RSC Advances

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Organic amine-f

aCenter for Nano Science and Nano Technolo

Technical Education & Research, Siksha

Bhubaneswar-751030, OR, India. E-mail: kubSchool of Chemistry & Physics, College

University of KwaZulu-Natal, Westville Cam

DIrjcuPOAonSs

international journal papers andinterests focus on metal-modiedtionalized mesoporous materialslytic and photocatalytic reactions.

Cite this: RSC Adv., 2014, 4, 57111

Received 2nd August 2014Accepted 14th October 2014

DOI: 10.1039/c4ra08005j

www.rsc.org/advances

This journal is © The Royal Society of C

unctionalized silica-basedmesoporous materials: an update of syntheses andcatalytic applications

Dharitri Rath,a Surjyakanta Ranab and K. M. Parida*a

Nowadays, inorganic–organic hybrid materials having pores in the mesoporous range are an intensively

studied new category of demanding materials. By template synthesis, the coupling of inorganic and

organic components gives pore sizes between 2 and 15 nm with very high surface area. The inorganic–

organic hybrid materials were prepared in two ways: one was by co-condensation and the other by

post-synthesis method. The inorganic part provides mechanical strength and the organic part shows

functional activities. This review gives an overview of the preparation, properties, and potential

applications of these materials in the areas of adsorption of pollutant gases like CO2 and heavy metals

and in catalysis. Their activity is found to be very impressive in all these fields and it is hoped to be

improved in the near future.

1. Introduction

Many porous materials have been widely studied with regard totechnical applications as excellent adsorbents and heteroge-neous catalysts. According to the IUPAC classication, porousmaterials are divided into three classes: microporous (pore

gy, Department of Chemistry, Institute of

‘O’ Anusandhan University, Khandagiri,

lamaniparida@soauniversity.ac.in

of Agriculture, Engineering & Science,

pus, Durban 4000, South Africa

r Dharitri Rath joined thenstitute of Minerals and Mate-ials Technology in 2005 as aunior research fellow. Sheompleted her PhD in 2011nder the guidance of Dr K. M.arida from Utkal University,disha. Now she is working assst Professor in the Departmentf Chemistry, Institute of Tech-ical Education and Research,iksha 'O' Anusandhan Univer-ity. She is the author of 11one book chapter. Her researchmesoporous materials, func-

and their applications in cata-

hemistry 2014

size < 2 nm), mesoporous (2–50 nm),1,2 and macroporous (>50nm) materials3 (Fig. 1).

The mesoporosity of silica-based materials has come to theforefront as a new and exciting research eld of great scienticand technological importance in heterogeneous catalysis. Theability to design both the size and wall surface characters of thepores is important towards imposing a framework for tailoringand ne-tuning catalytic activities and optoelectronic propertiesof further embedded clusters.4,5 It has become an exciting eldto be explored simultaneously by chemists, physicists, andengineers.

Dr Surjyakanta Rana was bornin 1982 and completed his PhDdegree at the Department ofChemistry, Utkal University,Odisha, India in 2013 under thesupervision of Dr K. M. Parida.He is the author of 15 interna-tional journal papers and onebook chapter. His researchinterest focuses on functional-ized mesoporous silica and itscatalytic applications.

RSC Adv., 2014, 4, 57111–57124 | 57111

Fig. 1 Types of porous materials depending on their pore sizes.

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2. Mesoporous materialsThe term zeolite was rst used by the Swedish mineralogist AxelFredrik Cronstedt in 1756. Zeolites are crystalline and micro-porous aluminosilicate minerals. A dening feature of zeolitesis that their frameworks are made up of four-connectednetworks of atoms. Just like a tetrahedron with a silicon atomin the middle and four oxygen atoms at the corners. Thesetetrahedra can then link together by their corners to form a richvariety of beautiful structures. The general formula of zeolite isMx/n[(AlO2)x(SiO2)y]ZH2O, where M represents a charge-compensating cation with valency n. The ratio y/x may haveany value ranging from one to innity. Z is the number of watermolecules, which can be reversibly adsorbed and desorbed intothe zeolite micropores.

Even though zeolites, having pore dimensions of 5 to 7 A,serve the purpose of many industrial reactions by providinghigh surface area, the pore dimensions are not sufficient toaccommodate a broad spectrum of large molecules. Theperformance of zeolite systems is limited by diffusionconstraints associated with smaller pores. So it is the aim of

Dr K. M. Parida is working asProfessor in Chemistry andDirector in the Centre for NanoScience and Nano Technology,Institute of Technical Education& Research, Siksha ‘O’ Anu-sandhan University, Bhuba-neswar, Odisha. He is the authorof more than 270 internationaljournal papers and 18 nationaland international patents. Hisresearch interest focuses on thedesign and development of

materials comprising a wide cross section such as metal oxides,metal phosphates, metal sulfates, cationic and anionic clays,perovskites, zeolites, nano metal/metal oxide/complex promotedmesoporous materials, naturally occurring materials such asmanganese nodules, manganese nodule leached residues, andmanganese oxides of natural origin for application in catalysis andphotocatalysis in the sector of energy and environment.

57112 | RSC Adv., 2014, 4, 57111–57124

industrial and scientic research to expand the pore size of zeo-type materials from the microporous to the mesoporous range.

In 1990, Yanagisawa et al.6 reported the synthesis of meso-porous materials characteristic of MCM-41 through intercala-tion of long-chain (typically C16) alkyltrimethylammoniumcations into the layered silicate kanemite, followed by calcina-tion to remove the organic species. Unfortunately no furthercharacterization data were available and therefore Yanagisawa'sresults have been ignored.

Typical mesoporous materials include some kinds of silicaand alumina that have ne mesopores of similar size. Meso-porous oxides of niobium, titanium, tantalum, zirconium,cerium and tin have also been reported.

