Synthesis and Investigations of Insoluble Keggin-Type Phosphomolybdovanadates with Cs+ Counterion

Vanadium-containing heteropoly compounds (HPCs), both in homogeneous and heterogeneous forms, represent a very promising class of materials with adjustable acid and oxidative properties. Herein, a catalytically active HPCs of H_3+xPMo_12–xV_xO₄₀ (x = 1–4) gross-composition were prepared by an environmentally friendly H₂O₂-affected approach and heterogenized in the form of insoluble salts with Cs⁺ counterion to obtain stable oxidation catalysts. Structural and textural characteristics of prepared compounds with Cs_2.5+xH_0.5PMo_12–xV_xO₄₀ (x = 1–4) gross-composition were investigated by XRD, TGA, IR-ATR, ICP-AES, and N₂ adsorption-desorption techniques, and their catalytic properties were tested in the oxidation of 5-hydroxymethylfurfural (5-HMF), a valuable biomass-derived platform chemical. The obtained materials were established to demonstrate retention of the initial Keggin structure during precipitation, low solubility in water, and high structural stability upon fourfold treatment with H₂O for 3 h, as well as thermal stability up to 400°C. It was shown that a simple method of replacing H⁺ ions with large Cs⁺ cations allows a traditional homogeneous catalyst to function heterogeneously, which simplifies the process of its separation and recycling. The catalytic performance was shown to decrease gradually with reducing x value. The attained Cs_5.5H_0.5PMo₉V₃O₄₀ and Cs_6.5H_0.5PMo₈V₄O₄₀ solids revealed high activity in 5-HMF transformation to 2,5-diformylfuran with a yield of up to 93%, which opens up prospects for their application in various important fields of synthetic chemistry as heterogeneous oxidation catalysts.