Effect of Ultrasound on the Green Selective Oxidation of Benzyl Alcohol to Benzaldehyde

Oxidation of alcohols plays an important role in industrial chemistry. Novel green techniques, such as sonochemistry, could be economically interesting by improving industrial synthesis yield. In this paper, we studied the selective oxidation of benzyl alcohol as a model of aromatic alcohol compound under various experimental parameters such as substrate concentration, oxidant nature and concentration, catalyst nature and concentration, temperature, pH, reaction duration, and ultrasound frequency. The influence of each parameter was studied with and without ultrasound to identify the individual sonochemical effect on the transformation. Our main finding was an increase in the yield and selectivity for benzaldehyde under ultrasonic conditions. Hydrogen peroxide and iron sulfate were used as green oxidant and catalyst. Coupled with ultrasound, these conditions increased the benzaldehyde yield by +45% compared to silent conditions. Investigation concerning the transformation mechanism revealed the involvement of radical species.

Does power ultrasound affect heterogeneous electron transfer kinetics?

Most of the observations seen in the application of power ultrasound in electrochemistry or also known as sonoelectrochemistry are due to enhanced mass-transport of electroactive species from the bulk solution to the electrode surface caused by efficient stirring, acoustic streaming and cavitation. However, fundamental studies on the effect of ultrasound on electrode kinetics i.e. on the electron-transfer are scarce. The main question still remains to be answered: Does power ultrasound affect heterogeneous electron transfer kinetics? This opinion paper discusses the effect of ultrasonic frequency and intensity upon the electrode kinetic parameters such as the half-wave potential (E_1/2) and the apparent heterogeneous rate constant (k_o). A few sonoelectrochemical studies have highlighted changes in half-wave potential and in apparent heterogeneous rate constant for both quasi-reversible and irreversible systems when the data were compared to silent conditions. These observations are thought to be due to the contribution of mass-transport and macroscopic temperature effects, as well as the continuous cleaning of the electrode surface caused by the collapse of high-energy cavitation bubbles and the production of high velocity jets of liquid. However, there still remains mechanistic controversy in assigning whether these findings could also be due to localised temperature increases, the contribution of sonolysis products or solely due to enhanced mass-transport at the electrode surface. Thus, the effect of stirring, macroscopic temperature and sonication time upon these electrode kinetic parameters is also shown to be important factors in comparing the validity of any sonoelectrochemical effects.

Sonochemical Degradation of Benzothiophene (BT) in Deionized Water, Natural Water and Sea Water

This paper deals with the sonochemical water treatment of polycyclic aromatic sulfur hydrocarbons (PASHs), one of the most common impurities found in waste water coming from petroleum industry. The best fit of the experimental data appears to be the kinetic parameters determined using the Michaelis-Mentonmodel in the concentrations range of the study. For the initial increase in the degradation rates, it is simply considered that the more the bulk concentration increases, the more the concentration in the interfacial region increases. This will be explained by Michaelis-Menton kinetics. The influence of organic compounds in the water matrix as a mixture with Benzothiophene (BT) was also evaluated. The results indicated that BT degradation is unaffected by the presence of bisphenol A (BPA). Finally, the results indicated that ultrasonic action is involved in oxidation rather than pyrolitic processing in the BT sonochemical degradation.

Enhanced sonochemical degradation of bisphenol-A by bicarbonate ions

Sonochemical elimination of organic pollutants can take place through two degradation pathways. Molecules with relatively large Henry's law constants will be incinerated inside the cavitation bubble, while nonvolatile molecules with low Henry's law constants will be oxidised by the OH(*) ejected from the bubble of cavitation. Taking bisphenol-A as a model pollutant, this study points out an alternate degradation route, mediated by bicarbonate ions, which is significant for the elimination of micro-pollutants at concentrations present in natural waters. In this process, OH(*) radicals react with bicarbonate ions to produce the carbonate radical, which, unlike the OH(*) radical, can migrate towards the bulk of the solution and therefore induce the degradation of the micro-pollutants present in the bulk solution. As a consequence, initial degradation rate is increased by a factor 3.2 at low concentration of bisphenol-A (0.022 micromol l(-1)) in presence of bicarbonate in water.

Sonochemical degradation of chlorinated organic compounds, phenolic compounds and organic dyes - a review

Sonochemical processes have been widely used in chemistry and chemical engineering field. Recently, these processes have found new applications in the environmental field, because of advantages in terms of operational simplicity, secondary pollutant formation and safety. Several studies have reported on sonochemical degradation of organic compounds that are toxic in nature. The objective of this review was to identify and examine some of the studies on sonochemical degradation of chlorinated organic compounds, phenolic compounds and organic dyes. This review also examines the basic theory of sonochemical reactions and the use of sonochemical reactors for environmental applications.

