Photooxidation of methane to methanol by perrhenate in water under ambient conditions

Article abstract of DOI:10.5560/ZNB.2013-3104

The oxidation of methane to methanol takes place selectively by the photolysis of perrhenate in aqueous solution in the presence of methane. This photoreaction is formally an oxygen atom transfer. Because the reoxidation of the reduced perrhenate is accomplished with hydrogen peroxide the overall process can be viewed as photocatalytic oxidation of methane to methanol: CH₄ +H₂O₂ ? CH₃OH+ H₂O.

Full text of DOI:10.5560/ZNB.2013-3104

Photooxidation of Methane to Methanol by Perrhenate in Water under
Ambient Conditions
Horst Kunkely and Arnd Vogler
Institut f u¨ r Anorganische Chemie, Universit a¨ t Regensburg, D-93040 Regensburg, Germany
Reprint requests to Prof. Dr. Arnd Vogler. E-mail: arnd.vogler@chemie.uni-regensburg.de
Z. Naturforsch. 2013, 68b, 891 – 894 / DOI: 10.5560/ZNB.2013-3104
Received April 3, 2013
The oxidation of methane to methanol takes place selectively by the photolysis of perrhen-
ate in aqueous solution in the presence of methane. This photoreaction is formally an oxygen
atom transfer. Because the reoxidation of the reduced perrhenate is accomplished with hydrogen
peroxide the overall process can be viewed as photocatalytic oxidation of methane to methanol:
CH + H O → CH OH+ H O.
4
2
2
3
2
Key words: Rhenium Complexes, Photochemistry, Methane, Catalysis
Introduction
In this context it is quite surprising that photochemi-
cal studies are still rare in this area. Although irra-
The activation and functionalization of alkanes re- diation has been used to generate active metal com-
mains to be a field of much activity [1]. This is not plexes [3 – 6] the photoconversion of alkanes them-
only of academic interest, but practical aspects of in- selves is largely unexplored. The light energy should
dustrial and commercial applications play an impor- not only promote photochemical reactions in general
tant role. In particular, the facile conversion of methane but in particular could supply the requested activation
to methanol could provide a basis for the “methanol energy. Selectivity may be preserved since the photo-
economy” [2] with novel utilizations in organic chem- chemistry can occur under ambient conditions. These
istry. The application of such systems for the chemi- considerations initiated our present study.
cal storage of solar energy and its simple manipulation
The thermal oxidation of alkanes by OsO in aque-
4
and transportation is a very attractive aspect. Unfortu- ous solution has been examined before [7, 8], but
nately, methane is characterized by a quite low reactiv- it proceeds only at higher temperatures, and over-
ity owing to its high C–H bond energy which amounts oxidation can apparently not be avoided. For an initial
to 104 kcal. Accordingly, high activation energies are attempt we selected this system but studied the photo-
required to facilitate functionalization. However, under chemistry under ambient conditions. This investigation
these conditions it is difficult to control any reaction was also facilitated by taking into account the results
and to achieve selectivity. In particular, oxidations can of previous work with OsO as an efficient photooxi-
4
frequently not be stopped when the requested product dant [9]. However, owing to the detrimental properties
is formed, and over-oxidation may take place termi- of OsO it was later replaced by the isoelectronic an-
4
−
nating at the thermodynamic sink, e. g. carbon dioxide. ion ReO which promotes a rather smooth photocon-
4
For that reason it is quite intriguing that Nature devel- version of methane to methanol.
oped the enzyme methane-monooxygenase which ac-
complishes the conversion of methane to methanol un- Results
der ambient conditions. Nevertheless, much progress
has been achieved in this research field by the investi-
Aqueous solutions of OsO are not light-sensitive as
4
gation of the interaction of alkanes including methane indicated by the absence of any spectral changes upon
with transition metal centers as an important branch of irradiation with white light or UV light (λ = 254 nm).
irr
organometallic chemistry [3 – 6].
However, when methane (we used natural gas consist-
©
2013 Verlag der Zeitschrift f u¨ r Naturforschung, T u¨ bingen · http://znaturforsch.com
Brought to you by | University of California
Authenticated
Download Date | 11/25/15 1:37 PM

