Synthesis and characterization of a novel ruthenium(ii) trisbipyridine complex magnetic nanocomposite for the selective oxidation of phenols

Article abstract of DOI:10.1039/c9ra05079e

Anchoring ruthenium(ii) trisbipyridine complex [Ru(Bpy)₃]²⁺ into a magnetic dendritic fibrous silica nanostructure produces an unprecedented strong nanocatalyst, FeNi₃/DFNS/[Ru(Bpy)₃]²⁺. Impressive oxidation of phenols to 1,4-benzoquinones catalyzed by FeNi₃/DFNS/[Ru(Bpy)₃]²⁺ is obtained in acetonitrile and water solution with molecular dioxygen as oxidant. Exclusively, apparently inert phenols such as phenol itself and mono-alkyl-substituted phenols are impressively oxidized to produce 1,4-benzoquinones through activation of the C-H bond in the position para to the carbon-oxygen bond under mild conditions. In addition, the production of industrially significant quinones that are known intermediates for vitamin combinations is investigated and studied FeNi₃/DFNS/[Ru(Bpy)₃]²⁺ magnetic nanoparticles were produced, and their properties were investigated by AFM, FTIR, XRD, TGA, SEM, TEM, and VSM.

Full text of DOI:10.1039/c9ra05079e

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Synthesis and characterization of a novel
ruthenium(II) trisbipyridine complex magnetic
nanocomposite for the selective oxidation of
phenols
Cite this: RSC Adv., 2019, 9, 28078
c
Zhengxin Fei,ab Feng Chen,a Mingqiang Zhong,a Jianping Qiu,b Weidong Li
and Seyed Mohsen Sadeghzadeh
*
de
Anchoring ruthenium(II) trisbipyridine complex [Ru(Bpy)3]2+ into a magnetic dendritic fibrous silica
nanostructure produces an unprecedented strong nanocatalyst, FeNi3/DFNS/[Ru(Bpy)3]2+. Impressive
oxidation of phenols to 1,4-benzoquinones catalyzed by FeNi3/DFNS/[Ru(Bpy)3]2+ is obtained in
acetonitrile and water solution with molecular dioxygen as oxidant. Exclusively, apparently inert phenols
such as phenol itself and mono-alkyl-substituted phenols are impressively oxidized to produce 1,4-
benzoquinones through activation of the C–H bond in the position para to the carbon–oxygen bond
under mild conditions. In addition, the production of industrially significant quinones that are known
intermediates for vitamin combinations is investigated and studied FeNi3/DFNS/[Ru(Bpy)3]2+ magnetic
nanoparticles were produced, and their properties were investigated by AFM, FTIR, XRD, TGA, SEM, TEM,
and VSM.
Received 4th July 2019
Accepted 14th August 2019
DOI: 10.1039/c9ra05079e
rsc.li/rsc-advances
functionalized metal oxides and polymers as well as antioxi-
dants. The coating of SiO2 with FeNi3 nanoparticles may
Introduction
impressively enhance the electrical contact of materials in high-
frequency applications. Currently, distinct approaches are
utilized for providing nanocomposites of magnetic alloys and
their nanoparticles. Nevertheless, in most cases, they need
excessive as well as inappropriate conditions such as vacuum,
inert atmosphere and sometimes hydrogen, high pressure and
temperature. Moreover, studying the risks associated with Fe
nanoparticles is a priority.8–14
Recently, half-sandwich complexes of organoruthenium
have been much studied because of their catalytic and phar-
macological characteristics.15,16 Their reactivity, biological
activity17,18 and also their catalytic activity may possibly be
modied using appropriate ligand selection.19–21 Loading them
onto macromolecular systems such as dendrimers, metal-
locages, liposomes, and nanoparticles equipped Ru(arene) and
other metal complexes with the capacity to target tumors.22 By
using catalysts on macromolecular supports, problems such as
separation of catalyst from the obtained products, and recycling
of the catalyst may be solved,23 as was observed in the study by
Recently, the use of surfactant on so template has led to the
production of mesoporous silica with dendritic silica ber
morphology (DFNS). Silicas possessing this morphology are
being tested experimentally as support materials for catalysis
and adsorption processes. The outward radial widening of these
silicas provides higher surface area for reactants to access the
functional materials more efficiently. In addition to that, DFNS
has intrinsic mesoporous properties, is thermally stable and
exhibits high activity. The synthesis of DFNS requires a micro-
emulsion system that contains surfactant, oil and water.
