Aerobic Oxidation of Thiols Catalyzed by Copper Nanoparticles Supported on Diamond Nanoparticles

Article abstract of DOI:10.1002/cctc.201200569

After purification by Fenton treatment, commercial diamond nanoparticles (NPs) are a suitable solid support for the deposition of Cu nanoparticles. Heating at 500°C under hydrogen proved to be a convenient annealing process for Fenton-purified diamond NPs that decreased the population of surface carboxylic acid groups and lead to samples with average Cu particle sizes of 3-4nm. The samples of Cu NPs supported on diamond NPs have been characterized by IR and X-ray photoelectron spectroscopy, as well as XRD and TEM. It was concluded that the samples contained Cu⁰ as well as Cu^I and Cu^II species. The resulting diamond-supported Cu NPs were highly active for the selective aerobic oxidation of aromatic thiols to the corresponding disulfides, whereas aliphatic thiols exhibited much lower reactivity because of some poisoning and catalyst deactivation produced by aliphatic thiols. The Cu catalysts used for thiophenol oxidation could be reused in four consecutive runs with 4% of decrease in the catalytic activity. This Cu catalyst exhibited similar catalytic activity, but is considerably more affordable, as an analogous diamond-supported Au catalyst.

Full text of DOI:10.1002/cctc.201200569

CHEMCATCHEM
FULL PAPERS
DOI: 10.1002/cctc.201200569
Aerobic Oxidation of Thiols Catalyzed by Copper
Nanoparticles Supported on Diamond Nanoparticles
Amarajothi Dhakshinamoorthy, Sergio Navalon, David Sempere, Mercedes Alvaro, and
Hermenegildo Garcia*[a]
After purification by Fenton treatment, commercial diamond
nanoparticles (NPs) are a suitable solid support for the deposi-
tion of Cu nanoparticles. Heating at 5008C under hydrogen
proved to be a convenient annealing process for Fenton-puri-
fied diamond NPs that decreased the population of surface
carboxylic acid groups and lead to samples with average Cu
particle sizes of 3–4 nm. The samples of Cu NPs supported on
diamond NPs have been characterized by IR and X-ray photo-
electron spectroscopy, as well as XRD and TEM. It was conclud-
ed that the samples contained Cu0 as well as CuI and CuII spe-
cies. The resulting diamond-supported Cu NPs were highly
active for the selective aerobic oxidation of aromatic thiols to
the corresponding disulfides, whereas aliphatic thiols exhibited
much lower reactivity because of some poisoning and catalyst
deactivation produced by aliphatic thiols. The Cu catalysts
used for thiophenol oxidation could be reused in four consecu-
tive runs with 4% of decrease in the catalytic activity. This Cu
catalyst exhibited similar catalytic activity, but is considerably
more affordable, as an analogous diamond-supported Au
catalyst.
