Mesostructure determination

Determination of the phase purity of mesoporous molecular sieves (MMSs) [1,2] is important in synthesis, modification and application of these materials [3-7]. Many of the synthesis procedures reported so far involved various phase transformations [8-20] and thus the desired MMS product may be contaminated with some mesostructured impurities. One of the possible impurities is a lamellar phase, which readily forms under various synthesis conditions [1,8-25]. Because of its layered structure, the lamellar phase collapses upon calcination [1] and therefore constitutes a disordered impurity of calcined MMS samples. 

Pure mesoporous silicas have little or no intrinsic catalytic activity, but catalytic centers can be introduced by grafting organic ligands onto the framework walls" or by incorporating metal ions into the framework12,13. Ti-functionalized derivatives are especially effective in demonstrating the importance of framework access in determining the catalytic activity of a mesostructure. 

Fig.2. Nitrogen adsorption-desorption isotherms determined at 77K on a series of samples of siliceous mesostructured materials (empty symbols adsorption, filled symbols desorption).

The relative number of incompletely condensed (Q, Q ) and fully condensed (Q ) silicon atoms in MCM-41, MCM-48 and MCM-50 (which represents a lamellar structur) can be determined by 29si MAS NMR, indicating the degree of the formed mesostructures. In other words, the Q /Q ratio measures the extent of silanol condensation, which means lower values for more condensed frameworks (40,48-50) (see Figure 20). 

Carbon (C)-aerogels have been investigated for one decade as a promising material for electrochemical applications in supercapacitors, fuel cells and waste water treatment [1,2], C-aerogels are nanoporous, electrically conducting and monolithic materials that provide the unique possibility to tailor the carbon properties on a molecular scale. The surface area and the degree of microporosity can be adjusted almost independently of the overall porosity for which mainly meso- and macropores are responsible. Whereas the mesostructure is determined by the stoichiometry of the reactants in the precursor solution, the pyrolysis conditions control the micropore structure of the material [3,4]. High pyrolysis temperatures will increase the electrical conduchvity [5], an important property for many electrochemical applications. 

The formation of surfactant-silica mesostructure can be considered as a result of the assembly of surfactant and silica species. This assembly involves many physical and chemical processes, such as the assembly of surfactant molecules, and hydrolysis and polymerization of inorganic species. The balance of all processes and all interactions between surfactant and inorganic species determines the final mesostructure. 

The synthesis parameters in determining the degree of mesostructure, mesophase composition, and morphologies of product have been emphasized and discussed on the... 

In a physically confined environment, interfacial interactions, symmetry breaking, structural frustration, and confinement-induced entropy loss can play dominant roles in determining molecular organization. Wu[295] studied the confined assembly of silica-copolymer composite mesostructures within cylindrical nanochannels of porous anodic... 

First preliminary variants of DDM were applied in the full-profile X-ray diffraction structure analysis of a series of new silica mesoporous materials and ordered nanopipe mesostructured carbons. DDM allowed stable back-ground-independent full-profile refinement of the structure parameters of these advanced nanomaterials, a result that was unattainable by any other method. To date, DDM has been applied to many various mesoporous and mesostructured substances. The structural parameters of a series of face-centred cubic (Fm3m), body-centred cubic Im3m), and two-dimensional hexagonal (pGmm) mesoporous silicates were determined by DDM from synchrotron XRD. A comprehensive structural analysis of mesoporous silicates SBA-16 (cage-type cubic Irriim), their carbon replicas, and silica/carbon composites was performed by applying DDM. The structure of MCM-48 mesoporous silicate materials was analysed in detail by DDM from different laboratory and synchrotron XRD data. The pore wall thickness of both as-made and... 

To determine the success of synthesis efforts and to provide feedback on quality control, the mesoporous silica films are characterized by several techniques. The ability to determine the continuity and quality of the films must be quick and accurate. Owing to this fact, x-ray diffraction has proven the easiest way to resolve the film structure. However, phase identification by x-ray diffraction alone is not always conclusive (e.g., when only one peak is observed). Other techniques must be employed to fully determine the phase of the mesostructure. 

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