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 ( 13 C and 1 H) 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.
Restoring the antibacterial properties of existing antibiotics is of great concern. Herein, we present, for the first time, the formation and deposition of stable antibiotic nanoparticles (NPs) on graphene oxide (GO) sheets by a facile one-step sonochemical technique. Sonochemically synthesized graphene oxide/tetracycline (GO/TET) composite shows enhanced activity against both sensitive and resistant Staphylococcus aureus (S. aureus). The size and deposition of tetracycline (TET) nanoparticles on GO can be controlled by varying the sonication time. The synthesized NPs ranged from 21 to 180 nm. Moreover, ultrasonic irradiation does not cause any structural and chemical changes to the TET molecule as confirmed by Fourier transform infrared spectroscopy (FTIR). The virtue of π-π stacking between GO and TET additionally facilitate the coating of TET NPs upon GO. A time dependent release kinetics of TET NPs from the GO surface is also monitored providing important insights regarding the mechanism of antibacterial activity of GO/TET composites. Our results show that the GO/TET composite is bactericidal in nature, resulting in similar values of minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC). This composite is found to be active against TET resistant S. aureus at a concentration four times lower than the pristine TET. The sensitive S. aureus follows the same trend showing six times lower MIC values compared to pristine TET. GO shows no activity against both sensitive and resistant S. aureus even at a concentration as high as 1 mg/mL but influences the biocidal activity of the GO/TET composite. We propose that the unique structure and composition manifested by GO/TET composites may be further utilized for different formulations of antibiotics with GO. The sonochemical method used in this work can be precisely tailored for the stable deposition of a variety of antibiotics on the GO surface to reduce health risks and increase the spectrum of applications.
Gallium oxide (β-Ga 2 O 3 ) nanorods were prepared by ultrasonic irradiation of molten gallium in warm water to form α-GaO(OH). This precursor was then subjected to calcination in air at 600 °C for 3 h to form β-Ga 2 O 3 , as confirmed by Xray diffraction (XRD). Field emission scanning electron microscopy (FE-SEM)/high-resolution transmission electron microscopy (HRTEM) micrographs revealed the formation of well-organized nanotubes/nanorods with homogeneous size distribution. The average length of the β-Ga 2 O 3 nanorods was affected by the temperature of the water during sonication, decreasing from 480 to 72 nm with an increase in the temperature from 25 to 50 °C. A sharp decline in the particle size was also observed when the temperature was above 35 °C. The catalytic activity of the β-Ga 2 O 3 nanorods was examined, as a model, during the dehydration reaction of xylose to furfural. Furfural is a versatile biomass-derived platform compound used for the synthesis of several strategic chemicals. This nanoscale catalyst has a large surface area, which enhances its catalytic activity and enables it to completely convert xylose to furfural at 150 °C within 12 h without any trace of byproducts, as confirmed by highperformance liquid chromatography (HPLC), 13 C nuclear magnetic resonance (NMR), and 1 H NMR. The XRD pattern of the used β-Ga 2 O 3 nanorods was identical to that of pristine Ga 2 O 3 , indicating the possible reusability of this catalyst. β-Ga 2 O 3 was reused for more reduction cycles, with similar results to the freshly prepared β-Ga 2 O 3 . HPLC analysis demonstrated that the selectivity of furfural is up to 94% compared to the 30% obtained with GaO(OH) as a catalyst.
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