Highly selective aerobic oxidation of alcohols to aldehydes over a new Cu(II)-based metal-organic framework with mixed linkers

Article abstract of DOI:10.1016/j.catcom.2017.02.024

A new Cu(II)-based metal-organic framework {[Cu₂(1,2-BDC)₂(Fbtx)₂]·3H₂O}_n (denoted as Cu-FMOF, CCDC-1519440, 1,2-BDC?=?1,2-benzenedicarboxylate, Fbtx?=?1,4-bis(1,2,4-triazole-1-ylmethyl)-2,3,5,6-tetrafluorobenenze) was hydrothermally synthesized and structurally characterized. The Cu-FMOF exhibits a 3D 4-connected coordination framework with the extremely rare (6⁵·8)-hxg-d-4-P2/c topology. Using air as the oxidant, the crystalline Cu-FMOF could be used as a highly efficient heterogeneous catalyst for the TEMPO-assisted oxidation of alcohols to aldehydes. Furthermore, the Cu-FMOF catalyst could be easily recovered and reused for several times without a significant loss of catalytic activity or structural deterioration.

Full text of DOI:10.1016/j.catcom.2017.02.024

Accepted Manuscript
Highly selective aerobic oxidation of alcohols to aldehydes over a
new Cu(II)-based metal-organic framework with mixed linkers
Sheng-Chun Chen, Sheng-Nan Lu, Feng Tian, Nan Li, Han-Yu
Qian, Ai-Jun Cui, Ming-Yang He, Qun Chen
PII:
S1566-7367(17)30075-4
doi: 10.1016/j.catcom.2017.02.024
CATCOM 4952
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
29 November 2016
24 January 2017
23 February 2017
Please cite this article as: Sheng-Chun Chen, Sheng-Nan Lu, Feng Tian, Nan Li, Han-
Yu Qian, Ai-Jun Cui, Ming-Yang He, Qun Chen , Highly selective aerobic oxidation
of alcohols to aldehydes over a new Cu(II)-based metal-organic framework with mixed
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ACCEPTED MANUSCRIPT
Catal. Commun.
Highly selective aerobic oxidation of alcohols to aldehydes over a new Cu(II)-based metal-organic framework
with mixed linkers
Sheng-Chun Chen , Sheng-Nan Lu , Feng Tian , Nan Li , Han-Yu Qian , Ai-Jun Cui , Ming-Yang Hea,b,*, Qun
a
a
b
a
c
a
Chena,*
a
Key Laboratory of Fine Petrochemical Technology, School of Petrochemical Engineering, Changzhou Univer-
sity, Changzhou 213164, China
b
School of Chemical Engineering, Nanjing University of Science & Technology, Nanjing 210094, China
c
School of Pharmacy, China Pharmaceutical University, Nanjing 211198, China

Corresponding author. Fax: 86-519-86330251. Tel: 86-519-86330251.
E-mail address: hemingyangjpu@yahoo.com (Prof. M.-Y. He).
Corresponding author. Fax: 86-519-86330251. Tel: 86-519-86330251.

E-mail address: chenqunjpu@yahoo.com (Prof. Q. Chen).
1

ACCEPTED MANUSCRIPT
Abstract
A new Cu(II)-based metal-organic framework {[Cu2(1,2-BDC)2(Fbtx)2]·3H2O}n (denoted as
Cu-FMOF, CCDC-1519440, 1,2-BDC 1,2-benzenedicarboxylate, Fbtx
,4-bis(1,2,4-triazole-1-ylmethyl)-2,3,5,6-tetrafluorobenenze) was hydrothermally synthesized
=
=
1
and structurally characterized. The Cu-FMOF exhibits a 3D 4-connected coordination framework
5
with the extremely rare (6 ·8)-hxg-d-4-P2/c topology. Using air as the oxidant, the crystalline
Cu-FMOF could be used as a highly efficient heterogeneous catalyst for the TEMPO-assisted
oxidation of alcohols to aldehydes. Furthermore, the Cu-FMOF catalyst could be easily recov-
ered and reused for several times without a significant loss of catalytic activity or structural dete-
rioration.
Keywords: Cu(II) MOF; Aerobic oxidation; Alcohol; Heterogeneous catalysis; Aldehyde
2

