Conventional and Upcoming Sulfur‐Cleaning Technologies ...

DOI: 10.1002/ente.201500475

Conventional and Upcoming Sulfur-Cleaning Technologiesfor Petroleum Fuel: A ReviewYahya S. Al-Degs,*[a] Amjad H. El-Sheikh,[a] Ramia Z. Al Bakain,[b] Alan P. Newman,[c]

and Mohammad A. Al-Ghouti*[d]

1. Introduction

Approximately 40 % of the worldÏs energy comes from pe-troleum, although this fraction can be expected to decreasewith the passage of time as new sources of energy are ex-plored. Both sulfur content and crude oil gravity (AmericanPetroleum Institute) are the most significant parameters thatreflect the quality of crude oil.[1] The level of sulfur, often ex-pressed as ppmw S or mgS kg¢1, is a significant property thatis dependent on the origin of crude oil. A quick examinationof the levels of sulfur in crude oils, taken from several re-gions in the world, showed that Middle East oil has a highsulfur level with a value of 1.71 %, which is somewhat higherthan the world average value.[1] Such a high S level should beconsidered with great care owing to the preponderance ofcrude oil from the Middle East in the world market. Accord-ingly, this will necessitate increased efforts from govern-ments, oil dealers, and oil refiners to reduce and monitorsulfur levels in diesel and gasoline, as these products arehighly consumed oil derivatives. Sulfur- and nitrogen-con-taining compounds are the most undesirable substances inpetroleum oil. Upon distillation from crude oil, alkyl sulfurcompounds distill with gasoline, whereas higher molar mass,aromatic, organosulfur compounds (OSCs) are associatedwith diesel.[2] Combustion of diesel as fuel will emit SOx

gases and sulfate particulate matter. Both have unwantedconsequences to health and the environment.[2] Whereasemission of SOx is clearly an example of anthropogenic influ-ence on the environment, S emissions also impair catalyticconverters and reduce engine performance by H2SO4 oxida-tion.[3,4] Accordingly, constant vigilance is needed to ensurethat fuel suppliers comply with regulations regarding sulfurlevels. In terms of regulatory limits, 10 mgS kg¢1 in liquidseems to be the lowest regulated level.[5] From an industrialpoint of view, sulfur-containing compounds also have a nega-

tive influence on oil-refining processes, including catalyst de-activation and corrosion of pipelines and refining plant. Themost common OSCs present in low-boiling (gasoline) andhigh-boiling (diesel) oil fractions are shown in Scheme 1.

As shown in Scheme 1, OSCs present in diesel are charac-terized by high molar masses and aromatic structures. Infact, dibenzothiophene (DBT) and other common derivativesoften constitute more than 50 % of the sulfur content indiesel.[6] In a recent study, the authors of this review reportedthat 40 % of the OSCs in commercial diesel (7055 mgS kg¢1)are present as dibenzothiophene.[7] Many of the published re-search indicates that removal of aromatic sulfur compoundsis rather limited if using the common hydrodesulfurization(HDS) method, which necessitates the pursuit of alternativemethods. Industrially, the most efficient S-cleaning procedure

Environmental regulations on the quality of liquid fuels con-tinue to create more requirements to produce a near-to-zerosulfur content. The hydrodesulfurization process has a highrunning cost and suffers from incomplete removal of sulfurcompounds, particularly alkylated dibenzothiophene. Recent-ly, oxidative and adsorption desulfurization proceduresshowed high potential to take part in commercial desulfuri-zation. The advantages and disadvantage of the newly pro-posed procedures are outlined and reviewed. Undoubtedly,

adsorptive-based methods have achieved many advances inthis area. Particularly, surface imprinted polymers andmetal–organic frameworks have manifested an excellent ten-dency to remove bulky sulfur compounds with minimum ex-perimental effort. This review presents information aboutthe workability, efficiency, and mechanisms of the current de-sulfurization methodologies for diesel as a common transpor-tation fuel. More attention is given to adsorptive desulfuriza-tion as a promising technology.

[a] Dr. Y. S. Al-Degs, A. H. El-SheikhChemistry DepartmentThe Hashemite UniversityP.O. Box 150459, Zarqa 13115 (Jordan)Fax: (+ 962) 53903349E-mail: [email protected]

[b] R. Z. Al BakainDepartment of Chemistry, Faculty of ScienceThe University of JordanAmman (Jordan)

[c] A. P. NewmanSchool of Energy Construction and EnvironmentCoventry UniversityPriory Street, Coventry CV1 5FB (UK)

[d] M. A. Al-GhoutiDepartment of Biological and Environmental SciencesCollege of Arts and SciencesQatar UniversityState of Qatar (Qatar)E-mail: [email protected]

Energy Technol. 2016, 4, 679 – 699 Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 679

for diesel is HDS.[8] This process can reduce sulfur from1.67 % down to 1670 mg kg¢1 (removal efficiency 90 %), butthis is achieved at 350 8C, 4.0 MPa H2, and by using NiMo-Al2O3 catalysts.[9] Indeed, the process is rapid and is oftenrecommended to reduce high levels of sulfur, but unfortu-nately, it suffers from a high running cost. A further disad-vantage of HDS is the poor performance towards derivativesof DBT, even under the best operating conditions.[7] The fail-ure of HDS to remove intransigent derivatives of DBT, par-ticularly 4,6-dimethyldibenzothiophene (4,6-DMDBT), hasinitiated new routes for removal including oxidation–extrac-tion and adsorptive desulfurization (AD) to achieve ultra-clean (<10 ppmw S) diesel. Interestingly, oxidation–extrac-tion is the oldest cleaning technology.[10] Here, the S atom isoxidized by suitable agents and the resulting polar sulfonecompounds are extracted from fuel into a polar solvent[10]

Adsorptive desulfurization is operated at ambient tempera-ture and pressure with a selective adsorbent employed to se-lectively adsorb the OSCs, whereas other constituents areleft in the fuel.[10] More details on the mechanism of HSD,oxidation–extraction, and adsorptive desulfurization are out-lined in the following sections.

2. Anthropogenic Influence of Sulfur Emissions:Environmental Perspectives

Once released into the environment, sulfur emissions under-go complex reactions that eventually affect the quality of theatmosphere and other parts of the environment.[11,12] AcidicSO2 gas is easily converted into H2SO4 and eventually formsacidic precipitation. The process of SO2 conversion into acidcan occur through either a homogenous or heterogeneouspathway.[11] The acidic precipitation will damage buildingsand negatively affect soil chemistry.[11] Alongside acid rain,sulfur emissions also play a role in smog and troposphericozone formation.[11] Furthermore, sulfate particulate mattercan contribute to serious health problems.[11] Removal ofOSCs from transportation fuel is thus an urgent environmen-tal and industrial requirement so as to, one, ensure longevityof catalytic converters and prevent oxidation by H2SO4

inside the engine and, two, reduce smog and acid rain forma-tion owing to SO2 emissions.

2.1. Safe levels of sulfur in liquid fuels

In most standards, the level of sulfur compounds is often ex-pressed as ppmw S. It would be highly desirable to combustzero-sulfur fuel; however, sulfur is present even in deeply de-sulfurized fuel. In 2006, the United States EnvironmentalProtection Agency (US EPA) set a maximum sulfur level to15 mg kg¢1 in diesel fuel.[1,13] A more stringent regulation issuggested by Euro V norms, for which the allowable level ofsulfur is set at 10 mg kg¢1.[13] Both USA and Canada have setthe maximum level of sulfur in transportation diesel and gas-oline to 15 and 30 mgS kg¢1, respectively, since mid 2006.[13] Amuch higher level (350 mgS kg¢1) is permitted in transporta-tion fuel in Jordan and some other countries.[7] To respond tonew and upcoming regulations, oil refineries face many chal-lenges to comply with statutory limits regarding sulfur levelsin liquid fuels.

2.2. Commercializing the upcoming sulfur-cleaning methods

The final evaluation stage for any sulfur-cleaning method isits applicability on a large scale, that is, can it be commercial-ized? The following criteria should be fulfilled so that anynew method or design can be practically applied.[14]

1) Capital cost: procedures that operate with less plant re-quirements are highly recommended

2) Running cost: running cost should be manageable (lessconsumption of solvents, gases, and other materials withminimum waste production)

3) Technical standpoint: procedures of less technical com-plexity are always demanded. Accordingly, the need forcarefully designed and expensive catalysts introducesmore technical complexity into HDS

4) Overall value to refineries5) Environmental impact: procedures that minimize envi-

ronmental harm (in the widest sense) should be chosen

Yahya S. Al-Degs was born in Jordan on

September 12, 1971. He obtained his

B.Sc. degree in chemistry from Yarmouk

University in 1991, his M.Sc. degree in

analytical chemistry from the University

of Jordan, and his Ph.D. degree from the

School of Chemistry and Chemical Engi-

neering, Queen’s University of Belfast in

2000. He has worked in the Department

of Chemistry at the Hashemite University

since 2002. He has published 65 scientific

papers in international journals, attended

many international conferences, and su-

pervised 10 M.Sc. theses.

Mohammad A. Al-Ghouti obtained his

Ph.D. degree in applied analytical and en-

vironmental chemistry from the Queen’s

University of Belfast, UK, in July 2004.

He held several posts at the Royal Scien-

tific Society (Jordan) from 1997 to 2010.

He also worked at the Fahad Bin Sultan

University, Saudi Arabia, as an assistant

professor in chemistry and a director of

the arts and sciences unit. Currently, he is

working at the University of Qatar,

Qatar, in biological and environmental

sciences as an assistant professor and as the undergraduate program co-

ordinator of the biology and environmental science programs. He has

published over 55 papers and conference publications, all of which have

been published in international journals; he has also attended many in-

ternational conferences.


Achieving the abovementioned criteria seems to presenta hard task. However, any new procedures should fulfillthese criteria if they are to compete with existing ones. Al-though HSD does not achieve all the above-mentioned crite-ria, the method still has large industrial applications owing toits excellent performance for cleaning sulfur-rich fuels. In ourview, adsorptive desulfurization seems to offer a reasonablesubstitute for highly advanced HDS, as it reasonably achievesall the above-mentioned criteria.

3. Hydrodesulfurization: Catalytic Pathway

With a high rate of commercial application, the HDS processhas been the most adopted desulfurization method duringthe past two decades.[15,16] In HDS, most OSCs are hydrogen-ated; this generates H2S, which is treated in a subsequentstage (see Scheme 2). Simple thiols, sulfides, and disulfides(see Scheme 1 for structures) are efficiently eliminated by

HDS. However, the reactivity of HDS toward bulkyaromatic S compounds (such as DMDBTs) in fuelis rather limited.[17] In fact, complete removal ofDBT and its derivatives by HDS requires operationat extreme conditions (i.e., high temperatures, highpressures, and with more H2 consumption), and thisis negatively reflected in octane number and othercombustion indicators of the resulting liquid fuel.[18]

This negative influence of HDS on diesel is mainlydue to hydrogenation of non-S-aromatic com-pounds, which constitute approximately 20 % of thefuel. Under the best commercially used HDS condi-tions, it is difficult to reach 50 ppmw S in the finalfuel, and this level is well above many statutorylimits.[13] Accordingly, new desulfurization methodshave been proposed to achieve the 10 ppmw leveland to reduce the high energy and H2 consumptioninvolved in this process. In large refineries, theHDS treatment procedure is performed in trickle-bed reactors.[13] The hydrogenation process is com-monly operated over the temperature range of 300to 450 8C and at H2 pressures between 3.0and5.0 MPa in the presence of a CoMo/Al2O3 orNiMo/A12O3 catalyst.[19] Under such extreme condi-tions, olefins and aromatic hydrocarbons become

involved in the hydrogenation process, and this leads to un-wanted consequences such as the loss of octane rating andintense H2 consumption, as previously mentioned. InScheme 2, the mechanism of DBT desulfurization underhigh-temperature and high-pressure conditions is outlined.

As indicated in Scheme 2, the final destiny of DBT is bicy-clohexyl, which remains in the final stream without furthertreatment. The most important advantage of HDS, in fact, isthe ability to treat different feeds (e.g., gasoline, diesel, andjet fuels) with extremely high levels of OSCs. For example,desulfurization of fuel oil containing 21 900 ppmw can beachieved with a 57 % cleaning efficiency.[20] Direct desulfuri-zation of extremely S-rich fuels by other noncatalytic meth-ods may not be possible. Accordingly, it is often suggested toapply newly developed procedures to supplement HDS thusto guarantee a reasonable and treatable S level in the fuel.The main advantage of hydrodesulfurization is the excel-lent performance for cleaning sulfur-rich fuels (up to

Scheme 1. Common organosulfur compounds in gasoline and diesel oil derivatives.[2]

Scheme 2. Mechanistic pathways for the complete hydrogenation of DBT at 300 8C and 10.3 MPa with the aid of a CoMo/Al2O3 catalyst.[9]


10 000 ppmw) to achieve the less than 100 ppmw level in onestep, and this performance is not reported for any othercleaning technology.

4. Oxidative Desulfurization: Noncatalytic path-way

Solvent extraction of sulfur and nitrogen impurities fromliquid fuel has previously been used by refineries.[21] Afterextraction, the solvent is recovered in a separate distillationstep.[21] However, this procedure is impractical, as the loss ofother useful compounds is possible. To improve desulfuriza-tion, the polarity of the OSCs can be increased to facilitatetheir extraction, and this is the basic idea of oxidative desul-furization (OD). Oxidative desulfurization is the oldest S-cleaning procedure originating as early as 1928.[22] The firstoxidants tested were nitric oxides/acids including HNO3 andNO2 gas.[23] In the following years, more selective oxidantswere proposed including tert-butyl hypochlorite,[24] toxicRuO4,

[25] and the H2O2–H2SO4 system.[26] Compared to HDS,OD is an attractive method to remove all types of organosul-fur compounds from diesel and gasoline with minimum op-erational effort. The OD method can operate at 50 8C and atambient atmospheric pressure.[10,27] Simply, the S moiety inthe organic compound is oxidized to a sulfone group bya suitable reagent without breaking S¢C or C¢C bonds.[10, 27]

Once the S moiety is oxidized, the compound becomes morepolar and more liable to extraction from diesel than the un-oxidized compound. On this basis, OD is composed of twomain steps: one, oxidation of S-containing compounds; two,extraction of these compounds. The extra “extraction” stepadds more cost and extra time. A representative demonstra-tion of the stepwise oxidation of DBT from diesel is given inScheme 3.[10,27, 28]

Selective oxidation of DBT and the other alkyl derivativesto sulfoxides/sulfones increases their polarity, which facili-tates their extraction or adsorption in some cases.[30,31] Theselective oxidation of OSCs is the central idea of the ODtechnique. Accordingly, it is viable to review oxidizing agentsthat are often employed in the process. However, the discus-sion is limited to oxidants that have been utilized for dieseloxidative desulfurization.

