Catalytic desulfurization of dibenzothiophene with palladium nanoparticles

Article abstract of DOI:10.1021/ic801550j

The thermal reduction of [(PEt₃)₂PdMe₂] (1 mol%), which was produced in situ from [(PEt₃)₂PdCl ₂] (1) and 2 equiv of MeMgBr in toluene solvent, yielded palladium nanoparticles that in conjunction with MeMgBr effected the desulfurization of dibenzothiophene (DBT). The reaction resulted in the generation of the sulfur-free compound 2,2′-dimethylbiphenyl, in high yields (60%). The use of several stabilizing agents such as sodium 2-ethylhexanoate and hexadecylamine was also addressed herein, their use resulting in a significant improvement of the desulfurization reaction that reached up to 90% conversion of DBT into the mentioned biphenyl. The palladium nanoparticles formed during the reaction were characterized by transmission electron microscopy and exhibited a smaller size and a lesser extent of agglomeration whenever stabilizers were used.

Full text of DOI:10.1021/ic801550j

Inorg. Chem. 2008, 47, 11429-11434
Catalytic Desulfurization of Dibenzothiophene with Palladium
Nanoparticles
Jorge Torres-Nieto, Alma Ar e´ valo, and Juventino J. Garc ´ı a*
Facultad de Qu ´ı mica, UniVersidad Nacional Aut o´ noma de M e´ xico, M e´ xico, D.F. 04510
Received August 13, 2008
3 2 2 3 2 2
The thermal reduction of [(PEt ) PdMe ] (1 mol%), which was produced in situ from [(PEt ) PdCl ] (1) and 2 equiv
of MeMgBr in toluene solvent, yielded palladium nanoparticles that in conjunction with MeMgBr effected the
desulfurization of dibenzothiophene (DBT). The reaction resulted in the generation of the sulfur-free compound
2,2′-dimethylbiphenyl, in high yields (60%). The use of several stabilizing agents such as sodium 2-ethylhexanoate
and hexadecylamine was also addressed herein, their use resulting in a significant improvement of the desulfurization
reaction that reached up to 90% conversion of DBT into the mentioned biphenyl. The palladium nanoparticles
formed during the reaction were characterized by transmission electron microscopy and exhibited a smaller size
and a lesser extent of agglomeration whenever stabilizers were used.
Introduction
Environmental regulations regarding the sulfur content in
fuels have become more stringent in many countries during
the past few years and because of this a considerable effort
has been expended to improve the efficiency of catalysts used
for the hydrodesulfurization (HDS) process in the oil
2
yldibenzothiophene (4,6-Me DBT), from oil (the latter being
the major causes for the high sulfur levels in it) pose
important challenges to the oil industry given their resistance
1
industry. The presence of sulfur compounds in fuels is
4
undesirable because their combustion results in formation
of sulfur oxides which, after being released into the
to use HDS. The use of noble metal sulfides generally
improves the catalytic activities in this reaction, according
2
5
atmosphere, cause acid rain. Normally, the HDS process
to model experiments using reactor vessels, and in the case
involves the use of cobalt or nickel doped molybdenum
sulfide catalysts supported over alumina that yield sulfur-
of heterogeneous HDS catalysts the use of platinum group
metals has indeed yielded better performances. The use of
ruthenium, rhodium, and iridium based catalysts has permit-
ted the desulfurization of a reasonable number of sulfur
2
free hydrocarbons and H S (eq 1), but the process requires
3
extensive use of hydrogen at high temperatures.
The removal of sulfur heterocyclic compounds such as
dibenzothiophene (DBT) and its more hindered analogues,
6
compounds in good yields and in the case of heterogeneous
catalysts those bearing palladium and platinum metal centers
have reached similar or even better HDS activities than
4
-methyldibenzothiophene (4-MeDBT) and 4,6-dimeth-
*
To whom the correspondence should be addressed. E-mail:
7
commercial catalysts.
juvent@servidor.unam.mx.
(
(
(
1) U.S. Environmental Protection Agency (http://www.epa.gov/otaq/
gasoline.htm). European Union, EU Directive 98/70/EC, 1998.
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Eds.; John Wiley & Sons: New York, 1994; p 1433.
