Microporous aluminoborates with large channels: Structural and catalytic properties

Article abstract of DOI:10.1002/anie.201106310

Channel zapping: PKU-1 and newly synthesized PKU-2 (Al₂B ₅O₉(OH)₃·n H₂O; see picture) possess microporous structures with 18-ring and 24-ring channels, respectively. They show high reactivity and size

Full text of DOI:10.1002/anie.201106310

DOI: 10.1002/anie.201106310
Microporous Catalysts
Microporous Aluminoborates with Large Channels: Structural and
Catalytic Properties**
Tao Yang, Agnieszka Bartoszewicz, Jing Ju, Junliang Sun, Zheng Liu, Xiaodong Zou,
Yingxia Wang, Guobao Li, Fuhui Liao, Belꢀn Martꢁn-Matute,* and Jianhua Lin*
Zeolites and related porous materials are widely used as acid
catalysts for a large number of reactions.[1,2] Furthermore, the
molecular dimensions of the pores can provide size selectivity
for certain chemical transformations of molecules that are
smaller than the pores or of comparable size to the pore
dimensions, which is a unique phenomenon happening in
porous materials.[2] The microporosity of catalysts, however,
may also restrict the diffusion rates of the reactants and
products, thereby limiting the activity.[3] It is well-known that
the diffusivity is proportional to the pore diameter. Large
pores increase the diffusion coefficients, thereby increasing
the potential of the material as an effective catalyst. It is
therefore highly desirable to prepare materials with large
pores. PKU-1 (HAl3B6O12(OH)4·nH2O), a porous alumino-
borate with channels formed by 18 octahedrally coordinated
atoms,[4] contains B and Al centers that can serve as Lewis
acid sites.[5] Herein we present the synthesis and structure
ologies of PKU-1 and PKU-2 are the same, but the former
consists of 18-ring channels, and the latter contains extra-large
pores of 24-ring channels, which makes the two materials
intriguing representative examples to study the reactivity and
selectivity versus the pore size by investigating the catalytic
performance.
PKU-2 was synthesized by direct reaction of AlCl3·6H2O
with H3BO3 heated to reflux at 2408C in a closed system. Only
very tiny needle-shaped crystallites were obtained (Figure S1
in the Supporting Information). Attempts to grow larger
single crystals by varying the B/Al ratio and extending the
reaction time were unsuccessful. Poor crystallization is also
indicated by the broad peaks observed in the powder X-ray
diffraction (XRD) pattern (Figure 1a), which can be easily
indexed to be trigonal using a hexagonal unit cell, a =
30.489(2) ꢀ and c = 7.013(1) ꢀ. The systematic absences
and intensity distribution in the electron diffraction (ED)
ꢀ
determination of
a
new aluminoborate,
PKU-2
patterns fit the space groups R3 or R3 (Figure S2 in the
(Al2B5O9(OH)3·nH2O), with larger channels formed by 24
octahedrally coordinated atoms. Both PKU-1 and PKU-2
possess large pores and Lewis acid centers, thus making them
potential candidates for heterogeneous catalysts. The top-
Supporting Information). Owing to the low quality of the
XRD data and the relatively large unit cell of PKU-2, the
detailed structure could not be solved directly. However, the
high-resolution transmission electron microscopy (HRTEM)
image taken along the [001] direction of PKU-2 (Figure 1b)
and the similarity of the cell parameters to PKU-1 (a = 22.038
and c = 7.026 ꢀ), allowed us to propose a reasonable struc-
ture model.
In our previous work, we described a structural rule that
applies to aluminoborate systems with octahedral-based
frameworks:[4,6] two types of unique connections between
AlO6 octahedra (cis and trans) were considered to be the
building units for this type of porous frameworks (Figure S3
in the Supporting Information). Herein, the similar c param-
eters of PKU-1 and PKU-2 indicate that the AlO6 octahedra
connected in a cis geometry form the same threefold helical
chains along the c axis, and the approximately 8.5 ꢀ larger a
parameter in PKU-2 suggests that the hexagonal channels are
larger. Therefore, by inserting an additional trans-AlO6 unit
into each of the six 18-ring edges in PKU-1, we proposed that
the AlO6 backbone of PKU-2 contains 24-ring channels
(Figure 1c). This assumption is supported by the HRTEM
image along the c axis, which agrees well with the image
produced by simulation by using such a structure model
(inserted in Figure 1b).
