Dispersed copper oxide: A multifaceted tool in catalysis

Article abstract of DOI:10.1016/j.ica.2011.10.001

The preparation of highly dispersed copper oxide over different supports by means of a simple and versatile technique makes available a family of catalysts active for different kind of transformations. The proper choice of the inorganic matrix used as the support shapes the electronic properties of the deposited phase and, as a consequence, its catalytic behavior. Cu(0), Cu(δ+) and CuO nanoparticles resulted to be very active, respectively in hydrogenation reactions, NOx selective catalytic reduction by hydrocarbons and acid catalyzed formation of asymmetrical ethers starting from alcohols.

Full text of DOI:10.1016/j.ica.2011.10.001

Inorganica Chimica Acta 380 (2012) 194–200
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Inorganica Chimica Acta
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Dispersed copper oxide: A multifaceted tool in catalysis
a
b
c,
⇑
Nicola Scotti , Damiano Monticelli , Federica Zaccheria
a
Dip. CIMA ‘‘Lamberto Malatesta’’, Università degli Studi di Milano, Via Venezian 21, 20133 Milano, Italy
Università degli Studi dell’Insubria, Dipartimento di Scienze Chimiche e Ambientali, Via Valleggio 11, 22100 Como, Italy
CNR Istituto di Scienze e Tecnologie Molecolari, Via Golgi 19, 20133 Milano, Italy
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 12 October 2011
The preparation of highly dispersed copper oxide over different supports by means of a simple and ver-
satile technique makes available a family of catalysts active for different kind of transformations. The
proper choice of the inorganic matrix used as the support shapes the electronic properties of the depos-
ited phase and, as a consequence, its catalytic behavior.
Young Investigator Award Special Issue
Cu(0), Cu(d+) and CuO nanoparticles resulted to be very active, respectively in hydrogenation reactions,
NOx selective catalytic reduction by hydrocarbons and acid catalyzed formation of asymmetrical ethers
starting from alcohols.
Keywords:
Copper nanoparticles
Dispersed copper species
Hydrogenation
Ó 2011 Elsevier B.V. All rights reserved.
Acid catalysis
1
. Introduction
Metal oxides represent a large class of heterogeneous catalysts
active materials still represents an important issue, particularly
in the perspective of a wide application of catalysis to industrial
purposes [3,9,10].
widely used in industrial processes, in particular in bulk and petro-
leum chemistry. The dispersion of simple metal oxides as small
particles onto inorganic matrixes gives the opportunity to take
the most from simple inorganic compounds, otherwise poorly
reactive if used in their bulk form. Firstly, chemical reactions pro-
ceed more rapidly on a nanostructured material, due to the higher
surface/volume ratio. Moreover, the small particle size and the
interaction with the support strongly affect the electronic state
of the dispersed phase, and consequently its chemical properties:
a highly dispersed metal oxide is not merely a highly active surface
system, but can express a completely different reactivity [1–3]. The
control of particle shape is an even finer tool in order to tune the
distribution of different coordination atom sites, such as faces,
edges and corners. Coordinatively unsaturated atoms, as well as
low-coordination and defective sites (kinks, steps or adatoms),
generally show electronic and chemical properties that differ from
flat surfaces and lead to higher activity [2,4–8]. For the same rea-
son high-index facets show higher catalytic activity than low-in-
dex (e.g. {730} versus {100}) ones: in fact the former expose
high densities of atomic steps.
Traditional coprecipitation and impregnation techniques are
based on uncomplicated preparation steps employing simple and
cheap precursors. As a drawback, catalysts prepared in this way
present quite low dispersion, particularly for high metal loadings
and lack in particle size control and uniformity.
Coprecipitation techniques allow one to prepare supported,
mixed and also single component catalysts, but the process may
be difficulty controlled and features low flexibility as components
in the starting homogeneous solution need to be simultaneously
precipitated in a single material.
Likewise, impregnation techniques are quite easy preparation
procedures, but often lead to a spectrum of catalytic sites different
in size, shape and support interaction.