Furthermore, the particle size and morphology of DFNS can be
easily manipulated by introducing co-surfactant and various
cosolvents.1–7
The alloy FeNi3 is magnetic and is utilized for its high
permeability, low energy losses, high temperature and high
saturation magnetism. Nevertheless, this nano adsorbent is
subject to rusting and so researchers commonly utilize
aCollege of Material Science and Engineering, Zhejiang University of Technology,
Hangzhou 310014, China
bJinhua Polytechnic, Jinhua 321007, China
¨
Suss-Fink et al., which was performed for Ru/
superparamagnetic iron oxide nanoparticles (SPIONs),24 as
well as that by Fan and colleagues relating to Ru(arene) organ-
ometallics.25 Moreover, such enhancement opens up opportu-
nities in this eld such as molecular nanomagnet development
and also single molecule magnet preparation.26,27
cPolytechnic Institute of Qianjiang College, Hangzhou Normal University, Hangzhou,
310018, China. E-mail: lwd@hznu.edu.cn
dNew Materials Technology and Processing Research Center, Department of Chemistry,
Neyshabur Branch, Islamic Azad University, Neyshabur, Iran
eYoung Researchers and Elite Club, Neyshabur Branch, Islamic Azad University,
Neyshabur, Iran
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The selective oxidation of phenols is of major signicance volume. In addition, it shows dramatically greater catalytic
for the production of 1,4-benzoquinones, main intermediates activity as well as a greatly enhanced lifetime in comparison
for the production of vitamins E and K. Different oxidant/ with amorphous SiO2 in the selective oxidation of phenols to
catalyst combinations like O2 or H2O2/CuCl2, particularly with 1,4-benzoquinones under moderate conditions, as seen in
different co-catalysts, are chosen in this oxidation approach. Scheme 2.
Nevertheless, the basic difficulty of this approach is that the use
of homogeneous catalysts causes issues concerning separation
Experimental
Materials and methods
and recycling of the catalyst as well as purication of products.
Therefore, the development of more impressive as well as
‘‘green’’ catalysts for the selective oxidation is currently a big
High purity chemicals were procured from Fluka and Merck.
challenge.28–32
2,3,6-Trimethylphenol (95%), 2,3,5-trimethylphenol (99%),
In previous studies, some teams showed that the C–H bond
reactivity of phenols in the para-position varied according to
3,5-dimethylphenol (>99%), 2,6-dimethylphenol (99%), 2-
methylphenol (>99%), 3-methylphenol (99%), 2-methox-
the substituents on the benzene ring.33,34 Commonly, when the
yphenol (98%), 2-tert-butylphenol (99%), 2-uorophenol
number of alkyl groups is higher than 3, the C–H bond of these
(98%), 2-chlorophenol (>99%) and 2-iodophenol (98%) were
phenols has amplied reactivity.34 The method for oxidation of
phenols to 1,4-benzoquinones is relatively straightforward.
obtained from Merck. The Electrothermal 9100 apparatus was
utilized for the determination of uncorrected melting points in
Multi-alkyl-1,4-benzoquinones are particularly reactive and
open capillaries. A VERTEC 70 spectrometer (Bruker) in
side-reactions can be occur under oxidation conditions, but
transmission mode was used for the determination of Fourier-
the C–H bond in the para-position of phenol and in mono-
transform infrared (FTIR) spectra. Samples were pulverized
alkyl-substituted phenols is generally inert. Few methods are
and pelleted with spectroscopic grade KBr. Determination of
impressive under mild conditions. Moreover, 1,4-benzoqui-
size and structure of nanoparticles was done using trans-
none as well as mono-alkyl-1,4-benzoquinones are commonly
mission electron microscopy (TEM; Phillips CM10) operated at
more active than phenol and mono-alkyl substituted phenols;
100 kV. The crystallographic structures of nanoparticles were
therefore, over-oxidation generally leads to reduced selectivity.
determined using powder X-ray diffraction (Bruker D8
However, it is desirable to develop effective catalysts for the
Advance model) with Cu ka radiation. Thermal gravimetry
optimized activation of the C–H bond in the para-position of
analysis (TGA) (NETZSCH STA449F3) was utilized under
inert phenols towards dioxygen. The synthesis of quinones
1
nitrogen atmosphere with a heating rate of 10 ꢀC minꢁ1. H
that are considered intermediates in the production of vita-
and 13C NMR spectra were determined with a Bruker DRX-300
mins is of popular concern in organic industry.35–38 The
AVANCE spectrometer and a Bruker DRX-400 AVANCE spec-
selective oxidation of appropriate arenes to their respective
trometer. The NMR spectra for the two elements were recorded
quinones is the basic synthetic stage in the production of
at 300.13 and 75.46 MHz, and 400.22 and 100.63 MHz,
vitamin K3 (menadione) and 2,3,5-trimethylbenzoquinone
respectively. A Heraeus CHN-O-Rapid analyzer was used to
(TMBQ), an intermediate for producing vitamin E, as shown in
perform elemental analyses for carbon, hydrogen and
Scheme 1. Both of the aforementioned vitamins are important
nitrogen. Thin-layer chromatography (TLC) was performed on
food additives, and are produced on a several thousand-ton
silica gel polygramSILG/UV 254 plates and used for the
scale yearly. Moreover, oxidation of arenes to quinones is of
determination of product purity and for monitoring reactions.
biological signicance,39–41 in photosynthesis as well as in the
respiratory chain.
