Monitoring of Remaining Thiophenic Compounds in Liquid Fuel Desulphurization Studies Using a Fast HPLC-UV Method
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separations
Article
Monitoring of Remaining Thiophenic Compounds in Liquid
Fuel Desulphurization Studies Using a Fast HPLC-UV Method
Vasiliki Kapsali 1 , Konstantinos Triantafyllidis 2 , Eleni Deliyanni 2 and Victoria Samanidou 1, *
1 Laboratory of Analytical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki,
GR-54124 Thessaloniki, Greece; vkapsali@chem.auth.gr
2 Laboratory of Chemical and Environmental Technology, Department of Chemistry, Aristotle University of
Thessaloniki, GR-54124 Thessaloniki, Greece; ktrianta@chem.auth.gr (K.T.); lenadj@chem.auth.gr (E.D.)
* Correspondence: samanidu@chem.auth.gr
Abstract: Thiophenic compounds constitute a class of sulfur compounds derived by thiophene,
containing at least one thiophenic ring. Their presence in fuels (crude oil, etc.) is important and
can reach 3% m/m. The combustion of fuels leads to the formation of sulfur oxides a severe source
of environmental pollution issues, such as acid rain with adverse effects both to humans and to
the environment. To reduce such problems, the EU and other regulatory agencies worldwide set
increasingly stringent regulations for sulfur content in fuels resulting in the necessity for intense
desulphurization processes. However, most of these processes are inefficient in the total removal of
sulfur compounds. Therefore, thiophenic compounds such as benzothiophenes and dibenzothio-
phenes are still present in heavier fractions of petroleum, therefore, their determination is of great
Citation: Kapsali, V.; Triantafyllidis, importance. Until now, all HPLC methods applied in similar studies use gradient elution programs
K.; Deliyanni, E.; Samanidou, V. that may last more than 25 min with no validation results provided. To fill this gap, the aim of the
Monitoring of Remaining Thiophenic present study was to develop and validate a simple and fast HPLC-UV method in order to be used as
Compounds in Liquid Fuel a useful monitoring tool in the evaluation studies of novel desulfurization technologies by means of
Desulphurization Studies Using a simultaneous determination of dibenzothiophene (DBT) and 4,6-dimethyl-dibenzothiophene and
Fast HPLC-UV Method. Separations dibenzothiophene sulfone in the desulfurization effluents.
2021, 8, 48. https://doi.org/10.3390/
separations8040048 Keywords: HPLC; thiophene compounds; fuels; desulphurization; dibenzothiophene (DBT);
4,6-dimethyl-dibenzothiophene; dibenzothiophene sulfone
Academic Editors:
George Zachariadis and
Rosa Peñalver
1. Introduction
Received: 1 March 2021
Accepted: 9 April 2021 Organosulfur compounds such as sulfides, disulfides, thiophenes, benzothiophenes,
Published: 11 April 2021 etc. contained in fossil fuels (e.g., petroleum, gasoline, diesel oil, etc.) can cause severe
environmental pollution issues upon combustion due to the formation of sulfur oxides,
Publisher’s Note: MDPI stays neutral which may cause air pollution and acid rain. Moreover, sulfur content may lead to the
with regard to jurisdictional claims in poisoning of automotive catalysts used to reduce emissions of pollutant hydrocarbons,
published maps and institutional affil- carbon monoxide, and nitrogen oxides from gasoline- and diesel-fueled vehicles [1].
iations. The growing environmental awareness resulted in a high necessity of efficient desulfu-
rization processes. Strict environmental regulations by the U.S. Environmental Protection
Agency (EPA) and other environmental regulatory agencies worldwide, require the total re-
moval of sulfur content which is not feasible to be achieved by conventional desulfurization
Copyright: © 2021 by the authors. procedures [2,3].
