S. Wang, et al.
MolecularCatalysis494(2020)111108
2.2. Synthesis of ZrO2 with different calcination temperature
Typically, 40 mL ZrCl4 aqueous solution (0.16 mol/L) was placed in
a beaker, and its pH was adjusted to 10 via dropwise addition of
0.1 mol/L NaOH aqueous solution with vigorous stirring and monitored
with a pH meter. After 72 h of aging, the suspension was filtered, wa-
shed with deionized water to pH = 7, and then the obtained solid was
dried under vacuum at 80 °C for 12 h. After milling, a Zr(OH)4 catalyst
was obtained. Then, the Zr(OH)4 powder was calcined respectively at
400, 500, 600, 700, 800 and 900 °C to obtain ZrO2-400, ZrO2-500,
ZrO2-600, ZrO2-700, ZrO2-800, and ZrO2-900. The ZrO2 with the 500 °C
and ZrO2 with 900 °C were named as t-ZrO2 and m-ZrO2 respectively.
Scheme 1. The reductive amination of carbonyl compounds.
It is well-known that there are three crystal forms of ZrO2 including
the cubic, tetragonal and monoclinic crystals. The monoclinic phase
keeps stable at room temperature, and is transformed to tetragonal one
at 1170 °C, and then transformed to cubic one at 2370 °C [22]. In these
forms, the metastable tetragonal one (t-ZrO2) exhibits particular in-
terest which probably exhibits preferable catalytic and mechanical
property compared to the cubic one (c-ZrO2) and monoclinic one (m-
ZrO2) [23]. In order to realize the synthesis of zirconia nanoparticles,
several efficient methods have been investigated which include the sol-
gel process [24], spray pyrolysis [25], magnetron sputtering [26], hy-
drothermal methods [27], carbon nanotube templated method [28],
and emulsion precipitation [29].
In this article, the m-ZrO2, t-ZrO2 and other oxides are prepared by
simple precipitation method, and employed as the heterogeneous cat-
alysts for the reductive amination of different ketones and aliphatic
aldehydes. As a result, the t-ZrO2 showed the highest catalytic activity
and provided the satisfactory yields of the target products. Based on the
DFT calculation and characterization results of catalysts using XRD,
BET, SEM, TEM, NH3-TPD, FT-IR, Pyridine-IR and TGA techniques, the
relationship between the structures and catalytic performances are
discussed.
2.3. General procedure for reductive amination of ketones and aliphatic
aldehydes
All the reductive amination experiments are performed in a 120 mL
autoclave equipped with the magnetic stirring and automatic tem-
perature control. A typical procedure for reductive amination of cy-
clohexanone is given as follows: the substrate cyclohexanone (0.20 g), t-
ZrO2 (0.050 g), H2O (0.500 g) and N, N-dimethylformamide (15 mL)
were charged into the reactor, and the atmosphere inside was replaced
by the pure nitrogen after the reactor is sealed. Under stirring, the ni-
trogen is charged to 0.3 MPa at room temperature, and the autoclave
was heated to 140 °C and was kept for 4 h. After reaction, the autoclave
is slowly cooled to room temperature and the solid catalyst is separated
by ultrafiltration membrane. The mixture is analyzed by an Agilent
7890A/5975C GC equipped with an HP-5MS column and FID detector.
2.4. Density functional calculations
2. Experimental section
Calculations were carried out in the framework of density functional
theory (DFT) by using CASTEP module in the Materials Studio of
Accelrys, Inc. The nonlocal exchange and correlation energies were
calculated with the Perdew-Wang (PW91) functional of the generalized
gradient approximation (GGA) [30,31]. A plane-wave basis set with a
cutoff energy of 400 eV was used to expand the valence electrons. The
geometries were optimized using a BFGS method [32] with spin po-
larization. The convergence criteria included threshold values of
2 × 10−5 eV/atom, 0.05 eV/Å and 0.002 Å for energy, maximum force
and maximum displacement, respectively, and the self-consistent-field
(SCF) density convergence threshold value of 2.0 × 10-6 eV/atom.
