Metallic short fibers for liquid-phase oxidation reactions

Article abstract of DOI:10.1016/j.molcata.2010.11.038

The application of metallic short fibers (MSF) for the liquid phase oxidation of alcohols and the epoxidation of alkenes has been investigated. Screening of various MSF lead to the conclusion that Cu₃Sn- and Cu64Ni28Mn7Fe1-fibers are the most promising candidates for oxidation reactions. Due to increased decomposition of H₂O₂ in presence of MSF containing Pt, Pd or Rh, lower yields were obtained with these catalysts. Oxidation reactions with the two above mentioned MSF are characterized by high selectivities for the oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones, respectively. Reactions are independent from reaction atmosphere and H₂O₂-dosage protocol, whereas parameters like solvent, temperature, and oxidant concentration had a significant influence.

Full text of DOI:10.1016/j.molcata.2010.11.038

Journal of Molecular Catalysis A: Chemical 335 (2011) 228–235
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Metallic short fibers for liquid-phase oxidation reactionsଝ
Achim Stollea,∗, Bernd Ondruschkaa, Ingrid Morgenthalb, Olaf Andersenb, Werner Bonrathc
a Institute for Technical Chemistry and Environmental Chemistry, Friedrich-Schiller University Jena, Lessingstraße 12, D-07743 Jena, Germany
b Fraunhofer Institute for Manufacturing and Advanced Materials, Dresden Branch Lab (IFAM-DD), Winterbergstraße 28, D-01277 Dresden, Germany
c R&D Chemical Process Technology, DSM Nutritional Products, P.O. Box 2676, CH-4002 Basel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
The application of metallic short fibers (MSF) for the liquid phase oxidation of alcohols and the
epoxidation of alkenes has been investigated. Screening of various MSF lead to the conclusion that
Cu3Sn- and Cu64Ni28Mn7Fe1-fibers are the most promising candidates for oxidation reactions. Due
to increased decomposition of H2O2 in presence of MSF containing Pt, Pd or Rh, lower yields were
obtained with these catalysts. Oxidation reactions with the two above mentioned MSF are characterized
by high selectivities for the oxidation of primary and secondary alcohols to the corresponding aldehy-
des and ketones, respectively. Reactions are independent from reaction atmosphere and H2O2-dosage
protocol, whereas parameters like solvent, temperature, and oxidant concentration had a significant
influence.
Received 23 August 2010
Received in revised form
23 November 2010
Accepted 28 November 2010
Available online 8 December 2010
Keywords:
Alcohol oxidation
Alloys
Hydrogen peroxide
Metallic short fibers
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
functional groups under mild conditions (e.g. in the production of
pharmaceuticals).
Compared to other chemical reactions oxidations in both gas
and liquid phase are in general challenging. Due to the thermody-
namics of this reaction type, it is always difficult to prevent the
substrates from undergoing over- or even total oxidation [1]. Thus,
the application of catalysts is often necessary for selective oxidation
processes in liquid phases. For gas phase reactions the application
of multi-component metal oxides is beneficial since those mate-
rials have a high thermal and chemical resistance. Undesired side
reactions can be suppressed by choosing the right catalyst or by
variation of the process parameters. However, in the gas phase oxi-
dants with a relative high hazardous potential (O2, ozone, NOx)
are applied requiring high regulatory efforts to prevent (thermal)
runaway. Working in liquid phase is advantageous since the sol-
vent can act as heat sink supporting heat and mass transfer and
therefore increase the process stability. Additionally, oxidants with
a lower hazardous potential like hydrogen peroxide can be applied
[2]. Compared to gas phase processes, reactions in liquid phase
generally have the disadvantage of lower throughput. Thus, oxi-
dations in liquid phase are often applied in selective oxidations of
The employment of catalysts in liquid phase oxidation is advan-
tageous due to the possibility for inherent control of selectivity and
conversion, thus making the overall reaction more affordable from
economic and ecologic point of view. However, the application
of bulk or supported catalysts requires special reactor technol-
ogy (e.g. fixed-bed). The combination of catalytic activity with
material design is therefore interesting, since hierarchical materi-
als/reactors could be designed. Within this topic the application of
metallic catalysts is promising due to excellent material properties
of metals and the possibility for preparation of various three-
dimensional (hierarchical) structures with distinct properties (e.g.
wire meshes, networks, metal foams). In line with this concept of
catalyst design a bottom-up concept was chosen to investigate the
possibilities for use of metal-alloys as oxidation catalysts. In the first
stage different alloys and intermetallic phases have been manufac-
tured with similar particle geometry: metallic short fibers (MSF)
[3].
