Ultrasound effect on the aromatic nucleophilic substitution reactions on some haloarenes

The sonochemical nucleophilic aromatic substitutions on haloarenes with different amines have been studied. A beneficial ultrasound effect was observed and high yields of the products were obtained after 15-30 min sonication. The reaction course of the nucleophilic aromatic substitution was found to be strongly dependent on nucleophilicity, bulkiness, and boiling point of amines as well as on the electron-withdrawing property of the substituents on the haloarenes.     

Kinetics analysis for development of a rate constant estimation model for ultrasonic degradation reaction of methylene blue

Ultrasound has been used as an advanced oxidation method for wastewater treatment. Sonochemical degradation of organic compounds in aqueous solution occurs by pyrolysis and/or reaction with hydroxyl radicals. Moreover, kinetics of sonochemical degradation has been proposed. However, the effect of ultrasonic frequency on degradation rate has not been investigated. In our previous study, a simple model for estimating the apparent degradation rate of methylene blue was proposed. In this study, sonochemical degradation of methylene blue was performed at various frequencies. Apparent degradation rate constant was evaluated assuming that sonochemical degradation of methylene blue was a first-order reaction. Specifically, we focused on effects of ultrasonic frequency and power on rate constant, and the applicability of our proposed model was demonstrated. Using this approach, maximum sonochemical degradation rate was observed at 490 kHz, which agrees with a previous investigation into the effect of frequency on the sonochemical efficiency value evaluated by KI oxidation dosimetry. Degradation rate increased with ultrasonic power at every frequency. It was also observed that threshold power must be reached for the degradation reaction to progress. The initial methylene blue concentration and the apparent degradation rate constant have a relation of an inverse proportion. Our proposed model for estimating the apparent degradation rate constant using ultrasonic power and sonochemical efficiency value can apply to this study which extended the frequency and initial concentration range.     

Comparison of ultrasonic degradation rates constants of methylene blue at 22.8 kHz, 127 kHz, and 490 kHz

Techniques such as solvent extraction, incineration, chemical dehalogenation, and biodegradation have been investigated for the degradation of hazardous organic compounds. We found ultrasound to be an attractive technology for the degradation of hazardous organic compounds in water. However, the effects of ultrasonic frequency on degradation rate constants were not investigated quantitatively. In this study, the degradation process of a model for hazardous organic compound methylene blue was investigated using ultrasonic irradiation. The study focused on the effects of ultrasonic frequency and ultrasonic power on the degradation rate constant. The apparent degradation rate constants were estimated based on time dependence of methylene blue concentration assuming pseudo-first-order kinetics for the decomposition. A linear relationship between the apparent degradation rate constant and ultrasonic power was identified. In addition, the apparent degradation rate constants at frequencies of 127 and 490 kHz were much larger than those at 22.8 kHz. A relationship between the apparent degradation rate constant and the sonochemical efficiency value (SE value) was also found. Based on these results, a simple model for estimating the apparent degradation rate constant of methylene blue based on the ultrasonic power and the SE value is proposed in this study.     

The Michael reaction catalyzed by KF/basic alumina under ultrasound irradiation

The Michael reaction of chalcones as acceptors with various active methylene compounds such as diethyl malonate, nitromethane, cyclohexanone, ethyl acetoacetate and acetylacetone as donors catalyzed by KF/basic alumina results in adducts in high yield within a shorter time under ultrasound irradiation.     

Sonochemistry on primordial Earth--its potential role in prebiotic molecular evolution

Sonochemical processes are known to occur in nature and have occurred ever since there was liquid water on Earth. We advance a hypothesis that complex carbonaceous compounds, the probable precursors of life, were produced from simple primordial molecules by sonochemical processes in breaking waves of primordial seas or oceans. Our calculations show that these processes were much more common on Earth than other pathways, suggested for the formation of prebiotic complex carbonaceous compounds. The high occurrence rate of sonochemical events in breaking sea waves and the qualitative difference between sonochemical processes and other shock phenomena might have enabled formation of a variety of complex carbonaceous compounds, including amino acids, nucleotides and nucleosides, the precursors of RNA or DNA.     

