Synthesis and characterization of cobalt-containing hydrated aluminophosphate molecular sieves CoAPO(4)-H3

Pure, highly crystalline AlPO4-H3 (structure type APC) and cobalt-substitut ed materials CoAPO(4)-H3 have been synthesized hydrothermally in the presen ce of dipropylamine. As-made materials are free from organic molecules, whi ch indicates that the amine does not act as a templating molecule but fathe r as a pH moderator. Under the synthesis conditions, cobalt could substitut e for framework aluminum up to an extent of approximately two cobalt atoms per unit cell. Spectroscopic characterization of the samples showed that co balt substitutes predominantly for four-coordinated aluminum species; i.e., in the 4.8(2) layers of the structure. The framework charge defect associa ted with the presence of Co2+ is compensated by protons, and the materials contain essentially Bronsted acid sites. As for pure AlPO4-H3, CoAPO(4)-H3 materials undergo structure modifications at elevated temperatures, thus le ading to CoAPO(4)-C, CoAPO(4)-D and CoAPO(4)-H6. These transformations have been followed by various spectroscopic techniques, including Al-27 two-dim ensional-five-quantum magic-angle spinning nuclear magnetic resonance spect roscopy, and occur without any modification of the cobalt coordination and the oxidation state. (C) 1998 Elsevier Science B.V. All rights reserved.