Within this paper the application of various MSF in liquid-
phase oxidation reactions has been assessed. The formation
of ketones from secondary alcohols is used as model reac-
tion and various reaction parameters are varied investigat-
ing their influence on the course of reaction. Furthermore,
the scope of reactions has been extended by incorporating
the selective oxidation of primary alcohols (benzyl alcohol)
and the epoxidation of alkenes (styrene, limonene) into this
study.
ଝ
Supplementary data for this article containing kinetic correlations for variation
of solvent, stirring speed, reaction atmosphere, H2O2-dosage protocol, and H2O2-
concentration are available.
∗
Corresponding author. Tel.: +49 3641 948413; fax: +49 3641 948402.
E-mail address: achim.stolle@uni-jena.de (A. Stolle).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.11.038

A. Stolle et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 228–235
229
Table 1
Metallic short fibers (MSF) used as catalysts for this study.
Catalyst
Composition (w%)
lfiber [mm]
dfiber [␮m]
Afiber [m2 g−1
]
V255
V288
V325
V327
V328a
V420
V423
V438b
V441a
V442
V447
V449
FeAl20Ce10Pt5
FeCrAlPt0.5
FeAl20Ce3Pt1
FeAl20Pt0.5
FeAl30Pt0,5
86
89
0.012
0.009 (optical)
11.4
6.1
4.7
5.9
12.9
5.3
10.3
10.3
11.6
59
87
62
0.017 (optical)
0.008
FeCrAl20.5Pt0.5
FeCrAl20.5
X5Al20Ce3(PtRh)0.5
Cu68.1Sn31.9 (=Cu3Sn)
Al68.5Cu25.5Fe6
Cu64Ni28Mn7Fe1
Cu88Ni10Mn1Fe1
105
0.008 (optical)
70
138
82
0.010 (BET)/0.006 (optical)
0.042 (BET)/0.010 (optical)
0.008 (optical)
98
0.006 (optical)
a
Intermetallic phase.
Alloyed steel.
b
2. Experimental
(30 ml). After addition of hydrogen peroxide (4 g aqueous, 60 w%),
the reaction mixture was heated up to 80 ◦C. The course of reaction
was monitored by samples taken from the reaction mixture and
analysis by GC after drying with MgSO4 and extraction with ethyl
acetate.
2.1. General remarks
All chemicals were purchased from commercial suppliers
(Sigma–Aldrich, Alfa Aesar) and used without further purification.
The purity of all substrates was checked prior to use with GC-FID.
Metallic short fiber (MSF) catalysts listed in Table 1 have been pre-
pared by crucible melt extraction [3b].
2.6. Product analyses
Analyses were carried out with a 6890 Series GC-MSD and a 7890
Series II GC-FID from Agilent Technologies. Products were identi-
fied by comparison of their retention times and their mass spectra
with those of pure reference compounds. Conditions GC-FID (com-
pound 1): Poraplot Q, 25 m × 0.32 mm × 10 ␮m, 5 psi H2; program:
160 ◦C (hold 2 min), 10 K min−1 up to 250 ◦C (hold 9 min); injector
temperature: 250 ◦C; detector temperature: 250 ◦C. Conditions GC-
FID (all other compounds): HP 5, 30 m × 0.32 mm × 0.25 ␮m, 5 psi
H2; program: 80 ◦C (hold 4 min), 20 K min−1 up to 250 ◦C; injec-
tor temperature: 250 ◦C; detector temperature: 300 ◦C. Conditions
GC-MSD: HP 5-MS, 30 m × 0.32 mm × 0.25 ␮m, 9 psi He; program:
50 ◦C (hold 5 min), 10 K min−1 up to 250 ◦C (hold 1 min); injector
temperature: 250 ◦C, detector: electron impact (70 eV).
2.2. Catalyst characterization
The SEM-images were recorded using a scanning electron
microscope DSM 940 (Carl Zeiss). Optical microscopy pictures were
taken with a Stemi 2000-C (Carl Zeiss; magnification: 6.5–50×; cold
light source KL 1500 LCD) in combination with a digital camera JVC
GZ-MC200E (2 mega pixel; 10× optical zoom). 6-Point BET mea-
surements were carried out with Autosorb1 (Quantachrome) using
N2 as sorbent gas and a heating temperature of 350 ◦C.
2.3. General reaction procedure for secondary alcohol
dehydrogenation
All data reported herein are based on peak-areas and are com-
parable with the isolated ones. The reported yields were corrected
by means of different FID-sensitivity for substrate and product. The
data represent values from at least two independent experimental
runs.
2-Propanol (1; 6 mmol) and the catalyst (Table 1; 0.5 g) were
suspended in acetonitrile (30 ml). After addition of an appropriate
amount of hydrogen peroxide (aqueous, 60 w%), the reaction mix-
ture was heated up to 80 ◦C. The course of reaction was monitored
by samples taken from the reaction mixture and analysis by GC after
drying with MgSO4. Reactions were performed using a six-place
carousel reactor under refluxing conditions. In case of deionized
water as solvent extraction with ethyl acetate was performed prior
to drying and analysis.
