Heterocyclic reaction inducted by Br?nsted–Lewis dual acidic Hf-MOF under microwave irradiation

Article abstract of DOI:10.1016/j.mcat.2020.111291

Use of green chemistry and alternative strategies has been explored to prepare diverse organic derivatives. The combination between heterogeneous catalyst, environmentally benign reaction and high-yielding methods is gaining momentum. Herein, a defective 6-connected Hf-MOF, named Hf-BTC, was efficiently synthesized and characterized for the heterogeneous catalysis under microwave irradiation. The MOF features including structural defect, porosity, acidity, and stability was analyzed by powder X-ray diffraction, N₂ sorption isotherms, acid-base titration, and thermal gravimetric analysis. In the catalytic studies, the Br?nsted-Lewis dual acidic Hf-BTC was efficiently applied for the synthesis of the heterocyclic compounds via the microwave-assisted cycloaddition and condensation reactions. The reactions proceeded smoothly in the presence of the Hf-MOF with a broad scope of substrates provided the expected products in high to excellent yields (up to 99 %) for few minutes and the catalyst could be easily recycle over many consecutive reactions without loss of its reactivity and structure.

Full text of DOI:10.1016/j.mcat.2020.111291

Molecular Catalysis xxx (xxxx) xxx
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Heterocyclic reaction inducted by Brønsted–Lewis dual acidic Hf-MOF
under microwave irradiation
,
b
,
,
,
Linh Ho Thuy Nguyena c, Trang Thi Thu Nguyena c, Minh-Huy Dinh Danga c, Phuong
,
,c,
Hoang Tranb c,*, Tan Le Hoang Doana
*
a Center for Innovative Materials and Architectures (INOMAR), VNU-HCM, Ho Chi Minh City, 721337, Viet Nam
b Faculty of Chemistry, University of Science, VNU-HCM, Ho Chi Minh City, 721337, Viet Nam
c Vietnam National University – Ho Chi Minh City (VNU-HCM), Ho Chi Minh City, 721337, Viet Nam
A R T I C L E I N F O
A B S T R A C T
Keywords:
Use of green chemistry and alternative strategies has been explored to prepare diverse organic derivatives. The
combination between heterogeneous catalyst, environmentally benign reaction and high-yielding methods is
gaining momentum. Herein, a defective 6-connected Hf-MOF, named Hf-BTC, was efficiently synthesized and
characterized for the heterogeneous catalysis under microwave irradiation. The MOF features including struc-
tural defect, porosity, acidity, and stability was analyzed by powder X-ray diffraction, N2 sorption isotherms,
acid-base titration, and thermal gravimetric analysis. In the catalytic studies, the Brønsted-Lewis dual acidic Hf-
BTC was efficiently applied for the synthesis of the heterocyclic compounds via the microwave-assisted cyclo-
addition and condensation reactions. The reactions proceeded smoothly in the presence of the Hf-MOF with a
broad scope of substrates provided the expected products in high to excellent yields (up to 99 %) for few minutes
and the catalyst could be easily recycle over many consecutive reactions without loss of its reactivity and
structure.
Metal–organic framework
Brønsted–Lewis dual acidic catalyst
Microwave irradiation
Cycloaddition reaction
Condensation reaction
Introduction
well as in pharmaceutical and agrochemical chemistry [14]. The
convenient and economic way to synthesize these heterocyclic classes by
Aromatic heterocyclic rings containing nitrogen, including benzox-
azole, benzimidazole, benzothiazole, quinazolinone, and their de-
rivatives are an important class of heterocyclic compounds due to
biological and pharmaceutical activities [1]. These compounds are also
valuable precursors for the synthesis of various commercial drugs [2–4].
Traditionally, these compounds could be prepared via the reactions of
2-aminophenol, o-phenylenediamine, 2-aminothiophenol and anthra-
nilamide with arylating reagents, for examples aldehydes, ketone, or
carboxylic acid derivatives in the presence of acidic catalysts [5–8].
Although the synthesis could be effective in the catalysis of Brønsted or
Lewis acids, such as ionic liquids [9], zeolites [10], resin [11], and
halogen salts [12], the several drawbacks including large amounts of the
catalysts, difficulty in recycling the catalysts, toxic and corrosive re-
actants, and a large amount of wastes from organic solvents and addi-
tives need to be overcome [13]. Besides that, the development of new
methods for the synthesis of nitrogen-containing heterocycles is still a
focus of intense and containing the interest in the organic chemistry, as
using microwave irradiation would be beneficial for improved product
yields and reduction of reaction time as compared to those observed for
the synthesis under conventional heating [15–18].
