Molybdenum Complexes as Catalysts for the Oxidation of Cycloalkanes with Molecular Oxygen

Article abstract of DOI:10.1007/s10562-016-1716-7

A group of new polymolybdates was synthesized and tested in the catalytic oxidation of cycloalkanes. Investigated compounds exhibit broad structural diversity including polymolybdates with isolated anionic clusters (0-dim), polymeric anions (1-dim), Mo-O layers (2-dim) as well as hexagonal and orthorhombic molybdenum oxides (3-dim). All studied molybdenum complexes were found to be active in the reaction conditions yielding ketones and alcohols as main products. In the case of layered compounds (2-dim), dicarboxylic acid was detected in the mixture of reaction products. The structure of investigated molybdenum compounds was shown to have influence on yields and selectivities of the investigated reactions. Anionic layers separation (in 2-dim materials) as well as type and charge of organic cations present in the compounds are prime examples of structural factors influencing the oxidation reactions efficiencies. On the basis of obtained results and literature reports a mechanism for the oxidation of cycloalkanes by polymolybdates has been proposed.

Full text of DOI:10.1007/s10562-016-1716-7

Catal Lett (2016) 146:998–1010
DOI 10.1007/s10562-016-1716-7
Molybdenum Complexes as Catalysts for the Oxidation
of Cycloalkanes with Molecular Oxygen
1
Anna Szymanska Wojciech Nitek2 Marcin Oszajca2 Wiesław Łasocha1,2
•
•
•
•
´
Katarzyna Pamin1 Jan Połtowicz1
•
Received: 23 January 2016 / Accepted: 12 February 2016 / Published online: 3 March 2016
Ó The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract A group of new polymolybdates was synthe-
sized and tested in the catalytic oxidation of cycloalkanes.
Investigated compounds exhibit broad structural diversity
including polymolybdates with isolated anionic clusters (0-
dim), polymeric anions (1-dim), Mo–O layers (2-dim) as
well as hexagonal and orthorhombic molybdenum oxides
(3-dim). All studied molybdenum complexes were found to
be active in the reaction conditions yielding ketones and
alcohols as main products. In the case of layered com-
pounds (2-dim), dicarboxylic acid was detected in the
mixture of reaction products. The structure of investigated
molybdenum compounds was shown to have influence on
yields and selectivities of the investigated reactions.
Anionic layers separation (in 2-dim materials) as well as
type and charge of organic cations present in the com-
pounds are prime examples of structural factors influencing
the oxidation reactions efficiencies. On the basis of
obtained results and literature reports a mechanism for the
oxidation of cycloalkanes by polymolybdates has been
proposed.
Graphical Abstract
Keywords Cycloalkanes oxidation Á Heterogeneous
catalysis Á Polyoxometalates synthesis Á X-ray diffraction Á
Single crystal Á Powder diffraction
1 Introduction
Molybdenum complexes are well known as active centers
of several enzymes such as oxotransferases [1–4]. Some
molybdates show antiviral, antitumoral or antibiotic
activity [5–7]. They are also known as efficient catalysts
for the oxidation of various organic compounds with
different oxygen donors like ROOH [8–20] and H2O2 [21–
29]. Molybdenum(VI) dioxo or oxodiperoxo compounds
are commonly encountered in oxidation catalysis. It has
been shown that mononuclear peroxo and mono-oxo
bridged dinuclear molybdenum complexes are active cat-
alysts for the epoxidation of olefins and nonfunctionalized
olefins with t-butylhydroperoxide TBHP as oxygen donor
[8, 13–15]. The epoxidation of olefins catalyzed by a large
family of complexes with MoO2X2(N–N)2 or MoO2X2(N–
O)2 formula where N–N or N–O are bidenate ligands and
X = Cl, Br or Me has been previously reported [10, 11,
15, 16]. All these catalysts show high activity and
selectivity.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-016-1716-7) contains supplementary
material, which is available to authorized users.
& Wiesław Łasocha
lasocha@chemia.uj.edu.pl
1
Jerzy Haber Institute of Catalysis and Surface Chemistry
PAS, Niezapominajek 8, 30-239 Krakow, Poland
2
Faculty of Chemistry, Jagiellonian University, Ingardena 3,
30-060 Krakow, Poland
123

