Solvent-free ketalization of polyols over germanosilicate zeolites: The role of the nature and strength of acid sites

Article abstract of DOI:10.1039/d0cy01662d

Isomorphic substitution of silicon for germanium affords germanosilicate zeolites with weak acid centers capable of catalyzing key reactions such as Baeyer-Villiger oxidation of ketones and etherification of levulinic acid. Herein, we show for the first time that UTL (Si/Ge = 4.2) and IWW (Si/Ge = 7.2) germanosilicate zeolites are active and selective catalysts of polyol (e.g., ethylene glycol, glycerol and 1,4 butanediol) ketalization to dioxolanes. Large-pore IWW outperformed the extra-large-pore UTL zeolite in the ketalization of polyols, thus indicating diffusion limitations in bulky platelet-like UTL crystals. FTIR spectroscopy of adsorbed pyridine revealed the Lewis acidity of the UTL zeolite, whereas the more active IWW catalyst was characterized by water-induced Br?nsted acidity. Increasing the activation temperature (200-450 °C) reduced the concentration of Br?nsted acid centers in the IWW germanosilicate (i.e., 0.16; 0.07 and 0.05 mmol g-1 for Tact = 200, 300 and 450 °C, respectively) but increased the number of Lewis acid sites in both zeolites. Under optimized reaction conditions (e.g., acetone/glycerol = 25, Tact = 300 °C), almost total transformation of glycerol into solketal was achieved within 3 h of reaction time over the IWW zeolite at room temperature (>99% yield of the target product). The results from the present study clearly show that weak acid centers of germanosilicate zeolites can serve as active sites in ketalization reactions.

Full text of DOI:10.1039/d0cy01662d

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DOI: 10.1039/D0CY01662D
ARTICLE
Solvent-free ketalization of polyols over germanosilicate zeolites: the role of the
nature and strength of acid sites
Iunia Podolean,a Jin Zhang, Mariya Shamzhy,b Vasile I. Pârvulescu,a* and Jiří Čejka,b#
Received 00th January 20xx,
Accepted 00th January 20xx
Isomorphic substitution of silicon for germanium affords germanosilicate zeolites with weak acid centers capable of
catalyzing key reactions such as Baeyer–Villiger oxidation of ketones and etherification of levulinic acid. Herein, we show
for the first time that UTL (Si/Ge = 4.2) and IWW (Si/Ge = 7.2) germanosilicate zeolites are active and selective catalysts
of polyol (e.g., ethylene glycol, glycerol and 1,4 butanediol) ketalization to dioxolanes. Large-pore IWW outperformed the
extra-large-pore UTL zeolite in the ketalization of polyols, thus indicating diffusion limitations in bulky platelet-like UTL
crystals. FTIR spectroscopy of adsorbed pyridine revealed the Lewis acidity of the UTL zeolite, whereas the more active
DOI: 10.1039/x0xx00000x
o
IWW catalyst was characterized by water-induced Brønsted acidity. Increasing the activation temperature (200 – 450 C)
reduced the concentration of Brønsted acid centers in IWW germanosilicate (i.e., 0.16; 0.07 and 0.05 mmol/g for Tact = 200,
300 and 450ºC, respectively) but increased the number of Lewis acid sites in both zeolites. Under optimized reaction
conditions (e.g., acetone/glycerol = 25, Tact = 300 oC), almost total transformation of glycerol to solketal was achieved
within 3 h of reaction time over the IWW zeolite at room temperature (> 99% yield of target product). The results from the
present study clearly show that weak acid centers of germanosilicate zeolites can serve as active sites in ketalization
reactions.
variable strength while enhancing the hydrolytic stability and
reducing the cost of such zeolites. Thus, for Ge-poor zeolites
(Si/Ge > 20), instability is no longer a critical issue.
Introduction
Large- and especially extra-large-pore zeolites had been
highly targeted but rarely obtained as synthesis products until
researchers recognized the structure-directing ability of
germanium towards the formation of frameworks with small
d4r and d3r units.1,2 Following this strategy, new low-
framework density structures such as BEC,3 IWS,4 -ITV5 and
UTL6,7, among others, were successfully synthesized. However,
Despite recent achievements in the design of germanosilicate
zeolites, these prospective materials have only been used as
catalysts in a limited number of applications. Among these
materials, germanosilicate zeolites with UTL, IWW and EWO
structures were found to catalyze the Baeyer–Villiger oxidation
of 2-adamantanone,22 esterification of levulinic acid,25 and
hydration of ethylene oxide to ethylene glycol.26
Notwithstanding these recent advances highlighting the
catalytic activity of germanosilicate zeolites, our knowledge of
the nature of active sites in these catalysts remains limited. In
addition, the scarce information on the acidity of specific
germanosilicate zeolites is controversial. For example, Kasian
et al detected both Brønsted and Lewis acid sites in UTL
zeolite,27 whereas other studies based on FTIR
characterizations suggested only the presence of Lewis
acidity.28,7
Glycerol acetalization with aldehydes and ketones is
especially important for using overproduced biodiesel glycerol
to synthesize cyclic acetals and ketals29-31 being prospective
fuel additives.32 Glycerol ketalization with acetone is generally
performed in the presence of a Brønsted or Lewis acid catalyst
(Scheme 1), yielding two products: the target 2,2-dimethyl-4-
hydroxy-methyl-1,3-dioxolane (solketal) and 2,2-di-methyl-1,3-
dioxan-5-ol.
germanosilicates have two significant limitations: the high cost
of Ge and their low hydrothermal stability.