A Shimadzu GCMS-QP5050 mass spectrometer was used to
record the mass spectra.
In the present study, a Ru(II) trisbipyridine complex is
introduced into the ber core of DFNS and used in the oxidation
of phenols and production of vitamin K3. This new material has
high hydrothermal stability, high surface area, and large pore
Scheme 1 Vitamins E and K3, and vitamin E precursor.
Scheme 2 Oxidation of phenols and 2-methylnaphthalene.
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concentrated aqueous ammonia (NH3$H2O, 2.0 mL) and nally
20 mmol of 2,20-bipyridinyl-4,40-diol was added. Aer intense
stirring, the suspension produced was ltered and washed
repeatedly several times, and then dried at a temperature of
Total approach for the synthesis of FeNi3 MNPs
The procedure for the preparation of FeNi3 magnetic nano-
particles (MNPs) involved dissolving 0.01 mol FeCl2$4H2O and
0.03 mol NiCl2$6H2O in 300 mL of distilled water and the
subsequent addition of 1.0 g of polyethylene glycol (PEG, MW
6000). To achieve the pH range of 12 # pH # 13, sodium
hydroxide (NaOH) was added to the solution. Next, hydrazine
hydrate (N2H4$H2O) at 80% concentration was added to the
suspension in varied quantity. The suspension was le to react
continuously for 24 h at room temperature. The pH of the
suspension was maintained at 12 # pH # 13 with periodic
dosing of NaOH. Aer that, the solid phase of the resultant
mixture was ltered out to obtain black FeNi3 MNPs and rinsed
with ionized water multiple times.42
ꢀ
50 C in the presence of air.
Total approach for the synthesis of FeNi3/DFNS/[Ru(Bpy)3]2+
MNPs
3.0 mg of RuCl3$3H2O and 70 mg of FeNi3/DFNS/Bpy (0.041
mmol) were dissolved into a wet DMF/EtOH 4/1 (10 mL) solu-
tion and the reaction mixture was heated to 70 ꢀC for 72 hours.
The reaction mixture was cooled to room temperature and
concentrated under vacuum. The residue was dissolved in
CH2Cl2 and also washed with distilled water. A saturated solu-
tion of 10 mL NH4PF6 was added and the solid obtained was
separated out by using an external magnet. Aer that it was
dried under vacuum.
Total approach for the synthesis of FeNi3/SiO2 MNPs
An aqueous solution containing 80 mL of ethanol, 20 mL of
deionized water and 2.0 mL of 28 wt% concentrated ammonia
aqueous solution (NH3$H2O) was prepared. FeNi3 MNPs (0.02
mol) were dispersed in the mixture and 0.20 g of tetraethyl
orthosilicate (TEOS) was added subsequently. The mixture was
vigorously stirred for 24 h. The solid phase of the resultant
mixture was ltered off and washed several times before being
Total approach for oxidation of phenol
The selected phenol (1 mmol) and 10 mg of FeNi3/DFNS/
[Ru(Bpy)3]2+ MNPs in 10 mL of acetonitrile/water 100/8 (v/v)
were placed in a ask which was then charged with pure
oxygen at a pressure of 2 MPa. The mixture obtained was stirred
ꢀ
dried in atmospheric conditions at 60 C.42
ꢀ
at a temperature of 40 C for 1 hour. The solution was diluted
with ethyl acetate aer the reaction was complete. The catalyst
was separated out using an external magnet. The obtained
solution was dried using sodium sulfate. Then, the excess
solvent was removed under reduced pressure to provide the
respective 1,4-benzoquinone. The substrate conversions and
product yields were quantied by gas chromatography (GC)
using biphenyl as the internal standard. Each experiment was
reproduced at least three times. The experimental error in the
determination of the substrate conversions and product yields
normally did not exceed 2%.
