{[Co2(btec)(2,2′-bipy)2]·H 2O}n metal-organic framework: Structure and activity in the solvent-free oxidation of cyclohexene with oxygen

Article abstract of DOI:10.1016/j.ica.2014.06.005

A Co (II) metal-organic framework (MOF) {[Co₂(btec)(2,2′- bipy)₂]·H₂O}_n (H₄btec: 1,2,4,5-benzenetetracarboxylic acid; 2,2′-bipy: 2,2′-bipyridine) was hydrothermally synthesized and characterized using X-ray crystallographic analysis, Fourier transform infrared spectroscopy, elemental analysis, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and N₂ adsorption/desorption. Its catalytic performance was examined for the allylic oxidation of cyclohexene with oxygen under solvent-free conditions. It acted as a heterogeneous catalyst, which was deactivated in catalyst recycling and regenerated through treatment with a scCO₂-expanded ethanol system. The inhibitive effect of H₄btec and other ligands on cyclohexene oxidation was detected, presumed to be caused by hydrogen-bonding interaction between the H₄btec and a 2-cyclohexene-1-hydroperoxide intermediate.

Full text of DOI:10.1016/j.ica.2014.06.005

Inorganica Chimica Acta 421 (2014) 246–254
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
{[Co2(btec)(2,20-bipy)2]ꢀH2O}n metal–organic framework: Structure
and activity in the solvent-free oxidation of cyclohexene with oxygen
⇑
Jianmin Hao , Sijia Li, Limin Han, Lin Cheng, Quanling Suo, Yang Xiao, Xiaoli Jiao, Xuemin Feng,
Weiwei Bai, Xiaofei Song
Chemical Engineering College, Inner Mongolia University of Technology, Hohhot 010051, PR China
a r t i c l e i n f o
a b s t r a c t
A Co (II) metal–organic framework (MOF) {[Co2(btec)(2,20-bipy)2]ꢀH2O}n (H4btec: 1,2,4,5-benzenetetra-
carboxylic acid; 2,20-bipy: 2,20-bipyridine) was hydrothermally synthesized and characterized using X-
ray crystallographic analysis, Fourier transform infrared spectroscopy, elemental analysis, X-ray diffrac-
tion, scanning electron microscopy, transmission electron microscopy and N2 adsorption/desorption. Its
catalytic performance was examined for the allylic oxidation of cyclohexene with oxygen under solvent-
free conditions. It acted as a heterogeneous catalyst, which was deactivated in catalyst recycling and
regenerated through treatment with a scCO2-expanded ethanol system. The inhibitive effect of H4btec
and other ligands on cyclohexene oxidation was detected, presumed to be caused by hydrogen-bonding
interaction between the H4btec and a 2-cyclohexene-1-hydroperoxide intermediate.
Article history:
Received 26 February 2014
Received in revised form 11 May 2014
Accepted 10 June 2014
Available online 18 June 2014
Keywords:
Metal–organic framework
Cobalt
Heterogeneous catalysis
Cyclohexene oxidation
Oxygen
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
and 343 K [8]. Chitosan-supported salophen Mn (III) complexes
exhibited high catalytic activity in cyclohexene oxidation with a
The selective oxidation of cyclohexene is an essential chemical
process for the current chemical industry because its products,
such as alcohols, ketones, epoxides, and acids, are chemical inter-
mediates used for synthesizing polymers, drugs, agrochemicals,
and surfactants [1]. Both the C@C double bond and the allylic C–
H bond in cyclohexene are in active positions, which are prone to
oxygenation for forming a great variety of products [2]. Because
of increased environmental concerns, solvent-free benign oxida-
tions that use heterogeneous catalyst and clean oxidants are
favored [3]. Notably, product distribution depends on the catalysts
and oxidants used [4,5]. As an oxidant, molecular oxygen has
attracted much more attention because it is inexpensive, readily
available, and environmentally benign compared with other oxi-
dants [6,7]. Recently, certain heterogeneous catalysts, including
metal-containing redox molecular sieves or metal complexes, oxi-
des, and nanoparticles were researched in the oxidation of cyclo-
hexene. For example, CrMCM-41 was discovered to be an
efficient catalyst for the oxidation of cyclohexene, in which the
conversion was 52.2% and the total selectivity of 2-cyclohexene-
1-ol (Cy-ol), 2-cyclohexene-1-one (Cy-one), and 2-cyclohexene-1-
hydroperoxide (Cy-HP) was 96.6% in the conditions of 1 atm O2
turnover number of 11.3 ꢁ 104, selectivity of 93.3% for Cy-ol, Cy-
one, and Cy-HP at 343 K and ambient oxygen pressure [9].
The design and synthesis of metal–organic frameworks is
increasingly crucial because of their special physical properties
and potential applications in electronic, magnetic, optical, absor-
bent, and catalytic materials [10–13]. In the field of crystal engi-
neering, the versatility of molecular chemistry enables the
production of a great variety of polytopic organic ligands with dif-
ferent functionalities [14]. One ligand, H4btec, is well known for
having four rigid and symmetrical carboxyl groups that can con-
struct stable, porous, and multidimensional frameworks through
various coordination modes through the complete or partial depro-
tonation of carboxyl groups [15–17]. The stability of metal–organic
frameworks containing H4btec and other ligands can be enhanced
by forming hydrogen bonds because H4btec can act as a hydrogen-
bond acceptor or hydrogen-bond donor. MOFs are promising mate-
rials for application in catalysis because they possess single-site
active species characteristic of homogeneous catalysts, combined
with the advantages of easy separation and recycling typical of het-
erogeneous catalysts. A new extended metal–organic framework
[Cu(H2betc)(bipy)] (bipy = 2,20-bipyridine) for cyclohexene oxi-
1
dation was hydrothermally synthesized by Brown et al., which pre-
sented a high conversion of 64.5% and selectivity of 73.1% for
cyclohexene epoxide [18]. The copper metal–organic framework
⇑
Corresponding author. Tel./fax: +86 471 6575675.