8
92
H. Kunkely – A. Vogler · Photooxidation of Methane to Methanol
ing of 97.5 vol-% methane, 1.1% ethane and 0.8% ni-
trogen) is blown through this solution during irradi-
ation spectral variations (Fig. 1) are observed which
2−
clearly indicate the formation of [Os(OH) (O) ]
4
2
(
1
λmax = 300 nm, ε = 1300; 515, 34 and 680 nm, sh,
4) [9, 10] as photoproduct according to the simple
stoichiometry shown in Eq. 1.
OsVIIIO + CH → Os O + CH OH
VI
(1)
4
4
3
3
The product OsO exists in aqueous solution as
3
2
−
Os(OH) (O) ] . Moreover, methanol was detected
4 2
[
by an enzyme-based method (alcohol dehydrogenase
or alcohol oxidase provided as UV test 340 nm from
Fig. 2. Spectral changes during the photolysis of
Roche or Sanelco), but only traces of CH OH were
3
−3
3
.6 × 10 M NH ReO in water and in the presence
4
4
found. In addition, some formaldehyde was iden-
of CH (see text) after 0 min (a) and 10 h (b) irradiation time
4
tified (Merckoquant 1.10036.0001 and Spectroquant at λirr = 254 nm (see Fig. 1), 1-cm cell.
.4500.0001). The lack of methanol as product is not
surprising because the thermal oxidation of methanol
by OsO is well known [11, 12]. It is quite interesting of rhenate(V) such as ReO . In previous studies at-
1
−
4
3
2−
that [Os(OH) (O) ] can be completely reoxidized tempts to isolate rhenate(V) failed owing to its subse-
4
2
photochemically by H O to OsO (λ = 254 nm) quent disproportionations [13]. However, the presence
2
2
4
irr
while this reaction does not take place thermally [11].
of weak bands of the photolysis products in the spec-
Aqueous solutions of perrhenate are also light- trum of Fig. 2 suggests that these originate from d−d
2
−
insensitive, but when methane is passed through this (or LF) transitions in analogy to [Os(OH) (O) ]
4
2
2
solution a photolysis takes place. The concomitant which is also a d species. Accordingly it was assumed
spectral changes (Fig. 2) showed a pattern which re- that the photolysis proceeds according to Eq. 2.
sembles that of Fig. 1. Rather weak bands in the visi-
ReVIIO + CH → Re O + CH OH
−
V
−
(2)
4
3
4
3
ble region at λmax = 430 nm and 570 nm appeared but
−
could not be assigned. They are attributed to some kind The loss of ReO4 was determined by measuring the
decrease of the absorbance at 300 nm taking into ac-
count the residual extinction of the photolysis products
at this wavelength. In addition, methanol was detected
in the photolyzed solution. The reliability of this se-
lective analysis was confirmed by control experiments
with measured amounts of added methanol prior and
after the photolysis. At longer irradiation times fur-
ther spectral changes did not occur but only 80% of
the methane which should have been reacted accord-
ing to Eq. 2 were recovered as methanol by quantita-
tive analysis. Irrespective of the precision of this anal-
ysis (±10%) the loss of some methanol takes certainly
place by its removal by the stream of methane passing
this solution. Formaldehyde was not detected in this
solution. An over-oxidation of methane is certainly not
Fig. 1. Spectral changes during the photolysis of 7.6 ×
−
expected owing to the fact that ReO4 is only a weak
oxidant compared to OsO4.
−
M OsO in water (pH = 9.0) and in the presence of CH
4 4
4
1
0
(
see text) after 0 min (a) and 3 h (b) irradiation time with
When H O was added to the photolyzed solution of
λ
= 254 nm (low-pressure mercury lamp, Hanau 6 W), 1-
2
2
irr
−
ReO /CH the original spectrum of ReO (see Fig. 2)
4
4
−
cm cell; (b): absorbance × 10.
4
Brought to you by | University of California
Authenticated
Download Date | 11/25/15 1:37 PM

