Catalytic behavior of melamine glyoxal resin towards consecutive oxidation and oxy-Michael addition

Article abstract of DOI:10.1007/s11164-010-0169-7

Synthesis of melamine glyoxal resin involves a catalyst-free, one pot reaction between melamine and glyoxal in DMF. The synthesized resins have a similar morphological arrangement to that of layered materials as depicted by their XRD pattern and Raman spectra. The catalytic behavior of melamine glyoxal resin (MGR) have been studied in allylic oxidation of cyclohexene and simultaneous Michael addition. The MGR/solvent/O2 oxidant system can be regarded as a metalfree, additive-free, cost-effective and environmentally benign catalytic system. The oxidative behavior of MGR is attributed to its ability to generate in situ organic peroxide species during the course of reaction. Generation of peroxide species is confirmed by the KI/starch test and further confirmed by the complete suppression effect of TEMPO (2,2,6,6- tetramethylpiperidine-1-oxyl) over oxidation. The activity for Michael addition can be attributed to the presence of a higher content of nitrogen atoms, which serves as the active site. In oxidation, 28.1% conversion of cyclohexene with 37.19 and 62.81% selectivities for cyclohexenol and cyclohexenone were observed, respectively. In consecutive oxidation and oxy-Michael addition, 31.5% conversion of cyclohexene was observed with selectivities of 61.6% for cyclohexenone and 38.4% for alkoxy product. Springer Science+Business Media B.V. 2010.

Full text of DOI:10.1007/s11164-010-0169-7

Res Chem Intermed (2010) 36:677–684
DOI 10.1007/s11164-010-0169-7
Catalytic behavior of melamine glyoxal resin
towards consecutive oxidation and oxy-Michael
addition
Mohd. Bismillah Ansari
Jun Lee Sang-Eon Park
•
Eko Adi Prasetyanto
•
•
Received: 17 January 2010 / Accepted: 29 March 2010 / Published online: 16 September 2010
Ó Springer Science+Business Media B.V. 2010
Abstract Synthesis of melamine glyoxal resin involves a catalyst-free, one pot
reaction between melamine and glyoxal in DMF. The synthesized resins have a
similar morphological arrangement to that of layered materials as depicted by their
XRD pattern and Raman spectra. The catalytic behavior of melamine glyoxal resin
(
MGR) have been studied in allylic oxidation of cyclohexene and simultaneous
Michael addition. The MGR/solvent/O oxidant system can be regarded as a metal-
2
free, additive-free, cost-effective and environmentally benign catalytic system. The
oxidative behavior of MGR is attributed to its ability to generate in situ organic
peroxide species during the course of reaction. Generation of peroxide species is
confirmed by the KI/starch test and further confirmed by the complete suppression
effect of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) over oxidation. The
activity for Michael addition can be attributed to the presence of a higher content of
nitrogen atoms, which serves as the active site. In oxidation, 28.1% conversion of
cyclohexene with 37.19 and 62.81% selectivities for cyclohexenol and cyclohexe-
none were observed, respectively. In consecutive oxidation and oxy-Michael
addition, 31.5% conversion of cyclohexene was observed with selectivities of
6
1.6% for cyclohexenone and 38.4% for alkoxy product.
Keywords Metal-free catalysis Á Melamine glyoxal resin Á Consecutive oxidation
and Michael addition Á Molecular oxygen
Mohd. B. Ansari Á E. A. Prasetyanto Á J. Lee Á S.-E. Park (&)
Laboratory of Nano-Green Catalysis and Nano Center for Fine Chemical Fusion Technology,
Department of Chemistry, Inha University, Incheon 402-751, Korea
e-mail: separk@inha.ac.kr
123

