reviewmicro meso soler illia 2002

8/12/2019 ReviewMicro Meso Soler Illia 2002

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Chemical Strategies To Design Textured Materials: from Microporous andMesoporous Oxides to Nanonetworks and Hierarchical Structures

Galo J. de A. A. Soler-Illia, Clement Sanchez,* , Benedicte Lebeau, and Joel Patarin

Laboratoire de Chimie de la Matiere Condensee, CNRS UMR 7574, Universite Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05,

France, and Laboratoire de Materiaux Mineraux, CNRS UMR CNRS-A 7016, ENSCMu, 3 rue A. Werner, 68093 Mulhouse Cedex

Received May 6, 2002

Contents

1. Introduction 40932. Crystalline Microporous Materials 4095

2.1. Introduction 40952.2. Porous Solids 40952.3. Zeolites and Related Microporous Solids 4095

2.3.1. Definition and Structural Delimitation 40952.3.2. Synthesis Strategies of Microporous

Crystalline Phases

4096

2.3.3. Formation Mechanisms of MicroporousCrystalline Phases

4097

2.3.4. Properties and Applications of ZeoliticMaterials

4098

3. Mesoporous Materials: Using SupramolecularTemplates To Enhance Pore Size

4099

3.1. Introduction 40993.2. Synthesis Tools for Mesostructure Production 4100

3.2.1. Synthesis Strategies 41003.2.2. Self-Assembling Templates 4102

3.3. Silica-Based Structures 41073.3.1. Evolution of the Research 4107

3.3.2. Formation of the Inorganic Network 41083.3.3. Formation Mechanisms 41103.3.4. Stability of the Inorganic Network 41143.3.5. Thermal Treatment and Porosity 4114

3.4. Non-Silica Mesostructured Materials 41143.4.1. Synthesis Strategies for Non-Silica

Oxide-Based Structures: Evolution of theResearch

4115

3.4.2. Control of the Formation of the InorganicNetwork

4115

3.4.3. Formation Mechanisms 41233.4.4. Stability of the Inorganic Network: from

Consolidated Hybrids to MesoporousPhases

4127

4. Multiscale Texturation 41284.1. Phase Separation, Cooperative

Autoassembly, and Topological Defects4128

4.2. Micromolding and Cooperative Self-Assembly 41294.3. Biotemplates and Cooperative Self-Assembly 41304.4. Organogel/Metal Oxide Hybrids 4130

5. Perspectives and Concluding Remarks 41326. References 4134

1. Introduction

Nanosciences will be, as biology, one of the fieldsthat will contribute to a high level of scientific andtechnological development along the 21st century.

Nan ostructured inorga nic, orga nic, or hybrid organic-inorga nic nanocomposites present pa ra mount a dvan -ta ges to facilita te integration and minia turiza tion ofthe devices (na notechnologies), thu s a ffording a directconnection betw een the inorga nic, orga nic, an d bio-logical worlds. The a bility to a ssemble and organizeinorganic, organic, and even biological componentsin a single mat erial represents an excit ing directionfor developing novel multifunctiona l ma terials pre-senting a wide r an ge of novel properties.1

Soft chemistry based processes (i.e., chemistry atlow temperatur es an d pressures, from m olecular orcolloidal precursors) clearly offer innovative strate-gies to obtain t ailored nanostr uctured ma teria ls. The

mild condit ions of sol-gel chemistry provide reactingsystems mostly under kinetic control. Therefore,slight chan ges of experimental para meters (i.e., pH,concentra t ions, tempera tures, na ture of t he solvent,counterions) can lead to substantial modifications ofthe r esult ing supramolecular assemblies. This ma ygive rise to inorga nic or hy brid solids wit h enormousdifferences in morphology and structure and, hence,in their properties.2-4 However, the result ing nano-st ru ct u re s, t h e ir de g re e o f o rg a n iza t io n , a n d t h u stheir properties certainly depend on the chemicalnature of their organic and inorganic components, butthey a lso rely on t he synergy between these compo-nents. Thus, the tuning of the nature, the extent, the

accessibility, and the curvature of the hybrid inter-faces is a key point in the design of new nan ostruc-tured ma terials. The growth of soft chemistry d erivedinorganic or hybr id networks templat ed by orga nizedsurfacta nt a ssemblies (str u ctu re d i r ecti n g a g en ts) al-lowed construction of a n ew fa mily of nanostructur edma terials in the mesoscopic scale (2-100 nm): thebest exam ple is the ever-growin g fa mily of meso-orga-nized hybrids or mesoporous inorganic ma teria ls.5-17

Moreover, recently, micromolding methods havebeen developed, in which the use of emulsion drop-

* Author to whom correspondence should be addressed [telephone+33(0)144275534;fax+33(0)144274769;[email protected]]. La boratoire d e C himie de la Mat iere Condensee. La boratoire de Mat eria ux Minera ux.

4093Chem. Rev. 2002, 102, 40934138

10.1021/cr0200062 CCC: $39.75 2002 American Chemical SocietyPublished on Web 10/25/2002


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le t s , la t e x be a ds, ba ct e ria l t h re a ds, co l lo ida l t e m-p la t e s , or or g a n og el a t or s l ea d s t o con t r ol of t h eshapes of complex objects in the micrometer scale. 18-25

These synt hesis procedures a re inspired by those

observe d t o t a ke p la ce in n a t u ra l syst e ms. In de ed,for some hundreds of million y ears, n at ure ha s beenproducing extremely performing ma terials (ma gne-totactical bacteria, ferritine, teeth, bone, shells, etc.)by making use of highly selective structures. Theconstruction of these complex structures is promotedby specific chemical links, as well as by a rich andva rie d se t of con forma t ion s a n d t o polog ies. Fo rexample, it is well-known that the highly efficientre cog n it ion p rocesse s in biolog y (e .g. , a n t ibody/an tigen or enzyma tic beha vior) depend on the spat ialdistribution (tert iary structures) at the nanometriclevel, as w ell as on t he molecular scale interactions.Learning t he savoir faire of these part icular living

syst e ms a n d o rg a n isms fro m u n de rst a n din g t h e irrules and tra nscription modes could lead us to be ablet o d es ig n a n d b u il d n ov el m a t e r ia l s . Th es e n ew compounds will bear improved or entirely new capa-bili t ies , fa r more e ff icien t t h a n t h e con ven t ion a lma t e ria ls t h a t w e a re a ble t o syn t h esize n owa da ys.An a mbitious project ha s been underta ken by severalresearch groups with diverse origin (biologists, chem-i st s , p h ys ici st s , . .. ), w h o a i m t o u n d er s t a n d t h econstruction processes of mineral objects that takeplace within hybrid interfaces that play a structuringan d functiona l role. The first r esults, concerning t hetailored design and construction of mineral-based ororganomineral hybrid-based frameworks, are indeed

Since 1998, Benedicte Lebeau has been a Centre National de laRecherche Scientifique (CNRS) research scientist at the Laboratoire deMateriaux Mineraux in Mulhouse (Universite de Haute Alsace, France).Her current research is focused on the synthesis of highly ordered poroussilica and aluminosilicate materials. Her primary interest is the modificationof these solids by covalent coupling of organic functions on the inorganicnetwork. She carried out graduate work with Dr. Clement Sanchez at theUniversity Pierre and Marie Curie in Paris, specializing in the synthesisand characterization of solgel hybrid materials for optics applications.After earning her Ph.D. degree in 1995, she worked on the control of themorphology of hydroxyapatite crystals in the Dr. Sanchezs research groupas a Rhodia-Lafarge-Bouygue postdoctoral fellow, after which she carriedout postdoctoral research in the biomimetism area with Prof. StephenMann at the University of Bath, U.K.

The first contact Galo Juan de Avila Arturo Soler-Illia (born in BuenosAires, May 31, 1970) had with chemistry was when he burned his parentsdining table with sulfuric acid, at the age of five. Since then, he obtainedhis Licenciatura en Quimica (1989/1993) at the University of Buenos Aires(UBA), where he also received his Ph.D. in chemistry (1994/1998),supervised by Dr. M. A. Blesa, in the precipitation mechanisms of mixedmetal oxide precursors from solution. During his postdoctoral research inParis with Dr. Clement Sanchez, he worked in nanocomposite hybrid ma-terials, particularly in the synthesis and formation mechanisms of transitionmetal oxide-based mesostructured and mesoporous phases. He alsoworked in applications for mesoporous thin films for St Gobain Recherche,Paris. At present, he is moving to the Chemistry Unit of the ComisionNacional de Energia Atomica, in Buenos Aires, Argentina, as a CONICETstaff scientist. His main current interests are the development of novelnano- and mesostructured multifunctional materials, as well as the explor-ation of their complex synthesis routes, which combine solgel chemistryand self-assembly. He has been fellow of CONICET, UBA, and FundacionAntorchas. He is coauthor of about 30 publications and illustrations for 3books. He is a member of the MRS, CONICET (Argentina), and Gabbos.

Clement Sanchez, born in 1949, is Director of Research at the FrenchCouncil Research (CNRS) and has a teaching professor position at lEcolePolytechnique (Palaiseau). He received an engineer degree from lEcoleNationale Superieure de Chimie de Paris in 1978 and a these detat (Ph.D.)in physical chemistry from the University of Paris VI in 1981. He didpostdoctoral work at the University of California, Berkeley, and is currentlyperforming research at the University Pierre and Marie Curie in Paris. Hespecializes in the field of chemistry and physical properties of transitionmetal oxide gels. He currently heads a research group of about 10scientists working on solgel chemistry of transition metal alkoxides andsynthesis of new hybrid organic inorganic materials. His main interestsare the study of the relationship between optical, mechanical propertiesand hybrid material structures. More recently he has focused a part ofhis research on the study of self-assembly processes to build hybridorganicinorganic materials textured at different length scales. He wasorganizer of several international meetings [The First European Meetingon Hybrid Materials, (1993); Materials Research Symposia: BetterCeramics Through Chemistry VI (1994); Hybrid OrganicInorganicMaterials B.C.T.C. VII (1996); Hybrid Materials (1998); and HybridOrganicInorganic Materials (2000 and 2002)]. He is also a member ofthe Materials Research Society and the Societe Chimique de France.Since 2001 he has been Editor in Chief of the New Journal of Chemistry(RSC). He is the author or coauthor of over 250 scientific publicationsand 15 patents. He has also given over 50 invited lectures in internationalmeetings. He has taken the direction of the Laboratoire de Chimie de laMatiere Condensee (University of Paris 6) since 1999.

