Isothermal Cyclic Conversion of Methane into Methanol over Copper-Exchanged Zeolite at Low Temperature

Article abstract of DOI:10.1002/anie.201511065

Direct partial oxidation of methane into methanol is a cornerstone of catalysis. The stepped conversion of methane into methanol currently involves activation at high temperature and reaction with methane at decreased temperature, which limits applicability of the technique. The first implementation of copper-containing zeolites in the production of methanol directly from methane is reported, using molecular oxygen under isothermal conditions at 200 °C. Copper-exchanged zeolite is activated with oxygen, reacts with methane, and is subsequently extracted with steam in a repeated cyclic process. Methanol yield increases with methane pressure, enabling reactivity with less reactive oxidized copper species. It is possible to produce methanol over catalysts that were inactive in prior state of the art systems. Characterization of the activated catalyst at low temperature revealed that the active sites are small clusters of copper, and not necessarily di- or tricopper sites, indicating that catalysts can be designed with greater flexibility than formerly proposed.

Full text of DOI:10.1002/anie.201511065

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International Edition: DOI: 10.1002/anie.201511065
German Edition: DOI: 10.1002/ange.201511065
Methane to Methanol Conversion Hot Paper
Isothermal Cyclic Conversion of Methane into Methanol over Copper-
Exchanged Zeolite at Low Temperature
Patrick Tomkins, Ali Mansouri, Selmi E. Bozbag, Frank Krumeich, Min Bum Park,
Evalyn Mae C. Alayon, Marco Ranocchiari, and Jeroen A. van Bokhoven*
Abstract: Direct partial oxidation of methane into methanol is
a cornerstone of catalysis. The stepped conversion of methane
into methanol currently involves activation at high temperature
and reaction with methane at decreased temperature, which
limits applicability of the technique. The first implementation
of copper-containing zeolites in the production of methanol
directly from methane is reported, using molecular oxygen
under isothermal conditions at 2008C. Copper-exchanged
zeolite is activated with oxygen, reacts with methane, and is
subsequently extracted with steam in a repeated cyclic process.
Methanol yield increases with methane pressure, enabling
reactivity with less reactive oxidized copper species. It is
possible to produce methanol over catalysts that were inactive
in prior state of the art systems. Characterization of the
activated catalyst at low temperature revealed that the active
sites are small clusters of copper, and not necessarily di- or
tricopper sites, indicating that catalysts can be designed with
greater flexibility than formerly proposed.
Herein we show a high yielding, low temperature, isothermal
cyclic procedure for the oxidation of methane into methanol
using molecular oxygen. In contrast with prior studies, we
found that copper-exchanged zeolites activated at low tem-
perature (2008C for example) are capable of converting
methane into methanol if elevated pressures of methane are
employed during the reaction. A stable yield was achieved
over several cycles. In situ extended X-ray absorption fine
structure (EXAFS) identified the active species as dehy-
drated copper oxide clusters. No spectroscopic signature
assigned to a m-oxo dicopper site was observed, which proves
the presence of an unprecedented reaction mechanism. Our
results indicate that materials found to be inactive using high
temperature activation, become active under isothermal
conditions at elevated methane pressures. The isothermal
and regenerable nature of the process is a major break-
through in the development of an applicable technology for
the transformation of methane into methanol. Based on this
study, new classes of catalysts may be developed and
a practicable solution for one of chemistryꢀs unsolved
challenges found.
[10]
D
irect production of methanol from methane and oxygen is
a long standing challenge, which has not yet been solved
satisfactorily because of the higher reactivity of primary
The effect of pressure on methanol yield was established
by screening pressures up to 6 bar of oxygen and up to 37 bar
of methane (Figure 1B–D) using copper-exchanged morden-
ite (Cu-MOR; Si/Al = 6, 4.7 wt% Cu). Activation at 1 bar of
oxygen and 4508C, followed by reaction at 50 mbar of
[
1]
oxidation products compared to methane. More expensive
oxidants, such as oleum, nitric oxide (NO), or hydrogen
[
2]
[3]
[
4]
[5]
peroxide may be used. Inspired by nature, methane has
been converted by a stepwise activation method, reaction
with methane, and extraction with water (Figure 1A), using
iron-, copper- , and cobalt-exchanged zeolites. In the
case of copper-exchanged zeolites, reactivity has been
À1
methane (5% in helium) at 2008C, yielded 14.4 mmolg of
[
6]
[3,7]
[8]
[7d]
methanol, which was comparable to reported values.
