ordered mesoporous materials

Microporous and Mesoporous Materials 27 (1999) 131–149

Review

Ordered mesoporous materials

Ulrike Ciesla a,b, Ferdi Schuth a,*a Johann Wolfgang Goethe-Universitat Frankfurt, Institut fur Anorganische und Analytische Chemie,

Marie-Curie-Str. 11, D-60439 Frankfurt-am-Main, Germanyb Department of Chemistry, University of California–Santa Barbara, Santa Barbara, CA, USA

Received 5 January 1998; received in revised form 13 March 1998; accepted 15 March 1998

Abstract

About six years after the first publication, ordered mesoporous oxides can be prepared by a variety of proceduresand over a wide range of compositions using various different surfactant templates. The mechanisms of formation,although still a matter of discussion, are understood in principle, and the macroscopic morphology as well as theorientation of the pores can be controlled in fortunate cases. However, still lacking are groundbreaking developmentsin the field of applications, either in catalysis or other areas. The state-of-the-art in the synthesis, characterizationand application of ordered mesoporous oxides will be covered in this review. © 1999 Elsevier Science B.V. Allrights reserved.

Keywords: Review; Mesoporous materials; MCM-41

1. Introduction the main aspects in zeolite chemistry. Larger poresare present in porous glasses and porous gels,

Porous solids are used technically as adsorbents, which were known as mesoporous materials at thecatalysts and catalyst supports owing to their high time of the discovery of MCM-41 [2]. However,surface areas. According to the IUPAC definition they show disordered pore systems with broad[1], porous materials are divided into three classes: pore-size distributions. Other mesoporous solidsmicroporous (<2 nm), mesoporous (2–50 nm) and were synthesized via intercalation of layered mate-macroporous (>50 nm). Well-known members of rials, such as double hydroxides, metal (titanium,the microporous class are the zeolites, which pro- zirconium) phosphates and clays. These also havevide excellent catalytic properties by virtue of their very broad mesopore-size distributions, as well ascrystalline aluminosilicate network. However, their additional micropores.applications are limited by the relatively small pore With MCM (Mobil Composition of Matter) 41openings; therefore, pore enlargement was one of the first mesoporous solid was synthesized that

showed a regularly ordered pore arrangement anda very narrow pore-size distribution. After the

* Corresponding author. Present address: MPI furdiscovery of MCM-41 in 1992 [2], the researchKohlenforschung, Postfach 10 13 53, 45466 Mulheim,interest focused on the following main subjects:Germany. Tel: + 49 208 306 2373; Fax: +49 208 306 2395;

e-mail: [email protected] (1) characterization; (2) the mechanism of forma-

1387-1811/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.PII: S1387-1811 ( 98 ) 00249-2

132 U. Ciesla, F. Schuth / Microporous and Mesoporous Materials 27 (1999) 131–149

tion; (3) the synthesis of new materials based onthe MCM-41 synthesis concept; (4) morphologycontrol; and (5) the technical applications ofMCM-41 and related mesoporous materials. Thesefields will be surveyed in this review.

2. Characterization

A procedure for the preparation of low-densitysilica was already described in a patent filed in1969 [3]. Di Renzo et al. [4] reproduced thesynthesis reported in the patent and found that itleads to a material identical to mesoporousMCM-41, which scientists of the Mobil OilCorporation patented in 1991 [5]. However, in theoriginal patent only few of the remarkable proper-

Fig. 1. X-ray diffraction pattern of high-quality calcinedties of the material were actually discovered. It MCM-41 made by Huo and Margolese [7].was the Mobil scientists who really recognized thespectactular features of these ordered mesoporousoxides. The question thus arises: why is the charac- to contain substantial amounts of MCM-41.

Corma recently attributed the apparently ‘‘worse’’terization of these mesoporous materials so diffi-cult? As the most investigated member of the XRD pattern to the formation of smaller although

no less ordered MCM-41 crystallites [10]. In ourM41S family, MCM-41 provides an excellentexample of the problems in characterizing meso- group the diffraction patterns of MCM-41 with

different degrees of defects were recently simulatedporous materials. MCM-41 has a honeycombstructure that is the result of hexagonal packing [11]. It was found that even when the hexagonal

pore structure contains a large number of defects,of unidimensional cylindrical pores. Reliable char-acterization of the porous hexagonal structure a hexagonally indexable three-reflection pattern

can be calculated. Decreasing domain size, how-requires the use of three independent techniques[6 ]: X-ray diffraction ( XRD), transmission ever, leads to loss of high-order reflections.

To elucidate the pore structure of MCM-41electron microscopy (TEM) and adsorptionanalysis. transmission electron microscopy is usually used.

Fig. 2 shows a TEM image of the hexagonalThe XRD pattern of MCM-41 shows typicallythree to five reflections between 2h=2° and 5° arrangment of uniform, 4 nm sized pores in a

sample of MCM-41. However, the exact analysis(Fig. 1), although samples with more reflectionshave also been reported [8,9]. The reflections are of pore sizes and thickness of the pore walls is

very difficult and not possible without additionaldue to the ordered hexagonal array of parallelsilica tubes and can be indexed assuming a hexago- simulations because of the focus problem. Chen

et al. [12] showed for MCM-41 that the thicknessnal unit cell as (100), (110), (200), (210) and(300). Since the materials are not crystalline at the of the features—the pore sizes and wall thick-

nesses—depend strongly on the focus conditions,atomic level, no reflections at higher angles areobserved. Moreover, these reflections would only and careful modeling is necessary for precise analy-

sis. Moreover, most MCM-41 samples not onlybe very weak in any case, owing to the strongdecrease of the structure factor at high angles. By show ordered regions but also disordered regions,

lamellar and fingerprint-like structures [13]. Themeans of X-ray diffraction it is not possible toquantify the purity of the material. Samples with existence of a lamellar phase after calciniation is

unlikely, because silicate layers are too distantonly one distinct reflection have also been found

133U. Ciesla, F. Schuth / Microporous and Mesoporous Materials 27 (1999) 131–149

the adsorption and desorption branches (Fig. 3).The adsorption at very low relative pressure, p/p0,is due to monolayer adsorption of N2 on the wallsof the mesopores and does not represent the pres-ence of any micropores [21,22]. However, in thecase of materials with pores larger than 4.0 nm[23] or using O2 or Ar as adsorbate [17], theisotherm is still type IV but also exhibits well-defined hysteresis loops of the HI type [20]. Thepresence and size of the hysteresis loops dependon the adsorbate [17], pore size [23] and temper-ature [24]. Non-local density functional theory(NLDFT) provides an accurate description of thethermodynamics of nitrogen confined in pores ofthis size and predicts the thermodynamic limits forthe adsorption–desorption hysteresis loops [25].Comparison of the theoretically predicted thermo-dynamic hysteresis loops with experimental dataFig. 2. Transmission electron micrograph of MCM-41 featuring

