MCM-41 and Na-magadiite silicates internal voids probed by pyrene fluorescence spectroscopy

Investigation of the properties of two different silicate materials, namely MCM-41, with one-dimensional, cylindrical pores (4.04 nm diameter), and Na-magadiite, with a bidimensional, layered structure (1.67 nm between layers) was carried out. These materials may be used as sorbents, and their polarity is determinative to the kind of applications they may be employed at. Pyrene, which presents a vibronically resolved emission spectrum, was used as a fluorescent probe to give information on the polarity of these silicates voids. The ratio of band intensity at 383 nm (band 3) to that at 373 nm (band 1) is known to diminish as the polarity of the medium increases. Pristine sorbents and pyrene-adsorbed ones were characterized by thermogravimetric analysis and X-ray diffraction. UV-VIS emission spectra of included pyrene were recorded at temperatures ranging from 10 to 300 K, and from comparing the I(3) : I(1) ratio it was found that Na-magadiite internal sites correspond to a less polar medium than those in MCM-41. This is in accordance with the thermogravimetric data, which shows that a larger amount of pyrene was included in Na-magadiite, despite its smaller dimensions. Yet, excimer emission, usually a concentration dependent phenomenon, was mainly observed for the MCM-41 samples. Upon temperature lowering, pyrene molecules move to other sites of different polarities, towards an intermediary value for both materials.