In situ replacement of Cu-DEN: An approach for preparing a more noble metal nanocatalyst for catalytic use

Article abstract of DOI:10.1039/d0nj04381h

The advantage of dendritic monodisperse macromolecules' dual templating ability was useful in the formation of silica-supported copper nanoparticles Cun@SiO2NPs. This was acquired by the initial synthesis of a silica framework (G4-PAMAM-NH2-SiO2) as a mes

Full text of DOI:10.1039/d0nj04381h

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In situ replacement of Cu-DEN: an approach
for preparing a more noble metal nanocatalyst
for catalytic use†
Cite this: New J. Chem., 2020,
44, 20322
Oluwatayo Racheal Onisuru, Charles O. Oseghale and Reinout Meijboom
*
The advantage of dendritic monodisperse macromolecules’ dual templating ability was useful in the
formation of silica-supported copper nanoparticles Cun@SiO2NPs. This was acquired by the initial
synthesis of
a silica framework (G4-PAMAM-NH2-SiO2) as a mesoporous support using amine-
terminated generation 4 PAMAM dendrimers (G4-PAMAM-NH2). The encapsulated Cun@SiO2 NPs,
calcined at 500 1C, were made to undergo a displacement reaction with Au3+ generated from the
equivalent molar addition of HAuCl4. This resulted in the formation of Aun@SiO2NPs upon in situ
replacement at a regulated pH. The synthesized nanoparticles were characterized and examined. The
catalysts were shown to be catalytically active in the hydrogenation of 4-nitrophenol, and oxidation of
rhodamine B as model reactions, before oxidation of styrene using TBHP. Also, the stability of the
catalyst was evaluated, and the catalytic activity was retained after three consecutive cycles.
Received 1st September 2020,
Accepted 27th October 2020
DOI: 10.1039/d0nj04381h
rsc.li/njc
The synthesis of dendrimer-encapsulated copper nano-
particles (CuDENs) stabilized by generation 4 amine-terminated
1. Introduction
The diverse applications of nanoparticles will continually make PAMAM (G4 PAMAM-NH2) dendrimers was carried out. An effort
them the subject of focus among researchers. This is due to the was also made to immobilize the colloidal CuDENs on a
uniqueness of their excellent performance in the enhancement mesoporous silica support to form heterogeneous catalysts.
of selectivity and productivity. Metal nanoparticles are making Heterogeneous catalysts have the advantage of recoverability
ways for imaginable innovations in the field of catalysis.1 The and separation from the reaction medium compared to homo-
crucial characteristics of large surface area to volume ratio geneous catalysts.3 Silica has been in use as a mesoporous
possessed by metal nanoparticles has an unparalleled implica- support for metal nanoparticles by researchers in the field.13–16
tion on their performance. Generally, under normal conditions, It has been reported that mesoporous silica materials are not
to prevent agglomeration, they are produced in the presence of only excellent in chromatographic, cosmetic, and photographic
stabilizers. This provides control over the particle size formed.2 use, but also for catalytic applications.17 Hence, much interest
Focus had long been on the use of dendrimers as stabilizers for has been shifted to its use. Yanqiu and coworkers18 reported
metal nanoparticles, which has led to extensive research being the fabrication of hollow MCM-41 (Mobil Composition of
carried out over the years on the formation of dendrimer- Matter No. 41) microspheres through the assemblage of tetra-
encapsulated nanoparticles.1–11 Dendrimers are macromole- ethyl orthosilicate (TEOS) coupled with a dual template. The
cules with a three-dimensional structure with a globular shape dual template acquired was prepared by dissolving cetyltri-
that can easily host metal nanoparticles for use in catalysis.1 methylammonium bromide (CTAB) in poly(styrene-methyl
However, the synthetic protocols of some metal nanoparticles methacrylate) microsphere soap-free latex and by consecutive
are not always easily and quickly achieved as anticipated. removal of the polymer core present in tetrahydrofuran.18
This is illustrated by dendrimer-encapsulated platinum nano-
In this study, we utilized the hydrolysis of TEOS together
particles (Pt-DENs), which take at least 72 hours before com- with hydrochloric acid at room temperature for the meso-
plete complexation between the metal ions and dendrimer can porous silica sphere synthesis, which has a different phase
be achieved.12 Hence, a faster alternative route is required.
