| dc.description.abstract | ABSTRACT
V-substituted and Ga-substituted hexagonal mesoporous silicas (HMS) with Si/V and Si/Ga ratios in the range of 15 ~ 200 prepared at ambient temperature by neutral (S0I0) surfanctant templating pathway are catalytically active. V-HMS and Ga-HMS silicas with various metal compositions were synthesized by using dodecylamine (DDA) as a template and tetraethylorthosilicate (TEOS) as a silica derivative. These materials were characterized by powder X-ray diffraction (XRD), N2 adsorption-desorption, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared absorption spectroscopy (FT-IR), ultraviolet-visible absorption spectroscopy (UV-vis), and energy dispersive X-ray spectroscopy (EDS).
V-HMS and Ga-HMS samples had high surface areas and uniform mesoporous channels, because they were similar to MCM-41. However, they differed from MCM-41 in presenting only a single peak in XRD patterns. They also possessed other characters of larger framework wall thicknesses, small crystallite domain sizes, and complementary textural mesoporosities in comparison with M41S materials. V-HMS and Ga-HMS materials had microporous pores and the hysteresis loops were obvious. These small crystallite size and complementary textural mesoporosity provided better access of the framework-confined mesopores.
Our efforts in preparing V-HMS and Ga-HMS specimens by templated synthesis have led to new mesoporous silica molecular sieves with catalytically active vanadium and gallium centers. These new mesoporous metallosilicates exhibited irregularly shaped mesoscale fundamental particles which aggregated into larger particles. The specific surface areas of Ga-HMS materials were larger than those of V-HMS materials. In reference to framework wall thickness, the wall thickness of V-HMS samples was larger than those of Ga-HMS samples. All of them should have better thermal stability than MCM-41. The textural pore volumes of V-HMS and Ga-HMS specimens could be up to 20 times as large as the framework volumes. The surfactant of V-HMS and Ga-HMS could be removed completely by calcination at 650 0C. Because these materials were metal-containing, an absorption band of FT-IR at ca. 960 cm–1 was assigned to the vibration of Si-O-Me (Me is V or Ga) linkages. These V-HMS and Ga-HMS samples also showed UV-visible absorbance band at about 255 nm and 250 nm, respectively. The V-HMS and Ga-HMS specimens can be widely used in a number of adsorption, ion-exchange and catalytic processes. | en_US |