STUDY OF THE STATE OF ALUMINUM IN ZEOLITE-BETA

The modifications of the state of aluminium when introducing protons into zeolite-beta have been investigated by combining several analytical techniques. Low temperature (350-degrees-C) thermolysis of ammonium cations or treatment with dilute nitric acid provokes the development of a Al-27 NMR signal at 0 ppm attributable to aluminium in octahedral symmetry. Calcination at 550-degrees-C generates NMR invisible aluminium and a strongly acidic hydroxyl group vibrating at 3780 cm-1 in the infra-red spectra. The neutralization of the protons by reaction with a solution of pyridine in ethanol at room temperature or with gaseous ammonia at 100-degrees-C allows for all the aluminium atoms of the solid to recover a tetrahedral symmetry. The Al(IV) --> Al(VI) transformation can be also reversed by replacing the protons by sodium or potassium cations. It is then concluded that the octahedrally coordinated and NMR invisible aluminium must be regarded as inherent parts of the framework of the zeolite. Their formation is explained by a distortion of aluminic sites caused by the high electron affinity of the proton.