Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis
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References 559
Acknowledgments
The author would like to thank Dr Maria - Luisa Palacios and Dr Richard Harris.
References
1 Allen , G.C. and Tempest , P.A. ( 1982 )
Journal of the Chemical Society, Dalton
Transactions , 2169 .
2 Allen , G.C. and Holmes , N.R. ( 1995 )
Journal of Nuclear Materials , 223 , 231 .
3 Cordfunke , E.H.P. ( 1969 ) The Chemistry
of Uranium , Elsevier , New York .
4 Lynds , L. , Young , W.A. , Mohl , J.S. and
Liebowitz , G.G. ( 1962 ) Nonstoichiometric
compounds , ACS Advances in Chemistry
Series (ed. R.F. Gould ), American
Chemical Society , vol. 39 , p. 58 .
5 Vanlierde , W. , Pelsmaekers , J. and
Lecocq - Robert , A. ( 1970 ) Journal of
Nuclear Materials , 37 , 276 .
6 Belbeoch , B. , Boivineau , J.C. and Perio ,
P. ( 1967 ) Journal of Physics and Chemistry
of Solids , 28 , 1267 .
7 Westrum , E.F. and Gronvold , F. ( 1962 )
Journal of Physics and Chemistry of Solids ,
23 , 39 .
8 Hoekstra , T.H.R. , Siegel , S. and
Gallagher , F.X. ( 1970 ) Journal of
Inorganic and Nuclear Chemistry , 32 ,
3237 .
9 Hoekstra , H.R. , Siegel , S. and Charpin ,
P. ( 1968 ) Journal of Inorganic and Nuclear
Chemistry , 30 , 519 .
10 Rundle , R.E. , Baeziger , N.C. , Wilson ,
A.S. and MacDonald , R.A. ( 1948 ) Journal
of the American Chemical Society , 70 , 99 .
11 Loopstra , B.O. ( 1964 ) Acta
Crystallographia , 17 , 651 .
12 Ball , R.G.J. and Dickens , P.G. ( 1991 )
Journal of Materials Chemistry , 1 , 105 .
13 Loopstra , B.O. ( 1970 ) Acta
Crystallographia , B26 , 656 .
14 Hoekstra , H.R. and Siegel , S. ( 1961 )
Journal of Inorganic and Nuclear
Chemistry , 18 , 154 .
15 Greaves , C. and Fender , B.E.F. ( 1972 )
Acta Crystallographia , 28 , 3609 .
16 Debets , P.C. ( 1966 ) Acta Crystallographia ,
21 , 589 .
17 Loopstra , B.O. , Taylor , J.C. and Waugh ,
A.B. ( 1977 ) Journal of Solid State Chemistry ,
20 , 9 .
18 Wait , E. ( 1955 ) Journal of Inorganic and
Nuclear Chemistry , 1 , 309 .
19 Kovba , L.M. , Viadavskii , L.M. and Labut ,
E.L. ( 1963 ) Zhurnal Strukturnoi Khimii , 4 ,
627 .
20 Siegel , S. , Hoekstra , H.R. and Sherry , E.
( 1966 ) Acta Crystallographia , 20 , 292 .
21 Allen , G.C. and Tempest , P.A. ( 1983 )
Journal of the Chemical Society, Dalton
Transactions , 267 , 7 .
22 Colmenares , C.A. ( 1984 ) Progress in Solid
State Chemistry , 12 , 257 .
23 Willis , B.T.M. ( 1964 ) Proceedings of the
British Ceramic Society , 1 , 9 .
24 Willis , B.T.M. ( 1978 ) Acta Crystallographia ,
A34 , 88 .
25
Catlow , C.R.A. ( 1977 ) Proceedings of the
Royal Society of London , A353 , 533 .
26 James , J.H. ( 1922 ) Chemical and
Metallurgical Engineering , 26 , 209 .
27 Hall , W.J. ( 1926 ) US Patent 1,595,299 ,
August 10, 1926.
