O. Ouariach et al. / Applied Catalysis A: General 503 (2015) 84–93
85
Table 1
Specific surface areas and chemical analysis of AlPO4 and Pd(x)/AlPO4.
to identify all the factors impacting the catalyst behaviour. More-
over, a reaction performed in oscillatory mode usually yields much
higher conversions than those achieved in steady state regimes
[19,20].
Al/P
Wt.% Pd
S (m2 g−1
)
AlPO4
1
0
141
138
135
127
Pd(0.5)/AlPO4
Pd(1)/AlPO4
Pd(2)/AlPO4
1
1
1.06
0.55
0.98
1.96
The production of methyl ethyl ketone by the oxidative dehy-
drogenation of butan-2-ol is an important industrial process [21]. It
was also used as a probe reaction for the investigation of catalysts
acid–base properties. Moreover, the reaction was widely studied
and numerous kinetic equations were established for the reaction.
Its mechanism possibly changes with the increase of temperature,
suggesting that instabilities might arise from the coupling of the
reaction kinetics and the experimental conditions [22,23].
The present work was devoted to the study of palladium loaded
AlPO4 behaviour in the butan-2-ol conversion. In situ UV–vis char-
acterisation of the catalyst samples was performed in presence of
the reaction mixture under dynamic conditions, wishing to collect
information about the state of the catalyst surface when working.
The choice of AlPO4 as a carrier of Pd was motivated by the crys-
tallographic similarities of this phosphate with the silica albeit the
existing differences in their acid–base properties [24]. Tempera-
ture programmed desorption (TPD) and temperature programmed
surface reaction (TPSR) were used to enlighten the interactions
between the adsorption sites of alcohol on AlPO4 and Pd/AlPO4.
While performing these investigations, a self-oscillatory behaviour
was observed at low temperatures (85–100 ◦C). The phenomenon
was studied in detail by varying the feed gas composition and the
reactor temperature. A fitting mechanism was setup for the spon-
taneous periodic activity observed in restricted conditions.
equipped with a cell allowing investigations in controlled temper-
ature and atmosphere.
Diffuse reflectance spectra were recorded between 190 and
2500 nm on a Varian Cary 5E spectrometer equipped with a double
monochromator and an integration sphere coated with polyte-
trafluoroethylene (PTFE). For the in situ experiments a special
optical accessory equipped with a stainless-steel chamber was
used. It allows to work under controlled atmospheres and tem-
peratures. The PTEF was used as the reference.
2.3. Catalytic tests
The catalytic oxidation of butan-2-ol was studied in a quartz
U-shaped fixed bed continuous-flow microreactor operated at
atmospheric pressure. Its internal diameter is 4 mm and the thick-
ness is 1 mm. The catalyst was maintained in the reactor between
two quartz wool plugs and the temperature was measured using a
K type thermocouple attached to the wall of the reactor or for more
accurate reading introduced inside the reactor between the grains
of the catalyst. The temperature of the oven was monitored by a
second thermocouple fixed on its internal wall. Butan-2-ol diluted
in pure N2 or air was supplied to the reactor at different partial
pressures by a saturator held at a selected temperature. The total
2. Experimental
flow rate of the feeding mixture was maintained at 100 mL min−1
.
2.1. Catalyst synthesis
Analysis of the products was performed using a FID chromato-
graph equipped with a 4 m (1/8ꢀꢀ) stainless-steel column containing
Carbowax 1500 (15%) on chromosorb PAW (60/80 mesh). Carbon
dioxide was analysed on a 2 m (1/8ꢀꢀ) stainless-steel column packed
with silica gel. The catalytic measurements were carried out on
about 100 mg of the catalyst sieved to a grain size ranging from
125 to 180 m and evacuated under N2 or air at 450 ◦C without
any prior reduction.
Aluminium phosphate (AlPO4) was synthesised by adding to
a solution of aluminium nitrate Al(NO)3. H2O (25 g dissolved in
133 mL of water) a stoichiometric proportion of phosphoric acid.
The pH was maintained at the end of the precipitation at 6 by adding
droop-wise a solution of ammonia. The precipitate was filtered,
washed with hot water and dried at 120 ◦C then calcined for 15 h
at 500 ◦C in a rotating reactor fed with air at 30 mL min−1
.
Aluminium phosphate loaded with 1 and 2 wt.% of palladium
(named in the following as Pd(1)/AlPO4 and Pd(2)/AlPO4 respec-
tively) were prepared by impregnating AlPO4 with a titrated
aqueous solution of Pd(NO3)2. The mixture was kept under stirring
at 80 ◦C until the complete evaporation of water. The resulting solid
was dried at 120 ◦C then calcined for 24 h at 500 ◦C. It displayed a
greyish colour.
2.4. Temperature programmed desorption (TPD) and
temperature programmed surface reaction (TPRS)
The TPD experiments were performed on AlPO4 and Pd(2)/AlPO4
at atmospheric pressure in the same system in which was car-
ried out the study of the catalytic reaction. Butan-2-ol diluted in
N2 (Pbutan-2-ol = 0.84 kPa) was adsorbed on 100 mg of the catalyst
at room temperature for 30 min. After the evacuation of the alco-
hol excess by pure N2 for 2 h, the temperature of the reactor was
raised at 3 ◦C min−1 up to 200 ◦C. The desorbed products were
analysed at regular intervals of time (around 4 min) by an online
chromatograph equipped with FID and catharometers. The TPSR
measurements were performed in the same conditions using air
instead of N2 as the carrier gas.
2.2. Techniques of characterisation
The BET specific surface areas of the samples calcined at 500 ◦C
and evacuated for 2 h at 300 ◦C, were measured using nitrogen
adsorption at −196 ◦C and a Micromeritics apparatus.
Chemical analyses of Pd(2)/AlPO4 were carried out using sev-
eral techniques. The phosphorus was titrated by the colorimetric
method. Palladium and aluminium were determined by atomic
absorption spectroscopy after dissolving the sample in HCl.
3. Results and discussion
X-ray powder diffraction patterns were recorded with
a
Siemens D5000 high resolution spectrometer using Cu K␣ radia◦-
˚
tion (ꢀ = 1.540598 A). The data were collected with a step of 0.02
(2ꢁ) at room temperature.
X-ray diffraction patterns showed that the synthesised AlPO4
with Al/P = 1 (Table 1) is amorphous and does not fully crystallise
until 1000 ◦C. At this temperature it exhibits the ␣-crystobalite type
structure [25]. AlPO4 calcined at 500 ◦C and loaded with palladium
IR spectra were obtained with a Perking-Elmer spectrometer
using self supporting disks of around 25–30 mg compressed at
200 kg cm−1 or samples diluted in KBr. The spectrometer is also