A mineralogically-inspired silver–bismuth hybrid material: Structure, stability and application for catalytic benzyl alcohol dehydrogenations under continuous flow conditions

Article abstract of DOI:10.1016/j.mcat.2020.111263

In the present contribution, we are reporting our findings on the structure, stability and synthetic applicability of a silver-containing hybrid material, which has recently been introduced by our research groups as a mineralogically-inspired novel heterogeneous catalyst. To determine how silver ions can be fixed into the structure of the catalyst, a set of experiments was designed with modification of the interlayer gallery under hydrothermal conditions. Subsequently, the stability of the material was examined in various solvents under demanding continuous flow conditions with the aim of achieving a clear picture of its applicability in organic syntheses. On the basis of the useful data obtained during the stability tests, a continuous flow methodology was developed for catalytic dehydrogenation of diversely substituted benzylic alcohols. As far as selectivity is concerned the catalyst performed superbly, while the conversions were varied from fair to extremely good.

Full text of DOI:10.1016/j.mcat.2020.111263

Molecular Catalysis 498 (2020) 111263
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
A mineralogically-inspired silver–bismuth hybrid material: Structure,
stability and application for catalytic benzyl alcohol dehydrogenations
under continuous flow conditions
a
b
e
,
c
,
d
g
,
e
d,e
¨
´
´ ´
´
¨
´
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¨ ¨
´
Rebeka Meszaros , Sandor B. Otvos
*, Gabor Varga , Eva Boszormenyi
,
,
e
f
,
f
d,
´
Marianna Kocsisd , Krisztina Karadi , Zoltan Konya , Akos Kukovecz , Istvan Palinko e,**,
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´
a,b,
¨
Ferenc Fülop
*
a
¨
¨
Institute of Pharmaceutical Chemistry, University of Szeged, Eotvos u. 6, Szeged, H-6720 Hungary
b
¨
¨
MTA-SZTE Stereochemistry Research Group, Hungarian Academy of Sciences, Eotvos u. 6, Szeged, H-6720 Hungary
c Institute of Chemistry, University of Graz, NAWI Graz, Heinrichstrasse 28, Graz, A-8010 Austria
d
´
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Department of Organic Chemistry, University of Szeged, Dom ter 8, Szeged, H-6720 Hungary
e
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Material and Solution Structure Research Group and Interdisciplinary Excellence Centre, Institute of Chemistry, University of Szeged, Aradi Vertanúk tere 1, Szeged, H-
6720 Hungary
f
´
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Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela ter 1, Szeged, H-6720 Hungary
g
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MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, Rerrich Bela ter 1, Szeged, H-6720 Hungary
A R T I C L E I N F O
A B S T R A C T
Keywords:
In the present contribution, we are reporting our findings on the structure, stability and synthetic applicability of
a silver-containing hybrid material, which has recently been introduced by our research groups as a
mineralogically-inspired novel heterogeneous catalyst. To determine how silver ions can be fixed into the
structure of the catalyst, a set of experiments was designed with modification of the interlayer gallery under
hydrothermal conditions. Subsequently, the stability of the material was examined in various solvents under
demanding continuous flow conditions with the aim of achieving a clear picture of its applicability in organic
syntheses. On the basis of the useful data obtained during the stability tests, a continuous flow methodology was
developed for catalytic dehydrogenation of diversely substituted benzylic alcohols. As far as selectivity is con-
cerned the catalyst performed superbly, while the conversions were varied from fair to extremely good.
