Mesoporous materials preparation

For each of the following compositions, name what would be a preferred hard template (mesoporous silica or carbon) for preparing mesoporous materials ... 

Table 8.2 summarizes the main synthetic approaches to prepare mesoporous materials. 

Extending the use of zeolites into larger dimensions, say to catalyse enzymatic reactions and for the purification of colloidal precious metals, was the aim of researchers at Mobil Corporation (USA), who in 1992 discovered a viable and versatile synthetic procedure to prepare mesoporous materials, i.e. materials with ordered porosity in the range between 20 and 500 A (2-50 nm Kresge et al. 1992). Their first material was termed MCM-41 (Mobil Composition of Matter) and the mechanisms involved templating of a silica sol-gel synthesis by an amphiphilic surfactant. 

The second case study. This involves all silica micro- and mesoporous SBA-15 materials. SBA-15 materials are prepared using triblock copolymers as structure-directing templates. Typically, calcined SBA-15 displays pore sizes between 50 and 90 A and specific surface areas of 600-700 m g with pore volumes of 0.8-1.2cm g h Application of the Fenton concept to mesoporous materials looks simpler since mass transfer would be much less limited. However, it is not straightforward because hydrolysis can take place in the aqueous phase. 

The TS-l/MCM-41 catalysts were synthesized in two steps [8]. The first step was involved with the preparation of TPAOH impregnate mesoporous materials and the second stq) was the DGC process. The TPAOH impregnated H-MCM-41 was prepare with calcine Ti-MCM-41, TPAOH (1 M solution of water) and ethanol under stirring by impregnation method. The parent gels were prepared with a TPAOH/Ti-MCM-41 ratio of 1/3 by weight. After 4 h, ethanol and water were removed in a rotary evaporator at room temperature and solid products were dried in a convention oven at 373 K for 48 h. The DGC process was carried out at 448 K for 3 h to obtain TS-1/MCM-41-A and for 6 h to obtain TS-1/MCM-41-B. However, the mesoporosity of Ti-MCM-41 was lost when the DGC process was carried out for 9 h. 

The pore size of most zeolites is <1.5nm. This microporosity limits their utility in most areas of chemistry, where the molecules used are much larger, and for which mesoporous materials would be necessary. Unfortunately, attempts to use larger template molecules in the zeolite synthesis, an approach which should in theory lead to larger pore size zeolites, have met with very little success. Indeed, some zeolitic materials have been prepared which have mesopores - none of these has ever displayed any real stability and most collapse on attempts to use them. A new methodology was thus required. 

Abstract A review of the thermolytic molecular precursor (TMP) method for the generation of multi-component oxide materials is presented. Various adaptations of the TMP method that allow for the preparation of a wide range of materials are described. Further, the generation of isolated catalytic centers (via grafting techniques) and mesoporous materials (via use of organic templates) is simimarized. The implications for syntheses of new catalysts, catalyst supports, nanoparticles, mesoporous oxides, and other novel materials are discussed. 

This chapter discusses the synthesis, characterization and applications of a very unique mesoporous material, TUD-1. This amorphous material possesses three-dimensional intercoimecting pores with narrow pore size distribution and excellent thermal and hydrothermal stabilities. The basic material is Si-TUD-1 however, many versions of TUD-1 using different metal variants have been prepared, characterized, and evaluated for a wide variety of hydrocarbon processing applications. Also, zeolitic material can be incorporated into the mesoporous TUD-1 to take the advantage of its mesopores to facilitate the reaction of large molecules, and enhance the mass transfer of reactants, intermediates and products. Examples of preparation and application of many different TUD-1 are described in this chapter. 

Preparation of chiral mesoporous materials has become a great interest for material scientists. Normally chiral property is introduced into chiral mesoporous material via an organic chiral templating component. But, by using a sonochemical method, Gabashvili et al. [36] have prepared mesoporous chiral titania using a chiral inorganic precursor and a non-chiral dodecylamine as a template. Size of the pores was 5.5 nm. 

Nanohybrids prepared from biomolecules and mesoporous materials can be used in various applications. In this section, examples of such applications are briefly introduced. 

Time-resolved in situ Small Angle Neutron Scattering (SANS) investigations have provided direct experimental evidence for the initial steps in the formation of the SBA-15 mesoporous material, prepared using the non-ionic tri-block copolymer Pluronic 123 and TEOS as silica precursor. Upon time, three steps take place during the cooperative self-assembly of the Pluronic micelles and the silica species. First, the hydrolysis of TEOS is completed, without modifications of the Pluronic spherical micelles. Then, when silica species begin to interact with the micelles, a transformation from spherical to cylindrical micelles takes place before the precipitation of the ordered SBA-15 material. Lastly, the precipitation occurs and hybrid cylindrical micelles assemble into the two-dimensional hexagonal structure of SBA-15. 

With the combined methods of 29Si NMR spectroscopy, X-ray diffraction, HRTEM and SAED we were able to characterize the Ti-Beta particle growth. 29Si NMR spectroscopy gave us an opportunity to see the formation of nanoparticles even before they were detectable with other techniques such as XRD. The above mentioned techniques enabled us to obtain sufficient knowledge to prepare Ti-Beta nanoparticles which were than successfully incorporated in novel micro/mesoporous materials [1],... 

In the present work, a different technique (pyrrole polimerisation and carbonisation) has been applied to prepare nitrogen-containing carbons from mesoporous materials. The influence of iron present in the matrix skeletons, introduced by impregnating the matrices with FeCl3, on the properties of the carbon replicas has been evaluated. 

The mesoporous materials reported above are usually prepared from relatively expensive surfactants. Some of them have poor hydrothermal stability. Furthermore, the MCM-41 host structure has a one-dimensional pore system with consequent pore blockage and diffusion limitations. Shan et al. (52) reported the synthesis of a three-dimensional and randomly connected mesoporous titano-silicate (Ti-TUD-1, mesopore wall thickness = 2.5-4 nm, surface area — 700-1000 m2/g, tunable pore size —4.5-5.7 nm) from triethanolamine (TEA). Ti-TUD-1 showed higher activity (about 5.6 times) for cyclohexene epoxidation than the framework-substituted Ti-MCM-41. Its activity was similar to that of the Ti-grafted MCM-41 (52). 

The most widely studied member of the M41S family is hexagonal MCM-41, which was first prepared by the I S+ liquid-crystal approach (168, 169). A charged surfactant like CTAB produces well-ordered mesoporous materials having ID... 

To extend the applicability of titania, the ease of production and reproducibility has to be secured in terms of production cost and product quality. The literature is abundant in methodologies to prepare Ti02 photocatalysts, and as a consequence a very large number of materials have been prepared and tested from colloidal to large surface area mesoporous materials. 

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