The most commonly employed oxidants in oxidative desul-furization are H2O2, H2SO4, NO2, CH3CO2H, and CH2O2.

[13]

As can be noted, organic acids (acetic and formic acids) takepart in the oxidation process. The best oxidant is the one

that selectively oxidizes sulfur compounds only, whereasother organic compounds are left unaltered. It is importantto mention that the oxidation process can be accelerated byadding various catalysts.[32] Owing to its relatively benigneffect on the environment, H2O2 is the most common oxi-dant.[13] With the aid of various catalytic systems, OSCs inliquid fuels are effectively oxidized by H2O2. The most em-ployed catalytic materials are HCOOH, CCl3COOH,CF3COOH, phosphotungstic acid, silicates, and solid bases.[13]

In a classical study, Shiraishi and co-workers have reportedthe excellent efficiency of H2O2 and acetic acid to oxidizesulfur compounds in oils.[33] In a similar study on H2O2, itwas found that the oxidation efficiency can be further im-proved by adding formic acid in the presence of a metaloxide loaded on a high surface area support to remove thio-phene derivatives from model gasoline.[34] The oxidizing sys-tems H2O2/CH3CO2H/Na2WO4, Na2WO4/Al2O3, and aceticacid/K2FeO4 exhibit excellent oxidative desulfurization effi-ciency to remove the hard-target 4,6-DMDBT from diesel ina short time.[35–37] To assess the overall performance of ODfor removing OSCs and its suitability for commercial applica-tions, some important and systematic studies are summarizedin Table 1. In the reviewed studies, oxidative desulfurizationof commercial fuel is tested, and details of the experimentalconditions (e.g., experimental mode, temperature, pressure,catalyst, and mixing time) are provided.

4.1. Classical oxidative desulfurization methodology

It is interesting to observe that some proposed OD proce-dures are applied without the need for a final extractionstep. Lu and co-workers[30] have applied H2O2/formic acid forthe efficient removal of DBT from diesel and the oxidizedbyproducts were favorably removed from diesel by carbonadsorption. Compared to other reported methods, the earlierprocedure suffers from a long treatment time: up to 12 h wasneeded to remove the byproducts.[30] In a similar study, DBTand other derivatives were removed from S-rich fuel by ini-tial oxidation with acetic acid, and the oxidized productswere then removed by using Mn-loaded activated carbon.[31]

Reportedly, a contact time as long as 300 min was needed toaccomplish desulfurization. In the earlier studies, there wasno need to apply the final extraction step to remove oxidizedbyproducts, but the methods required a long time for satis-factory desulfurization. The oxidation efficiency of H2O2/or-ganic acid can be further increased to achieve an efficient de-sulfurization procedure. The following catalysts have beentested to improve the oxidation process: phosphotungsticacid, K2FeO4, Na2WO4, Mo/Al2O3, andV2O5.

[32,36–41] The oxi-dation efficiency of H2O2 was improved by using phospho-tungstic acid/alumina to eliminate DBT and its derivativesfrom diesel in 1.0 h.[32] An extra step with the use of acetoni-trile was necessary to remove byproducts.[32] The oxidationefficiency of K2FeO4 for removing DBT proved to be asgood as that of more common H2O2.

[36] DBT was removedfrom diesel in 45 min and furfural was then used to extractthe byproducts.[36] The H2O2/CH3CO2H oxidation system was

Scheme 3. Stepwise oxidation of DBT prior to extraction from diesel.[29]


Tabl

e1.

Effic

ienc

yof

oxid

ativ

ede

sulfu

riza

tion

met

hods

unde

rdi

ffer

ent

expe

rim

enta

lcon

diti

ons.

Fuel

Targ

etco

mpo

und

Oxi

dant

/ext

ract

ant

Was

hing

solu

tion

/ad

-so

rben

tC

ondi

tion

s[a]

Tota

lSle

v-el

[ppm

v]S

rem

ov-

al[%

]R

ef.

dies

elD

BT

H2O

2/fo

rmic

acid

(ox)

acti

vate

dC

batc

h,60

8C,0

.1M

Pa,

acti

vate

dca

rbon

,12

h80

098

[30]

dies

elD

BT

and

deri

vati

ves

acet

icac

id/M

n-A

C(o

xan

dad

sorp

tion

)M

n-ac

tiva

ted

batc

h,25

8C,0

.1M

Pa,

acet

icac

id,3

00m

in71

0045

[31]

dies

elD

BT

and

deri

vati

ves

H2O

2(o

x)ac

eton

itri

leba

tch,

258C

,0.1

MPa

,ph

osph

otun

gsti

cac

id/a

lum

ina,

1.0

h

320

56[3

2]

dies

elD

BT

acet

icac

id/K

2FeO

4(o

x)fu

rfur

alba

tch,

258C

,0.1

MPa

,K

2FeO

4,45

min

475

97[3

6]

mod

eldi

esel

DB

T,4,

6-D

MD

BT

H2O

2an

dC

H3C

O2H

(ox)

met

hano

lba

tch,

30–9

08C

,0.1

MPa

,N

a 2W

O4,

90m

in11

0096

[37]

dies

elD

BT

and

deri

vati

ves

H2O

2(o

x)ac

eton

itri

leba

tch,

608C

,0.1

MPa

,M

o/A

l 2O

3,1.

0h

320

97[3

8]

dies

elB

T,D

BT,

4-M

DB

T,4,

6-D

MD

BT

tert

-but

ylhy

drop

erox

ide

orH

2O2

(ox)

acet

onit

rile

batc

h,60

8C,0

.1M

Pa,

V2O

5su

ppor

ted

ondi

ffer

ent

subs

trat

es,

1.0

h

1005

99[3

9–41

]

light

fuel

DB

T,4-

MD

BT,

4,6-

DM

DB

Thi

gh-p

ress

ure

mer

cury

UV

lam

p(3

00W

)aq

ueou

sso

luti

onba

tch,

508C

,0.1

MPa

,no

cata

lyst

,30

h10

5022

[42]

light

fuel

DB

TH

2O2,

high

-pre

ssur

em

ercu

ryU

Vla

mp

(300

W)

aque

ous

solu

tion

batc

h,50

8C,0

.1M

Pa,

noca

taly

st,

24h

2000

98[4

3]

mod

elfu

elD

BT

met

hylim

idaz

oliu

m-A

lCl 3-

base

dio

nic

liqui

d(e

xt)

–ba

tch,

258C

,0.1

MPa

,no

cata

lyst

,15

min

500

45[4

4]

dies

elD

BT

and

deri

vati

ves

3-m

ethy

lpyr

idin

ium

-bas

edIL

(ext

)–

batc

h,25

8C,0

.1M

Pa,

noca

taly

st,

15m

in97

60[4

5]

dies

elT,

BT,

DB

TN

-oct

ylpy

ridi

niu

mte

traf

luor

obor

ate

IL(e

xt)

–ba

tch,

258C

,0.1

MPa

,no

cata

lyst

,15

min

8347

[46]

mod

elfu

elB

T,D

BT,

DB

Tde

riva

tive

sH

2O2/

ioni

cliq

uid

(ox

and

ext)

–ba

tch,

258C

,0.1

MPa

,pe

roxo

phos

phom

olyb

date

s,15

min

1000

(as

DB

T)97

[47]

mod

elfu

elB

T,4,

6-D

MD

BT

H2O

2an

dIL

(ox

and

ext)

–ba

tch,

608C

,0.1

MPa

,de

catu

ngst

ates

,30

min

1000

99[4

8]

dies

elD

BT

and

deri

vati

ves

H2O

2an

dca

prol

acta

miu

mhy

drog

ensu

lfate

trifl

uoro

acet

icac

idIL

(ox

and

ext)

–ba

tch,

308C

,0.1

MPa

,no

cata

lyst

,2.

0h

660

(2cy

cles

)99

[49]

dies

elD

BT

H2O

2/ul

tras

ound

(20

kHz)

(ox)

acet

onit

rile

batc

h,75

8C,0

.1M

Pa,

phos

phot

ungs

tic

acid

,18

min

7744

98[5

0]

dies

el4,

6-D

MD

BT

H2O

2/ac

etic

acid

ultr

asou

nd(2

0kH

z)(o

x)m

etha

nol

batc

h,90

8C,0

.1M

Pa,

noca

taly

st,

9m

in18

099

[51]

[a]E

xper

imen

tal

mod

e,te

mpe

ratu

re,

pres

sure

,ca

taly

st,

tim

e.


efficiently catalyzed by Na2WO4 to remove the hard-target4,6-DMDBT from diesel.[37] The experimental procedure re-quired a gradual increase in the temperature up to 90 8C.[37]

The proposed OD procedure achieved 96 % removal of DBTand 4,6-DMDBT in a short time.[37] In a systematic study,Caero and co-workers extensively studied the selective oxi-dation of DBT and other derivatives in the presence ofV2O5.

[39–41] The authors also investigated the type of supportused to carry V2O5 during the desulfurization process.[39–41]

The initial investigations showed that V2O5 could improvethe overall oxidation of OSCs by H2O2 or tert-butyl hydro-peroxide. For all systems, the desulfurization trend of prob-lematic OSCs decreased in the following order:[41] DBT>4-methyldibenzothiophene (4-MDBT)>4,6-DMDBT>benzo-thiophene (BT). The interesting observation was the favora-ble removal of bulky 4,6-DMDBT over BT, which was oppo-site to that seen in commercial HDS. Furthermore, the typeof substrate was interestingly correlated to the overall desul-furization process.[41] The oxidation activity of DBT andother derivatives by V2O5 decreased according to the trendniobia>alumina>SBA-15> titania>ceria>Al¢Ti oxides.[41]

In a typical OD test, commercial diesel with an initial sulfurlevel of 1005 ppmw was efficiently desulfurized by usingH2O2/V2O5 supported on niobia, and acetonitrile was used asthe extraction solvent.[41]

4.2. Photo-oxidation of organosulfur compounds prior to extrac-tion

Photodecomposition of DBT and other derivatives by UVlight (l>280 nm to avoid C¢C bond cleavage) in the ab-sence[42] or presence[43] of H2O2 has been presented as a newmethodology for liquid-fuel cleaning. In a one-step desulfuri-zation process, DBT, 4-MDBT, and 4,6-DMDBT were oxi-dized under the action of UV irradiation, and S was removedas SO4

2¢ by water.[42] The earlier procedure has two draw-backs: long treatment time (30 h) and the fact that it can oxi-dize other non-S compounds.[42] The results indicated the fol-lowing desulfurization trend: DBT<4-MDBT<4,6-DMDBT, which is opposite to that observed in the commer-cial HDS process. One more advantage is that no organicsolvent was needed to clean up the oxidized byproducts.[52–55]

In a lengthy OD procedure, H2O2 combined with UV irradia-tion (4-phenylbenzophenone was added to improve photo-oxidation) was employed to remove DBT from 2000 mgkg¢1

light fuel with 98 % efficiency.[43]

4.3. Ionic liquids for the extraction of organosulfur compounds:New generation of liquid extractant

Ionic liquids (ILs) are composed of two basic components(organic cations and organic/inorganic anions) with meltingpoints lower than 100 8C in most cases.[56, 57] Nonvolatility,thermal stability, solubility for organic/inorganic compounds,and nonflammability are the most attractive properties ofILs.[56,57] Accordingly, ILs find many applications in liquid–liquid extraction procedures, including selective removal of S

compounds from fuel,[56,57] and one of the first applicationsof ILs was the selective removal of OSCs from liquid fuel.[44]

As indicated in Table 1, ILs were applied for direct removalof OSCs[44–46] or used to remove oxidized products after aninitial oxidation step.[47–49] The main components of ionic liq-uids that were commonly used to remove OSCs were imida-zolium and pyridinium as organic cations, whereas BF¢ ,PF6

¢ , and EtSO4¢ were the anionic parts.[44,58,59] Moreover,

a number of comprehensive reviews on the use of ionic liq-uids are available.[14,60] The workability of ILs for diesel de-sulfurization is presented in Table 1. The first IL tested toremove DBT was methylimidazolium–AlCl3.

[44] The finalresult was modest with 45 % desulfurization of a model fuelcontaining 500 ppmw as DBT. Direct liquid–liquid extractionof DBT and other derivatives by using a 3-methylpyridiniumIL showed promising results.[45] The notable factors regardingliquid extraction by IL is the short treatment time (15 min)and an extra extraction step is not required, as is the case inOD. In systematic work, the selective extraction of N-octyl-pyridinium tetrafluoroborate ionic liquid for thiophene (T),BT, and DBT was investigated, and the estimated partitioncoefficients were 0.79, 1.40, and 1.79, respectively.[46] Indeed,extraction by ILs would be applicable on a commercial scaleowing to high selectivity towards bulky DBT.[46] In recentwork, Nejad and Beigi reported that many alkyl thiols andaromatic thiophene compounds were removed from liquidfuel with the aid of imidazolium-based ionic liquids.[61] Thecomponents of the ionic liquids were 1-butyl-3-methylimida-zolium tetrachloroaluminate and 1-octyl-3-methylimidazoli-um tetrafluoroborate.[61] Under optimum extraction condi-tions, a desulfurization efficiency of 95 % was reported withan excellent efficiency towards benzothiophene compared tothiophene, 3-methylthiophene, and 2-methylthiophene.[61]

It seems that direct liquid–liquid extraction by ILs doesnot achieve the required target of reaching a trace level ofOSCs. Accordingly, to obtain better results an extra oxida-tion step (e.g., by H2O2) is proposed before IL extraction.He and co-workers outlined that initial H2O2 oxidation ofDBT and other derivatives just before IL extraction resultedin better desulfurization efficiency.[47] The reactivity of OSCsby the proposed method was in the following decreasingorder: DBT>4,6-DMDBT>BT.[47] BT and 4,6-DMDBTwere efficiently removed from diesel by using the H2O2/ILssystem, and the oxidation system was recyclable by addingfresh H2O2.