Activation in a flow of oxygen at 6 bar brought about
[7b,d,g,9]
À1
ascribed to m-oxo dicopper sites
a tricopper site, which required high temperatures (greater
, and more recently to
a decrease in yield to 7.5 mmolg . Activation of Cu-MOR
[7i]
at 1 bar of oxygen, using 1, 6, or 36 bar of methane pressure,
[7b, 10]
À1
than 2808C) for its formation.
The reaction must be
yielded 45.3, 84.1, or 103.3 mmolg of methanol, respectively.
carried out at lower temperature (up to about 2008C),
These results show that larger amounts of methanol can be
extracted when Cu-MOR is in contact with methane at higher
pressures following high temperature (4508C) activation. A
6 bar gas mixture of both oxygen and methane yielded
[7a,d]
because methane is otherwise combusted.
The tedious
heating and cooling procedure performed throughout cycles
limit applicability of the technique and prolong cycling time,
consequently isothermal conversion is highly desirable.
À1
68.4 mmolg of methanol, indicating that the oxygen activa-
tion step has a weak effect on the reaction performance. In
contrast, higher methane pressure increased the yield.
Based on the high yield of methanol after high temper-
ature activation, Cu-MOR activation and methane reaction
were carried out isothermally at 2008C. At 1 bar of oxygen
over a 13 h activation period, and after exposure to 50 mbar
[
*] P. Tomkins, A. Mansouri, Dr. S. E. Bozbag, Dr. F. Krumeich,
Dr. M. B. Park, Dr. E. M. C. Alayon, Prof. Dr. J. A. van Bokhoven
Institute for Chemistry and Bioengineering, HCI E 127
Vladimir-Prelog-Weg 1, 8093 Zürich (Switzerland)
E-mail: Jeroen.vanbokhoven@psi.ch
(
5%) of methane in helium, the methanol yield was very low
À1
(0.3 mmolg ), and comparable to the yield of methanol
P. Tomkins, A. Mansouri, Dr. S. E. Bozbag, Dr. M. B. Park,
Dr. E. M. C. Alayon, Dr. M. Ranocchiari, Prof. Dr. J. A. van Bokhoven
Paul Scherrer Institut; 5232 Villigen (Switzerland)
[3]
obtained over Cu-ZSM-5 after activation with NO at 1508C.
Similar to the high temperature system, methanol yield
À1
increased to 21.2 mmolg when 6 bar of methane was used
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201511065.
during the reaction. Methanol was the only product detected
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Figure 1. Catalytic cycle and the effect of oxygen and methane pressure on the yield of methanol. A) Catalytic cycle: activation of the catalyst,
reaction with methane, and methanol extraction. Presented experiments typically include activation in oxygen flow, purging with helium, reaction
with methane (30 min), and extraction with liquid water at room temperature. B) Methanol yields after activation at 4508C and off-line extraction
at different pressures of oxygen and methane. Yields were higher at higher methane pressures, whereas higher oxygen pressures lead to a slightly
decreased methanol yield. Pressure is stated in bar (absolute). C) The dependence of activation on oxygen pressure and its effect on methanol
yield; methanol yield decreases as oxygen pressure increases. D) Dependence of methanol yield on methane pressure after 13 h activation at
À1
1
bar of oxygen and off-line extraction. The yield of methanol increased to 56.2 mmolg at a methane pressure of 37 bar.
