Deep catalytic oxidative desulfurization of fuels by novel Lewis acidic ionic liquids
Introduction
Nowadays, SOx produced by combustion of sulfur-contained fuels in transportation and industry is the main cause of air pollution. As a response, worldwide environmental regulations towards transportation fuels have been increasingly strict, and many countries adopted standards to cut the S-content down to 10 ppm, which put forth a critical challenge to the refinery industry [1]. At present, the traditional hydrodesulfurization (HDS) is widely used for the deep desulfurization of fuels in petroleum processing industry. HDS is efficient for the removal of thiols, disulfides and thiols but less effective for that of heterocyclic S-compounds, such as benzothiophene (BT), dibenzothiophene (DBT) and their derivatives. Meanwhile, the energy consumption of HDS (350–400 °C, 30–100 bar hydrogen pressure) is ultra-high [[2], [3], [4]].
So far, alternative desulfurization methods have been wildly investigated, and the corresponding technologies such as extractive desulfurization (EDS) [[5], [6], [7], [8]], oxidative desulfurization (ODS) [[9], [10], [11], [12], [13], [14], [15]] and adsorptive desulfurization (ADS) [[16], [17], [18], [19], [20], [21]] are developed to approach deep desulfurization. Among these studies, ODS is regarded as one of the most promising processes due to its high desulfurization efficiency for refractory sulfides under mild conditions. In a typical ODS process, aromatic sulfides can be firstly oxidized to their corresponding sulfones, and then removed by the extraction [22,23]. H2O2 is the commonly employed oxidant due to its high reactivity, environmental compatibility and low price [24,25]. However, volatile and flammable organic solvents are usually utilized as the extractant, which may cause further environmental and security problems.
Ionic liquids (ILs), a class of new emerging green solvents, own exclusive properties, such as low melting point, negligible vapor pressure, excellent thermally stability and especially adjustable structure by changing components of cations and anions [26,27]. In recent research, some studies have reported that the extractive combined with catalytic oxidative desulfurization (ECODS) strategy, which could achieve ultra-deep desulfurization of model fuels with ILs as both the catalysts and extractants [[28], [29], [30], [31], [32]]. Various polyoxometalate-based ILs [11,13,28,29,33], Brønsted acidic ILs [[34], [35], [36]] and Lewis acidic ILs [27,[37], [38], [39], [40], [41]] have been explored, and led to good results. For instance, Li et al. prepared Lewis acidic IL C5H9NO·0.3FeCl3 by reacting N-methyl-pyrrolidone with anhydrous FeCl3 and employed it in ODS. (2015) [42]. Mokhtarani et al. (2016) synthesized Brønsted acidic ILs 1-octyl-3-methylimidazolium hydrogen sulfate ([Omim][HSO4]) for oxidative desulfurization [30]. Jiang and co-workers (2017) proposed a polyoxometalate-based ionic liquid (POM-IL) [N-(3-sulfonatepropyl)-pyridinium]3PMo12O40 for ODS process in the solvent-free system [28].
In addition, in order to evaluate the ILs performances for practical industrial application, many researchers have applied the ECODS process to real fuel. Gao et al. (2007) reported that the Brønsted ionic liquids [Hmim]BF4 could perform well in ODS of a commercial diesel. The S-content of the real diesel could be reduced from 1000 to 270 ppm (reducing 73.0%) under the condition of T = 90 °C, ILs/oil = 1.6/1 and H2O2/S = 40/1 [43]. Gao et al. (2010) also utilized [Bmim][HSO4] to the desulfurization of a diesel with initial S-content of 97 ppm. The sulfur removal was obtained as 85.8% in 2 h at room temperature with ILs/oil = 1/1 and H2O2/S = 5/1 [44]. Zhang and co-workers (2014) used Lewis acid IL [C6mim]Cl/2FeCl3 for the desulfurization with a gasoline. The S-content of the gasoline reduced from 440.3 ppmw to 62.4 ppmw under the conditions of ILs/oil = 1/1 at 25 °C [12]. Yu et al. (2015) synthesized 1-methyl-2-pyrrolidonium zinc chloride ([Hnmp]Cl/ZnCl2), a kind of Brønsted-Lewis acidic ILs, which was utilized in oxidative desulfurization of a FCC diesel fuel with the S-content of 224.6 ppm. The sulfur removal could reach 83.3% after a five-stage process at 75 °C [38]. A kind of Brønsted acid IL, caprolactamium trifluoracetate, was synthesized by Sun et al. (2016) and used in ODS of a hydrogenated diesel with 659.7 ppm. The sulfur removal was 98.7% after two-stage ECODS process under the conditions of T = 40 °C, ILs/oil = 1/1, H2O2/S = 6/1 [45].
