C–N cross-coupling on supported copper catalysts: The effect of the support, oxidation state, base and solvent

Article abstract of DOI:10.1016/j.jcat.2016.06.011

A series of supported copper catalysts at two different loadings (1 and 2?wt%) have been prepared by deposition precipitation on various supports including TiO₂, ZnO, Al₂O₃ and active carbon and submitted or not to reductive treatments to favor the increase in population of Cu(I). The samples have been characterized by textural measurements, electron microscopy and spectroscopic techniques including EPR and XPS, concluding the presence of dispersed copper oxides on the support with small particle size and contrasting prevalence of Cu(II) or Cu(I). The catalytic activity of all these catalysts for the C–N coupling of aniline and bromobenzene has been evaluated. A strong influence of the support, copper oxidation state, solvent, nature of the base was observed, the optimal conditions being the use of ZnO or TiO₂ as supports and toluene/dioxane as solvent and EtOK as base. t-C₅H₁₁OK as base in either THF or toluene give rise to the formation of t-C₅H₁₁ phenyl ether in some extent. The catalyst undergoes deactivation during the reaction, but about 88% of the activity of the fresh sample could be regained by dioxane washings before reuse. XPS indicates that the most likely origin of catalyst deactivation is adsorption on the copper catalyst surface of KBr and inorganic salts formed as byproducts during the reaction.

Full text of DOI:10.1016/j.jcat.2016.06.011

Journal of Catalysis 341 (2016) 205–220
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
C–N cross-coupling on supported copper catalysts: The effect of the
support, oxidation state, base and solvent
a
a
a
a
a
Alina Tirsoaga , Bogdan Cojocaru , Cristian Teodorescu , Florin Vasiliu , Maria Nicoleta Grecu ,
a
a,
⇑
b,
⇑
Daniela Ghica , Vasile I. Parvulescu , Hermenegildo Garcia
a
Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry, University of Bucharest, Bdul Regina Elisabeta 4-12, Bucharest 030016, Romania
Instituto de Tecnología Química CSIC-UPV, Universitat Politécnica de Valencia, Av. De los Naranjos s/n, 46022 Valencia, Spain
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of supported copper catalysts at two different loadings (1 and 2 wt%) have been prepared by
deposition precipitation on various supports including TiO , ZnO, Al O and active carbon and submitted
2 2 3
Received 5 April 2016
Revised 10 June 2016
Accepted 19 June 2016
or not to reductive treatments to favor the increase in population of Cu(I). The samples have been char-
acterized by textural measurements, electron microscopy and spectroscopic techniques including EPR
and XPS, concluding the presence of dispersed copper oxides on the support with small particle size
and contrasting prevalence of Cu(II) or Cu(I). The catalytic activity of all these catalysts for the C–N cou-
pling of aniline and bromobenzene has been evaluated. A strong influence of the support, copper oxida-
Keywords:
Heterogeneous catalysis
C–N cross coupling
Copper catalysts
Precipitation-deposition technique
Amination of bromobenzene
tion state, solvent, nature of the base was observed, the optimal conditions being the use of ZnO or TiO
supports and toluene/dioxane as solvent and EtOK as base. t-C 11OK as base in either THF or toluene
give rise to the formation of t-C 11 phenyl ether in some extent. The catalyst undergoes deactivation
2
as
5
H
5
H
during the reaction, but about 88% of the activity of the fresh sample could be regained by dioxane wash-
ings before reuse. XPS indicates that the most likely origin of catalyst deactivation is adsorption on the
copper catalyst surface of KBr and inorganic salts formed as byproducts during the reaction.
Ó 2016 Elsevier Inc. All rights reserved.
1
. Introduction
Cross-coupling is a very powerful strategy to generate aryl-
oride or silylamide) that plays an important role on the evolution
of the reaction, being involved in the deprotonation of the N-
substrate. A suitable base affords considerable benefits in the func-
tional group tolerance of Pd-catalyzed amination reactions. Classi-
cal solvents usually employed in the Buchwald-Hartwig amination
reaction [40] are nonpolar and aprotic being thus able to dissolve
all reactants with the exception of NaO-t-Bu (the traditional base
of this reaction).
carbon and aryl-heteroatom covalent bonds and may occur follow-
ing different routes [1–15]. Among these, the formation of the C–N
bonds elicited a large interest in the synthesis of compounds with
pharmaceutical, cosmetic, agrochemical and optical devices [16–
2
3] applications. Because of this it received a great industrial and
academic importance in the past two decades [24]. Initiated by
Migita et al. [25] using aminostannanes, the amination of aryl
halides is mostly known as a Buchwald-Hartwig reaction [26–31].
Classical Buchwald-Hartwig reaction represents a palladium-
mediated cross-coupling procedure [32,33] under homogeneous
catalytic conditions, using strong bases, aprotic and/or polar and
non-polar solvents and different ligands [34–39].
Although palladium [32,33] is the most widely used active
metal of the Buchwald-Hartwig cross-coupling reactions, copper,
nickel and iron have been also considered as alternatives because
they are less expensive, and require cheaper bidentate ligands such
as phenanthrolines, 1,3-diketones, imines,
a-amino acids, salicy-
lamides, diamines, lowering thus the overall synthesis procedure
costs [41].
Along with the choice of the ligand, an important component of
the C–N cross-coupling reaction is the nature of the base (e.g. car-
bonates, phosphates, alkaline hydroxides, alkoxides, inorganic flu-
The literature already reported a broad variety of nucleophiles
that were successfully used in homogeneous copper-mediated
cross-coupling reactions to form C(aryl)–N bond: amines, anilines,
amides, imides, ureas, carbamates, sulfonamides as well as aro-
matic heterocycles (imidazoles, pyrazoles, thiazoles, tetrazoles,
benzimidazoles, indazoles) [42,43].
⇑
Corresponding authors.
E-mail addresses: vasile.parvulescu@chimie.unibuc.ro (V.I. Parvulescu), hgar-
cia@qim.upv.es (H. Garcia).
http://dx.doi.org/10.1016/j.jcat.2016.06.011
0
021-9517/Ó 2016 Elsevier Inc. All rights reserved.