Benzylic alcohols
Continuous flow dehydrogenation
Silver-containing hybrid catalyst
Structural characterization
1. Introduction
forces, which often involves inadequate catalyst stability, particularly
under demanding reaction conditions [17–20]. Heterogeneous catalysts
In recent years, silver has gained significant importance as catalyst in
organic reactions [1–8]. Silver catalysts are environmentally benign and
more economical than other commonly used transition metal catalysts,
such as gold, palladium and platinum. Thus, silver-catalysed trans-
formations are gaining importance not only in academic research but
also in industry. In most cases, silver complexes [9–12] and commer-
cially available silver salts [13–16] are employed as homogeneous cat-
alytic sources. Since the robustness and reusability of such catalysts are
extremely beneficial as concerns process simplicity and sustainability,
heterogeneous silver catalysts are also known; however, in most cases
active silver species, such as nanoparticles, are immobilized via weak
containing silver play key role not only in organic syntheses but also in
environmental applications, such as catalytic elimination of pollutants
[21]. Numerous quick and simple methods can be found in the literature
for the heterogenization of the transition metal ions as well as nano-
particles [22–28]. Some of them proved to be useful tools to carry out
complex organic oxidations or multicomponent reactions [29–33].
Nevertheless, silver-containing heterogeneous catalysts are still less
represented than their soluble counterparts [34–36].
Continuous flow reaction technology has become an important tool
for the synthetic and medicinal chemistry fields [37–42], and has been
extensively investigated to improve the synthesis of active
¨
¨
* Corresponding authors at: MTA-SZTE Stereochemistry Research Group, Hungarian Academy of Sciences, Eotvos u. 6, Szeged, H-6720 Hungary.
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** Corresponding author at: Department of Organic Chemistry, University of Szeged, Dom ter 8, Szeged, H-6720 Hungary.
¨
¨
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E-mail addresses: sandor.oetvoes@uni-graz.at (S.B. Otvos), palinko@chem.u-szeged.hu (I. Palinko), fulop@pharm.u-szeged.hu (F. Fülop).
https://doi.org/10.1016/j.mcat.2020.111263
Received 17 August 2020; Received in revised form 23 September 2020; Accepted 12 October 2020
Available online 1 November 2020
2468-8231/© 2020 The Author(s).
Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

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R. Meszaros et al.
Molecular Catalysis 498 (2020) 111263
pharmaceutical ingredients [43–49], nanomaterials [50–52] or other
complex substances [53–55]. Transition metal-catalysed reactions
under flow chemistry conditions have been shown to have many ad-
vantages over traditional batch methods [56–61]. However, such re-
actions frequently require harsh conditions, such as high temperature
and high pressure, and the continuous flow of the reaction medium exert
a pronounced mechanical stress for solid materials, which together often
contribute to limited applicability and stability of heterogeneous tran-
sition metal catalysts in flow reactors [62].
nitric acid. After mixing, urea (7.05 g) dissolved in 100 mL of deionized
water was added to the solution and stirred for 72 h at 130 ◦C. As an
alternative way of the synthesis, after addition of the urea solution to the
mixture of the required salts, the reaction mixture was placed into an
oven for 24 h at 105 ◦C. The obtained material was next filtrated,
washed with aqueous thiosulfate solution, water and ethanol four times,
and dried at 60 ◦C to obtain the final product.
AgBi-HM was fully characterized by means of diverse instrumental
techniques as detailed earlier [59]. In the present study, the as-prepared
and the treated samples of the material were checked by X-ray diffrac-
tometry (XRD), Raman and IR spectroscopies as well as SEM-EDX
measurements. Powder XRD patterns were registered in the 2θ =
It is well-known that different solvents may exert significant effects
on the performance of homogeneous as well as heterogeneous catalysts
[63]. Therefore, in the course of a catalytic reaction, it is crucial to
choose the appropriate solvent [64,65], and this is especially true for
flow conditions to ensure reaction homogeneity and to avoid clogging in
reactor channels. One of the most important limitations of heteroge-
neous materials being employed as catalysts in flow systems is their
incompatibility with certain solvents [66]. In these cases,
temperature-dependent solvent interactions result in irreversible struc-
tural changes, which significantly reduce catalyst robustness and per-
formance. For example, amide type solvents delaminate the layers of
layered double hydroxides, which leads to the collapse of catalyst
structure and leaching under flow conditions [67].