[*] Dr. T. Yang, Dr. J. Ju, Prof. Y. Wang, Prof. G. Li, F. Liao, J. Lin
Beijing National Laboratory for Molecular Sciences, State Key
Laboratory for Rare Earth Materials Chemistry and Applications
College of Chemistry and Molecular Engineering, Peking University
Beijing 100871 (China)
E-mail: jhlin@pku.edu.cn
Dr. J. Sun, Dr. Z. Liu, Prof. X. Zou
Berzelii Center EXSELENT on Porous Materials and Department of
Materials and Environmental Chemistry, Stockholm University
SE-106 91 Stockholm (Sweden)
A. Bartoszewicz, Prof. B. Martꢀn-Matute
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University
SE-106 91 Stockholm (Sweden)
E-mail: belen@organ.su.se
Dr. Z. Liu
Nanotube Research Centre, National Institute of Advanced
Industrial Science and Technology (AIST)
Higashi 1-1-1, Tsukuba, 305-8565 (Japan)
[**] This work was supported by the Nature Science Foundation of
China. Financial support from the Swedish Research Council (VR),
the Swedish Governmental Agency for Innovation Systems
(VINNOVA), the Faculty of Natural Sciences at Stockholm Univer-
sity, the Knut and Alice Wallenberg Foundation, the Gçran-
Gustafsson Foundation, and the Berzelii Center EXSELENT is
gratefully acknowledged.
The BO3 borates are crucial for the charge balance that
stabilizes the Al octahedral framework.[4,6] The character-
ization of PKU-2 by 27Al and 11B magic-angle-spinning NMR
(MAS NMR) and IR spectroscopy (Figures S4 and S5 in the
Supporting Information) indicates that the Al and B atoms
are coordinated in octahedral and trigonal-planar geometry,
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106310.
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Considering the charge balance,
the formula should be
Al2B5O9(OH)3. Thermogravi-
metric analysis shows a two-
step weight loss (see the inset
of Figure S7 in the Supporting
Information). The first approx-
imately 17.4 wt% loss below
2008C corresponds to the loss
of guest water molecules in the
channels. The second weight
loss (ca. 8.6 wt%) at approxi-
mately 5008C corresponds to
the dehydration of the borate
groups (calculated 8.9 wt%).
The in situ high-temperature
XRD measurements show that
the framework of PKU-2 is
retained up to 4008C (Figure S7
in the Supporting Information).
The nitrogen adsorption iso-
therm of PKU-2 exhibits a typ-
ical microporous behavior with
a
large BET surface area
(900 m2 gÀ1, Figure S8 in the
Supporting Information).
The B and Al atoms of
PKU-1 and PKU-2 can act as
Lewis acid sites[5] and thus coor-
dinate the oxygen atom of alde-
hydes, which increases their
reactivity towards nucleophilic
attack. The Lewis acid catalyzed
cyanosilylation reaction of alde-
hydes was chosen to study the
catalytic activity of the two
frameworks (Scheme 1).[8,9] In
Figure 1. a) Rietveld refinement on powder X-ray diffraction of PKU-2; b) HRTEM image along the [001]
direction of PKU-2 showing the 24-ring channels (white features). Insets are the corresponding ED
pattern (right) and a simulated image based on the structure model (left); c) projected structure views of
both PKU-1 and PKU-2 along the c axis, showing the 18- and 24-ring channels, respectively.
