Surfactant templating

Surfactant templating chemistry involves the hydrolysis and condensation of solubilized precursors of metal oxides in the presence of surfactant molecules that form 

Surfactant templating chemistry can be extended to many nonsilicate compositions after modifications to the synthesis route. These materials are less structurally stable than the mesoporous silicates, which is attributed to the thinness of the amorphous pore walls ( 1 to 2 nm). Stucky and coworkers [85,86] showed that this problem could be mitigated by preparing the materials with thicker walls. To prepare mesoporous WO3, they dissolved a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer and WCle salt in ethanol, and dried the resulting solution in open air. The tungsten salt reacted with moisture to undergo hydrolysis and condensation reactions. These chemical reactions caused the eventual formation of amorphous WO3 around triblock copolymer micelle-like domains, and after calcination at 400 C, a mesoporous WO3 with thick, nanocrystalline walls ( 5 nm) and surface area of 125 m /g was formed. 

A mesoporous W0x/Zr02 was prepared in an analogous fashion, in which Z1CI4 salt was added with WCU prior to drying of the ethanoUc solution [85]. After calcination at 4(X)°C, the resulting mesoporous material was amorphous, with a pore wall thickness of 4.5 nm and a surface area of 170 m /g. The distribution of W and Zr atoms was not reported, although the W can be presumed to be distributed on the surface and throughout the pore walls. 

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Many studies on template thermal degradation have been reported on zeolites of industrial interest including ZSM5 [1-5], silicalite [1], and beta [6-8], as well as surfactant-templated mesostructured materials [9-13]. The latter are structurally more sensitive than molecular sieves. Their structure usually shrinks upon thermal treatment. The general practice is slow heating at 1 °C min (N2/air) up to 550 °C, followed by a temperature plateau. 

A non-surfactant templating route to mesoporous silica materials. Advanced Materials, 10, 313—316. 

After removal of the surfactant template, a regular arrangement of mesopores with a narrow pore size distribution is obtained in the replicated silicas. 

MCM-41 spheres were prepared modifying the procedure reported by Grim et. al. [9], using n-hexadecyltrimethylammonium bromide (Ci6TMABr) as surfactant template. Reactant molar ratio was 1 TEOS 0,3 C TMABr 0,129 NH3 144 H20 58 EtOH with pH about 9. The surfactant (Ci6TMABr) was dissolved into the mixture of distilled water, NH3 and EtOH tetraethylorthosilicate ([C2H50]4Si, TEOS) was then added to the surfactant solution and stirred for 2 hours at room temperature. 

Etienne, M. Cortot, J. Walcarius, A. 2007. Preconcentration electroanalysis at surfactant-templated thiol-functionalized silica thin films. Electroanalysis 19 129-138. 

Kavan, L. Rathousky, J. Gratzel, M. Shklover, V. Zukal, A. 2000. Surfactant-templated Ti02 (anatase) Characteristic features of lithium insertion electrochemistry in organized nanostructures. J. Phys. Chem. B 104 12012-12020. 

Ordered mesoporous materials of compositions other than silica or silica-alumina are also accessible. Employing the micelle templating route, several oxidic mesostructures have been made. Unfortunately, the pores of many such materials collapse upon template removal by calcination. The oxides in the pore walls are often not very well condensed or suffer from reciystallization of the oxides. In some cases, even changes of the oxidation state of the metals may play a role. Stabilization of the pore walls in post-synthesis results in a material that is rather stable toward calcination. By post-synthetic treatment with phosphoric acid, stable alumina, titania, and zirconia mesophases were obtained (see [27] and references therein). The phosphoric acid results in further condensation of the pore walls and the materials can be calcined with preservation of the pore system. Not only mesoporous oxidic materials but also phosphates, sulfides, and selenides can be obtained by surfactant templating. These materials have pore systems similar to OMS materials. 

Figure 4.5 TEM image of MCM-41, a surfactant templated silica with ordered mesopores. (Kindly provided by S. Dai, Oak Ridge National Laboratory, Oak Ridge, TN.)...

A combined microcalorimetry and adsorption stndy was nsed by Meziani et al. [49] to characterize the surface acidity of a series of MCM41 aluminosilicates (referred to as SiAl C , where x is the molar Si Al ratio and n the chain length of the surfactant template). With the exception of H-SiAl32Ci4 and SiAlgCi4, all samples were found to present low surface acidity. 

The early attempts to produce mesostructured metal chalcogenide involved surfactant-templated cross-linking polymerization of Zintl clusters (MQ4, M4Q10 and Sn2Qe M = Ge, Sn Q = S, Se, Te) with various metal ions (Hg, Cd, Pt, ... 

Recently, we demonstrated that the Zintl clusters [Geg]" react with chalcogen atoms (S, Se, and Te) in the presence of surfactant templates to form ordered mesoporous Ge-rich chalcogenides [74]. The mesostructured frameworks grow through a coupling reaction of (Ge9)-clusters with chalcogens in formamide/ethy-lenediamine mixture solution in an unusual reaction that seems to be a redox process (5). 

Few reports are available in the literature, in which cationic surfactants have been used to prepare the silica nanoparticles [82] and particularly those encapsulating an optical component. In a surfactant template approach Lin et al. [83] prepared Gd(III)-loaded mesoporous silica... 

TGA measurements show that cationic surfactant template can be removed between 162 to 385 °C with a large exothermic peak at 336 °C and total weight loss is about 31 %. 

Assembly of nanoporous silica ia amphoteric surfactant templating scheme... 

The sodium aluminate and appropriate amounts of surfactants/templates were dissolved in water using C(/Cl4 ratios of 85%/15% and 75%/25%, respectively, and the mixture was aged over night (16-20 hours) according to ref [6]. Subsequently, the usual gel preparation described earlier [4] were followed with no further changes, then the gels were transferred to PTFE-lined autoclaves and heated at 150-175°C for 6 days, quenched in cold water, washed 3-4 times in liberal amounts of water, decanted, filtered and dried overnight in ambient air. 

Independently, we reported [17] that the surfactant template can be directly exchanged by organochlorosilanes, which form a strong covalent bonding with the silica surface. A major advantage of this approach in comparison to the ion-exchange method [20] lies in its... 

Martin, J. D., Keary, C. L., Thornton, T. A., Novotnak, M. P, Knutson, J. W., Folmer, J. C. W., Metallotropic liquid crystals formed by surfactant templating of molten metal halides. Nature Materials 2006, 5, 271-275. 

Figure 15.4 SEM images showing diverse mesoporous silica shapes and patterns produced by surfactant templating (a) rope, (b) toroid, (c) discoid, (d) pinwheel, (e) wheel, (f) gyroid, (g) bagel, (h) shell, (i) knot, (j) clock, (k) eccentric 1 and (1) eccentric (reproduced by permission from Macmillan Publishers Ltd).

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