A recyclable cucurbit[6]uril-supported silicotungstic acid catalyst used in the esterification reaction

Article abstract of DOI:10.1016/j.ica.2021.120418

The esterification reaction used to prepare butyl paraben (BP) and propyl gallate (PG) catalyzed using a cucurbit[6]uril-supported Keggin-type silicotungstic acid (Q[6]-STA) catalyst has been investigated. The Q[6]-STA catalyst has been characterized using FTIR spectroscopy, XRD, SEM, EDS, thermal analysis, and surface area studies. Q[6]-STA was easy to prepare in high yield and exhibited some advantageous properties, such as high catalytic activity and its convenient recovery. Moreover, a reusability study showed that the Q[6]-STA catalyst was stable and active.

Full text of DOI:10.1016/j.ica.2021.120418

Inorganica Chimica Acta 523 (2021) 120418
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
A recyclable cucurbit[6]uril-supported silicotungstic acid catalyst used in
the esterification reaction
Wen Xia, Yu-Mei Nie, Na Lei, Zhu Tao, Qiang-Jiang Zhu, Yun-Qian Zhang*
Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province, Guizhou University, Guiyang 550025, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The esterification reaction used to prepare butyl paraben (BP) and propyl gallate (PG) catalyzed using a cucurbit
[6]uril-supported Keggin-type silicotungstic acid (Q[6]-STA) catalyst has been investigated. The Q[6]-STA
catalyst has been characterized using FTIR spectroscopy, XRD, SEM, EDS, thermal analysis, and surface area
studies. Q[6]-STA was easy to prepare in high yield and exhibited some advantageous properties, such as high
catalytic activity and its convenient recovery. Moreover, a reusability study showed that the Q[6]-STA catalyst
was stable and active.
Cucurbit[6]uril
Silicotungstic acid
Recyclable catalyst
Esterification reaction
1. Introduction
molecules, respectively and characterized two novel hybrid porous
complexes [28]. Cao and co-workers prepared a series of hybrid Q[n]–
Heteropolyacids (HPAs), as a class of oxy acids, contain heteroatoms
(such as P, Si, Fe and Co) and other metal atoms (such as Mo, W, V, Nb
and Ta), which adopt certain structures based on their oxygen coordi-
nation [1]. HPAs with a Keggin-type structure are widely used as acid
catalysts due to their novel structural properties and very strong
Brønsted acidity [2]. Keggin-type HPAs, such as phosphomolybdic acid
(H3PMo12O40), silicotungstic acid (H4SiW12O40) and phosphotungstic
acid (H3PW12O40), are readily available and most frequently used as
acid catalysts [3]. However, the main disadvantages of using HPAs as
catalysts are their relatively low surface area (1–10 m2∙gꢀ 1) and prob-
lematic separation from the reaction mixture [4,5].
HPA complexes [29–33] and demonstrated the photocatalytic activity of
4ꢀ
6ꢀ
water-insoluble Q[6]–[SiW12O40
]
and Me10Q[5]–[P2W18O62
]
to-
ward the degradation of methyl orange under visible light irradiation
[34,35]. Ion-dipole interactions formed between the electrostatic po-
tential positive outer surface of Q[n]s and HPAs anions, namely OSIQ
[26,27], can adequately explain the formation of these water-insoluble
materials. These results motivated our group to investigate Q[n]-sup-
ported HPAs as acid catalysts for the esterification reaction. In our
previous work, tetramethyl cucurbit[6]uril-supported phosphomolybdic
acid (TMeQ[6]-PMA) and perhydroxylated cucurbit[6]uril-supported
phosphomolybdic acid [(HO)12Q[6]-PMA] catalysts have been exam-
ined in the esterification reaction and showed good catalytic activity
[36,37].
