Vanadium phosphorus oxide as an efficient catalyst for hydrocarbon oxidations using hydrogen peroxide

Article abstract of DOI:10.1039/b209268a

Calcined vanadium phosphorus oxide (VPO) prepared by an organic route is found to be an active and effective catalyst for the oxidation of various alkanes such as cyclopentane, cyclohexane, n-hexane, cycloheptane, cyclooctane, cyclodecane and adamantane in acetonitrile solvent using the environmentally benign oxidant, hydrogen peroxide, where the oxidation mechanism is believed to involve a reversible V⁴⁺/V⁵⁺ redox cycle.

Full text of DOI:10.1039/b209268a

View Article Online / Journal Homepage / Table of Contents for this issue
Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Vanadium phosphorus oxide as an efficient catalyst for hydrocarbon
oxidations using hydrogen peroxide
Unnikrishnan R. Pillai and Endalkachew Sahle-Demessie*
National Risk Management Research Laboratory, Sustainable Technology Division, MS 443,
United States Environmental Protection Agency, Cincinnati, Ohio 45268, USA.
E-mail: Sahle-Demessie.Endalkachew@epa.gov; Fax: +1 513 569 7677
Received (in New Haven, CT, USA) 20th September 2002, Accepted 27th November 2002
First published as an Advance Article on the web 4th February 2003
Calcined vanadium phosphorus oxide (VPO) prepared by an organic route is found to be an active
and effective catalyst for the oxidation of various alkanes such as cyclopentane, cyclohexane, n-hexane,
cycloheptane, cyclooctane, cyclodecane and adamantane in acetonitrile solvent using the environmentally
benign oxidant, hydrogen peroxide, where the oxidation mechanism is believed to involve a reversible V4+/V5+
redox cycle.
characterized rather extensively by various techniques such
Introduction
as Raman, FT-IR, 31P NMR, XPS, LEIS, EPR, etc. The better
performance of this catalyst has been ascribed to the availabil-
ity and easy switching (reversible sequential reduction and re-
oxidation) between the different oxidation states of vanadium
on the catalyst surface. It is also known that the heterolytic
oxygen transfer catalyzed by vanadium can operate by either
high-valent peroxometal complexes or via oxometal species,
depending on the substrate. Most of the literature has been
focused only on n-butane oxidation, perhaps due to the com-
mercial importance and success of maleic anhydride produc-
tion using this catalyst. Amorphous microporous homo-
geneously mixed oxides of Ti, V, Fe, Zr, Cu, Mo, Co and Cr
on silica (TiO2–SiO2 , V2O5–SiO2 , Fe2O3–SiO2 , ZrO2–SiO2 ,
CuO–SiO2 , Mo2O5–SiO2 , Co2O3–SiO2 and Cr2O3–SiO2) have
been studied for cyclohexane oxidation using t-butyl hydroper-
oxide as the oxidant, however, with less than 10% conver-
sion.18 We have very recently accomplished an unprecedented
and efficient oxidation of cyclohexane over VPO catalyst using
H2O2 in acetonitrile under a nitrogen atmosphere.19 The cata-
lyst has not been well-explored for other hydrocarbon oxida-
tions, which is a very important and challenging area. This
paper focuses on this area, viz., the utility of a well-character-
ized catalyst for a new and wide-ranging application such as
oxidation of hydrocarbons, especially the C5–C10 cycloalk-
anes, in an environmentally benign oxidation protocol invol-
ving aqueous hydrogen peroxide in nitrogen atmosphere.
