CPO-27-M as heterogeneous catalysts for aldehyde cyanosilylation and styrene oxidation

Article abstract of DOI:10.1016/j.molcata.2014.06.040

A series of isostructural 3D metal-organic frameworks of 2,5-dihydroxyterephthalate with different metal ions, CPO-27-M (or MOF-74-M, M = Co, Mg, Mn, Ni and Zn), have been studied as catalysts for cyanosilylation of aldehydes with trimethylsilylcyanide an

Full text of DOI:10.1016/j.molcata.2014.06.040

Journal of Molecular Catalysis A: Chemical 394 (2014) 57–65
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
CPO-27-M as heterogeneous catalysts for aldehyde cyanosilylation
and styrene oxidation
Hong-Fei Yao, Yang Yang, Hui Liu, Fu-Gui Xi, En-Qing Gao∗
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of isostructural 3D metal–organic frameworks of 2,5-dihydroxyterephthalate with different
metal ions, CPO-27-M (or MOF-74-M, M = Co, Mg, Mn, Ni and Zn), have been studied as catalysts
for cyanosilylation of aldehydes with trimethylsilylcyanide and oxidation of styrene with tert-
butylhydroperoxide, and two mixed-metal Co-Mn MOFs also studied for cyanosilylation. All these MOFs
are active in promoting cyanosilylation, but for styrene oxidation, only the Co and Mn MOFs are active
while the others behave as initiators rather than catalysts. For both reactions, CPO-27-Mn exhibits the
highest activity, and the catalytic processes are heterogeneous. Radical mechanisms were proposed for
the styrene oxidation over CPO-27-Mn, which yields styrene oxide, benzaldehyde and a minor amount
of phenylacetaldehyde. The cyanosilylation over CPO-27-Mn shows size selectivity towards aldehyde
substrates, and the catalyst can be recycled without losing its structural integrity and catalytic activity.
It is also recyclable for styrene oxidation, though the structure changes after the catalytic reaction.
© 2014 Elsevier B.V. All rights reserved.
Received 22 March 2014
Received in revised form 19 June 2014
Accepted 30 June 2014
Available online 8 July 2014
Keywords:
Metal–organic frameworks
Heterogeneous catalysis
Cyanosilylation
Styrene oxidation
Manganese
1. Introduction
The isostructural CPO-27-M (or MOF-74-M) series, of general
formula M2(dhtp) (M = Co, Fe, Mg, Mn, Ni and Zn; H4dhtp = 2,5-
Metal–organic frameworks (MOFs) are a newly emerging class
of porous materials that bear some intriguing features such as
hybrid compositions, exceptionally high surface area, adjustable
pore size as well as modifiable functionality [1,2], so they have bril-
liant prospects for applications such as gas adsorption/separation
[3,4], drug delivery [5], chemical sensing [6], and heterogeneous
catalysis [7–10]. Heterogeneous catalysts are highly desirable for
several advantages in comparison with homogeneous ones, includ-
ing easier recovery and recycle, less wastes and cleaner products.
Furthermore, porosity can lead to enhanced reactivity and selectiv-
ity [10]. MOFs show great promise as new porous heterogeneous
catalysts that can be alternative or complementary to conventional
ones [10,11]. The catalytic sites of MOFs, either inherent in the
frameworks or generated by post-synthetic methods [10,12], can
be metal centers with unsaturated (or labile) coordination or any
active groups attached to the frameworks. MOFs may also serve as
host matrices encapsulating traditional catalysts such as nano-size
metals, metal oxides and molecular catalysts [13–15].
dihydroxyterephthalic acid), are 3D MOFs featuring 1D [M(␮-
COO)(␮-OH)]n chains and honeycomb-like hexagonal channels
with a free diameter of ∼12 A [16–23]. In the solvated forms, the
˚
channels are lined with coordinated water or DMF molecules. Upon
desolvation, the metal coordination changes from octahedral to
square pyramidal without compromising the framework integrity,
leaving coordinatively unsaturated metal sites open to channels.
The frameworks show good chemical stability, making them good
candidates for heterogeneous catalysts. These materials represent a
rare series of stable isostructural MOFs with a wide range of metals,
offering a good opportunity to test the catalytic properties of differ-
ent metal sites in almost identical environments. Furthermore, the
isomorphism of these MOFs facilitates the preparation of mixed-
metal catalysts with variable composition and tunable catalytic
properties. However, catalytic studies with CPO-27-M are still rare,
and the reactions studied have been limited to CO2 cycloaddition of
epoxides (M = Co, Mg) [24–26], aerobic or H2O2 oxidation of cyclo-
hexene (M = Co, Ni) [27,28], dehydrogenation of ammonia borane
(M = Mg, Zn) [29,30] and phenol hydroxylation (M = Fe) [31]. For
comparison with MIL-101-Sc, CPO-27-Ni has recently been stud-
ied for the intermolecular carbonyl ene reaction of ethylglyoxylate
and the Friedel–Crafts Michael addition of indoles to methylvinyl
ketone [32].
∗
Corresponding author at: Shanghai Key Laboratory of Green Chemistry and
Chemical Processes, Department of Chemistry, East China Normal University, 3663
North Zhongshan Rd., Shanghai 200062, China. Tel.: +86 21 62233404.