for the immobilization of CuDENs. This is an advantageous route
for preparing silica-supported gold nanoparticles (Aun@SiO2 NPs),
through an in situ galvanic exchange with gold after an initial
synthesis of Cun@SiO2 NPs. This was followed by the addition
of Au3+ metal atom (Au) to the presynthesized NPs as shown
Research Centre for Synthesis and Catalysis, Department of Chemical Sciences,
University of Johannesburg, P. O. Box 524, Auckland Park, Johannesburg 2006,
South Africa. E-mail: rmeijboom@uj.ac.za; Fax: +27 11 559 2819;
Tel: +27 72 894 0293
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nj04381h in Scheme 1. Au is nobler, having higher potential power
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Scheme 1 Schematic illustration of CuDEN immobilization on silica and subsequent in situ replacement.
electrochemically and capable of simultaneous reduction as the was performed. However, to evaluate the activity of this catalyst in
replacement takes place.19 Such a catalyst is likely to have a oxidation–reduction reactions, reduction of 4-nitrophenol, oxida-
considerable impact on catalysis.
tion of rhodamine B (RhB), and oxidation of styrene were used as
However, one drawback of Cu-based catalyst synthesis is model reactions. 4-Nitrophenol and RhB are two of the prevalent
their non-tolerance of oxygen.20 They are quick to oxidize in the pollutants commonly generated through industrial activities such as
presence of air. Therefore, the synthesis was carried out under textiles, pharmaceuticals, cosmetics, dyeing, food additives, plastics,
inert conditions before taking advantage of Cun@SiO2NPs leather, printing, and other associated industries.26,27 The degrada-
for the replacement reaction with Au for the formation of tion of 4-nitrophenol and RhB was observed at l 400 and 554 nm,
Aun@SiO2NPs. Meanwhile, this replacement was propelled by respectively, with time-resolved UV-vis spectroscopy.28
the smaller standard electrode potential of +0.340 E1 (volts)
possessed by Cu2+, which made the ease of exchange by Au3+ selective oxidation of styrene to styrene oxide, benzaldehyde,
having 1.50 E1 volts, a possibility.
phenylacetaldehyde, acetophenone, and benzoic acid.29–31
Over the years, studies have been repeatedly carried out on
Replacement reactions occur when a metal ion of higher Phenylacetaldehyde and benzaldehyde have their applications
oxidative potential meets another metal atom of smaller stan- in the synthesis of bulk chemicals, agricultural and fine
dard electrode potential. This results in the dissolution of the chemical products, as chemical intermediates in the manu-
surface metal atom by oxidation and consequent reduction of facturing of perfumes, pharmaceuticals, and dyes, and also,
the metal ion with higher oxidative potential.19 Hence, galvanic monitoring of insect control due to their aroma.30,32 Reaction
replacement is the displacement that takes place when there is conditions and catalyst type have been reported to be the
a replacement of one zero-valent metal by another more noble determinant of the product formation.33 Therefore, previous
metal to produce stable, well-dispersed, and compositionally reports, such as reaction temperature,31 solvent type,30
pure metal atom nanoparticles made by a reduced precursor.21 oxidant,34 and catalyst amount30 were selected as conventional
In situ replacement by galvanic exchange has been effectively reaction conditions. Hence with the result obtained, the selec-
utilized to synthesize metal nanoparticles in a homogeneous tivity of our catalysts was towards benzaldehyde, and phenyl-
phase.12,21–23 Herein, we designed a heterogeneous catalyst by acetaldehyde only, as shown in Scheme 2, without unwanted
performing a replacement reaction, because in situ replacement products.
is one of the exemplary methods in the synthesis of NPs.
Through it, researchers can produce different fully reduced
metal NPs with a specific atom at different times by maintaining
2. Experimental section
2.1. Chemicals and materials
the same number of metal precursors.12,24
One of the limiting factors associated with AuNPs is mobility
and sintering at higher temperatures due to thermal instability. Amine-terminated generation 4 PAMAM dendrimers (10 wt 5% in
Thermal stability of catalysts is of crucial importance to some methanol), tetraethyl orthosilicate (TEOS) (99%), [HAuCl4]ꢀ3H2O
industrially generated high-temperature reactions such as (99.9%), 4-nitrophenol (Z99.5%), rhodamine B (Z99.90%),
hydrocracking, partial oxidation, and complete combustion.25 styrene (Z99%), TBHP (70% in water), styrene oxide
Hence, an initial dual templating strategy of metal nanoparticles (Z97%), 2-phenylacetaldehyde (Z98%), benzaldehyde (Z99%),
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Scheme 2 Illustrative representation of the products formed from styrene oxidation.
