Fluorinated mesoporous materials

Xenon Xe)(I = 1/2). The authors report the variation of Xe chemical shift 6 vs. the pore size, the temperature and the xenon pressure of xenon adsorbed on mesoporous materials prepared in different synthesis conditions. The xenon and fluorine magnetic shielding tensors, a, of XeF2 are characterized using solid-state Xe and F NMR spectroscopy and nonrelativistic and spin-... 

Fluorinated surfactants can be very intereshng for the preparation of mesoporous materials, due to their good self-aggregation properties, allowing a more precise control of the mesostructure and properties of materials. In this context, the use of fluorocarbon surfactants allows the formahon of ordered mesoporous materials with a long-range organizahon [52, 53],... 

Nowadays, partially or fuUy fluorinated surfactants are available with a variety of chemical structure and chain lengths. Therefore hydrophilic-lipophilic properties can be controlled and these new surfactant molecules may provide a rich phase behavior, which could be used to obtain well-organized mesoporous materials. 

The use of fluorinated surfactants, in the preparation of mesoporous materials will be described in the following section. 

The above results demonstrate that partially fluorinated nonionic surfactants, having a fluorinated hydrophobic chain and a nonfluorinated poly(ethylene oxide) polar chain, can be successfully used to obtain mesoporous materials with controlled morphology and pore size. The specific surface areas can be high, similar to MCM-41 materials obtained using hydrocarbon surfactants. 

Figure 11.13 Mesoporous materials with a bimodal pore-size distribution prepared from mixtures of fluorinated and hydrocarbon surfactants, (a) TEM image (b) Nitrogen sorption isotherms, and (c) pore-size distributions. (Reproduced from Ref [60], with permission).

Mesoporous materials with cubic symmetry can also be obtained using fluorinated compounds. Tatsumi et al. described the preparation of large mesoporous... 

It should be pointed out that although most of the research works used silica as a model material, fluorinated surfactants can be used for the preparation of a very wide variety of different mesoporous materials, including TiOj [70], aluminophos-phates and Fe- aluminophosphates [71], and Fe-Si oxides [72]. Organic functionalizations can also be obtained in direct synthesis processes. An example is the preparation in a one-step precipitation process, by Rankin et al., of vinyl-functionalized porous silica materials using mixtures ofTEOS and vinyltriethox-ysilane in aqueous solutions of cationic fluorinated surfactants [73]. 

Table 11.2 Comparison between the main morphological characteristics of a mesoporous material prepared using the fluorinated surfactant and the typical values obtained for MCM-41 and SBA-15 materials.

Figure 11.19 clearly shows that the mesostructure of MCM-41, with thirmer pore walls, collapses during the hydrothermal treatment. However, the mesoporous material obtained using the fluorinated surfactant is more robust and remains practically unmodified after the hydrothermal treatment. 

In this context, silicone and fluorinated surfactants, which can impart peculiar properties to mesoporous materials, constitutes a research field that may be exploited to obtain new nanostructured materials for novel applications and many future developments may still arise in this field. The literature on the use of nonhydrocarbon surfactants for the preparation of mesostructured materials remains relatively scarce and many aspects are still unexplored. 

High degrees of dealumination are difficult to achieve using AHFS compared with those obtained via hydrothermal treatment, because of loss of framework crystallinity. The characterisation techniques used here have shown that silicon enrichment occurs during the AHFS treatment, leading to higher bulk Si/Al ratios. 7a1 MAS NMR appears to show the presence of aluminium species other than those teU a- or octahedrally co-ordinated. These may be the fluorinated aluminium species mentioned in earlier works. The textural properties of AHFS treated zeolites are not changed relative to the parent material in contrast to the steam dealuminated zeolites, where the introduction of secondary mesopores occurs. 

F.S. Xiao, Ordered Mesoporous Silica-based Materials Templated from Fluorocarbon-Hydrocarbon Surfactant Mixtures and Semi-fluorinated Surfactants. Curr. Opin. Colloid Interface Sci., 2005, 10, 94-101. 

CO2 modified with a methanol/dichloromethane mixture and showed that the template extracted in this manner could be re-used for further syntheses of mesoporous silica. Treatment with supercritical CO2 alone leads to pore swelling with retention of mesostructural ordering in 2D hexagonal phase silica triblock copolymer surfactant composites. Pore swelling with supercritical CO2 has also been observed for materials templated with fluorinated surfactants. ... 

Surfactant assemblies are generally utilized as a template to form mesoporous shell. Vesicle-like hollow sihca particles can be prepared by sol-gel reactions in the presence of surfactants. Although most of the materials reported so far have multilamellar structures, hollow mesoporous silica particles with a single-walled vesicle structure have been prepared by the reaction of TEOS in the presence of a fluorinated surfactant (lH,lH,2//,2//-perfluorodecylpyridinium chloride). With... 

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