Two alternative routes for 1,2-cyclohexanediol synthesis by means of green processes: Cyclohexene dihydroxylation and catechol hydrogenation

Article abstract of DOI:10.1016/j.apcata.2013.06.023

In this paper we compare two different reactions, aimed at the synthesis of 1,2-cyclohexanediol. Specifically: (a) the direct epoxidation and hydrolysis (dihydroxylation) of cyclohexene to trans-1,2-cyclohexanediol, with an aqueous solution of hydrogen peroxide, and (b) the hydrogenation of catechol to a mixture of cis and trans-1,2-cyclohexanediol, in an attempt to establish green protocols for the synthesis of diols. Both reactions, the dihydroxylation of cyclohexene and the hydrogenation of catechol, were carried out without organic solvents. In the former case, an unprecedented 97.4% yield to the glycol was obtained, by selecting proper reaction conditions and using a tungstic acid/phosphoric acid catalyst, in a biphasic system with a phase-transfer agent. In the second approach, a heterogeneous alumina-supported Ru(OH)_x catalyst was used, and a 90% yield to the glycol was obtained. A comparison of the two processes allowed to show the lower environmental impact of the catechol hydrogenation route.

Full text of DOI:10.1016/j.apcata.2013.06.023

Applied Catalysis A: General 466 (2013) 21–31
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Two alternative routes for 1,2-cyclohexanediol synthesis by means of
green processes: Cyclohexene dihydroxylation and catechol
hydrogenation
Claudia Antonettia, Anna Maria Raspolli Gallettia,∗∗, Pasquale Accorintib, Stefano Alinib,
Pierpaolo Babinib, Katerina Raabovac, Elena Rozhkoc, Aurora Caldarellic, Paolo Righic,
Fabrizio Cavanic,d,∗, Patricia Concepcione
a Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Risorgimento 35, 56126 Pisa, Italy
b Radici Chimica SpA, Via Fauser, Novara, Italy
c Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale Risorgimento 4, 41036 Bologna, Italy
d Consorzio INSTM, Unità di Ricerca di Bologna, Via G. Giusti, Firenze, Italy
e Instituto de Tecnología Química, UPV-CSIC, Campus de la Universidad Politécnica de Valencia, Avda. Los Naranjos s/n, 46022 Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper we compare two different reactions, aimed at the synthesis of 1,2-cyclohexanediol.
Specifically: (a) the direct epoxidation and hydrolysis (dihydroxylation) of cyclohexene to trans-1,2-
cyclohexanediol, with an aqueous solution of hydrogen peroxide, and (b) the hydrogenation of catechol
to a mixture of cis and trans-1,2-cyclohexanediol, in an attempt to establish green protocols for the syn-
thesis of diols. Both reactions, the dihydroxylation of cyclohexene and the hydrogenation of catechol,
were carried out without organic solvents. In the former case, an unprecedented 97.4% yield to the glycol
was obtained, by selecting proper reaction conditions and using a tungstic acid/phosphoric acid catalyst,
in a biphasic system with a phase-transfer agent. In the second approach, a heterogeneous alumina-
supported Ru(OH)x catalyst was used, and a 90% yield to the glycol was obtained. A comparison of the
two processes allowed to show the lower environmental impact of the catechol hydrogenation route.
© 2013 Elsevier B.V. All rights reserved.
Received 23 March 2013
Received in revised form 16 June 2013
Accepted 19 June 2013
Available online 28 June 2013
Keywords:
Cyclohexene dihydroxilation
1,2-Cyclohexanediol
Catechol hydrogenation
Tungstate catalyst
Supported ruthenium hydroxide
1. Introduction
several authors have reported the dihydroxylation of olefins with
H2O2 catalyzed by transition metal complexes, i.e. H2WO4, poly-
1,2-Diols are molecules widely employed as intermediates in
the perfume and fragrance industry, as well as in drug and lubricant
synthesis. Dihydroxylation of olefins may occur either via one-pot
epoxidation and water hydrolysis of the oxirane ring into the trans-
diol, or via formation of cyclic inorganic ester, for example, by
reaction with KMnO4 or OsO4, then hydrolyzed into the cis-diol.
While various conventional oxidants are typically used for syn- and
anti-dihydroxylation, including for instance KMnO4, OsO4, alkylhy-
droperoxides, and peracids [1–4], a greener oxidant such as hydro-
gen peroxide is desired, due to both better atom efficiency and the
coproduction of water [5–7]. For these reasons, intensive research
has been conducted with the aim of developing reaction systems
which may efficiently transform the olefin into 1,2-diols with H2O2,
possibly under organic solvent-free conditions. In this context,
oxometalates or CH3ReO3, and by various solid catalysts as well
[8–33]. The use of zeolites or other heterogeneous catalysts allows
dihydroxylation in the absence of organic solvents, but selectiv-
ity is no higher than 60% because of the formation of epoxides,
alcohols, ketones, and/or ethers: (NH4)10W12O41/hydrotalcite [14],
Ti-␤ [15,16], Ti-MCM [17,18], Nb-MCM-41 [19], Ti-MMM and Ce-
SBA [33] are examples of the catalytic systems investigated. In
cyclohexene epoxidation with H2O2 and Ti–silicates catalysts, the
obtained products are typical of both two-electron oxidation mech-
anisms, i.e., cyclohexene epoxide and trans-cyclohexane-1,2-diol,
and one-electron oxidation mechanisms (2-cyclohexene-1-ol and
2-cyclohexene-1-one) [9,20–31,33], even though the selectivity to
allylic oxidation products can be lowered by means of specific
approaches, e.g., the dropwise addition of hydrogen peroxide [24].
The use of a resin-supported sulfonic acid makes it possible to con-
duct the dihydroxylation of cyclohexene with 30% H2O2 without
any solvent, at 70 ◦C, with 98% yield to the diol [32].
∗
Correspondingauthorat:DipartimentodiChimicaIndustriale“TosoMontanari”,
Viale Risorgimento 4, 40136 Bologna. Tel.: +39 0512093680.
In literature one of the most investigated catalytic systems for
epoxide synthesis is based on tungstate anions, because the lat-
ter’s unique chemistry is more conducive to oxygen transfer over
∗∗
Corresponding author.
E-mail address: fabrizio.cavani@unibo.it (F. Cavani).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.06.023

22
C. Antonetti et al. / Applied Catalysis A: General 466 (2013) 21–31
peroxide disproportionation [34–53]. Systems based either on the
Venturello or the Ishii conditions, by using phase-transfer catalysis
in organic solvents, involve similar oxoperoxo anions [36–41,54],
which are well known as very efficient epoxidizing agents. Under
typical conditions, the Venturello system gives 96% conversion with
91% selectivity (81 epoxide + 10 glycol). In the case of Keggin P/W
polyoxometalates (POM), the latter are precursors of the true cat-
alytic complexes, [PO4{WO(O2)2}4]3− and [HPO4{WO(O2)2}4]2−
,
and/or other low nuclearity species [45,48,55,56]. Various authors
have investigated the use of amphiphilic quaternary ammonium
tungstophosphate in emulsion or microemulsion systems [57–61].
The same systems have also been suggested for the dihydroxyla-
tion of alkenes [28,54,58,62–65]. With regard to the mechanism of
H2O2 activation, it is believed that the key step of activation is the
heterolytic cleavage of the O O bond, because homolytic cleavage
generates free radicals in solution (Fenton-type chemistry), thus
leading to undesired reactions. The formation of cyclohexene epox-
ide may occur via two different pathways, a direct epoxidation by
H2O2 and an indirect epoxidation by cyclohexenyl hydroperoxide,
the formation of the latter compound occurring by involvement of
radicalic steps [22]. For all the systems described in literature for
cyclohexene epoxidation, the catalyst features and reaction con-
ditions affect the final selectivity into either the epoxide, diol, or
Scheme 1. Two different reaction strategies for the synthesis of 1,2-
cyclohexanediol.
catalytic system, the reaction being carried out in a biphasic system
by using an aqueous solution of H2O2, in the presence of a phase-
transfer-agent (PTA) and without any organic solvent; and (b) the
direct hydrogenation of catechol, by means of the heterogeneous
alumina-supported Ru(OH)x catalyst, once again by using water as
the solvent. Both catechol and cyclohexene are obtained from ben-
zene (Scheme 1), and the full sequence involves in both cases one
hydrogenation and at least one oxidation. 1,2-Cyclohexanediol can
be used as a starting material for the synthesis of adipic acid by
means of oxidative scission [86,87].
allylic oxidation products, or even the products of oxidative C
C
cleavage (mono or di-carboxylic acids); as a consequence, the selec-
tivity to a specific compound is often not very high.
2. Experimental
An alternative pathway for the synthesis of 1,2-cyclohexanediol
is the selective hydrogenation of catechol: when compared to the
oxidative route from cyclohexene, this process might offer the
advantage of a better availability of the starting material, as com-
pared to cyclohexene which, conversely, is currently supplied by
very few companies. As an example, Asahi Chemical, which devel-
oped the process for the selective hydrogenation of benzene into
cyclohexene [66], uses the olefin produced for its own transfor-
mation processes, i.e., for hydration into cyclohexanol and nitric
oxidation of the alcohol into adipic acid. Existing methods for aro-
matic ring reduction typically require excess amounts of reagents
and harsh reaction conditions because of the stabilization of the
aromatic ring. Catalytic hydrogenation with transition-metal cata-
lysts is a simple and sustainable method [67–70], especially when
easily separable and recyclable catalysts – based on, i.e., Ni, Rh, Ru,
Pt, and Pd – are used. However, conventional methods using het-
erogeneous catalysts generally require high pressures (>1 MPa) and
temperatures and/or strongly acidic or basic conditions. Only a few
examples of mild hydrogenation methods applicable for aromatic
nuclei have been reported [71,72]; therefore, a simple multipur-
pose catalytic approach is highly desirable. Some of us reported
the use of Ru-based catalysts for the hydrogenation of phenol
into cyclohexanone [73], driven by the increasing price difference
between Pd (typically used for phenol hydrogenation [74–76]) and
Ru. In the case of catechol hydrogenation, systems used in the lit-
erature include Pt and Rh [77] or Ru/C catalysts [78].