4094 Chemical Reviews, 2002, Vol. 102, No. 11 Soler-Illia et al.


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encouraging. These efforts should permit improvedunderstanding of the complex mechanisms involvedin biomineralization processes that remain currentlyunknown.9,26-30

Zeolites and related porous solids, silica, and hybridmesostructured composites, obta ined by combina tionof a l l t h e se me t h ods, a re t h e best e xa mp les of t h es t a t e o f t h e a r t o f t h e s e a p p r o a c h e s . 5,13,17,27,30,31-34

Without doubt, these strategies will give birth to aconstellat ion of innovat ive advan ced ma terials w ithpromising a pplicat ions in ma ny fields: optics, elec-tronics, ionics, mechanics, membranes, protectivecoatings, catalysis, sensors, and biological applica-

tions (immobilizat ion, recognit ion, drug delivery,etc.).35

2. Crystalline Microporous Materials

2.1. Introduction

A great number of natural ma terials are character-ized by a n e ga t ive ly ch a rg ed min era l f ra me wo rk,b ea r i n g ca v i t ie s, ca g e s, or t u n n el s w h e r e w a t e rmolecules or inorganic cations (as charge-compensat-ing ions) are occluded. Among these solids, zeolites(from the greek, zein, t o boil, a n d l i t h o s , stone) de-f in e t h e g re a t fa mily of cryst a l l in e microp oro us

aluminosilicates, presenting pore sizes of d 50 nm). Some illustrativeexamples are given in Figure 1. In this section, wewill focus on microporous materials, particularly onzeolites. Further on, we w ill discuss silica- and non-silica-ba sed m esoporous solids,37 which present sharppore distributions.

2.3. Zeolites and Related Microporous Solids

2.3.1. Definition and Structural Delimitation

Zeolites constitute by themselves one of the mostimportant families of crystalline microporous solids.T h e o r i g i n a l n a m e w a s i n i t i a l l y g i v e n t o n a t u r a l

aluminosilicates belonging to the class of tectosili-cates. The lat ter are built up from a three-dimen-sio n a l a rra y o f t e t ra h e dra l u n it s T O 4 (T ) Si, Al),each oxygen atom bridging two tetrahedra. However,the main difference between zeolites and the othermembers of the tectosilicat e fa mily (feldspar) is t hepresence of channels and cavit ies of molecular di-me n sio n s, wh ich a re in co n t a ct wit h t h e e x t e rn a lmedium. To preserve electroneutrality, alkaline (oralka line-eart h) cations ar e present w ithin th ese cavi-t ies, as well as water. The general formula of thesealuminosilicates can be considered to be M 2/nO,Al2O3,xS iO 2,yH 2O, where M is one cation of valence n a n dxg 2 .

Joel Patarin is a director of research at the National Center of ScientificResearch (CNRS) and head director of the Laboratory of InorganicMaterials at the Chemistry School, Universite de Haute-Alsace, Mulhouse(France). His research focuses on the synthesis of porous inorganicmaterials with controlled pore size such as crystalline microporous solids[zeolites and related materials (gallophosphates, zincophosphates, ...)]and organized mesoporous solids. In particular, he was involved in thesynthesis of the large-pore gallophosphate cloverite whose structuredisplays a three-dimensional channel system with pore openings delimitedby 20-membered rings. In the past few years, his group has devoted amajor effort in the synthesis of these porous solids in fluoride medium.Patarin studied chemistry at Nantes University, completing his diplomathesis on the synthesis of titanosilicates in 1985. He obtained a CNRSposition as researcher the same year and joined the group of ProfessorR. Wey at the Laboratory of Inorganic Materials of Mulhouse. In particular,he worked on the synthesis of iron-containing zeolites and obtained hisPh.D. degree in 1988. In 1995, he prepared his habilitation to followresearches and was named director of research in 1997. Since 1999, hehas been the head director of the laboratory. Dr. Patarin is the author orcoauthor of over 100 research papers, 13 patents, and more than 130communications. He is a member of the editorial board of Microporousand Mesoporous Materials, a member of the International ZeoliteAssociation, and a member of the French Zeolite Group. Dr. Patarin canbe reached at the Laboratoire des Materiaux Mineraux, Ecole NationaleSuperieure de Chimie de Mulhouse, Universite de Haute-Alsace, 3 rueAlfred Werner, 68093 Mulhouse, France, and by e-mail at [email protected].

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In t h e p a st 60 ye a rs, t h e re h a s be e n a su st a in e dinterest in the syn thesis of these mat erials, in viewof their properties and their applications in catalysisor sorp t ion . Th e f irst syn t h e sis p roce dure s we reperformed by alkaline treatment of aluminosilicategels, in th e presence of minera l ba ses. The intr oduc-tion of nitrogen-containing bases (or alkylammoniumcations) in t he reaction mixtures provided a n int er-esting brea kthrough. New zeolite-type st ructures

we re crea t e d, a s w e ll a s n e w fa mil ie s of ma t e ria ls ,bea rin g or n ot a n isost ru ct u ra l re la t ion sh ip wit hzeolites. This is the case of the microporous alumi-nophosphate molecular sieves, developed by UnionCarbide, AlPO4-n,38 or derived mat erials, obtained byincorporat ion of Si, 39 Me (Me ) Co, Fe, Mg, Mn , Zn),or El (El ) As, B , B e, Ga , Ti, Li).40 The classificat ionof these compounds is sketched in Figure 2. After

these findings, the family of crystalline microporousphosphates grew considerably. Some examples in-clude gallophosphates,41-43 zincophosphates,44,45 be -

ryllophosphates,46 vanadophosphates, 47 an d ferrophos-phates. 48 In some of these microporous phosphates,the ba ckbone element T (T ) Ga , Fe, . . .) associat edwit h P can be tetra -, penta- or hexacoordinated.

The expression zeolite has nowadays a broaderme a n in g , t o in clu de a l l microp oro us si lica -ba se dsolids presenting crystalline walls, including thosem a t e r ia l s w h e r e a f r a ct i on of S i a t om s h a s b ee nsubstitut ed by another element, T, such as a triva lent

(T ) Al , Fe , B , Ga , . . . ) or a t e t ra va len t (T )Ti, Ge,...) metal. The following categories have been estab-lished, a s a function of th e Si/T ra tio: zeolites, S i/T< 500; a nd zeosils, S i/T> 500; these compounds ar eessentia lly Si-based, but , contra ry t o clat hra sils, theporosity of these ma teria ls is accessible. B oth zeosilsa n d cla t h ra si ls de f in e t h e fa mily o f p o ro si l s i l ica -ba se d ma t e ria ls .

The crystalline microporous phosphates are identi-fied as related microporous solids.

In summary, microporous solids are distinguishedby a three-dimensional framework, result ing fromcorner-connected TO4 units (T ) S i , Al , P , G e , G a ,. .. ); ox y gen a t o m s b r id g e t w o m et a l ce nt e r s; a n d

channels or cavities of molecular dimensions, capableof communicat ing w ith t he outside.

Act u a l s t ru ct u re s ca n de via t e f rom t h e se idea ldefinitions, due to the presence of TX 4, TX5, or TX6polyhedra (X ) O, F) or nonbridging oxygen a toms(for example, terminal -OH groups).

A three-letter code is attributed to each structure,de fin ed by t h e S t ru ct u re Co mmission of t h e In t e r-national Zeolite Association (IZA).49 As an example,fa u ja si t e a n d t h e ir syn t h e t ic e qu iva le n t s X a n d Ycorrespond to the structural-type FAU. Nowadays,zeolit ic frameworks are classified in 135 differentstructure types.50

2.3.2. Synthesis Strategies of Microporous CrystallinePhases

Synthesis Methods.Crystalline molecular sievesare generally obtained by hydrothermal crystalliza-tion of a heterogeneous gel, which consists of a liquidand a solid phase. The reaction media contain thefol lowin g : sou rce s o f t h e ca t ion (s) t h a t form t h eframework (T) Si, Al, P , ...); sources of minera lizingagents (HO-, F-); minera l cat ions or orga nic species(cations or neutral molecules); and solvent (generallywa t e r) .

Zeolite synt hesis is generally performed in a lkalinemedia, T< 200 C . In the case of aluminophospha te

families, and derived compounds (SAPO, MeAPO,etc.), the reaction pH is between 3 and 10. Anionssu ch a s h ydrox ide o r f luoride colla bo ra t e in t h edissolution of the r eactive silica moieties in t he gela n d t h e ir t ra n sfe r t o t h e g ro win g cryst a ls . In a ddi-t ion, F- anions can play the role of a costructuringagent, by stabilizing certain building blocks of theinorga nic network.51

Nonaqueous routes have also been explored; theymay involve a nonaqueous solvent as ethylene gly-col52,53 or dry synthesis methods. 54 However, tracesof wa ter ma y be present in the solvents. 53 Water cana lso be g en e ra t e d in si t u , u p on e volu t ion of t h ereacting systems.54,55

Figure 1. Examples of micro-, meso-, and macroporousma terials, showing pore size doma ins an d ty pical pore sizedistributions.

Figure2. Classification of aluminophosphate a nd derivedphases (ada pted from F lanigen et a l .40).



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ZeoliteFormation and Growth.A great numberof studies have been focused on the crystallizat ionmechanism of zeolites. Two main hypotheses havebeen proposed. The first one, originally presented byFlanigen and co-workers,56 su g g e st e d t h a t cryst a l-l iza t io n t a ke s p la ce by a re o rg a n iza t io n o f t h e g e lnetwork; this process is not mediated by the liquidphase. This pioneering model has been practicallyaba ndoned a nd displaced by the hypothesis of Ba rrerand colleagues,57 wh o su pp ose d t h a t t h e fo rma t iono f ze o li t e cryst a ls t a ke s p la ce in so lu t io n . In t h ismodel, the n ucleation a nd gr owth of cryst alline nucleiare a result of condensation reactions between solubles pe ci es , t h e g el p er f or m i ng a l im i t ed r ol e a s areservoir of ma tter.

2.3.3. Formation Mechanisms of Microporous CrystallinePhases

Building Unit Model. Ba rre r a n d co-wo rkerssuggested that the elabora te zeolite fra meworks weremade up from more complex building blocks, whichwe re p re sen t in solu t ion : t h e secon da ry bu ildin gunits (SBU).57 Sixteen different kinds of SBU havebeen identified from structural studies.49 This modelset t le s a n in t ere st in g p oin t o f view: sol ids ma y bebuilt up from preformed building blocks. A major

l i m i t a t i o n o f t h i s a p p r o a c h i s t h a t n o t a l l o f t h eproposed SBUs have been found in solution. How-e ve r, t h e se bu ildin g blocks cou ld be p resen t inconcentrat ions below detection limits; these minorqua ntit ies could suffice to tr igger the generat ion ofmicroporous frameworks. Alternative models haveb ee n d e ve lop ed t o r a t i on a l i z e t h e f or m a t i on ofaluminosilicates,58-60 gallophosphates, 43 a n d a lu mi-nophosphates,61,62 b a s e d on m ol ec ul a r m od e li n gcoupled to structural analysis.

