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Proc. R. Soc. A (2012) 468, 1904–1926
doi:10.1098/rspa.2012.0056
Published online 14 March 2012
Rational design of single-site heterogeneous
catalysts: towards high chemo-, regio- and
stereoselectivity
BY VLADIMIRO DAL SANTO1 , MATTEO GUIDOTTI1, *, RINALDO PSARO1 ,
LEONARDO MARCHESE2 , FABIO CARNIATO2 AND CHIARA BISIO1,2
1 CNR-Istituto
di Scienze e Tecnologie Molecolari, Via C. Golgi 19,
20133 Milano, Italy
2 Dipartimento di Scienze e Tecnologie Avanzate and Nano-SiSTeMI
Interdisciplinary Centre, Università del Piemonte Orientale ‘A. Avogadro’,
Via Bellini 25G, 15100 Alessandria, Italy
The main methods for the design and preparation of single-site heterogeneous catalysts
on inorganic oxide supports are described and reviewed. Catalytically active metal sites
can be either introduced into the framework of porous materials via direct synthesis or
added to a pre-existing support by post-synthesis techniques. Particular attention is paid
to selected examples where the geometry, the nature and the chemical surroundings of the
active single site is a key factor to obtain catalytic systems with enhanced chemo-, regioand stereoselectivity. The ever-increasing capabilities of ‘nanoarchitecture’ at molecular
level enable chemists to build ideal catalysts for the sustainable transformation of bulky
and high added-value molecules.
Keywords: heterogeneous catalysis; single sites; selectivity; nanostructured catalysts;
epoxidation; hydrogenation
1. Introduction
In the last decade of the twentieth century and at the beginning of the twentyfirst century decade, a large scientific effort was focused on the design, synthesis
and characterization of a wide variety of nanoporous inorganic oxides with very
large channels for catalytic purposes (Thomas 1999; Davis 2001; Maschmeyer &
van de Water 2006; Climent et al. 2011; Xuereb & Raja 2011). Microporous
zeolites and zeotypes are optimal catalysts for the transformation of relatively
small molecules and have been largely applied for industrial catalytic processes.
However, microporous solids are unsuitable when bulkier and more functionalized
reactants are involved in catalytic processes. For these purposes, mesoporous
molecular sieves (porous materials whose pore diameters fall in the range
2–50 nm) could represent an ideal compromise to retain the advantages of a
*Author for correspondence ([email protected]).
One contribution of 14 to a Special feature ‘Recent advances in single-site heterogeneous catalysis’.
Received 16 January 2012
Accepted 14 February 2012
1904
This journal is © 2012 The Royal Society
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Rational design of catalysts
1905
porous structure, while minimizing mass-transfer limitations of large molecules,
such as those typically found in the fine chemicals industry (Guisnet & Guidotti
2006). The development of nanostructured mesoporous inorganic oxides paved the
way to the use of oxidic supports with very high specific surface areas as supports
for isolated metal centres (Thomas 1994), and led to the expansion of single-site
heterogeneous catalysis (Copéret et al. 2003; Thomas et al. 2005, 2009).
In a single-site heterogeneous catalyst (SSHC), active sites are well defined
and evenly distributed entities (single sites). They have distinct chemical
surroundings, as in conventional homogeneous systems, but they also show all
the advantages of heterogeneous systems, in terms of easy separation, recover and
recyclability. Single sites are usually (although not necessarily) located over solid
supports with high surface area, and they show the following general peculiarities:
(i) they consist of a limited and defined number of atomic species, i.e. one atom
(as in truly ‘single’ sites) or few atoms (as in the case of chemically defined
metal clusters), (ii) they are spatially isolated from each other, (iii) they all have
identical energy of interaction between the site itself and a reactant, and (iv) they
are structurally well characterized.
In the design of a new SSHC, by inserting (or depositing) metal single sites
into (or onto) inorganic oxides and by modifying the first-neighbour chemical
surroundings of the active centres, it is possible, in principle, to obtain the
inorganic equivalent of an enzyme and tune the properties of the catalyst
according to its applications (Lillerud et al. 2010; Xuereb et al. 2010). The
robust inorganic framework can indeed mimic the polypeptidic structure in
the enzyme protecting the catalytically active site and limiting the transfer
of reactants and products. Any modification of the internal surface of the
pores, in terms of acid/base, hydrophilic/hydrophobic or polar/apolar properties,
recalls the role of ancillary aminoacids in enhancing the activity and/or the
selectivity of the catalytic transformation. Then, the network of wide mesopores
can accommodate the bulky and complex intermediates found in the production
of fine chemicals, following bioinspired synthetic pathways.
The development of SSHCs, however, is still in its expansion phase, and several
aspects related to the preparation and the catalytic application of these systems
deserve deeper investigation and exploitation. The ever-increasing capabilities of
‘nanoarchitecture’ at a molecular level will provide chemists, in the next decades,
with new tools to design and build ideal catalysts for desired reactions rather than
adapt existing catalysts according to a conventional trial-and-error approach. In
this scenario, the most relevant synthesis techniques and some case studies of
SSHCs are described in the following sections.
2. Preparation of single-site heterogeneous catalysts on inorganic supports
Redox-active zeolitic structures, such as titanosilicalite-1 (TS-1), or transitionmetal-containing open-structure aluminophosphates (AlPOs) appeared in the
1980s and can be considered as the first examples of SSHC, as they fulfil the
main requirements of isolation, uniform distribution and controlled chemical
environment of the active sites (Wilson et al. 1982; Weisz et al. 1984; Chen et al.
1994; Marchese et al. 1994; Thomas et al. 1994; Corma 1997). During the design
and engineering steps that led to these catalytic systems, a major role was played
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V. Dal Santo et al.
