Tin and copper species dispersed on a metal-organic framework as a new catalyst in aerobic Baeyer-Villiger oxidation: An insight into the mechanism

Article abstract of DOI:10.1016/j.catcom.2020.105985

Metal-organic frameworks (MOF) containing tin and copper metal salts were used as catalysts in the Baeyer-Villiger oxidation of cyclohexanone. The outcome was a synergistic effect on the catalyst's efficiency, which resulted from the simultaneous presence of copper and tin species in the MOF. The catalyst with high metal content showed the intermolecular dehydration of benzoic acid and the formation of benzoic anhydride in a side reaction as well as the Baeyer-Villiger oxidation reaction with a decline in efficiency. The optimized catalyst promoted the Baeyer-Villiger reaction in a high yield without the formation of benzoic anhydride.

Full text of DOI:10.1016/j.catcom.2020.105985

Catalysis Communications 139 (2020) 105985
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Catalysis Communications
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Short communication
Tin and copper species dispersed on a metal-organic framework as a new
catalyst in aerobic Baeyer-Villiger oxidation: An insight into the mechanism
⁎
Masoumeh Karimi Alavijeh, Mostafa M. Amini
Department of Chemistry, Shahid Beheshti University, G.C., Tehran 1983963113, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords:
Metal-organic frameworks (MOF) containing tin and copper metal salts were used as catalysts in the Baeyer-
Villiger oxidation of cyclohexanone. The outcome was a synergistic effect on the catalyst's efficiency, which
resulted from the simultaneous presence of copper and tin species in the MOF. The catalyst with high metal
content showed the intermolecular dehydration of benzoic acid and the formation of benzoic anhydride in a side
reaction as well as the Baeyer-Villiger oxidation reaction with a decline in efficiency. The optimized catalyst
promoted the Baeyer-Villiger reaction in a high yield without the formation of benzoic anhydride.
Metal-organic framework
Baeyer–Villiger
Intermolecular dehydration
Synergistic effect
Tin
Copper
1. Introduction
2
silica [11], zeolites [12,13], TiO [14], and graphene oxide [15], in an
oxidation reaction. Among the works reported so far, only Huo et al.
used tin and copper simultaneously in the B-V reaction [11]. The aim of
the present study was to create an outstanding catalyst by integrating
copper and tin species in porous metal-organic framework (MOF),
which are potentially powerful platforms in catalysis [16–19], and to
take advantage of both species and the MOF simultaneously. A further
aim was to make this reaction economical and environmentally friendly
by designing a catalyst that is less cost- and time-consuming to syn-
thesize, decreasing the amount of metal loading and using less ben-
zaldehyde as the sacrificing agent.
Metal-organic frameworks are hybrid porous materials made of
organic linkers and metal clusters or ions that are suitable for stabi-
lizing metal nanoparticles [20]. In addition, the size of the metal par-
ticles can be adjusted by the cavities and windows of the MOF. The
control of the metal particle size allows for the access to active catalytic
sites and overcomes several obstacles, such as the hard recovery from
the reaction environment, agglomeration, and the accumulation of
nanoparticles [21]. Among the various MOFs previously reported re-
garding the loading of metal nanoparticles and metal ions, MIL-101(Cr)
has attracted particular attention because of its stability and high re-
sistance to hydrolysis [22,23].