3. Results and discussion
Recently the application of metallic short fibers (MSF) as cat-
alysts for the gas phase oxidative dehydrogenation of propane
affording propene and of 2-propanol (1) furnishing acetone (1a)
has been reported (Scheme 1) [4,5]. Comparison with other cata-
lysts revealed that the materials are interesting but the potential
applications are limited due to process instabilities and lower con-
version and selectivity rates. Thus, the concept of oxidation in
presence of MSF has been investigated in liquid phase reactions. As
2.4. Reaction procedure for the oxidation of benzyl alcohol
Benzyl alcohol (3; 6 mmol, 648 mg) and the catalyst (Table 1;
0.5 g) were suspended in water (30 ml). After addition of hydrogen
peroxide (4 g aqueous, 60 w%), the reaction mixture was heated up
to 80 ◦C. To avoid any side effects concerning autoxidation of 3, the
reactions have been performed in nitrogen-atmosphere. The course
of reaction was monitored by samples taken from the reaction mix-
ture and analysis by GC after drying with MgSO4 and extraction
with ethyl acetate.
OH
H2O2
O
catalyst
1
1a
H2O2
2.5. Reaction procedure for the epoxidation of styrene and
limonene
OH
O
catalyst
2
2a
Styrene (4; 6 mmol, 625 mg) or limonene (5; 6 mmol, 816 mg)
and the catalyst (Table 1; 0.5 g) were suspended in acetonitrile
Scheme 1. Model reactions.

230
A. Stolle et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 228–235
Fig. 1. Light microscopy image of the cross section of Cu3Sn-fibers (V441; crystalline
structure visible using polarized light).
Fig. 3. Scanning electron microscopy image of a Cu64Ni28Mn7Fe1-fiber surface
(V447).
model reactions the formation of 1a and cyclohexanone (2a) from
1 and cyclohexanol (2), respectively, have been chosen (Scheme 1)
[6–9]. Initially a range of MSF (Table 1) has been tested. The most
promising candidates are afterwards applied for the investigation
of various reaction parameters and also in the oxidation of other
substrates.
microscopy or scanning electron microscopic (SEM) characteriza-
tion (see Figs. 1–3). The SEM surface image of a Cu64Ni28Mn7Fe1
fiber (V447) reveals fine surface structures, e.g. roughness. Such a
rough surface structure was also found on AlCuFe fibers (V442).
This explains the difference between measured values of the spe-
cific surface area (BET or optical). By means of the BET-method
the morphology of the fiber surface is detected and, therefore, the
measured values are higher (see Table 1).
3.1. Preparation of metallic short fibers (MSF)
The process of crucible melt extraction allows the production
of MSF in almost arbitrary compositions [3b]. A rotating notched
wheel is wetted by the molten material. Since the wheel is water-
cooled, a high cooling rate is achieved resulting in short fibers
with a homogeneous distribution of the metals. Small grain sizes,
a high solubility of the constituents and metastable or amorphous
phases are possible [3,4]. Due to continuous development of the
melt extraction process at Fraunhofer IFAM, the production of fibers
with an average equivalence diameter of 50–150 ␮m is possible.
The length of the fibers can be varied from 3 to 12 mm. For this
study, different types of fibers were produced and characterized
(Table 1).
3.2. Catalyst screening
Results of screening experiments for oxidation of 2-propanol (1)
to acetone (1a) using aqueous H2O2 under refluxing conditions in
acetonitrile are described in Fig. 4. In all cases 1a was identified
as the only oxidation product. Blank reactions without the catalyst
afforded 1a in minor amounts (<5% after 24 h). Except for catalysts
V288, V420, V441, V442, and V447 the content of 20% was reached
within 8 h. The increase in yield of oxidation product after 24 or
32 h is usually not higher than 5%. In case of V288 and V420 the
MSF contains Pt as well as Cr but no Ce, which seems to be bene-
ficial for the reaction. Employment of the Ni-containing MSF V447
results in significantly higher yields of 1a within longer reaction
The melt extracted fibers typically possess a kidney shaped cross
section (see Figs. 1 and 2). The crystalline structure is visible by light
80
after 8 h
after 24 h
after 32 h
60
40
20
0
255 288 325 327 328 420 423 438 441 442 447 449
catalyst (cf. Table 1)
Fig. 4. Results of oxidation of 2-propanol (1; 6 mmol) to acetone (1a) using hydrogen
peroxide (2 g; 60 w%; aqueous) in 30 ml acetonitrile using catalysts listed in Table 1
(0.5 g; T = 80 ◦C).
