A Dual-Functional Catalyst for Cascade Meerwein–Pondorf–Verley Reduction and Dehydration of 4″-Methoxypropiophenone to Anethole

Article abstract of DOI:10.1002/cssc.201801340

Anethole is an ingredient in many flavours, fragrances and pharmaceutical formulations. To reduce the dependence of its supply on natural oils, a green route for anethole synthesis was designed on the basis of Meerwein–Pondorf–Verley (MPV) reduction and dehydration of 4′-methoxypropiophenone. The one-pot cascade reactions were heterogeneously catalysed by dual-functional Zr-MSU-3, a predominantly Lewis-acidic catalyst with a Si/Zr ratio of 10 and pores with sizes in the range of 3.2–4.2 nm. The use of 2-pentanol as solvent and hydrogen donor for the MPV reduction was advantageous, as its high boiling point enhances the rate of the reactions, especially the dehydration of the MPV product, 1-(4-methoxyphenyl)-propan-1-ol. This dispenses with the need for a strong acid catalyst that could result in by-products of acid-catalysed reactions. Anethole yields of 91 % with a trans/cis isomer ratio of about 92:8, similar to that of natural anethole, were obtained. In comparison, microporous Zr-beta (Si/Zr 12.5) gave lower activity owing to pore-size constraints. Hence, through design of the reactions and catalyst, 4′-methoxypropiophenone can be efficiently converted to anethole in a sustainable and green manner.

Full text of DOI:10.1002/cssc.201801340

Accepted Article
Title: A dual-functional Zr-MSU-3 catalyst for cascade MPV reduction
and dehydration of 4'-methoxypropiophenone to anethole
Authors: Hongwei Zhang, Candy Li-Fen Lim, Muhammad Zaki bin Ali,
Stephan Jaenicke, and Gaik-Khuan Chuah
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To be cited as: ChemSusChem 10.1002/cssc.201801340
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ChemSusChem
10.1002/cssc.201801340
FULL PAPER
A dual-functional Zr-MSU-3 catalyst for cascade MPV reduction
and dehydration of 4’-methoxypropiophenone to anethole
[
a]
[a]
[a]
[a]
Hongwei Zhang, Candy Li-Fen Lim, Muhammad Zaki, Stephan Jaenicke and Gaik Khuan
Chuah(s)*[a]
Dedication ((optional))
Abstract: Anethole is an ingredient in many flavour, fragrance and
pharmaceutical formulations. To reduce the dependence of its supply
from natural oils, a green route for anethole synthesis is designed
based on Meerwein-Pondorf-Verley reduction and dehydration of 4’-
methoxypropiophenone. The one-pot cascade reactions were
oil in the natural plant species is very low, e.g., ~1.5 – 5% in
aniseed and 2 – 6% in fennel seeds.[ The output and price of
anethole fluctuate greatly due to changing weather and climate
conditions so that there is a need to reduce the reliance on natural
sources of trans-anethole. Anethole can also be obtained by
rectification of crude sulfate turpentine, a by-product of the pulp
and paper industry, where the anethole concentration is ~0.5 %.
Despite the low concentration, the large quantity of turpentine
10]
heterogeneously catalysed by dual-functional Zr-MSU-3,
a
predominantly Lewis acid catalyst with Si/Zr ratio of 10 and pores in
the range of 3.2 – 4.2 nm. The use of 2-pentanol as solvent and
hydrogen donor for the MPV reduction was advantageous as its high
boiling point enhanced the rate of reactions, especially the
dehydration of the MPV product, 1-(4-methoxyphenyl)-propan-1-ol.
This dispenses with the need for a strong acid catalyst that could
cause acid-catalyzed by-products. Anethole yields of 90 % with
trans:cis isomer ratio ~ 92:8, similar to that of natural anethole, were
obtained. In comparison, use of microporous Zr-Beta (Si/Zr 12.5) gave
lower activity due to pore size constraints. Hence, through design of
the reactions and catalyst, 4’-methoxypropiophenone could be
efficiently converted to anethole in a sustainable and green manner.
leads to
a significant output. However, due to forestry
conservation, recycling of paper resources, and the trend towards
paperless communication, there is a decline in yield from this
source. It is also possible to obtain anethole by synthetic routes.
Bauer and Molleken reported a two-step synthesis for anethole
and 2-alkoxy-4-propen-1-yl phenols where condensation between
anisole (4-methoxybenzene) and propionaldehyde is followed by
acid-catalysed splitting at 100 – 300 C to form o- and p-
anetholes.[ De Pasquale reported 91 % overall yield of anethole
(cis:trans 13:87) from isomerization of estragole (1-allyl-4-
methoxybenzene) using an iron pentacarbonyl catalyst at 100 –
11]
1
40 C.[12] Two recent patents reported the reduction of 4’-
methoxypropiophenone to 1-(4-methoxyphenyl)propan-1-ol
followed by dehydration to anethole. Stoichiometric amounts of
Introduction
NaBH
4
and LiAlH
4
as reducing agent and concentrated sulfuric
Anethole (para-methoxyphenylpropene), the main ingredient of
[13]
acid for dehydration were required.
anise oil, is widely used as a flavour substance.[1] It is certified as
We investigated the feasibility of a green route for the
synthesis of anethole from 4’-methoxypropiophenone using
heterogeneous catalysts. Heterogeneous catalysts offer several
advantages including recyclability of the catalyst and easy
recovery of products.[14] 4’-Methoxypropiophenone can be readily
synthesised from anisole and propionic anhydride, using zeolite
H-Beta as an efficient solid acid catalyst.[15] Instead of using non-
4 4
recoverable reductants like NaBH or LiAlH , it is proposed to
reduce 4’-methoxypropiophenone to 1-(4-methoxyphenyl)
propan-1-ol by Meerwein-Ponndorf-Verley (MPV) transfer
hydrogenation with a secondary alcohol as the reductant (Fig. 1).
