s1-ln20503499-1463272660-1939656818Hwf-1279672816IdV-200098714120503499PDF_HI0001

Oxidative Desulphurization of Fuel Oils Using Ionic Liquids:

A Review

Journal: Green Chemistry

Manuscript ID: GC-TRV-05-2015-001114

Article Type: Tutorial Review

Date Submitted by the Author: 25-May-2015

Complete List of Authors: Bhutto, Abdul Waheed; Beijing University of Chemical Technology, , College of Chemical Engineering Beijing, China; Dawood College of Engineering and Technology, Department of Chemical Engineering Abro, Rashid; Beijing university Of Chemical Technology, Beijing Key Laboratory of Membrane Science and Technology & College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, P.

R. China abbas, tauqeer; Universiti Teknologi Petronas, Malaysia, Petronas Ionic Liquid Center, Department of Chemical Engineering Chen, Xiaochun; Beijing University of Chemical Technology, College of Chemical Engineering Yu, Guangren; Beijing University of Chemical Technology, College of Chemical Engineering

Green Chemistry

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Page 1 of 55 Green Chemistry

1

Dear Editor,

We highly appreciate your efforts to improve our manuscript.

Following those pretty helpful comments, we have significantly revised our

manuscript. The responses are as follows.

Referee 1

Q1. The authors summarized the oxidative desulfurization of fuel using ionic liquids.

However, this manuscript is a routine work. The authors only summarized the results

from the literatures, and did not put forward their own views deeply.

Re. In the revised version, we have deep discussed and put forward our own views, as

the highlighted text in yellow. In addition, we have added one new section 3.4.

Challenge and Perspective of ODS using ILs, to address the challenge and perspective

of ODS using ILs.

Q.2. In addition, at times it is difficult to understand in whole or in parts. The manuscript

requires proper writing.

Re. We have improved and polished the text, especially the writing.

Q.3. The literatures about oxidative desulfurization of fuel using ionic liquids are not

cited completely. According to the Web of Science, the number of literatures is about

70.

Re. We have re-carried out the literature survey. Now the number of literatures cited in

the manuscript is about 74.

Referee 2

Q1. The authors present an interesting review of the literature about the ODS with the

use ionic liquids. The manuscript concisely describes all aspects of the use of ionic

liquids in the ODS processes. From this point of view, this manuscript is eligible for

publication in Green Chemistry. However, the manuscript was very carelessly prepared.

In text can find numerous errors that absolutely must be removed. In current form, this

manuscript should not be submitted to Green Chemistry. Authors must make very

careful adjustment of the whole manuscript and remove many unacceptable errors:

Re. We really feel sorry for such errors. We have carefully checked and made the

manuscript free of writing mistakes, especially type errors. The sections have been also

Page 2 of 55Green Chemistry

2

re-arranged, and a new section 3.4. Challenge and Perspective of ODS using ILs was

added. Please read it again, and any further suggestions will be very welcome.

Q2. citation [12] and [13] is the same

Re. Correction has been made.

Q3.citation [31] is incomplete


Q4. Tables 2,3 and 4 (numbered as 3!!!) must by reedited


Q5. Figures 5 and 9 are have been taken from the cited papers and inserted into the

manuscript very carelessly. Both figures are far from original and must be reedited


Q6. In chemical abbreviations of ionic liquids indexes must be used in the correct form

(eg. PF4)


Q7. All abbreviations of ionic liquids must be used in uniform format (eg. bmim or BMIM)


Q8. In my opinion, based on common definition well described in literature, the

IL/POM/SiO2 compound isnt amphiphilic.


Q9. p. 16 [Otbim]Ac isnt correct, should be [Omim]Ac.


Q10.Citation [41] must by checked, probably is incorrect



1

Oxidative Desulphurization of Fuel Oils Using Ionic Liquids: A Review

Abdul Waheed Bhuttoa,b

, Rashid Abroa, Tauqeer Abbas

c,d , Xiaochun Chen

a and Guangren Yu

a,*

aBeijing Key Laboratory of Membrane Science and Technology & College of Chemical Engineering,

Beijing University of Chemical Technology, Beijing 100029, P. R. China

bDepartment of Chemical Engineering, Dawood University of Engineering and Technology, Karachi,

Pakistan.

cPetronas Ionic Liquid Center, Department of Chemical Engineering, Universiti Teknologi Petronas,

Malaysia

dDepartment of Chemical Engineering, COMSATS Institute of Information Technology, Lahore,

Pakistan

*Authors to whom correspondence should be addressed; E-mail:[email protected].

Tel./Fax: +86-10-6443-3570.

Abstract

Due to sterically-hindered adsorption of some thiophenic sulphur compounds (S-compounds) such as

thiophene, dibenzothiophene and their derivatives on catalyst surface, hydrodesulphurization (HDS) is not

effective to remove such thiophenic S-compounds in fuel oils. To produce clean fuel oils with lower S-content

(e.g., S

2

scientific findings about ODS using ILs have shown its good future. Here, we give a review for these interesting

results to illustrate the novelty, problem and perspective of such a new method.

1. Introduction

Desulphurization of fuel oils such as gasoline, jet fuel, kerosene, diesel and heating oil is an

important process in oil refining. Sulphur compounds (S-compounds) in fuel Oils are

undesirable as they create problems during refining as well as during their commercial use.

During refining, S-compounds tend to deactivate some catalysts used in crude oil processing

and cause corrosion problems in pipeline, pumping, and refining equipment. During the

commercial use of fuel oils, S-compounds are transferred to sulfur oxides in combustion, and

then to sulfates, acid rain, or other particulates which are harmful to the environment.

Furthermore, the S-compounds in the exhaust gases of diesel engines can significantly impair

the emission control technology designed to meet NOx and suspended particulate matter (SPM)

emission standards. With increasing environmental concerns, governments all over the world

are implementing stringent standards to limit the S-content in fuel oil, as shown in Figs. 1 and 2.

Fulfilling the required lower S-compounds specifications, represents a major operational and

economic challenge for the petroleum refining industry1. Therefore, intensively studied are

going to find new economically viable deep desulphurization technologies.

Hydrodesulphurization (HDS) is a widely used commercial method to remove S-compounds

in fuel oils. In HDS, S-compounds are catalytically converted into H2S and corresponding

hydrocarbons at elevated temperatures (300 to 340C) and pressures (20 to 100 atm of H2)

with Co-Mo/Al2O3 or Ni-Mo/Al2O3 catalyst, and H2S is subsequently separated from oils and

catalytically oxidized into elemental sulfur in the Claus process. HDS process is efficient in

removing thiols, sulfides, and disulfides but is less effective for heterocyclic S-compounds


3

such as thiophene (TS), dibenzothiophene (DBT) and their derivatives, especially 4, 6-

dimethyldibenzothiophene (4, 6-DMDBT) because of the ineffective adsorption of these

heterocyclic S-compounds on catalyst surface. Some typical S-compounds in fuel oils are

shown in Fig.3. To remove these heterocyclic S-compounds in HSD, the reactor size unit needs

to be increased by factors of 5 to 15 and the pressure and temperature have to be elevated

further more with more active catalysts. These steps result into significantly increase in the

operational and capital costs and also leads to undesired side reactions bringing saturation of

olefins and loss of octane number. To overcome drawbacks of HDS in deep desulphurization,

some alternative methods such as extraction (EDS), oxidation (ODS), adsorption (ADS) (Tab.1

shows different adsorbents used in ADS at optimized conditions), and bio desulphurization

(Tab.2 shows bio-desulfurization by different microorganisms) are under considerations.

