G Model
APCATA-15323; No. of Pages10
ARTICLE IN PRESS
W.F. Hoelderich, V. Ritzerfeld / Applied Catalysis A: General xxx (2015) xxx–xxx
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.2.5. Pyridine-FTIR measurements
The pyridine-FTIR adsorption spectra were recorded on a Pro-
For other loadings different amounts of the nitric solution are
used.
tégé 460 from Nicolet Comp. Before starting the measurement, the
sample is pressed in a mechanical press to form translucent pellets
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with a strength of 8 mg/cm . Afterwards the pellets were calcined
2.4. Experimental set up
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−3
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at 400 C and 10 Torr and cooled down in vacuum to 50 C within
2
h. During these 2 h the background was recorded at 400, 300, 200
All experiments were carried out batch wise in 75 ml autoclaves
whose operating limits are 300 C and 100 bar. A manometer, a
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◦
and 100 C. Then pyridine was added to the measuring cell and the
adsorption started. Finally the sample was heated up to 100, 200,
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valve, a tube for the temperature sensor and a rupture disk are fixed
on the autoclave stem. The reaction mixture was stirred with a mag-
netic stirring bar. The temperature was set by means a Eurotherm
temperature controller 2216e and a heating sleeve. Thereby, the
temperature sensor is in direct contact with the reaction mixture
and therefore the reaction temperature can be adjusted directly
in the liquid. In order to prevent heat loss the autoclaves were
insulated with glass wool. The autoclaves were filled with 30 ml
of acetone and various amounts of 4-hydroxybenzaldehyde. Over-
all, the molar ratios of acetone and 4-hydroxybenzaldehyde ranged
from 4:1 to 12:1 and the amount of catalyst varied between 0.3 g
and 1 g. Once the desired temperature was reached the experiments
were run for 1, 1.5, 2, 2.5 and 3 h, respectively. Different samples of
the liquid phase were taken by a syringe equipped with a syringe
filter (0.25 m pore diameter) to avoid catalyst contaminating the
sample.
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00, 400 C. Thereby, each temperature was hold for 1 h before the
FTIR spectra was recorded.
2.3. Preparation of the catalysts
2.3.1. Pure rare earth oxides
First of all 50 g of the rare earth oxides are dissolved in 68.39 ml
nitric acid and 1.5 l distilled water. In order to accelerate this pro-
cess the solution is stirred and slightly heated up. After cooling
down to room temperature, 1.5 equivalents oxalic acid, dissolved
in distilled water, was added to the constantly stirred solution.
Thereby, the rare earth oxalates precipitated. The precipitation
is filtered by suction filtration and washed with water until the
mother liquor gets neutral. Afterwards the rare earth oxalates are
dried at 120 C overnight and then calcined to their oxides at 900 C
for 15 h using a heating rate of 5 C/min. Finally the freshly cal-
cined rare earth oxides are stored in a desiccator over potassium
hydroxide to avoid moisture deposit on the oxides.
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The best catalyst tested in the autoclaves was afterwards inves-
tigated in a continuous plug flow reactor. The apparatus consists
of a standard tube reactor in form of a coil with 90 cm length and
6
cm diameter and a fixed catalyst bed. It is heated in an oven simi-
lar to a GC oven. The pre-heated starting material were pumped
at autogeneous pressure (up to 25 bar) using a Latec P402 high
pressure pump. After conversion, the products were cooled down
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.3.2. Partly stabilized tetragonal zirconia
Zirconium hydroxide and 10 weight percent of a rare earth
nitrate solution is stirred. Then the six fold amount of urea is added
as precipitant as well as 50 ml of distilled water and the resultant
mixture is heated under reflux for 16 h. Thereby a mixture of rare
earth hydroxide and zirconium hydroxide is obtained.
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to −20 C and the pressure was released using an one-way-high
pressure valve to keep the reactor itself at 25 bar.
Afterwards the precipitate was filtered by suction filtration and
washed until the washing water got pH-neutral. The solid material
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. Results
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is dried at 120 C, ground thoroughly and calcined at 700 C for 6 h
.1. Catalyst characterization
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at a heating ramp of 2 C/min. A partly stabilized tetragonal zirconia
was obtained as a carrier.
.1.1. XRD investigations
Several XRD measurements were carried out in order to deter-
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.3.3. Rare earth catalysts supported on zirconia or activated
charcoal
The partly stabilized tetragonal zirconia was loaded with e.g.
0 wt.% rare earth oxide in the following procedure.
0 g of a nitric solution consisting out of 200 g of the respective
mine the crystal structure of the different catalysts. The pure rare
earth metal oxides exhibit high crystallinity and always show a
single oxide phase. In the case of lanthanum, neodymium, samar-
ium and ytterbium the phases were X O , while the praseodymium
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oxide consisted of mixed valence metal ions (+III and +IV) resulting
rare earth oxide per kg of solution is added to 18 g of the carrier.
After diluting with 100 ml of de-ionized water, the six fold molar
in the oxide phase Pr O11. The cerium oxide even consisted of a
pure CeO2 phase. The XRD patterns which are in agreement with
those described in literature [12] are shown in Fig. 2
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amount of urea is added to the mixture and heated under reflux for
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1
6 h. The remaining solid is washed and dried at 120 C overnight.
The charcoal-supported catalysts calcined in high vacuum led
to the same characteristic XRD patterns as described in Fig. 2 for
the pure rare earth metal oxides. The only difference showed the
praseodymium catalyst for which a pure Pr O phase was detected
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The dried solid is ground and calcined at 700 C for 6 h. For other
loadings of the catalyst, a different amount of the nitric solution is
used.
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The same procedure was applied in the case of active carbon
instead of the mixed valence oxide.
as support. However, the solid is only calcined in high vacuum at
In the case of alumina supported catalysts very little crystallinity
were observed when the loading of rare earth metal is low. On
one hand, the diffraction pattern corresponding to alumina van-
ished with increasing loadings. On the other hand reflexes were
detected which could be assigned to the rare earth metal alumina
mixed oxide. At loadings with 50 wt.% of rare earth metal oxide the
crystallinity is quite high and the pure alumina phase disappeared.
Sample XRDs for different loadings of lanthanum oxide on alumina
is illustrated in Fig. 3.
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00 C for 3 h under nitrogen atmosphere to avoid oxidation.
2.3.4. Rare earth catalysts supported on alumina or titania
In order to obtain supported catalyst based on alumina or titania
as carriers with 10 wt.% loading of rare earth oxide, 10 g of a nitric
solution consisting out of 200 g of the respective rare earth oxide
per kg of solution is added to 18 g of the carrier and diluted with
1
00 ml of de-ionized water. After mixing all components, the paste
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Since the loadings of rare earth metal oxide on titania P25 were
very low, XRD measurements showed no indication of the corre-
sponding pure rare earth metal oxide or its titania mixed oxide.
in case of alumina.
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W.F.
Hoelderich,
V.
Ritzerfeld,
Appl.
Catal.
A:
Gen.
(2015),