Solid State Sciences 2 (2000) 405–420

Generalised syntheses of ordered mesoporous oxides: theatrane route

Saul Cabrera 1, Jamal El Haskouri, Carmen Guillem, Julio Latorre,Aurelio Beltran-Porter, Daniel Beltran-Porter, M. Dolores Marcos *,

Pedro Amoros *Institut de Ciencia dels Materials de la Uni6ersitat de Valencia (ICMUV), PO Box 2085, 46071-Valencia, Spain

Received 15 September 1999; received in revised form 22 April 2000; accepted 2 May 2000

Abstract

A new simple and versatile technique to obtain mesoporous oxides is presented. While implying surfactant-assistedformation of mesostructured intermediates, the original chemical contribution of this approach lies in the use of atranecomplexes as precursors. Without prejudice to their inherent unstability in aqueous solution, the atranes show a markedinertness towards hydrolysis. Bringing kinetic factors into play, it becomes possible to control the processes involved in theformation of the surfactant–inorganic phase composite micelles, which constitute the elemental building blocks of themesostructures. Independent of the starting compositional complexity, both the mesostructured intermediates and the finalmesoporous materials are chemically homogeneous. The final ordered mesoporous materials are thermally stable and showunimodal porosity, as well as homogeneous microstructure and texture. Examples of materials synthesised on account of theversatility of this new method, including siliceous, non siliceous and mixed oxides, are presented and discussed. © 2000Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Keywords: Mesoporous oxides; Atrane complexes; Mesostructures

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1. Introduction

‘Porous materials created by nature or by syn-thetic design have found great utility in all aspects ofhuman activity’. This pertinent sentence introducesan excellent publication which nicely introduces usto the field of nanoporous materials [1]. The attrac-

tion of new prospects for industrial utility explains toa great extent the impact that the discovery of thefirst family of silica-based mesoporous molecularsieves (denoted M41S) had in the scientific commu-nity. Following the announcement of the Mobil re-searchers’ success, based on the use of long-chainquaternary ammonium surfactants as ‘structural di-recting agents’ [2], a large amount of related workwas published in a very short time [3–5]. An attemptwas made to quickly adapt Mobil’s technology forthe preparation of non-silica-based mesostructuredand/or mesoporous oxides, but attempts to extendthe so-called ‘supramolecular templating method’met with limited and sporadic success [6–9]. Dealingwith doped silicate compositions (of interest in catal-

* Correspondence and reprints: Tel.: +34-96-3983617; fax: +34-96-3983633.

E-mail addresses: solidos@mail.megalink.com (S. Cabrera),daniel.beltran@uv.es (D. Beltran-Porter), loles.marcos@uv.es(M.D. Marcos), pedro.amoros@uv.es (P. Amoros).

1 Permanent address: Laboratorio de Solidos y Quımica Teor-ica, Instituto de Investigaciones Quımicas UMSA, Cota-Cota,Calle no. 27, La Paz, Bolivia.

1293-2558/00/$ - © 2000 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

PII: S1293 -2558 (00 )00152 -7

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420406

ysis), it has been stated that although the incorpora-tion of a metal to the silica system seems straightfor-ward a priori, ‘the amount of dopant incorporation,the reproduction of the mesophase that is obtainedin the pure silica system and the stability of theresulting mesoporous material cannot be predicted’[4]. Preparative difficulties, which extend from con-trol of amounts and dispersity of metals in dopedsilica to the obtaining of metal oxides, have evenprovoked a certain scepticism with regard to theapplied interest of research in this field [8].

We have recently reported a set of results thatshows how a significant portion of the outlinedproblems can be overcome by using alternativepreparative methods [10–13]. Our synthesis ap-proach is based on the use as reactant of an ade-quate ‘hydrolysis retarding agent’. Furthermore, weelaborate on this concept and also present new re-sults on mesoporous solids. Our aim is to show thatthe ‘atrane route’ is a powerful generic chemicaltechnique for the obtaining of nanoporous materials.

2. Processing routes to ordered mesoporous solids

All known processing routes to ordered meso-porous oxides imply, in practice, the intermediateformation of mesostructured materials. These can beconsidered crystal liquid-like organic–inorganiccomposites. Two different types of pathways canlead to such intermediates: the route of the liquid-crystal templating mechanism (LCT) and the onesbased on the cooperative organisation (self-assem-bling) of the respective organic and inorganicsupramolecular moieties [2–4,14]. In the case of theLCT mechanism, the inorganic phase grows andoccupies the intermicellar space of a previouslyformed ordered organic mesophase (‘supramoleculartemplate’). The other approach relies more on con-ventional colloid chemistry: regardless what the truemechanistic details may be, cooperative self-assem-bling procedures involve the formation of composite(inorganic–organic) micelles. These composite mi-celles are those which organise to form particlenuclei that flocculate after a more or less lengthygrowing stage. The main procedural difference be-tween both types of pathways is the surfactant con-centration in the reaction medium. Whereas theformation of liquid crystals (LCT mechanism) re-quires very high surfactant concentrations, coopera-

tive self-assembling processes occur in the presenceof surfactant concentrations around the critical(cmc) values.

The principal synthetic advances have resultedfrom developments of the chemistry that underliesthe cooperative self-assembling processes at low sur-factant concentrations. As indicated above, thepreparative chemistry of ordered mesoporous solidsis, in that case, closely bound to colloid chemistry.However, the solution chemistry of the starting inor-ganic precursor is an essential tool in the search forrational synthesis designs. This can be easily under-stood by considering that the solvolysis and conden-sation processes affecting the precursor are whatgenerate the inorganic component of the compositemicelles [15]. Indeed, besides the very nature andrelative concentration of the surfactant, otherparameters which can be deliberately tuned for de-signing mesoporous materials are the chemical char-acteristics of the precursor and its concentration, aswell as temperature and pH. The process as a wholecan also be modulated in the presence of an ade-quate cosolvent [16].

2.1. Precursors solution chemistry

Metal (or metalloid) alkoxides (in general, com-plexes involving alkoxy ligands) are very useful cleanoxide-precursors because, once displaced from themetal (or metalloid) environment (solvolysis), alco-hol groups do not participate in subsequent pro-cesses (condensation) or can be easily removed fromthe solution. Then, it is crucial in this context toknow about the sol–gel chemistry of alkoxides[17,18]. Alkoxides are thermodynamically unstable inaqueous solution, their lability increasing as theLewis acidity of the coordinated element as well asthe possibility of expanding its coordination sphere.Indeed, they usually react readily with water to formprecipitates, exception made from those of the rela-tively more electronegative elements. In fact, thehydrolytic inertness of the alkoxysilanes is the reasonfor which their chemistry has been the most widelyexploited in the preparation of gels, nanoparticlesand mesoporous materials. It seems clear, therefore,that to moderate the alkoxides reactivity towards thenucleophilic attack of water is an essential require-ment for their applicability as precursors of gels ormesostructured materials.