28 Craver , A.E. ( 1927 ) US Patent 1,636,954 ,
July 26, 1927.
29 Parks , W.G. and Katz , J. ( 1933 ) Industrial
and Engineering Chemistry , 28 , 3193 .
30 Craver , A.E. ( 1927 ) US Patent 1,636,952 ,
July 26, 1927.
31 Wietzel , R. and Pfaundler , C. ( 1932 ) US
Patent 1,844,998 , February 16, 1932.
32 Nozaki , F. and Ohki , K. ( 1972 ) Bulletin of
the Chemical Society of Japan , 45 , 3473 .
33 Hutchings , G.J. , Heneghan , C.S. , Hudson ,
I.D. and Taylor , S.H. ( 1996 ) Nature , 384 ,
341 .
34 Hutchings , G.J. , Heneghan , C.S. ,
Hudson , I.D. and Taylor , S.H. ( 1996 )
Heterogeneous Hydrocarbon Oxidation
(eds B.K. Warren and S.T. Oyama ), ACS
Symposium Series , ACS , Washington, DC ,
vol. 638 , p. 58 .
560 13 Heterogeneous Catalysis by Uranium Oxides
35 Taylor , S.H. , Heneghan , C.S. , Hutchings ,
G.J. and Hudson , I.D. ( 2000 ) Catalysis
Today , 59 , 249 .
36 Taylor , S.H. and O ’ Leary , S.R. ( 2000 )
Applied Catalysis B , 25 , 137 .
37 Harris , R. , Hutchings , G.J. , Boyd , V.J.
and Taylor , S.H. ( 2002 ) Catalysis Letters ,
78 , 369 .
38 Heneghan , C.S. , Hutchings , G.J. ,
O ’ Leary , S.R. , Taylor , S.H. , Boyd , V.J.
and Hudson , I.D. ( 1999 ) Catalysis Today ,
54 , 3 .
39 Hofer , L.J.E. and Anderson , R.B. ( 1964 )
US Patent 3,140,148 , July 7, 1964.
40 Madhavaram , H. and Idriss , H. ( 1997 )
Studies in Surface Science and Catalysis ,
110 , 265 .
41 Madhavaram , H. and Idriss , H. ( 1999 )
Journal of Catalysis , 184 , 553 .
42 Grasselli , R.K. , Suresh , D.D. and Knox ,
K. ( 1970 ) Journal of Catalysis , 18 , 356 .
43 Grasselli , R.K. and Suresh , D.D. ( 1972 )
Journal of Catalysis , 25 , 273 .
44 Grasselli , R.K. and Callahan , J.L. ( 1969 )
Journal of Catalysis , 14 , 93 .
45 Delobel , R. , Baussart , H. , Le Bras , M. , Le
Maguer , D. and Leroy , J.M. ( 1982 )
Journal of the Chemical Society, Faraday
Transactions , 78 , 485 .
46 Sykora , R.E. , King , J.E. , Illies , A.J. and
Albrecht - Schmitt , T.E. ( 2004 ) Journal of
Solid State Chemistry , 177 , 17170 .
47
Van der Baan , H.S. , Steenhof , J.G. , De
Jong , J.G. and Guffens , C.H.E. ( 1972 )
Journal of Catalysis , 26 , 401 .
48 Steenhof , J.G. , Guffens , C.H.E. and Van
der Baan , H.S. ( 1973 ) Journal of Catalysis ,
32 , 149 .
49 Collette , H. , Derouane , E.G. , Verbist , J.J. ,
Deremince - Mathieu , V. and Nagy , J.B.
( 1987 ) Journal of the Chemical Society,
Faraday Transactions , 83 , 1263 .
50 Ai , M. ( 1991 ) Heterogeneous Catalysis and
Fine Chemicals II , Elsevier Science
Publishers B.V , Amsterdam , pp. 423 – 30 .
51 Zhu , J. and Anderson , S.L.T. ( 1989 )
Journal of the Chemical Society, Faraday
Transactions , 85 , 3629 .
