X. Xia et al.
Applied Catalysis A, General 559 (2018) 138–145
addition, the presence of open metal sites and regular frameworks in
MIL-101 contributes to postfunctionalization via grafting of active
species [32]. The fully accessible porosity, the favorable structural
morphology, together with a high chemical and thermal stability make
MIL-101 an excellent potential for catalytic purposes.
and elemental mapping analysis were collected using a high-resolution
transmission electron microscope (HRTEM, TECNAI F20). Energy-dis-
persive X-ray (EDX) analysis was used to determine the chemical
composition of the samples. Thermogravimetric analysis (TGA) was
recorded on a Mettler-Toledo 1600HT thermoanalyzer under a nitrogen
atmosphere at a heating rate of 10 ℃ min . The concentration of alu-
minum was monitored by an atomic absorption spectrophotometer (Z-
2000, Hitachi, Japan).
-
1
In this paper, a novel strategy has been proposed for preparing MIL-
1
01(Cr/Al)A-f in a acid-free environment and a substitution process by
3
+
cation exchange was carried out to transform the Al
into MOFs. We
compared the catalytic properties of the prepared MIL-101(Cr/Al)A-f
with the MIL-101(Cr/Al)A-0.5 in hydroxyalkylation of phenol with for-
maldehyde. In addition, the effect of acidity on the morphology of the
framework during the preparation process was investigated. The MIL-
2.4. Catalytic activity tests
Hydroxyalkylation of phenol with formaldehyde to bisphenol F was
carried out in reactor with magnetic stirring. Typical experimental
processes were summarized as follows: 0.465 g (5.7 mmol) of for-
maldehyde solution, 8.12 g of phenol and 0.12 g of catalyst were added
into the reactor and the reaction mixture was heated to the desired
temperature. After a certain time interval, 0.03 g of the suspension was
taken out and then diluted with 10 mL of methanol. The composition of
the product was confirmed by HPLC with a Shimadzu LC-20AT system
connected with a SPD-20A UV/Vis detector and a Phenomenex Luna
C18 column (250 × 4.6 mm, 5 mm); A mixture of methanol and water
with 65:35 v/v was used as the mobile phase with a constant flow rate
of 0.6 mL/min. The injective volume of the sample was 100 μL.
The yield and selectivity of bisphenol F are calculated on the basis of
formaldehyde. The calculation equations are as follows:
101(Cr/Al)A-f exhibited a excellent catalytic activity due to its unique
morphology, which was supported by BET, SEM and TEM. Finally, a
possible mechanism for the synthesis of bisphenol F was proposed and
the hydroxyalkylation of phenol with formaldehyde was fitted by the
Langmuir-Hinshelwood kinetic model.
2. Experimental section
2.1. Chemicals
All chemicals were provided from commercially available resources
and were studied without further processing. Chromium nitrate non-
ahydrate (Cr(NO ·9H O), aluminum chloride hexahydrate, nitric acid,
3
)
3
2
terephthalic acid (BDC, 98%), N,N-dimethylformamide (DMF, 99%),
formaldehyde (37–40%) and phenol were obtained from Sinopharm
Chemical Reagent Co. Ltd., China.
Moles of bisphenol F formed
Expected moles of bisphenol F formed based on formaldehyde consumed
Yield(%) =
× 100%
Moles of bisphenol F formed
Moles of all the products
Selectivity(%) =
× 100%
∑
2.2. Synthesis of MIL-101(Cr/Al)A-X
The synthetic route for the formation of bisphenol F by hydro-
xyalkylation of phenol with formaldehyde is shown in Scheme 1.
Typical synthetic process was carried out via hydrothermal synth-
esis and described as follows: BDC (1.66 g, 10 mmol), Cr(NO
3
)
3
·9H
2
O
(
4.0 g, 10 mmol), deionized water (30 mL) and nitric acid from 0 to
3. Results and discussion
0.5 g were mixed at room temperature, and then sonicated for 30 min.