dichloromethane (DCM) (Z99.8%), acetonitrile (Z99.9%), phos- an inert environment. The pH of the obtained slurry was adjusted
phate buffer (pH B 7.87) with K2HPO4 (98%), hydrogen peroxide to 3.2 from 7.5 using 0.1 M HClO4 before the addition of HAuCl4
(H2O2) 30%, HCl (32%), sodium carbonate (Na2CO3) (90%) and (40 mL, 0.1 M, 4.0 mmol). The addition of HAuCl4 resulted in the
CuSO4ꢀ5H2O were all acquired from Sigma-Aldrich. NaOH color change from a brownish color to a light purple coloration.
(99.9%), KMnO4 (99.4%), and sodium bicarbonate (NaHCO3) However, it was stirred for about 30 min at 25 1C to ensure proper
(99.60%) were obtained from Promack Chemicals. Perchloric and complete replacement. The slurry was then centrifuged for
acid (HClO4) (70.0%), and NaBH4 (Z98.0%) were acquired from 20 min at 3500 rpm, and the separated supernatant was dried
Fluka. All chemicals were used as obtained. All experimental in vacuo at 70 1C to obtain powdered Aun@SiO2NPs.
analyses were performed using 18.2 MO cm Milli-Q water.
2.5. Characterization of the catalyst
2.2. Synthesis of SiO2 spheres
All spectrophotometric analyses obtained for monitoring the
The silica spheres were prepared following a previous report
with slight modification.3 The synthesis began by transferring
generation 4-NH2 dendrimer (10 wt% in MeOH, 0.18 g, 1.28 ꢁ
10ꢂ6 M) into a round-bottom, two necked flask, for the removal
of methanol in vacuo. Phosphate buffer, 60 mM was used to dissolve
the dendrimer and diluted with degassed Milli-Q water to make a
total volume of 8 mL and 0.16 mM. This was completed by a
dropwise addition of silicic acid (5 mL) prepared by hydrolyzing
TEOS in 1 mM HCl. The precipitate formed was centrifuged for
20 min at 3500 rpm. Finally, this was rinsed with degassed Milli-Q
water before overnight drying at 70 1C in vacuo.
absorbance were carried out on a Shimadzu UV-1800 in 3 mL
quartz cuvettes. The internal structure of the images was obtained
by high-resolution transmission electron microscopy (HR-TEM),
which was carried out using a JEOL JEM-2100F electron microscope
of 200 kV voltage accelerator. This was done by suspending the fine
powder of Cun@SiO2NPs and Aun@SiO2NPs in water ultrasonically.
A few drops of Cun@SiO2NPs and Aun@SiO2NPs were placed upon
Ni and Cu grids, respectively, and then set up for image capturing.
Nitrogen sorption analyses of the samples were performed using a
Micromeritics ASAP 2460 instrument. The samples were degassed
at 90 1C under nitrogen gas overnight. These samples were left to
cool under vacuum to make their surfaces and pores available for
probing. The data obtained were computed by the Brunauer–
Emmett–Teller (BET) method to determine the samples’ surface
area, while the pore size distribution was calculated using the
Barrett–Joyner–Halenda (BJH) method. A SDT Q600 thermogravi-
metric analyzer (TA Instrument) was used to determine the thermal
stability of the samples between 0 and 1000 1C at 10 1C minꢂ1
heating rate. A Shimadzu IRAffinity-1 was used to conduct the FTIR
analysis. Powder X-ray diffraction analysis was executed with a
Rigaku SmartLab at the 2y range from 51 to 801 angle with Cu Ka
radiation (l = 1.54056 Å). All pH adjustments were conducted using
an ORION model 520A pH meter with a Schott pH electrode
blueline 25.
2.3. Synthesis of Cun@SiO2NPs with G4-PAMAM-NH2
dendrimer as a stabilizer
In carrying out the synthesis, a procedure reported in the
literature was adapted.3 In summary, aqueous CuSO4 solution
made in a ratio of 16 : 1 of G4-PAMAM-NH2 dendrimer (40 mL,
0.1 M, 4.0 mmol) was pipetted to the preformed dendrimer in
60 mM phosphate buffer solution with consecutive addition of
degassed Milli-Q deionized H2O for a total volume of 8 mL.