2,8,9
Nevertheless,
the high lability of Si–O–Ge and Ge–O–Ge linkages in
acid/neutral aqueous medium and the preferential location of
Ge atoms in d4r units of UTL,10,11, 12 UOV,13,14 IWW15,16 and
*CTH17 have been recently exploited for controllable
framework disassembly, thereby enabling the top-down
synthesis of 2D precursors of new 3D zeolites.1 Moreover, the
18,19
post-synthesis substitution of Ge by other three-
tetravalent elements20-23 combined with recovery and
recycling of leached germanium24 generates acid sites of
or
a. University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry,
Biochemistry and Catalysis, 4-12 Regina Elisabeta Av., S3, 030018 Bucharest,
Romania.
b. Department of Physical and Macromolecular Chemistry & Charles University
Center of Advanced Materials, Faculty of Science, Charles University, Hlavova 8,
12843 Prague, Czechia.
c. *E-mail: vasile.parvulescu@chimie.unibuc.ro #jiri.cejka@natur.cuni.cz
Electronic Supplementary Information (ESI) available: S1: 1H and 13C NMR spectra
of reaction product (Figure 8); S2: FTIR spectra of IWW zeolite subjected to dose-
by-dose adsorption of water, followed by saturation with pyridine; S3: Variation of
conversion with molecule size. See DOI: 10.1039/x0xx00000x
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gently and carefully packed into the holderViebweAfroticrlee Ontlhinee
DOI: 10.1039/D0CY01662D
measurement.
The size and morphology of the zeolite crystals were
examined by scanning electron microscopy (SEM, JEOL JSM-
5500LV microscope). For these measurements, the crystals
were coated with a thin layer of platinum (~ 10 nm) in a BAL-
TEC SCD-050 instrument.
The chemical compositions of the zeolite samples were
determined by ICP/OES (ThermoScientific iCAP 7000) analysis.
In total, 50 mg of zeolite were mineralized in a mixture of 2 ml
of HF, 4 ml of HNO3, and 4 ml of HCl in a microwave oven.
After cooling, the excess of HF was eliminated by complexation
with 15 ml of a saturated solution of H3BO3, treating the final
mixture in a microwave oven. Then, the solutions under
analysis were collected and diluted in ultrapure water to a
total volume of 250 ml. The leached species in the liquid
phases was checked by ICP-OES (Agilent Technologies, Santa
Clara, CA, USA, 700 Series) after calibrating the instrument
with standard solutions.
Scheme 1: Glycerol ketalization with acetone.
Solketal is used as 1) an additive to improve oxidation
stability, and the octane number of liquid fuels, 2) a versatile
solvent and a plasticizer in the polymer industry and 3) a
solubilizing and suspending agent in pharmaceutical
preparations, as recently reported.32 To date, different types
of solid acids with strong acid sites such as aluminosilicate
zeolites,33 heteropolyacids34-36 and MOFs37 have proved their
ability to catalyze the acetalization of aldehydes and ketones.
The state of the art described above thus encouraged us to
further investigate the catalytic performance of weakly acidic
germanosilicate zeolites, including IWW and UTL. The
surprisingly high catalytic activity of both germanosilicates in
the liquid-phase ketalization of glycerol under solvent-free
conditions prompted us to perform this detailed investigation
of the zeolite structure and acidity effect on the reaction
outcome. For this purpose, here we address the catalytic
performances of IWW and UTL germanosilicates and of the
commercial large-pore aluminosilicate zeolites beta (BEA) and
mordenite (MOR) in the ketalization of different polyols in
relation to the structural and acidic properties of the catalysts
by XRD, N2 ad-/desorption, SEM, and FTIR spectroscopy of
adsorbed pyridine.
Nitrogen adsorption/desorption isotherms were measured
on an ASAP 2020 (Micromeritics) static volumetric apparatus
at –196 °C. Before the sorption measurements, all samples
were degassed with a turbo molecular pump at 300 °C for 8 h.
The t-plot method41 was applied to determine the volume of
micropores (Vmicro).
