Highly efficient and selective one-pot tandem imine synthesis via amine-alcohol cross-coupling reaction catalysed by chromium-based MIL-101 supported Au nanoparticles

Article abstract of DOI:10.1016/j.mcat.2020.111363

One-pot tandem synthesis of imines from alcohols and amines is regarded as an effective, economic and green approach under mild conditions. In this work, Au nanoparticles (NPs) dispersed on MIL-101 (Au/MIL-101) were demonstrated as highly active and selective bifunctional heterogeneous catalyst for production of various imine derivatives with excellent yields, via amine-alcohol cross-coupling reaction at 343 K in an open flask under an Ar atmosphere. Various physicochemical techniques, including inductively coupled plasma optical emission spectroscopy (ICP-OES), powder X-ray diffraction (P-XRD), X-ray photoelectron spectroscopy (XPS) transmission electron microscopy (TEM) and N₂ adsorption-desorption, were used to characterize of the Au/MIL-101 catalyst. The obtained bifunctional catalyst is highly active and selective towards one-pot imine formation and exhibited the highest TOF (30.15-51.47 h^?1) among all the ever-reported MOF-supported Au catalysts. The reaction mechanism of the imine formation from alcohol and amine over Au/MIL-101 catalyst was proposed. Mechanism experiment results demonstrate that Au NPs highly effective in activating oxidation of benzyl alcohol to benzaldehyde while the Lewis acid sites on MIL-101 catalyzed the second condensation step without interfering with the oxidation step. As a result, the excellent catalytic performance of Au/MIL-101 can be ascribed to the synergistic effect between Au NPs with Lewis acid sites in MIL-101.

Full text of DOI:10.1016/j.mcat.2020.111363

Molecular Catalysis 501 (2021) 111363
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Highly efficient and selective one-pot tandem imine synthesis via
amine-alcohol cross-coupling reaction catalysed by chromium-based
MIL-101 supported Au nanoparticles
a,
b,
,
1
c
c
c,1,
Ilkay Gumus
* , Adem Ruzgar , Yasar Karatas , Mehmet Gülcan
*
a
Advanced Technology Applied and Research Center, Mersin University, Mersin, 33343, Turkey
Department of Basic Sciences, Faculty of Maritime, Mersin University, Mersin, 33343, Turkey
Department of Chemistry, Van Yüzüncü Yıl University, Van, 65080, Turkey
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
One-pot tandem synthesis of imines from alcohols and amines is regarded as an effective, economic and green
approach under mild conditions. In this work, Au nanoparticles (NPs) dispersed on MIL-101 (Au/MIL-101) were
demonstrated as highly active and selective bifunctional heterogeneous catalyst for production of various imine
derivatives with excellent yields, via amine-alcohol cross-coupling reaction at 343 K in an open flask under an Ar
atmosphere. Various physicochemical techniques, including inductively coupled plasma optical emission spec-
troscopy (ICP-OES), powder X-ray diffraction (P-XRD), X-ray photoelectron spectroscopy (XPS) transmission
Au NPs
Catalyst
Cross-coupling reaction
Imine synthesis
MIL-101
2
electron microscopy (TEM) and N adsorption-desorption, were used to characterize of the Au/MIL-101 catalyst.
The obtained bifunctional catalyst is highly active and selective towards one-pot imine formation and exhibited
ꢀ 1
the highest TOF (30.15-51.47 h ) among all the ever-reported MOF-supported Au catalysts. The reaction
mechanism of the imine formation from alcohol and amine over Au/MIL-101 catalyst was proposed. Mechanism
experiment results demonstrate that Au NPs highly effective in activating oxidation of benzyl alcohol to benz-
aldehyde while the Lewis acid sites on MIL-101 catalyzed the second condensation step without interfering with
the oxidation step. As a result, the excellent catalytic performance of Au/MIL-101 can be ascribed to the syn-
ergistic effect between Au NPs with Lewis acid sites in MIL-101.
1
. Introduction
Tandem reactions have recently attracted increasing attention due to
8–16]. For example, the dispersed/encapsulated Au NPs on/inside MOFs
can have oxidation or reduction activity, while the metal nodes or the
functional organic linkers of support can have the second type of active
sites [17,18]. On the other hand, compared to other porous materials
such as mesoporous silica and zeolites, the properties of MOFs are
logically “designable” to examine the effect of the chemical environment
on the dispersed/encapsulated metal NPs. Because the metal nodes and
the functional organic linkers of MOFs can be fine-tuned either by uti-
lizing different groups or by the post-synthetic modification of
sustainable green processes that show the concepts of efficiency and
atomic economy. In these reactions two or more successive individual
reactions occur realized in a one-pot system without purifying and
separating the byproducts [1–6]. In tandem reactions, main aim is to
obtain high selectivity and activity towards a single product and these
reactions often require the oxidation or reduction steps to be combined
with others such as Knoevenagel condensation [1,2,7]. To achieve this
combination, tandem reactions are supported by two catalysts or a
bifunctional catalyst having two or more types of active sites [1].
MOFs are particularly appropriate materials for tandem reactions
because they can have two or more different active centres such as metal
nodes, guest species in the pores and functional organic linkers [1,5,6,
pre-synthesized MOFs [19–22]. The introduction of various groups such
t
as -NH
2
, -SO
3
H, LiOEt, LiO Bu, AcO to isoreticular MOFs can signifi-
cantly affect activity, stability, and mechanical properties of the MOFs
[23–27].
Recently, monometallic and bimetallic NPs encapsulated/dispersed
on/inside MOFs that catalyze various tandem reactions have been
*
Corresponding authors.
E-mail addresses: gumus.ilkay84@gmail.com (I. Gumus), mehmetgulcan65@gmail.com (M. Gülcan).
1
Website: https://avesis.yyu.edu.tr/mehmetgulcan.
https://doi.org/10.1016/j.mcat.2020.111363
Received 7 September 2020; Received in revised form 26 November 2020; Accepted 16 December 2020
Available online 5 January 2021
2
468-8231/© 2020 Elsevier B.V. All rights reserved.