Fig. 2. Light microscopy image of the cross section of a Cu64Ni28Mn7Fe1-fiber
(V447).

A. Stolle et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 228–235
231
Table 2
Table 3
Solvent influence on the oxidation of cyclohexanol (2) to cyclohexanone (2a)a,b
.
Oxidation of 2-propanol (1) and cyclohexanol (2) under various reaction
atmospheresa,b
.
Catalystc
V441
Solvent
Ycyclohexanone [%]
Yacetone [%] for MSFc,d
Ycyclohexanone [%] for
Ethyl acetate
Acetonitrile
Deionized water
Ethyl acetate
Acetonitrile
8
23
14
3
40
53
MSFc,e
Reaction atmosphere
V441
V447
V441
V447
V447
Air
N2
Ar
CO2
O2
63
65
61
57
62
89
87
63
33
62
32
29
34
9
38
37
35
38
35
Deionized water
a
Substrate: 6 mmol 2, oxidant: aqueous H2O2 (60 w%, 4 g), solvent: 30 ml, t = 5 h,
T = 80 ◦C.
27
b
For complete dependency from reaction time cf. SD-1.
0.5 g; cf. Table 1.
a
Substrate: 6 mmol, oxidant: aqueous H2O2 (60 w%, 4 g), solvent: 30 ml deionized
c
water, t = 6 h, T = 80 ◦C.
b
For complete dependency from reaction time cf. SD-II1.
Catalyst: 0.5 g MSF; cf. Table 1.
Substrate: 1.
Substrate: 2.
c
times than 8 h, indicating an induction period for the catalyst. With
V288 and V447 yields of 40–60% can be achieved. Performing reac-
tions with Cu-containing MSF V441 and V442, the conversion of
1 can reach 70%, whereby the latter shows a significant induction
period of almost 24 h [10,11].
d
e
catalysts it can be concluded that elevated T is advantageous for
the reaction of 2 to 2a. In case of the Cu–Sn-alloy V441 (Fig. 5, left),
variation of T effected the kinetics of the reaction resulting in a
higher production rate at higher T. At 50 ◦C the reaction rate was
almost zero. In contrast, T influences the reaction kinetics only at
the beginning of the reaction in presence of the Cu88Ni10Mn1Fe1-
fiber (V447; Fig. 5, right) reaching a plateau of approximately 50%
yield (T = 80–100 ◦C). Within the applied reaction time this plateau
cannot be reached for oxidation at 70 ◦C. Similar to the behavior of
catalyst V441, the reaction rates are significantly lower at T = 50 ◦C.
The influence of reaction atmosphere on the oxidation of both
1 and 2 to the corresponding ketones was investigated by purg-
ing different gases through the reaction mixtures (Table 3). Either
inert gases (N2, Ar) or reactive gases (air, CO2, O2) have been applied
for the investigations. The pressure was kept constant in all cases at
0.11 MPa and a flow rate of 1 ml min−1 was maintained. Comparison
of the results of reactions in air with those reported previously (no
purging but air-atmosphere) revealed similar results for both cata-
lysts and substrates after 6 h of reaction time. The product yields are
independent from the gas purged through the reaction mixtures,
since the levels of yields are similar. Although the kinetic profiles
revealed differences (especially in the presence of CO2), the termi-
nal selectivity and yield are comparable for the oxidation of 1 and
2 in presence of either V441 or V447 (SD-III). Thus, further reac-
tions have been performed under standard air-atmosphere without
purging.
Besides oxidation with hydrogen peroxide, experiments have
been conducted employing gaseous oxygen as the oxidant in
PTFE-shielded stainless-steel autoclaves (pO2 = 0.8 MPa). The reac-
tions afforded 1a as the only reaction product [7]. In contrast to
results summarized in Fig. 4 Pt-containing MSF lead to the high-
est conversions (Table 1). However, yields were restricted by the
external oxygen pressure. Due to this fact, the decreased hazardous
potential for application of H2O2 instead of O2, and the improved
mass transport (two-phase instead of three-phase reaction) further
experiments have been carried out in the presence of aqueous H2O2
as the oxidant.
3.3. Influence of reaction parameters
As with other chemical transformations, oxidations with aque-
ous hydrogen peroxide are processes with many (un-)constrained
variables and parameters. The most obvious ones: are solvent,
reaction temperature, reaction-atmosphere, and oxidant con-
centration, as well as the type of substrate [6]. Experimental
investigation of the influence of the parameters on the product
yield was performed with two model reactions: (i) oxidation of
2-propanol (1) to acetone (2) and (ii) formation of cyclohexanone
(2a) from cyclohexanol (2; Scheme 1). Due to lower solubility of 2
with aqueous H2O2 solvent effects are more pronounced in com-
parison to the application of 1 as substrate. The use of water or polar
organic solvents with a high hydrophilicity (acetonitrile) afforded
higher yields compared to the reaction in ethyl acetate (Table 2).
