polyoxometalates
ations/acylations, aldol condensations, Michael additions,
chain isomerizations, or oxidations such as epoxidation and
dehydrogenation (Boon et al., 1986; Wang et al., 2001; Pizzio et
al., 2003; Chang et al., 2010; Qiu et al., 2015).
support is a critical aspect because, depending on the type of
catalytically active species, the support can strongly affect,
enhance or reduce both its selectivity and/or its activity. This
fact has been theoretically analyzed in terms of strong metal–
support interactions (SMSI) (Tauster et al., 1978; Goodman,
1995) and, recently, also in terms of electronic metal support
interactions (EMSI) (Campbell, 2012). The basic concept
behind both SMSI and EMSI is that the chemical bonding
required to anchor the catalyst to the support can change the
electronic and/or chemical structure of the former, hence
modifying its catalytic behaviour.
Heteropoly acids (HPAs) stand out as one class of mol-
ecules with the highest potential to catalyse the H2O2-based
oxidation of organic substrates efficiently. The large family of
family of anionic metal–oxygen clusters known as poly-
oxometalates (POMs) comprises a number of species with
The latter oxidation reactions require reagents capable of
donating O atoms to the catalyst–substrate adduct, which
coordinate to the metal centres with Lewis acidity to form
oxometallic or peroxometallic intermediate species, and then
transfer to the substrate leading to the oxidized product
(Shinachi et al., 2005). In this regard, one of the most relevant
advances undergone by oxidation catalysis is the increased use
of cheaper and less environmentally harmful oxidants, such as
hydrogen peroxide (H2O2), the reduction product of which is
merely water in contrast to other inorganic oxidants that give
rise to the formation of pollutants (Bossmann et al., 1998). As
H2O2 usually lacks enough activity because of its low
decomposition temperature, catalysts are required to raise the
reaction rates (Babou et al., 1995), among which transition-
metal-containing molecular species (e.g. Ti, V, Mo, W, Cr, Mn,
Fe and Re) stand out as the most suitable because their ease in
forming oxo- or peroxometallic derivatives gives them excel-
lent activities and selectivities in H2O2-based oxidation reac-
tions. However, two major drawbacks are associated with such
transition-metal-based molecular catalysts. On the one hand is
their marked tendency to undergo deactivation through
oligomerization, which leads to polynuclear oxo-bridged
complexes that are catalytically inactive. On the other hand is
the fact that they are usually soluble in the reaction medium,
which makes the catalytic processes run in the homogeneous
phase when the optimization of such processes from the
industrial viewpoint demands insoluble solid catalysts able to
work under heterogeneous conditions.
¨
very diverse compositions, shapes and sizes (Pope & Muller,
1991) and HPAs specifically refer to the protonated forms of
the subclass of heteropolyanions, which incorporate additional
elements (heteroatoms) in geometrically well-defined central
sites (tetrahedral or octahedral) besides oxygen and metals
from groups 5 and 6. A thorough body of reports on the
catalytic applications of POM species can be found in the
literature (Kozhevnikov, 2002), many of which refer to the
Hn[XM12–xM0xO40] HPAs, with the well-known Keggin-type
structure as the most relevant representatives (X
=
heteroatom; e.g. SiIV or PV; M or M0 = MoVI, WVI or VV). As
HPAs show high solubility in a range of polar solvents, they
need to be immobilized on porous solid matrices for appli-
cation as supported catalysts under heterogeneous conditions,
and different supports have been successfully explored to this
ˆ
end, such as mesoporous MCM-41 silica (Tropecelo et al.,
2010), zirconia (Alcan˜iz-Monge et al., 2018), activated carbo-
naceous materials (Alcan˜iz-Monge et al., 2013) or alumina
(Endalew et al., 2011).
Here we report our studies on the applicability of a series of
HPA-immobilized porous solids as heterogeneous acid cata-
lysts for the green oxidation of organic substrates with H2O2 as
´
an eco-friendly oxidant. In a previous work (Martın-Caballero
et al., 2016), we tested the catalytic activity of some HPA
species (i.e. H3[PMo12O40] and H4[PVMo11O40] in a homo-
geneous phase), as well as that of V2O5, toward the oxidation
of adamantane using H2O2 as oxidant (see Scheme 1), and
found that the H3[PMo12O40] species, which is a well-estab-
lished oxidation catalyst for a range of organic substrates
(Mizuno & Kamata, 2011; Kozhevnikov, 2002), was in fact
remarkably inactive under our conditions. The replacement of
molybdenum(VI) centres with vanadium(V) atoms in the
Keggin-type framework has proven to be beneficial for redox
catalysis as it enhances the redox character of the POM cluster
(Molinari et al., 2011) and, accordingly, we obtained high
conversions instead (90%) by using the vanadium(V)-mono-
substituted H4[PVMo11O40] derivative (hereafter referred to
as VPMo). Following on from these studies, the VPMo species
has been selected in this work as the catalytically active probe
molecule among the different Keggin-type HPAs that have
been reported to date. Zirconia (ZrO2, hereafter abbreviated
These two major drawbacks can be overcome by making the
molecular catalysts insoluble in the reaction medium and,
therefore, the development of new solid acid catalysts with
high selectivity has become a focus of attention in catalysis, as
indicated by the remarkable number of scientific studies
produced on this topic in recent decades. One elegant
approach to the design of such a type of heterogeneous
catalyst is seen in so-called heterogenization, that is, the
immobilization of soluble species as the catalytically active
functionalities on a solid support that provides a hetero-
geneous character to the resulting material. Supported cata-
lysts are of great interest because of their many advantages,
including their facile separation from the reaction products,
which avoids any potential contamination of the latter, and the
possibility of recycling and reusing the catalysts in consecutive
reaction runs (Leofanti et al., 1998; Farhadi & Zaidi, 2009;
Alcan˜iz-Monge et al., 2014). The selection of the appropriate
ꢀ
Acta Cryst. (2018). C74, 1334–1347
Bakkali et al.
Zirconia-supported 11-molybdovanadophosphoric acid catalysts 1335