Key Stages in the Formation of AlPO4-11 via the Crystallization of a Boehmite-Based Aluminophosphate Gel

Abstract

The staged crystallization of aluminophosphate AlPO₄-11 from a commercially available aluminum source based on boehmite is studied for the first time by means of X-ray diffraction, ²⁷Al and ³¹P magic-angle spinning nuclear magnetic resonance, low-temperature nitrogen adsorption–desorption, and scanning election microscopy. It is shown that the synthesis of AlPO₄-11 proceeds via the formation of an intermediate phase based on crystalline aluminophosphate with a layered structure. It is found that AlPO₄-11 with a high degree of crystallinity and phase purity forms in 6–24 h at 200°C. Lengthening the time of crystallization at 200°C to more than 48 h results in the transformation of AlPO₄-11 into nonporous cristobalite. The results can be used to develop means for directional control of the phase purity and degree of crystallinity of commercially important silicoaluminophosphate molecular sieves SAPO-11 with desired properties. These can be used as a base for the synthesis of promising domestic catalysts for the commercial hydroisomerization of higher n-paraffins.