52 Dowden , D.A. and Walker , G.T. ( 1971 )
UK Patent 1,244,001 , August, 1971.
53 Dowden , D.A. , Schnell , C.R. and Walker ,
G.T. ( 1968 ) Proceedings of the 4th
International Congress on Catalysis,
Moscow , Paper 62, pp. 201 – 15 .
54 Walker , G.S. , Lapszewicz , J.A. and Foulds ,
G.A. ( 1994 ) Catalysis Today , 21 , 519 .
55 Corma , A. , Corber á n , V.C. and Kremeric ,
G. ( 1984 ) Industrial & Engineering Chemistry
Product Research and Development , 23 , 546 .
56 Taylor , S.H. , Hutchings , G.J. , Palacios , M.
and Lee , D.F. ( 2003 ) Catalysis Today , 81 ,
171 .
57 Teng , Y. and Kobayashi , T. ( 1998 ) Catalysis
Letters , 55 , 33 .
58 Lee , A.F. , Sears , P.J. , Pollington , S.D. ,
Overton , T.L. , Wells , P.B. and Lee , D.F.
( 2000 ) Catalysis Letters , 70 , 183 .
59 Choudhary , V.R. , Jha , R. and Jana , P.
( 2007 ) Green Chemistry , 9 , 267 .
60 Madhavaram , H. and Idriss , H. ( 2004 )
Journal of Catalysis , 224 , 3589 .
61 Madhavaram , H. , Buchanan , P. and Idriss ,
H. ( 1997 ) Journal of Vacuum Science and
Technology A , 15 , 1685 .
62 Heynen , H.W.G. and Van der Baan , H.S.
( 1974 ) Journal of Catalysis , 34 , 167 .
63 Heynen , H.W.G. , Van der Baan , H.S. and
Camp - van Berkel , C.G.M.M. ( 1977 ) Journal
of Catalysis , 48 , 386 .
64 Berry , F.J. , Murray , A. and Parkyns , N.D.
( 1993 ) Applied Catalysis A , 100 , 131 .
65 Berry , F.J. , Murray , A. and Steel , A.T.
( 1998 ) Journal of the Chemical Society,
Faraday Transactions , 84 , 2783 .
66 Pollington , S.D. , Lee , A.D. , Overton , T.L. ,
Sears , P.J. , Wells , P.B. , Hawley , S.E. ,
Hudson , I.D. , Lee , D.F. and Ruddock , V.J.
( 1999 ) Journal of the Chemical Society,
Chemical Communications , 725 .
67 Campbell , T. , Newton , M.A. , Boyd , V. , Lee ,
D.F. and Evans , J. ( 2005 ) Journal of
Physical Chemistry B , 109 , 2085 .
68 Campbell , T. , Newton , M.A. , Boyd , V. , Lee ,
D.F. and Evans , J. ( 2006 ) Journal of
Molecular Catalysis A , 245 , 62 .
69 Nicklin , T. and Burgess , K.H. ( 1970 ) GB
Patent 1,198,991 , July 15, 1970.
70 Nicklin , T. , Clack , J. and Burges , K.H.
( 1971 ) US Patent 3,630,967 , January 1,
1971.
71 Nicklin , T. and Farrington , F. ( 1974 ) US
Patent 3,847,836 , November 12, 1974.
72 Gavin , D.G. ( 1974 ) US Patent 3,974,098 ,
1974.
73 Gordeva , L.G. , Moroz , Y.I. Aristov , E.M. ,
Rudina , N.A. , Zaikovskii , V.I. , Tanashev ,
Y.Y. and Parmon , V.M. ( 1995 ) Journal of
Nuclear Materials , 218 , 202 .