Oxidation of hydrocarbons is a key process in the chemical
industry due to the wide ranging utility of the ensuing functio-
nalized compounds as raw materials and intermediates in
industrial and pharmaceutical chemistry.1 Small, partially
oxygenated hydrocarbons are used as building blocks in the
manufacture of plastics and synthetic fibers. For example,
oxidation products of cyclohexane, cyclohexanone and cyclo-
hexanol are important raw materials for the production of adi-
pic acid and caprolactam used for the manufacture of nylon.2
Current processes for the production of these highly desired
oxygenates in many cases involve stringent conditions, which
may include high temperatures and pressures, strong acids,
free radicals and corrosive oxidants.3,4 Although these pro-
cesses are currently being utilized they have low energy efficien-
cies as well as generating environmentally hazardous waste and
by-products. Increased environmental concerns in recent years
call for environmentally benign oxidants like molecular oxygen
or hydrogen peroxide rather than organic peroxides and stoi-
chiometric metal oxides, which have been widely employed
until now.4 The importance of hydrocarbon oxidation is evi-
denced by the large number of research papers appearing on
this subject over a variety of homogeneous and heterogeneous
catalytic systems and employing different oxidants such as
hydrogen peroxide, t-butyl hydroperoxide and molecular oxy-
gen. Notable catalysts for this oxidation include Na–GeX zeo-
lite, titanium-containing mesoporous materials such as TS-1
and Ti-MCM-41 and metal-containing AlPO redox molecular
sieves.5–8 A recent review article by Schuchardt et al. acknowl-
edges that efficient cyclohexane oxidation continues to be a
challenge in spite of several research attempts.9 Some attempts
have been made for the oxidation of cyclohexane10 and
cyclooctane11 under supercritical CO2 medium, but with not
much improvement in the conversion and selectivity. A few
other studies describe the oxidation of other cycloalkanes such
as cyclopentane, cycloheptane, cyclooctane and adaman-
tine.12–16 Therefore, a major challenge in this field is to find
reaction pathways that afford the primary product with high
selectivity at high conversion of the hydrocarbon.
Results and discussion
The VPO catalysts are prepared by a variety of methods, all of
which, however, eventually result in the same active phase. The
most important of the synthesis steps is the initial preparation
of the active phase precursor, vanadyl hydrogen phosphate
hemihydrate, (VO)HPO4ꢀ0.5H2O, which is then thermally
decomposed and activated inside a reactor. The two main
methods for the preparation of this precursor involve the
reduction of V5+ (V2O5) to V4+ either in water by HCl or
hydrazine or the reduction in an organic phase (2-butanol) in
the presence of another reducing agent such as benzyl alcohol.
The catalyst prepared by the organic route is rather more pop-
ular. Therefore, vanadium phosphorus oxide (VPO) with a P:V
Vanadium phosphorus oxide (VPO) is an efficient catalyst
for the vapor phase n-butane oxidation to maleic anhydride.17
This catalyst has been synthesized by various methods and
DOI: 10.1039/b209268a
New J. Chem., 2003, 27, 525–528
525
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
View Article Online
various cycloalkanes over any given catalyst has not been
ratio of 1.1 was prepared by the organic route. XRD spectra of
the catalyst after calcination showed high intensity peaks at 2y
values of 23.1ꢁ, 28.4ꢁ and 29.9ꢁ, characteristic of the presence
of the (VO)2P2O7phase, and also low intensity peaks at 2y
values of 22.0ꢁ, 26.0ꢁ and 28.9ꢁ, characteristic of the VOPO4
phase.17 This shows that the vanadium in the calcined catalyst
sample was predominantly in the V4+ state with a small
amount of V5+ species. The mechanism of formation of this
phase involves the solubilization of V5+ through the formation
of vanadium alcoholate followed by its reduction to V2O4 by
the organic alcohol (benzyl alcohol). On the addition of
H3PO4 , V2O4 reacts with H3PO4 to form (VO)HPO4ꢀ0.5
H2O at the solid–liquid interface, which on calcination forms
(VO)2P2O7 .20 The BET surface area of the calcined VPO cat-
reported earlier and hence is very promising. This oxidation
protocol is also much cleaner than the traditional oxidation
processes using stoichiometric oxidants and peracids, which
would produce copious amounts of undesired by-products.
