W. Wang et al.
MolecularCatalysis453(2018)113–120
supports for catalyst. The UiO-66-NH2 has the uncoordinated amino
groups which can be employed to anchor the vanadium oxyacetylace-
tonate by the post-grafted method. The not fully coordinated Zr ion in
the MOF can act as vacant site to provide the lewis acid [30]. The lewis
acid can promote the activation of CeH bond and suppress the further
oxidation of phenol to improve the selectivity [31–33]. In addition, the
aromatic ligands of the Zr-MOF favor the adsorption of the substrate
benzene by the π-π stacking interaction [34]. Therefore, it would be
ideal to develop a V-derived MOF material to serve as an efficient
benzene aerobic hydroxylation catalyst.
Scheme 1. Schematic illustration for the preparation of V/UiO-66-NH2 and V/
ZrO2.
Characterization
Herein, we reported an efficient catalyst of V/UiO-66-NH2 for direct
hydroxylation of benzene to phenol using molecular oxygen. V/UiO-66-
NH2 was prepared by the vanadium oxyacetylacetonate grafted on UiO-
66-NH2. The synergetic effect of the UiO-66-NH2 and the vanadium
complex contributed to the excellent catalytic performance, resulting in
the yield of 22.0% for phenol. The recyclable V/ZrO2 catalyst obtained
by the calcination of V/UiO-66-NH2 under the nitrogen gave the yield
of 16.2% for phenol. This is the first demonstration that V grafted onto
MOF catalyst can be used for direct hydroxylation of benzene to phenol
using O2 and gave the high yield of phenol with high selectivity.
The X-Ray Diffraction (XRD) patterns were obtained by an X-ray
diffractometer (Rigaku IV) operated with Cu-Kα radiation at 40 kV and
40 mA, scanning mode of 2 theta/theta, scanning type of continuous
scanning, and a scanning range from 3° to 90° at a scanning rate of 8°/
min. The X-ray photoelectron spectrometer (XPS) was carried out at
15 kV and 8 mA, with the binding energies calibrated at 284.8 eV from
C1s of the adventitious carbon. Asymmetrical XPS peaks were decon-
voluted by using the curve-fitting approach with CasaXPS software, as
well as by applying Shirley background subtraction and
Lorentzian–Gaussian functions (30% L, 70% G). Transmission electron
microscope (TEM, FEI Tecnai F20) operated at 200 kV. Fourier trans-
form infrared spectroscopy (FT-IR) was performed on a VECTOR-22
using the KBr pellet technique with a resolution of 2 cm−1 from 400 to
4000 cm−1 and consisted of 32 scans at room temperature. BET surface
area of the catalyst was conducted by the adsorption of N2 at −196 °C
(Gemini VII 2.00, Micromeritics Instrument Corporation).
Experimental section
Materials
Benzene, acetic acid, ethanol, dichloromethane, and DMF (dimethyl
formamide) were purchased from Tianjin Kemiou Chemical Reagent
Co. Ltd. The aminobenzoic acid, ZrCl4 (zirconiumy tetrachloride), VO
(acac)2 (vanadium oxyacetylacetonate) were obtained from Shanghai
Macklin Biochemical Co. Ltd. All the reagents and solvents were used as
received without further purification.
Catalytic reaction
The hydroxylation of benzene to phenol over the catalysts was
carried out as follow. Typically, 0.05 g catalyst, 1.0 mL benzene
(11.25 mmol), and 3.0 mL 70 vol.% acetic acid aqueous solution were
added into the reactor. The autoclave was flushed with 3.0 MPa O2 at
the room temperature. The mixture was stirred at 60 °C for 21 h. After
reaction, toluene was added into the product mixture as an internal
standard. The catalyst was separated by centrifugation and obtained
liquid. The liquid was analyzed by gas chromatography (GC,
Agilent7890), equipped with a HP-5 capillary column. Phenol was de-
tected as the product. The byproducts such as catechol, hydroquinone,
and benzoquinone were undetectable. The mass balance was de-
termined as about 97%–102%. The conversion of benzene and yield of
phenol were calculated by standard samples. The terms of reaction
performance were defined as follows:
Preparation of the UiO-66-NH2
The synthesis of nanoscale UiO-66-NH2 was achieved by using
modified literature procedures [35–37]. Typically, a solution of ZrCl4
(0.39 g, 1.7 mmol) in DMF (75 mL) was reacted with acetic acid
(2.85 mL) at 55 °C. A DMF solution (25 mL) of 2-aminoterephtalic acid
(0.307 g, 1.7 mmol) was added to the clear solution. Then, water
(0.125 mL, 0.007 mmol) was added to the solution and the solution was
sonicated at 60 °C and kept in bath at 120 °C for 24 h. After that, the
solution was cooled to room temperature and the precipitate was col-
lected by centrifugation. The obtained solid was wash with DMF
(2 × 10 mL) and ethanol (3 × 10 mL), and dried under reduced pres-
sure (80 °C and 3 h).
Conversion of benzene = [1 − (mole of benzene residue)/(initial mole
of benzene)] × 100%
Preparation of the catalysts
Yield of phenol = (mole of phenol produced)/(initial mole of ben-
zene) × 100%
The V/UiO-66-NH2 catalyst was obtained by the post grafted
method. Typically, the 5% loading capacity of the catalyst was prepared
as follow. 0.148 g UiO-66-NH2 was mixed uniformly in 20 mL CH2Cl2
(dichloromethane), and then the above mixture was stirred 30 min at
the room temperature. VO(acac)2 (0.053 g, 0.153 mmol) dissolved in
CH2Cl2 was added in the above mixture. The mixture was stirred at
room temperature for 18 h. Subsequently, it was filtered and then wa-
shed by ethanol. The V/UiO-66-NH2-5 catalyst was obtained after it was
dried in oven 90 °C for overnight. The different loading capacity cata-
lysts were prepared by the similar method, such as the V/UiO-66-NH2-
11 for 11% loading, and the V/UiO-66-NH2-22 for 22% loading.
V/ZrO2 was prepared by calcination of V/UiO-66-NH2. Typically,
the solid of V/UiO-66-NH2 sample was transferred into a tube furnace
and calcined in N2 at different temperature for 4 h with a heating rate of
3 °C/min to obtain the V/ZrO2-T catalyst, in which the T represents the
calcination temperature, for example V/ZrO2-500 for the temperature
of 500 °C. The preparation of the V/UiO-66-NH2 and V/ZrO2 was shown
Selectivity of benzene = (yield of phenol)/(conversion of ben-
zene) × 100%
Catalyst recycling
The reusability of the V/ZrO2-500 catalyst for the benzene hydro-
xylation reaction was tested under the optimized reaction condition. At
the end of one hydroxylation run, the catalyst was separated by cen-
trifugation, washed with methanol, and dried at 80 °C. Then, the cat-
alyst sample was used in the next run under the same reaction condi-
tions.
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