Study on thermal stabilities and symmetries of chemisorbed species formed on K-zeolites upon CO2 adsorption by TPD and in situ IR spectroscopy

Article abstract of DOI:10.1007/s10973-017-6811-5

In the current study, K-zeolites with different structure, Si/Al ratio and morphology have been prepared and then characterized by different techniques including in situ IR spectroscopy upon CO₂ sorption and CO₂–TPD with the aim of u

Full text of DOI:10.1007/s10973-017-6811-5

J Therm Anal Calorim
DOI 10.1007/s10973-017-6811-5
Study on thermal stabilities and symmetries of chemisorbed
species formed on K-zeolites upon CO adsorption by TPD
and in situ IR spectroscopy
2
1
1
2,3
1
Yadolah Ganjkhanlou
•
Roman Bul a´ nek
•
Oleg Kikhtyanin
•
Karel Frolich
Received: 3 August 2017 / Accepted: 29 October 2017
Ó Akad e´ miai Kiad o´ , Budapest, Hungary 2017
Abstract In the current study, K-zeolites with different
structure, Si/Al ratio and morphology have been prepared
and then characterized by different techniques including
in situ IR spectroscopy upon CO sorption and CO –TPD
almost all the bidentate species are desorbed at 80 °C,
while monodentate species are thermally stable at least up
to 130 °C. Based on combination of experimental data
obtained from TPD with IR spectroscopy results, origin
and assignment of the TPD peaks were discussed.
2
2
with the aim of understanding the nature of basic sites
present on their surface acting as catalytic sites in aldol
condensation reaction. Results showed that depending on
the zeolite structure, pore size and Si/Al ratio, two cate-
gories of basic sites could be present in potassium modified
Keywords TPD Á Zeolite Á CO adsorption Á Basic sites Á
2
Potassium Á IR spectroscopy
zeolites. Symmetries of chemisorbed CO on these sites are
2
different and comparing the results of TPD and in situ IR
spectroscopy; it can be concluded that highly symmetric
species (e.g., monodentate carbonates) have higher thermal
stability than low symmetric adsorbed species (e.g.,
bidentate carbonates). It was found that in the zeolite with
relatively smaller pore size or less accessible pores (e.g.,
MFI), second type of adsorbed species is more popular,
while highly symmetric species tend to form on large pore
zeolites and on materials with some mesoporosity (e.g.,
BEA or dealuminated FAU zeolites). It is observed that
Introduction
Strength and distribution of acidic or basic sites on the
surface of the zeolite catalysts are the most important
characteristic of them for catalytic application, and
numerous characterization techniques have been developed
for identification and investigation of these sites. In the
case of acidic catalyst, extensive researches have been
performed due to their importance in petroleum chemistry;
however, the research about basic catalyst is limited [1, 2].
Basic sites in the oxides and especially zeolites could have
different origin and they may be observed directly in the
zeolite framework [3] or more possibly as the extra-
framework species (e.g., oxides of alkali metals) [4]. It was
reported that the negatively charged framework oxygen
could act as basic site in the zeolites [2] and it is observed
in FAU zeolites [5, 6]. As mentioned, the extra-framework
species like clusters of the alkali metal oxides inside the
zeolite pores could also act as strong basic sites [7]. Strong
basic sites in the oxide materials have recently reported as
catalyst for different chemical reactions (e.g., transesteri-
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10973-017-6811-5) contains supple-
mentary material, which is available to authorized users.
&
Yadolah Ganjkhanlou
yadolah.ganjkhanlou@upce.cz
1
Department of Physical Chemistry, Faculty of Chemical
Technology, University of Pardubice, Studentsk a´ 573,
5
32 10 Pardubice, Czech Republic
2
3
Unipetrol Centre of Research and Education (UNICRE),
Chempark Litv ´ı nov, 436 70 Z a´ lu zˇ ´ı -Litv ´ı nov, Czech Republic
fication of glycerol [8], soot oxidation and NO reduction
x
[9], methylation of phenol with methanol [10], biomass
valorisation and aldol condensation [7, 11–17]). Usually,
the basic solution like NaOH was implemented as
Technopark Kralupy, University of Chemistry and
ˇ
Technology Prague, Zi zˇ kova 7, 278 01 Kralupy nad Vltavou,
Czech Republic
123