by GC-FID after extraction. A blank experiment under
identical reaction conditions, using non-copper-exchanged
MOR, yielded very little methanol (2.2 mmolg ), proving
An increase in methane pressure resulted in higher
methanol yields (Figure 1D), whereas low yields
(0.3 mmolg ) were obtained at low partial pressure. At
À1
À1
that copper is the active species (Supporting Information,
37 bar of methane, the yield increased gradually to
56.2 mmolg . Thus, a higher methane pressure is advanta-
À1
Table S1, entry 1). The methanol yield was very low
À1
(
0.8 mmolg ; Supporting Information, Table S1, entry 2)
geous for the reaction with copper oxide after low temper-
ature activation. Figure 1D illustrates that the maximum has
not yet been reached. The reaction was also carried out
isothermally after 13 h of activation at 1 bar of oxygen and
6 bar of methane at different temperatures. The yield of
when there was no gas flow during activation, suggesting
that dehydration is essential for activity. At elevated methane
pressure (6 bar), methanol yield decreased slightly from 21.2
À1
to 16.8 mmolg upon increase of the oxygen pressure from
À1
1
to 6 bar (Figure 1C), once again confirming the weak effect
methanol at 1758C was 11.7 mmolg (Supporting Informa-
of oxygen pressure on the yield of methanol during activation.
The slight decrease in methanol yield might be caused by less
efficient removal of volatiles. A longer activation period with
tion, Table S1, entry 3), slightly less than the yield at 2008C.
This may be due to insufficient activation or variable activity
[7b]
at different reaction temperatures. Little or no production
1
bar of oxygen led to an increase in the methanol yield. Upon
of CO is revealed by mass spectrometric detection during
2
reaction with 6 bar of methane immediately after reaching the
reaction temperature under a flow of oxygen, the methanol
yield was 10.4 mmolg (Supporting Information, Figure S1).
methane interaction at 1758C, suggesting that the lower yield
is not because of over-oxidation (Supporting Information,
À1
À1
Figure S2). The yield of methanol was 18.6 mmolg at 2258C
When activation was carried out for 4 h, the yield increased to
(Supporting Information, Table S1, entry 4), virtually the
same as that at 2008C. At 2008C, the isothermal reaction gave
the highest yield overall.
À1
À1
1
9.0 mmolg , and then more slowly to 25.5 mmolg after 42 h
of activation.
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UV/Vis measurements were performed to gain insight
for the small amount of methanol formed at the lowest
into the state of copper within the zeolite under isothermal
conditions at 2008C (Supporting Information, Figure S3). The
active site of Cu-ZSM-5 after high temperature activation was
methane pressure after high temperature activation. The
m-oxo dicopper core is known to completely react after
[7b]
treatment with 50 mbar of methane.
This observation
recently described as a mono-m-oxo dicopper site, revealed by
further suggests that the m-oxo dicopper core is not formed
under isothermal conditions, as additional methanol is
produced at higher methane pressures. Condensation of
À1 [7a,e, 9a]
a band in the UV/Vis spectrum at 22700 cm .
The same
[
7b,d]
feature was found in Cu-MOR.
After activation at 2008C
there was no band in the spectrum in the region of
attached hydroxy ligands takes place at significantly higher
À1
[10]
2
0000 cm , proving the absence of this site. Furthermore,
temperatures of 300–3508C.
EXAFS revealed that the
À1
a broad band appeared at 13500 cm and perhaps a shoulder
at 16750 cm , which can be assigned to Cu species at two
aluminum sites and one aluminum site,
average coordination number of oxygen is high (ca. 3.2) at
2008C, and the presence of distant copper neighbors suggests
a significant fraction of copper oxide clusters and particles,
where an average coordination number of four is expected for
À1
2+
[7h,9b]
respectively. The
finding that a m-oxo dicopper site was not present at 2008C,
indicates that a different copper species is responsible for
activity under isothermal conditions. These sites are appa-
rently less reactive than the m-oxo dicopper core and require
higher methane pressure to achieve reduction of copper
oxide, consistent with a larger methanol yield at higher
methane pressure. Other copper structures in molecular
[10]
the closest oxygen neighbors. Generally, after activation at
2
+
2008C Cu-MOR is characterized by dehydrated Cu -species,
[10]
which are present irrespective of the gas environment.
