Self-assembled tetramethyl cucurbit[6]uril-polyoxometalate nanocubes as efficient and recyclable catalysts for the preparation of propyl gallate

Article abstract of DOI:10.1039/d0nj01755h

The development of cucurbit[n]urils-polyoxometalate (Q[n]-POM) hybrids with the same microshape and nanoscale features is highly desirable, yet remains a great challenge. Herein, we design and synthesize a class of Q[n]-POM hybrids and tetramethyl cucurbit[6]uril-phosphomolybdic acid (TMeQ[6]-PMA) nanocubes (NCs) via a facile one-step self-assembly method, as heterogeneous acid catalysts for greatly boosting the catalysts in term of activity and stability for the esterification of gallic acid and n-propanol to propyl gallate (PG). The Fourier transform infrared (FTIR) spectroscopy reveals that the self-assembled mechanism of TMeQ[6]-PMA NCs based on the outer-surface interaction of Q[n]s. The temperature programmed desorption experiments with ammonia and FTIR analysis of the pre-adsorbed pyridine results confirm that the coexistence of medium and strong acid sites and a larger number of Lewis acid sites other than Br?nsted acidic sites on the catalyst surface. These new features make the as-prepared TMeQ[6]-PMA NCs exhibit a high PG conversion (95.6percent) and excellent stability, which epresents a better catalytic performance than other reported catalysts.

Full text of DOI:10.1039/d0nj01755h

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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
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Self-Assembled Tetramethylcucurbit[6]uril-Polyoxometalate NanocubeDsOaI: s10.E103f9f/iDc0iNeJ0n17t55H
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and Recyclable Catalysts for Preparation of Propyl gallate
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Shuang Li, Wen Xia, Yunqian Zhang*, Zhu Tao
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Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province,
Guizhou University,
Guiyang 550025, China
*Corresponding to: sci.yqzhang@gzu.edu.cn (Y. Q. Zhang)
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DOI: 10.1039/D0NJ01755H
Abstract: The development of cucurbit[n]urils-polyoxometalates (Q[n]-POMs) hybrids with the same
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microshape and nanoscale features is highly desirable, yet remains a great challenge. Herein, we design
and synthesize a class of Q[n]-POMs hybrids, tetramethyl cucurbit[6]uril-phosphomolybdic acid
(TMeQ[6]-PMA) nanocubes (NCs), via an facile one-step self-assembly method, as heterogeneous
acid catalysts for greatly boosting catalyst in term of activity and stability for the esterification of gallic
acid and n-propanol to propyl gallate (PG). The fourier transform infrared (FTIR) spectroscopy reveals
that the self-assembled mechanism of TMeQ[6]-PMA NCs based on the outer-surface interaction of
Q[n]s. The temperature programmed desorption experiments with ammonia and FTIR analysis of pre-
adsorbed pyridine results confirm that the coexistence of medium and strong acid sites and a larger
number of Lewis acid sites than the Brönsted acidic sites on the catalyst surface. These new features
make as-prepared TMeQ[6]-PMA NCs exhibit a high PG conversion (95.6 %) and excellent stability,
which represents the best catalytic performances than other reported catalysts.
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Keywords: tetramethyl cucurbit[6]uril; polyoxometalates; catalyst; propyl gallate
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1. Introduction
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Over the past few decades, catalysts, acid catalysts in particular, which can be divided into
homogeneous as well as heterogeneous, play an important role in petrochemical, fine chemical and
environmental industries.1-5 Among them, the homogeneous acid catalysts such as sulfuric acid,
phosphoric acid, hydrochloric acid, hydrofluoric acid as well as p-toluene sulfonic acid and so on, have
been extensively applied in acid-catalyzed reactions.6-10 Nevertheless, owing to their lethal drawbacks
including corrosive nature, difficulty of separation and recovery, high pollution, high energy
consumption and low water tolerance,2, 9 seeking heterogeneous acid catalysts with high catalytic
activity, no contamination, easy separation, as well as excellent stability is urgently desirable.
