This is a review paper which describes the advancements in techniques of diesel desulphurization.

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Advancements in Diesel Desulfurization Techniques
Saifullah Azam, Tahir Rasheed, Qumar Bilal, Ali Asghar
saifullahazam@yahoo.com
COMSATS Institute of Information Technology Lahore Pakistan.
ABSTRACT
Diesel is mainly consisted of paraffin and thio-aromatic compounds. The presence of sulfur in the diesel exhaust
is the major challenge for oil refineries. Sulfur is an oxidizing element that discharges in the form of SOx in the
environment which causes adverse effects on human and animal life. The scope of this review paper is to
discuss and highlight the recent development in process of desulfurization of diesel oil, and to to explore the less
energy intensive and most economical process. Several methods such as hydro-desulfurization (HDS), oxidative
desulfurization (ODS), ionic liquid desulfurization, bio-desulfurization & adsorptive desulfurization have been
used to reduce the concentration of sulfur from diesel fuel. Adsorptive desulfurization technique is found to be
environmental benign, less energy intensive and most economical as compared to others. Optimization of
adsorptive desulfurization technique may yield 100% desulfurization of diesel oil.
Key Words:
Diesel, Desulfurization, Oxidative Desulfurization, Ionic Liquid, Adsorption
1. Introduction:
Diesel exhaust is a common contaminant found in environment whether it is urban or rural. Its odor is
objectionable. Several environmental agencies even believe that it has potential carcinogenic effect and other
chronic diseases if inhaled for long time [1]. The major component of the diesel exhaust is sulfur that is allergic
to skin and can proliferate cancer [2, 3]. Sulfur is oxidized to SOx when oil is burned, which discharges into
environment in the form of fine particles resulting adverse effect on environment [4, 5]. There are many
compounds of sulfur present in the diesel, such as: thiophene, dibenzothiophene (DBT), benzothiophene (BT),
Dimethyl-dibenzothiophene (DMDBT) [6]. Therefore, it is needed to develop some processes to minimize the
toxic effects produced from diesel exhaust. Removal of sulfur from crude oil can reduce its lethal effects
significantly. Desulfurization of fuel oil is a big challenge for the current refineries to meet the standards set by
environment protection agencies regulations. There are different techniques reported for the desulfurization of

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diesel oil [7]. In addition, many are being developed: hydro-desulfurization (HDS), oxidative desulfurization
(ODS), adsorptive desulfurization, catalytic desulfurization and bio-desulfurization [8].
2. Chemical Methods for Diesel Desulfurization
2.1 Hydrodesulfurization
Hydrodesulfurization (HDS) is primarily used in petrochemical industries. Sulfur present in the form of thiols
and thiophenes, which is undesirable in diesel oil [9]. In HDS process, hydrogen combines with sulfur to form
hydrogen sulfide (H2S) [10, 11].
HDS is used in many oil refineries for reduction of SO2 emissions [12]. This technique offers some major
advantages. It is more cost effective due to the use of less hydrogen having sulfur removal efficiency up to 80-
98% with short reaction time. Furthermore, a vast variety of catalysts can be used in HDS process.
Hydrodeselfurization of diesel oil is catalyzed by Nickel and Molybdenum (Ni & Mo) nano powder. The
catalysts are prepared by mixing of metals with fibers, followed by ultrasonic oxidation. Their characteristics are
comparable to the Russian catalysts (CoMo/γAl2O3). Ni and Mo catalysts remove 180 and 100 ppm of sulfur
from diesel oil respectively., On the other hand, the Russian catalyst removes sulfur up to 190 ppm [13]. HDS,
with assistance of platinum (Pt) over alumina (Al) catalyst at atmospheric pressure and 290-350ºC temperature,
gives the favorable results for sulfur removal up to 60%, with low H2O2/fuel ratio [14]. HDS activity is reported
at 320ºC reaction temperature using Ni-Mo catalysts supported by Al, P modified hexagonal mesoporous silica
(HMS) substrate. Direct Desulfurization (DDS) and hydrogenation reactions removed 4, 6-DMDBT, while
thiophenes leads to the formation of butadiene and butane [15].
HDS is done in three stages by using Co-Mo catalyst in the first stage and Ni-Mo catalyst in the further two
stages under H2 at 2.9 MPa. Temperature is kept high in the first two stages for sufficient removal of BT and
DBT, while it is low at third stage to remove the fluorescent color and sulfur content is achieved <200 ppm [16].
