Post-synthesis modification of TUN zeolite: Textural, acidic and catalytic properties

Article abstract of DOI:10.1016/j.cattod.2010.11.090

Dealumination, desilication and silylation of TUN zeolite was investigated and related to the catalytic behavior in toluene disproportionation and its alkylation with isopropyl alcohol. Only negligible concentration of acid sites was identified on the "external" surface of parent TUN zeolite and this concentration increased with increasing desilication. Silylation resulted in channel narrowing and probably some plugging. As a result, significant increase in the para-selectivity in toluene disproportionation and its alkylation with isopropyl alcohol was achieved. No changes in toluene conversion were observed for all parent and modified samples in toluene alkylation confirming diffusion and transport of products as the rate determining step in 10-ring zeolites. Selectivity to cymenes and p-cymene decreased with dealumination as well as desilication. In contrast, silylation increased both cymene (80%) and p-cymene (90%) selectivity in toluene alkylation.

Full text of DOI:10.1016/j.cattod.2010.11.090

Catalysis Today 168 (2011) 63–70
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Post-synthesis modification of TUN zeolite: Textural, acidic and
catalytic properties
a
a
a,b,∗
ˇ
ˇ
Martin Kubu˚ , Nadeˇzˇda Zilková , Jirˇí Cejka
a J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejsˇkova 3, CZ-182 23 Prague 8, Czech Republic
b Center of Excellence in Refining and Petrochemistry, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
Dealumination, desilication and silylation of TUN zeolite was investigated and related to the catalytic
behavior in toluene disproportionation and its alkylation with isopropyl alcohol. Only negligible concen-
tration of acid sites was identified on the “external” surface of parent TUN zeolite and this concentration
increased with increasing desilication. Silylation resulted in channel narrowing and probably some plug-
ging. As a result, significant increase in the para-selectivity in toluene disproportionation and its alkylation
with isopropyl alcohol was achieved. No changes in toluene conversion were observed for all parent and
modified samples in toluene alkylation confirming diffusion and transport of products as the rate deter-
mining step in 10-ring zeolites. Selectivity to cymenes and p-cymene decreased with dealumination as
well as desilication. In contrast, silylation increased both cymene (80%) and p-cymene (90%) selectivity
in toluene alkylation.
Received 5 October 2010
Received in revised form
26 November 2010
Accepted 29 November 2010
Available online 12 January 2011
Keywords:
TUN
Dealumination
Desilication
© 2011 Published by Elsevier B.V.
Hierarchic system
Aromatic transformations
Internal and external acidity
1. Introduction
ment with appropriate zeolite type can provide a highly efficient
zeolite-based catalyst [4,5].
Zeolites represent one of the most important groups of het-
erogeneous catalysts with numerous applications in chemical
industry [1–3]. The interplay among textural, chemical and cat-
alytic properties is optimized for a high performance in these
processes. Despite that zeolites generally suffer from a low acces-
sibility of acid sites located in micropore channels for bulkier
reactants.
One of the possible ways to overcome low accessibility is
the fabrication of hierarchical zeolites, it means those success-
fully combining micropores and mesopores. Such systems are
much more flexible and transport of bulky reactants and products
proceeds with a higher efficiency [4,5]. Dealumination and particu-
larly desilication were recently intensively investigated to provide
mesopores in different structural types of zeolites. MFI [6], MOR
[7], FER [8], MWW [9], IFR [10], and STF [11] are some examples of
successful desilication. Desilication not only increased significantly
the mesoporosity of these hierarchic zeolites but also changed the
acidic properties by removing a part of silica material. Therefore,
it is believed that optimum combination of post-synthesis treat-
Recently, diquaternary ammonium structure directing-agents
(SDA) built from N-methyl pyrrolidine heterocycles connected
by methylene chain of different lengths [12] led to the discov-
ery of novel zeolites -SVR [13], IMF [14] and TUN [15]. Their
structures are rather complex and were recently finally solved
[16–18]. TUN is a new high-silica zeolite possessing 24 crystal-
lographically distinct tetrahedral sites (T-sites) in its monoclinic
unit cell. Its framework contains two distinct straight 10-ring
channels (0.52 nm × 0.60 nm and 0.51 nm × 0.55 nm). TUN crystal-
lizes under hydrothermal conditions at the expense of a lamellar
precursor over a very narrow range of Si/Al and NaOH/SiO2
ratios. The proton form of TUN exhibits high hydrothermal sta-
bility, strong acidity and unique shape selective properties in
acid-catalyzed reactions of monoaromatic hydrocarbons (benzene
and toluene alkylation, disproportionation of toluene and isomer-
ization of m-xylene [15,19,20]) or cracking of vacuum gas–oil
[21].