I. Gumus et al.
Molecular Catalysis 501 (2021) 111363
reported [28–31]. Chen et. al. reported monometallic and bimetallic
NPs@MOF (Pd@MIL-101 and PdAg@MIL-101) catalysts with tiny metal
NPs inside MOFs for the one-pot tandem synthesis of secondary aryl-
amine [32]. The results showed that MIL-101 provides Lewis acid sites
2. Experimental
2.1. Materials and reagents
(
chromium centres) and Pd offers hydrogenation activity while Ag
Gold (III) chloride trihydrate (HAuCl
4
∙3H
2
O), chromium trinitrate
), methanol
), sodium borohydride (NaBH ), benzyl
greatly improves selectivity toward the corresponding secondary amine.
Li et. al. reported bimetallic Pt–Ni frames into porous MOF-74 for the
one-pot tandem reductive imination of nitroarenes with carbonyl com-
nonahydrate (Cr(NO
3
)
3
⋅9H
3
2 8 6 4
O), terephthalic acid (C H O
(CH OH), acetone (CH
3
COCH
3
4
alcohol and amine derivatives were supplied from common commercial
sources such as Merck, TCI and Sigma-Aldrich®. Toluene dried by using
standard procedure. Water was purified by reversed osmosis with ion
exchange and used as a solvent for the preparation of reagents solution
and dilution (Human RO 180 brand water purifier). In addition, the glass
and other materials needed in all studies were used by washing with
chromic acid cleaning solution, water and acetone and drying.
2
pounds [33]. Qi et al. reported bifunctional Au/MIL-53(NH ) catalyst
for synthesis of 2-benzylidenemalononitrile and its derivatives via
one-pot alcohol oxidation/Knoevenagel condensation reactions. The
results display that Au NPs only promote the first oxidation step, and the
amino groups catalyze the second condensation step without interfering
with the first step [34].
Over the last decades, remarkable efforts have been devoted to the
direct synthesis of various imines, in particular via one-pot tandem
procedures because they can be further transformed into various bio-
logically and pharmaceutically active compounds such as β-lactams,
aziridines and benzoxazole benzimidazole [35,36]. According to the
widely accepted reaction mechanism of one-pot imines formation from
alcohols and amines, during cross-coupling reactions of alcohol-amine,
alcohol is first oxidized to corresponding aldehyde (under aerobic con-
dition) which is then reacted with unreacted amine to form the sec-
ondary imine (under aerobic condition) [31,37,38]. This process is very
challenging because if both alcohols and amines are in the same reaction
mixture, there are many possible side reactions and many byproducts
such as amides [39], acids [40], diazo compounds [41] and esters [42]
can be obtained depending on the nature of the catalyst used and the
reaction conditions selected (Scheme 1). Thereby, the design of an
effective heterogeneous catalyst to prevent the unnecessary side re-
actions requires a combination of a metallic functional moiety for
alcohol oxidation step and an acidic functional part for condensation
step.
2.2. Instrumentation
The exact Au loading was measured by the inductively coupled
plasma optical emission spectroscopy (ICP-OES) technique on an Perkin
Elmer DRC II model. The powder X-ray diffraction (P-XRD) analyses
were carried out on Rigaku Ultima-IV by using Cu-K radiation (wave-
α
length 1.54 Å, 40 kV, 55 mA). X-ray photoelectron spectroscopy (XPS)
analysis was performed under ultra-high vacuum on a Physical Elec-
tronics 5800 spectrometer equipped with a hemispherical analyzer and
using monochromatic Al- K
α
radiation (1486.6 eV, the X-ray tube
working at 15 kV and 350 W, and pass energy of 23.5 eV). The binding
energy shift due to the surface charging was adjusted using a reference
to the C 1s line at 284.5 eV. The surface morphology and microstructure
of Au/MIL-101 catalyst was determined by using conventional trans-
mission electron microscope (TEM, JEOL JEM-200CX, 120 kV), equip-
ped with energy dispersed X-ray detector (EDX) and high resolution
transmission electron microscope (HR-TEM, JEOL JEM-2010F, 200 kV).
Fourier transform infrared (FT-IR) spectra were performed on a Bio-Rad-
Win-IR spectrophotometer instrument in the range of between 4000 and
In this work, we have developed a simple and efficient method for
the selective synthesis of imines from alcohols and amines via one-pot
prosess using a new readily recoverable bifunctional catalyst that
combines the catalytic activity of Au NPs with the active centers of MIL-
ꢀ 1
3
400 cm . The NMR spectra were recorded in CDCl solvent on Bruker
Avance III 400 MHz NaNoBay FT-NMR spectrophotometer using tetra-
methylsilane as an internal standard. Gas chromatography (GC-FID)
analyses were performed were performed on Shimadzu GC-2010 Plus.
1
01. The Au/MIL-101 catalyst showed excellent activity, selectivity,
stability and substrate scope towards one-pot imine formation, offering
the highest TOFs among ever reported MOF-supported Au catalysts in
the literature. We also examined the effect of various reaction parame-
ters (such as temperature, base, oxygen) as well as Au NPs and MIL-101
solid support structure on catalytic activity.
2
The N adsorption-desorption experiment was carried out at 77 K using a
Micrometrics Surface Area and Porosity instrument (MicroActive for
TriStar II Plus 2.00).
2.3. Synthesis and purification of MIL-101(Cr) and MIL-101(Cr)
stabilized gold NPs (Au/MIL-101)
MIL-101 host matrix is prepared by hydrothermal synthesis method,
which is frequently used in the literature reports [43–45], and purified
Scheme 1. Possible pathways for the imine formation from alcohol and amine.
2

I. Gumus et al.
Molecular Catalysis 501 (2021) 111363
from impurities. Au NPs stabilized with MIL-101 support (Au/MIL-101)
were synthesized with a simple impregnation-reduction approach
[
45–47]. Detailed synthesis protocols for the synthesis of both MIL-101
host matrix and Au/MIL-101 catalyst are provided as Supplementary
Material.
