Selective oxidation of n-butane on a V-P-O catalyst: Study under fuel-rich conditions

Article abstract of DOI:10.1006/jcat.2000.2902

A vanadium pyrophosphate oxide (V-P-O) catalyst was tested for n-butane oxidation under fuel-rich conditions (O₂/C₄H₁₀ = 0.6:1). The solid, although selective toward maleic anhydride (MA) under standard feed conditions, (O₂/C₄H₁₀ = 12:1) was not adapted to work under the fuel-rich feed conditions, which prevail in the first layer of a membrane reactor. A rapid decrease in MA production was observed with time, which was associated with a rapid reduction of the catalyst surface based on XPS analysis. From the evolution of the distribution of products, it has been inferred that CO formation mainly occurred by the oxidation of adsorbed butane whereas CO₂ was formed by the overoxidation of MA. The n-butane activation occurred on V⁴⁺ sites while the insertion of oxygen in the C₄-intemediates leading to MA took place on V⁵⁺ sites. The deactivation in fuel-rich conditions was not so drastic when the V-P-O catalysts was previously oxidized at 500°C. In this case, oxygen was stored in the V-P-O surface region, hence, favoring the specific oxygen-inserting role of V⁵⁺.

Full text of DOI:10.1006/jcat.2000.2902

Journal of Catalysis 193, 308–318 (2000)
doi:10.1006/jcat.2000.2902, available online at http://www.idealibrary.com on
Selective Oxidation of n-Butane on a V–P–O Catalyst:
Study under Fuel-Rich Conditions
S. Mota, M. Abon, J. C. Volta, and J. A. Dalmon
Institut de Recherches sur la Catalyse, 2 Avenue Albert Einstein, 69626 Villeurbanne C e´ dex, France
Received January 12, 2000; revised April 24, 2000; accepted April 24, 2000
5+
vanadyl orthophosphate (V ) formed on the (100) face of
A vanadium pyrophosphate oxide (V–P–O) catalyst has (VO)2P2O7 (V4 ) crystallites under catalytic reaction con-
+
been tested for n-butane oxidation under fuel-rich conditions ditions (9, 10). It has been also considered that the defects
(
O2/C4H10 = 0.6). A rapid decrease in maleic anhydride produc-
in the (100) face play an important role in the reaction (9).
From an industrial point of view, a distinction is made
between a nonequilibrated (short-term catalytic test) and
equilibrated (long-term catalytic test) catalyst, and it has
been shown that, when working under fuel-lean conditions
(C4H10/air = 1–3% ) the catalytic conditions (C4H10/air mix-
tion was observed with time. XPS analysis showed that this was
associated with a rapid reduction of the catalyst surface. From the
evolution of the distribution of products, it has been inferred that
CO formation mainly occurs by the oxidation of adsorbed butane
whereas CO2 is formed by the overoxidation of maleic anhydride. It
4
+
has been concluded that the n-butane activation occurs on V sites
5+
4+
ture, temperature, time) tend to change the V /V ra-
while the insertion of oxygen in the C4-intermediates leading to MA
tio in such a way that the oxidation rate of V4 to V
+ 5+
takes place on V5 sites. The deactivation in fuel-rich conditions is
+
not so drastic when the V–P–O catalyst has been previously oxidized will be decreasing for the equilibrated catalyst (11). It has
�
at a relatively low temperature (500 C). In this case oxygen is stored been pointed out that this specific point requires further
in the V–P–O surface region, hence favoring the specific O-inserting investigation since the catalytic performances are directly
role of V5 responsible for MA formation.
+
�c 2000 Academic Press
connected to the changes occurring in the V–P–O catalyst
Key Words: n-butane partial oxidation; maleic anhydride; fuel- during activation and aging (11). The surface of the V–P–O
rich feed conditions.
catalyst is progressively reduced with time-on-stream as ob-
served by XPS (12). When increased C4/air ratios are used,
it is possible to detect the formation of olefinic intermedi-
ates (butenes, butadiene) from n-butane. In this case, V3
+
1
. INTRODUCTION
species are detected in the catalysts (13).
Maleic anhydride has been produced for a long time by
In industry, different types of reactors have been con-
the partial oxidation of benzene. The catalyst used has been sidered for this catalytic reaction (14), fixed-bed (15, 16),
based on V2O5–MoO3 mixed oxide deposited on a support. fluidized-bed (17), and recirculating solid (18, 19) reactors.
The conversion is almost total (97–99% ), and the selectiv- The fixed-bed reactor is a multiple tube heat exchanger re-
ity is near 75% with CO and CO2 by-products. During the actor. Under industrial conditions with 2% n-butane in the
1
980s, n-butane partial oxidation was developed for eco- feed, conversions range from 70 to 85% with a molar se-
nomical (low price and availability of butane) and environ- lectivity to MA of around 67–75% (1). In a fluidized-bed
mental (toxicity of benzene) reasons (1, 2).
reactor, the reaction gases flow upward through a bed of
n-Butane catalyticoxidation isstillthe onlyexample ofan fine catalyst particles. This system offers the advantage of
industrially performed oxidation reaction of a light alkane. operating at higher butane concentrations (up to 4% ), thus
V–P–O catalysts are known to be efficient for the produc- lower operating costs. The recirculating solid reactor de-
tion of maleic anhydride (termed MA) from n-butane veloped by DuPont de Nemours (17–19) allows the rapid
(
1– 4). A single (VO)2P2O7 phase has been inferred to be recycling of the V–P–O solid thanks to a high gas velocity.
present in the long-term activated catalyst (5–7). The cata- In this reactor, the oxidation of n-butane over the oxidized
lytic properties of the vanadyl pyrophosphate catalyst de- V–P–O catalyst and the regeneration (oxidation) of the re-
pends on the nanostructure and the morphology of the duced V–P–O catalyst are performed in separate zones. This
5+ 4+
(
VO)2P2O7 crystals (8). The V /V dimeric species in configuration results in an increase of the selectivity to MA
the topmost oxidized layer of vanadyl pyrophosphate have (up to 90% ) (19), because the oxidation state of the catalyst
been proposed as the active sites (3). Other authors suggest is optimized. Since the reduction stage occurs separately, a
that the active sites lie within microdomains of crystalline more concentrated n-butane stream can be used.