The nature and strength of acid sites in germanosilicate
zeolites were determined by FTIR spectroscopy of adsorbed
pyridine (FTIR-Py). For this purpose, the zeolites were pressed
into self-supporting wafers to a density of ~10 mg/cm2 and in
situ activated at T = 200, 300 or 450 °C and p = 5·10–5 Torr for 4
h. In particular experiments, water calibrated aliquots were
gradually introduced into the cell at 25°C, followed by pyridine
adsorption. An excess of pyridine (Py) was adsorbed at 25°C
for 20 minutes, followed by 20-minute desorption at the same
temperature. Thermodesorption of Py was performed at 25,
Experimental
Synthesis of zeolites
Structure-directing agents (SDA)
1,5-bis-(methylpyrrolidinium)
pentane
dihydroxide
(MPP(OH)2) and (6R,10S)-6,10-dimethyl-5-azoniaspiro decane
hydroxide (DMAD(OH)) were prepared according to Ref.38,39
Hydrothermal synthesis
The IWW zeolite sample was synthesized using MPP(OH)2
as the SDA according to Corma et al.38 The gel with a
composition of 0.80 SiO2:0.20 GeO2 : 0.25 MPP(OH)2:15 H2O
was transferred into a Teflon-lined autoclave and heated to
175 oC for 7 days.
The UTL zeolite was synthesized according to ref.40 A gel
with a composition of 0.67 SiO2:0.33 GeO2:0.25DMAD(OH):30
H2O was heated to 175 oC for 7 days under agitation (60 rpm).
The solid products were then recovered by filtration, washed
out with deionized water, and dried overnight at 70 oC. Finally,
the IWW and UTL zeolites were calcined in the air flow at 580
and 550 oC, respectively. The calcination time was 6 h,
whereas the temperature ramp was 1 oC·min-1.
o
50, 75, 100, 120 and 150 C for germanosilicate and at 150,
o
250, 350, 450 C for aluminosilicate zeolites for 20 min. FTIR
spectra were recorded using a Nicolet iS50 spectrometer with
a transmission MTC/B detector with a resolution of 4 cm-1 by
collecting 128 scans for
a single spectrum at room
temperature. The spectra were treated using Omnic 8.2
(Thermo Scientific) program. For baseline correction, the
spectrum of activated wafer was subtracted from the spectra
collected after Py adsorption/desorption. The concentrations
of Brønsted acid sites in germanosilicate and reference
aluminosilicate zeolites were evaluated from the integral
intensity of the band at 1545 cm−1 using the absorption
coefficient ε=1.7 cm/μmol.42 The concentrations of Lewis acid
sites in aluminosilicate zeolites were evaluated from the
integral intensities of the bands at 1454 cm−1 using absorption
coefficient ε(L) = 2.2 cm/μmol,42 whereas the number of Lewis
acid sites in germanosilicates was estimated based on the
integral intensity of the band at 1611 cm−1. To determine the
area of the peak characteristic for coordinatively bonded (1611
cm-1) and H-bonded (1596 cm-1) Py, the resulting spectral
curve was fitted using the Gaussian line shape. IR peak centers
Characterization
The phase purity of the zeolites was examined by X-ray
powder diffraction (XRD) using a Bruker AXS-D8 Advance
diffractometer with a graphite monochromator and a position-
sensitive detector (Våntec-1) using CuKα radiation (λ = 1.5418
Å, 40 kV, 40 mA) in Bragg-Brentano geometry at a scan rate of
0.25° 2θ min-1 in the 3 - 40° 2θ range. Samples were grinded
2 | J. Name., 2012, 00, 1-3
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were fixed within ±5 cm-1, and the full widths at half maxima Results and discussion
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were constrained between 5 cm-1 and 20 cm-1.
DOI: 10.1039/D0CY01662D
Physico-chemical properties of germanosilicate zeolites
The differential thermal and thermogravimetric analysis of
XRD patterns of both germanosilicate zeolites (Figure 1A)
neat and spent catalyst was carried out using a TG-DTA correspond to those reported in the literature6,7, confirming
analyser (Shimadzu Apparatus) on 4–6 mg samples in nitrogen their phase purity. Both IWW and UTL samples showed type I
flow of 10 ml min−1, under the heating rate of 10°C/min from isotherms characteristic of microporous materials (Figure 1B).
room temperature to 600°C, using Al crucible and alumina as The micropore volume of the medium-pore IWW was lower
reference.
than that of the extra-large pore UTL zeolite (0.11 versus 0.21
cm3·g–1, Table 2).
Catalytic tests
Before the catalytic tests, the germanosilicate zeolites
were activated by heating to different temperatures (200, 300,
450 ºC) with a rate of 10 ºC/min and maintaining the selected
temperature for 2 hours. Typically, 5mg of catalyst, 1mmol of
glycerol, ethylene glycol or 1,4-butanediol were mixed with an
excess of ketone (5 or 25 mmol). The catalytic tests were
performed in glass vials with magnetic stirring at room
temperature or 80 ºC for 3 hours. After the reaction time
elapsed, the reaction mixture was cooled, and a small amount
of ethanol (up to total volume of 1ml) was added to solubilize
the untransformed glycerol that usually formed a separate
phase. The mixture was then centrifuged, filtered and dried
over sodium sulphate. In total, 200 µl of final solution were
slowly evaporated at 45 ºC overnight to remove the solvent.
