O. Verdes, et al.
Catalysis Today xxx (xxxx) xxx–xxx
temperatures, below 200 °C and polyaromatic at high temperatures,
above 350 °C [17–19].
2.2. Characterization methods
Doping of solid acid catalyst with palladium enhances catalyst re-
generation by coke combustion for propene oligomerization on palla-
dium-doped HPW/SiO . The effect of Pd on coke combustion (in air)
2
was observed by TGA/TPO method. In the absence of palladium, the
coke burns at approximately 450 °C and by doping with palladium the
coke burns at 350 °C (below the temperature of decomposition for
The thermal analyses were carried out using a thermo analyzer
system Mettler TGA/SDTA 851/LF/1100. The measurements were
conducted in dynamic atmosphere of air (50 ml/min), using the alu-
mina plates crucibles of 150 μl. Heating rate was of 10 °C /min in the
range of temperature 25−600 °C and the mass samples were about
30 mg.
HPW). In the case of pure HPW/SiO
coke was form in contrast with palladium doped catalyst in which only
soft coke was detected [18,19].
The amount of total coke can be determined by recording the weight
changes by TG-DTA technique [20]. This method involves different heat
treatments to remove adsorbed water, as well as adsorbed reactants and
volatile products. The coke precursors are removed from the catalyst by
increasing the temperature up to 600 °C in N
hard coke is removed by burning in air.
2
catalyst a mixture of hard and soft
The FTIR absorption spectra were recorded with a Jasco 430 spec-
−1
trometer (spectral range 4000−400 cm
range, 256 scans, and re-
−
1
solution 2 cm ) using KBr pellets for the all synthesized heteropoly
compounds after their keeping in air at room temperature until to
constant mass.
Powder X-ray diffraction data were obtained with X’Pert PRO MPD
PANalytical diffractometer with the following measurement para-
meters: Theta/Theta PW3050/60 Goniometer, PixCEL detector, zero
2
atmosphere, while the
background sample holder (Si), Ni-filtered CuK
α
radiation, 45 kV/
This work concerns to a study of the optimal temperature reaction
and coke formation in the gas phase ethanol dehydration using pure
and palladium doped catalysts. The synthesized catalysts were sup-
ported on SBA-15 mesoporous molecular sieve in the concentration
loading of 30 wt. %. The deactivation and air regeneration of the
compounds were also examined. For the clarity of discussion, some data
obtained for pure and Pd doped heteropoly tungstate catalysts pre-
sented in our previous papers [11,12,21] are also given.
30 mA, continuous scanning, step size 0.0130 [2θ] in the 2θ range
2–60°.
The specific surface areas of samples were calculated from the ni-
trogen adsorption-desorption isotherms using a Quantachrome instru-
ment, Nova 2000 series. Prior to the measurements the samples were
preheated and degassed at 250 °C for 2 h.
The Brunauer-Emmet-Teller (BET) surface area and pore diameters
calculated by Barrett-Joyner-Halenda (BJH) method applied to the
desorption branches of the isotherms were determined [26]. From X-ray
peak broadening is possible to calculate the crystallite size (D) using
Scherrer equation:
2. Experimental
D = 0.9λ/(β-β
o
)cosθ
(1)
2.1. Catalysts preparation
where: λ is X-ray wavelength (CuK
α
) in angstroms (1.54 Å), θ the dif-
The Cs
denoted as CsxPW) were prepared by precipitation method from an
aqueous solution of the HPW (purchased from Merck) by adding a re-
x
H
3-xPW12
O
40 salt with Cs contents x = 1, 2, 2.25 and 2.5
fraction angle, β the line width (in radians) and β the instrumental line
o
(
width (in radians). The crystallite size was estimated from the full width
at half maximum of diffraction peak. It is well known that the cesium
salts of Keggin type heteropoly acids possess a porous structure and a
partial substitution of H+ from HPW with Cs+ will lead to an increase
in surface area and pore structure [4,5,26].
