Novel preparation of vanadyl pyrophosphate for selective oxidation of n-butane utilizing intercalation and exfoliation

Article abstract of DOI:10.1246/cl.2001.484

Intercalation-exfoliation of VOPO₄·2H₂O crystallites (20 (μm in size) in 2-butanol, followed by reduction with 2-butanol, brought about thin layers of precursor, VOHPO₄·O-5H₂O, with size of about 2 μm. The obtai

Full text of DOI:10.1246/cl.2001.484

484
Chemistry Letters 2001
Novel Preparation of Vanadyl Pyrophosphate for Selective Oxidation of n-Butane
Utilizing Intercalation and Exfoliation
Norihito Hiyoshi, Naoki Yamamoto, and Toshio Okuhara*
Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810
(Received February 28, 2001; CL-010170)
Intercalation–exfoliation of VOPO4·2H2O crystallites (20
µm in size) in 2-butanol, followed by reduction with 2-butanol,
brought about thin layers of precursor, VOHPO4·0.5H2O, with
size of about 2 µm. The obtained vanadyl pyrophosphate (27
m2 g–1) and the corresponding SiO2 composite were highly
active and selective for selective oxidation of n-butane.
Vanadium phosphate catalysts have extensively been stud-
ied for selective oxidation of n-butane to maleic anhydride
(MA), since they are well-characterized crystalline compounds
and at the same time they are only effective compounds.1
Vanadyl pyrophosphate, (VO)2P2O7, is a main component of
the commercial catalyst for this reaction. Since the yield of
maleic anhydride is still not sufficient, it is thus keenly desired
to improve the catalyst performance.
There are many reports about the preparation of vanadyl
pyrophosphates.2–5 VOPO4·2H2O is a possible starting material.
Johnson et al.2 demonstrated that the precursor VOHPO4·0.5H2O
was obtained by direct reduction of VOPO4·2H2O with alcohol.
Hutchings et al.3 reported that the morphology of the precursor
could be controlled by the choice of alcohol to perform the
reduction of VOPO4·2H2O.
On the other hand, VOPO4·2H2O has been known to be
intercalated by various molecules including alcohol.6
Intercalation of VOPO4·2H2O could be intriguing as a method of
preparing new nanostructurally modified catalysts. Benzinger et
al.4 claimed that VOHPO4·0.5H2O was intercalated with alky-
lamines and the resulting compounds catalyzed n-butane oxida-
tion, while the selectivity to MA was less than 50%.
Recently “exfoliation” technique has been developed from
intercalation for various layered materials like clays,7 zirconium
phosphate,8 niobates,9 and titanates,10 in which exfoliation is a
method of delaminating stacked inorganic sheets in a solvent by
infinite swelling of their interlayer spaces. We showed that an
intercalation compound of VOPO4 with 4-butylaniline was
exfoliated in polar solvents.11 Here we wish to report that a
novel vanadyl pyrophosphate prepared through the intercala-
tion, exfoliation, and reduction in 2-butanol exhibited a high
catalytic performance.
By the evaporation of the resulting 2-butanol homogeneous
solution at 333 K, the green powder was recovered. This solid
showed a shape like flower petals (Figure 1b), which are greatly
different from that of the starting VOPO4·2H2O (Figure 1a).
From the IR and XRD, the structure of the resulting solid was
shown to be the same as that of VOPO4·2H2O, indicating the
conservation of lattice structure during the exfoliation process.
Refluxing the exfoliation solution (378 K) in the presence
of small amount of VOHPO4·0.5H2O (5 mg) for 20 h produced
the precipitates which were confirmed to be the precursor
VOHPO4·0.5H2O. As Figure 1c shows, the precursor consists
of smaller thin platelets (~2 µm in length). This precursor is
denoted to EP(2-Bu). As a reference, another precursor was
separately prepared by direct reduction of VOPO4·2H2O with 2-
butanol at 423 K using 2.5 g of VOPO4·2H2O and 25 cm3 of 2-
butanol. The obtained precursor had a shape of large platelet (8
µm in length) as shown in Figure 1d. This precursor is denoted
to P(2-Bu).
Figure 2 shows the dependence of the conversion of n-
butane on the contact time (W/F; W = catalyst weight (g) and F
= flow rate of n-butane (mol·h–1)) (Figure 2A) and the selectivi-
ty as a function of the conversion (Figure 2B). The oxidation of
n-butane was performed in a flow reactor under an atmospheric
pressure at 703 K using a mixture consisting of n-butane 1.5
vol%, O2 17 vol%, and He (balance).14 The precursor was acti-
vated in the reactant gas by heating from room temperature to
703 K at a rate of 5 K·min–1. Since the stationary activity and
selectivity were obtained at about 100 h after the temperature
reached 703 K, the data were collected after at least 100 h of
reaction. The products were analyzed with gas chromatographs
VOPO4·2H2O was prepared according to the literature,12
and the structure was confirmed by IR and XRD. As Figure 1
shows, the VOPO4·2H2O consisted of square platelets with the
lateral dimensions of around 20 µm and the thickness of about
1 µm (Figure 1a). VOPO4·2H2O (1.0 g) was added to 50 cm3
of 2-butanol at room temperature, at which VOPO4·2H2O
remained as the solid state. When the suspension was heated at
323, 343, or 363 K for 1 h, the solution became homogeneous,
which is the result of exfoliation via the intercalation of 2-
butanol.11 We have already reported the evidence for the exfo-
liation of intercalation compounds of VOPO4.11,13
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
485
MA were 61 and 63% at about 60%-conversion over 13 wt%
and 31 wt%VPO-SiO2 composites, respectively. The slight
decrease in the selectivity by supporting is probably due to
interaction between VP species and the support. These selec-
tivities were higher than those reported so far for the supported
catalysts.18–20 These results demonstrate that the present
method utilizing intercalation and exfoliation of the layered
materials is promising for the preparation of efficient catalysts.
This work was partly supported by New Energy and
Industrial Technology Development Organization (NEDO).
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