Investigation of the mechanism of n-butane oxidation on vanadium phosphorus oxide catalysts: Evidence from isotopic labeling studies

Article abstract of DOI:10.1021/ja010285v

The selective oxidation of n-butane to maleic acid catalyzed by vanadium phosphates (VPO) is one of the most complex partial oxidation reactions used in industry today. Numerous reaction mechanisms have been proposed in the literature, many of which have butenes, butadiene, and furan as reaction intermediates. We have developed an experimental protocol to study the mechanism of this reaction in which ¹³C-isotopically labeled n-butane is flowed over a catalyst bed and the reaction products are analyzed using ¹³C NMR spectroscopy. This protocol approximates the conditions found in an industrial reactor without requiring an exorbitant amount of isotopically labeled material. When [1,4-¹³C]n-butane reacted on VPO catalysts to produce maleic acid and butadiene, the isotopic labels were observed in both the 1,4 and 2,3 positions of butadiene and maleic acid. The ratio of label scrambling was typically 1:20 for the 2,3:1,4 positions in maleic acid. For butadiene, the ratio of label scrambling was consistently much higher, at 2:3 for the 2,3:1,4 positions. Because of the discrepancy in the amount of label scrambling between maleic acid and butadiene, butadiene is unlikely to be the primary reaction intermediate for the conversion of n-butane to maleic anhydride under typical industrial conditions. Ethylene was always observed as a side product for n-butane oxidation on VPO catalysts. Fully ¹³C-labeled butane produced about 5-13 times as much isotopically labeled ethylene as did [1,4-¹³C]butane, indicating that ethylene was produced mainly from the two methylene carbons of n-butane. When the reaction was run under conditions which minimize total oxidation products such as CO and CO₂, the amounts of ethylene and carbon oxides produced from fully ¹³C-labeled butane were almost equal. This strongly suggests that the total oxidation of n-butane on VPO catalysts involves the oxidation and abstraction of the two methyl groups of n-butane, and the two methylene groups of n-butane form ethylene. An organometallic mechanism is proposed to explain these results.

Full text of DOI:10.1021/ja010285v

Published on Web 01/24/2002
Investigation of the Mechanism of n-Butane Oxidation on
Vanadium Phosphorus Oxide Catalysts: Evidence from
Isotopic Labeling Studies
Bin Chen and Eric J. Munson*
Contribution from the Department of Chemistry, UniVersity of Minnesota,
207 Pleasant Street SE, Minneapolis, Minnesota 55455
Received February 2, 2001
Abstract: The selective oxidation of n-butane to maleic acid catalyzed by vanadium phosphates (VPO) is
one of the most complex partial oxidation reactions used in industry today. Numerous reaction mechanisms
have been proposed in the literature, many of which have butenes, butadiene, and furan as reaction
intermediates. We have developed an experimental protocol to study the mechanism of this reaction in
which 13C-isotopically labeled n-butane is flowed over a catalyst bed and the reaction products are analyzed
using 13C NMR spectroscopy. This protocol approximates the conditions found in an industrial reactor without
requiring an exorbitant amount of isotopically labeled material. When [1,4-13C]n-butane reacted on VPO
catalysts to produce maleic acid and butadiene, the isotopic labels were observed in both the 1,4 and 2,3
positions of butadiene and maleic acid. The ratio of label scrambling was typically 1:20 for the 2,3:1,4
positions in maleic acid. For butadiene, the ratio of label scrambling was consistently much higher, at 2:3
for the 2,3:1,4 positions. Because of the discrepancy in the amount of label scrambling between maleic
acid and butadiene, butadiene is unlikely to be the primary reaction intermediate for the conversion of
n-butane to maleic anhydride under typical industrial conditions. Ethylene was always observed as a side
product for n-butane oxidation on VPO catalysts. Fully 13C-labeled butane produced about 5-13 times as
much isotopically labeled ethylene as did [1,4-13C]butane, indicating that ethylene was produced mainly
from the two methylene carbons of n-butane. When the reaction was run under conditions which minimize
total oxidation products such as CO and CO2, the amounts of ethylene and carbon oxides produced from
fully 13C-labeled butane were almost equal. This strongly suggests that the total oxidation of n-butane on
VPO catalysts involves the oxidation and abstraction of the two methyl groups of n-butane, and the two
methylene groups of n-butane form ethylene. An organometallic mechanism is proposed to explain these
results.
Introduction
maleic anhydride is around 65-70% under typical industrial
conditions (less than 2 molar % n-butane in air, with conversion
The selective oxidation of n-butane to maleic anhydride is
recognized as one of the most complex selective oxidation
reactions used in industry today.1-12 In this reaction, eight
hydrogen atoms are abstracted from n-butane, and three oxygen
atoms are added to form maleic anhydride. The selectivity to
from 70 to 85% between 400 and 450 °C).2 Only maleic
anhydride (or acid), carbon oxides, and a trace amount of acetic
acid are detected under such conditions. All catalysts used
industrially for the production of maleic anhydride from
n-butane are based on vanadium phosphorus oxides (VPO).2
Vanadyl pyrophosphate ((VO)2P2O7) is regarded to be the active
phase of the VPO catalysts since it is the single crystalline phase
that exists in equilibrated VPO catalysts (after more than 200 h
on stream).2 The average oxidation state of vanadium in
equilibrated VPO catalysts is typically 4.00-4.03.2
* To whom correspondence should be addressed.
(1) Birkeland, K. E.; Babitz, S. M.; Bethke, G. K.; Kung, H. H. J. Phys. Chem.
B 1997, 101, 6895-6902.
(2) Centi, G.; Trifiro`, F.; Ebner, J. R.; Franchetti, V. M. Chem. ReV. 1988, 88,
55-80.
(3) Centi, G. Catal. Today 1993, 16, 5-26.
(4) Cavani, F.; Trifiro`, F. Catalysis 1994, 11, 246-315.
(5) Abon, M.; Volta, J.-C. Appl. Catal. 1997, 157, 173-193.
(6) Hutchings, G. J.; Kiely, C. J.; Sananes-Schulz, M. T.; Burrows, A.; Volta,
J. C. Catal. Today 1998, 40, 273-286.
Despite extensive study, little is known about the reaction
mechanism for n-butane oxidation on VPO catalysts. One reason
is that none of the potential reaction intermediates, such as
butenes, butadiene, furan, etc., have been observed in the
products using typical industrial conditions.13 A common
approach has been to run the reaction under unusual reaction
conditions to detect possible reaction intermediates. Centi et
(7) Herrmann, J.-M.; Vernoux, P.; Be´re´, K. E.; Abon, M. J. Catal. 1997, 167,
106-117.
(8) Hutchings, G. J.; Sananes, M. T.; Sajip, S.; Kiely, C. J.; Burrows, A.;
Ellison, I. J.; Volta, J. C. Catal. Today 1997, 33, 161-171.
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J. R.; Volta, J. C.; Abon, M. J. Catal. 1998, 177, 224-230.
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1994, 368, 41-45.
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(12) Coulston, G. W.; Bare, S. R.; Kung, H.; Birkeland, K.; Bethke, G. K.;
Harlow, R.; Herron, N.; Lee, P. L. Science 1997, 275, 191-193.
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24, 32-37.
9
1638 VOL. 124, NO. 8, 2002 J. AM. CHEM. SOC.
10.1021/ja010285v CCC: $22.00 © 2002 American Chemical Society

n-Butane Oxidation on VPO Catalysts
A R T I C L E S
al.14 have shown that at high butane concentrations, butenes,
butadiene, and furan, but no maleic anhydride, were detected
using a traditional flow reactor. Another approach has been
nonequilibrium transient experiments which employ a temporal
analysis of products (TAP) reactor developed by Gleaves et
al.15-17 The reactor consisted of high-speed injection valves, a
microreactor, and a quadrupole mass spectrometer separated
from the reactor by differentially pumped chambers.15 Butenes,
butadiene, and furan were detected sequentially with respect to
their maximum intensity on equilibrated (VO)2P2O7 catalyst.
