Effect of the alkyl side chain of the 1-alkylpiperidinium-based ionic liquids on desulfurization of fuels

doi:10.1016/j.jct.2013.12.029

The Journal of Chemical Thermodynamics

Volume 72, May 2014, Pages 31-36

https://doi.org/10.1016/j.jct.2013.12.029 Get rights and content

Highlights

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Extraction of thiophene and benzothiophene from heptane.
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The ternary (liquid + liquid) equilibria using alkyl-piperidinium-based ILs.
•
High selectivity and solute distribution ratio for the extraction of sulphur compounds form alkanes.
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[PMPIP][NTf₂] was proposed as entrainer for the separation process.

Abstract

In this work the desulfurization ability of three alkyl-piperidinium-based ionic liquids (PIPILs) from heptane, which is used as a model of gasoline and diesel oils, has been developed. With this aim, ternary (liquid + liquid) phase equilibrium data (LLE) have been obtained for mixtures of {PIPIL (1) + thiophene (2) + heptane (3)} at T = 298.15 K and ambient pressure and for the best thiophene entrainer {[PMPIP][NTf₂] (1) + benzothiophene (2) + heptane (3)} at T = 308.15 K and ambient pressure. Three PIPILs have been studied: 1-propyl-1-methylpiperidinium bis{(trifluoromethyl)sulfonyl}imide [PMPIP][NTf₂], 1-butyl-1-methylpiperidinium bis{(trifluoromethyl)sulfonyl}imide [BMPIP][NTf₂] and 1-hexyl-1-methylpiperidinium bis{(trifluoromethyl)sulfonyl}imide, [HMPIP][NTf₂]. The suitability of PIPILs used as solvents for extractive desulfurization has been evaluated in terms of solute distribution ratio and selectivity. Immiscibility was observed in the binary liquid systems of (thiophene, or benzothiophene + heptane) with all PIPILs used. One of proposed PIPILs, [PMPIP][NTf₂] shows high selectivities and high distribution ratios for extraction of sulfur compounds. The data obtained have been correlated with the non-random two liquid NRTL model. The experimental tie-lines and the phase composition in mole fraction in the ternary systems were calculated with an average root mean square deviation (RMSD) of 0.0037.

Graphical abstract

Introduction

Deep desulfurization of diesel fuel has become an important research subject due to the new legislative regulations to reduce sulfur content in the USA and Europe [1], [2]. The emission of sulfur from petrol and diesel oils, which is linked to acid rain phenomena, plays a crucial role in pollution problems of large conglomerates. Due to this situation, the European Union approved a new Directive stating that the content of total sulfur in European gasoline and diesel fuels from 2010 onwards must be at a maximum concentration level of 10 · 10⁻⁶ [2].

Conventional hydro-desulfurization (HDS) processes have been employed by refineries to remove organic sulfur from fuels for several decades. However, to achieve low sulfur targets with current HDS technology, higher temperature, higher pressure, larger reactor volumes, and more active catalysts are needed [3]. Faced with continuing fuel quality challenges, scientists have begun to pay close attention to extraction of sulfur compounds with ionic liquids (ILs), which have the potential for alternative and future complementary technology for deep desulfurization [4], [5], [6], [7], [8], [9]. The hydro-desulfurization processes used in the petroleum industry is specifically not able to decrease the content of benzothiophene, methyldibenzothiophenes, 4,6-dibenzothiophenethiols, thioethers, and disulfides. In order to solve this problem, extractive (liquid + liquid) equilibrium (LLE) desulfurization with ILs has been proposed [4], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20].

Pyrrolidinium-based ILs with different anions [18], and 1-alkylcyanopyridinium-based ILs [19] have been recently studied in our laboratory in ternary LLE (IL + thiophene, or benzothiophene + heptane) with promising results. The best selectivities for extraction of thiophene and benzothiophene, however were obtained with 1-ethyl-3-methylimidazolium thricyanomethanide, [EMIM][TCM] IL [20]. High selectivities for the extraction of sulfur compounds from hydrocarbons with high solute distribution ratios were obtained with tricyanomethanide, [TCM]-, and dicyanomethane-based, [C(CN)₂]- ILs, [7], [18], [20]. Attractive extraction parameters were presented as well as for 1-ethyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [EMIM][NTf₂], [14], 1-ethyl-3-methylimidazolium acetate, [EMIM][OAc] [15], 1-ethyl-3-methylimidazolium thiocyanate, [EMIM][SCN], [4] and 1,3-dimethylimidazolium methylphosphonate [DMIM][MP] [4].