4◦–60◦ range on a Rigaku Miniflex II instrument using Cu K
α (λ = 1.5418
Å) radiation. FT-IR spectra were measured on a BIO-RAD Digilab Divi-
sion FTS-65A/896 spectrophotometer with 4 cmꢀ 1 resolution. 256 scans
were collected for each spectrum. The spectra of each sample were
recorded with diffuse reflection technique by fixing the incident angle in
45◦ position. Raman spectra were measured with a Thermo Scientific™
DXR™ Raman microscope at an excitation wavelength of 635 nm
applying 10 mW laser power and averaging 20 spectra with an exposure
time of 6 s. The actual silver-bismuth molar ratios in the samples before
and after treatment were determined by performing SEM-EDX mea-
surements with an S-4700 scanning electron microscope (SEM, Hitachi,
Recently, we reported on a silver–bismuth hybrid, beyerite-like [68]
material (AgBi-HM) with structurally-bound silver catalytic centres, and
¨
Japan) with accelerating voltage of 10–18 kV coupled with a Rontec
–
–
successfully employed it for heterogeneous catalytic C C bond activa-
QX2 energy dispersive microanalytical system.
–
tion to yield organic nitriles directly from terminal alkynes under batch
reaction conditions [69]. Although the as-prepared material was
completely characterized, the actual position of silver ions in the cata-
lyst lattice remained unresolved. In the present contribution, we report
our new findings on the structure of the AgBi-HM, and also on the sta-
bility of the material in different solvents under a variety of continuous
flow conditions. Finally, aided by the data acquired in elaborate stability
tests, we aimed for a simple continuous flow methodology for catalytic
dehydrogenation of benzylic alcohols to the corresponding aldehydes as
valuable substances; the results are presented herein.
2.3. Anion-exchange experiments under hydrothermal conditions
For hydrothermal treatment of the AgBi-HM, highly concentrated
iodide (cNaI = 4 M), chloride (cNaCl = 4 M) and carbonate (cNa2CO3 = 3 M)
aqueous solutions were prepared and used. The direct anion-exchange
tests were carried out in a Teflon-lined stainless steel autoclave at 120
◦C for 2 days. The obtained slurries were filtered, washed with water
several times and dried at 80 ◦C overnight. Treated samples were
characterized by XRD, Raman and IR spectroscopies, as well as SEM-
EDX measurements.
2. Experimental
2.4. Procedure for investigating the solvent compatibility of AgBi-HM
under flow conditions
2.1. General information
All fine chemicals, materials and reagents used were commercially
available, and were applied as received without further purification. The
benzyl alcohol substrates (with purity of ≥98 %) and the solvents used
for catalyst stability experiments and also for dehydrogenation reactions
were purchased from Sigma-Aldrich (Merck) and VWR. Analytical thin-
layer chromatography was performed on Merck silica gel 60 F254 plates
and flash column chromatography on Merck silica gel 60. Compounds
were visualized by means of UV or KMnO4. NMR spectra were recorded
on a Bruker Avance DRX 500 spectrometer, in CDCl3 as solvent, with
TMS as internal standard, at 500.1 and 125 MHz, respectively. GC–MS
analyses were performed on a Thermo Scientific Trace 1310 Gas Chro-
matograph coupled with a Thermo Scientific ISQ QD Single Quadrupole
Mass Spectrometer using a ThermoScientific TG-SQC column (15 m ×
To examine the stability of the as-prepared AgBi-HM, a simple
continuous flow set-up was assembled (Fig. 1). The system consisted of
an HPLC pump (JASCO PU-2085), a stainless steel cartridge with in-
ternal dimensions of 30 × 2.1 mm and a 10-bar backpressure regulator
(BPR; IDEX) to enable overheating of the solvents. The column was
charged with approximately 50 mg of the AgBi-HM, and was sealed with
compatible frits (0.5 μm pore size). Parts of the system were connected
with stainless steel capillary tubing (internal diameter 250 μm). The
catalyst bed was immersed into an oil bath for heating purposes. In each
0.25 mm ID × 0.25
μ film). Measurement parameters were as follows.