Appending HPAs on a suitable support is expected to circumvent the
above mentioned problems. As a result, many traditional acids have
been immobilized on SiO2, Al2O3, TiO2, active carbon, anion-exchange
resin [6,7], clay [3], ZrO2 [8] and Nb2O5 [9]. Cucurbit[n]urils (Q[n]s)
are a family of host molecules possessing a rigid hydrophobic cavity and
two identical carbonyl-fringed portals [10]. They have attracted a lot of
attention in supramolecular chemistry because of their superior mo-
lecular recognition properties in aqueous media [11–25]. In particular,
recently studies have revealed that the electrostatically positive outer
surface of Q[n]s can provide a driving force, known as the outer-surface
interaction of Q[n]s (OSIQ), which is capable of generating numerous
novel Q[n]-based supramolecular assemblies, including Q[n]-HPA-
Butyl paraben (BP) and propyl gallate (PG) are commercially avail-
able products obtained from an esterification reaction. Some mineral
acids, such as sulfuric and phosphoric acid, are conventionally used as
catalysts for industrial scale esterification reactions. Although these
mineral acid catalysts have many advantages, such as high catalytic
activity and selectivity, they also have some disadvantages, such as
being homogeneous reaction catalysts, difficult to recover, nonreusable,
difficult to separate from the desired product, corrosive and toxic
[38,39].
In this work, a cucurbit[6]uril-supported Keggin-type silicotungstic
acid (Q[6]-STA) catalyst has been prepared, characterized and applied
toward the synthesis of butyl paraben (BP) and propyl gallate (PG). The
¨
based porous structures [26,27]. Kogerler and co-workers first investi-
gated the interactions of [H2O@VIV18O42
]
with Q[6] and Q[8]
12ꢀ
* Corresponding author.
E-mail address: sci.yqzhang@gzu.edu.cn (Y.-Q. Zhang).
https://doi.org/10.1016/j.ica.2021.120418
Received 21 January 2021; Received in revised form 9 April 2021; Accepted 21 April 2021
Available online 23 April 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

W. Xia et al.
Inorganica Chimica Acta 523 (2021) 120418
structure of Q[6] and STA are shown in Fig. 1.
2. Experimental section
2.1. Materials
Reagent grade silicotungstic acid hydrate (H4SiW12O40⋅xH2O; STA),
p-hydroxybenzoic acid, gallic acid, butyl alcohol and propyl alcohol
were purchased from Shanghai Aladdin Biochem Technology Co. Ltd.
and used as received without further purification. Q[6] was synthesized
according to a literature procedure [40].
Fig. 1. Structure of cucurbit[6]uril (Q[6]) and silicotungstic acid (STA).
2.2. Preparation of the Q[6]-STA catalyst
The Q[6]-STA catalyst was prepared by simply mixing solutions
containing Q[6] and STA, respectively. A solution of STA (2.5 × 10ꢀ 5
mol∙Lꢀ 1) in H2O (10 mL) was added slowly to a solution of Q[6] (5.0 ×
10ꢀ 5 mol∙Lꢀ 1) in 3 M HCl (10 mL) with stirring. A precipitate imme-
diately appeared and after stirring for a further 2 h, the solvent was
removed under vacuum. The Q[6]-STA catalyst was dried at 353 K for 3
h to afford a yield of > 90% based on STA.
2.3. Characterization of the Q[6]-STA catalyst
The FTIR spectra of Q[6], STA and Q[6]-STA catalyst were measured
on a Bruker Vertex 70 FTIR spectrometer using KBr pellets. Powder X-
ray diffraction (XRD) analysis of Q[6]-STA was performed on a X’Pert-
Fig. 2. The outer-surface interaction of Q[n] with HPAs.
Pro MPD (CuKα radiation) and compared to mechanical mixtures of Q
Fig. 3. (a) FTIR spectra obtained for Q[6], STA and the Q[6]-STA catalyst. (b) XRD patterns obtained for a simple mixture of Q[6] and STA, and the Q[6]-STA
catalyst. (c) DTG and TG curves obtained for the Q[6]-STA catalyst. (d) Nitrogen gas adsorption–desorption isotherms obtained for the Q[6]-STA catalyst.
2

W. Xia et al.
Inorganica Chimica Acta 523 (2021) 120418
Fig. 4. (a,b) SEM images obtained for the Q[6]-STA catalyst. (c) The surface elemental analysis images and (d) corresponding composition of the Q[6]-STA catalyst
obtained using EDS.
