A novel modified MIL-101-NH2 ligand for CuI-catalyzed and air promoted oxidation of secondary alcohols

Article abstract of DOI:10.1039/c7ra00296c

An efficient Cu(i)-catalyzed aerobic alcohol oxidation system was developed utilizing a novel metal-organic framework (MOF) ligand at room temperature. Several relatively inert secondary alcohols were converted to their corresponding ketones in high yields and selectivities in the presence of a Cu(i) catalyst and air as the oxidant. This newly developed Cu(i)/MOF ligand system can also be easily extended to the aerobic oxidation of primary alcohols including aliphatic ones. Furthermore, the MIL-101-N-2-pyc ligand can be recycled several times without compromising reaction activity.

Full text of DOI:10.1039/c7ra00296c

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A novel modified MIL-101-NH2 ligand
for CuI-catalyzed and air promoted
oxidation of secondary alcohols†
Cite this: RSC Adv., 2017, 7, 22353
a
c
Hui Liu,a Daniele Ramella,b Peng Yu
and Yi Luan
*
*
An efficient Cu(I)-catalyzed aerobic alcohol oxidation system was developed utilizing a novel metal–organic
framework (MOF) ligand at room temperature. Several relatively inert secondary alcohols were converted to
their corresponding ketones in high yields and selectivities in the presence of a Cu(I) catalyst and air as the
oxidant. This newly developed Cu(I)/MOF ligand system can also be easily extended to the aerobic oxidation
of primary alcohols including aliphatic ones. Furthermore, the MIL-101-N-2-pyc ligand can be recycled
several times without compromising reaction activity.
Received 8th January 2017
Accepted 10th March 2017
DOI: 10.1039/c7ra00296c
rsc.li/rsc-advances
Furthermore, oxidation of secondary alcohols using oxygen or
air remains a major challenge.
Introduction
Metal–organic frameworks (MOFs) are attractive micropo-
rous materials for a wide range of applications in gas absorp-
tion, chemical sensing, molecular separation, drug delivery and
catalysis.12 Thanks to the ability of the organic linkers to be
post-synthetically modied, MOFs are versatile platforms for
a variety of transition metal-based catalytic systems.13 Stanley
and Huang has developed a bipyridine MOF for the Pd(II)
coordination and utilized the complex for an efficient Suzuki–
Miyaura cross-coupling reaction.14 Given its very versatile
nature, the N,N0-chelating ligand was employed in the oxidation
of several secondary alcohols.15 Due to the toxicity and carci-
nogenicity of the commonly used N,N0-bidentate ligands, recy-
clable MOF/N,N0-ligands are desirable from a green chemistry
point of view. Furthermore, their use lowers the cost of the
oxidation process.16 However, the synthesis of an ideal solid
ligand for copper salt immobilization featuring N,N0-chelating
Efficient and selective oxidation of alcohols to aldehydes or
ketones is a fundamental step in the synthesis of several natural
products and other useful compounds.1 In the past, there have
been several well-known selective oxidation processes realized
in an industrial setting, such as the preparation of cumene
peroxide, acetaldehyde, and propylene oxide. Among the several
oxygen sources available for the selective alcohol oxidation
reaction, molecular oxygen would be the most environmentally
and economically advantageous.2 During the past few decades,
copper complexes have overcome other and noble metals such
as Pd,3 Au,4 Ru5 and Rh6 as inexpensive, environmentally benign
and efficient transition metal catalysts for the aerobic alcohol
oxidation.7 Stahl and co-workers reported systems formed by
[Cu(MeCN)4]X (X ¼ TfO, BF4, or PF6) and 2,20-bipyridine (bpy)
ligand for primary alcohol oxidation in the presence of air8
´
(Fig. 1). Marko also developed a catalytic system, based on 1,10-
phenanthroline as the ligand, CuCl as the catalyst and DBADH2/
O2 as the oxidizer, which smoothly transformed secondary
alcohols into the desired ketones in toluene (Fig. 1).9 Several
other organic ligands were also studied in combination with
copper salts for similar processes.10 Although many catalytic
systems did display satisfactory efficiency for alcohol oxidation,
only a very limited number of successful examples of aerobic
oxidations of secondary alcohols has been reported to date.11
aScience College of Hunan Agricultural University, Hunan, 410128, China. E-mail:
pengy7505@hunau.net; Tel: +86-731-84617022
bTemple University-Beury Hall, 1901, N. 13th Street, Philadelphia, PA 19122, USA
cSchool of Materials Science and Engineering, University of Science and Technology
Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, P. R. China. E-mail:
yiluan@ustb.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra00296c
Fig.
