Synthesis and catalytic study of open metal site metal–organic frameworks of Cu3(BTC)2 microbelts in selective organic sulfide oxidation

Article abstract of DOI:10.1002/aoc.3528

The synthesis of open metal site metal–organic frameworks of Cu₃(BTC)₂ with microbelt morphology and a study of the catalytic oxidation of organic sulfides are reported. This CuBTC was found to be an efficient, selective and waste-fr

Full text of DOI:10.1002/aoc.3528

Full paper
Received: 11 March 2016
Revised: 22 April 2016
Accepted: 5 May 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3528
Synthesis and catalytic study of open metal site
metal–organic frameworks of Cu (BTC)
3
2
microbelts in selective organic sulfide oxidation
a
a†
a†
Sadegh Rostamnia *, Hassan Alamgholiloo , Maryam Jafari ,
a
b
Rezan Rookhosh and Amir Reza Abbasi *
The synthesis of open metal site metal–organic frameworks of Cu
oxidation of organic sulfides are reported. This CuBTC was found to be an efficient, selective and waste-free green heterogeneous
co-catalyst for the green H catalytic oxidation of sulfides. The catalyst can be isolated from the reaction mixture and reused at
3 2
(BTC) with microbelt morphology and a study of the catalytic
2 2
O
least five times. Copyright © 2016 John Wiley & Sons, Ltd.
Keywords: metal–organic framework; open metal site MOFs; microbelts (MBs); hydrogen peroxide; sulfide
biologically and industrially interesting corresponding
sulfoxides without any salts and additives with excellent to
good yields (Fig. 1).
Introduction
[
8]
Metal–organic frameworks (MOFs) are emerging as a new
class of nanoporous crystalline hybrid materials that are as-
sembled from two main components of inorganic metal ions
or clusters and organic linkers, through robust coordination Experimental Methods
[
1]
bonds.
MOFs as porous coordination polymers are
Chemicals and Apparatus
nanoporous materials prepared using the well-established
principles of coordination chemistry. MOF-based structures
demonstrate various advantages such as having high surface
areas and simplicity of processability, and being tuneable
All reagents were obtained from Merck (Germany) or Fluka
(Switzerland) and were used without further purification. The yields
and identification of products were determined using GC with a
Shimadazu model GC-14A instrument. Infrared (IR) spectra were
recorded with a Shimadzu IR-460 spectrometer.
[
1,2]
and stable alternative materials.
MOF nanoreactor-assisted
organic synthesis using organic–inorganic hybrid micro-
channels has been utilized not only to accelerate a number
of synthetic reactions, but also in green catalysis to increase
[
3]
3 2
Cu (BTC) Microbelt Preparation
reaction rate and yields.
There is much current interest in exploiting the properties of
MOFs in heterogeneous catalysis. Of course, transition metal com-
Microbelts of Cu
previously reported method with modifications. In a typical
method, Cu(NO O (2.00 g, 7.2 mmol) was dissolved into
Á5H
5 ml of deionized water, and H BTC (0.85 g, 4 mmol) was
dissolved into 25 ml of ethanol–water. The copper and
BTC solutions were transferred to a Teflon-lined stainless
3
(BTC)
2
were synthesized based on a
[7]
[4]
plexes are good catalysts for organic transformations. However,
typically the problem in this case is deactivation arising from com-
3
)
2
2
2
3
[
5]
plex degradation and metal aggregation as metal oxide particles.
As regards MOFs, their easily tuneable topology and composition,
with sometimes high content of unsaturated metal sites, combined
with their high porosity make these solids attractive candidates for
heterogeneous catalysis, complementing other porous solids such
H
3
steel autoclave and stirred for 1 h at room temperature. The
mixture was then kept at 110°C for 18 h. The reaction vessel
[4,5]
as zeolites and mesoporous metal oxides.
Recently, thermally and chemically stable MOFs have been
*
Correspondence to: Sadegh Rostamnia, Organic and Nano Group (ONG),
Department of Chemistry, University of Maragheh, PO Box 55181-83111,
Maragheh, Iran. E-mail: rostamnia@maragheh.ac.ir; srostamnia@gmail.com
Amir Reza Abbasi, Faculty of Chemistry, Razi University, Kermanshah, 67194, Iran.