Y. Ganjkhanlou et al.
homogenous catalyst for these reactions (e.g., aldol con-
densation [18]); however, heterogeneous catalyst as recy-
clable material offers green and economical pathways for
this purpose. Potassium modified zeolites as heterogeneous
recyclable basic catalysts have been recently implemented
to facilitate different chemical reactions (e.g., biomass
valorisation and aldol condensation [7], transesterification
of glycerol [8]) and promising results have been obtained
due to their numerous advantages like superior basic
activity and shape selectivity [7]. To optimize these
materials as basic catalyst, different active sites of them
should be well characterized. In previous studies by our
Experimental
Material synthesis
Commercial H-FAU, H-BEA and H-MFI zeolites with
different Si/Al ratio obtained from Zeolyst International
and were implemented as parent zeolite for further modi-
fication by potassium. Parent zeolites (list of them is
reported in Table 1) have been impregnated with potas-
sium nitrate with similar procedure to the earlier report of
our group [7]. Briefly, the as-received protonated zeolites
were impregnated using aqueous KNO3 solutions with
concentration of 0.7 M of salt (Fig. S1). The suspension
was stirred at 80 °C with the speed of 200 RPM for 2 h.
Then, the solids were filtrated using a Buchner funnel
without any washing. Resultant samples were then dried in
ambient environment overnight and heat treated at
T = 530 °C for 3 h. These samples were labeled as
K-zeolite type(xx) where xx stands for Si/Al ratio of parent
zeolite. Table 1 lists the descriptions of all prepared sam-
ples for the study.
group [7, 19], it was reported that extra-framework K O
2
clusters could be formed upon thermal activation of
potassium modified zeolites prepared by wet impregnation
procedure and these clusters are responsible for strong
basic behavior of the obtained materials. Proton-accepting
or electron pair donating sites (basic sites) in solid catalyst
could be identified by suitable acidic molecule and the
adsorption of acidic molecule or its reaction outcome could
be implemented for characterization and quantification of
basic sites. Many methods, e.g., vibrational spectroscopy
[
1, 20], microcalorimetry [2, 21], temperature programed
Material characterization
desorption (TPD) [22] could be utilized for characteriza-
tion of basic site upon sorption of the acidic probe mole-
cules. Carbon dioxide is well-known acidic probe molecule
for studying different basic sites in heterogeneous catalyst
The morphology of prepared materials was evaluated by
scanning electron microscopy images using a JEOL JSM-
7500F instrument with a cold cathode—field emission
(parameters of measurements: 1 kV, GB high mode). BET
surface area, micropore volumes and external surface were
determined from nitrogen adsorption/desorption experi-
ments performed at - 196 °C after sample activation in
vacuum at 300 °C for 12 h using a Micromeritics ASAP
2020 unit and applying BET model and t-plot calculated by
using of Harkins–Jura thickness curve equation. IR spectra
of the prepared samples were recorded on Nicolet 6700
FTIR spectrometer equipped with a MCT detector. Before
due to its abundance and less toxic properties. CO che-
2
misorbed in basic sites, while its interaction with acidic
sites is very weak and physi-sorption of CO occurred on
2
acidic sites [20, 23]. Current study is devoted to charac-
terize different basic sites formed in the potassium modi-
fied zeolites using CO as probe molecule. For this aim,
2
potassium modified zeolites have been prepared by wet
impregnation procedure and temperature programmed
desorption (TPD) of CO and in situ IR spectroscopy upon
2
CO desorption were implemented to characterize active
2
CO adsorption coupled with IR experiments, the self-
2
sites of prepared materials. Complementary textural and
supporting sample wafers were placed into a home-made
IR cell for transmission spectra measurement. Samples
were heat treated in a dynamic vacuum (residual pres-
microstructural analyses like N sorption and field emission
2
scanning electron microscopy (FE-SEM) techniques have
been performed on prepared samples and obtained results
implemented to describe the features of the prepared
K-zeolites and to relate microstructures and texture of
zeolites with distribution of different basic sites. The cat-
alytic activities of investigated materials are also included
to find the relation between distribution of different basic
sites with catalytic yield and selectivity to aldol conden-
sation of furfural and acetone.
