Characteristics of gallium-substituted hexagonal mesoporous silica: Effects of gallium content

Gallium-substituted hexagonal mesoporous silicas (Ga-HMS) with various Si/Ga ratios in the range of 15 and 200 were prepared at ambient temperature by neutral surfanctant templating pathway. The materials were synthesized by using dodecylamine as a template and tetraethylorthosilicate as a silicon source. They were characterized by energy dispersive X-ray spectroscopy, powder X-ray diffraction (XRD), N₂ adsorption-desorption, thermogravimetric analysis, differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, Fourier-transform infrared absorption spectroscopy and ultraviolet-visible absorption spectroscopy. Ga-HMS samples had high surface areas and uniform mesoporous channels, which are 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 thickness, small crystallite domain sizes, and complementary textural mesoporosities in comparison with M41S materials. Ga-HMS materials had micropores and the hysteresis loops were obvious. These small crystallite size and complementary textural mesoporosity provided better access of the framework-confined mesopores. These mesoporous Ga-HMS samples exhibited irregularly shaped mesoscale fundamental particles which aggregated into larger particles. They also demonstrated better thermal stability than MCM-41. The textural pore volumes of Ga-HMS specimens could be up to 20 times as large as the framework volumes. The surfactant could be removed completely by calcination at 650°C. An absorption band of FT-IR at ca. 960 cm^-1 was assigned to the vibration of Si-O-Ga linkages. These samples also showed an absorbance band at 255 nm and 250 nm in UV-vis spectra. The results show that gallium was incorporated into the structure of HMS. The efforts in preparing Ga-HMS specimens by neutral-template synthesis route had led to new mesoporous silica molecular sieves with catalytically active gallium centers.