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1
. Introduction
The selective oxidation of alcohols to the corresponding carbonyl compounds represents one
of the most important and fundamental transformations in both industry and academia [1–4]. In
these oxidation processes, stoichiometric oxidants such as metal-based oxidants, organic perox-
ides and hydrogen peroxide are often used because they generally provide the oxidation products
in good yield. However, the relatively high costs of these oxidants and their negative environ-
mental impacts have limited their applications in large scale syntheses [5–7]. For this reason, the
utilization of air or molecular oxygen as oxidant has recently attracted a growing interest in al-
cohol oxidation reactions on account of the increasingly environmental and economic concerns
[2, 8–11]. However, because the triplet state characteristics of molecular oxygen impedes its re-
action with hydrocarbons in a singlet ground state [12], the development of more active catalysts,
especially heterogeneous materials, would be of great value for such oxidation processes. In re-
cent years, heterogeneous catalysts based on noble metals, e.g. gold [13], palladium [14], plati-
num [15], and ruthenium [16], have been reported to be active and selective for aerobic oxidation
of alcohols. Given that the noble metals are expensive and scarce, it is important to develop
non-noble metal catalysts to promote alcohol oxidation. Consequently, non-noble transition met-
als immobilized on a variety of insoluble solid supports including clay, zeolite, silica, and hy-
drotalcite as potential catalysts for the selective oxidation of alcohols have been evaluated [17].
Despite these remarkable progresses, constructing an efficient heterogeneous catalyst for the
highly selective oxidation of alcohols with ideal air as the oxidant still remains a great challenge.
Metal-organic frameworks (MOFs), constructed by self-assembly of organic ligands and
metal ions, have emerged as a new type of promising heterogeneous catalysts or catalyst supports
for a broad range of organic transformations [18–21], not only because of their highly ordered
location of single-site active species, but also owing to their merits of separation and recyclability
in the liquid phase. The selective oxidation of alcohols with molecular oxygen under mild condi-
tions, which can be catalyzed by copper-containing MOFs, has been recently reported by several
research groups [22–30]. Garcia et al. presented a benchmark example of Cu-MOFs, using
Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate) as a catalyst in the TEMPO-assisted aerobic
3

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oxidation of primary alcohols [31]. More recently, Kitagawa et al. synthesized a
free-radical-decorated
,7-bis(4-pyridyl)-1,1,3,3-tetramethylisoindolin-2-yloxyl, which efficiently promoted the oxida-
tion of various alcohols with air as the ideal oxidant, achieving a high conversion and selectivity
32]. However, to date, the field of developing highly active MOFs with good stability in both
Cu-MOF
derived
from
4
[
chemical and thermal environments, which can be applied to such oxidation studies, remains
largely unexplored [33]. Moreover, in order for the alcohol oxidation reactions to be practical,
higher conversion and selectivity, lower catalyst loading, and desirable oxidant are also needed.
In this communication, we report the synthesis, structural characterization, and catalytic
properties of a new Cu-based MOF, {[Cu2(1,2-bdc)2(Fbtx)2]·3H2O}n [1,2-bdc
=
1
,2-benzenedicarboxylate, Fbtx = 1,4-bis(1,2,4-triazole-1-ylmethyl)-2,3,5,6-tetrafluorobenenze)],
namely Cu-FMOF. Single-crystal X-ray diffraction analysis revealed that Cu-FMOF features an
extremely rare 3D hxg-d topological network. When air was used as the oxidant, this crystalline
Cu-FMOF was verified to be a highly active, readily recyclable and reusable heterogeneous cat-
alyst for selective oxidation of primary alcohols to the corresponding aldehydes.
2
. Experimental
2
.1. Catalyst synthesis
In a typical preparation, a mixture of Cu(NO ) ·3H O (24.1 mg, 0.1 mmol), Fbtx (31.3 mg,
3
2
2
0
.1 mmol), 1,2-H2bdc (16.6 mg, 0.1 mmol), NaOH (8.0 mg, 0.2 mol/L) and water (6 mL) was
stirred for 30 min at room temperature and then sealed in a 15-mL Teflon-lined stainless steel
container, which was heated to 130 °C and held at that temperature for 3 days. After cooling
down to room temperature, green block-shaped crystals suitable for X-ray diffraction analysis
were obtained in ca. 52% yield (29.6 mg, based on copper nitrate). Anal. Calcd for
–
1
C40H30Cu2F8N12O11: C 42.37, H 2.67, N 14.82%; found: C 43.52, H 2.81, N 14.83%. IR (cm ,
KBr pellet): 3448 (br), 3104 (w), 2963 (w), 2927 (w), 1618 (s), 1577 (m), 1554 (m), 1492 (s),
1
453 (w), 1392 (m), 1371 (s), 1289 (s), 1125 (m), 1085 (w), 1039 (m), 1002 (w), 866 (m), 837
(w), 759 (m), 672 (m).
4