-
4
sure \ 10 mbar) overnight (12 h) at 450 °C. CO2
adsorption was performed at room temperature, and initial
pressure was 2 mbar (the heat treatment time was 24 h for
in situ IR study) after dosing CO , the system was equili-
2
brated for 20 min (30 min in the case of studying equili-
bration time effect) before evacuation. IR spectra recording
proceeded (accumulating 8 scans with the resolution of
-
1
2 cm ) during adsorption equilibrium and gradual evac-
uation procedure. To perform in situ IR study, after
adsorption of CO , the cell was first evacuated to the
2
pressure of 0.01 mbar and then the samples gradually
1
23

Study on thermal stabilities and symmetries of chemisorbed species formed on K-zeolites upon…
Table 1 List of the prepared samples and their descriptions
2
-1
3
/cm g
-1
2
-1
Zeolite framework type
BEA (beta)
Sample
Si/Al
S
BET/m g
V
l
S
ext/m g
K/mass%
a
a
H-BEA(11.5)
K-BEA(11.5)
H-BEA(19)
K-BEA(19)
H-MFI(21)
K-MFI(21)
H-FAU(15)
K-FAU(15)
H-FAU(40)
K-FAU(40)
11.5
11.5
19
653
257
714
542
545
387
847
349
761
319
0.181
0.059
0.226
0.165
0.172
0.150
0.320
0.144
0.286
0.137
203
114
138
134
30
–
5.52
–
19
3.0
–
MFI (ZSM-5)
21
21
29
3.88
–
b
FAU (Y)
15
84
15
82
4.56
–
40
81
40
64
4.03
a
Data adopted from the our recent work [7]
b
Data adopted from the our recent work [19]
heated up to 200 °C and IR spectra recorded at different
temperatures.
Results and discussion
The CO –TPD has been measured by Micromeritics
2
Textural and microstructural properties
of prepared K-zeolite
AutoChem II 2920 instrument. The desorbed products were
assessed by TCD calibrated for CO . Prior to TPD exper-
2
iments, the samples were heated to 530 °C in He flow and
SEM images of the main prepared samples are illustrated
in Fig. 1. K-MFI(21) sample (Fig. 1a) has submicron size
crystallite (ca 300–600 nm), while the K-BEA(11.5)
sample shows nanoparticle character with particle size in
the range of 20–30 nm. In case of the dealuminated Y
zeolites, SEM images in Fig. 1c, d show that samples
have micron size crystals (ca. 900–1100 nm). A pores
and surface roughness can be observed on the surface of
the Y zeolites crystals with sizes around 30 nm which are
formed during the dealumination by partial dissolution of
the zeolite framework. The textural properties of the
samples provided in Table 1 evidence good microporos-
ity of all parent zeolites (high values of micropore vol-
treated in O flow at the same temperature for 2 h for
2
activation. After cooling to the RT, the samples were sat-
-
urated with CO (10% CO /He; 25 mL min ) for 25 min.
1
2
2
The physisorbed CO was desorbed during He flow
2
-
25 mL min ) for 60 min. The end of both saturation and
1
(
flushing was checked by mass signal stabilization. The
CO –TPD proceeded with temperature ramp of
2
-
1
-1
1
0 °C min from 0 to 530 °C in He flow of 25 mL min
Catalytic activity of the K-MFI(21) and K-BEA(19)
samples was investigated in aldol condensation of furfural
Sigma-Aldrich) and acetone (Lach-Ner, s.r.o., Czech
.
(
Republic, pre-dried before the experiments with a molec-
ular sieve) implementing the same procedure of earlier
report [7]. Briefly, the catalytic tests were performed in a
ume (V ) typical for such type of zeolites). External
l
surface area of parent zeolites differs significantly. The
highest external surface area exhibits BEA zeolites [203
3
00 mL autoclave at 100 °C. In a catalytic run, 1 g of
2
and 138 m g
-1
freshly heat-treated (at 530 °C for 3 h) catalyst was mixed
together with a stirred mixture of 39.5 g acetone and 6.5 g
of furfural (acetone/furfural molar ratio 10/1). The desired
temperature was achieved in * 60 min after starting of the
heating, and the autoclave was then kept at T = 100 °C for
additional 2 h. Analysis of the reaction products was per-
formed after 2 h at the reaction temperature using an
Agilent 7890A GC equipped with a flame ionization
detector and an HP-5 capillary column (30 m/0.32 ID/
for BEA(11.5) and BEA(19), respec-
tively] as a consequence of the nanosize of zeolite
crystallites. FAU zeolites exhibit mediocre external sur-
2
-1
face area (slightly above 80 m g ) despite large crys-
tallites (see Fig. 1c, d). This is caused by secondary
mesoporosity formed during dealumination of the original
zeolite (usually synthesized with Si/Al ratio around 2.5).