Recently, polyoxometalates (POMs), an exciting type of subnanometer-sized transition metal-
oxygen clusters, have attracted extensive research interest as heterogeneous acid catalysts owing to
their strong electron-accepting ability.11-16 However, the major disadvantages of pristine POMs are
low surface area hindering the exposure of active sites, and the difficulty of separation due to high
solubility in most of polar solvents,12, 15 which inevitably restrict their practical applications in catalytic
reactions. Immobilizing POMs on suitable supports16-21 and constructing organic-POMs hybrids22-25
have been regarded as two effective strategies to overcome above problem, and the latter is more
promising than the former.
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Cucurbit[n]urils (Q[n]s), as an emerging class of cage-like supramolecular organic compounds
after cyclodetrin, calixarene, and crown ether, possess a rigid hydrophobic cavity, two identical
carbonyl-laced portals and positive electrostatic potential outer surface.26-29 They can act as ideal hosts
as well as ligands to derived various Q[n]-based hybrids, which, therefore, has sparked enormous
research attention recently, especially in POMs as guests.30-37 For instance, Fang et al. first
demonstrated that Q[6] and Q[8] as host can assemble with guest POMs through interaction of outer
surface and thus greatly expand the application of the hybrids.32 Simultaneously, Cao et al. successfully
synthesized Q[6]-POMs hybrids via hydrogen bonding that possessed outstanding photocatalytic
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activity and stability for the degradation of methyl orange.33 Furthermore, two isostructDuOraI:l10d.1e0c39a/mD0eNtJh0y17l55H
Q[5]-POMs composites were prepared and displayed reversible photochromic property and
photocatalytic activity.34 Very recently, we also revealed that perhydroxylated Q[6] could coordinate
with POMs through supramolecular interactions, thereby leading to formation of new supramolecular
assemblies.35 Inspired by the above cases, novel Q[n]-POMs hybrids with the same microshape and
nanoscale features for catalytic reaction is promising, but it has not been explored thus far, to the best
of our knowledge.
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Herein, we report an extremely facile strategy to synthesize a new Q[n]-POMs hybrids with the
same microshape and nanoscale features, tetramethyl cucurbit[6]uril-phosphomolybdic acid
(TMeQ[6]-PMA) nanocubes (NCs) (Scheme 1), by one-step self-assembly method. The field-
emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and
energy dispersive X-ray spectroscopy (EDX) results confirm that as-prepared TMeQ[6]-PMA NCs
possess a microshape and nanoscale features, and the homogeneous dispersion of PMA on the hybrid.
The fourier transform infrared (FTIR) spectroscopy reveals that the self-assembled mechanism of
TMeQ[6]-PMA NCs based on outer-surface interactions. The temperature programmed desorption
experiments with ammonia (NH3-TPD-MS) and FTIR analysis of pre-adsorbed pyridine (py-FTIR) on
catalyst under vacuum at different temperatures demonstrate that the coexistence of medium and strong
acid sites and a larger number of Lewis acid sites than the Brönsted acidic sites on the catalyst surface.
To evaluate the catalytic activity of as-prepared TMeQ[6]-PMA NCs, the synthesis of propyl gallate
(PG) which is widely applied in food antioxidant, cosmetics, adhesives, and lubricants industry,38-40
was carried out. As a result, the as-prepared TMeQ[6]-PMA NCs exhibit a high PG conversion
(95.6 %) and excellent stability, which represents the best catalytic performances than other reported
catalysts.
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Scheme 1. Preparative route for TMeQ[6]-PMA NCs.
