Desulfurization of diesel fuel with nickel boride in situ generated in an ionic liquid

Article abstract of DOI:10.1039/c4gc00695j

In order to improve the desulfurization efficiency, an ionic liquid (IL) was used as the solvent for the desulfurization of diesel fuel with nickel boride. The nickel boride prepared in IL-H₂O showed high specific surface area. The desulfurization efficiency of model organosulfur compounds in this work was higher than that in the previous studies. The desulfurization reactivity of model organosulfur compounds followed the order of BT (DBT) > 3-MBT > 4,6-DMDBT. Furthermore, the products of model organosulfur compounds after desulfurization were analyzed by GC/MS and their corresponding reaction routes were proposed. The effectiveness of nickel salts followed the order of NiCl₂ (Ni(OAc)₂) > NiSO₄ > Ni(NO ₃)₂. The desulfurization efficiency of model diesel fuels reached 90.6% under the conditions of B/S molar ratio = 9, Ni(OAc)₂/S molar ratio = 3, oil/IL volume ratio = 3, water content in IL = 5%, and reaction time = 50 min. ILs maintained their original structures after regeneration. Finally, the desulfurization of real diesel fuel was carried out and a desulfurization efficiency of 88.6% was obtained in 50 min. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00695j

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Desulfurization of diesel fuel with nickel boride
in situ generated in an ionic liquid†
Cite this: Green Chem., 2014, 16,
3881
Chenhua Shu, Tonghua Sun,* Qingbin Guo, Jinping Jia and Ziyang Lou
In order to improve the desulfurization efficiency, an ionic liquid (IL) was used as the solvent for the de-
sulfurization of diesel fuel with nickel boride. The nickel boride prepared in IL–H2O showed high specific
surface area. The desulfurization efficiency of model organosulfur compounds in this work was higher
than that in the previous studies. The desulfurization reactivity of model organosulfur compounds fol-
lowed the order of BT (DBT) > 3-MBT > 4,6-DMDBT. Furthermore, the products of model organosulfur
compounds after desulfurization were analyzed by GC/MS and their corresponding reaction routes were
proposed. The effectiveness of nickel salts followed the order of NiCl2 (Ni(OAc)2) > NiSO4 > Ni(NO3)2. The
desulfurization efficiency of model diesel fuels reached 90.6% under the conditions of B/S molar ratio =
9, Ni(OAc)2/S molar ratio = 3, oil/IL volume ratio = 3, water content in IL = 5%, and reaction time =
50 min. ILs maintained their original structures after regeneration. Finally, the desulfurization of real diesel
fuel was carried out and a desulfurization efficiency of 88.6% was obtained in 50 min.
Received 17th April 2014,
Accepted 9th June 2014
DOI: 10.1039/c4gc00695j
www.rsc.org/greenchem
nonpyrophoric nature, and simple removal from a reaction
Introduction
mixture by filtration make it
a particularly convenient
SOx emission from automobile exhausts not only pollutes air reagent.23 A lot of studies about the desulfurization of organo-
heavily, but also irreversibly poisons the metal catalysts in sulfur compounds with nickel boride have also been carried
automobiles. So many countries have implemented stringent out. For example, W. E. Truce et al. reported the desulfuriza-
legislation to regulate the sulfur content of transportation tion of mercaptans, sulfides and sulfoxides with nickel
fuels.1,2 To meet the increasingly stringent environmental boride.24 T. G. Back et al. used the nickel boride prepared with
regulations, many deep desulfurization processes have been NaBH4 and NiCl2 to desulfurize BT and its derivatives (BTs),
developed in the past few years such as alkylation,3 DBT and its derivatives (DBTs), alkylthio and arylthio com-
extraction,4–7 oxidation,8–12 adsorption,13–19 membrane separ- pounds.25,26 J. M. Khurana reported the desulfurization of
ation,20,21 and bio-desulfurization.22 In addition, reductive thioureas, thiobarbiturates and dithiolanes with nickel
desulfurization is also of importance both in the laboratory boride.27,28 However, the desulfurization efficiency is low for
and in industry. Hydrodesulfurization (HDS), for example, is a all the above studies. The reason for the low desulfurization
traditional industrial process for sulfur removal from transpor- efficiency can be explained from the desulfurization mechan-
tation fuels. However, HDS is not effective for removing hetero- ism of nickel boride. According to the previous reports,25,26,29