Total approach for the synthesis of FeNi3/DFNS MNPs
Solution A was prepared as 30 mL of aqueous solution con-
taining 0.3 g urea and a dispersion of 0.25 g of FeNi3/SiO2. The
mixture was placed in an ultrasonic bath for 1 h. Solution B was
prepared by adding 0.5 g cetylpyridinium bromide (CPB) into
0.75 mL of n-pentanol and 30 mL of cyclohexane. Solution A and
solution B were mixed and stirred at room temperature, and
1.25 g TEOS was added in a dropwise manner. The mixture was
continuously stirred for 1 h at room temperature and placed in
an oven at 120 ꢀC for 5 h to incite reaction. The completed
reaction was cooled to room temperature before applying strong
magnetic force for the isolation of FeNi3/DFNS core–shell
microspheres. The solid phase was then washed several times
with water and acetone. The washed solid was dried in a drying
oven at 40 ꢀC overnight and calcined at 550 ꢀC for 5 h in
atmospheric conditions.42
Total approach for the synthesis vitamin K3
2-Methyl naphthalene (10 mmol) compound, 2 mL glacial acetic
acid, and 0.1 mL H2O2 as well as 1 mg FeNi3/DFNS/[Ru(Bpy)3]2+
MNPs were stirred while being heated under reux in 20 mL
acetonitrile for around 8 hours. The catalyst was separated out
using an external magnet. Solvent evaporation of the ltrate at
decreased pressure provided the crude product. Vitamin K3 was
isolated using GC on silica gel, eluted with petroleum ether/
EtOAc (3 : 1).
Total approach for the synthesis of FeNi3/DFNS/3-
bromopropyl MNPs
FeNi3/DFNS (200 mg) was mixed via ultrasonication with 20 mL
of THF followed by the addition of 20 mmol of NaOH. Aer that,
22 mmol of 3-bromopropyl trimethoxysilane was added into the
mixture at room temperature and stirred for 16 h at 50 ꢀC. The
resultant mixture was ltered and washed with ethanol and
deionized water. The ltered solids were then dried under
Results and discussion
In the present work, magnetic DFNS solution was produced
according to reported methods and reacted with 3-bromopropyl
trimethoxysilane, followed by reductive amination to make the
corresponding co-immobilized [Ru(Bpy)3]2+. This process can
be observed in Scheme 3.
ꢀ
vacuum at 50 C for 3 h.
The morphology and the physical structure of FeNi3/DFNS
and FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs were analysed with eld-
Total approach for the synthesis of FeNi3/DFNS/Bpy MNPs
FeNi3/DFNS/3-bromopropyl (200 mg) was diffused into a solu- emission scanning electron microscopy (FESEM) and TEM
tion (80 mL) of ethanol, deionized water (20 mL) and 28 wt% (Fig. 1). As shown in Fig. 1a and c, the FeNi3/DFNS possesses
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5–8 nm ordered in three dimensions to create walls, which can
allow straightforward access to the large surface area available.
From FESEM and TEM analyses of FeNi3/DFNS/[Ru(Bpy)3]2+
MNPs, it can be seen that this arrangement is not altered aer
changing the morphology of the MNPs (Fig. 1b and d). Fig. 2
demonstrates the TGA of FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs. The
removal of the solvent chemisorbed and physisorbed on the
surface of the FeNi3/DFNS/[Ru(Bpy)3]2+ material led to weight
loss. In addition, weight loss of almost 29.1 wt% in the range
250–450 ꢀC was related to the organic group derivatives. The
reduction in weight at this range of temperature can be ratio-
nalized as being due to oxidation of ruthenium(II) complexes. In
fact, the residual mass aer the decomposition of FeNi3/DFNS/
[Ru(Bpy)3]2+ MNPs is due to the vacated FeNi3/DFNS NPs.
The X-ray diffraction patterns for FeNi3, FeNi3/DFNS, and
FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs are shown in Fig. 3. All spectra
have in common the (111), (200), and (220) diffraction peaks in
proper accord with the information in JCPDS card no. 19-0629
for the FeNi3 component of the samples (Fig. 3a). In addition to
these iron oxide peaks, the XRD patterns of nanoparticles of the
FeNi3/DFNS core–shell contained a wide peak at low diffraction
angle that related to amorphous silica (as can be seen in
Fig. 3b). Fig. 3c shows an XRD pattern for the FeNi3/DFNS/
[Ru(Bpy)3]2+ MNPs that was unchanged from that of FeNi3/
DFNS.
Scheme 3 The process of co-immobilization of [Ru(Bpy)3]2+ on 3-
bromopropyl
nanoparticles.
trimethoxysilane
functionalized
FeNi3/DFNS
The FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs surface roughness was
investigated by atomic force microscopy (AFM). Fig. 4 shows the
topographic picture for FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs. As can
be observed in Fig. 4, the highest regions, indicated by the
brighter yellowish white color, were enhanced by decreasing T/
W, suggesting an increase in the roughness of the catalyst
surface.
