B.D. Bankar, J.H. Advani and A.V. Biradar
Polyhedron 200 (2021) 115129
and selectivity by stabilizing the active metal intermediate species,
preventing the sintering of active metals, and providing an elec-
trophilic and nucleophilic environment [15,16]. Among the differ-
ent supports utilized, ZrO2 finds a special place in catalysis owing
to its amphoteric nature, thermal stability, variable phases, and
tuneable surface area. The high thermal stability of cubic ZrO2
(2400 °C) as compared to its analogues, i.e., monoclinic (1100 °C)
and tetragonal (1400 °C) [17] and the presence of both acidic and
basic sites on the surface, and it makes an excellent candidate for
redox type of catalysis [18]. Furthermore, zirconia was stabilized
into the cubic phase using copper and nickel oxide and used for
light alkane oxidation [19,20]. Considering these points, herein,
we have developed a facile synthetic method for the synthesis of
active Cu-containing Zr-supported metal oxide catalysts by a co-
precipitation-hydrothermal method at milder temperatures. The
catalyst is active towards the liquid-phase oxidation of alkanes,
alcohols, and amines under mild reaction conditions, which might
result from the structural aspect arising from the synthetic
procedure.
3. Results and discussion
For the synthesis of CuxZrO100-x mixed metal oxide catalyst, a
combination of co-precipitation followed by the hydrothermal
method was used. A co-precipitation method aids in exchanging
ligands giving metal complexes with catalytically active phases,
while hydrothermal processes yield crystalline metal oxides under
aqueous solutions, temperature, and autogenous pressure condi-
tions. Hydrothermal synthesis is usually carried out below
300 °C. Water in supercritical conditions favours particle growth
by increasing reaction rate and supersaturation based on the
nucleation theory [21]. The metal salt-containing aqueous solution
changes the reaction equilibrium, which results in the formation of
metal hydroxide or metal oxides. Scheme 1 shows the formation of
hydrated zirconia from zirconium nitrate aqueous solution [22]
followed by the addition of aqueous copper nitrate with stirring
to yield a hydrated complex of both metals. The addition of aque-
ous NH3 solution act as a co-precipitating agent by dissociating
hydroxyl complex (NH3 being a strong ligand) forming an ammo-
nium complex, which is stabilized by dissociated nitrate groups.
During the course of the reaction, it produces monomers, followed
by nucleation and crystal growth, and then finally calcined at high
temperature to remove impurities to produce a phase pure oxide
[23]. The material was characterized using various physicochemi-
cal methods (see SI, S3).
2. Experimental
See Supporting information for materials and methods (SI, S1-
S2).
The phase purity of the materials was confirmed by PXRD
(Fig. 1A). The diffraction peaks at 2h = 30.5, 35.3, 50.7, 60.2, 62.9
and 74.6° correspond to the crystal planes (111), (200), (220),
(311), (222), and (400) of cubic ZrO2 support (JCPDS No. 00-
027-0997). However, the diffraction peaks of Cu or CuO were not
observed in the case of 1–5 wt% Cu on ZrO2, maybe due to the
lower wt.% of Cu, non-crystalline nature of Cu/CuO or may be
due to Cu been repressed inside the zirconia support [24]. How-
ever, a shift in the diffraction peaks between the pure ZrO2 and
the Cu doped ZrO2 could be observed, which may be attributed
to the effect of copper loading in the ZrO2 samples. The average
crystallite particle size of the CuxZrO100-x catalyst calculated from
the (111) plane using the Debye-Scherer equation and was found
to be 8.60 nm (See SI, S4). The XRD profile of 20 wt% CuZrO2
showed the presence of the peaks originating from the monoclinic
CuO planes (200), (ꢀ200), (200), (202), (ꢀ113), (ꢀ311), (220)
and (311) along with the ZrO2 planes (See Fig. 1B or SI, S4). Thus,
monoclinic CuO ( JCPDS 04-005-4712) was loaded on ZrO2 lattice.
