
ARTICLE NO. 0123
A Comparison of the Reactivity of “Nonequilibrated” and “Equilibrated”
V–P–O Catalysts: Structural Evolution, Surface Characterization, and
Reactivity in the Selective Oxidation of n-Butane and n-Pentane
,1
ꢀ
ꢀ
ꢀ
ꢀ
S. Albonetti, F. Cavani, F. Trifiro`, P. Venturoli, G. Calestani,† M. Lo´pez Granados,‡ and J. L. G. Fierro‡
ꢀ
Dipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy; †Dipartimento di Chimica Fisica ed Inorganica, Viale
Risorgimento 4, 40136 Bologna, Italy; and ‡Instituto de Cata´lisis y Petroleoqu´ımica, CSIC, Campus UAM, Cantoblanco, 28049 Madrid, Spain
Received March 16, 1995; revised June 26, 1995; accepted December 28, 1995
treated under reaction conditions. In the latter case, the
calcination gives rise to mixtures of amorphous and crys-
talline VV and VIV phases, the relative amounts of which are
a function of the conditions of treatment (either oxidizing
or nonoxidizing) (5).
Changes occurring on thermal treatment of the precursor of
vanadium/phosphorus mixed oxide, the industrial catalyst for the
oxidation of n-butane, were studied. The precursor was mixed with
stearic acid, used as an organic binder for pelletization of the pow-
der. The calcination of the precursor leads to a partially oxidized
compound, constituted of an amorphous VIV–P mixed oxide and a
crystalline hydrated VV–P–O phase. The calcined compound, when
left in a 1% hydrocarbon/air stream for 100 h leads to a “nonequili-
brated” catalyst, and after 1000 h to the “equilibrated” catalyst. The
catalytic activity of the nonequilibrated and equilibrated catalysts
in n-butane and n-pentane oxidation was studied and compared;
the chemical–physical features of the two catalysts were studied by
means of XRD, FT-IR, chemical analysis, TGA, XPS, and TPD.
Only well crystallized (VO)2P2O7 was detected in the equilibrated
catalyst and a homogeneous distribution of surface centers seems
to be present on its surface. In the case of nonequilibrated cata-
lyst, a poorly crystallized (VO)2P2O7 is present together with an
amorphous VIV–P–O phase and ꢀ-VOPO4; these phases define a
heterogeneous distribution of at least two kind of surface centers.
This surface heterogeneity gives rise to a catalyst less selective in
n-butane oxidation to maleic anhydride and less specific in the con-
c
The nature of the phases present in the catalyst affects re-
markably the catalytic behavior in n-butane oxidation; this
effect has been studied by several authors, and the contem-
poraneous presence of oxidized VV–P–O and reduced VIV–
P–O phases has been pointed out as a necessary condition
in order to have an active and selective catalyst (12–14). The
relative amounts of VV and VIV under reaction conditions is
a function of the thermodynamic properties of the solid–gas
system represented by the catalyst surface and the reactant
mixture, which can be more or less oxidative depending on
the relative concentration of the oxygen and the hydrocar-
bon, on the temperature and pressure employed, and which
can accelerate the structural transformations of the catalyst
itself. VV–P–O and VIV–P–O phases exhibit structural sim-
ilarities that allow, under the reaction conditions, easy re-
dox cycles between different oxidation states of vanadium
(15–17). Other factors that affect the catalytic performance
are the surface dynamics of the hydrocarbon (18) and of
the oxygen (19) species on the catalyst surface. One of the
most important aspects is that the structural evolution of
the catalyst from the beginning of its lifetime (this has of-
ten been referred to as a “nonequilibrated” compound)
in the reaction environment finally leads to the structure
that is thermodynamically more stable under usual reac-
tion conditions in the equilibrated catalyst (20), i.e., the
vanadyl pyrophosphate, which is a VIV–P–O phase, even
though this does not exclude the presence of surface VIII
and VV species. This aging phenomenon may take several
hundred hours, or in some cases shorter times, depending
on the reaction conditions and on the structural composi-
tion of the nonequilibrated catalyst. The structural changes
and the modifications occurring under the reaction con-
ditions are accompanied by modifications in the catalytic
performance in n-butane oxidation (21–23).
version of n-pentane to phthalic anhydride.
ꢀ 1996 Academic Press, Inc.
INTRODUCTION
V–P–O based catalysts are known to be efficient in the
selective transformation of n-butane to maleic anhydride
(1–4) and are employed industrially for this reaction in
processes developed by several companies (5, 6). More-
over, the same system was found to oxidize n-pentane to
maleic and phthalic anhydrides (7–11).
The active phase in n-butane oxidation, (VO)2P2O7, is
prepared via a multistep transformation of a precursor,
VOHPO4 ꢁ 0.5H2O (5). The precursor can be either trans-
formed into the active phase in situ under reaction condi-
tions or calcined ex situ in either air or nitrogen and then
1 To whom correspondence should be addressed.
52
0021-9517/96 $18.00
c
Copyright ꢂ 1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.