2.1. Catalysts preparation
The catalyst for cyclohexene dihydroxylation was prepared
as follows: tungstic acid and phosphoric acid were dissolved
in H2O2/H2O; the hydrogen peroxide solution was prepared by
adding water to a 30 wt.% H2O2 aqueous solution, until the desired
H2O2 concentration (typically, 7.5 wt.%) was reached. The relative
amount of each component was varied for each reactivity experi-
ment, as described in detail in Tables 2–6; in a typical experiment,
the following quantities were used (e.g., in experiment 4 in Table 2):
190 g of H2O2 (7.5 wt.% aqueous solution), 0.96 g H2WO4 99%, and
0.22 g H3PO4 85%.
It is worth noting that the Venturello system [PO4{W2O2(␮-
O2)2(O2)2}2]3−, can be prepared either ex situ by means of
crystallization, as described in refs [11,57], and then added to the
reaction mixture, or can form in situ, by adding the various com-
ponents of the catalyst in aqueous solution directly to the reaction
system, as reported in refs [35,37,38]. It should also be noted that
the ratio between the reaction components, i.e., the substrate, the
tungstate anion, the phosphate anion and the PTA can change in
function of the reaction type. This is shown in Table 1, comparing
the molar ratios between substrate and catalysts components from
some literature sources. For our experiments, we decided to use
conditions closer to those used for cyclohexene epoxidation, but
with a lower amount of phosphate anion, with a W/P ratio close to
that of the Venturello system.
For our hydrogenation experiments, we decided to use catalysts
based on Ru(OH)x grafted over alumina, which in recent years was
reported to be an easily recoverable heterogeneous system. It is
active and selective for a variety of reactions, including catalytic
hydrogen transfer for allylic alcohol reduction [79], racemization
of alcohols and amines [80], oxidation of both alcohols and diols
[81] and of amines [82,83] with oxygen, ammoxidation of primary
alcohols to nitriles [84], synthesis of tertiary and secondary amines
directly from alcohols, and urea and N-alkylation of amines with
alcohols [85].
Supported Ru(OH)x catalysts were prepared according to the
procedure reported by Yamaguchi and Mizuno [80]. Specifically,
we first dissolved RuCl3·xH2O (38–42 wt.% Ru, Aldrich) in 60 mL
of distilled water (pH of the solution 2), in the amount necessary
to obtain the desired Ru content in the catalysts (i.e. 0.126 g to
obtain the 2.5 wt.% Ru on alumina); then, 2.0 g of ␥-Al2O3 were
added (precalcined at 500 ◦C for 3 h in static air; surface area
106 m2/g). The system was left at room temperature, under stir-
ring for 30 min; in these conditions, there was no precipitation
of Ru(OH)3. Afterwards, the pH of the slurry was increased by
means of anhydrous NaOH addition, until pH 13.2 was reached;
the slurry was left under stirring for 24 h. In these conditions, the
Here we report on the comparison between the two differ-
ent catalytic routes for the synthesis of 1,2-cyclohexanediol: (a)
the selective dihydroxylation (epoxidation + hydrolysis) of cyclo-
hexene by means of the homogeneous tungstate/phosphoric acid
Ru species in the solution, [RuClx(OH)y(H2O)6−x−y]
(3−x−y)+, react

C. Antonetti et al. / Applied Catalysis A: General 466 (2013) 21–31
23
Table 1
Molar ratios between substrate and catalyst components used with the Venturello-type system.
Ref.
Reaction
Substrate
PTA
Na2WO4 or H2WO4
H3PO4
[11]
[35]
[37]
[57]
Dihydroxilation of cyclohexene to 1,2-cyclohexanediol
Epoxidation of cyclohexene
Oxidative scission of 1,2-cyclohexanediol to adipic acid
Oxidative scission of cyclohexene to adipic acid
Oxidative scission of cyclohexene to adipic acid
Dihydroxilation of cyclohexene to 1,2-cyclohexanediol
100
100
100
100
100
100
–
0.5
–
–
0.27 [(nC8H17)3NCH3]3[PO4[WO(O2)2]4]
1.25
4
2.5
2
1.2 [(nC8H17)3NCH3]3[PO4[WO(O2)2]4]
[42]
1
1
1
–
This work (best conditions)
0.25–0.75
0.25–0.5
with the Al O− surface groups on alumina, to generate grafted Ru
the aqueous phase was analyzed to determine the dissolved car-
boxylic acids (ionic chromatograph Dionex 2000isp, column IonPac
ICE AS1e and suppressor AMMS ICE, eluent octanesolphonic acd
1 mM 2.5% in isopropanol), the remaining part was titrated with
KMnO4 to determine the residual H2O2; (b) a second part was
heated under vacuum to completely eliminate water; the organic
residue was weighed, dissolved in methanol, and then analyzed by
means of GC (Agilent 6850 GC, equipe with PTV injector, FID detec-
tor, capillary column HP-INNOWax Polyethlene Glycol, T ramp
from 50 ◦C to 230 ◦C), to determine the non-acid reaction products:
1,2-cyclohexanediol, 2-cyclohexe-1-ol, and 2-cyclohexe-1-one. In
some cases, the produced 1,2-cyclohexanediol was purified to
determine the effective weight yield; in this case, purification was
carried out by means of recrystallization in acetone. Specifically,
acetone was added to the reaction mixture after water evaporation
and concentration (acetone/organic residue 2.5/1 vol ratio), heated
until complete dissolution, and then cooled under mild stirring to
foster recrystallization. The solid was then separated, washed twice
with cold acetone, dried, and then weighed.
species: Al O− Na+ + [RuCl(OH)2] → Al
O Ru(OH)2 + NaCl. Finally,
the solution was filtered off, and the solid was dried at room tem-
perature for 2 days; the catalysts were used as such for reactivity
experiments, without any preliminary thermal or reactive treat-
ment.
The analysis of the Ru content in both catalysts and in the solu-
tion showed that this method permits the quantitative deposition
of the Ru species. Three Ru(OH)x/Al2O3 samples were prepared
with this procedure: Ru1.3 (1.3 wt.% Ru), Ru2.5 (2.5 wt.% Ru), and
Ru3.7 (3.7 wt.% Ru). A commercial catalyst containing metallic Ru
was also used for reference: the 5 wt.% Ru/Al2O3 catalyst was pur-
chased from Aldrich and used as received.
2.2. Catalytic experiments: cyclohexene dihydroxylation
Reactions were carried out using a 3-neck 250 ml glass reac-
tor, with magnetic stirring. The aqueous solution containing the
catalyst and hydrogen peroxide was heated to 50 ◦C for 15–30 min,
with stirring at 700 rpm. Then, a PTA/cyclohexene solution was pre-
pared; the following quantities were used (e.g., in experiment 4 in
Table 2): 31.7 g cyclohexene 99%, and 0.75 g PTA 98% (in most exper-
iments, Aliquat 336); this solution was then added dropwise to
the solution containing the catalyst and hydrogen peroxide (addi-
tion rate ca 5 g/min), while the stirring was raised up to 1000 rpm.
The reaction temperature spontaneously started to rise in a few
minutes; the heating bath was regulated to maintain the temper-
ature at the desired level (typically, 70 ◦C), and the reaction was
carried out for the desired reaction time (for example, 1 h). Lastly,
the reaction was completed in one more hour; during this final
period the released heat was negligible, and the bath tempera-
ture was the same as that of the reaction mixture, i.e. 68–70 ◦C.
We also conducted an experiment under optimal conditions, but
changing the way in which the reactants were mixed: the aque-
ous solution containing H2O2, tungstic acid and phosphoric acid
was added dropwise to the cyclohexene/PTA solution, under stir-
ring. The obtained results were similar to those achieved with the
standard procedure.
2.3. Catalytic experiments: catechol hydrogenation
The hydrogenation reactions were carried out in a 300 ml
mechanically stirred Parr 4560 autoclave equipped with a P.I.D.
4843 controller. In a typical procedure, the proper amount of the
chosen Ru-based catalyst was introduced into the autoclave under
inert atmosphere. Then the autoclave was closed, evacuated up to
0.5 mm Hg, and a solution containing the substrate (0.6 g of cat-
echol) in 50 ml of water was introduced by suction, followed by
pressurization with hydrogen to 0.2 MPa. Afterwards, the reactor
was heated up to the chosen temperature and, once it was reached,
the autoclave was pressurized with hydrogen to the desired pres-
sure, under stirring. The pressure value was held constant at the
chosen value manually by repeated hydrogen feeds. The course
of the reaction was monitored both by periodically sampling the
liquid from a sampling valve and by analyzing it through gas chro-
matography and GC–MS. When the reaction was completed, the
autoclave was rapidly cooled, the gas was discharged, and the liq-
uid mixture was immediately analyzed after dilution with acetone.