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420 407

Although with limited success, a technique thathas been often applied to get such a moderatingeffect is based on the use of precursors consisting ofalkoxide–bidentate ligand mixed complexes[17,19,20]. The functionality lowering in the mixedcomplex with regard to the pure alkoxide is relatedto the stability of the chelate. However, the addedligand is only partially eliminated in the condensa-tion step. Hence, this technique leads to gels andmesostuctured materials, but these do not go for-ward as mesoporous solids because the remainingorganic portions hinder the formation of the wantedcontinuous oxide network [21].

2.2. The ideal precursor concept

The above comments endeavour to hint at a con-cern: the obtaining of non silica-based (and also thatof ‘highly doped’ silica-based) ordered mesoporousmaterials on the basis of the intermediate prepara-tion of an inorganic–organic composite mesostruc-ture is certainly not easy a priori. There are reallymany competitive factors that have to be har-monised in the search for an adequate precursor.Thus, first of all it is necessary that the kinetics ofpolymerisation of the alkoxide complex can be con-trolled to allow the inorganic solutes condense onthe surfactant headgroups; the formation of criticalsize pure oxide particle nuclei must be avoided inany case. However, the inherent reactivity of thealkoxides works against this methodological require-ment for hydrolysing in the presence of the surfac-tant at relatively high dilution.

As suggested above, an adequate precursor wouldhave to show a moderate stability in neutral or basicsolution, but in turn, its very chemical nature wouldnot have to hinder the fact that the hydrolysis andcondensation processes go to completion. In addi-tion, when thinking about materials including two ormore oxide-forming elements, the reactivity of thedifferent precursors should be harmonised in orderto guarantee the compositional homogeneity of theproduct.

The most useful guide to overcome such a thermo-dynamic and kinetic challenge lies in the properalkoxysilanes chemistry: ‘an adequate molecular pre-cursor has to be constructed from ligands able tocoordinate the elements of interest to give complexeswhich are simultaneously unstable and inert inaqueous solution at neutral or basic pH values’. It is

known that the alkoxides inertness increases as boththe size of the organic radicals (steric effect) and thenumber (and relative closeness) of alcohol groups inthe ligand (chelate or cage effect). We can hope,therefore, that the best candidates for ligands toconstruct adequate precursors belong to the series offunctionalised polyalcohols.

In the present work, we report how the tri-ethanolamine ligand (2, 2%, 2%%-nitriletriethanol),hereinafter TEAH3, fulfils all the necessary require-ments when dealing with a wide set of elements. So,atrane complexes (i.e. complexes that include fullydeprotonated TEAH3-like species as ligands) verylikely are one of the best possible precursors for bothnon-siliceous and mixed-oxide mesostructured andmesoporous systems. Shown in Table 1 is a summaryof the systems (and compositions) that we have beenable to explore so far on account of the complexingproperties of such a polyalcohol.

3. TEAH3 as ligand: atrane complexes chemistry

In aqueous solution TEAH3 behaves as a tertiaryamine. It forms weak cationic complexes by acting asa neutral nitrogen-donor ligand [22]. Therefore,TEAH3, as well as other functionalised polyalcohols,has been used as a reagent in sol–gel processingmethods [23,24]. However, although described sincethe beginning of the 1930s, it is less widely knownthat, in the absence of water, TEAH3 is capable offorming more stable amine–trialkoxo complexes(with a diversity of elements) by acting as anionic(tetradentate) tripod ligand [25–31]. For instance,triethanolamine reacts easily and smoothly withboric acid to give the coordination complexN(CH2CH2O)3B, a crystalline solid soluble in a vari-ety of organic solvents, which slowly hydrolyses inaqueous solution [26]. Silicon atranes were the sub-ject of a certain interest in the 1970s [27]. Also, thechemistry of other similar species of stoichiometryN(CH2CH2O)3M or N(CH2CH2O)3M�Z (whereM=Si, Al, Sm, V, Ti, among others, and Z=H, O,OR, SR, NR or NR2, depending on M) has beenrecently revisited [32]. As a general rule, the synthe-ses of atrane complexes like these were carried outby means of transesterification reactions, startingfrom alkoxy derivatives in non-aqueous dried sol-vents under an inert atmosphere [33].

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420408

The most outstanding feature of the atrane com-plexes, with respect to the matter we are dealingwith, is their chemical inertness towards hydrolysis[27]. Indeed, once an atrane complex has been ob-tained in the absence of water, it may generateaqueous solutions that remain unchanged for long

periods (which can reach several weeks for a givenligand and metal). This is a relevant observation inour concern and, therefore, we have investigated theamazing ‘apparent stability’ of aqueous solutions ofthe TEAH3 derivatives of B, Al, Si and Ti, amongother elements. In order to identify the species in

Table 1Properties of selected mesoporous materials prepared by the atrane route a

BET surface aread [A, ] (XRD) Pore size [A, ] Type (TEM)Pure oxides Lowest Si/M[m2/g] reported and Ref.

340(15)AlO 33(3)69.0(4) WW35.5(3)TiO 606(10)56.2(4)

1120(15) 25.6(2) HoSiO(MCM-41) 37.5(3)

Doped silicaSi/M1, Si/M2

850(15)44.1(2) Ho/HdSi/Mn=3.03(1) 26.5(2) 11 [44]650(15) 23.1(2) Ho/HoSi/Co=23.23(3) 39.1(2)

44.1(21) 692(10)Si/Ni=12.02(3) 34.0(2) Ho/Hd

843(15) 28.5(2)47.0(3) Ho/HoSi/Cu=22.12(2)Si/Zn=7.50(2) Ho/Hd27.0(1)630(10)46.4(2)

25.5(2) Ho/Hd1070(15) 17.0 [4,44]37.2(2)Si/B=15.05(2)430(15)Si/Al=1.06(4) 20.1(2)41.8(2) Ho/Hd 1.03 [47]

