Post-synthesis incorporation of Al into germanosilicate ITH zeolites: The influence of treatment conditions on the acidic properties and catalytic behavior in tetrahydropyranylation

Article abstract of DOI:10.1039/c4cy01594k

Post-synthesis alumination of germanosilicate medium-pore ITH zeolites was shown to be an effective procedure for tuning their acidity. Treatment of ITH zeolites synthesized with different chemical compositions (i.e. Si/Ge = 2.5, 4.4 and 5.8) with aqueous Al(NO₃)₃ solution led to the formation of strong Br?nsted and Lewis acid sites and an increasing fraction of ultramicro- and meso-pores in Ge-rich ITH samples (Si/Ge = 2.5 and 4.4). The concentration of Al incorporated into the framework increases with decreasing Si/Ge ratio of the parent ITH. The increasing temperature of alumination from 80 to 175 °C (HT conditions) resulted in (1) a 1.5-2-fold increase in the concentration of Br?nsted acid sites formed and (2) a decreasing fraction of framework Al atoms detectable with base probe molecules (i.e. pyridine, 2,6-di-tert-butylpyridine), i.e. an increased concentration of the "inner" acid sites. The activity of prepared Al-substituted ITH zeolites in tetrahydropyranylation of alcohols is enhanced with increasing amount of accessible acid sites in bulky crystals (e.g. alumination at lower temperature) or with increasing total concentration of acid centres within tiny ITH crystals (e.g. alumination under HT conditions). This trend became more prominent with increasing kinetic diameter of the substrate molecules under investigation (methanol < 1-propanol < 1-hexanol).

Full text of DOI:10.1039/c4cy01594k

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Post-synthesis incorporation of Al into
germanosilicate ITH zeolites: the influence of
treatment conditions on the acidic properties and
catalytic behavior in tetrahydropyranylation
Cite this: Catal. Sci. Technol., 2015,
5, 2973
ab
ab
ac
Mariya V. Shamzhy, Maksym V. Opanasenko, Francisca S. de O. Ramos,
a
a
d
d
Libor Brabec, Michal Horáček, Marta Navarro-Rojas, Russell E. Morris,
Heloise de O. Pastore and Jiří Čejka*
c
a
Post-synthesis alumination of germanosilicate medium-pore ITH zeolites was shown to be an effective
procedure for tuning their acidity. Treatment of ITH zeolites synthesized with different chemical composi-
tions (i.e. Si/Ge = 2.5, 4.4 and 5.8) with aqueous AlIJNO
and Lewis acid sites and an increasing fraction of ultramicro- and meso-pores in Ge-rich ITH samples
Si/Ge = 2.5 and 4.4). The concentration of Al incorporated into the framework increases with decreasing
3 3
) solution led to the formation of strong Brønsted
(
Si/Ge ratio of the parent ITH. The increasing temperature of alumination from 80 to 175 °C (HT condi-
tions) resulted in (1) a 1.5–2-fold increase in the concentration of Brønsted acid sites formed and (2) a
decreasing fraction of framework Al atoms detectable with base probe molecules (i.e. pyridine, 2,6-di-
tert-butylpyridine), i.e. an increased concentration of the “inner” acid sites. The activity of prepared Al-
substituted ITH zeolites in tetrahydropyranylation of alcohols is enhanced with increasing amount of
accessible acid sites in bulky crystals (e.g. alumination at lower temperature) or with increasing total con-
centration of acid centres within tiny ITH crystals (e.g. alumination under HT conditions). This trend
became more prominent with increasing kinetic diameter of the substrate molecules under investigation
(methanol < 1-propanol < 1-hexanol).
Received 3rd December 2014,
Accepted 12th March 2015
DOI: 10.1039/c4cy01594k
www.rsc.org/catalysis
Zeolite ITH, possessing three sets of interconnecting
medium-pore 9- (4.0 × 4.9 Å), 10- (4.8 × 5.7 Å) and 10- (4.7 ×
1
. Introduction
The role of zeolites in the current petroleum industry can
hardly be overestimated. Aluminosilicate zeolites possessing
strong acid centres have been widely used as heterogeneous
5.1 Å) ring channels, was first synthesized as a pure silicate
IV
in concentrated reaction mixtures IJH O/T < 10, where T is
2
the zeolite framework tetrahedral atom) using hexametho-
1
9
catalysts for a great variety of processes. The application of
nium as the structure-directing agent (SDA). Introducing Al
medium-pore zeolites as industrial catalysts and the recogni-
tion of the ‘shape selective’ properties of these materials have
during the synthesis was reported to diminish the crystalliza-
2
tion rate of ITH while favouring the formation of an EU-1
1
0
stimulated synthetic work aimed at preparing new micropo-
phase. On the other hand, the smaller B–O–Si angles were
claimed to facilitate the introduction of B into the ITH frame-
work with respect to the EU-1 structure, although in relatively
3
–8
rous materials with various pore sizes and structures.
1
0
small amounts (Si/B ratios of >50). Similarly, Al present in
the initial gel was also reported to limit the crystallization
field of germanosilicate UTL zeolites in favour of an STF
a
J. Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences,
v. v. i., Dolejškova 3, CZ-182 23 Prague 8, Czech Republic.
1
1
phase, while B influences the selectivity of UTL zeolite for-
E-mail: jiri.cejka@jh-inst.cas.cz; Fax: +420 286 582 307; Tel: +420 266053795
1
2
b
mation to a lesser extent. Attempts to generate strong acid
centres by post-synthesis alumination of borosilicate ITH zeo-
lites resulted in an exchange of only a fraction of boron
atoms yielding ITH with a Si/Al molar ratio of 80 (45 and 105
L. V. Pisarzhevskiy Institute of Physical Chemistry, National Academy of Sciences
of Ukraine, pr. Nauki, 31 Kiev 03028, Ukraine; Fax: +380 445 256 216;
Tel: +380 445 254 196
c
Micro and Mesoporous Molecular Sieves Group, Institute of Chemistry,
University of Campinas, Rua Monteiro Lobato, 270, 13083-861, Cidade
Universitária Zeferino Vaz, Campinas, SP, Brazil
−
1
13
μmol g of Brønsted and Lewis acid sites, respectively). At
the same time, Al-containing ITH zeolites with an increased
d
EaStCHEM School of Chemistry, University of St. Andrews, St. Andrews KY16
−
1
9ST, United Kingdom
concentration of Brønsted acid sites (63 and 48 μmol g of
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Brønsted and Lewis acid sites, respectively) were synthesized
directly only in the presence of Ge, exploiting its propensity
to increase the nucleation rate of zeolites containing D4R
experiments. All reactants and solvents were obtained from
Sigma Aldrich and used as-received without any further
treatment.