FESEM analysis of the ZrO2 support (Fig. 2a) as well as 2 wt%
CuZrO2 (Fig. 2b), both showed an irregular morphology. The uni-
form distribution of the elements was confirmed by elemental
mapping of the catalyst, as shown in Fig. 2(c-f). HRTEM analysis
confirmed the loading of copper onto the ZrO2 support (Fig. 2g).
The average particle size of 2 wt% CuZrO2 catalyst was 5.3 nm
(See SI, S6 fig.3.4). The fringes relating to the support, as well as
CuO (111) and (200) phases, can be seen in (Fig. 2g(1ꢀ4)). The d-
spacing of the ZrO2 support, as well as CuO, matches well with
the JCPDS data. The CuO fringes from the (111) plane having a
d-spacing of 0.232 nm can be seen in Fig. 2g3. The SAED pattern
of the 2 wt% CuZrO2 (Fig. 2h) showed the presence of (220) and
(422) facets of cubic ZrO2 and (400) facets of Cu, supported in
cubic zirconia phases (JCPDS file No. 00–004-0836). The FESEM
and TEM analysis of other samples are given in SI, S6 Fig. S3.
The BET surface area and pore size of the series of CuxZrO100-x
catalysts measured by N2 gas adsorption and desorption are tabu-
lated in Table 1 (See SI, S7 Fig. S4). The surface area of pure ZrO2
was 131.5 m2/g with a pore size of 132.6 Å. The increase in the
loading wt.% of Cu from 1 to 5 wt% gave a decreasing trend in
the surface area while pore size was found to increase. The ICP-
OES analysis of the catalyst showed the presence of 1.95 wt% of
2.1. Synthesis of CuxZrO100-x
The CuxZrO100-x catalyst was synthesized by loading different
wt.% of copper (x = 1–20%) through a combination of the co-precip-
itation-hydrothermal method. In a typical synthesis, 10 g of ZrO
(NO3)2ꢁxH2O was added to 50 mL of distilled water and stirred
for 30 min at room temperature until complete dissolution. Then,
the required amount of Cu(NO3)2ꢁ3H2O was added to this solution.
After the solution became homogeneous, 5.5 mL ammonia solution
(25%) was added dropwise as a co-precipitating agent, maintaining
a constant pH 9. Further, the reaction mixture was aged by stirring
for 20 h at room temperature and transferred to a 100 mL stainless
steel hydrothermal reactor and kept in a muffle furnace at 180 °C
for 3.5 h. After cooling the reactor to room temperature, the solid
was collected by simple filtration and washed several times with
deionized water until the filtrate showed neutral pH and then
finally washed with 10 mL methanol. The obtained solid was dried
in an oven at 70 °C for overnight and calcined in a muffle furnace at
500 °C at the heating rate of 5 °C/min for 5 h.
2.2. Synthesis of ZrO2
The synthesis of pristine ZrO2 was also carried out using a sim-
ilar procedure as above without the addition of copper salt.
2.3. Catalytic activity
The liquid-phase oxidation reaction was performed in a two-
necked dry 25 mL round-bottom flask. Typically, 1 mmol of the
substrate, 3 mmol aq. TBHP, 15 mg of freshly prepared CuxZrO100-
catalyst, 2 mL acetonitrile as a solvent, and chlorobenzene as an
x
internal standard were mixed. The reaction mixture was heated
to the desired temperature using a temperature-controlled oil
bath. The progress of the reaction was monitored by withdrawing
reaction aliquots, which were analyzed by gas chromatography
(Agilent GC-7890B) and GC–MS (Shimadzu, QP-2010, Japan) with
HP-5 column (5% diphenyl and 95% dimethyl polysiloxane capil-
lary column). The catalyst was collected by filtration, washed twice
with acetone (2 ꢂ 10 mL), dried in an oven at 100 °C for 5 h, and
reused for the next cycle (See in S11 Table S4).
2