Sonochemical degradation of martius yellow dye in aqueous solution

The sonolytic degradation of the textile dye martius yellow, also known as either naphthol yellow or acid orange 24, was studied at various initial concentrations in water. The degradation of the dye followed first-order kinetics under the conditions examined. Based on gas chromatographic results and sonoluminescence measurements of sonicated aqueous solutions of the dye, it is concluded that pyrolysis does not play a significant role in its degradation. The chromatographic identification of hydroxy added species indicates that an OH radical induced reaction is the main degradation pathway of the dye. Considering the non-volatility and surface activity of the dye, the degradation of the dye most probably takes place at the bubble/solution interface. The quantitative and qualitative formation of the degradation intermediates and final products were monitored using HPLC and ESMS. The analytical results suggest that the sonolytic degradation of the dye proceeds via hydroxylation of the aryl ring and also by C-N bond cleavage of the chromophoric ring, either through OH radical attack or through another unidentified process. The identification of various intermediates and end products also imply that the degradation of martius yellow proceeds through multiple reaction pathways. Total organic carbon (TOC) analyses of the dye solutions at various times following sonication revealed that sonolysis was effective in the initial degradation of the parent dye but very slow in achieving mineralization. The slow rate of mineralization is likely to be due to the inability of many of the intermediate products such as, the carboxylic acids, to accumulate at the bubble (air/water) interface and undergo decomposition due to their high water solubility (low surface activity).

Effects of operating parameters on sonochemical decomposition of phenol

Ultrasonic removal of phenol under irradiation at 20, 300 and 520 kHz was investigated to assess the impacts of operating parameters on the efficiency of the systems. It was found under our experimental conditions that 20 kHz was the least effective frequency for ultrasonic decomposition of phenol, owing to the low volatility of phenol and the slow rate of OH radical ejection to the bulk solution at this frequency. Assessment of relative rates of destruction and ultrasonic yields showed that maximum efficiency was accomplished with 300 kHz employed in a reactor enclosed with an ultrasonic energy of 14.7 W. The same reactor and frequency was found to provide maximum ejection of hydroxyl radicals to the solution. Impacts of pH and initial concentration on the efficiency of phenol removal were such that acidic pH and high concentrations accelerated the process as related to the increased likelihood of phenol at these conditions to approach the cavity sheath. Separate injection of equivalent volumes of air and argon into the reactors showed that the decomposition was enhanced in the presence of air by virtue of the production of additional reactive species via the reaction of nitrogen with molecular oxygen.

Comparison of enhancement of pentachlorophenol sonolysis at 20 kHz by dual-frequency sonication

The comparison of enhancement effect of pentachlorophenol sonolysis at 20 kHz by different dual-frequency ultrasonic irradiations has been investigated. Dual-frequency (20 kHz/40 kHz, 20 kHz/530 kHz, 20 kHz/800 kHz and 20 kHz/1040 kHz) ultrasounds have been used. It has been found that the rate of pentachlorophenol degradation at dual-frequency ultrasonic irradiation is the highest compared to mono-frequency ultrasonic systems. The combination of dual-frequency systems has synergistic effect and the enhancement effect of sonochemical degradation of pentachlorophenol at 20 kHz by dual-frequency systems appears to be remarkable frequency sensitive. The order of contribution to the enhancement effect of sonochemical degradation of pentachlorophenol at 20 kHz is as follows: 530 kHz > 800 kHz > 40 kHz > 1040 kHz.

Ultrasonic destruction of phenol and substituted phenols: a review of current research

Phenol and some of its derivatives such as chlorophenol and nitrophenol have received considerable attention from environmental scientists and engineers, due to their undesirable effects in the water environment, where they end up as a result of improper disposal methods. As a consequence, research has been conducted all around the world with the common goal of reducing their concentrations to allowable limits or converting them to non-toxic, non-hazardous forms that may easily be handled by natural decay processes. Some of this research has focused on ultrasonic techniques, which currently appears to present a convenient but as yet unproven method for large scale water remediation. The goal of this study is to identify and review some of these studies that are directly related with the use of ultrasound in decontaminating effluents with phenol residuals and to summarize the main points of interest and problems encountered.

Synergy of combining sonolysis and photocatalysis in the degradation and mineralization of chlorinated aromatic compounds

Merits of using advanced oxidation processes such as sonolysis and photocatalysis as well as a combination of the two have been explored using model herbicides such as 2,4-dichlorophenoxy acetic acid and 2,4-dichlorophenoxypropionic acid and the chlorinated phenols 2,4-dichlorophenol and 2,4,6-trichlorophenol. Whereas sonolysis is quite effective in the initial degradation of chlorinated aromatic molecules, complete mineralization is difficult to achieve. Photocatalysis is selective toward the degradation of polar compounds but causes the build up of undesirable chemical intermediates. In contrast to sonolytic degradation, photocatalysis is very effective toward achieving complete mineralization. By simultaneously carrying out high-frequency sonolysis and photocatalysis we have succeeded in achieving faster and complete mineralization with no build up of toxic intermediates even at very low catalyst loadings. The synergy of combining the two advanced oxidation processes is discussed.

