Quantitative infrared spectroscopic studies and 2D COS analysis of xylenes isomerization over hierarchical zeolites

Article abstract of DOI:10.1016/j.cattod.2016.02.035

The microporous and hierarchical ZSM-5 zeolites were studied with respect to their interaction with o-, m- and p-xylene molecules as well as the catalytic activity in xylenes isomerization process followed by 2D COS analysis of IR spectra. The aspects of mechanism of xylene isomerization reaction have been discussed in the light of the results of 2D COS analysis of IR spectra and catalytic performance of studied zeolites. The ZSM-5 zeolites were characterized by different Si/Al ratio and by various types of generated mesoporosity. This selection assured ability to follow properly the changes in the catalytic performance related not only to acidic properties but also to porosity of the samples. It was shown that the microporous character of zeolites was crucial for high selectivity to the most desired product i.e. p-xylene. In the case of hierarchical zeolites it was confirmed that the formation of by-products as mesitylene and toluene occurred during ortho-xylene isomerization. Furthermore, for hierarchical zeolites the high amount of silanols groups prompted to the coke formation on highly developed external surface.

Full text of DOI:10.1016/j.cattod.2016.02.035

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Catalysis Today
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Quantitative infrared spectroscopic studies and 2D COS analysis of
xylenes isomerization over hierarchical zeolites
a,∗
a
b
a
Karolina A. Tarach , Kinga Goł a˛ bek , Minkee Choi , Kinga Góra-Marek
a
Faculty of Chemistry, Jagiellonian University in Kraków, 3 Ingardena St., 30-060 Kraków, Poland
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The microporous and hierarchical ZSM-5 zeolites were studied with respect to their interaction with o-,
m- and p-xylene molecules as well as the catalytic activity in xylenes isomerization process followed
by 2D COS analysis of IR spectra. The aspects of mechanism of xylene isomerization reaction have been
discussed in the light of the results of 2D COS analysis of IR spectra and catalytic performance of studied
zeolites. The ZSM-5 zeolites were characterized by different Si/Al ratio and by various types of generated
mesoporosity. This selection assured ability to follow properly the changes in the catalytic performance
related not only to acidic properties but also to porosity of the samples. It was shown that the microporous
character of zeolites was crucial for high selectivity to the most desired product i.e. p-xylene. In the case
of hierarchical zeolites it was confirmed that the formation of by-products as mesitylene and toluene
occurred during ortho-xylene isomerization. Furthermore, for hierarchical zeolites the high amount of
silanols groups prompted to the coke formation on highly developed external surface.
Received 31 October 2015
Received in revised form 14 January 2016
Accepted 18 February 2016
Available online xxx
Keywords:
2
D COS analysis
Xylene
Isomerization
Hierarchical zeolites
ZSM-5
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
wavenumbers and one for correlation level, expressed as intensity.
In synchronous correlation applied in this study the observed on
diagonal positive auto-peaks represent each varying band at given
wavenumber. The positive correlation between two separate bands
(of different wavenumbers) represents the simultaneous increase
or decrease in intensity of these bands. On the other hand, the nega-
tive correlation occurs when one of the bands changes its intensity
at the expense of the other one. The use of 2D COS analysis signif-
icantly enhanced the use of IR spectroscopy to following even the
slightest changes in IR spectrum during heterogeneous reactions at
high temperatures [6].
The synthesis of hierarchically structured zeolites are performed
according to two different approaches. Bottom-up approaches pro-
vide the control during synthesis and employ either various types
of templates (e.g. surfactants, porous carbon, cationic polymers,
etc.) [7–10] or concern zeolitization of mesoporous materials [11].
Also direct synthesis of nanosized crystals [12,13] is in the field of
interest of researchers. Hierarchical zeolites obtained in the pres-
ence of surfactants contain the acid sites of high concentration and
strength, and the additional uniform mesoporosity improves the
accessibility of acid sites to reactants. The tuning of both the shape
and the size of pores is a matter of template strategy. On the other
hand, a selective extraction of silica (via desilication) [14–16] or
alumina (via dealumination) [17] is the most common of top-down
strategies. The demetalation processes led to the formation of zeo-
The ZSM-5 zeolite is widely applied in several industrial
processes requiring the shape-selectivity, including alkylation,
isomerization and disproportionation [1–3]. During xylene trans-
formation process two main reactions can take place: (i)
isomerization, which is aimed to increase the relative content of
p-xylene and (ii) disproportionation leading to trimethylbenzene
and toluene in equal amounts. It well know that the isomeriza-
tion reaction occurs through the monomolecular mechanism while
the disproportionation involves the bimolecular mechanism with
bulky intermediates [4].
The 2D COS analysis of spectroscopy results was developed by
Noda [5] and initially applied to the studies of sinusoidal changes in
IR absorbance spectra of polymer upon mechanical strain. Further
development of 2D COS analysis allowed operating with systems
changed upon non-periodical variables as in the case of chemical
processes undergoing on surface of catalyst. The 2D COS analysis
is based on Fourier transform, which is used to extract spectral
correlations occurring upon external modulation of system. As a
result a 3D graph of changes in spectra is obtained with two axes for
∗ Corresponding author.
E-mail address: karolina.tarach@gmail.com (K.A. Tarach).
http://dx.doi.org/10.1016/j.cattod.2016.02.035
0
920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: K.A. Tarach, et al., Quantitative infrared spectroscopic studies and 2D COS analysis of xylenes
isomerization over hierarchical zeolites, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.035

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2
lites with bimodal porosity and are known as the most effective
approaches for obtaining the hierarchical zeolites. The steaming of
zeolite Y and desilication of ZSM-5 zeolite are well-known exam-
ples [18–20]. The number of framework silicon or aluminium atoms
that could be removed without structural damage is ruled by the
features inherent to zeolites (Si/Al ratio and framework topology)
as well as by the alkaline treatment conditions (type and concentra-
tion of agents used). Both processes not only create mesopores, but
also affect the acidity of hierarchical zeolite mainly by modifying
the nature of acidic sites [14,21].
