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