doi: 10.1098/rspa.2012.0056, 1904-1926 first published online 14 March 2012468 2012 Proc. R. Soc. A

 Carniato and Chiara BisioVladimiro Dal Santo, Matteo Guidotti, Rinaldo Psaro, Leonardo Marchese, Fabio stereoselectivitycatalysts: towards high chemo-, regio- and Rational design of single-site heterogeneous  

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Proc. R. Soc. A (2012) 468, 1904–1926doi:10.1098/rspa.2012.0056

Published online 14 March 2012

Rational design of single-site heterogeneouscatalysts: towards high chemo-, regio- and

stereoselectivityBY VLADIMIRO DAL SANTO1, MATTEO GUIDOTTI1,*, RINALDO PSARO1,

LEONARDO MARCHESE2, FABIO CARNIATO2 AND CHIARA BISIO1,2

1CNR-Istituto di Scienze e Tecnologie Molecolari, Via C. Golgi 19,20133 Milano, Italy

2Dipartimento di Scienze e Tecnologie Avanzate and Nano-SiSTeMIInterdisciplinary 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 catalystson inorganic oxide supports are described and reviewed. Catalytically active metal sitescan be either introduced into the framework of porous materials via direct synthesis oradded to a pre-existing support by post-synthesis techniques. Particular attention is paidto selected examples where the geometry, the nature and the chemical surroundings of theactive single site is a key factor to obtain catalytic systems with enhanced chemo-, regio-and stereoselectivity. The ever-increasing capabilities of ‘nanoarchitecture’ at molecularlevel enable chemists to build ideal catalysts for the sustainable transformation of bulkyand 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 twenty-first century decade, a large scientific effort was focused on the design, synthesisand characterization of a wide variety of nanoporous inorganic oxides with verylarge channels for catalytic purposes (Thomas 1999; Davis 2001; Maschmeyer &van de Water 2006; Climent et al. 2011; Xuereb & Raja 2011). Microporouszeolites and zeotypes are optimal catalysts for the transformation of relativelysmall molecules and have been largely applied for industrial catalytic processes.However, microporous solids are unsuitable when bulkier and more functionalizedreactants are involved in catalytic processes. For these purposes, mesoporousmolecular sieves (porous materials whose pore diameters fall in the range2–50 nm) could represent an ideal compromise to retain the advantages of a

*Author for correspondence (m.guidotti@istm.cnr.it).

One contribution of 14 to a Special feature ‘Recent advances in single-site heterogeneous catalysis’.

Received 16 January 2012Accepted 14 February 2012 This journal is © 2012 The Royal Society1904

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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 & Guidotti2006). The development of nanostructured mesoporous inorganic oxides paved theway to the use of oxidic supports with very high specific surface areas as supportsfor isolated metal centres (Thomas 1994), and led to the expansion of single-siteheterogeneous catalysis (Copéret et al. 2003; Thomas et al. 2005, 2009).

In a single-site heterogeneous catalyst (SSHC), active sites are well definedand evenly distributed entities (single sites). They have distinct chemicalsurroundings, as in conventional homogeneous systems, but they also show allthe advantages of heterogeneous systems, in terms of easy separation, recover andrecyclability. Single sites are usually (although not necessarily) located over solidsupports 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 definedmetal clusters), (ii) they are spatially isolated from each other, (iii) they all haveidentical energy of interaction between the site itself and a reactant, and (iv) theyare structurally well characterized.

In the design of a new SSHC, by inserting (or depositing) metal single sitesinto (or onto) inorganic oxides and by modifying the first-neighbour chemicalsurroundings of the active centres, it is possible, in principle, to obtain theinorganic equivalent of an enzyme and tune the properties of the catalystaccording to its applications (Lillerud et al. 2010; Xuereb et al. 2010). Therobust inorganic framework can indeed mimic the polypeptidic structure inthe enzyme protecting the catalytically active site and limiting the transferof reactants and products. Any modification of the internal surface of thepores, in terms of acid/base, hydrophilic/hydrophobic or polar/apolar properties,recalls the role of ancillary aminoacids in enhancing the activity and/or theselectivity of the catalytic transformation. Then, the network of wide mesoporescan accommodate the bulky and complex intermediates found in the productionof fine chemicals, following bioinspired synthetic pathways.

The development of SSHCs, however, is still in its expansion phase, and severalaspects related to the preparation and the catalytic application of these systemsdeserve 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 thanadapt existing catalysts according to a conventional trial-and-error approach. Inthis scenario, the most relevant synthesis techniques and some case studies ofSSHCs 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 transition-metal-containing open-structure aluminophosphates (AlPOs) appeared in the1980s and can be considered as the first examples of SSHC, as they fulfil themain requirements of isolation, uniform distribution and controlled chemicalenvironment 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 designand engineering steps that led to these catalytic systems, a major role was played

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1906 V. Dal Santo et al.

sol–gel orhydrothermalconditions

= O

= Si

= M

= C

= H

Figure 1. Schematic of a one-pot synthesis procedure starting from precursors of inorganicframework (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 mesoporousmolecular sieves, larger reactant molecules, which can enter the pore network,react there and leave it, can be transformed within the inner space of poroussolids (Xiao 2005; Yang et al. 2009). Mesoporous silica materials, such as MCM-41, MCM-48, HMS, KIT-6, SBA-15 or SBA-16, are, indeed, ideal supports forSSHCs, since they exhibit high surface area (600–1300 m2 g−1) and their innerand outer surfaces have a profusion of pendant silanol groups (generally from 1to 3 OH groups per square nanometre) that are optimal loci for immobilizingtransition metal catalytic centres (Rigutto et al. 2007). Among them, materialswith a good hydrothermal stability, such as HMS or SBA-15, owing to a largepore wall thickness and highly condensed frameworks with limited connectivitydefects, are preferable, as they can more easily withstand the strong conditionsoccasionally experienced during synthesis or catalysis (Zhang et al. 2005).

Catalytically active single sites can have a structural role, when they areintroduced into the framework of spatially ordered materials, via direct synthesis(figure 1). Otherwise, single sites can be added to a pre-existing support bypost-synthesis techniques.

Following a direct-synthesis approach, SSHCs with a microporous and/ormesoporous inorganic structure can be prepared. In this type of preparation,precursors of the metal active sites (e.g. salts and alkoxides) are added tothe synthesis gel of the inorganic support and part of the framework atomsare 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 whichdifferent metals (such as Co, Mg, Zn, Ti, etc.) are isomorphously substituted toframework Al, Si or P atoms. At the end, solid catalysts with an ordered arrayof pores and isolated acid and/or redox-active functionalities are obtained.

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Rational design of catalysts 1907

O

OOO

Si Si

Si

SiSi Si

Si

Si

Si Si Si

Si

Si

Si

Si

SiSi SiSiSiSiSiSi

Si

Si

Si

SiSi

Si

Si

Si

Si

Si

SiSiSi

Si

SiO

OO

OOO

OO

O

O O

O

O

O

OO

O

OO

O

O

OO

O

O

OO

O

OO

O

O

OO

OOO

OO

O O

OO

O

OO O O

OOO

O

O

O

O O

O

Si

Si

SiSi

Si

Si

Si

Si

Si O

OO

O O

O O O

O O

OO

O

OO

O

OO

O

O

OO

OO

O

OO

O

OOO

OO

OO

OO O O

O O

O

O

OO

O

O

O

OO

(b)(a)

(d )(c)

O

OOOO

O

OO O O O

O OO

Si

O

HH

MLn

M

NH3+MLn

–

MLn

H H

OO

O OO O O

H HHH H

HO

O OO

Si

O

H HO

Figure 2. Post-synthesis modifications of mesoporous silicates. Several strategies are chosento 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 areschematized 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, twomain approaches can be considered: post-synthesis covalent deposition andpost-synthesis non-covalent deposition.

In the first case, the active centre is added to the support (which can alsobe commercially available) as a precursor that can be deposited irreversibly(more precisely, anchored or grafted) onto the surface as it is, by the formationof 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 centresare immobilized onto the surface of solid supports by non-covalent interactions,such as hydrogen bonds, weak van der Waals interactions, which allow theirconfinement (trapping or encapsulation). In particular, encapsulation covers a

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1908 V. Dal Santo et al.

