A good performance VPO catalyst for partial oxidation of n-Butane to maleic anhydride

Article abstract of DOI:10.1246/cl.2001.696

A VPO catalyst prepared by the reaction of vanadium pentoxide and isobutyl alcohol/benzyl alcohol in the presence of polyethylene glycol with the molecular weight of 2000 (PEG2000) was found to be highly selective and active for the conversion of n-butane to maleic anhydride.

Full text of DOI:10.1246/cl.2001.696

696
Chemistry Letters 2001
A Good Performance VPO Catalyst for Partial Oxidation
of n-Butane to Maleic Anhydride
Xiaoshu Wang,†,†† Weiyan Nie,†† Weijie Ji,*†† Xuefeng Guo,†† Qijie Yan,*†† and Yi Chen††
†Center for Materials Analysis, Nanjing University, Nanjing, 210093, P. R. China
††Deparment of Chemistry, Nanjing University, Nanjing, 210093, P. R. China
(Received March 29, 2001; CL-010289)
A VPO catalyst prepared by the reaction of vanadium pen-
rate of 2K/min, in an atmosphere of 1.5% n-butane/air and acti-
vated at this temperature for 12 h (GHSV=1800 h–1). For the
VPO2, it was prepared in the similar way except that the
PEG2000 was not adopted.
toxide and isobutyl alcohol/benzyl alcohol in the presence of
polyethylene glycol with the molecular weight of 2000
(PEG2000) was found to be highly selective and active for the
conversion of n-butane to maleic anhydride.
The BET surface areas were measured by nitrogen adsorp-
tion at 77 K using a Micromeritics ASAP 2000 adsorption
apparatus. X-ray diffraction (XRD) was conducted on a
Shimadzu D/MAX-RA diffractometer with graphite filtered Cu
Kα radiation. XPS was performed on a VG ESCALAB MKII
spectrometer, with the X-ray energy of 1253.6 eV (Mg Kα).
The binding energy was referred to the C1s signal (284.6 eV).
Infrared spectra were collected on a Nicolet 170SX Fourier
transform infrared spectrometer. The catalyst evaluation was
carried out in a quartz micro-reactor and 0.5 g catalyst was
charged. The reaction mixture consisted of n-C4H10/O2/N2 =
1.5/17.2/81.3 (volume ratio) with GHSV=1200 h–1. The reac-
tion temperature was in the range of 593–698 K. Two on-line
gas chromatography systems were adopted to analyze the outlet
reaction mixture.
The vanadium phosphorus oxide catalysts (VPO) are wide-
ly used for selective oxidation of n-butane to maleic anhydride.
The typical performance for currently used industrial VPO cata-
lyst was the MA yield of 55–58%, with butane conversion of
around 85% and MA selectivity of ca. 65 mol%. The selectivi-
ty was not sufficiently high and large amount of butane was
burned to COX. It is commonly suggested that the efficient cat-
alysts are associated with the presence of vanadyl pyrophos-
phate, (VO)2P2O7, and the catalyst performance depends
strongly on the preparation history.1–3 In one standard method,
the catalysts can be prepared by using HCl as the reducing
agent, and the obtained catalyst typically had low surface areas
(< 10 m2/g).1,3 In another standard procedure, the VPO cata-
lysts with higher surface area (around 20 m2/g) can be prepared
by using alcohols as solvent and reducing agent, and they gen-
erally showed better catalytic performance than the former.3,4
Since the total surface area is closely related to the active sites
available for the reaction, it is a constant interesting project for
researchers if some novel methods can be applied to prepare
high-surface area VPO catalyst which still maintains the essen-
tial active and selective component or phase for the reaction.
Different approaches have been tried to prepare small particle,
high-surface area system, including mechanical milling5 and
super critical field drying (SCFD) process6 in our laboratory
However, these methods are generally not so easy to operate and
control the process, especially for large quantity production. In
this work, we first reported here that the VPO catalyst was pre-
pared in the presence of polyethylene glycol. The obtained cata-
lyst showed rather high surface area (41 m2/g), small particle
size (< 100 nm) with rather even particle size distribution and
well-crystallized (VO)2P2O7 phase. The catalytic performance
was enhanced notably on the so-obtained sample.
The BET surface area of VPO1 is 41 m2/g, much higher
than that of VPO2, 19 m2/g. The XRD results indicated that the
two precursors were all the well-crystallized VOHPO4·0.5H2O
phase. The XRD patterns for both the VPO1 and VPO2 cata-
lysts accords well to those of (VO)2P2O7 phase,1–3 seen in
Figure 1. Note that although the major phase composition was
almost the same for the two catalysts, the crystallinity and the
relative diffraction intensities for different crystal planes are