On basis of the experimental evidence (XRD results in Fig. 1), the (-1
1 1) surface of monoclinic zirconia (m-ZrO2) and (0 1 1) surface of
tetragonal zirconia (t-ZrO2) were employed in the DFT calculations in
this research. The 2 × 2 (m-ZrO2) and 3 × 2 (t-ZrO2) surface supercells
were used for surface slabs, respectively; the corresponding formulas
are Zr44O76 and Zr36O60. The bottom 3 layers were fixed to that of the
corresponding polarization bulk structures, while the top 8 layers for m-
ZrO2 (-1 1 1) and 5 layers for t-ZrO2 (0 1 1) were relaxed respectively.
The vacuum region thickness between slabs was 20 Å, which was large
enough to avoid the inter-planar interaction generated by periodic
boundary conditions. The surface Brillouin zone was sampled by a
2 × 2 × 1 Monkhorst-Pack grid [33]. All the adsorption molecules
were fully relaxed. The structures of m-ZrO2 (-1 1 1) and t-ZrO2 (0 1 1)
surface were given in Figure S11 of the Supporting Information.
In order to describe the interaction between adsorption molecules
and ZrO2 surface, the adsorption energy (Eads) was defined as following
2.1. Reagents and experimental equipment
The reagents are of analytical grade and were purchased from
commercial venders, which include N, N-dimethylformamide (DMF),
ZrCl4, ZrOCl2 NaOH, TiO2, Al2O3, CeO2, MgO, ZnO, KBr and formic
acid, pyridine, 2,2′-dipyridine, dimethylamine, cyclohexanone, cyclo-
butanone, cyclopentanone, cycloheptanone, cyclooctanone, butalde-
hyde, caproaldehyde, valeraldehyde, heptaldehyde, 2-heptanone, 2-
pentanone, 3-pentanone. The nitrogen stored in the high-pressure cy-
linder is used directly through the pressure reducing valve without
further treatment.
X-ray powder diffraction (XRD) patterns were obtained using a
Rigaku/Max 2500/PC powder diffractometer with Cu Kα radiation at a
scan range (2θ) of 5−80°. The surface morphology observation was
performed on scanning electron microscope (SEM: JSM-6301 F, JEOL)
and the interior morphology was characterized by transmission electron
microscope (TEM: JEM-2100, JEOL). The Brunauer-Emmett-Teller
(BET) surface area of the sample were measured by N2 adsorption using
an ASAP2020 M system. All the sample were degassed at 200 °C under
vacuum before N2 adsorption measurements. Fourier transform infrared
(FT-IR) and Pyridine infrared (Py-IR) spectra were obtained on a Bruker
Tensor 27 spectrometer with dried KBr or pyridine, respectively.
Thermogravimetric (TG) was performed by NETZSCH-TG209 F3,
heating in N2 at 10 ℃ min−1 rate. NH3-TPD was performed on a
Micromeritics’ ChemiSorb 2720 TPx system.
The analysis of liquid products was performed by gas chromato-
graphy-mass spectrometry (Agilent 7890A/5975C). The nuclear mag-
netic resonance (NMR) measurement was achieved by a Bruker 400
MHZ spectrometer. The conversion and selectivity were determined
using an internal standard method and area normalization method,
respectively.
Eads = (EDMF/ZrO − EDMF − EZrO
)
2
2
where EDMF/ZrO was the total energy of the ZrO2 slab with adsorbed
2
DMF, EDMF was the energy of isolated DMF molecule in gas phase, EZrO2
was the energy of a clean ZrO2 slab.
The EC-N/gas and EC−N/ZrO were used to describe the CeN bond
2
strength of DMF molecular in gas and adsorbed on the ZrO2 surface,
2