OH

1 2 3 4 5 6

OH

OH OH

OH

Scheme 1. a-terpineol (1), terpinen-4-ol (2), carvotanacetol (3), isopulegol (4), carveol (5) andlimonene (6).

large selection of methods for immobilizing catalytically active species within thepores of a solid, and it allows one to keep the optimal performances of the originalhomogeneous catalysts unaffected. In some cases, owing to a positive cooperativeeffect, it is also possible to have a final system with improved characteristics withrespect 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 areasilica as epoxidation catalysts

Titanium-containing mesoporous silica has been largely investigated in theliterature and synthesized following different synthetic approaches. Among thedifferent preparation methodologies proposed in the literature, the graftingreaction has largely been exploited to introduce the metal centres in thesiliceous framework, obtaining efficient SSHCs. For example, Ti centres weresuccessfully grafted onto the inner walls of MCM-41 mesoporous silica by usingan organometallic precursor, titanocene dichloride [Ti(Cp)2Cl2] (Cp = C5H5)(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 forthe selective epoxidation of bulky and richly functionalized alkenes, such as cyclicterpenes of interest for the flavour and fragrance industry, which often cannot beeasily 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 besuitable catalysts for the selective epoxidation of this class of compounds, and theresulting epoxides are useful intermediates for the synthesis of a broad series ofvaluable and high added-value products (van der Waal et al. 1998; Ravasio et al.2004). For instance, a series of cyclic unsaturated alcoholic terpenes, namelya-terpineol (1), terpinen-4-ol (2), carvotanacetol (3), isopulegol (4), carveol(5) and limonene (6) (scheme 1), was selectively epoxidized with tert-butylhydroperoxide (TBHP) over Ti-containing mesoporous silica catalysts withboth an ordered and non-ordered porous array. High conversions (up to 90%)and good selectivity to the desired epoxides (60–80%) were achieved in theliquid phase, in polar aprotic solvents (acetonitrile and ethyl acetate) underreflux 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 conversionrate of the terpene. In addition, a direct comparison between the turn-overfrequencies obtained over grafted Ti–MCM-41 (where Ti is deposited bypost-synthesis treatments) and in-framework Ti–MCM-41 (where Ti is inserted inthe catalyst’s matrix during synthesis) shows that the epoxidation rate over theformer is up to 10 times higher than over the latter, owing to the better expositionand 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 highon such reactants, and a diastereospecific epoxidation (100% diastereoisomericexcess (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 transferfrom the oxidant to one specific side of the six-membered ring. In fact, when theOH group in 1 is replaced by an acetyl moiety (a-terpinylacetate), the d.e. valuedecreases 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 anordered array of mesopores is not strictly necessary in order to have an efficientheterogeneous epoxidation catalyst. Provided that the pore diameter of the solidis large enough to accommodate the reactant molecules and that the titaniumactive sites are evenly dispersed and available on the silica support surface, themorphology of the surroundings of the titanium atoms does not considerablyaffect the catalytic performance. Indeed, when the pore diameters are too largeand no proper ‘sieving’ effect occurs, the siliceous materials act as simple supportsfor the active sites, and an ordered array of pores does not seem essential (Berliniet al. 2001). The performances obtained over Ti-containing mesoporous solidsare often fully comparable to those obtained over Ti-containing amorphous non-ordered 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 drawbackif relatively non-polar olefins have to be epoxidized: if a more hydrophobiccatalyst is needed, the silica surface silanols can be silylated. Catalysts whosesilanols were silylated with hexamethyldisilazane or triethoxyfluorosilane showa 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 ofsubstrates with intermediate polarity and with functional groups that are ableto interact with the catalyst surface, a hydrophilic environment around Ti(IV)sites can lead to good results. Thus, for instance, silylated Ti–MCM-41 catalystswere more active than non-silylated catalysts in limonene epoxidation. On thecontrary, in the epoxidation of a-terpineol, the surface silanols played a relevantrole in enhancing the chemoselective formation of the epoxide (Guidotti et al.2008a). Analogously, in the full epoxidation of methyl linoleate, the positiveinteraction of the already formed epoxide rings with the hydrophilic catalystsurface favoured the transformation of methyl epoxyoleate (the monoepoxide)into methyl diepoxystearate (the diepoxide). A direct correlation between thepolar character (expressed as surface density of silanols per gram of catalyst) andthe selectivity to diepoxides after 24 h was indeed found in the epoxidation ofsoy-bean oil fatty acid methyl ester (FAME) mixtures over titanosilicate catalystswith various textural properties (Guidotti et al. 2006).

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OHO OH

Ocitronellal isopulegol

Ti–MCM-41

PhCH3

+ TBHP

+ CH3CN

isopulegolepoxide

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, insome situations, a partial silylation of the solid can also be envisaged, in order totune accurately the hydrophilic/hydrophobic character of the catalyst, in orderto maximize the yield of the desired product (Guidotti et al. 2011a).

The grafting of Ti(IV) species over a mesoporous silica can give riseto a bifunctional acid and redox-active oxidation catalyst (the Tid+ sitewith free coordination sites) (Bhaumik & Tatsumi 1999; Bisio et al. 2011).Isopulegol epoxide (a compound with fungicidal and insect-repellent activity) wassynthesized over Ti-grafted MCM-41 under one-pot two-step conditions (Guidottiet al. 2000; Ravasio et al. 2004). Citronellal was first converted into isopulegol, byacid-catalysed cyclization (ene reaction), which was then epoxidized to isopulegolepoxide, 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 completecyclization of citronellal into isopulegol. With this approach, a global yield of 68per cent in isopulegol epoxide was obtained.

Titanocene-grafted silicas showed promising results too in the epoxidation withTBHP 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 andtrans configurations, respectively, it was found that the structural features ofan ordered solid, such as Ti–MCM-41, are necessary to attain very high yieldsinto trans-epoxystearate. So, with these substrates, by choosing titanosilicatemesoporous materials with an ordered hexagonal array of pores, such as Ti–MCM-41, it was possible to obtain very high yields in epoxidized FAMEs (upto 95% yield of epoxidized methyl linoleate or 96% yield in epoxidised castor oilmethyl 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 amain advantage, by this method, unsaturated FAMEs can be epoxidized with nouse of peracids (that are instead used industrially in the Prilezhaev epoxidationprocess), hence under completely acid-free reaction conditions.

Finally, the use of aqueous hydrogen peroxide is typically difficult over mostmesoporous titanosilicates (Clerici 2009). Actually, specific experiments on theinteraction of TBHP and H2O2 with Ti(IV) centres have clarified that a 1-aliquotaddition of aqueous H2O2 leads to a rapid and irreversible transformation ofTi(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 reportedthe synthesis of Ti(IV)–silica catalysts by grafting Ti(OiPr)4 onto a commercialnon-ordered mesoporous silica. This catalyst showed promising results in theepoxidation of non-functionalized alkenes, dienes and allylic alcohols, not onlyusing TBHP as oxidant (Cativiela et al. 1996), but also with dilute aqueoushydrogen peroxide under mild conditions (23% epoxide yield after 1 h), owingto a slow drop-wise addition of oxidant (Fraile et al. 2000). In the latter case,the contribution of direct epoxidation to the productive H2O2 consumptionincreases considerably with respect to one-shot addition (from 21% to 56%),improving the selectivity to epoxide. The role of the slow addition of H2O2is crucial to avoid, or at least minimize, the useless decomposition of theoxidant and to keep the local water concentration as low as possible. The sideproduction of allylic oxidation side-products (cyclohexenol and cyclohexenone) isthus negligible and the selectivity to the desired epoxide is very high (Fraile et al.2001b, 2003). In addition, specific immersion calorimetry studies to evaluate theinteraction between the reactant and the catalyst evidenced a very good affinityof cyclohexene towards titanosilicate, which is a crucial factor in the epoxidationreaction (Vernimmen et al. 2011).

The protocol of the controlled drop-wise addition of aqueous H2O2(4.17 mmol H2O2 h−1 g−1

cat) was applied to titanocene-grafted silica catalysts,and it was thus possible to attain excellent selectivity (greater than 98%)and interesting yields (from 44% to 63%) in cyclohexene epoxide (Guidottiet al. 2009). Even higher yields in epoxide were found over Ti–MCM-48than over Ti–MCM-41 or Ti–SiO2 owing to better isolation, dispersion andstability of Ti(IV) sites. Recently, the drop-wise addition of H2O2 has alsobeen extended successfully to the epoxidation of methyl oleate (Guidottiet al. 2011b). All the tested titanosilicate molecular sieves showed goodcatalytic activity and comparable behaviour in terms of conversion, selectivityand yield of methyl 9,10-epoxystearate, notwithstanding the morphology andthe texture of the silica support. By optimizing the reaction conditions(especially, the catalyst to reactant ratio), high yields of up to 91 per centin epoxide were obtained for Ti–MCM-41, with a high stereoselectivity (80%)towards the cis-epoxide.