For recycle studies, after each catalytic cycle, the catalyst was
immediately centrifuged, washed several times with ethanol
and acetone, dried at 40 ºC and subjected to another catalytic
run, thus avoiding a longer contact with the moisture. To
identify possible leaching of active species, the reaction
mixture was removed after the 1-hour catalytic test, filtered
off, and the separated liquid was investigated under the same
reaction conditions for another 2 or 12 hours.
Figure 1: XRD patterns (A) and nitrogen ad-/desorption isotherms (B) of IWW (−) and
UTL (−) zeolites.
Table 1: Chemical composition and crystal sizes of germanosilicate zeolites.
Chemical composition
Crystal size,
Sample
mol. %
mmol·g–1
Ge
µm
Si/G
e
Si
Ge
Silylation of reaction products before the injection into
chromatographic column was required to increase their
volatility and to derivatize the free hydroxyls of polyols. For
this purpose, 150 μL of derivatization agent (1 % w/w of
IWW
UTL
87.8
80.8
12.2
19.2
1.9
2.8
7.2
4.2
0.5×0.5×0.5
30×25×1
Figure 2 depicts the SEM images of IWW (A) and UTL (B)
zeolites illustrating important differences between the two
samples. IWW possesses homogeneous 0.5×0.5×0.5 µm-sized
rectangular crystals, whereas UTL zeolite shows quite uniform
thin rectangular 30×25×1 µm-sized crystals (Table 1).
In line with previously reported results, the FTIR spectra of
both IWW and UTL zeolites (Figure 3 A) display characteristic
bands of silanol (3740 cm−1) and germanol (3660-3680 cm-1)
groups.28,43-45 The remarkably more intensive band at ca. 3740
cm−1 in the spectrum of IWW sample is consistent with its
smaller crystal size/higher external surface (Table 1, 2) bearing
terminal silanol groups. A weak and broad band apparent at
3600−3400 cm-1 in the spectra of zeolites activated at 200 and
300 ºC is indicative of hydrogen-bonded OH groups attributed
to adsorbed water.
trimethylchlorosilane
in
N,O-
Bis(trimethylsilyl)trifluoroacetamide) was mixed with 50 μL of
pyridine as catalyst. The derivatization occurred at 60 ºC for
30min. The derivatization agent-to-substrate molar ratio was
3:1. The final products were analyzed using a GC-MS (THERMO
Electron Corporation, ISQ LT Single Quadrupole GC-MS system
TRACE
1310,
equipped
with
TG-5SilMS
column
30m×0.25mm×0.25μm) and NMR spectroscopy (Bruker
Advance III UltraShield 500 MHz spectrometer, operating at
1
500.13 MHz for H NMR, 125.77 MHz for 13C NMR). For GC
analysis, the injector was set up at 250 ºC. The temperature in
the oven was kept at 50 ºC for 1 min and then increased to 250
ºC at a rate of 7 ºC min-1.
Reference commercial aluminosilicate zeolites were used
for comparison with UTL and IWW germanosilicate catalysts.
Commercial BEA zeolites with different Si/Al included BEA-12.5
(Si/Al = 12.5, CP814E, purchased by Zeolyst), BEA-25 (Si/Al =
25, CP814Q, Zeolyst), BEA-75 (Si/Al = 75, CZB-150 from
Clariant). The commercial MOR zeolite was MOR-10 (Si/Al =
10, CBV-20A, Zeolyst).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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are much weaker than those of aluminum-containing zeolites
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(Figure 5A and Ref28).
DOI: 10.1039/D0CY01662D
B
A
25 o
75 o
C
C
50 o
C
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
A
100 o
150 o
C
C
120 o
C
40%
Figure 2: SEM images of IWW (A) and UTL (B) zeolites.
20%
14%
Tact = 450 o
Tact = 200 o
C
Tact = 300 o
C
C
The adsorption of pyridine on both IWW and UTL zeolites
gave rise to absorption bands (a.b.) characteristic of: 1)
pyridine coordinatively bonded to Ge Lewis acid sites, LAS-Py,
at υ8a = 1611 and υ19b = 1452 cm–1; 2) H-bonded pyridine, H-
Py, at υ8a =1596 and υ19b =1443 cm–1 and 3) physically
adsorbed pyridine, phys-Py, at υ8b =1577 and υ19b =1438 cm–1
(Figure 3B).46,47
6
5
4
3
2
1
0
C
30%
C
20%
Tact = 200 o
C
Tact = 450 o
C
Tact = 300 o
Figure 3: FTIR spectra of UTL and IWW zeolites after (A) activation at Tact = 200 (−), 300
(−), 450ºC (−) and (B) ad-/desorption of Py at T = 25 ºC. Region of OH- (A) and Py-ring
(B) vibrations.
Figure 4: Number of Brønsted acid sites in IWW zeolite vs. Tact (A) and of Lewis acid
sites in UTL (B) and IWW (C) zeolite vs. Tact as determined from the desorption of
pyridine at different temperatures. x% values correspond to the fraction of acid sites
keeping adsorbed pyridine after desorption at 100 ºC (A) or 150 ºC (B).
