Porous aggregates of unidirectionally oriented microcrystallites of heteropoly compounds

Abstract

The microstructures of aggregates of heteropoly compounds Cs 3PW 12O 40 and (NH 4) 3PW 12O 40 precipitated at various temperatures were characterized by SEM, XRD, TEM, ED, and N 2 adsorption. The microstructures were remarkably different according to the precipitation temperature and the kind of cation. Cs 3PW 12O 40 precipitated at 298 K formed dispersed or loosely aggregated fine particles of ca. 10 nm with random crystal orientation. As the temperature increased to 368 K, Cs 3PW 12O 40 self-assembled to form microporous aggregates in which the crystal planes of microcrystallites were oriented unidirectionally, but were not much connected epitaxially with each other. (NH 4),PW 12O 40 precipitated at 273 K formed round aggregates similar to Cs 3PW 12O 40 precipitated at 368 K, while the NH 4 salt precipitated at 368 K formed dodecahedral aggregates in which microcrystallites were epitaxially connected with each other (epitaxial self-assembly). These observations indicate that microstructure is controlled by the solubility at the precipitating conditions; with increasing precipitation temperature or decreasing ionic radii of cation, the microstructure of aggregates varies from the random aggregates to unidirectionally oriented microcrystallites and the epitaxial connection between microcrystallites develops.