Hydrodesulfurization of dibenzothiophene using Pd-promoted Co-Mo/Al2O3 and Ni-Mo/Al2O3 catalysts coupled with ionic liquids at ambient operating conditions

Article abstract of DOI:10.1039/c9ra00095j

Sulfur compounds in fuel oils are a major source of atmospheric pollution. This study is focused on the hydrodesulfurization (HDS) of dibenzothiophene (DBT) via the coupled application of 0.5 wt% Pd-loaded Co-Mo/Al₂O₃ and Ni-Mo/Al₂O₃ catalysts with ionic liquids (ILs) at ambient temperature (120 °C) and pressure (1 MPa H₂). The enhanced HDS activity of the solid catalysts coupled with [BMIM]BF₄, [(CH₃)₄N]Cl, [EMIM]AlCl₄, and [(n-C₈H₁₇)(C₄H₉)₃P]Br was credited to the synergism between hydrogenation by the former and extractive desulfurization and better H₂ transport by the latter, which was confirmed by DFT simulation. The Pd-loaded catalysts ranked highest by activity i.e. Pd-Ni-Mo/Al₂O₃ > Pd-Co-Mo/Al₂O₃ > Ni-Mo/Al₂O₃ > Co-Mo/Al₂O₃. With mild experimental conditions of 1 MPa H₂ pressure and 120 °C temperature and an oil:IL ratio of 10:3.3, DBT conversion was enhanced from 21% (by blank Ni-Mo/Al₂O₃) to 70% by Pd-Ni-Mo/Al₂O₃ coupled with [(n-C₈H₁₇)(C₄H₉)₃P]Br. The interaction of polarizable delocalized bonds (in DBT) and van der Waals forces influenced the higher solubility in ILs and hence led to higher DBT conversion. The IL was recycled four times with minimal loss of activity. Fresh and spent catalysts were characterized by FESEM, ICP-MS, EDX, XRD, XPS and BET surface area techniques. GC-MS analysis revealed biphenyl as the major HDS product. This study presents a considerable advance to the classical HDS processes in terms of mild operating conditions, cost-effectiveness, and simplified mechanization, and hence can be envisaged as an alternative approach for fuel oil processing.

Full text of DOI:10.1039/c9ra00095j

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PAPER
Hydrodesulfurization of dibenzothiophene using
Pd-promoted Co–Mo/Al O and Ni–Mo/Al O
2
3
2 3
Cite this: RSC Adv., 2019, 9, 10371
catalysts coupled with ionic liquids at ambient
operating conditions†
ab
b
a
c
Yaseen Muhammad,
Ata Ur Rahman, Haroon Ur Rashid, Maria Sahibzada,
ab
a
Sidra Subhan and Zhangfa Tong
*
Sulfur compounds in fuel oils are a major source of atmospheric pollution. This study is focused on the
hydrodesulfurization (HDS) of dibenzothiophene (DBT) via the coupled application of 0.5 wt% Pd-loaded
ꢀ
Co–Mo/Al
pressure (1 MPa H
Cl, [EMIM]AlCl , and [(n-C
former and extractive desulfurization and better H
simulation. The Pd-loaded catalysts ranked highest by activity i.e. Pd–Ni–Mo/Al
Ni–Mo/Al > Co–Mo/Al . With mild experimental conditions of 1 MPa H
2
O
3
and Ni–Mo/Al
). The enhanced HDS activity of the solid catalysts coupled with [BMIM]BF
17)(C P]Br was credited to the synergism between hydrogenation by the
transport by the latter, which was confirmed by DFT
2
O
3
catalysts with ionic liquids (ILs) at ambient temperature (120 C) and
2
4 3 4
, [(CH ) N]
4
8
H
4 9 3
H )
2
O
2 3
> Pd–Co–Mo/Al
2
O
3
ꢀ
>
2
O
3
O
2 3
2
pressure and 120
C
temperature and an oil : IL ratio of 10 : 3.3, DBT conversion was enhanced from 21% (by blank Ni–Mo/
Al ) to 70% by Pd–Ni–Mo/Al coupled with [(n-C 17)(C P]Br. The interaction of polarizable
O
2 3
2
O
3
8
H
4 9 3
H )
delocalized bonds (in DBT) and van der Waals forces influenced the higher solubility in ILs and hence led
to higher DBT conversion. The IL was recycled four times with minimal loss of activity. Fresh and spent
catalysts were characterized by FESEM, ICP-MS, EDX, XRD, XPS and BET surface area techniques. GC-MS
analysis revealed biphenyl as the major HDS product. This study presents a considerable advance to the
classical HDS processes in terms of mild operating conditions, cost-effectiveness, and simplified
mechanization, and hence can be envisaged as an alternative approach for fuel oil processing.
Received 5th January 2019
Accepted 20th March 2019
DOI: 10.1039/c9ra00095j
rsc.li/rsc-advances
ꢀ
poor safety and harsh temperature (380–400 C) and pressure (8
MPa) operating conditions.
1
. Introduction
11–13
Similarly, expensive catalytic
Organosulfur compounds leading to the production of delete- promoters like Pt, Pd, and Ir further add to the uncontrolled
rious and hazardous sulfur oxides, i.e. SO , from the combus- process costs. Thus, an HDS process operated at mild temper-
x
tion of fuel oils has led to the permissible limits of sulfur ature and pressure over a low-cost catalyst without compro-
1
becoming much more stringent over the years. Among the mising on the process efficiency could offer an alternative route
many desulfurization approaches, such as oxidative desulfur- for fuel oil processing.