In agreement with Ref. 46, the υ19b absorption bands of H-
Py and LAS-Py overlapped, while respective υ8a bands were
well-resolved and thus used here for semi-quantitative
estimation of Ge Lewis acid centers. Nevertheless,
determination of the molar absorption coefficient for υ8a band
and hence quantification of LAS in both IWW and UTL was
precluded by unavoidable contribution of H-Py. The markedly
higher intensity of the υ8a-LAS band in UTL vs. IWW (Figure 3B)
The concentration of Brønsted acid sites in IWW zeolite
evaluated from the integral intensity of the band at 1545 cm−1
(Figure 3B) decreases with the increase in activation
temperature: 0.16 mmol/g (Tact = 200ºC), 0.07 mmol/g (300ºC)
0.05 mmol/g (450ºC). The weakness of Ge-associated Brønsted
acid sites and the variation of their concentration as a function
of the activation conditions may be related to the water-
induced nature of such acid centers. To validate this
assumption, IWW zeolite activated at T=450°C was subjected
to dose-by-dose adsorption of water, followed by saturation
with pyridine monitored using FTIR spectroscopy. The gradual
adsorption of water resulted in progressive diminishing a.b. of
LAS-Py at υ8a = 1611 cm−1 with simultaneously growing
intensity of BAS-Py a.b. at υ19b = 1545 cm−1 (Figure S2).
suggested an enhanced number of Lewis acid sites in the
27
former. Although Ref.
reported Brønsted acidity was
characteristic of UTL germanosilicate, no sign of Brønsted acid
centers was detected in the UTL zeolite activated at different
temperatures (Figure 3B). In contrast to UTL, the FTIR-Py
spectra of IWW germanosilicate indicated the presence of the
Brønsted acid centers (a.b. of BAS-Py at υ19b = 1545 cm−1 and
υ8a = 1637 cm−1). The evolution of the band at ca. 1545 cm-1
with the temperature of pyridine desorption (Figure 4B)
revealed that the Brønsted acid sites of IWW germanosilicate
Similarly, the water-induced formation of Brønsted acid
sites in Sn-BEA and their reactivity was recently reported.47
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Decreasing a.b. of LAS-Py in the spectra of IWW zeolite with hydrolyze Ge-O-Si linkages decreasing the amount of
View Article Online
DOI: 10.1039/D0CY01662D
decreasing activation temperature/increasing water loading framework Ge atoms and hence detectable LAS.
accompanying by the increase in the intensity of the
Noticeably, the number of acid sites detected by FTIR-Py
absorption band of BAS-Py (Figure 3B, S2) may suggest a (Figure 4) can hardly be correlated to the chemical
similar mechanism of Lewis-to-Brønsted acid site conversion composition of germanosilicates under the study (Table 1). The
for Sn- and Ge-containing zeolites: adsorption of water on result can be explained, considering (i) sterical limitations for
tetrahedrally coordinated Ge atoms (Lewis acid centers, ≡Si– probe molecule interaction with neighboring acid sites located
O–Ge(OSi)3) followed by formation of bridging OH groups in close proximity to each other - the situation which is
(Brønsted
acid
centers,
≡Si–(OH)–Ge(SiO)3(OH)(H2O)). characteristic of germanosilicate zeolites known for
Conversely, the increase in activation temperature positively preferential location of Ge atoms in d4r units of the
affected the number of Lewis acid sites in the UTL framework;48 (ii) low strength of Ge acid centers (i.e., the shift
germanosilicate (~3 times higher integral intensity of υ8a
1611 cm–1 for zeolite activated at 450 vs. 200 ºC, Figure 4B).
=
of pyridine adsorption-desorption equilibrium to the
desorption even at low temperature).
The XRD patterns of reference commercial BEA and MOR
Table 2: Chemical composition, textural and acidic properties reference zeolites highlight the lack of any crystalline admixtures (Figure
aluminosilicate zeolites.
5A), whereas the results of N2 ad-/desorption reveal
reasonable values of micropore volumes characteristic of
those materials (Table 2).
FTIR-Py showed the presence of both Brønsted and Lewis
acid sites in aluminosilicate zeolites. The total number of acid
centers decreased with the increase in the Si/Al ratio (Table 2).
Importantly, all aluminosilicate zeolites had much stronger
acid sites than IWW and UTL germanosilicates (Figure 5B
shows the results of FTIR-Py for BEA-12.5 and MOR-10 to
exemplify this general trend).
Concentration of acid sites,
mmol/g
Sext,
Vmicro
,
m2·g–1
Sample
Si/Al
cm3·g–1
B
L
Σ
BEA-12.5
BEA-25
BEA-75
12.5
25
75
0.31
0.33
0.05
0.56
0.16a
0.07b
0.05c
-
0.32
0.19
0.07
0.35
0.63
0.52
0.12
0.91
0.16
0.25
0.20
0.14
220
210
220
70
MOR-10
10
IWW
UTL
-
n.d.d
n.d.d
n.d.
n.d.
0.11
0.21
94
40
The crystal size of BEA-12.5, BEA-25 and MOR-10 samples
was comparable to that of IWW germanosilicate, while BEA-75
showed bigger crystals (Figures 6 and 2).