NONEQUILIBRATED AND EQUILIBRATED V–P–O CATALYSTS
53
The formation of phthalic anhydride from n-pentane is 1.8- 2.7-eV linewidths and a Gaussian/Lorentzian mix with
a unique property of the V–P–O system; other vanadium- a Lorentzian contribution of 30% were employed for the
based systems lead only to the formation of maleic anhy- V 2p envelope. Spectra were corrected for sample charg-
dride(24). Fromthispointofview, thereactionofn-pentane ing by referencing photoelectron peaks to the C 1s peak at
oxidation is a more suitable probe than that of n-butane 284.9 eV. Samples were also sputtered in the pretreatment
oxidation. In fact, the formation of a wider range of prod- chamber using an Ar+ beam of 3 keV and a current density
ucts allows a better characterization of the reactivity of the of 120 nA/cm2. In the V 2p core level region, the spectra
sites. The production of phthalic anhydride requires not were complicated by the interference of an O 1s peak in the
only oxidation sites but also condensation centers in or- region of the V 2p1/2 component. This is due to the contri-
der to obtain a C8 molecule starting from a C5 reactant. bution of secondary exciting photons (Mg Kꢀ3,4) present in
Therefore the formation of phthalic anhydride must be re- the non-monochromatic X-ray source.
lated to some particular property of the surface structure
Thermogravimetric experiments were conducted using a
of V–P mixed oxides which makes this reaction a probe to Cahnmicrobalanceconnectedtoagashandlingsystem. The
achieve more information on the surface properties of the samples (35 mg) were heated under a flow of 15 cm3/min dry
V–P–O system. The aim of the research work reported here air at a rate of 4ꢃC/min. The microbalance was interfaced
was to study and compare the bulk properties, reactivity, with a data station which allowed weight changes to be
and surface properties of nonequilibrated and equilibrated recorded at intervals of 0.5 min.
V–P–O catalysts in the selective oxidation of n-butane and
TPD tests were carried out as follows. The catalyst sam-
n-pentane. The different phases present in these samples, ples were first pretreated in flowing dry air (30 cm3/m) at
their surface properties, and the comparison of their cat- 340ꢃC (heating rate 5ꢃC/min) for 1 h and then cooled down
alytic activity in both reactions should be useful in clarify- to 300ꢃC and evacuated for 1 h to P = 10ꢄ4 mbar. Succes-
ing the requirements for the formation of anhydrides on sively, the temperature was raised to 340ꢃC and 100 Torr of
the surface sites.
hydrocarbon was kept in contact with the sample for 1 h.
The sample was then cooled down and when room tem-
perature was reached evacuation for 1 h was carried out
to P = 10ꢄ4 mbar. For the TPD tests, the temperature was
raised at a rate of 5ꢃC/min to 720ꢃC under vacuum. Several
m/z fragments were followed, using a quadrupole device
connected to the reactor outlet. The most important frag-
ments were CO and CO2, H2, O2 and H2O.
The catalytic tests were carried out at atmospheric pres-
sure and were performed in a tubular stainless steel flow re-
actor at atmospheric pressure. The reaction products were
collected by bubbling the outlet stream into pure acetone-
containing condensers and analyzed on a 1-h-reaction ba-
sis; permanent gases were collected by a gas-tight syringe.
All products were analyzed by means of Hewlett-Packard
5890 II gas chromatography, equipped with FID and TCD.
Carbon oxides and oxygen were separated and analyzed by
means of a packed Carbosieve S column, with oven temper-
atureprogrammedfrom40to240ꢃC. Condensableproducts
and unreacted hydrocarbons were separated and analyzed
in a 0.53 mm OV-17 column, with the oven temperature
programmed from 80 to 200ꢃC.
METHOD
The catalyst was prepared in organic medium by the fol-
lowing procedure. V2O5 (15 g) was suspended in 90 ml of
isobutyl alcohol and 60 ml of benzyl alcohol. The suspen-
sion was stirred continuously under reflux for 3 h, cooled
to room temperature, and then 16.2 g of 99% o-H3PO4 was
added. The slurry was heated under reflux with constant
stirring for 2 h, cooled, filtered, washed with isobutyl alco-
hol, and the filtrate was dried at 150ꢃC for 24 h.
XRD patterns were recorded using a computer con-
trolled Philips PW 1050 diffractometer and Cu Kꢀ radi-
ation.
FT-IR spectra were recorded at a resolution of 4 cmꢄ1
on a Nicolet 57DX spectrometer with the sample powder
diluted with KBr (2%) and pressed into self-supporting
wafers.
Chemical analyses were carried out to determine the va-
lence state of vanadium following the usual titration proce-
dure with KMnO4 for VIV and Mohr salt for VV (12).
XPS spectra were recorded using a Fisons Escalab MkII
spectrometer employing Mg Kꢀ radiation (photon energy
1253.6 eV) and an electron take-off angle of 45ꢃ. The base
pressure of the spectrometer was typically 10ꢄ9 mbar. The
X-ray gun was operated at 12 kV and 10 mA, corresponding
to a power of 120 W. The O 1s, P 2p, V 2p, and C 1s energy
regions were recorded for each sample. Data analysis was
carried out using a VGS 5000 data system based on the DEC
PDP11 computer. The methodology employed for peak fit-
ting and peak deconvolution was the following: typically
Three grams of catalyst was used for catalytic tests, sieved
intoparticleswithsizerangingfrom0.3to0.5mm. Theusual
experimental conditions for catalytic tests were the follow-
ing: n-pentane, 1 mol% in air; n-butane, 1.7 mol% in air;
temperature, 340ꢃC; W/F, 3 g s/ml. Yields were calculated
according to the following equations:
C5H12 + nO2 → C4H2O3 + COx + 5H2O
2C5H12 + nO2 → C8H4O3 + 2COx + 4H2O
C5H12 + nO2 → 5COx + 6H2O

54
ALBONETTI ET AL.
Yield% to maleic anhydride (MA) =
(mol MA produced) ꢅ 100/(mol n-C5H12 fed)
Yield% to phthalic anhydride (PA) =
(mol PA produced) ꢅ 2 ꢅ 100/(mol n-C5H12 fed)
Yield% to carbon oxides (COx) =
{(mol COx produced/5)ꢄ(mol MA produced)ꢄ2(mol
PA produced)} 100/(mol n-C5H12 fed).
Carbon oxides which are considered in the calculation
of the yield to COx are therefore only those produced by
complete overoxidation, while those formed as coproducts
in the formation of maleic and phthalic anhydrides are not
taken into account.
