Advances in Colloid and Interface Science 189–190 (2013) 21–41

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Soft templating strategies for the synthesis of mesoporous materials: Inorganic,organic–inorganic hybrid and purely organic solids

Nabanita Pal, Asim Bhaumik ⁎Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata, 700 032, India

⁎ Corresponding author. Tel.: +91 33 2473 4971; faxE-mail address: msab@iacs.res.in (A. Bhaumik).

0001-8686/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.cis.2012.12.002

a b s t r a c t

a r t i c l e i n f o

Available online 2 January 2013

Keywords:Self-assemblySoft templatingMesoporous materialsSurfactantsPorous organic materials

With the discovery of MCM-41 by Mobil researchers in 1992 the journey of the research on mesoporous mate-rials started and in the 21st century this area of scientific investigation have extended into numerous branches,many of which contribute significantly in emerging areas like catalysis, energy, environment and biomedical re-search. As a consequence thousands of publications came out in large varieties of national and internationaljournals. In this review,we have tried to summarize the publishedworks on various synthetic pathways and for-mation mechanisms of different mesoporous materials viz. inorganic, organic–inorganic hybrid and purely or-ganic solids via soft templating pathways. Generation of nanoscale porosity in a solid material usually requiresparticipation of organic template (more specifically surfactants and their supramolecular assemblies) calledstructure-directing agent (SDA) in the bottom-up chemical reaction process. Different techniques employedfor the syntheses of inorganic mesoporous solids, like silicas, metal doped silicas, transition and non-transitionmetal oxides,mixed oxides,metallophosphates, organic–inorganic hybrids aswell as purely organicmesoporousmaterials like carbons, polymers etc. using surfactants are depicted schematically and elaborately in this paper.Moreover, some of the frontline applications of thesemesoporous solids, which are directly related to their func-tionality, composition and surface properties are discussed at the appropriate places.

© 2012 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212. Synthesis pathway and mechanism of formation of mesoporous materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.1. Role of template in generating the porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.2. Surfactants as structure-directing agent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.3. Interaction pathways of surfactants and inorganic metal precursors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.4. Factors affecting the surfactant self-assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.5. General formation methods of mesoporous materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.5.1. Synthesis method and mechanism of purely inorganic mesoporous materials . . . . . . . . . . . . . . . . . . . . . . . . 282.5.2. Synthesis method and mechanism of organic–inorganic hybrid mesoporous materials and Periodic Mesoporous Organosilicas (PMOs) 322.5.3. Synthesis method and mechanism of purely organic mesoporous materials . . . . . . . . . . . . . . . . . . . . . . . . . 35

3. Properties and applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395. Future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

: +91 33 2473 2805.

rights reserved.

1. Introduction

Since “the overwhelming tendency for solids to minimize void spacewithin their structures” is inherent, porous materials are difficult tomake naturally. But Albert Einstein said “In the middle of difficulty

22 N. Pal, A. Bhaumik / Advances in Colloid and Interface Science 189–190 (2013) 21–41

lies opportunity.” This impossible task is made possible in 1992 by thescientists of Mobil Oil Corporation (USA), when they first successfullysynthesized MCM (Mobil Composition of Matters)-41/48 throughsurfactant-mediated self-assembly method [1]. This surfactant-assisted templating pathway or soft templating strategy opens anew class of materials named as mesoporous materials. The history ofporous materials began with zeolites having aluminosilicate frame-works, which are conventionally synthesized via non-surfactantassisted route employing single molecule template and having smallpores (micropore) inside it [2]. The most generalized definition of aporous material is a continuous and solid network material filledthrough voids. In the case of nanoporous materials the pore or voidsare of the order of≈1–100 nm. Amaterial can be recognized as porousif its internal voids can befilledwith gas. Sometimes, the voidsmight befilledwith a liquid or even a solid [3]. Generally, pores are classified intotwo types: open pores which connect to the outside of thematerial andclosed pores which are isolated from the outside. Porosity providesmaterials with lower density and higher surface area compared todense materials. For most industrial applications including separation,catalysis and bioreactors open pores can play a crucial rule. Closedporous materials are used mainly for thermal insulation and lowdensity structural components [4]. According to IUPAC convention,depending upon size of the pore, the porous materials are of threetypes: mesoporous, microporous and macroporous. Whereas, basedon the framework building blocks porous materials can be classifiedas: purely inorganic, hybrid organic–inorganic and all-organicporous polymers and carbonaceous materials. The classificationof porous materials is given in a flowchart (Scheme 1). This reviewmainly highlights the soft templating strategies involved in thesynthesis of purely inorganic, organic–inorganic hybrid and all-organic porous polymers and carbons materials, which are mesoporousin nature. We do not discuss in this review about metal–organicframework (MOF) [5] and metal phosphonates [6] since those solidsare commonly synthesized via non-templating route and having mi-cropores in the framework.

Meso, the Greek prefix, meaning “in between”, has been adoptedby IUPAC to define porous materials having dimension of pores inbetween micropore and macropore i.e. typically between 2 and50 nm [7]. Owing to their high stability, surface areas and large

Scheme 1. Schematic representation of the

pore volumes they are used technically as adsorbents, ion-exchangers,catalysts, catalyst supports and in many other related applications[8–10]. According to IUPAC classifications, mesoporous materialscan be ordered or disordered in nature. Ordered mesoporous mate-rials such as MCM-41/48, SBA-15 have uniform and regular arrange-ments of pore (or channel) widths whereas, atomic arrangementsare not ordered [1]. Disordered mesoporous solids like KIT-1 hasuniform channel width but they are not regularly arranged i.e. nei-ther the pore arrangements nor the atomic arrangements are or-dered. Both the pore and atomic arrangements are observed orderedin microporous zeolites [11,12]. Mobil scientists employed long chainorganic surfactant molecules as the structure-directing agent (SDA)during the synthesis of highly ordered mesoporous material MCM-41instead of using small organic molecules, which are conventionallyused as the single molecule template as in the case of zeolites or relatedmicroporous solids [2]. Rather than individual molecule being usedas void filler in the ordering of the reagents to form the microporousmaterials, assemblies ofmolecules dictated by solution energetic are re-sponsible for the formation of these pore systems. During the synthesisprocess, the SDAmolecules, in the form of a lyotropic liquid-crystallinephase, lead to the assembly of an ordered mesostructured compositeduring the condensation with the silica precursors. The mesoporousmaterials are obtained by subsequent removal of the surfactant byextraction or calcination. Since the discovery of mesoporous materialsthrough this type of supramolecular templating pathway, this researcharea has tremendously expanded and has been reviewed intensively inthe recent time [13–15]. Surfactant templating routes have also beenused successfully in the preparation of non-silica based mesoporoustransition and non-transition metal oxides [16], metal sulfides [17],metal phosphates [18] etc. In the following section a brief account ofthe synthetic chemistry as well as mechanism behind the surfactantassisted synthesis along with the various applications regarding thesemesoporous materials has been reviewed minutely. That is, the scopeof this review is mainly focused on various soft templating strategiesemployed (discussed in Section2.5) in synthesizingdifferentmesoporoussolids. However, we have taken a chance to discuss in brief aboutthe hard templating approach (Section 2.5) in order to differenti-ate, and highlight the merits and demerits of it from that of softtemplating strategy.

classification of nanoporous materials.

23N. Pal, A. Bhaumik / Advances in Colloid and Interface Science 189–190 (2013) 21–41

2. Synthesis pathway and mechanism of formation ofmesoporous materials

In the synthesis of mesoporous material organic surfactant mole-cules play a decisive role to generate porosity within the buildingblocks and thus act as template or structure directing agent (SDA).This soft templating method is the most successful pathway for thesynthesis of ordered and disordered mesoporous matrices. Beforegoing to the main topic it is essential to know what is template orwhat kind of interaction occurs between a template and precursorspecies forming the mesoporous framework during the synthesis.

2.1. Role of template in generating the porosity

In broad sense, template means a stencil, pattern or overlay usedin graphic arts (drawing, painting, etc.) and sewing to replicate let-ters, shapes or designs. In nanoporous regime template representsthat particular molecule, which helps to generate or design porosityin the matrix. Thus a template acts as structure directing agent(SDA) in the synthesis of porous materials [19]. The SDAs can be ofdifferent types. Surfactant can be employed as a SDA, the essentialfeature of which is the coexistence of chemically bonded hydrophobic(non-polar) hydrocarbon ‘tail’ and a hydrophilic (polar) ‘head’ groupin a molecule. These molecules have high molecular weight and theyaggregate in solvent to form self-assembled micelle [1,20]. There aresome other SDAs bearing hydrophobic-hydrophilic groups in a singlemolecule, which are not surfactants but play the role of template indesigning mesopores in a material. These template molecules mayor may not form self-assembly [21]. Another type of SDA is dendrimeror polymer, which can be macromolecular single molecule havinghigh molecular weight [22,23]. All the above templates are soft tem-plates (discussed in Section 2.5).