-
b
c
aTact = 200ºC; Tact = 300ºC; Tact = 450ºC. dn.d. – not determined due to the
restrictions of FTIR-Py.
0.6
B
0.5
0.4
150oC
250oC
0.3
0.2
0.1
0.0
350oC
450oC
)
S
)
)
)
S
S
S
A
A
A
L
A
B
(
B
L
(
(
0
(
5
0
.
5
.
1
-
1
-
2
Figure 6: SEM images of BEA-12.5 (A), BEA-25 (B), BEA-75 (C) and MOR-10 (D) zeolites.
2
1
-
1
-
R
R
O
A
O
A
E
M
M
E
B
B
Figure 5: XRD patterns (A) and concentrations of Brønsted (BAS) and Lewis (LAS) as
determined from the desorption of pyridine at different temperatures (B) of the
reference aluminosilicate zeolites. The samples were activated at T = 450 ºC for 2 h
Catalytic behavior of germanosilicate zeolites in the ketalization of
polyols
The main issue of ketalization reaction is its low
equilibrium constant and the need to remove the water
The result is in agreement with lower hydrolytic stability of produced in the reaction or to use an excess of ketone to shift
16
UTL vs. IWW zeolite previously reported in Ref. 15-16.15,
the equilibrium to the product side. The research performed in
Thus, the amount of Lewis acid sites detected by FTIR-Py in this study used the second approach. Table 3 compiles
UTL activated at 450 °C and lacking adsorbed water (Figure 3A) comparative results assessed with IWW and UTL
reflects the number four-coordinated Ge atoms accessible for germanosilicates for the ketalization of different polyols.
pyridine molecules. In turn, adsorbed water observed after
UTL activation at 200 – 300 °C (Figure 3A) is expected to
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DOI: 10.1039/D0CY01662D
Table 3: Catalytic performance of IWW and UTL germanosilicates in ketalization of polyols.
IWW
UTL
Nr.
Polyol
Ketone
Main product
T oC
Selectivity
(%)
Selectivity
Conversion (%)
Conversion (%)
(%)
OH
O
HO
O
O
O
1
2
80
80
>99
69
>99
>99
>99
59
>99
OH
OH
O
O
99
O
OH
OH
HO
OH
3
80
63
97
56
>98
O
O
O
OH
O
O
HO
O
O
4
5
80
80
96
51
>99
98
98
49
>99
>99
OH
OH
O
OH
OH
OH
O
O
O
HO
OH
OH
6
80
19
96
7
98
O
O
OH
HO
7
8
25
25
56
89
98
27
90
97
O
O
O
O
OH
HO
>99
>99
Reaction conditions: 1mmol polyol, 5mmol ketone, 3h, solvent-free, 5mg catalyst. The catalysts were activated at 300 °C.
Table 4: Comparison of the catalytic performances of IWW and UTL activated at different temperatures with commercial aluminosilicate zeolites.
Catalyst
(activation temperature)
Nr.
Polyol
Ketone
Main product
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
8
11
12
UTL (450)
IWW (450)
UTL (300)
IWW (300)
UTL (200)
IWW (200)
BEA-12.5 (450)
BEA-25 (450)
BEA-75 (450)
MOR 10 (450)
17
41
27
56
36
46
53
57
49
15
96
96
97
98
97
98
98
96
96
95
OH
O
OH
O
HO
O
OH
Reaction conditions: 1mmol polyol, 5mmol ketone, 3h, solvent-free, RT, 5mg catalyst.
Depending on the nature of the reagent, polyol conversion both germanosilicates (Table 3; S2). The result may indicate
ranged from 7 to >99 % at T = 80 oC after 3 h, whereas diffusion limitations for reagents in IWW and UTL
selectivity was higher than 99 % for both catalysts in reactions germanosilicates; the trend was more pronounced for UTL
with diols. When glycerol is used a side-product 2,2-di-methyl- featuring bigger crystals. Decreasing the reaction temperature
1,3-dioxan-5-ol can be formed and as a result, in this case, to room temperature, as expected, decreased the conversion
selectivity vary between 96 and 99%. Increasing the ketone for both glycerol and ethylene glycol ketalization with acetone
(e.g., 2-butanone vs. acetone) or polyol (1,4 butanediol vs. (Table 3, entry 7,8). Even at room temperature, ketalization
glycerol vs. ethylene glycol) size decreased the conversion for proceeded with satisfactory conversions (Table 3).
6 | J. Name., 2012, 00, 1-3
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efficiency of solvent-free glycerol-to-solketal conversion at room
Table 4 compares conversions of polyols achieved with IWW
and UTL germanosilicates activated at different temperatures
and those of commercial aluminosilicate zeolites.