[48] Again, the most interesting point of ILs is thatthere is no need for an extra extraction step owing to the ex-tractive efficiency of the ionic liquids for all possible byprod-ucts. Jiang and co-workers have applied H2O2/caprolactami-um hydrogen sulfate–trifluoroacetic acid (as an IL) toremove 99 % of DFBT and other derivatives from diesel con-taining 660 ppmw.[49] The extraction process seemed to beslow, as two successive cycles were needed to end up withthe ideal 100 % removal efficiency.[49]


4.4. Ultrasound-assisted oxidation of organosulfur compoundsprior to removal from fuel

Recently, ultrasonic waves were utilized to improve reactionefficiency under phase-transfer conditions.[62] The ultrasonicwaves improve the liquid–liquid interfacial area by formingviscous films of cavitation bubbles through emulsification.[62]

The fine emulsions or microemulsion significantly improvethe contact area for the reaction, which thus increases the ef-fective local concentration of the reacting species. Simply,the waves penetrate the body of the reaction and increasethe reaction rate under phase-transfer conditions.[62] More-over, extreme local conditions with high temperatures andhigh pressures are generated upon sonication, and this cre-ates active intermediates that make the reaction occur in-stantaneously. The influence of ultrasound on DBT oxidationby H2O2 is well demonstrated.[50] With sonication for 18 min,98 % removal of DBT was achieved to clean diesel samplecontaining 7744 ppmw.[50] As in the case of classical OD, anextra extraction step with the use of acetonitrile was necessa-ry to remove unwanted oxidized byproducts.[50] In a similarstudy, ultrasound combined with H2O2 was found to be anexcellent tool to remove problematic 4,6-DMDBT fromdiesel.[51] The process was fast, but a high temperature wasneeded and (as usual) an extra extraction step was necessaryto remove unwanted byproducts.[51]

4.5. Biological oxidation of organosulfur compounds prior to ex-traction

An attractive and safe option to achieve selective oxidationis the utilization of biological pathways. Biological oxidationis achieved by using bacterial species that have the ability toconsume DBT and other derivatives.[14,63] This process is alsocalled biodesulfurization, and it has received significant at-tention owing to its specificity for treating DBT and its deriv-atives in liquid fuels.[13] The most common bacterial speciesthat are capable of biodegrading DBT and its derivatives

(i.e., DBT is used as a sulfur source) are Brevibacterium, Ar-throbacter, Gordona, and Pseudomonas.[13] There are twobiological oxidation mechanisms for DBT and its derivatives.In the Kodama mechanism, the initial attack is directed toone of the carbon atoms in the target molecule.[64] In the 4Smechanism, the initial attack is directed to the S atom.[65,66]

As an illustration for biodesulfurization, a simple sketch isprovided in Scheme 4.

Initially, the oxidation reaction is started in the presenceof H2O and O2 at normal temperature and pressure to pro-duce hydroxybiphenyl sulfonate. Under the action of bacte-ria, the conversion of hydroxybiphenyl sulfonate into 2-hy-droxybiphenyl is achieved.[66] Although biodesulfurizationseems to be attractive from a practical point of view, reportsindicate that desulfurization efficiency is rather modest. An86 % removal efficiency of DBT was reported by using My-cobacterium sp.X7B (a bacterial species) in diesel. The pro-cess was accomplished in batch mode at 45 8C.[66]

4.6. Practical assessment of different oxidative-based proceduresfor fuel desulfurization

It is essential to note that the experimental procedures formost OD reactions start by mixing liquid fuel with an organ-ic solvent (usually 1:1) in the reaction vessel. The tempera-ture is carefully controlled whilst the oxidizing agents andcatalysts are added at suitable doses under continuous stir-ring. In many cases, the OD system is composed of two im-miscible layers, of which vigorous agitation is necessary toachieve better results.[49] The level of OSCs is monitored bytaking samples from the reaction for analysis. In some cases,the organic/fuel layer is separated after completion of the re-action, and then the final sulfur level is measured. Owing tomany analytical problems associated with the detection ofOSCs in diesel, the most adopted method for quantificationis an X-ray fluorescence sulfur analyzer.[7] This method offersa very low detection limit and a wide dynamic range in manypetroleum products.[7] Detection and quantification of DBT

Scheme 4. Biological oxidation of DBT by a 4S mechanism.[66]


and other derivatives is also performed by using gas chroma-tography with flame ionization detection.[7] Moreover,13C NMR spectroscopy is used to detect OSCs in liquid fuel.After completion of desulfurization, the diesel layer is sepa-rated and further washed with solvent to remove oxidizedbyproducts. Finally, polar solvents are used to remove oxi-dized byproducts (sulfones) from the fuel. The extractantshould be immiscible with fuel and absorb all oxidized by-products and unreacted compounds. As indicated in Table 1,acetonitrile and methanol are often applied to eliminate by-products. Although the polarity of methanol is good enoughto extract oxidized sulfoxides/sulfones from fuel, the densityof this solvent (0.79 g mL¢1) is comparable to that of mostfuels such as diesel (0.84 g mL¢1[67]) and this makes it less fa-vorable for extraction. Although acetonitrile is the better sol-vent for most extractions, its high toxicity may retard its ap-plication on a large scale. In most cases, the extraction sol-vent is separated from the fuel by decantation or mild centri-fugation. To ensure that the fuel does not contain any un-reacted oxidant, washing with water is performed. Anotherpractical procedure is to pass the treated fuel over a solid ad-sorbent (e.g., silica gel or aluminum oxide) to remove sulfox-ides/sulfones. In addition to acetonitrile, dimethyl sulfoxide,furfural, and dimethylformamide may be utilized for com-plete extraction of oxidized byproducts.[13] It should not beforgotten that HDS is an excellent commercial procedurewith some disadvantages as outlined earlier. However, imple-mentation of HDS is not affordable in many refineries owingto its high running costs. On the basis of the information pro-vided in Table 2, the practical and industrial applications ofdifferent OD procedures can be ascertained. Ideally, the bestprocedure is the one that achieves the highest desulfurizationwith minimum experimental efforts. Table 2 provides a back-ground to evaluate the best procedure from a practical pointof view.

There are two important issues that need to be consideredto realize acceptable desulfurization results: one, the appliedoxidant should selectively oxidize S compounds only undermild conditions; two, the oxidized byproducts should becompletely removed from the fuel by using a suitable ex-tracting solvent. As with any industrial process, OD may notbe accomplished ideally because significant oxidation of non-S compounds may occur and because the combustion proper-ties of fuel may become poorer as a result of incomplete re-moval of oxidized byproducts. Despite these disadvantages;OD is achievable under moderate temperatures and pres-sures. Selective, safe, and recycled catalysts are often re-quired for OD. As shown in Table 1, almost all developedOD procedures achieve high desulfurization and are selec-tive to DBT and other unwelcome derivatives may be ob-tained. Regarding industrial applicability, photo-oxidationdesulfurization should be used with great care because of theoxidation of non-S organic compounds. Normal OD exhibitshigh desulfurization efficiency; for better applicability, thefollowing issues are suggested: one, developing selective ex-traction solvents to reduce the treatment time and to pro-duce fuel of high combustion quality; two, long-life catalysts

are always necessary. To a large extent, ILs coupled with ODseem to be the best candidates for industrial purposes. Thehigh selectivity of ILs toward oxidized DBT (and otherforms) is the reason behind the intense application of ionicliquids in fuel desulfurization in recent years.[14,60] As ODshows excellent selectivity toward DBT and other deriva-tives, it has a high potential to be combined with HSD toachieve deep desulfurization. Biodesulfurization is an attrac-tive process and more successful results are expected in thenear future. Notably, the first commercial introduction ofOD was announced in 2002.[13] The project was demonstratedin 2004.[14] Basically, the process used H2O2/formic acid asthe oxidant, and the generated sulfones were directly re-moved from diesel by Al2O3 adsorption. The adsorbent waswashed with methanol in a later stage. The overall produc-tion rate was 1000 tondiesel day¢1, whereas 1 ton day¢1 was gen-erated as sulfone species.[14]

5. Adsorptive Desulfurization: The Upcoming De-sulfurization Technique

Adsorption is a general term that describes the transfer ofa molecule (from liquid or gaseous phase) to a solid surfaceor adsorbent. Many factors control the adsorption process,including specific surface area, porosity, and surface chemis-try (functional groups).[68] The most common forms of ad-sorption are physisorption, chemisorption, and reactive ad-sorption.[13] Clearly, adsorption is a common wastewatertreatment method, in which physisorption is the preferredform of adsorption so that the adsorbent can be simplyreused after reactivation.[68] Adsorption has also been em-ployed to remove OSCs from liquid fuels. Selective removalof DBT and other derivatives (particularly 4,6-DMDBT) hasbeen assessed by using activated carbon, modified activatedcarbon, zeolites, aluminosilicates, zinc oxide, alumina, andmany other adsorbents.[2–4,6,7, 31,69–80] Among the tested ad-sorbents, only a few exhibit high selectivity for removal ofthe intransigent compound 4,6-DMDBT from fuel. In thissection, the most promising adsorbents for removing OSCsare reviewed and the desulfurization efficiency of the adsorb-ents is also critically discussed.

5.1. Carbon-based adsorbents and modified forms

Activated carbons (ACs) are common and efficient adsorb-ents for removing many types of pollutants from differentmatrices.[12] The high adsorptive capacity of activated carbonsis mainly attributed to the high density of surface functionalgroups, its well-developed porous structure, and its large spe-cific surface area.[12,81] Accordingly, ACs show high adsorp-tion for both OSCs and other non-S compounds, and this canaffect the quality of the final product in some cases.[12,82]

Therefore, surface modification of ACs to improve removalof OSCs is a hot research area.[82,83] However, the applicationof ACs to minimize OSCs has not received much attention,and this may be attributed to the high price of commercialACs relative to that of other adsorbents, and the favorable


uptake of non-S compounds ispossible owing to the nonselec-tive nature of ACs. In a recentstudy, Muzic and co-workerstested commercial ACs and zeo-lites to remove DMBT andDMDBT from diesel.[84] The au-thors outlined that the adsorp-tion process of OSCs could betreated as a single-solute ad-sorption process, which facilitat-ed the application of commonisotherms and kinetic models.[84]

Activated carbon preparedfrom the pits of dates and acti-vated by ZnCl2 proved to be aneffective material to removeOSCs from diesel.[85] Removalof OSCs was also evaluated byusing commercial ACs, acid-ac-tivated carbon, and Ni-modifiedACs.[86] In an interesting study,AC modified with 28 % Nishowed a higher breakthroughuptake capacity than the un-modified form, which indicateda high selectivity of the loadedmetal towards the target sol-utes.[87] Surface modification ofactivated carbon by depositionof transition metals or oxidationhas been proven to have a posi-tive influence on the removal ofDBT and its derivatives fromliquid fuel.[88, 89] The selective re-moval of OSCs by metal-loadedactivated carbon is attributed tofavorable p–p interactions be-tween a partially filled d orbitaland the slightly basic S atom.[90]

Moreover, the selective removalof DBT by metal-loaded acti-vated carbon is also attributedto the catalytic oxidation of theOSCs just before removal bypolar functional groups.[91,92]

Acid–base interactions be-tween slightly basic S-contain-ing compounds and the acidicoxygen atom increase the polar-ity of the surface groups, andthe associated redox reactionsare proposed to explain selec-tivity toward target solutes.[92]

Beside chemical interactions,activated carbons of high sur-face areas and developed poros-Ta

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ity (10 è diameter) are preferred towards bulky DBT andother derivatives.[89] The amount of adsorbed DBT is directlyrelated to the total pore volume of the 10 è pores.[93] In a sys-tematic study, the selective removal of DBT from other aro-matic hydrocarbons after diesel acidification was reported.[69]

The competitive adsorption test indicated that DBT was pref-erentially removed over di-, tri-, and tetraaromatic hydrocar-bons after diesel acidification by 5 % acetic acid.[69] Addingacetic acid to the fuel enhanced DBT adsorption. The aceticacid catalyzed oxidation of the S moiety by activated carbon,before leading to more adsorption of the oxidized product.[69]

The importance of surface oxidation for the selective re-moval of DBT and other derivatives has been demonstratedin many studies.[93,94] Upon surface oxidation, the removal ofOSCs was improved against other aromatics, and this con-firmed that surface groups were involved in the oxidation ofS compounds, which then improved their uptake by the polarsurface. Hence, surface oxidation of DBT improved its affini-ty to the surface through specific adsorption centers, whereasthe other aromatic hydrocarbons were left in the liquidphase.[93,94]

5.2. Inorganic adsorbents and modified forms

In fact, inorganic materials (e.g., metal oxides, aluminosili-cate minerals, zeolites, and metal admixtures), metal–organicframeworks, and molecular imprinted polymers have beentested for the selective removal of DBT with the other com-pounds left unaffected in the fuel. Surface modification of in-organic adsorbents has also been investigated to create selec-tive adsorbers.

Silica gel is the most tested adsorbent, as it has a modifia-ble surface. In an interesting study, selective desulfurizationof diesel was performed by using Ni nanoparticles loaded onamorphous silica gel.[79] An adsorption value of 1.7 mg g¢1

was observed in column tests starting from a 240 ppmw Slevel feed.[79] In our laboratory, acid-activated bentonite (alu-minosilicate mineral) proved to be a reasonable substitutefor commercial desulfurization adsorbents.[3] Both equilibri-um and column tests indicated the selective removal of Scompounds from commercial diesel. However, removal ofother organics was not monitored.[3] Selective removal byacid-modified bentonite was attributed to its large surfacearea, developed porosity, and the favorable interaction be-tween the basic OSCs and the acidic surface.[3]

Commercial activated alumina (crystalline form) can beconverted into the acidic form simply by direct treatmentwith strong acids, after which the acidic adsorbent exhibitsgood affinity toward OSCs. In an interesting study on activat-ed alumina, Larrubia and co-workers showed that aluminawas a more selective adsorber toward BT, DBT, and 4,6-DMDBT than zirconia and magnesia.[72] Relatively speaking,4,6-DMDBT had the weakest affinity, which was attributedto structural considerations.[72] In a similar study, Srivastavand Srivastava showed that activated alumina was a reasona-ble adsorbent that could be used to remove DBT, and theadsorption kinetics indicated that the process attained results

in a reasonable time.[95] Surface modification of inorganic ad-sorbents has been investigated by many researchers. Zirconialoaded with 3 % Cu has been shown to have a high affinitytoward thiophene under normal experimental conditions.[80]

The enhancement in the adsorption of thiophene upon sur-face modification was attributed to p–p interactions and tothe possible oxidation of thiophene followed by favorableuptake of the oxidized products by the adsorbent.[80]

Compared to other aluminosilicates, zeolites have receivedgood attention in desulfurization studies. Zeolites (crystallinealuminosilicate materials composed of tetrahedral units ofSiO4 and A1O4) have stable and regular three-dimensionalcrystalline frameworks.[2,13] It was shown that natural zeolite(mainly phillipsite, as confirmed by XRD) has promising re-moval of OSCs from diesel with a maximum retention valueof 7.15 mg g¢1.[2] During the removal process, liberation ofH2S was observed. A catalytic pathway was proposed for thisprocess by Mustafa et al., as shown in Scheme 5.[2]