Molybdenum Complexes as Catalysts for the Oxidation of Cycloalkanes with Molecular Oxygen
999
Molybdenum oxo-peroxo complexes were applied as
catalysts for selective oxidation of arylalkyl, diaryl and
dialkylsulfides to corresponding sulfoxides with TBHP as
oxidant [30–33].
polyoxomolybdates of aromatic amines (three compounds
of 4-methylaniline, six compounds of other aniline
derivatives, two polyoxomolybdates of 4-methylpyridine).
Five of them, namely: A3, B1, B2, B4 and C4 are new
compounds, not previously reported. Consult Table 1, for
compounds description including chemical formulas,
symbols used henceforward and graphical representations
of their structures. Crystal structures of investigated cata-
lysts, in particular new compounds, are described in
chapter 3.1.
Recent studies have shown that oxo-peroxo molybde-
num complexes in combination with H2O2 can convert a
wide range of hydrocarbons like alkanes and alkylbenzenes
to the corresponding oxidized products [34]. Molybdenum
oxo and oxo-peroxo complexes have been used for selec-
tive oxidation of amines [13, 30–33] and for oxidation of
allylic alcohols with H2O2 [34]. Dioxomolybdenum(VI)
complexes with a N-salicylidene amino acid hydrazide [28]
or oxazoline [16] ligand were applied for oxidation of
sulfides with high selectivity to sulfoxides. Dioxomolyb-
denum(VI) complexes of the MoO2L2 type (L = (20-piry-
dyl)alcoholate) were used as catalysts for the selective
terminal oxidation of n-alkanes with molecular oxygen
[35]. The catalytic data indicate an important influence of
ligand structure on the catalysts activity and product dis-
tribution [35].
Chemical syntheses of new compounds were carried out
in aqueous solutions containing MoO3 or molybdic acid
and the appropriate amine. Full description of the syntheses
procedures, yields, results of chemical analyzes has been
placed in Supporting Information (SI).
The details of X-ray diffraction experiments and results
of crystal structure determination are also available in the
SI. Crystal structures of A3 and B1 were studied by single
crystal methods, other catalysts by ‘ab initio’ powder
diffraction methods. B2 and C4 were investigated with the
use of synchrotron radiation while B4 with a laboratory
diffractometer. Data concerning structure determination
and structure description are listed in Tables SI-1,2,3
(single crystal data) SI-4,5,6,7 (powder data), structures are
depicted on accompanying Figs (SI-A3, B1, B2, B4, C4).
While selecting catalysts, thermal stability data for
prospective compounds were considered, and if reliable
data were not available temperature dependent XRPD
studies (in air, temp = 25–600 °C) were performed.
Results are presented in SI. In general, all compounds were
found to be stable in reaction conditions and the most
active compounds B1, B3, C2–C4 were stable up to
250 °C.
Selective catalytic oxidation of cycloalkanes with
molecular oxygen is one of the most important and
attractive transformations in organic synthesis [36–38]. It is
evident that the use of molecular oxygen from air which is
an inexpensive, abundant and readily available oxidant in
place of other oxygen donors is clearly desirable on eco-
nomic and environmental grounds.
Very recently, we have reported the synthesis, physico-
chemical properties and catalytic activity of new
oxodiperoxomolybdenum complexes with nicotinic acid in
the oxidation of cyclooctane [39].
This paper is a result of combined studies on syntheses
of new polymolybdate complexes together with research
on their catalytic activity in cycloalkanes oxidation. Cat-
alytic properties are discussed in relation to structural
dimensionality of presented polyoxomolybdate complexes,
various amounts and types of oxygen atoms (in terms of
bonding arrangements) in their structures and ligands used
in syntheses.
Excluding samples obtained as large single crystals
(A1–A3), surface areas for investigated samples were
determined applying BET technique. Results of these
experiments are presented in Table 2.
Selected catalysts (B2, C2) after 6 h of reaction time
were recovered by filtration, washed with propanol, dried
in air, and reused in catalytic experiments. Obtained results
are discussed in chapter 3 (see also Fig. 3).
2 Experimental
2.1 Synthesis and Structural Characterization
3 Catalytic Investigation
For the purpose of this work, a group of 13 polyoxo-
molybdenum compounds was selected. Three of them
represent compounds with polyoxo moieties in the form of
isolated clusters (denoted as An; n = 1,2,3), 4 contain
polymeric chains (Bn), we also investigated 4 layered
compounds (Cn) and two ‘molybdenum oxides’. To
determine the influence of structural features on catalytic
activity, all selected compounds (except for MoO3) are
The liquid-phase oxidation of cycloalkanes (cyclopentane,
cyclohexane or cyclooctane) was performed in a stainlees-
steel batch reactor system at the optimum temperature of
393 K and under the pressure of 10 atm. with the reagents
molar ratio CnH2n (where n = 5, 6, 8):O2 = 13:2. A Te-
flon-lined reactor of 1 L volume equipped with a magnetic
stirrer was used. In a typical experiment, molybdenum
based catalyst having the concentration of 3.3 9 10-4
M
123

´
A. Szymanska et al.
1000
123

Molybdenum Complexes as Catalysts for the Oxidation of Cycloalkanes with Molecular Oxygen
1001
123

´
A. Szymanska et al.
1002
123

Molybdenum Complexes as Catalysts for the Oxidation of Cycloalkanes with Molecular Oxygen
1003
Table 2 Cyclooctane oxidation with molecular oxygen and polyoxomolybdates as catalysts
Mo catalyst Yield of ketonea (%) TONd
Yield of alcohola (%) TONd
Ketone ? alcohol (%) Ketone/alcohol Surface area m2/g
A1
A2
A3
B1
B2
B2b
B3
B4
C1c
C2c
C3c
C4c
D1
D2
a
13.3
10.2
10.7
34.3
29.6
Traces
34.7
25.6
11.7
34.1
34.0
23.4
3.9
4990
3830
4010
12,860
11,100
–
9.3
3490
1650
3530
6230
5810
–
22.6
14.6
20.1
50.9
45.1
–
1.4
2.3
1.1
2.1
1.9
–
4.4
9.4
16.6
15.5
Traces
13.6
15.9
5.5
4.56
2.91
13,010
9600
4390
12,790
12,750
8780
1460
1280
5100
5960
2060
4050
3975
5740
1010
900
48.3
41.5
17.2
44.9
44.6
38.7
6.6
2.6
1.6
2.1
3.2
3.1
1.5
1.4
1.4
12.47
9.59
4.34
1.91
4.66
3.76
10.8
10.6
15.3
2.7
3.4
2.4
5.8
Isolated yields calculated on the basis of oxygen quantity in batch reactor
Reaction with free radical scavenger 2,6-di-tert-butyl-p-cresol
Suberic acid as by-product is observed (see Fig. 4)
Mol product 9 mol catalyst-1 9 h-1
b
c
d
3.1 Catalysts Characterization—Crystal Structure
Description
was introduced into the reaction mixture. The catalyst
together with the substrate were closed in the air-free
autoclave and were heated together with the whole system
until temperature of 393 K was reached. In due time air was
added to the hot reaction mixture in such amount that final
pressure of 10 atm was obtained and the reaction started.
After 6 h of reaction time the oxidation was stopped by
immersing the hot reactor in a cold water bath. Reaction
products (cycloketone and cycloalcohol) were analyzed by
means of Agilent Technologies 6890 N gas chromatograph
equipped with Innowax (30 m) column in the presence of
chlorobenzene as internal standard. Amounts of CO and
CO2 were determined chromatographically using Perkin-
Elmer Clarus 500 with methanizer.
Considering the structural diversity of presented poly-
molybdates, in particular their dimensionality, the com-
pounds were divided into four main categories.
Octamolybdates A1–A3 contain isolated molybdenum-
oxygen clusters (0-dim). Polymeric materials B1-B4 are
built of infinite molybdenum-oxygen chains (1-dim).
Compounds C1–C4 are formed by Mo–O layers with
amines in between (2-dim). Two types of three-dimen-
sional molybdenum trioxide D1 and D2 represent the last
group of compounds (3-dim). The main subject of the
presented research was to correlate catalytic activity with
the structure of investigated polymolybdates. The number
of terminal (oxo) and bridging oxygen atoms (ln) was also
taken into account. In general, all investigated catalysts are
Mo(VI) isopolyoxo compounds, with all Mo atoms in
octahedral coordination surrounded only by oxygen or by
oxygen and nitrogen atoms.
The amount of cycloalkane hydroperoxide in the reac-
tion mixture was evaluated by the iodometric titration and
Shul’pin method [40–42]. The yield of cycloalkyl
hydroperoxide (yield \0.1 %) observed after 6 h of the
reaction is too small to have an impact on the results of the
subsequent GC analysis.
The basic parameters of the compounds with structure
drawings are summarized in Table 1. Main structural fea-
tures are described below; data for new compounds are
highlighted with bold font.
Stability of catalysts in reaction conditions was tested in
the following manner. Compounds B1 or C2 were mixed
with cyclooctane and boiled (Tb = 149 °C) under reflux
(in air) for 24 h. When the mixture was cooled down,
catalysts were separated from cyclooctane by filtration and
dried in air. Diffraction patterns and IR spectra of fresh
catalysts and catalysts after cyclooctane treatment are
presented in the Supporting Information (see Fig. SI-11,12
and Fig. SI-13,14).
As representatives of isolated (0-dim) compounds we
have selected c- and b-octamolybdates of 4-methylpyridine
A1- A2, recently investigated by us [39]. These compounds
were supplemented by
a new b-octamolybdate of
4-methylanilinium A3 obtained within this study.
123

