Catalytic desulfurization of dibenzothiophene using nickel supported on cross-linked polystyrene-PPh3 catalysts

Article abstract of DOI:10.1016/j.jorganchem.2008.12.019

Nickel catalysts supported on a cross-linked polystyrene-PPh₃ resin were easily prepared in different oxidation states (0 and II) and used (0.1 mol% Ni) in conjunction with MeMgBr to achieve the desulfurization of dibenzothiophene (DBT), result

Full text of DOI:10.1016/j.jorganchem.2008.12.019

Journal of Organometallic Chemistry 694 (2009) 780–784
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Catalytic desulfurization of dibenzothiophene using nickel supported on
cross-linked polystyrene-PPh catalysts
3
Jorge Torres-Nieto, Juventino J. García *
Facultad de Química, Universidad Nacional Autónoma de México, 04510 México, D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Nickel catalysts supported on a cross-linked polystyrene-PPh resin were easily prepared in different oxi-
3
Received 6 November 2008
Received in revised form 28 November 2008
Accepted 3 December 2008
Available online 16 December 2008
dation states (0 and II) and used (0.1 mol% Ni) in conjunction with MeMgBr to achieve the desulfurization
of dibenzothiophene (DBT), resulting in the formation of 2,2 -dimethylbiphenyl as the sulfur-free product
0
(100%). The recycling of these catalysts was addressed in consecutive runs, from which a decrease in cat-
alytic activity was observed within the recycling cycles. Comparison of SEM micrographs of the catalysts
before and after the desulfurization process confirmed a decrease in the size of the polymeric resin par-
ticles as a result of their progressive dissolution into the organic phase, the latter being the major cause of
the decrease in the catalytic activity, associated with gradual loss of the nickel catalysts by leaching.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
Nickel
Desulfurization
Cross-coupling
Dibenzothiophene
1
. Introduction
The removal of sulfur from petroleum and other fossil feedstocks
good yields, which leads to the formation of substituted biphenyls
via C–C cross-coupling reactions [5] (Eq. (2))
R
is a very important global issue in order to decrease atmospheric
pollution caused by sulfur oxides, which are released during the
combustion processes [1]. As a result of this, a large number of
countries have restricted the amount of acceptable sulfur content
present in fuels to very low limits [2]. The process that is used to re-
move sulfur-compounds is known as catalytic hydrodesulfurization
1% [Ni]
RMgX
S
R
ð2Þ
(
HDS) which typically uses cobalt or nickel doped molybdenum sul-
fide catalysts supported over alumina and requires both large
amounts of hydrogen and high temperatures to operate at indus-
trial scale (Eq. (1)) [3]. Additionally, commercial HDS processes of-
ten encounter difficulties in succeeding with the complete removal
of dibenzothiophene (DBT) and its more hindered analogues, 4-
methyldibenzothiophene (4-MeDBT) and 4,6-dimethyldibenzothi-
ophene (4,6-Me DBT); the latter compound exhibit a very low
2
reactivity being in fact the major residual organosulfur component
after HDS present in fuels such as diesel [4]
It is well known that two important disadvantages that homo-
geneous catalysts have with respect to their heterogeneous coun-
terparts are related with the separation and recovery procedures
that are required for these at the end of each process. For these rea-
sons, several supporting materials have been systematically tested
in the intent of allowing the preparation of heterogenized systems
that could be used in the same processes for which the homoge-
neous systems were developed [6]. As particular cases, several
cross-coupling reactions have been reported to be successful when
using palladium catalysts supported on polymeric materials [7]
and also, different types of heterogenized metal complexes have
been employed in a variety of reactions such as olefin metathesis
using ruthenium [8], oxidations using osmium [9] and hydrofor-
mylation and isomerization of olefins using rhodium catalysts
[Cat]
+
H
2
+
H S
2
ð1Þ
S
Recently, our group reported the use of homogeneous nickel
and platinum catalysts that in the presence of alkyl Grignards drive
the desulfurization reaction of DBT, 4-MeDBT and 4,6-Me DBT in
2
[
10]. In all these examples, the recovery of the metal compounds
was achieved readily, also finding a sustained catalytic perfor-
mance upon consecutive recycling. In the case of the HDS reac-
tions, only one example using a polymer supported rhodium
catalyst has been reported to the best of our knowledge. The
catalyst was able to drive the hydrogenolysis of benzothiophene
*
Corresponding author. Tel.: +52 55 56223514; fax: +52 55 56162010.