1
3
SBUs (e.g., ITH). Al-ITH was shown to be a shape selective
1
4
catalyst for conversion of aromatics and catalytic cracking
2.2 Preparation of hexamethonium dihydroxide
1
3
of vacuum gas oil producing propylene.
HexamethylenebisIJtrimethylammonium) dibromide was pre-
pared according to ref. 19 and was transformed into the
hydroxide form using a Bio-Rad AG-1X8 anion exchange
resin. The solution of hexamethonium dihydroxide was con-
centrated (evaporated at p = 25 Torr, T = 35 °C) until the
Pure germanosilicate ITH zeolites characterized by Si/Ge
ratios in the range 6–∞ were prepared using hexamethonium
1
5
as a SDA. Ge was found to preferentially occupy T-sites in
1
5
the D4Rs. Recently, Ren et al. reported the synthesis of
Ge-enriched ITH zeolites (2 < Si/Ge < 6) in dilute medium
−
1
IV
hydroxide concentration became 1.0 mol L .
IJH
2
O/T
=
22–66)
using
N,N,N′,N′-tetramethyl-1,6-
1
6
hexanediamine (TMHDA) as the template.
The possibility of tuning the strength of acid centres by
post-synthesis substitution of B and Ge in the framework of
borogermanosilicate large-pore IWR zeolites with Al and Ga
was shown and confirmed by means of X-ray powder diffrac-
tion, chemical analysis and infrared spectroscopy of adsorbed
2.3 Synthesis of ITH zeolites
The zeolites prepared in this study were designated as ITH-n,
where n is the Si/Ge ratio in the reaction mixture.
Ge-poor zeolite ITH-6 was synthesized according to ref. 10
using hexamethonium dihydroxide as the SDA. The starting
1
7
pyridine.
The catalytic performance of prepared iso-
gel had the following composition: 0.86 (SiO
2 2
) : 0.14 (GeO ) :
morphously substituted IWR zeolites in Friedel–Crafts acyla-
0
.25 IJSDAIJOH) ) : 5 (H O). Typically, a certain amount of ger-
2
2
1
7
tion was also evaluated.
manium oxide (GeO ) was dissolved in
a solution of
2
In this paper, we successfully extended this approach to
generate strong acid centres by post-synthesis alumination of
medium-pore germanosilicate ITH zeolites. The formation of
ultramicropores and mesopores at the expense of Ge extrac-
tion was observed when Ge-rich ITH zeolites with Si/Ge = 2.5
or 4.4 as starting materials were used. It was shown that the
temperature of post-synthesis treatment strongly influenced
the concentration of Brønsted acid sites formed and their
accessibility. The improved catalytic behaviour of the
aluminated ITH zeolites containing strong acid centres with
respect to the initial germanosilicates was demonstrated in a
tetrahydropyranylation reaction (Scheme 1) commonly used
for protection of hydroxyl groups in peptide, nucleotide, car-
−1
SDAIJOH)
2
(1.0 mol L ). Then, TEOS was added and the mix-
ture was gently stirred at room temperature until complete
evaporation of the alcohol occurred. The resulting fluid gel
was charged into a 25 ml Teflon-lined autoclave and heated
at 175 °C for 20 days under agitation (~60 rpm). The solid
product was separated by filtration, washed with distilled
water and dried overnight at 65 °C. Occluded hexametho-
nium was removed from the samples by heating the products
from room temperature to 300 °C at a rate of 1 °C min , and
this temperature was maintained for 3 h. The next step
involved increasing the temperature at a rate of 1 °C min
−1
−1
up to 580 °C; this temperature was maintained under air flow
−1
(
200 mL min ) for 3 h to remove the remaining organic
1
8
bohydrate, and steroid chemistry.
residues.
Ge-rich zeolites ITH-1 and ITH-2 were prepared according
to ref. 16. N,N,N′,N′-Tetramethyl-1,6-hexanediamine (TMHDA)
was used as the structure directing agent. The synthesized
2
. Experimental part
2
.1 Materials
suspension had a composition of (1−x) SiO : x GeO2 : 7
2
2
TMHDA : 1.4 HF : 44 H O, where x = 0.5 for ITH-1 and x =
1
,6-Dibromohexane (96%), trimethylamine solution (31–35
0
6
.33 for ITH-2. The reaction mixture was heated at 175 °C for
days under static conditions.
wt.% in ethanol), N,N,N′,N′-tetramethyl-1,6-hexanediamine
(
(
99%), germanium oxide (99.99%), tetraethylorthosilicate
TEOS, 98%), and aluminium nitrate nonahydrate (≥98.5%)
The as-synthesized samples ITH-1 and ITH-2 were washed
with distilled water, dried at 65 °C for 12 h and calcined at
were used for synthesis and post-synthesis treatment of zeo-
−
1
6
50 °C for 8 h with a temperature ramp of 1 °C min under
lites. 3,4-Dihydro-2H-pyran (DHP, 97%), methanol (≥99.9%),
−
1
air flow (200 mL min ).
1
-propanol (≥99.9%), 1-hexanol (≥99.5%), mesitylene
(≥99%) and hexane (≥97%) were used for catalytic
2.4 Post-synthesis treatment
Post-synthesis alumination of ITH zeolites was performed by:
−
1
•
stirring of the parent ITH zeolite in a 1 mol L solution
of AlIJNO ) (1 g of zeolite per 100 ml of solution) at T = 80 °C
3
3
and pH = 2.0 for 96 h.
treatment of the parent ITH zeolite in a 1 mol L solu-
tion of AlIJNO ) (1 g of zeolite per 100 ml of solution) at T =
−
1
•
3
3
Scheme 1 Tetrahydropyranylation of alcohols.
175 °C and pH = 2.0 for 24 h in an autoclave.
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The solid product was centrifuged, extensively washed
with distilled water until the filtrate became neutral and then
dried overnight at room temperature.