308
0021-9517/00 $35.00
Copyright �c 2000 by Academic Press
All rights of reproduction in any form reserved.

n-BUTANE ON V–P–O CATALYST UNDER FUEL-RICH CONDITIONS
309
However, these different types of reactors present some of the O2/C4H10 ratio, from fuel-rich conditions at the inlet
disadvantages. In the fixed-bed, due to the exothermicity of of the reactor to oxygen-rich at the outlet. Therefore, the
the reaction, the extent of the reaction must be limited in implementation of this membrane reactor requires a cata-
order to avoid the formation of hot spots and the possibility lyst able to work under conditions where reactants are fed
of explosive mixtures. For the fluidized-bed reactor and the with different ratios.
recirculating fluidized-bed reactor, the major constraint is
The present work has been performed in a conventional
the attrition resistance of the catalyst. Furthermore in the fixed-bed reactor. The performance of a standard V–P–O
latter, large amountsofcatalyst have to be recirculated from catalyst has been investigated as a function of the composi-
the reactor to the regenerator. In conclusion, whatever the tion of the reactant gas mixture (O2/C4H10 ratio). In order
type of reactor, the maleic anhydride yield in industrial pro- to simulate the reducing conditions prevailing in the begin-
cesses is below 60% .
ning of the catalytic bed of the membrane tubular reactor, a
In order to improve the maleic anhydride yield, it has fuel-rich mixture was fed. A specific oxygen-pretreatment
been proposed that researchers work with a high n-butane of the V–P–O catalyst has been considered in order to adapt
concentration and a low O2/C4H10 ratio (fuel-rich condi- the solid to this reducing atmosphere. Indeed it was previ-
tions) to limit total oxidation (13, 20, 21). By comparing the ously observed that such a pretreatment improved the se-
effect of butane concentration on maleic anhydride selec- lectivity to maleic anhydride under conventional fuel-lean
tivity, Centi et al. (20) showed that decreasing the oxygen conditions (25, 26).
concentration from 20 to 5% vol at 1.5% vol of n-butane led
to a 5% increase in the maximum yield of maleic anhydride
2. EXPERIMENTAL
�
(
from 51 to 56% ) in the reaction range from 300 to 340 C.
In another study, also performed under fuel-rich conditions, 2.1. Catalyst Preparation
Hutchings showed that it is possible to achieve a higher MA
The precursor, VO(HPO4),0.5 H2O was prepared ac-
cording to the method given by Johnson et al. (28). V2O5
11.8 g) was added to isobutanol (250 ml) and refluxed with
selectivity (80 mol% ) at 9% butane conversion (21) with a
feed composition of 20.3 mol% butane, 18.9 mol% oxygen
(
�
� 1
at 360 C and a GHSV of 1000 h . Furthermore, even with a
low n-butane conversion per pass, the exit gas contained sig-
nificantly higher MA concentrations (1.5–2.0 mol% ) than
under conventional fuel-lean operating conditions. In this
way, substantial yield can be recovered by condensation
rather than by using the conventional water scrubbing re-
covery technique. However, operating under fuel-rich con-
ditions necessitates a recycling process (21).
H3PO4 (85% ) for 16 h. The blue suspension was then sep-
arated from the organic solution by filtration and washed
with isobutanol and ethanol. The resulting solid was re-
�
fluxed in water, filtered hot and dried in air at 110 C for 16 h.
Pellets from the fine powder of precursor was made using
3
.4% graphite, giving cylindrical particles of 6 mm diameter
and ca. 8 mm length.
In order to obtain the (VO)2P2O7 catalyst, hereafter de-
We are presently developing a new type of reactor based
on a ceramic porous tubular membrane. The V–P–O cata-
lyst will be placed as a fixed bed in the inner volume of the
tube. Butane will be directly fed at the inlet of the tube,
and oxygen will be gradually distributed from the shell side
along the tube. The membrane allows control of the oxy-
gen supply to the catalyst bed, and in this case a low, con-
stant partial pressure of O2 can be thus maintained on the
catalyst. It is anticipated that this feature will improve the
maleic anhydride selectivity by limiting the total oxidation
reaction. This membrane reactor concept has already been
proposed for various selective oxidation processes (oxida-
tive coupling of methane, oxidation of ethylene to acetalde-
�
noted PYRO, the precursor was calcined under N2 at 550 C
for 72 h.
2
.2. Oxidizing Pretreatment
The PYRO catalyst (ca. 0.5 g) was heated to 500 C
�
�
(
5 C/min) under a flow of oxygen (20 ml/min) and then
kept at this temperature for 2 h. This procedure follows
that proposed by A ¨ı t Lachgar et al. (25). The resulting solid
was denoted PYRO.OX.
2
.3. Experimental Setup
The catalytic experiments were carried out in a fixed-bed
hyde and of propene to acrolein) (22). Moreover, the ability reactor at atmospheric pressure. The reactor, a classical
to distribute the oxygen feed through a membrane may al- tubular Pyrex microreactor, was equipped with a thermo-
low a better management of the heat produced and also the couple inside the catalytic bed for continuous temperature
use of global oxygen/hydrocarbon ratios corresponding to control. Five hundred milligrams of catalyst was commonly
explosive mixtures in conventional reactors (23). Due to used.
its geometry (the oxygen partial pressure is kept low but
Sampling valves were inserted in a hot box heated at
�
constant along the fixed bed, when the n-butane concentra- 150 C to avoid condensation of maleic anhydride. The flow
tion is decreasing progressively through conversion), the of gas reactants was regulated by mass flow controllers. The
membrane reactor will operate with a continuous change feed consisted ofa mixture ofn-butane, oxygen, and helium.

3
10
MOTA ET AL.
