Direct methane oxidation over Pt-modified nitrogen-doped carbons

Article abstract of DOI:10.1039/c2cc36232e

Nitrogen-doped carbons derived from biomass precursors were modified with Pt²⁺ and successfully tested as solid catalysts in the direct oxidation of methane in fuming sulfuric acid. Remarkably, the catalytic performance was found to be substantially better than the Pt-modified Covalent Triazine Framework (CTF) system previously reported, although deactivation is more pronounced for the biomass derived catalyst supports.

Full text of DOI:10.1039/c2cc36232e

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Direct methane oxidation over Pt-modified
nitrogen-doped carbons†
Cite this: DOI: 10.1039/c2cc36232e
a
b
a
Mario Soorholtz, Robin J. White,z* Tobias Zimmermann,
Received 28th October 2011,
Accepted 12th November 2012
b
b
ac
Maria-Magdalena Titirici, Markus Antonietti, Regina Palkovits* and
a
Ferdi Sch u¨ th*
DOI: 10.1039/c2cc36232e
www.rsc.org/chemcomm
1
,10
Nitrogen-doped carbons derived from biomass precursors were these approaches.
Alternative pathways for oxy-functionalization
2+
12
modified with Pt and successfully tested as solid catalysts in the of C–H bonds were recently reviewed. Nevertheless, currently
10,12
4,13–16
17,18
direct oxidation of methane in fuming sulfuric acid. Remarkably, neither homogeneous
the catalytic performance was found to be substantially better catalysts allow a viable industrial application.
than the Pt-modified Covalent Triazine Framework (CTF) system Recently, we reported the development of a solid catalyst,
previously reported, although deactivation is more pronounced for which mimicked the molecular Periana system by coordinatively
nor heterogeneous
or enzymatic
2
+
the biomass derived catalyst supports.
binding a Pt species to a nitrogen-rich Covalent Triazine Frame-
work (CTF), acting as a solid ligand. This material achieved
19
The direct utilization of methane via C–H activation remains a turnover numbers comparable to the molecular Periana catalyst
challenge as the development of efficient catalysts with sufficient with stable catalytic performance over at least five recycling steps.
activity and selectivity is hindered by the high binding energy of the Extending this approach to poly(benzimidazole) materials
1–4
methane C–H bond. Thus, a viable methane-to-liquid process resulted in even higher TON, but significant Pt leaching was
20
still suffers from ineffective catalytic systems. The C–H activation of observed. However, these results point towards strategies for
hydrocarbons by transition metal complexes presents a promising further material development.
5
–7
pathway. One of the most active catalytic systems is that reported
Herein, we report on the progress of catalyst development
8–10
by Periana and co-workers.
The dichlorobipyrimidyl platinum(II) for methane oxidation in the Periana system. Specifically, we
complex (i.e. Pt(bpym)Cl ) performs the catalytic oxidation in present a novel material that has higher activity than the
2
fuming or concentrated sulfuric acid, achieving high yields of recently published Pt@CTF system, although more pronounced
methanol with selectivities Z 90%. Methylbisulfate is initially deactivation is observed.
formed, which can then be hydrolyzed to methanol in high
The novel solid catalysts reported here are based on nitrogen-
overall yields. In this process the solvent is used, simulta- doped carbons (NDC), which have gained significant interest in
1
0,11
21
neously, as both a protecting and an oxidizing agent.
The research due to their thermal/oxidation stability. We have recently
limited catalytic activity as well as difficult product and catalyst reported simple and low-cost routes to produce nitrogen-enriched
22–25
separation pose challenges regarding industrial application of carbonaceous scaffolds derived from biomass resources.
The
hydrothermal carbonization and further thermal treatment of
chitin-based precursors (i.e. the crustacean exoskeleton of lobster)
generates highly porous nitrogen-doped carbons (ExLOB) with a
a
b
c
Max-Planck-Institut f u¨ r Kohlenforschung, Kaiser-Wilhelm-Platz 1,
5470 M u¨ lheim an der Ruhr, Germany. E-mail: schueth@mpi-muelheim.mpg.de;
4
2
ꢀ1
Fax: +49 208 306 2995; Tel: +49 208 306 2373
high surface area (>400 m g ) and tunable chemical properties
Scheme 1). In this study, such NDC materials were coordinatively
Max Planck Institute of Colloids and Interfaces, Am M u¨ hlenberg 1,
(
1
4476 Potsdam, Germany. E-mail: robin.white@mpikg.mpg.de;
Fax: +49 331 567 9502; Tel: +49 331 567 9508
RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany.