Although the atom economy of transfer hydrogenation is lower
than direct hydrogenation using gaseous hydrogen, it is a safer
“GRAS” (Generally Recognized as Safe) by the US Food & Drug
administration (FDA) and the Flavor Extract Manufacturers
Association (FEMA). It exists as the cis- and trans-form with the
latter being more abundant and preferred for use. Other
applications of anethole include use as a masking agent in
cosmetics, soap, and oral rinses.[1a, 2] Recent studies have
focused on its medicinal properties because trans-anethole was
found to exert anti-inflammatory[3] and anti-metastatic activity
against fibro-sarcoma tumour cells.[4]
Natural anethole is obtained from essential oils extracted from
[5]
a number of plant species by steam distillation or supercritical
[
5a, 6]
The main sources are anise oil,[2b, 7] star
fluid extraction (SFE).
anise oil [
2b, 8]
and fennel oil [9], which contain 55 – 90 % anethole
with 90 % in the trans-form [2a]. However, the content of essential
alternative. Furthermore, it has the added advantage that
precious metal catalysts need not be used.[16] For the subsequent
dehydration step, a solid acidic catalyst advantageously replaces
hazardous concentrated sulfuric acid. A dual functional catalyst
capable of MPV transfer hydrogenation and dehydration should
enable a direct transformation of 4’-methoxypropiophenone to
anethole in one pot, eliminating the need for work-up steps to
obtain the intermediate, 1-(4-methoxyphenyl)propan-1-ol.[17]
[
a]
H. Zhang, C. L.-F Lim, M. Zaki, Prof. S. Jaenicke, and Prof. G. K.
Chuah*
Department of Chemistry
National University of Singapore
3
Science Drive 3, Kent Ridge, Singapore 117543
E-mail: chmcgk@nus.edu.sg
Supporting information for this article is given via a link at the end of
the document.
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ChemSusChem
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FULL PAPER
Fig. 1. Proposed cascade MPV reduction and dehydration of 4’-
methoxypropiophenone to trans-anethole.
Zirconium-based catalysts are highly active for MPV
reduction.[18] In particular, zirconium incorporated into the silica
framework of zeolite beta shows excellent MPV reduction activity
for a diverse range of substrates.[18c-g] Therefore, the feasibility of
using Zr-Beta for the cascade MPV and dehydration reaction was
first investigated in this study. Instead of the fluoride-assisted
hydrothermal synthesis where the Si/Zr is limited to 75 or higher,
a two-step post synthesis method was used to form the Zr-Beta
with a high Zr content (Si/Zr 12.5). In the first step, Al-Beta with
Si/Al 12.5 was dealuminated followed by introduction of zirconium
via wet impregnation in the second step. This method allows the
formation of zeolites with a high density of the catalytically active
zirconium.[18a] The reaction mechanism for the MPV reduction is
generally accepted to involve a cyclic six-membered transition
state in which both the reducing alcohol and the carbonyl
compound are coordinated to the same metal ion. Electron
transfer from the oxygen of the carbonyl group to the lowest
Fig. 2. (a) Low and (b) high angle X-ray diffractograms of Zr-MSU-3 with varying
Zr content.
The x-ray diffractograms of Zr-Beta show the characteristic
peaks of zeolite beta (Fig. S1A). The position of the (302) peak at
2 ~ 22.6° is particularly sensitive to composition changes of the
material. Dealumination of Al-Beta caused the (302) peak to shift
towards higher angles as the spacing between the (302) planes,
d
302, decreased. Subsequently, with incorporation of zirconium
into the Si tetrahedral framework, the d-spacing was increased to
3.921 Å. No diffraction planes corresponding to bulk ZrO
observed.
2
were
The low angle x-ray diffractogram of the pure silica MSU-3
sample showed distinct and well resolved (100), (110) and (200)
peaks, indicative of a 2D-hexagonal p6mm structure (Fig. 2a).[
The peak intensity decreased with zirconium loading so that for
SiZr-10-1D (Si/Zr 10), only the (100) peak was observed. The
broadness of the peaks are typical of the MSU-X samples and are
23]
2
unoccupied molecular orbital of zirconium (dz ), activates the
molecule and initiates a hydride transfer from the alcoholate.[19]
Our previous study showed that Zr-Beta obtained by the two-step
post synthesis method is more hydrophilic than the fluoride-
assisted sample, which is favourable for interaction with polar
substrates. In view of the size of 4’-methoxypropiophenone, a
catalyst with larger pores was synthesized for comparison.
Mesoporous MSU-3 (in the nomenclature of MSU materials, the
indicative of the formation of 3-dimensional disordered wormhole
pore structure.[
22, 24]
A similar trend was observed for the samples
formed in the presence of NaF as a condensation agent. The high
angle x-ray diffractograms of the samples did not show any peaks
of ZrO , indicating that most of the zirconium was successfully
2
number “3” indicates that polypropylene oxide (PPO)
–
incorporated into the silica framework (Fig. 2b). However, for the
samples synthesized with the aid of NaF, peaks corresponding to
zircon, ZrSiO (PDF 03-0443), were observed. The intensity of the
4
zircon diffraction peaks increased with zirconium loading.