�
The reaction was carried out at 400 C with a total flow rate
TABLE 1
3
� 1
� 1
.
of 25 cm min and at a constant GHSV of 3000 h
Two different feed compositions were chosen:
Catalytic Performance of PYRO and PYRO.OX Catalysts
under Standard Feed Conditions
—
Standard oxidizing conditions: O2/C4 = 12 correspond-
Catalyst
C(C4H10)
C(O2)
S(MA)
S(COx)
Y(MA)
ing to C4/O2/He = 0.8 : 10 : 92 to mimic the situation at the
PYRO
PYRO.OX
20
12
6
1
76
91.4
22
8.6
15.2
10.9
outlet of the catalytic bed in the membrane reactor.
—
reducing conditions: O2/C4 = 0.6 corresponding to
C4/O2/He = 16.6 : 10 : 73.4 to mimic the situation at the inlet
� 1
�
Note. O2/C4 = 12; GHSV = 3000 h ; T = 400 C. C(C4H10) and C(O2):
of the catalytic bed in the membrane reactor. This low value
n-butane and oxygen conversions. S(MA) and S(COx): selectivity for MA
has been roughly estimated as that existing in the first tenth and COx. Y(MA): MA yield.
of a tubular zeolite membrane acting as oxygen distributor
in the catalyst bed. It has been calculated on the basis of PYRO.OX was more selective (S(MA) = 91.4% instead of
the O2 permeance of the zeolite membrane (24) and a ratio 76% ) but with a lower conversion (12% instead of 20% ).
(
total distributed) of O2/C4 of 12. Oxygen feed has been With increasing MA selectivity, the formation of COx de-
kept constant.
creased, particularly for CO . Note that these catalytic re-
2
sults are stable with reaction time, at least for 24 h (duration
of the run).
Reactants and reaction products were analyzed by
on-line gas chromatography using double FID detection
(
HP5890A series II) with HP-PLOT/Al2O3 (C4H10) and
3
.2. Catalytic Results under Reducing Feed Conditions
HP-INNOWAX (MA) columns followed by a TCD detec-
tor (Intersmat GC IGC 120 MB) with a CTR1 column (O2,
In order to be near the feed conditionsfor the first catalyst
CO, CO2). A computer was used with HP Chemstation layer of the membrane reactor, both catalysts, PYRO and
software.
PYRO.OX, were tested with an excess of butane (O /C =
2
4
0
.6), i.e., under fuel-rich conditions.
Figures 1a and 1b show the changes in the catalytic
2
.4. Catalyst Characterization
The specific surface area of the samples was measured by performance of the PYRO catalyst as a function of time.
the BET method with N2 adsorption at 77 K automatically
controlled by volumetry with a homemade apparatus.
X-ray diffraction (XRD) patterns were collected with a
Siemens D500 diffractometer using Cu K� radiation.
The 31P NMR spectra were recorded with a Bruker
DSX400 spectrometer at 161.9 MHz, equipped with a stan-
dard 4-mm probe head. The 31P Spin Echo Mapping (SEM)
spectra were obtained with a sweep width of 2 MHz,
�
31
t = 20 �s, and a 90 pulse length of 1.5 �s. The P MAS
spectra were recorded at 12 kHz spinning speed with a P/2
acquisition sequence, a pulse length of 1.5 �s, and a recycle
delay of 60 s.
XPS analysis was performed in a VG Escalab 200R ma-
chine using Mg K� radiation. The electrical charge was cor-
rected by setting the binding energy (BE) of adventitious
carbon (C1s) at 284.5 eV. For quantitative analysis, the in-
tegrated area under V2p3/2, O1s, P2p, and C1s peaks after
smoothing and subtraction of a nonlinear Shirley back-
ground was used. Analysis of the V2p3/2 level allowed us
to measure the changes in the surface oxidation state of
vanadium (V5 , V , and V ) from a peak decomposition
+
4+
3+
and curve fitting technique, as already described (12).
3
. RESULTS
3
.1. Catalytic Results under Standard Feed
Conditions (O2/C4 = 12)
Table 1 compares the catalytic performance of the
FIG. 1. Catalytic performance of PYRO catalyst under reducing feed
conditions (O2/C4 = 0.6), T = 400 C: (a) conversions, C(C4H10) and C(O2);
�
PYRO catalyst before and after an oxidizing pretreatment. (b) selectivities, S(MA), S(unsatC4), and S(COx).

n-BUTANE ON V–P–O CATALYST UNDER FUEL-RICH CONDITIONS
311
(
24.2% ); cis-2- and trans-2-butenes were produced in sim-
ilar amounts (respectively, 7.1 and 10.5% ), and selectivity
toward butadiene was low (2.5% ). The steady state was
reached after 7 h on stream: the PYRO catalyst was equi-
librated and its performances remained stable: C(C4H10) =
3
8.7% , C(O2) = 100% , S(MA) = 0% , S(unsatC4) = 43.4% ,
and S(COx) = 40.4% . All the oxygen present in the gaseous
phase is consumed and therefore the surface of the catalyst
is most likely reduced. This should be the reason for the for-
mation on unsaturated C4 and the absence of maleic anhy-
dride (12). In Fig. 3b, the different selectivities for butenes
have been normalized with respect to the selectivity for cis-
2
-butene. This figure will be discussed later.
Figures 4a and 4b show the variation of the PYRO.OX
catalytic performances under the same feed conditions as
before. From these results, we observed that the PYRO.OX
behaved similarly to the PYRO catalyst when compared
with Fig. 1, but over a different time scale: the steady state
for PYRO.OX catalyst was reached only after 25 h. The
C4H10 and O2 conversions increased more slowly, and the
catalyst remained selective to mild oxidation for a longer
time (almost 10 h): S(MA) goes first to a maximum (64.2%
at t = 5 h) and then decreases to zero after 15 h of run.
At steady state, the catalytic performances were close to
those ofthe PYRO catalyst:C(C4) = 38.5% , C(O2) = 100% ,
FIG. 2. CO and CO2 selectivities for PYRO (a) and PYRO.OX
�
(
b) catalysts under reducing feed conditions (O2/C4 = 0.6), T = 400 C.