Recycling experiments of the solid catalyst were carried out in a
similar manner, but after removing the liquid reaction mixture
through the sample valve, the autoclave containing the solid cat-
alysts was evacuated and again charged with the fresh catechol
When the reaction time was completed (overall reaction time
2–2.5 h), the reaction mixture was weighed, and then filtered twice
(first with paper filter and then with a 0.45 ␮ syringe filter), to sep-
arate suspended particles or particles in emulsion of the PTA. The
filtered solution was then divided into two parts: (a) one part of
Table 2
Reaction conditions and results of experiments of cyclohexene dihydroxylation into trans-1,2-cyclohexanediol: effect of the PTA and phosphoric acid concentration.
Exper.
Molar ratio of reactants with respect to H2WO4
Reaction
Time, h
Y CHD
mol%
Y DA
mol%
Y others
mol%
Conv.
CH, %
Residual
HP, %
N
H2WO4
H3PO4
PTA
CH
HP
1
2
3
4
5
1
1
1
1
1
1
0.5
0.5
0.5
0.5
0
0
100.4
100.3
99.5
100.5
100.4
100.0
111.5
110.2
110.6
110.1
112.6
113.1
3.4
2.6
2.7
2.7
3.3
3.0
n.d.
90.3
96
97.2
78.1
97.4
0.7
0.6
0.6
0.4
1.9
0.2
n.d.
9.1
3.4
2.4
2.9
2.4
7.6
100
100
100
82.9
100
97.8
3.5
0.9
0.4
4.9
0.2
0.25
0.32
0.5
0.48
0.74
24
0.27
PTA, phase-transfer-agent; Y, yield; CH, cyclohexene; HP, H2O2; CHD, trans-1,2-cyclohexanediol; DA: dicarboxylic acids. Temperature 70 ◦C; HP concentration 7.5 wt%.

24
C. Antonetti et al. / Applied Catalysis A: General 466 (2013) 21–31
solution for a subsequent catalytic cycle. Catechol, trans and cis-
1,2-cyclohexanediol were purchased from Sigma Aldrich and used
as received. 2-Hydroxycyclohexanone was not commercially avail-
able and was synthesized by oxidation of 1,2-cyclohexanediol with
NaBrO3/NaHSO3 as previously reported [88].
GC analyses of reactant and products were carried out with
a HP 5890 gas chromatograph equipped with a HP 3396 inte-
grator, a flame ionization detector, and a PONA capillary column
(50 m × 0.25 mm × 0.25 ␮m) with a 100% dimethylpolysiloxane
stationary phase (carrier gas nitrogen, flow 1 mL/min). The repro-
ducibility of repeated catalytic runs was within 5%. GC–MS
analyses were carried out using the instrument Hewlett-Packard
HP 6890 with one MSD HP 5973 detector with a Phenomenex
Zebron G.C. column characterized by a stationary phase of 100%
dimethylpolysiloxane (length of the column: 30 m, inner diame-
ter: 0.25 mm, and thickness of the stationary phase: 0.25 ␮m). The
transport gas was helium 5.5 and the flow rate was 1 ml/min.
of 1,2-cyclohexanediol overoxidation, such as 6-hydroxyhexanoic
and 6-oxohexanoic acids (overall yield 2.4%) and dicarboxylic acids
(overall yield 0.4%). We also carried out an experiment by using
a phosphate/tungstate ratio close to ¼, which corresponds to the
stoichiometry of the Venturello system (experiment 24 in Table 2);
with an amount of PTA even greater than that used in experiment 4,
we could achieve 97.4% yield to 1,2-cyclohexanediol, thanks to the
slightly decreased formation of dicarboxylic acids. Worth of note,
under the conditions shown in Table 1, Venturello and Gambaro
[11] obtained the best yield of 87% in cyclohexene dihydroxyla-
tion to trans-1,2-cyclohexanediol, at 70 ◦C, in a two-phase mixture
of acidified (pH 1.5) aqueous hydrogen peroxide and a benzene
solution of the alkene, containing the catalyst.
Decreasing the PTA concentration led to a progressively lower
yield to 1,2-cyclohexanediol, and a greater yield to monocarboxylic
acid by-products; in the absence of PTA, the reaction did not
progress (experiment no. 1). Therefore, the role of the PTA was
not only that of bringing the activated form of the tungstate cata-
lyst from the aqueous H2O2 layer to the organic cyclohexene layer,
where cyclohexene epoxide was formed – which is the well-known
role of a PTA in this phase-transfer catalysis – but also that of
limiting the consecutive oxidation of 1,2-cyclohexanediol, by keep-
ing it in the organic layer, as long as some unconverted cyclohexene
remained. Finally, at total cyclohexene conversion, a single phase
was obtained, but H2O2 was also completely converted; in these
conditions, the 1,2-cyclohexanediol produced was stable because
H2O2 was no longer available. The protecting effect of the PTA
on 1,2-cyclohexanediol was probably effective only as long as a
biphasic system was present; in fact, experiments carried out start-
ing from 1,2-cyclohexanediol (see below) have shown that in a
monophasic aqueous system the reactivity of the diol with regard
to consecutive oxidation reactions was similar both in the presence
and in the absence of the PTA component (Table 5, see below). This
behavior was also observed at 90 ◦C, in the presence of an excess
H2O2. On the other hand, the hydrolysis/ring opening of cyclohex-
ene epoxide may occur at the interface between the organic layer
and the water layer.
2.4. Catalysts characterization
Ru(OH)x/Al2O3 catalysts were characterized by means of XPS,
surface area measurements (BET) and Raman spectroscopy. X-ray
photoelectron spectroscopy (XPS) measurements were performed
on a SPECS spectrometer with a 150 MCD-9 detector and using a
non monochromatic AlK␣ (1486.6 eV) X-ray source. Spectra were
recorded using analyzer pass energy of 30 V, an X-ray power of
200 W, and under an operating pressure of 10−10 MPa. Only the
Ru3p peak was used for the evaluation of the Ru oxidation states
since the more intense Ru3d peak is completely masked by the C1s
peak of contaminating carbon. Spectra treatment was performed
using the CASA software. Binding energies (BE) were referenced to
Al2p at 74.5 eV. Laser Raman spectra were recorded using a Ren-
ishaw 1000 spectrometer, with laser source Argon ion (514 nm),
equipped with a Leica DMLM microscope.
3. Results and discussion
We expected the phosphoric acid to have also the role of accel-
erating the hydrolysis of cyclohexene epoxide, but this was not
observed. Indeed, the acid apparently had the role of accelerat-
ing the cyclohexene and H2O2 conversion, but it did not show an
important effect on selectivity. Therefore, the acidity of the aque-
ous phase (containing the tungstic acid) was sufficient to foster the
oxirane ring opening.
3.1. The direct hydroxylation of cyclohexene into
1,2-cyclohexanediol
The performance of most homogeneous epoxidation systems
is greatly enhanced by adding molecules that coordinate to some
component in the reaction mixture; these additives may consist
of tertiary heterocyclic amines (i.e. pyridines, pyrazoles, and imid-
azoles) or carboxylates (i.e. acetates, benzoates, or glyoxylates); for
tungsten-based epoxidations, an optimal additive for tungstic acid
is phosphate anion [11,34,35,37,57]. We investigated the reactivity
behavior of cylohexene by using H2WO4 as the active component,
in the presence of phosphoric acid as co-catalyst and Aliquat 336
as the PTA.
We first investigated the role of both the PTA and phosphoric
acid. Results are summarized in Table 2, reporting the effect of
the molar ratio between the PTA and cyclohexene, while keep-
ing the concentration of all other reactants constant; experiment
5 was carried out with the optimal amount of PTA, but in the
absence of phosphoric acid. It is shown that in the absence of
either PTA (experiment 1) or phosphoric acid (experiment 5), the
cyclohexene conversion was lower than in the presence of both
components, even though the effect of PTA was much greater than
that shown by phosphoric acid. With an equimolar amount of
PTA and phosphoric acid, it was possible to obtain a 97.2% yield
to trans-1,2-cyclohexanediol, with a very low amount of uncon-
verted H2O2, which was used in 10% molar excess only with
respect to the stoichiometric requirement for cyclohexene dihy-
droxylation. By-products mainly consisted of acidic compounds
The peculiarity of the reaction described is that a very high selec-
tivity to 1,2-cyclohexanediol was obtained using only 1.1 mole
H2O2 per mole of cyclohexene, with minimal or nil formation of
both cyclohexene epoxide and acid by-products. This is an excel-
lent result, especially if we consider that low H2O2/alkene ratios are
typically used in cyclohexene epoxidation; in this way, high selec-
tivity to cyclohexene epoxide + 1,2-cyclohexanediol with respect
to both cyclohexene and H2O2 is obtained, but the conversion of
cyclohexene is low [89,90]. On the other hand, in the oxidation of
cyclohexene with stoichiometric H2O2, such a high selectivity to
1,2-cyclohexanediol in the reaction catalyzed by tungstate-based
catalysts has never been reported up to now [33]. In general,
with tungstate-based systems the catalyst acidity may lead to oxi-
rane ring opening [27]. However, considerably different results are
reported in the literature, depending on the reaction conditions
used; for example, with Ti-containing W-based polyoxometalates,
cyclohexene yielded cyclohexene epoxide with up to 98% selectiv-
ity and 84% conversion (or even 100% selectivity at 50% conversion)
when 1 eq of H2O2 was used, but was oxidized up to adipic acid
by using 4 equivalents of H2O2, working at 50–70 ◦C. In this case,
the mechanism included the hydrolysis of cyclohexene epoxide to
glycol, the oxidation of the latter into the 2-hydroxyketone, the

C. Antonetti et al. / Applied Catalysis A: General 466 (2013) 21–31
25
Table 3
Reaction conditions and results of experiments of cyclohexene dihydroxylation into trans-1,2-cyclohexanediol: effect of the H2O2 concentration.