Licensee MDPI, Basel, Switzerland. European Community Member States shall ensure that gas oils are not used within
This article is an open access article their territory if their sulfur content exceeds 0.10% by mass; heavy fuel oils are not used if
distributed under the terms and their sulfur content exceeds 3% by mass; marine fuels are not used within their territory
conditions of the Creative Commons if their sulfur content exceeds 0.50% by mass, except for fuels supplied to ships using
Attribution (CC BY) license (https:// emission abatement methods as reported in Directive (EU) 2016/802 [4] and Directive (EU)
creativecommons.org/licenses/by/ 2015/1513 [5].
4.0/).
Separations 2021, 8, 48. https://doi.org/10.3390/separations8040048 https://www.mdpi.com/journal/separationsSeparations 2021, 8, x FOR PEER REVIEW 2 of 8
Separations 2021, 8, x FOR PEER REVIEW 2 of 8
To that end, efficient desulfurization is an essential step in the petroleum refining
Separations 2021, 8, 48 2 of 8
process. Hydrodesulfurization (HDS) is the main process widely used to eliminate sulfur
content. HDS processes are not economical and have a high energy demand since H2 and
To that
catalysts suchend, efficient 2desulfurization
as CoMo/Al O3 and/or NiMo/Al is an essential step in the petroleum refining
2O3 are used at temperatures from 300 to
process. To Hydrodesulfurization
that end, efficient (HDS)
desulfurization is
400 °C and high pressures [6]. Besides, among thiophenicthe
is main
an process
essential stepwidely
in the used
compounds tothat
petroleum eliminate
refining sulfur
are derived
content.
fromprocess. HDS processes areat
Hydrodesulfurization
thiophene containing not economical
(HDS)
least oneis the andprocess
main
thiophenic have a widely
ring, high
DBTenergy todemand
usedespecially
and eliminate since
sulfurH2 and
4,6-DMDBT,
content.such
catalysts HDS asprocesses
CoMo/Al are2not
O economical
3 and/or and have
NiMo/Al 2O3 aare
high energy
used at demand since Hfrom
temperatures 2 and300 to
the sulfur-containing compounds in diesel oil that are considered as refractory species
400catalysts
since °C and
they
such
highasordinary
◦ Cresist
CoMo/Al2[6].
pressures O3 and/or
methods Besides, NiMo/Al O3 are used at
among2 thiophenic
of treatment, cannot be
temperatures
compounds
completely
from
that are300
derived
to 400 and high pressures [6]. Besides, among thiophenic compounds thatremoved
are derived due to
from
their thiophene containing at least one thiophenic ring, DBT and especially 4,6-DMDBT,
from thiophene containing at least one thiophenic ring, DBT and especially 4,6-DMDBT, cur-
structure (their chemical structures are shown in Figure 1). This fact makes the
the
rent sulfur-containing
target of a low level compounds
of 30 ppmw in sulfur
diesel for oil diesel
that are considered
fuel, set byas the as refractory species
Environmental
the sulfur-containing compounds in diesel oil that are considered refractory species Pro-
since
tection they
sinceAgency resist ordinary
(EPA)
they resist methods
requirements,
ordinary of treatment,
to be a difficult
methods of treatment, cannot
cannot be completely removed removed
task. be completely due to theirdue to
their structure
structure (their
(their chemical
chemical structures
structures are shown
are shown in Figurein 1).
Figure
This 1).
factThis
makesfactthe
makes the cur-
current
rent
target of a low level of 30 ppmw sulfur for diesel fuel, set by the Environmental Protection Pro-
target of a low level of 30 ppmw sulfur for diesel fuel, set by the Environmental
tection
Agency Agency (EPA) requirements,
(EPA) requirements, to be atask.
to be a difficult difficult task.
Dibenzothiophene 4,6-Dimethyldibenzothiophene Dibenzothiophene sulfone
(DBT) (4,6-DMDBT) (DBT sulfone)
Figure 1. Chemical structures
Dibenzothiophene of the studied thiophenic compounds:
4,6-Dimethyldibenzothiophene dibenzothiophene
Dibenzothiophene sulfone(DBT), 4,6-
dimethyl-dibenzothiophene (4,6-DMDBT), and dibenzothiophene sulfone.
