L. Ning et al. / Journal of Catalysis 364 (2018) 1–13
3
At last, the sample was dried in a vacuum oven at 100 °C for 12 h to
afford pure UiO-66.
room temperature so that Au colloids can be completely deposited
on the UiO-66. The solid was collected by centrifugation (8000 r/
min, 5 min), and washed using distilled water for three times,
and then dried under vacuum at room temperature overnight,
and finally calcined at 300 °C in air for 2.0 h to obtained the
Au@UiO-66 catalyst.
In addition, the Au@UiO-66-NH2, Au@UiO-66-NO2, Au@UiO-66-
COOH and UiO-66-NH3Cl catalysts were synthesized with I-H
method, respectively.
The UiO-66-NH2 was synthesized with a modified method [36].
ZrCl4 (0.0203 g, 0.087 mmol) and 2-aminoterephthalate acid
(0.0118 g, 0.065 mmol) were dissolved in 15 mL DMF. The mixture
was stirred in a glass vial for 10 min, and the resulting solution was
transferred into a 20 mL teflon-lined stainless steel autoclave and
heated at 120 °C for 24 h. After being cooled to room temperature
naturally, the precipitate was collected by centrifugation at 4000
rpm for 10 min, and further washed with ethanol for three times,
and dried under vacuum at 100 °C for 12 h to afford pure UiO-
66-NH2.
2.3. Characterization of the catalysts
The UiO-66-NO2, UiO-66-COOH and UiO-66-NH3Cl were syn-
thesized according to the literatures [37,38].
X-ray single-crystal diffraction data were collected on a Rigaku
SCX-mini diffractometer at 293(2) K with Mo KR radiation (k = 0.
71073 Å) by
(XRD) were recorded on Rigaku D/Max 2400 diffractometer with
Cu/K radiation. Fourier transform infrared spectra (FT-IR) were
x scan mode. The powder X-ray diffraction patterns
2.2.2. Synthesis of Au@UiO-66-X catalysts (X = –H, –NH2, –NO2, –
COOH)
a
For the synthesis of Au@UiO-66, the impregnation-reduction-
H2 (I-H), the impregnation-reduction-NaBH4 (I-S), deposition–pre
cipitation-carbonization (D-C), deposition–precipitation-H2 (D-H)
and colloid-immobilization (C-I) methods were used, respectively
[39–43]. The experimental procedures were given as followings.
In a typical procedure of I-H method, 100 mg of activated UiO-
66 was immersed in 1 mL solution of HAuCl4 (0.01 g/mL), and then
2 mL of deionized water was added under N2 atmosphere. After
being sonicated for 6 h at room temperature, the mixture was
placed in a vacuum oven and dried at 100 °C for 12 h. Finally, the
dried precursor of the catalyst was treated in a fixed-bed stainless
steel reactor with an inner diameter of 6 mm under H2 with a total
flow rate of 50 mL minꢁ1 and maintained at 250 °C for 2 h to obtain
4.7 wt% Au@UiO-66 catalyst.
recorded using Bruker EQUINOX55 infrared spectrometer. The gold
contents of all the samples were quantitatively determined by the
inductively coupled plasma optical emission spectrometry (ICP-
OES: Varian 700-ES). High resolution scanning electron microscopy
(HRSEM) was performed in a JSM 6490LV JEOL microscope at 25
kV. Transmission electron microscopy (TEM) was performed in a
Phillips CM200 at 200 kV. X-ray photoelectron spectroscopy
(XPS) was carried out in a 5700 model Physical Electronics appara-
tus. Temperature programmed reduction (TPR) analysis were car-
ried out using
a
Micromeritics ChemiSorb 2720 Pulse
Chemisorption System, 50 mg of each catalyst was pre-treated in
5% H2/N2 (100 mL/min) at 70 °C for 1 h, and then TPR was per-
formed over the sample with the temperature increasing from
70 °C to 700 °C at a speed of 10 °C /min.