=O
= Si
sol–gel or
hydrothermal
conditions
=M
=C
=H
Figure 1. Schematic of a one-pot synthesis procedure starting from precursors of inorganic
framework (i.e. tetraethoxysilane) and metal active species (M). (Online version in colour.)
by in situ characterization methods for solid catalysts (Thomas 1997). Then,
starting from the 1990s, owing to the development of silica-based mesoporous
molecular sieves, larger reactant molecules, which can enter the pore network,
react there and leave it, can be transformed within the inner space of porous
solids (Xiao 2005; Yang et al. 2009). Mesoporous silica materials, such as MCM41, MCM-48, HMS, KIT-6, SBA-15 or SBA-16, are, indeed, ideal supports for
SSHCs, since they exhibit high surface area (600–1300 m2 g−1 ) and their inner
and outer surfaces have a profusion of pendant silanol groups (generally from 1
to 3 OH groups per square nanometre) that are optimal loci for immobilizing
transition metal catalytic centres (Rigutto et al. 2007). Among them, materials
with a good hydrothermal stability, such as HMS or SBA-15, owing to a large
pore wall thickness and highly condensed frameworks with limited connectivity
defects, are preferable, as they can more easily withstand the strong conditions
occasionally experienced during synthesis or catalysis (Zhang et al. 2005).
Catalytically active single sites can have a structural role, when they are
introduced into the framework of spatially ordered materials, via direct synthesis
(figure 1). Otherwise, single sites can be added to a pre-existing support by
post-synthesis techniques.
Following a direct-synthesis approach, SSHCs with a microporous and/or
mesoporous inorganic structure can be prepared. In this type of preparation,
precursors of the metal active sites (e.g. salts and alkoxides) are added to
the synthesis gel of the inorganic support and part of the framework atoms
are isomorphously substituted by metal species. Beside zeolite-based catalysts,
one-pot direct synthesis was largely used to develop microporous AlPOs,
silicoaluminophosphates (SAPOs) and framework-substituted variants, in which
different metals (such as Co, Mg, Zn, Ti, etc.) are isomorphously substituted to
framework Al, Si or P atoms. At the end, solid catalysts with an ordered array
of pores and isolated acid and/or redox-active functionalities are obtained.
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Rational design of catalysts
(a)
(b)
M
MLn
H
H
O
O
O
O
O
O
Si
Si
H
H
O
O
Si
Si
O
O O
O
O
O
Si
O
O Si
Si
O O
O
O
O
O O O O
O
Si
Si
O
O Si O Si
O Si O
O
O
O
Si
O
O
O
H
O
O
O
O Si O
Si O Si
Si
Si
O
O O
O
O
O
O
O
O
O
Si
O
O Si
Si
O O
O
O
O
O O O O
O
Si
Si
O
O Si O Si O
O
NH3+MLn–
MLn
(c)
H
O
(d )
H
H
H
O
Si
O
O O
O Si
O O
O
Si O
O
Si
O
Si
O
O
O
Si
O
O
O
O
O
H
O
O
O
Si
H
O Si O
O
O
O
Si
O
Si
O O O
Si
Si
O
O
O
O
O
H
O
O
O
O Si O
Si O Si
Si
Si
O
O O
O
O
O
O
O
O
O
O
O Si
Si
Si
O O
O
O
O
O O O O
O
Si
Si
O
O Si O Si O
Figure 2. Post-synthesis modifications of mesoporous silicates. Several strategies are chosen
to introduce catalytic functions onto the support: (a) grafting, (b) anchoring, (c) impregnation,
and (d) electrostatic interactions. (Online version in colour.)
Different methods used to produce SSHCs via post-synthesis modification are
schematized in figure 2. In such approaches, the isolated sites can be deposited,
heterogenized, tethered or linked to a pristine matrix.
Then, with regard to the methods of creation of single-site centres, two
main approaches can be considered: post-synthesis covalent deposition and
post-synthesis non-covalent deposition.
In the first case, the active centre is added to the support (which can also
be commercially available) as a precursor that can be deposited irreversibly
(more precisely, anchored or grafted) onto the surface as it is, by the formation
of covalent bonds, or after a functionalization with a side chain (a tether;
Kolasinski 2007).
In the second case, the homogeneous precursors of the single-site active centres
are immobilized onto the surface of solid supports by non-covalent interactions,
such as hydrogen bonds, weak van der Waals interactions, which allow their
confinement (trapping or encapsulation). In particular, encapsulation covers a
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OH
OH
OH
OH
OH
1
2
3
4
5
6
Scheme 1. a-terpineol (1), terpinen-4-ol (2), carvotanacetol (3), isopulegol (4), carveol (5) and
limonene (6).
large selection of methods for immobilizing catalytically active species within the
pores of a solid, and it allows one to keep the optimal performances of the original
homogeneous catalysts unaffected. In some cases, owing to a positive cooperative
effect, it is also possible to have a final system with improved characteristics with
respect to the parent precursor.
3. Selected examples and applications of single-site heterogeneous catalysts
(a) Grafted Ti single-site transition metals supported onto high surface area
silica as epoxidation catalysts
Titanium-containing mesoporous silica has been largely investigated in the
literature and synthesized following different synthetic approaches. Among the
different preparation methodologies proposed in the literature, the grafting
reaction has largely been exploited to introduce the metal centres in the
siliceous framework, obtaining efficient SSHCs. For example, Ti centres were
successfully grafted onto the inner walls of MCM-41 mesoporous silica by using
an organometallic precursor, titanocene dichloride [Ti(Cp)2 Cl2 ] (Cp = C5 H5 )
(Maschmeyer et al. 1995), and tested as catalysts in the olefin epoxidation.
Such materials proved to be active in the epoxidation of model substrates (i.e.
cyclohexene) to the related epoxides.
Mesoporous titanosilicate molecular sieves can also be suitable catalysts for
the selective epoxidation of bulky and richly functionalized alkenes, such as cyclic
terpenes of interest for the flavour and fragrance industry, which often cannot be
easily carried out for microporous titanosilicate molecular sieves (Sheldon et al.
1998; Ratnasamy et al. 2004).
Data in the literature reported that mesoporous titanosilicate samples can be
suitable catalysts for the selective epoxidation of this class of compounds, and the
resulting epoxides are useful intermediates for the synthesis of a broad series of
valuable and high added-value products (van der Waal et al. 1998; Ravasio et al.
