Issue 18/2014 - Application of ionic liquids in deep desulfurization of fuels

Application of ionic liquids in deep desulfurization of fuel oil

□ Hou Fuguo, Qingdao University of Science and Technology

Today's world has huge fuel consumption, and the sulfide produced by combustion has caused serious damage to the environment, and deep desulfurization of fuel has become one of the important scientific research topics of
mankind.
Ionic liquids are a new class of compounds, and have excellent characteristics such as green environmental protection, stable performance, wide electrochemical window and designable structure, and have been initially applied to fuel desulfurization experiments
.

ionic liquid (IL) is a liquid composed entirely of ions, usually composed of organic cations and inorganic anions, and is a salt
that is liquid at low temperatures (generally below 100 ° C).
Ionic liquids that are liquid at room temperature, also known as room temperature ionic liquids, room temperature molten salts, organic ionic liquids, etc
.
Ionic liquid desulfurization is the use of ionic liquids as extractants or oxidants, or in combination with chemical desulfurization
.
The earliest report of ionic liquid desulfurization appeared in 2001, when the Wasserscheid experimental group of RWTH Aachen University in Germany carried out deep desulfurization experiments
on diesel fuel with a variety of ionic liquids composed of imidazole cations and AlCl4-, PF6-, CF3SO3-, BF4-, Cl-, CH3SO4-, CH3SO3-.
The results showed that after multistage treatment of A1C13 ionic liquid, the sulfur content of diesel fuel could be reduced from 500mg/L to 235mg/L, and the desulfurization rate could reach 53%.

At present, the desulfurization methods of ionic liquids that are more studied at home and abroad mainly include direct extraction, oxidation-extraction combination method, and electrochemical polymerization method
.

Direct extraction desulfurization

The ionic liquid and the fuel oil are stirred at a certain temperature to extract sulfur-containing compounds from the oil into the
ionic liquid.
This method is to extract sulfur-containing compounds into ionic liquids using the properties of different partition coefficients in fuel oil and ionic liquids, and most of the relevant literature reports such methods
.
The advantage of this method is that the reaction time is short, and the disadvantage is that the desulfurization rate is not too high
.

Akzo-Nobel Chemical Company conducted a process study
for fuel oil desulfurization using ionic liquids [BMIM]BF4 and [EMIM]PF6.
At room temperature, these ionic liquids are mixed with the fuel oil, and then separated after absorbing sulfur, which can remove 10%~30% of the sulfur
in the fuel at one time.

Zhou Hancheng et al.
of Lanzhou Institute of Chemistry, Chinese Academy of Sciences applied [EMIM]BF4, [EMIM]PF6, [BMIM]BF4, [BMIM]PF6, [DMIM]BF4, [DMIM]PF6 to simulate gasoline desulfurization, and found that the [DMIM]BF4 ionic liquid with a longer carbon chain was the best extraction and desulfurization effect among the six, and the sulfur content of simulated gasoline was reduced from 1.
5×10-3mg/ during the first extraction.
L was reduced to 4.
72×10-4mg/L, and the desulfurization rate reached 68.
53%.


Zhang Shuyan of Liaoning Shihua University and others used chloroaluminic acid ionic liquid as an extractant to extract and desulfurize
catalytic cracking (FCC) gasoline.
Under the protection of nitrogen, when the molar ratio of AlCl3 to [BMIM]Cl is 2:1, the reaction temperature is 30 °C, and the reaction time is 50min, the chloroaluminic acid ionic liquid can effectively reduce the sulfur content and iodine value of FCC gasoline, while the octane number is basically unchanged
.

Huang Chongpin of China University of Petroleum et al.
investigated in detail the desulfurization effects
of three Lewis acidic ionic liquids [BMIM][Cu2Cl3], [BMIM][AlCl4] and [BMIM][BF4] on fuel oil.
The experimental results show that under the same conditions of agent-oil ratio of 0.
2, room temperature and reaction time of 30min, the single-pass desulfurization rates are 23.
4%, 16.
0% and 11.
0%,
respectively.