The general trend in new heterogeneous catalyst design has
been driven towards the production of well defined and uniform
active sites, in order to combine the advantages of homogeneous
catalysts with an easy product separation, recover, recycling and
a good stability of the catalyst [2,10–14].
The latest cutting-edge of single site heterogeneous catalysis
(SSHC) is nothing but the extreme expression of this strategy. This
particular approach aims at the preparation of catalytic systems
where the active sites are well-defined, isolated, evenly distributed
entities (single sites) with defined chemical surroundings, as in
conventional homogeneous catalysts or enzymes, while showing
all of the advantages of heterogeneous systems [3,15].
A simpler alternative to SSHC is chemical vapor deposition
(CVD) and related methods. CVD is an useful technique for the
preparation of well dispersed catalysts with quite high metal
Consequently, the synthesis of nanostructured materials sup-
ported on different matrixes is a well set and continuously pursued
topic in heterogeneous catalysis. To find reliable and easy prepara-
tion techniques, enabling the preparation of robust and highly
⇑
Corresponding author. Tel.: +39 0250314384; fax: +39 0250314405.
E-mail address: f.zaccheria@istm.cnr.it (F. Zaccheria).
0
020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.10.001

N. Scotti et al. / Inorganica Chimica Acta 380 (2012) 194–200
195
loadings, avoiding several step of the traditional method, such as
washing, drying, calcination and reduction [12,16–18].
are a direct consequence of the preparation method and are very
interesting tools for catalytic applications.
Unfortunately, preparation methods like SSHC and CVD, though
affording the synthesis of highly active heterogeneous systems,
frequently require sophisticated and tricky techniques or appara-
tus that lead to low reproducibility or scarce practical applicability,
particularly when large scale productions are sought. The search
for a particular morphology in the catalytic site calls for highly con-
trolled conditions in the preparation method, as much as the use of
elaborate, costly and unstable complexes as metal precursors
The road is therefore open to the preparation of a new family of
catalysts with improved performances with respect to the tradi-
tional ones.
2
. Experimental
2.1. Materials
[
3,15,17,19].
2
SiO2. A (Si A, BET = 320 m /g, PV = 1.79 mL/g), SiO
2
B (Si B,
(SiAl, 13wt.%
2 3
Al O , BET = 485 m /g, pv = 0.79 = mL/g) used were kindly supplied
The preparation of colloidal metal nanoparticles and their depo-
2
2 2 3
BET = 580 m /g, PV = 0.75 mL/g), and SiO –Al O
sition on a support is an alternative persecuted route. It usually al-
lows a good control of the shape and size of the particles at the
expenses of the handiness of the protocol. The synthetic procedure
is frequently complicated, leading to the coverage of the colloid
surface by organic polymers that can interfere with the catalytic
activity of the system: moreover the removal of these polymers
may affect the stability of the nanoparticles [2,20].
The use of copper, copper oxide and salts in catalysis is tradi-
tionally related with the use of systems with a very high metal
content (up to 40%) for gas phase heterogeneous reactions, such
as CO oxidation, conversion of syn-gas into methanol and water–
gas shift [9,21–25].
On the other hand, the spread of innovative preparation tech-
niques for supported nanoparticles has enabled the application of
copper catalysts to a wide variety of reactions due to the enhanced
activity provided by the high dispersion.
Metal vapor synthesis (MVS) has been used to obtain materials
with high metal dispersion, used for Ullman reactions [26] as well
as for oxygen activation [27,28].
2
2
by Grace Davison (Worms, Germany); TiO
was purchased by Degussa.