2004). For instance, a series of cyclic unsaturated alcoholic terpenes, namely
a-terpineol (1), terpinen-4-ol (2), carvotanacetol (3), isopulegol (4), carveol
(5) and limonene (6) (scheme 1), was selectively epoxidized with tertbutylhydroperoxide (TBHP) over Ti-containing mesoporous silica catalysts with
both an ordered and non-ordered porous array. High conversions (up to 90%)
and good selectivity to the desired epoxides (60–80%) were achieved in the
liquid phase, in polar aprotic solvents (acetonitrile and ethyl acetate) under
reflux conditions.
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Rational design of catalysts
1909
The catalyst activity is mainly governed by the hydroxyl-function position,
and the closer the hydroxyl group to the unsaturation, the higher the conversion
rate of the terpene. In addition, a direct comparison between the turn-over
frequencies obtained over grafted Ti–MCM-41 (where Ti is deposited by
post-synthesis treatments) and in-framework Ti–MCM-41 (where Ti is inserted in
the catalyst’s matrix during synthesis) shows that the epoxidation rate over the
former is up to 10 times higher than over the latter, owing to the better exposition
and availability of Ti centres in the grafted solid (Sankar et al. 1994; Oldroyd et al.
1996; Berlini et al. 2000; Thomas & Raja 2004). Stereoselectivity can be high
on such reactants, and a diastereospecific epoxidation (100% diastereoisomeric
excess (d.e.)) was obtained on homoallylic and bishomoallylic terpenes, 1 and 2,
respectively, owing to the role of the OH function in directing the oxygen transfer
from the oxidant to one specific side of the six-membered ring. In fact, when the
OH group in 1 is replaced by an acetyl moiety (a-terpinylacetate), the d.e. value
decreases abruptly from 100 per cent down to 66 per cent (Guidotti et al. 2002).
However, in the epoxidation of these terpenes, the use of materials with an
ordered array of mesopores is not strictly necessary in order to have an efficient
heterogeneous epoxidation catalyst. Provided that the pore diameter of the solid
is large enough to accommodate the reactant molecules and that the titanium
active sites are evenly dispersed and available on the silica support surface, the
morphology of the surroundings of the titanium atoms does not considerably
affect the catalytic performance. Indeed, when the pore diameters are too large
and no proper ‘sieving’ effect occurs, the siliceous materials act as simple supports
for the active sites, and an ordered array of pores does not seem essential (Berlini
et al. 2001). The performances obtained over Ti-containing mesoporous solids
are often fully comparable to those obtained over Ti-containing amorphous nonordered silicas, in terms of conversion, selectivity and activity (Fraile et al. 2001a;
Guidotti et al. 2003a).
In general, the hydrophilic nature of MCM-41-based solids can be a drawback
if relatively non-polar olefins have to be epoxidized: if a more hydrophobic
catalyst is needed, the silica surface silanols can be silylated. Catalysts whose
silanols were silylated with hexamethyldisilazane or triethoxyfluorosilane show
a higher activity in the epoxidation of simple alkenes, such as cyclohexene,
cyclooctene or linear alkenes, compared with unsilylated ones (Corma et al.
1998; Peña et al. 2001; Kamegawa et al. 2011). Nevertheless, in the case of
substrates with intermediate polarity and with functional groups that are able
to interact with the catalyst surface, a hydrophilic environment around Ti(IV)
sites can lead to good results. Thus, for instance, silylated Ti–MCM-41 catalysts
were more active than non-silylated catalysts in limonene epoxidation. On the
contrary, in the epoxidation of a-terpineol, the surface silanols played a relevant
role in enhancing the chemoselective formation of the epoxide (Guidotti et al.
2008a). Analogously, in the full epoxidation of methyl linoleate, the positive
interaction of the already formed epoxide rings with the hydrophilic catalyst
surface favoured the transformation of methyl epoxyoleate (the monoepoxide)
into methyl diepoxystearate (the diepoxide). A direct correlation between the
polar character (expressed as surface density of silanols per gram of catalyst) and
the selectivity to diepoxides after 24 h was indeed found in the epoxidation of
soy-bean oil fatty acid methyl ester (FAME) mixtures over titanosilicate catalysts
with various textural properties (Guidotti et al. 2006).
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+ TBHP
Ti–MCM-41
O
PhCH3
OH
OH + CH CN
3
O
citronellal
isopulegol
isopulegol
epoxide
Scheme 2. One-pot two-step conversion of citronellal into isopulegol epoxide.
Each pair of reactant/catalyst, however, should be studied case by case and, in
some situations, a partial silylation of the solid can also be envisaged, in order to
tune accurately the hydrophilic/hydrophobic character of the catalyst, in order
to maximize the yield of the desired product (Guidotti et al. 2011a).
The grafting of Ti(IV) species over a mesoporous silica can give rise
to a bifunctional acid and redox-active oxidation catalyst (the Tid+ site
with free coordination sites) (Bhaumik & Tatsumi 1999; Bisio et al. 2011).
Isopulegol epoxide (a compound with fungicidal and insect-repellent activity) was
synthesized over Ti-grafted MCM-41 under one-pot two-step conditions (Guidotti
et al. 2000; Ravasio et al. 2004). Citronellal was first converted into isopulegol, by
acid-catalysed cyclization (ene reaction), which was then epoxidized to isopulegol
epoxide, without isolation of the intermediates (scheme 2).
In order to avoid the direct epoxidation of the starting unsaturated aldehyde
(to citronellal epoxide), the oxidant TBHP was added only after the complete
cyclization of citronellal into isopulegol. With this approach, a global yield of 68
per cent in isopulegol epoxide was obtained.
