Isomeric effects on the acidity of Al13Keggin clusters in porous ionic crystals

Article abstract of DOI:10.1039/d1cc03600a

We demonstrate a facile synthesis method of a porous ionic crystal (PIC) composed of the little-known δ-Keggin-type cationic polyoxoaluminum cluster ([δ-Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺, δ-Al₁₃) with an oppositely-charged polyoxometalate, which enabled us to investigate the activity as a solid acid. The δ-Al₁₃based PIC exhibited much higher activity in pinacol rearrangement, a typical acid-catalyzed reaction, than the PIC based on the well-known and thermodynamically stable rotational isomer (ε-Al₁₃). This work is a rare example of rotational isomers of polyoxoaluminum clusters exhibiting remarkably different catalytic activities.

Full text of DOI:10.1039/d1cc03600a

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Isomeric effects on the acidity of Al Keggin
1
3
clusters in porous ionic crystals†
Cite this: DOI: 10.1039/d1cc03600a
a
a
a
a
b
Wei Zhou, Naoki Ogiwara,
Li-Kai Yan
Zhewei Weng, Nanako Tamai, Congcong Zhao,
b
a
Received 5th July 2021,
Accepted 30th July 2021
and Sayaka Uchida
*
DOI: 10.1039/d1cc03600a
rsc.li/chemcomm
We demonstrate a facile synthesis method of a porous ionic crystal instead of three shared edges resulting in C molecular
3
v
6
,9
(
PIC) composed of the little-known d-Keggin-type cationic polyoxoa- symmetry. To the best of our knowledge, there are no reports
luminum cluster ([d-Al13O (OH)24(H O)12] , d-Al13) with an oppositely- on the utilization of d-Al , while d-Al is expected to offer
4 2
13 13
7+
charged polyoxometalate, which enabled us to investigate the activity interesting catalytic applications associated with the char-
as a solid acid. The d-Al13 based PIC exhibited much higher activity in acteristic rotated [Al 13] trimer. Herein, we present the first
3
O
pinacol rearrangement, a typical acid-catalyzed reaction, than the PIC investigation of the catalytic activity of d-Al13 as an example for
based on the well-known and thermodynamically stable rotational its function and isomer effects of Keggin-type Al13 moieties.
isomer (e-Al13). This work is a rare example of rotational isomers of
To address the catalytic activity of d-Al13, we focus on a porous
polyoxoaluminum clusters exhibiting remarkably different catalytic ionic crystal (PIC) with a formula of [d-Al O (OH) (H O) ]
1
3
4
24
2
12
19
activities.
[a-CoW O ](OH)ÁnH O (d-Al -CoW ), in which d-Al13 cations
12
40
2
13
12
are stabilized by an a-Keggin-type polyoxometalate (POM) anion
6À
Aluminum is the third most abundant element in the earth’s ([a-CoW12O ] , CoW12) by electrostatic interactions and hydro-
40
crust and has complex solution chemistry. The hydrolysis of gen bonds, so that d-Al13 and CoW12 are alternately arranged in
aluminum salts in water produces an array of polyoxoalumi- the crystal lattice (Fig. 1a and b). Specifically, the three non-
1
–5
num clusters with various sizes, shapes, and compositions.
rotated [Al
interactions and hydrogen bonds, and two of the three trimers
Al ) has five possible rotational isomers of the Baker–Figgis– interact with adjacent d-Al13 by multiple hydrogen bonds. As a
3
O13] trimers are connected to CoW12 by electrostatic
7
+
4 2
Among the polyoxoaluminum clusters, [Al13O (OH)24(H O)12]
(
1
3
6
–13
Keggin (Keggin) structure: a, b, g, d, and e.
