Mechanistic Features of Oxidative Desulfurization Using Sono-Fenton–Peracetic Acid (Ultrasound/Fe²⁺–CH₃COOH–H₂O₂) System

This article attempts to identify the links between the chemistry of oxidative desulfurization and cavitation physics for an ultrasound-assisted oxidative desulfurization (UAOD) process using Fenton–peracetic acid as the oxidant. The model system employed was dibenzothiophene (as a model sulfur compound) and toluene (as a model gasoline/diesel). Experiments were performed to assess the role of each component of the oxidant in the chemistry of the process. H₂O₂ was found to be the key component of the oxidant that balances between several competing pathways and reactions in overall oxidative desulfurization process. Addition of Fe²⁺ to peracetic acid has a beneficial effect, whereas excess H₂O₂ has an adverse effect on the process. This article also highlights the physical and mechanistic features of the UAOD process. Transient cavitation is revealed to play a negative role in the desulfurization process, whereas ultrasound has a positive effect. The former effect is a consequence of the scavenging of HO₂^• radicals in the aqueous phase by radicals generated by cavitation bubbles, whereas the latter effect is attributed to the generation of a fine emulsion between the oxidant and toluene phases as a result of strong micromixing generated by ultrasound. The results of this study clearly point out that less scavenging and effective interphase transfer of HO₂^• radicals are more crucial to the utilization of HO₂^• radicals for desulfurization than mere generation

Mechanistic Features of Ultrasound-Assisted Oxidative Desulfurization of Liquid Fuels

A new technology for the removal of sulfur compounds from liquid fuels is oxidative desulfurization. Although several studies have reported the enhancement effect of ultrasound irradiation on oxidative desulfurization, the exact mechanism underlying this enhancement is not known yet. In this study, we have addressed this issue with dual approach of coupling experiments with mathematical model for cavitation. Results of this study have given interesting revelation of interaction between mechanism of ultrasound, cavitation, and oxidation system. Isolation of cavitation phenomenon helps to increase the extent of oxidation. This effect is attributed to formation of hydrogen and carbon monoxide during transient collapse of cavitation bubbles due to thermal dissociation of hexane vapor entrapped in the bubble, which hamper the action of O species generated from the oxidation system. Transient cavitation itself does not give rise to radical formation, because of rather low temperature peaks reached during collapse. Therefore, cavitation does not enhance the oxidation process, but in fact, has an adverse effect on it. Current study has established that the beneficial effect of ultrasound on oxidative desulfurization system is merely of a physical nature (i.e., emulsification due to intense micromixing), with no involvement of a sonochemical effect

Mechanistic Features of Ultrasound-Assisted Oxidative Desulfurization of Liquid Fuels

A new technology for the removal of sulfur compounds from liquid fuels is oxidative desulfurization. Although several studies have reported the enhancement effect of ultrasound irradiation on oxidative desulfurization, the exact mechanism underlying this enhancement is not known yet. In this study, we have addressed this issue with dual approach of coupling experiments with mathematical model for cavitation. Results of this study have given interesting revelation of interaction between mechanism of ultrasound, cavitation, and oxidation system. Isolation of cavitation phenomenon helps to increase the extent of oxidation. This effect is attributed to formation of hydrogen and carbon monoxide during transient collapse of cavitation bubbles due to thermal dissociation of hexane vapor entrapped in the bubble, which hamper the action of O species generated from the oxidation system. Transient cavitation itself does not give rise to radical formation, because of rather low temperature peaks reached during collapse. Therefore, cavitation does not enhance the oxidation process, but in fact, has an adverse effect on it. Current study has established that the beneficial effect of ultrasound on oxidative desulfurization system is merely of a physical nature (i.e., emulsification due to intense micromixing), with no involvement of a sonochemical effect

Sonochemical Synthesis and Characterization of Manganese Ferrite Nanoparticles

This paper reports sonochemical synthesis and characterization of Mn–ferrite nanoparticles using acetates precursors. Mn–ferrite synthesis requires external calcination of oxide precursors formed by sonication. pH does not play a dominant role in the synthesis. Collisions between metal oxide particles induced by shock waves generated by transient cavitation are unable to cross the activation energy barrier for the formation of ferrite. The calcination temperature is a significant parameter that influences the magnetic properties of ferrites. The size, coercivity, and saturation magnetization of ferrite particles increases with the calcination temperature. Ferrites formed at calcination temperatures of 650, 750, and 850 °C show ferromagnetic behavior with easy axis magnetization. Calcination at 950 °C leads to the formation of rods with grain growth that introduces large shape anisotropy. The magnetization curve for rods does not reach saturation, indicating paramagnetic behavior. The cause leading to this effect is nonalignment of the easy axis of magnetization with the direction of the applied magnetic field, resulting in hard axis magnetization

Mechanistic Investigation in Ultrasound-Assisted (Alkaline) Delignification of <i>Parthenium hysterophorus</i> Biomass

Delignification of biomass is a primary step in biomass pretreatment in fermentation based synthesis of alcoholic biofuels. This paper attempts to give mechanistic insight into ultrasound-assisted delignification of biomass. <i>Parthenium hysterophorus</i> (carrot grass) has been used as the model biomass. The approach of study is to couple simulations of cavitation bubble dynamics to the experiments on delignification. Best values of delignification parameters with ultrasound have been identified as temperature = 303 K, NaOH concentration = 1.5% w/v, and biomass concentration = 2% w/v. Characterization of delignified biomass has been carried out using FTIR spectroscopy and XRD and FESEM techniques. Both physical and chemical effects of transient cavitation contribute to delignification. The physical effect of shock waves leads to depolymerization of lignin matrix through homolytic cleavage of phenyl ether α–O–4 and β–O–4 bonds. The chemical effect of radical generation causes hydroxylation/oxidation of the aromatic moieties and side chain elimination. Due to these peculiar mechanisms, ultrasound treatment gives effective delignification at ambient temperature and with lesser requirement of delignifying agents. Cavitation also causes decrystallization of cellulose due to partial depolymerization. Kinetic analysis of delignification at best values of parameters has revealed 2-fold enhancement with ultrasound as compared to mechanically agitated treatment