1 General reaction scheme for the aerobic oxidation of
secondary alcohols.
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ligand for efficient aerobic secondary alcohol oxidation is still
a challenge.17
In this study, we wish to report a novel MOF/CuI combina-
tion for the effective oxidation of secondary alcohols using air as
the stoichiometric oxidant. A newly designed iminopyridine
derived MIL-101-N-2-pyc MOF was utilized as the solid and
recyclable ligand; it complexes with the copper ion from CuI to
form the CuI/MIL-101-N-2-pyc catalyst in situ. To our knowl-
edge, this is the rst example of utilization of an MOF solid
ligand in the oxidation of alcohols using air as a free and stoi-
chiometric oxidant. Air is a more challenging oxidant than
others because it is a low concentration form of molecular
oxygen. The CuI/MIL-101-N-2-pyc MOF developed as a highly
efficient aerobic oxidation catalyst for secondary alcohols could
also be easily utilized for the oxidation of primary alcohols. In
addition, the MIL-101-N-2-pyc solid ligand offers the great
advantage of being recyclable.
Fig. 3 SEM images of (a and b) MIL-101-NH2 and (c and d) MIL-101-N-
2-pyc.
Results and discussion
The post-synthetically modied MIL-101-NH2 was charac-
terized by proton nuclear magnetic resonance (1H NMR) to
determine its modication ratio (Fig. 5 and S2†). Nuclear
magnetic resonance studies of the digested sample indicated
that 24% of the amino groups are functionalized as aromatic
iminopyridine. The amount of ligand usage is calculated
according to the amount of functionalized amino groups based
on the 1H-NMR results. However, FT-IR spectra failed to provide
any evidence for internal structural alternation of MIL-101-NH2;
this is probably due to the weakness of the peak corresponding
to the iminopyridine group and to its low modication ratio
(Fig. S3†).
The MIL-101-NH2 used for post-synthetic modication in this
project was obtained through a one-step synthetic procedure. This
procedure furnished large quantity of MIL-101-NH2 directly from
a chromium salt and 2-aminoterephthalic acid (NH2-H2BDC).
Then, the facile synthesis of MIL-101-N-2-pyc was accomplished
by reacting MIL-101-NH2 with 2-pyridinecarbaldehyde; a similar
strategy was recently developed by Yaghi and co-workers (Fig. 2).18
As shown in Fig. 1 and S1,† the iminopyridine moiety on MIL-101-
N-2-pyc bears no electron-withdrawing group, providing an ideal
coordination environment for a copper salt.
Scanning electron microscopy (SEM) of MIL-101-NH2 and
MIL-101-N-2-pyc are shown in Fig. 3. The crystals of MIL-101-N-
2-pyc MOF appear to have a spherical morphology with diam-
eters of around 150 nm, which was retained aer the post-
synthetic modication with 2-pyridinecarbaldehyde. The
powder X-ray diffraction (pXRD) pattern of MIL-101-NH2 was in
agreement with the literature data.19 As expected, the pXRD
patterns observed for the MIL-101-N-2-pyc material possessed
the same crystalline reections as its precursor MIL-101-NH2.
The high similarity in pXRD patterns indicated the structural
topology of MIL-101-N-2-pyc throughout the post-synthetic
modication was maintained (Fig. 4). Not surprisingly, salicy-
laldehyde modied MIL-101-NH2 also showed the same XRD
pattern as expected (Fig. 4c).
The MIL-101-NH2 and MIL-101-N-2-pyc were then examined
by thermal gravimetric analysis (TGA) to conrm the thermal
and structural stability of iminopyridine derived MOF ligand.
Both MIL-101-NH2 and MIL-101-N-2-pyc showed good thermal
ꢁ
stability with a weight loss at ꢀ372 C as observed on the TGA
Fig. 2 Schematic illustration of the synthesis of the MIL-101-N-2-pyc Fig. 4 pXRD of (a) MIL-101-NH2, (b) MIL-101-N-2-pyc and (c) MIL-
ligand.
101-N-sal.
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Fig. 5 1H NMR spectra of MIL-101-NH2 and MIL-101-N-2-pyc.
curve. This observation indicates that the post-synthetic modi-
cation with 2-pyridinecarbaldehyde does not result in struc-
tural instability (Fig. 6).