G4-PAMAM-NH2/Cu2+ was stirred for 1 hr under an inert atmo-
sphere to obtain thorough complexation at a fixed pH of 7.5. This
was followed by a total reduction of the Cu2+ ions in solution
through 10 molar excess of NaBH4 (400 mL, 0.1 M, 0.5 mmol) for
CuDEN formation. After that, preformed silicic acid was added
slowly to the CuDENs, while stirring continuously under an inert
atmosphere for 15 min. This leads to silica precipitation, which
was collected by centrifuging the precipitate and drying overnight
at 70 1C. Finally, calcination was done for 3 h at 500 1C using a
1 1C minꢂ1 heating rate to remove the template.
2.6. Catalytic reactions
A Shimadzu UV-1800 spectrophotometer was used to monitor
the decrease in absorbance with the aid of quartz cuvettes
(3 mL). The reduction was monitored at room temperature
from l 600 nm to l 200 nm at an intermission of 3 min and
1 min, respectively, for both Cun@SiO2NPs and Aun@SiO2NPs.
Nitrophenol (4-NP; 0.2 mmol) was dissolved in 0.1 M NaOH to
2.4. Preparation of Aun@SiO2NPs by in situ replacement
This was obtained using the stepwise description of Zhao and form a stock solution. A freshly made NaBH4 solution (0.1 M,
Crooks.22 To start with, fine powder of Cun@SiO2 obtained after 0.32 mM) was added to a portion of 4-NP alongside different
calcination in the air was suspended in Milli Q water, stirred aliquots of Cun@SiO2NPs and Aun@SiO2NPs. UV-vis measurement
into slurry formation. This was reduced appropriately due to the was then performed to monitor the time-resolved absorbance at
possibility of oxidation since calcination was not carried out under each degradation step consecutively.
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The oxidation of RhB was also investigated using a UV-vis and alkalinity have a remarkable effect on structural materials
spectrophotometer. An aqueous mixture of RhB (0.1 M) was like silica.36 Hence, the synthesis of the silica spheres under
prepared, and the concentration of the oxidant was determined ambient temperature and at a pH of 7.87 brings about the
through titration using potassium permanganate. The reaction spherical structure of the silica. This permits the ease of
was performed by first varying the amount of Aun@SiO2NPs NP encapsulation. The particle size obtained (283.3 nm) is
within the range of 0.3–0.7 mg, while the amount of substrate comparable to that which has been described under similar
and oxidant remained constant in the presence of a freshly conditions.3
prepared carbonate buffer. The procedure was then repeated by
The TEM images presented in Fig. 1b and c revealed the
varying the amount of substrate and oxidant at different times, internal morphology of both Cun@SiO2NPs and Aun@SiO2NPs
while other parameters also remained constant. Adsorption of prepared by the templating method and displacement method,
the substrate on the catalyst surface was carried out between respectively. The histographical plot obtained for Cun@SiO2NPs
0.012 mM and 0.020 mM, while analysis of the oxidant adsorp- revealed the particle size to be 3.7 ꢃ 0.7 nm, which is an indication
tion on the catalyst was between 24 mM and 32 mM. A stable of narrowly dispersed NPs. The image obtained for Aun@SiO2NPs
buffer (sodium carbonate and sodium hydrogen carbonate) revealed that the spherical structure of the silica framework was
medium of 7.5 pH was ensured throughout the process.35 All still preserved after a replacement had taken place. This also shows
verification was done with 3 mL quartz cuvettes.
the successful encapsulation of the particles with an increase in the
Catalytic oxidation of styrene was performed with the use of size distribution to 10.2 ꢃ 3.7 nm. The notable increase could be as
a carousel TM multi-reactor (Radley Discovery Technologies) a result of calcination temperature at 500 1C for 3 h and decom-
fitted to a reflux condenser. Aun@SiO2NPs (0.1 g), 4.7 mmol position of the organic dendrimer stabilizers that were present
(300 mL) of decane as an internal standard, 2 mmol (214 mL) before calcination. Calcination has been reported to be responsible
styrene, and 3 mmol (415.2 mL) of TBHP were all pipetted into a for a change in particle size25 as a result of gradual growth in the
50 mL vial containing 10 mL acetonitrile (solvent). This was number of atoms.37 But it is interesting to note that this increase in
stirred at 450 rpm/80 1C. Samples for analysis were taken Aun@SiO2NP particle size did not hamper the catalyst effectiveness.
repeatedly at an interval of 1 h, for 6 h. Analyses of the products The EDX analysis shown in Fig. S1 (ESI†) is evidence for the
were performed using a Shimadzu GC-2010 Plus fitted with a successful replacement of Cun@SiO2NPs with Aun@SiO2NPs.
flame ionization detector (FID) and a Restek-800-356-1688
capillary column (30 m ꢁ 0.25 mm ꢁ 0.25 mmol of film
3.2. FTIR measurement of silica sphere before and after
calcination
thickness). The injection port was set at 250 1C, while the FID
temperature remained at 300 1C. The column temperature was
regulated to 300 1C and the carrier gas was nitrogen gas.