The prepared samples are designated in the following
way: ITH-n/Al/temperature (°C).
at 150 °C for 15 min at an equilibrium vapour pressure of the
probe molecule. Desorption proceeded at the same tempera-
ture for 1 h followed by collection of spectra at room temper-
ature. The extinction coefficients determined in ref. 20 were
used for the evaluation of c adsorbing DTBP.
B
2.6 Tetrahydropyranylation of alcohols
2
.5 Characterization
The catalytic experiments were performed in the liquid phase
under atmospheric pressure at room temperature (25 °C) in a
multi-experiment workstation StarFish (Radleys Discovery
Technologies). Before use, the catalyst (100 mg) was activated
The crystallinity of all samples under investigation was deter-
mined by X-ray powder diffraction (XRD) using a Bruker AXS-
D8 Advance diffractometer with a graphite monochromator, a
position sensitive detector (Våntec-1) and CuKα radiation in
Bragg–Brentano geometry at a scan rate of 0.25° (2θ) min .
Nitrogen adsorption/desorption isotherms were measured
using an ASAP 2020 (Micromeritics) static volumetric appara-
tus at liquid nitrogen temperature (−196 °C). Prior to sorp-
tion measurements, all samples were degassed with a
turbomolecular pump at 300 °C for 8 h.
The size and shape of zeolite crystals were examined by
scanning electron microscopy (SEM, JEOL JSM-5500LV micro-
scope). For the measurement, the crystals were coated with a
thin layer of platinum (~10 nm) in a BAL-TEC SCD-050
instrument.
−1
at 450 °C for 90 min at a rate of 10 °C min . Typically, alco-
−
1
hol (i.e. methanol, 1-propanol or 1-hexanol,
9 mmol),
mesitylene (0.4 g, internal standard), hexane (10 ml, solvent)
and the catalyst (100 mg) were placed in a two-necked vessel
equipped with a thermometer. DHP (15 mmol) was then
added into the vessel. Samples of the reaction mixture were
taken periodically and analyzed by using an Agilent 6850 GC
equipped with a nonpolar DB-5 column (length 20 m, diame-
ter 0.180 mm, and film thickness 0.18 mm) and a flame ioni-
zation detector.
The reaction products were identified by using a Thermo
Finnigan Focus DSQ II single quadrupole GC/MS.
The concentration of Al, Ge and Si in the zeolites was
determined by energy dispersive X-ray spectroscopy (EDX)
using a Jeol JSM 5600 instrument.
3
. Results and discussion
2
7
Solid-state Al NMR spectra were obtained using a Bruker
Advance III spectrometer equipped with a 9.4 T wide-bore
The X-ray diffraction patterns of germanosilicate ITH zeolites
(Fig. 1) used as starting materials for alumination match well
1
22
superconducting magnet ( H Larmor frequency of 400.13
with those reported in the literature. In contrast to ITH-1
MHz). The samples were packed into a conventional 4 mm
zirconia rotor, and rotated at a MAS rate of 12.5 kHz using a
and ITH-2 possessing platelet-like crystals, ITH-6 is character-
ized by needle-like crystals (Fig. 2). While samples ITH-1 and
ITH-2 present separated crystals of remarkably higher size
(Fig. 2A and D, Table 1), the Ge-poor ITH-6 sample shows
agglomerates of tiny crystals (Fig. 2G, Table 1). This result
may be connected with a higher rate of crystal growth in the
presence of TMHDA acting as a SDA for the ITH-1 and ITH-2
samples. The broader and less intense diffraction lines char-
acteristic of the ITH-6 zeolite, in comparison to those of the
ITH-1 and ITH-2 samples (Fig. 1), are consistent with the
smaller size of ITH-6 crystals (Fig. 2). ITH-1 and ITH-2 zeolites
exhibit type I isotherms (Fig. 3A, B) characteristic of micropo-
rous solids and are characterized by micropore volumes of
0.112 and 0.125 cm g , respectively (Table 3). ITH-6
germanosilicate representing quite small crystals (ca. 5.0 ×
s
0.5 × 0.5 μm) showed uptake in the range of p/p = 0.8–1.0
(Fig. 3C) most probably attributed to filling of the inter-
crystalline space.
Chemical analysis of the ITH zeolites agrees with the com-
position of the corresponding gels showing increasing Si/Ge
ratio in the solids with decreasing Ge concentration in the
reaction mixture (Table 1). However, while the chemical com-
position of the Ge-poor ITH-6 zeolite (Si/Ge = 5.8) corre-
sponds to Si/Ge in the initial gel, Ge-rich ITH-1 (Si/Ge = 2.5)
and ITH-2 (Si/Ge = 4.4) samples are characterized by ca. twice
as low Si/Ge ratios compared to those in the reaction mixture
(Table 1). This agrees with the results obtained in ref. 15 and
Bruker 4 mm HFXY probe. A pulse of 1.5 μs (ν ≈ 100 kHz)
1
was applied. Signal averaging was carried out for 200 tran-
sients with a repeat interval of 2 s. The spectra were
referenced to 1.1 M AlIJNO3)3 in D O using solid AlIJacac)
2
3
(
δ
iso = 0 ppm, centre of gravity = −4.2 ppm at 9.4 T) as a secondary
reference.
The concentration of Lewis (c ) and Brønsted (c ) acid
L
B
sites was determined after adsorption of pyridine (Py) by
FTIR spectroscopy using a Nicolet Protégé 460 Magna with a
transmission MCT/A detector. The zeolites were pressed into
−
2
self-supporting wafers with a density of 8.0–12 mg cm and
−
5
3
−1
activated in situ at T = 450 °C and p = 5 × 10 Torr for 4 h.
Pyridine adsorption was carried out at 150 °C and a partial
pressure of 3.5 Torr for 20 min, followed by desorption for 20
min at the same temperature. Before adsorption, pyridine
was degassed by freeze–pump–thaw cycles. All spectra were
−
1
recorded with a resolution of 4 cm by collecting 128 scans
for a single spectrum at room temperature. The spectra were
−
2
recalculated using a wafer density of 10 mg cm . c and c
L
B
were evaluated from the integral intensities of bands at
−
1
−1
1
454 cm (c ) and 1545 cm (c ) using extinction coeffi-
L B
−
1
−1 20
cients, εIJL) = 2.22 cm μmol and εIJB) = 1.67 cm μmol .