Organic Templating Agents. As previouslys t a t e d , t h e u s e o f or g a n i c s p eci es a s t e m pl a t i n gagents ha s w idened the number and nat ure of micro-porous crystalline solid phases. So far, amines and

related compounds (quaternary ammonium cations),linear or cyclic ethers, and coordination compounds(organometallic complexes) have been the most com-monly used organic t emplat es. The templat e is sup-posed to keep its integrity in the sy nthesis medium(chemical and thermal stability). Under an alterna-tive approach, th e template species can be genera tedin sit u by cont rolled decomposition of orga nic precur-sors. This ha s been adva nta geously used to preparenovel microporous a luminophosphat es in the pres-ence of a lkylforma mides. The par tia l degra dat ion ofthese compounds leads to the corresponding alkyl-amines, which are kept in the cavit ies of the mate-ria l .62,63

Table 1 presents some templating agents of w ide-sp re a d u se in t h e syn t h e sis o f p o ro si ls , a s we ll a stheir standard codes.

Roleof the Organic Species. The organic speciesare frequently occluded in the microporous voids oft h e syn t h e size d ma t e ria l , co n t ribu t in g t o t h e st a -bility of the mineral backbone. The guest-f ra me -work stabilizing interactions can be of Coulombic,H-bonding, or van der Waals type. However, guest-g u e st in t e ra ct io n s ca n a lso co n t ribu t e t o t h e t o t a lenerg y. This is t he ca se of 18-crow n-6/Na+ complexes,in t h e syn t h e sis of EMC-2 zeolit e ,64 o r o f t h e p-

di oxa ne /Na

+

complex, in the synthesis of MAZ-likezeolites. 65 Th e t ra p pe d org a n ic mo ie t ie s p erformseveral roles: Co ulombic ba la n ce of t h e n e ga t ive ly ch a rg ed

framework (e.g., aluminosilicates). filling of the microporous cavities. structuring by the template effect; that is, the

mineral species present a degree of preorganizationa ro u n d t h e org a n ic molecules, a n d/or t h e cryst a lgrowth is oriented by the sha pe and symmetry of thetemplate. chemical a ct ion, by modifying the properties of

the solution or t he result ing gel (the organ ic speciesis mostly hydrophobic).

t h ermo dyn a mic a ct ion , by st a bi l iz in g a g ive nbuilding block of the m ineral fra mework.Silica-based molecular sieves and silica-rich zeo-

lites (5 < S i/Al < ) a re t h e simp le st syst e ms inwh ich these template effects ha ve been observed.

Template E ffect of the Organic Species. I nmo st ca se s, a n a de qu a t e ma t ch in g e x ist s be t we e nthe geometries of the organic species and those ofthe microporous cage or channel network. The mol-ecules play t hus a real t em pl ate effect, around w hichthe minera l fram ework is built. The local str ucturingof wa ter might be a n importa nt issue; in fact , a closea n a lo g y be t we e n cla t h ra t e s (a rra n g e me n t s co n st i-t u t ed m a i nl y b y w a t e r s t r uct u r ed a r ou nd ot h er

molecules) a nd th e family of cla thr as ils can be noted.Th i s i s t h e ca s e of t e t r a m et h y l a m m on i um cl a t h -rate [(CH 3)4NO H 5H 2O], w hich presents the sodalite-like structure. This zeolite has been also obtainedin the presence of tetramethylammonium (TMA+)ca t io n s. In t h e ca se o f s t ru ct u ra l ly re la t e d so l ids(SOD type), such a s zincophospha te (Zn, O, P fra me-work) and aluminophosphat e (Al, O, P fram ework),TMA+ ca t i on s a r e a l s o p er f ect l y a d a p t ed t o t h esodalite-like cage size (Figure 3). Wiebke66 h a s s y n -t h e size d a n e w fa mily o f mix e d cla t h ra t e-silicatema t e ria ls , r e in forcin g t h e a n a lo gy bet w e en t h e t wofamilies of compounds. In clat hra tes, wa ter is str uc-tured around big cations or anions, presenting low

Table 1. Templating Agents Used in the Synthesis of Porosils (SiO2 Polymorphs)

organic template

Porosils t r u c t u r al

code or ga n ic t empla t e

Porosils t r u c t u r al

code

1-a m in oa da m a n t a ne D D R N-ben zy l-1-a zon iu m[2.2.2]bicy clooct a ne I FRN,N,N-t r ime t hy l-1-ad a mant ammoniu m c at ions AF I 1,3,3,6,6-p ent am e t hy l-6-azoniu m[3.2.1]b icy clooct ane I TEdiben zyldim et h yla mm onium ca t ions B E A coba lt iciniu m ca t ions NONet h ylene gly col or t r ioxa n e S OD 3,5-dim et hy l-N,N-d iet hy lpiper id in iu m ca t ion s M ELt et ra pr opyla mm onium ca t ions MF I 4,4-t r imet hy lenedipiper idin e MTWdiet hy la m ine TON q uinuclidinium ca t ion s AS T

di -n-pr opyla mine MTT t et r a m et h yla mm onium ca t ions MTN



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charge an d hydrophobic chara cter. These nonpolaror sl ig h t ly p ola r molecules [a t yp ica l e xa mp le is(C 3H 7)4N+] are rejected by the solvent and isolatedin polyhedra l cavit ies.

The presented analogies between clathrates andsilicat es, including t he existence of mixed compoundsa n d t h e s t r u ct u r i ng of w a t e r i n t h e p r e se nce of

hydrophobic molecules, led to the elaboration of aporous mineral fra mework in term s of a r eplicationw a t e r-inorganic species process. The latter is thenfollowed by condensation of the result ing organic-inorga nic species. This template effect will be il-lustrated by an example: silicalite-1 (MFI structuraltype).

Two main components constitute the MFI structure(MFI for Mobil Five): ZSM-5, wh ere the S i/Al ra tiovaries between 10 and 500, and silicalite-1, a puresiliceous form (S i/Al > 500). MFI st ru ct u re67 ischara cterized by t he presence of tw o types of inter-connected cha nnels, t he opening of which is delimit edby 10 tetra hedra l units (Figur e 4). Silicalit e-1 ca n be

obtained in the presence of a great variety of organicspecies. However, the pure phase can be obtainedonly by performing the synth esis in th e presence oftetra propyla mmonium cat ions (TP A+) cations. In t heas-synthesized solids, four TPA+ p e r u n it ce l l a reoccl ud ed w i t h i n t h e i n or g a n i c f r a m e w or k , a t t h eintersection of both systems of channels (Figure 4C).It is clear tha t t here is a geometrical tuning betweent h e se ca t io n s a n d t h e ch a n n e l syst e m, a s o n ly o n ecation per intersection is present.

Solid-state NMR studies of the reaction gels alongthe synt hesis performed by Bur kett a nd co-w orkers 68

confirmed the van der Waals interactions betweent h e TP A+ a n d t h e in org a n ic si lica sp ecie s. Th eexperiments were carried out using 1H-29Si cross-polarizat ion (CP) techniques, and the reaction mix-t u r e s w e r e p r e p a r e d i n t h e p r e s e n c e o f D 2O. Anorg a n o min e ra l e n t i t y wa s e viden ce d, e ven in t h eabsence of an y crysta lline phase, by the substit utionof the w a ter belonging t o the (hydrophobic) solvationla ye r of TP A+ by silica-containing moieties. This

replication of the water structure by a silica copyshould be favorable from a thermodynamical pointof view, the van der Waals interactions and the de-structuring of the wa ter m olecules contributing to th eentha lpic a nd entropic terms, respectively. The a s-sembly of the organominera l entit ies and t he growthof the nuclei thus formed should lead to silicalite-1.The different steps involved in the formation of thiss ol id p ha s e a r e s ch em a t i z ed i n F i gu r e 5. Th es ea u t h o rs h a ve sh own t h a t t h e cryst a l l iza t ion did n o tta ke place in the presence of tetra etha nolammoniumcat ions (geometr ically simila r to TP A+). In th is case,t h e st ro n g H-bo n din g in t e ra ct io n s t h a t t a ke p la cebe t we e n t h e e t h a n o l g ro u p s a n d wa t e r mo le cu le s

hinder the replication process.More recently, t his mechanism ha s been va lidated

by de Moor a nd co-w orkers usin g sm a ll (SAXS)-, w ide(WAXS)-, a nd ultra sma ll (US AXS)-a ngle X-ra y Scat -tering.69 Other authors have independently demon-st ra t e d t h e fo rma t ion of a n org a n o min era l sp eciescon t a i n in g 33 S i a t o m s, b en t a r o un d TP A+, a n dp rese n t in g a con n e ct ivit y sch e me simila r t o t h a tfound in the MFI structure.70

2.3.4. Properties and Applications of Zeolitic Materials

Because of their perfectly controlled porous struc-ture with molecular size pores, zeolitic materials are

genuine s ha pe-selective molecula r sieves. The pres-ence of charge compensation cations (alkaline, alka-line-earth, protons, etc.) within the porosity of theinorga nic frameworks gives to these ma terials ionicexchange an d cata lyt ic properties, wh ich a re widelyu sed in t h e in du st ry. More ove r, t h e h ydrop h obic(zeosils, SiO2) or hydrophilic (aluminosilicat es) na tureo f t h e t a i lo ra ble in o rg a n ic f ra me wo rk ma ke t h e sesolids useful as specific adsorbents of organic mol-ecules or water in the gas or liquid phase. The threema in a pplica tions of zeolitic mat erials a re ad sorptiona n d dryin g , ca t a lysis , a n d de t erg e n cy.

Figure 3. TMA+ cations occluded into the sodalite cage(cage). In this scheme, the network-forming elements T(T ) Si, Al, . . .) are located at the corners, defined by theintersect ion of thr ee edges. O a toms can be found a t thecenter of each edge.

Figure 4. Structure-type MFI: (a) crystals of silicalite-1; (b) overview of the channel system; (c) scheme of the location ofthe TPA+ cations at the channel intersection.



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To i llu st ra t e t h e se a p plica t ion s, on e ca n qu ot ead sorption of wa ter molecules in double-glazin g, gas

drying, catalytic cracking (production of fuel, increaseof the gasoline octane number), and non-phosphate-containing laundry soaps (trapping of calcium andma gnesium). The w orld consumption (103 tons) an ddist ribu tion (vol %) of use of th ese ma ter ia ls in 1988a r e g i v e n i n F i g u r e 6 .71 Re ce n t da t a 72 r e l a t e d t osynth etic zeolite production show t ha t the t onnagesf or t h e t h r ee m a i n a p pl ica t i on s h a v e g r ea t l y i n -creased in the past 10 years. Indeed, in 1998 theyrose 1.05 106, 1.6 105, a n d 1 105 tons fordetergency, catalysis, and adsorption drying, respec-tively. D etergency represents 70% of t h e ma rket ;some la u n dry soa p s ca n con t a in u p t o 40 w t % ofzeolite A (str uctu re-like LTA).