Introduction
Disulfides have been extensively used in chemistry[1] and bio-
logical studies.[2–4] In biological systems disulfides prevent oxi-
dative damage.[5] Disulfides are also used as reagents for or-
ganic syntheses to sulfenylate various anions[6] and as a protect-
ing group for thiols.[7] In industry, disulfides are commonly
used to vulcanize rubber and elastomers.[4]
Herein, we report the preparation of copper nanoparticles
(NPs) supported on diamond NPs and their catalytic activity for
the aerobic oxidation of thiophenols to their corresponding di-
phenyldisulfide. The main advantages of this method are low
catalyst loading, short reaction time, cost effectiveness of the
catalyst and high product selectivity. In recent precedent, we
have shown that the inertness of diamond NP surface makes
this carbonaceous material suitable as support of active metal
NPs in oxidation reactions.[20]
Oxidative coupling of thiols is the most common method
for disulfide synthesis. Reagents that have been used to per-
form the SÀS oxidative coupling include permanganate,[8] halo-
gens,[9] iron(iii) chloride,[10] organometallic complexes,[11,12] and
peroxides.[13] Transition metal complexes have also been used
as catalysts for the conversion of thiols to disulfides.[14–16] One
of the main problems associated with these methods is the Results and Discussion
need of stoichiometric reagents, long reaction times and high
Characterization of Cu/D and Cu/DH
catalyst loading as well as undesirable side products arising
from over-oxidation of disulfides. Further, tedious workup pro-
cedure has to be implemented to isolate the desired products
from the oxidizing reagents. For this reason, aerobic catalytic
oxidation of thiols to disulfides is a good alternative to those
using other oxidizing reagents. The aerobic oxidation of thiols
to disulfides has been reported with heterogeneous catalysts
like metal-organic frameworks,[17] graphite oxide[18] and gold
supported on CeO2.[19]
BET surface area of the commercial diamond NPs was
250 m2 gÀ1, whereas the areas of the Fenton-treated diamond
NPs (D) and hydrogen-annealed supports (DH) slightly in-
creased to 282 and 296 m2 gÀ1, respectively. This increase was
attributed to the removal of amorphous soot matter present in
the commercial diamond samples.[20]
FTIR spectra of the commercial, Fenton-treated, and an-
nealed diamond NPs with or without Cu deposition are pre-
sented in Figure 1. As previously reported,[20] commercial dia-
mond functionalization is needed to introduce surface hydrox-
yl groups that will further interact with the metal NPs.[20,24] The
sample D presented a broad band from 3674 to 2832 cmÀ1 cor-
responding to ÀOH groups of alcohol and acid carboxylic func-
tionalities. In addition, the presence of carboxylic groups is re-
vealed by the C=O band appearing around 1754 cmÀ1. Also,
a characteristic band at 1132 cmÀ1 corresponding to the CÀO
bond is observed.
[a] Dr. A. Dhakshinamoorthy, Dr. S. Navalon, D. Sempere, Dr. M. Alvaro,
Prof. Dr. H. Garcia
Instituto Universitario de Tecnologꢀa Quꢀmica CSIC- UPV and
Departamento de Quꢀmica, Univ. Politꢁcnica de Valencia
Av. De los Naranjos s/n, 46022 Valencia (Spain)
Fax: (+34)9638-7807
E-mail: hgarcia@qim.upv.es
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201200569.
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 241 – 246 241

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
Figure 3. Diffuse reflectance spectra (plotted as the Kubelka–Munk function
of the reflectance, R) of the catalysts a) Cu/D and b) Cu/DH.
Figure 1. FTIR spectra of diamond NP samples. a) DH, b) DH with Cu depos-
ited (Cu/DH), c) D, and d) D with Cu deposited (Cu/D).
Further, annealing treatment at 5008C under H2 atmosphere
of the D samples produced a more symmetric band from 3674
to 3000 cmÀ1 (indicating the prevalence of alcohol OH groups),
together with a clear decrease of the carbonyl band intensity
and shifting to 1726 cmÀ1. Also, a more symmetric and less in-
tense CÀO band at 1132 cmÀ1 is observed. In addition, DH
sample presents the vibration corresponding to CÀH vibrations
2917 and 1384 cmÀ1. These facts indicate that thermal treat-
ment under H2 produces the re-
sizes similar to those of Cu/DH were obtained for Au/DH. In
particular, Au/DH presented a particle size distribution cen-
tered at 5–6 nm with a maximum particle size of 10 nm (Fig-
ure S2).
Shown in Figure 3 are the diffuse reflectance spectra for the
different copper catalysts. As it can be seen there, the band at
550–600 nm corresponds to the surface plasmon band of met-
allic Cu NPs[25] that we propose to correspond to the small
duction of some carboxylic func-
tionalities to alcohol groups or
even further to aliphatic carbons,
decreasing the density of surface
carboxylic acid groups. After Cu
deposition, the ÀOH vibration
band in both Cu/D and Cu/DH
samples diminished indicating
the interaction of the copper
species with hydroxyl groups as
reported previously for the case
of gold supported on D.[20]
TEM analysis reveals an influ-
ence of the copper particle size
depending on the support,
namely D or DH. Figure 2 shows
representative TEM images and
particle size distribution of Cu/D
and Cu/DH (see the Supporting
information, Figure S1). As it can
be seen, Cu/D presents a broad
particle size distribution up to
19 nm with the main population
centered at 6–7 nm. On the
other hand, Cu/DH samples
show a narrower particle size
distribution centered at 3–4 nm
with a maximum size of 10 nm.