MFI zeolite exhibits the lowest surface area because of
large crystallites with only internal microporosity. Pres-
ence of potassium in the zeolites leads to drop in total
surface area and volume of micropores by about 30–55
rel.% compare to parent zeolite. It is caused by partial
0
.25 lm). The products were identified based on the
standard reference compounds as well as additional GC–
MS analyses. Finally, the selectivity to product and con-
version of furfural could be calculated by obtained data. In
case of Y zeolites, the catalytic data for comparison and
discussion were adopted from our recent work [19] and for
K-BEA(11.5) sample data obtained in earlier report [7].
pore blocking by the K O clusters.
2
123

Y. Ganjkhanlou et al.
Fig. 1 SEM images of the a K-MFI(21), b K-BEA(11.5), c K-FAU(15), d K-FAU(40) samples
IR spectroscopy and TPD upon CO adsorption
2
IR spectroscopy during CO sorption
2
IR spectroscopy before and after thermal activation
To quantify the density of formed basic sites as well as
characterization of their strength, CO as probe molecule
2
IR spectra of samples before and after thermal activation at
has been utilized. Interaction of the CO with acidic sites is
2
4
50 °C over a night (12 h) are provided in Fig. S2 of
supplementary information (SI). IR spectra shows that
weak which results in molecularly physisorbed CO char-
2
acterized by stretching (m ) vibration band at about
3
-
1
before calcination the bands related to trapped KNO in the
3
2350 cm [29, 30], while it has strong interaction with
basic sites and consequently, it is good probe molecule for
characterization of basic sites [1, 20]. IR spectra of pre-
pores of sample could be detected (bands at 1370 and
-
1
1
0
405 cm
for BEA structure related to splitted m (e )
3
vibration bands arise from the effect of the cation field on
the nitrate ion removing the threefold axis of symmetry and
producing a common symmetry between nitrate and cation
field of C2v [24], small shift observed in other structures
due to different dispersion interaction and same result
observed in [25]); however, after thermal activation at
pared samples after CO adsorption are provided in Fig. S3
2
of SI. It could be seen that the bands related to physisorbed
-
CO (main band at 2350 cm ) are disappeared upon short
1
2
evacuation at room temperature, while the bands with
-
1
lower wavenumbers (\ 1800 cm ) falling in the ranges
related to carbonates and chemisorbed species are persist
even after dynamic evacuation. These bands could be uti-
lized for characterization of basic sites in the samples.
Basically, free carbonate ion has a simple stretching band
4
50 °C over a night, this band disappeared which indicated
that nitrates are decomposed and K O clusters formed or
2
the potassium is exchanged with the zeolitic Brønsted
-
1
protons by means of solid state reaction [26–28]. Results of
?
the previous research [7, 19] indicate that exchanged K
(m ) at 1415 cm . However, upon adsorption of carbonate
3
ion on the surface, its symmetry lowered from D3h to C2m
cations do not form strong basic sites in BEA or FAU
zeolites; however, in potassium impregnated zeolites with
[31] due to covalent bonding through one or two oxygen
and this band (m ) is splitted into two other bands in either
3
-
1
sides of 1415 cm with lower and higher wavenumbers.
excess of potassium, the stable clusters of K O as strong
2
basic site could be formed.
The splitting wavenumber, Dm , can be implemented to
3
measure the strength of the base sites: the lower splitting is
1
23

Study on thermal stabilities and symmetries of chemisorbed species formed on K-zeolites upon…
associated with the stronger the basic site [1]. Dm values of
K-zeolites at room temperature considering the splitting
3
-
1
about 100, 300, and 400 cm are generally ascribed to
monodentate, chelating bidentate, and bridging bidentate
structures, respectively [23]. Figure 2 shows the observed
wavenumber is bidentate species (these species itself can
also be divided into two subgroups in some cases like BEA
zeolite considering the Dm and they can be attributed to
3
bands in IR spectra of samples after CO adsorption and
2
different chelating and bridging bidentate species [1].).
subsequent evacuation. Interestingly, the formed bands are
changing with the change of the zeolite type; however, in
case of the samples with same structure and different Si/Al
ratio, just the intensities of the bands are different (Fig. 2b, c).