cyclic sulfur compounds such as benzothiophene (BT), the desulfurization mechanism of nickel boride can be
dibenzothiophene (DBT) and their derivatives.1,2 Furthermore, described as follows. An electron-rich state of nickel exists in
it needs high investment and operating costs, and suffers from nickel boride by transferring a part of electrons of boron to
a significant loss in the octane number caused by saturation of nickel, so the active hydrogen can be absorbed on the surface
olefins.1,2
of nickel boride. Meanwhile, the organosulfur compounds
Nickel boride, a fine, black solid that is easily prepared by also can be absorbed on the surface of nickel boride by com-
the reduction of nickel salts with sodium borohydride (NaBH4) plexation of their sulfur atoms with nickel boride. Then the
in protic solvents, is a useful reducing agent for a variety of active hydrogen and nickel on the surface of nickel boride
functional groups. Its low cost, easy preparation and handling, would form a kind of nickel hydride intermediate. Finally, the
oxidative addition of the C–S bond of organosulfur compounds
to the nickel atom of the nickel hydride intermediate was fol-
lowed by the reductive elimination of C–H, which resulted in
the C–S bond cleavage. Based on this mechanism, it can be
inferred that the high specific surface area of nickel boride can
School of Environmental Science and Engineering, Shanghai Jiaotong University,
Dongchuan Road 800, Shanghai, 200240, People’s Republic of China.
E-mail: sunth@sjtu.edu.cn
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
help to absorb active hydrogen and organosulfur compounds
c4gc00695j
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on the surface of nickel boride and accordingly increase the terized by transmission electron microscopy (TEM) and
desulfurization efficiency. However, the solvents used in the Fourier transform infrared spectrometry (FTIR). The desulfuri-
previous studies were methanol, ethanol or methanol–tetra- zation reactivity of different organosulfur compounds was
hydrofuran (THF) in which the nickel boride in situ generated investigated and their corresponding reaction routes were pro-
easily agglomerated together to form larger clusters. Conse- posed. Furthermore, the effects of different nickel salts, the
quently, the generated nickel boride showed low specific dosage of NaBH4, the oil/IL volume ratio and the water content
surface area, which resulted in low desulfurization efficiency.
In this work, in order to improve the desulfurization
efficiency, an ionic liquid (IL) was used as the solvent instead
of methanol, ethanol or methanol–THF. The use of IL has
several advantages. On the one hand, the nickel boride pre-
pared in IL can be dispersed uniformly to form tiny particles.
Furthermore, some ILs can be adsorbed on the surface of
nickel boride by coordination reaction, which can stabilize the
nickel boride by preventing its agglomeration.30,31 Therefore,
the nickel boride prepared in IL showed high specific surface
area and will bring about high desulfurization efficiency. On
the other hand, IL, an excellent extractant, can be used as an
effective extractant in the desulfurization of fuel oils.4–6 That is
to say, IL will play two roles in the stabilization of nickel
boride and extractive desulfurization during the course of
desulfurization. Fig. 1 shows the present desulfurization
process. Nickel boride is prepared with NaBH4 and nickel salts
(e.g. 4NaBH4 + 2Ni(OAc)2 + 9H2O → Ni2B + 3H3BO3 + 4Na(OAc) +
12.5H2), and then the freshly prepared nickel boride is stabilized
through coordination with IL. Simultaneously, the organosulfur
compounds extracted from the oil phase and the active hydrogen
coming from the hydrolysis of NaBH4 (NaBH4 + H2O → NaBO2 +
H2) are also absorbed on the surface of nickel boride. Then the
organosulfur compounds are desulfurized and transformed into
the corresponding hydrocarbons. Finally, the produced hydrocar-
bons return to the oil phase due to its lessened polarity.
in IL on the desulfurization efficiency were also examined.
Experimental section
Materials
Model diesel fuel with a sulfur content of 500 ppmw was pre-
pared by dissolving BT, 3-methylbenzothiophene (3-MBT),
DBT or 4,6-dimethyldibenzothiophene (4,6-DMDBT) in
n-octane. Real diesel fuel with a sulfur content of 458 ppmw
was supplied by Sinopec Shanghai Petrochemical Company.