Titanocene-grafted silicas showed promising results too in the epoxidation with
TBHP of a wide variety of unsaturated FAMEs: methyl oleate, methyl elaidate,
methyl linoleate and mixtures of methyl esters obtained from high-oleic sunflower,
castor, coriander and soy-bean oils (Guidotti et al. 2003b, 2006). In particular,
in the comparison between methyl oleate and methyl elaidate, with cis and
trans configurations, respectively, it was found that the structural features of
an ordered solid, such as Ti–MCM-41, are necessary to attain very high yields
into trans-epoxystearate. So, with these substrates, by choosing titanosilicate
mesoporous materials with an ordered hexagonal array of pores, such as Ti–
MCM-41, it was possible to obtain very high yields in epoxidized FAMEs (up
to 95% yield of epoxidized methyl linoleate or 96% yield in epoxidised castor oil
methyl ester in 24 h) with a relatively small excess of TBHP oxidant (only 10%
more than the stoichiometric amount needed) (Guidotti et al. 2007, 2008b). As a
main advantage, by this method, unsaturated FAMEs can be epoxidized with no
use of peracids (that are instead used industrially in the Prilezhaev epoxidation
process), hence under completely acid-free reaction conditions.
Finally, the use of aqueous hydrogen peroxide is typically difficult over most
mesoporous titanosilicates (Clerici 2009). Actually, specific experiments on the
interaction of TBHP and H2 O2 with Ti(IV) centres have clarified that a 1-aliquot
addition of aqueous H2 O2 leads to a rapid and irreversible transformation of
Ti(IV) centres into TiO2 -like domains, which are then catalytically inactive in
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Rational design of catalysts
1911
epoxidation (Gianotti et al. 2007). Nevertheless, Mayoral and co-workers reported
the synthesis of Ti(IV)–silica catalysts by grafting Ti(Oi Pr)4 onto a commercial
non-ordered mesoporous silica. This catalyst showed promising results in the
epoxidation of non-functionalized alkenes, dienes and allylic alcohols, not only
using TBHP as oxidant (Cativiela et al. 1996), but also with dilute aqueous
hydrogen peroxide under mild conditions (23% epoxide yield after 1 h), owing
to a slow drop-wise addition of oxidant (Fraile et al. 2000). In the latter case,
the contribution of direct epoxidation to the productive H2 O2 consumption
increases considerably with respect to one-shot addition (from 21% to 56%),
improving the selectivity to epoxide. The role of the slow addition of H2 O2
is crucial to avoid, or at least minimize, the useless decomposition of the
oxidant and to keep the local water concentration as low as possible. The side
production of allylic oxidation side-products (cyclohexenol and cyclohexenone) is
thus negligible and the selectivity to the desired epoxide is very high (Fraile et al.
2001b, 2003). In addition, specific immersion calorimetry studies to evaluate the
interaction between the reactant and the catalyst evidenced a very good affinity
of cyclohexene towards titanosilicate, which is a crucial factor in the epoxidation
reaction (Vernimmen et al. 2011).
The protocol of the controlled drop-wise addition of aqueous H2 O2
−1
) was applied to titanocene-grafted silica catalysts,
(4.17 mmol H2 O2 h−1 gcat
and it was thus possible to attain excellent selectivity (greater than 98%)
and interesting yields (from 44% to 63%) in cyclohexene epoxide (Guidotti
et al. 2009). Even higher yields in epoxide were found over Ti–MCM-48
than over Ti–MCM-41 or Ti–SiO2 owing to better isolation, dispersion and
stability of Ti(IV) sites. Recently, the drop-wise addition of H2 O2 has also
been extended successfully to the epoxidation of methyl oleate (Guidotti
et al. 2011b). All the tested titanosilicate molecular sieves showed good
catalytic activity and comparable behaviour in terms of conversion, selectivity
and yield of methyl 9,10-epoxystearate, notwithstanding the morphology and
the texture of the silica support. By optimizing the reaction conditions
(especially, the catalyst to reactant ratio), high yields of up to 91 per cent
in epoxide were obtained for Ti–MCM-41, with a high stereoselectivity (80%)
towards the cis-epoxide.
Anyway, even in the presence of aqueous H2 O2 , if the reaction is rapid
enough, the oxidation can occur before the aggregation and deactivation
of Ti sites takes place. This is the case with the oxidation of substituted
phenols into benzoquinones and, for instance, very good yields (96%)
in 2,3,5-trimethyl-1,4-benzoquinone, a vitamin E precursor, were obtained from
2,3,6-trimethylphenol after 1 h, with no remarkable deactivation of the catalyst
and a good recyclability (Kholdeeva et al. 2007, 2009a). In this class of reactions,
the catalysts prepared by grafting titanocene dichloride revealed a strong
dependence of the product selectivity on the surface concentration of titanium
active centres. Mesoporous materials with titanium surface concentration
in the range 0.6–1.0 Ti nm−2 were identified as optimal catalysts for the
transformation of alkylphenols into benzoquinones, whereas catalysts having less
than 0.6 Ti nm−2 produced a mixture of benzoquinones and dimeric by-products.
In fact, several spectroscopic evidences showed that dinuclear (Ti–O–Ti-like
species) and/or subnanometre-size clusters evenly distributed on the silica surface
are the best active sites for this reaction. This phenomenon can be explained
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H
2H2O
ArOH
O
Ti
OH2
HO
Ti
Ti
OH
H2O
Ti
O
Ar
H
OH
Ti
OH
Ti
O
H
quinone
OH2
O
Ti
O
R
O
OH2
O
H2O
Ti
Ti
R
O
H OH
H2O
Ar
H2O
Ti
2H2O2
O
OH
Ti
OOH
HOO
Ti
H2O
HOO
R
H
Scheme 3. Suggested oxidation mechanism for the conversion of alkylphenols into benzoquinones
over dinuclear Ti sites.
by the particularly favourable interaction between the phenolic substrate and
(at least) two contiguous Ti hydroperoxo groups, as depicted in the suggested
oxidation mechanism (scheme 3).
Over such catalysts, the target of 100 per cent selectivity was attained in
the oxidation of both trimethylphenol and dimethylphenol, and this is the first
example that clearly demonstrates the advantages of Ti cluster-site catalysts over
Ti single-site catalysts in hydrogen peroxide-based oxidation reaction. Moreover,
by using several mesoporous silica supports, it was possible to find a correlation
displaying the dependence of the selectivity on surface Ti density and to use it
as a predictive tool for the preparation of catalysts with optimized performance
for the chemospecific oxidation of substituted phenols to related quinones with
hydrogen peroxide (Kholdeeva et al. 2009b).