2.1 Silica-based mesoporous materials

The pore size constraint (15 A) of microporous zeolite wasovercome with the rst successful report on the synthesis ofmesoporous materials (M41S) by Mobil researchers, with well-dened pore sizes of 20–500 A. The high surface area andprecise tuning of the pores are among the desirable propertiesof these materials. These materials are mainly used in a newsynthetic approach where, instead of a single molecule astemplating agent as in the case of zeolites, self-assemblies ofmolecular aggregates or supramolecular assemblies areemployed as templating agent. They have a uniform hexagonalarrangement of pores with size ranging between 1.5 and 10 nm.7

This material was named Mobil Composition of Materials no.41 or MCM-41.8 Researchers at the University of California inSanta Barbara produced silica nanoparticles with much largerpores of size 4.6 to 30 nm.9 This material was named SantaBarbara Amorphous type material or SBA-15. In this materialparticles are also arranged in a hexagonal array. Yanagisawaand co-workers6 prepared another mesoporous silica fromkanemite (NaHSi2O5$3H2O) and CTA cation designated as FSM-16. FSM-16 is structurally similar to but functionally differentfrom MCM-41. It shows better adsorption properties andsurface chemistry. MCM-48 is another mesoporous materialwith a cubic structure and a three-dimensional pore system.The two other phases of mesoporous silica are lamellar (MCM-50) and molecular organic octomer (a surfactant–silicacomposite) and are unstable (Fig. 2).10

In 2007, a new porous material was formulated by Nan-diyanto et al. in Hiroshima University. They prepared the mes-oporous material by a simple sol–gel method11 with controllablepore size from 3 to 15 nm and outer diameter from 20 to 100nm. This material was named Hiroshima Mesoporous Material,and abbreviated as HMM. These materials show excellentadsorption properties for large molecules.12 Physical propertiesof various mesoporous silica materials are shown in Table 1.

2.2 Mechanistic pathways for the formation of silica-basedmesoporous materials

A variety of synthetic procedures can be adopted for the prep-aration of mesoporous silica. However, there is one thing allthese procedures have in common next to the obvious presenceof a source of silica: a templating agent. A template is a

This journal is © The Royal Society of Chemistry 2014

Fig. 2 Structures of mesoporous silica materials: (a) MCM-41 (hexagonal), (b) MCM-48 (cubic), (c) MCM-50 (lamellar), and (d) octomer.Reproduced from ref. 10. “Reprinted with permission from (P. Selvam, S. K. Bhatia, C. G. Sonwane, Ind. Eng. Chem. Res., 2001, 40, 3237).Copyright (2014) American Chemical Society”.

Table 1 Physical properties of various mesoporous silica materials

Sample code

Structural data

Mean pore size (nm) ReferenceDimensionality,crystal system, space group Unit cell dimension (nm)

MCM-41 2D hexagonal (P6mm) a ¼ 4.04 3.70 7MCM-48 Cubic (Ia3hd) a ¼ 8.08 3.48 7FSM-16 2D hexagonal (P6mm) a ¼ 4.38 2.80 13SBA-1 Cubic (Pm3hn) a ¼ 7.92 2.00 14SBA-2 3D hexagonal (P63/mmc) a ¼ 5.40, c ¼ 8.70 2.22 15SBA-3 2D, hexagonal (P6mm) a ¼ 4.75 2.77 14SBA-8 2D rectangular (cmm) a ¼ 7.57, b ¼ 4.92 1.87 16SBA-11 Cubic (Pm3hm) a ¼ 10.64 2.50 17SBA-12 3D hexagonal (P63/mmc) a ¼ 5.40, c ¼ 8.70 3.10 17SBA-14 Cubic (Pm3hn) a ¼ 4.47 2.40 17SBA-15 2D hexagonal (p6mm) a ¼ 11.6 7.80 18SBA-16 Cubic (Im3hm) a ¼ 17.6 5.40 17HMM 2D hexagonal (P6mm) a ¼ 5.70 3.10 19

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structure-directing agent, which is usually a relatively simplemolecule or ion, around which a framework is built up. Due totheir large hydrophobic alkyl chains the template ions willaggregate together in order to minimize energetically unfavor-able interactions of the apolar alkyl chains with the very polarwater solvent molecules. The resulting aggregates of ions arecalled micelles. It follows that these micelles have a hydro-phobic core, containing the large alkyl chains, and a hydro-philic surface, due to the ionic character of the ammoniumhead groups.

Spherical geometry is the mostly favorable form of micellesbecause in this geometry the surface energy is minimized mostefficiently. Moreover, this conrmation allows the largest numberof micelles to be formed out of a certain amount of template,which is attractive considering the entropy of the system.

The silica source and nature of surfactant decide the natureof interaction as illustrated in Fig. 3. By varying the synthesisconditions like the silica source and the type of surfactant used,many other mesoporous materials can be synthesized.20–22 Inaddition to the co-operative pathway, which is discussed above,

This journal is © The Royal Society of Chemistry 2014

also the true liquid crystal templating pathway23 and nano-casting are used to form ordered mesoporous materials ashard templates.24,25

In this method there must be an attractive interactionbetween the silica source and the template, in which thestructure-directing agent (SDA) is added without any change inits phase. The various interactions between the inorganicprecursors and the head groups of the SDA are shown in Fig. 3.Huo et al.21,22 suggested four possible interactions between thesilica source and the head groups of the SDA. (a) When the SDAhas a +ve head group, in basic medium the interaction is rep-resented as S+I� (Fig. 4a; S: surfactant; I: inorganic species). (b)When the SDA has a +ve head group, in acidic medium (belowthe iso-electric point of Si–OH group) a mediator ion X� (usuallya halide) is required to bring about the interaction and is rep-resented as S+X�I+ (Fig. 4b). (c) When the SDA has a �ve headgroup, in basic medium a mediator metal ion M+ is required tobring about the interaction and is represented as S�M+I�

(Fig. 4c). (d) When the SDA has a �ve head group, in acidmedium the interaction is represented as S�I+ (Fig. 4d).