E-mail: ar.abbasi@razi.ac.ir
[
3,4,6]
used as heterogeneous catalysts by our group and others.
Among the various types of these MOFs, open metal site
MOFs as promising catalysts are of interest. Open metal site
Cu
system and accessible nanocavities.
results for the synthesis of open metal site MOF of Cu
BTC) with a new microbelt morphology and its catalytic
application as a suitable and efficient catalyst for the green
oxidation (30% of organic sulfides, to achieve
3 2
(BTC) has a three-dimensional square-shaped channel
[
5,7]
†
We report herein our
These authors contributed equally
3
a Organic and Nano Group (ONG), Department of Chemistry, University of
(
2
Maragheh, PO Box 55181-83111, Maragheh, Iran
H
2
O
2
)
b Faculty of Chemistry, Razi University, Kermanshah, 67194, Iran
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

S. Rostamnia et al.
mmol), when the reaction was completed, the catalyst was easily
separated from the product (after the addition of EtOAc) by
decantation of the reaction solution. The catalyst was washed with
dichloromethane and diethyl ether to remove residual product and
dried under vacuum at 150°C for 18 h and reused in a subsequent
reaction.
Results and Discussion
In spite of more interest in MOFs, few stable and active MOFs for
[
4]
catalytic purposes have been reported. Among the MOF catalytic
systems, the Cu (BTC) family with usual hexagonal morphology
have been employed. In our method, the Cu
3
2
[
7]
3
2
(BTC) material with
3 2
Figure 1. Cu (BTC) -catalysed green sulfide oxidation.
a microbelt morphology and open metal site framework was
prepared from H BTC and copper nitrate in EtOH–water with a
3
was very quickly cooled to ambient temperature, washed with
water and methanol, centrifuged and the solid product was
vacuum dried at 120–150°C to afford Cu (BTC) . The MOF
Teflon-lined autoclave using a hydrothermal method (Fig. 2).
In the IR spectrum of Cu (BTC) , the characteristic absorption
3
2
À1
3
2
peaks at 481 and 727 cm can be assigned to the Cu O stretching
vibration. The characteristic peak at 1382 cm is attributed to
was then characterized using a variety of techniques.
[9]
À1
the stretching vibration of C O in carboxyl. Compared with the
À1
absorption band of 1720 cm for H BTC, the absorption peak at
3
General Procedure for MOF-Catalysed Oxidation of Sulfides to
Sulfoxides
À1
1650 cm
which corresponds toνC O stretching shows a
2+
bathochromic effect suggesting that the Cu coordinates with
À1
Into a round-bottomed flask (25 ml) equipped with a magnetic stirrer,
the oxygen atom. The O H bending vibration at 1447 cm
indicates the presence of a carboxylic acid group. The band at
1617 cm is attributed to the H O H bending vibration, which
[9]
sulfide (1 mmol) was added in the presence of 4 mmol of H O (30%)
2
2
À1
and Cu (BTC) microbelts (5 mol%) in 3 ml of solvent. The reaction
3
2
was stirred at room temperature for an appropriate time (Table 1).
After completion of the reaction, the product was extracted from
the reaction mixture with chloroform. The solvent was then
evaporated and the product purified using column chromatography
indicates that CuBTC contains crystal water. The band at 1115
cm corresponds to C O Cu stretching of CuBTC. The presence of
these characteristic peaks provides clear evidence of the successful
synthesis of CuBTC (Fig. 3(a)).
À1
(n-hexane–ethyl acetate (10:1)) if necessary. The isolated catalyst
Three diffraction peaks at around 2θ = 10° are readily recognized
in the X-ray diffraction (XRD) pattern (Fig. 3(b)). The observed
could be reused by addition of new portions of substrate.
[
4]
diffraction peaks agree with Cu
3
2
(BTC) .
All peaks are very sharp,
[
7,9]
indicating that this MOF has a highly crystalline nature.