Metal-organic frameworks (MOFs), constructed by metal-connecting
nodes and polydentate bridging linker, have attracted much attention of
the scientific community for the past decades [19]. With many
outstanding features such as large pores, thermal and chemical stability,
and ease of tailoring the original material structure as well as
post-synthesis on MOF materials, MOFs can be used for many applica-
tions such as adsorption and gas separation [20,21] catalysis [22–31],
sensors [32–35], nanocarriers [36,37], electrochemical [33,38], and
environmental treatment [39–41]. The water-stable MOFs, for examples
Zr- and Hf- based MOFs, with structural stability at high temperature
and in water at a wide range of pH have been good candidates for
catalyst [23,42,43]. Moreover, these MOFs consist of Zr6-octahedra
capped with by μ3-O and μ3ꢀ OH groups which can attribute as the
Brønsted acid catalytic sites in their structure [14,44,45]. Among them,
* Corresponding authors.
E-mail addresses: thphuong@hcmus.edu.vn (P.H. Tran), dlhtan@inomar.edu.vn (T.L.H. Doan).
https://doi.org/10.1016/j.mcat.2020.111291
Received 1 October 2020; Received in revised form 23 October 2020; Accepted 24 October 2020
2468-8231/© 2020 Elsevier B.V. All rights reserved.
Please cite this article as: Linh Ho Thuy Nguyen, Molecular Catalysis, https://doi.org/10.1016/j.mcat.2020.111291

L.H.T. Nguyen et al.
Molecular Catalysis xxx (xxxx) xxx
Fig. 1. a) PXRD patterns of Hf-BTC, simulated and activated. b) N2 isotherms of Hf-BTC (The filled and opened symbols represent the adsorption and desorption
processes respectively).
Hf-BTC is
a
defect MOF, constructed from Hf6(μ3-O)4
by centrifugation. The sample was washed with fresh DMF (3 × 20 mL)
for 3 days and dispersed in deionized water (3 × 20 mL). Then, the white
solid was immersed in 20 mL of anhydrous acetone for once times,
replaced nine successive times in 3 days. Finally, the materials were
dried under reduced pressure at room temperature for 24 h and acti-
vated at 150 ◦C for 24 h.
(
μ3ꢀ OH)4-(HCOO)6 clusters and tricarboxylate linkers (BTC3ꢀ ), which
have OH groups and formic acid on Hf cluster to play an active sites for
catalysis [17,46–49].
This work deals with the synthesis of 2-substituted benzoxazoles,
benzimidazoles, benzothiazoles, quinazolinone derivatives through the
reaction of o-amino aromatics coupled with different arylating reagent
employing a Hf-MOF, named Hf-BYC, as an efficient Brønsted and Lewis
dual acidic catalyst under microwave irradiation and solvent-free con-
ditions. The MOF could be recycled and reused several times without
significant decrease operation of catalyst.
Evaluation of brønsted acidity
The Brønsted acidity of material is defined by potentiometric acid-
base titrations protocol [52]. 50 mg of each MOFs were immersed in
60 mL of 0.01 M aqueous NaNO3 in an Erlenmeyer flask with the stopper
and allowed to equilibrate in 18 h. Then, 15 mL were withdrawn from
each solution and added to each 25 mL titration flask. Using 0.1 M
aqueous HCl to adjust the pH of the solution to 3 and then titrating with
0.1 M aqueous NaOH until the pH reach approximately 10. Titration
curves were repeated for three times to get an average value.
Experimental
Materials
All materials and reagents were purchased from Merck and Acros
Company and used without further purification. UiO-66 (Zr, Hf), Zr-BTC
were prepared based on previously reported method [47,50].
Typical procedure for the cycloaddition reaction
General methods
A mixture of Hf-BTC (1 mol %), anthramide (0,5 mmol) and ketone
(0.5 mmol) was heated under microwave irradiation in a CEM Discover
apparatus monitored by TLC. After completion of the reaction, the
catalyst was filtered from the reaction mixture and extract with ethanol
(50 mL). The solvent was removed on a rotary evaporator and the crude
product was purity by flash chromatography (90:10 acetone/petroleum
ether to give a corresponding product. The purity and identity of the
products were confirmed by GC–MS spectra, which were compared with
the spectra in the NIST library, and by 1H and 13C NMR spectroscopy.