The analysis made by us for activity coefficients at infinite dilution for the selectivity for separation of thiophene from hexane using pyrrolidinium-based IL (n = 3–10) [21], [22] and piperydinium-based ILs (n = 3–6) [23], [24] with different alkane chains and the bis{(trifluoromethyl)sulfonyl}imide anion revealed better results in the latest study.

This work is a continuation of our studies on the extraction abilities of piperidinium-based ionic liquids, PIPILs. We measured the activity coefficients at infinite dilution [23], [24], the (solid + liquid) (SLE) and (liquid + liquid) (LLE) phase equilibrium in binary systems of PIPILs with many solvents [25], [26], [27], as well as the enthalpy of mixing of PIPILs with alcohols [28]. As a result we proposed new Modified UNIFAC interaction parameters for the PIPILs [29]. The experimental and modified UNIFAC (Dortmund) model predicted infinite dilution selectivity and capacity of piperidinium ILs (n = 3–6) in the thiophene/heptane separation problem at T = 328.15 K was shown as very attractive [29]. The selectivities for the separation of aromatic hydrocarbons from aliphatic hydrocarbons obtained for PIPILs are much better than for many other ILs with the [NTf₂]-anion [30].

The solubility of [PMPIP][NTf₂] in water was also recently measured [30], [31], and the density [32], [33], isothermal compressibility [32] and viscosity [33] of different ILs, including the PIPILs.

In this work, we report experimental ternary LLE data for three alkyl-PIPILs in the systems {PIPIL (1) + thiophene (2) + heptane (3)} at T = 298.15 K and for one chosen IL in a system {PIPIL (1) + benzohiophene (2) + heptane (3)} at T = 308.15 K.

The experimental tie lines for the three ternary mixtures of 1-propyl-1-methylpiperidinium bis{(trifluoromethyl)sylfonyl}imide [PMPIP][NTf₂], 1-butyl-1-methylpiperidinium bis{(trifluoromethyl)sylfonyl}imide [BMPIP][NTf₂] and 1-hexyl-1-methylpiperidinium bis{(trifluoromethyl)sylfonyl}imide, {[HMPIP][NTf₂] (1) + thiophene (2) + heptane (3)} and one ternary mixture for {[PMPIP][NTf₂] (1) + benzothiophene (2) + heptane (3)} have been determined at ambient pressure. From the experimental results, the extraction selectivity and the solute distribution ratio were determined and are discussed.

Section snippets

Chemicals and materials

The PIPILs studied: [PMPIP][NTf₂], [BMPIP][NTf₂] and [HMPIP][NTf₂] were purchased from IoLiTec (see table 1) and the solvents from Merck or Sigma Aldrich (the origins of the chemicals, CAS numbers, purity, water content and densities are listed in table 1S in the Supplementary Material). The samples of PIPILs were dried for 24 h at T = 300 K under reduced pressure to remove volatile impurities and trace amounts of water. Thiophene and benzothiophene were stored over freshly activated molecular

Results and discussion

The LLE data for the experimental tie-line ends of the three ternary systems {PIPIL (1) + thiophene (2) + heptane (3)} at T = 298.15 K and ambient pressure are reported in table 3 and for one ternary system {PIPIL (1) + benzothiophene (2) + heptane (3)} at T = 308.15 K and ambient pressure in table 4. Experimental solubilities are reported for the PIPILs [PMPIP][NTf₂], [BMPIP][NTf₂] and [HMPIP][NTf₂] in heptane and thiophene at T = 298.15 K and for [PMPIP][NTf₂] in heptane and benzothiophene at T = 308.15 K. In

Data correlation

The ternary LLE data measured in this study were correlated (the tie-line correlation) using the well-known non-random liquid equation, NRTL [35]. The equations and algorithms used for the calculation of the compositions in both phases followed the method described by Walas [36]. The objective function F(P), was used to minimize the difference between the experimental and calculated compositions: $F (P) = \sum_{i = 1}^{n} {[x_{2}^{I \exp} - x_{2}^{Icalc} (PT)]}^{2} + {[x_{3}^{I \exp} - x_{3}^{Icalc} (PT)]}^{2} + {[x_{2}^{II \exp} - x_{2}^{IIcalc} (PT)]}^{2} + {[x_{3}^{II \exp} - x_{3}^{IIcalc} (PT)]}^{2},$ where P