4.0 nm sized pores, hexagonally arranged. We should like to on MCM-41 supports the classical physical sce-thank BASF AG, Ludwigshafen for recording the image. nario of capillary condensation in an open cylindri-

cal capillary. In contrast, there is still no clearexplanation for those cases where the hysteresisfrom one another to preserve the spacing in the

silicate organic phase and collapse without addi- loops are absent. Calculations by Ravikovitch et al.[22,25] showed that the absence cannot betional post-treatments. Chenite et al. [14] showed

that the equidistant parallel lines observed in the explained by the capillary critical temperaturebeing achieved, which was previously assumedmicrographs are related to the hexagonal repeat

between tubules. The honeycomb structure is [26 ].To determine the pore-size distributions in cylin-sufficiently regular to give fringes in projection

under proper orientation of the specimen. Feng drical pores, several methods are known based ongeometrical considerations [27], thermodynamicset al. [15] investigated the curved, fingerprint-like

structures in more detail, and observed two dislo-cation and two disclination defect structures whichare similar to those detected in pure liquid-crystalphases. They proposed that since the mesoporousstructure cannot shear without fracturing, thesedefects must have formed in the unpolymerizedliquid-crystal-like silicate precursor phase.

Adsorption of probe molecules has been widelyused to determine the surface area and to charac-terize the pore-size distribution of solid catalysts.Soon after the preparation of MCM-41 the phy-sisorption of gases such as N2, O2 and Ar hadbeen studied to characterize the porosity [16–19].The nitrogen adsorption isotherm for MCM-41with pores of around 4.0 nm, which is type IV inthe IUPAC classification [20], shows two distinctfeatures: a sharp capillary condensation step at a Fig. 3. Adsorption isotherm of nitrogen on MCM-41 with

4.0 nm pores at 77 K [16 ].relative pressure of 0.4 and no hysteresis between


[20] or a statistical thermodynamic approach [25]. independent studies showed in the case of theoriginal MCM-41 synthesis that the wall thicknessIn addition, freezing point depression can be usedof around 1.0 nm remains constant if the poreand was followed by nuclear magnetic resonancesizes are varied in the range from 2.5 to 10.0 nm(NMR) spectroscopy in the case of MCM-41 [28].[32,33].The traditional method for analyzing pore-size

The characterization of the pore walls focusesdistributions in the mesopore range is theon two aspects: (1) structural properties; and (2)Barrett–Joyner–Halenda (BJH) method [29,30]the surface chemistry. From X-ray diffraction atwhich is based on the Kelvin equation and, thus,high angles it is known that mesoporous MCM-41has a thermodynamic origin. However, comparedmaterials are amorphous. Therefore, the walls arewith new methods that rely on more localizedbest analyzed in terms of local atomic ordering. Adescriptions such as density functional theoryvery powerful method for the characterization of(DFT) [22,25] and Monte Carlo simulation (MC)framework locations is solid-state NMR spectro-[31], the thermodynamically based methods over-scopy. Uncalcined MCM-41 samples show threeestimate the relative pressure at desorption anddifferent 29Si-NMR peaks which can be assignedtherefore underestimate the calculated pore diame-to Q2, Q3 and Q4 silicon species [21]. After calcina-ters by ca. 1.0 nm. Moreover, the theoretical basistion Q4 environments are formed at the expensefor the BJH analysis becomes fairly weak if theof Q3 and/or Q2. The degree of this transformationstep for N2 at 77 K lies below p/p0=0.42, becausedepends on the severity of reaction conditionsthis is considered to be the stability limit of theapplied to the condensation of Si–OH groups.meniscus. Pore sizes calculated in such cases areBecause of the formation of catalytically activestill probably in the right range, but a soundacid sites, the incorporation of aluminum intotheoretical foundation for such values is missing.silica frameworks is of special interest. The alumi-So far, the method of determination of pore-sizenum-containing as-made samples show tetra-distributions in cylindrical pores based on thehedrally coordinated aluminum as well as octahe-

results of NLDFT calculations seems to be thedrally coordinated atoms [34,35]. However, some

most accurate one [25]. Maddox et al. [31] recently authors could achieve the incorporation of onlysuggested a new ‘‘heterogeneous interaction exclusively tetrahedrally coordinated aluminum bymodel’’ based on MC simulations. This method varying the aluminum source [36–38]. Janickeconsiders that the capillary condensation pressure et al. [39] observed that the octahedral aluminumused to calculate the pore size is sensitive to can be converted into species withsolid–fluid interactions. According to the authors, tetrahedral coordination upon calcination.other methods, including the density functional Tetrahedrally coordinated aluminum is of specialtheory, seem to underpredict the pore size by a interest and the desirable species, because it issignificant amount because the approaches assume assumed that only these are incorporated into thea homogeneous surface of the wall, which under- framework and therefore are responsible for theestimates the binding energy of the submonolayer formation of the acid sites, while the octahedralatoms adsorbed at low pressures. aluminum species are occluded in the pores or

The wall thickness can be calculated by deter- exist as an amorphous byproduct. Two-dimen-mining the difference between the lattice parame- sional (2D) NMR experiments show clearly thatter (a=2d(100)/