Anyway, even in the presence of aqueous H2O2, if the reaction is rapidenough, the oxidation can occur before the aggregation and deactivationof Ti sites takes place. This is the case with the oxidation of substitutedphenols into benzoquinones and, for instance, very good yields (96%)in 2,3,5-trimethyl-1,4-benzoquinone, a vitamin E precursor, were obtained from2,3,6-trimethylphenol after 1 h, with no remarkable deactivation of the catalystand a good recyclability (Kholdeeva et al. 2007, 2009a). In this class of reactions,the catalysts prepared by grafting titanocene dichloride revealed a strongdependence of the product selectivity on the surface concentration of titaniumactive centres. Mesoporous materials with titanium surface concentrationin the range 0.6–1.0 Ti nm−2 were identified as optimal catalysts for thetransformation of alkylphenols into benzoquinones, whereas catalysts having lessthan 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-likespecies) and/or subnanometre-size clusters evenly distributed on the silica surfaceare the best active sites for this reaction. This phenomenon can be explained

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1912 V. Dal Santo et al.

TiO

TiO

H

H

H2O

2H2O2

Ti

OH

Ar

TiO

Ti

O

Ti

HOO

H2OOH2

O

H

OTi

OTi

O

H2OOH2

O

H OH

Ti

OH2

OHTi

H2O

HO

2H2O

quinone

TiOH

Ar

OOHTi

HOO

OH

H

R

R

R

H2O

ArOH

Scheme 3. Suggested oxidation mechanism for the conversion of alkylphenols into benzoquinonesover dinuclear Ti sites.

by the particularly favourable interaction between the phenolic substrate and(at least) two contiguous Ti hydroperoxo groups, as depicted in the suggestedoxidation mechanism (scheme 3).

Over such catalysts, the target of 100 per cent selectivity was attained inthe oxidation of both trimethylphenol and dimethylphenol, and this is the firstexample that clearly demonstrates the advantages of Ti cluster-site catalysts overTi single-site catalysts in hydrogen peroxide-based oxidation reaction. Moreover,by using several mesoporous silica supports, it was possible to find a correlationdisplaying the dependence of the selectivity on surface Ti density and to use itas a predictive tool for the preparation of catalysts with optimized performancefor the chemospecific oxidation of substituted phenols to related quinones withhydrogen peroxide (Kholdeeva et al. 2009b).

(b) Anchored organometallic complexes as single-site heterogeneous catalysts

Different approaches have been introduced to immobilize on solid surfaces theactive organometallic complexes that are designed and optimized to work underhomogeneous conditions in the liquid phase.

One possible method is based on the surface organometallic chemistry, in whichmost of the know-how developed for homogeneous organometallic complexescan be transferred to the study of novel organometallic species where the solidsurface itself acts as a ‘special’ ligand to the metal. Owing to this methodologicalapproach, Basset and co-workers have produced a number of organometalliccomplexes 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 oxidesupport (silica, alumina, etc.) via either a covalent/ionic bond or a Lewis–acid–Lewis–base interaction. In these systems, the coordination sphere of the metalinvolves not only the ancillary ligands that influence the stability, the activityand the selectivity as in homogeneous catalysis, but also the surface.

Another strategy of immobilization is to link the transition metal complexcatalyst to a mesoporous material via a spacer chain (a tether). Thisimmobilization can be carried out by substituting one of the ligands of thehomogeneous catalyst by a similar moiety that contains a linker and which isable to anchor, typically via a covalent bond, to the support/carrier (oxide,polymer and dendrimer). According to this approach, most of the syntheticeffort is devoted to preserve as much as possible, the molecular structure andthe chemical environment after anchoring (Copéret & Basset 2007).

Owing to the anchoring strategy, large and bulky moieties can be covalentlyattached onto the surface of mesoporous silicas, whose mesopores are largeenough to accommodate this kind of guest species. As an example, Ti-containingpolyhedral oligomeric silsesquioxane (Ti–POSS), which is an excellent model ofisolated and well-dispersed Ti sites in the SiO2 matrix, was first successfullydeposited (Krijnen et al. 1998; Smet et al. 2000; Zhang et al. 2007) and thenrecently anchored via covalent bonds to the surface of an ordered (SBA-15) anda non-ordered silica support (Carniato et al. 2008, 2011) (figure 3). In the latterexample, Ti–POSS moieties were, therefore, mainly accommodated as dinucleardimer species on the external surface of the ordered mesoporous silica and in thelarge 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 closeproximity of silanol groups. These solids can thus be considered as model systemsto study the behaviour of isolated Ti centres and the extent of interaction betweenthe silanol-rich silica surface and the metal species. The final Ti–POSS–SBA-15catalyst revealed a good dispersion of Ti sites and an interesting catalytic activity.Epoxidation tests on unsaturated terpenes (limonene, carveol and a-pinene) withTBHP as oxidant were performed to monitor the activity and accessibility ofTi(IV) sites in the anchored samples and compared with conventional titanocene-grafted Ti/SBA-15 and Ti/SiO2 systems. In terms of activity, anchored catalystsdisplayed comparable conversion and turn-over number values almost identicalto grafted ones. On the contrary, in terms of selectivity to epoxide products,the anchored Ti–POSS-containing materials showed slightly better results thanthe grafted ones owing to the absence of proximal silanols surrounding the Ticentres 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 acovalent-linking approach, some examples of non-covalently bound homogeneouscatalysts 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 andthe possibility to easily recover the anchored complex for further characterizationstudies by standard liquid-phase techniques.

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1914 V. Dal Santo et al.

R

SiO

OO

OO

OOO

OR

R

R´

O

OO

O

OO O

ROR

R

iPr

R

R

SiSi

Si

SiTiTi

OO

O

OOO

RSi

Si

R

RR

iPr

CH3

CH3

CHH2

R =

R´ =

O OEtSiOEt

OEtNH

NH

C

R´

SiSi

Si

SiSi

Si

Si

OO

O

Figure 3. Anchoring of a Ti-containing polyhedral oligomeric silsesquioxane (Ti–POSS) onto thesurface of silica supports. For the sake of clarity, only one tether is shown here to be directly boundto the surface. (Online version in colour.)

In this field, the groups of Bianchini & Psaro introduced, in 1999, a non-covalent method of preparing silica-immobilized metal catalysts for use in solid–liquid reactions in apolar solvents (Bianchini et al. 1999, 2000). The procedureinvolved the formation of hydrogen bonds between the terminal silanols of thesilica surface and the oxygen atoms of sulphonate groups from phosphine and/orfrom trifluoromethansulphonate counteranion. This heterogenization protocolgives rise to supported hydrogen-bonded (SHB) catalysts and is alternative tothe supported liquid-phase (SLP) method developed by Davis that involves thedissolution of a homogeneous catalyst in a hydrophilic solvent adsorbed as athin 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 = −O3S(C6H4)CH2C(CH2PPh2)3; cod = cycloocta-1,5-diene) was achi-eved by a straightforward solvent impregnation method using anhydrous

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Rh

MeCN

Ru PP

MeCN

Ph2

PPPh

2

Rh

+

+

S OO

O

P

MeCN 2+

P PP

SOOO

HH H H H H

1 2 3

–

F F F F FFCSO

OO

CSO O

O

––

OO O O O OH H HO O O

H H HO O O

–

Scheme 4. Supported hydrogen-bonded (SHB) Rh and Ru complexes on silica. (Online versionin colour.)

dichloromethane solutions of the catalyst precursors that are then stirred inthe presence of the support material until all the metal complex has beenchemisorbed onto the silica surface. Reproducible 1–2 wt.% metal loadings weregenerally obtained. Once grafted to silica, the Rh complex was not extractedback into CH2Cl2, whereas stirring the SHB complex in alcohols at roomtemperature resulted in its complete desorption from silica and its dissolution intothe solvent.

The immobilized catalyst (sulphos)Rh(cod)/SiO2 (1/SiO2 in scheme 4) isactive for the hydrogenation of alkenes in either flow reactors (ethene andpropene) or batch reactors (styrene) in hydrocarbon solvents. Moreover, in allthe cases investigated, there was no evidence whatsoever of the formation ofcontiguous Rh–Rh sites, indicating that the catalytic active sites are truly isolatedRh atoms (i.e. real SSHCs), as in the homogeneous phase. A further interestingfeature of SHB catalysis is provided by the facile and quantitative extraction ofthe grafted metal products with alcohols. This allows the study of metal productsof single-site catalytic reactions by applying standard spectroscopic techniquesin solution, as occurs when the heterogenization of organometallic complexes isachieved by the ion-pairing technique.

The SHB methodology has been extended to cationic metal complexes, ofwhich 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](OSO2CF3) can be mentioned. On the basis ofspectroscopic characterization, a structure can be proposed for Ru(II)/SiO2, inwhich both the complex cations and the triflate anions are tethered to silica byhydrogen-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 representationof the reality, as there will be cations and anions hydrogen bonded to silica viatwo or even one ≡Si–OH units.

