Design and synthesis of a versatile cooperative catalytic aerobic oxidation system with co-immobilization of palladium nanoparticles and laccase into the cavities of MCF

Article abstract of DOI:10.1016/j.jcat.2019.12.023

We have designed a versatile reusable cooperative catalyst oxidation system, consisting of palladium nanoparticles and laccase with unprecedented reactivity. This biohybrid catalyst was synthesized by the stepwise immobilization of laccase as an enzyme and Pd as a nanometallic component into the same cavity of siliceous mesocellular foams (MCF). MCF and nanobiohybrid catalyst were characterized by BET, SAXS, SEM, EDX elemental mapping, ICP-OES, TEM, TGA, FT-IR, and XPS techniques and the stepwise immobilization of laccase enzyme and Pd onto MCF was evaluated through several compelling electrochemical studies. The present catalytic system exhibits high activity toward (i) aerobic oxidation of alcohols to the corresponding carbonyl compounds, (ii) aerobic oxidation of cyclohexanol and cyclohexanone to phenol and (iii) aerobic dehydrogenation of important N-heteocyclic compounds (tetrahydro quinazolines, quinazolonones, pyrazolines and 1,4-diydropyridines) in the presence of catalytic amount of hydroquinone (HQ) as mediator in phosphate buffer (0.1 M, pH 4.5, 4 mL)/THF (4%, 1 mL) as solvent under mild conditions. The immobilization of both oxygen-activating catalyst (laccase) and oxidizing catalyst (Pd) onto the same support makes the present catalyst system superior to other currently available heterogeneous palladium based catalytic aerobic oxidation systems.

Full text of DOI:10.1016/j.jcat.2019.12.023

Journal of Catalysis 382 (2020) 305–319
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Design and synthesis of a versatile cooperative catalytic aerobic
oxidation system with co-immobilization of palladium nanoparticles
and laccase into the cavities of MCF
⇑
Sirvan Moradi, Zahra Shokri, Nadya Ghorashi, Aso Navaee, Amin Rostami
Department of Chemistry, Faculty of Science, University of Kurdistan, Zip Code 66177-15175, Sanandaj, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
We have designed a versatile reusable cooperative catalyst oxidation system, consisting of palladium
nanoparticles and laccase with unprecedented reactivity. This biohybrid catalyst was synthesized by
the stepwise immobilization of laccase as an enzyme and Pd as a nanometallic component into the same
cavity of siliceous mesocellular foams (MCF). MCF and nanobiohybrid catalyst were characterized by BET,
SAXS, SEM, EDX elemental mapping, ICP-OES, TEM, TGA, FT-IR, and XPS techniques and the stepwise
immobilization of laccase enzyme and Pd onto MCF was evaluated through several compelling electro-
chemical studies. The present catalytic system exhibits high activity toward (i) aerobic oxidation of alco-
hols to the corresponding carbonyl compounds, (ii) aerobic oxidation of cyclohexanol and cyclohexanone
to phenol and (iii) aerobic dehydrogenation of important N-heteocyclic compounds (tetrahydro quinazo-
lines, quinazolonones, pyrazolines and 1,4-diydropyridines) in the presence of catalytic amount of hydro-
quinone (HQ) as mediator in phosphate buffer (0.1 M, pH 4.5, 4 mL)/THF (4%, 1 mL) as solvent under mild
conditions. The immobilization of both oxygen-activating catalyst (laccase) and oxidizing catalyst (Pd)
onto the same support makes the present catalyst system superior to other currently available heteroge-
neous palladium based catalytic aerobic oxidation systems.
Received 1 October 2019
Revised 13 December 2019
Accepted 16 December 2019
Keywords:
Reusable cooperative catalytic system
Palladium
Laccase
Hydroquinone
Aerobic oxidative dehydrogenation
Ó 2020 Elsevier Inc. All rights reserved.
1
. Introduction
The selective oxidation of organic compounds is an easy way to
functionalization, but over the past two decades, the oxidation
reactions in the presence of homogeneous palladium catalysts
have led to many other reactions, including oxidation of alcohols,
dehydrogenation of C-C bonds and oxidative C-H functionaliza-
tion’s [1]. Although these reactions proceed in good to high yields,
in most of aerobic oxidation reactions catalyzed by palladium,
organic solvents, high pressure and/or temperature [3], expensive
transform simple precursors into valuable products. Among the
various reactions of organic chemistry, oxidation reactions have
always been a challenge and have been discussed. Since most of
these reactions have low selectivity and by-products are seen
when oxidizing stoichiometric values are used. In this regard, the
development of aerobic oxidation in a liquid phase with high
chemoselectivity, regioselectivity and stereoselectivity can have a
great impact on the synthesis of drugs, agrochemicals, and fine
chemicals [1]. Also, the industry’s need for environmentally
friendly processes has increased demand for eco-friendly oxidants
ligands [1] and strongly basic pH are required. As well, because
0
of the unfavorable electron transfer between Pd and
2
O ,
palladium-catalyzed oxidation reactions with direct reoxidation
0
of Pd by molecular oxygen sometimes fail. Thus, the use of Elec-
tron Transfers Mediators (ETMs) for many Pd-catalyzed aerobic
oxidation reactions are required to obtain high selectivity and high
efficiency [2,4].
2 2 2
today, with H O and O meeting this need [2]. Aerobic oxidation
in the presence of palladium dates back to the late 1950s when
Wacker process was discovered. In the decades following the dis-
covery of the Wacker process, these reactions have been consid-
ered and developed often in the direction of oxidative alkene
Recently, cooperative catalyst systems have been developed as
highly promising sustainable alternatives to traditional catalysts.
In these catalysts, two or more catalytic centers cooperate to
reduce the energy of chemical transformations. In nature, such sys-
tems are abundantly seen in metalloenzymes that use a metal and
an organic cofactor [5]. Several research groups designed bioin-
spired oxidation cooperative catalytic systems to carry out reac-
⇑
Corresponding author.
E-mail address: a.rostami@uok.ac.ir (A. Rostami).
https://doi.org/10.1016/j.jcat.2019.12.023
021-9517/Ó 2020 Elsevier Inc. All rights reserved.
0