E3) determined by X-ray diffraction interfacial framework aluminum is incorporatedand the pore size obtained by nitrogen adsorption into the MCM-41 material and that the frameworkanalysis. However, one should bear in mind that aluminum is preserved following calcination treat-the values are only estimates, because so far no ment [40]. Moreover, by using 2D NMR, Janickereliable means for pore-size analysis exists. et al. [40] observed that the surfactant species areMoreover, the lattice parameters are often calcu- dipole–dipole coupled to both tetrahedrally coor-lated from quite broad reflections and therefore dinated aluminum atoms and adsorbed water

molecules. Therefore, it is suggested that bothdo not correspond to an exact value. However,


four- and six-coordinated aluminum species are showing either cylindrical [46 ] or hexagonal [47]pores.present in the framework of the hydrothermally

synthesized MCM-41 sample.The surface properties of the pore walls were

3. Formation mechanismstudied by adsorption of molecules on the surfaceand by using Fourier transform infrared (FTIR)

The original MCM-41 synthesis was carried outanalysis. By adsorbing polar or unpolar moleculesin water under alkaline conditions 1. Similar toon surfaces it is possible to measure the hydrophiliczeolite syntheses, organic molecules—surfac-or hydrophobic properties of surfaces. Therefore,tants—function as templates forming an orderedby carrying out the adsorption of cyclohexaneorganic–inorganic composite material [49]. Via[21], benzene [32] and water [41], the relativelycalcination the surfactant is removed, leaving thehydrophobic character of siliceous MCM-41 hasporous silicate network. However, in contrastbeen clearly demonstrated. The water adsorptionto zeolites, the templates are not single organicisotherm [23] corresponds to a type V in themolecules but liquid-crystalline self-assembledIUPAC classification [20], which is an indicationsurfactant molecules. The formation of the inor-of the hydrophobic character in the low-pressureganic–organic composites is based on electrostaticregion of the adsorption isotherm. Moreover, ainteractions between the positively charged surfac-small amount of surface OH sites is detected andtants and the negatively charged silicate species.at least three different silanol groups [42,42] canSeveral studies have investigated the buildingbe distinguished by using pyridine adsorption:mechanism of MCM-41 and seem to be at firstsingle, hydrogen-bonded and geminal silanolinconsistent. However, the ‘‘liquid-crystal templat-groups. A fourth possible silanol group has alsoing’’ (LCT) mechanism suggested by Beck et al.been reported [43]. The presence of silanol groups[32] early after the discovery of MCM-41 seemsis important for surface modifications like silyla-to include all these proposed mechanisms,

tion for hydrophobization of the surface [32].although the details were not specified in that

Even aluminum-containing MCM-41 is fairly publication (Fig. 4). They proposed two mainhydrophobic as shown in a comprehensive sorption pathways, in which either the liquid-crystal phasestudy [44]. Because of its high sorption capacity is intact before the silicate species are added (path-for hydrocarbons, MCM-41 might thus be used way 1), or the addition of the silicate results infor removing hydrocarbons from moist gas the ordering of the subsequent silicate-encasedstreams. surfactant micelles (pathway 2). The reason for

Aluminum-containing MCM-41 materials were the apparently different reaction pathways resultsinvestigated for their acidic sites on the surface, from changes in surfactant properties, dependingwhich is important for catalytic reactivity. on the surfactant concentration in water and theTherefore, adsorption studies using bases such as presence of other ions [50,51]. MCM-41 can beammonia [21,45] and pyridine [45] were carried synthesized with surfactant concentrations as lowout. Based on temperature-programmed desorp- as the critical micelle concentration (CMC) up totion (TPD) and FTIR results, aluminum-contain- concentrations where liquid crystals are formeding MCM-41 possesses an acidity similar to that [52]. In very dilute aqueous solutions (~10−3 toof amorphous aluminosilicate. This result is in 10−2 mol l−1 surfactant concentration) the existinggood agreement with the absence of X-ray reflec- species are spherical micelles. Monnier et al. [53]tions at high angles and NMR results indicating

1 A large number of different synthesis procedures exist. It seemsthe presence of amorphous walls in the case ofthat almost every single group has developed their special prepa-pure silica as well as aluminosilicate. Combiningration method. Since such a large synthesis variety leads toall results from the different characterization meth- MCM-41 and a comparison between these different method is

ods, two structural models with amorphous walls almost impossible, we did not want to discuss the synthesis indetail. An overview was recently given in [48].have been constructed for the hexagonal MCM-41


Fig. 4. Liquid-crystal templating (LCT) mechanism proposed by Beck et al. [32] showing two possible pathways for the formationof MCM-41: (1) liquid-crystal-initiated and (2) silicate-initiated.

studied this situation in detail using concentrations tion was investigated by Chen et al. [56 ] using14N-NMR spectroscopy. The randomly orderedof around 1 wt%. They proposed that three steps

are involved in the formation of the surfactant– rod-like micelles interact with the silicate speciesto yield tubular silica arranged around the externalsilica composite. First, the oligomeric silicate poly-

anions act as multidentate ligands for the cationic surface of the micelles. These composite speciesspontaneously form the long-range order indica-surfactant head groups, leading to a strongly inter-

acting surfactant/silica interface with lamellar tive of MCM-41. A similar mechanism was pro-posed on the basis of in situ electron paramagneticphase. In the second step preferential polymeriza-

tion of the silicate in the region of the interface resonance (EPR) measurements with surfactantconcentrations below 2 wt% [57]. The first step isoccurs, which leads to the reduction of the negative

charge at the interface. The following charge- the formation of domains, which consist of micel-lar rods encapsulated with silicate ions showingdensity matching between the surfactant and the

silicate leads to a phase transformation, forming hexagonal order. Second, the silicate ions polymer-ize at the interface, resulting in hardening of thethe hexagonal surfactant–silicate composite. In the

following, it was recognized that the layered inter- inorganic phase.At high concentrations surfactants form lyo-mediate is not always necessary, but that the

charge-density matching could directly lead to a tropic mesophases [50,51]. Depending on thenature of the surfactant, the concentration and thehexagonal or a cubic phase. In particular, the work

of Chmelka, Stucky and co-workers [54] with temperature, these mesophases show differentphases with, e.g., hexagonal, cubic or lamellarcubic octamers of silica and thus decoupled self-

assembly and silica polymerization showed that structure. Attard et al. [58] used this approach tosynthesize MCM-41 as monoliths. However, inunder conditions when no condensation occurs

( low temperature) the surfactant self-assembly contrast to the original MCM-41 synthesis, non-ionic instead of cationic surfactants were used.governs the system, but as soon as the silica

polymerizes, the resulting structure is controlled Therefore, in this case not ionic interactions buthydrogen bonding and hydrophilic/hydrophobicby the inorganic framework. Also, an initially

formed layered structure observed by Steel et al. interactions are responsible for the formation ofthe inorganic–organic composite. However, the[55] was proposed as the ‘‘puckering layer model’’.

The silicate species in aqueous solution form a use of non-ionic surfactants and their perspectivesin mesophase preparation will be discussed in morelayered structure, further ordering resulting in

puckering of the silicate layers and the formation detail in the following section.Almost at the same time as the synthesis ofof the hexagonal channels.