Nanostructured TiO2 cavitation agents for dual-modal sonophotocatalysis with pulsed ultrasound

Current sonochemical methods rely on spatially uncontrolled cavitation for radical species generation to promote chemical reactions. To improve radical generation, sonosensitizers have been demonstrated to be activated by cavitation-based light emission (sonoluminescence). Unfortunately, this process remains relatively inefficient compared to direct photocatalysis, due to the physical separation between cavitation event and sonosensitizing agent. In this study, we have synthesized nanostructured titanium dioxide particles to couple the source for cavitation within a photocatalytic site to create a sonophotocatalyst. In doing so, we demonstrate that site-controlled cavitation from the nanoparticles using pulsed ultrasound at reduced acoustic powers resulted in the sonochemical degradation methylene blue at rates nearly three orders of magnitude faster than other titanium dioxide-based nanoparticles by conventional methods. Sonochemical degradation was directly proportional to the measured cavitation produced by these sonophotocatalysts. Our work suggests that simple nanostructuring of current sonosensitizers to enable on-site cavitation greatly enhances sonochemical reaction rates.     

Sonochemical procedures; the main synthetic method for synthesis of coinage metal ion supramolecular polymer nano structures

During the last two decades, supramolecular polymers have received great attention and the number of their synthesized compounds is still growing. Although people have long been interested in their crystalline network form it was only until 2005 that the first examples of nano- or microscale coordination polymers particles be demonstrated. This review tries to give an overview of all nano supramolecular compounds which were reported from coinage metal ions, their attributed synthetic procedures and to investigate the relation between the dimensions of coinage metal ions (Cu, Ag and Au) coordination and supramolecular polymers with their nano-structural morphologies and dimensions. Eleven compounds (from twenty compounds) with nano-structure morphology were prepared by sonochemical process and Ag(I) coordination and supramolecular polymer nano-structures can be easily prepared by sonochemical procedures.     

Simple sonochemical synthesis of Ho2O3-SiO2 nanocomposites as an effective photocatalyst for degradation and removal of organic contaminant

In this work, highly photocatalytically active Ho₂O₃-SiO₂ nanocomposites have been designed and applied for decomposition of methylene blue pollutant. Ho₂O₃-SiO₂ nanocomposites have been produced by new, quick and facile sonochemical process with the aid of tetramethylethylenediamine as a novel basic agent for the first time. The effect of the kind of basic agent, ultrasonic time and dosage of Ho source on the grain size, photocatalytic behavior and shape of the Ho₂O₃-SiO₂ nanocomposites have been evaluated for optimization the production condition. FESEM, EDX, FT-IR, DRS, XRD and TEM have been applied to characterize the as-produced Ho₂O₃-SiO₂ nanocomposites. Use of the as-produced Ho₂O₃-SiO₂ nanocomposites as photocatalyst via destruction of methylene blue pollutant under UV illumination has been compared. It was observed that SiO₂ has notable impact on catalytic activity of holmium oxide photocatalyst for destruction. Introducing of SiO₂ to holmium oxide can enhance destruction efficiency of holmium oxide to methylene blue pollutant under ultraviolet light.     

Synthesis of 1,1-disubstituted-2,6-diarylcyclohexane-4-ones catalyzed by KF/basic Al2O3 under ultrasound

1,1-Disubstituted-2,6-diarylcyclohexane-4-ones have been synthesized via double Michael addition of 1,5-diaryl-1,4-pentadien-3-one with various active methylene compounds such as dimethyl malonate, diethyl malonate, methyl cyanoacetate and ethyl cyanoacetate catalyzed by KF/basic alumina under ultrasound irradiation to give good yields within a short time.     

Ultrasound in synthesis: natural products and supersonic reactions?