E-mail address: juvent@servidor.unam.mx (J.J. García).
0
022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.12.019

J. Torres-Nieto, J.J. García / Journal of Organometallic Chemistry 694 (2009) 780–784
781
0
producing 2-ethyl-thiophenol in moderate yields. The sulfur-free
product (ethylbenzene) was obtained only in poor yields (3%) dur-
ing the same process [11].
In the current report we disclose our findings on the catalytic
desulfurization of DBT using nickel catalytic precursors in different
oxidation states (0 and II) which were supported on a cross-linked
of the supported catalyst 1 or 2, selectively producing 2,2 -dimeth-
ylbiphenyl as the sole sulfur-free product (entries 1 and 5). These
results are very similar to those obtained with the homogeneous
systems that were reported before by our group [5], therefore sug-
gesting that the catalytic activity of the heterogenized systems used
herein is maintained when compared with those exhibited by the
homogeneous ones. To note, no desulfurization activity was ob-
served when using THF as the solvent regardless of the use of
greater catalyst loadings (entries 2 and 6), the latter results being
similar to the one depicted by the homogenous systems when used
in the presence of polar solvents that result in significant diminish-
ment or even halting of, the catalytic activity. In both cases, the
inhibition of the catalytic process may be due as a result of THF
coordination to the metal centers, that in the case of the heterog-
enized systems is rationalized as a passivation of the catalyst active
sites within the polymer matrix. Consistent with this conclusion,
the activity of the supported systems was not decreased when
using non-polar higher boiling point solvents such as o-xylene (en-
tries 3 and 7) or mesitylene (entries 4 and 8), indicating that it is
indeed the greater coordinating capability of THF that ultimately
inhibits the process.
3
polystyrene-PPh resin. This heterogenized systems showed recy-
0
0
cling ability to produce 2,2 -dimethylbiphenyl (2,2 -DMBP) as the
sole sulfur-free product.
2
. Results and discussion
The nickel catalysts were easily synthesized by stirring a solu-
tion of 1 equiv. of the nickel precursor, [(COD)
1) or [(PPh NiCl ] for the Ni(II) (2) catalyst, additionally adding
equiv. of the cross-linked polystyrene-PPh resin which yielded
2
Ni] for the Ni(0)
(
3
)
2
2
4
3
a colored solid (red-wine and brown, respectively) that contained
the supported nickel center (Eq. (3)). The nickel content was deter-
mined by atomic absorption spectroscopy (AAS)
(
COD) Ni
2
C H PPh Ni(0)
(1, 3.67 %Ni)
6
4
2
THF, R.T.
2
DBT
.2. Catalyst recycling experiments for continuous desulfurization of
(
Wine)
PPh3
The recycling capability of the nickel supported catalysts was
also addressed for all the such conditions in which DBT desulfuriza-
tion was found to be successful. The experiments were conducted
using 1 equiv. of catalyst, 1000 equiv. of DBT and 2000 equiv. of
MeMgBr, heating the reaction mixture during 5 days in refluxing
toluene. After such time, the heating was stopped and the solution
filtered via cannula to analyze conversions by means of GC–MS,
leaving the insoluble catalyst and other solid residues which are
(
PPh ) NiCl
3 2 2
C H PPh NiCl
2
(2, 2.61 %Ni)
6
4
2
(
Acetone:EtOH), R.T.
(
Brown)
ð3Þ
mainly composed by MgBr
the final fate of the sulfur. Fresh loadings of 1000 equiv. of DBT and
000 equiv. of MeMgBr were then added to them and the resulting
2
and MgS [5], the latter compound being
2.1. Catalytic desulfurization of DBT with nickel supported catalysts
2
The catalytic desulfurization reactions using the supported
mixtures heated again for 5 days. This process was consecutively
repeated until the complete loss of catalytic activity was encoun-
tered. The results for the recycling capability of both catalysts 1
and 2 in toluene are presented in Fig. 1.
