Nature of the shift in the absorption band of the molecular forms of azo dyes adsorbed onto the surface of aluminosilicate catalysts

Abstract

For the prediction of the catalytic properties of aluminosilicatc catalysts, we must be able to obtain an objective determination of their acid properties, i.e., the nature and strength of the acid sites. The most promising method for the study of the acidity of catalyst surfaces is the investigation of the electronic spcctra of adsorbed Hammctt indicator dycs [ 1-3]. The use of this method is limited by the difficulties in interpreting the observed adsorption bands of the adsorbed molecules interacting with the active sites of the catalysts. In studies on the nature and strength of the acid sites of various acid-base catalysts, the shift in the maxima of the adsorption bands of dye molecules adsorbed on the catalyst surface is usually determined relative to their spectral position in nonpolar solvents such as hexane and cyclohexane [ 1-71. It has been considered that small shifts of ",,30 nm in the absorption bands in the spectrum of a dye on a catalyst surface relative to its spectrum in a nonpolar solvent result from the reaction of the dye molecules with "weakly acidic" sites (surface hydroxyl groups) with the formation of hydrogen bonds [6, 8, 9]. In the present work, a new explanation is proposed for the phenomenon of the shift in the maxima of the absorption bands of the molecular forms of azo dyes adsorbed on catalyst surfaces without implicating hydrogen bonding. In analyzing the absorption spectra of dye molecules adsorbed on catalysts and comparing them with the spectra in solution, we must take account of the difference in phase state in which the molecules studied are located. The surface of a solid catalyst on which the dye is adsorbed is not equivalent to an isotropic liquid in which the dye is dissolvcd. The effect of such a transition on the spectral characteristics of molecules has been the object of very inadequate study, while the effect of the gasliquid phase transition has been studied rather well [ 10]. Bayliss and McRae [ 11 ] have shown that the shift in the absorption band of a compound in solution relative to the gaseous state results from a difference in the interaction energies (so-called stabilization energy) of the molecules with the environment in its ground state and corresponding excited electronic state after the absorption of a light quantum. In an analysis of the shift of absorption bands of the molecular forms of dyes adsorbed on a catalyst surface, we employcd the same approach as Bayliss and McRae [ I I ]. The interaction energy of the dye molecule with an isotropic solvent may bc broken down into the sum of the energies of coulombic, dipole, quadrupolc and higher multipolar interactions l l0l. To study the effect of the nature of the solvent on the spectra of dyes, we studied the behavior of the most intense absorption bands. These transitions arc accompanied by the greatest change in dipole moment of thc dye molecule. A series of azo dyes was studied: azobcnzcne (AB, pK = --2.3), benzcncazodiphenylamine (BAD, pK = +1.5), p-dimethylaminoazobenzenc (DMAAB, pK = +3.3), and methyl orange (MO, pK = +3.4). The dielectric constants of the solvent were varied. In Fig. I, curves are shown for the dependence of the band shift of these azo dyes relative to their position in isoamyl alcohol (all the dyes studied dissolve in this solvent). The dependence of the band shift on c was studied since the cnergy of the dipole-, dipole interaction which makes the major contribution to the stabilization energy in this case is dctermined in polar liquids by the magnitude of the dielectric constant. The results obtained (Fig. I) indicate a virtually linear dependence of the shift on the dielectric constant of the medium. This implies that the change in the dipole moment upon the adsorption of light for the dye molccules studied is constant over the entire range of e studied. It should also be noted that the observed band shifts vary in the same direction as thc dye pK (Fig. 2). The band shift in a solvent (Av) may bc given as l l0l: Av - :XPEeff/h. where &P is the change in the molecular dipole moment upon the absorption of a light quantum, and E,r f is the strcngth of the electrical field acting on the dye molecule from the liquid. Let us examine the spectra of the dye molecules adsorbed on catalyst surfaces on the assumption that the shift in this case is also described by the equation presented. Wc studied the band shift for azo dye molecules adsorbed onto Na, K, Mg. Ca, Sr, and Ba monocation-substitutcd forms of Glukhov kaolinite relative to their position in isoamyl alcohol analogously to the procedure for solutions with different e values. The catalyst samples were prepared as films obtained by precipitation from aqueous suspensions of the monocation-substituted forms of kaolinite on quartz plates. The dye adsorption on the catalyst surface was carried out from the vapor phase at 100~ The absorption spectra were taken on a Specord UV-VIS spectrometcr. In Fig. 3, the spectra are given for these dyes adsorbed on the strontium form of Glukhov kaolinite. In Table I, a classification is given for the bands observed based on their correlation with the absorption spectra of these dyes in neutral and acid media. Analysis of the spectra obtained showed that the nature of the cxchangc cation does not have a marked effcct on the position of the absorption band of the azo dyes adsorbed in the molecular state. Thc average shifts are 350 cm -t for