A relatively large probe molecule 2,6-di-tert-butylpyridine
(
DTBP) was used to determine the accessibility of acid sites
2
1
in the prepared zeolites. The adsorption of DTBP took place
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Fig. 1 XRD patterns of ITH-1 (A), ITH-2 (B), and ITH-6 (C) zeolites: initial germanosilicates (black), ITH-n/Al/80 (blue) and ITH-n/Al/175 (red).
Fig. 2 SEM images of ITH zeolites under investigation: ITH-1 (A); ITH-1/Al/80 (B); ITH-1/Al/175 (C); ITH-2 (D); ITH-2/Al/80 (E); ITH-2/Al/175 (F);
ITH-6 (G); ITH-6/Al/80 (H); ITH-6/Al/175 (I).
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Table Chemical composition and textural characteristics of
Paper
1
the IWR zeolite in ref. 17 and may be connected with rapid
healing of defects formed in the course of hydrolysis of
germanosilicate ITH zeolites under investigation
3 3
Ge–O–Si bonds with Al. No diffraction lines of AlIJNO ) or
Chemical
composition
of zeolites,
mol.%
other crystalline admixtures were observed in the XRD pat-
terns of ITH zeolites after employing the alumination proce-
dures described in this study. The decrease in the intensity
of diffraction lines of ITH zeolites subjected to alumination,
which is especially pronounced in the Ge-rich ITH-1 sample
Si/Ge
Reaction
mixture
Crystal size,
μm
Sample
Si
Ge
Zeolite
ITH-1
ITH-2
ITH-6
71.4
81.5
85.6
28.6
18.5
14.5
1
2
6
2.5
4.4
5.8
10.0 × 2.5 × 1.0
40.0 × 5.0 × 5.0
5.0 × 0.5 × 0.5
(
Fig. 1A), likely originates from diminishing of the framework
density caused by leaching of the framework Ge atoms under
the conditions used (Table 2). The aluminated samples
showed drastically higher Si/Ge ratios (12.6–158) compared
with parent ITH zeolites (Si/Ge = 2.5–5.8). The magnitude of
Ge extraction depends on both the temperature of treatment
and the size of zeolite crystals. While ITH-2/Al/80 and ITH-2/
Al/175 characterized by the bulkiest crystals (40.0 × 5.0 × 5.0
μm, Table 1) show close Si/Ge ratios (Table 2), a higher drop
in Ge concentration at 175 vs. 80 °C was observed for ITH-1
(10.0 × 2.5 × 1.0 μm) and especially ITH-6 zeolites (5.0 × 0.5 ×
1
6 and may be related with “saturation” of T-sites in D4Rs
1
2 2
and ij4 5 6 ] cages.
It should be noted that Ge preferentially occupies posi-
tions in D4Rs forming [4Si;4Ge] domains regularly located
in-between silica layers in ITH-1 and ITH-2, while both pure
silica [8Si] and Si-rich D4Rs [7Si;1Ge] were detected in Ge-poor
2
3
(
Si/Ge > 5) ITH zeolites.
0.5 μm). This result indicates the role of diffusion in the
degermanation process and is in agreement with previous
observations on the influence of zeolite crystal size on the
dealumination and desilication outcome.
3
.1 The structure and textural properties of post-synthetically
treated ITH zeolites
24
25
As it was shown in ref. 23, acidic treatment of ITH zeolites
having appropriate amounts of Ge in D4R SBUs (i.e. Si/GeD4R
While the results of chemical analysis indicate increasing
concentration of Al (Table 2), the solid-state Al MAS NMR
spectra of all aluminated samples (Fig. 3) show the presence
2
7
≤
1, like in the ITH-1 sample) results in zeolite disassembly,
e.g. the formation of layered materials possessing the same
topology of the layers as the initial ITH zeolite. In contrast,
all ITH zeolites subjected to alumination in acidic medium
4
of both tetrahedral AlO (shift ranges of 50–80 ppm) and
2
6
octahedral AlO6 species (−10–15 ppm). The Ge-rich ITH-1
and ITH-2 zeolites aluminated at 80 or 175 °C and ITH-6/Al/
175 sample show a dominant peak at −54 ppm (~90% of the
integrated signal intensity) evidencing the incorporation of
most aluminium atoms into the framework of the zeolites
(Fig. 3). In contrast, ITH-6/Al/80 shows a dominant reflex at
0 ppm (76% of the integrated intensity, Table 2) attributed to
(pH = 2.0) at T = 80 °C or 175 °C still display sharp diffrac-
tion lines at the characteristic 2-theta positions, which proves
the preservation of the structure ordering of the samples
(Fig. 1). This indicates that Al acts as a “stabilizer” preventing
the disassembly of the Ge-rich ITH zeolite (i.e. ITH-1) in
acidic medium. The mentioned effect was also observed for
6
AlO species. It seems that the output of alumination is
2
7
Fig. 3
Al MAS NMR spectra of ITH-1 (A), ITH-2 (B), and ITH-6 (C) zeolites: ITH-n/Al/80 (blue) and ITH-n/Al/175 (red).
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Table 2 Chemical composition of ITH zeolites subjected to alumination
ITH-6 zeolites possessing tiny crystals when aluminated at
80 °C (Table 2).
Chemical composition,
The shape of ITH platelet-like crystals remained intact
mol.%
a
AlTh
,
during alumination (Fig. 2), while ITH-1/Al/80 (Fig. 2B) and
ITH-2/Al/80 (Fig. 2E) showed an increased fraction of cracked
tiny crystals (1–3 μm) in comparison with the parent ITH-1
(Fig. 2A) and ITH-2 (Fig. 2D) zeolites. In contrast, ITH-1/Al/
Sample
Al
Ge
Si
Si/Ge
mol.%
ITH-1
—
28.6
6.8
2.2
18.5
7.0
7.2
14.5
4.3
0.6
71.4
90.6
93.5
81.5
91.5
90.7
85.6
89.0
95.0
2.5
—
92
88
—
91
84
—
24
89
ITH-1/Al/80
ITH-1/Al/175
ITH-2
ITH-2/Al/80
ITH-2/Al/175
ITH-6
2.6
4.3
—
1.5
2.1
—
13.3
42.5
4.4
13.0
12.6
5.8
1
75 (Fig. 2C) and ITH-2/Al/175 (Fig. 2F) zeolites treated under
hydrothermal conditions showed crystals of uniform size
close to that of the parent germanosilicates. This result is
probably connected with acceleration of the dissolution/
precipitation processes at elevated temperature via Ostwald
ripening of ITH crystals under hydrothermal conditions. It
seems that alumination at higher temperature resulted in
accelerated and more complete recrystallization of ITH-n
zeolites, while low temperature treatment led to deterioration
of the crystal morphology (destruction or conglutination,
Fig. 2).