◦
◦
Column oven temperature: from 50 to 300 C at 15 C/min, injection
temperature: 240 ◦C, ion source temperature: 200 ◦C, electrospray
ionization: 70 eV, carrier gas: He at 1.5 mL min–1, injection volume: 2
μ
L, split ratio: 1:33.3, mass range: 50–500 m/z.
2.2. Synthesis and characterization of the AgBi-HM
AgBi-HM was prepared by using the urea hydrolysis method
following our previously reported procedure [70]. In this way, the pH
could precisely be controlled through the temperature of the
co-hydrolysis. In a typical synthesis, the adequate amounts of AgNO3
(3.73 g) and Bi(NO3)3⋅5H2O (5.36 g) were dissolved in 50–50 mL 5 wt%
Fig. 1. Experimental setup for the continuous flow experiments.
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R. Meszaros et al.
Molecular Catalysis 498 (2020) 111263
test, the selected solvent was continuously pumped through the column
for 90 min under the appropriate conditions. The treated hybrid mate-
rial sample was then removed from the cartridge, and was examined by
XRD and IR spectroscopy.
2.5. Procedure for AgBi-HM-catalysed alcohol dehydrogenations under
flow conditions
The silver-catalysed dehydrogenation reactions were carried out by
using the same flow system as the one used for the solvent compatibility
tests (Fig. 1). As catalyst bed, a stainless steel cartridge with internal
dimensions of 100 × 4.6 mm was used, which encompassed approxi-
mately 2 g of AgBi-HM. For each reaction, the corresponding benzyl
alcohol (c = 0.075 M) was dissolved in toluene, and the solution was
pumped continuously under the selected conditions. In each run, 4 mL of
product solution was collected. Between two experiments, the system
was washed for 20 min by pumping toluene at a flow rate of 0.5 mL
minꢀ 1. The crude products were checked by NMR spectroscopy, to
determine the conversions and the selectivities. If necessary, column
chromatographic purification was carried out with mixture of hexane
and EtOAc as eluent. The reaction products were characterized by NMR
and MS techniques. The characterization data can be found in the Sup-
porting Information (SI) file.
Fig. 2. XRD patterns of AgBi-HM samples: as-prepared material (A), material
treated with iodide- (B), chloride- or (C) carbonate-containing solutions (D).
3. Results and discussion
Additionally, by using carbonate as guest anions, pure bismutite
(PDF#41-1488) was obtained. It could be assigned to orthorhombic
crystal structure similarly to the AgBi-HM. However, because of the
theoretical reasons, more expanded interlayer gallery was detected for
bismuth subcarbonate without silver ions, which was indicated by the
shift of the [002] Bragg reflection into lower 2θ values.
3.1. Indirect proof of the structure of the AgBi-HM
From our earlier investigations [70], it was clear that the AgBi-HM
contained Ag(I) and Bi(III) cationic and carbonate anionic components
with silver ion as the minor cationic component. The material exhibited
layered structure resembling that of a mineral called beyerite (CaB-
i2O2(CO3)2) [69], which contains Ca(II) ions fixed between BiO(CO)3
layers. In the hybrid material, Bi(III) ions are connected via one of the
oxygen atoms of the carbonate ions and they are constituents of the
layers. At that point, we speculated that Ag(I) ions as minor cationic
component are residing among the Bi(III)-containing layers, and are
strongly coordinated to the carbonate ions. As concerns the catalytic
applicability of the hybrid material, this structural information is of
crucial importance. For example, it is essential to know how leaching of
the active component can occur, and whether it takes place in a way that
the lamellar structure remains intact. On the other hand, the inserted
lamellar (Aurivillius) structure may not only be formed by intercalating
silver cations among the bismutite layers, but also via incorporating
those into the framework of the structure [71]. Accordingly, the removal
of silver content without definitive termination of the long-range order
of the lamellar structure is only possible from the interlayer-modified
bismutite.
