Alkaline earth-tuned polyoxometalate frameworks: enhanced selectivity in styrene epoxidation

Abstract

Two novel compounds based on high-nuclear metal–oxygen clusters, copper–amine complexes and alkaline earth metals were successfully synthesized under hydrothermal conditions, with chemical formulae [Cu(en)₂]_3.5Ba₂[H_2.5K_0.5SiNb₁₈O₅₄]·18H₂O (1) and [Cu(en)₂]₅Ba₂K₂[H₁₃K₂Nb₃₁O₉₃]·24.5H₂O (2) (en = ethylenediamine), respectively. The compounds were characterized by single-crystal X-ray diffraction, infrared spectroscopy (IR), powder X-ray diffraction (PXRD), and ultraviolet-visible spectroscopy (UV-Vis). Interestingly, both compounds exhibit 3D grid structures constructed from high-nuclear polyoxoanions, copper–amine complexes, and alkaline earth metals. Beyond synthesis and structural elucidation, the catalytic performance of these compounds in styrene oxidation was investigated, revealing that the incorporation of barium ions plays a critical role in modulating the conversion and selectivity toward styrene oxide.