2
06
A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
Converting the C–N cross-coupling reaction from homogeneous
2. Experimental
to heterogeneous conditions is very attractive because of the mul-
tiple advantages offered by the heterogeneous catalysis: (a) easy of
product separation, (b) use of a small amount of metal catalyst
supported on large surface area materials, (c) possibility of recy-
cling the catalyst, (d) minimal metal contamination of the product.
To combine the advantages of homogeneous and heterogeneous
catalysis, supported homogeneous complexes were designed
allowing covalent immobilization in order to efficiently recover
and reuse the catalyst, but strong basic conditions and mono/
bidendate ligands were still required to generate a ligand-
2.1. Catalysts synthesis
All starting materials were of analytical purity, and used as
received without any further purification: Cu(NO ) ꢀ3H O
3
2
2
(Sigma–Aldrich) as copper precursor salt, NaOH (Sigma–Aldrich),
hydrazine monohydrate solution (64–65%, in water) (Sigma–
Aldrich) as reducing agent, and TiO2 – Degussa P25 (Sigma–
Aldrich), active carbon – NORIT A SUPRA EUR, alumina – Alfa
Aesar/Johnson Matthey, ZnO – NanoTek, Alfa Aesar as supports.
The preparation of the heterogeneous copper catalysts was per-
formed via a deposition–precipitation method (DP) onto the above
commercial supports. Aqueous 1.57 mM Cu(NO ) ꢀ3H O (for 1 wt%
2
stabilized metal catalyst. In this scope a fiber-supported PdCl -
triphenylphospine catalyst was reported for the reaction of p-
bromotoluene with piperazine in the presence of a strong base.
The catalyst facilitated the desired amination, but side products
were present, as in homogeneous conditions. Moreover the cata-
lyst was unstable in the reaction conditions, releasing active com-
ponents from the polymer support [44]. Anchoring a homogeneous
copper (II) catalyst on an insoluble polymer (reported for the C–N
cross-coupling reaction of primary amines with aryl iodides [45])
provided a more stable catalytic system, but the yield was small
3
2
2
copper catalysts) and 3.21 mM Cu(NO ) ꢀ3H O (for 2 wt% copper
3
2
2
catalysts) solutions were adjusted at pH = 9–9.5 (with NaOH,
added drop-wise) and then heated at 80 °C for 1 h in the presence
of 1 g support. After that, the catalysts were washed with double
distilled water till a conductibity of 10 lS, and dried at 110 °C
under vacuum for 10 h. Following this procedure samples with 1
and 2 wt% Cu were obtained. Table SI1 compiles the values of the
nominal concentration of copper determined by ICP-OES compared
to the deposited amount of copper.
Activation of the catalysts has been carried out following two
different routes: (i) calcination at 300 °C with the purpose of gen-
erating supported CuO species (samples denoted as CuO/support);
(ii) reduction under different reductive atmospheres in order to
[
46]. Chaudhari et al. [47] developed heterogenized copper com-
plexes on solid supports in the presence of bidentate N-ligands
and phosphotungstic acid (using encapsulation and tethering pro-
tocols) for the coupling of aryl amines with iodobenzene. The reac-
tion was carried out using over stoichiometric quantities of strong
bases, as well as 4–8% copper-containing catalysts, but the selec-
tivity in diphenylamine was low, due to diffusion-limitation occur-
ring into the zeolite supports. Lipshutz et al. [48,49] used a high
content of nickel and copper oxide (10 wt%) on activated charcoal
matrix (stabilized with bidentate ligands) for aromatic amination
of activated aryl halides with secondary alkylamines or aniline
where the metal leaching at the end of the reaction was
considerable.
Several Cu(I)/Cu(II) ‘ligand-free’ catalytic systems have also
been described with the aim to develop more attractive systems
for industry. Park and co-workers [50] used catalytic amounts of
acetylene-carbon-immobilized CuO-hollow nanospheres for N-
arylation reactions of various N-containing heterocycles with aryl
halides at high temperature, using very strong bases. However,
even for a large content of the copper catalyst (5%), the efficiency
for the N-arylation of aniline with iodobenzene was low. A
dependence of the reaction rate on the morphology and size of
the Cu(I)/(II) oxides was also reported [51,52]. Punniyamurthy
and co-workers [53,54] exploited the high surface area and reac-
tive morphology of the CuO nanoparticles for C–N, C–O, and C–S
cross-coupling reactions showing that the catalytic properties of
Cu(II) oxide were improved for the N-arylation of pyrazole with
generate supported Cu O species. Thus, the reduction of the cata-
lysts was carried out following two routes: (a) the treatment of
the samples at 300 °C under a flow of N2 (10 mL min ) (samples
2
ꢁ
1
denoted as Cu O_N /support); and (b) the treatment of the sam-
2
2
ples under ambient temperature with a hydrazine monohydrate
solution for 15 min under vigorous stirring. Then, the samples
were dried for 12 h at 40 °C, for 6 h at 120 °C, under vacuum, and
ꢁ
1
heated at 300 °C under an Ar flow (10 mL min ) (samples denoted
as Cu O_N H /support).
2
2
4
For reference commercial anhydrous CuCl , and CuO and Cu O
2
2
(all from Sigma–Aldrich, over 99.9% purity) were used as homoge-
neous and heterogeneous catalysts, respectively, without any addi-
tional treatment.
2.2. Catalysts characterization
The prepared catalysts were characterized using different tech-
niques. Textural characteristics (surface area and pore diameter)
were determined from the adsorption–desorption isotherms of nitro-
gen at ꢁ196 °C using a Micromeritics ASAP 2020 Surface Area and
Porosity Analyzer. DRUV–Vis spectra were collected under ambient
conditions with a Specord 250 (Analytic Jena) equipment. The equip-
ment is provided with a measuring device in the reflectance mode
using Spectralon as reference material. The collected spectra were
transformed using the Kubelka–Munk F(R) function. The final spectra
were averaged from 400 scans at a resolution of 4 cm . Raman spec-
tra were collected with a Horiba Jobin Yvon – Labram HR UV–Visible–
NIR (200–1600 nm) Raman spectra were recorded using a Raman
Microscope Spectrometer, and a laser with the wavelength of
aryl halides by the presence of [Fe(acac)
Correa and Bolm [56] also reported a ligand-free catalyst system
for N-arylation of heterocycles using 10 mol% Cu O under argon
atmosphere where the role of FeCl was again emphasized [57].
O as a suitable catalyst
2
] as co-catalyst [55].
2
3
ꢁ1
DFT calculations also recommended Cu
for this reaction [58].
2
Our interest is to convert C–N cross coupling into a truly
heterogeneous process. Previous work in amination of bromoben-
zene using titania-supported gold catalysts [59] indicated that
this reaction can occur indeed with very high selectivity, but only
for moderate conversions. The base plays a very important role
also under heterogeneous conditions. Based on this state of the
art the aim of the present work was to investigate the catalytic
activity of copper catalysts deposited on various supports (differ-
6
2
32 nm. The spectra were collected from 10 scans at a resolution of
cm . Powder X-ray Diffraction patterns were collected at room
ꢁ1
temperature using a Shimadzu XRD-7000 apparatus and the Cu K
a
monochromatic radiation k = 1.5406 Å, 40 kV, 40 mA with a scanning
ꢁ1
rate of 0.1 2h min , in the 2h range of 5–80. XPS spectra were
recorded at room temperature using a SSX-100 spectrometer, Model
2
ent Cu(I)/Cu(II) ratios on TiO , active carbon, alumina, or ZnO) in
amination of an unactivated aryl bromide (bromobenzene), as
reaction test.
2
06 from Surface Science Instrument. The pressure in the analysis
chamber during the analysis was 1.33 mPa. Monochromatized Al
radiation (h = 1486.6 eV) generated by bombarding the Al anode
K
a
m