These data reveal the benefits of
copper NP deposition on ther-
mally H2-annealed DH sample
with a lower population of car-
boxylic acid groups. Particle Figure 2. TEM images and particle size distributions of a) Cu/D, b) Cu/DH, and c) Cu/DH after reuse.
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 241 – 246 242

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
metal Cu NPs as revealed by TEM.[26] Further, the absorption
band at approximately 640 nm and a more intense band
above 700 nm could indicate the presence of a residual popu-
lation of Cu2+ ions.[25] Notably, although the polyol preparation
method would reduce Cu2+ ions to Cu0, the metallic copper
can still undergo partial oxidation under atmospheric
conditions.
easily undergo oxidation, at least near the surface, under aero-
bic conditions.
These data show that Cu NPs interact mainly with the ÀOH
groups of D and DH supports, the thermal annealing under H2
leading to Cu NPs with sizes centered at 3–4 nm that are small-
er than the sizes of the unannealed D NPs at approximately 6–
7 nm. In addition, the presence of metallic and oxidized
copper species has been confirmed by XPS measurements.
The X-ray diffractograms of Cu/D and Cu/DH catalysts
showed the presence of a low-intensity peak at 2q=50.88 cor-
responding to Cu NPs (Figure 4) in accordance with the good
Catalytic activity of Cu/DH
Thiophenol (1) was selected as a model substrate to optimize
the reaction conditions. Aerobic oxidation of 1 to diphenyldi-
sulfide (2) in the absence of a catalyst resulted in 7% conver-
sion. The aerobic oxidation of 1 to 2 is promoted by the Cu
catalysts. Thus, the use of Cu/D resulted in 60% conversion of
1 to 2 in 4 h. Using Cu/DH as a catalyst, quantitative conver-
sion of 1 was achieved in 4 h in the presence of oxygen with
a selectivity of 2 of >99% (Figure 6). The better catalytic per-
Figure 4. X-ray diffractograms of the catalysts a) Cu/D and b) Cu/DH.
dispersion observed in the TEM measurements (Figure 2 and
Figure S1). Other diffraction peaks of metallic Cu NPs at 2q=
43.6 and 74.48 corresponding to the Cu (111) and Cu (202)
facets, respectively,[26] should be masked by the diffraction pat-
tern of diamond NPs centered at approximately 43.6 and 74.88.
In addition, the presence of a weak diffraction peak at 2q=658
attributable to Cu2O was also observed.
The presence of oxidized CuII on the surface of the catalyst
was determined by X-ray photoelectron spectroscopy (XPS,
Figure 5). Typical Cu0 and CuI peaks are indistinguishable here
because they are separated by only 0.1 eV; therefore, we at-
tributed the deconvoluted peak at 932.4 eV to a combination
of these reduced copper species.[27,28] In contrast, the binding
energy of CuII can be easily distinguished from the energies of
Cu0 and CuI and, thus, the peak at approximately 934.0 eV was
assigned to oxidized CuII species.[28] It is known that Cu NPs
Figure 6. Time–conversion plot for the aerobic oxidation of 1 to 2 using Au/
DH and b) Cu/DH catalysts. c) Hot-filtration test under the same experimen-
tal conditions with Cu/DH as the catalyst. For reaction conditions, see
Table 1, entry 3.
formance of Cu/DH over Cu/D is in agreement with the gener-
al trend of the beneficial influence of small particle size on the
catalytic activity and indicates the advantages of H2 annealing
reducing the population of surface carboxylic groups before
deposition Cu NPs on diamond. Thus, the following experi-
ments were all conducted with this Cu/DH catalyst.
