β-cyclodextrin as inverse phase transfer catalyst on the electrocatalytic hydrogenation of organic compounds in water

Article abstract of DOI:10.1016/j.electacta.2011.10.094

The optimum conditions for the electrocatalytic hydrogenation (ECH) of benzaldehyde in water, using a nickel sacrificial anode (SA) (referred to as ECH-SA) and β-cyclodextrin (β-CyD) as inverse phase transfer catalyst (IPTC) were determined. Four parameters were investigated: the morphology of the nickel deposited on the cathode matrix (Cu, Fe, Ni or Fe/Ni alloy (64:36)) during a pre-electrolysis, the size of the CyD cavity, the concentration of β-CyD, the supporting electrolyte concentration and the current density applied. The results showed that a Ni matrix together with ultrasound pre-electrolysis treatment allowed a nanostructured nickel deposit on the cathode surface. Under the best electrolysis conditions (2.8 mmol dm ^-3 of β-CyD, 1.0 mol dm^-3 of NH₄Cl and a current density of 330 mA dm^-2), the yield of benzyl alcohol (99%) was 27% higher than that obtained under the same conditions but in the absence of β-CyD. Taking into account the hydrophobic character of the β-CyD, the best conditions of the ECH-SA method were applied to the hydrogenation of a variety of organic substrates. Excellent yields and current efficiencies were obtained with arylbenzaldehydes and acetophenone. ECH-SA of styrene gave moderate yield and current efficiency, and the hydrogenation of a terminal non-conjugated olefin (safrole) was not efficient.

Full text of DOI:10.1016/j.electacta.2011.10.094

Electrochimica Acta 59 (2012) 270–278
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
␤
-cyclodextrin as inverse phase transfer catalyst on the electrocatalytic
hydrogenation of organic compounds in water
a
b,∗
Márcio Vilar , Marcelo Navarro
a
Instituto Federal de Educa c¸ ão, Ciência e Tecnologia de Pernambuco, Cidade Universitária, CEP 50740-540 Recife PE, Brazil
Departamento de Química Fundamental, C.C.E.N, Universidade Federal de Pernambuco, Cidade Universitária, CEP 50670-901 Recife PE, Brazil
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 August 2011
Received in revised form 14 October 2011
Accepted 22 October 2011
Available online 6 November 2011
The optimum conditions for the electrocatalytic hydrogenation (ECH) of benzaldehyde in water, using a
nickel sacrificial anode (SA) (referred to as ECH-SA) and ␤-cyclodextrin (␤-CyD) as inverse phase trans-
fer catalyst (IPTC) were determined. Four parameters were investigated: the morphology of the nickel
deposited on the cathode matrix (Cu, Fe, Ni or Fe/Ni alloy (64:36)) during a pre-electrolysis, the size of
the CyD cavity, the concentration of ␤-CyD, the supporting electrolyte concentration and the current
density applied. The results showed that a Ni matrix together with ultrasound pre-electrolysis treatment
allowed a nanostructured nickel deposit on the cathode surface. Under the best electrolysis conditions
Keywords:
Cyclodextrin
Electrocatalytic hydrogenation
Inverse phase transfer catalyst
Inclusion effect
−
3
−
3
−
2
(2.8 mmol dm of ␤-CyD, 1.0 mol dm of NH4Cl and a current density of 330 mA dm ), the yield of
benzyl alcohol (99%) was 27% higher than that obtained under the same conditions but in the absence of
␤-CyD. Taking into account the hydrophobic character of the ␤-CyD, the best conditions of the ECH-SA
method were applied to the hydrogenation of a variety of organic substrates. Excellent yields and current
efficiencies were obtained with arylbenzaldehydes and acetophenone. ECH-SA of styrene gave moderate
yield and current efficiency, and the hydrogenation of a terminal non-conjugated olefin (safrole) was not
efficient.
Ultrasound
©
2011 Elsevier Ltd. All rights reserved.
1
. Introduction
phase [6,7]. A PTC system offers numerous advantages over tra-
ditional methodologies, e.g.: (1) an increase of the reaction rate
under mild conditions; (2) the use of inexpensive, recoverable,
and non toxic solvents, (3) commercially available catalysts, (4)
inexpensive inorganic bases for anion generation, (5) improve-
ment of the yield and enantioselectivity of the products, and (6)
application in continuous operations [6,7]. PTC also can be widely
applied in the organic synthesis laboratory [8–11] and industrial
processes, e.g., pharmaceutical and agrochemical industries, as well
as in monomer synthesis and polymer modification [7–12].
Substrates like olefins [13], arenes [14], aromatic and aliphatic
nitriles [15], saturated aldehydes [16] and ketones [17] have been
hydrogenated under such conditions. However, the use of insol-
uble compounds leads to an overall yield drop of the method,
due to mass-transport limitation between the layers. Conventional
techniques can be used to circumvent this mutual insolubility prob-
lem: (1) rapid agitation, (2) surface active agents [18], (3) water
soluble coordination complex supported on silica or mesoporous
glass beads [19–21], and (4) the use of a co-solvent [22] which
exhibits both lipophilic and hydrophilic properties. The addition
of co-solvent may eliminate the phase separation and provide a
homogeneous mixing state for the reaction taking place. Conse-
quently, the use of co-solvent is generally the only alternative for
the industrial chemist wishing to hydrogenate highly hydrophobic
substrates in water. Although, co-solvents might solve the mutual
The difficulty to separate the products from expensive metal cat-
alysts through an easy and clean pathway is the main disadvantage
of the homogeneous catalysis versus heterogeneous catalysis. There
are some approaches described in the literature to overcome this
problem. In a first approach, called supported liquid phase catalysis,
a porous solid support is impregnated with an organic or aqueous
solution containing the dissolved homogeneous catalyst [1,2]. In a
second one, the organometallic catalyst is impregnated into inor-
ganic supports or organic polymers [3]. However, the catalysis in
aqueous/organic two-phase system is the most effective, safe and
economical approach, containing water soluble transition metal
complexes commonly named phase transfer catalysis (PTC) [4–6].
Normally, phase transfer reactions are carried out in hetero-
geneous two-phase systems, in which exists a negligible mutual
solubility of the phases, and a transfer agent (catalyst) is used to
cause or accelerate the reaction between substances that are dis-
solved in different phases. The catalyst, a source of lipophilic cations
located in the organic phase, continuously introduces reagent
anionic species in the form of lipophilic ion pairs into the organic
∗ Corresponding author. Tel.: +55 81 21267460; fax: +55 81 21268442.
E-mail address: navarro@ufpe.br (M. Navarro).