In the last few decades, metal nanoparticles have become
an important alternative for heterogeneous catalysis given
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Science and Technology; Springer-Verlag: Berlin, 1996. (b) Kabe, T.;
Ishihara, A.; Qian, W. Hydrodesulfurization and Hydrodenitrogena-
tion: Chemistry and Engineering; Kondasa-Wiley-VCH: Tokyo, 1999.
4) (a) Sanchez-Delgado, R. A. Organometallic Modeling of the Hy-
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Chem. Eng. J. 2006, 123, 15. (c) Ishihara, A.; Dumeignil, F.; Lee, J.;
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(
3
073.
1
0.1021/ic801550j CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 23, 2008 11429
Published on Web 10/16/2008

Torres-Nieto et al.
8
the variety of reactions in which they can be used. Several
the above-described case and yield the formation of disub-
stituted biphenyls. Results are significant as they illustrate
one of the very few existing cases in which palladium
catalysts are used for this reaction.
examples of C-C coupling reactions catalyzed by palladium
8
a,b,9
nanoparticles have been reported.
The use of metal
nanoparticles in HDS reactions has been studied with a
variety of transition metals such as Mo, Ni, Ni/Fe, Ni/W,
Mo/W, and Mo/Co, which are able to hydrodesulfurize
thiophene (T) and DBT in good yields, although only poor
HDS yields have been afforded in the case of 4,6-Me
Recently, Schlaf et al. have shown that palladium nanopar-
ticles produced by reduction of palladium salts such as PdCl
Pd(dba) , and Pd(OAc) are able to cleave the C-S bond of
aromatic thioethers in the presence of silanes. The reaction
Results and Discussion
10
2
DBT.
Desulfurization of Dibenzothiophene with Palladium
Nanoparticles. The reaction of cis-[(PEt
equiv of MeMgBr at room temperature resulted in the
quantitative formation of cis- and trans-[(PEt PdMe ], in
( P{ H} NMR signals for cis- and trans-
PdMe ] being δ 10.42 (s) and 18.56 (s), respectively).
The thermal decomposition of the two methylated com-
3 2 2
) PdCl ] (1) and 2
2
,
3
)
2
2
3
2
3
1
1
toluene-d
[(PEt
8
1
1
produces thiosilanes and silthianes in DMA (eq 2).
3
)
2
2
3
1
1
pounds was followed by P{ H} NMR spectroscopy. The
followup confirmed the disappearance of the corresponding
3
1
1
P{ H} NMR signals after continuous heating at 100 °C,
which coincided with the apparition of a black metallic
precipitate in the NMR tube, unequivocally identified as
palladium black. Catalytic desulfurization experiments over
DBT were then carried out assuming 1 to behave as a
nanoparticle precursor. The catalytic desulfurization reaction
of DBT in the presence of two additional equivalents of
MeMgBr and complex 1 (1 mol%) under toluene reflux
yielded in all cases the cross-coupling product 2,2′-dimeth-
ylbiphenyl as the sole product, although variable amounts
of DBT were still found to remain in solution in these
experiments. Table 1 summarizes the catalytic experiments
that were carried out. Indications for compounds 2, 3, and 4
in the table refer to the use of additional palladium nano-
We recently reported the use of soluble nickel and
platinum compounds that allow the desulfurization of DBT
and its more hindered analogues 4-MeDBT and 4,6-Me DBT.
2
The reaction occurred in the presence of Grignard reagents
which yield both substituted and unsubstituted biphenyls in
1
2
high yields (eq 3).
particle precursors, the latter being [(PPh
(dippe)PdCl ] (3), and [(dppe)PdCl ] (4), respectively.