2
P25 (Ti, BET = 50 m /g)
2
.2. Catalyst preparation
Copper catalysts, with a 7–8% metal loading, were prepared as
follows: the support (10 g) was added to a 0.7 M [Cu(NH
tion prepared starting from Cu(NO
of water and adding NH OH until pH 9. After 20 min under stirring,
the slurry, held in an ice bath at 0 °C, was slowly diluted in order to
allow hydrolysis of the copper complex and deposition of the finely
dispersed product to occur. Under these conditions, no dissolution
of silica was detected. The solid was separated by filtration,
washed with 0.5 L of water, dried in oven overnight at 120 °C,
and calcined in static in air at 350 °C for 4 h. In all cases the Cu con-
tent was ca. 8 wt.% (Table 1). Metal loadings were determined by
inductively coupled plasma (ICP)-QMS (X series II, Thermo Fisher
Scientific) and an external calibration methodology, after micro-
2+
3
)
4
]
solu-
3
)
2
2
ÁH O (4 g) dissolved in 25 mL
4
A different approach has been employed in the precipitation-gel
PG) technique, involving the addition of an aqueous NaOH solu-
3
wave digestion of fresh and used catalysts in HNO .
(
The Incipient Wetness (IW) sample was prepared by impregnat-
ing the support with a copper nitrate solution of proper concentra-
tion and volume in order to obtain a 8% loaded catalyst.
tion and of colloidal silica to the precipitate suspension, in order
to stabilize the microparticles of the precipitate and simulta-
neously form a gel. These materials have been proposed for the
glycerol hydrogenolysis into 1,2-propandiol [29].
A microwave assisted protocol introduced for noble metals has
been successfully extended to copper catalysts and used for C–S
coupling reactions [30].
2.3. TPR
TPR profiles were recorded with a modified version of the
Micromeritics Pulse Chemisorb 2700 apparatus; catalysts (25 mg)
were diluted with an equal amount of quartz, calcined at 500 °C
Here we report on the use of an unconventional technique,
hereafter called chemisorption–hydrolysis (CH), which enables to
obtain highly versatile copper based catalytic systems in an easy
and reproducible way.
2
under O (40 mL/min) and then reduced at 8 °C/min with a 8%
2
H /Ar mixture at 15 mL/min.
Chemisorption–hydrolysis represents a fruitful trade-off, as it
combines the simplicity of impregnation techniques, both in terms
of handiness (easy experimental procedure, simple apparatus) and
cheapness, with the high metal dispersion obtained by anchoring
techniques.
The possibility to prepare highly dispersed samples by means of
a particular preparation technique smoothes the way to diverse
applications of the metal oxide. Here we will show that the high
dispersion of a low loading (7–8%) copper oxide on silica and sil-
ica-alumina results in (i) easy reducibility into Cu(0) phase, (ii)
CuO species in strong interaction with the support very active in
deNOx reaction, (iii) unusual acidic activity. These peculiar features
2.4. Voltammetry
Cyclic voltammetric experiments were performed employing an
AMEL 433 polarograph (Amel s.r.l., Milan Italy) equipped with a Pt
wire auxiliary electrode and an Ag/AgCl saturated reference elec-
trode using a 5 mV/s sweep rate from +400 to À700 mV. The pow-
ders were immobilized on the electrode as follows. The three
starting materials (the two catalysts Cu ZSM-5, Cu/SiAl and the
bulk CuO) were suspended in water (100 mg in 10 mL) and ultra-
sonically stirred for 10 min. Fifty microliters of the suspensions
were deposited onto the surface of three cleaned glassy carbon
Table 1
2 2 2 3
Hydrogenation of 3-methylcyclohexanone over Cu/SiO and Cu/SiO –Al O at 60 °C (270 °C activation temperature).
2
TOF 10 (mmol/mmolCu h)a
À2
TOF 104 (mmol/mmolCuexp h)b
Entry
Catalyst
V
in (mmol/h)
17
0.5
6.0
Productivity (mmol/gcat h)
SSACu(0) (m /g)
1
2
3
Cu/Si A
Cu/SiAl
1.30
0.07
0.55
97
3.6
21
93
5
34
7.0
10
12
Cu/TiO
2
a
Turnover frequency calculated with respect to the total amount of copper.