0
013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.10.094

M. Vilar, M. Navarro / Electrochimica Acta 59 (2012) 270–278
271
Scheme 1. (A) ␣-1,4 bond representation in CyDs. (B) General structure of ␣-, ␤-, and ␥-CyD (n = 6, 7 and 8, respectively).
insolubility problem, they can also cause certain disadvantages
such as competing hydrolysis pathways and difficulties in their
purification and removal [7].
reacts with the unsaturated organic molecules (Y Z) (Eqs. (4) and
(5)) adsorbed on the catalyst (cathode) surface, followed by des-
orption of the hydrogenated product (Eq. (6)) and regeneration of
the adsorption sites.
The use of cyclodextrins (CyDs) as inverse phase transfer catalyst
(
IPTC) is a promising methodology to promote both electrocat-
H O + e− + M → MHads + H O
+
(Volmer)
(1)
(2)
(3)
(4)
(5)
(6)
3
2
alytic and catalytic hydrogenation of insoluble substrate in water.
Accordingly to Harada [23], CyDs can act as counter-phase agents,
working in the reverse manner of PTC. CyDs are cyclic oligosaccha-
rides made up of six, seven or eight (␣-, ␤-, and ␥-CyD, respectively)
glucopyranose units linked by ␣-1,4 bonds (Scheme 1) and are often
described as a torus, but are somewhat more realistic pictured as a
shallow truncated cone and have approximate Cn symmetry [24].
As a result of its structural shape, a CyD presents both large and nar-
row cavities, where the wider side is formed by secondary 2- and
2
MHads → M + H2
(Tafel)
H O + MHads + e− → M + H + H O
+
(Heyrovsky)
3
2
2
Y
Z + M → M(Y Z)ads
2MHads + M(Y Z)ads → M(YH–ZH)ads
M(YH–ZH)ads → M + YH–ZH
3
-hydroxyl groups (responsible for macrocyclic stability), whereas
In this paper we describe an ECH system using an undivided
cell and Ni as sacrificial anode (ECH-SA), water as solvent and ␤-
CyD as IPTC. Benzaldehyde was adopted as the standard substrate.
Moreover, factors as IPTC type and amount, pre-electrolysis condi-
tions, current density effect, supporting electrolyte concentration
and structural variations of water-insoluble organic substrates on
both encapsulation and conversion processes were evaluated. This
study is relevant from both electrocatalytic and green standpoint
because no studies have yet been reported concerning the use ␤-
CyD as IPTC (in water) on the ECH-SA technique.
the narrower side is structured by primary 6-hydroxyl groups. The
inside of the cavity is lined up with oxygen atoms involved in the
glycosidic ring (C1 and C4) and hydrogen atoms connected to C3
and C5, thus conferring to the cavity a hydrophobic character. The
presence of free hydroxyl groups on the outside gives to the CyD
a hydrophilic character [25–27]. This structural character allows
the use of these compounds as hosts and consequent formation of
inclusion (host/guest) complexes in water.
The first use of such inclusion agent has been reported by
Zahalka et al. [28] and Harada et al. [29] who employed CyD in
the oxidation of terminal alkenes to the corresponding methyl
ketones. Concerning catalytic hydrogenation, Monflier and co-
workers [30] reported the use of both ␤-CyD and dimethyl-␤-CyD
in the hydrogenation of water insoluble aldehydes at water/toluene
solution. The authors showed clearly that ␤-CyD was more effi-
cient (34–72% of conversion) than common co-solvents such as
methanol, tetrahydrofuran, dimethylformamide and acetonitrile,
when performing the hydrogenation of aldehydes containing a
larger number of carbon atoms. In the field of electrocatalysis, the
number of related articles with the use of ␤-CyD is limited. For
instance, Maiorova et al. [31] have applied ␤-CyD in the electrore-
duction of acetophenone in both water and water/ethanol solutions
reaching 90% of conversion.
2. Experimental
2.1. Chemicals
Benzaldehyde, 2-methyl-benzaldehyde, 4-methyl-benzal-
dehyde, 4-ethoxy-benzaldehyde, 2-fluoro-benzaldehyde, 4-fluoro-
benzaldehyde, 2-chloro-benzaldehyde, 4-chloro-benzaldehyde,
3-hydroxy-4-methoxy-benzaldehyde, 2-furfuraldehyde, thio-
phene-3-carbaldehyde and safrole were purchased from Aldrich.
Acetophenone, styrene, 4-isopropyl-benzaldehyde and 4-N,N-
dimethylamino-benzaldehyde were purchased from Fluka. All
reagents were 99+% purity and used as received. Ammonium
chloride was purchased from Vetec. Water was of Milli-Q® grade.
␣- and ␤-CyD (Aldrich) were used as received.
The examples above show that ␤-CyD can be advantageously
used to tailor the reactivity of organic compounds. Furthermore, the
electrochemical hydrogenation approach provides an interesting
means to hydrogenate organic compounds under mild conditions
2.2. Electrocatalytic hydrogenation methodology
(
(
room temperature and pressure). Electrocatalytic hydrogenation
ECH) has been carried out since the beginning of the last cen-
ECH-SA controlled current preparative electrolyses were car-
ried out with Autolab PGSTAT30 potentiostat/galvanostat, using
tury [32]. In the ECH process, the hydrogen source at the metal
M) surface comes from the reduction of the solvent or the sup-
porting electrolyte (Eq. (1)); unwanted competing (Eq. (2) and/or
3)) reactions, which reduce the current efficiency, may occur lead-
an undivided cell of 10.0 mL capacity. A cylindrical nickel rod was
(
used as sacrificial anode (0.1 dm diameter; 0.078 dm2 area). Fe/Ni
alloy (64:36), Cu, Fe and Ni rods (all 0.1 dm diameter; 0.094 dm2
area) were used as working electrodes, which can be reused sev-
eral times after removing the nickel deposit through abrasion and
(
i
n
g
t
o
t
h
e
f
o
r
m
a
t
i
o
n
o
f
H
g
a
s
.
T
h
e
c
h
e
m
i
s
o
r
b
e
d
h
y
d
r
o
g
e
n
(
M
H
)
2
a
d
s

2
72
M. Vilar, M. Navarro / Electrochimica Acta 59 (2012) 270–278
◦
◦
−1
polishing processes. Water was the solvent in all electrochemical
experiments.
OV-5 column, using a 60–200 C temperature range (10 C min ).
Comparisons with authentic samples were performed by gas
chromatography/mass spectrometry in order to identify the hydro-
genation products and reagents. Toluene (0.067 mmol) was used as
an internal standard to quantify the products and reagents.
Scanning electron microscopy (SEM) was used for the mor-
phologic analyses of the electrodeposited nickel particles on the
cathode and was carried out with a Quanta 200 FEG instrument at
the Electron Microscopy and Microanalysis Laboratory of the Center
of Strategic Technologies of the Northeast, Brazil.