However, when the catalyst was first pulsed extensively with
oxygen at reaction temperature (420 °C) and subsequently pulsed
with n-butane, maleic anhydride was observed, but the proposed
intermediate products were not.15 The following reaction
pathway was proposed on the basis of these results:4,14
methyl group, with the concerted formation of two strong
Cterminal-Osurf bonds so that the molecule is anchored long
enough for the reaction to be completed. However, experiments
with deuterium-labeled n-butane revealed that the first step of
butane oxidation is the irreversible activation of a methylene
C-H bond in butane on the catalyst surface.20
In situ Fourier transform infrared (FTIR) spectroscopy studies
have been performed to investigate butane oxidation on VPO
catalysts.21,22 Wenig and Schrader used an in situ FT-IR cell to
study the interaction of n-butane with VPO catalysts.22 These
authors reported evidence for the presence of n-butane, maleic
anhydride, carbon oxides, and reactive surface species (maleic
acid and olefins) on the catalyst at temperatures of 200-400
°C. Recently, Xue and Schrader developed a technique called
“transient FTIR” which uses special operation techniques such
as pulse reaction and reactant feed cycling to observe the
evolution of the IR spectra as a function of time.23 Results from
transient FTIR studies suggested that unsaturated noncyclic
carbonyl species may be precursors to maleic anhydride and
butane might be adsorbed on the VPO catalyst to form olefinic
species at low temperatures (50 °C). The latter point contradicts
general observations that the activation of butane to butenes is
the rate-limiting step and occurs at much higher temperature
(>350 °C).2
Recently, we have developed an experimental protocol which
utilizes selective 13C isotopic labeling and examination of the
reaction products by 13C NMR spectroscopy to investigate the
mechanism of n-butane oxidation on VPO catalysts. The
advantage of this protocol is that the fate of the 13C label can
be monitored after the reaction and therefore gives insights into
the reaction mechanism. Previously we have shown that the label
in butadiene produced from [1,4-13C]n-butane is completely
scrambled, but in maleic acid, also produced from [1,4-13C]n-
butane, the label is largely unscrambled. This makes it unlikely
that maleic acid is formed predominantly by a butadiene
intermediate.24
However, several arguments have been made against butenes,
butadiene, and furan as reaction intermediates:2,4
1. These compounds were detected under very unusual
conditions, such as low oxygen and very high n-butane
concentrations and at very low contact times, or under high
vacuum in the TAP reactor, or in the oxidation of n-butane under
anaerobic conditions in a pulse reactor.
2. There was no desorption of intermediates during n-butane
oxidation in the presence of available oxygen (either molecular
oxygen or lattice oxygen associated with V5+).
3. The oxidation of n-butane and the intermediate compounds
on VPO catalysts yielded different product distributions. As
mentioned above, only maleic anhydride, carbon oxides, and a
trace amount of acetic acid were detected for n-butane oxidation.
By contrast, acetaldehyde, crotonaldehyde, and other partial
oxidation products were detected in the case of the oxidation
of C4 olefins.2
Kinetic methods have also been used to study the reaction
mechanism. Zhang-Lin et al.18 conducted kinetics studies of the
oxidation of n-butane, butadiene, furan, and maleic anhydride
on various VPO phases to investigate the mechanism of n-butane
oxidation on VPO catalysts. They concluded that the main route
from butane to maleic anhydride is an “alkoxide route” in which
the precursors to maleic anhydride are alkoxide species. These
alkoxides maintain a σ-bond between the substrate and the
catalyst surface, and there is no desorption in the gas phase of
butenes, butadiene, and furan. By using crystallochemical
models of active sites and examining the energetics and
geometries of butane oxidation on the (100) face of (VO2)2P2O7,
Ziolkowski et al.19 reported that the active site for the direct
oxidation of butane to maleic anhydride is situated between four
protruding, undersaturated oxygens (2 × V-O, 2 × P-O). The
activation of butane consists of the abstraction of a H from each
In this paper, we describe our experimental results for all
aspects of this reaction obtained using this protocol. We have
found that ethylene was always a side product for the reaction
of n-butane on VPO catalysts. When fully 13C-labeled butane
was used instead of [1,4-13C]butane, the 13C NMR peak intensity
of ethylene increased by 5-13 times, showing that ethylene
was produced mainly from the two methylene carbons of
n-butane. Moreover, the yields of carbon oxides and ethylene
were roughly equal on catalysts with phosphorus:vanadium
ratios slightly higher than the stoichiometric ratio (P:V ) 1.2
and 1.1). These catalysts had higher selectivities for maleic acid
production. This result suggests that the total oxidation of
n-butane on the selective VPO catalysts involves mainly the
oxidation and abstraction of the two methyl carbons, and the
two methylene groups are left to form ethylene. The ratio of
label scrambling in maleic acid changed significantly when
n-butane reacted on the VPO catalysts with higher P:V ratios.
When the P:V ratio changed from 0.9 to 1.2, the ratio of label
scrambling changed from 1:30 to 1:6 for the 2,3:1,4 positions
(14) Centi, G.; Fornasari, G.; Trifiro`, F. J. Catal. 1984, 89, 44-51.
(15) Gleaves, J. T.; Ebner, J. R.; Kuechler, T. C. Catal. ReV. Sci. Eng. 1988,
30, 49-116.
(16) Rodemerck, U.; Kubias, B.; Zanthoff, H.-W.; Baerms, M. Appl. Catal. A:
General 1997, 153, 203-216. Rodemerck, U.; Kubias, B.; Zanthoff, H.-
W.; Baerms, M. Appl. Catal. A: General 1997, 153, 217-231.
(17) Kubias, B.; Rodemerck, U.; Zanthoff, H.-W.; Meisel, M. Catal. Today 1996,
32, 243-253.
(20) Pepera, M. A.; Callahan, J. L.; Desmond, M. J.; Milberger, E. C.; Blum,
P. R.; Bremer, N. J. J. Am. Chem. Soc. 1985, 107, 4883-4892.
(21) Busca, G. Catal. Today 1996, 457-496.
(22) Wenig, R. W.; Schrader, G. L. J. Phys. Chem. 1987, 91, 5674-5680.
(23) Xue, Z.-Y.; Schrader, G. L. J. Catal. 1999, 184, 87-104.
(24) Chen, B.; Munson, E. J. J. Am. Chem. Soc. 1999, 121, 11024-11025.
(18) Zhang-Lin, Y.; Forissier, M.; Sneeden, R. P.; Ve´drine, J. C.; Volta, J. C.
J. Catal. 1994, 145, 256-266. Zhang-Lin, Y.; Forissier, M.; Ve´drine, J.
C.; Volta, J. C. J. Catal. 1994, 145, 267-275.
(19) Ziolkowski, J.; Bordes, E.; Courtine, P. J. Catal. 1990, 122, 126-150.
9
J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002 1639

A R T I C L E S
Chen and Munson
in maleic acid. Concurrently, more ethylene was observed when
[1,4-13C]butane reacted on the VPO catalysts with higher P:V
ratios. The correlation between the ratio of label scrambling
and the peak intensity of ethylene when [1,4-13C]butane was
the reactant indicates that there might be an association between
the formation of ethylene and the occurrence of label scrambling.
There was no ethylene formed and no label scrambling was
observed when butadiene reacted on the VPO catalysts to
produce maleic acid. From these results we are able to propose
a plausible mechanism for this reaction. We propose that
n-butane is absorbed on the VPO catalysts via an organometallic
interaction between the two methyl carbons of butane and the
vanadium atom on the surface of the catalyst during the reaction,
thus forming a chelating configuration. The chelating config-
uration serves to hold the butane molecule on the surface of
the catalyst while hydrogens of butane are removed from it and
oxygens are added to it until maleic anhydride is formed. The
chelating configuration of the reaction intermediates is used to
explain label scrambling, formation of ethylene, and other
observations.
Experimental Section
Synthesis of VPO Catalysts. The catalysts used in this study can
be divided into two groups: catalysts with constant P:V ratio (1:1) but
different average vanadium oxidation states, and catalysts with different
P:V ratios. Catalysts are denoted either by their average vanadium
oxidation states, e.g., VPO3.92, or the P:V ratio, e.g., P:V ) 0.9. The
procedures for making these catalysts are described as follows.
(i) Catalysts with Constant P:V Ratio (1:1). Precursor 1.
Stoichiometric amounts (P:V ) 1:1) of V2O5 and H3PO4 (85%) were
mixed in ethanol and refluxed for 16 h. The resulting precursor was
blue and identified by XRD as VOHPO4‚0.5H2O.
Figure 1. Pseudo-flow reactor.
stirring. The suspension was then transferred to a 1000-mL beaker and
heated gently on a hot plate to near dryness. The residue was then
dried at 110 °C for 24 h. The resulting precursor was calcined at 380
°C for 3 h in air to obtain the catalyst.
Characterization of the Catalysts. The average valence state of
vanadium in the VPO catalysts was determined using the potentiometric
method described by Niwa and Murakami.26 Specific surface areas of
the VPO catalysts were measured using the BET method (Micromeritics
ASAP 2000). The XRD patterns of the catalysts were obtained using
a X-ray diffractometer (SIEMENS D5005).