Gas chromatography-mass spectrometry measurements were carried
out on an Agilent GC System 7890 equipped with a mass selective de-
tector (Agilent 5973 N) and a capillary DB-5MS column (30 m × 250 μm
× 0.25 μm). Analytical thin-layer chromatography (TLC) was performed
on F-254 silica gel coated aluminum plates from Merck. 1H and 13C
nuclear magnetic resonance (NMR) spectra were recorded on a Bruker
Advance II 500 MHz NMR spectrometer. Powder X-ray diffraction
(PXRD) patterns were recorded using a D8 Advance diffractometer
equipped with a LYNXEYE detector (Bragg–Brentano geometry, Cu K
α
Typical procedure for the condensation reaction
radiation λ = 1.54056 Å). Fourier Transform infrared (FT-IR) spectra
were recorded from KBr pellets using a Bruker Vertex 70 system. Ther-
mal gravimetric analysis (TGA) was performed on a TA Q500 thermal
analysis system with the sample held in a platinum pan in a continuous
airflow. Microwave irradiation was performed on a CEM Discover
BenchMate apparatus.
A mixture of Hf-BTC (1 mol %), benzoyl chloride (0.140 g, 1 mmol)
and an 2-aminophenol (0.119 g, 1 mmol) was heated under microwave
irradiation at 120 ◦C for 15 min in a CEM Discover apparatus monitored
by TLC. After completion of the reaction, the catalyst was filtered from
the reaction mixture. The filtrate was diluted with ethyl acetate (50 mL),
washed with H2O (3 × 20 mL), aqueous NaHCO3 (2 × 20 mL), and brine
(20 mL), and dried over Na2SO4. The solvent was removed on a rotary
evaporator. The crude product was purified by flash chromatography
(90:10 acetone/petroleum ether to give a corresponding product. The
purity and identity of the products were confirmed by GC–MS spectra,
Synthesis and preparation of MOF
Synthesis of Zr-BTC and Hf-BTC were prepared according to methods
reported in the literature [51]. An equimolar solution (8.5 mM) of HfCl4
salt (or ZrCl4 salt) and the tricarboxylic acid H3BTC in 320 mL solvent
mixture of DMF and formic acid (v/v = 1:1) in 500-mL capped bottle
was heated in an oven at 120 ◦C for 3 days under static conditions. After
cooling the bottle to room temperature, the precipitates were collected
which were compared with the spectra in the NIST library, and by 1
H
and 13C NMR spectroscopy. They were obtained by the same microwave
irradiation procedure as that for preparation of 2-aryl substituted
benzimidazole and 2-aryl substituted benzothiazole using 1,2-
2

L.H.T. Nguyen et al.
Molecular Catalysis xxx (xxxx) xxx
Fig. 2. Acid–base titration curve of Hf-BTC (blue) and first derivative (red). b) TGA curve of activated Hf-BTC. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article).
phenylenediamine (0.108 g, 1 mmol) or 2-aminothiophenol (0.125 g, 1
Table 1
mmol) instead of 2-aminophenol.
Study of the effect of various catalysts in the solvent-free synthesis of 2,2-
diphenyl-2,3-dihydroquinazolinone-4(1H)-one.
Entry
Result and discussion
MOF
Types of active
sites
Average pore
GC yield
(%)
TOF
(hꢀ 1
catalyst
size (Å)
)
Catalysts characterization
1
2
None
–
0
0
UiO-66
(Zr)
12-connected
Zr6
10.5 [54]
10.5
14
36
1.08
1.95
5.10
5.76
The Hf-BTC was synthesized from tricarboxylic acid (H3BTC) and
hafnium tetrachloride according to method reported in the literature
with the high yield after a simple the purification step [51]. The phase
purity of synthesized Hf-BTC was further confirmed by PXRD, TGA, and
N2 isothermal. Powder X-ray diffraction (PXRD) patterns between
experimental and the simulated pattern has been recognized in Fig. 1a to
show the match well with reported literature [51]. The characteristic
peaks of Hf-BTC at 2θ values of 8.3, 8.7, 10.1, 11.0 and 13.1 corre-
sponding to (311), (222), (400), (331), and (511) index planes respec-
tively. N2 isotherm of Hf-BTC at low relative pressures and 77 K
exhibited type1with Brunauer–Emmett–Teller of 1540 m2/g, a pore
volume of 0.13 cm3/g and an average pore diameter of about 14 Å.