In order to evaluate the CyD influence on the electrocatalytic
−
3
system, aqueous stock solutions of ␣- (7.0 mmol dm ) and ␤-CyD
(
−3
3.0, 5.0, 7.0 and 9.0 mmol dm ) were prepared. A 4.0 mL aliquot of
the stock solution was mixed with an appropriated amount of stan-
dard substrate (1.0 mmol benzaldehyde) and stirred for about 30 h.
After that, the electrolytic cell containing the working electrode
(
cathode: Fe/Ni alloy (64:36), Cu, Fe or Ni) and the sacrificial anode
−3
was filled with 6.0 mL of 0.33 mol dm (2.0 mmol) solution of the
supporting electrolyte (NH Cl). The general procedure used in this
4
work was preceded by a pre-electrolysis of the water/supporting
electrolyte solution under stirring and in the absence of substrate,
aiming to activate the cathode surface by eliminating induction
periods and making a thin nickel deposit layer on the cathode
matrix. In addition, the pre-electrolysis was performed in an ultra-
sound bath (Branson 1510) with the application of a constant
3. Results and discussion
3.1. ECH-SA process optimization using ˇ-CyD and benzaldehyde
as substrate
3.1.1. Comparision of the effect of ˛- and ˇ-CyD
−2
current density (j = 280 mA dm ), during 15 min (consumption of
4 C).
After pre-electrolysis the solution containing both ␣- or ␤-CyD
and substrate was added to the electrochemical cell. At this point,
The carbonyl group of molecules in which it is conjugated to
an aromatic ring can be selectively hydrogenated under mild con-
ditions, which is useful for obtaining the corresponding aromatic
alcohol. For instance, p-isobutyl acetophenone hydrogenation
leads to 1-(p-isobutylphenyl)ethanol, a key step for Ibuprofen®
synthesis [34]. In addition, the hydrogenation of benzaldehydes has
industrial importance in the synthesis of pharmaceuticals, flavors,
fragrances, chemical intermediates and photographic chemicals
[35]. The results concerning the ECH-SA of benzaldehyde in the
presence of different ␤-CyD concentrations are summarized in
Table 1.
As can be seen, the ␤-CyD concentration increase (Table 1,
entries 2–4) is proportional to the hydrogenation product yield,
furnishing a gain of 18% when compared to the same reaction in
the absence of the IPTC (Table 1, entry 1), which is related to the
better solubilization of the benzaldehyde in the reaction medium.
Indeed, the current efficiency of the ECH-SA is directly proportional
to the ratio of the rate of hydrogenation (Eqs. (4)–(6)) over the rate
of H2 generation (Eqs. (2) and (3), and mainly Eq. (3) under cata-
lyst polarization), as was demonstrated by Mahdavi et al. [36]. The
maximum ratio (maximum current efficiency) should correspond
to a complete saturation of both organic substrate adsorption sites
and Hads sites. Part of the ␤-CyD effect is due to the organic sub-
strate solubilizaton, which is low soluble in water. As the ␤-CyD
concentration increases (Table 1, entries 1–4), more Y Z should be
solubilized and diffuse to the cathode, which is the main reason for
the increase of the hydrogenation yield.
2
−
3
the final concentrations of species in solution were: 2.8 mmol dm
− −3
3
␣
-CyD or 1.2, 2.0, 2.8, 3.6 mmol dm ␤-CyD; 0.2 mol dm sup-
−3
porting electrolyte and 0.1 mol dm substrate. Electrolyses were
−
2
carried out by using a constant current density of 330 mA dm
and stirring until total consumption of the charge required for the
complete hydrogenation of the C O or C C double bond. The use
of the nickel sacrificial anode during the electrolysis allows a slow
but constant renewal of the active adsorption sites of the catalyst
surface avoiding its poisoning [33].
The electrolysis current density effect was investigated in the
−2
range of 175–580 mA dm , under the experimental conditions
described above. The effect of the supporting electrolyte concen-
−
3
tration was also investigated, using 0.2, 0.5 and 1.0 mol dm of
NH Cl.
4
The ultrasound influence on the electrodeposited nickel mor-
phology, during the pre-electrolysis step, was another parameter
studied. For this purpose, the pre-electrolysis procedure was
performed in two ways: (1) the system was introduced in the
ultrasound bath for 15 min by applying a current density of
−
2
2
80 mA dm , and (2) the system was stirred without the ultra-
sound irradiation (silent condition) by application of the same
current density. Both electrolyses were performed under the exper-
imental conditions described above. After determination of the best
ECH-SA conditions, the reliability, reproducibility and suitability of
the proposed method were evaluated by electrolysis of different
substrates.
Another effect is related to the weak adsorption of ␤-CyD on
the catalyst (cathode) surface, which ensure a high concentration
of Y Z near the surface and its efficient diffusion, furnishing a high
hydrogenation yield and current efficiency. However, when the
␤-CyD concentration becomes too high (Table 1, entry 5), the com-
plexation equilibrium (␤-CyD/benzaldehyde) is shifted towards the
encapsulation of species in solution, thus more Y Z remains in the
At the end of all electrolyses, the water solution was extracted
with diethyl ether, dried with Na SO4 and analyzed by gas
2
chromatography (GC). The chromatograms were recorded with a
Thermo Finnigan Focus GC model fitted with a 30 m × 0.25 mm
Table 1
CyD effect on the ECH-SA of benzaldehyde.a,b
Entry
Cyclodextrin
[CyD]final (mmol dm−3)
Hydrogenation yield (%)
−
1
−1
−1
3.0 F mol
2.0 F mol
2.5 F mol
1
2
3
4
5
6
␤
␤
␤
␤
␤
0.0
1.2
2.0
2.8
3.6
2.8
72
82
87
90
80
82
83
92
92
98
92
90
91
98
98
100
98
96
c
␣
a
−3
−2
Pre-electrolysis conditions: 6.0 mL of water; 0.2 mol dm of NH4Cl; j = 280 mA dm ; Cu cathode matrix; ultrasound; Ni sacrificial anode.
b
−3
−3
−2
Electrolysis conditions: addition of benzaldehyde (0.1 mol dm ) + 4.0 mL of ␤-CyD solution (0.0, 3.0, 5.0, 7.0 and 9.0 mmol dm ) to the electrolytic cell; j = 330 mA dm
2
(
current density (j) was determined from surface area of the electrode (0.094 dm )); Ni/Cu cathode; Ni sacrificial anode; stirring.
c
Both pre-electrolysis and electrolysis conditions were the same as those applied to the ␤-CyD system.