Feng Jie et al.
from the School of Chemical Engineering of Beijing University of Chemical Technology used phosphate ester ionic liquids [EMIM][DEP], [BMIM][DBP] and [MMIM][DMP] as extractants to simulate the desulfurization experiment
of gasoline.
The results show that the order of desulfurization capacity is: [EMIM][DEP]>[BMIM][DBP]>[MMIM][DMP].

When the best ionic liquid [EMIM] [DEP] is used as the extraction agent, the agent-oil ratio is 1:1, and the extraction time is 5 times, the removal rate of p-dibenzothiophene (DBT) can reach 99.
5%.

Oxidation-extraction combined with desulfurization

The oxidation-extraction combined with desulfurization of fuel oil is to first oxidize sulfur-containing compounds such as thiophene into sulfone, and then selectively extract sulfone with polar solvents or extractive ionic liquids; Or sulfur-containing compounds such as thiophene are first extracted into ionic liquids, and then oxidized by oxidants in ionic liquids, and finally separated from ionic liquids and oxidized sulfides
.
Traditional oxidation-extraction desulfurization uses a large number of volatile organic solvents, which seriously pollutes the environment
.
Because ionic liquids have the characteristics of non-volatile and low vapor pressure, which reduces environmental pollution, scientific researchers are vigorously studying in order to replace general oxidants with oxidizing ionic liquids and extractive ionic liquids instead of organic extractants
.

Huang Weixia and others from Sinopec Research Institute applied the new AlCl3-tertiary amine ionic liquid catalyst to the desulfurization of FCC gasoline
.
The results show that the new AlCl3-tertiary amine ionic liquid catalyst has excellent catalytic desulfurization effect, the desulfurization rate can reach more than 80%, the olefin content of oil samples is significantly reduced after treatment, the content of naphthenes and isoparaffins increases, and the octane number does not change much
.

Professor Wen-Hen Lo et al.
of Chung Cheng University in Taiwan used ionic liquids [BMIM][BF4] and [BMIM][PF6] as extractants, H2O2-acetic acid system as oxidant, and carried out oxidation-extraction desulfurization experiments on the simulated oil composed of DBT and tetradecane, DBT was first extracted into the ionic liquid phase, and then oxidized to more polar sulfone in the ionic liquid phase and retained in the ionic liquid.
This disrupts the extraction equilibrium of DBT in the two phases, allowing more DBT to be extracted into the
ionic liquid phase.
Experiments show that [BMIM][PF6] is used as the extractant to have the best desulfurization effect, and the desulfurization rate can reach 73%.


Professor Zhao Dishun et al.
of Hebei University of Science and Technology used alkyl pyrrole ionic liquid [Hnmp]BF4 as both an extractant and catalyst to investigate its oxidation-extraction desulfurization effect
on simulated fuel when it coexists with oxidant H2O2.
Experiments show that thiophene sulfides are successfully oxidized to polar sulfides such as sulfone or sulfoxide, which increases the depth of extraction, and the ionic liquid can completely remove DBT from the simulated fuel, and the desulfurization rate of the fuel can still reach 95%
after repeated use for 7 times.

Zhu Wenshuai et al.
of Jiangsu University with a vanadium peroxide complex containing 1,10-phenanthroline ligand[VO(O2)(phen)]· H2O was the catalyst, the ionic liquid [BMIM]BF4 and [OMIM]BF4 were the extractants, and hydrogen peroxide was used as the oxidant.

Since the [BMIM]BF4 and H2O2 aqueous solutions are miscible as one phase, and are two-phase systems with simulated oil, the desulfurization effect is better.
However, [OMIM]BF4, H2O2 aqueous solution and simulated oil are not miscible and form a three-phase reaction system, resulting in poor
desulfurization effect.
However, experiments can still prove that the desulfurization effect of ionic liquid extraction coupled with catalytic oxidation is excellent, and [VO(O2)(phen)]· After 5 cycles of H2O-H2O2-[BMIM]BF4 system, the desulfurization rate was still above
97.
3%.