The FTIR spectra demonstrate the presence of silanols,
phosphate groups, and hydroxyl organic groups in both FeNi3/
DFNS MNPs (Fig. 5a) and FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs
(Fig. 5b). In the case of FeNi3/DFNS the wide absorption zones at
1099 and 3561 cmꢁ1 are ascribed to Si–O–Si asymmetric
stretching and OH respectively. Two peaks at 799 and 467 cmꢁ1
are present in Fig. 5a, for Si–O–Si symmetrical stretching and
bending, respectively. These peaks show the linkage of
[Ru(Bpy)3]2+ onto the surface of FeNi3/DFNS. The composite
FeNi3/DFNS/[Ru(Bpy)3]2+ shows bands at 1195, 799 and
Fig. 1 FESEM images of FeNi3/DFNS MNPs (a) and FeNi3/DFNS/
[Ru(Bpy)3]2+ MNPs (b); TEM images of FeNi3/DFNS MNPs (c) and FeNi3/
DFNS/[Ru(Bpy)3]2+ MNPs (d).
a core FeNi3 particle, a nonporous silica layer and silica bres,
and the samples comprise spheres of uniform size with diam-
eters of ꢂ300 nm with a wrinkled radial structure. Scrutiny of
TEM and FESEM images indicated that the FeNi3/DFNS/
[Ru(Bpy)3]2+ consists of dendrimeric bers with thicknesses of
Fig. 2 TGA diagram of FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs.
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467 cmꢁ1. In the 3000–3550 cmꢁ1 range, a strong and wide
absorption band can be seen, which corresponds to –OH
stretching vibrations. The peaks at 3120 and 2980 cmꢁ1 that can
be observed in Fig. 5b are due to stretching of the aromatic and
aliphatic carbon-hydrogen bonds.
Vibrating sample magnetometry (VSM) was used for the
determination of the nanoparticles' magnetic properties, with
the magnetization curves of the obtained nanocomposite
recorded at 300 K. In Fig. 6, it is shown that no residual
magnetism is detected, thus paramagnetic characteristics are
exhibited by the nanocomposites. Saturation magnetization
values of 53.8 and 24.1 emu gꢁ1 were determined for FeNi3 and
FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs, respectively. Responsivity
towards an external magnetic eld and the ability for quick
redispersion upon removal of the magnetic eld are charac-
teristic of paramagnetic nanocomposites with high magnetiza-
tion values. The resultant nanocomposite exhibited good
magnetic responsivity, suggesting potential application for
targeting and separation.
Fig. 3 XRD analysis of (a) FeNi3, (b) FeNi3/DFNS, and (c) FeNi3/DFNS/
[Ru(Bpy)3]2+ MNPs.
The UV-vis spectra of neat complex and the supported Ru(II)
catalysts are given in Fig. 7. The spectra of the supported
ruthenium catalysts showed features similar to those of the neat
complex. The bands at 210 and 325 nm can be attributed to the
charge transfer transition of salen ligand. The band at 475 nm is
due to ligand-to-metal charge transfer transition of Ru(II) salen
complex. On immobilization of Ru salen complex, all the
characteristic bands appeared in its spectra. The UV-vis spectra
conrmed the immobilization of Ru salen complex on the
supports.
The Brunauer–Emmett–Teller (BET) nitrogen physisorption
analysis showed that the specic surface areas of FeNi3/DFNS
and FeNi3/DFNS/[Ru(Bpy)3]2+ are around 721, and 403 m2 gꢁ1
,
respectively. The lower surface area of FeNi3/DFNS/[Ru(Bpy)3]2+
compared with FeNi3/DFNS may be due to the [Ru(Bpy)3]2+.
Fig. 8 indicates the nitrogen adsorption as well as desorption
isotherms of FeNi3/DFNS supported catalysts. The compound
FeNi3/DFNS shows a type IV isotherm and also a hysteresis loop
of H1-type which suggests the presence of mesopores. The pore
size estimated using the desorption section of the nitrogen
Fig. 4 Three-dimensional AFM image of FeNi3/DFNS/[Ru(Bpy)3]2+
MNPs.
Fig. 5 FTIR spectra of (a) FeNi3/DFNS MNPs, and (b) FeNi3/DFNS/
[Ru(Bpy)3]2+ MNPs.
Fig. 6 Room-temperature magnetization curves of (a) FeNi3 and (b)
FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs.
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Fig. 7 UV-vis spectrum of the catalyst.
Fig. 9 Oxidation of phenol by FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs in
different solvents.
production of 1,4-benzoquinone. The yield of 1,4-benzoquinone
obtained was close to 93% in a mixture of acetonitrile and
water. No 1,2-benzoquinone or chlorophenol was made in this
reaction (only different unidentied compounds). The product
yield was enhanced from 79% to 93% when the volume of water
was increased from 4 to 8 mL (CH3CN/H2O [100 : 8 (v/v)]).