E-mail address: haojmin@gmail.com (J. Hao).
http://dx.doi.org/10.1016/j.ica.2014.06.005
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

J. Hao et al. / Inorganica Chimica Acta 421 (2014) 246–254
247
[Cu(bpy)(H2O)2(BF4)2(bpy)]
(bpy = 4,40-bipyridine)
exhibited
obtained using Micromeritics a ASAP 2020 instrument after an
in situ automatic degassing procedure at 473 K. The density func-
tional theory (DFT) calculations for Cy-HP and H4btec interaction
were performed using the GAUSSIAN 03 suite of programs. Since the
Hybrid methods (one type of the density functional methods), such
as B3LYP, tend to be the most commonly used methods, the
proposed geometries of the hydrogen bonds between Cy-HP and
H4btec were optimized at the gas phase using B3LYP functional
and the standard 6-31G(d) basis set.
promising catalytic activity and a high selectivity of 90% for Cy-
HP in the allylic oxidation of cyclohexene with molecular oxygen
as the only oxidant in the absence of solvent [19].
In this study,
a
Co (II) metal–organic framework
{[Co2(btec)(2, 20-bipy)2]ꢀH2O}n was synthesized using a hydrother-
mal method in accordance with the literature [20–22]. Its compo-
sition and structure was characterized and its catalytic activity in
the selective oxidation of cyclohexene with molecular oxygen
under solvent-free conditions was discussed in detail.
2.3. Cyclohexene oxidation
2. Experimental
Cyclohexene (Aladdin CP) and high-purity oxygen (99.999%)
were used as-delivered. In typical reactions, a certain amount of
substrate and catalyst were charged into a 50-mL stainless steel
autoclave with a Teflon inner liner at room temperature. The reac-
tor was heated to the desired temperature in an oil bath and a
quantity of O2 gas was then introduced into the reactor. The reac-
tion runs were conducted whiles simultaneously stirring, using a
magnetic stirrer. At the end of the reaction, the autoclave was
cooled to room temperature and then depressurized. The catalyst
was filtered and the product solution was diluted with ethanol.
The main oxidation products, Cy-ol and Cy-one were identified
by comparing with standard samples (retention time in GC), Cy-
HP was analyzed using triphenylphosphine reduction because it
is difficult to analyze by using GC [19]. The qualitative analysis of
other by-products was examined using GC–MS. The composition
of the reaction mixture was analyzed using a gas chromatograph
(Shimadzu GC-14C, column RTX-50). The conversion was calcu-
lated as the moles of products formed (cyclohexene and the major
product in a mole ratio of 1:1) divided by the initial moles of cyclo-
hexene, and selectivity was calculated as the moles of a certain
product divided by the total moles of products formed. Safety
warning: Using compressed O2 in the presence of organic sub-
strates requires appropriate safety precautions and must be carried
out in suitable equipment.
2.1. Catalysts preparation and regeneration
{[Co2(btec)(2,20-bipy)2]ꢀH2O}n was prepared under hydrother-
mal conditions. All the reagents were purchased commercially
and used as delivered. In a typical synthesis, the reaction mixture
of Co(NO3)2ꢀ6H2O (P99%, 0.291 g), H4btec (98%, 0.254 g), 2,20-
bipyridine (AR, 0.156 g) and redistilled water (15 mL) in a molar
ratio of 1:1:1:833 was loaded in a 25-mL Teflon-lined stainless
steel autoclave and the pH was adjusted to 8–9 by using NH3ꢀH2O.
The autoclave was heated at 433 K for 120 h and slowly cooled to
room temperature at 2.5 K/h. The product was filtered, washed
with distilled water, and air-dried at room temperature. The prod-
uct was in the form of wine block crystals with average quality of
0.2883 g per autoclave (yield: 82.6% based on Co). The product that
was crushed was named Co-MOF-A and the product that was
crushed, washed with ethanol (99%), and air-dried was named
Co-MOF-B.
The amount of Co-MOF-B catalyst samples used after a catalytic
cyclohexene oxidation run was collected. The used catalyst sam-
ples and 5 mL of ethanol were added to a 50-mL stainless steel
batch reactor. After the reactor was heated to 308 K for 0.5 h,
CO2 was introduced into the reactor to 8 MPa, using a high-pres-
sure liquid pump. The mixture was stirred continuously, using a
Teflon-coated magnetic stir bar for 24 h. Subsequently, the reactor
was cooled to room temperature and depressurized. The solid
product was filtered and then dried at room temperature.
3. Results and discussion
3.1. Catalysts characterization
2.2. Catalysts characterization
X-ray crystallographic analysis revealed that {[Co2(btec)(2,20-
bipy)2]ꢀH2O}n was crystallized in the orthorhombic space group
C222(1). Fig. 1(a) shows that the Co (II) center displays a distorted
octahedral geometry arrangement coordinating to two nitrogen
atoms of one 2,20-bipy ligand, three oxygen atoms of two different
carboxylate groups, and one oxygen atom of a water molecule.
Fig. 1(b) exhibits an infinite 2D lamellar structure. The four carbox-
ylate groups of H4btec present two types of coordination mode
with the Co atoms: Two carboxylate groups on the same side offer
four O atoms to form two bidentate chelating structures, and two
carboxylate groups on the other side each offer one O atom to form
two mono-dentate bridging structures. The bridging action of H4-
btec resulted in the formation of zig–zag chains running along
the a axis, as shown in Fig. 1(c). The coordinated H2O molecule
acted as a bridge connecting two Co atoms, which resulted in the
repeated emergence of symmetrical units along the c axis, as
shown in Fig. 1(d).
The structural measurements of a single 0.12 ꢁ 0.1 ꢁ 0.08-mm
wine crystal of the compound was performed using X-ray diffrac-
tion on a Bruker SMART 1000 CCD diffractometer with Mo Ka radi-
ation (k = 0.71073) at 296 K in the range of 2.69 < h < 24.98. The
structures were solved using direct methods and refined by imple-
menting the full-matrix least-squares method on F2 using the
SHELX-97 crystallographic software package [23,24]. The crystallo-
graphic details of the structure of {[Co2(btec)(2,20-bipy)2]ꢀH2O}n
are summarized in Table S1 and the selected bond lengths and
angles are given in Table S2. Fourier transform infrared (FTIR) spec-
tra were recorded on a Nicolet FTIR spectrometer, using pellets of
the materials diluted with KBr in the range of 4000–400 cmꢂ1
.
