Desulfurization of gasoline by condensation of thiophenes with formaldehyde in a biphasic system using aqueous phase of acids☆

Miao He,Yingxia Li*,Jie Zhang,Biaohua Chen

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

1.Introduction

Sulfur that is present in transportation fuels leads to sulfur oxide(SOx)emissions into the air and inhibits the performance of pollution control equipment on vehicles[1–4].Therefore,to minimize the negative health and environmental effects from automobile exhaust,many countries recently have mandated a reduction in the sulfur content in motor fuel and increasingly severe regulations are being imposed to reduce the S-content to a very low level such as(10–20)× 10?6[5].Consequently,the deep desulfurization of motor fuels has attracted increased attention in the research community worldwide.

In the petroleum industry,low-sulfur fuels are often obtained from hydrocracking processes or hydrotreating processes[6].Although hydrotreating processes have been highly effective for the reduction of sulfur levels,further improvement of the hydrodesulfurization efficiency is limited to increasingly severe operational conditions at escalated cost.Moreover,when the deep hydrodesulfurization of motor fuels is needed,not only the energy and hydrogen consumption will be evidently increased,but undesired side reactions(such as the saturation of more ole fins)will be induced.Such side reactions result in a decrease in the octane number of the gasoline.Therefore,in the past two decades,such as ole finic alkylation of thiophenic sulfur[7–9],extraction[10–12],oxidation[13–15],precipitation[16]and adsorption[17–19]have been investigated extensively,among which alkylation has drawn wide attention.In the alkylation desulfurization,the thiophenic compounds were converted to the compounds with higher boiling pointviaalkylation with ole fins and then removed from the light fractions by means of fractional distillation conveniently[20].However,the alkylation of thiophenic compounds with ole fins is catalyzed by strong acids such as AlCl3and BF3.Under the catalysis of these strong acids,the side reactions—alkylation of aromatic hydrocarbons and ole fin polymerization in the gasoline can limit the efficiency of the alkylation desulfurization and lead to obviously decrease of gasoline yield[1].

There is another approach to shift the boiling points of thiophenic compounds to higher value.In this approach,the condensation of thiophenes with formaldehyde is employed.The typicalreaction of condensation of thiophene with formaldehyde is described in the following scheme(Fig.1).In the previous process,the condensation carried out in the presence of AlCl3or H2SO4also leads to significant aromatic hydrocarbons and ole fin losses and much acid sludge[21].Since the reactivity of formaldehyde is higher than that of ole fin,we anticipate that the desulfurizationviacondensation of thiophenes with formaldehyde could be catalyzed by the catalysts with much lower acidity than AlCl3or concentrated H2SO4.The low acidity of the catalyst could obviously reduce the loss of gasoline.

In this work,we studied the desulfurization of the model gasoline by condensation reaction catalyzed by liquid organic acids and heteropolyacids.The desulfurization performances of these catalysts were evaluated and the effects of the operating variables were investigated.The performances of the recycled catalysts were studied and the possible integration of the condensation desulfurization into the existing refinery structure was brie fly discussed.

Fig.1.Condensation of thiophene with formaldehyde.

2.Experimental

2.1.Materials

2,2,4-Trimethylpentane(99%),n-heptane(99%),cyclohexane(99%),toluene(99.5%),paraxylene(99%),orthoxylene(99%),1-hexene(98%),1-octene(98%),thiophene(99%),paraformaldehyde(99%),formic acid(88 wt%in water)and acetic acid(99.5%)were purchased from Beijing Chemical Reagents Company,China.2-Methylthiophene(98%)and 3-methylthiophene(97%)were obtained from Acros Organics,China.All the regents mentioned above were used without further purification.Heteropolyacid of H3PW12O40in the solid state was obtained from Beijing Jiayousheng New Technology Development Center,China.The aqueous solutions of H3PW12O40with molar concentrations of 0.8 mol·L?1and 0.1 mol·L?1were prepared.

Two kinds of model gasoline were prepared for the desulfurization experiments.The model gasoline of No.1 was made by simply adding thiophene inton-heptane with a sulfur content of 600× 10?6.The other model gasoline(No.2)contained 0.1 wt%of thiophene,0.05 wt%2-methylthiophene and 0.05 wt%3-methylthiophene,corresponding to sulfur contents of 381×10?6,163×10?6and 163×10?6,respectively.No.2 model gasoline also contained 10.0%aromatic hydrocarbons and 10.0%ole fin hydrocarbons.The detailed composition of the fuel is listed in Table 1.

