A Cu-Doped ZIF-8 metal organic framework as a heterogeneous solid catalyst for aerobic oxidation of benzylic hydrocarbons

Article abstract of DOI:10.1039/c9nj03698a

Mixed-metal metal organic frameworks have received considerable attention in recent years and it has been shown that the activity of the parent metal organic framework (MOF) is often enhanced upon doping with external metal ions within the framework. In this context, Cu²⁺ ions with different loadings were incorporated within the ZIF-8 framework to obtain a series of Cu-doped ZIF-8 materials and their activity was examined in the aerobic oxidation of hydrocarbons. The as-synthesized Cu-doped solids were characterized by powder X-ray diffraction (XRD), ultraviolet diffuse reflectance spectroscopy (UV-DRS), scanning electron microscopy (SEM), Fourier Transform infrared (FT-IR), electron paramagnetic resonance (EPR) and inductively coupled plasma (ICP) analysis. The experimental results revealed that the activity of Cu-doped ZIF-8 is much higher than that of the parent ZIF-8 in all the tested substrates at 120 °C. Furthermore, the activity of the Cu-doped ZIF-8 with the highest Cu loading was eight fold higher than that of the parent ZIF-8 in the aerobic oxidation of cyclooctane (1) at 120 °C with more than 80% selectivity to the corresponding cyclooctanol/cyclooctanone (ol/one) mixture. Cu-doped ZIF-8 was reused two times with no significant drop in its activity under identical conditions. Furthermore, comparison of the two times reused solid with that of the fresh solid by powder XRD and SEM analysis revealed identical structural integrity and morphology, respectively during the oxidation reactions.

Full text of DOI:10.1039/c9nj03698a

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A Cu-Doped ZIF-8 metal organic framework as a
heterogeneous solid catalyst for aerobic oxidation
of benzylic hydrocarbons†
Cite this: New J. Chem., 2019,
43, 18702
Nagarathinam Nagarjun and Amarajothi Dhakshinamoorthy
*
Mixed-metal metal organic frameworks have received considerable attention in recent years and it has
been shown that the activity of the parent metal organic framework (MOF) is often enhanced upon
doping with external metal ions within the framework. In this context, Cu2+ ions with different loadings
were incorporated within the ZIF-8 framework to obtain a series of Cu-doped ZIF-8 materials and their
activity was examined in the aerobic oxidation of hydrocarbons. The as-synthesized Cu-doped solids
were characterized by powder X-ray diffraction (XRD), ultraviolet diffuse reflectance spectroscopy
(UV-DRS), scanning electron microscopy (SEM), Fourier Transform infrared (FT-IR), electron paramagnetic
resonance (EPR) and inductively coupled plasma (ICP) analysis. The experimental results revealed that the
activity of Cu-doped ZIF-8 is much higher than that of the parent ZIF-8 in all the tested substrates at
120 1C. Furthermore, the activity of the Cu-doped ZIF-8 with the highest Cu loading was eight fold
higher than that of the parent ZIF-8 in the aerobic oxidation of cyclooctane (1) at 120 1C with more than
80% selectivity to the corresponding cyclooctanol/cyclooctanone (ol/one) mixture. Cu-doped ZIF-8 was
reused two times with no significant drop in its activity under identical conditions. Furthermore,
comparison of the two times reused solid with that of the fresh solid by powder XRD and SEM analysis
revealed identical structural integrity and morphology, respectively during the oxidation reactions.
