Atomic arrangements in amorphous sodium titanosilicate precursor powders

Infrared, Raman and 29Si MAS-NMR spectroscopic methods are applied to analyse the relationship between the chemical compositions of the initial synthesis gels, the predominant type of atomic arrangement in the amorphous precursors obtained from the dried gels and the favoured crystalline titanosilicate phases formed upon hydrothermal treatment. Alkali titanosilicate gels of various molecular ratios were hydrothermally treated at 200 °C, thus leading to formation of microporous ETS-4, GTS-1 and sitinakite, layered AM-1 and AM-4, and dense natisite and paranatisite. The results show that the increase in the Na2O content enhances the formation of Si-O-Ti linkages on the account of Si-O-Si and Ti-O-Ti bridges in the ternary Na2O-TiO2-SiO2 system, prior to the hydrothermal treatment. Occurrence of precursor clusters with medium range order resembling that of the subsequently crystallised titanosilicate phases was detected in the initial amorphous substances. The ratio Na2O/TiO2 is vital for the type of incipient SiO4-TiOn clustering and thus for the formation of crystalline titanosilicate phases of desired framework topology. Frameworks with five-coordinated titanium are obtained from gels with the lowest and highest values of Na2O/TiO2 ratio (smaller than 4 and higher than 8), while six-coordinated Ti4+ is characteristic for gels prepared at intermediate values of Na2O/TiO2 ratio. Layered and microporous titanosilicates crystallise at a lower Na2O/TiO2 ratio, while dense titanosilicates crystallise preferentially at higher alkalinity.