VPO3.92, VPO4.75, and VPO4.95 were made from precursor 1:
Precursor 1 550 °C, 3 h8 VPO3.92
(2)
(3)
(4)
N2
Pseudo-Flow Reaction. A pseudo-flow reactor was designed to
prepare samples for NMR analysis (Figure 1). VPO catalyst (∼0.2 g)
was placed in the center of the glass reactor. The reactor was attached
to a high-vacuum line and evacuated to <5 × 10-3 Torr. The reactants
(butane or butadiene, ∼50 µmol) were condensed in one end of the
reactor with liquid N2. After the catalyst was preheated in a furnace to
the desired temperature, liquid N2 was removed from the reactants and
moved to the other end of the reactor. The reactants then boiled off
and passed through the catalyst due to the pressure gradient induced
by the liquid N2 on the other end. After the pseudo-flow process was
complete, the glass ampule containing the reaction products and the
remaining reactant was flame-sealed and analyzed using NMR spec-
troscopy. All of the reactions reported here were run using the pseudo-
flow reactor under anaerobic conditions unless otherwise denoted.
1,4- and fully 13C-labeled n-butane were obtained from Isotec Inc.
2-13C-labeled butadiene was obtained from Cambridge Isotope Labo-
ratories. These materials are of 99% purity, and the trace amounts of
impurities did not interfere with the interpretation of the experimental
results.
NMR Spectroscopy. 13C NMR spectra were acquired on a home-
built 200 spectrometer operating at 50.197 MHz for 13C. The glass
ampule from the pseudo-flow reaction was placed in a zirconia rotor
(7.5 mm), packed with diatomaceous earth, and sealed with Kel-F plugs.
There was no detectable background signal from the diatomaceous earth.
The Kel-F plugs and the Vespel spinning module give a very broad
background signal from approximately 75 to 150 ppm, but this caused
no difficulties in spectral interpretation. Single-pulse 13C excitation
(Bloch decay) with proton decoupling (pulse delay ) 1-3 s, 10 000-
Precursor 1 380 °C, 3 h8 VPO4.75
air
Precursor 1 450 °C, 24 h8 VPO4.95
air
Precursor 2. The procedure described by Cavani et al.25 was
modified slightly. The catalysts were prepared by suspending 45 g of
V2O5 in 90 mL of isobutyl alcohol and 60 mL of benzyl alcohol. The
suspension was stirred continuously and boiled for 1 h under reflux.
H3PO4 (85%) emulsified in 100 mL of isobutyl alcohol was then added
to the cold slurry in a quantity necessary to obtain the 1:1 P:V atomic
ratio, and then the slurry was boiled and stirred for 1 h. After cooling,
the slurry was filtered and the powder dried at 150 °C for 10 h.
VPO4.34, VPO4.56, and VPO4.95 were made from precursor 2:
Precursor 2 550 °C, 3 h8 380 °C, 3 h8 VPO4.34
(5)
(6)
(7)
N2
air
Precursor 2 380 °C, 3 h8 VPO4.56
air
Precursor 2 450 °C, 3 h8 VPO4.84
air
(ii) Catalysts with Different P:V Ratios (0.9-1.2). Catalysts with
various P:V ratios were made using the following procedure: 5 g of
V2O5 and a stoichiometric amount of H3PO4 (85%) (P:V ) 0.9-1.2)
were mixed in 100 g of isobutanol and refluxed for 24 h with continuous
(25) Cavani, F.; Centi, G.; Trifiro`, F. Appl. Catal. 1984, 9, 191-202.
(26) Niwa, M.; Murakami, Y. J. Catal. 1982, 76, 9-16.
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1640 J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002

n-Butane Oxidation on VPO Catalysts
A R T I C L E S
Table 2. 13C Chemical Shift Assignmentsa for Reactants,
Table 1. Physical and Chemical Properties of the VPO Catalysts
Products, and Intermediates Observed in This Study
av valence
state of V in crystalline phases
fresh catalyst detected by XRD
BET
product
observed 13C chemical shiftb
surface area,
2
catalyst
color
brown
brownish green
green
green
green
yellowish green
dark green
dark green
green
yellowish green
yellowish green
m g-1
methane
ethane
-10.8c
3.5c (6.4)
VPO3.92
VPO4.34
VPO4.56
VPO4.75
VPO4.84
VPO4.95
P:V ) 1.2
P:V ) 1.1
P:V ) 1.0
P:V ) 0.95
P:V ) 0.90
3.92
4.34
4.56
4.75
4.84
4.95
4.47
4.57
4.86
4.96
4.97
(VO)2P2O7
unknown
unknown
unknown
unknown
unknown
12.68
10.34
13.80
8.38
10.34
5.85
2-cis-butene
1-butene
9.1 (12.1), 124.8 (123.0)
10.7 (13.1), 26.1 (27.1), 110.4 (113.1),
139.7 (140.3)
n-butane
2-trans-butene
propene
acetylene
methandiol
10.8 (13.5), 24.4 (25.4)
14.8 (17.7), 124.9 (126.3)
16.1 (18.6), 112.8 (115.0), 132.0 (133.2)
67.3c
83.0c
furan
butadiene
ethylene
107.3 (109.6), 142.0 (143.0)
113.6 (117.1), 137.3 (138.0)
120.0 (122.3)
benzene
127.7 (128.2)
maleic acid, disodium salt
fumaric acid, disodium salt
maleic acid (satd aq solution)
fumaric acid (satd aq solution) 135.0, 170.0
CO2
CO
131.6, 176.5
131.8, 176.7
132.1, 170.1
100 000 transients) was used to obtain all spectra presented in this
contribution. Unless otherwise indicated, the spectra shown are Bloch
decay spectra. All spectra were acquired at room temperature. Magic-
angle spinning was used to help eliminate susceptibility effects
associated with observing samples containing both gases and liquids.
125.6c
183.5c
a Based on literature, running reference compounds and multiplicity
patterns obtained without proton decoupling and/or with 13C fully labeled
materials. All chemical shift values were obtained by running reference
compounds at room temperature unless noted. Values in parentheses refer
to chemical shifts in the liquid phase. b ppm from TMS. c From literature.
Results
Catalyst Characterization. The physical and chemical
properties of the VPO catalysts are listed in Table 1. Only the
average vanadium oxidation state was determined for the
catalysts with P:V ratios of 0.9-1.2. The XRD pattern of
VPO3.92 matches very well with that of vanadyl pyrophosphate
((VO)2P2O7). No match to a crystalline phase was found for
the other catalysts, as all of these catalysts were basically
amorphous. The surface areas of all of these catalysts was
around 10 m2/g, which is typical of VPO catalysts made in
organic solvent.16 For the catalysts with different P:V ratios,
the oxidation state increased as the P:V ratio decreased, although
their precursors had the same vanadium valence state (4.0) and
the precursors were calcined under identical conditions. This is
consistent with the observations in the literature that addition
of phosphorus prevents the oxidation of vanadyl pyrophosphate
to VOPO4.2,3
NMR Peak Assignments. Table 2 lists the 13C chemical shift
assignments for the reactants, products, and intermediates
observed in this study. Most of the chemical shift values listed
in Table 2 were obtained from analysis of pure compounds under
conditions similar to those used to analyze the pseudo-flow
reaction products. While most of the chemical shifts were found
to be in agreement with the previously reported values, a few
discrepancies arose because no chemical shift has been reported
for the species in the gas phase or in aqueous solution. For
example, the literature values27 for the chemical shifts of maleic
acid (solvent: CDCl3 + DMSO-d6) are 167.2 (C1 and C4) and
130.6 ppm (C2 and C3), while the values we measured using
saturated aqueous solution of maleic acid were significantly
different (170.1 and 132.1 ppm). Moreover, the chemical shifts
of maleic acid changed slightly with concentration (Figure 2),
which might be attributed to the larger degree of dissociation
at smaller concentrations. When [2,3-13C]maleic acid was heated
at 110 °C for 5 h (Figure 2e), another peak appeared at 135
ppm, which corresponds to the chemical shift of the 2,3 carbons
of fumaric acid, indicating that part of maleic acid was converted
to the trans configuration. The chemical shift assignments of
Figure 2. Chemical shift of the 2,3 carbons of maleic acid at different
concentrations: (a) 1.0 M, (b) 0.27 M, (c) 0.10 M, (d) 0.016 M, (e) same
concentration as (b) but heated at 110 °C for 5 h.
maleic acid and fumaric acid were further confirmed by the
1H-13C coupling patterns in the absence of proton decoupling
and the 13C-13C coupling patterns when fully 13C-labeled
materials were analyzed.