E-mail address: eqgao@chem.ecnu.edu.cn (E.-Q. Gao).
We are interested in the catalytic properties of the CPO-
27-M series for two important organic reactions: carbonyl
http://dx.doi.org/10.1016/j.molcata.2014.06.040
1381-1169/© 2014 Elsevier B.V. All rights reserved.

58
H.-F. Yao et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 57–65
diffractometer equipped with Cu-K␣ at a scanning rate of 10◦ min−1
in the 2Â range 3–50◦. The accelerating voltage and the applied
current were 35 KV and 25 mA, respectively. SEM study and the
energy dispersive X-ray spectroscopy (EDS) analyses were car-
ried out with a S-4800 HITACHI scanning electron microscope. Gas
chromatography (GC) was conducted using a LingHua GC 9890E
instrument equipped with an FID detector and an SE-54 capillary
column (30 m × 0.25 mm × 0.25 ␮m).
cyanosilylation and styrene oxidation. Cyanosilylation of carbonyl
compounds provides a convenient route to cyanohydrins, which
are versatile intermediates that can be readily converted into
industrially important compounds [33,34]. Cyanosilylation of
aldehydes does not require strong Lewis acidity, and it is one of
most frequently used model reactions for the study of MOFs as
Lewis acid catalysts [10,35–39]. Catalytic oxidation of alkenes
is a fundamental reaction that finds important applications in
chemical industry. The oxidation of various substrates has been
accomplished using metal-containing homo- or heterogeneous
catalysts, including an increasing number of MOFs [27,28,40–48].
In this paper, we present a systematic study on the catalytic
properties of different CPO-27-M catalysts (M = Co, Mg, Mn, Ni
and Zn) for aldehyde cyanosilylation with trimethylsilylcyanide
and styrene oxidation with tert-butylhydroperoxide. After com-
parative investigations on these different MOFs, we are focused
on the most active catalyst to study the heterogeneity, size selec-
tivity, recyclability and the possible mechanisms. The studies
on two mixed-metal catalysts, CPO-27-Mn0.57Co0.43 and CPO-27-
Mn0.10Co0.90, are also included to confirm the influence of different
metal ions.
2.3. Catalytic reaction
Generally, before a catalytic test, the as-synthesized MOFs were
activated by the following solvent-exchange and heating proce-
dures, if not specified otherwise. The MOFs were immersed and
stirred continuously in methanol for 12 h, and then the liquid was
decanted off. This methanol exchange procedure was repeated
twice. The solid thus obtained was filtered, dried in air, transferred
into a flask, heated at 170 ◦C for 4 h under dynamic vacuum, and
cooled under nitrogen atmosphere to an appropriate temperature
for the catalytic reaction. n-Dodecane was used as the internal
standard for GC analysis. The temperature program for GC analysis
was set as follows: the temperature was held at 40 ◦C for 1 min, then
raised to 260 ◦C at 30 ◦C/min and held for 5 min. Inlet and detector
temperatures were 280 ◦C.
2. Experimental
2.1. Materials and catalyst preparation
2.3.1. Cyanosilylation of benzaldehyde
The catalytic reactions were carried out in dichloromethane
(DCM) solutions under nitrogen atmosphere. The solvent was
dried and distilled before use. In a typical catalytic experiment,
DCM (15 mL), TMSCN (10 mmol), n-dodecane (5 mmol, as inter-
nal standard) and benzaldehyde (5 mmol) were added into the
flask containing activated CPO-27-M (0.5 mmol). The mixture was
stirred and refluxed in an oil bath. The reaction conversion was
monitored by GC at different time intervals.
The starting materials for catalyst synthesis [2,5-
dihydroxyterephthalic acid (H4dhtp) and various metal salts]
and those for catalytic reactions [trimethylsilylcyanide (TMSCN),
benzaldehyde, styrene, tert-butylhydroperoxide (TBHP, 5–6 M
in decane)] were all used without further purifications. All of
these materials except for TBHP (Sigma) were purchased from
Sinopharm.
CPO-27-M catalysts were synthesized by the solvothermal pro-
cedures described in the literature. For M = Co, Ni, Mg, Zn, a mixture
of tetrahydrofuran (THF) and water was used as solvent, and the
metal sources are metal acetates or nitrates [21,49]; CPO-27-Mn
was synthesized from manganese chloride with a mixture of N,N’-
dimethylformide (DMF), ethanol and water as solvent [22]. The
method for CPO-27-Mn was also successfully applied to CPO-27-Co.
Two mixed-metal MOFs (solid solutions) were prepared as follows.
CPO-27-Mn0.57Co0.43. This compound was prepared according
to the literature method for CPO-27-Mn. H4dhtp (0.34 mmol,
67 mg), MnCl2.4H2O (0.56 mmol, 110 mg), and CoCl2.6H2O
(0.56 mmol, 132 mg) were dissolved in a mixture of DMF, ethanol
and water (13/1/1, v/v; 15 mL) in a 23 mL Teflon lined stainless
steel autoclave and heated at 135 ◦C for 24 h. After cooling to room
temperature, the solid was filtered out, washed with methanol
three times, and dried in air. Yield: 90%. The metal contents in the
product were determined by EDS analysis
2.3.2. Styrene oxidation
In a typical reaction, the flask containing the activated catalyst
was charged with a given amount of styrene (5 mmol), TBHP (5–6 M
in decane, 15 mmol) and solvent (5 mL, if needed). The reaction
mixture was stirred and kept at the given temperature under nitro-
gen atmosphere. Aliquot samples were withdrawn at different time
intervals and immediately detected by GC.