Nickel and Cobalt catalysts with the combination of tungsten, molybdenum and sulfur, are prepared using
impregnation of zirconium doped mesoporous silica. HDS shows higher performance at low temperature
(340ºC) and moderate pressure of 3MPa. Ni containing catalysts are more efficient as compare to Co catalysts.
The most appropriate catalyst is 5 wt% of Ni and 20 wt% of tungsten (W) that shows the remarkable stability of

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HDS against removal of DBT [17]. The selectivity of biphenyl is increased by Co-Mo catalyst with support of
Al-HMS in the HDS of DBT [18].
CoMo/Al2O3 and NiMo/Al2O3 are the conventional catalysts used in HDS process. CoMo/MCM-41 activity is
higher as compared to CoMo alumina catalyst for HDS activity [19]. HDS is carried out using impregnated
alumina supported Mo and Co catalysts at 400ºC and 400Psi for 3h reaction time. DBT conversion is achieved
up to 69-77% while biphenyl product selectivity obtained up to 60-64% [20]. Impregnating of V2O5 on -Al2O3
or TiO2-ZrO2 affected the morphological structure of CoMoSx to enhance the efficiency of HDS for removal of
4, 6-DMDBT [21]. HDS is carried out under action of two mixed matrix NiMo/Al2O3 with addition of nano and
micro-sized zeolite (Y). Nano sized zeolite containing catalyst showed higher desulfurization and higher rate
constant as compare to micro sized catalyst [22]. Fluid catalytic cracking (FCC) diesel desulfurization is
increased using NiW/AMTB catalyst with the addition of TiO2 due to higher hydrogenation activity. The sulfur
content can be reduced up to 10 ppm at 350ºC temperature and 5MPa pressure [23, 24].
Ni, Cu and Mo catalysts are prepared by the chemical precipitation method with ammonium heptamolybdate.
The sulfur compound 4, 6-DMDBT is removed up to 80.6% by Ni9.5Cu0.5Mo10 [25]. Hydro-desulfurization is
incorporated with NiMoS/Al2O3 as catalyst. Rate of desulfurization is found proportional to the amount of
catalyst, temperature, pressure and hydrogen flow rate with the variations in the parameters [26]. Pd-Pt USY
zeolite is also used for the HDS activity. At 200ºC calcination temperature nearly at the rate of 2.72/h, sulfur
products are removed [27]. Sulfur content is reduced up to 10 ppm by utilizing carbon supported Ni-Mo-sulfide
catalyst of 916 to 3075 m2
/g surface area to remove the 4, 6-DMDBT at 337ºC [28]. Ultra deep HDS can be
enhanced with the help of nano alumina that has greater surface area, pore size and pore volume than
conventional alumina. The addition of boron and phosphorus on nano alumina boost the removal of sulfur up to
99.92% with Co-P-B-Mo/nano Al2O3 [29]. The combination of metal promotes the desulfurization. These metals
can be Ru, Mo, Ni and Co which combine and increase the activity of desulfurization. DBT conversion is
achieved up to 73% by using only 1 wt.% of Ru at 38 ºC [30].
Hydro desulfurization of 4, 6-DMDBT is performed using flow reactor. Decalin is used as a solvent in this
reactor. The reactor is operated at 613K and 4MPa. After reaction, Varian 3400 chromatograph is used for
analysis of liquid sample at temperature range of 373-503K. Sulfur content is removed up to 80% [31].
Haldor-Topsoe develops the TK-554 and TK-574 active catalyst which promotes the efficiency of HDS to
reduce the sulfur products up to 60ppmw and 30ppmw respectively [32].

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Addition of sodium on rhodium phosphide (Rh2P) is utilized to know the effect on HDS activity. The results of
temperature programmed reduction (TPR) and XRD show that HDS activity is higher at low reduction
temperature [33].
HDS could be enhanced by using different catalysts and their modifications. Ni and Co are the most promising
catalysts that can increase the efficiency of desulfurization process. The combination of other metals like boron
and phosphorus may also increase the diesel desulfurization up to 99.92%.
2.2 Oxidative Desulfurization:
In oxidative desulfurization (ODS), molecular oxygen is used to remove the sulfur compounds from diesel oil
which is supplied by different types of oxidizing agents such as hydrogen and organic peroxides. Sulfur is
removed in the form of sulfones which are polar compounds and can easily be extracted at the end of reaction.