There are also hard templates like porous silica or colloidal silicaspheres, polystyrene etc. to generate porosity within the matrix [24].Strictly speaking, a template is a structure (usually organic, thoughhard template may be inorganic) around which a material (ofteninorganic) nucleates and grows in a “skin-tight” fashion, so thatupon the removal of the templating structure, its geometric andelectronic characteristics are replicated in the (inorganic) materials[25,26]. The classification of soft template SDAs has been illustratedin the flowchart Scheme 2. Sustainable design and fabrication ofmesoporous materials can provide breakthroughs in the advance-ment of energy conversion and storage devices. In this contextblock copolymers has emerged as a powerful and affordable tool in

Template or strdirecting ag

Depending upon functionality

Smmoassso

e.g. CTAB, CPC, etc.Cationic

Surfactants Non-surfactantLarge molecule, high molecular wt, form micelle, e.g. CTAB, SDS etc. Single molecule

template, no self-assembly e.g. TPA etc.

Scheme 2. Schematic representation of v

directing the structure of nanomaterials for photovoltaics, batteriesand fuel cells, where the morphologies of the mesoporous hybrids canranged from hexagonally arranged cylinders to three-dimensionalbi-continuous cubic networks [27]. In this review we will mainly em-phasize upon the surfactant type template associated in the synthesisof mesoporous materials.

2.2. Surfactants as structure-directing agent

Amphiphilic surfactant templating pathway has been defined as aprocess in which an organic species functions as a central structureabout which precursor moieties organize to form long range orderedmaterial [25]. The molecules have both hydrophobic and hydrophilicparts and the hydrophobic component of surfactant can solubilize or-ganic species, while the hydrophilic or ‘water-loving’ component in-teracts with charged inorganic precursors to direct the formation ofthe inorganic framework (Fig. 1A).

Surfactants have both hydrophobic and hydrophilic components inthemolecule, which could behave distinctly in polar and non-polar sol-vents. In aqueous media the polar part of the molecule interacts withwater, while the non-polar part stays away from the water molecules.There are two ways by which such amphilic molecule stabilizes inwater. The amphiphilic surfactant molecule (Fig. 1A) can arrange itselfat the surface of the water such that the polar part interacts with thewater and the non-polar part is held above the surface (either in theair or in a non-polar liquid) as shown in Fig. 1B. The presence of thesemolecules on the surface disrupts the cohesive energy at the surfaceand thus lowers the surface tension. That's why, such molecules arecalled ‘surface active’ molecules or surfactants. Another arrangementof these molecules can allow each component to interact with itsfavored environment. Molecules can form aggregates in which the hy-drophobic portions are oriented within the cluster and the hydrophilicportions are exposed to the solvent [28]. Such aggregates are calledmicelles (Fig. 1C). The proportion of molecules present at the surfaceor as micelles in the bulk of the liquid depends on the concentrationof the amphiphile. At low concentrations surfactants will favor arrange-ment on the surface. As the surface becomes crowded with surfactantmore molecules will arrange into micelles. At certain concentrationthe surface becomes completely loaded with surfactant and any furtheradditionsmust arrange asmicelles. This concentration is called the Crit-ical Micelle Concentration (CMC). Different SDAs has different CMCvalue in water [29]. Beyond the CMC value the self-assembly of micelleoccurs to form 3D spherical or 2D rod like array with further increasingconcentration and this self-assembly helps in the pore generation. This

ucture ents

Depending upon the charge

all molecule, low lecular wt, self-embly, e.g. urea,

dium salicylate etc.

e.g. P123, F127 etc.e.g. SDS, lauric acid etc. Anionic Nonionic

Single macromolecule template, high molecular wt, do not form micelle, e.g. dendrimers.

arious categories of soft templates.

Water

Air

Water

Hydrophobic (non-polar)

hydrocarbon ‘tail’

Hydrophilic (polar) ‘water

loving’ head gr.

A B C

Fig. 1. Surfactant molecule and its behavior in aqueous media.

24 N. Pal, A. Bhaumik / Advances in Colloid and Interface Science 189–190 (2013) 21–41

self-assembled micelle formed by the association of individual amphi-philic templating molecules bonded through weak forces like vander Waals, hydrogen bonding etc. but there is no covalent linkagebetween those amphiphiles [30]. Actually, these SDA molecules arethe ‘placeholder’, what becomes the void space to produce nanoporousmaterial. They not only allow controlling the variation of pore size butalso the shape of the pores i.e. the total architecture of the templatemolecule, its size and shape are imprinted in the porous solid (Fig. 2)[3].

Depending upon the dissociation of surfactant molecule in aqueousmedia there are three types of amphiphilic surfactant molecules [31].

a. Cationic surfactants dissociates in water into an amphiphilic cationand an anion, most often halide type. A large portion of this classbelongs to nitrogenous compounds such as amine salts and qua-ternary ammoniums, with one or several long chain alkyl group,often coming from natural fatty acids. The common examples arecetyltrimethylammoniumbromide or hexadecyltrimethylammoniumbromide (CTAB), cetylpyridinium chloride (CPC), N-Dodecylpyridinium chloride etc.

b. Anionic surfactants dissociates in water into an amphiphilicanion, and a cation, which is in general an alkaline metal (Na+,K+) or a quaternary ammonium ion. Anionic surfactants includealkylbenzene sulfonates (detergents), (fatty acid) soaps, laurylsulfate (foaming agent), di-alkyl sulphosuccinate (wetting agent),lignosulfonates (dispersants) etc. Few common examples are sodi-umdodecyl sulphate (SDS), sodiumdodecylbenzene sulphonate etc.

c. Nonionic surfactants are another type which does not ionize inwater, because their hydrophilic components are non-dissociablelike alcohol, ether, ester, amide etc. A large variety of this type sur-factant is made hydrophilic by the presence of polyethylene glycolchain obtained by the polycondensation of ethylene oxide. Polycon-densation of propylene oxide results a polyether which is slightlyhydrophobic. This polyether chain is used as lipophilic group i.e.non-polar group in the so-called poly-EO-poly-PO block copoly-mers. These copolymers are often included in different classnamed polymeric surfactants. Common examples of non-ionicsurfactants are Pluronic P123, F127 etc [31].

Fig. 2. A common pathway for the fo

Beside these, there is another type of surfactant which is calledamphoteric or zwitterionic surfactants. In this case a single surfactantmolecule shows both cationic and anionic dissociations. Synthetic prod-ucts like betaines, sulphobetaines, natural substances such as aminoacids and phospholipids etc. belong to this category [32]. Dodecyl beta-ines, lauryl betaines are common examples. The structures of somecommonly used surfactants are depicted here for clarity (Fig. 3).

2.3. Interaction pathways of surfactants and inorganic metal precursors

Six different synthesis pathways that reflect the metal-templatemolecular interaction have been readily employed to prepare orderedmesoporous materials under wide range of pHs, temperatures andsurfactant nature and their concentrations [33]. The pathways areS+I−, S−I+, S−M+I−, S+X−I+, S0I0 and N0I0, where S is the surfac-tant, I is the inorganic phase and X is the mediating anionic species,M is intermediate cation, S0 is neutral amine, I0 is hydrated inorganicoligomer and N0 is non-ionic template (Table 1).

For pathways involving electrostatic interaction, charge of theinorganic source is controlled by the pH and isoelectric point. ThepH at which the charge of molecules is zero is called isoelectric pH.Silica species (isoelectric point=2) has a positive charge at pHb2(acidic conditions), neutral charge at pH=2 and a negative chargeat pH>2 (alkaline conditions) (Fig. 4). M41S silica molecular sieveswere originally fabricated at alkaline condition when anionic inor-ganic species (I−) stabilize with cationic surfactant (S+) throughthe S+I− strong interaction [1]. Likewise, silica mesostructure canbe formed with anionic surfactant (S−) at acidic condition (inorganicspecies I+) using the S− I+ approach [34]. The synthetic pathwaysinvolving a mediating ion (e.g., S+X−I+) have a weaker electrostaticinteraction between the cationic precursor I+ and the S+X− activesites which leads to products with rich morphologies [35]. In case ofS−M+I− type the metal salt of aliphatic acid are generally used astemplate. Hence, in highly basic media metal hydroxo species fore.g. Zn(OH)2−, Zn(OH)42−, Zn2(OH)62− components from zinc precur-sor generated which interact with anionic template through metalcounter ion to form lamellar oxide [36]. Non-ionic template and aminesare also used as templates which interact withmetal precursor through

rmation of mesoporous solid [3].

Fig. 3. Some common surfactants used in different field of science.

25N. Pal, A. Bhaumik / Advances in Colloid and Interface Science 189–190 (2013) 21–41

H-bonding in acidic media (S0I0/N0I0 interaction). MSU-X, hexagonalmesoporous silica HMS are synthesized in these fashions [37–40]. Allthese types of interactions are beautifully shown in Fig. 5. Besides, thereare some examples of interaction between organic and inorganic speciesthrough coordination and covalent bond formation. Mesostructured nio-bium oxide synthesized in non-aqueous media is a good example of theformer case whereas hydrolysable silane group containing surfactanttype precursor can condensed through covalent interaction between or-ganic and inorganic parts of samemolecule to form orderedmesoporoussolidwithout template [41,42]. Stucky et al. have developed awide rangeofmesostructured opticalmaterials using a one-step synthesis procedurewhere the inorganic/surfactant/optically active species co-assemble dur-ing the synthesis [43]. The corresponding regularly arranged pores foundin mesoporous metal oxide frameworks provide a high surface area forbetter dispersion of optically active components and allow for rapid dif-fusion, which are essential for optical applications.

