Angewandte
Chemie
DOI: 10.1002/anie.201200015
Methane Activation
Direct Conversion of Methane into Formaldehyde Mediated by
+
[
Al O ]C at Room Temperature**
2
3
Zhe-Chen Wang, Nicolas Dietl, Robert Kretschmer, Jia-Bi Ma, Thomas Weiske,
Maria Schlangen,* and Helmut Schwarz*
Dedicated to Professor George A. Olah on the occasion of his 85th birthday
As the major component of natural gas, methane (CH ) is
mainly used as fuel for power generation and as a chemical
feedstock for the industrial production of syngas. The
commercial catalyst is available (even at high temperature)
for the one-step conversion CH !CH O with acceptable
4
4
2
yields, a result of the interdependence of conversion and
[
4b,6]
selective functionalization of CH to more valuable chemicals
selectivity.
As has been demonstrated, CH can be activated catalyti-
4
under environmentally benign and economically feasible
conditions constitutes one of the major contemporary chal-
4
[
7]
cally with unexpected low activation energies, and reactive
Al–O Lewis acid/Lewis-base pairs are found to act as the key
[1]
lenges in chemistry, and value-added products include
[
2]
[8]
3.
methanol (CH OH) and formaldehyde (CH O). As one of
species on the (110) face of g-Al O One efficient approach
3
2
2
the top 25 most-produced chemicals worldwide, the annual
to improve the performance of existing catalysts is to
understand the detailed mechanisms by which single-site
catalysts operate. This understanding can ideally be achieved
by gas-phase studies in which models of the active site are
prepared, mass-selected, and studied under (near) single-
collision conditions in their reactions with various sub-
production of CH O amounts to more than 20 million tons
2
[3]
per year, and the major large-scale process is the conversion
of CH OH at high temperature using iron- or silver-based
3
[3]
catalysts, Reaction (1).
[
9]
strates. Sierka et al. reported the unexpected structure of
2
CH OH þ O ! 2 CH O þ 2 H O
ð1Þ
3
2
2
2
+
[
(Al O ) ]C which has a terminal AlꢀOC single bond by using
2
3 4
[10]
gas-phase infrared-photodissociation spectroscopy,
reactivity studies revealed that [(Al O ) ]C clusters (x = 3, 4,
and
Currently, the industrial production of CH O starting from
+
2
2
3 x
CH proceeds in several steps and requires high pressure and
4
5
) engage in H-atom abstraction from methane under thermal
high temperature, thus consuming a large amount of energy;
[11]
conditions; this reactivity pattern can be explained by the
spin density located at terminal oxygen atoms which turned
out to be crucial for hydrogen-atom transfer (HAT) proc-
[4]
further, the byproducts are harmful to the environment.
Despite extensive efforts, the direct conversion of CH into
4
[
5]
CH O remains a great challenge because formaldehyde is
much more easily oxidized than methane. To date, no
[2,9a,12]
2
esses.
[
3]
In previous studies, different types of methane activation
[
2,9,12f,h,13]
by gaseous oxide clusters have been described,
[
*] Dr. Z.-C. Wang, Dipl.-Chem. N. Dietl, Dipl.-Chem. R. Kretschmer,
J.-B. Ma, Dr. T. Weiske, Dr. M. Schlangen, Prof. Dr. H. Schwarz
Institut fꢀr Chemie, Technische Universitꢁt Berlin
Strasse des 17. Juni 135, 10623 Berlin (Germany)
E-mail: maria.schlangen@mail.chem.tu-berlin.de
including HAT, oxygen-atom transfer, carbene as well as
formaldehyde formation [Eq. (2)–(5)].
½
½
½
½
ðM
x
O
y
ÞOꢁþ þ CH
4
! ½ðM
x
O
y
ÞOHꢁþ þ CH
3
C
ð2Þ
ð3Þ
ð4Þ
ð5Þ
þ
þ
ðM O ÞOꢁ þ CH ! ½M O ꢁ þ CH OH
x
y
4
x
y
3
Prof. Dr. H. Schwarz
Chemistry Department, Faculty of Science
King Abdulaziz University
Jeddah 21589 (Saudi Arabia)
E-mail: hschwarz@kau.edu.sa
ðM
x
O
y
ÞOꢁþ þ CH
4
! ½ðM
x
O
y
ÞCH
2
ꢁþ þ H
2
O
þ
þ
ðM O ÞOꢁ þ CH ! ½M O H ꢁ þ CH O
x
y
4
x
y
2
2
Dr. Z.-C. Wang
Department of Chemistry, Colorado State University
Fort Collins, CO 80523-1872 (USA)
Reaction (3) is the gas-phase analogue of the formation of
methanol from methane. Formaldehyde can be generated
under these conditions by the rarely observed direct oxidation
[
**] This work is supported by the Fonds der Chemischen Industrie, the
Deutsche Forschungsgemeinschaft (DFG), and the Cluster of
Excellence “Unifying Concepts in Catalysis” (coordinated by the
Technische Universitꢁt Berlin and funded by the DFG). For
computational resources, the Institut fꢀr Mathematik at the
Technische Universitꢁt Berlin is acknowledged. Z.-C.W. is grateful to
the Alexander von Humboldt Stiftung for a postdoctoral fellowship.
R.K. acknowledges the Stiftung Stipendien-Fonds des Verbandes
der Chemischen Industrie for a Kꢂkulꢂ scholarship. We thank Dr.
Jianwen Liu (Humboldt Universitꢁt zu Berlin) for helpful discus-
sions.
+
+
of CH , Reaction (5), and [PtO ] and [CrO2] have been
4
2
reported to bring about this conversion; however, the chemo-
[13j,k]
selectivity is with 2% and 30%, respectively, rather poor.
Herein, we show that this direct oxidation is possible with
+
good chemoselectivity in the reaction of gaseous [Al O ]C
2
3
clusters with CH at room temperature.
4
Figure 1 shows the thermal reactions of mass-selected
+
[Al O ]C with CH , CD , and CH D , at a pressure of 8 ꢀ
2
3
4
4
2
2
Angew. Chem. Int. Ed. 2012, 51, 3703 –3707
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3703