Sulfur compounds (BT, DBT, 2,4-DMT, 4,6-DMDBT) which are difficult to remove using HDS process while
could be easily removed by oxidative desulfurization (ODS). ODS is performed at mild temperature pressure
conditions (100ºC, 1 atm) and no special arrangement of reactor nor hydrogen is required for this process.
Oxidative desulfurization is more beneficial method as compared to HDS and extraction method due to its low
operational cost. The electronegativity difference between S-O (1.0) is higher than S-C (0.03) which enhances
the reaction activity [34]. DBT and their derivatives are removed with the help of biphasic system in which n-
octane/acetonitrile is used as phase transfer agent. The reaction is occurred at 60ºC in the presence of H2O2
oxidant. It is examined that 95% of sulfones are removed in acetonitrile [35].
TiO2 anatase over vanadium pentaoxide (V2O5) is utilized for the oxidation of BT, DBT and 4,6-DMDBT from
diesel oil in the presence of H2O2. It is observed that more than 80% of these compounds converted into sulfones
for 60 min. A controlled amount of oxidant is required to hinder the formation of water which can impede the
process. Only 20% of DMDBT, 40% of DBT and 50% of BT removal of sulfones is possible by extraction [36].
The further modification in this technique is investigated using alumina over V2O5 as a catalyst and by changing
the phase transfer agents like acetonitrile, N, N-Dimethylformamide (DMF) and gamma-butyrolactone. It is
observed that 100% DBT is oxidized in the process while 85% is removed by physical DMF extraction [37]. 4,
6-DMDBT is removed by using Na2CO3/H2O2 as a catalyst assisted by 23 kHz ultrasound radiations. The
feasible temperature limit is kept lower than the boiling point of acetonitrile.Using these conditions >90%
removal of 4, 6-DMDBT is obtained [38]. Na2CO3 is used with the addition of hydrogen peroxide and formic

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acid. The hydrogen peroxide and formic acid react with each other and form a peroxyformic acid for the
oxidation of BT, DBT from diesel [39].
Complex compounds such as iron containing tetra amid macrocyclic legend (Fe-TAML) [40], vanadoperiodate
[C8H17N(CH3)3]3HIV9O28 [41] and chromium terephthalate metal-organic framework MIL-101 and the
tetrabutyl ammonium (TBA) salt of a zinc-substituted polyoxotungstate anion, TBA4.2H0.8[PW11Zn(H2O)O39]
(denoted PW11Zn) [42] are used for the removal of sulfur from diesel oil. Fe-TAML is used as catalyst in
slightly alkaline medium (pH 8.0) with a tert-butanol as co-solvent which is responsible to soluble the DBT
from solution. The process in executed in the aqueous phase so there is no mixing of catalyst and oxidant with
remaining diesel. Only 75% sulfur is removed at 50ºC temperature and 3h reaction time [40]. The
vanadoperiodate complex compound is synthesized for the removal of DBT, 4,6-DMDBT from diesel. It acts as
an oxidizing agent as well as catalyst at mild reaction conditions. There is no need of oxidant and catalyst
separately due to dual nature of vanadoperiodate complex. DBT & 4,6-DMDBT are removed (99%) at 90o
C.
The reaction time can be decreased from 6 to 2h by increasing temperature to 100ºC [41]. A new composite
compound PW11Zn is prepared for the removal of total sulfur (BT, DBT, 4,6-DMDBT). It gives 100% reduction
of sulfur from diesel at 50ºC in 2h in the presence of H2O2 oxidant. This catalyst shows the high compatibility
with ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6). Furthermore, this catalyst
showed the better stability with acetonitrile and gives maximum catalyst recovery [42].
Phosphotungstic acid (H3PW12O40) is prepared for the oxidation of DBT in the presence of H2O2 oxidant. 98%
of DBT is removed at 70ºC by using tetra octyl ammonium bromide as a phase transfer agent [43]. Keggin type
polyoxomatalates are used for the deep desulfurization of diesel fuel. H3PW6Mo6O40 catalyst has potential to
convert the DBT up to 99.79% at operating condition of 70ºC temperature, Oxygen / Sulphur ration 15 and 6.38
gL-1
amount of catalyst [44]. 1-butyl-3-methylimidazolium dodecatungstophosphate over silicon dioxide
([Bmim]3PW12O40/SiO2) has hydrophilic and hydrophobic nature. This catalyst is used for the removal of BT,
DBT and 4,6-DMDBT in the presence of H2O2 as an oxidizing agent. 98.2% reduction of 4, 6-DMDBT and
DBT in 60 min is achieved by using DMF as phase transfer agent. This catalyst can be reused for seven times
with slightly decreasing its activity [45].