2
.4. General experimental procedure for one-pot tandem synthesis of
imines
t
Typically, benzyl alcohol (0.625 mmol), amine (0.625 mmol), KO Bu
0.20 mmol), 5 ml of toluene and Au/MIL-101 (20 mg) was added
(
successively. Then, the reaction mixture was stirred (900 rpm) at 343 K
for 8 h in open system (to allow air to pass into the reaction medium)
under an Ar atm. After cooling, ethyl acetate was added to dilute the
reaction mixture and the catalyst was centrifuged from the solution.
Then, solution filtered through a short pad of SiO
2
column. The filtrate
was analysed by GC and NMR techniques (Figs. S1-S20).
2
.5. Reusability experiments
The reaction between 4-chlorobenzylamine and benzyl alcohol was
monitored as a model reaction for reusability tests. After the completion
of the first reaction, the catalyst was separated by centrifugation (6000
rpm for 15 min), washed with ethanol, ethyl acetate and deionized
◦
water, dried at 70 C for 12 h under vacuum, and was used for the next
runs under the same reaction conditions.
2
.6. Leaching test
The Au/MIL-101 catalyst was separated from the reaction mixture
after 3 h. The filtrate transferred to a new vial and the reaction was
carried out under the same conditions for an additional 5 h.
3
. Result and discussion
3
.1. Characterization of MIL-101(Cr) and Au/MIL-101 catalyst
The detailed morphology of Au/MIL-101 catalyst was investigated
via TEM, TEM-EDX and HR-TEM. Fig. 1 shows the low resolution TEM
images of Au/MIL-101 catalyst in different scale. As depicted in Fig. 1,
the Au NPs are well dispersed on MIL-101 host matrix surface and large-
sized gold agglomerates were not formed on the MIL-101 surface. Fig. 2
(
a) and (b) demonstrates the TEM image in 50 nm scale and associated
Fig. 1. Low resolution TEM images in different scale of Au/MIL-101 catalyst.
particle size histogram of Au/MIL-101 catalyst. According to the particle
size analysis using the NIH image program [48], the mean particle size
of the Au (0) NPs in the Au/MIL-101 catalyst was determined to be 1.94
suggesting the crystalline structure of MIL-101 is stable during the
catalyst preparation. Additionally, the wide-angle XRD pattern of
±
0.16 nm. The corresponding EDX analysis of Au/MIL-101 catalyst
Au/MIL-101 catalyst shows a broad and weak diffraction peaks at 2θ =
further indicates the existence of Au, Cr, C, and O elements (Fig. 2 (c)).
The HR-TEM image (Fig. 2 (d)) shows the lattice fringes of Au/MIL-101
catalyst with D-spacing of 0.237 nm, confirming the formation of Au
◦
3
8.34, 44.76, 65.14 and 77.96 , assigned to (111), (200), (220) and
(
(
311) planes of a face-centered cubic (fcc) crystal structure of gold
(Joint Committee on Powder Diffraction Standards-JCPDS no. 04-0784,
(
111) planes [49,50].
USA) [54,55].
XPS measurements were performed to reveal the elemental compo-
Characteristic IR bands of MIL-101 and Au/MIL-101 catalyst are
compared in Fig. 5. When the spectrum is examined, two bands resulting
sitions and electronic structure of Au/MIL-101 catalyst. The survey XPS
spectrum for Au/MIL-101 catalyst shows the co-existence of Au, Cr, C,
and O (Fig. 3 (a)). As shown in Fig. 3 (b), the binding energies for Au 4f7/
ꢀ 1
from stretching and bending are seen at 3433 and 1625 cm . The sig-
nals seen in these band intervals are caused by the adsorbed water on the
2
and Au 4f5/2 at 82.5 and 86.1 eV are clearly observed in Au 4f XPS
ꢀ 1
surface of MIL-101. The band observed at 1402 cm is caused by
spectrum, which could be assigned to states of Au (0) [51,52] In addi-
tion, the binding energies for Au 4f7/2 and Au 4f5/2 at 83.7 and 87.0 eV
are corresponded to Au (III), which could be explained by the surface
oxidation of Au during the sample preparation process for XPS test
and/or unreduced species of Au [51,53].
symmetrical (O C O) vibrations in the structure of terephthalic acid.
– –
The presence of this band in the IR spectrum confirms the presence of
terephthalic acid in the structure of MIL-101. The signals observed be-
ꢀ 1
ꢀ 1
tween 600 cm and 1600 cm result from the benzene ring in the
–
structure. The bond structure in the benzene ring (C
–
C) causes tension
The crystal structures of MIL-101 and Au/MIL-101 samples were
characterized by P-XRD (Fig. 4). The characteristic diffraction peaks of
MIL-101 can be apparently observed in Fig. 4, indicating the successful
formation of MIL-101 crystals [43–45]. The characteristic diffraction
peaks of MIL-101 crystal are still retained after loading of metal NPs,
ꢀ 1
vibrations at 1508 cm . Other signals from the benzene ring are seen in
ꢀ 1
1
107, 1020, 749 and 663 cm [56–61]. When comparing IR spectrum
of Au/MIL-101 catalyst with MIL-101 spectrum; In both spectra, the
peaks appear in the same band regions. These results are considered to
3

I. Gumus et al.
Molecular Catalysis 501 (2021) 111363
Fig. 2. (a) TEM image in 50 nm scale, (b) associated particle size histogram, (c) related EDX spectrum and (d) HR-TEM image in 5 nm scale of Au/MIL-101 catalyst.
be evidence of no degradation in the structure of the MIL-101 during the
preparation of the Au/MIL-101 catalyst [61].
of imines from alcohols and amines, alcohol is first oxidized to corre-
sponding aldehyde (oxidation step under aerobic conditions and cata-
lysed step) which is then reacted with amine to form the secondary
imine (condensation step under anerobic conditions and sometimes
uncatalysed step) [31,37,38,56–58,61]. Thereby, design and production
of active centers for selectively oxidize alcohols to the related aldehydes
remains no doubt crucial for obtaining high yield of imines. The selec-
tive oxidation of benzyl alcohol to benzaldehyde is the key step for this
tandem reaction, and in this step, by-products such as benzoic acid and
benzyl benzoate may occur depending on the nature of the catalyst used
and the reaction conditions selected. To achieve the maximum conver-
sion and selectivity in this step, we first optimized the conversion of
benzyl alcohol (Step 1), which is the rate-determining step for this
one-pot reaction. Meanwhile, the control experiment was carried out
without catalyst and considerable bezaldehyde yield was not observed
(Table 1, entry 1).