Turnover frequency calculated with respect to exposed metallic copper.
b

1
96
N. Scotti et al. / Inorganica Chimica Acta 380 (2012) 194–200
electrodes (3 mm diameter): the suspensions were allowed to dry
at room temperature (12 h). The modified electrodes were subse-
quently covered with a Nafion film by direct deposition of a 5%
Nafion solution in ethanol (Aldrich): the solvent was slowly evap-
orated at 18–19 °C to allow the formation of a uniform film.
CuO and Cu-ZSM-5 were utilized for comparison purposes in
CYV experiments. CuO (Aldrich Cat. No. 241741-powder) was used
as purchased. Cu-ZSM-5 (1.51 wt.% Cu loading, 80% exchange level)
was prepared starting from H-ZSM-5 (Si/Al = 25) by ion exchange
with a 0.1 M copper acetate solution. H-ZSM-5 (Si/Al = 25) was ob-
The high dispersion grants on one hand the possibility to easily
reduce the oxide phase and on the other hand the formation of
well-formed copper crystallites, already shown to be highly active
in hydrogen activation [39–41] and alcohol dehydrogenation [42].
The unique hydrogenation activity observed for catalysts pre-
pared in this way can be conveniently highlighted by results ob-
tained in the carbonyl reduction carried out with CuO/MOx
prereduced with hydrogen.
2
In particular reduced Cu/SiO reveals outstanding performances
in the reduction of 3-methylcyclohexanone into the corresponding
mixture of cyclohexanols (Scheme 1) under very mild conditions,
namely, atmospheric pressure of hydrogen and 60 °C. It is worth
to underline that Pt and Ru systems promote hydrogenation of ali-
4
tained from NH -ZSM-5 (PQ Corp.(CBV5020)) by a thermal treat-
ment at 773 K in air for 5 h. Full details about the preparation
procedure are reported elsewhere [31].
The Cu(0) surface area was determined at Micromeritics USA
2
phatic ketones at 25–60 °C and 1–5 atm H , whereas Cu chromites
laboratories by using the N
2
O adsorption–decomposition method
and Ni catalysts require about 50 atm pressure and temperatures
with a AutoChem II 2920 instrument. The Cu(0) surface area was
computed by using a surface coverage factor (moles of oxygen
atoms per moles of surface Cu(0) atoms) Q = 0.35 and a mean sur-
face area for a copper site of 7.41 Å.
higher than 100 °C [43].
The comparison with copper catalysts prepared with the same
technique over titania and silica alumina as supports, allows to
put in light some interesting points related to the active site in-
volved in the hydrogenation reaction. In Table 1 are reported some
diagnostic parameters for the evaluation of the catalytic perfor-
mances in relation to the textural properties of the catalyst, namely
specific surface area of the metallic phase, initial reaction rate,
turnover frequencies and productivity. The clearest evidence is
the much higher activity of Cu/Si with respect to Cu/Ti and Cu/SiAl
in terms of rate and productivity, also confirmed by reaction pro-
files obtained for the complete conversion of the substrate (Fig. 1).
However, in spite of a difference of more than one order of mag-
nitude in the initial reaction rate (0.48 versus 6 and 17) and in
turnover frequencies per total amount of copper, the difference
in TOF evaluated per active site smoothes down. The use of turn-
over frequency as a measure for the catalytic activity is a common
practice in catalysis, as it allows one to directly compare different
systems normalizing performances with respect to the active site,
that rarely correspond to the total amount of metal.
2.5. Catalytic tests
Hydrogenation reactions were carried out with prereduced cat-
alysts, that were treated for 20 min in air at 270 °C (or 400 °C) and
for 20 min under rotary vacuum and thereafter reduced in static at
2
the same temperature with 1 atm of pure H . In a typical hydroge-
nation test, the prereduced catalyst (100 mg, Cu 8 wt.%), 3-methyl-
cyclohexanone (0.9 mmol) and 6 mL of n-heptane as solvent were
stirred in a glass reaction vessel under hydrogen (1 atm) at 60 °C.