[6] and the STA. The XRD patterns of the samples were recorded in the
2θ range of 5 ꢀ 50◦. TG-DTA analysis of the samples was carried out on
an Netzsch STA499C thermal Analyzer using ~ 20 mg of sample in a
platinum crucible. SEM and EDS were carried out using an Hitachi
SU8010 FESEM instrument. The samples were suspended in ethanol,
subjected to ultrasonication for 30 min, filtered and dried prior to
measurement. The specific surface area, pore volume, and pore size
distribution of the samples were measured on a ASAP 2020 HD88
automatic adsorption instrument using low temperature N2 adsorp-
tion–desorption isotherms. Prior to measurement, the samples were
degassed in situ at 393 K for 6 h under vacuum.
acid,0.015–0.210 mol of n-propyl alcohol and 5 mL of toluene as a
water-carrying agent were added to a 50-mL three-necked flask equip-
ped with a reflux condenser and water separator, and the resulting re-
action mixture heated at reflux for 1–5 h. The resulting mixture was
filtered while hot to separate the solid catalyst and the filtrate subjected
to vacuum distillation to remove the excess n-propyl alcohol. The crude
product was placed into a beaker, water added and the solution was left
to stand at room temperature for 12 h. The resulting colorless crystals
were filtered, washed with water (3 × ) and recrystallized from ethanol
(75% v/v) to give PG.
The Q[6]-STA catalyst was recovered by filtration, washed with
water, and vacuum–dried. The activated catalyst was reused for further
experiments.
2.4. Catalytic tests
3. Results and discussion
Q[6]-STA was used as a catalyst for the synthesis of butyl paraben
(BP) to investigate its catalytic activity. 20–100 mg of freshly activated
Q[6]-STA catalyst (dried at 353 K for 3 h), 0.036 mol of p-hydrox-
ybenzoic acid and 0.072–0.432 mol of butyl alcohol were added to a 50-
mL three-necked flask equipped with a reflux condenser and water
separator, and the resulting reaction mixture was heated at reflux for
1–5 h. The resulting mixture was filtered while hot to separate the solid
catalyst and the filtrate subjected to vacuum distillation to remove the
excess butyl alcohol. The crude product was placed into a beaker, water
added and the solution was left to stand at room temperature for 12 h.
The resulting colorless crystals were filtered, washed with water (3 × )
and recrystallized from ethanol (75% v/v). The BP product was obtained
via filtration, dried, weighed and the yield calculated.
3.1. Interaction of Q[6] and HPA
It is common that Q[n]s interact with HPAs in aqueous media to form
water-insoluble hybrid compounds, in which the driving force is derived
from the so-called OSIQ[26]. That is to say, the electrostatically positive
outer surface of the Q[n] interacts with the HPAs anions via ion–dipole
interactions, which generally involve the protruding oxygen atoms of
the HPAs and portal carbonyl carbon atoms or methine/methylene
protons on the back of the Q[n] (Fig. 2) [28]. Previous studies have
revealed that some Q[n]s interact with selected HPAs to form hybrid
compounds via OSIQ[23,28–35]. Generally, Q[n] molecules interact
with HPAs in simple ratios, such as 1:1 [28,31,33,34] or 2:1
[29,30,32,33,35]. This was corroborated in the present work when we
prepared a Q[6]-supported STA catalyst. The as-obtained precipitate
exhibited a fixed composition and was obtained in high yield (>90%
In order to further investigate the catalytic performance of Q[6]–STA
during the esterification reaction, PG was synthesized using a similar
method to that used to prepare BP. A total of 20–150 mg of freshly
activated Q[6]-STA catalyst (dried at 353 K for 3 h), 0.015 mol of gallate
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W. Xia et al.
Inorganica Chimica Acta 523 (2021) 120418
Fig. 5. (a) The influence of the molar ratio of the reactants on the yield of BP. (b) The influence of the catalyst loading on the yield of BP. (c) The influence of the
reaction time on the yield of BP. (d) The synthesis of BP was repeated 5 times under the optimal reaction conditions.
based on STA). The elemental analysis results showed that the Q[6]:STA
stoichiometry was consistently 2:1 and the chemical formula was
C72H76N48O64SiW12⋅nH2O.
analysis was carried out on the Q[6]-STA catalyst (Fig. 3c), which
showed a mass loss of 12.02% at 108.24 ◦C. This can be attributed to the
loss of free water from the compound. The following two endothermic
bands corresponded to decomposition of Q[6] and STA at 378.74 ◦C and
760 ◦C. These experimental results suggested that the interactions
formed between STA and Q[6] render STA with increased thermal sta-
bility. Nitrogen gas adsorption measurements were applied to charac-
terize the porosity of the Q[6]-STA catalyst (Fig. 3d). The surface area
was calculated from the isotherms using the multi-point Brunauer-
Emmett-Teller (BET) method. The pore size distribution was determined
using the Barrett-Joyner-Halenda (BJH) method from the desorption
parts of the isotherms. The results showed that the BET surface area,
micropore volume and pore size of the Q[6]–STA catalyst were 55.41
m2∙gꢀ 1, 0.018 cm3∙gꢀ 1 and 27.66 nm, respectively. Evidently, the
surface area of the Q[6]–supported STA was greater than that of the
unsupported STA [4,5].
