Mechanochemical synthesis of graphene oxide-supported transition metal catalysts for the oxidation of isoeugenol to vanillin

Article abstract of DOI:10.3762/bjoc.13.141

Vanillin is one of the most commonly used natural products, which can also be produced from lignm-derived feedstocks. The chemical synthesis of vanillin is well-established in large-scale production from petrochemical-based starting materials. To overcome this problem, lignin-derived monomers (such as eugenol, isoeugenol, feruhc acid etc.) have been effectively used n the past few years. However, selective and efficient production of vanillin from these feedstocks still remains an issue to replace the existing process. In this work, new transition metal-based catalysts were proposed to investigate their efficiency in vanillin production. Reduced graphene oxide supported Fe and Co catalysts showed high conversion of isoeugenol under mild reaction conditions using H₂O₂ as oxidizing agent. Fe catalysts were more selective as compared to Co catalysts, providing a 63% vanillin selectivity at 61% conversion in 2 h. The mechanochemical process was demonstrated as an effective approach to prepare supported metal catalysts that exhibited high activity for the production of vanillin from isoeugenol.

Full text of DOI:10.3762/bjoc.13.141

Mechanochemical synthesis of graphene oxide-supported
transition metal catalysts for the oxidation of
isoeugenol to vanillin
Ana Franco1, Sudipta De1,2, Alina M. Balu1, Araceli Garcia1 and Rafael Luque*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 1439–1445.
1Departamento de Química Orgánica, Universidad de Cordoba
Campus de Rabanales, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km
396, E14014, Cordoba, Spain and 2Department of Chemical and
Biomolecular Engineering, National University of Singapore, 4
Engineering Drive 4, 117585, Singapore
doi:10.3762/bjoc.13.141
Received: 06 April 2017
Accepted: 20 June 2017
Published: 21 July 2017
Email:
This article is part of the Thematic Series "Green chemistry".
Guest Editor: L. Vaccaro
Rafael Luque* - q62alsor@uco.es
* Corresponding author
© 2017 Franco et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
H2O2; isoeugenol; mechanochemical synthesis; non-enzymatic
process; vanillin
Abstract
Vanillin is one of the most commonly used natural products, which can also be produced from lignin-derived feedstocks. The
chemical synthesis of vanillin is well-established in large-scale production from petrochemical-based starting materials. To over-
come this problem, lignin-derived monomers (such as eugenol, isoeugenol, ferulic acid etc.) have been effectively used in the past
few years. However, selective and efficient production of vanillin from these feedstocks still remains an issue to replace the existing
process. In this work, new transition metal-based catalysts were proposed to investigate their efficiency in vanillin production.
Reduced graphene oxide supported Fe and Co catalysts showed high conversion of isoeugenol under mild reaction conditions using
H2O2 as oxidizing agent. Fe catalysts were more selective as compared to Co catalysts, providing a 63% vanillin selectivity at 61%
conversion in 2 h. The mechanochemical process was demonstrated as an effective approach to prepare supported metal catalysts
that exhibited high activity for the production of vanillin from isoeugenol.
Introduction
Vanillin is the main flavor and aroma compound in vanilla. It is cosmetic, and fine chemical industries. Therefore much atten-
an aromatic compound (4-hydroxy-3-methoxybenzaldehyde) tion has been paid to research on the improvement of its pro-
containing two reactive functional groups that are useful for the duction [5].
synthesis of thermoplastic polymers [1-4].
At the present time only 1% of total vanilla production is from
Vanillin is one of the most important chemicals in the aroma extraction of natural material. This extraction is a very long and
industry, because it is abundantly used in food, pharmaceutical, expensive process [6]. The remaining 99% is being produced
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Beilstein J. Org. Chem. 2017, 13, 1439–1445.
via chemical and biochemical routes. Biotechnology-based ap- as oxidant. The catalytic support, RGO, a graphene derived ma-
proaches, particularly enzymatic processes, have been well terial are normally produced by chemical reduction of graphene
known for many years for vanillin production and are consider- oxide (GO) [22,23].
ably less harmful to the environment. However, they have
inherent disadvantages including comparatively high costs, The materials were prepared using a simple and effective ball
slowness, difficult purification and the requirement of selected milling approach and were characterized by different tech-
strains of microorganisms [7-9]. Major quantities (85%) of the niques.
world supply are still produced from petroleum-based interme-
diates, especially guaiacol and glyoxylic acid using the most Results and Discussion
employed Riedel process [10,11]. The classical synthetic routes The supported RGO materials were characterized by using
are not “environment friendly” and the vanillin produced by several techniques including BET, SEM, TEM, XRD, and IR
these methods is considered to be of lower quality because it spectroscopy. N2 adsorption/desorption isotherms of the
does not contain some trace components that contribute to the reduced graphene sample (Figure 1a) can be classified as type
natural vanilla flavor.