Heteropolyoxometallate Catalysts for Partial Oxidation
Jacques C. V é drine and Jean - Marc M. Millet
561
Metal Oxide Catalysis. Edited by S. David Jackson and Justin S. J. Hargreaves
Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31815-5
14
14.1
Introduction
Polyoxometallates ( POMs ) are a large class of nanosized transition metal – oxygen
clusters [1] . Their structures, sizes and properties correspond to compounds inter-
mediate between molecules and oxides. They are composed of anions having
metal – oxygen octahedra as the basic structural unit. The octahedra are linked
together to yield a stable and compact skeleton of polymeric oxoanions formed by
the condensation of mononuclear oxoanions (isopolyanions) or of more than two
different mononuclear oxoanions (heteropolyanions) in acidic media as shown in
the equation:
pq z z
r
xr
n
mn
pqs
px mq zs
XO MO H X M O H O
[]
+
[]
+→ +
()
−−
+⋅+⋅−⋅
()
22
2
2
with M = metal (designated as addenda atom), X = heteroatom, x = the valency of
the heteroatom, m = the valency of the addenda atom and s + ( z /2) = n · q + r · p
(oxygen balance). The elements M that can act as addenda atoms in heteropoly or
isopolyanions are limited to those with both a favorable combination of ionic
radius and charge and the ability to form d π – p π M
–
O bonds. However, there are
no such restrictions for the heteroatom X. The most common addenda atoms are
molybdenum or tungsten and, less frequently, tantalum, vanadium and niobium,
or mixtures of these elements in their highest oxidation state (d
0
) [2] . In principle
all the elements of the periodic table could act as heteroatoms, though the most
usual are P
V
, As
V
, Si
IV
, Ge
IV
, Ce
IV
, Th
IV
, B
3+
, Co
3+
, Al
3+
, Cr
3+
, and so on. The counter
ions, required to charge balance the anions, may be protons, alkali metals or other
metal cations.
More than twenty types of structure, incorporating from four to forty metal
atoms and from one to nine heteroatoms are known. There are fi ve general types
of POM, namely (i) the “ Lindqvist ” structure ( O
h
symmetry of six MoO
6
edge -
shared octahedra) [M
6
O
19
]
n −
; (ii) the “ Keggin ” structure, α - [(XO
4
)M
12
O
36
]
n −
( T
d
sym-
metry of four M
3
O
13
groups of three MO
6
octahedra), (iii) the “ Dawson ” structure
562 14 Heteropolyoxometallate Catalysts for Partial Oxidation
α - [(XO
4
)
2
M
12
O
54
]
n −
( D
3 h
symmetry of two “ Keggin ” fragments α - XM
9
O
34
), (iv) the
“ Anderson ” structure [H
x
(XO
6
)M
6
O
18
]
n −
( D
3 h
symmetry planar arrangement of
seven edge - shared MO
6
octahedra), (v) the [(XO
12
)M
12
O
30
] structure ( I
h
symmetry
with an XO
12
icosahedron surrounded by six equivalent M
2
O
9
groups of face -
shared MO
6
octahedra, linked together by corner - sharing tetravalent cations). A
schematic drawing of the fi rst four types is presented in Figure 14.1 .
The most common compounds, particularly for catalytic application, belong to
the 12 series (M/X = 12) and are Keggin - type heteropolyacid s ( HPA ), as they are
the most stable, are more easily available and have been studied in more detail.
The well defi ned Keggin structure comprises a central XO
4
tetrahedron surrounded
by twelve edge - and corner - sharing metal – oxygen octahedra (MO
6
) units as shown
in Figure 14.2 . They contain heterododecametallate anions with the formula
Figure 14.1 Types of polyoxometallates formed by different
arrangements of MO
6
octahedra. (a) Lindqvist, (b) Anderson,
(c) Keggin, (d) Dawson.
Figure 14.2 Schematic representations of the Keggin anion
showing the four distinct oxygen sites. (a) Polyhedral, (b) ball
and stick.
Figure 14.3 Primary, secondary and tertiary structures. ( Taken from fi gure 14.2 in Ref. [3] ).