Table 1Types of interaction between template head group and inorganic precursor.

Surfactant type Interactionpathway

Interactiontype

Examples

Cationic S+ Electrostaticinteraction

S+I− Tungsten oxide, MCM-41

S+X−I+ Lamellar zinc phosphate, cubic Pm3nsilica

Anionic S− Electrostaticinteraction

S−I+ Lamellar Iron oxide

S−M+I Lameller aluminium oxide, zinc oxide

Neutral S0/N0 H-bondinginteraction

S0I0 Hexagonal silica HMS

N0I0 Silica MSU-X

2.4. Factors affecting the surfactant self-assembly

As it is already discussed that the formation of pore and its size,shape and dimension are entirely dependent on the surfactant supra-molecular self-aggregation, depending upon the various factors likenature of template molecule, temperature, presence of counter ionsin the reaction media, the concentration of surfactants, role of sol-vents etc. the size and shape of the surfactant micelle varies as wellas that of the pore system of mesoporous solids. The structure or ge-ometry of the template micelle may be of different patterns such asspherical, cylindrical, lamellar, bilayer and so on [29] (Fig. 6). CMC,which is the determining parameter of the formation and stabilityof micelle are governed by all these factors mentioned above and isa characteristic of a particular surfactant.

The counter ions like intermediate anions or cations have strongeffect on the metal–template interaction. Generally anions have stron-ger effect than cation and as in majority of cases for the synthesisof mesoporous solids are carried out in the presence of cationic ornon-ionic surfactants [44]. The transformation of spherical micelle ofcationic surfactants to rod like geometry is more favorable when thehydrated size of anion present in themedia decreases and polarizabilityincreases. Upon neutralization the surfactant charge by the counter ion,rod like micelle formation observed and thus small hydrated anionwith high polarizability are more effective in this case. Well knownHofmeister series of anions, SCN−>NO3

−>Br−>Cl−>OH−>F− arefound to induce the sphere to rod transition of alkyltrimethylammoniumsurfactants [45]. On the other hand high concentration of salts like NaCl,KCl, NaBr, Na2SO4 etc. may lead spherical micelle to transform intolamellar form (Fig. 6). The CMC of non-ionic surfactants is also

Si(OR)4Silica

precursor

pH < 2

pH > 2

Ionic inorganic species (I)

Si

H2O

HO OH2

OH

Si

O

HO O

OH

Hydrolysis

Controlled condensation

Si

O

O O

O

Si

O

O

Si

O O

Fig. 4. Silica precursor at variable pH condition.

O

Si

Si

O

Si

HO

OH2

HO

O

Si

OH2HO

OH2

O

OH2HO

-

-

-

-S+X-I+ Interaction

O

Si

Si

O

Si

HO

O

HO

O

Si

OHO

O

O

OHO

S+I- Interaction

O

M

M

O

M

HO

O

O

M

O

O

O

O

O

H

MHO

H

H

H

H

S-I+ Interaction

O

O

Si

O

OH

Si

O O

Si

O

Si

OH

OH

O

S0I0/ N0I0 Interaction

S-M+I- Interaction

+

+

+

O

M

M

O

M

HO

O

O

M

HO

O

O

MHO

O

O

O+

Fig. 5. Different types of metal surfactant interactions.

26 N. Pal, A. Bhaumik / Advances in Colloid and Interface Science 189–190 (2013) 21–41

27N. Pal, A. Bhaumik / Advances in Colloid and Interface Science 189–190 (2013) 21–41

greatly influenced by anions, which lower the respective CMC valueleading to increment of the attractive interactions between non-ionicmicelles [46].

The nature of surfactants and the concentration of it in aqueousmedia have large impact on the size and shape of the self-assembledsuperstructure. In aqueous media when the concentration of amphi-philes changes from low to high, growth of spherical to rod shape mi-celle results though more high concentration leads to lamellar phase.Double-tailed surfactants generally furnish bilayer phase and lamellaraggregate can also be formed from mixed surfactants aqueous solution[46]. Surfactants with large hydrophilic group and small hydrophobicchain form normal spherical micelle whereas if surfactants have smallpolar head group but large brunched tail then spherical reverse micellewill be formed i.e. the head group is inside and tail is outside (Fig. 6). Ifoil is present in aqueousmedia the shape and size of all the self-assemblychanges and if more water is present surfactants organize themselves aslamellar type [47]. Beside this normal micelle can be reversed in totallynon-polar media.

Thermal effect on the salt induced spherical to rod like micelle for-mation of different alkyltrimethylammoniumand non-ionic surfactantswere studied by Zieliński et al. and Lindman et al., respectively [48,49].It has been found experimentally that the size of ionic surfactantmicelledecreases with rise of temperature although for non-ionic surfactantsreverse trend is observed. The temperature effect is more prominentin case of rod like micelle than spherical one [50].

2.5. General formation methods of mesoporous materials

Thorough investigation and continuous research to understand theformation mechanism for the mesostructured materials is going on tilldate since 1992 [35,51]. Surfactant-assisted synthesis of mesoporousmaterials can occur in two pathways: in endotemplate methods(“soft-matter templating”) and in exotemplate methods (“nanocasting”).

In endotemplate (‘endo’ means within) or soft-templating route,around the self-assembly of SDA in liquidmedium inorganic precursors

Fig. 6. Different types of s

arrange themselves in regular ordered array to form an inorganic–organic composite solid i.e. template self-assembly remains withinthe inorganic materials. In this method usually there are three steps toform a mesoporous solid. First, the surfactant self-assembly (aboutwhichwe have discussed in the previous section), second is the orga-nization of inorganic precursor over this surfactant self-assemblyand formation of a stable inorganic–organic hybrid and third or lastis the successful subsequent removal of organic template to get themesoporous solid. Detailed study about the formation process ofthese inorganic-surfactant composite materials has revealed thattwo different mechanisms are involved in endotemplate route. Oneis cooperative self-assembly (CSA) mechanism and another is ‘true’liquid crystal template (TLCT) mechanism [13,52,53]. In former case(CSA) there is a simultaneous aggregation of self-assembled SDAalong with the already added inorganic species and a liquid–crystalphase with hexagonal, cubic, or laminar arrangement containing boththe organic micelle and inorganic precursor can be developed(Fig. 7a). On the other hand, the TLCT mechanism first proposed byMobil scientists is ‘true’ since this pathway covers all the possibility be-hindmesophase formation [1,20]. Here, the concentration of the surfac-tant is so high that under the prevailing conditions (temperature, pH) alyotropic liquid-crystalline phase is formed without requiring the pres-ence of the precursor inorganic framework materials (Fig. 7b) [54].After the formation of the inorganic–organic nanocomposite materialand its further condensation it is necessary to remove the templatefrom this composite (called as-synthesized) to obtain the porosity. Cal-cination in aerial atmosphere is the most familiar method to eliminatethe organic template completely and usedwidely formesoporous silica,oxides and phosphates. Different temperatures are required for remov-ing different templates. Further, high temperature treatment increasesthe degree of crystallinity in the mesoporous material [55]. But in caseof organic modified and unstable, air sensitive materials this methodis not suitable owing to loss of porosity. This problem has been solvedby introduction of solvent extraction method which is mild but veryeffective for removal of surfactant without disturbing the framework

urfactant structures.

28 N. Pal, A. Bhaumik / Advances in Colloid and Interface Science 189–190 (2013) 21–41

structure. Solvents like ethanol, THF with small quantity of dilute hy-drochloride acid, ethylene diammine or ammonium acetate are usedin solvent extraction methods [56,57].

Exotemplate (‘exo’means outside) (“nanocasting”) method is thatwhere a porous solid like silica or carbon or sometimes colloidal crys-talline silica microsphere, polystyrene beads etc. is used as the tem-plate in place of the surfactant. Thus, this method is also known as“hard-matter templating” or hard-templating. In this case templateremains outside the inorganic material and the hollow space thatprovided by the exotemplate framework (usually mesoporous silicaor carbon) are filled with that inorganic precursor, which is thentransformed (cured) under suitable conditions. In this way, thepore system of the template is copied as a “negative image” in thematerial. The now-filled exotemplate is removed by HF or NaOHsolution or by high temperature [58]. After removal of the templateframework, the incorporatedmaterial is obtained with a large specif-ic surface area. This replication method was used for first time byRyoo et al. [59,60] for the synthesis of mesoporous carbon (CMK-1)(Fig. 8).

