Self-supported nickel nanoparticles on germanophosphate glasses: Synthesis and applications in catalysis

Article abstract of DOI:10.1039/c9ra02927c

The development of supported catalysts based on simple procedures without waste products and time-consuming steps is highly desirable. In this paper, self-supported nickel-based nanoparticles were obtained at the surface of the germanophosphate glasses by

Full text of DOI:10.1039/c9ra02927c

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Self-supported nickel nanoparticles on
germanophosphate glasses: synthesis and
applications in catalysis
Cite this: RSC Adv., 2019, 9, 17157
ab
c
Guilherme Felipe Lenz,
Rodrigo Schneider,
Kelen M. F. Rossi de Aguiar,b
Rafael A. Bini,b Juliano Alexandre Chaker,d Peter Hammer,
e
f
g
b
*
Giancarlo V. Botteselle,
Jorlandio F. Felix
and Ricardo Schneider
The development of supported catalysts based on simple procedures without waste products and time-
consuming steps is highly desirable. In this paper, self-supported nickel-based nanoparticles were
obtained at the surface of the germanophosphate glasses by bottom-up process and evaluated as
potential catalysts for the benzyl alcohol oxidation and bis(indolyl)methanes synthesis. A classical melt-
quenching technique was used for preparing the nickel-doped germanophosphate glasses, followed by
annealing under a hydrogen atmosphere at 400 ꢀC for two different times. The approach enabled the
synthesis of self-supported nanoparticles as a homogeneous film, covering the glass surface. The
physical and chemical properties of synthesized glasses were characterized by UV-vis and Raman
spectroscopies and thermal analysis. Scanning electron microscopy (SEM) and X-ray photoelectron
spectroscopy (XPS) were performed to monitor the growth process, morphology and chemical bonding
structure of the nanoparticles surface.
Received 18th April 2019
Accepted 26th May 2019
DOI: 10.1039/c9ra02927c
rsc.li/rsc-advances
liquid-phase methods are also employed, chemical reactions in
solvents lead to the formation of colloidal solutions. Other
Introduction
methods involve solvent-based colloidal or liquid-phase
synthesis, molecular self-assembly and mechanical processes
of size reduction such as grinning, milling, and alloying.3,4
Therefore, the development of a simple and efficient
approach for the synthesis of immobilized nanoparticles is
desirable. In this sense, over the last few years, our research
group has developed a non-conventional route for the growth of
copper and silver nanoparticles on borophosphate, oxide and
oxyuoride glasses. We have also evaluated their performance
in catalysis, Surface-enhanced Raman Spectroscopy (SERS), and
antibacterial applications.5–9
Phosphate-based glasses can be obtained at relatively low
temperature by melt-quenching technique using affordable
chemical reagents (e.g., KH2PO4, P2O5, NaPO3 or NaH2PO4). In
addition, this type of glass becomes even more interesting
because it can be easily doped with transition metal ions, alkali,
and rare earth oxides to provide the desired physical and/or
chemical characteristics.10 Moreover, phosphate glasses can
act as a host material for nanoparticle synthesis enabling the
growth of nanoparticles onto glass surface when annealed
under a reductive atmosphere (e.g., hydrogen gas). In general,
silver, gold, and copper are the transition metals more reported
which can be precipitated inside glasses by chemical reactions,
annealing or radiation exposure.11–13 So et al.14 reported the
synthesis of PbS quantum dots in glasses containing silver
Catalysis presents a signicant role in industrial processes.