(DBT) (4,6-DMDBT) (DBT sulfone)
Figure 1. these
For
Figure Chemical
1. structures
reasons,
Chemical ofofthe
alternative
structures thestudied
and/orthiophenic
studied compounds:
supplementary
thiophenic dibenzothiophene
methods
compounds: (DBT),
are needed,(DBT),
dibenzothiophene 4,6-
including
dimethyl-dibenzothiophene
hydrodesulfurization (4,6-DMDBT),
on Co/Ni
4,6-dimethyl-dibenzothiophene and
catalysts and
(4,6-DMDBT), dibenzothiophene
[7,8], sulfone.
oxidation/photo-oxidation
dibenzothiophene sulfone. [9,10], and ad-
sorption [11] with adsorptive desulfurization to be of great importance especially when a
Forthese
For these reasons, alternative
alternative and/or supplementary methods are needed, including
removal target reasons,
of a level less than 30 and/or
ppmwsupplementary methods
sulfur is considered. are needed, including
hydrodesulfurizationon
hydrodesulfurization on Co/Ni
Co/Nicatalysts
catalysts [7,8],
[7,8], oxidation/photo-oxidation
oxidation/photo-oxidation [9,10], [9,10], andand ad-
ad-
Among adsorbent
adsorption [11] with
materials,
adsorptive
activated carbons
desulfurization to be of
have
great
been proven
importance
to be suitable
especially when
sorptionof
sorbents [11] with adsorptive desulfurization to be of great their
importance especially when a
a removaldibenzothiophenic
target of a level less compounds
than 30 ppmw[12–19]. sulfur isBesides,
considered. acidic oxygen functional
removalontarget
groups of a levellead
theiradsorbent
surface lessto
than 30 ppmw
specific sulfur of
adsorption is considered.
dibenzothiophenic species via oxy-
Among materials, activated carbons have been proven to be suitable adsor-
Among
gen–sulfur adsorbent
bents of dibenzothiophenic materials,
interactions [20] and compoundsactivated
acid-base carbons
interactions
[12–19]. have
Besides, withbeen
their the proven
slightly
acidic oxygen to be suitable
basic thiophenes
functional ad-
sorbents
groups of
[21]. ondibenzothiophenic compounds
their surface lead to specific [12–19].
adsorption Besides, their acidic
of dibenzothiophenic speciesoxygen functional
via oxygen–
groups
sulfur on their
interactions surface
[20] lead
and to specific
acid-base adsorption
interactions with of
thedibenzothiophenic
slightly
Additionally, either oxygen of these carbons’ surface oxygen functional groups basic species
thiophenes via oxy-
[21]. or
gen–sulfur interactions
Additionally, either[20] and
oxygen acid-base
of these interactions
carbons’ surfacewith
oxygen
chemisorbed oxygen may take part in reactions that lead to the oxidation of adsorbed DBTthe slightly
functional basic
groups thiophenes
or
chemisorbed oxygen may take part in reactions that lead to the oxidation of adsorbed DBT
[21].
and 4,6-DMDBT on the carbon’s surface to their sulfoxides and finally sulfones with the
and 4,6-DMDBT on
Additionally, the carbon’s
either oxygensurface to their sulfoxides andoxygen
finally sulfones withgroups
the
portion
portion
of of
4,6-DMDBT
4,6-DMDBT
oxidation
oxidation
toofbe
to
these
be less
carbons’
lesscompared
compared surface
to
to
DBT
DBT oxidation
oxidation
functional
[17,18,20,22].
[17,18,20,22]. The
or
The
chemisorbed
DBT sulfone oxygen
is isformed may take part in reactions that lead to the oxidation of adsorbed DBT
DBT sulfone formedaccording
according toto the
theoxidation
oxidationreaction
reaction route
route shown
shown in Figure
in Figure 2 [23,24].
2 [23,24].
and 4,6-DMDBT on the carbon’s surface to their sulfoxides and finally sulfones with the
portion of 4,6-DMDBT oxidation to be less compared to DBT oxidation [17,18,20,22]. The
DBT sulfone is formed according to the oxidation reaction route shown in Figure 2 [23,24].
Figure 2. DBT sulfone formation.
Figure 2. DBT sulfone formation.