2
N adsorption-desorption isotherms of MIL-101 and Au/MIL-101 are
given in Figs. S21 and S22. MIL-101 and Au/MIL-101 show type I shape
nitrogen adsorption-desorption isotherms, which is a characteristic for
microporous materials [62]. The micropore volumes and surface areas
for MIL-101 and Au/MIL-101 were established by the t-plot method
[
63]. On passing from MIL-101 to Au/MIL-101, both the micropore
3
3
volume (from 1.26 cm /g to 1.17 cm /g) and BET surface area (from
2
2
2
399.62 m /g to 2104.79 m /g) are prominently reduced. The consid-
erable decrease observed in the micropore volume and surface area for
Au/MIL-101 can be attributed to the encapsulation of gold nanoparticles
in MIL-101 or their deposition on the cage windows of MIL-101.
Furthermore, hysteresis loop was not observed in the
N
2
adsorption-desorption isotherm of Au/MIL-101 and pore size distribu-
tion was found to be changed in the same manner that of MIL-101
framework which reveals that the synthesis protocol followed in the
preparation of Au/MIL-101 did not create any mesopores in MIL-101
framework [44,45].
Initially, for comparison, the selective oxidation of benzyl alcohol to
benzaldehyde was evaluated using the Au/MIL-101 catalysts with
different Au content (0.5, 1.0, 2.0, 3.0 and 4.0 wt % Au) by keeping the
other reaction parameters constant (i.e. 0.625 mmol of benzyl alcohol,
t
3
.2. The catalytic efficiency of Au/MIL-101 catalyst in the imine
0.20 mmol of KO Bu, 20 mg of catalyst, 343 K, air atmosphere). As the
production from the coupling of alcohols and amines
Au content of catalyst is increased up to wt 3%, the yield of benzalde-
hyde was gradually increased. Interestingly, when the amount of Au
content increases to 4%-Au/MIL-101, the yield of the related aldehyde
also decreased to 76 %. This decrease in activity might be explained by
the many Au NPs aggregated in the surface of the MIL-101 support [31].
Thereby, 3%-Au/MIL-101 is an optimum catalyst with 99 % conversion
of benzyl alcohol and with 99 % selectivity of benzaldehyde (Table 1,
entries 2–6).
To understand the effect of oxygen on reactivity, the same reaction
was also investigated in the Ar atmosphere (sealed system) under tested
conditions. As shown in Table 1, the yield of benzaldehyde product was
trace amount (Table 1, entry 7). Therefore, it can be said that the
After the successful preparation of Au/MIL-101 catalysts with
different Au content, their catalytic performances were evaluated for the
one-pot tandem imine synthesis via amine-alcohol cross-coupling reac-
tion. The large surface area, structure stability, rich Lewis acid active
region of MIL-101 caused us to choose it as the host matrix for Au NPs [2,
1
2,14]. Because the porous cage structure and large surface area can be
used to stabilize the ultra-small Au NPs (ca. 1.92 nm).
Reactivity of the catalysts in one-pot tandem reaction was studied
using benzyl alcohol and 4-chlorobenzylamine as model substrates.
According to the widely accepted reaction process for one-pot formation
4

I. Gumus et al.
Molecular Catalysis 501 (2021) 111363
Fig. 4. Wide angle P-XRD patterns of MIL-101 and Au/MIL-101 catalyst (5.0 wt
%
Au) in the range of 2θ = 5-80◦.
Fig. 3. (a) Survey and (b) Au 4f core level high resolution XPS spectra of Au/
MIL-101 catalyst.
presence of oxygen is important for this reaction. Similarly, the reaction
did not proceed in the absence of the base (Table 1, entry 8). Thus, we
searched the influence of various bases such as Na
3
PO
4
, Cs
2 3 2 3
CO , K CO ,
t
and KO Bu with different dissociation constants (pK
a
) values and the
results are summarized in Table 1 (entries 9–11). As can be seen from the
Table 1, the benzyl alcohol conversion after 7 h of reaction follows
t
almost the same order as the pK
a
: KO Bu > Na
3
PO
4
> K
2
CO
3
≈ Cs
2 3
CO ,
this most probably associated to the concentration of benzyloxide spe-
cies that are involved in the first step of the reaction [64].
It is generally accepted that the role of the base is to form an alco-
holate that will bind to the unsaturated Au atoms [2,59]. Although
elaborations about this subject are still not known, the key point in the
oxidation step is proposed to be the transfer of a -H from the alcoholate
α
to Au. To confirm this and understand the effect of Au NPs in oxidation
step, we screened the oxidation of benzyl alcohol with bare MIL-101 as
catalyst without Au NPs, and only 5% of benzaldehyde was obtained
Fig. 5. FT-IR spectra of MIL-101 and Au/MIL-101 catalyst in the range of 4000-
ꢀ 1.
4
00 cm
(
Table 1, entry 12). As a result, it can be said that Au NPs are crucial for
the catalytic reactivity in the selective oxidation reaction of benzyl
alcohol and the oxidation reaction proceeds through the Au atoms in the
presence of a base.
chemical process. Especially, thermal conductivity of solvent influences
catalyst NPs activation and eventually conversion. Solvent with high
thermal conductivity leads increase in the particle thermal conduction
which results in the activation of catalyst and hereby needs lower acti-
vation energy for conversion [65]. In the oxidation reactions, activity
may also be affected by the solubility of oxygen in the solvent. The
oxidation of benzyl alcohol to benzaldehyde increases with increased
oxygen demand, so the solvent that can dissolve more oxygen will lead
to high conversion. The highest conversion was achieved with nonpolar
Subsequently, with the optimal 3.0 %-Au/MIL-101 as catalyst and
t
optimal KO Bu as base, various solvents were also examined for this
transformation. Because, the catalytic activity is considerably affected
by the solvent. Every solvent has specific properties like acid–base
behavior, chemical constitution, boiling point, solute–solvent interac-
tion, thermal stability, oxygen’s solubility etc. which are important to a
5

I. Gumus et al.
Molecular Catalysis 501 (2021) 111363
Table 1
a
Optimization of benzyl alcohol oxidation with Au/MIL-101 .