Catalytic materials/nanomaterials show applications in chem-
ical manufacturing, energy, biological and environmental
elds.1,2 The outstanding properties of nanocatalysts are asso-
ciated with high catalytic activity and easy separation. Nano-
particles (NPs) from noble metals are extensively studied for
organic transformations and applied in pharmaceutical
process, for example.3
Nanoparticles can be synthesized by a variety of conventional
methods using gas-, liquid-, or solid-phase processes. These
methods include some undesirable variables such as gas-phase
reactions of ame pyrolysis, high-temperature evaporation,
plasma, microwave irradiation, etc.4 In addition, colloidal or
a
´
Universidade Federal do Parana - UFPR, Departamento de Engenharias e Exatas,
Palotina, PR, 85950-000, Brazil
bUniversidade Tecnologica Federal Parana
UTFPR, Group of Polymers and
´
´
-
Nanostructures, Toledo, PR, 85902-490, Brazil. Tel: +55 45 33796850
c
˜
´
˜
Universidade Federal de Sao Carlos, Departamento de Quımica, Sao Carlos, SP,
13565-905, Brazil
d
´
´
´
Universidade de Brasılia - UNB, Instituto de Quımica, Brasılia, DF, 70904-970, Brazil
e
˜
´
Universidade do Estado de Sao Paulo - UNESP, Instituto de Quımica, Araraquara, SP,
14800-060, Brazil
f
´
´
Universidade Estadual do Oeste do Parana - UNIOESTE, Departamento de Quımica,
Toledo, PR, 85903-000, Brazil
g
´
´
´
´
Universidade de Brasılia - UNB, Instituto de Fısica, Nucleo de Fısica Aplicada,
`
nanoparticles using laser illumination, while Estournes
´
Brasılia, DF, 70910-900, Brazil. E-mail: jorlandio@unb.br
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et al.15,16 reported the precipitation of nickel nanoparticles in 2p, and O 1s were subtracted using the Shirley method.18,19 The
soda-lime-silicate glass, when annealing under hydrogen composition (at%) of the surface layer (<5 nm) was determined
atmosphere at 600 ꢀC. Their results showed that the annealing by the relative proportions of peak areas corrected for Scoeld's
process produced nickel nanoparticles embedded in silica- atomic sensitivity factors to an accuracy of ꢃ5%.20 The binding
based glass. In terms of catalytic applications, the nano- energy calibration was performed by referring the C 1s
particles should be accessible for the reactions and, therefore, component of aliphatic carbon species to 285.0 eV. The spectra
embedded nanoparticles (i.e. surrounded by support) are were deconvoluted using a Voigtian type function, with
undesirable. Nickel-based catalysts are applied for heteroge- combinations of 70% Gaussian and 30% Lorentzian. The width
neous catalysis with practical applications in the industry (e.g., at half maximum varied between 1.1 and 2.0 eV, and the posi-
methanation reaction, oxidation of carbon monoxide, tion of the peaks was determined with an accuracy of ꢃ0.1 eV.
hydrogen-transfer reductions).17
The Raman spectra were recorded using a micro-Raman
In this work, we report the synthesis and application of Renishaw InVia, laser power 8 mW, 633 nm excitation wave-
germanophosphate glass, which acts as a host and a support length and CCD detector. All samples were measured without
material for the growth of nickel nanostructures onto its surface any additional treatment. The deconvolution analysis of Raman
(not embedded in glass matrix). The self-supported nickel- spectra was obtained with Voigt functions using Fityk program
based nanoparticles were obtained at relatively low tempera- (version 1.3.1). The UV-vis absorption spectra of glasses
ture and short annealing time in form of a homogeneous layer (z0.5 mm thick foils) were recorded in the 200–1000 nm range
on the glass surface. Then, the supported nanoparticles were using a T80+ spectrometer from PG instruments with 1 nm of
applied as catalyst in benzyl alcohol oxidation by sodium step and air as baseline.
hypochlorite and in Friedel–Cras alkylation reaction of indole
and benzaldehyde. The structural characterization was per-
formed using a combination of techniques such as differential
Catalytic measurements for benzyl alcohol oxidation
ꢀ
thermal analysis, UV-vis spectroscopy, X-ray photoemission Nickel-doped germanophosphate glass was annealed at 400 C
spectroscopy, scanning electron microscopy and Raman during 60 minutes under H2 atmosphere (100 mL minꢁ1) and
spectroscopy.
tested as catalyst for benzyl alcohol (BnOH) oxidation by
sodium hypochlorite (NaOCl) solution. The sample annealed
during 60 minutes was chosen for the catalytic tests due to its
larger number of nickel nanoparticles at the glass surface, as
will be shown later in the SEM results (Fig. 4). The sample ob-
tained with annealing time of 60 minutes was chosen for the
catalytic tests due to its larger number of nickel nanoparticles at
the glass surface, as will be shown later from SEM results.