3. Results and discussion
3.1. Characterization of the catalysts
The XRD patterns (Fig. 1a) of all as-synthesized products with
different metal ions (Co, Ni, Mg, Zn, and Mn) are similar to one
another and in good agreement with those reported elsewhere
[21,22,49], indicating that the products are isostructural and have
the desired CPO-27-type structures. The XRD profiles (Fig. 1b) of the
samples activated by methanol exchange and subsequent vacuum
heating at 170 ◦C showed no appreciable changes compared with
those for the as-synthesized MOFs, confirming that the structural
integrity of the frameworks is retained after activation.
Initially, CPO-27-Mn was synthesized by a solvothermal reac-
tion for 24 h, according to a literature method [22]. However, we
found that the compound can be obtained within a much shorter
time, under otherwise identical conditions. The XRD pattern and
the SEM picture of the sample obtained by reacting for 4 h are
provided as Fig. S1 in Supplementary material. It is obvious that
the crystalline phase can be well formed within the reduced time.
Catalytic tests (vide infra) indicated that the catalysts synthesized
within 4 and 24 h showed similar activity.
CPO-27-Mn0.10Co0.90. This compound was prepared accord-
ing to the literature method for CPO-27-Co. H4dhtp (0.34 mmol,
67 mg) in THF (1.5 mL) and a solution of Co(OAc)2.4H2O (0.56 mmol,
138 mg) and Mn(OAc).4H2O (0.56 mmol, 136 mg) in H2O (2 mL)
were combined in the Teflon lined steel autoclave. The autoclave
was sealed and heated in a pre-heated oven at 110 ◦C for 3 days.
After cooling to room temperature, the solid was filtered out,
washed with methanol three times, and dried in air. Yield: 85%. The
metal contents in the product were determined by EDS analysis.
2.2. Characterization
The FT-IR spectra were recorded in the range 400–4000 cm−1
using KBr pellets on a Nicolet NEXUS 670 spectrophotometer. Pow-
der X-ray diffraction data were collected on a Rigaku Ultima IV

H.-F. Yao et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 57–65
59
Scheme 1. Cyanosilylation of benzaldehyde.
Fig. 1. XRD patterns of the as-synthesized (a) and activated (b) CPO-27-M MOFs.
We have also synthesized two mixed-metal MOFs, CPO-27-
Mn0.57Co0.43 and CPO-27-Mn0.10Co0.90. XRD analysis indicated that
they are isostructural with the parent MOFs (Fig. 2). Comparing the
XRD patterns of the Co, Mn and MnxCo1−x MOFs, one can see that
the reflection peaks show systematic shifts towards low angles as
the Mn content increases, consistent with the fact the ionic radius
of Mn(II) is larger than that of Co(II). No splitting or broadening was
observed for the peaks of mixed-metal systems compared with the
single-metal compounds. The results suggest that the mixed-metal
materials are essentially single phases (solid solutions with ran-
domly distributed metal sites), rather than mechanical mixtures of
phase-separated Co and Mn MOFs.
Fig. 3. Conversion vs. time plots for benzaldehyde cyanosilylation. (a) Reactions
with and without CPO-27-M catalysts. (b) CPO-27-Mn-catalyzed reactions with
and without filtering off the catalyst. Conditions: benzaldehyde, 5 mmol; TMSCN,
10 mmol; DCM, 15 mL, reflux; cat., 0.5 mmol (if used).
3.2. Catalytic properties for cyanosilylation
different CPO-27-M MOFs (M = Co, Ni, Mg, Zn, Mn). The time
dependence of benzaldehyde conversion is shown in Fig. 3a and
selected data are given in Table 1.
The addition reactions of TMSCN to benzaldehyde (Scheme 1)
were carried out to compare the catalytic performance of the
In all cases, the reactions of benzaldehyde with 2 molar equiva-
lent of TMSCN in the presence of activated CPO-27-M yielded only
the cyanosilylation product. Clearly, the catalytic activity is strongly
dependent upon the nature of the metal ion in the catalyst. CPO-
27-Co and -Ni show comparable activity with a conversion of ∼27%
within 48 h, which is higher by only 10% than the conversion for
the blank test in the absence of any catalyst under otherwise iden-
tical conditions. CPO-27-Zn and -Mg are more active, giving much
higher conversions within 48 h (83% and 96%, respectively).
Remarkably, CPO-27-Mn showed much higher activity than the
above materials, the reaction reaching almost completion within
1 h. The CPO-27-Mn catalyst was prepared by a solvothermal
method using DMF as solvent (with a small amount of ethanol
and water), while the other catalysts used in the above compar-
ison were obtained by the solvothermal synthesis in a mixture of
THF and water. Therefore, it should be clarified whether the excep-
tional activity of the Mn MOF is related to its special synthetic
Fig. 2. XRD patterns for mixed-metal MOFs and the parent materials.