This contribution focuses on the post-synthesis treatments of
TUN zeolite via dealumination, desilication and silylation. Textu-
ral, acidic, and catalytic properties of parent and modified TUN
samples were investigated. The relationship between acidity (both
internal and external) and catalytic activity in toluene dispropor-
tionation and its alkylation with isopropyl alcohol is discussed as
well.
∗
Corresponding author.
E-mail address: Jiri.cejka@jh-inst.cas.cz (J. Cejka).
ˇ
0920-5861/$ – see front matter © 2011 Published by Elsevier B.V.
doi:10.1016/j.cattod.2010.11.090

64
M. Kubu˚ et al. / Catalysis Today 168 (2011) 63–70
2. Experimental
2.1. Synthesis
2.3. Characterization
The crystallinity of all TUN samples was checked by powder X-
ray diffraction (XRD) using a Bruker AXS D8 Advance diffractometer
with a graphite monochromator and a position sensitive detector
Våntec-1 using CuK␣ radiation in Bragg–Brentano geometry. The
size and shape of zeolite crystals were examined by scanning elec-
tron microscopy (SEM) on a JEOL, JSM-5500LV microscope. For the
measurement, crystals were coated with a thin platinum layer by
sputtering in vacuum chamber of a BAL-TEC SCD-050.
TUN zeolite was synthesized according to Ref. [15]. AlCl3·6H2O
(>99%, Fluka) was used as a source of aluminum and fumed silica
(Aldrich) as a source of silica. The organic SDA cation 1,4-bis(N-
methylpyrrolidinium) butane (1,4-MPB) was prepared according
to the literature [22]. The synthesis was carried out under agi-
tation in 90-mL Teflon-lined stainless steel autoclaves heated
at 160 ◦C and autogenous pressure for 12 days. After that, the
autoclaves were cooled down to the ambient temperature, the
solid product was filtered off, washed out with excess of dis-
tilled water and dried in the oven at 80 ◦C overnight. As-made
TUN zeolite was calcined under flowing air at 580 ◦C for 12 h with
a heating rate 1 ◦C/min. Calcined TUN sample was then treated
four-times with 1.0 M NH4NO3 solution (99%, Lach-Ner) for 4.5 h
at room temperature using 100 mL of solution per 1 g of zeolite.
Ammonium forms of TUN samples were activated before each mea-
surement to remove ammonia and to provide proton form of the
zeolite.
For the determination of the chemical composition of synthe-
sized and modified zeolites, X-ray fluorescence analysis with a
Philips PW 1404 spectrometer equipped with an analytical pro-
gram UniQuant was used. The samples were mixed with dentacryl
as a binder and pressed on the surface of cellulose pellets.
Concentration of Lewis (L) and Brønsted (B) acid sites was deter-
mined after adsorption of pyridine (PYR) by FTIR spectroscopy
(Nicolet Protégé 460 Magna). Zeolites were pressed into self-
supporting wafers with a density of 8.0–12 mg/cm2 and activated
in situ at 450 ◦C overnight. Before adsorption pyridine was degassed
by freezing and thawing cycles. Pyridine adsorption was carried out
at 150 ◦C for 20 min at partial pressure 600–800 Pa, followed by des-
orption for 20 min. All spectra were recorded with a resolution of
2 cm−1 by collecting 128 scans for a single spectrum at room tem-
perature. Spectra were recalculated on wafer density of 10 mg/cm2.
Concentration of Lewis and Brønsted acid sites were evaluated from
the integral intensities of bands at 1454 cm−1 (Lewis acid sites) and
at 1545 cm−1 (Brønsted acid sites) using extinction coefficients, ε
(L) = 2.22 cm/␮mol, and ε (B) = 1.67 cm/␮mol [24].