Butane and oxygen conversions (Fig. 1a) first increased
rapidly to reach a steady state, C(C4H10) from 2.8 to 38.7%
and C(O2) from 38.4 to 100% . MA selectivity (Fig. 1b),
reached a maximum S(MA) = 49.4% after 1 h on stream;
then S(MA) dropped to zero after 2 h on stream. Con-
versely, the selectivity for unsaturated C4 (1-butene, cis-2-
butene, trans-2-butene), low at the beginning of the reac-
tion (S(unsatC4) = 17.5% at t = 0 and 7% at t = 1 h), sharply
increased later on (S(unsatC4) = 51.2% at t = 2 h).
Figure 2a describes the evolution of CO and CO2 as a
function of time for the PYRO catalyst. CO and CO2 selec-
tivities were similar (respectively, 21.6 and 19.7% ) during
the first hour on stream, then S(CO2) decreased to 4.3% ,
while S(CO) increased up to 36.1% at steady state (18 h).
Consideration of the carbon balance, which is 83.8% at
steady state, shows that there is a deposit of carbonaceous
polymeric species at the surface of the catalyst. This has
been confirmed by the formation of COx when the used
catalyst (at the end of the test) was treated under an oxy-
�
gen flow at 400 C. The evolution of the different selec-
tivities for unsatC4 are shown in Fig. 3a. At the begin-
ning of the reaction, the total unsatC4 selectivity is low
(
Fig. 1b, 17.5% at t = 0 and 6% at t = 1 h) and no differ-
ences between their distribution could be noticed. How-
ever, after 18 h on stream, there was a significantly dif-
FIG. 3. Unsaturated-C4 selectivities (a) and normalized butene selec-
tivities (b) (as refered to cis-2-C4H8) for PYRO catalyst under reducing
�
ferent distribution: 1-butene selectivity increased strongly feed conditions (O2/C4 = 0.6), T = 400 C.

3
12
MOTA ET AL.
3
.3. Characterization of the Catalysts
.3.1. XRD. Figure 6 compares the XRD spectra of
3
the PYRO catalyst (a), the PYRO.OX before (b) and the
PYRO after (c) test under fuel-rich conditions. The pattern
of PYRO (Fig. 6a) is characteristic of the (VO)2P2O7 phase
with the characteristic (200), (024), and (032) reflections.
�
The reflection at 26.5 (2�) corresponds to graphite, which
has been added for pelletizing the catalyst.
The spectrum of PYRO.OX (Fig. 6b) is also typical of a
poorly crystallized (VO)2P2O7 phase, which displays a more
disorganized structure as compared to PYRO (Fig. 6a). This
is obvious from a decrease of the intensity of the diffrac-
tion peaks and an increase of the background. It can also be
stressed that there are no characteristic lines of any VOPO4
5+
(
V
) phase which could have been formed during the
PYRO oxidation. These observations are in agreement with
previous results obtained when oxidizing a PYRO catalyst
�
prepared at a higher temperature (750 C) (25) and there-
fore likely to be less prone to oxidation following O2 pre-
�
treatment at 500 C. The XRD spectrum of PYRO.OX is
almost not changed by the test under fuel-rich conditions
(
compare Figs. 6b and 6c). This catalyst remains, then, as
poorly crystallized as before the test.
FIG. 4. Catalytic performance of PYRO.OX catalyst under reducing
�
feed conditions (O2/C4 = 0.6), T = 400 C: (a) conversions, C(C4H10) and
C(O2); (b) selectivities, S(MA), S(unsatC4), and S(COx).
S(MA) = 0% , S(unsatC4) = 43.6% , and S(COx) = 34.9% .
Concerning COx (Fig. 2b), it appears that the selectivities
for CO and CO2 exhibit the same trends for the first 12 h
on stream. This goes in parallel with the production of MA
while the increase of CO after 12 h when CO2 decreases
may be associated with the production of unsaturated C4 as
previously observed for the PYRO catalyst.
Figure 5a shows the evolution of the selectivities for un-
saturated C4 with time-on-stream. Up to 15 h, butenes were
produced in same amount. At 37 h, 1-butenes reached
2
3.3% , cis-2- and trans-2 butenes reached 7.4 and 10.6% ,
respectively, and, finally, butadiene reached 2.3% , values
similar to those of the PYRO catalyst. However the kinetics
necessary to reach the steady state are different. In Fig. 5b,
the different selectivities for butenes have been normalized
with respect to the selectivity for cis-2-butene. This figure
will be discussed later.
The production of maleic anhydride does occur for a
longer period under fuel-rich conditions in the case of
PYRO.OX (15 h) as compared to PYRO (2 h). This obser-
vation suggests that the oxidizing pretreatment has stored
oxygen species active in selective oxidation within the
V–P–O surface region. This conclusion has been supported
by the following physico-chemical investigations.
FIG. 5. Unsaturated-C4 selectivities (a) and normalized butene selec-
tivities(b) (asrefered to cis-2-C4H8) for PYRO.OX catalyst under reducing
feed conditions (O2/C4 = 0.6), T = 400 C.
�

n-BUTANE ON V–P–O CATALYST UNDER FUEL-RICH CONDITIONS
313
The PYRO.OX spectrum (Fig. 7b) shows a displace-
ment of the peak characteristic of (VO)2P2O7 from 2472 to
2
500 ppm, indicative of a better organization of the ma-
terial due to the O2 pretreatement. The intensity of the
corresponding peak decreases. An important peak appears
around 0 ppm, which corresponds to a significant percent-
age of P bonded to V5 centers, which illustrates the fact
+
that the PYRO catalyst has been effectively oxidized. The
31
P NMR MAS spectrum of the corresponding catalyst
(
Fig. 7e) shows a band at � 13.4 ppm with a broad spin-
ning side band pattern. In a previous publication (26), this
signal has been attributed to species characteristic of iso-
lated V5 microdomains located at the surface of the (100)
+
faces of (VO)2P2O7 crystallites.