Niobium pentachloride (NbCl5) is used with assistance of azacrown ether in the ODS with oxidizing media
hydrogen per oxide at 80ºC. Sulfur products are removed from 80% to 40% and can be decreased up to 13% by
increasing the concentration of H2O2 [46].

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Oxidation of sulfur can be used with the assistance of ultrasound radiation, gamma radiations [47] in the
presence of hydrogen peroxide oxidant. Oxidative desulfurization is explored with MoOx/Al2O3 and hydrogen
peroxide as an oxidizing agent with the assistance of ultrasound radiations that have an ability to activate the
chemical compounds for rapid reaction. The reactivities of sulfur compound increases in this order
BT>>DMDBT>DBT with help of 20 kHz ultrasound radiation. ODS can be used to oxidize the sulfur
compounds with assistance of ultrasound radiations using acetic acid and hydrogen per oxide [48]. Gamma
radiations are induced for removal of organically bonded sulfur but the removal low desulfurization is achieved
through this process. Only 1.392% sulfur compounds are removed with the help of 50kGy gamma radiations
which show that it is not suitable process [47]. Oxidative desulfurization using organic acids with the help of
ultrasound waves is a useful method for desulfurization of diesel. Acetic acid and formic acid gives 93% and
93.4% efficiency for 20 min in response of 20-50 kHz and 70% amplitude of ultra sound waves. But there is
only 36% efficiency achieved without using ultra sound waves [49]. Ferric chloride (FeCl3) and copper sulfate
(CuSO4) are used as catalysts with hydrogen peroxide (H2O2) assisted with ultrasonic treatment. These
chemicals achieved 70% reduction of sulfur contents in 7.5 min [50]. Commercial ferrate (VI) is used as an
oxidizing agent for desulfurization process. For the oxidation of sulfur compounds, ultrasound radiations are
applied in this process and examine the trend of % sulfur removal by the variation in time of sonication,
concentration of ferrate and phase transfer agent (PTA). The result show the removal of DBT up to 91% and BT
up to 85.7% in 30min by using 123mg of PTA [51].
Photo-catalytic desulfurization is a process to oxidize the 2,4 dimethylthiophene (DMT). DMT can be converted
up to 97.4% with assistance of photo irradiation on TiO2-pillared montmorillonite catalyst in oxidizing media at
298K [52].
Fenton-like catalysts are also used in oxidative desulfurization. Heterogeneous catalysts like Fe/activated carbon
(AC) and Fe/AC-H2 are prepared for this process. It is observed that Fe/AC-H2 act as dual nature like catalyst
and adsorbent. Acetonitrile gives about 94.5% removal of sulfur. This catalyst can be recycled at the end of this
process but after recycling 4 times, the activity is decreased up to 78.5% [53].
Un-extracted SOx are removed after oxidizing diesel sample with H2O2, with the assistance of porous zeolite
catalyst (composed of alumina and silica) supported with single walled carbon nano tubes (SWCNT’s). Total
sulfur is removed up to 80% by using this method, here SWCNT’s are used to increase adsorption capacity of
zeolite as carbon nano tubes provide large specific surface area [54].

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ODS is an efficient process for removal of sulfur content up to 80-97% by using different phase system and
ultrasonic assistances. The gamma radiations assisted oxidative desulfurization has been reported in more
efficient processes.
2.3 Desulfurization by Micro-Organisms (Bio-Desulfurization):
Bio-desulfurization (BDS) is a new technique in which bacterial strain is isolated from industrial sludge or
wastes having sulfur eating properties and grown in sulfur-free environment introduced to the fuel oil [55].
These strain use sulfur content selectively and convert DBT, BT and thiophenic compounds into 2-hydroxyl
biphenyl (HBP) and other compounds which are less hazardous than former ones. The desulfurization is
dependent on the activity of the strains. Ultra-low sulfur content in HDS treated fuels can be achieved by using
this technique. The strain can be utilized several times [56]. Several strains such as Pantoeaagglomerans
D23W3 [57], Achromobacter sp [58], Pseudomonas delafieldii [59], Rhodococcuserythropolis LSSE8-1-vgb
[60], Rhodococcus sp and A. Sulfureus [61] , and Gordonia sp C-6 [62] can be used to desulfurize fuel oils.