2
3
4
ization, biodesulfurization and extractive desulfurization,
Apart from HDS, a newer approach to fuel oil desulfurization
14,15
hydrodesulfurization (HDS) ranks higher, attributed to its research is extraction using ionic liquids (ILs).
diverse nature, high practicability and efficiency. HDS gener- as ease of synthesis, high recycling ability, low volatility, high
ally utilizes a Co or Ni sulde phase added to Mo impregnated thermal stability and environmental friendliness are credited
Aspects such
5
6
–8
16
over Al O -supported catalysts. Furthermore, incorporation of for the widespread application of ILs. However, few studies on
2
3
a third promoter metal, i.e. Pt, Pd, and Ir, can further enhance the integrated application of ILs with solid catalysts have been
5,9,10
17
the HDS activity of classical catalysts.
Despite its efficient reported in hydrogenation reactions, which emphasizes that
and versatile nature, HDS suffers from complex mechanization, ILs coupled with solid catalysts should be tested for HDS of fuel
oils.
In previous work, we reported 52% dibenzothiophene (DBT)
conversion in a HDS process with a mild operating temperature
a
School of Chemistry and Chemical Engineering, Key Laboratory of Petrochemical
Resource Processing and Process Intensication Technology, Guangxi University,
Guangxi, 530004, P. R. China. E-mail: zhong@sina.com
b
and pressure by the coupled application of selected ILs with Ce–
Institute of Chemical Sciences, University of Peshawar, Peshawar, 25120, KP, Pakistan Ni–Mo/Al
O catalysts. However, to further improve the effi-
2 3
18
c
Department of Chemistry, Umea University, Umea, Sweden
ciency of solid catalyst-coupled ILs HDS processes, more
exploration is required for new types of catalysts and ILs.
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra00095j
1
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Pd has been reported as a more promising promoter for inductively coupled plasma mass spectrometry (ICP-MS), energy
classical HDS catalysts than many of its counterparts, e.g. Ir and dispersive X-ray (EDX), X-ray diffraction (XRD) and BET surface
19,20
Ru.
However, to our knowledge, no studies on the HDS of area techniques. The reaction products were quantitatively and
DBT coupled with ILs using Pd-promoted Co–Mo/Al O and Ni– qualitatively analyzed via high-pressure liquid chromatography
2
3
2 3
Mo/Al O catalysts at mild operating conditions have been re- (HPLC) and gas chromatography coupled with mass spectrom-
ported so far. Extractive desulfurization by ILs can augment the etry (GC-MS), respectively, and a suitable reaction mechanism
hydrogenation by solid catalysts in their blended application at was proposed.
mild operating conditions. Low Pd loading and mild operating
conditions can concomitantly lead to cost-effectiveness and
2
. Materials and methods
process safety. Thus, in this work, HDS of DBT was performed at
ꢀ
2.1. Chemicals
1
20 C temperature and 1 MPa H pressure over low (0.5 wt%)
2
Pd-loaded Co–Mo/Al O and Ni–Mo/Al O sulde phase cata-
2
3
2
3
All the reagents used in this study were of analytical reagent
lysts coupled with selected ILs. The fresh and spent catalysts
were characterized by X-ray photoelectron spectroscopy (XPS),
grade and were used without further purication. DBT, n-
octane, the Al
2
O
3
support and Ni(NO
3
)
2
$4H
2
O were purchased
$6H

eld emission scanning electron microscopy (FESEM),
from Sinopharm Chemical Reagent Co. Ltd. Co(NO
3
)
2
2
O
Fig. 1 Design of the batch autoclave reactor.
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2 3 2 3 2 3 2 3 2 3
Fig. 2 FESEM images of (a) Al O and the fresh catalysts (b) Co-M/Al O , (c) Ni–Mo/Al O , (d) Pd–Co–Mo/Al O and (e) Pd–Ni–Mo/Al O .
was obtained from Tian Jin Shifu Chen Chemical Reagent En Chemical Technology, Co. Ltd. Shang Hai, China. Five
Factory, China. Ammonium heptamolybdate ((NH ) - different ILs were purchased from Shanghai Cheng Jie
4
6
Mo
7
O
24$4H
2
O) was purchased from Jin Mao Chemical Reagent Chemical Co. Ltd. China. Pure H
) was provided by Sa genation experiments.
2
gas was used in all hydro-
Co. Ltd, while palladium chloride (PdCl
2
ꢀ
Table 1 Surface porosity data for fresh and spent catalysts used in HDS under 1 MPa H
2
pressure, 4 h reaction time and 120 C temperature
coupled with [(C
8
H
17)(C
4
H )
9
3
P]Br
BET surface area
Langmuir surface
Pore volume
2
ꢁ1
2
ꢁ1
3
ꢁ1
(
m g
)
area (m g
)
(cm g
)
Pore diameter (nm)
Sample
Fresh
Spent
Fresh
Spent
Fresh
Spent
Fresh
Spent
Al
Co–Mo/Al
Ni–Mo/Al
Pd–Co–Mo/Al
Pd–Ni–Mo/Al
2
O
3
200.8
109.6
120.5
115.1
108.4
—
291.5
156.6
172.5
164.2
155.0
—
0.30
0.21
0.22
0.22
0.22
—
6.0
7.7
7.4
8.0
8.2
—
2
O
O
2 3
3
99.1
91.9
72.5
79.9
148.5
138.0
109.4
119.8
0.19
0.17
0.16
0.17
7.6
7.5
9.0
8.6
2 3
O
2 3
O
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2
.2. Experimental
.2.1. Catalyst preparation. Four types of catalysts were
prepared using the incipient impregnation method reported
(PerkinElmer NexION™ 350D) and EDX techniques. BET
surface area measurements were performed via N adsorption
2
2
method using a Micromeritics Gemini VII surface area and
porosity analyzer. Crystal structure and phase characterizations
were achieved via XRD (Rigaku Smartlab, X-ray diffractometer,
2
2 3
elsewhere. A known amount of Al O support was sequentially
impregnated via stirring with a known amount of precursor salt
solution of Co, Ni, Mo, or Pd, each corresponding to 2, 4, 8, and
ꢀ
ꢁ1
Japan) operated at 9 kW with a scan speed of 10 min in the 2q
ꢀ
angular range of 5–80 . The surface elemental composition and
0
2
.5 wt%, respectively, at 200 rpm and room temperature for
4 h. The solutions were then dried in an oven at 110 C for 12 h
ꢀ
ꢀ
electronic states of various metals in the catalysts were char-
acterized by XPS (Thermo Electron Corporation, USA).