The Co content in the catalyst samples was determined by con-
ducting inductively coupled plasma optical emission spectroscopy
(ICP-OES) using a PerkinElmer Optima 7000 DV. Elemental analy-
ses (C, H, and N) were performed using an Elementar VarioEL III
elemental analyzer. Structural studies of the catalysts were per-
The FT-IR spectra of H4btec, Co-MOF-A, and Co-MOF-B are pre-
formed using X-ray diffraction (XRD) on
a
Bruker-AXS D8
sented in Fig. 2. The H4btec revealed one
m
(CO) bond at 2017 cmꢂ1
ADVANCE with Cu K in the 2h range of 10–30°. A scanning elec-
a
and distinct
m
(O–H) bonds between 3540 and 2520 cmꢂ1, which
tron microscope (SEM; Hitachi S-3400 N) was used to observe
the surface morphology of the catalyst samples. Transmission elec-
tron microscopy (TEM) images were obtained using a FEI Tecnai G2
F20. The N2 adsorption–desorption isotherms at 77 K were
were also visible in Co-MOF-A but invisible in Co-MOF-B. This indi-
cated that an amount of unreacted H4btec was mixed in Co-MOF-A.
This result was further confirmed when the filtered solution
obtained after washing Co-MOF-A with ethanol was analyzed

248
J. Hao et al. / Inorganica Chimica Acta 421 (2014) 246–254
Fig. 1. A ball and stick representation of {[Co2(btec)(2,20-bipy)2]ꢀH2O}n structure: (a) The local coordination environment of the compound. The intra-chain hydrogen bonding
interaction is shown in broken lines; (b) The network of the compound along the b axis; (c) The network of the compound along the a axis; (d) The network of the compound
along the c axis.
Table 1
100
Chemical composition of the catalyst samples.
Entry
Catalyst
Co (%)
C (%)
H (%)
N (%)
80
60
40
20
0
1
2
calculated
Co-MOF-A
Co-MOF-B
Co-MOF-B
Co-MOF-B
16.88
15.16
15.37
13.63
12.18
51.60
51.21
51.25
48.40
44.30
2.89
2.84
3.00
2.78
2.76
8.02
8.21
8.19
7.32
6.07
(a)
(b)
(c)
(d)
(e)
3
4a
5b
a
After one cyclohexene oxidation run
After the fifth cyclohexene oxidation run.
b
the fresh Co-MOF-A and Co-MOF-B catalyst samples. The Co, C, N
and H contents in Co-MOF-B after catalytic reactions diminished
simultaneously, which suggested that the O content increased. So
it was suggested that certain oxidative organic residue were pro-
duced and adsorbed on the surface of MOF catalyst during the cat-
alytic oxidation reactions. Compared with C, N and H contents, the
Co content in Co-MOF-B catalyst after the reactions decreased in
large proportion, which proved that Co leaching might occur dur-
ing the reactions. However, this evidence was not reliable as a
quantitative analysis of the Co leaching amount for the Co-MOF-
B catalyst.
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber cm-1
Fig. 2. FT-IR spectra of (a) H4btec, (b) fresh Co-MOF-A catalyst, (c) fresh Co-MOF-B
catalyst, (d) Co-MOF-B catalyst after one cyclohexene oxidation run and (e) Co-
MOF-B catalyst after the fifth cyclohexene oxidation run.
using LC–MS (Shimadzu LCMS-2020) as shown in Fig. S1. The IR
spectra of the fresh Co-MOF-B catalyst, Co-MOF-B catalyst after
one cyclohexene oxidation run, and Co-MOF-B catalyst after the
fifth cyclohexene oxidation run were the same.
Regarding the chemical composition of the catalyst samples in
Table 1, the error of actual C, H, and N content was < 0.4% for
(Fig. 3) shows the XRD patterns of Co-MOF-A and Co-MOF-B
catalyst samples. The fresh Co-MOF-A catalyst and fresh Co-MOF-
B catalyst exhibit the same characteristic diffraction peaks, which
illustrates that the residual H4btec did not affect the crystalline
structure. When the residual H4btec was removed by washing with
ethanol, the diffraction peaks of the fresh Co-MOF-B catalyst

J. Hao et al. / Inorganica Chimica Acta 421 (2014) 246–254
249
formed and the surface of the particles was smooth and intact, as
shown in Fig. 4(a) and (b). Comparing the results in Fig. 4(a) and
(c), crystals damage was observed on the surface of the catalyst
after one cyclohexene oxidation run. It was suggested that Co2+
leaching from the catalyst during the reaction lead to the partial
collapse of the framework. During continuous stirring, partial col-
lapsed framework agglutination occurred to form a filamentous
organic residue coating on the surface of some catalyst particles
during the catalyst recycling tests, as shown in Fig. 4(d), which is
consistent with the results of the elemental analysis. The organic
residue was physically adsorbed on the surface of the catalyst
because the IR vibration frequency of the catalyst did not shift after
the oxidation reaction.
The TEM images of the Co-MOF-B catalyst samples are pre-
sented in Fig. 5, which shows a layered 2D microstructure com-
posed of nanoflakes. The nanoflakes were compact and did not
exhibit a porous structure. Furthermore, the nanoflakes became
smaller after one cyclohexene oxidation run because the crystal
particles were crushed during the stirring process. The microstruc-
ture of the Co-MOF-B catalyst was mainly determined by the coor-
dination structure and the crystal growth characteristics. The Co-
MOF-B crystals exhibited a 2D (6, 4)-network because the chains
were connected by coordinated H2O molecule, and the adjacent
(d)
(c)
(b)
(a)
10
15
20
25
30
2 Theta (degree)
Fig. 3. XRD patterns of catalyst samples: (a) fresh Co-MOF-A catalyst; (b) fresh Co-
MOF-B catalyst; (c) Co-MOF-B catalyst after one cyclohexne oxidation run; (d) Co-
MOF-B catalyst after the fifth cyclohexene oxidation run.
intensified, suggesting that the inherent crystallinity of the catalyst
is excellent. After cyclohexene oxidation, the characteristic diffrac-
tion peaks of Co-MOF-B were retained, but the intensity dropped
and the reflection peaks broadened. This indicated that the crystal-
line structure of the catalyst was preserved during the reactions,
but that the exterior surface was either covered by an amorphous
substance or partially destroyed, which was revealed by the SEM
images.
