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Gold Redox Catalysis for Selective Oxidation of Methane to Methanol.

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Highlights
Homogeneous Catalysis
Gold Redox Catalysis for Selective Oxidation of
Methane to Methanol
Dirk E. De Vos* and Bert F. Sels
Keywords:
gold · homogeneous catalysis · methane ·
oxidation · selenium
The challenge posed by the activation
and selective oxygenation of inert CH
bonds, as in methane, has haunted
chemists for many years. Gas-phase
reactions, such as the oxidative coupling
of methane to ethane and ethylene, and
the direct conversion of methane into
oxygenates, have met with limited success.[1] The need for high selectivity has
incited scientists to investigate liquidphase reactions at more moderate temperatures.
The origins of transition-metal catalyzed CH4 oxidation chemistry can be
traced back to the early work of Snyder
and Grosse.[2] They oxidized methane
with fuming sulfuric acid at 263 8C
employing an HgSO4 catalyst, and obtained oxygenated and sulfonated methane derivatives in a summed yield of
44 %. In an evolved version of this
process, it was demonstrated that the
reaction is mediated by the HgII/HgI
redox couple (Table 1).[3] At the same
time, Shilov discovered that PtII/PtIV is
capable of oxidizing alkanes to alcohols;[4] with PtII chelated by a bipyrimidine ligand and SO3 as the oxidant,
methane was oxidized to a 72 % yield of
methanol and its esters.[5] In a recent
paper, Periana and co-workers now
report that the combination of gold as
a catalyst and H2SeO4 as the oxidant
results in a 94 % selectivity for CH3OSO3H at 28 % CH4 conversion.[6] As a
[*] Prof. Dr. D. E. De Vos, Prof. Dr. B. F. Sels
Centre for Surface Chemistry and Catalysis
K.U.Leuven
Kasteelpark Arenberg 23, 3001 Leuven
(Belgium)
Fax: (+ 32) 1632-1998
E-mail: dirk.devos@agr.kuleuven.ac.be
30
common feature of all these reactions,
the first step is the formation of a M
CH3 intermediate, possibly by electrophilic substitution on a HgII or PtII
species.[4b,c, 7, 8] With this knowledge, the
step towards gold then seems straightforward, since AuI is isoelectronic with
HgII, and AuIII shares its electronic
configuration with PtII.
This new example of homogeneous
gold catalysis joins a series of discoveries made largely in the last decade.[9]
Ionic gold, either as AuI or AuIII, has
been found to catalyze asymmetric aldol
reactions and carbonylations.[10] The
Lewis acid properties of Aun+ have been
exploited to promote the addition of
oxygen nucleophiles, such as water, to
alkynes, or to catalyze CC bond formations using alkynes.[11] Ionic gold also
catalyzes the hydroarylation of alkynes
with electron-rich aromatics or furans.[12]
This reaction is strongly reminiscent of
analogous PdII-catalyzed couplings,
which have been proposed to start with
the activation of the aromatic CH bond
by an electrophilic PdII compound.[13]
While it is tempting to propose that
with gold, auration of the aromatic
compound is the first step, it seems
more likely that gold-catalyzed hydroarylation is a classical Friedel–Crafts
reaction.
While the previous list is by no
means exhaustive, it is clear that none
of these reactions involves changes of
the gold redox state, even if two redox
states of gold, that is, AuI or AuIII, are
readily accessible. In the case of the
coupling of arenes, one might project
that, as with PdII/Pd0, an oxidative
coupling of aromatics and olefins to
alkenylaromatics might proceed, but
attempts to devise such a redox process
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200461561
with gold have not yet been successful.[12a]
Only a small number of homogeneous gold-mediated redox reactions have
been reported. Ionic gold catalyzes the
oxidation of alkanes with H2O2 to alkyl
hydroperoxides, for example, but this is
probably a free radical process.[14a]
[AuCl2(NO3)] promotes the oxidation
of thioethers to sulfoxides at room
temperature and 1 atm of O2. In this
case, the thioether, in a slow step,
reduces AuIII to AuI, which is quickly
reoxidized by O2 to AuIII, without any
Au0 formation (Scheme 1). This process
demonstrates the feasibility of AuIII/AuI
redox cycling in organic reactions.[14b] In
addition, ionic gold catalyzes the oxidative carbonylation of aniline to phenylcarbamates with O2 as the oxidant.[15]
The same reaction is known to be
catalyzed by the PdII/Pd0 redox couple;[15c] however, in the gold case, it is
not fully clear whether ionic gold or
colloidal zerovalent gold is the actual
catalyst.
The new successful application of
gold in the catalytic methane oxidation
critically depends on the use of selenic
acid as an oxidant. Periana et al. previously showed that AuIII stoichiometrically oxidizes methane to methanol.[3]
Only with H2SeO4 as the oxidant, the
system actually turns over. As selenic
acid is one of the few known reagents to
dissolve Au0, it may even re-dissolve
colloidal Au0 particles if these were to
form in the drastic reaction conditions.