The spectra of PYRO and PYRO.OX after the catalytic
test under fuel-rich conditions are both very similar. They
4+
are typical of P atoms surrounded by V in a poorly crys-
5
+
4+
tallized (VO)2P2O7 phase. All V coming from the V –
V
5+
5+
dimers (for PYRO) and from isolated V species (for
PYRO.OX) have been consumed during the catalytic test.
Note also the absence of any signal around 4600–4700 ppm,
FIG. 6. XRD spectra. Main characteristic (VO)2P2O7 lines are in-
�
dexed. The line at 26.5 2� corresponds to graphite added for pelletizing:
(
a) PYRO catalyst; (b) PYRO.OX catalyst; (c) PYRO.OX catalyst after
test under fuel-rich conditions.
3
.3.2. 31P NMR. Figure 7comparesthe Spin Echo Map-
ping spectra of PYRO (Fig. 7a) and PYRO.OX (Fig. 7b)
catalysts. The spectra obtained with the used catalysts after
the test under reducing conditions are shown in Figs. 7c and
7
d for the PYRO and the PYRO.OX catalysts, respectively.
The PYRO spectrum (Fig. 7a) shows a main peak at
472 ppm characteristic of (VO)2P2O7. This peak position is
2
typical of P atoms in a poorly crystallized (VO)2P2O7 phase
12, 25, 26). Recall that a more crystallized (VO)2P2O7 ex-
(
hibits a peak around 2600 ppm (25, 29). The lack of any
signal around 0 ppm indicates that the total V5 concentra-
tion is very low and impossible to detect by this technique,
which is bulk sensitive. Note, however, a small contribution
in the 1000-ppm range which has been previously attributed
+
FIG. 7. 31P NMR spectra: (a) spin echo mapping (SEM) of PYRO;
b) SEM of PYRO.OX; (c) SEM of PYRO; (d) SEM of PYRO.OX after
to V4 –V pairs (12) since this contribution is never ob-
+
5+
(
served on pure (VO)2P2O7 samples (26).
reducing feed conditions; (e) magic angle spinning of PYRO.OX.

3
14
MOTA ET AL.
indicative of the absence of V3 species (25), which should
+
TABLE 2b
have been developed by the test under fuel-rich reducing
Estimation of the Relative Percentages of Surface
5+
conditions. Then the reduction only affects the V sites
4+
5+
and V by XPS
V
and stops at V4 oxidation.
+
%
3
.3.3. XPS. Table 2a compares the superficial concen-
Catalyst
V4
+
V
5+
V
4+
5+
/V
trations of V, P, O, and C measured on both PYRO and
PYRO.OX catalysts before and after reaction under fuel-
rich conditions.
The C observed on the PYRO and PYRO.OX (fresh PYRO.OX (fresh catalyst)
catalysts) corresponds to graphite, which has been added
PYRO (fresh catalyst)
PYRO (after reaction)
41
72
—
75
59
28
100
25
0.7
2.6
—
3
a
a
PYRO.OX (after reaction)
for pelletizing. The O2 pretreatment involves a decrease
of the superficial C content due to oxidation. In contrast,
the catalytic test under fuel-rich conditions is associated
with a strong superficial C deposit as it is observed on
both PYRO and PYRO.OX after reaction. In order to fol-
low only the relative superficial distribution of V, P, and
O, the relative percentages have been recalculated without
the carbon content (values in parentheses in Table 2a). It
appears that these percentages do not change for PYRO
and PYRO.OX. The main evolution principally concerns
the composition after reaction: V strongly decreases from
a
�
At T = 400 C and O2/C4H10 = 0.6 after 20–40 h working.
percentage. The carbonaceous species could be polymeric
organic chains enriched in oxygen, and, as a consequence,
this could be the reason for the increase of the oxygen per-
centage.
The V2p3/2 peak may be decomposed into two peaks with
5+
a binding energy of 518.0 eV for V and 516.9 eV for
4+
3+
V
5
(12). There is no evidence for any V contribution at
15.7 eV (Table 2b), in agreement with the absence of these
31
entities as observed with P NMR by spin echo mapping.
1
2.5% down to 10.7% for PYRO and from 12.2% down
The relative amounts of the V4 and V species are given
+
5+
to 9.3% for PYRO.OX when O moderately increases from
5+
in Table 2b. Note that only the presence of V is detected
on the fresh PYRO.OX catalyst. After reaction under fuel-
6
6
6.4% up to 68.2% for PYRO and strongly increases from
7.1% up to 71.5% for PYRO.OX. This evolution does not
rich conditions, there is a strong reduction of V5 to V
+
4+
affect the superficial composition of P, which almost does
not change (21.0% as compared to 21.1% for PYRO and
and the vanadium oxidation state is almost identical for the
two catalysts, about 75% V4 and 25% V
+
5+
.
2
0.6% as compared to 19.2% for PYRO.OX). The decrease
of the V percentage and the increase of the O percentage
have to be considered in parallel with the strong increase
of the carbon content: indeed, the catalytic test under fuel-
rich conditions results in a high deposition of carbonaceous
species at the surface of the PYRO and PYRO.OX cata-
lysts. This is responsible for a progressive masking of the
active vanadium species which results in a decrease of the V
4
. DISCUSSION
Figures 1 and 4 show that the reaction under fuel-rich
conditions (O2/n-C4H10 = 0.6) induced dramatic changes
in the catalytic performance on both the PYRO and the
PYRO.OX catalysts. Let us first discuss the changes in the
conversion of the reactants (Figs .1a and 4a).
The initially low n-butane conversion increased with the
time of reaction and then stabilized at the same level (about
TABLE 2a
3
8% ) for both the PYRO and the PYRO.OX catalysts.
