The effects of ultrasound on micromixing

The Villermaux–Dushman reaction is a widely used technique to study micromixing efficiencies with and without sonication. This paper shows that ultrasound can interfere with this reaction by sonolysis of potassium iodide, which is excessively available in the Villermaux–Dushman solution, into triiodide ions. Some corrective actions, to minimize this interference, are proposed. Furthermore, the effect of ultrasonic frequency, power dissipation, probe tip surface area and stirring speed on micromixing were investigated. The power and frequency seem to have a significant impact on micromixing in contrast to the stirring speed and probe tip surface area. Best micromixing was observed with a 24 kHz probe and high power intensities. Experiments with different frequencies but a constant power intensity, emitter surface, stirring speed, cavitation bubble type and reactor design showed best micromixing for the highest frequency of 1135 kHz. Finally, these results were used to test the power law model of Rahimi et al. This model was not able to predict micromixing accurately and the addition of the frequency, as an additional parameter, was needed to improve the simulations.     

Process intensification using cavitation: optimization of oxidation conditions for synthesis of sulfone

Cavitation can be effectively used for intensification of chemical reactions due to the production of free radicals and conditions of high temperatures and pressures locally. In the present work, use of cavitation for the intensification of the synthesis of sulfone has been explored. The oxidation of thioether or sulfide to synthesize corresponding sulfone with 30% H₂O₂ as an oxidant was studied under acoustic cavitation and the results have been compared with the conventional approach based on the use of mechanical agitation. The aim has been also to optimize the different operating conditions viz. molar ratio of reactants to the oxidizing agent, type of the catalyst as well as its concentration, type of the solvent and the reactant concentration, so as to maximize the degree of intensification. It was observed that under the optimized conditions of sonication, the yield of sulfone was about five to six times higher as compared to the conventional approach of using mechanical agitation only.     

Sonochemical synthesis of amide-functionalized metal-organic framework/graphene oxide nanocomposite for the adsorption of methylene blue from aqueous solution

Graphene oxide-[Zn₂(oba)₂(bpfb)]·(DMF)₅ metal-organic framework nanocomposite (GO-TMU-23; H₂oba = 4,4′-oxybisbenzoic acid, bpfb = N,N′-bis-(4-pyridylformamide)-1,4-benzenediamine, DMF = N,N-dimethylformamide) is prepared through a simple and large-scale sonochemical preparation method at room temperature. The obtained nanocomposite is characterized by Field Emission Scanning Electron Microscopy (FE-SEM), powder X-ray diffraction (PXRD) and FT-IR spectroscopy. Additionally, the absorption ability of GO-TMU-23 nanocomposite toward cationic dye methylene blue was also performed. Significantly, GO-TMU-23 nanocomposite exhibits remarkably accelerated adsorption kinetics for methylene blue in comparison with the parent materials. The adsorption process shows that 90% of the dye has been removed and the equilibrium status has been reached in 2 min by using the nanocomposites as the adsorbent.     

Simulation of the spatial distribution of the acoustic pressure in sonochemical reactors with numerical methods: A review

Numerical methods for the calculation of the acoustic field inside sonoreactors have rapidly emerged in the last 15 years. This paper summarizes some of the most important works on this topic presented in the past, along with the diverse numerical works that have been published since then, reviewing the state of the art from a qualitative point of view. In this sense, we illustrate and discuss some of the models recently developed by the scientific community to deal with some of the complex events that take place in a sonochemical reactor such as the vibration of the reactor walls and the nonlinear phenomena inherent to the presence of ultrasonic cavitation. In addition, we point out some of the upcoming challenges that must be addressed in order to develop a reliable tool for the proper designing of efficient sonoreactors and the scale-up of sonochemical processes.     

Practical selective monohydrolysis of bulky symmetric diesters: Comparing with sonochemistry

The conditions of the practical selective monohydrolysis of symmetric diesters we previously reported have been modified and applied to selective monohydrolysis of bulky symmetric diesters. While ultrasound is generally considered effective for two-phase reactions, its effect actually turned out to be rather marginal. Instead, use of a larger proportion of a polar aprotic co-solvent, DMSO, and aqueous KOH helped enhance the reaction rates and improve the yields of the half-esters. The reactions are simple, mild and practical without special devices.     

