Jianwei Li,Xiang Li,Yan Liu,Jie Zhang*

State Key Laboratory of Chemical Resource Engineering,Beijing University of Chemical Technology,Beijing 100029,China

1.Introduction

Recently,worldwide environmental policies have gradually decreased the allowed limits of sulfur content in the light oils in order to reduce atmospheric pollution and derived effects such as acid rain[1,2].Therefore,it is an important process to remove sulfur compounds from the light oil.Among these sulfur compounds that exist in the light oil,the mercaptan is toxic,malodorous and corrosive contributing to damage chemical instruments and pipelines and may cause the deactivation of catalyst during the reactions[3–5].The most widely used method for removing the mercaptan present in the light oil and natural gas is Merox technology.The classic Merox technology uses caustic solution to extract mercaptans from the light oil by the formation of mercaptides,and followed by the conversion to disul fides by the oxidization in order to accelerate the mass transfer of mercaptans from oil phase to the extractant[6–9].Owing to the limitation of the mass transfer of mercaptans from the oil phase to the lye phase and the long chains of mercaptans with the lower solubility,many researchers added conventional organic solvents into the lye phase to improve the mass transfer by the increase of solubility of mercaptans[10,11].However,these traditional organic solvents have mainly two drawbacks:(i)the poor selectivity as traditional organic solvents can extract plenty of ole fins as well as removing sulfur compounds,which causes the loss of oil quality;(ii)the safety risks,including toxicity,flammability and volatility[12,13].

Ionic liquids have attracted great attention on the application in biocatalysis,catalysis,electrochemistry,and polymerizations as safe solvents in the last years[14–16].Because of the higher thermal/chemical stabilities and higher density than organic solvents,ionic liquids have been employed to dissolve and extract inorganic or organic compounds[17–20].Considering the imidazolium-based ionic liquids are advantageous as they are easy to be manufactured in a commercial scale with very high yield[21–23],we added the imidazolium-based ionic liquids into NaOH(aq)in the extraction process of mercaptans instead of traditional organic solvents to accelerate the mass transfer in this study.We also investigated the effect of the ionic liquid species and cation alkyl chain length imidazolium-based ionic liquids on performance of mercaptan removal.

2.Experimental

2.1.Chemicals

The chemical materials used were purchased from different companies separately.1-Methyl imidazole(99 wt%),ethyl bromide(98 wt%),1-butyl bromide(99 wt%),1-hexyl bromide(99 wt%)and 1-octyl bromide(98 wt%)from Shanghai Aladdin Bio-Chem Technology Co.,Ltd.,KOH(AR),NaOH(AR),ethyl acetate(99.5 wt%),isopropyl alcohol(99.5 wt%)and ammonium fluoroborate from Tianjin Guangfu Fine Chemical Research Institute,1-hexene(97 wt%)and 1-butyl mercaptan(97.5 wt%)from Lark Technology Co.,Ltd.,methylbenzene(99.5 wt%)and acetone(99.5 wt%)from Beijing Chemical Works,andn-hexane from Sinopharm Chemical Reagent Co.,Ltd.All reagents were used as received.

2.2.Preparation of ionic liquids

The syntheses of imidazolium-based ionic liquids were prepared by two-step method according to the previous studies[24–26].In a typical experiment,1-methylimidazole(0.44 mol)was added into the solution of methyl benzene(50 ml)in a flask with three necks,followed by the slow addition of 1-butyl bromide(0.4 mol)into the flask.The mixture was stirred magnetically for 48 h at 70°C,in order to achieve a perfect homogenization of the product.The process was kept under a dry nitrogen atmosphere in the flask with three necks to prevent raw materials from becoming oxidized.The product was distilled off under reduced pressure,then we got an intermediate product—[BMIM][Br].Then,ammonium fluoroborate,potassium hydroxide and potassium acetate were added into[BMIM][Br],respectively,and stirred magnetically for 24 h atroom temperature.The final products,[BMIM][BF4],[BMIM][OH]and[BMIM][Ac]were obtained after the removal of water and volatile impurities under vacuum at 80°C for 48 h.And the other ionic liquids of different lengths of cation,such as[EMIM],[HMIM]and[OMIM],we just change 1-butyl bromide into ethyl bromide,1-hexyl bromide and 1-octyl bromide,respectively.The prepared ionic liquids were stored in a dry inert atmosphere.

