On the Controllable Soft-Templating Approach to Mesoporous

Volume 107, Number 7

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, Shanghai 200433, P. R. China Received September 11, 2006

Contents 1. Introduction 2. Synthesis Mechanism and Pathway 2.1. Surfactants 2.2. Cooperative Self-Assembly of Surfactant and Silica Source To Form Mesostructure 2.3. Liquid-Crystal Template Pathway 3. Synthesis of Mesoporous Silicate Molecular Sieves 3.1. Hydrothermal Method 3.1.1. Basic Synthesis 3.1.2. Acidic Synthesis 3.1.3. Synthesis (Reaction) Temperature 3.1.4. Hydrothermal Treatment 3.1.5. Separation and Drying 3.1.6. Removal of Template 3.2. Nonaqueous Synthesis 3.3. Postsynthesis Treatment 3.3.1. Secondary Synthesis 3.3.2. Recrystallization 4. Controllable Synthesis on Mesoscale 4.1. Mesophase Tailoring 4.1.1. Micellar Mesostructure 4.1.2. 2D Mesostructures 4.1.3. 3D Mesostructures 4.1.4. Lamellar and Disordered Mesostructures 4.1.5. Other Mesostructures 4.2. Pore Size Control 5. Morphology Control 6. Summary and Outlook 7. Acknowledgment 8. References

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1. Introduction Technical advances in various fields, such as adsorption, separation, catalysis, drug delivery, sensors, photonics, and * To whom correspondence should be addressed. E-mail: dyzhao@ fudan.edu.cn. Tel: 86-21-5566-4194. Fax: 86-21-6564-1740.

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 processes involving large molecules, for example, biology and petroleum products.1-4 Zeolites or microporous materials, whose pore sizes are less than 1.2 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 the 1980s. 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.1 Moreover, the pore sizes are widely distributed, and the arrangement of pores is disordered. The significance of “template” was not realized. In the early 1990s, 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)3Br, 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 mesoporous silicate materials. The synthesis of mesoporous molecular sieves is mainly concerned with “building mesopores”. In general, 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 organicinorganic assembly by using soft matter, that is, organic molecules or supramolecules (e.g., 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

10.1021/cr068020s CCC: $65.00 © 2007 American Chemical Society Published on Web 06/20/2007

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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 2002 with 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 1990 from 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 Professor (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 250 international scientific publications.

important in the fields of catalysis, sensors, electronic devices, biology, nanodevices, separation, etc. The organicinorganic self-assembly is driven by weak noncovalent bonds such as hydrogen bonds, van der Waals forces, and electrovalent 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 particularly 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 controlled.

Wan and Zhao

The emergence of ordered mesoporous materials provides not only a series of novel materials that possess large uniform pore sizes (1.5-10 nm), highly ordered nanochannels, large surface areas (∼1500 m2/g) and attractive liquid-crystal structures but also an idea of the design of periodic arrangements 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 p6mm, three-dimensional (3D) hexagonal P63/mmc, 3D cubic Pm3hm, Pm3hn, Fd3hm, Fm3hm, Im3hm, bicontinuous cubic Ia3hd, etc.) and compositions (silica, metal oxides,8-15 metal sulfides,16-18 metals,19-21 and even polymers and carbons22,23) have been synthesized. Owing to the elaborate studies on sol-gel chemistry of silicates, mesoporous silicate molecular sieves from surfactant 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 diffraction spots on the mesoscale. (2) Classical zeolites are strictly constructed by aluminosilicate tetrahedron (TO4) networks. The pore walls of mesoporous materials are amorphous. Many polyhedrons, such as hexa-coordination octahedron (TO6) and penta-coordination trigonal bipyramid (TO5) are allowed. Diverse compositions can then be constituted of mesoporous molecular sieve frameworks. (3) TO4 units 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-5 and JDF-20.24,25 The number of surface hydroxyl groups is low. However, not all SiO4 units in mesoporous silicates are four-connected. In other words, three-connected and even two-connected SiO4 units 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 6 h or in steam (100% water) at 800 °C for 4 h.26 (5) Although many efforts have been devoted to the synthesis of mesostructured materials with zeolite-type walls,27-29 there 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 from the hydrothermal treatment of MCM-like starting solutions by using doublefour-ring (D4R) or double-three-ring (D3R) silicates as a precursor.30 This work implies that the amorphous frameworks 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

Controllable Soft-Templating Approach

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

molecular sieves.5,31 An 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 crystallization temperature for zeolites is much higher, that is, 80300 °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.32 The resultant silicates exhibited high cross-linking degrees, and thus high hydrothermal stability. 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.11,33-36 Water is however, necessary in the preparation of zeolites. Without water, zeolites cannot be fully crystallized. 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 0 to 12 for 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.1,35,37-39 Many 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 difficult to control because zeolites are a kind of perfect crystal. In contrast, mesoporous silicates exhibit various morphologies, such as thin films, spheres, monoliths, fibers, etc.40-42 Several excellent reviews have summarized the synthesis, characterization, and applications of mesoporous silicates.1,35,37,43-69 On the consideration of characteristics for

mesoporous materials, this review will present the recent developments in the syntheses of ordered mesoporous materials by the surfactant assembly, especially for mesoporous silicates. We summarize the synthesis pathways, the corresponding 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 morphologies. 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 include almost all possibilities.5,6 Two main pathways, that is, cooperative self-assembly and “true” liquid-crystal templating 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, CnH2n+1N(CH3)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, multiheadgroup surfactants, and recently reported cationic fluorinated surfactants can also be used as templates to prepare various mesostructures.8,73-75 Frequently used cationic quaternary ammonium surfactants are shown in Figure 2. In the


Wan and Zhao

Figure 2. Molecular formula of frequently used cationic surfactants.

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 (CSDAs).76 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 selfassembling of nonionic surfactants produces mesophases with different geometries and arrangements. They become more and more popular and powerful in the syntheses of meso-


Figure 3. Anionic surfactants. Reprinted with permission from ref 76. Copyright 2003 Nature Publishing Group.

porous solids. Attard and co-workers directly synthesized ordered mesoporous silica structures in acidic C12H25EO8 and C16H33EO8 systems.77 The pore sizes are limited to 3 nm. Other classes of highly ordered mesoporous materials with uniform pore sizes larger than 5 nm were synthesized by employing poly(ethylene oxide)-b-poly(propylene oxide)-bpoly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers as templates under acidic aqueous media.31,78 The syntheses that largely promote the development of mesoporous materials are simple and reproducible. A family of mesoporous silica materials has been prepared with various mesopore packing symmetries and well-defined pore connectivity. Figure 4 lists 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 mechanism) was postulated by Kuroda, Inagaki, and co-workers, according to which the mesostructure is created from a layered kanemite precursor.7,79 In the synthesis, the FSM16 and KSW-2 mesostructures were obtained from the layered inorganic precursor kanemite.7,79,80 Such a motif was also suggested by Stucky and co-workers.81 However, 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-16 generated by layered kanemite and MCM-41, are identical. Davis and co-workers proposed a “silicate rod assembly” mechanism.82 Two or three monolayers of silicate species first deposit on isolated surfactant micellar rods. The long surfactant-silicate rods spontaneously aggregate and eventually pack into a long-range ordered hexagonal arrangement. Zhou and co-workers gave weak evidence for this mechanism, because they observed a single rod on the edge of samples in different synthetic periods using TEM.83 This 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 researchers.8,84 Silicate polyanions such as silicate oligomers interact with positively charged groups in cationic surfactants driven by Coulomb forces. The silicate species at the interface polymerize 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 assembly process. The final mesophase is the ordered 3D arrangement with the lowest interface energy. The transformation 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.85,86 The mechanism 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 mechanism of mesoporous silica SBA-15, which are templated by triblock copolymer P123 (EO20PO70EO20) by using direct imaging and freeze-fracture replication cryo-TEM techniques, in situ electron paramagnetic resonance (EPR) spectroscopy, and electron spin-echo envelope modulation (ESEEM) experiments.88,89 They found a continuous transformation 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/P123 molar ratio. The formation dynamics of SBA-15 was studied by Flodstrom et al.90 on 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.91 proposed a structure with a hydrophobic PPO core and a PEO-watersilicate 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 surfactant/inorganic species interfaces. Monnier and Huo et al.81,84 gave a formula of the free energy in the whole process:

∆G ) ∆Ginter + ∆Gwall + ∆Gintra + ∆Gsol in which ∆Ginter is the energy associated with the interaction between inorganic walls and surfactant micelles, ∆Gwall is the structural free energy for the inorganic frameworks,


Wan and Zhao

Figure 4. Classical commercial nonionic surfactants.

∆Gintra is the van der Waals force and conformational energy of the surfactant, and ∆Gsol is the chemical potential associated with the species in solution phase. For the surfactant-templating assembly of mesostructured silicates, ∆Gsol can be regarded as a constant in a given solution system. Therefore, the key factor is the interaction between surfactant and inorganic species, such as the matching of charge density. The more negative ∆Ginter is, the more easily the assembly process can proceed. Elaborate investigations on mesoporous materials have been focused on understanding and utilizing the inorganicorganic interactions.8,35,51 Table 1 lists the main synthesis routes and the corresponding surfactants and classical products. Stucky and co-workers proposed four general synthetic routes, which are S+I-, S-I+, S+X-I+, and S-X+I- (S+ ) surfactant cations, S- ) surfactant anions, I+ ) inorganic precursor cations, I- ) inorganic precursor anions, X+ ) cationic counterions, and X- ) anionic counterions).8,84 To yield mesoporous materials, it is important to adjust the chemistry of the surfactant headgroups, which can fit the requirement of the inorganic components. Under basic conditions, silicate anions (I-) match with surfactant cations

(S+) through Coulomb forces (S+I-). The assembly of polyacid anions and surfactant cations to “salt”-like mesostructures also belongs to S+I- interaction. In contrast to this, one of the examples of S-I+ interaction occurs between cationic Keggin ion (Al137+) and anionic surfactants like dodecyl benzenesulfonate salt. The organic-inorganic assembly of surfactants and inorganic precursors with the same charge is also possible. However, counterions are necessary. For example, in the syntheses of mesoporous silicates by the S+X-I+ interaction, S+ and I+ are cationic surfactants and precursors, and Xcan be halogen ions (Cl-, Br- and I-), SO42-, NO3-, etc. In a strongly acidic medium, the initial S+X-I+ interaction through Coulomb forces or more exactly, double-layer hydrogen bonding interaction, gradually transforms to the (IX)-S+ one. It was the first time that mesoporous silica was synthesized under a strongly acidic condition. Here anions affect the structures, regularity, morphologies, thermal stability, and porosities of mesoporous silicas. The Hofmeister series of the anions are one of the possible reasons that change the hydrolysis rates of the silicate precursors and the micellar structures.42,110



Table 1. Synthesis Routes to Mesoporous Materials with the Emphasis on Silicates route

interactions

symbols

conditions

I

electrostatic Coulomb force

S , cationic surfactants (frequently used cationic surfactants are shown in Figure 2); I-, anionic silicate species

basic

S-I+

electrostatic Coulomb force

aqueous

S+X-I+

electrostatic Coulomb force, double layer H bond electrostatic Coulomb force

S-, anionic surfactants, CnH2n+1COOH, CnH2n+1SO3H, CnH2n+1OSO2H, CnH2n+1OPO2H; I+, transition metal ions, such as Al3+ S+, cationic surfactants (Figure 2); I+, silicate species; X-, Cl-, Br-, I-, SO42-, NO3-

acidic

SBA-1,8 SBA-2,95 SBA-392

S-, anionic surfactants (lab-made) (Figure 3); N+, cationic amino group of TMAPS or APS; I-, anionic silicate species S-, anionic phosphate surfactants CnH2n+1COOH, CnH2n+1SO3H, CnH2n+1OSO2H, CnH2n+1OPO2H; I-, transition metal ions, WO42-, Mo2O7-; X+, Na+, K+, Cr3+, Ni2+, etc. S0, nonionic surfactants, oligomeric alkyl PEO surfactants, and triblock copolymers; N0, organic amines, CnH2n+1NH2, H2NCnH2n+1NH2; I0, silicate species, aluminate species S0, nonionic surfactants (Figure 3); I+, silicate species; X-, Cl-, Br-, I-, SO42-, NO3-

basic

AMS-n76,96-100

basic

W, Mo oxides8,51

neutral

HMS, MSU, disordered worm-like mesoporous silicates101,102

acidic, pH < ∼2 acidic

SBA-n (n ) 11, 12, 15, and 16),31,78 FDU-n (n ) 1, 5, and 12),103-105 KIT-n (n ) 5 and 6)106,107 Nb, Ta oxides8,51

basic

mesoporous silica108,109

S-N+-I-

S-X+I-

electrostatic Coulomb force, double layer H bond

S0I0 (N0I0)

H bond

S0H+X-I+

electrostatic Coulomb force, double layer H bond coordination bond covalent bond

N0‚‚‚I+ S+-I-

+

classical products

S+ -

N0, organic amines; I+, transition metal (Nb, Ta) S+, cationic surfactants containing silicate, e.g., C16H33N(CH3)2OSi(OC2H5)3Br; I-, silicate species

Figure 5. Schematic illustration of the two types of interactions between APS (A) or TMAPS (B) and anionic surfactant headgroups. Reprinted with permission from ref 76. Copyright 2003 Nature Publishing Group.