The nitrogen isotherms provide valuable information on
the textural properties of aluminated ITH zeolites when
compared with the parent ones. In contrast to parent
germanosilicate ITH-1 and ITH-2 zeolites (Fig. 4A, B)
exhibiting type I isotherms, zeolites ITH-1/Al/80 and ITH-2/Al/
80 aluminated at 80 °C show isotherms of type I with a hys-
teresis loop of H4 characteristic of microporous solids with a
broad pore size distribution. The increase in the average pore
ITH-6/Al/80
ITH-6/Al/175
6.7
4.0
21.0
158
a
Calculated based on the integral intensities of peaks at 0 and
2
7
6
0 ppm in the Al NMR spectra.
dependent on the rates of 3 processes, i.e. 1) extraction of Ge
leading to the formation of silanol defects (vide infra), 2)
healing of formed defects with Al leading to the formation of
Brønsted acid sites, and 3) precipitation of oxo-Al species in
the internal/intercrystalline voids resulting in deposition of
extra-framework species. The total amount of Al as well as
the ratio between framework and extra-framework Al species
(
Table 2) depends on both the treatment conditions and
intrinsic properties of the initial germanosilicate (i.e. crystal
size, Si/Ge ratio). Increasing the temperature of treatment
results in an increase in the concentration of Al that is incor-
porated into the framework of ITH-1 and ITH-6 zeolites. In
contrast, the concentration of Al in ITH-2 zeolites possessing
the bulkiest crystals does not depend significantly on the
temperature of treatment. This result is likely connected
with either diffusional restrictions for Ge extraction or Al
penetration into bulky crystals. On the other hand, the con-
tamination of extra-framework Al species is favourable for
3 3
size of ITH-1 and ITH-2 zeolites treated with AlIJNO ) solu-
tion in acidic medium is most probably connected with
breaking of hydrolytically unstable Ge–OIJSi) bonds resulting
in the increasing void volume. A similar process of mesopore
formation in the course of the treatment of germanosilicate
IWW zeolites with hydrochloric acid at elevated temperature
was also observed in ref. 27. The increase in the alumination
temperature leads to an increased fraction of mesopores in
Fig. 4 Nitrogen adsorption (●) and desorption (○) isotherms of ITH-1 (A), ITH-2 (B), and ITH-6 (C) zeolites: initial germanosilicates (black), ITH-n/
Al/80 (blue) and ITH-n/Al/175 (red).
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−
1
Table 3 Textural characteristics of ITH zeolites subjected to alumination
3745 cm
attributed to silanol groups increased with
a
b
c
a
decreasing crystal size of germanosilicate ITH zeolites (ITH-2 <
ITH-1 < ITH-6 (Fig. 5A, Table 1)), a broad absorption band at
V
micro
,
V
meso
,
S
BET
,
ext
S ,
2
3
−1
3
−1
2
−1
−1
Sample
cm g
cm g
m g
m g
−1
28
3633–3653 cm attributed to external Ge–OH groups grew
ITH-1
0.112
ITH-1/Al/80 0.103
ITH-1/Al/175 0.080
0.125
ITH-2/Al/80 0.124
ITH-2/Al/175 0.101
0.138
ITH-6/Al/80 0.141
ITH-6/Al/175 0.130
0.007
0.05
0.12
0.005
0.07
0.17
0.13
0.18
0.13
285
330
207
313
400
275
321
345
303
23
with increasing Ge content in the following sequence: ITH-6 <
ITH-2 < ITH-1.
143
62
ITH-2
28
−1
The increased absorption band at 3745 cm in the spectra
166
90
of the ITH-n/Al/80 zeolite with respect to the parent
germanosilicate indicates formation of silanol defects under
the conditions used (Fig. 5A). In contrast, ITH-n/Al/175 and
ITH-6
68
86
64
−1
ITH-n zeolites showed absorption bands at 3745 cm of com-
a
parable intensities (Fig. 5A). This indicates acceleration of Al
incorporation into the framework of ITH zeolites accompa-
nied with some healing of silanol defects with increasing
temperature of treatment. In agreement with this assump-
tion, the ITH zeolites aluminated at 175 °C are characterized
by more intensive absorption bands assigned to bridging
The micropore volume and external surface area were evaluated
b
c
using the t-plot method. Evaluated using the BJH method. The
surface area was evaluated using adsorption branch in the range p/p
.05–0.25.
s
=
0
−
1
ITH-1/Al/175 and ITH-2/Al/175 both exhibiting combined type
I and type IV isotherms typical of hierarchical micro/meso-
porous materials. It should be noted that a contribution of Si
extraction in the formation of mesopores at 175 °C cannot be
excluded, since ITH-2/Al/80 and ITH-2/Al/175 characterized by
close Si/Ge ratios (Table 2) showed drastically different nitro-
gen adsorption/desorption isotherms (Fig. 4). In contrast,
alumination does not significantly impact the pore system of
the Ge-poor ITH-6 zeolite. Both aluminated ITH-6/Al/80 and
ITH-6/Al/175 zeolites or the parent ITH-6 germanosilicate pos-
sessed similar adsorption/desorption isotherms of type I
hydroxyl groups (ca. 3620 cm , Fig. 5A) compared with the
samples treated at 80 °C.
The incorporation of Al atoms into the framework of ITH
zeolites in the course of post-synthesis treatment was con-
firmed by FTIR spectroscopy of adsorbed pyridine. The
−
1
appearance of the absorption band at 1545 cm assigned to
the pyridinium ion in the spectra of the aluminated ITH zeo-
lites clearly evidences the incorporation of aluminium into
the framework positions (Fig. 5B). In addition, alumination
caused the increase in the intensity of the absorption band at
−
1
1455 cm , which is obviously connected with the increased
concentration of Lewis acid centres [25] in the aluminated
ITH zeolites in comparison with the initial germanosilicates.