3.2. Characterization of the Q[6]-STA catalyst
Fourier-transform infrared (FTIR) spectroscopy provides an insight
into the interaction of Q[6] with STA (Fig. 3a). Generally, Q[6] exhibits
characteristic IR bands at 2929 cmꢀ 1
[ν
(–CH2-)] and 1728 cmꢀ 1 (ter-
–
minal C O). Meanwhile, STA exhibits four characteristic IR bands at
–
1080 cmꢀ 1 (Si-O in central tetrahedron), 980 cmꢀ 1 (terminal W = O),
890 cmꢀ 1 (corner-sharing oxygen), and 800 cmꢀ 1 (W-O-W edge-sharing
bridging oxygen) [1]. When compared to the spectra obtained for the
two pure compounds (Q[6] and STA), the characteristic IR bands of Q[6]
and STA can still be observed in the Q[6]-STA catalyst. In particular, the
bands due to the portal carbonyl groups showed no obvious changes,
whereas the bands due to the bridging –CH2- showed different shapes,
suggesting that the STA molecules may reside on the outer surface via
weak intermolecular interactions. It is conceivable that the electrostat-
ically negative carbonyl-fringed portals of the Q[6] molecules may
prevent the approach of the STA anions. Evidently, Q[6] and STA
maintained their initial structures in the Q[6]-STA catalyst without
being destroyed. The XRD patterns of Q[6]-STA was completely
different form that of a simple mixture of Q[6] and STA (Fig. 3b),
indicating that a new phase was formed between Q[6] and STA in so-
lution, that is, Q[6] formed interactions with STA. Thermogravimetric
The morphologies and elemental distribution of the Q[6]-STA cata-
lyst were investigated using SEM and EDS. The SEM image shows that
the Q[6]-STA catalyst was uniform with a particle size of ~100 nm
(Fig. 4a.b) and the surface of the composite was rough, which was
beneficial for the enhanced contact area between the catalyst and
reactant. Furthermore, the surface elemental analysis (Fig. 4c) and
strong peaks attributed to C, N, O, Si and W were observed in the EDS
spectrum (Fig. 4d), and confirmed that the composition of the sample
was consistent with the Q[6]-STA catalyst.
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Inorganica Chimica Acta 523 (2021) 120418
Fig. 6. (a) Influence of the molar ratio of the reactants on the yield of PG. (b) The influence of the catalyst loading on the yield of PG. (c) The influences of the
reaction time on the yield of PG. (d) The synthesis of PG was repeated five times under the optimal reaction conditions.
Fig. 7. The reusability of the Q[6]-STA catalyst in the esterification reaction used to prepare BP and PG.
3.3. Catalytic activity tests using the Q[6]–STA catalyst
factors, including catalyst loading, reaction time and molar ratio of acid
to alcohol, on the synthesis of BP and PG were investigated. BP was
synthesized heating different molar ratios of butyl alcohol to p-
hydroxybenzoic acid with 100 mg Q[6]–STA catalyst at reflux for 3 h
In order to evaluate the catalytic activity of the Q[6]-supported STA
catalyst, it was applied in the esterification reaction. The influencing
5

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Inorganica Chimica Acta 523 (2021) 120418
(Fig. 5a). The optimal molar ratio of butyl alcohol to p-hydroxybenzoic
acid was identified to be 6:1, which gave the maximum yield of BP
(95.6%). Fig. 5b shows the influence of the catalyst loading on the yield
of BP using a fixed molar ratio of butyl alcohol to p-hydroxybenzoic acid
of 6:1 at reflux for 3 h. The yield of BP increased upon increasing the
catalyst loading with a maximum yield of 95.8% obtained when using
80 mg of the Q[6]–STA catalyst. To investigate the influence of reaction
time on the yield of BP, the molar ratio of butyl alcohol to p-hydrox-
ybenzoic acid was fixed at 6:1, the catalyst loading was fixed at 80 mg
and the reaction heated at reflux. Fig. 5c shows the yield of BP increased
upon increasing the reaction time up to 3 h, after which it levelled off at
95.8%. Under optimal conditions (catalyst loading, 80 mg; molar ratio
of butyl alcohol to p-hydroxybenzoic, 6:1; reaction temperature, 391 K,
reaction time, 3 h), the reaction was repeated 5 times. The results
showed that the yield of BP was maintained at ~ 95% (Fig. 5d).