and subsequently calcined in a muffle furnace at 500 C for 5 h.
The calcined catalysts were then presulded in a tubular
furnace at 500 C for 6 h using 20% CS
in a N
presulded catalysts were stored under N
.2.2. Catalyst characterization. Fresh and spent catalysts
were characterized for surface morphology, elemental compo-
sition and distribution via eld emission scanning electron
microscopy (FESEM SU-8220N, Hitachi, Japan), ICP-MS
2.2.3. Measurement of HDS catalytic activity. Catalytic
ꢀ
activity tests were performed in a 200 mL stainless steel batch
autoclave reactor heated by an electrical furnace (Fig. 1) and
connected to a temperature and stirrer console. In a typical
experiment, the reactor was charged with 30 mL of 1000 ppm
DBT solution in n-octane, 0.1 g of presulded catalyst and
a xed mass of IL. Aer sealing, the reactor was evacuated of air
2
solution in cyclohexane
ꢁ1
2
ow at a rate of 100 mL min and 2 MPa pressure. The
2
atmosphere.
2
2
via a vacuum pump for 15 min three times with purges of H at
every step at a pressure of 3 MPa. Finally, the reactor was
charged with H gas to the desired pressure and then heated to
2
ꢀ
ꢁ1
a known temperature at a heating rate of 5 C min . The
different reaction parameters, i.e. type of catalyst, type of IL, H
2
Fig. 4 High-resolution XPS spectra of Mo 3d (A) Co–Mo/Al
Fig. 3 XRD patterns of (A) support and fresh catalysts and (B) spent Pd–Co–Mo/Al and (B) Ni–Mo/Al and Pd–Ni–Mo/Al
catalysts. catalysts.
2 3
O and
O
2 3
2
O
3
2 3
O
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pressure, temperature, and oil : IL ratio, were separately Al
optimized.
2
O
3
catalysts possessed less compact morphology (Fig. 2(b
and c)) than those of trimetallic Pd–Co–Mo/Al O and Pd–Ni–
2
3
The catalytic activity in terms of DBT conversion (%) was Mo/Al O (Fig. 2(d and e)). The spent catalysts (Fig. S2(a–d)†)
2
3
calculated using eqn (1):
(tested in HDS reaction at optimized conditions of 4 h, 1 MPa H2
pressure at 120 C using an oil : IL ratio of 10 : 3.3) suffered
(1) a signicant decrease in porosity, resulting in the appearance of
ꢀ
ꢁ
ꢀ
C
o
ꢁ C
i
DBT conversion ð%Þ ¼
ꢂ 100
C
o
clots with an overall increase in particle size and pore diameter
24
where C
reaction, respectively.
.2.4. Product analysis. HDS products were quantitatively during the HDS reaction. Compared to the spent Co–Mo/Al O3
o
and C
i
are the DBT concentrations before and aer the (Table 1). This could be due to the deposition of DBT or HDS
products, and preferential utilization of micro- and meso-pores
25
2
2
analyzed via HPLC (Agilent 1100 using a Zorbax SB-C18 column and Ni–Mo/Al O catalysts in Fig. S2(a and b),† the spent Pd-
2
3
with dimensions of 4.6 ꢂ 150 mm) with a UV detector at promoted (Pd–Co–Mo/Al O and Pd–Ni–Mo/Al O ) catalysts in
2
3
2 3
2
a wavelength of 320 nm applying the calibration curve shown Fig. S2(c and d)† exhibited a much more compact and packed
in Fig. S1.† Qualitative analyses were performed on a GC-MS morphology, which might be the result of more sulfur species
chromatograph (Agilent 7890A with a DB-5MS stainless steel deposited being deposited owing to their much higher DBT
column 30 m in length with an inner diameter of 0.25 mm) conversion than Co–Mo/Al O and Ni–Mo/Al O (to be dis-
2
3
2 3
coupled with a mass spectrometer (MS-5975C). Each GC-MS run cussed in the following sections).
utilized 2 mL of sample and was heated from room temperature
The ICP-MS results in Table S1† advocate lower concentra-
to 80 C in one minute, then increased to 250 C at a heating tions of the impregnated metals in the four types of catalysts
ꢀ
ꢀ
ꢀ
ꢁ1
rate of 10 C min till the end of the run.