RESULTS
Thermal Evolution of the Precursor in Air
Thermal changes in the pellets. The catalyst precursor,
VOHPO4 ꢁ 0.5H2O, was prepared according to the “anhy-
drous” procedure in an organic medium (5); details of the
preparation are given under Method. Before the calcina-
tion and activation procedures, the precursor powder was
mixed with 5 wt% stearic acid, which is necessary for easy
tabletting of the powder. Cylindrical pellets (200 g) were
then calcined at 380ꢃC for 5 h. The X-ray pattern of the
sample after this treatment is displayed in Fig. 1a (patterns
of compounds in Figs. 1b and 1c will be discussed later),
while its FT-IR spectrum is given in Fig. 2a. Other features
of this sample are listed in Table 1; the average oxidation
degree of vanadium (4.40) indicates the presence of both
VV and VIV phases. The absence of reflections relative to
any known VIV crystalline phase in the XRD pattern sug-
gests that an amorphous phase holds the VIV. The surface
area is 11 m2/g, remarkably lower than the one typical of the
vanadyl pyrophosphate (usually, in the range 20 to 30 m2/g).
Neither the XRD pattern nor the FT-IR spectrum show the
features typical of the (VO)2P2O7. These results were un-
expected; in fact, it is known that a dehydration in air of
the precursor prepared in organic medium usually leads to
a compound in which the prevailing phase is the vanadyl
pyrophosphate, though often it is of low crystallinity and
mixed with a V5 + phase, and that in order to achieve such
a wide extent of vanadium oxidation it is necessary to carry
out the calcination at temperatures higher than 450ꢃC.
FIG. 1. XRD patterns of the precursor mixed with stearic acid and
calcined at 380ꢃC. (a) Calcined catalyst; (b) nonequilibrated catalyst; (c)
equilibrated catalyst.
formation of the (VO)2P2O7 phase. Broadening of the (h00)
reflections indicates that the sample is not well crystallized.
Leaving the sample for 5 h leads to the appearance of the
same reflections also observed in the case of the sample in
Fig. 1a;however, thisphasebecomestheprevailing onewith
respect to the vanadyl pyrophosphate only for very long pe-
riods of calcination (60 h). On the contrary, in the case of
the calcination carried out with a large amount of catalyst
Thermal changes in the powder. In order to evaluate shaped in pellets (Fig. 1a) this phase was formed after a
whether the observed phenomena were due to the use of few hours; therefore, in this case a mass effect was clearly
the precursor shaped in the form of pellets, we carried out a present, which accelerated the formation of this phase.
study of the changes in the precursor in the form of powder,
The effect of stearic acid addition was confirmed by ex-
but mixed with stearic acid. Figure 3 shows the ex situ XRD amining the structural evolution of the precursor powder
pattern (samples kept under room atmosphere) of the sam- at 380ꢃC when this was not preliminarily mixed with the
ples after increasing periods of elapsed time at 380ꢃC. The stearic acid (Fig. 4). The two following effects are observed:
reflections from the hemihydrate vanadyl orthophosphate (i) the disappearance of the reflections of the hemihydrate
progressively disappear (none are left after 1 h), with an vanadyl orthophosphate is completed in a longer period of
evident contemporaneous amorphization and subsequent time than with the mixed precursor; (ii) only the reflections

NONEQUILIBRATED AND EQUILIBRATED V–P–O CATALYSTS
55
FIG. 2. FT-IR spectra of (a) the calcined sample, (b) the nonequili-
brated catalyst, and (c) the equilibrated catalyst.
which developed after 10 h of calcination at 380ꢃC corre-
spond to those of a poorly crystallized vanadyl pyrophos-
phate; also an amorphous phase seems to be present. No
other crystalline phase was obtained even after 30 h of calci-
nation. The reflections relative to the crystalline phase dis-
playedinFig. 1awereformedonlyaftercalcinationat450ꢃC
for 20 h at least (Fig. 5a). Therefore, the presence of stearic
acidacceleratesthestructuraltransformationsandalsolow-
ers the temperature at which they occur. It is likely that the
combustion of the stearic acid locally increases the effec-
tive temperatures; with large amounts of catalyst shaped
in pellets the release of the heat is less efficient, probably
causing a marked increase in the local temperature.
FIG. 3. XRD patterns of the precursor mixed with 5 wt% stearic acid
TABLE 1
and calcined at 380ꢃC for increasing periods of time.
Valence State as Determined by Chemical Analysis and
BET Surface Area of Samples
Figure 5b shows the pattern of the unmixed precursor
when the latter is stored at room temperature in dry air
after calcination at 450ꢃC; when the sample was left at
room atmosphere for some days, the spectrum displayed
in Fig. 5a was restored. Therefore it seems that the two
compounds whose patterns are shown in Fig. 5 are in a
hydration–dehydration equilibrium.
Vanadium average
oxidation state
Surface area
(m2/g)
Sample
Calcined
4.40
4.36
11
11
23
Nonequilibrated
Equilibrated
ꢆ4.00

56
ALBONETTI ET AL.
FIG. 5. XRD patterns of the precursor calcined at 450ꢃC for 20 h and
stored under either (a) room atmosphere or (b) anhydrous conditions.
˚
˚
are two main reflections at d = 4.17 A and at d = 3.10 A;
the assignment of these peaks is uncertain. They might be
attributed to the (004) and (040) reflections, respectively, of
ꢁ -VOPO4, which are reported to be very intense in the ref-
erence compound (15, 17) (even though other very intense
reflections typical of this phase are lacking in our spectrum,
FIG. 4. XRD patterns of the precursor not mixed with stearic acid
˚
for instance the one at d = 4.90 A). Indeed, intense peaks
and calcined at 380ꢃC for increasing periods of time.
˚
at around d = 3.10 A are reported for all VOPO4 phases,
and this makes a definite attribution a hard task.