3
06
S. Moradi et al. / Journal of Catalysis 382 (2020) 305–319
tions under mild conditions [4,6]. Although these protocols repre-
sent considerable advances, there are still other drawbacks, such as
homogeneity of the catalyst systems, and the use of transition
metal complexes and expensive ligands (Scheme 1A). Therefore,
the development of intramolecular heterogeneous cooperative cat-
alytic systems with simple separation and recycling in the ligand-
free and benzoquinone-free aerobic oxidation of organic com-
pounds under mild conditions is highly desirable.
Recently, several one-pot tandem catalytic systems have been
developed, which combine the reactivity of transition metal catal-
ysis and the selectivity of enzymatic catalysis [7]. Despite these
developments, there has been no report on the design of heteroge-
neous cooperative catalyst systems that make use of both biocatal-
ysis and transition metals being employed in the oxidation of
organic compounds.
Laccases, multi-copper-containing oxidoreductase enzymes, are
highly attractive biocatalysts in modern organic synthesis. They
catalyze the oxidation of various compounds such as benzenediols,
aminophenols, polyphenols, polyamines, and lignin-related mole-
cules using oxygen as an electron acceptor and producing water
as by-product. The most efficient strategy to extend the range of
laccase substrates is the simultaneous use of the enzyme and redox
mediators [8].
2. Experimental
2.1. Material and instrumentation
All substances, reagents and solvents except N-heterocyclic
compounds were bought from the Merck and Aldrich Chemical
Companies and used without further purification. The N-
heterocyclic compounds were synthesized using previous methods
1
13
in our research lab [13]. FT-IR and H, CNMR spectra were
recorded on Thermo Nicolet Nexus 670 and BrukerAvance spec-
trometers (300 MHz). The physical properties of mesoporous
materials were determined from N2 adsorption/desorption iso-
therms using an ASAP 2010 instrument. The particle size was
determined by SEM using FESEM-TESCAN. The chemical composi-
tion of the prepared nanocatalyst was measured by EDX (Energy
Dispersive X-ray Spectroscopy) performed in SEM. The loading
and size of the Pd nanoparticles on the support was determined
by inductively coupled plasma-optical emission spectrometry
(ICP-OES, Optima 7300D) and Transmission Electron Microscopy
(Microscope an acceleration voltage of 80 kV), respectively. X-ray
photoelectron spectroscopy (XPS) was used to determine the oxi-
dation states of the Pd nanoparticles and the elements types on
Pd-Laccase@MCF.
Recently, Bäckvall and co-workers reported Pd@MCF as an effi-
cient nanocatalyst for aerobic oxidation of alcohols [9] and bifunc-
tional biomimetic catalyst in which Pd nanoparticles and lipase are
co-immobilized on MCF for dynamic kinetic resolution of an amine
The electrochemical tests were performed by a Zahner electro-
chemical workstation equipped with a conventional three elec-
trode system. A glassy carbon electrode (GCE) with diameter of
2 mm was used as a support electrode. The desired materials were
well dispersed in ethanol with concentration dispersion of
[
10]. These works and our previously published results on the appli-
ꢀ1
cation of laccase and laccase-mediated catalytic system in organic
reactions [11] prompted us to design a heterogenous cooperative
catalyst system, consisting of palladium nanoparticles and laccase
enzyme for biomimetic oxidation of organic compounds
1 mg mL . For each experiment, 3 mL of dispersed suspension
were cast on the electrode surface and dried at room temperature.
The voltammetry and electrochemical impedance spectroscopy
(EIS) were carried out in the 0.1 M KCl solution containing
1 mM K [Fe(CN) ]/K [Fe(CN) ] as redox probe. EIS was taken in
(Scheme 1B). In this work, we used mesocellular foams (MCF) as a
3
6
4
6
support because of features such as simple preparation, large
channel size, high surface areas, and the presence of a large
number of silanol groups that contribute to high catalyst loading.
Also, it is important to keep in mind that we use catalytic amount
of hydroquinone [12] instead of stoichiometric amount of benzo-
quinone, a toxic mediator (Scheme 1B).
the frequency range of 1 Hz to 100 kHz using a modulation voltage
of 5 mV with applying the E of redox peaks as constant voltage.
The Zview modeling program (version 3.5f) was used to fit the far-
adaic impedance spectra. All electrochemical measurements were
performed at room temperature.
D
2.2. Synthesis of siliceous mesocellular foams (MCF)
MCF particles were prepared according to a previously reported
method [14] with slight modifications. P123 (2.0 g, 0.4 mmol) was
dissolved in an acidic HCl solution (1.6 M, 75 mL) at room temper-
ature. Then, NH
4
F (23 mg, 0.6 mmol) and 1,3,5-trimethylbenzene
(
2.0 g, 17 mmol) were added. After stirring for 45 min at 35–
4
0 °C, tetraethoxysilane (4.4 g, 21 mmol) was added and stirring
continued for 20 h. Subsequently, the mixture was placed at
1
00 °C for 24 h. The resulting product was filtered, washed and
dried. Finally, the obtained sample was calcined at 550 °C in air
for 6 h to give the MCFs. The synthesized MCFs exhibited the fol-
lowing characterstics: window size = 7 nm, Cell size = 24 nm,
specific pore volume = 1.1 cm g and BET surface area = 503 m2
g
3
ꢀ1
ꢀ1
(Fig. S1 in the Supplementry information). The characterstics of
the synthesized MCFs in this work are in agreement with those
previously reported in the literatures (Table S1, p. S4). Further-
more, the prepared MCFs also have been analyzed using small-
angle X-ray scattering (SAXS). Based on this technique, the cell size
was obtained about 26 nm, which is in good agreement with the
value obtained from nitrogen sorption.
2.3. Synthesis of aminopropyl-functionalized MCF (AmP-MCF)
The synthesized MCFs (1 g) was dispersed in 20 mL dry toluene,
Scheme 1. Cooperative catalytic aerobic oxidation systems.
then a solution of 3-aminopropyltrimetoxysilane (2.7 mL) in