Increasing the surfactant concentration results MCM-41 was published, Yanagisawa et al. [59]described an alternative synthesis pathway forin the self-assembly of surfactant rods. This situa-


preparing mesoporous silicate from a layered sili- syntheses based on ionic interactions, the liquid-crystal approach has been further extended tocate, kanemite, which consists of single layers of

SiO4 tetrahedra. This material is designated as include two additional pathways showing organic–inorganic interaction other than ionic. Under neut-FSM-16 (Folded Sheet Mesoporous Materials)2.

The preparation is similar to MCM-41 using a ral conditions mesostructures are formed by usingneutral (S0) [66 ] or non-ionic surfactants (N0)layered silicate as silica source. Instead of a liquid-

crystal templating mechanism a ‘‘folded sheet’’ [67]. In this approach (S0/N0I 0) hydrogen bondingis considered to be the driving force for the forma-mechanism [60] is proposed: the layered organic–

inorganic composites are formed by intercalation tion of the mesophase. In the so-called ligand-assisted liquid-crystal templating mechanism,of the layered silicate using surfactants [61]. The

transformation to the hexagonal phase occurs covalent bonds are formed between the inorganicprecursor species and the organic surfactant mole-during the hydrothermal treatment by condensa-

tion of the silanol groups. However, some authors cules followed by self-assembling of the surfactant[68]. Interestingly, Zhao et al. [69] showed thatproposed dissolution of the kanemite under the

reaction conditions [62]. The dissolution rate of the liquid-crystal templating approach is not lim-ited to amphiphilic molecules. By use of non-kanemite is dependent on the quality of the starting

material, and such deviations might explain vary- amphiphilic mesogens, which show the ability toform liquid-crystal structures, lamellar meso-ing results from different groups. In this case the

kanemite would act as a silica source comparable structured inorganic–organic composites were syn-thesized. Following the principle of charge match-to the MCM-41 synthesis. MCM-41 and FSM-16

are similar but show slightly different properties ing, the mechanism is based on the (S−I+)pathway since the template molecules were nega-in adsorption [62] and surface chemistry [63].tively charged.

Since the liquid-crystal structures of the surfac-tant serve as organic template, rather than single4. Alternative synthesis pathways and new

materials molecules commonly proposed as templates inzeolite chemistry3, the behavior of the surfactantin binary surfactant/water systems is the key forThe liquid-crystal-controlled synthesis of

MCM-41 opened a wide variety of synthesis the controlled preparation of silica mesostructures[54,70]. According to a microscopic model intro-approaches for developing new materials. The

successful preparation of new porous materials duced by Israelachvili et al. [71], the relativestabilities of different aggregate shapes and thecould be achieved by developing new synthesis

pathways and by taking advantage of the liquid- corresponding mesophase structures can be pre-dicted. The preferred shape of self-assembled sur-crystal chemistry provided by the surfactant.

So far, we have described the organization of factant molecules above the CMC depends on theeffective mean molecular parameters that establishcationic quaternary ammonium surfactants and

anionic silicate species (S+I−) to produce three- the value of a dimensionless packing parameter g,which is defined as: g¬V/a0lc, where V is thedimensional periodic biphase arrays. However,

cooperative interactions between inorganic and effective volume of the hydrophobic chain, a0 isthe mean aggregate surface area per hydrophilicorganic species based on charge interaction can

also be achieved by using reverse charge matching head group and lc is the critical hydrophobic chain(S−I+) [64] or by mediated combinations of cat-ionic or anionic surfactants and corresponding

3 Although the label ‘‘template’’ is commonly used in zeoliteinorganic species (S+X−I+, X−=halides; orchemistry, most ‘‘templates’’ do not act in the sense of a mold,S−M+I−, M+=alkali metal ion) [65]. Besides thelike the liquid-crystalline surfactants, but rather direct the syn-thesis to a particular structure by mostly unknown mechanisms.

2 The number depends on the length of the surfactant alkyl It would thus be more appropriate to use the term ‘‘structuredirector’’.chain, in this case a C16 surfactant was used.


length [71]. The parameter g depends on the the preparation of mesoporous oxides other thansilica. The following sections discuss how suchmolecular geometry of the surfactant molecules,

such as the number of carbon atoms in the hydro- materials can be prepared in line with the conceptsdescribed above.phobic chain, the degree of chain saturation and

the size and charge of the polar head group. Inaddition, the effects of solution conditions, includ- 4.1. Silica polymorphs from ionic synthesis

pathwaysing ionic strength, pH, co-surfactant concentrationand temperature, are included implicitly in V, a0and lc [72]. In classical micelle chemistry, meso- Similar to the original MCM-41 synthesis by

using cationic surfactants and anionic silicatephase transition occurs when the g value isincreased above critical values (Table 1). species (S+I−), MCM-48, the cubic member of

the M41S group of mesoporous materials, wasMoreover, the phase transitions also reflect adecrease in surface curvature from the cubic first synthesized by scientists of the Mobil Oil

Corporation [32]. MCM-48 can be synthesized(Pm3n) over the hexagonal to the lamellar phase.Spherical aggregates are preferentially formed by either by adjusting the silica/CTAB ratio and vary-

ing synthesis conditions [75,76 ], or by using geminisurfactants possessing large polar head groups. Onthe other hand, if the head groups are small and surfactants [7]. It has been suggested [53] and

later confirmed [77] that the structure contains apacked tightly, the aggregation number increases,resulting in rod-like or lamellar aggregates. By three-dimensional channel network with channels

running along [111] and [100] directions.including the inorganic component, Stucky et al.[73,74] expanded the model to the ternary NaOH/ Calculations from X-ray diffraction and TEM

show that the structure of the so-called bicontinu-cetyltrimethylammoniumbromide (CTAB)/tetra-ethoxysilane (TEOS) system and created a synthe- ous phase has the space group Ia3d, which has

also been found in the binary water/CTAB systemsis-space diagram of mesophase structures. Byusing XRD, NMR and polarized microscopy, the [78]. Moreover, Alfredsson et al. [79] studied

MCM-48 samples by both scanning electronbinary and ternary systems were recently investi-gated in great detail by Firouzi et al. [54]. microscopy (SEM) and TEM, and realized that

the particles have a crystal or ‘‘cubosome’’ shapeAdditionally, Vartuli et al. [75] studied the effectof surfactant/silica ratios on the formation of associated with them. Although all the particles

are not perfect polyhedra, they noticed that amesostructures. Obviously, the ratio is a criticalvariable in the mesophase formation and by vary- prevailing shape is the truncated octahedron.