14.1 Introduction 563
[XM
12
O
4
]
x − 8
where x is the oxidation degree of the central heteroatom X. The
addenda atoms, M (Mo
VI
, W
VI
), can be partly replaced by other metals, in particular
by Ta
V
, V
V
and Nb
V
, resulting in a change of the anionic charge (one per substituted
metal atom) and thus also in the number of counter - cations. These compounds
are nearly always negatively charged and their negative charge density is variable
depending on their elemental composition and molecular structure.
Solid POMs have a hierarchical structure and can be divided into three
sub - structures [3] , namely: primary, secondary and tertiary, as shown in Figure
14.3 . These structures are important for understanding their heterogeneous cata-
lytic properties. The primary structure is the structure of the heteropolyanion
itself, that is the metal oxide cluster. The secondary structure is the three - dimen-
sional arrangement consisting of polyanions, counter - cations and additional mol-
ecules (in particular organic molecules or water clusters such as
HO
52
+
, hydrated
species, or dimethyl sulfoxide (DMSO) in H
4
SiW
12
O
40
· 9DMSO and pyridine in
PW
12
O
40
· [(C
5
H
5
N)
2
H]
3
). The tertiary structure is the arrangement by which the
secondary structure assembles into solid particles, and this relates to properties
564 14 Heteropolyoxometallate Catalysts for Partial Oxidation
such as particle size, surface area and porosity. Based on this hierarchical
structure, it was demonstrated that there are three different modes of catalysis:
surface - type catalysis, pseudo - liquid bulk - type catalysis and bulk - type catalysis, as
represented in Figure 14.4 . Surface - type catalysis corresponds to ordinary hetero-
geneous catalysis, which takes place on the outer surface of the solid. The other
two types of catalysis are bulk - type and occur when the diffusion of reactant mol-
ecules in the solid (diffusion into the lattice rather than into the pores) is faster
than the reaction. The solid bulk forms a pseudo - liquid phase in which catalytic
reaction can proceed. Thus the catalyst appears solid but behaves like a liquid
(solvent). As the active sites in the bulk, for example protons or transition metals,
take part in catalysis, very high catalytic activity is observed. Owing to the fl exible
nature of the solid structure of some POMs (variable secondary structure), reac-
tants having polarity or basicity are readily absorbed into the solid lattice, between
the polyanions in the lattice, sometimes causing expansion of the lattice, and reac-
tion occurs therein. In other words, the reaction becomes three - dimensional as in
a solution. Owing to this behavior, POM catalysts often exhibit high catalytic activi-
ties and unique selectivity.
Heteropolyacids generally have a small surface area (10 m
2
g
− 1
) and no porosity.
However, microporosity has been observed in the presence of alkali metal counter -
cations, especially in cases involving large cations such as K
+
and Cs
+
as well as
with NH
4
+
, and so on. A full chapter detailing the microporosity of POMs can be
found in Ref. [4] . For instance, the pore size of acidic Cs
+
salts (Cs
x
H
3 − x
PW
12
0
40
)
has been shown to be controlled by the Cs
+
content, for example Cs
2.2
H
0.8
PW
12
0
40
has micropores in the range 0.62 – 0.72 nm diameter compared to less than 0.59 nm
for Cs
2.1
H
0.9
PW
12
O
40
and > 0.85 nm for Cs
2.5
H
0.5
PW
12
O
40
. Correspondingly, there is
a great difference in surface area values, for example ∼ 10 m
2
g
− 1
for H
3
PW
12
O
40
and
130 m
2
g
− 1
for its Cs
2.5
salt. The microporosity of NH
4
+
and Cs
+
salts of H
3
PW
12
,
H
3
PMo
12
and H
4
SiW
12
Keggin - type heteropolyacid s ( HPA ) has been investigated
Figure 14.4 Types of catalysis in solid heteropoly compounds.
(a) Surface, (b), pseudo - liquid bulk type (I), (c) bulk - type (II).
( Taken from Ref. [3] ).
[5] by
129
Xe NMR. It has been observed that ammonium salts had a pore size of
0.9 nm regardless of the anion, while for Cs
+
salts pore size varies between 0.7 and
1.0 nm depending upon the nature of the anion. However, this porosity has fre-
quently been observed to be lost upon catalytic application.