To ascertain the bonding between the organic dendrimer and
the silica sphere, the FTIR analysis was carried out within 500–
4000 cmꢂ1 vibration frequency. From Fig. 2, we observed weak
C–N stretching/N–H bending vibrations on the uncalcined
silica sphere. This can be ascribed to amide and amine groups
at n 1641 and 1551 cmꢂ1, respectively. Besides, dendrimer
interaction with the silica sphere was also confirmed.38 This
3. Results and discussion
3.1. TEM Characterization of silica spheres, Cun@SiO2NPs
and Aun@SiO2NPs
The morphological appearance of the silica spheres prepared in observed interaction was only found in the silica sphere before
a 60 mM phosphate buffer solution is presented in Fig. 1a. calcination. After calcination at 500 1C, organic dendrimer
It has been reported that parameters such as pH, temperature, decomposition had taken place. However, the appearance of
Fig. 1 The TEM images and histographical plots of (a) G4-PAMAM-NH2 templated silica sphere (G4-PAMAM-NH2-SiO2) (b) Cun@SiO2NPs and
(c) Aun@SiO2NPs.
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3.4. Brunauer–Emmett–Teller nitrogen sorption analysis
The nitrogen adsorption–desorption isotherm capacity measure-
ment of the surface area and pore structure of the samples that
were taken are shown in Fig. 4a–d. With an exception of Cun@
SiO2 before calcination, all the hysteresis loops appeared in the
range of 0.55–0.82P/P0 with high surface areas. The isotherm
curves of this form of hysteresis loop are particular to the type
IV adsorption isotherm, which is a unique property of uniform
mesopore distribution.18,47 However, Cun@SiO2 before calcina-
tion also possesses a type IV H3 hysteresis loop with no limit to
adsorption even at high P/P0 in between 0.8 and 1.0 including the
desorption closure.48 Moreover, the presence of a small hysteresis
loop is an indication that the material possesses the characteristic
porosity of which its surface area is 221.16 m2 gꢂ1. This surface
area drastically increased after calcination to 744.04 m2 gꢂ1 due to
dendrimer removal at 500 1C.46 Also, an increase in trend from
0.02731 cm3 gꢂ1 to 0.07150 cm3 gꢂ1 was seen in pore volume,
while the surface area of the calcined Cun@SiO2 is higher than the
surface area of the calcined silica sphere. We believe this enlarged
surface area might be caused by the increase in the adsorption
sites on the surface of the calcined Cun@SiO2 after the removal of
the organic dendrimer during the calcination process. The surface
area data, pore-volume, and pore size distribution obtained are
outlined in Table 1.
Fig. 2 FTIR analysis of dendrimer and silica sphere interaction.
bands at n 1068, 790, and 452 cmꢂ1 can be ascribed to the Si–O–Si
stretching band.36 The strong absorption at n 969 cmꢂ1 depicts
Si–OH, which indicates that the silica spheres were free from
moisture.39
3.3. PXRD characterization
The PXRD characteristic patterns obtained for the catalysts are
shown in Fig. 3. The peaks for Cun@SiO2NPs and Aun@SiO2NPs
displayed in Fig. 3a were indexed according to JCPDS number
3.5. Thermogravimetric Analysis (TGA)
(89-2838)40,41 and (04-0784),42 respectively. The diffractogram The thermal stability of the samples measured over time
of Cu at 2y = 34.31 can be ascribed to the 110 phase. The other peaks concerning the temperature change is presented in Fig. 5.
