On the Controllable Soft-Templating Approach to Mesoporous Silicates

Ying Wan and Dongyuan Zhao*

Department of Chemistry,Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials,Key Laboratory of Molecular Engineering of Polymers,and Advanced Materials Laboratory,Fudan University,Shanghai200433,P.R.China

Received September11,2006

Contents

1.Introduction2821

2.Synthesis Mechanism and Pathway2823

2.1.Surfactants2823

2.2.Cooperative Self-Assembly of Surfactant and

Silica Source To Form Mesostructure

2825

2.3.Liquid-Crystal Template Pathway2828

3.Synthesis of Mesoporous Silicate Molecular

Sieves

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3.1.Hydrothermal Method2829

3.1.1.Basic Synthesis2830

3.1.2.Acidic Synthesis2830

3.1.3.Synthesis(Reaction)Temperature2831

3.1.

4.Hydrothermal Treatment2832

3.1.5.Separation and Drying2832

汉语听力3.1.6.Removal of Template2833

3.2.Nonaqueous Synthesis2834

3.3.Postsynthesis Treatment2835

3.3.1.Secondary Synthesis2835

3.3.2.Recrystallization2836

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4.Controllable Synthesis on Mesoscale2836

4.1.Mesophase Tailoring2836

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4.1.1.Micellar Mesostructure2836

4.1.2.2D Mesostructures2841

4.1.3.3D Mesostructures2843

4.1.4.Lamellar and Disordered Mesostructures2848

4.1.

5.Other Mesostructures2849

4.2.Pore Size Control2849

5.Morphology Control2851

6.Summary and Outlook2855

7.Acknowledgment2856

8.References2856 1.Introduction

Technical advances in various fields,such as adsorption, separation,catalysis,drug delivery,sensors,photonics,and nanodevices,require the development of ordered porous materials with controllable structures and systematic tailoring pore architecture.The structural capabilities at the scale of a few nanometers can meet the demands of the growing applications emerging in proce

sses involving large molecules, for example,biology and petroleum products.1-4Zeolites or microporous materials,whose pore sizes are less than1.2 nm,are far away from these demands.These motivations spark the proliferation of mesoporous materials.

In fact,mesoporous materials were developed a couple of decades ago.Pillared clays,which possess mesopore sizes, have been extensively investigated since the1980s.However, their rectangular pores could not be fully opened.The reagents and products could not easily pass through the pores, leading to coking in catalytic processes.1Moreover,the pore sizes are widely distributed,and the arrangement of pores is disordered.The significance of“template”was not realized. In the early1990s,Japanese scientists and Mobil scientists separately reported the synthesis of mesostructued silicates.5-7 In Mobil’s report,quaternary ammonium cationic surfactants such as cetyltrimethylammonium bromide(C16H33N(CH3)3-Br,CTAB)were first used as templates to prepare highly ordered M41S mesoporous silicate molecular sieves under hydrothermal,basic conditions.This kind of attractive material extends the uniform pore sizes from the range of micropore to mesopore.More importantly,the concept of “template”was first postulated in the synthesis of mesopo-rous silicate materials.

The synthesis of mesoporous molecular sieves is mainly concerned with“building mesopores”.In gen

eral,two classes of materials have often been integrated as components in this mesoporous family,including mesoporous molecular sieves with open framework structures,mesoporous silicate replicas constructed by nanowire arrays,etc.Mesoporous molecular sieves,which are obtained from the organic-inorganic assembly by using soft matter,that is,organic molecules or ,amphiphilic surfactants and biomacromolecules),as templates,clearly contribute the main mesoporous family members.Surfactants are mostly used as templates.The open frameworks and tunable porosities endow mesoporous materials with accessibility to metal ions and reagents.These characteristics are extremely

*To whom correspondence should be addressed.E-mail:dyzhao@ fudan.edu.Tel:86-21-5566-4194.Fax:

86-21-6564-1740.