Note that the terms endo- and exotemplate are formally derivedfrom the terms endo- and exo-skeleton used in biology. In addition,there is another method named evaporation induced self-assembly(EISA) (discussed later) for synthesis of mesostructured silicates andtransition metal oxides (TMOs) [61–63]. Brinker et al. have reportedthe synthesis of mesostructured thin silica films derived frommethyltriethoxysilane (MTES) and/or tetraethyl orthosilicate (TEOS)silica precursors and polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymers via EISA process [61]. The meso- and micro-structure of the calcined films consists of cubic-ordered arrays of spher-ical mesopores of ca. 5-7 nm in diameter, together with PEO-inducedinterconnected micropores of ca. 1 nm in diameter, as determinedby HRTEM, N2 sorption, gas permeation and grazing incidencesmall-angle X-ray scattering studies. Mesoporous organic polymerresin and carbon based solids are also synthesized in this strategy[64]. Beside these general methods each type porous solids like sili-ca, phosphates, mesoporous polymers, carbons, oxides etc. have in-dividual specific methods which are applied to synthesize thoseunder variable conditions. In addition Ariga et al. have reviewedthe recent research to design nanomaterials into organized struc-tures, called nanoarchitectonics which involve the syntheses ofmesoporous silicas, metal oxides, semiconductive materials, metals,alloys, organic composites, biomaterial composites, carbons, carbonnitrides, and boron nitrides, as innovative components [65]. Allthese materials are being discussed in the following section under

Fig. 7. Formation of mesoporous structures: (a) via co-operative

the category of purely inorganic, organic–inorganic hybrid andpurely organic porous materials (Scheme 1).

2.5.1. Synthesis method and mechanism of purely inorganic mesoporousmaterials

Syntheses of silica, non-silica transition and non-transition metaloxide, metallophosphate, sulphide based materials are illustrated underthis topic.

2.5.1.1. Silica basedmesoporousmaterials.Due to several advantages likea great variety of possible structures (flexibility of tetra-coordinatedsilicon atom), a precise control of the hydrolysis-condensation reac-tions (because of lower reactivity), enhanced thermal stability of theobtained amorphous networks (no crystallization upon thermal treat-ment) and strong grafting of organic functions as well as for the appli-cations in numerous promising fields extensive work is going on oversilica-based mesoporous materials since the discovery of M41S familyof mesoporous silicas. Mesoporous pure silica and silica based mate-rials are generally prepared in endotemplate method under hydrother-mal condition using basic or acidic media.

Hydrothermal condition is actually a sol-gel process consisting of anumber of steps. Initially, formation of surfactant self-assembly occursvia TLCTmechanism to form a homogeneous surfactant solution in com-mon solvent media (usually aqueous). Then silicate precursor, such astetraethyl or tetramethyl orthosilicate (TEOS or TMOS) or inorganic so-dium silicate is added to the surfactant solutionwhen it gets hydrolyzedunder acidic or basic condition to form a silicate oligomer sol. Next,these oligomers are condensed with surfactant micelle via coopera-tive assembly and aggregated to form inorganic–organic hybrid,which finally precipitates in the form of a gel. The gel is treated hy-drothermally for further condensation, solidification and reorganiza-tion of the material to an ordered arrangement [51,66]. Finally, aftercertain time the resultant product is cooled, filtered, washed anddried (Fig. 9). Ordered mesostructured silica material is obtained fromthis as-synthesized solid after the removal of surfactant through calci-nation or solvent extraction [67,68].

In basic condition during hydrolysis silica precursor remains as sili-cate anion (Fig. 9) and condensation, polymerization as well as cross-linking take place in the pH range of 9.5 to 12.5 [51]. This is the mostwell-known, old and convenient method of silica synthesis. MesoporousMCM-41, MCM-48 [69], etc. and various transition as well as non-transition metal doped silica [70,71] are synthesized using this strategy.Commonly, CTAB, SDS are used as surfactants [70], sodium hydroxide(NaOH), potassium hydroxide (KOH), ammonia solution (NH3.H2O),

self-assembly, (b) via true liquid–crystal templating process.

Fig. 8. Schematic representation of ‘exotemplate’ method.

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tetramethylammonium hydroxide or TMAOH [(CH3)4NOH] andtetraethylammonium hydroxide or TEAOH [(C2H5)4NOH] are used asbase to control the pH of the synthesis gel. There is also example ofmesoporous silica (HMS-3) synthesis in basic media at freezing tem-perature [72]. The discovery of SBA type ordered mesoporous silicamaterials having thick pore walls and large pores synthesized withthe use of non-ionic triblock copolymers like Pluronic P123 andF127 as SDAs under acidic conditions is an important milestone inthis context [73]. In highly acidic media the oxide linkage of Pluronicsurfactants breaks through protonation and micelle formation oc-curs. Silica precursor forms cationic species (Fig. 9) which condenseswith surfactant micelle [74]. It is believed that acidic condensation ofsilica is slower than basic. Hydrochloric acid (HCl), nitric acid(HNO3), sulphuric acid (H2SO4) sometimes weak acids like phospho-ric acid (H3PO4), acetic acid (CH3COOH) etc. are also used as catalyst

Fig. 9. Stepwise formation of mes

for silica synthesis under this condition. The hydrothermal tempera-ture for silica remains in the range of 263 K to 303 K depending uponthe surfactant nature [51]. Though medium room temperature ispreferred for gaining high crystallinity owing to slow hydrolysis. Alarge mesopore of 6–7 nm with consecutive generation of small mi-cropore on the amorphous pore wall of SBA type material is themain characteristic of using this type of Pluronic surfactant.Miyazawa et al. has also beautifully established that this microporevolume within mesoporous pore wall can be controlled by varyingtemperature as well as Si/surfactant ratio [75]. Following the abovestrategies other heteroelements like Ti, V, Co, Fe, Mn, Cr, Zn, Cu, Wetc. which have similar cationic size and coordination sites same asSi, are experimentally incorporated into the silicate framework,which showed high thermal stability and high surface area togetherwith multipurpose applications [76–79]. Most of these doping with

oporous silica material [68].

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high loading occurs in basic media having S+I− interaction whereasthere are few reports of acidic media synthesis with lowmetal incor-poration [80].

2.5.1.2. Non-silica based mesoporous materials. The surfactant templatingstrategy to the synthesis of non-silica based mesostructure; mainlymetal oxides have been commenced since 1993 [81]. Other severaltypes of non-siliceous mesostructuredmaterials like phosphate, sulphidebased materials and also different types of mesoporous metals areknown [82–85]. Sayari has summarized the synthesis of mesoporousmolecular sieves (pure and modified silicates, other metallic oxidesand sulfides aswell as aluminophosphates ofMCM-41,MCM-48, SBA-n,MSU-n structures) under a wide range of conditions in the presence ofcationic, anionic, gemini, or neutral surfactants [86]. The pore size ofthese mesostructured materials may be adjusted from ca. 2–10 nmusing different strategies. Because of their unique flexibility in termsof synthesis conditions, pore size tuning and framework composition,these materials have been targeted for a number of potential applica-tions, particularly in catalysis.

Mesoporous oxides have the great potential to various fields suchas heterogeneous liquid phase catalysis [87], photocatalysis [56],magnetism [88], optoelectronics [89], integrated optics [90] and bio-technology [63] etc. It is often observed for transition metal ions thatthe multitude of possible coordination numbers and oxidation stateshas a pronounced influence on the observed composite mesoporousmaterials as expected. After the first successful approach towards thesynthesis of mesostructured titania [16] various mesoporous metaloxides and mixed oxides of Nb, Ta, V, W, Zr, Sn, Hf, Al, Zn, Cu, Ni,Al etc. have been synthesized through the mediation of electrostatic,hydrogen-bonding, covalent and van der Waals interactions be-tween metal and template [91–94].

Mesoporous oxides having same type of extended frame-work containing metal-oxygen bonds like silica and silica basedmesostructured materials should have similar synthetic conditions.However, very little attention was paid on oxide based mesostructuredmaterials compare to other silica based composites. The reason behindis mainly [34]:

A. The high reactivity toward hydrolysis and condensation of transi-tion metal oxide leads to uncontrolled phase separation betweenorganic and inorganic components, yielding non-mesostructuredmaterials, but porous gels.

B. The redox reactions, the possible phase transitions and the crystalli-zation processes are often accompanied by the collapse of the struc-tural integrity.

C. Synthesis procedures are extremely sensitive to many external pa-rameters, leading to irreproducible results in some cases.

The first stable oxide (mesoporous titania) was prepared bycontrolled hydrolysis of titanium isopropoxide in the presence oftetradecylphosphates through ‘modified sol-gel method’, usingacetylacetone as a chelating agent [16]. Though, IR spectroscopy mea-surements revealed the presence of phosphate groups after calcinationof the sample at 623 K. This result prompted the researchers todesign other mesostructured transition metal oxides via differentsynthetic routes.

The synthesis of porous oxides can be carried out via both‘exotemplate’ and ‘endotemplate’ methods. For all the methods it wasfound that in order to form a successfulmesophase of oxides, three con-ditions should be satisfied [30]. First is the ability of formationpolyanions or polycations by the inorganic metal precursor toallow multidendicity. Second must be the ability of polyanions orpolycations to condense into rigid walls (Fig. 10A). Finally, a chargedensity matching, which is an essential to explain the interactionbetween the surfactant and the inorganic species and thus to directthe formation of a particular mesophase [94]. Recently, P. Bruceand his co-workers have reviewed different template synthesis and

applications of ordered mesoporous metal oxides supplying clear in-formation about the mechanism [95].

