PENTACOORDINATED ALUMINUM IN NONCALCINED AMORPHOUS ALUMINOSILICATES, PREPARED IN ALKALINE AND ACID-MEDIUM

Amorphous aluminosilicates prepared in acid and alkaline medium are dried at moderate temperatures (<150 degrees C) and submitted to Al-27 MAS-NMR (after hydration) and surface area measurements by N-2 adsorption. The occurrence of a NMR peak between the octa- and tetrahedral signal is ascribed to the presence of pentahedral Al. Variation of the synthesis and the washing and drying procedures changes the intensity of the Al-5 peak and indicates the interfacial character of this Al species, located between the octahedral alumina and the tetrahedral aluminosilicate phase. Relating the relative Al-5 content with the specific surface area is only possible in limited cases. The dispersion of the octahedral phase and the presence of cations (Na+, NH4+) also play an important role with respect to Al-5 formation. The effect of hydration on the amount of Al-5 detected is noticed to be smaller in dried than in calcined samples.