HC-SCR experiments was carried out as already reported [32]. A
typical etherification reaction was carried out by filling the reactor
containing the unreduced catalyst with 2-propanol (6 mL) as the
solvent with a solution of 4-methoxyphenylethanol (0.65 mmol)
under nitrogen.
The reaction mixtures in both cases were analyzed by GC–MS
(
Agilent 5975C) by using a HP-5 column and the products were
Comparable TOFs per exposed metal site, as was found to be the
case in 3-methylcyclohexanone reduction, strongly support the
hypothesis that hydrogenation reaction occurs on the same active
site, that is Cu(0), whatever is the support used for the catalyst
characterized by NMR.
3
. Results and discussion
3.1. Cu(0) species
O
OH
Most of the industrial scale hydrogenation processes are carried
out with molecular H
2
in the presence of a catalyst (heterogeneous
Cu cat
or homogeneous), rather than with an appropriate hydrogenation
agent (e.g. metal hydride). The most used catalysts are based on
noble metals (such as Pd, Pt, Rh, Ir and Ru) as well as non noble
ones like Ni, Cr, Fe, Co and Cu. However, the leading role is tradi-
tionally up to noble metal based catalysts and nickel ones,
although the use of these systems often implies safety concerns
and high costs.
H
2
(1 atm), 60°C
Scheme 1. Reduction of 3-methylcyclohexanone into 3-methylcyclohexanol.
Hydrogenation reactions with copper catalysts mainly rely on
the use of copper chromites, particularly for reduction of esters, ke-
tones and aldehydes into alcohols and for methanol synthesis.
These catalysts are known for their selectivity toward the hydroge-
nation of the carbonyl group, leaving almost unaffected the C–C
double bond: for that reason copper chromite catalysts find impor-
tant industrial application for the partial hydrogenation of vegeta-
ble oils and fatty acids. However deactivated copper chromite
catalysts raise serious environmental concerns, and therefore they
must either be recycled or reclaimed. This led to the need for the
preparation of inexpensive Cr free copper catalysts that could re-
place the traditional Cr-containing catalysts [13,24,33–38].
Chemisorption hydrolysis technique leads to highly dispersed
copper oxide, thus representing an easy access to finely dispersed
metallic copper particles.
Fig. 1. Hydrogenation profiles of 3-methylcyclohexanone with Cu/SiO
2 2
, Cu/SiO –
Al and Cu/TiO
2
O
3
2
.

N. Scotti et al. / Inorganica Chimica Acta 380 (2012) 194–200
197
preparation. In the case of Cu/SiO
cific surface area (SSACu) measured by N
icantly lower than the one measured for Cu/SiO
initial reaction rate. Notwithstanding, the single active site has
comparable hydrogenation capacity.
2
–Al
2
O
3
the metallic copper spe-
O chemisorption is signif-
, thus giving lower
Nevertheless, as shown by data reported in Fig. 3, the reduction
temperature does not influence to a significant extent the hydroge-
nation activity of copper supported over silica alumina, thus con-
firming that even if reduced at higher temperature the pure
metallic copper exposed does not increase.
2
2
Such a marked difference in the fraction of copper(0) sites ob-
served when using catalysts prepared on different supports is
strongly related with the interaction of the parent copper oxide
phase with the oxidic matrix used.
On the other hand a slight decrease in activity was observed
when Cu/SiO is reduced at higher temperature, probably due to
2
some aggregation phenomena. Mobility of highly dispersed metal
particles when treated at high temperature is a well known phe-
nomenon in highly dispersed metallic systems, already shown to
In fact, on SiO
duced after calcination, easily converted into metallic Cu small par-
ticles by reduction in H , whereas on SiO –Al mainly isolated
2
, small and well-dispersed CuO particles are pro-
2
take place in the case of Cu/SiO [44].
2
2
2 3
O
d+
copper ions, likely belonging to a surface Cu-aluminate-like phase
and strongly resistant to reduction, are obtained [44].
3.2. Cu( ) species
These features are clearly highlighted by TPR analysis (Fig. 2).