Furthermore, neither the FT-IR nor the Raman spectra of bismuth
oxyiodide and oxychloride contained any relevant vibrational bands
related to the carbonate species of the raw material (Fig. 3) [72]. In the
halogen-containing systems, the broadened stretching vibrations at
around 1040 cm–1 may be attributed to the Bi–I/Cl band in the BiOI/Cl
structure [73]. In addition, the Raman spectra confirmed that the
treated materials had oxyiodide/oxychloride structure with the same
Raman peaks as in the literature. The characteristic Raman bands at
around 180 and 150 cm–1 could be associated with A1g internal Bi–Cl
stretching and Eg external Bi–Cl stretching modes of bismuth oxy-
chloride upon intercalation of chloride anions [74]. Moreover, two
incisive characteristic bands at 160 and 81 cm–1 of the iodine-treated
samples were assigned to A1g and Eg stretching modes of bismuth oxy-
iodide [75]. Additionally, by means of SEM-EDX measurements, the loss
of the silver cations from the treated structures could also be illustrated
(Fig. 4).
These results lead to the conclusion that by performing hydrothermal
heat treatment on the Aurivillius structures in the presence of highly
concentrated anions, partial or complete anion exchange could be ach-
ieved. At the same time, the silver content also diminished. Conse-
quently, it has been proven that silver cations are fixed among the
bismutite-like layers of the hybrid material in the form of silver-
containing anionic species.
To prove the presence or absence of exchangeable interlayer anions
of silver species, anion exchange reactions were attempted in the pres-
ence of highly concentrated ‘entry’ anions under hydrothermal condi-
tions. As-prepared AgBi-HM samples were thus treated with
◦
concentrated NaI, NaCl or Na2CO3 aqueous solutions at 120 C for 2
days, and the treated samples were compared to the as-prepared one.
As can be seen in Fig. 2, the XRD patterns of the treated samples
showed large differences from the as-prepared material. The significant
shifts in the characteristic peaks as well as the change in the intensities
of the reflections indicated that notable anion exchange took place.
Moreover, by finding perfect agreement in the JCPDS (Joint Committee
of Powder Diffraction Standards – International Centre for Diffraction
Data) database, the produced structures could be undoubtedly identified
as bismuth oxyiodide (P(owder)D(iffraction)F(ile) #73-2062) and oxy-
chloride (PDF#06-0249) with tetragonal matlockite crystal phase,
which is a layered structure. The framework consists of fluorite-like
[M2O2] layers sandwiched between double halogen layers upon
applying iodide and chloride anions, respectively, as entry anions.
3.2. Investigating the stability and solvent-compatibility of the AgBi-HM
under continuous flow conditions
To obtain valuable data on the applicability of the AgBi-HM for
organic synthesis in continuous flow mode, the effects of various sol-
vents and reaction conditions on the structure of the material were
examined. A simple flow reactor set-up was assembled as detailed in the
Experimental section. A comprehensive list of solvents was compiled for
the stability tests comprising polar, non-polar, protic and aprotic ones.
This included CH2Cl2, CHCl3, EtOAc, acetone, diethyl ether, MeOH,
EtOH, iPrOH, H2O, MeCN, tetrahydrofuran (THF), N-methyl-2-
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R. Meszaros et al.
Molecular Catalysis 498 (2020) 111263
Fig. 4. EDX spectra of the as-prepared AgBi-HM and the one treated with
carbonate-containing solution.