The adsorption of 2,6-di-tert-butylpyridine (DTBP) took place at
150 ◦C, and at equilibrium probe vapour pressure with the zeolite
wafer for 15 min. Desorption proceeded at the same temperature
for 1 h followed by collection of spectra at room temperature.
Extinction coefficients for pyridine [24] were used for the quantita-
tive analysis evaluation of the concentration of Lewis and Brønsted
acid sites.
Nitrogen sorption isotherms were measured on a Micromeritics
ASAP 2020 volumetric instrument at liquid nitrogen temperature
(−196 ◦C) to determine surface area, pore volume and pore size dis-
tribution of TUN. Prior to the sorption measurements, all samples
were degassed at 250 ◦C for at least 24 h. Detailed description can
be found in [25].
2.2. Post-synthesis treatments
Ammonium form of TUN was calcined under flowing air at
450 ◦C with a heating rate 10 ◦C/min to obtain H+ form of zeolite.
Dealumination was performed via treatment with different con-
centrated solution of HNO3 (Lachema, CZE) at 100 ◦C under reflux
using 25 mL/g of zeolite. After that, the solid product was recov-
ered by filtration, washed with distilled water until neutral pH was
reached and finally dried overnight at 60 ◦C.
Desilication procedure was based on Ref. [23]. In 100-mL flask,
2.5 g of TUN was stirred in 77 mL of 0.2 M NaOH aqueous solu-
tion (Lach-Ner) at 65 ◦C under reflux for different periods of time
(Table 1). Subsequently, the flask was cooled down, the solid prod-
uct was filtered and washed out with distilled water to reach
neutral pH and then dried at 60 ◦C overnight. Na+ form of TUN was
converted to NH4+ form by four-fold treatment with 1.0 M NH4NO3
solution at room temperature for 4.5 h.
The silylation procedure proceeded in this way: ammonium
form of TUN was activated in argon stream at 450 ◦C with a heating
rate 10 ◦C/min for 2 h followed by cooling to ambient tempera-
ture under argon. Dry toluene (25 mL/g of zeolite) and defined
amount of tetraethyl orthosilicate (TEOS, 1.0 wt.%, Aldrich) were
added with exclusion of air moisture. Afterwards, the mixture
was stirred at room temperature overnight, followed by heat-
ing under reflux for 4 h. Evaporation of liquid phase in vacuum
closed the procedure. The silylated sample was calcined in air
at 450 ◦C for 8 h. Table 1 summarizes the abbreviations of all
samples studied and the conditions of post-synthesis treatments
applied.
2.4. Catalytic experiments
The disproportionation and alkylation of toluene with isopropyl
alcohol were investigated in a down-flow glass microreactor with
a fixed bed of catalyst under atmospheric pressure. Zeolite cata-
lyst was pressed and sieved and the fraction between 0.750 and
0.355 mm was used for the reaction. Before the catalytic run, zeolite
catalysts were activated at 500 ◦C in nitrogen stream for 120 min.
Table 1
Experimental conditions of post-synthesis treatment and respective textural properties.
Marking
HNO3 conc. (M)
NaOH conc. (M)
Si (wt.%)
Time (h)
Vmic (cm3/g)
Vmeso (cm3/g)
SBET (m2/g)
ꢀw (%)a
TUN
–
–
–
–
–
0.2
0.2
0.2
–
–
–
–
–
–
–
–
2
2
2
0.25
0.5
2
0.167
0.172
0.168
0.169
0.141
0.140
0.133
0.160
–
–
–
–
418
442
430
429
435
473
474
406
–
–
–
–
TUN/A
TUN/B
TUN/C
TUN/D
TUN/E
TUN/F
TUN/G
0.5
1.0
6.0
–
–
–
0.234
0.298
0.340
–
12.7
27.7
42.6
–
–
–
1.0b
–
a
Weight reduction after desilication.
TEOS.
b

M. Kubu˚ et al. / Catalysis Today 168 (2011) 63–70
65
The disproportionation of toluene was carried out at 450 and 500 ◦C
with WHSV 1.9 and 18.9 h−1. The alkylation of toluene was per-
formed at 250 ◦C and toluene to isopropyl alcohol molar ratio of 9.6
and weight hour space velocity (WHSV) based on toluene equal to
500
TUN/G
10 h−1
.