The catalytic evaluation was initially performed in aqueous
media using NaOCl 5 wt% as oxidant solvent at pH 10 (10 mL,
6.7 mmol). 100 mg of the catalyst was rst dispersed in the
NaOCl solution, followed by the addition of 1.50 mmol of
Experimental
Glass synthesis and Ni-based nanoparticles growth
Germanophosphate glasses (NaH2PO4–GeO2–Al2O3) were ob-
tained by melting-quenching technique using high purity
reagents (Aldrich Co.) with NaH2PO4/GeO2 ¼ 2 and 3 mol% of
Al2O3. The nickel-doped glass was prepared with addition of
1.5 mol% Ni2O3 (3 mol% Ni2+ ions). A typical synthesis was
performed by homogenizing 2 g of the aforementioned
compounds in an Agate mortar. The mixture was transferred to
ꢀ
ꢀ
BnOH. The reaction was performed at 20.0 C ꢃ 0.5 C during
1 h. Posteriorly, the reaction condition was adapted by using
acetonitrile (ACN) as solvent (10 mL) and sodium hypochlorite
(NaOCl) 10–12 wt% applied only as oxidant (3 mL, 4.8 mmol).
100 mg of the catalyst was rst dispersed in ACN, followed by
the addition of 0.75 mmol of BnOH. Oxidant was added in
portions of 1 mL at the beginning and within the rst and
second hour of reaction. The reaction was performed at 20.0 ꢀC
ꢀ
a covered Pt/Au crucible and heated at 950 C for 1 h, in a pre-
heated resistive oven. Aerwards, the molten sample was
quenched at room temperature in a graphite mold. Finally, the
glasses were crushed in an agate mortar and sieved on a 325-
mesh sieve. Nickel-based nanoparticles were obtained by
annealing the nickel-doped glasses at 400 ꢀC for 30 and 60
minutes under hydrogen (H2(g)) ow of 100 mL minꢁ1
.
ꢀ
ꢃ 0.5 C during 8 h.
Glass and nanoparticles characterization
The unconverted benzyl alcohol (BnOH) and the products
The characteristic glass temperatures of the samples were benzaldehyde (BnCHO), benzoic acid (BnCOOH) and benzyl
determined by Differential Thermal Analysis (DTA-50, Shi- benzoate (BnBz) were determined using high performance
madzu) using the powdered glass (325 mesh) in a Pt pan with liquid chromatography (HPLC) in a Thermo Scientic Ultimate
nitrogen gas ow (50 mL minꢁ1
)
and heating rate of 3000 equipment. The separation was performed at 30 ꢀC using
octadecylsilane C18 column (Ace ltd.) in gradient elution at
ꢁ1
ꢀ
10 C min
.
