Mesoporous Composites

Hybrid Inorganic–Organic Mesoporous MaterialsGuido Kickelbick*

Keywords:mesoporous materials · organic–inorganic hybridcomposites · self-assembly · silanes ·sol–gel processes

Periodic mesoporous materials havegained a lot of interest in recent years,because of the possibility of tailoring thepore structure, framework composition,and morphologies over a wide range.Many potential applications arise fromthe promising properties of these high-surface-area materials, including sepa-ration technology (chromatography,membranes, etc.), catalysis, nanoelec-tronics, sensors, and spatially definedhost materials for substances or reac-tions.[1]

The synthesis of hexagonally or-dered silica or aluminosilica mesopo-rous materials (the MCM-41 materials)using liquid-crystalline phases of ionicsurfactants as templates was describedfor the first time in 1992 by Kresgeet al.[2, 3] Soon it became clear that thisnew approach could be expanded to-wards the synthesis of other porousinorganic materials, such as variousoxides,[4–6] metal sulfides,[7, 8] phos-phates,[9] and metals.[10, 11] A variety oflyotropic liquid-crystalline surfactantsand phase-separated block copolymerswere used as templates for the assemblyof the inorganic frameworks, which wereformed as disordered, hexagonal, orcubic structures with a high control ofthe channel diameter between2–100 nm.

Organic functionalization soon be-came a major topic of research becauseit offered a further possibility to tailorthe chemical properties of the porousmaterials. The addition of organic

groups by post-synthetic grafting ofRSi(OR’)3, RSiCl3, or R3SiCl onto thesurface of the pores resulted in func-tional mesoporous inorganic–organichybrid materials (Figure 1). This post-synthetic grafting process has beenwidely employed to anchor variousorganic groups onto the surface of thepores, including organometallic species,amino- and thiol groups, and epoxides.However, this method often led to aquite low loading, an inhomogeneousdistribution of the functional groups,and a decrease of the of pore volume.The alternative route of applying orga-nosiloxanes directly during the synthesisby co-condensing them with tetraalkoxy-silanes (Figure 1) led to a higher load-ing but above a limit of around 25%RSiO3 groups the mesostructure col-lapsed. Functional groups, such as vi-nyl,[12, 13] 3-sulfanylpropyl,[14, 15] phenyl,[16]

aminopropyl,[14,17] cyanoethyl,[17] diphe-nylphosphanylpropyl,[18] were incorpo-rated by this method into the inorganicnetwork. Recently, based on this ap-proach, amphiphiles with a cleavablealkyl chain and a condensable headgroup were used as templates for the

synthesis for mesoporous silica materi-als.[19] After the synthesis, the cleavageand removal of the alkyl chains leftnanopores with a densely functionalizedsurface.

An expansion of the above men-tioned concepts was the incorporationof bifunctional organosiloxane precur-sors, also referred to as bridged silses-quioxanes, of the general formula[(R’O)3Si]mR (m� 2) as the network-forming species (Figure 2). The resultingmaterials, called periodic mesoporousorganosilicas (PMOs), contain the or-ganic groups as an integral part of theinorganic-oxide framework and the in-organic and organic moieties are cova-lently linked to each other. Hence, the“chemistry of the void space” exploredby the classical MCM-41-type materialswas extended to the “chemistry of thewalls”.[20] This novel route offers mate-rials with some important features thatcannot be obtained by other approaches.First, the organic moieties are homoge-neously dispersed within the channelwall and a maximum loading of up to100% is obtained. Second, organicgroups incorporated into the channel

Figure 1. Synthesis of hybrid mesoporous materials containing functional groups that dangleinto the channels.

[*] Dr. G. KickelbickInstitute of Materials ChemistryVienna University of TechnologyGetreidemarkt 9, 1060 Vienna (Austria)Fax: (+43)1-58801-15399E-mail: guido.kickelbick@tuwien.ac.at

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3102 � 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/anie.200301751 Angew. Chem. Int. Ed. 2004, 43, 3102 –3104

walls do not block the pores. Integratingthe “soft” organic groups should delivermechanical properties different fromthose of the classical, purely inorganicmesoporous materials.[21] And last butnot least, changing the organic precur-sors can result in pore walls with differ-ent chemical and physical properties.[22]