than the theoretical values, which could be attributed to the
leaching of some metal species during the catalyst synthesis or
improper dissolution of the catalysts for ICP analysis, especially
for the Pd-based catalysts. The concentration of all metals in the
3
3
. Results and discussion
.1. Characterization of catalysts
Fig. 2 shows the FESEM images of pure Al
Al , Ni–Mo/Al , Pd–Co–Mo/Al
catalysts while Fig. S2† summarizes the FESEM images of spent DBT. The EDX spectrum (Fig. S3(a)†) of the Al
versions of these catalysts. The data in Table 1 and FESEM abundant Al and O species. Fig. S3(b–e)† conrms the
images in Fig. 2a reveal the highly porous nature of the Al
successful impregnation of Co, Ni, Mo and Pd onto the Al
spent catalysts was considerably decreased, which could be
2
O
3
and fresh Co–Mo/ attributed to their agglomeration (FESEM images in Fig. S2†) or
2
O
3
2
O
3
2 3 2 3
O , and Pd–Ni–Mo/Al O
disposition of HDS products aer being tested in the HDS of
O
2 3
support shows
2
O
3
2 3
O
26
support. Fig. 2b shows that uniformly deposited Mo and Co support. Furthermore, the EDX elemental mappings for the
species led to a decrease in the porosity and surface area and an fresh catalysts in Fig. S4A† suggest the uniform distribution of
increase in the particle size compared to those of the pure Co, Ni, Mo and Pd over the support surface, which was
21–23
support (Table 1).
The bimetallic Co–Mo/Al
2
O
3
and Ni–Mo/ considerably decreased for the spent catalysts (Fig. S4B†), which
ꢀ
Fig. 5 DBT conversion by 0.1 g of presulfided catalyst at 3 MPa H
2
pressure, 160 C and 4 h reaction time using 30 mL of 1000 ppm DBT solution.
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was attributed to the blockage of the surface by DBT molecules Table 2 Molecular and structural formulae of ILs tested in catalytic
HDS of DBT
in the latter. Moreover, the EDX elemental mapping results
further conrmed that Mo had the densest distribution
Molecular formula
Structural formula
(
(Fig. S4A(g–j) and S4B(r–u)†) while Pd had the lowest distri-
bution density (Fig. S4A(e and f) and S4B(p and q)†). Al present
in the bulk in the Al support possessed the densest distri-
bution (Fig. S4A(k)†).
Porosity, specic surface area (SSA) and Langmuir surface
area data for the Al support and various catalysts are
provided in Table 1. The SSA of the support was considerably
2 3
O
[
EMIM]BF
4
2 3
O
7,27
decreased upon the impregnation of Co, Ni, Mo and Pd. The
hysteresis loop at higher relative pressure (P/P > 0.6) in Fig. S5†
o
indicates the predominant presence of micropores and meso-
2
8
pores. Spent catalysts (tested in HDS reaction for 4 h, 1 MPa H
2
ꢀ
pressure at 120 C using an oil : IL ratio of 10 : 3.3) exhibited
[
BMIM]BF
4
a signicantly high decrease in SSA, which could be attributed
29
to the deposition of DBT or HDS products. The decrease in the
SSA and increase in the pore diameter aer the addition of Co or
Ni to the Al O support and Pd to the Co–Mo/Al O or Ni–Mo/
2
3
2 3
Al O catalysts could be attributed to the increase in meso-
2
3
30
porosity. In the case of the spent catalysts (Table 1), the net
increase in the pore diameter and decrease in the pore volume
could be owing to the preferential utilization of micro- and
meso-pores during the HDS reaction, leaving macropores [EMIM]AlCl4
30
behind with their larger pore diameter.
The XRD patterns of Al O and fresh catalysts compiled in
2
3
Fig. 3 suggest a high degree of dispersion with a minimum
degree of crystallization. Fig. 3A shows strong peaks for Al O
2
3
ꢀ
ꢀ
31,32
at 2q of 47 and 67 (black rectangles),
which were weak-
ened by the incorporation of Co, Ni, Mo or Pd, suggesting that
the catalysts maintained the proper pore structure required for
ꢀ
ꢀ
the HDS reaction. The peaks at 2q of 33 and 59 were ascribed
[
3 4
(CH ) N]Cl
3
3,34
to MoS
2
species in all catalysts
while Ni showed a weak
peak at 2q of 60 in Ni–Mo/Al O and Pd–Ni–Mo/Al O . The
ꢀ
35
2
3
2 3
XRD patterns of the spent catalysts shown in Fig. 3B indicate
that there was little change in the bulk structure compared
with those of the fresh catalysts (Fig. 3A) suggesting their
3
6
stable nature under the current HDS reaction conditions.
However, from the XRD patterns in Fig. 3B, one can see the
decrease in the dispersion of metallic species as the peaks
become sharper compared to the fresh catalysts (Fig. 3A),
which could be due to the deposition of sulfur moieties aer [(n-C H )(C H ) P]Br
8
17
4
9 3
the HDS reaction. It can be concluded from Fig. 3(A and B) that
with low metal loading, specically of Pd, and a highly amor-
phous nature with a high degree of dispersion of the catalyst
(
Fig. S4A and S4B†), the XRD data provided little information
about the exact composition and crystallinity of the individual
1
8,33
species
. Furthermore, the low metal loading of Pd
(
0.5 wt%) was below the detection limit of the XRD instrument
and hence no clear peak appeared for Pd in the Pd–Co–Mo/
Al O or Pd–Ni–Mo/Al O catalysts.
binding energies (BE) of 231.2 eV and 234.5 eV ascribed to Mo
2
3
2 3
38,39
3
d
5/2 and Mo 3d3/2 as MoS
3
phase, respectively.