The concentrations of Brønsted and Lewis acid sites deter-
mined from the integral intensities of the bands at 1545 and
showing interparticle adsorption in the range of p/p
.0 (Fig. 4C). The formation of extra porosity not observed
during alumination of the Ge-poor ITH-6 zeolite with
AlIJNO solution is likely caused by “unsuitable” distribution
of extractable Ge atoms in the framework (the absence of
0
= 0.8–
1
3 3
)
−
1
20
1455 cm using extinction coefficients are given in Table 4.
2
3
−1
[4Si;4Ge] domains according to the NMR data ).
The ITH-6/Al/80 (c = 97, c = 194 μmol g ) and ITH-6/Al/175
(c = 239, c = 150 μmol g ) samples having the smallest
B L
B
L
−
1
Both ITH-6 zeolite and its aluminated derivatives ITH-6/Al/
8
0 and ITH-6/Al/175 are characterized by high Vmeso values,
crystals showed the highest concentrations of incorporated
acid centres despite a relatively low concentration of hydrolyt-
ically unstable Ge–OIJSi) bonds (Table 1). For zeolites of com-
parable crystal size (e.g. ITH-1, ITH-2), the amount of incorpo-
rated acid centres increased with decreasing Si/Ge ratio in
the parent germanosilicates.
This may be connected with 1) increasing concentration
of hydrolytically unstable Ge–OIJSi) bonds being the source of
defects in the second step, condensing with aluminium ions
to form acid centres and 2) increasing average size of pores
developed in the course of hydrolysis of available Ge–OIJSi)
bonds facilitating diffusion of aluminium ions to the formed
defects. In this way, alumination of the ITH-1 zeolite containing
which is related to adsorption of nitrogen in the interparticle
space of the crystals (Table 3). Alumination of ITH-1 and ITH-2
zeolites decreased the micropore volume but increased the vol-
ume of mesopores in the range of 0.3–0.8 of p/p (Table 3).
0
Increased mesopore volume corresponds to the formation of
extra pores in the course of Ge extraction during alumination
of ITH-1 and ITH-2 enhancing with increasing temperature of
the treatment. The higher values of external surface area Sext
for ITH-1/Al/80 and ITH-2/Al/80 with respect to the initial
germanosilicates (Table 3) agree with the results of SEM
showing the decreasing average crystal size after alumination
of Ge-rich ITH zeolites at 80 °C (Fig. 2).
2
8.6 mol.% Ge (Table 1) generated a higher amount of acid
3
.2 The nature and concentration of acid sites
−1
centres (200 and 240 μmol g , Table 4), compared with ITH-2
−
1
FTIR spectroscopy was carried out to investigate the evolution
of OH groups during alumination of ITH zeolites with differ-
ent chemical compositions; the relevant spectra are depicted
in Fig. 5A. The parent germanosilicates showed two absorp-
tion bands (at ca. 3745 and 3633–3653 cm ) in the region
of 3800–3500 cm . While the intensity of the band at
(18.9 mol.% Ge)-derived samples (157 and 175 μmol g ,
Table 4). A remarkably increased concentration of formed
Brønsted acid centres (1.5–2.5 times) and a slightly decreased
amount of Lewis acid sites (Table 3) with increasing tempera-
ture of alumination of ITH zeolites from 80 to 175 °C should
be noticed.
−
1
−
1
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Fig. 5 IR spectra of ITH-1 (a), ITH-2 (b), and ITH-6 (c) zeolites (initial germanosilicates (black), ITH-n/Al/80 (blue) and ITH-n/Al/175 (red)): (A)
region of hydroxyl vibrations; (B) region of pyridine vibrations. Bottom/thin lines show spectra before adsorption of pyridine, while top/bold lines
show spectra after adsorption of a base molecule.
At first sight, successful incorporation of Al atoms into the
framework of medium-pore ITH zeolites in the course of
post-synthesis treatment seems to contradict the literature
SBUs) in ITH-1 and ITH-2 zeolites seems to create the impor-
tant prerequisites for the incorporation of Al atoms. Since the
conditions for treatment (pH = 2.0, T = 80 or 175 °C) are suf-
ficient for extraction of most Ge atoms from the framework
(Table 2), it is reasonable to assume that the first stage of the
2
9,30
results,
thoroughly disproving aluminium incorporation
into medium-pore boralites in acidic medium with the inner
void being inaccessible to bulky hydrated aluminium cations.
However, these studies focused only on (boro)silicate zeolites.
The presence of hydrolytically unstable “domains” (i.e. D4R
2
3
overall process is the breaking of Si–O–Ge bonds in D4Rs.
This leads to the formation of silanol defects in all ITH zeo-
lites under investigation (Fig. 5A) as well as the creation of
ultramicro- and meso-pores in ITH-1 or ITH-2 zeolites
(
Fig. 4). Thus, the development of extra porosity in the course
of ITH-1 and ITH-2 alumination seems to facilitate the pene-
tration of hydrated aluminium cations into the inner volume
of zeolite crystals followed by their condensation with Si–OH
groups. At the same time, the “unsuitable” distribution of
extractable Ge atoms in the framework (the absence of
Table 4 The concentration of Lewis and Brønsted acid sites within ITH
zeolites determined by means of FTIR spectroscopy of adsorbed Py and
DTBP
L
c ,
μmol g
−1
−1
c
B
, μmol g
P
a
P
c
,
AlFR
,
c
%
/AlFR
,
2
3
−
1
−1
[4Si;4Ge] domains according to the NMR data ) of the ITH-6
Sample
Pyridine DTBP Pyridine μmol g
μmol g
zeolite seems to prevent the formation of extra pores
ITH-1
ITH-1/Al/80 60
ITH-1/Al/175 140
ITH-2
ITH-2/Al/80 90
ITH-2/Al/175 127
ITH-6
ITH-6/Al/80 97
ITH-6/Al/175 239
—
—
24
4
—
21
1
—
10
8
26
26
—
—
53
38
—
72
62
—
78
65
(
Fig. 4C). For the latter sample, 75% of Al forms inter-
140
100
20
67
48
13
194
150
200
240
20
157
175
13
381
624
—
216
279
—
crystalline extra-framework species with octahedrally coordi-
nated aluminium (Fig. 3), whereas the formation of Brønsted
acid sites (Table 4) likely proceeds mainly on the outer sur-
face of the crystals.