The two types of nickel catalysts exhibited very similar perfor-
mances during the recycling experiments promoting 100% desul-
furization of DBT, only in the first run. After the first recycle the
activity of the catalysts dropped to 70% and 50%, respectively, pro-
nickel catalysts 1 and 2 were performed in a variety of conditions,
the representative results of which are presented in Table 1.
As indicated in Table 1, the results obtained with both nickel
catalysts (1 and 2) were very similar regardless the difference in
the oxidation state of the nickel center in each of these (0 and II,
vide infra). The complete desulfurization of DBT was observed
when the reaction was carried out in toluene using 0.1 mol% Ni
Table 1
Catalytic desulfurization of dibenzothiophene with nickel supported catalysts.
PS
C H
PPh2 (Ni)
6
4
Me
(
0.1 mol% Ni)
+
2 MeMgBr
S
Me
Entry
Catalyst (equiv.)
Thiophene (equiv.)
Grignard (equiv.)
Solvent
Organics (%)
1
2
3
4
5
6
7
8
1 (1)
1 (1)
1 (1)
1 (1)
2 (1)
2 (1)
2 (1)
2 (1)
DBT (1000)
DBT (100)
DBT (1000)
DBT (1000)
DBT (1000)
DBT (100)
DBT (1000)
DBT (1000)
MeMgBr (2000)
MeMgBr (200)
MeMgBr (2000)
MeMgBr (2000)
MeMgBr (2000)
MeMgBr (200)
MeMgBr (2000)
MeMgBr (2000)
Toluene
THF
o-Xylene
Mesitylene
Toluene
THF
MePh–PhMe (100)
DBT (100)
MePh–PhMe (100)
MePh–PhMe (100)
MePh–PhMe (100)
DBT (100)
o-Xylene
Mesitylene
MePh–PhMe (100)
MePh–PhMe (100)
All reactions were carried out under reflux of their corresponding solvent for 5 days, typically using 6.600
by GC–MS, after work up.
lmol of the corresponding nickel catalyst. All yields were quantified

7
82
J. Torres-Nieto, J.J. García / Journal of Organometallic Chemistry 694 (2009) 780–784
100
100
90
80
70
60
50
40
30
20
10
0
A
90
80
70
60
50
40
30
20
10
0
B
1
2
3
4
1
2
3
4
Cycle
Cycle
Fig. 1. Catalytic desulfurization of DBT using nickel supported catalysts (0.1 mol% Ni), in toluene. (A) Ni(0) catalyst (1); (B) Ni(II) catalyst (2).
1
00
A rationale for the similarities in performance exhibited by both
catalysts can be established in terms of the reactivity that the nick-
el(II) centers exhibit when exposed to the presence of Grignards,
from which the nickel (II) catalyst (2) may produce a dimethylated
compound by metathesis with 2 equiv. of MeMgBr that decom-
poses to an active nickel(0) entity, namely 1, as a result of reductive
elimination of ethane (Eq. (4)), implying that both systems evolve
to common intermediates for the desulfurization process, thereaf-
ter. To note, a closely related proposal for the synthesis of Ni(0)
moieties from Ni(II) precursors has been reported by Pörschke
and co-workers [12].
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
1
2
3
4
5
6
MeMgBr
Δ
(
C H )-PPh NiCl
2
(C H )-PPh NiMe
(C H )-PPh Ni(0)
Cycles
6
4
2
6
4
2
2
-
3 3
(CH -CH )
6
4
2
(
2)
(1)
Fig. 2. Catalytic desulfurization of DBT using the nickel(0) supported catalyst, 1, in
mesitylene.
ð4Þ
Also worth to note, the use of solvents with higher boiling point
such as mesitylene for the catalytic desulfurization experiments
starting with compound 1 (0.1 mol% Ni) allowed us to increase
gressively resulting in smaller conversions of 33% and 20% and 12%
and 3% in the third and fourth runs.
Fig. 3. SEM images of the supported nickel catalysts before, (A) 1, (B) 2, and after 4 desulfurization cycles using toluene as solvent, (C) 1 (D) 2.

J. Torres-Nieto, J.J. García / Journal of Organometallic Chemistry 694 (2009) 780–784
783
the number of recycled runs to a total of six (see Fig. 2), a result
4.1. Preparation of 1
that may be derived from a temperature-enhanced mass transfer
process that adds for the overall system’s efficiency.