Replacing oxygen by nitrogen led to incomplete oxidation
of 1 that is probably promoted by residual oxygen still present
in the system. Typically, oxidation of 1 with Cu/DH was per-
formed at a molar ratio 1/Cu of 666, achieving complete con-
version in 4 h. Doubling the amount of 1 from 1 mmol to
2 mmol also resulted in complete conversion in 4 h under the
same experimental conditions. Thus, we were interested in
probing the catalyst stability in the presence of large excess of
1 (10 mmol). Under these conditions the catalyst lost its activi-
ty after reaching 90% conversion (Figure S3). In this case with
a large excess of substrate (molar ratio 1/Cu of 5772), the turn-
over number and the turnover frequency was approximately
5700 cycles and 825 hÀ1, respectively. We attribute catalyst de-
activation to the oxidation of metallic Cu0 or CuI to CuII and to
Figure 5. X-ray photoelectron spectra of Cu 2p3/2 line measured for the Cu/
DH sample. The plot also shows the best deconvolution of the experimental
peaks.
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 241 – 246 243

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
the poisoning effect of sulfur-containing products or to a com-
bination of both.
51% conversion of 1, Cu/DH catalyst was removed from the re-
action mixture by hot filtration and the resultant reaction mix-
ture was allowed to react further in the absence of solid
(Figure 6). As conversion did not increase after removal of Cu/
DH, the possibility that oxidation was promoted by any soluble
Cu species was ruled out.
In addition, aerobic oxidation of 1 to 2 in the presence of
Cu/DH with 2,2’,6,6’-tetramethylpiperidine N-oxide (TEMPO)
and 2,6-di-tert-butylhydroxytoluene as co-catalysts resulted in
high conversion of 1, which agrees with the role of TEMPO as
a co-catalyst in some copper catalysts promoting benzyl alco-
hol oxidation and with the absence of free-radical oxidation.[29]
Hence, herein TEMPO rather acts as a cocatalyst trapping the
radicals formed during the course of the reaction.
Reusability of Cu/DH for the aerobic oxidation of 1 to 2 was
studied by performing four consecutive runs with Cu/DH cata-
lyst. A 4% conversion decrease was observed (entries 4–6,
Table 1). Furthermore, inductively-coupled plasma mass spec-
trometry analysis after the reuse experiments revealed that
copper leaching after the first use was about 2.1% of the initial
Cu content of the Cu/DH catalyst, whereas in the subsequent
reuse experiments negligible Cu leaching was observed. In ad-
dition, TEM analysis of the reused sample (see Figure 2) re-
vealed a slight Cu NP size increase to an average of 5–6 nm, al-
though large Cu NPs of 17 nm diameter, which were not pres-
ent in the fresh catalyst sample,
To gain understanding on the role of diamond as support,
copper NPs were also supported on activated carbon (AC) and
graphite (G) and their catalytic activity tested in the aerobic ox-
idation of 1. The initial reaction rates and final conversion of
1 with these catalysts were lower than those achieved with
Cu/DH as a catalyst (Table 1 and Figure S4). These experimental
results show the advantage of an inert diamond surface over
other types of carbons.
were also observed.
Based on these results, we pro-
Table 1. Aerobic oxidation of 1 to 2 using different catalysts.[a]
pose that the reaction of 1 with
Cu/DH takes place through the in-
Run
Catalyst
t
[h]
Conv.
[%][b]
Selectivity
[%][b]
termediacy of surface-bound thiyl
radicals that undergo self-coupling
1
2
3
4
5
6
7
8
4
4
4
4
4
4
1
4
1
24
2
4
1
4
1
4
3
7
60
100
99
97
96
89[c]
100[c]
13[d]
92[d]
96[e]
99[f]
16
98
99
99
99
99
99
99
99
98
99
98
98
98
98
99
99
99
to give 2. Upon the generation of
each thiyl radical, partially oxidized
Cu/D
Cu/DH
Cu/DH 1st reuse
Cu/DH 2nd reuse
Cu/DH 3rd reuse
Cu/DH
Cu/DH
Cu/DH
Cu/DH
Cu/DH
Cu/DH
Cu/AC
Cu/AC
Cu/G
Cu/G
Au/DH
copper species is reduced by 1, as
shown in Scheme 1.
