Metal-free phenanthrenequinone cyclotrimer as an effective heterogeneous catalyst

Article abstract of DOI:10.1021/ja9046735

(Chemical Equation Presented) A phenanthrenequinone macrocyclic trimer was synthesized and used as a heterogeneous catalyst for oxidative dehydrogenation of ethylbenzene. This model molecule under comparable kinetic conditions is up to 47 times more active than extended solid catalysts including nanocarbons, metal phosphates, and oxides, confirming the hypothesis that diketone -like groups can serve as active sites.

Full text of DOI:10.1021/ja9046735

Published on Web 07/23/2009
Metal-Free Phenanthrenequinone Cyclotrimer as an Effective Heterogeneous
Catalyst
Jian Zhang,† Xuan Wang,‡ Qi Su,‡ Linjie Zhi,*,| Arne Thomas,§ Xinliang Feng,‡ Dang Sheng Su,*,†
Robert Schlo¨gl,*,† and Klaus Mu¨llen*,‡
Fritz Haber Institute of the Max Planck Society, Berlin 14195, Germany, Max Planck Institute for Polymer
Research, Mainz 55128, Germany, Max Planck Institute of Colloids and Interfaces, Golm 14476, Germany, and
National Center for Nanoscience and Technology of China, Beijing 100190, China
Received June 8, 2009; E-mail: dangsheng@fhi-berlin.mpg.de; zhilj@nanoctr.cn; acsek@fhi-berlin.mpg.de;
muellen@mpip-mainz.mpg.de
Scheme 1. Schematic Synthesis of the MCT Oligomera
Since the invention and industrialization of the Haber-Bosch
NH3 process, almost all technical heterogeneous catalysts have been
developed by empirical “trial-and-error” experiments. The enormous
progress of surface science and theory provides fundaments for
designing catalysts. A key challenge remains, however, the
identification and realization of active sites for desired functions.
Such sites are dynamical minority species in an “active matrix”.
They form through reaction of the active matrix with reactants and
are capable of regenerating themselves in each catalytic cycle.
a Compounds: (1) phenanthrenequinone, 1; (2) 3,6-dibromo-phenan-
threnequinone, 2; and (3) macrocyclic trimer of phenanthroquinone, MCT,
3. Bromination: Br2, nitrobenzene, 100 °C; Yamamoto coupling: bis(cy-
clooctadiene)nickel, cyclooctadiene, 2,2′-bipyridine, DMF, 75 °C.
For styrene synthesis from ethylbenzene, the exothermic oxidative
dehydrogenation (ODH) is an energy-saving candidate for an industrial
process.1 Carbon materials including cokes, pyrolyzed polyacrylonitrile,
molecular sieves, activated carbons, and graphite are as efficient as
metal catalysts.2-5 Recent studies revealed that carbon nanomaterials
analysis (Figure S2) and infrared spectra (data shown later). The
with graphitic structure provide high activity over a long period of
time due to the controlled homogeneity of the support structure and
the chemical uniformity of the active sites.6-8 On the basis of ex situ
and quasi in situ characterizations, the structures of active sites have
been proposed to consist of diketone- and/or ketone-like groups, which
are initially generated during the synthesis and maintained in their
oxidation states by contact with the reactant O2.9,10 Mechanistic
investigations suggest that breaking C-H bonds may be kinetically
limiting in the reaction sequences.6 Styrene desorbs from the surface,
while the produced hydrogen is oxidized to water. However, so far,
there is still no direct evidence for the chemical structure of the active
sites. We report herein the unique activity of a phenanthrenequinone
based cyclotrimer with R-diketone subunits, serving as a model catalyst
with excellent performance and as a molecular analogue to the carbon-
based systems. A potential approach toward tailoring the active sites
in a catalyst is thus demonstrated in the area of heterogeneous catalysis.
The macrocyclic trimer (MCT, 3) was synthesized via a
nickel(0)-mediated Yamamoto-type coupling reaction,11,12 as de-
picted in Figure 1. After bromination under light illumination and
subsequent recrystallization,13 the commercially available phenan-
threnequinone, 1, was converted into the 3,6-dibromophenanthrene-
quinone, 2, an orange needle-like crystal. Further Yamamoto
coupling reaction of compound 2 and subsequent purification (see
Supporting Information) produced the MCT oligomer as an
insoluble powder with an overall yield of 84%. The poor solubility
of the oligomer precluded direct structural verification by solution
1H and 13C NMR. The macrocyclic structure was evidently
confirmed by MALDI-TOF mass spectrometric characterization,
while no obvious signal of other oligomers was detected (Figure
S1).14 The R-diketone structure was further supported by elemental
high yield of MCT in the synthesis may arise from the intrinsic
angle of the phenanthrenequinone unit, as well as from the poor
solubility in common organic solvents serving as the driving force
during the crystallization process. A similar analysis has been
reported for the synthesis of the macrocyclic trimer of dialkyl
substituted phenanthrene.11,12 As the consequence of the macrocycle
formation, the melting point of the compound increased above 410
°C after trimerization. Such a high melting point certainly benefits
the application of MCT oligomer as a heterogeneous catalyst.