The high porosity of the parent MIL-101-NH2 was conrmed
by nitrogen adsorption isotherms. Also, the porosity is retained
aer post-synthetic modication, even though the surface area
decreased due to occupation by the imine functional groups of
the pores. As a result, MIL-101-N-2-pyc has a calculated surface
area of 1352.14 m2 gꢂ1 as determined by the Brunauer–Emmett–
Teller (BET) method. Although the BET surface area is reduced
from the measured value of 1998.58 m2 gꢂ1, for the parent
chromium derived MIL-101-NH2 (Fig. 7), the surface area
reduction of MIL-101-N-2-pyc has no negative impact on the
excellent catalytic activity of the MIL-101-N-2-pyc ligand.
A solvent screening was performed in order to determine the
optimal reaction conditions for the oxidation of 1-phenylethan-
1-ol at room temperature. It was not surprising to observe the
low yield of the reaction when run in toluene or ethanol since
such solvents do not usually function well under aerobic
oxidation reaction conditions (Table 1, entries 1 and 2).22 It was
noticed that acetonitrile is the most suitable solvent for this
Fig. 7 Nitrogen adsorption/desorption isotherms of MIL-101-NH2
(left) and MIL-101-N-2-pyc (right).
transformation, even though DMF offered better catalytic
results in terms of conversion and yield (Table 1, entries 3 and
4). Although not surprising, it was highly interesting to observe
the superior catalytic result obtained with the use of DMF as
solvent, since excellent results of DMF as a solvent for different
catalytic systems has been reported in literature,20 although the
strong basic and ligand effects of DMF were not thoroughly
studied. In addition, the disadvantage of the use of DMF as
solvent is signicant since the removal of high boiling point
DMF during the catalytic reaction is tedious, and because of
DMF's known carcinogenicity. With the optimal reaction
conditions in hand, a variety of organic ligands was further
evaluated, either in homogeneous or heterogeneous form. 2,20-
Bipyridine L1 functioned as an excellent homogeneous organic
ligand in the reaction and its role and mechanism were studied
in detail by Stahl (Table 1, entry 5).21 The reaction is sluggish in
the absence of the 2,20-bipyridine ligand, indicating the crucial
chelating effect the ligand has on CuI during the catalytic
process (Table 1, entry 6). Unfortunately, the electron-withdraw
functional group on the 2,20-bipyridine ligand diminishes the
yield of the desired product (L2, Table 1, entry 7). Since the
carboxylate moiety is crucial for the formation of the MOF
structure, we turned our attention towards looking for other
possible ligands. N,O ligands, such as proline L3 and 2-pyr-
idinecarbaldehyde L4 showed not to be suitable ligands for
Cu(I) promoted aerobic oxidation reaction (Table 1, entries 8
and 9). Salicylaldehyde L5 also functioned poorly in the
Fig. 6 TGA of MIL-101-NH2, MIL-101-N-2-pyc and MIL-101-N-sal.
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Table 1 Copper(I) ligand and solvent optimization for secondary
carboxylate electron-withdraw groups. As a result, the M3 (Cr)
ligand, which utilized 2-pyridinecarbaldehyde for the PSM of
MIL-101-NH2, gave the best yield (95%) in presence of air under
room temperature (Table 1, entry 13). This observation strongly
supported our catalyst design strategy, which furnishes the
successful and efficient solid ligand M3 (Cr). In the end, sali-
cylaldehyde functionalized MIL-101-NH2 was not able to ach-
ieve a similarly high yield, presumably because of the poor
coordination ability originated from the phenol group (Table 1,
entry 14). CuBr and CuCl were much less efficient than CuI and
they were excluded from further catalytic studies (Table 1,
entries 15 and 16). In summary, MIL-101-N-2-pyc (M3 (Cr)) was
chosen as the most optimal solid ligand, in combination with
CuI for further catalytic studies. Our catalytic system showed
great efficiency under extremely mild reaction conditions and
using air as the oxidant. The reaction time and catalyst loading
can be further lowered under elevated reaction temperatures;
the observed catalytic activity is superior to that of several
systems reported in literature for processes run under similar
reaction condition (Table S1†).
alcohol oxidationa
As shown in Table 2, various substituted secondary alcohols
were evaluated using the MIL-101-N-2-pyc/CuI system in
combination with TEMPO and N-methylimidazole (NMI).