The XRD pattern in Fig. 1c is obtained after 1000 h un-
der reaction conditions. This catalyst will be referred to as
equilibrated catalyst. The pattern agrees very well with that
Structural Evolution of the Calcined Compound in the
Reaction Environment
Reported in Figs. 1b and 1c are the ex situ XRD pat- of well crystallized (VO)2P2O7 (25).
terns of samples used for catalytic tests; after unloading,
Figure 2 reports the IR spectra of the nonequilibrated
the samples were stored under room atmosphere. In par- (Fig. 2b) and equilibrated (Fig. 2c) catalysts. The spec-
ticular, Fig. 1b shows the pattern of the “calcined” sam- trum of nonequilibrated sample is quite complicated; some
ple (whose pattern is given in Fig. 1a) after approximately bands can be attributed to (VO)2P2O7, but other bands are
100 h time-on-stream in n-pentane oxidation; hereinafter present, which confirm the contemporaneous presence of
this sample is referred to as nonequilibrated catalyst. It is other phases. A clear attribution to a specific phase is im-
worth mentioning that the biggest variations in the catalytic possible. The FT-IR spectrum for equilibrated catalyst is
performance are observed within the first 100 h time-on- typical of (VO)2P2O7 (1, 12).
stream, while after this period the behavior, even though
Table 1 gives the average valence state of the vanadium,
not definitely constant, is much more stable. The pattern asdeterminedbychemicalanalysisandthevaluesofspecific
exhibits diffraction lines assignable to poorly crystallized surface area. The nonequilibrated catalyst is only slightly
(VO)2P2O7; most of the sample must be amorphous as in- more reduced (average degree of oxidation, 4.36) than the
dicated by the low signal-to-noise ratio; in addition, there calcined catalyst. Therefore, under the reaction atmosphere

NONEQUILIBRATED AND EQUILIBRATED V–P–O CATALYSTS
57
FIG. 7. n-Butane conversion as a function of the reaction temperature
over nonequilibrated and equilibrated catalysts. Reaction conditions: 1.7
mol% n-butane in air, W/F 3 g s/ml.
FIG. 6. Weight changes in air at increasing temperatures for (a) the
calcined sample, (b) the nonequilibrated catalyst, and (c) the equilibrated
catalyst.
Comparison between Nonequilibrated and Equilibrated
Catalysts in Paraffin Oxidation
Reactivityinn-butaneoxidation. ReportedinFigs. 7and
8 are the n-butane conversion as a function of reaction tem-
perature and the selectivity vs conversion plot, respectively,
for the two samples. The equilibrated catalyst displays a
higher activity than the nonequilibrated one; the higher
activity can be justified by the higher surface area; how-
ever, the areal activity (activity per unit surface area) is still
higher for the equilibrated catalyst. The apparent activation
energy for the nonequilibrated catalyst is 20.5 kcal/mol and
for the equilibrated catalyst is 18.5 kcal/mol.
The selectivity dependence upon conversion is very dif-
ferent for the two samples. For the nonequilibrated catalyst
the selectivity to maleic anhydride is lower (around 50%),
and is fairly constant over the entire range of temperatures
examined, whereas the equilibrated catalyst exhibits an ini-
the reduction of vanadium is a very slow process, slower
than the transformation of the amorphous VIV/P/O phase
into crystalline (VO)2P2O7. It is not unlikely that some
amorphous phase is still left as indicated by the XRD pat-
tern. The “equilibrated” sample is instead completely re-
duced, at least within the sensitivity limits of the method
employed for the determination of the valence state. The
surface area of the calcined sample is 11 m2/g; after 100 h
time-on-stream (nonequilibrated sample) the surface area
remains substantially unaltered. The surface area of equili-
brated sample is about 23 m2/g.
Thermogravimetric tests were carried out on the cal-
cined sample, and for nonequilibrated and equilibrated
catalysts. The results are reported in Fig. 6. The weight loss
at below 400ꢃC can be ascribed to a dehydration process.
This weight loss for the nonequilibrated sample (Fig. 6b)
is much lower than for the calcined sample (Fig. 6a). The
equilibrated sample (Fig. 6c) does not lose weight at all,
which agrees with the bulk characterization; in fact, only
(VO)2P2O7 was detected in this sample. The weight gain at
above 400ꢃC can be assigned to the oxidation of VIV phases
to VVOPO4 phases. The weight increase of the equilibrated
sample is much higher than that for the other two samples,
in agreement with the results of the chemical analysis,
which indicated a lower average oxidation degree for the
equilibrated catalyst. Another observation concerns the
onset temperature for the oxidation of vanadium, which
is the highest in the equilibrated catalyst. This confirms
that the well crystallized (VO)2P2O7 is oxidized with
more difficulty than the amorphous or poorly crystallized
VIV phase present in the calcined sample and in the
nonequilibrated catalyst, respectively.
FIG. 8. Selectivity to maleic anhydride as a function of the n-butane
conversion; conditions as in Fig. 7.

58
ALBONETTI ET AL.
FIG. 9. n-Pentane conversion and yields as functions of the reaction
temperature on equilibrated catalyst. Reaction conditions: 1% n-pentane
in air, W/F 3 g s/ml.
FIG. 11. Rates of formation of the products and of n-pentane deple-
tionasfunctionsofthen-pentaneconcentrationinfeedonnonequilibrated
catalyst. Reaction conditions: O2 20%; temperature, 340ꢃC; W/F 3 g s/ml.
