Aqueous aerobic oxidation of alkyl arenes and alcohols catalyzed by copper(ii) phthalocyanine supported on three-dimensional nitrogen-doped graphene at room temperature

Article abstract of DOI:10.1039/c4cc01406e

Copper(ii) tetrasulfophthalocyanine supported on three-dimensional nitrogen-doped graphene-based frameworks was synthesized and introduced as a bifunctional catalyst for selective aerobic oxidation of alkyl arenes and alcohols to the corresponding carbonyl compounds. The ease of catalyst separation, high turnover, low catalyst loading and recyclability could potentially render it applicable in industrial setting. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01406e

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Aqueous aerobic oxidation of alkyl arenes and
alcohols catalyzed by copper(II) phthalocyanine
supported on three-dimensional nitrogen-doped
graphene at room temperature†
Cite this:DOI: 10.1039/c4cc01406e
Received 23rd February 2014,
Accepted 27th May 2014
Mojtaba Mahyari, Mohammad Sadegh Laeini and Ahmad Shaabani*
DOI: 10.1039/c4cc01406e
www.rsc.org/chemcomm
Copper(II) tetrasulfophthalocyanine supported on three-dimensional the catalytic oxidation reaction, chemical modification of the
nitrogen-doped graphene-based frameworks was synthesized and surface with nitrogen doping and transition metals are promising
introduced as a bifunctional catalyst for selective aerobic oxidation of methods. The solvent-free aerobic oxidation of alkyl arenes and
alkyl arenes and alcohols to the corresponding carbonyl compounds. alcohols with Pd@N-doped carbon was presented by Zhang et al.,25
The ease of catalyst separation, high turnover, low catalyst loading and in which the reactions were carried out at 120 1C, with low yields.
recyclability could potentially render it applicable in industrial setting.
Also using Pd, an expensive metal, as the active site in the catalyst
limited its application. Because of the above-mentioned reasons
The selective oxidation of organic compounds as a pivotal and also our interest in the synthesis of phthalocyanines and new
transformation in organic synthesis is one of the most challenging catalysts,5,26–31 herein we report the successful preparation and
reactions both on the laboratory and industry scale.1,2 Removing the excellent catalytic activity of copper(II) tetrasulfophthalocyanine
need for the activators such as inorganic bases and accomplishing supported on three-dimensional nitrogen-doped graphene-based
the reaction without any activation at ambient temperature and frameworks (CuTSPc@3D-(N)GFs). It should be noted that the
in a green solvent such as water are three important aspects presented catalyst is the first example of synthesis and using
which should be considered in the aerobic oxidation reactions.3–7 3D-(N)GFs-supported phthalocyanine as a novel bifunctional
The combination of these aspects is highly challenging and (nitrogen acts as a base and copper(II) as the active site) catalyst
will provide more benefits for these chemical transformations with unique properties for the selective aerobic oxidation of alkyl
in industry.8–10 As the key for this challenge is catalyst, studies arenes and alcohols to the corresponding carbonyl groups. The
are still ongoing to obtain appropriate catalysts, which have the selective aerobic oxidation reactions were carried out under
above-mentioned aspects.11–14
environmentally-friendly conditions with excellent yields in short
In recent years, the design of catalysts focusses on the reaction time at room temperature. Moreover, the effect of nitrogen
development of new catalytic materials based on biomimetic doping on the catalytic activity of CuTSPc decorated on 3D-(N)GFs
and green principles.2,15–21 Due to this fact, metal porphyrins was investigated. The synthesis of CuTSPc@3D-(N)GFs is
and metal phthalocyanines as biomimetic catalysts have been schematically shown in Scheme 1.
widely employed in several oxidation reactions.19,21 Compared
to porphyrins, phthalocyanines have improved thermal and
chemical stability, low cost and more availability.