TPR profile of CuO/Si shows the presence of a single symmetric
peak centered at 223 °C; such a low temperature is diagnostic of
the presence of a highly dispersed copper oxide phase in a single
and reducible state. The reduction of CuO bulk starts indeed at
low temperature (around 190 °C), with the maximum shifted to
The particular nature of the unreduced oxide obtained over
silica alumina can conversely be exploited for other kind of
2
76 °C due to the broadness of the whole peak, related to CuO
dimension. The peak obtained with CuO /Ti can be ascribed mainly
to the reduction of CuO particles having little interaction with the
support. CuO /SiAl shows two main peaks with maxima located,
respectively at 251 and 283 °C, compatible with the presence of
two different CuO species, the one referred to the higher temper-
ature being CuO in strong interaction with the support. The cata-
x
x
x
x
lyst prepared by simple impregnation technique (IW) is less easily
reducible (Tmax = 332 °C) due to the presence of species interacting
with the support as well [45,46].
Fig. 3. Conversions obtained in 3-methylcyclohexanone hydrogenation with Cu/
SiO and Cu/SiO –Al O prereduced at 270 and 400 °C.
2 2 2 3
Fig. 4. CYV cyclic voltammograms of Cu ZSM-5, Cu/SiAl and CuO immobilized on a
Fig. 2. TPR profiles of CuO/Si, CuO
x
/SiAl, CuO
x
/Ti, CuO/Si (IW) and CuO bulk.
glassy carbon electrode (scan speed 5 mV/s).

1
98
N. Scotti et al. / Inorganica Chimica Acta 380 (2012) 194–200
copper can migrate to the electrode through an ion exchange
mechanism. This mechanism cannot be active with bulk CuO
which accordingly shows a single broad peak. It is worth noting
that this behavior is observed only in the presence of chloride ions
because, as suggested by El Murr et al. [50], they stabilize the inter-
mediate Cu+ species.
The voltammetric analysis therefore supported the evidence
that copper was present as exchanged ion in the oxidized Cu/SiAl
catalyst. The formal Cu(II) oxidation state predicted by voltammet-
+
ric analysis is not in contrast with the Cud state observed with the
previously reported spectroscopic investigations as they mainly re-
fer to an electron density state rather than to the number of elec-
trons necessary to fully reduce the copper ion.
The particular electronic properties of the oxidic phase when
dispersed over this silica alumina can be conveniently exploited
for catalytic applications other than hydrogenation. In particular,
the similarity with copper-exchanged zeolites prompted to test
these systems in the selective catalytic reduction in oxidizing
atmosphere (HC-SCR) for NOx abatement.
According to Márquez-Alvarez et al., HC-SCR can be performed
in any Cu-based system containing small cupric oxide particles and
some acidity [51]. The catalyst preparation method is a critical fac-
tor for improving the de-NOx activity of copper dispersed catalysts
together with the choice of a suitable support with acidic proper-
ties and a wide surface to disperse the copper phase.
The superior performances observed for copper-exchanged zeo-
lites (particularly ZSM-5) compared to amorphous supported sys-
tems has been ascribed to the capacity of zeolites in dispersing
the active copper at atomic level [52]. On the other hand, high
mechanical and hydrothermal stability are desirable properties
for the support.
Fig. 5. Conversion of NOx to N
function of reaction temperature in SCR (initial concentrations: ca. 1500 ppm of
NOx and of C and 15000 ppm of O ).
2 2 4
and conversion of C H to carbon oxide as a
2
H
4
2
Chemisorption–hydrolysis revealed to be an adequate method
to prepare nanodispersed catalysts when supported over SiO
Al to be used for this kind of catalytic application. Thus, 35%
of NOx conversion into N and 80% of C conversion to carbon
oxides in the HC-SCR can be obtained over Cu/SiO –Al at
conversions
2
–
2 3
O
applications. Activity of Cu/SiO
2
–Al
2
O
3
in the carbene insertion
2
2 4
H
coming from methyl phenyldiazoacetate, into one C–H bond of
THF has been recently reported [47], thus constituting the first
example of this reaction promoted by a purely inorganic catalyst.
