G Model
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Y. Liang et al. / Chinese Chemical Letters xxx (2019) xxx–xxx
a modulation approach to create linker vacancies within UiO-66 in
the presence of trifluoroacetic acid and HCl during the synthesis,
which significantly promote the conversion of citronellal to 3-
methyl-1,3-butanediol compared with UiO-66 synthesized with-
out the modulator [15]. Feng et al. [19] employed a hemilabile
linker (4-sulfonatobenzoate) to synthesize a hemilabile UiO-66,
the defects of which could be created through a postsynthetic
treatment with H2SO4. Liang F. et al. [20] promoted the formation
of ultrafine metal oxide nanoparticles by thermal decomposition of
the linker, ultrasmall metal oxide nanoparticles immobilized in an
open framework that exhibits high catalytic activity for Lewis acid-
catalyzed reactions. Nonetheless, the postsynthetic treatment is
not achievable for directly producing defects within pristine UiO-
66 by consideration of the robust stability even within acidic
solutions. Now, we develop a microwave-assisted acid-induced
post-treated method for formation of linker vacancies within Zr-
based metal organic frameworks. The physical characterization
proves that the MOF materials with abundant unsaturated metal
active sites can be obtained by this post-treatment method.
Therefore, these MOF materials have better catalytic performance.
The following is the synthesis method of UiO-66 and MOF-808.
citronellal to Zr is 15:1. After the reaction mixture is introduced,
the flask is connected with the reflux condensing device, placed in
an oil bath at 110 ꢀC and stirred. Every certain time, the reaction
liquid of about 0.2 mL was taken and filtered through a 200 nm
filter to obtain the samples to be tested and analyzed by gas
chromatography (Shandong Lunan SP-7890, FID).
Motivated by the principle of microwave interaction, we
expected that strong coordination bonds between Zr and O could
be broken upon the strong oscillation of acidic polar molecules
(such as HCl) generated with microwave. Herein, we demonstrated
that the microwave irradiation could promote effectively the acid-
induced formation of linker vacancies within UiO-66. The number
of linker vacancies can be regulated through changing the
irradiation time and the concentration of HCl solution. It is found
that increasing the irradiation time and the concentration of HCl
solution can increase the number of linker defects, and the optimal
condition is 0.5 mol/L HCl solution for 1 h under microwave
irradiation, where defective UiO-66 displays the largest specific
surface area with desirable linker vacancies (number of linker
vacancies is 5.0 vs.1.8 for pristine UiO-66). The optimized defective
UiO-66 contains more exposed unsaturated Zr4+ active sites, which
can serve as an effective Lewis acid for cyclization of citronella to 3-
methyl-1,3-butanediol (Fig. 1). This defective UiO-66 show
significantly improved catalytic activity with a conversion of
68.1% compared with that of 5.4% for pristine UiO-66. Whatever,
the microwave-assisted acid induction strategy represents an
effective method to create rich linker vacancies to expose more
Lewis acid active sites for heterogeneous catalysis.
We selected UiO-66 as an initial model to create defective Zr-
based MOFs with linker vacancies. Defective UiO-66 was synthe-
sized with a two-step procedure. UiO-66 was first prepared with a
typical solvothermal reaction (hereafter abbreviated as UiO-66ST),
which was subsequently treated with an HCl aqueous solution
under the microwave irradiation at 100 ꢀC to produce defective
UiO-66 (noted as UiO-66MW-mMnh, where M and h represent the
concentration of the HCl aqueous solution and the irradiation time,
respectively). The number of defects (linker vacancies) was
controllable through regulation of the concentration of the HCl
aqueous solution and the irradiation time. The morphology of as-
prepared samples was characterized with scanning electron
microscopy (SEM). As shown in Figs. 2 a and b, UiO-66ST shows
a regular octahedron shape with an average diameter of around
240 nm. After the microwave irradiation, no obvious change of
particle size was observed except that the particles turned to a
truncated octahedron shape with a rough surface, indicative of the
etching phenomenon. The crystal structures of UiO-66MW-0.5M1h
and UiO-66ST were determined with powder X-ray diffraction
(PXRD). Figs. 2c and d shows that both UiO-66MW-0.5M1h and UiO-
66ST have the same crystal system and cell parameters without any
impurity, which match very well with the simulated PXRD pattern
of a pure crystalline UiO-66 built-up in a cubic unit-cell with a
Fm3m space group [11]. It is also mentioned that varying the
concentration of the HCl aqueous solution and the irradiation time
did not change the crystal structures of samples with slight
decrease of the intensity possibly because of the increase of the
defects (Fig. S1 in Supporting information).