Increasing the duration of the hydrothermal treatment from 0 to 4
days for samples with Si/Zr 10 did not yield any significant
changes in the x-ray diffractograms (Fig. S2).
Transmission electron micrographs of the pure Si-MSU-3
reveal the presence of well-ordered hexagonal channels with
~9.15 nm spacing (Fig. 3). This agrees well with the measured
polyethylene oxide (PEO) block copolymers were used as
porogen) was chosen as the core material for substitution of
zirconium into the silica framework. MSU-x mesostructures
exhibit three-dimensional worm-like pore channels in contrast to
the highly ordered mesoporous silicates such as M41S, SBA-15,
KIT, etc.[ It has been speculated that the interconnected random
network of pores of MSU-3 could minimize diffusion limitations.[21]
The synthesis of MSU materials requires neutral or mildly acidic
conditions.[22] This is beneficial for the incorporation of zirconium
as under more acidic conditions, the metal oxo-cation remains
highly soluble, making it difficult to condense with the silica
framework. The conditions necessary for zirconium incorporation
into Zr-MSU-3 (hydrothermal treatment and addition of NaF) and
the effect on catalytic activity are investigated in this study. The
samples are denoted as SiZr-X-YD, where X and YD represent
the Si/Zr molar ratio and hydrothermal duration (days),
respectively.
20]
d
100 ~9.20 nm from the x-ray diffractogram. Zirconium
incorporation led to disruption of the long channels by increasing
domains of disorder. For SiZr-10-1D, disordered wormhole pore
structure was predominant with only small pockets of ordered
channels. In contrast to the fluoride-free samples, the addition of
NaF during synthesis resulted in better ordering of the channels
with less disordered domains (Fig. S3).
The surface areas of the samples were 520 – 730 m2 g-1
(
Table 1). In comparison, the Zr-Beta has a smaller surface area
2
-1
of 427 m g with ~59 % due to the micropores. The nitrogen
adsorption/desorption isotherms of the MSU-3 samples, with
hysteresis loops between P/P of 0.4 to 0.7, can be classified as
Results and discussion
o
a Type IV isotherm following the IUPAC convention (Fig. 4). The
Physicochemical properties
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ChemSusChem
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Fig. 4. (a) Nitrogen adsorption-desorption isotherms and (b) pore size
distribution of mesoporous Zr-MSU-3 samples.
Table 1. Textural and chemical properties of Zr-MSU-3, Zr- and Al-Beta.
2
3
B/L[a]
n.d.
0.24
0.17
n.d.
0.30
0.23
n.d.
Sample
S
520
730
713
615
611
600
665
BET (m /g)
V
0.86
0.93
1.13
0.56
0.56
0.76
0.71
pore (cm /g)
Si/Zr
-
19.9
20.8
11.1
10.3
9.6
Fig. 3. TEM images for (a, b) Si-MSU-3 (c, d) SiZr-20-1D and (e, f) SiZr-10-
Si-MSU-3
SiZr-20-1D
SiZr-20-1D-F
SiZr-10-0D
SiZr-10-1D
SiZr-10-1D-F
SiZr-10-3D
SiZr-10-4D
Zrβ-12.5
1D.
pore sizes were calculated using the BJH model on the desorption
isotherms. For SiZr-20-1D, the pores were in the range of 3.3 
9.9
583
0.64
10.2
12.0

n.d.
0.25
0.80
4.2 nm but decreased to 3.2  4.0 nm for SiZr-10-1D. The addition
[
b]
[b]
[b]
427 (251)
568 (362)
0.84 (0.14)
1.04 (0.19)
of NaF to the synthesis gel resulted in a bimodal pore distribution.
[b]
Alβ-12.5
This can be clearly seen in SiZr-10-1D-F where pores of ~3.2 
[
[
a] Brønsted/Lewis acid sites from pyridine IR after evacuation at 200 C.
4
.0 nm and 5.2  6.1 nm formed. Furthermore, hydrothermal
b] microporous area and volume.
treatment of three days or longer led to larger pores with a wider
distribution. The increase in the average pore diameter can be
explained by the expulsion of water from the hydrophilic shell of
the hybrid micelle.[ With longer hydrothermal treatment, more
water is lost from the hydrophilic PEO shells (from P123) of the
hybrid micelles, stretching the PEO chains and resulting in larger
pore sizes.[26]
significantly affect the Si/Zr ratio. These results confirm that the
synthesis condition for MSU-3 is suitable for introduction of
zirconium ions into the silica framework even to Si/Zr ~10.
Elemental mapping analysis with SEM/EDS showed that the
zirconium was uniformly dispersed throughout the particles (Fig.
S4). Using diffuse UV/vis spectroscopy, the isolated nature of Zr4+
in the tetrahedral configuration was inferred from the absorption
at ~203–210 nm which has been ascribed to ligand-to-metal
25]
Hydrothermal treatment of the synthesis gel was important for
the incorporation of zirconium into the siliceous framework.
