Synthesis and Characterization of Composites of MgAl-based Layered Double Hydroxide and Nb Oxide Clusters

Abstract

Polyoxometalates (POMs), called as metal oxide clusters, show unique properties including superacidity and superbasicity. Among them, Nb- and Ta-based POMs have attracted attention as homogeneous catalysts due to their water-resistant superbasicity and catalytic activity in CO₂ fixation. To utilize these POMs as solid base catalysts, compositing of POMs with layered double hydroxides (LDHs) is a promising approach. In this study, composites of Lindqvist-type [Nb₆O₁₉]⁸⁻ (Nb6) and MgAl-based LDH were synthesized using the reconstruction method. Various structural analysis confirmed the successful intercalation of Nb6 into the LDH while Nb6 maintained its Lindqvist-type structure. X-ray diffraction showed an increased interlayer distance consistent with the size of Nb6, and energy dispersive X-ray spectroscopy confirmed the high dispersion of the Nb6 within the LDH. Knoevenagel condensation reaction showed that the Nb6-LDH composite achieved yields of 33 % and 30 % for substrates with pK_a of 19.5 and 21.9, respectively, which was significantly higher than bare LDH without Nb6 (3 %). Furthermore, the Nb6-LDH composite showed reusability as a boomerang-type catalyst.