The Baeyer-Villiger (B-V) oxidation reaction is one of the most im-
portant processes in the field of organic transformation chemistry. In
this reaction, ketones become lactones or esters, which are valuable
intermediates in a wide range of chemical industries [1,2]. ε-Capro-
lactone, which is a chemical produced in the B-V reaction, plays a major
role in the pharmaceutical and medical fields as a monomer in the
preparation of biodegradable poly-ԑ-caprolactone [3–5]. In this context,
the evolution of improved productive and selective catalysts in the B-V
oxidative reaction has attracted the attention of many scientists
working in the catalyst field. Consequently, several catalysts, including
mesoporous silica [6], Cu-Fe
Keggin-type heteropoly salts [8], graphite [9], and carbon nanotubes
10] have been introduced. However, these catalysts have some lim-
3 4
O supported mesoporous silica [7],
[
itations and drawbacks, including high metal loading, prolonged reac-
tion time, relatively low activity, and difficult synthesis. Another issue
in some previously reported catalyst systems is the necessary use of
expensive oxidizers or environmentally harmful oxidizers, such as hy-
drogen peroxide and peracids. The use of hydrogen peroxide results in
the formation of water as a byproduct in the reaction medium, which
leads to the hydrolysis of the product. To overcome this problem, hy-
drogen peroxide was replaced by oxygen or air in the presence of al-
dehyde as a sacrificing agent, which is known as the Mukaiyama
method. However, this method is problematic because it requires the
use of pure oxygen and benzaldehyde greater than the stoichiometric
ratio. Earlier studies showed the beneficial effects of deposited tin and
copper species, as the Lewis acid sites on various supports, including
Despite all conditions provided for the intermolecular dehydration
of benzoic acid in previous studies of the B-V reaction in the presence of
II
IV
2
Cu and Sn under air/O [24,25], the formation of benzoic anhydride
was not observed. However, the results of this work showed that the
benzoic anhydride is produced by the intermolecular dehydration of
II
IV
benzoic acid in the presence of Cu and Sn under air as an oxidant,
⁎
Corresponding author.
E-mail address: m-pouramini@sbu.ac.ir (M.M. Amini).
https://doi.org/10.1016/j.catcom.2020.105985
Received 3 January 2020; Received in revised form 25 February 2020; Accepted 13 March 2020
Available online 14 March 2020
1566-7367/ © 2020 Elsevier B.V. All rights reserved.

M.K. Alavijeh and M.M. Amini
Catalysis Communications 139 (2020) 105985
and the formation of benzoic anhydride could be controlled and
eliminated by selecting the optimal metal loading. Therefore, it was
deemed beneficial to investigate the effects of metal species loading on
the conversion of benzaldehyde as the sacrificing agent to benzoic an-
hydride by the dehydration reaction among the benzoic acid molecules
produced by benzaldehyde oxidation. In this study, MIL-101(Cr) was
used as a support to fabricate a potent catalyst. To the best of our
knowledge, this is the first study to use an MOF to prepare hetero-
geneous catalysts for the B-V oxidation reaction. In addition, to obtain
further insights and clarify the reaction mechanism, the effects of metal
loading were investigated. The results showed that by using the present
catalyst, the amount of benzaldehyde in the reaction decreased by 25%
compared with previous studies. These results indicate that the re-
markable performance of this catalyst in the critical cyclohexanone
oxidation reaction and the production of ε-caprolactone could open a
new direction in the use of different MOFs to promote the B-V reaction
and introduce a new and reliable alternative that would reduce both the
cost and the environmental risks associated with this reaction.
2.3. General procedure for catalytic reactions and the regeneration of
catalyst
The aerobic B-V oxidation of cyclohexanone was performed in a
flask equipped with a refluxing condenser. Cyclohexanone (0.196 g,
2 mmol) and benzaldehyde (0.318 g, 3 mmol) were poured into a flask
containing 5 mL of dichloroethane (DCE), and the reaction was carried
out at 50 °C for 5 h in the presence of 50 mg of the activated catalyst
while air was passed continuously through the reaction mixture. After
II
IV
each catalytic cycle, Cu -Sn @MIL-101(Cr)/C was recovered by cen-
trifugation, and after washing with dichloromethane, it was dried under
vacuum at 150 °C and then used in the subsequent step of the catalytic
test. The reaction conversion and the identification of side products
were performed by GC and GC–MS analyses, respectively.
2.4. Leaching test
After 3 h of reaction, the catalyst was separated from the hot re-
action matrix. The reaction then was continued for 6 h with the filtrate
in the absence of the catalyst. After the catalyst was removed, the yield
of the reaction product did not increase. This result indicated that the
catalytically active sites, which were Cu and Sn species, remained
intact in the catalyst and did not leach into the reaction solution.