Although the adsorbent was selective to OSCs, the authorsdid not test the removal of other diesel components or theability to regenerate the exhausted zeolite.[2] The work ofYang and Takahashi (and their co-workers) on zeolite is themost renowned in this field. Cu- and Ag-modified zeolite ex-hibited excellent uptake of T relative to commercialACs.[96, 97] The selective removal of OSC by Cu-zeolite en-couraged the researchers to mix it with AC to achieve thebest desulfurization result.[98] The combined adsorbent hada high equilibrium and dynamic retention capacity of 18.9and 10.9 mg g¢1, respectively, upon treating 430 ppmw liquidfuel.[98] Gallium-modified Y-zeolite also showed selectiveuptake under dynamic conditions with an adsorption valueof 14.5 mg g¢1 for 4,6-DMDBT.[78] The preferred removal ofDBT by Ga-zeolite is attributed to p–p interactions betweenthe S compounds and the partially filled d orbital in themetal.[78] In a systematic study, Ma and co-workers[99] testedvarious transition metals to achieve the best removal of thio-phene compounds with the other aromatic compounds left inthe fuel. The proposed adsorptive desulfurization method issuitable to many types of liquid fuel.[99]

Scheme 5. A proposed catalytic-based mechanism for the removal of cyclic Scompounds and liberation of H2S.[2]


5.3. Traditional and surface molecular imprinted polymers

Selective adsorbents are needed for many applications, in-cluding solute separation, preconcentration, and pollutionminimization. Using molecular imprinted polymer (MIP)technology, many selective adsorbents have been pre-pared.[100,101] In a later stage, more advanced materials can befabricated by loading MIPs on a solid surface. These areknown as surface molecular imprinted polymers (SMIPs).[100]

Both MIPs and SMIPs have been tested for selective remov-al of DBT from liquid fuel.[100] As will be discussed below,the performance of SMIPs seems to be promising, and theyoutperform MIPs and other commercial adsorbents. A shortsummary on the fabrication of MIPs and SMIPs is thus pro-vided. MIPs are easily prepared by copolymerizing a func-tional monomer with a cross-linker in the presence ofa target solute (the template or DBT in the current case). In-itially, the monomers are bonded by covalent or noncovalentforces with the template. Then, a suitable cross-linker isadded with the initiator to start copolymerization.[100] Afterpolymerization, the template is eluted and a stable polymericmatrix containing cavities is left behind; it can selectivelyrebind the template in a later stage.[100] SMIPs are simply ob-tained by grafting a thin imprinted polymer film with an in-tense number of binding sites onto a support.[100,102] The sup-port is often added after the initial interaction between thetemplate and the functional monomer.[100] Only a few inor-ganic materials have been used as supports (e.g., SiO2, TiO2,K2Ti4O9, and carbon microspheres) to prepare SMIPs as de-sulfurizing agents. The following sketch depicts the prepara-tion steps of both MIPs and SMIPs (Scheme 6).

5.3.1. MIPs as selective adsorbents for DBT and its derivatives

As shown in Scheme 6 a, specific molecular recognition cavi-ties for DBT can be created by molecular imprinting meth-odology. Accordingly, the DBT-imprinted polymer should beselective for removing DBT from liquid fuel. In an interest-ing study, three MIPs were prepared to selectively bind DBTfrom model fuel.[103] The prepared MIPs were made by usingthree different functional monomers including methacrylicacid.[103] As a control, non-imprinted polymers were also syn-thesized in the same procedure but without the template.[103]

The final results showed that the MIPs selectively boundDBT, but the non-imprinted polymers did not. Methacrylicacid based MIP with divinylbenzene as a cross-linker had themaximum retention capacity of 14.8 mgDBT g¢1 at 25 8C.[103]

Chang and co-workers prepared a DBT-imprinted polymerbased on natural chitosan with epichlorohydrin or glutaric di-aldehyde as the cross-linker.[104] The material prepared withglutaric dialdehyde as the cross-linker demonstrated an ex-cellent adsorption with a retention capacity of22.69 mgDBT g¢1 by using model fuel. The removal processhad a physical nature and was attained in 300 min with animprinting factor of 2.45.[104] The overall process was sponta-neous at 25 8C. The performance of the adsorbent was signifi-cantly reduced upon treating real fuel with a final retention

capacity of 3.5 mgDBT g¢1 and regeneration of the adsorbentwas possible.[104]

5.3.2. SMIPs as selective adsorbents for DBT and its derivatives

Compared to normal MIPs, SMIPs show excellent desulfuri-zation capabilities for DBT and its derivatives from liquidfuel. In addition to the highly selective desulfurization effi-ciency, SMIPs have a promising retention capacity for DBTand its derivatives, good mechanical strength, and high ther-mal stability.[100] As mentioned earlier, SMIPs are often pre-pared by grafting a thin imprinted polymer film onto a sup-port that acts as an excellent adsorbent.[100] For desulfuriza-tion purposes, silica gel, TiO2,K2Ti4O9, and carbon micro-spheres are the most commonly applied supports for MIPs.An effective SMIP with an adsorption value of 57.4 mgS g¢1

was prepared by using methacrylic acid/ethylene glycol dime-thacrylate supported on SiO2 particles.[105] The adsorbentmaintained the same retention capacity after many regenera-tion cycles.[105] Liu and co-workers prepared a double-tem-plate molecularly imprinted polymer supported on carbonmicrospheres.[102] Simply, the SMIP was selectively able tobind BT and DBT at the same time. Adsorption isothermsplots indicated that the retention capacities of BT and DBTincreased fivefold after surface imprinting.[102] The authorssuggested the possible application of this double-templatemolecularly imprinted polymer to achieve ultratrace S-levelliquid fuel.[102]

5.4. Metal–organic frameworks

Adsorbents of high selectivity, high retention capacity, highmechanical strength, and high thermal stability are oftenneeded. In the last decade, significant progress in the fabrica-tion of selective adsorbents has been achieved through inte-gration of different disciplines. Clearly, metal–organic frame-works (MOFs) are one of the most attractive materials andare utilized in many industrial applications.[106–108] Indeed,MOFs of developed porosity (mesoporous and microporous)have been prepared for many purposes, including gas storageand fuel desulfurization.[108] Structurally, MOFs are hybrid in-organic–organic framework systems that consist of two basiccomponents: a metal ion (or a cluster of metal ions) and anorganic connector that links the metal ions to create a well-designed porous material.[109] The organic molecules that linkthe metals are, in fact, chelating agents. Benzene-1,2-dicar-boxylic acid, imidazole, and benzene-x,y,z-tricarboxylic acidare typical organic linkers.[109] The adsorbent HKUST-1 (alsoknown as MOF-199) has been shown to be a better adsorb-ent for 4,6-DMDBT than for BT and DBT.[110] In Scheme 7,the preparation of HKUST-1 is presented.[111]

Initially, two Cu2+ ions and four benzene-1,3,5-tricarboxy-late linkers are bonded to form Cu–linker dimers (each Cuatom is coordinated by four oxygen atoms), and upon elimi-nation of water molecules bonded with Cu ions a veryporous 3 D structure is generated.[111] Nitrogen adsorptiontesting revealed that HKUST-1 has an extremely high sur-


face area of 2260 m2 g¢1 and a high pore volume of0.79 cm3 g¢1.[110] The high porosity of HKUST-1 was also con-firmed by CO2 adsorption tests.[111] Compared to activatedcarbon and other inorganic adsorbents, MOFs exhibit many

advantages, including high porosity, extremely high surfacearea, and modifiable surfaces, all of which further extendtheir industrial use.[112] Except for instability at high tempera-tures, MOFs are amongst the best adsorbents ever prepared.

Scheme 6. Preparation of a) MIP and b) SMIPs (on SiO2). Monomer: methyl methacrylate; cross-linker: ethylene glycol dimethacrylate; initiator: 2,2-azobis-2-methylpropionitrile; solvent : toluene.

Scheme 7. HKUST-1 (MOF-199) preparation from Cu2+ and benzene-1,3,5-tricarboxylate (the linker). Cu2+ ion (dark balls), oxygen (gray balls), and carbon(light gray balls).[111]


5.4.1. MOFs as selective adsorbents for DBT and its derivatives

Adsorptive desulfurization by MOFs and modified-MOFshas been investigated by many authors.[110,113] In general, re-moval of OSCs by MOFs is correlated to various factors in-cluding type of metal and surface functionality.[110] The firstsystematic work on this topic was reported in 2008 by Cy-chosz and co-workers to test the removal of BT, DBT, andDMDBT by different types of MOFs including Cu-BTC,MOF-5, MOF-505, and MOF-177.[110] Of these, MOF-5showed a promising adsorption value for DMDBT(40 mgS gMOF

¢1), and it was more active than Na-Y-zeolite.[110]

With an opposite trend, this research confirmed that MOFsof higher surface areas and porosities showed modest remov-al of OSCs from fuel, for which surface functionality wasmore controlling. Achmann and co-workers showed thatHKUST-1 had an excellent desulfurization efficiency (86 %)for thiophene compounds from model fuel.[114] In the sameline, Khan and co-workers claimed that the metal employedin the MOF significantly affected the removal of OSCs fromfuel.[115] Vanadium-based MOFs [MIL-47(V)] outperformedboth Cr and Al-MOFs with an adsorption value of51 mgS gMOF

¢1, and this value was notably higher than that ofother classical adsorbents.[115] Moreover, many attempts havebeen made to create selective MOFs towards OSCs, and thishas been achieved by surface modification.[116] Vanadium-based-MOFs [MIL-47(V)] were loaded with Cu2+ ions tocreate a selective adsorbent for BT, and the better adsorp-tion was attributed to favorable p interactions between Cu+

(Cu2+ was reduced on the MOF by V3+) and BT.[116] Uponsurface modification, the adsorption capacity notably in-creased from 51 to 73 mgS gMOF

¢1.[116] In an interesting study,a combination of ionic liquid and MOFs technologies was ac-complished by loading an ionic liquid on MIL-101.[117] Thecombined IL–MOF adsorbent showed excellent desulfuriza-tion performance for the removal of BT from liquid fuel.[117]

For better removal of OSCs, Shi and co-workers modifiedMOF-5 by depositing Mo(CO)6 to create a selective adsorberfor DBT from model fuel containing a high fraction of aro-matic hydrocarbons.[118] With a MOF loaded with 20 wt%Mo (surface area 1800 m2 g¢1), DBT was efficiently removedwith a column adsorption capacity (at breakthrough point)of 16 mgS g¢1. The Mo–MOF-5 had reasonable efficiency forremoving DBT from diesel and gasoline.[118]

6. Efficiency of Adsorptive Desulfurization forLiquid Fuel

To assess the AD of different adsorbents towards practicalapplication, a good piece of published research is reviewedand summarized in Table 3. In this table, desulfurization ofcommercial or model diesel is assessed, and details of the ex-perimental conditions (i.e., experimental mode, temperature,pressure, agitation time, etc.) are explicitly provided. The re-tention capacities of the adsorbents in the original paperswere provided as mgT g¢1, mgDBT g¢1, and mgDMDBT g¢1; in thistable, the retention capacities of adsorbents are expressed as

mgS g¢1 so that fair comparisons can be made. Moreover, ex-perimental conditions (which are not often provided indetail) are also summarized in the table to provide a faircomparison between the adsorbents. In OD studies, desulfur-ization efficiency is assessed by estimating the percentage ofS removed (Table 1); however, in AD studies the best ad-sorbent is the one that has the highest retention capacity,particularly for DBT. Furthermore, dynamic retention ca-pacity (obtained from a column test) is more informativethan a batch test. More discussion is provided for those stud-ies focusing on 4,6-DMDBT removal. The results are sum-marized in Table 3.

6.1. Activated Carbon, aluminosilicate, and inorganic adsorb-ents

Before discussing the results summarized in Table 3, itshould be mentioned that although more literature was re-viewed in the previous sections, only the most significant re-sults are provided in Table 3. Probably, activated carbonalong with its modified forms were the first adsorbents testedto remove DBT and its derivatives from liquid fuel. Activat-ed carbons with high surface areas and variable functionalityhave been used in combination with HDS to remove OSCs(1200 ppmw) and nitro compounds from gas oil.[71] As notedin Table 3, activated carbon and modified forms show varia-ble affinity for OSCs, including BT, DBT, and 4,6-DMDBT,with overall retention capacities of 57–166 mgS g¢1. The over-all capacity was reported as mgS g¢1 as outlined earlier. Incertain cases, retention capacity was reported as a combinedcapacity for DBT and 4,6-DMDBT.[102] Notably, in most stud-ies the researchers did not monitor removal of non-S organiccompounds,[2–4,102, 110,120, 121, 123] whereas in a few studies remov-al of both S and non-S compounds was carefully moni-tored.[12,69,83, 88,119] Al-Ghouti and co-workers reported a highadsorption for OSCs from commercial diesel (57 mgS g¢1);however, the nature of the removed compounds was notidentified and removal of non-S compounds was not moni-tored.[4] In a systematic study, adsorption of DBT and othercommon organics by activated carbon was studied, and naph-thalene showed the best affinity:[69] naphthalene>DBT>an-thracene>chrysene. After fuel acidification by acetic acid,DBT was preferably removed over other organics:[69] DBT>chrysene>anthracene> naphthalene. Although removal ofDBT was high (166 mgS g¢1), the removal of other organicswas still significant, and this can affect the combustion quali-ty of treated diesel. Indeed, activated carbon showed excel-lent performance in removing OSCs. With three adsorptioncycles with the use of fresh adsorbent, the level of S was re-duced from 7044 to 43 mgS g¢1.[31] In the same line, Bu andco-workers demonstrated that commercial activated carbon(acidic form) removed not only DBT and 4,6-DMDBT butalso phenanthrene, anthracene, fluorine, and naphthalene.[12]

The adsorption affinity decreased in the following order:phenanthrene>anthracene>2,4-DMDBT>DBT> fluo-rene>naphthalene.[12] Competitive adsorption tests onmodel diesel clearly indicated the favorable uptake of non-S


Tabl

e3.

Des

ulfu

riza

tion

effic

ienc

yof

diff

eren

tad

sorb

ents

.

Fuel

Targ

etco

mpo

und(

s)A

dsor

bent

Con

diti

ons[a

]R

eten

tion

capa

city

[mg S

g¢1 ]

Ref

.

Act

ivat

edca

rbon

dies

elO

SCs

(DB

Tan

dde

riva

tive

s)co

mm

erci

alA

cba

tch,

500

ppm

was

OSC

s,25

8C,2

4.0

h57

.0[4

]di

esel

OSC

s(D

BT

and

deri

vati

ves)

Mn-

AC

batc

h,70

55pp

mw

asO

SCs,

258C

,24.