Compared with that of the cationic surfactants, the repulsive interaction between anionic surfactants and silicate species fails to organize ordered mesostructures. Concerning the charge matching effect, Che et al.76 demonstrated a synthetic route to create a family of mesoporous silica structures (AMS-n) under basic conditions by employing anionic surfactants (Figure 3) as SDAs and APS or TMAPS as CSDAs. This route can be described as an “S-N+-I-” pathway, where N+ are cationic amino groups of organoalkoxysilanes. Figure 5 gives the schematic illustration of interactions between amino groups and anionic surfactant headgroups. The negatively charged headgroups of the anionic surfactants interact with the positively charged ammonium sites of APS or TMAPS electrostatically through

MCM-41,6 MCM-48,6 MCM-50,6 SBA-6,73 SBA-2,92 SBA-8,93 FDU-2,94 FDU-11,74 FDU-13,74 etc. mesoporous alumina, etc.8

neutralization. The most efficient surfactant is possibly terminal carboxylic acid.99 The co-condensation of tetraethoxysilane (TEOS) with APS or TMAPS and assembly with surfactants occur to form the silica framework.98 Hydrogen-bonding interaction mechanisms, namely, S0I0 or N0I0, were proposed by Pinnavaia and co-workers for preparing mesoporous silicates under neutral conditions.101,102 S0 are neutral amines, N0 are nonionic surfactants, and I0 are hydrated silicate oligomers from TEOS. It should be noted that amines and PEO-derived molecules are different. Organic long-chain alkyl amines, such as primary alkylamines (dodecylamine, hexadecylamine, etc.) and N,Ndimethylalkylamines (N,N-dimethyldodecylamine, N,Ndimethylhexadecylamine, etc.) have hydrophobic hydrocarbon chains and hydrophilic amine groups, similar to surfactants.102 However, ethanol has to be added in the synthesis batch for mesoporous silicas due to the insolubility of these amines.102,111 Only disordered worm-like mesoporous silicas were obtained. Silicate oligomers are negatively charged in neutral solution. Neutral amines (N0) and nonionic surfactants (S0) are probably partially protonated or charged. Later on, the synthesis of mesoporous silica SBA-15 was carried out under a strongly acidic conditions by using triblock copolymer P123 as a template. It is more likely a double-layer hydrogenbonding S0H+X-I+ interaction.31,78 The interaction between organic and inorganic species can also be coordination bonds. For example, mesostructured niobium oxides can be prepared from niobium ethoxide and long-chain alkylamines in nonaqueous systems.51


Another organic-inorganic interaction can be through covalent bonds. If surfactants containing hydrolyzable silane groups are used, they can react with either each other or other silicate species and thus form mesoporous materials.108,109 A unique precursor can assemble to highly ordered mesostructure without any templates. It consists of an oligosiloxane segment acting as both the hydrophobic head and the cross-linking unit and a long carbon chain performing as the “surfactant”. Besides the direct interactions between inorganic species and surfactants, other interactions, such as the crucial surfaces between silicate species and water adsorbed on the micellar surfaces may also be somewhat important to the final mesostructures.112 In addition, Wong and Ying reported that the reactive oxygen atoms in the anionic phosphate surfactants could covalently interact with zirconium, resulting in mesoporous zirconia.113 Inorganic oligomers or nanoparticles (4 M) is, however, not recommended, because low-quality products are often obtained. In contrast, using low acidic concentration (0.5 M HCl) and n-butanol additive slows the condensation rate of the inorganics and can easily yield a highly ordered 3D cubic SBA-16 mesostructure.134 Figure 7A exhibits the relatively large synthesis range of SBA-16. In this medium, bicontinuous cubic and face-centered cubic mesostructures have also been synthesized by using different triblock copolymers (Figure 7B).106,107,135 It is claimed that the low concentration of acidic catalyst favors the slow condensation kinetics of the inorganics. Besides HCl, strong acids such as HNO3, HBr, HI, and H2SO4 can be used as catalysts. Seldom cases adopt weak acids, like H3PO4 and acetic acid (HAc), due to the low-quality products. It is unknown whether the products contain phosphor in H3PO4-containing systems. The precipitation of mesoporous silicates is extremely slow in the pH value range from 1 to 2, probably because this is around the isoelectric point of silica. On account of this character, Prouzet and co-workers designed a method to investigate the formation mechanism of mesostructures templated by nonionic surfactants. The stable prehydrolyzed-silicate sols in the solutions at pH ) 2 make the detailed analysis possible. The main components, soluble silicate oligomers and nonionic surfactants micelles, do show interactions at this stage.57,136 (2) Controllable morphology. The acidic synthesis is suitable for the formation of mesoporous silicates with diverse morphologies, such as “single crystals”, thin films, fibers, spheres, etc.40 It may be related to the sol-gel chemistry of silicates. Linear silicate oligomers are the main products from the hydrolysis of silicates under acidic conditions that favor various regular morphologies. Base catalysis leads to a fast polymerization and condensation of silicates, yielding 3D silicate networks. The morphology is sometimes difficult to control. Spherical particles are the most common product. (3) Irreversible reaction. The irreversible polymerization of silicate species will lead to failure of the synthesis once



Figure 7. (A) Diagram of mesophase structures established according to the XRD measurements. Each sample is prepared with a molar ratio of 0.0035 F127/x TEOS/y BuOH/0.91 HCl/117 H2O. Reprinted with permission from ref 134. Copyright 2006 American Chemical Society. (B) Synthesis space diagram of mesophase structures established according to the XRD measurements. Each sample is prepared with a molar ratio of 0.017 P123/x TEOS/y BuOH/1.83 HCl/195 H2O. Reprinted with permission from ref 135. Copyright 2005 American Chemical Society.

the gel forms. In contrast, the hydrolysis of silicates is reversible under basic conditions. Ordered mesostructures can be synthesized even if a gel appears. (4) Simple silica source. Siliceous oligomers and monomers are suitable precursors owing to the irreversible polymerization of silicates under acidic conditions. TEOS is the optimal choice. Sodium metasilicate (Na2SiO3) that can generate small siliceous oligomers in a fast acidification process can also be used as a precursor.66,137,138 The simultaneously generated sodium salts facilitate the formation of mesostructures. (5) Low processing temperature. The synthesis of mesoporous silica (SBA-3) is carried out at room temperature by using cationic surfactant as a SDA. Heating or hydrothermal treatment is not adopted. (6) Only few examples for phase transformation. A surfactant generally templates one mesostructure; for example, CTAB yields the 2D hexagonal mesostructure SBA-3 and C16TEABr gives the 3D simple cubic mesostructure SBA-1. Compared with basic synthesis, it is much more difficult to change the mesostructure by simply adjusting concentration, temperature, etc. Only few examples for transformations are observed. For example, in the synthesis of 3D SBA-12 with mixed hexagonal close packing (hcp) and cubic close packing (ccp) phases, a hydrothermal treatment at 100 °C for 3 days can transform it to the 2D hexagonal mesostructure. (7) There is no need for a washing step. (8) The addition of inorganic salts like KCl, NaCl, Na2SO4 and K2SO4 can accelerate and improve the synthesis while organic solvent additives may reduce the formation rate in acidic nonionic surfactant systems.40,139,140 With the addition of inorganic salts, Yu et al.139,140 found that high-quality SBA-15 can be synthesized even at low temperature (∼ 10 °C) and low triblock copolymer P123 concentration.

3.1.3. Synthesis (Reaction) Temperature In the solution synthesis, the reaction temperature is relatively low, ranging from -10 to 130 °C. The most convenient temperature is room temperature. Two factors can be referred to, CMT and cloud-point (CP), to select the temperature. The temperature is normally higher than the CMT values of the surfactants. The CMT values are relatively low for cationic surfactants. In the cationic surfactant templating cases, the assembly rate of the templating surfactants slows down with the decrease of the synthetic temperature that facilitates the preparation of highquality mesoporous silicates. Heating is unnecessary and room-temperature synthesis is feasible. When nonionic surfactants are used as templates, the reaction temperature is normally higher than room temperature because of their higher CMT values. But also a low-temperature strategy has been developed to synthesize highly ordered mesoporous silicates FDU-12 with exceptionally large pore sizes (Figure 8).141 Figure 8A exhibits well-resolved SAXS patterns of FDU-12s, and Figure 8B shows the unit cell parameters of FDU-12s as a function of reaction temperature. The unit cell and the pore size can be as large as 44 and 27 nm, respectively. It should be noted that the synthesis is assisted by 1,3,5-trimethylbenzene (TMB) and inorganic salts, like KCl. In a relatively low-temperature region (Figure 8C, process II), the smaller association number and less tight aggregation of surfactant micelles lead to a weaker hindrance of the hydrophilic PEO moiety to the penetration of TMB. This can, in turn, cause a high content of the TMB swelling agent and a subsequent pore size expansion. Many nonionic surfactants have the problem that they become insoluble in water (CP value) at elevated temperature. All of a sudden the solutions become cloudy due to phase separation and the surfactants begin to precipitate. So the synthesis temperature must be lower than the CP value of a surfactant.


Figure 8. (A) SAXS traces of mesoporous silica LP-FDU-12 samples (S-x-y represents the FDU-12 sample synthesized at x °C, followed by hydrothermal treatment at y °C. (B) The dependence of lattice parameter of mesoporous silica FDU-12 on synthesis temperature. (C) Model illustrating the behavior of TMB penetration into block copolymers at different synthesis temperatures. Deep blue and pale blue represents that the association number is relatively larger and lower or the aggregation of PEO in micelles is tight and loose in the process I and II, respectively. Reprinted with permission from ref 141. Copyright 2005 American Chemical Society.

A common idea is to decrease the synthetic temperature, which reduces the reaction rate and thereby improves the crystalline regularity. In the synthesis of SBA-15 templated by triblock copolymer P123, the optimal synthetic temperature is 35-40 °C, due to the solubility limit and the CMT value for the formation of micelles.78,142 The reaction temperature is high when using block copolymers with high CMT and CP values. It is found that ordered mesoporous silicates can only be obtained at the temperature higher than 90 °C in the triblock copolymer P85 (EO26PO39EO20) and P65 (EO20PO30EO20) systems, both of which have a CP value of 82 °C in water.143 This phenomenon can be explained by the fact that the CP values of surfactants may be influenced by the solvents. In the presence of 2 M HCl and ethanol, the CP values of P85 and P65 are higher than 95 °C. Because the hydrolysis of TEOS generates ethanol, the reaction temperature in the acidic P85 and P65 systems is a little higher than their CP values in water.143 The nature of the inorganic precursor is not a decisive factor that would restrict the synthetic temperature. However, high temperature is adopted in the cases of inorganic precursors with high polymerization degrees. When silica aerogel or water glass is used as silica precursor, a high temperature of 100 °C favors the rehydrolysis, cross-linking, and “uniform distribution” of silicate species, the assembly of mesostructures, and the formation of ordered stable mesoporous silicates.

3.1.4. Hydrothermal Treatment Hydrothermal treatment is one of the most efficient methods to improve mesoscopic regularity of products.56,92 After the solution reaction, the mesostructures undergo reorganization, growth, and crystallization during hydrothermal treatment. The treating temperature is relatively low,

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between 80 and 150 °C, in which the range of 95-100 °C is mostly used. High temperature would result in the degradation of ordering and the decomposition of surfactants, which may direct the formation of microporous materials (for cationic surfactants), for example, ZSM-5 and silicalite1.144 Only a few instances adopt a temperature higher than 150 °C, which are based on high-temperature stable surfactant micelles including fluorocarbon-hydrocarbon surfactant mixtures and cationic phase transfer catalysts.32 In general, the hydrothermal temperature is higher when cationic quaternary ammonium salts are used as templates than in case of nonionic surfactants. This phenomenon may be related to the ordered microdomains of the surfactants and the interactions between surfactants and silica species. Cationic surfactants (S+) have comparatively strong Coulomb interactions with electronegative silicate species (I-). The hydrothermal temperature can be higher than in the case of nonionic surfactants, which have weak double-layer hydrogenbond interactions with silicate species. Because the mesostructures have assembled before the hydrothermal treatment and the regularity is improved during this process, a long treatment is necessary, ranging from days to weeks. The range of 1-7 days is efficient. When microwave is involved in this step, hydrothermal treatment time can be shortened to 2 h or even shorter.145,146 The hydrolysis and cross-linkage of inorganic species and assembly further proceed during this step. High alkaline conditions where MCM-41 is formed with a low degree of polymerization allow the phase transition from MCM-41 to MCM-48 during the hydrothermal treatment. It is the loosely condensed silicate species that facilitate the formation of cubic bicontinuous Ia3hd phase through ongoing silica polymerization and enhanced cross-linking.147 2D hexagonal MCM-41 materials are the usual products in basic CTAB surfactant systems at room temperature. A direct hydrothermal treatment of the mother liquor at 110 °C for 3 days can cause the mesophase transformation to 3D cubic bicontinuous MCM-48.148 It is the easiest way to synthesize MCM-48 when using a low amount of surfactants. Prolonging hydrothermal time at a certain temperature (e.g., 135 or 140 °C) causes the similar continuous phase transformation from MCM-41 to MCM-48 and to layered mesostructure.149,150 The adsorptive and structural properties of mesoporous silicates can also be tailored to some degree, by varying hydrothermal treatment time and temperature, which will be discussed in the section on pore size control.

3.1.5. Separation and Drying Separated from the mother liquor, as-synthesized mesostructured materials can be obtained after washing and drying. Mesoporous materials with good crystallinity normally have large particles, about 0.1 mm, which are easily filtrated. Centrifugation can sometimes be helpful. Water is used in the washing step. Alcohol can also be added. In the case of mesoporous silicates synthesized under basic conditions, sufficient washing to neutrality is necessary to avoid the effect, or more seriously the destruction, of the mesostructure upon calcination by the residual NaOH. However, the washing step can be skipped in the acidic synthesis because volatile HCl does not affect the quality of the products and can be totally removed together with surfactants upon calcination.151 In addition, the mesostructures are formed at low temperature (room temperature) under acidic conditions. The washing step may cause the destruction of


partially cross-linking frameworks. The drying process for as-synthesized mesostructured materials is usually carried out at room temperature. Heating may reduce the mesoscopic regularity to some extent. However, no systematic studies have been carried out on the effect of this step. According to sol-gel chemistry, wet silica gels prepared under acidic conditions have many unreacted silanols, which can further condense upon aging.152 It is currently believed that partially condensed silica species can further cross-link during the drying process.

3.1.6. Removal of Template The porosity can only be obtained after the removal of templates from as-synthesized inorganic-organic composites. Different removal methods certainly influence the characteristics of mesoporous materials. The most common method to remove templates is calcination owing to the easy operation and complete elimination. Organic surfactants can be totally decomposed or oxidized under oxygen or air atmosphere. This method is mostly applied into the cases of mesoporous silicates, aluminosilicates, metal oxides, and phosphates. The temperature programming rate should be low enough to prevent the structural collapse caused by local overheating. A two-step calcination was adopted by Mobil’s scientists, the first 1 h under nitrogen to decompose surfactants and the following 5 h in air or oxygen to burn them out.5 This complicated procedure was then simplified. The first calcination step under nitrogen can be substituted by heating in air with a low rate. Heating the as-synthesized SBA-15 materials with a rate of 1-2 °C/min to 550 °C and keeping this temperature for 4-6 h can completely remove triblock copolymer templates. Calcination temperature should be lower than the stable temperature for mesoporous materials and higher than 350 °C to totally remove PEO-PPO-PEO type surfactants or 550 °C for long-chain alkyl surfactants. Higher calcination temperature would lead to lower surface areas, pore volumes, and surface hydroxyl groups and higher cross-linking degrees of mesoporous materials. But these materials possess higher hydrothermal stability due to the higher cross-linking degrees.153 During the calcination, cationic cetyltrimethylammonium surfactants undergo several decomposition steps.154 The majority of the surfactants first break down to form hexadecene and a trimethylamine species in the temperature range of 100 to 220 °C. At the same time, a small quantity of surfactant molecules, which are more strongly bonded to the inorganic hosts, decomposes between 195 and 220 °C, via the mechanism analogous to the thermal decomposition of the pure surfactant under the same experimental conditions. Several decomposition residues remain within the inorganic hosts, and the surface is notably hydrophobic. Upon continuous heating at high temperatures (to 550 °C), the carbon chain fragments are eliminated and the surface available becomes hydrophilic. The drawbacks of calcination are the unrecovery of surfactants and the sacrifice of surface hydroxyl groups. Moreover, it is unsuitable for thermally unstable and air-sensitive materials, such as sulfides and organic frameworks. Extraction is a mild and efficient method to remove surfactants and get porosities without distinct effects on frameworks.151 Ethanol or THF can be used as an organic extracting agent. A small amount of hydrochloric acid is added in the extracting agent to improve the cross-linkage of frameworks and to minimize the effects on mesostruc-