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Mohd. B. Ansari et al.
Introduction
Metal-free organic catalysts have attracted much attention because of their unique
greenness [1]. The design and development of a green oxidative catalytic system is
challenging for catalytic chemist. The selective oxidation at the allylic position into
the corresponding alcohols and ketones is undoubtedly one of the most important
organic transformations. Conventional oxidants used for oxidation are notably based
on transitional metal ions, like chromium, magnesium, etc., are often toxic, and their
use presents significant environmental issues [2–5]. As a consequence, the
development of catalytic systems of high atom efficiency based upon renewable
clean oxidants such as molecular oxygen, hydrogen peroxide or hypochlorite is
preferred, both in terms of environmental and economical perspectives. Thus, the
use of oxygen is atom efficient and economic [6, 7], and so can be designated as
greener process.
In order to carry out a heterogeneous reaction, a range of insoluble supports like
silica materials [8–10] and polymers [11] have been reported for reagent or catalyst
immobilization. In the late 1960s, much attention was paid to polymer support
because of their flexible applicability and unique microenvironment. In general, the
resins are mainly used for heterogenization of homogeneous catalysts [11], though
using resin itself as a catalyst is a challenging task. Recently, polyethylene glycol
resin-initiated oxidation of benzylic alcohols was reported by He and co-workers
[
12]. For decades, melamine resins have been utilized in the paper industry [13],
thermostating materials [14] and water-containing embedding medium for electron
microscopy [15]. In the past few years, melamine resins have drawn great attention
due to their surface properties such as high surface area [16] and basicity. Bjorn
Weber et al. reported self-condensation of hexa methoxy methyl melamine layered
material for creating new materials [17]. To the best of our knowledge, until now
there is no report on utilization of melamine glyoxal resin (MGR) as a catalyst.
Here, we report MGR catalyzed allylic oxidation of cyclohexene by molecular
oxygen and consecutive oxidation and oxy–Michael addition of alcohols. The
oxy-Michael addition catalyzed by triphenylphosphine [18] and aromatic amine
has been reported recently [19]. The heteroatom and amine functionalization on
triazine ring are responsible for Michael addition behavior in MGR. The catalyst
shows bifunctional behavior as it poses catalytic sites for oxidation and Michael
addition.
Experimental
Materials
The glyoxal (40% solution in H O) and cyclohexene were purchased from Aldrich,
2
O and N with a purity of 99.999% were commercially available. The other organic
2
2
compounds were purchased from Tokyo Chemical Industries and were used without
further purification except for the solvents, which were distilled by the known
method prior to use.
1
23