that were also observed at 39.41 and 57.81 were as a result of the The thermal profile obtained for both the silica sphere and
oxidation of CuNPs43–45 as the sample got exposed to air prior to Cun@SiO2 in Fig. 5a and b respectively exhibit three different
analysis. These aligned with the JCPDS number 05-661. Moreover, decomposition stages. The first degradation below 170 1C is
four major characteristic peaks associated with Au at 2y = 38.11, due to the presence of adsorbed moisture, while the subse-
44.41, 64.51, and 77.61 were ascribed to (111), (200), (220), and (311) quent loss at 495 1C can be attributed to the decomposition of
diffraction planes of the face-centered-cube (fcc). These charac- residues of the organic dendrimer.49,50 The final step above
teristic peaks specific to metallic Au have been previously 495 1C is ascribed to decomposition due to silanol condensa-
reported.42 The diffraction peak around 2y = 231 specific to all the tion that formed oxides on the surface of the materials.51–53
measurements in Fig. 3a and b is ascribed to amorphous silica.46
Breaking of siloxane bonds (RSi–O–SiR) can occur in
Fig. 3 PXRD patterns of (a) Cun@SiO2 and Aun@SiO2 and (b) calcined and uncalcined silica spheres.
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Fig. 4 N2 adsorption–desorption isotherms of (a) silica sphere before and after calcination, and (b) Cun@SiO2 before and after calcination, and
Aun@SiO2. Pore distribution patterns of (c) silica sphere before and after calcination, and (d) Cun@SiO2 before and after calcination, and Aun@SiO2.
Table 1 Surface area data and pore structure parameters of silica sphere, Cun@SiO2 before and after calcination, and Aun@SiO2
All samples
BET surface area (m2 gꢂ1
)
BJH pore volume (cm3 gꢂ1
)
BJH pore diameter (nm)
Silica sphere
Calcined silica sphere
Cun@SiO2
Calcined Cun@SiO2
Aun@SiO2
136.69
632.55
221.16
744.04
563.36
0.40829
0.07110
0.02731
0.07150
0.83954
11.945
6.3257
3.6261
3.8441
5.9610
amorphous silica to form soluble silanols (RSi–OH) through Cun@SiO2 and Aun@SiO2 on the reduction of 4-nitrophenol
hydrolysis.53 This is displayed in the FTIR spectra shown earlier. revealed good performance. An aqueous yellow solution of
The Aun@SiO2 appeared to have two decomposition steps 4-nitrophenol displayed a maximum absorption peak at lmax
between 120 1C and 620 1C after which stability occurred, as 400 nm. The addition of the catalyst and protonation of the
shown in Fig. 5c. Based on the thermogram, the first thermal reaction medium by NaBH4 caused a change in coloration from
occurrence is as a result of adsorbed moisture loss and the second yellow to a colorless solution. The simultaneous reduction also
decomposition event occurred due to the emergence of the SiO2 + followed this as new absorbance intensity at l 300 nm emerged.
Si reaction forming volatile 2SiO on the silica framework.54
This is an indication of the reduction of 4-nitrophenol to form
3.5.1. Catalytic evaluation of the synthesized Cun@SiO2 4-aminophenol.25 Fig. 6a and b revealed the time-dependent
and Aun@SiO2 on 4-nitrophenol reduction. The reduction of decline in the UV-vis absorption spectra obtained for the catalytic
4-nitrophenol using NaBH4 is a model reaction for investigating reduction of Cun@SiO2 and Aun@SiO2, respectively. Reaction with
catalytically active metal nanoparticles.55 Catalytic investigation of Aun@SiO2 was completed within 10 min, while it took 30 min for
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Fig. 5 Thermogravimetric analysis of (a) silica spheres, (b) Cun@SiO2, and (c) Aun@SiO2.
reduction to be achieved with Cun@SiO2. Considering the time shown in Fig. 7b. This revealed that an increase in catalytic
taken for the reaction to be completed by the two catalysts, it is performance is directly proportional to an increase in catalyst
evident that Aun@SiO2 is more effective. The non-linear regression amount. Considering the mathematical expression of the
with respect to time presented in Fig. 6c and d aligned with a kinetic pseudo-first-order used in eqn (1),57 A0, and At represent
pseudo-first order reaction. However, the reduction reaction con- an initial concentration of RhB and concentration at time t,
ducted in the absence of both Cun@SiO2 and Aun@SiO2 catalysts respectively. AN is the final concentration of RhB, while kobs
(result not shown) displayed no change in the peak intensity. This denotes the observed rate constant.
confirmed that the reduction was catalytically enhanced.