For XPS studies the samples were pressed into tablets with a few
tenth of mm thickness and 1 cm diameter. Sample treatments were
performed in a high pressure cell directly attached to the analysing
chamber. SPECS instrument was equipped with a PHOIBOS 150 MCD-9
hemispherical electron energy analyser operated in the FAT mode. Kα
quired stoichiometric quantity of counter-ion salts as CsNO
stirring as it was described in detail earlier [21].
3
under
The HPW doped with 0.15, 0.20 and 0.25 at Pd/Keggin Unit (KU)
namely (Pd0.15 P W),
Pd0.2 P W) and Pd0.25 40 (Pd0.25 P W) were prepared
through the reaction between Pd(NO and HPW 0.1 M aqueous so-
Pd0.15
H2.7PW12
O
40
Pd0.20
H
2.6PW12
O
40
(
H
2.5PW12O
3 2
)
lutions in the proper ratio. The samples were stirring at 60–70 °C, until
a paste was obtained.
The Cs
Pd0.25CsxPW) were prepared by adding an amount of Pd(NO
aqueous solution 0.1 M into the HPW aqueous solution 0.1 M and after,
the required stoichiometric quantity of CsNO (aqueous solution) was
poured drop by drop under continues stirring. The pH was kept under
x
H
3-xPW salts doped with 0.25 at Pd/KU (denoted as
radiation of a magnesium anode (h = 1252.6 eV) was used as ex-
citation source. X-ray gun was operated at 180 W power and the pass
energy was set to 20 eV. The binding energies were calibrated with
respect to the position of C 1s to compensate for possible charging ef-
fect. Commonly five scans were added to get a single spectrum.
ν
3
)
2
as
3
1
6
.5 during all syntheses. The suspensions of precipitate were heated at
0−70 °C under stirring until a paste was obtained.
The synthesized compounds were: Pd0.25Cs
Pd0.25Cs1PW), Pd0.25Cs (Pd0.25Cs2 P W),
40 (Pd0.25Cs2.25 P W) and Pd0.25Cs2.5 PW12
Pd0.25Cs2.5 P W). Finally, all the catalysts were heated at 250 °C, 1 h
in air, for nitrate anion total decomposition.
SBA-15 was synthesized according to Zhao et al. [22] by the hy-
drolysis of tetraethyl orthosilicate (TEOS) using Pluronic P123 block
copolymer as surfactant. The HPW and PdyPW (y = 0.15, 0.20 and
1
H
1.5PW12
O
40
2.3. Catalytic reaction
(
2 40
H0.5PW12O
Pd0.25Cs2.25
(
H
0.25PW12
O
O
40
The ethanol dehydration was performed in a flow microreactor
(stainless steel tubular reactor of 10 mm inner diameter), which was
placed into an electric furnace. The microreactor temperature was ad-
justed by a temperature controller within ± 1 % in the temperature
range of 200−350 °C. The catalysts sample was placed in the middle of
the reactor and supported by quartz packing at both ends. The amounts
of unsupported and silica-supported catalysts introduced in reactor
were 100 and 330 mg, respectively.
Liquid ethanol (99.8 % Riedel de Haën) was introduced by a
Hamilton syringe pump at a flow rate of 1.2 ml/h into an evaporator
heated at 150 °C. The nitrogen flow was kept constant at 30 ml/min.
The composition of the reactor effluent stream was analysed by gas
chromatograph equipment with a flame ionization detector (FID). For
separation of the products were used a stainless steel packed columns of
0.25) were impregnated by aqueous incipient wetness onto SBA-15
mesoporous molecular sieve as loading of 30 wt. % concentration. To
obtain a monolayer array of HPC on SBA-15 the size of Keggin unit was
considered. According to the literature data, the size of the Keggin unit
has been reported as 10 Å [23], 12.5 Å by X-ray crystallography [24]
and 11.7 Å by STM [25]. In our calculation, the diameter value of the
Keggin-type molecule was 12 Å.
3
m long and 1.5 inner diameter packed with Porapak QS 80–100 mesh.
2