M.S. Esmaeili, et al.
MolecularCatalysis492(2020)111037
3.6. TGA
The TGA was performed to examine the thermal feature of the na-
nocatalyst. As indicated in Fig. 8, the TGA weight loss thermogram of
the Fe3O4@raffinose-Cu2O NPs exhibits three significant weight losses
in the range of 50–800 °C which is equal to 15% weight of the nano-
catalyst. The first small amount of weight loss around 100 °C was at-
tributed to the desorption of adsorbed water. The second and third
weight losses are related to the successful grafting of raffinose on the
surface of the Fe3O4 NPs. These weight losses were showed in two parts
in the range of 200−350 °C and 350−800 °C that the first range was
relatively attributed to depolymerization and dehydration of oligo-
saccharide rings and the second was appointed to separation of the
biopolymer matrix and cross-links among biopolymer chain.
3.7. Application of the Fe3O4@raffinose-Cu2O NPs for the selective
oxidation of PBA to BAD
A variety of substrates of electron-donating and electron-with-
drawing groups were applied to study the performance and effective-
ness of green Fe3O4@raffinose-Cu2O NPs. The result of yield and re-
action time for the selective oxidation of PBA to BAD is presented in
Table 1. All the substrates were successfully converted to the corre-
sponding BAD at high conversions of 87–97% without any byproducts
such as aromatic acids within reaction times of 3–6.5 h.
3.8. Optimization of the selective oxidation of PBA to BAD
The refluxing conditions in the acetonitrile as a co-reactant was
considered as a model reaction for the synthesis of the 4-methoxy
benzaldehyde (2f) to evaluate the efficiency of Fe3O4@raffinose-Cu2O
NPs oxidation of 4-methoxybenzyl alcohol (1f) in the presence of TBHP.
At first, the effects of various amounts of catalyst loading are in-
vestigated. Initial experiments depicted that the trace product of 2f was
converted in the absence of any catalyst even when the reaction sub-
strate was stirred in the acetonitrile under reflux conditions for 3 h
(Entry 1, Table 2). Interestingly, when a specified amount of a catalytic
was added to the reaction mixture in the acetonitrile under refluxing
conditions, the desired product of 2f was obtained. According to the
results of Table 2 entry 3, 4, 5 and 6, when 0.03 g amount of catalyst
was added, the rate of the reaction and conversion values were higher
for desired products at fewer reaction times. During the optimization
examinations, different temperatures and co-reactant were investigated
and it was found that the catalyst activity under refluxing conditions in
the acetonitrile co-reactant is the best temperature and co-reactant
amount between other tested conditions in terms of produced desired
product of 2f at a high conversion of 97% and lower reaction time of 3
h.
Fig. 7. The XRD pattern of the Fe3O4@raffinose-Cu2O NPs.
the previous studies, the magnetic saturation value for Fe3O4 is 75 emu
−1 [36]. The reason for a reduction in magnetization value is typically
g
attributed to the existence of newly loaded functional groups on the
surface of the MNPs. As can be seen in Fig. 6 the magnetic saturation
value of Fe3O4 is approximately 75 emu g−1 (Fig. 6a) that in compar-
ison with a magnetic saturation value of Fe3O4@raffinose with 64 emu
(Fig. 6b) and Fe3O4@raffinose-Cu2O NPs (Fig. 6c) with 56 emu g−1 was
decreased. Furthermore, the magnetic saturation value of Fe3O4@raf-
finose-Cu2O NPs showed that novel nanocatalyst have
a super-
paramagnetic action at room temperature and the VSM curve can
confirm this effect.
3.9. Proposed mechanism for the selective oxidation of PBA to BAD using
Fe3O4@raffinose-Cu2O NPs
3.5. X-ray diffraction (XRD)
The proposed mechanism for the selective oxidation of PBA to BAD
in the presence of catalyst is shown in Fig. 9. In the first step, the copper
NPs (I) has interacted with the acetonitrile (II) as a co-rectant to formed
active nitrile intermediate (III). Then in the presence of TBHP (IV),
intermediate III converted to the peroxyimidic acid (V) that is known as
an active oxidant reagent and plays a considerable role during the
oxidation reactions [39]. In the next step, most active part of the per-
oxyimidic acid reacts with PBA and formed intermediate VII. In order
to form a sustainable product from intermediate VII, intramolecular
exchange occurs to the formation of product VIII.
The XRD pattern of Fe3O4@raffinose-Cu2O is presented in Fig. 7. Six
diffraction peaks at 2θ = 30.16˚, 35.45˚, 43.25˚, 53.55˚, 56.78˚, and
62.73˚ were corresponded to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 5 1) and
(4 4 0) diffraction planes of magnetite (Fe3O4) NPs with cubic phase
that was in excellent agreement with the reported JCPDS card no.
03‐0863 [37]. Four distinguished diffraction peaks at 2θ = 29.58˚,
36.42˚, 42.34˚, and 61.40˚ were related to (1 1 0), (1 1 1), (2 0 0) and (2
2 0) diffraction planes of Cu2O that was in good acceptance with the
reported JCPDS card no 78-2076 [38]. The XRD conclusion for the
Fe3O4@raffinose-Cu2O NPs expresses that the crystal arrangement of
the Fe3O4 core does not convert throughout the functionalization route.
3.10. Reusability of Fe3O4@raffinose-Cu2O NPs
The reusability of the nanocatalyst is an important issue based on
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