Elemental
The full survey XPS spectra of the four types of fresh catalysts
39
Mo (at 227.5 or 230.6 eV) was not detected in any of the
are shown in Fig. S6(I),† conrming the presence of Co, Mo, Ni
and Al. In Fig. S6(I),† the presence of a large amount of C 1s
could be owing to the presence of organic compounds or the C
samples, indicating the complete suldation of Mo species. The
shi towards higher BE of Mo 3d5/2 (231.5 eV) and Mo 3d3/2
37
(235.0 eV) (Fig. 4A) for Pd–Co–Mo/Al O compared to those in
2 3
le from cyclohexane in the presuldation step. The Co–Mo/
Al catalyst in Fig. 4A exhibited two prominent peaks at
Co–Mo/Al
2
O
3
could be attributed to the stronger interaction of
2 3
O
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ꢀ
Fig. 6 Comparison of DBT conversion by Pd–Ni–Mo/Al
O
2 3
(0.1 g) combined with 4 g of each IL at 160 C, 3 MPa H
2
pressure, and 4 h reaction
time with 30 mL of 1000 ppm DBT solution.
Mo with Pd in the former. On the contrary, the lower BE for Mo
5/2 and 3d3/2 (230.9 and 234.5, respectively) in Pd–Ni–Mo/ incorporation. Four types of sulde phase catalysts i.e. Co-M/
than those for the same two states in Ni–Mo/Al (231.8 Al O , Ni–Mo/Al O , Pd–Co–Mo/Al O and Pd–Ni–Mo/Al O3
3.2.1. HDS activity of different catalysts and effect of Pd
3d
Al
2
O
3
2 3
O
2
3
2
3
2
3
2
and 235.3, respectively) in (Fig. 4B) was indicative of a weaker (each 0.1 g) were separately tested for HDS activity in the
ꢀ
interaction between Mo and Pd species in Pd–Ni–Mo/Al
2
O
3
,
absence of IL at 160 C, 4 h reaction time, 3 MPa H pressure,
2
leading to better dispersion of the active phase, which can play and 200 rpm stirring speed utilizing 30 mL of a 1000 ppm DBT
an active role in enhancing the catalytic activity towards DBT model solution. The results in Fig. 5 indicate that the Ni-based
conversion. The degree of interaction of Pd with Mo in the Pd– catalysts exhibited higher catalytic activity than the Co-based
Co–Mo/Al O and Pd–Ni–Mo/Al O catalysts could be justied catalysts, which is attributed to the higher reduction potential
2
3
2 3
8
0
by a charge compensation model in which Mo loses d-electrons of Ni with 3d 4S conguration (higher affinity towards the
7
02
and gains sp-electrons while Pd gains d-electrons and loses sp electron-rich sulfur center in DBT) than that of Co with 3d 4S
40
electrons. The weaker interaction of Pd with Ni in the Pd–Ni– conguration. Furthermore, the lower adsorption strength over
Mo/Al catalyst (lower BE in Fig. 4B) could be attributed to the Al O support of Ni compared to Co allows the Ni to remain
stronger Ni–Mo interaction owing to the 3d 4S conguration on the surface without forming aluminate ions and ultimately
2 3
O
2
3
8
0
42
7
0
than that in the Pd–Co–Mo/Al
2
O
3
catalyst (with 3d 4S cong- facilitates higher chances of DBT hydrogenation. The enhanced
41
uration). In brief, XPS analysis indirectly conrmed the activity of the Pd-promoted catalysts was obviously due to the
extremely low loading of Pd (0.5 wt%) in Pd–Co–Mo/Al O and extra Pd active phase, better dispersion of Co, Ni and Mo
43
2
3
Pd–Ni–Mo/Al
2 3
O catalysts, which was not achieved by XRD species by the incorporation of Pd, and hindrance of sintering
44
analysis. A similar trend can be observed in Fig. S6(II and III)† and clotting. From Fig. 5, the HDS activity order of the various
showcasing the S 2p3/2 and S 2p1/2 states in various catalysts.
41
catalysts was found to be: Pd–Ni–Mo/Al O > Pd–Co–Mo/Al O >
2 3 2 3
Ni–Mo/Al O > Co–Mo/Al O .
2
3
2 3
3
.2.2. Process and mechanism of HDS by the solid catalyst-
3.2. Catalytic HDS activity tests
IL coupled system. The selected ILs (4 g; Table 2) coupled with
0
1
.1 g of Pd–Ni–Mo/Al
000 ppm DBT solution at 160 C temperature, 3 MPa H
2
O
3
catalyst in the HDS of 30 mL of
HDS catalytic activity tests of the presulded catalysts were
performed in a 200 mL batch autoclave reactor (Fig. 1) as
detailed in Section 2.2.3. Different reaction parameters were
independently optimized, as discussed below.
ꢀ
2
pressure, 200 rpm stirring speed and 4 h reaction time were
separately tested. The results in Fig. 6 indicate that [BMIM]BF4,
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ꢀ
Fig. 7 Comparison of DBT conversion by different catalysts (0.1 g) combined with 4 g of each IL at 160 C, 3 MPa H
2
pressure and 4 h reaction
time using 30 mL of 1000 ppm DBT solution.
