Depolymerization using sonochemical reactors: A critical review

Ultrasonic irradiation has been proposed as a novel approach for degradation of polymer compounds, especially considering the fact that the reduction in the molecular weight (also the intrinsic viscosity) is simply by splitting the most susceptible chemical bond without causing any changes in the chemical nature. An overview of the application of ultrasound for the polymer degradation has been presented in this work, discussing the mechanism of degradation, kinetic modeling, effect of operating parameters and the type of reactors generally used for depolymerization. The effect of important operating parameters such as initial polymer concentration, presence of functional groups in the polymer chain, reaction volume, initial molecular weight, temperature, operating frequency, power dissipation and use of process intensifying additives have been discussed also giving guidelines about selection of the optimum parameters. It has been observed that the low concentrations and higher power dissipation (till an optimum) are favorable resulting in higher extents of degradation. Typically low frequency is recommended but for the case of water soluble polymers, higher frequencies would also give similar results due to the dominant action of chemical effects of cavitation. It has been demonstrated that the alkyl group substituent also affects the degradation rate of polymer. An overview of degradation using combined approach based on ultrasound and additives with comparison with individual approach has also been presented. It has been observed that the main contributing factor for the synergy of the combined approach is the selection of optimum loading of additives. Overall, it has been observed that efficient polymer degradation can be achieved using combined process based on the use of ultrasound.     

Rapid sonochemical synthesis of MCM-41 type benzene-bridged periodic mesoporous organosilicas

Benzene-bridged periodic mesoporous organosilicas (PMOs) with the MCM-41 were synthesized by a rapid sonochemical process via co-condensation of tetraethoxysilane (TEOS) and 1,4-bis(triethoxysilyl) benzene (BTEB) under basic conditions within a few minutes using cetyltrimethylammoniumbromide (CTMABr) as a structure-directing agent. The molar ratio of the silicon precursors and the synthesis time were varied in order to investigate their influence on the structural ordering of the materials. The characteristics of the materials were evaluated by X-ray diffraction (XRD), N₂-sorption, transmission electron microscopy (TEM) and solid-state NMR spectroscopy. The resultant materials exhibited well-ordered hexagonal mesostructures with surface areas in the range of 602–1237 m²/g, pore volumes of 0.37–0.68 cm³/g, and pore diameters in the range of 2.5–3.5 nm. Two dimensional ²⁹Si{¹H} heteronuclear correlation (HETCOR) NMR spectra confirmed the formation of a single mesophase with various Q (from TEOS) and T (from BTEB) silicon species located randomly within the pore walls due to the co-condensation of BTEB and TEOS, which excluded the possibility of formation of island or two separate phases within such a short synthesis time. The prime advantage of the present synthesis route is that it can effectively reduce the total synthesis time from days to a few minutes, much shorter than the conventional benzene-bridged PMOs synthesis methods.     

Efficient sonochemical synthesis of novel 3,5-diaryl-4,5-dihydro-1H-pyrazole-1-carboximidamides

An ultrasound-assisted preparation of a series of novel 3,5-diaryl-4,5-dihydro-1H-pyrazole-1-carboximidamides that proceeds via the efficient reaction of chalcones with aminoguanidine hydrochloride under clean conditions is described.     

Simultaneous sonication assistance for the synthesis of tetrahydropyridines and its efficient catalyst ZrP2O7 nanoparticles

In this research, a general synthetic method for the synthesis of tetrahydropyridines were developed using ZrP₂O₇ nanoparticles under ultrasonic irradiations. Firstly by a simple and green process, nano zirconium pyrophosphate was prepared via sonication. Subsequently, this nanoparticle was used as an efficient catalyst for the synthesis of highly functionalized tetrahydropyridines via five-component reaction of aromatic aldehyde, amine and ethyl acetoacetate in ethanol under ultrasound irradiation. The present approach offers several advantages such as high yields, environmentally benign, simple work-up, excellent yield of products, short reaction times as well as recoverability and reusability of the catalyst.     

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.     

One-pot sonochemical synthesis of 1,3-thiazolidin-4-ones using nano-CdZr4(PO4)6 as a robust heterogeneous catalyst

An efficient three-component synthesis of 1,3-thiazolidin-4-ones is described by one-pot condensation of aldehydes, aniline and thioglycolic acid with nano-CdZr₄(PO₄)₆ as a robust heterogeneous catalyst under ultrasonic irradiation. Use of simple and readily available starting materials, experimental simplicity, applying the sonochemical methodology as an efficient method and innocuous means of activation in synthetic chemistry are some advantages of this protocol.     