Received 16th July 2019,
Accepted 31st October 2019
DOI: 10.1039/c9nj03698a
rsc.li/njc
energy of C–H bonds and increased reactivity of primary oxidation
products. A great deal of effort has been devoted to developing
1. Introduction
Selective oxidation of hydrocarbons to their respective alcohols/ heterogeneous catalysts that can selectively oxidize the C–H bond
ketones (ol/one) with high selectivity is one of the most challenging in hydrocarbons with very high selectivity of ol/one under
tasks in organic transformations due to the possible secondary sustainable reaction conditions.14,15
reactions of the initially formed ol/one mixture.1–3 In spite of a
MOFs are crystalline porous materials whose crystal structure
large number of catalytic systems developed, one of the issues to is constructed between metal ions or clusters interconnected by
be addressed in this area is the formation of low selectivity of the rigid organic linkers to form one, two, or three dimensional
ol/one products.4,5 In contrast, a highly selective and efficient networks.16,17 MOFs are characterized by their promising properties
C–H bond oxidation is a favorable process using enzymes and such as high specific surface area, high porosity, high transition
biomimetic systems.6,7 On the other hand, large numbers of metal content, low density and chemically tunable structures.16,18–22
transition metal-based homogeneous catalysts have been reported These promising properties have encouraged researchers to employ
for the oxidation of C–H bonds in the past few decades.8–12 A wide these solid materials for a wide range of applications including gas
range of metal catalysts was employed for oxidation of hydro- separation,23 gas storage,24 catalysis,15,25 drug delivery,26–28 electro-
carbons using KHSO5, KMnO4, PhIO, dichromate, peroxides, chemical applications29 and oxidation reactions.30,31
molecular oxygen and peroxy acids as oxidants. Among them,
Among the various robust MOFs reported so far, zeolite
oxygen and hydrogen peroxide were considered to be beneficial imidazolate frameworks (ZIFs) have emerged as a unique and
since they are environmentally benign and water is formed as the intriguing class of MOFs, combining the properties of both
only by-product.13 The selective oxidation of a C–H bond is a from zeolites and MOFs.32 Out of the various ZIF-based MOFs,
difficult reaction in catalysis due to the high bond dissociation ZIF-8 is one of the most representative materials in which ZnN4
tetrahedra are composed of divalent zinc cations linked through
four imidazolate based anion linkers leading to the formation of
School of Chemistry, Madurai Kamaraj University, Madurai-625 021, Tamil Nadu,
cages around 11.6 Å in diameter which are accessible through
India. E-mail: admguru@gmail.com
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj03698a narrow six ring pores (3.4 Å) with sodalite zeolitic topologies.33–35
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Interestingly, ZIFs have been shown to possess many advantages used as received. Solvents were also received from Sigma
for catalytic applications which include high stability in water, Aldrich and used as received without any further purification.
high thermal stability, large surface area (1700 m2 À1) and
g
high chemical stability in various solvents.36–38 These physical
2.2 Instrumentation
properties and high stability of ZIFs have expanded the applications FT-IR spectra were measured with a Bruker tensor 27 series
in various industrial applications such as gas storage, separation FT-IR spectrometer in the region of 400–4000 cmÀ1 with a
and catalysis.35,39–41
resolution of 2 cmÀ1. UV-DRS spectra were recorded by a
One of the recent strategies in using MOFs as heterogeneous Shimadzu UV-2700; ISR-2600 plus instrument in the range
catalysts is to prepare mixed-metal MOFs in which two different between 200 and 900 nm and the samples were dispersed on
metal ions having identical ionic radii are incorporated within the surface of BaSO4. Powder XRD diffraction patterns were
the framework to enhance the activity of mixed metal MOFs recorded in refraction mode using a Philips X’Pert diffracto-
compared to the parent MOF.42 The synthesis of mixed-metal meter with CuKa radiation (l = 1.5417 Å) as the incident beam,
MOFs has attracted considerable interest in recent years due to and a PW3050/60 (2 theta) goniometer. SEM images were
the synthetic flexibility offered by MOFs to allow incorporation of measured with a Hitachi S-3000H scanning electron micro-
two different metals without affecting the framework structure.43 scope. Gas chromatography (GC) and gas chromatography with
The activity of mixed-metal MOFs was comparatively superior to mass spectrometry (GC-MS) were measured using 7820 A and 5890B
the parent MOFs due to the intrinsic activity of incorporated model Agilent instruments, respectively, to determine conversion,
metal atoms in the parent MOF as evidenced from the pioneering selectivity and confirmation of the oxidation products.