2.3.Desulfurization experiments

The extractions of 1-butyl mercaptan from mixtures ofn-hexane and 1-hexene were used as a model for the desulfurization process.The model oil used is a binary mixture ofn-hexane and 1-hexene,with 1-butylmercaptan being 200×10?6.Since 1-butylmercaptan is volatile,extraction experiments were carried out in a sealed bottle with a volume of approximately 200 ml,which is equipped with a magnetic agitator and a glass thermometer.For each experiment,a certain amount of different ionic liquids and a fixed volume of NaOH(aq,10%)at 30 ml were added directly into the model oil under magnetic stirring at a room temperature in the sealed bottle.The extraction experiments were generally controlled within 10 min.Then,the system was allowed to settle for about 2 h and the samples were collected for analysis.

2.4.Analysis method

The S-contents with respect to 1-butyl mercaptan in the oil phase were measured by a gas chromatography detector(GC-4000A,East&West Analytical Instruments Ltd.,China).The temperatures of detector,injector and column oven were 200 °C,150 °C and 60 °C,respectively.For each sample,the analysis was repeated five times to obtain the average 1-butyl mercaptan-content.The maximum analysis errors of S-content were controlled within 1%.

The characterizations of ionic liquids were investigated by Fourier transform infrared(FT-IR)at room temperature on a Bruker Tensor 27 FT-IR spectrometer with a MCT detector,using the method ofKBr pellet.The results are shown in Fig.1.

The desulfurization efficiency(DE)was defined as in Eq.(1).

whereC0is the initialsulfur concentration in the model oil,Cfis the final sulfur concentration in the model oil after extraction experiment.

Fig.1.FT-IR spectra of prepared ionic liquids.

3.Results and Discussion

3.1.Characterization of ionic liquids

FT-IR spectra of different prepared ionic liquids were characterized and the results are shown in Fig.1.It is found that most of the characteristic bands of imidazolium-based ionic liquids give clear evidence that products have been synthesized.A broad peak in the range of 3200–2800 cm?1is due to the stretching vibrations of C--H from the imidazole ring.Both the samples of the ionic liquids exhibited a band at 1600–1450 cm?1,which is attributed to the stretching vibrations of C=N on the imidazole ring.It can also be found that there is a band at 1160–1170 cm?1,which is attributed to the stretching vibrations of imidazole ring.Whereas,the characteristic bands at 700–550 cm?1,originate from the swing bending vibrations of C--H on the imidazole ring[27–29].

Besides,the B--F stretching bands of BF4?group are at 1018 cm?1(see Fig.1(b)),the O--H stretching bands of OH-group exhibited a band at 3405 cm?1(see Fig.1(c)),and peak at wave number 1600 cm?1is due to C=O stretching vibration(see Fig.1(d)).

3.2.The effect of anion of ionic liquids on 1-butyl mercaptan removal

The results of extractive desulfurization performance are shown in Fig.1.When NaOH(aq)was added into the model oil,the desulfurization rate can be reached to 86.95%.However,ionic liquids and NaOH(aq)were puttogether into the model oil to remove 1-butyl mercaptan,the extraction efficiency had been significantly improved.The desulfurization rate is 93.2%,98.3%,98.4%and 100%for[BMIM][BF4]-NaOH,[BMIN][Br]-NaOH,[BMIM][OH]-NaOH,and[BMIM][Ac]-NaOH,respectively.Owing to the good solubility of1-butylmercaptan in ionic liquids,adding them has a great enhancement on the mass transfer of 1-butyl mercaptan from the oil phase to the lye phase,which improved the extraction ability[30].

Among these ionic liquid–NaOH,[BMIM][Ac]-NaOH exhibits the best 1-butyl mercaptan-removal performance,the final desulfurization rate of which is almost100%.As shown in Fig.2,the desulfurization rate follows[BF4]?＜ [Br]?≈ [OH]?＜ [Ac]?.The ionic liquids with basic anion functional groups as extractants in the extraction experiment have better desulfurization efficiency than the ordinary ones.The probable reason for this result may be that 1-butyl mercaptan is a weak acid,which can release the hydrion,while[OH]?and[Ac]?are both alkalescent anions,which can form stronger affinity than the other anions.The more alkaline the ionic liquids are,the more the extracting power they have.So,the anions of ionic liquids have a great influence on the extraction efficiency.