To a 50 mL Schlenk flask was added [(COD)
2
Ni] (0.11 g,
0
3
.4 mmol) and dissolved with 10 mL of dry THF. The PPh polymer
2
.3. Scanning electron microscopy studies of the catalysts
bound resin (1 g, 1.6 mmol) was added to the mixture under con-
stant stirring and was left to stir for 4 h, during which time the col-
or of the solution changed from pale-yellow to becoming almost
colorless, the suspended resin also turning red-wine. The latter
was filtered and washed 5 times with dry THF (5 mL) and then
dried for 5 h under vacuo. The nickel content (3.87%) was deter-
mined by AAS digesting the sample in a microwave oven reactor
Scanning electron microscopy (SEM) studies of the nickel sup-
ported catalysts, 1 and 2, were carried out before and after the
desulfurization experiments in order to understand the decrease
in the catalytic activity encountered during the recycling experi-
ments. Fig. 3 illustrates representative micrographs of the resem-
blance of polymer particles in these two circumstances.
2 4 3
using H SO and then HNO .
Before the desulfurization process was undergone, the two cat-
alysts exhibited a particle size distribution ranging between 60 and
4.2. Preparation of 2
1
50 lm, labeled as A and B in Fig. 3. However, after the recycling
process (4 cycles) using toluene as solvent, the same catalyst par-
ticles presented a dramatically different particle size, decreasing to
A 50 mL Schlenk flask was charged with [(PPh
0.4 mmol) and dissolved with 10 mL of a mixture of acetone:EtOH
(2:1) to give a green solution. The PPh polymer bound resin (1 g,
3 2 2
) NiCl ] (0.26 g,
less than 20
lm in both cases (indicated as C and D). The change in
3
particle size has been attributed to the dissolution of the support-
ing polymer in the organic phase while undergoing desulfurization,
thereby also enabling the loss of active metal centers as a result of
leach of nickel into the reaction media, ultimately resulting in the
progressive diminishment of the catalytic activity that is reflected
over time after continuous recycling runs.
1.6 mmol) was added to the solution under constant stirring fol-
lowing the procedure described above, during which the resin
turned dark-brown and the remaining solution became almost col-
orless. The resin was filtered and washed 5 times with a mixture of
acetone:EtOH (2:1 v/v, 5 mL) and then dried for 5 h in vacuo. The
nickel content (2.61%) was determined by AAS following the previ-
ous procedure.
3
. Conclusions
4.3. Catalytic desulfurization experiments
The current report presents the catalytic desulfurization process
A 50 mL Schlenk flask was charged in a glovebox with the corre-
sponding supported nickel catalyst (6.59 lmol Ni) and suspended
of DBT using polymer supported nickel catalysts in two different
oxidation states (0 and II), which in conjunction with MeMgBr
in 8 mL of dry solvent. Then, to it was added DBT (6.59 mmol)
and allowed to stand at room temperature for 30 min under con-
stant stirring. After complete mixing a solution of the Grignard re-
agent (13.18 mmol) was added; no changes in color were observed.
The reaction mixture was heated to reflux under argon in a gas/vac-
uum line, constantly stirred during 5 d. During this time a beige
precipitate was observed. The heating was stopped and the sample
hydrolyzed at room temperature with HCl (10 mL, 10% vol.). A
strong effervescence was observed with the immediate release of
0
yield the formation of 2,2 -dimethylbiphenyl, selectively. The recy-
cling ability of these systems was assessed, allowing at least four
consecutive catalytic runs before complete loss of activity. The
use of higher boiling point non-coordinating solvents such as
mesitylene permitted to extend the effective lifetime of the cata-
lysts, resulting in a larger amount of recycles. SEM studies of the
catalysts before and after the desulfurization process allowed to
conclude that a reduction of the particle’s size takes place during
the desulfurization process as a result of progressive dissolution
of the polymer matrix, which is directly related with the loss of
catalytic activity exhibited by the heterogenized catalysts within
consecutive runs in recycling experiments. Studies are underway
to extend this methodology to other sterically hindered DBT
analogues.
H
2
S (Caution, poisonous gas!), which was usually bubbled into a trap
with 50 mL of an aqueous concentrated solution of Pb(CH COO)
After bubbling and venting all the gases, all organics proceeding
from the reaction mixture were extracted using CH Cl
3
2
.