IV corresponding to the mesoporous materials. The RGO sam-
ple showed a BET surface area of 103 m2 g−1 with a pore diam-
Nowadays, 15% of the overall vanillin production comes from eter of 39 nm and pore volume of 0.74 cm3 g−1 (Table 1). After
lignin, more precisely from lignosulfonates. Different products the ball milling with metal precursors, the mesoporous struc-
can be synthesized by lignin oxidation being vanillin the most ture of RGO was found to be partially collapsed as observed
well and valuable product. Recently, eugenol, isoeugenol and from BET isotherms in Figure 1b and c. BET surface areas of
ferulic acid have been used as substrates for vanillin manufac- metal supported RGO materials consequently decreased, with
turing due to their economic and commercial availability. These increased pore diameter and pore volume as a consequence of
compounds are easily derived from lignin and have the common the structure deterioration observed after milling. Additional
structural unit with that of vanillin, being potentially useful for macroporosity (interparticular) was created upon milling, which
vanillin production via simple oxidation pathways [12-14]. Pho- increased both pore diameter and volume. SEM results also
tocatalytic oxidation has been reported for the production of support the observation from BET analysis. The mesoporous
vanillin where TiO2-based materials have been used as effec- nature of the RGO can be easily observed from SEM images
tive catalysts in recent years [15-18]. Although the conversion (Figure 2a and b), whereas metal-supported RGO materials
was high in some cases, vanillin selectivity was never signifi- show a smooth surface with decreased crystallinity.
cant. Another problem related to the slow reaction rates, unsuit-
able for commercial production. As a result, chemical oxida- TEM images of RGO materials with different thickness show a
tion pathways were also followed. To achieve faster kinetics sheet like morphology with different transparencies (Figure 3).
and better selectivity of vanillin, homogeneous catalysts based Dark areas result from the superposition of several graphene
on different transition metal salts/complexes were employed oxide and/or graphene layers containing oxygen functional
[14,19-21]. However, the selectivity of vanillin still remains an groups. Most transparent areas are from thinner films composed
important issue.
of a few layers of reduced graphene oxide from stacking nano-
structure exfoliation. A significant collapse of the structure
In this work, we report the mechanochemical design of transi- could be observed upon metal incorporation (see Figure 3,
tion-metal-based catalysts supported on reduced graphene oxide images c and d), although several domains remained to be
support for the oxidation of isoeugenol into vanillin using H2O2 almost unchanged as compared to those of RGO (see Figure 3f).
Table 1: Textural properties of RGO and NPs supported RGO materials.
Material
SBETa (m2 g−1)
DBJHb (nm)
VBJHc (cm3 g−1)
RGO
103
<10
<15
39
0.74
1.46
2.04
1% Fe/RGO
1% Co/RGO
205
190
aSBET: specific surface area was calculated by the Brunauer–Emmet–Teller (BET) equation. bDBJH: mean pore size diameter was calculated by the
Barret–Joyner–Halenda (BJH) equation. cVBJH: pore volumes were calculated by the Barret–Joyner–Halenda (BJH) equation.
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Beilstein J. Org. Chem. 2017, 13, 1439–1445.
Figure 1: N2 isotherms of (a) RGO, (b) Fe/RGO, and (c) Co/RGO.
Figure 2: SEM images of (a and b) RGO, (c) 1% Fe/RGO, and (d) 1% Co/RGO.
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Beilstein J. Org. Chem. 2017, 13, 1439–1445.
Figure 3: TEM micrographs at different magnifications of (a and b) RGO, (c and d) 1% Fe/RGO, and (e and f) 1% Co/RGO.
X-ray diffraction patterns of RGO-supported materials are Table 2 summarizes the experimental results for the oxidation
shown in Figure 4. Two characteristic peaks at 2θ = 26° and of isoeugenol using supported RGO catalysts. Reaction condi-
2θ = 43° correspond to the typical RGO material. The broad tions were optimized under various conditions. Blank runs (in
nature of the peak confirms the highly amorphous nature of the absence of catalysts) were also performed, with a low conver-
RGO support. A closer look at the figures pointed out the pres- sion in the systems, which could be attributed to the effect of
ence of iron in the form of a FeO/Fe2O3 mixture (mixed phases) the strong oxidizing agent H2O2. However, the reaction pro-
as compared to a more pure CoO phase in the case of Co. Due duced a higher amount of ether compounds with a very low
to the amorphous nature of RGO and low metal loading, the selectivity to vanillin. When RGO was used as catalyst, the
corresponding metal oxide peaks could not be well resolved. conversion increased but the selectivity of vanillin was still
lower than other side products. Importantly, metal incorpora-
Additionally, IR spectra (Figure 5) showed that there is no such tion on RGO support significantly increased both conversion
peak in the range of 1700–1740 cm−1, indicating the absence of and vanillin selectivity in the systems (Table 2, entries 3 and 4).
any oxidized groups such as carbonyl or carboxylic acid groups.
One peak at around 1600 cm−1 could be observed that corre- The optimum results were obtained after 2 h of reaction as seen
sponds to C=C from aromatic groups.
in results from Table 2. The Fe-containing catalysts were found
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Beilstein J. Org. Chem. 2017, 13, 1439–1445.