In a typical procedure of I-S method, 100 mg of UiO-66 and 1
mL solution of HAuCl4 (0.01 g/mL) were mixed in a three-neck flask
and 2 mL of deionized water was added. The reaction mixture was
ultrasonicated for 6 h. Then, the reaction mixture was heated to
105 °C, and 5 mL fresh solution of NaBH4 (20 mg/mL) was added
dropwise and the mixture was refluxed for 5 h. Next, the resulting
solid was filtered and further washed with deionized water for sev-
eral times. After being dried under vacuum at 100 °C for 12 h, 4.7
wt% Au@UiO-66 catalyst was prepared.
In a typical procedure of D-C method, the solution of HAuCl4 (1
mL, 0.01 g/mL) was charged into a beaker where the pH value was
adjusted to 9 with 0.1 M NaOH under vigorous agitation. Then,
100 mg of UiO-66 was added to the above solution under the stir-
ring, and the pH value of the suspension was re-adjusted to 9 using
0.1 M NaOH. Then, the as-obtained suspension was heated to 65 °C
and kept for 1 h. After the precipitate was filtered, the solid was
washed with deionized water for three times and dried at 60 °C
overnight, the resulting Au@UiO-66 precursor was calcined at
300 °C for 3 h to prepare the final catalyst.
The procedure of D-H method was similar with D-C except that
the as-prepared catalyst precursor was further treated in a fixed-
bed stainless steel reactor with an inner diameter of 6 mm under
H2 with a total flow rate of 50 mL minꢁ1 and maintained at 250
°C for 2 h to obtain Au@UiO-66 catalyst. Herein, the reduction tem-
perature of Au particles has been confirmed by the H2-TPR result of
catalyst precursor (shown in Fig. S3 of supporting information).
In a typical procedure of C-I method, 10 mg HAuCl4 was dis-
solved in 100 mL distilled water, and 2 mL 0.3 wt% PVP aqueous
solution was added (PVP/Au = 1.2/1.0 in weight ratio). After stir-
ring at room temperature for 1.0 h, 2.5 mL 0.1 moL/L NaBH4 aque-
ous solution was promptly added into the mixture, and Au(III)
species were immediately reduced to metallic Au species, and Au
colloids were formed (0.24 mmol/L). After further being stirred at
room temperature for 1.0 h, 0.1 g UiO-66 was added into the Au
colloid solutions. The suspension was further stirred overnight at
2.4. Catalytic reaction
Catalytic experiments were carried out in a 120 mL autoclave
equipped with a magnetic stirrer and automatic temperature con-
trol. After the FUR, Au catalyst, K2CO3 and the alcohol were added,
the reactor was sealed, and purged with pure O2 for three times to
remove the air. Then, the pressure of oxygen was charged to 0.3
MPa and the reaction mixture was heated to 140 °C and kept for
4 h. When the reaction was finished, the solution was diluted with
acetonitrile after the reactor was cooled to room temperature. The
products were qualitatively detected by an Agilent 7890A/5975C
gas chromatography-mass spectrometry (GC–MS). The conversion
of FUR and selectivity of the product were quantitatively obtained
by GC instrument with the FID detector.
2.5. Recycling test
After first cycle was completed, the catalyst was filtered
through the high speed centrifugation (8000 r/min, 5 min), and
washed with ethanol (3 ꢂ 30 mL) and then dried at 80 °C in the
vacuum oven overnight, and then was directly used for the next
catalytic cycle. After being recycled five times, the catalyst was
respectively characterized by XRD, HRSEM, TEM and other
techniques.
3. Results and discussion
3.1. Oxidative esterification of FUR with methanol in the presence of
molecular oxygen
Firstly, the catalytic performances of Au@UiO-66 catalysts with
different preparation methods were investigated in the ‘‘FUR-
methanol-O2” system (Scheme 2). In order to find a suitable load-
Ning, Liangmin