In ‘endotemplate’ or soft-templating route mesoporous oxide ma-terials are generally synthesized in hydrothermal method [91], at lowtemperature (freezing) [90] or at room temperature [96]. In all theabove methods template molecules self-assemble prior or simulta-neously with the inorganic metal precursor (Fig. 7) which form ahydroxo species and interact with template molecules in electro-static way to form metal–template composite in aqueous sol–gelprocess [34]. After the removal of template by calcination orsolvent-extraction we get the desired solid porous metal oxide(Fig. 10B). A large variety of metal oxides have been synthesizedthrough supramolecular self-assembly method by proper selectionof a cationic, anionic or non-ionic surfactants. Hydrothermal synthe-sis of hydrous crystalline mesoporous RuO2 is reported by Oh et al.using cationic surfactant hexadecyltrimethylammonium chloride(C16TMA+Cl−) via chloride anion mediated S+X-I+ interactionof cationic surfactant and ruthenium nitrosyl precursor [97].Mesoporous mixed oxides can be prepared through hydrothermalmethod using Pluronic P123 template which is observed in case ofmesoporous titania-silica or Ga-Nb mixed oxide (synthesized viaself-assembly hydrothermal assisted approach or SAHA method)[98,99]. Anionic template SDS is also employed for the synthesis of high-ly orderedmesoporousmixed oxide TiO2-Fe2O3 phase [100]. In the EISAmethod [101,102] nonaqueous sol–gel route is employed, where organ-ic solvents like ethanol, propanol etc. are used as reaction medium. Thismethod for the synthesis of mesostructured materials has proven to bean extremely useful process for both controllingmacroscopic form (thinfilms,membranes, andmonoliths) and enabling the synthesis of a seriesof mesostructured metal oxides [62]. Because of using non-aqueousmedia and block copolymer SDA the hydrolysis rate as well as theredox reaction, phase transformation etc. have been minimized in thisnonaqueous sol–gelmethod [35]. Thismethod based on supramolecularself-assembly of surfactant has several advantages than aqueous sol–gelprocess for achieving better control over particle formation, high crys-tallinity at relatively moderate reaction temperature (373–573 K) andhomogeneous particle morphology within one reaction system [103].Here, the evaporation of the volatile solvent (usually ethanol) followingdeposition concentrates the surfactant molecules and inorganic precur-sors, driving their co-assembly to form ordered mesophases. Subse-quent aging, heat or chemical treatments induce inorganic precursorcondensation and lock in the mesostructure (Fig. 11). The addition ofmetal chlorides to an alcohol generates HCl in situ, yielding stablechloroalkoxy precursors. The slow introduction of moisture from theambient environment causes these precursors to partially hydro-lyze. These stable, hydrophilic precursors are arranged by theblock copolymer SDA. Different mesoporous transition and non-transition metal oxide and mixed oxides with semicrystallineframeworks have been synthesized in EISA route in highly acidicalcoholic solutions in presence of non-ionic Pluronic surfactantsF127, P123 etc. [64,104,105]. Mesoporous titania thin film with con-trolled size and wall thickness is produced recently in this methodusing a new triblock copolymer template PEO–PB–PEO structure(PEO = poly(ethylene oxide) and PB = polybutadiene) [106]. ThisEISA method is widely employed for the synthesis of a huge numberof mesoporous binary and ternary mixed oxides [107–109]. A delayedhydrolysis and condensation of the reacting precursors in order toachieve sufficiently ordered arrangement and homogeneous mixtureof multiple oxides via EISA method is observed in case of silica–germa-nia mixed oxide [110].

Another process for mesoporous oxide synthesis is ‘exotemplate’or hard-templating or nanocasting method (Fig. 8) pioneered by thegroup of Ryoo [59,60]. It is one of the most important strategies forsynthesizing mesostructured transition pure or multi metal oxides.Recently, many interesting porous metal oxides and mixed oxideshave been synthesized using this method [111,112]. The method is

Fig. 10. A: Stabilized highly reactive metal precursor for metal oxide synthesis and B: interaction of metal-surfactant to form mesostructure.

Fig. 11. Schematic model for EISA method.

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not discussed here elaborately since this is beyond the scope of thisreview.

Although many open-framework and layered metal phosphateswith diverse compositions and properties have been developed[113–115] their applications are constrained by the lack of large sur-face areas and the dominance of small pores or interlayer spaces.Mesoporous metallophosphates having good surface area are ofkeen attention in this respect, due to their applications in the fieldsof catalysis, ion exchange, proton conductivity, intercalation chemis-try, photochemistry, and materials chemistry [116]. For playing acrucial role as active solid catalysts in various acid-base catalysis,redox catalysis and photocatalytic processes [117,118] much workshave been devoted to the synthesis of mesoporous metal phosphatematerials [119,120]. The discovery of aluminophosphates (AlPO,similar to zeolite), AlPO4, in 1980s with reasonable thermal stabilityand novel structures is an important development in this area [121].But the material was microporous in nature synthesized using non-surfactant organic amine salt. Probably the first mesoporous phos-phate synthesized in surfactant mediated pathway was aluminiumand gallophosphate reported by Holland et al. [119]. In this contexta rich varities of metal phosphates/phosphonates with mesoporousframework have been reported, synthesized via ionic, non-ionictemplating routes [122–125].

Porous inorganic phosphates are oxide type materials with openframework and the synthesis of those phosphates can be carried outfollowing the same procedures utilized for mesoporous silica as wellas oxides i.e. the same supramolecular self-assembly of surfactantsfollowed by the interaction of ionic or non-ionic inorganic metal phos-phate with charged or neutral template and further condensation toform mesoporous solid. Mesoporous phosphates generally have theframework structure alike porous oxide consisting tetrahedral or octa-hedral MO4 or MO6 unit, but here is alternation of MO4 and PO4 unitesin the moiety. Mesoporous aluminophosphate shows an alternativetetrahedral AlO4 and PO4 unites, whereas silicoaluminophosphate hasanother additional SiO4 unit connected by shared oxygen molecule[126]. Similar to oxides, phosphates can be synthesized both in aqueousandnonaqueous sol–gel routes. In aqueous sol-gel route generally, CTAB[126], SDS, tetradecyltrimethylammoniumbromide (TTBr) [127] etc. areused as surfactants, inorganic phosphorous sources are phosphoric acid(H3PO4) [126], disodiumhydrogen phosphate (Na2HPO4), diammoniumhydrogen phosphate etc. [127] and pH is adjusted by liquid ammonia,TMAOH, NaOH etc, though preferably TMAOH results more stable struc-ture [126]. For e.g. surfactant template approach using CTAB has beensuccessfully employed for the synthesis of mesoporous highly acidiczirconium phosphate at room temperature in aqueous sol–gel method[128]. Nonaqueous sol–gel route is also important to synthesize wellarranged mesoporous phosphates where ethanol, propanol etc. areused as nonaqueous solvents [129]. Hydrothermal treatment or some-times room temperature helps to form the desired mesostructure[126,127]. Sometimes EISA method is effective to form mesoporousmetallophosphates and bioactive glasses [130–132] In Fig. 12 we haveprovided a schematic presentation of the synthesis of mesoporousmetallophosphate by using EISA method. Kapoor et al. has establishedconvenient room temperature sol–gel method for the synthesis ofmesoporous high surface area zirconium–titanium mixed phosphateusing cationic surfactant octadecyltrimethylammonium chloride(C18TA+Cl−) [133]. Synthesis of mesoporous metallophosphatevia hard-template route or exotemplate' method are also very commonnow-a-days. A report by Luo et al. for the synthesis of cubic orderedyttrium phosphate using KIT-6 as hard template [134] and by Xia etal. for mesoporous hydroxyapatite (Ca10(PO4)6(OH)2) using CMK-3[135] is worthy to mention here.

Analogous to the oxide-based porous materials there are inorganicchalcogenides (sulfides and selenides), which have received an in-creasing amount of attention in modern time after Stupp et al. first re-port of mesoporous cadmium sulfide and selenide in the year 1996

[136]. Use of porous sulfides as semiconductors, sensing material,electronics [137] etc. makes those an imperative in nanoscience andnanotechnology for which many sulfides and selenides of Zn, Sn, Cd,Mo, Fe, Ga and so on are synthesized in surfactant mediated supramo-lecular pathway [138] similar to silica. In case of porous sulfide syn-thesis hydrogen sulfide (H2S) is used as sulfide source along withthe respective metal salt precursor and ionic or non-ionic organictemplate leads to the formation of the particular metal sulfides viaLCT mechanism [94].

Zero valent transition metals are very important in many catalyticprocesses and hence the synthesis of nanostructured metals with highsurface areas and controlled porosity imposes a significant challengeon materials scientists. In 2008, Yamauchi et al. reported the aqueoussyntheses of mesostructured metallic platinum with large pore diame-ter exceeding 10 nm, i.e. giant mesocages by surfactant liquid–crystaltemplating pathway [84]. Hao et al. has nicely summarized mainprinciples governing the usage of soft vesicle in the synthesis ofhard materials (mainly porous metals and metalic nanoparticles)and detailed procedures for vesicle templating and the characteriza-tion of the synthetic mechanisms [139].