In this synthetic application, Cu/SiAl leads to better results regard-
ing yield and catalyst recovery than Cu/Si, due to the in situ reduc-
tion of Cu(II) into Cu(I) by the diazocompound, confirming that
different redox properties of the supported Cu phase influence
the behavior of the catalyst depending on the support.
2
2 3
O
350 °C (see [32] and Fig. 5). Comparable NO to N
(41%) were reported over Cu-ZSM-5 at 250 °C [52].
2
3.3. CuO species
A step forward in exploiting as much as possible the versatility
of this kind of copper catalysts is represented by the use of dis-
persed copper oxide as an acidic catalyst, even if supported over
non acidic matrixes.
In fact, the main feature of the Cu-aluminate-like phase present
on SiO –Al O , as emerged from previous spectroscopic investiga-
2 2 3
tions (EXAFS, EPR, XPS, and IR of adsorbed NO and CO), is the pres-
ence at the surface of isolated copper species with oxidation state
ranging from (II) to (I), with a significant decrease in the amount of
the Cu(II) species in favor of Cud+(1 < d < 2).
Acidic properties in heterogeneous catalysts usually derive from
conventional acidic functions such as – OH groups in molecular
sieves or clays or exchanged metal ions as Lewis acid [53].
Dispersed copper over a non acidic silica obtained with the
chemisorption–hydrolysis technique can be used as a heteroge-
neous acid catalyst, by virtue of its high dispersion.
As an example of this behavior, already reported for alcoholysis
reactions of epoxides with different alcohols [54], we report the
activity of copper oxide over silica in the direct etherification of
4-methoxyphenylethanol with 2-propanol (Scheme 2 and Table 2).
Ether formation is actually an important reaction in organic
synthesis, for both bulk and fine chemicals preparation, and it is
traditionally performed with the Williamson reaction starting from
an alcohol and a halide by using a strong base for the alkoxide
formation.
+
To better understand the nature of these Cud species, cyclic
voltammetric measurements have been performed. The voltam-
mograms have been registered using a glassy carbon electrode
on which a thin powder film of the calcined catalyst is deposited
and protected with a Nafion film (see the experimental section
for details). In Fig. 4 the cyclic voltammograms of Cu/SiAl are
shown together with the ones of Cu/ZSM-5 (80% i.e.) and of bulk
CuO which represent typical ionic and oxidic systems, respectively.
The figure clearly shows that the Cu/SiAl system strongly resem-
bles the electrochemical behavior of the ionic Cu/ZSM-5, showing
two well separated reduction and oxidation peaks. According to
the interpretation by Brendlé and Louati [48], the cathodic reduc-
tion signals can be ascribed to the two monoelectronic reductions
Cu++ ? Cu+ ? Cu(0). This mechanism is normally operative only if
Cu(II) is present in ionic form: since both ZSM-5 and SiAl are insu-
lating materials, it is likely that the reduction (and reoxidation) of
copper occur at the surface of the electrode [49], meaning that
Nonetheless, Brønsted acid catalysts are known to promote
ether formation starting from the corresponding alcohols by means
of a dehydration process [55]. In a recent communication, Sn-Beta
has been reported as active heterogeneous Lewis acid for the
asymmetric etherification of benzylic alcohols and aliphatic ones
[56].

N. Scotti et al. / Inorganica Chimica Acta 380 (2012) 194–200
199
OH
OH
O
+
H
3
CO
3
H CO
Scheme 2. Asymmetrical ether formation directly starting from two alcohols.
Table 2
Etherification reaction of 1-phenylethanol and 2-propanol over different catalysts.
dissolution of the support is also expected and both unsupported
CuO and a disordered, amorphous surface layer containing Cu, Ti,
oxygen and nitrogen will be produced after calcination. Upon
reduction an amorphous overlayer covering in a continuous way
the TiO crystallites is formed, thus resulting in a low metallic sur-
2
face area and therefore in a poor catalytic activity.