However, among the above studies, some shortcomings still exist and restrict the application of functionalized ILs in a large scale. Some expensive raw materials were used, which significantly increased the cost of the process. Some ILs did not possess good desulfurization performances, thus resulting in large usage of oxidant and multi-step ODS procedure. Poor stability of some protic or Brønsted acid ILs further led to the poor reusability [46]. Therefore, the development of stable, low-cost and high-efficient ILs to overcome such shortcomings is of great importance for further industrial application.
In this work, novel Lewis acidic ionic liquids [ODBU]Cl/nZnCl2 (n = 1, 2, 3, 4 and 5) (Scheme 1) with alkylated 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) cation and ZnCl2-based complex anion were developed via a simple two-step synthesis. The structures were confirmed by using various characterization techniques, including 1H nuclear magnetic resonance (1H NMR), fourier transform infrared spectroscopy (FT-IR) and mass spectrometry (MS). Then, the ILs were employed as catalysts in ODS of model oil with H2O2 as the oxidant, and the optimum composition of ILs was determined. Some important reaction parameters such as reaction temperature, molar ratio of O/S, mass ratio of IL/oil, sulfide species were systematically investigated. The stability as well as recyclability of the ILs were evaluated by repeating experiment as well as characterizations. Finally, a hydrogenated diesel was employed to actually investigate its sulfur removal performance.
Section snippets
Chemicals and materials
Dibenzothiophene (DBT, 98%), benzothiophene (BT, 98%), 4,6-dimethyldibenzothiophene (4,6-DMDBT, 98%), hydrogen peroxide (30 wt%) and zinc chloride (98%) were purchased from Aladdin Reagent Co., Ltd. n-Octane (99%), ethyl acetate (99%) and 1-chlorooctane (99%) were purchased from Tianjin Jiangtian Chemical Technology Co., Ltd. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) was purchased from Heowns Biochemical Technology Co., Ltd. Hydrogenated diesel was provided by SINOPEC Tianjin. Petrochemical Co.,
Effect of ZnCl2 on desulfurization
In order to evaluate the desulfurization performance of [ODBU]Cl/nZnCl2 (n = 1, 2, 3, 4 and 5) with different proportion of ZnCl2, the ODS experiments of model oil were conducted and DBT was chosen as a typical refractory S-compounds. As shown in Table 1, with the increasing of ZnCl2 proportion, the EDS performance was firstly enhanced and then basically unchanged, while similar trend could also be observed for the ODS efficiency. When the molar ratio of [ODBU]Cl to ZnCl2 was 1:3, the IL
Conclusion
In this work, a series of novel Lewis acidic ILs [ODBU]Cl/nZnCl2 (n = 1, 2, 3, 4 and 5) were synthesized and utilized in the ECODS of both model oil and diesel fuel, with hydrogen peroxide (H2O2, 30 wt%) as the oxidant. The superbase-derived ILs can be simply synthesized with inexpensive raw materials and possess excellent thermal stability. When the mole ratio of [ODBU]Cl to ZnCl2 was 1:3, the optimal desulfurization performance could be obtained by achieving complete sulfur removal under
Acknowledgement
We are grateful for the financial support from National Key R&D Program of China (No. 2016YFC0400406 and 2017YFB06022702-02).
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