´
A. Szymanska et al.
1004
Compounds A1–A3 are built of isolated octamolybdate formula. These rigid, zigzag chains built of distorted
clusters (0-dim). Eight isomers of octamolybdates are octahedra are linked by ionic interactions and hydrogen
known: a, b, c, d, e, f, g and h. Among them the b-isomer bonds with 3,5-dimethylanilinium cations. In this com-
is the most common, while isomer c, considered an inter- pound each MoO6 octahedron contains two terminal oxy-
mediate between a and b forms is also an important and gen atoms (Ooxo) and four l3-O oxygen atoms, that are
frequently occurring ‘building block’ of many poly- linking 3 adjacent octahedra. Very similar structure was
molybdate structures [52]. Compound A1 is c-octamolyb- determined in the case of B4. Crystallographic data for B1,
date of 4-methylpyridine. The anion includes 14 terminal B2, B4 is listed in SI.
oxygen atoms (Ooxo
˚
)
(average Mo=O distance is
Four pentamolybdates C1–C4 belonging to a group of
1.7104(49) A), 6 oxygens l2-O, 4 l3-O and 2 l4-O. What is layered materials (2-dim) have been examined. As a
more, in A1 complex a molybdenum-nitrogen bond occurs starting point, we selected aniline and 4-methylaniline
˚
(Mo–N = 2.249(2) A) which is characteristic for this pentamolybdates C1, C2 [46, 47]. To investigate the
octamolybdate type.
influence of substituent, we extended the group of inves-
Both A2 and A3 are b-type isomers, where A2 is octa- tigated compounds adding pentamolybdate of 4-bro-
molybdate of 4-methylpyridine and A3 is octamolybdate of moaniline [48] and a new compound with a larger
4-methylaniline. The arrangement of b-octamolybdate substituent, that is 4-ethylanilinum pentamolybdate (C4).
anion is somewhat different than in the case of A1. The Mo
All discussed compounds have infinite anionic layers
atoms are surrounded only by O atoms. Each anion [(Mo5O16)2-]n, formed by distorted MoO6 octahedra and
includes 14 terminal oxygen atoms (like in the c-octa- are surrounded by different cations: C1—anilinium, C2—
˚
molybdate) average Mo=O distance is 1.7007(88) A in the 4-methylanilinium (p-toluidinium), C3—4-bromoanilinium
˚
case of compound A2 and 1.6981(49) A for A3), 6 oxygens and C4—4-ethylanilinium. In the structures of the layered
l2-O, 4 l3-O and 2 l5-O. The b isomer is thermally more pentamolybdates nearly flat, centro-symmetric blocks of 6
stable and more compact than c form. An b-octamolybdate deformed MoO6 octahedra connected by their edges are
anion is surrounded by organic cations and molecules of present. Two boundary octahedra on both sides of each
water, connected by a network ofhydrogen bonds. Crys- block are common to the two adjacent blocks, so each
tallographic data for A3 is listed in the SI part.
formal unit consists of 5 MoO6 individuals. Two oxygen
˚
Group of one-dimensional (1-dim) polymeric com- atoms at a distance of * 1.735 A are bound to each of the
pounds (with fibrous morphology) consists of compounds molybdenum atoms. These O atoms are always in cis
B1-B4. Since new compounds B1, B2 turned out to be position relative to each other. The (Mo5O16)2- anion
chemically different, we tested additional compounds contains 10 oxygen atoms of this type, among them 6 are
belonging to a family of polymeric molybdates. To verify terminal atoms, wherein 4 are perpendicular to the plane of
the effect of the type and charge of cation, 1,3- the layers. Each of the remaining four, strongly bonded
phenylenediammonium trimolybdate B3 [45] was selected. oxygen atoms are common for two coordination polyhedra
˚
To test the effect of organic cation we have undertaken of neighbouring Mon and Mon’ atoms (Mon-O *1.70 A,
˚
efforts to synthesise trimolybdate of 4-methylaniline (B4). O-Mon, *2.42 to 2.74 A). Octahedral coordination of four
Both B3 and B4 have the same type of anion as B2 but Mo atoms in each pentamolybdate unit is achieved through
˚
different organic cation (cation charge is 2? in the case of an oxygen atom placed at a long distance of *2.4 to 2.7 A
B3 and 1? in B4).
from Mo atom. We believe that the fragment: Mon-
˚
In the case of B1 the infinite chain (without electric Om…Mon’ (where Mon-Om *1.74 A, ] Mon-Om…Mon,
˚
charge) is formed by Mo and O atoms, with a molecule of *165 8, Om-Mon, *2.60 A) is a characteristic feature of
1,2-phenylenediamine (pda) attached to every molybde- pentamolybdates. In (Mo5O16)2- unit 6 l2-O and 4 l3-O
num atom. Formally, the compound may be treated as a atoms are present (see Fig. 1 and SI for details for C4).
hybrid molybdenum oxide with the composition of
Three-dimensional hexagonal molybdenum trioxide D1
MoO3(pda). Each of the Mo atoms is coordinated by two (3-dim) forms a system of parallel channels with a diameter
˚
nitrogen atoms of pda bringing the coordination number of of *5.8 A, which allows penetration of the channels by
Mo to 6. In each MoO4N2 octahedron two terminal oxygen monovalent ions (H?, Ag?, K?, Rb?, etc.) [49]. For
atoms, in cis relative position, are present, Mo=Ooxo bond comparison purposes, we have also tested D2, a commer-
˚
length is 1.7264(11) A. Two opposite vertices in each cially available orthorhombic MoO3 (POCh Gliwice). It is
octahedron are occupied by l2-O atoms shared with adja- a layered compound with the distance between thick dou-
˚
ble layers of about 6.5 A, whereas the distances between
cent octahedra.
New 3,5-dimethylanilinium trimolybdate hydrate (B2) is atoms from adjacent layers, interacting through van der
composed of infinite, anionic chains with (Mo3O10)2n- Waals forces, are about 2.9 to 3.5 A [50].
˚
123