The desulfurization of DBT using THF (entry 1) did not
3 2 2
) PdCl ] (2),
[
2
2
Herein we report the use of palladium nanoparticles
formed by the thermal reduction of (PEt PdMe in the
desulfurization of DBT, also using MeMgBr. The desulfu-
)
3 2
2
result in any catalytic activity, although the use of toluene
did result in formation of the C-C cross-coupled product,
2,2′-dimethylbiphenyl, in 45% yield (entry 2). The difference
in reactivity was attributed to the more coordinating nature
of THF, which lowers the desulfurization activity as observed
in earlier experiments concerning this reaction when using
both nickel and platinum based catalysts, under homogeneous
conditions. In particular, the latter behavior was explained
at the time in terms of a decoordination of the bound
thiophene from the metal center, which is driven by solvent
rization occurs via C-C cross-coupling reaction similar to
(
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(
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1
2
displacement. The same explanation is proposed in the
current case. Noteworthy, a severe decrease of the catalytic
activity was observed whenever solvents with higher boiling
points, such as o-xylene and mesitylene, were used (entries
(
(
8) See for instance: (a) Astruc, D. Inorg. Chem. 2007, 46, 1884. (b)
Moreno-Ma n˜ as, M.; Pleixats, R. Acc. Chem. Res. 2003, 36, 638. (c)
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Tetrahedron 2005, 61, 8633.
3
-4). The use of 300 equiv of MeMgBr (entry 5) improved
in the extent of desulfurization to 60%. Use of an even more
concentrated system (400 equiv) did not result in any further
(11) (a) Chung, M. K.; Schlaf, M. J. Am. Chem. Soc. 2004, 126, 7386. (b)
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(
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1
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11430 Inorganic Chemistry, Vol. 47, No. 23, 2008

Catalytic Desulfurization of Dibenzothiophene
a
Table 1. Desulfurization of DBT using Palladium Nanoparticles and Grignard Reagents
entry
complex (equiv)
thiophene (equiv)
Grignard (equiv)
solvent
THF
organics (%)
DBT (100%)
MePh-PhMe (45%); DBT (55%)
MePh-PhMe (10%); DBT (90%)
MePh-PhMe (8%); DBT (92%)
MePh-PhMe (60%); DBT (40%)
MePh-PhMe (60%); DBT (40%)
DBT (100%)
DBT (100%)
DBT (100%)
DBT (100%)
DBT (100%)
1
2
3
4
5
6
7
8
9
1 (1)
1 (1)
1 (1)
1 (1)
1 (1)
1 (1)
1 (1)
1 (1)
2 (1)
3 (1)
4 (1)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
MeMgBr (200)
MeMgBr (200)
MeMgBr (200)
MeMgBr (200)
MeMgBr (300)
MeMgBr (400)
EtMgBr (300)
toluene
o-xylene
mesytilene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
i
PrMgCl (300)
MeMgBr (300)
MeMgBr (300)
MeMgBr (300)
1
1
0
1
a
All reactions were carried out under solvent reflux for 5 days, typically using 0.012 mmol of the corresponding palladium nanoparticle precursor. All
yields in the table were determined by GC-MS, after work up.
a
Table 2. Desulfurization of DBT Using Palladium Nanoparticles and Grignard Reagents in the Presence of Stabilizing Agents
entry
precursor (equiv)
stabilizer (equiv)
thiophene (equiv)
Grignard (equiv)
solvent
organics (%)
1
2
3
4
5
6
7
8
9
1 (1)
1 (1)
1 (1)
1 (1)
1 (1)
1 (1)
1 (1)
1 (1)
1 (1)
1 (1)
5 (0.1)
6 (5)
7 (5)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
DBT (100)
MeMgBr (300)
MeMgBr (300)
MeMgBr (200)
MeMgBr (300)
MeMgBr (300)
MeMgBr (300)
MeMgBr (200)
MeMgBr (300)
MeMgBr (300)
MeMgBr (300)
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
MePh-PhMe (15%); DBT (85%)
MePh-PhMe (25%); DBT (75%)
MePh-PhMe (70%); DBT (30%)
MePh-PhMe (90%); DBT (10%)
MePh-PhMe (17%); DBT (83%)
MePh-PhMe (2%); DBT (98%)
MePh-PhMe (70%); DBT (30%)
MePh-PhMe (90%); DBT (10%)
MePh-PhMe (16%); DBT (84%)
DBT (100%)
7 (5)
7 (2.5)
7 (10)
8 (5)
8 (5)
8 (2.5)
8 (10)
1
0
a
All reactions were carried out under solvent reflux for 5 days, typically using 0.012 mmol of the corresponding palladium nanoparticle precursor. All
yields were determined by GC-MS, after work up.