The SHB Ru(II)/SiO2 system is a selective catalyst for the hydrogenationof benzonitrile to benzylidenebenzylamine in n-octane (35% conversion and99% selectivity after 1.5 h). No appreciable ruthenium leaching into thehydrocarbon phase was observed, even after three recycles. Notably, the reductivetransformation of an organic nitrile into a secondary imine has never been

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reported to occur in a heterogeneous fashion. The results obtained in this workare thus quite interesting, as they outline a new and clean route to the preparationof secondary bis(aryl)imines, which are important organic synthons.

The Ru(II) complex immobilized on silica has been also successfully appliedin hydrodenitrogenation/hydrodesulphurization catalysis of model hetero- andpolyaromatics (Bianchini et al. 2003). The immobilised Ru(II) catalyst wasactive and highly selective in the hydrogenation of indole to indoline (turn-overfrequency 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 observedusing the homogeneous catalyst in either CH2Cl2 or tetrahydrofuran, even forreaction times as long as 5 h (Barbaro et al. 2002). The presence of a protic acidis necessary to generate an equilibrium concentration of the 3H -indolium cationthat contains a localized, hence reducible, C=N bond. The use of protic acidsto promote the hydrogenation of indole is a common procedure in heterogeneouscatalysis, 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 theSHB Ru(II)/SiO2 system, as it needs strong acids: even trifluoroacetic acid andp-toluenesulphonic acid give incomplete protonation of indole, leading to anequilibrium concentration of 3H -indolium (Jackson & Smith 1964). Therefore,neither isolated silanols of silica (pKa > 9) nor hydrogen-bonded silanols (pKa5–7) should be able to protonate indole.

The hydrogen-bond interactions between the N–H proton of indole and theoxygen atoms of the silanols groups (either isolated or, much more likely, engagedin hydrogen bonding to the sulphonate groups) may activate the heterocyclicring 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 indolehydrogenation by Ru(II)/SiO2 was provided by the use of methylindole in theplace of indole. The protection of the nitrogen atom with a methyl group didnot allow for substrate hydrogenation. Silica surface-mediated reactions are arelatively new synthetic protocol, leading to interesting results in terms of activityand selectivity, as well as environmentally friendliness (Kabalka & Pagni 1997).The reactions can be conducted by simple mixing of silica, reagent and substratein the dry state followed by heating, while the products are generally extractedfrom the adsorbent with an appropriate solvent. Mediation by silica was foundto involve either isolated or associated silanols. Similar examples of silica surface-mediated 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 chelatingphosphines and, as such, it is the simplest procedure ever reported for thesynthesis of chiral heterogeneous catalysts, as commercially available chiralligands may be employed with no modification of their structure (Wan &Davis 1994).

Two different types of SHB chiral catalysts, all containing rhodium, havebeen prepared: neutral complexes, such as [Rh(nbd)((R)-(R)-BDPBzPSO3)]((R)-(R)-BDPBzPSO3 = (R)-(R)- 3 -(4-sulphonate)benzyl-2,4-bis(diphenylphos-phino)pentane, nbd = norbornadiene), and cationic complexes containing triflatecounter-anions, such as [Rh(nbd)((+)-DIOP)]OTf and [Rh(nbd)((S)-BINAP)]Proc. R. Soc. A (2012)

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OTf (3/SiO2 in scheme 4), where (+)-DIOP = (+)-2, 3-O-isopropylidene-2,3-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-bondinginteraction between the metal complex and surface silanols and, therefore, thesulphonate group must be previously introduced into the chiral ligand.

In contrast, the cationic complexes reside close to the silica surface byelectrostatic interaction with the triflate ions that, in turn, are immobilized onthe support via hydrogen bonds. Consistently, no immobilization whatsoever wasobserved when the triflate anion was replaced by counter-anions such as BPh−

4or BF− that are not capable of hydrogen-bonding interaction with silica.

Chiral SHB rhodium catalysts have been used in the enantioselectivehydrogenation of prochiral olefins (Bianchini et al. 2001), particularly itaconatesand 2-acetamido acrylates, in n-heptane or n-hexane. As a general trend,the immobilization of the chiral precursors on silica did not reduce theirenantioselectivity when compared with analogous homogeneous reactions inmethanol, a beneficial effect was observed in some cases (cf. hydrogenation ofdimethylitaconate, homogeneous versus SHB [((S)-BINAP)Rh(nbd)]OTf, whichshow 100% conversion, with 25% of enantiomeric excess (e.e.) (S) and 100%conversion, with 30% of e.e. (S), respectively). Moreover, the conversion valueswere generally comparable with or higher than those in the homogeneous phase.

Irrespective of the immobilization procedure of the metal complex (SHB or ionpairing to SHB counter-anion), no appreciable Rh leaching was observed withinseveral consecutive heterogeneous runs, and an effective catalyst recycling withno loss of activity or enantioselectivity was accomplished by standard procedures.

When complexes are supported inside the pores (of the proper size) ofmesoporous 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 poresizes, were tested in the asymmetric hydrogenation of methyl benzoylformateto methyl mandelate showing excellent e.e. values (i.e. [Rh(cod)PMP]CF3SO3/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]CF3SO3, where L-L can be: (S ,S ,S)-[2-(4,5-dihydro-4′,5′-diphenyl-oxazol-2′-yl)ferrocenyl] phosphine ((S ,S ,S)-dipof ); L-tryptophanbenzyl ester;(−)-(S)-2-(aminomethyl)-1-ethylpyrrolidine or (+)-(1R, 2R)-1,2-diphenylethane-1,2-diamine) were immobilized on MCM-41 and used for the hydrogenation of(E)-(a)-phenylcinnamic acid to 2,3-diphenylpropionic acid showing similar goodperformances, up to 90 per cent conversion, 84 per cent selectivity and 97 percent e.e. (Rouzaud et al. 2003).

SHB catalysts also found interesting applications in other reactions, differentfrom the hydrogenation ones, such as cyclopropanation, Diels–Alder additions orepoxidation reactions.

Preformed polypyrazolylborate copper(I) complexes BpCu, Tp∗Cu and[B(pz)4]Cu (Bp = dihydrobispyrazolylborate, H2B(pz)2; Tp∗ = hydrotris(3,5-dimethyl)pyrazolylborate, HB(pz∗)3; B(pz)4 = tetrakispyrazolylborate) wereimmobilized by classical and non-classical hydrogen-bond ligand-to-supportinteraction involving the hydroxyl groups of the silica surface and the borohydrideBH group and/or the pyrazolyl nitrogen atoms on silica gel. The resulting

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catalysts were employed for the styrene cyclopropanation reaction. The catalyticactivity is very similar to that observed under homogeneous conditions (i.e. 76%yield and 75 : 25 anti/syn ratio, under homogenous conditions versus 90%, yieldand 54 : 46 anti/syn ratio, under heterogeneous conditions). The strength ofsuch interactions lets the recovery and recycling of the supported catalysts for anumber of cycles, even if some leaching was observed using solvents other thanpetroleum 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 effectivecatalysts for the Diels–Alder reaction between 3-((E)-2-butenoyl)-1,3-oxazolin-2-one and cyclopentadiene allowing up to 93 per cent e.e. at room temperatureto be obtained with the heterogeneous bis(oxazoline) complexes. Moreover, thecatalysts can be recycled without significant loss of enantioselectivity (Wanget al. 2006).

Again, sulphonato-(salen)Mn(III) complex hydrogen bonded to silicaswas revealed to be an effective and recyclable catalyst in the enatioselectiveepoxidation of styrene and a-methylstyrene (Tang et al. 2008). Enantiomericexcess 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/H2O).

Finally, a recent development of the SHB grafting methodology worthmentioning 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). Thesesystems showed an enhanced activity, greater than the sum of the activities ofthe two single components, in aromatic hydrogenation reactions and for them,a synergistic effect of the single-site molecular metal complex on the metallicnanoparticles has been evoked.

(d) Single-site heterogeneous catalysts prepared by one-pot approaches

Microporous solids, such as zeolites and AlPOs, possess a well-organizedinorganic structure composed of pores and channels with regular dimensions,where single-site catalytic entities can be introduced, aiming at the preparationof a wide range of catalysts for selective oxidation and acid-catalysedtransformations (Hartmann & Kevan 1999; Albuquerque et al. 2006; Raja et al.2008). The high performances in terms of activity and selectivity inoxidation reactions have been mainly attributed to the particular localstructure and coordination geometry of the active metals when introduced inframework positions.

