Spectroscopic Studies of Disorder in the Microporous Titanosilicate ETS-10

The microporous titanosilicate ETS-10 is a potential photocatalytic material with a disordered crystal structure. The disordered stacking of repeat units in the structure gives rise to defects that strongly influence the reactivity. This paper describes the spectroscopic study of a series of ETS-10 samples containing different levels of defects. All samples are well crystalline by X-ray powder diffraction, but they show significant variations in other spectroscopic properties that are sensitive to short-range order. Raman spectroscopy was particularly sensitive to disorder, and the 724 cm(-1) stretching mode of the -Ti-O-Ti-chains broadened and shifted with increasing disorder through the series. Changes in the UV absorbance spectrum also indicated disruptions of the delocalized charge-transfer transitions along the chains. We propose that stacking defects interrupt the -Ti-O-Ti-chains and that differences in the relative nucleation and growth rates lead to different concentrations of these defects. Hydroxyl groups associated with defect sites were observed by FTIR and solid-state cross-polarization H-1-Si-29 NMR. Disorder is also correlated with the occurrence of faulting in the crystal morphology. No new cation exchange sites associated with the defects were observed by 3Q Na-23 NMR.