Without any hydrothermal treatment, the Si/Zr ratio in SiZr-10-0D
was only 11.1, less than in the synthesis gel. On the other hand,
the Si/Zr molar ratio of the samples that had been subjected to
hydrothermal treatment was between 9.6 – 10.3, close to the
expected value of 10 (Table 1). The duration of hydrothermal
treatment on the synthesis gel from 1 to 4 days did not
2-
4+
charge transfer from
configuration.[19a, 27] The absence of bulk ZrO
samples is supported by the lack of a sharp absorption edge at
O
to isolated Zr
in tetrahedral
2
in Zr-MSU-3
~
240 nm (Fig. S5). The local environment of zirconium was
deduced from 29Si MAS NMR and XPS. The Si spectrum of Si-
29
MSU-3 shows two peaks, at ~ -110 ppm and -103 ppm in the Q4
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ChemSusChem
10.1002/cssc.201801340
FULL PAPER
3
and Q range, which can be assigned to Si(4Si) and Si(3Si, 1OH),
respectively (Fig. S6). The incorporation of zirconium in the
framework is supported by a higher Q /Q4 intensity and the
3
2
[28]
appearance of a small signal at - 91 ppm, in the Q range. The
state of zirconium at the surface was studied using XPS. In bulk
ZrO
2
, the binding energies of Zr 3d5/2 and Zr 3d3/2 are observed at
For Zr-Beta and Zr-MSU-3, these
182.2 and 184.6 eV (Fig. S7)
.
peaks are shifted to higher binding energies, in agreement with
reported values for zirconium in silica framework.[29] The formation
of Zr-O-Si bonds results in a more electropositive Zr4 as
compared to zirconium in Zr-O-Zr due to a more electronegative
+
4+
4+
Si . Consequently, the binding energy of Zr in the silica
framework is higher than for ZrO
2
.
The substitution of zirconium into silica is known to create
acidic sites which can be detected using ammonia TPD.[30] The
desorption of ammonia occurred in a broad peak with onset at
Fig. 6. FTIR spectra of pyridine after evacuation at 200 C for SiZr-20-1D, SiZr-
20-1D-F, SiZr-10-1D, SiZr-10-1D-F and Zr-Beta-12.5.
~
120 °C and extending to ~450 °C for the Zr-MSU-3 and Zr-Beta
samples, showing that the samples had similar acid strengths.
The broad desorption profiles indicate that the sites had a wide
range of acid strength (Fig. 5). The density of acid sites for the Zr-
similar acidic properties. In comparison, Zr-MSU-3 samples
synthesized with NaF had lower acid density and smaller B/L
ratios than those synthesized under fluoride-free conditions. After
evacuation at 300 C, the intensity of the pyridine peaks were
reduced, indicating that the Zr-containing samples were of weak
to moderate acidity. This contrasts with Al-Beta-12.5 which has
pronounced Brønsted acidity with a high B/L ratio of 0.80. The
intensity of the pyridine adsorption peaks remained high even
-
1
MSU-3 samples was between 0.44–0.57 mmol g while it was
only 0.31 mmol g- for Zr-Beta. In contrast, only a very small
desorption peak could be observed for Si-MSU-3 and
dealuminated Beta samples, reflecting the low acidity of silica.
The pyridine IR measurements showed the presence of both
Brønsted (B) and Lewis (L) acid sites in the samples (Figs. 6 and
S8). The adsorption of pyridine at Lewis acid sites is indicated by
1
-1
after heating at 300 C (Fig. S8a). Furthermore, the 1445 cm
peak shifted to higher wavenumbers with increase of evacuation
-
1
-1
bands at ~1447–1460 cm , ~1575, and 1600–1630 cm , while
the pyridinium ion formed by adsorption of pyridine at Brønsted
temperature, showing the presence of strong Lewis acid sites.
-
1
acid sites shows bands at ~ 1540–1550 and 1640 cm . The peak
-
1
[31]
at ~1490 cm is attributed to both Brønsted and Lewis acidities.
Catalytic Activity
The relative density of Brønsted/Lewis acid sites were obtained
from the areas of the 1545 and ~1447 cm-1 peaks, respectively,
after normalizing with the respective molar extinction
coefficients.[31b] The B/L ratio for Zr-Beta was 0.25 while SiZr-20-
Effect of mesoporosity
The initial MPV reduction and dehydration of 1 mmol 4’-
methoxypropiophenone in 10 mL of pentanol was carried out at
120 °C with 200 mg of an Al-Beta zeolite (Si/Al=12.5). However,
1D and SiZr-10-1D have ratios of ~0.24 and 0.30, respectively
the conversion of 4’-methoxypropiophenone was below 1 % even
(Table 1). The close B/L ratios suggest that the samples have very
after 45 h (Table 2). Instead, autoetherification of 2-pentanol to 2-
(
pentan-2-yloxyl)pentane occurred which can be attributed to the
presence of strong acid sites on the catalyst. In contrast, Zr-Beta-
2.5, showed much better performance (Fig. 7a). The desired
1
cascade reaction occurred with anethole forming the bulk of the
products. After 45 h, 87.1% of 4’-methoxypropiophenone was
converted with 90.5 % selectivity to anethole (3) (78.8 % yield).
The trans:cis isomers were formed in the ratio of 92:8, similar to
that for natural anethole. Besides anethole, about 2.4 % of the
ether, 1-methoxy-4-(1-(pentan-2-yloxy) propyl)benzene (2), was
detected (Scheme 1). This could be formed by etherification
between the MPV product, 1-(4-methoxyphenyl)propan-1-ol (1),
and 2-pentanol and/or by addition of 2-pentanol to anethole.
