A new layered metal-organic framework as a promising heterogeneous catalyst for olefin epoxidation reactions

Article abstract of DOI:10.1039/c2cc18127d

A new layered MOF material [Co(Hoba)₂·2H₂O] (1) (H₂oba = 4,4′-oxybis(benzoic acid)) has been synthesized and used as a highly recyclable heterogeneous catalyst for olefin epoxidation reactions. Both high conversion (96%) and high selectivity of epoxide products (96%) are achieved.

Full text of DOI:10.1039/c2cc18127d

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ARTICLE TYPE
A New Layered Metal-Organic Framework as a Promising
Heterogeneous Catalyst for Olefin Epoxidation Reactions
a
‡
a
‡b
a
Jingming Zhang , Ankush V. Biradar , Sanhita Pramanik , Thomas J. Emge , Tewodros
Asefa , Jing Li
*
b
*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A new layered MOF material [Co(Hoba)
2
∙2H
2
O] (1) (H
2
oba =
as self-supported heterogeneous catalysts with built-in active
metal centers that can readily be modified. MOFs containing
unsaturated metal coordinate sites are of particular interest and
have been employed in numerous catalytic reactions because of
their Lewis acidity and associated catalytic functionality,
well-organized and well-maintained active sites.
4
,4’-oxybis(benzoic acid) has been synthesized and used as a
5
6
6
7
7
8
5
0
5
0
5
0
highly recyclable heterogeneous catalyst for olefin
epoxidation reactions. Both high conversion (96%) and high
selectivity of epoxide products (96%) are achieved.
1
0
14a,14d,15
To the best of our knowledge, only very few MOFs have
been reported as self-supported catalysts for epoxidaion of
Selective oxidation of hydrocarbons to higher-value
chemical feedstocks represents an important petrochemical
16
olefins. Spodine and co-workers obtained a 24% conversion of
styrene and 71% selectivity to styrene oxide using a 2D MOF
1
process; however, it remains a great challenge to achieve high
16d
1
2
2
3
3
4
4
5
5
0
5
0
5
0
5
0
performance of these reactions. In particular, the selective
oxidation of olefins into epoxides (or olefin epoxidation) has
attracted tremendous attention in the scientific community
because epoxides are widely used as industrially important
intermediate chemicals for making valuable products such as
epoxy resins, paints and surfactants. Epoxides are also versatile
(
[Cu(H
2 ∞
btec)(bipy)] as the catalyst. In another recent work by
Koner and co-workers, 1D lanthanide MOF catalysts gave a 92%
16e
conversion of styrene with 41% yield of styrene oxide in 24 h
.
While these examples illustrate that MOFs are capable of
catalyzing olefin epoxidation reactions, most of them are unable
to simultaneously give both high conversion and high selectivity
to epoxide product. We have laid our focus on the development
of the Co(II) MOF catalyst not only because Co(II) cations are
well known for their catalytic functions in the oxidation of
2
precursors for pharmaceutical synthesis. Over the past a few
3
4
decades, many homogeneous and heterogeneous catalysts have
been developed and used to catalyze the epoxidation reactions of
various olefins. Major research efforts also include the
development of effective heterogeneous catalysts that can be
easily recovered, reused, and separated from reaction mixtures,
making the processes more cost effective. The dispersion and
attachment of catalytically-active metallic nanoparticles or
organometallic complexes onto polymeric or inorganic solid
support materials via non-covalent or covalent interactions can
generally produce effective heterogeneous catalysts for numerous
7,17
18
olefins because of their excellent oxygen transferring ability,
relatively high abundance and low cost, but also because very
few Co(II) based MOF catalysts have been tested for olefin
19
epoxidation. Here, we report a new two-dimensional (2D)
2 2 2 2
layered MOF structure, [Co(Hoba) (H O) ](H oba = 4,4'-
oxydibenzoic acid) (1) and its catalytic activity in olefin
epoxidation reactions.
The pure-phase crystals of
hydrothermal conditions from
Co(NO ·6H O and H oba in 1:2 molar ratio in 10 mL distilled
1
were grown under
5
reactions including olefin epoxidation. This approach has also
a
reaction between
commonly been used for homogeneous epoxidation catalysts
3
)
2
2
2
3+
3+
3+
6+
such as Co , Fe , Mn and Mo complexes. Such an
immobilization process has successfully produced heterogeneous
epoxidation catalysts that are as effective as their homogeneous
water and in the presence of triethylamine (TEA). The latter was
used to adjust the pH to ~7.00. The reaction at 120 C for 3 days
afforded plate-like pink crystals, which were characterized by
single crystal X-ray diffraction method. Compound 1 crystalizes
in triclinic crystal system, space group P-1. The primary building
unit (PBU) is shown in Figure 1a. All the cobalt atoms in 1 are
octahedrally coordinated to four oxygen atoms from carboxylate
groups in the equatorial positions and two oxygen atoms from
water molecules in the axial positions. Each cobalt atom is
connected to the four neighboring cobalt atoms via the bidentate
5c,6
counterparts.