The reaction products were analyzed using an “on-line” gas
chromatograph (Agilent 6890 Plus) with flame ionization detector
and a high-resolution capillary column (Innowax for the dispro-
portionation and DB-5 for the alkylation). The first analysis was
performed after 15 min of time-on-stream (T-O-S) and the other
followed in approx. 55 min interval.
TUN/E
TUN/B
TUN
3. Results and discussion
0
10
20
30
40
50
60
2 Theta, °
3.1. Characterization of TUN zeolites
Fig. 1. XRD patterns of TUN.
Combination of different experimental techniques has been
applied to characterize in detail the chemical and textural proper-
ties of parent and post-synthetically modified TUN zeolites. Fig. 1
shows X-ray diffraction patterns of parent TUN zeolite and TUN
modified by dealumination, desilication and surface silylation. All
four X-ray patterns evidence high crystallinity and phase purity
of these samples confirming that no structural changes proceeded
during the post-synthesis treatments. The same XRD patterns were
obtained also for remaining TUN samples under study (not shown
here).
Scanning electron images of parent and modified TUN zeolites
are given in Fig. 2 under the same resolution. From the images it
is seen that the size of TUN crystals is about 2 ␮m × 1 ␮m × 1 ␮m
with a few larger agglomerates. There is no sign of another phase
or amorphous species in the TUN zeolite. The applied synthesis
resulted in rather homogeneous distribution of crystals sizes and
shapes of TUN zeolites. The size and shape is practically not changed
after individual modification treatments. Neither dealumination,
nor desilication and nor silylation led to dramatic changes of the
crystals habitus.
pared with the parent zeolite (cf. Fig. 3A and Table 1). Samples
TUN/A, TUN/B and TUN/C were prepared by dealumination carried
out under reflux for 2 h by varying the concentration of nitric acid.
The adsorption isotherms did not practically change their shape
and all samples posses practically the same micropore volume
(0.167–0.172 cm3/g), which is the value confirming a high quality
of the TUN samples studied (Table 2). The silylation treatment led
to a little decrease in the micropore volume as well as BET surface
area. Thus, we can expect that silica material used for modification
is mainly located on the external surface or in channel entrances of
TUN crystals without a significant penetration into the TUN channel
system. Results differ from those obtained on STF zeolite [11] where
a substantial decrease in the micropore volume after the silylation
was reported. The main reason is probably a special structure of
STF zeolite with individual 18-ring STF cages being separated by
only one 10-ring. Thus, larger molecules can more easily penetrate
to the channel system of STF despite 1D channel structure.
In contrast, desilication changed significantly the textural prop-
erties of TUN zeolites. With increasing time of the treatment
at the same concentration of NaOH (Table 1) micropore vol-
Nitrogen adsorption isotherms provided valuable information
on the textural properties of individual TUN samples when com-
Fig. 2. SEM images of parent and treated TUN.

66
M. Kubu˚ et al. / Catalysis Today 168 (2011) 63–70
A
B
240
220
200
180
160
140
120
100
80
240
220
200
180
160
140
120
100
80
TUN/F
TUN/E
TUN/D
TUN
TUN/A
TUN/B
TUN/C
TUN/G
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
relative pressure / p/p0
Fig. 3. Nitrogen isotherms. Dealuminated and silylated TUN (A), parent and desilicated (B). For better view, following isotherms were shifted along y axis (in cm3/g): TUN/G
+0,TUN/C +10, TUN/B +25, TUN/A +40 (A), TUN −5, TUN/D +5, TUN/E +10, TUN/F +30 (B).
ume decreased very slightly, however, volume of mesopores and
BET areas increased dramatically. After 0.25 h mesopore volume
reached the value 0.234 cm3/g and after 2 h 0.340 cm3/g was
achieved. In parallel, BET surface area increased from 418 (parent
TUN) to 474 (TUN/F). Formation of mesopores is nicely visible from
the shape of the adsorption isotherms exhibiting large hysteresis
loop, the size of which increases with prolongation of the time of
desilication (Fig. 3B). The shape of the hysteresis loop evidences a
broad range of mesopores of different sizes and shapes. Desilica-
tion treatment was accompanied by a large weight loss reaching
42.6 wt.% after 2 h of desilication treatment.
Brønsted acid sites in TUN regardless the way of modification are
accessible for pyridine. In the region of pyridine vibration bands of
pyridine interacting with Brønsted and Lewis acid sites appeared.
