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Angewandte
Communications
Chemie
Alkane Oxyhalogenation
Hot Paper
Halogen-Dependent Surface Confinement Governs Selective Alkane
Functionalization to Olefins
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´
Guido Zichittella , Matthias Scharfe , BegoÇa Puꢀrtolas, Vladimir Paunovic, Patrick Hemberger,
Andras Bodi, Lꢁszlꢂ Szentmiklꢂsi, Nfflria Lꢂpez, and Javier Pꢀrez-Ramírez*
Abstract: The product distribution in direct alkane function-
alization by oxyhalogenation strongly depends on the halogen
of choice. We demonstrate that the superior selectivity to olefins
over an iron phosphate catalyst in oxychlorination is the
consequence of a surface-confined reaction. By contrast, in
oxybromination alkane activation follows a gas-phase radical-
chain mechanism and yields a mixture of alkyl bromide,
cracking, and combustion products. Surface-coverage analysis
of the catalyst and identification of gas-phase radicals in
operando mode are correlated to the catalytic performance by
a multi-technique approach, which combines kinetic studies
with advanced characterization techniques such as prompt-
gamma activation analysis and photoelectron photoion coin-
cidence spectroscopy. Rationalization of gas-phase and surface
contributions by density functional theory reveals that the
molecular level effects of chlorine are pivotal in determining
the stark selectivity differences. These results provide strategies
for unraveling detailed mechanisms within complex reaction
networks.
reactions for direct hydrocarbon upgrading, which can
provide design criteria for active, selective, and stable
catalysts. In the case of light-alkane functionalization—
generally requiring high temperatures and/or aggressive
reactants—the level of complexity is further amplified by
the possibility of gas-phase reaction pathways, in which highly
reactive radical species and/or radical initiators are liberated
from the surface to generate desired and undesired prod-
ucts.[2] A high level of selectivity control could be achieved if
alkane activation were confined on a catalyst surface. How-
ever, to unravel reaction pathways, as well as reactive
intermediates in gaseous and solid phases, and to model
complex and dynamic surfaces at the atomic level, the
combined use of strong experimental evidence with advanced
theoretical approaches is required. Catalytic oxychlorination,
which involves the reaction of an alkane with HCl and O2, has
recently demonstrated selective (ꢀ 95%) generation of
ethylene from ethane over a wide range of catalyst families,[3]
while the use of HBr as a halide source results in a range of
products comprising alkyl bromide, CH4, and carbon oxides,
among others.[4] To understand the mechanistic origin of such
selectivity control, we combined kinetic studies with oper-
ando surface-coverage quantification by prompt-gamma
activation analysis (PGAA) and monitoring of gas-phase
radicals by photoelectron photoion coincidence spectroscopy
(PEPICO), ultimately rationalized at the molecular level by
density functional theory (DFT) calculations (Figure 1). An
iron phosphate catalyst was chosen because of its ability to
selectively (ꢁ 97%) generate ethylene and propylene via
alkane oxychlorination.[3] Performance assessments in ethane
oxychlorination (EOC) and oxybromination (EOB), under
variable temperatures (573–853 K), revealed that the light-off
curve for oxybromination was shifted to about 150 K—
a lower temperature compared to that obtained in oxy-
chlorination (Figure 2a) and in agreement with previous
observations on other materials.[4b] Characterization of the
material before and after catalysis by means of N2 sorption, X-
ray diffraction (XRD), and Raman spectroscopy revealed
that the textural properties and the crystallographic structure
were preserved (Supporting Information, Figure S1,
Table S2). A comparison of the selectivity patterns obtained
at a similar alkane conversion level (ca. 20%) showed that
C2H4 is the major product (selectivity ca. 95%) when HCl is
used as a halide source, while oxybromination led to the
formation of C2H5Br, CH4, carbon oxides, and C2H4 (Fig-
ure 2b; Figure S2). Both reactions are believed to follow
a consecutive mechanism, where the alkyl halide is the
intermediate to the olefin (Figure S2). Nevertheless, the
observed selectivity differences might be caused by the
T
he development of novel technologies for the selective
functionalization of light alkanes is a critical step to enable
the utilization of natural gas as an energy vector in the
transition between the oil and the renewables era.[1]
This pivotal advancement is limited by our understanding of
the mechanisms governing the heterogeneously catalyzed
[*] G. Zichittella,[+] M. Scharfe,[+] Dr. B. Puꢀrtolas, Dr. V. Paunovic,
´
Prof. J. Pꢀrez-Ramꢁrez
Institute for Chemical and Bioengineering, Department of Chemistry
and Applied Biosciences, ETH Zurich
Vladimir-Prelog-Weg 1, 8093 Zurich (Switzerland)
E-mail: jpr@chem.ethz.ch
Dr. P. Hemberger, Dr. A. Bodi
Laboratory of Femtochemistry and Synchrotron Radiation
Paul Scherrer Institute
5232 Villigen (Switzerland)
Dr. L. Szentmiklꢂsi
Nuclear Analysis and Radiography Department, Centre for Energy
Research, Hungarian Academy of Sciences
Konkoly-Thege Miklꢂsi fflt 29–33, 1121 Budapest (Hungary)
Prof. N. Lꢂpez
Institute of Chemical Research of Catalonia
The Barcelona Institute of Science and Technology
Av. Països Catalans 16, 43007 Tarragona (Spain)
[+] These authors contributed equally to this work.
Supporting information, including catalyst preparation, character-
ization, and evaluation, descriptions of the operando PGAA and
PEPICO techniques, DFT calculations, and the ORCID identification
number(s) for the author(s) of this article can be found under:
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!