At ꢂ A
1
ꢂ ln
¼ kobs ꢁ t
(1)
3.5.2. Catalytic investigation on oxidation of RhB. Oxida-
tion of RhB carried out in the presence of H2O2 can also be
utilized as a model reaction to access the catalytic influence of
A0 ꢂ A
1
The catalytic reaction order conformed to
a
pseudo-
the metal nanoparticles.56 With the establishment of Aun@SiO2 first-order reaction. The plot is shown in Fig. S2 (ESI†).
as an effective catalyst in 4-nitrophenol reduction, the UV-vis Also, a carbonate buffer mixture with a pH of 7.5 containing
spectrum of RhB oxidation obtained at an interval of 3 min for Na2CO3 and NaHCO3 added ensured H2O2 stability and
its degradation using Aun@SiO2 is presented in Fig. 7a. The enhanced adsorption all through the reaction.33 This was
catalytic decomposition was observed within 30 min at each because the force due to repulsion between the catalyst
degradation process using different amounts of the catalyst as and the dye was limited.58 The observed rate constant versus
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Fig. 6 UV-vis spectra showing the degradation of 4-nitrophenol using (a) Cun@SiO2 and (b) Aun@SiO2; non-linear regression against time with (c)
Cun@SiO2 and (d) Aun@SiO2.
Fig. 7 (a) UV-vis spectrum of RhB oxidation by using Aun@SiO2, and (b) non-linear fits of data.
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Fig. 8 Concentration variations of (a) RhB and (b) H2O2.
concentration of RhB adsorption on the catalyst surface in (BzA) as the primary products. The obtained result is presented in
Fig. 8a, decreased with an increase in substrate amount due Fig. 9a. A record of progressive reaction and a proportional
to dye saturation on the catalyst surface. This is an indication increase in conversion as time proceeds with maximum styrene
that the reaction rate is a function of catalyst availability. In conversion of 54.3% is obtained. Conversion of styrene is
Fig. 8b, the analysis of H2O2 adsorption shows a concurrent expressed as the percentage converted into a product to the total
increase with respect to the oxidant due to an increase in the percentage initially present in the reaction mixture. The reaction,
supply of oxygen from H2O2, thereby hastening the decomposi- which was found to be selective towards both benzaldehyde and
tion of the substrate.
phenylacetaldehyde produced a higher selectivity of 64.4%
3.5.3. Catalytic evaluation of Aun@SiO2 in the oxidation of towards phenylacetaldehyde and 35.6% for benzaldehyde. More-
styrene to benzaldehyde and phenylacetaldehyde. The catalytic over, the reaction mixture did not display the presence of other
evaluation of the as-synthesized Aun@SiO2 catalyst was also products from the analysis, especially acid formation that results
carried out on the oxidation of styrene in the liquid phase using from co-oxidation of benzaldehyde.59 The selectivity of the
TBHP as an oxidant. At the same time, acetonitrile served as the catalyst is suspected to be enhanced by the surface electronic
solvent at 80 1C. Heterogeneous catalytic oxidation of styrene distribution, surface morphology, and particle size of Au.37,60 It
gave selectivity towards phenylacetaldehyde (PA) and benzaldehyde has been reported that the electronic nature of Au catalyst sites
Fig. 9 (a) Plots revealing conversion and selectivity of Aun@SiO2 at 80 1C: 0.1 g catalyst, 0.59 wt% metal loading, 214 mL (2 mmol) styrene, 415.2 mL
(3 mmol) TBHP, and 6 h and (b) catalyst recyclability.
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Table 2 Oxidation comparison with other previous reports on Au-based catalysts
Selectivity (%)
Catalysts
Au (wt%)
Time (h)
Temp. (1C)
Oxidant
TOF (hꢂ1
)
Conv. (%)
PA
BzA
SO
Others
Ref.
Ti–Au-s
Ti–Au-w
Au/SrO
Au/MgO (HDP)
1Au25/AC
1Au25/PGO
1Au25/SiO2-300
Au/BaTNT
Au/BaTNT
Au25/SiO2
Aun@SiO2
1.3
1.8
0.6
4.1
1.0
1.0
1.0
1.0
0.5
2.0
0.59
15
15
24
24
16
16
16
15
15
24
6
70
70
80
80
80
80
80
80
80
82
80
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
—
—
—
—
—
—
—
41.0
23.0
21.4
44.6
35.3
44.8
46.9
46.8
31.0
15.1
54.3
—
—
26.0
16.8
—
—
—
4.8
3.4
—
61.0
67.0
11.7
10.8
—
—
—
18.9
14.0
6.6
39.0
33.0
44.8
54.3
4.3
—
—
17.4
18.1
—
—
—
5.4
6.8
—
63
63
65
65
64
64
64
66
5.6
14.7
70.9
75.8
93.4
—
182
273
103 ꢃ 10
306
66
25
64.4
35.6
—
This work
Reaction conditions: 0.1 g of catalyst, 0.59 wt% metal loading, 214 mL (2 mmol) of styrene, 415.2 mL (3 mmol) of TBHP, temperature 80 1C, and 6 h;
conversion = (converted styrene)/(initial amount of styrene) ꢁ 100.