Figure 4: Powder XRD patterns of RGO supported Fe and Co NPs.
to be more selective than the Co-containing catalysts at similar
conversions under otherwise identical reaction conditions. After
prolonged reaction times, Fe/RGO remained selective towards
vanillin, but Co/RGO experienced a significant drop in selec-
tivity (although the conversion increased). This could be ex-
plained by the strong oxidizing nature of Co that might
facilitate further reactions of vanillin to other compounds. To
investigate the stability of the Fe/RGO and Co/RGO the materi-
als were subjected to different reuses. The results showed a sig-
nificantly decrease in the catalytic activity due to material deac-
tivation.
Conclusion
A simple mechanochemical ball milling process was used to
prepare highly active transition-metal-supported reduced
graphene oxide catalysts. The catalysts were used to produce
the highly useful aromatic compound vanillin, by oxidizing
naturally abundant isoeugenol. The catalysts showed good ac-
Figure 5: IR spectra of 1% Fe/RGO and 1% Co/RGO catalysts
collected by using diffuse reflectance infrared transform spectroscopy
(DRIFT) at room temperature.
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Beilstein J. Org. Chem. 2017, 13, 1439–1445.
Table 2: Results for the catalytic oxidation of isoeugenol.a
Entry
Catalyst
Time (h)
Conversion (mol %)
Selectivity (mol %)
Vanillin
Diphenyl ether
Others
1.
2.
blank
RGO
2
2
2
2
3
3
3
3
5
5
5
5
7
7
7
7
18
39
61
60
19
41
64
70
20
54
64
75
22
59
62
81
7
84
47
8
9
26
63
32
8
27
29
59
13
28
29
67
16
51
33
75
14
51
34
79
3.
1% Fe/RGO
1% Co/RGO
blank
4.
9
5.
79
47
13
6
6.
RGO
25
58
27
11
19
54
21
16
26
52
19
7.
1% Fe/RGO
1% Co/RGO
blank
8.
9.
73
30
13
4
10.
11.
12.
13.
14.
15.
16.
RGO
1% Fe/RGO
1% Co/RGO
blank
70
23
14
2
RGO
1% Fe/RGO
1% Co/RGO
aReaction conditions: 5 mmol isoeugenol, 1.2 mL H2O2, 8 mL acetonitrile, 0.1 g catalyst, 90 °C.
tivity and vanillin selectivity at mild reaction conditions using Micromeritics ASAP 2000 volumetric adsorption analyzer. The
H2O2 as oxidizing agent. A better selectivity was observed for samples were degassed for 24 h at 30 °C under vacuum
the Fe-based catalyst.
(P0 < 10−2 Pa) and subsequently analyzed. Surface areas were
calculated according to the BET (Brunauer–Emmet–Teller)
equation. Mean pore size diameter and pore volumes were
measured from porosimetry data by using the BJH
Materials and Methods
Preparation of materials
In a typical synthesis of ball-milled materials, reduced graphene (Barret–Joyner–Halenda) method. Wide-angle X-ray diffrac-
oxide (RGO) support, together with an appropriate amount of tion experiments were performed on a Pan-Analytic/Philips
the iron precursor (FeCl2∙4H2O) to reach a theoretical 1% iron X`pert MRD diffractometer (40 kV, 30 mA) with Cu Kα (λ =
loading, was ground by using a Retsch-PM-100 planetary ball 0.15418) radiation. Scans were performed over a 2θ range be-
mill with a 25 mL reaction chamber and 8 mm stainless steel tween 10–80° at step size of 0.0188 with a counting time per
ball. Milling was conducted at 350 rpm for 10 min. The same step of 5 s. TEM images of the samples were recorded on JEM
protocol was used to design a 1% Co catalyst using the Co pre- 2010F (JEOL) and Phillips Analytical FEI Tecnai 30 micro-
cursor Co(NO3)2∙6H2O. Graphene oxide was kindly donated by scopes. SEM micrographs were recorded on a JEOL-SEM JSM-
Nano Innova Technologies SL (http://www.nanoinnova.com).
6610 LV scanning electron microscope in backscattered elec-
tron model at 3/15 kV. DRIFT spectra were recorded on a PIKE
Technologies MB 3000 ABB at room temperature.
Characterization of materials
Materials were characterized by using N2 physisorption, powder
X-ray diffraction (XRD), transmission electron microscopy Catalytic activity tests
(TEM), scanning electron microscopy (SEM) and diffuse reflec- In a typical experiment, isoeugenol (5 mmol) and 0.1 g catalyst,
tance infrared Fourier transform spectroscopy (DRIFT). N2 H2O2 (1.2 mL) and acetonitrile (8 mL) were heated at 90 °C
adsorption measurements were performed at 77 K by using a under continuous stirring in a carrusel place reaction station.
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Beilstein J. Org. Chem. 2017, 13, 1439–1445.
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Acknowledgements
Rafael Luque gratefully acknowledges Consejeria de Ciencia e
Innovacion, Junta de Andalucia for funding project P10-FQM-
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