[
EMIM]AlCl , [(CH ) N]Cl, and [(n-C H )(C H ) P]Br coupled anion interaction being looser than those of the other ILs, while
4 3 4 8 17 4 9 3
with the solid catalyst exhibited higher HDS activity than blank the longer cation chain of [(n-C
catalysts, which could be attributed to certain electronic factors. [(CH N]Cl resulted in higher desulfurization efficiency of the
The nature and structure of the anions and cations of ILs play former. In addition, the higher DBT conversion by [(n-
8 4 9 3
H17)(C H ) P]Br compared to
3 4
)
45
4
5
crucial roles in their desulfurization efficiency. For example, in
BMIM]BF and [EMIM]BF , the higher desulfurization effi- between phosphonium and its anion than that of ammonium
ciency of the former could be attributed to the longer cation with the corresponding anion and the higher thermal stability
8 4 9 3
C H17)(C H ) P]Br could also be owing to the looser bond
[
4
4
45
chain. The higher HDS of DBT by the coupled application of of the former than the latter. Under identical experimental
ILs with the solid catalyst was owing to the synergistic effect of conditions, the other three types of catalyst, i.e., Co–Mo/Al O ,
2
3
1
8,46–48
extraction by the ILs
and hydrogenation by the solid Ni–Mo/Al
2
O
3
and Pd–Co–Mo/Al
2
O
3
, followed a similar activity
17)(C P]Br was selected
catalyst. The extraction of DBT by IL is owing to the insertion of trend (Fig. 7) to that in Fig. 6. [(n-C
8
H
4 9 3
H )
sulfur from DBT (with conjugation between the lone pair on S for subsequent experiments owing to its maximum DBT
and the p electrons of the aromatic ring) into the dynamic conversion efficiency coupled with the solid catalyst.
49
ꢁ
ꢁ
structure of the IL. [EMIM]AlCl
4
produces AlCl
4
and Al
2
Cl
7
ions in the solution phase, which drastically enhances its
4
6
extractive efficiency towards DBT. Similarly, [(CH ) N]Cl Table 3 Catalytic/extractive performance in terms of DBT conversion
3
4
45
8 17 4 9 3
by 4 g of [(n-C H )(C H ) P]Br coupled with HDS reaction (in the
possesses outstanding extraction ability and hence led to
enhanced DBT conversion coupled with the solid catalyst. The
ꢀ
absence of a solid catalyst) at 160 C, 4 h reaction time, 200 rpm and
MPa H pressure
3
2
highest DBT conversion by [(n-C
8 4 9 3
H17)(C H ) P]Br coupled with
Pd–Ni–Mo/Al in Fig. 6 and 7 was accredited to its superior
2
O
3
Type of
H
2
DBT
extraction ability, higher thermal stability, higher solubility of experiment Type of IL
polarizable delocalized bonds of DBT and van der Waals forces
pressure (MPa) conversion (%)
50
Extraction
Extraction
with HDS
[(n-C
[(n-C
8
H
H
17)(C
17)(C
4
4
H
H
9
)
)
3
3
P]Br
P]Br
—
13
22
than the other tested ILs. Similarly, the higher conversion by
8
9
3
quaternary cation-based ILs, i.e. [(CH ) N]Cl and [(n-
3
4
C H )(C H ) P]Br, could also be attributed to their cation–
8
17
4
9 3
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The catalytic role of ILs coupled with solid catalysts was mere [(n-C
8
H
17)(C
4
H
9
)
3
P]Br (13%) was enhanced by
further analyzed by performing experiments in the same combining it with HDS (22%). From these results, one can
autoclave reactor without the addition of Pd–Ni–Mo/Al O conclude that [(n-C H )(C H ) P]Br along with extraction also
2
3
8
17
4
9 3
over 4 g of [(n-C H )(C H ) P]Br IL with 200 rpm stirring played a catalytic role to a certain extent in combination with
8
17
4
9 3
ꢀ
speed and 160 C temperature for 4 h using 30 mL of 1000 ppm HDS. This synergistic effect was further enhanced by the
DBT solution. Aer the reaction, the concentration of DBT in introduction of the Pd–Ni–Mo/Al catalyst to the reaction
2 3
O
the oil phase (separated by decantation) was analyzed by medium (Fig. 6 and 7), enhancing the DBT conversion from
HPLC. The results in Table 3 suggest that DBT extraction by 26% to 44% (Fig. 6).
2 3 4 4 8 4 9 3 4
Fig. 8 DFT simulation indicating interaction of H with (a) n-octane, (b) [(CH ) N]Cl, (c) [BMIM]BF , (d) [(n-C H17)(C H ) P]Br and (e) [EMIM]AlCl
in terms of binding energy and relative distance between atoms.
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Table 4 DFT simulation data in terms of binding energy of H
2
interaction with n-octane and different types of IL
Binding energy
(uncorrected) kJ mol
BSSE correction
factor
Binding energy
(corrected) kJ mol
ꢁ1
ꢁ1
Sample
n-Octane
0.029
ꢁ3.77
ꢁ2.64
ꢁ2.75
ꢁ1.19
0.004
0.321
0.378
0.091
0.366
0.033
ꢁ3.449
ꢁ2.262
ꢁ2.659
ꢁ0.824
[
[
[
[
(CH
BMIM]BF
(n-C 17)(C
EMIM]AlCl
3 4
) N]Cl
4
8
H
4 9 3
H ) P]Br
4
The effect of the ILs-solid catalyst coupled system on HDS were much smaller than those between H
was further validated via DFT simulation using Gaussian 3 rming the higher solubility of H in IL than in n-octane. This
soware (Fig. 8) and the resulting BE are provided in Table 4. can facilitate easier mass transfer of H through the IL and
2
and n-octane, con-
2
2
51
The geometries of the reacting species were optimized at the hence better chances of activated H atom generation which
B3LYP/6-311+g(d,p) level, both with and without BSSE correc- can, in turn, enhance the HDS activity. Better H solubility in ILs
2
tions, while the geometry parameters of each IL with H are is envisaged in Fig. 9. A heterogeneous catalytic process, e.g.,
2
shown in Table S2.† The relative distance between H
terminal C atom of n-octane (Fig. 8a) was much larger (5.8 A˚ ) of the solid catalyst. As illustrated in Fig. 9, H
than that between H and the anionic center of the ILs (Fig. 8b–e into the reactor faces and penetrates through the barrier of the
and Table 4), suggesting a stronger interaction between the oil phase (DBT solution) followed by the lm between the oil
latter pair. Furthermore, the BE between H and ILs in Table 4 phase and the solid catalyst and nally reaches and spreads
2
and the HDS, is controlled by the adsorption of H
2
gas onto the surface
2
gas introduced
2
2
2
Fig. 9 HDS of DBT process demonstrating transport of H through oil and IL phase reaching the solid catalyst surface.