Entry
Catalyst
Base
Time
h)
Temp.
(K)
Conditions
Conv.
(%)
(
t
1
2
3
4
5
6
7
8
9
None
KO Bu
24
7
343
343
343
343
343
343
343
343
343
343
343
343
343
343
343
r.t.
Air
trace
29
t
0.5%-Au/MIL-101
1.0%-Au/MIL-101
2.0%-Au/MIL-101
3.0 %-Au/MIL-101
4.0%-Au/MIL-101
3.0 %-Au/MIL-101
3.0 %-Au/MIL-101
3.0 %-Au/MIL-101
3.0 %-Au/MIL-101
3.0 %-Au/MIL-101
MIL-101
KO Bu
Air
t
KO Bu
7
Air
51
t
KO Bu
7
Air
68
t
KO Bu
7
Air
99
t
KO Bu
7
Air
76
t
KO Bu
24
24
7
Sealed flask/Ar
trace
trace
32
None
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Cs
2
CO
CO
Na PO
3
10
11
12
13
14
15
16
17
18
K
2
3
7
44
3
t
4
7
62
KO Bu
24
20
20
20
7
5
3.0 %-Au/MIL-101
3.0 %-Au/MIL-101
3.0 %-Au/MIL-101
3.0 %-Au/MIL-101
3.0 %-Au/MIL-101
3.0 %-Au/MIL-101
1,4-dioxane
DMSO
26
31
IPA
6
t
KO Bu
14
t
KO Bu
7
323
363
74
t
KO Bu
7
99
a
t
Reaction conditions: 3.0 %-Au/MIL-101 (20 mg), benzyl alcohol (0.625 mmol), KO Bu (0.20 mmol), toluene (5 mL), 343 K, Air.
t
aprotic toluene, which has high thermal conductivity and high oxygen
solubility. Conversely, when polar protic IPA (isopropyl alcohol) with
high thermal conductivity and good oxygen solubility was used as a
solvent, the benzaldehyde yield was quite low. This may be due to the
exchange of the labile Bronsted acidic protons with the solvent mole-
cules and hence retarding the progress of the reaction [66] (Table 1,
entries 13–15). Also, high selectivity to benzaldehyde of benzyl alcohol
in toluene can be ascribed to the low solubility of water in this nonpolar
solvent [6]. Because water is one of the products in this tandem prosess
and the low solubility of water in the reaction medium could prevent the
formation of over-oxidation products such as benzoic acid and benzyl
benzoate. After that, we examined the influence of temperature on the
this step (Table 1 entries 16–18). The yield of benzaldehyde was very
low at room temperature (Table 1, entry 16). When we reduced the
temperature from 343 K to 323 K, the yield of benzaldehyde was still low
the presence 0.625 mmol of benzyl alcohol and 0.20 mmol of KO Bu at
343 K within 7 h.
After optimizing the reaction conditions for the oxidation step, we
examined the reaction between benzyl alcohol and 4-chlorobenzyl-
amine for the one-pot tandem synthesis (Step 1 + 2) of the corre-
sponding imine (i.e. 0.625 mmol of benzyl alcohol, 0.625 mmol of 4-
t
chlorobenzylamine, 0.20 mmol of KO Bu, 20 mg of catalyst, 343 K, air
atmosphere). The yield of related imine was 77 % after 8 h (Table 2,
entry 1). This demonstrates the powerful multistep transformation ac-
tivity of 3.0 %-Au/MIL-101. On the other hand, it is generally accepted
that the condensation step proceeds in anerobic conditions. Therefore,
although the presence of excess oxygen facilitates the oxidation step, it
may be inhibiting the condensation step. Because, in the presence of
excess oxygen, the amine in reaction can be oxidized. To control the
effect of oxygen during the one-pot tandem reaction, we examined the
reaction between benzyl alcohol and 4-chlorobenzylamine under
(
Table 1, entry 17). In contrast, when we raised the temperature from
43 K to 363 K, there was no significant increase in the yield of benz-
3
2
different atmospheres; open flask/Ar, O and air. When the reaction is
aldehyde (Table 1, entry 18). Therefore, optimum temperature for high
yield of benzaldehyde product was chosen as 343 K. Finally, the best
result was observed when the benzyl alcohol oxidation reaction was
carried out in toluene by using 20 mg of 3.0 %-Au/MIL-101 catalyst in
carried out in an open flask under an Ar atmosphere, the the yield of
related imine was 96 % after 8 h (Table 2, entry 2). In contrast, when the
2
reaction is carried out under the O atmosphere, the yield of imine was
very low after 8 h and benzyl benzoate formation was observed as a by-
Table 2
a
Optimization of one-pot imine synthesis from alcohols and amines with 3.0 %Au/MIL-101 .
Entry
Reactions
Catalyst
Conditions
Time
h)
Yield of imine
(%)
Selectivity
of imine
(
(
%)
1
2
3
4
5
6
7
8
Steps 1 + 2
Steps 1 + 2
Steps 1 + 2
Steps 1 + 2
Steps 1 + 2
Step 2
3.0 %-Au/MIL-101
3.0 %-Au/MIL-101
3.0 %-Au/MIL-101
None
Air
8
8
8
8
8
1
1
1
77
96
10
trace
4
>99
>99
55
Open flask/Ar
O
2
Open flask/Ar
Open flask/Ar
Open flask/Ar
Open flask/Ar
Open flask/Ar
>99
>99
>99
>99
>99
MIL-101
MIL-101
96
89
64
Step 2
3.0 %-Au/MIL-101
None
Step 2
a
t
Reaction conditions: 3.0 %-Au/MIL-101 (20 mg), amine (0.625 mmol), benzyl alcohol (0.625 mmol), KO Bu (0.20 mmol), Toluene (5 mL), 343 K, Open flask/Ar.
