H.M. Yusoff et al.
Inorganic Chemistry Communications 130 (2021) 108749
palladium, copper, nickel and gold are favoured, contributing to their
fast depletion known as resource deficits. Considering the ill-effect
contributed by the unsustainable use of elements on our environment,
society and the economy, scientists have put forth a vital move to protect
these “endangered elements”. Elemental Sustainability is a philosophy
that is coined to protect these endangered elements, and at the same
time, encourage the balance use of elements in periodic table to achieve
industrial development and sustainable synthesis [16].
with hydrogen peroxide 30% wt/v (3 mL) as oxidant, and samarium
oxide (10 mmol) as catalyst. The mixture was then heated at about 80 ◦C
for 4 h. Product formation was monitored by using thin layer chroma-
tography (TLC). The reaction mixture was left to cool at room temper-
ature and the catalyst was separated from the reaction mixture by
filtration. Subsequently, the organic layer was separated, washed with
sodium bicarbonate solution and the resulting aqueous layer was
washed three times with ethyl acetate (3 × 10 mL). The combined
organic layers were concentrated under reduced pressure to obtain the
dried crude product. Lastly, the single pure product was afforded by
using the precipitation method. The recycled samarium oxide was
employed to synthesize different phenols under the same condition.
As part of our continuous effort in the search for efficient and sus-
tainable catalyst for organic transformations, herein, we would like to
demonstrate the use of samarium oxide (Sm2O3) as an alternative
method in achieving sustainable production of phenols via phenyl-
boronic acids in the presence of H2O2 (Fig. 1). Lately, the use of
lanthanide metals has gained much attention among scientists due to its
unique electron configuration, robustness and high catalytic activity of
surface structure in catalysing chemical processes [17]. One such
example is the recently developed samarium nanoparticles as sustain-
able catalysts in Pechmann and coupling reactions [18,19]. To date, our
group is the first to report on the utilisation samarium oxide to produce
phenols. The current method is of significant and useful in the chemical
industry for manufacturing chemical intermediates as it encompasses
the use of non-endangered metal for organic transformation.
3. Results and discussion
The Sm2O3 catalyst were characterized by XRD and SEM analyses as
shown in Fig. 2. The synthesized precipitates transformed from amor-
phous intermediate phase to crystalline Sm2O3 phase when calcined at
750 ◦C. The Sm2O3 phase was found to be cubic single phase without any
presence of secondary phase (Fig. S1). This highly crystalline Sm2O3
phase matched with Inorganic Crystal Structure Database (ICSD) 98-
001-2948. Based on the SEM image (Fig. S2), it can be observed that
most of the particles are spherical with a size distribution between 120
and 300 nm. In order to investigate the uniformity of element distri-
bution in Sm2O3, EDX mapping was carried out. The inset of Figure S2
show the elemental mapping of O and Sm, demonstrating homogeneous
distribution of Sm2O3 with high purity. FTIR spectrum as shown in
Fig. S3 indicated that the stretching vibrations of Sm3+-O are located at
877 cmꢀ 1, 522 cmꢀ 1 and 439 cmꢀ 1, respectively. The strong band at
1427 cmꢀ 1 was ascribed to the Sm-O-Sm deformation vibration whereas
the weak band at 3788 cmꢀ 1, 2323 cmꢀ 1 and 2968 cmꢀ 1 corresponded
to the stretching and bending vibrations of –OH. These weak –OH bands
were observed due to the marginal presence of water within the Sm2O3
nanoparticles.
2. Experimental
2.1. Chemicals and instrument
All chemicals and solvents utilized in this study were purchased from
Sigma-Aldrich Malaysia. They were used without purification unless
stated. The phase formation and nanostructure of Sm2O3 were per-
formed using an X-ray diffractometer (X’Pert Pro PW3040 MDP/Pan-
alytical) with Cu-K
α radiation source and a scanning electron
microscope (JEOL JSM-6360LA). The X-ray diffraction (XRD) pattern
was collected within 20–80◦ in with increment step size of 0.033◦. In
addition, a Bruker Nuclear Magnetic Resonance (NMR) spectrometer
was employed to analyze samples and all the spectra were recorded on
proton (1H) (400 MHz) and carbon-13 (13C) (100 MHz). All chemical
shifts (δ) were quoted in the NMR as parts per million (ppm). The mo-
lecular mass of the samples was analyzed over the Shimadzu QP2010SE
Gas Chromatography Mass Spectrometry (GC–MS). The values of all
molecular mass were recorded in spectra as unit of mass over charge
ratio (m/z).
In the optimisation study of reaction condition, a round bottom flask
(50 mL) were equipped with phenylboronic acid (1 mmol) as the model
substrate, hydrogen peroxide 30% (3 mL) as oxidant and Sm2O3 (10
mmol) as catalyst. The reaction mixture was subject to heating at 60 ◦C
for about 1 h. The formation of 3a was monitored using TLC analysis. At
this stage, about 54% reaction’s yield was observed for 3a after 1 h
(Table 1, entry 1). Increasing the reaction temperature has also
increased the yield of 3a (Table 1, entry 3). Furthermore, it was observed
that amount of Sm2O3 used could also affect the catalytic performance
on 3a synthesis. The increased amount of Sm2O3 has resulted a better
yield of 3a with 10 mmol Sm2O3 as the optimum amount of catalyst in
3a synthesis (Table 1, entry 7. In addition, different amounts of
hydrogen peroxide used also have an effect on the yield of 3a formation.
Laboratory result showed that 3 mL of H2O2 gave the best result in the
formation of 3a (96%), when employed for the oxidation of
2.2. Synthesis of Sm2O3 catalyst
The Sm2O3 nanoparticles were synthesized via hydrothermal method
starting from 99.9% Sm(NO3)3⋅6H2O. A Sm(NO3)3 solution was pre-
pared and mixed with oleic acid (OA) and tert-butylamine (TA) in a fixed
ratio of 1:1:1 for Sm:OA:TA. The clear homogeneous solution was
adjusted to pH 12 with NaOH. The mixture was transferred into a Teflon
lined stainless steel autoclave and heated at 200 ◦C for 72 h in an oven.
The obtained precipitate was washed with ethanol for several times
before calcination at 750 ◦C in a furnace to obtain the Sm2O3
nanoparticles.
2.3. General method for phenols synthesis
Phenylboronic acid was added to a round bottom flask suspended
Sm2O3, H2O2
80
B(OH)2
OH
°
C
R1
R1
Fig. 1. Conversion of phenylboronic acids to phenols using samarium oxide as
catalyst and hydrogen peroxide as oxidant.
Fig. 2. Recyclability experiment using 3a as a model product.
2