RSC Adv., 2014, 4, 57111–57124 | 57113

Fig. 4 Electrostatic interactions between the inorganic species andthe head group of the surfactant in acidic or basic media. Reproducedfrom ref. 10. “Reprintedwith permission from (P. Selvam, S. K. Bhatia, C.G. Sonwane, Ind. Eng. Chem. Res., 2001, 40, 3237). Copyright (2014)American Chemical Society”.

Fig. 5 (a) Mechanism for the formation of mesoporous materials bystructure-directing agents: true liquid-crystal templatemechanism, (b)cooperative liquid-crystal template mechanism. Reproduced from ref.10. “Reprinted with permission from (P. Selvam, S. K. Bhatia, C. G.Sonwane, Ind. Eng. Chem. Res., 2001, 40, 3237). Copyright (2014)American Chemical Society”.

Fig. 3 Basic interaction between silica and surfactant. Reproducedfrom ref. 10. “Reprintedwith permission from (P. Selvam, S. K. Bhatia, C.G. Sonwane, Ind. Eng. Chem. Res., 2001, 40, 3237). Copyright (2014)American Chemical Society”.

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Hence the ruling interactions in pathways (a–d) are of anelectrostatic nature. Experimentally it was observed that thesurfactant to silica ratio has a signicant impact on thecomposite structure obtained.26 The use of anionic surfactantvia S�I+ or S�M+I� interaction results mainly in lamellar anddisordered mesostructures which is also demonstrated by Cheet al.27,28 The pore size of these materials can be controlled bythe length of the alkyl chain of the surfactant used.29 Also themesopore size can be expanded by the use of some auxiliaryorganic molecules like organic alkanes30 or fatty acids.31 Thenegatively charged head group of the anionic surfactant (i.e.palmitic acid or N-lauroyl-L-glutamic acid) interacts with the

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positively charged ammonium groups of additives for co-condensation of TEOS (tetraethyl orthosilicate). Two alky-lammonium surfactants with different alkyl chain length can bemixed to ne-tune the pore size between those of the long- andthe short-chain surfactants.32

The proposed mechanistic pathways for the formation ofmesoporous silica structures are illustrated in Fig. 5 and6.7,24,25,33 In the rst, the presence of liquid-crystal mesophaseprior to the addition of the reagents, i.e., pre-existence ofsurfactant aggregates (rod-like micelles), followed by themigration and polymerization of silicate anions, results in theformation of the MCM-41 structure.

The geometry of the micelle formed depends on theconcentration of the template used. Initially the geometry isspherical which gradually transforms into long tubes. Finallythe tube-like micelles aggregate to form a hexagonal liquidcrystalline structure which is the framework for MCM-41. Againon further increasing the template concentration, the hexag-onal geometry is transformed to cubic and lamellar structureswhich form the MCM-48 and MCM-50 materials respec-tively.21,22,26,27 The liquid crystalline structures act as the actualtemplates for the different mesoporous materials. The mecha-nistic pathways are shown in Fig. 6.

3. Scope of the review

Mesoporous silica MCM-41 and various organic amine-modied MCM-41 provide a good base for the dispersal ofdifferent organic molecules and nano metal oxides. Moreoverthe review will discuss detail about the synthesis proceduresand widespread applications of these materials. The mostimportant elds are the adsorption of heavy metals and harmfulgases and industrially viable organic reactions. This will providea new outlook to researchers for carrying out work in theseelds.

4. MCM-41

Among the above mentioned mesoporous silica materials,MCM-41 has an eminent place due to some of its special

This journal is © The Royal Society of Chemistry 2014

Fig. 6 Proposedmechanistic pathways for layered silica by (1) stackingof silicate surfactant rods, via the formation of an initial (2) lamellarintermediate and (3) silicate bilayer. Reproduced from ref. 10.“Reprinted with permission from (P. Selvam, S. K. Bhatia, C. G. Son-wane, Ind. Eng. Chem. Res., 2001, 40, 3237). Copyright (2014) Amer-ican Chemical Society”.

Fig. 7 Transmission electron micrograph of MCM-41. Reproducedfrom ref. 10. “Reprintedwith permission from (P. Selvam, S. K. Bhatia, C.G. Sonwane, Ind. Eng. Chem. Res., 2001, 40, 3237). Copyright (2014)American Chemical Society”.

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properties. It is an ordered mesoporous material, displaying ahoneycomb-like structure of uniform mesopores runningthrough a matrix of amorphous silica. Because of its structuralsimplicity and ease of preparation with negligible pore-networking and pore-blocking, it is the most studied amongthe numerous mesoporous silica materials studied so far. Thetransmission electron micrograph of MCM-41 depicted in Fig. 7shows the beautiful honeycomb-like structure. In this gure wecan see directly inside the uniform mesopores, which areseparated from each other by thin walls of amorphous silica,approximately 1–1.5 nm thick.

Themost remarkable features ofMCM-41, and in generalmostperiodic mesoporous materials, are as follows: well-dened poreshapes (hexagonal/cylindrical), narrow distribution of pore sizes,negligible pore networking or pore blocking effects, veryhigh degree of pore ordering, possibility of tailoring and ne-tuning of the pore dimensions (1.5–20 nm), large pore volumes(>0.6 cm3 g�1), exceptional sorption capacity as a result of thelarge pore volume, very high surface area (�700–1500 m2 g�1),large amount of internal hydroxyl (silanol) groups (�40–60%),high surface reactivity, ease of modication of the surfaceproperties, enhanced catalytic selectivity in certain reactions,and excellent thermal, hydrothermal, chemical, and mechan-ical stability.