6

I. Gumus et al.
Molecular Catalysis 501 (2021) 111363
product. (Table 2, entry 3). As a result, it can be said that amount of
oxygen plays an important role in the one-pot tandem the synthesis of
imine and so, all further experiments were performed in an open flask
under an argon stream. Meanwhile, control experiments were per-
formed under the optimized conditions to understand the function of
Au/MIL-101 in the one-pot prosess. As expected, when using MIL-101 as
the catalyst, only 4% of the corresponding imine was obtained because
there were no Au NPs to promote the first oxidation step. Similarly,
considerable imine formation was not observed without catalyst
electron-withdrawing group (i.e. -Cl). Furthermore, it is found that the
position of the substituent on the phenyl group of amines is also sig-
nificant for the yield of related imine. For example, for o-, m- and p-Cl/
3
OCH substituents, the yields of related imines are in the order of para >
meta > ortho substitution, indicating an influence of a steric effect. The
conversion rate of 2,6-diisopropylaniline and 1-naphthylamine into the
corresponding imines are lower than those of other substituted amines,
but the selectivities are very good (Table 3). This decrease might be due
to the limited diffusion of large-sized substrates in MIL-101. As a result,
it can be said that Au/MIL-101 is a highly effective and selective catalyst
for this one-pot reaction.
(
Table 2, entries 4 and 5).
On the other hand, we separately investigated the condensation re-
action between of the in situ generated benzaldehyde and 4-chloroben-
zylamine (Step 2). When separately investigating the condensation
reaction, MIL-101 with Lewis acidic sites showed excellent catalytic
activity which is confirming the synergistic effects of acidic sites
A important advantage of heterogeneous catalysis is the ability to
smoothly remove the catalyst from the reaction and reuse it for next
runs. In this regard, the reusing and recycling capability of 3.0 %-Au/
MIL-101 were investigated for the one-pot imine synthesis, under opti-
mum reaction conditions. After each catalytic run, the catalyst was
collected by centrifugation and exhaustively washed with various sol-
vents. After that, it was dried and used directly for the further runs under
the same reaction conditions. After being used five cycles in successive
reactions, the reaction conversion was still up to 97 % with 100 %
selectivity (Fig. 6). Au/MIL-101 sample harvested from the fifth cata-
lytic run was analyzed by TEM. The sum of its results showed that no
obvious changes in the morphology of Au NPs (Fig. S23). The slight
decrease (~3%) in the activity of Au/MIL-101 catalyst can be attributed
to increase in the average size of Au NPs from 1.94 ± 0.16 nm to 2.12 ±
0.17 nm (Fig. S23). XRD analysis was also performed to determine the
stability of the Au/MIL-101 catalyst. In Fig. S24, the XRD patterns of the
fresh catalyst and the product obtained at the end of the 5th catalytic
cycle are compared. It was observed that there was no significant change
in the crystallinity of the Au/MIL-101 catalyst after reuse. A new
diffraction peak observed at 2θ = 27.46◦ in the product isolated after
reuse experiment. It is estimated that this new diffraction peak may arise
(
Table 2, entry 6).In the presence of 3.0 %-Au/MIL-101, the activity
slightly decreased, while in the absence of catalyst, yield of the corre-
sponding imine was only 64 % (Table 2, entries 7 and 8). Thus, we
suggest that MIL-101 promoted the condensation step. These results
show that the active sites for both the oxidation and the condensation
step were on the Au/MIL-101 catalyst.
By determining the optimized reaction conditions, the scope of this
one-pot tandem reaction was examined by structurally and electroni-
cally diverse amines (Table 3). As is formerly disclosed the selective
oxidation of benzyl alcohol is the key step in the one-pot tandem reac-
tion. Thereby, the structure of chosen amines may likely have an effect
on the conversion and product yield. The Au/MIL-101 catalyst shows a
high imine yield (87–99 %) with the substituted anilines. On the other
hand, located groups on the phenyl group of amines could change the
nucleophilicity of aniline derivatives and thus promote the nucleophilic
attack due to the nature and the position of substituents. As excepted,
the reaction of benzyl alcohol with amines modified by electron-
t
donating groups (i.e. ꢀ CH
3
and ꢀ OCH
3
) was faster than those with
from the base (KO Bu) used in the catalytic study. These results indicate
Table 3
3
.0 %-Au/MIL-101- catalysed one-pot imine formation from benzyl alcohol and various amines.
a
t
Reaction conditions: 3.0 %-Au/MIL-101 (20 mg), amine (0.625 mmol), benzyl alcohol (0.625 mmol), KO Bu (0.20 mmol), Toluene (5 mL), 343 K, Open flask/Ar. The
selectivity of corresponding imine is given in parentheses.
7

I. Gumus et al.
Molecular Catalysis 501 (2021) 111363
synthesize imine derivatives via oxidative coupling of amines and al-
cohols. However, most of them suffer from harsh reaction conditions
(
high gase pressure and high temperature), long reaction time, limited
substrate scope and low turnover number/frequency (TON and TOF). A
comparison between the catalytic activities of prepared catalyst and
other reported catalysts systems in the synthesis of N-benzylidenaniline
from aniline and benzyl alcohol was made, and the results are given in
Table 4 [56,60,67–78]. Long et al. have developed a base-free catalytic
2
system using CeO immobilized on magnetic core-shell microparticles as
catalyst. The alkalinity of the prepared catalysts enabled this tandem
reaction to run in a without base environment, but it suffers from low
ꢀ 1
turnover frequency (TOF =3.96 h ) [79] (Table 4, entry 1). Tan and
co-workers reported Ni (OH)(COO)
3
6
ꢀ based MOF catalyst system,
through which imines can be obtained in the presence of KOH under
solvent free conditions although it requires high temperature (373 K) to
promote the reaction and long reaction time (12 h, the selectivity of
imine 88 %) [80] (Table 4, entry 2). Chen et. al. and Mandal et. al. have
x x 2
reported active 5.0 wt% MnO /HAP [81] and MnO /TiO @SBA-15 [75]
Fig. 6. The reusability of Au/MIL-101 catalyst along the one-pot
as catalysts, respectively, any base was not required for imine synthesis
and reactions carried out ambient conditions (Table 4, entries 3–4). But
high imine yields were achieved only with a high amount of catalyst and
long reaction time (24 h). Although the active catalytic species of
MnOx/HAP have not yet been fully clarified by Chen et al., both HAP
and MnOx/HAP were found to obviously promote the condensation
step, which can be attributed to the utility of Lewis acid sites of the
catalyst. On the other hand, in the reaction pathway proposed by
imine synthesis.
that Au/MIL-101 catalyst was stable under the current one-pot imine
formation from benzyl alcohol and amines conditions and could be
reused for multiple runs of the cross coupling of alcohols and amines.