Moreover, in some samples (for instance K-FAU(15) and
-
1
K-FAU(40)), additional bands (e.g., band at 1447 cm
)
-
1
with lower splitting wavenumber (100–200 cm , Fig. S3
-
in SI, the counter part of the band at 1447 cm is in right
1
-
1
The chemisorbed CO in impregnated K-BEA(11.5) and
2
shoulder of the band at 1690 cm considering Fig. S4 in
SI) more possibly related to monodentate species are also
observed [23, 33]. Additional experiment with longer time
of thermal activation (24 h) and longer time of CO2
adsorption (30 min) equilibration (see Fig. S5 in SI)
revealed that formation of chemisorbed species has slow
kinetic and the density of species with higher symmetry
-
1
K-BEA(19) could be detected by bands at 1688 cm
-1
(
moved to 1680 cm
at low coverage), 1354 and
-1
1
1
325 cm (Fig. 2b, c). The species related to the bands at
-1
354 and 1680 cm are very stable even after dynamic
-1
vacuum. Dm is about 363 cm (1688–1325 cm ) and
-1
3
-1
-1
decreased to 326 cm (1680–1354 cm ) at low coverage
for BEA structure. Considering the extent of splitting, it can
be concluded that two sets of bidentate species may be
present in K-BEA zeolite and upon evacuation the one with
higher stability and lower splitting persists. Bands of che-
misorbed carbonate-like species in K-MFI(21) at room
temperature are very stable and they are observed at
(lower Dm ) increased with longer activation or equilibra-
3
tion time.
Temperature programmed desorption of CO2
CO –TPD curves of the prepared K-zeolites are depicted in
2
-
1
-1
-1
1
675 cm (1672 cm in lower coverage) and 1368 cm
Fig. 3. Two different desorption peaks could be detected in
the most of the cases; however, in case of K-MFI(21), the
high temperature desorption peak is negligible. It should be
-
1
(
1365 cm in lower coverage) (Fig. 2a). In this sample,
-1
splitting Dm is about 307 cm at room temperature (lower
3
Dm in comparison with BEA zeolite result in higher sta-
considered that the observed peaks in CO –TPD patterns
2
3
bility at room temperature). It should also considered that
are different than what observed for basic mixed oxide [34]
and the peaks are well differentiated in this case. The low-
temperature desorption peak is observed at & 50 °C,
while the high temperature peak observed at & 130 °C for
different samples. Considering this results, it can be con-
cluded that two categories of the basic sites may present in
the samples which agrees with IR results. It seems that the
smaller pore size of MFI zeolite (10 membered
CO adsorption on alkali modified MFI zeolite result in the
2
formation of sole bridged CO adsorption considering the
2
microcalorimetry and volumetry analysis [32] which is
somehow in agreement with current result.
In FAU (Y zeolites) samples with Si/Al ratios of 15 and
4
0 (see Fig. 2d, e), the bands related to chemisorbed spe-
-
1
cies could be detected and their splitting is about 362 cm
1
-
˚
(
340 cm
at lower coverage). A very weak band at
rings, * 5.5 A) in comparison with BEA (12 membered
-
447 cm could also be detected in IR spectra of these
1
˚
1
rings, * 6.68 A) and super-cage of Y zeolite (12 mem-
-
1
˚
˚
samples which exhibits the lowest Dm (100–200 cm
)
bered entrance window * 7.4 A with cavity of 11 A in
diameter) is the main reason that K O clusters would not
3
which is close characteristics of highly symmetric mon-
odentate species. Summarizing this part, it could be con-
cluded that the main type of adsorbed species on the
2
form inside the MFI pores. The lattice parameter of the
˚
crystalline K O with cubic structure is 6.436 A [35] which
2
Fig. 2 IR spectra of
3
2
1
0
1
2
before evacuation
after evacuation
(
(
(
(
a) K-MFI(21),
b) K-BEA(11.5),
c) K-BEA(19), (d) K-FAU(15),
e) K-FAU(40) sample after
0
.4
(a)
0.3
0.2
2
CO adsorption at room
Δν3
temperature (P = 1 mbar,
equilibration time = 20 min)
(
d)
(b)
(
blue curve labeled by legend 1)
0
.1
and then evacuation in dynamic
vacuum (red curves labeled by
legend 2). (Color figure online)
(
c)
(
e)
0.0
2
000
1800
1600
1400
1
950
1800
1650
11500
1350
–
–1
Wavenumber/cm
Wavenumber/cm
123

Y. Ganjkhanlou et al.
–
–
3
3
3
3
3
3
4
3
.5 × 10
35
Y = 284.8 X
R = 0.98
3
2
.0 × 10
30
(
a)
–
.5 × 10
–
–
25
20
2
1
1
5
.0 × 10
(
(
b)
c)
.5 × 10
–
.0 × 10
1
5
0
(d)
–
.0 × 10
(
e)
1
0
.0
0
100
200
300
400
500
°
0.04
0.06
0.08
TPD area/a.u.