In the PG synthesis, the reaction conditions were similar to that used
to prepare BP and the details listed in Table S1–S4. The optimum re-
action conditions were catalyst loading, 100 mg; molar ratio of n-
propanol to gallic acid, 8:1; reaction temperature, 370 K; reaction time,
4 h. The yield of PG was >94% over five repeated reactions (Fig. 6). BP
and PG were characterized using FTIR and NMR spectroscopy, and the
results shown in Figs. S1 and S2.
4. Conclusion
In this work, a cucurbit[6]uril-supported silicotungstic acid (Q[6]-
STA) catalyst has been prepared. Q[6] and STA not only exhibited good
catalytic activity in the esterification reaction, but are also insoluble
solids, which could be easily isolated from the reaction system. The Q
[6]-supported STA enhanced the stability of STA and improved its cat-
alytic efficiency.
CRediT authorship contribution statement
Wen Xia: Conceptualization, Investigation, Writing - original draft.
Yu-Mei Nie: Investigation. Na Lei: Investigation. Zhu Tao: Resources.
Qiang-Jiang Zhu: Resources. Yun-Qian Zhang: Resources, Conceptu-
alization, Supervision.
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.
Q[6] has no catalytic activity in esterification reaction, and its role is
to formed a stable solid catalyst through the self-assembly interaction of
Q[6] and STA. The active species in Q[6]-STA is STA. However, control
experiments revealed the low efficiency of unsupported STA. When 47
mg of STA was used as a homogeneous catalyst, the yield of BP was
found to be 85%. And the yield of PG became as low as 83% using 59 mg
of pristine STA. The results showed that the catalytic performance of Q
[6]-STA is better than that of unsupported STA.
Acknowledgements
We acknowledge the support of the National Natural Science Foun-
dation of China (No. 21761007, 21871064).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2021.120418.
Q[n]s-HPAs self-assembled systems often have the characteristics of
porous structure [28,36]. In addition, the morphology of Q[6]-STA
nanoparticles also increase its surface area and active site on the cata-
lyst surface. The greater the specific surface area of the catalyst, the
more dispersed the active species, and the larger the number of active
sites. The high porosity of the catalyst makes it easy to adsorb the re-
actants, thus enhancing its catalytic performance.
References
[1] E.B. Wang, C.W. Hu, L. Xu, Introduce in Polyacid Chemistry (in Chinese), Chemical
Industry Press, Beijing, 1997, p. 4.
[2] J. Haber, K. Pamin, L. Matachowski, D. Mucha, Appl. Catal. A 256 (2003) 141–152,
https://doi.org/10.1016/S0926-860X(03)00395-8.
Since STA is a Brønsted acid [2,41], we speculate that the afore-
mentioned reaction can be explained by acid-catalysed Fischer esterifi-
cation mechanism [42]. When the carbonyl oxygen atom of carboxylic
group binds with a proton of STA, the density of positive charge on
carbonyl carbon atom increases. The nucleophillic attack of the alcohol
on the protonated carbonyl group gives a tetrahedral intermediate
bearing two hydroxyl groups, which undergoes a water and proton
elimination process to produce the ester.
[3] S.K. Bhorodwaj, D.K. Dutta, Appl. Clay Sci. 53 (2011) 347–352, https://doi.org/
10.1016/j.clay.2011.01.019.
[4] D.P. Sawant, A. Vinu, J. Justus, P. Srinivasu, S.B. Halligudi, J. Mol. Catal. A: Chem.
276 (2007) 150–157, https://doi.org/10.1016/j.molcata.2007.07.001.
[5] M.N. Timofeeva, M.M. Matrosova, T.V. Reshetenko, L.B. Avdeeva, A.A. Budneva,
A.B. Ayupov, E.A. Paukshtis, A.L. Chuvilin, A.V. Volodin, V.A. Likholobov, J. Mol.
Catal. A: Chem. 211 (2004) 131–137, https://doi.org/10.1016/j.
molcata.2003.10.019.
[6] Y. Wu, X.K. Ye, X.G. Yang, X.P. Wang, W.L. Chu, Y.C. Hu, Ind. Eng. Chem. Res. 35
(1996) 2546–2560, https://doi.org/10.1021/ie950473s.