Furthermore, the M41S family has been extendeding the surfactant/silica molar ratio from 0.5 to2.0 products with hexagonal, cubic (Ia3d ), lamellar by the lamellar mesostructure MCM-50 [80]. The

lamellar phase can be represented by sheets orand uncondensed cubic octamer composite struc-tures were obtained. bilayers of surfactant molecules with the hydrophi-

lic heads pointed towards the silicate at the inter-The use of these new synthetic concepts hasfocused on two main areas: (1) the synthesis of face. Removal of the surfactant from between the

silicate sheets results in structure collapse and losssilicates with new structural properties; and (2)of porosity. However, by means of a post-treat-ment with TEOS, it is possible to produce ther-Table 1

Surfactant packing parameter g, expected structure and exam- mally stable lamellar materials. So far, theples for such structures structure has not been solved successfully, but two

structural models are proposed: first, the structureg Expected structure Exampleis like the classical pillared layer silicate, or second,

1/3 cubic (Pm3n) SBA-1 the structure may be composed of a variation in1/2 hexagonal ( p6) MCM-41, FSM-16, SBA-3 the stacking of surfactant rods. Huo et al. [7,81]1/2–2/3 cubic (Ia3d ) MCM-48 made great progress in preparing high-quality meso-1 lamellar MCM-50

structures by using surfactants other than CTAB


in the S+I− system. Sophisticated modification of 4.2. Metal oxide mesophases from ionic synthesisapproachthe surfactant by varying the chain or the head

group made it possible to prove the qualitativeMonnier et al. [53] suggested the possibility ofmodel of Israelachvili [71] for silica/liquid crystal

substituting the silicate by metal oxides that aresystems. Moreover, gemini surfactants, with twoable to form polyoxoanions, because the charge-quaternary ammonium head groups separated bymatching model is not specific for silica but appli-a methylene chain of variable length and with eachcable to all other systems in which condensablehead group attached to a hydrophobic tail, can bepolyions are possible. Subsequently, mesostruct-used to prepare a mesophase that has three-dimen-ured surfactant composites of tungsten oxide,sional hexagonal (P63/mmc) symmetry, showingmolybdenum oxide and antimony oxide have beenregular supercages that can be dimensionally tai-synthesized [64,65]. In the case of tungsten oxidelored and a large inner surface. This mesostructureand antimony oxide hexagonal structures could beis called SBA-2 (Santa Barbara No. 2) [81].obtained, whereas molybdenum oxide only formedInterestingly, the P63/mmc mesophase is so far thelamellar structures. Furthermore, the approachonly one that has no lyotropic surfactant/watercould be extended to the charge reversed systemmesophase analog. A new, very fascinating phase(S−I+) by using polyoxocations and to the medi-could be developed by McGrath et al. [82] whichated combinations of S+X−I+ and S−M+I−.is based on the surfactant L3 phase by usingHowever, besides the two hexagonal phases andcetylpyridinium chloride as surfactant. The silicatean impure hexagonal phase in the case of leadL3 phase is similar to aerogels or xerogels, butoxide, only lamellar phases could be obtained atpossesses two distinct, continuous, interpenetratingfirst for the following ions: Fe2+, Mg2+, Mn2+,channel networks that traverse the entirety of theFe3+, Co2+, Ni2+, Zn2+, Al3+ and Ga3+ [64,65].material. In this case calcination is not requiredSince vanadium oxides are catalytically very inter-because the channels contain water, which can beesting, several groups investigated vanadium-based

removed at low temperatures. Moreover, in con-mesostructures using the S+I− approach. Lamellar

trast to other mesoporous materials described so [85] as well as hexagonal [86,87] mesostructuredfar, the pore sizes are determined by the relative phases could be successfully prepared. Moreover,concentration of the surfactant rather than the Janauer et al. [88] reported the crystal structuresurfactant chain length. of the first lamellar vanadium oxide phase contain-

In a different approach, using the mediated ing discrete vanadium oxide clusters and not asynthesis pathway S+X−I+, the cooperative continuous transition metal oxides lattice. In anal-assembly of cationic silicate species with cationic ogy to the extension of concepts from zeolitesurfactant mediated by chloride ions in strongly chemistry to mesoporous alumosilicates, anotheracidic solutions leads to hexagonal MCM-41 meso- main interest is focused on the preparation ofstructure [65]. However, since the material pre- mesostructured aluminophosphates. Using cat-pared from the acid synthesis is not identical with ionic surfactants, lamellar [89], hexagonal [90,91]MCM-41 synthesized in alkaline medium, it is and even the cubic [91] mesostructured alumino-often labeled SBA-3 [7,83] or APM [84] (acid- phosphate phase and, in the case of silicoalumino-prepared mesostructure). Additionally, a large- phosphates (SAPO), the hexagonal phase couldcage (~30 A) cubic silica structure with the space be synthesized [92]. Also, Rao et al. [93] recentlygroup Pm3n has been obtained using the acid succeeded in the preparation of mesostructuredsynthesis approach. The globular surfactant lamellar, hexagonal and cubic aluminoborates bymicelle has the largest surface curvature of all using the (S−I+) approach. However, a mainlyotropic liquid crystals, and the micelle surface problem of all the mesostructures mentioned sohas the lowest charge density [65,83]. So far, the far is thermal stability and, therefore, the removalso-called SBA-1 mesophase can only be obtained of the surfactant. None of these structures could

be obtained as MCM-41 or MCM-48 analogousin the acid synthesis approach.