Reaction of n-Butane on VPO Catalysts with Constant
P:V Ratio (1:1) but Different Average Vanadium Oxidation
States. The oxidation state of vanadium is the key factor that
controls the product distribution of n-butane oxidation on VPO
catalysts. Figure 3 shows the 13C MAS NMR spectra of the
reaction products of either [1,4-13C]butane or fully 13C-labeled
butane flowed over a catalyst with an intermediate oxidation
(27) Pouchert, C.; Behnke, J. The Aldrich Library of 13C and 1H FT NMR
Spectra, 1st ed.; Aldrich Chemical Co., Inc.: Milwaukee, WI, 1993; Vol.
1.
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J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002 1641

A R T I C L E S
Chen and Munson
was open to air prior to sealing. The largest peak corresponds
to carbons in the 1,4 positions of maleic acid, because other
major components such as CO, CO2, ethylene, and butane were
all gaseous species and were largely removed when the ampule
was open to air. The peaks at 132 and 134 ppm remained,
confirming that these peaks were produced by a nonvolatile
compound. When [1,4-13C]butane was reacted on VPO4.56 at
380 °C and the ampule evacuated prior to analysis (Figure 3e),
all the gaseous and volatile species mentioned above were totally
removed, leaving only peaks for maleic acid, fumaric acid, and
methanediol in the spectrum. The peak assigned to [2,3-13C]-
maleic acid is bracketed by two smaller peaks assigned to the
2,3 carbons of maleic acid at natural abundance. The natural
abundance carbons correspond to a doublet due to 13C-13C
couplings to the 13C labels in the 1 and 4 positions of maleic
acid. No correlation between the relative peak intensity of maleic
acid to fumaric acid was found in our studies of this reaction.
Figure 3f shows the 13C MAS NMR spectrum of fully 13C-
labeled butane run under the same conditions as in Figure 3b.
The product distribution was the same as that of the sample
shown in Figure 3b, except that the peaks for ethylene and
acetylene are much larger than those observed for [1,4-13C]-
butane, suggesting that ethylene and acetylene were produced
mainly from the two methylene carbons of n-butane. Figure 3f
also shows that the parameters used to acquire the spectrum
resulted in the carbonyl resonance of maleic acid being ∼6.5
times smaller than the olefinic resonance, although the ratio
should be 1:1, because NMR spectra, especially single-pulse
spectra, should be quantitative. Spectra acquired with longer
repetition times between acquisitions showed that the intensities
of all of the other peaks observed in our experiments were
quantitative. From the integrated intensity ratios of 1:5 for the
carbonyl and olefinic resonances of maleic acid, respectively,
in Figure 3e and using a scaling factor of 6.5, we estimate that
the amount of label exchange for the sample in Figure 3e was
∼3%. This is consistent with the intensities of the natural
abundance carbons (1.1%) observed in Figure 3e.
Figure 3. 13C MAS NMR spectra of the reaction products of n-butane on
VPO4.56 after reaction at various temperatures: (a) [1,4-13C]butane, 330
°C; (b) [1,4-13C]butane, 380 °C; (c) [1,4-13C]butane, 450 °C; (d) [1,4-13C]-
butane, 500 °C, ampule open to air prior to sealing; (e) [1,4-13C]butane,
380 °C, ampule evacuated prior to sealing; (f) uniformly 13C-labeled butane,
380 °C. Spectra in (a), (b), and (c) were expanded vertically at a factor of
8, 8 and 32, respectively. The expansions for the peaks in (e) and (f) are
not uniform.
state of vanadium (VPO4.56) at different temperatures. Figure
3a-c shows the reaction products of [1,4-13C]butane obtained
after reaction at 330, 380, and 450 °C, respectively. The product
distribution was relatively independent of temperature. The
largest peaks in Figure 3a,b are due to the 13C-labeled C1 and
C4 carbons of butane in both the liquid and gas phases. The
small peaks due to natural abundance 13C in the 2 and 3 positions
of butane were also observed around 25 ppm in Figure 3a,b.
Only CO2, CO, and maleic acid (170 ppm, corresponding to
the 1 and 4 positions) were detected as the major reaction
products, which is consistent with observations in the literature.2
Maleic acid, rather than maleic anhydride, was detected because
of excess water produced from both the reaction and possibly
the release of adsorbed water on the catalyst. Besides the major
peaks, two minor peaks corresponding to ethylene (120 ppm)
and methanediol (83 ppm) were also detected. In addition, two
small peaks at 132 and 134 ppm were observed. They cor-
respond to maleic acid (132 ppm) and fumaric acid (134 ppm),
in which the labeled carbons are in the 2,3 positions.
The reaction was performed on VPO catalysts with average
oxidation states of vanadium ranging from 4.95 to 4.34 in order
to investigate the effect of the oxidation state of the catalyst on
n-butane oxidation. The product distributions were very similar
to that of VPO4.56. Label scrambled maleic acid and fumaric
acid, ethylene, and methanediol were produced on all the
catalysts.
n-Butane oxidation was also carried out on VPO3.92, which
contains only V4+ and V3+, to compare product distributions
on this catalyst with those on catalysts which contain V5+ 13C
.
MAS NMR spectra for the reaction of [1,4-13C]butane on
VPO3.92 at different temperatures are shown in Figure 4. The
activity of the catalyst was quite low compared to that of
catalysts containing V5+, as evidenced by the very small peaks
of the reaction products at 450 and 500 °C (Figure 4a,b).
Moreover, there were no peaks corresponding to maleic acid
in all of the spectra, indicating that V5+ was necessary for the
production of maleic acid.6,12,16 Instead, butenes, butadiene, and
furan were detected.
Several experiments were performed to validate the assign-
ment of the peaks at 132 and 134 ppm to maleic and fumaric
acid, respectively. Figure 3d shows the spectrum of the reaction
products of [1,4-13C]butane at 500 °C when the glass ampule
The most interesting reaction product was butadiene for
several reasons. First, the ratio of [1,4-13C]butadiene to [2,3-
13C]butadiene was ∼60:40. This ratio did not change signifi-
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n-Butane Oxidation on VPO Catalysts
A R T I C L E S
Figure 4. 13C MAS NMR spectra of the reaction products of n-butane on VPO3.92 after reaction at various temperatures: (a) [1,4-13C]butane after reaction
at 450 °C; (b) [1,4-13C]butane after reaction at 500 °C; (c) [1,4-13C]butane after reaction at 550 °C; (d) [1,4-13C]butane after reaction at 600 °C; (e) fully
13C-labeled butane after reaction at 600 °C. A, B, C, D, and E are the expansions of a, b, c, d, and e, respectively.
cantly with temperature (Figure 4a-d). If butadiene produced
by this mechanism was a reaction intermediate for maleic acid
production, then the amount of label scrambling observed in
maleic acid should be ∼40%. Because the amount of label
scrambling observed for maleic acid was typically only 1-7%,
this suggests that butadiene is unlikely to be a reaction
intermediate in the mechanism responsible for most of the
conversion of n-butane to maleic anhydride. Second, the peaks
for butadiene are not split by 13C-13C spin-spin couplings.
This indicates that the observed label scrambling is probably
not produced by the formation of cyclobutane, which would
produce 13C labels in the 1,2; 2,3; and 1,4 positions with a 2:1:1
peak ratio. The resulting coupling pattern should be composed
of a doublet due to 1,2 coupling and a singlet due to 1,4 and
2,3 carbons. Multiple couplings are observed in the spectrum
of fully 13C-labeled butane (Figure 4e). Because only a singlet
is observed, a concerted reaction mechanism, in which the
carbons in the 1,4 positions are transferred to the 2,3 positions,
is more reasonable.
Ethylene was the second major product in all of the spectra
shown in Figure 4. The intensity of the ethylene peak did not
increase significantly when [1,4-13C]butane was substituted with
fully 13C-labeled butane (compare spectra d and e in Figure 4).
This result is quite different from that of n-butane oxidation on
VPO4.56 (Figures 3), where only a trace amount of ethylene
was detected when [1,4-13C]butane was the reactant. The
coincidence of a higher yield of ethylene and a higher degree
of label scrambling suggests that there might be an association
between label scrambling and the formation of ethylene.
Methane, ethane, and propene are likely to be the cracking
products of n-butane because they were detected in significant
amounts only at 550 and 600 °C.
n-Butane Oxidation on VPO3.92 in the Presence of
Gaseous Oxygen. All the reactions mentioned above were run
under anaerobic conditions, which is quite different from
industrial conditions (less than 2% n-butane in air). The
industrially used VPO catalysts contain only a single crystalline
phase, vanadyl pyrophosphate, which is regarded as the active
phase for the selective oxidation of n-butane. The XRD pattern
of VPO3.92 matches well with that of vanadyl pyrophosphate.