—
—
291
389
371
594
Noticeably, the concentration of framework Al (AlFR,
2
7
Table 4) calculated based on the results of Al NMR spectro-
scopy exceeds the total concentration of acid sites deter-
mined using Py as the probe molecule. This is obviously
a
Calculated based on the results of chemical analysis and relative
2
7
integral intensity of the peak at 60 ppm in the Al NMR spectra.
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connected with different accessibilities of the incorporated Al
atoms being dependent on the treatment conditions and
crystal size of the aluminated ITH zeolites. A lower fraction
of acid centres detectable with Py is characteristic of ITH-n/
Al/175 (38–65%, Table 4) vs. ITH-n/Al/80 zeolites (53–78%,
Table 4). This result may indicate the incorporation of a
higher amount of Al into the “inner” domains of ITH crystals
with increasing temperature of alumination. Remarkably
higher concentrations of surface Brønsted acid centres for
ITH-1/Al/80 to ITH-1/Al/175 and similarly for ITH-2-derived
samples (Table 4) were also detected using FTIR spectroscopy
of adsorbed DTBP (the size of the DTBP molecule is about
While practically no conversion of substrates was observed
over the initial germanosilicate ITH zeolites possessing only
a small amount of weak Lewis acid centres (Fig. 6, Table 4),
the only products of alcohol transformation formed in the
reaction mixture over the Al-containing zeolites under investi-
gation were targeted tetrahydropyranyl ethers (Scheme 1).
The relative amount of dimers formed in competitive bimo-
lecular dimerization of DHP did not exceed 6 mol.%.
The yield of tetrahydropyranyl ether obtained in the reac-
tion of methanol and DHP after 1200 min was found to
increase in the following order of catalysts: ITH-2/Al/175
(25%) < ITH-1/Al/175 (58%) < ITH-1/Al/80 (85%) < ITH-6/Al/
80 (100%) = ITH-6/Al/175 (100%) = ITH-2/Al/80 (100%)
(Fig. 6). Noticeably, ITH-n/Al/80 (n = 1, 2) samples character-
ized by higher accessibility of acid sites performed better
than ITH-n/Al/175 (n = 1, 2) zeolites. In particular, ITH-2/Al/
175 possessing a higher concentration of acid centres than
2
1
7
.9 Å, which is higher than the diameter of 10-ring chan-
nels in zeolites). The higher accessibility of acid centres in
ITH-2-derived samples (62 and 72%, Table 4) vs. ITH-1 deriva-
tives (38 and 53%, Table 4) is probably connected with the
larger fraction of mesopores within ITH-2/Al vs. ITH-1/Al
−
1
(Table 3). Naturally, ITH-6/Al zeolites characterized by tiny
ITH-2/Al/80 (175 vs. 157 μmol g , Table 4) showed a remark-
crystals (Fig. 2) showed the highest accessibility of acid cen-
tres (65 and 78%, Table 4) among the zeolites under investi-
gation. According to the results of FTIR spectroscopy of
adsorbed DTBP, ITH-6 zeolites aluminated at different tem-
peratures insignificantly differ in the concentration of surface
ably lower initial rate in methanol tetrahydropyranylation
−
5
−5
−1
(80 × 10 vs. 430 × 10 mmol h , Fig. 7). At the same time,
2
−1
while ITH-1/Al/80 (Sext = 143 m g , Table 3) and ITH-1/Al/
2
−1
175 (Sext = 62 m g , Table 3) were characterized by similar
initial reaction rates in methanol transformation (110–150 ×
−
1
−5
−1
Brønsted acid sites detected with DTBP (8–10 μmol g ,
10 mmol h , Fig. 7A). ITH-1/Al/80 with a larger Sext (accessi-
Table 4).
bility of acid sites) showed a higher yield of the target ether
(
Fig. 7B). The obtained results may indicate that diffusion
3
.3 Catalytic performance of Al-substituted ITH zeolites in
constraints exist in the prepared micro/mesoporous ITH zeo-
lites possessing big crystals (i.e. ITH-1 and ITH-2 aluminated
derivatives) even when methanol is used as a substrate.
Diffusion limitations do not seem to play the decisive role
in tetrahydropyranylation of methanol over aluminated ITH-6
zeolites having the smallest crystals of 5.0 × 0.5 × 0.5 μm size
tetrahydropyranylation of alcohols
The activity of various heterogeneous catalysts in the tetra-
hydropyranylation of alcohols may be attributed to both
Brønsted and Lewis acid sites.
aluminated ITH zeolites was investigated in the tetra-
hydropyranylation of methanol, 1-propanol and 1-hexanol.
1
8,31
The catalytic behavior of
2
−1
(Table 1, Fig. 2H, I). In this way, ITH-6/Al/80 (Sext = 86 m g ,
Fig. 6 Yield of tetrahydropyranyl ether versus time obtained in the reaction of methanol and DHP at 25 °C over ITH-1 (A), ITH-2 (B), and ITH-6
C)-derived zeolites: initial germanosilicates (black), ITH-n/Al/80 (blue) and ITH-n/Al/175 (red).
(
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Fig. 7 Initial rate (A) and yield of tetrahydropyranyl ethers (B) formed in the reaction of methanol (a), 1-propanol (b) and 1-hexanol (c) over ITH
zeolites aluminated at 80 °C ( ) and at 175 °C ( ).
Table 3) showed an initial rate 4 times higher in methanol
substituted ITH zeolites cannot be correlated only with their
textural characteristics. For instance, ITH-1/Al/175 (Sext = 62
tetrahydropyranylation (Fig. 7A) in comparison with ITH-1/Al/
2
−1
2
−1
8
0 (Sext = 143 m g , Table 3) (Fig. 7A). The superior perfor-
m g , Table 3) showed higher yields, despite having a lower
2
−1
mance of ITH-6/Al/80 having a higher total concentration of
Sext than ITH-2/Al/175 (Sext = 90 m g , Table 3). This agrees
with the higher concentration of surface Brønsted acid sites
in ITH-1/Al/175 (Table 4).