tures.155 More than 95% triblock copolymer P123 from assynthesized SBA-15 can be extracted and recovered to reuse.78 SBA-15 materials templated by the recovered P123 have good quality, indicating a minor destruction of triblock copolymer surfactants during the solvent extraction. With the aid of sulfuric acid, triblock copolymers in SBA-15 and SBA-16 mesostructures can be removed.156-158 Tailored pore channels and structures can then be achieved. Figure 9 shows that the mesopores and micropores of SBA-15 can be vacated step by step, mesopores by the partial decomposition of templates via ether cleavage by sulfuric acid (Figure 9A(i)) and micropores by the combustion of triblock copolymers inserting into silicate walls in air at 200 °C (Figure 9A(ii)). 13 C cross polarization/magic angle spinning (CP/MAS) NMR spectra (Figure 9B) were used to monitor this process and confirmed the above conclusion. Microporous volumes are larger than those of the materials from traditional hightemperature calcination (Figure 9C,D). Kawi and co-workers reported an extraction method to remove surfactants using SC CO2 as an extractant.159 Supercritical fluids have better solubility than normal solvents and therefore are more efficient. Both the removal and recovery of the surfactants exceed 90%. The mild conditions exhibit minor effects on the mesostructures, and the products are highly ordered. These characteristics, together with the good solubility and fluidity, show that SC fluid extraction is an ideal method to remove the surfactants in most mesostructures with diverse components and morphologies. Compared with calcination, extraction can get silicate materials with larger pore sizes in some cases. Much more surface hydroxyl groups can also be kept, enhancing the hydrophilic property and modifying the reactive ability of pore channels.160 However, the application of extraction is limited by the fact that surfactants cannot be completely (100%) removed. Gallis et al.161 utilized microwave irradiation to remove templates. Microwave-sensitive materials such as activated carbon can generate instantaneous high temperature, which facilitates the entire elimination of surfactants in a very short period ranging from 10 to 30 min. It normally takes about 10 h upon calcination. A moderate heating power is required. Over-high power would decrease the regularity of mesoporous materials and result in carbon deposition. Surfactants can also be removed by irradiation using a high-energy ultraviolet lamp.162 Ultraviolet irradiation can break C-C bonds in organic surfactants and decompose them. Simultaneously, strong oxidants ozone (O3) and oxygen atom (O), which are generated by ultraviolet excitation, can further oxidize the organic species to remove the surfactants. In comparison with calcination, this method can be carried out at room temperature and produce products with better ordering. But the operation is time-consuming and ineffective, which is unsuitable for the large-scale production. Tian et al.163 first adopted microwave digestion (MWD) in the removal of surfactants. This is achieved for the example of SBA-15 by placing the as-synthesized SBA-15 and an appropriate amount of HNO3 and H2O2 in a reactor. The instantaneous high temperature (∼200 °C) and pressure (∼1.3 MPa) generated by microwave radiation facilitates the oxidation of surfactants by HNO3 and H2O2. Surfactants in the mesopores can be totally eliminated. It is fairly quick, only taking 3-10 min. In contract to calcination, the MWD technique is facile and effective to remove surfactants without any sacrifice of the silanols on the silicate pore walls and without distinct framework shrinkage. It can be expected that


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Figure 9. (A) Schematic representation of the stepwise generation of mesopores and micropores in SBA-15 by treatment with concentrated H2SO4 at 95 °C (i) and subsequent calcination at 200 °C (ii). (B) 13C CP/MAS NMR spectra of as-synthesized SBA-15 (a) and of the same sample after treatment with 60 wt % H2SO4 (b) and subsequent calcination at 200 °C (c). (C) N2 sorption isotherms and (D) t-plots of Ar sorption at 87.29 K for calcined SBA-15 (a), 48 wt % H2SO4-treated SBA-15 (b), and the same sample after calcination at 200 °C (c). Reprinted with permission from ref 156. Copyright 2003 American Chemical Society.

mesoporous silica with large pore volumes, high surface areas, and, most important of all, abundant silanols serve as ideal hosts for nanocasting replica mesostructures. Highly ordered metal oxide nanowire arrays can be replicated with various compositions including Cr2O3, MnxOy, Fe2O3, Co3O4, NiO, CeO2, and In2O3.164 This method can be widely applied in porous materials, for example, zeolites and macroporous silicates templated by polystyrene (PS) nanospheres, but except for those that are either easily oxidized or sensitive to acids, such as organic-containing frameworks and mesoporous titania.

3.2. Nonaqueous Synthesis Nonaqueous synthesis is a very convenient method to prepare ordered mesoporous materials especially for mesoporous thin films, membranes, monoliths, and spheres. This method has become more and more powerful. Most of the syntheses conducted in the nonaqueous media adopt the wellknown EISA process.33 For the preparation of mesostructured silica films, TEOS is dissolved in the organic solvent (normally ethanol, THF, and acetonitrile) and prehydrolyzed with stoichiometric quantities of water (catalyzed by acids, such as HCl) at a temperature of 25-70 °C. Then lowpolymerized silicate species can randomly assemble with surfactants. Upon solvent evaporation, the silicate species further polymerize and condense around the surfactants. The polymerization rate is gradually increased due to the increas-

ing acid concentration during the solvent evaporation. Simultaneously, templating assembly in the concentrated surfactant solution occurs, resulting in the formation of ordered mesostructures. The process is very fast and needs only several seconds.120 Mostly solvents with weak polarity are used. Surfactants lose the hydrophilic/hydrophobic properties in the weakpolarity solvents because both hydrophilic and hydrophobic segments can interact with these solvents. The surfactant selfassembly would be inhibited. However, the assembly can be induced upon the solvent evaporation. Nonpolar and oily solvents are seldom adopted. In toluene or xylene solution, silica nanowires with adjustable diameters were synthesized with P123 and F127 (EO106PO70EO106) by the EISA approach.165 The formation of this kind of arrays corresponds to the reversed mesophases of surfactants in oily solvents. Hollow sphere silicates can also be obtained by tuning the ratios of oil/water.166 The synthetic conditions are, however, quite strict. In addition, the possible products include silica mesostructures, reversed mesostructures, and their mixtures because a little water (sometimes from wet air) incorporates in the process upon the evaporation of the oily solvent and the reversed micelles turn back. Relatively wide diffraction peaks at 2θ of 3-5° are detected in the XRD patterns of the SBA-15 samples prepared by using P123 as a template from the EISA method. Apparently, the mesostructure regularity is quite low. TEM


measurements reveal, however, large domain ordering. One can see ordered arrays everywhere. The lack of XRD diffraction peaks can be attributed to the extremely fast formation rate of the mesostructure that causes nonuniform micelles. The nonuniformity is difficult to observe by TEM images due to the minor discrepancy. However, this is magnified by the powder X-ray diffraction. At the same time, the samples normally require certain substrates for controlled deposition, which imposes a strain field, generating a uniaxial lattice distortion. This distortion lowers the mesostructure symmetry that can be qualitatively and quantitatively analyzed.35,63,167,168 This phenomenon is also observed in the example of largepore 3D cubic mesoporous silicates (FDU-5) synthesized under acidic conditions by using triblock copolymer P123 as template and a small amount of organic agents such as 3-mercapropropyltrimethoxysilane (MPTMS) and TMB as additives via the EISA strategy.104 The resultant materials have highly ordered bicontinuous cubic Ia3hd symmetry that is analogous to the structure of MCM-48 materials prepared in the basic cationic surfactant-templating systems. The XRD pattern of FDU-5 is not as well-resolved as MCM-48 (Figure 10A). However, TEM images show large domains of ordered 3D bicontinuous mesostructure (Figure 10B,C). Mesoporous materials prepared by the EISA strategy generally have lower surface areas than those from the hydrothermal method. This has not been fully understood up to now. But one of the reasons may be the lack of microporosity, which is possibly due to either a lower extent of inclusion of PEO segments into the inorganic frameworks or the retraction of PEO chains under the present selfassembly conditions of the EISA process.168-173 The nonaqueous solvents may screen the charge coupling or other interactions between the inorganic species and the hydrophilic corona. This fact may also lead to dense inorganic frameworks. Template-silica interpenetration has been clearly observed by NMR in the fabrication of organized silica monoliths by slow EISA. When the starting concentrations of the block copolymers exceed 50%, the obtained mesostructures are ordered. This can be attributed to the increasing degree of microphase separation, as well as the reduced interfacial contact between the PPO and PEO blocks and the rigid matrix.169-171 SBA-15 mesoporous silica synthesized via EISA has much larger pore size (9.0 nm) than that (4.6 nm) from hydrothermal synthesis under similar conditions. EISA does not require stringent selection of SDAs. For example, it is very difficult to use the triblock copolymers F108 (EO132PO50EO132) and F98 (EO123PO47EO123) for the formation of mesostructures under aqueous conditions, while they are good templates in the EISA process. It is also confirmed that EISA is an easy and convenient strategy to get 3D cubic silica mesophase (Im3hm). By using block copolymers with large PEO segments, for example, F127, F108, or F98, or mixed surfactants, cubic SBA-16 mesostructure can be easily obtained.120

3.3. Postsynthesis Treatment 3.3.1. Secondary Synthesis To improve thermal stability of mesoporous silicates, secondary synthesis is generally used to increase the pore wall thickness or enhance the local ordering, which can be achieved by either grafting or hydrothermal treatment. After


Figure 10. (A) XRD patterns for as-synthesized (a), ethanolextracted (b), and calcined (c) mesoporous FDU-5. (B, C) TEM images for calcined FDU-5 recorded along the [100] and [111] directions, respectively. Insets are corresponding Fourier diffractograms. FDU-5 samples were synthesized at room temperature under acidic conditions with P123 as a template and MPTMS as an additive. Reprinted with permission from ref 104. Copyright 2002 Wiley.

MCM-41 materials are treated by AlCl3 vapor or react with AlCl3 solution, they exhibit better mechanical and hydrothermal stability than the parent materials. This is related to the increase of pore wall thickness and cross-linking degree and the reparation of defects in frameworks.174 Immersing mesoporous silica thin films in ethanol vapor before the air drying process promotes structural regularity.175 Postsynthesis treatment by ammonia gas is beneficial for the thermal stability of mesoporous silicate thin films, perhaps due to


the improvement of the cross-linkage of silica networks under alkaline conditions.176 The improvement of stability or the expansion of pores of mesoporous silica solids is distinct after the secondary hydrothermal treatment in the presence of ammonia or N, N-dimethyldecylamine aqueous solution.177,178 TEOS can also be utilized in the secondary synthesis. It should be noted that this kind of secondary synthesis is more effective for as-synthesized materials than calcined ones. Mokaya and co-workers reported a method to restructure calcined mesoporous silica MCM-41 by utilizing it as the “silica source” in a synthesis gel consisting CTAB, TMAOH, and water.179 After the secondary hydrothermal treatment, MCM-41 material exhibits extremely high hydrothermal stability, which is ascribed to the improvement of condensation degrees of silicates.

3.3.2. Recrystallization Recrystallization is a very efficient method to improve the regularity of mesoporous materials. However, only a few research groups realize this method, which is easily confused with the hydrothermal treatment. In fact, both processes are largely different. Recrystallization is a procedure in which as-synthesized powder samples without washing are placed into deionized water at 100-150 °C for several days (sometimes even 1 week). The quality (ordering, thermal stability, etc.) can be improved for most materials, sometimes accompanied with the enlargement of pore sizes.92,180 This process is quite complicated. Dissolution and crystallization of silicate species and reorganization of mesostructures may take place. In comparison with the hydrothermal treatment, the reorganization rate in this process may be slower and more localized by reason of separated surfactants and unreacted silicate species. For the recrystallization, unwashed samples are favorable, because residues of acid or base catalysts, silicate oligomers, and surfactants would facilitate the reorganization of mesostructures. Huo et al.92 recrystallized mesoporous silica MCM-41 from a basic CTAB surfactant system and found that it had more than seven XRD diffraction peaks. If mesoporous silica thick membranes templated by P123 from the EISA approach are recrystallized at 100 °C for 3 days, the mesostructure regularity is much improved as evidenced by at least three well-resolved XRD peaks. The resultant product has a highly ordered 2D hexagonal mesostructure with much larger surface area (840 m2/g), pore size (9.0 nm), and pore volume (1.12 cm3/g). The employment of nonpolar solvents (such as hexane) instead of water as the heating media plays a decisive role in the phase transformation.168 A postsolvothermal procedure induced a phase transition from 2D hexagonal to 3D bicontinuous cubic FDU-5 mesostructure. The beginning materials are the mesostructured silica membranes prepared by the EISA method. The main reasons for the phase transition are the low polymerization degrees of inorganic silicate frameworks, the relatively high contents of organic templates, and the flexible nature of hybrid matrices.

4. Controllable Synthesis on Mesoscale The controllable synthesis on mesoscale mainly includes mesophases (interface curvatures, connecting manners, and symmetries of mesochannels) and pore sizes.

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4.1. Mesophase Tailoring The derived mesostructures are remarkably influenced by the rational control of organic-inorganic interactions and cooperative assembly of silica species and surfactants. Therefore, final mesostructures are dependent on the surfactant liquid-crystal phases or silica-surfactant liquidcrystal-like phases. The decisive factors on mesophases are liquid-crystal phase diagram, which can be drawn as a function of surfactant concentration and temperature, and packing parameter (g value) or hydrophilic/hydrophobic volume ratio (VH/VL) of template molecule.181

4.1.1. Micellar Mesostructure 4.1.1.1. Critical Micelle Concentration. A surfactant with low critical micelle concentration (CMC) value is an important criterion toward increasing the regularity of mesostructure.182 Ordered mesostructures are always obtained if the CMC values of surfactants are between 0 and 20 mg/ L. Strategies can be used to decrease the CMC values to yield ordered mesostructures when surfactants have CMC values between 20 and 300 mg/L. Surfactants with large CMC values generally give cubic mesostructures. If the CMC values further increase, it is difficult to produce ordered mesostructures.182 4.1.1.2. The Packing Parameter. The packing parameters of ionic surfactants are widely used in predicting and explaining the final mesostructures.92 The calculation of g value is simple but of great significance and guidance: g ) V/(a0l). Here, V is the total volume of surfactant hydrophobic chains plus any cosolvent (organic molecules) between the chains, a0 is the effective hydrophilic headgroup area at the aqueous-micelle surface, and l is the kinetic surfactant tail length. The expected mesophase sequence as a function of g value is cubic (Pm3hn, etc.) and 3D hexagonal (P63/mmc) with g < 1/3, 2D hexagonal (p6mm) with 1/3 < g < 1/2, cubic (Ia3hd) with 1/2 < g < 2/3, and lamellar with g ≈ 1. Table 2 gives examples for mesostructures from cationic surfactants with different g values. Many facts can be explained by g values, such as the effects of inorganic and organic additives, phase transformation, etc. Some successful predictions can be made for the packing model and mesostructure in a hydrothermal synthesis. (1) The headgroup repulsion in the ionic surfactant is decreased by adding inorganic salts. This results in a decrease of the effective headgroup area at the aggregate interface, and hence, an increase of the g value. The resultant 3D cubic mesostructure transforms to the 2D hexagonal mesostructure. Different inorganic salts show effects on ionic surfactants, depending on radii of hydrated anions and cations. This is great for small hydrated ions. The micellization and selfassembly of cationic surfactants accord to the Hofmeister series of anions.110 For example, under an acidic synthesis process, an order for increasing the packing parameter of C16TEABr surfactant was found as NO3- > Cl- > SO42-, inducing the mesophase transformation from a higher to a lower curvature.183 However, anions affect nonionic surfactants in intricate ways. Some other factors, like the solubility of nonionic surfactants containing ether groups, which decreases upon dehydration, should be considered. Tang et al.184 found a different order, neither the Hofmeister series nor the reverse, which is SO42- (HSO4-) > NO3- > Cl-, in an acidic solution to cause p6mm to transform to Ia3hd mesostructure when triblock copolymer P123 is employed