Conformational study of CdS nanoparticles prepared by ultrasonic waves

CdS nanoparticles of about 5 nm in size have been prepared with the aid of ultrasound irradiation to ethylenediamine solution of cadmium acetate dehydrate and elemental S in presence of 1-decanthiol under air and normal laboratory conditions. X-ray diffraction (XRD) and selected area electron diffraction (SAED) studies indicate that the products are nanocrystallites in hexagonal structure. High resolution transmission electron microscopy (HRTEM) image reveals that lattice fringes are clearly visible, conforming their crystallinity with lattice space of 0.27 nm corresponding to (1 0 2) plane of hexagonal CdS. Energy-dispersive X-ray analysis (EDAX) shows that the product are entirely pure and atomic percentage ratio of Cd to S is about 53:47. UV–vis absorption spectroscopy of the as prepared nanoparticles reveals an energy band gap of about 3.8 eV compared to 2.42 eV corresponding to its bulk value; a blue shift of about 1.38 eV, which is understood as quantum size effect due to confinement of electron and hole in a small volume.     

Numerical simulation of cavitation bubble dynamics induced by ultrasound waves in a high frequency reactor

The use of high frequency ultrasound in chemical systems is of major interest to optimize chemical procedures. Characterization of an open air 477 kHz ultrasound reactor shows that, because of the collapse of transient cavitation bubbles and pulsation of stable cavitation bubbles, chemical reactions are enhanced. Numerical modelling is undertaken to determine the spatio-temporal evolution of cavitation bubbles. The calculus of the emergence of cavitation bubbles due to the acoustic driving (by taking into account interactions between the sound field and bubbles' distribution) gives a cartography of bubbles' emergence within the reactor. Computation of their motion induced by the pressure gradients occurring in the reactor show that they migrate to the pressure nodes. Computed bubbles levitation sites gives a cartography of the chemical activity of ultrasound. Modelling of stable cavitation bubbles' motion induced by the motion of the liquid gives some insight on degassing phenomena.     

Catalytic performance of Mn3O4 and Co3O4 nanocrystals prepared by sonochemical method in epoxidation of styrene and cyclooctene

A simple sonochemical method was developed to synthesis uniform sphere-like Co₃O₄ and Mn₃O₄ nanocrystals. Epoxidation of styrene and cyclooctene by anhydrous tert-butyl hydroperoxide over the prepared Co₃O₄ and Mn₃O₄ nanocatalysts was investigated. The results of conversion activity were compared with bulk Co₃O₄ and Mn₃O₄. Under optimized reaction conditions, the nanocatalysts showed a superior catalytic performance as compared to the bulk catalysts. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and BET surface area, were used to characterize and investigate the nanocatalysts.     

Ionic Liquid in Synthesis: A Phase Transfer Reagent and a Redox Mediating Medium

Ionic liquids (ILs) with chloride anion or bromide anion were attempted as a phase transfer reagent to depolymerize MX₂ (M = Pt, Pd; X = Br, Cl) in chloroform for reaction with 2,2′‐bipyridine (= bpy) to give the (bpy)MX₂ product. Supersonic irradiation of equiv‐molar bpy, PdX₂, and IL in CHCl₃ produced almost quantitative precipitate of (bpy)PdX₂ in a short time at ambient temperature, where IL is either [BEIm]Br or [BMIm]Br. That is, ILs and sono‐techniques assisted greatly on the synthesis of (bpy)PdX₂. For preparation of (bpy)PtX₂, nonetheless, free bpy always remained in mixture even after a long time of supersonic treatment. The system of equiv‐molar bpy, PtBr₂, and IL in CHCl₃, produced yellow (bpy)PtBr₂ and orange (bpy)PtBr₄, both being characterized with ¹H NMR and ¹⁹⁵Pt NMR in d₆‐Me₂SO as well as with single crystal X‐ray diffraction. Overall for PtX₂ reactions with halide anions in highly polar environments attributable to ILs, the redox pathway becomes important in that Pt(II) transforms to Pt(IV) to yield (bpy)PtBr₂ and (bpy)PtBr₄.     

Catalytic wet air oxidation of phenol over ultrasound-assisted synthesized Ni/CeO2–ZrO2 nanocatalyst used in wastewater treatment

Non-noble metal Ni with different loadings was coated on precipitated CeO₂–ZrO₂ support by the sonochemistry method and examined for catalytic wet air oxidation of phenol. The structure of the nanocatalysts was determined by BET, FESEM, XRD, and FTIR analyses. The results showed non-uniform morphology of the nanocatalyst at lower Ni contents changed to homogenous morphology with spherical nano particles at higher Ni contents. While the size of NiO crystals remained constant with rising Ni content, the crystallinity of NiO significantly increased. If the crystallinity of NiO was 100% in 20% wt Ni/CeO₂–ZrO₂, the crystallinity of NiO in 5% wt Ni was found to be 41.13%. The average particle size in Ni(15%)/CeO₂–ZrO₂ was 77 nm in which 85.71% of particle diameters were less than 100 nm. Catalytic wet air oxidation of phenol with different Ni loadings indicated improvement of phenol destruction at higher amounts of active phase. Removal of phenol increased with increasing catalyst loading from 4 to 9.0 g/l but further increase to 10 g/l declined the catalyst reactivity.