The unfavorable solvent properties of ethyl acetate resulted in low
conversions due to the poor mixing between organic and aqueous
phases containing the substrate and oxidant, respectively. How-
ever, the solvent effect is more pronounced in case of catalyst V447.
The kinetics of the oxidation in water or acetonitrile is similar for
both catalysts (cf. Supplementary Data: SD-I). Unfavorable mass
transport due to the low solubility of 2 in water may hinder the
reaction. Uniform dispersion of oxidant, catalyst, and substrate is
necessary for high yields and can be realized by more intensive
stirring. However, variation of the stirring frequency between 100
and 600 rpm revealed identical kinetic reaction profiles for both
catalysts (SD-II).
Generally, lower local oxidant concentrations are assumed to
afford higher yields since over-oxidation is suppressed as well as
the effectiveness of oxidant consumption increases due to suppres-
sion of H2O2 decomposition. Thus, different dosage-protocols for
the oxidant have been applied in presence of catalysts V441 or
V447 (Table 4). The overall mass of 60 w% aqueous H2O2 added to
the reaction mixture was 4 g (↔2.4 g = 72 mmol H2O2). The amount
was either added in one portion at the beginning of the reaction
Table 4
Variation of the H2O2-dosage protocol for the oxidation of cyclohexanol (2)a,b
.
Addition of aqueous H2O2 [g]after. . . Ycyclohexanone [%] for
MSFc
Dosage protocol
0 h
1 h
2 h
3 h
V441
V447
The application of H2O2 as terminal oxidant is problematic,
since the oxidizing agent may undergo self-decomposition ini-
tiated either by higher temperature T, light, the presence of
acids and bases or by contact with metal-surfaces [2b,c]. Titration
experiments during the conversion of 1 and 2 revealed that the
decomposition of H2O2 is not as significant as thought. However,
variation of T from 50 to 100 ◦C afforded a tremendous change in
reactivity and activity of the catalysts (Fig. 5) [12]. For both model
A
B
C
D
4
2
2
1
14
12
12
12
53
44
58
41
2
1
2
1
1
a
Substrate: 6 mmol 2, oxidant: aqueous H2O2 (60 w%, ꢀm = 4 g;
n(H2O2) = 72 mmol), solvent: 30 ml deionized water, t = 5 h, T = 80 ◦C.
b
For complete dependency from reaction time cf. SD-IV.
Catalyst: 0.5 g MSF; cf. Table 1.
c

232
A. Stolle et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 228–235
catalyst V441
catalyst V447
75%
50%
25%
0%
50
70
80
90
100
60%
45%
30%
15%
0%
50
70
80
90
100
0
2
4
6
0
2
4
6
t
/ h
t / h
Fig. 5. Influence of reaction temperature T on the oxidation of cyclohexanol (2; 6 mmol) to cyclohexanone (2a) using hydrogen peroxide (4 g; 60 w% aqueous) in 30 ml
deionized water in presence of MSF V441 and V447 (0.5 g, cf. Table 1).
or addition in smaller portions at different time periods during
the reaction. Apparently, the final product yields are independent
from the employed catalysts and the reaction protocol. It might be
expected, that the dosage protocol does not influence the terminal
results but reaction kinetics. However, time dependency of prod-
uct yield for the conditions shown in Table 4 revealed no significant
invariance as provided in SD-IV. Furthermore, the selectivities are
>99% and also the reaction plateau typical for catalyst V447 (com-
pare Fig. 5, right) is reproduced. For further reactions the oxidant is
added in one portion at the beginning of the reactions.
Another important reaction parameter influencing the
course of reaction is the substrate-to-oxidant ratio, which
can be varied in different ways: (i) variation of noxidant
(nsubstrate = constant), (ii) variation of the oxidant concentra-
tion (noxidant and nsubstrate = constant), and (iii) variation of nsubstrate
(noxidant = constant).
In order to investigate the influence by variation according to
case (i), the oxidation of 6 mmol 2 in presence of catalyst V447
was performed by variation of n(H2O2) from 18 to 72 mmol. Thus,
the oxidant-to-substrate ratio ranges from 3 to 12, respectively
(Table 5). Comparison of the yields after 2 and 6 h revealed a direct
proportionality between oxidant-to-substrate ratio and product
yield, thus assuming that these parameters are constrained vari-
ables [13]. The slopes of the linear regression functions are 0.0477
and 0.0480 and the cubic proximity levels R2 are 0.9939 and 0.9912
for the results after 2 and 6 h of reaction time, respectively (compare
SD-VI).
w(H2O2,aq), thus assuming that dilution of the oxidant by water has
no influence on the chemical transformations shown in Scheme 1.