To simulate industrial reaction conditions, [1,4-13C]butane was
oxidized on VPO3.92 at 380 °C in the presence of oxygen.
Specific ratios of oxygen and n-butane were co-condensed in
one end at the pseudo-flow reactor. Oxygen was allowed to
boil off first to oxidize the catalyst while keeping n-butane
condensed. The gaseous oxygen was then condensed in the other
end of the reactor, and [1,4-13C]butane was allowed to react on
the catalyst. The amount of oxygen in the pseudo-flow reactor
was varied, with the mole ratio of oxygen/n-butane being 1-22.
The pressure of oxygen in the pseudo-flow reactor was
calculated to be in the range of ∼0.1-2 atmospheric pressure.
The 13C MAS NMR spectra for the products of these reactions
are shown in Figure 5. No maleic acid was detected when the
mole ratio of oxygen/n-butane was 1:1. For all the other oxygen/
n-butane ratios, maleic acid was formed, and label scrambling
was observed. However, the ratio of label scrambling did not
change consistently with the ratio of oxygen/n-butane. When
the ratio of oxygen/n-butane increased from 3:1 to 10:1, the
ratio of label scrambling decreased, which agrees with our
observation that label scrambling decreases with increased
oxidation state of the catalyst (vide infra). However, label
scrambling increased dramatically when the oxygen/n-butane
ratio increased from 10:1 to 22:1, and label scrambling at
oxygen/n-butane of 22:1 was almost at the highest ratio obtained
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A R T I C L E S
Chen and Munson
Figure 6. 13C MAS NMR spectra of the reaction products of [1,4-13C]-
butane on VPO3.92 after reaction at 380 °C in the presence of gaseous
oxygen (mole ratio of oxygen/butane ) 10).
anediol and label scrambling were consistent with previous
results, except that the percent conversion of the reaction was
much higher, showing that the presence of gaseous oxygen
during the reaction enhanced the activity of the catalyst.
Effect of Oxidation State of the Catalyst on Label
Scrambling. The most unexpected result that we observed was
13C label scrambling in maleic acid. While the amount of label-
scrambled maleic acid might be very low, it could provide
valuable insight into the mechanism of the reaction. To further
understand the mechanistic significance of label scrambling, the
effects of various factors, such as oxidation state and P:V ratio
of the catalyst, reaction temperature, etc., on label scrambling
were investigated. The percentage of label exchange for maleic
acid varied between 1% and 5% when the initial oxidation state
of vanadium changed between 4.34 and 4.95. Because these
catalysts were made from different precursors and showed
different activities, it is hard to estimate the effect of the
oxidation state of vanadium on label scrambling using the data
from these catalysts. To simplify the situation, sequential
reactions of [1,4-13C]butane on VPO4.95 were performed to
investigate the effect of reduction of the catalyst on label
scrambling. Five sequential reactions were performed on
VPO4.95 at 380 °C, and the ratio of label scrambling changed
from 1:100 to 1:20 (2,3:1,4 in maleic acid) as the catalyst was
reduced, showing that the lower oxidation state of the catalyst
favors label scrambling.
Figure 5. 13C MAS NMR spectra of the reaction products of [1,4-13C]-
butane on VPO3.92 after reaction at 380 °C. The catalyst was oxidized
under different pressures of oxygen at 380 °C for 30 min before flowing
butane through the catalyst.
so far for maleic acid. However, the selectivity of the reaction
toward maleic acid showed an opposite trend. The selectivity
increased with the ratio of oxygen/n-butane and reached a
maximum when oxygen/n-butane ) 10, and then the selectivity
decreased as oxygen/n-butane increased from 10 to 22. The
cause of the increase in label scrambling at higher oxygen/n-
butane ratios is not clear. It is likely that on the catalysts that
have been oxidized by a higher concentration of oxygen
(oxygen/n-butane > 15), total oxidation of n-butane dominated
the reaction at the beginning. Total oxidation of n-butane
significantly decreased the concentration of V5+ on the surface
of the catalyst, which favored label scrambling in the subsequent
selective oxidation of n-butane to maleic acid. Again, ethylene
and methanediol were always detected. Methanediol became a
major partial oxidation product at oxygen/butane ratios of 20:1
and 22:1.
The drawback of the above-mentioned approaches is that
oxygen and n-butane react sequentially with the catalyst, which
is not exactly the case with industrial reactions in fixed-bed
flow reactors. Another experiment was performed to better
mimic true industrial conditions. A vacuum valve was added
to the pseudo-flow reactor between the reactant chamber and
the catalyst. Oxygen and [1,4-13C]butane (O2/butane ) 10) were
mixed before the valve was opened. The gaseous mixture was
then allowed to flow through VPO3.92 at 380 °C. Shown in
Figure 6 is the 13C MAS NMR spectrum of the reaction
products. The formation of maleic acid, ethylene, and meth-
Effect of P:V Ratio on Label Scrambling. The phosphorus-
to-vanadium (P:V) ratio is a key parameter in determining the
selectivity and activity of VPO catalysts.2-4,8,28,29 A slight excess
of phosphorus with respect to the stoichiometric amount (an
(28) Wenig, R. W.; Schrader, G. L. Ind. Eng. Chem. Fundam. 1986, 25, 612-
620. Wenig, R. W.; Schrader, G. L. J. Phys. Chem. 1986, 90, 6480-6488.
(29) Hannour, F. K.; Martin, A.; Kubias, B.; Lu¨cke, B.; Bordes, E.; Courtine,
P. Catal. Today 1998, 40, 263-272.
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n-Butane Oxidation on VPO Catalysts
A R T I C L E S
Figure 7. 13C MAS NMR spectra of the reaction products of [1,4-13C]-
butane on the VPO catalysts with different P:V ratios. The average oxidation
state of vanadium in these catalysts was 4.47-4.97 (Table 1). The reaction
temperature was 380 °C.
Figure 8. 13C MAS NMR spectra of the reaction products of [1,4-13C]-
butane on the precursors of the VPO catalysts with different P:V ratios.
The reaction temperature was 550 °C.
atomic P:V ratio in the range of 1.01-1.10) was necessary to
obtain an optimal catalyst.4 For this reason the effect of P:V
ratio on label scrambling was investigated. [1,4-13C]Butane was
allowed to react at 380 °C on VPO catalysts with P:V ratios
ranging from 0.9 to 1.2. 13C MAS NMR spectra of the products
of these reactions are shown in Figure 7. The ratio of label
scrambling in maleic acid and fumaric acid changed consistently
as the P:V ratio changed from 1.2 to 0.9. When P:V ) 1.2, the
ratio of label scrambling was estimated to be 1:6 (2,3:1,4 in
maleic acid and fumaric acid); when P:V ) 0.9, the peaks from
the label scrambled carbons were very small, though the signal/
noise ratio does not allow an accurate estimation of the ratio of
label scrambling. The precursors of the catalysts with different
P/V ratios (0.9-1.2) have essentially the same oxidation state
(∼4.0). However, large variations (4.47-4.97) in the oxidation
state of the catalysts were obtained after the precursors were
calcined under identical conditions (Table 1). It has been shown
that higher oxidation states result in less label scrambling, which
makes it difficult to attribute changes in the ratio of label
scrambling to P:V ratio only.
When [1,4-13C]butane reacted at 550 °C on the precursors
of the catalysts with different P:V ratios to produce butadiene,
the percentage of label-scrambled butadiene was consistently
40-45% (Figure 8). This result is consistent with our previous
experiments on VPO3.92.
Formation of Carbon Oxides and Ethylene. Previous
experiments showed that ethylene is always a byproduct in the
oxidation of n-butane on the VPO catalysts. Figure 9 shows
the 13C MAS NMR spectra of the reaction products of fully
13C-labeled butane and [1,4-13C]butane on VPO catalysts with
different P:V ratios. The ratios of the integrated peak intensities
of ethylene and carbon oxides (CO and CO2) from either fully
13C-labeled butane or [1,4-13C]butane were calculated and are
shown in Figure 10. The peak intensity of ethylene increased
Figure 9. 13C MAS NMR peaks of maleic acid (and fumaric acid), carbon
dioxide, and ethylene produced from the reaction of [1,4-13C]butane and
fully 13C-labeled butane on VPO catalysts with different P:V ratios at 380
°C: (A) fully 13C-labeled butane and (B) [1,4-13C]butane.
by a factor of 5-13 when fully 13C-labeled butane was used
instead of [1,4-13C]butane. A similar result has been observed
on VPO4.56 (Figure 3b,f) and other VPO catalysts. This result
suggests that ethylene was likely produced mainly from the two
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A R T I C L E S
Chen and Munson
Figure 11. Selectivity to maleic and fumaric acids and conversion of
n-butane on VPO catalysts with different P:V ratios. The reaction temper-
ature was 380 °C.