Recovery Study
To investigate the surface morphologies and crystalloid
structures of the samples, scanning electron microscopy (SEM)
characterization was conducted and the corresponding images
are shown in Fig. 4. Based on previous reports, Cu (BTC) crystals
To investigate the possibility of recycling of the MOF microbelt
catalyst, model reactions in the presence of Cu (BTC) microbelts
3
2
were studied. For this study (scale of the reaction selected as 5
3
2
[
9]
have regular octahedral geometry, but for our method the
synthesized Cu (BTC) crystals have regular microbelt geometry,
a
3
2
Table 1. Optimized reaction conditions for the oxidation of sulfides
with smooth surfaces, the average particle width and thickness
being about 60 and 16 μm, respectively (Fig. 4(a)). Electron
dispersive spectroscopy (EDS) of Cu (BTC) microbelts was also
3 2
1
R
2
R
Entry
Time (min) Yield (%)
1
2
3
4
5
6
7
8
9
C
6
H
5
–
C
6
H
4
CH
2
–
60
120
60
80
84
97
91
92
95
80
82
94
93
90
C
6
H
5
–
6 5
C H –
4-BrC
6
H
4
–
6 4 2
C H CH
–
C
6
H
5
–
CH
2-Py–
3-NO
3
–
30
C
6
H
5
–
40
3-NO
2
C
4
H
4
–
2 4
C
H
4
–
90
C
6
H
5
–
3-MeOC
3-MeOC
4
H
4
–
120
160
20
3-MeOC
CH
4-NO
4-BrC
4
H
4
–
4
H
4
–
C
6
H
4
2
–
C
6
H
4
CH
4-NO
4-MeOC
2
–
10
1
2
C
4
H
4
CH
2
–
2 4
C
H
4
CH
2
–
20
1
4
H
4
–
4
H
4
–
60
a
2 2
Reaction conditions: sulfide (1 mmol), H O (4 mmol).
3 2
Figure 2. Schematic synthetic process of Cu (BTC) microbelts.
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Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2016)

Open metal site Cu (BTC) microbelts for sulfide oxidation
3
2
below 160–300°C can be attributed to elimination of coordinated
[9,10]
water.
A weight loss between 300 and 480°C can be assigned
to removal of organic motif from the structure (Fig. 5).
Oxidation of sulfides is the most straightforward alternative to
furnish pharmaceutically and industrially interesting sulfoxide
[
8]
intermediates. One of the problems encountered with this
method is over-oxidation of sulfoxides to sulfones, and the absence
of selectivity sometimes leads to undesired compounds. Herein, the
3 2
catalytic activity of the Cu (BTC) microbelts in the oxidation
reaction of sulfides to corresponding sulfoxides without using any
additives was investigated. The oxidation of phenylmethylsulfide
to corresponding sulfoxide and sulfone was selected as a model
reaction. To obtain optimized conditions, various parameters were
taken to consideration.
First, due to the fact that the nature of the MOF or its synthetic
component may play an important role in the oxidation process,
the initial model reaction was carried out using 5 mol% of copper
nitrate (Cu(NO ) ), benzene-1,3,5-tricarboxylic acid (trimesic acid,
3
2
H BTC), activated Cu (BTC) microbelts and non-activated Cu3
3
3
2
(
BTC) (H O) MOF at 30°C in 4 ml of acetonitrile for 2 h. The acti-
2 2 3
vated open metal site Cu (BTC) microbelt MOF was assessed for
3
2
its catalytic activity in the sulfide oxidation by studying the model
reaction to produce phenylmethylsulfoxide as the principal product
3 2
Figure 3. Cu (BTC) microbelts: (a) IR spectra; (b) XRD pattern.
(Fig. 6).
The effect of the amount of open metal site Cu (BTC) microbelts
3
2
as the catalyst was examined in the reaction under similar
conditions including the same temperature (30°C) and identical
time (2 h). Several amounts of catalyst were added to reaction mix-
tures and among them 5 mol% of MOF is found to be more efficient
from the point view of yield in each reaction (Fig. 7).
The amount of hydrogen peroxide (H O ) as oxidant was also op-
2
2
timized for the model oxidation reaction. Based on this study, the
best amount of hydrogen peroxide is 4 mmol for each millimole
of sulfide. Figure 8 shows the results for five different amounts of
H O
2 2
in the reaction performed under similar conditions.
Also investigated was the effect of various solvents on the yield
and selectivity towards sulfoxide product through the model reac-
tion. In this case, polar and nonpolar solvents were compared under
3
similar conditions. Among the solvents investigated, CH CN (91%)
and toluene (80%) show better results in terms of both yield and
selectivity. However, because acetonitrile is an available, cheaper
and therefore greener solvent and also leads to a slightly higher
3 2
Figure 4. (a) SEM micrograph of Cu (BTC) microbelts and (b) EDS
spectrum of Cu (BTC)
3
2
.
performed to determine the elements present, and specifically the
purity of the MOF (Fig. 4(b)).