2
. Experimental
II
IV
2.1. Materials and instrumentation
All the chemicals and solvents used in this study were acquired from
3
. Results and discussion
Merck, and they were applied as received. The infrared spectra were
obtained on a Bomem MB-Series FTIR spectrometer. The powder X-ray
diffraction (XRD) measurements were performed at room temperature
3.1. Characterization of catalysts
by a STOE diffractometer using monochromatized Cu K
α
radiation
Various analyses were performed to identify the structure and
(
λ = 0.15418 nm). The nitrogen physisorption technique was applied
morphology of the prepared catalysts. Compared with the FTIR spec-
to measure the surface area and textural properties of the prepared
catalysts at −196 °C using a Micromeritics Tristar II 3020 apparatus.
The scanning electron micrographs (SEM) were obtained in the sec-
ondary electron contrast using a MIRA3 TESCAN field-emission in-
strument. The same instrument was used to carry out the energy-dis-
persive X-ray (EDX) analyses. The concentrations of tin and copper
were measured using an Espectro Arcos (AMETEK) inductively coupled
plasma optical emission spectrometer (ICP-OES). The conversion of the
reaction was estimated by an Agilent 7890 A (G3440A) GC. The by-
products were identified by GC–MS (TRACE MS/ ThermoQuest-
Finnigan) with a DB-5 column.
II
IV
trum of MIL-101(Cr), the FTIR spectrum of Cu -Sn @MIL-101(Cr) did
not show new absorption bands. For this reason, ICP and EDX analyses
were carried out to establish the presence of metal salts in the MOF.
The XRD patterns of the catalysts that were prepared using MIL-
1
01(Cr) as the support are illustrated in Fig. 2. The XRD results con-
firmed that the prepared catalysts possessed crystalline structures ex-
pected in the relevant MOF, and the metal loading on them had no
effect on their crystalline structures. Because of the low amounts of
loaded metal species and their uniform distribution on MIL-101(Cr),
II
IV
reflections of the loaded metal salts did not emerge in the Cu -Sn
@
MIL-101(Cr) diffraction pattern.
The representative nitrogen adsorption-desorption isotherms of the
Cu -Sn @MIL-101(Cr) catalyst and the bare support are shown in Fig.
II
IV
II
IV
2
.2. Synthesis of MIL-101(Cr) and Cu -Sn @MIL-101(Cr)
S1 (Electronic Supplementary Information, ESI). The BET surface area
2
−1
3 −1
of 2214 m g and pore volume of 1.13 cm g was determined for the
synthesized MIL-101(Cr). After impregnation with different amounts of
copper and tin salts in MIL-101(Cr), the BET surface area and total pore
The synthesis of MIL-101(Cr) was carried out following the pre-
II
IV
viously reported method [26]. To synthesize Cu -Sn @MIL-101(Cr)
with different percentages of metal, first different amounts of copper
nitrate and tin chloride were completely dissolved in 5 mL ethanol and
added to 0.5 g of MIL-101(Cr) that had been pre-activated at 160 °C.
The resulting mixture was stirred for 24 h, and then heated to 50 °C
until complete dryness of the solvent (Fig. 1). The prepared catalysts
with different percentages 0.4, 0.8; 0.6, 1.2; 0.8, 1.6; 0.8, 4.8; 2.4, 1.6;
2
−1
3
−1
II
IV
volume were decreased to 1943 m g and 0.99 cm g for Cu -Sn
MIL-101(Cr)/C and 1713 m g
@
2
−1
3
−1
II
IV
and 0.88 cm g
for Cu -Sn @MIL-
1
01(Cr)/F. These results confirmed the successful loading of metal salts
inside the cavities.
Fig. 3 (insert a) shows the SEM image of the Cu -SnIV@MIL-
II
II
IV
1
01(Cr)/C catalyst. As the micrograph shows, the octahedron mor-
and 2.4, 4.8 wt% of copper and tin ions were designated as Cu -Sn
MIL-101(Cr)/A, B, C, D, E, and F, respectively, in Table 1.