Even the typical yield plateau n case of fiber V447 is independent
from the substrate and w(H2O2,aq), indicated by kinetic reaction
profiles (SD-V). The higher rates of conversions reported for the
oxidation of 1 compared to the reaction of 2 (Tables 3 and 6) is
attributed to the higher molecular flexibility of 1 leading to a higher
reactivity of this substrate.
Kinetic reaction profiles for the oxidation of 2 furnishing 2a
with variation of nsubstrate from 2 to 12 mmol are reported in Fig. 6.
Independent from reaction time and catalyst the highest yields
are afforded in case of 3 mmol 2 (oxidant-to-substrate ratio = 24).
However, plotting of the absolute product yields in dependence
of reaction time reveals strong similarities for different nsubstrate
.
Thus, the oxidant-to-substrate ratio shows no visible influence
on the absolute substrate conversion, when keeping noxidant con-
stant [14,15]. Comparing the results with those reported in Table 5
for variation of noxidant revealed significant differences. Whereas
in the former case a linear time-independent relationship was
found, variation of nsubstrate strongly influences the reaction kinetics
(Fig. 6). Notwithstanding those differences, the functional correla-
tion between oxidant-to-substrate ratio and yield of 2a for both
modes of variation is linear as indicated by linear regression func-
tions (SD-VI). Concluding that the reaction rates are of zero- and
first-order with respect to the substrate and oxidant, respectively.
3.4. Substrate screening
The assessment of results from the variation of exper-
iments with different oxidant concentrations (noxidant and
nsubstrate = constant) is more difficult. The concentration of aque-
ous H2O2, indicated as w(H2O2,aq), has been varied between 30 and
60 w%. The overall amount of oxidizing agent is constant (72 mmol),
thus an oxidant-to-substrate ratio of 12 has been employed for
the reactions of 1 and 2 in the presence of catalysts V441 and
V447 (Table 6). Similar to the variation of reaction atmosphere (cf.
Table 3) the yields after 6 h reaction time are independent from
For all tested substrates blank reactions in absence of the metal-
lic shortfibers havebeen conductedresulted in eitherinferior yields
(<3%; secondary alcohols) or conversion rates below 3% (3–5) after
6 h of reaction time. In addition to the oxidation of primary (3)
Table 6
Influence of H2O2-concentration on the oxidation of 2-propanol (1) and cyclohex-
anol (2)a,b
.
Yacetone [%] for
Ycyclohexanone [%]
Table 5
MSFc,d
for MSFc,e
Influence of the oxidant-to-substrate ratio for the oxidation of cyclohexanol (2)a.
w(H2O2,aq) [w%]
m(H2O2,aq) [g]
V441
V447
V441
V447
Xcyclohexanone [%]
after:
30
40
50
60
8.0
6.0
4.8
4.0
65
68
66
64
58
51
65
61
29
29
32
32
37
43
35
38
m(H2O2,aq)
n(H2O2)
n(H2O2)/
t = 2 h
t = 6 h
[g]
[mmol]
n(substrate)
2.1
4.1
6.1
8.0
18
36
54
72
3
6
9
18
35
49
61
15
32
47
58
a
Substrate: 6 mmol, oxidant: aqueous H2O2 (n = 72 mmol, m(H2O2) = 2.4 g), sol-
vent: 30 ml acetonitrile, t = 6 h, T = 80 ◦C.
b
For complete dependency from reaction time cf. SD-V.
Catalyst: 0.5 g MSF; cf. Table 1.
Substrate: 1.
Substrate: 2.
12
c
a
d
Substrate: 6 mmol 2, oxidant: aqueous H2O2 (30 w%), solvent: 30 ml acetonitrile,
catalyst: 0.5 g V447 (cf. Table 1; T = 80 ◦C).
e

A. Stolle et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 228–235
233
catalyst V441
catalyst V447
100%
75%
50%
25%
0%
50%
40%
30%
20%
10%
0%
3 mmol
6 mmol
9 mmol
12 mmol
3 mmol
6 mmol
9 mmol
12 mmol
0
2
4
6
0
2
4
6
t
/ min
t
/ min
catalyst V441
catalyst V447
3.0
2.0
1.0
1.5
1.0
0.5
0.0
3 mmol
3 mmol
6 mmol
9 mmol
12 mmol
6 mmol
9 mmol
12 mmol
0.0
0
0
2
4
6
2
4
6
t
/ min
t / min
Fig. 6. Influence of substrate concentration on the oxidation of cyclohexanol (2; 3–12 mmol) to cyclohexanone (2a) using hydrogen peroxide (4 g, n(H2O2) = 36 mmol; 30 w%
aqueous) in 30 ml deionized water in presence of MSF V441 and V447 (0.5 g, cf. Table 1; T = 80 ◦C). The top diagrams display relative yields and the bottom ones absolute
yields.
and secondary alcohols, the epoxidation of alkenes (4, 5) has been
investigated.
ane ring, respectively. Due to the structural rigidity in case of 2,
its reactivity is significantly lower than in case of the more flexible
aliphatic alcohols. Similar to the reaction of 2-decanol, the oxida-
tion of cyclooctanol afforded side reaction products (formation of
carboxylic acids indicated by drop of pH) lowering the selectivity
for the formation of cyclooctanone to 43 and 38% with V441 and
V447 as catalysts, respectively.