Scheme 1. Proposed mecha-
nism for aerobic oxidation of
1 to 2 catalyzed by Cu/DH.
The scope of this catalyst was
further studied by using different
9
aromatic and aliphatic thiols under
10
11
12
13
14
15
16
13
the optimized conditions (Table 2).
4-Fluoro-, 4-chloro-, 4-methyl-, 4-methoxy-, and 2-aminothio-
phenol showed 99% conversion with more than 99% selectivi-
ty to the corresponding disulfide. On the other hand, 2-mer-
34
43
78
100
Table 2. Aerobic oxidation of thiols to disulfides catalyzed by Cu/DH.[a]
[a] Reaction conditions: Thiophenol (1 mmol), catalyst (20 mg, 0.5 wt.%),
ethanol (4 mL), oxygen purged through balloon, 708C. [b] Determined by
GC. [c] Thiophenol (2 mmol), catalyst (40 mg, 0.5 wt.%), ethanol (8 mL),
oxygen purged through balloon, 708C. [d] Thiophenol (10 mmol), catalyst
(20 mg), ethanol (10 mL), oxygen purged through balloon, 708C. [e] With
TEMPO. [f] With 2,6-di-tert-butylhydroxytoluene.
Run
Thiol
t
Conv.
[%][b]
Selectivity
[%][b]
[h]
1
2
3
4
5
6
7
8
thiophenol
4
2
2
2.5
2
1.5
3
>99
>99
>99
>99
>99
>99
57
5
–
17
12
99
99
99
99
98
98
98
96
–
98
98
98
98
Finally, Au/DH was used as a catalyst for the aerobic oxida-
tion of 1 to 2, and its activity was compared with that of Cu/
DH (Figure 6). Although the initial reaction rate and the final
conversion were somewhat higher for Au/DH than for Cu/DH,
both metals exhibited high activity in this oxidation experi-
ment. The higher activity of Au is not totally unexpected with
regard to the strong interaction of thiols with gold and the
high oxidation activity of Au catalysts.[19] However, considering
the cost and availability, the development of such Cu catalysts
would be more competitive than that of analogous catalysts
based on precious metals.
4-fluorothiophenol
4-chlorothiophenol
4-methylthiophenol
4-methoxythiophenol
2-aminothiophenol
2-mercaptopyridine
1-napthylthiol
thiobenzoic acid
1-hexanethiol
cyclohexyl mercaptan
benzyl mercaptan
3-chloro-1-propanethiol
8
9
24
29
29
29
24
10
11
12
13
35
44
[a] Reaction conditions: Thiol (1 mmol), catalyst (20 mg), ethanol (4 mL),
oxygen purged through balloon, 708C. [b] Determined by GC.
Heterogeneity of the reaction and absence of Cu leaching
was addressed by a hot-filtration test. After achieving about
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 241 – 246 244

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
captopyridine showed 57% conversion in 3 h, attributable to
the competing coordination of oxidizing copper sites with the
N atom of the pyridine ring. 1-Napthylthiol showed very poor
reactivity because of the low solubility of this substrate in eth-
anol. Further, thiobenzoic acid showed no reactivity. Aliphatic
thiols were less reactive than aromatic thiols. For example, 1-
hexanethiol showed 17% conversion in 29 h. Cyclohexyl mer-
captan and benzyl mercaptan showed 12 and 35% conversion,
respectively, in 29 h. Finally, 3-chloro-1-propanethiol showed
44% conversion in 24 h. To compare the activity of Cu/D, Au/D
was used as a catalyst under the same conditions and exhibit-
ed a reactivity similar to that of Cu/D (Figure 7).
enced a slight deactivation that has been attributed to a minor
Cu leaching and an increasing of the average Cu NP size. In
the case of aliphatic thiols, the Cu/DH catalyst became deacti-
vated before high conversions were reached.