However, the syntheses of these catalysts are
8
9
9
5
0
5
often costly. In addition, some of them have poor selectivity and
possess randomly dispersed catalytic groups with geometries less
conducive for the active sites in the solid matrix.
4,7
Very recently, promising heterogeneous catalytic activities
of metal organic frameworks (MOFs) have been reported for a
number of reactions. These reactions are catalyzed by: (1) build-
8
9
in active sites (metal centers or functional organic linkers ); and
4
carboxylate groups from oba ligands to form a 4 brick-like 2D
(2) grafted active sites (MOFs as solid support for grafting metal
layer (Figure 1b). Only one carboxylic acid of each oba ligand is
deprotonated (Figure 1a). The undeprotonated carboxylic groups
point outward from both sides of the Co(Hoba) layer and form
2
10
11
nanoparticles or post-synthesis method, which can generate
metal binding sites). Some great advantages of MOFs as
1
2
heterogeneous catalysts include interesting regioselectivity or
hydrogen bonds between the adjacent layers (Figure 1c).
13
enantioselectivity, which is often attributed to their large
surface area, high metal content and coordination unsaturation,
Thermogravimetric (TG) analysis shows a weight loss of 6.0
wt% for 1 in the range of 110 C to 160 C upon heating. The
weight loss can be assigned to the coordinated water molecules in
13-14
chirality, systematically tunable structures and composition.
These multifunctional properties make MOFs especially suitable
1
00 1 and matches very well with the theoretically calculated amount
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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(
5.9 %) based on the crystal structure. The water-free sample (1’)
and 3). When ethyl cinnamate was used as a substrate, lower
is thermally stable up to 300 C, and starts to decompose at
higher temperatures. 1’ can also be generated by heating the as-
made sample 1 at 150 C under vacuum for 2 h. The powder X-
ray diffraction (PXRD) pattern of the sample taken immediately
conversion (69%) compared to styrene but high selectivity
(ca.100%) was obtained (Table 1, Entry 4). When trans-stilbene
was used as substrate, the reaction gave the same selectivity
65 (ca.100%) as ethyl cinnamate but higher conversion (86%) (Table
5
1, Entry 5). The difference in the conversion between Entries 1
and 3, as well as between Entries 4 and 5, may be attributed the
electronic and steric effects of the substutients at the double
21
bond.
1
0
70
i
Table 1. Epoxidation of various olefins under "solvent-free" condition by 1’.
Conversion
%)
Selectivity (%)
Entry
Substrates
j
k
(
A
B
1
2
96
96
96
65
4
(a)
(b)
35
3
4
5
47
69
86
94
6
0
0
ca.100
ca.100
1
5
(
c)
i
Reaction conditions: Olefins: 50 mmol; TBHP (5.5 M in decane): 100 mmol;
catalyst (1’): 15 mg (0.5 mol%); Chlorobenzene (internal standard): 15 mmol;
4
Figure 1. (a) The PBU and the coordination around Co(II); (b) the 4 brick-
like 2D layer parallel to the ab plane (the light turquoise polyhedral represent
j
k
Temperature: 75 ºC; and Reaction time: 6 h. a = Epoxide products. b =
5 Aldehyde products. The selectivity is only based on the two major products a
and b.
7
8
2
2
0 the octahedral coordination around Co(II). Benzene rings of the oba ligands
are deleted for clarity; (c) the layered structure packed along the c axis and the
outlined area indicates the hydrogen bonding section between the two adjacent
layers. Color scheme: Co turquoise; C gray; O red; H orange; and all the
hydrogen atoms are deleted for clarity except those on the undeprotonated
5 carboxylate groups).
22
Metal leaching from a solid matrix is an unwanted
phenomenon that is not uncommon in many heterogeneous
catalytic systems. Thus, to confirm the heterogeneous behaviour
of 1’, a hot filtration experiment was conducted. As a result, a 55%
conversion was obtained after 2 h of reaction and another 16% of
conversion was observed after removing the catalyst and running
the reaction solution for another 4h (see SI, Figure S8). The latter
0
5
0
5
after removal of water resembles that of the as-made sample,
except a shift of the major peaks to lower angles (see Supporting
Information, SI, Figure S2), which indicates an expansion of the
3
3
4
4
5
5
6
0
5
0
5
0
5
0
framework in the c direction. This suggests that the overall
framework is very robust due to the strong hydrogen bonding
between carboxylic groups from the adjacent layers (Figure 1c),
while the 2D layers are fairly flexible, making them readily
accessible to the substrate molecules. A color change was
observed from the as-made sample (1, pink) to the water-free
sample (1’, purple). The former can be readily regenerated from
the latter by simply immersing the sample in water and stirring
for 6 h at room temperature. Thus, the conversion between the
two can be confirmed by either the change in color or by the
PXRD patterns (see SI, Figure S1).