The isomers have result, d-Al -CoW possesses 1D channels along the b-axis with a
13 12
an [AlO ] tetrahedral center surrounded by four trimeric minimum aperture of ca. 13 Å Â 7 Å, and the rotated [Al O ]
4
3 13
3 6 2 3
aluminum-hydroxide ([Al (OH) (H O) ]) caps with aluminum trimers are exposed to the pore surface of d-Al13-CoW12, which
coordinated in an [AlO
6
] octahedral fashion.
may serve as catalytically active sites. While the crystal structure of
Among the five Al13 isomers, the e-Al13 isomer with a T
d
d-Al13-CoW12 is already known, there are no studies on its func-
19
molecular symmetry is the most thermodynamically favored, tions due to its very low yield (1.5%). In this work, we demon-
and four [Al 13] trimers are linked through edge-sharing of strate a facile synthesis method to apply d-Al13-CoW12 in a typical
3
O
hydroxyl groups. The e-Al13 isomer has been widely used as a Brønsted acid-catalyzed reaction, pinacol rearrangement, for
building block of functional materials, especially for catalysis which an e-Al -based PIC ([e-Al O (OH) (H O) ][a-CoW O ]
13
13
4
24
2
12
12 40
and water treatment, owing to the high cationic charge and (OH)ÁnH O, e-Al -CoW ) is known to show moderate catalytic
2
4
13
12
1
large numbers of hydroxyl/aqua ligands on the molecular activity. In the crystal structure of e-Al13-CoW12, e-Al13 is con-
1
4–18
surface.
In contrast, the d-Al13 isomer is less stable and nected to two POMs and e-Al13 in the nearest neighbors by
13] trimers is rotated electrostatic interactions and hydrogen bonds, resulting in the
01 to the remaining part of Al13 to form six shared vertices formation of one-dimensional channels along the a-axis with a
differs from e-Al13 in that one of the [Al
6
3
O
minimum aperture of ca. 12 Å Â 6 Å (Fig. 1c and d). The same
cation/anion stoichiometric ratio and similar pore sizes of the two
PICs with different rotational isomers would enable an appro-
priate evaluation of the possible isomeric effects as solid acids.
It is well-known that dissolution–recrystallization with the aid
of inorganic salts such as NaCl assists the growth of protein
crystals because the salts can fine-tune the Coulomb interactions
a
Department of Basic Science, School of Arts and Sciences, The University of Tokyo,
Komaba, Meguro-ku, Tokyo 153-8902, Japan. E-mail: csayaka@g.ecc.u-tokyo.ac.jp
Institute of Functional Material Chemistry, Faculty of Chemistry,
b
Northeast Normal University, Changchun 130024, P. R. China
†
Electronic supplementary information (ESI) available: Experimental details,
NBO charges, PXRD, TGA, IR, AAS, NMR, XPS, and catalytic results. See DOI:
0.1039/d1cc03600a
1
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Scheme 1 Pinacol rearrangement. Pinacolone is the main product. 2,3-
Dimethyl-1,3-butadiene is formed via dehydration as a by-product.
Pinacol rearrangement was carried out with d-Al -CoW or
1
3
12
e-Al -CoW as solid acid catalysts. In the pinacol rearrange-
1
3
12
ment reaction, elimination of water from pinacol (a diol) gives
pinacolone (a ketone) via 1,2-methyl shift as a major product
and 2,3-dimethyl-1,3-butadiene (an olefin) via simple dehydra-
1
4
tion as a minor product (Scheme 1). As shown in Table 1, d-
Al13-CoW12 exhibited remarkably high catalytic activity (conv.
1
00% and 75% yield to pinacolone, Table 1) at 373 K and 10 h
1
(see Fig. S8 and S9, ESI† for the GC chart and H-NMR,
respectively).‡ These values were much higher than those for
e-Al13-CoW12 under the same reaction conditions (conv. 48%
Fig. 1 (a) Local structure of d-Al13 in d-Al13-CoW12 in the bc-plane. (b)
Crystal structure of d-Al13-CoW12 in the ac-plane. (c) Local structure of
e-Al13 in e-Al13-CoW12 in the ac-plane. (d) Crystal structure of and yield 13% to pinacolone, Table 1). The reaction was
e-Al13-CoW12 in the bc-plane. [AlO
rotated [Al 13] trimers are shown by light blue and purple polyhedrons,
respectively. [AlO ] and [WO ] units are shown by dark blue and light green
6
] units of the non-rotated and 601-
completely stopped by the removal of d-Al13-CoW12 (Fig. S10,
ESI†), indicating that the observed catalysis is truly heteroge-
neous. The PXRD pattern of d-Al13-CoW12 after the reaction was
analogous to that before the reaction (Fig. S11, ESI†), showing
3
O
4
6
polyhedrons, respectively. Possible hydrogen bonds (O–O o 3.01 Å) are
shown by the red dashed lines. Minimum apertures of d-Al13-CoW12 and
e-Al13-CoW12 are those of the channels running perpendicular to the plane that the crystal structure of the catalyst was maintained during
of the paper and are ca. 13 Å Â 7 Å and 12 Å Â 6 Å, respectively.