E-mail: palkovits@itmc.rwth-aachen.de; Fax: +49 241-80-22177;
Tel: +49 241 80 26497
†
Electronic supplementary information (ESI) available: Detailed experimental
procedures, N
catalytic data. See DOI: 10.1039/c2cc36232e
Current address: Technische Universit ¨a t Berlin, Fakult ¨a t II, Institut f u¨ r
2
sorption data, N 1s and Pt 4f XPS data, TGA, SEM and additional
‡
Chemie, Sekretariat BA 2, Hardenbergstraße 40, 10623 Berlin, Germany. E-mail:
robin.j.white@tu-berlin.de; Fax: +49 30 314 29271; Tel: +49 30 314 28767.
Scheme 1 Preparation of coordinatively modified Pt@NDC materials derived
from crustacean exoskeleton of lobsters (ExLOB).
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2+
modified with Pt species. Elemental analysis (EA) confirmed a Table 1 Conversions, yields, selectivities to methanol (methylbisulfate) and
catalytic activities (TOFs) achieved under given reaction conditions
stable Pt loading of B6 wt% (Table S4, ESI†), while XPS investiga-
tions indicated metal coordination similar to Pt@CTF (Fig. S2 and
S3, ESI†). Additionally, thermal stability and robustness towards
acidic solvents make such materials very interesting as a possible
solid catalyst in the Periana system.
a
ꢀ1
b
ꢀ1
Entry Catalyst
X (%) Y (%) S (%) TOF (h
)
TOF (h
)
1
2
3
Pt@CTF
Pt(bpym)Cl
7.0
17.9
6.0
17.2
31.7
85.4
96.3
174
912
94.0 2074
95.2 1938
91.6 1826
233
779
1227
2
Pt@ExLOB-900 33.8
1st run)
Pt@ExLOB-900 18.5
2nd run)
Pt@ExLOB-900 6.0
3rd run)
(
Catalytic testing was initially performed in a simple batch
autoclave set-up, using reaction conditions similar to those of
4
5
a
17.6
5.6
1516
1802
b
(
8
Periana and co-workers (ESI†). This set-up was suitable to inves-
(
tigate material stability and to screen catalytic materials in terms of
methanol productivities. However, due to different ranges of
achieved conversion and conversion before reaching the reaction
temperature it could not be used for a true comparison of catalytic
activities. Therefore, the most promising catalyst in terms of
stability and activity, i.e. Pt-modified ExLOB carbonized at 900 1C
Determined from a pressure drop from 69 to 67.5 bar (ESI). Deter-
mined from the amount of methanol produced within 30 minutes.
differently active catalysts, different pressure drops are reached over
the 30 minutes studied, which influences the rate by, e.g. pressure
dependency of reaction rates and different consumption of sulfur
trioxide. Nevertheless, deviations between the two methods did not
exceed 40% and typically were much smaller.
Fig. 1 shows the comparison of different catalysts containing
the same amount of platinum. The achieved pressure curves
indicate significant differences in activity between the catalytic
materials. Calculated reaction rates from the pressure drop
from 69 to 67.5 bar confirm this observation and show that the
turnover frequency of Pt@ExLOB-900 is substantially higher
(Pt@ExLOB-900), was tested in a modified set-up to investigate the
catalytic behaviour in detail in order to determine the reaction
rates, expressed as apparent turnover frequencies (TOFs).
Pt-modified CTF as well as the molecular Pt(bpym)Cl catalyst
2
were tested under identical conditions as references. The set-up
consisted of a Hastelloy autoclave that was filled partially with
oleum and catalyst, and then flushed with argon. After heating
the autoclave to reaction temperature, preheated methane from
a second autoclave was added to start the reaction (ESI†). The
pressure drop as a result of conversion of methane to methyl-
bisulfate could be monitored via pressure indicators, and this
pressure drop provides a measure of the reaction rate. Accord-
ingly, pressure-time plots show autogenous pressure (B10 bar)
of oleum and a sharp increase of pressure due to the addition
of preheated methane (Fig. 1). Methane oxidation was carried
out for 30 minutes before the reactor was quenched in a water
bath. Subsequently, gas phase products were quantified by
FT-IR, while the liquid phase was filtrated and hydrolysed to
obtain methanol. Using this methodology, the carbon balance
for the autoclave set-up could be closed within at least 95%,
with carbon dioxide as the only by-product resulting from
overoxidation of methylbisulfate or methane. Turnover frequencies
could be calculated either from a pressure drop from 69 to 67.5 bar
or as an integral value for the first 30 minutes from the amount
of methanol produced (ESI†). The first method is preferred,
since this allows comparison of performance in the same range
of conversion. The latter method gives less accurate values, since for
2
than that of the molecular Pt(bpym)Cl catalyst (Table 1).
Remarkably, the novel solid catalyst is significantly more active
than previously developed Pt-modified CTF materials, indicat-
ing a strong influence of the carbon support. However, the
structural difference at the active center is difficult to access
especially for solid single-site catalysts since the acquisition of
reliable information at the atomic level is challenging.