Entry
Catalyst
t (h)
Conv%
Sel%
1
2
3
Si B
CuO/Si B
CuO/Si B IW
24
5
24
0
60
12
-
100
25
a
On the contrary when preparing the catalyst by the CH method
a
The main by-product reasonably being the symmetrical ether.
2
+
the pH of the Cu(NH
3
)
4
]
solution is well above the zero charge
point of the support, the titania surface is negatively charged favor-
ing the adsorption of Cu cations , while nitrate ions are removed by
washing (Fig. 6). In this case the ionic exchange reaction can ac-
count for the high dispersion of CuO observed on calcined CH sam-
ples. The reduction of this well dispersed phase gives rise to well
formed Cu crystallites. This translates into much higher metallic
surface area and hydrogenation activity, due to the presence of
low index facets, whereas the dispersion of the CuO phase in the
unreduced sample accounts for its acidic behavior.
The bare silica resulted to be completely inactive under the
reaction conditions used (entry 1), whereas the unreduced copper
catalyst supported over the same silica is able to promote the con-
densation reaction with excellent selectivity and good activity (en-
try 2).
These results unravel the unexpected acidity of this material,
where none of the partners shows acidic activity itself. Thus,
although the use of Cu(II) salts as Lewis acids is known [57], re-
ports on CuO are lacking. On the other hand, the use of chromato-
graphic silica ensures the inertness of the support used under these
conditions.
4
. Conclusions
Among the preparation methods used in heterogeneous cataly-
The sample prepared by traditional Incipient Wetness tech-
nique with the same copper loading resulted to be almost inactive
under the reaction conditions used, confirming the peculiar prop-
erties given by the particular preparation method used.
sis, chemisorption–hydrolysis represents a powerful technique in
order to combine high activity and handiness. This protocol is reli-
able and versatile, giving the opportunity to properly choose the
support in order to tune the catalytic activity or selectivity, thus
ranging over very different kind of purposes, aimed both to fine
chemicals preparation and environmental remediation [60].
Thus, results reported in the present paper show that the same
preparation technique leads to different catalytic sites just by vary-
ing the inorganic matrix, while always keeping a remarkable dis-
persion. On the other hand the reductive treatment allows one to
switch from an acidic catalyst to an hydrogenation one.
3
.4. The influence of the preparation method
The examples reported in this work highlight the positive effect
of the high dispersion of the supported phase in these materials,
which is a direct consequence of the catalysts preparation method.
The difference between the CH and the IW method was deeply
elucidated comparing, by means of different spectroscopic tech-
Moreover the use of a non noble, non toxic and non pyrophoric
metal shelters from several economical and safety concerns.
2
niques of two sets of Cu/TiO samples with a Cu loading of 2%,
4
% and 8% prepared according to the two methods [58]. A highly
significant difference was already evident by comparing the XRD
patterns of the two 8% samples. Only incipient formation of CuO
crystallites was observed on the CH sample whereas the presence
Acknowledgements
of gerhardtite, Cu
2
NO
3
(OH)
3
was evident on the IW (8% Cu) sample
Dr. Nicoletta Ravasio is warmly acknowledged for her stimulat-
ing and precious teaching. The authors thank Prof. Sandro Recchia
for his help in cyclic voltammetry analysis and the Italian Ministry
of Education, University and Research for financial support through
the Project ‘‘Ital- NanoNet’’ (Rete Nazionale di Ricerca sulle Nano-
scienze; prot. no. RBPR05JH2P).
with an estimated particle size of this phase of 65.5 nm.
The formation of a gerhardite phase can be due to the low pH of
the impregnating solution (pH 3–4) which is below the zero charge
point for TiO
2
(around 6). Accordingly, the positively charged sur-
face will adsorb anionic species, that is copper nitrates [59]. Some
Fig. 6. Formation of copper nanoparticles over silica by means of the chemisorption–hydrolysis technique.

2
00
N. Scotti et al. / Inorganica Chimica Acta 380 (2012) 194–200
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