M. Vilar, M. Navarro / Electrochimica Acta 59 (2012) 270–278
273
Table 2
Table 3
a,b
Current density effect on the ECH-SA of benzaldehyde using ␤-CyD as IPTC.
Supporting electrolyte influence on the ECH-SA of benzaldehyde using ␤-CyD as
a,b
IPTC .
Entry
jc (mA dm−2
)
Hydrogenation yield/current efficiencyd (%)
Entry
[NH4Cl] (mol dm−3)
Hydrogenation yield (%)
2
.0 F mol 1
−
2.5 F mol
−1
−1
3.0 F mol
−
1
−1
−1
3.0 F mol
2.0 F mol
2.5 F mol
1
2
3
4
5
6
7
175
250
330
400
450
515
580
81/81
86/86
90/90
87/87
76/76
71/71
68/68
89/71
96/77
98/78
95/76
92/73
88/70
84/67
95/64
98/66
100/67
97/65
97/65
95/64
94/63
1
2
3
0.2
0.5
1.0
90
91
98.5
98
98
–
100
–
–
a
−3
Pre-electrolysis conditions: 6.0 mL of water; 0.2, 0.5 and 1.0 mol dm NH4Cl;
−
2
j = 280 mA dm ; Cu cathode; ultrasound; Ni sacrificial anode.
b
−3
Electrolysis conditions: addition of benzaldehyde (0.1 mol dm ) + 4.0 mL of
a
−3
−3
−2
−3
Pre-electrolysis conditions: 6.0 mL of water; 0.2 mol dm
of NH4Cl;
␤-CyD solution [(7.0 mmol dm ) − final concentration of 2.8 mmol dm ] to the
−2
j = 280 mA dm ; Cu cathode; ultrasound; Ni sacrificial anode.
electrolytic cell; j = 330 mA dm ; Ni/Cu cathode; Ni sacrificial anode; stirring.
b
−3
Electrolysis conditions: addition of benzaldehyde (0.1 mol dm ) + 4.0 mL of
−3
−3
␤
-CyD solution [(7.0 mmol dm ) − final concentration of 2.8 mmol dm ] to the
−2
electrolytic cell; j = 175–580 mA dm ; Ni/Cu cathode; Ni sacrificial anode; stirring.
c
molecules, transition metals and some inorganic salts like per-
chlorates, thiocyanates, chlorides, nitrates, etc. [40,41]. It has been
proposed that some salts can be included within the cavity of
CyDs, competing with the organic guest for binding sites and,
consequently, lowering its binding strength [42]. Yi et al. [41],
for instance, showed the influence of inorganic salts (KF, K2SO4,
NaAc, KCl and NaClO4) on the equilibrium constants. The authors
Current density (j) was determined from surface area of the electrode
2
(
0.094 dm ).
d
Current efficiency = (Qtheoretical/Qexperimental) × conversion.
␤
-CyD cavity and the effective concentration of free Y Z should
decrease. Also, when the concentration of ␤-CyD is too high, the
weak ␤-CyD layer on the catalyst surface can become thicker, mak-
ing the Y Z adsorption difficult with, as a consequence, a drop of
the hydrogenation yield.
−
2−
−
observed that hydrophilic anions (F , SO4 and Ac ) do not form
inclusion complex (1:1) with the ␤-CyD whereas lipophilic anions,
−
−
such as ClO4 and Cl , do form inclusion complex with the cav-
ity of the ␤-CyD. We have also investigated the influence of the
supporting electrolyte on the inclusion of benzaldehyde in the
hydrophobic cavity of the ␤-CyD [38], by monitoring the cathodic
peak current in the cyclic voltammogram as a function of time.
Experimental data revealed that alkali cations such as K+ and Na+
does not affect the magnitude of the current cathodic peak and
hence the inclusion phenomena. Additionally, the nature of the
salts did not influence the inclusion phenomena in water since the
During the formation of the CyD/guest complex, the stereoselec-
tive discrimination arises from the substrate structural differences
and is related to its adaptation to the CyD cavity [37]. Hence, small
molecules form complexes with both ␣- and ␤-CyD due to the
existing compatibility between the substrate volume and the CyD
cavities. In the case of ␥-CyD, if the substrate is very small the fit-
ting becomes unfavorable due to the larger size of the CyD cavity.
Indeed, the use of the ␣-CyD in the ECH-SA process leads to the
production of 82% of benzyl alcohol (Table 1, entry 6), which rep-
resents an hydrogenation yield increase of 10%, when compared to
that obtained in the absence of the IPTC (Table 1, entry 1), however
−
current peak intensity for lipophilic and hydrophilic anions (Cl ,
2
−
−
SO4
ment.
and Ac ) remains practically the same during the experi-
8
% lower than the hydrogenation yield observed in the presence
of ␤-CyD (Table 1, entry 4). Therefore, the smaller cavity of the
-CyD (0.53 and 0.47 nm) does not favor the complexation equilib-
rium (CyD/benzaldehyde), which is better supported by the larger
-CyD cavity (0.65 and 0.60 nm). This observation was confirmed
in a recent paper [38], in which the inclusion constant value deter-
In previous papers, the best ECH-SA experimental conditions
have been determined by varying the following parameters:
inert gas, supporting electrolyte, co-solvent, temperature, cathodic
matrix, etc. [43–45]. The best supporting electrolyte was NH4Cl,
which was explained by the complexation of Ni2+ generated dur-
ing the sacrificial anode oxidation with NH3, thus avoiding the
formation of insoluble Ni(OH)2. Indeed, NH4+ ions act as a buffer
producing the soluble [Ni(NH3)6]Cl2 salt which is reducible at a
␣
␤
−
1
3
mined for the ␤-CyD/benzaldehyde system (1086 ± 84 mol dm )
was three times that observed for the system ␣-CyD/benzaldehyde
−1
3
2+
(
322 ± 27 mol dm ).
higher potential than the free Ni . Table 3 shows the effect of
NH Cl concentration on the ECH-SA process of benzaldehyde using
4
␤
-CyD as IPTC. The increase of the supporting electrolyte concen-
3
.1.2. Current density and supporting electrolyte influence in the
−
3
tration from 0.2 to 1.0 mol dm (Table 3, entries 1 and 3) enhances
the hydrogenation yield from 90 to 99% for a charge of 2 F mol
which can be correlated to an increase of the solution conductivity
and to the complexation of Ni2+ cations by NH3 [43]. The yield of
hydrogenation in entry 3 (99%) corresponds to an increase of 27%
over that obtained in the absence of ␤-CyD (72%, Table 1, entry
presence of ˇ-CyD
−
1
,
The current density effect on both conversion and current effi-
ciency of the ECH-SA for benzaldehyde hydrogenation is shown
in Table 2. As can be noticed, these parameters increase concomi-
tantly with the increase of current density from 81% (entry 1) to
9
0% (entry 3) showing that the hydrogenation process (Eqs. (4)–(6))
1
).