Experimental Section
Hydrogen peroxide solution in water (30%, v/v), nitric acid (65%),
hydrochloric acid (37%, ACS reagent), sulfuric acid (98%),
HAuCl4·3H2O, Cu(NO3)2·2H2O, NaOH (ACS reagent) and diamond
nanopowder (ref: 636444, 95%) were commercial samples from
Sigma–Aldrich. Milli-Q water was used in all the experiments. The
other reagents used were of analytical or HPLC grade. Other start-
ing materials were obtained commercially from Aldrich and used
without any further purification unless otherwise noted.
Fenton treatment of commercial diamond NPs was performed by
suspending raw diamond nanopowder (0.5 g) in 150 mL H2O2
(30%, v/v) in a 500 mL open flask. The pH was adjusted at 3 using
HNO3 (0.1m) and maintained at this value during all the process.
This slurry was sonicated in an ice-refrigerated ultrasound bath
and held at 1–58C for 20 min. Then, a freshly prepared aqueous so-
lution of Fe(SO4)·7H2O (mgmLÀ1) at pH 3 was slowly dropped for
1 h while observing intense gases evolution. (caution: the Fenton
reaction is a highly exothermic reaction and occurs with evolution
of heat and gases. The process must be carried out with care in
a well-ventilated fume hood whilst wearing goggles and appropri-
ate personal safety items). After 1 h, additional amounts of
H2O2·(50 mL) and Fe(SO4)·7H2O were added until complete H2O2
decomposition as evidenced by titration with titanyl oxalate.
&
*
Figure 7. Aerobic oxidation of 1-hexanethiol using Au/DH ( ) and Cu/DH ( )
as catalysts.
After the Fenton treatment, the suspension was diluted with dis-
tilled water and allowed to reach RT. Then, several washings were
made using an aqueous solution of sulfuric acid (0.1m) until the
absence of iron detected by colorimetric method using KSCN. The
excess of acid was removed by performing five consecutive centri-
fugation–redispersion (14000 rpm) cycles with Milli-Q water. Dia-
mond NPs sediment at the bottom of the centrifuge tube under
these conditions and can be easily recovered and re-suspended.
The pH value of the supernatant at the fifth centrifugation–redis-
persion cycle was neutral. Finally, the Fenton-treated diamond NPs
were submitted to overnight freeze-drying to give dry, purified dia-
mond NPs (D).
To understand the reasons of the lower catalytic activity of
Cu/DH for aliphatic thiols compared to that for thiophenols,
the coupling reaction of cyclohexanethiol to dicyclohexyldisul-
fide was performed for prolonged time to ensure the maxi-
mum yield. A conversion of 12% was achieved after 29 h. At
this time an extra amount of fresh Cu/DH catalyst was added,
whereby the conversion increased to 27%. This suggests that
the limitation in the product yield of aliphatic thiols is not
based on thermodynamics but owing to catalyst deactivation.
Probably the strong adsorption of aliphatic disulfides or other
sulfur-containing byproducts on the copper reaction sites was
causing their deactivation. For this reason the addition of fresh
catalyst could increase the product yield. Similarly, the strong
adsorption of thiobenzoic acid on the copper sites would be
responsible of the failure of Cu/DH to promote the oxidation
of this compound, whereas the other aromatic thiols tested
were easily oxidized.
Additionally, D was submitted to a subsequent annealing treat-
ment under continuous H2 flow. In particular, D powder was
placed in a quart reactor under H2 flow (100 mLminÀ1) and heated
using a ramp of 88CminÀ1 until the temperature reached 5008C
and maintained for 6 h. Then, the sample was cooled at RT and the
support labeled as DH. This reduction treatment decreases the
number of surface defects by increasing the population of ÀOH
groups.[21]
Preparation of Cu/D, Cu/DH, and Au/DH catalysts was accom-
plished by using the polyol method.[22,23] Briefly, 200 mg of the dia-
mond support (D or DH) were suspended in 80 mL of ethylene
glycol and sonicated for 30 min. Then, the corresponding amount
of gold or copper salts dissolved in water was added to the dia-
mond suspension to achieve 0.5 wt.% loading. Under vigorous stir-
ring the suspension was heated up to 858C and allowed to react
for 4 h. After cooling the reaction at RT the powder suspended
was recovered by centrifugation at 14000 rpm. Then the superna-
tant was removed and the catalyst dispersed in ethanol and
washed by performing three consecutive centrifugation–redisper-
sion cycles with acetone and subsequently other three with water.