X-ray photoelectron spectroscopy (XPS) measurements indi-
2+
cated that a significant fraction of Cu was reduced during
interaction with 6 bar of methane at 2008C after 13 h
activation at 2008C (see Supporting Information).
[11]
[7b,h]
catalysts
from methane. TEM measurements of the sample activated at
008C show finely dispersed particles (Figure 2A), some of
and solids
are reported to yield methanol
Substituting oxygen with helium during the first cycle of
À1
2
activation resulted in an identical yield (20.0 mmolg ;
which are smaller than 1 nm (Supporting Information, Fig-
ure S4), that are probably generated during activation
Supporting Information, Table S1, Entry 5). These findings
suggest that the active sites are small, dehydrated copper
oxide clusters. These clusters are less reactive than the m-oxo
dicopper site, and their reduction is only enabled by high
methane pressure. The comparable yield obtained after
helium activation shows that no oxidation of autoreduced
copper species is required and that the oxidation power stems
(
Supporting Information, Figure S5 and S6). Only reflections
of the MOR framework are visible in the PXRD pattern,
confirming the absence of larger (ꢀ 3 nm) crystalline copper
particles (Supporting Information, Figure S7). Heating of
Cu-ZSM-5 to 2008C induces a loss of bulk water in the
[10]
2+
hydration sphere of copper.
In situ XAS measurements
from initially available Cu . Cu-MOR with lower copper
showed that heating above 2508C results in rather small
loading (that is, 0.64 and 3.0 wt%) gave methanol yields of 7.9
and 33.1 mmolg , respectively (Supporting Information,
Table S1, Entries 6 and 7). The amount of methanol formed
per copper atom decreases from 0.078 to 0.071 and 0.029, at
copper loadings of 0.64, 3.0, and 4.7 wt%, respectively.
Smaller, highly dispersed copper clusters are expected to
À1
changes in the local geometric coordination sphere of copper
2+
+
and there is only a little autoreduction of Cu into Cu at
[
10]
2
008C. The m-oxo dicopper core is formed after autore-
duction of copper oxide and subsequent reaction with
[
10]
oxygen. In low amounts this species may be responsible
Figure 2. Cycling of the isothermal process at 2008C. A) TEM image of Cu-MOR activated at 2008C, showing the intact pore structure of the
À1
zeolite and small copper oxide particles. B) Methanol yields after consecutive cycles. In each cycle the yield (about 20 mmolg ) was stable. A
cycle consisted of activation for 8 h at 1 bar of oxygen, reaction with methane at 6 bar, and extraction with steam. The liquid was collected by
condensation of the reactor effluent.
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form with lower catalyst loadings. Thus, the presence of small
copper clusters enhances the activity of the catalyst.
on-line with steam) and analysis of the corresponding condensate
using GC-FID with acetonitrile as an external standard.
To prove the principle of activating additional sites at
elevated methane pressures, and to show applicability in other
systems, an investigation of Cu-ZSM-5 (Si/Al = 15, 2.8 wt%
Cu) was carried out under isothermal conditions at 2008C.
After activation in oxygen at 2008C, no spectroscopic
signature ascribed to the m-oxo species was observed (Sup-
Acknowledgements
We would like to thank Dr. Marco Taddei for PXRD
measurements and discussions. We thank Dr. Luca Artiglia
for XPS discussions. Electron microscopy was performed at
the Scientific Center for Optical and Electron Microscopy
[7b]
porting Information, Figure S8); the catalyst was inactive at
[
3]
a 10 mbar methane partial pressure (1% in argon). After
activation in oxygen for 13 h and reaction under 32 bar of
(ScopeM) of ETH Zürich. S.E.B. acknowledges the financial
À1
support of the Turkish Scientific and Technological Research
Council (TUBITAK) under the international postdoctoral
scholarship program (2219).
methane, 17.7 mmolg of methanol (Supporting Information,
Table S1, entry 8) was obtained, slightly higher than values
[7a,h]
reported involving high temperature activation.