S. Moradi et al. / Journal of Catalysis 382 (2020) 305–319
307
toluene (10 mL) was added to the reaction mixture. The mixture
was stirred under argon (10 min) and then refluxed for 24 h. The
resulting solid was filtered and washed several times with toluene,
ethanol, and dichloromethane and dried [9].
loon) or in an open-air round-bottom flask at room temperature for
the time specified (Scheme 2). After completion of the reaction
(monitored by TLC), the catalyst was separated using filtration
and washed with CH
Cl
(2 ꢁ 5 mL). The organic phase was dried over anhydrous
MgSO . After evaporation of the solvent under reduced pressure,
2 2 2
Cl and the product was extracted with CH -
2
2.4. Synthesis of Pd nanoparticles onto AmP-MCFs (Pd(0)-AmP-MCF)
4
the crude product was purified by recrystallization from ethanol
The aminopropyl-functionalized MCF (500 mg) was dispersed
or chromatography on silica gel (n-hexane-EtOAc = 3:1).
in pH-adjusted deionized water solution by adding 15 mL LiOH
0.1 N, pH 8). Then, the mixture was stirred at room temperature
for 5 min. On the other hand, Li PdCl was prepared by mixing
PdCl (145.41 mg, 0.82 mmol) and LiCl (69.52 mg, 1.64 mmol) in
0 mL distilled water, the suspension was stirred at 80 °C until a
homogeneous solution was achieved. The resulting solution was
filtered, pH-adjusted (pH 8), and then added to the mixture of
AmP-MCFs in water. The reaction mixture was stirred for 24 h.
Then, the suspension was centrifuged, and the resulting solid
(
2.9. General procedure for the aerobic oxidation of cyclohexanol and
cyclohexanone
2
4
2
1
A 25 mL round-bottomed flask equipped with a magnetic stirrer
was charged with substrate (1 mmol), Pd-Laccase@MCF (0.25 g,
0.34 mmol Pd), HQ (0.34 mmol) and NaPBS/THF (5 mL, 4/1 v/v).
The reaction mixture was stirred under the O2 (balloon) at room
temperature for the time specified (Scheme 3). After completion
of the reaction (monitored by TLC), the catalyst was separated
using filtration and washed with CH Cl and the product was
(
(
the Pd(II)-precatalyst) was washed with distilled water. The Pd
II)-precatalyst was then re-dispersed in water (15 mL), and a solu-
2
2
tion of NaBH
4
(310.2 mg, 8.2 mmol) in distilled water (5 mL) was
extracted with CH Cl (2 ꢁ 5 mL). The organic phase was dried over
2
2
added slowly to reduce Pd(II) to Pd(0). After the completion of
the reduction, the Pd(0)-AmP-MCFs was isolated by centrifugation
and washed with water, acetone, and dried overnight [9].
2 4
anhydrous Na SO . After evaporation of the solvent under reduced
pressure, the crude product was purified by chromatography on
silica gel (n-hexane-EtOAc = 5:1).
2.5. Synthesis of glutaraldehyde-functionalized Pd(0)-AmP-MCF
2.10. General procedure for the aerobic dehydrogenation of N-
heterocycle compounds (2-substute-1,2,3,4-tetrahydroquinazolines,
Pd(0)-AmP-MCF (0.5 g) was dispersed in 30 mL of sodium phos-
1,4 dihydropyridines and pyrazolines)
phate buffer (100 mM, pH 8.0), and allowed to stir with glutaralde-
hyde solution (50% in H O, 0.23 g, 0.99 mmol) at r.t. for 24 h. The
2
To a mixture of substrate (1 mmol), catalyst (0.2 g, 0.27 mmol
reaction mixture was then centrifuged and the resulting solid was
washed with phosphate buffer (3 ꢁ 45 mL, 100 mM, pH 8.0) and
acetone (3 ꢁ 45 mL), and finally dried under reduced pressure [8].
Pd), and HQ (0.27 mmol), NaPBS/THF (4/1 mL) were added. The
reaction mixture was stirred under O (balloon) at room ambient
for an appropriate time (Scheme 4). After being finished (moni-
tored by TLC), the catalyst was separated using filtration and
2
2.6. Immobilization of laccase enzyme on glutaraldehyde-
washed with CH
2
Cl
2
and the product was extracted with CH
2 2
Cl
functionalized Pd(0)-AmP-MCF (Pd-Laccase@MCF)
(
2 ꢁ 5 mL). The organic layer was dried over anhydrous Na
2
SO , fil-
4
tered and evaporated under reduced pressure. The crude product
was purified by chromatography on silica gel using n-hexane/
ethyl acetate (3:1).
Glutaraldehyde-functionalized Pd(0)-AmP-MCF (0.5 g) was dis-
persed in sodium phosphate buffer (1 mL/100 mg support,
1
00 mM, pH 7.2), followed by a solution of laccase enzyme (30
U) in distilled water (3 mL). The reaction mixture was stirred at
room temperature for 12 h. The final product was then separated
by centrifugation, washed with phosphate buffer (2 ꢁ 5 mL,
2
2
.11. General procedure for aerobic dehydrogenation of 2-substute-
,3-dihydroquinazolin-4(1H)-ones
1
00 mM, pH 7.2) and dried under reduced pressure.
To a mixture of substrate (1 mmol), catalyst (0.25 g, 0.34 mmol
Pd), and HQ (0.34 mmol), NaPBS/THF (4/1 mL) were added. The
2.7. Activity assays of immobilized laccase in Pd-Laccase@MCF
The activity of laccase in Pd-Laccase@MCF was assayed spec-
trophotometrically with 2,2-azino-bis-3-ethylbenzothiazoline-6-s
ulfonic acid (ABTS) as substrate (5 mM) in 100 mM Na-acetate buf-
fer (pH 5) by measuring absorbance increase at 420 nm at a tem-
perature of 25 °C. Suitable amount of Pd-Laccase@MCF in Na-
acetate buffer (100 mL) was added to the mixture and the initial
rate was immediately measured as increase in optical density at
Scheme 2. Pd-Laccase@MCF catalyzed the aerobic oxidation of alcohols to the
corresponding carbonyl compounds.
4
20 nm [15]. The molar extinction coefficient for the oxidation of
4
ꢀ1
ꢀ1
ABTS at 420 nm is 3.6 ꢁ 10
M
cm . One unit of activity is
defined as the amount of enzyme required to oxidize 1 mmol of
ABTS per minute. Based on this procedure, the amount of immobi-
lized Laccase onto support was obtained 24 U per 0.5 g of solid
support.
2.8. General procedure for the aerobic oxidation of alcohols
A 25 mL round-bottomed flask equipped with a magnetic stirrer
was charged with alcohol (1 mmol), Pd-Laccase@MCF (0.2 g,
.27 mmol Pd), hydroquinone (HQ, 0.27 mmol) and NaPBS/THF
5 mL, 4/1 v/v). The reaction mixture was stirred under the O (bal-
0
(
Scheme 3. Pd-Laccase@MCF catalyzed the aerobic oxidation of cyclohexanol or
cyclohexanone to phenol.
2