mesoporous materials. The thermal instability isprobably due to the different oxo chemistry of themetals in comparison with silicon. It was assumedthat the existence of several relatively stable oxida-tion states of the metal centers results in oxidationand/or reduction during calcination [64]. In addi-tion, incomplete condensation of the species insidethe pore walls could also be responsible for thethermal instability as was assumed in the case ofantimony oxide [65]. Furthermore, Stein et al. [94]showed in the case of mesostructured tungstenoxide that the walls consist of uncondensed Kegginions and a complete condensation does not seempossible. The first porous transition metal oxidereported by Antonelli and Ying [95] was TiO2showing a hexagonal structure. The mesostructurewas prepared by using an anionic surfactant withphosphate head groups and titaniumalkoxy pre-cursors, which were stabilized with bidentateligands such as acetylacetone. After calcination at350°C, porous TiO2 with surface areas of around200 m2 g−1 was obtained. The formation mecha-nism has not been well studied yet. Froba et al.[96 ] suggested in a recent study that the mechanismis analogous to the mediated S−M+I− approachsince potassium ions are proposed to be the medi-ating ions. However, the existence of anionic tita-nium precursors under the reaction conditions of Fig. 5. Transmission electron micrograph of calcined zirconium

oxo phosphate showing the hexagonal pore arrangement ana-pH 5–6 is not likely according to titanium sol–gellogous to MCM-41 materials. We should like to thank Drchemistry [97]. Parallel to the work of AntonelliUshida and Prof. Dr Schlogl from the Fritz Haber-Institut,and Ying, we investigated the preparation of meso-Berlin for recording this image.

structured zirconias in our group. By using zirco-nium sulfate as precursor, which is able to interactwith cationic surfactant molecules, mesostructured lization is well known for sulfated [100] and

phosphated [101] zirconias due to the anions. Thesurfactant composites with hexagonal phase couldbe prepared [98]. By a post-synthesis treatment new zirconia-based materials show high surface

areas and pore sizes in the range between micro-with phosphoric acid or by reducing the amountof sulfate in the composite, two porous materials and mesopores. A similar approach, stabilization

using the phosphoric acid treatment, has also beenwith different compositions, thermally stable up to500°C, were obtained. The new materials are called successfully used in the case of hafnium oxide and

leads to microporous hafnium oxide, which iszirconium oxide sulfate and zirconium oxo phos-phate, and show MCM-41 analogous structures as structurally analogous to MCM-41 [102].

Additionally, Kim et al. [103] treated zirconiumcan be clearly seen in the TEM images (Fig. 5).The high thermal stability is due to a crystallization sulfate surfactant mesostructures with chromate

solutions to obtain a binary transition metaldelay caused by the sulfate or phosphate groupsin the zirconia structures so that the disordered (Zr–Cr) oxide framework. After calcination, the

porous product exhibits surface areas of up towall structure, which is favorable for these meso-porous materials, is retained [99]. Delay of crystal- 374 m2 g−1. Moreover, Stein and co-workers [104]


used the post-synthesis treatment with phosphate work wall thickness (1.7–3.0 nm) but less long-range hexagonal order [111,112]. Extendingions to link Al13 clusters of mesostructured Al13

cluster–surfactant salts. During the linking reac- the neutral route to a biomimetic templatingapproach by using neutral ‘‘bola-amphiphiles’’tion the clusters break up to form a network of

AlOh–O–PTd linkages, as well as a fraction of H2N(CH2)nNH2, lamellar silicas were preparedthat have vesicular particle morphology, denotedtetrahedral aluminum sites. The procedure can

also be used for the preparation of galloalumi- MSU-V (Michigan State University) [113,114].The MSU-V materials are thermally stablenophosphates.

By using cationic surfactants, but also cationic and, after calcination, porous lamellar silicas withhigh surface areas were obtained. Moreover,octamers as zirconium precursors, Hudson and

Knowles [105,106 ] succeeded in the formation of Pinnavaia’s group was the first to use non-ionicsurfactants, poly(ethylene oxide) monoethers ashigh-surface-area zirconia. Although first a

scaffolding mechanism was assumed, it seems to well as Pluronic- and Tergitol-type surfactants inneutral aqueous media, which are able to formbe more likely that the high surface areas are due

rather to the formation of very small particles than liquid crystals [64]. The mesoporous products,denoted MSU-X, show worm-like disordered meso-to a porous structure. Moreover, two other

approaches using anionic [107] and amphoteric pores with pore sizes of 2.0–5.8 nm [115]. Thesynthesis could also be successfully used in the[108] surfactants lead to mesostructured hexagonal

zirconia surfactant composites. However, so far it case of alumina, which shows the worm-like struc-ture as well [116 ]. Attard et al. [58] improved thisdoes not seem to be clear whether these structures

are also stable after calcination. synthesis approach by using non-ionic surfactantconcentrations at which liquid crystals are formed.Furthermore, the surfactant-controlled synthesis

approach is not limited to oxide-based materials This technique is termed ‘‘true liquid-crystal tem-plating’’ (TLCT ). Highly ordered hexagonal, cubicand recently novel mesostructures based on

tin(IV ) sulfide [109] and a lamellar mesostructure and lamellar mesoporous silica showing sharppore-size distributions could be obtained. Withof thiogermanates [110] have been synthesized.

However, these structures could only be obtained this approach they even recently succeeded in thesynthesis of pure mesoporous metals of palladiumas surfactant composites and calcination leads to

structural collapse of the mesophase. [117] and platinum [118,119] and mesoporousplatinum films [120] by chemical reduction ofmetal salts dissolved in the aqueous domains of a4.3. N0/S0I 0: silica polymorphs and metal oxidehexagonal lyotropic liquid-crystalline phase. Usinga liquid-crystalline phase as template formed fromThe advantage of using neutral or non-ionic

surfactants over the ionic route is the possibility non-ionic organic amphiphiles, Braun et al. [121]synthesized stable semiconductor–organic super-of recovery of the surfactant by extraction, which

can be attributed to the relatively weak assembly lattices based on cadmium sulfide and cadmiumselenide. They described the material as a compos-forces due to hydrogen bonding. Tanev and

Pinnavaia [66 ] developed the neutral templating ite solid in which organic structures are molecularlydispersed in the inorganic lattice. Great progressroute for mesoporous materials which is based on

hydrogen bonding and self-assembly between neut- in the preparation of mesoporous silicas was veryrecently made by Zhao et al. [122], who usedral amine surfactants and neutral inorganic precur-

sors. The materials prepared by using the S0I 0 triblock polyoxyalkylene copolymers for the syn-thesis of large-pore materials, so-called SBA-15.approach, so-called HMS (hexagonal mesoporous

silica) materials, exhibit single d100 reflections However, the mechanism is proposed to be(S+X−I+) since the block copolymer is positivelyaccompanied by more or less pronounced diffuse

scattering centered at ~1.8 nm. Compared with charged under the reaction conditions. SBA-15can be prepared with pore sizes between 4.6 andthe electrostatically templated MCM-41 silicas,

HMS materials show a consistently larger frame- 30.0 nm and wall thicknesses of 3.1 to 6.4 nm.