Zeolite M-ZSM-5 was obtained via a direct synthesis route using
the amphiphilic organosilanes as a mesopore-directing agent [9].
For the synthesis of mesoporous zeolite M-ZSM-5 of Si/Al = 29,
the TPOAC (3-(trimethoxysilyl) propyl) octadecyldimethylammo-
nium chloride) (Aldrich, 42 wt% in methanol) was added to a
conventional ZSM-5synthesis gel containing TPABr (tetrapropy-
lammonium bromide). The molar synthesis composition was 43
Na O: 5 Al O : 200 SiO :20 TPABr:10 TPOAC:15 H SO :18000 H O.
2
2
3
2
2
4
2
In a typical synthesis, 1.77 g NaOH, 2.75 g TPABr, 6.09 g TPOAC,
and 130 g H O were dissolved in a polypropylene bottle. Then,
2
The studies concerning the use of 2D COS analysis of IR spec-
tra to follow the heterogeneous reaction of xylene isomerization
are quite limited. The results presented by several authors have
concerned the o-xylene isomerization process over H-ZSM-5 zeo-
lites. The role of porosity and acidity during isomerization has
been mostly studied in context of coke deposition [6,22,23]. This
work was attempted to offer an insight into acidity and poros-
ity properties of microporous and hierarchical ZSM-5 zeolites in
respect to their interaction with xylene molecules and also their
catalytic performance in xylene isomerization process. The range
of materials studied in this paper, typically microporous ZSM-5
and their hierarchical analogues obtained by different methods,
however representing at some points similar properties, allowed
following the influence of acidity and porosity on xylene isomer-
ization process. The application of 2D COS analysis to the results of
time-resolved in-situ IR spectroscopy enabled following even the
slightest changes in the spectra of reagents adsorbed over studied
zeolites. The correlation either positive or negative between diag-
nostic bands of xylene isomers provided valuable information on
processes taking place during catalytic reaction. Both acidity and
porosity of studied zeolites were significantly different depending
on synthesis or modification procedure however they found the
reflection in catalytic reactions.
21.5 g TEOS (tetraethylorthosilicate) was added into the solution
and stirred for 1 h. Then, 1.72 g Al (SO ) · 18H O dissolved in 37.0 g
2
4
3
2
H O was added to the aforementioned solution and then homog-
2
enized by handshaking. The synthesis gel was stirred at room
temperature for 24 h for aging. The resultant gel was hydrother-
◦
mally treated in a tumbling autoclave at 170 C for 72 h. After the
hydrothermal crystallization, the precipitated products were fil-
◦
tered, washed with distilled water, dried at 110 C and calcined at
550 C [9].
◦
2.2. Characterisation techniques
2.2.1. X-ray diffraction (XRD)
Wide-angle XRD patterns were taken with a Rigaku Multiflex
diffractometer equipped with Cu K␣ radiation (40 kV, 40 mA).
◦
2.2.2. Nitrogen sorption measurements at −196 C
The nitrogen sorption measurements were performed on a
◦
Quantachrome Autosorb-1-MP gas sorption system at −196 C.
Prior to the measurements, all samples were degassed under high
◦
vacuum conditions for duration of 16 h at 200 C. The microp-
ore volume was calculated based on the t-plot method, while the
Brunauer–Emmet–Teller (BET) method was applied to determine
the apparent specific surface area, taking into account the rec-
ommendations of Rouquerol et al. [24]. Distributions of mesopore
diameters were calculated via the Barrett–Joyner–Halenda (BJH)
algorithm using the adsorption branch [25].
2
. Material and methods
2.1. Studied materials
Parent microporous ZSM-5 zeolite denoted as B-ZSM-5 of
2.2.3. STEM investigations
Si/Al = 32 was supplied by Zeolyst (CBV 5524G). The sample
denoted as D-ZSM-5 was obtained by desilication in the 0.2 M solu-
tion of NaOH&TBAOH (tetrabutylammonium hydroxide) mixture
The STEM micrographs were obtained using transmission elec-
tron microscope (JEOL 2100F) working at 200 kV, with Field
Emission Gun (FEG), EDX analysis and STEM detectors for bright
and dark mode.
◦
TBAOH/(NaOH + TBAOH) = 0.1 at the temperature of 80 C for 5 h.
The 100 ml of solution was added to 3.0 g of zeolite. After desil-
ication the suspension was cooled down in ice-bath, filtered and
washed until neutral pH. Finally, the zeolites were again filtered,
washed with distillate water, dried overnight at room temperature
2.2.4. IR spectroscopic studies
Prior to FTIR studies, the materials were pressed into the form of
2
self-supporting discs (ca. 5–10 mg/cm ) and pre-treated in situ in
◦
◦
and calcined at 550 C.
an IR cell at 500 C under vacuum conditions for 1 h. Time resolved
Carbon-black templated zeolite (denoted as C-ZSM-5) was syn-
FT-IR spectra were recorded with a Bruker Vertex 70 spectrome-
ter equipped with a MCT detector. The spectral resolution was of
2 cm . All the spectra presented in this work were normalized to
10 mg of sample. The 2D-COS graphs were prepared with use of
OPUS 3D software from Bruker Optics.
Total concentration of the Brønsted and Lewis acid sites in
calibration materials was determined in quantitative IR studies
of pyridine (hereafter denoted as Py) sorption according to the
procedure described in Refs. [21,26]. Pyridine was supplied by
Sigma-Aldrich (≥99.8%).
thesized using the gel composition of 2.0 NaCl: 1.67 Al O : 100
2
3
−
1
SiO : 20 TPA O: 200H O. The added carbon-black was 100 wt%
2
2
2
of the added silica. In a typical synthesis, 0.69 g NaCl and
.0 g aluminium isopropoxide were dissolved in 212 g TPAOH
tetrapropylammonium hydroxide) solution (TCI, 20–25% in H O).
