Sol-gel preparation and properties of amorphous iron-containing aluminosilicate

The objective of the present study includes a modification of synthetic aluminosilicate with iron cations and an estimation of the modificator influence at structure and properties of the aluminosilicate. The iron-containing aluminosilicate (Si/Al = 4.72) with an Al/Fe molar ratio of 5:1 was prepared by the sol-gel method at pH = 1-2. Amorphous aluminosilicate (Si/Al = 4.72), which was synthesized by analogous procedure, was applied as a reference sample. By scanning electron microscopy, it was found that the powders had particles of 1-20 micrometers in size. The results of low-temperature adsorption-thermal desorption of nitrogen showed that the modification with Fe3+ ions affected the specific surface area and porosity of the material under study. The iron-containing sample has a higher specific surface area and pore volume comparing to the initial aluminosilicate. The Hammett indicator method was used to evaluate the surface centers of the samples. It was found that the materials under study were characterized by the presence of active sites with pKax values in the range from - 4.4 to 12.8 with a pronounced maximum at pKax = 1.02. For an iron-containing sample, the concentration of acid sites significantly decreases at pKax = 1.02, while at pKax 0.80, 1.03, 2.10, 2.50, 4.10, 5.00, 8.00, and 12.80, an increase in the number of acid sites is observed. The values of the Hammett function are practically the same for the studied samples and characterizes them as materials of medium acidity. For iron-containing aluminosilicate, the larger number of active sites was noted, it amounted to 313.5 mmol/g.