Si/Fe=10.01(2) 36.5(2) 859(10) 24.5(2) Ho/Hd 15.0 [44]Si/Ti=1.96(5) 34.5(2) 595(15) 20.0(2) Ho/W 5.0 [45]

5.0 [4]Ho/W18.6(2)Si/Zr=3.01(3) 687(10)31.5(2)625(15)Si/Sn=5.23(4) 25.0(2)42.0(2) Ho/Hd 42 [44]433(10) 27.1(2) Ho/W 20 [44]Si/V=4.02(4) 39.1(2)

37.5(2) 780(20) Hd21.0(2)Si/Ti=5.50(1),Si/Al=3.90(1)

707(20) 21.5(2)38.1(2) HdSi/Ti=2.70(1),Si/Al=5.03(1)

35.8(2) HdSi/Ti=4.02(1),Si/Zr=6.02(3)

25.6(2) HdSi/Zr=1.01(2),Si/V=10.01(2)

Hd41.0(2)Si/Ni=45.23(2),Si/Zn=8.43(1)

41.0(2)Si/Ni=60.08(2), Hd

Si/Mo =6.51(1)

ALPO/SAPOP/M1, P/M2

P/Al=0.17(3) 13.0(2)30.0 480(10) W0.64 [48]Hd36.0(2)650(15)51.0P/Al=0.74(3)

37.1(2) Hd622(15)47.0P/Al=1.03(4)33.2 730(10) 38.4(2) HdP/Al=0.59(3),

P/Ti=4.68(3)31.6(2) 640(15) 21.0(2) HdP/Al=0.75(3),

P/Si=0.43(3)

a Ho=Orderer hexagonal. Hd=Disordered hexagonal. W=Wormhole.

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420 409

Table 2Main species in solution from FAB-MASS and NMR spectroscopies a

2M:7TEAH3:xNaOH:yH2O

x=0 x=1 x=3

y=180 y=0 y=180y=0 y=0

B(TEA)2H4+B B(TEA)

B(TEA)2H3B(TEA)2H3

BTEAH+

Al(TEA)2H3 AlNa(TEA)2H2Al Al(TEA)2H3[Al(TEA)]n, n=2, 3, 4 AlNa(TEA)2H2

Al2Na(TEA)2+ AlNa(TEA)2H2 AlNa2(TEA)2H

Al(TEA)(OH)− Al2TEA2,... Al2TEA2

[Al(TEA)]n, n=2. (w)Al(TEA)(OH)−

Si(TEA)2H2(H3O)]+ SiNa(TEA)2H2+Si Only sodatrane speciesSi(TEA)2H2 SiNa(TEA)2H2

+

(w) Si(TEA)2H2Si2(TEA)3H [Si(TEA)2H2.

Si(TEA)(OH) Si2Na(TEA)3H+ Si2Na(TEA)3H+Si3(TEA)4

Si3Na(TEA)4+ Si3Na(TEA)4

+

Ti(TEA)2H2Ti Ti(TEA)2H2

TiNa(TEA)2H TiNa(TEA)2H2+

Ti2Na(TEA)3HTiNa2(TEA)2H(OH)Ti2(TEA)3H2

M1:M2:7TEAH3:xNaOH:yH2Ox=1, y=0

B�Si B(TEA), BNa(TEA)2H2, Si(TEA)2H2, SiNa(TEA)2H2+

AlNa(TEA)2H3+, Si(TEA)2H2Si�Al

Si2Na(TEA)3H2+, (*) SiAlNa(TEA)3H2

+

M(TEA)2H2, MNa(TEA)2H2, (M=Ti o Si)Ti�SiSiTiNa(TEA)3H+, SiTiNa2(TEA)3

+, SiTi2(TEA)3H+

Ti�Al Ti(TEA)2H2, TiNa(TEA)2H2+, AlNa(TEA)2H2

AlTi(TEA)3H3+, AlTiNa(TEA)3H2

+, AlTiNa2(TEA)3H+, AlTi2(TEA)4H2+, AlTiNa(TEA)3H

a Bold lettering indicates predominant species and normal lettering main secondary ones). (w) Derived mainly from some fragmentsfound in FAB-MASS spectrum and from hydrolysis mechanism. (*) Probable specie as it has the same m/e ratio as Si2Na(TEA)3H2

+.

solution, we have used mainly both analytical (FastAtomic Bombardment, FAB, coupled with MassSpectroscopy) and structural (multinuclear NMR)spectroscopic techniques. The main procedural vari-ables have been temperature, time and pH. All thecorresponding experiments have been performed inthe presence of an excess of TEAH3 with respect tothe involved metal or metalloid. The most significantresults are summarised in Table 2. In general, theresults are consistent with the metathetic formationof atrane species by displacing the correspondingalcohol from the initial alkoxides [34].

In anhydrous solution, the dominant species de-pend on the charge and acidity of the involvedcation. In the case of trivalent elements, the majorityspecies detected in anhydrous medium are of the[N(CH2CH2O)3M]n (n=2–4, M=Al; n=1, M=B)type, which is consistent with the literature results[33]. Tetravalent elements (M=Si, Ti, Zr) form spe-cies like [N(CH2CH2O)3M�OR] (Type I) [27]. In ourcase, the OR groups come from either the startingalkoxide or another TEAH3 molecule. We havedetected the formation of oligomers consisting oflinked Type I species in the case of Ti. However, in

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420410

the case of Si, the majority species are[N(CH2CH2O)3Si](3−x)[N{(CH2CH2O)(3−x)(CH2CH2

OH)x}] (Type II).The presence of NaOH in anhydrous medium

modifies the system not only by generating [N{(CH2

CH2O)(3−x)(CH2CH2OH)x}](3−x)− alkoxo anions,but also originating atrane anions like [N(CH2

CH2O)3SiN{(CH2CH2O)2(CH2CH2OH)}]−. In asimilar way, in the case of trivalent elements like Al,the anionic dominant species are [N(CH2CH2

O)3AlN{(CH2CH2O)(1+x)(CH2CH2OH)(2−x)}](1+x)−

atrane complexes. Very likely, all these anionic spe-cies become stabilised in anhydrous medium by in-corporating Na+ cations. In fact, FAB-MASSresults indicate the presence of sodium–silicon orsodium–aluminium mixed atranes in significantamounts.

