Angewandte
Chemie
DOI: 10.1002/anie.200702466
Metal-Free Catalysis
Nanocarbon as Robust Catalyst: Mechanistic Insight into Carbon-
Mediated Catalysis**
Jian Zhang, Dangsheng Su,* Aihua Zhang, Di Wang, Robert Schlögl, and CØcile HØbert
Dedicated to Süd-Chemie on the occasion ofits 150th anniversary
[
4,5]
Metal-free nanostructured elemental carbons and carbon-
based composites (e.g. C N ) have proven to be attractive
ethylbenzene.
However, this model is incorrect and with-
[
6]
out physical relevance. Therefore there is an urgent need to
describe the reaction pathway by a physically relevant model.
Ordered nanocarbon is chemically homogeneous and thus
could be seen as the most suitable platform for a mechanistic
investigation.
3
4
alternatives to conventional metal-based catalysts for several
important reactions, such as dehydrogenation of aromatic
[
1]
hydrocarbons or alkanes, Friedel–Crafts Reaction. Carbon
as the catalytic substance has significant advantages over the
conventional metal-supported systems owing to the unique
controllability of both its surface acidity/basicity and p-
electron density through surface functionalization. In a
carbon material it is the short- and long-range ordering of
atomic carbon that essentially determines the macroscopic
properties (e.g. thermal and electronic conductivities, com-
bustibility) and thus its long-term performance in any
potential industrial process. However, the lack of basic
knowledge on the nature of carbon-mediated reactions
remains the most critical restriction for the development of
carbon-based catalysis.
To date, all such investigations have been confined to pure
2
[4,7]
or mostly sp -hybridized carbons.
In particular, conven-
tional activated carbon which has long-range disorder and
high porosity has been thoroughly studied and claimed to be
the efficient catalyst for ODH reaction. Nanodiamond is an
[
4]
3
sp -hybridized carbon and carbon nanotubes are an inter-
2
3
mediate state between sp -and sp -hybridization. Each differ-
ent hybridization produces a distinct electronic structure for
the surface carbon atoms and completely different bulk
properties. Moreover, carbon nanotubes are catalytically
active but there are no reports on the structure–activity
relationships, for example, the effects of tube length, diam-
eter, or thickness.
We report herein, a novel mechanistic understanding of
the carbon-mediated ODH of ethylbenzene, in which acti-
vated carbon, nanodiamond, and carbon nanotubes with
various geometric parameters were tested. We found that the
order in the microstructure of the carbon material essentially
determines its long-time performance and only nanocarbon
could robustly catalyze the ODH reaction. In addition, our
model is supported by isotope tracer work.
For oxidative dehydrogenation (ODH) reactions, surface
quinone-type oxygen functional groups have been proposed
as the active sites and the reaction has been assumed to
[
2,3]
proceed by a redox mechanism.
However, no quantitative
description of the elementary steps, or kinetic data can be
derived from the literature. The few mechanistic studies
reported were conducted either with remarkable secondary
[
4]
oxidation and deactivation or over “impure” surfaces, for
[
2]
example, Pd- or Fe-coordinated polynaphthoquinone or
[
5]
pre-coked metal phosphates or oxides. More detailed and
reliable information is expected to be obtained over a pure
carbon surface in the kinetic reaction region. Most impor-
tantly, the Mars–van Krevelen model for redox reactions is
widely accepted based on previous work on the ODH of
Intrinsic reaction rates are measured over four commer-
cial nanocarbons: 1) nanodiamond (3–6 nm); 2) nanotube-1:
long (3–14 mm) and thick-wall (15 Æ 10 walls); 3) nanotube-2:
long (1–10 mm) and thin-wall (8 Æ 4 walls); and 4) nanotube-
3
: short (0.1–1 mm) and thin-wall (7 Æ 3 walls). Several
experiments were carried out separately by varying catalyst
pellet size while changing the catalyst loading and flow rates
to achieve different liquid hourly space velocity (LHSV). As
shown in Figure 1a, the ODH rates over nanotube-1
approach the same value when the LHSV is higher than
[*] Dr. J. Zhang, Dr. D. S. Su, Dr. A.H. Zhang, Dr. D. Wang,
Prof. R. Schlögl
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4–6, 14195 Berlin (Germany)
Fax: (+49) 30-8413-4401
À1
E-mail: dangsheng@fhi-berlin.mpg.de
8 h , indicating the absence of mass and heat diffusion
Dr. C. HØbert
artifacts.
Institut für Festkörperphysik
Technische Universität Wien
Wiedner Hauptstrasse 8–10/138, 1040 Wien (Austria)
Figures 1b and 1c show the performance of all the tested
carbons with time on stream at 723 K. For nanocarbons, major
byproduct is CO and the concentrations of ethylbenzene,
2
[
**] This study was supported by European Union as part of the
CANAPE and EnerChem projects. The ELNES part was financially
supported by USTEM. The authors thank Dr. J. Carlsson for helpful
discussion and J. Kröhnert, U. Wild, J. Mizera, and Dr. J. Delgado for
assistance with experiments.
styrene, and CO contribute a closed carbon balance of 100 Æ
2
2
%. After a short induction period, each nanocarbon stably
catalyzes styrene formation over 1500 minutes. The steady-
state styrene selectivity obtained is as high as 95% which
indicates success in depressing secondary oxidation. Only the
ordered nanocarbon materials perform outstandingly during
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2007, 46, 7319 –7323
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7319