However, the latter route can be ruled out as the ether
concentration reached a maximum of ~ 4.5 % before decreasing,
suggesting reversibility of the reaction. If the ether was formed by
the second pathway, its concentration would be expected to grow
with time as the concentration of anethole increased. At longer
reaction times, 1-methoxy-4-propylbenzene (4) was detected
which was due to transfer hydrogenation of C=C bond in the
formed anethole. Notably, no side products due to acid-catalyzed
Fig. 5. Ammonia TPD of (a) Si-MSU-3 (b) SiZr-20-1D (c) SiZr-20-1D-F (d) SiZr-
0-1D (e) SiZr-10-1D-F (f) dealuminated Beta and (g) Zr-Beta-12.5.
1
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ChemSusChem
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Table 2. Catalytic activity of Zr-MSU-3 and zeolite-Beta catalysts.
etherification or dehydration of 2-pentanol could be detected.
Another striking feature is that the MPV product, 1-(4-
methoxyphenyl)propan-1-ol (1), was not observed, showing that
once formed, it was rapidly dehydrated to anethole. The presence
of Zr is necessary for the reaction as the siliceous Si-MSU-3
showed no activity.
Sample
TOF
Conv.
(%)
Selectivity (%)[b]
-
1
[a]
[b]
(
h )
Anethole (trans:cis)
2
4


6.7

8.1
2.4
4.4
2.6
deAl-Beta
Alβ-12.5
Zrβ-12.5
Si-MSU-3
SiZr-20-1D
SiZr-20-1D-F
SiZr-10-1D
SiZr-10-1D-F
0

0




< 1
87.1
0
98.5
79.5
98.1
90.5
0.11
0
0.14
0.13
0.14
0.13
90.5 (92:8)

89.1 (93:7)
74.2 (90:10)
91.5 (92:8)
77.1 (90:10)
2.8

2.8
23.4
4.1
20.3
Despite its activity, the rate of reaction over Zr-Beta is rather
low.This is possibly due to the microporous zeolitic structure
which limits access of bulky substrates to the active sites within
the channels. The rate was increased when the Zr-MSU-3
material, SiZr-20-1D, was used as a catalyst. Despite a smaller
zirconium content (Si/Zr = 20), the initial TOF (based on 4’-
methoxy propiophenone converted after 3 h per Zr site) was
[a] after 3 h
[b] after 34 h for SiZr-10-1D and SiZr-10-1D-F and 45 h for others.
some anethole underwent further transfer hydrogenation at the
C=C bond to form 1-methoxy-4-propylbenzene.
-
1
-1
0
.14 h as compared to 0.11 h for Zr-Beta (Table 2). After 45 h,
As the duration of hydrothermal synthesis led to larger pore
diameters as well as a wider distribution of pore sizes (Fig. 4),
these effects on the catalytic activity were examined (Fig. S9).
However, the most active catalyst was SiZr-10-1D, which had the
smallest average pore size (3.6 nm) as well as the narrowest
spread of pore size distribution of the samples. The initial rate of
reaction (within 3 h) and anethole yield after 30 h were highest
with this sample, suggesting an optimum pore size for the reaction.
The results show that the combination of Lewis and Brønsted
acid sites on Zr-Beta and Zr-MSU-3 effectively catalysed the
a higher conversion of 98.5 % was obtained (Fig. 7b). The
selectivity to anethole remained high, 89.1 %, of which 93 % was
trans-anethole. With SiZr-10-1D where zirconium content was
closer to that of Zr-Beta, the reaction time was significantly
shortened, so that close to 99 % conversion was obtained after
34 h (Fig. 8a). The yield of anethole increased to 89.7 % (92:8
trans:cis). The ether side product 2 was slightly higher, 4.3 %,
than for Zr-Beta which is most probably caused by the larger pore
size. In addition,
cascade
MPV
reduction
and
dehydration
of
4’-
4
+
methoxypropiophenone. The tetrahedrally-sited framework Zr
serves as the Lewis acid site for coordination of the carbonyl
compound and the reducing alcohol for hydride transfer.[ The
presence of Brønsted acid sites with moderate strength facilitates
the dehydration of 1-(4-methoxyphenyl)propan-1-ol to anethole,
without competing acid-catalyzed side products. Although
dehydration can be catalysed by Lewis acid sites, studies have
shown that Brønsted acid sites are more active for alcohol
dehydration.[ In mixed oxides, protons associated with hydroxyl
groups bridging Si and Zr form Brønsted acid sites which can be
detected by pyridine IR spectroscopy. In addition, Brønsted acid
sites can be created during the reaction as shown by Kung’s
group in a recent study on supported Sn complexes.[ The
binding of an alcohol to the Sn center forms an alkoxide and the
released hydrogen is transferred to the neighbouring Sn-O-Si
bond to form a Brønsted acid site.
18]
32]
Fig. 7. Cascade MPV reduction and dehydration over (a) Zr-Beta-12.5 and (b)
SiZr-20-1D. Reaction conditions: 1 mmol 4’-methoxypropiophenone, 10 mL 2-
pentanol, 200 mg catalyst, 120 °C. () 4’-methoxypropiophenone () trans-
anethole (▲) cis-anethole () ether and (■) 1-methoxy-4-propylbenzene.