Catalytic behavior of melamine glyoxal resin
679
Synthesis of MGR
Melamine (0.4 mol) was introduced with stirring into (1.6 mol) glyoxal solution
40% in H O) and the temperature was brought up to 60 °C. After it was dissolved, a
(
2
gradual thickening of the medium takes place and a yellow resin was formed which
further changes its color to brown. The reaction mixture was allowed to cool to
room temperature. The precipitated MGR was isolated by filtration over a Buchner
funnel and washed with excess acetone, tetrahydrofuran and dichloromethane. The
obtained MGR was dried overnight at 60 °C. IR, Raman, and XRD were used for
the characterization of synthesized MGR material.
Oxidation reaction
In a typical reaction procedure, 50 mg catalyst was mixed with 1.5 mL of
cyclohexene in DMF (10 mL) in a 100-mL steel autoclave maintained at 80 psi
(
varying O and N concentration as shown in Table 1) at room temperature. The
2 2
reaction mixture was gradually heated up to 100 °C with constant stirring at
00g and maintained for a period of 10 h. The aliquots of the reaction mixture were
withdrawn and subjected to GC–MS analysis.
3
Test for formation of organic peroxide
The presence of organic peroxide was tested with Whatman starch iodide paper,
which is sensitive to peroxide concentrations below 100 ppm. A low concentration
of peroxide present in the sample turns the paper yellow, whereas a high
concentration of peroxide turns the paper blue. After the 9 h reaction, the autoclave
was opened and the test strip was immersed immediately in the reaction mixture for
1
s, slowly blow air on the test strip for 15 to 30 s till the color stabilizes.
Table 1 Oxidation of cyclohexene at different O
2
concentration and solvents
a
a
S. no.
Solvent
P
O2/PN2 (psi)
Conversion (%)
Selectivity (%)
Cyclohexenol
Cyclohexenone
1
2
3
4
5
6
7
8
DMF
0.143
13
39.1
60.9
DMF
0.2307
0.3333
0.4545
0.3333
0.3333
0.3333
0.3333
17.12
27.89
28.10
20.12
25.30
26.18
25.22
40.60
38.12
37.19
34.08
40.60
37.10
39.2
59.40
61.88
62.81
65.92
59.40
62.90
60.80
DMF
DMF
Chloroform
Acetonitrile
b
DMF
c
DMF
Total pressure maintained at room temperature by O
a
2
/N
2
in appropriate ratio to 80 psi, 100 °C
Analyzed by GC
b
2nd cycle
c
3rd cycle
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Mohd. B. Ansari et al.
Characterization
Fourier transform infrared (FT-IR) spectroscopy (Nicolet 6700) and Raman
spectroscopy (HR 800; Horiba/Jobin-Yvon) were employed to analyze the chemical
structure. Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku
miniflex X-ray diffractometer with Cu Karadiation. The Brunauer–Emmett–Teller
(
BET) surface area was measured with an ASAP2020 (Micromeritics Instruments).
The aliquot of reaction mixture was analyzed by GC–MS (Agilent Technologies
975).
5
Results and discussion
-
1
The IR spectra of melamine in Fig. 1 depict peaks at 3,417 and 3,468 cm
corresponding to NH groups of melamine. However, in the spectra of MGR, the
2
peaks disappear as a result of the condensation of the -NH groups with the -CHO
2
groups of the glyoxal. Additionally, the new imines bond (-HC=N) formed after
-
1
the condensation reactions appears at 1,661 cm . These results are in agreement
with those reported earlier [20]. The XRD pattern of MGR in Fig. 2 showed well-
resolved peaks with a very intense diffraction peak at 2h = 27.5. The strongest peak
at 27.5° (d = 0.324) is a characteristic interplanar stacking peak of aromatic
systems, indexed as the (002) peak. The higher packing is attributed to localization
of the electrons and strong binding due to the presence of heteroatom. The 10.68°
(
001) peak presumably relates to an in-plane structural packing motif, such as the
hole-to-hole distance of nitride pores in the crystal [21–23]. The MGR XRD pattern
resembles that of highly stacked materials like carbon nitride and graphite, which is
-
1
further confirmed by the Raman spectra (Fig. 3) depicting the G band at 1,584 cm
associated with variation of the optically allowed zone (E ) of aromatic rings and
2
g
Fig. 1 FT-IR spectra of MGR and melamine
1
23

Catalytic behavior of melamine glyoxal resin
681
Fig. 2 XRD pattern of MGR
Fig. 3 Raman spectra of MGR
-
1
the D band at 1,336 cm due to the disorder-induced mode corresponding to an
imperfect single crystal [23]. The Raman spectra and XRD pattern demonstrate that
the structural arrangement of MGR is similar to that of the highly stacked layered
material. The BET isotherm was measured from N2 adsorption–desorption
2
experiments and the surface area of resin is found to be 45 m /g with uniform
pore size distribution around 3.5 nm.
Catalysis
The catalytic behavior of MGR was evaluated in the oxidation reaction, consecutive
oxidation and Michael addition. The resin shows selective oxidative behavior for
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Mohd. B. Ansari et al.
2
Fig. 4 Effect of O concentration on oxidation of cyclohexene
allylic oxidation of cyclohexene. Variations of oxygen concentration and solvent
effect were also studied. In alcohol, the MGR showed different catalytic behavior
leading to consecutive oxidation and oxy-Michael addition of alcohols.
Effect of O and solvent on reaction
2
The influence of the O concentration on oxidation of cyclohexene to cyclohexe-
2
none and cyclohexenol was examined with varying O content and maintaining total
2
pressure to 80 psi using N . As shown in Fig. 4, with the increase in O pressure,
2
2
there is increase in conversion, but after 20 psi of O , no significant increase in
2
conversion was observed due to saturation.
The conversion is higher when DMF (Table 1, entry 3) is used as solvent in
comparison with chloroform and acetonitrile; this may be attributed to higher
solubility of O in the particular solvent. In DMF, 28% conversion of cyclohexene
2
was observed. The selectivities in DMF for cyclohexenol and cyclohexenone were
found to be 37.19 and 62.81%, respectively. Analyses of insitu-generated peroxide
in reaction media have been the area of major interest [24–26]. The starch iodide
paper test was performed to check the presence of peroxide. The occurrence of
yellow color in the starch iodide paper test inferred that there is formation of a low
concentration of organic peroxide. The activity of MGR towards oxidation can be
attributed to in situ generation of organic peroxide during the course of the reaction
[
12]. The formation of peroxide during the course of the reaction is further
confirmed by complete suppression of the reaction by TEMPO (2,2,6,6-tetramethyl
piperidine-1-oxyl) [12]. The catalyst was also tested for reusability up to three
cycles, and it was found that there is no significant change in activity. Prior to reuse,
the catalyst was washed several times with water followed by excess acetone,
tetrahydrofuran and dichloromethane and dried overnight at 60 °C (Scheme 1).
Consecutive oxidation and Michael addition in alcohols
The oxidation carried out in different alcohols yielded consecutive oxidation and
oxy-Michael addition products (Scheme 2). The oxy-Michael addition of methanol
1
23