Table 2. Relationship between the Packing Parameter of Cationic Surfactant and Mesostructure g ) V/a0l f 1 reversed micelles double-chain surfactants with small groups

MCM-50 (lamellar structure)

Acidic Synthesis

as a template. This phenomenon was attributed to the balance between the dehydration and the radii effects. (2) When the degrees of ionization get larger, the g values of anionic surfactants (for example, acylglutamate) vary. Liquid crystalline phases of anionic surfactants change from lamellar to cubic and to hexagonal phases.185 The ionization of the surfactant can be readily controlled by the pH value of the solution. Therefore, several mesostructures can be obtained in an anionic surfactant system by precisely controlling the alkalinity of the solution.186 (3) The enlargement of hydrophilic heads results in decreased g values and, hence, the formation of spherical mesostructures with high curvatures. For example, cationic surfactants CnH2n+1N(CH3)3Br (n ) 10-18) generally induce the formation of 2D hexagonal mesostructure, while a 3D cage mesostructure, for example, SBA-1, is synthesized by using CnH2n+1N(CH2CH3)3Br as a SDA. (4) If two surfactants have the same headgroup, namely, the same equilibrium area a0, but different (single and double) tails, the one with double tails has a g value twice that of the one with a single tail. Bilayers instead of spherical or globular micelles are formed. In fact, Gemini surfactant C16-2-16 favors the formation of the lamellar mesostructure. (5) Each hydrophilic headgroup in Gemini surfactant (Cm-s-m) is linked by a hydrocarbon chain. The hydrophilic area can therefore be adjusted by the length of the hydrocarbon chain. For example, when s ranges from 2 to 12 in the Gemini surfactant C16-s-16, the templated product changes from lamellar to 2D hexagonal and to cubic bicontinuous mesostructures under basic conditions. (6) Organic additives also play a role in the properties of surfactant micelles. The impurities that can dissolve in micelles alter the surface energy. The formation of micelles are thus either improved or inhibited. In some cases, lyophilic organics can be solubilized within the micelles, and the CMC values of the surfactants are reduced. Small molecules are preferentially located near the micelle-water interface, while large molecules are absorbed in the core. This results in the change of micellar shapes and g values and, in turn, the phase transformation, the enlargement of mesopores, or the variation of morphologies of the final products. These organic additives are merely effective at low concentrations. Otherwise, the solubility of monomeric surfactants would be

SBA-4 (lamellar structure)

enhanced by highly concentrated agents such as dioxane, short-chain alcohols, and ethylene glycol, and the micellization would be opposed. The synthesis of cubic MCM-48 mesostructure with Ia3hd symmetry is a good example to explain the effects of g values on the final mesostructures. In the basic synthesis, with the increase of g values and the reduction of surface curvatures around the surfactant micelles, the general tendency of the phase transformation is the hexagonal p6mm to the cubic Ia3hd.92,137,187 Therefore, the enlargement of surfactant hydrophobic volume or the enhancement of hydrophobic property can easily induce the formation of MCM-48 mesostructure: (1) TEOS is the best silica precursor in the synthesis of MCM-48, which generate ethanol during the hydrolysis. Polar ethanol tends to enter the hydrophobic zone but cannot penetrate into the surfactant micelle core.188,189 The enlarged surfactant hydrophobic volume decreases the interfacial curvature around the surfactant micelles and thus results in the formation of MCM-48. Although Ryoo and co-workers synthesized MCM-48 using colloidal silica as a source; adding ethanol is also necessary.137 (2) Moderate polar molecules, for example, triethanolamine, favor the retention of periphery in the hydrophobic zone of surfactant micelles, which gives rise to MCM-48 mesostructure.188 (3) Gemini surfactants and the large headgroup cationic surfactant C16H33(CH3)2N(CH2)(C6H5) can easily template MCM48 mesostructure. The alkyl chains or benzene groups that link with the polar heads of surfactants favor staying in the outer shell of hydrophobic regions of micelles due to their hydrophobicity and large volumes and hence reduce the curvature. Gemini surfactant C22-12-22 is very efficient in directing MCM-48 mesostructure even at room temperature.92 (4) Cosurfactants with negative charge (e.g., CnH2n+1COOH and CnH2n+1SO3H) can interact with a small amount of cationic surfactants to form ion pairs. The mixture shows low hydrophilicity, increases g value, and assists the phase transformations from p6mm to Ia3hd.190,191 4.1.1.3. The Hydrophilic/Hydrophobic Volume Ratio. The hydrophilic/hydrophobic volume ratios (VH/VL) are suggested especially for nonionic surfactant templating systems to account for the formation of different mesophases.192 Generally, block copolymers with high VH/VL ratios (such as F108, F98, F127, and Brij700 (C18H37(OCH2-


CH2)100OH, C18EO100)) can direct the syntheses of cage-type cubic mesoporous materials, whose topological curvatures are rather high. Block copolymers with medium hydrophilic/ hydrophobic ratios (e.g., P123 and B50-1500 (BO10EO16, BO ) butylene oxide)) favor the formation of mesostructures with medium curvatures (e.g., 2D hexagonal or 3D bicontinuous cubic (Ia3hd)).78 Aggregation of block copolymers in water is sensitive to the additives. The addition of “salting out” electrolytes, such as KCl, K2SO4, and Na2SO4, would decrease both CMC and CMT vales. Micellization is favored. Highly ordered mesoporous silicates can be therefore prepared even with low block copolymer concentrations or at low temperature, resorting to these inorganic salts.193 Highly hydrophilic block copolymers (e.g., F127, F108, F98, and Brij700) are ideal SDAs for the formation of caged mesoporous materials from the viewpoint of their intrinsic packing symmetries and mesophases, although they are somewhat difficult for preparing ordered mesostructured solids practically. This conflict can be well overcome by adding the “salting out” inorganic salts to the synthetic batches.140,193 It is because the aggregates of surfactant-silica composites are initially formed in a dilute solution from a colloidal point of view. These colloidal particles are energetically unfavorable to approach each other because of electrostatic repulsion, especially for the highly hydrophilic block copolymers templated hybrid colloid. The PEO segments of these SDAs are highly protonated/hydrated and hence are rather difficult to assemble with each other. The energy barrier can be decreased with the increase of the ionic strength in the solution. At a certain high ionic strength, the aggregation of colloidal particles becomes energetically favorable. In contrast, if the energy required is increased by adding electrolytes, the “salting in” effect occurs. The CMC values of block copolymers increase, and the mesophase curvature can be increased.194 The addition of inorganic salts such as NaI was found to be a choice for the phase transformation from basically multilamellar vesicles to the bicontinuous cubic Ia3hd mesoporous silica. The addition of ionic surfactants (especially anionic) exhibits the most striking effects on the micellization of block copolymers. For example, the addition of sodium dodecylsulfate (SDS) to the copolymer solution successively reduces the formation of micelles. The interaction of SDS with the hydrophobic PPO segments leads to a decreased tendency for copolymer self-assembly. By adjusting the hydrophilic/ hydrophobic volume (VH/VL) ratios of the mixed templating agents, one can “rationally” design and prepare mesoporous materials with different symmetries.192 Cubic bicontinuous Ia3hd mesostructure can be templated by blending surfactants of triblock copolymer P123 and anionic SDS surfactant.195 Interestingly, adding anionic surfactants such as sodium dioctyl sulfosuccinate (AOT) and organic swelling agent TMB in the triblock copolymer F127 surfactant assembly system causes an expansion of the hydrophobic volume and hence a consecutive phase transformation from face-centered cubic (Fm3hm) to body-centered Im3hm then toward 2D hexagonal p6mm and eventually to cubic bicontinuous Ia3hd symmetries (Figure 11).196 The interaction of some block copolymers with ionic surfactants may lead to the formation of hybrid aggregates. This interaction is mainly employed in the establishment of hierarchical pores and will be mentioned in the following section. The reciprocity between organic additives and surfactants increases the hydrophobic portion of micelles, decreases the

Wan and Zhao

VH/VL values, and hence results in the phase transformation from p6mm to Ia3hd. For example, the first cubic bicontinuous Ia3hd mesoporous silica with large pore size was templated by triblock copolymer Pluronic P123 in the presence of MPTMS or nonpolar organic molecules, for example, toluene, benzene, xylene, TMB, etc.104 TMB seems to be one of the most effective additives, possibly due to the spherically symmetric molecule. Later on, large-pore Ia3hd mesostructure was also reported by adding MPTMS or vinyltriethoxysilane (VTES) and inorganic salts into the acidic Pluronic P123 and TEOS coassembling system.197,198 The expansion behavior of n-butanol with amphiphilic triblock copolymers is unique due to its rather similar hydrophilic and hydrophobic properties. Mesoporous silica with the Ia3hd structure could be prepared by adding n-butanol into the weakly acidic, dilute P123 solution.107,135 When the SDAs are highly hydrophilic, for example, Brij 700 and F127, the addition of TMB can improve the ordering of mesostructures.105,199 Reversed PPO-PEO-PPO copolymers are seldom used as templates to get ordered mesostructures due to the difficulty in the formation of oil-in-water micelles.200 Micelle clustering and networks have been reported.201 Key to the use of this kind of reverse triblock copolymers for the fabrication of ordered mesoporous materials is the solubility in water and the optimal balance between the hydrophobic and hydrophilic sections, namely, VH/VL values. The appearance of either irregular aggregation or precipitation would lead to a failed synthesis. If the PPO chains are fixed, an increase in the PEO blocks stabilizes the cubic and the hexagonal phases.202 Therefore, the use of reversed triblock copolymers with long PEO chains may give rise to new mesostructures that have even been reported in the triblock copolymer-templating systems. For example, the facecentered 3-D cubic (Fd3hm) mesostructure can be synthesized by using reverse amphiphilic triblock copolymer PO53EO136PO53 as a template. 4.1.1.4. Surfactant Phase Diagram. Surfactant phase diagrams can determine mesoporous silicate structures in the case of the “true” liquid-crystal templating approach. A liquid-crystal phase is formed when the concentration of surfactant is extremely high, caused by solvent evaporation or aggregation and condensation of inorganic precursors. The surfactant concentration can be tuned in a relatively wide range. The derived mesostructures can be diverse according to the ordered microdomains. Normally, the higher concentration leads to mesostructures with lower mesophase curvatures.120,203 In this case, even water-insoluble surfactants can be used as SDAs. Yu et al. 172,204 reported that waterand ethanol-insoluble polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymers can template regular cubic and reversed mesophases, as well as multilayer vesicular mesostructures, via the EISA strategy. The solvent is THF, which can dissolve PS-b-PEO diblock copolymers. In a hydrothermal process, the concentration and temperature of the surfactant, which determine the phase diagram, affect the final mesostructure.5,205 A typical example is a CTAB surfactant system under basic conditions. If the molar ratio of surfactant/TEOS is as low as 0.11-0.5, 2D hexagonal MCM-41 are the products in most cases. 3D cubic MCM-48 can be formed after hydrothermal treatment at 100 °C if the ratio increases to 0.5-0.8. Further increasing the ratio results in the formation of unstable lamellar mesostructures.5 This phenomenon is



Figure 11. Schematic representation of the mesophase transformation induced by cotemplate AOT and swelling agent TMB in the amphiphilic triblock copolymer F127 assembly system. With the increase of anionic surfactant AOT and/or organic additive TMB concentration, the interface curvature of F127-AOT mixed micelles reduces, resulting in the mesophase transformation from cubic closed packing (facecentered structure) to loose packing (body-centered bicontinuous structure). (A-D) SAXS patterns of the cubic Fm3hm (A), cubic Im3hm (B), 2D hexagonal p6mm (C), and cubic Ia3hd (D) mesoporous silicates. From: Chen, D. H.; Li, Z.; Wan, Y.; Tu, X. J.; Shi, Y. F.; Chen, Z. X.; Shen, W.; Yu, C. Z.; Tu, B.; Zhao, D. Y. J. Mater. Chem. 2006, 16, 1511. s Reproduced by permission of the Royal Society of Chemistry.


Figure 12. Synthesis-space diagram of mesostructures established by XRD measurements. Reprinted with permission from Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger, P.; Huo, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138 (http:// www.sciencemag.org). Copyright 1995 AAAS.

obviously related to the ordered microdomains of a surfactant to some extent (Figure 12). The phase diagram is a very useful guide to the hydrothermal synthesis, but the formation of mesophase does not exactly follow it. This is because the interaction between silicate oligomers and the hydrophilic segments of the surfactant contributes the hydrophilicity. Therefore, the hydrophobic/hydrophilic properties of the system continuously change during the polymerization of silicate species. Other parameters, like temperature, inorganics/water solubility, and alkalinity, which affect the hydrolysis and cross-linking degree of silicates, also alter the formation of mesophases.85,206 The surfactant concentration is limited to a certain range for synthesizing ordered mesoporous molecular sieves in water media. The regularity of mesostructures can be improved especially when the surfactant concentration is decreased,8 possibly due to the slow assembly of silicate oligomers and surfactant molecules in the formation of mesostructures. But normally the concentration should be higher than the CMC value of the surfactant. Increasing temperature can decrease the steric repulsion of the PEO segments in triblock copolymers. This corresponds to a reduction of effective hydrophilic headgroup area at the aqueous-micelle surface. Therefore, the spherical micelles change to cylindrical micelles. It is also reported that at higher concentrations and temperatures, the arrangements of triblock copolymers vary from cylinders or rods in cubic or hexagonal arrangements to lamellae to other structures. In one ternary phase diagram, nine different structures can be obtained.69,200 Increasing the concentration and temperature of block copolymers may be a choice to obtain multi-mesostructures in a single template system. 4.1.1.5. Other Templates. The amphiphilic compounds that possess liquid-crystalline properties, such as Vitamin E TPGS 1000 (C33O5H54(CH2CH2O)23) and ionic liquid crystals, can also serve as templates.207-209 From a true liquidcrystal template strategy, Zhou et al.208 prepared monolithic mesoporous silicas with worm-like pore structures by using 1-butyl-3-methyl-imidazolium-tetrafluoroborate (C4MIMBF4) as the template. On the consideration that the short-chain organic (butyl) group cannot perform preferential selfassembly into an ordered micellar structure or liquid-crystal phase, they supposed that the π-π stacking of imidazolium