Sonochemical syntheses of a number of useful reagents such as samarium diiodide, sodium phenylselenide, tertiary alkoxides and the reducing agent [(PPh₃P)CuH]₆ are discussed. The rates of these sonochemical reactions can be increased by the addition of electron-transfer agents, such as benzophenone ketyls. These results would not be expected on the basis of mechanical effects alone and lend support to proposals that ultrasound acts preferentially on single electron-transfer processes. Ultrasound also allows the rapid and efficient generation of tetracarbonyliron, which can be trapped by simple organic compounds, such as conjugated dienes, vinyl epoxides and allylic alcohols. In the latter cases, this allows the facile preparation of π-allyltricarbonyliron lactone and lactam complexes that can be converted to give a range of biologically active β-, γ- and δ-lactones, including a number of alkaloids and antibiotics. In addition, the δ-lactone products have been incorporated into several complex multi-step syntheses of macrocyclic natural products. The synthesis of key fragments of the potent antiparasitic agent avermectin B1a demonstrates that the sonochemical reaction conditions are fully compatible with the synthesis of highly functionalized molecules.     

Potassium sorbate as an efficient and green catalyst for Knoevenagel condensation

Potassium sorbate has been utilized as a novel, efficient and green catalyst for the Knoevenagel condensation of aromatic aldehydes with active methylene compounds to afford substituted ole-fins through the conventional stirring or under ultrasound irradiation. Improvements were observed by carrying out the reactions under ultrasound irradiation. The advantages of this procedure are mild reaction conditions, high yields, cleaner reaction profiles and operational simplicity.     

Advancement of high power ultrasound technology for the destruction of surface active waterborne contaminants

The current paper explores recent advances in sonochemical techniques to improve the ultrasound-mediated degradation efficiency of surface active, waterborne contaminants. Sonochemical degradation efficiency of surface active contaminants generally has a strong dependence on the concentration of contaminant at the gas/solution surface of cavitation bubbles. This in turn depends on the thermodynamic and diffusion/kinetic-controlled adsorption properties of the surfactant at the rapidly pulsating gas/solution surface of acoustic cavitation bubbles. The adsorption properties of surfactants can be exploited to enhance their sonochemical decomposition by varying ultrasound exposure parameters such that changes in the nature of the bubble population (especially the bubble life-time and rate of pulsations) cause changes in the amount of surfactant that adsorbs to the gas/solution interface of cavitation bubbles. Herein we describe recent results on the effect of ultrasound frequency and pulsing mode on sonochemical degradation of surfactants in aqueous solutions and show how the exposure parameters can be adjusted in ways to produce more efficient decomposition of contaminants, even under exposure conditions where seemingly poor sonochemical activity is detected in the bulk solution. The relevance of these results to scale-up of ultrasound decontamination processes is discussed.     

Method for determining the chemically active zones in a high-frequency ultrasonic reactor

In order to determine the optimal geometry of the high-frequency sonoreactors to be used for degradation of organic compounds in aqueous solutions, a method based on the chemiluminescence of luminol (which reacts specifically with OH radicals) is applied to visualize the zones where the sonochemical reactions are the most active. The spatial distribution of the light intensity emitted by these reactions is measured inside the reactor with an optical fibre fitted on a photomultiplier. To determine the relative ultrasonic energy distribution, the temperature is measured at different points of the reactor with a thermocouple probe embedded in an absorbing material. The zones where the energy density is the highest were shown not to correspond with those where chemiluminescence reactions are the most efficient.     

A phenomenological investigation into the opposing effects of fluid flow on sonochemical activity at different frequency and power settings. 2. Fluid circulation at high frequencies

Sonochemical activity is dependent on flow patterns within the reactor and either no affect or a decrease in activity was observed at 376, 995, and 1179 kHz from overhead stirring. The interaction of fluid flow with ultrasound was further investigated in this study with circulatory flow. The effect of fluid circulation on radical production was investigated at two circulation speeds, with and without surface stabilisation. The sonochemical activity was determined by the yield of hydrogen peroxide, measured by iodide dosimetry. The sonochemically active region was pictured using sonochemiluminescence imaging and the flow fields were visualised with dyed flow videos. At 376 and 995 kHz, an increase in sonochemical activity was observed with the slower flow rate; however at 1179 kHz, the sonochemical activity was either not affected or decreased. The observed changes in sonochemical activity were attributed to an increase in asymmetry of the bubble collapse brought about by fluid motion.     