60
H.-F. Yao et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 57–65
Table 1
Table 2
Comparison of different catalysts for the cyanosilylation of benzaldehyde.a
The influence of synthetic time and post-synthetic treatment on the catalytic activity
of CPO-27-Mn.a
Entry
Catalyst
Time (h)f
Conv. (%)f
Entry
Synthetic
time (h)b
Methanol
exchange
Heating
Time (h)c
Conv. (%)c
1
2
3
4
5
6
7
8
9
10
11
12
13
Without catalyst
CPO-27-Cob
CPO-27-Ni
CPO-27-Zn
CPO-27-Mg
2, 48
2, 48
2, 48
2, 48
2, 48
1
2, 48
1
1, 2
1
∼1, 17
∼2, 27
5, 27
14, 83
19, 96
100
3, 25
65
5, 9
1
2
3
4
5
24
24
24
24
4
−
+
−
−
+
+
+
10
10
3
0.2, 1
0.2, 1
90
>99
>99
58, >99
60, >99
−
CPO-27-Mn
+
CPO-27-Coc
+
CPO-27-Mn0.57Co0.43
CPO-27-Mn0.10Co0.90
CPO-27-Mn + pyd
CPO-27-Mn+ pyOe
a
Conditions: benzaldehyde, 5 mmol; TMSCN, 10 mmol; DCM, 15 mL, reflux; cat.,
0.5 mmol (10 mol%). “+”, the treatment was applied; “−”, the treatment was not
applied.
38
24
44
1
1
b
The time duration of solvothermal reactions for the synthesis of the catalyst.
For some entries, more than one conversion data obtained for different time
g
MnCl2
c
H4dhtp
1, 48
∼0, 17
periods (separated by commas) are provided for convenience of comparison.
a
Conditions: benzaldehyde, 5 mmol; TMSCN, 10 mmol; DCM, 15 mL, reflux; cat.,
0.5 mmol (10 mol%).
b
methanol treatment before use leads to enhanced activity, with
almost complete conversion after 10 h (entry 2). The heating treat-
ment is more efficient in enhancing the activity: the reaction time
required to achieve completion was dramatically reduced to 3 h
(entry 3). The reaction time is further reduced to 1 h when both
methanol exchange and subsequent heating treatment are applied
to the catalyst (entry 4). These results indicate that the successive
post-synthetic treatments can efficiently activate the catalyst and
that the heating procedure is especially important in generating
open metal sites. Hereafter in this paper, the fresh catalysts were
all subjected to the general activation treatments. Table 2 also gives
the catalytic data for the CPO-27-Mn sample synthesized by per-
forming the solvothermal reaction for only 4 h (entry 5). It seems
that the catalytic activity is not sensitive to the synthetic time, at
least in the range of 4–24 h. Therefore, the CPO-27-Mn catalyst can
be synthesized by a relatively time-saving procedure and activated
by mild and simple treatments.
To demonstrate whether the catalytic activity of the Mn MOF is
due to active sites in the solid phase or to any active species leach-
ing into the liquid phase, a control filtration test was carried out.
After a reaction over CPO-27-Mn proceeded for 12 min (58% con-
version), the catalyst was removed by hot filtration, and the filtrate
was refluxed again and monitored by GC analysis at different time
intervals. The plot of benzaldehyde conversion vs. time is given in
Fig. 3b, compared with the data for the reaction without filtration.
The very small increase of the conversion after filtration could be
attributed to a thermally activated slow reaction. The results sug-
gest that there is no appreciable leaching of active species into the
liquid phase and that the catalytic mechanism of the Mn MOF is
heterogeneous in nature.
The CPO-27-Co catalyst was prepared by a solvothermal procedure with THF-
water as solvent.
c
The CPO-27-Co catalyst was prepared by a solvothermal procedure with DMF-
ethanol-water as solvent.
d
In the presence of pyridine (5 mmol).
In the presence of pyridine N-oxide (3.5 mmol).
For some entries, more than one conversion data obtained for different time
e
f
periods (separated by commas) are provided for convenience of comparison.
g
The amount of MnCl2 was 1.0 mmol so that the metal content is the same as that
of the MOF catalysts.
method. For this purpose, it is desirable to compare the CPO-27-
Mn catalysts prepared by different methods. However, our efforts
to synthesize CPO-27-Mn using the THF-water method proved to
be unsuccessful. Nevertheless, the DMF method for CPO-27-Mn
has been successfully used to prepare CPO-27-Co, making it possi-
ble to compare the CPO-27-Co catalysts obtained by two different
methods. It proved that the two samples led to similar conver-
sions (Table 1, entries 2 and 7), so it may be said that the different
synthetic methods do not significantly influence the catalytic per-
formance.
To confirm that the remarkable difference in catalytic activ-
ity of the CPO-27-M MOFs is associated with the different metal
centers, two mixed-metal MOFs, CPO-27-Mn0.57Co0.43 and CPO-27-
Mn0.10Co0.90, which had been confirmed to be isostructural with
the parent Co and Mn MOFs (Fig. 2), were tested for the same cat-
alytic reaction. As can be seen from Table 1 (entries 8 and 9), the
catalytic activity of the mixed-metal catalysts lies between the Co
and Mn catalysts and increases with the Mn content. This confirms
that it is the Mn(II) ion that is responsible for the remarkably high
activity of CPO-27-Mn.