Scheme 1. Stabilizers Used for the Catalytic Desulfurization of DBT Employing Palladium Nanoparticles
improvement (entry 6). In terms of the nature of the particular
Grignard used, it is worth noting that with the more bulky
the ones, EtMgBr and PrMgCl, no desulfurization was
catalysts, this precedent encouraged us to use a number of
commercially available stabilizing agents in the catalytic des-
ulfurization of DBT using palladium nanoparticles. In principle,
the improvement in activity is mostly due to the coordination
of these stabilizers to the nanoparticles, which inhibit ag-
glomeration and prevent their precipitation from the reaction
i
observed (entries 7-8). Assessment of different nanoparticle
precursors to 1, such as 2, 3, or 4 (mentioned above), did
not result in any desulfurization activity (entries 9-11),
thereby confirming compound 1 as the best choice of catalyst
precursor. All suspensions produced from desulfurization
experiments were centrifuged (10 min at 5000 cycles/min)
in order to recover the corresponding palladium precipitates
and analyze them by TEM, the supernatant solutions were
tested in desulfurization experiments, but no activity was
observed. The TEM determinations were done with the aim
of obtaining proof of the formation of palladium nanopar-
ticles as the likely catalysts for the desulfurization reactions.
The results obtained are discussed further in the text (vide
infra).
13-15
media.
Scheme 1 illustrates the structures of the stabilizers
that were used and Table 2summarizes the catalytic desulfur-
ization experiments undertaken in their presence.
(13) See for instance Schmid, G. In Encyclopedia of Inorganic Chemistry,
2
nd ed; King, R. B., Eds.; John Wiley & Sons: New York, 2005.
(
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Mater. Chem. 2002, 12, 934. (h) Niu, Y.; Yeung, L. K.; Crooks, R. M.
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Y. L.; Jang, S. W.; Chou, P. T.; Wang, C. R. C.; Yu, S. J. J. Catal.
Desulfurization of DBT Using Palladium Nanopar-
ticles and Stabilizer Agents. Considering the fact that
stabilizer agents have permitted notable improvements in
activity whenever metal nanoparticles have been used as
2
000, 195, 336. (j) Chen, S.; Huang, K.; Stearns, J. A. Chem. Mater.
2000, 12, 540.
Inorganic Chemistry, Vol. 47, No. 23, 2008 11431

Torres-Nieto et al.
Figure 1. TEM images (20 nm scale) of palladium nanoparticles recovered from desulfurization processes undergone in the absence of a stabilizing agent.
Nanoparticles formed: (A) from 1, (B) from 2, and (C) from 3.
As presented in Table 2, the desulfurization of DBT using
polyvinylpyrrolidone (PVP, 5) and sodium dodecylsulfate
nanoparticles recovered from a series of experiments under-
taken in the absence of stabilizing agents.
(
6) as the stabilizing agents did not show any enhancement
According to the micrographs shown in Figure 1, a
conclusion that follows is that there are not important
differences between palladium nanoparticles generated from
either precursor 1 or 2 (labeled as A and B in the figure), as
they exhibit diameter averages of 4 nm and in general, a
very similar shape, even though the catalytic activity that is
ultimately displayed from these was in fact different (see
entries 5 and 9 of Table 1). It is therefore thought that the
difference in catalytic activity among these two systems may
be adduced to important differences in the degree of
stabilization that is afforded over the nanoparticle-related
catalytic intermediates, probably affected by the basicity of
the particular phosphine that was present in the catalyst
in the reaction yield. In fact, the two reactions presented a
severe decrease in the catalytic activity when compared with
the nonstabilized system (entries 1-2). The result in turn
was attributed to the presence of reactive groups in the
stabilizers, which may be subject to nucleophilic attack by
MeMgBr, ultimately leading to poorly active nanoparticles.
The use of 5 equiv of sodium 2-ethylhexanoate (7) resulted
in an improved catalytic system when compared to the
nonstabilized ones (entries 3-4). A 90% desulfurization of
DBT was achieved when using this agent. The use of a
smaller amount of 7 (2.5 equiv) resulted in a dramatic
decrease in desulfurization (entry 5), which was also
observed when concentrating it, instead (10 equiv; entry 6).