The advantages of the double-sub-stituted molecules are that bonding isalready established between the organicand inorganic species and thus thestoichiometry between silicon and car-bon in the framework is assured. Exam-ples of organosiloxanes that were suc-cessfully incorporated into the porewalls are bis(triethoxysilyl)methane,[20]

bis(triethoxysilyl)ethane,[23,24] bis(tri-ethoxysilyl)ethene,[25] bis(triethoxysilyl)-acetylene,[22] 1,4-bis(triethoxysilyl)ben-zene and derivatives thereof,[22,26–28] 2,5-bis(triethoxysilyl)thiophene,[22] 1,4-bis-(triethoxysilyl)ferrocene,[22] and bis(tri-ethoxysilyl)bithiophene[22] (Figure 3).These organic precursors were eithermixed with classical inorganic-oxidenetwork builders, such as tetraethoxysi-lane (TEOS), or were used as single-source precursors. The nature of theorganic group greatly influences thehydrolytic stability of the Si�C bond,whereas the simple precursors contain-

ing methylene, ethylene, or ethenylenegroups show high stability in the syn-thesis, the use of more complicatedgroups results in a significant Si�C bondcleavage as well as a decrease in thedegree of order in the material.[22]

Generally, the resulting products arecomparable to MCM-41 materials inthat they have high Brunauer–Em-mett–Teller (BET) surface areas ofbetween 600 and 1700 m2g�1, averagepore diameters—depending on the sur-factant and reaction conditions used—from 3 nm up to 10 nm, wall thicknessesof 0.7–3.3 nm, and narrow pore sizedistributions.

The chemical reactivity of the incor-porated organic groups was demonstrat-ed, for example, by the bromination ofunsaturated spacers where by the struc-tural integrity and order of the materialwas maintained.[23, 25] Furthermore, mes-oporous aminosilica materials were ob-tained by the thermal ammonolysis ofmethylene or ethylene PMOs.[29]

An interesting property was ob-served for the mesoporous silica materi-als that contain bridging methylenegroups in the walls. This materialshowed a so-called metamorphosis uponheating under inert-gas atmosphere,that is, the bridging methylene groups

are transformed into terminal methylgroups and the mesoordered structureremains intact.[20]

Structural order in the pore walls, onthe molecular scale, was usually notobserved, however, if aromatic systemswere used as bridging groups the pres-ence of arylsilica ordering arising fromp–p stacking was observed leading to akind of “double self-assembly” processinvolving simultaneous preorganizationand co-assembly of both surfactant mi-cellar and the organosilica precursorspecies.[26, 27,30] Hence, a hierarchicallyordered mesoporous solid is producedwith a molecular-scale pore-surface pe-riodicity.

A further important step in the useof organosilioxane precursors was thechange from bis(trialkoxysilane)-substi-tuted organic groups, whose only func-tion was to incorporate organic groupsinto the framework, to tri- or multi-alkoxysilane substituted units, whichalso act as cross-linkers in the networkformation.[31, 32]

Recently, a novel ring-shaped pre-cursor was introduced, that is, 1,3,5-tris[diethoxysila]cyclohexane [{(EtO)2-SiCH2}3],

[33,34] in which each silicon atomhas two organic substituents comparedto one in all previous systems. Thisarrangement allows for a higher degreeof organic functionalization in the walls.It forms networks through the six hydro-lyzable groups in the ring and allows thepreparation of mesoporous powders andoriented films. The resulting materialcontains mesoscale channels with aspacing of about 4.5 nm, mesopores witha diameter of approximately 2.2 nm, andan estimated wall thickness of 2.3 nm.Unlike other complex precursors, noSi�C bond cleavage was observed in thefinal material. The framework showedquite high thermal stability, and a ther-mal transformation of bridging CH2 intoterminal CH3 groups was observed as inother methylenesilica systems. The thinfilms obtained by spin coating werecalcined at 300 8C under a nitrogenatmosphere without cracking or loss ofmesostructure and also the Si�C bondsremained intact. Hence, these materialsare interesting low-dielectric-constant(k) materials for use in future integratedcircuits and have a high thermal and agood mechanical stability. Films with avarying organic content were synthe-

Figure 2. Synthesis of the periodic mesoporous organosilicas (PMOs).

Figure 3. PMO building blocks.

AngewandteChemie

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sized from mixtures of tetramethoxysi-lane (TMOS) and the silsesquioxanering. In these films a linear decrease ofthe k value with increasing organic con-tent was observed. While the films madeentirely of [{(EtO)2SiCH2}3] had k� 2.5,this value could be decreased to 2 bythermal treatment up to 400 8C. Aunique feature of the described precur-sor is the possibility to functionalize oneof the bridging methylene groups by alithiation reaction followed by a nucle-ophilic substitution and the use of thefunctionalized ring thus formed for theformation of PMOs. The resulting ma-terials contained functional groups, suchas I and Br, which were still intact aftersynthesis and extraction. The chemicalmodification of these groups will allow aplethora of novel materials to be pre-pared. This novel precursor is a proto-type of a new class of mesoporoushybrid materials and we can look for-ward to future developments in thisclass.

Published Online: April 30, 2004

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