Molybdenum-doped α-MnO2 as an efficient reusable heterogeneous catalyst for aerobic sulfide oxygenation

Article abstract of DOI:10.1039/c5cy01552a

Oxygenation of sulfides to sulfoxides and/or sulfones is an important transformation, and the development of efficient heterogeneous catalysts for oxygenation, which can utilize O₂ as the terminal oxidant, is highly desired. In this study, we have successfully developed manganese oxide-based efficient heterogeneous catalysts for aerobic oxygenation of sulfides. Firstly, we prepared four kinds of manganese oxides possessing different crystal structures, such as α-MnO₂, β-MnO₂, γ-MnO₂, and δ-MnO₂, and their structure-activity relationships were examined for the aerobic oxygenation of thioanisole. Amongst them, α-MnO₂ showed the best catalytic performance for the oxygenation. Moreover, α-MnO₂ was highly stable during the catalytic oxygenation possibly due to the tunnel K⁺ ions. In order to further improve the catalytic performance of α-MnO₂, substitutional doping of transition metal cations, such as Mo⁶⁺, V⁵⁺, Cr³⁺, and Cu²⁺, into the framework was carried out. Undoped α-MnO₂ possessed a fibrous morphology. When high-valent transition metal cations were doped, especially Mo⁶⁺, the lengths of the fibers drastically shortened to form grain-like aggregates of ultrafine nanocrystals, resulting in an increase in specific surface areas and the numbers of catalytically active surface sites. In the presence of Mo⁶⁺-doped α-MnO₂ (Mo-MnO₂), various kinds of sulfides could efficiently be oxidized to the corresponding sulfoxides as the major products. The observed catalysis was truly heterogeneous, and Mo-MnO₂ could repeatedly be reused while keeping its high catalytic performance. Besides sulfide oxygenation, Mo-MnO₂ could efficiently catalyze several aerobic oxidative functional group transformations through single-electron transfer oxidation processes, namely, oxygenation of alkylarenes, oxidative α-cyanation of trialkylamines, and oxidative S-cyanation of benzenethiols.

Full text of DOI:10.1039/c5cy01552a

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DOI: 10.1039/C5CY01552A
Molybdenum-doped α-MnO2 as an efficient reusable
heterogeneous catalyst for aerobic sulfide oxygenation
Tsubasa Uematsu, Yumi Miyamoto, Yoshiyuki Ogasawara, Kosuke Suzuki, Kazuya Yamaguchi
and Noritaka Mizuno*
Department of Applied Chemistry, School of Engineering, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
Abstract
Oxygenation of sulfides to sulfoxides and/or sulfones is an important transformation, and the
development of efficient heterogeneous catalysts for the oxygenation, which can utilize O2 as the
terminal oxidant, is highly desired. In this study, we have successfully developed manganese
oxide-based efficient heterogeneous catalysts for aerobic oxygenation of sulfides. Firstly, we
prepared four kinds of manganese oxides possessing different crystal structures, such as α-MnO2,
β-MnO2, γ-MnO2, and δ-MnO2, and their structure-activity relationships were examined for the
aerobic oxygenation of thioanisole. Amongst them, α-MnO2 showed the best catalytic performance
for the oxygenation. Moreover, α-MnO2 was highly stable during the catalytic oxygenation possibly
due to the tunnel K+ ions. In order to further improve the catalytic performance of α-MnO2,
substitutional doping of transition metal cations, such as Mo6+, V5+, Cr3+, and Cu2+, into the
framework was carried out. Undoped α-MnO2 possessed a fibrous morphology. When doping of
high-valent transition metal cations, especially Mo6+, the lengths of fibers drastically shortened to
form grain-like aggregates of ultrafine nanocrystals, resulting in increasing the specific surface areas
and the numbers of surface catalytically active sites. In the presence of Mo6+-doped α-MnO2
(Mo-MnO2), various kinds of sulfides could efficiently be oxidized to the corresponding sulfoxides
as the major products. The observed catalysis was truly heterogeneous, and Mo-MnO2 could
repeatedly be reused with keeping its high catalytic performance. Besides sulfide oxygenation,
Mo-MnO2 could efficiently catalyze several aerobic oxidative functional group transformations
through single-electron transfer oxidation processes, namely, oxygenation of alkylarenes, oxidative
α-cyanation of trialkylamines, and oxidative S-cyanation of benzenethiols.
1