Quantitative Analysis in Percentages of V, P, O, and C
on the Catalyst Surface by XPS
However, these changes were rapid for PYRO (about 5 h)
but were much slower (about 25 h) for PYRO.OX. As also
31
%
proposed in previous studies (25, 26, 29), P-NMR (Fig. 7b)
and XPS (Table 2b) showed that the PYRO.OX catalyst
accumulated a reservoir of active lattice oxygen bonded
Catalyst
V
P
O
C
to V5 species. A reduction of V to V occurs as evi-
+
5+
4+
PYRO (fresh catalyst)
9.4
12.5)
5
5.8
(21.1)
9.8
(21.0)
(18.1)
(20.6)
7.2
49.8
(66.4)
31.8
(68.2)
59
25
b
(
31
denced by P-NMR (Figs. 7a to 7d) and XPS (Table 2b).
XPS and P-NMR analysis performed after reaction have
shown that the vanadium oxidation state in the PYRO and
the PYRO.OX samples is identical, and this observation fits
PYRO (after reaction)a
PYRO.OX (fresh catalyst)
PYRO.OX (after reaction)b
53.4
12.2
31
(
10.7)
10.8
12.2)
3.5
(
(67.1)
26.8
62.5 with the same conversion (about 38% ) measured at steady
state for the two samples.
(
9.3)
(19.2)
(71.5)
a
�
The consumption of oxygen (Figs. 1a and 4a) during the
course of the reaction followed the same trend on both
PYRO and PYRO.OX catalysts. As already stated, the
At T = 400 C and O2/C4H10 = 0.6 after 20–40 h working.
b
Values in parentheses correspond to the atomic percentages when
considering only V, P, and O.

n-BUTANE ON V–P–O CATALYST UNDER FUEL-RICH CONDITIONS
315
TABLE 3
steady-state activity. This observation may be explained by
the leveling effect exerted by the reaction mixture on the
degree of oxidation and the surface state of the catalyst.
However, the catalyst working for a long time in a reducing
atmosphere becomes roughly twice more active than the
same catalyst working in an oxidizing atmosphere. There-
fore, the present results do support the idea that the acti-
�
Steady-State Intrinsic Activities in mol m 2
Calculated for the Same n-Butane Partial Pres-
sure) of the PYRO and PYRO.OX Catalysts Work-
� 1
h
(
ing in an Oxidizing (O2/C4 = 12) or a Reducing
(
O2/C4 = 0.6) Gas Mixture
O2/C4H10
12
0.6
vation of n-butane occurs on V4 sites, as already proposed
in several studies (13, 31–33). Indeed the reaction (the n-
butane conversion) readily occurs as soon as the alkane is
activated. Furthermore, the activation step likely requires
+
5
5
� 5
� 5
PYRO
PYRO.OX
4.6 � 10�
3.7 � 10�
9.2 � 10
12.6 � 10
a limited number of active V4 sites, possibly a single V
+
4+
difference in the time scales of reduction must be related species, and is therefore not very sensitive to the formation
to the storage of lattice oxygen (created by the oxygen of carbonaceous deposits which are expected to poison the
�
treatment at 500 C) in the PYRO.OX sample. It is quite vanadium surface sites.
striking to observe that the activity of both samples at
As reducing conditions yield a more active catalyst, they
first increases substantially in parallel with the reduction also change the selectivity pattern and suppress selective
of V5 to V . Furthermore the steady state n-butane con- oxidation to MA (Figs. 1b and 4b). Apart from the reduc-
version stays high even though the reaction is limited by ing atmosphere, the total consumption of oxygen may also
oxygen supply (100% oxygen consumption), and in spite contribute to the complete drop of MA formation. The oxy-
+
4+
�
of important carbon deposits as shown by XPS analysis gen pretreatment at 500 C was quite advantageous since
(
Table 3).
the formation of MA on the PYRO.OX catalyst was ob-
It is interesting to compare the steady-state intrinsic ac- served for about 15 h (Fig. 4b) compared to only 2 h on the
tivity of the PYRO and PYRO.OX catalysts working either PYRO catalyst (Fig. 1b). This is again in agreement with
under the standard (oxidizing) conditions (Table 3) or un- the idea that the storage of active lattice oxygen is associ-
der reducing conditions (Figs. 1 and 4). The comparison ated with selective V5 species in the surface region of the
is not straightforward since, in order to change the (O2/n- PYRO.OX catalyst. These observations support the view
C4H10) ratio, the n-butane partial pressures were very dif- that the presence of selective V5 species is crucial for the
ferent under the two experimental conditions. However, as oxygen-insertion reaction steps which lead to MA forma-
stressed by Centi et al. (1), most kinetics studies agree on tion, as previously proposed (26, 27, 30, 33–35). Besides
a nearly first-order reaction with respect to the n-butane their number, the nature of V5 species is also important
partial pressure. Assuming this first-order rate, the intrinsic since most orthophosphate phases are known to be very
steady-state activities under reducing conditions have been poorly selective (33, 36, 37). In this respect, it is worth re-
calculated in Table 4 for the same n-butane partial pressure calling that, in the present work, XRD analysis does not
+
+
+
used under standard conditions. For a given feed condition detect the formation of any crystalline VOPO4 phases even
�
(
O2/C4H10 ratio), the two catalysts display about the same after the oxygen pretreatment at 500 C; however, NMR
studies do show the presence of V5 species (Fig. 7b). With
respect to MA selectivity, Figs. 1b and 4b show a very inter-
esting feature. MA selectivity first increases with time-on-
+
TABLE 4
Catalytic Results for PYRO.OX after Different Regeneration Steps stream, goes through a maximum, and then drops to zero.