The local bonding structure of the samples was evaluated by a ow rate 1 mL minꢁ1, with mobile phase composed by the
X-ray photoelectron spectroscopy (XPS) using a commercial mixture of acidied water (0.01% v/v phosphoric acid, pH 2.75
spectrometer UNI-SPECS UHV, at a base pressure of 5 ꢂ 10ꢁ7 ꢃ 0.05) and acetonitrile (ACN, J.T. Baker HPLC grade): initially
Pa. Al Ka radiation source was used (hn ¼ 1486.6 eV) and the 30% ACN was increase to 60% aer 10 min., keeping this
pass energy of the analyzer was adjusted to 10 eV. The inelastic condition until 25 minutes. The detection was made by a diode-
noise of the high resolution spectra of Ni 2p3/2, Ge 3p, Na 1s, P array detector at 210 nm. Aliquots of the reaction medium were
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collected at the beginning, during, and at the end of the reac-
tion, and then diluted 10 times with mobile phase and ltered
in a 0.22 mm hydrophilic PVDF syringe lter. Analytical stan-
dards (Sigma Aldrich, Supelco) were used as reference for
sample concentration determination. To verify the proportion
of compounds (mmols) in the formed phases, the organic
reaction media was centrifuged (3400 rpm, 6 minutes) and
a fraction of both organic and aqueous phases was collected for
analysis.
Synthesis of bis(indolyl)methane (3)
For synthesis of bis(indolyl) methane the Ni-based nano-
particles (25 mg), indole (0.50 mmol, 58.5 mg) and benzalde-
hyde (0.25 mmol, 26.5 mg) were added into a tube test (5 mL).
The tube was then immersed in an oil-bath at 60 ꢀC and stirred
during 1.5 h. The organic compounds were then directly
extracted with ethyl acetate (3 ꢂ 3 mL), dried over Na2SO4 and
concentrated under vacuum. The crude product was puried by
ash column chromatography on silica gel using a mixture of
hexane and ethyl acetate (85 : 15) as the eluent. The identica-
tion of the product was conrmed by 1H and 13C nuclear
magnetic resonance (NMR) according to the following data.
3,30-(Phenylmethylene)bis(1H-indole) (3) (Section Glass catalyst
performance): 1H NMR (CDCl3, 200 MHz): d ¼ 7.67 (br, 2H);
7.37–6.93 (m, 13H); 6.51 (s, 2H); 5.84 (s, 1H); 13C NMR (CDCl3, 50
MHz): d ¼ 144.3, 137.0, 129.0, 128.5, 127.4, 126.3, 123.8, 122.1,
120.1, 119.9, 119.4, 111.2, 40.3.
Fig. 1 Differential thermal analysis (DTA) of glasses without annealing
(a) 3 mol% Ni-doped, and (b) undoped sample.
peaks at 582 cmꢁ1 and 757 cmꢁ1 (the dashed lines (Fig. 3)) are
attributed to d(Ge–O–P) bending vibrations and n(Ge–O–P) and/
or ns(P–O–P) (Q1 units) stretching frequencies, respectively.26–29
In turn, Kamitsos et al.30 ascribed the peak/shoulder at
z628 cmꢁ1 to the formation of connected GeO6 octahedral
units. Alternatively, Kumar et al.27 ascribed the shoulder at
z643 cmꢁ1 to Ge6–O–P bending modes in Li2O–GeO2–P2O5 (P/
Ge ¼ 1.6) glasses. The region above 810 cmꢁ1 can be deconvo-
luted with a series of four Voigts line shapes. The broad band is
composed of peaks at z930, 1075, 1135 and 1230 cmꢁ1. The
peaks at 1135 cmꢁ1 and 1230 cmꢁ1 are assigned to symmetric
ns(PO2) (Q2), and to the asymmetric nas(PO2) (Q2) stretchings.31
The peak at 1075 cmꢁ1 is ascribed to nas(PO3) in pyrophosphates
(Q1).26,29,32 At last, the shoulder at z930 cmꢁ1 is associated with
nas(PO43ꢁ) (Q0) isolated orthophosphate units and to asym-
metric stretching of nas(P–O–P).32–34
Results and discussion
Glass characterization
Fig. 1 shows the DTA analysis of the NaH2PO4–GeO2–Al2O3
germanophosphate glasses. The glass transition temperature
(Tg) was noticed at z550 ꢀC for the undoped sample (Fig. 1(b))
and 558 ꢀC for Ni-doped sample (Fig. 1(a)). The liquidus