The heterogeneous catalytic nature of Au/MIL-101 was confirmed by
the hot leaching test for both the oxidation and condensation steps. In
both reactions, the conversion in the presence Au/MIL-101 as a catalyst
gradually increased from 0 to 3 h. To demonstrate that no Au NPs were
dissolved in the solvent during the reactions, the Au/MIL-101 catalyst
was removed after 3 h and reaction was carried out under the same
conditions for an additional 5 h (Figs. S25-26). The reaction no longer
proceeded when the catalyst was removed. Consequently, during the
reactions, there were no Au NPs leaching into the reaction medium, and
MIL-101 as a carrier perfectly keeped the loading Au NPs.
Mandal et al., the role of MnO
2
is to produce O
2
- species without oxygen
3+
splitting by adsorbing oxygen to the Mn
2
region. This O - species
4+
adsorbed on Ti gives rise to Ti-O-O- species. This Ti-O-O- species acts
as an active oxidizing species and benzyl alcohol gets oxidized in pres-
ence of Ti-O-O- species and produce benzaldehyde. The interaction of
aniline on weak acid site (Siꢀ OH) of SiO
81].
Moreover, there are many other heterogeneous catalysts such as Au/
HAP [49], Au/TiO [67,77,78], Au/ZnAl [56], Au-Pd/ZrO [69],
2
lead to formation of imine
[
Until now, a number of catalyst systems have been employed to
2
2
O
3
2
Table 4
One-pot synthesis of N-benzylidenaniline from benzyl alcohol and aniline with reported various heterogeneous catalyst.
ꢀ
1
Entry
Catalyst
Base
Conditions
Temp.
Time
(h)
Yield of imine
(%)
TOF (h
)
Ref.
◦
(
C)
a
1
Fe
Ni
3
O
4
@X@CeO
2
–
Air
60
12
12
24
24
12
25
10
8
–
3.96
[76]
[80]
[81]
[75]
[83]
[73]
[74]
[56]
[60]
[67]
[77]
[18]
[18]
[78]
[78]
[68]
[69]
[70]
[71]
[72]
2
3
3
(OH)(COO)
6
ꢀ based MOF
KOH
Air ballon
100
80
88
93
93
94
100
53
99
99
7
–
MnOx/HAP
MnOx/TiO
TiO @PMHSIPN
Ag/Al
CdS@MIL-101
–
–
Air ballon
–
b
4
2
@SBA-15
Air
80
–
5
2
K
2
CO
CO
3
Ar
160
125
0
–
c
6
d
2
O
3
Cs
–
–
–
CH
–
–
2
3
Air
–
7
Air, λ > 420 nm
Air
–
8
Au/ZnAl
Au/HAP
O
2 3
60
39.1
9
O
2
O
2
O
2
bubbling
60
3
e
1
1
1
1
1
1
1
1
1
1
2
2
0
1
2
3
4
5
6
7
8
9
0
Au/TiO
Au/TiO
Au/TiO
Au/TiO
Au/TiO
Au/TiO
2
2
2
2
2
2
3
OK
20
24
36
22
22
14
14
12
8
–
–
–
–
–
–
–
bubbling
60
88
45
22
≈ 50
7.5
70
94
5
f
0.5 MPa O
0.1 MPa N
2
120
110
120
120
30
g
h
i
Cs
Cs
–
2
CO
CO
3
3
2
2
5 atm N
5 atm N
2
2
j
PICB-Au/Pd
NaOH
O
2
k
l
c
Au–Pd@ZrO
2
–
–
Air
, light irradiation
40
79
PdAu@MIL-100(Fe)
Pd ꢀ Au@Mn(II)-MOF
Cu-MOF nanorod/Au–Pd
Au/MIL-101
N
2
R.T.
110
110
70
24
30
18
8
–
KOH
Air
99
91
99
4.4
–
m
–
Air
t
1
KO Bu
Open flask/Ar
51.47
This study
a
b
c
Amine (2.0 mmol), alcohol (1.0 mmol), Fe
mmol), 2.4 wt.%Ag/Al (165 mg), Cs CO
3
O
4
@CN@CeO
2
(20 mg); Amine (1.0 mmol), Alcohol (1.0 mmol), catalyst (100 mg); Amine (1.5 mmol), Alcohol (3
d
e
2
O
3
2
3
(100 mg); Amine 0.1 mmol, Alcohol (0.15 mmol), CdS(2 wt.%)@MIL-101 100 mg; Alcohol (20 mmol), Amine (20
f
g
mmol), base (2 mmol) and methanol (50 mmol) and Au/substrate molar ratio of 1/4500; Azobenzene was formed as a by-product, Amine, ester, amide were as by-
h
i
j
products; N-phenylbenzylamine (ca.50 %) was formed as a by-product, Main product was N-phenylbenzylamine (ca. 93 %); Amine (0.368 mmol), Alcohol (0.368
k
3 2
mmol), NaOH (0.095 mmol), PICB-Au/Pd (0.0057 mmol, 1.5 mol%), THF/CF CH OH (9:1, 0.5 mL); The reaction was carried out in a one-pot, two-step method. In the
first step, benzyl alcohol was oxidized to benzaldehyde. The second step reaction commenced by adding stoichiometric amount of aniline after the first step reaction
l
m
was completed. TOF is measured at t =1 h, Amine (0.1 mmol), Alcohol (3 mmol), CH
3
CN (2 mL), catalyst (10 mg); Alcohol (1.04 mmol), aniline (1.24 mmol) and
catalyst (4 mg).