Molybdenum Complexes as Catalysts for the Oxidation of Cycloalkanes with Molecular Oxygen
1005
30
ketone
alcohol
25
20
15
10
5
0
Fig. 2 Oxidation of different cycloalkanes by Mo catalyst B2
Fig. 1 Pentamolybdate building block and infinite layer, view along
a axis
Cyclooctane was determined to be the most reactive of
all investigated cycloalkanes and the following order of
catalytic reactivity was established: cyclooctane [ cyclo-
hexane [ cyclopentane. Consequently, cyclooctane was
selected for further investigations. All the catalytic results
of cyclooctane oxidation with molecular oxygen are gath-
ered in Table 2 and presented in Figs. 2 and 3. All applied
polyoxomolybdates were active as catalysts in the oxida-
tion of cyclooctane.
4 Catalytic Results
Three different cycloalkanes, viz. cyclopentane, cyclo-
hexane or cyclooctane were applied as substrates
(Scheme 1) with molybdenum complex B2 as a represen-
tative catalyst.
The main products of cycloalkanes oxidation with
molecular oxygen (from air) were cycloketone and
cycloalcohol. No oxidation occurred at 393 K in the
absence of the catalyst. For cyclopentane, only 2.9 % yield
of ketone (TON = 1090) and 0.8 % yield of alcohol
(TON = 300) was obtained in the presence of Mo catalyst
B2 after 6 h (Fig. 2). Under analogous conditions, cyclo-
hexane was converted to ketone and alcohol with yields of
5.2 % (TON = 1950) and 2.2 % (TON = 830), respec-
tively. Finally, the oxidation of cyclooctane catalyzed by
B2 resulted in 29.6 % yield for ketone (TON = 11,100)
and 15.5 % yield for alcohol (TON = 5812).
The A1-A3 compounds (0-dim) built of isolated octa-
molybdate anions show moderate activity towards desired
products. All compounds demonstrate quite similar selec-
tivity towards ketone but A2 reaches only 50 % of selec-
tivity towards alcohol in comparison with two other
members of the series.
Compounds of the B series (1-dim) demonstrate high
catalytic activity. Compound B1 shows the highest cat-
alytic activity among all the studied catalysts. The
50
40
30
20
10
0
O
O
OH
+
cyclopentane
Mocatalyst
cyclopentanone
cyclohexanone
cyclopentanol
OH
+
O
2
cyclohexane
cyclohexanol
O
OH
+
cyclooctane
cyclooctanone
cyclooctanol
B2
C2
Scheme 1 Oxidation of cycloalkanes
Fig. 3 Recycling and reuse of B2 and C2 molybdenum catalysts
123

