Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser The role of ionic liquids in desulfurization of fuels: A review Muna Hassan Ibrahim a,b , Maan Hayyan b,c, ⁎ , Mohd Ali Hashim a,b , Adeeb Hayyan b,c,⁎⁎ a Department of Chemical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia b University of Malaya Centre for Ionic Liquids (UMCiL), University of Malaya, Kuala Lumpur 50603, Malaysia c Institute of Halal Research University of Malaya (IHRUM), University of Malaya, Kuala Lumpur, 50603 Malaysia ARTICLE INFO Keywords: Desulfurization Ionic liquid Deep eutectic solvent Superoxide ion Petroleum refining Energy ABSTRACT Sulfur compounds in transportation fuels are a pressing issue currently due to the more stringent limits of sulfur content. Sulfur compounds lead to SO x emissions which cause many environmental and health problems. The conventional refinery desulfurization process, hydrodesulfurization (HDS), is conducted at elevated tempera- tures 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. 1. Introduction Sulfur in fossil fuels such as gasoline, diesel and kerosene is the biggest challenge facing oil refiners today. Sulfur is found in fuels in organic sulfur compounds (e.g. sulfides, disulfides, and thiophene) [1,2]. Upon combustion of these sulfur compounds in fuels, SO x compounds are released [3]. These SO x compounds poison catalytic converters, cause respiratory problems and lead to acid rain [2,4,5]. Thus, regulatory bodies are imposing increasingly stringent regulations with regard to the maximum level of sulfur in transportation fuels [6– 8]. The conventional method to remove sulfur compound involves the reaction of these compounds with hydrogen gas at high pressures and temperatures, with the use of expensive catalysts [1,9]. It also reduces the octane rating of gasoline and releases toxic hydrogen sulfide (H 2 S) gas [6,10]. Refractory sulfur compounds (e.g. dibenzothiophene) are particularly challenging to remove using this method [11,12]. To reduce the sulfur content further using HDS would require even more severe conditions than those currently employed, which would result in an increase in capital and operating costs [3,12,13]. Therefore, there is an intense research effort to find an alternative means of desulfuriza- tion. Many methods have been explored including bacteria use, microwaves, ultrasound, electricity, adsorption and extraction [5,14– 16]. Recently, the unique solvents called ionic liquids (ILs) have become increasingly popular for these applications and for electrochemical investigations in general [17–19]. ILs are salts which melt at 100 °C or below [20,21] usually composed of a combination of a cation which is organic with an anion which is organic or inorganic [22–24]. Their highly desirable properties which have attracted the attention of researchers include their wide electrochemical windows, thermal stability and low volatility [25]. There are 10 18 ILs can theoretically be synthesized [26]. Furthermore, ILs are highly tunable and can be designed with a specific task in mind; hence the term ‘designer solvents’ is used to describe them [25,27]. 2. Overview on fuels desulfurization technologies The desulfurization of fuels has become an area of great interest to researchers in recent years. This is due to increasingly stringent regulations imposed by regulatory agencies and a decrease in the quality of extracted petroleum due to the depletion of oil reserves. Sweet (low sulfur) crude reservoirs which are most easily accessible have been depleted. The extraction of sour (high sulfur) crudes has become necessary due to increasing demand [28,29]. Crude oil (petroleum) is a brown or black flammable liquid formed from the ancient remains of plants and animals resulting in a complex mixture of different chemical compounds [30]. It comprises of Carbon (85– 89%), Hydrogen (10–13%), Sulfur (0.25–4%) as well as Nitrogen and Oxygen (less than 1% combined) [31], as well as trace amount of metals [32]. Sulfur is naturally found in crude oil in the form of free elemental sulfur or organosulfur compounds. The sulfur compounds that must be removed from fossil fuels can be found in crude oil, or http://dx.doi.org/10.1016/j.rser.2016.11.194 Received 14 October 2015; Received in revised form 29 July 2016; Accepted 12 November 2016 ⁎ Corresponding author. ⁎⁎ Corresponding author at: University of Malaya Centre for Ionic Liquids (UMCiL), University of Malaya, Kuala Lumpur 50603, Malaysia. E-mail addresses: maan_hayyan@yahoo.com, maan.hayyan@gmail.com (M. Hayyan), adeeb.hayyan@yahoo.com (A. Hayyan). Renewable and Sustainable Energy Reviews xx (xxxx) xxxx–xxxx 1364-0321/ © 2016 Elsevier Ltd. All rights reserved. Available online xxxx Please cite this article as: Ibrahim, M.H., Renewable and Sustainable Energy Reviews (2016), http://dx.doi.org/10.1016/j.rser.2016.11.194