Spectroscopic and quantum chemical study of the Brønsted acid sites in zeolite L channels with acidochromic cyanine dyes

Four pH sensitive cyanine dyes with pKa values (in aqueous solution) of 3.4, 5.0, 6.8, and 8.6 were inserted into the pores of zeolite L to estimate the acidity of the zeolite channels. The photophysical properties of the dyes were investigated first in solution via UV–vis and fluorescence spectroscopy. The absorption and the emission maxima of the protonated and deprotonated forms vary significantly for all dyes, with the protonated forms absorbing at around 350 nm, and the deprotonated forms at around 450 nm. In the channels of zeolite L the two dyes with the higher pKa values were completely protonated, while the two dyes with lower pKa values were partially deprotonated. We found a pH value between 3.0 and 3.5 inside the pores of a fully hydrated potassium exchanged zeolite L. Preliminary results show that the zeolite particles can also be used to probe gaseous amines from aqueous solutions.DFT cluster optimizations were carried out to study the Brønsted acidity of the zeolite pore. Acidity was related to the hydrogen bond strength of the zeolite, which was characterized by geometric parameters, and by adsorption energy. The optimizations showed an interaction between the dye and the acid site of the zeolite. This interaction caused a change in the dye structure and the position of the proton.