Mild, aqueous, aerobic, catalytic oxidation of methane to methanol and acetaldehyde catalyzed by a supported bipyrimidinylplatinum-polyoxometalate hybrid compound

Article abstract of DOI:10.1021/ja0493547

We have demonstrated that a bipyrimidinylplatinum-polyoxometalate, [Pt(Mebipym)Cl₂]⁺[H₄PV₂Mo₁₀O₄₀]^-, supported on silica is an active catalyst for the aerobic oxidation of methane to methanol in water under mild reaction conditions. Further oxidation of methanol yields acetaldehyde. The presence of the polyoxometalate is presumed to allow the facile oxidation of a Pt(II) intermediate to a Pt(IV) intermediate and to aid in the addition of methane to the Pt catalytic center. Copyright

Full text of DOI:10.1021/ja0493547

Published on Web 07/30/2004
Mild, Aqueous, Aerobic, Catalytic Oxidation of Methane to Methanol and
Acetaldehyde Catalyzed by a Supported
Bipyrimidinylplatinum-Polyoxometalate Hybrid Compound
Itsik Bar-Nahum, Alexander M. Khenkin, and Ronny Neumann*
Department of Organic Chemistry, Weizmann Institute of Science, RehoVot, Israel 76100
Received February 5, 2004; E-mail: Ronny.Neumann@weizmann.ac.il
Scheme 1. Proposed Catalytic Cycle for Aerobic Methane
Oxidation Consisting of Electrophilic Activation of Methane (1),
Aerobic Oxidation of Pt(II) to Pt(IV) (2), Nucleophilic Cleavage of
the Methyl-Pt(IV) Bond (3), and Ligand Exchange (4) Steps
The selective transformation of alkanes leading to oxygenated
petrochemicals such as alcohols, aldehydes, and carboxylic acids
is important for improved utilization of petroleum and natural gas-
based resources. In this context, controlled aerobic oxidation of
methane is a key research objective. Although methane can be
aerobically oxidized to methanol using methane monoxygenase
enzymes, the oxidation requires the use of sacrificial reducing agents
for oxygen activation.1 Thus, there is significant interest in
nonbiological systems for selective activation of saturated C-H
bonds.2 Particularly enticing is electrophilic alkane activation and
oxidation by platinum(II) complexes originally reported by Shilov
and co-workers using Pt(IV) salts as the stoichiometric oxidant.3
In recent years, advances in these Pt(II)- and also Pd(II)-catalyzed
reactions have been made from both synthetic (practical)4 and
mechanistic5 points of view. The mechanistic work can be sum-
marized by a probable catalytic cycle for aerobic oxidation of
methane to methanol2 (Scheme 1). The cycle has not, however,
been definitively demonstrated, although some alkane4c and methane4d
aerobic catalytic oxidation has been observed at high oxygen
Scheme 2. Bipyrimidinylplatinum-Polyoxometalate Catalyst and
the Catalytic Transformation Observed
pressure, usually PO > 20-25 bar.
2
We now report on the synthesis of a bipyrimidinylplatium-
polyoxometalate (H5PV2Mo10O40) hybrid complex and its use as a
supported (silica) catalyst for the mild, aerobic (50-60 °C, 1-2
bar O2), aqueous oxidation of methane to methanol and then
partially further to acetaldehyde (Scheme 2). Although methane
has been oxidized directly to acetic acid with SO3,6 and acetic acid
has also been formed from methane and CO and CO2 (methanol
over-oxidation products),7 the significant formation of acetaldehyde
without formation of acetic acid has not been previously reported.