A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
207
with an electron gun operated with a beam current of 12 mA and
acceleration voltage of 10 kV has been used. The spectrometer energy
scale was calibrated using the Au 4f7/2 peak centered at 83.98 eV.
Charge correction was made with the C1s signal of adventitious car-
bon (C–C or C–H bonds) located at 284.8 eV. The atomic surface com-
positions were calculated using the sensitivity factors included in the
machine software, applied tothe surface below thecorresponding fit-
ted XPSsignals. Anestimatederrorof±0.1 eVhasbeenassumed forall
measurements. Tominimizethereduction ofcopper intheXPScham-
ber during analysis, this element has been firstly analyzed [60]. Spec-
imens for electron microscopy were prepared by suspending them in
acetone and transferring to a copper grid coated with an amorphous
carbonsupport. TEM and HRTEM imageswererecorded on a JEOLJEM
ARM 200F electron microscope operated at 300 keV. The copper and
support crystallite sizes were established from the measurement of
over 100 particles for each sample. The X (9.1–9.5 GHz)- and Q
metal nanoparticles. Samples at two different copper loadings (1
or 2 wt%) were prepared. After the DP preparation and considering
that DFT calculations predict that Cu(I) should be the most active
species for C–N crosscoupling, a subset of samples were submitted
to reductive treatments, i.e., thermal treatment under N or chem-
2
ical reduction with hydrazine at room temperature. Heating Cu(II)
oxide at 300 °C results in spontaneous partial reduction to Cu(I)
due to the absence of O . Table 1 presents the textural characteris-
2
tics of the reference oxides and catalysts containing 1 and 2 wt% Cu
and the corresponding supports. The samples prepared via the
route (i) preserved the textural characteristics of the support. How-
ever, the samples prepared via the route (ii), i.e. under the reduc-
tion treatments, showed a diminution of all these characteristics
suggesting a re-dispersion of copper with a partial blockage of
the pores. Noteworthy, this occurred even for the samples reduced
under nitrogen. The increase of the copper loading to 2 wt% did not
change these characteristics (Table 1).
(
34 GHz)-band EPR spectra were performed at room temperature
on weighed powered samples, in the same conditions of modulation
amplitude and microwave power (depending of the frequency band)
using CMS 8400Adaniand ELEXSYS500 Bruker spectrometer, respec-
Fig. 1 shows comparative XRD patterns of titania and catalysts
2
containing 1 and 2 wt%CuO/TiO . These patterns are typical for
TiO -P25 showing a mixture of anatase (JCPDS card: 21-1272)
2
2
+
tively. The relative concentrations of paramagnetic copper (Cu , the
only EPR active valence) were estimated from double integrated EPR
spectra; the absolute concentration measurements are quite difficult
to determine and these sometimes lack precision, asthe specific spec-
tra intensities depend on many factors, which are experimentally dif-
ficult to control exactly. While the X-band EPR spectra consist of
broad superposition of lines, the Q-band EPR spectra exhibit more
well resolved anisotropic lines characteristic to Cu2 ions and show
the presence of some paramagnetic defects [61,62].
and rutile (JCPDS card: 34-180), in which anatase is the dominant
component. No reflection plane of copper oxide has been identified
in these patterns confirming the high dispersion of copper.
No change of XRD patterns was observed for the series of N
2
or
N H reduced catalysts (Figs. SI1 and SI4). Although the deposition
2 4
of copper was found to produce a decrease of all the textural prop-
erties (surface area, pore size and pore volume) no agglomeration
of copper at a level that might have make it XRD visible (i.e. crys-
tallites larger than 3.5 nm) has occurred. Thus, the XRD analysis
has proved also the stability of the catalysts. No agglomeration
was produced during the catalyst reaction (Fig. SI2–3).
+
2.3. Catalytic tests
Table 2 and Fig. 2 show the optical absorption spectra of the
investigated catalysts and supports. TiO
Vis spectrum of titania P-25 with the absorption band onset at
04 nm [63]. Accordingly, the absorption edge at 404 nm is attrib-
uted to the charge transfer from O 2p to the Ti 3d orbitals [64].
Deposition of copper species on the support changed the profile
2
shows a typical DR-UV–
All starting compounds were obtained from commercial suppli-
ers and used without any further purification: bromobenzene
Sigma–Aldrich) and aniline (Fluka) as coupling reagents, potas-
sium ethoxide, EtOK, (Acros Organics) and potassium 2-methyl-
-butoxide, t-C H11OK in toluene (Sigma–Aldrich) as bases,
5
,4-dioxane (Sigma–Aldrich), dimethylformamide (Chempur) and
-methyltetrahydrofuran (Sigma–Aldrich) as solvents. The compo-
4
(
2
1
2
Table 1
sition of copper precursors was checked by ICP-OES. As expected
the started materials did not contain even traces of palladium or
other metals able to catalyze the reaction.
Bromobenzene (1.2 mmol), aniline (1 mmol), base potassium
ethoxide or potassium 2-methyl-2-butoxide (2 mmol), catalyst
Textural characteristics of the reference oxides and catalysts containing 1 and 2 wt%
Cu and the corresponding supports.
Catalyst
BET surface
Pore size from
BJH desorption
(nm)
Pore volume from
BJH desorption
2
area (m /g)
3
(cm /g)
(
0.1 g), and solvent (1,4-dioxane, dimethylformamide or 2-
CuO
Cu O
2
1.5
1.4
–
–
–
–
methyltetrahydrofuran (5 mL)) were added to a 50 mL stainless
steel reactor from Hell. The reaction mixture was heated to 200 °C
under continuous magnetic stirring (1300 rpm) for 24–44 h. After
that the autoclave was cooled at room temperature, the catalyst
was recovered by filtration, and the products analyzed by CG-MS
using a Trace GC 2000 chromatograph coupled with a DSQ MS from
Thermo Electron Corporation. The chromatograph was equipped
with a non-polar GC separation column (Trace TR-1MS) operated
with He as carrier gas. The content of the leached metal in the reac-
tion liquid was determined by ICP-OES analysis (Agilent Technolo-
gies, 700 Series). In addition to the catalysts synthesised in this
1
wt% Cu
TiO2 Degussa P25
CuO/TiO
51.8
52.8
48.7
48.7
45.8
45.7
40.0
41.1
0.32
0.32
0.28
0.28
2
Cu
Cu
2
O_N
O_N
2
/TiO
/TiO
2
2
2
2
H
4
ZnO
CuO/ZnO
Cu O_N /ZnO
Cu O_N H /ZnO
29.3
27.9
27.4
27.1
30.1
30.0
29.8
29.7
0.31
0.30
0.30
0.30
2
2
2
2
4
Al
2
O
3
110.3
106.5
103.2
101.7
3.9
3.7
3.2
3.1
0.16
0.14
0.09
0.08
CuO/Al O3
2
study, CuCl
were used as reference catalysts.
2 2
, CuO and Cu O (>99.9% purity) from Sigma–Aldrich
Cu
Cu
2
O_N
O_N
2 2 3
/Al O
2
2
H
4
/Al
2 3
O
Active C
CuO/C
952.0
941.8
936.0
931.1
>1.0
>1.0
>1.0
>1.0
0.34
0.28
0.27
0.21
3
. Results and discussion
Cu
Cu
2
O_N
O_N
2
/C
2
2
H
4
/C
3
.1. Catalysts characterization
2 wt% Cu
CuO/TiO
2
50.8
46.4
46.3
45.5
39.8
40.7
0.31
0.26
0.25
Cu
Cu
2
O_N
O_N
2
/TiO
/TiO
2
2
All the supported copper catalysts were prepared by the DP
2
2
H
4
method, a general procedure for the preparation of supported

2
08
A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
2 2 2
Fig. 1. XRD patterns of 1 wt%CuO/TiO , 2 wt%CuO/TiO and TiO support.
of the spectra. Such a behavior may be explained by contributions
2+
assigned to electronic transitions of Cu ((i) a transfer of charge
2
ꢁ
2+
from O to Cu (band at 210–270 nm) and (ii) the presence of
+
oxo-Cu(II) species (band at ꢂ320–440 nm) [65]) and Cu species
(
the absorption band at about 350 nm attributed to existence of
+
nanooxide Cu species (band at 368–584 nm) [66] and band-to-
band transitions [67]). Interaction of the copper nanoparticles with
the support is also well proved by the measured band gap energies
(Fig. 2, Table 2). The deposition of copper led to a shift to higher
energies irrespective of the nature of the support. Then, the
increase of the copper loading from 1 to 2 wt% enhanced this shift.
The nature of the reducing agent also influenced this interaction as
resulted from the differences in the band gap of the samples trea-
ted in nitrogen or with hydrazine.
Figs. 3–6 show TEM and SAED images, and the particle size his-
tograms of the dispersed copper oxide particle for 1 wt%CuO/TiO ,
2
1
wt%Cu
2
2 2 2 2 4 2
O_N /TiO and 1 wt%Cu O_N H /TiO catalysts. 1 wt%
CuO/TiO
2
catalyst (Fig. 3) presented a high dispersion of copper
oxide with a narrow size distribution in which most of the particles
were smaller than 3 nm (Fig. 3B). This is in a perfect agreement
with the characteristics of the deposition-precipitation preparation
methodology [68] and XRD results. SAED analysis (Fig. 3C) presents
polycrystalline rings accounting to the presence of (101), (004),
(
2
200), (105), (204), (116) and (215) TiO -anatase planes (fisa
2
2
2
1-1272) and of (ꢁ111), (111) and (200) CuO planes (fisa 41–
54). The last ones correspond to interplanar distances d of
.524 Å, 2.323 Å and 2.311 Å, respectively.
Treatment of these catalysts under reducing atmosphere led to
a different morphology (Figs. 4 and 5). The heating under nitrogen
preserved as dominant particles those with size smaller than 4 nm
Fig. 2. DR-UV–Vis spectra of (A) 1 wt%CuO/TiO
Cu O_N /TiO ; (B) 1 wt%CuO/ZnO and 1 wt%Cu
Al and 1 wt%Cu O_N /Al catalysts and those of the corresponding supports.
2
, 1 wt%Cu
2 2 2
O_N /TiO and 1 wt%
(Fig. 4B). The copper oxide nanoparticles are still very well dis-
2
2
H
4
2
2
O_N /ZnO; and (C) 1 wt%CuO/
2
2
O
3
2
2
2 3
O
Table 2
Optical band-gap of support (TiO
2
, ZnO, Al
2
O
3
) and 1 and 2 wt% Cu catalysts.
Catalyst
Estimated band-gap (eV)
persed and XRD silent. However, the measured histogram evi-
denced co-existing particles with sizes till 14 nm (Fig. 4B). SAED
TiO
2
Degussa P25
3.068
3.086
3.382
3.371
3.175
3.398
3.383
1
1
1
2
2
2
wt%CuO/TiO
wt%Cu
wt%Cu
wt%CuO/TiO
wt%Cu
wt%Cu
2
analysis (Fig. 4C) besides, intense (101) plane of TiO
-anatase (fisa
2
2
O_N
O_N
2
/TiO
/TiO
2
2
1-1272) showed very weak reflexions assigned to (111) plane of
2
2
H
4
2
2
Cu
2
O (fisa 5-667) with an interplanar distance d of 2.46 Å.
Treatment of the catalysts with hydrazine led to more evident
rearrangements of copper nanoparticles on the support surface
Fig. 5B). SAED analysis indicated (Fig. 5C), besides the intense
(101) plane of TiO -anatase (fisa 21-1272), more intense reflexions
2
2
O_N
O_N
2
/TiO
/TiO
2
2
2
H
4
(
ZnO
2.870
3.036
3.014
1
1
wt%CuO/ZnO
wt%Cu O_N /ZnO
2
2
2
2
assigned to (111) plane of Cu O (fisa 5-667) with an inter-planar
Al
2
O
3
3.244
3.375
3.573
distance d of 2.46 Å.
1
1
wt%CuO/Al
wt%Cu O_N
2
O
3
Fig. 6 shows TEM, SAED image, and the histogram of the 1 wt%
O_N /TiO catalyst recovered from the reaction of aniline
2
2 2 3
/Al O
Cu
2
2
H
4
2