Exper.
Molar ratio of reactants with respect to H2WO4
Reaction
Time, h
HP
Y CHD
mol%
Y DA
mol%
Y others
mol%
Conv.
CH, %
Residual
N
H2WO4
H3PO4
PTA
CH
HP
wt.%
HP, %
4
6
7
8
9
1
1
1
1
1
1
1
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
100.5
100.1
100.3
100.9
99.8
110.1
110.3
110.6
218.8
327.2
220.9
329.2
2.7
2.5
2.6
2.8
2.7
2.8
2.7
7.5
15.2
19.9
7.7
7.8
15.1
20
97.2
96.3
94.5
94.9
92.7
86.6
83.1
0.4
0.4
0.5
1
1.5
2.8
7
2.4
2.9
5.0
4.1
5.8
100
100
100
100
100
100
100
0.4
1.2
2.7
38.8
61.7
32.5
54.5
10
11
100.7
100.5
19.6
9.9
PTA, phase-transfer-agent; Y, Yield; CH, cyclohexene; HP, H2O2; CHD, trans-1,2-cyclohexanediol; DA, dicarboxylic acids. Temperature 70 ◦C.
Table 4
Experiments carried out by reacting cyclohexene epoxide.
Exper.
Molar ratio of reactants with respect to H2WO4
HP
Y CHD
mol%
Y DA
mol%
Y others
mol%
Conv.
Residual
HP, %
N
H2WO4
H3PO4
PTA
CEP
HP
wt.%
CEP,%
12
13
14
1
1
1
0.6
0
0.5
0.5
0.5
0
107
100.1
99.9
109.6
111.9
111.2
7.7
7.6
7.5
82.4
84.6
80.2
10.9
9.0
12.3
6.5
6.4
7.5
99.8
100
100
79.7
68.3
65.6
PTA, phase-transfer-agent; Y, yield; CEP, cyclohexene epoxide; HP, H2O2; CHD, trans-1,2-cyclohexanediol; DA, dicarboxylic acids. Temperature 70 ◦C; reaction time 3 h.
formation of adipic anhydride, and final hydrolysis into adipic acid
[28,91].
epoxide is very fast at the conditions used (the epoxide turned
out to be much more reactive than cyclohexene), and that the
1,2-cyclohexanediol produced is stable; in fact, the majority of the
added H2O2 was found to be unconverted at the end of the exper-
iment. With regard to the role of the PTA and phosphoric acid, the
two compounds showed only a minor effect, with minimal varia-
tions in conversion and selectivity in the experiments carried out
either without phosphoric acid (entry 13) or without the PTA (entry
14), as compared to the experiment carried out with both compo-
nents (entry 12). With regard to the nature of the dicarboxylic acids
formed, adipic acid was again the prevailing one, but with a signif-
icant amount of glutaric acid (for instance, in experiment 12: 7.0%
adipic acid, 3.9% glutaric acid, traces of succinic acid).
These results indicate that 1,2-cyclohexanediol is stable under
the conditions used, and does not undergo consecutive oxidative
cleavage; the relatively low reaction temperature is likely to be
the major factor contributing to high selectivity. In order to further
confirm this hypothesis, we carried out experiments by reacting
trans-1,2-cyclohexanediol; the results are summarized in Table 5.
It is shown that:
The low selectivity to dicarboxylic acids experimentally
observed in our case is attributable to both the low reaction temper-
ature (70 ◦C) and the stoichiometric amount of H2O2 used; under
our conditions, the consecutive reactions leading to the oxidation of
1,2-cyclohexanediol into adipic acid are kinetically unfavored. With
regard to this point, we carried out experiments using different
H2O2 concentrations and molar ratios between H2O2 and cyclo-
hexene. Table 3 summarizes the results obtained. It is shown that
an increase in H2O2 concentration had a moderate effect on 1,2-
cyclohexanediol yield (with only a minor increase in the selectivity
to by-products). The same minor effect was shown when a large
excess of H2O2 with respect to the stoichiometric requirement was
used; indeed, most of the H2O2 remained unconverted, an indica-
tion that, at the used reaction conditions, any consecutive oxidation
upon 1,2-cyclohexanediol was kinetically unfavored. A more sig-
nificant effect was observed when both a large H2O2 excess and a
higher H2O2 concentration were used (experiments 10 and 11).
It was not possible to study the effect of temperature, because
at 75 ◦C the mixture started refluxing; only once the organic phase
containing cyclohexene had disappeared, could the temperature
be raised up to 90 ◦C. As expected, the final result was similar
to that obtained in the experiment carried out at 70 ◦C: 1,2-
cyclohexanediol yield 96.9%, unconverted H2O2 0.48%, dicarboxylic
acids yield 0.36%.
(a) The reactivity of 1,2-cyclohexanediol was much lower
than that of the olefin; products obtained were both 2-
hydroxycyclohexanone and monocarboxylic acids (lumped into
“Others” in the table) and dicarboxylic acids. These “Others”
were in lower amount after reaction at 90 ◦C, being intermedi-
ates in the transformation of the diol into adipic acid.
(b) The PTA had negligible effects on 1,2-cyclohexanediol conver-
sion (experiments 15 and 16 for tests at 70 ◦C; experiments 20
and 21 at 90 ◦C).
Some experiments were carried out by reacting cyclohexene
oxide aimed at observing the effect of PTA and phosphoric acid
on epoxide hydrolysis into 1,2-cyclohexanediol (Table 4). The
achieved results demonstrate that the hydrolysis of cyclohexene
Table 5
Experiments carried out by reacting trans-1,2-cyclohexanediol.
Exper.
Molar ratios of reactants with respect to H2WO4
T, ◦
C
Reaction
Time, h
HP
Y DA
mol%
Y others
mol%
Conv.
Residual
HP, %
N
H2WO4
H3PO4
PTA
CHD
HP
wt.%
CHD, %
15
16
17
18
19
20
21
1
1
1
1
1
1
1
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0
101.1
100.8
101.4
99.7
99.9
100.5
100.5
111.1
111.5
333.6
337
332.1
333.7
333.8
70
70
70
70
90
90
90
3.2
3.1
2.9
3
3.1
6
7.6
7.6
7.6
31.1
31.1
31.1
31.1
6.4
3.5
2.8
16.7
16
14.4
12.4
5.6
23.1
19.5
17.2
26.4
97.4
97.4
98.1
17.5
14.1
79.5
37.7
1.5
0.5
0.5
0.5
0.5
0
14
91.8
93.3
93.9
4.1
4.2
0.05
0.03
6.2
PTA, phase-transfer-agent; Y, yield; HP, H2O2; CHD, trans-1,2-cyclohexanediol; DA, dicarboxylic acids.

26
C. Antonetti et al. / Applied Catalysis A: General 466 (2013) 21–31
Table 6
Experiments of cyclohexene oxidation carried out under selected conditions, in order to replicate tests reported in literature.
2−
Exper.
Molar ratio of reactants with respect to WO4
T, ◦
C
Reaction
Time, h
HP
Y CEP
mol%
Y CHD
mol%
Y DA
mol%
Y others
mol%
Conv.
CH, %
Resid.
HP, %
N
Catalyst
H3PO4
PTA
CH
HP
Solvent
wt.%
24
22
23
1 (H2WO4)
1 (Na2WO4)
1 (Na2WO4)
0.27
2.01
0.51
0.74a
0.38a
0.49b
100.0
184.5
50.8
113.1
60.5d
75.2
–
70
70
74–79
3.0
1
4
7.6
7.9
30
0
23.1
0.2
97.4
6.5
46.2
0.2
0.4
15.7
2.4
5.2
7.5
100
35.1
69.6
0.2
2.8
2.7
CH3Clc
–
PTA, phase-transfer-agent; Y, Yield; CH, cyclohexene; HP, H2O2; CHD, trans-1,2-cyclohexanediol; DA, dicarboxylic acids; CEP, cyclohexene epoxide.
a
Aliquat 336.
Methyltrioctylammonium hydrogensulfate.
Solvent/cyclohexene weight ratio 0.6/1.
The pH of the H2O2/H2O solution has been adjusted to 3.2 with NaOH.
b
c
d
(c) At 70 ◦C, the adoption of a H2O2/1,2-cyclohexanediol molar
Table 7
Hydrogenation of catechol to 1,2-cyclohexanediol in the presence of sample Ru2.5.
ratio close to 3 had no important effect on conversion (com-
pare experiment 15 with 17); however, when the greater H2O2
amount was used in more concentrated conditions (experiment
18), the highest 1,2-cyclohexanediol conversion (26.4%) with
14% yield to dicarboxylic acids was obtained.
T, ◦
C
P H2, MPa
Time, h
Catechol conv., %
Sel. to CHD, mol%a
90
190
90
5
5
3
8
0.5
5
99.6
100
35.9
90.2
87
78
(d) At 90 ◦C and H2O2/1,2-cyclohexanediol molar ratio 3, with H2O2
concentration 31% (that is, under conditions closer to those
reported in literature for the one-step oxidative cleavage of
cyclohexene into adipic acid), almost complete diol conversion
was achieved with very high yield to dicarboxylic acids in 3 h
reaction time (experiment 19); in these conditions, the PTA had
no role (experiments 20 and 21).