studies.44–46 Furthermore, Dinca and coworkers have reported
the introduction of metal cations such as Fe2+, Mn2+ and Ti3+ into
2.3 Synthesis of Cu-doped ZIF-8
the MOF-5 framework by creating a redox active MOF from inert Cu-doped ZIF-8 solids were synthesized according to a slightly
MOF-5 to activate NO.47 In another precedent, Li and coworkers modified procedure.66 Solutions of zinc(II) nitrate hexahydrate
have reported that substitution of Co2+ in Ni-MOF-74 can and copper(II) nitrate trihydrate (total 2 mmol) in 22.6 mL
transform the inert Ni-MOF-74 into a highly active solid catalyst methanol and Hmim (1320 mg, 16 mmol) in the same volume
for cyclohexene oxidation due to the intrinsic role of the of methanol were prepared separately (Table S1, ESI†). Then, in
incorporated Co2+ for the oxidation reaction.48
a two-neck flask, these two solutions were mixed by dropwise
ZIF-8 has been reported as a heterogeneous solid catalyst for addition of the Hmim solution to the Zn2+ and Cu2+ solution
a broad range of organic reactions like polymerization,49 oxidation under nitrogen flow at room temperature with stirring for 1 h
of limonene,50 chemical fixation of CO2,51 Knoevenagel and then kept for 24 h at room temperature. The Cu-doped
condensation,52,53 and Michael addition.54 Furthermore, ZIF-8 ZIF-8 crystals were separated by centrifugation (4500 rpm, 15 min)
was conveniently used as a support for the deposition of metal and washed with methanol (3 Â 30 mL). The material was dried at
nanoparticles and the resulting solid exhibited high activity in 60 1C for 6 h before analysis. The as-synthesized sample was stored
C–C cross coupling reactions.55–64 Furthermore, iron-doped in a tightly capped vial at room temperature.
ZIF-8 (Fe-ZIF-8) was employed as a heterogeneous photocatalyst
for the degradation of rhodamine B dye.65 However, no attempts
2.4 General procedure for the aerobic oxidation of 1
were made to use mixed metal MOFs using ZIF-8 and especially In a typical aerobic oxidation reaction, a 25 mL round bottom
for oxidation reactions. Hence, in this work, Cu2+ metal ions are flask was charged with 20 mg of Cu-doped ZIF-8 solid followed
doped at different loadings within the framework of ZIF-8 and by the addition 1 (1 mL) using a micropipette. To this mixture,
the resulting solids are characterized by adequate techniques to 1 mL of solvent was added and sealed with a silicon septum.
ascertain the formation of the ZIF-8 framework and the presence Later, this heterogeneous mixture was placed on a preheated oil
of Cu2+ within the framework. The activity of these mixed-metal bath maintained at 120 1C. This heterogeneous reaction mixture
MOFs was studied in the aerobic oxidation of benzylic hydro- was purged with molecular oxygen (99.99% purity) continuously
carbons since the number of reports involving mixed metal ZIF-8 through a balloon. This reaction mixture was stirred for the
MOFs as heterogeneous catalysts for the oxidation reaction is required time as shown in Table 1. The reaction temperature
scarcely reported. Some of the salient features of this method are and pH of the slurry were monitored throughout the reaction.
easy synthesis of these solids, mild reaction temperature to The temperature of the reaction was maintained at 120 1C and
perform the oxidation reaction, the absence of radical initiators/ no pH change was noticed during the course of the reaction. A
co-catalysts/noble metals and it being reusable in consecutive known amount of aliquot (25 mL) was periodically taken, diluted
cycles without much decay in its activity.
with acetonitrile (0.5 mL) and filtered using a syringe filter.
This sample was analyzed by GC (operation conditions: HP-5
capillary column, oven temperature 50 to 300 1C at ramp rate
10 1C minÀ1 and inlet temperature of 250 1C) to determine
conversion and selectivity using nitrobenzene as an internal stan-
dard method. After the required time, the reaction was stopped
1 and and the reaction flask was allowed to cool to room temperature.
2. Experimental procedure
2.1 Materials
Cu(NO3)2Á3H2O, Zn(NO3)2Á6H2O, 2-methylimidazole,
other hydrocarbons were purchased from Sigma Aldrich and Later, the mixture was diluted with acetonitrile (3 mL) and filtered.