The most active CPO-27-Mn was compared with a simple salt,
manganese chloride. Under the same conditions, the salt is also
active for cyanosilylation but led to a much lower conversion than
CPO-Mn-27 (Table 1, entry 12. A comparison of the conversion
vs. time plots is given as Fig. S2 in Supplementary material). It is
obvious that the Mn MOF is superior to the salt in catalyzing the
cyanosilylation reaction. The organic precursor, H4dhtp, was also
tested. It turned out that the weak acid does not promote the reac-
tion (Table 1, entry 13): no conversion was detected after 1 h, and
the conversion after 48 h was the same as that for blank test.
The catalysts used in the above catalytic experiments were acti-
vated by a general post-synthetic procedure (methanol exchange
followed by heating treatment, see the experimental section). The
CPO-27-Mn samples obtained by different post-synthetic proce-
dures were also tested (Table 2, entries 1–4). As can be seen, even
the as-synthesized catalyst (no methanol exchange and no heating
treatment before the catalytic test) shows significant activity for
the cyanosilylation reaction, the conversion reaching 90% after 10 h
(entry 1). This observation may indicate that the solvent molecules
coordinated to the metal centers are labile to some degree. The
To provide further evidence for the existence of Lewis acidic
sites (open metal centers) in CPO-27-Mn, control experiments were
performed in the presence of CPO-27-Mn and catalyst poisons
(Table 1, entries 10 and 11). The conversions of benzaldehyde after
1 h were dramatically decreased from 100% (in the absence of poi-
sons) to 38% and 24% in the presence of pyridine and pyridine oxide,
respectively. These results can be easily explained by the poison-
ing of the Lewis acid sites by strong interactions with pyridine or
pyridine oxide.
With the above experimental results, on can safely say that the
cyanosilylation reaction over CPO-27-Mn is a Lewis-acid catalyzed
process. The mechanism could be similar to that proposed else-
where [34,39]. The open Mn(II) center interacts with the oxygen
atom of benzaldehyde, thus the carbonyl group is further polarized
to facilitate the nucleophilic addition by the cyanation agent. The
size effects of aldehyde substrates (see below) indicates that the
reaction mainly occurs within the channel of the catalyst.
The recyclability of the CPO-27-Mn catalyst was also tested
(Fig. 4). After the first reaction run (complete conversion, 1 h), fresh
benzaldehyde (5 mmol) and TMSCN (5 mmol) were added into the

H.-F. Yao et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 57–65
61
Table 3
CPO-27-Mn catalyzed cyanosilylation of various aldehydes and ketones.a
Entry
1
Carbonyl substrate
Time (min)
Conv. (%)
CHO
60
∼100
∼100
CHO
2
3
30
35
NO
2
CHO
Fig. 4. Recycling tests of CPO-27-Mn for benzaldehyde cyanosilylation. Conditions:
∼100
benzaldehyde, 5 mmol; TMSCN, 10 mmol; DCM, 15 mL, reflux; cat., 0.5 mmol; 1 h.
NO
2
CHO
4
5
5
∼100
O N
2
CHO
CHO
60
89
6
7
8
60
60
60
97
66
61
CHO
CHO
CHO
9
60
21
O
10
11
5
∼100
O
60
11
O
12
60
17
Fig. 5. IR spectra (a) and XRD profiles (b) of the CPO-27-Mn catalyst before and after
the cyanosilylation reaction.
aConditions: substrate, 5 mmol; TMSCN, 10 mmol; DCM, 15 mL, reflux; cat., 0.5 mmol
(10 mol%).
refluxing reaction mixture without separating out the catalyst and
the product. It proved that the catalyst remained highly active with
100% conversion of benzaldehyde after 1 h. If the procedure was
repeated for the third run, the conversion after 1 h was 86%. The
reduced conversion in the third run could be due to the accumula-
tion of product in the voids of the MOF. To support this assumption,
the IR spectrum of the catalyst isolated after the third run was
recorded and compared with that of the fresh catalyst (Fig. 5a).
runs are compared with that of the fresh catalyst in Fig. 5b. The
crystalline phase of the catalyst is essentially retained after four
cycles of reactions.
Furthermore, the CPO-27-Mn catalyst was applied to the
cyanosilylation of other carbonyl substrates (Table 3). It proved that
CPO-27-Mn is highly active towards mono-substituted benzalde-
hydes. The three nitrobenzaldehydes show much higher reactivity
than benzaldehyde, reflecting the strong electron-withdrawing
effects of the nitro group. The weak electron-donating effects of the
methyl group were evidenced by the fact that the three methylben-
zaldehydes show somewhat lower reactivity than benzaldehyde. It
is noted that the electronic effects are more significant for para-
substituted substrates, with p-nitro- and p-methylbenzaldehydes
showing the highest and lowest reactivity, respectively, among the
mono-substituted benzaldehydes studied. p-Nitrobenzaldehyde
can reach complete conversion within as short as 5 min. The effects
of substituent positions are open to further investigations.
The catalyst shows size selectivity towards the substrates. In
comparison with the complete conversion of benzaldehyde, the
The used catalyst shows two new bands at 2192 and 2145 cm−1
,
attributable to the ꢀ(C≡N) absorption of the TMSCN reactant and
the cyanosilylation product, respectively. The used catalyst after
the third run was washed several times with DCM (until no signals
except that of DCM were detected by GC) and then reused for the
fourth run. The conversion in 1 h was increased to 91%. The some-
what lower conversion compared with the fresh catalyst could be
due to the solid loss during the treatment. The recycling experi-
ments indicate that the CPO-27-Mn catalyst can be reused with no
need of heating reactivation and with no significant loss of activity.