Reactivity in n-pentane oxidation. Reported in Fig. 9 are
the effects of the reaction temperature on the n-pentane
conversion and product yields for the equilibrated cata-
lyst. The yield in maleic anhydride increases over the range
of temperatures examined, leveling at 370ꢃC; phthalic an-
hydride instead reaches a maximum at 350ꢃC and then
decreases, with a corresponding enhancement in COx for-
mation. The effect of residence time is reported in Fig. 10
at 315ꢃC. The trend of the yields is typical of parallel re-
actions: the maleic and phthalic anhydrides are formed
through independent pathways, and at this temperature the
two products are stable.
tially higher selectivity (higher than 80%), which then de-
creases as the n-butane conversion increases. This occurs
even though the operating temperatures are lower than
that of the nonequilibrated catalyst. The decrease, there-
fore, can not be attributed to the reaction conditions, but
rather may well reflect a different reaction network. A fairly
constant selectivity with conversion means the presence of
two parallel reactions (one selective to maleic anhydride,
the second one unselective) which are characterized by ap-
proximately comparable apparent activation energies. In
the case of the equilibrated sample the selectivity, extrap-
olated to zero conversion, is very high, suggesting that at
low temperature the surface sites are highly selective to the
formation of maleic anhydride and that on increasing the
temperature the formation of COx is preferred.
The rates of n-pentane depletion (expressed with respect
tothespecificsurfacearea)andproductformationasafunc-
tion of n-pentane concentration for the two samples are re-
ported in Figs. 11 and 12, while the trends of the selectivities
FIG. 10. n-Pentane conversion and yields as functions of the residence
FIG. 12. Rates of formation of the products and of n-pentane deple-
time on equilibrated catalyst. Reaction conditions: 0.46% n-pentane in air, tion as functions of the n-pentane concentration in feed on equilibrated
temperature, 315ꢃC.
catalyst. Reaction conditions as in Fig. 11.

NONEQUILIBRATED AND EQUILIBRATED V–P–O CATALYSTS
59
FIG. 15. Yields to maleic anhydride (ma) and to phthalic anhydride
(pa) from n-butane and n-pentane on nonequilibrated and equilibrated
catalysts. Reaction conditions: 1.7% hydrocarbon in air; temperature,
340ꢃC; W/F 3 g s/ml.
FIG. 13. Selectivity to the products as a function of the n-pentane
conversion on nonequilibrated catalyst. Reaction conditions as in Fig. 11.
are shown in Figs. 13 and 14. The relative order of activity
depends on the level of n-pentane concentration at which
the two catalysts are compared. The rates of product for-
mation reach a “plateau” at low hydrocarbon concentration
for the equilibrated sample (Fig. 12), indicating saturation
of the surface sites. The nonequilibrated catalyst, in con-
trast, exhibits a continuous increase in the rates of product
formation, and reaches a higher value than the equilibrated
sample at high n-pentane concentration (Fig. 11). In both
samples an increase in n-pentane concentration leads to
an increase in the phthalic to-maleic anhydride ratio; how-
ever, in the equilibrated sample this increase is much more
marked.
dicating that the same active sites are involved in the two
reactions), while in the nonequilibrated catalyst the overall
amount of anhydride from n-pentane is far higher than the
maleic anhydride from n-butane.
Comparison between Nonequilibrated and Equilibrated
Catalysts in Surface Properties
XPS characterization. The results of XPS analysis for
the three samples are summarized in Table 2. Reference
spectra for VOHPO4 ꢁ 0.5H2O (where vanadium is VIV) and
for VOPO4 ꢁ 2H2O (a VV phase), were also studied; Fig. 16
reports the spectra for the reference compounds and for
the three samples. The spectrum of the calcined sample
confirms the presence of both VIV and VV. The signal at
BE 518 eV corresponds well to that of the V 2p3/2 in the
Figure 15 compares the yield to anhydrides from the two
paraffins. In the equilibrated catalyst the amount of maleic
anhydride from n-butane is the same as the amount of an-
hydrides (phthalic plus maleic) from n-pentane (thus in-
TABLE 2
Binding Energies (eV) of Core Electrons and XPS Intensity
Ratios of the Samples (Evacuated at 20ꢃC) and of Reference Com-
pounds (Evacuated at 100ꢃC) in the V 2p Region
Sample
O 1s
P 2p
V 2p3/2
Calcined
531.3 (70)
532.6 (30)
531.3 (70)
532.6 (30)
531.3 (75)
532.5 (25)
531.2 (73)
532.6 (38)
531.2 (73)
532.1 (27)
133.6
516.9 (50)
518.2 (50)
516.9 (50)
518.2 (50)
517.0 (100)
Nonequilibrated
133.7
133.7
132.7
132.4
Equilibrated
Precursor, VOHPO4 ꢁ 0.5H2O
VOPO4 ꢁ 2H2O
516.8 (100)
518.1 (100)
FIG. 14. Selectivity to the products as a function of the n-pentane
conversion on equilibrated catalyst. Reaction conditions as in Fig. 11.
Note. Figures in brackets indicate the approximate percentage (ꢇ10%)
of each signal as detected by peak deconvolution.

60
ALBONETTI ET AL.
FIG. 17. TPD spectra of CO and CO2 on nonequilibrated catalyst
after adsorption of (a) n-butane and (b) n-pentane.
temperature for the catalysts are represented in Figs. 17
and 18.
In the case of the equilibrated sample (Fig. 18), CO and
CO2 were released mainly at high temperatures. Moreover,
the behavior from n-butane and n-pentane was very similar
in terms of the temperature at which the COx desorption oc-
curred and of the amount of released COx. Completely dif-
ferentwasthecaseforthenonequilibratedcatalyst(Fig. 17).