different (for VPO1, I(200)/I(042) = 0.83 and FWHM(200) = 0.56;
for VPO2, I(200)/I(042) =1.14 and FWHM(200) = 0.40), reflecting
the deviations in microstructure between these two samples.
The infrared spectra showed that both VPO1 and VPO2 had the
characteristic IR features of the vanadyl pyrophosphate,8 name-
ly, the bands of 1248 cm–1, 1141 cm–1, 1080 cm–1, and 969
cm–1, but the 1248 cm–1 band was rather weak for the VPO2
sample (not shown). The C–H and C–O vibration bands were
not observed in the infrared spectrum of the freshly activated
Two catalysts were prepared for comparison: for the VPO1,
the preparation procedure was detailed in a Chinese patent.7
Typically, 3.2 g of V2O5 were suspended in a mixture of 70 mL
of isobutyl alcohol and 50 mL of benzyl alcohol under reflux-
ing, then PEG 2000 and H3PO4 (85%) were added under further
refluxing, with a molar ratio of the repeating unit (CH2CH2O)
of PEG2000 to the vanadium being 2 and the P/V molar ratio
being 1.2. The resulting suspension was cooled down to room
temperature and filtered. The precipitate was washed with
isobutyl alcohol and acetone, and then dried at 393 K for 24 h.
The precursor was heated from room temperature to 673 K, at a
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
697
VPO1 sample, suggesting the PEG agent had been removed
thoroughly after the activation procedure. The XPS data
demonstrated that P element was enriched on the surfaces of
these two catalysts, with the surface P/V ratio of 1.6 for VPO1
and 1.5 for VPO2 respectively, and this was in good agreement
with the other observation.9 The binding energies of V2p3/2
(517.2 eV for VPO1 and 516.9 eV for VPO2) and O1s (531.6
eV for VPO1 and 531.2 eV for VPO2) are very close to those
of (VO)2P2O7 reported.9
of PEG in the activation procedure is also beneficial in generat-
ing the high surface area sample. This kind of catalyst appears
more active and selective than that prepared in a normal route
without PEG. This could be due to the more available active
sites for MA formation per mass of catalyst (as revealed by its
significantly higher surface area) and its unique structural fea-
ture or morphology (partly evidenced by XRD and FT-IR results
as mentioned above). Thus the method developed in this study
generates a more environmentally friendly and productive VPO
catalyst for partial oxidation of n-butane to MA. Another inter-
esting feature of the PEG treated sample is its good low-temper-
ature performance. At 633 K, the MA yield can reach 60 mol%,
however, the catalyst prepared without PEG showed much lower
performance at the same temperature, seen in Figure 2.
The catalytic performances of these two catalysts are
shown in Figures 2 and 3. In the temperature range of 593–698
K, the selectivity of maleic anhydride (MA) on the VPO1 cata-
lyst was decreased more notably than that on VPO2, while the
conversion of n-butane on VPO1 was increased drastically with
increasing temperature. At rather low reaction temperature, that
is, 593 K, the catalyst VPO1 showed only 43% of conversion
and 92 mol% of selectivity. The catalyst showed its best per-
formance at 658 K: the conversion of 90% with the MA selec-
tivity of 74 mol%, resulting in the MA yield of 67 mol%.
Comparatively, only 50 mol% MA yield was obtained on the
VPO2 at the same temperature, with the conversion of 71% and
the selectivity of 71 mol%. At the typical reaction temperature,
namely, 673 K, the VPO2 catalyst showed its best performance:
the conversion of 84% and the MA selectivity of 66 mol%,
yielding 55 mol% MA. Note that on the former catalyst, even
the reaction temperature is 15 K lower, the performance so
obtained is considerably better than that of the latter obtained at
673 K. As shown in Figure 3, both VPO1 and VPO2 catalysts
become less selective with increasing conversion, but the for-
mer showed a faster drop in selectivity in the conversion range
of 40–90%.
The authors would like to acknowledge the financial sup-
ports from the NSFC and SINOPEC (Grant: 29792073-1), and
the MSTC (grant: 2000049008). This work was also partly
supported by Center for Materials Analysis, Nanjing
University.
References and Notes
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The use of PEG in preparation seems to provide an easy
and effective media to generate high surface area VPO catalyst.
The presence of PEG could decrease the surface energy of
nucleation of precursor particles, and the adsorbed PEG on the
freshly formed precursor surfaces can effectively hinder the
agglomeration of the precursor particles. The gradual removal