3
08
S. Moradi et al. / Journal of Catalysis 382 (2020) 305–319
Scheme 4. Pd-Laccase@MCF catalyzed the dehydrogenation of N-heterocycle compounds.
reaction mixture was stirred under O
appropriate time (Scheme 5). After being finished (monitored by
TLC), the catalyst was separated using filtration and washed with
2
(balloon) at 45 °C for an
particles had discrete spherical morphology in the size range of
8–96 nm. Also, based on this technique the particle size of the
Pd-Laccase@MCF has determined to be 44–83 nm.
CH
2
Cl
2
and the product was extracted with CH
2
Cl
SO
2
(2 ꢁ 5 mL).
The morphology and distribution of Pd nanoparticles onto the
surface of the catalyst was surveyed by TEM analysis. As indicated
in Fig. 2, the Pd-Laccase@MCF catalyst contained many small well-
dispersed spheres with a particle size range of 2–3 nm due to the
presence of Pd nanoparticles.
The organic layer was dried over anhydrous Na
2
4
, filtered and
evaporated under reduced pressure. The crude product was puri-
fied by recrystallization from ethanol or chromatography on silica
gel using n-hexane/ethyl acetate (3:1).
In order to determine the elemental composition of the Pd-
Laccase@MCF nanocomposite, EDX technique was applied. The
EDX spectrum confirmed the presence of Pd, C, N, O, Si and Cu ele-
ments in the synthesized nanocomposite (Fig. 3). The presence of
Cu atom in this analysis verified the successful immobilization of
laccase. Further, the presence of Pd, C, N, O, Si and Cu elements
was approved using the elemental mapping images (Fig. 4). Also,
the inductively coupled plasma-optical emission spectrometry
3
. Results and discussion
3.1. Catalyst preparation
In order to prepare the Pd-Laccase@MCF, firstly palladium
nanoparticles were immobilized in aminopropyl-functionalized
MCF. Then, further functionalization of the aminopropyl groups
that are uncoordinated to Pd was carried out with glutaralde-
hyde. Finally, laccase was covalently linked to the support by
the bond formation between the remaining ACHO group of glu-
(
ICP-OES) analysis was used to determine the Pd and Cu contents
of the synthesized nanocomposite. This analysis indicated that
the weight percentage of Pd and Cu was 14.5 and 0.035%,
respectively.
2
taraldehyde and ANH group situated on the enzyme surface.
Fig.
functionalized MCF (AmP-MCF), glutaraldehyde-functionalized Pd
0)-AmP-MCF (GA-Pd-AmP-MCF) and Pd-Laccase@MCF. As can be
5
displays FT-IR spectra of MCF, aminopropyl-
The synthetic route was depicted in Scheme 6. The Pd-
Laccase@MCF was characterized using FESEM, TEM, EDS, ICP-
OES, FT-IR, TGA, EIS and XPS.
(
ꢀ
1
seen, MCF exhibit two intense bonds at 3465 and 1132 cm cor-
respond to the O-H stretching vibrations from silanol and stretch-
ing vibration Si-O-Si respectively. Also, the bonds at 1654 and
3.2. Catalyst characterization
ꢀ1
8
21 cm probably attributed to adsorbed water molecules and
The SEM characterization images of MCF and Pd-Laccase@MCF
Si-O bending vibrations respectively [16].
are shown in Fig. 1. According to the SEM images, the bare MCF
Scheme 5. Pd-Laccase@MCF catalyzed the dehydrogenation of 2-substute-2,3-dihydroquinazolin-4(1H)-ones.

S. Moradi et al. / Journal of Catalysis 382 (2020) 305–319
309
Scheme 6. Synthetic diagram of the Pd-Laccase@MCF nanocatalyst.
Fig. 2. TEM image of Pd-Laccase@MCF.
Fig. 1. SEM images of (a) MCF and (b) Pd-Laccase@MCF.
Fig. 3. EDX spectrum of Pd-Laccase@MCF.

3
10
S. Moradi et al. / Journal of Catalysis 382 (2020) 305–319
In FT-IR spectrum of AmP-MCF, all the peaks of MCFs can be
seen. Also, the presence of anchored aminopropyl groups was con-
weight loss appeared between 200 and 800 °C corresponds to the
decomposition of the organic functional groups, palladium
nanoparticles and laccase grafted on MCF nanoparticles.
ꢀ1
firmed by the appearance of several peaks at 2960 cm
,
ꢀ
1
ꢀ1
2
881 cm and 1484 cm attributed to –CH
propyl chain [9].
In FT-IR spectrum of GA-Pd-AmP-MCF, the Schiff-base forming
reaction between free ANH group of Pd-AmP-MCF and –CHO
group of glutaraldehyde was confirmed by the C@N stretching
2
groups of the amino-
Electrochemical Impedance Spectroscopy (EIS) can be used as a
powerful technique to reveal the proficiency of biomolecules
immobilization on the solid surfaces [18]. The EIS analysis clearly
demonstrated the existence and the effects of each material on
the electrode resistance. Hence, different steps of biocatalyst
preparation process were evaluated by recording the Nyquist plots
of EIS. As schematically shown in Fig. 7A, the resulted material in
each step of catalyst synthesis was assembled on an electrode
and examined in presence of a redox probe. Fig. 7B shows the
impedance Nyquist plots of GCE before (a) and after assembling
with AmP-MCF (b), Pd-Amp-MCF (c), GA-Pd-AmP-MCF (d) and
Pd-Laccase@MCF (e) in 0.1 M KCl electrolyte and presence of
2
ꢀ1
vibration that appeared at 1658 cm [17].
Based on the above observations, we think that FT-IR is a useful
technique for characterizing MCF, Amp-MCF, GA-Pd-AmP-MCF, but
in the case of FT-IR spectrum of Pd-Laccase@MCF (Fig. 5d), no
indicative peaks for laccase were observed and this spectrum
was quite similar to the FT-IR spectrum of GA-Pd-AmP-MCF
spectrum.
The TGA curves for MCF and different synthesis steps of Pd-
Laccase@MCF (AmP-MCF, Pd-AmP-MCF, GA-Pd-AmP-MCF and
Pd-Laccase@MCF) are shown in Fig. 6. In the all samples, the
weight loss below 200 °C is owing to the removal of the surface
hydroxyl groups or physically adsorbed water, and the other
4
ꢀ/3ꢀ
1 mM Fe(CN)
6
. The corresponding cyclic voltammograms are
also given in Fig. 7C, in which for each of modified electrode a pair
redox peak is seen. However, through increasing the surface resis-
tance, the anodic and cathodic peaks are shifted to the higher over-
potentials. The resulted Nyquist plots were fitted with a Randles
Fig. 4. Elemental mapping of the Pd-Laccase@MCF.
Fig. 5. FT-IR spectra of (a) MCF, (b) AmP-MCF, (c) GA-Pd-AmP-MCF, (d) Pd-Laccase@MCF.