2D layers construct a 3D framework through
p–p interactions
between the aromatic groups of the 2,2’-bipy ligands [21].
3.2. Catalytic performances in cyclohexene oxidation
In the oxidation of cyclohexene, Cy-ol, Cy-one, and intermediate
Cy-HP were produced along with several by-products such as
cyclohexene oxide, 1,2-cyclohexanediol, hexanediol, adipic acid,
and cyclohexanone, among others, as shown in Scheme 1. The
potential reaction pathways in the catalytic oxidation of
Using SEM provided some intuitive information regarding the
surface properties of the Co-MOF-B catalyst during the catalytic
reaction process. Small and irregularly shaped particles were
Fig. 4. SEM images of Co-MOF-B catalyst samples: (a) and (b) fresh catalyst; (c) catalyst after one cyclohexene oxidation run; (d) catalyst after the fifth cyclohexene oxidation
run.

250
J. Hao et al. / Inorganica Chimica Acta 421 (2014) 246–254
Fig. 5. TEM images of Co-MOF-B catalyst samples: (a) fresh catalyst; (b) catalyst after one cyclohexene oxidation run.
OOH
OH
O
{[Co2(btec)(2,2'-bipy)2]·H2O}n
Solvent-free
O2
+
others
+
+
+
Scheme 1. Cyclohexene oxidation over {[Co2(btec)(2,2’-bipy)2]ꢀH2O}n catalyst.
fraction of them could be oxidized into deep oxygenated products.
These reaction pathways were the source of by-products.
O
OH
HO
OH
H2O
Side products such as cyclohexene oxide, 1,2-cyclohexanediol,
hexanediol, adipic acid, and cyclohexanone were identified by
comparison with standard samples (retention time in GC) and
by-products were detected by GC–MS.
+
Step 4
The effect of ligands on catalyst activity was examined for the
oxidation of cyclohexene with oxygen in solvent-free conditions,
and the results are listed in Table 2. For the blank test without
the catalyst samples, small amounts of products were detected
and the conversion was 2.7% (Entry 1). The conversion was only
12.3% with the Co-MOF-A catalyst, but using the Co-MOF-B cata-
lyst yielded a superior conversion of 30.6% under the same reaction
conditions (Entries 2 and 3). The Co-MOF-B catalyst exhibited an
excellent catalytic activity and the decomposition of Cy-HP was
accelerated, leading to the decrease of its selectivity. The difference
between Co-MOF-A and Co-MOF-B was that Co-MOF-A was mixed
with some residual H4btec. A series of experiments were designed
to obtain more information regarding the effects of ligands on the
catalytic activity in the oxidation of cyclohexene. Co(NO3)2ꢀ6H2O
was used as the catalyst with a conversion of 37.4% (Entry 4).
The homogeneous Co2+ ions presented slightly improved catalytic
activity than Co-MOF-B did. When using Co(NO3)2ꢀ6H2O, the con-
version decreased to 7.9% and 2.4% after the addition of H4btec
and 2,20-bipy, respectively, and decreased to 2.1% when H4btec
and 2,20-bipy were simultaneously added (Entries 5–7). These
results indicated that H4btec and 2,20-bipy inhibited the catalytic
activity of Co(NO3)2ꢀ6H2O in the oxidation of cyclohexene and that
2,20-bipy has a stronger influence than H4btec. Although H4btec
and 2,20-bipy exert the same inhibitive effects, the products distri-
butions vary greatly, which implied that the acting mechanisms of
the two ligands might be different. In addition to H4btec, other aro-
matic carboxylic acids were also analyzed. Remarkably, the con-
version decreased to 8.1% and 8.3% after the addition of 1,2-
benzenedicarboxylic acid and 1,3,5-benzenetricarboxylic acid,
respectively, but increased to 42.6% and 48.7% after addition of
benzoic acid and 1,4-benzenedicarboxylic acid, respectively
(Entries 8–11). When 2,20-bipy was subsequently added, the
OOH
Co-MOF
Step 1
Co-MOF
O2
+
OH
O
Step 2
O
H2O
+
Scheme 2. Potential reaction pathways in the catalytic oxidation of cyclohexene
with oxygen over {[Co2(btec)(2,20-bipy)2]ꢀH2O}n (Co-MOF) catalyst.
cyclohexene with oxygen over {[Co2(btec)(2,20-bipy)2]ꢀH2O}n were
presumed in Scheme 2 according to the literature [3,7,9,25]. It is a
complex radical-chain reaction and cyclohexenyl peroxyl radical
(Cy-OOꢀ) is the main chain propagator because the intermediate
product of Cy-HP was detected in the reactions. Cy-HP was initially
formed as the key primary product, which was prone to directly
decompose to Cy-ol and Cy-one or to Cy-one and water in the pres-
ence of {[Co2(btec)(2,20-bipy)2]ꢀH2O}n catalyst cycling between the
Co(II) and Co(III) oxidation states (step 1 and 2). These reaction path-
ways provide the principal source of main products. Cy-HP can also
change to Cy-ol and cyclohexene oxide through the epoxidation of
cyclohexene, and cyclohexene oxide can react with water to pro-
duce 1,2-cyclohexanediol (step 3 and 4). When Cy-ol, Cy-one,
cyclohexene oxide and 1,2-cyclohexanediol were obtained, a small

J. Hao et al. / Inorganica Chimica Acta 421 (2014) 246–254
251
Table 2
Effect of ligands on catalytic performance in cyclohexene oxidation.