The IL [C4mpyr][OTf] was prepared based on the previous
paper.32 The molecular structure of the IL and the nuclear
magnetic resonance (NMR) spectra and spectroscopic data of
the produced IL are given in the ESI.† BT, 3-MBT, DBT,
4,6-DMDBT and n-octane (AR) were purchased from Aladdin
Reagent Co. Ltd (Shanghai, China). NaBH4 (>96%), nickel
acetate tetrahydrate (Ni(OAc)2·4H2O, >98%, AR), nickel chlor-
ide hexahydrate (NiCl2·6H2O, >98%, AR), nickel sulfate hexa-
hydrate (NiSO4·6H2O, >98%, AR) and nickel nitrate
hexahydrate (Ni(NO3)2·6H2O, >98%, AR) were purchased from
Sinopharm Chemical Reagent Co. Ltd (Shanghai, China).
Mutual solubility of the ionic liquid and real diesel fuel
The mutual solubility is an important factor in evaluating the
applicability of an extractant because the noticeable solubility
of ILs in diesel fuels may contaminate the fuel and further
lead to possible NOx pollution. On analyzing the IL-saturated
real diesel fuels by high performance liquid chromatography
(HPLC), no IL peak was found. Therefore, the IL [C4mpyr][OTf]
used here has negligible solubility in real diesel fuels. The
solubility of real diesel fuels in ILs was measured using a gravi-
metric method by weighing the mass difference of a given
amount of ILs and the corresponding ILs saturated with real
diesel fuels. Results showed that the solubility of real diesel
fuels in [C4mpyr][OTf] was 4.3 wt%.
In the current study, we aim to investigate the effect of the
use of IL on the desulfurization performance of nickel boride.
Model and real diesel fuels were desulfurized with nickel
boride prepared from nickel salts and NaBH4. The IL 1-butyl-1-
methylpyrrolidinium trifluoromethanesulfonate ([C4mpyr]-
[OTf]) and a small amount of H2O was selected as the solvent.
The nickel borides prepared in different solvents were charac-
Desulfurization process
A typical desulfurization experiment was carried out in a two-
neck flask. Model or real diesel fuels, IL and NaBH4 were
added into the two-neck flask in turn and the magnetic stirrer
was turned on. A few minutes later, the nickel salts dissolved
in H2O were dripped into the two-neck flask slowly and the
reaction began immediately. All desulfurization experiments
were conducted at ambient temperature (10–30 °C) and
pressure. The desulfurization efficiency was calculated by the
following equation:
TS1 ꢀ TS2
Desulfurization efficiency ðwt%Þ ¼
ꢁ 100%
ð1Þ
TS1
Fig. 1 Schematic diagram of the present desulfurization process.
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where TS1 is the sulfur content in the original diesel fuel
sample and TS2 is the sulfur content in the desulfurized diesel
fuel sample.
Analytical methods
The structures of original and recycled ILs were analyzed by
NMR (Bruker Avance III 400 NMR spectrometer). The IL-satu-
rated real diesel fuel was analyzed by HPLC (Waters, C-18
column, 80/20 methanol–water mobile phase) equipped with a
UV-Vis detector. The nickel borides prepared in different sol-
vents were characterized by TEM (JEM-2010HT, 200KV) and
FTIR (Nicolet-6700, using a thin KBr disc). The sulfur content
in model diesel fuel after desulfurization was determined
Fig. 2 TEM images of nickel borides prepared (a) in methanol, (b) in
methanol–THF (3 : 1) and (c) in IL–H2O.
It is obvious that the nickel borides prepared in methanol and
in methanol–THF agglomerated together to form larger clus-
ters whereas that prepared in IL–H2O dispersed uniformly to
form tiny particles. Therefore, the nickel boride prepared in
IL–H2O showed higher specific surface area compared with
that prepared in methanol or methanol–THF. But why the
nickel boride prepared in IL–H2O did not agglomerate? It can
be explained by the FTIR spectra of IL and nickel boride.
