Structural and surface properties of siliceous and titanium-modified HMS molecular sieves

Siliceous and titanium-modified HMS molecular sieves were synthesized and characterized by means of X-ray diffraction, atomic absorption, diffuse reflectance UV-visible spectroscopy and nitrogen adsorption at 77 K in a wide range of relative pressures. It was shown that the introduction of relatively small amounts of titanium does not lead to appreciable changes in the structure of the synthesized HMS materials. However, for higher titanium loadings, a significant decrease in the primary mesopore volume, the specific surface area and the pore size uniformity was observed. High pressure adsorption behavior of the HMS samples was compared to the adsorption properties of previously studied MCM-41 materials and the differences were attributed to differences in size and possibly shape of their particles. The primary mesopore sizes for the HMS materials were estimated by means of several methods and the results were compared and discussed. Low pressure adsorption behavior of the samples was also studied and it was shown that the incorporation of titanium in the HMS materials leads to a small but noticeable enhancement of nitrogen adsorption in the lowest pressure region. The nitrogen adsorption energy distributions were calculated and found to be similar for all samples. However, a small increase in the adsorption energy with increasing titanium content in the HMS samples was observed.