H. Kunkely – A. Vogler · Photooxidation of Methane to Methanol
893
were not successful. The photoactivity was restricted
to the absorption of perrhenate. Accordingly, any mea-
sured quantum yield can not be attached any signif-
icance, but with regard to the light absorbed by per-
rhenate Φ at λ = 255 nm it is estimated to be lower
irr
−6
than 10
.
In terms of organometallic chemistry the photoreac-
−
tivity of OsO /CH and ReO /CH is a further ex-
4
4
4
Scheme 1.
4
ample of the significance of CT states which induce
photoreactions of organometallic compounds [16]. In
was completely restored. Rhenate(V) was apparently this context it is of interest that CH ReO is character-
3
3
−
VII
reoxidized in a thermal reaction according to Eq. 3.
ized by a very reactive (CH → Re ) LMCT excited
3
V
−
VII
−
state [17, 18]. The stability of the CH3–Re bond may
Re O + H O → Re O + H O
(3)
2
2
2
3
4
be close to that of the H C–H bond in methane.
3
It follows that the sum of reactions 2 and 3 simply
yields Eq. 4.
A further interesting analogy should be mentioned.
−
The photoreactions of OsO and ReO with CH ac-
4
4
4
cording to Eqs. 1 and 2 can be considered as an oxygen
atom transfer process. A related photolysis has been
observed before (Eq. 5) [19].
CH + H O → CH OH+ H O
(4)
4
2
2
3
2
Discussion
VII
CH Re O (PPh )−hν/LMCT →
3
3
3
The overall reaction can be also expressed as
a cyclic process which can be viewed as a photocat-
alytic oxidation according to Scheme 1.
V
MeRe O + O=PPh
(5)
2
3
The tetraoxo complexes OsO and ReO4 are d0
−
The suitability of water as reaction medium may be
favored by the ability of alkanes to form gas hy-
drates [20].
4
systems which have available only LMCT transitions.
Both complexes can expand their coordination sphere
by accepting a further ligand. However, CH4 does
not provide a free electron pair for bonding to the
metal. Accordingly, complexes such as OsO (CH ) or
In conclusion, in aqueous solution perrhenate pho-
tooxidizes methane to methanol in a rather selec-
tive process. Because the reduced perrhenate is reox-
idized by H O the overall reaction CH + H O →
4
4
−
ReO (CH )] are certainly not very stable. Moreover,
4 4
2
2
4
2
2
[
CH OH + H O can be conducted as a cyclic process
3
2
the electronic interaction which could lead to a (CH4
to metal) LMCT transition would be also very weak.
which represents a kind of photocatalysis.
As an alternative, the CT interaction of CH and these
tetraoxo complexes may be also of the outer-sphere
4
Experimental Section
(
OS) CT type [14] as it is well known for OsO and
OsO4 and NH ReO (Puratrem) were commercially
4
4
4
certain aromatic molecules [15]. Again, this type of available (Strem) and used as obtained. Natural gas was pro-
vided through the pipeline of the local gas supplier. Absorp-
tion spectra were measured with a Varian Cary 50 spec-
trophotometer. The light sources used for irradiation were
a low-pressure mercury lamp (Hanau, 6W) or a high-pressure
mercury lamp (Osram HBO 200 W/2). Monochromatic light
was obtained using Schott PIL/IL interference filters and
Schott cutoff filters to avoid short-wavelength and second-
order irradiation. In all cases the light beam was focused on
a photolysis cell by a quartz lens. The photolyses were per-
formed in 1-cm spectrophotometer cells.
OSCT interaction should be very weak for CH as
4
OSCT donor. Accordingly, it is not surprising that the
absorption spectrum of perrhenate in water is not af-
fected by methane passing this solution. In addition,
−3
the limited solubility of methane in water (∼ 10 M)
will certainly also contribute to a rather low station-
ary state concentration of any species containing OsO4
−
or ReO4 and CH . So it is not unexpected that the
4
photolysis (Eqs. 1 and 2) requires a long irradiation
time, also in view of the low light intensity which is
available at λirr = 254 nm in our set up. Owing to the
presumably very low concentration of the photoactive
Acknowledgement
We are grateful for financial support by DFG (grant
species, attempts to identify its absorption spectrum Vo 211/19-1).
Brought to you by | University of California
Authenticated
Download Date | 11/25/15 1:37 PM