Electron-rich phenylethanols were oxidized to their corre-
sponding ketones in excellent yields and selectivities (Table 2,
entries 1–3). Electron-decient phenylethanols instead afforded
lower yields even under extended reaction times; this is likely
due to the lower electron-density of the benzylic carbon (Table
2, entries 4–6). Furthermore, para-phenylphenylethanol was
evaluated as an electron-rich bulky alcohol; excellent yield and
selectivity were observed (Table 2, entry 7). This suggests that
our catalytic system is compatible with relatively larger alcohols.
However, biphenylmethanol gave a lower yield due to its bulk-
iness (Table 2, entry 8). The decrease in yield for bulky
substrates was also observed in other MOF catalytic studies; it
was explained as due to the insufficient pore size of the 3D
framework structure.
Entry
Copper
Ligand
Solvent
Yieldb
1
2
3
4
5
6
7
8
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuBr
CuCl
L1
L1
L1
—
L1
—
L2
L3
PhCH3
EtOH
DMF
27%
34%
99%
99%
97%
23%
<5%
3%
19%
10%
25%
34%
95%
72%
28%
17%
DMF
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
9
L4
L5
10
11
12
13
14
15
16
M1 (Zr)
M2 (Cr)
M3 (Cr)
M4 (Cr)
M3 (Cr)
M3 (Cr)
a
5 mol% Cu catalyst, 5 mol% ligand, 5 mol% TEMPO, 10 mol% NMI in
b
the presence of air at room temperature for 6 h. Yield was calculated
based on the GC-MS conversion and selectivity.
Table 2 Alcohol oxidation employing MIL-101-N-2-pyc/CuI systema
oxidation of secondary alcohols (Table 1, entry 10). As a result,
only N,N0-compounds were determined to be efficient ligands
for CuI catalysis and we focused our efforts on synthesizing
a solid MOF ligand bearing a N,N0 bidentate ligand.
MOF M1 was synthesized according to the literature proce-
dure.23 It was not surprising to observe an extremely poor
reaction yield provided by M1 ligand (Table 1, entry 11), since
the electron-withdrawing effect was investigated in entry 7 of
Table 1. This observation excludes the further utilization of M1
ligand for aerobic oxidation studies of secondary alcohols. MIL-
101-NH2 M2 only provided a moderate yield since the aromatic
amino group is not a good coordinating functional group for
Cu(I) (Table 1, entry 12). With all this information in hand, we
intended to synthesize a MOF structure as the heterogeneous
Cu(I) ligand, in which the pyridine moiety was not affected by
Entry
R1
R2
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
4-CH3C6H4
4-CH3OC6H4
3-CH3OC6H4
4-FC6H4
4-ClC6H4
3-CF3C6H4
4-PhC6H4
Ph
CH3
CH3
CH3
CH3
CH3
CH3
CH3
Ph
6
6
6
12
12
12
6
94
99
99
90
85
71
93
67
12
a
Reaction condition: 5 mol% CuI, 5 mol% ligand, 5 mol% TEMPO, 10
mol% NMI in the presence of air at room temperature for 6 h.
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Under similar reaction conditions, the MIL-101-N-2-pyc/CuI N-2-pyc ligand and CuI). Compromised yield and similar
system was further utilized in the aerobic oxidation of several selectivity was observed for the 1-phenylethan-1-ol oxidation
primary alcohols over a shorter reaction time (Fig. 8). The reaction. As we expected, the yield of acetophenone product
aromatic alcohols, either bearing electron-donating or electron- remained to be 65% aer ve reaction cycle, which is almost
withdrawing groups, reacted smoothly using the MIL-101-N-2- identical as the yield aer the rst reaction cycle. The strong
pyc/CuI system in presence of air for 1 h (Fig. 8, aldehydes covalent bond between the iminopyridine moiety and the
a1–a3). In addition, (E)-3-phenylprop-2-en-1-ol was evaluated as aromatic group ensures stability to the ligand during the reac-
an enol and a yield of 98% was achieved (Fig. 8, aldehyde a4). tion. A hot ltration test was performed and the oxidation
Cyclohexylmethanol and octan-1-ol were tested as aliphatic reaction stopped aer the solid ligand was isolated from the
primary alcohols but only moderate yields were observed (Fig. 8, reaction solution (Fig. S6†). This observation indicated that the
aldehydes a4 and a5). It can be concluded that the MIL-101-N-2- MIL-101-N-2-pyc ligand is crucial for the reaction and that there
pyc/CuI system is highly efficient for the oxidation of primary was no leaching of the iminopyridine moiety. Also, the X-ray
alcohols under extremely mild reaction conditions.
powder diffraction pattern of the MIL-101-N-2-pyc ligand
The MIL-101-N-2-pyc solid ligand was recycled for ve times remains unvaried even aer ve catalytic cycles (Fig. S4†). These
in order to examine its recyclability and chemical stability. The facts clearly demonstrate that there was no alternation in
ligand was centrifuged from the reaction mixture and then internal MOF structure or decomposition over the aerobic
washed with CH3CN. In this study, the high efficiency of the oxidation process (Fig. S5†).