Wan and Zhao

groups be responsible for the self-assembly. Lin and coworkers found that increasing the organic chains in 1-alkyl3-methyl-imidazolium bromide (CnMIMBr, n ) the number of carbons in the alkyl chain) from C14 to C16 to C18 results in mesoporous silica materials with pore structures varying from the MCM-41 type of hexagonal mesopore to the rotational moire type of helical channels.209 Releasing antibacterial ionic liquids against Escherichia coli K12 brings these materials into controlled release delivery nanodevices.209 Amine-terminated dendrimers, which are analogous to alkylamine surfactants and carbosilane dendrimers can also participate in the formation of disordered mesostructured silica.210,211 4.1.1.6. Mixed Surfactants. Mixed surfactants with unique aggregation behaviors have many advantages over a single surfactant, such as changing CMT and CMC values, adjusting the interactions with inorganic silicate species, tuning pore sizes, getting hierarchical pore structures, and causing phase transitions, etc. The latter three objectives have been and will be discussed in the former and latter sections, respectively. To obtain ordered mesostructures, the surfactants should be compatible. During the hydrothermal synthesis process, uniform micelles are important for controlling a uniform surface curvature.192,212 Tian et al.203 found that the cotemplates of triblock copolymers and alkyl poly(ethylene oxide)s can increase the efficiency of cooperative assembly of organic and inorganic species. This can result in the formation of highly ordered silica mesostructures, especially for the 3D cubic mesostructures. M41S type and vesiclelike mesoscopically ordered silica materials can also be synthesized in a cationic-anionic double surfactant systems.213 Hydrothermally stable mesoporous silica-based materials have been prepared by using mixtures of cationic fluorocarbon and hydrocarbon polymer surfactants through a high-temperature hydrothermal process.32 Lee et al.214 found that the pore wall thickness of 2D hexagonal mesostructures can be tuned by changing the ratio of cationic and nonionic surfactants in an acid solution. Mixed surfactants can also be used to obtain mesostructured nanoparticles. Utilizing different interaction mechanisms between cationic and nonionic surfactants and silicates, Imai and co-workers obtained silica nanoparticles with ordered mesostructures.215,216 This is due to the balance between the ordered assembly of anionic silicates from acidcatalyzed hydrolysis with a cationic surfactant through the electrostatic interaction and the inhibition of grain growth with a nonionic amphiphilic agent through the hydrogen bonding interaction. Han and Ying217 found that cationic fluorocarbon surfactants like FC-4 (C3F7O(CFCF3CF2O)2CFCF3CONH(CH2)3N(C2H5)2CH3I), though they cannot template mesostructured solids by themselves, can exhibit an effect of confined growth on triblock copolymer templating mesostructured particles to form ultrafine particles. In some other cases where the mixed surfactants are separated, several factors should be considered, such as the self-assembling nature of the surfactants, the influence of micellar phases by other surfactants, the competition of electrostatic forces and hydrogen bonding interactions between separated surfactants with silicate species, the ionic strength of ionic liquids or other small-molecular cationic surfactants, etc. Hierarchical pore structures, which will be discussed later, are the special examples.



Figure 13. Pore models of mesostructures with symmetries of (A) p6mm, (B) Ia3hd, (C) Pm3hn, (D) Im3hm, (E) Fd3hm, and (F) Fm3hm. Reprinted with permission from refs 73, 98, 218, and 219. Copyright 2000 Nature Publishing Group and Copyright 2002, 2004, and 2006 American Chemical Society.

By use of the present techniques, various mesoporous silicates have been synthesized with different symmetries (Figure 13), or the same mesostructures with different cell parameters and pore sizes have been obtained, indicating the controllable synthesis on will.

4.1.2. 2D Mesostructures 2D mesostructured materials with hexagonal symmetry are most easily produced, the classical products being MCM41, FSM-16, SBA-3, SBA-15, etc. (Table 3). The ideal models for these structures are hexagonally close packed cylindrical pore channels belonging to the p6mm space group. Typical TEM images can show two features: hexagonal structures along the channel system and parallel stripes if viewed perpendicular to the channel directions. Figure 14A shows the hexagonally symmetric pore arrays. MCM-41 is the simplest and most extensively investigated mesoporous silica molecular sieve. It can be synthesized in a wide range of conditions, with the most popular synthesis using CTAB as a SDA in a basic solution. The cell parameter of MCM-41 (∼4.0 nm) can be easily obtained from XRD and TEM analysis. The pore channels in MCM-41 are often simply approximated as cylinders,230 although at least three kinds of shapes have been proposed. The other two models are the hexagonal prism and a so-called “cucurbit”, which can be envisaged as a string of connected spherical cages along the [001] direction.231,232 Typical isotherms of MCM41 show no obvious hysteresis loop. The pore wall thickness is estimated to be about 1 nm, and the Brunauer-EmmettTeller (BET) surface area is generally higher than 1000 m2/ g. In addition, micropores are not detected. The second important 2D hexagonal mesostructure is SBA-15, which is normally synthesized using PEO-PPOPEO triblock copolymer as a SDA under acidic conditions. The optimal template is triblock copolymer P123. Compared

with the synthetic system for MCM-41 involving a cationic surfactant, the concentration of triblock copolymer is higher. SBA-15 materials prepared from P123 at 40-100 °C have uniform pore sizes from ∼6.5 to 10 nm. The pore walls range from 3.1 to 4.8 nm in thickness, much thicker than that of MCM-41, which result in higher thermal stability and hydrothermal stability. SBA-15 with small pore sizes can be templated by nonionic oligomeric surfactants, for example, Brij 56 (C16H33EO10), in acidic solutions after hydrothermal treatment at 100 °C.31,78 Another feature of SBA-15 is the disordered micropore system in the silicate walls. Micropores in SBA-15 mesostructure were noticed by Lukens et al.233 in 1999, although it was known from the pore expending phenomena by the hydrothermal treatment when the initial discovery of SBA15 in 1998. However, the quantitative analysis and the reason were not given. Other experimental results reveal that the wall structure of SBA-15 is quite different from that of MCM-41, although the two materials have the same space group of p6mm (Figure 13A). Connections exist between the mesopore channels of SBA-15.233,234 Evidence for this is obtained by the fact that metal oxides, metal sulfides, carbons, and even metal nanowire or nanorod arrays cast from SBA-15 mesoporous silicas can retain the ordered 2D hexagonal mesostructure.235 The final nanowire or nanorod arrangements are connected and supported by smaller nanorod pillars. Only separated nanowires are obtained if MCM41 is used as a hard template.236 Quantitative XRD analysis of the SBA-15 diffraction pattern supports the model of hexagonal pore channels, surrounded by a corona of micropores.237 It is found that the microporosity of SBA-15 can be controlled by the treating method.238,239 Complementary porosity of SBA-15 can be retained to a significant extent even after calcination at 900 °C, but probably completely


Wan and Zhao

Table 3. List of Typical Mesoporous Silicate Materials with 2D Mesostructures space group p6mm

researchers or materials MCM-41, FSM-16 MCM-41

Attard and co-workers CMI-1 SBA-3 UK-1 JLU-14 and 15 SBA-15 MSU-H IBN-4 JLU-20 Feng et al. Chen et al. Wiesner and co-workers Cho et al. DAM-1 Kramer and co-workers

c

SDAs

remarks

CTAB CnTMA+ (n ) 12 - 18) C16-n-16 (n ) 4, 6, 7, 8, 10) CTAB and C20-3-1 bolaform surfactant R12 C12EO8, C16EO8 Brij56 (C16EO10) CnTMA+ (n ) 14 - 18) C16-n-16 (n ) 3, 4, 6, 7, 8, 10, 12) FSPCl-1a FSO-100 (CF3(CF2)4(EO)10) P123, P85, P65 B50-1500 (B010EO16) Brij 97(C18H35EO10) P123 (EO20PO70EO20) P123 (EO20PO70EO20) P123 (EO20PO70EO20) F127 (EO106PO70EO106) F127 (EO106PO70EO106) PI-b-PEO PEO-b-PLGA-b-PEOb vitamin E TPGS 1000 d8PS-b-P2VPc

refs

first reported synthesis basic conditions

5-7 92

with hydrothermal treatment liquid-crystal templating mechanism

93 77 220 8 8,92 221

first reported acidic synthesis acidic synthesis first single cationic fluorinated surfactant template highly ordered large pore neutral pH, adding inorganic salts with FC-4, nanoparticle with FC-4, stable mesoporous silica monolith, with organic additives with AOT, large pore organically modified aluminosilicate thin film, with low to moderate cross-linking degree of template

222 31,78 223 192 224 217 32 41 196 225 226 207 227

cmm

SBA-8

bolaform surfactant R12 CTAB

without hydrothermal treatment silatrane derivatives of triethanolamine as the silica source, triethanolamine as the solvent

93 228

mesh phase

CK-2

HFDePCd

elongated particle, layers orientation orthogonal to main axis

75

layered

MCM-50 KSW-2

CTAB CTACle

kanemite as silica source, folded sheets mechanism

5,6 229

a 1,1,2,2-Tetrahydroperfluorooctylpyridinium chloride. b Poly(ethylene oxide)-block-poly(DL-lactic acid-co-glycolic acid)-block-poly(ethylene oxide). Poly(d8-styrene)-block-poly(2-vinylpyridne). d 1,1,2,2-Tetrahydroperfluorodecylpyridinium. e Cetyltrimethylammonium chloride (C16H33N(CH3)3Cl).

Figure 14. TEM images of (A) traditional 2D hexagonal SBA-15, (B) 3D SBA-15, and (C) 3D SBA-15-like material derived from the mesoporous carbon-silica nanocomposite with a “steel-bar-concrete” structure. Insets are corresponding Fourier diffractograms. Reprinted with permission from refs 240 and 242. Copyright 2001 and 2006 American Chemical Society.

disappears at 1000 °C. Adding a cosolvent like ethanol and a small amount of salts manipulates the P123 micellar environment and hence eliminates the microporosity of SBA15 after the microwave hydrothermal treatment. Modification of SBA-15 surface by octyldimethylsilyl groups can also block these small pores.239 It is difficult to image the disordered micropores and small mesopores by the highresolution transmission electron microscope (HRTEM) technique. By comparison, the relatively large mesopores inside the pore walls can be directly observed. The enlargement of

mesotunnels in the pore walls can be of help to realize it. Before this, understanding the origin of micropores is necessary. Although the structure of micropores inside the SBA-15 pore walls is unclear, it arises from the partial occlusion of the PEO chains of triblock copolymers into the silica matrix, which is also true for most PEO-containing surfactanttemplated mesostructures.57 A small amount of small mesopores inside the frameworks is possibly caused by the distortion of surfactant micelles upon heating. In contrast to the


shrinkage of micropores, small mesopores on the walls become larger with the increase in hydrothermal treatment temperature.240 Fan et al.240 developed an approach to form a 3D SBA-15 material by introducing TMB into the embryo of mesostructured SBA-15. The average mesostructure is still hexagonal p6mm. The void space in the walls is attacked by the swollen P123 micelles assisted by TMB molecules and becomes expanded. At high-temperature such as 130 °C, micropore regions in the complementary pores almost disappear, leaving only relatively large (2-8 nm) connections or mesotunnels with random distributions as evidenced by TEM images (Figure 14B). Galarneau et al.241 also observed the elimination of microporosity and the existence of secondary mesoporosity of SBA-15 upon hydrothermal treatment. Very recently, a kind of 3D SBA-15-like mesoporous silicate was derived from the carbon-silica nanocomposites.242 The hybrid materials are fabricated via the EISA strategy by using resols as an organic precursor, prehydrolyzed TEOS as an inorganic precursor, and F127 as a SDA and have interpenetrating networks. Upon calcination in air, carbons can be burned out and ordered mesoporous silicas are left. Mesotunnels can be clearly observed from the TEM images (Figure 14C) of mesoporous silicas whose parent nanocomposites have a high content of carbon. The complementary porosity, including both micropores and smaller mesopores, of SBA-15 is important not only for nanocasting, but also for the diffusivity of molecules, which is significant for catalysis, separation, etc. Kaliaguine and co-workers investigated the diffusion of small probe molecules such as n-heptane through the pore structure of intrawalls.238,243 The relative contents of micropores and secondary mesopores have strong effects on the diffusivity and activation energy. In the case of a high content of micropores, the diffusion is low. This phenomenon suggests a micropore diffusion controlled process. If the secondary mesopores are dominant in the intrawall structures, the process will be controlled by this kind of pore. The diffusivity is 3-4 times larger than in the former case. The silica frameworks are amorphous and possess a large number of surface hydroxyl groups on the pore surface. These characteristics offer these materials great opportunities in further modification.244 Water molecules confined inside the mesochannels show dramatically slow molecular dynamics compared with that of bulk water. This is possibly caused by the interaction of water molecules with surface OH groups through hydrogen bonds.245,246 The SBA-3 mesostructure, which is synthesized using cationic surfactants such as CTAB as a SDA under acidic conditions, possesses the same symmetry as MCM-41.8 They are easily mixed up with each other. Different from assynthesized MCM-41, as-synthesized SBA-3 contains a large number of halogen ions like Cl- as counterions of the cationic surfactants. Although calcined SBA-3 has the same composition (silica SiO2) and symmetry (p6mm) as MCM41, it is not clear whether differences in the pore wall structures would discriminate them. At least their hydrothermal stability and surface acidity after the incorporation with the same amount aluminum are different. Another 2D mesostructure worthy of mention is KSW-2, which is transformed from a layered mesostructure.229 The domain structure was observed during the phase transformation. The folded sheets mechanism may explain the formation of KSW-2. Beginning with the composites of layered kanemites and surfactants, rectangular-arranged square or


rhombic pore channels are generated after the treatment by weak acid. Accordingly, the pore walls of KSW-2 are tabulate. TEM images indeed show that kanemite layers are folded but disconnected with the adjacent sheets. By using bolaform quaternary cationic surfactants as SDAs, SBA-8 with the 2D centered rectangular mesostructure (space group cmm) has been synthesized at room temperature under basic conditions.93 This mesophase is unusual, either a distortion of the 2D hexagonal mesostructure or an intermediate during the synthesis of MCM-41. The phase transformation to high-quality MCM-41 is easily observed during hydrothermal treatment at 70 °C.