A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
209
2
Fig. 3. TEM picture (A), copper oxide size histogram (B), and SAED (C) for fresh 1 wt%CuO/TiO .
2 2 2
Fig. 4. TEM picture (A), copper oxide size histogram (B), and SAED (C) for fresh 1 wt%Cu O_N /TiO .
2 2 4 2
Fig. 5. TEM picture (A), copper oxide size histogram (B), and SAED (C) for fresh 1 wt%Cu O_N H /TiO .
with bromobenzene, after washing with dioxane by percolation.
Both the histogram (Fig. 6B) and SAED confirmed the fact that
the catalyst preserved the morphology during the reaction.
1 wt% copper on the investigated supports followed by calcina-
tion corresponded to specific XPS spectra (Fig. 8A, Table 3)
[71,72].
Fig. 7 shows the HRTEM images of 1 wt%Cu
2
O_N
2
H
4
/TiO
2
and
For titania as support, the Cu 2p main component for the sam-
ple before reaction (i.e. fresh catalyst) occurs at an unusually low
binding energy (930.57 ± 0.02 eV), and thus it is ascribed to nega-
tive Cu moieties (Fig. 8A, Table 3). Similar components were
detected also for Au deposited on other oxides [73]. The other com-
ponent, at 932.27 ± 0.03 eV, may be attributed to neutral or posi-
1
wt%CuO/TiO catalysts. They confirm the conclusions of the
2
TEM analysis concerning the size of supported copper nanoparti-
cles and crystallinity of the catalysts.
Table 3 and Fig. 8 present the results of the XPS analysis of
these catalysts. The supports exhibited the typical binding ener-
gies expected for the constituent elements (see Table 3), binding
energies for the Ti2p and O1s levels [69,70]. The deposition of
2
tively ionized Cu (e.g. in Cu O – like state). After reaction, only
one component subsists (the low binding energy one); the high

2
10
A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
2 2 4 2
Fig. 6. TEM picture (A), copper oxide size histogram (B), and SAED (C) for 1 wt%Cu O_N H /TiO separated from the reaction.
2 2 4 2 2
Fig. 7. HRTEM images of 1 wt%Cu O_N H /TiO (A) and 1 wt%CuO/TiO (B).
Table 3
XPS binding energy of the Cu2p, M2p (M = Ti, Zn, Al) and O1s levels.
Catalyst
Binding energy (eV)
Cu2p3/2
XPS high/low binding
energy ratio
Cu2p1/2
M2p3/2
M2p1/2
O1s
CuO
Cu
2
O
TiO
2
Degussa P25
–
–
458.4
457.7
464.0
464.0
528.8
528.0, 529.0
1
1
wt%CuO/TiO
2
933.4
952.6
0.12
0.25
0.17
0.22
0.31
0.14
0.27
0.32
4
58.6
457.7
58.6
457.7
58.6
457.7
58.6
457.7
58.6
457.7
58.6
457.7
58.6
457.7
58.6
wt%Cu O_N
2
2
/TiO
2
fresh
930.8
930.8
932.0
929.8
933.6
930.7
929.7
950.6
950.6
951.6
949.6
952.8
950.5
949.7
464.0
464.0
464.0
464.0
464.0
464.0
464.0
528.2, 529.7
528.7, 530.5
528.5, 530.3
528.2
4
After washing with dioxane
After reaction
4
4
1
2
2
2
wt%Cu
2 2 4 2
O_N H /TiO
4
wt%CuO/TiO
2
530.1
4
wt%Cu
wt%Cu
2 2
O_N /TiO
2
528.2
4
2 2
O_N H
4
/TiO
2
528.2
4
ZnO
–
–
1021.6
1021.6
1021.6
1044.5
1044.5
1044.5
530.7
530.3
530.3
1
1
wt%CuO/ZnO
wt%Cu O_N /ZnO
933.3
931.0
952.4
951.2
0.14
0.21
2
2
Al
2
O
3
–
–
74.3
74.3
74.3
74.7
74.7
74.7
531.1
530.7
530.7
1
1
wt%CuO/Al
wt%Cu O_N
2
O
3
933.6
931.2
953.1
951.4
0.10
0.17
2
2 2 3
/Al O

A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
211
binding energy component disappears completely (its amplitude
revealed by the fit is negligible). After washing, the high binding
energy component occurs again, with a more elevated binding
energy (933.30 eV), thus it may be attributed to CuO – like environ-
ments. The ratios between the high and the low binding energy
component for the Cu 2p spectra are quite similar before reaction
and after washing, 0.25–0.22. The fact that the respective spectra
look different is due to a higher broadening of the high binding
energy component in the spectrum taken before reaction.
Obviously, the nature of the support is influencing the interac-
tion with the metal particles thus controlling their oxidation state.
The XPS investigation of the catalysts in the present study also
showed a SMSI effect. For titania, the Ti 2p components are well
4
+
3+
in the both Ti
and Ti
states, with binding energies
4
58.56 ± 0.16 eV and 457.41 ± 0.29 eV, respectively (Fig. 8B,
3
+
4+
Table 3). The ratio [Ti ]:[Ti ] is 0.59 for the sample before reac-
tion, and increased to 1.00 after reaction, and drops to 0.78 after
washing. Expectedly, under these conditions the XPS binding
energy of Cu(I) species should be shifted to higher energies for
2
TiO support than for ZnO or alumina, that is not the case.
The O 1s spectrum is also deconvoluted with two components
with binding 528.42 ± 0.35 eV and 529.87 ± 0.75 eV; the spectrum
before reaction has also a weak component at about 534.3 V bind-
ing energy (Fig. 8C, Table 3). The ratio between the high and low
binding energy component is initially low (0.43), after reaction
becomes 3.05, and after washing is reduced to 1.54.
The increase of the copper loading from 1 to 2 wt% had no con-
sequence on the oxidation state of the investigated elements irre-
spective of the state of the catalyst (calcined, reduced, spent or
after washing with dioxane) (Table 3).
The binding energies of Zn, Al and O in these oxides fitted the
values reported in the literature for these supports [74–77]. Also,
the evolution of the binding energy of the Cu2p level followed
2
the same trend as commented in the case of the TiO supported
catalysts. Thus, binding energies of Cu(II) and Cu(I) for CuO/ZnO
catalysts were as well very close with those previously reported
in the literature [78].
The localization of Cu2 ions was determined by analyzing the
+
EPR spectrum in terms of the well-known spin-Hamiltonian
[
61,62]
H ¼ bB ꢀ g ꢀ S þ S ꢀ A ꢀ I þ I ꢀ Q ꢀ I ꢁ g b B ꢀ I
ð1Þ
n
n
where the corresponding terms have the known significance, i.e. the
electron Zeeman interaction, the electron-nuclear hyperfine inter-
action, the quadrupole interaction and nuclear Zeeman interac-
tions; b, b are electron and nuclear magneton, B is the applied
n
magnetic field, S and I are electron and nuclear angular moments,
and g, A, Q are g, hyperfine and quadrupole interaction tensors.
These tensors contain all the physical information about the elec-
tronic properties and localization of Cu(II) ion and a full information
could be obtained only for slightly doped single crystals. In poly-
crystalline powders, due to the orientation disorder of microcrys-
tals, the spectrum corresponds to an envelope of all oriented
spectra of microcrystals. For extracting the principal values of spin
Hamiltonian tensors spectra simulation must be done. If the lines
are broadened by several factors like: g and A strain [79], material
non-homogeneities, variation of local symmetry and coordination
around Cu(II) ion, superposition of different cluster spectra, the
spectrum is only partially resolved. For Cu(II) ions this case is very
often encountered. Even in such cases, when the spectrum is
resolved in the parallel band, by using empirical g||–A|| correlation
[
80,81], some conclusions about local symmetry and first neighbor
Fig. 8. XPS binding energies of the Cu2p (A), Ti2p (B) and O1s (C) levels of 1 wt%Cu/
ligand ions can be drawn. The experimental spectra have been
2
TiO catalysts in different stages.