Since refractory heterocyclic S-compounds such as TS, DBT and their derivatives are easily

removed by the oxidative desulphurization (ODS)2, 3

, therefore ODS is currently being

explored as a promising strategy to achieve an ultra-low S-level in fuel oil because it is simple

in processing and high efficiency4. No use of expensive hydrogen is another advantage of ODS.

In subsequent operation, oxidized compounds can be extracted from fuel oils through

contacting oxidized fuel oils with non-miscible polar solvents. Many solvents have been

explored such as N,N-dimethylformamide, acetonitrile, methanol and dimethylsulfoxide.

However, these solvents are volatile which result in not only solvent volatile loss and

contamination but also make the solvent recovery difficult.

Ionic liquids (ILs), just emerging in the past few decades, are a class of new solvents, which

are promising alternatives to traditional volatile solvents used in ODS. ILs, composed entirely

of organic cation and inorganic or organic anion, have been explored as green reaction media,


4

owing to their unique properties such as negligible volatility, excellent thermal and chemical

stability, good solubility characteristics, and the variety of structures available.

ILs has been investigated as extractive reagents in extractive desulfurization, as has been

reviewed in our recent publication5. Recently, researchers have been attempting to use ILs as

solvents in ODS as well as both catalysts and extractive reagents in coupled extractive-

oxidative desulfurization6. Such ODS with ILs shows very positive results and also eliminates

the loss of volatile solvent and contamination/pollution. ODS with ILs also ensure easy

regeneration and good stability. In addition to this, it also ensure that those heterocyclic S-

compounds unreactive to HDS are efficiently removed in only single stage to obtain clean fuel

oils with S

5

Oxidization brings dramatic changes in physical properties of S-compounds. For example

methyl sulfide (CH3SCH3) is a light ( = 0.846 g/mL), water-insoluble, low boiling liquid (bp

= 36oC), whereas methyl sulfoxide (CHSOCH3) is a heavy ( = 1.481g/mL), high boiler

(189oC) and water-miscible liquid

7.

2.1. Oxidant in ODS

The oxidation of S-compounds with organic hydroperoxides occurs in the presence of

catalysts. The oxidants that can be used include peroxy organic acids, hydroperoxides, nitrogen

oxides, peroxy salts and ozone, etc. These oxidants donate oxygen atoms to the S-compounds

in mercaptans (thiols), sulfides, disulfides and thiophenes to form sulfoxides or sulfones.

Tab.3 gives the list of some common oxidants and their active oxygen content.

Among different chemical oxidants, the best option is H2O2. Table 3 indicates that H2O2 gives

the highest percentage of active oxygen. The other advantages for the use of H2O2 are: (1) low

cost, (2) non-polluting, (3) non-strongly corrosive, and (4) commercial availability8. The main

challenge using H2O2 as an ODS oxidant is the slow reaction rate because of mass-transfer

limitation in the biphasic oxidation reaction and the subsequent energy-consuming biphasic

separation process.

2.2. Catalyst in ODS

Oxidation reactions are effective in the catalysis of organic acids or transition metal salts.

Most studies have been focused on molybdenum (Mo) catalysts, and they usually employ

heterogeneous Mo/Al2O3 catalysts. Mo catalyst supported on Al2O3 presented higher catalytic


6

activity compared with Mo catalyst supported on TiO2 and SiO29. However, molybdenum

tends to leach into the reaction medium, where the catalysts are not very stable and the main

part of the catalytic activity is due to the solubilized molybdenum. The oxidation activity of

each S-compound increased with the increasing O/S molar ratio up to the O/S molar ratio of 3

and then leveled off beyond this value. The oxidative reaction of S-compound can be treated as

a first-order reaction and apparent activation energies of oxidative reaction are 281 kJ/mol9.

The polyaromatic S-compounds show higher reactivity in ODS than TSs and BTs, in the

reverse reactivity order of HDS10

. In the oxidation of S-compounds catalyzed by

polyoxometalates, the mass transfer across the interface of aqueous phase and oil phase is the

rate-limiting step. Consequently, a phase transfer agent (PTA) is usually added to the reaction

system in order to enhance the mass transfer. 100% oxidation of DBT in a gas oil was achieved

with H2O2 as oxidation agent using phosphotungstic acid as the catalyst and

tetraoctylammonium bromide as the phase transfer agent, at optimal conditions11

. Zhang and

co-workers12

suggested that [C4mim]3PMo12O40/SiO2 could effectively catalyze the oxidation

of BT, DBT, and 4,6-DMDBT using H2O2 as the oxidant under mild conditions. Complete

DBT conversion was obtained at 60C, an O/S molar ratio of 3.0, and thermal equilibrium in

100 min. The oxidation reactivity decreased in the order: DBT > 4,6-DMDBT > BT.

[C4mim]3PMo12O40/SiO2 and water phase were easily separated together from the oil phase by

centrifugation, and no deactivation was observed after seven runs if the spent catalyst was

dried to remove the H2O retained on the catalyst surface. Carbazole and quinoline had positive

effects on DBT oxidation, probably because they help to slow down the thermal decomposition

of H2O2. The presence of indole inhibited DBT oxidation due to the strong adsorption of

indigo generated by indole oxidation. The [C4mim]3 PMo12O40/SiO2-H2O2 oxidation system is

effective in removing bulky S-compounds from diesel fuels.


7

2.3. Separation Operations

The second step in ODS is the removal of the oxidized compounds from fuel oil. The

oxidized S-compounds can be extracted from the light oil by contacting oxidized light oil with

non-miscible polar solvents. Depending on the solvents used for extraction, the oxidized

compounds and solvent are separated from the light oil by gravity separation or centrifugation.

Sulfones are extracted by polar solvents such as N-methyl pyrrolidone (NMP)13

, methanol 13

,

Dimethylformamide (DMF) , dimethyl sulfoxide (DMSO), and acetonitrile in a simple process

under milder conditions with low equipment and operation costs. The most attractive solvent

for the extraction of oxidized organic S-compounds is DMSO14

. However these polar solvent

are volatile and harmful to environment and human health15

.

Fig 4 shows the flowchart of biphasic simultaneous oxidation/extraction ODS unit.

Oxidation is accomplished by contacting an oxidant with fuel oil under optimum conditions

and continuing the reaction until the oxidized S-compounds are formed. The reaction

conditions are selected in such a way that reaction is stopped before the oxidant attacks other,

less reactive, fuel oil components. Down the reaction, oxidants are regenerated for re-use.

Washing, extraction and chemical post-treatment can remove any unused oxidant that remains

in the light oil.