This phenomenon has been observed previously (26) and
clearly illustrates the model developed by Cavani et al. (38)
O2/C4H10
C(C4H10)
C(O2)
S(MA)
S(C4H8)
S(COx)
(
1)
explaining that MA selectivity goes through a maximum
0
2
0
0
.6
38.5
29.9
38.3
8.9
100
12.2
100
37.4
0
58.8
0
43.6
0
47.9
14.9
34.9
36.8
30.6
39.2
5+ 4+
(
1)
1
for a suitable V / V balance. For the oxidized sample,
(
(
2)
3)
5+
4+
.6
.6
the V / V ratio is first too high and therefore the MA
selectivity steadily increases with the course of the reac-
41.7
tion (favoring the reduction of V5 ) for about 7–8 h up to
+
Note. C(C4H10) and C(O2): n-butane and oxygen conversions. S(MA),
a maximum (about 65% ) before slowly decreasing down
S(C4H8), and S(COx): selectivity for MA and COx. The steps are as fol-
lows. (1) O2/C4H10 = 0.6: PYRO.OX results after 25 h of run, O2/C4 = to zero. One would expect then the absence of V5 species
+
(
1)
0
.6 . (2) O2/C4H10 = 12: reduced PYRO.OX results under fuel-lean
on the surface at this stage. However, XPS analysis still
(1) (1)
conditions, O2/C4 = 0.6 → 12 . (3) O2/C4H10 = 0.6: PYRO.OX results
after reducing–oxidizing–reducing cycle, O2/C4 = 0.6 → 12 → 0.6
4) O2/C4H10 = 0.6: reduced PYRO.OX reoxidized under pure O2 and
after 2 h of run (O2/C4 = 0.6), O2/C4 = 0.6 → 12 → 0.6 → O2 500 C
h → O2/C4 = 0.6
5+
shows the presence of a significant amount of V (24–28% )
(
1)
(1)
(2)
.
(
Table 2). This puzzling observation could be explained by
(
(
1)
(1)
(2)
�
the fact that the whole multistep mechanism leading to the
formation of MA demands several adjacent sites (39– 41),
(
3)
2
.

3
16
MOTA ET AL.
known to be vanadium dimers. XPS shows (Table 2) that
Besides the butenes isomers, 1,3-butadiene was detected
the presence of carbonaceous deposits were favored un- after a few hours of reaction (but at a very low level,
der the reducing conditions. These deposits are expected to about 2.5% ) for PYRO and PYRO.OX. The selectivity to
mask the V5 sites located in the surface. Hence, oxidation 1-butene steadily increased with time whereas the selec-
to MA (41) would be stopped at the first step (formation of tivities to cis-2-butene and trans-2-butene followed similar
butenes and COx) insofar as the probability of finding free changes and reached a maximum. It is known (14) that the
+
ensembles of selective V5 sites becomes too low.
+
formation of butene isomers is in line with the initial activa-
It is indeed striking to observe that unsatC4 are the main tion of n-butane by dissociation of a methylene C–H bond
products when the reaction reaches a steady state under to form a secondary butyl radical. The further loss of a sec-
fuel-rich conditions (Figs. 1b and 4b). This is in agreement ond hydrogen occurseither from a methylgroup or from the
with Centi et al. (14) and Busca (42), who claimed that other methylene group, and a statistical distribution of the
(
VO)2P2O7 is substantially an oxidative dehydrogenation isomers is then expected: 1-butene : cis-2-butene : trans-2-
catalyst that also contains active sites to insert oxygen. In butene = 3 :1 :1. This distribution is actually observed pro-
the present case, the V5 sites responsible for selective ox- vided the hydrogen loss is rapid and statistically occurs.
+
idation are either reduced to V4 or poisoned by carbona- Moreover, the residence time of adsorbed butenes should
+
ceous deposits. be very short (43, 44). Otherwise, the surface isomeriza-
Let us now consider in more detail the nature of the main tion occurs and the equilibrium ratio 1 :1 :1.1 is observed
products (CO, CO2, and the butene isomers) formed in a (44).
reducing atmosphere. The CO/CO2 distribution during the
It is interesting to look more closely at the type of distri-
course of the reaction is shown in Figs. 2a (PYRO catalyst) bution which is observed in the course of the reaction on
and 3b (PYRO.OX catalyst). It can be observed that CO PYRO and PYRO.OX catalysts. The experimental infor-
and CO2 selectivities are at first nearly identical. Then the mation is indeed in Figs. 3a and 5a, but the data are difficult
CO2 formation drops and CO becomes by far prevalent. to analyze in this respect. Therefore, in Figs. 3b and 5b, the
These changes are rapid on the PYRO catalyst (Fig. 2a) different selectivities have been normalized with respect to
but occur over a longer lapse of time on PYRO.OX catalyst the selectivity for cis-2-butene. Now it can be clearly seen
(
about 15 h, Fig. 2b). If one considers the usual simplified that the general trends for PYRO (Fig. 3b) and PYRO.OX
reaction scheme depicted in Fig. 8, the present observations
strongly suggest that CO mainly comes from the oxidation
of adsorbed butane and butenes whereas CO2 is mainly
produced by the overoxidation of MA. It is already known
that under standard conditions, the selectivity for MA is
mainly limited by the overoxidation of adsorbed MA (1).
According to Busca (42), the n-butane is first activated
at C(2) giving rise to an adsorbed secondary butoxy group.
These species can be further dehydrogenated oxidatively
on the same carbon atom to the adsorbed ketone leading
then to acetate species and COx. Conversely, the selec-
tive way involves the formation of a mixture of adsorbed
butenes which are normally rapidly converted to butadi-
ene and eventually to MA, thanks to the acidic and oxidiz-
ing properties of the V–P–O catalysts. Butenes are evolved
in the gas phase only on a surface that is nonreactive to-
wards olefins, because the suitable selective oxidizing sites
are lacking. Another reason could be that the surface is not
acidic enough, like for Mg vanadates (43, 44).
(
5b) are similar with, however, again a longer time scale for
PYRO.OX. In first approximation, the isomers distribution
is at first almost statistical, then at equilibrium, and again
statistical. Since there are no theoretical explanations for
the higher selectivity for trans-2-butene compared to that of
cis-2-butene, (1.5 : 1) instead of (1 : 1), this difference might
well result from the experimental accuracy in the analysis
of the reaction products. We therefore propose the follow-
ing general analysis of the observed changes of selectivity
with the course of the reaction under reducing conditions.