On the other hand, the spontaneous
reaction between H2SeO4 and methane
should not be neglected. Comparison
with blank experiments shows that
about 20 % of the CH3OH formation
may be due to the background reaction.
Angew. Chem. Int. Ed. 2005, 44, 30 –32
Angewandte
Chemie
Table 1: Reaction characteristics of metal-catalyzed CH4 oxidation in the liquid phase.
Catalyst
Solvent + oxidant
T [K]
CH4 conversion [%]
Hg(OSO3H)2
100 % H2SO4
453
50
Selectivity [%]
[Oxygenates][a]
85 % CH3OSO3H
1.5 m
CH4/Cat.[b] TON[c] TOF [h1][d]
36
[Pt(bpym)Cl2][f ]
102 % H2SO4
493
90
81 % CH3OSO3H
1m
29
Au0
3 m SeO3 in 96 % D2SO4
3 m SeO3 in 96 % D2SO4
+ 2 % SO3
453
8
28
77 % CH3OSO3D
94 % CH3OSO3D
0.18 m
0.63 m
580
29
15
24
500[e]
32
8
3.6[e]
10
36[e]
3.6
2.7
Ref.
[3]
[3]
[5]
[5]
[6]
[6]
[a] Eventual concentration of methanol and derivatives. [b] Molar ratio CH4 :catalyst. [c] TON = turnover number = moles of product per mole of
catalytic metal. [d] TOF = turnover frequency = moles of product per mole of metal per time. [e] Maximal values, obtained in separate experiments.
[f] bpym = 2,2’-bipyrimidine.
Scheme 1. AuIII/AuI redox cycling in organic reactions.
Such a spontaneous reaction does not
necessarily decrease the selectivity: protection of methanol as methyl bisulfate
greatly reduces the reactivity of the CH3
group, not only for electrophilic activation, but also for radical processes
initiated by hydrogen-atom abstraction.[16]
As can be expected based on both
experiment and calculation, the catalytic
cycle comprises formation of an Au
CH3 compound, oxidation of the metal,
and product elimination, though not
necessarily in this order. In the analogous platinum reactions, the oxidation
of the metal precedes the product elimination;[4b, 5, 8] in the case of mercury
catalysis, the product is eliminated before reoxidation of HgI.[3] What will be
the order of the steps in case of gold
catalysis? This sequence depends on
whether methane is activated on AuI
(by oxidative addition, 3!4 in
Scheme 2) or on AuIII (by electrophilic
substitution, 1!2 in Scheme 2). While
the majority of gold is present as AuIII, a
pathway starting with oxidative addition
of CH4 on AuI is energetically favored
and therefore should be considered as
well.
In evaluating the future applications
of this discovery, environmental issues
should be considered. While selenium is
needed in small quantities in food as a
micronutrient, it is quite toxic at high
concentrations. This means that the
selenium has to be confined to the
reactor; selenium leaks, for example, as
selenite in waste water or as SeO2 in offgases must be avoided. Secondly, the
reaction is performed in H2SO4 ; methanol recovery implies dilution with water, and next, the sulfuric acid must be
concentrated again. It is therefore
worthwhile to consider other inert solvents, such as water. Sen and co-workers
and Sames and co-workers have demonstrated that catalytic CH activation
on electrophilic PtII is possible in water,[17] but the very low solubility of
methane still needs to be dealt with
Scheme 2. Possible pathways for the gold-catalyzed methane oxidation.
Angew. Chem. Int. Ed. 2005, 44, 30 –32
www.angewandte.org
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
31
Highlights
Scheme 3. Examples of gold-, platinum-, and palladium-catalyzed reactions.
(0.25 g L1 at 100 8C and 2 MPa). Finally,
it is possible to recycle the selenic acid,
by oxidation of the H2SeO3 with Cl2 or
H2O2. To obtain a commercially viable
process, TOFs larger than 3000 h1, and
a 5 m concentration of oxygenates would
probably be needed (compare values in
Table 1). Particularly regarding CH4
conversion and TON, platinum catalysis
seems still more promising than gold
catalysis. In any case, major advances
are still required before the metal-catalyzed methane oxidations can result in
an industrial process.
Strikingly, both in heterogeneous
and homogeneous oxidation catalysis,
gold is no longer considered inert, but
presents itself as an intriguing alternative to platinum and palladium, with
often
superior
characteristics
(Scheme 3). Just like metallic supported
Pt0 and Pd0, Au0/C oxidizes alcohols to
acids, but the gold catalysts are less
affected by metal leaching or by poisoning by strong adsorption of acids.[18] In
the direct production of H2O2 out of H2
and O2, supported gold can be used
instead of palladium; the formed hydroperoxide can be employed in situ for
propene epoxidation.[19] The similar role
of palladium and gold in oxidative
carbonylation of amines has been discussed before.[15] With the discovery by
Periana and co-workers for the Aucatalyzed methane oxidation,[6] gold
claims its place in this domain as well,
alongside other noble metals.
Published Online: November 26, 2004
32
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