(Fig. 1b)
3
4
5
UiO-66
(Hf)
12-connected
Hf6
65
85
96
Zr-BTC
6-connected
Zr6
Hf-BTC
6-connected
Hf6
14
Reaction conditions: anthranilamide (0.5 mmol), benzophenone (0.5 mmol) and
the catalyst (0.2 mmol %) were stirred under microwave irradiation without
using solvent at 160 ◦C for 50 min. TOF (turnover frequency) was moles of
product formed per mol of catalyst per hour.
6HfO2, attributed to formate groups, was lower than the calculated form
the crystal structure (1.40 and 1.45 wt % over 6HfO2 for the experi-
mental and theoretical, respectively) [45,52]. In the second step above
450 ◦C, the weight loss over HfO2, corresponded to the complete
decomposition of the linkers, decreased lower than the expected value
for the crystal structure (1.24 and 1.28, respectively). It is note that TGA
supported for identification the mixing linker and defect sites in the
structure.
To quantify the defects in Hf-BTC, potentiometric acid-base titration
method was performed for Hf-BTC and Zr-BTC as a verification material.
Accordingly, the pKa value at 3.67 was recorded by acid-base titration of
activated Zr-BTC at distinct equivalent point (5.8) (Fig. S3, Section S2,
SI), corresponding with appear representative of MOF with Zr nodes
with μ3ꢀ OH proton in the previous report [52]. For Hf-BTC, the pKa
value was 3.48 at equivalent calculated point (5.9) (Fig. 2a). Comparing
with Zr-BTC, the more flexibility of proton of Hf-μ3ꢀ OH than Zr-μ3ꢀ OH
is understood since the coordination between Hf and oxygen is stronger
than Zr and oxygen, leading to pKa of Hf-BTC is lower than Zr-BTC and
Brønsted acidic of Hf-BTC is higher than Zr-BTC. These results opened up
the potential of Hf-BTC for catalytic activity in diverse organic reactions.
TGA measurements were carried out for the Hf-BTC sample under air
flow and TGA curve was built per 100 % of the residual hafnium oxide
(Fig. 2b). Fig. 2b shows two distinct weight loss steps, corresponding to
the presence of defect sites (HCOOꢀ ) and the decomposition of BTC3-
linkers. In the first step (200ꢀ 300 ◦C), a weight percentage loss over
Catalytic performance
Synthesis of cycloaddition reaction
In our previous study, the synthesis 2,3-dihydroquinazolin-4(1H)-
ones in the presence of UiO-66 catalyst under traditional heating
required the reaction time prolonged to 8 h to get the desired product
[53]. According to the structural characterization of Hf-BTC, our
expectation is using Hf-BTC which higher Brønsted acidity in catalyst
and microwave irradiation method for the preparation of this reaction to
scale down the reaction time and enhance the yield of the products.
Scheme 1. The cyclization reaction between anthranilamide and benzophenone.
3

L.H.T. Nguyen et al.
Molecular Catalysis xxx (xxxx) xxx
Scheme 2. Catalytic application of Hf-BTC for the preparation of quinazolinone derivatives.
Table 2
GC yields for reaction of the Hf-BTC catalyzed synthesis of various functionalization from the anthranilamide and ketone.
3a
3b
3c
3d
5 min, 50 ◦
C
40 min, 60 ◦
C
50 min, 80 ◦
C
50 min, 100 ◦
C
Yield: 95 % MP (oC): 185ꢀ 188
Yield: 96 % MP (oC): 223ꢀ 188
Yield: 83 % MP (oC): 167ꢀ 170
Yield: 91 % MP (oC): 165ꢀ 172
3e
3f
3g
3h
50 min, 50 ◦
Yield: 75 %
C
50 min, 100 ◦
Yield: 85 %
C
30 min, 100 ◦
Yield: 85 %
C
40 min, 80 ◦
Yield: 93 %
C
MP (oC): 223ꢀ 225
MP (oC): 165ꢀ 168
MP (oC): 195ꢀ 203
MP (oC): 201ꢀ 204
3i
3j
3k
3 L
50 min, 160 ◦
C
15 min, 60 ◦
C
40 min, 80 ◦
C
40 min, 60 ◦
C
Yield: 96 % MP (oC): 208ꢀ 211
Yield: 95 % MP (oC): 202ꢀ 205
Yield: 92 % MP (oC): 205ꢀ 207
Yield: 70 % MP (oC): 202ꢀ 205
With the goal of evaluating the catalytic behavior of MOF based on
the node connectivity, the model reaction between anthranilamide and
benzophenone in the catalysis of Zr-, Hf-UiO-66 (12-connected), and
Zr-/ Hf-BTC (6-connected) under microwave irradiation and solvent-
free at 160 ◦C for 50 min (Scheme 1) was applied to evaluate the cata-
lytic behavior. As shown in Table 1, a clear distinction was made among
the various catalysts in this reaction. The failure in the formation of this
product was observed in the absence of the catalyst (Table 1, entry 1).