Fig. 3. (a) IR and (b) Raman spectra of AgBi-HM samples: as-prepared material
(A), material treated with iodide- (B) or chloride-containing solutions (C).
pyrrolidone (NMP), n-hexane, toluene, N,N-dimethylformamide (DMF),
dimethyl sulfoxide (DMSO) and N,N-dimethylacetamide (DMA). In each
test, the appropriate solvent was pumped for 90 min at 0.5 mL min–1
flow rate through a catalyst bed encompassing a sample of AgBi-HM.
The tests were carried out at 25, 50, 100, 150 and 200 ◦C as tempera-
tures typically used in catalytic flow reactions. It should be noted that
according to TG and DTG measurements, the structure of the hybrid
material is thermally stable until 380 ◦C [70].
Fig. 5. XRD patterns of AgBi-HM samples: as-prepared material (A), sample
treated with toluene at 25 ◦C (B), sample treated with H2O at 150 ◦C (C),
sample treated with iPrOH at 50 ◦C (D) and sample treated with diethyl ether at
25 ◦C (E).
The XRD patterns of the treated AgBi-HM samples verified that none
of the tested conditions caused collapse or even damage of the structure
of the as-prepared material. This was also found for DMF and DMA,
which are known as delaminating solvents for certain layered materials
[67]. For toluene, DMSO, H2O, MeCN and EtOAc, the XRD patterns of
the treated samples were analogous with that of the as-prepared mate-
rial at most of the temperatures investigated (Fig. 5, diffractograms B
and C as examples). The calculated basal spacings related to the lamellar
structures were exactly the same before and after the solvent treatment.
In certain solvents, such as acetone, CH2Cl2, diethyl ether and alcohols,
the XRD patterns were still typical of the beyerite-like hybrid material
with basal reflections in the 2θ = 9–38◦ region, but doubling of char-
acteristic reflections appeared on their diffractograms at different tem-
peratures (Fig. 5, diffractograms D and E as examples). This observation
is very similar to the well-known staging effect of lamellar structures
[76], and may be explained by partial recrystallization and the forma-
tion of a new phase with increased interlayer distances. IR spectra of
samples with doubled reflections showed the typical characteristics of
the non-treated material, the new absorption bands appeared at around
2900 cm–1 can be identified as C H stretching vibrations of organic
–
solvent traces (Fig. S18) [77]. The results of the stability tests are
summarized in Table 1, and the complete collection of diffractograms is
presented in the SI file.
3.3. Catalytic dehydrogenation of benzylic alcohols under flow conditions
Encouraged by the promising results on the stability of the AgBi-HM
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Molecular Catalysis 498 (2020) 111263
reaction further under continuous flow conditions using a simple fixed-
bed system as shown in Fig. 1. Considering that the AgBi-HM was proven
to be compatible with wide variety of solvents even at high temperatures
(Table 1), extensive solvent screening was carried out at 180 ◦C
(Table 2). The reaction outcome proved to be strongly dependent on the
solvent applied. In acetone, CH2Cl2, EtOAc and MeCN, conversions of
20–46 % were detected; however, selectivity was very low (entries 1–4).
Contrarily, in DMSO, MeOH, THF or toluene, selective aldehyde for-
mation was observed (entries 5–8). The highest conversion was achieved
in toluene (92 %, entry 8), which was therefore chosen as solvent for
further parameter optimization.
Table 1
Summary of the results of the AgBi-HM stability tests under flow conditions. (A:
XRD patterns analogous with that of the as-prepared material, B: doubled
characteristic reflections.).