(b) Anchored organometallic complexes as single-site heterogeneous catalysts
Different approaches have been introduced to immobilize on solid surfaces the
active organometallic complexes that are designed and optimized to work under
homogeneous conditions in the liquid phase.
One possible method is based on the surface organometallic chemistry, in which
most of the know-how developed for homogeneous organometallic complexes
can be transferred to the study of novel organometallic species where the solid
surface itself acts as a ‘special’ ligand to the metal. Owing to this methodological
approach, Basset and co-workers have produced a number of organometallic
complexes anchored onto non-porous silica surfaces (Basset & Choplin 1983;
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Rational design of catalysts
1913
Basset & Ugo 2009). The organometallic complex is attached directly to the oxide
support (silica, alumina, etc.) via either a covalent/ionic bond or a Lewis–acid–
Lewis–base interaction. In these systems, the coordination sphere of the metal
involves not only the ancillary ligands that influence the stability, the activity
and the selectivity as in homogeneous catalysis, but also the surface.
Another strategy of immobilization is to link the transition metal complex
catalyst to a mesoporous material via a spacer chain (a tether). This
immobilization can be carried out by substituting one of the ligands of the
homogeneous catalyst by a similar moiety that contains a linker and which is
able to anchor, typically via a covalent bond, to the support/carrier (oxide,
polymer and dendrimer). According to this approach, most of the synthetic
effort is devoted to preserve as much as possible, the molecular structure and
the chemical environment after anchoring (Copéret & Basset 2007).
Owing to the anchoring strategy, large and bulky moieties can be covalently
attached onto the surface of mesoporous silicas, whose mesopores are large
enough to accommodate this kind of guest species. As an example, Ti-containing
polyhedral oligomeric silsesquioxane (Ti–POSS), which is an excellent model of
isolated and well-dispersed Ti sites in the SiO2 matrix, was first successfully
deposited (Krijnen et al. 1998; Smet et al. 2000; Zhang et al. 2007) and then
recently anchored via covalent bonds to the surface of an ordered (SBA-15) and
a non-ordered silica support (Carniato et al. 2008, 2011) (figure 3). In the latter
example, Ti–POSS moieties were, therefore, mainly accommodated as dinuclear
dimer species on the external surface of the ordered mesoporous silica and in the
large mesopores of the non-ordered silica support. In both cases, isolated Ti(IV)
single sites were deposited on the silica surface, but they were not located in close
proximity of silanol groups. These solids can thus be considered as model systems
to study the behaviour of isolated Ti centres and the extent of interaction between
the silanol-rich silica surface and the metal species. The final Ti–POSS–SBA-15
catalyst revealed a good dispersion of Ti sites and an interesting catalytic activity.
Epoxidation tests on unsaturated terpenes (limonene, carveol and a-pinene) with
TBHP as oxidant were performed to monitor the activity and accessibility of
Ti(IV) sites in the anchored samples and compared with conventional titanocenegrafted Ti/SBA-15 and Ti/SiO2 systems. In terms of activity, anchored catalysts
displayed comparable conversion and turn-over number values almost identical
to grafted ones. On the contrary, in terms of selectivity to epoxide products,
the anchored Ti–POSS-containing materials showed slightly better results than
the grafted ones owing to the absence of proximal silanols surrounding the Ti
centres and, hence, to the less marked formation of acid-catalysed side products
(Carniato et al. 2011).
(c) ‘Supported hydrogen-bonded’ complexes as hydrogenation catalysts
Even though most of the heterogenized anchored catalyst are obtained via a
covalent-linking approach, some examples of non-covalently bound homogeneous
catalysts show specific advantages, such as remarkably mild grafting procedures
(anchoring at room temperature, with no need of high-temperature treatments),
with minimal or no ligand modification of the parent homogeneous catalyst and
the possibility to easily recover the anchored complex for further characterization
studies by standard liquid-phase techniques.
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1914
V. Dal Santo et al.
R
R
Si
i
Pr O O
Si O
O OR
R
O Si
R
Si OR Ti Ti O
O
Si O Si O
O O
O
O
OR
i
O Si O Si
Pr
O R
Si
Si
O
R
R´
CH3
H2
C CH
R=
CH3
O
R´ =
N
H
N
H
R´
O
Si
R O
Si O
O Si
O O R
Si
R
OEt
Si OEt
OEt
Figure 3. Anchoring of a Ti-containing polyhedral oligomeric silsesquioxane (Ti–POSS) onto the
surface of silica supports. For the sake of clarity, only one tether is shown here to be directly bound
to the surface. (Online version in colour.)
In this field, the groups of Bianchini & Psaro introduced, in 1999, a noncovalent method of preparing silica-immobilized metal catalysts for use in solid–
liquid reactions in apolar solvents (Bianchini et al. 1999, 2000). The procedure
involved the formation of hydrogen bonds between the terminal silanols of the
silica surface and the oxygen atoms of sulphonate groups from phosphine and/or
from trifluoromethansulphonate counteranion. This heterogenization protocol
gives rise to supported hydrogen-bonded (SHB) catalysts and is alternative to
the supported liquid-phase (SLP) method developed by Davis that involves the
dissolution of a homogeneous catalyst in a hydrophilic solvent adsorbed as a
thin layer onto a porous material with a hydrophilic surface (e.g. silica or glass)
(Davis 1992).
The immobilization of the zwitterionic Rh(I) catalyst [(sulphos)Rh(cod)]
(sulphos = − O3 S(C6 H4 )CH2 C(CH2 PPh2 )3 ; cod = cycloocta-1,5-diene) was achieved by a straightforward solvent impregnation method using anhydrous
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1915
Rational design of catalysts
Me
C
Me
CN N
2+
MeCN Ru P
P P
+
Rh P
P P
+
Ph2
P
Rh
P
Ph2
–
–
O
O S
O
H H
H
O O
O
1
F F F
C
S O
O
O
H
H H
O O
O
–
O S O
O
H H
H
O O
O
2
FF F
C
O S O
O
H H
H
O O
O
–
3
Scheme 4. Supported hydrogen-bonded (SHB) Rh and Ru complexes on silica. (Online version
in colour.)
dichloromethane solutions of the catalyst precursors that are then stirred in
the presence of the support material until all the metal complex has been
chemisorbed onto the silica surface. Reproducible 1–2 wt.% metal loadings were
generally obtained. Once grafted to silica, the Rh complex was not extracted
back into CH2 Cl2 , whereas stirring the SHB complex in alcohols at room
temperature resulted in its complete desorption from silica and its dissolution into
the solvent.