Table 1Composition of No.2 model gasoline

2.2.Experimental procedure

The general procedure in the batch operation was to charge 50 g(about 57 ml)the prepared model gasoline into a custom-designed 100 ml stainless autoclave with a temperature controller.Following the additions of1 g paraformaldehyde and 10 g acid catalyst,the system was heated to a certain temperature,and then was vigorously stirred with a magnetic stirrer for a certain time.After that time,the system was cooled to room temperature and then the product was discharged out.The aqueous phase was separated from the product in a separatory funnel.The oil phase was distilled to obtain the fraction of the boiling range of 35–205 °C.The residue of the distillation was then dissolved in acetone and collected for analysis.

2.3.Analysis and characterization

The sulfur content in the oil samples was determined by using a Varian CP 3800 gas chromatograph with a sulfur selective pulsed flame photometric detector(PFPD)with a 0.25 mm,50 m PONA capillary column.A CP 3800 equipped with a hydrogen flame ionization detector with a same capillary column mentioned above analyzed the model oil composition.A Shimadzu QP5000 gas chromatograph-mass spectrometer(with an ion source temperature of 180°C,in the range 40–400 amu,with a scan rate of 1.4 s?1)was used to analyze the products of condensation.

3.Results and Discussion

3.1.Sulfur removal in the presence of thiophene

The No.1 model oil was treated to remove sulfur compound in the presence of different acids in order to select a suitable catalyst for condensation desulfurization.Formic acid,acetic acid and two aqueous solutions of H3PW12O40were employed as the catalysts.In these experiments,the treating temperature was 90°C and the treating time was 4 h.The typical results are listed in Table 2.It can be seen that formic acid showed good performance for removal of sulfur from the model oil.After treated by formic acid,92%sulfur in the No.1 model gasoline was removed,whereas the sulfur removal rates for acetic acid were only 22%.By means of the qualitative analysis of a mass spectrometer,it was found that the main component in the residue of distillation was di-thienylmethane whose boiling point(＞220 °C at atmospheric pressure)was out of the distillation range of gasoline.These results revealed that the thiophenes were converted to the sulfur compound with higher boiling point and were separated from the modeloilreadily by fractional distillation.The sulfur removal rate for aqueous solutions of H3PW12O40(0.8 mol·L?1)was also above 90%.However,the sulfur removal performance of aqueous solutions of H3PW12O40declined obviously with the decrease of the concentration.Similarly,dithienylmethane was found in the residue of distillation in the case of aqueous solutions of H3PW12O40as the catalyst.

Table 2Sulfur removal from the No.1 model gasoline with different catalysts

In the condensation,the acidity of the catalyst is one of the key factors.Owing to the electron-donating effect of methyl,the acidity of formic acid is stronger than acetic acid,resulting that the thiophene conversion in condensation catalyzed by formic acid was higher than that by acetic acid.H3PW12O40showed high activity in the condensation of benzene with formaldehyde[22],therefore,the good sulfur removal performance in the treatment of the gasolineviathe similar reaction was not surprising.

3.2.Sulfur removal in the presence of aromatics and ole fi ns

In the desulfurization processviaalkylation with ole fin,the competing reactions such as alkylation of aromatic hydrocarbons and polymerization of ole fins might limit the efficiency the process[1].Thus,the effect of these hydrocarbons also should be investigated in the desulfurization processviacondensation of thiophenes with formaldehyde.In these experiments,the treating temperature was 90°C and the treating time was 4 h.The typical results are listed in Table 3.In sulfur removal treatment of the No.2 model gasoline,formic acid and aqueous solution of H3PW12O40(0.8 mol·L?1)still exhibited high desulfurization rates of 90%and 93%.After distillation,nearly a quantitative yield of 100%of the model gasoline was obtained.Neither detectable products of the condensation of aromatic hydrocarbon with formaldehyde nor the products of polymerization of ole fins were found in the residue of distillation.Thus,a conclusion can be drawn that the side reactions such as the condensation of aromatic hydrocarbon with formaldehyde and polymerization of ole fins under the condition of these experiments were negligible.

Table 3Sulfur removal from the No.2 model gasoline with different catalysts

In the presence of water,the polymerization of ole fins was almost inhibited completely[23].The other competing reaction,i.e.the condensation of aromatic hydrocarbon with formaldehyde,was more important in this study.The acidity of formic acid was not strong enough to catalyze the condensation of aromatic hydrocarbon with formaldehyde[24].However,owing to the presence of sulfur atom,the π-cloud of thiophene is stronger than that of benzene,indicating that the reactivity of thiophene for the reaction of electrophilic addition is higher than that of benzene.Thus,thiophene conversion on condensation with formaldehyde under the catalysis of formic acid was high.The condensation of aromatic hydrocarbon with formaldehyde catalyzed by the aqueous solution of H3PW12O40was performed at 140 or 160°C[22].In this study,the condensation of aromatic hydrocarbon with formaldehyde was inhibited remarkably at relative low temperature.

