Selective oxidation has an important role in environmental and green chemistry (e.g., oxidative desulfurization of fuels and oxidative removal of mercury) as well as chemicals and intermediates chemistry to obtain high-value-added special products (e.g., organic sulfoxides and sulfones, aldehydes, ketones, carboxylic acids, epoxides, esters, and lactones). Due to their unique physical properties such as the nonvolatility, thermal stability, nonexplosion, high polarity, and temperature-dependent miscibility with water, ionic liquids (ILs) have attracted considerable attention as reaction solvents and media for selective oxidations and are considered as green alternatives to volatile organic solvents. Moreover, for easy separation and recyclable utilization, IL catalysts have attracted unprecedented attention as “biphasic catalyst” or “immobilized catalyst” by immobilizing metal- or nonmetal-containing ILs onto mineral or polymer supports to combine the unique properties of ILs (chemical and thermal stabili...

Ionic Liquids in Selective Oxidation: Catalysts and Solvents

Sulfur compounds in transportation fuels are a pressing issue currently due to the more stringent limits of sulfur content. Sulfur compounds lead to SOx emissions which cause many environmental and health problems. The conventional refinery desulfurization process, hydrodesulfurization (HDS), is conducted at elevated temperatures and pressures, using expensive hydrogen gas and catalysts. This review aims to discuss the merits and drawbacks of the major areas of alternative desulfurization technologies, including biodesulfurization, adsorption, extractive and oxidative desulfurization, with special emphasis on the role of ionic liquids (ILs) as distinctive multi-task fluids. With the rapid development of ILs in the last decades, more effective and reliable desulfurization techniques are foreseeable.

The role of ionic liquids in desulfurization of fuels: A review

Harvesting clean energy from fuel feedstocks is of paramount significance in the field of environmental science. In this dynamic area, desulfurization provides a valuable contribution by eliminating sulfur compounds from fuel feedstocks to ensure the utilization of fuels without the emission of toxic sulfur oxides (SOx gases). Nonetheless, the inadequacy of the current industrial technique (hydrodesulfurization, HDS) in the removal of refractory sulfur (RS) compounds and the stringent rules imposed on the fuel sulfur level have kindled research on other desulfurization methods like oxidative desulfurization (ODS). With the capacity of eliminating RS compounds under mild conditions, ODS is endorsed as a suitable replacement or complementary to HDS. ODS, in general, consists of two steps: (i) oxidation and (ii) extraction. The oxidation of sulfur compounds is carried out using a suitable catalyst (hereafter termed as an ODS catalyst) in the presence of an oxidant. Choosing a suitable ODS catalyst for industrial applications is still a quest among the various types of catalysts reported so far. With this outline, herein, all the types of ODS catalysts along with their synthetic methods, reactivity and mechanistic insights are reviewed. The activity of ODS catalysts could be influenced by factors like the type of RS compound, solvent, fuel, etc. and those factors are reviewed. The effects of ionic liquids, light, and ultrasound on the performance of ODS catalysts are also briefly summarized. The opportunities and challenges for ODS catalysts are comprehensively explicated in the end. Through this review, systematic information about the types of ODS catalysts including the basic definition, preparative methods, reactivity and mechanism can be comprehended. Furthermore, this review reveals the merits and demerits related to highlighting catalytic ODS as a replacement or complementary to HDS.

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A comprehensive review on oxidative desulfurization catalysts targeting clean energy and environment

Polyoxometalates (POMs) are a unique class of molecular metal oxides with a tunable structure at the atomic level. They have demonstrated various potential applications in various fields. In particular, POMs have been widely used as catalysts, due to their facilely modified acid–base properties and redox potential through molecular designing. Normally, POMs can be utilized as both homogeneous and heterogeneous catalysts, where the former favors high activity, while the latter benefits facile separation of the catalysts. Faced with the requirement for sustainable development, significant effort has been made in the preparation of POM-based heterogeneous catalysts. This review focuses on the recent developments in heterogeneous strategies of POM-based catalysts and their applications in liquid organic reactions.