4.1.3. 3D Mesostructures Many cubic mesostructures have been reported. The MCM-48 mesostructure with interesting and complicated pore channels has been most often studied. In addition, SBAn, KIT-n, AMS-n, and FDU-n series of silicas have several characteristic mesostructures, as shown in Table 4. 4.1.3.1. Bicontinuous Cubic Mesostructures. MCM-48 is defined by a so-called minimal surface, the gyroid (Gsurface), which was first proposed by Monnier et al.81 The minimal surface divides the space into two enantiomeric separated 3D helical pore systems, forming a cubic bicontinuous structure (Ia3hd) (Figure 13B). Only a few materials have such a structure in nature. The structure factors have been determined by phase-contrast HRTEM.257,258 This attractive mesostructure stimulated intensive research efforts. MCM-48 mesostructure with the 3D uniform bicontinuous mesochannels shows type-IV sorption isotherms and a narrow pore size distribution at about 2 nm. Disordered micropores ranging from 0.5 to 0.8 nm are found to form interconnections between two main channels. In a combination of comprehensive quantitative XRD analysis with the continuous density function and the derivative difference minimization methods, the wall thickness of MCM-48 have been determined to be 0.80 nm with a precision of 0.01 nm.259 The mesoporous silica FDU-5 structure is the bicontinuous cubic mesostructure prepared by using triblock copolymer P123 as template and some organic agents as additives under acidic conditions.104 Ryoo and co-workers reported that n-butanol may produce a distinct change of the interfacial curvature of the triblock copolymer P123 micellar system, leading to the phase transition to Ia3hd mesostructure (KIT6). The XRD pattern and N2 sorption isotherms of KIT-6 are shown in Figure 15, indicating highly ordered Ia3hd mesostructure and large mesopores.107,135 Large-pore 3D cubic bicontinuous KIT-6 has pore diameter and wall thickness of about 8 and 3.5 nm, respectively. Complementary pores of about 1.7 nm connect the two-group helical mesochannels, which are confirmed by TEM modeling and investigating nanowire replicas cast from KIT-6.257 Other triblock copolymers that can direct the 2D hexagonal mesostructure can also be used as templates to prepare Ia3hd mesostructure.107 By using a mixture of triblock copolymer F127 and an anionic surfactant such as AOT or P123 and SDS as templates, high-quality cubic bicontinuous mesoporous silicates can be easily synthesized with large pore sizes.195,196 Therefore, large-pore mesoporous silicas with cubic bicontinuous structure (Ia3hd) can only be prepared in the presence of some additives when triblock copolymers are used as templates under acidic conditions. The additives have a strong influence on the VH/VL values of block copolymers and hence the packing symmetries of the organic/ inorganic mesophase hybrids, as described above.


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Table 4. List of Typical Ordered 3D Mesostructured Silicates space group Fm3hm

Im3hm

Pm3hm Pm3hn

researchers or materials

SDAs

remarks

FDU-12

F127 (EO106PO70EO106)

KIT-5 Chen et al. FDU-1 IBN-2

F127 (EO106PO70EO106) F127 (EO106PO70EO106) B50-6600 (EO39BO47EO39) F127 (EO106PO70EO106)

SBA-16 IBN-1 Kleitz et al. Chen et al. Yu et al. FDU-1 ST-SBA-16 Wiesner and co-workers SBA-11 SBA-1

F127 (EO106PO70EO106) F127 (EO106PO70EO106) F127 (EO106PO70EO106) F127 (EO106PO70EO106) F108 (EO132PO50EO132) B50-6600 (EO39BO47EO39) Brij 700 (C18EO100) PI-b-PEO Brij 56 (C16EO10) C16TEABr C16TEABr C16TEABr CnTAB (n ) 14, 16) C18TMACl C16-3-1

Fd3hm

SBA-6 FDU-2

18B4-3-1 Cm-2-3-1 (m ) 14, 16, 18)

P63/mmc

AMS-8 SBA-2

FDU-5

C12GlyA Cn-3-1 (n ) 12-18), Cn-6-1 (n ) 16, 18) Brij 76 (C18EO10) Cn-3-1 (n ) 12-18), Cn-6-1 (n ) 16, 18), baloform surfactants, e.g., 18B4-3-1 CnTMA+ (n ) 14-18) C22-12-22, C16-12-16, C16H33(CH3)2N(CH2C6H5) CTAB and CnH2n+1COONa (n ) 11, 13, 15, 17) CnTAB and C12(EO)m (n ) 12-18; m ) 3, 4) P123 (EO20PO70EO20)

Che et al. Schuth and co-workers Flodstrom et al. KIT-6 Chen et al. Chen et al. Chan et al. Chan et al.

P123 (EO20PO70EO20) P123 (EO20PO70EO20) P123 (EO20PO70EO20) P123 (EO20PO70EO20) P123 (EO20PO70EO20) P123 (EO20PO70EO20) F127 (EO106PO70EO106) PI-b-P(PMDSS)-b-PIa EO17MA23

SBA-12 SBA-7

Ia3hd

a

MCM-48

ultralarge caged, with TMB and inorganic salt such as KCl with n-butanol, low acidic concentration large caged, mixed hcp and ccp phases nanoparticle, with FC-4 and TMB, mixed hcp and ccp phases TMOS is preferable to TEOS with FC-4, ultrafine particles with n-butanol, low concentration of acid with AOT highly ordered “single crystal” large-caged pore small pore, thick wall “the plumber’s nightmare” first acidic synthesis with short-chain alcohols with D-fructose with poly(acrylic acid) with sodium silicate as silica source, pH ) 1.0-2.0 basic synthesis, highly ordered mesostructure low temperature of 17 ˚C, narrow synthesis range with TMAPS as the CSDA acidic synthesis, mixed hcp and ccp phases mixed hcp and ccp phases basic synthesis, highly ordered mesostructure, without detailed study on mesostructure first bicontinuous cubic mesoporous silica recrystallization

refs 105 106 196 247 217 78 217 134 196 193 103 248 249 78 8 189 250 251 252 253 73 94 98 92 78

5,6 92 190,191 254

first acidic synthesis of large-pore silica, with MPTMS, EISA method post-solvothermal synthesis with MPTMS with TEVS and inorganic salts with inorganic salts (NaI) with n-butanol, low concentration of acid with SDS, low concentration of acid with AOT and TMB silicon-containing triblock copolymer acidic synthesis

104 168 197 198 194 107 195 196 255 256

Poly(isoprene)-block-poly(pentamethyldisilylstyrene)-block-poly(isoprene).

By using C14GluA as the self-assembling anionic surfactant and TMAPS as the CSDA, a bicontinuous cubic mesoporous silica with Pn3hm symmetry (AMS-10) was prepared.100 It is a minimal D-surface structure, the counterpart to the original MCM-48 with the minimal G-surface. This mesostructure is produced only if the pH value of the solution is tightly controlled. The synthesis region is relatively narrow. 4.1.3.2. Cage-Type Mesostructures. Besides the above mesostructures that have 1D and 3D uniform mesochannels, most mesoporous materials have 3D cage-type pores (Figure 13C-F). Characterization of pore structures is attractive. The ink-bottle model can account for such cage-type mesopores. The hysteresis in N2 sorption isotherms can be explained by

three assumptions.260-264 (1) The most popular one is a poreblocking effect, which is also applied in a constricted cylindrical pore. Desorption from a cavity is delayed until the vapor pressure is reduced below the equilibrium desorption pressure from the pore windows. (2) A large-scale evaporation process in the large cavity on desorption cannot be formed, provided that a sufficient expansion of pore together with the local density fluctuation leads to cavitation. (3) Near-equilibrium evaporation occurs at unblocked cavities that have access to the vapor phase. Cage sizes and window sizes are calculated from the adsorption and desorption branches of nitrogen sorption isotherms, respectively. On the consideration that the Barrett-Joyner-Halenda (BJH) model


Figure 15. (A) XRD pattern for calcined cubic Ia3hd silica (KIT6) sample. (B) N2 adsorption-desorption isotherms at 77 K for KIT-6. From: Kleitz, F.; Choi, S. H.; Ryoo, R. Chem. Commun. 2003, 2136. s Reproduced by permission of The Royal Society of Chemistry.

is inaccurate in calculating the pore sizes of spherical cages, Ravikovitch and Neimark263 developed the nonlocal density functional theory (NLDFT). However, the hysteresis loops are sometimes undetectable for the pores with small sizes (1000 m2/g) can be obtained at room temperature even without the hydrothermal treatment.


Wan and Zhao

Figure 18. (A) XRD patterns of as-synthesized and calcined FDU-12. (B) Experimental (solid line) and simulated (dashed line) synchrotron XRD profiles obtained for as-synthesized and calcined KIT-5 mesoporous silica samples. Reprinted with permission from refs 105 and 106. Copyright 2003 Wiley and American Chemical Society.

Although SBA-2 has a special cage-type mesostructure, research efforts on this material are not as high as expected because the actual structure is far more complicated than described above.253,271 TEM images show an intergrowth of hcp with ccp mesostructures viewed along the [110] direction of the cubic phase or the [100] direction of the hexagonal phase. The intergrowth of hcp with ccp can form either a layer structure along the [001] direction of the hexagonal phase and the [111] direction of the cubic phase or a domain structure along any direction of 3D space. The corresponding pure ccp phase should be a face-centered cubic Fm3hm mesostructure. SBA-12 has the same mesostructure as SBA-2. Their syntheses are similar, such as the same silicate precursor of acid-catalyzed TEOS and the same reaction temperature of room temperature, except with different SDAs.78 Nonionic oligomeric surfactants are used as SDAs for the SBA-12 mesostructure. The minimum wall thickness of the adjacent cages is 1.3 nm in a Brij 76 (C18H37EO10)-templated SBA12 with a cell parameter of 7.43 nm.263 It is much thicker than that of SBA-2, indicative of higher thermal and hydrothermal stability. Soon after its original synthesis, SBA12 was also observed as an intergrowth of hexagonal with cubic close packed mesophases. The diffraction peaks of SBA-12 in the XRD patterns are not as evident as those of SBA-2, due to either the increase of the cubic phase or less ordered mesostructure. Fortunately, large domains of the cubic phase were found in SBA-12 modified with mercapto groups. Electron crystallography resolved that it is a facecentered structure with a unit cell parameter of 8.2 nm.272 Each spherical cage is connected with 12 adjacent cages by small windows.

For spherical cage mesopores, pore sizes, as well as entrance sizes, are important. The entrance size is a key factor for the applications in which mass transportation and diffusion are necessary. Considering the large cage sizes, the window sizes of cage-type mesostructures are too small. Expanding entrance sizes benefits the immobilization of enzymes and the fabrication of cubic mesoporous carbon replicas with ordered mesopores. The resulting carbon replicas cast from mesoporous silicas with too small entrances have extremely thin rods to connect the spheres for the cage-type structures and, hence, are unstable. Wright and co-workers found that the window size of the SBA-2 mesostructure is dependent upon the synthesis parameters. At a low synthetic temperature (-4 °C), an acidic solution generates a window size larger than 0.8 nm, while in a basic solution, the cages of SBA-7 are linked through windows of sizes smaller than 0.8 nm, sometimes even smaller than 0.4 nm. (SBA-7).253 Prolonging the hydrothermal time and increasing the hydrothermal temperature can enlarge the entrance sizes of SBA-16.273 Other methods for the uniform expansion of pore entrances include blending triblock copolymers of P123 and F127 in the synthesis of mesoporous silica SBA-16 and the post-treatment by H2SO4.157,273 The latter method can even get pseudo-cylindrical mesopores while maintaining the overall cubic symmetry of SBA-16.

4.1.4. Lamellar and Disordered Mesostructures Lamellar mesostructures are templated by surfactants with large g values or low hydrophilic/hydrophobic volume ratios. The lamellar mesostructures can be templated by doublechain surfactants with small headgroups or rigid, immobile



Table 5. List of Disordered Mesostructures researchers or materials

CmH2m+1NH2, m ) 8-22 CmEOn, m ) 11-15

MSU-2 MSU-3 MSU-J

IBN-5 KIT-1

C8PhEOn EO13PO30EO13 H2NCH(CH3)CH2[OCH2CH(CH3)]xNH2, x ) 33, 68 C16EO10, C18EO10 dodecylamine, hexadecylamine, N,N-dimethyldodecylamine, N,N-dimethylhexadecylamine F108 (EO106PO50EO106) CTACl

TUD-1

triethanolamine

Al-MMS Aramendia et al.

hexadecylamine Tween 20, Tween 40, Tween 60

Su and co-workers Cassiers et al.

a

SDAs

HMS MSU-1

remarks N0I0 mechanism, pore size of 2-10 nm first hydrogen-bonding interaction, pore size of 2-4.5 nm pore size of 4.9-14.3 nm

refs 102 101 101 101 278

pore size of 4-10 nm acidic synthesis, pore size of 2.5-9 nm

279, 280 111

with FC-4 with EDTANa4,a highly thermally stable pore size of 2.5-25 nm, high hydrothermal and thermal stability doping with Al with NH4F as a catalyst

217 274 275 276 281

Ethylenediaminetetraacetic acid tetrasodium salt.

chains, such as C20H41N(CH3)3Br and C16-2-16.92 It should be noted that MCM-50 is pillared silicate mesostructure and thermally stable. The MCM-50 mesostructure is derived from lamellar mesostructured silica by using hydrolyzed TEOS as pillars. The nonionic surfactants with low hydrophilic EO groups usually direct the synthesis of lamellar mesostructure, for example, C12H25EO3.192 Some other mesostructures, including HMS,102 MSU,101 KIT-1,274 TUD-1,275 and Al-MMS276 mesoporous silicates (Table 5), show disordered mesopores and amorphous pore walls. Dissimilar to amorphous silica, 3D pores in these disordered mesostructures are interpenetrating, uniform, and adjustable. The pore system is probably best described as randomly packed short interconnected 1D pores.277 As evidence for the full connectivity of the pore system, carbon replicas can be cast. These materials are named foam-like or worm-like mesoporous molecular sieves. HMS mesoporous silica was first reported by Tanev and Pinnavaia. It was fabricated by hydrogen bonding interactions between silicate precursors and organic amines under neutral conditions.102 XRD patterns of HMS mesoporous silicas only show one broad diffraction peak, because of the disordered mesostructure. However, the pore size distributions are uniform. Pinnavaia and co-workers developed disordered MSU mesostructures by using cheap and environmentally benign nonionic alkyl poly(ethylene oxide) surfactants (CmH2m+1EOn) and triblock copolymers as templates under near neutral conditions.101 The pore sizes can be tuned by surfactants with different chain lengths. In particular, the pore sizes strongly depend on the synthesis temperature because the hydrogen bonds between inorganic species and surfactants under neutral conditions are sensitive to temperature.279,280 In addition, the hydrothermal and thermal stabilities of these mesostructures are high. Disordered mesostructures cannot be defined by unit cells, symmetries, and space groups. However, the characteristics, such as uniform pores, high surface areas, and easy modification offer good opportunities in catalysis, adsorption, separation, and immobilization. Which structure is more beneficial in applications, ordered or disordered? It is still hard to pass a verdict due to the complexity of the problem.

4.1.5. Other Mesostructures Shen et al.74 reported two well-ordered 3D mesoporous silicas with low-symmetry tetragonal (FDU-11, space group P4/mmm) and orthorhombic (FDU-13, space group Pmmm) structures. Such low symmetries for 3D mesostructure had not been observed before in amphiphilic liquid crystals. The formation can be traced back to the unique character of tetraheadgroup bolaform surfactants under basic conditions. The high charge density, rigid biphenyl rod-like unit, and flexible hydrophobic coil segment can increase the interaction between inorganic silicate precursors and organic surfactant molecules. The other factor that may control the formation of these structures is the aggregation force via π-π interaction. The electrostatic repulsion among anionic surfactant C12GluA, APS (CSDA), and silicate precursor is the driving force for the formation of the AMS-9 mesostructure with low symmetry (space group P42/mnm).99 The time delay between adding APS and TEOS adjusts the interaction and assists the phase transition with the formation of an intermediate P42/mnm mesostructure. Employing a microemulsion lyotropic liquid-crystal templating route, El-Safty et al.282,283 reported several 3D mesostructures with symmetries of P63/mmc, Pm3hm, Pn3hm, Pm3hn, etc. by using Brij 56 and Brij 76 as templates and alkanes as organic additives under acidic conditions. However, the XRD patterns are not well-resolved. More detailed characterization is required to identify these mesostructures.