0h

19.6

[7]

mod

eldi

esel

DB

T,4,

6-D

MD

BT

com

mer

cial

Ac

batc

h,up

to40

0pp

mw

asS,

308C

38.0

[12]

dies

elD

BT

com

mer

cial

Ac

batc

h,25

78pp

mw

asD

BT,

300

8C,2

4.0

h78

.0[6

9]di

esel

DB

Tco

mm

erci

alA

cba

tch

and

fuel

acid

ifica

tion

,25

78pp

mw

asD

BT,

300

8C,

24.0

h16

6.0

[69]

mod

eldi

esel

DB

TH

2SO

4-A

Cfix

ed-b

edflo

w,2

20pp

mw

asS,

258C

47.1

(no

aren

e)32

.4(3

0%

aren

e)[8

3]

mod

eldi

esel

DB

T,4,

6-D

MD

BT

Cu-

AC

fixed

-bed

flow

,20

ppm

w,2

58C

10.3

(no

aren

es)

9.5

(wit

har

enes

)[8

8]

mod

eldi

esel

DB

Tca

rbon

aero

gels

(nan

osiz

e)ba

tch

and

fuel

acid

ifica

tion

,25

0pp

mw

asD

BT,

258C

,24.

0h

15.1

[119

]

Alu

min

osili

cate

and

inor

gani

cad

sorb

ents

dies

elO

SCs

(DB

Tan

dde

riva

tive

s)na

tura

lzeo

lite

batc

h/co

lum

n,12

00pp

mw

asO

SCs,

258C

batc

h7.

15co

lum

n4.

45[2

]

dies

elO

SCs

(DB

Tan

dde

riva

tive

s)ac

id-a

ctiv

ated

bent

onit

eco

lum

n,78

00pp

mw

asO

SCs,

258C

5.4

[3]

mod

elfu

elB

TC

u-ze

olit

eba

tch,

1800

ppm

w,3

08C

,15

h63

.1[1

20]

mod

elfu

elB

TC

u-zi

rcon

iaco

lum

n,20

00pp

mw

asS,

258C

15.7

[121

]

MIP

san

dSM

IPs

mod

eldi

esel

BT,

DB

TSM

IP[b

]ba

tch,

BT:

13–1

34pp

mv,

DB

T:1

8–18

4pp

mv,

258C

,90

min

(cen

trifu

gati

on)

25.2

[102

]m

iner

aloi

land

gaso

line

BT,

DB

TSM

IP[b

]ba

tch,

BT:

11.2

ppm

v,D

BT

:10.

9pp

mv,

258C

,90

min

(cen

trifu

gati

on)

2.8

[102

]m

odel

dies

elD

BT

MIP

batc

h,3.

69g

L¢1

asD

BT,

258C

,24

h11

.61

mg

g¢1

[103

]m

odel

fuel

(n-h

exan

e)D

BT

SMIP

[b]

batc

h,1–

5m

mol

L¢1

DB

T,25

8C,6

.0h

SMIP

:20.

2bl

ank

poly

mer

:15.

7[1

22]

MO

Fsm

odel

fuel

(iso

octa

ne)

BT,

DB

T,4,

6-D

MD

BT

MO

F(U

MC

M-1

50)[c

]ba

tch,

upto

2000

ppm

wS

for

BT

and

DB

Tan

dup

to70

0pp

mw

Sfo

r4,

6-D

MD

BT,

258C

,2h

for

BT,

3.5

hfo

rD

BT,

12h

for

DM

DB

T14

.0(a

sB

T)34

.0(a

sD

BT)

37.0

(as

4,6-

DM

DB

T)

[110

]

mod

elfu

el(i

sooc

tane

)D

BT,

4,6-

DM

DB

TM

OF

(UM

CM

-152

)[d]

batc

h,up

to20

00pp

mw

Sfo

rD

BT

and

upto

600

ppm

wS

for

4,6-

DM

DB

T59

(as

DB

T)82

(as

4,6-

DM

DB

T)[1

23]

mod

elfu

el(i

sooc

tane

)D

BT,

4,6-

DM

DB

TM

OF

(UM

CM

-153

)[d]

batc

h,up

to20

00pp

mw

Sfo

rD

BT

and

upto

600

ppm

wS

for

4,6-

DM

DB

T89

(as

DB

T)40

(as

4,6-

DM

DB

T)[1

23]

[a]E

xper

imen

tal

mod

e,co

ncen

trat

ion

,te

mpe

ratu

re,

shak

ing

tim

e.[b

]Sup

port

:ca

rbon

mic

rosp

here

s;fu

ncti

onal

mon

omer

:m

etha

cryl

icac

id;

cros

s-lin

ker:

ethy

lene

glyc

oldi

met

hacr

ylat

e.[c

]Met

al:

Cu2

+;

cros

s-lin

ker:

biph

enyl

-3,4

’,5-t

rica

rbox

ylic

acid

.[d]

Met

al:C

u2+

;lin

ker:

deri

vati

ves

ofte

trac

arbo

xyla

ted

trip

heny

lben

zene

.


aromatics (i.e., phenanthrene and anthracene) over 2,4-DMDBT and DBT.[12] An interesting point in the work of Buand co-workers is the good performance of a carbon fixed-bed adsorber for DBT.[12] Carbons with an acidic nature aremore effective toward OSCs owing to favorable interactionwith the slightly basic S atom, as confirmed by Haji andErkey.[119] These authors reported a high adsorption of DBT(15.1 mgS g¢1) by using nanosize carbon aerogels with a selec-tivity factor of 1.61 against naphthalene.[119] Yang and co-workers created a selective adsorbent for DBT by activatingthe surface with H2SO4.

[83] Competitive adsorption tests indi-cated that adding an arene (i.e., tetrahydronaphthalene) re-duced the removal of DBT from 47.1 to 32.4 mgS g¢1, whichreflected the high selectivity of the adsorbent towardDBT.[83] The carboxylic groups created upon acid treatmentincreased the interaction with the basic S atom.[83] Interactionof the S atom with the acidic functional groups was con-firmed by IR spectroscopy.[7] The adsorption and catalyticperformance of activated carbon was improved toward DBT-rich diesel with a reasonable capacity of 19.1 mgS g¢1.[7] Un-fortunately, many issues, including removal of non-S com-pounds, regeneration, and column test, were not investigat-ed.[7] Cu-modified AC showed excellent performance witha column capacity of 10.3 mgS g¢1 (for DBT and 4,6-DMDBT).[88] The column capacity was slightly affected uponadding large fractions of arenes (i.e., naphthalene and 1-methylnaphthalene) to the fuel.[88] The authors outlined thesignificance of Cu atoms, which created a selective adsorberfor 4,6-DMDBT.[88] Unfortunately, aluminosilicates and inor-ganic adsorbents showed only modest efficiency in the re-moval of OSCs (average capacity 1–5 mgS g¢1) and surfacemodification was necessary for better performance.[2–4,13, 14]

Natural zeolite and acid-activated bentonite were tested byMustafa et al. to remove DBT-rich diesel. Unfortunately,both adsorbents had poor capacity in batch and column testswith a maximum retention of 7.15 mgS g¢1.[2] The main ad-vantage of aluminosilicate materials is their availability. Onthe basis of earlier observations, modifying aluminosilicatesand inorganic adsorbents with Cu is recommended to pro-vide selective adsorbers for DBT and its derivatives. Cu-modified zeolite showed excellent performance and outper-formed some commercial carbons towards BT with a reten-tion capacity 63.1 mgS g¢1.[120] As discussed earlier, the favora-ble interaction between BT and Cu-zeolite is attributed to p–p interactions between the S atom and partially filled d orbi-tals in the metal.[120] Although Cu-zeolite is selective for BT,the removal of non-S aromatics was not monitored.[120] Cu-zirconia seems to be a promising adsorbent for BT witha column retention capacity of 15.7 mgS g¢1.[121]

6.2. Molecular imprinted polymers and surface imprinted poly-mers

The second generation of adsorbents includes MIPs andSMIPs. In relative terms, SMIP adsorbents are more efficientand selective for DBT and its derivatives than other adsorb-ents, as shown in Table 3. The first demonstration was for

DBT removal and was reported by Castro and co-workers byusing model diesel. The tested MIP removed up to11.61 mgS g¢1 (as DBT).[103] Relative to that removed by theblank (non-imprinted) polymer, MIP removed more DBT.However, its effect on non-S compounds and the possibilityof its commercial application on diesel were not investigat-ed.[103] A selective SMIP adsorbent was prepared to removeDBT from model fuel.[122] The high selectivity towards DBT(owing to molecular recognition) may retard uptake of othernon-S compounds, and this property is not available in acti-vated carbon adsorption. Liu and co-workers prepareda SMIP that was able to remove both BT and DBT from fuel(i.e., double function adsorbent).[102] The combined removalcapacity was as high as 25.2 mgS g¢1 (for BT and DBT). Theprepared SMIPs showed high selectivity for BT and DBTwith selectivity factors of 7.4 and 2.5, respectively.[102] Al-though the prepared SMIP was effective for cleaning modelfuel, a modest efficiency for both targets was observed(2.8 mgS g¢1).[102] The significant reduction in BT/DBT remov-al was mainly attributed to negative competition with otheraromatics toward the surface. Unfortunately, the removal ofnon-S compounds by MIP and SMIP was not reported, andthis is an important issue that calls for more research.[102]

Moreover, the performance of MIPs/SMIPs in column andthe possibility of reusability also need more investigation.Preparation of MIPs and SMIPs and their application in theremoval of OSCs is reviewed elsewhere.[100]

6.3. Metal–organic frameworks

The third generation of adsorbents is MOFs. Their extremelyhigh surface area, variable surface functionality, and intenseporosity are, indeed, attractive enough to test them for theirability to remove DBT and other bulky derivatives fromdiesel. Cychosz and co-workers studied the individual remov-al of BT, DBT, and 4,6-DMDBT from model fuel by usingfive MOFs.[110] The adsorbent UMCM-150 (specific surfacearea 3100 m2 g¢1 and pore volume 1.11 cm3 g¢1) showed rea-sonable retention for OSCs with an interesting trend: 4,6-DMDBT>DBT>BT (see Table 3 for adsorption capacity).UMCM-150 had higher affinity for hard-to-remove 4,6-DMDBT than for DBT and BT, and this is the opposite tothe trend observed in the commercial HDS process.[13,14, 31]

An excellent S-cleaning result would be obtained by combin-ing MOFs with HDS. Unexpectedly, removal of OSCs byMOFs was not logically correlated with a large surface areaand intense porosity.[110] In addition to UMCM-150, MOF-177 was also tested for the same purpose.[110] The basic unitsof MOF-177 are Zn2+and 1,3,5-tris(4-carboxyphenyl). Al-though MOF-177 has a large surface area (5640 m2 g¢1) andintense porosity (1.86 cm3 g¢1), it absorbed only 5.0 mgS g¢1

for BT and DMDBT and 15.0 mgS g¢1 for DBT.[110] The poorcapacity of MOF-177 was attributed to the surface function-ality of the material rather than to the surface area or porosi-ty. It is highly possible that Cu atoms in UMCM-150 attractmore OSCs than Zn atoms in MOF-177; however, this postu-lation may need further experimental confirmation. In the


same field, Schnobrich and co-workers also confirmed thatremoval of DBT and 4,6-DMDBT by two different MOFs(i.e., UMCM-152 and UMCM-153) could not be correlatedto either surface area or porosity.[123] The interesting pointwas the high retention of 4,6-DMDBT (82 mgS g¢1) byUMCM-152, which was not previously reported. Althoughboth MOFs have comparable BET surface areas (3370 and3480 m2 g¢1 for UMCM-153 and UMCM-152, respectively),UMCM-153 removed only 40 mgS g¢1 (as 4,6-DMDBT), asshown in Table 3. An opposite trend was observed for the re-moval of DBT.[123] The dramatic differences in the retentionof DBT/DMDBT by both MOFs provide indisputable evi-dence that liquid-phase adsorption is not dependent on sur-face area, linker, or identity of the metal cluster.[123] Accord-ingly, adsorption of DBT and 4,6-DMDBT by MOFs ismainly controlled by acid–base, hydrogen-bonding, and p-complexation interactions. Generally, the retention capacitiesfor DBT and 4,6-DMDBT are higher for MOFs than forSMIPs, and this may give MOFs a better chance for practicalapplication. However, the fact that MOFs are good adsorb-ents for 4,6-DMDBT is not reason enough for them to beseen as the best choice. Removal of non-S compounds anddesulfurization of commercial fuels should be thoroughly in-vestigated.

6.4. Practical assessment of different adsorbents for fuel desul-furization

As mentioned earlier, HDS is the most adopted procedureto remove OSCs from diesel, but the running costs are highand removal of DBT and its derivatives is not so efficient.

Accordingly, new procedures have been developed, includingoxidative extraction and adsorptive desulfurization.[14] Onthe basis of the provided discussion of Table 4, the practica-ble and commercial application of different adsorbents canbe identified. Ideally, the best adsorbent is the one that hasthe highest retention capacity with minimum removal ofnon-S compounds. Removal of OSCs from fuel should berapid. Moreover, regeneration of the adsorbent should beachieved with minimum solvent consumption.

Amongst the tested adsorbents, activated carbon is theone that is usually involved on commercial scale. Sano andco-workers combined activated carbon with HDS to achievedeep desulfurization.[71] The feed fuel had 1200 mgS g¢1, andupon treatment in the HDS unit the level of S was signifi-cantly reduced to 300 mgS g¢1. In a separate unit, the fuel wasfurther treated with fresh activated carbon to produce almostultraclean fuel with a S level of only 10 mgS g¢1.[71] The desul-furization unit was also efficient in removing N compoundswith a final nitrogen level down to zero.[71] The exhaustedcarbon was simply regenerated by toluene extraction at am-bient temperature.[71] One of the best advantages of adsorp-tive desulfurization of liquid fuel (in comparison to HDS andoxidative extractions routes) is the affordable operating con-ditions, including low temperature and normal pressure, to-gether with a minimum amount of generated byproducts.Another advantage is that no H2 is needed, and this leavesmore H2 available for fuel-cell applications. Furthermore, thelevel of S in liquid fuel can reach near-to-zero values owingto removal of DBT and its derivatives that are hard-to-elimi-nate by conventional HDS. Two major challenges are associ-ated with adsorptive desulfurization: one, developing simple-

Table 4. Assessment of different adsorbents toward practical applications.