Volume107,Number7

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important in the fields of catalysis,sensors,electronic devices,biology,nanodevices,separation,etc.The organic -inorganic self-assembly is driven by weak noncovalent bonds such as hydrogen bonds,van der Waals forces,and electro-valent bonds between the surfactants and inorganic species.Instead of a simple superposition of the weak interaction,an integrated and complex synergistic reaction facilitates the process.Cooperative assembly between organic surfactants and inorganic precursors is generally involved,forming inorganic/organic mesostructured composites.Mesoporous molecular sieves can be obtained after the removal of surfactants.Therefore,the surfactant self-assembly is par-ticularly essential for the formation of highly ordered mesostructures.On the basis of the current knowledge on the surfactant self-assembly,the mesoporous materials can be rationally designed and the synthesis can be con-trolled.

The emergence of ordered mesoporous materials provides not only a series of novel materials that possess large uniform pore sizes (1.5-10nm),highly ordered nanochannels,large surface areas (∼1500m 2/g)and attractive liquid-crystal structures but also an idea of the design of periodic arrange

ments of inorganic-organic composite nanoarrays.Tremendous research effort was put into the syntheses and applications of these materials.A large variety of mesoporous materials with different mesostructures (two-dimensional (2D)hexagonal,space group p 6mm ,three-dimensional (3D)

hexagonal P 63/mmc ,3D cubic Pm 3

h m ,Pm 3h n ,Fd 3h m ,Fm 3h m ,Im 3h m ,bicontinuous cubic Ia 3h d ,etc.)and compositions

(silica,metal oxides,

8-15metal sulfides,16-18metals,19-21and even polymers and carbons 22,23)have been synthesized.Owing to the elaborate studies on sol -gel chemistry of silicates,mesoporous silicate molecular sieves from surfac-tant self-assembly are most amply investigated.

A common thought here is to compare mesoporous silicates with zeolite molecular sieves,both of which have open pore framework structures.Besides pore size,at least five discrepancies can result from the viewpoints of structure and composition.(1)Zeolites,which are crystalline silicates or aluminosilicates with 3D framework structures,are perfect inorganic crystals on the molecular scale.Mesoporous crystals possess a periodic arrangement of a moiety and give well-defined diffract

ion spots on the mesoscale.(2)Classical zeolites are strictly constructed by aluminosilicate tetrahedron (TO 4)networks.The pore walls of mesoporous materials are amorphous.Many polyhedrons,such as hexa-coordination octahedron (TO 6)and penta-coordination trigonal bipyramid (TO 5)are allowed.Diverse compositions can then be constituted of mesoporous molecular sieve frameworks.(3)TO 4units constructed by Si and Al atoms in zeolites are generally four-connected by covalent bonds.Only a few zeolite structures,which have surface defects or large rings,possess three connections,like VPI-5and JDF-20.24,25The number of surface hydroxyl groups is low.However,not all SiO 4units in mesoporous silicates are four-connected.In other words,three-connected and even two-connected SiO 4units can be detected,which generate a hydrophilic surface with more hydroxyl groups (Si -OH).(4)Zeolites show high hydrothermal stability,whereas mesoporous materials,in particular silicates,are unstable in water or steam.The structures would be destroyed by treating mesoporous silica in boiling water for 6h or in steam (100%water)at 800°C for 4h.26(5)Although many efforts have been devoted to the synthesis of mesostructured materials with zeolite-type walls,27-29there has been no major success in reproducibility.Moreover,ordered mesostructures with zeolite nanocrystal walls could not be validated by transmission electron microscope (TEM)images.This is mainly due to the fragility of the amorphous silica frameworks.An interesting work by Chmelka and co-workers mentioned meso-layered silicas with zeolite-type walls,derived fro

m the hydrothermal treatment of MCM-like starting solutions by using double-four-ring (D4R)or double-three-ring (D3R)silicates as a precursor.30This work implies that the amorphous frame-works inherent to mesoporous silicates are indeed fragile.The hydrothermal method similar to that for zeolites was used to synthesize mesoporous silicates by Mobil scientists.However,the dissimilarity is evident in the preparation of these two kinds of molecular sieves due to their structural differences.(1)The synthetic temperature is rather low (from room temperature to 130°C)for mesoporous silicate