Firstly, the synthesis of UiO-66 is 207 mg ZrOCl2 8H2O was
Á
weighed and dissolved in 20 mL N,N-dimethylformamide (DMF),
then 5 cm3 acetic acid was added and ultrasonic treatment was
conducted until the solution was completely dissolved. Then weigh
120 mg of terephthalic acid into the mixture and conduct
ultrasound for 20 min until it is completely dissolved. The mixture
was transferred into a bottle with bag cover of 30 cm3 and reacted
in the oven at 120 ꢀC for 24 h. After it was naturally cooled to room
temperature, the white sample was obtained by centrifugation.
After being washed in DMF for three times, the mixture was soaked
in acetone for three times, each time for 12 h. After vacuum drying
at 60 ꢀC for 12 h, white UiO-66 powder was obtained. Vacuum
drying at 200 ꢀC for 24 h removed the adsorbed high boiling
solvent molecules from the sample.
Then, MOF-808 was synthesized according to existing reports.
1.042 g H3BTC and 1.94 g ZrOCl2 8H2O were dissolved in the
Á
mixture of formic acid/DMF (45 cm3/45 cm3). After the mixture
was completely dissolved and mixed evenly, the mixture was
divided into two sealed reaction reactors of 100 cm3 and reacted in
the oven at 130 ꢀC for 48 h. After cooling to room temperature,
wash with DMF three times, soak in acetone, and replace acetone
once for 12 h, a total of three times. After drying at 60 ꢀC for 12 h,
the white powder was dried again at 200 ꢀC for 12 h.
Next, the process of microwave acid treatment of MOF was HCl
solutions of different concentrations were prepared, which were
0.01, 0.1, 0.5, 1 mol/L, respectively. Weigh 100 mg of UiO-66 sample
powder and pour it into the microwave tube. Then measure 20 mL
0.5 mol/L hydrochloric acid solution into the microwave tube and
stir magnetically for 10 min until the UiO-66 powder is completely
dispersed in the hydrochloric acid solution. Finally, the closed
microwave tube was put into the reactor, and the reaction
conditions were set as 100 W, 100 ꢀC, and the reaction time was
from 30 min to 3 h. After cooling, the solution was centrifuged,
washed with ultra-pure water, and ultrasonic. The operation was
repeated for several times until the solution became neutral.
Vacuum drying at 60 ꢀC for 12 h, and then again drying at 200 ꢀC
and 12 h for activation. Similarly, MOF-808 was treated with a
similar method. hydrochloric acid solution of 0.1 mol/L was
selected and microwaved for 30 min, 1 h and 2 h respectively to
obtain samples with different treatment times. The microwave
reactor used in this experiment is Discover SP (CEM, U. S. A.).
Finally, the catalytic performance of different materials were
tested by cyclization of citronella aldehyde. A solution of toluene
containing citronellal was added to a three-mouth flask containing
10 mL glass containing MOF. For each catalyst, the ratio of
Fig. 1. Conversion of citronellal to 3-methyl-1,3-butanediol.
Please cite this article in press as: Y. Liang, et al., Microwave-assisted acid-induced formation of linker vacancies within Zr-based metal organic
Liang, Yu