Reactions conditions: H2O 50 ml, catechol 0.6 g, Ru: 5.1 mg.
CHD, 1,2-cyclohexanediol.
a
The by-products were cyclohexanol, cyclohexanone, and traces of phenol.
4. The hydrogenation of catechol into 1,2-cyclohexanediol
in aqueous medium
(e) The prevalent dicarboxylic acid was, in all cases, adipic acid;
for example, in experiment 21 the following distribution was
observed: yield to adipic acid 92.1%, glutaric acid 1.6%, succinic
acid 0.2%.
Catechol hydrogenation was first studied using a Ru2.5 cata-
lyst; under the conditions used, the alumina-supported Ru(OH)x
produced almost total catechol conversion after 8 h reaction time
(Table 7). The main reaction was the selective hydrogenation
of catechol to 1,2-cyclohexanediol (a mixture of cis and trans
isomers), with 90% selectivity; the other by-products were the
consecutive hydrogenolysis or dehydration compounds, cyclohex-
anol and cyclohexanone, respectively, with only traces of phenol
(Scheme 2). No formation of the partially hydrogenated product, 2-
hydroxycyclohexanone, occurred. When the reaction was carried
out at 190 ◦C, total catechol conversion was achieved already after
0.5 h, but 1,2-cyclohexanediol selectivity was 87%, with 13% selec-
tivity to hydrogenolysis compounds. Conversely, when the reaction
was carried out at 3 MPa H2 pressure and 90 ◦C, as expected, cat-
echol conversion was 36% only after a 5 h reaction. On the basis
of these results, further investigation was carried out at 90 ◦C, but
using the higher pressure of 5 MPa of hydrogen.
In conclusion, it is evident that temperature is the main factor
responsible for the absence of further transformation of 1,2-
cyclohexanediol into dicarboxylic acids when cyclohexene reacts
with H2O2, even with a large excess of the latter. Furthermore, the
PTA had no role in the two consecutive steps of cyclohexene epox-
ide hydrolysis into 1,2-cyclohexanediol and of 1,2-cyclohexanediol
oxidation into dicarboxylic acids (in this step also, phosphoric acid
played no role). Due to the fact that the two compounds both show
a non-negligible effect on the rate and selectivity of cyclohexene
transformation into 1,2-cyclohexanediol, we may conclude that the
promotional effect of these components (especially of the PTA) is
mainly on the first step of cyclohexene oxidation into cyclohexene
epoxide.
Final experiments were carried out under reaction conditions
similar to those reported by Venturello (experiment 22) [35], and
those described in Noyori’s papers (experiment 23) [92], both
designed to obtain high yield to cyclohexene epoxide. Results
are summarized in Table 6 and compared with the best ones
obtained in our reaction conditions (experiment 24). It is shown
that by using conditions similar to those reported by Venturello,
the extent of cyclohexene epoxide hydrolysis was much lower
than under our conditions; differences between the two proce-
dures concerned the presence of a solvent, the type of catalyst
and of PTA used, and the relative amount of the components.
With regard to the Noyori’s conditions, we could not exactly repli-
cate those reported in the literature. We used phosphoric acid
instead of aminomethylphosphonic acid, and were unable to raise
the temperature above 79 ◦C (the reflux T of the mixture). In this
case, under our conditions, the absence of the solvent led to the
hydrolysis of the epoxide (cyclohexene epoxide formed with a very
low yield); however, because of the excess H2O2 and the high
temperature used, the yield to dicarboxylic acids (mainly adipic
acid) was higher than that obtained using our standard condi-
tions.
With the aim of better understanding the suitability of
Ru(OH)x/Al2O3 catalysts in catechol hydrogenation, samples at
different Ru loadings were prepared and tested (see Section
2). Hydrogenation tests were carried out at 90 ◦C under 5 MPa
of H2, keeping the catechol/Ru initial molar ratio constant at
Scheme 2. Products obtained in catechol hydrogenation.

C. Antonetti et al. / Applied Catalysis A: General 466 (2013) 21–31
27
Fig. 2. Catechol conversion (%) (open symbols) and selectivity to 1,2-
cyclohexanediol (full symbols) based on the reaction time. Catalyst: fresh
sample Ru3.7 (ꢀꢁ), and sample Ru3.7 recovered and reused without any regenera-
tion treatment (ꢁ᭹). Reaction conditions: T 90 ◦C, 5 MPa H2, H2O: 50 ml, catechol
1.2 g, Ru: 10.0 mg.
this catalyst. The obtained results were just the opposite of what
we expected; the characterization of used samples will help clarify
this point (see below).
On the basis of these encouraging results, sample Ru3.7 was
recovered by filtration after use and reused without any regenera-
tion procedure, by adding a fresh substrate: the results achieved are
compared with those obtained with the fresh catalyst in Fig. 2. It can
be seen that the recovered catalyst not only maintained a good effi-
ciency, but also showed a higher activity than the fresh one at the
beginning of the recycle test. However, the increase of conversion
over time was slower than that observed with the fresh sample.
Some differences were also observed with regard to the selectivity
to 1,2-cyclohexanediol; in fact, the reused catalyst showed a better
1,2-cyclohexanediol selectivity, and also did not show any decline
for longer reaction times, as was the case for the fresh sample.
Overall, these results highlight some changes in the active phase
features which had likely occurred during the first experiment (on
the fresh catalyst), changes which were responsible in the end for
the modified catalytic behavior shown by the reused catalyst.
We also carried out the reaction using the reaction mixture, after
filtering off the catalyst; this experiment was carried out by stop-
ping the reaction after 2 h (catechol conversion being close to 60%
at that time), by carrying out the “hot filtration” of the reaction mix-
ture, by adding fresh reactant and starting again the reaction at the
same conditions previously used with the fresh catalyst. No reac-
tion took place in the absence of catalyst, thus allowing us to rule
out the presence of any active species leached from the catalyst to
the liquid phase.
Lastly, an experiment was carried out using sample Ru2.5,
with a very high initial catechol/Ru molar ratio (catechol/Ru:
3.65 × 103 mol/mol), with the aim of achieving moderate catechol
conversion and thus identifying possible reaction intermedi-
ates; results are shown in Fig. 3. Actually, in this experiment,
it was possible to identify by GC/MS the intermediate 2-
hydroxycyclohexanone by comparison with the pure substance;
in fact, due to the very high catechol/Ru molar ratio, the reaction
progressed more slowly, and consequently the intermediate
2-hydroxycyclohexanone desorbed from the catalyst surface and
dissolved into the liquid phase, thus being identified in the reaction
mixture. In literature it is reported that this two-step reaction
pathway is unfavorable for Ru, while the amount of intermediate
2-hydroxycyclohexanone is scarce, even at the beginning of the
catalytic run [93]. Huang et al. reported that the reaction pathway,
Fig. 1. Catechol conversion (%) based on reaction time (top) and selectivity to 1,2-
cyclohexanediol (%) based on catechol conversion (bottom) with Ru(OH)x/Al2O3
catalysts: sample Ru2.5 (ꢀ), sample Ru1.3 (♦) and sample Ru3.7 (ꢀ). Reactions
conditions: 90 ◦C, 5 MPa of H2, H2O: 50 ml, catechol 0.6 g, Ru: 5.1 mg.
1.1 × 10−2 mol/mol; Fig. 1 shows the results. It can be seen that,
despite the similar overall Ru content used for the experiments,
sample Ru1.3 (containing the lower Ru content) was the least active
catalyst, whereas the sample showing the highest activity was
that containing the higher amount of Ru (sample Ru3.7), with an
intermediate behavior shown by sample Ru2.5. With regard to 1,2-
cyclohexanediol selectivity, samples Ru1.3 and Ru3.7 both showed
a drop in selectivity to 1,2-cyclohexanediol depending on the cat-
echol conversion; with sample Ru1.3, the decline in selectivity
started already for catechol conversion higher than 40%, whereas
with sample Ru3.7 the selectivity declined above 80% catechol con-
version. The progressive decrease in selectivity with hydrogenation
advancement was due to the undesired hydrogenolysis reaction,
which increased with the longer reaction times. However, in the
case of sample Ru2.5, which contained an intermediate amount of
Ru loading, a slight increase in 1,2-cyclohexanediol selectivity was
observed in the 20–60% catechol conversion interval, after which
the selectivity was almost stable at 90% until total catechol conver-
sion.
The differences in catalytic behavior experimentally observed
were quite unexpected, especially with regard to the catalyst activ-
ity; in fact, we expected that in the sample containing the lowest
Ru content (sample Ru1.3) a better dispersion of the active species
over alumina would have led to a better intrinsic activity (referred
to the amount of Ru). For the same reason, in the sample contain-
ing the highest Ru content (sample Ru3.7), we expected that the
formation of either Ru(OH)x aggregates or oligomeric species over
the support would lead to a lower availability for the Ru species in

28
C. Antonetti et al. / Applied Catalysis A: General 466 (2013) 21–31
Fig. 3. Hydrogenation of catechol to 1,2-cyclohexanediol in the presence of sam-
ple Ru2.5. Reactions conditions: 90 ◦C, 5 MPa of H2 H2O: 50 ml, catechol 15.0 g,
Ru: 3.8 mg. Conversion of catechol (ꢂ), selectivity to 1,2-cyclohexanediol (ꢃ), to
2-hydroxycyclohexanone (᭹) and to cyclohexanol + cyclohexanone (ꢁ).