The catalyst was prepared by the following synthetic sequence
(details in Supporting Information): (a) reaction of 2,2′-bipyrimidine
(bipym) with PtCl2(DMSO)2 to give Pt(bipym)Cl2, (b) methylation
of Pt(bipym)Cl2 with Me2SO4 at one of the non-coordinated
aromatic nitrogen atoms to yield the cationic methylated [Pt-
(Mebipym)Cl2]+HSO4-, (c) an anion exchange reaction in aceto-
Table 1. Aerobic Oxidation of Methane Catalyzed by
a
[Pt(Mebipym)Cl2]+[H4PV2Mo10O40]-/SiO2
products, µmol
CH OH
3
HCHO
CH CHO
3
acid (µmol)
TON
3
30
24
46
1
12
19
9
13
48
49
41
none
H2SO4 (150)
H5PV2Mo10O40 (60)
H5PV2Mo10O40 (30)
6
31
32
33
-
nitrile between [Pt(Mebipym)Cl2]+HSO4 and (POM) H5PV2-
Mo10O40 to yield [Pt(Mebipym)Cl2]+[H4PV2Mo10O40]-, which was
then wet impregnated on a silica matrix. The H5PV2Mo10O40
polyoxometalate was chosen as counteranion to the cationic [Pt-
(Mebipym)Cl2]+ species because of (a) its advantageous oxidation
potential8 needed to mediate the oxidation of a Pt(II) complex to a
Pt(IV) complex (Scheme 1, step 2) and (b) its ability to very
efficiently oxidize hydride species to protons and reduced poly-
oxometalate that in the presence of dioxygen are easily reoxidized
under mild conditions with formation of water. 8,9 See, for example,
the oxidation of hydrogen (eq 1).
a Reactions conditions: 2.88 µmol of catalyst, 2 mL of H2O, 0-150
µmol of acid, 30 bar CH4, 2 bar O2, 50 °C, 4 h in a 14 mL Parr autoclave.
TON ) moles of products per mole of catalyst.
Oxidation of methane was carried out as noted in Table 1. The
results clearly show that there was significant turnover in the
oxygen-mediated catalytic oxidation of methane (∼1% conversion)
under mild conditions. As is known for these reactions,3 excess
acid was also needed. No significant formation of CO or CO2 was
monitored (GC-MS analysis of both the gas and liquid phases).
No formation of acetic acid was measured. Probably, acetaldehyde
is not further oxidized as a result of the mild reaction conditions
coupled with the anti-autoxidation properties of the polyoxometa-
late.10 TEM analysis (1 nm resolution) of the supported catalyst
O2
98
H2 + H5PVV Mo10O40
9
M8 H7PVIV2Mo10O40
2
H2O + H5PVV Mo10O40 (1)
2
9
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J. AM. CHEM. SOC. 2004, 126, 10236-10237
10.1021/ja0493547 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. Possible Reactions Involved in Acetaldehyde
Formation
inhibited the reaction and such a step has a ∆Gr ) 12 kcal/mol;
other coupling reactions are driven by water formation. (iv) The
most likely scenario for acetaldehyde formation is the oxidation of
methane to formaldehyde via methanol, followed by its coupling
with methane to yield the product, possibly occurring entirely in
the coordination sphere of the catalyst (Scheme 3c). Accumulation
of HCHO and then CO leads to deactivation (high concentrations
of these inhibit the reaction).
The presence of the polyoxometalate in the [Pt(Mebipym)-
Cl2]+[H4PV2Mo10O40]- hybrid catalyst is key in enabling mild
aerobic oxidation of methane and possibly functions to facilitate
both (a) oxidation of Pt(II) to Pt(IV) intermediates and (b) the
addition of methane (also methanol) to a Pt(II) center by providing
a conduit for improved oxidation of intermediate hydride species.
after the reaction showed no formation of Pt aggregates due to
reduction of the Pt complex. ICP-MS analysis also gave no
indication of leaching of Pt into the solution. An active carbon
support instead of a silica matrix was equally effective; hydrophobic
silica was an inferior support. Simple Pt compounds such as cis-
Pt(II)(NH3)2Cl2 and K2PtCl4 showed no catalytic activity.