MCM-41 has a very large void fraction, due to the presence ofthe mesopores, and concomitantly a rather low density. As aresult MCM-41 displays a very large specic surface area ofapproximately 1300 m2 g�1. Therefore, MCM-41 is in principleideally suited to be used as a support material for heteroge-neous catalysts. Moreover, MCM-41 exclusively contains meso-pores which can provide access for large molecules and improve

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diffusion problems, which are frequently encountered inmicroporous materials such as zeolites.

Unfortunately there is one pronounced drawback associatedwith MCM-type materials, i.e. limited stability, which is a resultof very thin, amorphous pore walls. Because of the very largemesopore surface area, the pore walls are extremely reactivetowards a number of agents, resulting in the collapse of the thinwalls upon exposure to these agents. The silica-supportedmaterials are not suitable for a large range of processes due totheir instability towards steam. Steam either co-fed as diluent orproduced during catalysis results in the chemical evaporation ofsilica. Also, MCM-41 material is unstable towards hydroxideand uoride solutions as they can dissolve silica. Hence thestability of MCM-41 material is restricted to pH # 7 in aqueoussolutions.

4.1 Synthesis of MCM-41

The synthesis of MCM-41 requires four essential components:structure-directing agent (template), solvent, silica source andmineralizing agent. The most commonly used template forMCM-41 is hexadecyl (or cetyl) trimethylammonium bromide(or chloride). It is a template with an alkyl chain containingsixteen –CH2– moieties. This template yields MCM-41 with auniform pore size of approximately 2.7 nm. By using templateswith longer or shorter alkyl chains the pore size can becontrolled. Nevertheless, due to the limited range of alky-lammonium ions suitable for the preparation of MCM-41, thepore size can be adjusted to a small extent only.34–36 Someauxiliary organics like 1,3,5-trimethylbenzene34,35 can be intro-duced to adjust the pore size of the material to a remarkableextent. Being apolar, the organics cannot be dissolved in waterbut they can be absorbed in the hydrophobic core of thetemplate micelles. Due to this absorption the micelles swell,

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thus increasing the average size of themesopores inMCM-41 upto values of approximately 8–10 nm in diameter. Mixtures of twosurfactants can also be used to ne-tune the pore size of MCM-41. Cheng et al.37 proposed a synthesis procedure for MCM-41using the C18TABr–C14TABr mixed system. Next to a structure-directing agent and water as a solvent, an important ingre-dient is a source of silica. Various sources of silica can be usedfor syntheses, i.e. water glass, amorphous silica and kanemite (alayered silicate structure consisting of anionic silica sheets withcharge-compensating sodium ions present in the interlayers).Furthermore, organic silicon alkoxides are also frequently used.The most widely used one is tetraethyl orthosilicate (TEOS).38–52

Silica sources can be dissolved in a mineralizing agent likeNaOH or concentrated NH4OH or HF. Aer dissolution of silica,small silicon oxy anions are produced. There is electrostaticattraction between the micelles and the silicate anions and theymove towards the micelles. As a result the anion concentrationincreases on the micelle surface. Again there is repulsiveinteraction among the silicate ions. In order to assuage theinteraction the silicate ions start to condense with each otherand form a monolayer of silica over the micelles. At this stagethe silica “coated” micelles can start to cluster together bycondensation reactions between the silica layers of individualmicelles, thus generating the MCM-41 framework. As a result ofthese processes the pore walls of MCM-41 are amorphous andonly 2–3 monolayers thick.53–75

Aer formation of MCM-41 the pores are lled with thetemplate molecules. These micelles have to be removed to get aperfect mesoporous material. One of the simple methods fortemplate removal is calcination. This is the process of heating asample in the presence of air. During the removal process thetemplate is decomposed into CO2, NOx and steam. The steamquantity is too low to cause damage to the MCM-41 framework.A schematic representation of this process is shown in Fig. 5.

4.2 Functionalized MCM-41

Siliceous mesoporous materials in neat form lack active sites ontheir surface. Hence their applications are restricted and theirsurface has to be modied according to the requirements. Inorder to utilize the unique properties of the mesoporousmaterials for specic applications like catalysis, sorption,sensing, ion exchange etc., introduction of reactive organicfunctional groups is required. The incorporation of organiccomponents as part of the silicate walls or trapped within thechannels to form inorganic–organic hybrid materials (IOHMs)remains the main issue. The advantages of IOHMs arise fromthe fact that the inorganic components can provide mechanical,thermal or structural stability, while the organic features canreadily be modied for various specic applications.76–99

IOHMs represent the natural interface between the twoworlds of chemistry, each with signicant contributions andcharacteristic properties. The advantages and limitations totheir eld lead to a creative alternative for obtaining newmaterials with unusual features. The main idea of developinghybrid materials is to take advantage of the best properties ofeach component and try to decrease their drawbacks, hence

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resulting in a unique advanced material with potential appli-cations. The possibility of combining the properties of inor-ganic and organic compounds to get a unique material is achallenging task in recent years. Although we do not know theoriginal birth of hybrid materials exactly, it is clear that mixingof inorganic and organic components was carried out in ancienttimes. However, it was only at the end of the 20th and thebeginning of the 21st century, 7 years aer the discovery ofMobil Composition of Materials, that the concept of hybridmaterials came to the scene. Then researchers consideredIOHMs as innovative and advance materials having potentialapplication in various elds including catalysis.100–112

Functionalized mesoporous oxides possess exceptionallyhigh surface areas, which allow the binding of a large number ofsurface chemical moieties and hence they can be used asadsorbents and catalysts. Mainly the large diameter andwormhole nature of the pore channels in these oxides areadvantageous for the formation of functional materials inwhich the reactive species are highly dispersed. Hence these arequantitatively accessible for reaction with adsorbatemolecules.130–133

A wide range of different materials come under the classi-cation of hybrid materials. These are highly crystalline co-ordination polymers and amorphous sol–gel compounds, withand without interactions between the inorganic and organicunits. The most wide ranging denition is that a hybrid mate-rial includes two moieties blended on the molecular scale.Commonly, one of these compounds is inorganic and the otherone is organic in nature.