Conclusions
Fenton-treated diamond nanoparticles (NPs) have been found
to be suitable solid supports for copper NPs that catalyze the
selective aerobic oxidation of aromatic thiols to disulfides. Al-
though analogous diamond-supported gold catalysts exhibited
a somewhat higher activity in this reaction, copper is a more
economical metal. Hydrogen annealing improved the perfor-
mance of the diamond support by decreasing the population
of carboxylic groups, thus leading to smaller Cu NPs that ex-
hibit higher catalytic activity. Upon reuse, the catalyst experi-
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 241 – 246 245

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
Finally, the catalysts (Cu/D, Cu/DH, and Au/DH) were freeze-dried.
The same procedures were followed for the preparation of Cu/AC
and Cu/G by replacing D or DH either by AC or G.
Keywords: aerobic oxidation
nanoparticles · thiophenol
·
copper
·
diamond
·
By using inductively coupled plasma atomic emission spectroscopy
(ICP-AES) chemical analysis, the metal loading (0.5 wt.%) deposited
on the functionalized diamond NPs was confirmed. FTIR spectra of
the different samples were recorded on a Nicolet 710 FTIR spectro-
photometer by using KBr disks of the samples prepared by com-
pression at 10 Ton for 2 min. Diffuse reflectance optical spectra
were recorded by using a CARY 5G UV-Vis-NIR spectrophotometer
adapted with an integrating sphere. X-ray diffractograms (XRD)
were recorded by using a Philips X-Pert diffractometer equipped
with a graphite monochromator, operating at 40 kV and 45 mA
and employing Ni filtered CuKa radiation (l=0.1542 nm). TEM and
high resolution TEM (HRTEM) of the different samples were ob-
tained by using a TECNAI G2 F20 (FEI) instrument operating at
200 kV with a resolution of 0.24 nm. The particle size distribution
was estimated by counting over 300 particles. The percentage con-
version, purity, and relative yields of the final products were deter-
mined by using a Hewlett Packard 5890 series II gas chromato-
graph with an FID detector and high-purity helium as the carrier
gas. The products were identified by GC-MS by using a Hewlett
Packard 6890 series spectrometer.
[1] E. Zysman-Colman, D. N. Harpp, J. Org. Chem. 2005, 70, 5964.
[2] R. J. Cremlyn in An Introduction to Organosulfur Chemistry, Wiley, New
York, 1996.
[3] G. H. Whitham in Organosulfur Chemistry, Oxford University Press,
Oxford, 1995.
[4] S. Ghammamy, M. Tajbakhsh, J. Sulfur Chem. 2005, 26, 145.
[5] H. Karami, M. Montazerozohori, M. H. Habibi, J. Chem. Res. 2006, 8, 490.
[6] L. Bischoff, C. David, L. Martin, H. Meudal, B.-P. Roques, M.-C. Fournie-Za-
luski, J. Org. Chem. 1997, 62, 4848.
[7] F. Hosseinpoor, H. Golchoubian, Catal. Lett. 2006, 111, 165.
[8] N. A. Noureldin, M. Caldwell, J. Hendry, D. G. Lee, Synthesis 1998, 1587.
[9] M. H. Ali, M. McDermott, Tetrahedron Lett. 2002, 43, 6271.
[10] A. R. Ramesha, S. Chandrasekaran, J. Org. Chem. 1994, 59, 1354.
[11] D. K. Y. Tan, J. W. Kee, W. Y. Fan, Organometallics 2010, 29, 4459.