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This sample was analyzed by GC to determine the final conver- plasma-optical emission spectrometry was used to determine
sion, and the selectivity and products were confirmed by GC-MS the final copper loading of Cu-ZIF-8 solids. This analysis
with identical conditions used for GC. A similar experimental indicated that the final copper loadings in Cu-ZIF-8 solids are
procedure was followed for reusability experiments except the 0.8%, 1.6% and 2.8% for those solids named as Cu-1-ZIF-8,
use of recovered catalyst rather than a fresh solid. After com- Cu-2-ZIF-8 and Cu-3-ZIF-8, respectively (Fig. 1).
pleting the reaction, the catalyst was filtered, washed well with
Power XRD patterns of the as-synthesized ZIF-8 and
acetonitrile and dried at 100 1C for 3 h. This sample was used for Cu-doped ZIF-8 solids are shown in Fig. 2. The as-synthesized
reusability experiments with fresh reactants. Furthermore, the Cu-doped ZIF-8 showed a similar pattern corresponding to ZIF-8
above procedure was repeated with TEMPO (20 mg) or hydro- which indicates the formation of a body-centered cubic crystal
quinone (20 mg) as radical scavengers to identify the nature lattice symmetry.67 Furthermore, the addition of a Cu2+ ion to
of the active sites. Similarly, control experiments were also the reaction mixture during the growth of ZIF-8 crystals did not
performed with Cu(NO3)3Á3H2O (38 mg) and CuI (26 mg) as homo- affect the framework structure in the ZIF-8 materials and the
geneous catalysts under identical conditions with similar copper structural integrity is retained due to the similar ionic size
loading as in the case of Cu-3-ZIF-8.
between Cu2+ (0.71 Å) and Zn2+ (0.74 Å) ions. UV-DRS studies
were performed to confirm the presence of Cu2+ in the ZIF-8
framework and the observed results are shown in Fig. 3. ZIF-8
shows an absorption band only in the UV region due to its band
gap energy Eg = 4.9 eV.68 Doping of Cu2+ ions in ZIF-8 crystals
3. Results and discussion
Different Cu-doped ZIF-8 samples were prepared by appropriately made it possible to engineer its band gap and also shift its
selecting three loadings of a Cu precursor during the synthesis as absorption to the visible region and it exhibited a well-defined
shown in Scheme 1 by adapting an earlier procedure reported by band located at 490 nm which corresponds to d–d transition of
Schneider and coworkers.66 The as-synthesized Cu-doped ZIF-8 Cu2+, confirming the presence of the heterobimetallic nature of
was characterized by powder XRD, SEM, EDX, EPR, UV-DRS and the ZIF-8 material. Furthermore, the intensity of the absorption
FT-IR techniques and these data are in agreement with pre- edge increases with increasing Cu2+ loading (Fig. 3). In addition,
viously reported results.66 Furthermore, inductively coupled these samples were also characterized by EPR spectra and the
Scheme 1 Schematic representation of the synthetic route and structure of Cu-doped ZIF-8.
Fig. 1 Visual color change observed during the Cu loading in ZIF-8: (a) ZIF-8, (b) Cu-1-ZIF-8, (c) Cu-2-ZIF-8 and (d) Cu-3-ZIF-8.
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ZIF-8 material and the signal intensity increases with respect to
doping of Cu2+ into the ZIF-8 material.
SEM images were measured for ZIF-8 and Cu-doped ZIF-8
samples to compare the morphology between ZIF-8 and
Cu-doped ZIF-8 solids (Fig. 5). These images clearly illustrate
that both samples exhibit a well-defined truncated rhombic
dodecahedron structure which is the typical morphology reported
for ZIF-8. These results suggest that the surface morphology and
the framework structure of ZIF-8 are not altered during the doping
of Cu2+ ions. The elemental composition of Cu-doped ZIF-8
samples was analyzed by EDX spectra and the attained results
are given in Fig. 6. These results are in close agreement with the
copper content observed from ICP analysis. Furthermore, the
elemental mapping of Cu-3-ZIF-8 is shown in Fig. 7.
Fig. 2 Powder XRD patterns of (a) ZIF-8, (b) Cu-1-ZIF-8, (c) Cu-2-ZIF-8
and (d) Cu-3-ZIF-8.
