Fe-Modified Zeolite BETA as an Active Catalyst for Intramolecular Prins Cyclization of Citronellal

Article abstract of DOI:10.1007/s10562-020-03456-w

Abstract: Prins intramolecular cyclization of citronellal giving desired product isopulegol was performed using different modified zeolites BETA (Si/Al ratio 25, 38 and 75 modified with iron or zinc of 1, 5 or 10?wt.% by wet impregnation method). In case of materials BETA Si/Al 38 and 75 led material impregnation with metal to increase of material catalytic activity (accompanied with increase of amount of weak acid sites detected using temperature programmed desorption). Material BETA 38 with loading 1?wt.% of Fe provided 97% citronellal conversion and 94% selectivity of isopulegol formation (90?°C, toluene, 24?h). Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03456-w

Catalysis Letters
https://doi.org/10.1007/s10562-020-03456-w
Fe‑Modifed Zeolite BETA as an Active Catalyst for Intramolecular Prins
Cyclization of Citronellal
Eva Vrbková1 · Tomáš Prejza1 · Miloslav Lhotka2 · Eliška Vyskočilová1 · Libor Červený1
Received: 18 June 2020 / Accepted: 4 November 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Prins intramolecular cyclization of citronellal giving desired product isopulegol was performed using diferent modifed zeo-
lites BETA (Si/Al ratio 25, 38 and 75 modifed with iron or zinc of 1, 5 or 10 wt.% by wet impregnation method). In case of
materials BETA Si/Al 38 and 75 led material impregnation with metal to increase of material catalytic activity (accompanied
with increase of amount of weak acid sites detected using temperature programmed desorption). Material BETA 38 with
loading 1 wt.% of Fe provided 97% citronellal conversion and 94% selectivity of isopulegol formation (90 °C, toluene, 24 h).
Graphic Abstract
Keywords Isopulegol · Fe-BETA zeolite · Citronellal cyclization · Prins cyclization · Heterogeneous catalysis
1 Introduction
intermediate for menthol synthesis. Menthol (with char-
acteristic peppermint odor) is further used as an impor-
tant ingredient in various pharmaceuticals, cosmetics
and special products [1–3]. Citronellal, which serves as a
main source for obtaining isopulegol, is renewable com-
pound, abundantly present in a large number of essential
oils (e.g. lemongrass). Process of citronellal cyclization
might be catalyzed, either homogeneously or heterogene-
ously. In the second half of the twentieth century, the usage
of the especially homogeneous catalysts was studied. As
homogeneous catalysts tris(triphenylphosphine)rhodium
[4], molybdenum or tungsten carbonyl complexes [5] are
Isopulegol is a well-known and important monoterpene
alcohol, which can be obtained as a product of citronellal
cyclization, is often used in a fragrance industry as a part
of blossom composition fragrances, and is an important
* Eva Vrbková
eva.vrbkova@vscht.cz
1
Department of Organic Technology, University of Chemistry
and Technology, Prague, Czech Republic
2
Department of Anorganic Technology, University
of Chemistry and Technology, Prague, Czech Republic
Vol.:(0123456789)
1 3