@
phology of MIL-101(Cr) was preserved after the metal salts were
II
IV
loaded. The elemental composition of the Cu -Sn @MIL-101(Cr)
samples was determined by EDX analysis, and the Cu and Sn loadings
Fig. 1. Preparation of the Cu -SnIV@MIL-101(Cr) for
II
B-V oxidation of cyclohexanone.
2

M.K. Alavijeh and M.M. Amini
Catalysis Communications 139 (2020) 105985
Table 1
The results of the cyclohexanone oxidation catalyzed by various catalysts under air atmosphere at 50 °C.
Entry
Catalyst
Metal loading (%) Cu-Sn
Yield of ԑ-caprolactone (%)
Yield of benzoic acid (%)
Yield of benzoic anhydride (%)
TONb
1
2
3
4
5
6
7
8
9
Blank
–
–
12
35
23
44
42
72
100
65
60
19
19
0
100
100
100
100
100
100
100
85
82
70
86
63
–
–
–
–
–
–
–
5
–
–
MIL-101(Cr)
II
Cu @MIL-101(Cr)
1.55–0
0–1.54
38
131
136
151
154
69
46
15
10
–
IV
Sn @MIL-101(Cr)
II
IV
Cu -Sn @MIL-101(Cr)/A
0.37–0.79
0.59–1.18
0.78–1.57
1.57–1.56
0.78–4.74
2.36–1.54
2.36–4.71
2.36–4.71
2.30–0
II
IV
Cu -Sn @MIL-101(Cr)/B
II
IV
Cu -Sn @MIL-101(Cr)/C
II
IV
Cu -Sn @MIL-101(Cr)/C
II
IV
Cu -Sn @MIL-101(Cr)/D
7
II
IV
10
11
12
13
14
Cu -Sn @MIL-101(Cr)/E
10
14
24
8
II
IV
Cu -Sn @MIL-101(Cr)/F
a
II
IV
Cu -Sn @MIL-101(Cr)/F
II
Cu @MIL-101(Cr)
17
86
73
92
18
86
IV
Sn @MIL-101(Cr)
0–4.75
4
Reaction conditions: cyclohexanone (2 mmol), benzaldehyde (3 mmol), and catalyst (50 mg) in DCE (5 mL), 50 °C, 5 h, under pumping air (20 mL min−1).
a
Without cyclohexanone.
Turnover number is calculated based on the amount of copper and tin for ԑ-caprolactone.
b
3.2. Evaluation of factors effecting the cyclohexanone oxidation reaction
The aerobic B-V oxidation of cyclohexanone was carried out in the
presence of benzaldehyde as a sacrificial agent under various conditions
to determine the optimal conditions. A main problem in the B-V reac-
tion under Mukaiyama conditions is the need to use large amounts of
benzaldehyde, which results in substantial amounts of benzoic acid in
the reaction media. To resolve this issue, different quantities of ben-
zaldehyde in the reaction were examined (Table S1), which revealed
that 2:3 mol was the optimal ratio of cyclohexanone to benzaldehyde.
The investigation of temperature as an active parameter on the
catalytic reaction demonstrated that by increasing the temperature
from 30 °C to 50 °C, a significant increase in the yield of the ԑ-capro-
lactone (35–100%) occurred, whereas increasing the temperature to
70 °C resulted in a downward trend in the product yield (Table S2).
Furthermore, the effect of the catalyst amount on the reaction pro-
gression was investigated and results are listed on Table S3. As the
results show by increasing the amount of the catalyst to 50 mg, the
reaction yield increased, and the further addition of the catalyst did not
increase the yield.
Fig. 2. XRD patterns of (a) simulated MIL-101(Cr), (b) synthesized MIL-
II
IV
II
IV
101(Cr), (c) Cu -Sn @MIL-101(Cr)/C, and (d) Cu -Sn @MIL-101(Cr)/F.
were also determined by ICP analysis. As shown in Fig. 3, the results of
the EDX analysis revealed that the as-prepared catalyst contained Cr,
Cu, Sn, C, O, N, and Cl. The EDX elemental mapping (Fig. 3, insert b)
results also implied that the Cu and Sn species were homogeneously
dispersed on the surface of MIL-101(Cr). The results of the ICP analysis
The reaction was carried out in the presence of various catalysts to
evaluate the role of MIL-101(Cr) and the metal species loaded on it in
the cyclohexanone oxidation process. The results are listed in Table 1.