0.10
0.12
Temperature/ C
Fig. 3 TPD of CO
c) K-BEA(19), (d) K-FAU(15), (e) K-FAU(40) samples after thermal
activation at 530 °C
2
recorded on (a) K-MFI(21), (b) K-BEA(11.5),
Fig. 4 Correlation of area under TPD curves and area of bands
related to chemisorbed species in the range of 1300–1800 cm
(
-1
derived mixed oxides using calorimetry and FTIR spec-
is bigger than the channel size of MFI zeolite (two channels
exist in MFI structure, both of them consist of 10 tetrahedra
troscopy upon CO adsorption, while in some old litera-
2
˚
with the sizes 5.5 9 5.1 A and 5.3 9 5.6 A) [36]. It should
˚
tures [1, 37] opposite trend is presented and authors
believed that monodentate complex is less stable. Com-
paring with many other basic solids (e.g., hydrotalcite like
materials [23]), K-zeolite shows quite unique behavior of
CO2 adsorption and the obtained IR spectra and TPD
results are simpler than other basic solid. Consequently,
thermal stabilities of different chemisorbed species could
be easily determined in this case.
also be considered that based on molecular dynamics
simulations [35] with decreasing the size of the K O
2
clusters its stability decreasing (free energy increased).
Therefore, it can tentatively be claimed that the formed
K O clusters in MFI channels are thermodynamically
2
unstable due to high surface area. Another explanation
would be diffusion constrain which caused the inaccessi-
bility of possibly formed K O clusters in MFI channels. In
2
In situ IR study during the CO desorption
2
any case, it can be assumed that accessible basic species
(
K O clusters) in MFI structure are mainly formed on the
2
Figure 5a shows the in situ IR spectra recorded on
surface of the zeolite crystals (SEM image in Fig. 1), while
in samples with larger micropores the accessible basic sites
could probably be present inside the zeolite channels as
K-FAU(15) upon desorption of CO . It could be seen that
2
the bands with higher splitting disappear first and the bands
with lower splitting are stable (even slightly grows in the
intensity) up to 80 °C and then they start to diminish
mainly at 130–150 °C. This means that species with lower
evidenced by TPD and CO -IR data.
2
To further proof the harmony of the TPD data and
results of IR study of CO sorption on samples, the cor-
2
-
1
symmetry (higher splitting, Dm [ 300 cm ) are less
relation between the area under the TPD curves and the
area of IR bands related to chemisorbed species (bands
3
stable and therefore these species (mainly bidentates con-
-
1
-
1
sidering the splitting wavenumbers, Dm [ 300 cm ) are
observed in the range of 1300–1800 cm ) after normal-
ization of IR spectra by weight of prepared self-supporting
3
removed/transformed to species with higher symmetry,
most probably monodentate type, considering the splitting
-
pellets is displayed in Fig. 4. TPD and CO -IR data are
2
1
wavenumber (Dm * 100–200 cm ). The bands with
well correlated and show linear behavior. Outcome of this
observation is that in situ IR spectroscopy upon desorption
3
-
1
lower splitting (Dm \ 200 cm ) grow up in intensity/
3
stable upon heating up to 80 °C (e.g., observed bands at
of CO could be implemented for unraveling the peaks
2
-
1
635, 1590, 1431 and 1441 cm , Fig. 5a) which indicate
1
observed in CO –TPD curves. Considering literatures,
2
that bidentate species are desorbed by heating and some of
them adsorbed as monodentate species with higher sym-
metry). In case of K-BEA(11.5) sample (Fig. 5b) upon
heating, the bands with lower splitting are formed (bands at
there are controversial reports about thermal stability of
different types of chemisorbed CO on basic sites due to
2
complexity of basic sites in investigated materials. For
instance in [23], it was reported that bidentate species are
desorbed upon heating up to 50 °C, while monodentate