Entry
Catalysts
Ligands
Conversion (%)
Selectivity (%)
Cy-ol
Cy-one
Cy-HP
Others
1a
–
–
–
–
–
2.7
12.3
30.6
37.4
7.9
2.4
2.1
8.1
8.3
42.6
48.7
39.9
2.8
8.5
8.1
9.8
21.8
22.3
26.8
38.3
29.7
47.3
57.6
26.5
42.3
43.3
41.6
42.1
49.5
33.7
68.0
64.9
48.4
1.4
1.7
4.7
15.0
18.1
5.4
6.7
7.4
4.7
1.9
15.4
16.7
17.5
5.5
2a
Co-MOF-A
Co-MOF-B
3a
4b
Co(NO3)2ꢀ6H2O
Co(NO3)2ꢀ6H2O
Co(NO3)2ꢀ6H2O
Co(NO3)2ꢀ6H2O
Co(NO3)2ꢀ6H2O
Co(NO3)2ꢀ6H2O
Co(NO3)2ꢀ6H2O
Co(NO3)2ꢀ6H2O
Co(NO3)2ꢀ6H2O
Co(NO3)2ꢀ6H2O
Co(NO3)2ꢀ6H2O
42.2
17.7
26.9
22.5
11.2
18.2
26.5
16.7
25.5
26.9
11.7
5b
H4btec
2,20-bipy
H4btec and 2,20-bipy
1,2-benzenedicarboxylic acid
1,3,5-benzenetricarboxylic acid
benzoic acid
1,4-benzenedicarboxylic acid
benzoic acid and 2,20-bipy
1,4-benzenedicarboxylic acid and 2,20-bipy
–
47.2
19.1
12.5
57.6
37.6
14.8
25.0
14.9
18.1
47.3
6b
7b
8b
9b
10b
11b
12b
13b
14c
21.8
7.3
a
b
c
Reaction conditions: cyclohexene 5 mL, molar ratio of Co and cyclohexene 1:250, oxygen 2 MPa, temperature 343 K, time 6 h.
Reaction conditions: cyclohexene 5 mL, molar ratio of Co and cyclohexene 1:125, molar ratio of Co and ligands 1:1, oxygen 2 MPa, temperature 343 K, time 6 h.
Reaction conditions: cyclohexene 5 mL, Co2+ content in solution 24.7 mg/L, oxygen 2 MPa, temperature 343 K, time 6 h.
conversion changed slightly for benzoic acid, but decreased to 2.8%
for 1,4-benzenedicarboxylic acid (Entries 12 and 13). These results
suggested that for aromatic carboxylic acids containing two or
more carboxyl groups, when two carboxyl groups are in ortho-
and meta-positions, the ligand exerted a marked inhibitive effect;
using 2,20-bipy, also exerted the same inhibitive effect, but the
influence can be eliminated by adding benzoic acid. To explain this
complicated phenomenon, H4btec was further studied. After the
catalytic oxidation of cyclohexene, conducting ICP-OES analysis
revealed that the concentration of the Co2+ ion in the filtered prod-
uct solution was 136.3 mg/L by using Co(NO3)2ꢀ6H2O (Entry 4) and
24.7 mg/L by using Co(NO3)2ꢀ6H2O and H4btec (Entry 5) (photo-
graphs of the products are displayed in Fig. S2). The Co2+ ion con-
centration in the solution decreased significantly after addition of
H4btec, which demonstrated that the inhibitive effect of H4btec
on Co(NO3)2ꢀ6H2O was primarily caused by the immobilization
and encapsulation of homogeneous Co2+ for the formation of a che-
lating structure. Subsequently, Co(NO3)2ꢀ6H2O was used alone
with the same residual Co2+ content (24.7 mg/L) in the aforemen-
tioned solutions, and the conversion was 21.8% (Entry 14), which
is higher than 7.9% (Entry 5), which indicated that the residual
Co2+ content in solutions essentially plays a role in the catalytic
reactions; however, catalytic behavior was also hindered by H4btec
through another mechanism.
in Table 3. A {[Co(2,20-dpa)(2,20-bipy)2]ꢀ2H2O}n (2,20-dpa: 2,20-
diphenic acid; 2,20-bipy: 2,20-bipyridine) metal–organic framework
was prepared under hydrothermal conditions and a Cu-ELD/Co was
prepared using copper electroless deposition as nanoparticles
adsorbed on Co metal powders. For {[Co(2,20-dpa)(2,20-bipy)2]ꢀ2H2-
O}n the conversion was 39.0%, and it decreased to 21.1% and 19.1%
after addition of 0.5 and 1.0 mg H4btec, respectively, and it
decreased slightly when the amounts of H4btec used increased
(Entry 1–4). Similar changes in the conversions presented when
combinations of the Cu-ELD/Co catalyst and H4btec were used
(Entry 6–9). These results indicated that when the amount of H4-
btec used was equal in the cyclohexene oxidation using the two
types of aforementioned heterogeneous catalysts, the conversion
decrease was approximately equivalent regardless of the type
and dosage of the catalysts. No interactions with H4btec such as
coordination, adsorption, and encapsulation were detected, partic-
ularly for the Cu-ELD/Co catalyst with a metal–metal alloy compo-
sition. Although H4btec might interfere with the contact between
the substrate and active sites of the catalyst, the conversions were
not dramatically increased after the amount of catalysts used was
doubled under the same reaction conditions (Entries 5 and 10). A
conclusion is drawn that the inhibitive effect of H4btec is indepen-
dent of the heterogeneous catalysts but dependent on the cyclo-
hexene oxidation process. Adding H4btec presented low
conversion with high Cy-HP selectivity, which suggests that the
chemical changes involving Cy-HP were hindered by H4btec and
lead to the deceleration of the overall oxidation rate because of
its aforementioned dominative role in cyclohexene oxidation. It
To obtain more information regarding the inhibitive mechanism
of H4btec, other heterogeneous catalysts were investigated in
cyclohexene oxidation by excluding the possibility of physical
adsorption and H4btec residue, and the results are summarized
Table 3
Effect of H4btec on catalytic performance in cyclohexene oxidation.