As can be observed in Table 2, the optimum temperature
required for performing the reaction was investigated. Aer
studying various temperatures, it was found that 40 ꢀC was the
most appropriate temperature for this reaction. Temperatures
higher than around this did not increase production. Although
increasing the temperature promoted the reaction, serious side-
reactions occurred at over 40 ꢀC. Another factor that can be
examined for the demonstrated reaction is the amount of FeNi3/
DFNS/[Ru(Bpy)3]2+ MNPs to be used. The reaction was
Fig. 8 Adsorption–desorption isotherms of the FeNi3/DFNS NPs (a),
and FeNi3/DFNS/[Ru(Bpy)3]2+ (b); and BJH pore size distributions of the
FeNi3/DFNS NPs (c), and FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs (d).
isotherm using the Barrett–Joyner–Halenda (BJH) method
showed a narrow pore distribution peaking at around 11 nm for
FeNi3/DFNS (see Table 1). The large mesopore amount of FeNi3/
DFNS along with its high capacity can accomodate [Ru(Bpy)3]2+,
which possesses a comparatively big molecular size.
Different solvents were examined for the oxidation of phenol
in the presence of FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs, with the
outcomes indicated in Fig. 9. Usual solvents such as ethyl
acetate, water, isopropanol, and dichloromethane had no
inuence on the reaction. Methanol as a solvent produced
a higher yield than did acetonitrile. It is important to note that
though no reaction occurred in water, mixing water with
Table 2 Oxidation of phenol by FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs for
different temperatures, amounts of catalyst, O2 pressures, and reac-
tion times
O2 pressure
Entry Temperature (ꢀC) Catalyst (mg) Time (h) (MPa)
Yielda (%)
1
2
3
4
5
6
7
8
r.t.
30
40
50
40
40
40
40
40
40
40
40
40
40
40
40
14
14
14
14
12
10
8
—
10
10
10
10
10
10
10
10
2
2
2
2
2
2
2
2
1
0.5
1
1
1
1
1
1
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
0.5
1
—
71
93
93
93
93
80
—
93
39
11
52
81
93
93
91
acetonitrile caused
a considerable enhancement in the
9
10
11
12
13
14
15
16
Table
1 Structural parameters of FeNi3/DFNS and FeNi3/DFNS/
[Ru(Bpy)3]2+ MNPs
1.5
2.0
2.5
3.5
Catalyst
SBET (m2 gꢁ1
)
Va (cm3 gꢁ1
)
DBJH (nm)
FeNi3/DFNS
721
403
3.7
2.9
11
7
FeNi3/DFNS/[Ru(Bpy)3]2+
Isolated yield.
a
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performed at 2 h at the temperature of 40 ꢀC, in the presence of tailoring chemical and physical characteristics such as
catalyst (8, 10, 12, and 14 mg). As can be seen in Table 2, in the composition, shape, and size, as well as morphology. To
absence of catalyst, no 1,4-benzoquinone was produced. Using investigate the exact effect of the presence of DFNS in the
different amounts of FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs as catalyst catalyst, the MNPs of FeNi3/DFNS/[Ru(Bpy)3]2+ were compared
led to variation in yield. The reaction using 8 mg of catalyst with FeNi3/SiO2/[Ru(Bpy)3]2+ MNPs, which possess identical
produced an 80% yield aer 2 h duration. Increasing the compositions but distinct structures. When FeNi3/SiO2/
amount of catalyst to 10 mg increased the yield of 1,4-benzo- [Ru(Bpy)3]2+ was utilized as the catalyst, the yield of the desired
quinone to about 93%. Amounts of FeNi3/DFNS/[Ru(Bpy)3]2+ product was near to the mean value; however, the product for
MNPs higher than 10 mg did not further increase the yield. FeNi3/DFNS/[Ru(Bpy)3]2+ was better.
Entries 6, 9 and 10 indicate the effects of time. The product yield
Different phenols were tested, and the outcomes are pre-
increased on changing the reaction time from 0.5 h to 1 h. In sented in Table 3. Generally, the reactivity of phenols depended
addition, Table 2 indicates the inuences of oxygen pressure. on the number of alkyl groups in the benzene ring. Full
The yield of 1,4-benzoquinones was highly enhanced when conversion of 2,3,5,6-tetramethylphenol to yield 95%
oxygen pressure was increased from 0.5 bar to 2.0 bar owing to tetramethyl-1,4-benzoquinone was obtained at a temperature of
the increased oxygen concentration in the reaction mixture. By 25 ꢀC and a pressure of 2.0 bar for 1 hour. When 3,5-dime-
increasing the oxygen pressure to 3.5 bar, the phenol concen- thylphenol was utilized, 90% production of 2,6-dimethyl-1,4-
tration is diluted and the reaction activity decreased.