For this reason, DBT, 4,6-DMDBT, and DBT-sulfone can be found in final effluent after
adsorption
For this of dibenzothiophene
reason, (DBT) and
DBT, 4,6-DMDBT, 4,6-dimethyldibenzothiophene
and DBT-sulfone can be found(4,6-DMDBT)
in final effluent
in
Figuredesulfurization
2. DBT studies,
sulfone making
formation. their analytical determination during desulfurization
after adsorption of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-
studies and the use of fast validated HPLC methods [25–27] of great importance.
DMDBT) in desulfurization studies, making their analytical determination during desul-
For this
furization reason,
studies andDBT,
the 4,6-DMDBT, and DBT-sulfone
use of fast validated can be found
HPLC methods in of
[25–27] final effluent
great im-
after adsorption
portance. of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-
DMDBT) in desulfurization studies, making their analytical determination during desul-
furization studies and the use of fast validated HPLC methods [25–27] of great im-
portance.Separations 2021, 8, 48 3 of 8
Therefore, in order to evaluate the effectiveness of new technologies, an analyt-
ical method is necessary to determine the concentration of initial as well as adsorp-
tion/oxidation products. Although these compounds have been determined in several
fuel samples during desulfurization studies there is a vacancy in the analytical field con-
cerning the development, optimization, and validation of a fast analytical method for their
monitoring [28–33]. Various GC and HPLC methods based on UV detection are reported
using gradient elution programs that may last more than 25 min. A comprehensive review
has listed most of the published methods. However, all these methods are reported as
monitoring tools where neither analytical performance characteristics are provided, nor
validation criteria are applied [25,34].
The aim of this study was to develop and validate a simple and fast HPLC method
for the simultaneous determination of DBT, DBT-sulfone, and 4,6-DMDBT in extracts after
the oxidative desulfurization process of fuels. To the best of our knowledge, there is no
published method for the determination of the examined thiophenic compounds.
2. Materials and Methods
2.1. Instrumentation
The separation of the target compounds was performed by the High-Performance
Liquid Chromatography (HPLC) method, using an HPLC column 250 × 4.6 mm (PerfectSil
Target ODS-3 5 µm, MZ-Analysentechnik GmbH, Wohlerstrasse, Mainz, Germany) with
a mobile phase consisted of a solution of acetonitrile and water (90% CH3 CN: 10% H2 O
v/v) delivered isocratically at a flow rate of 1.0 mL/min, using a Shimadzu (Kyoto, Japan)
LC-10AD pump. The pressure observed was 80 bar. Sample injection was performed via a
Rheodyne 7125 injection valve (Rheodyne, Cotati, CA, USA) with a 20 µL loop. Detection
was achieved by an SSI 500 UV-Vis detector (SSI, State College, PA, USA) at a wavelength
of 313 nm and a sensitivity setting of 0.002 AUFS. DAQ Software for Chemists, developed
by Emeritus Professor P. Nikitas (Dept. of Chemistry, Aristotle University of Thessaloniki).
The degassing of the mobile phase was performed in an ultrasonic bath Transonic
460/H (35 kHz, 170 W, Elma, Germany). All samples were filtered prior to HPLC analysis
using Q-Max RR Syringe Filters 0.22 µm Nylon, 13 mm Frisenette, Knebel, Denmark.
2.2. Chemicals and Reagents
HPLC grade acetonitrile and methanol were purchased from PanReac AppliChem (Ot-
toweg, Darmstadt, Germany). High purity water, obtained by a Milli-Q purification system
(Millipore, Bedford, MA, USA), was used throughout the entire study. Dibenzothiophene
sulfone 97%, 4,6-dimethyldibenzothiophene (4,6 DMDBT, C14 H12 S), and dibenzothiophene
(DBT, C12 H8 S) were supplied by JKchemical 1 (Beijing, China).
2.3. Preparation of Standard Solutions
Standard solutions of each analyte were prepared in acetonitrile (200 ng µL−1 ),
stored at 4 ◦ C in the dark. Working standards solutions were prepared by further di-
lution in acetonitrile covering the linear range from 0.1 to 30 ng µL−1 . Aliquots of 20 µL
were injected into the column and quantitative analysis was performed based on peak
area measurements.