33]
O
OH
+
O
Effect of zircon
4
'-methoxypropiophenone
Attempts to synthesize Zr-MSU-3 with even higher zirconium
loading by this method were unsuccessful due to loss of ordered
mesoporosity. Therefore, samples were synthesized with NaF as
condensation catalyst. Although the materials had a bimodal pore
distribution with bigger pores, the rate of reaction was decreased
for both SiZr-10-1D-F and SiZr-20-1D-F and lower yields of
anetholes were obtained as compared to the catalysts
synthesized without NaF (Figs. 8b and S10). From the kinetic
profiles, the concentration of the ether side product 2 built up to
~25 % before decreasing slowly with time. This clearly shows that
the fluoride-synthesized catalysts were less efficient in the
hydrolysis of the ether. From ammonia TPD, it was found that
these samples have lower acid concentration which could be
[
Zr4+
]
MPV
OH
OH
O
H2O
H2O
OH
O
O
1
-(4-methoxyphenyl)propan-1-ol (1)
1-methoxy-4-(1-(pentan-2-yloxy)
propyl)benzene (2)
H+
-H2O
[
]
OH
O
+
O
O
O
trans-anethole (3a)
cis-anethole (3b)
1-methoxy-4-propylbenzene (4)
Scheme 1. Cascade reactions of p-methoxypropiophenone to anethole and
side products.
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Table 3. Influence of solvent types on conversion of 4’-methoxypropiophenone
[a]
[b]
in open and closed systems.
Solvent
T
Conv.
(%)
Selectivity (%)
(
°C)
Anethole
1
2
4
3
2
2
2
2
2
-propanol[a]
-propanol[b]
-butanol[a]
-butanol[b]
-pentanol[a]
82
11.0
51.2
29.1
45.4
90.6
30.8
45.1
78.3
45.9
86.8
90.1
90.4
52.9
17.3
0
0
0
2.0
4.4
2.2
4.1
3.9
0
120
100
120
120
120
38.5
9.1
6.0
9.6
0
0
Fig. 8. MPV reduction and dehydration over (a) SiZr-10-1D and (b) SiZr-10-1D-
F. Reaction conditions: 1 mmol 4’-methoxypropiophenone, 10 mL 2-pentanol,
2-pentanol[b]
0
2
00 mg catalyst, 120 °C. () 4’-methoxypropiophenone () trans-anethole (▲)
cis-anethole () ether and (■) 1-methoxy-4-propylbenzene.
Reaction condition: 1 mmol 4’-methoxypropiophenone, 200 mg SiZr-10-1D;
solvent (10 mL), after 30 h of reaction.
attributed to the presence of crystalline zircon (Fig. 2b). The poor
acidity of zircon has been reported previously by Melero et al.[34]
dimerization of anethole occurred. Such products have been
reported to form on acidic HY zeolites or under photoirradiation.
[35]
The dimerization of anethole in the presence of an aprotic solvent
suggests a built-up of the molecule at the catalyst surface. Hence,
for the designed cascade reactions to occur, it is best to use the
alcohol as the reductant and solvent. Apparently, the high
concentration of the alcohol helps in desorbing the anethole once
it is formed, enabling a high selectivity to the desired product.
Effect of hydrogen donor
In the MPV reaction, the solvent also serves as the hydrogen
donor. Hence, different solvents were investigated. Normally,
isopropanol is the preferred reducing agent as the evaporation of
the acetone formed in the reaction shifts the equilibrium towards
product formation. However, the conversion was only 11.0 % after
3
0 h with 45 % selectivity to anethole (Table 3). Instead a
Optimization of reaction
substantial amount of 1-(4-methoxyphenyl)propan-1-ol was
present, indicating that its dehydration to anethole was hindered
The effect of substrate to catalyst ratio (S/C) was investigated
using SiZr-10-1D. With a S/C ratio of 1.64, the conversion
reached 84.7 % after 45 h. However, only about 50 mole % of the
products formed was anethole. Of the side products, a significant
fraction was ether 2 as its concentration reached a maximum of
34.4 mole % at 30 h before decreasing slightly to 33.2 mole % at
45 h (Fig. 9a). Lowering the S/C ratio increased the reaction rate
and the ether concentration decreased. At a S/C ratio of 0.41, the
ether concentration was < 1.3 % at 100 % conversion (Fig. 9b).
However, the higher catalyst loading resulted in further
hydrogenation of anethole to 1-methoxy-4-propylbenzene. Its
concentration increased from < 1.5 to 36 mole % when the S/C
ratio decreased from 0.66 to 0.16. While the enhancement in
reaction rate can be attributed to an increase in the density of
Lewis acid active sites, its effect on selectivity of the products is
interesting. At the lowest catalyst loading, the high ether
concentration indicates that there are insufficient acidic sites for
(Fig. S11a). In an open system, the use of isopropanol as the
hydrogen donor restricts the reaction temperature to 82 °C. As
dehydration is an endothermic process, this temperature is
probably too low for dehydration to occur as well as for removal
of any water formed. To increase the reaction temperature, linear
secondary alcohols with higher boiling points such as 2-butanol
and 2-pentanol were used. With 2-butanol, the conversion of 4’-
methoxypropiophenone was slightly higher, 29.1 % (Fig. S11b). It
is notable that in 2-butanol
at 100 °C, 1-(4-
methoxyphenyl)propan-1-ol was not observed showing that its
dehydration was no longer limited. The use of 2-pentanol at
1
20 °C resulted in a significant rate increase and the conversion
was 90.6 % after 30 h. Due to the higher boiling point of 2-
pentanol, energy for the reaction as well as removal of water
drove the reaction forward. For comparison, the reactions were
also conducted in an autoclave. At 120 C, the conversions were
higher for 2-propanol and 2-butanol than under reflux. However,
for 2-pentanol, only 30.8 % conversion was obtained as the 2-
pentanone and water formed remained in the closed system.