The non-calcined silica-based mesophases are regarded asIOHM. However their applications are very limited. Thesematerials are either calcined or solvent-extracted for furtherapplications. Sugi et al.,114 however, found that non-calcinedMCM-41 silica exhibits high catalytic activity for the Knoeve-nagel condensation. From 29Si NMR data, the authors came tothe conclusion that the catalytic activity is attributable to basic(SiO)3SiO

� sites, which are present in large amounts in non-calcined silica mesophases. According to Tolbert and co-workers,113–115 an ordered silica/surfactant MCM-41 mesophaseexhibits remarkable behavior under high pressure. In additionto the as-synthesized amphiphile or silica hybrids, the followingmaterials t a broad denition of silica-based IOHM: (1) mes-oporous silicas with organically modied surfaces, (2) expandedmesoporous silicas, (3) mesoporous organosilicates, and (4)mesoporous silica with occluded organic materials such aspolymers.

4.3 Synthetic approaches for functionalized MCM-41

Surface functionalization of inorganic supports with manyorganic moieties provides organic–inorganic hybrids, where theinorganic and organic components are linked via strong-typeinteractions (i.e. covalent or iono-covalent bonds). Manyresearch efforts, which have focused on preparing the organic–inorganic hybrids through functionalization of the exterior and/or interior surfaces, prompted the utilization of the resultant

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materials in the elds of catalysis, separation, sensor design,adsorptions, and nanoscience.

According to the location of the organic fragments in theinorganic network, organically modied MCM-41 solids can bedivided into three main classes. In the rst, the organic groupsare rooted to the surfaces of the inorganic walls and protrudeinto the pores by (i) post-synthetic graing of X–Si(OR)3compounds (–X: functional group) to a calcined MCM-41silica,114,115 (ii) co-condensation of a functionalized organo-siloxane with the main source of the inorganic constituent,followed by acid extraction to remove the surfactant moleculesfrom the pores116,117 and (iii) displacement of the surfactantmolecules from the mesopores by an organosiloxane throughinterfacial reactions.118 In the second class, the organic moietiesare fused within the silica walls through co-condensation oforganosiloxanes of the type (RO)3Si–X–Si(OR)3 (X: organicspacer) with the prime silicon alkoxide, followed by extractionof the surfactant molecules (periodic mesoporous organo-silicas, PMO).119–123 Finally, the third class comprises a combi-nation of the aforementioned classes of inorganic–organichybrids, e.g., some organic groups reside outside and otherswithin the walls of the inorganic network.

4.3.1 Post-synthetic (“graing”) method. The process of“graing” means the subsequent modication of the innersurfaces of mesostructured silica phases with organic groups(Fig. 8). In this process, organosilanes [(R0O)3SiR], chlorosilanes[ClSiR3] or silazanes [HN(SiR3)3] are treated with the free silanolgroups of the pore surfaces. Functionalization with a variety oforganic groups can be done by varying the organic residue R.This method of modication has the advantage that, under thesynthetic conditions used, the mesostructure of the startingsilica phase is usually retained, whereas the lining of the walls isaccompanied by a reduction in the porosity of the hybridmaterial. If the organosilanes react preferentially at the poreopenings during the initial stages of the synthetic process, thediffusion of further molecules into the center of the pores can

Fig. 8 Schematic representation of grafting (post-synthetic func-tionalization) for organic modification of mesoporous pure silicaphases with terminal organosilanes of the type (R0O)3SiR (R ¼ organicfunctional group). Reproduced from ref. 129. With permission fromCopyright © 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim,license no. 3472940511823.

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be restricted, which can in turn lead to a non-homogeneousdistribution of the organic groups within the pores and alower degree of occupation. In extreme cases (e.g., with verybulky graing species), this can lead to complete closure of thepores (pore blocking).

The process of graing is frequently erroneously calledimmobilization, which is a term that we believe should bereserved for adsorptive methods (e.g., the removal of toxic orenvironmentally relevant contaminants by adsorbent materials,or the separation of proteins and biocatalysts by restriction ofthe freedom of movement). Secondary and higher order modi-cations consist of further reactions of the previously graedspecies to create new functionalities.

4.3.2 Co-condensation (direct synthesis) method. Analternative method to synthesize organically functionalizedmesoporous silica phases is the co-condensation method (one-pot synthesis) (Fig. 9). Mesostructured silica phases can beprepared by the co-condensation of tetraalkoxysilanes, (RO)4Si(TEOS or TMOS), with terminal trialkoxyorganosilanes of thetype (R0O)3SiR in the presence of structure-directing agents. Byusing structure-directing agents known from the synthesis ofpure mesoporous silica phases (e.g., MCM or SBA silica phases),organically modied silicas can be prepared in such a way thatthe organic functionalities project into the pores. Since theorganic functionalities are direct components of the silicamatrix, pore blocking is not a problem in the co-condensationmethod. Furthermore, the organic units are generally morehomogeneously distributed than in materials synthesized withthe graing process. However, the co-condensation methodalso has a number of disadvantages: in general, the degree ofmesoscopic order of the products decreases with increasingconcentration of (R0O)3SiR in the reaction mixture, which ulti-mately leads to totally disordered products. Consequently, thecontent of organic functionalities in the modied silica phases

Fig. 9 Schematic representation of co-condensation method (directsynthesis) for the organic modification of mesoporous pure silicaphases. Reproduced from ref. 129. With permission from Copyright ©2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, license no.3472940511823.