benzoquinone was achieved. A yield of 85% was obtained for
In order to study the performance of the catalyst, various 2,6-dimethylphenol under identical conditions. For 2,3,6-tri-
control experiments were done; the data obtained are presented methylphenol, 92% production of trimethyl-1,4-benzoquinone
in Fig. 10. Firstly, the reaction was performed using FeNi3/ was achieved. A similar outcome (94% yield) was determined
DFNS, and it was found that none of the favorable product was for 2,3,5-trimethylphenol under identical conditions. A yield of
made in 1 h reaction time. In addition, when FeNi3/DFNS/Bpy 91% tert-butyl-1,4-benzoquinone were found for 2-tert-butyl-
was utilized as the catalyst, a reaction could not be seen. The phenol. Under the same conditions, 89% of 2-methoxyphenol
Bpy cannot catalyze the reaction under moderate reaction was converted to methyl-1,4-benzoquinone. In experiments
conditions. Following these negative results, we carried on the using phenols with electron-withdrawing groups, such as
investigation to enhance the amount of product by adding the halogen, no reaction was observed even aer prolonged time.
ruthenium(II). It is important to note that there is not a high
variation between the yields obtained when the reaction was was investigated. For polar protic solvents, like water, ethanol,
performed with MNPs and methanol and isopropanol, none of the desired product was
FeNi3/DFNS/[Ru(Bpy)3]2+
The effect of various solvents for production of vitamin K3
[Ru(Bpy)3]2+ catalyst, but [Ru(Bpy)3]2+ is not reusable. These formed. Hence, the production of the cross-coupling is
results prove that the reaction is basically catalyzed using comparatively poor for polar aprotic solvents like CHCl3, DMF,
[Ru(Bpy)3]2+ functional groups within the nanostructure of DMSO and CH2Cl2 as well as EtOAc. In the present paper, it was
FeNi3/DFNS. The nano-sized particles increase the presented determined that heating with acetonitrile is very effective
surface of the active catalyst, therefore increasing the contiguity compared with utilizing organic solvents and produces green
between catalyst and reactants, mimicking homogeneous conditions for vitamin K3 production (see Table 4).
catalysts. Nanoparticles of FeNi3/DFNS/[Ru(Bpy)3]2+ were
In this paper, the crucial role of temperature was investi-
utilized in the subsequent studies due to their appropriate high gated for vitamin K3 production in the presence of the MNPs of
selectivity, reactivity, and easy separation. In addition, the FeNi3/DFNS/[Ru(Bpy)3]2+ as catalyst. The most appropriate
activity and selectivity of the nano-catalyst may be managed by temperature in the case of this reaction was obtained by heating
under reux to improve the performance of the reaction. This
indicated the catalytic activity is sensitive to the temperature of
reaction. The amount of catalyst is a signicant parameter in
the considered reaction to obtain vitamin K3. No yield was ob-
tained in the absence of the catalyst. When 0.2–0.8 mg of FeNi3/
DFNS/[Ru(Bpy)3]2+ MNPs was used in this reaction, production
of vitamin K3 was achieved under mild conditions. The best
result was obtained when the model reaction was performed in
the presence of 1 mg of FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs. Further
increasing the amount of catalyst did not enhance the yield in
the model reaction (Fig. 11). Under the optimized conditions,
GC (gas chromatography) was used to analyse the minimum
reaction time necessary in the presence of FeNi3/DFNS/
[Ru(Bpy)3]2+ MNPs (1 mg, showing that good yields of vitamin
K3 may be obtained at 8 h Fig. 12).
We consider that the following hypothesis based on phenol
is reasonable (Fig. 13): FeNi3/DFNS/[Ru(Bpy)3]2+ is oxidized by
Fig. 10 Oxidation of phenol by different FeNi3/DFNS nanoparticles.
28084 | RSC Adv., 2019, 9, 28078–28088
dioxygen as ruthenium(II), which oxidizes phenols to produce
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Table 3 The results for aerobic oxidation of various phenols into 1,4- Table 3 (Contd.)
benzoquinonesa
Entry
Substrate
Product
Yield (%)
Entry
1
Substrate
Product
Yield (%)
95
12
—
13
—
2
3
4
92
94
90
a
Isolated yield.
Table 4 Synthesis of vitamin K3 by FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs in
different solventsa
Entry
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
H2O
MeOH
EtOH
—
—
—
—
—
—
17
61
35
38
43
93
23
67
—
5
6
7
85
89
87
i-PrOH
Dioxane
n-Hexane
THF
Toluene
CH2Cl2
CHCl3
DMSO
CH3CN
EtOAc
9
10
11
12
13
14
15
DMF
Solvent-free
a
Reaction conditions: 2-methyl naphthalene (10 mmol), glacial acetic
acid (2 mL), H2O2 (0.1 mL) and FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs (2 mg)
were stirred together while being heated under reux in solvent (20
mL) for 10 hours. b Isolated yields.