A key feature of a Shilov-type catalytic cycle is the formation
of methanol via a nucleophilic cleavage of a Pt(IV)-methyl
intermediate (Scheme 1, step 3), rather than a direct oxidation of
an intermediate with O2. Three experiments were carried out that
support a nucleophilic cleavage pathway. (i) Oxidation of methane
in the presence of HCl yielded 63 µmol of CH3Cl, 11 µmol of
CH3OH, 2 µmol of HCHO, and 62 µmol of CH3CHO (48 TON).11
(ii) A reaction in the presence of 18O-labeled water gave 18O-labeled
CH3OH, HCHO, and CH3CHO (∼73 ( 3% enrichment).12 Enrich-
ment is reduced due to exchange of H218O with the acid. (iii)
Reaction in the presence of 18O-labeled O2 (96.3% enrichment)
under standard reaction conditions (Table 1) yielded no 18O-labeled
products. Methyl chloride formation in the presence of HCl and
the 18O-labeling experiments support a nucleophilic cleavage
pathway.13
The significant formation of acetaldehyde is a novel feature of
this reaction. A time course profile (Figure 1S) of the reaction
showed no induction period; methanol and acetaldehyde were
accumulated immediately. The accumulation of formaldehyde was
delayed and coincided with the inhibition of the reaction. Also,
the oxidation of methane upon addition of 250 µmol of formalde-
hyde (conditions given in Table 1) was almost totally inhibited.
Conceivable reactions to consider for acetaldehyde formation are
outlined in Scheme 3. To gain further insight into these possibilities,
various additional experiments were carried out. (i) A reaction with
CH4/13CH3OH gave labeled CH313CHO only at the carbonyl
position,14 but mostly unlabeled product was formed (CH3CHO/
CH313CHO ∼3/1). (ii) In the absence of methane, the oxidation of
methanol to formaldehyde was very slow (Figure 2S); neither the
polyoxometalate nor the [Pt(Mebipym)Cl2]+ ligand alone showed
activity for alcohol oxidation. (iii) Similarly, the oxidation of ethanol
was also slow (Figure 2S). (iv) Addition of a small amount of 13CO
to a methane oxidation reaction showed no formation of CH313CHO,
and only traces of CH3OH (∼1 TON), but no CH312CHO was
observed.15
Acknowledgment. The research was supported by Israel
Science Foundation and the Minerva Foundation. R.N. is the
Rebecca and Israel Sieff Professor of Organic Chemistry.
Supporting Information Available: Experimental details, NMR
spectra of the [Pt(Mebipym)Cl2]+ ligand, and time course profiles for
oxidations. This material is available free of charge via the Internet at
http://pubs.acs.org.
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(9) Authentic hydride species are immediately oxidized by H5PV2Mo10O40
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(11) Conditions: 2.9 µmol of catalyst, 300 µmol of HCl, 2 mL of H2O, 30 bar
CH4, 2 bar O2, 50 °C, 4 h. A control experiment showed no formation of
CH3Cl from CH3OH.
(12) Conditions: 0.29 µmol of catalyst, 0.2 mL of H218O (95.2% 18O), 50 µmol
of H2SO4, 30 bar CH4, 2 bar O2, 50 °C, 4 h. In a control experiment,
<1.5% exchange between CH3OH and CH318OH was observed.
(13) It is possible that a H-PtIV-CH3 intermediate is formed by oxidative
addition of CH4 to a PtII species. In such a scenario, the hydride may be
oxidized by the polyoxometalate (ref 9) along with the nucleophilic
cleavage step.
From the results described above, one can draw the following
conclusions. (i) Coupling of methanol and formaldehyde is not
likely (Scheme 3e) because addition of 13CH3OH to a reaction
yielded labeled acetaldehyde only at the carbonyl position. (ii) A
pathway involving oxidative coupling of coordinated methanol and
methane to yield ethanol that is then oxidized to acetaldehyde
(Scheme 3b) is also unlikely since ethanol is only slowly oxidized
under the reaction conditions and ethanol was not observed as an
intermediate. (iii) Formation of acetaldehyde by reaction of methane
with CO (Scheme 3d) is also improbable since CO strongly
(14) Conditions: 2.88 µmol of catalyst, 1 mmol of 13CH3OH (99% 13C),
2 mL of H2O, 60 µmol of H5PV2Mo10O40, 30 bar CH4, 2 bar O2, 50 °C,
4 h. TON ) 31.
(15) Conditions: 2.88 µmol of catalyst, 2 mL of H2O, 60 µmol of H5PV2-
Mo10O40, 30 bar CH4, 0.2 bar 13CO (CH4/CO ) 150), 2 bar O2, 50 °C,
4 h.
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