´
A. Szymanska et al.
1006
following order of catalytic activity was established:
B1 [ B3 [ B2 C B4. B2 was twice recovered from the
reaction mixture and reused in the catalytic reaction. Yield
of ketone and alcohol decreases insignificantly in the
subsequent runs which is important for the possibility of
recycling and reuse of molybdenum catalysts (Fig. 3).
Higher catalytic activity of B3 in comparison with B2 and
B4 catalysts is intriguing taking into account the identical
polymeric structure of the trimolybdate anion.
narrow channels in three-dimensional covalent structure
that may be too small for the reactants to enter into, seem
to be the reason behind such poor results.
Summarizing, the best catalytic performance for oxida-
tion of cyclooctane was observed for the polymeric B1, B3
and layered C2, C3 materials. The results obtained indicate
that the catalytic activity is not only dependent on the
structure of the inorganic anion, but also on other factors,
e.g. type of organic cation and its charge, crystal structure
etc.
Layered compounds: C1–C4 (2-dim) demonstrate
diverse catalytic activity (Fig. 4). C2 and C3 pentamolyb-
dates show similar catalytic activity and selectivity towards
the desired products. For C4 the selectivity to ketone
slightly decreases while selectivity to alcohol increases
significantly in comparison to C2 and C3 catalysts. C2
catalyst was recycled and reused with only a small change
of catalytic activity (Fig. 3) preserving the original crystal
structure as indicated by XRPD studies.
5 Discussion
Structural differences between the b and c-octamolybdate
anions are relatively small (the same amount of oxo oxy-
gens, and l2-O atoms), which may explain quite similar
catalytic activity. Slightly higher activity is observed for c-
octamolybdate A1 in which 2 nitrogen atoms are directly
connected to Mo atoms. Also, in the case of c-octa-
molybdates average Mo = O distance is slightly longer
than in the case of b-octamolybdates. The presence of
nitrogen atoms directly connected to Mo atoms, which is
also observed in one of the most active B1 catalyst, seems
to be a factor contributing to a higher catalytic activity.
However, the high activity of B3, C2, C3 (without Mo–N
bonds) proves the existence of additional factors needed to
be taken into account when discussing catalytic activity.
A very interesting observation is the high conversion of
cyclooctane for hybrid polymeric molybdenum oxide B1. It
seems that organic part of the compound improves liofility
(wettability), whereas rigid inorganic-chain ensures the
availability of catalytic centers (of yet unknown type)
along the polymeric chain. In comparison, anionic poly-
meric core in typical trimolybdates reduces the availability
of catalytic centers along one-dimensional chain, by the
presence of cations compensating its negative charge. This
may be an explanation of the much lower activity of B2
and B4 in comparison to B1. These assumptions are con-
firmed by high activity of the compound B3 (with exactly
the same polymeric anion as in B2 and B4). In this case
[NH3–C6H4–NH3]2? cations are present, and the number of
these cations needed to neutralize the charge of anionic
chain is halved with respect to B2 or B4. In this way the
availability of the active centers increases by providing
‘‘free space’’ along anionic chain unoccupied by the
cations.
It is important to state that for the whole series of C
catalysts an additional product, suberic acid, was identified.
Its formation is connected with the presence of cycloalcoxy
radical formed during the oxidation of cyclooctane
(Scheme 2) and stabilized between layers of C catalysts.
The formation of suberic acid was found by XRPD analysis
of the solid phase left in the batch reactor after the oxi-
dation reaction with layered C1-C4 catalysts.
Considering the surface area values for compounds C1–
C4, one can see that they are rather small, and it appears
that surface is not the main factor determining catalytic
activity (Table 2). For example in case of C1 the biggest
surface area was determined, whereas it is the worst cata-
lyst in Cn family; exactly the opposite observation can be
made in the case of C2. Similar observation can be made in
case of compounds from B series.
Molybdenum oxides: hexagonal molybdenum oxide D1
and orthorhombic MoO3 D2 show the lowest catalytic
activity among the tested catalysts. Low availability of
molybdenum-oxygen catalytic centres and a system of
ketone alcohole conversion ketone/alcohole*10 suberic acid
50
40
30
20
10
0
The high conversion of cyclooctane for C2 and C3 and
its significantly higher value compared to C1 is surprising
due to high similarity in chemical composition, structure of
the anion and location and arrangement of the cations.
Steric reasons may give a clue. For the reaction to proceed,
cyclooctane molecules must be absorbed in the inter-layer
C1
C2
C3
C4
d(200)=13.60Å d(200)=14.36Å d(200)=14.81Å d(200)=15.18Å
Fig. 4 Oxidation of cyclooctane with layered C1–C4 compounds. In
the lowest row interplanar d(200) spacings are given. Conversion
indicates yield of ketone ? alcohole
123

Molybdenum Complexes as Catalysts for the Oxidation of Cycloalkanes with Molecular Oxygen
1007
Scheme 2 Mechanism of
cycloalkanes oxidation in the
presence of polymolybdates
catalysts
H MoVIcomplex
.
+
H
(1)
(2)
+
.
OO
.
+
O
2
.
OO
H
OOH
.
+
(3)
(4)
(5)
+
.
.
.
OO
OO
OH
O
+
O
+
+
2
OOH
.
MoVIcomplex
OO
H+
+
.
OOH MoVcomplex
O
.
(6)
(7)
+
OH
.
O
H
OH
.
+
+
.
layered
.
.
MoVIcomplex
O
H
O
H
(8)
(9)
O
OH
OH
O
O
OH
H
MoVIcomplex
H O
+
1/2 O
2
+
2
space of pentamolybdates. Cyclooctane seems to require
slightly more space than anilinium ion can provide, hence
the low activity of the compound C1. Further, more
detailed considerations included the examination of the
impact of the distances between the layers on the catalytic
activity. The yield of products for 4-methylanilinium
oxidation reaction is more difficult to proceed and the
formation of oxidation products (along with suberic acid) is
suppressed. Increasing the distance between layers above
the optimal size does not provide a clear benefit. It may
even result in a decrease in catalytic activity, as is revealed
for the C4 compound for which the distance
˚
pentamolybdate C2 [d(200) = 14.36 A] and 4-bro-
˚
d(200) = 15.18 A. Interplanar d(200) spacings used in
˚
moanilinium pentamolybdate C3 [d(200) = 14.81 A] is
Fig. 4 are derived from lattice constants and formulas of
geometric crystallography. This value gives an estimation
of space available between neighbouring layers in catalyst.
Based on the yields of obtained products, correlation
existing between d(200) and catalytic performance for very
similar. Figure 4 shows that there is an optimal distance
between layers [corresponding to the inter-planar spacing
d(200)] for the best catalytic performance. If the interlayer
˚
distance is too small, like in C1 [d(200) = 13.6 A], the
123