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2
1
.2. Catalytic reaction
In a typical experiment, 4-methoxybenzyl alcohol (12 uL, 0.1 mmol), Cu-FMOF (11.3 mg,
0 mmol %), TEMPO (7.8 mg, 0.05 mmol) and NaCO3 (10.6 mg, 0.1 mmol) in 1 mL of air satu-
rated acetonitrile were taken in a 15-mL three-necked round-bottom flask. The solution was
magnetically stirred for 16 hours at 75 °C under air atmosphere. The progress of the reaction was
monitored via gas chromatography (Shimadzu GC-2010AF) involving a Chromopak capillary
column and flame ionization detector. The products were further confirmed by using gas chro-
matography-mass spectroscopy (GC-MS) (Shimadzu GCMS-2010). The concentration of
4
-methoxybenzyl alcohol and 4-methoxybenzylaldehyde were calibrated by external standard
method with standard samples (see Fig. S1).
3
. Results and discussion
Since 1,2-H2BDC and Fbtx are both insoluble in water at room temperature, the preparation
of Cu-FMOF was indeed achieved by means of hydrothermal reaction. It was necessary to intro-
duce NaOH to deprotonate for 1,2-H2BDC. The reaction was performed at 130 °C, which can
lead to a moderate yield.
3
.1. Structure of {[Cu2(1,2-BDC)2(Fbtx)2]·3H2O}n (Cu-FMOF)
Single-crystal X-ray diffraction analysis reveals that Cu-FMOF crystallizes in a monoclinic
system with the P21/c space group (see Table S1), and shows an extended three-dimensional (3D)
coordination framework. The asymmetric unit is composed of one Cu(II) ion, one 1,2-BDC anion,
two halves of the Fbtx ligands, and one and a half free water molecules. As shown in Fig. S2, the
Cu(II) ion is five-coordinated by three oxygen atoms from two 1,2-BDC anions [Cu1–O1 =
2
.401(2) Å, Cu1–O2 = 2.015(2) Å, Cu1–O3 = 1.928(2) Å] and two nitrogen atoms from two
Fbtx ligands [Cu1–N1 = 1.990(3) Å, Cu1–N4 = 1.980(3) Å] (Table S2), exhibiting a nearly ideal
square-pyramid geometry with a τ value of 0.01. The V-shaped 1,2-BDC ligand adopts a
2-bridging coordination mode connecting the neighboring Cu(II) ions to form the left- and
right-handed helical [Cu(1,2-BDC)] chains with a pitch of 6.217(1) Å along the b-axis (Fig. S3).
n
These helical chains with equal numbers are alternatively arranged. Then the Fbtx ligands having
5