Compared to the oxidation of secondary alcohols (Table 7),
the similar reaction with primary alcohols is much more difficult,
due to the possibility over-oxidation furnishing the corresponding
carbonic acid or further reaction products. However, high selec-
tivities for the partial oxidation product benzaldehyde (3a) have
been found for the oxidation of benzyl alcohol (3; Scheme 2) in
the presence of fiber catalysts V441 and V447 (Fig. 7) [14–21].
Whereas in the former case the reaction was selective (>99%), appli-
cation of V447 afforded also the formation of benzoic acid (3b)
at higher conversion rates. It is remarkable that this side reac-
tion does not affect the overall yield of 3a. Rather the formation
of 3b resulted from direct oxidation of 3, which is accompanied
by catalyst destruction affording a brownish reaction solution.
This indicates dissolution of Mn from the Cu64Ni28Mn7Fe1-alloy
during the oxidation process, affording MnO2 as final oxida-
tion product. Additionally, decomposition product benzene was
formed at t > 4 h with minor selectivities of 1 and 2% for MSF
Table 7 summarizes the results for the oxidation of secondary
alcohols in presence of two different fiber catalysts: V441 and V447.
In almost all cases the reaction furnishes the corresponding ketones
with excellent selectivity (>99%). The yields strongly depend on
the nature and reactivity of the substrate. Lower acyclic alcohols
(<C6) lead to higher conversions compared to longer-chain educts.
In contrast to the lower homologues, 2-decanol afforded side reac-
tion products also (nonanoic acid) decreasing the selectivity for
2-decanone to 46 and 67% for oxidation in presence of V441 and
V447, respectively. The comparison of the reaction rates of 1 with
2 and 3-pentanol is interesting, because the latter contains 1 as
substructure flanked by methyl groups or fixated in a cyclohex-
Table 7
Oxidation of secondary alcohols in the presence of 0.5 g metallic short fibers (cf.
Table 1).a
Yketone [%] for MSF
Secondary alcohol
V441
V447
2-Propanol (1)
2-Butanol
24
7
58
11
2-Pentanol
30
61
3-Pentanol
64
54
2-Hexanol
14
18
2-Octanol
38
44
2-Decanolb
Cyclopentanol
Cyclohexanol (2)
Cyclooctanol
12 (X = 26%)
32
13
24 (X = 36%)
47
38
CHO
+
COOH
H2O2
OH
catalyst
28 (X = 65%)
26 (X = 68%)
3
a
3a
3b
Substrate: 6 mmol, oxidant: aqueous H2O2 (60 w%, 4 g), solvent: 30 ml acetoni-
trile, t = 4 h, T = 80 ◦C.
b
Scheme 2. Oxidation of benzyl alcohol (3) with metallic short fibers.
Formation of unidentified side products (X = conversion).

234
A. Stolle et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 228–235
3 (V441)
3a (V441)
3b (V441)
3 (V447)
100%
75%
50%
25%
0%
O
O
3a (V447)
H2O2
+
+
3b (V447)
catalyst
O
O
5
5a
5c
5b
Scheme 4. Oxidation of limonene (5) with metallic short fibers.
Table 8
Epoxidation of limonene (5; cf. Scheme 4) with H2O2 and in presence of metallic
short fibersa.
Catalystb
V441
t [h]
X(5) [%]
S(5a) [%]
S(5b) [%]c
S(5c) [%]c
2
4
6
2
4
6
1
3
60
42
34
0
39
41
40
58
19
>99
12
16
0
0
47
0
49
42
15
1
18
58
0
2
4
6
V447
t / h
Fig. 7. Time dependency of the reaction mixture composition for oxidation of benzyl
alcohol (3; 6 mmol) using aqueous hydrogen peroxide (60 w%, 4 g) in 30 ml deionized
water in presence of MSF V441 and V447 (0.5 g, cf. Table 1; T = 80 ◦C).
a
Substrate: 6 mmol 5, oxidant: aqueous H2O2 (60 w%, 4 g), solvent: 30 ml ace-
tonitrile, T = 80 ◦C.
b
Catalyst = 0.5 g (cf. Table 1).