S. Moradi et al. / Journal of Catalysis 382 (2020) 305–319
311
hindrance of species against the electron transfer from probe Fe
4
ꢀ/3ꢀ
(
CN)
from the semicircle of Nyquist plot. As illustrated, the GCE displays
a Nyquist plot with Rct of 1.0 K , where after assembling the
amino-propyl-functionalized-MCF (AmP-MCF) on the surface of
GCE, Rct is increased to 6.2 K . The assembling the Pd-Amp-MCF
on the surface of GCE, Rct is twice increased to 19.2 K , indicating
6
to the electrode surface. Rct can be visually estimated
X
X
X
the increase of electron transferee resistance at the electrode sur-
face. Interestingly, after incubation of electrode surface with
Glutaraldehyde-functionalized Pd-AmP-MCF (GA-Pd-AmP-MCF),
Rct value is decreased from 19.2 KX (in Pd-Amp-MCF) to 10 KX,
which it may be explained as follows: GA linkers can diffuse
between Pd nanoparticles to interact with amino moieties around
of MCF and consequently, lead to the distributed arrangement of
Pd nanoparticles on the MCF core. The well-arrangement of Pd
nanoparticles leads to decrease in electron transfer resistance at
electrode surface. Finally, a significant increase in Rct is seen
through incubation of GCE with Pd-Laccase@MCF, where Rct
Fig. 6. TGA curves of different synthesis steps of Pd-Laccase@MCF.
equivalent circuit model, as shown in the inset of Fig. 7B to obtain
the fitting parameters. The resulted equivalent circuit is involved
reaches to 27.7 KX, indicating the hindrance of electron transfer
kinetics because of presence a nonconductive species such as Lac-
case (see Fig. 7A). These variations in Rct (in EIS) and over-potential
(in voltammetric peaks) noticeably clarify the immobilization of
laccase enzyme on glutaraldehyde-functionalized Pd-AmP-MCF.
The XPS technique is demonstrated to be extremely useful not
only to ascertain the elemental composition of the Pd-
s
four elements; solution resistance (R ), charge transfer resistance
(
R
ct), Warburg element (W , related to the diffusion resistance of
o
double layer at electrolyte/electrode interface) and constant phase
element (CPE) related to the double layer capacitance. Among
them, Rct is a key parameter in EIS since it directly related to the
4
6
ꢀ=3ꢀ
Fig. 7. (A) Schematic of electrochemical tests and electron transfer (ET) of FeðCNÞ
at GCE; (B) Electrochemical impedance spectra of bare GCE before (a); and after
ꢀ
1
assembling of AmP-MCF (b); Pd-Amp-MCF (c); GA-Pd-AmP-MCF (d); Pd-Laccase@MCF (e) on it; C) Cyclic voltammograms with scan rate 0.1 V s corresponding to the EIS
studies which presented in A.

3
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S. Moradi et al. / Journal of Catalysis 382 (2020) 305–319
Laccase@MCF, but also to determine the oxidation state of the Pd.
The XPS of the reduced Pd nanocatalyst (Pd - Laccase@MCF) is
cyclohexane and aerobic oxidative dehydrogenation of N-
heterocycle compounds.
0
given in Fig. 8. In Fig. 8a, silicon, carbon, oxygen, nitrogen, and pal-
ladium peaks are clearly detected in elemental survey scan. As
shown in Fig. 8b, the surface of the nano-Pd particles have both
Pd(0) and Pd(II) atoms.
3
.4. The aerobic oxidation of alcohols to the corresponding carbonyl
compounds
Two strong peaks at 335 and 340 eV, are due to the Pd(0)
absorptions and two peaks at 343 and 338 eV, which are consistent
with Pd(II) [9]. As it was indicated, the major part of the Pd(II) has
been reduced to Pd(0).
The selective oxidation of alcohols to the corresponding car-
bonyl compounds is one of the most fundamental reactions in
the synthetic organic chemistry [19]. However, many oxidizing
reagents have been traditionally employed to accomplish this
transformation, their use is associated with serious drawbacks,
such as use of toxic and/or hazardous reagents, and generation of
large amount of waste. To overcome these drawbacks, catalytic
3.3. Investigation of the catalytic activity of the Pd-Laccase@MCF
nanoparticles
2 2 2
protocols employing O or H O as the terminal oxidant have been
The catalytic activity of Pd-Laccase@MCF was investigated in
the most important oxidation reactions such as aerobic oxidation
of alcohols, the production of phenol from cyclohexanone and
developed [20]. Recently, heterogeneous Pd catalysts have
attracted more attention for the aerobic oxidation of alcohols
[9,21]. Although these procedures have been successfully applied
0
Fig. 8. Survey XPS spectrum of Pd -Laccase@MCF (a) Pd3d (b) Orbitals.