Three-dimensional graphene-based frameworks (3D-GFs)
like graphene aerogels and foams are important types of new
generation porous carbon materials, which can be employed as
robust matrices for accommodating metals, metal oxides, and
active polymers for different applications, especially catalytic
systems.22–24 On the other hand, to increase the performance
and interaction of oxygen with the support and the substrate in
Faculty of Chemistry, Shahid Beheshti University, G.C., P.O. Box 1983963113,
Tehran, Iran. E-mail: a-shaabani@sbu.ac.ir
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 Synthesis of CuTSPc@3D-(N)GFs.
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Table 1 Comparison of the results obtained from CuTSPc@3D-(N)GFs
with GO, CuTSPc, and 3D-(N)GFs for the oxidation reactionsa
Yieldb Selectivityb
Entry Catalyst
Product
(%)
(%)
1
2
3
4
5
CuTSPc@3D-(N)GFs 2,3-Dihydro-1H-indene 98
499
499
499
499
499
499
97
98
98
99
Benzaldehyde 98
2,3-Dihydro-1H-indene 24
Benzaldehyde 30
2,3-Dihydro-1H-indene 33
Benzaldehyde 35
2,3-Dihydro-1H-indene 86
Benzaldehyde 88
CuTSPc@MCM-41 2,3-Dihydro-1H-indene 75
Benzaldehyde 81
GO
Fig. 1 SEM images of 3D-(N)GFs.
3D-(N)GFs
CuTSPc
The morphologies, and sample surfaces of the suspended
single layers of GO have been shown in the SEM and TEM
images of Fig. S1A and B and Fig. S2A and B (ESI†), respectively.
The 3D morphology and size of as-prepared 3D-(N)GFs were
confirmed by SEM images. Fig. 1A and B show an interconnected
framework of ultrathin graphene nanosheets with porous structure.
a
Reaction conditions: 2,3-dihydro-1H-indene or benzyl alcohol (1.0 mmol),
b
catalyst, H2O (4.0 mL), air (1 atm), r.t., 3 h. Yield and selectivity
determined by GC analysis.
As can be seen, the pore size ranges from a few hundred We examined GO (0.005 g) as the catalyst for the oxidation
nanometers to several micrometers. The surface area in porous reaction, which showed low yields under our mild reaction
materials is an important factor affecting the loading of catalytic conditions (Table 1, entry 2). It has been proposed that the use
sites. So, the Brunauer–Emmett–Teller (BET) method was used of 3D-(N)GFs as a support can promote the adsorption of
to estimate the surface area of the as-prepared 3D-(N)GFs. aromatic reactants due to the formation of p–p interaction
Nitrogen adsorption–desorption analysis reveals a typical BET between the benzene skeleton of the reactants and graphene.
surface area of up to 266.0 m2 gÀ1 for 3D-(N)GFs. Therefore, the Moreover, 3D-(N)GFs enhance oxygen adsorption on the bridge
synthesized support has a high surface area to embed CuTSPc as sites of graphene with a migration energy of over 1.0 eV and lead
active catalytic sites.
to activation of the oxygen molecules due to their interaction
The chemical composition of 3D-(N)GFs and CuTSPc supported with the nitrogen-doped support. The existence of mobile oxygen
on the 3D-(N)GFs was confirmed by elemental analysis (CHNS), on graphene can promote the elimination of hydrogen and thus
FT-IR, thermogravimetric analysis (TGA), and X-ray powder promote the aerobic oxidation of organic compounds.32 Actually
diffraction (XRD). Moreover, X-ray photoelectron spectroscopy the improvement of the results, in comparison with the homo-
(XPS) was employed for the surface characterization of geneous catalyst (Table 1, entries 1 and 4), is due to a couple of
CuTSPc@3D-(N)GFs (see ESI,† Fig. S3–S6).