[7] R.M. Ladera, M. Ojeda, J.L.G. Fierro, S. Rojas, Catal. Sci. Technol. 5 (2015)
484–491, https://doi.org/10.1039/C4CY00998C.
´
´
[8] J.G. Hernandez-Corteza, M.a. Manríquezb, L. Lartundo-Rojasc, E. Lopez-Salinasa,
Catalysis Today 220–222 (2014) 32–38, https://doi.org/10.1016/j.
cattod.2013.09.007.
3.4. Recyclability of the Q[6]-HPA catalyst.
We previously mentioned that the Q[6]-STA catalyst can be easily
prepared in high yield and shows high stability because it is insoluble in
both acidic aqueous solutions and organic solvents. As an esterification
reaction catalyst, Q[6] ꢀ STA is characterized by its high catalytic ac-
tivity when compared to its classical counterparts and it may be easily
isolated from reaction media via filtration due to its insolubility. An
additional important advantage of the Q[6]-STA catalyst is its stability
with respect to leaching of the acid sites during the esterification reac-
tion. Thus, the catalyst can be collected using simple filtration after
completion of the reaction. The catalyst can be regenerated by washing
with water several times, followed by drying at 373 K in an oven under
an air atmosphere. Thereafter, it can be reused in a new esterification
reaction system. Fig. 7 shows the results of 5 consecutive reactions under
the optimal conditions used to prepare BP and PG with the Q[6]-STA
catalyst. The normalized yields suggested the catalyst retained its orig-
inal catalytic activity. XRD shows that the composition and structure of
Q[6]-STA remained unchanged after repeated use (Fig. S3).
[9] S. An, D.Y. Song, Y.N. Sun, Y.H. Guo, Q.K. Shang, Microporous Mesoporous Mater.
226 (2016) 396–405, https://doi.org/10.1016/j.jcat.2016.07.004.
[10] H. Cong, X.L. Ni, X. Xiao, Y. Huang, Q.J. Zhu, S.F. Xue, Z. Tao, L.F. Lindoy, G. Wei,
Org. Bio Mol. Chem. 14 (2016) 4335, https://doi.org/10.1039/c6ob00268d.
[11] J.W. Lee, S. Samal, N. Selvapalam, H.J. Kim, K. Kim, Acc. Chem. Res. 36 (2003)
621, https://doi.org/10.1002/chin.200346280.
[12] K. Kim, Chem. Soc. Rev. 31 (2002) 96, https://doi.org/10.1039/a900939f.
[13] J. Lagona, P. Mukhopadhyay, S. Chakrabarti, L. Isaacs, Angew. Chem. Int. Ed. 44
(2005) 4844, https://doi.org/10.1002/anie.200460675.
[14] K. Kim, N. Selvapalam, Y.H. Ko, K.M. Park, D. Kim, J. Kim, Chem. Soc. Rev. 36
(2007) 267, https://doi.org/10.1039/b603088m.
[15] R.N. Dsouza, U. Pischel, W.M. Nau, Chem. Rev. 111 (2011) 7941, https://doi.org/
10.1021/cr200213s.
[16] V. Sindelar, S. Silvi, S.E. Parker, D. Sobransingh, A.E. Kaifer, Adv. Funct. Mater. 17
(2007) 694, https://doi.org/10.1002/adfm.200600969.
[17] S. Gadde, A.E. Kaifer, Cur. Org. Chem. 15 (2011) 27, https://doi.org/10.2174/
138527211793797800.
[18] L. Isaacs, Acc. Chem. Res. 47 (2014) 2052, https://doi.org/10.1021/ar500075g.
[19] K.I. Assaf, W.M. Nau, Chem. Soc. Rev. 44 (2015) 394, https://doi.org/10.1039/
c4cs00273c.
[20] M.N. Sokolov, D.N. Dybtsev, V.P. Fedin, Russ. Chem. Bull., Int. Ed. 52 (2003) 1041,
https://doi.org/10.1023/A:1024771902420.
6

W. Xia et al.
Inorganica Chimica Acta 523 (2021) 120418
[21] O.A. Gerasko, M.N. Sokolov, V.P. Fedin, Pure Appl. Chem. 76 (2004) 1633, https://
doi.org/10.1351/pac200476091633.