8

I. Gumus et al.
Molecular Catalysis 501 (2021) 111363
Ir-Zr-MOF [82], etc. The Au/ZnAl
2
O
3
catalyst presented the high per-
for efficient one-pot imine formation using Au/MIL-101 can be
explained in Fig. 7. First, alcohol forms an alcoholate with the assistance
of the base, and it binds to unsaturated Au atoms bearing positive charge
density located at corners, edges and steps of the NPs. Then, Au-H and
formance among the reported Au catalysts at mild temperatures with
ꢀ 1
high TOF =39.1 h , due to the active surface oxygen species and the
acidic-basic property of support, but it suffer from limited substrate
scope (Table 4, entry 8) [56]. Similarly, Au/HAP as catalyst displayed
Au-alkoxide species form with the transfer of a
α
-H (ꢀ CH
2
-) from the
◦
high performance at 60 C and after 3 h the yield and selectivity of imine
alcoholate to Au atoms. This is generally regarded as the initial step
along alcohol oxidation over Au surfaces [2,34]. The role of oxygen in
the reaction medium in this stage is to remove the hydrogen from cor-
ners/edges/steps of the Au NPs, leading to the catalytic cycle. After that,
by removing a hydrogen from the Au-alkoxide, the desired benzalde-
hyde product is produced (Step 1).
was 99 %, but in this reaction substrat/catalyst molar ratio was 100. In
this base-free system, a delicate cooperation between metallic gold and
the acid/base sites of the hydroxyapatite (HAP) surface was play a key
role in determining the efficiency and compatibility of the Au/HAP
catalyst for not only the rate-determining alcohol oxidation but also for
the subsequent condensation step of the tandem reaction (Table 4, entry
After the benzaldehyde formed, the reaction can proceed in two
possible pathways: (i) the produced benzaldehyde with a water mole-
cule to form gem-diol, which can be oxidized into carboxylic acid in-
termediate or (ii) the produced benzaldehyde with amine molecule to
form instable carbinolamine intermediate. We predicted that the reac-
tion pathway proceeds through an unstable carbinolamine intermediate
because carboxylic acid was not observed during the reaction in our
system. MIL-101 has a high density of open chromium centres, which
can act as Lewis acid sites for catalysis and along possible pathway, the
9
) [60].
On the other hand, the catalytic activity of Au/TiO
2
in oxidative
coupling of amines and alcohols has been studied by several groups in
the absence and/or presence base (Table 4, entries 11–15) [67,77,78].
According to these studies, product selectivity (imine, amine and others)
depends on significantly to reaction conditions such as temperature,
base and oxygen (Table 4, entries 4–7). In the examples reported by Sun
et al. and Ishida e. al., when the reactions carried out without base at 60
◦
–
and 120 C under O
2
atmosphere, the yield of imine was 88 (after 36 h)
C
–
O group of the benzaldehyde coordinates with this Lewis acid sites,
–
–
and 45 % (after 22 h), respectively. Azobenzene was formed as
by-product due to the direct aerobic oxidation of aniline in the presence
of excess oxygen (Table 4, entries 4 and 7). Ishida et al. also reported
which increases the electrophilicity of the C O group [84]. Probably
this nucleophilic attack effectively prevents the formation of
over-oxidized by-products. Then, as a result of the nucleophilic addition
–
reaction to the C O group of the unreacted amine, an carbinolamine is
that when the reactions performed with a base under 0.1 MPa N
2
–
pressure, the imine selectivity was low (secondary amine, ester, amide
were formed as by-products). The results of He et al. were interesting for
formed. The formed unstable carbinolamine undergoes dehyration by
acid-catalyzed a pathway to form the corresponding imine (Step 2).
Consequently, Au/MIL-101 catalyst is appropriate material for the one
pot tandem synthesis of imines because Au NPs offer oxidation activity
while MIL-101 provides Lewis acid sites during the condensation step.
this tandem reaction. According to the report, in a base-free system
◦
under 5 atm N
2
at 120 C, the main product was secondary amine, while
imine was formed only as a byproduct. The adding of base to the reac-
tion lead to formation of the imine product (after 14 h, ca. 50 %) [78].
There is a very limited number of MOF-based catalysts for the one-
pot imine synthesis from alcohols and amines (Table 2, entries
4
. Conclusions
1
3
8–20). Among all the ever-reported MOF-based catalysts, the prepared
In conclusion, an efficient and stable bifunctional Au/MIL-101
ꢀ 1
.0 %-Au/MIL-101 catalyst exhibited the highest TOF (51.47 h ) for
catalyst was successfully developed by using the liquid phase impreg-
nation method which showed excellent catalytic performance when
applied in the one-pot tandem synthesis of various imines. The effects of
various factors on the catalytic activity such as reaction atmosphere,
solvent, temperature and base have been also investigated. We found
that amount of oxygen and base are necessary for the high selectivity to
benzaldehyde of benzyl alcohol which is the rate-determining step for
one-pot synthesis of N-benzylidenaniline from benzyl alcohol and
aniline.
3
.3. Mechanism study of the reactions
The above experimental findings suggest that the catalytic pathway
Fig. 7. Possible reaction pathway for the one-pot imine synthesis (yellow colored balls represent Au atoms) (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article).
9

I. Gumus et al.
Molecular Catalysis 501 (2021) 111363
this one-pot reaction. A plausible catalytic reaction mechanism was
proposed and it was found that Au NPs were highly effective in acti-
vating selective oxidation of benzyl alcohol to benzaldehyde, while the
Lewis acid sites of MIL-101 catalyze the second condensation step
without interfering with the oxidation step. Furthermore, prepared
bifunctional catalyst was recyclable and could be reused for at least five
times, demonstrating its excellent capacity in industrial catalysis. To our
knowledge, bifunctional Au/MIL-101 catalyst has the highest TOF
values among all the ever-reported MOF-supported catalysts.
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