Thus, the overall amount of COx desorbed when the n-
butane was first adsorbed was smaller than the amount
arising from n-pentane adsorption. Moreover, in the case
of n-pentane, CO and CO2 desorbed in two temperature
FIG. 16. XPS spectra of (a) the “calcined” sample, (b) the nonequi-
librated catalyst (c) the equilibrated catalyst (d) the reference compound
VOPO4 ꢁ 2H2O, and (e) the precursor VOHPO4 ꢁ 0.5H2O.
reference VV phase. The spectrum of the nonequilibrated
catalyst shows the presence of VV at the surface, together
with VIV, while the spectrum of the equilibrated catalyst
shows only the signal relative to the VIV. It is worth noting
that the VIV on the surface of the samples cannot arise
from the evacuation treatment which the samples undergo
before the XPS spectra are recorded: VOPO4 ꢁ 2H2O does
not exhibit any VIV signal even when evacuated at 100ꢃC.
TPD characterization. When the n-pentane and n-
butane interacted with the catalyst surface at 340ꢃC sev-
eral products were detected when the temperature was
increased above room temperature under vacuum. CO,
CO2, H2O, H2, and O2 were the main fragments detected.
Fragmentswithm/zvaluesassignabletohydrocarbonswere
also seen but the intensity of these signals was much less
(around 1000 times weaker). CO and CO2 were the most
intense fragments with C atoms and they were chosen as
probes for describing the hydrocarbon interaction with the
surface. TPD signals for CO and CO2 as a function of
FIG. 18. TPD spectra of CO and CO2 on equilibrated catalyst after
adsorption of (a) n-butane and (b) n-pentane.

NONEQUILIBRATED AND EQUILIBRATED V–P–O CATALYSTS
61
ranges; the amount of desorbed COx at low temperature reported has been reported by Schneider (30), by Harrouch
was similar to that desorbed at high temperatures.
Batis et al. (13) after catalytic tests with VOPO4 ꢁ 2H2O, by
Moser (31) for V–P–O catalysts prepared by spray drying at
high temperature and by Miquel and Katz (29) for samples
prepared by flame hydrolysis. These authors attributed the
pattern to VOHxPO4 ꢁ yH2O (according to Schneider (30)
a VOH0.16PO4 ꢁ 1.9 H2O phase, with V4.84 + , formed during
reduction of the VOPO4 ꢁ 2H2O), even though the reflec-
DISCUSSION
Structural Evolution in Air of the Precursor
When Mixed with Organic Binders
˚
The active phase in n-butane and n-pentane oxidation,
(VO)2P2O7, can be formed either through the direct trans-
formation of the precursor, VOHPO4 ꢁ 0.5H2O, inside the
reactor in the reaction environment, or by first dehydrating
the precursor by calcination in air or in nitrogen, and then
aging the catalyst inside the reactor (4, 5). The heating of the
precursor in air, when the latter is prepared in an organic
medium and contains a slight excess of phosphorus, first
leads to the dehydration of the VOHPO4 ꢁ 0.5H2O (at tem-
peratures ranging from 330 to 380ꢃC), with development
of an amorphous compound, which is then transformed
into a poorly crystallized (VO)2P2O7, the active phase in
n-butane oxidation (5, 12). The complete crystallization of
the vanadyl pyrophosphate takes several hundreds of hours
to be completed in the reaction environment (20). Initially,
it was hypothesized that the transformation of the precur-
sor to the vanadyl pyrophosphate was a topotactic reaction,
thus occurring very quickly and with only a small struc-
tural rearrangement (26, 27). However, even though the
two crystalline structures have structural similarities, the
formation of the crystalline (VO)2P2O7 is not so easy, at
least when a precursor of lower crystallinity (as is the case
for those prepared in organic medium) is used.
tion at d = 3.04 A (observed in our case, and also reported
by Miquel and Katz (29)) does not match with any other
known V–P–O phase. Several VOPO4 phases possess re-
˚
flections close to this (15, 17): ꢁ -VOPO4 at d = 3.06 A, ꢂ-
˚
˚
VOPO4 at d = 3.07 A, ꢀII-VOPO4 at d = 3.06 A; however,
in all cases the other major reflections of each of these
phases are not observed in our spectrum. A possible at-
˚
˚
tribution of the reflections at d = 3.55 A and d = 3.04 A is
to the (101) and (111) planes of ꢀII-VOPO4; however, in
this case, too, the other very intense reflection typical of
this compound, at d = 4.42 A, is not observed in our spec-
˚
trum. In addition, patterns in Fig. 5 clearly indicate that
these reflections belong to a hydrated phase rather than
to a simple VOPO4; this hydrated phase can be reversibly
dehydrated, giving the compound whose pattern is shown
in Fig. 5b. Recently, Ben Abdelouahab et al. (32), reported
that all VOPO4 phases, with the exception of ꢂ-VOPO4,
can be reversibly hydrated, leading to the formation of
VOPO4 ꢁ 2H2O. To sum up, we prefer to assign the observed
pattern to VOH0.16PO4 ꢁ 1.9H2O.
The spectrum in Fig. 5b shows intense reflections at
2ꢃ = 18.8ꢃ, 21.6ꢃ, 22.9ꢃ, 27.9ꢃ, 30.0ꢃ, and 34.1ꢃ, plus some
less intense peaks at higher values of 2ꢃ. Some of these re-
flections might correspond to the most intense ones of the
ꢄ-VOPO4 (15, 17) (which shows peaks at 2ꢃ = 19.6ꢃ, 22.1ꢃ,
24.2ꢃ, 28.6ꢃ, 30.3ꢃ, 34.8ꢃ), even though all corresponding
peaks in Fig. 5b are shifted with respect to these values.
Other reflections correspond well to those of the vanadyl
pyrophosphate, also present in Fig. 5a.
The transformation of the precursor when it is calcined
in air is shown in Fig. 3 (for the powder mixed with stearic
acid) and in Fig. 4 (for the pure powder of precursor).