The performance of MCT oligomer was tested in a quartz microre-
actor under atmospheric pressure. Figure 1A shows the catalytic
function of MCT at 350 °C along with the time-on-stream. The sample
was immediately active and quickly reached a steady state, in which
the reaction rate and styrene selectivity remained above 0.34 mmol
g-1 h-1 and 86%, respectively. Increasing the O/C ratio from 2 to 5
enhanced the reaction rate to 0.44 mmol g-1 h-1 at constant styrene
selectivity (83%). COx was the major byproduct. The time-independent
carbon balance of 100 ( 4% indicated oxidative stability of the
hydrocarbon catalyst. A comparison in performance between MCT
and nonmolecular catalysts15-22 is shown in Figure 1b, in which the
activity was represented by the amount of styrene generated per square
meter of BET surface area per hour (µmol m-2 h-1). MCT exhibits
already at low temperature the highest activity of 84.5 µmol m-2 h-1,
which is up to 47 times the activity of other catalysts including metal
phosphates and oxides.
Kinetic parameters were derived in a differential reactor without
mass and heat transport limitations (Figure S3). The reaction rate
(R) displays (Figure 1c) a positive dependency on the partial
pressures of ethylbenzene and O2 (Pi), revealing reaction orders of
0.39 and 0.25 for ethylbenzene and O2, respectively. An activation
energy (Ea) of 77.4 kJ mol-1 was calculated from the slope of an
Arrhenius plot (Figure 1d). All kinetic parameters are close to those
determined from carbonaceous materials in previous studies,6
confirming the similarity of both mechanism and structure of active
† Fritz Haber Institute of the Max Planck Society.
‡ Max Planck Institute for Polymer Research.
§ Max Planck Institute of Colloids and Interfaces.
| National Center for Nanoscience and Technology of China.
9
11296 J. AM. CHEM. SOC. 2009, 131, 11296–11297
10.1021/ja9046735 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
TPO and related DSC signals remained unchanged, but the positions
shifted slightly being consistent with a modification of crystallinity
of the material during thermal stress in the reaction. The IR spectra
(Figure 2d)23,24 reveal retention of the terminating geometry (CdO
bonds above 1550 cm-1 and CsH ones below 1000 cm-1) and
only slight modification of the crystal packing as felt sensitively25
by the CsC stretch vibrations between 1000-1500 cm-1 in
agreement with the thermal analysis data. A negligible change in
long-range ordering is observed in the XRD patterns (Figure S4)
of MCT before and after catalysis.
In summary, we have shown that MCT is a suitable model to
study ODH catalysis under conditions being rather severe for a
molecular catalyst. Its outstanding performance confirms the
previous hypothesis that the carbon-catalyzed ODH process may
be mediated by diketone- and/or ketone-like functional groups.
Potential application is possible by supporting the molecular active
component in submonolayer amounts.
Figure 1. (a) Reaction rate (mmol g-1 h-1) and styrene selectivity on MCT
oligomer. Conditions: 20 mg, 2.1%EB, O/EB ) 2-5, 12.5 mL min-1, 350
°C. (b) Comparison of activities with typical catalysts in literature. Dependencies
of reaction rate on partial pressures of each reactant (c) and temperature (d).
Acknowledgment. This work was supported by the Max Planck
Society (EnerChem), the Deutsche Forschungs-gemeinschaft (SFB
625), and NAIMO (NMP4-CT-2004-500355) projects. The authors
gratefully thank Dr. J. J. Delgado and Mrs. G. Weinberg for
experimental assistance.
sites (Table S1). Under the same conditions, MCT gives an areal
rate 5-9 times those of the nanocarbons. The superior activity can
be thus related to the abundance of diketone groups for styrene
formation. The nonzero reaction orders for both reactants indicate
the matched rates for the organic transformation and the oxidation
of hydrogen in contrast to systems where lattice oxygen decouples
the regeneration of the active site from the organic transformation.
The MCT sample after ODH reaction was studied by using
nitrogen physisorption (BET surface area 5 m2 g-1), scanning
electron microscopy (SEM) with EDX (only C, O, Figure S2),
temperature-programmed oxidation/thermogravimetric analysis (TPO/
TG), and transmission infrared spectroscopy. The solid-state
MALDI-TOF mass spectrometric characterization on fresh and
reacted samples shows that the molecular structure changed to a
negligible degree (Figure S1). No obvious difference in the color
of the substance can be found after reaction. TPO results reveal a
weight loss less than 1% at the reaction temperature (Figure 2a) to
indicate the oxidative stability of the molecule. The profiles of the
produced CO2 and H2O (Figure 2b) show no additional features
that could point to additional carbon structures from coke deposition
or decomposition of the molecular catalyst. The peak profiles of
Supporting Information Available: Synthesis, reaction, and char-
acterization details. This material is available free of charge via the
Internet at http://pubs.acs.org.