MCM-41 materials prepared by the cationic sur- combining niobium (or tantalum) alkoxides withone equivalent of long-chain amines, niobium (orfactant CTAB commonly have pore sizes of around

3.0 to 4.0 nm. In the conventional synthesis cosol- tantalum) alkoxide–amine complexes are formedand, by adding water, the condensation of niobiumvents such as trimethylbenzene (TMB) [32] and

also some special synthesis variations [123,124] (tantalum) centers is forced. The hexagonal phaseis formed by self-assembly of the metal alkoxide–have been used to expand the pore sizes up to

around 10.0 nm. Even bigger pores of up to 50 nm amine complexes. After removal of the organic byextraction, the porous oxides show surface areaswith wall thicknesses of up to 10 nm, although not

as regularly arranged as the ones prepared with of up to 500 m2 g−1.the triblock copolymers, were synthesized byGoltner, Antonietti and co-workers [125,126 ] whoused cationic as well as anionic block copolymers 5. Morphology controland alkoxysilanes as silica precursors. The silicasresulting after calcination are supposed to be exact The mesoporous silica-based materials obtained

from the original MCM-41 synthesis by Beck et al.replicas of the mesophases. Inspired by the surfac-tant-controlled syntheses, even mondisperse [2] consist of aggregates and loose agglomerates

of small particles. However, for many applicationsmacroporous materials could be prepared recentlyby Imhof and Pine [127] using an emulsion of in catalysis, chemical sensoring or as optical

devices, defined morphologies are required. Theequally sized droplets as templates around whichthe inorganic material deposits through a sol–gel research has focused particularly on four develop-

ments: thin films, fibers, spheres and monoliths.process. Silica, titania and zirconia materials havebeen prepared. The organic can be removed while Yang et al. [131] showed the potential of acid

synthesis (S+X−I+) in the preparation of a largepreserving the macroporous structure, althoughphase transformations into the crystalline oxide variety of differently curved, highly interesting

morphologies. However, no details of how tophases have been observed.obtain desired morphologies or why these mor-phologies were obtained were given in that publica-4.4. S–I approachtion. Nevertheless, the acidic route pioneered byHuo et al. [65] seems to be best suited for morphol-There are only a few studies concerning the

preparation of covalently linked hybrid organic ogy control. Therefore, for most morphologicallycontrolled syntheses, the (S+X−I+) approach wasnetworks, which, however, could be an interesting

approach to a wide range of functionalized hybrid successfully used. Thin films have been preparedat the air/water [132,133] and oil/water [84] inter-materials with ordered mesopores. Huo et al. [7]

and Burkett et al. [128] successfully prepared face as free-standing films, on both mica [134] andgraphite [135,136 ] surfaces and by using dip- orhexagonal mesostructured silicas by this method.

This approach has been more important in the spin-coating [137,138] techniques. Except for thespin- or dip-coated films and the film from thepreparation of mesoporous transition metal oxides.

Ying’s group used it for the preparation of meso- oil/water interface, the grown films all show con-tinuous hexagonal, mesoporous silica with poresporous hexagonal niobia [68] and tantalum oxide

[129], which are called Nb- and Ta-TMS1 (trans- parallel to the film surface. Tolbert et al. [139]recently reported the synthesis of silicate surfactantition metal oxide mesoporous molecular sieve

No. 1). In the case of niobia, also the hexagonal films with the structure P63/mmc both on micaand as unsupported films. The phase P63/mmc hasP63/mmc (Nb-TMS2), the cubic (Nb-TMS3) and

the layered phase (Nb-TMS4) have been reported the advantage that it allows easy transport into orthrough the film. Moreover, selected area growth[130]. The materials are formed via a ‘‘ligand-

assisted liquid-crystal templating’’ mechanism of mesoporous thin film silica could be achievedby using self-assembly of alkane thiols on goldinvolving the hydrolysis of long-chain amine com-

plexes of niobium (or tantalum) alkoxides. After surfaces [140]. The preparation of fiber morpholo-


gies was first carried out in oil-in-water emulsions sis, destroys the liquid-crystalline mesophase,removal of the methanol is important to reformunder acidic (S+X−I+) conditions. Schacht et al.

[84] demonstrated fiber formation at the oil/water the mesophase. Chmelka and co-workers [147]used a sophisticated approach to macroscopicallyinterface by using low stirring rates. In a similar

system, Huo et al. [141] observed slow fiber growth orient the silicate surfactant liquid crystals.Anisotropic silica bulk materials were synthesizedinto the aqueous phase. The fibers are transparent

and show excellent order, as revealed by parallel through alignment of an unpolymerized hexagonallyotropic silicate surfactant liquid crystal in a highpores that follow the fiber-axis curvature with high

fidelity. Very recently also a spinning process for magnetic field. Further, the macroscopic aligmentcan by controlled by tuning the synthesis composi-the generation of fibers was introduced, by which

the fiber dimensions can be mechanically con- tion: the lamellar or hexagonal mesophases adoptdifferent orientations according to the diamagnetictrolled (Fig. 6) [142]. Through emulsion biphase

chemistry, the preparation of hollow [84] as well susceptibilities of different organic additives, likebenzene, present in the synthesis mixture [148].as hard spheres [143] (Fig. 7) of mesoporous silicas

has been achieved. Grun et al. [144] modified the After calcination the aligned pore orientation isretained, leading to ordered mesoporous aniso-well-known Stober synthesis [145] of monodisperse

silica spheres and succeeded in the preparation of tropic silica monoliths (Fig. 8). In this experimenta monolith was mounted on a diffractometer andmonodispersed mesoporous MCM-41 spheres.

For several applications the preparation of the X-ray pattern was recorded in two dimensionsafter the beam had passed the sample. If theremonoliths would be of great interest. So far,

monolithic periodic mesoporous silicas have been were no preferred orientation, rings around theprimary beam would be expected, similar to theprepared in different systems by using ionic and

non-ionic surfactants as well [58,146 ]. Ordered case from a powder in a Debye–Scherrer experi-ment. The fact that the intensity of the scatteredliquid-crystalline mesophases of non-ionic surfac-

tants, octaethylene glycol monodecyl ether X-rays is inhomogeneously distributed around theprimary beam proves the preferred orientation of(C12EO8) and octaethylene glycol monohexadecyl

ether (C16EO8), have been used by Attard et al. the pore system as indicated in the sketch in Fig. 8.A different approach to align the structure was[58] to template mesoporous silica, leading to the

first monolithic MCM-41 samples. As silica source used by Raimondi et al. [149]. The macroscopicallyaligned mesostructures were made from TLCTtetramethoxysilane (TMOS) was used and since

the methanol, which is released from the hydroly- systems [58] by confining the synthesis mixtures ina capillary. The coating contains coils of alignedmesopores, lining the walls of the capillary. Aninteresting morphology of ‘‘tubules-within-a-tubule’’ hierachical order has been reported by Linand Mou [150]. They designed a strategy forproducing hollow tubular forms of MCM-41 basedon the sequential separation of self-organizationof template silicates and the polymerization ofsilicates.

