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 H3+xPMo12–xVxO40 (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 Cs2.5+xH0.5PMo12–xVxO40 (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 Cs5.5H0.5PMo9V3O40 and Cs6.5H0.5PMo8V4O40 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.