Adsorbent Advantages Disadvantages Practical application/Remarks

activated carbon - modifiable surface towardOSCs- high retention- capacity for DBT and deriv-atives- easily recycled- stable at high temperature

- expensive- high adsorption for non-S compounds- gradual to slow removal rate for DBT andderivatives[7]

- AC is used commercially with HDS[71]

- new modifiers should be tested to improveuptake of DBT and derivatives against non-Scompounds

aluminosilicate and inor-ganic adsorbents

- easily modified to createselective surface for DBT andderivatives- easily available- stable at high temperature

- modest intrinsic capacity for DBT and deriv-atives[13]

- hard to separate from diesel due to its col-loidal nature

- compared to AC, utilization of aluminosilicateand inorganic adsorbents is rather limited com-mercially- more research is needed to utilize aluminosili-cate and inorganic adsorbents for real applica-tions

molecular imprinted poly-mers and surface imprintedpolymers

- very selective due to molec-ular imprinting- easily prepared with con-trolled textural properties

- modest performance for commercial fuel[102]

- regeneration of the MIP/MIPs is questiona-ble due to strong interaction with DBT and itsderivatives[100]

- not stable at high temperatures

- it is highly recommended for practical applica-tion due to excellent selectivity toward DBT andderivatives- more research is needed to assess adsorptionin column, regeneration, and affinity toward non-S compounds

metal–organic frameworks - distinguished retention ca-pacity for DBT and 4,6-DMDBT- easily prepared relative tocommercial AC- modifiable surface towardDBT and derivatives

- not tested for cleaning commercial fuels- not stable at high temperature

- due to the high affinity toward 4,6-DMDBT,MOFs are strongly recommended for practicalapplications[123]

- more research is needed to assess adsorptionin column, regeneration, and affinity toward non-S compounds


to-regenerate adsorbents with a high retention capacity forDBT and its derivatives; two, developing adsorbents that areselective towards DBT and its derivatives over non-S aromat-ic compounds present in hydrocarbon fuel. Whereas the re-moval process is highly efficient in most cases, the tested ad-sorbents are difficult to regenerate and solvent extraction isoften needed. The adsorption capacity of aluminosilicates israther modest at <5 mgS g¢1, and thus a large amount of theadsorbent is required, which is an impractical solution. Theinteresting point regarding aluminosilicate and other inor-ganic adsorbents is the possible creation of an excellent ad-sorber after surface modification. Stability at high tempera-ture is another advantage of aluminosilicate adsorbents, asthis is necessary for reactive adsorption, for which a hightemperature is necessary for desulfurization.[13,14] Owing totheir colloidal nature (particle diameter 1–10 mm), it is ratherhard to remove clay particles from fuel, and filtration underhigh pressure is needed. Relative to AC, the commercial uti-lization of aluminosilicate and inorganic adsorbents is ratherlimited. In fact, more investigation is needed to utilize alumi-nosilicate and inorganic adsorbents for real applications, andthis should include removal of DBT and its derivatives ina fixed-bed adsorber and recycling of the exhausted adsorb-ent. Recently, research was focused on developing adsorb-ents that are low costing, have a high surface area, and areporous and selective to DBT and its derivatives. SMIPs andMOFs offer good solutions for the removal of DBT and itsderivatives from liquid fuel; however, more research isneeded at this stage, as outlined in Table 4. The main draw-back of SMIPs is their poor performance toward DBT andits derivatives in cleaning commercial fuels. Moreover, regen-eration of the SMIPs is questionable, owing to its strong in-teraction with DBT and its derivatives (due to molecular im-printing), besides their instability at high temperatures. Atthis stage, adsorption in column, regeneration, and affinitytowards non-S compounds are the main issues to be re-searched. Owing to their high affinity towards 4,6-DMDBT,MOFs are strongly recommended for practical applications.At this stage, more research work is recommended to evalu-ate the performance in fixed-bed mode, regeneration, and af-finity for non-S compounds. For better adsorptive desulfuri-zation, the main issue is to create an adsorbent that is selec-tive for DBT and its derivatives with no affinity for non-Scompounds. To a large extent, SMIPs are able to fulfil thiscondition. The mechanism of desulfurization and the rate ofdesulfurization over different adsorbents is an interesting re-search area that deserves more attention.

7. Impact of Desulfurization Processes on the En-vironment

Although they play a very important role, the capital and on-going costs of the potentially available desulfurization meth-ods are not the only consideration that should be taken intoaccount in selecting a technology. Another important factorrelates to the environmental impact of the competing ap-proaches, because the real environmental costs are not truly

reflected in the cost of an industrial process[124] and, particu-larly, not to the extent at which the local financial costs ac-tually reflect the environmental costs. Financial costs willvary from place to place and from time to time and, in par-ticular, will depend on the price of energy. Although a “pol-luter pays” principle is adopted as policy in many jurisdic-tions, it rarely, if ever, works effectively; furthermore, it hasbeen pointed out[125] that decision support techniques that donot depend exclusively on monetization may be preferred.Various techniques have been used to allow both policymakers and businesses to make decisions between competingproduction methods, of which the most widely used is “lifecycle analysis (LCA)”.[126,127] A thorough review of academicliterature, government publications, and gray literature sour-ces produced very little in the way of formal efforts to try toaccount for, or even to produce an inventory of, the environ-mental impacts of the emerging methods of desulfurizationthat have been discussed in this paper. The hydrodesulfuriza-tion process has received slightly more attention in this area,but even this is really underrepresented. One advantage oftaking the formal LCA approach to this is the well-definedinventory creation process, and even if the LCA is not takento completion, the inventory of environmental effects can bea useful first step in making a broader environmental ap-praisal of a new process.

Possible reasons for the rarity of such studies are the diffi-culties in defining the process boundaries,[128] as for sucha study to be meaningful the process must take into accountfactors that are highly variable from refinery to refinery. Theterm “well to wheel” is commonly used in life cycle analysisstudies of hydrocarbon-based fuel production, from whichthe entire fuel-production process either from an individualrefinery or within the entire industrial base of a country isstudied. Alternatives are the “well to gate” and “well totank” approaches. However, whereas the contribution of thedesulfurization processes of diesel and other fuels are com-monly highlighted at the start of the documents, and includ-ed in the process flow diagrams and tables, details of the con-tribution to environmental harm (whether relative or abso-lute) of that part of the process is usually lost. A good exam-ple of this is the literature review of LCAs for petrol anddiesel by Erikson and Ahlgren, which fails to consider desul-furization as a distinct operation.[129] This is because of thedifficulties in assigning contributions to the carbon footprintand both aquatic and atmospheric emissions to intimatelyconnected and inter-reliant processes. The difficulties ofdoing this are highlighted by Wang and co-workers,[130] whowere able to produce a method to assign energy usage to in-dividual subprocesses within a refinery. Researchers whowish to proceed along an environmental assessment ofa novel desulfurization process would find this paper an ex-cellent starting point.

However, although it may be complex, it is not an impossi-ble task, and a detailed life cycle analysis has recently beenpublished by the IMOA,[131] which considers the use of mo-lybdenum catalysts in the production of low sulfur dieselthrough hydrodesulfurization. Usefully, this study is not lim-


ited to the energy/carbon footprint aspects but focuses ona wider range of environmental effects.

A separation is made between effects generated in the useof the diesel and the refining of the petroleum, and impor-tantly, it also takes separate account of the effects specificallygenerated through the hydrodesulfurization process. Bothdirect and indirect effects are taken into account, as are theenvironmental consequences of the creation of the catalyst.In addition, credits are applied for the avoided production ofprimary molybdenum trioxide, cobalt, and nickel through therecycling process. These factors are important to consider, asthe process studied made use of increased catalyst amountsto achieve their desired targets. In this study the followingparameters were taken into account:

1) Acidification potential, primarily from released sulfurand nitrogen oxides or generated from the oxidation ofreduced forms by oxidation in the atmosphere or soil

2) Global warming potential, including CO2 from fuel com-bustion in vehicles and energy use on the plant andduring transportation, but also including release of othercompounds that may be more potent greenhouse gases

3) Smog creation potential, mainly associated with hydrocar-bons (in combination with nitrogen oxides), but there isalso a contribution from sulfur

4) Particulate matter/respiratory inorganics, including partic-ulate sulfates and sulfuric acid aerosol, plus particulatesgenerated by fuel consumption in the use phase andduring various plant operations

5) Eutrophication potential, mainly from atmospheric re-lease of N-containing compounds, but also by the aqueousroute

7.1. Primary Energy Demand

Whereas the desulfurization stage makes a measurable con-tribution to all these parameters, an important observation isthe dominance of the “use phase”, which in particular ac-counts for 74 % of the total acidification for 10 ppmw Sdiesel and 80 % of the impact of 2000 ppmw S diesel. The au-thors illustrated that processes and materials related to thehydrodesulfurization contribute only 0.3 % of the overallimpact and that the production, regeneration, and recyclingof catalysts account for only 0.06 % of the overall acidifica-tion impact for 10 ppmw diesel.

For particulate matter, the impact of hydrodesulfurizationrepresents only 0.1 % of the total amount. Probably, themost important parameter for many stake holders is globalwarming potential. This study reported that the contributionof the hydrodesulfurization step to this parameter is only0.7 % of the total. This study is, apparently, not peer re-viewed and is published by an organization that aims to pro-mote molybdenum use. Thus, the results presented must beinterpreted in light of both the origin of the document andthe starting point adopted in terms of both the concentrationof S in the un-desulfurized feedstock and the lack of infor-

mation on the compound distribution. However, in terms ofall the parameters, including the important relative contribu-tion to global warming, this study suggests that even theenergy-demanding hydrodesulfurization process has verylittle impact relative to the effects of the rest of the life cycleof diesel production and use. Given the primary importanceof hydrocarbon transport fuels to global warming, we mustrealize that a relative value of 0.7 % represents a very largeabsolute number, and this is indicative of the importance oftechnology selection in considering alternative or supplemen-tary techniques.

Despite the previously mentioned limitations, the studydoes present an approach that could serve as a useful tem-plate for further studies on the emerging desulfurization ap-proaches that will be necessary if newly emerging technolo-gies are to be selected on a sound environmental basis.

As mentioned previously, detailed LCA studies on emerg-ing desulfurization technologies are rare. One example is thework of Alves and co-workers, which examined biodesulfuri-zation.[132] Two biodesulfurization process designs were com-pared in terms of energy consumption and greenhouse-gasemissions by following a life cycle assessment methodology.The hydrodesulfurization process was used as the referencetechnology. Different theoretical scenarios were considered,and the best biodesulfurization results were scaled up toevaluate a case study of providing ultralow sulfur diesel toan urban taxi fleet; the process was found to be a relativelyenvironmentally benign supplementary process capable of re-moving the compounds that are recalcitrant to hydrodesulfu-rization under mild operating conditions. A life cycle costanalysis was also included, which indicated that the processcould also be cost effective.

In generating life cycle analysis inventories for a “well towheel” LCA, it is necessary to choose a particular referenceyear to account for production and use rates. The problem isthat if a year is chosen in which product use is high, the con-tribution of any part of the refining process that has a fixedelement to their environmental impacts tends to be dilutedin relation to the use phase. The need to have a referenceperiod over which the LCA applies can lead to some very in-ventive mechanisms in trying to model the implementationof technology yet to be adopted.

For example Mart�nez-Gonz�lez et al.[133] performed a lifecycle analysis comparison of the potential impacts on theproduction and use of, what these authors termed, high andlow sulfur diesel on the basis of an assessment of the Barran-cabermeja refinery in Santander, Columbia, which now usesthe hydrodesufurization process. However, the requirementsover the reference period were to comply with Colombianlaw, which required a 500 ppmw S diesel, and this was previ-ously produced by mixing 10 ppmw imported product with1200 ppmw diesel available from the refinery. In their work,they proposed a scenario in which their 500 ppmw diesel wasproduced by blending with product from a hydrodesulfuriza-tion plant that came on stream in 2010, some two years afterthe reference year used in their calculations These authorsconcluded that the use of the process to produce low sulfur


diesel added approximately 3.8 % to the overall carbon foot-print of diesel production and utilization somewhat higherthan that proposed by the previously mentioned LCA.[131]

Clearly, the inherent sulfur content (both in terms of quanti-ty of S and distribution of various compounds) of the dieselfraction being treated, itself a function of the crude oil beingrefined, would be highly influential on both the S contentachievable by hydrodesulfurization and the contribution tooverall environmental impact of the desulfurization step. Ifalternative or supplementary desulfurization techniques areproposed, then these factors become even more important.

It is important to realize that on using life cycle analysisand related techniques to compare relative contributions ofthe desulfurization process to the total “well to wheel” lifecycle, the uncertainties in the data and the poor transferabili-ty may result in the contribution of a particular desulfuriza-tion process becoming lost in the noise. What is required,and to a large extent lacking, are detailed comparative stud-ies of existing and emerging techniques in which the systemboundaries are drawn narrow enough to allow reasonablecomparisons to be made. What will be very important onceone starts to compare the environmental impacts of compet-ing processes is how one accounts for the impacts of treatingthe sulfur-containing waste streams produced and the poten-tial methods available for dealing with them. It is worthwhileto consider the waste streams produced in the various meth-ods previously outlined. In the hydrodesulfurization process,the primary S-containing “waste” is H2S, which can be easilyconverted into useful products such as sulfuric acid or ele-mental sulfur (with much of the generated heat being recov-erable).[134] One must also remember that the sulfur recoveryprocesses themselves are capable of improvement[135,136] andthat data will be very much time and place dependent. Ona plant by plant assessment of environmental effects, oneshould take into account the environmental costs of achiev-ing these preferred products, but it would be reasonable togive credit for the environmental costs avoided by not pro-ducing these compounds by traditional means. In adsorptiveand oxidative desulfurization, however, the sulfur-containingwastes directly exiting the processes are in the form of orga-nosulfur compounds, either in the original form or partiallyoxidized to increase polarity. The processes of removingthese compounds from the phase into which they have beeninitially partitioned and the additional steps required to con-vert this potentially wide range of compounds into substan-ces that either have a reuse value or are suitable for environ-mentally sound disposal would need to be taken into ac-count. An assessment would also need to be made as to thepotential for release into the aquatic environment, and inany cost calculations, account must be taken of any addition-al load made on wastewater treatment plants to maintaincompliance with effluent discharge limits. Essentially, thesedemands are technologically capable of being solved, but theenvironmental as well as the financial costs must be incorpo-rated into the calculations used in process evaluation. Theseaspects of the study would provide a fertile research oppor-tunity.

8. Conclusions

Distillates of petroleum oil, especially gasoline and dieselfractions, contain significant amounts of alkylated and aro-matic organosulfur compounds. The levels of organosulfurcompounds are much greater in diesel. This includes diben-zothiophene and other derivatives that are hard to removeby the commercial hydrodesulfurization process. Whereashydrodesulfurization is highly involved in reducing organo-sulfur compounds in liquid fuel, the real participation ofnewly developed oxidative and adsorptive methods in desul-furization is highly possible. For oxidative desulfurizationroutes, a drastic reduction in S compounds is reported byusing ultrasound-assisted procedures. Using ionic liquids toremove preoxidized S compounds is a novel approach to-wards ultraclean fuel with high combustion quality. Handlingwastes of oxidation desulfurization methods would be a chal-lenging environmental task. Undoubtedly, adsorption-basedmethods have achieved many advances in this area. So far,surface molecular imprinted polymers and metal–organicframeworks have shown excellent tendency to remove prob-lematic 4,6-dimethyldibenzothiophene from liquid fuel withminimum experimental efforts. Merging adsorptive desulfuri-zation methods with hydrodesulfurization has been demon-strated to be a promising pathway to reach “zero-level-S”fuel; however, individual use of adsorptive or oxidativemethods in practical desulfurization seems to be an achieva-ble task.