Bacterium which reduces sulfur either by eating or converting sulfur into separable species are mostly extracted
from petroleum oil fields, soils and waste oil sewage or pools.“Pantoeaagglomerans D23W3”, isolated from
contaminated soils that is collected from refinery is utilized for desulfurization of diesel oil. This bacterium
gives 22% desulfurization for lignite. It can also be used for the diesel desulfurization [57].
“Achromobacter sp” is extracted from a sample of contaminated petroleum oil. Bacteria consume the DBT and
4-MDBT and converts into 2-HBP which is further converted into 2-MBP by methylation. The experiment takes
96h for desulfurization process with 7.1% reduction in total sulfur [58]. “Pseudomonas delafieldii” is achieved
from an oil field of China and then it is immobilized in calcium alginate beads. The thiophene and DBT is
simultaneously reduced up to 40% and 25% respectively. The immobilized cell can be utilized for 15 cycles for
bio-desulfurization. Total desulfurization time is taken up to 450h [59].
Bacterial strain termed as “Gordona strain CYKS1” is successfully used for the conversion of DBT into 2-HBP.
The strain is introduced to diesel oil for 12h, the sulfur content is significantly reduced (0.15% w/w to 0.06%
w/w) [63]. “Gordonia sp (Strain ZD-7)” extracted from sludge is grown in fuel oil for removal of DBT. Bacteria
strain is introduced into a reactor of volume 500mL with 100mL model fuel oil. After 24h, DBT is converted to
2-HBP. The strain can be utilized up to 193.5h for desulfurization process reducing DBT from 2.82mM to
0.23mM [64]. “Rhodococcuserythropolis LSSE8-1-vgb” is modified by the introduction of nano -Al2O3 on the

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magnetic immobilized Rhodococcuserythropolis. This modification increases the strain efficiency and
desulfurization rate approaches to 20% which was only 5% earlier [60].
Bacterium are isolated and grown in sulfur free environment, then after growth they are introduced to diesel oils
for desulfurization. For ultra-low sulfur content, they are often introduced after HDS treatment. “Rhodococcus
sp” and “A. sulfureus” microorganisms are grown in oil media to remove the sulfur products. These bacteria
strains convert the DBT into 2-HBP and give the promising result of desulfurization (50% by Rhodococcus sp &
53% by A.sulfureus) [61].
“Rhodococcuserythropolis DS-3” and “Gordonia sp C-6” are adapted for biodesulfurization to remove sulfur
from HDS treated diesel oil. The synergistic approach leads to remove most of heterocyclic sulfur compounds
from the diesel oil. The strain converts the BT and DBT into 2-HBP and successfully removes 86% of total
sulfur [62]. Desulfurization of light cycle after HDS is done with bacterial strain. Bio-desulfurization is carried
out by growing “Rhodococcus sp Strain ECRD-1” in sulfur free environment and further combined with
vitamins and minerals. 20 ml of culture is added to model compound of 1L diluted oil. Culture extracts are
concentrated with the help of nitrogen gas stream after incubation and shaking. The sulfur content is reduced
from 669ppm to 56ppm [65].
Selective removal of DBT is done with the help of newly isolated bacteria ZD-M2 (Micro bacterium). Sulfur is
converted into two products of 2-methoxybiphenyl (2-MBP) and 2-hydroxybiphenyl (2-HBP). The strain is also
capable of total degradation of 4, 6-DMDBT, thiophene, BT and 70% diphenylsulfide [66]. The culture
condition of bacterial strain is improved with the help of computer aided bioreactor. The growth of strain is
enhanced with the addition of carbon at constant pH. The sulfur content is reduced up to 90.3% [67].
“Rhodococcuserythropolis DS-3” and “Gordonia sp C-6” bacteria have the potential to reduce the sulfur content
up to 14% while ZD-M2 (Micro bacterium) is also a best candidate to remove the sulfur content from the diesel
oil up to 90.3%.