The immobilized catalyst (sulphos)Rh(cod)/SiO2 (1/SiO2 in scheme 4) is
active for the hydrogenation of alkenes in either flow reactors (ethene and
propene) or batch reactors (styrene) in hydrocarbon solvents. Moreover, in all
the cases investigated, there was no evidence whatsoever of the formation of
contiguous Rh–Rh sites, indicating that the catalytic active sites are truly isolated
Rh atoms (i.e. real SSHCs), as in the homogeneous phase. A further interesting
feature of SHB catalysis is provided by the facile and quantitative extraction of
the grafted metal products with alcohols. This allows the study of metal products
of single-site catalytic reactions by applying standard spectroscopic techniques
in solution, as occurs when the heterogenization of organometallic complexes is
achieved by the ion-pairing technique.
The SHB methodology has been extended to cationic metal complexes, of
which very few heterogenized examples, obtained via the ion-pairing technique,
are known (Beckler & White 1986; Scott et al. 1994). For this purpose, the Ru(II)
complex [(sulphos)Ru(NCMe)3 ](OSO2 CF3 ) can be mentioned. On the basis of
spectroscopic characterization, a structure can be proposed for Ru(II)/SiO2 , in
which both the complex cations and the triflate anions are tethered to silica by
hydrogen-bond interactions involving the oxygens from the −SO−
3 groups and the
−OH groups from the terminal silanols on the silica surface (2/SiO2 in scheme 4)
(Bianchini et al. 2000). Obviously, the sketch is only a schematic representation
of the reality, as there will be cations and anions hydrogen bonded to silica via
two or even one ≡Si–OH units.
The SHB Ru(II)/SiO2 system is a selective catalyst for the hydrogenation
of benzonitrile to benzylidenebenzylamine in n-octane (35% conversion and
99% selectivity after 1.5 h). No appreciable ruthenium leaching into the
hydrocarbon phase was observed, even after three recycles. Notably, the reductive
transformation of an organic nitrile into a secondary imine has never been
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V. Dal Santo et al.
reported to occur in a heterogeneous fashion. The results obtained in this work
are thus quite interesting, as they outline a new and clean route to the preparation
of secondary bis(aryl)imines, which are important organic synthons.
The Ru(II) complex immobilized on silica has been also successfully applied
in hydrodenitrogenation/hydrodesulphurization catalysis of model hetero- and
polyaromatics (Bianchini et al. 2003). The immobilised Ru(II) catalyst was
active and highly selective in the hydrogenation of indole to indoline (turn-over
frequency of 14 h−1 , 100% selectivity to indoline), even in the absence of acids.
It is worth noting that for this substrate, almost no hydrogenation was observed
using the homogeneous catalyst in either CH2 Cl2 or tetrahydrofuran, even for
reaction times as long as 5 h (Barbaro et al. 2002). The presence of a protic acid
is necessary to generate an equilibrium concentration of the 3H -indolium cation
that contains a localized, hence reducible, C=N bond. The use of protic acids
to promote the hydrogenation of indole is a common procedure in heterogeneous
catalysis, for example, with Raney Ni or copper chromate catalysts (Adkins &
Coonradt 1941; Smith & Utley 1965).
The formation of 3H -indolium cation is very unlikely to occur with the
SHB Ru(II)/SiO2 system, as it needs strong acids: even trifluoroacetic acid and
p-toluenesulphonic acid give incomplete protonation of indole, leading to an
equilibrium concentration of 3H -indolium (Jackson & Smith 1964). Therefore,
neither isolated silanols of silica (pKa > 9) nor hydrogen-bonded silanols (pKa
5–7) should be able to protonate indole.
The hydrogen-bond interactions between the N–H proton of indole and the
oxygen atoms of the silanols groups (either isolated or, much more likely, engaged
in hydrogen bonding to the sulphonate groups) may activate the heterocyclic
ring to expose a more localized, hence reducible, C2 −C3 bond to ruthenium.
Strong support for the positive influence of N−H . . . O(H)−Si < bonds on indole
hydrogenation by Ru(II)/SiO2 was provided by the use of methylindole in the
place of indole. The protection of the nitrogen atom with a methyl group did
not allow for substrate hydrogenation. Silica surface-mediated reactions are a
relatively new synthetic protocol, leading to interesting results in terms of activity
and selectivity, as well as environmentally friendliness (Kabalka & Pagni 1997).
The reactions can be conducted by simple mixing of silica, reagent and substrate
in the dry state followed by heating, while the products are generally extracted
from the adsorbent with an appropriate solvent. Mediation by silica was found
to involve either isolated or associated silanols. Similar examples of silica surfacemediated reactions include the oxidation of sulphides and sulphoxides by TBHP
(Kropp et al. 2000).
Like the SLP immobilization (Fache et al. 2000; Bianchini & Barbaro 2002),
the SHB technique has been extended to chiral metal complexes with chelating
phosphines and, as such, it is the simplest procedure ever reported for the
synthesis of chiral heterogeneous catalysts, as commercially available chiral
ligands may be employed with no modification of their structure (Wan &
Davis 1994).