Effects of ultrasonic frequency and liquid height on sonochemical efficiency of large-scale sonochemical reactors

The sonochemical efficiency of a cylindrical sonochemical reactor has been investigated as a function of frequency and liquid height. The irradiation frequencies were 45, 129, 231 and 490 kHz. The liquid height was varied from 10 to 700 mm. The sonochemical efficiency of the cylindrical reactor was evaluated by potassium iodide (KI) dosimetry and calorimetry. In our study, the sonochemical efficiency depended on the frequency and liquid height; further, the plots of sonochemical efficiency against liquid height exhibit one or two peaks for each frequency. The sonochemical efficiency up to the first peak increased monotonically with the logarithm of the frequency, and the liquid height for the first peak was inversely proportional to the frequency. From these results, the optimum frequency for a sonochemical reactor can be determined if the liquid height is specified for scale-up of the sonochemical reactor.     

Insights into ultrasound-promoted degradation of naphthenic acid compounds in oil sands process affected water. Part I: Accelerated H-abstraction and decarboxylation of aromatic and alicyclic compounds

Propelled by enormous increase in demand for fuel sources, Canadian oil sands are becoming increasingly important as a fuel source due to their abundance and upgrading capability. However, extraction of bitumen, a high acid crude (HAC) oil, requires 2–3 units of water per unit of oil resulting in naphthenic acid (NA)-rich oil sands process affected water (OSPW) collected in effluent ponds. This study illustrates the role of sonochemistry in the accelerated degradation through H-abstraction and subsequent decarboxylation of aromatic and alicyclic naphthenic acid model compounds. Benzoic acid and 3-methylcyclohexane carboxylic acid were selected as model NA compounds to investigate the mechanism of hydroxyl radical (OH^•) initiated carboxylic acid degradation in 378 KHz sonochemical reactor. Established FTIR methods with low resolution LCMS spectroscopy confirmation were applied to determine the extent of carboxylic acid degradation and identify the formation of products. FTIR monitoring showed a non-linear degradation of carboxylic acids with formation of many intermediates highlighting the shift from cyclic carboxylic acids to cyclic alcohols during BA degradation. Subsequent decrease in carboxylic acid groups signifies scission of cyclic structures before complete mineralization. This is confirmed with the LCMS identification of products such as: 3-hydroxybenzoic acid and phenol. This study postulated new breakdown pathways for degradation of benzoic acid with complete mineralization at a sonochemical reaction time (SRT) of 4 h. A radical quenching process was also inferred through the formation of conglomerates during sonochemical degradation of BA. Extension of the study to 3-methylcyclohexane carboxylic acid (3mCHA) shows similar non-linearity with an increase in carboxylic acid groups indicating H-abstraction followed by ring-opened compounds. However, due to the complex nature of 3mCHA’s ring-opened compounds, complete mineralization is not achieved. The putative role of sonochemistry is a promising and sustainable degradation method for mitigating NAs in OSPW, but sonication periods need to be considered carefully to ensure adequate mineralization of their constituents and combinatorial methods with other advanced oxidation methods may be needed to enhance industrial application.In Part II, an in silico screening approach using first principles is reported to identify the breakdown of the organic compounds and determine molecular rates of reaction to confirm the mechanistic origins of the compounds formed.     

Sonochemical and photochemical oxidation of organic matter

Recent developments in sonochemistry have led us to study its use to treat water and wastewater. The effects of ultrasound wave in hydrophilic chemical oxidations are mainly due to hydroxyl radical production during the cavitation-induced water decomposition. Currently, the sonochemical destruction of aromatic compounds in water solution is obtained with low rates. The aim of this work is to evaluate the efficiency of the sonochemical effect in conjunction with a photochemical irradiation. Taking phenol as an example, the combined action of sonochemistry and photochemistry has been considered in a ‘sonuv’ reactor. An important enhancement of the degradation rate of phenol has been observed. It may be the result of three different oxidative processes: direct photochemical action, high frequency sonochemistry and reaction with ozone (produced by UV irradiation of air). The process has been successfully tested to lower the chemical oxygen demand of a municipal wastewater.     

