The sonolysis of methanol in diluted aqueous solutions: product yields

Abstract

Diluted aqueous methanol solutions (0.001–0.5 M) were exposed to 724 kHz ultrasound irradiation under argon atmosphere at 26 ± 3°C. As measured with the argon-saturated Fricke dosimeter, the γ-equivalent dose rate of ultrasound in the sonochemical cell was 2.6 kGy/h (1.6 × 10 22 eV dm −3 h −1). The yields of the following products were measured: H 2, H 2O 2, CO, CH 2O, ethylene glycol, CH 4, C 2H 2, C 2H 4, C 2H 6. It was found that over the entire investigated concentration range the yield of methane was strictly proportional to the methanol concentration, the yields of CO and H 2 increased and that of H 2O 2 decreased with increasing the concentration of methanol in the solutions. In very diluted solutions the yields of formaldehyde and ethylene glycol were measured to be, respectively, 1.5 and 0.64 molecules per 100 eV chemically active acoustic energy and did not depend upon the concentration of methanol unless that reached 0.02 M in case of formaldehyde and 0.1 M in case of ethylene glycol. On the further increase of the concentration their yields started to grow. An examination of the concentration dependences of the product yields leads to the assumption that in the sonolysis of methanol in very diluted aqueous solutions the H atoms and .OH radicals generated inside the imploding cavitation bubbles from thermal water decomposition causes the transformations of methanol. In doing so, H atoms pass into the bulk and react with methanol to form formaldehyde and ethylene glycol, whereas .OH radicals probably cannot escape from the hot region; they react there with methanol to give CO or recombine to give H 2O 2. At higher methanol concentrations the decomposition of methanol due to the thermolysis of its vapors in the cavitational hot-spots becomes significant. The experimental data are discussed in terms of distinction and similarity between the sonolysis and the radiolysis of methanol in aqueous solutions.