3 2
Thermogravimetric analysis of Cu (BTC) indicates the synthesis
of a hybrid organic–inorganic structure. A significant weight loss
Figure 5. Thermogravimetric analysis of Cu (BTC) microbelts.
3
2
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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S. Rostamnia et al.
Figure 6. Reaction scale (2 mmol based on sulfide).
Figure 9. Optimizing the solvent in the model reaction.
work are: 30°C with 5 mol% of H O (30% wt) and 4 ml of acetoni-
2
2
trile in each millimole of sulfide.
The catalytic activity of open metal site Cu (BTC) microbelts in
3
2
the chemoselective oxidation of sulfide to sulfoxide and sulfone
was also studied under the optimized conditions. When we exam-
ined the reaction under the optimal conditions, the yield is 68%
for phenylmethylsulfone after 3 h, and under the same reaction
conditions the yield for diphenylsulfone is just 17% (Scheme 1).
3 2
The possibility of recycling the open metal site Cu (BTC)
microbelt catalyst was also studied. When the oxidation of the
model reaction is complete, the catalyst is easily separated from
the products. As illustrated in Fig. 10, the catalyst can be reused
over at least five consecutive runs. After the fifth run, the yield
decreases to 58%.
[
11]
Based on investigations of Mori and co-workers, addition of H
to the Cu complex copper(II) trans-1,4-cyclohexanedicarboxylate,
Cu (O CC O, produces dinuclear copper(II) peroxo com-
]ÁH
plex of H CC (O O, by intermolecular bridging
2 2
O
[
2
2
6
H
[Cu
10CO
2
)
2
2
Figure 7. Effect of amount of catalyst. Reaction conditions: 1 mmol of
phenylmethylsulfide, 4 mmol of hydrogen peroxide, in acetonitrile (4 ml)
at 30°C during 2 h. Blue (conversion based on sulfide); red (sulfoxide as
main product); green (sulfone as side product).
2
2
(O
2
6
H10CO
2
)
2
2
)]ÁH
2
of μ-1,2-trans-Cu OO Cu species, and the colour of the reaction
changes from blue to green. Also, the pH of the reaction mixture
changes from neutral to acidic that suggests the occurrence of
heterolytic H OOH bond cleavage, which confirms the suggestion
3 2
Scheme 1. Use of Cu (BTC) microbelts in chemoselectivity study.
2 2
Figure 8. Optimizing the amount of H O in the model reaction.
product yield, it was selected as an optimized solvent for the selec-
tive synthesis of sulfoxide (Fig. 9).
Thus, after optimization of the selective oxidation of
phenylmethylsulfide to the corresponding sulfoxide, the reaction
was extended to the other derivatives of sulfides and the results ob-
tained are summarized in Table 1. The optimized conditions in this
Figure
10. (a)
Recyclability
study
of
model
compound
phenylmethylsulfoxide and (b) SEM image of the MOF catalyst after the
fifth run.
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Appl. Organometal. Chem. (2016)

Open metal site Cu (BTC) microbelts for sulfide oxidation
3
2
Acknowledgments
The authors thank Prof. M. R. Yaftian and his group for the chemical
analysis of the synthesized compounds (Zanjan University). The
authors are grateful for financial support from the University of
Maragheh.
References
[
[
[
[
1] J. Reboul, S. Furukawa, N. Horike, M. Tsotsalas, K. Hirai, H. Uehara,
M. Kondo, N. Louvain, O. Sakata, S. Kitagawa, Nat. Mater. 2012, 11, 717.
2] a) H. Li, M. Eddaoudi, M. O’Keeffe, O. M. Yaghi, Nature 1999, 402, 276. b)
S. Rostamnia, E. Doustkhah, RSC Adv. 2014, 4, 28238.
3] a) J. L. C. Rowsell, O. M. Yaghi, Micropor. Mesopor. Mater. 2004, 73, 3. b)
M. L. Foo, R. Matsuda, S. Kitagawa, Chem. Mater. 2014, 26, 310.
4] a) J. Gascon, U. Aktay, M. D. Hernandez-Alonso, G. P. M. van Klink,
F. Kapteijn, J. Catal. 2009, 261, 75. b) S. Couck, J. F. M. Denayer,
G. V. Baron, T. Remy, J. Gascon, F. Kapteijn, J. Am. Chem. Soc. 2009,
Scheme 2. Feasible mechanisms for sulfide oxidation using Cu
3 2
(BTC)
microbelt catalyst.
