EXAMINATION OF THE SOLVENTLIKE NATURE OF ZEOLITES BASED ON A SOLVATOCHROMIC INDICATOR

Zeolites are crystalline aluminosilicates with cages and channel systems that can host a variety of organic transformations. This intracrystalline space is akin to a "solvent", and description of this space in terms of solventlike properties is appropriate. The concept of solvatochromic indicators has been successfully used to define the physicochemical properties of organic solvents. In this study, we have investigated the electronic and Raman spectroscopy of the molecule N-(2-hydroxybenzylidene)aniline and established a quantitative correlation between the spectral intensities of the benzenoid and zwitterionic forms of this molecule and the alpha-value of various hydroxylic solvents. The alpha value is a measure of the hydrogen bond donor ability of the solvent. This correlation has been used to establish an alpha value scale for a series of faujasitic zeolites with varying Si/Al ratios. It was found that the alpha value of the zeolite increased with Si/Al ratio to reach a maximum around 7.8, followed by a decrease at higher Si/Al ratios. Since Na+-exchanged zeolites were examined in all cases, the interaction of the anil molecule in its zwitterionic form with Lewis acids (Na+) and bases (oxygen of the framework) was considered to be responsible for its formation. The Si/Al ratio of the framework determines the acid-base character of the zeolite and is reflected in a quantitative manner by the alpha value determined in this study.