Ying Wan completed each of her academic degrees in Industrial Catalysis from the East China University of Science and Technology,receiving her Ph.D.in 2002with Professor Jianxin Ma.Then,she joined Shanghai Normal University.From 2005−2007,she was a postdoctoral member in Fudan University working with Professor Dongyuan Zhao.She is currently a Professor in the Department of Chemistry at Shanghai Normal University,working on porous materials in catalysis and electrodes.

Dongyuan Zhao was born in 1963.He received his B.S.and M.S.in chemistry from Jilin University.He obtained his Ph.D.in 1990from Jilin University and Dalian Institute Chemical Physics.In 1992−1993,he was a visiting scholar at the Department of Chemistry,University of Regina,and later carried out his postdoctoral research at the Weizmann Institute of Science (1993−1994),University of Houston (1995−1996),and University of California at Santa Barbara (1996−1998).He is now a Profes

sor (Cheung Kong Professorship)in the Department of Chemistry at Fudan University.His current research interests include synthesis,structural characterization and applications on ordered porous materials,such as mesoporous materials,zeolites,and coordination polymers.He has contributed to about 250international scientific publications.

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molecular sieves.5,31An operation temperature can be -10°C.The hydrothermal treatment temperature should be lower than 130°C (in general,100-130°C)even after the precipitation of mesoporous materials,which implies the formation of mesostructures or gels.In contrast,the crystal-lization temperature for zeolites is much higher,that is,80-300°C.The synthesis of mesoporous materials can thus not be considered a “true”hydrothermal synthesis.A surfactant containing fluoride was used to increase the hydrothermal temperature of mesoporous silicate materials up to 170°C.32The resultant silicates exhibited high cross-linking degrees,and thus high hydrothermal sta

bility.Unfortunately,the mesostructure regularity was low,and the reason was not given.(2)In comparison with zeolites,mesostructured materials show much faster formation rates.It takes only several seconds to minutes for the crystallization as solid precipitation.The crystallization of zeolites generally requires several days and even months.(3)Mesoporous molecular sieves can be formed in nonaqueous media.In many polar organic solvents,like alcohols and tetrahydrofuran (THF),mesostructures can be formed through solvent evaporation induced self-assembly (EISA)or the solvothermal synthesis method.11,33-36Water is however,necessary in the preparation of zeolites.Without water,zeolites cannot be fully crystal-lized.A large amount of water must be added in the batch even in the solvothermal method.(4)In contrast to a very wide pH value ranging from 0to 12for the synthesis of mesoporous silicates,most zeolites are prepared in basic media.Despite the reduction in the pH values of the synthetic media by the addition of fluoride ions,successful syntheses of zeolites are carried out only in neutral and weak acidic media.1,35,37-39Many expectations are left in the hearts of synthesis scientists,one of which is the preparation of zeolite crystals in acidic media (pH <2).(5)The morphologies of zeolites are strongly related to their structures and are dif-ficult to control because zeolites are a kind of perfect crystal.In contrast,mesoporous silicates exhibit various mor-phologies,such as thin films,spheres,monoliths,fibers,etc.40-42

Several excellent reviews have summarized the synthesis,characterization,and applications of mesoporous sili-cates.1,35,37,43-69On the consideration of characteristics for

mesoporous materials,this review will present the recent developments in the syntheses of ordered mesoporous mater-ials by the surfactant assembly,especially for mesoporous silicates.We summarize the synthesis pathways,the corre-sponding mechanisms,and the key factors for controllable synthesis,which include the selection of surfactants,the hydrothermal method,the EISA strategy,the control of mesostructures,and the adjustment of pore sizes and mor-phologies.High-quality mesoporous products will be easily obtained provided that these factors can be fully understood by researchers.We hope that a beginner can easily grasp the preparation skills,obtain high-quality mesoporous silicate materials upon understanding the present issues,and promote their applications.