The behaviour of the atrane species towards wateraddition depends on the characteristics of the ele-ment involved (acidity and preferential coordina-tion), being particularly sensitive to the features ofthe coordination sphere of the element in the starting

atrane complex. Thus, in the absence of NaOH,aluminium-containing oligomers evolve tomonomers such as [N(CH2CH2O)3Al�Z] (where Z=OH−, N(CH2CH2OH)3 or H2O). Besides to be con-sistent with the FAB-MASS data analysis, this resultalso is supported by 27Al NMR experiments (Fig.1a). The chemical shift value corresponding to theNMR peak indicates the absence of[N(CH2CH2O)3Al]n (n53) oligomers after water ad-dition [17,33]. Alumatrane hydrolysis (and subse-quent condensation processes) can be forced byraising the temperature. Temperature variable 27AlNMR experiments (Fig. 1a) clearly show that theatrane species progressively transform by gentleheating into hydrolysed species, but hydrolysis pro-ceeds all the time through [N(CH2CH2O)3Al�Z] enti-ties. The marked asymmetry of the NMR peakindicates the apparition of low-coordinated alu-minium environments at the initial hydrolytic steps.On the other hand, the hydrolysis of the anionicalumatranes (stabilised as mixed sodium–aluminiumatranes in the presence of NaOH) goes, as FAB-MASS analysis shows, through [N(CH2CH2O)3AlN{(CH2CH2O) (CH2CH2OH)2}]−, an amine–te-tralkoxo anion. The different reactivity of this spe-cies is proved by temperature variable 27Al NMR(Fig. 1b). As can be noted, for the same experimen-tal conditions (concentration, temperature and time),the intensity lowering of the d= −65 ppm mainpeak is now much less pronounced, this indicating aretarded hydrolysis rate with respect to that occur-ring in the absence of NaOH. Moreover, the NMRspectra include a second peak, at d= −85 ppm,which corresponds to the apparition of a secondaluminium coordination environment (probably witha lower coordination number). The position of thissecond peak is coincident with that of the asymmetryin the aluminium signal observed in the spectraregistered for samples exempt from sodium. In con-trast to the enhanced sensitivity towards hydrolysisdisplayed by the aluminium trialkoxides, the aluma-trane hydrolytic inertness can be understood in thelight of the formation of anionic pentacoordinatedspecies, in which the acidity and coordination ofaluminium are sufficiently saturated. In short, theintermediate formation of [N(CH2CH2O)3AlN{(CH2CH2O)(CH2CH2OH)2}]− or N(CH2CH2O)3

Al�OH]− would account for the alumatrane’s ‘ap-parent stability’. Probably, the hydroxy species is anecessary intermediate for further condensation pro-

Fig. 1. Evolution of the 27Al NMR spectra of solutions containing2 Al:7 TEAH3:×NaOH: 180 D2O molar ratios. (a) x=0.0. (b)x=1.0.

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420 411

cesses, which might lead to dihydroxy,[N{(CH2CH2OH)(CH2CH2O)2)} Al�(OH)2]−, or m-oxo, [N(CH2CH2O)3Al�O�AlN(CH2CH2O)3]2−, spe-cies. In turn, species such as these lasts may explainthe apparition of the second peak (or signal asymme-try) in the 27Al NMR spectra registered as the hy-drolysis progresses. In a similar way to that above,the evolution in water of the silatrane oligomerswould lead to species like [N(CH2CH2O)3

SiN{(CH2CH2O)(CH2CH2OH)2}] or [N(CH2CH2

O)3Si(H3O)N{(CH2CH2O)(CH2CH2OH)2}]+. In-deed, the system is more inert toward hydrolysisthan the conventional silicon tetralkoxides, as it wasreported for the similar 1-ethoxysilatrane(CH3CH2O)�SiN(CH2CH2CH2O)3 [27]. On the otherhand, the mixed sodium–silicon atranes must hy-drolyse in aqueous medium to give the neutral spe-cies [N(CH2CH2O)3SiN{(CH2CH2O)(CH2CH2

OH)2}] together with an equivalent concentration ofOH− groups. Then, the nucleophilic displacement ofthe monodentate alkoxo group (to give the 1-hy-droxysilatrane species) becomes possible. Our resultsindicate that the system then evolves readily (basecatalysed hydrolysis and condensation), in such away that only sodatrane species are detected byFAB-MASS spectroscopy. Considered as a whole, itcan be stated that the silatranes behave as a TEOS-type system. Titanatranes behaviour is similar tothat of silatranes.

In summary, all occurs as if the nucleophilic at-tack of water on the inert atrane species wouldfinally lead to reactive hydroxy-atranes,[N(CH2CH2O)3M(OH)]n−, being these intermediatespecies what initiate the condensation processes.

4. A generalised synthetic approach to mesoporousmaterials: the atrane route

As we just have seen, the atrane complexes ofelements such as B, Al, Si and Ti are inert towardshydrolysis, their inertness being controllable as afunction of the pH. Such behaviour is not exclusiveto these elements, and the atrane derivatives ofmetals like Zr, V, Mo, Fe, Co, Ni and Zn behavesimilarly. At this point, it should be emphasised that,when prepared in aqueous solution, the complexspecies are weak cationic N-bonded aminecom-plexes, {M[N(CH2CH2OH3)x(OH2)y ]}z+. In con-trast, atrane complexes per se are aminetrialkoxy

species, like N(CH2CH2O)3M or N(CH2CH2

O)3M�Z. Very likely, it is a combination of elec-tronic, steric and thermodynamic factors what makesthem inert towards hydrolysis. In short, this is asimilar behaviour to that shown by the alkoxysilanes(without prejudice to the enhanced inertness of thesilatrane with respect to conventional alkoxides). Inpractice, it is such a balance between thermodynamicweakness and kinetic inertness towards hydrolysisthat allows controlling the inorganic-polyanion/sur-factant interactions, in single systems as well asmixed ones. In this last case, it then becomes possi-ble to guarantee the homogeneous intimate mixtureof the components. Therefore, the idea of ‘prepara-tive design’ for a given mesostructured system hav-ing a complex composition, which might progress tomesoporous, is not an entelechi.

Let us see how a typical sample preparation turnsout [35]:

(i) preparation of the respective atrane complexby transesterification of an alkoxide derivative of therequired element with TEAH3 (in the presence ornot of an inert solvent), according to a reactionscheme such as:

nM(OR)3+ (n+x)(HOCH2CH2)3N

l [M(OCH2CH2)3N]x+3nROH

(such a reaction might involve various M elements,simultaneously or one by one)

(ii) surfactant addition(iii) dilution with water, which may be followed

by a thermal treatment to control hydrolysis andcondensation (this is actually the key steep for theformation of M�O�M% links in mixed systems)

(iv) precipitation and separation of the mesostruc-tured material

(v) final thermal treatment for the obtaining ofthe mesoporous material.