E. Vrbková et al.
reported to be used. As another efcient catalyst scandium
trifuoromethanesulfonate was reported (under low tempera-
ture (−78 °C) [6]). Traditional Lewis acids as ZnCl2, ZnBr2,
AlCl3, SbCl3, SnCl4 and TiCl4 might be also used for cycli-
zation of citronellal [7]. Heterogeneous catalysts are also
widely reported to be used for this cyclization, since they
can ofer some advantages, as avoiding handling corrosive
mixtures and producing large amounts of waste water or
can ofer possibility of catalyst reuse. Study [8] described
the use of solid materials SiO2-Al2O3, FeSO4, NiSO4,
Ti(SO4)2, Zr(SO4)2 and Al2O3, which did not decompose
by hydrolysis (compared to traditional Lewis acids) and
led to the conversion 61–90% with selectivity higher than
90%. Citronellal cyclization was also led in gas phase on
ion-exchanged faujasites [9]. Another used catalysts were
based on ZrO2 or its sulphated alternatives (99% citronellal
conversion, 99% selectivity of isopulegol formation) [10]. In
study [11] Zr-impregnated β-zeolites were used with maxi-
mum impregnated amount 2.4 wt.% of Zr. Another possible
catalysts were zeolite ZSM-5 in benzene (total citronellal
conversion,>97% selectivity of isopulegol formation) [12],
mixed oxides as TiO2-(SiO2)x, ZrO2-(SiO2)x [13] or mixed
gels HfO2-(SiO2)8 [14]. Using of BETA zeolites with low
Si/Al ratio was reported with high citronellal conversion
(>99%) and>95% selectivity of isopulegol formation [15,
16]. Silica or MCM-41 modifed with phosphothungstic acid
can be also used to catalyze this reaction (>97% citronellal
conversion,>74% selectivity of isopulegol formation) [17,
18]. Most recently, WO3-SiO2 materials are reported to be
used in this reaction [19].
Int. For zeolite modifcations Fe(NO3)3·9H2O from Lachema
and Zn(NO3)2·6H2O from Lach-Ner (both p.a.) were used.
2.2 Catalyst Preparation
Fe- and Zn- modifed zeolites were prepared as described
in [20]. Zeolite (1 g) was modifed with 1, 5 or 10 wt.% of
metal by wet impregnation method. In 15 cm3 of deminer-
alized water the needed amount of metal nitrate was dis-
solved and then 1 g of zeolite was added. The suspension
was stirred for 12 h at 60 °C. After that, water was evapo-
rated and modifed zeolite was dried overnight at 100 °C.
Material was then calcined (450 °C, air, 12 h). Materials
were denoted with names according to β-zeolite type and Fe/
Zn amount used (e.g. BE25Fe1 – material BETA25 modifed
with 1 wt.% of Fe).
2.3 Catalyst Characterization
Prepared catalysts were characterized using diferent meth-
ods. Non-modifed and modifed materials were analyzed
on UV–VIS Spectrometer Lambda 35 (Perkin Elmer). XRD
powder data was measured at RT on θ-θ powder difractom-
eter X’Pert PRO in Bragg-Brentan parafocusing geometry
with CoKα radiation (λ=1.7903 Å, U=35 kV, I=40 mA).
XRF analysis was performed on WD-XRF ARL 9400 XP
Spectrometer. The specifc surface area was determined
using nitrogen adsorption (3Flex volumetric analyzer,
Micromeritics, USA). The specifc surface area of materi-
als was calculated via the BET equation and t-plot method.
Temperature-programmed desorption (TPD) of pyridine
was measured on AutoChem II 2920 (Micromeritics, USA).
Details of TPD measurement were provided in our previous
works [19, 22]. Powders for SEM analysis were applied on
carbon adhesive tape and then goldened by 5 nm of gold via
Quorum Q150 ES (Quorum Technologies Ltd., Laughton,
UK). Footage were made by scanning electron microscope
TESCAN VEGA 3 LMU (Tescan Brno, Czech republic) in
regime low vacuum (UniVac) at pressure in working cham-
ber 1 Pa, accelerating voltage 20 kV and mod BSE. For
elemental analysis, EDS analyzer OXFORD Instrument
X-max 20 mm2 with software Aztec (Oxford Instruments,
Abingdon, UK) was used.
Fe-modifed zeolites BETA are known for its catalytic
activity and were recently used e.g. in perillyl alcohol prepa-
ration from β-pinene oxide, with the range of impregnated
iron from 0 to 10 wt.% [20]. In another study [21] the authors
used β-zeolites modifed by iron for α-pinene isomerization
to important substance trans-carveol. In this case β-zeolites
were modifed with 0.625–10 wt.% Fe.
In this work we would like to present catalytic activity
of iron and zinc modifed β-zeolites with Si/Al molar ratio
25, 38 and 75. The infuence of these metals on the course
of citronellal cyclization was not previously described. Iron
and as well zinc are available, cheap and efcient catalysts
in many acid catalyzed reactions.
2.4 Typical Experiment
2 Experimental
Catalyst (5 wt.% to citronellal) was inserted into round
bottom 25 cm3 fask with magnetic stirrer and then 3 cm3
of toluene was added. Suspension was stirred and heated
to 50 °C and then 1 g of citronellal was added and reac-
tion continued at 50 °C for 24 h. Hot fltration test was
used to confrm that reaction is only heterogeneous cata-
lyzed – reaction was carried out using typical reaction
2.1 Materials
Citronellal of purity>95% was obtained from Aroma Praha,
toluene (p.a.) was purchased from Penta and β-zeolites with
Si/Al molar ratio 25, 38 and 75 were purchased from Zeolyst
1 3