2. Results and discussions
2.1. Morphological and structural characterization
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TMeQ[6]-PMA NCs were synthesized through a simple one-step self-assembly process. The
morphology and elemental distribution of as-synthesized TMeQ[6]-PMA NCs, are investigated by
FESEM, TEM as well as EDX, as shown in Figure 1. The low-magnification SEM image displays
that the TMeQ[6]-PMA NCs are uniform, with a particle size of 200 nm (Figure 1 a). The enlarged
SEM image reveals that the surface of the composites is very rough, which is beneficial to the enhanced
contact area between the catalyst and reactant, as shown in Figure 1 b. Moerover, the TEM image
reveals that the TMeQ[6]-PMA nanocubes possess a particle size of 200 nm which is in good
agreement with SEM results (Figure 1 c). Figure 1d presents a typical TMeQ[6]-PMA NCs and the
corresponding elemental mapping based on EDX is shown in Figure 1e-i. The C, N, O, Mo, and P
elements are uniformly distributed on TMeQ[6]-PMA samples, suggesting the successful decoration
of PMA into the composites. To confirm the chemical formula of as-prepared TMeQ[6]-PMA NCs,
the elemental analysis was carried out (Table S1). The result indicates that chemical formula of as-
prepared TMeQ[6]-PMA NCs is C80H97N48O144P3Mo36nH2O.
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Figure 1. a, b) FESEM images and c) TEM image of the as-synthesized TMeQ[6]-PMA NCs. d)
Typical SEM image of TMeQ[6]-PMA NCs and corresponding elemental mapping of e) C, f) N, g) O,
h) Mo, and i) P elements.
The crystal structures of TMeQ[6], PMA, and as-prepared TMeQ[6]-PMA NCs are characterized
by the powdered X-ray diffraction (PXRD) (Figure 2a). The results indicate that the both TMeQ[6]
and PMA have obvious crystal characteristic peak. Furthermore, in the XRD pattern of the TMeQ[6]-
PMA NCs, it is easy to observe the characteristic peaks of TMeQ[6], indicating that the introduction
of PMA does not affect the crystal structure of TMeQ[6]. However, no evident peaks for PMA
crystalline phases are discovered from XRD patterns of TMeQ[6]-PMA NCs, suggesting PMA clusters
are successfully dispersed in the TMeQ[6] framework rather than existing as free solid acid. To further
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confirm the self-assembled mechanism of composites, FTIR spectroscopy of pure TMDeOQI:[160.]1,03P9M/D0ANJ,0a17s55H
well as TMeQ[6]-PMA NCs in the range of 400-4000 cm-1 are characterized (Figure 2b). In the FTIR
spectrum of the pure TMeQ[6], there are four distinct characteristic peaks at 2995-2930 cm-1 region (-
CH2), ca. 1747 (-C=O), and 1469 cm-1 (-CH2). For the pure PMA with type Keggin structure displays
several typical characteristic peaks at ca. 1065, 967, 871, and 790 cm-1, which are attributes to the
symmetric stretching vibrations of (P-Oa) in the central PO4 tetrahedron, (Mo-Od) in the exterior
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MoO6 octahedron, (Mo-Ob-Mo) in the corner shared octahedron and (Mo-Oc-Mo) in the edge shared
octahedron, respectively, in accordance with previous reports.20 Moreover, the above characteristic
peaks correspond to PMA could still be observed in the TMeQ[6]-PMA sample, suggesting that the
initial Keggin structures of PMA preserve intact after the formation of the composites. In comparison
with the FTIR spectrum of pure TMeQ[6], the peaks assigned to the bridged -CH2 in the TMeQ[6]-
PMA NCs have a red-shift of ca. 9 and 6 cm-1, respectively, which is the result of the strong
electrostatic interactions and formation of hydrogen bonds between TMeQ[6] and [PMAO40]3-. The
thermogravimetry and differential scanning calorimetry (TG-DSC) were employed to investigate the
thermal stability of TMeQ[6]-PMA NCs (Figure 2c). The result reveals impressive thermal stability,
with a decomposition temperature up to 630 oC in N2. Furthermore, the weight losses from 30 to 240
oC could be attributed to the removal of water molecules absorbed in the surface of TMeQ[6]-PMA
NCs. From the nitrogen adsorption and desorption measurement (Figure 2d), the TMeQ[6]-PMA NCs
shows mixed adsorption isotherms of type I and II adsorption-desorption isotherms.41 The specific
surface areas (SSA) and pore size distribution of TMeQ[6]-PMA NCs are characterized by the
Brunauer-Emmett-Teller (BET) analysis and nonlocal density functional theory (NL-DFT),
respectively. TMeQ[6]-PMA NCs possess a large SSA of 49.76 m3 g-1 and a high pore volume of 0.055
cm3 g-1 with a micropore diameter of around 0.95 nm.