Nu mero us ot h e r a p p lica t io n s, le ss import a n t int o n n a g e a n d m a i n l y b a s e d o n t h e u s e o f n a t u r a lzeolites, ha ve been also developed: w ast ewa ter trea t-ment, t rea tment of nuclear effluents, pet food, or soilimprovement.

R ece nt p a t en t s h a v e cl a i m ed t h e u s e o f t h es ematerials for antibacterial properties 73,74 or as com-bustion delaying agents.

I n ca t a l y si s, m os t of t h e z eol it es a r e u sed i nrefining and petrochemical material. In petroleumre fin in g , t h e ma in a p p lica t io n s a re f lu id ca t a lyt iccracking (FCC ), hyd rocracking, isomerizat ion of prod-u ct s f rom t h e de comp osit ion of C5-C 6 c u t s , a n d

dewa xing. In petrochemicals, developed rea ctions ar ear oma tic tra nsforma tions (isomerization, alkylat ion,disproportionation, and transalkylat ion reactions).

The use of zeolites in fine chemistry processes isa n d w i ll b e m or e a n d m or e i mp or t a n t . I n deed ,homogeneous catalysis processes (contrary to hetero-geneous catalysis processes) act usually in a stat ic

bed, limit or avoid corrosion problems, and allowre t u rn t o t h e ca t a lyst .I n t h i s a r e a , a m o n g t h e r e a c t i o n s c a t a l y z e d b y

zeolites, one can quote double-bond isomerization,skeleton isomerization, dehydration, dehydrogena-tion, halogena tion, acylation or a lkylation of ar omaticcompounds, selective oxidation, selective hy drogena-tion, etc.75-77 One of the most rema rkable examplesin the past decade is the industrial development ofTS-1 (silica-rich and titanium-doped MFI zeolite)78

as a catalyst for phenol hydroxylat ion. 79

I t i s n ot ew or t h y t h a t a m on g t h e 135 t y pes ofidentified zeolites, only about a dozen a re used in theindustry or have a high industria l potentia l. Table 2

p re se n t s a l is t o f t h e ma in ze o li t ic ma t e ria ls . Re -cently, a more detailed review on the applications ofthese solids in cat alysis ha s been published.80

3. Mesoporous Materials: Using SupramolecularTemplates To Enhance Pore Size

3.1. Introduction

Regardless of the great am ount of work dedicatedto zeolites a nd related crysta lline molecular sieves,the dimensions and accessibility of pores were re-str ained t o the sub-na nometer scale. This limited th eapplication of these pore syst ems to sma ll molecules.

Figure 5. S cheme of the proposed format ion mecha nismof silicalite-1 (adapted from ref 68).

Figure 6. Consumption (103 tons) (a) and distributionin vol %of the use of zeolitic ma teria ls in the w orld in 1988(b) (adapted from ref 71).



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During the past decade, an important effort has beenfocused on obtaining molecular sieves showing largerpore size.

The introduction of supra molecular assemblies(micellar aggr egates, ra ther t han molecular species)a s t e m pl a t i n g a g e n t s p er m i t t ed a n ew f a m i ly ofmesoporous silica an d aluminosilicat e compounds

(M41S) to be obta ined, first developed by a resear chgroup at Mobil Oil.81,82 These solid phases are char-a ct e rized by orde red me sop ore s p rese n t in g sh a rppore s ize dispersions.

The M41S family includes a bidimensiona l h ex-agonal phase (MCM-41, for Mobil C omposit ion ofMatter), a cubic phase, MCM-48, and several lamel-lar phases; part icularly, MCM-50 is reported to bet h e rma lly st a ble.

The synthesis of large-pore molecular sieves wasn ot t h e on ly con se qu en ce of t h is discovery. Th isbreakthr ough permitt ed confirma tion of several idea sa n d con cep t s p rop ose d by a la rg e commu n it y ofresear chers, focused on biomineraliza tion processes.

As described by S. Mann27

the importa nce of this neworganized matter soft chemistry synthesis is con-tinuously increasing. The sy nthesis of inorga nic orhybrid materials presenting complex architecturesover a multiscale range should be possible by control-l in g con st ru ct ion , morp h olog y, a n d h iera rch y inprecipita tion rea ctions. Another r eleva nt outcome isthe a pproach experienced by the communities study -ing biomineralizat ion and the synthesis and designof advanced materials. The close relat ionships be-tween biology and chemistry of organized matterconverge in the terms m olecul ar tectoni cs or nano-tectonics. Nature employs macromolecules and mi-crostructures to control the nucleation and growth

of mineral compounds or orga nomineral h ybrid com-p os it e s; s im i la r a p pr oa c h es a r e n ow a d a y s b ei n gdeveloped in the synthesis of advanced materials.

In pa rticular, a perma nent effort is m ade t o developtextured inorgan ic or hybrid pha ses. These mat erialsare potential candidates for a variety of applications,in the fields of catalysis,34 optics, photonics, sensors,sep a ra t ion , dru g de livery,83 sorption, acoustic ore le ct rica l in su la t io n , u l t ra l ig h t s t ru ct u ra l ma t e ri-a ls ,24,25 etc.

In t he case of porous mat erials, increasing t he poresize has been one of the goals of st ructura l control,to permit the penetra tion of lar ge size molecules intothe host porous structure. Macroporous materials

(i.e., Lpore > 50 nm) ar e part icularly interesting, dueto their improved transport properties. Organizedmacroporous arrays should present optimal f luxes,a nd diffusion should not be a limit ing issue for th esematerials. This is a central point for any processesconcerning accessibility, such as catalysis, sorption,delivery, or sensors.

T h e ch o ice o f t h e o rg a n ic t e mp la t e t o sp a t ia l lycon t ro l t h e min era l iza t ion p rocess a lo n g va riou s

scales, ranging from t he angst roms to micrometers,is a key issue in th e synt hesis of textured or porousma t e ria ls . In t h e ca se o f me sop orou s ox ide s, t h etemplating relies on supramolecular arr ay s: micellars y s t em s f or m ed b y s u r fa c t a n t s or b lock cop ol y -mers.81,82

In t h e fol lowin g se ct ion s, we wil l de scribe t h echemical tools th at ar e necessary to construct silica-and non-silica-based organized hybrid or inorganicstructures by soft chemistry processes. The m ostrelevant w ork produced in th is field from 1992 to thepresent will be reviewed a nd t horoughly discussed.

3.2. Synthesis Tools for Mesostructure

Production

3.2.1. Synthesis Strategies

Chimie Douce (soft chemistry) is indeed an in-t e rest in g st a rt in g p o in t for t h e de ve lop men t of abiomimetic approach of mesostructured ma terials,i n v ie w of t h e t y p ica l s y n t h es is con d it i on s : l ow temperatures; coexistence of inorganic, organic, ande ve n biolog ica l ly a t t ra ct ive moiet ie s; wide spre a dchoice of precursors (monomers or condensed s pecies);a nd possibilities of controlled sha ping (i.e., pow ders,gels, films, ...). Exploration in this field is persistentlyg r ow i n g , a n d a n u m ber of b iom i m et i c s y n t h e si sstr a tegies ha ve been recently developed:27,28 The la rgeset of sm a r t m a t er i a l s , ranging from nan ostructuredma t e ria ls (such as ordered dispersions of inorganicb r i cks i n h yb ri d ma tr i ces, me sostr u ctu red i n o rg a n i c

networks, or dual networks) to more complex ma teri-als ha ving hierarchical ar chitectures, reported duringt h e p a s t 10 y e a r s , i s t e st i m on y t o t h e s ci en t i fi csuccess of this field.2-14

F i gu r e 7 p r es en t s a n i ll us t r a t i on of t h e m a i ngenera l synth esis stra tegies used to construct thesema t e ria ls . In a l l o f t h e se syn t h e sis s t ra t e g ie s, t h ech e mica l , sp a t ia l , a n d st ru ct u ra l p ro p e rt ie s o f t h etexturing agent, or the reaction pockets, must bethoroughly adjusted by controlling the rates of chemi-

ca l re a ct ion s, t h e n a t u re of t h e in t erfa ces, a n d t h ee n ca p su la t ion of t h e g rowin g in org a n ic p h a se. Anadequa te ta iloring of the orga nominera l interface isof utmost importa nce to obta in w ell-defined texturedphases. The chemical, spatia l, a nd tempora l controlof t h i s h y b r id i n t er f a ce i s a m a j or t a s k i n t h echallenge of developing cooperatively assembled in-organic-organic integrated systems.

These synthesis strategies (Figure 7) can be cat-egorized following two principal approaches:

(1) The m olecular /supra molecula r templa tes a represent in the synthesis media from the beginning;the self-assembly process of the templates is followedb y (or s y n ch r on i ze d w i t h ) t h e f or m a t i on of t h e

Table 2. Zeolitic Structures Used or with a HighIndustrial Potential (Adapted from Reference 62)

st r uct ur a l t ype a pplica t ion

LTA det er gency, dr ying, a nd sepa ra tionF AU , MOR a dsorpt ion a nd ca t aly sisLTL ca t a ly sis: a r om a t iza t ionMF I a dsorpt ion a nd ca t a ly sisB E A ca t a ly sisCHA (silicoalumino-

phosphate)cata lysis: conversion of metha nol

into olefins

F E R ca ta ly sis: fr a mew or k isom er iza tionof n-butenes

AEL (silicoa lumino-phosphate), TON

cata lysis: isomerizat ion of para ffins;decrease of the flowing pointfor diesel oils

MTW ca t a ly sis



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min era l n e t wo rk, de posit ed a ro u n d t h e self-a s-sembled substrate. Inorganic replication occurs at

accessible interfaces built by preorganized or self-assembled molecular or supramolecular templates,w h i ch cr e a t e t h e m es os t r u ct u r e i n t h e m a t e r ia l .These templates can be organic compounds (surfac-tant molecules, amphiphilic block copolymers, den-drimers, etc.) or biomolecules, forming micellar as-semblies an d/or liquid cry sta l mesophases. They ca nalso be preformed objects having submicronic, mi-cronic, or macroscopic sizes, colloids (latex, silica),bacteria or virus, or even mesoporous silica frame-w o r ks t h a t ca n b e u s ed a s a t e m pl a t e (n a n o- ormicroca st in g ) t o e mbe d a n y ot h e r comp on e n t ormaterial, being commonly used examples (route A).

In many cases a cooperative self-assembly can

t a k e p l a ce i n s it u b et w e en t h e t e m p la t e s a n d t h emineral network precursors yielding the organizedar chitectures (route B ).

