FTIR SPECTROSCOPIC STUDY OF GALLIUM-BETA

The gallosilicate analog of zeolite beta has been synthesized and characterized by FTi.r. spectroscopy, temperature-programmed desorption (t.p.d.) of NH3, thermogravimetric (t.g.) analysis, and adsorption of probe molecules. Four types of hydroxyl groups absorbing at 3740, 3670, 3622, and 3540 cm-1 have been identified and assigned to terminal/internal Si-OH hydroxyl groups associated with Ga, bridging framework hydroxyls [Si (OH) Ga], and hydrogen-bonded (OH) groups, respectively, The influence of adsorption of NH3, D2O, benzene, and pyridine on these hydroxyl groups has been evaluated. The results are compared with those obtained with [Al]-beta, Y, [Al]-ZSM-5, and [Ga]-ZSM-5. The following decreasing order for the acidic strength of the framework hydroxyls (bridging hydroxyls) is observed: [Al]-ZSM-5 > [Ga]-ZSM-5 > [Al]-beta > [Gal-beta > Y. The hydroxyl groups of [Ga]-beta are also thermally less stable than those of [Al]-beta. Both Bronsted and Lewis acid sites are present on [Ga]-beta, as could be inferred from the spectra of adsorbed pyridine molcules.