Liu Haixia of Liaoning Shihua University and others synthesized alkyl imidazole type and pyridine type bis(trifluoromethylsulfonyl)imide ionic liquids, and used them as extractants and hydrogen peroxide as oxidizing agents, and the two formed an oxidation system for oxidative extraction and desulfurization experiments
of simulated oils.
The results show that under the conditions of V (ionic liquid): V (simulated oil) ∶V (oxidant) = 1:30:1, the reaction temperature is 60 °C, and the reaction time is 1h, the desulfurization rate of the primary oxidation extraction of the simulated oil can reach 65%.


Zhang Cun et al.
from the College of Chemistry and Chemical Engineering of Yangzhou University used the acidic ionic liquid N-carboxymethylpyridine bisulfate [CH2COOHPy]HSO4 as the extractant and catalyst and hydrogen peroxide as the oxidant for the extraction-oxidative desulfurization experiment
of simulated oils.
The results show that under the conditions of oxygen-sulfur molar ratio of 6, simulated oil dosage of 10mL, ionic liquid dosage of 0.
6mL, reaction temperature of 50°C and reaction time of 40min, the desulfurization rate can reach 99.
7%, and the ionic liquid can be recycled
.

Electrochemical polymerization desulfurization

The high ionic conductivity, low vapor pressure, and wide electrochemical window of ionic liquids create conditions
for their application in electrochemical reactions.
Therefore, ionic liquids have been used as electrolyte solvents, and a lot of research has been carried out on electrochemical polymerization, and it has been found that ionic liquids can be recycled in them, can replace organic solvents, and are easy to separate from the product, which opens up a new way
for the application of ionic liquids in gasoline desulfurization.
The electrochemical process can remove thiophene-sulfur-containing compounds that are easily polymerized in the fuel oil in the ionic liquid, while the octane number of the fuel oil remains unchanged
.

Shi Jiahua et al.
of the College of Chemistry and Chemical Engineering of Henan University used [BMIM]PF6 as electrolyte and solvent, and used constant current, constant potential, cyclic voltammetry and other electropolymerization experiments
on thiophene.
The experimental group successfully used the method of electrochemical polymerization to polymerize thiophene-sulfur-containing compounds in fuel oil into insoluble substances, and then removed them by filtration or distillation.
Moreover, ionic liquids and fuel oils are not miscible with each other, and after the end of the reaction, ionic liquids are easy to separate and can also be recycled
.

Qi Ximin et al.
of Shanghai Key Laboratory of Green Chemistry and Chemical Process Greening used [HMIM]BF4, [BMIM]BF4 and [BMIM]PF6 as extractants and electrolytes, and used electrochemical methods to polymerize
thiophene sulfides in simulated gasoline.
The results show that there is a certain rule, that is, the more electrolytic electricity of thiophene simulation sample, the better the desulfurization effect; When the electrolytic charge and current density are the same, the higher the concentration of the thiophene simulation sample, the better
the desulfurization effect.
Experiments also proved that [BMIM]PF6 is the best desulfurization effect among the three, and thiophene can polymerize in the three, but the optimal current density of desulfurization is different, but the removal effect of thiophene can reach 50%~90%.

epilogue

At present, ionic liquids are initially used in the desulfurization of fuels, which to a certain extent overcome some shortcomings of traditional solvents, catalysts and extractants, and ionic liquids have structural designability, and ionic liquids that meet the needs can be designed as appropriate to complete the fuel desulfurization process, which has great development and application prospects
.
But we still need to be clear that real fuel is far more complex than simulated fuel composition and has different properties, so there are still process technologies that need to be optimized and improved
.
With the deepening of people's research on ionic liquids, the technology and technology of desulfurization using ionic liquids are becoming more and more mature, and it will become a reality
for low-priced, environmentally friendly, high-sulfur and stable ionic liquids for deep desulfurization of fuel.