Figure 10. Ratios of the integrated peak intensity of COx (sum of CO and
CO2) and ethylene produced from the reaction of either [1,4-13C]butane or
fully 13C-labeled butane on the VPO catalysts with different P:V ratios.
The reaction temperature was 380 °C. COx and C2H4 correspond to carbon
oxides and ethylene produced from [1,4-13C]butane. COx* and C2H4*
correspond to carbon oxides and ethylene produced from fully 13C-labeled
butane.
The cleavage of the C2-C3 bond of butane resulted in the
formation of ethylene from either C1-C2 or C3-C4 of butane.
This is the minor mechanism for the formation of ethylene.
Ethylene formation by this mechanism might be related to the
mechanism of label scrambling, because more ethylene was
observed at higher ratios of label scrambling.
methylene carbons of n-butane, which was not observed in the
13C NMR spectra when [1,4-13C]butane was the reactant.
By further examining the 13C MAS NMR spectra for the
oxidation of fully 13C-labeled butane (Figure 9), it was found
that the ratio of peak intensity of ethylene to carbon oxides (sum
of CO2 and CO) is close to 1 for P:V ) 1.2 and 1.1. This result
suggests that there might be a relationship between the formation
of ethylene and the formation of carbon oxides on the catalysts
with P:V ) 1.2 and 1.1. Also, the peak intensity of carbon
oxides did not change significantly (fully 13C-labeled butane
versus [1,4-13C]butane) for the catalysts with P:V ratios of 1.2
and 1.1 (Figure 10), suggesting that the total oxidation of
n-butane involves mainly the two methyl groups on these two
catalysts.
Our results also suggest that there is more than one mecha-
nism for the total oxidation of butane, especially on the
nonselective catalysts (P:V ) 0.90-1.0). Fully 13C-labeled
butane yielded more carbon oxides than [1,4-13C]butane on the
catalysts with lower P:V ratio (Figure 10), which were also less
selective (Figure 11). When P:V ) 0.90, which was the least
selective catalyst, the peak intensity of carbon oxides almost
doubled for fully 13C-labeled butane compared to those for [1,4-
13C]butane, indicating that the predominant mechanism for total
oxidation of butane on this catalyst was the conversion of the
entire butane molecule to carbon oxides:
The combination of these results suggests that the formation
of ethylene and carbon oxides might be associated with each
other via the following mechanism on the catalysts with P:V
ratios of 1.2 and 1.1:
Further evidence is that the ratio of peak intensity of ethylene/
COx decreased with P:V ratio and dropped to <30% when
P:V ) 0.9 (Figure 10). This result shows that the contribution
to the yield of carbon oxides from the mechanism described in
eq 8 decreased with P:V ratio, and correspondingly, the
contribution from the mechanism involving the oxidation of all
four carbons of butane increased with P:V ratio.
Another explanation that might account for the increase of
yield of carbon oxides on the less selective but more active
catalysts is the further oxidation of ethylene and maleic acid.
Maleic acid and ethylene were reacted at 380 °C on VPO
catalyts with P:V ratios of 0.9-1.2 to investigate the possible
contributions from these secondary oxidations. Ethylene was
almost unreactive on these catalysts at 380 °C (conversion
0-6.2%). A significant amount of maleic acid (20-50%) was
oxidized to carbon oxides on the nonselective catalysts.
However, for the catalyst with P:V ) 0.9, the selectivity to
maleic acid was only 15%, which indicates that the oxidation
of maleic acid contributes only a small proportion (<10%) to
the total yield of carbon oxides, assuming 50% conversion of
maleic acid to carbon oxides. Therefore, the 30-90% increase
in peak intensity of carbon oxides due to the substitution of
fully 13C-labeled butane for [1,4-13C]butane on the catalyst with
P:V ) 1.0-0.90 can only be attributed to a total oxidation
mechanism in which all of the four carbons of butane are
The total oxidation products (CO and CO2) were formed
primarily from the two methyl groups of butane, and the two
methylene groups formed ethylene.
The P:V ratio plays a key role in determining the selectivity
and activity of the VPO catalysts. Catalysts with P:V slightly
greater than 1 had a much higher selectivity and much lower
activity than the catalysts with P:V ) 1 or P:V < 1 (Figure
11). Our results suggest that the mechanism described in eq 8
might be the major mechanism for the total oxidation of
n-butane on the more selective VPO catalysts.
Equation 8 is not the only mechanism for the formation of
ethylene, because ethylene was always detected in the 13C NMR
spectra when [1,4-13C]butane was the reactant. The observed
ethylene in the case of [1,4-13C]butane oxidation was likely
produced from the following mechanism:
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n-Butane Oxidation on VPO Catalysts
A R T I C L E S
Figure 12. 13C MAS NMR spectra of the reaction products of [2-13C]butadiene on the VPO catalysts with constant P:V ratio (P:V ) 1) but different
average oxidation state of vanadium: (a) [2-13C]butadiene; (b) VPO3.92 after reaction at 550 °C; (c) VPO4.34 after reaction at 330 °C; (d) VPO4.56 after
reaction at 330 °C; (e) VPO4.95 after reaction at 330 °C.
oxidized to carbon oxides, whereas on the catalysts with higher
P:V ratios (P:V ) 1.2 and 1.1), total oxidation of butane is
likely to involve mostly C1 and C4 of butane only.
been identified. Only maleic acid and fumaric acid were detected
as the major partial oxidation products of butane. Ethylene,
methanediol, acetylene, and acetic acid are the byproducts for
the oxidation of n-butane, but they were not seen in butadiene
oxidation. All these results suggest that the mechanisms for the
oxidation of n-butane and butadiene on VPO catalysts are
different and that butadiene is unlikely to be a primary reaction
intermediate for the oxidation of n-butane to MA.
Reaction of [2-13C]Butadiene on the VPO Catalysts.
Butadiene is one of the proposed intermediates for the conver-
sion of n-butane to maleic anhydride on VPO catalysts. We
have seen that butadiene is unlikely to be a primary reaction
intermediate because ∼40% label scrambling was observed
when n-butane reacts on VPO catalysts to form butadiene,
whereas typically only 1-7% of the maleic acid was label
scrambled. The oxidation of [2-13C]butadiene was performed
on some of the VPO catalysts to further compare the reaction
behavior of butadiene with n-butane. The 13C MAS NMR
spectra of the reaction products are shown in Figure 12. No
label scrambling in maleic acid was observed for all the catalysts
tested. The catalysts showed much higher activity for the
oxidation of butadiene compared to that for n-butane, especially
for the VPO catalysts with higher valence states. Conversion
of 20-90% was achieved at 330 °C for butadiene, while for
n-butane, the conversion was typically less than 1% at this
temperature. The selectivity of the oxidation of n-butane
decreased with increased oxidation state of the VPO catalysts,
whereas for butadiene, the selectivity to maleic acid increased
with oxidation state of the catalysts. Furthermore, the product
distribution for the oxidation of butadiene was quite different
from that of n-butane. Besides that for maleic acid, many other
peaks were also detected for the oxidation of butadiene, though
the compounds corresponding to some of these peaks have not
Summary of Results. Following is the summary of the results
obtained using our experimental protocol:
1. Maleic acid, fumaric acid, carbon oxides, ethylene, and
methanediol were detected when n-butane reacted on VPO
catalysts containing V5+. Butenes, butadiene, and furan were
formed when n-butane reacted on the VPO catalysts containing
V4+ and V3+ only.
2. Label scrambling was observed in the products when [1,4-
13C]butane reacted on VPO catalysts. When n-butane reacted
on the VPO catalysts to produce maleic acid and fumaric acid,
the percentage of label scrambled products was ∼1-15%, and
was typically around 5%. Higher P:V ratios of the catalyst and
lower reaction temperatures favored label scrambling. When
[1,4-13C]butane reacted on VPO catalysts to form butenes,
butadiene, and furan, the percentage of label-scrambled products
was around 40%, and this value did not change with P:V ratio
and reaction temperature.