Fig. 3a shows the FTIR spectra of [C4mpyr][OTf]. The absorp-
tion peaks at 3036 cm−1, 2968 cm−1 and 2880 cm−1 can be
assigned to the stretching vibration (CH). 1633 cm−1 can
be assigned to the stretching vibration (CC). 1471 cm−1 can be
assigned to the scissoring vibration (CH3). 1384 cm−1 can be
assigned to the twisting vibration (CH2). 1261 cm−1, 1157 cm−1
and 1031 cm−1 can be assigned to the stretching vibration
using
a
gas chromatography-flame ionization detector
(GC-FID, GC-2010, Shimadzu) equipped with a DB-FFAP capil-
lary column (0.25 mm × 30 m). Analysis conditions were as
follows: the injector temperature was 340 °C and the detector
temperature was 250 °C, and the column temperature was pro-
grammed from 100 °C to 300 °C (8 min) at 15 °C min−1. The
injection sample was 1 µl for all samples. The sulfur content
in real diesel fuel was determined using a sulfur–nitrogen ana-
lyzer (Antek 9000, Antek). The data of the sulfur content for
each sample were presented as the average of three replicates
(data error <5%). The products of various model organosulfur
compounds after desulfurization were analyzed using a gas
chromatograph/mass spectrometer (GC/MS, 7890A-5975C,
Agilent) equipped with a DB-5MS column (30 m × 0.25 mm ×
0.25 µm). Helium was used as the carrier gas at a constant
flow of 1 mL min−1. The injector temperature was 270 °C and
the oven temperature was programmed from 60 °C (5 min) to
300 °C at 20 °C min−1. The split ratio was 200 : 1 and the injec-
tion sample was 0.1 µl for all samples. Mass spectrometry con-
ditions were as follows: ionization voltage 70 eV; ion source
temperature 230 °C; quadrupole temperature 150 °C; full scan
mode in the m/z range 20–400. Compounds were identified by
the use of National Institute of Standards and Technology
(NIST) 2011 Library of Mass Spectra. The organosulfur com-
pounds in original real diesel fuel and desulfurized real diesel
fuel were analyzed by gas chromatography-flame photometric
detection (GC-FPD, GC-2010, Shimadzu) with a DB-5MS capillary
column (30 m × 0.32 mm × 0.25 µm). Analysis conditions were
as follows: the injector temperature was 300 °C and the detector
temperature was 250 °C, and the column temperature was pro-
grammed from 50 °C to 300 °C at 5 °C min−1. The split ratio
was 20 : 1 and the injection sample was 1 µl for all samples.
Results and discussion
Characterization of nickel boride
In order to study the effect of IL on the surface structure and
stabilization of nickel boride, nickel borides were prepared in
methanol, methanol–THF and IL–H2O, respectively. The pre-
pared nickel boride was washed with distilled water many
times and dried under vacuum and then was characterized by
Fig. 3 FTIR spectra of (a) [C4mpyr][OTf], (b) nickel boride prepared in
TEM and FTIR. Fig. 2 shows the TEM images of nickel boride. IL–H2O and (c) nickel boride prepared in methanol.
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(SO2 and CF). 966 cm−1, 930 cm−1 and 825 cm−1 can be compounds compared with that in the previous studies,25,26
assigned to the vibration of the pyrrolidinium ring. 756 cm−1 e.g. the desulfurization efficiency of BT increased from 82% (in
and 639 cm−1 can be assigned to the scissoring vibration the previous studies) to 98% (in this work), the desulfurization
(CF3 and OSO).33,34 All these peaks confirm the structure of efficiency of DBT increased from 83% (in the previous studies)
[C4mpyr][OTf]. Fig. 3b shows the FTIR spectra of the nickel to 97% (in this work) and the desulfurization efficiency of 4,6-
boride prepared in IL–H2O. It is obvious that the nickel boride DMDBT increased from 29% (in the previous studies) to 72%
prepared in IL–H2O has the same characteristic absorption (in this work). These results indicated that the use of IL really
peaks as the IL, which indicates that some IL still remained on improved the desulfurization efficiency. The desulfurization
the surface of nickel boride though nickel boride was washed efficiency of DBT was almost equal to that of BT. But the desul-
with distilled water many times. However, [C4mpyr][OTf] is furization efficiency of 4,6-DMDBT was much less than that of
miscible with water and can be washed off with distilled water. BT and DBT. The desulfurization reactivity of model organosul-
Furthermore, the characteristic absorption peaks of IL could fur compounds followed the order of BT (DBT) > 3-MBT > 4,6-
not be observed in the spectra of nickel boride prepared in DMDBT. This result showed that the desulfurization reactivity
methanol (Fig. 3c). Therefore, it can be inferred that IL was is significantly affected by steric hindrance. On the other
combined with nickel boride by some strong chemical bonds hand, the products of various model organosulfur compounds
instead of simple physical adsorption. In addition, according after desulfurization were analyzed by GC/MS and their corres-
to the previous studies,30,31 the [C4mpyr] cation in IL can coor- ponding reaction routes were proposed. As shown in Fig. 5, the
dinate with nickel boride through the lone pair electrons of reaction routes consisted of direct desulfurization (DDS) and
functional groups on them, so the chemical bonds should be hydrogenation (HYD). The detailed reaction routes of various
coordinate bonds.