Entry
Solvent
25 ◦
C
50 ◦
C
100 ◦
C
150 ◦
C
200 ◦
C
1
acetone
DMA*
toluene
THF*
A
A
A
A
A
B
A
B
B
B
B
B
B
A
B
B
A
B
A
A
A
A
B
B
B
B
B
B
B
A
A
B
B
A
B
A
A
B
A
A
B
B
B
A
B
B
A
A
A
A
A
B
B
A
B
A
A
B
B
B
B
B
B
A
A
A
B
A
B
B
A
B
A
A
A
B
B
A
B
A
A
A
A
A
A
2
3
4
5
DMSO*
CHCl3
iPrOH
CH2Cl2
MeOH
NMP*
6
7
The reaction temperature was found to have significant role on the
8
◦
9
conversion and the selectivity (Fig. 6a). For example, at 100 C, con-
10
11
12
13
14
15
16
17
version of merely 13 % occurred, which was improved remarkably upon
gradual increase of the temperature. Gratifyingly, at 180 ◦C, 92 %
conversion and 100 % selectivity were observed. However, further in-
crease to 200 ◦C triggered the benzylation of toluene with the substrate
as competing side reaction, and thus provoked a momentous decrease in
selectivity to 62 % (besides a marginal increase in conversion). Such
benzylations are known from the literature, and can be accessed in the
presence of various metal catalysts typically at higher temperatures [92,
93]. The residence time also had significant effects on the reaction
diethyl ether
EtOH
H2O
MeCN
hexane
DMF*
EtOAc
*
tetrahydrofuran
(THF),
(DMF),
dimethylacetamide (DMA).
N-methyl-2-pyrrolidone
dimethyl sulfoxide
(NMP),
N,N-
N,N-
dimethylformamide
(DMSO),
outcome. Upon decreasing the flow rate from 150
μ
L min–1, the con-
version gradually improved from 64 % until it finally reached 100 % at
L min–1 (Fig. 6b), which corresponded to a residence time of
under demanding conditions, we next turned our attention to potential
synthetic applications in continuous flow mode. As a consequence of the
ubiquity of carbonyl compounds, catalytic oxidation/dehydrogenation
of alcohols are of fundamental importance in synthetic organic chem-
istry and, accordingly, in the fine chemical and pharmaceutical in-
dustries [78–82]. Oxidant-free dehydrogenation processes are cleaner,
more atom-efficient, and due to the lack of over-oxidation, generally
more selective than catalytic oxidations [83–85]. However, such re-
actions typically require harsh conditions, and hence pose a significant
synthetic challenge. In fact, besides well-established industrial processes
for gas-phase alcohol dehydrogenations [86,87], liquid-phase catalytic
methodologies are quite limited, and often suffer from significant
drawbacks, such as low activity, limited substrate scope and the neces-
sity of various additives [88–91]. Inspired by these factors, we selected
catalytic dehydrogenation of benzylic alcohols as a benchmark to
evaluate the synthetic capability of the AgBi-HM catalyst under
demanding flow conditions.
20
μ
approximately 75 min. However, at 20
μ
L min–1 flow rate, the
competing benzylation appeared as side reaction, and reduced the
selectivity to 56 %. As optimum flow rate, 30
μ
L min–1 was selected
(corresponding to 50 min residence time), which ensured an excellent
conversion of 92 % and fully selective aldehyde formation. The effects of
substrate concentration were also examined (Table S1), and 0.075 M
was found to be the best as higher concentrations led to lower selectivity
due to side product formation.
Based on the findings of the parameter optimization, the reaction
was operated most efficiently at 30
μ
L min–1 flow rate and 180 ◦C re-
action temperature with a substrate concentration of 0.075 M. With
these conditions in hand, we finally set out to investigate the substrate
scope of the process (Table 3). Excellent conversions were found for
substituted benzyl alcohol derivatives containing methyl, methoxy,
bromo and nitro groups in the para position. The reactions also worked
well with meta-substituted benzyl alcohols, and furnished conversions
in the range of 36–82 %. The multisubstituted 3-methyl-4-nitrobenzyl
alcohol gave an acceptable conversion of 40 %. Dehydrogenation of 2-
bromobenzyl alcohol was also attempted, but due to the significant
steric hindrance generated by the ortho bromo moiety, the conversion of
the substrate was merely 18 %. It is remarkable that in all reactions, fully
selective aldehyde formation was observed, side products were not
detected.