As a result, these experiments showed that the relative ratio
by which stabilizers are used with respect to the palladium
nanoparticle precursor may severely affect the final outcome
of catalysis. The use of hexadecylamine (8) showed results
very similar to those of 7 (entries 7-10). An improvement
in the desulfurization was achieved with the use of 5 equiv
of this compound, which also exhibited a decrease in catalytic
activity when modifying its concentration (2.5 or 10 equiv,
likewise; entries 9 and 10). All the palladium nanoparticles
that were formed during these reactions were also analyzed
by TEM, following the procedure described in the previous
section. Their micrographs are depicted later in the text (vide
infra).
1
5
precursor. Only the precursor containing the more basic
phosphine (PEt ) exhibited any catalytic activity. In the case
3
of compound 3, the nanoparticles that were formed exhibited
a larger average size of 6 nm and may be the probable cause
for the null activity encountered with this system. To note,
all the catalytically active systems exhibited nanoparticles
with an average diameter of 4 nm or less. Related to this
observation, samples recovered from reactions assessed at
different reaction times permitted the observation of nano-
particle-agglomeration, which may also be reflected in a
decrease in catalytic activity. Figure 2 illustrates a series of
micrographs which depict the appearance of aggregates of
nanoparticles after 1, 3, and 5 days.
The agglomeration of palladium nanoparticles takes place
as a function of time and a considerably larger size of the
total entity may be found after 5 days of reaction (C). The
latter is consistent with a less active system over time,
confirmed by the presence of unreacted DBT in all the
experiments in which no stabilizer was used. Complementary
to this conclusion, the same type of TEM analysis performed
over nanoparticles produced from 1 in the presence of
stabilizing agents confirmed the extent of agglomeration to
be smaller, accompanied by a better catalytic performance
of the catalysts in the desulfurization process overall
Analysis of Palladium Nanoparticles Formed during
Desulfurization Processes. Precipitates formed during des-
ulfurization reactions were analyzed at the end of these
processes with the use of TEM, which allowed confirmation
of the presence of palladium nanoparticles as the actual
catalysts. Figure 1 illustrates the TEM images of palladium
(
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2
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11432 Inorganic Chemistry, Vol. 47, No. 23, 2008

Catalytic Desulfurization of Dibenzothiophene
Figure 2. TEM images (20 nm scale) of palladium nanoparticles produced from 1, taken at different reaction times: (A) 1 day, (B) 3 days, (C) 5 days.
Figure 3. TEM images of palladium nanoparticles produced from 1 after the desulfurization process, in the presence of a stabilizer. (A) Sodium 2-ethylhexanoate
and (B) hexadecylamine.
(
compare results in Table 2 with those in Table 1). Figure 3
of nanoparticle agglomeration. Studies are underway to
expand the scope of this reaction to closely related S- and
N-containing substrates.
illustrates two of the TEM images obtained from the systems
in which a stabilizer was used, comparing between (A)
sodium 2-ethylhexanoate with (B) hexadecylamine.
As noted in Figure 3, the use of stabilizers promoted the
formation of smaller diameter nanoparticles (average diam-
eter, 3 nm), unlike to the agglomerates obtained over time
in the case of unstabilized systems. The use of stabilizers
prevented the agglomeration of the metal particles and as
such the better performance of the catalytic systems that was
encountered in the presence of the former is consistent with
this fact.
Experimental Section
All manipulations were carried out using standard Schlenk and
1
6
glovebox techniques, under argon (Praxair 99.998). THF (J. T.
Baker) was dried and distilled from dark purple solutions of sodium/
benzophenone ketyl. Toluene, o-xylene and mesitylene (J. T. Baker)
were dried and distilled from sodium. MeMgBr (3.0 M in diethyl
ether solution) was purchased from Aldrich and was used as
2
received. DBT, (COD)PdCl , PVP, sodium dodecylsulfate, sodium
2
-ethylhexanoate and hexadecylamine were purchased from Aldrich,
were dried in vacuo, and were used without further purification.
PEt , PPh , and dppe were purchased from Aldrich and were also
used without further purification. 1,2-Bis(diisopropylphosphino)et-
Conclusions
3
3
The current work demonstrates an efficient palladium
nanoparticle system that is useful for the catalytic desulfu-
rization of DBT. The reaction allows the selective formation
of 2,2′-dimethylbiphenyl by means of a cross-coupling
reaction with MeMgBr. The use of stabilizing agents permits
the onset of a greater catalytic performance of the nanopar-
ticles as a result of a smaller diameter size, and a lesser extent
1
7
hane (dippe) was prepared as reported. Deuterated solvents were
(16) Vicic, D. A.; Jones, G. D. In ComprehensiVe Organometallic Chemistry
III; Crabtree, R. H.; Mingos, D. M. P., Eds. Elsevier: Amsterdam,
2
006; Vol. 1; p 197.