(2) In t he second approach, a n a nometric inorga niccomponent is formed (by inorganic polymerization orprecipitation reactions). Nanoparticle formation cantake place not only in solution but also in micelleinteriors, emulsions, or vesicles, leading to complexsh a p e d ma t e ria ls . T h e co n t ro l o f t h e dyn a mics o fprecipitat ion of this na nometric building block (NBB )is a key point w hen synth eses a re performed underthese condit ions. These NB B can be subsequentlyassembled and linked by organic connectors or byta king adva nta ge of orga nic functions da ngling on the

particle surface (route C). The synthetic strategiesa n d rou t e s u sin g NB B lea din g t o o rdere d o r disor-

dered hybrid netw orks have been recently reviewed.4

All o f t h e se st ra t e g ie s ba se d on t ra n script ion ,synergic assembly, and morphosynth esis can also besimultan eously combined (integra tive sy nthesis) togive rise to hierarchical materials. 27 We w ill brieflydiscuss the main parameters that control mesostruc-tured assemblies in the following paragraphs.

The key featu re in th e synt hesis of mesostr ucturedmaterials is to achieve a well-defined segregation oforganic (generally hydrophobic) a nd inorga nic (hy-drophilic) domains a t th e nan ometric scale; here, then a t u re o f t h e h ybrid in t e rfa ce p la ys a fu n da me n t a lrole. The most relevant thermodynamic factors af-fecting t he forma tion of a hy brid inter face hav e been

first proposed by Monnier et al.84

and discussed ind e p t h b y H u o e t a l .33 in t h e ir de scrip t ion of t h echa rge m a tchin g model (see section 3.3.4). The freeenergy of mesostructure formation (Gms ) is com-posed of four ma in t erms, w hich represent, respec-t ive ly, t h e co n t ribu t io n s o f t h e in o rg a n ic-organicinterface (Ginter), the inorganic framework (Ginorg),the self-assembly of the organic molecules (Gorg ),an d t he contr ibution of t he solution (Gsol ).

In t he clas sical liquid cryst al t emplat ing (route A),t h e con t r i bu t i on d u e t o t h e or g a n i za t i on of t h e

Figure 7. Ma in synt hetic approaches for mesostructured m at erials. The mesostructure can be previously formed (routeA), or a cooperat ive process (route B ) can ta ke place. Route C ma kes use of preformed na nobuilding blocks (NBB ).

Gm s ) Ginter + Ginorg + Gor g + Gso l (1)



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amphiphilic molecules prevails over the other inter-actions. In the coopera tive a ssembly route (B), t em-plate concentrations may be well below those neces-sa ry for obt a in in g l iqu id cryst a l l in e a sse mblies oreven micelles. Thus, the creation of a well-definedand compatible hybrid interface between the inor-ganic walls and the organic templates (i .e. , Ginter)is centr al to the genera tion of a well-ordered hybr idst ru ct u re wit h a de qu a t e cu rva t u re . T h is h a s be e ndemonstra ted for silica systems in st rongly alkalineme dia (p H 13) a t a mbie n t t e mpe ra t u re , w h e re e x-tended silica polymerization is not possible (i .e. ,|G inorg| f 0). In these condit ions, hydrolysis andin org a n ic con de n sa t io n a re se pa ra t e e ven t s .85 Th eforma tion of the inorganic phase can be subsequentlytriggered as a subsequent process, directed to differ-ent mesophases. A similar strategy has been appliedfor t h e con st ru ct ion of t i t a n ia-su rfa ct a n t h ybridassemblies in st rongly a cidic medium (vide infra ).

Fr om the kinetic point of view, th e forma tion of a norganized hybrid mesostructure is the result of thedelica te bala nce of tw o competitive processes: pha sesepara tion/organ izat ion of the templa te and inorga nicpolymerization. This issue, well-known in microscalephase segregation,86 is essentia l wh en one is working

wit h syst e ms w h e re in org a n ic con de nsa t io n is fa st .In condit ions where condensation is slow (i.e. , pHn e a r t h e p H iep of silica), the kinetic constants (ki) ofthe different processes should be ordered as follows:

Thus, the formation of ordered phases is controlledby the self-assembly involving the hybrid interface.En ha nced hydr olysis by add ition of F- anions87 helpsto obtain w ell-defined mesostructur es even in condi-t ions w here condensat ion is fa ster (pH, presence ofF-, which also induces condensation).

Hence, two aspects are essential to fine-tune theself-assembly and the construction of the inorga nicfra me wo rk: t h e reacti vit y of th e in organic precur sors(polymerization rate, isoelectric point, etc.) and thei n t era cti o n s to g en era te a w el l -d efi n e d h yb ri d i n ter-

face. These central points are not only relevant formesostructured silica but can a lso be tra nslat ed intothe doma in of the more reactive non-silica systems,as will be shown in the next section.

3.2.2. Self-Assembling Templates

The main organic templates used in the elaborationof mesostructured hybrids or mesoporous solids canbe classified in three cat egories: molecular-based

organized systems (MOS), polymeric templates, andother texturing agents.

Molecular-Based Organized Systems. Su rfa c- ta n t-Ba sed M OS. St ructuring is the consequence ofthe combinat ion of st eric effects an d r epulsive forces,yielding a different result in pure or multicomponentmedia. In part icular, in liquid media, the presenceof components wit h a strong a nisotropy ma y inducean organization on the nano- to micrometric scale,in t h e form of n a n o met ric a g g re g a t e s (mice lles),extended layers (membranes), or solvent-containing

bilay ers (liposomes). Amphiphilic or su rfa cta nt mol-ecules, displaying a polar head and a nonpolar tail ,t e n d t o a g g re g a t e in so lve n t s wh e re o n e o f t h e sedoma in s is in solu ble. Th is fru st ra t e d s i t u a t ionforces the amphiphilic molecule to adopt a compro-mise situation, from the energetic point of view, andaggregates are formed. Beyond the crit ical micellarconcentration (cmc), the amphiphilic molecules formmicelles in solvents of a marked polar or nonpolarcharacter.

A combination of molecular geometry and inter-molecula r (solvophilic/phobic, Coul ombic, H -bondin g)and entropic interactions drives these solutions toself-assemble into colloidal systems, presenting dif-

ferent microstructures: spherical, cylindrical, plana r,cellular,88etc. The assembly depends on the natureand morphology of the discrete molecules. The nu-merous micellar assemblies and aggregates, consti-tuted by the associat ion of amphiphilic molecules,l in ked by we a k forces (va n de r Wa a ls , H -bon ds,electrostatic, etc.) rather than by covalent bonds, arecollectively kn own a s a ssociat ion colloids or m olecularorganized systems.

The ar chitecture of the fina l ma terials (the man -n er i n w h i ch coe xi st i n g p h a s es a r e a r r a n g e d i nspace89) w i ll d ir ect l y r el y on t h e n a t u r e o f t h esurfacta nt molecules, that is, the m orphology of th e

m icel la r a g g reg a t es a n d t h e i nt er a ct i on s a t t h einorganic-organ ic interfa ce (solvent-micelle interac-tion, in the case of solutions). Thus, knowledge of thep ola r h e a d g e ome t ry a n d ch a rg e o f t h e su rfa ct a n t sis essential.

Role of Surfactant Geometry. Micellar aggregatesorganize according to different shapes (spherical orcyl in drica l mice lles, la me lla e , . . .), p ermit t in g t h ecoexistence of tw o incompa tible phas es. Some ty pica lmice lla r s t ru ct u re s a re p re se n t e d in Fig u re 8. Insome colloida l syst ems, a more complex beha vior ha sbeen evidenced, an d other a rra ngements, such as thespongelike bicontinuous structure (Figure 8E), ar epossible.

Figure 8. Micellar structures (A ) sphere, B ) cylinder, C ) planar bilayer, D ) reverse m icelles, E ) bicontinuousphase, F ) liposomes). Reprinted wit h permission from ref 88. C opyright 1994 J ohn Wiley a nd Sons, I nc.

kinter > kor g > kinorg (2)



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U pon progressive increase of surfacta nt concentr a-t io n in t h e a qu e o u s so lu t io n , a n u mbe r o f p h a se sa p p ea r, a lwa ys fo llowin g t h e sa me o rder: direct spheres, direct cylinders, lamellae, inverse cylin-ders, a nd inverse spheres; th is order correspondsto a monotonic var iat ion of the interfacial curvat ure.

Different models have been proposed to explainthese experimenta l facts, the ma in para meters ta keninto account being (1) the hydrophobic interactionsbetw een orga nic chains, (2) geometric restrictions dueto molecular pa cking, (3) molecule excha nge betw eenag grega tes, (4) entha lpy and ent ropy of packing, and(5) electrostatic repulsion between polar heads.

A relatively simple model proposed by Israelachvili

and colleagues,90,91 based on geometrical consider-a t i on s , e xp la i n s a n d p r ed ict s t h e r es u lt i n g s el f-assembled structures of each type. This model con-siders a hyd rophobic liquid -like core, th e cont ribut ionof wh ich is me rely du e t o g eome t rica l con st ra in t s .These geometr ical considera tions rely on t he ra tio ofthe polar head surface to the hydrophobic volume.The amphiphilic molecules are thus modeled like aconical fra gment (the hydrophobic par t) at ta ched t oa spherical (hydrophilic) head. Two main shapes arepossible (Figur e 9): direct conica l, or i ce-cream cone(on e h ydro ph obic ch a in ), a n d in verse con ica l , orcham pagne cor k (two hydrophobic chains).

The st eric hindran ce of t he hy drophobic chain is

ch a ra ct e rize d by t h e ra t io v/l, wh e re v is t h e ch a involume an d l is the chain length; in the case of thepolar head, its contribution is given by the effectiveoptimal surface, a0. To ensure chain fluidity, l mustveri fy t h a t l < lc, wh e re lc is the length of th e fullyextended cha in; lc ca n b e e a s il y es t im a t ed a s afunction of the number of C atoms in the chain, n.The value of the packing para meter, g) v/ lca0, linkst h e mo le cu la r s t ru ct u re o f t h e a mp h ip h il ic mo le -cule to the architecture of the aggregates. The limit-i n g v a l ue s of g ca n b e e a s i ly ca l cu la t e d f or a nagg regat e of know n geometry , by using th e conditiono f ch a in f lu idi t y (l < lc) a n d a n e s t i m a t i o n o f t h eaggrega tion n umber (number of molecules forming

the micelle). The latter can be obtained from two rela-t ion sh ips: t h e a g g re g a t e su rfa ce t o a0 a n d t h e a g -gregate volume to v. Table 3 sum ma rizes the differ-ent micellar st ructures compat ible wit h a given g.90-93

Huo et al.94 were the first to ta ke into account t heg p a ra me t e r t o e x p la in t h e fo rma t io n o f di f fe re n tsurfacta nt-templated oxide mesostructures. I n prin-ciple, the str ucture of the mesopha se depends on th epacking propert ies of the sur fa cta nt m olecules, hence,on the va lue of g. The va lidity of th is concept ha s beenillu st ra t e d by a comp le t e st u dy, w h ich t o ok in t oa ccou n t va riou s p a ra me t ers, su ch a s su rfa ct a n t n a -ture, pH , presence of cosolvents or cosurfa ctan ts, a ndtheir influence on t he observed pha se tra nsitions, for

fixed synt hesis condit ions. For s ilica sy stems, it ha sbeen shown that an increase of g(i.e., a decrease inthe curvature of the micellar motif) leads to phasetra nsitions a long t he sequence micella r cubic (Pm3 n )f hexagonal (P6 m) f bicontinuous cubic (I a 3 d ) fla me lla r .95