The desired product was achieved with the performance when using Hf-
BTC as a heterogeneous catalyst (Table 1, entry 5) while much lower
efficiency was observed in the reaction catalyzed by other MOFs
(Table 1, entries 2–4). This mainly results from the acidic features of the
Hfꢀ OH/Hfꢀ OH2 facing the pores and the large pore of Hf-BTC which
made its catalytically active sites accelerate the cycloaddition reaction
[16,45].
Table 3
The comparison of the microwave irradiation method and solvent-free with
previous literatures in the synthesis of 2,3-dihydroquinazolin-4(1H)-one.
Entry
Catalyst
H2SO4
Conditions
Yield
(%)
Ref.
1
2
3
4
5
6
Few min
68ꢀ 78
[55,
50]
2-morpholinoethanesulfonic
acid (MES)
10 min, 60 ◦
C
95
[56,
51]
Co-CNTs
59 min, 100 W
50
[57,
52]
Amberlyst-15
Catalyst free
Hf-BTC
3 min, 130 ◦C,
360 W
78
[58,
53]
3 min, 60 ◦C,
300 W
91
[55,
50]
15 min, 60 ◦C,
80 W
85
This
work
We next proceeded to evaluate the scope and limitation of Hf-BTC in
the cyclization reaction with a variety of ketones and aldehydes (Scheme
2). The investigation results indicated that the best result obtained in
two cases ketone such as acetone and cyclohexanone could effort the
2,2-dimethyl-2,3-dihydroquinazolin-4(1H)-one and spiroquinazolin-4-
(1H)-one with excellent yields of 95 %. The effect of the cumbersome
structure of cyclohexanone required increasing temperature reaction to
60 ◦C for a quantitative transformation into expected the yield of
spiroquinazolin-4-(1H)-one. Similarly, the benzaldehyde has a certain
effect on the yields of the desired products. The resonance effect of
benzene on carbonyl group lead this carbonyl less reactivity in cyclo-
addition reaction.
We expanded the reaction scope over many ketones in the formation
of quinazolin-4(1H)-one. The summarized data in the Table 2 illustrated
4

L.H.T. Nguyen et al.
Molecular Catalysis xxx (xxxx) xxx
Scheme 3. Catalytic application of Hf-BTC for the preparation of 2-phenylbenzoxazole, 2-phenylbenzimidazole and 2-phenylbenzithiazole.
Table 4
GC yields for reaction of the Hf-BTC catalyzed synthesis of various functionali-
zation from the 2-aminophenol, 1,2-phenylenediamine, 2-aminothiophenol and
aromatic carbonyl chloride.
4b
4c
4d
Yied: 85 %
Yied: 51 %
Yied: 30 %
MP 103ꢀ 104 (oC)
MP 99ꢀ 100 (oC)
MP 156ꢀ 157 (oC)
5b
5c
5d
Yield: 90 %
Yield: 55 %
Yield: 62 %
MP 203ꢀ 204 (oC)
MP 253ꢀ 254 (oC)
MP 234ꢀ 236 (oC)
6b
6c
6d
Yield: 98 %
Yield: 98 %
Yield: 95 %
MP 123ꢀ 124 (oC)
MP 110ꢀ 111 (oC)
MP 206ꢀ 207 (oC)
Reaction conditions: 2-aminophenol, 1,2-phenylenediamine, or 2-aminothio-
phenol (1 mmol), aromatic carbonyl chloride (1 mmol) and the catalyst (1
mmol %) were stirred under microwave irradiation without using solvent at 120
◦C for 15 min.
Fig. 3. PXRD patterns of Hf-BTC before (blue) and after (red) 5 consecutive
condesation reactions in comparison to the simulated pattern (black). (For
interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article).
that all reactions performed under short time with high reaction yields.