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the trans configuration link with Cu(II) to form line-shaped [Cu(Fbtx)]n chains, which further
connected with the helical [Cu(1,2-BDC)]n chains to result in a 3D framework (Fig. S4). Topo-
logically, each Cu(II) ion is a 4-connected node, and both 1,2-BDC and Fbtx are 2-connected
5
linkers, so the whole framework displays the rare (6 ·8)-hxg-d-4-P2/c topology (Fig. 1) [34].
(Insert Fig. 1 here)
3
.2. Catalytic performance
To study the catalytic activity of Cu-FMOF to identify suitable reaction conditions, initial
oxidation reactions were performed on a 0.1 mmol scale with 4-methoxybenzyl alcohol as the
mode substrate and with TEMPO as oxidizing agent in conventional batch equipment.
Fig. 2 shows the dependence of conversion on the effect of reaction temperature and reac-
tion time (GC analysis is shown in Fig. S5). It was observed that the reaction almost could not
occur at 25 °C with only 3% conversion being detected after 16 h. The reaction carried out at
5
0 °C could afford 28% conversion after 16 h. Increasing the temperature to 75 °C led to full
conversion of 4-methoxybenzyl alcohol after 16 h. Obviously, higher temperature could enhance
the reaction rate, and 75 °C was a relatively optimized temperature.
(Insert Fig. 2 here)
The influence of Cu-FMOF catalyst concentration on the conversion of 4-methoxybenzyl
alcohol was investigated, as illustrated in Fig. 3 (GC analysis is shown in Fig. S6). Initially, the
reaction in the absence of Cu-FMOF catalyst gave a low conversion of 8%. Significantly en-
hanced conversions were observed using Cu-FMOF as catalyst for the aerobic oxidation of alco-
hols. Addition of 10.0 mol% gave complete conversion of 4-methoxybenzyl alcohol after 16 h.
Nearly equal conversion was obtained when 15.0 mol% Cu-FMOF was employed. Thus, the cat-
alyst percentage higher than 10% does not increase the reaction rate. Garcia and co-workers pre-
viously employed 24 mol% Cu3(BTC)2 as the catalyst for the similar oxidation of
4
-methoxybenzyl alcohol, where relatively lower reaction activity was observed [31]. Wang et al.
reported the catalytic oxidation of 4-methoxybenzyl alcohol with 17 mol% nanoscaled
Cu3(BTC)2 proved facile and was accomplished in high conversion (>99%) within 16 h at 75 °C
6

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[24]. The outstanding catalytic performance of Cu-FMOF may be related to the characteristic of
the five coordinated Cu(II) ions and mixed linkers [35].
(Insert Fig. 3 here)
.
In our next study, the influence of base additives (Na2CO3, NaCl, Cs2CO3, K3PO4 3H2O, and
t-BuOLi) on the reaction was investigated, as shown in entries 1–5 (see Table 1). It was found
.
that Na2CO3, Cs2CO3 and K3PO4 3H2O appear to be the preferred bases over NaCl and t-BuOLi,
resulting in the formation of the desired product with high conversion (88–99%) and selectivity
(79–95%). Furthermore, experiments with various solvents, including acetonitrile, 1,4-dioxane,
dimethylformamide (DMF), N-methyl pyrrolidone (NMP), and toluene, were conducted with
Cu-FMOF and the results (see Table 1, entries 1 and 6–9) indicate that acetonitrile (both conver-
sion and selectivity of 99%) is the best solvent, whereas the worst one is toluene (conversion of
2
7% and selectivity of 88%). In our previous work, we reported the efficient oxidative coupling
reactions using the Cu-doped zeolite X as heterogeneous catalyst [36]. In this contribution, com-
parison of catalytic activities of Cu-doped zeolites including Cu-Ets-10, Cu-ZSM-5 and Cu-Beta
in the above-mentioned model reaction with that of Cu-FMOF revealed that these zeolite cata-
lysts have lower performance (see Table 1, entries 10–12).
(Insert Table 1 here)
Under the optimized conditions in hand, we subsequently evaluated the substrate scope of
the TEMPO-assisted selective alcohol oxidation over the Cu-FMOF material under air atmos-
phere (Table 2). The results showed that a large variety of benzylic alcohols are effective sub-
strates in this oxidation system. Various benzylic alcohols bearing electron-donating or
–
withdrawing functional groups afforded the corresponding substituted aldehydes in good to ex-
cellent yields within 16 h, although the electron-withdrawing substituents have a negative effect
on the reactivity. In addition, somewhat lower oxidation yield were observed when 1- and
2
-naphthalenemethanol were employed as the substrates under the same conditions. Compared
with primary alcohols, the aerobic oxidation of secondary alcohols gave much lower oxide yields
under similar reaction conditions. For example, the oxidation of
-methoxy-α-methyl-benzenemethanol resulted in only 37% conversion even by extending the
4
7