2.5.2. Synthesis method and mechanism of organic–inorganic hybridmesoporous materials and Periodic Mesoporous Organosilicas (PMOs)

In addition to the purely inorganic porousmaterials, another impor-tant way of modifying the framework structure of nanoporous materialis the incorporation of organic components, either on the silicate sur-face, inside the silicate wall, or trapped within the channels. Organicfunctionalization of these solids permits the tuning of surface properties(e.g. hydrophilicity, hydrophobicity, binding to guest molecules), alter-ation of the surface reactivity, protection of the surface from attackalong with modification of the bulk properties of the materials, and atthe same time stabilization of the materials towards hydrolysis [140].In view of the advantages and properties of these materials, they canfind wide range of applications in catalysis [141], adsorption chemistry[142], electronics [143], sensors [144], environmental technology [145]and bioapplication [146] etc. introducing suitable functional groupsinto the walls. Stein et al. have described the generalized methodsof preparing organic–inorganic hybrid mesoporous silicates withuniform channel structures bearing both reactive and passiveorganic groups in the porous solids by grafting methods or by co-condensation under surfactant control [147]. Functional groupscould be placed selectively on the internal or external pore surfaces oreven within walls of the mesoporous solids. Such functionalized hybridmesoporous silicates are especially important in catalysis, sorption ofmetals anions, and organics reactors for polymerization, fixation of bio-logically active species, and optoelectronics.

After successful introduction in 1999 [148], a number of organic–inorganic hybrid silica or periodic mesoporous organosilica (PMO)materials with various organic bridging groups have been synthe-sized [149–157]. Owing to the stability, selectivity, high density ofbinding sites and ease of modification these organic-inorganic hybridmaterials have versatile potential in different fields of research. ThesePMOs can be synthesized by using a large variety of organosilanecompounds [(R′O)3Si\R\Si(OR′)3, R=CH2CH2, C6H4, etc., R′=CH3,C2H5 etc.], where two trialkoxysilyl groups connected by an organicbridge. These materials have a homogeneous distribution of organicfragments and silica moieties within the framework. Thus these ma-terials can be designed to carry from simple to bulky functionalities.A large variety of organic bridging groups starting from ethylene,benzene to complex large heterocyclic bridging groups [150] as wellas fluorophore moieties [157] can be incorporated inside themesporous pore walls of these PMOs using soft-templating routes.Corma et al. have developed a chiral periodic mesoporous organosilicaChiMO with 2D-hexagonal periodicity in the PMO framework usingchiral vanadyl salen complex bearing two peripheral trimethoxysilylgroups as organosilane precursor [155]. This chiral PMO gave 30%

Fig. 12. Schematic representation of mesoporous phosphate synthesis in EISA method [132].

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enantioselectivity in the cyanosilylation of benzaldehyde. Self-organization in these organic-inorganic hybrid materials leads tomolecular-scale and mesoscale periodicities. Inagaki et al. has de-scribed the synthesis of organosilicas with molecular periodicityof organic units formed by self-organization of bissilylated organicmonomers without employing templates, as well as PMOs with meso-scale periodicity in the presence of soft templates [156]. The uniquestructural features of these materials make them potentially useful ina wide range of advanced applications, such as selective separation,sensing, catalysis, optoelectronics, light-harvesting, etc [156].

Organic–inorganic hybrid materials comprise mainly the silicabased materials though there are some reports on hybrid phosphates[6,158], microporous metal organic frameworks or MOFs [159] andpolymers (discussed later). Hybrid phosphates and phosphonatesare mainly synthesized in the absence of template or by using singlemolecule templates. Kimura has synthesized few organic-inorganichybrid mesoporous alumnophosphates via surfactant templatingroute [160]. One of the aluminum organophosphonates (AOPs) withmethylene groups inside the inorganic-organic hybrid frameworkshave been synthesized by using oligomeric surfactants (C(16)EO(10)

and C(16)EO(20)) and triblock copolymers (EO(80)PO(30)EO(80), EO(106)

PO(70)EO(106), and EO(20)PO(70)EO(20)). Some of these materials haveperiodicity in pore arrangement and are potential candidates foradsorption, ion-exchange and catalytic applications.

Organic functionalized silica molecules can be synthesized inthree pathways depending upon supramolecular templating mecha-nism (Fig. 13). First one is the “grafting” method where the pore sur-face of a purely inorganic silica material is ‘grafted’ by subsequentmodification with other organic groups or organosilica precursors.Second one is the “co-condensation” method where simultaneouscondensation of corresponding silica and organosilica precursors‘co-condensed’ to form hybrid silica. If uniform surface coverage withorganic groups is desired in a single step, the direct co-condensationmay be the first choice which provides better control over the amountof organic groups incorporated in the structure. However, productsobtained by post-synthesis grafting are often structurally better

defined and hydrolytically more stable. The third type is periodicmesoporous organosilica (PMO) formation method i.e. bridging or-ganic units are directly incorporated in the three-dimensional net-work structure of the silica matrix through two covalent bondsgenerally in “co-condensation” route [161–163] and thus distributedhomogeneously in the pore walls, which could effectively bind anactive catalytic site at the mesopore surface.

At the very beginning organic functionalized mesoporous silicaswere synthesized via post-synthetic modification or post-grafting ofthe inner surfaces of mesostructured silica materials. In this proce-dure, generally the condensation of organosilanes is carried outwith free surface silanol groups of the mesostructured silica resultingin an inorganic framework with an organic layer grafted onto thisframework. Different organosilane precursors commercially availablecan be used in this purpose (Fig. 14). On proper modification withother organic functional groups before or after grafting a great varietyof organo functional groups can be incorporated into the silica frame-work. For example\SH group on oxidation by dilute aqueous hydro-gen peroxide can be converted into reactive \SO3H group, \NH2 or\Cl group on condensation with aldehydes, other amines can gener-ate newly functionalized silica. Synthesis of an \COOH functionlizedSBA-15 material [164] using APTES as organosilane source has beendepicted schematically in Fig. 15A. Utilization of azide functionalizedSBA-15 to form a new free amine based organosilica via [3+2] Azide–alkyne cycloaddition ‘click’ reaction is an effective strategy for post-functionalization of the surface of mesopores by a reactive organic moi-ety [165]. Post-grafting method has several advantages. Firstly, theinitial mesostructure of the silica material is generally preserved,depending on the amount of grafted organosilane. Secondly, owing toa huge number of suitable organosilanes, a wide range of organic func-tionalized silicas can be synthesized with different chemical and physi-cal properties. Again if bulky organosilanes are used, these groups canselectively be grafted at the pore openings, leading to completeclosure of the pores thus sealing the air within and giving rise tolow-кmaterials [162]. Many large highly reactive, chiral organic groupslike 2,2′-bis(diphenylphosphinooxide)-1,1′-binaphthyl (I-BINAPO) has

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been successfully utilized to synthesize highly ordered 2D-hexagonalPMO [166]. Using these post-synthesized modification procedures or-ganic functional groups incorporated in the silica matrix may be foundboth on surface i.e. inner and outer surface of the silica pore wall. Avery useful technique for controlling the site-selective functionalizationof mesoporous silica is the condensation organosilane with templatefilled as-synthesized silica particles in order to minimize the innerwall functionalization of silica to a great extent. Here the amount anddensity of organic group on the outer surface of silica can be controlledby adjusting organosilane to silane ratio [167].

Compared to the post-grafting technique, the co-condensationmethod is a one-pot synthesis procedure. Here, the hybrid materialis prepared through co-condensation of tetraalkoxysilanes withterminal trialkoxyorganosilanes or bis trialkoxysilane (some ofwhich are shown in Fig. 14) in the presence of a structure directingorganic template (Fig. 15B). There are also a number of interestingorganosilanes which can be prepared in the laboratory to synthe-size new PMO [154]. The organic functionalities of these materialsare incorporated into the framework during the formation ofmesostructure and hence more regularly distributed throughoutthe network compared to the organosilica materials obtained viathe grafting method [168]. However, the homogeneous distributionof the organic units is largely dependent on the hydrolysis and con-densation rates of the silica and organosilica precursors. Rate differ-ences for the hydrolysis or condensation process of the differentprecursors can lead to a significant amount of self condensation.

Fig. 13. Synthetic pathways of organic–inorganic hybrid mesoporous silica: 1. Post-synthesisbridged periodic mesoporous silica.

The integration of organic bridges in the pore channel walls makeorganic-inorganic hybrid materials very unique as in the case of PMO.Having a high organic loading these PMOs can have very narrow poresize distributions, high specific surface areas, large pores, thick porewalls and large pore volumes. Furthermore, as the organic units areembedded in the channel walls, these functionalities are easily acces-sible for further modification. The most obvious advantage is theirpore sizes, which can be engineered from 2 to 30 nm, while retainingnarrow pore size distributions. This broadens the applicability ofPMOs in industrial as well as pharmaceutical world. The higher cata-lytic activity is attributed to the textural and morphological featuresof these materials in addition to the improved hydrophobicity insidethe pore channels.