The oxidized compounds are then extracted from the light oil by contacting oxidized light oil

with a non-miscible solvent. This solvent is selective for the relatively polar oxidized S-

compounds. The oxidized compounds and solvent are separated from the light oil by

decantation. The light oil is water washed to recover any traces of dissolved extraction solvent

and polished using other methods, such as by absorption using silica gel and alumina. The

oxidized compounds and solvent are separated from each other by a simple distillation for


8

recycling. The desulphurization efficiency for light oils lies in the order straight-run light gas

oil > commercial light oil > light cycle oil and demonstrates that high-aromatic-content light

oil is difficult to desulfurize.

2.4. Oxidative Reaction Mechanism

Generally, in the ODS reactions, the divalent sulfur atom of S-compounds undergoes

electrophilic addition of oxygen atoms from the hydrogen peroxide to form the sulfone, i.e., a

hexavalent sulfur9, 16-18

. Fig. 5 shows the overall ODS reaction and a schematic diagram of the

process.

Several mechanisms of ODS reactions have been previously proposed17-19

. The

homogeneous biphasic ODS system is simple and uses no PTA. Fig. 6 presents a detailed

reaction mechanism. Once the catalyst mixes with the H2O2 and the diesel fuel in acetic acid,

the biphasic catalyst system starts to form at room temperature. Farhan Al et.al 20

suggested

that WO42

anion reacts in two steps with two molecules of H2O2 in sequential substitution

reactions and in each step a W = O group is replaced by a W(O2) group and H2O is displaced.

The resulting peroxotungstate [(O)2WO2)2] anion then reacts by sequential oxygen atom

transfer to the sulfur of R2S to form sulfoxide (R2SO) then the sulfone (R2SO2), which can be

extracted with the aqueous phase. The peroxotungstate can be regenerated on the interface

between the two layers or in the aqueous phase in presence of adequate supply of H2O2.

Sulfones are known to be slightly more polar than S-compounds; so they will form a white

precipitate. The whole process will result in a biphasic solution by which the upper layer

becomes almost S-free diesel20

.

2.5. Kinetics Study of Catalytic Oxidation on S-Compounds


9

Zhu et al,21

investigated ODS using surfactant-type polyoxometalate-based ILs, such as

[(nC8H17)3NCH3]3{PO4[MoO(O2)2]4},[(n-C12H25)3NCH3]3{PO4[MoO(O2)2]4}, [(n-

C8H17)3NCH3]3{PO4[WO(O2)2]4} and [(n-C12H25)3NCH3]3{PO4[WO(O2)2]4} . The study

proposed overall mechanism of catalytic desulfurization in four steps as shown in Fig. 7,

which are; (1) the active peroxo species (II) (abbreviated as Mo(O2)) is regenerated from the

reaction of (I), when surfactant-type polyoxometalate-based ionic liquids (SPILs), reacting

with excess H2O2. (2) An oxygen transfers from the active Mo-peroxo species (II) to the

sulfide; then there is a formation of the transition state. (3) A complete O-transfer to the sulfide

takes place, which yields the sulfoxide and the Mo(O) species (I). (4) The active species (II)

takes part in the ODS, which leads to the corresponding sulfone and the regeneration of the

Mo(O) species (I).

The reaction sequence that may be involved in this oxidation as proposed by Zhu et al21

is

shown in Fig 8.The first step of the reactions (1) must be reversible to regenerate the catalyst,

followed by other oxidation steps which lead to the R2SO and R2SO2. Furthermore, k2 is the

rate constant of the rate determining step, which was given on the basis of the concentration of

the active species. So the rate of the formation of sulfoxide can be expressed by Eq. (1):

r =

= kMoORS (1)

= kHOMoO kMoO kMoORS k MoORSO=0 (2)

MoT = MoO + MnO (3)

r =

=

$%$&'()

$*%+$%(+$+$, (4)

The initial rate can be obtained as the following Eq. (5):


10

r- = /

=

$%$&'(/)

$*%+$%(/+$/ (5)

According to the experimental condition, k-1 + k1[H2O2]> > k2[R2S] is reasonable for the

stability of [MoO2]. In this case, the oxidation rate is the first-order. So Eqs. (6) and (7) were

obtained from Eq. (5).

r =

= KRS (6)

K = $%$&(

$*%+$%( (7)

Eq. (6) suggests that there is a straight linear relation between the plot of [R2S] and t, which

goes through the origin point. Therefore, the catalytic oxidative desulfurization can be treated

as the pseudo-rst-order rate with SPILs catalysts.

Reaction kinetics was great important parameters to explain the oxidation of DBT.

Experiments were performed using SPIL A to obtain kinetic parameters of the oxidation of S-

compounds. The rate constant for the apparent consumption of S-compound was obtained from

the pseudo-first-order Eq. (6) as follow 22

.

1= kC (8)

In 1415= kt (9)

t/ = 0.693/k (10)

Where Co and Ct were the S-concentrations at time zero and time t (min) and k was the rst-

order rate constant (min1

). The plot of ln(Co/Ct) against t, a straight line with slope k was

obtained (Fig 9). Half-life was calculated by Eq. (9), which was derived from Eq. (8) by

replacing Ct with Co/2.

The apparent rate constant k of DBT was 0.0408 min1

and the half-life was 17.0 min. The

kinetic data of the DBT oxidation were in accordance with an apparent rst-order kinetic rate21

.


11

The kinetics for oxidative desulfurization of DBT and BT by [C63MPy]Cl/FeCl3 is given in

Fig.10. It shows that the rate constant was 1.0161 and 0.3147 min1

for DBT and BT at 298 K,

respectively23

. It is observed that the rate constant values of bulkier S-compounds is higher in

ODS when compared to the rate constant for the same compounds in HDS process24

.

3. ODS using Ionic Liquids

3.1. Ionic Liquids (ILs)

ILs, termed as green solvents, can be used for desulfurization of liquid fuel due to their very

low vapor pressure and wide range of applications with unique physical and chemical

properties as mentioned above25

. Their ability to be recycled without any impact on the

environment provides impressive advantage in the present era of environmental concern34

. In

the past few years, significant literatures have become available in the area of preparation,

characterization and application of ILs for synthesis, catalysis and separation.

Commonly used ILs consist of inorganic anions and organic cations with alkyl side chains

and aromatic moieties26

. Different ILs can be synthesized having a wide range of physical and

chemical properties that can be fine-tuned by using different cations and anions to meet the

requirement of specific applications. ILs are good solvents for a wide range of both inorganic

and organic materials, and unusual combinations of reagents can be brought into the same

phase. They are often composed of poorly coordinating ions, so they are highly polar yet non-

coordinating solvents. They are immiscible with a number of organic solvents and provide a

non-aqueous, polar alternative for two-phase systems. Hydrophobic ILs can also be used as

immiscible polar phases with water. ILs are nonvolatile, hence they may be used in high-

vacuum systems and eliminate many contaminant problems27

. Well-known cations are


12

imidazolium, pyridiniuim, isoquinolonium, ammonium, phosphonium and sulfonium as shown

in Fig 11.

