General Strategy for Preparing Uniform Polyoxometalate Microcrystals with Selectively Exposed Low-Index Facets

Abstract

Polyoxometalate (POM) microcrystals have unique molecular structures and abundant acid–base sites, making them good candidates as crystalline catalysts. The facets of POM materials can greatly affect their catalytic activity due to the different atomic arrangements in various facets. Many factors affect the nucleation and growth processes of crystalline materials; therefore, preparing uniform POM microcrystals with selectively exposed facets is a challenge. In this work, we report a general strategy for preparing polyoxometalate microcrystals of ammonium salts with selective exposure of low-index facets by the hydrothermal decomposition of urea. The obtained NH4–PMo ([PMo12O40]3–) microcrystals show uniform dodecahedral cubes with selective exposure of (110) facets. A series of controlled experiments demonstrates that the rate and amount of NH4+ production play key roles in the growth of POM microcrystals. NH4–PMo microcrystals exhibit excellent activity in the Knoevenagel condensation reaction, and their catalytic activities have a positive correlation with the exposed (110) facets of NH4–PMo. NH4–PMo microcrystals can provide more Lewis acid–base sites due to their high specific surface area and selectively exposed (110) facets. Moreover, NH4–PW ([PW12O40]3–) microcrystals with selectively exposed (110) facets were also successfully prepared by the same method, indicating that this preparation strategy has great potential to be applied to the synthesis of other valuable POM microcrystals