FT-IR spectra of ZIF-8 and copper-doped ZIF-8 are provided
in Fig. 8. A sharp peak at 423 cmÀ1 is attributed to a Zn–N
stretching mode which reveals that zinc is linked to nitrogen
atoms of the linkers to form a porous ZIF coordination structure.69
The stretching frequencies from 1100 to 1300 cmÀ1 are assigned to
C–H vibrations while the peaks located at 1466 and 1470 cmÀ1 are
assigned to CQN stretching vibration. Furthermore, the peaks at
1229 and 1231 cmÀ1 confirm the trembling of C–N in the
imidazole ring. The Cu–N bonding vibration is assigned in the
range of 550–620 cmÀ1 in Cu-doped ZIF-8 samples which are in
agreement with earlier reports.70
In a seminal contribution, Corma and coworkers have shown
that MOFs with Cu2+ centers coordinated to nitrogen atoms like
pyrimidine and imidazole are active catalysts for aerobic oxidation
of alkanes such as cumene, ethylbenzene and tetralin without the
requirement of any radical initiators under mild reaction
conditions.71,72 Continuing in this line of research and considering
the high stability of ZIF-8, the present work aims to convert
inactive ZIF-8 to active ZIF-8 upon doping with Cu2+ ions within
the framework of ZIF-8 in promoting the aerobic oxidation of 1
without the requirement of any radical initiators/co-catalysts.
The catalytic performance of Cu-doped ZIF-8 catalysts was
optimized in the aerobic oxidation of 1. Aerobic oxidation of 1
resulted in various oxidized products like cyclooctanone (2),
cyclooctanol (3), cyclooctanediol (4) and cyclooctanedione (5). A
blank control experiment in the absence of catalyst showed
negligible conversion of 1 at 120 1C in DMF as solvent after
24 h. On the other hand, aerobic oxidation of 1 in the presence
of ZIF-8 under identical conditions afforded negligible conversion,
thus showing the inert nature of the ZIF-8 catalyst. In contrast,
Cu-1-ZIF-8 afforded 7% conversion of 1 with 80% selectivity to
3/2 under similar reaction conditions. Similarly, Cu-2-ZIF-8 and
Cu-3-ZIF-8 afforded 11 and 16% conversions of 1 with 82 and
85% selectivity to 3/2, respectively under identical conditions.
Fig. 9 shows the time conversion profile for the aerobic oxidation
of 1 using Cu-doped ZIF-8 catalysts with DMF as solvent at 120 1C.
These results clearly indicated that the conversion of 1 gradually
increases with respect to the amount of Cu2+ doping to ZIF-8,
suggesting the positive role of Cu2+ in promoting the oxidation
reaction. In order to confirm this hypothesis, control experiments
were performed with Cu(II) and Cu(I) salts as homogeneous
catalysts under identical conditions. For instance, Cu(NO3)2Á3H2O
Fig. 3 UV-DRS spectra of (a) Cu-1-ZIF-8, (b) Cu-2-ZIF-8 and (c) Cu-3-
ZIF-8.
results are given in Fig. 4. ZIF-8 is EPR inactive due to the
presence of solely Zn2+ ions in its crystal structure. On the other
hand, doping of Cu2+ in ZIF-8 shows an intense signal in the
high field region which corroborates the presence of Cu2+ in the
Fig. 4 EPR spectra of (a) Cu-1-ZIF-8, (b) Cu-2-ZIF-8 and (c) Cu-3-ZIF-8.
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Fig. 5 SEM images of (a) ZIF-8, (b) Cu-1-ZIF-8, (c) Cu-2-ZIF-8, and (d) Cu-3-ZIF-8.
Fig. 6 EDX spectrum of Cu-3-ZIF-8.
and CuI exhibited 8 and 6% conversions of 1 with 69 and 71% of
3/2 selectivity, respectively. Fig. 10 shows the time conversion
profile for the aerobic oxidation of 1 using Cu-3-ZIF-8 as a solid
catalyst and homogeneous catalysts. These results clearly suggest
that Cu2+-doped in ZIF-8 is responsible for promoting the oxidation
reaction and interestingly the ol/one selectivity is superior with
Cu-doped ZIF-8 catalysts than these homogeneous control catalysts.
The aerobic oxidation of 1 using Cu-3-ZIF-8 was also optimized
with respect to the reaction temperature and nature of solvents.