carbonate are thermally stable and they may persist upon
heating even up to 400 °C on the surface of hydrotalcite-
-
1
1629, 1580, 1472, 1436 and 1404 cm ) which can be
explained by same concept. In case of K-MFI(21), negli-
gible amount of monodentate species are present as
1
23

Study on thermal stabilities and symmetries of chemisorbed species formed on K-zeolites upon…
(a)
RT (CO -1mbar)
2
Vacuum RT
°
5
8
1
1
2
0 C
°
0 C
30 C
50 C
00 C
°
°
°
Δν
1800
1700
1600
1500
1400
1300
–
1
Wavenumber/cm
(b)
(
c)
CO 2 mbar
CO 2mbar
2
2
Vacuum (0.01 mbar)
Vacuum (0.01 mbar)
80
°
C
80 C
°
Dynamic vauum at 80
°
C
Dynamic vacuum at 80 C
°
1800
1700
1600
1500
1400
1300
1800
1700
1600
1500
1400
1300
–
1
–1
Wavenumber/cm
Wavenumber/cm
Fig. 5 In situ IR spectra upon thermal desorption of CO
could be implemented to describe the observed desorption peak in the
2
(the results
thermal activation at 450 °C (24 h) and CO
20 min). (Color figure online)
2
adsorption (1 mbar,
TPD curves). a K-FAU(15), b K-BEA(11.5), c K-MFI(21) after
evidenced by the absence of the bands with low splitting in
IR spectra (Fig. 5c) and as it could be observed, almost all
the bands are removed upon heating up to 80 °C, which
agree with the presence only low-temperature peak in
group [7]. The main possible products of the reaction with
their IUPAC names are listed in Table 2. For sake of
brevity, the abbreviations will be used in the following text.
For K-BEA(11.5) and K-FAU samples, the data were
adopted from [7, 19], respectively. The results are sum-
marized in Fig. 6a, b and Table 3. Considering Table 3,
K-BEA(11.5) and K-FAU(15) samples show high furfural
CO –TPD.
2
All in all, CO –TPD and IR results indicate that two
2
different types of the basic sites may be present in K-ze-
olites and upon CO adsorption, they form chemisorbed
2
conversion and selectivity to long chain product (F Ac) is
2
carbonate-like species with different symmetries.
Depending on the zeolite pore size and morphology, the
distribution of these two types is also different. It is also
observed that the highly symmetric species have higher
thermal stability in comparison with lower symmetric
species.
higher on these samples in comparison with other samples.
BEA(19) and K-FAU(40) samples with same structure and
higher Si/Al ratio show lower activity most probably due to
less basic sites and potassium content (Table 1).
K-MFI(21) also shows less activity and selectivity to F Ac.
2
To correlate these results with characterization data, CO2–
TPD data were compared with catalytic activity. Conver-
Catalytic test
sion of furfural increases with the increasing CO –TPD
2
area (Fig. 6a) and even have better correlation with area of
HT desorption peak in TPD curves (inset of Fig. 6a) which
indicates that basic sites detected by TPD (mainly those
form HT peak in TPD) are the active species catalyzing the
reaction. What is more, considering Table 3, selectivity of
The aldol condensation of furfural and acetone as impor-
tant reaction for biomass valorization [11, 15, 16] was
performed on K-BEA(19) and K-ZSM(21) samples in
according to the instruction given in earlier report of our
123

Y. Ganjkhanlou et al.
Table 2 Main possible products obtained after aldol condensation of furfural and acetone on heterogeneous catalyst
Product abbreviation
IUPAC name
Other names/naming
FAc
4-(2-Furyl)-3-buten-2-one
Furfuryl acetone
FAcOH
4-(2-Furyl)-4-hydroxy-butan-2-one
1,5-Di-2-furanyl-1,4-pentadien-3-one
4-(2-Furyl)-5-(2-furylmethyl)-3-heptene-2,6-dione
(Furfuryl hydroxide)acetone
Difurfuryl acetone
2
F Ac
(
FAc)
2
Bis-furfuryl acetone
(
b)100
(
a)100
100
8
0
0
80
60
40
20
5
0
6
0
0
.00
0.04
0.08
HT TPD area/a.u.
40
20
0
0
0
0.00
0.02
0.04
0.06
0.08
0.10
.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
TPD area/a.u.
Area of HT in TPD/a.u.