The catalyst preparation is also relatively simple and does
not involve the use of any expensive precursor materials. The
percentage of H2O2 consumed is determined by iodometric
titration22 after the end of each experiment and found to be
in the range of 93–95%. This suggests that more than a stoi-
chiometric amount of H2O2 is utilized in the reaction, which
could be due to the non-selective thermal decomposition of
H2O2 to oxygen and water at the reaction temperature.23
The VPO catalyst is, however, soluble in the reaction mix-
ture, forming a homogeneous solution and therefore not reco-
verable for recycling. Nevertheless, we have demonstrated that
cyclohexane oxidation could be successfully carried out repeat-
edly by the addition of new batches of substrate along with the
solvent and oxidant to the previous reaction mixture.19 The
same is found to be true also in the case of the other substrates
studied here. The activity is found to decrease with successive
batches, however, the reaction could be successfully completed
by extending the reaction time.
XRD analyses show that the calcined VPO catalysts is
composed mainly of the vanadyl pyrophosphate ((VO)2P2O7)
phase with a small amount of the VOPO4 phase as if the latter
is dispersed on a vanadyl pyrophosphate (VPP) support. P/V
ratios in the precursor higher than the stoichiometric one sta-
bilize the (VO)2P2O7 phase from re-oxidation in the reactant
atmosphere as well as during calcination in air at high tem-
peratures.20 VPO catalysts have a higher surface P/V ratio
than the bulk P/V ratio, suggesting that they may be ter-
minated by a distorted VPP structure, where the excess
amount of phosphorus is positioned between the vanadyl units
and the phosphate groups.24 This may lead to the generation
of more Lewis (V4+) and Bronsted (P–OH) acid sites on the
catalyst surface, which are stabilized due to the large cohesion
of VO6 pseudo-octahedra in the bi-dimensional framework at
the surface. This stability is more pronounced in catalysts pre-
pared in organic media. It is also observed that maximum cat-
alytic performance requires a certain degree of disorder in
the VPP lattice.25 This is further supported by the improved
catalytic efficiency observed for the tribomechanically treated
crystallized VPP catalysts.26 Catalysts calcined at lower tem-
peratures ( < 750 ꢁC) contain V5+ microdomains on the surface
and isolated V5+ sites in the sublayers of the vanadyl pyrophos-
phate structure.27 It may also be possible that different phases
present in the calcined catalyst can cooperate to enhance the
catalytic activity.
It is therefore expected that the mechanism of oxidation
over VPO catalysts may involve a reversible V4+/V5+ redox
cycle as illustrated in Scheme 1.19,28 Excess phosphorus at
the surface stabilizes the pyrophosphate framework (V4+) so
that the V4+/V5+ redox cycle is maintained in a dynamic equi-
librium. This is why the reaction works better under nitrogen
atmosphere than in the presence of air/O2 as the availability
of oxygen hinders the reduction of V5+ species to V4+. The
decrease in the conversions for methyl cyclohexane (entry 3,
Table 1) and isopropyl cyclohexane (entry 4, Table 1) is in
accordance with the expected cyclic intermediate on the cata-
lyst surface as per the above mechanism and the effect of steric
restrictions to form the transition state complex leading to the
oxygenated products. The presence of excess phosphorus pre-
vents the oxidation of the surface (and the bulk) to the
b-VOPO4 phase, which has a deleterious effect on catalyst per-
formance.29 The redox mechanism is also evident from the
color of the reaction mixture wherein the initial green color
of the reaction mixture (V4+) slowly changes to greenish-
yellow to brown (V5+), which further changes to green
(V4+). In other words, it appears that dispersed V5+ species
alyst is found to be 10 m2 gꢂ1
.
We have earlier found that calcined VPO catalyst prepared
by the organic route is highly active for the oxidation of cyclo-
hexane using hydrogen peroxide and acetonitrile at 65 ꢁC
under nitrogen atmosphere where the maximum activity is
obtained at a substrate-to-catalyst ratio of 385.19 Acetone is
also found to be another successful solvent. However, the reac-
tion is much slower in solvents such as 1,4-dioxane and
methanol whereas other solvents such as dichloromethane,
methyl t-butyl ether, tetrahydrofuran and dimethyl sulfoxide
are not found to be useful. Acetonitrile activates H2O2 by
forming a perhydroxyl anion (OOHꢂ), which nucleophilically
attacks the nitrile to generate a peroxycarboximidic acid inter-
mediate (Scheme 1, I), which is a good oxygen transfer agent.21
Acetone also activates H2O2 in a similar way but to a lesser
extent than acetonitrile. In addition, acetonitrile and acetone
have a comparatively good solubility power for both the
organic substrate as well as the aqueous H2O2 .