View Article Online
temperature.51 In the case of MFI zeolites increase of
DOI: 10.1039/D0CY01662D
hydrophobicity lead also to higher TOF.52 As result strong acidity in
this type of biphasic reaction is not always an advantage. In contrast
to Al, Ge incorporation into the frameworks of some silica zeolites
decreased water uptake, thus allowing the preparation of weak
solid acids characterized by moderate hydrophilicity.53,54 Moreover,
water adsorption on Ge Lewis sites of both germanosilicate zeolite
may promote the in situ generation of Brønsted acid centers, which
are more active in the ketalization of glycerol than Lewis acid
sites.55
The internal diffusion effect on the rate of glycerol ketalization
cannot be ruled out for UTL germanosilicate, which has the largest
crystals (Figure 2B) and the lowest glycerol conversion values (16 –
39 %) vs. IWW and commercial aluminosilicate BEA zeolites (Table
4). Conversely, the conversions of glycerol achieved over IWW
germanosilicate (40
– 50 %) were comparable to those of
commercial aluminosilicate BEA (45 – 48 %) and exceeded the
conversion over MOR (14 %) zeolite, which showed the highest
concentration of strong acid sites, both Brønsted and Lewis (Table
2).
100
90
80
70
60
50
40
30
The crystallite size of zeolites was earlier reported as one of an
important factors for ghe glycerol conversion.49 Smaller crystal sizes
seam to improve the diffusion of reactants due to short path.
Accordingly, diffusion limitation generated in mordenite (MOR)
pores results in a lower glycerol conversion. According to data
presented in Tables 1, 2 and 3, 4 a correlation between the volume
of the micropores, surface area of the catalysts and conversion can
be seen for commercial zeolites, especially the BEA ones. The
conversions (Table 4) for commercial zeolites increase with increase
of the volume of micropores and surface area. The surface area of
germanosilicate zeolites is lower than for commercial ones, while
the yields are comparable or even higher than for commercial
zeolites. UTL catalyst has a higher surface area, larger crystals, but
smaller volume of micropores comparative to IWW. In accordance,
the conversion generated by UTL zeolite is comparable with that for
MOR also possess smaller volume of micropores and larger crystals.
However, only the textural properties of catalysts are not enough to
explain the similar activity of germanosilicate zeolites, which have
small surface area and weaker acid sites, with BEA. In this context,
the acidic properties of the catalyst also play an important role and
the collected results suggest that even the weak acid sites of
germanosilicate zeolites serve as active centers in the ketalization
reaction. Conversion values provided by commercial BEA zeolites
(Table 4) cannot be correlated only with Si/Al ratio or with the
20
10
0
BEA-25 (450)
IWW (300)
UTL (200)
15min
30min 1h
1.5h
2h
4h
12h
time
A
B
Figure 7: (A) Time-on-stream dependence of glycerol conversion (square) and solketal
selectivity (circle) over zeolite catalysts. (B) appearance or reaction mixture after 15 and
60 minutes of reaction (B) Reaction conditions: 1mmol glycerol, 5mmol acetone,
solvent-free, RT, 5mg catalyst.
Conversion to solketal in time for BEA-25, IWW and UTL
catalysts is presented in Figure 7. For the germanosilicate
zeolites the increase of the activity is more evident in the first
two hours: for IWW the conversion increases from around 30
to more than 50% in the first two hours leading after that to
slightly higher conversion than BEA-25. The same trend is
observed for the UTL as well, although for smaller conversions.
The commercial zeolite led to a 50% conversion in the first 15
overall concentration of acid sites. However, at some extent a minutes when it reaches a plateau. The single phase formation
after 15 minutes of reaction (Figure 7 right). Taking into
consideration the difference existing between the textural
properties, Si/Ge ratio and acidity of IWW and UTL catalysts,
the similar time needed to achieve a highest conversion
correlation exists between Brønsted acid sites and conversion or
B/L acid sites ratio. Varying from 0,97 to 1,7 and 0.7 B/L ratio in BEA
zeolites lead to 53, 57 and respectively 49% conversion. The results
suggest that even the weak acid sites of germanosilicate zeolites
serve as active centers in the ketalization reaction. Consistently
with the increase in the number of Brønsted acid sites of the IWW
zeolite and with the enhanced strength of Lewis acid sites of UTL
with the decrease in activation temperature (Figure 4), the optimal
o
temperature for IWW is 300 C, whereas the optimal temperature
for UTL is 200 oC. The decrease in the activity of the zeolite catalyst
with the increase in the concentration of aluminum acid centers
(Table 2 and 4) is consistent with the high affinity of framework Al
towards water formed in the reaction (Scheme 1), which may
compete with reagents for adsorption on active sites.50 Indeed,
hydrophobization of Al-rich HY zeolite (CBV600, Si/Al ratio=2.6) with
organosilane surfactant was recently reported to improve the
Figure 8: Ketalization of acetone to ketal. Reaction conditions: 5 mg catalyst IWW, RT,
3h magnetic stirring, solvent-free.
suggests that some induced species are responsible for
increase of catalytic activity and the formation of those species
is a time-factor. The concentration of Brønsted acid sites per
surface unit is slightly higher in IWW zeolite, compared to BEA-
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25. However, the faster plateau reached in the presence of the after the catalyst washing. The differential thermal and
View Article Online
DOI: 10.1039/D0CY01662D
thermogravimetric analysis of fresh and spent BEA-25 and
BEA catalyst should be assumed to a larger surface area.