−
1
acid sites (291 vs. 200 μmol g , Table 4) in transformation of
methanol can be explained by taking into account the possi-
bility of substrate activation by the “inner” acid centres. In
contrast, ITH-1/Al/80 (67–84% yield, Fig. 7B) performed better
than ITH-6/Al/80 (52–64% yield, Fig. 7B) when using bulkier
In contrast, the total acidity likely determines the catalytic
behavior of ITH-6 aluminated derivatives when 1-propanol
and 1-hexanol are used as substrates. ITH-6/Al/80 possessing
P
−1
1
-propanol and especially 1-hexanol as substrates. This result
a lower total number of acid sites (c = 291 μmol g ) shows
is likely connected with the increased concentration of external
acid sites in ITH-1/Al/80 possessing higher external surface area
lower initial rates and yields in tetrahydropyranylation of
P
−1
1-propanol and 1-hexanol than ITH-6/Al/175 (c = 389 μmol g )
(Fig. 7) despite having a similar number of external Brønsted
acid sites (Table 4) and a higher Sext (Table 3). This result can be
rationalized through considering the lower pass length of
reacting molecules into and off the tiny crystals of
Al-containing ITH-6 zeolites and assuming activation of sub-
strate molecules (kinetic diameters <6.2 Å) with acid centres
inaccessible to DTBP (kinetic diameter = 7.9 Å) within the
ITH-6/Al catalysts.
2
−1
2
−1
(
143 m g , Table 3) vs. ITH-6/Al/80 (86 m g , Table 3).
A decrease in the activity of all aluminated ITH samples
Fig. 7) was observed when the size of the reactants increased
methanol (kinetic diameter 3.6 Å) < 1-propanol (4.7 Å) <
(
(
1
3
2
-hexanol (6.2 Å)).
Again, ITH-1/Al/80 and ITH-2/Al/80
showed higher initial rates of alcohol transformation
Fig. 7A) and yields of target ethers (Fig. 7B) vs. ITH-1/Al/175
(
and ITH-2/Al/175 possessing lower Sext (accessibility of acid
sites). It should be noted that the particular results of DTBP
The ITH-2/Al/80 zeolite possessing the highest value of Sext
3
3
2
−1
(a selective probe molecule for Brønsted sites ) adsorption
(166 m g , Table 3) showed the highest initial rates when
−
5
−1
and activity of the prepared ITH-n/Al samples in tetra-
hydropyranylation of alcohols can hardly be correlated with-
out considering the total concentration of accessible acid
sites (both Brønsted and Lewis) formed on the external sur-
face. In particular, by comparing ITH-1/Al/80 (Sext = 143
0 0
1-propanol (r = 330 × 10 mmol h ) and 1-hexanol (r = 250 ×
−
5
−1
10 mmol h ) were used as substrates (Fig. 7). However, it
should be noted that the yields achieved over ITH-2/Al/80
possessing relatively high Sext and ITH-6/Al/175 having the
highest concentration of acid sites are comparable (Fig. 7).
This agrees with the higher concentration of acid centres
activating the substrate molecules at the initial time within
ITH-2/Al/80 (e.g. surface acid centres), while involving the
“inner” acid sites of ITH-6/Al/175 in the catalytic process as it
progresses.
2
−1
2
−1
m g , Table 3) and ITH-2/Al/80 (S = 166 m g , Table 3), a
ext
very similar amount of external Brønsted acid sites (Table 4)
gives massively different activities (Fig. 7), which may be
related to the higher Sext of more active ITH-2/Al/80. On the
other hand, the catalytic activity of the prepared Al-
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4
. Conclusions
Acknowledgements
The authors thank Dr. Daniel Dawson (University of St.
Andrews) for providing the NMR data. M. S. thanks the Czech
Science Foundation for support through the project 14-
Alumination of the Ge-rich medium-pore ITH zeolites (Si/Ge =
.5 and 4.4) in acidic medium (pH = 2) was found to result not
2
only in the formation of strong Brønsted and Lewis acid sites
but also in the development of extra porosity, related to extrac-
tion of Ge from D4Rs SBUs regularly arranged in the initial
germanosilicates. In contrast, no extra pores were formed in the
course of alumination of the Ge-poor ITH zeolites (Si/Ge = 5.8).
The chemical composition and crystal size of the initial
zeolites as well as the temperature of post-synthesis treat-
ment were shown to determine the total concentration and
accessibility of formed acid centres as well as the ratio
between Brønsted and Lewis acid sites.
30898P. M. O. acknowledges the Czech Science Foundation
for the project 13-17593P. R. E. M. thanks the EPSRC for
funding (EP/K025112/1 and EP/L014475/1).
Notes and references
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For Ge-rich ITH zeolites showing bulky crystals (e.g. 10.0 ×
2
.5 × 1.0 μm), the amount of incorporated acid centres
increased with decreasing Si/Ge ratio in the parent
germanosilicate, which is likely connected with: 1) increased
concentration of hydrolytically unstable Ge–OIJSi) bonds
being the source of defects further condensed with alumin-
ium ions to form acid centres and 2) increased average size
of pores developed during hydrolysis of available Ge–OIJSi)
bonds, which facilitate diffusion of aluminium ions to the
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5 A. Corma, M. J. Diaz-Cabanas, J. L. Jorda, F. Rey, G. Sastre
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−
1
tration of incorporated acid centres (291 and 389 μmol g
for the samples aluminated at 80 and 175 °C, respectively).
The formation of larger pores at 175 °C enhanced the dif-
fusion of Al species as well as the acceleration of the rates of
condensation reactions leading to the formation of Si–O–Al
bonds. This led to a remarkable increase (1.5–2.5 times) in
the concentration of formed Brønsted acid centres and a
slightly decreased amount of Lewis acid sites in the
aluminated ITH zeolites independent of the chemical compo-
sition of the parent germanosilicates.
In contrast to alumination at 80 °C, treatment of
germanosilicate ITH zeolites at 175 °C was accompanied with
a decreasing fraction of framework Al atoms detectable with
base probe molecules (i.e. pyridine, 2,6-di-tert-butylpyridine),
i.e. the increased concentration of the “inner” acid sites.
The activity of the prepared Al-substituted ITH zeolites
possessing big crystals (e.g. 10.0 × 2.5 × 1.0 μm) was found to
be not dependent on the total concentration of acid centres,
indicating diffusion constraints existing in the prepared
micro/mesoporous ITH zeolites even when methanol was
used as a substrate in the tetrahydropyranylation reaction. In
general, Ge-rich ITH zeolites (Si/Ge = 2.5 and 4.4) aluminated
at 80 °C and having higher Sext and concentration of accessi-
ble acid sites performed better in tetrahydropyranylation of
alcohols than the samples aluminated at 175 °C. In contrast,
the activity of aluminated derivatives of Ge-poor ITH zeolites
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