This
finding confirms that there are different types of copper
species involved in the conversion of methane into methanol
at elevated pressure. Cu-Y (Si/Al = 2.7–3.0, 7.6 wt% Cu),
Keywords: copper zeolites · isothermal reaction · methane ·
methanol · partial oxidation
À1
which yields less than 1 mmolg of methanol after activation
How to cite: Angew. Chem. Int. Ed. 2016, 55, 5467–5471
Angew. Chem. 2016, 128, 5557–5561
at 4508C with 1 bar of oxygen followed by reaction under
[
7b]
5
1
0 mbar of methane (5% methane in nitrogen), produced
À1
0.5 mmolg (Supporting Information, Table S1, entry 9) of
methanol under isothermal conditions at 2008C and 32 bar of
methane. This initial screening suggests that not only copper
and zeolites, but also many other catalysts, might be active in
selective conversion of methane into methanol.
Repeated cycles were carried out with Cu-MOR to
demonstrate applicability of the newly developed procedure
described herein. The reaction consisted of activation for 8 h
with 1 bar of oxygen and subsequent reaction under 6 bar of
methane at 2008C, followed by on-line extraction of the
[
[
1] R. Horn, R. Schlçgl, Catal. Lett. 2015, 145, 23 – 39.
2] a) R. A. Periana, D. J. Taube, E. R. Evitt, D. G. Lçffler, P. R.
Wentrcek, G. Voss, T. Masuda, Science 1993, 259, 340 – 343;
b) R. A. Periana, D. J. Taube, S. Gamble, H. Taube, T. Satoh, H.
Fujii, Science 1998, 280, 560 – 564; c) R. Palkovits, M. Antonietti,
P. Kuhn, A. Thomas, F. Schüth, Angew. Chem. Int. Ed. 2009, 48,
6
909 – 6912; Angew. Chem. 2009, 121, 7042 – 7045.
[
[
3] T. Sheppard, C. D. Hamill, A. Goguet, D. W. Rooney, J. M.
Thompson, Chem. Commun. 2014, 50, 11053 – 11055.
4] M. H. Ab Rahim, M. M. Forde, R. L. Jenkins, C. Hammond, Q.
He, N. Dimitratos, J. A. Lopez-Sanchez, A. F. Carley, S. H.
Taylor, D. J. Willock, D. M. Murphy, C. J. Kiely, G. J. Hutchings,
Angew. Chem. Int. Ed. 2013, 52, 1280 – 1284; Angew. Chem.
2013, 125, 1318 – 1322.
5] a) R. Balasubramanian, S. M. Smith, S. Rawat, L. A. Yatsunyk,
T. L. Stemmler, A. C. Rosenzweig, Nature 2010, 465, 115 – 119;
b) M. Merkx, D. A. Kopp, M. H. Sazinsky, J. L. Blazyk, J. Müller,
S. J. Lippard, Angew. Chem. Int. Ed. 2001, 40, 2782 – 2807;
Angew. Chem. 2001, 113, 2860 – 2888.
produced methanol with water at the same temperature
À1
(
Figure 2B). A stable methanol yield of about 20 mmolg
was achieved in each cycle, in close agreement with the yield
À1
(
20.8 mmolg ) obtained by liquid extraction, demonstrating
[
the efficiency of the on-line extraction method and the
feasibility of a continuous cyclic process. This experiment also
showed that the catalyst is stable and reusable. Powder XRD
patterns (Supporting Information, Figure S7) and nitrogen
physisorption isotherms (Supporting Information, Figure S9
and Table S2) hardly changed, showing that the zeolite
framework is stable in this reaction. TEM images reveal
finely dispersed particles after activation and subsequent
reaction cycles. There was no significant change after
successive cycles compared to the activated catalyst (Sup-
porting Information, Figure S6).
In summary, these results constitute a major step towards
simplification of the methane to methanol conversion process,
opening up new opportunities for catalyst optimization and
academic and engineering avenues for one of the most
challenging reactions in chemistry and catalysis.
[6] a) G. I. Panov, V. I. Sobolev, K. A. Dubkov, V. N. Parmon, N. S.
Ovanesyan, A. E. Shilov, A. A. Shteinman, React. Kinet. Catal.