2.4. Method Validation
Method validation was performed following the criteria set by Commission Decision
2002/657/EC [35].
In that aspect, selectivity, linearity, accuracy, precision (repeatability and between-day
precision), as well as limits of detection (LODs) and quantification (LOQs) were taken
into consideration. Calibration curves were constructed by three replicates of standard
solutions in the working interval of 0.1–30 ng µL−1 .Limits of detection (LODs) were calculated based on the following equation
3.3 S/N, where S = signal and N = noise. Limits of quantitation (LOQs) were calcul
Separations 2021, 8, 48 4 of 8
the equation LOQ = 10 S/N.
Accuracy was evaluated at three concentration levels of 0.5, 2, and 10 ng/μL
sion within-day (intra-day, n = 3) and between-day (inter-day n = 4 days by du
Limits of detection (LODs) were calculated based on the following equation LOD = 3.3 S/N,
analyses) was studied at three concentration levels 0.5, 2, and 10 ng/μL.
where S = signal and N = noise. Limits of quantitation (LOQs) were calculated by the
Peak purity
equation LOQ was checked by standard addition of respective standards.
= 10 S/N.
Accuracy was evaluated at three concentration levels of 0.5, 2, and 10 ng/µL. Precision
2.5. Adsorption
within-day of DBT
(intra-day, and
n = 3) 4,6-DMDBT
and between-day (inter-day n = 4 days by duplicate analyses)
was studied at three concentration levels 0.5, 2, and 10 ng/µL.
For the
Peak adsorption
purity was checked experiments a solution
by standard addition containing
of respective the same concentrations
standards.
and 4,6-DMDBT in acetonitrile, was prepared. The concentration of each compo
2.5. Adsorption
solution wasof20
DBT and 4,6-DMDBT
ppmwS. The corresponding total sulfur concentration was 40 p
For the adsorption experiments
The mass of the carbon adsorbent a solution
was containing
1.25 g/L the
andsame
theconcentrations
adsorption of DBT was
process
and 4,6-DMDBT in acetonitrile, was prepared. The concentration of each compound in
out at ambient temperature. The remaining concentrations of thiophenes after ads
solution was 20 ppmwS. The corresponding total sulfur concentration was 40 ppmwS. The
were determined
mass of by HPLC
the carbon adsorbent at1.25
was a wavelength of 231 nm.
g/L and the adsorption process was carried out at
ambient temperature. The remaining concentrations of thiophenes after adsorption were
3. Results by HPLC at a wavelength of 231 nm.
determined
3.1. HPLC Method
3. Results
3.1. HPLC Method
Isocratic elution was optimized in terms of mobile phase constituents and rat
Isocratic
etonitrile elution
as the was optimized
organic modifierinledterms of mobile
to better peakphase
shape.constituents
The ratioand ratios. v/v of a
of 90:10
Acetonitrile as the organic modifier led to better peak shape. The ratio of 90:10 v/v of
trile and water mixture provided the sufficient peak resolution and fast analysis.
acetonitrile and water mixture provided the sufficient peak resolution and fast analysis.
Atypical
A typical HPLC
HPLC chromatogram
chromatogram at twophase
at two mobile mobile phase
ratios ratios isinillustrated
is illustrated Figure 3. in Fig
Figure 3.3.AAtypical
Figure HPLC
typical chromatogram
HPLC under different
chromatogram under eluent conditions:
different eluent(90% CH3 CN: 10%
conditions: H2CH
(90% O 3CN: 1
v/v,
H 2O peaks: DBT-sulfone
v/v, peaks: 3.89 min, DBT
DBT-sulfone 3.899.59
min,min,
DBTand9.59
4,6-DMDBT
min, and18.26 min) and (80%
4,6-DMDBT CH3 CN:
18.26 20%
min) and (80%
H O v/v, peaks: DBT-sulfone 3.33 min, DBT 6.25 min, and 4,6-DMDBT 10.20 min).