Various examples of single-site microporous catalysts prepared by the one-pot procedure and used for catalytic oxidation reactions are reported in theliterature. Metal-containing AlPOs were extensively studied as catalysts foroxidation and oxidative hydrogenation reactions, obtaining interesting results(Thomas et al. 1999). In different cases, the catalytic performance of one-potprepared materials was compared with that of solids containing the same activespecies 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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As an example, aluminophosphate Co–APO-5 solid was proposed as a catalystfor selective catalytic oxidation of organosulphur compounds (Chica et al. 2006).The catalyst was prepared by introducing the synthesis gel used for the AlPOpreparation, Co(acetate), as a precursor of the redox site. A Co/H-Y zeolitematerial (containing a similar amount of metal species) was prepared by classicalion-exchange reactions to obtain a comparison.

The oxidation of different sulphur compounds (sulphides, benzothiophenesand thiophenes) with tert-butyl hydroperoxide occurs on both Co–APO-5 andCo/H-Y. Co–APO-5 gave the highest organosulphur oxidation rates (per Coatom), reflecting the role of tetrahedral Co framework cations that undergo facileredox cycles. Indeed, when Co3+ is in the framework position, it can be easilyreduced 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. Thegood conversion obtained with the largest organosulphur compounds (diphenylsulphide and 4-methyldibenzothiophene) suggests that site accessibility is not amajor problem within the channels of Co–APO-5, even for bulky substrates. Inaddition, this type of SSHC proved to be less prone to metal leaching underreaction conditions than ion-exchanged analogues.

In the recent literature, examples of the preparation of multi-functionalmicroporous catalysts through one-pot reactions have been given. Raja andco-workers focused attention on bimetallic multi-functional solids, obtained byisomorphously replacing Al(III) and P(V) framework cations with tetrahedrallycoordinated Co(III) and Ti(IV) ions, respectively (Paterson et al. 2011). Thehighly active catalysts exhibited an interesting synergistic effect between Coand Ti sites and an enhancement of the catalytic performance, when comparedwith results obtained from their corresponding monometallic analogues in thecatalytic epoxidation of olefins. This behaviour was associated with the factthat the active metal ions are located not only in the AlPO framework, butalso in close proximity with each other. It was also hypothesized that thesimultaneous isomorphous substitution of Al(III) and P(V) in the AlPO-5framework with Co(III) and Ti(IV) metal ions results in a higher fraction ofCo(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 suitablesites for the generation of free-radical intermediates involved in the catalyticepoxidation reactions.

Other interesting examples of SSHCs were obtained by introducing redoxmetal centres in mesoporous solids through one-pot reactions. Although metal-containing mesoporous materials are often prepared by post-synthesis reactions,in some cases, the one-pot preparation approach is useful to obtain high-surface area solids with highly dispersed active species and with readilyreducible/oxidizable metal centres. Very recently, the peculiar catalytic activityof one-pot prepared V–SBA-15 and V−SiO2 towards dichloromethane conversionwith respect to impregnated solids was reported. This was associated withthe presence of isolated V species incorporated inside silica walls producedduring one-pot synthesis, while impregnated species especially contains poly-vanadate (or microcrystalline V2O5) phases that are not so active forgas-phase volatile organic compound oxidation purposes (Piumetti et al.2010, 2011).

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4. Conclusions

The examples presented in this brief overview show how SSHCs can be appliedto different transformations of interest for the synthesis of richly functionalizedfine chemicals. Redox-active metal-containing solid catalysts can be used as viablealternatives to stoichiometric non-catalytic processes (still widely used, especiallyin oxidation reactions) that are based on hazardous and environmentallyunfriendly reactants. SSHCs proved, in many cases, to be robust, easily recyclableand potential candidates for developing new chemo-, regio- and stereoselectivesynthetic processes at a large-scale level. Nevertheless, the development ofbioinspired inorganic equivalents of enzymes based on nanostructured molecularsieves 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 atan industrial level, even if the high added value of the product could justify theuse of valuable inorganic oxide-based SSHCs, often prepared by expensive andtime-consuming synthetic routes.

Once again, a multi-disciplinary approach, based on a strict cooperationbetween chemists with different cultural backgrounds (in heterogeneous catalysis,organic synthesis, biochemistry, inorganic chemistry, materials sciences, molecularmodelling, etc.) can be the only winning strategy to obtain successful results inthis field in the next future.

The financial support of the European Community’s 7th Framework Programme through the MarieCurie Initial Training Network NANO-HOST (grant agreement no. 215193) and of the ItalianMinistry of Education, University and Research through the project ‘ItalNanoNet’ (Rete Nazionaledi Ricerca sulle Nanoscienze; prot. no. RBPR05JH2P) is acknowledged.

References

Adkins, H. & Coonradt, H. L. 1941 The selective hydrogenation of derivatives of pyrrole, indole,carbazole and acridine. J. Am. Chem. Soc. 63, 1563–1570. (doi:10.1021/ja01851a020)

Albuquerque, A., Marchese, L., Lisi, L. & Pastore, H. O. 2006 [V,Al]-MCM-22: a novelacid/redox bifunctional molecular sieve. J. Catal. 241, 367–377. (doi:10.1016/j.jcat.2006.05.012)

Barbaro, P., Bianchini, C., Meli, A., Moreno, M. & Vizza, F. 2002 Hydrogenation of indoleby phosphine-modified rhodium and ruthenium catalysts. Organometallics 31, 1430–1437.(doi:10.1021/om011005n)

Barrett, P. A., Sankar, G., Catlow, C. R. A. & Thomas, J. M. 1996 X-ray absorption spectroscopicstudy of Bronsted, Lewis, and redox centers in cobalt-substituted aluminum phosphate catalysts.J. Phys. Chem. 100, 8977–8985. (doi:10.1021/jp953034f)

Basset, J. M. & Choplin, A. 1983 Surface organometallic chemistry: a new approach toheterogeneous catalysis. J. Mol. Catal. 21, 95–108.

Basset, J. M. & Ugo, R. 2009 Modern surface organometallic chemistry (eds J. M. Basset, R. Psaro,D. Roberto & R. Ugo), pp. 1–22. Weinheim, Germany: Wiley-VCH.

Beckler, R. K. & White, M. G. 1986 Polynuclear metal complexes as model mixed oxide catalysts:an ion exchange support interaction. J. Catal. 102, 252–256. (doi:10.1016/0021-9517(86)90160-0)

Berlini, C., Guidotti, M., Moretti, G., Psaro, R. & Ravasio, N. 2000 Catalytic epoxidation ofunsaturated alcohols on Ti-MCM-41. Catal. Today. 60, 219–225. (doi:10.1016/S0920-5861(00)00338-2)

Proc. R. Soc. A (2012)

on May 25, 2012rspa.royalsocietypublishing.orgDownloaded from

Rational design of catalysts 1921

Berlini, C., Ferraris, G., Guidotti, M., Moretti, G., Psaro, R. & Ravasio, N. 2001 A comparisonbetween [Ti]-MCM-41 and amorphous mesoporous silica-titania (MST) as catalysts for theepoxidation of bulky unsaturated alcohols. Microp. Mesop. Mater. 44–45, 595–602. (doi:10.1016/S1387-1811(01)00239-6)

Bhaumik, A. & Tatsumi, T. 1999 Intramolecular rearrangement of epoxides generated in situ overtitanium silicate molecular sieves. J. Catal. 182, 349–356. (doi:10.1006/jcat.1998.2355)

Bianchini, C. & Barbaro, P. 2002 Recent aspects of asymmetric catalysis by immobilized chiralmetal catalysts. Topics Catal. 19, 17–32. (doi:10.1023/A:1013825114636)

Bianchini, C., Burnaby, D. G., Evans, J., Frediani, P., Meli, A., Oberhauser, W., Psaro, R.,Sordelli, L. & Vizza, F. 1999 Preparation, characterization, and performance of tripodalpolyphosphine rhodium catalysts immobilized on silica via hydrogen bonding. J. Am. Chem.Soc. 121, 5961–5971. (doi:10.1021/ja983940g)

Bianchini, C., Dal Santo, V., Meli, A., Oberhauser, W., Psaro, R. & Vizza, F. 2000Preparation, characterization, and performance of the supported hydrogen-bonded rutheniumcatalyst [(sulphos)Ru(NCMe)3](OSO2CF3)/SiO2. Comparisons with analogous homogeneousand aqueous-biphase catalytic systems in the hydrogenation of benzylideneacetone andbenzonitrile. Organometallics 19, 2433–2444. (doi:10.1021/om000309r)

Bianchini, C., Barbaro, P., Dal Santo, V., Gobetto, R., Meli, A., Oberhauser, W., Psaro, R. &Vizza, F. 2001 Immobilization of optically active rhodium-diphosphine complexes on poroussilica via hydrogen bonding. Adv. Synth. Catal. 343, 41–45. (doi:10.1002/1615-4169(20010129)343:1<41::AID-ADSC41>3.0.CO;2-P)

Bianchini, C., Dal Santo, V., Meli, A., Moneti, S., Moreno, M., Oberhauser, W., Psaro, R.,Sordelli, L. & Vizza, F. 2003 A comparison between silica-immobilized ruthenium(ii) single sitesand silica-supported ruthenium nanoparticles in the catalytic hydrogenation of model hetero-and polyaromatics contained in raw oil materials. J. Catal. 213, 47–62. (doi:10.1016/S0021-9517(02)00027-1)

Bisio, C., Gatti, G., Marchese, L., Guidotti, M. & Psaro, R. 2011 Design and applicationsof multifunctional catalysts based on inorganic oxides. In Design and applications ofmultifunctional catalysts based on inorganic oxides (eds G. Rios, N. Kannellopoulos & G. Centi),pp. 13–54. Singapore: Pan Stanford Press.