Recent advances in polyoxometalate-based heterogeneous catalytic materials for liquid-phase organic transformations

The selectivity for sulfur removal from oils is an important topic. In this work, the selectivity for different sulfur removal methods has been studied by conceptual density functional theory (CDFT) at the B3LYP/6-311++G(3df,2p) level of theory. In principle, the selectivity is directly related to the mechanisms of sulfur removal. It cannot be precisely elucidated until the mechanisms are totally known. However, current work shows that relationships can be constructed between CDFT and the selectivity. That is, for hydrodesulfurization, good descriptors will be ionization energy, hardness, and bond lengths of SC; for adsorptive desulfurization, the hardness is a good descriptor; for oxidative desulfurization, good descriptors are electron density and Fukui function. And for extractive desulfurization (nonmetal-based ionic liquids), electron affinity and electrophilicity may be good descriptors. In addition, structures and frontier orbitals of various sulfides have also been discussed. It is hoped that these relationships between CDFT and selectivity can give useful information to develop highly efficient sulfur removal methods for specific sulfides, like 4,6-dimethyldibenzothiophene, and 4-methyldibenzothiophene. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2087–2100, 2016

The Selectivity for Sulfur Removal from Oils: An Insight from Conceptual Density Functional Theory

Ionic liquid extraction and catalytic oxidative desulfurization of fuels using dialkylpiperidinium tetrachloroferrates catalysts

A heterogeneous catalyst system was synthesized by immobilizing phosphotungstic acid on ionic liquid-modified mesoporous silica SBA-15 and applied in oxidative desulfurization. Structure and properties of catalyst were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), N2 adsorption–desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and the contact angle. The results demonstrated that the synthesized catalyst possessed ordered mesopore structure and high special surface area. Due to the introduction of imidazole-based ionic liquid, the catalyst exhibited good wettability for model oil, which had significant contribution to desulfurization activity. Both DBT and 4,6-DMDBT could be removed completely at mild conditions (60 °C, 40 min). The removal of BT also can reach 81.3% within 60 min. Furthermore, the catalyst was recovered and reused in four reaction runs with a slight decrease in activity.

Phosphotungstic Acid Immobilized on Ionic Liquid-Modified SBA-15: Efficient Hydrophobic Heterogeneous Catalyst for Oxidative Desulfurization in Fuel

Boosting aerobic oxidative desulfurization performance in fuel oil via strong metal-edge interactions between Pt and h-BN

Hexagonal boron nitride: A metal-free catalyst for deep oxidative desulfurization of fuel oils

A novel reaction-controlled foam-type catalyst has been designed by pairing 1-hexadecyl-3-methyl-imidazolium cation with peroxomolybdate anion. This catalyst switched from the powder to the foam-type active species, exhibiting high catalytic activity in the oxidative desulfurization process. After the reaction was finished, the foam became brittle and returned to powder form, which could be easily separated and reused. Reasons for this change were detailed by experiments. The removal of dibenzothiophene (DBT) could reach 98.4% under the optimal conditions of n(DBT)/n(catalyst)/n(H2O2) = 30:1:180, at 50 °C for 1 h. The catalyst could be recycled six times, and the sulfur removal still remained to be about 93.9%.

Fengxia Zhu

Papers

Ionic liquid extraction and catalytic oxidative desulfurization of fuels using dialkylpiperidinium tetrachloroferrates catalysts

Phosphotungstic Acid Immobilized on Ionic Liquid-Modified SBA-15: Efficient Hydrophobic Heterogeneous Catalyst for Oxidative Desulfurization in Fuel

Boosting aerobic oxidative desulfurization performance in fuel oil via strong metal-edge interactions between Pt and h-BN

Hexagonal boron nitride: A metal-free catalyst for deep oxidative desulfurization of fuel oils

A Novel Reaction-Controlled Foam-Type Polyoxometalate Catalyst for Deep Oxidative Desulfurization of Fuels