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Fig. 10 Effect of reaction temperature on DBT conversion utilizing 0.1 g of each catalyst at 3 MPa H
2
pressure, 4 h reaction time, 200 rpm stirring
8 4 9 3
speed, 30 mL of 1000 ppm DBT solution and 4 g of [(n-C H17)(C H ) P]Br.
ꢀ
Fig. 11 Effect of H
2
pressure on the HDS activity of different catalysts (0.1 g each) in the presence of 4 g of [(n-C
8
H17)(C
H
4 9
)
3
P]Br at 120 C and
4
h reaction time and 30 mL of 1000 ppm DBT solution.
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over the surface of the Pd–Ni–Mo/Al
2
O
3
catalyst. Over the cata-
3.2.5. Effect of the amount of IL on the HDS activity of the
lyst surface, H is activated into H atoms and then performs the solid catalyst. The effect of the amount of [(n-C H )(C H ) P]Br
2
8
17
4
9 3
HDS reaction of DBT. In this scenario, the solubility of H in the (3–10 g) using 30 mL of 1000 ppm DBT solution was tested using
2
51
liquid phase is the major activity-controlling factor. The DFT the optimized reaction conditions. The results compiled in
results in Fig. 8 show the much higher solubility of H in all of Fig. 12 suggest a corresponding increase in DBT conversion
the four ILs than in the DBT model oil, which is conducive to with increasing IL content. A higher IL content provides more
better H transport through the reaction mixture in the pres- sulfur extraction sites and increased H solubility (DFT results
ence of IL, allowing the H
surface of the Pd–Ni–Mo/Al
2
2
2
2
to reach and then be activated on the in Fig. 8) and hence the maximum DBT conversion (70%) was
, ultimately performing the HDS recorded by the Pd–Ni–Mo/Al O catalyst with an oil : IL ratio of
2 3
O
2
3
reaction. Thus, one can conclude that ILs concomitantly act as 10 : 3.3. These results suggest a 4.5-fold increase in the HDS
DBT extractants as well as enhancing the solubility of H , which activity versus blank Co–Mo/Al O and 2.5-fold versus blank Pd–
2
2 3
augmented the HDS activity of the solid catalyst and hence Ni–Mo/Al O
8 17 4 9 3
aer coupling with [(n-C H )(C H ) P]Br.
2
3
recorded better activity than that of the blank catalysts.
Furthermore, at 3-fold milder operating conditions than those
59,60
3
.2.3. Effect of reaction temperature on the HDS activity. for the classical HDS process,
a DBT conversion of 70% with
ꢀ
The effect of mild reaction temperatures (90–200 C) on the a low-cost (0.5 wt%) Pd-loaded Pd–Ni–Mo/Al O3 catalyst
2
catalytic HDS of DBT coupled with [(n-C
8 4 9 3
8 17 4 9 3
H17)(C H ) P]Br IL (4 g) coupled with [(n-C H )(C H ) P]Br credits this approach with
shown in Fig. 10 indicates the higher activity of the IL-coupled great promise for industrial applications.
system than of the blank catalysts owing to the synergistic effect
3.2.6. Recycling of IL. The uncontrolled release of used ILs
61
of the catalytic desulfurization reaction (by H ) and extraction to the ecosystem can concomitantly damage the environment
2
18
ꢀ
62
(
by IL). Increasing the temperature from 90 to 120 C leads to and increases process costs. Thus, [(n-C H )(C H ) P]Br was
8 17 4 9 3
2
a decrease in the viscosity of the IL, facilitating better H solu- separated and recycled from the HDS products using vacuum
52
bility, and can also provide the activation energy for the distillation and was tested for four consecutive cycles coupled
reaction, thus leading to a corresponding increase in DBT with Pd–Ni–Mo/Al O using the optimized conditions. Fig. 13
2
3
ꢀ
conversion. However, temperatures beyond 120 C may lead to shows that [(n-C H )(C H ) P]Br remained highly stable for
8
17
4
9 3
reverse migration of DBT to the solution media according to four consecutive cycles with net DBT conversions from the rst
50
Van't Hoff law, hence decreasing the DBT conversion. Apart to the last cycle of 70, 68, 68 and 67%, respectively. The Fourier
ꢀ
from this, increasing the temperature beyond 120 C may lead transform infrared (FT-IR) spectra (aer four cycles, obtained
1
8
1
to the dissociation of the IL, hence diminishing the synergistic via Nicolet iS50 FT-IR spectrometer) and the H NMR (ASCEND-
effect of the IL coupled with the solid catalyst, and a decrease in 600, Bruker) spectra of the fresh and recycled ILs shown in
DBT conversion is observed. Another reason could be the Fig. S7† revealed minimal changes in the structures of the fresh
ꢀ
53
decreased solubility of H
2
in the IL or n-octane beyond 120 C.
