イギリスの王立化学会発行の学術雑誌 Nanoscale 2021年11月号 Front Coverに選ばれました。
28 November 2021, Issue 44
Zhewei Weng, Naoki Ogiwara, Takashi Kitao, Yuji Kikukawa, Yu Gao, Likai Yan and Sayaka Uchida
Polyoxometalates (POMs) are oxide cluster anions composed of high-valence early transition metals and are widely used as catalysts. Yet base catalysis of POMs remains an ongoing challenge; group V (V, Nb, and Ta) elements form more negatively charged POMs than group VI (Mo and W) elements, and in particular, polyoxoniobates and polyoxotantalates are known to show strong basicity in solution due to the highly negative surface oxygen atoms. Herein, we report for the first time porous ionic crystals (PICs) comprising Nb or Ta. The PICs are composed of Dawson-type Nb/W or Ta/W mixed-addenda POMs with oxo-centered trinuclear CrIII carboxylates and potassium ions as counter cations to control the crystal structure. Among the PICs, those with Nb or Ta tri-substituted POMs exhibit the highest yield (78–82%) and selectivity (99%) towards the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate (353 K, 6 h), which is a typical base-catalyzed reaction, as reusable solid catalysts, and they can also catalyze the reaction of other active methylene compounds. A detailed investigation into the crystal structures together with DFT calculations and in situ IR spectroscopy with methanol as a basic probe molecule shows that the exposure of [Nb3O13] or [Ta3O13] units with highly negative surface oxygen atoms to the pore surface of PICs is crucial to the catalytic performance. These findings based on the composition–structure–function relationships show that Nb- and Ta-containing PICs can serve as platforms for rational designing of heterogeneous base catalysts.