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reaction time to 72 h (GC analysis is shown in Fig. S7). Wang et al. reported the oxidation of
secondary alcohols to the corresponding ketones catalyzed by a trinuclear Cu(II) complex with
H2O2 as the source of oxygen [37]. Very recently, efficient Cu(II)-MOF-based catalytic system
has been reported by Pombeiro et al., where aerobic and peroxidative (with
tert-butylhydroperoxide) oxidations of various secondary alcohols have been realized by a sol-
vent-free microwave assisted procedure [38]. Nevertheless, applying MOF-based heterogeneous
catalysts in aerobic oxidation of secondary alcohols to the corresponding ketones still remains a
challenge.
(Insert Table 2 here)
3
.3. Catalyst stability and recycling
The thermal stability of Cu-FMOF was examined by thermogravimetric analysis (TGA) in
the N2 atmosphere from 30–800 °C, as shown in Fig. S8. Up heating, Cu-FMOF was stable up to
00 °C, which was confirmed by the temperature-variable PXRD analyses (Fig. S9). To assess
2
the catalyst stability and recyclability, the alcohol oxidation reactions were performed under the
same conditions as described above except for the use of the recovered catalyst. After the reac-
tion, the resulting mixture was cooled and the Cu-FMOF catalyst was collected by centrifugation
and washed with water and ethanol. After drying at 100 °C for 6 h in a vacuum oven, the recy-
cled Cu-FMOF can be reused as catalyst for the next run under the same reaction conditions. The
results included in Fig. 4 indicate that the recovered Cu-FMOF catalyst almost maintained its
high activity with almost no decrease after five consecutive cycles of experiments. After 4 h of
reaction, the catalyst was filtered out and the results showed that no further product was formed
even after 16 h under the same conditions (see Fig. S10), inferring that no leaching occurred to
the catalyst and the process should be truly heterogeneous. To further confirm the stability of the
catalyst, FT-IR and PXRD analyses were made. The similarity of the FT-IR spectrum (Fig. S11)
and PXRD patterns (Fig. S12) of the recycled and fresh catalyst strongly suggested catalyst sta-
bility under the reaction conditions. To determine the oxidation state of copper before and after
reactions, XPS experiments have been carried out (Fig. S13). The observation of the Cu 2p3/2
peak at 934.50 eV demonstrated no change in the oxidization state for copper, further implying
8

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Cu-FMOF is stable during the reaction processes.
Insert Fig. 4 here)
(
3
.4. Proposed mechanism
As shown in Fig. 5, the proposed mechanism was raised based on previous literature. First,
the alcohol substrate reacts with the Cu(II)-FMOF A to form the Cu(II)-FMOF intermediate B,
involving initial coordination of alcohol to the Cu(II) ion. Second, intermediate B can react with
the TEMPO radical to form the radical adduct C. Consequently, H-atom abstraction from the
α-carbon atom by TEMPO (from C to D) should be the rate-determining step (RDS) in the reac-
tion, resulting in the formation of the aldehyde product together with Cu(I)-FMOF E and
1
-hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH). The Cu(II)-FMOF A is regenerated by the
TEMPO-assisted oxidation of Cu(I) to Cu(II) frameworks after the spontaneous oxidation of
TEMPOH back to TEMPO under O2, facilitating the completion of the catalytic cycle.
(Insert Fig. 5 here)
In addition, the SEM image reveals that the Cu-FMOF particles have block morphology
with size particle sizes of 50 ×100 m (Fig. S14). BET surface area and pore size distribution
2
have been measured. The results show that the Cu-FMOF has a very small surface area of 1.6 m
–
1
g
and such framework has no accessible pore distribution which is consisted with the void
space analysis as calculated by PLATON. Thus, only exterior reaction occurred in this system.
4
. Conclusion
In summary, a new Cu(II)-based framework FMOF with mixed linkers was synthesized by a
hydrothermal method, and characterized by several techniques including single-crystal diffrac-
tion, PXRD, FT-IR, TG, XPS, SEM and nitrogen adsorption measurements. The FMOF with co-
ordination centers not only presented a rare 3D 4-connected hxg network, but also showed a
highly efficient catalyst for the selective oxidation of alcohols to aldehydes assisted by TEMPO
under air. The stability and recyclability of the FMOF catalyst were also studied. Further inves-
tigations on the mixed-linker MOFs in various organic transformations are currently under way
in our laboratory.
9

