Copper (0) nanoparticles onto silica
XPS graph (figure S1 in Supplementary Information). surface of silica was determined by AAS analysis. The
A peak due to 2p3/2 observed at 932 eV confirmed the catalyst was stirred in dil. HNO3 and then subjected to
presence of copper (0) state in silica supported copper AAS analysis. It was found that SiO2-Cu(0) contained
(0) nanoparticles. In addition to this, a shakeup peak 0.0129 g of Cu per gram of catalyst.
at 952 eV arises when the outgoing electron interacts
with a valence electron and excites it (shakes it up) to
a higher energy level. Another low intensity peak at
934 eV was recorded which may due to the oxidized
surface of the catalyst (oxidized surface protects the
metal (0) state in the core of the catalyst). But this peak
is very insignificant. The stability of the catalyst is
determined by thermogravimetric analysis (TGA). The
curve (figure S2 in SI) showed an initial weight loss
of 7.5% at 158◦C which may be due to the residual
water trapped onto the silica. Further, there is a slight
weight loss of approx. 10% up to 321◦C and followed
by a major weight loss of about 18% at 465◦C which
shows that the catalyst is stable up to 200◦C and there-
fore it is safe to carry out the reaction at 80◦C which
is our desired reaction temperature. The EDX spec-
trum shows the presence of Si, O and Cu in the sil-
ica supported Cu (0) nanoparticles which is in consis-
tent with the expected elemental composition of the
catalyst (figure S3 in SI). The morphology and dis-
tribution of the catalyst was studied using SEM and
TEM images. The SEM image shows that the copper
(0) nanoparticles are finely distributed onto the surface
and there is no bulk aggregation of the copper (0) parti-
cles onto the surface (figure 1). The TEM micrographs
gave detailed information about the internal structure
of the catalyst like the morphology and particle size. It
has been found that the size of copper (0) particles was
5 nm (figure 2). The amount of copper loaded onto the
3.2 Catalyst testing for the synthesis of
arylmethylene-bis-(3-hydroxy-2-cyclohexene-1-one)
To select the appropriate reaction conditions, benzalde-
hyde (entry 1, table 1) was selected as test substrate and
the reaction was carried out under different set of con-
ditions with respect to different solvents, different cat-
alysts and temperature. To select the most appropriate
heterogenous solid supported copper (0) catalyst, the
reaction was carried out using different supported cop-
per (0) catalysts such as silica supported copper (0),
HAP supported copper (0), cellulose supported cop-
per (0) and basic alumina supported copper (0) oxide
at 80◦C using ethanol as solvent. The results are sum-
marized in table 1. It can be seen from the Table that
SiO2-Cu(0) gave best results and thus SiO2-Cu(0) was
selected as the optimum reaction catalyst at which the
reaction proceeded smoothly and furnished the prod-
ucts in good to excellent yields. The use of different sol-
vents (water, ethanol, toluene, acetonitrile, DCM) has
been investigated using the test substrates and silica
supported copper (0) catalyst under refluxing conditions
and it was found that ethanol gave the best results with
respect to yield, selectivity and reaction time. To study
the catalytic role of silica supported copper (0) catalyst,
the reaction has also been carried out using test sub-
strates catalyzed with activated silica, Cu(0) nanopar-
ticles, silica supported copper (0) and in the absence
of any catalyst (table 2), it has been inferred from the
Table that silica supported copper (0) furnished the
products in good yields in less time. To study the gen-
erality of the developed protocol for the synthesis of
cyclohexen-1-one, various aldehydes having electron
releasing and electron withdrawing groups were cho-
sen and it was found that the reaction proceeds effi-
ciently with all the aldehydes and there is a formation
of arylmethylene bis(3-hydroxy-2-cyclohexene-1-one)
exclusively without the formation of 9-aryl-1,8-dioxo-
1
octahydroxanthene which is confirmed by H NMR,
13C NMR and mass spectrometry.
3.3 Post-characterization of SiO2-Cu(0)
nanoparticles
Silica supported copper (0) nanoparticles which were
obtained after the reaction work-up has been dried
and subjected to post-characterization to examine any
change in its morphology and its activity. For this, TEM
Figure 1. SEM image of SiO2-Cu(0).