8
9
91
89
phenoxy radicals; the produced radical intermediates react with
O2 to form 1,4-benzoquinone and H2O. Since phenol and o-
mono-alkyl substituted phenols are inert, strong conditions
such as higher temperature are required for the activation of the
C–H bond in the para-position of these phenols. Because the
radical intermediates of phenol and inert o-alkyl phenols
produced during reaction are really active, a high pressure of
dioxygen is necessary to keep dioxygen at a higher concentra-
tion in solution, which scavenges the intermediates to form the
nal products. That too high a pressure of dioxygen gives no
clear improvement is evidence for this.
10
11
—
—
The reusability of a catalyst is a signicant property in the
case of green chemistry. Hence, the reusability of MNPs of
FeNi3/DFNS/[Ru(Bpy)3]2+ was investigated under optimal
conditions for the aerobic oxidation of phenol. Aer performing
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Fig. 14 The reusability of catalysts for oxidation of phenol.
Fig. 11 Effect of increasing amount of FeNi3/DFNS/[Ru(Bpy)3]2+ MNPs
on the preparation of vitamin K3.
ltration experiment on the oxidation of phenol under
optimum conditions and determined that around 39% of the
catalyst was magnetically removed in situ aer 30 min of reac-
tion. In addition, the reactants were able to tolerate more
reaction. The outcome was that, aer the heterogeneous cata-
lyst had been removed, the free catalyst remaining was feebly
active, and a conversion of about 42% was obtained aer 1 hour
of the oxidation of phenol. This proved that the catalyst worked
heterogeneously during the reaction and showed just slight
leaching. Secondly, to conrm the heterogeneous form of the
catalyst, mercury poisoning was additionally performed.
Mercury(0) was used directly or synthesized in situ, and
dramatically deactivated the metal catalyst on the active surface,
so reducing the catalytic activity. The experiment conducted is
proof of the heterogeneous nature of the catalyst. This test was
performed with the aforesaid model reaction under optimal
conditions. Aer 30 min of the reaction, about 300 molar
mercury was released into the reaction mixture. The reaction
mixture was stirred for more than 1 hour, during which period
no further conversion was seen. The kinetic scheme of the
reaction in the presence of Hg(0) is shown in Fig. 16. The
negative outcomes obtained from both of the heterogeneity
experiments (Hg(0) poisoning as well as hot ltration) sug-
gested that the solid catalyst is really heterogeneous and no
measurable ruthenium is leached upon oxidation of phenol.
Finally, we checked whether the structure of the recovered
catalyst was maintained aer 10 runs for the catalytic oxidation
Fig. 12 Effect of time on yield of vitamin K3.
Fig. 13 Aerobic oxidation of phenol into 1,4-benzoquinone over
FeNi3/DFNS/[Ru(Bpy)3]2+
.
the reaction, the solid MNPs of FeNi3/DFNS/[Ru(Bpy)3]2+ were
directly separated from the liquid reaction zone, magnetically,
as when the reaction mixture is placed near a magnet, the solid
readily separates from the solution in a few seconds. Aer
cleaning with solvent, the catalyst can quickly be reused. Fig. 14
indicates the results of reusing the catalyst for 10 consecutive
runs. The yield of product obtained was 90% at the tenth run,
which means only a 3% drop in performance was seen in
comparison with the fresh catalyst (93%). In addition, the
amount of ruthenium leached into the solution for oxidation of
phenol aer each run was evaluated using ICP. The catalyst
indicated very little leaching in each run: 0.6% metal leaching
was discovered aer the tenth run, so showing the durability of
the catalyst under the reaction conditions, as illustrated in
Fig. 15.
In addition, two studies were performed to clarify the
heterogeneous nature of the catalyst. Firstly, we did a hot
Fig. 15 Recyclability of the catalyst for oxidation of phenol.
28086 | RSC Adv., 2019, 9, 28078–28088
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as for eco-friendly production of vitamin K3. This green
approach has some noteworthy eco-friendly properties, such as
the simplicity of catalyst reclamation from the reaction mixture
using a magnet, the use of molecular O2 as a non-toxic oxidant,
and the synthesis of water as a by-product. Bio-catalysts
(heterogeneous form) may improve the performance of the
catalytic method in green organic synthesis, particularly
accompanied by environmentally friendly solvents.
Conflicts of interest
There are no conicts to declare.
Fig. 16 Reaction kinetics, Hg(0) poisoning, and hot filtration studies
for oxidation of phenol.
Acknowledgements
This work was funded by the Zhejiang Provincial Natural
Science Foundation of China (Grant No. LQ15E080007), the
Jinhua Science and Technology Development Foundation of
China (Grant No. 2018-3-002), and the Qianjiang College
Natural Science Foundation of China (2019QJJL03).
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