The surfactant directing agent used in all thepreparations quoted in this work was cetyltrimethy-lammonium bromide (CTAB), and the pH was ad-justed to the working range adequate for theformation of inorganic polyanions (depending on theinvolved elements). [17] Also, reaction parameterslike temperature, time and concentrations of thereagents, must be optimised in the function of thesystem under study. Summarised in Table 3 arethose corresponding to the materials listed in Table1.

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420412

Table 3Synthesis conditions of selected mesoporous materials prepared by the atrane route a

Pure oxidesM :TEA :CTAB :W Aging:Type Temp.(°C) Time (h)

2 :7.2AlO :0.5 :1.06 H* 120 722 :7.5 :0.7TiO** :332 n-H* 20 402 :7 :0.6 :333 n-H*SiO 45 72

Doped silicasSi/M Si :M :TEA :CTAB :W Aging:Type Temp.(°C) Time(h)

3.03(1) 1 :0.30 :3.5Si/Mn :0.2 :155 n-H* 25 2423.23(3) 1 :0.10 :3.5 :0.2 :155 n-H*Si/Co 25 2423.23(3) 1 :0.17 :3.5 :0.2Si/Ni :155 n-H* 25 24

Si/Cu 22.12(2) 1 :0.12 :3.5 :0.2 :155 n-H* 25 24Si/Zn 7.50(2) 1 :0.15 :3.5 0.2: 155 n-H* 25 24

15.05(2) 1 :0.10 :3.5 :0.2:Si/B 155 n-H* 25 24Si/Al 1.06(4) 1 :1 :6.8 :0.54 :303 n-H* 25 24

10.01(2) 1 :0.16 :3.5 :0.2Si/Fe :155 n-H* 25 241.96(5) 1 :0.50 :3.5Si/Ti :0.2 :155 n-H* 25 243.01(3) 1 :0.30 :3.5 :0.2Si/Zr :155 n-H* 25 245.23(4) 1 :0.20Si/Sn :3.5 :0.2 :155 n-H* 25 244.02(4) 1 :0.20 :3.5 :0.2 :155 n-H*Si/V 25 24

Doubly doped silicasSi/M2 Si :M1 :M2 :TEASi/M1 :CTAB :W Aging:Type Temp.(°C) Time (h)

3.90(1) 1 :0.18 :0.25 :3.8Si/Ti/Al :0.25.05(1) :155 n-H* 25 245.03(1) 1 :0.37Si/Ti/Al :0.202.70(1) :3.8 :0.2 :155 n-H* 25 246.02(3) 1 :0.025 :0.16 :3.84.02(1) :0.2Si/Ti/Zr :155 n-H* 25 24

10.01(2) 1 :1 :0.10 :7.0Si/Zr/V :0.41.01(2) :303 n-H* 25 248.43(1) 1 :0.05 :0.13 :3.845.23(2) :0.2Si/Ni/Zn :155 n-H* 25 246.51(1) 1 :0.02 :0.20 :3.8 :0.2 :155 n-H* 25 24Si/Ni/Mo 60.08(2)

ALPOS/SAPOSP/M2 P :M1 :M2 :TEAP/M1 :CTAB :W Aging:Type Temp.(°C) Time(h)

0.12 :1P/Al :00.17(3) :3.8 :0.3 :130 n-H* 25 241.50 :1 :0 :3.80.74(3) :0.3P/Al :130 n-H* 25 242.00 :1 :0 :3.8 :0.3 :130 n-H*P/Al 251.03(4) 24

4.68(3) 2 :2 :0.4 :8.00.59(3) :0.5P/Al/Ti :303 n-H* 25 240.43(3) 1.32 :2 :0.86 :10 :0.6 :325 n-H* 25 24P/Al/Si 0.75(3)

a *, H: hydrothermal treatment; n-H: no hydrothermal treatment. **, Surfactant removal is achieved after thermal treatment at 550°Cduring 5 h. In the TiO2 case, the treatment is stepwise: 2 h at 120°C and 5 h at 350°C under flowing argon, followed by calcination during120 h at 350°C under flowing air atmosphere.

5. Synthesised mesoporous materials: some selectedexamples

As indicated above, it is possible to prepareatranes of B, Al, Si and Ti, as well as their mixtures,as kinetically stable aqueous solutions. Learningfrom experience, we proceeded analogously

to achieve similar solutions for a wide varietyof metals. This facilitated the extension ofour synthetic approach to, at least, the diversity ofmesoporous materials listed in Table 1.Furthermore, our aim is to shorten report on theforemost results concerning the synthesised materi-als.

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420 413

5.1. Silica materials of the MCM-41 type

Silatranes hydrolysis is highly dependent on theworking pH and temperature, which affect both thenature of the resulting polyanionic species and thereaction rate. This non-surprising complexity allows,however, the obtaining of mesostructured materialscovering the entire range of possible typologies:hexagonal (ordered or disordered), cubic and lamel-lar. Shown in Fig. 2 are representative TEM micro-graphs of these materials. In every case, the finalmesoporous material consists of homogeneous parti-

cles, both in size and morphology. The atrane routeallows the preparation of high surface area (1130 m2

g−1) hexagonal mesoporous silica of the MCM-41type without the need for the usual hydrothermaltreatments required when other precursors areused.

5.2. Mesoporous alumina: ICMUV-1 materials

To avoid the formation of lamellar materials is achallenge in the preparation of mesostructured alu-mina [36–39]. When applying the conventionalpreparation approaches, such a difficulty is likely toarise because of the stabilisation in the processingcourse of highly anisotropic particles, which areprecursors of boehmite-like phases [17,18]. In con-trast, the alumatrane route allowed us to isolatethermally stable mesoporous alumina with con-trolled pore diameter (denoted ICMUV-1) [10]. Anoticeable result is that the features of the finalmesoporous alumina can be tuned by the sole adjust-ment of the water/TEAH3 molar ratio in the reac-tion medium. Thus, by changing this relation from 7to 50, there was initially obtained a series ofmesostructured materials having continuously ad-justable homogeneous micelle sizes (d-spacing ap-proximately varies from 70 to 90 A, ). Uponcalcination these mesostructured intermediates be-come stable mesoporous alumina, with variable uni-modal pore size distributions (ranging from 30 to 60A, ), and surface areas of approximately 250–340 m2

g−1. Shown in Fig. 3 are TEM micrographs corre-sponding to the two alumina synthesised with thelimit water/TEAH3 ratios. The high absolute valuesof both the pore sizes and thickness of the pore walls(compared with those of silica) might be related withthe previously mentioned aluminium trend to formboehmite seeds of the type [Al4O(OH)10(H2O)5]n[18,40], whose adaptability in order to achieve thenecessary charge density matching at the interfacewith the surfactant should be limited. 27Al MASNMR results indicate that nearly all the aluminiumatoms are octahedrally coordinated.