Fig.2.Desulfuration efficiency of ionic liquid–NaOH for 1-butyl mercaptan.

3.3.The effect of cation alkyl chain length

The effects of alkyl chain length of imidazolium cation([CnMIM]+,withn=2,4,6 and 8)are illustrated in Fig.3.For ionic liquids based on[BF4]?,[Br]?and[OH]?,the desulfurization rate for 1-butylmercaptan follows the trend:[C2MIM]+＜[C4MIM]+＜[C6MIM]+＜[C8MIM]+.The effects of cation alkyl chain length are similar to those of the cation family and can be explained in this way.With the cation alkyl chain length increase,the symmetry of imidazolium-based ionic liquids reduce,and it can increase the polarity of the ionic liquid and enhance the interaction force between 1-butyl mercaptan and ionic liquids,which improve the desulfurization rate of 1-butyl mercaptan.As for Ac?-based ionic liquids,the cation alkyl chain length has little effect on the desulfurization rate in our study.Different from the in fluence of cations,the effect of anions is greater.

Fig.3.Desulfurization rate of[C n MIM]+based ionic liquids for 1-butyl mercaptan at 25°C.

3.4.Extractive equilibrium time

Extractive equilibrium time is another very important aspect of the extraction process because a short time favors high product yield in the practical application.As shown in Fig.4,the extractive equilibrium time of desulfurization by adding NaOH(aq)and ionic liquids together into the model oil is less than by adding NaOH(aq)alone.Among these four ionic liquids,the extraction process of[BMIM][Br]-NaOH,[BMIM][OH]-NaOH and[BMIM][Ac]-NaOH can get extractive equilibrium in 1 min.However,the extraction process of[BMIM][BF4]needs 8 min to reach desulfurization equilibrium.

Fig.4.1-Butyl mercaptan-content vs.extraction time for the desulfuration by NaOH+ionic liquids.

3.5.Ionic liquid–oil volume ratio

The ionic liquid–oil volume ratio is an operation condition for the desulfurization results.Take the cost of ionic liquids into consideration,it is preferred that using minimum quantity can get better extraction efficiency.However,most of literature show that the sulfur extraction efficiency decreases with a decreasing ionic liquid–oil volume ratio.In this experiment,the volume of NaOH(aq)was fixed at 10 ml,and the volume ratio of model oil and ionic liquids were varied.It was found that the most effective ionic liquid—[OMIM][Ac].The results are shown in Fig.5.As the oil–ionic liquid volume ratio decreases,the contents of1-butylmercaptan oilin the modeloilreduce.However,according to the national regulations,S-contents should not be more than 10×10?6.Owing to expensive prices of ionic liquids,we should take process cost into account.So the best ratio of desulfurization experiment isV(oil):V([HMIM][Ac])=40:1.

Fig.5.Comparison of optimizing experiments under different volume ratios.

4.Conclusions

In this work,imidazolium-based ionic liquids were applied in the extraction process ofmercaptan removalinstead ofconventionalorganic solvents,and the results have proven that the imidazolium-based ionic liquids possess excellent extraction performance of mercaptans.Owing to the better dispersion of ionic liquids in light oils,adding them has a great enhancement on the mass transfer of 1-butyl mercaptan from the oilphase to the lye phase.100%desulfurization rate of1-butylmercaptan was achieved by the anion of[Ac]?ionic liquids at a volume ratio of 40:1:1(V(oil):V(NaOH):V(ionic liquid))and a short equilibrium time within 10 min.Meanwhile,the anion species of ionic liquids on the efficiency of extraction for mercaptans exhibited the order of[Ac]?＞[OH]?≈ [Br]?＞ [BF4]?.Moreover,with the increase of alkyl chain,the desulfurization rate of 1-butyl mercaptan increased.Therefore,it is possible to tune the extractability and the selectivity of ionic liquids on the extraction of mercaptans by a proper choice of the ionic liquid cation and anion.

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