4
(
1
2
22 g TEOS was added under vigorous stirring and the reaction mix-
ture was stirred for 3 h at room temperature. Then, 35.2 g carbon
black (HIBLACK 900 L, Evonik, average particle size: 15 nm, surface
2
−1
area: 270 m g ) was added and the mixture was stirred for 1 h
◦
for homogenization. The resultant mixture was heated at 90 C on
The concentrations of the Brønsted acid sites accessible for bulky
pivalonitrile (hereafter denoted as Pn) was achieved from quantita-
tive studies previously described in Refs. [27,28]. The pivalonitrile
(98%, Sigma Aldrich) was adsorbed on the zeolites at room temper-
ature followed by 20 min evacuation at the same temperature in
order to remove the physisorbed probe. The concentration of the
Brønsted and Lewis acid sites detected by Pn was calculated from
the maximum intensities (the maximum heights) of the respective
a hot plate under stirring to evaporate ethanol and extra H O to
2
obtain the aforementioned gel composition. The resultant synthe-
sis gel was transferred into a Teflon-lined stainless-steel autoclave
◦
and hydrothermally crystallized at 180 C for 72 h. After cooling
to room temperature, the zeolite product was filtered and washed
thoroughly with deionized water. The product was dried in an oven
◦
◦
at 130 C and subsequently calcined in air at 600 C for 6 h.
Please cite this article in press as: K.A. Tarach, et al., Quantitative infrared spectroscopic studies and 2D COS analysis of xylenes
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3
−
2
1
−1
bands at 2277 cm and 2305 cm and their extinction coefficients
(
−1
0.11 and 0.15 cm ␮mol , resp.).
3
. Results and discussion
3.1. Structural and textural characterization
The samples chosen for study were characterized by Si/Al ratio
in range between 22 and 32, as well as by different level and type of
developed mesoporosity (Table 1). This selection assured the abil-
ity to properly follow the changes in catalytic performance related
not only to acidity but also to porosity of the samples (Table 1). The
XRD patterns of the bulk B-ZSM-5 and carbon templated C-ZSM-
5
zeolites demonstrated well resolved reflections representative
for ZSM-5 structure (Fig. 1). Similar observation was made for
hierarchical zeolite D-ZSM-5 obtained via top-down approach, i.e.
desilication. Thus it can be concluded that the alkaline treatment
did not perturb the crystallinity of resulting D-ZSM-5 material. The
XRD pattern also confirmed formation of the ZSM-5 structure in
the case of M-ZSM-5 zeolite synthesized by bottom-up route; how-
ever its lowered crystallinity can be considered in the terms of both
highly developed mesoporosity and small size of zeolite grains. The
synthesis process of M-ZSM-5 zeolite led to the generation of reg-
ularly distributed mesopores and enhanced surface area (Table 1
Fig. 2), thus high contribution of disordered surface.
Fig. 1. XRD patterns of studied materials.
The textural characteristics of studied hierarchical zeolites were
derived from the low-temperature nitrogen sorption studies. Both
C-ZSM-5 zeolite showed that the synthesis procedure did not suc-
ceed in fabricating substantial mesoporosity, only the interparticle
macroporosity is present (Fig. 3). The enhanced surface area SBET
and high contribution of micropore surface area point to the well-
developed microporosity. This important characteristic of C-ZSM-5
should be underlined. The STEM images of B-ZSM-5 and C-ZSM-5
B-ZSM-5 and C-ZSM-5 zeolites demonstrated the type-I N adsorp-
2
−1
3
tion isotherm (Fig. 2a). The value of Vmicro = 0.17 cm g point to
the presence of microporosity typical of the MFI structure. Minor
mesoporosity in both samples results from the interparticle space
between the zeolite grains (Table 1). Indeed, the STEM images of
Fig. 2. (a) Low temperature N2 isotherms and (b) BJH pore size distributions of studied materials.
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Table 1
The Si/Al ratios and the Al concentrations determined by chemical analysis (AlICP), as well as the textural parameters from low temperature N2 adsorption/desorption
measurements for studied zeolites.
Material
Si/Al
AlICP
SBET
[m2
Smeso
Vmicro
Vmeso
[␮mol g−1]
g
−1
]
[m2
g
−1
]
[cm3
g
−1
]
[cm3
g ]
−1
[-/u.c.]
B-ZSM-5
C-ZSM-5
D-ZSM-5
M-ZSM-5
32
30
22
29
2.95
3.10
4.19
3.20
478
502
680
518
377
459
342
520
40
44
150
187
0.17
0.17
0.09
0.16
0.06
0.08
0.30
0.35
Fig. 3. The STEM images of bulk and micro/mesoporous zeolites.
additionally provide information on various sizes of their grains, for
B-ZSM-5 the grains size is in range of 150–300 nm while for C-ZSM-
also showed a type-IV N -isotherm with hysteresis loop at interme-
2
diate pressures, corresponding to the mesopores with maximum
diameter at 9.2 nm (Fig. 2b). It is worth noticing that despite simi-
lar values of mesopore surface the D-ZSM-5 and M-ZSZM-5 samples
significantly differ in the PSDs curves both in the maximum value
reached and the pore diameters broadening. The STEM image of
M-ZSM-5 zeolites (Fig. 3) indicated highly porous structure with
small grains agglomerated in globular structure.
5
grains are formed from aggregates of a smaller size. The significant
difference between the C-ZSM-5 and B-ZSM-5 samples is also seen
in the shape of the zeolite crystal grains. The C-ZSM-5 sample is
characterized by spherical grains with high surface roughness, on
the other hand the B-ZSM-5 sample possess grains of bigger size
and more rectangular shape. The top-down modified hierarchical
zeolite D-ZSM-5 demonstrated a type-IV N -isotherm and showed
The selection of the studied materials, from microporous ZSM-
5 to micro/mesoporous analogues obtained by top-down and
bottom-up methods, allows following the influence of porosity
character and acidity on xylene isomerization process.