The XRD profiles of the catalysts isolated after the first and fourth

62
H.-F. Yao et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 57–65
Table 4
Influence of catalyst dose and temperature on benzaldehyde cyanosilylation over Mn-MOF and the comparison with selected MOFs studied elsewhere.a
Entry
Catalystb
Catalyst dose (mol%)c
Temp. (◦C)
Time (h)d
Conv. (%)d
Ref.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
CPO-27-Mn
CPO-27-Mn
CPO-27-Mn
CPO-27-Mn
2
5
10
4.5
10
20
10
15.8
11
4
4.5
4.5
4.5
4.5
13
5
40
40
40
r.t.
r.t.
40
r.t.
40
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
25
r.t.
1
1
1
63
90
∼100
30, 78
54, 94
77
69
50
98
45, 90
99
This work
This work
This work
This work
This work
[36]
[40]
[41]
[42]
[43]
[44]
[44]
[44]
[44]
[45]
[38]
2, 5
1, 4
24
16
48
9
5, 12
2
2
5
5
24
14
CPO-27-Mn
[Cd(bpy)2](NO3)2
[Sm(H2L)(H3L)]
Cu3(BTC)2
Mn3[(Mn4Cl)3(BTT)8]2
Sc2(C4O4)3
[Nd(BTC)]
[Ho(BTC]
[Er(BTC)]
[Yb(BTC)]
76
61
57
22
[Zn3(bpy)3.5(O2CH)4](ClO4)2
[Cd(L’)2(ClO4)2]
92
a
All the catalytic reactions were performed with a 1:2 molar ratio of benzaldehyde to TMSCN in DCM.
b
H4L = 2,2ꢀ-diethoxy-1,1ꢀ-binaphthalene-6,6ꢀ-bisphosphonic acid, BTC = benzene 1,3,5-tricarboxylate, H3BTT = 1,3,5-benzenetristetrazol-5-yl, Lꢀ = bis(4-imidazol-1-yl-
phenyl)diazene, bpy = 4,4ꢀ-bipyridine.
c
The catalyst doses all correspond to the molar amount of the metal ion relative to benzaldehyde.
For some entries, more than one conversion data obtained for different time periods (separated by commas) are provided for convenience of comparison.
d
conversion of 1-naphthaldehyde dropped dramatically to 61%
under the same conditions, and for the still larger substrate 9-
anthrylaldehyde, the conversion was further decreased to only
21%. The size effect could suggest that the cyanosilylation reaction
occurs mainly within the open channels of the MOF catalyst.
For acetaldehyde, the catalyst shows high activity similar to
that for p-nitrobenzaldehyde, the conversion reaching completion
within as short as 5 min. For acetone and acetophenone, the activity
is much lower due to the low reactivity of ketones compared with
aldehydes.
A number of MOFs have been studied as catalysts for cyanosily-
lation of benzaldehyde and its derivatives. It is desirable to compare
the performance of CPO-27-Mn with that of other MOFs stud-
ied previously. However, the reaction conditions used (catalytic
dose, temperature, solvent, reaction time, the reactant ratio and
even the substrate) are diverse so that it is impossible to make a
meaningful comparison covering all the MOFs studied. To make
the comparison to cover more (but not exhaustive) MOFs, we
have performed additional reactions of benzaldehyde with TMSCN
by varying the catalytic dose and the reaction temperature. As
expected, the conversion within 1 h increases with the amount
of the catalyst and with the temperature (Table 4, entries 1–5).
The data reported for other MOF catalysts studied under compa-
rable conditions [35,37,50–55] are also collected in Table 4. As can
be seen, CPO-27-Mn is among the most active MOF catalysts for
cyanosilylation addition. It is more active than most of the cata-
lysts listed, including the Cd(II)/Zn(II) coordination polymers with
neutral N-donor ligands (entries 6, 15 and 16), some MOFs with
1,3,5-benzenetricarboxylate (BTC) (entries 8, 13 and 14), the Sm(III)
compound with a phosphonate ligand (entry 7), a Sc(III) species
with squarate (entry 10) and the Mn(II) MOF with a tritetrazolate
ligand (entry 9). Only the Nd(III) and Ho(III) MOFs with BTC (entries
11 and 12) show higher activity than CPO-27-Mn.
Fig. 6. Conversion vs. time plots for styrene oxidation. (a) Reactions with and with-
out CPO-27-M catalysts. (b) CPO-27-Mn-catalyzed reaction and the control tests
with catalyst filtration, with radical quenching, and without catalyst. Conditions:
cat., 0.25 mmol (if used); styrene, 5 mmol; TBHP (5–6 M in decane), 15 mmol; 75 ◦C.