is favored over hydrogen evolution (Eqs. (2) and (3)). Conversely,
−
2
current density values higher than 330 mA dm (Table 2, entries
–7) decreases the performance of the process. In fact, at a higher
4
3.1.3. Ultrasound and cathodic matrix effects on the nickel
deposit morphology
current density, the potential of the cathode is shifted to more nega-
tive values. This affects the rate of H generation by electrochemical
The application of ultrasonic irradiation is one of the most
promising tools in heterogeneous catalysis. The method has been
applied to polymerization [46], sonoeletrosynthesis [47], sono-
chemical hydrogenation, etc. [48–50]. The acoustic irradiation
during the pre-treatment of the catalyst has been shown to
enhance the durability, enantioselectivity and activity of the cata-
lyst [50–52]. The cause of the reaction rate increase has frequently
been associated to cavitation effects near the catalyst surface. A
powerful jet is produced when the cavitation bubble collapses,
2
desorption of Hads (Eq. (3)) without affecting the rate of hydrogena-
tion (Eqs. 4–6) since the sites are fully occupied by (Y Z)ads and
Hads. Hence, the increase of the current density favors the H2 evo-
lution over the hydrogenation in agreement with published data
[
33,35,39].
Before discussing the supporting electrolyte influence in the
ECH-SA process, we would like to make some considerations. It is
well known that CyDs are able to complex with several organic

2
74
M. Vilar, M. Navarro / Electrochimica Acta 59 (2012) 270–278
3.2. ECH-SA of arylaldehydes, acetophenone, and olefinic
compounds in the presence of ˇ-CyD
Table 4 shows the comparison between different ECH proce-
dures described in the literature and the ECH-SA process described
in this work for arylaldehydes, acetophenone, styrene, and a
4
-arylpropene. The hydrogenation yields for the ECH-SA of ben-
zaldehyde using ␤-CyD as IPTC (Table 4, entry 1) are comparable
to those obtained for the ECH carried out with a well-established
platinized Pt catalyst (Table 4, entry 4). It is noteworthy that, for
the ECH-SA of benzaldehyde on a Ni cathode, the current effi-
ciency is much higher in water containing ␤-CyD (99%, entry 1)
than in water without ␤-CyD (72%, Table 1, entry 1) as already seen.
In water–methanol without ␤-CyD, the current efficiency is even
lower (50% and 45%, Table 4, entries 2 and 5) than in water despite
the poor solubility of benzaldehyde in water. The positive influence
of the presence of ␤-CyD has been observed also for the ECH-SA of
2
-methyl- and 4-methylbenzaldehyde in water (compare entries 6
and 7, and entries 8 and 9, respectively).
Fig. 1. Influence of the catalyst pre-treatment on the cathodic matrices. Pre-
The ECH-SA of the 2-methylbenzaldehyde (Table 4, entry 6)
was less efficient than that of benzaldehyde one (Table 1, entry
1). Accordingly to Vilar et al. [45], the steric and inductive/hyper-
conjugative effects of the methyl group at the ortho position has
a small influence in the reactivity or adsorption of the carbonyl
group on the catalyst surface. Moreover, the ␤-CyD enhances the
hydrogenation yield and the current efficiency to 99% and 66%,
respectively (Table 4, entry 6), when compared to water/methanol
ECH-SA procedure, 86% and 43%, respectively (Table 4, entry 7). This
behavior can be ascribed to the increase of the substrate concen-
tration in the reaction medium as previously discussed.
−
2
electrolysis conditions: 6.0 mL of water; 2.0 mmol NH4Cl; j = 280 mA dm ; Ni,
Cu, Fe or Fe/Ni alloy (64:36) cathode matrix; Ni sacrificial anode; ultrasound
irradiation or silent condition. Electrolysis conditions: addition of 1.0 mmol of
−
3
benzaldehyde + 4.0 mL of 7.0 mmol dm
2
␤-CyD solution (final concentration of
.8 mmol dm 3) to the pre-electrolysis cell; j = 330 mA dm−2; Ni/(Fe–Ni alloy), Ni/Fe,
−
Ni/Cu, Ni/Ni cathode; Ni sacrificial anode; stirring.
causing a catalyst damage and an increase of the number of edges,
crevices and kinks that expose fresh and highly active surfaces
[
53,54]. Beside these effects, the sonic treatment can change the
catalyst morphology and particle size [49,55], as well as circumvent
the mass transfer limitation of electro-active species [56,57].
Fig. 1 shows the ultrasound effect during the pre-electrolysis on
the hydrogenation yield for different cathode matrices. The hydro-
genation yields for the cathode matrices of Fe–Ni alloy, Fe, and
Cu were not affected, remaining within the experimental error:
Fe–Ni alloy (89%), Fe (88–89%), and Cu (90–91%). In the case of the
nickel matrix, the effect of ultrasounds leads to an increase of 5% in
the hydrogenation yields (88–93%). In order to get a better under-
standing of this effect, a study of the nickel deposit morphology
Interestingly, the ␤-CyD has no pronounced effect on the
ECH-SA methods of the 2-fluorobenzaldehyde, as shown by the
hydrogenation yields described in Table 4, entries 10 and 11.
Since the inclusion constant value, K , for 2-fluorobenzaldehyde
i
−1
3
(
2650 ± 175 mol dm ) is higher than that observed for the ben-
−1
3
zaldehyde (1086 ± 84 mol dm ) [38], the ␤-CyD should also favor
the ECH-SA of 2-fluorobenzaldehyde. The hydrogenation of 2-
chlorobenzaldehyde and 4-chlorobenzaldehyde (Table 4, entries
1
3 and 14) were even more difficult than the respective fluoroben-
(
shape, spacing, homogeneity and size of the particles) was carried
zaldehydes (Table 4, entries 10 and 12), confirming the evidences
discussed above.
The current efficiency for the hydrogenation of 2-
chlorobenzaldehyde and 4-chlorobenzaldehyde (Table 4, entries
out using SEM analyses of the electrodes after the pre-electrolysis
process.
The SEM of the Ni deposit on Cu electrode matrix revealed that
the acoustic irradiation has a smoothing effect on the nickel parti-
cles, as can be seen in Fig. 2A and B. However, this smoothing effect
does not promote an enhancement of the hydrogenation yield. In
the case of the Fe/Ni cathode matrix, no significant differences
on the shape and particle size distribution were observed when
comparing the silent (Fig. 2C) and the ultrasound (Fig. 2D) treat-
ment. However, the clusters of electrodeposited nickel are slightly
smaller on the matrix treated by ultrasound as previously observed
1
3 and 14) was lower than that of the analogous fluorobenzalde-
hydes (Table 4, entries 10 and 12). The effect of the substituent
on the current efficiency of 2-substituted benzaldehydes is most
probably due to an interplay of steric and electronic factors.