2
the significant mesoporosity development upon alkaline treatment
with preservation of the crystallinity. Upon alkaline treatment the
development of mesopore surface area occurred at the expense
of micropore area, thus the drop of Vmicro was observed. Fur-
thermore, in D-ZSM-5 zeolite the plaguing of the micropores by
extra-framework aluminium material is also expected, in line with
IR acidity studies presented in Section 3.2.2. The STEM micrographs
3
3
.2. IR acidity characterization
.2.1. Analysis of the hydroxyls bands
(Fig. 3) confirmed the formation of secondary mesoporosity directly
Several well resolved bands of hydroxyl groups can be distin-
guished in the IR spectra of microporous and hierarchical zeolites
inside the zeolite grains. Truly micro/mesoporous zeolite M-ZSM-5
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Information on the strength of acid sites was based on the com-
parison of (i) the values of the ꢀn obtained in CO sorption
OH·CO
and (ii) the results of pyridine thermodesorption experiments. The
increasing strength of acidic Si(OH)Al groups is represented by
more pronounced downshift of the Si(OH)Al group band perturbed
by hydrogen bonding with CO molecule (the increasing ꢀꢁ
OH·CO
value—Table 2). Among studied zeolites the highest ꢀꢁ
values
OH·CO
were found for microporous zeolites B-ZSM-5 and C-ZSM-5, thus
the most acidic bridging hydroxyls are highly populated in those
zeolites. Similar conclusion was withdrawn from Py thermodes-
−
1
+
orption experiments. The conservation of the 1545 cm
(PyH )
−
1
and 1450 cm (PyL) bands under the desorption temperature at
50 C was taken as a measure of the acid strength of the acid sites
◦
3
(Table 2). The lower strength of sites, the more disposed are Py
molecules for desorption from acid sites. The reduced acid strength
of Si(OH)Al groups was again observed for the micro/mesoporous
zeolites. This observation is in line with literature data reporting
that the external sites are less acidic than those in solely microp-
orous channels.
3.2.3. Accessibility of protonic sites
Regarding the catalytic applications the material with model
acidic properties i.e. the high concentration and strength of the
acid sites as well as with the shape selective properties should
also meets the proper accessibility of acid sites. The detailed char-
acterization of accessibility of acid sites in studied zeolites was
performed in fully quantitative manner using Pn (pivalonitrile)
Fig. 4. IR spectra of studied materials in the region of O H vibration.
[
28] as probe molecule. Even at room temperature the pivaloni-
(
Fig. 4). The highest frequency band (3745 cm−1) represents the
trile molecules are able to reach through the 10-MR channels of
zeolite the first available acid sites within the micropores mouths
[
isolated silanols groups located on mesopore surface and on exter-
nal surface of crystallites. Indeed, the intensity of isolated silanols
band is significantly raised up for the hierarchical D-ZSM-5 and
M-ZSM-5 zeolites. The 3725 cm
silanols was found only in the spectra of B-ZSM-5 and C-ZSM-5
zeolites. Additionally, the presence of 3690 cm Al(OH) band of
extra-framework non-acidic Al-species was detected in bulk zeolite
B-ZSM-5. The acidic Si(OH)Al groups are represented by 3610 cm
band. Both the intensities and the positions of these bands indicated
the differentiated acidity of materials studied with regard to both
the amount of sites and their strength.
28,29]. Consequently, the quantification of the acid sites exposed
on mesopore surface and those hidden near to micropore mouths
can be assessed. Furthermore, the sorption of Pn enables distin-
guishing among the accessibility of the Brønsted and Lewis acid
sites. The values of the accessibility factors of pivalonitrile (AFPn)
for Brønsted and Lewis acid sites were calculated as the ratio of
the concentrations of acid sites accessible to Pn molecules to those
determined from sorption of pyridine (Table 2).
For all the studied materials the accessibility factor values were
significantly higher for micro/mesoporous zeolites than for purely
microporous ones. Even though the C-ZSM-5 sample is composed
from small and highly agglomerated grains no substantial acces-
sibility of acid sites was evidenced. The highest accessibility of
protonic sites (AFPn = 60%) was observed for mesoporous M-ZSM-5
−
1
band attributed to the vicinal
−
1
−
1
3
.2.2. Concentration and strength of acid sites
The concentrations of Brønsted and Lewis acid sites were cal-
−
1
culated using the maximum heights of the 1545 cm
pyridinium ions (PyH ) and of the 1455 cm
natively bonded pyridine to Lewis sites (PyL), by applying the
extinction coefficients equal 0.07 cm /␮mol and 0.1 cm /␮mol,
respectively [21,26]. The quantitative studies of Py sorption indi-
cated that the sum of Brønsted and Lewis acid sites concentrations
is comparable with the total Al concentration from chemical anal-
ysis for all studied samples (Table 2). Purely microporous zeolites
contained comparable amount of Brønsted acid sites; the C-ZSM-
band of
B
zeolite. Lower AFPn factors of both Brønsted and Lewis acid sites
+
−1
band of coordi-
were recognized for desilicated zeolite. Additionally, enhanced
accessibility of Lewis acid sites in hierarchical zeolites points to
their high dispersion on mesopore surface.
2
2
3.3. Analysis of IR spectra of xylenes interacting with
microporous and hierarchical zeolites
5
additionally maintained significant amount of Lewis acid sites.
3.3.1. Interaction of meta-xylene, ortho-xylene and para-xylene
with zeolite
Desilication process considered as selective extraction of silicon
from zeolite framework assured increased concentration of acid
sites. Indeed, the alkaline treated D-ZSM-5 revealed enhanced
Brønsted and Lewis acidity in comparison with its bulk B-ZSM-5
counterpart. Bottom-up synthesized mesoporous zeolite M-ZSM-5
possessed slightly lowered amount of Brønsted acid sites and rela-
tively high contribution of Lewis acid sites. As mentioned, the total
concentration of acid sites detected with pyridine fully reproduced
the aluminium concentration derived from chemical analysis, thus
at our experimental conditions the diffusion of pyridine molecules
was not limited even though the presence of high content of extra-
framework species.