3.2. ODS using ILs

Lo et al28

were first to combine chemical oxidation and solvent extraction using ILs in a one-

pot operation for deep desulfurization of light oils. Simultaneous extraction and oxidation of S-

compounds from light oil in ILs increases the desulfurization yield by about an order of

magnitude relative to that of merely extracting with ILs28

. The ILs acts as both extraction

media for the S-compounds and provide the oxidation environments for the conversion of S-

compounds to sulfones. An organic acid or a transition metal is needed as catalyst in the ODS

process. The same study found water-immiscible IL [C4mim]PF6 more-effective solvent than

the water-soluble [C4mim]BF4 for providing an environment that results in a higher rate of

chemical oxidation28

. Xu et al have carried out a detailed theoretical study to better understand

the mechanism of oxidative-extractive desulfurization of DBT from fuels by [C6mim]BF4 IL in

the presence of H2O229

. These ILs containing halogen atoms such as the anion BF4- and PF6

-

easily yield white fumes of hydrogen fluoride or hydrate precipitates, which will lead to

potential environmental and safety problems.

Some other ODS processes using H2O2 as oxidant and ILs as both solvent and extractant

were reported30-33

. Nie et al used Iron-containing ILs used as extractant in ODS process to

remove 100% S and reached reaction equilibrium fast34

with an apparent rate constant of

0.9951 min-1

at 298 K. Cooper and co-workers 35

observed that when a coordination compound

is generated between H2O2 and an amide, such as urea; then the H2O2 on the coordination

compound decomposes to produce hydroxyl radicals that are strong oxidizing agents.More

than 95% S-removal could be obtained by carrying out the extraction with [C4mquin]N(CN)2


13

at 30oC at 500 rpm with 1:5 IL:diesel weight ratio and 20 minutes of extraction time36. Zhao et

al37

reported that the S-contents of DBT dissolved in n-octane could be dramatically reduced

using H2O2formic acid as an oxidant and pyridinium-based ILs as phase-transfer catalysts

(PTCs). Jiang et al38

showed that [C8bim]CH3COO containing eight-carbon side chain has of

best catalytic activity for ODS. 87.5% of TS in the model oil was removed under the optimal

conditions of oxidation temperature at 70 C, oxidation time of 180 min, simulated oil dosage

of 10 ml, and ILs/H2O2 volume ratio of 1:1.1. When acetic acid-based ILs were used as both

PTCs and extractants, which showed good oxidative ability31

.

Jiang et al38

achieved high desulfurization efficiency of DBT both in hydrophilic IL

[C4mim]BF4 (94.9%) as well as in hydrophobic IL [C8mim]BF4 (97.2%) by using ChFeCl4 as a

catalyst under mild conditions. Electron spin resonance measurements suggested that the active

oxygen species generated by ChFeCl4 and H2O2 in IL were involved in the catalytic oxidation

of DBT. Wilfred et al36

achieved desulfurization efficiency up to 99.9% through catalytic

oxidation using 1- butyl-6-methylquinolinium dicyanamide [C4mquin]N(CN)2.

Kulkarni and co-workers39

also applied this oxidation/extraction system to actual light oil

containing a S-content of 8040 ppm. After 10 h, in the C4MIM+PF6

extraction/oxidation

system, the decrease of S-content was observed from 8040 to 1300 ppm. The Bronsted acid IL

[Hnmp]BF4 is effective on the ODS of actual diesel fuel. The results show that 99.4% sulfur-

containing compounds which are present in the actual diesel fuel can be removed40

.

Use of acidic ILs have led to very good results in ODS of fuels without the need of addition

of an external catalyst to the medium41

. The intrinsic acidic character of the ILs allows them to

act as both catalysts and solvents. It also eliminates the requirement of additional catalyst. The

acidity of protons is mainly determined by their solvation. The acetic acid-based ILs act as


14

both phase-transfer catalysts and extractants with good oxidative ability. In this regard, study

by Thomazeau et al42

has proposed an acidity scale that correlations the catalytic activities

measured in various acidic reactions. Fanget al43

suggests that with imidazolium as the cation,

the desulfurization capability of ILs decrease in the following order, TFA> HSO4

>

COO>AlCl4

> AcO

. The study also shows that when the acidity of IL is much stronger, the

catalytic and extractive capability is better.When the anion is HSO4, the order of the

desulfurization capacity of different cationtypes is [C5mim]+> [C3Py]

+>[C4mim]

+> [C3mim]

+>

[HSO3C3 mim]+> [HSO4C3EA]

+> [HSO3C5Py]

+. It shows that ILs with cation

[Cnmim]+(including [C2mim], [C3mim], [C4mim], and [C5mim]) have better extraction

capability than those with [HSO3C3mim]+, [HSO4C3EA]

+, and [HSO3C5Py]

+43. Zhao et al

44

optimized ODS of DBT in the model oil with Brnsted acidic ILs N-methyl-pyrrolidonium

phosphate [C6nmp]H2PO4 as catalytic solvent and H2O2 as oxidant. 99.8% DBT in the model

oil was removed under the optimal conditions of molar ratio of H2O2 to sulfur of 16:1, reaction

temperature of 60oC, reaction time of 5 h, and volume ratio of model oil to ILs of 1:1. The

desulfurization efficiency of actual diesel was 64.3% under the optimized conditions. The

Brnsted acidic IL [C6nmp]H2PO4 can be recycled six times without a significant decrease in

activity.

There are very few examples of the use of Fenton-like reagents in oxidative desulfurization38

.

Fenton-like ILs [(C8H17)3CH3N]Cl/FeCl3H2O2 system showed that the S-removal of DBT-

containing model oil reached 97.9% and that the S-level of FCC gasoline could be reduced

from 360 ppm to 110 ppm45

.

Dong el al 23

studied the activity of [C63mpy]Cl/ FeCl3 on BT and DBT. The results suggest

S-removal followed the order of DBT> BT. Xiong el al46

investigated the catalytic

performance of Fenton-like IL supported on meso-porous material MCM-4,1 with BT, DT,


15

DBT, and 4,6-DMDBT.The results shown in Fig. 12 suggest that the S-removal decreased in

following order: DBT>DT>BT>4,6-DMDBT.

The difference in removal efficiency mainly came from aromatic -electron density of S-

compounds. The electron density on the sulfur atom of DBT, BT, and 4,6 DMDBT is 5.758,

5.739, and 5.760, respectively 49.

The relatively lower electron density of TS made ILs

extraction after ODS process ineffective when attempting to desulphurize gasoline. S-

extraction is not the only parameter to take into account. Co-extraction of (un)desired

compounds, integration of the process in the refinery, and life cycle analyses, among others,

are further aspects to take into account in the decision-making prior to implementation of the

proposed alternatives.

3.2.1. Effect of Reaction Temperature on S-Removal

The influence of reaction temperature on removal of DBT by [C63mpy]Cl/FeCl3 in the

presence of H2O2 is shown in Fig. 12 and 13. It can be seen that when the reaction temperature

for [C63mpy]Cl/FeCl3 increases S-removal increases. However, when the temperature increases

above 308K, the S-removal decreases sharply. DBT is easier to remove than BT at 298 K by

[C63mpy]Cl/FeCl3

23. For [C4mim]Cl/3ZnCl2, the highest S-removal efficiency of 99.9% is

obtained at 45C after 7h, but only 65.7% at 30C and 89%at 60C; whats more, the S-

removal is 99.9% after 3h at 45C, 47.4% at 30Cand 62.1% at 60C 47

. According to

Sachdeva and Pant48

the formation of peroxide becomes more rapid at higher temperature,

leading to increased conversion of S-compounds to sulfones.