MIL-101-N-2-pyc ligand was maintained beyond the h reac-
tion cycle (Fig. 9). Furthermore, the recycling experiment was
Conclusions
also conducted at lower catalyst loading (2.5 mol% of MIL-101-
In conclusion, a novel 2-pyridinecarbaldehyde functionalized
amino MOF was synthesized and utilized as a solid and recy-
clable ligand for Cu(I) catalysis. The newly synthesized MIL-101-
N-2-pyc showed comparable efficiency with regard to synthetic
ease and for the convenience of using the 2,20-bipyridine ligand.
An efficient aerobic alcohol oxidation system was designed for
secondary alcohols based on the combination of CuI and of the
solid MIL-101-N-2-pyc ligand under mild reaction conditions. A
variety of primary and secondary alcohols was successfully
oxidized to ketones under the optimized reaction conditions in
high yields and selectivities. To our knowledge, it is the rst
example of secondary alcohol oxidation using a copper salt,
a solid ligand and molecular oxygen in air as the oxidant. This
study also highlights the use of air as a green and inexpensive
oxidant and the recyclability of the MOF ligand.
Fig.
8 Oxidation of primary alcohol using MIL-101-N-2-pyc/CuI
system.
Experimental section
All the chemicals were used without further purication.
Preparation of MIL-101-N-2-pyc ligand
The synthesis of amino-functionalized MIL-101 was done
according to the literature procedure.17 1.39 g of MIL-101-NH2
(1.0 mmol based on MW of 1392 g molꢂ1, with 7 amino groups)
were suspended in 15 mL of CH3CN; 21.0 mmol of 2-pyr-
idinecarbaldehyde (2.25 g, 21.0 mmol) were then added. The
mixture was stirred slowly at 40 ꢁC for 12 h, aer which the
solvent was decanted. Fresh CH3CN (10 mL) was used to rinse
the crystals once a day for three days. The crystals were dried
ꢁ
under vacuum at 80 C before use.
Catalytic selective oxidation of alcohols
In general, the catalytic properties of the catalysts were exam-
ined for the alcohol oxidation with air in a 25 mL round-bottom
ask. In a typical process, a mixture of acetonitrile (5.0 mL) and
alcohol (1.0 mmol) was added into a 25 mL round-bottom ask,
Fig. 9 The recycling evaluation of MIL-101-N-2-pyc ligand in the
oxidation of 1-phenylethan-1-ol.
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together with 5 mol% of the chosen copper species, 5 mol% of
ligand, 5 mol% of TEMPO, and 10 mol% of NMI. Aer a certain
reaction time, n-dodecane (0.2 mmol) was added as an internal
standard for the determination of yield and selectivity. The
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1
ltered liquid samples were analyzed by GC-MS and H-NMR.
MIL-101-N-2-pyc ligand recycling
At the end of each oxidation reaction cycle, the MIL-101-N-2-pyc
ligand was recovered by centrifugation of the solution mixture
followed by washing with 5–10 mL of CH3CN. Aer being
immersed in the solvent for 12 h and dried at 40 ꢁC under
vacuum for 12 h, the MIL-101-N-2-pyc solid ligand was reused.
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Material characterization
Scanning electron microscope (SEM) images of samples were
obtained with a ZEISS SUPRA 55 (the samples for the SEM
measurements were rst dispersed in ethanol, sonicated for
a few minutes, and then supported onto the silicon slice and the
holey carbon lm on a Cu grid, respectively). The BET surface
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AUTOSORB-1C analyzer. The phase composition of the prod-
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radiation, l ¼ 0.1542 nm) via a M21X diffractometer. Fourier
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6700 spectrometer. Thermogravimetric analysis (TGA) was
conducted by using a Netzsch STA449F at a heating rate of 10 ꢁC
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Acknowledgements
This work is supported by Beijing Natural Science Foundation
(2172037) and National Natural Science Foundation of China
(No. 51503016). P. Y. thanks the State Key Laboratory of
Chemical Resource Engineering and Y. L. thanks the Funda-
mental Research Funds for the Central Universities (Grant No.
FRF-TP-16-004A3) for funding support.
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