In addition, recycling experiments were performed to investigate
the catalyst stability under the harsh reaction conditions. Pt@Ex-
LOB-900 was used in three subsequent runs under identical reaction
conditions. However, catalyst amounts in subsequent runs were
lower, due to losses by filtration and elemental analysis after each
experiment, which led to decreasing conversions and thus decreas-
ing pressure drops in subsequent runs (Fig. 2a). Pt@ExLOB-900
ꢀ
1
achieved a high TOF of 2074 h in the first run, which decreased to
826 h for the third run (Table 1 and ESI†). Thus, decreasing
ꢀ1
1
Fig. 1 Pressure–time plots for catalyst comparison (reaction conditions: 10 mmol
Pt equivalents catalyst, 215 1C, 1000 rpm, 15 mL oleum (20 wt% SO
3
), 30.4 mL
Fig. 2 (a) Pressure–time plots for Pt@ExLOB-900 recycling experiments. (b)
Pressure–time plots for fresh and recycled catalyst (Pt@ExLOB-900 (3rd run)).
Hastelloy autoclave).
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TOFs from recycling experiments indicate catalyst deactivation. activity in comparison to the molecular catalyst and the loss of
This becomes more obvious when comparing pressure drops of activity are still under investigation. However, the remarkable
Pt@ExLOB-900 (3rd run) and the same amount of fresh catalyst catalytic performance may provide new insight for further
(
Fig. 2b). Since no leaching of Pt species and no significant targeted material development which in turn could bring
changes in porosity could be observed the reason for deactiva- catalytic methane utilization closer to technical feasibility.
tion remains as yet unclear (ESI†). Deactivation may result from Financial support by the ‘‘ENERCHEM’’ project house of the
the recycling procedure, i.e. from washing of the solid catalyst, or Max Planck Society and the Fonds der Chemischen Industrie
from changes of the catalyst during the reaction. It should also (Kekul ´e -stipend to Tobias Zimmermann) is gratefully acknowl-
be mentioned that activity resulting from homogeneous species edged. We thank Dr C. Weidenthaler for XPS measurements
in solution under reaction conditions cannot fully be excluded, and discussions.
since hot filtration tests are so challenging under the applied
reaction conditions that they could not as yet be performed.
However, as Pt loading does not change during recycling experi-
Notes and references
ments, irreversible leaching can be excluded.
1 H. Arakawa, M. Aresta, J. N. Armor, M. A. Barteau, E. J. Beckman,
A. T. Bell, J. E. Bercaw, C. Creutz, E. Dinjus, D. A. Dixon, K. Domen,
D. L. DuBois, J. Eckert, E. Fujita, D. H. Gibson, W. A. Goddard,
D. W. Goodman, J. Keller, G. J. Kubas, H. H. Kung, J. E. Lyons,
L. E. Manzer, T. J. Marks, K. Morokuma, K. M. Nicholas, R. Periana,
L. Que, J. Rostrup-Nielson, W. M. H. Sachtler, L. D. Schmidt, A. Sen,
G. A. Somorjai, P. C. Stair, B. R. Stults and W. Tumas, Chem. Rev.,
Furthermore, it was observed that TOFs were dependent on
catalyst concentration. Specifically, TOFs increased with a
decreasing amount of Pt. Mass transfer limitation of methane
into the solution could be excluded (Fig. S7, ESI†), and the
reason for this behaviour remains unclear. Thus, the TOFs
during recycling experiments are not fully comparable, since
catalyst deactivation and rate dependency on the amount of
platinum counteract each other. Since the TOF decreased
although less platinum was present in subsequent runs,
catalyst deactivation is probably even more pronounced than
expressed by the TOFs presented in Table 1.
2001, 101, 953–996.
2 J. H. Lunsford, Catal. Today, 2000, 63, 165–174.
3
4
R. H. Crabtree, Dalton Trans., 2001, 2437–2450.
M. C. Alvarez-Galvan, N. Mota, M. Ojeda, S. Rojas, R. M. Navarro and
J. L. G. Fierro, Catal. Today, 2011, 171, 15–23.
5 A. E. Shilov and G. B. Shul’pin, Chem. Rev., 1997, 97, 2879–2932.
6
A. E. Shilov and G. B. Shul’pin, Activation and catalytic reactions of
saturated hydrocarbons in the presence of metal complexes, Kluwer,
Dordrecht, 2000.