The effect of co-solvent was also investigated, keeping the
ratio of 4’-methoxypropiophenone:2-pentanol between 1:1 to 1:10.
However, it was difficult to find a suitable co-solvent. With anisole
(4-methoxybenzene), the MPV product, 1-(4-methoxyphenyl)
propan-1-ol (1), unexpectedly underwent C-C coupling and
formed 1,1-bis(methoxylphenyl) propanes (Table S1). Increasing
the alcohol to substrate ratio from 1:1 to 10:1 resulted in higher
conversion, showing that excess alcohol did not inhibit the MPV
reduction (Fig. S12). In polar aprotic DMF, the conversion was <
Fig. 9. Cascade MPV reduction and dehydration using S/C ratio of (a) 1.64 and
(
b) 0.41 in 10 mL 2-pentanol at 120 °C. () 4’-methoxypropiophenone; ()
trans-anethole (▲) cis-anethole () ethers and (■) 1-methoxy-4-propylbenzene.
Catalyst: SiZr-10-1D.
3
% even after 45 h. With dichlorobenzene, o- and p-xylene,
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ChemSusChem
10.1002/cssc.201801340
FULL PAPER
the hydrolysis. In contrast, at high catalyst loading, there are
enough acid sites so that transfer hydrogenation at the C=C bond
of anethole also occurred. Thus, an optimal S/C ratio is needed
to ensure high selectivity to anethole.
The substrate concentration was next investigated. The
reaction rate dropped drastically when the substrate
concentration was decreased from 0.10 to 0.07 or 0.05 mol/L (Fig.
The Si/Zr mole ratio of the sample recovered after three runs
remained at 10.4, indicative that no leaching of Zr had occurred
during reaction. Therefore, the mesoporous SiZr-10-1D is a useful
recyclable heterogenous catalyst for the one-pot cascade
transformation of 4’-methoxypropiophenone to anethole.
1
0). After 45 h, the conversion was only ~66 % due to the more Conclusions
dilute reaction conditions. A substantial proportion, 18 – 37
mole %, of the products comprised of the ether (2) rather than the
desired anethole (Fig. S13). These results lend support to the
hypothesis that etherification of the MPV product, 1-(4-
methyoxyphenyl)propan-1-ol, competes with its dehydration to
anethole. Although the etherification reaction is reversible,
A direct one-pot synthesis of anethole was successfully carried
out by MPV reduction of 4’-methoxypropiophenone to 1-(4-
methyoxyphenyl)propan-1-ol followed by its dehydration. The
cascade reactions were efficiently catalyzed by mesoporous Zr-
containing MSU-3 with a high density of zirconium active sites.
Hydrothermal treatment of the synthesis gel at 100 C for one day
resulted in quantitative incorporation of zirconium into the silica,
forming Zr-MSU-3 with Si/Zr of 10. The material possessed a
disordered wormhole structure which allowed easier access to
active sites within the pore channels. In comparison, the cascade
reactions were slower when a microporous Zr-Beta zeolite was
used as catalyst. Compared to lower alcohols, the use of 2-
pentanol as solvent and hydrogen donor in the MPV reaction
enabled a high reaction rate and also facilitated the dehydration
of 1-(4-methyoxyphenyl)propan-1-ol. A high anethole yield of
excessive
amounts
of
2-pentanol
(2-pentanol:4’-
methoxypropiophenone > 140: 1) shifted the equilibrium to the
ether formation. At 0.20 mol/L substrate concentration, a higher
reaction rate and a higher selectivity to anethole were achieved.
After 23 h, 100 % conversion was reached with 91.2 % anethole
yield.
~
90 % was obtained over SiZr-10-1D. The catalyst could be
reused with no observable loss of activity after three cycles.
Experimental Section
Synthesis of Zr-beta and Zr-MSU-3
The two-step post-synthesis method for Zr-Beta zeolite was carried as
follows.[18a] Commercial Al-Beta zeolite (Si/Al 12.5, Zeolyst CP814E) was
first calcined at 500 C for 6 h before dealumination in 12 M nitric acid at
Fig. 10. Anethole yield versus 4’-methoxypropiophenone concentration: () 0.20
-
1
(
--) 0.10 (--) 0.07 and () 0.05 mol/L at 120 C. Catalyst: SiZr-10-1D, 200 mg,
80 C for 20 h (10 mL g ). The dealuminated zeolite was recovered by
S/C 0.82.
filtration, washed with deionized water and dried overnight at 80 °C. It was
3 4
suspended in water and a calculated amount of Zr(NO ) to give Si/Zr 12.5
was added. The slurry was heated under stirring to remove the water, dried
overnight at 80 C in an oven and calcined at 600 C for 8 h.
Recyclability of Zr-MSU-3
After the reaction, SiZr-10-1D was recovered by centrifugation,
washed with ethanol and dried in an oven at 80 C. It was
recalcined at 500 °C for 5 h to remove any remaining organic
residues. The conversion for three successive runs remained high
with no significant change in the selectivity to anethole (Fig. 11).