Comparing with the traditional heating method, microwave method
exhibited as a powerful to enhance catalytic activity of Hf-BTC in the
reaction with high yield and short reaction times. Interestingly, the re-
action time under microwave irradiation for a few minutes could obtain
2,2-diphenyl-2,3-dihydroquinazolin-4(1H)-one whereas using the con-
ventional heating required hours for obtained the same yield of this
product.
obtained optimized set of reaction time, the promising results under
microwave irradiation and solvent free conditions, at 120 ◦C in the
presence of 1 mol % of Hf-BTC (Scheme 3). The increasing yield of
products in this model reaction were 98 %, 95 % and 85 % for 2-phenyl-
benzoxazole, 2-phenylbenzothiazole and 2-phenylbenzimidazole,
respectively. While 2-aminothiophenol was able to undergo the aryla-
tion with the same ease as 2-aminophenol, o-phenylenediamine trans-
formation maintained in the prolonging reaction time with up to 30 min.
We explored the reaction scope over many aromatic carbonyl chlo-
rides in the formation of 2-arylbenzoxazoles and other analogues
(Table 4). In general, a series of 2-substituted benzaldehyde smoothly
reacted with 2-aminophenol, 1,2-phenylenediamine, and 2-aminothio-
phenol provide 2-arylbenzoxazole, 2-arylbenzimidazole and 2-arylben-
zothiazole in excellent yields. As a common trend, the reactions
between 2-aminothiophenol and aromatic carbonyl derivatives
It is essential to compare our work with previous literature using
acidic catalysis. A comparative study of the microwave irradiation
method with previous literatures was reported in Table 3, efficiency of
Hf-BTC catalyzed condensation between anthranilamide and benzalde-
hyde afforded the 2,3-dihydroquinazolin-4(1H)-one product under of
lower temperature and power MWI heating.
Synthesis of condensation reaction
After investigating the catalytic efficiency of Hf-BTC in the cyclo-
addition reaction, the MOF was used as catalyst for solvent-free prepa-
ration of heterocyclic analogues including 2-arylbenzoxazole, 2-
Table 5
Reusability of Hf-BTC in the synthesis of 2-phenylbenzoxazole.
arylbenzimidazole, and 2-arylbenzothiazole derivaties through
a
Run
1
2
3
4
5
condensation reaction under microwave irradiation. The effect of
important parameters such as amount of catalyst, temperature and sol-
vent free conditions was examined for the model reaction. Under the
Isolated yield (%)
98
97
97
95
95
5

L.H.T. Nguyen et al.
Molecular Catalysis xxx (xxxx) xxx
Scheme 4. Proposed mechanism for the synthesis of 2-aryl benzoxazole, 2-aryl benzimidazole, 2-aryl benzothiazole using Hf-BTC as catalyst.
proceeded smoothly to give the corresponding products in excellent
yields. Intuitively, the electronic properties of substituents of benzal-
dehydes exhibited a little effect on the reaction. Electron-donating group
on benzoyl chloride, such as 4-methoxybenzoyl chloride, gave the ex-
pected products in excellent yield (85–98 %). Electron-withdrawing
groups on benzoyl chloride, such as 4-nitrobenzoyl chloride, was all
effectively reactive and provided the corresponding products in good
yields. Benzoyl chloride bearing halogen substituents could also react
smoothly to attain the expected products in high yields (51–98 %).
The recyclability of Hf-BTC was surveyed on the optimized
condensation reaction of a 2-aminophenol and benzoyl chloride. Upon
completion of the reaction, the recovered catalyst was easily separated
by centrifuging and washing many times with ethyl acetate. The catalyst
was collected by filtration, washed with acetone, reactivated under
vacuum at 120 ◦C for 24 h before the reuse for consecutive cycles. PXRD
patterns corresponding to the fresh and used Hf-BTC after 5 times were
observed without a changed structure (Fig. 3). Furthermore, no loss of
catalytic activity in the recycling test of this catalyst is the most prom-
inence of microwave method (Table 5).
Validation. Phuong Hoang Tran: Validation, Resources, Supervision,
Conceptualization, Methodology, Writing - review & editing, Visuali-
zation, Project administration. Tan Le Hoang Doan: Validation, Re-
sources, Supervision, Conceptualization, Methodology, Writing - review
& editing, Visualization, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgement
The work was supported by the Vietnam National University, Ho Chi
Minh City under grant number NCM2019-50-01.
Appendix A. Supplementary data
According to the literatures and our experiment results, we suggest a
mechanism for the formation of 2-arylbenzoxazole, in Scheme 4. The
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.111291.
μ
3ꢀ OH and format groups act both as Brønsted acid and Lewis acid to
protonate of the carbonyl oxygen from benzoyl chloride. The interme-
diate A was formed through the reaction of the amino group on the
substrate with activated carbonyl. Then the termination, cyclization and
dehydration take place to form the desired product.
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