CH3CN: 20% H2O v/v, peaks: DBT-sulfone 3.33 min, DBT 6.25 min, and 4,6-DMDBT 10.20 m
2
3.2. Calibration Curves
3.2. Calibration Curveswere constructed by least-squares linear regression analysis. All
Calibration curves
calibration data obtained
Calibration curvesby were
standard solution analyses
constructed are shown in Table
by least-squares 1. Regression
linear regression analy
equations presented satisfactory correlation coefficients ranging between 0.9925 and 0.9993
calibration data obtained by standard solution analyses are shown in Table 1. Reg
over the examined range.
equations presented satisfactory correlation coefficients ranging between 0.99
0.9993
Table 1. over thedata
Linearity examined range.
of developed method.
Analyte Slope Intercept Correlation Coefficient (R)
Table 1. Linearity data of developed method.
DBT 0.0136 0.009 0.9925
4,6-DMDBT 0.0135 0.0019 0.9953
Analyte
DBT Sulfone
Slope
0.0670
Intercept
−0.0414
Correlation
0.9993
Coefficient
DBT 0.0136 0.009 0.9925
4,6-DMDBT 0.0135 0.0019 0.9953
DBT Sulfone 0.0670 −0.0414 0.9993
The limit of detection was found to be 0.16 ng/μL, while the limit of quantitatiSeparations 2021,
Separations 2021, 8,
8, 48
x FOR PEER REVIEW 55 of
of 88
Selectivity was studied by the absence of peaks in the same Rt of analytes.
The limit of detection was found to be 0.16 ng/µL, while the limit of quantitation was
Accuracy and precision data at three concentration levels are summarised in Tables
found 0.5 ng/µL.
2 and 3 with regards to within-day and between-day repeatability respectively.
Selectivity was studied by the absence of peaks in the same Rt of analytes.
Accuracy and precision data at three concentration levels are summarised in Tables 2 and 3
Table 2. Accuracy and repeatability using CH3CN-H2O 90:10 v/v as mobile phase n = 3.
with regards to within-day and between-day repeatability respectively.
Analyte Added (ng/μL) Found ± SD (ng/μL) R% RSD%
Table 2. Accuracy and repeatability
0.5 using CH3 CN-H0.462 O 90:10 phase n = 3. 8.7
± 0.04v/v as mobile92.0
DBT
Analyte 2
Added (ng/µL) Found ± 1.94 ± 0.20
SD (ng/µL) R% 97.0 RSD%10.3
10 9.8 ± 1.1 98.0 11.2
0.5 0.46 ± 0.04 92.0 8.7
DBT 2 0.5 0.47
1.94 ± 0.20± 0.05 97.0 94.0 10.6
10.3
4,6-DMDBT 10 2 9.8 ±2.0
1.1± 0.2 98.0100.0 10.0
11.2
0.510 ± 0.05
0.47 10.2 ± 0.87 94.0102.0 10.68.5
4,6-DMDBT 2 0.5 ± 0.2± 0.06
2.0 0.52 100.0104.0 10.0
11.5
10 10.2 ± 0.87 102.0 8.5
DBT Sulfone 0.5
2 1.88 ± 1.3
0.52 ± 0.06 104.0
94.0 6.9
11.5
DBT Sulfone 2 10 9.8
1.88 ± 1.3± 1.1 94.0 98.0 11.2
6.9
10 9.8 ± 1.1 98.0 11.2
Table 3. Accuracy and between-day precision using CH3CN-H2O 90:10 v/v as mobile phase (du-
plicate3.analyses
Table Accuracyin and
fourbetween-day
days). precision using CH CN-H O 90:10 v/v as mobile phase (dupli-
3 2
cate analyses
Analytein four Added
days). (ng/μL) Found ± SD (ng/μL) R% RSD%
Analyte 0.5
Added (ng/µL) Found ±0.53
SD ±(ng/µL)
0.06 R% 106.0 RSD%11.3
DBT 2 2.08 ± 0.25 104.0 12.0
0.5 0.53 ± 0.06 106.0 11.3
DBT 102 2.089.49 ± 0.84
± 0.25 104.0 94.9 12.08.8
0.5
10 9.490.5 ± 0.06
± 0.84 94.9 100.0 8.812.0
4,6-DMDBT 0.5
2 ± 0.06
0.5 2.16 ± 0.3 100.0108.0 12.013.9
4,6-DMDBT 102 ± 0.3
2.169.79 ± 0.7 108.0 97.9 13.97.2
10 9.79 ± 0.7 97.9 7.2
0.5
0.5
0.49 ±
0.49 ± 0.05
0.05 98.0
98.0 10.2
10.2
DBT
DBTSulfone
Sulfone 22 1.92 ± 0.150.15
1.92 ± 96.0 96.0 7.87.8
10
10 9.4 9.4 ± 0.8
± 0.8 94.0 94.0 8.58.5
3.3. Application in Real Sample
The method
methodwaswasapplied
applied to to samples
samples derived
derived fromfrom desulfurization
desulfurization studies
studies as de-
as described
scribed in the experimental
in the experimental part. part.