Fe‑Modifed Zeolite BETA as an Active Catalyst for Intramolecular Prins Cyclization of…
conditions for 30 min, then the catalyst was separated from
the reaction mixture and this mixture was heated for 24 h.
The fact, that the reaction did not take place after catalyst
removal confrmed heterogeneity of this reaction. Analy-
sis of product from citronellal cyclization was performed
on gas chromatograph Shimadzu GC-17A, equipped with
polar column Stabil Wax with length 30 m, inner diameter
0.32 mm and thickness of stationary phase 0.5 µm. Car-
rier gas was helium. Citronellal conversion (xcitronellal) and
selectivity of isopulegol (sisopulegol) formation were calcu-
lated according to Eqs. 1 and 2.
3.2 X‑Ray Difraction
X-Ray difraction was used to confrm the catalyst struc-
ture (Fig. 1). In the case of pure BETA38 material charac-
teristic bands for zeolite were observed – 7.9, 16.8, 17.9,
21.6, 22.6, 25.5, 27.0, 29.0, 30.0, 33.6, 34.9 and 44.0
2Theta°. These bands were present also in the case of Fe
or Zn modifed materials, which confrmed the fact, that
incorporating Fe/Zn ion into the zeolite did not destroyed
its structure. In BE38Fe1 material characteristic bands for
iron oxides were not present, which could mean, that their
intensity was either bellow detection limit or iron oxide
was homogeneously distributed in zeolite material. In the
case of BE38FE5 and BETA38Fe10 several bands charac-
teristic for Fe2O3 – 35.6, 40.9 and 49.5 2Theta° and Fe3O4
– 30.5, 37.4, 53.8 and 66.3 2Theta° were present [23, 24].
In the case of Zn modifed material the zeolite structure
was not afected, which fact was already described in lit-
erature [25].
rel.concentrationcitronellal
xcitronellal
=
× 100
(1)
initial rel.concentrationcitronellal
Equation 1 Calculation of citronellal conversion (x)
rel.concentrationisopulegol
sisopulegol
=
(2)
xcitronellal
Equation 2 Calculation of selectivity of isopulegol
formation.
3.3 Difuse UV–VIS Refectance
UV–VIS spectra for Fe-modifed zeolites showed char-
acteristic local maxima at 420 nm and 890 nm, which
intensity increased with increasing Fe-loading (Fig. 2).
The signifcant refectance increase was also observed in
the area of 500–600 nm with increasing metal loading.
The band in area 400–450 nm can be assigned to Fe-OH
interaction, 500–600 nm to α-Fe2O3 and 850–900 nm to
α-Fe-OOH interaction [20, 26]. Spectra of BE38Zn5 mate-
rial did not have any specifc maxima, only in the range
of 300–600 nm a signifcant decrease of refectance was
observed compared to nonmodifed BETA38.
3 Results and Discussion
3.1 Material Characterization
X-Ray fuorescence (XRF) was performed to determine
the composition of prepared materials (Table 1). Fe/Zn
content calculated from XRF results showed to be in good
correlation with desired amount of metal in the prepared
materials (1, 5 or 10 wt.%).
Table 1 XRF results showing catalyst composition (wt.%)
Catalyst
SiO2 Al2O3 Fe2O3 (ZnOa) Others Fe (Zna)
(calcu-
lated)
BE25Fe1
BE25Fe5
BE25Fe10 81.1 5.9
BE25Zn5
BE38Fe1
BE38Fe5
91.3 6.8
85.9 6.3
1.8
7.7
13.0
6.0a
1.8
0.1
0.1
0.1
0.3
0.1
0.1
0.1
0.2
0.3
0.3
0.3
1.3
5.4
9.1
4.8a
1.3
5.6
9.7
5.7a
1.1
5.7
9.9
86.7 7.0
93.5 4.6
87.7 4.2
8.0
BE38Fe10 82.2 4.0
13.8
7.1a
1.6
8.2
14.1
BE38Zn5
BE75Fe1
BE75Fe5
88.5 4.3
96.7 1.4
90.1 1.4
BE75Fe10 84.3 1.3
Fig. 1 XRD difractograms of nonmodifed BETA38 and this mate-
rial modifed with diferent Fe/Zn loading
aZnO/Zn content
1 3