Stereoisomers are summarized.
c
V441 and V447, respectively. The presence of both catalysts led
to an intermediate start of the oxidation reaction indicated by
the relative high amount of 3a formed in the beginning of the
reaction.
forms, which is ignored in the calculation of selectivity reported in
Table 8. In case of catalyst V447 excellent selectivities for 5b have
been found at very low conversions. Thus, the regioselectivity is
comparable to the application of ␥-alumina or Na2WO4-PhPO3H2-
[Me(n-C8H17)3N]HSO4 as the catalysts and also 60 w% aqueous
H2O2 as the terminal oxidant [25c,e]. Increase of the reaction time
and therefore of conversion afforded predominantly the formation
of 5a and 5c. Application of metallic fiber V441 revealed lower reac-
tion rates resulting in a maximum conversion of 15% after 6 h. The
selectivity ratio is similar to that reported for the employment of
V447. The complete absence of over-oxidation products is dedi-
cated to the absence of any (de)stabilizing groups (e.g. aromatic
ring in 4).
Beside the oxidation of secondary and primary alcohols the
possibility for alkene epoxidation in presence of metallic short
fibers has been assessed. Styrene (4) and limonene (5) have
been applied as model substrates. With the same reaction condi-
tions as described previously (Table 7, Fig. 7) styrene oxide (4a;
Scheme 3) was formed in 1% yield only [16,22,23]. This find-
ing was independent from reaction time, conversion or type of
catalyst (V441, V447). As major oxidation product 3a has been
identified in addition to minor amounts of acetophenone (4b;
<3%) [16]. After 6 h reaction a conversion of 25 and 42% has
been found and a product selectivity for 3a of 38 and 90% has
been calculated for the employment of V441 and V447, respec-
tively. Reaction under similar experimental prerequisites with
4a afforded isomerization to 4b with selectivities of 69 (X = 66%)
and 94% (X = 55%) for oxidation with V441 and V447 (t = 6 h),
respectively. Also the formation of 3a was observed. Thus, the
formation of 3a from 4 is a competing reaction pathway to
epoxidation (4a) and subsequent isomerization (4b). The for-
mation of epoxide ring opening products 1-phenylethan-1,2-diol
and 2-phenylacetaldehyde has not been observed throughout the
experiments.
3.5. Mechanistic considerations
The mechanism of the Cu3Sn-catalyzed (V441) oxidation reac-
tion with H2O2 as the oxidant is different to the activation in metal
complexes (e.g. CuII, TiIV or MoVI) via coordination of perooxo-
anions to the metal center [26]. However, it is well known that
oxygen is able to oxidize copper very rapidly to its oxides; whereas
the formation of cuprous (Cu2O) and cupric oxide (CuO) is a dif-
fusion limited process [27]. With increasing layer thickness the
relative content of Cu2O-to-CuO increases. The presence of hydro-
gen peroxide as oxidant is also able to furnish the oxidation of
Cu-metal in acidic media [28] and reduction of NO to N2 is reported
also [29]. Both concepts afford the formation of Cu2O preferably.
Thus, the reaction is expected to form Cu2O by direct oxidation of
the metallic fiber and consecutive reaction of the alcohol with the
oxide layer furnished the reaction products.
Concerning the relation to over-oxidation and epoxidation the
latter is definitely more pronounced in case of 5 than reported
for similar reaction of 4 (Scheme 3). In case of 5 epoxidation
is the predominant reaction pathway affording different epox-
idation products: 7,8-limonene oxide (5a), 1,2-limonene oxide
(5b), and 1,2;7,8-limonene dioxide (5c; Scheme 4 and Table 8)
[22–25]. Products 5b and 5c exist in different stereoisomeric
As discussed above reactions with catalyst V447
(Cu64Ni28Mn7Fe1) afforded the formation of MnO2 under loss
of the structural integrity of the alloyed metal fiber. Apparently,
in those case hydrogen peroxide also oxidizes the manganese.
The formation of (per)manganate species and of MnO2 would
account for the higher reactivity of V447 compared to V441, since
manganese in those oxidation states is a potent oxidant for alcohol
dehydrogenation [30].
O
O
CHO
H2O2
catalyst
+
+
4
4a
4b
3a
Scheme 3. Oxidation of styrene (4) with metallic short fibers.

A. Stolle et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 228–235
235
4. Conclusion
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Results indicated that the fibers are suitable catalysts for the for-
mation of ketones from the corresponding secondary alcohols
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In general noble-metal free MSF are significantly more active for
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Acknowledgements
The assistance of G. Gottschalt (Institute for Technical Chem-
istry and Environmental Chemistry) for performing the oxidation
experiments is strongly acknowledged. AS and BO are thankful to
Dr. P. Scholz (Institute for Technical Chemistry and Environmental
Chemistry) for interesting discussions.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2010.11.038.
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