6. Application

Silica-based mesoporous materials have beenextensively studied in several catalytic reactions[151]. A very recent, comprehensive review on theFig. 6. Scanning electron micrograph of calcined mesoporous

silica fibers [142]. catalytic properties and applications of MCM-41


Fig. 7. As-made MCM-41 featuring spherical morphology [143].

type materials was given by Corma [10]. We into silica structures is known to form acid sitestherefore outline here only the major trends in this in the framework. Therefore, mesoporous alumi-field and refer the reader to this review for further nosilicates were tested in acid catalysis [152–155].information. However, independent of the aluminum content in

When MCM-41 was first discovered there were the framework, MCM-41 materials show onlygreat expectations for applications in the petro- weak acidity which is comparable to amorphousleum industry, for instance in processing of heavy aluminosilicates. Therefore, they are only promis-residues. It was assumed that the aluminosilicate ing for reactions that do not require very strongmaterials exhibited acid site strengths comparable acidity. To enhance the catalytic potential, severalto that of zeolites. The incorporation of aluminum metal ions have been isomorphously substituted:

titanium [156–161], zirconium [162], vanadium[163–166 ], iron [167–169], cobalt [170], boron[171–174], tin [175] and platinum [176 ]. Promisingproperties have been observed for Ti-MCM-41 ascatalysts for the selective oxidation of large organicmolecules in the presence of tert-butyl hydroperox-ide or dilute H2O2. It was found that Ti-MCM-41is more active than Ti-Beta, a large-pore zeolite,in these reactions because of lower diffusion limita-tions. However, except for Ti-MCM-41, the prop-erties of metal-incorporated MCM-41 leave muchto be desired. Although isomorphous substitutionis the method to tailor zeolites for special desired

Fig. 8. Two-dimensional XRD data acquired from calcined hex- reactions, this does not seem to be effective in theagonal mesoporous silica derived from aligned crystal interme- case of the mesoporous materials, which may bediate [147]. For isotropic pore orientation a ring around the

due to the amorphous walls. Therefore, new meth-primary beam would be expected. The inhomogeneous intensitydistribution proves the preferential ordering of the pores. ods to incorporate the metals have to be developed


beyond the traditional ones such as ion exchange Wu and Bein used the one-dimensional regularchannel structure to prepare conducting polyani-or incipient wetness inpregnation. Maschmeyer

and Thomas [177,178] prepared a catalyst by line filaments [183] and conducting carbon wires[184] inside the channels. Charge carriers in nano-grafting metallocene complexes onto mesoporous

silica. Large concentrations of accessible, well- meter channels are very interesting for the develop-ment of nanometer electronic devices. To preparespaced and structurally well-defined active sites

could thus be achieved. This approach has also semiconductor quantum wire structures, Leonet al. [185] filled the mesopores of MCM-41 withbeen successfully used for vanadium incorporation

[179]. Another way of ensuring a high dispersity germanium. They succeeded in filling the pores byusing a vapor-phase loading technique.of the catalytically active sites onto the catalyst

support MCM-41 has been achieved in our group Finally, in a very different approach, MCM-41has been used for immobilization of small enzymes[180]. Well-defined metal clusters like Pd561 and

Au55 have been used as precursors of the active in the mesopore structure [186 ]. The loading effi-ciency of the immobilized enzymes showed a clearcomponents. Since the metal clusters are stabilized

by hydrophobic ligands, an in situ syntheses was correlation with the enzyme size, suggesting thatthe mesopores participate in the immobilizationpossible to incorporate the clusters, which interact

with the hydrophobic surfactant chain. After process.calcination to remove both the surfactant and theligands, very small (2 to 4 nm) metal clusters havebeen found to be highly dispersed on the MCM-41 7. Summaryinner and outer surfaces. However, so far it is notclear whether MCM-41 as a support for noble Considering the fact that MCM-41 type materi-

als were introduced only about six years ago, ametals will have advantageous properties com-pared with optimized amorphous silica supports. high level of understanding regarding their forma-

tion and the control of material properties hasExcept for catalytic applications, some otherinteresting approaches have been reported that use been reached. Also, generalization of the concepts

to other classes of liquid-crystal-forming mole-the unique pore structure and high surface areasof mesoporous materials. Feng et al. [181] reported cules, on the one hand, and inorganic frameworks

other than silica, on the other, can be consideredrecently a route for the modification of the silicasurface. Functional groups, thiol groups in their as a major achievement. However, a major driving

force for research in such an application-orientedcase, can be introduced onto the pore surface ofmesoporous silica as terminal groups of an organic field are the applications. To our knowledge, there

is no commercial process or product in whichmonolayer. This material, called ‘‘functionalizedmonolayers on mesoporous supports’’ (FMMS), ordered mesoporous oxides are used to date,

despite high expectations at the time when thecan efficiently remove mercury or other heavymetals from contaminated solutions. Moreover, material was introduced. When thinking about

possible applications, one has to bear in mind thatthis method can be generalized and used to synthe-size materials with tailor-made surfaces. in many cases cheaper alternatives are available.

For the majority of applications one could envis-In high-performance liquid chromatography(HPLC) silica is the predominant packing mate- age, especially in catalysis, one does not need

either the perfect order of pores or even a narrowrial. Therefore, Unger and co-workers [182] haveinvestigated the properties of MCM-41 packings pore-size distribution. Shape selectivity due to size

restrictions cannot be expected for typical porein normal-phase HPLC and compared its retentionbehavior and selectivity with those of other oxides. sizes around 4 nm. What then remains as the

crucial advantage for most applications is the veryThe most interesting feature of MCM-41 is theability to separate all types of analytes (basic, acid high surface areas. In particular, oxides other than

silica with high surface areas could become impor-and neutral ) within acceptable retention times andgood peak shapes. tant, since they are not easily accessible, or even


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