factors, on the one hand the presence of basic sites on the solid
To optimize the required amount of catalyst and solvent, the and, on the other hand, p–p interactions of the aromatic groups
oxidation of 2,3-dihydro-1H-indene was performed as a model with the support favoring the close proximity of the reagents to
reaction and the results are summarized in Table S1 (ESI†). The the catalytic sites, and the active role of this support in aerobic
optimum conditions were 0.4 mol% Cu and H2O as the solvent oxidations, as shown in the results described in Table 1. Given
at room temperature for both reactions.
that the better behavior of the supported catalyst may be due to
The advantages of the support were clearly presented in the other factors, for instance site isolation has been shown to be
revised version of the manuscript as follows: some important advantageous in some other oxidation reactions, this catalyst
advantages of the 3D-(N)GFs as support are: (i) having macroporous was compared with the same Cu(II)-complex immobilized, using
structures, low mass density, excellent chemical and physical stability the same strategy, in a different support, silica, to be sure of the
and high surface area to embed the active sites; (ii) easy synthesis; advantages related to the use of this particular solid. At first,
(iii) having a bifunctional heterogeneous catalyst (copper(II) as the MCM-41-NH2 was synthesized according to the literature,33 then
active site and N as the base) and therefore removing the need for copper(II) tetrasulfophthalocyanine was introduced to the sup-
activators such as inorganic bases and carrying out the reaction port via the same process as that used for CuTSPc@3D-(N)GFs
without any activation at ambient temperature; (iv) increasing the (see experimental section). The SEM image of the synthesized
performance and interaction of oxygen with the support and CuTSPc@MCM-41 is presented in Fig. S7 (ESI†). The as prepared
the substrate in the catalytic oxidation reaction via the chemical catalyst was used for the catalytic aerobic oxidation of benzyl
modification of the surface with nitrogen doping; and (v) being alcohol and 2,3-dihydro-1H-indene as a model reaction, which
hydrophilic to have good dispersion in water as the reaction solvent. gave lower yields compared to CuTSPc@3D-(N)GFs (Table 1,
To show the role and performance of the support, the catalytic entry 5). So, we concluded that 3D-(N)GFs as the support play
oxidation reactions of 2,3-dihydro-1H-indene and benzaldehyde an important role in the catalytic activity. Moreover, increasing
were accomplished using GO, 3D-(N)GFs, and CuTSPc under the stability of the catalyst and facilitating the recovery process
the same reaction conditions and the results are shown in are the other advantages of using 3D-(N)GFs as the support.
Table 1 (entries 2–4). It should be noted that to have a correct
The presence of nitrogen is necessary to make ionic interaction
comparison, the metal contents of CuTSPc@3D-(N)GFs, CuTSPc, between the complex and the support and also easy functionaliza-
and CuTSPc@MCM-41 were the same for model reactions. tion of the support with CuTSPc. The contact between the active
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copper(II) tetrasulfophthalocyanine sites and the supporting efficient oxidation reactions. The catalyst was easily recovered
material plays a vital role in the activity of the catalyst. and reused several times without loss of activity. Further
Copper(II) tetrasulfophthalocyanine was introduced into the investigations of the application of this catalyst for other
3D-(N)GFs by ionic interaction which also increases the hydro- oxidation processes and also other organic transformations
philic properties of the catalyst and consequently improves are currently in progress.
the dispersity and stability of the catalyst in aqueous solution
We gratefully acknowledge financial support from the
(Fig. S8, ESI†). The good dispersity of CuTSPc@3D-(N)GFs in Research Council of Shahid Beheshti University and Catalyst
the aqueous solution seems to be an important factor that Center of Excellence (CCE).
affects the efficiency.28,34,35 The interaction between the support
and the complex also has a critical effect on the true heterogeneity
of the catalyst, and the possible contribution by leached, homo-
geneous species. To examine the leaching, in a separate experiment
Notes and references
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content in the solution of the residue mixture is below 0.1 ppm.
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catalyst is negligible and excellent heterogeneity is displayed
by CuTSPc@3D-(N)GFs. Exchange of anionic phthalocyanines
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