Keywords: adsorption · hydrodesulfurization · sulfur ·sustainable chemistry · transportation fuels

[1] R. C. Selley, S. A. Sonnenberg, Elements of Petroleum Geology,3rd ed., Academic Press, San Diego, CA, 2015, ch. 2, pp. 13 –39.

[2] F. Mustafa, M. A. Al-Ghouti, F. I. Khalili, Y. S. Al-Degs, J. Hazard.Mater. 2010, 182, 97 –107.

[3] F. I. Khalili, M. Sultan, C. Robl, M. Al-Ghouti, J. Ind. Eng. Chem.2015, 28, 282 –293.

[4] M. A. Al-Ghouti, Y. S. Al-Degs, F. I. Khalili, Chem. Eng. J. 2010,162, 669 –676.

[5] T. J. Bandosz, Interface Sci. Technol. 2006, 7, 231 –292.[6] S. H. D. Lee, R. Kumar, M. Krumpelt, Sep. Purif. Technol. 2002, 26,

247 – 258.[7] M. A. Al-Ghouti, Y. S. Al-Degs, Energy Technol. 2014, 2, 802 –810.[8] C Kwak, J. J. Jung, J. S. Bae, K. Choi, S. H. Moon, Appl. Catal. A

2000, 200, 233 –242.[9] M. Houalla, N. K. Nag, A. V. Sapre, D. H. Broderick, B. C. Gates,

AIChE J. 1978, 24, 1015 – 1021.[10] F. Zannikos, E. Lois, S. Stournas, Fuel Process. Technol. 1995, 42,

35– 45.[11] G. W. vanLoon, S. J. Duffy, Environmental Chemistry: A Global Per-

spective, 2nd ed., Oxford University Press, Oxford, 2005.[12] J. Bu, G. Loh, C. G. Gwie, S. Dewiyanti, M. Tasrif, A. Borgna,

Chem. Eng. J. 2011, 166, 207 – 217.[13] V. Chandra Srivastava, RSC Adv. 2012, 2, 759 – 783.[14] E. Ito, J. A. R. van-Veen, Catal. Today 2006, 116, 446 –460.[15] O. Gonz�lez-Garc�a, L. Ceden"-Caero, Catal. Today 2009, 148, 42 –

48.[16] S. Bruneta, D. Meya, G. P¦rot, C. Bouchy, F. Diehl, Appl. Catal. A

2005, 278, 143 –172.


http://dx.doi.org/10.1016/j.jhazmat.2010.06.003




http://dx.doi.org/10.1016/j.jiec.2015.03.004




http://dx.doi.org/10.1016/j.cej.2010.06.019




http://dx.doi.org/10.1016/S1573-4285(06)80014-1

http://dx.doi.org/10.1016/S1573-4285(06)80014-1

http://dx.doi.org/10.1016/S1573-4285(06)80014-1

http://dx.doi.org/10.1016/S1383-5866(01)00174-5

http://dx.doi.org/10.1016/S1383-5866(01)00174-5

http://dx.doi.org/10.1016/S1383-5866(01)00174-5

http://dx.doi.org/10.1016/S1383-5866(01)00174-5

http://dx.doi.org/10.1002/ente.201402033



http://dx.doi.org/10.1016/S0926-860X(00)00635-9

http://dx.doi.org/10.1016/S0926-860X(00)00635-9

http://dx.doi.org/10.1016/S0926-860X(00)00635-9

http://dx.doi.org/10.1016/S0926-860X(00)00635-9

http://dx.doi.org/10.1002/aic.690240611



http://dx.doi.org/10.1016/0378-3820(94)00104-2

http://dx.doi.org/10.1016/0378-3820(94)00104-2

http://dx.doi.org/10.1016/0378-3820(94)00104-2

http://dx.doi.org/10.1016/0378-3820(94)00104-2




http://dx.doi.org/10.1039/C1RA00309G



http://dx.doi.org/10.1016/j.cattod.2006.06.040






http://dx.doi.org/10.1016/j.apcata.2004.10.012




[17] R. T. Yang, A. J. Hernandez-Maldonaldo, F. H. Yang, Science 2003,301, 79 –81.

[18] T. H. Park, K. A. Cychosz, A. G. Wong-Foy, A. Dailly, A. J. Matzg-er, Chem. Commun. 2011, 47, 1452 –1454.

[19] R. Shafi, G. J. Hutchings, Catal. Today 2000, 59, 423 –442.[20] U. T. Turaga, C. Song, Catal. Today 2003, 86, 129 –140.[21] I. V. Babich, J. A. Moulijn, Fuel 2003, 82, 607 – 631.[22] J. Williams, Trans. Faraday Soc. 1928, 24, 245 – 255.[23] Y. Zhao, R. Hao, T. Wang, C. Yang, Chem. Eng. 2015, 273, 55 –65.[24] L. Skatteboel, B. Oulette, S. Solomon, J. Org. Chem. 1967, 32,

3111 – 3114.[25] C. Schaeffer-Reiss, P. Schaeffer, A. Putschew, Org. Geochem. 1998,

29, 1857 –1873.[26] E. Ahnonkitpanit, P. Prasassarakich, Fuel 1989, 68, 819 –824.[27] S. Otsuki, T. Nonaka, N. Takashima, W. Qian, A. Ishihara, T. Imai,

T. Kabe, Energy Fuels 2000, 14, 1232 –1239.[28] W. Y. Liu, Z. L. Lei, I. K. Wang, Energy Fuels 2001, 15, 38 –43.[29] M. H. Ali, A. Al-Maliki, B. El-Ali, G. Martinie, M. N. Siddiqui, Fuel

2006, 85, 1354 –1363.[30] G. Yu, S. Lu, H. Chen, Z. Zhu, Carbon 2005, 43, 2285 – 2294.[31] M. Al-Ghouti, Y. Al-Degs, Korean J. Chem. Eng. 2015, 32, 685 – 693.[32] J. Garc�a-Guti¦rrez, G. C. Laredo, P. Garc�a-Guti¦rrez, F. Jim¦nez-

Cruz, Fuel 2014, 138, 118 –125.[33] Y. Shiraishi, Y. Taki, T. Hirai, I. Komasawa, Ind. Eng. Chem. Res.

2001, 40, 1213 –1224.[34] L. Chen, S. Guo, D. Zhao, Chin. J. Chem. Eng. 2007, 15, 520 –523.[35] Z. A. Abdalla, B. Li, A. Tufail, J. Ind. Eng. Chem. 2009, 15, 780 –

783.[36] S. Liu, B. Wang, B. Cui, L. Sun, Fuel 2008, 87, 422 –428.[37] F. Al-Shahrani, T. Xiao, A. Simon, S. Barri, Z. Jiang, H. Shi, G.

Martinie, L. H. M. Green, Appl. Catal. B 2007, 73, 311 – 316.[38] J. L. Garc�a-Guti¦rrez, A. G. Fuentes, M. E. Hernandez-Teran, P.

Garcia, F. Murrieta-Guevara, F. Jimenez-Cruz, Appl. Catal. A 2008,334, 366 –373.

[39] L. Caero, E. Hern�ndez, F. Pedraza, F. Murrieta, Catal. Today 2005,107 – 108, 564 – 569.

[40] L. CedeÇo-Caero, F. Jorge, A. Navarro, A. Guti¦rrez-Alejandre,Catal. Today 2006, 116, 562 –568.

[41] L. CedeÇo-Caero, H. Gomez-Bernal, A. Fraustro-Cuevas, D. H.Guerra-Gomez, R. Cuevas-Garcia, Catal. Today 2008, 133 –135,244 – 254.

[42] T. Hirai, K. Ogawa, I. Komasawa, Ind. Eng. Chem. Res. 1996, 35,586 – 589.

[43] T. Hirai, Y. Shiraishi, K. Ogawa, I. Komasawa, Ind. Eng. Chem. Res.1997, 36, 530 –533.

[44] A. Bçsmann, L. Datsevich, A. Jess, A. Lauter, C. Schmitz, P. Was-serscheid, Chem. Commun. 2001, 2494 – 2495.

[45] H. S. Gao, Y. G. Li, J. M. Wu, Y. Wu, M. F. Luo, Q. Li, J. M. Xing,H. Z. Liu, Energy Fuels 2009, 23, 2690 –2694.

[46] H. S. Gao, M. F. Luo, J. M. Xing, Y. Wu, Y. G. Li, W. L. Li, Q. F.Liu, H. Z. Liu, Ind. Eng. Chem. Res. 2008, 47, 8384 –8388.

[47] L. He, H. M. Li, W. S. Zhu, J. S. Guo, X. Jiang, J. D. Lu, Y. S. Yan,Ind. Eng. Chem. Res. 2008, 47, 6890 – 6895.

[48] H. M. Li, X. Jiang, W. S. Zhu, J. D. Lu, H. M. Shu, Y. S. Yan, Ind.Eng. Chem. Res. 2009, 48, 9034 – 9039.

[49] B. Jiang, H. Yang, L. Zhang, R. Zhang, Y. Sun, Y. Huang, Chem.Eng. J. 2016, 283, 89– 96.

[50] H. Mei, B. W. Mei, T. F. Yen, Fuel 2003, 82, 405 –414.[51] F. A. Duarte, P. Mello, C. Bizzi, M. Nunes, E. Moreira, M. Alencar,

H. Motta, V. Dressler, E. Flores, Fuel 2011, 90, 2158 – 2164.[52] H. Tao, T. Nakazato, S. Sato, Fuel 2009, 88, 1961 –1969.[53] A. Ibrahim, S. B. Xian, Z. Wei, Pet. Sci. Technol. 2003, 21, 1555 –

1573.[54] Y. Shiraishi, T. Hirai, I. Komasawa, Ind. Eng. Chem. Res. 2001, 40,

293 – 303.[55] A. Ibrahim, S. B. Xian, Z. Wei, Pet. Sci. Technol. 2004, 22, 287 – 301.[56] G. Yu, X. Li, X. Liu, C. Asumana, X. Chen, Ind. Eng. Chem. Res.

2011, 50, 2236 –2244.[57] K. Kendra-Krolik, M. Fabrice, J. N. Jaubert, Ind. Eng. Chem. Res.

2011, 50, 2296 –2306.

[58] L. Alonso, A. Arce, M. Francisco, A. Soto, Fluid Phase Equilib.2008, 263, 176 –181.

[59] L. Alonso, A. Arce, M. Francisco, A. Soto, J. Chem. Thermodyn.2008, 40, 966.

[60] L. A. Aslanov, A. V. Anisimov, Pet. Chem. 2004, 44, 65–69.[61] F. Nejad, M. Beigi, Pet. Sci. 2015, 12, 330 – 339.[62] J. L. Luche, Synthetic Organic Sonochemistry, Plenum Press, New

York, 1998.[63] K. Tawara, T. Nishimura, H. Iwanami, T. Nishimoto, T. Hasuike,

Ind. Eng. Chem. Res. 2001, 40, 2367 – 2370.[64] M. Soleimani, A. Bassi, A. Margaritis, Biotechnol. Adv. 2007, 25,

570 – 596.[65] D. J. Monticello, D. Bakker, W. R. Finnerty, Appl. Environ. Micro-

biol. 1985, 49, 756 – 760.[66] F. L. Li, P. Xu, C. Q. L. L. Ma, X. S. Luo, FEMS Microbiol. Lett.

2003, 223, 301 –307.[67] M. Al-Ghouti, Y. Al-Degs, F. Mustafa, Fuel 2010, 89, 193 – 201.[68] S. D. Faust, O. M. Aly, Adsorption Processes for Water Treatment,

Butterworths, Boston, MA, 1987.[69] Y. S. Al-Degs, Mohammad A. Al-Ghouti, Sep. Purif. Technol. 2015,

139, 1 –4.[70] C. O. Ania, J. B. Parra, A. Arenillas, F. Rubiera, T. J. Bandoszand,

J. J. Pis, Appl. Surf. Sci. 2007, 253, 5899 –5903.[71] Y. Sano, K. Sugaraha, K. Choi, Y. Korai, I. Mochida, Fuel 2005, 84,

903 – 910.[72] M. A. Larrubia, A. Gutierrez-Alejandre, J. Ramirez, G. Busca,

Appl. Catal. A 2002, 224, 167 – 178.[73] O. Etemadi, T. F. Yen, J. Colloid Interface Sci. 2007, 313, 18– 25.[74] J. Weitkamp, M. Schwark, S. Ernst, J. Chem. Soc. Chem. Commun.

1991, 1133 – 1134.[75] X. Liu, J. Guo, Y. Chu, D. Luo, H. Yin, M. Sun, R. Yavuz, Fuel

2014, 123, 93–100.[76] J. Liao, W. Bao, L. Chang, Fuel Process. Technol. 2015, 140, 104 –

112.[77] K. A. Abu Safieh, Y. S. Al-Degs, M. S. Sunjuk, A. l. Saleh, M. A. Al-

Ghouti, Environ. Technol. 2015, 36, 98 –105.[78] K. Tang, K. L. Song, L. Duan, X. Li, J. Gui, Z. Sun, Fuel Process.

Technol. 2008, 89, 1 –6.[79] J. G. Park, K. C. Hyun, K. B. Yi, J. H. Park, S. Han, S. H. Cho, J. N.

Kim, Appl. Catal. B 2008, 81, 244 –250.[80] P. Baeza, G. Aguila, G. Vargas, J. Ojeda, P. Araya, Appl Catal B:

Environ. 2012, 111 – 112, 133 – 140.[81] M. T. Timko, J. Wang, J. Burgess, P. Kracke, L. Gonzalez, C. Jaye,

D. A. Fischer, Fuel 2016, 163, 223 –231-[82] M. Muzic, K. Sertic-Bionda, Z. Gomzi, Chem. Eng. Technol. 2008,

31, 355 –364.[83] Y. Yang, H. Lu, P. Ying, Z. Jiang, C. Li, Carbon 2007, 45, 3042 –

3044.[84] M. Muzic, K. Sertic-Bionda, Z. Gomzi, S. Podolski, S. Telen, Chem.