The changes in the patterns shown in Fig. 3 indicate that
the destruction of the VOHPO4 ꢁ 0.5H2O generates a prod-
uct which is in part amorphous, and in part constituted of
poorly crystallized vanadyl pyrophosphate. The significant
enlargement of the reflection at approximately 2ꢃ = 23ꢃ
can be assigned either to the contemporaneous formation
of a ꢄ- or a ꢁ -VOPO4 phase (which are characterized by
intense reflections in this position), as indicated by other
authors (33), or by assuming particular structural or mor-
phological features of the vanadyl pyrophosphate along
the corresponding (200) crystallographic plane. This latter
hypothesis seems to us more likely, due to the following
considerations: (i) the other intense reflections of ꢁ - and
ꢄ-VOPO4 are absent in the spectrum; (ii) it is known that
The assignment of all reflections shown in Fig. 1a (whose
pattern is the same as the one displayed in Fig. 5a and is also
observed in Fig. 3) is rather difficult. Three main reflections
˚
are observed: the most intense is at d = 7.12 A, and two less
˚
intense ones appear at d = 3.55 and 3.04 A; this compound
is obtained either by calcination of the pellets at 380ꢃC
(Fig. 1a), or by calcination of the pure powder at 450ꢃC
at least (Fig. 5a), or is formed together with the vanadyl
pyrophosphate after prolonged calcination of the powder
mixed with the stearic acid at 380ꢃC (Fig. 3). In addition,
patterns in Fig. 5 indicate that this phase is hydrated and
can be reversibly dehydrated yielding the sample in Fig. 5b;
this is also confirmed by thermogravimetric tests, which in-
dicate a considerable weight loss for the calcined sample.
Tetragonal VOPO4 ꢁ 2H2O is characterized by a diffraction
˚
peak at d = 7.45 A, attributed to the (001) reflection (17,
˚
28), which is shifted toward lower d-spacings (7.3 A) by
progressive removal of water molecules (29); this peak is
reported by some authors as the most intense peak (29), by
others as a weak peak (17). Other reflections typical of this
˚
compound are found at d = 4.75, 3.10, and 1.55 A. There-
fore, the pattern shown in Figs. 1a, 3, and 5a cannot be as-
signed to VOPO4 ꢁ 2H2O. A pattern similar to that one here

62
ALBONETTI ET AL.
many factors, such as the nature of the precursor and the shows the reflections relative to a poorly crystallized
procedure employed for the dehydration, can remarkably (VO)2P2O7 and of a VV–P–O which we can, with some
affect the morphology and the crystallinity of the corre- uncertainty, indicate as a ꢁ -VOPO4. The calcined sample,
sponding vanadyl pyrophosphate (i.e., in the absence of VV instead, shows reflections relative only to the hydrated,
in the compound) and that these aspects mainly condition oxidized phase, which we have suggested corresponds to
the shape of the reflection relative to the (200) crystallo- VOH0.16PO4 ꢁ 1.9H2O (29, 30).
graphic plane. This reflection can be either very intense
and narrow, or very broad (12, 34–36), depending, for in-
stance, on the development of the basal (100) face. Re-
cently, Thompson et al. (37, 38) have been confuting the
importance attributed to broadening of the (200) reflec-
tion; the authors reported that this effect does not depend
on vanadium disorder, as initially stated (12), but rather
might be due to more subtle variations in the position
of oxygen atoms; indeed, it was reported that this reflec-
tion is invariant with respect to “linear disorder” effects
in the structure, such as columnar disorder of the metal
atoms.
Finally, Fig. 3 shows that after 5 h of calcination, also the
oxidized hydrated phase discussed above is formed in the
sample mixed with stearic acid; the same does not occur
with the pure, unmixed precursor (Fig. 4). Therefore, the
data indicate that the presence of large amounts of organic
compounds can remarkably affect the structural evolution
of the precursor in air; it is likely that the burning of these
compounds (such as that of the binder necessary to allow
tableting of the powder for fixed-bed applications) causes
a local overheating and leads to the formation of VV–P–O
phases whose formation would otherwise need much higher
temperatures (at least 450ꢃC, as shown in Fig. 5); in addi-
tion, all the structural transformations (destruction of the
precursor and production of the intermediate amorphous
phase to the vanadyl pyrophosphate) are accelerated.
During aging, the following transformations occur:
(1) Dehydration of the VOH0.16PO4 ꢁ 1.9H2O com-
pound, with formation of ꢁ -VOPO4: this is in agreement
with indications given by other authors (32).
(2) Reduction of VV to VIV: this is a slow reaction, as indi-
cated by the fact that after 100 h time-on-stream the average
oxidation degree is only slightly changed, from 4.40 to 4.36.
It has to be considered that under our experimental con-
ditions (1–2% hydrocarbon in air, thus with a large excess
of oxygen with respect to the stoichiometric requirement)
the gas phase probably possesses a rather weak reducing
power.
(3) Crystallization of amorphous VIV/P oxide into
(VO)2P2O7: the comparison of FT-IR spectra and XRD
patterns of the calcined and nonequilibrated samples show
clearevidenceofthisevolution;however, thecrystallinityof
the vanadyl pyrophosphate is still poor, and some amount
of an amorphous phase is present. This transformation oc-
curs without any significant variation in the surface area,
which remains approximately 11 m2/g.
The complete reduction of the vanadium and crystalliza-
tion of the vanadyl pyrophosphate need much longer times;
the equilibrated catalyst only possesses VIV, as indicated by
both XPS and chemical analysis (even though the presence
of small amounts of VV on the catalyst surface cannot be ex-
cluded), only exhibits the XRD reflections and IR spectrum
typical of crystalline (VO)2P2O7, and possesses a surface
area of 23 m2/g.