3.3. Catalytic properties for styrene oxidation
First, we compared the catalytic ability of five CPO-27-M MOFs
with different metal ions under “solvent-free” conditions (except
for the decane introduced together with TBHP, no solvent was
added into the reaction mixture). The time dependence of styrene
conversion is shown in Fig. 6a and selected data of conversion and
selectivity are collected in Table 5. As can be seen, the Mn and Co
MOFs show different activities for the oxidation reaction, but the
Mg, Zn and Ni MOFs are inactive and even have inhibition effects
because the conversions in the presence of these MOFs are lower
than that for the thermally activated reaction in the blank test (no
catalyst). The Co MOF also plays an inhibiting role in the initial stage
but becomes active after a long induction time. Although the mech-
anism of the inhibition effects is unclear, the activity of the Mn and

H.-F. Yao et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 57–65
63
Table 5
Catalytic data for the oxidation of styrene over different CPO-27-M MOFs.a
Table 6
Effect of solvents on the oxidation of styrene.a
Entry
Catalyst
Time (h)
Conv (%)
Selectivity (%)
Entry
Solvent
Conv. (%)
Selectivity (%)
A
B
C
A
B
C
1
2
3
4
5
6
7
CPO-27-Mn
CPO-27-Co
CPO-27-Ni
CPO-27-Mg
CPO-27-Zn
No catalyst
2
24
24
24
24
24
2
95
86
41
46
21
67
74
38
68
61
65
55
63
49
55
26
26
23
29
31
47
7
6
13
12
16
6
1
2
3
4
5
6
7
8
“Solvent free”
Acetonitrile
DCE
n-Decane
Benzene
n-hexane
DMF
95
69
58
51
44
31
12
11
38
27
33
29
30
31
36
32
55
70
60
64
59
64
63
67
7
3
7
7
11
5
b
MnCl2
4
1
1
Ethanol
a
Conditions: catalyst, 0.25 mmol (5 mol%); styrene, 5 mmol; TBHP
(5–6 M in decane), 15 mmol; 75 ◦C. A = styrene oxide, B = benzaldehyde,
C = phenylacetaldehyde.
a
Conditions: catalyst, 0.25 mmol; styrene, 5 mmol; TBHP (5–6 M in decane),
15 mmol; solvent, 5 mL; 75 ◦C, 2 h; DCE = 1,2-dichloroethane, DMF = N,N’-
dimethylformide; A = styrene oxide, B = benzaldehyde, C = phenylacetaldehyde.
b
The amount of MnCl2 was 0.5 mmol so that the metal content is the same as that
of the MOF catalysts.
ability and lead to the blocking of metal sites. For the rest of
the solvents, it seems that a more polar solvent lead to a higher
conversion (acetonitrile > 1,2-dichloroethane (DCE) > benzene > n-
hexane). Nevertheless, the conversion of styrene in the presence
of any of these solvents is significantly lower than that for the
“solvent-free” reaction. Hereafter the oxidation was carried out
under ‘solvent-free’ conditions.
Scheme 2. Oxidation of styrene catalyzed by CPO-27-M.
The heterogeneous character of the styrene oxidation over CPO-
27-Mn was checked. After a catalytic reaction was carried out for
0.5 h, the solid catalyst was filtered off from the hot mixture, and
the filtrate was heated at the same temperature for some time. As
shown in Fig. 6b, the reaction in the filtrate was much slower than
the reaction without filtration. The slight increase of the conversion
from 41% to 55% upon heating for 5.5 h after filtration is most likely
due to non-catalytic thermal oxidation. As also shown in Fig. 6b, the
blank reaction in the absence of any catalyst reaches a conversion
of 23% after 6 h, confirming the occurrence of thermally activated
oxidation. The above results suggest that the high reaction rate in
the presence of CPO-27-Mn arises from the solid phase rather than
leached species in the solution.
The possible mechanism for the oxidation reaction was also
investigated. To probe if radicals were involved, a radical scavenger,
butyl hydroxy toluene (BHT, 2,6-di-tert-butyl-4-methylphenol),
was added after the CPO-27-Mn catalyzed reaction proceeded for
0.5 h. As can be seen from Fig. 6b, no further conversion of styrene
was observed after the addition of BHT, indicating that both cat-
alytic and non-catalytic processes were completely inhibited. The
test suggests that the oxidation reaction proceeds via radicals. The
radical mechanism for the catalytic formation of benzaldehyde and
styrene oxide could be proposed as follows, similar to that for a
Co(II) catalyst [48]. As shown in Scheme 3, TBHP reacts with the
Mn(II) MOF to generate a tert-butylperoxy radical. Then the radi-
cal was added to the styrene double bond to generate a benzylic
radical derivative, which undergoes oxygen migration to form the
epoxide (and the t-BuOH byproduct) (route I) or reacts with the tert-
butylperoxy radical to give benzaldehyde via oxidative C–C bond
cleavage (route II).
Benzaldehyde may also be a secondary product coming from
styrene oxide, the mechanism involving the ring-opening of
styrene oxide with peroxides followed by C–C cleavage (route
III) [56,57]. Concerning the formation of phenylacetaldehyde, it is
often attributed to the Lewis-acid promoted ring-opening isom-
erization of styrene oxide [45,56,57], which involves a hydride
migration (route IV) [58,59]. To gain information about the mech-
anism of aldehyde formation, we checked the product distribution
of the above radical-inhibiting test and found no indications of
the conversion of the epoxide product to aldehyde products (the
amount of each product remained constant with time) after the
styrene conversion was stopped by BHT. Furthermore, an indepen-
dent control experiment was carried out, for which a mixture of
Co species and the inactivity of the Zn, Mg and Ni analogs are in
agreement with the different ability of these divalent metal ions in
changing their oxidation state. A previous study has also revealed
that the same Co MOF can catalyze the aerobic oxidation of cyclo-
hexene while the Ni analog is totally inactive [27]. Notably, our
results suggest that the Mn MOF is much more active (95% conver-
sion after 2 h) than the Co analog (86% conversion after 24 h). This
trend is qualitatively consistent with the fact that Mn(II) is easier
to be oxidized than Co(II).