(
17) Cloke, F. G. N.; Gibson, V. C.; Green, M. L. H.; Mtetwa, V. S. B.;
Prout, K. J. Chem. Soc., Dalton Trans. 1988, 2227.
Inorganic Chemistry, Vol. 47, No. 23, 2008 11433

Torres-Nieto et al.
purchased from Cambridge Isotope Laboratories and were stored
over 3 Å molecular sieves in an MBraun glovebox (<1 ppm H
of reaction, accompanied by formation of a beige precipitate.
Heating was stopped and the sample hydrolyzed at room temper-
ature using HCl (10 mL, 10% v). A strong effervescence was
2
O
1
31
1
and O
2
). H, and P{ H} NMR spectra were recorded at room
temperature on a 300 MHz Varian Unity spectrometer in toluene-
observed with the immediate release of H S (caution poisonous
gas!), which was bubbled into a trap with 50 mL of an aqueous
2
1
d
8
. H chemical shifts (δ) are reported relative to the residual proton
3
1
1
resonances in the deuterated solvent. P{ H} NMR spectra were
concentrated solution of Pb(CH COO) inside a fume hood. After
3
2
recorded relative to external 85% H PO . All NMR spectra were
3
4
bubbling and venting all the gases, all organics proceeding from
carried out using thin wall (0.38 cm) WILMAD NMR tubes with
J. Young valves. GC-MS determinations were performed using a
Varian Saturn 3 on a 30 m DB-5MS capillary column. All
nanoparticles produced were characterized by TEM at 200 kV using
a Jeol-2010 electron microscope.
the reaction mixture were extracted using CH Cl (3 × 5 mL) and
2
2
the organic layer analyzed by GC-MS. The samples prepared for
TEM analysis were not hydrolyzed but centrifuged at 5000 cycles/
min for 10 min. After this time, the supernatant solution was
removed leaving a grayish solid which was dried for 5 h in vacuo
and then analyzed.
Nanoparticle Precursors. Complexes [(PEt
3
)
2
PdCl
2
] (1),
[
(PPh PdCl ] (2), [(dippe)PdCl ] (3), [(dppe)PdCl
3
)
2
2
2
2
] (4) were
1
8
prepared following the procedures reported in the literature. The
methylated compounds were prepared in situ by reaction of
chlorinated compounds with excess of MeMgBr.
Acknowledgment. We thank CONACyT (080606) and
DGAPA-UNAM (IN-202907-3) for their financial support
to this work. We thank Iv a´ n Puente-Lee (USAI-UNAM) for
transmission electron microscopy determinations and Dr.
Marco G. Crestani for technical support. J.T.-N. also thanks
CONACyT for a graduate studies grant.
1
9
Catalytic Desulfurization Experiments. A 50 mL Schlenk flask
was charged in a glovebox with the corresponding chlorinated
catalytic precursor (0.012 mmol) dissolved in 8 mL of dry toluene
and to it was added DBT (1.2 mmol) with constant stirring, during
3
0 min. After complete mixing, a solution of the Grignard reagent
(
2.4 mmol) was added. No changes in color were observed. The
Supporting Information Available: Includes detailed tables for
the desulfurization experiments, typical chromatographic plots, and
bar graphs with the nanoparticle size distribution of representative
experiments. This material is available free of charge via the Internet
at http://pubs.acs.org.
reaction mixture was heated to reflux under argon in a gas/vacuum
line and constantly stirred during the whole time of reaction. A
color change from yellow to brown was observed during the course
(
(
18) Cygan, Z. T.; Bender IV, J. E.; Litz, K. E.; Kampf, J. W.; Holl,
M. M. B. Organometallics 2002, 21, 5373.
19) Calvin, G.; Coates, G. E. J. Chem. Soc. 1960, 2008.
IC801550J
11434 Inorganic Chemistry, Vol. 47, No. 23, 2008