131, 6326. c) P. Serra-Crespo, E. V. Ramos-Fernandez, J. Gascon,
F. Kapteijn, Chem. Mater. 2011, 23, 2565.
[
5] a) A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Catal. Sci. Technol.
2011, 1, 856. b) A. Dhakshinamoorthy, M. Alvaro, H. Garcia, ACS
Catal. 2011, 1, 48.
of Baiker and co-workers of generation of A by addition of CuBTC
[12]
and H
2
O
2
.
Note that both copper(II) peroxo complex A and H
2
[
Cu (O CC H CO ) (O )]ÁH O are active for oxidation of organic
2
2
6
10
11,12]
2 2
2
2
[6] a) S. Rostamnia, H. Xin, N. Nouruzi, Micropor. Mesopor. Mater. 2013, 179,
99. b) S. Rostamnia, A. Morsali, RSC Adv. 2014, 4, 10514. c) S. Rostamnia,
A. Morsali, Inorg. Chim. Acta 2014, 411, 113. d) S. Rostamnia, H. Xin,
Appl. Organometal. Chem. 2014, 28, 359. e) S. Rostamnia, Z. Karimi,
Inorg. Chim. Acta 2015, 428, 133.
[7] a) S. S. Y. Chui, S. M. F. Lo, J. P. H. Charmant, A. G. Orpen,
I. D. Williams, Science 1999, 283, 1148. b) C. Prestipino, L. Regli,
J. G. Vitillo, F. Bonino, A. Damin, C. Lamberti, A. Zecchina,
P. L. Solari, K. O. Kongshaug, S. Bordiga, Chem. Mater. 2006, 18,
[
molecules.
In order to gain some insight into whether the
reaction proceeds via peroxo copper intermediate A or B mecha-
nism, we dispersed our Cu (BTC) microbelts (Scheme 2, test tube
3
2
1
) in acetonitrile and added H O . After 24 h the colour changes
2 2
to green (Scheme 2, test tube 3) and also the pH of the mixture
changes from neutral to acidic (Scheme 2, litmus paper test) which
support our assumed mechanism as indicated in Scheme 2.
1337. c) A. R. Abbasi, M. Rizvandi, A. Azadbakht, S. Rostamnia,
J. Colloid Interface, Sci. 2016, 471, 112.
[
8] a) M. Carmen Carreno, Chem. Rev. 1995, 95, 1717. b) S. Rostamnia,
E. Doustkhah, K. Bahrami, S. Amini, J. Mol. Liq. 2015, 207, 334.
9] a) S. L. Serna, L. L. Núñez, J. Flores, R. López-Simeon, H. I. Beltrán, RSC
Adv. 2013, 3, 10962. b) Q. Luoa, X. Songa, M. Jia, S.-E. Parkb, C. Hao,
Y. Lia, Appl. Catal. A 2014, 478, 81. c) K. Huang, Y. Xu, L. Wang, D. Wu,
RSC Adv. 2015, 5, 32795.
Conclusions
[
We have synthesized a hybrid material with microbelt morphology
based on the structure of the MOF Cu (BTC) . This open metal site
MOF shows remarkable catalytic activity in the green and selective
sulfide oxidation of aromatic and aliphatic sulfides. It is believed
that the hybrid nature of the MOF affords the efficient mass transfer
3
2
[
[
10] B. Zhang, J. Zhang, C. Liu, X. Sang, L. Peng, X. Ma, T. Wu, B. Han, G. Yang,
RSC Adv. 2015, 5, 37691.
11] a) C. N. Kato, M. Hasegawa, T. Sato, A. Yoshizawa, T. Inoue, W. Mori,
J. Catal. 2005, 230, 226. b) C. N. Kato, W. Mori, C. R. Chim. 2007, 10, 284.
3
of starting substrates to the active sites within or on the Cu (BTC)2
microbelts.
[12] S. Marx, W. Kleist, A. Baiker, J. Catal. 2011, 281, 76.
Appl. Organometal. Chem. (2016)
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