Angewandte
Communications
Chemie
Radical Reactions
Bent Carbon Surface Moieties as Active Sites on Carbon Catalysts for
Phosgene Synthesis
Navneet K. Gupta, Anastasia Pashigreva, Evgeny A. Pidko,* Emiel J. M. Hensen,
Leslaw Mleczko, Stefan Roggan,* Erika E. Ember,* and Johannes A. Lercher*
Abstract: Active sites in carbon-catalyzed phosgene synthesis
from gaseous CO and Cl2 have been identified using C60
fullerene as a model catalyst. The carbon atoms distorted
tration of active sites, but also to minimize production of
chlorinated side products.
In a first step, transmission electron microscopy has been
used to exemplify the characteristic features of a typical
activated carbon catalyst, that is, the ample presence of bent
carbon layers, semi-spheres, and carbon cages (Figure 1).
Bending of the carbon structures is usually induced by the
presence of 5-membered rings or the combination of 5- and 7-
2
from sp coordination in non-planar carbon units are con-
cluded to generate active Cl . Experiments and density func-
2
tional theory calculations indicate the formation of a surface-
bound [C ···Cl ] chlorine species with radical character as key
6
0
2
intermediate during phosgene formation. It reacts rapidly with
physisorbed CO in a two-step Eley–Rideal-type mechanism.
[5]
membered rings (Stone–Wales defects).
T
he synthesis of phosgene (COCl ) by a catalyzed reaction
2
between gaseous chlorine (Cl ) and carbon monoxide (CO) is
2
one of the oldest, most reliable, and efficient large-scale
[1]
chemical processes. Since catalytic COCl2 synthesis has
[2]
replaced earlier processes based on photolysis, the activated
carbon used as catalyst has remained basically unchanged.
While several kinetic studies on phosgene formation have
[
3]
been reported, mechanistic insight into carbon-catalyzed
COCl synthesis and the associated active sites is largely
2
[
4]
lacking. This may in part be associated with the diversity and
chemical complexity of carbon surface structures rendering
the identification of the active sites on carbon catalysts highly
challenging.
Figure 1. HR-TEM image of an activated carbon catalyst for COCl2
synthesis from CO and Cl showing curled carbon layers (left) and
carbon semi-spheres that resemble fullerene-type structures (right).
To initiate the development of a new generation of stable
and highly active catalysts, the nature of active sites and of the
elementary steps of COCl synthesis from CO and Cl on
2
2
2
activated carbon have been studied. Insight is critical not only
to optimize catalytic properties by maximizing the concen-
The diameters of these bent structural units (Figure 1,
blue bar) approximate the diameter of fullerenes, which range
[6,7]
[
*] N. K. Gupta, Dr. A. Pashigreva, Dr. E. E. Ember, Prof. J. A. Lercher
Technische Universität München, Department of Chemistry and
Catalysis Research Center
from 0.71 (C ) to 1.2 nm (C176).
The structural analogy
60
between the curved surfaces of activated carbon and the
molecular structure of fullerenes stimulated us, in conse-
quence, to select fullerenes as well-defined model catalysts.
These models lack defect sites, edges, and functional groups. It
Lichtenbergstrasse 4, 85748 Garching (Germany)
E-mail: erika.ember@mytum.de
allows the mechanism of carbon-catalyzed Cl activation and
2
Dr. E. A. Pidko, Prof. E. J. M. Hensen
conversion with CO to be explored in the absence of various
more reactive but less defined carbon species.
Laboratory of Inorganic Materials Chemistry, Schuit Institute of
Catalysis, Eindhoven University of Technology
PO Box 513, 5600 MB Eindhoven (The Netherlands)
Table 1 compares DFT-computed reaction energies for
COCl formation and the Cl addition to unsaturated moieties
Dr. E. A. Pidko
2
2
Institute for Complex Molecular Systems
Eindhoven University of Technology
PO Box 513, 5600 MB Eindhoven (The Netherlands)
E-mail: e.a.pidko@tue.nl
in several model structures with distinct structural character-
istics, that is, a planar C H graphene model, 2-butene, and
32
14
C . Only in the case of C , the reaction energy is lower than
60
60
that of the reaction of Cl with CO. The reaction of Cl with
2
2
Prof. L. Mleczko, Dr. S. Roggan
Technology Development, Bayer Technology Services GmbH
graphene is highly endothermic, and addition to graphene has
[8]
not been observed. Chlorination of butene is strongly
exothermic, suggesting the formation of strong and stable
CÀCl bonds. We hypothesize, therefore, that the curved
5
1368 Leverkusen (Germany)
E-mail: stefan.roggan@bayer.com
structure of the conjugated fullerene p-system provides the
appropriate balance between the loss of conjugation energy
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1728 –1732