S. Moradi et al. / Journal of Catalysis 382 (2020) 305–319
313
in the field of aerobic oxidation of alcohols, the use of catalysis that
allows processes to occur under mild reaction conditions is inten-
sely encouraged. Therefore, in the present study, we report for the
first time the aerobic oxidation of various alcohols in the presence
of Pd-Laccase@MCF under mild conditions.
(Table 1, entry 15). This result shows that turnover number
(TON) could be increased by raising temperature to 60 °C.
However, we have found that in terms of time and product
yield, the best result was obtained for aerobic oxidation of 1-
phenyl ethanol using MCF@Pd-Lacasse (0.2 g), HQ (0.27 mmol) in
To optimize the reaction conditions, the catalytic activity of Pd-
Laccase@MCF was first surveyed for the oxidation of 1-phenyl
ethanol using molecular oxygen as a terminal oxidant. When the
reaction was performed in the presence of Pd-Laccase@MCF
phosphate buffer (0.1 M, pH 4.5, 4 mL)/THF (4%, 1 mL) under O
at room temperature (Table 1, entry 2).
2
Additional control studies were also performed to demonstrate
palladium nanoparticles and laccase immobilized into the cavities
of MCF (heterogeneous catalyst) and HQ obtain a stepwise electron
transfer from substrate to molecular oxygen and act cooperatively
to facilitate the aerobic oxidation under mild conditions. We exam-
ined the aerobic oxidation of 1-phenyl ethanol with HQ in the
absence of Pd-Laccase@MCF and found that the desired product
was not formed (Table 2, entry 1). With argon instead of air only
10% of the desired product was obtained (Table 2, entry 2). These
results clearly demonstrate that this transformation requires the
triple action of Pd-Laccase@MCF, HQ and air. We also examined
other catalysts such as bare MCF, Pd@MCF, and Laccase@MCF in
the aerobic oxidation of 1-phenyl ethanol (Table 2, entries 3–7)
under same reaction conditions. As expected, MCF failed to cat-
alyze the desired oxidative reaction (Table 2, entry 3). It is worth
mentioning that similar results were obtained using Pd@MCF as
a catalyst in the presence or absence of HQ as a mediator (Table 2,
entries 4–5). Using Laccase@MCF as catalyst led to 10% conversion
into the desired product (Table 2, entry 6). Increasing the temper-
ature to 60 °C in the presence of Laccase@MCF increased the yield
only to 20% (Table 2, entry 7).
With the optimal reaction conditions in hand, we then investi-
gated the scope of this aerobic oxidation protocol for structurally
diverse alcohols (Table 3). As shown in Table 3, various types of
primary benzylic alcohols were selectively oxidized to the corre-
sponding benzaldehyde derivatives without any over-oxidation
to corresponding carboxylic acids (Table 3, entries 1–5). The pre-
sent catalyst system showed a higher reactivity for primary ben-
zylic alcohols with electron-donating (Table 3, entry 2) than for
the electron-withdrawing ones (Table 3, entries 3–5). Also, the pre-
sent catalyst system was applicable for the aerobic oxidation of
substituted secondary benzylic alcohols, to afford the respective
(
0.2 g) in phosphate buffer (0.1 M, pH 4.5, 4 mL)/THF (4%, 1 mL)
at room temperature, the corresponding product was isolated in
5% (Table 1, entry 1). In order to increase the yield of product,
4
the aerobic oxidation of 1-phenyl ethanol was investigated in the
presence of different mediators including hydroquinone (HQ),
3
,5-di-tert-butylcatechol (DTBC) and p-benzoquinone (BQ). It was
found that the yield of the oxidation of 1-phenyl ethanol could
be improved to 99% in the presence of all of them (Table 1, entries
2
–4). Although the reaction time in the presence of DTBC and BQ
were shorter, HQ was applied as an electron transfer mediator
for the subsequent experiments because of its low cost and toxicity
and easily handled. These results indicated that both the heteroge-
neous Pd-Laccase@MCF catalyst and HQ are essential to complete
the aerobic oxidation of 1-phenyl ethanol under mild conditions.
Also, a number of solvents were tested for this reaction (Table1,
3
entries 2 and 5–7). Both mixtures of THF/NaPBS and CH CN/NaPBS
led to high yields of the product, but THF/NaPBS was selected as
the reaction medium due to less toxicity. Afterward, the effect of
the amount of the Pd-Laccase@MCF and mediator was studied
upon the reaction. When the amount of catalyst or HQ was
reduced, the GC yield dropped (Table 1, entries 8–10).
Further experiments devoted to the influence of the tempera-
ture (Table 1, entries 11–15) revealed that when the reaction tem-
perature was increased to 40 and 60 °C, the reaction time was
decreased to 9 and 6 h, respectively (Table 1, entries 11 and 12).
A further increase of the temperature to 80 °C leads to a decrease
in the yield of the product, which could be attributed to the
enzyme deactivation in high temperatures (Table 1, entry 13).
The oxidation of 1-phenyl ethanol at 60 °C tolerates the reduction
of the amount of catalyst and mediator without any loss of yield
Table 1
Optimization of reaction conditions for aerobic oxidation of 1-phenylethanol. [3a,b]
Entry
1
Catalyst (g)
Solvent
Mediator (mmol)
Temperature (°C)
Time (h)
GC Yield (%)
Pd-Laccase@MCF (0.2)
Pd-Laccase@MCF (0.2)
Pd-Laccase@MCF (0.2)
Pd-Laccase@MCF (0.2)
Pd-Laccase@MCF (0.2)
Pd-Laccase@MCF (0.2)
Pd-Laccase@MCF (0.2)
Pd-Laccase@MCF (0.2)
Pd-Laccase@MCF (0.1)
Pd-Laccase@MCF (0.15)
Pd-Laccase@MCF (0.2)
Pd-Laccase@MCF (0.2)
Pd-Laccase@MCF (0.2)
Pd-Laccase@MCF (0.08)
Pd-Laccase@MCF (0.08)
THF/NaPBS
THF/NaPBS
THF/NaPBS
THF/NaPBS
–
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
40
60
80
40
60
10
10
8
45
99
99
99
98
87
68
87
81
90
99
99
38
73
99
a
2
HQ (0.27)
DTBC (0.27)
BQ (0.27)
HQ (0.27)
HQ (0.27)
HQ (0.27)
HQ (0.15)
HQ (0.27)
HQ (0.27)
HQ (0.27)
HQ (0.27)
HQ (0.27)
HQ (0.108)
HQ (0.108)
3
4
5
6
7
8
9
9
CH
THF
CH
3
CN/NaPBS
10
10
10
10
10
10
9
3
CN
THF/NaPBS
THF/NaPBS
THF/NaPBS
THF/NaPBS
THF/NaPBS
THF/NaPBS
THF/NaPBS
THF/NaPBS
1
1
1
1
1
1
0
1
2
3
4
5
6
10
10
10
a
Reaction conditions unless stated otherwise: 1-phenylethanol (1 mmol), phosphate buffer (0.1 M, pH 4.5, 4 mL)/THF (4%, 1 mL), O
2
(balloon), r.t.

3
14
S. Moradi et al. / Journal of Catalysis 382 (2020) 305–319
Table 2
Control studies for aerobic oxidation of 1-phenylethanol.a
Entry
1
Catalyst (g)
Solvent
Mediator (mmol)
Temperature (°C)
Time (h)
GC Yield (%)
–
THF/NaPBS
THF/NaPBS
THF/NaPBS
THF/NaPBS
THF/NaPBS
THF/NaPBS
THF/NaPBS
HQ (0.27)
HQ (0.27)
HQ (0.27)
HQ (0.27)
–
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
60
10
10
10
10
10
10
10
–
10
–
30
30
10
20
b
2
Pd-Laccase@MCF (0.2)
MCF (0.2)
Pd@MCF (0.2)
Pd@MCF (0.2)
Laccase@MCF (0.2)
Laccase@MCF (0.2)
3
c
4
c
5
6
d
HQ (0.27)
HQ (0.27)
d
7
a
b
c
Reaction conditions unless stated otherwise: 1-phenylethanol (1 mmol), phosphate buffer (0.1 M, pH 4.5, 4 mL)/THF (4%, 1 mL), O
Under an atmosphere of argon.
The amount of Pd@MCF (0.2 g, 27 mol% Pd).
2
(balloon).
d
The amount of Laccase@MCF (0.2 g, 40 U laccase).
Table 3
Aerobic oxidation of structurally diverse alcohols.a
Entry
1
Substrate
Product
Time (h)
14
Isolated yield (air) (%)b
86 (78)
2
3
4
5
11
36
23
18
89 (80)
80 (69)
83 (75)
88 (81)
6
10
95 (89)
7
8
9
13
19
83 (77)
84 (71)
24
21
75 (61)
81 (63)
1
1
0
1
20
23
80 (70)
81 (73)
1
2