A typical
typical chromatogram
chromatogram of ofsample
sampleobtained
obtainedasasdescribed
describedininSection
Section2.5
2.5after
after
2424 h
h is
is shown
shown in Figure
in Figure 4, which
4, which illustrates
illustrates the peaks
the peaks of remaining
of remaining DBT DBT andDMDBT
and 4,6 4,6 DMDBT com-
compounds,
pounds, as well
as well as the
as the peak
peak of of
thethe polar
polar DBTsulfone,
DBT sulfone,which
whichisispresent
present in
in the effluent
eluted in the polar acetonitrile solvent.
Figure 4. HPLC
HPLC chromatogram
chromatogramafter
afteraa24
24hhadsorption
adsorptionstudy
studywhere
wherepeaks
peaksofof
remaining
remaining DBT,
DBT,4,6-
4,6-
DMDBT,
DMDBT, as as well
well as
asthe
thepolar
polarDBT
DBTsulfone
sulfonepresent
presentininthe
theeffluent
effluentdue
duetoto elution
elution byby acetonitrile,
acetonitrile, peaks:
peaks: DBT-sulfone
DBT-sulfone 3.31
3.31 min, min, unknown
unknown peak
peak 3.76, 3.76,
DBT DBT
6.23 6.23
min, min,
and and 4,6-DMDBT
4,6-DMDBT 10.24 min.
10.24 min.Separations 2021, 8, 48 6 of 8
3.4. Comparison with Other Methods
So far various GC and HPLC methods based on UV detection are reported using
gradient elution programs that may last more than 25 min. All previously reported methods
were applied as monitoring tools where neither analytical performance characteristics are
provided, nor validation criteria are applied. The herein developed method is faster since
the analysis is completed within 10 min and no equilibration time is necessary in the
intervals as a benefit of isocratic elution. Therefore, six samples can be analyzed per hour
increasing the productivity of the method. The cost of the analysis is also lower since no
highly sophisticated instrumentation is needed.
4. Conclusions
It is vital to reduce the content of benzothiophene in liquid fuels—a process known
as desulfurization. Adsorption and catalytic oxidation can be used for the removal of
dibenzothiophene (DBT). Since this process is not complete, some amounts are still present
in the final product, thus, it is necessary to quantitate the remaining as well as other
thiophenic products in the final result.
Herein, a fast HPLC method was developed and validated to serve as an analytical
tool in desulfurization studies. The analysis is completed in ca 10 min and gives results for
three thiophenic compounds. The proposed method has been proved to be a useful tool in
the environmental technology field providing accurate, repeatable, and reliable analytical
results when applied in the respective evaluation studies.
Author Contributions: Conceptualization, K.T., E.D. and V.S.; methodology, E.D., K.T. and V.S.;
validation, V.K. and V.S.; investigation K.T., E.D. and V.S.; experimental: V.K.; writing—original draft
preparation, E.D. and V.S.; writing—review and editing, E.D., V.S. and K.T.; supervision, E.D. and V.S.
All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data are available upon request.
Conflicts of Interest: The authors declare no conflict of interest.
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