Control of the shape and size of pores (micro - and meso - pores) in polyoxometal-
lates is an important objective for the future. Pioneering studies have shown that
such control may be applied to the primary structure with generation of pores
between the anionic species [6] or to the tertiary structure with generation of pores
between self - assembled (NH
4
)
3
PW
12
O
40
micro - crystallites [7] . However, attempts
to control the pore size in primary structures with the use of organic templates
have failed so far, mainly because of the diffi culties encountered in the template
removal step, during which the structure collapses. Success has been reported in
the control of pore size in tertiary structures for applications at low temperatures;
however, for applications at high temperature, procedures need to be improved,
as the aggregates are thermally unstable. Alkali and alkaline earth substituted
Keggin - type phosphomolybdic or phosphotungstic acids have been shown to form
nano - crystallites composed of alkaline salt particles (10 – 20 nm) thickly covered by
the acid after precipitation [8] . Upon heat treatment, these particles are converted
to particles having a size similar to those before heat treatment with a more or
less uniform composition. These particles are connected epitaxially to each other
to form aggregates that can develop an important internal porosity when the ratio
of alkaline earth cations to protons is greater than or equal to 2.5 [8] . The complete
homogeneity of the composition of this type of polyoxometallate after heat treat-
ment is rather diffi cult to prove, since the acid phase coating the salt particles is
not detectable by X - ray diffraction and can only be observed by X - ray photoelectron
spectroscopy or Raman spectroscopy [9] .
14.2
History of Polyoxometallates
POMs have been known for about two centuries. The discovery of the fi rst ammo-
nium salt of the dodecamolybdophosphate (NH
4
)
3
PMo
12
O
40
(12 : 1 composition)
was reported by Berzelius [10] in 1826 and its tungsto counterpart by Galissard de
Marignac [11] in 1864. However, the fi eld remained undeveloped. The fi rst attempts
to understand the composition of heteropolyanions were based on Werner ’ s co -
ordination theory [12] . Structure determination for heteropoly compounds was not
possible until the advent of X - ray diffraction techniques. A hypothesis proposed
in 1908 by Miolati and Pizzighelli [13], based upon sharing metal – oxygen polyhe-
dra, was adopted and developed by Rosenheim [14] and Pauling [15] , during the
following 25 – 30 years. Rosenheim was probably the most productive and infl uen-
tial researcher in the fi eld of polyanion chemistry. According to the Miolati –
Rosenheim theory, heteropoly acids are based on six - coordinate heteroatoms
bound with MO
4
. It was Keggin [16] who solved the structure of the most impor-
tant 12 : 1 heteropoly compound H
3
PW
12
O
40
· 5 H
2
O by X – ray diffraction. Evans [17]
14.2 History of Polyoxometallates 565
566 14 Heteropolyoxometallate Catalysts for Partial Oxidation
determined the structure of the Anderson compound (6 : 1 series for Te
6+
[Mo
6
O
24
]
6 −
)
and Dawson [18] determined that of the 18 : 2 heteropoly compound [P
2
W
18
O
62
]
6 −
.
Souchay and his group brought clarity in the fi eld and have provided a major
contribution to polyoxometallate chemistry [1a, 19] . Since the early 1990s, polyo-
xometallate chemistry has expanded tremendously, following fundamental
dis coveries by M ü ller and his group at Bielefeld University [20] . These scientists
have shown that aqueous solutions of molybdates result in a huge variety of
linkable units and a large variety of clusters, among them amazing wheels
and spheres such as the giant anions [La
16
As
12
W
148
O
524
]
76 −
(mass ∼ 4000, diameter
4 nm) or {Mo
132
}, an inorganic superfullerene built up from 12 pentagonal
(Mo)Mo
5
units (one pentagonal MoO
7
unit sharing edges with fi ve MoO
6
octahe-
dra) and 30 different Mo
V
2
linkers or Mo Mo O CH COO H O
VI V
72 60 372 3
30
2
72
42
()()
[]
−
or
Mo Mo O H H O
VI V
126 28 462 14 2
70
14
()
[]
−
, and so on. These wheels can be considered as
nanostructures mimicking oxide surfaces and allowing reactions to occur on well
defi ned sites. The history of polyoxometallate chemistry and its applications for
nano - chemistry has been published [21] . Today, POMs still represent an exciting
area of research and technology, and new compounds are discovered every month.