On the other hand, the variations of the X-raydiffraction d-spacing, pore sizes, and disorder in thepore arrangement, depending on the TEAH3 relativeconcentration, follow similar patterns to those ob-served in the case of silica–CTAB–water–S (S=al-cohol or formamide) systems [41]. In these lastsystems, the micelle sizes decrease as the S/water

Fig. 2. TEM micrographs of pure silica materials. (a) lamellarmesophase (pH 13.5). (b) Ordered hexagonal mesoporous silica(pH 10.85). (c) Disordered hexagonal mesoporous silica (pH 9.5).The insets show enlarged (×2) images of selected area.

Fig. 3. TEM micrographs of mesoporous ICMUV-1 aluminamaterials. The samples were synthesized with water/TEAH3 mo-lar ratios of 7.4 (image a, pore size 33 A, ) and 51.3 (image b, poresize 65 A, ). The insets show enlarged (×2) images of selected area.

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420414

Fig. 4. Nitrogen adsorption–desorption isotherms of mesoporousICMUV-4 titania. Shown in the inset is a representative TEMimage, together with an enlarged (×2) small area. The wormhole-like pore motif is clearly recognized.

Moreover, the materials synthesised in this way in-corporate phosphorus. In any case, their very naturehas been the subject of a certain controversy [21,42].The failure of the usual strategies can be readilyunderstood on the basis of the raised reactivity to-wards water of the titanium alkoxides. Systems likeTiO2 make it possible to assess the opportunitiesoffered by the synthetic methodology we aredescribing.

On the basis of the kinetic stability of the titana-tranes aqueous solutions, we were able to synthesisea mesostructured titania showing wormhole-like to-pology. Calcination at 500°C of this intermediategave thermally stable high surface area (around 610m2 g−1) pure mesoporous titania (ICMUV-4)with a relatively narrow pore size distribution(centred at 35 A, ) and homogeneous microstructure[13]. As in the case of ICMUV-1 alumina, thetitania pore packing motif can be described aswormhole-like (Fig. 4). The thickness of thepore walls, which include nanocrystalline anatase(25 A, sized), is approximately 30 A, . The anal-ysis of the UV–Vis spectra of these solidsindicates that all Ti atoms display octahedral coordi-nation.

Apart from surfactant-assisted procedures, there isonly one recent report on the synthesis of puremesoporous titania. The synthesis was carriedout in non-aqueous medium by using block-copoly-mers as templates [43]. As in the case of ICMUV-4,the described material contains anatase nano-crystals in the pore walls. Otherwise, this meso-porous titania differs significantly from our material,as indicated by its hexagonal topology, wall thick-ness (51 A, ), pore size (65 A, ) and surface area (205m2 g−1).

5.4. Doped silica of the M-MCM-41 type

Following examples of the prospects opened bythe atrane route, we must refer to the possibility ofobtaining materials of the M-MCM-41 type in whichthe amount of the M dopant element can be easilycontrolled along a large compositional range. Thus,we have prepared doped silica incorporating repre-sentative elements (B, Al, Sn and Zn) or transitionmetals (Ti, Zr, V, Mo, Mn, Fe, Co, Ni). In most ofthe examples listed in Table 1, the contents in do-pant elements are higher by one order of magnitudethan those previously reported in the literature-

ratio increases; so, the previously mentioned depen-dence is ultimately attributed to the so-called cosol-vent effect (by modifying the nature of thereaction medium, both the nucleation and growingconditions of the surfactant/inorganic species pri-mary units become altered) [16]. TEAH3includes both alcohol and amine functional groupsthat could synergically act to modify the size of themicelles, which should finally correlate with the low-ering of the unit cell size. Thus, the above-mentionedcontrol of the pore sizes in ICMUV-1 materialsseems to be explainable in terms of the cosolventeffect. The inorganic walls of the pores have anapproximately constant width of 35 A, in all IC-MUV-1 materials (i.e. independent of theconcentration of TEAH3), thus indicating that thegrowth of aluminium species perpendicularly to theinterface with the surfactant is constant and mainlydepends on the composition of the inorganic precur-sor.

5.3. Mesoporous titania: ICMUV-4 materials

Only a few papers have reported on surfactant-as-sisted synthesis of ordered mesoporous titania.

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420 415

Fig. 5. Representative nitrogen adsorption–desorption isothermsfor mesoporous Al-MCM-41 materials having Si/Al molar ratiosof (a) �, (b) 8.17, (c) 4.51 and (d) 1.06.

[4,8,44,45]. In adopting the term M-MCM-41 we areopting for brevity, but we will return on this pointlater on.

5.4.1. Al-MCM-41: aluminium rich materials(Si/Al=1)

After the discovery of silica-based MCM-41 mate-rials, a key scientific objective was the attainment ofincreasing content of tetrahedral Al in the frame-work of materials of this type. However, the achieve-ment of this objective has been limited due to thehuge reactivity difference between the aluminiumand silicon precursors normally used in typical sur-factant-assisted procedures. The best typical contentof tetrahedral Al was Si/Al=2 [46], although it hasbeen recently described a low silica material withSi/Al=1.03 [47]. A clear advantage of the atraneroute lies in the possibility of harmonising the hy-drolytic rates of the alumatrane and silatrane precur-sors. By relatively slowing down the aluminiumprecursor hydrolysis, the formation of Si�O�Al linksbecomes optimised. Moreover, the Si/Al ratio can bemodulated down to a minimum Si/Al value of1.06(4) (i.e. close to the theoretic Loewenstein limit)[12]. The surface area of the synthesised mesoporousmaterials is high, but it decreases as the Al contentincreases (Fig. 5). The materials are chemically ho-mogeneous, and, what is more important, only tetra-hedral Al is detected by means of 27Al MAS NMR.Incorporation of Al leads to increasing disorder (thepore packing motif is disordered hexagonal in allcases; see Fig. 6) and decreasing pore sizes, whereasthe pore wall thickness continuously increases. Thesetrends are consistent with the gradual replacement ofSi by Al. In fact, the size of the inorganic polyanionadequate to compensate the surfactant-head chargewould increase as the Al content. Moreover, in asmuch as the system involves two different elements,there will be a diversity of inorganic polyanions ableto contribute to such a charge balance. If so, thegrowth of heterogeneous micelles (both in diameterand shape) can be expected, which, in turn, wouldlead to the lowering of the observed order (due todifficulties for close packing).