Carniato, F., Bisio, C., Boccaleri, E., Guidotti, M., Gavrilova, E. & Marchese, L. 2008Titanosilsesquioxane anchored on mesoporous silicas: a novel approach for the preparation ofheterogeneous catalysts for selective oxidations. Chem. Eur. J. 14, 8098–8101. (doi:10.1002/chem.200801241)

Carniato, F., Bisio, C., Sordelli, L., Gavrilova, E. & Guidotti, M. 2011 Ti-POSS covalentlyimmobilized onto mesoporous silica. A model for active sites in heterogeneous catalyticepoxidation Inorg. Chim. Acta 380, 244–251.(doi:10.1016/j.ica.2011.11.051)

Cativiela, C., Fraile, J. M., Garcia, J. I. & Mayoral, J. A. 1996 A new titanium-silica catalyst forthe epoxidation of alkenes. J. Mol. Catal. A 112, 259–267. (doi:10.1016/1381-1169(96)00131-8)

Chen, J. et al. 1994 SAPO-18 catalysts and their Brönsted acid sites. J. Phys. Chem. 98,10 216–10 224. (doi:10.1021/j100091a042)

Chica, A., Gatti, G., Moden, B., Marchese, L. & Iglesia, E. 2006 Selective catalytic oxidationof organosulfur compounds with tert-butyl hydroperoxide. Chem. Eur. J. 12, 1960–1967.(doi:10.1002/chem.200500858)

Clerici, M. G. 2009 Titanium silicalite-1. In Metal oxide catalysis (eds S. D. Jackson &J. S. J. Hargreaves), pp. 705–754. Weinheim, Germany: Wiley-VCH.

Climent, M. J., Corma, A. & Iborra, S. 2011 Heterogeneous catalysts for the one-pot synthesis ofchemicals and fine chemicals. Chem. Rev. 111, 1072–1133. (doi:10.1021/cr1002084)

Copéret, C. & Basset, J. M. 2007 Strategies to immobilize well-defined olefin metathesis catalysts:supported homogeneous catalysis versus surface organometallic chemistry. Adv. Synth. Catal.349, 78–92. (doi:10.1002/adsc.200600443)

Copéret, C., Chabanas, R., Saint-Arroman, R. P. & Basset, J. M. 2003 Homogeneous andheterogeneous catalysis: bridging the gap through surface organometallic chemistry. Angew.Chem. Int. Ed. 42, 156–181. (doi:10.1002/anie.200390072)

Proc. R. Soc. A (2012)

on May 25, 2012rspa.royalsocietypublishing.orgDownloaded from

1922 V. Dal Santo et al.

Corma, A. 1997 From microporous to mesoporous molecular sieve materials and their use incatalysis. Chem. Rev. 97, 2373–2420. (doi:10.1021/cr960406n)

Corma, A., Jorda, J. L., Navarro, M. T. & Rey, F. 1998 One step synthesis of highly activeand selective epoxidation catalysts formed by organic–inorganic Ti containing mesoporouscomposites. Chem. Commun. 1998, 1899–1900. (doi:10.1039/a804801k)

Dal Santo, V., Liguori, F., Pirovano, C. & Guidotti, M. 2010 Design and use of nanostructuredsingle-site heterogeneous catalysts for the selective transformation of fine chemicals. Molecules15, 3829–3856. (doi:10.3390/molecules15063829)

Davis, M. E. 1992 Supported aqueous-phase catalysis. ChemTech. 22, 498–502.Davis, M. E. 2001 Evolution of extra-large pore materials. In Zeolites and mesoporous materials at

the dawn of the 21st century, vol. 135 (eds A. Galarneau, F. Di Renzo, F. Fajula & J. Vedrine),pp. 29–36. Amsterdam, The Netherlands: Elsevier.

Fache, F., Schulz, E., Tommasino, M. L. & Lemaire, M. 2000 Nitrogen-containing ligandsfor asymmetric homogeneous and heterogeneous catalysis. Chem. Rev. 100, 2159–2232.(doi:10.1021/cr9902897)

Fraile, J. M., Garcia, J. I., Mayoral, J. A. & Vispe, E. 2000 Silica-supported titanium derivatives ascatalysts for the epoxidation of alkenes with hydrogen peroxide: a new way to tunable catalyticactivity through ligand exchange. J. Catal. 189, 40–51. (doi:10.1006/jcat.1999.2701)

Fraile, J. M., Garcia, J. I., Mayoral, J. A., Vispe, E., Brown, D. R. & Naderi, M. 2001a Is MCM-41really advantageous over amorphous silica? The case of grafted titanium epoxidation catalysts.J. Chem. Soc. Chem. Commun. 2001, 1510–1511. (doi:10.1039/B103057B)

Fraile, J. M., Garcia, J. I., Mayoral, J. A. & Vispe, E. 2001b Effect of the reaction conditionson the epoxidation of alkenes with hydrogen peroxide catalyzed by silica-supported titaniumderivatives. J. Catal. 204, 146–156. (doi:10.1006/jcat.2001.3355)

Fraile, J. M., Garcia, J. I., Mayoral, J. A. & Vispe, E. 2003 Optimization of cyclohexene epoxidationwith dilute hydrogen peroxide and silica-supported titanium catalysts. Appl. Catal. A 245,363–376. (doi:10.1016/S0926-860X(02)00643-9)

Gianotti, E., Bisio, C., Marchese, L., Guidotti, M., Ravasio, N., Psaro, R. & Coluccia, S. 2007Ti(IV) catalytic centers grafted on different siliceous materials: a spectroscopic evaluation ofthe surface properties. J. Phys. Chem. C 111, 5083–5089. (doi:10.1021/jp067506+)

Guidotti, M., Moretti, G., Psaro, R. & Ravasio, N. 2000 One-pot conversion of citronellalinto isopulegol epoxide on mesoporous titanium silicate. Chem. Commun. 2000, 1789–1790.(doi:10.1039/b003397i)

Guidotti, M., Conti, L., Fusi, A., Ravasio, N. & Psaro, R. 2002 Diastereoselective epoxidation ofhydroxy-containing unsaturated terpenes on heterogeneous titanium-catalyst. J. Mol. Catal. A.182–183, 151–156. (doi:10.1016/S1381-1169(01)00484-8)

Guidotti, M., Ravasio, N., Psaro, R., Ferraris, G. & Moretti, G. 2003a Epoxidation on titanium-containing silicates: do structural features really affect the catalytic performance? J. Catal. 214,242–250. (doi:10.1016/S0021-9517(02)00152-5)

Guidotti, M., Ravasio, N., Psaro, R., Gianotti, E., Marchese, L. & Coluccia, S. 2003b Heterogeneouscatalytic epoxidation of fatty acid methyl esters on titanium-grafted silicas. Green Chem. 5,421–424. (doi:10.1039/b303171c)

Guidotti, M., Ravasio, N., Psaro, R., Gianotti, E., Marchese, L. & Coluccia, S. 2006 Epoxidationof unsaturated FAMEs obtained from vegetable source over Ti(IV)-grafted silica catalysts: acomparison between ordered and non-ordered mesoporous materials. J. Mol. Catal. A Chem.250, 218–225. (doi:10.1016/j.molcata.2006.01.032)

Guidotti, M., Psaro, R., Sgobba, M. & Ravasio, N. 2007 Catalytic processes for the selectiveepoxidation of fatty acids: more environmentally benign routes. In Catalysis for renewables:from feedstock to energy production (eds G. Centi & R. A. van Santen), pp. 257–272. Weinheim,Germany: Wiley-VCH.

