Deep Desulfurization of Fuel Diesels Using Alkyl Sulfate and Nitrate Containing Imidazolium as Ionic Liquids

Journal of Applied Chemical Research, 7, 1, 75-85 (2013)
Deep Desulfurization of Fuel Diesels Using Alkyl Sulfate and Nitrate Containing Imidazolium as Ionic Liquids
Ali Mehdizadeh, Amir N. Ahmadi* , Fakhrosadat Fateminassab
Chemistry and Petrochemical Research Division, Research Institute of Petroleum Industry, Tehran, Iran.
Abstract
1-ethyl-3-methylimidazolium ethyl sulfate [Etmim][EtSO4],1, 3-dimethylimidazolium methyl sulfate [Mmim][MeSO4] and 1-octyl-3-methylimidazolium nitrate [Omim][NO3] ionic liquids have been used in thermal oxidative desulfurization of model fuel oils consisting of 500 ppm solutions of benzothiophene (BT) and dibenzothiophene (DBT) in dodecane. The efficiency of sulfur removal from BT and DBT solutions by these ILs as well as the effect of anion and cation chain length have also been investigated. The ILs studied performed pretty efficiently in removal of sulfur containing compounds and sulfur contents have been reduced to 18-27% following the order below: [Mmim][MeSO4] > [Etmim][EtSO4] > [Omim][NO3]. To further study the performance of the ILs studied, multiple extractions of BT and DBT solutions were also carried out to reduce sulfur content to 81% and 68% for DBT and BT solutions, respectively, after 5 extraction cycles. These environmentally friendly ILs could also be quantitatively regenerated and were shown to be as efficient in sulfur removal after 5 regeneration cycles.
Introduction
Over the last three decades, sulfur removal from diesel fuel has attracted much attention due to major air pollution and acid rain resulting from sulfur compounds, which are present in such fuels. In the oil refinery industry, hydrodesulfurization (HDS) has long been the method of choice for removal of acyclic and cyclic sulfur compounds from diesel fuel. This process, which employs mixed catalysts such as Co-Mo, converts sulfur containing compounds to H2S [1]. A major drawback associated with HDS method, which leads to very low sulfur contents, is the high expense of the process due to harsh reaction conditions including high hydrogen pressure, reaction temperature and the necessity of application of a very highly active catalyst.
In addition, some sulfur containing compounds in fuels such as benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dialkyl substituted DBTs are difficult to convert into H2S on the Co-Mo or Ni-Mo catalyst surfaces because of the steric hindrance associated with them [2,3]. Thus, alternative methods for deep desulfurization of diesel oil are becoming attractive. Organic compounds with steric hindrance such as BT, DBT and their derivatives can be selectively oxidized to their corresponding sulfoxides and sulfones, which are then removed by adsorption on the catalyst surface through a competitive technology known as oxidative desulfurization (ODS) process [4]. Taking advantage of the polar nature of these oxidation products, the oxidation process is then followed by extraction with polar solvents such as dimethyl sulfoxide in the second step [5].
In order to improve the efficiency of HDS and ODS technologies, research has been focused on better reactor design and new catalysts based on noble metals have been developed for this purpose. These modifications yield diesel fuels of low sulfur contents (<100 ppm) and are therefore applicable for deep desulfurization [6]. Although ODS is an effective method in this regard, large quantities of flammable and volatile organic compounds (VOCs) are required in the process.
On the other hand, there are other more competitive deep desulfurization techniques such as extraction desulfurization process (EDS). The latter process does not involve hydrogen consumption, catalyst, high temperature or pressure and requires mild and simple operation conditions [6,7]. In addition, sulfur compounds are not chemically altered during this process and can thus be reused as raw materials. Though still not an industrially established process for deep desulfurization, the attractive features of EDS make it worthwhile to consider alternative solvents to use in this process. Examples for solvents employed in EDS process, all of which have proved to perform undesirably, include polyalkylene glycol, imidazolidinone, pyrimidinone and DMSO [8].
Room Temperature Ionic Liquids (RTILs) can be thought of as alternative solvents for conventional EDS solvents since they are nonvolatile, non-explosive, recyclable, easy to handle, thermally and hydrothermally stable, non-moisture sensitive, and most importantly polar in nature, which makes them good candidates for extraction of polar, oxidized sulfur containing compounds [8]. In fact, many imidazolium based ILs containing BF4, PF6, and Cl anions have been investigated in EDS [9]. Some ILs previously used in EDS are given in Table 1
However, some of these ILs exhibit some undesirable characteristics including release of corrosive hydrofluoric acid (HF) caused by decomposition of fluorinated anions (BF4 and PF6) and the non-moisture stability and high viscosities of all these ILs make their handling difficult. Furthermore, the efficiency of sulfur removal using ILs is generally rather low, in the range of 10-30% because of the similar polarities of competing alkenes, aromatic and sulfur compounds present in the fuels [8]. We herein report application of other nonfluorinated, low viscosity ILs, 1-octyl- 3-methylimidazolium nitrate [Omim] [NO3], 1-ethyl-3-methylimidazolium ethyl sulfate [Etmim] [EtSO4], and 1,3-dimethylimidazolium methyl sulfate [Mmim][MeSO4], prepared from relatively cheap starting materials, in the oxidative desulfurization of BT and DBT solutions in dodecane as model fuel oils. The desirable characteristics of these ILs such as their negligible miscibility with fuels, high affinity for sulfur containing compounds, and ease of regeneration make them potential alternative solvents in EDS process.
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