In a generalized synthesis of a PMO via co-condensation route, thebridging organosilane precursor of the type (R1O)3Si\R\Si(OR1)3 isallowed to hydrolyze along with TEOS under acidic or basic pH condi-tions in the presence of a soft template. This is followed by hydrother-mal treatment to yield the as-synthesized hybrid material, which ontemplate removal by acid extraction results PMO material graftedwith organic group (R) in the pore wall (Fig. 16). Calcination is notsuitable to remove the template molecules as in this case the materi-al itself bearing organic group at its pore wall, which could be burntout from the material during calcination. Initially, bridging R waslimited to simple organic groups such as ethylene, phenylene andthiophene etc. [148,170], but recent improvement of synthetic pro-cedures of organosilane precursors has enhanced the possibility of

functionalization or post grafting, 2. Co-condensation or in situ grafting, and 3. Organic

Fig. 14. Different organosilane precursors for synthesizing organic–inorganic hybrid mesoporous silicas including PMOs.

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incorporation of a variety of organic groups ranging from hydrocar-bon and metal complexes to heteroaromatics in the silica framework[150]. Multifunctional PMO can be synthesized using two or moredifferent organosilane precursors. Beside these organically function-alized silicas or organosilicas synthesized via any of the above proce-dures, introduction of metal or metal complex in the organicfunctionalized mesoporous silica are an interesting material for ca-talysis, adsorption study, optics, magnetism or sensors. Mesoporousorganosilica having large tunable porosity, high surface area as wellas containing donor sites like \NH2, \SO3H, \COOH, \OH, \CNgroups offer a good possibility of metal incorporation and their stabi-lization on the silica pore wall via coordination or covalent bonding.Various transitionmetal ion or complexes of Mn, Fe, Co, Cu, Pd, Pt etc.are extensively used in this purpose to introduce new property in thePMOmaterial. Periodic mesoporous silica containing diimine moietyfound an excellent scavenger for grafting of oxo-vanadium Schiffbase complex and the resulting material showed excellent catalyticefficiency in the liquid phase partial oxidation reactions [169].

2.5.3. Synthesis method and mechanism of purely organic mesoporousmaterials

Porous organic polymers are attracting increasing attentions in re-cent time for their wide scope in surface chemistry, especially in

adsorption and catalysis [171–174]. Carbon based porous materialsare also indispensable due to their abundance and sustainability.Their applications vary widely, e.g. as electrode materials for batte-ries, fuel cells, and supercapacitors, as sorbents for separation pro-cesses and gas storage, bioapplication and as supports for manyimportant catalytic processes [175–179]. Their use in such diverse ap-plications is directly related not only to their superior physical andchemical properties, such as electrical and thermal conductivity,chemical stability, and low density, but also to their wide availability.Extensive applications of these porous ordered carbons as ‘hard tem-plate’ for designing highly ordered mesoporous transition metal ox-ides and mixed oxides, zeolites, nitrides are also well established[180–182].

2.5.3.1. Mesoporous polymers. Only a few studies have appeared sofar on the soft templating strategy for the synthesis of orderedmesoporous polymers [183–187]. The choice of the polymer precursoris the key to the successful organization of the organic-organicmesostructures. The conditions for polymer formation which have tobe fulfilled are (a) dissolution in the same medium as the surfactants,(b) interactionwith the template molecules, (c) organizing itself neatlyaround the template, (d) further polymerization, preferably forming arigid polymer structure, without losing the interaction with the

Fig. 15. A: Schematic diagram for the synthesis of an acid (\COOH) functionalizedsilica SBA-15 (AFS-1). B: Schematic pathway of synthesis of a \NH2 functionalizedsilica in co-condensation method.

36 N. Pal, A. Bhaumik / Advances in Colloid and Interface Science 189–190 (2013) 21–41

template and (e) finally the removal of template without destroyingthe polymer mesostructure. Prior to the development of self-assembly soft-templating syntheses, mesoporous polymers weremain-ly synthesized by a hard-templating approach. In this approach, amonomer is infiltrated into a hard template (for example, anopal-like colloidal assembly of silica), which is removed after polymeri-zation to give mesoporous polymer networks [188]. The soft-templatingself-assembly approach represents a breakthrough allowing the efficientsynthesis of mesoporous polymers (Fig. 17). A review on the designedsynthesis of mesoporous solids, including mesoporous polymers, via thenonionic surfactant templating method, has been published recently[189]. Most of themesoporous polymers are prepared by the EISA (evap-oration induced self-assembly)method, typically using ethanol as solvent(Fig. 17). However, similar materials can also be prepared in basic aque-ous media by a liquid crystal templating or cooperative assemblingpathway [190].

Most of the reportedmesoporous polymers are phenol-formaldehyde(resol) resins (template polymer), mainly synthesized as a precursor formesoporous carbon. Mesoporous phenol-formaldehyde resin can bereadily prepared through the assembly of oligomers with surfactant,followed by polymerization and surfactant removal. More specifically,the phenolic oligomers are prepared readily by heating phenol andformaldehyde in a molar ratio of 1:2 in a basic aqueous medium at343 K for 1 h. After the reaction, the solution is neutralized withhydrochloric acid and the water is removed under vacuum, formingthe oligomers. This oligomer and a Pluronic surfactant on evaporationinduced self-assembly (EISA) process lead to formation of an or-dered surfactant-oligomer nanocomposite thin film which on sub-sequent heating at 373 K for 24 h polymerizes further and formsmesostructured surfactant-polymer nanocomposite containingliquid-crystalline surfactant [64]. Decomposition of the surfactantby heating at 623 K for 5 h in nitrogen inert atmosphere or by

sulphuric acid (H2SO4) extraction method produces mesoporous poly-mer thin films (Fig. 17). Mesoporous polymers with ordered two di-mensional hexagonal (p6mm), three dimensional cubic (Im3m) orlamellar structures [64] can be synthesized using Pluronic surfactantF127 (EO107PO70EO107) or P123 (EO20PO70EO20, EO = ethylene oxide,PO = propylene oxide). In similar fashion, phloroglucinolic oligomerscan be prepared by heating phloroglucinol with formaldehyde inacidic water-ethanol at room temperature. Like silicate clusters,these oligomers contain a large number of hydrophilic hydroxylgroups, which allow them to form hydrogen bond with a Pluronicsurfactant.

2.5.3.2. Mesoporous carbons.Mesoporous carbons are often producedby using a mesoporous ordered silica material as a hard-template[191]. More recently, also polymers (e.g. polyacrylonitrile) synthe-sized in situ in the pores or voids of mesoporous silicas have beenused as carbon precursors, leading to mesoporous carbons whichare in some cases even partially graphitic [192]. Microporous carboncan be prepared using zeolite hard template as reported by Kyotaniet al. [193]. Polymer beads, anodic aluminium oxide, silica/alumino-silicate gel etc. are also reported hard-template for synthesizingmesoporous carbon [194]. The process of producing mesoporous car-bon using exocasting is indirect and needs aggressive chemicals to re-move the silica matrix. This hard-template route used to synthesizemesoporous carbon materials are also time-consuming or producescarbon with a disordered pore structure [195]. Therefore it would bemore elegant and desirable to produce mesoporous carbon by a di-rect templating mechanism using amphiphilic molecules.

But, the preparation of mesoporous carbon materials with orderedopen pore structures via a supramolecular templating approach isdifficult, due to the intrinsic characteristics of organic moleculesand high formation energy of C\C bonds. The basic requirementsfor synthesizing porous carbon using soft-template are (a) highability of the precursors to self-assemble within the nanostructures,(b) simultaneous presence of minimum one pore-generating as wellas one carbon-generating precursors, (c) pore-generating componentshould have the stability to maintain the temperature required for curingthe carbon-yielding component but must be readily decomposed duringcarbonization having least carbon yield and 4) carbon-yielding com-ponent must have the ability to form a highly cross-linked polymericmaterial which retains its nanostructure during the removal of thepore-generating template [195]. There are very few precursorswhich fulfill the above requirements. Although, some very interestingstudies on the soft-templated synthesis ofmesoporous carbons have re-cently been reported [196,197]. One of the most successful ways inachieving mesoporous carbon through this soft-templating pathway isto synthesize mesoporous polymers, with high content of carbon.Mesoporous polymers obtainedbyusing various precursors (as discussedearlier) can be converted to their corresponding carbons with controlledpore structures by simply carbonizing the polymer template in inertatmosphere (nitrogen or argon) applying temperature up to 1173 K.In presence of block copolymer F127 mesoporous polymer is derivedfrom self-assembled resorcinol-formaldehyde precursors mediatedby the I+X−S+ and H-bonding interaction in highly acidic media. Fi-nally mesoporous carbon is obtained from this polymer in a well repro-ducible approach [198]. A very interesting pathway to producemesoporous carbon with increased microporosity is reported byJaroniec and co-workers, where KOH is used for the activation of phe-nolic resin polymer derived porous carbon in order to simultaneouspreservation of micro and mesopores [199].