[12] K. Y. D. Tan, G. F. Teng, W. Y. Fan, Organometallics 2011, 30, 4136.
[13] A. R. Hajipour, S. E. Mallakpour, H. Adibi, J. Org. Chem. 2002, 67, 8666.
[14] H. Golchoubian, F. Hosseinpoor, Catal. Commun. 2007, 8, 697.
[15] S. M. S. Chauhan, A. Kumar, K. A. Srinivas, Chem. Commun. 2003, 2348.
[16] K. Tanaka, K. Ajiki, Tetrahedron Lett. 2004, 45, 25.
[17] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Chem. Commun. 2010, 46,
6476.
[18] D. R. Dreyer, H.-P. Jia, A. D. Todd, J. Geng, C. W. Bielawski, Org. Biomol.
Chem. 2011, 9, 7292.
[19] A. Corma, T. Rodenas, M. J. Sabater, Chem. Sci. 2012, 3, 398.
[20] S. Navalon, R. Martin, M. Alvaro, H. Garcia, Angew. Chem. 2010, 122,
8581; Angew. Chem. Int. Ed. 2010, 49, 8403.
A 50 mL round-bottomed flask was charged with the catalyst
(20 mg) in ethanol (4 mL) and the substrate (1 mmol). This reaction
mixture was purged with oxygen through balloon. Then it was
stirred for the required time at 708C. The reaction was monitored
periodically by analyzing the sample with GC until the completion
of reaction. The mass balances of the recovered reaction mixture
accounted for more than 95% of the initial substrate, as confirmed
by GC using nitrobenzene as the external standard. The yields of
the product were determined by using nitrobenzene as the exter-
nal standard considering the response factors unity.
[21] S. Osswald, G. Yushin, V. Mochalin, S. Kucheyev, Y. Gogotsi, J. Am. Chem.
Soc. 2006, 128, 11635.
[22] K. J. Carroll, J. U. Reveles, M. D. Shultz, S. N. Khanna, E. E. Carpenter, J.
Phys. Chem. C 2011, 115, 2656.
Reuse experiments were performed as given in entry 7, Table 1.
After the catalytic oxidation the catalyst was recovered by centrifu-
gation. Then, the catalyst was washed in ethanol, acetone and
water three times each solvent, and dried under air at RT. Then the
isolated catalyst was treated with ethylene glycol at 858C for 5 h
followed by different cycles of washing/centrifugation with ethanol
and water. The recovered catalyst was used for the next run with
appropriate substrate and solvent.
[23] R. J. Joseyphus, K. Shinoda, D. Kodama, B. Jeyadevan, Mater. Chem.
Phys. 2010, 123, 487.
[24] R. Martin, S. Navalon, J. J. Delgado, J. J. Calvino, M. Alvaro, H. Garcia,
Chem. Eur. J. 2011, 17, 9494–9502.
[25] C. M. Chanquꢁa, L. Andrini, J. D. Fernꢂndez, M. E. Crivello, F. G. Requejo,
E. R. Herrero, G. A. Eimer, J. Phys. Chem. C 2010, 114, 12221.
[26] H. Lei, Y. Tang, J. Li, J. Luo, Appl. Phys. Lett. 2007, 91, 113.
[27] K. Yoshida, C. Gonzalez-Arellano, R. Luque, P. L. Gai, Appl. Catal. A 2010,
379, 38.
[28] T.-Y. Dong, H. H. Wua, C. Huang, J. M. Song, I. G. Chen, T. H. Kao, Appl.
Surf. Sci. 2009, 255, 3891.
[29] P. Gamez, I. W. C. E. Arends, J. Reedijk, R. A. Sheldon, Chem. Commun.
2003, 2414.
Acknowledgements
Financial support by the Spanish Ministry of Science and Educa-
tion (Consolider MULTICAT, CTQ-2009-11856) is gratefully
acknowledged.
Received: August 20, 2012
Published online on November 9, 2012
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 241 – 246 246