RSC Adv., 2014, 4, 57111–57124 | 57117

Fig. 10 Schematic representation of general synthetic pathway toPMOs that are constructed from bis-silylated organic bridging units.Reproduced from ref. 129. With permission from Copyright © 2006Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, license no.3472940511823.

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does not normally exceed 40 mol%. Furthermore, the propor-tion of terminal organic groups that are incorporated into thepore-wall network is generally lower than would correspond tothe starting concentration of the reaction mixture. Theseobservations can be explained by the fact that an increasingproportion of (R0O)3SiR in the reaction mixture favors homo-condensation reactions at the cost of cross-linking co-condensation reactions with the silica precursors. Thetendency towards homo-condensation reactions, which iscaused by the different hydrolysis and condensation rates of thestructurally different precursors, is a constant problem in co-condensation because the homogeneous distribution ofdifferent organic functionalities in the framework cannot beguaranteed. Moreover, an increase in loading of the incorpo-rated organic groups can lead to a reduction in the porediameter, pore volume, and specic surface areas. A further,purely methodological disadvantage that is associated with theco-condensation method is that care must be taken not todestroy the organic functionality during removal of the surfac-tant, which is why commonly only extractive methods can beused, and calcination is not suitable in most cases.

4.3.3 Displacement of surfactant molecules by an organo-siloxane. The displacement of surfactant molecules by neutralorganosiloxanes is accomplished under rather intense condi-tions, requiring reux of the as-synthesized MCM-41 solid inpure organosiloxane for a prolonged period of time.124 Themethod relies on simple cation exchange reactions between thecations electrostatically attached to the mesoporous silicasurfactant and a positively charged organosiloxane. Themethoddiffers from the others in the following respects: (1) the drivingforce causing the surfactant displacement from the MCM-41walls is the positively charged organosiloxane species insertedinto the pores, (2) the surfactant detachment and graing of theorganosiloxanes onto the surfaces of the MCM-41 materialproceed simultaneously under so conditions, (3) the methodmakes possible the uniform loading of the silica matrix with ahigh amount of modier without destroying the organizedarchitecture of the parent MCM-41 material, and (4) the pres-ence of the majority of the surfactant molecules within the porechannels of the silica guarantees the same location for most ofthe inserted organic modier. Nevertheless, the method is onlyapplicable to charged organosiloxanes or amino-functionalizedorganosiloxanes that acquire a positive charge by protonation.Themechanism of the exchange reactions and the potentialitiesof such functionalized solids are also discussed.

4.3.4 Periodic mesoporous organosilicas (PMOs). Thesynthesis of organic–inorganic hybrid materials by hydrolysisand condensation reactions of bridged organosilica precursorsof the type (R0O)3Si–R–Si(OR')3 has been known for a long timein sol–gel chemistry125,126 (Fig. 10). In contrast to the organicallyfunctionalized silica phases, which are obtained by post-synthesis or direct synthesis, the organic units in this case areincorporated in the three-dimensional network structure of thesilica matrix through two covalent bonds and thus are distrib-uted totally homogeneously in the pore walls. These materials,which are obtained as porous aerogels and xerogels, can havelarge inner surface areas of up to 1800 m2 g�1 as well as high

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thermal stability but generally exhibit pore systems with arelatively wide distribution of pore radii. The transfer of theconcept of the structure-directed synthesis of pure silica mes-ophases by surfactants to the bis-silylated organosilica precur-sors described above allows the construction of a new class ofmesostructured organic–inorganic hybrid materials: periodicmesoporous organosilicas (PMOs) in which the organic bridgesare integral components of the silica network. In contrast toamorphous aero- and xerogels, PMOs are characterized by aperiodically organized pore system and a very narrow poreradius distribution. The rst PMO was synthesized in 1999 bythree research groups working independently of oneanother.127,128 PMO materials are considered as highly prom-ising candidates for a series of technical applications, forexample, in the areas of catalysis, adsorption, chromatography,nanoelectronics or the preparation of active compound releasesystems.129

4.3.5 Organic amine-functionalized MCM-41. In order toimprove the basic properties of MCM-41, the silica frameworkneeds to be modied with some functional group having basicproperty. However, solid base catalysis, particularly on micro-porous and mesoporous materials, is still insufficiently inves-tigated compared to the corresponding acid catalysis.Immobilization of organic functional groups on internalsurfaces of porous silicates to obtain organic-functionalizedmolecular sieves is a conventional and common idea fordeveloping heterogeneous catalytic systems.

Importantly, the nature and the content of organic groupsdetermine the specic properties of these nanocomposites, likethe surface hydrophobicity, and hydrothermal, thermal and

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mechanical properties. However, the uniformity of the organicgroups inside the pore channels affects the surface properties,the functionalized organic group reactivity and the accessibilityof the porous network for further modications.

The basicity in Si-MCM-41 molecular sieves is achieved invarious ways. One such way is dispersing with alkali metaloxides.130,131 The drawback of this method is that the high pHused in the impregnation may damage the structure. Anotherway is by modifying the surfaces with organic compounds,particularly amines, by the impregnation method. In thismethod, the cationic surfactant present in the pores is removedby calcination before functionalization. Recently Kubota et al.used MCM-41 molecular sieve as a catalyst in Knoevenagelcondensation reaction at room temperature still containing itsstructure-directing organic cation: the cetyltrimethylammoniumoccluded in its pores.132,133 Martins et al. have proposed that theactive sites in this catalyst are the basic siloxy SiO� anions andthe reaction only occurs at the pore mouths.134,135

The impregnation method for the organic amine amino-propyltriethoxysilane (APTES) typically results in inhomoge-neous surface coverage because the introduced organicmoieties congregate near the entries to the mesoporous chan-nels and on the exterior surfaces. In the co-condensationmethod the amine and silica sources are added to the reac-tion mixture simultaneously.136 The surfactant is removed byacid extraction. In this method the pores of functionalizedMCM-41 are occluded by organic amine templates, whichprovide high activity as a base in mild conditions. The orga-nosilane is more evenly distributed on the silica surface in thecase of the co-condensationmethod than in the case of the post-synthesis method. So the material obtained from the lattermethod is more efficient as a base catalyst than that obtainedfrom the former. These APTES-modied MCM-41 samples(Fig. 11) show very good catalytic activity towards various base-catalyzed organic transformation reactions such as Knoevena-gel condensation, Michael condensation, Henry reaction etc.