The basic requirement in quantitative IR studies is the determi-
nation of the molar extinction coefficient of the diagnostic band of
probe molecule interacting with the specific adsorption site. Thus,
there is need for an elaboration of experimental conditions in which
the probe molecule reacts selectively with acid site according to
known stoichiometry. The simplest and the most desired situation
is when an each introduced probe molecule, for example one of
the xylene isomer, reacts selectively with the one kind of the acid
site solely, for example the Brønsted acid site. The sorption of the
xylenes molecule seems to obey this condition. The IR spectra in the
region of cycle vibrations of the xylenes and by-products of isomer-
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Table 2
Concentration (␮mol g−1) of the Al determined by chemical analysis (AlICP), of the Brønsted (B) and Lewis (L) acid sites measured in quantitative experiments of pyridine (Py),
pivalonitrile (Pn) together with respective accessibility factor (AFPn). Acid strength of Brønsted sites measured as the downshift of the Si(OH)Al band perturbed by hydrogen
bonding with CO molecule, ꢀꢁOH·CO, as well as the strength of Brønsted and Lewis sites derived from Py-thermodesorption experiments.
Concentration [␮molg 1
−
]
Acid strength of sites
Material
Si/Al
AlICP
Py
B
Pn
B
AFPn
B
ꢀꢁOHCO
Py
B
L
B + L
L
L
[cm−1]
L
B-ZSM-5
C-ZSM-5
D-ZSM-5
M-ZSM-5
32
30
22
29
471
502
680
518
390
390
582
342
34
424
491
682
449
48
7
0.12
0.10
0.40
0.60
0.21
0.18
0.63
0.56
315
314
309
312
0.95
1.00
0.90
0.90
1.00
1.00
1.00
1.00
100
100
107
38
233
206
18
63
60
[
6], confirming the correctness of performed quantitative proce-
dure. To the best of our knowledge the absorption coefficient of
−
1
1
515 cm band representing the interaction of para-xylene with
Brønsted acid sites has not been previously reported. The calcu-
lated absorption coefficients will be further used for quantitative
evaluation of the products of catalytic reaction performed in IR
quartz cell. The xylene isomerization process can also lead to for-
mation of by-products, thus the absorption coefficients of 1610
−
1
and 1495 cm bands representative respectively for mesitylene
and toluene interaction with Brønsted acid site were derived. The
attained values of the absorption coefficients based on the heights
−
1
−1
of the 1610 cm and 1495 cm bands attributed to the mesity-
lene and toluene bonded to Brønsted acid sites were found to be
2
2
equal of 0.093 ± 0.002 cm /␮mol and 0.172 ± 0.001 cm /␮mol.
3
.3.2. 2D-COS analysis of isomerization of o-xylene over studied
◦
zeolites at 250 C
Fig. 5. IR spectra in region of cycle vibrations of the xylenes and by-products of
isomerization (toluene and mesitylene) adsorbed on the B-ZSM-5 zeolite in room
temperature.
The ortho-xylene isomerization over zeolites in the function of
time can be considered as the external perturbation suitable for
2
D COS analysis. For purposes of this study the series of spectra
were collected during 15 min reaction time. Fig. 6 presents the
results of synchronous correlation of series of spectra in the region
ization (toluene and mesitylene) adsorbed on the B-ZSM-5 zeolite
in room temperature are shown in Fig. 5.
−
1
of cycle vibrations of products of isomerization (1650–1360 cm
)
To identify the diagnostic bands of xylenes interacting with
Brønsted acid sites the sorption of each isomer was carried out
on zeolite B-ZSM-5 possessing only negligible amount of Lewis
acid sites. The interaction of the xylenes with Brønsted acid
sites n B-ZSM-5 zeolite at room temperature resulted in the
over purely microporous B-ZSM-5 and C-ZSM-5 zeolites. The anal-
ysis for hierarchical D-ZSM-5 and M-ZSM-5 is shown in Fig. 7. For
clarity, the resulting 2D maps for each sample were divided into two
separate graphs with positive and negative correlation peaks. Addi-
tionally, the 3D representations of correlation levels for all studied
zeolites were included in Supporting information. The positive cor-
relation between two separate bands represents the simultaneous
increase or decrease in intensity of these bands. On the other hand,
the negative correlation occurs when one of the bands changes its
intensity at the expense of the other one. Thus, during ortho-xylene
isomerization process following features should be observed: (i)
the positive auto- and cross-peaks of bands of each isomer present
in reaction (ii) the negative cross-peaks representing the transfor-
mation of bands of o-xylene to m-xylene or p-xylene.
−1
decrease of the Si(OH)Al band (3615 cm ) and the appearance
−
1
−1
−1
of the 1605 cm , 1515 cm
and 1465 cm bands attributed
respectively to the meta-xylene, para-xylene and ortho-xylene
bonded to Brønsted sites (Fig. 5). The selection of diagnostic
bands was based on the results presented in paper of Thibault-
Starzyk et al. [6], where the most intense negative covariance
for these bands has been reported. The conditions required for
quantitative measurement were ensured by both sorption of rel-
atively small amounts of xylene isomers and a specified 10 min
time delay before collecting the spectra enabling the diffusion of
xylenes through 10 MR channel system of B-ZSM-5 zeolite. The
second aspect worth underlining is the fact that the sorption of
more bulky ortho- and meta-xylene isomers led to the interac-
tion of probes not only with acidic Si(OH)Al groups but also with
free Si(OH) groups. However, this interaction was negligible if
enough small portion of probe molecule was introduced to IR cell.