Absorption band at 1545 cm−1 is characteristic for pyridine inter-
acting with Brønsted acid sites while bands around 1450 cm−1 are
attributed to pyridine being in interaction with Lewis acid sites or
silanol groups (Fig. 4B, D and F). Extinction coefficients and integral
areas of these bands have been used to quantitatively determine the
concentrations of individual sites according to Ref. [26].
Dealumination procedure resulted in a slight decrease in the
concentration of Lewis acid sites while concentration of Brønsted
sites remained practically unchanged (Fig. 4E and F). It is in a good
agreement with only a slight increase in Si/Al ratio. The concen-
tration of Brønsted and Lewis acid sites was changed very slightly
(Table 2). This dealumination procedure under the same conditions
was recently applied for STF zeolites possessing Si/Al ratio 29 [11].
STF exhibited much larger extent of dealumination when compared
with TUN although having a higher Si/Al ratio. One can speculate
about a higher resistance to acidic treatment in the case of TUN
zeolite. Interaction of 2,6-di-tert-butyl-pyridine with parent and
dealuminated TUN samples showed only a negligible concentration
of accessible Brønsted sites (Fig. 5B and F). It evidences that prac-
tically no acid sites are located on the external surface or mouth
openings and expectedly dealuminated samples are also almost
without acid sites on the crystal surface (Table 2).
3.2. Acidic properties of TUN zeolites
With relation to the catalytic behavior of TUN zeolite and its
modified analogues, acidity assessment is of critical importance. To
understand in more detail the acidic properties of zeolites studied,
the concentration and type of acid sites were studied using FTIR
with pyridine and 2,6-di-tert-butyl-pyridine as probe molecules
[26–29]. The infrared spectra of parent TUN zeolite (Fig. 4A) exhibit
three absorption bands in the region of hydroxyl vibrations, more
specifically, a band at 3745 cm−1 belonging to terminal silanol
groups, a relatively weak absorption band at 3670 cm−1 due to
the presence of OH groups attached to extra-framework alumina
species and finally strong absorption band at 3610 cm−1 assigned
to acidic bridging Si–OH–Al groups [30].
In contrast, silylated TUN/G possesses substantially lowered
concentration of Brønsted sites (from 0.35 to 0.20 mmol/g) while
the decrease in the amount of Lewis acid sites was smaller
(0.22–0.17 mmol/g). No acid sites were distinguished on the
external surface after silylation and the increase in Si/Al ratio cor-
Pyridine absorption on parent TUN and all modified TUN sam-
ples resulted in complete disappearance of the absorption band
of Si–OH–Al groups and that of OH groups attached to extra-
framework alumina (Fig. 4A, C and E). This confirms that all
Table 2
Lewis and Brønsted acid site concentration. Adsorption of pyridine and 2,6-di-tert-butylpyridine (I).
Marking
Si/Al
cB (mmol/g)
cL (mmol/g)
cB (%)
cL (%)
cB (I) (mmol/g)
Si/Al (CA)
TUN
20.2
21.9
22.2
23.0
16.7
16.4
15.8
22.4
0.35
0.34
0.34
0.34
0.30
0.29
0.28
0.20
0.22
0.20
0.19
0.18
0.32
0.33
0.33
0.17
62
63
65
66
49
46
44
55
38
37
35
34
51
54
56
45
–
–
–
–
16.1
16.3
16.5
17.9
13.8
12.5
11.3
16.4
TUN/A
TUN/B
TUN/C
TUN/D
TUN/E
TUN/F
TUN/G
0.026
0.033
0.038
–
CA, chemical analysis.