Zhao, et al44

found out the optimal temperature at 60C, for ODS of DBT in n-octane as

model oil with Brnsted acidic IL [C6nmp]H2PO4 as catalytic solvent and H2O2 as oxidant.

Zhang, et al49

used [C4mim]HSO4 in ODS of DBT in model oil. In their work complete


16

conversion at 60C was realized in 30 min. When temperature was raised to 70 or 80C, the

desulfurization decreased to 99% and 96%, respectively. These studies suggest a high

temperature is not beneficial for the removal of DBT. This might be because the

decomposition of H2O2 was accelerated at high temperatures50

. Therefore, ODS using ILs

could be easily carried out at moderate (40-600C) temperature. The optimal temperatures

depends upon nature of ILs33

. Mostly, low viscous ILs follow a slight decreasing or no

effecting trend for S-removal efficiency with increasing temperature from ~ 400C. On the other

hand, the trend becomes reversed for high viscosity ILs, such as [C4mim]Cl/3ZnCl2,

[C6nmp]H2PO4 and [C4min][PF6], where an increase in temperature from ~ 400C corresponds

to a slight or high increase in the efficiency. The reason for these variations between high and

low viscosity ILs can be understood from the variation in the viscosity of ILs with increasing

temperature. With an increase of temperature for high viscosity ILs, the viscosity of ILs is

decreased, and as a result, the ILs and the S-content of the oil have more opportunities to

interact with each other, leading to the high S removal efficiency.

3.2.2. Mass Ratio of IL to Model Fuel

The ILoil mass ratio is one important factor for the selectivity of ILs for ODS

desulfurization. As the cost of ILs is high, it is preferred that a minimum quantity is utilized in

fuel desulfurization, but it is observed from the literature that the S-removal efficiency

decreases with a decreasing ILoil mass ratio. For instance, the mass ratios of

[C4mim]Cl/3ZnCl2 to oil were tested at 60 minutes and the order of S extraction in ODS

efficiency follows the order 1 : 5 < 1 : 4 < 1 : 3 < 1 : 2 < 1 : 1 < 2 : 1 (w/w) ILoil. Tab.45

indicates that, with the increase of IL/oil mass ratio, the S-removal efficiency increases both

for [C4mim]Cl/2ZnCl2 and [SO3H_C4mim]HSO4. Compared with the molar ratio of O/S, the


17

effect of mass ratio of IL/oil is limited; for example, when the mass ratio of IL/ oil is increased

from 1:5 to 1:1, the S-removal is increased from 92.2% to 100% for [C4mim]Cl/2ZnCl2 and

from 93.9%to 98.1% for [SO3H_C4mim]HSO4. Meanwhile, Similar results were obtained

when a series of [BF4] and [MeSO4]-based ILs were used to investigate the influence of the

mass ratio. Despite the increasing efficiency with an increasing ILoil mass ratio, it is found

that the increase does not have the same rate for all ILs. This typically depends on how the

individual chemical natures of ILs are related to their extraction abilities. According to the

study by Mochizuki and Sugawara51

the extraction yield of DBT increased linearly with an

increase in the length of alkyl chains and the mass ratio of the IL to the model fuel.

Apart from the positive effects of increasing the ILoil mass ratio, the ILoil mass ratio has

to be carefully selected based on a compromise between S removal and oil recovery. Moreover,

the regeneration of ILs may also be considered for the process as this highly affects the overall

process cost.

3.2.3. Regeneration of used ILs

After each ODS process finished, the IL is recovered and reused without significant loss of activity.

Regeneration of ILs is an important feature for selection of ILs. It can compensate for the high cost of

ILs. The methods of regeneration of S-loaded ILs may vary on the basis of the nature of the ILs and S-

compounds. The regeneration of used hydrophilic ILs is carried out by dilution with water followed by

simple distillation41

. According to the study by Zhang et al 49

[C4mim]HSO4 can be recycled five

times with a slight loss in activity, e.g., DBT removal dropped from 100% to 95% under the same

experimental conditions. The study by Zhao et al40

suggest that IL [C6nmp]BF4 can be recycled 7

times without a significant decrease in activity. [CH2COOHPy]HSO4 can be recycled 9 times without

a significant decrease in the sulfur removal52

.


18

Wang et al53

suggested that during the recycling some IL was wasted when IL was separated

and regenerated which result in little drop in DBT removal from the oil phase with the increase

of the cycle. Zhang et al54

attributes the decrease of S-removal to the increase of oxidation

product DBTO2 that accumulates in the IL phase.

The desulfurization activity of [C63mpy]Cl/FeCl3 decreased from 100% to 94.5% after five

cycles. Dong et al23

investigated the recycling of the used IL phase with and without

distillation. Results are shown in Tab. 5 which suggest that the used IL [C63mpy]Cl/FeCl3 with

distillation has a better recycling performance than[C63mpy]Cl/FeCl3 without distillation in the

ODS process. Presence of white solid DBTO2 also affects the performance of recycled ILs23

.

3.2.4. Concentration of H2O2 as Oxidant

The concentration of H2O2 has a significant influence on desulfurization efficiency. Mostly, the S-

removal efficiency increases with oxidant to S- compound (O/S) molar ratio. The effect of O/S molar

ratio on desulfurization for [C4mim]Cl/3ZnCl2 was determined by Chen et al47

, the S-removal

efficiency increases from 18.6% when the molar ratio of O/S is 2, to the maximum value of 99.9%

when the molar ratio of O/S is 8; but exceeding the O/S molar ratio of 8, the S-removal efficiency

decreases with increasing O/S molar ratio, e.g., the value is reduced to 49.3% when the molar ratio of

O/S is 9. According to the stoichiometry of the ODS reaction, 2 mol of H2O2 is consumed for every 1

mol of DBT to give the corresponding sulfone, i.e., DBT-sulfone (DBTO2), thus the molar ratio of O/S

is 2; however, the highest S-removal of 99.9% is obtained at the O/S molar ratio of 8, instead of 2, as

shown in Fig.14, i.e., the stoichiometric H2O2 is not enough to oxidize all the DBT extracted into the

IL phase; this is because of the fact that the decomposition of some H2O2 occurs in ODS process and

excess H2O2 is required to ensure the DBT oxidization, which are also indicated in several papers in

literature.53

At the same time, it is neither desirable when the amount of H2O2 solution is too much,


19

because excessive H2O2 solution will dilute the IL and influence the extraction of DBT from diesel

fuel41

. This is consistent with the result in Fig.14, where the S-removal efficiency is reduced from 99.9%

to 49.3% when the O/S molar ratio increases from 8 to 9.