Roleof thePolar Head Charge. Both surfactant a ndin org a n ic solu ble sp ecie s direct t h e syn t h e sis ofmesostructur ed MCM-41-ty pe mat erials. The hy bridso lids t h u s fo rme d a re st ro n g ly de p e n de n t o n t h ein t e ra ct io n be t we e n su rfa ct a n t s a n d t h e in o rg a n icp r ecu r s or s . I n t h e ca s e of i on i c s u r f a ct a n t s , t h eformation of the mesostructured material is mainlygoverned by electrostatic interactions. In the simplestca s e, t h e ch a r g es of t h e s ur fa c ta n t (S) a n d t h emineral species (I) a re op posit e, in t h e syn t h e sisconditions (pH). Two main direct synthesis routesha ve been identified: S+I- a nd S-I+.96,97 Two othersynth esis paths, considered to be i n d i r ect , a lso yieldhybrid mesophases from the self-assembly of inor-g a n ic a n d s u rf a ct a n t s peci es b ea r i n g t h e s a m echar ge: count erions get involved as charg e compen-sating species. The S+X-I+ p a t h t a ke s p la ce u n dera cidic condit ions, in th e presence of halogenide an ions(X- ) C l-, B r-); the S-M+I- route is characterist icof basic media, in t he presence of alka line ca tions (M+

) Na+, K+). The different possible hybrid inorganic-

organic interfaces are schemat ized in Figure 10.Ot h e r syn t h e sis ro u t e s re ly o n n o n io n ic su rfa c-t a n t s , w h e r e t h e m a i n i n t er a c t ion s b et w e en t h etempla te an d the inorga nic species a re H-bonding ordipola r , g ivin g birt h t o t h e so-ca l led n e ut ra l p a t h :S0I0,98,99 N0I0,100,101 a n d N0F-I+.102

Table 4 gives different examples of mesostructuredin o rg a n ic ma t e ria ls o bt a in e d fo l lo win g t h e a bo ve -mentioned paths.33,94,103-114

Or ganogelat or -Based M OS Systems. Organogela-tors (low -w eight orga nic molecules) a re a ble to formthermoreversible physical gels in a variety of solventsin very low concentrat ions (10-3 mol dm-3, 3 inverse spher ica l m icelles

Figure 9. Schematic representation of amphiphilic mol-ecules, adopting conical shape (i cecream cone, A) or inverseconical (champagne cork, B).



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str ongly anisotropic str uctures ar e formed, mostly inthe sha pe of f ibers, but also a s ribbons, plat elets, orcylinders.115-118

Several families of orga nogelators exist ; however,t h e p h ysica l-ch emica l p h en ome n a lea din g t o t h eg el a t i on of or g a n i c l i q ui d p ha s e s i s n ot y e t w e llunderstood, a nd most organogelat ors have been ser-endipitously dis covered. These molecules a re m ostlyclassified according t o the ma in forces present in th egel formation step, although different interactionsca n be re sp o n sible fo r t h e o rg a n iza t io n a t t h e su -pramolecular level: H-bonding, van der Waals forces,dipole-dipole intera ctions, charge tra nsfer, electro-sta t ic intera ctions, coordination bonds, etc.

Figure 11 presents some typical examples of orga-n og e la t o rs belon g in g t o t h e di ffere n t fa mil ie s: ca -pable of performing H-bonding, based on a steroidalor organometallic skeleton, or other molecules suchas phthalocyanines or 2,3-bis-n-decyloxyanthracene(DDOA).

Recent AFM studies demonstra ted t he role of th esolvent in t he forma tion of fibrous orga nogels, ba sedin cholesterol derivatives. These gels are composedby fibers imprisoning 30%of t he solvent molecules;e ve ry org a n o g ela t o r molecule is t h u s a ble t o f ix

103 solvent molecules. The rest of the solvent isplaced betw een t hese fibers, bear ing a wea k interac-tion with the organogelators; this weakly bonded

s ol ve nt m a y a l s o b e a cos ol ve nt , n ot ca p a b le offorming gels.118

Org a n o ge ls a re a lre a dy bein g u se d in t h e p h o t o-gra phic, cosmetic, oil, and food industries. B eing ableto reversibly form fibrous networks, with well-definedgeometry and shape, they have been recently useda s t e mpla t e s fo r t h e syn t h e sis o f n a n o- a n d micro-st ru ct u red m a t e ria ls , a s w il l be sh own below.

Polymeric Templates. D en d r i m ers.119-121 Den-drimer s a re ma cromolecules composed of monomerst h a t a re a ssocia t e d in a f ra ct a l-l ike ma n n e r a ro un da mu lt i fu n ct ion a l cen t ra l core . Two syn t h e sis a p -proaches (convergent or divergent) ha ve been de-scribed. After successive reactions, an nth-generation

polymer (Figure 12) is obtain ed, resulting in a highlybra n ch e d a rra n g e me n t o f fu n ct io n a lize d ch a in s o foverall spherical sha pe. The termina l functions in theperiphery can be adequately tailored, as well as thena ture of the inner cavit ies, closer to t he dendrimercore.

De n drimers a re p olymers of ve ry we ll-de fin edstructure, isomolecular an d mult ifunctional, present-ing characterist ic solubility, viscosity, and thermalsta bility. This high st ructura l definit ion, a ssociat edwit h t h e ir f le x ibi l i t y in size a n d fu n ct io n s, ma ke sdendrimers a very promising template for the syn-thesis of novel materials. The most interesting stud-i es s h ou ld a i m t o t h e s y nt h es is of n ew h y br id

Figure 10. S chemat ic representat ion of the different typesof silica-surfacta nt interfaces. S represents the surfactantm olec ule a n d I , t h e in or ga n ic f r a m ew o rk . M+ a n d X-

represent t he corresponding counterions. S olvent moleculesar e not shown, except for the I0S0 case (triangles); da shedlines correspond to H-bonding intera ctions. For a deta iled

explanation, refer to the text.

Table 4. Examples of Mesostructured InorganicMaterials Showing Different Interactions betweenthe Surfactant and the Inorganic Framework

surfactanttype

interactiontype

example ma terials(structure)a re f

cationic S+ S+I- s ilica : M CM -41 (h ex) 37MC M-48 (cub) 37MC M-50 37tungsten oxide (lam, hex) 33, 103Sb oxide (V) (lam, hex, cub) 33

t in sulfur (la m) 33, 104aluminum phosphat e(lam , hex)

105, 106

S+X-I+ s i li ca : S B A-1 (cub Pm 3 a ) 3 3S B A-2 (hex 3D ) 33, 94S B A-3 (hex) 33z in c ph osph at e (la m) 33zirconium oxide (lam, hex) 108t it a n iu m d iox id e (h ex) 283

S+F-I 0 silica (hex) 102

anionic S- S-I+ Mg, Al, Ga, Mn, Fe, Co, Ni,Zn (lam ) oxides

33

lea d oxid e (la m , h ex) 33a l um in u m ox id e (h ex ) 109t in oxide (hex) 110t it a niu m oxid e (h ex) 111

S-M+I- zinc oxide (la m) 33

a lumina (la m ) 33

neutral S 0 S 0I 0 silica : H MS (hex) 99or N0 N0I 0 MS U -X (hex) 100

s ilica (la m , cu b, h ex) 112Ti, Al, Zr, Sn (hex) oxides 100, 110

N0X-I+ s ilica : S B A-15 (h ex) 107N0F-I+ silica (H ex) 102(N0Mn+)I 0 silica (H ex) 148S -M e(O E t ) N b, Ta (h ex ) ox id e 113, 114

a hex, hexagonal; la m, la mellar ; cub, cubic.



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nanocomposites; all of these strategies can in prin-ciple be tra nsposed for indu str ial pur poses to hyper-branched polymers, which are cheaper, although notso m onodisperse.

Dendrimers have already found numerous applica-tions including meta l complexation122 a n d a l s o u se

a s ca t a lyst s ,123 an d t heir incorpora tion wit hin silicaallows production of supported catalysts,124 chromato-gra phic supports, 125 or porous membran es.2c

Recently, dendrimers ha ve been used a s buildingblocks for nanostructured materials 126 a n d a s t e m -plates to prepare mesoporous silica. 127

Figure 11. Examples of organogelators. Description and examples are given in the text.



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Bl ock Copolym er s. Amphiphilic block copolymers(ABC) represent a new class of functional polymers,wit h a st rong application potentia l, mainly due to theh ig h e n e rg e t ic a n d st ru ct u ra l co n t ro l t h a t ca n bee xe rt ed on t h e ma t e ria l in t e rfa ces. Th e ch emica ls t r u ct u r e of AB C s ca n b e p r og r a m m ed t o t a i l orin t erfa ces bet w e en ma t e ria ls of t o t a l ly di ffere n tchemical natures, polarities, and cohesion energies.128

AB Cs ar e a ble to self-assemble in var ied morpholo-gies (Figure 13), like their molecular counterparts,

the tra dit ional surfa ctants. P olymer organized sys-tems (POS) formed by ABC polymers are excellenttemplates for the str ucturing of inorga nic netw orks;t h ey h a v e b een a l s o u s ed f or g r ow t h con t r ol ofdiscrete mineral particles.8,129 Dib lock (AB ) or t riblock(ABA) block copolymers are generally used, in whichA represents a hydrophilic block [polyethylene oxide

(P EO) or polya crylic acid (P AA)] a nd B , a hy drophobicblock [polystyr ene (P S), polypropylene oxide (P P O),polyisoprene (P I), or polyvinylpyr idine (P VP)].