Fig. 4. Raman spectra of Ru1.3 and Ru3.7, both fresh and after use in hydrogenation
of catechol to 1,2-cyclohexanediol.
are shown in Fig. 4. The Ru3+ species which strongly interacts
with La2O3 is reported to show a broad band centered at around
670 cm−1 [97]; in our samples, such a band was not observed (nor
was it seen in fresh samples either). On the other hand, bands
observed in both Ru1.3 and Ru3.7, not corresponding to any typi-
cal vibration observed in Ru oxides (i.e., RuO2, RuO3 or RuO4) [98],
can be attributed to Ru3+ species chemically interacting with the
support. Indeed, in literature, a band at 470 cm−1 (also visible in
spectra shown in Fig. 4) is attributed to hydrated RuO2 (together
with the band at 670 cm−1 which, however, is not observed in our
case) [99], whereas the band at 510 cm−1 is close to that reported
for crystalline RuO2 dispersed over zirconia support [100]; typi-
cally of RuO2, however, other bands should also be seen at 640 and
705 cm−1, which instead are not present.
The relative intensity of the two main bands observed in fresh
samples, at around 460 and 510 cm−1, was a function of the Ru
loading. In sample Ru3.7, the latter was more intense than the
former band, whereas the opposite was true in sample Ru1.3;
therefore, based on these observations and by analogy with sim-
ilar bands reported in literature, we can tentatively attribute the
lower wavenumber band to an isolated Ru3+ species which is chem-
ically grafted to the support, while attributing the band at around
510 cm−1 to a less dispersed, oligomeric Ru3+ species. It is inter-
esting to note that the Raman spectrum of sample Ru3.7 showed
the most significant changes after reaction, compared with the
spectrum before reaction; more specifically, the band at 510 cm−1
almost disappeared in the spectrum of the used sample. The same
relevant change was not observed in sample Ru1.3. These results
can be attributed to the fact that mainly the oligomeric Ru3+ species,
especially present in the Ru3.7 sample, underwent reduction down
to the metallic state during contact with hydrogen in catechol
hydrogenation. This evidence agrees with the development of a
more active metallic Ru species during reaction in the presence of
Ru3.7 catalyst.
either consecutive or direct hydrogenation, is influenced by the
strength of oxygen affinity with the catalyst [93]. In the case of
weak oxygen affinity catalysts, such as Pt and Pd, the intermediate
2-hydroxycyclohexanone can more easily desorb and dissolve
in the liquid phase, and thus the consecutive hydrogenation
mechanism takes place. Conversely, in the case of strong oxygen
affinity catalysts, such as with Ru and Rh, the desorption of the
intermediate is more difficult, and the contribution of the direct
hydrogenation mechanism prevails over the two-step one.
5. Characterization of Ru(OH)x/Al2O3 catalysts after use
For samples recovered after reaction, the XPS results referred to
the Ru3p3/2 peak are summarized in Table 8. Attribution of the Ru
BE to different oxidation states is quite controversial in literature
[80,94–96]. In general, it is recognized that on increasing the Ru
oxidation state, the BE value shifts from around 458 eV (for metal-
lic Ru), to around 466 eV for Ru6+. In our case, for all the samples a
single broad band was centered at around 462.5 eV, which seems to
suggest the presence of Ru3+ as the main species. However, a shoul-
der centered at ca 458.5 eV, attributable to Ru0, was present in the
sample at higher Ru loading (Ru3.7), and this could suggest that for
this sample Ru3+ is partially reduced during the reaction; by means
of peak deconvolution, the relative amount of the metallic Ru was
estimated to be around 10 3% of the Ru content. The contribution
of this lower BE peak was instead negligible for Ru1.3 and Ru2.5.
This might explain the highest activity experimentally observed for
sample Ru3.7, and also the change of catalytic behavior shown by
the recycled catalyst. The formation of metallic Ru in sample Ru3.7
may be due to the presence of Ru species which develop a weaker
interaction with the support, and are more easily reducible from
Ru3+ to Ru0. Conversely, in samples Ru2.5, and especially in sample
Ru1.3, the higher dispersion of Ru3+ species led to a stronger inter-
action with the alumina support, which makes them less reducible
and, in the end, less intrinsically active in catechol hydrogenation.
Catalysts were also characterized by means of Raman spec-
troscopy; spectra of both fresh and used Ru1.3 and Ru3.7 samples
Because of the suggested important role of the in situ-generated
metallic Ru, we compared the reactivity of the Ru2.5 catalyst with
that of a commercial catalyst having 5 wt.% Ru/Al2O3, with Ru in the
metallic form. This catalyst was characterized in a previous work
[73]; it shows an average Ru particle size of 4.9 2.3 nm, greater
than that shown by Ru(OH)x nanoparticles in fresh catalysts pre-
pared in the lab [101]. Results are reported in Fig. 5, which shows
catechol conversion and selectivity to 1,2-cyclohexanediol depend-
ing on reaction time. In the two experiments, the same catechol/Ru
ratio was used (1.08 × 102 mol/mol, with 5.1 mg Ru), working at
90 ◦C under 5 MPa of hydrogen. In both cases, by-products were
cyclohexanol and cyclohexanone, with only traces of phenol. It is
Table 8
XPS results of samples Ru1.3, Ru2.5 and Ru3.7 after use in hydrogenation of catechol
to 1,2-cyclohexanediol at 90 ◦C, 5 MPa of H2.
Sample
Ru3p3/2 (BE 462.5 eV), %
Ru3p3/2 (BE 458.5 eV), %
Ru1.3
Ru2.5
Ru3.7
100
100
0
0
3
90
3
10

C. Antonetti et al. / Applied Catalysis A: General 466 (2013) 21–31
29
Fig. 5. Conversion of catechol (open symbols) and selectivity to 1,2-cyclohexanediol
(full symbols) in the presence of sample Ru2.5 (ꢀꢃ), and a commercial 5 wt.%
Ru/Al2O3 catalyst (ꢀꢁ). Reactions conditions: 90 ◦C, 5 MPa of H2, H2O: 50 ml, cate-
chol 0.6 g, Ru: 5.1 mg.
Fig. 7. Comparsion of E and EI factors for the processes of 1,2-cyclohexanediol
synthesis by means of catechol hydrogenation and of cyclohexene dihydroxylation.
shown that despite the low fraction of metallic Ru present in the
spent Ru2.5 catalyst, differences in activity between the two sam-
ples were small. The TON, calculated with respect to the overall Ru
loading (catechol conversion after 2 h reaction time was taken), is
shown to be lower for sample Ru2.5 than for the commercial sam-
ple [0.0080 molcatechol/(gRu h) for sample Ru2.5, vs 0.0144 for the
commercial sample]. However, if we consider that the amount of
metallic Ru in the spent Ru2.5 sample was no more than 10 3%
of the overall Ru loading, the TON calculated with respect to the
metallic Ru species was by far greater with Ru2.5 than with the
commercial sample. It can be speculated that the significant activity
of the alumina-supported Ru(OH)x catalysts was due either to some
synergic interaction between the different Run+ species, which led
to an enhanced activity of metallic Ru, or to a direct participation of
the oxidized Ru3+ species. The latter hypothesis is more likely, due
to the activity shown by Ru1.3 and Ru2.5 catalysts, which showed
no metallic Ru species in used samples.
6. Comparing the sustainability of the two processes
Common aspects of the two processes investigated were the
excellent yield to the desired product (especially in the case of
cyclohexene dihydroxylation), and the absence of organic solvents;
in both cases, the reactions were carried out in water solvent. On
the other hand, the main disadvantage of the tungstate-based sys-
tem is the recovery of the catalyst and of the PTA. Recently some
papers reported about procedures for the recovery of the soluble
W catalyst [102], or for the use of heterogeneous, hence more eas-
ily recovered, WO3 catalyst [103]. Instead, the Ru-based catalyst is
easily recoverable and reusable.
Concerning the starting reactant, catechol and cyclohexene,
both are obtained from benzene. The current world production
for cyclohexene is around 40,000 tons/y, mainly used by Asahi for
hydration into cyclohexanol, precursor for the synthesis of adipic
acid. Catechol is produced by phenol hydroxylation.
Finally, we compared Ru2.5 and the commercial Ru/Al2O3 cat-
alysts, using a greater concentration of the substrate (2.5 g instead
of 0.6 g), and a greater amount of catalyst (20.5 mg Ru instead of
5.1 mg). The results are shown in Fig. 6; for both catalysts, we
observed a lower conversion rate compared to the more diluted
conditions (Fig. 5), whereas the selectivity to 1,2-cyclohexanediol
remained unchanged.
Fig. 7 presents a comparative assessment of the two alterna-
tive routes to 1,2-cyclohexanediol. The assessment is normalized
to 1 kg of product and was made with the aid of the EATOS [104]
tool. It takes into consideration both the masses (E-factor) as well as
environmental impact of the substances released by the processes
(Sheldon’s Q) [105]. This assessment compares the two routes via
oxidation of cyclohexene (Table 2, experiment 4) and via hydro-
genation of catechol (Table 7, entry 1). The following assumption
was made: the catechol route is a heterogeneous reaction and
therefore both the solvent (water) and the catalyst have been con-
sidered easily recyclable at the 95% amount with only 5% of losses.
Comparison of E-factors shows that the route from cyclohex-
ene produces 0.39 kg of wastes while route from catechol produces
0.13 kg. Inspection of EATOS graphical output allows also to see
where these wastes come from. Much of the wastes produced in
cyclohexene route (0.31 kg out of 0.39 kg; blue block) are aque-
ous waste and come from the aqueous reagents. Further inspection
of this segment with EATOS tool shows that the remaining waste
is unreacted reagents (green block) and catalyst (magenta block).