3. Ethylene was always a side product for n-butane oxidation
to maleic acid on VPO catalysts. The peak intensity of ethylene
increased by a factor of 5-13 when [1,4-13C]butane was
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A R T I C L E S
Chen and Munson
substituted with fully 13C-labeled butane for the reaction on VPO
catalysts containing V5+. The yield of ethylene was almost equal
to the yield of carbon oxides for the oxidation of fully 13C-
labeled butane on VPO catalysts with P:V ratios ) 1.2 and 1.1.
For the less selective and more active catalysts (P:V )1.0, 0.95,
and 0.9), more carbon oxides were produced than ethylene.
When [1,4-13C]butane reacted on VPO catalysts to form maleic
acid, a larger ethylene peak was usually observed when there
was a relatively higher amount of label scrambling.
mechanism for C-C bond formation via organometallic inter-
mediates is reductive elimination:33
LnMR2 f LnM + R-R
(11)
M can be many transition metals, such as Ni, Pd, Au, etc.
Sometimes the reductive elimination mechanism results in a ring
closure reaction, such as the intramolecular hydrocylation of
4-pentenal catalyzed by Rh(PPh3)3Cl:34
4. Acetylene was always detected when fully 13C-labeled
butane reacted on VPO catalysts to form maleic acid but was
almost never detected when [1,4-13C]butane was the reactant.
5. Methanediol was always a byproduct when n-butane
reacted on VPO catalysts to produce maleic acid. The 13C NMR
peak intensity did not change significantly when [1,4-13C]butane
was replaced with fully 13C-labeled butane.
6. Butadiene reacted on the VPO catalysts to produce high
yields of maleic acid at much lower tempeartures than n-butane.
No ethylene, methanediol, or label scrambling was observed
for butadiene oxidation. On the other hand, many other partial
oxidation products were detected. The selectivity and conversion
for butadiene oxidation increased with the oxidation state of
the catalyst, whereas for n-butane, the selectivity and conversion
decreased with the oxidation state of the catalyst.
Transition metal complexes act as homogeneous catalysts for
these reactions. Little evidence has been reported in the literature
that similar mechanisms could operate in heterogeneous cataly-
sis, partially due to the difficulty in detecting organometallic
intermediates under heterogeneous catalytic conditions.
Discussion
Mechanism of Label Scrambling. The most interesting result
reported here is that label scrambling occurs in the products
when [1,4-13C]butane reacts on the VPO catalysts to produce
maleic acid and butadiene. Label scrambling could occur in
several different ways! The most intuitive would involve a cyclic
intermediate, which suggests that [1,4-13C]butane would produce
1,2-13C, 1,3-13C, or 2,3-13C products. It is unlikely that the label-
scrambled products are 1,2-13C compounds since no 13C-13C
coupling was observed in the 13C NMR spectra for either maleic
acid or butadiene. The label-scrambled products could be either
1,3-13C or 2,3-13C compounds since the 13C-13C coupling for
both of them is negligible. It is difficult to envision a mechanism
that produces 1,3-13C that does not also produce a significant
amount of 1,2-13C. We will therefore assume that most of the
label-scrambled products are 2,3-13C.
Neither of the two representative reaction pathways proposed
in the literature, denoted the “olefinic” route and the “alkoxide”
route,18 suggest the possibility or provide a mechanism for label
scrambling. In the olefinic route, butane is oxidized to maleic
anhydride via butenes, butadiene, and furan sequentially. No
cyclic species are proposed which might result in label
scrambling. In the alkoxide route, reaction intermediates are
bound to the catalyst via either the C-O-V or C-O-P
bond18,19 until desorption as maleic anhydride. Neither of these
mechanisms explains the observed label scrambling.
As stated previously, it is unlikely that a cyclic intermediate
is formed because of the isotopic label distribution. There is,
however, evidence that the formation of a four-membered ring
might be occurring. When C1 and C4 of the activated butane
approach each other during the formation of the new C1-C4
bond, tension along the ringlike structure begins to accumulate.
The accumulated tension is distributed along the three C-C
bonds in butane: C1-C2, C2-C3, and C3-C4. It is possible
that the C2-C3 bond of butane might break before the formation
of the C1-C4 bond, or concertedly with the formation of the
C1-C4 bond, which results in different products (Scheme 1).
If the cleavage of C2-C3 happens first, two ethylene molecules
are formed (Scheme 1a). If the cleavage of C2-C3 happens
concertedly with the formation of C1-C4, then the observed
label scrambling is achieved (Scheme 1b). In either case, no
four-membered ring is really formed. This explains why ethylene
is always observed as a side product of the reactions and usually
more ethylene was observed in the 13C NMR spectra of the
reaction products when the ratio of label scrambling was higher.
Mechanism for the Selective Oxidation of Butane on VPO
Catalysts. There have been controversies in the literature over
whether the olefinic route or the alkoxide route is the dominant
mechanism in the conversion of n-butane to maleic anhy-
dride.2,4,18 We have shown that the olefinic route is unlikely to
be the dominant reaction pathway due to the inconsistency in
the percentage of label-scrambled products when butane reacts
on the VPO catalysts to produce butadiene and maleic acid.24
Label scrambling in the products of butane oxidation on VPO
catalysts must involve the cleavage of an old C-C bond and
the formation of a new C-C bond when label scrambling
occurs. C-C bond formation is facile in certain organometallic
compounds via many mechanisms such as cyclopropanations,
vinylation, metathesis, cross-coupling, etc.30-32 A common
(32) Keim, W. In Transition Metals for Organic Synthesis; Beller, M., Bolm,
C., Eds.; Wiley-VCH Press: New York, 1998; Vol. 1, p18.
(33) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals;
John Wiley & Sons Press: New York, 1994; p 151.
(30) Brouwer, D. M.; Hogeveen, H. Prog. Phys. Org. Chem. 1972, 9, 179-
240.
(31) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic
Processes; McGraw-Hill Book Co.: New York, 1979; p 23.
(34) Sakai, K.; Ide, J.; Oda, O.; Nakamura, N. Tetrahedron Lett. 1972, 13, 1287-
1290.
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n-Butane Oxidation on VPO Catalysts
A R T I C L E S
Scheme 1
This gives some credence to an alkoxide route, but this
mechanism does not explain many of our results.
Here we propose a different mechanism for the selective
oxidation of butane based on our results. It is a single-site
organometallic mechanism. We want to be extremely clear that
we have observed no surface-bound species that would cor-
roborate our results. However, we present this mechanism as
an alternative to the alkoxide and olefinic mechanisms that have
been previously proposed. This mechanism consists of the
following steps:
1. Butane is activated, which involves the cleavage of a
methylene carbon-hydrogen bond on the catalyst surface.20
2. The activated butane rearranges to coordinate with a
vanadium ion via C1 and C4 of butane, forming a chelating
configuration as shown in Scheme 2.
3. Label scrambling and the formation of ethylene result from
the chelating configuration. It is most likely that label scrambling
occurs at the beginning of the formation of the chelating
configuration and before the activated butane is further oxidized,
as illustrated in Scheme 2a,b.
4. If there is enough V5+ in the catalyst, the activated butane
will be further oxidized by the lattice oxygen while it remains
in the chelating configuration until it is desorbed as maleic
anhydride. This is a single-site reaction, which means the
reaction intermediates do not move to other sites during the
reaction. However, the oxidation of butane to maleic anhydride
involves the insertion of 3 oxygen atoms, abstraction of 8
hydrogen atoms, and transfer of 14 electrons. These steps are
achieved on a single site by the theory proposed by Weiss et
al.35 for the catalytic oxidation of hydrocarbons on metal oxide
catalysts, though it was originally proposed for the reaction of
unsaturated hydrocarbons. The theory can be represented by
Scheme 3, where SR represents the substrate to be oxidized and
SO stands for the oxidation product. SR is adsorbed on a site
M1 having the character of an electron acceptor, generally the
cation with the highest valences.35 Oxidation of the substrate
occurs on this site. Lattice oxygen atoms are transferred to this
site to complete the oxidation. The electrons produced by the
reaction are transferred to another site, Mn, which undergoes
Figure 13. Changes in the average oxidation state of vanadium in the VPO
catalysts after reaction with [1,4-13C]butane at 380 °C.
reduction and becomes an electron-donor site, on which
molecular oxygen can be adsorbed for the reoxidation of the
system. This is actually a detailed description of the Mars and
Van Krevelen mechanism.36 Because there is no molecular
oxygen in our pseudo-flow reaction system, the above scheme
needs to be modified: there is no reoxidation of the catalyst
but just flow of electrons and oxygen atoms within the catalyst.