temperature (Tl) of the doped glass was noticed at z942 ꢀC,
while for the undoped glass the Tl was not observed within the
analysis range. The addition of nickel ions increased the Tg
indicating that the metallic ions inside the glass matrix act as
a network intermediate.21,22
The main advantage of the germanophosphate glass is the
ability of nickel-based nanoparticles to grow on its surface when
annealed at relatively low temperature. Basically, the annealing
The UV-vis absorption spectra for undoped and Ni-doped
samples are shown in the Fig. 2. The undoped glass was
colorless, (Fig. 2(b)) transmitting the radiation above z340 nm,
while the Ni-doped germanophosphate glass shows an amber-
colored aspect, evidencing the doping process. The band
observed at 436 nm (Fig. 2(a)) was assigned to a spin-allowed
3
transition A2g(F)
coordination.23,24
/
3T1g(P) of Ni2+ ion in octahedral
Fig. 3 shows the results of the Raman analysis of the
synthesized germanophosphate glass samples. The Raman
spectrum is mainly composed of two complex broad bands
located below and above of z810 cmꢁ1. The band located below
810 cmꢁ1 is an overlapping of phosphate- and germanium-
based groups. The band at z350 cmꢁ1 is attributed only to
O–P–O bending.25,26 The band at z523 cmꢁ1 is assigned to
Fig. 2 UV-vis absorption spectra for bulk (foils with z0.5 mm of
thickness) 3 mol% Al2O3 germanophosphate glasses without annealing
symmetric stretching (ns) of three-membered GeO4 rings.26 The (a) 3 mol% Ni-doped, and (b) undoped.
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be carried out in lead-germanate glasses under a nitrogen
atmosphere.8 On the other hand, the negative point of lead-
germanate glasses is that they have heavy-metal in their
matrix. Fig. 4(b) and (c) show the effect of the annealing time in
the nanoparticle growth. For an isothermal process, the amount
of nanoparticles on the glass surface increases with the
annealing time. Moreover, the nanostructures can be observed
covering homogeneously the glass surface (Fig. 4(d)).
To win an insight into the nanoparticle growth onto the glass
surface, XPS measurements have been performed. The attribu-
tion of the tted component and the corresponding tting
parameters obtained from XPS results are summarized in Table
1. Annealing in air was also performed to evince the benecial
effect of hydrogen for nanoparticle growth. The tting of the
spectrum of Fig. 5(a) and (b), was performed using four
components. Only a small contribution (<4.5%) of the NiO
phase was found at 854.6 eV, while the predominant Ni2O3
phase at 586.5 eV contributes with about 60%. Moreover, there
are two shake up satellite relative to NiO (Ni II) at about 560 eV,
and Ni2O3 (Ni III) at 563 eV.
Fig. 3 Raman spectrum for germanophosphate 3 mol% Al2O3 ger-
manophosphate glasses without annealing (a) 3 mol% Ni-doped, and
(b) undoped.
The Ni 2p3/2 spectrum of Fig. 5(c) shows clearly that the
sample annealing under H2 favors the reduction of nickel
species when compared to the reference sample (Fig. 5(a)) and
the sample annealed under air (Fig. 5(b)). Besides the formation
of approximately 10% of metallic nickel, the reductive H2
atmosphere results in a strong increase of the amount of NiO by
up to 50%, and a reduction of Ni2O3 species (Table 1 and
Fig. 5(c)). Furthermore, the small shi of the latter component
from 856.6 eV to 856.3 eV indicates at this binding energy the
presence of Ni(II) species in form of Ni(OH)2, as revealed by the
weak intensity of the Ni(III) shake-up and the analysis of the O 1s
spectra (not shown).