´
A. Szymanska et al.
1008
similar compounds (C1–C4) seems to be reasonable and
justified.
A hypothetical set of reactions is presented in Scheme 2.
Except cycloketone and cycloalcohol, which are the major
products of this reaction, small amounts of cyclohy-
droperoxide are observed but the formation of ring-opened
side-products like di-carboxylic acids is also possible
through this mechanism. It is well-known that the selective
catalytic oxidation of C-H bonds results in the formation of
hydroperoxides [36–38]. For the investigated polymolyb-
dates only traces of hydroperoxide (\0.1 %) were detected
after 6 h of reaction which is indicative of polymolybdates
as good catalysts for hydroperoxides decomposition.
The first step of the whole process is a slow reaction of
the cycloalkane with polymolybdate resulting in the
abstraction of hydrogen from hydrocarbon to form
cycloalkyl radical R• (Reaction 1), which acts as the ini-
tiation of chain reaction [57–59]. The newly formed spe-
cies can react further with molecular oxygen in the chain-
propagation step to the cycloperoxyl radical ROO• (Reac-
tion 2). This in turn reacting with cycloalkane gives
cyclohydroperoxide ROOH and cycloalkyl radical R•
(Reaction 3). Recombination of cycloperoxyl radicals
ROO• is a first source of cycloketone and cycloalcohol
constituting the major chain-termination step (Reaction 4).
Cyclohydroperoxide can be decomposed in a heterolytic
[56, 60] or homolytic [56, 61] manner. The heterolytic
decomposition of cyclohydroperoxide catalyzed by
molybdenum catalyst produces cycloperoxyl radical
ROO•(Reaction 5) while cycloalkoxyl radical RO• and
hydroxyl radical OH• are produced in homolytic decom-
position (Reaction 6). Majority of cycloalcohol is produced
in Reaction 4. Additional amount of cycloalcohol origi-
nates from abstraction of hydrogen from cycloalkane by
cycloalkoxyl radical (Reaction 7). Cycloalkoxyl radicals
can also undergo a b-cleavage of C–C bond [62–65]
yielding the ring-opened x-formyl radical CHO–(CH2)6–
CH2 which is the precursor for obtaining di-carboxylic
acid (Reaction 8). However, the literature dealing with
cycloalkane oxidation claims that cycloketone is the most
important precursor of ring–opened by-products [56–61].
In all our catalytic tests the yield of cycloketone is higher
than the yield of cycloalcohol. This is a clear indication
that cycloketone can be formed not only in Reaction 4 but
also in the oxidation of the cycloalcohol (Reaction 9).
To sum up the discussion, we have to consider all the
data which characterize the investigated compounds (also
data gathered in SI). In particular:
Pentamolybdate layers are open-framework construc-
tions with variable thickness, which may allow for the
oxygen to pass through the layers. Adsorbed molecule
trapped in the oxidizing environment, may undergo further
oxidation reactions leading to formation of suberic acid.
For compounds A1–A3 (0-dim) or polymeric B1–B4 (1-
dim), after the occurrence of a single process of oxidation,
molecules of cyclooctanone or cyclooctanol can easily drift
away from the catalyst surface through diffusion. In a
qualitative manner, this also explains the lower ratio of
ketone/alcohol for these catalysts (see Table 2), and the
lack of suberic acid in the reaction products.
Oxidation of alkanes was widely studied by many
researchers. Oxidation of cyclohexane and cyclooctane
with hydrogen peroxide as oxygen donor was performed in
the presence of the CH3ReO3-PCA system. Formation of
the cycloalkyl hydroperoxide, which was the main product
in the cycloalkane oxidation, proceeded via free-radical
mechanism [36].
Radical mechanism of cyclooctane oxidation was also
observed for the reaction performed with H2O2 at low
temperature [41, 42]. In contrast to our catalytic system,
cyclooctyl hydroperoxide was the main reaction product.
Non-radical reaction mechanism of cycloalkane oxida-
tion was described by [53] using t-butyl hydroperoxide and
peracetic acid as oxygen donors and ruthenium catalyst.
The reaction proceeds through the oxo-ruthenium species.
In this case the reaction was carried out in the presence of
radical sacavenger .
Similar catalytic systems based on polyoxometalates as
catalysts were studied in the cyclooctane oxidation with
molecular oxygen [54]. Introduction of cobalt ion into
heteropoly structure significantly influences the catalytic
activity and selectivity in the studied reaction. The amount
of cyclooctyl hydroperoxide formed during the reaction
was negligible.
Salen metallocomplexes were also investigated in the
same reaction conditions [55]. Catalytic activities of salen
complexes and polymolybdates are similar, selectivity to
cycloketone is different, though.
Based on the literature [56–61] and our investigations,
we suggest the following oxidation mechanism in the
presence of polymolybdate catalysts (Scheme 2).
According to Sheldon and Kochi [56] the oxidation of
cycloalkanes with molecular oxygen proceeds via free-
radical mechanism. The radical chain reaction mechanism
of this reaction was experimentally confirmed by us,
showing almost complete inhibition of cyclooctane oxida-
tion in the presence of a free radical scavenger 2,6-di-tert-
butyl-p-cresol (DBPC).
–
–
Formation of CO and CO2 is not observed during the
oxidation of cycloalkanes.
Stability of compounds in reaction conditions was
indicated by XRPD studies versus temperature (see
Supporting Information).
123