2
12
A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
simulated using Bruker-Simphonia software. The EPR spectra are
noted specifying the sample type and indexes a, b, c, where a – fresh
prepared sample, b – sample after reaction, c – sample after reac-
tion washed with dioxane.
The EPR spectra of untreated supports are given in Fig. SI5. No
paramagnetic centers are in TiO
ZnO a strong narrow line (g = 1.96,
attributed to shallow donors in the semiconductor oxides [82–
4]. For active-C, like in many types of amorphous carbons, a wide
complex spectrum is observed; it is associated with free radicals
dangling bonds of the carbon network) and/or other paramagnetic
2
, a very week signal in Al
2
O3. In
D
B = 0.6 mT) is present. It is
8
(
impurities in the material. As such an initial paramagnetic spec-
trum interferes strongly with Cu(II) spectra, no more EPR measure-
ments were carried out for this support material.
Experimental data are presented in the following figures. In
Fig. 9 A and B are the spectra for CuO/TiO
respectively, for the fresh (a), used (b) and used and washed (c)
samples. A comparison of Cu O/TiO spectra reduced by N and
respectively N routes is given in Fig. 10 (notice that for a better
visualization the spectra are not scaled to the same mass).
Fig. 11A and B present the spectra for CuO/ZnO and Cu
ZnO, respectively, for samples (a, c), while Fig. 12 shows the spec-
2 2 2 2
and Cu O_N /TiO ,
2
2
2
2
H
4
2 _ 2
O N /
Fig. 10. X-band EPR spectra of Cu
2
O_N
2
/TiO
2
(A) and Cu
O_N
2
O_N
2 4 2
H /TiO (B) samples
(
c). In the inset is shown Q-band spectrum for Cu
2
2
H
4
/TiO .
2
tra of CuO/Al
2
O
3
and Cu
2
2
O_N2_Al O
3
, samples (a, c). To compare
the behavior of CuO and Cu
2
O on TiO
2
and ZnO supports, under cat-
alytic reaction, their spectra are given in Fig. SI6(A and B).
The spectra are strongly dependent on the support nature,
degree of reduction, and chemical treatment in the catalytic reac-
tion. For as prepared CuO catalysts, broad asymmetrical lines with
the region of such g|| and A|| values, will appear in the spectra of
reacted samples for all catalysts. In accordance with g||–A|| correla-
tion [80,81] the isolated Cu clusters are very probable of Cu(OH)2
ꢁ
4
type.
Significant changes were induced in all samples by the catalytic
reaction. They strongly depend on support and valence state of Cu
ions. An essential reduction of Cu(II) concentration after reaction is
g
av = 2.1–2.15 are observed (see Figs. 9A, 11A and 12A). Such lines
are interpreted as due to complex clusters of Cu(II) ions [85–87].
The reduction in N and N in general reduces the intensities
2
2 4
H
of Cu(II) spectra and narrows the lines. That is in accordance with
the general idea of reducing the valence of Cu ions to +1, valence
state silent for EPR spectroscopy; by decreasing the concentration
of Cu(II) ions dipole-dipole interaction becomes smaller and the
lines are less broadened.
2
observed for CuO/TiO samples (Fig. 9A samples), and the spectra
are not normalized to the same mass; their DIA reported to unit
mass are in the ratios 17/6/1. Therefore the reaction reduces the
Cu(II) concentration about 3 times, and after washing with dioxane
a further 6 time decrease takes place. That illustrates the fact that
The first comment is dealing with the effect of reducing CuO by
CuO deposited on TiO
Cu O_N /TiO (Fig. 9B, samples a, b, c) the same ratios are
/1.2/1.2, demonstrating that after reduction the Cu(II) ions
2
surface is very unstable. On the contrary for
treatment in N
as prepared samples reported to the unit mass have been calcu-
2 3
lated. The ratios DIA(CuO)/DIA(Cu O_N ) for TiO , ZnO, and Al O
2 2 4
and N H . For this the double integral areas for the
2
2
2
1
2
2
2
remaining on the support surface are very stable. Significant
modifications appear also in spectra shapes. Resolved structures
supports have the following values: 2.5, 1.5, 1.9. The degree of
reducing Cu(II) state to Cu(I) is therefore not very high, but evident.
in parallel band appear. Taking as example Cu
2 2
O_N /ZnO
It is interesting to notice that even for Al
ing a much higher specific surface than ZnO and TiO
is similar. In Cu O/ZnO superimposed on the broad spectrum a
weak spectrum with resolved structure in the parallel band was
observed (see Fig. 11A) having g|| = 2.33, g = 2.07, A|| = 11.4 mT.
2
O
3
support, material hav-
(Fig. 11A and B) the parallel band with g|| = 2.29, A|| = 11.0 mT is
2
, the reduction
clearly evidenced. As such changes are not associated with a signif-
icant reduction of Cu(II) concentration, the only explanation is a
change in Cu ion neighborhood due to reorganization processes
on support surface. As it was mentioned previously this type of
2
\
It corresponded to isolated Cu(II) ions, located at more stable posi-
tions on support surface. As it will be seen further, such spectra, in
4
Cu complex is very probably Cu(OH) ; it does appear also on
(a)
(
c)
(c)
b)
(
b)
(
(a)
A
B
240
280
320
360
240
280
320
360
Magnetic field (mT)
Magnetic field (mT)
Fig. 9. X-band EPR spectra of CuO/TiO (A) and Cu O_N /TiO (B) samples, fresh (a), after reaction (b), and washed with dioxane (c).
2
2 2 2

A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
213
Fig. 11. EPR spectra for fresh CuO/ZnO and Cu
dioxane by percolation (B). In the inset is EPR Q-band spectrum for sample Cu
2
O_N
2
/ZnO (A), and separate from the reaction of aniline with bromobenzene working with EtOK as base, after washing with
O_N /ZnO (c).
2
2
2 3 2 2 2 3 2 2 3 2 3 2 2 4 2 3
Fig. 12. X-band EPR spectra for (A) CuO/Al O and Cu O_N /Al O fresh samples; CuO_N /Al O base EtOK, and (B) Q-band spectra for CuO/Al O and Cu O_N H /Al O
5
t–C H
11OK base, (c) samples.
TiO
2
supported catalysts. Q-band measurements allow separation
EPR spectrum, an orthorhombic spectrum superposed on a single
line, which decreases on washing in dioxin, localized probable on
of spectra having close g factors, but also make differences
between paramagnetic centers having a disordered structure and
those with a more stable one. As the structural disorder reflects
in a larger distribution of EPR parameters, their spectra are spread
over a large magnetic field domain. In Q-band such PC might have
spectra spread over a large field domain so that are no more
observable. Hence, in Q-band, only centers with well-defined
structure give observable spectra. In Fig. 12B spectra in Q-band
the surface. In the case of ZnO and Al
line at g = 1.96 with p–p = 1.5 mT is worth noting in Q-band mea-
surements (inset of Figs. 11 and 12), associated with a donor like
center (shallow donor type). It is known that treatment in N atmo-
sphere induces such centers in other oxides [62]. Other defects are
observed for CuO/Al (Fig. 12) having g = 1.99 and g = 2.03; their
nature is not elucidated.
2 3
O supports, a strong single
D
B
2
2 3
O
of CuO respectively Cu
solved lines at g|| = 2.34 and 2.12 (Cu
2
2
O on Al
2
O
3
are given; Two groups of unre-
O), respectively g|| = 2.33 and
.15 are observed. According to their g-values a most probable
2
3.2. Catalytic C-N cross coupling
association is with Cu(OH)
4
2
Cu(OH )
4
complexes, respectively.
To establish the reaction conditions, experiments were carried
Their existence is not observable in X-band spectra, dominated
by broad lines. A very well resolved spectrum, quite weak in inten-
out in the temperature range 150–250 °C and for reaction times
from 12 till 48 h. Thus, in the case of the Cu O/TiO catalyst tem-
2 2
sity, is observed for Cu
parameters are: g|| = 2.23, g
2
O_N
2
H
4
/TiO
2
in Q-band (Fig. 10 inset). Its
= 2.9 mT.
peratures of 150 °C afforded yields smaller than 5%, while at
250 °C the increase in the yields in DPA was smaller than 5% due
to an increased selectivity in TPA. Also, at 200 °C, the increase in
the reaction time led to a decrease in the yield of DPA of around
3% due to the same increase in the selectivity to TPA.
\
= 2.05, A|| = 18.0 mT, A
\
Such a narrow spectrum is quite unexpected for a catalyst material,
and no assignment was made for it.
The two procedures of reducing Cu2+, with N
and N
2
2 4
H , leave
the catalyst surfaces with different disorder. The N
2
H
4
treatment
Diphenylamine (DPA) is the target reaction product of the C–N
coupling of bromobenzene and aniline, and it was indeed the dom-
inant product (in selectivity) in all the experiments performed in
dioxane and in the presence of EtOK, as base (see Scheme 1). How-
ever, by changing some parameters of the reaction conditions, the
catalytic C–N coupling reaction may proceed further to tripheny-
lamine (TPA), as secondary reaction product, or follow a second
route with the formation of biphenyl and aminobiphenyl.
produces a less disorder distribution of Cu2 ions, so the ordering
effect of catalytic reaction is more pronounced (see Fig. SI6).
A second important effect of reducing process and catalytic
reaction is the generation of paramagnetic defects. Ti3 centers
+
+
(
(
othorhombic g
x
= 1.93, g
y
= 1.97, g
z
= 1.98) and oxygen vacancy
sup-
g
av = 1.99) related centers are produced, in the case of TiO
2
port [82,88]. The former centers are generated in the nanoparticle
bulk, as their spectrum intensities do not change in dioxane
washed samples. The oxygen vacancies centers give a complex
Also, in the preliminary screening commercial homogeneous
anhydrous CuCl and heterogeneous CuO and Cu O catalysts were
2 2