The use of the VPO catalysts for the oxidation of other simi-
lar substrates was explored by extending the reaction to a vari-
ety of other alkanes such as cyclopentane, methyl cyclohexane,
isopropyl cyclohexane, n-hexane, cycloheptane, cyclooctane, n-
octane, cyclodecane and adamantane; the results are presented
in Table 1. Apparently, the VPO catalysts are effective for the
oxidation of many of these substrates, which are usually diffi-
cult to accomplish. The oxidation protocol using VPO catalyst,
however, is not useful for the selective oxidation of linear
alkanes. For example, n-hexane gets over-oxidized (entry 5)
whereas n-octane undergoes negligible oxidation (entry 8).
On the other hand, cycloalkanes show very encouraging
results. Approximate turnover numbers are also estimated on
the assumption that all the vanadium in the catalyst sample
was in the form of (VO)2P2O7 . The presence of a substituent
in the carbon ring decreases the reactivity (entries 3 and 4).
The higher the size of the substituent, the lower is the conver-
sion. Most of the substrates studied formed their correspond-
ing ketones as the major product with negligible or no
formation of the respective alcohol, except cyclohexane (entry
2) and adamantane (entry 10). To the best of our knowledge,
this kind of effective oxidation protocol for the oxidation of
Scheme 1
526
New J. Chem., 2003, 27, 525–528

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
View Article Online
a
Table 1 Oxidation of various hydrocarbons over VPO catalysts using H2O2 under N2
% Selectivity
Entry
1b
Alkane
Products
T/ꢁC
Time/h
% Conversion
TON
A
B
50
4
8
23
35
48
70
84
96
16
27
36
2
22
33
46
135
162
185
31
52
69
4
–
–
–
100
100
100
51
20
4
2
3
4
65
65
70
49
8
50
50
50
52
49
50
50
20
4
50
8
48
51
20
4
–
–
–
100
100
100
00
12c
100
90d
93d
76d
78d
78d
–
8
4
8
20
4
8
15
19
154
88
106
108
87
90
90
–
5
6
65
70
10
80
46
55
56
45
47
47
0
100
22
–
8
4
8
–
20
4
–
7
8
70
70
12
–
8
20
4
–
–
8
4
8
53
49
–
47
51
20
4
6
12
40
46
54
25
33
48
9
21
24
28
13
17
25
100
100
100
27
31f
30f
70
70
8
–
20
4
–
10e
73
69
49
8
20
a
Substrate ¼ 12.5 mmol, acetonitrile ¼ 10 mL, catalyst ¼ 10 mg, H2O2 ¼ 7 g. b Catalyst ¼ 20 mg. c Remaining acids (formic acid ¼ 30%, acetic
d
e
f
acid ¼ 11%, propanoic acid ¼ 26%). Remaining acid products. Acetonitrile ¼ 20 mL. Remaining adamantane-1,3-diol, TON ¼ turn over
number, number of molecules of product formed per active site.
in combination with (VO)2P2O7 (V4+) is important for the oxi-
dation of cyclohexane whereas the V5+ act as a dynamic oxi-
dizing center. Calcination of the catalyst at high temperature
results in more b-VOPO4 phase, which is detrimental to the
catalytic activity.
In summary, the calcined VPO catalyst with a P/V ratio 1.1
is found to be a very active catalyst for the oxidation of
alkanes, especially cycloalkanes such as cyclopentane, cyclo-
hexane, cycloheptane, cyclooctane, cyclodecane and adaman-
tane, to their respective oxygenates using hydrogen peroxide
under nitrogen atmosphere. The active center for the oxidation
is believed to be (VO)2P2O7 sites (V4+) in combination with
dynamic V5+ sites involving a reversible redox cycle. The
VPO catalyst can also be successfully reused for the reaction.
of the catalyst was determined using N2 adsorption at ꢂ196 ꢁC
(77 K) using a Micromeritics Auto Chem. II instrument
(Model 2920).