2.Synthesis Mechanism and Pathway

A large number of studies have been carried out to investigate the formation and assembly of mesostructures on the basis of surfactant self-assembly.The initial liquid-crystal template mechanism first proposed by Mobil’s scientists is essentially always “true”,because the pathways basically includ

e almost all possibilities.5,6Two main pathways,that is,cooperative self-assembly and “true”liquid-crystal tem-plating processes,seem to be effective in the synthesis of ordered mesostructures,as shown in Figure 1.1,37,51,70

2.1.Surfactants

Generally a clear homogeneous solution of surfactants in water is required to get ordered mesostructures.Frequently used surfactants can be classified into cationic,anionic,and nonionic surfactants.Until now,few amphoteric surfactants were used in synthesis.71,72

Quaternary cationic surfactants,C n H 2n +1N(CH 3)3Br (n )8-22),are generally efficient for the synthesis of ordered mesoporous silicate materials.Commercially available CTAB is often used.Gemini surfactants,bolaform surfactants,multi-headgroup surfactants,and recently reported cationic fluori-nated surfactants can also be used as templates to prepare various mesostructures.8,73-75Frequently used cationic qua-ternary ammonium surfactants are shown in Figure 2.In the

Figure 1.Two synthetic strategies of mesoporous materials:(A)cooperative self-assembly;(B)“true”liquid-crystal templating pro-cess.

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first reports of mesoporous silicates from Mobil Company,cationic surfactants were used as structure-directing agents (SDAs).Cationic surfactants have excellent solubility,have high critical micelle temperature (CMT)values,and can be widely used in acidic and basic media.But they are toxic and expensive.

Anionic salt surfactants include carboxylates,sulfates,sulfonates,phosphates,etc.Recently,a kind of lab-made anionic surfactant terminal carboxylic acids (salts)(Figure 3)is used to template the synthesis of mesoporous silicas with the assistance of aminosilanes or quaternary amino-silanes such as 3-aminopropyltrimethoxysilane (APS)and N -trimethoxylsilylpropyl-N ,N ,N -trimehylammonium chloride (TMAPS)as co-structure-directing agents (CS-DAs).76

爱碧论坛Nonionic surfactants are available in a wide variety of different chemical structures.They are widely used in industry because of attractive characteristics like low price,nontoxicity,and biodegradability.In addition,the self-assembling of nonionic surfactants produces mesophases with different geometries and arrangements.They become more and more popular and powerful in the syntheses of meso-

Figure 2.Molecular formula of frequently used cationic surfactants.

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porous solids.Attard and co-workers directly synthesized ordered mesoporous silica structures in acidic C 12H 25EO 8and C 16H 33EO 8systems.77The pore sizes are limited to 3nm.Other classes of highly ordered mesoporous materials with uniform pore sizes larger than 5nm were synthesized by employing poly(ethylene oxide)-b -poly(propylene oxide)-b -poly(ethylene oxide)(PEO -PPO -PEO)triblock copolymers as templates under acidic aqueous media.31,78The syntheses that largely promote the development of mesoporous materi-als are simple and reproducible.A family of mesoporous silica materials has been prepared with various mesopore packing symmetries and well-defined pore connectivity.Figure 4lists the classical commercial nonionic surfactants.The main members include oligomeric alkyl PEO surfactants,amphiphilic block copolymers (e.g.,PEO -PPO -PEO),sorbitan esters,etc.

2.2.Cooperative Self-Assembly of Surfactant and Silica Source To Form Mesostructure

This pathway is established on the basis of the interactions between silicates and surfactants to form inorganic -organic mesostructured composites.