For instance, the development of functionalized polyoxometallate [22] and poly-
oxometallate - based polymers [23] with creation of giant two - dimensional networks
has appeared recently and constitute new perspectives in the fi eld of organic –
inorganic frameworks.
14.3
Properties and Applications of Polyoxometallates
There are more commercial applications of polyoxometallates than of any other
class of cluster compound and there is also much potential for additional applica-
tions in areas ranging from catalysis and medicine to proton conductors, materials
and micro - device technology [24] . These applications stem from the wide range of
molecular architectures, surface charge density and chemical and electronic prop-
erties exhibited by POMs, as described above. Varying in size from one to a few
nanometers, they are used as electrode functionalization agents [25] , anti - retroviral
agents [26] and for detection, separation and quantifi cation. The latter uses are
based on properties such as their high molecular weight ( > 2000), their solubility
in water or organic solvents (depending on the size of the counter - ion size –
solubility decreases with size), the fact that a wide variety of elements can be
incorporated into the polyanion framework changing the properties, their electro-
chemical activity and their reducibility to form colored species depending on the
substitution element and extent (e.g. from yellow to orange when Mo is substituted
by V, while reduction leads to the famous “ heteropoly blues ” ). A vast range of
applications has been described in fi elds as different as synthesis of organic
and organometallic derivatives of POMs, absorbents (ion - exchange materials and
alcohol or carboxylic acid radical detectors), electron - transfer reactions, photo - and
electro - chromism, fl ame retardants and smoke suppressants, together with appli-
cations in metallurgy (corrosion inhibitors and coatings on steel and aluminum),
medicine and catalysis. Nevertheless, catalysis is by far the most important fi eld
of application (representing > 80% in terms of patent applications). Most of the
pioneering work was performed in Japan (Izumi, Misono, Mizuno, Ono, Otake,
Yoneda and coworkers), in Russia (Matveev, Kozhevnikov, Kholdeeva and cowork-
ers) and in France (Souchay, Herv é and coworkers).
POMs can display very strong Br ø nsted acidity and are effi cient oxidants, exhibit-
ing fast and reversible multi - electron redox transformations under mild condi-
tions. This acidity is very strong, as the negative charge is delocalized over much
larger anions than for mineral acids or solids such as zeolites. Consequently, the
electrostatic interaction between protons and the anion is much less for POMs.
The general trend in acidity among the most common HPAs is a function of the
hetero atom [27] with Co < B < Si, Ge < P, falling into the following order [28]
determined in an ammonia desorption study:
HPWO HSiWO HGeWO HPMoO
HSiMo O HG
3 12 40 4 12 40 4 12 40 3 12 40
412404
>>>~
~eeMo O
12 40
with NH
3
desorption maxima being at 865, 805, 736 and 696 K, respectively. Recent
studies report a decrease in the quantity of acid sites, but not in their strength,
when dispersing POMs on silica - type supports [29 – 31] such as HMS, MCM41 and
Kenyaite.
The acid – base and redox properties can be controlled across a broad spectrum
by varying the chemical composition, the extent of hydration, the type of support
for supported samples, the thermal treatment, and so on. For instance, the sub-
stitution of M
VI
by M
n +
of close valence ( n = 5, 4, 3 or 2) has been studied and leads
to additional negative charges on the HPA and thus to additional protons (from
one to four respectively) for charge balance, thus generating a higher acid density.