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DOI: 10.1039/D0NJ01755H
a
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TMeQ[6]-PMA
TMeQ[6]-PMA
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PMA
-CH2
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PMA
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Figure 2. a) PXRD and b) FTIR patterns of TMeQ[6], PMA, and TMeQ[6]-PMA NCs. c) TG-DSC
curves of TMeQ[6]-PMA NCs in N2 at a heating rate of 10 oC min-1. d) Nitrogen adsorption-desorption
isotherms and pore size distribution (the inset) of TMeQ[6]-PMA NCs.
2.2. Surface acidic property
Surface acid property, including acid site, intensity and density, is a significant parameter, closely
linked with the catalyst activity, which could be evaluated by NH3-TPD-MS. The desorption
temperature profiles and the amounts of desorption ammonia of the TMeQ[6]-PMA catalysts from 323
to 973 K are displayed in the Figure 3a and Figure S1, where three temperature zones correlated to
the acid sites intensity (weak, medium and strong) can be differentiated according to the previous
literature.42,43 The result depicts that the TMeQ[6]-PMA NCs has three obvious desorption peaks at
temperature of 633.5, 662.3, and 853.5 K with ammonia adsorption capacity was 11.3, 3.7 as well as
5.9 mmol g-1, respectively (whereas the total number of acid sites was 20.9 mmol g-1), which reveals
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the co-presence of medium and strong acid sites on the catalyst surface. Moreover, thDeOBI: 1r0ö.1n03s9t/eDd0NaJc0i1d755H
sites and Lewis acid sites of as-prepared materials were further investigated by FTIR analysis of pre-
adsorbed pyridine on catalyst under vacuum at different temperatures within the frequency range 1700-
1400 cm-1 (Figure 3b). The spectrum of TMeQ[6]-PMA sample at 298 K displays five bands: band at
1447, 1576, and 1606 cm-1 correspond to the characteristic of pyridine adsorbed at Lewis acidic sites;
bands at 1642 cm-1 is generally assigned to the coordinated pyridine bound to Brönsted acidic sites
(PyH+); the another band appeared at 1490 cm-1 reflects a both Brönsted acidic and Lewis acidic site
toward pyridine adsorption.44,45 As a result, the TMeQ[6]-PMA NCs exhibit a larger number of Lewis
acid sites than the Brönsted acidic sites, and the frequencies for pyridine at Lewis acid sites are
appeared at all temperatures, only changing in relative intensity between the bands, which indicate
pyridine is not desorbed even at high temperatures.
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Figure 3. a) Temperature programmed desorption experiments with ammonia of TMeQ[6]-PMA NCs.
b) FTIR analysis of pre-adsorbed pyridine on catalyst under vacuum at different temperatures of
TMeQ[6]-PMA NCs.