4.4 Amine-functionalized mesoporous silica other thanMCM-41

Other mesoporous silica materials are also susceptible tosurface modication by organic amines. A lot of work has beendone on amine-functionalized SBA-15, SBA-16, MCM-48, silicagel etc. A recent work by Melendez-Ortiz et al.137 explained theroom temperature synthesis of APTES-MCM-48 and its appli-cation towards CO2 (ref. 138) and H2S capture in the natural gassweetening process. MCM-41 was found to be more inclined topore blocking compared to MCM-48. SBA-15 is among the most

Fig. 11 Functionalization of MCM-41 with APTES.

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attractive mesoporous silicas due to its adjustable pores andthick pore walls. Different amine-modied SBA-15 materials arestudied by various research groups. The surface of SBA-15 canbe modied with APTES, APTMS, polyethylene imine, poly-propylamine and also different primary, secondary and tertiaryamines for the adsorption of CO2, NO2, various metal pollutantslike Na, K, Ca, and Cu, VOC etc.139–144 Gao et al.145 synthesized anovel kind of spaced amine-modied SBA-15 through Diels–Alder reaction. The condensation of SBA-15-Cp and fumaroni-trile was followed by reduction of cyano groups. By this method,the distance between two consecutive amines was found to besame as that of the n-butyl group. The material was found to bean excellent catalyst for Knoevenagel condensation.

4.5 Other amine-functionalized oxides

In addition to mesoporous silica, other mesoporous oxides arealso able to undergo amine modication. Parida et al. haveworked extensively on amine-modied metal oxides likezirconia, titania, titania–silica mixed oxides,146 montmoril-lonite, layered double hydroxide147 etc. The amine-functionalized zirconia materials were used as effective cata-lysts for Knoevenagel condensation, Henry reaction and C–Scoupling reactions.148–150 The amine-functionalizedmontmorillonite-supported Cu, Ni catalyst showed synergeticand co-operative effectiveness towards C–S coupling reac-tions.151,152 Different amine-functionalized TiO2 and ZrO2 canalso act as good adsorbents for CO2 and other pollutantgases.153,154

5. Applications of amine-modifiedMCM-415.1 Adsorption of CO2

The various amine-functionalized materials act as effectivecatalysts and adsorbents in several catalytic reactions. Carbondioxide (CO2) removal is increasingly important because a highCO2 concentration in the atmosphere leads to global climatechanges. In addition, the removal of CO2 is also required incryogenic plants to prevent CO2 solidication. The removal canbe achieved by using methods such as liquid absorption, solidadsorption cryogenic techniques, and selective diffusionthrough polymer, ceramic, or metallic membranes. At present,chemical absorption using liquid amine is commercially usedin large-scale separation. However, a relatively high heat ofabsorption causes a high cost of regenerating primary andsecondary amines. In addition, solvent leakage and corrosionare also a problem. So an alternative is to use the various amine-functionalized materials. Adsorption of pollutant gases like CO2

can be done by these functionalized materials. Zhang et al.155

studied the adsorption properties of CO2 on MCM-41 meso-porous materials impregnated with ethylenediamine (EDA),tetraethylenepentamine (TEPA) and two kinds of polyethyleneimines (PEI600 and PEI1800). According to the study, the EDA-impregnated samples showed a low CO2 adsorption capacitydue to volatilization of EDA compared to the TEPA-impregnatedones. The adsorption capacity decreased with an increase in

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molecular weight of the amines. A maximum CO2 adsorptioncapacity of 2.7 mmol g�1 was achieved on TEPA-MCM-41 with40% TEPA. The work of Kamarudin et al.156 discusses CO2

adsorption on solid adsorbent in a pressure swing adsorptionsystem and its regeneration performance. According to thatstudy, among monoethylamine (MEA)- and diethylamine (DEA)-modied MCM-41, 25 wt% MEA has high CO2 removal evenaer 10 cycles of operation. Various polyethylene imine (PEI)-modied mesoporous silicas such as MCM-41, MCM-48 andSBA-15 were studied for the CO2 adsorption phenomena bySharma et al.157 All the PEI-loaded pelletized materials exhibitedsubstantially high reversible CO2 adsorption–desorptionbehaviors with >99% recovery. The results indicate that pelletscontaining methyl cellulose and activated carbon show bettermechanical strength and CO2 adsorption. That study alsoproved that MCM-48 is a better material as compared to MCM-41 and SBA-15 for pelletization and loading of PEI. Lopez-Aranguren et al.158 examined the functionalization of silicasupports via supercritical CO2 graing of aminosilanes, whichis an important step in the preparation of materials used assolid sorbents in CO2 capture. Four materials have beenconsidered as solid supports: two commercially available silicagels (4.1 and 8.8 nm pore diameter), mesoporous silica MCM-41(3.8 nm pore diameter) and a microporous faujasite of the Ytype. Mono- and diaminotrialkoxysilane were chosen for thisstudy. Through various characterizations it was found that theaminosilane groups were covalently attached to the amorphoussilica surface in the mesoporous supports, but not in themicroporous zeolite. The amine-functionalized MCM-41 andthe 8.8 nm silica gel exhibited a signicantly higher uptake ofCO2 at low pressures compared with the bare supports. On thecontrary, for the 4 nm silica gel and the zeolite the adsorptiondecreased aer impregnation.