Consequently, the rule of the selective reaction of xylenes with
Brønsted acid sites was fulfilled. The values of the absorption coef-
The 2D COS analysis of the spectra recorded during the o-
◦
xylene isomerization at 250 C allowed concluding that positive
correlation (auto- and/or cross-peaks) for all studied samples,
both microporous and hierarchical ones, can be observed for the
bands representative for m-xylene (1623, 1605, 1592, 1484 and
−
1
−1
1382 cm ), o-xylene (1497, 1465, 1428 and 1414 cm ) and p-
−
1
xylene (1515 cm ). For all studied samples the positive correlation
was also found between bands of p-xylene (1515 cm ) and m-
xylene (1623, 1605, 1592 cm ) pointing to coincident formation
of both isomers. Only for microporous B-ZSM-5 and C-ZSM-5 the
positive correlation was also found for additional bands of p- and
−1
−
1
−
1
−1
ficients attained from the heights of the 1605 cm , 1515 cm
−
1
and 1465 cm bands attributed to the meta-xylene, para-xylene
and ortho-xylene bonded to Brønsted acid sites were found to
−
1
−1
m-xylene (1515 cm ∼1484 cm ). What is more, in the case of
hierarchical D-ZSM-5 and M-ZSM-5 zeolites the positive correla-
tion between the bands of both p-xylene and m-xylene (1515 and
2
2
be equal of 0.029 ± 0.001 cm /␮mol, 0.224 ± 0.004 cm /␮mol and
2
0
.024 ± 0.001 cm /␮mol, respectively. The absorption coefficient
−
1
−1
values obtained for meta and ortho-xylene are in good agree-
1623 cm ) and the band of mesitylene (1610 cm ) confirmed the
formation of mesitylene as by-product formed by xylenes dispro-
ment with results presented in paper of Thibault-Starzyk et al.
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isomerization over hierarchical zeolites, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.035

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◦
Fig. 6. Positive (right) and negative (left) part of the 2D synchronous correlation map of ortho-xylene isomerization reaction at 250 C for microporous zeolites B-ZSM-5 and
−
1
C-ZSM-5. Top and right projection represent the change of correlation level at 1515 cm
.
portionation. Xylenes disproportionation leads to the production
of toluene as the second product. However, the diagnostic bands of
For all studied zeolites the negative correlation was also found
−
1
between the bands of o-xylene (1465 cm ) and bands of both p-
−
1
−1
toluene (1604 and 1495 cm ) overlap with the bands of m-xylene
and o-xylene thus the toluene production is difficult to be detected
in IR spectra. However the formation of toluene can be tentatively
xylene and m-xylene (1605 and 1515 cm , resp.). It can be noticed
that the C-ZSM-5 sample with highly agglomerated small grains
demonstrated more pronounced than B-ZSM-5 sample correlation
level between the above mentioned bands, what is well presented
on 3D representations of correlation levels (Supporting informa-
tion). The textural properties of C-ZSM-5 sample seem to facilitate
the occurrence of isomerization process. These conclusions are sup-
ported by the results of quantitative evaluation of reaction products
presented in Section 3.3.3. Less selective character of hierarchical
zeolites in the case of xylene isomerization reaction has been also
presented in work of Fernandez et al. [23], where authors widely
discussed the formation and nature of coke species formed during
reaction.
−
1
confirmed by negative correlation between the 1495 cm band of
−1
toluene and 1623 and 1515 cm bands of m-xylene and p-xylene,
respectively. Still however, the negative correlation between the
−
1
−1
o-xylene (1465 cm ) and mesitylene (1610 cm ) bands observed
only for hierarchical zeolites confirms that mesoporous zeolites as
catalysts in isomerization of xylene are less selective and the forma-
tion of bulky by-products as mesitylene (or others in no detectable
amounts) is also effective. Consequently, the formation of toluene
was derived from the presence of mesitylene and above mentioned
−
1
negative correlation between the 1495 cm band of toluene and
the bands of m-xylene (1623 cm 1) and p-xylene (1515 cm−1).
−
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◦
Fig. 7. Positive (right) and negative (left) part of the 2D synchronous correlation map of ortho-xylene isomerization reaction at 250 C for hierarchical zeolites D-ZSM-5 and
−
1
M-ZSM-5. Top and right projection represent the change of correlation level at 1515 cm
.
3
.3.3. 2D-COS analysis of isomerization of m-xylene over studied
The 2D COS analysis of the spectra recorded in 15 min reaction
◦
◦
zeolites at 250 C
time of the m-xylene isomerization at 250 C over microporous and
hierarchical zeolites clearly revealed their porous characteristic.
The positive correlations seen as auto- and/or cross-peaks indepen-
dently from the type of zeolite were found for the bands of m-xylene
Similarly as presented above the meta-xylene isomerization was
carried out for 15 min in quartz IR cell and the collected spectra
were analysed with the use of 2D COS method. The resulting 2D
maps of synchronous correlation in the region of cycle vibrations
−
1
−1
(1623, 1605, 1484 and 1382 cm ), o-xylene (1497 and 1465 cm
)
−1
−1
of products of isomerization (1650–1360 cm ) were divided into
the positive and negative parts and they are presented in Fig. 8
for microporous zeolites and in Fig. 9 for hierarchical ones. Addi-
tionally, the 3D representations of correlation levels for all studied
zeolites were included in Supporting information. In the case of
m-xylene isomerization process the following features should be
observed: (i) the positive auto- and cross-peaks of bands of each iso-
mer present in reaction (ii) the negative cross-peaks representing
the transformation of bands of m-xylene to o-xylene or p-xylene.
The m-xylene isomer being the bulkiest of all xylene isomers retains
in zeolite structure and its diffusion through 10 MR channels is
limited.
and p-xylene (1515 cm ). However, the overall correlation level
for auto-peaks differed among samples depending on porosity type.
Desilication process did not influence the character of m-xylene
interaction with zeolite surface i.e. the obtained 2D COS maps of
purely microporous B-ZSM-5 and hierarchical D-ZSM-5 are quite
similar, a strongly different behaviour was observed for hierarchical
M-ZSM-5 zeolite. This zeolite possesses enhanced mesopore sur-
face area Smeso resulting from small (diameter of around 10 nm)
and regularly distributed mesopores within zeolite grains (Table 1,
Fig. 2) and the highest amount of silanols groups on surface grains
(Fig. 4). These textural and spectroscopic properties did not sig-
nificantly affect the o-xylene isomerization described above and
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isomerization over hierarchical zeolites, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.035

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◦
Fig. 8. Positive (right) and negative (left) part of the 2D synchronous correlation map of meta-xylene isomerization reaction at 250 C for microporous zeolites B-ZSM-5 and
−
1
C-ZSM-5. Top and right projection represent the change of correlation level at 1515 cm
.
similar results were found for M-ZSM-5 and D-ZSM-5 samples.