Notably, in the absence of the catalyst, the yield of ԑ-caprolactone was
II
IV
showed that the contents of copper and tin in the Cu -Sn @MIL-
1
0
01(Cr)/A, B, C, D, E, and F were 0.37, 0.79; 0.59, 1.18; 0.78, 1.57;
.78, 4.74; 2.36, 1.54; and 2.35, 4.71 wt%, respectively.
12%. In addition, as Table 1 shows, the Lewis acid centers present in the
MIL-101(Cr) had a positive effect on the cyclohexanone aerobic
Fig. 3. EDX analysis, SEM image (insert a), and elemental mapping (insert b) of Cu -SnIV@MIL-101(Cr)/C.
II
3

M.K. Alavijeh and M.M. Amini
Catalysis Communications 139 (2020) 105985
study. The formation of these byproducts was confirmed by GC–MS
analysis. As shown in Table 1, by increasing the metal loading above a
certain amount (entry 11), conditions were provided for the inter-
molecular dehydration reaction, and benzaldehyde was involved in
both the B-V oxidation reaction and the intermolecular dehydration
reaction, which resulted in a decrease in the ԑ-caprolactone yield (yield
of ԑ-caprolactone: 19, benzoic acid: 86 and benzoic anhydride: 14%).
Although carboxylic anhydrides are an important class of the highly
active organic compounds that are involved as intermediates in organic
synthesis, especially in the preparation of drugs and peptides, they were
considered a byproduct in this reaction. In addition, the results showed
that tin and copper had a synergistic effect on the formation of benzoic
anhydride (Table 1, entry 11), and removal of either from the catalyst
led to a decrease in benzoic anhydride yield (entries 13 and 14). As
Table 1 shows, the limit value of Cu content in catalyst structure for the
absence of benzoic anhydride in the reaction mixture was 0.78%
(Table 1, entry 7) and by increasing the Cu content to 1.57% (entry 8),
Fig. 4. Effect of reaction time on the conversion of the B-V oxidation of cy-
II
IV
clohexanone with Cu -Sn @MIL-101(Cr)/C as catalyst (a) and leaching test
b).
(
5
% benzoic anhydride produced. This limit for Sn is 1.57% (entry 7),
and by increasing it to 4.74%, 7% benzoic anhydride produced (entry
). Notably, by decreasing the metal species loading, particularly the
oxidation reaction [27] because, as can be seen, in the absence of MIL-
01(Cr), the reaction efficiency is reduced to 12% (entry 1) from 35%
entry 2). However, copper nitrate by blocking chromium sites through
the coordination of nitrate groups, deactivated them and decreased the
reaction efficiency (entry 3). The use of copper or tin solely in the
catalyst results in low catalyst performance (entries 3 and 4). In con-
trast, the simultaneous presence of copper and tin species in MIL-101
9
1
(
amount of copper, the production of benzoic anhydride was prevented,
consequently, leading to the direct reaction to ԑ-caprolactone forma-
tion. For further insights into the formation of benzoic anhydride, the
reaction was performed in the absence of cyclohexanone with a higher
metal loading catalyst (Table 1, entry 12) with respect to the optimized
catalyst (entry 7). In the absence of cyclohexanone (entry 12), the
competing factor in the use of benzaldehyde was eliminated from the
reaction medium. Thus, there was a substantial amount of benzalde-
hyde in the medium, and it was converted to the benzoic anhydride
through intermolecular dehydration of benzoic acid that produced by
oxidation of benzaldehyde; consequently, the yield of benzoic anhy-
dride increased from 14 to 24%. To enable the better understanding of
the issues discussed above, the proposed mechanism is explained
below.