In the study by Jiang et al55

the S-removal was 76.3% with 30 wt % H2O2 which increased to

97.9% with 7.5 wt % H2O2.In the above study [C4mim]3Fe(CN)6was used as catalyst and

[C4mim]BF4 as an extractant. The results of the ODS of model oil catalyzed by Fenton-like ILs

at room temperature by Jiang, Y., et al 45

are shown in Tab.6.

3.2.5. Influence of Molar Ratio of ILs on S-removal.

Chen, et al 47

investigated the effect of IL/oil mass ratio on S-removal. Their results are given

in Fig.15 which indicates that the quantity of IL has an important effect on the removal of DBT.

The S-removal efficiency was increased from 49.1% to 99.9% when the mass ratio of IL/oil

was increased from 1:5 to 1:2. However, the S-removal efficiency came down to 87.8% when

mass ratio of IL/oil was further increased to 1:1. The reason being that when O/S molar ratio

was reduced with the increase in IL/oil mass ratio, the IL get capacity to extract more DBT56

,

but the amount of H2O2 was kept constant so the O/S molar ratio is reduced, less S-compounds

were oxidized. Similar results were obtained by both Zhang et.al 57

and Chen, X., et al41

.

3.2.6. Influence of viscosity of ILs on S-removal.

ILs are prepared from two solid materials, which would lead to the high dynamic viscosity

of the obtained ILs which limits their extraction ability58

. Due to the much higher viscosity of

ILs59

, the overall sulfur removal rate is limited by the mass transfer between phases with very

active catalysts. For this reason, an increase in the mass transfer clearly improves the overall

sulfur removal rate. With viscous IL, the overall sulfur removal rate is limited by the mass


20

transfer between two phases which led to reduction in reaction rates and may also causes

competitive unimolecular side reactions. Viscous IL also causes handling difficulties during

filtration, decantation, and dissolution.ODS process can be more effective if the shortcomings

of ILs on high cost and viscosity are overcome31-33

. Ionic liquids incorporating the

bis(trifluoromethanesulfonyl)imide, [NTf2], anion are favoured for their low viscosities

59.

According to Li, et al C5H9NO0.3FeCl3shows remarkable extraction ability for

dibenzothiophene (DBT) with Nernst partition coefficients (kN) above 7.560

. Low viscosity IL

allows extraction of oxidized sulphur compound in relatively short timeat room temperature58

.

Increasing temperature reduces the viscosity of IL which increases the catalytic performance of

IL. However, the decomposition of H2O2 accelerates at higher temperatures, leading to reduced

desulfurization efficiency61

.

3.3. Other Pattern of ODS with ILs

3.3.1. Supported ILs as Catalytic Solvent

Processes that focused on homogeneous catalytic systems need large amount of ILs that are

costly and may affect the economic viability of a potential process. The recovery of the ILs is

another problem. Many groups have committed to heterogeneous catalysis system and turned

to studying supported ILs. Immobilizing a certain amount of ILs on solid carriers could obtain

ILs supported catalysts. The obtained catalysts combined the advantages of ILs with the easy

separation and recovery performances of the supported materials. Xun et al 62

in their study of

the synthesis of metal-based IL supported catalyst and its application in catalytic oxidative

desulphurization of fuels prepared the heterogeneous catalyst by embedding [C4mim]FeCl4 IL

in silica gel. Scheme given in Fig. 16was followed to synthesize the [C4mim]FeCl4/silica gel


21

catalysts. The amount of [C4mim]FeCl4 decreased sharply to reach deep desulphurization and

solved the problems of recycle. The catalytic oxidation reaction of different S-containing

compounds decreased in the order of DBT > BT > 4,6-DMDBT62

.

Although heterogeneous catalysts showed excellent performance in catalyst recovery and

reuse, the catalytic sites were lower exposure to the reactants. Thus, their activities were

usually lower than those of homogeneous catalysts. Additionally, heterogeneous catalyst also

required relative long reaction time and easily leached of active species into the reaction

medium63

.

3.3.2. Catalytic ODS using IL Emulsion System

Ge et al64

have developed a catalytic oxidation IL emulsion desulphurization system, which

composed of water-immiscible IL ([C4mim]PF6), 30 wt % H2O2, and an amphiphilic catalyst

[C18H37N(CH3)3]7[PW11O39]. Here [C18H37N(CH3)3]7[PW11O39] behaves both an emulsifying

agent and a catalyst instead of only a surfactant. This kind of catalyst not only maintains the

emulsion droplets stable but also provides higher interfacial surface area where the oxidation

of S-compounds takes place. During the reaction, the IL emulsion functions as highly

dispersed microreactors. The S-compounds in the model oil were first extracted into the IL

emulsion phase and then oxidized to their corresponding sulfones by

[C18H37N(CH3)3]7[PW11O39] in the H2O2/[C4mim]PF6 interface. The sulfones accumulated in

the [C4mim]PF6 phase. After reaction, the desulphurization system quickly divided into two

layers; the deep desulphurization can be achieved in this way. After the reaction, the reaction

system was still a biphasic system, so the oil could be separated by simple decantation from the

biphasic system of the IL easily. The used ILs, whose structure does not change and the

recovered IL can be reused for further emulsion catalytic oxidation cycles. For example


22

[C4mim]PF6 emulsion desulphurization system could be recycled five times with an

unnoticeable decrease in catalytic activity.

The milky-white IL emulsions are formed when H2O2 and [C18H37N(CH3)3]7[PW11O39] were

added into [C4mim]PF6. The catalyst is distributed in the interface of H2O2 and water-

immiscible ILs. In this emulsion reaction system, the catalyst molecule acts as an emulsifying

agent, could be uniformly distributed in the interface of H2O2IL, and forms a film around the

dispersed IL droplets (Fig 17). Consequently, the lipo philic quaternary ammonium cations of

the amphiphilic catalyst would lie on the oil side and the hydrophilic hetero poly anions would

lie on the H2O2 side. Since hetero poly anions ([PW11O39]7-) depolymerized into several

smaller active species including [(PO4)(WO(O2)2)4]3-(PW4), [(PO4)(WO(O2)2)2(WO(O2)2

(H2O))]3-(PW3), and [(PO3(OH)) (WO(O2)2)2]2-(PW2) in the presence of H2O2, because the IL

emulsion was immiscible with n-octane and formed a biphasic system. The S-compounds in

the model oil were first extracted from oil phase into IL phase and oxidized to their

corresponding sulfones by the active species simultaneously; the sulfones accumulated in IL

phase. After reaction, the desulphurization system quickly divided into two layers. In this way,

the catalyst in the emulsion droplets can be readily separated from the oil and recycled, and the

IL emulsion system could be recycled five times without an obvious decrease in activity65

.