Oxidations of various alkanes were conducted in a 100 mL
round-bottomed flask fitted with a reflux condenser and a
magnetic stirrer. In a typical reaction procedure, 12.5 mmol
substrate was mixed with 10 mg of calcined VPO catalyst, 10
mL solvent and 50 mmol of 30% hydrogen peroxide and the
mixture was kept at 50–70 ꢁC using a heating mantle with
vigorous stirring under nitrogen atmosphere. The reaction
mixture was sampled at regular intervals and the aliquot
extracted with ether. The organic layer was analyzed by a
Hewlett–Packard 6890 gas chromatograph using an HP-5
5% phenyl methyl siloxane capillary column (30 m ꢃ 20
mm ꢃ 0.25 mm) and a quadrupole mass filter equipped HP
5973 mass selective detector. The aqueous phase was also ana-
lyzed to detect any acid formation using an HPLC (Finnigan-
mat-LCQ) system using a Supelcogel2 H 59346 column (25
cm ꢃ 4.6 mm) with a water–phosphoric acid solution as the
eluent. The conversion and product selectivities reported are
based on the GC yield. Quantification of the oxygenated
products was obtained using a multi-point calibration curve
for each product.
Experimental
Vanadium phosphorus oxide (VPO) catalyst with a P/V of 1.1
was prepared by refluxing an appropriate quantity (10 g) of
V2O5 in a mixture of isobutanol (30 mL) and benzyl alcohol
(15 mL) for 12 h, followed by addition of the required quantity
of 85% H3PO4 (P/V ¼ 1.1) and refluxing for a further 6 h to
give a light green precipitate.17,20 The precipitate was filtered
off, dried at 110 ꢁC overnight and then calcined in air at
400 ꢁC for 4 h.
The VPO catalyst was characterized by X-ray diffraction
analysis using a Siemens D5000 diffractometer with a Cu-Ka
radiation running at 40 kV/30 mA in the 2y range of 10ꢁ to
80ꢁ with a step size of 0.05ꢁ. The single-point BET surface area
Acknowledgements
URP is a postgraduate research participant at the National
Risk Management Research Laboratory administered by the
Oak Ridge Institute for Science and Education through an
New J. Chem., 2003, 27, 525–528
527

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
View Article Online
13 J. K. Beattie, S. Macleman and A. F. Masters, Inorg. Chim. Acta,
1999, 294, 99.
14 J. Perutka and A. E. Martell, Anal. Chim. Acta, 2001, 435, 385.
15 I. Muzart and A. N. Ajjou, J. Mol. Catal., 1993, 84, L15.
16 M. M. T. Khan and R. S. Shukla, J. Mol. Catal., 1992, 77, 221.
17 (a) G. Centi (Ed.), Vanadyl Pyrophosphate Catalysts, Special issue
of Catal. Today, 1993, 16, 1; (b) F. Cavani and F. Trifiro, Chem-
tech, 1994, 18; (c) F. Trifiro, Catal. Today, 1993, 16, 91.
18 (a) S. Klein, S. Thorimbert and W. F. Maier, J. Catal., 1996, 163,
476; (b) R. S. da Cruz, M. M. Dauch, U. Schuchardt and R.
Kumar, Stud. Surf. Sci. Catal., 2000, 130, 1037.
interagency agreement between the US Department of Energy
and the US Environmental Protection Agency. We also thank
Mr. Julius Enriquez for his technical assistance.
References
1
(a) D. H. R. Barton, M. J. Gastinger and W. B. Motherwell,
J. Chem. Soc., Chem. Commun., 1983, 731; (b) J. M. Thomas,
Nature (London), 1985, 314, 669; (c) T. Ito and J. H. Lunsford,
Nature (London), 1985, 314, 721.