Catalytic behavior of melamine glyoxal resin
683
Scheme 1 Oxidation of cyclohexene
Scheme 2 Consecutive Michael addition in alcohols
Table 2 Consecutive oxidation and oxy-Michael addition of alcohols
a
a
Solvent
Conversion (%)
Selectivity (%)
Cyclohexenone
3-Alkoxy cylohexanone
1-Alkoxycylohex-1-ene
Methanol
Ethanol
31.5
29.2
25.1
61.6
61.7
59.9
22.8
21.5
22.0
15.6
16.8
18.1
Propanol
Total pressure maintained at room temperature to 80 psi, O
a
analyzed by GC
/N
2 2
ratio .333, 100 °C
to a, b-unsaturated ketones gave a better selectivity and higher conversion
compared to other alcohols. In methanol, 31.5% conversion was observed for
cyclohehexene, with 61.6 and 22.8% selectivities for cyclohexenone and 3-methoxy
cyclohexanone, respectively (Table 2, entry 1). It is assumed that the Michael
addition occurred consecutively after oxidation. The catalytic activity towards the
Michael addition can be attributed to the presence of nitrogen atoms in the resin
which can serve as catalytic centers as reported by Maruoka [19], Macquarrie and
co-workers. [27]. The results are in agreement with those reported earlier [19].
Conclusions
MGR resin was synthesized by one pot reaction and characterized by IR, Raman,
and XRD. The XRD pattern, D and G band in Raman spectra of MGR depicts the
structural arrangement of resin similar to highly stacked materials. The catalytic
activity of MGR was elucidated towards cyclohexene oxidation and attributed to in
situ generation of organic peroxide, confirmed by a starch iodine test. The resin
shows bifunctional catalytic behavior for oxidation and Michael addition. In the
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Mohd. B. Ansari et al.
oxidation reaction, 28.1% conversion of cyclohexene with 37.19 and 62.81%
selectivities for cyclohexenol and cyclohexenone, respectively, were observed. In
alcohols, the consecutive oxidation and oxy-Michael addition was observed with
3
3
1.5% conversion of cyclohexene with selectivities of 61.6% for cyclohexenone and
8.4% for the alkoxy product.
Safety warning
Experiments using large amounts of compressed gases, especially molecular
oxygen, are potentially hazardous and must only be carried out by using the
appropriate equipment and under rigorous safety precautions. In particular, oxygen
is introduced into the substrate-loaded reactor after N . To avoid an explosive
2
regime, the following order should be used: substrate [ N [ oxygen.
2
Acknowledgments We acknowledge KOSEF for A3, NRL (36379-1), BK21 projects and MOCIE for
Nano Center for Fine Chemicals Fusion Technology.
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