Communications
CNTs were chosen as they are a compromise between high
3
2
reactivity (sp , activated carbon) and high stability (sp ,
graphite). The compromise is motivated by the poor perfor-
mance in ODH provided by a reference sample of activated
carbon loaded with 5 wt% B O (Supporting Information,
2
3
Figure S3).
Microscopic, spectroscopic, and thermoanalytic techni-
ques were employed to investigate the structural and surface
properties of catalysts before and after reaction. Energy-
filtered transmission electron microscopy (EF-TEM) shows
that both boron and oxygen are highly dispersed throughout
the CNT substrate (Supporting Information, Figure S4). The
composite is stable under reaction conditions, and all of the
B O (1.7 atom% B for 5B-oCNT) could be detected by
2
3
electron energy-loss spectroscopy (EELS) after use in ODH
Supporting Information, Figure S5). The CNT framework
(
was not affected by modification by heteroatoms or the
oxidative reaction atmosphere, as shown by subsequent
characterizations by Raman spectroscopy, scanning and
transmission electron microscopy (SEM and TEM), and N2
physisorption (Supporting Information, Figures S1 and S5).
As an indication of the disorder of carbon atoms, the ratio of
the D band to the G band decreases slightly, and it varies
between 2.0 and 2.1 (Supporting Information, Table S1). The
SEM and TEM images show that fresh and used samples have
a similar morphology. The position of the major peak in the
TPO curves is almost the same; however, a low-temperature
peak at around 550 K disappears, which is probably due to the
removal of some combustible functionalities in the oxidative
reactant flow. The qualitative and quantitative change in
oxygen surface functionalities was monitored by temper-
ature-programmed desorption (TPD) and thermogravimetric
analysis (TGA; Supporting Information, Figure S6).
Figure 1. Catalytic performance of CNT catalysts in the ODH of
[
14]
propane, with VO -Al O as a reference. a) Apparent activation
x
2
3
energies E of propane conversion, oxygen conversion, and propene
A
formation (bars), and normalized activity rnorm and propene selectivity
S (in %) at 5% propane conversion (symbols); b) propene selectivity
at 5% propane conversion (
B O loading w . Reaction conditions for CNT catalysts: 1 g, C H /
&
) and reaction rate r (*) as a function of
2
3
B2O3
3
8
À1
O /N =1:1:4, 10–140 mL min , 673 K.
2
2
n
The initial evolution of catalytic performance of pristine
and modified CNT samples is illustrated in the Supporting
Information, Figure S7. During the time on-stream, pristine
and oxidized CNTs increase in activity. This result can be
explained by the in-situ generation of active groups and/or
elimination of labile functional groups and amorphous carbon
order
CNTs < oCNTs ! 2B-oCNTs ꢀ 5B-oCNTs < 5P-
oCNTs (Supporting Information, Figure S1). The onset
temperature and peak position shift to higher temperature
with an increasing amount of boron oxide. The modification
with borate and phosphate is indeed reported to block the
combustion sites serving as a point of attack for gaseous
oxygen, and thus to suppress the combustion of the carbon
framework at elevated temperatures.
point of view, this protective effect of boron oxide modifica-
tion against O activation is reflected by an approximately
[
16]
debris in the oxidizing feed-gas atmosphere, which is in
agreement with TPO and TPD results. A lower degree of
disorder accompanied by a loss of near-surface oxygen is
observed by Raman spectroscopy and also by ex-situ X-ray
photoelectron spectroscopy (XPS) and TPD, respectively
(Supporting Information, Table S1). In general, the TPD
analysis of fresh and used CNT samples shows a loss of labile
carboxy, anhydride, and lactone groups that release CO2
between 400 and 800 K, and the formation of ketonic and
quinoidic functionalities that release CO in the range 800–
[12,13]
From a kinetic
2
À1
4
0 kJmol higher activation energy of O conversion (Fig-
2
ure 1a). However, the activation of propane is not affected;
À1
the activation energy remains at (130 Æ 5) kJmol .
The modified CNTs are much more selective than a
variety of supported vanadia catalysts (Supporting Informa-
[14]
[2]
tion, Figure S2).
For propene selectivity at 673 K, the
1200 K (Supporting Information, Figure S6). The latter
following trend could be observed: VO -Al O > 2B/5B-
species dominate the active surface together with high-
temperature-stable lactone and anhydride groups. The
oCNT samples have a higher initial activity than pristine
CNTs. However, after five hours, the difference in structure
or catalysis becomes negligible. The activation period can be
simulated by further oxidation of oCNTs (1 h, 773 K, air). A
superior stable performance of the oCNTs is observed from
x
2
3
oCNTs ꢀ VO -TiO > 5P-oCNTs > VO -CeO > VO -SiO >
x
2
x
2
x
2
VO -ZrO > (o)CNTs. The decreasing selectivity with ongo-
x
2
ing conversion is due to the weaker CÀH bonds in propene
[
9]
and its resulting combustion. Total oxidation of propane is
suppressed by boron and phosphorus modification, as seen by
an almost 100% propene selectivity by extrapolating to zero
2
propane conversion. Only an sp -hybridized carbon atom
the start of the reaction. Regarding the N physisorption
2
[15]
allows stable and selective ODH catalysis. For this study,
results, a general rise in surface area, and in particular within
6
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 6913 –6917