4.2. Pore Size Control Several methods can be relied on to adjust the pore sizes of mesoporous molecular sieves. Table 6 illustrates the pore sizes obtained by various methods. The pore sizes of mesoporous materials mainly depend on the hydrophobic groups in surfactants. Cationic quaternary surfactants with longer alkane chains can yield mesoporous materials with larger pore sizes. When the surfactant chain length increases from C8 to C22, the BJH pore diameter of MCM-41 increases from 1.6 to 4.2 nm.284 Adjusting the carbon chain length in a cationic Gemini surfactant Cn-12-n can tune the pore size of MCM-48 mesostructure obtained by the hydrothermal method in the range of 1.6 to 3.8 nm.285


Wan and Zhao

Table 6. Pore Sizes of Ordered Mesostructures Obtained by Various Methods pore size (nm) 2-5 4-7 5-8 2-8 4-20 4-11 10-27

method surfactants with different chain lengths including long-chain quaternary cationic salts and neutral organoamines long-chain quaternary cationic salts as surfactants high-temperature hydrothermal treatment charged surfactants with the addition of organic swelling agents such as TMB and midchain amines nonionic surfactants triblock copolymer surfactants secondary synthesis, for example, water-amine postsynthesis high molecular weight block copolymers, such as PI-b-PEO, PIB-b-PEO and PS-b-PEO triblock copolymers with the addition of swelling agents TMB and inorganic salts low-temperature synthesis

Figure 19. TEM images of (A) mesoporous aluminosilicates templated by PI-b-PEO (the molecular weight is nearly 10 kg/mol with the volume fraction of the PEO block of ∼15%) [Reprinted with permission from Templin, M.; Franck, A.; DuChesne, A.; Leist, H.; Zhang, Y. M.; Ulrich, R.; Schadler, V.; Wiesner, U. Science 1997, 278, 1795 (http://www.sciencemag.org). Copyright 1997 AAAS], (B) mesostructured silica nanocasted from PIB85-EO79 [Reprinted with permission from ref 286. Copyright 2005 Wiley], and (C) a mesostructured silica thin film templated by PS215-EO100 [Reprinted with permission from ref 204. Copyright 2001 American Chemical Society].

As for the conventional PEO-PPO-PEO triblock copolymers, the pore sizes are enlarged with increasing molecular weights of the hydrophobic blocks rather than those of copolymers.182 In general, the block copolymer micelles (in most cases, spherical) are larger than those aggregated by low-molecularweight surfactants. For example, the spherical micelles of PEO-PPO-PEO block copolymers contains several tens of molecules (15-60) per aggregate, several times larger than low-molecular-weight surfactants. The hydrodynamic radii range from 6 to 10 nm. Therefore, the pore sizes of mesoporous silicas templated by block copolymers are larger than those from low-molecular-weight surfactant systems. Diblock copolymers always direct larger pore sizes compared with triblock copolymers with similar molecular weights or PPO chains because the latter tend to bending aggregation. High-molecular-weight block copolymers are of great interest owing to the relatively large mesopores in the resultant aluminosilicates and silicates, which were first demonstrated by Wiesner and co-workers.225 These lab-made copolymers include poly(isoprene)-block-poly(ethylene oxide) (PI-b-PEO), PS-b-PEO, and poly(isobutylene)-blockpoly(ethylene oxide) (PIB-b-PEO) that can template mesopores with sizes larger than 20 nm (TEM images are shown in Figure 19).172,204,225,249,286 TEM image (Figure 19C) shows the mesoporous silica thin films templated by PS-b-PEO block copolymers with “large mesopores”. However, the expected large mesopore sizes are not obtained. N2 adsorption measurements show that the BET surface areas are close to zero. This is probably due to the isolated sphere packing model, which results in thick pore walls and no micropore connection between mesopores.173 The insertion of long PEO

chains inside the silica pore walls may facilitate the formation of tunnels connecting primary mesopores of silicates. One may assume that the use of block copolymers with high molecular weights and long PEO chains provides a facile pathway to give ordered silica-type mesostructures that extend the accessible pore with the width scale by about an order of magnitude. Changing the hydrothermal temperature can tailor the pore size.224 During hydrothermal treatment at high temperature, the PEO blocks become hydrophobic and retract from the silicate walls.287,288 As mentioned above, the hydrolysis and cross-linkage of inorganic species and assembly with surfactant continue to react in this stage. The enlarged surfactant micelles result in large-pore SBA-15, thin pore walls, and low micropore volumes.241,289 The mesopore sizes of SBA15 can be easily tuned from 4.6 to 10 nm and from 9.5 to 11.4 nm by increasing the hydrothermal temperature from 70 to 130 °C and by prolonging the hydrothermal time from 6 h to 4 days, respectively. Similar results were obtained from the mesoporous silicates with body-centered cubic Im3hm mesostructure by using F127 as a template and the cubic bicontinuous Ia3hd mesostructure by using triblock copolymer P123 as a template and n-butanol as a cosolute.135,265 Increasing the hydrothermal treatment for SBA16 from 45 °C and 1 day to 100 °C and 2 days also thins pore walls, reduces intrawall micropores, and gains large primary mesopores. The tunable pore size of mesoporous silica with Ia3hd symmetry ranges from 4 to 10 nm when the hydrothermal temperature increases from 65 to 130 °C.135 Adding organic swelling agents is a significant way to expand pore sizes. The hydrophobic organic species can be solubilized inside the hydrophobic regions of surfactant


micelles, which leads to a swelling of the micelles.290 The solubility of organic additives in the aqueous mixtures should be considered for the swelling roles. The pore sizes are expanded by the additives of large organic hydrocarbons such as dodecane, TMB, triisopropylbenzene, tertiary amines, and poly(propylene glycol).177,291-294 With the aid of TMB molecules, the pore sizes can be enlarged to 40 nm in acidic triblock copolymer systems or to 10 nm in basic CTAB surfactant systems. However, the resulting mesoporous silica materials are rather disordered. The pore sizes of ordered mesostructures can only be increased to 13 and 6 nm for SBA-15 and MCM-41, respectively, by adding TMB as swelling agent. A large amount of TMB in the synthesis system for SBA-15 results in the formation of mesocellular siliceous foams (MCF).291 With the addition of AOT and TMB, highly ordered 2D hexagonal mesoporous silicates with large pores of 11 nm are the products templated by the amphiphilic F127.196 Ultra large pore sizes (27 nm) result in an F127-templating system in the presence of TMB and KCl when the synthesis temperature is lowered to 15 °C.141 These phenomena give us a hint. Other substances that can be dissolved in the micelle cores may also expand pore diameters. Hanrahan et al.295 demonstrated a method for tailoring the pore size of hexagonal mesoporous silica using SC CO2 as the swelling agent during the silicate hydrolysis process. On the consideration of the solubility, substantial and undetectable swelling effects were found for mesoporous silicas templated by triblock copolymers, such as P123, F127, and P85, and cationic surfactant CTAB, respectively. Somorjai and co-workers found the pre-existence of a certain concentration of metallic nanoparticles in the coassembling system of P123 and TEOS can expand the mesochannels. These nanoparticles can be encapsulated in mesoporous silica SBA-15.296 In contrast, a high concentration of TEOS in the synthesis leads to a fraction of ordered mesopores exhibiting porous plugs within SBA-15 mesochannels.297 Binary surfactant systems can give products with tunable pore sizes and bimodal or trimodal pores. Blending two quaternary cationic surfactants with different carbon chains together (e.g., C12TAB and C16TAB, C16TAB and C22TAB) can change the pore sizes of MCM-41 mesostructures to intermediate values between the values templated by a single surfactant.284 Hierarchical pore structures are beneficial for special applications. Multimicellar systems and the assemblies of nanoparticles with small mesopores can give bimodal and even trimodal pore architectures. It should be emphasized that the creation of hierarchical micellar systems is usually unfavorable from a general thermodynamic point of view.298 The mixture of most cationic surfactants, whose products have small mesopores, and block copolymers, which template comparatively large mesopores, always results in either phase separation or compound micelles. The latter favors the formation of uniform large mesopores. The properties of surfactants themselves should be thoroughly considered in the mixture, for example, the hydrophobic/hydrophilic balance of the block copolymer by the addition of ionic surfactants, the interactions such as hydrogen bonding and polar interactions between the headgroups of ionic surfactant and block copolymer micelles, and the comparability between them, etc.299-301 Ionic liquids and small fluorinated cotemplates are employed in triblock copolymer-templating systems to create bimodal mesoporous materials.299,300 Smarsly and co-workers reported that the ionic liquid 1-hexadecyl-


3-methylimidazolium chloride (C16MIMCl) exhibits an advantageous templating role, with small mesopores (2-3 nm) being located between block copolymer mesopores. Imai and co-workers found that an excess amount of the surfactants inhibits the growth of silicate grains. The assembly of the small-sized grains so obtained produces a bimodal pore structure with framework mesopores of 2-3 nm and textual mesopores of 10-100 nm.215 A mixture of cationic fluorocarbon surfactant C8F17CH2OHCH2NH(C2H5)2Cl (IC-11) and triblock copolymer F127 can direct the formation of a coreshell structure in silica particles. The mesoporosities in shells and cores are templated by IC-11 and F127, respectively.302 Macro-mesoporous structures are always generated from a dual-templating system with the macropore region defined by a range of organized media at this scale length including polymer spheres, large poly(ethylene glycol) (PEG) molecules and bacterial threads, and colloidal crystals, as well as CTAB, triblock copolymers, or other surfactants for mesopores.303-305 Foaming spray technique is also utilized to produce hierarchical architecture with pores from surfactant micelles and macrovoids.306

5. Morphology Control The morphology of mesoporous silicates is important for industrial applications, for example, films in catalysis and separation, monoliths in optics, and uniformly sized spheres in chromatography. Mesoporous molecular sieves are made of amorphous walls. Controllable synthesis on both the mesoscale (mesostructure) and macroscale (morphology) is therefore possible. Mesostructure assembly and morphology growth influence each other. It has been found that several factors can affect the morphology of the final materials: hydrolysis and condensation of silicate species, the shapes of surfactant micelles, the interactions between them, and the additives (inorganic salts, organic swelling agents, cosolvents and cosurfactants) are important.40,307 Tuning them can lead to the formation of mesostructured silica fibers, thin films, monoliths, spheres, “single crystals”, etc. For example, a colloidal phase separation mechanism (CPSM) is proposed based on the investigations of the formation of mesoporous crystals templated by nonionic surfactants.140 The meso/macro topological evolution includes cooperative assembly, colloidal-like interaction, and multiphase energy competition. The first stage takes place at the molecular level to form surfactant-silica composite aggregates. Further coalescence and condensation of these nano-building blocks give a liquid-crystal-like phase made up of the block copolymer-silica hybrid species. As the silica species further condense, the liquid phase grows denser with time and finally separates from the water phase. At the final stage, the mesostructure assembly is still under way, and the separated liquid-crystal-like phase is further growing into the final solid mesostructure. Although the free energy of the mesophase formation (∆G) is responsible for the final mesostructure, the competition between ∆G and the surface free energy (∆Gsurf) of this liquid-crystal-like phase determines the morphology of final mesoporous materials. In a basic CTAB template system, liquid droplets tend to the spherical shape in order to minimize the ∆Gsurf value as proposed by Chan et al.307 However, whether a spherical morphology or a more regular form is obtained is dependent on the balance between the rate of inorganic species polymerization and the rate of mesostructure formation.


Huo et al.308 first synthesized mesostructured fibers by using TEOS as a silica source in a biphase solution with the oil phase of long chain alcohols or hexane and the acidic CTAB water phase. The oil phase can dissolve TEOS and separate it from the water phase to reduce the hydrolysis and condensation rates of TEOS. Therefore, a controllable assembly can occur. TBOS with low hydrolysis and condensation rates can replace TEOS as a silica source to grow the fibers in the absence of oil phase. Under static conditions, mesostructured thin films are first formed on the interface. Then mesostructured fibers grow in the water solution, with 1-10 µm diameters and 100 µm to 0.05 m lengths. This structure in its early stage was proposed to consist of hexagonally arranged pores parallel to the axis of the fibers under acidic conditions. Subsequently, a more general model for the mesoporous silica fibers and even hollow fibers was suggested as channels whirl around the center of fibers.309,310 Wang et al.311 prepared mesoporous silica fibers with diameters ranging from 50 to 250 nm and lengths up to millimeters in a one-phase strongly acidic cationic surfactant system. Hexagonally arranged mesopores can either be parallel to or concentrically circling around the axis of mesostructured fibers, as shown in Figure 20, depending on the synthesis temperature and the addition of inorganic salts. Single-crystal mesoporous silica ribbons can grow in the mixture of TEOS and cationic CTAB or hexadecylpyridinium surfactant.312 They are 50-250 nm in thickness, 0.4-1.5 µm in width, and hundreds of micrometers in length. Interestingly, the hexagonally arranged pore channels are perpendicular to the long axis of the ribbons. Well-ordered SBA15 fibers were prepared by using TMOS as a silica source and triblock copolymer P123 as a template under acidic conditions.40 Fabrication of hollow and capillary MCM-41 fibers was also carried out on bacteria and glass capillary, respectively.313,314 Nanofibers from the mechanically drawing approach have diameters of 30-100 µm and consist of multiply ordered domains.315 Mesostructured silica fibers can also grow within a confined space, like in anodic aluminum oxide (AAO) membrane. Interfacial interactions, symmetry breaking, structural frustration, and confinement-induced entropy loss can play dominant roles in determining the surfactant assembly. Here we only give one example to describe the growth of mesostructured fibers in AAO membranes although much research has been done in this area.316-318 Wu et al.316 studied the confined assembly of silica-surfactant composite mesostructures within cylindrical nanochannels of various diameters. The confined space affects the solvent evaporation and, therefore, the formation of mesoporous silicates. Figure 21A shows unprecedented silica mesostructures with chiral mesopores such as singleand double-helical geometries inside individual alumina nanochannels. A transition in the mesopore morphology occurs from a coiled cylindrical to a spherical cage-like geometry upon the reduction of nanochannel sizes. Selfconsistent field calculations for the mesostructures accord well with the experiments (Figure 21B). Oriented mesoporous silica films can be arranged on the interface of water and air, mica surfaces, and crystalline substrates with anisotropic surfaces.117-119 Re-dispersion of as-collected silica-surfactant hybrid mesophases in organic solvents can also produce films.319 Relying on the similarity principle known from biomineralization, Lu et al.33 developed a method of dip-coating or spin-coating to synthesize highly ordered mesoporous silica thin films. Mikaya et al.320 utilized

Wan and Zhao

Figure 20. Mesostructured nanofibers with the circular (A-C) and longitudinal (D-F) pore architecture, respectively. (A, D) SEM images; the insets are TEM images. (B, E) High-magnification TEM images of a 214-nm-diameter (B) and 93-nm-diameter nanofiber (E); the double-arrowed line indicates the fiber axis direction. (C, F) TEM images taken on microtomed nanofiber samples. Reprinted with permission from ref 311. Copyright 2003 American Chemical Society.