Ultrasound mediated,iodine catalyzed green synthesis of novel 2-amino-3-cyano-4H-pyran derivatives

An efficient synthesis of a novel series of twelve substituted 2-amino-3-cyano-4H-pyran derivatives was achieved by a one-pot three-component cyclocondensation reaction of heteroaryl aldehydes, malononitrile and active methylene compounds catalyzed by iodine in aqueous medium under ultrasound irradiation. In comparison with conventional methods, our protocol is convenient and offers several advantages, such as shorter reaction time, higher yields, milder conditions and environmental friendliness. We have herein successfully demonstrated the synergistic outcome of multi-component reaction (MCR) and sonication to offer a facile route for the design of these derivatives.     

La2Sn2O7/g-C3N4 nanocomposites: Rapid and green sonochemical fabrication and photo-degradation performance for removal of dye contaminations

The deficiency of drinking water sources has become a serious crisis for the future of the world that the photocatalytic process is one of the most favorable methods for removal of artificial dyes and poisonous organic impurities. In the present study, rapid ultrasonic treatment was performed to obtain La₂Sn₂O₇/Graphitic carbon nitrides (LSO/CN) nanocomposites with advanced photo-catalytic performance. Broccoli extract was utilized as a natural surfactant with active surface groups to control nucleation and growth of formed crystals with the creation of spatial barriers around the cations, and finally prevent nano-product agglomeration. Changing experimental parameters in synthesis reaction in turn offers a virtuous control over the nano-products size and shape. The shape and size distribution of particles was considered via diverse characterization techniques of microscopic and spectroscopic. The photocatalytic behaviors along with a kinetic study of the nanoparticles were examined by elimination and degradation of different artificial dyes under the UV waves. Effect of particle size, weight ratio of LSO:CN, type of dye, scavenger kind, dye and catalyst loading was designated on altering proficiency of nano-catalyst function. Also, the probable mechanism of removal dye by photocatalytic function was studied.     

A parametric review of sonochemistry: Control and augmentation of sonochemical activity in aqueous solutions

In this review the phenomenon of ultrasonic cavitation and associated sonochemistry is presented through system parameters. Primary parameters are defined and considered, namely; pressure amplitude, frequency and reactor design; including transducer type, signal type, vessel-transducer ratio, liquid flow, liquid height, liquid temperature and the presence of a reflective plate. Secondary parameters are similarly characterised and involve the use of gas and liquid additives to influence the chemical and physical environments. Each of the parameters are considered in terms of their effect on bubble characteristics and subsequent impact on sonochemical activity. Evidence suggests that via parametric variation, the reaction products and efficiency may be controlled. This is hypothesised to occur through manipulation of the structural stability of the bubble.     

The ex-situ and in-situ ultrasonic assisted oxidation of corn starch: A comparative study

The objective of this study was to evaluate the differences in ex-situ (starch treated by ultrasonication and oxidation sequentially, U-OS) and in-situ (starch treated by ultrasonication and oxidation simultaneously, UOS) ultrasonic assisted oxidation process of corn starch, which were studied in contrast to the traditional oxidized starches (OS). Fourier-transform infrared spectra confirmed the successful oxidation of all modified starches samples. In comparison to the OS, the carboxyl contents of U-OS and UOS increased by 56% and 112%, respectively. The same increase trend was also found for the carbonyl contents. The significance raise was attributed to the great increase of pores and specific surface areas in the starch granule after ultrasonic irradiation which promoted the penetration of the sodium hypochlorite into the starch granules with higher chances for chemical reactions. SEM and pore size distribution characterizations further verified this result. However, the method of in-situ ultrasonic assisted oxidation can simultaneously accelerate the increase of pores and the penetration process. Consequently, the starches with higher oxidation degree can be more efficiently prepared by the strategy of in-situ ultrasonic assisted oxidation.     

An investigation of electron and oxygen ion damage in Si npn RF power transistors

Abstract

The effects of 8 MeV electrons and 60 and 95 MeV oxygen ions on the electrical properties of Si npn RF power transistors have been investigated as a function of fluence. The dc current gain (h _FE), displacement damage factor, excess base current (Δ I _B=I _Bpost?I _Bpre), excess collector current (Δ I _C=I _Cpost?I _Cpre), collector saturation current (I _CS) and deep level transient spectroscopy trap signatures of the irradiated transistors were systematically evaluated.     