2
14
A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
Scheme 1. Products observed in the amination of bromobenzene with aniline on the investigated supported Cu catalysts.
tested as reference catalysts. The reactions were carried out at
00 °C in 1,4-dioxane as solvent and in the presence of EtOK as
base. Under these conditions the yield of aniline was below 5%
with no diphenylaniline formation on CuCl
With few exceptions, the homogeneous catalysis on copper also
reported much smaller yields than catalysis on palladium. Most of
them were smaller than 60% [89], but very few peaks in between
Yields in DPA
Yileds in TPA
Selectivity in DPA
2
10 0S electivity in TPA
Selectivity in biphenyls
A
2
.
80
60
40
6
0-and 90% were also reported for selected conditions and sub-
strates [90]. However, these homogeneous syntheses have the
advantages of working at much lower temperatures and the disad-
vantage of impossibility to recover/recycle the catalyst.
Catalytic reports on CuO nanoparticles in the C–N coupling
were somehow controversial. Using bromobenzene as substrate
their reported yields were from zero till 60–80% [53,54]. In this
2
0
0
TiO2
ZnO
C
Al2O3
support
Yields in DPA
00
Yields in TPA
Selectivity in DPA
Selectivity in TPA
2
study, using the commercially available CuO and Cu O the results
confirmed a very poor behavior of these bulk oxides. Thus, after
a screening of the reaction conditions, CuO (50 mg) afforded yields
1
B
8
0
0
0
0
of 5% in dioxane and EtOK, 4% in THF/dioxane and t-C
toluene/dioxane and t-C 11OK, 3% in 2-Me-THF/THF and t-
11OK, and 4% in DMSO/THF and t-C 11OK, while Cu
50 mg) afforded yields of 8% in dioxane and EtOK, 4% in THF/diox-
ane and t-C 11OK, 3% in 2-Me-THF/THF and t-C 11OK, and 4% in
DMF/THF and t-C 11OK. These results emphasize both the influ-
ence of the particle size (very large particles for CuO and Cu O),
5
H11OK, 7%
5
H
6
4
2
5
C H
5
H
2
O
(
5
H
5
H
5
H
2
and the importance of the interaction of the dispersed particles
with the support. As generally accepted, the particle size of the
metal dispersed catalysts is an important parameter influencing
both catalytic activity and selectivity [51,52]. Somehow, these
0
TiO2
ZnO
C
Al2O3
support
Fig. 13. Aniline yields and selectivity to DPA/TPA, as a function of support, working
with 1 wt%CuO/support catalysts in the presence of (A) 1 mL t-C 11OK (2 M in
THF) and (B) 1.2 mL t-C 11OK (1.7 M in toluene) (reaction conditions: aniline
1 mmol):bromobenzene molar ratio of 1:1.2, 4 mL 1,4-dioxane, 100 mg catalyst,
00 °C, 24 h).
results also demonstrated that even in a bulky state Cu
slightly more active than CuO.
2
O is
5
H
5
H
(
2
Then, we attempted to optimize the reaction conditions using
wt%CuO/TiO as heterogeneous catalyst by investigating the
1
2
influence on the aniline conversion and selectivity to DPA of sev-
eral parameters such as the catalyst loading, substrate ratio, cata-
lyst amount, reaction temperature and time.
After this screening it was found that for 1.0 mmol aniline, the
optimal aniline-to-bromobenzene ratio was of 1:1.2, the mass of
catalyst of 100 mg, the temperature of 200 °C and the reaction time
The results show an influence on both support and solvent, in
where the base is dispersed. Thus, yields in THF/dioxane
Fig. 13A) are higher than in toluene/dioxane (Fig. 13B). While
working in THF/dioxane TiO and Al afforded the highest con-
versions, in toluene/dioxane the most active catalysts were those
supported ZnO and C, indicating an interaction of the solvent with
the support. As a general rule, higher yields corresponded to lower
DPA selectivities. Thus the selectivity to DPA was also related to
support and solvent. In THF/dioxane the most active catalysts
(
2
2 3
O
2
4 h. A blank control showed that no aniline conversion occurs in
the absence of the catalyst.
3.3. The influence of the catalyst support
(
2 2 3
i.e. 1 wt%CuO/TiO or 1 wt%CuO/Al O ) led to high selectivities
Fig. 13 shows the variation of the conversion of aniline and
in TPA. Also, biphenyl by-product was produced as well under
these conditions. In toluene/dioxane selectivities to DPA were
much higher exceeding 80%. In toluene/dioxane using CuO/ZnO
selectivity in DPA and TPA as a function of support for CuO-
based catalysts using t-C 11OK as base in either THF or toluene.
5
H

A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
215
as catalyst, DPA yield approached 40%. Under the investigated con-
ditions, working in THF/dioxane, the base is also reacting with bro-
Yields in DPA
00Selectivity in TPA
Yields in TPA
Selectivity in biphenyls
Selectivity in DPA
1
5
mobenzene producing the t-C H11 phenyl ether in percents smaller
A
than 30% with respect to the amount of bromobenzene introduced
in the reaction (not shown in figure). A slight increase of yield (less
than 5%) also occurred for higher reaction times. However,
extended reaction times were accompanied by a decrease of the
selectivity to DPA in the favor of TPA.
8
0
0
0
0
6
4
2
In a previous study considering a TiO
2
supported Au catalyst
[
59] the best yields were in between 30% and 40%. However, the
present results indicate that replacing Au with Cu led to slightly
higher yields with the important advantage of a cheaper metal.
The comparison with the literature also evidences the perfor-
mances of the investigated catalysts. Lipshutz et al. [48] and co-
workers reported the amination of aryl halides with alkyl-/
arylamines on copper and nickel oxide supported within charcoal
catalysts working also at 200 °C but under more energetic condi-
tions, i.e. additional exposure to the microwave irradiation. Amina-
0
TiO2
ZnO
C
Al2O3
support
Yields in DPA Yields in TPA Selectivity in DPA Selectivity in TPA
1
00
B
8
0
0
0
0
2
tion of bromobenzene on heterogeneous Au/TiO catalysts was also
reported at 200 °C and 24 h as more favorable conditions working
in both toluene and dioxane [59].
6
4
2
The reduction of copper under nitrogen or by N
changes in the catalytic behavior of the materials in THF/dioxane.
On Cu O_N /support the yields were slightly higher than on CuO/-
support catalysts Fig. 14A. However, in this series the treatment
under N exhibited a notable beneficial effect for Cu O_N /C on
which both the yield and DPA selectivity increased. More evident
was the effect of the reduction with N Fig. 14B. Except titania,
2 4
H led to some
2
2
2
2
2
0
TiO2
ZnO
Al2O3
support
2 4
H
which produced a slight increase in the activity, for all other sup-
ports the yields decreased. This decrease was accompanied by an
important increase in the selectivity to DPA, reaching almost com-
Fig. 14. Aniline yields and selectivity to DPA/TPA, as a function of support using (A)
wt%Cu O_N /support catalysts and (B) 1 wt%Cu O_N /support catalysts in the
presence of t-C 11OK (1 mL of 2 M t-C 11OK in THF), aniline (1 mmol):
bromobenzene molar ratio of 1:1.2, 4 mL 1,4-dioxane, 200 °C, 24 h.
1
2
2
2
2 4
H
5
H
5
H
2 2 4
plete selectivity for ZnO. No reaction was observed for Cu O_N H /
Scheme 2. By-products detected in the reaction of aniline with DMF in the presence of the catalysts and EtOK as base, using GC–MS analysis.