To investigate the effects of reaction conditions on the catalytic
dehydrogenation, 4-methylbenzyl alcohol was selected as a model
substrate. Initially, the AgBi-HM catalyst was tested under batch con-
ditions and, in parallel, another reaction was performed under the same
conditions using AgNO3 as catalyst (Scheme 1). After stirring for 24 h in
refluxing toluene (110 ◦C), 62 % conversion was detected with the
hybrid material, whereas AgNO3 gave only 15 % conversion (selectivity
was 100 % in both reactions). No conversion was detected without the
presence of catalyst. The enhanced activity of the catalyst – compared to
silver nitrate salt – may be assigned to the promoter roles of the bismuth
centres and the oxide surface. A possible reaction mechanism was sug-
gested accordingly (Scheme S1). Moreover, in our opinion, the exclusive
selectivity is the contribution not only of Ag(I) centres but the shape-
selective effect of the layered structure.
Table 2
Investigating the effects of various solvents on the dehydrogenation reaction of
4-methylbenzyl alcohol under continuous flow conditions.
Entry
Solvent
Conv.a (%)
Sel.a (%)
The promising preliminary results encouraged us to explore the
1
2
3
4
5
6
7
8
acetone
CH2Cl2
EtOAc
MeCN
DMSO
MeOH
THF
20
36
46
27
8
traces
traces
25
40
100
100
100
100
4
22
92
toluene
Scheme 1. Catalytic dehydrogenation of 4-methylbenzyl alcohol under
a
batch conditions.
Determined by 1H NMR analysis of the crude product.
5

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R. Meszaros et al.
Molecular Catalysis 498 (2020) 111263
fixed among the layers of the material as silver-containing anionic
species. The stability of the material was investigated in a wide variety
of organic solvents under continuous flow conditions to aid potential
synthetic applications. It was found that none of the tested solvents
resulted in the collapse or even damage of the structure of the material,
◦
even at temperatures as high as 200 C. As proof of concept, a novel
continuous flow protocol was established for catalytic dehydrogenation
of benzylic alcohols using the hybrid material as catalyst. The effects of
reaction temperature, residence time, substrate concentration and
various solvents were explored to achieve high conversions and selective
aldehyde formation without the need for any additives. The scope and
applicability of the flow protocol was also demonstrated.
CRediT authorship contribution statement
´
´
Rebeka Meszaros: Investigation, Data curation, Writing - original
¨
´
¨
draft. Sandor B. Otvos: Conceptualization, Supervision, Writing - re-
´
´
view & editing. Gabor Varga: Investigation, Data curation. Eva
¨
¨
´
Boszormenyi: Investigation. Marianna Kocsis: Investigation. Krisz-
´
´
´
´
tina Karadi: Investigation. Zoltan Konya: Methodology. Akos Kuko-
´
´
´
vecz: Methodology. Istvan Palinko: Conceptualization, Supervision,
¨
Validation, Writing - review & editing. Ferenc Fülop: Supervision,
Resources.
Declaration of Competing Interest
Fig. 6. Investigating the effects of temperature (a) and flow rate (b) on the
AgBi-HM-catalysed dehydrogenation of 4-methylbenzyl alcohol under contin-
uous flow conditions. (Reaction conditions: c = 0.075 M, toluene as solvent, 30
The authors report no competing interest.
Acknowledgements
μ
L min–1 flow rate for the temperature study, 180 ◦C temperature for the flow
rate study.).
Financial support was obtained from the ÚNKP-19-3 New National
Excellence Program of the Ministry for Innovation and Technology
¨
(Hungary). SBO acknowledges the Premium Post Doctorate Research
Table 3
AgBi-HM-catalyzed dehydrogenation of diversely substituted benzyl alcohols
under continuous flow conditions.
Program of the Hungarian Academy of Sciences. All these supports are
highly appreciated.
Entry
Substrate
Conv.a (%)
Sel.a (%)
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.111263.
1
2
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