Sonochemical Transformations of Methane and Ethylene in Aqueous Solutions under Conditions of Cavitation
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ICAL CHEMISTRY OF SOLUTIONS
Sonochemical Transformations of Methane and Ethylene in Aqueous Solutions under Conditions of Cavitation S. D. Arsenteva,* a
Nalbandyan Institute of Chemical Physics, National Academy of Sciences of the Republic of Armenia, Yerevan, 0014 Armenia *e-mail: [email protected] Received October 1, 2019; revised January 25, 2020; accepted February 11, 2020
Abstract—The conversion of methane, ethylene and mixtures of them in aqueous solutions was studied using ultrasonic vibrations with a frequency of 22 kHz under conditions of cavitation. It is found that formaldehyde, the main product, forms even if there is no dissolved oxygen in the initial solution. It is shown that the rate of accumulation of formaldehyde depends on the power of the ultrasound and the amount of molecular oxygen introduced into the system. Keywords: sonochemistry, cavitation, methane, ethylene, formaldehyde DOI: 10.1134/S0036024420090022
INTRODUCTION A variety of means are used to intensify chemical, physicochemical, and technological processes. One way of influencing chemical reactions is the use of ultrasound. In addition, most chemical reactions in ultrasonic fields occur in aqueous solutions when there is cavitation. It is known that under the influence of cavitation, the direction and rate of chemical reactions that proceed in a solution can vary substantially [1–4]. Processes that are not feasible under ordinary conditions can in some cases occur [4–7]. It should be emphasized that ultrasonic radiation does not directly interact with reagent molecules. The range of ultrasonic vibration frequencies is much lower than those of the natural vibrations of the molecules, and the specific energies are much less than the activation energy. It is therefore obvious that sonochemical transformations are caused by cavitation phenomena. The effect ultrasound has on chemical reactions in solutions is a consequence of the concentration of relatively low energy emitted into the solution in small volumes of vapor–gas bubbles. The kinetic energy of the liquid is in this case converted into the energy of heating and compression of the contents of the collapsing bubble, i.e., reagents in the gas–vapor phase. The high temperatures and pressures attained during the collapse of vapor–gas bubbles, combined with the high cooling rate of reaction products and the occurrence of shock waves [8–10] suggest a certain similarity of processes occurring in vapor–gas bubbles formed in solutions during cavitation with processes in shock tubes [11–15]. One possible reason for the effect ultrasound has on chemical processes in aqueous solu-
tions is the emergence of electric discharges in gasvapor bubbles [16, 17]. Ample opportunities can arise when hydrocarbon oxidation is performed under similar conditions. Unfortunately, there are virtually no experimental data in this field of research. It is known only that the formation of formaldehyde and acetaldehyde is possible under the action of ultrasound with a frequency of 850 kHz in aqueous solutions
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