2.4 Ionic Liquid Desulfurization:
Ionic liquids (ILs) are the salts in liquid state that have low melting point at work at room temperature. Many
ILs are generated and being used as solvent since last decade [68]. Different types of ILs are used for the
desulfurization of diesel, such as pyridinium [69], dialkyl-pyridinium tetrachlorferrates [70], butyl-pyridinium
tetrafluoroborate [71], thiazolium [72], imidazolium containing alkyl sulfate and nitrogen compounds [73], 1-

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butyl-3-methylimidazolium [74], 1-butyl-4-methylpyridinium dicyanamide and 1-butyl-1-methylpyrrolidinium
dicyanide [75, 76], n-methyl-pyrrolidonium phosphate [77], quaternary ammonium coordinated ionic liquid [78]
and Piperazinium [79]. ILs are widely used due to their greater extraction power for the desulfurization of diesel
oil.
Thiazolium (IL) is used in extractive desulfurization. The selective removal of DBT using [BMTH]SCN is
81.2% in single extraction step and 88.5% when ultrasonic assistance is applied for 30 min. This IL can be used
up to 5 times, after which its efficiency of removing DBT decreases [80]. N-butyl-pyridinium tetrafluoroborate
is used for the removal of DBT as an ionic liquid. 97.5% DBT selective removal is reported using above
mentioned IL [71]. Dialkyl-pyridinium tetrachlorferrates IL is used for DBT removal from model diesel oil
using H2O2 as an oxidant and 99.9% desulfurization is achieved at room temperature for 10 min reaction time
[70].
Desulfurization is dependent of several parameters; temperature, solvent, reaction time and reactivity of
thiophene compounds with oxidant. The removal of desulfurization of DBT is increased from 76.1% to 99.8%
using catalytic solvent of acidic ionic liquid N-methyl-pyrrolidonium phosphate [(Hnmp).H2PO4] in H2O2
oxidizing media by variation in temperature from 40-60ºC and volume ratio from 1:4 to 1:1 [77]. Nickel boride
is a catalyst used for the reduction of sulfur in diesel oil. Nickel boride in situ is generated ILs. It is observed
that 88.6% desulfurization of DBT and its derivatives from diesel fuel is done. The regeneration of nickel boride
can be done with the addition of water and distillation at 110ºC and can be utilized up to 8 times by achieving
same efficiency [81]. Four types of thiazolium based ILs are used to remove DBT from diesel oil. The ILs 3-
butyl-4-mehylthiazolium dicyanamide removed the DBT (64%) at 25ºC for 20 min and ILs/oil (mass) ratio is
1:1 [72]. Imidazolium containing alkyl sulfate and nitrogen compounds are potential substances to remove the
sulfur compounds of BT and DBT. [Etmim][EtSO4] and [Mnim][MeSO4] have greater efficiency of
desulfurization as compared to [Omim][NO3]. Removal of DBT and BT is 81% and 68% after 5 extraction
cycles [73].
Pyridinium ionic liquids containing 1-(butyl, hexyl, octyl)-3, 5-dimethylpyridinium tetra fluoroborate are more
efficient for decomposition of aromatic heterocyclic compounds from diesel. The increase in number of carbon
atoms for Pyridinium ionic liquids can enhance the desulfurization. The percentage efficiency of sulfur removal
for C8, C6 and C4 is 47.1%, 38.8% and 28.2% respectively. Moreover the sulfur removed by these ILs are in
order of DBT˃BT˃4,6DMDBT˃TS [69]. Desulfurization of diesel oil is accomplished under catalytic oxidation

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and extraction through ionic liquid. Sulfur percentage removal of short alkyl chain with
peroxophosphomolybdate and H2O2 is low which can be increased to 97.3% by adding 1-butyl-3-
methylimidazolium IL [74]. Piperazinium based ionic liquid (IL) with addition of lactic acid anion is used for
desulfurization. The sulfur content is removed from 24.3ppmw to 11.7ppmw in single extraction. This IL can be
regenerated for 5 times. It is also more economic as compared to pyridinium and imidazolium based ILs [79].
Some complex compounds are also used as ionic liquids for the most appropriate choice for the extraction of
SOx up to 50-78% from aromatic compounds. There are many ionic liquid used for desulfurization process but
1-butyl-4-methylpyridinium dicyanamide [BmPYR][DCA] and 1-butyl-1-methylpyrrolidinium dicyanide
[BmPYR][DCA] are the more promising ILs that can be used for extractive desulfurization [75]. A new coupled
oxidative-extraction desulfurization method is introduced for the removal of sulfur. For this purpose new ionic
liquids [(CH2)4SO3HMIm][Tos] and [(CH2)4SO3HMIm][ZnCl3] are used as a catalyst and extractant in the
acidic medium with 30% H2O2. The S-content removed by this ILs is 98% [76]. Ionic liquids are used for
oxidative desulfurization of thiophene from diesel. Quaternary ammonium coordinated ionic liquid
[(C4H9)4NBr2C6H11NO] is used as a catalyst in H2O2 over acetic acid oxidizing media. Sulfur compound is
removed up to 98.8% at 40ºC, when H2O2 is used for 30 min with assistance of ionic liquid up to 0.20g [78].