5.4.2. Ti-MCM-41: titanium rich materials(Si/Ti=1.9)

As in the case of Al, incorporation of Ti in thesilica-based hexagonal mesoporous framework im-

Fig. 6. Characteristic TEM images of mesoporous Al-MCM-41materials with different Al content. (a) Si/Al=� (ordered hexag-onal MCM-41; Ho). (b) Si/Al=1.45 (disordered hexagonal Al-MCM-41; Hd). (c) Si/Al=1.06 (highly disordered Al-MCM-41).The insets show enlarged (×2) images of selected area.

Fig. 7. Representative TEM micrographs of mesoporous titanosil-icates with increasing different Ti content. (a) Si/Ti=49.9, (b)Si/Ti=19.0 (c) Si/Ti=1.9. The evolution from hexagonal orderedTi-MCM-41 to highly disordered or wormhole-like pore arrays isclearly observed.

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420416

Fig. 8. Diffuse reflectance UV-visible spectra of mesoporous ti-tanosilicates having different Ti content. (a) Si/Ti=49.9, (b)Si/Ti=34.9, (c) Si/Ti=19.0, (d) Si/Ti=8.0 and (e) Si/Ti=1.9.The spectra of the solids having a Si/Ti molar ratio higher than34.9 exhibit an absorption band centered at 220 nm characteristicof Ti atoms in tetrahedral coordination. For samples having aSi/TiB34.9 molar ratio, the spectra present an aditional band at300 nm characteristic of octahedral Ti environmets.

dopant Al is tetrahedral Al, the characteristics of theresulting material continuously vary in a smoothway with the Al/Si molar ratio (see Fig. 9b). More-over, in Al-MCM-41 the pore packing motif is disor-dered hexagonal in all cases (see Fig. 6), withoutprejudice to the increasing disorder as the Al contentincreases. On the other hand, the progressive disor-der associated with dopant incorporation results in alowering of the surface area in both types of materi-als. This is a reasonable observation in the light ofthe continuous increase of the pore wall thicknesstogether with the loss of uniformity in the poresystem.

It is in this context in which we are maintainingthe M-MCM-41 term, mainly referred to as apreparative methodology, although the very topo-logic/structural characteristics of the original Mobilsilica are sometimes lost.

5.4.3. Other materials of the M-MCM-41 typeAs indicated in Table 1, the atrane route allowed

the preparation of a relatively wide variety of doped-silica mesoporous materials. Dopant elements in-clude transition metals, and our synthesis approach

Fig. 9. Evolution of the XRD d-spacings (open circles) and BETsurface areas (filled circles) with the M/Si molar ratio. (a) M=Ti.(b) M=Al.

plies a progressive lowering of the order (Fig. 7). Inany case, for a given Si/M ratio, the degree of orderin Ti-MCM-41 oxides is significantly lower than inAl-MCM-41 oxides. UV–Vis spectra (Fig. 8) showthat Ti occupies tetrahedral sites in the skeletal netuntil reaching the Si/Ti=35 molar ratio, whereas forSi/Ti ratios lower than 19 it becomes evident thepresence of both tetrahedrally and octahedrally co-ordinated Ti atoms. It is noteworthy that the appari-tion of octahedral Ti is associated with a significantchange in the characteristics of the material (see Fig.9a). As can be noted, whereas the measured surfacearea is practically constant while Ti remains in atetrahedral environment, it suddenly drops for theSi/Ti=35 molar ratio and then decreases smoothly.In a parallel way, there is also a drastic change in thevariation of the X-ray diffraction d-spacing with theSi/Ti molar ratio. As shown in Fig. 7, these changesoccur as the disordered hexagonal typology charac-teristic of the MCM-41-type materials is lost infavour of a wormhole-like porosity. These resultsmark a clear difference with regard to the Al-MCM-41 oxides. In the Al-MCM-41 case, where all the

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420 417

Fig. 10. Representative nitrogen adsorption-desorption isothermsfor M-MCM-41 mesoporous materials. (a) B-MCM-41 (Si/B=15), (b) Fe-MCM-41 (Si/Fe=10), (c) Zn-MCM-41 (Si/Zn=7.5)and (d) Sn-MCM-41 (Si/Sn=5.2).

characteristics are progressively lost as the dopantcontent increases. Independent of the nature of thedopant element, its valence or preferential coordina-tion, there exists a limit to the dopant content fromwhich the material changes from more or less disor-dered hexagonal to wormhole-like. Taking into ac-count the high dopant contents we have reached, theexperimental values of the surface area in the finalmaterials can be considered satisfactory (see Fig. 10).Moreover, all these materials show one well-definedstep in their N2 adsorption isotherms, which is in-dicative of unimodal pore size distributions.

Finally, it must be stressed that all the materialslisted in Table 1 are chemically homogeneous (ac-cording to systematic EPMA analyses), independentof the number and nature of the involved dopants.

5.5. Mesoporous aluminophosphates (ALPOs) andsilicoalumino-phosphates (SAPOs)

The atrane route was successfully applied to thepreparation of ALPOs and SAPOs by using, phos-phoric acid as source of phosphate groups. Thestudied compositional range extends from pure alu-mina to systems whose (P+Si)/Al ratio is equal to 1,including the denoted ICMUV-3 materials [11]. Theprocedure allowed incorporating into the final solidpractically the same P/Al or Si/Al molar ratiospresent in the initial mixture. Thus, for instance,mesoporous wormhole-like ALPOs with the P/Al=1 ratio have been prepared for the first time, signifi-cantly improving previous reports [48,49]. InALPOs, the average pore diameter varies inversely tothe phosphate content in the material, this effectbeing stronger the higher is the pH. Indeed, even fora fixed P/Al=0.35 molar ratio, it was possible tomodulate both the pore size and surface area from33 A, /300 m2 g−1 to 15 A, /500 m2 g−1. In practice,this family of mesoporous ALPOs can be consideredas derived from mesoporous alumina by insertion ofphosphate groups. 27Al MAS NMR shows that theproportion of Al occupying coordination environ-ments lower than octahedral (essentially tetrahedralAl) increases as the phosphate group content (Fig.11). This last effect is still more pronounced in thepresence of silicon. Thus, the mesoporous SAPO ofrelative composition Al: P: Si=1: 0.66: 0.44 containsonly tetrahedral Al. In short, in comparison with thepure mesoporous alumina case, all occurs as if theinsertion of tetrahedral phosphate or silicate groups,