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Acknowledgements
We gratefully acknowledge the financial support from the National Natural Science Founda-
tion of China (U1463203 and 21676030), and the project of Industry-University-Research in
Jiangsu Province, China (BY2014037-17 and BY2016029-08).
Appendix A. Supplementary data
Supplementary data associated with this article can be found in the online version at
http:dx.doi.org/10.1016/j.catcom.2016.xx.xxx.
CCDC 1519440 contains the supplementary crystallographic data for FMOF. These data can
be obtained free of charge via www.ccdc.can.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Centre, 12 Union Road, Cambridge CB21EZ, UK; Fax: (+44) 1223–336033; or
deposit@ccdc.cam.ac.uk).
1
0

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Table 1
[
a]
Optimization of the reaction conditions.
Entry Catalyst
Base
Solvent
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Dioxane
DMF
Conv. (%) Select. (%)
1
2
3
4
5
6
7
8
9
Cu-MOF
Cu-MOF
Cu-MOF
Cu-MOF
Cu-MOF
Cu-MOF
Cu-MOF
Cu-MOF
Cu-MOF
Na2CO3
NaCl
Cs2CO3
K3PO4
t-BuOLi
Na2CO3
Na2CO3
Na2CO3
Na2CO3
>99
36
88
95
9
>99
89
85
96
24
87
61
90
88
16
53
28
81
49
99
27
15
73
38
NMP
Toluene
CH3CN
CH3CN
CH3CN
[
b]
1
1
0
Cu-Ets-10 Na2CO3
Cu-ZSM-5 Na2CO3
Cu-Beta
[
c]
1
[
d]
1
2
Na2CO3
[
a] Reaction conditions: 4-methoxybenzyl alcohol (1 mmol), catalyst (113 mg, 10 mol%), TEMPO (0.5
equiv), base (1 equiv), acetonitrile (5 mL), 16 h, 75 °C, air atmosphere. [b] With 7.6 mg Cu-Ets-10 (1.9 %
mmol Cu). [c] With 6 mg Cu-ZSM-5 (1.9 % mmol Cu). [d] With 9.5 mg Cu-Beta (1.9 % mmol Cu).
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Table 2
[
a]
Aerobic oxidation of alcohol by using Cu-FMOF.
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[
2 3
a] Reaction conditions: alcohol (1 mmol), Cu-FMOF (113 mg, 10 mol%), TEMPO (0.5 equiv), Na CO (1
equiv), acetonitrile (5 mL), 16 h, 75 °C, air atmosphere. [b] Isolated yield.
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Caption to figures
Fig. 1
Fig. 2
Fig. 3
Perspective view of the topological network of Cu-FMOF.
Effect of reaction temperature on reaction conversions.
Effect of catalyst concentration on reaction conversions.
Catalyst recycling studies.
Fig. 4
Fig. 5
Proposed reaction mechanism for the TEMPO-assisted oxidation of alcohol as-
suming the five-coordinated Cu(II) ions of Cu-FMOF as the active species.
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Figure 1
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Figure 2
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8

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Figure 3
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9

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Figure 4
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0

ACCEPTED MANUSCRIPT
Figure 5
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1

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Graphical abstract
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Highlights


A new Cu(II)-based MOF with mixed linkers is hydrothermally synthesized.
Highly selective aerobic oxidation of primary alcohols to aldehydes over the Cu-FMOF
is presented.

Excellent stability and reusability of Cu-FMOF as a heterogeneous catalyst is demon-
strated.
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