However, in the case of more bulky m-xylene molecule the different
type of porosity in M-ZSM-5 sample seems to have an impact on the
isomerization process. As it can be seen from 2D correlation maps
isomerization, what is also well presented on 3D representations
of correlation levels (Supporting information). For M-ZSM-5 sam-
−
1
ple the most significant alteration was observed for 1605 cm of
m-xylene band, which intensity gradually dropped during the reac-
tion time. Thus it can be concluded that the highest amount of
m-xylene was sorbed on the surface of M-ZSM-5 zeolite, what is
justified in the high surface area with large amount of silanol groups
as sorption sites for xylene.
The 2D correlation related to the formation of p-xylene and
o-xylene was also found for B-ZSM-5, C-ZSM-5 and D-ZSM-5
samples. The positive correlation between the bands of p-xylene
−
1
the newly arising band at 1634 cm negatively correlates with the
−
1
−1
1
605 and 1484 cm bands of m-xylene as well as the 1497 cm
−
1
band of o-xylene. Thus, we assume that the band of 1634 cm is
representative for coke precursors. The textural properties of M-
ZSM-5 i.e. large mesopore surface area with high amount of silanols
groups prompted the formation of coke precursors. The differences
between the M-ZSM-5 sample and the other ones were noticed also
in positive part of 2D correlation maps. The highest correlation level
of auto-peaks for B-ZSM, C-ZSM-5 and D-ZSM-5 is observed for p-
−
1
−1
(1515 cm ) and o-xylene (1497 and 1465 cm ) indicates a
simultaneous formation of both isomers. However, the posi-
tive correlation was also found between the bands of m-xylene
−1
xylene band at 1515 cm . On the other hand, for M-ZSM-5 sample
the highest correlation level in auto-peaks is seen for 1605 cm
−
1
−1 −1
(1484, 1605 cm ) and both of p-xylene (1515 cm ) and o-xylene
−
1
band of m-xylene. Thus, it can be concluded that in the case of
former zeolites the increase of intensity of p-xylene band is the
most dominating variation in the spectrum during the m-xylene
(1465 cm ), what demonstrates that in our experimental condi-
tions not only isomerization process but also the on-going sorption
of m-xylene take place. This phenomenon was not observed for o-
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isomerization over hierarchical zeolites, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.035

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0
◦
Fig. 9. Positive (right) and negative (left) part of the 2D synchronous correlation map of neta-xylene isomerization reaction at 250 C for hierarchical zeolites D-ZSM-5 and
−
1
M-ZSM-5. Top and right projection represent the change of correlation level at 1515 cm
.
xylene isomerization process. Thus it can be anticipated that the
3.3.4. Quantitative evaluation of xylene isomerization process
over studied zeolites
diffusion of m-xylene molecules is largely restricted by size limita-
◦
tions and even at temperatures as high as 250 C the transport to
In situ IR spectroscopy quantitative studies of ortho- and
meta-xylene isomerization processes were performed in order to
determine the influence of acidity and porosity of zeolites on their
catalytic performance. Despite obvious limitations of in situ IR stud-
ies, the presented results provide noteworthy insight in xylene
isomerization reaction over hierarchical zeolites. The xylene iso-
active sites through 10 MR channel of zeolite is limited and requires
longer period of time.
Diverse observations on the basis of 2D COS maps were done
for M-ZSM-5 sample, where the positive correlation between the
−
1
−1
bands of m-xylene (1605 cm ) and o-xylene (1497 and 1484 cm )
◦
points to simultaneous transformation of these molecules either
to p-xylene or to mentioned earlier coke precursors. As can be
expected the negative correlation between the band of p-xylene
merization process was performed in quartz IR cell at 250 C for
15 min, after this period of time the quartz cell was cooled down
to room temperature and then the IR spectra of reaction products
were gathered. Conversion, selectivity and yield were calculated
on the basis of IR quantitative measurements of reaction products
with employ of the presented earlier extinction coefficients.
The 2D COS analysis results (Sections 3.3.1 and 3.3.2.) proved
that isomerization of o-xylene over hierarchical zeolites led to the
formation of by-products. The formation of by-products was even
detected at 1D IR spectra after isomerization process (Fig. 10). It
is clearly seen that for hierarchical zeolite the mesitylene (band at
−
1
−
1
(
1515 cm ) and the bands of both m-xylene (1605 cm ) and
−
1
o-xylene was also found (1497 and 1484 cm ). This negative
correlation was only evidenced for M-ZSM-5 sample. Contrary
to other samples, on the surface of M-ZSM-5 sample isomeriza-
tion process occurs without on-going sorption of m-xylene. This
result can be related with highly porous character of M-ZSM-5
with enhanced accessibility of sites being the centres of xylene
sorption.
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isomerization over hierarchical zeolites, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.035

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◦
Fig. 10. IR spectra in region of cycle vibrations of the xylenes and by-products of isomerization (toluene and mesitylene) after o-xylene isomerization at 250 C for all studied
zeolites.
1
610 cm−1) is easily produced. The quantitative IR studies (Table 3)
mers and (ii) the transformation of the o-xylene to both p-xylene
and m-xylene (negative correlation between the bands of o-xylene
revealed that the zeolites offered moderate and comparable activity
in xylene isomerization, however in this study the main attention
will be paid on the differences in selectivity to desired products. It
can be noticed that for purely microporous B-ZSM-5 and C-ZSM-5
zeolites higher selectivity to the most desired product i.e. p-xylene,
was achieved. This result can be directly related to high abun-
dance of microporous environment giving rise to shape selectivity.
The another important factors influencing the activity of C-ZSM-
−
1
(1465 cm ) and bands of both p-xylene and m-xylene (1605 and
−
1
1515 cm , resp.)) it can be concluded that at given experimental
conditions the monomolecular mechanism witch direct conver-
sion between the o-xylene and p-xylene occurred in 10 MR pores.