The observed results are given in Table 1. The conversion of 1 was
Fig. 7 EDX elemental mapping of Cu-3-ZIF-8.
reduced significantly to 8% when the reaction temperature
decreased to 100 1C and further lowering the reaction temperature
to 80 1C, the conversion of 1 was drastically declined to 2% under
identical conditions. The observed results are shown in Fig. 11. On chlorobenzene and p-xylene under similar reaction conditions
the other hand, the aerobic oxidation of 1 was not successful in suggesting that the reaction is highly specific to the nature of
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Table 1 Aerobic oxidation of 1 using Cu-doped ZIF-8 solids as catalystsa
Selectivityb (%)
(2 + 3) (4 + 5)c
T
Conversion
Entry Catalyst
Solvent
(1C) (%)
1
2
3
4
5
6
7
8
—
ZIF-8
Cu-1-ZIF-8
Cu-2-ZIF-8
Cu-3-ZIF-8
DMF
DMF
DMF
DMF
DMF
120
120
120
120 11
120 16
120
120
100
80
1
2
7
—
—
80
82
85
69
71
81
—
—
—
—
79
72
63
—
—
20
18
15
21
29
19
—
—
—
—
21
28
37
Cu(NO3)2Á3H2O DMF
8
6
8
2
CuI
DMF
DMF
DMF
DMSO
Chlorobenzene 120 —
p-Xylene
DMF
Cu-3-ZIF-8
Cu-3-ZIF-8
Cu-3-ZIF-8
Cu-3-ZIF-8
Cu-3-ZIF-8
9
10
11
12
120 —
Fig. 8 FT-IR spectrum of (a) ZIF-8, (b) Cu-1-ZIF-8, (c) Cu-2-ZIF-8 and (d)
Cu-3-ZIF-8.
120 —
120 13
120 17
120 23
13d Cu-3-ZIF-8
14e
15f
—
—
DMF
DMF
a
Reaction conditions: 1 (1 mL), catalyst (20 mg), solvent (1 mL), O2
b
purged, 24 h. Conversion and selectivity were determined by GC.
c
Others correspond to the combined selectivity of products (4 + 5).
d
e
After second reuse of Cu-3-ZIF-8. 0.14 mmol of t-butylhydroperoxide
f
(TBHP). 0.14 mmol of azobis(isobutyronitrile) (AIBN).
Fig. 9 Time conversion profile for the aerobic oxidation of 1 in the
presence of (a) Cu-1-ZIF-8, (b) Cu-2-ZIF-8 and (c) Cu-3-ZIF-8 catalysts.
Fig. 11 Aerobic oxidation of 1 using Cu-3-ZIF-8 at (a) 80 1C, (b) 100 1C
and (c) 120 1C.
the solvent. Further, the aerobic oxidation of 1 was also performed
under similar conditions in DMSO and the catalytic data indicate
that no oxidation products are formed. This may be due to the
radical scavenging ability of DMSO by trapping the radical species
and not facilitating the interaction with molecular oxygen.
In order to gain some insights into the use of solid catalysts
in terms of higher selectivity than conversions, a series of radical
initiators were employed instead of a Cu-3-ZIF-8 solid catalyst
under identical conditions. For instance, AIBN and TBHP were
used as radical initiators in the absence of Cu-3-ZIF-8 solid.
Fig. 10 Time conversion profile for the aerobic oxidation of 1 in the
presence of (a) Cu-3-ZIF-8, (b) Cu(NO3)2Á3H2O and (c) CuI.
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The loading of TBHP or AIBN was identical to the moles of Cu2+ TBHP and Cu-3-ZIF-8 is the product selectivity of 3/2. As it can
metal ion present in the Cu-3-ZIF-8 solid catalyst. The catalytic be seen in Table 1, the ratio of 3/2 was lower with organic radical
results have shown that the conversion of 1 is higher using initiators than the Cu-3-ZIF-8 MOF catalyst. For instance, the
TBHP and AIBN than that of Cu-3-ZIF-8. However, the main conversion of 1 was 17% with 3/2 selectivity of 72% with TBHP as
difference between the use of radical initiators like AIBN or a radical initiator. Furthermore, the conversion of 1 was slightly
higher with AIBN as a radical initiator reaching to 23% with 3/2
selectivity of 63% under identical conditions. These catalytic
results clearly prove that organic radical initiators provide higher
conversion of 1 than the Cu-3-ZIF-8 solid catalyst, but the solid
catalyst affords higher selectivity to 3/2 which is an important
requirement in the aerobic oxidation reactions.
