Mesoporous template removal

Third Concept in Catalyst Design. Fenton Detemplation. Mild Organic Template Removal in Micro- and Mesoporous Molecular Sieves... 

Mesoporous zirconia has been prepared using anionic surfactants containing reactive oxygens that could bind Zr2+ (224-226). Mesoporous zirconia was obtained using alkyl phosphate amphiphiles but they were not stable to template removal. 

Isoelectronic mesoporous Ti02 has been prepared by the same method but it is also not stable to template removal (224, 227). Hexagonal and cubic manganese oxide mesostructures (MOMS) have been prepared (228). Layered Mn(OH)2 is combined with CTAB and stirred at 75°C for 12 h. Depending on the CTAB concentration, either hexagonal MOMS-1 or cubic MOMS-2 is formed. The MOMS phases are apparently stable to calcination and exhibit semiconducting properties. 

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. 

XRD and TEM analysis on template-removed MSU-Ge-2 evidenced the presence of a well-defined, long-range periodicity of the hexagonal pore structure (Fig. 3). The low-angle powder XRD pattern of as-prepared and template-removed mesoporous MSU-Ge-2 indicates a pore periodicity of 4.8 and 4.0 nm, respectively. The pore-to-pore distance (4.0 nm) determined from XRD... 

Preparation of MCM-41 materials [1] usually includes a high temperature calcination step that in fact opens mesopores by removing structure directing molecules (template) as well as results in the structure shrinkage due to high temperature condensation of silanol groups [2], Functionalization of mesoporous materials with organosilanes is often employed to synthesize materials of desired surface properties for advanced adsorption and catalytic applications [3] as well as to improve their stability [4]. 

Figure 1. Schematic representation of a single-step process that involves a simultaneous template removal from the MCM-41 mesopores and their surface functionalization [17].

Porous carbons were prepared using MCM-48 and SBA-15 mesoporous templates and various carbon precursors (see Chapter 3 for preparation description). Figure 8.11 displays the nitrogen adsorption isotherms at 77 K of SBA-15 and of the corresponding templated carbon obtained by carbonization of sucrose in the template. Both isotherms show a bimodal porosity in the templated carbon, mesopores are generated by the removal of the silica walls, and micropores are present in... 

A desire to design high capacity adsorbents resulted in the synthesis of hybrid materials with multifunctional groups introduced via direct co-condensation of proper organosilanes instead of less effective post-synthesis si lanization.19,20 One-pot synthesis through self-assembly mechanism allows the creation of active sites inside mesopores, that after template removal become accessible for adsorption.21 An alternative method for chemical grafting of OMSs, known as template displacement synthesis, permits a simultaneous template removal and attachment of functional groups.22... 

Kleitz et al. (2002), Krivoruchko et al. (2001) TEM, TGA Zirconium oxophosphate Mesoporous structuring, template removal + + n.a Oxidative template removal, phase formation... 

Ad Figure 2.1,2. Zeolite layers can be grown by hydrothermal synthesis onto porous supports (clay, alumina, sintered metal). Especially layers of MFI-type zeolite have been studied [e.g. 5-7]. Such MFI-layers were shown to survive template removal and subsequent thermal cycles up to 350 °C, which is taken as a strong indication for chemical bonding [8] at the support interface. To understand chemical attachment to metals one has to take into consideration that metals - by exposure to air - will be covered with a thin (1-2 nm) oxide film. Sometimes an intermediate mesoporous layer has been applied, e.g. a metakaolin film on clay or on zirconia [5] or metal wool on sintered metal [6]. 

Dong JH, Lin YS, Hu MZC, Peascoe RA, and Payzant EA. Template-removal-associated microstmctural development of porous-ceramic-supported MFI zeolite membranes. Micropor Mesopor Mater 2(K)0 34(3) 241-253. 

In 2002, Chiang and coworkers[124] developed a new scheme for the confined synthesis of TPA-silicalite nanocrystals. The surfactant cetyltinmethylammonium bromide (CTAMeBr) (in ethanol solution) was added to the single- and double-heated TPA-silicalite precursor sols (SHPS and DHPS), and the mixture was flocculated at a certain pH value to collect the nano-size silicate species in the precursors, and then dried. The dried precursor/surfactant hybrid was pressed into pellets and then steamed in a stainless steel autoclave at 110 150°C for 7 36 h. Finally, the product was calcined to remove the surfactants and TPA. The particle sizes of silicalite-1 produced in this method are about 30 nm. The study indicates that the nanoparticles collected by surfactants already exhibit the structural features of MFI. They crystallize entirely to form silicalite-1 nanocrystals after steam treatment at 110 150 °C. This new solid-phase approach provides a way to synthesize MFI nanocrystals without the problem of separation and collecting nanocrystals from suspension, and it also avoids the large consumption and cost of special mesoporous templates used in the confined-synthesis methods. 

O. Pachtova, M. Kocirik, A. Zikanova, B. Bernauer, S. Miachon, and J.-A. Dalmon, A Comparative Study of Template Removal from Silicalite-1 Crystals in Pyrolytic and Oxidizing Regimes. Microporous Mesoporous Mater., 2002, 55, 285-296. 

The mesoporous aluminas synthesized using a nonionic templating method are thermally stable not only to template removal, but also to prolonged heating at elevated temperature. Therefore, these aluminas would be able to maintain their unique structural features in fairly demanding catalyst preparations and catalytic applications. Unlike sol-gel-derived aluminas, the synthesis temperature used for the hydrolysis and condensation of the aluminum alkoxide did not affect the resulting thermal evolution from the aluminum hydroxide to transitional alumina and the subsequent thermal stability of the transitional alumina. The only observed effect of synthesis temperature was the impact on median pore diameter and pore volume.[231]... 

D and 3-D metal nanowire thin films[263] with tunable 3-10 nm wire diameters have been obtained by electrodeposition into mesoporous silica thin-film templates, resulting in nanowire arrays that reflect the pore structure of the template. Removal of silica is achieved via annealing followed by etching to leave mechanically strong freestanding metal nanowire films. 

Ordered mesoporous carbons (OMC) of various structures, designated as CMK-1 5, have been synthesized by carbonization of sucrose, furfuryl alcohol or other carbon sources inside silica or aluminosilicate mesopores that are interconnected into three-dimensional networks such as in MCM-48, SBA-1 and SBA-15. The mesoporous carbon molecular sieves, obtained after template removal, show TEM images and patterns characteristic of the ordered arrangement of uniform mesopores. The OMC, which are opening up a new area of the nanoporous materials, exhibit high BET specific surface areas, excellent thermal stability in inert atmospheres and strong resistance to attack by acids and bases. 

After template removal, a mesoporous material is obtained in which the pores can be arranged in either a hexagonal or a cubic fashion. 

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