2
16
A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
2
C. As in the case of CuO/support catalysts, also for Cu O/support
Yields in DPA
Yields in TPA
Selectivity in DPA
Selectivity in TPA
Selectivity in C-C
catalysts increasing the reaction time from 24 to 48 h decreases
DPA selectivity with just a minor increase in yields. In addition,
the non-catalytic reaction of the base with bromobenzene occurred
in the same extent regardless the nature of the catalyst present in
the medium.
In fact, in all the investigated catalysts Cu(I) exists in tandem
with either Cu(II) (see alumina) and more reduced species (see tita-
nia). Besides deNOx catalysis [91] Cu has been applied in different
reactions in which Cu(I) was supposed to participate in the cat-
alytic process [78].
1
00
A
80
6
4
0
0
20
0
TiO2
ZnO
C
Al2O3
support
The nature of the support is influencing the interaction with the
metal particles thus controlling their oxidation state. Titania is
known for the induction of SMSI effects [92,93], while alumina
may react with the supported metals producing aluminates. ZnO
and carbons are less reactive but a metal-support interaction may
also occur in these cases as well. Giamello et al. [94] had shown that
copper ions deposed on ZnO are more resistant to reduction than
bulk CuO and remain in a partially oxidized state Cu(I).
The present study, by the XPS investigation of the catalysts con-
firmed the SMSI effect in the case of titanium. Expectedly, in the
presence of SMSI the XPS binding energy of Cu(I) species should
Yields ini DPA
Yileds in TPA
Selectivity in DPA Selectivity in TPA
100
80
60
40
20
0
B
be also shifted to higher energies for the TiO
ZnO or alumina, that was not the case. The absence of the Cu(II)
in the initial XPS spectra of Cu/TiO catalysts corroborated to the
2
support than for
2
TiO2
ZnO
Al2O3
existence of the negative-like charged species, might be assigned
to the preparation method. The formation of copper nanoparticle
prone to be reduced under the high vacuum produced during the
XPS measurements may provide such an effect. However, as TEM
and EPR measurements showed the support is not innocent in
the preparation of the catalysts and its effect is merely due to its
semiconductor properties. Even using the same preparation proce-
support
Fig. 16. Aniline yields and selectivity to DPA/TPA, as a function of support using (A)
wt%Cu O_N /support and (B) 1 wt%Cu O_N /support catalysts in the presence
of EtOK as base (2 mmol), aniline (1 mmol):bromobenzene molar ratio of 1:1.2,
mL 1,4-dioxane, 200 °C, 24 h.
1
2
2
2
2 4
H
5
dure, TiO
2
and ZnO afforded a higher dispersion. For TiO
2
the dis-
2 M t-C
in the conversion with almost no effect on the selectivity. In DMF
(1 mL 2 M t-C 11OK in THF and 4 mL DMF), the yields were much
5
H11OK in THF and 4 mL 2Me-THF) led to a slight decrease
persion of Cu ions is certainly larger as observed by TEM, and the
broad EPR component must be associated with Cu2 ions localized
on oxide surface. Spin–spin interaction and Jahn–Teller effect con-
tribute also to widening the spectral line. A smaller fraction of Cu
ions is placed on a specific surface site. Their spectrum has axial
symmetry, with g|| = 2.29, A|| = 12.5 mT, and g = 2.059. The pres-
ence of this second population of Cu ions is also observed for
the samples separated from the reaction of aniline with bromoben-
zene, after washing with dioxane (see for example Fig. 9A).
+
5
H
smaller (e.g. 5.3% for CuO/ZnO and 12.6% for Cu
no enhancement in the selectivity to DPA.
2 2 4
O_N H /ZnO) with
2
+
Furthermore, under the investigated conditions, DMF was found
to react with aniline in the presence of the catalysts leading to the
by-products described in Eqs. (2)–(4) (Scheme 2).
1
2+
The reaction between aniline, bromobenzene, DMF and inter-
0
mediates led to the formation of N,N-dimethyl-N -phenyl-formami
dine (M/z: 148 (M, 100), 149 (10.5), 147 (61.9), 133 (38), 106 (14),
104 (40), 78 (9.6), 77 (77), 51 (19)); N-phenyl ethylester metha-
nimidic acid (M/z: 149 (M, 28.5), 121 (47.2), 93 (100), 104 (44.3),
77 (43.2), 51 (34.4), 65 (30.5)) and N,N-dimethyl-aniline (M/z:
120 (M+1, 100), 121 (M, 81), 105 (11.7), 104 (13.5), 93 (14.3), 77
(24.6)). The formation of N,N-dimethyl-N -phenyl-formamidine in
the presence of EtOK was also previously confirmed by Pettit and
Garson [95].
3
.4. Influence of the solvent
In addition to dioxane, dimethylformamide (DMF) and
-methyltetrahydrofuran (2Me-THF) are other solvents commonly
2
0
used for C–N coupling. For all the investigated catalysts (i.e. CuO/-
support and Cu O/support catalysts) working with 2Me-THF (1 mL
2
Both aniline and bromobenzene may act as individual solvents
[96] and exhibit a high miscibility with the solvents investigated
in this study: 1,4-dioxane, dimethylformamide, 2-methyltetra-
hydrofuran or toluene. However, 1,4-dioxane, dimethylformamide,
Yields in DPA Yields in TPA Selectivity in DPA Selectivity in TPA
1
00
8
6
4
2
0
0
0
0
0
2
-methyltetrahydrofuran or toluene were used with the scope to
dilute the reactants in the interaction with the active sites and,
to avoid any additional competition. Depending on its polarity
the solvent may compete with the substrates for the catalytic sites.
Therefore the solvent dielectric constants may provide an estima-
tion of their capability to accommodate charged species. Thus,
the dielectric constants (e) of the investigated solvents around
TiO2
ZnO
C
Al2O3
Support
room temperature [97] varied in the same order with the catalytic
performances in the investigated conditions that might suggest,
indeed, such an influence (1,4-dioxane, 2.22, toluene, 2.38, 2-
methyltetrahydrofuran, 6.37, dimethylformamide 37.6). Further-
more, working with DMF as solvent, by-products obtained as a
Fig. 15. Aniline yields and selectivity to DPA/TPA, as a function of support using
wt%CuO/support catalysts in the presence of EtOK (2 mmol EtOK, aniline
1 mmol):bromobenzene molar ratio of 1:1.2, 5 mL 1,4-dioxane, 200 °C, 24 h). Note
that no reaction occurs for CuO/Al
1
(
2 3
O .