Fig. 11. 27Al-MAS NMR spectra of mesoporous phosphates (a)ALPO (P/Al=0.7), (b) SAPO (P/Al=0.66, Si/Al=0.43).

also facilitates the incorporation of several dopantsin a given matrix. The examples listed in Table 1correspond to materials having high dopant con-tents. As in the Ti case, the MCM-41 structural

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420418

which relatively furnish high negative charge densi-ties, would result in a lowering of the bidimensional-ity of the inorganic fragments which get adjusted tothe surfactant, so improving the electrostatic match-ing effectiveness.

6. Discussion and conclusions

It has become usual to state that the surfactant-as-sisted preparative chemistry of mesoporous materialsis a case in point of the ‘template-based’ synthesisapproach [16]. The results described in the presentreport can be rationalised in terms of the cooperativeself-assembling mechanism, in which the nature ofthe interactions between the ‘templating’ and theembedding inorganic matrix is essentially electro-static [4,16]. The micelles resulting from these inter-actions are built up from inorganic–organic primaryaggregates, which arise as Br−

aq anions, initially asso-ciated to CTA+

aq cations, are replaced by multiden-tate bulky inorganic polyanions [14,15]. Thesenegatively charged species enfold the cationic head-groups of the surfactant and drastically modify itsphysical behaviour in solution, in as much as thecationic headgroups are transformed into dipoleforming ion pairs [16]. The enhanced hydrophobiccharacter of these new entities changes the verynature of the double layer and surface potential andgreatly reinforces the micelle originating interactions.Indeed, the cmc values in ternary inorganic phase–CTAB–water systems can become reduced by sev-eral orders of magnitude with regard to thosecorresponding to the binary CTAB–water system[15]. This general argument accounts for the forma-tion of mesostructured materials at low surfactantconcentrations.

Our synthetic approach is based on the inertnessof the aqueous solutions of the atrane complexes,without prejudice to their inherent thermodynamicinstability. It is in this context in which we havereferred to the TEAH3 reactant as a ‘hydrolysisretarding agent’ [10–13]. In this way, the evolutionof each one of the involved inorganic systems isorchestrated by the surfactant (i.e. by the manytimes broadly so-called ‘cationic template’): all oc-curs as a ‘heterogeneous nucleation’, and the ‘criticalsize’ of the inorganic nuclei becomes determined bythe best possible charge density matching at theinorganic-species/surfactant interface. The presence

of the surfactant avoids ‘homogeneous nucleation’,which would result in the segregation of the pureinorganic phase.

All the mesostructured and mesoporous materialsreported in this work are chemically homogeneous,and are also homogeneous in microstructure andtexture. Moreover, the mesoporous final oxidesshow, in general, unimodal narrow pore size distri-butions. The capability of the inorganic matrix toevolve and generate polyanions adaptable to thesurfactant depends on its composition. The experi-mental values of pore sizes and thickness of the porewalls show this fact. When we studied A/B mixed-oxide solids, a result systematically observed is theprogressive increase of disorder and pore wall thick-ness as one of the components (A or B) is incorpo-rated in the matrix of the other (B or A); disorderand thickness are maxima when both componentsare in equimolar proportions (A/B=1/1). We pro-pose that the explanation to this result is based onthe diversity of inorganic primary units due to thecompositional variation, as well as on the differentpreferential coordination for each one of the ele-ments (A and B). For mesoporous materials both thethickness of the skeletal phase and the pore size itselfare typically only a few angstroms. Consequently, inmixed systems, the macroscopic compositional ho-mogeneity cannot be reproduced at the atomic levelin the pore walls. Thus, in addition to the inherent(entropy-driven) variety of inorganic polyanionsavailable to form primary aggregates, there is aconstraint at structural scale working against theattainment of an unimodal micellar distribution.Therefore, it can be expected that relatively disor-dered mesoporous materials be formed.

On the other hand, the inorganic walls of the poresystem in the final materials are amorphous. There-fore, it is not surprising that glass-forming elementsare also the best candidates for constructing frame-works of mesoporous materials [14]. Even more sig-nificant is the agreement between the different rolesassigned in the glass-maker jargon (forming, stabilis-ing, modifier) and the observed behaviour in oursystems, as shown by the compositional tolerance (inrelation to the dominant coordination) and theadaptation of the systems to excesses of elementsthat are not glass-forming. However, our work alsoproves that, by kinetically controlling the hydrolysisof the precursors (atranes), oxides that are not glass-forming (Al2O3, TiO2, ZrO2, etc.) can also be pre-pared as ordered mesoporous materials.

S. Cabrera et al. / Solid State Sciences 2 (2000) 405–420 419

In summary, the results reported here show thatatrane complexes are excellent precursors of orderedmesoporous mixed oxides. Moreover, it is evidentthat the procedure is also applicable to the obtainingof particulate solids, gels and microporous materials.In most cases, the high surface area in these materi-als confers on them an inherent thermodynamicmetastability. Control of their preparative chemistryessentially is kinetic. This means that the syntheticand design strategies clearly are ‘non-abelian’. Forthis reason, it is usual that the synthesis of a giventype of micro- or mesoporous material appears asextraordinarily specific and difficult to generalise[4,8,50]. To a great extent, it has also caused the lossof the original meaning of the so-called ‘templatingeffect’, which frequently is invoked to justify ourlack of understanding about the role played by areagent in a preparative route to nanoporous materi-als [51]. By thinking on kinetic grounds, it is per-fectly possible to select a ‘hydrolysis retarding agent’adequate to harmonise the hydrolytic processes in-volving metals of different basicities. In this sense,TEAH3 is just one of an amount of chemicals ableto yield unstable–inert atrane complexes. Beyond alldoubt, the atrane versatility will open new prospectsfor improving the preparative chemistry and, there-fore, performances of nanoporous materials.

Acknowledgements

This research was supported by D.G.E.S. undergrants PB95-1094 and PB98-1424-C02-01. S.C. andJ.E.H. thank the A.E.C.I. for doctoral grants.

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