M. Kubu˚ et al. / Catalysis Today 168 (2011) 63–70
67
A
B
A
0.03
0.2
B
1491
0.2
TUN-a
0.4
TUN-a
TUN-a
1546
TUN-a
TUN-b
TUN-b
3737
3741
TUN/G-a
TUN/G-a
TUN/G-a
3782
3614
3743
TUN/G-a
TUN/G-b
0.4
3614
3670
TUN/G-b
3670
0.4
TUN/F-a
C
E
0.4
C
D
F
TUN/F-a D
1483 1467
TUN/F-a
1531
3745
3363
3616
1438
TUN/F-a
TUN/F-b
TUN/E-a
TUN/F-b
TUN/E-a
TUN/E-a
TUN/D-a
TUN/E-b
TUN/D-a
TUN/E-b
TUN/D-a
TUN/E-a
1454
1446
TUN/D-a
3745
3616
0.05
TUN/D-b
TUN/D-b
0.2
0.2
0.03
E
F
0.4
TUN/C-a
TUN/C-a
TUN/C-b
TUN/C-a
TUN/C-a
TUN/B-a
TUN/C-b
TUN/B-a
TUN/B-a
TUN/A-a
TUN/B-a
TUN/B-b
TUN/A-a
TUN/B-b
TUN/A-a
TUN/A-b
TUN/A-a
TUN/A-b
4000
3800
3600
3400
1550
1500
1450
1400
4000
3800
3600
3400
1550
1500
1450
1400
Wavenumber / cm-1
Wavenumber / cm-1
Fig. 4. FTIR spectra of TUN. Adsorption of pyridine. Region of hydroxyl vibration (A,
C and E), region of pyridine vibration (B, D and F). After (a) and before (b) adsorption.
Fig. 5. FTIR spectra of TUN. Adsorption of 2,6-di-tert-butylpyridine. Region of
hydroxyl vibration (A, C and E), region of 2,6-di-tert-butylpyridine vibration (B, D
and F). After (a) and before (b) adsorption.
responds to the amount of Si added during the post-synthesis
treatment (Table 1). It indicates that silylation closed or nar-
rowed some channel entrances and pyridine is no longer able
to reach the acid sites. It is supported by only a slight decrease
in micropore volume (0.167 cm3/g for TUN and 0.160 cm3/g for
TUN/G).
Desilication then results in the removal of silica from these domains
while the surface composition is not significantly changed.
3.3. Catalytic behavior of TUN zeolites
Desilication led not only to the substantial increase in the
mesopore volume (Table 1) but also to substantial changes in the
concentration of acid sites and their location. Concentration of
Brønsted sites decreased from 0.35 to 0.28–0.30 mmol/g depend-
ing on the length of treatment but simultaneously the desilication
led to an increase in the concentration of Lewis acid sites from 0.22
to 0.33 mmol/g. Consequently, higher concentrations of Lewis acid
sites were observed than of Brønsted sites. Fig. 5C and D shows
spectra of desilicated samples before and after adsorption of 2,6-
di-tert-butyl-pyridine and adsorbed 2,6-di-tert-butyl-pyridine on
Brønsted acid sites, their concentrations are provided in Table 2.
Absorption band at 1531 cm−1 is typical for interaction of 2,6-
di-tert-butyl-pyridine with Brønsted acid sites and the absence
of a band at 1545 cm−1 confirms no dealkylation of the probe
molecules used. The concentration of acid sites on the external
surface of zeolite crystals increases with prolongation of the desil-
ication treatment and simultaneously with increasing mesopore
volume. As the differences in mesopore volume and weight loss
during the desilication treatment for individual TUN samples are
substantial in contrast to a slight increase in the concentration of
Brønsted sites at the external surface, it can be inferred that there
could be a large siliceous domains in the structure of TUN zeolite.
In addition to the general interest in textural and acidic prop-
erties of micro/mesoporous zeolites we tried also to evaluate their
effects on the catalytic behavior in different reactions. Transfor-
mations of aromatic hydrocarbons are particularly sensitive to
textural/structural and acidity changes as documented in numer-
ous literature [2,3,31–33].
Toluene disproportionation belongs to the reactions, the rate of
which is supposed to be controlled by the concentration of acid sites
even in the case of medium-pore zeolites [3]. MFI is the only zeolite
being able to provide para-selectivity higher than that related to
the thermodynamic equilibrium due to the transport phenomena.
Therefore, this reaction was chosen for comparison of parent and
post-synthesis modified TUN zeolite.
Surprisingly, no big differences in toluene conversions in its
disproportionation were obtained for parent and modified TUN
zeolites at 450 ◦C (Fig. 6A) and 500 ◦C (not shown here). This is
most probably related to similar concentrations of acid sites and
it indicates that diffusion is not a critical factor. These results are in
contrast to recently reported behavior of mesoporous ZSM-5 zeo-
lites prepared using carbon particles as secondary template [34,35].