Entry
Catalysts
Catalyst amount (mg)
H4btec amount (mg)
t (h)
Conversion (%)
Selectivity (%)
Cy-ol
Cy-one
Cy-HP
Others
1a
2a
3a
4a
5a
6b
7b
8b
9b
10b
{[Co(2,20-dpa)(2,20-bipy)2]ꢀ2H2O}n
{[Co(2,20-dpa)(2,20-bipy)2]ꢀ2H2O}n
{[Co(2,20-dpa)(2,20-bipy)2]ꢀ2H2O}n
{[Co(2,20-dpa)(2,20-bipy)2]ꢀ2H2O}n
{[Co(2,20-dpa)(2,20-bipy)2]ꢀ2H2O}n
Cu-ELD/Co
Cu-ELD/Co
Cu-ELD/Co
Cu-ELD/Co
Cu-ELD/Co
3.0
3.0
3.0
3.0
6.0
11.7
11.7
11.7
11.7
23.4
-
6
6
6
6
6
6
6
6
6
6
39.0
21.1
19.1
14.1
23.1
44.0
26.9
19.3
16.3
20.1
13.4
9.7
10.5
6.9
10.3
9.0
6.3
5.5
6.8
6.2
33.0
26.2
27.4
19.7
28.0
26.2
21.2
18.0
19.8
19.7
36.0
53.0
50.7
63.7
50.5
47.1
60.6
67.0
62.8
63.0
17.6
11.1
11.4
9.7
11.2
17.7
11.9
9.5
0.5
1.0
10.0
1.0
-
0.5
1.0
10.0
1.0
10.6
11.1
a
Reaction conditions: cyclohexene 5 mL, oxygen 1.5 MPa, temperature 340 K.
Reaction conditions: cyclohexene 5 mL, oxygen 2.0 MPa, temperature 349 K.
b

252
J. Hao et al. / Inorganica Chimica Acta 421 (2014) 246–254
50
100
80
60
40
20
0
40
30
20
10
0
1
2
5
6
Rea3ction tim4e (h)
Fig. 6. Optimized geometries of the hydrogen bonds between Cy-HP and H4btec.
Fig. 8. Effect of reaction time on conversion and selectivity in cyclohexene
oxidation. Reaction conditions: cyclohexene 5 mL, Co-MOF-B catalyst 57.3 mg,
oxygen 2 MPa, temperature 343 K. h: Conversion of cyclohexene; N: Selectivity of
Cy-ol; .: Selectivity of Cy-one; ꢁ: Selectivity of Cy-HP; d: Total selectivity of Cy-ol,
Cy-one and Cy-HP.
is assumed that Cy-HP and H4btec interact through a hydrogen-
bonding network, and theoretical modeling and calculation were
performed to preliminarily assume the reasonable configuration.
The optimized geometries are illustrated in Fig. 6. The calculated
results revealed that the oxygen atom connecting with cyclohexe-
nyl in a Cy-HP –OOH group forms a hydrogen bond with the hydro-
gen atom in one H4btec –COOH group and that the hydrogen atom
in a Cy-HP –OOH group forms a hydrogen bond with carbonyl oxy-
gen in another H4btec –COOH group on one side. The length of the
O–Hꢀ ꢀ ꢀO hydrogen bond was 1.823 Å and 1.945 Å. A symmetrical
construction was displayed on the other side of H4btec. The hex-
atomic ring network built with the atoms to incorporate the two
hydrogen bonds strengthened the stability of Cy-HP and the steric
effect was also beneficial for protecting the Cy-HP –OOH group
from catalyst attack.
(Fig. 7) shows the effects of reaction temperature on cyclohex-
ene oxidation. When the temperature was increased from 323 to
343 K, the conversion of cyclohexene increased from 4.8% to
39.2%, but the total selectivity of Cy-ol, Cy-one and Cy-HP
decreased from 98.2% to 82.1%, because of the formation of by
products such as cyclohexene oxide, 1,2-cyclohexanediol, and hex-
anediol. The selectivity of Cy-HP decreased from 66.8% to 41.3%,
whereas the selectivity of Cy-ol increased from 8.7% to 11%, and
that of Cy-one increased from 22.7% to 29.8%, which indicated that
Cy-HP rapidly decomposed into Cy-ol and Cy-one at high reaction
temperatures. That the change in selectivity of Cy-HP was contrary
to those of Cy-ol and Cy-one and that the selectivity of Cy-one was
always higher than that of Cy-ol verified the aforementioned reac-
tion pathways. The reaction temperature is one of the essential
parameters in cyclohexene oxidations using the Co-MOF-B cata-
lyst. The occurrence of deep oxidation leaded to carbonization at
high temperatures, whereas no reaction occurred at low tempera-
tures. Thus, the feasible reaction temperature is between 338 and
343 K.
(Fig. 8) illustrates the conversion and selectivity changes with
different reaction times catalyzed using the Co-MOF-B catalyst at
a temperature of 343 K. The conversion increased from 2.8% to
39.2% when the reaction time was extended from 1 h to 6 h. The
selectivity of Cy-HP was 65.5% for 1 h, which suggested that Cy-
HP was the primary product at the initial stage of reaction; how-
ever, it decreased to 41.3% when the reaction time was 6 h. Simul-
taneously, the selectivity of Cy-ol increased from 8.5% to 11% and
the selectivity of Cy-one increased from 23.5% to 29.8%. The con-
centration of free radicals accumulated as the reaction time
increased, which lead to the gradual increase of the reaction rate,
the Cy-HP decomposition rate and the Cy-ol and Cy-one formation
rate were simultaneously enhanced. The total selectivity of Cy-ol,
Cy-one, and Cy-HP decreased linearly because peroxidation
occurred as the reaction time increased. Therefore, in the following
experiments, the oxidation was operated at 6 h in consideration of
both the conversion and selectivity under the present reaction
conditions.