Similar results of the conversion between the o-xylene and p-
xylene was observed by Goncalves et al. [30] and Young et al. [31],
◦
when temperature of reaction above 200 C has been applied. On
5
sample are the textural properties. It seems that the spherical
the other hand, the mesoporosity fabricated by desilication and
gemini-surfactant assisted rout did not significantly affect the cat-
alytic performance but at the same time led to decrease of p-xylene
selectivity. It is worth underlining that acid sites easily available
from external surface can privilege bimolecular reactions leading
shape of small aggregated grains with limited mesopore surface
area assured higher conversion and higher selectivity to desired p-
xylene product. Considering the results of 2D COS analysis pointing
to (i) coincident formation of both p-xylene and m-xylene iso-
Please cite this article in press as: K.A. Tarach, et al., Quantitative infrared spectroscopic studies and 2D COS analysis of xylenes
isomerization over hierarchical zeolites, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.035

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1
2
◦
Fig. 11. IR spectra in region of cycle vibrations of the xylenes and by-products of isomerization (toluene and mesitylene) after m-xylene isomerization at 250 C for all studied
zeolites.
to by-products or catalyst coking [32,33]. The highest contribution
of by-products was found for M-ZSM-5 zeolite, what is in agree-
ment with the most mesoporous character of this sample, thus the
highest accessibility of acid sites.
merization better activity of C-ZSM-5 zeolite is a result of not only
the specific textural properties but also the optimal acidity. Higher
values of conversion should be related to the increase of the con-
centration and the strength of acid sites. Thus, it can be concluded
that the moderate amount of Brønsted acid sites in C-ZSM-5 sam-
ple together with considerably high amount of Lewis acid sites are
optimal for enhanced conversion. Significantly higher concentra-
tion of acid sites (as for D-ZSM-5) or enhanced mesopore surface
area (as for D-ZSM-5 and M-ZSM-5) led to decrease in selectiv-
ity and conversion of isomerization process. The work of Cejka
and Wichterlova [3] showed that zeolites with lower acidity can
provide higher activity under specific reaction condition. Further-
more, in paper of Perez-Pariente et al. [34] it has been proved that
maximum activity of studied zeolite Beta was observed for zeo-
In the case of m-xylene isomerization only the presence of
xylene isomers was confirmed, the products originating from dis-
proportionation reaction were detected neither in 2D COS analysis
nor in 1D IR spectrum after reaction (Table 4, Fig. 11). The highest
activity was found for C-ZSM-5 with almost twice higher con-
version in comparison with mesoporous zeolites D-ZSM-5 and
M-ZSM-5. Similar trend was observed when considering the selec-
tivity to p-xylene i.e. the most desired product. The highest values
of p-xylene selectivity were observed for microporous B-ZSM-5
and C-ZSM-5 zeolites. It seems that in the case of m-xylene iso-
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13
Table 3
was crucial for high selectivity to the most desired product i.e.
p-xylene. However, it was found that even for purely microp-
orous zeolites the size and shape of zeolite grains were crucial
for activity and selectivity during catalytic reaction. In the case of
mesoporous zeolites the formation of mesitylene and toluene as
by-products was confirmed. Furthermore, for hierarchical zeolites
the high amount of silanols groups prompted coke formation on
highly developed external surface. The isomerization of m-xylene,
the bulkiest among xylene isomers, performed over hierarchical
zeolites was affected by thermodynamics of xylene isomerization
process rather than the presence of shape selective micropore envi-
ronment.
Catalytic performance of studied zeolites in o-xylene isomerization process per-
formed in quartz IR cell at 250 C for 15 min. The results obtained from IR spectra
recorded after reaction at RT.
◦
Material
Product
Yield [%]
Selectivity [%]
Conversion [%]
35%
B-
ZSM-
5
m-xylene
p-xylene
o-xylene
30%
4%
65%
87%
13%
C-
ZSM-
5
m-xylene
p-xylene
o-xylene
33%
7%
60%
83%
17%
40%
35%
D-
ZSM-
5
m-xylene
p-xylene
o-xylene
mesitylene
Toluene
25%
3%
65%
3%
74%
8%
10%
9%
Acknowledgement
3%
M-
ZSM-
5
m-xylene
p-xylene
o-xylene
mesitylene
Toluene
27%
4%
60%
5%
67%
9%
40%
The work was financed by Grant No. 2014/13/D/ST5/02761 from
the National Science Centre, Poland.
11%
13%
5%
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.02.
035.
Table 4
Catalytic performance of studied zeolites in m-xylene isomerization process per-
formed in quartz IR cell at 250 C for 15 min. The results obtained from IR spectra
recorded after reaction at RT.
◦
Material
Product
Yield [%]
Selectivity [%]
Conversion [%]
28%
References
B-
ZSM-
5
m-xylene
p-xylene
o-xylene
72%
8%
20%
[
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m-xylene
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62%
10%
27%
38%
22%
24%
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18%
19%
81%
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m-xylene
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76%
6%
19%
23%
77%
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lites with Si/Al ratios between 14 and 15, what corresponded to
the optimum amount of Brønsted and Lewis acid sites. The differ-
ences in catalytic performance between the C-ZSM-5 and B-ZSM-5
sample can be associated both with the lower amount of Lewis acid
sites enhancing Brønsted acidity and textural properties especially
the shape of zeolite grains. The presence of mesopores in studied
samples strongly influenced their catalytic performance favouring
non-selective reactions and the most probably leading to coking.
The isomerization of m-xylene, as the most bulky among all xylene
isomers, performed over hierarchical zeolites with high accessibil-
ity of acid sites seems to be strongly affected by thermodynamics
of xylene isomerization process [35]. Low conversion of m-xylene
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Please cite this article in press as: K.A. Tarach, et al., Quantitative infrared spectroscopic studies and 2D COS analysis of xylenes
isomerization over hierarchical zeolites, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.035

G Model
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isomerization over hierarchical zeolites, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.035