Mesoporous ZSM-5 exhibits increasing toluene conversion with
increasing mesopore volume. A tentative explanation of differences

68
M. Kubu˚ et al. / Catalysis Today 168 (2011) 63–70
A B
40
20
48
46
44
42
0
0.90
0.85
0.80
40
30
C D
28
26
24
22
0
60
120
180
0
60
120
180
T-O-S (min)
T-O-S (min)
Fig. 6. Time-on-stream dependence of toluene conversion (A), selectivity to xylenes (B), xylene/benzene molar ratio (C) and p-xylene selectivity (D) in toluene dispropor-
tionation at 450 ◦C and WHSV = 20 h−1 for TUN ( ), TUN/A (᭹), TUN/C ( ) and TUN/G ( ).
between mesoporous TUN and mesoporous MFI (albeit prepared by
different procedures) might be in their different pore sizes. Slightly
smaller pores of MFI (0.54 nm × 0.56 nm) invoke diffusional restric-
tions being much less important for TUN (0.52 nm × 0.60 nm).
Toluene conversions decreased from about 30–35% after 15 min
of time-on-stream (T-O-S) to about 10% after 180 min for WHSV
20−1 and to about 20% for WHSV 2 h−1 most probably due to the
coke formation. This was accompanied by a color change of the cat-
alyst during the reaction. The lowest toluene conversion observed
under all reaction conditions used was for TUN/G zeolites evidenc-
ing that narrowing of pores and plugging of some them is important
for mass transport efficiency. This decrease in the toluene conver-
sion was accompanied by an increase in the selectivity to xylenes,
xylene/benzene molar ratio and p-xylene selectivity (Fig. 6B–D).
Selectivity to xylene isomers reached about 46–48% at the reac-
tion temperature of 450 ◦C and decreased to 44–45% at 500 ◦C. In
parallel, we observed similar behavior of xylene/benzene molar
ratio. At 450 ◦C xylene/benzene ratio was close to 0.9 while this
values decreased to 0.8 at 500 ◦C as a result of the increased reac-
tion rates of dealkylation and transalkylation reactions. Selectivity
to p-xylene was close to 24–25% for TUN catalysts with exception of
TUN and TUN/G. Parent TUN samples exhibited a slight increase in
the p-xylene selectivity to about 26% after 180 min of T-O-S due to
continuous deactivation by coke formation. In contrast, p-xylene
selectivity over TUN/G, silylated samples, increased to 30.5% at
450 ◦C and 29.2% at 500 ◦C. Both values are after 180 min of T-O-S.
Reaction mechanism of toluene alkylation with C3 alcohols or
olefin having desorption and transport of products as the rate
determining step was deeply investigated [36,37]. The kinetic and
labelling studies over MFI and MEL zeolites provided clear evidence
on the bimolecular transfer of propyl group from one reacting ben-
zene ring to another one implicating the critical role of the zeolite
structure. With TUN, the channel size of which zeolite is slightly
larger than those of MFI and MEL, we wonder about the role of
textural properties on conversions and selectivites. The toluene
conversion after 15 min of time-on-stream (T-O-S) is influenced by
toluene disproportionation, which is almost negligible after 55 min
of T-O-S. For longer T-O-S values toluene conversion is close to
8% (toluene to isopropyl alcohol molar ratio is 9.6) for all TUN
catalysts despite the way of modification (Figs. 7A and 8A). This
confirms the importance of mainly desorption and/or diffusion of
products in a similar way as for MFI catalyst although the channel
size of TUN is slightly larger than that of MFI (0.52 nm × 0.60 nm vs.
0.54 nm × 0.56 nm). Based on the comparison of catalytic behavior
of mesoporous TUN and MFI zeolites in toluene disproportionation,
one can infer that differences in their pore sizes are much more sig-
nificant for MFI zeolite. In contrast, when both zeolites are applied
for the reaction, the rate of which is controlled by the product des-
orption (toluene alkylation with isopropyl alcohol [36]), catalytic
behavior of both zeolites is rather similar and is not substantially
changed by the presence of mesopores.