3.3.The effect of the operating variables on the desulfurization rates

3.3.1.Reaction temperature

The sulfur removal performances of formic acid and aqueous solution of H3PW12O40(0.8 mol·L?1)on treatment of No.2 model oil at different temperatures were listed in Table 4.The treating time was 4 h.It can be seen that,for the two catalysts,the sulfur removal rates increased with the increase of the treating temperature.When the treating temperature increased from 50 °C to 75 °C,the increase of desulfurization was obvious,whereas in the range of 75 °C to 105 °C,a little improvement of sulfur removal was observed.Moreover,at the treating temperature of 105°C,the condensation of aromatic hydrocarbon with formaldehyde present with the yield of the model oil was 97%.Dimethyl diphenylmethane and its alkyl derivatives were detected in the residue of distillation.

3.3.2.Treating time

The effect of treating time on sulfur removal rates was also investigated.The treatments were performed under the temperature of 90°C.As shown in Table 5,with the treating time of 1 to 4 h,the rates of sulfur removal increased with the increase of the treating time.However,the sulfur removal rates in the cases of treating time at 5 and 6 h were not improved in comparison with those at 4 h,indicating that the catalytic system achieved equilibrium at a treating time of about 4 h.Thus,the 4 h was the proper treating time for the sulfur removal from the aromatic hydrocarbons.

3.3.3.Recyclability of catalysts

The sulfur removal performances of the recycled catalysts were summarized in Table 6.The treating temperature was 90°C and treating time was 4 h.It can be seen that,when the treatment was repeated using recycled catalysts,the sulfur removal rates were almost maintained at a constant level.After the fifth treatment,the sulfur removal rate was slight lower than that of the fresh one.The decrease of the sulfur removal rate might be due to the slight loss of the catalysts.

Table 4Sulfur removal under different temperatures

Table 5Sulfur removal under different treating times

Table 6Sulfur removal with the recycled catalysts

Fig.2.Integration of desulfurization of gasoline by condensation with formaldehyde in a biphasic system.1,2:condensation reactor;3,4:oil/acid separator;5:distillator.

3.4.Integration into existing re fi nery process

Fig.2 shows the possible process for the integration of condensation desulfurization into the existing refinery process for the production of gasoline with low sulfur content.In the condensation desulfurization unit,two stirred reactors were installed by means of parallel connection.The sulfur-rich gasoline was treated in the reactor under catalysis of the liquid catalyst.The treatment should be switched between the two reactors in order to assure the continuous operation.A proper temperature(e.g.about 90°C)and treating time(e.g.4 h)were necessary.After the treatment,the reaction system was allowed to settle for a certain time,and the catalyst phase was separately recovered for the next treatment.The gasoline phase was sent to a conventional distillation column where it was separated into a light sulfur-free gasoline and a heavy sulfur-rich stream.The light stream was directly sent to the gasoline-blending unit and the heavy stream was blended into diesel oil.The sulfur compounds in diesel oil could be removed by means of hydro-treatment.In the process,a certain amount of fresh catalyst and paraformaldehyde should be fed into the reactors.

4.Conclusions

In this study,desulfurization of two different types of model gasoline with formaldehydeviacondensation in a biphasic system using aqueous phase of acids was investigated.Comparing with acetic acid and H3PW12O40(0.1 mol·L?1),the formic acid and aqueous solution of H3PW12O40(0.8 mol·L?1)showed good performance for removal of sulfur from the model gasoline.The desulfurization rates were above 90%.On the model gasoline of No.2 containing ole fin and aromatic,the effects of the reaction temperature,treating time and recycle times of catalyst on the desulfurization rate were also investigated.The sulfur removal rates increased(from 25%to 92%for the formic acid and from 41%to 95%for the H3PW12O40(0.8 mol·L?1))with the increase of the temperature(from 50 °C to 105 °C)and 4 h was the proper treating time for the sulfur removal.These catalysts can be recycled at least 4 times without decreasing the desulfurization rate.Finally,based on the investigation of reaction conditions,the possible process for the integration of condensation desulfurization into the existing refinery process for the production of gasoline with low sulfur content was proposed.

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