Applications of ultrasound in total synthesis of bioactive natural products: A promising green tool

Total synthesis is frequently compared to climbing as it provides a suitable route to reach a high point from the floor, the complex natural product from simple and commercially available materials. The total synthesis has a privileged position of trust in confirming the hypothetical complex structures of natural products despite sophisticated analytical and spectroscopic instrumentation and techniques that are available presently. Moreover, total synthesis is also useful to prepare rare bioactive natural products in the laboratory as several bioactive secondary metabolites are obtained in small quantities from natural sources. The artistic aspect of the total synthesis of bioactive natural products continues to be praised today as it may provide environmental protection through the concept of green or clean chemistry. The use of ultrasound waves as a non-polluting source of energy is of great interest in the field of sustainable and pharmaceutical chemistry as it differs from conventional energy sources in terms of reaction rates, yields, selectivities, and purity of the products. The present review highlights the application of ultrasound as a green tool in the total synthesis of bioactive natural products as well as this article is also aimed to offer an overview of natural sources, structures, and biological activities of the promising natural products for the first time from 2005 to 2020 elegantly.     

An experimental study on the motion of water droplets in oil under ultrasonic irradiation

The motion of a single water droplet in oil under ultrasonic irradiation is investigated with high-speed photography in this paper. First, we described the trajectory of water droplet in oil under ultrasonic irradiation. Results indicate that in acoustic field the motion of water droplet subjected to intermittent positive and negative ultrasonic pressure shows obvious quasi-sinusoidal oscillation. Afterwards, the influence of major parameters on the motion characteristics of water droplet was studied, such as acoustic intensity, ultrasonic frequency, continuous phase viscosity, interfacial tension, and droplet diameter, etc. It is found that when the acoustic intensity and frequency are 4.89 W cm⁻² and 20 kHz respectively, which are the critical conditions, the droplet varying from 250 to 300 μm in lower viscous oil has the largest oscillation amplitude and highest oscillation frequency.     

Ultrasound-assisted synthesis of pyrimidines and their fused derivatives: A review

The pyrimidine scaffold is present in many bioactive drugs; therefore, efficient synthetic routes that provide shorter reaction times, higher yields, and site-selective reactions are constantly being sought. Ultrasound (US) irradiation has emerged as an alternative energy source in the synthesis of these heterocyclic scaffolds, and over the last ten years there has been a significant increase in the number of publications mentioning US in either the construction or derivatization of the pyrimidine core. This review presents a detailed summary (with 140 references) of the effects of US (synergic or not) on the construction and derivatization of the pyrimidine core through classical reactions (e.g., multicomponent, cyclocondensation, cycloaddition, and alkylation reactions). The main points that were taken into consideration are as follows: chemo- and regioselectivity issues, and the results of conventional heating methods compared to US and mechanistic insights that are also presented and discussed for key reactions.     

Poly(4-vinylpyridine) supported acidic ionic liquid: A novel solid catalyst for the efficient synthesis of 2,3-dihydroquinazolin-4(1H)-ones under ultrasonic irradiation

A novel poly(4-vinylpyridine) supported acidic ionic liquid catalyst was synthesized by the reaction of 4-vinylpyridine with 1,3-propanesultone, followed by the polymerization and the addition of the heteropolyacid. Due to the combination of polymer features and ionic liquid, it acted as a heterogeneous catalyst to effectively catalyze the cyclocondensation reaction of anthranilamide with aldehydes under ultrasonic irradiation and afforded the corresponding 2,3-dihydro-4(1H)-quinazolinones compounds in good to excellent yields. In addition, the catalyst could be easily recovered by the filtration and reused six times without significant loss of catalytic activity. More importantly, the use of ultrasonic irradiation can obviously accelerate the reaction.     

Green in water sonochemical synthesis of tetrazolopyrimidine derivatives by a novel core-shell magnetic nanostructure catalyst

A green approach for the one-pot four-component sonochemical synthesis of 5-methyl-7-aryl-4,7-dihydrotetrazolo[1,5-a]pyrimidine-6-carboxylic esters from the reaction of 2-cyano-guanidine, sodium azide, various aromatic aldehydes and methyl or ethyl acetoacetate in the presence of a catalytic amount of Fe₂O₃@SiO₂-(CH₂)₃NHC(O)(CH₂)₂PPh₂ as a new hybrid organic–inorganic core–shell nanomagnetic catalyst is described. This is the first design, preparation, characterization and application of the present nanomaterial and also the first ultrasound irradiated synthesis of the biologically and pharmaceutically important heterocyclic compounds in water as a green solvent. This novel sonocatalysis/nanocatalysis protocol offers several advantages such as high yields, short reaction times, environmentally-friendly reaction media, easily isolation of the products, simple preparation, full characterization and recoverability of the nanocatalyst by an external magnet and reusing several times without significant loss of activity.     