8
94
H. Kunkely – A. Vogler · Photooxidation of Methane to Methanol
[
[
1] M. P. Doyle, K. I. Goldberg, Acc. Chem. Res. 2012, 45, [11] T. Osako, E. J. Watson, A. Dehestani, B. C. Bales,
7
77.
J. M. Mayer, Angew. Chem. Int. Ed. 2006, 45,
2] G. A. Olah, A. Goeppert, G. K. S. Prakash, Beyond Oil
7433 – 7436.
and Gas: The Methanol Economy, Wiley-VCH, Wein- [12] T. E. Geswindt, W. J. Gerber, H. E. Rohwer, K. R.
heim 2009.
Koch, Dalton Trans. 2011, 8581 – 8588.
[13] G. Rouschias, Chem. Rev. 1974, 74, 531 – 566.
[14] A. Vogler, H. Kunkely, Top. Curr. Chem. 1990, 158,
3 – 27.
[15] J. M. Wallis, J. K. Kochi, J. Am. Chem. Soc. 1988, 110,
8207 – 8223.
[
3] R. H. Crabtree, J. Organomet. Chem. 2004, 689, 4083 –
4
091.
4] J. A. Labinger, J. E. Bercaw, Nature 2002, 417, 507 –
08.
5] R. G. Bergman, Science 1984, 223, 902 – 908.
[
5
[
[
6] R. H. Crabtree, J. Chem. Soc., Dalton Trans. 2001, [16] A. Vogler, H. Kunkely, Coord. Chem. Rev. 2004, 248,
437 – 2450. 273 – 278.
7] B. C. Bales, P. Brown, A. Dehestani, J. M. Mayer, J. [17] H. Kunkely, T. T u¨ rk, C. Teixeira, C. de Meric de
2
[
[
Am. Chem. Soc. 2005, 127, 2832 – 2833.
Bellefon, W. A. Herrmann, A. Vogler, Organometallics
8] J. M. Mayer, E. A. Mader, J. P. Roth, J. R. Bryant,
1991, 10, 2090 – 2092.
T. Matsuo, A. Dehestani, B. C. Bales, E. J. Watson, [18] W. A. Herrmann, F. E. K u¨ hn, D. A. Fiedler, M. R. Matt-
T. Osako, K. Valliant-Saunders, W. H. Lam, D. A. Hro-
vat, W. T. Borden, E. R. Davidson, J. Mol. Cat. A:
Chem. 2006, 251, 24 – 33.
ner, M. R. Geisberger, H. Kunkely, A. Vogler, S. Steen-
ken, Organometallics 1995, 14, 5377 – 5381.
[19] H. Kunkely, A. Vogler, J. Organomet. Chem. 2000,
606, 207 – 209.
[
9] H. Kunkely, A. Vogler, Inorg. Chem. Commun. 1998, 1,
7
– 9.
10] K. A. K. Lott, M. C. R. Symons, J. Chem. Soc. 1960,
73 – 976.
[20] C. A. Koh, Chem. Soc. Rev. 2002, 31, 157 – 167.
[
9
Brought to you by | University of California
Authenticated
Download Date | 11/25/15 1:37 PM