5.2 Adsorption of heavy metals and organic pollutants

Due to their high surface area, MCM-41 materials can act asexcellent adsorbents for heavy metals. The amine-modiedsamples can easily extract various heavy metals from water byforming covalent bonds with them. Speciation and separationof chromium(VI) and chromium(III) from aqueous solutionswere investigated bymany research groups. Idris et al.159 studiedaminopropyl-functionalized mesoporous silica (AP-MCM-41) asan adsorbent for Cr(VI) and Cr(III). The as-synthesized adsorbentwas produced following a simple synthesis method at roomtemperature prior to template removal using microwave diges-tion. AP is a simple chelate, yet it can extract Cr(VI) exclusivelyfrom solutions containing other mixed metal ions simply bytuning the solution pH. Recovery of Cr(VI) from loaded AP-MCM-41 is also easy to perform with 100% extraction efficiencies. Thematerials can be reused several times without losing theiractivity. The ability of various as-prepared and organicallymodied MCM-41 and HMM mesoporous silica materials tobehave as efficient adsorbents for organic pollutants in aqueoussolution was investigated by using different surface function-alization procedures, so as to adjust their hydrophilic/hydrophobic balance. For highly organically functionalized

57120 | RSC Adv., 2014, 4, 57111–57124

samples, the residual supercial silanol groups (<50%) aresufficiently isolated from each other so as to prevent watercapillary condensation within the pores, thereby leading to anincreased hydrophobic character of the resulting mesoporoussilica. The adsorption and storage capacity of the MCM-41materials increased by 20 times aer amine functionalizationtowards the organic pollutant N,N-diethyl-m-toluamide.160 Par-ida et al.161 showed that a 12.8 wt% APTES-modied MCM-41acted as the best catalyst for the adsorption of Cu fromaqueous solution. The adsorbed Cu can again be used in theepoxidation of styrene with good conversion and selectivity.

5.3 Catalysis of liquid-phase reactions

The organic amine-modiedMCM-41materials are mainly usedas base catalysts in liquid-phase reactions. According to Paridaet al.,162NH2-MCM-41 is an efficient catalyst for the Knoevenagelcondensation reaction. The effect of amine group can bestudied by varying the percentage of APTES. Again the NH2-MCM-41material is further modied with Cumetal and utilizedin single-step amination of benzene.163 Cu/NH2-MCM-41 withSi/Cu ¼ 20 showed a maximum 72% conversion with 100%selectivity for aniline. Dıaz et al.164 studied dialkylsilane-functionalized MCM-41 for the esterication of glycerol withfatty acids such as lauric and oleic acids. According to Choudaryet al.,165 diamine-functionalized MCM-41 materials show goodactivity towards Knoevenagel and Aldol condensation reactions.The covalently bonded amine-modied MCM-41 materials canact as excellent CO2 capture agents. Again, CO2 reacts with 2-aminobenzonitriles to form a wide variety of quinazoline-2,4(1H,3H)-dione derivatives. Further, the intermediates areused for the synthesis of biologically active derivatives such asprazosin, bunazosin and doxazosin.166,167 The amine-functionalized materials can be further modied by variousother active groups (i.e. heteropoly acids, transition metals,sulphonic acid etc.) for further catalytic applications.168,169

6. Thermal stability of amine-functionalized mesoporous silica

In the industrial application of these materials, thermalstability plays an important role. The surface of mesoporoussilica materials is hydrophilic in nature due to the presence ofsilanol groups. Aer various amine modications it becomeshydrophobic as alkyl chains replace the silanol groups. Thesesurface modications inuence the thermal and hydrothermalstability of the functionalized materials. Wei et al.170 discussedthe variation of thermal stability with amine modication ofSBA-15 and SBA-16 materials. They showed the thermal stabilityof N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPS)-modied SBA-16 in both He and air atmosphere. AEAPS-SBA-16 was found to be stable up to 200 �C in He and also in air.Ethylenediamine-SBA-15 was stable up to 300 �C in He and200 �C in air.171 Varying the amine to APTES, the APTES-SBA-15material was found to be stable up to 250 �C.172 Huang et al.173

also showed that APTES-MCM-48 was thermally stable up to200 �C. AEAPS-modied SBA-16 can be used as a good adsorbent

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for CO2 gas.174 The thermal stability of this material was studiedby TG-DTG measurement and found to be stable up to 200 �C.The thermal stability of amine-functionalized materialsincreased with an increase in molecular weight of the amine,although the adsorption capacity decreased with molecularweight of the amine. Zhi-lin et al.175 studied various amine-modied MCM-41 materials and found that PEI-modiedmaterials are maximally stable up to 175 �C. Hence thermalstability plays a vital role in the applications of thesematerials.176

7. Conclusions

As there are increasing economic, social and environmentaldemands for safe and green research, the contribution of multi-active amine-modied MCM-41 materials becomes a prominentarea of research. Simply mixing homogeneous and heteroge-neous catalysts is not the solution for the search for newmaterials. The harmful effects of catalysts cannot be avoided bythe traditional methods. Hence the design of inorganic–organichybrid materials can be an important solution to deal withthese challenges.

In this review the synthesis and modication of differenthybrid materials, especially amine-modied mesoporous silicamaterials, and the impact of these modications on theircatalytic activities are discussed through various examples. Thesole aim of this review is to attract the attention of more andmore researchers towards this versatile material. Also some newand interesting applications can be designed with furtherinnovative research in this eld.

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