A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
217
Yields in DPA
Yields in TPA
Selectivity in DPA
Selectivity in TPA
loading to 2 wt% led to an increase of the conversion with no effect
on the selectivity. For the Cu O catalysts, the loading increase from
to 2 wt% decreased the catalytic activity, although DPA selectivity
was somehow preserved.
1
00
2
1
8
0
0
0
0
6
4
2
3.6. Extension of the scope of the reaction
2 2 2
Tables 4 and 5 show the activity of the CuO/TiO and Cu O_N -
H
4
/TiO
amines taking aniline as reference. The selection of EtOK was pre-
ferred to avoid the formation of t-C 11 phenyl ether observed
using t-C 11OK as base in either THF or toluene. However, work-
2
catalysts using EtOK as based in the coupling of a series of
0
1
%CuO
2%CuO
1%Cu2O_N2
2%Cu2O_N2 1%Cu2O_N2H4 2%Cu2O_N2H4
5
H
Titania
5
H
Fig. 17. The influence of copper content, on aniline yields and selectivity to DPA/
TPA, as a function of the catalyst, in the presence of EtOK (2 mmol) aniline
ing with alkoxides as base, it is difficult to avoid the formation of
an inorganic side-product (i.e. KBr) which would block the activity
of the catalysts. Reduced Cu O_N H /TiO showed a superior activ-
2 2 4 2
(
1 mmol):bromobenzene molar ratio of 1:1.2, 5 mL 1,4-dioxane, 200 °C, 24 h.
ity and selectivity to the product of the C–N mono coupling, con-
firming the results presented above. It is also noticeable the fact
that the selectivity to the target C–N mono coupling product
exceeded 94% in all the examples.
side-reaction with aniline, were detected using the GC–MS
analysis.
3.5. Influence of the base
3.7. Recycling tests and deactivation
Fig. 15 shows the performance of the various CuO/support cat-
alysts using EtOK as base. In the presence of this base, aniline con-
versions were significantly lower and even no reaction is observed
The ICP–OES analysis of the reaction products indicated no cop-
per leaching, i.e. concentrations smaller than 5 ppm. However, in
order to check the reusability of the catalysts, they were tested
in eights successive runs. Simple separation of the catalyst and
recycle were not effective, the catalyst being almost inactive. How-
ever, by treating the catalysts in dioxane under percolation at
105 °C the catalytic activity was recovered with a loss of 12% in
activity and no loss in selectivity. To confirm the causes of deacti-
vation additional electron paramagnetic resonance (EPR) and XPS
measurements were carried out. XRD patterns showed no changes
of the catalysts during the reaction (Figs. SI1–SI4).
2 3
for CuO/Al O . The increase in the amount of EtOK from 0.8 to
2
mmol even led to a decrease of aniline yields from 43% (0.8 mmol
EtOK) to 17% (2.0 mmol EtOK) due to the direct reaction of EtOK
with bromobenzene.
In total contrast with the negative influence that EtOK as base
plays for CuO/support catalyst, for the series of N
2
or N
H
2 4
reduced
catalysts, EtOK as base increases the activity. For Cu
2
2
O_N /catalysts
the yield was more than doubled with only a very small decrease in
the selectivity to DPA (Fig. 16A). Under these conditions the yields
on Cu
2
O_N
2
/TiO
2
catalyst were close to 40%. The selectivities to
Although comparison of the EPR spectra of the samples after
being used as catalysts revealed differences with those of the fresh
materials including the generation of paramagnetic species in the
support (both in the bulk and on the surface) and a decrease in
the intensity of Cu signal (specially for Al O ) with redistribution
2 3
among various localized sites, no conclusive indication of the ori-
gin of the deactivation could be gained.
TPA were very small and neither side product nor ether by the
reaction of bromobenzene with the base was produced. The exper-
iments with Cu
influence of EtOK for Cu
although still Cu O_N /Al
any catalytic activity (Fig. 16B). Like for the Cu
2
O_N
2
H
4
/catalysts showed a similar beneficial
O_N /TiO and Cu O_N /ZnO,
and Cu O_N /C did not exhibit
O_N /catalysts nei-
2
+
2
H
2 4
2
2
H
2 4
2
2
H
4
2
O
3
2
H
2 4
2
2
ther side products nor ether by the reaction of bromobenzene with
the base were produced.
In contrast, XPS analysis provided important information show-
ing the presence in the spectra of the K2s and Br3p1/2 levels for a
Fig. 17 shows the influence of the copper loading and reduction
2 2 2
Cu O_N /TiO , catalysts collected after reaction and percolated
treatment for the TiO
base. For non-activated CuO catalysts the increase of the copper
2
supported copper catalysts using EtOK as
with dioxane (Fig. 18). These spectra show that during the reaction
both K and Br remain chemisorbed on the catalyst surface, most
Table 4
2
Conversion of various amines and selectivities to the corresponding C–N products in dioxane using CuO/TiO as catalyst and EtOK as base.
Amine
Conversion (%)
Yields to C–N coupling (%)
Mono coupling
Selectivity to C–N coupling (%)
Mono coupling
Bis coupling
Bis coupling
Aniline
16.1
11
6.8
4.7
15.7
10.9
0.8
0.2
0.06
–
98
1.1
0.5
–
4
-Et-aniline
Morpholine
-Me-piperazine
99.5
100
100
1
4.7
–
–
Table 5
Conversion of various amines and selectivities to the corresponding C–N coupling products in dioxane, using Cu
2
O_N
2
4
H /TiO
2
as catalyst and EtOK as base.
Amine
Conversion (%)
Yields to C–N coupling (%)
Mono coupling
Selectivity to C–N coupling (%)
Mono coupling
Bis coupling
Bis coupling
Aniline
41.5
29.1
21
39.1
28.6
21
2.4
0.5
–
94.4
99.2
100
100
5.6
1.8
–
4
-Et-aniline
Morpholine
-Me-piperazine
1
7.6
7.6
–
–

2
18
A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
through
a Cu(I)-nucleophile formation, followed by Cu(I)-
mediated aryl halide activation that has been assigned as a rate
determining step. The aryl halide activation step involving an
oxidative-addition process either through a Cu(III)/Cu(I) redox-
couple, or via an electron-transfer process involving all three
oxidative states of copper (I, II, III) [98,99].
Heterogeneous approaches suggested quite similar routes in
which the (i) Ar–X substrate suffers a dissociative chemisorption
followed by a subtraction of X by the base and chemisorption of
the nucleophile and the production of Ar–Nu coupled product
[
54], or (ii) reversely, the first step corresponds to the chemisorp-
tion of the amine followed by the second chemisorption of the Ar–
X substrate [53]. Our DFT calculations [58] taking into account the
Cu O:CuO (111) surface suggested that aniline undergoes a stron-
2
ger adsorption than bromobenzene, thus suggesting that the for-
mation of Cu(I)-nucleophile corresponds to the first step of the
reaction mechanism. The presence of the strong base determines
the deprotonation of the aniline (which is already activated by
adsorption on Cu species), whereas the breakage of C-aryl–Br bond
is activated due to adsorption of bromobenzene on the copper
(
111) surface [100]. The dehydrogenated aryl amine attacked bro-
mobenzene with the formation of diphenylamine, which is the
main reaction product. In accordance with literature results and
these experimental results and kinetic investigation [101], the
cross-coupling reaction of aryl bromide with aniline takes place
following the Scheme 3.
4
. Conclusions
2
Copper oxide nanoparticles prepared by DP on ZnO or TiO can
be a suitable catalyst to promote C–N cross-coupling of aniline and
bromobenzene, although conversion and selectivity values are
strongly dependent on the nature of the support, solvent and nat-
ure of the base, without a simple rationalization of the optimal
conditions. Deposition of copper using the precipitation-
deposition route allowed the formation of highly-dispersed reduci-
ble species. For the fresh catalysts, under the high vacuum of the
measuring chamber, neutral or positively ionized Cu have been
detected. The ratios between the high and the low binding energy
component for the Cu 2p spectra are quite similar before reaction
and after washing, 0.25–0.22. However, the investigation of the
titanium binding energy levels confirmed the existence of the SMSI
effect by the presence of Ti 2p components in the both Ti4+ and Ti
states. The effect was also confirmed by the sensibility of the ratio
3+
3
+
4+
[
Ti ]:[Ti ] to the different environments: i.e. before reaction, after
reaction, and after washing.
Supplementary confirmation of the influence of the support
nature, degree of reduction, and chemical treatment in the cat-
alytic reaction were provided by the ESR measurements. They
show that the degree of reducing Cu(II) state to Cu(I) was not very
high, but evident. They also show the fact that CuO deposited on
Fig. 18. XPS spectra of the K2s (A) and Br3p1/2 (B) levels for the reaction separated
and percolated Cu O/TiO catalyst.
2
2
TiO
2 2
surface is very unstable, while, on the contrary, Cu O species
probably blocking the active sites. Although part of these ions was
found even after the percolation with dioxane, washings with this
solvent contributes to a substantial cleaning of the surface. The
remaining ions may also exhibit a negative role, decreasing the
activity with 12%. Thus, the most reasonable cause of catalyst deac-
tivation is the adsorption of KBr which is an inorganic byproduct
on the copper sites of the catalyst.
produced after reduction the Cu(II) ions were very stable. The two
procedures of reducing Cu , with N
surfaces with different disorder. The N
less disorder distribution of Cu ions, so the ordering effect of cat-
alytic reaction was more pronounced. A second important effect of
2+
2
and N
2
4
H , leave the catalyst
2
H treatment produces a
4
2+
reducing process and catalytic reaction was the generation of para-
magnetic defects. Ti centers (othorhombic g = 1.93, g = 1.97,
x y
3+
g
z
= 1.98) and oxygen vacancy (gav = 1.99) related centers are
3.8. Suggested reaction mechanism
produced.
Thus, the reduction treatments afforded the increase of the pop-
ulation of Cu(I), with a direct effect on the increase of the conver-
sion and very small changes in the selectivity. Among the supports
Under homogeneous conditions using copper as active metal,
Buchwald and co-workers suggested that the C–N coupling reac-
tion is initiated by a nucleophilic substitution which takes place
2
ZnO and TiO led to the better performances.

A. Tirsoaga et al. / Journal of Catalysis 341 (2016) 205–220
219
x
Scheme 3. Heterogeneous amination of bromobenzene with aniline in the presence of CuO /support catalysts and base.
The role of the base in this reaction is also very important.
11OK is a more efficient base, but using the formation of t-
11 phenyl ether was observed in either THF or toluene. A corre-
lation between the solvent and the nature of the support was also
evidenced. THF/dioxane is better for some supports (TiO , Al ),
while toluene/dioxane is more suited medium for others (ZnO
and active carbon). The catalyst undergoes deactivation upon reuse
that can be mitigated by washings. The most likely origin of this
deactivation is poisoning by the inorganic salts generated in the
reaction. Except this, the characterization of the catalysts indicates
that the other supplementary changes in the catalyst structure
were reversible. After reaction, only one component subsist (the
low binding energy one) and the high binding energy component
disappears completely. After washing, the high binding energy
component occurs again, with a more elevated binding energy
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