[32] L.W. Han, J. Lu, X.J. Lin, R. Cao, Dalton Trans. 41 (2012) 10080, https://doi.org/
10.1039/c2dt30559c.
[22] V.P. Fedin, J. Russ, Coordin. Chem. 30 (2004) 151, https://doi.org/10.1023/B:
RUCO.0000022114.76939.16.
[33] N.C. Kasuga, M. Umeda, H. Kidokoro, K. Ueda, K. Hattori, K. Yamaguchi, Cryst.
Growth Des. 9 (2009) 1494, https://doi.org/10.1021/cg801007k.
[34] M. Cao, J. Lin, X.J. Lu, Y. You, T. Liu, R. Cao, J. Hazard. Mater. 186 (2011) 948,
https://doi.org/10.1016/j.jhazmat.2010.10.119.
[23] J. Lü, J.X. Lin, M.N. Cao, R. Cao, Coord. Chem. Rev. 257 (2013) 1334, https://doi.
org/10.1016/j.ccr.2012.12.014.
[24] X.L. Ni, X. Xiao, H. Cong, L.L. Liang, K. Chen, X.J. Cheng, N.N. Ji, Q.J. Zhu, S.
F. Xue, Z. Tao, Chem. Soc. Rev. 42 (2013) 9480, https://doi.org/10.1039/
c3cs60261c.
[35] J. Lu, J.X. Lin, X.L. Zhao, R. Cao, Chem. Commun. 48 (2012) 669, https://doi.org/
10.1039/c1cc16268c.
[36] X. Xia, W.W. Ge, H.Y. Chen, Z. Tao, Y.Q. Zhang, G. Wei, K. Chen, New J. Chem. 43
(2019) 10297–10304, https://doi.org/10.1039/C9NJ01116A.
[37] S. Li, W. Xia, Y.Q. Zhang, Z. Tao, New J. Chem. 44 (2020) 11895–11900.
[38] T.A. Peters, N.E. Benes, A. Holmen, J.T.F. Keurentjes, Appl. Catal. A 297 (2006)
182–188, https://doi.org/10.1016/j.apcata.2005.09.006.
[25] X.L. Ni, S.F. Xue, Z. Tao, Q.J. Zhu, L.F. Lindoy, G. Wei, Coord. Chem. Rev. 287
(2015) 89, https://doi.org/10.1016/j.ccr.2014.12.018.
[26] X.L. Ni, X. Xiao, H. Cong, Q.J. Zhu, S.F. Xue, Z. Tao, Acc. Chem. Res. 47 (2014)
1386, https://doi.org/10.1021/ar5000133.
[27] Y. Huang, R.H. Gao, X.L. Ni, X. Xiao, H. Cong, Q.J. Zhu, K. Chen, Z. Tao, Preprint
at, Angew. Chem. Int. Ed. (2020), https://doi.org/10.1002/anie.202002666.
[28] X.K. Fang, P. Kogerler, L. Isaacs, S. Uchida, N. Mizuno, J. Am. Chem. Soc. 131
(2009) 432, https://doi.org/10.1021/ja807751b.
[39] J.A. Dias, E. Caliman, S.C.L. Dias, M. Paulo, A.T.C.P. de Souza, Catal. Today 85
(2003) 39–48, https://doi.org/10.1016/S0920-5861(03)00192-5.
[40] W.A. Freeman, W.L. Mock, N.Y. Shih, Cucurbituril, J. Am. Chem. Soc. 103 (1981)
7367, https://doi.org/10.1021/ja00414a070.
[29] J.X. Lin, J. Lu, R. Cao, J.T. Chen, C.Y. Su, Dalton Trans. (2009) 1101–1103,
https://doi.org/10.1039/B817537N.
[41] M.N. Timofeeva, Appl. Catal. A: Gen. 256 (2003) 19, https://doi.org/10.1016/
S0926-860X(03)00386-7.
[30] J.X. Lin, J. Lu, H.X. Yang, R. Cao, Cryst. Growth Des. 2010 (1966) 10, https://doi.
org/10.1021/cg1000528.
[42] C.B. Vilanculo, L.C.A. Leles, M.J. Silva, Waste Biomass Valorization 2020 (1895)
11, https://doi.org/10.1007/s12649-018-00549-x.
[31] J.X. Lin, J. Lu, M.N. Cao, R. Cao, Cryst. Growth Des. 11 (2011) 778, https://doi.
org/10.1021/cg101355d.
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