the reaction. Moreover, d-Al -CoW can be easily recovered
1
3
12
from the reaction mixture by simple filtration and washing with
toluene and can be reused at least once (conv. 100% and 76%
yield to pinacolone, Table 1).
2
0
among the solutes, solvent, and precipitate. We adopted the
inorganic salt-assisted dissolution–recrystallization approach to
To verify the catalytically active sites in d-Al13-CoW12, the
reaction was carried out in the presence of 2,6-lutidine, which
can selectively interact with Brønsted acid sites and cannot
obtain d-Al -CoW crystals with high yield. In a typical synthesis,
13
12
an aqueous solution containing d-Al13 was prepared by mixing
AlCl and NaOH in distilled water at 353 K and aging at 368 K.
Then, the d-Al13 solution was mixed with an aqueous solution
containing K [a-CoW12 40] and NaCl at room temperature,
2
1–23
3
interact with Lewis acid sites due to steric hindrance.
When 2,6-lutidine was added to the reaction solution after
h, no further reaction proceeded (Fig. 2b). This result
6
O
2
followed by immediate and spontaneous precipitation. Blue rod-
shaped crystals of d-Al13-CoW12 grew via dissolution–recrystalliza-
tion of the precipitate with a high yield of 70% based on POMs.
We note that the crystals do not grow without the addition of NaCl
nor the post-addition of NaCl to the synthetic solution.
indicates that Brønsted acid sites, attributable to the protons
of the aqua ligands of d-Al13 which are exposed to the pore
surface of d-Al13-CoW12, contribute to the reaction. Notably, the
removal of water molecules (i.e., both water of crystallization
and aqua ligands of d-Al ) by drying at 373 K completely
1
3
As shown in Fig. S1 and S2 (ESI†), the Le Bail fitting of the
powder X-ray diffraction (PXRD) pattern of the ground powder
sample of d-Al13-CoW12 indicates that the lattice parameters of
suppresses the catalytic activity (Table 1). These observations
1
9
the powder are consistent with those previously reported,
Table 1 Pinacol rearrangement at 373 K
confirming that the crystal structure of d-Al13-CoW12 shown in
Fig. 1a and b represent the whole bulk solid. The compound is
formulated as [d-Al O (OH) (H O) ][a-CoW O ] (OH)Á
Yield of
Yield of
pinacol
(%)
2,3-dimethyl-
1,3-butadiene
(%)
1
3
4
24
2
12
12 40
Conv.
(%)
2
22H O with inductively coupled plasma atomic emission spec-
Catalyst
trometry (ICP-AES) and thermogravimetry analysis (TGA)
d-Al13-CoW12
e-Al13-CoW12
d-Al13-CoW12
100
48
100
75
13
76
26
6
24
(
(
Fig. S3, ESI†). The results of atomic absorption spectrometry
AAS), ion-exchange chromatography (Cl), and X-ray photoelec-
tron spectroscopy (XPS) reveal that d-Al13-CoW12 does not (2nd run)
d-Al13-CoW12
dried at 373 K)
CoW12
o1
o1
o1
contain Na or Cl, which may originate from NaCl used in the
(
2
7
dissolution–recrystallization process. IR and Al solid-state
magic angle spinning (MAS) NMR spectroscopies show that Blank
0
0
0
0
0
0
6
À
the molecular structures of d-Al13 and [a-CoW12
maintained in d-Al13-CoW12 (Fig. S4–S7, ESI†).