The stability of Cu-3-ZIF-8 was confirmed by performing
leaching tests under the optimized reaction conditions. The
aerobic oxidation of 1 was initiated with the Cu-3-ZIF-8 solid
catalyst and it was removed from the reaction mixture by
filtration under the reaction temperature after 3 h. Then, the
reaction mixture in the absence of a solid was stirred up to 24 h.
The kinetic profiles for the aerobic oxidation of 1 with and
without catalysts are shown in Fig. 12. These data suggest that
the aerobic oxidation is inhibited upon removal of catalyst,
thus suggesting the absence of leached Cu in the reaction
mixture. Furthermore, Cu-3-ZIF-8 was reused two times with
minor decrease in the conversion of 1 and selectivity to 3/2.
Fig. 12 Time conversion plot for the aerobic oxidation of 1 (a) in the
This may be due to the loss of solid catalyst during the recovery
from the reaction mixture. In addition, powder XRD of the
two times reused Cu-3-ZIF-8 exhibited an identical crystalline
pattern compared to the fresh solid catalyst. These results
indicate the retainment of structural integrity during the aerobic
oxidation of 1 (Fig. 13). Furthermore, the surface morphology of
the two times reused solid did not show any notable changes
compared to the fresh solid catalyst (Fig. 14).
presence of Cu-3-ZIF-8 and (b) Cu-3-ZIF-8 removed by filtration after 3 h
(leaching test).
To gain some insights into the reaction mechanism, a series
of control experiments were performed and the observed results
are shown in Scheme 2. Aerobic oxidation of 1 using Cu-3-ZIF-8
was performed under a nitrogen atmosphere at 120 1C with DMF
as solvent and observed no conversion of 1. This experiment
suggests that the molecular oxygen purged during the oxidation
reaction is essential and is acting as an oxidant. In another
experiment, the aerobic oxidation of 1 was also performed using
Fig. 13 Powder XRD patterns of (a) fresh and (b) two times reused Cu-3-
ZIF-8.
Fig. 14 SEM images of (a) fresh and (b) two times reused Cu-3-ZIF-8.
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present work and previous mechanistic proposals, it is believed
that Cu(II) ions located within the framework of ZIF-8 react with
molecular oxygen to provide CuII-superoxo species which can
activate the C–H bond in 1 to afford a cyclooctyl radical.
This radical intermediate reacts with molecular oxygen to give
an oxygen based radical intermediate, which upon hydrogen
abstraction from 1 gives the corresponding hydroperoxide.
Finally, this hydroperoxide decomposes to the expected ol/one
products as shown in Scheme 3.
The catalytic data in Table 2 provides the comparison between
Cu-3-ZIF-8 and other catalysts reported in the literature for the
aerobic oxidation of 1. Some of the notable features of the present
work are the absence of co-catalysts/additives, radical initiators
and noble metals. Furthermore, the aerobic oxidation of 1 was
performed at relatively low reaction temperature by employing
molecular oxygen as a green oxidant than previous reports with
peroxides and NaIO4 as oxidants. On the other hand, the
conversion of 1 with the present catalyst is lower than with
other catalysts due to the lack of co-catalysts/additives, but,
however, the ol/one selectivity is similar to these reported data.
These results indicate the superiority of using mixed metal
MOFs in the aerobic oxidation without using any radical initiators
and it is expected that this catalyst can be extended to oxidize other
substrates with high ol/one selectivity.
Scheme 2 Control runs to elucidate the reaction mechanism in the
aerobic oxidation of 1 using Cu-3-ZIF-8 as a catalyst.
Cu-3-ZIF-8 with TEMPO as radical quencher under identical
conditions. The experimental result showed no conversion of 1.
In another control experiment, Cu-3-ZIF-8 did not provide any
conversion of 1 in the presence of hydroquinone under identical
conditions. These experiments with radical scavengers clearly
suggest that the reaction proceeds through the involvement of
radical intermediates. Furthermore, the presence of these scavengers
in the reaction mixture facilely traps these radical intermediates,
inhibiting the aerobic oxidation.