Fig. 6 a Correlation of observed furfural conversion on different
samples with calculated TPD area, b plot the selectivity to different
products upon the aldol condensation of furfural and acetone on the
samples versus the area of high temperature desorption peak in TPD
curves (Inset) Correlation of observed furfural conversion on different
samples with area of high temperature peak in TPD curves. (Color
figure online)
-
1
OH group stretching vibrational band at about 3600 cm
,
Table 3 Furfural conversion (X%) and selectivity (S) to different
products measured on different samples after catalyst testing for aldol
condensation of furfural and acetone
cf. Fig. S2 in SI) and therefore this product is not formed.
The integrated area of high temperature (HT) desorption
peak in TPD curves plotted against selectivity of product
Sample
X/%
S
FAc/%
S
FAcOH/%
S
F2Ac/%
S
(FAc)2/%
(Fig. 6b) clearly showed that with increasing of the area
K-MFI(21)
K-BEA(11.5)
K-BEA(19)
8
85
35
42
39
32
29
46
36
41
70
2
7
0
0
0
0
0
(increasing the number of strong basic sites), amount of
a
longer chain product (F Ac) increased, while the selectivity
2
69
40
16
14
to FAcOH decreased. The selectivity to FAc is almost
constant indicating that FAc production is not strongly
14
48
45
b
K-FAU(15)
b
related to basic sites characterized by HT in CO –TPD
2
K-FAU(40)
curves. From Table 3, it can also be observed that BEA and
dealuminated Y zeolites exhibit much higher furfural
conversion than MFI zeolite which could be correlated
with CO –TPD and CO –IR results. It is also clearly vis-
a
Data adopted from the our recent work [7]
b
Data adopted from the our recent work [19]
2
2
product is also function of the distribution and density of
different basic sites as well as pore size of zeolites. The
ible that BEA zeolite exhibits the highest selectivity to
F2Ac product from all investigated zeolites. Considering
the selectivity data in Fig. 6b, it can be observed that the
correlation is not always straightforward and the trend of
change is more complex because not only character of
basic sites but also inner void space around these sites
influenced products of the reaction.
longer chain products F Ac (C13), which is favorable
2
product for biodiesel application [17], is formed in samples
with strong basic sites and larger pores (those formed
monodentate carbonate upon CO adsorption on BEA and
2
FAU samples). More interestingly, no (FAc)2 products
were detected in the reaction mixture. Formation of (FAc)2
products needs presence of acid site, as documented on
acidic form of zeolites MWW [12], BEA [7, 13], MFI [13]
and USY [13, 19]. Impregnated samples do not possess any
acidic site (due to exchange of proton with potassium
during the impregnation, as documented by the absence of
Schematics of the formed basic sites on different zeolite
are shown in Fig. 7. Summarizing all the results one could
be noted that morphology and structure of zeolite particles
(crystal size, porous microstructure and Si/Al ratio) are the
main factors affecting the distribution and types of formed
basic sites. The possible inter/intra K O clusters are the two
2
1
23

Study on thermal stabilities and symmetries of chemisorbed species formed on K-zeolites upon…
Fig. 7 Schematic of formed
basic sites on different samples
Beta (BEA)
ZSM-5 (MFI)
K
2
O clusters
Exchanged K sites
(
two major type of basic sites
most probably inter or intra K
clusters could be formed in
2
O
Ring size:
2 T atoms
1
K-zeolite and their distribution
is function of morphology)
Ring size:
10 T atoms
On surface of zeolite
crystals
Y (FAU)
Ring size:
2 6 4 T atoms
1
major types of formed basic site and their distribution plays
important role on the basicity of the samples. It seems that
basic sites formed inside the pore of the zeolite are highly
active and responsible for formation of more symmetric
pore zeolites and materials with some mesoporosity (e.g.,
BEA or dealuminated FAU zeolites). Moreover, it is
observed that almost all the bidentate species are desorbed
at 50–80 °C, while monodentate species are thermally
stable at least up to 130 °C. Catalytic tests also showed that
sample with stronger basic sites (those formed monoden-
monodentate carbonate-like species upon CO adsorption.
2
The higher activity of these small clusters may be related to
confined space effect which alters the symmetry of adsor-
bed species.
tate carbonates upon CO adsorption) is more active for
2
aldol condensation of furfural and acetone and forms
longer chain products.
Acknowledgements Financial support from the Czech Science
Foundation for the project of the Centre of Excellence (P106/12/
G015) is gratefully acknowledged.
Conclusions
Study of the formed basic sites in the K-zeolites with dif-
ferent structure and Si/Al ratio by CO as probe molecule
2
indicates that K-zeolite shows quite unique behavior of
CO adsorption and the obtained IR spectra and CO –TPD
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