Development of Ga Salt of Molybdophosphoric Acid for Biomass Conversion to Levulinic Acid

Efficient catalytic methods for biomass conversion present a challenge for the sustainable production of fuels and chemicals. Ga salt of molydophosphoric acid (GaHPMo) is prepared using a sonochemical irradiation method. Simultaneous formation of GaHPMo and entrapment of polyoxometallate in gallium micro-/nanoparticles (Ga@HPMo) is achieved. The amount of entrapped hetropoly acid in Ga particles is estimated to be ∼3 wt %, as determined from thermogravimetric analysis (TGA) and ultraviolet-visible (UV-vis) analysis. The preparation of GaHPMo is accomplished by subjecting an ethanolic solution of polyoxometallate [molybdophosphoric acid (HPMo)] and molten Ga metal to sonication at 50°C for 12 min. Physicochemical properties of GaHPMo are studied using X-ray diffraction, TGA, temperature-programmed desorption, differential scanning calorimetry, Fourier transform infrared spectroscopy, UV-vis spectroscopy, scanning electron microscopy, transmission electron microscopy, dynamic light scattering, and energy-dispersive X-ray spectroscopy analysis. GaHPMo was successfully used as a catalyst for the conversion of various carbohydrates (glucose, starch, and cellulose) and rice straw into levulinic acid via a hydrothermal process. Reaction conditions for obtaining an optimum yield of levulinic acid from glucose are deduced (time, 10 h; temperature, 423 K; and mole ratio of the catalyst/reactant, 1:5). The reaction products are analyzed qualitatively using nuclear magnetic resonance (¹³C and ¹H) spectroscopy and quantified using high-performance liquid chromatography analysis. The maximum yield of levulinic acid obtained from glucose is 56 wt %. Apart from the major product, levulinic acid, other minor byproducts, such as formic and lactic acids, are also observed.