E. Vrbková et al.
Fig. 3 TPD-MS signal for (non)modifed BETA38 (m/z=79 frag-
Fig. 2 UV–VIS spectra for nonmodifed BETA38 and this material
modifed with diferent Fe/Zn loading
ment)
all studied BETA materials. Therefore using certain Fe load-
ing had a positive efect on the material acidity. Too high
amount of Fe probably caused the blockage of the pores and
decrease the availability of acid sites for acidity measure-
ment. The efect of modifcation, visible in case of BETA25
and BETA38 above all, was caused by the fact, that a signif-
cant decrease in number of strong acid sites occurred after
modifcation. In all cases of modifed materials, the acidity
decreased with higher Fe loading. This could be caused by
the fact, that using higher Fe loading resulted in signifcant
crystallite of iron oxides formation, which could cause par-
tial pore blockage as mentioned above.
3.4 Temperature Programmed Desorption
of Pyridine
Temperature programmed desorption of pyridine was
performed to determine the acidity of prepared materials
(Table 2).
All materials contained two temperature maxima repre-
senting weak and strong acid sites (Table 2, Fig. 3). In the
case of BETA25 and BETA38 a signifcant decrease in the
material acidity was observed after modifcation. On the
other hand, in the case of BETA75, the increase of acidity
was observed using 1 and 5 wt.% of Fe. This phenomenon
was probably caused by fact, that BETA75 possessed by only
low acidity itself, because it has the highest Si/Al ratio from
3.5 Nitrogen Physisorption
Table 2 Characterization of materials obtained using temperature
Table 3 summarizes the results of the textural analyses of
the catalysts. All samples of catalysts contain a mixture of
micropores and mesopores, but the samples difer in the
ratio of micropores and mesopores. The total pore volume
of pores less than 40- nm diameter at p/p°=0.95 (Vtotal) of
catalysts was found in the range from 0.31 to 0.34 cm3 g−1,
but the micropore volume (t-plot method) was in the range
from 0.181 to 0.21 cm3 g−1. Modifcation of material with
iron or zinc led to decrease of total micropore volume.
Modifcation of zeolite with diferent iron loading>5 wt.%
resulted in decrease of specifc surface area (using 10 wt.%
of iron led to decrease from 540 to 483 m2/g which equals to
total 10% decrease of total specifc surface area).
programmed desorption of pyridine
Tmax1 (°C)
Tmax2 (°C)
Acid sites npyridine
ads
−1
(µmol. gcat
)
∑
Weak
Strong
BETA38
BE38Fe1
BE38Fe5
BE38Fe10
BETA25
BE25Fe1
BE25Fe5
BE25Fe10
BETA75
BE75Fe1
BE75Fe5
BE75Fe10
245
254
252
249
257
249
267
251
241
245
253
239
617
566
510
501
606
567
511
533
578
498
504
499
821
604
488
390
2229
540
455
406
198
227
220
169
318
388
331
272
432
358
284
253
162
166
155
133
503
215
157
118
1797
182
171
153
35
All samples of catalysts show adsorption isotherms of
type Ib and IVa (IUPAC classifcation), so samples consist
of micropores and mesopores with classical type hysteresis
loop H1 (IUPAC classifcation), which corresponds to nar-
row range of uniform mesopores. Figure 4 shows the N2
61
65
35
1 3