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Introduction
DOI: 10.1039/C5CY01552A
The development of highly efficient catalytic systems for oxygenation of sulfides has attracted much
attention because the corresponding oxygenation products, both sulfoxides and sulfones, have widely
been utilized as oxygen-transfer reagents, ligands in asymmetric catalysis, and important synthetic
intermediates for natural products, biologically important molecules, and functional materials.1 In
addition, oxidative desulfurization has emerged as a promising method for removal of sulfur
compounds from fuels and industrial effluents.1g,1h Generally, oxygenation of sulfides has been
performed using metal oxide-, peracid-, or oxoacid-based stoichiometric oxidants. Instead of using
such stoichiometric oxidants, the choice of greener oxidants, such as H2O2 and O2, in combination
with suitable catalysts has recently become the mainstream for oxygenation of sulfides.
Up to the present, a number of efficient transition metal catalysts have been developed for
H2O2-based oxygenation of sulfides.2 Importantly, the use of ubiquitous and the greenest O2 is more
desirable from the standpoint of green chemistry. Although several catalytic systems for aerobic
oxygenation of sulfides have been reported to date,3 they have several disadvantages in some cases;
for example, (i) sacrificial reductants, e.g., aldehydes, are required, (ii) photo-irradiation is required,
and/or (iii) most of them are homogeneous systems, and recovery and reuse of catalysts are very
difficult.3 As far as we know, there are only a few reports on heterogeneously catalyzed oxygenation
of sulfides using O2 as the terminal oxidant without any additives and photoirradiation;
polyoxometalates,4a MoS2/Ta3N5,4b and Au/MnO2‒x 4c are the examples of the reported heterogeneous
catalysts. However, they have quite a bit of room for improvement of their catalytic activities,
selectivities, substrate scopes, and in particular reusabilities. Therefore, the development of reliable
heterogeneous catalysts is still an important and challenging research subject.
Manganese oxides, including mixed oxides, have attracted tremendous research interests
because of their potential use as electrode materials, magnetic materials, adsorbents, oxidants,
catalysts, electrocatalysts, and catalyst supports.5 In laboratory scale organic synthesis, manganese
oxides have traditionally been utilized as stoichiometric oxidants. In recent years, the use of
manganese oxides as heterogeneous catalysts for aerobic oxidation reactions has been receiving a lot
of attention, and several efficient systems have been developed to date.6 For example, α-MnO2,
including a manganese oxide octahedral molecular sieve (OMS-2), has typically utilized for
oxidative dehydrogenation and single-electron transfer oxidation processes, such as oxidative
dehydrogenation of alcohols and its related transformations,6c‒6e oxygenation of alkylarenes,6f
oxidative α-cyanation of tertiary amines,6g and oxidative homocoupling of thiols and its related
transformations.6h,6i We have recently developed aerobic oxidative amidation of primary alcohols
using aqueous NH3 as the nitrogen source.6j‒6l For the oxidative amidation, α-MnO2 (OMS-2)
showed the high catalytic performance, while β-MnO2, γ-MnO2, and δ-MnO2 were not effective. For
the oxidation of formaldehyde, the structure-activity relationships were carefully examined using
α-MnO2, β-MnO2, γ-MnO2, and δ-MnO2, and amongst them, δ-MnO2 was found to be the most
active catalyst.6m In these manners, dramatic differences in catalytic performances among manganese
2

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oxides with different crystal structures have been observed with considerable frequDeOnIc: 1y0..10T3h9/uCs5,CYt0h1e552A
choice of kinds of manganese oxides for the target reaction is very crucial. In addition, the redox
stability is also a very important factor for the catalytic use and repeated reuse of manganese oxides.
In this study, we prepared four kinds of manganese oxides with α-, β-, γ-, and δ-phase
structures, and their structure-activity relationships were examined for the aerobic oxygenation of
thioanisole. The difference in the catalytic performance was clearly observed among them, and
α-MnO2 showed the best catalytic performance and was highly stable during the oxygenation under
aerobic and even anaerobic conditions. Furthermore, the performance of α-MnO2 could be much
improved by the substitutional doping of Mo6+ into the octahedral framework. In the presence of
Mo6+-doped α-MnO2 (Mo-MnO2), various kinds of structurally diverse sulfides including aromatic
and aliphatic ones could efficiently be oxidized to the corresponding sulfoxides as the major products.
The observed catalysis was truly heterogeneous, and Mo-MnO2 could be reused for the oxygenation
of thioanisole at least four times without an appreciable loss of its high catalytic performance. The
structure of Mo-MnO2 was intrinsically preserved after the repeated reuse for the oxygenation.
Besides sulfide oxygenation, Mo-MnO2 could act as an efficient heterogeneous catalyst for several
aerobic oxidative functional group transformations through single-electron transfer oxidation
processes, such as oxygenation of alkylarenes, oxidative α-cyanation of trialkylamines, and oxidative
S-cyanation of benezenethiols.
Results and discussion
Structure-activity relationships for thioanisole oxygenation
To begin with, we prepared four kinds of manganese oxides with α-, β-, γ-, and δ-phase structures
according to the previously reported procedures with minor modifications (see the Experimental
section).6k,6m,7 Their X-ray powder diffraction (XRD) patterns are shown in Fig. 1. The XRD pattern
of α-MnO2 could be well indexed to a cryptomelane-type manganese oxide with a 2 × 2 tunnel
(4.6 Å × 4.6 Å) (JCPDS 29-1020). The XRD patterns of β-MnO2 and γ-MnO2 were in good
agreement with those of pyrolusite-type (JCPDS 24-0735) and nsutite-type (JCPDS 14-0644)
manganese oxides, respectively. The XRD pattern of δ-MnO2 was characteristic of a birnessite-type
manganese oxide with a layered structure (JCPDS 43-1456). Therefore, four types of manganese
oxides with different crystal structures were successfully prepared. The peak widths and intensities
of these XRD patterns imply that α-MnO2 and β-MnO2 possess relatively high crystallinities in
comparison with γ-MnO2 and δ-MnO2. The specific surface areas of α-MnO2, β-MnO2, γ-MnO2, and
δ-MnO2 were 80 m2 g–1, 18 m2 g–1, 73 m2 g–1, and 124 m2 g‒1, respectively (Table 1). The manganese
contents in α-MnO2 (52.5 wt%) and δ-MnO2 (47.4 wt%) were somewhat lower than those in β-MnO2
(64.3 wt%) and γ-MnO2 (62.0 wt%) (Table 1), which is mainly due to the accommodation of K+ ions
into the 2 × 2 tunnel of α-MnO2 or into the interlayer of δ-MnO2.8 The average oxidation state (AOS,
determined by redox titration) of α-MnO2 was 3.77 and slightly lower than those of the others
3