2
2
(3 ꢁ 5 mL)
and the organic layer analyzed by GC–MS. The samples prepared
for SEM analysis were not hydrolyzed but centrifuged during a per-
iod of 10 min at 5000 cycles/min. Then, the supernatant solution
was decanted leaving a grayish solid residue that was further dried
for 5 h in vacuo.
4
. Experimental
All manipulations were carried out using standard Schlenk and
glovebox techniques under argon (Praxair, 99.998). THF (J.T. Baker)
was dried and distilled from dark purple solutions of sodium/ben-
zophenone ketyl. Toluene, o-xylene and mesitylene (J.T. Baker)
were dried and distilled from sodium. Acetone was dried and dis-
4.4. Recycling desulfurization experiments
A 50 mL Schlenk flask was charged in a glovebox with the cor-
responding nickel catalyst (6.59 lmol Ni) and suspended with
tilled over K
Triphenylphosphine polystyrene bound resin (cross-linked with 2%
DVB; 200–400 mesh; 1.6 mmolPPh /g resin), DBT and [(COD) Ni]
were purchased from Aldrich, dried in vacuo and used without fur-
ther purification. MeMgBr (3.0 M in diethyl ether solution) was pur-
2
CO
3
. Ethanol was dried and distilled over magnesium.
8 mL of dry solvent, adding DBT (6.59 mmol) with constant stirring
during 30 min. After complete mixing a solution of the Grignard re-
agent (13.18 mmol) was added; no changes in color were observed.
The reaction mixture was heated to reflux under argon in a gas/
vacuum line, constantly stirred during 5 d; a beige precipitate
was observed during this time. The heating was stopped and the
solution filtered via cannula and the solid residue washed
3 ꢁ 10 mL with the corresponding solvent (toluene or mesitylene).
The solution was analyzed by GC–MS. Then, the reaction was
reconstituted with DBT (6.59 mmol), and MeMgBr (13.18 mmol)
using 8 mL of dry solvent and heated again to reflux under argon
in a gas/vacuum line, constantly stirred for 5 d. The same proce-
dure was repeated until complete loss of catalytic activity.
3
2
chased from Aldrich and used as received. PPh
3
and NiCl
2
2
ꢀ 6H O
were purchased from Aldrich and were also used without further
purification. GC–MS determinations were performed using a Varian
Saturn 3 on a 30 m DB-5MS capillary column. Catalysts were char-
acterized by SEM analyses, undertaken using a Jeol JSM 5900 LV
microscope. Atomic absorption spectroscopy (AAS) analyses of the
nickel content were undertaken on a Varian Spectra AA 220
instrument.

7
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J. Torres-Nieto, J.J. García / Journal of Organometallic Chemistry 694 (2009) 780–784
(
(
(
c) R.J. Angelici, Organometallics 20 (2001) 1259;
d) D.D. Whitehurst, T. Isoda, I. Mochida, Adv. Catal. 42 (1998) 345;
e) R.J. Angelici, Polyhedron 16 (1997) 3073;
Acknowledgements
We thank CONACyT (080606) and DGAPA-UNAM (IN-202907-
(f) B.C. Gates, H. Topsoe, Polyhedron 16 (1997) 3213;
(g) X. Ma, K. Sakanishi, I. Mochida, Ind. Eng. Chem. Res. 33 (1994) 218.
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García, Organometallics 23 (2004) 4534;
3
) for their financial support to this work. We thank Iván Puente-
Lee (USAI-UNAM) for SEM determinations, and Araceli Tovar
USAI-UNAM) for AAS determinations. J. T.-N. also thanks CONA-
[
(
(
b) J. Torres-Nieto, A. Arévalo, J.J. García, Organometallics 26 (2007) 2228.
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413;
b) B.E. Hanson, R.B. King, Encyclopedia of Inorganic Chemistry, 2nd ed., John
Wiley & Sons, New York, 2005;
CyT for a Ph. D. studies grant.
1
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Appendix A. Supplementary material
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e) C.A. McNamara, M.J. Dixon, M. Bradley, Chem. Rev. 102 (2002) 3275.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2008.12.019.
[7] (a) See for instance: L. Bai, J.X. Wang, Adv. Synth. Catal. 350 (2008) 315;
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