a rubbing treated polyimide coating to produce mesoporous silica films with a single-crystalline 3D hexagonal (P63/mmc) mesostructure. The surface micelle alignment can be directed by both the uniaxial alignment on the rubbing surface and the interfacial hydrophobic interaction between the alkyl chain of the surfactant and the elongated polymer chains. The development of pore alignment methods in one direction has also been carried out by shear flows, oriental transfer from the surfaces of oriented polymer films or LangmuirBlodgett films, and photoalignment using photo-crosslinkable polymer liquid crystals.320-323 1D pores are always parallel to the surfaces of thin films, which could inhibit the mass transfer in three dimensions. Stucky and co-workers reported 3D hexagonal mesostructured films with the c-axis perpendicular to the growth interface, which should facilitate mass transfer.324 The recent results indicate that both outer fields and cosurfactants can align the pores in the direction perpendicular to the substrates. Tolbert et al. developed the method to orientate the pores of mesoporous silica by using


Figure 21. (A) Representative TEM images of silica fiber mesostructures formed inside alumina nanochannels with differing confinement dimensions. The confining nanochannel diameter is indicated underneath each image. (a-i) Silver inverted mesostructures prepared by backfilling the confined mesoporous silica; (jk) free-standing mesoporous silica fibers; (l) mesoporous silica embedded inside the alumina nanochannels obtained using a focused ion beam for sample preparation. (B) Summary of the experimentally observed confined mesostructural evolution with varying alumina nanochannel diameters. Reprinted with permission from ref 316. Copyright 2004 Nature Publishing Group.

an applied magnetic field with the alignment of the pores on average parallel to it (Figure 22A,B).325,326 The mesoporous silica films with the orientation of the pores perpendicular to the substrates can be fabricated, assisted with a high magnetic field of 12 T (Figure 22C,D).327 Confined growth of mesoporous silicate between two chemically neutral modified slides may also facilitate the perpendicular orientation.328 Transparent and rigid monoliths are of great interest in optics. Mesoporous silica monoliths can be prepared in the presence of lyotropic liquid-crystal mesophases or microemulsion liquid crystal of triblock copolymers and nonionic alkyl PEO surfactants.41,283,329 Melosh et al.169 found a large ordered area of 10 × 1 × 1 mm3 for the ordered hexagonal mesostructures of F127-SiO2 monoliths (Figure 23A). The phase separation in the presence of triblock copolymer P123 and high-content salt solution can lead to 3D foam-like monoliths. Nanocasting a polymer hydrogel monolith oriented by external electrical fields allows the internal patterning of silica monoliths with hierarchical arrays of mesopores and macropores.330 To yield transparent and crackfree mesostructured silica monoliths, Yang et al.331 demonstrated a liquid paraffin medium assisted solvent evaporation method. It shortens the traditional solvent evaporation process of silica gels to get highly ordered silica monoliths. The


resulting mesostructured silica monoliths are crack-free and optically transparent and can fully copy the shape of the reaction vessel. Figure 23B shows the examples of the peptop column shape 0.9 cm high and 0.8 cm in diameter. In addition, metal ions can be easily doped into pure silica monoliths in this one-step process. Schacht et al.332 synthesized 2D and 3D hexagonal mesostructured spheres in acidic conditions with tunable sizes ranging from 10 to 50 µm. Metal-containing sphere-like catalysts can also be produced in one step. A fast aerosol assisted methodology is developed to produce ordered mesoporous silica spheres with hexagonal, tetragonal, and vesicular structures.333 The whole process only lasts 6 s. Cosolvents, such as alcohols, are efficient in producing mesoporous silica spheres.187,334 Mesostructured silica spheres were precipitated in the ethanol/water/ammonia solution with cationic surfactants.187 When n-butanol is employed as a cosolvent, hollow mesoporous silicate spheres and macrospheres with sizes of ∼15 mm are the products of a cationic surfactant and a triblock copolymer system, respectively.334 An emulsion of triblock copolymer F68, TMB, and sodium silicate gives rise to silica hollow spheres.335 Hard, transparent, mesostructured silica spheres (50 µm to 0.02 m in diameter) were prepared by using long-chain alkoxysilanes like TBOS as a silica source.336 The expression “single crystals” for mesoporous silicas may be not be exactly correct, and “polyhedral” is more appropriate. The macroscopical morphologies are closely related to the mesostructures. Kim and Ryoo used sodium silicate as a silica source, CTAB as a template, and organic additives like methanol, ethanol, or butanol to prepare highquality MCM-48 with the truncated rhombic dodecahedral morphology. SBA-1 materials with “single-crystal” morphology of a large number of facets were also synthesized.252,337 Yu et al.193 have demonstrated the preparation of cubic mesoporous silica (Im3hm) with the uniform rhombdodecahedral shape, using block copolymer F108 as a template and an inorganic salt (KCl) as an additive under acidic conditions. Park and co-workers found that microwave irradiation induces the formation of polyhedral shape of SBA-16.338 Recently, Che et al.96 reported the synthesis of mesoporous silica materials with chiral channels and helical morphology by using chiral anionic surfactants as templates. Lab-made chiral cationic surfactants or liquid-crystal cellulose nanorod gels can also be used to template helical nanofibers.339,340 A structural model of twisted, hexagonally shaped rods with 2D chiral channels running inside and parallel to the ridge edge is shown in Figure 24.96,341 TEM images show two kinds of fringes indicated by arrows and arrowheads, which correspond to the interplanar spacing 10 and 11, respectively. The distance between two sets of 10 fringes is one-sixth of one pitch (along the rod axis was estimated to be ∼1.5 µm). The pitch sizes make great distinction between the mesostructures and chiral molecular structures. The application in optics is still under-discussed. In fact, this kind of helical mesostructure is independent of the chiral templates. It is not surprising that achiral surfactants can template mesoporous materials with mesoscale chirality. Recently, helical tubes with highly ordered hexagonal mesostructures were also found in an achiral CTAB or SDS surfactant system.342-345 Tang and co-workers reported the synthesis of helical MCM41 mesostructured silica nanofibers in a basic CTAB surfactant system.342 The self-assembly of sodium silicate and CTAB into hexagonal mesostructure is driven by the


Wan and Zhao

Figure 22. Two-dimensional XRD data acquired from (A) a polymerized hexagonal silica-surfactant composite and (B) calcined hexagonal mesoporous silica, both derived from the aligned liquid crystal intermediate. The nonuniform distribution of scattering intensity within the patterns indicates that the samples are oriented, as shown in the accompanying schematic diagrams and that the materials have mesoscopic hexagonal periodicity. (C) XRD patterns of as-prepared mesostructured silica films: (a) film prepared without magnetic field, (b) under the magnetic field parallel to the substrate, and (c) film under the magnetic field perpendicular to the substrate. (D) Cross sections of the as-prepared film under the magnetic field perpendicular to the substrate. Parts are reprinted with permission from Tolbert, S. H.; Firouzi, A.; Stucky, G. D.; Chmelka, B. F. Science 1997, 278, 264 (http://www.sciencemag.org). Copyright 1997 AAAS. Parts are from: Yamauchi, Y.; Sawada, M.; Noma, T.; Ito, H.; Furumi, S.; Sakka, Y.; Kuroda, K. J. Mater. Chem. 2005, 15, 1137. s Reproduced by permission of The Royal Society of Chemistry.

Figure 23. (A) A mesostructurally ordered transparent block copolymer F127/silica monolith. The monolithic disk is 2.5 cm in diameter and 3 mm thick. (B) Photographs of ordered transparent mesostructured block copolymer-silica composites doped with transition metal ions (none, Co2+, Fe3+, and Cu2+, from left to right) of the pep-top column shape prepared by liquid paraffin medium protected solvent evaporation. Reprinted with permission from refs 169 and 331. Copyright 1999 and 2003 American Chemical Society.

hydrolysis of ethyl acetate. The chiralty of the MCM-41 nanofiber may originate from a chiral aggregation of symmetric building blocks. Yang et al.344 prepared helical mesoporous materials with chiral channels in the presence of perfluorooctanoic acid and CTAB. The driving force is proposed by the morphological transformation accompanied by a reduction in the surface area and the surface free energy. After the helical morphology is formed, an increase in the bending energy together with the derivation from the perfect hexagonal mesostructure may limit the curvature of helices. This kind of helical structure has chiral characteristics. Lefthanded and right-handed products are normally half and half (50:50) and are enantiomeric. Until now, only Che et al.96 reported more left-handed fibers than right-handed fibers in

one batch of helical mesoporous silicates. It should be noted that in most cases the helical mesoporous silicate structures were synthesized by using small cationic or anionic surfactants under basic conditions, which may be related to the large reduction of surface areas originated from smalldiameter channels. Lamellar silicas with hierarchical vesicular structures were reported by Pinnavaia and co-workers.346,347 Electrically neutral and unsymmetrical Gemini surfactants of the type CnH2n+1NH(CH2)mNH2 (n ) 12, 14 and m ) 3, 4) are used as the templates. This kind of material is expected to provide optimal access to the framework walls under diffusionlimited conditions. Several other special morphologies of MCM-41, such as tubules within tubules and pillars within



Figure 24. (A) SEM image of chiral mesoporous silicates. (B-D) Schematic drawings of a structural model with cross-section (C) and one of the chiral channels (D) in the chiral mesoporous material. (E-G) TEM images with different enlargements, showing two types of fringes (indicated by arrows and arrowheads) with different spacings. (H) A simulated TEM image, showing good correspondence with the observed image. Reprinted with permission from ref 96. Copyright 2004 Nature Publishing Group.

spheres, have been reported by Lin and Mou.42,348 Soft lithography can also be utilized to make patterned and hierarchical ordered mesostructures.349

6. Summary and Outlook Strategy aimed at the controllable synthesis has been focused on the control of micro-, meso-, and macroscale, including synthetic methods, architecture concepts, and fundamental principles that govern the rational design and synthesis. In this review, synthesis mechanisms and the corresponding pathways are first demonstrated for the

synthesis of mesoporous silicates from the surfactanttemplating approach. Virtually all mesoporous silicates begin with an understanding of the interactions between organic surfactants and inorganic species, as well as among themselves. In combination with some other synthetic techniques, the EISA method, which was known before a method for preparing mesoporous silica thin films, are shown here as a facile method to fabricate highly ordered mesostructured materials. Synthesis factors are essential for a beginner to start their research work, including surfactant selection, hydrothermal method (pH value, synthesis temperature, and


hydrothermal treatment), nonaqueous synthetic technique, separation, drying, postsynthesis (secondary synthesis and recrystallization), and removal of templates. We show here a step-by-step choice according to their principles and recent developments. High-quality mesoporous products will be easily obtained provided that these factors can be fully understood. The controllable synthesis on mesoscale mainly includes mesophases (interface curvatures and arrangement of symmetries), pore sizes, and connecting manners. Besides the general discussion on the mesophase tailoring, the control of the pore size and the pore connecting manner is also presented. Typical mesostructures are introduced by dividing them into 2D, 3D, disordered, and other mesostructures. Soft-templating approach is one of the most general strategies now available for creating nanostructures. The assembly of surfactants and silicate species is normally carried out in solutions or at the interface to allow the required driving force for the formation of nanostructures. Relying on sol-gel, solution, and surface chemistry, there is great potential to explore novel strategies for mesostructures, especially a strategy that can utilize interfacial tension. Items that attract attention also include the control of weak interactions such as the hydrophobic interaction between the assembling components. In view of the fact that surfactant self-assembly can occur with components larger than molecules, the assembly of nanoparticles larger than 1 nm continues to be a challenge and an interest in condensed matter science. The creation of novel mesoporous silica mesostructures is in its fantasy, including mesophases, pores, etc. Novel synthesis strategies that are simple and mild, as well as new surfactants will be much in demand in the future. For example, low-temperature solid-state reaction methods and chemical vapor deposition (CVD) on interfaces could be used for the preparation of ordered mesoporous materials. Amphoteric surfactants, multihead quaternary ammonium ions, chiral surfactants, anionic surfactants, block copolymers (diblock copolymers A-B and triblock copolymers A-BC) and noncovalently bonded A + B type polymers are very useful for the synthesis. Amphoteric surfactants, multihead quaternary ammonium ions, and anionic surfactants are used to control the surface charge matching with silicate oligomers. The adjustment of the surface charges in silicate species also strongly influences Coulombic interactions. Both the charges on the silicate and those on the surfactants offer possibilities for new mesostructures and pore topologies. The mesostructures templated by amphiphilic block copolymers are limited compared with their rich lyotropic mesophases. For example, until now 3D cubic silicate mesostructures with space groups of Pm3hn, Pn3hm, Fd3hm, and Pm3hm have not been obtained by using block copolymers as templates. ABC triblock copolymers possess much richer ordered microdomains and more diverse components compared with ABA triblock copolymers. The hydrophobic/ hydrophilic properties of ABC copolymers strongly depend upon the interaction factors (χAB, χAC, χBC) and the component factors (×c4A, ×c4B). Therefore, dream mesostructures such as Q214 (I4132), Q230 (Ia3h2), O70 (Fddd), and spongelike L3 lamellar structures can be formed that have so far not been realized in mesoporous silicates. Significant work in block copolymers is expected for the manufacturability of diverse mesostructures.

Wan and Zhao

Recent progress on the large-pore mesoporous silicates templated by high-molecular-weight block copolymers (e.g., PS-b-PEO) has led to important advances in the synthesis. We believe that future efforts will be focused on seeking suitable block copolymers to directly synthesize highly ordered mesoporous silicates with pore sizes even larger than 50 nm. Moreover, bimodal and hierarchical pores and chiral pore channels are also on the way. To meet the practical applications, more complex mesoscopic structures, vesicles, single crystals, or large crystals (>20 µm) for structural solutions, etc. are desired. In addition, the functionality of organic modification either in surface or in pore wall matrixes by using siloxane or nanocrystals is worthy to be exploited.350-355 The mesostructure with crystalline pore walls has long been a hot topic. On the consideration that crystallization always accompanies the collapse of mesostructures, the interactions between SDAs and inorganic silicate species or the roles of SDAs get more prominence. If the crystallization can occur at a condition that SDAs are stable inside the mesopores, confined crystallization on inorganic pore walls may come into being. On the other hand, the use of ionic block copolymers with amine headgroups is expected. The block copolymer backbones and the amine headgroups may be used to induce the formation of mesostructures and crystalline (microporous) frameworks, respectively. These advances must be made in conjunction with analytic and resolved studies that will further elucidate the true space on mesoscale. The full-scale characterization of mesostructures, pore sizes, pore connection manners, and morphologies will guide the practical applications in protein separation, catalysis, environment protection, and photonic crystals.

7. Acknowledgment This work was supported by NSF of China (Grants 20233030, 20421303, 20407014, and 20521140450), State Key Basic Research Program of PRC (Grants 2006CB202502 and 2006CB0N0302), Shanghai Sci. & Tech. and Edu. Committees (Grants 03QF14037, 04JC14087, 05DZ22313, 055207078, and 06DJ14006), Shanghai Nanotech Promotion Center (Grant 0652 nm024), Shanghai HuaYi Chemical Group, and Unilever Research Institute of China. We thank Y. F. Shi and D. H. Chen for their assistance. Y.W. thanks China Post-Doc Scientific Fund.

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