In order to generalize the activity of Cu-3-ZIF-8 in the aerobic
oxidation of hydrocarbons under the present experimental
conditions, a wide range of benzylic hydrocarbons were tested
and the observed data are summarized in Table 3. In general,
the activity of ZIF-8 in the aerobic oxidation of hydrocarbons
was almost negligible for all the hydrocarbons tested whereas
the activity of Cu-3-ZIF-8 was notably higher under identical
conditions. Aerobic oxidation of ethylbenzene using Cu-3-ZIF-8
showed 8% conversion (Fig. S27, ESI†) with 93% selectivity to
ol/one. Similarly, aerobic oxidation of tetralin afforded 8%
conversion (Fig. S27, ESI†) with 72% selectivity to ol/one and
28% selectivity to 1,2-dihydronaphthalene. Under identical
conditions, diphenylmethane was converted around 6% (Fig. S27,
ESI†) to afford exclusively benzophenone. Similarly fluorene was
oxidized to flurenone with 11% conversion (Fig. S27, ESI†) and
99% selectivity. On the other hand, the aerobic oxidation of
benzylacetate using Cu-3-ZIF-8 exhibited 6% conversion to
provide exclusively the corresponding ketone as a product.
Cu(II) containing homogeneous and heterogeneous catalysts
have been reported to activate molecular oxygen to CuII-superoxo
species that can readily interact with alkenes and alkanes.73–75
Aerobic oxidation of alkenes, alkanes and alcohols was studied
with heterogeneous Co(II) and Cu(II) catalysts using DMF as a
solvent.76–78 Considering the catalytic results observed in the
Scheme 3 A proposed mechanism for the aerobic oxidation of 1 using
Cu-3-ZIF-8 as a solid catalyst.
Table 2 Comparison of the present catalytic data with earlier precedents for the oxidation of 1
S. no. Catalyst T (1C) Oxidant Time (h) Reuses Conversion (%) Selectivity of ol/one (%) Ref.
1
2
3
4
5
6
7
8
9
NHPI/Co-BTT
NHPI/Fe(BTC)
MOF-253
Copper-5-(4-pyridyl)tetrazolate
{(TBA)7H3[Co4(H2O)2(PW9O34)2]}/MIL-101
(CuPhen)/MIL-100(Al)
Mn(TPyP)OAc/CM-MIL-101/imidazole
Au–Pd/MIL-101
Fe(BTC) and TBAB
Cu-3-ZIF-8
120
120
150
RT
80
70
O2
O2
O2
H2O2
H2O2
H2O2
NaIO4
O2
O2
O2
29
24
4
10
24
3
6
4
24
24
—
2
—
4
3
—
3
—
2
2
48
71
70.8
22
5
78
61
66
22
72
—
58
23a
84
85
79
80
81
82
83
84
85
86
9
RT
72
51.8
73
16
150
120
120
87
10
This work
a
Selectivity corresponds to cyclooctanone. In addition, 58.2% of 1,3-dihydroxycyclooctane was also reported.
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Table 3 Aerobic oxidation of various hydrocarbons catalyzed by Cu-3-
ZIF-8 has significantly improved the catalytic activity and in
particular, eight-fold enhancement in the activity is observed
for Cu-3-ZIF-8 in the aerobic oxidation of 1 at 120 1C with more
than 80% selectivity to the corresponding ol/one mixture. The
activity of Cu-3-ZIF-8 was retained for two reuses. Furthermore,
the two times reused Cu-3-ZIF-8 catalyst retained its structural
integrity as well as morphology as evidenced by powder XRD
and SEM images, respectively than with the fresh solid.
ZIF-8a
Selectivityb
(Cu-3-ZIF-8)
Conversion Conversion
Entry Hydrocarbon
(ZIF-8) (%) (Cu-3-ZIF-8) (%) ol + one Others
1
2
3
—
8
8
6
93
7
28
—
o1
—
72c
99c
Conflicts of interest
There are no conflicts to declare.
4
5
—
—
11
6
99c
99c
—
—
Acknowledgements
AD thanks the University Grants Commission, New Delhi, for the
award of an Assistant Professorship under its Faculty Recharge
Programme. AD also thanks the Department of Science and
Technology, India, for the financial support through Extra Mural
Research Funding (EMR/2016/006500).
6
2
2
9
78d
76d
22
24
7
8
11
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