Evidence has been reported in the literature that all oxygen and
hydrogen atoms in the surface layer of VPO catalysts are in
fast exchange relative to the time scale of the conversion of
butane to maleic anhydride.20 This supports the single-site
mechanism in that enough oxygen can be transported to the
reaction site during butane oxidation. Our experiments also
showed that the catalysts were significantly reduced after the
reaction (Figure 13), indicating that not only the surface oxygen,
but also oxygen in the bulk structure of the catalyst, is available
to the reaction. The single-site mechanism also explains why
no intermediate products can be detected when there is sufficient
available oxygen for the oxidation of butane on VPO catalysts,2
since there is no desorption of the reaction intermediates until
maleic anhydride is formed under such reaction conditions.
5. The peak intensities of COx and ethylene are almost equal
when fully 13C-labeled butane reacts on the selective VPO
catalysts (Figures 9 and 10), which indicates that the formations
of COx and ethylene are associated with each other. We suggest
(35) Weiss, F.; Marion, J.; Metzger, J.; Cogion, J.-M. Kinet. Catal. (Engl. Transl.)
1973, 14, 32-53.
(36) Mars, P.; Van Krevelen, D. W. Chem. Eng. Sci. 1954, 3, 41-59.
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J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002 1649

A R T I C L E S
Chen and Munson
Scheme 2
Scheme 3
that this is also the result of the chelating configuration. Oxygen
insertion on C1 and C4 might be the next step after butane has
been activated (Scheme 2c). The tendency still exists for C1
and C4 to form a C-C bond after oxygen insertion has occurred,
because the molecule still maintains the chelating configuration.
However, at this point the insertion of oxygen on C1 and C4 of
butane weakens the C1-C2 and C3-C4 bonds. Therefore, when
C1 and C4 approach each other, the C1-C2 and C3-C4 bonds
are easier to break than the C2-C3 bond, which results in the
Figure 14. Label scrambling in butenes, butadiene, and furan when [1,4-
13C]butane was reacted on VPO catalyst with P:V ) 0.95 at 550 °C.
formation of an ethylene molecule consisting of C2 and C3,
and two formaldehyde or methanediol molecules (Scheme 2c).
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1650 J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002

n-Butane Oxidation on VPO Catalysts
A R T I C L E S
Figure 15. Proposed scheme for the interaction of n-butane and butadiene with the surface of the VPO catalyst. (a) Idealized vanadyl pyrophosphate
structure.2 (b) Activated n-butane in the chelating configuration, molecule forming a σ-complex with the vanadium ion. (c) Butadiene forms π-complex with
the vanadium ion on the surface of the catalyst.
The methanediol peak is always observed for the reaction of
butane on VPO catalysts containing V5+ and can be exception-
ally high under certain circumstances (Figure 10). Formaldehyde
was not detected since formaldehyde exists completely as its
hydrate (methanediol) in aqueous solution.37 This gives the
second mechanism and the main mechanism for the formation
of ethylene, because the peak intensity of ethylene increased
by a factor of 5-13 when 1,4-13C-labeled butane was substituted
by fully 13C-labeled butane. Methanediol is not a stable
compound under the reaction conditions. Most of it will be
further oxidized to CO or CO2. Because the ratio of the peak
intensity of ethylene/COx is close to 1:1 on the more selective
catalysts (Figure 9), it is likely that total oxidation of butane
on these catalysts involves mainly the oxidation and abstraction
of C1 and C4 of butane, and ethylene is the remaining part of
butane (C2 and C3) after C1 and C4 have been oxidatively
removed. It is likely that formation of acetylene also occurs by
a similar mechanism. When butane is further oxidized so that
the two methylene groups have become methine groups (Scheme
2d), the cleavage of the C1-C2 and C3-C4 bonds will result
in the formation of acetylene from C2 and C3 of butane.
Therefore, acetylene is unlikely to be detected in the 13C NMR
spectra of the reaction products using [1,4-13C]butane but was
always detected when fully 13C-labeled butane was used.
indicates the relationship between the formation of ethylene and
label scrambling as shown in Scheme 2a,b.
Differences between the Oxidations of n-Butane and
Butadiene on VPO Catalysts. The proposed mechanism for
the selective oxidation of butane on VPO catalysts is unlikely
to be applicable to butadiene due to the differences in their
reaction behavior. It has been suggested the active phases for
the selective oxidation of butane and 1-butene are (VO2)2P2O7
and â-VOPO4, respectively, and different mechanisms operate
for the oxidations of butane and 1-butene.38 On the basis of
current results, different mechanisms for the interactions of
butane and butadiene with VPO catalysts are proposed (Figure
15). The activated butane is adsorbed on the surface of the
catalyst via a chelating configuration (σ-complex, Figure 15b),
whereas butadiene forms a π-complex with V5+, which does
not need as high a temperature to form as that needed for the
activation of butane (Figure 15c). The activation of butane
involves the cleavage of a C-H bond, which apparently has a
much higher activation energy than that for the formation of a
π-complex and needs a much higher temperature for the reaction
to occur. The chelating configuration of butane results in label
scrambling and formation of ethylene, which are unlikely to be
produced by the π-complex when butadiene interacts with the
catalyst. The chelating configuration ensures that the reaction
intermediates do not move to other sites during the oxidation
until maleic anhydride is formed, which explains why no other
partial oxidation products are detected for butane oxidation. The
π-complex is likely to be much more flexible and mobile on
the surface of the catalyst. When one end of the molecule
(butadiene or butenes) forms a π-complex with a vanadium ion,
the other end of the molecule may interact with different sites
on the catalyst, which might be the reason various partial
oxidation products can be formed. The relative ease for the
formation of π-complexes means the catalyst may lose its
activity quickly if there are only a few V5+ sites on the surface
of the catalyst due to the strong absorption of olefinic species
on the surface of the catalyst.2 Therefore, a higher oxidation
state of the catalyst favors the oxidation of butenes or butadiene.
On the other hand, selective oxidation of n-butane requires V5+
to be isolated on the surface of the catalyst, so that once butane
6. Under oxygen-lean conditions (no gaseous oxygen or no
V
5+ in the catalyst), the catalyst is unable to oxidize butane to
maleic anhydride,15,16 and the reaction intermediates may desorb
as butenes, butadiene, and furan. The ratios of products
containing 1,4-13C to 2,3-13C for butenes, butadiene, and furan
were almost identical (Figure 14), suggesting that label scram-
bling occurs at the first step of butane oxidation: butane f
butenes. The observed label scrambling in butadiene and furan
is likely to result from the subsequent oxidation of label-
scrambled butenes. The same chelating configuration mechanism
as that shown in Scheme 2 is proposed to be responsible for
label scrambling of these compounds. The large amount of
ethylene observed in the 13C NMR spectra of the reaction
products when [1,4-13C]butane reacted on the VPO catalysts to
produce butenes, butadiene, and furan (Figure 4) further
(37) Ternay, A. L. Contemporary Organic Chemistry; W. B. Saunders Co.:
Philadelphia, 1979; p 682.
(38) Bordes, E. Catal. Today 1993, 16, 27-38.
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J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002 1651

A R T I C L E S
Chen and Munson
is activated by a V5+, it forms the chelating configuration in
situ with the specific vanadium ion, and is further oxidized to
maleic anhydride via the single-site organometallic mechanism.
The activated butane can be oxidized to carbon oxides by the
surrounding V5+ sites before the chelating configuration is
formed. Therefore, the oxidation state for the VPO catalysts of
best performance was found to be within 4.00-4.03,2,3,39 at
which V5+ is well isolated on the surface of the catalyst, or in
other words, V5+ sites exist as VOPO4 islands on a (VO2)2P2O7
matrix to minimize the total oxidation, as described by Centi.3
NMR studies of the reaction. The activited butane is proposed
to form a chelating configuration on the surface of the catalyst
via the coordination of C1 and C4 of butane with the vanadium
ion. The chelating configuration is maintained during the
reaction until the formation of maleic anhydride. Label scram-
bling, the formation of ethylene, and other observations are
explained using this reaction model. Butadiene is proposed to
form π-complexes with the catalyst.
Acknowledgment. The authors acknowledge the financial
support given by the National Science Foundation (CHE-
9624161) and by the Petroleum Research Foundation (PRF
50384-G5). We also thank Dr. John Bemis for making VPO3.92,
and Dr. Pinghua Liu for suggesting the possibility of organo-
metallic mechanisms.
Summary
A single-site organometallic mechanism is proposed for the
selective oxidation of n-butane on VPO catalysts based on 13
C
(39) Albonetti, S.; Cavani, F.; Trifiro`, F. Catal. ReV.-Sci., Eng. 1996, 38, 413-
438.
JA010285V
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1652 J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002