Lett. 1997, 61, 251 – 258; b) G. I. Panov, K. A. Dubkov, V. I.
Sobolev, E. P. Talsi, M. A. Rodkin, N. H. Watkins, A. A. Shtein-
man, J. Mol. Catal. A 1997, 123, 155 – 161; c) E. V. Starokon,
M. V. Parfenov, L. V. Pirutko, S. I. Abornev, G. I. Panov, J. Phys.
Chem. C 2011, 115, 2155 – 2161.
[7] a) M. H. Groothaert, P. J. Smeets, B. F. Sels, P. A. Jacobs, R. A.
Schoonheydt, J. Am. Chem. Soc. 2005, 127, 1394 – 1395; b) P. J.
Smeets, M. H. Groothaert, R. A. Schoonheydt, Catal. Today
2
005, 110, 303 – 309; c) E. M. C. Alayon, M. Nachtegaal, E.
Kleymenov, J. A. van Bokhoven, Microporous Mesoporous
Mater. 2013, 166, 131 – 136; d) E. M. Alayon, M. Nachtegaal,
M. Ranocchiari, J. A. van Bokhoven, Chem. Commun. 2012, 48,
4
2
04 – 406; e) N. Beznis, B. Weckhuysen, J. Bitter, Catal. Lett.
010, 138, 14 – 22; f) E. M. C. Alayon, M. Nachtegaal, A. Bodi,
J. A. van Bokhoven, ACS Catal. 2014, 4, 16 – 22; g) J. S. Woer-
tink, P. J. Smeets, M. H. Groothaert, M. A. Vance, B. F. Sels,
R. A. Schoonheydt, E. I. Solomon, Proc. Natl. Acad. Sci. USA
2009, 106, 18908 – 18913; h) M. J. Wulfers, S. Teketel, B. Ipek,
R. F. Lobo, Chem. Commun. 2015, 51, 4447 – 4450; i) S. Grund-
ner, M. A. C. Markovits, G. Li, M. Tromp, E. A. Pidko, E. J. M.
Hensen, A. Jentys, M. Sanchez-Sanchez, J. A. Lercher, Nat.
Commun. 2015, 6, 7546.
Experimental Section
Copper-exchanged zeolites were prepared by three-fold ion-exchange
of the sodium-zeolite using copper acetate hydrate (0.01m,
7
8 mL/gNa-zeolite). Reactions were carried out in stainless steel auto-
À1
claves: an oxygen flow (70 mLmin ) was employed during activa-
tion, followed by purging with helium, subsequent reaction with
methane, and concluding with extraction (either with liquid water or
5
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Communications
Chemie
[
[
8] a) N. Beznis, B. Weckhuysen, J. Bitter, Catal. Lett. 2010, 136, 52 –
6; b) N. V. Beznis, A. N. C. van Laak, B. M. Weckhuysen, J. H.
Bitter, Microporous Mesoporous Mater. 2011, 138, 176 – 183.
9] a) P. Vanelderen, J. Vancauwenbergh, M.-L. Tsai, R. G. Hadt,
E. I. Solomon, R. A. Schoonheydt, B. F. Sels, ChemPhysChem
[11] S. I. Chan, Y.-J. Lu, P. Nagababu, S. Maji, M.-C. Hung, M. M.
5
Lee, I. J. Hsu, P. D. Minh, J. C. H. Lai, K. Y. Ng, S. Ramalingam,
S. S. F. Yu, M. K. Chan, Angew. Chem. Int. Ed. 2013, 52, 3731 –
3735; Angew. Chem. 2013, 125, 3819 – 3823.
2
014, 15, 91 – 99; b) A. Sainz-Vidal, J. Balmaseda, L. Lartundo-
Rojas, E. Reguera, Microporous Mesoporous Mater. 2014, 185,
13 – 120.
1
Received: December 8, 2015
Revised: February 22, 2016
Published online: March 24, 2016
[
10] E. M. C. Alayon, M. Nachtegaal, A. Bodi, M. Ranocchiari, J. A.
van Bokhoven, Phys. Chem. Chem. Phys. 2015, 17, 7681 – 7693.
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