A novel approach for the synthesis of superparamagnetic Mn3O4 nanocrystals by ultrasonic bath

In this study, the synthesis of Mn₃O₄ (husmannite) nanoparticles was carried out in two different alkali media under sonication by ultrasonic bath and conventional method. Manganese acetate was used as precursor, sodium hydroxide and hexamethylenetetramine (HMT) as basic reagents in this synthesis. An ultrasonic bath with low intensity was used for the preparation of nanomaterials. The as prepared samples were characterized with X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (HRTEM, TEM), energy-dispersive spectrum (EDS), and superconducting quantum interference device (SQUID) analysis. The XRD patterns exhibit the nanocrystals are in pure tetragonal phase. The chemical composition was obtained by EDS analysis and confirmed the presence of Mn and O in the sample. According to the TEM and HRTEM results, both nanorods and nanoparticles of Mn₃O₄ were obtained in the presence of ultrasonic irradiation. The average size of nanoparticles was 10 nm, and the size of nanorods was 12 nm in diameter and 100-900 nm in length for the samples prepared in basic medium with sodium hydroxide. In the conventional method with the same basic medium, the nanorod was not observed and the nearly cubic nanoparticles was appeared with an average size of 2.5 nm. The selected area electron diffraction (SAED) patterns revealed that the nanocrystals are polycrystalline in nature. When HMT was used as a basic reagent in the presence of ultrasonic irradiation, it was led to a higher size of nanoparticles and nanorods than when sodium hydroxide was used as a basic reagent. The average size of nanoparticles was about 15 nm and its shape was nearly cubic. The diameter for nanorods was 50 nm and the length was about a few micrometers.The magnetic measurements were carried out on the sample prepared in sodium hydroxide under ultrasonic irradiation. These measurements as a function of temperature and field strength showed a reduction in ferrimagnetic temperature (T_c = 40 K) as compared to those reported for the bulk (T_c = 43 K). The superparamagnetic behavior was observed at room temperature with no saturation magnetization and hysteresis in the region of measured field strength.     

Ultrasonic-assisted environmentally-friendly synergetic synthesis of nitroaromatic compounds in core/shell nanoreactor: A green protocol

An efficient sonochemical protocol for the nitration of aromatic compounds was described in the presence of a catalytic amount of sulfuric acid-functionalized silica-based core/shell magnetic nanocomposite at room temperature in an eco-friendly and recyclable media, deep eutectic solvent (DES), based on choline chloride and urea. The particle size, morphology and elemental analysis of the core/shell nanocatalyst were carried out by TEM, SEM, EDX and XRD analyses. The nanocatalyst and DES were easily recovered from the reaction mixture quantitatively and reused several times.     

Semiconducting In2S3 quantum dots in thin film form: sonochemical versus conventional chemical synthesis and investigation of their structural and optical properties

Indium(III) sulfide quantum dots were deposited in thin film form using both a conventional chemical bath deposition method and a sonochemical route. The developed routes allow deposition of cubic α-In₂S₃ nanocrystals in thin film form. The as-deposited films produced by the conventional and sonochemical approaches are highly nanocrystalline, with average crystal sizes of 2.5 and 2.0 nm correspondingly (as determined from the Scherrer formula), which increase to 4.1 nm upon annealing treatment, due to coalescence and crystal-growth processes. Refinement of the lattice-constant value in the case of as-deposited and annealed films was performed using linear regression analysis. Blue-shifted band-gap energy values of as-deposited films with respect to those corresponding to bulk specimen, accompanied with the red shift of absorption onset upon annealing, strongly indicate the quantum-dot behavior of the synthesized nanocrystals. The detected three-dimensional quantum-confinement effects in the synthesized nanocrystals were discussed in terms of the Brus model. On the basis of optical spectroscopic data, we estimated the Bohr's excitonic radius value in this semiconductor. Presented at the X-th Symposium on Suface Physics, Prague, Czech Republic, July 11–15, 2005.     

A novel method for the preparation of III-V semiconductors: sonochemical synthesis of InP nanocrystals

A novel method for the preparation of III–V semiconductor has been provided in this paper. At room temperature, InP nanocrystals with diameter of ≈9 nm were successfully obtained under high-intensity ultrasonic irradiation for 4 h from the reaction of InCl₃·4H₂O, yellow phosphorus and KBH₄ in the mixed solvents of ethanol and benzene. Changing some parameters can effectively control the size of the products and possible explanations were offered. The products were characterized by X-ray powder diffraction, transmission electron microscope and electron diffraction pattern. The ultrasonic irradiation and the solvents are both important in the formation of the product.