The very high selectivity to methylbisulfate or methanol,
respectively, is unaffected by catalyst deactivation (Table 1). More
precisely, a selectivity between 92 and 95% at high reaction rates and
7 J. A. Labinger and J. E. Bercaw, Nature, 2002, 417, 507–514.
8
R. A. Periana, D. J. Taube, S. Gamble, H. Taube, T. Satoh and
H. Fujii, Science, 1998, 280, 560–564.
9
D. Wolf, Angew. Chem., Int. Ed., 1998, 37, 3351–3353.
conversions was observed, which emphasizes the great advantage of 10 B. L. Conley, W. J. Tenn, K. J. H. Young, S. Ganesh, S. Meier,
V. Ziatdinov, O. Mironov, J. Oxgaard, J. Gonzales, W. A. Goddard
the Periana system for methane activation in comparison to other
and R. A. Periana, Methane Functionalization, Wiley-VCH Verlag
GmbH & Co. KGaA, 2006.
13,14
approaches of heterogeneous catalytic systems.
One might expect that the synthesis of the catalysts would 11 I. H. Hristov and T. Ziegler, Organometallics, 2003, 22, 1668–1674.
1
2 J. R. Webb, T. Bola n˜ o and T. B. Gunnoe, ChemSusChem, 2011, 4,
7–49.
not be reproducible due to the fact that natural raw materials
are used for the preparation. The reproducibility of catalytic
results was thus carefully examined for Pt@ExLOB-900 and
was found to be excellent (within a few percent deviation in
produced methanol) for catalysts derived from the same batch
of lobster shells. In addition, catalysts were prepared from a
NDC support, which had been obtained from lobster shells
from different regions, and also the produced amount of
methanol of these samples deviates by less than ꢁ5% from
that of the other samples with no Pt leaching observed. The
natural variability of the raw materials thus does not seem to be
3
13 B. Lee, Y. Sakamoto, D. Hirabayashi, K. Suzuki and T. Hibino,
J. Catal., 2010, 271, 195–200.
1
1
4 B. Hereijgers and B. Weckhuysen, Catal. Lett., 2011, 1–6.
5 C. Hammond, M. M. Forde, M. H. Ab Rahim, A. Thetford, Q. He,
R. L. Jenkins, N. Dimitratos, J. A. Lopez-Sanchez, N. F. Dummer,
D. M. Murphy, A. F. Carley, S. H. Taylor, D. J. Willock,
E. E. Stangland, J. Kang, H. Hagen, C. J. Kiely and G. J. Hutchings,
Angew. Chem., Int. Ed., 2012, 124, 5219–5223.
1
6 M. M. Forde, B. C. Grazia, R. Armstrong, R. L. Jenkins, M. H. A.
Rahim, A. F. Carley, N. Dimitratos, J. A. Lopez-Sanchez, S. H. Taylor,
N. B. McKeown and G. J. Hutchings, J. Catal., 2012, 290, 177–185.
7 F. E. Zilly, J. P. Acevedo, W. Augustyniak, A. Deege, U. W. H ¨a usig and
M. T. Reetz, Angew. Chem., Int. Ed., 2011, 50, 2720–2724.
1
a major problem, although results for lobsters from two different 18 M. Merkx, D. A. Kopp, M. H. Sazinsky, J. L. Blazyk, J. M u¨ ller and
S. J. Lippard, Angew. Chem., Int. Ed., 2001, 40, 2782–2807.
9 R. Palkovits, M. Antonietti, P. Kuhn, A. Thomas and F. Sch u¨ th,
Angew. Chem., Int. Ed., 2009, 48, 6909–6912.
regions certainly do not allow full generalization.
1
In conclusion, hydrothermal treatment and subsequent
thermal carbonization of nitrogen containing biomass-derived 20 R. Palkovits, C. von Malotki, M. Baumgarten, K. M u¨ llen, C. Baltes,
M. Antonietti, P. Kuhn, J. Weber, A. Thomas and F. Sch u¨ th, Chem-
SusChem, 2010, 3, 277–282.
1 D. Mang, H. P. Boehm, K. Stanczyk and H. Marsh, Carbon, 1992, 30,
391–398.
2 R. J. White, N. Yoshizawa, M. Antonietti and M.-M. Titirici, Green
Chem., 2011, 13, 2428–2434.
3 R. J. White, M. Antonietti and M. M. Titirici, J. Mater. Chem., 2009,
19, 8645–8650.
4 N. Baccile, M. Antonietti and M. M. Titirici, ChemSusChem, 2010, 3,
precursors, followed by Pt coordination, gives access to novel
highly active solid catalysts for direct methane oxidation via
2
2
2
2
2
C–H activation. The initial catalytic activity of this material is
superior to the molecular benchmark originally described by
Periana and significantly better than that of the previously
reported solid catalysts. Recycling experiments revealed that
this novel solid looses activity to some extent. This deactivation,
however, is not associated with loss of platinum, since the
platinum leaching is negligible. The reason for the high initial
246–253.
5 C. Falco, M. Sevilla, R. J. White, R. Rothe and M.-M. Titirici,
ChemSusChem, 2012, 5, 1834–1840.
This journal is c The Royal Society of Chemistry 2012
Chem. Commun.