A layer-to-hexagonal mechanism (folded sheets mecha-nism)was postulated by Kuroda,Inagaki,and

co-workers,according to which the mesostructure is created from a layered kanemite precursor.7,79In the synthesis,the FSM-16and KSW-2mesostructures were obtained from the layered inorganic precursor kanemite.7,79,80Such a motif was also suggested by Stucky and co-workers.81However,this mechanism is not general,and the layered intermediate is unnecessary in the formation of hexagonal mesostructure MCM-41.It is also unclear whether the two hexagonal mesostructures,namely,FSM-16generated by layered kane-mite and MCM-41,are identical.

Davis and co-workers proposed a “silicate rod assembly”mechanism.82Two or three monolayers of silicate species first deposit on isolated surfactant micellar rods.The long surfactant -silicate rods spontaneously aggregate and eventu-ally pack into a long-range ordered hexagonal arrangement.Zhou and co-workers gave weak evidence for this mecha-nism,because they observed a single rod on the edge of samples in different synthetic periods using TEM.83This mechanism is however unconvincing due to the difficulty of assembling long rods.It is also not as popular as a

cooperative formation mechanism,which was first proposed by Stucky and co-workers and accepted by most research-ers.8,84

Silicate polyanions such as silicate oligomers interact with positively charged groups in cationic surfa

ctants driven by Coulomb forces.The silicate species at the interface poly-merize and cross-link and further change the charge density of the inorganic layers.With the proceeding of the reaction,the arrangements of surfactants and the charge density between inorganic and organic species influence each other.Hence the compositions of inorganic -organic hybrids differ to some degree.It is the matching of charge density at the surfactant/inorganic species interfaces that governs the as-sembly process.The final mesophase is the ordered 3D arrangement with the lowest interface energy.The transfor-mation of the isotropic micellar solutions of CTAB into hexagonal or lamellar phases when mixed with anionic silicate oligomers in highly alkaline solutions was indeed detected through a combination of correlated solution state 2H,13C,and 29Si nuclear magnetic resonance (NMR)spectroscopy,small-angle X-ray scattering (SAXS),and polarized optical microscopy measurements.85,86The mech-anism in different surfactant systems has been studied using NMR techniques.87

This cooperative formation mechanism in a nonionic surfactant system was investigated by in situ techniques.Goldfarb and co-workers investigated the formation mech-anism of mesoporous silica SBA-15,which are templated by triblock copolymer P123(EO 20PO 70EO 20)by using direct imaging and freeze -fracture replication cryo-TEM tech-niques,in situ electron paramagnetic resonance (EPR)

spectroscopy,and electron spin -echo envelope modulation (ESEEM)experiments.88,89They found a continuous trans-formation from spheroidal micelles into threadlike micelles.Bundles were then formed with dimensions that are similar to those found in the final materials.The elongation of micelles is a consequence of the reduction of polarity and water content within the micelles due to the adsorption and polymerization of silicate species.Before the hydrothermal treatment,the majority of PEO chains insert into silicate frameworks,which generate micropores after the removal of templates.Moreover,they found that the extent of the PEO chains located within the silica micropores depended on both the hydrothermal aging temperature and the Si/P123molar ratio.The formation dynamics of SBA-15was studied by Flodstrom et al.90on the basis of time-resolved in situ 1H NMR and TEM investigations.They observed four stages during the cooperative assembly,which are the adsorption of silicates on globular micelles,the association of globular micelles into floes,the precipitation of floes,and the micelle -micelle coalescence.Khodakov et al.91proposed a structure with a hydrophobic PPO core and a PEO -water -silicate corona in the first stage.Then the cylindrical micelles pack into large domains.At the same time,solvents are replaced by condensed silicate species.

These mechanisms consider the interactions on the sur-factant/inorganic species interfaces.Monnier and Huo et al.81,84gave a formula of the free energy in the whole pro-cess:

in which ∆G inter is the energy associated with the interaction between inorganic walls and surfactant micelles,∆G wall is the structural free energy for the inorganic frameworks,

∆G )∆G inter +∆G wall +∆G intra +∆G sol

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