S. Moradi et al. / Journal of Catalysis 382 (2020) 305–319
315
Table 3 (continued)
Entry
Substrate
Product
Time (h)
24
Isolated yield (air) (%)b
1
3
65 (50)
1
1
4
5
12
16
91 (87)
91 (80)
1
1
6
7
14
30
86 (75)
45 (–)
a
General procedure: Substrate (1 mmol), Pd-Laccase@MCF (0.2 g), HQ (0.27 mmol), O
Isolated yields in an open-air round-bottom flask are shown in parentheses.
2
(balloon), phosphate buffer (0.1 M, pH 4.5, 4 mL)/THF (1 mL), r.t.
b
ketones (Table 3, entries 6–8). Similarly, both primary and sec-
ondary aliphatic alcohols were selectivity oxidized to the respec-
tive carbonyl compounds in good to high yields (Table 3, entries
alyst was centrifuged followed by washing and drying. The recy-
cled catalyst exhibited consistent efficiency in 8 (corresponding
to a total TON = 29) and 12 (corresponding to a total TON = 111)
subsequent reaction runs without any significant change in activity
under optimized conditions and 60 °C, respectively (Fig. 9).
9
–12). Remarkably, we found that our catalyst system is also
highly effective for the oxidation of heterocyclic alcohols and gives
the corresponding products in good to excellent yields with high
selectivity (Table 3, entries 13–15). It is noteworthy that in the case
of 1-phenylethane-1,2-diol containing secondary benzylic alcohol
and primary aliphatic alcohol, a perfect selectivity was observed.
The secondary benzylic alcohol group in this compound was selec-
tively converted into ketone whereas the primary aliphatic alcohol
remained untouched (Table 3, entry 16). We have found that the
reactions in an open-air round-bottom flask give the desired prod-
ucts in medium to good yields in most cases (Table 3).
3.5. The aerobic oxidation of cyclohexanol and cyclohexanone to
phenol
Phenol, an important industrial chemical, is produced mainly
using the three-step cumene process [24]. However, the develop-
ment of a novel alternative approach for producing phenol is
highly desirable. In this perspective, dehydrogenation of cyclohex-
In order to gain further insight into the mechanism concerning
if the reaction is catalyzed by homogenous or heterogeneous palla-
dium, we have carried out heterogeneity tests including leaching
test (hot filtration test) and three-phase test in the presence of
the Pd-Laccase@MCF catalyst. Experimental details and results
and discussion of these tests were provided in the Supplementary
Information. As mentioned in the Supplementary Information (p.
S5), the hot filtration and three phase tests confirmed that there
was no significant leaching of Pd species during the reaction, which
rules out that leached Pd species catalyze the reaction as a homo-
geneous catalyst [22]. Generally, immobilization of enzyme on
support via covalent binding prevents enzyme leaching.
However, to evaluate the possibility of the laccase leaching, the
Pd-Laccase@MCF catalyst was shook with the phosphate buffer/
THF for 2 h. The catalyst was separated by filtration and ABTS
(
500 lL of 1 mM) as substrate was added to detect laccase in the
phosphate buffer/THF solution. UV–Vis analysis indicated that
the absorbance at 420 nm was not increased [23]. This result
shows that no leaching of laccase has taken place from the Pd-
Laccase@MCF into the liquid phase.
For practical purposes, the recovery and reusability of the cata-
lyst is highly desirable. Therefore, to evaluate the long-term stabil-
ity and reusability of Pd-Laccase@MCF, we performed the aerobic
oxidation on a 3 mmol scale of 1-phenyl ethanol to give acetophe-
none in the presence of Pd-Laccase@MCF as catalyst and HQ as
mediator under both optimized conditions (Table 1, entry 2) and
Fig. 9. Recycling runs for aerobic oxidation of 1-phenyl ethanol (3 mmol) using: (a)
Pd-Laccase@MCF (0.6 g), HQ (0.81 mmol) in phosphate buffer (0.1 M, pH 4.5,
12 mL)/THF (4%, 3 mL) at room temperature (b) Pd-Laccase@MCF (0.24 g) and HQ
6
0 °C (Table 1, entry 15). After completion of the reaction, the cat-
(
0.324 mmol) in phosphate buffer (0.1 M, pH 4.5, 12 mL)/THF (4%, 3 mL) at 60 °C.