ILs increase the efficiency of diesel desulfurization process. The most appropriate ionic liquid is dialkyl-
pyridinium that gives about 99.9% desulfurization of diesel oil. N-methyl-pyridinium phosphate is also another
ionic liquid that increase gives about 99.8% desulfurization in the presence of H2O2 oxidizing media.
2.5 Adsorptive Desulfurization:
Adsorption is a technique in which one or more selective components from a liquid or gas are sorbed on a solid
surface. Adsorbents have porous structure, the molecules of component to be separated by penetration into /
onto these porous structures. Normally, adsorption involves four steps. Addition of adsorbent, mixing,
separation of solute and solvent from bulk and removal of adsorbent from adsorbent’s surface. Different
adsorbents such as Cu (I)-Y zeolite [82], NaY/â & CeY/â zeolites [83], CuI
CeIV
Y zeolite [32], Clinoptilolite and
-Zeolite [84], metal impregnated activated carbons [85], montmorillonite, kaolinite, vermiculite and
palygorskite clays [86], and mixed metallic oxides [87] are reported as potential sorbents for desulfurization of
diesel.

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Commercialized fuel is desulfurized by adsorption process using -complexation with Cu (I)-Y Zeolite. The
experiment is performed in a fix bed absorber at room temperature. A diesel sample containing 335ppmw sulfur
content required 1g of Cu (I)-Y zeolite for making 14.7cm3
of gasoline sulfur free. When activated carbon (AC)
is used as guard bed, 19.6cm3
of gasoline is made sulfur free using 1g of AC. Thus, a thin layer of activated
carbon increases the capacity of adsorption of -complexation sorbent [82]. Modification of NaY/ zeolite (i.e
CeY/ and CaY/ zeolites) composite gives higher desulfurization as compared to unmodified NaY/
zeolite. In diesel fuel, thiophene compounds have special structure of lone pair electrons in sulfur atoms. If
zeolite is loaded with heavy metals then sulfide adsorption capacity of an adsorbent can be increased. Sulfur
removal is found 94.9% using CeY/ which is highest among NaY/ and CaY/ having efficiencies 26.8%
and 42.1% respectively [83]. CuI
CeIV
Y zeolite when used for adsorptive desulfurization, showed that 99.2%
sulfur is adsorbed at 50 C which is due to strong -complexation with sulfur compounds [32]. A composite
zeolite NaY modified with 0.09M caustic soda solution is used as adsorbent which has potential to remove
sulfur at comparatively enhanced level (99.9%) which is may be due to the structural change of catalyst from
micro to mesopores which not only removes thiophene but may also removes the big derivatives of thiophene
such as DBT and BT [88]. Desulfurization is perofrmed with the synergetic action of photo degradation and
adsorption to remove the BT from an organic solution. Photo degradation occurs under ultraviolet irradiation in
the presence of photo catalyst, 92% of BT decomposes at this stage. Clinoptilolite and -Zeolite are used to
remove rest of all the degraded sulfur [84].
Thiophenic compounds are removed with the assistance of Cu-zirconia adsorbent. These compounds adsorbed
more sulfur when there is an increase in the Cu content (maximum at 3% Cu). One gram of metal impregnated
adsorbent is found sufficient for the removal of 0.49 mmole of thiophene [89]. Activated carbon is impregnated
with AgCl, PdCl2 & CuCl2 separately and is used as an adsorbent in a fixed bed column. Palladium (Pd)-AC
adsorptive capacity is higher than AC by 448%, 698% higher than Ag-AC and 338% higher than Cu-AC due to
ð-complexation that provides high adsorption capacity [85]. Metal ion impregnated AC’s (Cu2+
/AC, Fe3+/
AC,
Ni2+
/AC), chitosan coated bentonite (CHB), aluminum oxide (ALU) and granular activated carbons (GAC) are
used for removal of benzothiophene sulfone (BTO) and dibenzothiophene sulfone (DBTO) from diesel sample.
Least removal of BTO is obtained using ALU, while Fe3+
/AC gives highest adsorption capacity from others.