Two different types of SHB chiral catalysts, all containing rhodium, have
been prepared: neutral complexes, such as [Rh(nbd)((R)-(R)-BDPBzPSO3 )]
((R)-(R)-BDPBzPSO3 = (R)-(R)- 3 -(4-sulphonate)benzyl-2,4-bis(diphenylphosphino)pentane, nbd = norbornadiene), and cationic complexes containing triflate
counter-anions, such as [Rh(nbd)((+)-DIOP)]OTf and [Rh(nbd)((S )-BINAP)]
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Rational design of catalysts
1917
OTf (3/SiO2 in scheme 4), where (+)-DIOP = (+)-2, 3-O-isopropylidene2,3-dihydroxy-1,4-bis(diphenylphosphino)butane and (S )-BINAP = (S )-(−)-1,1 binaphthalene-2,2 -diyl)bis(diphenylphosphine).
The immobilization of the neutral precursors implies a direct hydrogen-bonding
interaction between the metal complex and surface silanols and, therefore, the
sulphonate group must be previously introduced into the chiral ligand.
In contrast, the cationic complexes reside close to the silica surface by
electrostatic interaction with the triflate ions that, in turn, are immobilized on
the support via hydrogen bonds. Consistently, no immobilization whatsoever was
observed when the triflate anion was replaced by counter-anions such as BPh−
4
or BF− that are not capable of hydrogen-bonding interaction with silica.
Chiral SHB rhodium catalysts have been used in the enantioselective
hydrogenation of prochiral olefins (Bianchini et al. 2001), particularly itaconates
and 2-acetamido acrylates, in n-heptane or n-hexane. As a general trend,
the immobilization of the chiral precursors on silica did not reduce their
enantioselectivity when compared with analogous homogeneous reactions in
methanol, a beneficial effect was observed in some cases (cf. hydrogenation of
dimethylitaconate, homogeneous versus SHB [((S )-BINAP)Rh(nbd)]OTf, which
show 100% conversion, with 25% of enantiomeric excess (e.e.) (S ) and 100%
conversion, with 30% of e.e. (S ), respectively). Moreover, the conversion values
were generally comparable with or higher than those in the homogeneous phase.
Irrespective of the immobilization procedure of the metal complex (SHB or ion
pairing to SHB counter-anion), no appreciable Rh leaching was observed within
several consecutive heterogeneous runs, and an effective catalyst recycling with
no loss of activity or enantioselectivity was accomplished by standard procedures.
When complexes are supported inside the pores (of the proper size) of
mesoporous silicas, the resulting catalysts may show superior enantioselectivities,
if compared with the homogeneous counterparts, owing to the confinement effect.
Chiral Rh and Pd catalysts supported on commercial silicas, with different pore
sizes, were tested in the asymmetric hydrogenation of methyl benzoylformate
to methyl mandelate showing excellent e.e. values (i.e. [Rh(cod)PMP]CF3 SO3 /
SiO2 (PMP = (S )-(+)-1-(2-pyrrolidinylmethyl)-pyrrolidine) allows up to 94%
e.e. to be obtained) (Raja et al. 2003). Analogous Rh complexes
([Rh(cod)L-L]CF3 SO3 , where L-L can be: (S ,S ,S )-[2-(4,5-dihydro-4 ,5 -diphenyloxazol-2 -yl)ferrocenyl] phosphine ((S ,S ,S )-dipof ); L-tryptophanbenzyl ester;
(−)-(S )-2-(aminomethyl)-1-ethylpyrrolidine or (+)-(1R, 2R)-1,2-diphenylethane1,2-diamine) were immobilized on MCM-41 and used for the hydrogenation of
(E)-(a)-phenylcinnamic acid to 2,3-diphenylpropionic acid showing similar good
performances, up to 90 per cent conversion, 84 per cent selectivity and 97 per
cent e.e. (Rouzaud et al. 2003).
SHB catalysts also found interesting applications in other reactions, different
from the hydrogenation ones, such as cyclopropanation, Diels–Alder additions or
epoxidation reactions.
Preformed polypyrazolylborate copper(I) complexes BpCu, Tp∗ Cu and
[B(pz)4 ]Cu (Bp = dihydrobispyrazolylborate, H2 B(pz)2 ; Tp∗ = hydrotris(3,5dimethyl)pyrazolylborate, HB(pz∗ )3 ; B(pz)4 = tetrakispyrazolylborate) were
immobilized by classical and non-classical hydrogen-bond ligand-to-support
interaction involving the hydroxyl groups of the silica surface and the borohydride
BH group and/or the pyrazolyl nitrogen atoms on silica gel. The resulting
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V. Dal Santo et al.
catalysts were employed for the styrene cyclopropanation reaction. The catalytic
activity is very similar to that observed under homogeneous conditions (i.e. 76%
yield and 75 : 25 anti/syn ratio, under homogenous conditions versus 90%, yield
and 54 : 46 anti/syn ratio, under heterogeneous conditions). The strength of
such interactions lets the recovery and recycling of the supported catalysts for a
number of cycles, even if some leaching was observed using solvents other than
petroleum ether (Mar Díaz-Requejo et al. 2000).
Analogously, chiral bis(oxazoline) complexes of Cu(II), Zn(II) and Mg(II)
immobilized on silica support via hydrogen-bonding interactions are very effective
catalysts for the Diels–Alder reaction between 3-((E)-2-butenoyl)-1,3-oxazolin-2one and cyclopentadiene allowing up to 93 per cent e.e. at room temperature
to be obtained with the heterogeneous bis(oxazoline) complexes. Moreover, the
catalysts can be recycled without significant loss of enantioselectivity (Wang
et al. 2006).
Again, sulphonato-(salen)Mn(III) complex hydrogen bonded to silicas
was revealed to be an effective and recyclable catalyst in the enatioselective
epoxidation of styrene and a-methylstyrene (Tang et al. 2008). Enantiomeric
excess values up to 100 per cent (with isolated epoxide yield of 87%)
were obtained with a silica gel supported Mn complex in biphasic systems
(BMImPF6 /H2 O).
Finally, a recent development of the SHB grafting methodology worth
mentioning is the so-called ‘hybrid’ or transition complexes supported metal
(TCSM) catalysts that consist of the combination, on the same support material,
of dispersed metal nanoparticles (usually Pd) and grafted molecular complexes
(mainly rhodium phosphine or amine complexes) (Dal Santo et al. 2010). These
systems showed an enhanced activity, greater than the sum of the activities of
the two single components, in aromatic hydrogenation reactions and for them,
a synergistic effect of the single-site molecular metal complex on the metallic
nanoparticles has been evoked.