Cymenes are primary alkylation products and their selectivity
increases with T-O-S at the extent of products of toluene dispropor-
tionation for shorter T-O-S and n-propyltoluenes for longer T-O-S
(Figs. 7B and 8B). The highest selectivity to cymene was achieved for
TUN and silylated TUN/G (80%) while all dealuminated and desil-
icated TUN catalysts showed lower cymene selectivity (50–70%)
for T-O-S 180 min. Both dealumination and desilication led to a
decrease in the selectivity to p-cymene. As the extent of dealu-
mination was relatively low, p-cymene selectivity decreased from
80 to 70% after 180 min of T-O-S (Fig. 6D). In contrast, desilica-
tion creating large mesopores, resulted in p-cymene selectivities
between 30 and 50% depending on the harshness of the treatment
(Fig. 7D). In contrast, silylation of the external surface, TUN/G cat-
alyst, enhanced p-cymene selectivity to about 85%. This finding
points to the fact that the channel entrances are narrowed or par-
tially plugged as a result of silylation preferring the p-cymene to
leave easily the inner zeolite volume. This evidences that silyla-
tion of the crystals can enhance para-selectivity even for zeolites
with channel dimensions slightly larger than those of MFI. The
increase in para-selectivity can be explained in terms of a com-
bination of both annihilation of acid sites on the external surface
restricting consecutive and unselective reactions as well as by
increasing differences in diffusion rates of individual cymene iso-
mers. While the size of channels is increasing from MFI to TUN,
which should decrease the differences among individual cymene
isomers, it can be inferred that narrowing and partial plugging of
channel entrances is critical for increasing para-selectivity.
Formation of undesired n-propyltoluenes leads to iso-/n-
propyltoluene values about 3–7 for different TUN catalysts

M. Kubu˚ et al. / Catalysis Today 168 (2011) 63–70
69
A
B
80
60
40
20
11
10
9
8
7
8
C
D
80
60
40
20
6
4
2
0
0
60
120
180
0
60
120
180
T-O-S (min)
T-O-S (min)
Fig. 7. Time-on-stream dependence of toluene conversion (A), selectivity to cymenes (B), iso-/n-propyltoluene ratio (C) and p-cymene selectivity (D) in toluene alkylation
with isopropyl alcohol at 250 ◦C for TUN ( ), TUN/A (᭹), TUN/C ( ) and TUN/G ( ).
A
B
80
60
40
20
11
10
9
8
7
8
C
D
80
60
40
20
6
4
2
0
0
60
120
180
0
60
120
180
T-O-S (min)
T-O-S (min)
Fig. 8. Time-on-stream dependence of toluene conversion (A), selectivity to cymenes (B), iso-/n-propyltoluene ratio (C) and p-cymene selectivity (D) in toluene alkylation
with isopropyl alcohol for TUN/D ( ), TUN/E ( ) and TUN/F (ꢀ).
(Figs. 7C and 8C). These values are slightly larger than for MFI zeo-
lite [36], which is in agreement with a larger reaction volume in the
channel intersections of TUN compared with MFI.
2) the structure of TUN was preserved after all treatments and no
significant changes were observed in the individual XRD pat-
terns,
3) the decrease in the micropore volume by desilication, which was
accompanied by a substantial increase in mesopore volume of
TUN zeolites,
4. Conclusions
4) the increase in the concentration of Brønsted acid sites on the
“external” surface of zeolites determined by the adsorption of
2,6-di-tert-butyl pyridine,
5) desilication resulted in a simultaneous decrease in the concen-
tration of Brønsted acid sites and increase in the concentration
of Lewis acid sited. This evidences that desilication procedure
changes partially the type of acid sites,
Post-synthesis treatment of parent TUN zeolite was carried out
using dealumination, silylation and desilication procedures. These
treatments and detailed characterization of acidic and textural
properties of these zeolites resulted in:
1) no changes of the shape and size of TUN crystals as confirmed
by SEM images,

70
M. Kubu˚ et al. / Catalysis Today 168 (2011) 63–70
ˇ
6) toluene disproportionation, silylation of TUN zeolite provided a
higher selectivity towards xylenes, higher xylene/benzene ratios
as well as higher selectivity to p-xylene,
7) toluene alkylation with isopropyl alcohol, both dealumination
and desilication decreased the selectivity to p-cymene,
8) the highest selectivity to p-cymene of about 90% was achieved on
silylated TUN zeolite with a simultaneous increase in selectivity
to cymenes.
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that TUN zeolite is very interesting for transformation of aro-
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hydrocarbons.
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