2.3. Catalytic performance
Catalytic activity of the as-prepared TMeQ[6]-PMA catalysts were investigated in the
esterification of gallic acid and n-propanol to propyl gallate, which is an important gallic acid ester
with an excellent antioxidation capacity.38-40 As illustrated in Figure 4a, the influence of reaction time
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on conversion of PG was examined. The result suggests that the yield of PG underwenDtOaI:d10r.a10m39a/tDi0cNrJ0is17e55H
at the initial stage followed by a slow decrease with the increase of reaction time. The maximum
conversion (91.2 %) could be achieved at 5 h, which was selected as the suitable reaction time for in-
depth studies. Surprisingly, for this specific esterification reaction, one of the reactant (gallic acid) is
solid at room temperature, which possess high solubility in the other liquid reactant (n-propanol),
meaning that n-propanol is both a reactant and a solvent. Therefore, it is indispensable to investigate
the effect of molar ratio of n-propanol to gallic acid on PG conversion, as exhibited in Figure 4b. As
a result, only 39.5 % yield of PG was achieved at the molar ratio of 2:1, and the PG yield was increased
sharply to 93.6 % at the molar ratio of 8:1, but no further improve of PG yield was observed when the
molar ratio was increased to 10:1. Thence, the molar ratio of 8:1 for n-propanol and gallic acid was
optimal for further study based on the economic benefits. Generally, the catalyst amount has a great
effect on the esterification reaction in activity. Figure 4c depicts the yield of PG rised evidently from
46.7 % to 95.6 % as the catalyst amount improving from 10 to 40 mg, indicating the best catalyst
amount was 40 mg, which represents the best yield of PG than other reported catalysts to our
knowledge (Table S2).46-51 Moreover, the cyclical stability of TMeQ[6]-PMANCs was also evaluated
and is displayed in Figure 4d. Notably, negligible reduction of the catalytic activity was observed even
after recycling for six times, suggesting that the TMeQ[6]-PMA catalysts has excellent stability during
esterification reaction. The slight reduce of catalytic activity was ascribed to the no change of crystal
structure and morphology without obvious aggregation or agglomeration as supported by XRD pattern
and SEM images of TMeQ[6]-PMA NCs (Figure S2 and S3). In addition, the products were further
characterized by FTIR and liquid-state 1H magnetic resonance (NMR) spectra, as shown in Figure S4
and S5. The results confirm that product is only PG, no extra by-products are formed, indicating the
selectivity of PG is 100%.
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Figure 4. a) Effect of reaction time on the yield of PG. Reaction conditions: catalyst 50 mg, molar rate
of reactants 10:1, 80 oC. b) Effect of molar rate of reactants on the yield of PG. Reaction conditions:
catalyst 50 mg , 80 oC, 5h. c) Effect of catalyst loading on the yield of PG. Reaction conditions: molar
rate of reactants 8:1, 80 oC, 5h. d) Reusability of catalyst. Reaction conditions: catalyst 40 mg , molar
rate of reactants 8:1, 80 oC, 5h.
3. Conclusion
In summary, we prepared a novel Q[n]-POMs hybrid with same microshape and nanoscale
features, tetramethyl cucurbit[6]uril-phosphomolybdic acid (TMeQ[6]-PMA) NCs, through an facile
one-step self-assembly method. A combination investigation (FESEM, TEM and EDX) confirms that
as-prepared TMeQ[6]-PMA NCs have a microshape and nanoscale features, and the homogeneous
dispersion of PMA on the hybrid. FTIR spectroscopy reveal that the self-assembled mechanism of
TMeQ[6] and PMA based on outer-surface interactions. NH3-TPD-MS and py-FTIR results
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demonstrate that the coexistence of medium and strong acid sites and a larger number of LeVwiewisArtaiccleiOdnline
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sites than the Brönsted acidic sites on the catalyst surface. Consequently, the TMeQ[6]-PMA NCs have
achieved a high PG conversion (95.6 %) and excellent stability, which represents the best catalytic
performances than other reported catalysts.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We acknowledge the support of National Natural Science Foundation of China (No. 51663005,
21761007), Science and Technology Plan Project of Guizhou Province (No. 20175788 and 20185781).
4. Experimental Section
Experimental details are provided in the Supporting Information.
5. Notes and references
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Table of Contents Entry
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novel class of cucurbit[n]urils-polyoxometalates (Q[n]-POMs) hybrids, tetramethyl
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cucurbit[6]uril-phosphomolybdic acid (TMeQ[6]-PMA) nanocubes (NCs), are synthesized via
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one-step self-assembly method.
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