Molybdenum Complexes as Catalysts for the Oxidation of Cycloalkanes with Molecular Oxygen
1009
–
–
Recoverability and reusability of catalysts (B2, C2)
with only a small change of catalytic activity of fresh
and spent samples were confirmed (Fig. 3).
The most active compounds B1, B3, C2, C3 are
stable up to about 200 °C (temperature dependent XRPD,
see Supporting Information). What is more, catalysts are
recoverable and reusable, which indicates the possibility of
practical applications.
Compounds B1 and C2 boiled in cyclooctane (Tb
higher than the temperature of reaction) remain
unchanged (see Supporting Information). Furthermore,
compounds of C series were synthesized from the
reaction mixture containing boiling water solution (pH
2). Since the catalysts were synthesized in rather severe
conditions, their solubility in cycloalkane in given
reaction conditions is doubtful.
The catalytic properties of these materials are complex
features resulting from their crystal structure and chemical
composition. The interesting catalytic activity can be
observed for one-dimensional (polymeric) or two dimen-
sional (layered) materials. For the layered materials the
highest catalytic activity is obtained after adjusting proper,
optimal distance between layers. For the anionic polymers
B2, B4 higher blocking of molybdenum catalytic centers
(by cations) decreases the activity of the catalyst.
–
–
–
Rather obvious dependence of catalytic activity on
cationic charge in compounds B2–B4 was observed.
Dependence of catalytic properties on interplanar
d(200) spacing in the case of C1–C4 was demonstrated.
Low surface area for all the investigated compounds
and no obvious correlation between surface and
catalytic activity indicate the significance of factors
other than the surface area.
Acknowledgments The research has been partly supported by the
EU Human Capital Operation Program, Polish Project No.
POKL.04.0101-00-434/08-00. The research was carried out with the
equipment purchased thanks to the financial support of the European
Regional Development Fund in the framework of the Polish Inno-
vation Economy Operational Program (contract no. POIG.02.01.00-
12-023/08). The authors also wish to acknowledge the synchrotron
centres: DIAMOND and DESY for diffraction measurements.
CCDC—1041613, 909664, 1042866, 1042865, 1041614 contain the
supplementary crystallographic data for compounds A3, B1, B2, B4,
C4, respectively. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
All these observations allow us to draw a final conclu-
sion that the crystal structures of the catalysts investigated
by us are important factors in the catalytic process. This
also supports the heterogeneous character of the catalytic
reaction, at least in the case of the most stable B1, B3, C1–
C4 catalysts.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://crea
tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
made.
6 Conclusions
The first example of oxidation reaction of cycloalkanes
with molecular oxygen without solvent and co-reductor
that involves polymolybdate complexes as catalysts is
reported.
Polymolybdates with anions in the form of isolated
clusters A (0-dim), compounds with polymeric anions
B (1-dim), layered materials C (2-dim) as well as
hexagonal and orthorhombic molybdenum oxides D (3-
dim) were selected, synthesized and physico-chemically
characterized (if needed) and applied in the process of
catalytic oxidation of cycloalkanes. Among them A3,
B1, B2, B4, C4 polymolybdates were synthesized for the
first time.
References
1. Enemark JH, Cooney JJA, Wang J-J, Holm RH (2004) Chem Rev
104:1175–1200
2. Lyashenko G, Saischek G, Judmaier ME, Volpe M, Baumgartner
¨
J, Belaj F, Jancik V, Herbst-Irmer R, Mosch-Zanetti NC (2009)
Dalton Trans 29:5655–5665
3. Yin GC (2010) Coord Chem Rev 254:1826–1842
4. Hille R, Hall J, Basu P (2014) Chem Rev 114:3963–4038
´
´
5. Cindric M, Novak TK, Krajlevic S, Krajl M, Kamenar B (2006)
Inorg Chim Acta 359:1673–1680
All the synthesized compounds were active in the
investigated reaction. For series of B and C catalysts high
catalytic activity was observed. Moreover, for the series of
layered C catalysts the oxidation of cyclooctane to
cyclooctanone and cyclooctanol is accompanied by the
formation of suberic acid (COOH)–C6H12–(COOH). For-
mation of suberic acid is observed for layered poly-
molybdates only. Product yields for C series seem to be
affected by inter-layer distance between [Mo5O16]n2-
anions.
¨
6. Cartuyvels E, Van Hecke K, Van Meervelt L, Gorller-Walrand C,
Parac-Vogt TN (2008) J Inorg Biochem 102:1589–1598
´
7. Cindric M, Veksli Z, Kamenar B (2009) Croat Chem Acta
82(2):345–362
8. Djordjevic C, Puryear BC, Vuletic N, Abelt CJ, Sheffield SJ
(1988) Inorg Chem 27:2926–2932
9. Dickman MH, Pope MT (1994) Chem Rev 94:569–584
10. Thiel WR (1997) J Mol Catal A: Chem 117:449–454
11. Jia M, Thiel WR (2002) Chem Commun 20:2392–2393
12. Pereira CCL, Balula SS, AlmeidaPaz FA, Valente AA, Pillinger
M, Klinowski J, Goncalves IS (2007) Inorg Chem 46:8508–8510
123