DBTO removal is least with GAC, where Cu2+
/AC removed maximum DBTO. The difference in sulfur removal
is due to the adsorption capacity per unit area and surface area of adsorbents [90].

12. Page | 12
Granulated carbons produced from date stones and activated by ZnCl2 are used for the desulfurization of DBT.
It is reported that 86% of DBT is recovered in 3h of shaking adsorbent with model diesel oil, whereas 92.6%
DBT is removed if this process stayed for 48h. After 48h, no more desulfurization takes place [91]. Four
different activated carbons (CAA, CAB, CAC, CAD) prepared commercially (by Dutch company) are used for
sulfur removal. The 18h stirring process is executed at 1atm, 303K. The sulfur content is reduced from 72ppmw
to 15ppmw [92]. Activated date palm kernel powder is used with addition of activated carbon for adsorptive
desulfurization that reduced sulfur content from 410 ppm to 251 ppm and 184.6ppm using 5% and 10%
adsorbent material respectively [93].
Montmorillonite, kaolinite, vermiculite and palygorskite clays are used for desulfurization. At room
temperature, 1g of adsorbent material is dissolved into 20 mL of diesel sample for different time span (1, 3 and
6h) using batch process at continuous stirring. Kaolinite removed 66% sulfur in 6h process which is the highest
desulfurization percentage among other adsorbents (montmorillonite, vermiculite and palygorskite) [86].
Commercially prepared carbon samples (A and B) modified by HNO3 treatment and Ni supported system is
used for desulfurization of BT, 4-MDBT and 4, 6-DMDBT. The adsorptive capacity of carbon sample is
increased up to 1.18g(s)/m2
/g of adsorbent by modification at 35ºC [94]. Activated carbon modified by MnO2 is
used for selective removal of DBT. After surface modification of activated carbon, DBT adsorption capacity
becomes 43.8 mg(s)/g [95].
BT and DBT are selectively removed from gasoline using double template molecularly imprinted polymers (D-
MIP). The reduction in concentration of BT and DBT using D-MIP is found 57.16 and 67.19 mg/g respectively
[45].
Desulfurization is carried out with the assistance of activated alumina adsorbent (aluminum oxide). The small
surface area is achieved in desulfurization process due to loading of DBT on adsorbent surface [96]. Adsorbent
made up of mixed metallic oxides (NiO/ZnO-Al2O3-SiO2) is used in a fixed fluidized bed reactor to remove
sulfur from FCC gasoline sample at 380 and 1.5 MPa pressure. The sulfur content is decreased up to 11.6
mgL-1
from 180 mgL-1
[87].
Adsorptive desulfurization is found to be the cheapest technique among other methods. It does not require
special environment, expensive catalyst along with high energy consuming conditions. Moreover, emissions and
wastes are also minimum and it is an environmental friendly method. In addition to this, most of the adsorbents
are cheap and biodegradable [97].

13. Page | 13
3. Conclusion
Adsorptive desulfurization is one of the best adopted technique reported in the literature. It is green technique as
most of the left overs are biodegradable. In addition to its environment friendly nature, it doesn’t require any
expensive catalyst or energy intensive requirements for sulfur reduction from diesel oil. The sulfur content may
remove up to 99-100% using various efficient adsorbents hence it is feasible for ultra-low desulfurization.
Though there is no evidence of using adsorptive desulfurization at industrial scale which reduces its viability at
commercial level. Ionic liquid desulfurization and bio desulfurization are the new growing techniques for sulfur
removal from diesel oil. The cost factor lemmatize the use of these techniques in the industry desulfurization.
Oxidative desulfurization also improves the efficiency of diesel oil using H2O2, Cl2, O3, KMnO4 and K2FeO4 as
oxidants. Acetonitrile is the most effective phase transfer agent that is utilized in ODS. There is also a variety of
organic complexes and transition metals used for ODS. Hydrodesulfurization is the most adopted technique used
in refineries. This technique is costly and operated at elevated temperatures and high pressures. Emission of
greenhouse gasses while performing HDS, use of heavy metallic catalysts make this technique an environmental
hazard. It is not easy to use new techniques like adsorptive desulfurization and bio-desulfurization at industrial
scale therefore, HDS technique is being improved with addition of catalytic adsorptive technique to make the
diesel oil more sulfur free oil.
Acknowledgement
The authors would like to acknowledge COMSATS Institute of Information Technology (CIIT, Lahore) for
providing a good research forum.
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