Eng. Res. Des. 2010, 88, 487 – 495.[85] Y. A. Alhamed, H. S. Bamufleh, Fuel 2009, 88, 87- 94.[86] V. Selvavathi, V. Chidambaram, A. Meenakshisundaram, B. Sairam,

B. Sivasankar, Catal. Today 2009, 141, 99–102.[87] S. P. Hernandez, D. Fino, N. Russo, Chem. Eng. Sci. 2010, 65, 603 –

609.[88] M. Seredych, T. J. Bandosz, Fuel Process. Technol. 2010, 91, 693 –

701.[89] H. J. Jeon, C. H. Ko, S. H. Kim, J. N. Kim, Energy Fuels 2009, 23,

2537 – 2543.[90] A. J. Hern�ndez-Maldonado, G. Qi, R. T. Yang, Appl. Catal. B 2005,

61, 212 –218.[91] C. O. Ania, T. J. Bandosz, Carbon 2006, 44, 2404 –2412.[92] Z. Jiang, Y. Liu, X. Sun, F. Tian, F. Sun, C. Liang, W. You, C. Han,

C. Li, Langmuir 2003, 19, 731 –736.[93] M. Seredych, T. J. Bandosz, Carbon 2011, 49, 1216 –1224.[94] M. Seredych, T. J. Bandosz, Energy Fuels 2010, 24, 3352 –3360.[95] A. Srivastav, V. C. Srivastava, J. Hazard. Mater. 2009, 170, 1133 –

1140.[96] R. T. Yang, A. Takahashi, F. H. Yang, Ind. Eng. Chem. Res. 2001,

40, 6236 –6239.


http://dx.doi.org/10.1126/science.1085088




http://dx.doi.org/10.1039/C0CC03482G



http://dx.doi.org/10.1016/S0920-5861(00)00308-4

http://dx.doi.org/10.1016/S0920-5861(00)00308-4

http://dx.doi.org/10.1016/S0920-5861(00)00308-4

http://dx.doi.org/10.1016/S0920-5861(03)00463-2

http://dx.doi.org/10.1016/S0920-5861(03)00463-2

http://dx.doi.org/10.1016/S0920-5861(03)00463-2

http://dx.doi.org/10.1016/S0016-2361(02)00324-1

http://dx.doi.org/10.1016/S0016-2361(02)00324-1

http://dx.doi.org/10.1016/S0016-2361(02)00324-1

http://dx.doi.org/10.1039/tf9282400245

http://dx.doi.org/10.1039/tf9282400245

http://dx.doi.org/10.1039/tf9282400245




http://dx.doi.org/10.1021/jo01285a038




http://dx.doi.org/10.1016/S0146-6380(98)00175-2

http://dx.doi.org/10.1016/S0146-6380(98)00175-2

http://dx.doi.org/10.1016/S0146-6380(98)00175-2

http://dx.doi.org/10.1016/S0146-6380(98)00175-2

http://dx.doi.org/10.1016/0016-2361(89)90114-2

http://dx.doi.org/10.1016/0016-2361(89)90114-2

http://dx.doi.org/10.1016/0016-2361(89)90114-2

http://dx.doi.org/10.1021/ef000096i



http://dx.doi.org/10.1021/ef000039p



http://dx.doi.org/10.1016/j.fuel.2005.12.006




http://dx.doi.org/10.1016/j.carbon.2005.04.008



http://dx.doi.org/10.1007/s11814-014-0303-0

http://dx.doi.org/10.1007/s11814-014-0303-0

http://dx.doi.org/10.1007/s11814-014-0303-0




http://dx.doi.org/10.1021/ie000547m




http://dx.doi.org/10.1016/S1004-9541(07)60118-9

http://dx.doi.org/10.1016/S1004-9541(07)60118-9

http://dx.doi.org/10.1016/S1004-9541(07)60118-9







http://dx.doi.org/10.1016/j.apcatb.2006.12.016



















http://dx.doi.org/10.1021/ie9503407




http://dx.doi.org/10.1021/ie960576q




http://dx.doi.org/10.1039/b108411a



http://dx.doi.org/10.1021/ef900009g



http://dx.doi.org/10.1021/ie800739w



http://dx.doi.org/10.1021/ie800857a



http://dx.doi.org/10.1021/ie900754f








http://dx.doi.org/10.1016/S0016-2361(02)00318-6

http://dx.doi.org/10.1016/S0016-2361(02)00318-6

http://dx.doi.org/10.1016/S0016-2361(02)00318-6







http://dx.doi.org/10.1081/LFT-120023238



http://dx.doi.org/10.1021/ie000707u











http://dx.doi.org/10.1016/j.fluid.2007.10.010




http://dx.doi.org/10.1016/j.jct.2008.01.025

http://dx.doi.org/10.1016/j.jct.2008.01.025

http://dx.doi.org/10.1021/ie000453c



http://dx.doi.org/10.1016/j.biotechadv.2007.07.003







http://dx.doi.org/10.1016/j.seppur.2014.10.027




http://dx.doi.org/10.1016/j.apsusc.2006.12.065







http://dx.doi.org/10.1016/S0926-860X(01)00769-4

http://dx.doi.org/10.1016/S0926-860X(01)00769-4

http://dx.doi.org/10.1016/S0926-860X(01)00769-4

http://dx.doi.org/10.1016/j.jcis.2007.04.033



http://dx.doi.org/10.1039/c39910001133

http://dx.doi.org/10.1039/c39910001133

http://dx.doi.org/10.1039/c39910001133

http://dx.doi.org/10.1039/c39910001133





http://dx.doi.org/10.1016/j.fuproc.2015.08.036



http://dx.doi.org/10.1080/09593330.2014.938125

http://dx.doi.org/10.1080/09593330.2014.938125

http://dx.doi.org/10.1080/09593330.2014.938125

















http://dx.doi.org/10.1002/ceat.200700341







http://dx.doi.org/10.1016/j.cherd.2009.08.016








http://dx.doi.org/10.1016/j.ces.2009.06.050






http://dx.doi.org/10.1021/ef801050k











http://dx.doi.org/10.1021/la020670d






http://dx.doi.org/10.1021/ef9015087










[97] A. Takahashi, F. H. Yang, R. T. Yang, Ind. Eng. Chem. Res. 2002,41, 2487 –2496.

[98] A. J. Hern�ndez-Maldonado, R. T. Yang, AIChE J. 2004, 50, 791 –801.

[99] X. Ma, L. Sun, C. Song, Catal. Today 2002, 77, 107 –116.[100] Y. Yang, X. Liu, B. Xu, New Carbon Mater. 2014, 29, 1 –14.[101] Y. S. Al-Degs, A. S. Abu-Surrah. K. A. Ibrahim, Anal. Bioanal.

Chem. 2009, 393, 1055 –1062.[102] W. Liu, X. Liu, Y. Yang, Y. Zhang, B. Xu, Fuel 2014, 117, 184 – 190.[103] B. Castro, M. Whitcombe, E. Vulfson, R. Vazquez-Duhalt, E.

B�rzana, Anal. Chim. Acta 2001, 435, 83– 90.[104] Y. Chang, L. Zhang, H. Ying, Z. Li, H. Lv, P. Ouyang, Appl. Bio-

chem. Biotech. 2010, 160, 593 –603.[105] T. Hu, Y. Zhang, L. Zheng, G. Fan, J. Fuel Chem. Technol. 2010, 38,

722 – 729.[106] I. Ahmed, S. H. Jhung, Mater. Today 2014, 17, 136 –146.[107] S. Qiu, M. Xue, G. Zhu, Chem. Soc. Rev. 2014, 43, 6116 –6140.[108] J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang, C.-Y. Su, Chem. Soc.

Rev. 2014, 43, 6011 –6061.[109] C. Petit, T. J. Bandosz, Dalton Trans. 2012, 41, 4027 –4035.[110] K. A. Cychosz, A. G. Wong-Foy, A. J. Matzger, J. Am. Chem. Soc.

2008, 130, 6938 –6939.[111] S. Bordiga, L. Regli, F. Bonino, E. Groppo, C. Lamberti, B. Xiao,

P. S. Wheatley, R. E. Morris, A. Zecchina, Phys. Chem. Chem. Phys.2007, 9, 2676 –2685.

[112] S. Furukawa, J. Reboul, S. Diring, K. Sumida, S. Kitagawa, Chem.Soc. Rev. 2014, 43, 5700 – 5734.

[113] L. Wu, J. Xiao, Y. Wu, S. Xian, G. Miao, H. Wang, Z. Li, Langmuir2014, 30, 1080 –1088.

[114] S. Achmann, G. Hagen, M. H�mmerle, I. Malkowsky, C. Kiener, R.Moos, Chem. Eng. Technol. 2010, 33, 275 –280.

[115] N. A. Khan, J. W. Jun, J. H. Jeong, S. H. Jhung, Chem. Commun.2011, 47, 1306 –1308.

[116] N. A. Khan, S. H. Jhung, Fuel Process. Technol. 2012, 100, 49– 54.[117] N. A. Khan, Z. Hasan, S. H. Jhung, Chem. Eur. J. 2014, 20, 376 – 380.[118] F. Shi, M. Hammoud, L. T. Thompson, Appl. Catal. B 2011, 103,

261 – 265.[119] S. Haji, C. Erkey, Ind. Eng. Chem. Res. 2003, 42, 6933 –6937.[120] M. Jiang, F. T. Ng, Catal. Today 2006, 116, 530 – 536.[121] P. Baeza, G. Aguila, F. Gracia, P. Araya, Catal. Commun. 2008, 9,

751 – 755.[122] Y. Yang, X. Liu, S. Li, W. Liu, B. Xu, Colloids Surf. A 2011, 377,

379 – 385.

[123] J. K. Schnobrich, O. Lebel, K. A. Cychosz, A. Dailly, A. G. Wong-Foy, A. J. Matzger, J. Am. Chem. Soc. 2010, 132, 13941 –13948.

[124] A. B. Jaffe, R. G. Newell, R. N. Stavins, A Tale of Two Market Fail-ures: Technology and Environmental Policy, Resources for theFuture, Discussion Paper RFF DP 04-38, Pub Resources for theFuture, Washington, DC, 2004.

[125] M. O’Connor, Int. J. Environ Pollut. 1997, 7, 450 – 482.[126] A. S Williams, Life Cycle Analysis: A Step by Step Approach, TR-

040 ISTC Report TR-040, Institute of Natural Resource Sustainabil-ity, University of Illinois, Champaign, IL, 2009.

[127] B. Keesom, J. Blieszner, S. Unnasch, EU Pathway Study: Life CycleAssessment of Crude Oils in a European Context, Alberta Petrole-um Marketing Commission by Jacobs Consultancy, Calgary, Alberta,2012.

[128] S. Suh, M. Lemzen, G. J. Treloar, H. Hondo, A. Horvath, G.Huppes, O. Jolliet, U. Klann, W. Krewitt, Y. Moriguchi, J. Munks-gaar, G. Norris, Environ. Sci. Technol. 2004, 38, 657 –664.

[129] M. Eriksson, S. Ahlgren, LCAs of Petrol and Diesel : A LiteratureReview, Report 2013:058, ISSN 1654-9406, SLU, Swedish Universityof Agricultural Science, Department of Energy and Technology, Up-psala, 2013.

[130] M. Wang, H. Lee, J. Molburg, Int. J. Life Cycle Assess. 2004, 9, 34–44.

[131] PE International, Benefits of Molybdenum Use: Hydrodesulfuriza-tion Catalysts, International Molybdenum Association, London,2015.

[132] L. Alves, S. M. Paixao, R. Pacheco, A. F. Ferreira, C. M. Silva, RSCAdv. 2015, 5, 34047 –34057.

[133] A. Mart�nez-Gonz�lez, O. Casas-Leuro, J. Acero-Reyes, and E.Castillo-Monroy, CT&F-Ciencia, Tecnolog�a y Futuro. 2011, 4, 123 –136.

[134] AP42: Compilation of Air Pollutant Emission Factors, Vol. 1: Sta-tionary Sources, 5th ed., US Environmental Protection Agency, Cin-cinatti, OH, 2009, Chapter 8, Section 13.

[135] M. P. Elsner, M. Menge, C. Mîller, D. W. Agar, Catal. Today 2003,79– 80, 487 – 494.

[136] R. El-Bishtawi, N. Haimour, Fuel Process. Technol. 2004, 86, 245 –260.

Received: November 19, 2015Revised: February 4, 2016Published online on March 24, 2016









http://dx.doi.org/10.1016/S0920-5861(02)00237-7

http://dx.doi.org/10.1016/S0920-5861(02)00237-7

http://dx.doi.org/10.1016/S0920-5861(02)00237-7

http://dx.doi.org/10.1016/S1872-5805(14)60121-9

http://dx.doi.org/10.1016/S1872-5805(14)60121-9

http://dx.doi.org/10.1016/S1872-5805(14)60121-9

http://dx.doi.org/10.1007/s00216-008-2502-1

http://dx.doi.org/10.1007/s00216-008-2502-1

http://dx.doi.org/10.1007/s00216-008-2502-1

http://dx.doi.org/10.1007/s00216-008-2502-1




http://dx.doi.org/10.1016/S0003-2670(01)00799-1

http://dx.doi.org/10.1016/S0003-2670(01)00799-1

http://dx.doi.org/10.1016/S0003-2670(01)00799-1

http://dx.doi.org/10.1007/s12010-008-8441-7

http://dx.doi.org/10.1007/s12010-008-8441-7

http://dx.doi.org/10.1007/s12010-008-8441-7

http://dx.doi.org/10.1007/s12010-008-8441-7

http://dx.doi.org/10.1016/S1872-5813(11)60006-6

http://dx.doi.org/10.1016/S1872-5813(11)60006-6

http://dx.doi.org/10.1016/S1872-5813(11)60006-6

http://dx.doi.org/10.1016/S1872-5813(11)60006-6

http://dx.doi.org/10.1016/j.mattod.2014.03.002



http://dx.doi.org/10.1039/C4CS00159A



http://dx.doi.org/10.1039/C4CS00094C




http://dx.doi.org/10.1039/c2dt12017h



http://dx.doi.org/10.1021/ja802121u




http://dx.doi.org/10.1039/b703643d




http://dx.doi.org/10.1039/C4CS00106K




http://dx.doi.org/10.1021/la404540j














http://dx.doi.org/10.1002/chem.201304291













http://dx.doi.org/10.1016/j.catcom.2007.08.020




http://dx.doi.org/10.1016/j.colsurfa.2011.01.032




http://dx.doi.org/10.1021/ja107423k



http://dx.doi.org/10.1021/es0263745



http://dx.doi.org/10.1007/BF02978534

http://dx.doi.org/10.1007/BF02978534

http://dx.doi.org/10.1007/BF02978534

http://dx.doi.org/10.1039/C4RA14216K




http://dx.doi.org/10.1016/S0920-5861(03)00071-3

http://dx.doi.org/10.1016/S0920-5861(03)00071-3

http://dx.doi.org/10.1016/S0920-5861(03)00071-3

http://dx.doi.org/10.1016/S0920-5861(03)00071-3

http://dx.doi.org/10.1016/S0920-5861(03)00071-3

http://dx.doi.org/10.1016/S0920-5861(03)00071-3




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