B iological systems ma ke use of a mphiphilic poly-meric systems, such as proteins and polysaccharides,to solve problems of heteropha se sta bilizat ion. Nowa -da ys, a g re a t va rie t y of p o lyme rs ba se d on n u cleicacids, amino a cids, an d sa ccha rides are being devel-oped by biological and biochemical methods. Thesemacromolecules, which are used for medicinal ap-plications, could also be a pplied a s building blocksor t emplat es of adva nced mesostructured ma terials.A very interesting example has been recently pre-sented, w here synt hetic polypeptide-ba sed AB Cs a re

ca p a ble o f a ct in g simu lt a n e ou sly a s t e mpla t e s a n dcata lysts of the formation of an ordered silica frame-work.130

Other Texturing Agents. C ol l oi d a l C rysta l s.Colloidal suspensions of polystyrene (PS) spheres(colloidal latex) can lead to ordered st ructures int h e su bmicron ic ra n g e u p on slow sedimen t a t ionfollowed by solvent eva pora tion. The optical proper-ties of these systems, known as colloidal crystals,

h a ve bee n t h o rou g h ly st u died.131 These orga nicopals h ave been also studied a s templat es, to yieldma croporous ma terials. 24,25 Th e in t e rst i t ia l sp a cebe t we e n t h e sp h e re s is f irst imp re g n a t e d wit h a ninorganic sol (generally obtained by the hydrolysis-condensation of inorganic precursors, such as alkox-ides, or related compounds). A second step impliesthe elimination of the organic template, by heatingor wa shing. This permits in t urn t he revelat ion of atridimensional porous netw ork, the periodicity ofw hich can be cont rolled by controlling the size of thePS spheres. Three-dimensional ordered macroporoustita nia (ana ta se) structures ha ve also been obta inedby a cooperat ive method, where the fabr ication of thet e mpla t in g a g e nt a n d t h e imp reg n a t io n a re ca rr iedout simultan eously.132

Monodisperse hexa gonal m esoporous silica s pheresh a ve bee n p rep a re d by micro moldin g in in vert e dpolymer opals. The inverted polymer opal was madeby first t he infiltra t ion of monomer w ithin t he voidsof a s i lica op a l . Aft e r p olymeriza t ion , t h e si licasp h e re s we re re mo ve d by e t ch in g wit h HF.133 Th eu n iform a n d in t ercon n e ct e d voids of t h e p oro uspolymer can be subsequently used to genera te a w ideva riety of highly m onodisperse inorgan ic, polymeric,an d meta llic solid and core-shell colloids, a s w ell ashollow colloids with controllable shell thickness, as

colloidal crystals.134

Bi ological System s. B iolog ica l syst e ms su ch a sproteins or other supra molecularly orga nized entities(viruses, ba cteria) can a lso be used in order to obtaintextured or structured inorganic frameworks. Somere ce n t wo rks de scribe t h e u se of t h e se syst e ms a sdirect templates of the minera l pha se (tobacco mosaicvirus, Ba ci l l u s su b ti l i s , other ba cterial thr eads, cells,e t c. ) or a s h os t ca v i t ies t o con t r ol t h e g r ow t h ofinorga nic objects (ferrit ine).23,135-138

At the ma croscopic level, th e organic ma trix of acuttlebone of a Sep i a o ff i c i n a l i s (cuttlefish) wa s useda s a t e mpla t e t o ma ke ma crop orou s ch it in-silicacomposites.138 Bu t t e rfly w in g s a n d sp ide r si lk h a vebeen also used to templa te siliceous ma tr ix producedby chemical vapor deposition.139

Phase Separ ati on. Texturation methods that takea dva n t a g e of p h a se se pa ra t ion p roce sse s h a ve a lsobeen described.24,140,141 In this approach, the precipi-tat ion of the inorganic phase is performed within aw a ter/oil or polar /nonpolar solvent microemulsion.The inorganic phase, often hydrophilic, is formed inthe polar phase of the emulsion. Recently, thermo-dynamically stable microemulsions have been pre-pared w ith am phiphilic block copolymers an d usedto produce mesostructured cellular foam materialswith uniformly sized and shaped pores.142

Figure 12. Schematic representation of a dendrimer.

Figure 13. Main morphologies of AB C polymers: sphericalmicelles (MIC), cylindrical micelles (CYL), lamellar struc-tures (LAM), modulated lamellar (MLAM), hexagonalpinhole layers (HPL), gyroids (I a 3 d ), ordered cylinders(HEX), and body-centered cubic (BCC). Reprinted from ref129. C opyright 1998 Wiley-VCH .



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Mixtur es of hyd rosoluble/hyd rophobic polymersh a v e a l s o b e en u s ed a s p r ef or m ed t e mp la t e s t ogenera te a hierarchically contr olled morphology.86

Microst ru ct u ra l con t ro l is a t t a in e d by con t ro ll in ggelation a nd precipita tion processes; the phase sepa-ra tion step is via a spinodal decomposit ion.

In contra st , minera l gels (based on silica or orga -n os il a n es ) c a n a l so b e u s ed i n t h e g r ow t h a n dorienta tion cont rol of organ ic nanocryst a ls. The linear

and nonlinear optical properties of these materialsa re n o wa da ys bein g st u died.143

3.3. Silica-Based Structures

3.3.1. Evolution of the Research

Sil ica -ba se d ma t e ria ls a re t h e mo st s t u die d sys-tems, for severa l reasons: a grea t va riety of possiblestructures (flexibility of tetracoordinated Si), a pre-cise control of the hydrolysis-condensa tion rea ctions(due to a lower reactivity), enhan ced th ermal sta bilityof t he obtained a morphous netw orks (no crysta lliza-t ion upon thermal tr eatment), and str ong graft ing oforg a n ic fu n ct ion s. In a ddit ion , a g re a t n u mber ofstructures found in nature presenting complex ar-ch it e ct u re s (t h e ca se o f d ia t oms or ra diola ria ) a resilica-based.

The dis covery of mesoporous silica or a luminosili-ca t e m olecu la r s ieves in 1992 h a d a n e n ormo usimpa ct in di f fere n t do ma in s, su ch a s ca t a lysis , a d-sorption, optics, and electronics. The novel M41Sf a m i ly w a s or i gi n a l ly ob t a i n ed b y h y d r ot h e rm a lsynthesis, in basic media, from inorganic gels con-taining silicate (or aluminosilicate), in the presenceof quat ernary t rimethylamm onium cations, C nH 2n+1-(CH 3)3N+ (C nTMA+, 8 < n


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structure. Moreover, a controlled oxidation of -S H

groups belonging to the C ys fra gments permits va riedf in a l s t ru ct u res: f rom h a rd si lica sp h ere s (t o t a l lyreduced Cys) to well-defined silica columns (totallyoxidized form of the copolymer). These r esults il-lustra te t he importa nce of the self-ass embled copoly-me r a rch it e ct u re in t h e o bt a in ed t e rt ia ry st ru ct u reand thus in the m orphology.

In the synth esis of template-mediated siliceousma terials w ith complex architecture, t he use of softtemplates has been extended to microemulsions 140

and organogelators.156

Porous Organically M odif ied Sil ica M atr ices.Apar tfro m t h e syn t h e sis o p t imiza t io n a n d t h e qu e st fo rnew a rchitectures in MC M-41 an d relat ed solids, animportant effort has been set to develop materialswith well-defined functionalities. Once the pore sizea n d s h a p e h a v e b e e n m a s t e r e d , i t i s t h e t u r n o fa d d i n g f u n c t i o n s t o t h e i n t e r n a l s u r f a c e o f t h e s epores, to modify the surface properties or to providea part icular property to the material.157,158 The firstapproa ch is to postfunctiona lize a mesoporous oxidephase (calcined or extracted, to eliminate the tem-p la t e ), by g ra f t in g org a n ic159,160 or organometa llicgroups.161 Ma n n e t a l . h a ve sh own t h a t t h e g ra f t in gcan be directly made by co-condensation of an orga-nosilane and a silicon alkoxide, in the presence ofsu rfa ct a n t s .162 This synthesis route presents severaladva nta ges, such a s t he high control of the concen-tra t ion and dispersion of the gra fted functions.157,163

The most representa tive resear ch works concerningthe synthesis of meso- and macrostructured hybridsilicas are presented in Table 5.

3.3.2. Formation of the Inorganic Network

Pure Silica Frameworks: Role of the HybridInterface. MCM-41-type m at erials ar e char acterizedby a re g ula r h e xa g o n a l a rra n g e me n t o f cylin drica lp ore s, p re sen t in g a sh a rp p ore dist r ibu t ion . Th einorganic walls are generally microporous (nonorga-nized microporosity) a nd constitut ed by am orphoussilica.

Originally, the M41S fa mily wa s synt hesized from

different silica sources (such as TEOS, Ludox col-loida l silica , fumed silica, sodium silicate), a cat ionicsu rfa ct a n t (C 16TMAB r), a base (NaOH , TMAOH),a n d w a t e r .

In these alkaline condit ions, the interactions be-tween surfactant molecules (S +) a n d t h e in o rg a n icframework (I-) are ma inly electrosta tic (S+I-). Ana lo-gous solids can be syn thesized in a cidic media (SB A-1, SBA-3). In this case, the mineral-template inter-a c t ion s a r e d i ff er e nt , a n d a m or e r e la x ed h y b r idinterface (S+X-I+ type) is created, w here the char ge-compensating anion X- (for example, C l- from HCl,used to adjust pH) permits an electrostatic coupling

between the equally charged surfactant a nd inorga nicspecies.96

These procedures ha ve been extended t o nonionicsurfacta nts. The HMS family is obta ined by precipi-tation at neutral pH, using TEOS as the silica sourceand primary amines as templates.98,99 In these condi-tions, the formation of the silica network is probablycata lyzed by th e am ine functions [am ines are excel-lent cat a lysts of Si(IV) hydr olysis and condensat ion].The MS U -x fa mily of mesoporous silica has beenp re p a re d by h ydro lysis-condensat ion of TEOS instrong acidic media, in the presence of PEO-basedtemplates.100,101 In t hese systems, thicker w alls (an dh e n ce a n improved t h e rma l st a bi l it y) a re obt a in e da s a con s eq u en ce o f b o t h a ci d c a t a l y s is a n d t h einteractions betw een the t emplat e and t he inorgan icframework.

Fluoride is a well-known cata lyst for hydr olysis a ndpolymeriza tion of silica species15 and has been usedin the sy nth esis of mesoporous silica ma teria ls undervar ious conditions to improve structura l order.100,102,164

A two-step process based on the use of fluoride hasbeen developed to pr oduce mesoporus siliceous ma -terials. This synthetic pathway presents the advan-ta ges of being easy an d highly reproducible.165,166 Aone-step synthesis leading to highly ordered materi-als h as been developed in a w ide pH ra nge (0-9).167

Table 5. Main R esearch C oncerning the Synthesis of Micro-, Meso-, and Macrostructured Silica in the Presenceof Organic Templates

fr a mew or k st r uct u ring a gent s st ru ct ur ea por osit y ref

S iO 2 and SiO2/Al2O3 C nTMA+ hex, la m , cub 15-100 82

S iO 2 C nTMA+ hex (F S M) 20-40 144C nNH 2, C nE O hex (H MS , MS U ) 20-50 98-101AB C disordered 70-150 8-129AB C (P 123) hex (S B A-15) 100-300 107la t ex hc 200-500 19

ba ct er ia l t hr ea ds hex bimoda l 23la t ex/AB C disordered bimoda l 152la tex /C 16TMAB r hc/hex bimoda l 22elect roly t e/AB C foa m/hex bimoda l 317P DM S/la tex/ABC -/hc/cub poly moda l 151la t ex/zeolit es hc/MF I bimoda l 154co-polypept ide spher es/colum ns 130m icr oemulsion disordered 140orga nogela t or hollow fiber 500 330

functiona lized SiO 2indir ect pa th C 16TMAB r hex 20 159, 160

functiona lized SiO 2dir ect pa t h C 16TMAB r hex 20 162

a hex, hexagonal; lam, lamellar ; hc, compact hexagonal; cub, cubic; MFI , zeolite-type str ucture.



