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DOI: 10.1039/C6CC02813F
COMMUNICATION
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conditions. Their practical relevance was proven in a series of isobutylene and STY of this olefin. Oxides of yttrium, gallium
30 dehydrogenation/regeneration cycles at 550°C and 600°C and chromium are the most attractive dopants in terms of
lasting in total over 110 hours.
isobutylene productivity.
To check if the high activity of Cr10Zr90Ox is related to the
presence of well-known DH active CrOx, we prepared three
additional catalysts. Two of them were binary oxides
synthesized according to the same protocol as for Cr10Zr90Ox
but contained 2 and 5 times lower amount of chromium. They
are abbreviated as Cr5Zr95Ox and Cr2Zr98Ox and were composed
of monoclinic and tetragonal phases with the former fraction
increasing with a decrease in Cr loading (Figure S1 in ESI). The
third material was a supported catalyst with 0.5 wt.% Cr2O3 on
La8Zr92Ox possessing stabilized tetragonal ZrO2 phase; an
apparent Cr surface density was around 0.3 Cr nm-2 as
calculated from the nominal concentration of Cr and the
sample specific surface area. Since the binary CrZrOx materials
contain Cr not only on their surface but also in the bulk, this
calculation method is not applicable for these catalysts.
Therefore, we used the below procedure to estimated surface
concentration of Cr in the binary materials.
Table 1 Selected physico-chemical properties, isobutane conversion (X), space time
yield (STY) of isobutylene and isobutylene selectivity (S). Test conditions: 550°C, iso-
C4H10:N2=40:60, contact time of 0.24 g·s·ml-1
.
Catalyst
Dopant
SBET
/ m2·g-1
38
X
S
STY
/ kg·h-1·m-3
695
/ mol.%
/ -
/ -
ZrO2
0
8
0.04
0.07
0.15
0.36
0.02
0.01
0.13
0.94
0.83
0.91
0.84
0.88
0.78
0.80
La8Zr92Ox
Y9Zr91Ox
120
72
803
9
1883
5861
310
Cr10Zr90Ox
Al10Zr90Ox
In10Zr90Ox
Ga10Zr90Ox
10
10
10
10
135
75
51
177
60
2037
Pure ZrO2 and binary oxides with exception of La8Zr92Ox and
Y9Zr91Ox were prepared from of ZrOCl2·8H2O and nitrates (or
chloride in case of Ga) of corresponding second metal using an
aqueous solution of ammonia as a precipitating agent. The
formed precipitate was washed several times with deionized
water until chloride ions were not identified in the filtrate
(reaction with AgNO3). All solids were finally calcined at 550°C
for 4h. La8Zr92Ox and Y9Zr91Ox are commercial materials from
MEL Chemicals. According to X-ray diffraction analysis, bare
ZrO2 was composed of monoclinic main phase and a small
amount of tetragonal modification while all binary materials
with a ratio of Zr/dopant ≤ 11.5 were stabilized in the
tetragonal phase after calcination at 550°C (Figure S1 in the
Electronic Supporting Information (ESI)). A similar stabilization
effect of oxide dopants in ZrO2-based materials has been
We initially used XPS to determine atomic ratio of Zr/Cr in the
near-surface region. It was 11 and 17 for Cr10Zr90Ox and
Cr5Zr95Ox and close to the nominal values of 9 and 19
respectively. The concentration of Cr on the surface of
Cr2Zr98Ox was too low to be determined precisely.
Nevertheless, the XPS results exclude any enrichment of Cr at
the surface and indicate that Cr is incorporated in the ZrO2
structure forming a solid solution. On the basis of previous
characterisation study of Morterra et al.15, the (101) face is the
most preferentially exposed plane in polycrystalline tetragonal
ZrO2. Under this assumption, around 8.6 surface atoms of Zr
are located in one square nanometre. Using this value and the
nominal concentration of Cr, we calculated an apparent
surface density of this metal in the binary CrZrOx oxide
materials. Despite the fact that Cr5Zr95Ox and Cr2Zr98Ox are
composed of monoclinic and tetragonal modifications, our
estimation should also be valid for these materials because the
number of surface Zr atoms in one square nanometre of
tetragonal ZrO2 is close to 8.1 and 9.0 on the most stable (111)
and ( 111) surfaces of monoclinic zirconia respectively.16, 17
Figure 1 illustrates the effect of Cr surface density on the
apparent turnover frequency (TOF) of isobutane conversion,
i.e. the activity related to one surface Cr atom (Equations S4-
S6 in ESI). Essentially, the TOF values over the CrZrOx catalysts
decreased with Cr loading. Such behaviour can only be
explained when assuming that Cr-containing sites should not
be the only active ones in these catalysts. From a mechanistic
viewpoint, there are no doubts that CrOx species on the
14
documented in several previous studies.13, It is related to
formation of oxygen vacancies for charge compensation or due
to distortion of ZrO2 lattice.
The developed materials were initially tested for their activity
and selectivity in the non-oxidative dehydrogenation of
isobutane to isobutylene at 550°C using a feed with 40 vol.%
isobutane in nitrogen. The latter feed component was used as
an internal standard to take into account reaction-induced
changes in number of moles when calculating the isobutane
conversion, selectivity and space time yield (STY) of
isobutylene formation (Equations S1-S3 in ESI). Isobutylene, 1-
and 2- butenes, methane as well as C2- and C3-hydrocarbons
were the detected carbon-containing gas-phase products. For
all catalysts, the conversion of isobutane decreased within 45
min DH cycle (Figure S2 in ESI). Selected initial (after 9 min on
stream) catalytic data are summarized in Table 1. It is obvious
that the catalysts strongly differ in their activity for isobutane
dehydrogenation. Doping ZrO2 with In2O3 or Al2O3 has a
negative effect on the activity, while other doped catalysts
performed superior to bare ZrO2; the highest conversion of
around 0.36 was obtained over Cr10Zr90Ox. Such positive effect
of the dopants cannot be simply explained by the difference in
specific surface area (Table 1) thus highlighting the importance
of the kind of metal oxide dopant for isobutane
dehydrogenation. Doping also influences the selectivity to
surface
of
CrOx/La8Zr92Ox
catalyse
isobutane
dehydrogenation.1 This statement is actually supported by the
fact that this catalyst is significantly more active than La8Zr92Ox
(Table 1 and Figure 1).
It is also important to mention that the binary CrZrOx catalysts
showed higher STY values than the supported reference
material despite the fact that one of them had lower surface
density of Cr (Figure 1). This result clearly demonstrates the
advantage of bulk doping of ZrO2 over surface modification or
2 | Chem. Commun, 2016, 00, 1-3
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