Zr-MSU-3 with Si/Zr molar ratio of 10 and 20 were synthesized by a one
pot hydrothermal method. In a typical synthesis, solution A was prepared
by dissolving 1 g of Pluronic® P-123 (Sigma Aldrich) in a 100 mL beaker
containing 50 mL deionised water at 35 °C, followed by the addition of
zirconyl oxychloride octahydrate, ZrOCl
containing 5 g of tetraethyl orthosilicate (TEOS) (Sigma Aldrich), 0.5 mL of
M HCl and 5 mL deionized water was stirred at room temperature till the
2 2
·8H O (Merck). Solution B
2
TEOS was fully hydrolysed. Subsequently, solution B was added to
solution A, stirred for 22 h and placed in a Teflon-lined stainless autoclave.
The mixture was hydrothermally treated at 100 °C for various duration (0
–
4 days). The resulting precipitate was recovered by centrifugation,
washed with deionized water and dried overnight in an oven at 90 °C. The
sample was calcined in a muffle furnace at 500 °C for 5 h. The samples
were denoted as SiZr-X-YD, where X and YD represent the Si/Zr molar
ratio and hydrothermal duration (days), respectively. To investigate the
effects of sodium fluoride as a condensation catalyst, 0.02 g of NaF was
added to the solution A and the final calcined samples were denoted as
SiZr-X-YD-F.
Fig. 11. (a) XRD of fresh and recycled SiZr-10-1D and (b) conversion () and
selectivity to anethole () using recycled catalyst.
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ChemSusChem
10.1002/cssc.201801340
FULL PAPER
Characterization
2-pentanol, other secondary alcohols such as 2-propanol and 2-butanol
were tested as well. For comparison, the reaction was also carried out in
a 25 ml Parr autoclave at 120 C. Other parameters investigated include
X-ray diffractograms were measured using
a Bruker D8 x-ray
(
i) 4’-methoxypropiophenone to catalyst ratio (S/C, g/g) from 0.41 to 1.64
in 10 mL 2-pentanol and (ii) 4’-methoxypropiophenone concentration from
.05 to 0.2 mol/L, using SiZr-10-1D as catalyst. Aliquots were removed at
diffractometer. A step size of 0.005° and a dwell time of 0.02 s was used
in the low-angle range, 2 ~ 0.5 to 4°, with the aid of a knife edge accessory
to cut off the direct beam. High angle measurements for 2 ~ 10 to 80°
were taken using a step size of 0.02° and a dwell time of 0.02 s. The
surface area and porosity were determined by nitrogen
adsorption/desorption (Micromeritics Tristar 3000). Pore size distributions
were calculated using the Barrett-Joyner-Halenda (BJH) model. Prior to
the measurement, samples were thoroughly degassed at 300 °C under
nitrogen flow for 5 h. 29Si MAS NMR spectra were measured on a Bruker
Avance Neo 400 WB spectrometer with a 4 mm double resonance MAS
0
different intervals and analyzed with an Agilent HP 6890 gas
chromatograph equipped with a HP-5 capillary column and a flame
ionization detector. The reaction products were identified by gas
chromatography-mass spectrometry (GC-MS).
Acknowledgements
probe operating at
a resonance frequency of 79.49 MHz. The
measurement parameters are: spinning rate of 12 kHz, 20 ms acquisition
time, 593 ppm sweep width, 3 s pulse length, 100 kHz 1H decoupling and
a recycle delay of 20 s. The chemical shifts were referenced to DSS (4,4-
dimethyl-4-silapentane-1-sulfonic acid) as the secondary standard at -10.2
ppm. The Si/Zr molar ratio was determined by inductively coupled plasma
atomic emission spectroscopy (ICP-AES). The samples were dissolved
We are very grateful to Dr. Gerhard Althoff-Ospelt of Bruker
BioSpin GmbH, Germany for his help in the NMR measurements.
H. Zhang thanks NUS for a research scholarship. This work is
supported by FOS ARC Tier 1 grants R-143-000-667-114 and R-
1
43-000-603-112.
3 2
with HNO /HF/H O (1 mL/0.5 mL/1 mL) using microwave digestion.
Temperature programmed desorption (TPD) of ammonia was used to
evaluate the strength and density of acid sites of the samples.
Approximately 0.15 g of sample was placed in a quartz reactor and pre-
treated under a flow of helium at 500 °C for 2 h. The sample was cooled
Keywords: Sustainable chemistry • Domino reactions •
Anethole • Lewis acid • Zirconium •
3
to 100 °C before dosing with NH for 15 minutes. After flushing with helium
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ChemSusChem
10.1002/cssc.201801340
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Zirconium-incorporated MSU-3 is an
active dual functional catalyst with
accessible 3-dimensional wormhole
pore structure and Lewis acidic sites.
Through a cascade of MPV transfer
hydrogenation and dehydration, 4’-
methoxypropiophenone is efficiently
transformed to anethole with yields
up to 90 %.
Hongwei Zhang, Candy Li-Fen Lim,
Muhammad Zaki, Stephan Jaenicke
and Gaik Khuan Chuah(s)*
Page No. – Page No.
A dual-functional Zr-MSU-3 catalyst
for cascade MPV reduction and
dehydration of 4’-
methoxypropiophenone to anethole
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