Guidotti, M., Batonneau-Gener, I., Gianotti, E., Marchese, L., Mignard, S., Psaro, R., Sgobba,M. & Ravasio, N. 2008a The effect of silylation on titanium-containing silica catalysts forthe epoxidation of functionalised molecules. Microp. Mesop. Mater. 111, 39–47. (doi:10.1016/j.micromeso.2007.07.010)

Proc. R. Soc. A (2012)

on May 25, 2012rspa.royalsocietypublishing.orgDownloaded from

Rational design of catalysts 1923

Guidotti, M., Psaro, R., Ravasio, N., Sgobba, M., Gianotti, E. & Grinberg, S. 2008b Titanium–silica catalysts for the production of fully epoxidised fatty acid methyl esters. Catal. Lett. 122,53–56. (doi:10.1007/s10562-007-9345-9)

Guidotti, M., Pirovano, C., Ravasio, N., Lázaro, B., Fraile, J. M., Mayoral, J. A., Coq, B.& Galarneau, A. 2009 The use of H2O2 over titanium-grafted mesoporous silica catalysts:a step further towards sustainable epoxidation. Green Chem. 11, 1421–1427. (doi:10.1039/b903302e)

Guidotti, M., Psaro, R., Batonneau-Gener, I. & Gavrilova, E. 2011a Heterogeneous catalyticepoxidation: high limonene oxide yields by surface silylation of Ti-MCM-41. Chem. Eng. Tech.34, 1924–1927. (doi:10.1002/ceat.201100243)

Guidotti, M., Gavrilova, E., Galarneau, A., Coq, B., Psaro, R. & Ravasio, N. 2011b Epoxidationof methyl oleate with hydrogen peroxide. The use of Ti-containing silica solids as efficientheterogeneous catalysts. Green Chem. 13, 1806–1811. (doi:10.1039/c1gc15151g)

Guisnet, M. & Guidotti, M. 2006 Problems and pitfalls in the applications of zeolites and othermicroporous and mesoporous solids to catalytic fine chemical synthesis. In Catalysts for finechemical synthesis: micro and mesoporous solid catalysts, vol. 4 (eds E. Derouane & S. M.Roberts), pp. 39–67. Weinheim, Germany: Wiley-VCH Verlag.

Hartmann, M. & Kevan, L. 1999 Transition-metal ions in aluminophosphate andsilicoaluminophosphate molecular sieves: location, interaction with adsorbates and catalyticproperties. Chem. Rev. 99, 635–663. (doi:10.1021/cr9600971)

Jackson, A. H. & Smith, A. E. 1964 The protonation of tryptamine derivatives in acidic media.J. Chem. Soc. 1964, 5510–5517. (doi:10.1039/jr9640005510)

Kabalka, G. W. & Pagni, R. M. 1997 Organic reactions on alumina. Tetrahedron 53, 7999–8065.(doi:10.1016/S0040-4020(97)00264-0)

Kamegawa, T., Suzuki, N., Tsuji, K., Sonoda, J., Kuwahara, Y., Mori, K. & Yamashita,H. 2011 Preparation of hydrophobically modified single-site Ti-containing mesoporous silica(TiSBA-15) and their enhanced catalytic performances. Catal. Today 175, 393–397.(doi:10.1016/j.cattod.2011.04.012)

Kholdeeva, O. A., Ivanchikova, I. D., Guidotti, M. & Ravasio N. 2007 Highly efficient production of2,3,5-trimethyl-1,4-benzoquinone using aqueous H2O2 and grafted Ti(IV)/SiO2 catalyst. GreenChem. 9, 731–733. (doi:10.1039/b617162a)

Kholdeeva, O. A., Ivanchikova, I. D., Guidotti, M., Pirovano, C., Ravasio, N., Barmatova,M. V. & Chesalov, Y. A. 2009a Highly selective oxidation of alkylphenols to benzoquinoneswith hydrogen peroxide over silica-supported titanium catalysts: titanium cluster site versustitanium single site. Adv. Synth. Catal. 351, 1877–1889. (doi:10.1002/adsc.200900109)

Kholdeeva, O. A., Ivanchikova, I. D., Guidotti, M., Ravasio, N., Sgobba, M. & Barmatova, M. V.2009b How to reach 100% selectivity in H2O2-based oxidation of 2,3,6-trimethylphenol totrimethyl-p-benzoquinone over Ti,Si-catalysts. Catal. Today 141, 330–336. (doi:10.1016/j.cattod.2008.06.005)

Kolasinski, K. W. 2007. Surface science: foundations of catalysis and nanoscience, 2nd edn. NewYork, NY: J. Wiley.

Krijnen, S., Abbenhuis, H. C. L., Hanssen, R. W. J. M., Van Hooff, J. H. C. & vanSanten, R. A. 1998 Solid-phase immobilization of a new epoxidation catalyst. Angew. Chem.Int. Ed. 37, 356–358. (doi:10.1002/(SICI)1521-3773(19980216)37:3<356::AID-ANIE356>3.0.CO;2-5)

Kropp, P. J., Breton, G. W., Fields, J. D., Tung, J. C. & Loomis, B. R. 2000 Surface-mediatedreactions. 8. Oxidation of sulfides and sulfoxides with tert-butyl hydroperoxide and OXONE.J. Am. Chem. Soc. 122, 4280–4285. (doi:10.1021/ja9940569)

Lillerud, K. P., Olsbye, U. & Tilset, M. 2010 Designing heterogeneous catalysts byincorporating enzyme-like functionalities into MOFs. Top. Catal. 53, 859–868. (doi:10.1007/s11244-010-9518-4)

Marchese, L., Chen, J., Thomas, J. M., Coluccia, S. & Zecchina, A. 1994 Brönsted, Lewis andredox centres on CoAPO-18 catalysts. Vibrational modes of adsorbed water. J. Phys. Chem.98, 13 350–13 356. (doi:10.1021/j100101a040)

Proc. R. Soc. A (2012)

on May 25, 2012rspa.royalsocietypublishing.orgDownloaded from

1924 V. Dal Santo et al.

Mar Díaz-Requejo, M., Belderrain, T. R., Nicasio, M. C. & Pérez, P. J. 2000 From homogeneousto heterogeneous catalysis: novel anchoring of polypyrazolylborate copper(I) complexes onsilica gel through classical and nonclassical hydrogen bonds. Use as catalysts of the olefincyclopropanation reaction. Organometallics 19, 285–289. (doi:10.1021/om990780o)

Maschmeyer, T. & van de Water, L. 2006 An overview of zeolite, zeotype, mesoporous solidschemistry: design synthesis and catalytic properties. In Catalysts for fine chemical synthesis:micro and mesoporous solid catalysts, vol. 4 (eds Derouane, E. & Roberts, S. M.), pp. 1–38.Weinheim, Germany: Wiley-VCH Verlag.

Maschmeyer, T., Rey, F., Sankar, G. & Thomas, J. M. 1995 Heterogeneous catalysts obtainedby grafting metallocene complexes onto mesoporous silica. Nature 378, 159–162. (doi:10.1038/378159a0)

Oldroyd, R. D., Thomas, J. M., Maschmeyer, T., MacFaul, P. A., Snelgrove, D. W., Ingold,K. U. & Wayner, D. D. M. 1996 The Ti(IV)-catalyzed epoxidation of alkenes by tert-alkylhydroperoxides. Angew. Chem. Int. Ed. 35, 2787–2790. (doi:10.1002/anie.199627871)

Paterson, J., Potter, M., Gianotti, E. & Raja, R. 2011 Engineering active sites for enhancingsynergy in heterogeneous catalytic oxidations. Chem. Commun. 47, 517–519. (doi:10.1039/c0cc02341h)

Peña, M. L., Dellarocca, V., Rey, F., Corma, A., Coluccia, S. & Marchese, L. 2001 Elucidatingthe local environment of Ti(IV) active sites in Ti-MCM-48: a comparison between silylated andcalcined catalysts. Microp. Mesop. Mater. 44–45, 345–356. (doi:10.1016/S1387-1811(01)00201-3)

Piumetti, M., Bonelli, B., Armandi, M., Gaberova, L., Casale, S., Massiani, P. & Garrone, E.2010 Vanadium-containing SBA-15 systems prepared by direct synthesis: physico-chemical andcatalytic properties in the decomposition of dichloromethane. Microp. Mesop. Mater. 133, 36–44.(doi:10.1016/j.micromeso.2010.04.011)

Piumetti, M., Bonelli, B., Massiani, P., Dzwigaj, S., Rossetti, I., Casale, S., Armandi, M., Thomas,C. & Garrone, E. 2011 Effect of vanadium dispersion and support properties on the catalyticactivity of V-SBA-15 and V-MCF mesoporous materials prepared by direct synthesis. Catal.Today 176, 458–464. (doi:10.1016/j.cattod.2010.10.066)

Raja, R., Thomas, J. M., Jones, M. D., Johnson, B. F. G. & Vaughan, D. E. W. 2003 Constrainingasymmetric organometallic catalysts within mesoporous supports boosts their enantioselectivity.J. Am. Chem. Soc. 125, 14 982–14 983. (doi:10.1021/ja030381r)