19 U. R. Pillai and E. Sahle-Demessie, Chem. Commun., 2002, 2142.
20 F. Trifiro, Catal. Today, 1998, 41, 21.
´
U. Schuchardt, W. A. Carvalho and E. V. Spinace, Synlett, 1993,
2
3
4
5
6
10, 713.
(a) K. U. Ingold, Aldrichim. Acta, 1989, 22, 69; (b) J. C. Russell,
US Pat. 3 665028, 1972 (Halcon).
R. A. Sheldon and J. K. Kochi, Metal-Catalyzed Oxidation of
Organic Compounds, Academic Press, New York, 1981.
21 (a) S. Ueno, K. Yamaguchi, K. Yoshida, K. Ebitani and K.
Kaneda, Chem. Commun., 1998, 295; (b) K. Yamaguchi, K.
Ebitani and K. Kaneda, J. Org. Chem., 1999, 64, 2966; (c)
K. Yamaguchi, K. Mori, T. Mizugaki, K. Ebitani and K. Kaneda,
J. Org. Chem., 2000, 65, 6897; (d ) U. R. Pillai, E. Sahle-Demessie
and R. S. Varma, Tetrahedron Lett., 2002, 43, 2909.
22 A. I. Vogel, A Text Book of Quantitative Inorganic Analysis
Including Elementary Instrumental Analysis, John Wiley and Sons
Inc., New York, 1961, p. 363.
ˆ
V. I. Parvulescu, D. Dumitriu and G. Poncelet, J. Mol. Catal. A:
Chem., 1999, 140, 91.
´
(a) E. V. Spinace, H. O. Pastore,U andSchuchardt, J. Catal., 1995,
157, 631; (b) M. H. Zahedi-Niaki, M. P. Kapoor and S.
Kaliaguine, J. Catal., 1998, 177, 231.
ˆ
23 V. I. Parvulescu, D. Dumitriu and G. Poncelet, J. Mol. Catal. A:
Chem., 1999, 140, 91.
7
8
9
W. A. Carvalho, P. B. Varaldo, M. Wallau and U. Schuchardt,
Zeolites, 1997, 18, 408.
R. Raja, G. Sankar and J. M. Thomas, J. Am. Chem. Soc., 1999,
121, 11 926.
U. Schuchardt, D. Cardoso, R. Sercheli, R. Pereira, R. S. da Cruz,
24 W. P. A. Jansen, M. Ruitenbeck, A. W. D. von der Gon, J. W.
Geus and H. H. Brongersma, J. Catal., 2000, 196, 379.
25 A. Bruckner, B. Kubias and B. Lucke, Catal. Today, 1996,
¨
32, 215.
¨
´
M. C. Guerreiro, D. Mandelli, E. V. Spinace and E. L. Pires,
Appl. Catal. A: Gen., 2001, 211, 1.
26 J. Haber, V. A. Zazhigalov, J. Stoch, L. V. Bogutskaya and I. V.
Batcherikova, Catal. Today, 1997, 33, 39.
27 M. Abon, J. M. Herrmann and J. C. Volta, Catal. Today, 2001,
71, 121.
28 D. Wang, H. H. Kung and M. A. Barteau, Appl. Catal. A: Gen.,
2000, 201, 203.
10 (a) E. Shale-Demessie, M. A. Gonzalez, J. Enriquez and Q. Zhao,
Ind. Eng. Chem. Res., 2000, 39, 4858; (b) P. Srinivas and M.
Mukhopadhyay, Ind. Eng. Chem. Res., 1994, 33, 3118.
11 N. Theyssen and W. Leitner, Chem. Commun., 2002, 410.
12 J. Haber, L. Matachowski, K. Pamin and J. Poltowicz, J. Mol.
Catal. A: Chem., 2000, 162, 105.
29 E. A. Lombardo and L. M. Cornaglia, Appl. Catal. A: Gen., 1995,
127, 125.
528
New J. Chem., 2003, 27, 525–528