3.3.3. Photo-catalytic ODS using ILs

Photo-catalytic ODS (PODS) is also considered as aneective alternative method. In the

PODS process, S-compounds in nonpolaroil phase were extracted to polar extractant phases,

such as water, acetonitrile, or ILs, in which they were photo-oxidized to the corresponding

sulfones in the presence of O2, H2O2, or photosensitizers66-68

. With the development of photo-

catalysis, TiO2 become the most used photo-catalyst due to its nontoxicity, chemical stability,


23

low cost, and high photo-catalytic ability. TiO2 was also used in PODS69, 70

.Xiao et.al71

prepared Nano-TiO2 in [C4mim]BF4 IL via microwave radiation. In this process, IL was used

not only as microwave absorption medium for preparation of TiO2 but also as extractant for

DBT. The eect of microwave time on the photo-catalytic desulfurization ability of prepared

TiO2 in the presence of O2 was investigated. 98.2% and 94.3% sulfur could be removed from

model oil and actual diesel oil, respectively, in 10 h UV irradiation under the conditions that

V(IL)/V(oil) = 1:5, air flow = 200 mL/min. The [C4mim]BF4 IL was recycled five times with a

slight decrease in desulfurization eciency.

The in situ photo-catalytic oxidation process for DBT was proposed by Xiao et.al71

as shown

in Fig 18. TiO2 was prepared from titanium tetraisopropoxide (TTIP) in [C4mim]BF4 IL. When

[C4mim]BF4 IL containing TiO2 was mixed with model oil that contained DBT, DBT was

extracted from oil phase to the IL phase. When the TiO2 in the IL was irradiated by UV light,

the photogenerated holes reacted with water or hydroxyl groups on the surface of TiO2 to form

OH radicals, which oxidized DBT to DBTO2. Then, the DBT was transferred from oil to IL

and was degraded continuously.

Mohd Zaid et al72

studied the Photo-oxidativeextractive deep desulfurization of diesel

using CuFe/TiO2 and eutectic ionic liquid. First they synthesized series of bimetallic Cu

Fe/TiO2 photo-catalysts using solgel hydrothermal method and then photo-catalysts were

evaluated for photo-oxidative extractive deep desulfurization of model oil. The study

identified that 2.0 wt% CuFe/TiO2 photo-catalyst can efficiently oxidize sulphur species

under mild conditions in the presence of hydrogen peroxide (H2O2:S molar ratio of 4) as

oxidant and eutectic based ionic liquid as extractant.

3.4. Challenge and Perspective of ODS using ILs


24

Aiming at the replacement or supplement of the present inefficient and expensive HDS, to

remove those heterocyclic S-compounds to produce clean ultra-low S or S-free fuel oils, ODS

using ILs have been intensively studied recently. The research results presented and discussed

above indicate a good perspective for such a new method, though some problems accompanied,

the advantages and possible limitations are listed in Tab. 7.

Temperature, reaction/extraction time, molar ratio of O/S, mass ratio of IL/oil, etc., have

been thoroughly investigated, showing exciting results. Those heterocyclic S-compounds in

fuel oils, which are unreactive to traditional HDS, can be expected to be removed completely

at mild conditions such as lower temperature (e.g., room temperature) and atmospheric

pressure after few minutes of contacting. Using ILs instead of traditional volatile organic

solvents in ODS excludes the loss and contamination of solvents and possible safety issue,

makes the regeneration and separation of solvents or ILs easier, and the regenerated ILs can be

recycled without any noticeable activity loss. It is worthy of noting that the properties of ILs

vary enormously as a function of their molecular structure, that is to say that the property of

ILs can be tailored through elaborately selecting the paring of cation and anion or modifying

the structure of cation or anion. Therefore, there is still a very enormous room left to optimize

the ILs in their specific application in ODS.

Compared with the ODS using ILs as solvent and extractive reagent where one acidic

catalyst is required generally, the ODS using some functional acidic ILs as both extractive

reagent and catalyst where no additional catalyst is required is a better option, because there

does not exist some problems such as difficult regeneration and recycling of catalyst in

homogeneous catalysis and possible contamination of oils by catalyst. Some other options such

as supported ILs as catalytic solvent, ILs emulsion and ILs photo-catalysis are also worthy of

being investigated.


25

There are some issues to be considered such as treatment of waste sulphones as oxidation

product of S-compounds, cost of ILs and selectivity of ODS. In addition, most studies

performed currently are focused on the investigation of model fuel oils, and researchers are

encouraged to investigate the desulfurization of real feedstock such as gasoline and diesel fuels.

Compared with many studies on desulfurization experiments, the desulfurization mechanism

are rather scarce, and some computer simulations coupled with spectrum characterization

studies will favor uncovering the mechanism. In near future, to promote the final pilot test or

industrial application of such a new technology, some studies are desired, e.g., development of

cheaper and more active ILs; set-up of kilogram/ton-scale of desulfurization process;

optimization of operation conditions; investigations of loss, lifetime/stability, regeneration and

reusability of ILs; evaluations on oil yield and oil quality.

Concluding Remarks

S-compounds are the major harmful impurity in fuel oils, which result into the severe

environmental issues in the combustion of fuel oils. Desulfurization of fuel oils is a key

process in oil refining. Traditional HDS, due to its ineffectiveness to remove some heterocyclic

S-compounds in fuel oils, is facing some challenges in producing clean ultra-low S or S-free

fuel oils. Alternative methods are desired, among which ODS using ILs has been intensively

studied recently. In this work, we reviewed these research results of ODS using ILs since 2003

when Loet al reported the first results, also the traditional ODS. This review clearly indicates

that ODS using ILs can effectively remove those heterocyclic S-compounds in fuel oils and

reduce the S-content to < 10 ppm under mild conditions with a very good perspective for the

application in future, although some issues have to be also addressed in future such as


26

treatment of waste sulphones, cost of ILs, selectivity of ODS and economic-technological

evaluations at large scale of process.

The development of functional ILs and the ODS using such functional ILs as both extractive

reagent and catalyst, also supported ILs as catalytic solvent, ILs emulsion and ILs

photocatalysis, are expected to be studied further in future.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China

(21176021, 21276020). We extend our appreciation to the Deanship of Scientific Research at

King Saud University for funding the work, through Research Group Project No. RG-1436-

026.

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28

Fig.1. Maximum diesel sulfur limits, January 201473

.


29

Fig.2. Maximum gasoline sulfur limits, January 201473

.


30

Fig.3. S-compounds in fuel oils.


31

Fig.4. Flow chart of biphasic simultaneous oxidation/extraction ODS unit.


32

Fig.5. ODS reaction, biphasic system31

.


33

Polar Phase

Non-polar phase

Fig.6. Homogeneous biphasic ODS system without PTA.20


34

Fig.7. Mechanism proposed for oxidation of S-compound to sulfone over the active species21

.


35

+ Mo=O

k1 O

OMo + H2O

O

OMo

O

OMo

+

+

R2S

R2SO

Mo=O

Mo=O

+

+

R2S

R2SO

k2

k3

H2O2

Fig.8. Sequence involved in the oxidation reaction21

.


36

Fig.9. Time-course variation of DBT removal and ln(C0/Ct)21

.


37

Fig.10. Kinetics for oxidative desulfurization of DBT and BT by [C63MPy]Cl/ FeCl3. Conditions: moil=9g, mIL=

3 g, O/S molar ratio = 4, T = 298 K23

.


38

N+ N

CH3

CH33HCN

CH3

CH3

3HC

CH3

N+

3HC CH3

S+

CH3

3HC CH3

P+

CH3

3HC

CH3

CH3N+

CH3

3HC

CH3

CH3

ImidazoliumPyridiniuim

Pyrrolidinium

Sulfonium PhosphoniumAmmonium

Fig.11. Different structures for IL cations.