Metal ions as Lewis acid centers play a crucial role in advancing the
B-V reaction [11,14,24]. Initially, the coordination of benzaldehyde to
metal ions results in the formation of benzoyl radical, PhCO%, and then
in the presence of molecular oxygen and benzaldehyde, perbenzoic acid
forms. Then, Lewis centers by activating cyclohexanone make it sus-
ceptible to nucleophilic attack by the peracid to form the Criegee
complex. Subsequently, the Criegee adduct rearrangement results in the
formation of ԑ-caprolactone and benzoic acid [11,14,24].
(
Cr) had a synergistic effect on the reaction efficiency (entry 7). It was
also observed that by increasing the amount of loaded metal species to a
certain value (entry 11), both the efficiency and selectivity of the cat-
alyst in the formation of ԑ-caprolactone were significantly decreased.
Instead, benzoic anhydride started to form as a side product in another
pathway.
For further insight, a nitrogen adsorption-desorption analysis of the
prepared catalysts with different amounts of metal loading (Fig. S1,
ESI) was carried out to determine the role of the surface area and the
size of the catalyst cavities in the B-V oxidation performance of cyclo-
hexanone. The outcomes of these analyses revealed that the decreases
in the surface area and the size of the cavities in catalysts with higher
metal species loadings were not significant enough to contribute to the
decrease in catalytic activity. The results showed that as the metal
percentage increased, the oxidative strength of the catalyst increased,
and part of the benzaldehyde was oxidized to benzoic acid [28] before
entering into the main reaction pathway and the production of ԑ-ca-
prolactone. Finally, by decreasing the amount of benzaldehyde in the
reaction medium as a sacrificial agent in the B-V reaction, the yield of ԑ-
caprolactone was declined. In contrast, large amounts of benzoic acid
were produced in the reaction medium, which was converted to benzoic
anhydride during the intermolecular dehydration process [29–31]. It
should be noted that according to all previous reports of the B-V oxi-
dation reaction in the presence of benzaldehyde as a sacrificial agent,
In the present study, the cyclohexanone oxidation reaction was
performed at different times to evaluate the effect of time on the re-
action. As shown in Fig. 4a, when the reaction time increased from 1 to
5
h, the yield of the product increased from 34% to 100% and then
remained constant over longer a duration. A hot filtration test was
performed to confirm the heterogeneity of the catalyst (Fig. 4b). As
shown, after the removal of the catalyst from the reaction medium, the
catalytic process did not progress, which indicated the heterogeneous
nature of the catalyst and the absence of leaching in the catalytically
active sites.
the formation of benzoic acid as
a byproduct was inevitable
[
9–11,24,25,32,33]. However, the formation of benzoic anhydride
during the B-V oxidation reaction was observed for the first time in this
Table 2
Comparative chart of catalytic activity of Cu -SnIV@MIL-101(Cr)/C with other solid catalysts in the B-V oxidation of cyclohexanone using air/O
II
and benzaldehyde.
2
Entry
Catalyst (mg)
substrate/aldehydea
T (°C)
Time (h)
Yield (%)
Oxidant Ref.
1
2
3
4
5
6
7
8
Ketjen Black (5)
MnALPO-36 (150)
Graphite (20)
c-MWCNTs (20)
Fe − Cu bimetal oxide (25)
10:20
1:3
4:4
2:4
2:5
2:4
2:4
2:3
50
50
120
50
50
50
50
50
6
6
3
8
4
3
6
5
91
76.4
99
> 99
> 99
99
> 99.9
100
O
O
O
O
air
O
2
2
2
2
[32]
[33]
[9]
[10]
[25]
[24]
[11]
This work
Cu-MCM-41 (50)
2
-l-dopa-Cu /SnIV@m,mSiO
II
(50)
air
air
Fe
3
O
4
2
II
IV
Cu -Sn @MIL-101(Cr) (50)
a
The mole ratio of cyclohexanone to benzaldehyde.
4

M.K. Alavijeh and M.M. Amini
Catalysis Communications 139 (2020) 105985
study.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.catcom.2020.105985.
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II
IV
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3
4
MIL-101(Cr)/C was successful in several recovery cycles without a
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