Mesoporous phases

M. Eswaramoorthy, S. Neeraj, and C. N. R. Rao, High Catalytic Efficiency of Transition Metal Complexes Encapsulated in a Mesoporous Phase, J. Chem. Soc. Chem. Commun. 5, 615-616... 

When the synthesis is carried out at room temperature for 3 days, a solid containing only disordered mesoporous phase with a BET surface area of over 1000 m2/g is obtained. Its XRD pattern displays only a single [100] peak at low 20 angle. But if the crystallization time is prolonged to about 22 days, [100], [110] and [200] peaks except the absence of [210] peak can be distinctly resolved. This result shows that the synthesis time is important in transforming the ill-defined disordered mesoporous silica framework into integrated hexagonal MCM-41 framework. 

When the temperature is increased to 100 °C, the formation period of regular hexagonal mesostructure can be shortened dramatically to about 3 5 days. Between 100-150 °C, the XRD patterns of the obtained solids clearly show all four diffraction peaks. However, when the temperature is increased to around 160 °C, the resulting solid shows only one [100] diffraction peak which is typical of disordered mesoporous phases. When the temperature is further elevated to 185 °C, only amorphous phase is produced, regardless of the period of the synthesis. 

The pore arrangement in MCM-41 could be determined by XRD due to its relatively simple structure. For other mesoporous phases with much more complicated structures, such as SBA-2, determination of a complete mesopore system by XRD becomes extremely difficult. SBA-2 was first reported in 1995 [19] and was believed to consist of discrete large cages obeying the symmetry of space group P63/mmc [20,21], However, the pore system connecting these supercages had not been determined until the TEM technique was applied [10],... 

The existence of these stable, open-frameworks in the specific case of manganese is presumably related to the high ligand field stabilization energy associated with the t2g electronic configuration of Mn +. This should render the materials kinetically stable with respect to their collapse into more condensed phases. It is also, no doubt, the reason why it has been possible to make stable mesoporous phases based upon Mn02, and indeed to remove the surfactant templates in order to access their microporosity [110]. 

From their discovery in 1992, the field of mesoporous materials has grown in an exponential way with increasing successful synthetic methods for siHca, metal oxides, phosphates, and so on. The relatively low hydrothermal stabiHty of mesoporous phases is perhaps the most critical consideration for their appH-cation, in particular for heterogeneous catalysis. Likewise, the incorporation of chemical functionalities into mesoporous materials has enlarged their appHca-tions. Future research efforts will likely address the development of novel synthetic methods to better control such functionaHzation while avoiding the loss of textural and structural properties of these materials. The morphological control of mesoporous siliceous solids and the incorporation of chemical functionaHties are also of great interest to yield materials with novel properties and prospective applications. 

Extraction of the materials containing PTMOS is possible with preservation of the mesostructure. After extraction a modified mesoporous phase is isolated which still carries organic phenyl groups coupled to the pore walls. In which way and to what extent the phenyl groups have been covalently attached to the pore walls has to be tested by further characterization. 

By analogy to the aforementioned, porphyrin-doped mesoporous silica films have been used for the detection of TNT vapors by quenching of the porphyrin emission.The authors demonstrated that the use of mesoporous phases shows distinct advantages in terms of response when compared with functionalized amorphous silica. 

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