The results of the tted O 1s, shown in Table 1 conrmed the
presence of the NiO phase at 529.4 eV, PO4, Ni2O3 and Ni(OH)2 at
531.5 eV as well as O–Ge and O–C bonds at 532.6 eV.35 The O–C
and carboxyl groups (O–C]O), identied at 533.7 eV, can be
related to a weak contamination of the sample surface by adven-
titious carbon (hydrocarbons). Moreover, the P 2p3/2/2p1/2 spin–
orbit spectra of phosphorus conrmed the presence of the PO4
phase at 133.6 eV, and the position of the Na 2s spectra of the
sodium at 63.6 eV is indicative for the NaH2PO4 phase (not shown).
Fig. 4 SEM images of powder Ni-doped germanophosphate glasses
after different annealing times at 400 ꢀC under H2 atmosphere of (100
mL minꢁ1) (a) unannealed (scale bar ¼ 1 mm), (b) 30 min (scale bar ¼ 1
mm), (c) 60 min (scale bar ¼ 1 mm) and (d) large area – 60 min (scale bar
¼ 4 mm).
Glass catalyst performance: benzyl alcohol oxidation
under hydrogen atmosphere enables the reduction of the nickel
ions, thus inducing the growth of nickel nanoparticles at the
glass surface, without additional steps. The mobility of nickel
ions in the glass matrix increases with temperature rise, which
favors their migration from the bulk to the glass surface for
sequential reaction with the hydrogen atoms.
Fig. 4 shows the SEM images of nanostructures obtained on
the glass surface aer annealing. As expected, for the sample
unannealed, we did not notice the nanoparticle (Fig. 4(a)) since
the bottom-up process is thermally activated. It is worth
mentioning that the nickel-based nanostructures growth
requires a reductive atmosphere (i.e. H2(g)). In contrast to ger-
manophosphate matrix, the growth of silver nanostructures can
Fig. 6 shows benzyl alcohol (BnOH) conversion and product
yields, according to the reaction time, in aqueous NaOCl 5% pH
10, using as catalyst nickel-based nanoparticles obtained aer
60 minutes of annealing: BnOH conversion achieved 55.7 mol%
in 30 minutes, maintaining it constant until 60 minutes.
Benzaldehyde (BnCHO) was the major product of reaction –
29 mol% at 60 minutes, while benzoic acid yield reached
22 mol%. The benzyl benzoate, z5 mol%, was noticed as
product of esterication between benzyl alcohol and benzoic
acid. The catalytic activity and effectiveness of the nickel-based
nanoparticles were evinced testing the reaction without the
catalyst, where only 2.6 mol% of conversion was achieved aer
60 min reaction.
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Table 1 XPS fitting parameters for nickel-doped glass samples
Referencea
400 ꢀC, airb
Position
400 ꢀC, H2
Position
c
Intensity
(%)
Intensity
(%)
Intensity
(%)
Component
Position
Ni 2p3/2
Ni0
NiO
Ni2O3
Ni(III) Sat.1
Ni(III) Sat.2
—
—
—
—
853.4
854.3
856.3
860.1
863.0
9.8
49.1
29.2
8.2
854.6
856.5
859.7
862.9
4.48
56.39
12.99
26.14
854.6
856.6
860.0
862.9
3.90
62.40
17.70
16.00
3.3
O 1s
NiO
529.4
531.5
532.7
533.8
1.9
529.3
531.5
532.6
533.7
1.8
529.4
531.4
532.6
533.7
1.4
59.2
19.8
3.8
PO4, Ni2O3, Ni(OH)3
GeO2, O–C
O–C]O
60.3
20.4
4.8
56.5
23.5
4.6
a
Fig. 5(a). b Fig. 5(b). c Fig. 5(c).