At the beginning of the run, butenes were very quickly con-
verted to butadiene and to further reaction intermediates
which led to MA and COx because of the presence of se-
5+
lective V species. Furthermore, the overall selectivity for
butenes was low (Figs .1b and 4b). As the catalyst was pro-
gressively reduced, butenes remained adsorbed for a longer
5+
time on the acidic sites insofar as oxidizing V sites became
scarce and an equilibrium ratio was observed. This step also
coincides with the formation of a small amount of butadiene
by oxydehydrogenation. Then the catalyst led to the forma-
tion of increasing amounts of butenes, especially 1-butene
(
Figs. 3a and 5a). Enough adsorption sites were then no
longer available for the numerous butene molecules. The
average residence time of individual butene molecules be-
came again very small, and the statistical ratio was observed
once more. Besides desorption, the adsorbed butenes
produced in large amounts could only undergo crack-
ing reactions leading to either carbonaceous deposits or
FIG. 8. Schematic representation of the reaction mechanism.

n-BUTANE ON V–P–O CATALYST UNDER FUEL-RICH CONDITIONS
317
oxidation to CO and, to a small extent, dehydrogenation lyst surface. This improvement of the catalyst properties
to butadiene. must be related to the storage of lattice oxygen created by
Let us now come back to the complete inhibition of the the O2 treatment of the PYRO catalyst.
selective oxidation route to MA which occurred when a
The butane activation would occur on single V4 actives
+
low O2/C4H10 ratio was used (Figs. 1 and 4). We believe that sites, and therefore the butane conversion is not very sen-
under these conditionsthe totalconsumption ofoxygen also sitive to the development of carbonaceous deposits occur-
playsa major role. In order to maintain the MA formation, it ring under reducing conditions. Conversely, ensembles of
is crucial to avoid a total conversion of oxygen by all means, several surface V5 species would be required to obtain the
+
for example, by reducing the n-butane partial pressure or selective sites responsible for the crucial oxygen-insertion
by changing the GHSV.
steps leading to MA. The presence of carbonaceous de-
4+
The reduction of the catalyst and the loss of MA selectiv- posits originating from V species reduces the access to
5+
ity was significantly delayed for the PYRO.OX catalyst. The
reversibility of this reduction (reactivation of the catalyst) duced.
has been investigated. In order to look for the ability of the The highly reducing test conditions of reaction provide
V
sites. Consequently, butenes and COx are again pro-
PYRO.OX catalyst to work under a large range of feed con- information on the mechanism of butenes and COx for-
ditions, different steps of regeneration have been chosen. mation. Results suggest that CO comes mainly from the
First, reducing–oxidizing–reducing treatments were per- oxidation of adsorbed butane and butenes whereas CO2 is
formed on the PYRO.OX catalyst by changing the O2/C4 mainly produced by the overoxidation of MA. Concerning
ratio, 0.6 in a first step, 12 in a second step, and again 0.6 in the butenes, their distribution in the gas phase depends on
a third step.
the acidity and the redox properties of the solid surface.
Although the reduced PYRO.OX catalyst is selective Initially, when the selectivity to butenes was low, the C4
with O2/C4 = 12, the catalyst is likely not reoxidized enough isomer distribution was statistical, then it went to equilib-
to form MA during the second fuel-rich step.
rium (surface highly reduced), and finally again statistical
Therefore the catalyst was submitted to a treatment at (adsorption sites for butenes not available, CO formation).
�
5
00 C under an oxygen flow for 2 h. Then this catalyst was
Finally, this study shows that for highly reducing con-
able to work again with a ratio O2/C4H10 = 0.6, and the se- ditions (O2/C4H10 = 0.6) the key parameter is the oxygen
lectivity for MA was again fairly high: S(MA) = 41.7% . This consumption: the redox cycle does not operate when
last step shows that the catalytic properties of the PYRO C(O2) = 100% . However, the reduction of the catalyst is
catalyst can be largely recovered following a reactivation reversible, which indicates that the active surface closely
step under pure O2.
follows the redox influence of the reactant mixture. How-
This observation is in agreement with the redox charac- ever, when strongly reduced, the catalyst requires a pure
ter of this reaction also demonstrated by DuPont (20) using oxygen regeneration in order to be selective again toward
a recirculating solid reactor: the oxidation of n-butane is MA under fuel-rich conditions (O2/C4 = 0.6).
carried out in two steps. In the first step, the alkane is oxi-
This study provides important information which will
dized by the lattice oxygen of the catalyst (in the absence of have to be taken into account in the development of a mem-
gaseous oxygen), and in the second step the reduced cata- brane reactor presently in progress in our laboratory. It ap-
lyst is regenerated with air.
pears that enough oxygen should be co-fed with butane to
limit nonselective catalysis at the inlet of the membrane
reactor. The results also show that some catalyst develop-
ment, such as improvement of the resistance of the active
phase toward reduction, should be worthwhile when mem-
brane reactors are considered. This will be the subject of
a second paper dealing with the effect of doping a V–P–O
catalyst working under oxygen-lean conditions.
5
. CONCLUSIONS
This study underlines the effects of the O2/n-C4H10 ratio
on the catalytic performance of a V–P–O (PYRO) catalyst.
It shows that the solid, although selective toward maleic
anhydride under standard feed conditions, O2/C4H10 = 12,
is not adapted to work under the fuel-rich feed conditions
ACKNOWLEDGMENTS
(
O2/C4 = 0.6) which prevail in the first layer of a membrane
reactor. These reducing conditions lead to a rapid reduction
This work was conducted in the framework of the contract Brite BRPR-
5+
of the catalyst surface, and the V species are reduced to CT95-0046. The authors are indebted to the support of the European
4+
Community. We gratefully acknowledge Dr. P. Delich e` re and M. Brun for
XPS analysis and Dr. A. Tuel and M. C. Durupty for NMR analysis.
V
and masked and/or poisoned bycarbonaceousdeposits.
Therefore, butenes and COx formation are favored under
reducing conditions.
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�
An oxidizing pretreatment under pure O2 at T = 500 C
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4