However, all catalysts provided similar selectivities after 2 IWW catalyst is shown in Figure 10.
h of reaction. The secondary product, acetal (2,2-dimethyl-1,3-
After 3 catalytic cycles the weight loss for IWW catalyst is
dioxan-5-ol), is formed onto the IWW catalyst during the first around 3% and for BEA-25 around 5% indicating larger
hour, then subsequently glycerol converts only to solketal.
deposits of coke. Also, for BEA-25 a new DTA peak occurs at
temperatures between 120-130 ºC related to these deposits. It
was also observed a higher deactivation of UTL and IWW
catalysts during recycle, when a higher amount of acetone was
Figure 9: Catalyst recycling for solketal synthesis (Reaction conditions: 1mmol polyol,
5mmol ketone, 3h, solvent-free, RT, 5mg catalyst).
Noticeably, when increasing the ketone-to-alcohol molar
ratio to 25, the solketal yield reached 99 % over the IWW
catalyst, even at room temperature (Figure 8). In addition, no
side-products, such as dioxane (2,2-dimethyl-1,3-dioxan-5-ol),
or acetone condensation products (mesityl oxide) were
identified when using IWW and UTL or even commercial
zeolites under these conditions. This high selectivity is usually
explained by a mechanism involving the rearrangement of
dioxane to dioxalane, which is catalyzed by Brønsted acid sites
and favored by an increase in pore volume and by a decrease
in acidity. The literature on selectivity is, however,
contradictory, increasing in some cases by dealumination56 or
desilication29.
Framework germanium atoms are able to not only change
the local environment (reflected in the transformation of
Lewis-to-Brønsted acid sites) but also leach to the liquid phase
by coordination with water, as by-products of the ketalization
reaction. The results from the recycling test showed that both
UTL and IWW catalysts can be reused at least 3 times without
selectivity and conversion losses, when 1:5 glycerol:acetone
molar ratio is used (Figure 9). The type of acid sites and
hydrolysis reaction are two factors affecting the most the
selectivity to solketal.56 During the recycle, the selectivity
increases slightly for UTL and BEA catalyst. An enhancement
can be explained by adsorption of some water on Brønsted
acid sites during the catalyst recycle that can favour the
rearrangement to solketal.52
Figure 10: The differential thermal and thermogravimetric analysis of fresh and spent
BEA-25 and IWW catalyst (Reaction conditions: 1mmol glycerol, 5mmol acetone,
solvent-free, RT, 5mg catalyst, 3 cycles).
used (reaction conditions from Figure 8). However, even for
ultrastable Y zeolites, washing and drying did not prevent the
decrease in catalytic activity.31
Leached Ge species in solution after 1h were under 0,1ppm
as determined by ICP-OES. Nevertheless, the leached Ge
species were not active during the ketalization reactions, as
demonstrated for glycerol (Figure 11), with only a negligible
increase in the conversion after removing the catalyst, even
after 12 hours of reaction.
The commercial zeolite BEA-25 was also recycled 3 times
without considerable changes in terms of conversion or
selectivity. However, after the second recycle of BEA-25 it was
noticed a change of the catalyst colour in yellow, persisting
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commercial BEA-25 zeolite. Thus, the results ofVitehweArtpicrleeOsnelinnet
DOI: 10.1039/D0CY01662D
study demonstrate the catalytic potential of unmodified
germanosilicate zeolites, thereby encouraging further research
on the catalytic behavior of these materials, particularly on
their stability and deactivation and regeneration modes.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
I.P. and V.P .acknowledge the support of the Government
of Romania, Ministry of Research and Innovation, project PN-
III-P1-1.1-PD-2016-1054 Nr. 29/2018. M.S. and J.Č.
acknowledge OP VVV “Excellent Research Teams” project
No.CZ.02.1.01/0.0/0.0/15_003/0000417– CUCAM. J.Č. thanks
for the support of the Czech Science Foundation to the project
EXPRO (19-27551X). M.S. gratefully acknowledges the financial
support of the Czech Science Foundation to the project 20-
12099S. J.Z. acknowledges the support of the Grant agency of
Charles University for the project 1398119 and Czech Science
Foundation for the project 19-21534S.
Figure 11: Results of leaching tests over the IWW and UTL catalysts (Reaction
conditions: 1mmol glycerol, 5mmol acetone, solvent-free, RT, 5mg catalyst. The
catalysts were activated at 300 °C).
The comparison of the performance of germanosilicate
zeolites in ketalization reactions with the results reported in
the literature shows that similar conversion/selectivity values
were documented for glycerol ketalization with acetone when
using other homogeneous or more complex heterogeneous
catalysts, such as Lu(OTf)3,57 MoPO/SBA-15SiO2,58 Re/SiO2,59
60
ZrO2/SO4 or MOR,30 MFI, hydrophobic HY51 or BEA zeolites,
30, 56,49
Notes and references
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at higher temperatures.61
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