50
40
30
20
10
0
100
80
60
40
20
0
50
40
30
20
10
0
100
80
60
40
20
0
320
325
330
335
340
345
0
20
40
60
80
Quality of catalyst (mg)
Temperature (K)
Fig. 7. Effect of reaction temperature on conversion and selectivity in cyclohexene
oxidation. Reaction conditions: cyclohexene 5 mL, Co-MOF-B catalyst 57.3 mg,
oxygen 2 MPa, time 6 h. h: Conversion of cyclohexene;N: Selectivity of Cy-ol; .:
Selectivity of Cy-one; ꢁ: Selectivity of Cy-HP; d: Total selectivity of Cy-ol, Cy-one
and Cy-HP.
Fig. 9. Effect of Co-MOF-B catalyst amounts on conversion and selectivity in
cyclohexene oxidation. Reaction conditions: cyclohexene 5 mL, oxygen 2 MPa,
temperature 343 K, time 6 h. h: Conversion of cyclohexene; N: Selectivity of Cy-ol;
.: Selectivity of Cy-one; ꢁ: Selectivity of Cy-HP; d: Total selectivity of Cy-ol, Cy-
one and Cy-HP.

J. Hao et al. / Inorganica Chimica Acta 421 (2014) 246–254
253
(Fig. 9) depicts the effects of using different Co-MOF-B catalyst
amounts on conversion and selectivity. The cyclohexene conver-
sion increased as the amount of catalyst used increased. The con-
version was 4.6% when 5.0 mg of catalyst was used and it
reached a maximum value of 39.2% with 57.3 mg of catalyst. The
conversion decreased by further increasing the amount of catalyst
used, and it was 27.8% when the amount of catalyst used was
86.0 mg. The selectivity of Cy-HP presented a minimum value of
41.3% when 57.3 mg of catalyst was used, and the selectivity of
Cy-ol and Cy-one simultaneously reached the maximum value.
This indicates that the substrates adsorption in catalysts was bal-
anced and that the catalytic activity was the most effective when
57.3 mg of catalyst was used. Enhancing the catalyst quality can
inhibit reactions, which was identified as the so-called ‘‘catalyst
inhibitor conversion’’ phenomenon already known for certain
auto-oxidation processes [26]. A similar phenomenon has also
been observed in previous studies [27,28]. Therefore, the optimal
molar ratio of Co content and cyclohexne was 1:300.
Catalyst recycling is a crucial aspect of practical applications.
The reusability of the Co-MOF-B catalyst in the oxidation of cyclo-
hexene was examined under the conditions of 343 K and 2 MPa O2.
After a cyclohexene oxidation run, the solid catalyst was separated
through centrifugation from the product solution and washed with
ethanol several times, then dried at room temperature and reused
for the next run under the same conditions. In the process of cata-
lyst recycling, losing certain amounts of the catalyst is unavoid-
able. To maintain the same catalyst and substrate molar ratio,
the amount of cyclohexene was decreased accordingly. As the
results shown in Table 4, the conversion significantly decreased
from 33.2% (the first run; Entry 1) to 19.7% (the second run; Entry
2), but did not considerably change in the subsequent runs (Entries
3–5). According to the ICP-OES analysis, the concentration of Co in
the filtered product solution was 0.1385 mg/L after the first run
and 0.024 mg/L after the fifth run, which means that
approximately 0.89 and 0.32 wt.% of Co in the Co-MOF-B catalyst
leached from the matrix, respectively. The leached Co amount
was as minimal as the leached Au amount from Au/La-OMS-2
(0.24) catalyst in the solvent-free system [7]. To prove the hetero-
geneous nature of the catalytic reaction, the conventional filtration
experiment was carried out over Co-MOF-B catalyst. As the results
shown in Table 5 (Entries 1–5), when the solid catalyst was filtered
off from the reaction mixture after reacting for 4 h, the conversion
of cyclohexene in the filtrate increased from 12.0% to 13.8% in 1 h
and to 16.3% in 2 h respectively, which indicated that the leached
Co species were not responsible for the obtained catalytic activity
over Co-MOF-B catalyst. So it is confirmed that the present reac-
tions are truly performed over the Co-MOF-B catalyst surface het-
erogeneously. Therefore, it was suggested that the Co-MOF-B
catalyst surface was surrounded by organic residue after the cyclo-
hexene oxidation runs, as shown in Fig. 4, which affected the acces-
sibility of reactant molecules to metal active sites and decreased
the catalytic activity in the recycling tests. Because of this possibil-
ity, the catalyst was collected after a cyclohexene oxidation run,
treated using scCO2-expanded ethanol system, and reused in a sec-
ond run. Remarkably, the conversion of cyclohexene was 53.6%
(Entry 6) and the activity was higher than that of the fresh catalyst,
which suggested that this method not only realized catalyst regen-
eration, but also increased the activity of catalyst. More details
were obtained from the textural properties (Table 6). The BET sur-
face areas and pore volumes of the Co-MOF-B samples were deter-
mined by analyzing the N2 adsorption–desorption data. The Co-
MOF-B samples exhibited low surface areas and small pore vol-
umes, which can be attributed to the high crystallinity of the layer
microstructure, which is consistent with the results of the TEM
analysis (Fig. 5). The textural properties of the Co-MOF-B catalyst
are similar to those of the Mg/Al reconstructed hydrotalcite layered
crystals prepared by Pavel et al. [29,30]. Based on the results, it can
be assumed that the accessibility of substrate and catalytic active
Table 4
Recycle and regeneration of Co-MOF-B catalyst in cyclohexene oxidation.
b
Entry
Run times
Conversion (%)
TOF (hꢂ1
)
Selectivity (%)
Cy-ol
Cy-one
Cy-HP
Others
1
2
3
4
first
second
third
fourth
fifth
second
33.2
19.7
15.1
15.2
19.5
53.6
16.6
9.9
7.6
7.6
9.8
9.9
11.7
10.5
13.3
12.8
13.0
31.6
31.3
28.1
34.1
33.6
38.6
48.5
49.7
55.9
50.4
48.9
28.8
10.0
7.3
5.5
2.2
4.7
5
6a
26.8
19.6
Reaction conditions: cyclohexene 5 mL, molar ratio of Co and cyclohexene 1:300, oxygen 2 MPa, temperature 343 K, time 6 h.
a
The catalyst was collected after one cyclohexene oxidation run and treated with a scCO2-expanded ethanol system.
b
Turnover Frequency (TOF) is calculated by expression of (moles of cyclohexene consumed)/[(mole of total Co2+ ions in catalyst samples used) ꢁ time (h)].