39

Fig.12. The effect of different S-compounds on S-removal efficiency. Experimental conditions: 5-mL model oil,

n(H2O2) 0.39mmol, m([pmim]FeCl4-MCM-41)50.06 g, V([Omim]BF4)51 mL, T=30oCat 30

oC, time =1 h

46.

20 40 60 80 100 1200

10

20

30

40

50

60

70

80

90

100S

ulf

ur

rem

oval eff

icie

ncy (

%)

Time (min)

4,6DMDBT

BT

DT

DBT


40

0 2 4 6 8 10 12 14 16 18 20 2240

50

60

70

80

90

100

Su

lfu

r re

mo

val eff

icie

ncy (

%)

Time (min)

DBT

BT

Fig.13. Effect of different S-compounds on S-removal by [C63MPy]Cl/FeCl3. Conditions: initial S-content 1000

ppm, moil=9g,mIL= 3 g, O/S molar ratio = 4, T = 298 K 23

.


41

2:1 4:1 6:1 7:1 8:1 8.5: 1 9:10

20

40

60

80

100

S-r

em

oval effic

iency (%

)

Molar ratio of O/S

Fig.14. Influence of the molar ratio of O/S on S-removal efficiency by [C4mim]Cl/3ZnCl2-H2O2(temperature,45C; time,

3h; initial S-content, 505ppm; mass ratio of IL/oil, 1/2)47

.


42

Fig.15. Influence of the mass ratio of IL/oil on S-removal efficiency by [C4mim]Cl/3ZnCl2-H2O2 (temperature, 45

C; time, 3 h; initial S-content, 505 ppm; molar ratio of O/S, 8)47.


43

Fig.16. The synthesis process of [C4mim]FeCl4/silica gel catalyst62

.


44

Fig.17. Catalytic Oxidation of DBT in IL Emulsion System64

.


45

Fig.18. In Situ Photo-catalytic Oxidative Desulfurization Process of DBT71

.


46

Table.1. Different adsorbents used in ADS at optimized conditions5.

Adsorbents Model

oil

S-

comp.

Initial S: (ppm) Temp. (K) Pressure

(atm)

S-removal

Activated carbon gas oil DBT 178 198 1 95%

gas oil TS 300 243 1.5 88%

Alumina gas oil DBT 700 393 n/a 30%

Zeolites from coal hexane TS, BT 500 303 1 63%

NiMoP/Al2O3 hexane DBT 450 600 40 56%

Gallium +Y -zeolite nonane DBT 500 333 n/a 97%

Cu-zirconia octane TS 2000 180 n/a 99%

Ruthenium complexes hexane DBT 40 298 1 55%


47

Table.2. Bio-desulfurization by different microorganisms5.

Microorganism Model oil Initial S: (ppm) Temp (K) S-removal

Gordoniaalkanivorans RIPI90A hexadecane 320 303 90%

Mycobacterium sp. ZD-19 hexadecane 92 303 100%

Mycobacterium goodii X7B tertradecane 200 313 99%

Rhodococcuserythropolis IGTS8 hexadecane 100 303 80%

Gordoniaalkanivorans strain 1B heptane 100 308 63%

Bacillus subtilis WU-S2B tridecane 100 323 50%

Pseudomonas stutzeri UP-1 hexadecane 500 304 74%

Sphingomonassubarctica T7b gas oil 280 300 94%

Bacterium, strain RIPI-22 hexadecane 100 303 77%

Pseudomonas delafieldii R-8 diesel oil 591 303 47%


48

Table.3. List of some common oxidants and their active oxygen content 74

.

Oxidant Active oxygen (wt %) By-product

H2O2 47.1 H2O

t-BuOOH 17.8 t-BuOH

HNO3 25.0 NOx, N2O, N2

N2O 36.4 N2

NaClO 21.6 NaCl

NaClO2 35.6 NaCl

NaBrO 13.4 NaBr

C5H11NO2a 13.7 C5H11NO

KHSO5b 10.5 KHSO4

NaIO4 29.9c NaI

PhIO 7.3 PhI

H2O2 47.1 H2O

a N-Methylmorpholine N-oxide (NMO).

b Stabilized and commercialized as the triple salt: 2KHSO5KHSO4K2SO4.

c Assuming all 4 oxygen atoms are used.


49

Table.4. S-removal efficiency vs mass ratio of IL/oil for [C4mim]Cl/ 2ZnCl2 and [SO3H_C4mim] HSO447

.

ILs Oil O/S

Ratio

Time

(h)

Temp.

(K)

ODS efficiency for IL-oil mass ratio

1:5 1:3 1:2 1:1 2:1

[C4mim]Cl/ 2ZnCl2 Model

diesel

8:1 1 363 92.2 97 98.5 100 n/a

[SO3H_C4mim]HSO4 8:1 6 333 93.9 95 96 98.1 n/a


50

Table.5. Recycling of [C63MPy]Cl/FeCl3 in desulfurization of model oil

23.

Cycle Cycle S-removal [%](by ILs

with distillation)

S-removal [%](by ILs without

distillation)

1 100 100.0

2 100 99.8

3 100 97.4

4 97.8 92.9

5 94.5 85.4

Conditions: initial sulfur (as DBT) content 1000 ppm, moil = 9 g,

mIL= 3 g, O/S molar ratio = 4, t = 20 min, T = 298 K.


51

Table.6. Variation of types of ILs in the desulfurization system45

.

Entry ILs S-removal [%]

Without H2O2[a] With H2O2[b]

1 [(C8H17)3CH3N]Cl/FeCl3 30.6 97.9

2 [(C8H17)3CH3N]Cl/CuCl2 18.6 26.2

3 [(C8H17)3CH3N]Cl/SnCl2 22.1 25.8

4 [(C8H17)3CH3N]Cl/ZnCl2 20.5 19.8

5 [(C4H9)3CH3N]Cl/FeCl3 15.3 95.8

6 [C10H21(CH3)3N]Cl/FeCl3 17.2 93.4

7 [(C10H21)2(CH3)2N]Cl/FeCl3 29.0 98.7

[a] Experimental conditions: model oil (5 mL), IL (0.702 mmol), T=25C, t=1 h. [b] Experimental conditions: model oil (5 mL), IL (0.702 mmol), T=25C, t=1 h, n(H2O2)/n(S)=14:1.


52

Table.7. Advantages and limitations of ODS using ILs.

Advantages Limitations

mild conditions such as room temperature and atmospheric

pressure withconventional reaction and separation refinery

equipment

no consumption of expensive hydrogen or noble catalyst (in

some cases, functional ILs can act as solvent and catalyst)

high efficiency to remove heterocyclic S-compounds

immune to HDS

easy regeneration, recycling, separation of solvent or ILs

excluding of solvent loss and contamination

enormous room left to design and prepare functional ILs

with better desulfurization performances

other advantages such as low-cost and high selectivity

waste treatment of S-compound oxidized product of

sulfones

cost of ILs

possible side reaction in ODS

quantity and yield of fuel oil


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