Inspection of the wastes produced in the catechol route shows that
most of them come from the by-products (0.11 kg out of 0.13 kg;
yellow block).
For the assessment of the environmental impact of chemical
processes, EATOS can take into consideration up to ten different
substances ecotoxicological and human toxicological parameters,
and each parameter can be given a different weight. Such param-
eters are then normalized (each parameter is made to vary from
1 to 10), and then combined to afford an environmental quotient
Fig. 6. Conversion of catechol (open symbols) and selectivity to 1,2-cyclohexanediol
(full symbols) in the presence of sample Ru2.5 (ꢀꢃ), and a commercial 5 wt.%
Ru/Al2O3 catalyst (ꢀꢁ). Reactions conditions: 90 ◦C, 5 MPa of H2, H2O: 50 ml, cate-
chol 2.5 g, Ru: 20.5 mg.

30
C. Antonetti et al. / Applied Catalysis A: General 466 (2013) 21–31
EI (much the same of Sheldon’s Q). So each different component of
the waste can be assigned a quantitative potential environmental
impact PEIout (much the same of Sheldon’s environmental quotient
EQ), defined as the product of its mass (relative to the product unit
mass) with its EI. In this work the parameters taken into account
for the assessment of EIout of the waste were: MAK, TLV, Hazard
Symbol, or LX50 for human acute toxicity; any suspect carcinogen,
mutagen or teratogen for chronic human toxicity; and WGK or LC50
to fish for ecotoxicology. These data were obtained from substances
Material Safety Data Sheets. The quantitative comparison of poten-
tial environmental impact of waste (PEIout) is also reported in Fig. 7.
The figure allows to see immediately that the route from cyclohex-
ene is at 0.74 PEI unit/kg product while the route from catechol is
at 0.62 PEI unit/kg product, that is the catechol route achieves a
net 20% reduction in waste potential environmental impact. It is
worth of note that a significant contribution to the environmen-
tal impact of the cyclohexene route comes from the unrecovered
homogeneous catalyst (magenta block).
[10] T. Oguchi, T. Ura, Y. Ishii, M. Ogawa, Chem. Lett. 18 (1989) 857–860.
[11] C. Venturello, M. Gambaro, Synthesis 4 (1989) 295–297.
[12] W.A. Herrmann, R.W. Fischer, D.W. Marz, Angew. Chem. Int. Ed. 30 (1991)
1638–1641.
[13] R. Noyori, M. Aoki, K. Sato, J. Chem. Soc. Chem. Commun. 16 (2003) 1977–1986.
[14] T. Tatsumi, K. Yamamoto, H. Tajima, H. Tominaga, Chem. Lett. 21 (1992)
815–818.
[15] A. Corma, M.A. Camblor, P. Esteve, A. Martinez, J. Perez-Pariente, J. Catal. 145
(1994) 151–158.
[16] Q.-H. Xia, X. Chen, T. Tatsumi, J. Molec Catal. A 176 (2001) 179–193.
[17] A. Corma, M.T. Navarro, J. Perez-Pariente, J. Chem. Soc. Chem. Commun. 147
(1994).
[18] T. Tatsumi, K.A. Noyano, N. Igarashi, J. Chem. Soc. Chem. Commun. 3 (1998)
325–326.
[19] J. Xin, J. Suo, X. Zhang, Z. Zhang, New J. Chem. 24 (2000) 569–570.
[20] T. Blasco, A. Corma, M.T. Navarro, J. Perez Pariente, J. Catal. 156 (1995) 65–74.
[21] D.C. Dutoit, M. Schneider, R. Hutter, A. Baiker, J. Catal. 161 (1996) 651–658.
[22] J.M. Fraile, J.I. Garcia, J.A. Mayoral, E. Vispe, J. Catal. 189 (2000) 40–51.
[23] P. Wu, T. Tatsumi, T. Komatsu, T. Yashima, J. Catal. 202 (2001) 245–255.
[24] M. Guidotti, C. Pirovano, N. Ravasio, B. Lázaro, J.M. Fraile, J.A. Mayoral, B. Coq,
A. Galarneau, Green Chem. 11 (2009) 1421–1427.
[25] O.A. Kholdeeva, A.Yu. Derevyankin, A.N. Shmakov, N.N. Trukhan, E.A. Pauk-
shtis, A. Tuel, V.N. Romannikov, J. Mol. Catal. A 158 (2000) 417–421.
[26] N.N. Trukhan, A.Yu. Derevyankin, A.N. Shmakov, E.A. Paukshtis, O.A. Khold-
eeva, V.N. Romannikov, Microporous Mesoporous Mater. 44–45 (2001)
603–608.
[27] O.A. Kholdeeva, T.A. Trubitsina, M.N. Timofeeva, G.M. Maksimov, R.I. Maksi-
movskaya, V.A. Rogov, J. Mol. Catal. A 232 (2005) 173–178.
[28] N.V. Maksimchuk, K.A. Kovalenko, S.S. Arzumanov, Y.A. Chesalov, M.S. Mel-
gunov, A.G. Stepanov, V.P. Fedin, O.A. Kholdeeva, Inorg. Chem. 49 (2010)
2920–2930.
[29] T. Yamase, E. Ishikawa, Y. Asai, S. Kanai, J. Mol. Catal. A 114 (1996) 237–245.
[30] F. Chiker, F. Launay, J.P. Nogier, J.L. Bonardet, Green Chem. 5 (2003) 318–322.
[31] A.C. Dengel, W.P. Griffith, B.C. Parkin, J. Chem. Soc. Dalton Trans. 18 (1993)
2683–2688.
[32] Y. Usui, K. Sato, M. Tanaka, Angew. Chem. Int. Ed. 42 (2003) 5623–5625.
[33] M.N. Timofeeva, O.A. Kholdeeva, S.H. Jhung, J.-S. Chang, Appl. Catal. A 345
(2008) 195–200.
[34] B.S. Lane, K. Burgess, Chem. Rev. 103 (2003) 2457–2474.
[35] C. Venturello, E. Alneri, M. Ricci, J. Org. Chem. 48 (1983) 3831–3833.
[36] C. Venturello, R. D’Aloisio, J.C. Bart, M. Ricci, J. Mol. Catal. 32 (1985) 107–110.
[37] C. Venturello, M. Ricci, J. Org. Chem. 51 (1986) 1599–1602.
[38] C. Venturello, R. D’Aloisio, J. Org. Chem. 53 (1988) 1553–1557.
[39] Y. Ishii, K. Yamawaki, T. Ura, H. Yamada, T. Yoshida, M. Ogawa, J. Org. Chem.
53 (1988) 3587–3593.
[40] Y. Ishii, Y. Sakata, J. Org. Chem. 55 (1990) 5545–5547.
[41] S. Sakagucchi, Y. Nishiyama, Y. Ishii, J. Org. Chem. 61 (1996) 5307–5311.
[42] K. Sato, M. Aoki, R. Noyori, Science 281 (1998) 1646–1647.
[43] K. Sato, M. Aoki, R. Noyori, J. Org. Chem. 61 (1996) 8310–8311.
[44] C. Aubry, G. Chottard, N. Platzer, J.M. Brégeault, R. Thouvenot, F. Chauveau, C.
Huet, H. Ledon, Inorg. Chem. 30 (1991) 4409–4415.
[45] L. Salle, C. Aubry, R. Thouvenot, F. Robert, C. Doremieux-Morin, G. Chottard,
H. Ledon, Y. Jeanin, J.M. Bregeault, Inorg. Chem. 33 (1994) 871–878.
[46] L. Salle, J.Y. Piquemal, R. Thouvenot, C. Minot, J.M. Bregeault, J. Mol. Catal. A
117 (1997) 375–387.
7. Conclusions
Two different strategies for 1,2-cyclohexanediol synthesis were
adopted, with the aim of developing a high-yield and sustainable
method for the production of glycol, which can act as a start-
ing material for a new oxygen-based oxidation process for adipic
acid. The one-pot epoxidation and hydrolysis (dihydroxylation)
of cyclohexene to trans-1,2-cyclohexanediol was achieved with
97.4% yield, in the absence of a solvent using an aqueous solu-
tion of hydrogen peroxide, a phase-transfer-agent, and a tungstic
acid/phosphoric acid catalytic system. One drawback of the synthe-
sis was the difficulty in the recovery and reuse of the homogeneous
catalyst and of the PTA.
The alternative route investigated is the direct hydrogenation of
catechol to a mixture of cis and trans-1,2-cyclohexanediol, carried
out in a water solvent, using a Ru(OH)x-alumina heterogeneous cat-
alyst. Glycol was obtained with 90% yield; furthermore, the catalyst
could be recovered by filtration and reused, without any regenera-
tion. The reactivity of the Ru-based catalyst was influenced by the
Ru content; in fact, the in situ generation of metallic Ru during the
hydrogenation reaction led to a greater intrinsic activity for the cat-
alyst containing the greater Ru content. A comparison of the two
processes allowed to show the lower environmental impact of the
catechol hydrogenation route.
[47] C. Bolm, O. Beckmann, O.A.G. Dabard, Angew. Chem. Int. Ed. 38 (1999)
907–909.
[48] D.C. Duncan, R.C. Chambers, E. Hecht, C.L. Hill, J. Am. Chem. Soc. 117 (1995)
681–691.
