Abstract
The solid base catalysis of sodium salts of Lindqvist-type metal oxide clusters was investigated using a Knoevenagel condensation reaction. We successfully synthesized the sodium salts of Ta and Nb mixed-oxide clusters Na8−nHn[(Ta6−xNbx)O19]·15H2O (Na-Ta6−xNbx, n = 0, 1, x = 0–6) and found them to exhibit activity for proton abstraction from nitrile substrates with a pKa value of 23.8, which is comparable to that of the conventional solid base MgO. The Ta-rich Na-Ta6 and Na-Ta4Nb2 exhibited high activity among Ta and Nb mixed-oxide clusters. Synchrotron X-ray diffraction (SXRD) measurements, Fourier-transform infrared (FT-IR) spectroscopy, and X-ray absorption spectroscopy (XAS) revealed the structure of Na-Ta6−xNbx: (1) The crystal structure changed from Na7H[M6O19]·15H2O to Na8[M6O19]·15H2O (M = Ta or Nb) by the anisotropic expansion of the unit cell with an increase in Ta content; (2) Highly symmetrical Lindqvist [Ta6−xNbxO19]8− was generated in Na-Ta4Nb2 and Na-Ta6 because of the symmetrical association of Na+ ions with [Ta6−xNbxO19]8− in the structure. DFT calculation revealed that the Lindqvist structures with high symmetry have large NBO charges on surface oxygen species, which are strongly related to base catalytic activity, whereas the composition hardly affects the NBO charges. The above results showed that the Brønsted base catalysis was sensitive to the symmetry of the Lindqvist [Ta6−xNbxO19]8− structure. These findings contribute to the design of solid base catalysts composed of anionic metal oxide clusters with alkaline-metal cations.
Highlights
Metal oxide clusters (MOCs), referred to as polyoxometalates, are well known as super acid catalysts [1,2,3,4], photocatalysts [5,6], and redox catalysts [7,8]
We found that the alkali-metal MOC salts showed superior Brønsted base catalytic properties compared with the conventional metal oxide base catalysts
Na-Ta6−xNbx salts acted as solid base catalysts without pretreatment, and their strong basicity is comparable to that of activated MgO
Summary
Metal oxide clusters (MOCs), referred to as polyoxometalates, are well known as super acid catalysts [1,2,3,4], photocatalysts [5,6], and redox catalysts [7,8]. Biomass conversion using the acidity and redox property is one of the attractive applications of MOCs [9,10]. The base catalysis of MOCs attracted substantial attention because negatively charged MOCs proved to have base catalytic properties [11,12,13]. The density functional theory (DFT) revealed that the basicity of MOCs is related to the negative charges of the oxygen atoms within them. They achieved the strong basicity of MOCs by increasing the negative charge of the clusters with lacunary Keggin-type structures. Our group and Ge et al found that group V (Nb, Ta) MOCs such as [Nb6O19]8− [14,15],
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