Comparison of sonochemiluminescence images using image analysis techniques and identification of acoustic pressure fields via simulation

One common method to determine the existence of cavitational activity in power ultrasonics systems is by capturing images of sonoluminescence (SL) or sonochemiluminescence (SCL) in a dark environment. Conventionally, the light emitted from SL or SCL was detected based on the number of photons. Though this method is effective, it could not identify the sonochemical zones of an ultrasonic systems. SL/SCL images, on the other hand, enable identification of ‘active’ sonochemical zones. However, these images often provide just qualitative data as the harvesting of light intensity data from the images is tedious and require high resolution images. In this work, we propose a new image analysis technique using pseudo-colouring images to quantify the SCL zones based on the intensities of the SCL images and followed by comparison of the active SCL zones with COMSOL simulated acoustic pressure zones.     

Effects of Na2SO4 or NaCl on sonochemical degradation of phenolic compounds in an aqueous solution under Ar: Positive and negative effects induced by the presence of salts

Sonochemical degradation of 4-chlorophenol, phenol, catechol and resorcinol was studied under Ar at 200 kHz in the absence and presence of Na₂SO₄ or NaCl. The rates of sonochemical degradation in the absence of salts decreased in the order 4-chlorophenol > phenol > catechol > resorcinol and this order was in good agreement with the order of log P (partition coefficient) value of each phenolic compound. The effects of salts on the rates of sonochemical degradation consisted of no effect or slight negative or positive effects. We discussed these unclear results based on two viewpoints: one was based on the changes in pseudo hydrophobicity and/or diffusion behavior of phenolic compounds and the other was based on the changes in solubility of Ar gas. The measured log P value of each phenolic compound slightly increased with increasing salt concentration. In addition, the dynamic surface tension for 4-chlorophenol aqueous solution in the absence and presence of Na₂SO₄ or NaCl suggested that phenolic compounds more easily accumulated at the interface region of bubbles at higher salt concentration. These results indicated that the rates of sonochemical degradation should be enhanced by the addition of salts. On the other hand, the calculated Ar gas solubility was confirmed to decrease with increasing salt concentration. The yield of H₂O₂ formed in the presence of Na₂SO₄ or NaCl decreased with increasing salt concentration. These results suggested that sonochemical efficiency decreased with decreasing gas amount in aqueous solution: a negative effect of salts was observed. Because negative and positive effects were induced simultaneously, we concluded that the effects of salts on the rates of sonochemical degradation of phenolic compounds became unclear. The products formed from sonochemical degradation of 4-chlorophenol were also characterized by HPLC analysis. The formation of phenol and 4-chloro-1,3-dihydroxy benzene was confirmed and these concentrations were affected by the presence of salts.     

A phenomenological investigation into the opposing effects of fluid flow on sonochemical activity at different frequency and power settings. 1. Overhead stirring

The effect of flow in an ultrasonic reactor is an important consideration for practical applications and for the scale-up of ultrasonic processing. Previous literature on the influence of flow on sonochemical activity has reported conflicting results. Therefore, this work examined the effect of overhead stirring at four different frequencies, 40, 376, 995 and 1179 kHz, in two different reactor configurations. Comparable power settings were utilised to elucidate the underlying mechanisms of interactions between the flow and sonochemical activity. The sonochemical activity was determined by the yield of hydrogen peroxide, measured by iodide dosimetry, and the active region was visualised with sonochemiluminescence imaging. The overhead stirring in the low frequency reactor altered the yield of hydrogen peroxide so it produced the maximum yield out of the four frequencies. The increase in hydrogen peroxide yield was attributed to a reduction in coalescence at 40 kHz. However at the higher frequencies, coalescence was not found to be the main reason behind the observed reductions in sonochemical yield. Rather the prevention of wave propagation and the reduction of the standing wave portion of the field were considered.