3. Properties and applications

The physical and chemical properties of the mesoporous materialsdepend on the specific surface area, tunable pore diameter, porevolume and cubic, hexagonal arrangements of pores as well as

Fig. 16. General synthetic pathway for periodic mesoporous silica (PMO). R = bridging organic group.

Fig. 17. Surfactant directed synthesis of a mesoporous phenol–formaldehyde polymer from oligomer building blocks. Mesoporous carbon is obtained after carbonization.

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Table 2Various applications of mesoporous materials.

Field Type ofproperties

Name of the material Applications References

Adsorption Biomolecularadsorptionproperties

MCM-41, MCM-48, SBA-15, CMK-3, CMK-1, OMS Separation of biochemicals, adsorption of amino acid, vitamins,proteins, CO2

[178,206,207]

Toxic ionadsorptionproperties

Mesoporous alumina, PMO, titanium-silico phosphate Fluoride, As(III/V), Cd(II), Hg(II), Fe(III), Cu(II), Zn(II) etc.adsorption to purify water

[130,154]

Gas adsorptionproperties

Mesoporous polymers, Organically functionalizedsilica

N2, H2, CO2 gas adsorption and storage [72,142,173,208]

Adsorption andreleaseproperties

Mesoporous SiO2\ZrO2, silica nanoparticles, calciumphosphate, Au capped silica,

Drug delivery of bisphosphonate, adsorption and desorption ofDNA, dyes and drugs in human cell

[209–212]

Catalysis Redox properties Mesoporous titanium–phosphorous mixed oxide,Ti\Al\SBA-15, mesoporous mixed oxides

Partial oxidation of styrene, oxidation of cyclohexene to adipicacid, reduction of nitroarenes

[91,130,213]

Acid-baseproperties

Mesoporous zinc-titanate, Zn doped silica, \COOHfunctionalized SBA-15, zirconium phosphate

Friedel-Crafts benzylation, esterification, transesterification, basecatalyzed Knoevenagel condensation, Xanthenes preparation,biofuel production via esterification

[55,57,70,104,141]

Other syntheticname reactions

Mesoporous NiO\ZrO2, Pd doped porous polymer,polyoxometalate doped silica, Fe3O4@silica

C-S cross coupling, C-C cross coupling, Mannich reaction,Biginelli condensation reaction

[93,187,214,215]

Photocatalyticproperties

Mesoporous TiO2, ZnO\CeO2 binary oxide, tantalumoxide

Photodegradation of dye, decomposition of dye, photosplitting ofwater

[6,216,217]

Optics,electricity,magnetism

Magneticproperty

Mesoporous Fe3O4 Ferromagnetism [218]

Opticalproperties

Luminescent PMO, rare earth doped silica, tungstenoxide

Chemosensing [157,219,220]

Electricalproperties

Nanostructured SnO2 Dielectric effect [221]

Optoelectronics Mesoporous TiO2, ZnO, TiO2 nanoparticles Photocurrent enhancement on dye doping, DSSC [56,90]

Sensing Gas sensingproperty

Mesoporous SnO2, TiO2 materials, orderedmesoporous ZnO, In2O3, WO3

CO, NH3, H2, NO2, CH4 etc. gas sensing, [222–224]

Ion sensingproperty

Azo compound functionalized organosilica,fluorescent grafted silica, dye doped silica, porouscarbon nitride

Optical and electro-chemical sensing of cations like Fe(III),Zn(II), Cu(II), \NH2 gr. etc. and anions like \CN, citrate, borate,carboxylate etc.

[144,225–229]

Biosensingproperty

Mesoporous titania MSN, organically functionalizedsilica,

Detection and sensing of glucose oxidase, protein, glucose etc. [230,231]

pH sensing role Functionalized silica pH sensor of a particular medium [227,232]

38 N. Pal, A. Bhaumik / Advances in Colloid and Interface Science 189–190 (2013) 21–41

on the framework building metals and non-metals present inthe mesostructures. As a consequence, versatile applications ofmesoporousmaterials in different fields highlight it an indispensabletopic of research in nanoscience and technology. For example thefield of heterogeneous catalysis is highly benefited from thesurfactant-assisted synthesis of novel mesoporous materials andwill grow day by day with ongoing increased surface modificationand new composition of mesoporous framework [15]. The semicon-ducting properties of Ti, Zn etc. basedmesoporous oxides have foundtremendous importance in dye sensitized solar cell (DSSC), opticaldevice, photocatalysis etc. Simultaniously, electronics, magnetism,biotechnology, medicinal, adsorption chemistry, sensors and so onhave got the chance of visualizing the crucial role of these materials[137,143]. Magnetic nanocomposites of different shape and size canbe derived by embedding monodisperse magnetic nanocrystals inmesoporous nanospheres or inside the porous channels and cagesof ordered mesoporous materials [200]. These nanocomposites based

on mesoporous structures have huge potentials in the areas of heathcare, catalysis, and environmental separation.

Moller and Bein have reviewed different aspects of inclusionchemistry in ordered mesoporous host materials, which includes sorp-tion, ion exchange, immobilization followed by reduction, grafting of re-active metal alkoxides, halides etc., grafting of silane coupling agents(sometimes followed by subsequent reactions), grafting of reactivemetal complexes, and polymerization in the channels [201]. Co-condensation of a reactive species during the mesopore synthesisis described as an effective strategy to incorporate a reactive func-tionality into the walls of the mesoporous channel system. Impor-tant applications of these modified and functionalized mesoporoussystems are heterogeneous catalysis, photocatalysis, ion exchangeand separations, removal of heavy metals, chromatography, stabili-zation of quantumwires, stabilization of dyes, and polymer compos-ites. Surface-functionalized mesoporous silica nanoparticle (MSN)materials have been extensively studied as efficient drug delivery

39N. Pal, A. Bhaumik / Advances in Colloid and Interface Science 189–190 (2013) 21–41

carriers [202–204]. Slowing et al. have described the synthesis of dif-ferent class of MSN materials and their biocompatibility in vitro,where various contemporary methods for controlling the structuralproperties and chemical functionalization for biotechnological andbiomedical applications are given [202]. Progress made on usingMSN to penetrate various cell membranes in animal and plant cells,the concept of gatekeeping in the design of a variety of stimuli-responsive nanodevices suggest that MSN-based systems have agreat potential for the site-specific delivery and intracellular con-trolled release of drugs, genes, and other therapeutic agents.

Further, due to the necessity of refining industry for heavier feed-stocks, there is considerable demand for zeolites with mesopores,which can utilize its accessible surface areas and higher pore volumes.In this context different synthesis strategies involving ‘destructive’structure breaking together with ‘constructive’ structure buildingsynthesis pathways have been developed to synthesize mesoporouszeolitic materials combining micropores with mesopores [205]. InTable 2 we have summarized some of the important applications ofmesoporous materials synthesized through soft templating pathwayin a tabular form.

4. Summary

Frompurely inorganic, organic-inorganic hybrid to purely organic—anoverview of the surfactant-assisted synthetic pathways of all these or-dered and disordered mesoporous materials are depicted elaborately inthis review. Since there is a large number of research and synthesisworks randomly reported on a wide spectrum of mesoporous materials,a good review on the formationmechanism of thesematerials is essentialfor further study and development in this frontier area of research. Hencea deep insight on the formation of mesopores via self-assembly of surfac-tants is discussed here with well recognized schematic models and goodfitting references. Along with this respective interaction pathways in-volved in synthesis of mesoporous oxide, silica, organic–inorganic hybridsilica, phosphate, sulphides, carbon and polymer using high molecularweight large amphiphilic molecules, are systematically demonstratedhere in tabular form. Further, potential applications of these mesoporoussolids in the field of surface science, nanotechnology andmaterials chem-istry in general have been highlighted.

5. Future prospects

All themechanistic pathways described herein to createmesoporositywithin silica, metal oxide, phosphate or carbon based materials are asuccessful approach towards the additional effort in the mesoporousworld. A variety of mesoporous solids can be prepared employing thesesynthesis protocols like, organically functionalized mesoporous silicas,PMOs, new lanthanide and actinidemetal doped silicas, oxides or organicpolymers and their carbons. When a variation of metals is able generatenew porous mixed oxides similarly variation in condensation ofnew organosilane precursor with TEOS or other sources can producemulticomponent hybrid silica. Organic monomers synthesized in oureveryday research can be employed to synthesize porous polymers ofdifferent types which are not yet been prepared. An apparent idea ofmetal-template interaction will help to generate porous network inthesematerials aswell as conventional nonporousmaterials using surfac-tants molecules will open multiple branches in the research area ofnanoporous solids. Moreover, there are numerous porous oxides, mixedoxides, phosphates, organic-inorganic hybrid silica and carbons whichhave been fabricated and characterized till date but their applicationpotentials are not being explored so far. So in future the in-depth studyon catalysis, sensing, adsorption, optoelectronics, light-harvesting behav-ior may open good opportunity for the further research on thesematerials.

Acknowledgments

AB wishes to thank Department of Science & Technology, New Delhifor providing the instrumental facilities throughDSTUnit onNanoscience.NP expresses gratitude to Council of Scientific and Industrial Research(CSIR), New Delhi for a senior research fellowship.

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