14b
8
9
9
is most likely due to non-catalytic thermal oxidation. In order
to varify this, a blank reaction was carried out without adding any
catalyst. After reaction for 4h, the conversion reached 18%,
which matches very well with the additional amount of
conversion after removal of the catalyst (16%). Likewise, the
selectivity for the epoxide product in a blank reaction is ca. 80%,
similar to the values obtained after the CoMOF was removed
from the reaction. To further confirm the heterogeneous nature of
1
’, the catalyst was recovered and reused four times. All repeated
reactions gave identical % conversion, with only slightly lower
selectivity (see Table S4). The PXRD patterns of the recovered 1’
clearly suggest that its structure is well maintained after several
cycles of reactions (see Figure S6). The PXRD patterns also
confirm that coordination unsaturated Co(II) metal sites remain
open after the reactions (see Figure S7). The small decrease in
00 the selectivity is likely due to the reduced crystallinity of the
MOF catalysts after repeated use (see Figure S6). Inductively
coupled plasma optical emission spectroscopy (ICP-OES)
performed on the filtrate after the 1 and the 4 reaction cycle
gave a trace amount of 0.8 and 30 ppm of Co(II), respectively.
05 The calculated weight percent as a result of possible leaching of
the catalyst is ~2 wt% (after 4 cycles) and thus, may be
considered negligible.
20
The catalytic activity of 1’ in the "solvent-free"
epoxidation reactions of styrene (Scheme S1) yielded the highest
styrene conversion (96%) and selectivity (96% to epoxide)
compared to reactions in various other solvents (see Table S2).
This is due to the better interactions between the catalyst and
substrates when the concentration of reactants is very high under
such conditions. Also, 1’ gives better performance than 1 because
of the presence of highly reactive open Co(II) centers (Lewis acid
sites) in 1’ (see Table S1). In order to further explore the
versatility of the 2D MOF as a selective epoxidation catalyst and
the effect of substituents on the epoxidation reaction, different
types of olefins were used as substrates under "solvent-free"
conditions. The results are compiled in Table 1. When an
electron-withdrawing group, chloride, was used as a substituent at
the para-position on styrene (4-chlorostyrene), similar conversion
as styrene, but lower selectivity to the epoxide product, was
obtained. On the other hand, when an electron-donating group,
tert-butyl, was used as a substituent on styrene at the para-
position (4-tert-butylstyrene), the reaction produced lower %
conversion than styrene, but similar selectivity (Table 1, Entries 2
1
1
1
1
st
th
11c,23
.
Finally, the possible mechanism for the reaction was also
investigated. Previous reports show that two types of mechanisms
10 were most commonly found in metal-ion catalyzed epoxidation
24
reactions: concerted addition of oxygen and radical epoxidation.
To probe if radicals were involved in our reactions, a radical
scavenger, butyl hydroxy toluene (BHT), was added at the
beginning of the reaction, and the reaction progress was
15 monitored. It was observed that the reaction stopped immediately
2
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7
8
24-25
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7
0
presence of oxygen clearly indicated the formation of benzoic
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5
0
5
(
see Table S6). Thus, the proposed reaction mechanism is shown
in SI, Scheme S2. First, reaction between Co(II)-MOF and
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2 2
(H O)2∞ (1).
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1
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Notes and references
1
3
a
Department of Chemistry and Chemical Biology, Rutgers, The State
95 14
2
5
University of New Jersey, 610 Taylor Rd., Piscataway, NJ 08854. Fax:
+1-732-445-5312; E-mail: Jingli@rutgers.edu (J.L.)
b
Department of Chemical and Biochemical Engineering, Rutgers, The
State University of New Jersey, 98 Brett Rd., Piscataway, NJ 08854. Fax: 100
+1-732-445-5312; E-mail: tasefa@rci.rutgers.edu (T.A.)
§
3
3
4
4
0
5
0
5
Electronic Supplementary Information (ESI) available: syntheses and
1
5
selected crystallographic data of 1, PXRD, TG, catalytic activity,
recyclability test and mechanism study results, GC-MS spectra of
1
1
05 16
products and additional figures.
CCDC 777415. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b000000x
3
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‡These authors contributed equally to this work.
T.A. gratefully acknowledges the partial financial assistance by the US
National Science Foundation (NSF), CAREER Grant Nos. CHE-
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004218, NSF DMR-0968937, and NSF NanoEHS-1134289 for this
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3