respectively. The geometry of the octahedral framework in
PKU-2 seems able to accommodate dimeric and trimeric
borate groups B2O5 and B3O7. Simulated annealing was
performed using the TOPAS software[7] with the borate
groups treated as rigid fragments. A good fit of the Rietveld
refinement (Figure 1a) can be achieved using the above-
mentioned structure model (Figure 1c); the refined parame-
ters are given in the Supporting Information. As expected, all
oxygen atoms in the Al octahedral frameworks in PKU-2 are
coordinated to boron through two different 3-ring units,
either 2AlO6 + BO3 or AlO6 + 2BO3, which is the same as in
PKU-1 (Figure S6 in the Supporting Information). The slight
difference is that in PKU-1 only the borate group B2O5 is
present, while both B2O5 and B3O7 can be found in PKU-2.
This difference is consistent with the expansion of the channel
size. Although the structure is deduced by a chain of soft
evidence, the final refined structure agrees well with the
structural and chemical principles, that is, with reasonable
this transformation, highly versatile cyanohydrin trimethyl-
silyl ethers are produced, which can be readily converted into
important compounds such as a-hydroxycarboxylic acids or b-
amino alcohols.[10]
Scheme 1. Cyanosilylation of aldehydes catalyzed by PKU-1 or PKU-2.
Both PKU-1 and PKU-2 were first washed with water at
408C for four hours to remove the remaining boric acid
species from the pores. After filtration, the samples were
dried under vacuum at 1108C for another four hours. After
cooling to ambient temperature under a nitrogen atmosphere,
the frameworks were tested as heterogeneous catalysts. The
results are shown in Table 1. Benzaldehyde gave high yields of
the corresponding cyanohydrin trimethylsilyl ether after 18 h
at ambient temperature in the presence of either PKU-1 or
PKU-2 (1.5–2 mol%; Table 1, entry 1). More sterically
À
À
À
À
Al O, Al Al, Al B, and O O distances (Table S3 in the
Supporting Information).
On the basis of the crystallographic study, the framework
composition of PKU-2 is determined to be Al2(B2O5)(B3O7).
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crude mixtures by 1H NMR spectroscopy revealed a 25%
conversion for PKU-1 and 48% for PKU-2. Thus, after three
days in the absence of the solid frameworks the conversions
remained almost the same as those obtained after four hours
in the presence of the catalysts. This result demonstrates that
the reaction is only catalyzed by the heterogeneous catalysts
and not by homogeneous species present in the reaction
media. Moreover, both heterogeneous catalysts PKU-1 and
PKU-2 could be recycled and reused up to five times (Table 2)
without any decrease of the catalytic activity, thus suggesting
high stabilities of the catalysts.
Table 1: Cyanosilylation of a variety of aldehydes (1a–e) catalyzed by
PKU-1 and PKU-2.[a]
Entry
Aldehyde (1)
Yield of 3 [%][b]
PKU-1
PKU-2
1
2
93
100
23/75[c]
34/86[c]
47
3
10
Table 2: Recycling and reuse of PKU-1 and PKU-2 in the cyanosilylation
of benzaldehyde (1a).[a]
4
5
27
9
98
26
Catalyst
Yield of 3 [%][b]
run 1
run 2
run 3
run 4
run 5
PKU-1
PKU-2
86
97
72
100
80
100
88
100
84
100
[a] PKU-1 or PKU-2 (20 mg, 0.020 mmol, 1.5–2 mol%) was dried under
vacuum for 4 h at 1108C. After cooling the sample to 238C under a
nitrogen atmosphere, a solution of trimethylsilyl cyanide (140 mL,
1 mmol) and aldehyde (0.5 mmol) in degassed dichloromethane (1 mL)
was added. The reaction mixtures were stirred at 238C. [b] Unless
[a] See the Experimental Section. [b] The yield was determined by
1H NMR spectroscopy of the crude mixture after 20 h.