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Antoniett i an d colleagues proposed a synth esispat h tha t relies on ABC polymeric templat es.8,129 Th esyn t h e se s a re g e n e ra l ly ca rrie d o u t in a cid me dia ,using silicon alkoxide precursors. The formation oft h e min e ra l n e t wo rk ma y t a ke p la ce e i t h e r in t h ehydrophilic domains or at the interface of the self-as sembled block copolymer. The ut ilizat ion of AB Csp ermit s in cre a se d p ore size in a bou t a n orde r ofmagnitude and t hicker w alls, enhancing thermal a nd

mecha nical sta bility. Moreover, ma croscopic mono-liths (from millimeters to centimeters) can be ob-ta ined by this pat hw ay , due to the enhan ced ductilitya n d e la st icit y impa rt e d by t h e p olymer. Fo llowin gt h is a p p ro a ch , St u cky a n d co lle a g u e s syn t h e size dmesoporous silica presenting large size pores, up to300 .107 Even larger pores can be created by resort-ing to more complex organic or biological texturingagents (see below).151,154

Doped Silica and Silica-Based Mixed Struc-tures. The first synthesis methods of mesoporousma t e ria ls we re a ime d a t s i l ica t e s a n d a lu min o sil i-cates, as a consequence of their potential implicationsin t h e ca t a lysis f ie ld . T h is in i t ia l e f fo rt wa s so o nfollow ed by t he syn thesis of oth er mixed oxides, suchas vanadosilicates, borosilicates, zirconosilicates, ti-ta nosilicat es, an d ga llosilicat es.168-172 The insertiono f me t a l ca t io n s in t o t h e si l ica f ra me wo rk ca n bea t t a in e d e i t h e r by a p o st syn t h e sis t re a t me n t o r bythe mixing of the adequate precursors in the init ialreacting systems.

The int erest of doping relies in t he creat ion of novelcata lyt ic ma terials. The incorpora tion of metal cen-ters seems simple. How ever, a deeper an aly sis of theabundant methods described so far in the literaturere ve a ls a la ck o f re produ cibil i t y of t h e obt a in e dma terials, n am ely in terms of the effective incorpora -

tion of the heteroelement into the silica fr am eworka n d i t s loca l iza t io n a n d dispe rsion t h ro u gh ou t t h emat erial. Other importa nt ga ps are the nature of theme so st ru ct u re a n d t h e st a bi l i t y o f t h e ma t e ria l . As y n t h es i s m e t h od b a s ed on a r et a r d i n g a g e nt ,trietha nolam ine (TEA), circumvents these limita-tions.173 B y m a k in g u se of a t r a n e com pl ex es a sinorga nic precursors, it ha s been possible to obtaindoped M-MCM-41 (M ) B , Al, Sn, Zn, Ti, Zr, V, Mo,Mn, Fe, C o, and Ni) with a n M/Si r at io improved byan order of magnitude.

It ha s also been shown t ha t improved dopant ra tiosfor trivalent cat ions can be at tained by resorting toa n e u t r a l S0I0 syn t h e sis p a t h , ra t h e r t h a n a n e le c-trostat ic approach.174 In these cases, the incorpora -t i on of t h e M (I I I ) i s s t r on g ly d ep en d en t on t h esynth esis condit ions, part icularly on pH.

Recently, va rious meth ods to dope siliceous MC M-41 ma t rice s w it h t i t a n iu m h a ve be e n comp a re d.175

The direct synth esis route genera lly leads to a lessordered porous network. Postsynthesis doping of Tiallows high Ti loadings. The incorporation of Ti byimpregnation results in a phase separa tion. Althougha small decrease in pore diameter is observed, theMCM-41 st ru ct u re is p re se rve d wh e n t i t a n iu m ispostgrafted. The result ing solids present an evendispersion of t ita nium centers.

Hybrid Compounds. The high control of MCM-41 and r elated pha ses (in terms of highly accessiblesurfa ce, a w ide choice of pore size, and pore uniform-ity a nd distribution) ma kes these ma terials par t icu-larly interesting as supports. Or ganic functions canbe g ra f t e d o n t o t h e o x ide wa lls , le a din g t o h ybridme sost ru ct u red ma t e ria ls , wit h t u n a ble su rfa ces.This is an indeed promising issue in the design ofa d v a n c e d i n t e g r a t e d m a t e r i a l s , s u c h a s c a t a l y s t s ,

membranes, sensors, and nanoreactors.176

An import a n t n u mber of t e ch n iqu es h a ve bee ndeveloped or adapted to add organic functions to thewalls of mesoporous silica, 157 combining the proper-t ie s o f a me so p o ro u s in o rg a n ic st ru ct u re wit h t h esu rfa ce o rg a n ic g ro u ps. Th e min era l f ra me wo rke ns u r es a n or d er e d s t r u ct u r e i n t h e m es os ca l e,therma l a nd mecha nical sta bility. The organic speciesintegrat ed to the ma terial permit f ine control of theinterfa cial a nd bulk propert ies, such a s hy drophobic-ity, porosity, a ccessibility , optical, electr ical, or ma g-netic properties. The incorporation of the organicf un ct i on s ca n i n p r in ci pl e b e ca r r i ed ou t i n t w owa ys: (a) by covalent binding on the inorganic wa lls

of t h e ma t e ria l (p ost -t re a t me n t ) a n d (b) by directin corp ora t ion of t h e org a n ic fu n ct ion s, u p on t h esynthesis process (one-pot).

In the first approach, organochlorosilanes or orga-n oa l k ox y si la n e s h a v e b ee n w i d el y u s ed t o g r a f tspecific organic groups, by condensation reactionswit h si la n o l or S i-O-Si gr oups of the silica fram e-work (Figure 14 a). 159,160,177 The mesoporous hosts

must be thoroughly dr ied before the a ddit ion of theorganosilane precursors t o a void t heir a utoconden-sation in the presence of water. The concentrat ionand distribution control of the organic functions isrestricted by th e surface silanols and their a ccessibil-i t y . Th e g ra f t in g ra t io de pe n ds o f t h e p recursorreactivity, being also limited by diffusion a nd st ericfactors.

Figure 14. Incorporation of organic functions in mesopo-rous silica: (a) surface graftin g of orga nic functions on themesopore walls by postsynthesis; direct incorporation oforganic functions by co-condensa tion of organosilanes (b)or bridging silsesquioxanes (c).



18/46

An alternative approach for pore functionalizationrelies on a direct syn th esis, bas ed on the co-conden-sat ion of siloxane an d organosiloxane precursors insitu to y ield modified MCM-41 in one st ep (Figure14b).162,178 Whereas siloxane precursors ensure t heforma t ion of t h e min era l n e t wo rk, org a n o silox a n em oi et i es p la y a d ou b le r ol e: t h e y con t r i bu t e a sbuilding blocks of the inorganic structure and theyprovide the organic groups. This one-pot pathway

presents several a dvan ta ges, such a s high modifica-tion rat ios, homogeneous incorporation, and shortp rep a ra t ion t ime s.163 Ho we ver, t h e ch oice of t h emodified precursor is constrained by the synthesisconditions. Alkaline m edia, hydrotherm al conditions,a n d so lve n t e x t ra ct io n l imit t h e ch o ice t o o rg a n icfra g me n t s p re sen t in g S i-C bonds sta ble to nucleo-philic a t t ack. B ridged silsesquioxanes [(RO)3S i-R -Si(OR)3] h a v e a l s o b ee n u t i l iz ed i n t h i s k in d ofsynthesis, a lso yielding organically modified meso-porous silica, a lso known as periodic mesoporousorganosilicas (PMO).179-182 I n t h es e m a t e ri a l s, t h eorgan ic groups (R ) are h omogeneously incorporat edi n si de t h e m es op or ou s w a l ls , w h i ch p er m i t s t h e

a ddit ion of n e w fu n ct ion s wit h o u t p ore blockin g(Figure 14c). Moreover, these structures present anextremely well-defined mesostructure. A great vari-ety of bridging groups have been incorporated, sucha s -(CH 2)n-or -C 6H 4-. Mesoporous particles pre-senting different mesostruct ures (2D-hex, 3D-hex,an d micellar cubicPm3 n ) and controlled morphology(rodlike, spherical, a nd decaocta hedra l, respectively)have been obtained by r esorting to R ) -C H 2C H 2-in different synth esis condit ions.183 When R is ap-disubstituted phenyl, part icular ly w ell-ordered m a-terials have been synthesized, which combine meso-p ore s wit h wa lls p re sen t in g a la ye red st ru ct u re inthe m olecular ra nge (5.7 ), giving rise t o a hierar-

chica lly ordered mesostructur e in a one-pot sy nt hesis.I n t h i s c a s e , t h e p h e n y l-p h en yl in t e ra ct ion s a rebelieved t o be responsible for the cryst alline a rra nge-m e n t s o f t h e p o r e w a l l s . 184 B u l k y g r o u p s a b l e t ochelate metal centers, such as 1,4,8,11-tetraazacy-clotetradecane (cyclam), ha ve been incorpora ted inthe walls of large-pore Pluronics-templated silica. 185

3.3.3. Formation Mechanisms

After the discovery of M41S and related solids, animporta nt number of resear ch teams focused on theu n derst a n din g o f t h e ir forma t ion me ch a n isms.186

Most of the work was devoted to MCM-41 silica in

a lka l in e m e diu m, u sin g ca t ion ic a lkyla mmo n iu mhalide templates. In this section, we will present acrit ical review of these ad vances. Although there isa sust ained interest in th e forma tion pat hs of meso-structured silica films, we will not present the worksdevoted to film formation, as a detailed discussion isout of the scope of this review. The essentia l feat uresof the evaporation-induced self-assembly approachhave been addressed by Brinker and colleagues,187

and we will describe some interesting applicationsfor non-silica systems in the corresponding section.

Several models of MCM-41 formation have beenproposed,5 a very important task in order to rational-ize the na ture a nd str ucture of the obta ined materi-

a ls . F in din g a corre la t ion bet we e n t h e syn t h e sismethod and t he final structure is a key point t o thede sig n of me sop oro us ma t e ria ls , a n in dee d mo ree leg a n t s t r a t e gy t h a n t h e u s ua l com b in a t o r ia l /a n a lyt ica l a p p roa ch .

Al l m od el s p r op os ed s o f a r a r e b a s ed on

reviewmicro meso soler illia 2002

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