has a tuning effect on their catalytic activity and selectivity.61 reaction time was much shorter, and the Au loading amount
Hence the structure of Aun@SiO2 permits a rearrangement was lower than every other catalyst. Although there was no
of hydrogen through ring-opening, thereby giving rise to selectivity towards styrene oxide (SO) using Aun@SiO2, which
phenylacetaldehyde,30 while benzaldehyde formation occurred was seen in every other catalyst, an outstanding feature of this
through oxidative cleavage.62
catalyst is its selectivity towards phenylacetaldehyde and
An important parameter of heterogeneity in catalysis is benzaldehyde. We propose this may have originated from the
the ease of recoverability and recyclability from the reaction encapsulation of Au in the silica sphere after direct replace-
medium. This economic attribute makes them preferable over ment and change in particle size, pore size, and surface area.
homogeneous catalysts. Aun@SiO2 was evaluated for its recycl-
ability after recovering it from the reaction medium as shown
in Fig. 9b. The catalyst displayed no severe deterioration of
4. Conclusion
activity, and it is shown to be continuously selective towards
The systematic protocol of the CuDEN templating approach
both benzaldehyde and phenylacetaldehyde with stable higher
enabled the encapsulation of small-sized CuNPs into the silica
selectivity to phenylacetaldehyde. In respect of the Au loading
sphere as Cun@SiO2. The in situ replacement by galvanic
amount and the possibility of catalyst leaching into the reaction
exchange controlled by the difference in the electrode potential
solution, inductively coupled plasma-optical emission spectro-
was successfully achieved. Both the preliminary catalytic
scopy (ICP-OES) measurements revealed the Au loading
activity of Cun@SiO2 and Aun@SiO2 on the reduction of
amount to be 0.59 wt%. It also showed that an insignificant
4-nitrophenol using NaBH4 showed a distinctive pattern of a
amount of 0.03 mg Lꢂ1 of Au was present in the solution after
pseudo-first-order reaction. Aun@SiO2 possessed an improved
the reaction. These reports, including the turnover frequency
catalytic performance and also revealed an efficient activity on
(TOF) of 306 hꢂ1 are all indications that the Au NPs are well
oxidation of RhB using H2O2. This also aligned to a pseudo-
first-order reaction. An increase in the concentration of RhB
brought about a decrease in kobs with a subsequent increase in
kobs as peroxide concentration increased.
immobilized and encapsulated on the silica sphere.
The effect of air involvement on the mechanistic trans-
formation of styrene to phenylacetaldehyde and benzaldehyde
was considered since the reaction was performed in an open-air
Furthermore, concerning the reaction conditions, the
condition. The reaction carried out in the absence of TBHP
catalyst also efficiently catalyzed the styrene oxidation with
oxidant showed that less than 0.5% conversion was recorded
54.3% conversion. This gives 35.6% and 64.4% selectivity
(graph not shown). From this result, it is evident that there was
towards benzaldehyde and phenylacetaldehyde, respectively.
just a little and insignificant effect of oxygen on the reaction for
The yield is both selective and quantitative, while the stability
the observed period of 6 h. However, when the reaction was
of the catalyst was also retained after three consecutive cycles.
verified with only the aqueous TBHP oxidant, we found only
8.2% conversion (Fig. S3, ESI†). This confirmed TBHP as the
principal oxidant,63 which provided a substantial oxidative Conflicts of interest
environment for the reaction.64
The authors declare no competing financial interest.
A comparison of the catalytic activity of Aun@SiO2 on styrene
oxidation was made with previously reported gold catalysts.
As shown in Table 2, confirmation of this catalyst being in fair
comparison in terms of TOF and conversion with other pre-
Acknowledgements
viously reported gold-based catalysts can be made because it We acknowledge the support of the National Research Founda-
has the highest TOF value and conversion percentage (%). The tion of South Africa {Grant specific unique reference number
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