1
In summary, we have synthesized the 3D porous octahe-
dral framework PKU-2 (Al2B5O9(OH)3·nH2O), which con-
tains extra-large 24-ring channels. The structure model of
PKU-2 was deduced from its topological similarity to PKU-1
and further refined by powder X-ray diffraction. The model
was also confirmed by 27Al and 11B MAS NMR and IR
spectroscopy as well as HRTEM. The catalytic activity of
PKU-1 and PKU-2 as Lewis acids was investigated in the
cyanosilylation reaction of aldehydes under mild reaction
conditions. Both frameworks are highly active for the
cyanosilylation of small substrates, whereas only PKU-2
with larger channels formed by 24 octahedrally coordinated
atoms catalyzes the reaction for larger substrates.
otherwise noted, the yield was determined by H NMR spectroscopy of
the crude mixture after 18 h. [c] Yield after three days.
demanding substrates, such as 1b–e (Table 1, entry 2–5), gave
lower conversions after 18 h in the presence of either PKU-1
or PKU-2. Only after three days, good conversions were
obtained with both catalysts (Table 1, entry 2). With PKU-2
as the catalyst higher conversions were obtained in all cases,
which may be explained by the larger pore dimensions of this
framework. However, such differences in activity could also
be caused by the different particle size of the two materials:
the average particle size of PKU-1 is 2 ꢁ 2 ꢁ 20 mm, which is
much larger than that of PKU-2 (50 ꢁ 50 ꢁ 200 nm). To
minimize the difference in particle size between the two
materials we grinded a sample of PKU-1 to the size of (150 Æ
50) nm, which is comparable to the size of PKU-2. We then
performed the cyanosilylation reaction of substrate 1d in two
parallel experiments under identical reaction conditions using
both frameworks (now with similar average particle size). The
reaction conditions were identical to those of Table 1, and
very similar results were obtained (i.e. 13% using PKU-1,
97% using PKU-2). This experiment indicates that the
activity of PKU-1 is not significantly affected by the particle
size, and that PKU-2 shows a significantly higher catalytic
activity than PKU-1.
To confirm that the reaction was catalyzed by the
frameworks and not by homogeneous Al or B species leached
into the solution, further control experiments were per-
formed. The reaction of benzaldehyde (1a) with TMSCN (2)
catalyzed by PKU-1 and PKU-2 was started as described in
Table 1, and after four hours the catalysts were removed by
filtration. Conversions of 21% and 38% had been reached
with PKU-1 and PKU-2, respectively, after these four hours.
The reaction mixtures were stirred for additional three days
after removal of the heterogeneous catalysts. Analysis of the
Experimental Section
Synthesis of PKU-2: Typically, AlCl3·6H2O (5 mmol) and H3BO3
(100 mmol) were sealed into a 50 mL Teflon autoclave and the
mixture was heated at 2408C for 15 days. After cooling to room
temperature, the solids (containing the PKU-2 and residue H3BO3)
were washed extensively with hot water (508C) until the residual
boric acid was completely removed (yield 90% with respect to Al).
Since PKU-1 and PKU-2 can be synthesized under similar conditions,
the dependence of the products on reaction temperatures, time, and
the B/Al ratio in the systems were extensively studied. PKU-1 formed
easily at relatively low temperatures and in the first few days. Once
the reaction temperature was elevated to at least 2408C with longer
reaction time (more than 10 days), the chance to form PKU-2 either
as an admixture with PKU-1 or a pure phase increased. Chemical
analysis carried out by using the ICP method on an ESCALAB2000
analyzer showed that the B/Al ratio (ca. 2.3) was consistent with the
proposed formula Al2B5O9(OH)3·nH2O.
Characterizations: Powder-XRD data were collected at room
temperature with a Bruker D8 diffractometer with Bragg–Brentano
geometry with a curved germanium primary monochromator (Cu Ka1
l = 1.5406 ꢀ). The tube voltage and current were 50 kV and 40 mA,
respectively. Scan step size and time: 0.02 (2q) and 30 s. For TEM
investigations, the PKU-2 powder was crushed in a mortar and
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Communications
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Keywords: aluminoborates · borates · cyanosilylation ·
heterogeneous catalysis · microporous materials
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