3
16
S. Moradi et al. / Journal of Catalysis 382 (2020) 305–319
Table 4
The effect of the amount of catalyst on the aerobic oxidation of cyclohexanol.a
HQ (0.34 mmol) in phosphate buffer (0.1 M, pH 4.5, 4 mL)/THF
4%, 1 mL) at room temperature (Table 4, entry 2). Under the same
(
reaction conditions, aerobic oxidative aromatization of cyclohex-
anone was also studied. It was observed that phenol was formed
exclusively in 87% yield (Scheme 7).
3.6. The aerobic oxidation of N-heterocycle compounds
Entry
Pd-Laccase@MCF (g)
Solvent
HQ(mmol)
GC Yield (%)
1
2
3
0.2
0.25
0.3
THF/NaPBS
THF/NaPBS
THF/NaPBS
0.27
0.34
0.41
65
85
87
N-Heterocycles are versatile intermediates in the preparation of
pharmaceuticals, natural products, and synthetic materials. An
effective procedure for synthesizing these compounds is the dehy-
drogenation of the corresponding saturated N-heterocycles [27].
Although many catalytic systems for dehydrogenation of N-
heterocycles have been developed [28], it is highly desirable to
investigate more convenient and environmentally friendly
approaches. Therefore, we sought to accomplish this transforma-
tion with Pd-Laccase@MCF.
a
Reaction conditions: cyclohexanol (1 mmol), Pd-Laccase@MCF (0.25 g), HQ
0.34 mmol), O (balloon), phosphate buffer (0.1 M, pH 4.5, 4 mL)/THF (4%, 1 mL),
1 h, r.t.
(
2
2
Under the optimized conditions for oxidation of alcohols, sev-
eral N-heterocyclic compounds including 2-aryl-1,2,3,4-tetrahydro
quinazoline, 1,4-dihydropyridine and pyrazoline were successfully
dehydrogenated to the respective products with high yields. The
results are summarized in Table 5.
Scheme 7. Aerobic oxidative dehydrogenation of cyclohexanone.
The catalytic activity of Pd-Laccase@MCF was also investigated
for aerobic dehydrogenation of 2- substituted-2,3-dihydroquinazo
lin-4(1H)-ones. The oxidation of 2-phenyl-2,3-dihydroquinazolin-
anone and cyclohexanol has attracted considerable attention
25,26]. Regardless of notable progresses in the dehydrogenation
[
4
(1H)-one was selected as a model reaction. Under optimized reac-
of cyclohexanone and cyclohexanol to phenols, the development
of reusable catalytic systems to accomplish this transformation
with useful yields under mild conditions is of great importance.
It should be mentioned that the oxidation of cyclohexanol under
the optimized conditions for the oxidation of alcohols exclusively
delivered phenol in 65% yield; no cyclohexanone formation was
detected. This result promoted us to investigate the effect of the
amount of catalyst on the reaction yield (Table 4). The best result
was obtained in the presence of Pd-Laccase@MCF (0.25 g) and
tion conditions for oxidative dehydrogenation of other heterocyclic
compounds (Table 5), the yield of the product was 65% (Table 6,
entry 1). This result led us to optimize the reaction conditions
required for this conversion. For this purpose, the effect of temper-
ature on the reaction yield was studied, and it was found that rais-
ing the temperature to 40 °C increasing the yield of the product to
7
7% (Table 6, entry 2). Additional experiments dedicated to the
influence of the amount of the catalyst and HQ obviously showed
that the reaction was accomplished successfully in the presence
Table 5
Aerobic oxidative aromatization of N-hetrocyclic compounds in the presence of Pd-Laccase@MCF.a
Entry
1
Substrate
Product
Time (h)
10
Isolated yield (%)
94
2
3
8
91
89
12
4
5
4
6
96
90
a
2
General procedure: substrate (1 mmol), Pd-Laccase@MCF (0.2 g), HQ (0.27 mmol), O (balloon), phosphate buffer (0.1 M, pH 4.5, 4 mL)/THF (4%, 1 mL), r.t.

S. Moradi et al. / Journal of Catalysis 382 (2020) 305–319
317
Table 6
Optimization of reaction conditions for oxidative dehydrogenation of 2-phenyl-2,3-dihydroquinazolin-4(1H)-one.
a
o
GC yield (%)b
Entry
Catalyst amount (g)
HQ (mmol)
Temperature ( C)
1
2
3
4
Pd-Laccase@MCF (0.2)
Pd-Laccase@MCF (0.2)
Pd-Laccase@MCF (0.25)
Pd-Laccase@MCF (0.25)
0.27
0.27
0.34
0.34
25
40
40
50
65
77
98
98
a
Reaction conditions: Pd-Laccase@MCF (0.25 g), HQ (0.34 mmol), O
The conversion was determined by GC.
2
(balloon), phosphate buffer (0.1 M, pH 4.5, 4 mL)/THF (4%, 1 mL), 25 h.
b
of Pd-Laccase@MCF (0.25 g), HQ (0.34 mmol) in phosphate buffer
one derivatives, carrying the electron-withdrawing groups gave
the corresponding 2-substituted quinazolin-4(3H)-ones in excel-
lent yields (Table 7, entries 8–10).
(0.1 M, pH 4.5, 4 mL)/ THF (4%, 1 mL) at 40 °C with 98% yield
Table 6, entry 3).
(
Under the optimized conditions (Table 6, entry 3), we studied
On the basis of previously reported mechanisms for the oxida-
tion reactions in presence Pd@MCF/BQ system [22,9] and our
reported mechanisms for the application of laccase in the aerobic
oxidation of HQ in tandem reactions [11], it is proposed that Pd
(II) atoms in the Pd-Laccase@MCF/HQ catalytic system are actual
oxidizing agents. As shown in the XPS of Pd-Laccase@MCF, the
dehydrogenation of various 2-substituted ꢀ2,3-dihydroquinazo
lin-4(1H)-ones. The results are summarized in Table 7. It was
observed that the presence of the electron-donating groups
increased the reaction rate, regardless of their positions (Table 7,
entries 2–7). Also, 2-substituted-2,3-dihydroquinazolin-4(1H)-
Table 7
a
Aerobic dehydrogenation of 2-substituted ꢀ2,3-dihydroquinazolin-4(1H)-ones.
Entry
1
Substrate
Product
Time (h)
25
Isolated yield (%)
92
2
3
24
22
95
93
4
5
22
20
94
95
6
24
91
(
continued on next page)

3
18
S. Moradi et al. / Journal of Catalysis 382 (2020) 305–319
Table 7 (continued)
Entry
7
Substrate
Product
Time (h)
26
Isolated yield (%)
93
8
9
30
92
36
34
87
85
1
0
a
2
Reaction conditions: substrate (1 mmol), Pd-Laccase@MCF (0.25 g), HQ (0.34 mmol), O (balloon), phosphate buffer (0.1 M, pH 4.5, 4 mL)/THF (4%, 1 mL), in 40 °C.
nano-Pd particles continue both Pd(0) and Pd(II) atoms. Also, the
active Pd (II) atoms required for the oxidation reaction are gener-
ated through adsorption of BQ to the palladium nanoparticles in
Pd-Laccase@MCF [22]. The BQ is generated from aerobic oxidation
of HQ in presence of laccase catalyst and laccase can be reoxidized
by molecular oxygen, thus completing the catalytic cycle.
Acknowledgements
We gratefully acknowledge financial support of this research by
University of Kurdistan and the Iranian National Science Founda-
tion (INSF, Grant Number: 96016233). We are also thankful to Prof.
T. Hudlicky, Brock University, for editing the paper.
Appendix A. Supplementary material
4
. Conclusions
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2019.12.023.
In conclusion, we have successfully synthesized a heteroge-
neous reusable artificial metalloenzyme by co-immobilization of
palladium nanoparticles and laccase into the cavities of the
mesocellular foams. This hybrid catalyst was applied in aerobic
oxidative dehydrogenation of CAO, CAC and CAN bonds. The
major advantages of these procedures are as follows: (1) this is
the first report of Co-immobilization of laccase and palladium for
use as a robust and highly efficient heterogeneous cooperative
oxidative nanocatalyst system for aerobic oxidation of organic
compounds, (2) this is the first time that a ligand-free and hetero-
geneous palladium- based catalytic system has been used for aer-
obic oxidative dehydrogenation of nitrogen heterocycles, (3)
oxidation of important organic functional groups was performed
using an ideal oxidant with good to high yields at ambient temper-
ature and (4) the immobilization of the oxygen-activating catalyst
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