Fe‑Modifed Zeolite BETA as an Active Catalyst for Intramolecular Prins Cyclization of…
Table 3 Textural characteristic
SBET m2/g
St-plot m2/g
Total Pore vol-
ume cm3/g
t-Plot micropore
volume cm3/g
Ratio of
micropores
(%)
of catalysts
BETA38
BE38Fe1
BE38Fe5
BE38Fe10
BE38Zn5
540
543
493
483
503
136.2
145.1
117.5
115.0
144.8
0.34
0.34
0.32
0.31
0.31
0.21
0.21
0.20
0.19
0.18
63.4
60.8
60.7
59.5
58.8
Fig. 4 Nitrogen physisorption isotherms for (non)modifed BETA38 zeolite
(77 K) adsorption and desorption isotherms (red and black
symbols) for all samples.
activity. In all reactions a high selectivity of isopulegol
formation (> 95%) was observed, the only identifed by-
product was a product of citronellal autocondensation.
Activity of nonmodifed BETA zeolites with difer-
ent Si/Al ratio was compared (Fig. 7). The most active
material in citronellal cyclization was shown to be zeo-
lite BETA25. BETA25 material possessed by the lowest
Si/Al ratio, which means that it was the most acidic one
from the studied zeolite BETAs (its highest acidity was
also confrmed by TPD measurement). Mentioned acid-
ity signifcantly infuenced catalyst activity as it is visible
from Fig. 7 – the increase of acidity lead to increase of the
achieved citronellal conversion after 24 h.
3.6 Scanning Electron Microscopy (SEM)
SEM image for BETA38Fe5 (Fig. 5) with element mapping
images showed homogeneous distribution of Fe on catalyst
surface.
3.7 Catalytic Testing
Prepared materials were used as catalysts in intramolecular
Prins reaction – citronellal cyclization forming isopulegol
as a desired product (Fig. 6) to evaluate their catalytic
1 3

E. Vrbková et al.
activity was observed. This is in correlation with TPD
results for Fe modifed materials (Table 2) where a notable
decrease in acidity was observed after modifcation. Com-
paring the same modifcator amount in case of Fe or Zn
modifcation – the more considerable decrease of material
activity was observed in the case of Zn-modifed materials.
From studied zeolites BETA (25, 38 and 75), BETA25 has
the highest number of acid sites and its modifcation could
cause blocking of these acid sites, which could induce above
mentioned decrease of activity after its modifcation.
Activity of pure zeolite BETA38 was notably lower com-
pared to BETA25 material (Fig. 9). In the case of BETA38
material the modifcation was advantageous – material activ-
ity increased after modifcation in the case of all Fe-loading
and in case of Zn 1 wt.% metal loading. The behavior of Fe
modifed BETA38 was surprising looking at the total acidity
of the materials. The decrease of the acidity after modifca-
tion of pure BETA38 was observed (Table 2). Nevertheless,
the amount of weak acid sites increased after modifcation.
This might lead to the conclusion that in the citronellal cycli-
zation participates mainly weak acid sites. On the other side,
the role may also play a fact the availability of acid sites. In
the case of nonmodifed BETA38 the sites responsible for
acidity are incorporated in the catalyst pores mainly. In Fe
modifed catalysts, the decreased specifc surface confrmed
the partial pore blockage by the Fe oxide particles. These
particles are probably more accessible for catalysis.
Both Fe and Zn modifed BETA38 with 1 wt.% of metal
exhibited signifcantly higher activity than nonmodifed zeo-
lite (Table 4). Fe-modifed materials with metal loading 5
and 10 wt. % showed also higher activity than using non-
modifed zeolite. On contrary in case of Zn-modifed mate-
rials using of 5 or 10 wt.% metal loading was disadvanta-
geous and lead to lower citronellal conversion than using
nonmodifed zeolite (Table 4). Lower activity of BE38Zn1
compared to BE38Fe1 can be explained by the fact, that
Zn is weaker Lewis acid compared to Fe. Comparing the
infuence of total acidity of Fe and Zn modifed materials
on achieved conversion the expected trend in the case of Zn
modifed materials was observed. With increasing material
Fig. 5 SEM image of BE38Fe5 material and element mapping
images for Fe and Si (magnifcation 1000x)
Activity of Fe and Zn modifed zeolite BETA25 was
tested (Fig. 8) fowingly.
From graph (Fig. 8) it is clear, that modifed materials
were signifcantly less efcient in citronellal cyclization
comparing the nonmodifed BETA25. In both cases with
increasing modifcator amount, the decrease of catalytic
Fig. 6 Reaction scheme of isopulegol formation (a-isopulegol; b-autocondensate of citronellal)
1 3