Conclusions

It has been demonstrated that 1-alkyl-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide ILs are effective for extraction of sulfur compounds from alkanes. In particular, (liquid + liquid) phase equilibrium data were determined in this study for the extraction of thiophene, or benzothiophene from heptane using PIPILs. Four ternary systems {IL + thiophene, or benzothiophene + heptane} were analytically determined using a GC for the composition analysis at temperature T = 298.15 K or T = 308.15 K at ambient

Acknowledgments

This work has been supported by the project of National Science Center in Poland 011/01/B/ST5/00800 and SA-Polish International Scientific Cooperation.

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Citation Excerpt :
As a continuation of our previous works for separating N-compounds from complicated mixtures, also make a review and evaluation of the extraction analysis obtained by different kinds of ILs’ cations and anions, a detailed literature review about separating representative N-compounds were finished. Such as alkyl (ethyl/ butyl/ hexyl) substituted imidazolium-based ILs with Cl, BF4, PF6, OcSO4, H2PO4, C(CN)3, N(CN)2, AlCl4, Cl/ZnCl2, Cl/2ZnCl2, ZnCl2(EtSO4), EtSO4, TOS, NO3, TFA, HSO4, MeSO3 etc. [15–17] anions, different alkyl side chain substituent, [pmpip][NTf2], [bmpip][NTf2], [hmpip][NTf2], piperidinium-based ILs [18], [bmpyr][FAP], [bmpyr][TCB], [bmpyr][TCM] etc. alkyl substituted pyrrolidinium-based ILs [19,20], and there also have some studies by adding the functional groups, EtMe2S, 3-mebupy, 4-mebupy, empy, Qcpy, (CH3CH2)3N(CH2)3SO3H, 1B3M5M2Ppy, S2 in the cations with N(CN)2, SCN, EtSO4, Cl, HSO4, N(CN)2 etc. anions [21], to enhance the extraction selectivity. The effectiveness of the above IL extractants were all verified for the separation process.
Ionic liquids have acted as a hopeful solvent in separation field over the past decade due to its functional groups contributed in separation process. Thus, to test the effect of the phenolate anion on extracting typical heterocyclic N-compound, pyrrole was selected as the representative compounds due to its high content from diverse sources. In this work, the simple structure and easy prepared functional phenolate-based ionic liquid, 1-ethyl-3-methylimidazolium phenolate ([emim][Phe]), is evaluated to separate pyrrole with the polarity analysis by COSMO-SAC model. After understanding the H-bond donor or acceptor relationship of the studied regents, the interaction mechanism between the phenolate based ionic liquid extractant itself and its complex system with pyrrole are visual demonstrated by the RDF, SDF plots etc. analysis and experimental research, which can contribute to understand the H-bonds interaction, hydrogen atom connected to the heteroatoms in pyrrole and oxygen atom in phenolate anion, more indicative. Additionally, the liquid–liquid phase behavior of the methylbenzene + pyrrole + [emim][Phe] system was determined to verify the effect at the perspective of phase equilibrium, the values of D and S was calculated and a highly selectivity for pyrrole could be confirmed. This will lay a solid foundation for developing a new separation technology about the extraction of heterocyclic N-compounds such as pyrrole.

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Effect of the alkyl side chain of the 1-alkylpiperidinium-based ionic liquids on desulfurization of fuels

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Chemicals and materials

Results and discussion

Data correlation

Conclusions

Acknowledgments

Appl. Catal. A: Gen.

Sep. Purif. Technol.

Fluid Phase Equilib.

Fluid Phase Equilib.

J. Chem. Thermodyn.

Fluid Phase Equilib.

J. Chem. Thermodyn.

Fluid Phase Equilib.

J. Chem. Thermodyn.

Fluid Phase Equilib.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

Fluid Phase Equilib.

Fluid Phase Equilib.

Fluid Phase Equilib.

J. Chem. Thermodyn.