Sodium and calcium hypochlorite are known as strong
Many researchers have tried to nd reactions conditions and
oxidants, being able to convert primary alcohols to its respective catalysts that favor benzaldehyde production, mainly due to its
esters, secondary alcohols to ketones and aldehydes to acids/ extensive industrial application.40–42 Therefore, this study
esters.36–38 Nevertheless, commercial NaOCl solution (bleach) focuses on the test of glass-based catalyst and reaction condi-
was used with a nickel(II) salt to oxidize benzyl alcohol to ben- tions that provide selectivity to benzaldehyde. Fig. 7 shows
zoic acid, as reported by Grill and co-workers.39 The authors BnOH conversion when acetonitrile (ACN) was used as solvent
reported benzaldehyde as an intermediary of the reaction, easily and NaOCl 10–12% as oxidant, with the reaction being con-
oxidized to acid. However, applying our reaction conditions, ducted at 20 ꢀC under agitation. The BnOH conversion achieved
benzaldehyde was the major product instead of benzoic acid, 72 mol% aer 8 hours of reaction when the catalyst was grown
achieving 51% of selectivity to the aldehyde.
during 60 minutes, while only 5.7 mol% of conversion was ob-
tained without the catalyst.
The BnOH oxidation, carried out in aqueous medium, lead
to the formation of three products (mixture): benzaldehyde,
benzoic acid and benzyl benzoate. Water favours the oxidation
of BnCHO due to the hydration, generating BnCOOH,43 and the
basic pH (10.0) allows the esterication (i.e. generating BnBz).
Nevertheless, the reactions carried out using ACN as solvent
conducted to the formation of benzaldehyde as major product
(selectivity > 99%). Water and ACN are miscible in any
Fig. 6 Benzyl alcohol conversion at 20 ꢀC under agitation, and ob-
Fig. 5 Fitted high resolution XPS Ni 2p3/2 spectra (a) reference tained products in aqueous NaOCl 5% pH 10 using self-supported
(unannealed), (b) annealed under air, (c) annealed under H2.
nickel-based nanoparticles as catalyst.
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Fig. 7 Benzyl alcohol conversion at 20 ꢀC using acetonitrile as solvent,
NaOCl as oxidant (4.8 mmol) and self-supported nickel-based nano-
particles as catalyst.
proportion when pure,44 whereas a biphasic system is formed
when salts45 or monosaccharides/disaccharides46,47 are added in
a water/ACN mixture.
During the reactions, the nickel-based catalyst remains
dispersed in the lower oxidant aqueous phase (Fig. 8(a)). BnOH
conversion and BnCHO formation along the time was deter-
mined collecting a fraction of the superior organic phase. Table
2 summarizes the distribution of compounds (mmols) in both
phases at the end of the reaction.
According to Table 2, BnOH and BnCHO are predominantly
present in organic phase in both reactions and only a small
percentage of these products is present in aqueous phase, with
a maximum value of 0.8% and 0.3% for BnOH and BnCHO,
respectively. On the other hand, BnCOOH was only determined
in the aqueous phase and in low concentration (4.2 ꢂ 10ꢁ3
mmol), which allowed us to infer that this compound is
a byproduct. Thus, we have proposed a scheme for BnOH
oxidation by NaOCl using the nickel-based catalyst (Fig. 8).
We propose the following steps for the reaction: BnOH is
introduced in ACN and dissolved (Fig. 8(c) (steps 1 and 2)). The
biphasic system is formed immediately when NaOCl 10–12% is
introduced in the reaction. ACN forms the upper phase while
the catalyst dispersed on the oxidant form the lower phase. The
solubility of BnOH in water is z40 g L,48 allowing the phase
transference from organic to aqueous phase (Fig. 8(c) (step 3)),
where the oxidation occurs (Fig. 8(c) (step 4)). The solubility of
BnCHO in water is z0.3 g L,48 inducing its phase transference
to ACN (Fig. 8(c) (step 5)). The BnCHO diffusion from aqueous
to ACN phase not only avoids a further oxidation, but also
provides a high selectivity. In turn, BnCOOH is produced in the
aqueous phase as a byproduct of the reaction (Fig. 8(c) (step 6)).