and recycled ILs. This indicates the highly stable nature of [(n-
At the optimized 120 C, a maximum DBT conversion of 50% C H )(C H ) P]Br under the current experimental conditions,
ꢀ
8
17
4
9 3
was recorded for Pd–Ni–Mo/Al
2
O
3
coupled with 4 g of [(n- which could further help in controlling process costs and
C H )(C H ) P]Br.
facilitating the industrial applicability of the proposed process
8
17
4
9 3
3.2.4. Effect of hydrogen pressure on the HDS activity. for fuel oil desulfurization.
Fig. 11 shows a relatively mild effect of H2 pressure in the
range of 1–7 MPa on the HDS activity of the four types of
ꢀ
catalysts coupled with 4 g of [(n-C
8
H
17)(C
4
H
9
)
3
P]Br at 120 C 4. Product analysis and proposed
with a 4 h reaction time. A higher partial pressure of a gas can
lead to its better solubility in liquid medium and hence
provides better chances of H -catalytic active site interactions. Qualitative analyses of the reaction products were performed
reaction mechanism
5
4
2
However, at the same time, HDS of DBT results in bulk via GC-MS and the chromatograms (pre- and post-HDS reaction)
production of H S, which can poison the catalyst and hence are presented in Fig. S8(a and b).† The DBT peak at the reten-
2
5
5–57
8
inhibit the HDS process at higher pressures.
autoclave type reactor in this study did not allow the escape of post reaction GC-MS chromatogram in Fig. S8b† (performed
produced H S from the reaction system. An increase in H over Pd–Ni–Mo/Al O catalyst at optimized conditions using
pressure (1 MPa onward) consequently leads to a correspond- 10 g of [(n-C H )(C H ) P]Br), a new peak for biphenyl (BP) was
The closed tion time of 14 min was recorded in both the samples. In the
2
2
2
3
8
17
4
9 3
2
ing increase in H S partial pressure, which retarded the HDS observed at 9.2 min. This evidenced the preferential direct
5
6
8,20
process, as shown in Fig. 11. Another reason for the decline desulfurization (DDS) pathway for the HDS of DBT.
This is
ꢀ
in DBT conversion could be the high solubility of H S in ILs at because at the low temperature of 120 C, hydrogenation of the
higher pressure decreasing the extraction efficiency of the IL.
2
5
8
benzene ring is not feasible and hence C–S scission occurs
A DBT conversion of 51% by the Pd–Ni–Mo/Al O catalyst at preferentially via the DDS pathway. In the rst step, H attacks
2
3
2
ꢀ
1
MPa pressure and 120
H
2
C coupled with [(n- the C–S bond, resulting in biphenyl-2-thiol, which is immedi-
C
8
H
17)(C H ) P]Br is superior to an earlier report in terms of ately hydrogenated to an unstable hydrogenated BP thiol
4 9 3
1
8
catalyst cost, operating pressure and temperature. Based on intermediate, and which upon extraction of H S leads to BP. No
2
these results, 1 MPa pressure was selected for onward cyclohexylbenzene (CHB) was found in the product stream
experiments.
(Fig. S8(b)),† which could be due to insufficient catalytic active
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ꢀ
2
Fig. 12 Effect of amount of IL on DBT conversion using 30 mL of 1000 ppm DBT solution, 1 MPa H pressure, 120 C reaction temperature,
200 rpm stirring speed and 4 h reaction time.
Fig. 13 Effect of four consecutive recycles on DBT conversion using 10 g of [(n-C
8
H
17)(C
3
H
7
)
3
P]Br coupled with 0.1 g of Pd–Ni–Mo/Al
2
O
3
ꢀ
catalyst at 1 MPa H
2
pressure, 120 C temperature, 4 h reaction time, 200 rpm stirring speed and 30 mL of 1000 ppm DBT solution.
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conventional HDS process. At genial operating conditions of
ꢀ
1
MPa H pressure, 120 C temperature, oil : IL ratio of 10 : 3.3
2
and 4 h reaction time, 70% DBT conversion was achieved over
Pd–Ni–Mo/Al O coupled with [(n-C H )(C H ) P]Br. Textural
2
3
8
17
4
9 3
characterization revealed that Pd incorporation decreased the
surface area while increasing the pore diameter of the pristine
catalysts. DFT simulations validated that enhanced HDS activity
of the solid catalysts coupled with ILs was owing to the syner-
gistic effect of extraction by IL and hydrogenation by the solid
catalyst. The IL could be recycled four times with minimal loss
of HDS activity. GC-MS results conrmed that the HDS reaction
under the current setup followed the DDS pathway. The present
approach with its cost-effectiveness (0.5 wt% Pd loading),
extremely mild operating conditions with good catalytic activity,
and simplied mechanization could be deemed as an alterna-
tive approach for the HDS of fuel oils on an industrial level.
Conflicts of interest
The authors declare no conict of interest.
Acknowledgements
The authors are thankful to the Natural Science Foundation of
Guangxi (2017GXNSFDA198047), the Higher Education
Commission of Pakistan (Project No: 3365), the Dean Project of
Guangxi Key Laboratory of Petro-chemical Resource Processing
and Process Intensication Technology (2017Z001 & 2015Z010)
and the Postdoctoral Fund of Department of Human Resources
and Social Security of Guangxi Zhuang Autonomous Region for
the nancial support.
Scheme 1 Proposed reaction mechanism for HDS reaction by blank
Pd–Ni–Mo/Al O and Pd–Ni–Mo/Al O coupled with IL.
2 3 2 3
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