Table 5
Catalytic oxidation of cyclohexene with oxygen in different reaction conditions.
Entry
Catalyst
Time (h)
Conversion (%)
TOF (hꢂ1
)
Selectivity (%)
Cy-ol
Cy-one
Cy-HP
Others
1a
{[Co2(btec)(2,20-bipy)2]ꢀH2O}n
4
5
6
5
6
20
24
24
12.0
21.9
39.2
13.8
16.3
32.8
48.0
52.2
6.0
11.0
19.6
6.9
8.2
8.0
–
8.5
8.9
11.0
8.0
23.0
24.4
29.8
28.4
28.4
51.2
44.0
71.2
63.0
56.7
41.3
53.0
56.8
–
5.5
10.0
17.9
10.6
7.5
–
2a
{[Co2(btec)(2,20-bipy)2]ꢀH2O}n
3a
{[Co2(btec)(2,20-bipy)2]ꢀH2O}n
4a,b
5a,c
–
–
7.3
6d [3]
7e [7]
8f [8]
[Co2(DOBDC)(H2O)2]ꢀ8H2O
39.3
40.3
11.2
Au/La-OMS-2
–
14.2
–
–
Cr-MCM-41
–
a
b
c
d
e
f
Reaction conditions: cyclohexene 5 mL, molar ratio of Co and cyclohexene 1:300, oxygen 2 MPa, temperature 343 K.
The catalyst was filtered off from the reaction mixture after reacting for 4 h and then kept reacting for 1 h.
The catalyst was filtered off from the reaction mixture after reacting for 4 h and then kept reacting for 2 h.
Reaction conditions: catalyst 50 mg, cyclohexene 5 mL, temperature 353 K, oxygen balloon.
Reaction conditions: catalyst 0.2 g, cyclohexene 20 mL, temperature 353 K, oxygen 0.4 MPa.
Reaction conditions: catalyst 20 mg, cyclohexene 1 g, temperature 343 K, oxygen 1 atm.

254
J. Hao et al. / Inorganica Chimica Acta 421 (2014) 246–254
Table 6
Acknowledgments
Textural properties of Co-MOF-B samples.
Samples
BET surface area (m2 gꢂ1
)
Pore Volume (cm3 gꢂ1
)
The authors gratefully acknowledge the financial support from
the Natural Science Foundation of China (NSFC 21163011), the
Natural Science Foundation of Inner Mongolia (2013MS0208),
and the Science Research Projects of Inner Mongolia University
(NJ10072).
Fresh Co-MOF-B
Co-MOF-Ba
1.6
2.0
3.1
0.0084
0.0126
0.0151
Co-MOF-Bb
a
The catalyst sample was collected after one cyclohexene oxidation run.
b
The catalyst sample was collected after one cyclohexene oxidation run and
treated with a scCO2-expanded ethanol system.
Appendix A. Supplementary material
sites mainly occurred on the catalyst surface because of the poor
porous structure. Therefore, the regeneration and active enhance-
ment of catalysts by treating with scCO2-expanded ethanol were
attributed to these aspects: first, the surface area of the Co-MOF-
B catalyst samples increased because the crystal particles were
crushed in the treatment process, and the crystal particle size is
an essential factor affecting the catalytic activity; second, the
organic residue on the surface of catalyst samples after a reaction
was extracted using scCO2-expanded ethanol; using scCO2-
expanded liquid aids separation because it possesses unique
properties, such as low viscosity, high diffusivity, near-zero surface
tension, and high solubility for a wide range of organic compounds
[31], and also excess CO2 weakens the interactions between certain
polymers and matrices [32]. The regeneration of a deactivated cat-
alyst through supercritical CO2 extraction has also been reported in
the literature [33,34]. Furthermore, the TOF values of the Co-MOF-
B catalyst were relatively low, which was calculated according to
the mole of total Co2+ ions in the catalyst samples used. It was
revealed that only the Co sites at the outer surface of catalysts
are accessible and involved in the catalytic reactions, so the real
TOF values must be higher than the calculated TOF values
evidently. Finally, in comparison with the activities of heteroge-
neous catalysts such as [Co2(DOBDC)(H2O)2]ꢀ8H2O Co-MOF
(DOBDC = 2,5-dihydroxyterephthalic acid), Au/La-OMS-2, and Cr-
MCM-41 in the similar reaction conditions reported in the litera-
ture (Table 5 Entries 6–8), comparable reaction conversion and
TOF value were obtained with the present {[Co2(btec)(2,20-bipy)2]ꢀ
H2O}n Co-MOF catalyst.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2014.06.005.
These data include MOL files and InChiKeys of the most important
compounds described in this article.
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In summary, the Co (II) metal–organic framework {[Co2(-
btec)(2,20-bipy)2]ꢀH2O}n exhibited effective performance in the
allylic oxidation of cyclohexene with oxygen under solvent-free
conditions. It exhibited high crystallinity with a compact layered
microstructure and poor porous texture, which was determined
by the coordination modes and crystal growth characteristics. It
behaved as a heterogeneous catalyst, which was deactivated in cat-
alyst recycling because the formation of organic residue adsorbed
on the surface covered the active Co2+ sites. Catalyst regeneration
was achieved and the catalytic activity was enhanced by treating
with a scCO2-expanded ethanol system, which was attributed to
extracting organic residue and crushing crystal particles. The
inhibitive effects of H4btec and other ligands on cyclohexene oxi-
dation was detected, regarding aromatic carboxylic acids contain-
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were in ortho- and meta-positions, the ligand exerted a marked
inhibitive effect. The research results confirmed that the inhibitive
effect of H4btec is independent of the heterogeneous catalyst but
dependent on cyclohexene oxidation reactions, which is explained
by the presumed hydrogen bonds between H4btec and the 2-cyclo-
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