Fig. 8 The pictures (a) and (b) show the reaction media before and
after centrifugation for catalyst sedimentation, respectively. (c) The
scheme shows the proposed reaction process for the oxidation of
BnOH in the biphasic system: BnOH is introduced on ACN (1) and
dissolved (2). With the addition of the oxidant and the formation of
a biphasic system, fractions of BnOH are transfered to aqueous phase
(3) where it is oxidized to BnCHO (4). The low aqueous solubility of
BnCHO conducted to the diffusion of this specie to organic phase (5).
Low concentration of BnCOOH are formed on aqueous phase (6) as
a byproduct.
and benzaldehyde 2 for the synthesis of bis(indolyl)methane 3
under solvent-free conditions (Table 3). Initially, the reaction
ꢀ
was carried out at 60 C being the progress of reaction moni-
tored by thin layer chromatography (TLC) analysis. Aer 3 h of
reaction the total consumption of the indole 1 was observed and
the desired product 3 was obtained with a yield of 90% (Table 3,
entry 1). However, when the reaction time was reduced to 1.5 h
no signicant decrease in yield was observed (Table 3, entry 2).
On the other hand, a temperature reduction provided the
product 3 in only 35% of yield (Table 3, entry 3).
To examine the inuence of Ni-based nanoparticles in the
catalysis, the undoped germanophosphate glass and unan-
nealed Ni-doped were evaluated. However, even aer 3 h of
reaction the starting materials were not consumed, providing
the desired product in 65% and 68% yield, respectively (Table 3,
entries 4 and 5). Furthermore, it is notable that in the absence
Glass catalyst performance: Friedel–Cras alkylation reaction
of indole and benzaldehyde
Besides the investigation of the activity of nickel nanoparticles
based catalyst for benzyl alcohol oxidation, its catalytic perfor-
mance was tested for the Friedel–Cras alkylation of indole 1
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Table 2 BnOH, BnCHO and BnCOOH concentrations presents in organic (O) and aqueous (A) phases after reaction using Ni-based catalyst
nanoparticlesa,b
BnCOOH mmols
Phasec
BnOH mmols (ꢂ10ꢁ3
)
BnCHO mmols (ꢂ10ꢁ3
)
(ꢂ10ꢁ3
)
Organic
Aqueous
250
510
N.D.
4.2
2.0 (0.8%)d
1.7 (0.3%)d
a
b
c
d
N.D. ¼ not determined. Ni-doped glass annealed at 430 ꢀC during 60 min under H2. Aer centrifugation, 3400 rpm, 6 min. The % of the
species in aqueous phase related to organic phase.
Table 3 Ni-based nano catalyzed Friedel–Crafts alkylation reaction of
reactions. For benzyl alcohol oxidation, the catalyst shows high
selectivity to benzaldehyde employing a strong oxidant and
indole and benzaldehyde under solvent-free conditionsa
acetonitrile at mild conditions. Furthermore, the application of
the catalyst has been successfully extended to Friedel–Cras
alkylation reaction of indole and benzaldehyde.
Conflicts of interest
Entry
Catalyst
T
ꢀC
Time h
Yieldb %
There are no conicts to declare.
1
2
3
4
5
6
7
Ni-basedc
Ni-basedc
Ni-basedc
Undoped
Unannealed
—
60
60
r.t.
60
60
60
60
3
90
85
35
65
68
42
45
1.5
1.5
3
3
1.5
1.5
Acknowledgements
We would to like to thank the Brazilian agencies CNPq (grant
number: 430470/2018-5), CAPES, FAPDF (grant number:
193.001.757/2017), for nancial support and the research
scholarship.
Silica glass
a
Reaction conditions: indole (0.5 mmol), benzaldehyde (0.25 mmol)
b
and catalyst (25 mg). Yield of pure product isolated by column
chromatography and identied by 1H and 13C NMR. Nickel-doped
c
germanophosphate glass annealed at 400 ꢀC under H2(g) during
Notes and references
60 min. r.t. ¼ room temperature (25 ꢀC).
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