Transformation of the Calcined Compound
in the Reaction Environment
Correlation between Catalytic Activity and
Bulk/Surface Features
When the fresh catalyst is loaded in the reactor (after pre-
cursor dehydration) and left in the reaction environment,
thepoorlycrystallizedcompoundisslowlytransformedinto
crystalline (VO)2P2O7. The structural evolutions occurring
during these transformations lead to variations in the cat-
alytic performance, as also described by Sola et al. (22), by
Kubias et al. (21), and by Zhang-Lin et al. (14, 23). It has
been claimed that reaching a steady catalytic performance
and a stable catalytic composition and structure (equili-
The structural and surface differences between the equi-
librated and the nonequilibrated catalysts also correspond
to differences in catalytic behavior in both n-butane and
n-pentane oxidation. The correlation between the catalytic
performance and the chemical-physical features indicates
the following:
(1) The equilibrated catalyst is more active in the oxida-
brated catalyst) can take from a few hundred hours to more tion of n-butane. This higher activity can be attributed to its
than 1000 h (20). The time necessary to complete this trans- higher specific surface area, even though the areal activity
formation is a function of the reaction conditions employed (that referred to the surface area) is higher than that of the
(temperature, pressure, hydrocarbon-to-oxygen ratio) and nonequilibrated catalyst. It possesses surface sites that are
of the physico-chemical features of the fresh catalyst.
specific in the oxidation of the n-butane to maleic anhydride
The results reported here indicate that when the start- (at low temperature the selectivity is very high), so yielding
ing compound is highly oxidized, a long period of aging the desired product with a higher selectivity.
is needed for catalyst equilibration. The XRD pattern of
Different is the case for the n-pentane oxidation. The
the nonequilibrated sample (after 100 h time-on-stream) two catalysts possess a comparable activity at low hydro-

NONEQUILIBRATED AND EQUILIBRATED V–P–O CATALYSTS
63
carbon concentration; however, the equilibrated catalyst
The TPD results are in agreement with the catalytic re-
does not increase further the rate of product formation sults. The surface of the equilibrated catalyst is homoge-
when the hydrocarbon concentration is increased above neous: there is only one type of site able to interact with
0.5 mol% (Fig. 12), while the nonequilibrated one shows both hydrocarbons. In the case of nonequilibrated cata-
a continuous increase in the rate of n-pentane conversion lyst, instead, the amount of n-butane activated and des-
(Fig. 11). This indicates that the equilibrated catalyst pos- orbed in the form of COx is very low, much lower than the
sesses a lower number of surface active sites per unit surface amount formed from n-pentane interaction. Moreover, the
area. However, these sites are specific for the conversion of nonequilibrated surface is heterogeneous from the point
n-pentane to phthalic anhydride (Fig. 14); in fact, this cat- of view of n-pentane interaction since the hydrocarbon is
alyst yields the highest phthalic-to-maleic anhydride ratio released as COx in two distinct temperature ranges.
under all conditions.
This heterogeneity of surface centers corresponds to
Significantly, the productivity to maleic anhydride from a structural heterogeneity; the nonequilibrated sample is
n-butane is the same as that to anhydrides (maleic plus ph- characterized by the presence of two crystalline phases at
thalic) from n-pentane (see Fig. 15). These results suggest least, namely poorly crystallized (VO)2P2O7 and VOPO4
the existence of a single type of active center on the cat- (likely to be ꢁ -VOPO4), while the equilibrated catalyst only
alytic surface, which is able to activate both hydrocarbons possesses well crystallized (VO)2P2O7.
and therefore point to a homogeneous surface. The two an-
A further indication arising from the data reported is
hydrides form through two parallel pathways (Fig. 10); this that the conditions necessary to obtain a good catalyst
indicates that the conversion of activated n-pentane either for n-butane and n-pentane oxidation are different; the
to the maleic or to the phthalic anhydride is not a func- (VO)2P2O7 is the most active and selective phase in the
tion of the nature of the active site, but is likely to be due n-butane oxidation, while the nonequilibrated catalyst is
to other factors, such as the way the paraffin has been ad- active and selective in the oxidation of the n-pentane. How-
sorbed and activated and the probability of finding other ever, as the (VO)2P2O7 is more specific to the oxidation of
activated molecules in the surrounding area.
n-pentanetothephthalicanhydride, itislikelythatthepres-
(2) The nonequilibrated catalyst is less active in n-butane ence of the crystalline vanadyl pyrophosphate guarantees
oxidation and less specific in the formation of maleic anhy- the geometric constraints necessary to direct the activated
dride. It has surface centers which are not saturated at high n-pentane along the pathway leading to phthalic anhydride.
n-pentane concentration (Fig. 11), indicating the presence
of higher number of surface sites which are able to activate
the n-pentane. These sites are less specific to phthalic an-
hydride (the phthalic-to-maleic ratio is lower than that of
the equilibrated sample) (Fig. 13), even though the overall
selectivity to the anhydrides is higher than in equilibrated
catalyst (80% against 70%). With n-pentane the yield in an-
hydrides is higher than the yield in maleic anhydride from
n-butane (see Fig. 15). The phthalic-to-maleic ratio also in-
creases with increasing n-pentane concentration, but the
formation of maleic anhydride is always preferred to that
of phthalic anhydride.
The different behavior in n-butane and n-pentane oxi-
dation indicates that the surface of this catalyst discrimi-
nates between the interaction with the two hydrocarbons;
in other words, the surface possesses at least two kinds of
sites (or ensembles of active sites): these are (i) sites that
convert n-butane to maleic anhydride and which are also
active in the transformation of n-pentane to both anhy-
drides, more preferentially to maleic anhydride, and (ii)
other sites which are inactive in n-butane oxidation but are
able to oxidize n-pentane to the anhydrides, with higher se-
lectivity to the maleic anhydride. This justifies the observed
behavior: the slightly lower specific activity of the nonequi-
librated catalyst in n-butane oxidation, and the higher activ-
ity in n-pentane oxidation, especially at high hydrocarbon
concentration.
ACKNOWLEDGMENT
This work was carried out within the framework of the Human Capital
Mobility Fund (EEC), contract CHRX-CT92–0065.
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