The same would be true for the effect of the para substituent,
the current efficiency for the ECH-SA of para substituted ben-
zaldehydes. The current efficiency decreases in the order:
–
CH > –F > –Cl > –OEt > –CH(CH ) > –N(CH ) (Table 4, entries
3 3 2 3 2
[
48,50]. The same behavior was observed for the Ni deposits on a
8
,12,14–17 and 19). The current efficiency observed for the ECH-
Fe matrix surface (Fig. 2E and F). As reported by Navarro and co-
workers [43], the sprouting of iron oxide particles is formed on this
electrode matrix. SEM of Ni deposit on Ni cathode matrix treated
under silent condition revealed more regular nickel clusters, the
dimensions of which become nanometric upon ultrasonic irradia-
tion (Fig. 2G and 2H, respectively). In fact, the smaller particles have
more edges and kinks, which improve the surface reactivity. Indeed,
according to Ponec and Bond [58], this is the most important effect
accompanying the reduction of metal particle size.
SA of 4-ethoxybenzaldehyde (Table 4, entry 17) is slightly superior
to that obtained in the absence of the ␤-CyD (Table 4, entry 18).
Interestingly, the hydrogenation yield and current efficiency for
4
-methoxy-3-hydroxybenzaldehyde (Table 4, entry 19) are exactly
the same as those observed for 4-ethoxybenzaldehyde (Table 4,
entry 17).
The ECH of the styrene has been extensively studied under dif-
ferent experimental conditions and on several electrode materials
[
38,44,59]. Styrene may be hydrogenated to ethylbenzene, ethyl-
From the results obtained, it can be concluded that the best
conditions for ECH-SA of benzaldehyde in water are: (i) nickel
cathode matrix, (ii) ultrasound pre-electrolysis treatment, (iii)
cyclohexane and may also polymerize on the surface, deactivating
the active sites of the catalyst. Navarro et al. [38], for instance, car-
ried out the ECH-SA of styrene through the electrodeposition of Ni
over an iron cathode matrix, reaching 59% of hydrogenation yield
−
3
−
2
2
.8 mmol dm of ␤-CyD, (iv) current density of 330 mA dm and
−3
(
v) 1.0 mol dm of NH Cl.
−1
at 2.5 F mol and a current efficiency of 23% (Table 4, entry 22).
4

M. Vilar, M. Navarro / Electrochimica Acta 59 (2012) 270–278
275
Table 4
Comparison among literature ECH procedures and the ECH-SA process described in this work.
a
Substrate [0.1 mol dm 3
−
]
Cathode matrix
Charge (F mol
−1
)
Hydrogenation yield (%)
Current efficiencyb (%)
Ref.
Entry
1
2
3
4
Ni
Ni
Ni
Pt
2.0
4.0
2.8
2.0
99
99
89
96
99
50
32
75
This work
[44]
[33]
O
[59]
5
Ni
4.0
90
45
[45]
O
O
6
7
Ni
Ni
3.0
4.0
99
86
66
43
This work
[45]
8
9
Ni
Ni
2.0
4.0
95
94
95
47
This work
[45]
O
1
0
1
Ni
Ni
4.0
4.0
92
95
46
47
This work
[45]
1
F
O
F
1
2
3
Ni
Ni
3.0
6.0
98
91
65
31
This work
This work
O
Cl
1
O
Cl
1
4
Ni
4.0
97
49
This work
O
1
1
5
6
Ni
Ni
6.0
8.0
95
94
32
22
This work
This work
O
N
O
1
1
7
8
Ni
Ni
5.0
6.0
98
98
39
32
This work
[45]
O
O
O
OH
1
9
Ni
5.0
98
62
–
59
97
39
21
–
23
65
This work
20
21
22
23
Ni
Pt
Ni
Ni
6.0
–c
2.5
3.0
This work
[59]
[33]
c
c
[44]
2
2
2
2
2
2
3
3
4
5
6
7
8
9
0
1
Ni
Pt
Ni
Ni
Ni
Pb
3.0
2.0
6.0
5.2
7.0
2.0
2.0
2.0
95
99
93
85
90
90
100
33
55
92
63
85
31
33
This work
[59]
[44]
[45]
[33]
[31]
[60]
[60]
[60]
[61]
O
25
not available
100
32
59
46
Raney Ni
Raney Co
Devarda Cu
3
3
2
3
2.0
4.0
d
Ni

2
76
M. Vilar, M. Navarro / Electrochimica Acta 59 (2012) 270–278
Table 4 (Continued)
−
−1
)
Current efficiencyb (%)
Entry
4
Substrate [0.1 mol dm 3
]
Cathode matrix
Ni
Charge (F mol
7.0
Hydrogenation yield (%)
98
Ref.
S
O
3
28
This work
O
O
3
5
Ni
3.0
97
65
This work
O
O
3
6
Ni
14.0
30
5
This work
a
−3
−2
Pre-electrolysis conditions: 6.0 mL of water; 1.0 mol dm of NH4Cl; j = 280 mA dm ; Ni cathode; ultrasound; Ni sacrificial anode. Electrolysis conditions: addition of
substrate (0.1 mol dm ) + 4.0 mL of ␤-CyD solution [(7.0 mmol dm−3) − final concentration of 2.8 mmol dm−3] to the electrolytic cell; j = 330 mA dm ; Ni/Ni cathode; Ni
−3
−2
sacrificial anode; stirring.
b
Current efficiency = (Qtheoretical/Qexperimental) × conversion.
c
Substrate not reduced.
d
Microparticles dispersed in poly(pyrrole-alkylammonium) films.
−
3
Fig. 2. SEM micrographs of nickel deposit after pre-electrolysis in 1.0 mol dm NH4Cl solution under silent condition for Cu, Fe/Ni alloy (64:36), Fe and Ni electrode matrices
−
3
(
A), (C), (E) and (G), respectively. SEM micrographs of nickel deposit after pre-electrolysis in 1.0 mol dm NH4Cl under ultrasound irradiation for Cu, Fe/Ni alloy (64:36), Fe
and Ni cathode matrices (B), (D), (F) and (H), respectively.