(d) Single-site heterogeneous catalysts prepared by one-pot approaches
Microporous solids, such as zeolites and AlPOs, possess a well-organized
inorganic structure composed of pores and channels with regular dimensions,
where single-site catalytic entities can be introduced, aiming at the preparation
of a wide range of catalysts for selective oxidation and acid-catalysed
transformations (Hartmann & Kevan 1999; Albuquerque et al. 2006; Raja et al.
2008). The high performances in terms of activity and selectivity in
oxidation reactions have been mainly attributed to the particular local
structure and coordination geometry of the active metals when introduced in
framework positions.
Various examples of single-site microporous catalysts prepared by the onepot procedure and used for catalytic oxidation reactions are reported in the
literature. Metal-containing AlPOs were extensively studied as catalysts for
oxidation and oxidative hydrogenation reactions, obtaining interesting results
(Thomas et al. 1999). In different cases, the catalytic performance of one-pot
prepared materials was compared with that of solids containing the same active
species in extra-framework positions (prepared by post-synthesis approaches, e.g.
by ion-exchanging zeolites or zeotype materials) and proved to be better.
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Rational design of catalysts
1919
As an example, aluminophosphate Co–APO-5 solid was proposed as a catalyst
for selective catalytic oxidation of organosulphur compounds (Chica et al. 2006).
The catalyst was prepared by introducing the synthesis gel used for the AlPO
preparation, Co(acetate), as a precursor of the redox site. A Co/H-Y zeolite
material (containing a similar amount of metal species) was prepared by classical
ion-exchange reactions to obtain a comparison.
The oxidation of different sulphur compounds (sulphides, benzothiophenes
and thiophenes) with tert-butyl hydroperoxide occurs on both Co–APO-5 and
Co/H-Y. Co–APO-5 gave the highest organosulphur oxidation rates (per Co
atom), reflecting the role of tetrahedral Co framework cations that undergo facile
redox cycles. Indeed, when Co3+ is in the framework position, it can be easily
reduced to Co2+ with a parallel formation of OH surface groups. On the contrary,
Co2+ cations at exchange sites in H-Y catalysts do not undergo redox cycles. The
good conversion obtained with the largest organosulphur compounds (diphenyl
sulphide and 4-methyldibenzothiophene) suggests that site accessibility is not a
major problem within the channels of Co–APO-5, even for bulky substrates. In
addition, this type of SSHC proved to be less prone to metal leaching under
reaction conditions than ion-exchanged analogues.
In the recent literature, examples of the preparation of multi-functional
microporous catalysts through one-pot reactions have been given. Raja and
co-workers focused attention on bimetallic multi-functional solids, obtained by
isomorphously replacing Al(III) and P(V) framework cations with tetrahedrally
coordinated Co(III) and Ti(IV) ions, respectively (Paterson et al. 2011). The
highly active catalysts exhibited an interesting synergistic effect between Co
and Ti sites and an enhancement of the catalytic performance, when compared
with results obtained from their corresponding monometallic analogues in the
catalytic epoxidation of olefins. This behaviour was associated with the fact
that the active metal ions are located not only in the AlPO framework, but
also in close proximity with each other. It was also hypothesized that the
simultaneous isomorphous substitution of Al(III) and P(V) in the AlPO-5
framework with Co(III) and Ti(IV) metal ions results in a higher fraction of
Co(III) sites in the bimetallic catalyst with respect to the parent Co(III)–
ALPO-5 (Barrett et al. 1996). These centres are supposed to be suitable
sites for the generation of free-radical intermediates involved in the catalytic
epoxidation reactions.
Other interesting examples of SSHCs were obtained by introducing redox
metal centres in mesoporous solids through one-pot reactions. Although metalcontaining mesoporous materials are often prepared by post-synthesis reactions,
in some cases, the one-pot preparation approach is useful to obtain highsurface area solids with highly dispersed active species and with readily
reducible/oxidizable metal centres. Very recently, the peculiar catalytic activity
of one-pot prepared V–SBA-15 and V−SiO2 towards dichloromethane conversion
with respect to impregnated solids was reported. This was associated with
the presence of isolated V species incorporated inside silica walls produced
during one-pot synthesis, while impregnated species especially contains polyvanadate (or microcrystalline V2 O5 ) phases that are not so active for
gas-phase volatile organic compound oxidation purposes (Piumetti et al.
2010, 2011).
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V. Dal Santo et al.
4. Conclusions
The examples presented in this brief overview show how SSHCs can be applied
to different transformations of interest for the synthesis of richly functionalized
fine chemicals. Redox-active metal-containing solid catalysts can be used as viable
alternatives to stoichiometric non-catalytic processes (still widely used, especially
in oxidation reactions) that are based on hazardous and environmentally
unfriendly reactants. SSHCs proved, in many cases, to be robust, easily recyclable
and potential candidates for developing new chemo-, regio- and stereoselective
synthetic processes at a large-scale level. Nevertheless, the development of
bioinspired inorganic equivalents of enzymes based on nanostructured molecular
sieves with comparable activity and selectivity is still a challenge for researchers.
This results in a main hurdle to an effective expansion of this class of catalysts at
an industrial level, even if the high added value of the product could justify the
use of valuable inorganic oxide-based SSHCs, often prepared by expensive and
time-consuming synthetic routes.
Once again, a multi-disciplinary approach, based on a strict cooperation
between chemists with different cultural backgrounds (in heterogeneous catalysis,
organic synthesis, biochemistry, inorganic chemistry, materials sciences, molecular
modelling, etc.) can be the only winning strategy to obtain successful results in
this field in the next future.
The financial support of the European Community’s 7th Framework Programme through the Marie
Curie Initial Training Network NANO-HOST (grant agreement no. 215193) and of the Italian
Ministry of Education, University and Research through the project ‘ItalNanoNet’ (Rete Nazionale
di Ricerca sulle Nanoscienze; prot. no. RBPR05JH2P) is acknowledged.
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