The mixture was transferred into a 50 mL Teflon-lined autoclave and
heated at 220 °C for 18 h. The suspension was centrifuged and the
precipitate was washed with DMF and ethanol. The obtained product
was dried at 150 °C for 12 h. 1 g of sample was refluxed in 50 mL of
3.1. Catalyst characterizations
The morphological evolution of MIL-101(Cr/Al)A-X was conducted
by scanning electron microscopy (SEM) and transmission electron mi-
croscopy (TEM). MIL-101(Cr/Al)A-0.5 (Fig. 1a and b) exhibits single
octahedron morphology with a size around 300 nm, while the flower-
like morphology of MIL-101(Cr/Al)A-0.3 begins to appear in the vicinity
of the octahedron morphology (Fig. S1a–c). With the acidity weak-
ening, more deprotonated carboxyl groups are generated to accelerate
coordination with metal clusters. The appearance of mixed octahedron/
flower-like morphologies may be attributed to the heterogeneous nu-
cleation in the patterns of MIL-101(Cr/Al)A-0.3. During the hetero-
geneous nucleation process, the increased deprotonated carboxyl
groups firstly form flower-like morphology and the remained deproto-
nated carboxyl groups with low concentration form octahedron mor-
phology. The particle size of octahedron morphology in MIL-101(Cr/
Al)A-0.3 (∼150 nm) is about half smaller than that of MIL-101(Cr/Al)A-
0.5 with single octahedron morphology, which may due to the presence
of flower-like morphology in their adjacent areas. In addition, the
particle size of original MIL-101(Cr)A-X is investigated. The particle size
of flower-like MIL-101(Cr)A-f is slightly larger than that of octahedron
MIL-101(Cr)A-0.5 (Fig. S2), which is consistent with the reference [33].
As shown in Table S1 and Fig. S3, the average particle size of MIL-
101(Cr)A-X is analysed using a GaussAmp model. With the decrease of
acidity, the number of the deprotonated carboxyl groups of terephthalic
acid increases, which speeds up the rate of coordination with metal
ions. The increase of the coordination rate makes the coordination tend
to irregular and outward growth, leading to larger particles. With the
decrease of acid from 0.3 to 0.1 g, the growth of flower-like morphology
is easier (Fig. S1d–f). When there is no nitric acid, we get a nearly
perfect flower-like morphology (Fig. 1c–e). The HRTEM images (Fig. 1f)
3
0.3 m/L AlCl aqueous solution at 70 °C for 24 h. Then solid was se-
parated by centrifugation and washed twice with ethanol. Finally, the
samples were vacuum-dried at 150 °C for 12 h, and labeled as MIL-
101(Cr/Al)A-X, where X represents the quality of nitric acid in the
preparation process (0.5 g, 0.3 g, 0.1 g and free).
2.3. Characterization techniques
The crystalline phases of the products were examined by X-ray
diffraction (Bruker D8 Advance diffractometer) with Cu Ka radiation
working at 40 kV and current 40 mA). Diffraction data were recorded
(
in the 2 h range from 1° to 8° and 10° to 80° (increment: 0.5°). The unit
cell parameters were calculated with a standard least squares refine-
ment technique. NH
3
-TPD analysis was performed with a Micromeritics
AutoChem II 2920 V3.05 instrument. Prior to analysis, the catalyst
(
100 mg) was enclosed in a quartz tube and treated at 300 ℃ under
−
1
helium flow of 30 mL min
for 1 h. The magic angle spinning (MAS)
NMR analysis was performed by a Bruker Avance-400 with a 5 mm
zirconia rotor and a spinning frequency of 11 kHz. Spectra for 27Al was
2
obtained. The porosity was analyzed by N adsorption at 77 K using an
ASAP 2010 sorption system. UV–vis diffuse reflectance spectra were
recorded at a Perkin–Elmer Lambda 900 spectrophotometer under
2
00–800 nm. The binding energies of C, O, Cr and Al of the composite
microspheres were detected on X-ray photoelectron spectroscopy
Thermo Fisher, USA) using an Al-KX-ray source. Scanning electron
(
microscopy (SEM) micrographs were obtained on a Hitachi S4800 mi-
croscope operated at 30 kV. Transmission electron microscopy (TEM)
139