2
96
L. Matachowski et al. / Applied Catalysis A: General 469 (2014) 290–299
Table 2
The crystallite sizes of the K3 core and HPW in the K2 salt after different treatment.
state. The HPW spread again over the K3 core forming the sur-
face layer similarly to the phenomenon described previously for
the ‘as-synthesized’ and next aged K2 salt [28].
For all potassium salts after different treatments the surface
area and porosity were determined. Their nitrogen adsorption-
desorption isotherms were compared to those obtained for the
Sample
Treatment
K3 core
nm]
HPW
[nm]
[
K2HPW12O40
“As-synthesized”, drying at
22
23
20
22
–
313 K
After annealing at 398 K in
air with RH of 25%
After annealing at 473 K in
air with RH of 25%
42
14
17
‘as-synthesized’ potassium salts (Fig. 6). According to the IUPAC’s
classification, the isotherms obtained for the ‘as-synthesized’ K2.5
and K3 salts represent the type II. The shape of these isotherms
practically does not change after catalytic reaction.
After catalytic reaction at
473 K
Data in Table 3 show that the specific surface area of K2.5
salt decreased only slightly after catalytic reaction (from 105.8 to
2
2
9
4.8 m /g) as well as after annealing at 473 K (to 95.3 m /g). These
In contrast, the XRD pattern for the K2 sample exhibits additional
reflections apart from those coming from the K3 core (Fig. 5B). They
originate from the crystalline HPW, which lattice parameter was
estimated to be 1.213 nm, close to that of the H PW12O40·6H O
data confirm earlier suggestions about the stability of the secondary
structure of the K2.5 salt observed by the XRD measurements. The
surface area of the K3 salt also decreased after catalytic test (from
2
114.3 to 81.5 m /g, Table 3), which pointed out the influence of
3
2
phase. However, it should be underlined that the XRD measure-
ment for the K2 sample after reaction was performed in air with
relative humidity (RH) of ca. 25%, what could cause the hydration of
protons existing in the HPW. Despite that this result clearly shows
that the secondary structure of the K2 salt is unstable in the reaction
conditions.
temperature on its porosity. At the same time, the share of meso-
pores area in the total surface area decreased from 52.8% to 34.2%
for the K2.5 salt and from 62.3% to 22.2% for the K3 salt. Thus, it
can be concluded that the porous structure of both salts become
more microporous after catalytic reaction. Moreover, the average
pore size in the K2.5 salt (1.4 nm) did not change after reaction,
whilst in the K3 sample it reduced only slightly from 1.8 to 1.5 nm
(Table 3).
In order to find out what is the reason of such instability of the
secondary structure of the K2 salt, additional experiments were
performed. To investigate the influence of water, one sample of
the K2 salt was annealed at 473 K in flowing helium with RH of
In the case of the K2 salt after annealing at 473 K, the shape of
the isotherm changed from the type II to type IV with the hysteresis
H2, characteristic for the mesopores structure with the cylindrical
and spherical pores. At the same time, the average pore diameter
remained practically unchanged whilst the share of the mesopores
area in the whole surface area remarkably decreased from 71.8%
for the ‘as-synthesized’ K2 sample to 38.0%. It clearly shows that
the rising temperature strongly influenced the porosity of the K2
salt. The shape of the isotherm obtained for the K2 sample after cat-
alytic test is also similar to that obtained for the ‘as-synthesized’ K2
salt. However, its surface area decreased dramatically from 100.6 to
2
% (reaction conditions), whereas the other was annealed in air
with RH of 25%. Before the heating, the samples were stabilized in
appropriate atmosphere at ambient temperature for 1 h.
The XRD pattern obtained for the K2 sample annealed at 473 K in
helium with RH of 2% was the same as that for the ‘as-synthesized’
K2 salt (Fig. 2). In contrast, the XRD patterns of the K2 sample
annealed at 398 and 473 K in air with RH of 25% exhibited addi-
tional reflections (Fig. 5B), similarly to those observed for the K2
salt after catalytic reaction. It shows that the water can cause the
rearrangement of surface layer of the HPW covering the K3 core
into the crystalline (bulk) HPW. Thus, it can be suggested that dur-
ing the reaction performed in flowing helium (RH of 2%) at 398 K
2
3.9 m /g whereas the share of mesopores area in the whole surface
area remained practically unchanged, 71.8% for the ‘as-synthesized’
K2 salt vs. 69.2% for the K2 sample after catalytic reaction (Table 3).
This can be related to the two processes: (i) the rearrangement
of the surface layer of HPW covering the K3 core into the bulk
(crystalline) HPW due to rising temperature (Fig. 6), and (ii) the dis-
placement of formed HPW crystallites, resulting in partial blockage
of the micro- and meso-pores. It was showed earlier (Table 2) that
the sizes of HPW crystallites diminished with the rising tempera-
ture what could facilitate their displacement. As a result the specific
surface area of the K2 sample decreased strongly after the catalytic
reaction.
(
ethanol conversion of ca. 94%) on the K2 salt, the water originating
from the reaction also induces the same phenomenon.
The influence of the annealing of the K2 sample in air with RH of
2
5% at 398 and 473 K on crystallite sizes of two phases, namely the
K3 core and bulk HPW, was tested. As seen in Table 2, the crystallite
sizes of the K3 core (20–23 nm) practically remained unchanged,
independently of the temperature of annealing of the K2 salt. In
turn, the size of HPW crystallites after annealing at lower tempera-
ture (398 K) was determined to be 42 nm, whereas after annealing
at higher temperature (473 K) the crystallites were smaller, 14 nm
in size. Likewise, after the catalytic test (473 K) the crystallite size
of the HPW was calculated to be 17 nm. It shows that the size of
HPW crystallites can diminish with increasing temperature due to
the recrystallization process, which proceeds under the influence of
water originating from the reaction. Thus, obtained results clearly
indicate that the water can be a main factor influencing the stability
of the secondary structure of the K2 salt. It is interesting that the
same phenomenon does not occur in the case of the K2.5 salt, mak-
ing this salt promising candidate as a stable catalyst for the ethanol
dehydration.
The textural characteristics were also determined for two recov-
ered K2 samples. The specific surface area of the recovered K2
2
sample (after reaction) increased from 3.9 to 16.0 m /g and the sur-
2
face area of mesopores increased from 2.7 to 15.1 m /g (Table 3).
The surface area of micropores practically remained unchanged.
Hence, it can be supposed that the crystallites of HPW, which filled
the mesopores, can partially cover the K3 core according to the phe-
nomenon described previously [28] whereas this process does not
occur in the microporous structure. Thus, it seems that the meso-
porous structure of the K2 salt can be only partially recovered. In
the case of the K2 sample, which was annealed at 473 K and then
aged in air for 1 month, the specific surface area slightly decreased
In order to investigate the ability of the K2 salt to restore its
secondary structure, the obtained two-phase samples were placed
in desiccator in contact with air of relative humidity of 25% for a
month at ambient temperature. In both cases, the results were the
same. The Keggin structure of the K2 samples remains unchanged
2
from 84.8 to 72.9 m /g. It may be related to the increase of the share
of mesopores area from 38.0% to 60.6% (Table 3) and the decrease
of the share of micropores area from 52.6% to 28.7%, in total sur-
face area. Thus, it can be concluded that the secondary structure of
the K2 salt is not stable because the surface layer of HPW changes
under the influence of temperature as well as of ethanol or water
(Fig. 5A) whereas the HPW reflections disappeared (Fig. 5B) indi-
cating that the secondary structure of K2 salt reverted to its initial