O
40
]
are
Reaction conditions: toluene, 2 mL; catalyst, 0.067 mmol; pinacol,
0.667 mmol; naphthalene (internal standard), 0.267 mmol.
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Fig. 2 (a and b) Time courses of pinacol rearrangement catalyzed by d-
Al13-CoW12 at 373 K. In (b), 2,6-lutidine was added to the reaction solution
at 2 h. Black, red, and blue circles show the conversion of pinacol, the yield
of pinacolone, and the yield of 2,3-dimethyl-1,3-butadiene, respectively.
strongly support the idea that the Brønsted acid sites generated
from the water molecules in d-Al -CoW play a critical role in
1
3
12
the pinacol rearrangement.
To gain molecular-level insights into the isomer effect of Al13
on the observed catalytic activities, we examined the crystal-
lographic parameters of d-Al13 and e-Al13 in the PICs with a
specific focus on Al–O distances (O atoms are those of the aqua
ligands). The average distance of the twelve Al–O bonds in e-Al13
1
4
of e-Al13-CoW12 is 1.948 Å. As for d-Al13 in d-Al13-CoW12, the
average distance of the nine Al–O bonds in the non-rotated
[
Al O ] trimers is 1.924 Å, and that of the three Al–O bonds in Fig. 3 (a) Calculated natural bond orbital (NBO) charges of terminal
3 13
1
9
oxygen atoms of [Al O13] trimers in d-Al13 of d-Al13-CoW12 (left) and
3
the rotated [Al O ] trimer is 1.903 Å. Therefore, the Al–O
bond distances decrease in the order of e-Al13 4 d-Al13 (non-
rotated trimer) 4 d-Al13 (rotated trimer), and is the shortest for
3
13
1
e-Al13 of e-Al13-CoW12 (right). (b) solid-state H MAS NMR spectra (MAS
rate = 14 kHz) and (c) IR spectra of d-Al13-CoW12 (black) and e-Al13-CoW12
treated with pyridine. BAS: pyridine adsorbed on Brønsted acid site; LAS:
the characteristic rotated trimer, which is exposed to the pore pyridine adsorbed on Lewis acid site; PyH: hydrogen-bonded pyridine.
surface of d-Al13-CoW12. As is well known for the trend in the
n+ 24
2 6
acidity of aqua acids [M(OH ) ] , short Al–O distance would
elongate the O–H distance and weaken the O–H bond of the e-Al -CoW , those values were estimated as À0.982 to À0.983
13
12
aqua ligand, which may contribute to the increased Brønsted (see Fig. 3a and Tables S2–S4, ESI†). The slightly increased
acidity of d-Al -CoW . Furthermore, natural bond orbital negative charge in d-Al -CoW is probably related to the shorter
1
3
12
13
12
(
NBO) analysis was performed by utilizing the density func- Al–O distances, which would affect the states of protons of not
2
5,26
tional theory (DFT) method.
The NBO charges of the O only aqua ligands but also the water of crystallization.
13] trimer in
To investigate the states of the protons experimentally, we
d-Al13-CoW12 were estimated as À0.990 to À0.994. As for carried out solid-state H MAS NMR measurements. The NMR
3
atoms of the aqua ligands of the rotated [Al O
1
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spectrum of d-Al13-CoW12 shows a single signal at 4.9 ppm Notes and references
(
Fig. 3b), which has a slightly higher chemical shift than that of
‡
Pinacol to pinacolone rearrangement over various solid catalysts and
e-Al -CoW (4.6 ppm). Given the fact that a higher chemical
1
3
12
conditions are summarized in Table S1. Note that there are many
shift corresponds to a decrease in electron density of the superior catalysts for this reaction. In this work, catalytic reaction was
2
7,28
utilized as a characterization.
proton,
than e-Al13-CoW12, which is in line with the catalytic results
Table 1). The difference in Brønsted acidity was also character-
ized by IR spectroscopy using pyridine as a basic probe
d-Al -CoW would exhibit higher Brønsted acidity
13 12
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1
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1
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There are no conflicts to declare.
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