[49] L.J. Csanyi, K. Jaky, J. Catal. 127 (1991) 42–50.
Acknowledgements
[50] G. Gelbard, F. Raison, E. Roditi-Lachter, R. Thouvenot, L. Ouahab, D. Grandjean,
J. Mol. Catal. 114 (1996) 77–85.
Radici Chimica SpA is acknowledged for financial support.
Regione Piemonte is acknowledged for financial support through
POR-FESR Asse I “Innovazione e transizione produttiva, I.1.3 Inno-
vazione e P.M.I., Aiuti ai soggetti aggregati ai poli di innovazione”.
The ISA (Institute of Advances Studies), University of Bologna, is
acknowledged for financial support to KR. MIUR is acknowledged
for the PhD grant to ER.
[51] M. Quenard, V. Bonmarin, G. Gelbard, Tetrahedron Lett. 28 (1987) 2237–2238.
[52] D. Prat, R. Lett, Tetrahedron Lett. 27 (1986) 707–710.
[53] F. Gao, T. Yamase, H. Suzuki, J. Mol. Catal. A 180 (2002) 97–108.
[54] C.L. Hill, C.M. Prosser-McCartha, Coord. Chem. Rev. 143 (1995) 407–455.
[55] R. Neumann, Prog. Inorg. Chem. 47 (1998) 317–370.
[56] O.A. Kholdeeva, R.I. Maksimovskaya, J. Mol. Catal. A 262 (2007) 7–24.
[57] E. Antonelli, R. D’Aloisio, M. Gambaro, T. Fiorani, C. Venturello, J. Org. Chem.
63 (1998) 7190–7206.
[58] A. Lambert, P. Plucinshi, I.V. Kozhevnikov, Chem. Commun. 6 (2003) 714–715.
[59] J. Kaur, I.V. Kozhevnikov, Catal. Commun. 5 (2004) 709–713.
[60] R. Neumann, A.M. Khenkin, J. Org. Chem. 59 (1994) 7577–7579.
[61] W. Zhu, H. Li, X. He, Q. Zhang, H. Shu, Y. Yan, Catal. Commun. 9 (2008) 551–555.
[62] T. Sakamoto, C. Pac, Tetrahedron Lett. 41 (2000) 10009–10012.
[63] Z. Xi, N. Zhou, Y. Sun, K. Li, Science 292 (2001) 1139–1141.
[64] K. Kamata, K. Yonehara, Y. Sumida, K. Yamaguchi, S. Hikicchi, N. Mizuno,
Science 300 (2003) 964–966.
References
[1] K.B. Sharpless, K. Akashi, J. Am. Chem. Soc. 98 (1976) 1986–1987.
[2] H.C. Kolb, M.S. VanNieuwenhze, K.B. Sharpless, Chem. Rev. 94 (1994)
2483–2547.
[3] M. Schröder, Chem. Rev. 80 (1980) 187–213.
[65] R. Neumann, M. Cohen, Angew. Chem. Int. Ed. 36 (1997) 1738–1740.
[66] H. Nagahara, M. Ono, M. Konishi, Y. Fukuoka, Appl. Surf. Sci. 121 (1997)
448–451.
[67] P.N. Rylander (Ed.), Catalytic Hydrogenation in Organic Synthesis, Academic
Press, New York, 1979, p. 175.
[68] S. Nishimura (Ed.), Handbook of Heterogeneous Catalytic Hydrogenation for
Organic Synthesis, Wiley, New York, 2001, p. 414.
[69] E.L. Muetterties, J.R. Bleeke, Acc. Chem. Res. 12 (1979) 324–331.
[4] K. Bergstad, S.Y. Jonsson, J.E. Bäckvall, J. Am. Chem. Soc. 121 (1999)
10424–10425.
[5] F. Cavani, J.H. Teles, ChemSusChem 2 (2009) 508–534.
[6] V. Nardello, J.-M. Aubry, D.E. De Vos, R. Neumann, W. Adam, R. Zhang, J.E. ten
Elshof, P.T. Witte, P.L. Alsters, J. Mol. Catal. A 251 (2006) 185–193.
[7] I.W.C.E. Arends, R.A. Sheldon, Appl. Catal. A 212 (2001) 175–187.
[8] V. Singh, P.T. Deota, Synth. Commun. 18 (1988) 617–624.
[9] M. Schwegler, M. Floor, H. Van Bekkum, Tetrahedron Lett. 29 (1988) 823–826.

C. Antonetti et al. / Applied Catalysis A: General 466 (2013) 21–31
31
[70] A.F. Borowski, S. Sabo-Etienne, B. Chaudret, J. Mol. Catal. A 174 (2001) 69–79.
[87] F. Cavani, J. Chem. Technol. Biotechnol. 85 (2010) 1175–1183.
[71] H. Yang, H. Gao, R.J. Angelici, Organometallics 19 (2000) 622–629.
[72] M. Takasaki, Y. Motoyama, K. Higashi, S.-H. Yoon, I. Mochida, H. Nagashima,
Chem. Asian J. 2 (2007) 1524–1533.
[73] A.M. Raspolli Galletti, C. Antonetti, I. Longo, G. Capannelli, A.M. Venezia, Appl.
Catal. A 350 (2008) 46–52.
[88] S. Sakaguchi, D. Kikuchi, Y. Ishii, Bull. Chem. Soc. Jpn. 70 (1997) 2561–2566.
[89] X. Zuwei, Z. Ning, S. Yu, L. Kunlan, Science 292 (2001) 1139–1141.
[90] J.M. Fraile, J.I. García, J.A. Mayoral, E. Vispe, J. Catal. 204 (2001) 146–156.
[91] O.A. Kholdeeva, B.G. Donoeva, T.A. Trubitsina, G. Al-Kadamany, U. Kortz, Eur.
J. Inorg. Chem. 34 (2009) 5134–5141.
[74] S. Velu, M.P. Kapoor, Appl. Catal. A 245 (2003) 317–331.
[75] K.V.R. Chary, D. Naresh, V. Vishwanathan, M. Sadakane, W. Ueda, Catal. Com-
mun. 8 (2007) 471–477.
[76] N. Mahata, K.V. Raghavan, V. Vishwanathan, C. Park, M.A. Keane, Phys. Chem.
Chem. Phys. 3 (2001) 2712–2719.
[77] H.A. Smith, B.L. Stump, J. Am. Chem. Soc. 83 (1961) 2739–2743.
[78] T. Maegawa, A. Akashi, K. Yaguchi, Y. Iwasaki, M. Shigetsura, Y. Monguchi, H.
Sajiki, Chem. Eur. J. 15 (2009) 6953–6963.
[79] J.W. Kim, T. Koike, M. Kotani, K. Yamaguchi, N. Mizuno, Chem. Eur. J. 14 (2008)
4104–4109.
[92] K. Sato, M. Aoki, M. Ogawa, T. Hashimoto, D. Panyella, R. Noyori, Bull. Chem.
Soc. Jpn. 70 (1997) 905–915.
[93] M. Huang, C.Y. Chen, J. Chin. Chem. Soc. 27 (1980) 151–159.
[94] M. Zawadzki, J. Okal, Mater. Res. Bull. 43 (1991) 3111–3121.
[95] M. Lenarda, L. Storaro, R. Ganzerla, R. Bertoncello, J. Mol. Catal. A 144 (1999)
151–158.
[96] B. Varughese, S. Chellamma, M. Lieberman, Langmuir 18 (2002) 7964–7970.
[97] B. Faroldi, C. Carrara, E.A. Lombardo, L.M. Cornaglia, Appl. Catal. A 319 (2007)
38–46.
[98] H. Chan, C.G. Takoudis, M.J. Weaver, J. Catal. 172 (1997) 336–345.
[99] G.G. Yan, T. Wu, W. Weng, H. Toghiani, R.K. Toghiani, H.L. Wan, C.U. Pittman,
J. Catal. 226 (2004) 247–259.
[80] K. Yamaguchi, T. Koike, J.W. Kim, Y. Ogasawara, N. Mizuno, Chem. Eur. J. 14
(2008) 11480–11487.
[81] K. Yamaguchi, J.W. Kim, J. He, N. Mizuno, J. Catal. 268 (2009) 343–349.
[82] N. Mizuno, K. Yamaguchi, Chem. Eur. J. 9 (2003) 4353–4361.
[83] N. Mizuno, K. Yamaguchi, Catal. Today 132 (2008) 18–26.
[84] T. Oishi, K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 48 (2009)
6286–6288.
[100] W. Li, H. Liu, E. Iglesia, J. Phys. Chem. B 110 (2006) 23337–23342.
[101] E. Rozhko, K. Raabova, F. Macchia, A. Malmusi, P. Accorinti, S. Alini, P. Babini,
G. Cerrato, M. Manzoli, F. Cavani, ChemCatChem 5 (2013) 1998–2008.
[102] Y. Wen, X. Wang, H. Wei, B. Li, P. Jin, L. Li, Green Chem. 14 (2012) 2868–2875.
[103] Z. Bohström, I. Rico-Lattes, K. Holmberg, Green Chem. 12 (2010) 1861–1869.
[104] M. Eissen, J.O. Metzger, Chem. Eur. J. 8 (2002) 3580–3585.
[105] R.A. Sheldon, Chem. Soc. Rev. 41 (2012) 1437–1451.
[85] J.W. Kim, K. Yamaguchi, N. Mizuno, J. Catal. 263 (2009) 205–208.
[86] F. Cavani, Catal. Today 157 (2010) 8–15.