Fe‑Modifed Zeolite BETA as an Active Catalyst for Intramolecular Prins Cyclization of…
Fig. 7 Isopulegol concentra-
tion using diferent nonmodi-
fed zeolites BETA(left) and
infuence of material acidity on
citronellal conversion at 24 h of
reaction (right); (3 cm3 toluene,
1 g (1.17 cm3) citronellal,
0.05 g catalyst, 50 °C)
Fig. 8 Isopulegol concentra-
tion using diferent Fe-modifed
(left) and Zn-modifed (right)
zeolite BETA25 (3 cm3 toluene,
1 g (1.17 cm3) citronellal,
0.05 g catalyst, 50 °C)
acidity, the achieved conversion also increased. The difer-
ent was the behavior of Fe modifed BETA. As the material
with 1% loading was the most acidic of all three compared,
it was also the most active. The other two materials achieved
comparable conversion (and the reaction course was almost
the same) although their acidity was diferent. In this case,
the specifc surface (that was almost the same for both of
these materials) played a main role.
Activity of zeolite BETA75 modifed by diferent Fe load-
ing was also tested (Fig. 10). In this case, the same trend
1 3

E. Vrbková et al.
Fig. 9 Isopulegol concentra-
tion using diferent Fe-modifed
(left) and Zn-modifed (right)
zeolite BETA38 (3 cm3 toluene,
1 g (1.17 cm3) citronellal,
0.05 g catalyst, 50 °C)
Table 4 Citronellal conversion using BETA38 based materials (3 cm3 toluene, 1 g (1.17 cm3) citronellal, 0.05 g catalyst, 50 °C, 24 h)
Material
BE38
30
BE38Fe1
68
BE38Fe5
45
BE38Fe10
47
BE38Zn1
47
BE38Zn5
26
BE38Zn10
21
Conversion (%)
as using modified BETA38 was observed – the highest
activity was obtained using BE75Fe1 material, activity of
BE75Fe5 and BE75Fe10 was lower comparing the material
with 1 wt.% of Fe, but still higher than nonmodifed zeolite.
Higher Fe loading resulted in lower material acidity and this
was connected with lower catalytic activity of the material
(Fig. 10).
and N-methylpyrrolidone were not suitable solvents for cit-
ronellal cyclization – only low conversions were obtained
after 24 h (10–39%). This may be explained by the fact,
that all these solvents belong to the group of aprotic basic
solvents represented by high or medium donor number. Low
conversions might be caused by the blockage of acid sites of
catalyst by the solvents. The high basicity may also explain
the lower selectivity to isopulegol formation. In basic media
citronellal autocondensation may occur more probably. The
highest selectivity of isopulegol formation was obtained
using toluene (99%) and heptane (95%) – non-polar non-
basic solvents.
BETA38 and BETA75 have higher Si/Al ratio com-
pared to BETA25 with the resulting lower material acidity.
Modifcation using 1 wt.%of Fe can add acid sites, which
results in higher catalytic activity of materials. In case of 5
or 10 wt.% of Fe crystallites are formed, which can cause
the pore blocking accompanied by material loss of activity
compared to material with only 1 wt.% metal loading.
Selectivity of isopulegol formation was in all cases>94%
and was not dependent on material acidity (Fig. 11) or on
citronellal conversion.
Infuence of reaction temperature on reaction course was
tested (Table 6). As expected, reaction rate increased with
increasing temperature. Selectivity of isopulegol formation
was not dependent on temperature and was in the range 93
– 98%.
Infuence of used solvent on the reaction course was
tested using BE38Fe1 material (Table 5).
In the case of diferent solvents, selectivity of isopulegol
formation decreased by the formation of citronellal auto-
condensate or ethoxycitronellal in case of solvent ethanol.
N,N’-Dimethylformamide, dimethylsulfoxide, acetonitrile
3.8 Comparison with Data in Literature
Comparison with published data was performed (Table 7).
Fe-modifed BETA zeolite showed to be cheap alternative
1 3

Fe‑Modifed Zeolite BETA as an Active Catalyst for Intramolecular Prins Cyclization of…
Fig. 10 Isopulegol concentra-
tion using diferent Fe-modifed
zeolite BETA75(left) and
citronellal conversion depend-
ence on material acidity (right)
(3 cm3 toluene, 1 g (1.17 cm3)
citronellal, 0.05 g catalyst,
50 °C)
Fig. 11 Selectivity of isopulegol
(24 h) formation based on cata-
lyst acidity (left) and depend-
ence of selectivity on conver-
sion (right) using materials with
1wt.% Fe (3 cm3 toluene, 1 g
(1.17 cm3) citronellal, 0.05 g
catalyst, 50 °C)
1 3