M. Vilar, M. Navarro / Electrochimica Acta 59 (2012) 270–278
277
The same reaction (Table 4, entry 23), using NH Cl as supporting
cheap IPTC, the ␤-CyD is an interesting biphasic catalyst for solving
4
electrolyte and water/methanol (1:1) medium [44], gave a higher
yield (97%) and current efficiency (65%). Casadei and Pletcher [59],
using a platinized Pt cathode in basic methanol medium, found
that the styrene was not reduced (Table 4, entry 21). The use of
mass-transport limitations also in electrochemistry.
Acknowledgements
␤
-CyD promoted the ECH-SA of styrene in water giving a yield
The authors would like to acknowledge CNPq, CAPES and
PETROBRAS granting for financial support.
of 62% of ethylbenzene with a current efficiency of 23% (Table 4,
entry 20).
The ECH of the acetophenone to 1-phenylethanol has been stud-
ied with different electrode materials [31,38,44,45,59–61]. Casadei
and Pletcher [59] have also carried out the ECH of acetophenone on
platinized Pt electrodes and obtained a 99% hydrogenation yield
References
[1] J.A. Moulijn, P.W.N.M. Van Leeuwen, R.A. Van Saten, Catalysis: An Integrated
Approach to Homogeneous, Heterogeneous and Industrial Catalysis, vol. 79,
Elsevier, Amsterdam, 1993, pp. 1–8.
−
1
at 2 F mol
(theoretical charge) and a current efficiency of 85%
[
[
2] I.T. Horváth, Catal. Lett. 6 (1990) 43–48.
3] F.R. Hartley, Supported Metal Complexes: A new Generation of Catalyst, D.
Reidel, Dordrecht, 1985, pp. 11–14.
(
Table 4, entry 25). Cheong et al. [60] found that, for a variety
of Raney electrodes in water/methanol (7:93) medium (Table 4,
entries 30–32), the C O bond hydrogenation and the current effi-
ciency were higher on Raney-nickel electrode (Table 4, entry 30)
than on Raney-cobalt and Devarda-cooper electrodes (Table 4,
entries 31 and 32). Zouaoui et al. [61] described the use of nickel
microparticles dispersed in poly(pyrrole-alkylammonium) films
also for the ECH of acetophenone in a 93% yield and 46% current
efficiency (Table 4, entry 33). Navarro et al. [38,44,45] studied the
ECH-SA of acetophenone under different conditions and obtained
hydrogenation yields of 85–90% with current efficiencies of 25–33%
[4] B. Cornils, W.A. Herrmann, R.W. Eckl, J. Mol. Catal. A: Chem. 116 (1997) 27–33.
[5] B. Cornils, E.G. Kuntz, J. Organomet. Chem. 502 (1995) 177–186.
[6] A.M. Lucchese, L. Marzorati, Quim. Nova 23 (2000) 641–652.
[7] M. Makosza, M. Fedorynski, Interfacial Catalysis, Marcel Dekker, Inc., New York,
003, chapter 8.
2
[
8] G. Bram, J. Sansoulet, H. Galons, Y. Bensaid, C. Combet-Farnoux, M. Miocque,
Tetrahedron Lett. 26 (1985) 4601–4602.
[9] Y. Kimura, P. Kirszensztejn, S.L. Regen, J. Org. Chem. 48 (1983) 385–386.
10] H.D. Durst, Tetrahedron Lett. 15 (1974) 2421–2426.
11] J. Huang, Y. Huang, J. Organomet. Chem. 414 (1991) 49–53.
12] H.H. Freedman, Pure Appl. Chem. 58 (1986) 857–868.
[
[
[
[13] C. Larpent, R. Dabard, H. Patin, Tetrahedron Lett. 28 (1987) 2507–2510.
[14] J. Schulz, A. Roucoux, H. Patin, Chem. Commun. 6 (1999) 535–536.
(
Table 4, entries 26–28). The ECH-SA of acetophenone with ␤-CyD
[
15] Y.P. Xie, J. Men, Y.Z. Li, H. Chen, P.M. Cheng, X.J. Li, Catal. Commun. 5 (2004)
37–238.
provided the highest yield, 95%, and current efficiency, 63% (Table 4,
entry 24). In the absence of ␤-CyD, the current efficiency was much
lower (31%, Table 4, entry 26). The hydrogenation of acetophenone
2
[16] E. Fache, C. Santini, F. Senocq, J.M. Basset, J. Mol. Catal. 72 (1992) 337–350.
[17] K. Jothimony, S. Vancheesan, J.C. Kuriacose, J. Mol. Catal. 32 (1985) 11–16.
[18] F. Robert, G. Oehme, I. Grassert, D. Sinou, J. Mol. Catal. 156 (2000) 127–132.
[19] J.P. Arhancet, M.E. Davis, J.S. Merola, B.E. Hanson, Nature 339 (1989) 454–455.
[20] J.P. Arhancet, M.E. Davis, J.S. Merola, B.E. Hanson, J. Catal. 121 (1990) 327–339.
(
Table 4, entry 24, 63% current efficiency) was more difficult than
that of benzaldehyde (Table 4, entry 1, 99% current efficiency) in
agreement with the fact that ketones are generally less reactive
than aldehydes for steric reasons mainly.
[21] B. Cornils, W.A. Herrmann, Aqueous Phase Organometallic Catalysis – Concepts
and Applications, Wiley-VCH, New York, 1998, pp. 241–251.
[
[
22] Y. Dror, J. Manassen, J. Mol. Catal. 2 (1977) 219–222.
23] A. Harada, J. Synth. Org. Chem. Jpn. 48 (1990) 517–521.
The ECH-SA with ␤-CyD of 2-furfural and thiophene-3-
carbaldehyde has never been reported. The current efficiencies
show that the hydrogenation of the first (65%, Table 4, entry 35)
was more efficient than the latter (28%, Table 4, entry 34), but less
efficient than that of benzaldehyde (99%, Table 4, entry 1).
It is well known that the hydrogenation of a non-conjugated
double bond is difficult even if it is monosubstituted [62]. Indeed,
the ECH-SA of safrole, a natural allylbenzene which possesses a
terminal olefin, in the presence of ␤-CyD (Table 4, last entry), was
not very efficient producing the corresponding alcohol in a 30%
yield, but in a very low current efficiency of 5%.
[24] J. Hernandez-Benito, S. Gonzalez-Mancebo, E. Calle, M.P. Garcia-Santos, J.
Casado, J. Chem. Educ. 76 (1999) 419–421.
[
[
25] K.A. Connors, Chem. Rev. 97 (1997) 1325–1357.
26] W. Saenger, J. Jacob, K. Gessler, T. Steiner, D. Hoffmann, H. Sanbe, K. Koizumi,
S.M. Smith, T. Takaha, Chem. Rev. 98 (1998) 1787–1802.
[
[
[
27] J. Szejtli, Chem. Rev. 98 (1998) 1743–1753.