The effect of ultrasound on photochemical reactions

Sonochemistry and photochemistry are based on very different physical phenomena, which a priori seems to exclude any possibility to observe more than a simple superposition of their respective effects. Literature data are examined, first the few studies devoted to sonochemical and photochemical reactions in cases where a direct comparison is possible. The second part is devoted to an analysis of experiments in which both irradiations are effected simultaneously. Cases exist from which it can be concluded, at least provisionally, that sonication is able to quench long-lived excited states, because of the intense streaming which enhances strongly the probability of collisional deactivations.

Sonochemical degradation for toxic halogenated organic compounds

This paper describes the degradation of p-chlorophenol using three different ultrasonic devices. The dissipated power in the reaction matrix was measured based on calorimetric method. The study showed that hydrogen peroxide can improve the sonochemical reaction and gases dissolved in reaction matrix can affect the process to a small extent. The reaction mechanism and kinetics of degradation were also investigated.

Sonoelectrochemical and sonochemical effects of cavitation: correlation with interfacial cavitation induced by 20 kHz ultrasound

Sonoelectrochemical measurements at macro-electrodes under extreme conditions with a very short distance between ultrasonic horn tip and electrode and different ultrasound intensity levels are shown to result in violent cavitation detected in form of current peaks superimposed on the average limiting current. Analysis of the current data obtained for the oxidation of ferrocene in dimethylformamide (0.1 M NBu4PF6) at a 4 mm diameter Pt disc electrode and for the reduction of Ru(NH3)6(3+) in aqueous 0.1 M KCl at a 6 mm diameter Pt disc electrode consistently indicate a change of the physicochemical nature of sonoelectrochemical processes under extreme conditions. The sonoelectrochemical measurement of the rate constant for the carbon bromide bond cleavage of a 3-bromobenzophenone radical anion electrogenerated at a glassy carbon electrode in dimethylformamide solution in the presence of power ultrasound is shown to yield evidence for a breakdown of the conventional mass transport model of a planar diffusion layer under extreme conditions. The change can be correlated to the number of current data points deviating more than 10% from the mean of the current due to violent cavitation processes superimposed onto the average limiting current. Further, a study of the sonochemical destruction of aqueous dilute cyanide solution (in 0.1 M NaOH) demonstrates a correlation between the electrochemically detected cavitation violence and the sonochemical activity. Factors that govern the violence of interfacial cavitation appear to be directly proportional to the factors that make cavitation in the bulk solution chemically efficient.

Degradation of pentachlorophenol aqueous solutions using a continuous flow ultrasonic reactor: experimental performance and modelling

The degradation of aqueous solutions of pentachlorophenol (PCP) in a three-stage sonochemical reactor operating in the continuous flow mode has been investigated. The experimental reactor may be considered as a series of three high-frequency ultrasonic units. The influence of several parameters such as ultrasonic power, reactor volume and volumetric feed flow rate on the reactor performance is reported. Application of classical basic chemical engineering principles leads to a model that enables us to predict the PCP concentration within the reactor. In steady state, experimental conversion rates are shown to be in good agreement with model predictions.

Experimental study of the hydrodynamic behaviour of a high frequency ultrasonic reactor

In relation to design and modeling of sonochemical reactors, the hydrodynamic behaviour of a high-frequency ultrasonic reactor has been investigated. Residence time distribution (RTD) measurements have been performed by means of a tracer method. The influence of ultrasound on the response to an inlet pulse was evidenced. It was shown that the reactor behaves like a completely stirred tank reactor (CSTR) as soon as ultrasonic irradiation operates. Preliminary observations on acoustic streaming occurring within the reactor will also be presented.

Ultrasound in photoelectrochemistry: a new approach to the enhancement of the efficiency of semiconductor electrode processes

A novel photoelectrochemical experiment which simultaneously allows the illumination of a TiO2 semiconductor electrode surface and the application of power ultrasound emission is described. The horn probe of an immersion horn transducer is modified by an oxide coated titanium tip and placed in a conventional three electrode electrochemical cell which allows light from a monochromated source to be focussed onto the electrode surface. Well-defined photocurrents are observed in aqueous media and for the photoinduced oxidation of water in acetonitrile and of 2.4-dichlorophenol in acetonitrile. The effect of ultrasound is to shift the observed photocurrent responses to more negative potentials and therefore to enhance the observed processes. Several possible interpretations associated with the complex effects induced by ultrasound are considered and a model based mainly on the extreme change of mass transport at the semiconductor/solution interface is suggested. Considerably enhanced performance for non-Nernstian processes, such as those observed in photoelectrochemical reactions at semiconductor electrodes, can be achieved in the presence of ultrasound.