E. Vrbková et al.
Table 5 Infuence of used solvent on reaction course (24 h, 3 cm3 solvent, 1 g (1.17 cm3) citronellal, 0.05 g catalyst BE38Fe1, 50 °C)
Solvent
DMF
DMSO
ACN
NMP
Heptane
Toluene
1,4-dioxane
Acetone
Ethanol
Citronellal conversion (%)
10
51
13
82
26
82
39
82
53
95
68
99
97
89
98
51
98
77
Selectivity of isopulegol
formation (%)
DMF N,N-dimethylformamide, DMSO dimethylsulfoxide, ACN acetonitrile, NMP N-methylpyrrolidone
Table 6 Infuence of temperature on reaction course (24 h, 3 cm3 tol-
and it was shown that in order 10 < 5 < 1 wt.% Fe load-
ing materials acidity increased. This acidity increase was
in accordance with material catalytic activity increase.
Modifed zeolites BETA possessed by high selectivity of
isopulegol formation (> 93% in all cases). In the case of
zeolites BETA38 and BETA75, the increase of catalytic
activity was observed after materials modifcation with
Fe. On the other hand, the same modifcation in case of
BETA25 zeolite lead to decrease of material catalytic
activity. Material BETA 38 with loading 1 wt.% of Fe
was used to optimize the reaction conditions – using tem-
perature 90 °C, 5 wt.% of catalyst, 1 g of citronellal and
3 cm3 of toluene – 97% citronellal conversion and 94%
selectivity of isopulegol formation (24 h) was obtained,
which showed that this catalyst can be successfully used
in the studied reaction. The higher catalytic activity of
Fe-modifed BETA38 zeolite compared to BETA75 can
be attributed to its higher material acidity, which has a
positive efect on the reaction course.
uene, 1 g (1.17 cm3) citronellal, 0.1 g catalyst BE38Fe5)
Temperature (°C)
50
70
90
Citronellal conversion (%)
Selectivity
68
99
89
94
98
94
providing both high conversion and selectivity to already
tested heterogeneous catalysts.
4 Conclusion
Several zeolite BETAs modifed with diferent metal load-
ings were prepared and intensively characterized. X-Ray
difraction confrmed that zeolite structure was preserved
after modifcation and X-ray fuorescence confrmed the
desired metal loading. Acidity of materials was measured
using temperature programmed desorption of pyridine
Table 7 Comparison of catalysts used in cyclization of citronellal (CA)
Reference
Catalyst
Citronellal
conversion
(%)
Selectivity of
isopulegol forma-
tion (%)
Time (h) Reaction conditions
Current work
Fe-BETA (Si/Al=25)
Fe-BETA (Si/Al=38)
H3PW12O40/SiO2
20%HPW/MCM-41
H-Beta-11
99
96
94
98
74
95
95
76
98
97
5
90 °C, 5wt.% of cat., CA 1 g,
3 cm3 of toluene
Current work
98
24
0.5
3
90 °C, 5wt.% of cat., CA 1 g,
3 cm3 of toluene
da Silva [17]
97
25 °C, 1wt.% of cat., CA
0.15 mmol, cyclohexane
Braga et al. 18]
Mäki-Arvela et al. [15]
Shah et al. [27]
100
100
80
25 °C, CA 1 mmol, 10 wt% of
cat., 5 cm3 CH2Cl2
3
90 °C in cyclohexane, CA 3 g,
200 mg cat
ZSM-5
1
60 °C, CA 4.5 mmol, 5 cm3
benzene, 0.1 g cat
Rasero-Almansa and Sánchez
[28]
Plößer et al. [16]
Zr(Ti)MOF
99
24
1
150 °C in cyclohexane, CA
0.3 mmol, 5 mg cat
H-BEA (Si/Al=25, 35, 150) 99
WO3-SiO2 91
80 °C, CA 0.5 cm3, 50 mg cat.,
10 cm3 hexane
Vrbková et al. [19]
24
50 °C, 10wt.% of cat., CA 1 g,
3 cm3 of toluene
1 3

Fe‑Modifed Zeolite BETA as an Active Catalyst for Intramolecular Prins Cyclization of…
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Conflict of interest All authors declare that they have no confict of
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