28] H.A. Zahalka, K. Januskiewicz, H. Alper., J. Mol. Catal. 35 (1986) 249–253.
29] A. Harada, Y. Hu, S. Takahashi, Chem. Lett. 15 (1986) 2083–2084.
[30] S. Tilloy, H. Bricout, E. Monflier, Green Chem. 4 (2002) 188–193.
[31] N.A. Maiorova, A.A. Stepanov, M.K. Grachev, G.I. Kurochkina, V.A. Grinberg,
Russ. J. Electrochem. 40 (2004) 1248–1253.
[
32] S.Z. Fokin, Elektrochem 12 (1906) 749–762.
[33] D.S. Santana, G.O. Melo, M.V.F. Lima, J.R.R. Daniel, M.C.C. Areias, M. Navarro, J.
Electroanal. Chem. 569 (2004) 71–78.
[
34] V. Elango, M.A. Murphy, B.L. Smith, K.G. Devenport, G.N. Mott, G.L. Moss, US
Patent 4, (1991) 981–995, to Hoechst Celanese Corp.
4
. Conclusions
[
[
35] P. Gallezot, D. Richard, Catal. Rev.-Sci. Eng. 40 (1998) 81–126.
36] B. Mahdavi, P. Chambrion, J. Binette, E. Martel, J. Lessard, Can. J. Chem. 73 (1995)
8
46–852.
The best conditions for the use of the ␤-CyD as IPTC for
[
[
37] K. Harata, Chem. Rev. 98 (1998) 1803–1827.
38] M. Vilar, M. Navarro, Electrochim. Acta 56 (2010) 305–313.
[39] D. Robin, M. Comtois, A. Martel, R. Lemieux, A.K. Cheong, G. Belot, J. Lessard,
Can. J. Chem. 68 (1990) 1218–1227.
40] Q.-X. Guo, L. Liu, J. Inclusion Phenom. Macrocycl. Chem. 42 (2002) 1–14.
the ECH-SA in water of some arylaldehydes, ketones and olefins
have been determined using benzaldehyde as substrate. These
are: Ni deposited on a Ni cathode matrix under ultrasound
[
−
3
−3
␤-CyD; 1.0 mol dm NH Cl as
pre-electrolysis; 2.8 mmol dm
4
[41] Z. Yi, C. Zhao, Z. Huang, H. Chen, J. Yu, Phys. Chem. Chem. Phys. 1 (1999)
441–444.
−
2
supporting electrolyte; and a current density j = 330 mA dm . The
yield of hydrogenation of benzaldehyde in the presence of ␤-CyD
was 27% higher than in the absence of ␤-CyD. This higher yield
can be associated with a better solubility of the hydrophobic ben-
zaldehyde encapsulated in the hydrophobic cavity of ␤-CyD and,
hence, a larger amount of benzaldehyde delivered to the cathode
surface. In addition, the nanostructured Ni deposit on the cathode
surface probably allows a larger concentration of adsorbed species,
[
42] H.J. Schneider, R. Kramer, S. Simova, U. Schneider, J. Am. Chem. Soc. 110 (1988)
442–6448.
6
[
43] M.V.F. Lima, F.D. Menezes, B.B. Neto, M. Navarro, J. Electroanal. Chem. 613
(2008) 58–66.
[44] D.S. Santana, G.O. Melo, M.V.F. Lima, J.R.R. Daniel, M. Navarro, J. Tetrahedron
Lett. 44 (2003) 4725–4727.
45] M. Vilar, J.L. Oliveira, M. Navarro, Appl. Catal. A: Gen. 372 (2010) 1–7.
[46] D.J. Donaldson, M.D. Farrington, P. Kruus, J. Phys. Chem. 83 (1979) 3130–3135.
[47] M. Atobe, T. Tonoi, T. Nonaka, Electrochem. Commun. 1 (1999) 593–596.
48] G. Szöllösi, I. Kun, B. Török, M. Bartók, Ultrason. Sonochem. 7 (2000) 173–176.
49] W. Huang, H. Li, B. Zhu, Y. Feng, S. Wang, S. Zhang, Ultrason. Sonochem. 14
(2007) 67–74.
[50] B. Toukoniitty, E. Toukoniitty, P. Mäki-Arvel, J.-P. Mikkola, T. Salmi, D.Yu.
Murzin, P.J. Kooyman, Ultrason. Sonochem. 13 (2006) 68–75.
51] I. Kun, B. Török, K. Felföldi, M. Bartók, Appl. Catal. A: Gen. 203 (2000) 71–79.
52] A. Tai, T. Kikukawa, T. sigimura, Y. Inoue, T. Osawa, S. Fujii, J. Chem. Soc., Chem.
Commun. (1991) 795–796.
[
[
[
(
Y Z)ads and Hads, increasing the probability of collisions between
them. All these factors favor the hydrogenation of (Y Z)ads, over
the electrochemical desorption of Hads. The use of ␤-CyD in the
ECH-SA process allows the hydrogenation of lipophilic organic sub-
strate in water with good to excellent yields in many cases. Finally,
being a biodegradable water-soluble substance, a non-toxic and a
[
[

2
78
M. Vilar, M. Navarro / Electrochimica Acta 59 (2012) 270–278
[
53] T.J. Mason, J.P. Lorimer, Applied Sonochemistry – The Uses of Power Ultrasound
in Chemistry and Processing, Wiley-VHC, Weinheim, 2002, pp. 7–8.
54] K. Suslick, Science 247 (1990) 1439–1445.
[58] V. Ponec, G.C. Bond, Catalysis by Metal and Alloys, Series Surface Science and
Catalyst, vol. 96, Elsevier, Amsterdam, 1995, pp. 35–48.
[59] M.A. Casadei, D. Pletcher, Electrochim. Acta 33 (1988) 117–120.
[60] A.K. Cheong, Y. Bolduc, J. Lessard, Can. J. Chem. 71 (1993) 1850–1856.
[61] A. Zouaoui, O. Stephan, A. Ourari, J.C. Moutet, Electrochim. Acta 46 (2000)
49–58.
[
[
55] K.S. Suslick, D.J. Casadone, M.L.H. Green, M.E. Thompson, Ultrasonics 25 (1987)
5
6–59.
[
[
56] R.G. Compton, J.C. Eklund, F. Marken, Electroanalysis 9 (1997) 509–522.
57] R.G. Compton, J.C. Eklund, S.D. Page, T.J. Mason, D.J. Walton, J. Appl. Electrochem.
[62] S. Nishimura, Handbook of Heterogeneous Catalytic Hydrogenation for Organic
Synthesis, John Wiley & Sons Inc., 2001, pp. 65–68.
2
6 (1996) 775–784.