´
A. Szymanska et al.
1010
´
13. Brito JA, Teruel H, Muller G, Massou S, Gomez M (2008) Inorg
Chim Acta 361:2740–2746
39. Szymanska A, Nitek W, Mucha D, Karcz R, Pamin K, Połtowicz
J, Łasocha W (2013) Polyhedron 60:39–46
14. Abrantes M, Amarante TR, Antunes MM, Gago S, Paz FAA,
Margiolaki I, Rod-rigues AE, Pillinger M, Valente AA, Gonca-
lves IS (2010) Inorg Chem 49:6865–6873
40. Mair RD, Graupner AJ (1964) Anal Chem 36:194–204
41. Shulpin GB, Kozlov YN, Shulpina LS, Kudinov AR, Mandelli D
(2009) Inorg Chem 48:10480–10482
15. Pisk J, Agustin D, Vrdoljak V, Poli R (2011) Adv Synth Catal
353:2910–2914
42. Shulpin GB, Kozlov YN, Shulpina LS, Petrovskiy PV (2010)
Appl Organometal Chem 24:464–472
_
43. Szymanska A, Nitek W, Rutkowska-Zbik D, Łasocha W (2014)
16. Pisk J, Prugovecki B, Matkovic-Calogovic D, Poli R, Agustin D,
Vrdoljak V (2012) Polyhedron 33:441–449
´
Monatsh Chem 145:921–929
17. Amarante TR, Neves P, AlmeidaPaz FA, Pillinger M (2012) AA
Valente. I Inorg Chem Comm 20:147–152
´
´
´ ´
44. Roman P, Gutierrez-Zorrilla JM, Martınez-Ripoll M, Garcıa-
Blanco S, Cryst J (1987) Spectrosc. 17:109–119
18. Rayati S, Rafiee N, Wojtczak A (2012) Inorg Chim Acta
386:27–35
19. Masteri-Frhani M, Kashef Z (2012) J Magn Magn Mater
324:1431–1434
20. Grivani G, Akherati A (2013) Inorg Chem Comm 18:90–93
21. Djordjevic C, Puryear BC, Vuletic N, Abelt CJ, Sheffield SJ
(1997) Inorg Chem 36:1798–1805
45. Łasocha W, Rafalska-Łasocha A, Grzywa M, Gaweł B (2008)
Catal Today 137:504–509
46. Łasocha W, Schenk H (1997) J Appl Cryst 30:909–914
47. Oszajca M, Smrcok L, Łasocha
69:1367–1372
W (2013) Acta Cryst
48. Łasocha W, Grzywa M, Oszajca M, Kristallogr Z (2009) Suppl
30:387–394
22. Das S, Bhowmick T, Punni-Yamurthy T, Dey D, Nath J,
Chaudhuri MK (2003) Tetrahedron Lett 44:4915–4917
23. Gharah N, Chakraborty S, Mukherjee AK, Bhattacharyya R
(2004) Chem. Comm. 22:2630–2632
24. Bregeault J-M, Vennant M, Salles L, Piquemal J-Y, Mahha Y,
Briot E, Baka-la PC, Atlamsani A, Thouvenot R (2006) J Mol
Catal A Chem 250:177–189
25. Mancka M, Plass W (2007) Inorg Chem Commun 10:677–680
26. Gharah N, Chakraborty S, Mukherjee AK, Bhattacharyya R
(2009) Inorg Chim Acta 362:1089–1100
49. Guo J, Zavalij P, Whittingham M (1994) Eur. J. Sol. State Inor.
31:833
50. Gruber H, Krautz E (1980) Phys Status Solidi A 62:615
51. Brandenburg K, Putz H, Diamond: crystal and molecular struc-
ture visualization crystal impact. GbR, Postfach 1251. D-53003
Bonn
52. Allis DG, Rarig RS, Burkholder E, Zubieta J (2004) J Mol Struct
688:11–31
53. Murahashi Sh-I, Komiya N, Oda Y, Kuwabara T, Naota T (2000)
J Org Chem 65:9186–9193
27. Madeira F, Barroso S, Namorado S, Reis PM, Royo B, Martinsm
AM (2012) Inorg. Chim Acta 83:152–156
54. Pamin K, Jachimska B, Onik K, Połtowicz J, Grabowski R (2009)
Catal Lett 127:167–174
28. Bagherzadeh M, Amini M, Parastar H, Jalali-Heravi M, Ellern
AA, KeithWoo L (2012) Inorg Chem Commun 20:86–89
29. Carrasco CJ, Montilla F, Alvarez E, Herbert M, Galindo A (2013)
Polyhedron 54:123–130
30. Sensato FR, Custodio R, Longo E, Safont VS, Andres J (2003) J
Org Chem 68:5870–5874
55. Połtowicz J, Pamin K, Tabor E, Haber J, Adamski A, Sojka Z
(2006) Appl Catal A 299:235–242
56. Sheldon RA, Kochi JK (1981) Metal catalyzed oxidations of
organic compounds. Academic Press, London, pp 34–70
57. Hermans I, Jacobs PA, Peeters J (2006) J Mol Catal A: Chem
251:221–228
31. Pontes da Costa A, Reis PM, Gamelas C, Romao CC, Royo B
(2008) Inorg Chim Acta 361:1915–1921
58. Mishra GS, Frausto de Silva JJR, Pombeiro AJL (2007) J Mol
Catal A Chem 265:59–69
32. Bagherzadeh M, Tahsini L, Latifi R, Ellern A, KeithWoo L
(2008) Inorg Chim Acta 361:2019–2024
33. Bagherzadeh M, Mehdi Haghdoost M, Amini M, Derakhshandeh
PG (2012) Catal Commun 23:14–19
59. Hermans I, Peeters J, Jacobs PA (2008) Top Catal 48:41–48
60. Traylor TG (1987) Feng Xu. J Am Chem Soc 109:6201–6202
61. Mansuy D, Bartoli J-F, Momenteau M (1982) Tetrahedron Lett
23:2781–2784
34. Bagherzadeh M, Amini M, Ellern A, KeithWoo L (2012) Inorg
Chem Commun 15:52–55
62. Barclay LRC, Lusztyk J, Ingold KU (1984) J Am Chem Soc
106:1793–1796
35. Herrmann WA, Lobmaier GM, Priermeier T, Mattner MR,
Scharbert B (1997) J Mol Catal A Chem 117:455–469
36. Shulpin GB (2002) J Mol Catal A Chem 189:39–66
37. Połtowicz J, Haber J (2006) J Mol Catal A Chem 250:177–189
38. Duprez D, Cavani F (2014) Handbook of advanced methods and
processes in oxidation catalysis. Imperial College Press, London,
pp 320–333
63. Druliner JD, Krusic PJ, Lehr GF, Tolman CA (1985) J Org Chem
50:5838–5845
64. Beckwith ALJ, Hay BP (1989) J Am Chem Soc 111:230–234
65. Olivella S, Sole A (1991) J Am Chem Soc 113:87–94
123