model organosulfur compounds were as follows: (i) BT was
The results of TEM and FTIR indicated that the nickel transformed into ethylbenzene (EB) by DDS; (ii) 3-MBT was
boride prepared in IL–H2O showed higher specific surface area transformed into isopropylbenzene (IPB) by DDS; (iii) the
because its agglomeration was prevented through coordinating desulfurization of DBT proceeded by DDS and HYD. The
with IL. High specific surface area can help to absorb active DDS route proceeded over direct cleavage of the C–S bond of
hydrogen and organosulfur compounds on the surface of nickel DBT to biphenyl (BP), followed by hydrogenation to cyclohexyl-
boride and accordingly improve the desulfurization efficiency.
benzene (CHB). The HYD route proceeded over hydrogenation
of DBT to tetrahydrodibenzothiophene (THDBT) and hexa-
hydrodibenzothiophene (HHDBT), followed by cleavage of the
C–S bond to CHB; (iv) the desulfurization of 4,6-DMDBT also
proceeded by DDS and HYD. The DDS route proceeded over
direct cleavage of the C–S bond of 4,6-DMDBT to 3,3′-dimethyl-
biphenyl (3,3′-DMBP), followed by hydrogenation to 3,3′-dimethyl-
cyclohexylbenzene (3,3′-DMCHB). The HYD route proceeded
over hydrogenation of 4,6-DMDBT to 4,6-dimethyltetrahydrodi-
benzothiophene (4,6-DM-TH-DBT) and 4,6-dimethylhexahydro-
dibenzothiophene (4,6-DM-HHDBT), followed by cleavage of
the C–S bond to 3,3′-DMCHB. These results are almost in
agreement with the previous reports.35–40
Desulfurization of different organosulfur compounds
To investigate the effectiveness of the present process for
different organosulfur compounds in diesel fuel, the desulfuri-
zation efficiency of four model organosulfur compounds such
as BT, 3-MBT, DBT and 4,6-DMDBT was tested under similar
conditions. Fig. 4 shows the desulfurization efficiency of four
model organosulfur compounds. Higher desulfurization
efficiency was obtained for all the model organosulfur
Effect of nickel salts
The surface structure of nickel boride varied with preparation
methods. There are three forms of nickel – nickel metal, nickel
oxide and boron bound nickel – on the surface of nickel
boride, but only the boron bound nickel plays a key role in the
desulfurization of organosulfur compounds.41 Furthermore,
different precursors of nickel salts used will result in different
proportions of surface nickels and accordingly affect the desul-
furization activity of nickel boride. Fig. 6 shows the desulfuri-
zation efficiency of model organosulfur compounds by
different nickel salts. As shown in Fig. 6, the desulfurization
efficiency is high for any organosulfur compounds when NiCl2
and Ni(OAc)2 were used as the precursors of nickel salts,
whereas the desulfurization efficiency is low when Ni(NO3)2
was used. The effectiveness of nickel salts followed the order
of NiCl2 (Ni(OAc)2) > NiSO4 > Ni(NO3)2. According to the pre-
vious reports,41 more boron bound nickel could be obtained
Fig. 4 Desulfurization efficiency of BT, 3-MBT, DBT and 4,6-DMDBT.
Conditions: various model diesel fuels with sulfur contents of
500 ppmw were prepared by dissolving BT, 3-MBT, DBT or 4,6-DMDBT
in n-octane, respectively. B/S molar ratio = 9, Ni(OAc)2/S molar ratio = 3,
oil/IL volume ratio = 3, and water content in IL = 5%.
3884 | Green Chem., 2014, 16, 3881–3889
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