Raja, R., Thomas, J. M., Greenhill-Hooper, M., Ley, S. V. & Paz, F. A. A. 2008 Facile,one-step production of niacin (Vitamin B(3)) and other nitrogen-containing pharmaceuticalchemicals with a single-site heterogeneous catalyst. Chem. Eur. J. 14, 2340–2348. (doi:10.1002/chem.200701679)

Ratnasamy, P., Srinivas, D. & Knözinger, H. 2004 Active sites and reactive intermediatesin titanium silicate molecular sieves. Adv. Catal. 48, 1–169. (doi:10.1016/S0360-0564(04)48001-8)

Ravasio, N., Zaccheria, F., Guidotti, M. & Psaro, R. 2004 Mono- and bifunctionalheterogeneous catalytic transformation of terpenes and terpenoids. Topics Catal. 27, 157–168.(doi:10.1023/B:TOCA.0000013550.28170.6a)

Rigutto, M. S., van Veen, R. & Huve, L. 2007 Application of mesoporous molecular sieves incatalysis and separation. Stud. Surf. Sci. Catal. 168, 837–854. (doi:10.1016/S0167-2991(07)80811-1)

Rouzaud, J., Jones, M. D., Raja, R., Johnson, B. F. G., Thomas, J. M. & Duera, M. J. 2003Potent new heterogeneous asymmetric catalysts. Helv. Chem. Acta. 86, 1753–1759. (doi:10.1002/hlca.200390145)

Sankar, G., Rey, F., Thomas, J. M., Greaves, G. N., Corma, A., Dobson, B. T. & Dent, A. J. 1994Probing active sites in solid catalysts for the liquid-phase epoxidation of alkenes. J. Chem. Soc.Chem. Commun. 1994, 2279–2280. (doi:10.1039/c39940002279)

Scott, S. L., Dufour, P., Santini, C. C. & Basset, J.-M. 1994 Surface-mediated organometallicsynthesis of [≡SiO]–[RhH2(PMe3)4]+: the first example of a cationic organometallic complexattached to the silica surface by ion pairing. J. Chem. Soc. Chem. Commun. 17, 2011–2012.(doi:10.1039/c39940002011)

Proc. R. Soc. A (2012)

on May 25, 2012rspa.royalsocietypublishing.orgDownloaded from

Rational design of catalysts 1925

Sheldon, R. A., Arends, I. W. C. E. & Lempers, H. E. B. 1998 Heterogeneous catalysts forliquid phase oxidations. In Supported reagents and catalysts in chemistry (eds B. K. Hodnett,A. P. Kybett, J. H. Clark & K. Smith), pp. 37–46. London, UK: The Royal Society of Chemistry.

Smet, P., Riondato, J., Pauwels, T., Moens, L. & Verdonck, L. 2000 Preparation andcharacterization of a titanium(IV) silsesquioxane epoxidation catalyst anchored into mesoporousMCM-41. Inorg. Chem. Commun. 3, 557–562. (doi:10.1016/S1387-7003(00)00128-3)

Smith, A. & Utley, J. H. P. 1965 The catalytic hydrogenation of indoles. Chem. Commun. 18,427–428.

Tang, X., Tang, Y., Xu, G., Wei, S. & Sun, Y. 2008 Highly enantioselective epoxidation of styreneand a-methylstyrene catalyzed by new doubly-immobilized chiral (salen)Mn(III) catalysts.Catal. Commun. 10, 317–320. (doi:10.1016/j.catcom.2008.09.011)

Thomas, J. M. 1994 The chemistry of crystalline sponges. Nature 368, 289–290.(doi:10.1038/368289a0)

Thomas, J. M. 1997 The ineluctable need for in situ methods of characterizing solid catalystsas a prerequisite to engineering active sites. Chem. Eur. J. 3, 1557–1562. (doi:10.1002/chem.19970031004)

Thomas, J. M. 1999 Design, synthesis and in situ characterization of new solid catalysts. Angew.Chem. Int. Ed. 38, 3588–3628. (doi:10.1002/(SICI)1521-3773(19991216)38:24<3588::AID-ANIE3588>3.0.CO;2-4)

Thomas, J. M. & Raja, R. 2004 Catalytic significance of organometallic compounds immobilizedon mesoporous silica: economically and environmentally important examples. J. Organomet.Chem. 689, 4110–4124. (doi:10.1016/j.jorganchem.2004.07.052)

Thomas, J. M., Greaves, G. N., Sankar, G., Wright, P. A., Chen, J., Dent, A. J. & Marchese, L.1994 On the nature of the active sites in a CoAPO-18 solid acid catalyst. Angew. Chem. Int.Ed. 33, 1871–1873. (doi:10.1002/anie.199418711)

Thomas, J. M., Raja, R., Sankar, F. & Bell, R. G. 1999 Molecular-sieve catalysts for the selectiveoxidation of linear alkanes by molecular oxygen. Nature 398, 227–230. (doi:10.1038/18417)

Thomas, J. M., Raja, R. & Lewis, D. W. 2005 Single-site heterogeneous catalysts. Angew. Chem.Int. Ed. 44, 6456–6482. (doi:10.1002/anie.200462473)

Thomas, J. M., Hernandez, J. C., Raja, R. & Bell, R. G. 2009 Nanoporous oxidic solids: theconfluence of heterogeneous and homogeneous catalysis. Phys. Chem. Chem. Phys. 11, 2799–2825. (doi:10.1039/b819249a)

van der Waal, J. C., Rigutto, M. S. & van Bekkum, H. 1998 Zeolite titanium beta as a selectivecatalyst in the epoxidation of bulky alkenes. Appl. Catal. A 167, 331–342. (doi:10.1016/S0926-860X(97)00323-2)

Vernimmen, J. et al. 2011 Immersion calorimetry as a tool to evaluate the catalytic performanceof titanosilicate materials in the epoxidation of cyclohexene. Langmuir 27, 3618–3625.(doi:10.1021/la104808v)

Wan, K. & Davis, M. E. 1994 Design and synthesis of a heterogeneous asymmetric catalyst. Nature370, 449–450. (doi:10.1038/370449a0)

Wang, H., Liu, X., Xia, H., Liu, P., Gao, J., Ying, P., Xiao, J. & Li, C. 2006 Asymmetric Diels–Alder reactions with hydrogen bonding heterogeneous catalysts and mechanistic studies on thereversal of enantioselectivity. Tetrahedron 62, 1025–1032. (doi:10.1016/j.tet.2005.09.148)

Weisz, P. B., Haag, W. O. & Lago, R. M. 1984 The active site of acidic aluminosilicate catalysts.Nature 309, 589–591. (doi:10.1038/309589a0)

Wilson, S. T., Lok, B. M., Messina, C. A., Cannan, T. R. & Flanigen, E. M. 1982 Aluminophosphatemolecular sieves: a new class of microporous crystalline inorganic solids. J. Am. Chem. Soc. 104,1146–1147. (doi:10.1021/ja00368a062)

Xiao, F. S. 2005 Ordered mesoporous materials with improved stability and catalytic activity. Top.Catal. 35, 9–24. (doi:10.1007/s11244-005-3809-1)

Xuereb, D. & Raja, R. 2011 Design strategies for engineering selectivity in bio-inspiredheterogeneous catalysts. Catal. Sci. Technol. 1, 517–534. (doi:10.1039/c0cy00088d)

Xuereb, D., Dzierzak, J. & Raja, R. 2010 Biomimetic single-site heterogeneous catalysts: designstrategies and catalytic potential. In Heterogenized homogeneous catalysts for fine chemicals

Proc. R. Soc. A (2012)

on May 25, 2012rspa.royalsocietypublishing.orgDownloaded from

1926 V. Dal Santo et al.

production, vol. 33, (eds P. Barbaro & F. Liguori), pp. 37–63. Berlin, Germany: SpringerScience+Business Media B.V.

Yang, Z., Lu, Y. & Yang, Z. 2009 Mesoporous materials: tunable structure, morphology andcomposition. Chem. Commun. 2009, 2270–2277. (doi:10.1039/b820539f)

Zhang, F., Yan, Y., Yang, H., Meng, Y., Yu, C., Tu, B. & Zhao, D. 2005 Understanding effect ofwall structure on the hydrothermal stability of mesostructured silica SBA-15. J. Phys. Chem.B 109, 8723–8732. (doi:10.1021/jp044632+)

Zhang, L. et al. 2007 An efficient hybrid, nanostructured, epoxidation catalyst: titaniumsilsesquioxane–polystyrene copolymer supported on SBA-15. Chem. Eur. J. 13, 1210–1221.(doi:10.1002/chem.200600752)

Proc. R. Soc. A (2012)

on May 25, 2012rspa.royalsocietypublishing.orgDownloaded from