References
(1) Cavani, F.; Trifiro`, F. Appl. Catal., A 1995, 133, 219–239.
(2) Fiedorow, R.; Przystajko, W.; Sopa, M.; Lana, I. G. D. J. Catal. 1981, 68,
33–41.
(3) Emig, G.; Hofmann, H. J. Catal. 1983, 84, 15–26.
´
(4) Pereira, M. F. R.; Orfa˜o, J. J. M.; Figueiredo, J. L. Appl. Catal., A 2001,
218, 307–318.
(5) Mestl, G.; Maksimova, N. I.; Keller, N.; Roddatis, V. V.; Schlo¨gl, R. Angew.
Chem., Int. Ed. 2001, 40, 2066–2068.
(6) Zhang, J.; Su, D. S.; Zhang, A. H.; Wang, D.; Schlo¨gl, R.; He´bert, C. Angew.
Chem., Int. Ed. 2007, 46, 7319–7323.
(7) Zhang, J.; Liu, X.; Blume, R.; Zhang, A. H.; Schlo¨gl, R.; Su, D. S. Science
2008, 322, 73–77.
(8) Resasco, D. E. Nat. Nanotechnol. 2008, 3, 708–709.
´
(9) Pereira, M. F. R.; Orfa˜o, J. J. M.; Figueiredo, J. L. Appl. Catal., A 2001,
184, 153–160.
(10) Macia´-Agullo´, J. A.; Cazorla-Amoro´s, D.; Linares-Solano, A.; Wild, U.;
Su, D. S.; Schlo¨gl, R. Catal. Today 2005, 102-103, 248–253.
(11) Yang, C.; Scheiber, H.; List, E. J. W.; Jacob, J.; Mu¨llen, K. Macromolecules
2006, 39, 5213–5221.
(12) Staab, H. A.; Bra¨unling, H. Tetrahedron Lett. 1965, 6, 45–49.
(13) Bhatt, M. V. Tetrahedron 1964, 20, 803–821.
(14) Although the direct solution characterization of MCT was restricted, the
functional soluble macrocycle derivatives through the condensation reaction
of MCT with 1,2-diamino-4,5-dialkylbenzene were synthesized in good
yields, enabling the NMR tests and thus indirectly proving its structure
and purity. Takase, M.; Feng, X.; Mu¨llen, K., unpublished.
(15) Reddy, B. M.; Lakshmanan, P.; Loridant, S.; Yamada, Y.; Kobayashi, T.; Lo´pez-
Cartes, C.; Rojas, T. C.; Ferna´ndez, A. J. Phys. Chem. B 2006, 110, 9140–
9147.
(16) Bautista, F. M.; Campelo, J. M.; Luna, D.; Marinas, J. M.; Quiro´s, R. A.;
Romero, A. A. Appl. Catal., B 2007, 70, 611–620.
(17) Yoo, J. S. Appl. Catal., A 1996, 142, 19–29.
(18) Wang, L. F.; Zhang, J.; Su, D. S.; Ji, Y. Y.; Cao, X. J.; Xiao, F.-S. Chem.
Mater. 2007, 19, 2894–2897.
(19) Li, P.; Li, T.; Zhou, J.-H.; Sui, Z.-J.; Dai, Y.-C.; Yuan, W.-K.; Chen, D.
Microporous Mesoporous Mater. 2006, 95, 1–7.
´
(20) Pereira, M. F. R.; Figueiredo, J. L.; Orfa˜o, J. J. M.; Serp, P.; Kalck, P.;
Kihn, Y. Carbon 2004, 42, 2807–2813.
(21) Delgado, J. J.; Su, D. S.; Rebmann, G.; Keller, N.; Gajovic, A.; Schlo¨gl,
R. J. Catal. 2006, 244, 126–129.
(22) Delgado, J. J.; Vieira, R.; Rebmann, G.; Su, D. S.; Keller, N.; Ledoux,
M. J.; Schlo¨gl, R. Carbon 2006, 44, 809–812.
(23) Crowley, P. J.; Haendler, H. M. Inorg. Chem. 1962, 1, 904–909.
(24) Floriani, C.; Henzi, R.; Calderazzo, F. J. Chem. Soc., Dalton Trans. 1972,
23, 2640–2642.
Figure 2. (a) TG/DTG/DSC profiles and (b) evolution of CO2 and H2O
during TPO/TG of MCT samples before and after ODH reaction. Conditions:
10 mg, 21%O2 in Ar, 100 mL min-1, 5 K min-1. (c) Infrared spectra of
fresh MCT sample and the one after ODH.
(25) Bauschlicher, C.W., Jr. Chem. Phys. 1998, 233, 29–34.
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