An example is HPVM O
VVI
31240+−xx x
, which can be synthesized with x = between 1 and
3. Other metal elements can also be substituted. The problem is that during syn-
thesis one could have either framework substitution of the M element or cationic
exchange between protons and the added metal cation, as is well known for zeolite
materials. It follows that the number of protons could be less than 3 + x in the
above formula, so that the balance between redox and acid properties of such
materials is diffi cult to control during synthesis, since they depend on tempera-
ture, pH conditions, and so on.
POMs are promising catalysts for acid, redox and bifunctional catalysis. In many
structures, the transition metal addenda atoms such as Mo or W exist in two oxida-
tion states, which results in different redox properties as determined by polarog-
raphy. The exceptional ability of heteropolyanions to act as electron reservoirs has
been demonstrated by the preparation and characterization of numerous reduced
derivatives [32] . They also exhibit high solubility in polar solvents, which means
that they can be used in homogeneous catalysis. The wide range of applications
of heteropoly compounds are based on their unique properties which include size,
mass, electron and proton transfer (and hence storage) abilities, thermal stability,
14.3 Properties and Applications of Polyoxometallates 567
568 14 Heteropolyoxometallate Catalysts for Partial Oxidation
lability of “ lattice oxygen ” and the high Br ø nsted acid strength of the correspond-
ing acids. An added advantage is that the HPAs can be separated and can be
extracted into organic solvents.
Transition metal substituted polyoxometallate s ( TMSP ) are synthesized using
transition metals as addenda ions. Most commonly, fi rst row transition metals are
incorporated into the framework. The transition metal element fi ts into the lacu-
nary vacancy created in the Keggin structural unit by the loss of an MO
6
octahe-
dron, which is equivalent to the stoichiometric loss of an MO
n +
unit, resulting in
the formation of the [XM
11
O
39
]
n −
anion. This involves the partial degradation of the
Keggin unit by lowering the pH of the solution upon the addition of a suitable
buffer. The solubilities of the TMSP complexes can be controlled by selecting an
appropriate counter - cation. Usually alkali metals and tetra - alkyl ammonium
groups are the counter - cations of choice. There are two methods for the synthesis
of a TMSP. The fi rst method involves the addition of a transition metal salt to the
already prepared isolated lacunary salt under appropriate synthetic conditions,
while the second method corresponds to the generation of in situ lacunary vacan-
cies followed by the addition of the transition metal salt.
Applications of POMs to catalysis have been periodically reviewed [33 – 40] .
Several industrial processes were developed and commercialized, mainly in Japan.
Examples include liquid - phase hydration of propene to isopropanol in 1972, vapor -
phase oxidation of methacrolein to methacrylic acid in 1982, liquid - phase hydra-
tion of isobutene for its separation from butane – butene fractions in 1984, biphasic
polymerization of THF to polymeric diol in 1985 and hydration of n - butene to 2 -
butanol in 1989. In 1997 direct oxidation of ethylene to acetic acid was industrial-
ized by Showa Denko and in 2001 production of ethyl acetate by BP Amoco.
Since the application of POMs is so widespread across areas such as homoge-
neous and heterogeneous catalysis, as well as acid and redox catalysis, it is not
possible to exhaustively review all the applications. Thus the rest of this chapter
focuses only on the catalytic properties of the polyoxometallates in heterogeneous
gas - or liquid - phase oxidation reactions, and reviews the most recent progress in
the knowledge of their properties and working process, underlining both their
potential and their limitations.
14.4
Catalytic Applications in Partial Oxidation Reactions
It is known [41] that partial oxidation reactions in heterogeneous catalysis involves
redox properties of the solid catalysts, allowing the well known Mars – van Krevelen
mechanism [42] to occur, or at least to be facilitated. Acid – base properties are also
an important feature, as they play a determining role in the activation of the reac-
tants and in the desorption of the intermediate compounds. For instance, an acid
surface will favor desorption of acid products, thus avoiding further over - oxidation,
while a basic surface will favor desorption of basic products as olefi ns. It follows
that heteropolyoxometallate compounds, in particular TMSP, appear as potential