The most active Mn MOF has been compared with MnCl2 for the
oxidation reaction (Table 5, entry 7. A comparison of the conver-
sion vs. time plots is given in Fig. S3 in Supplementary material).
Ii turned out that he chloride salt is less active than CPO-27-Mn.
During the reaction using the salt, partial dissolution of the solid
was observed, and a filtration control test suggests that the cataly-
sis with the salt is homogeneous (see Fig. S3 in the supplementary
material).
As shown in Table 5, the oxidation of styrene under the
given conditions yielded styrene oxide (A), benzaldehyde (B), and
phenylacetaldehyde (C) (Scheme 2). The time dependence of the
conversion and yields is shown in Fig. S4 (see Supplementary mate-
rial) for CPO-27-Co and -Mn. In both cases, phenylacetaldehyde is
a minor product (less than 10%). With CPO-27-Mn, the selectiv-
ity for benzaldehyde is always higher than that for styrene oxide,
while CPO-27-Co favors styrene oxide. The effects of temperature,
catalyst dose and oxidant dose on the styrene oxidation over CPO-
27-Mn were tested (see Fig. S5 in Supplementary material). As
expected, the conversion increases with these reaction parame-
ters. As far as selectivity is concerned, benzaldehyde is favored at
lower temperature. Using a large amount of the catalyst (3 mol% or
more) or the oxidant (3 equivalents) also increases the selectivity
of benzaldehyde.
The CPO-27-Mn catalyzed oxidation reactions were performed
in various solvents for comparison with the “solvent-free” reac-
tion. The results are compiled in Table 6. In all solvents used, the
selectivity for different products decreases in the order of ben-
zaldehyde > styrene oxide ꢁ phenylacetaldehyde, as observed in
the solvent-free case. The results obtained for decane (Table 6,
entry 4) suggest that dilution has a strong negative effect on the
catalytic activity. The conversions in highly polar solvents such
as ethanol and DMF are much lower than those in the other sol-
vents, perhaps because ethanol and DMF have a strong coordination

64
H.-F. Yao et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 57–65
three successive reaction cycles (Fig. 7a) though XRD analysis indi-
cated that the catalyst has lost the original structure and changed
into an unknown phase after the first run (Fig. 7b). This represents
a rare example of MOFs retaining the high catalytic activity after
structural degradation. The catalytically active new phase derived
from CPO-27-Mn is open to further investigations.
4. Conclusions
In summary, we have presented a systematic study of using the
isostructural CPO-27-M MOFs (M = Co, Mg, Mn, Ni, Zn, Mn0.57Co0.43
,
and Mn0.1Co0.9) as catalysts for two organic reactions, cyanosi-
lylation of aldehydes with trimethylsilylcyanide and oxidation of
styrene with tert-butylhydroperoxide. The catalytic performance
is strongly dependent on the metal ion contained in the MOF. CPO-
27-Mn is the most active for both reactions, while the Mg, Zn and Ni
MOFs act as inhibitors rather than catalysts for the oxidation reac-
tion. The CPO-27-Mn catalyzed cyanosilylation is heterogeneous
and size selective towards aldehyde substrates, characteristic of
a porous catalyst, and the catalyst retains the structural integrity
and catalytic activity when recycled. Heterogeneity and reusability
of CPO-27-Mn have also been confirmed for the oxidation reac-
tion, though the structure changes after the reaction. After some
intentional control tests, radical mechanisms were proposed for the
formation of styrene oxide, benzaldehyde and phenylacetaldehyde
from styrene. An extension of the study to more reactions is under-
way in this lab to further understand the great effects of different
metal ions on catalytic performance of MOFs. Studies along this line
may also open the possibility of using mixed-metal MOFs to tune
catalytic properties and to develop catalysts for multicomponent
or cascade reactions.
Scheme 3. Possible mechanism for the styrene oxidation over CPO-27-Mn.
styrene oxide, TBHP and CPO-27-Mn was allowed to react under
the conditions used for styrene oxidation. No conversion of styrene
oxide to aldehydes was detected. These results clearly suggest that
both aldehydes are the primary products of styrene oxidation via
radical-involving processes, rather than originating from the epox-
ide product, so routes III and IV could be ruled out. A possible
mechanism for the formation of phenylacetaldehyde is that the
benzylic radical immediate undergoes migration of a ␤-hydrogen
to the benzylic carbon and simultaneous (or subsequent) O-O cleav-
age (route V).
Acknowledgements
This work is supported by the National Natural Science founda-
tion of China (NSFC no. 21173083) and the Research Fund for the
Doctoral Program of Higher Education of China.
Finally, the recyclabilityof CPO-27-Mnfor the oxidationreaction
was confirmed. The conversion of styrene remains at a high level for
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2014.06.040.
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