PKU-3: An HCl-Inclusive Aluminoborate for Strecker Reaction Solved by Combining RED and PXRD

Article abstract of DOI:10.1021/jacs.5b03685

A novel microporous aluminoborate, denoted as PKU-3, was prepared by the boric acid flux method. The structure of PKU-3 was determined by combining the rotation electron diffraction and synchrotron powder X-ray diffraction data with well resolved ordered

Full text of DOI:10.1021/jacs.5b03685

Communication
pubs.acs.org/JACS
PKU-3: An HCl-Inclusive Aluminoborate for Strecker Reaction Solved
by Combining RED and PXRD
Hong Chen,†,⊥ Jing Ju,‡ Qingpeng Meng,† Jie Su,† Cong Lin,‡,§ Zhengyang Zhou,‡,§ Guobao Li,‡
Weilu Wang,§ Wenliang Gao,§ Chunmei Zeng,§ Chiu Tang,∥ Jianhua Lin,*,‡ Tao Yang,*,‡,§
and Junliang Sun*,†,‡
†Berzelii Centre EXSELENT on Porous Materials, Department of Materials and Environmental Chemistry, Stockholm University,
106 91 Stockholm, Sweden
‡College of Chemistry and Molecular Engineering, Peking University, 100871 Beijing, China
§College of Chemistry and Chemical Engineering, Chongqing University, 400044 Chongqing, China
∥Diamond Light Source Ltd., Didcot, Oxfordshire OX11 ODE, U.K.
⊥Faculty of Materials Science and Chemistry, China University of Geosciences, 430074 Wuhan, China
S
* Supporting Information
diffraction as an alternative method to SC-XRD could be used to
ABSTRACT: A novel microporous aluminoborate, de-
noted as PKU-3, was prepared by the boric acid flux
method. The structure of PKU-3 was determined by
combining the rotation electron diffraction and synchro-
tron powder X-ray diffraction data with well resolved
ordered Cl− ions in the channel. Composition and crystal
structure analysis showed that there are both proton and
chlorine ions in the channels. Part of these protons and
chlorine ions can be washed away by basic solutions to
activate the open pores. The washed PKU-3 can be used as
an efficient catalyst in the Strecker reaction with yields
higher than 90%.
solve the structures with tiny crystal sizes as demonstrated in our
former studies in PKU-1,13 PKU-5,23 PKU-6,19 PKU-8,24 etc.
However, there are still a lot of reported aluminoborates without
knowing atomic structures.17,18,21,22 One typical example is
PKU-3 in this work, which was synthesized more than 10 years
ago without a structure model. The sample shows preferred
orientation with a needle-like morphology, and the severe peak
overlapping due to the trigonal symmetry (P3c1) and relatively
̅
large unit cell dimensions. Fortunately, the recently developed
rotation electron diffraction (RED) method can be used to
collect the 3D electron diffraction data for nanosized crystals,25
and it requires significantly lower electron dose compared to the
HRTEM imaging. However, one remaining problem in RED
data is the residue dynamical effects, which sometimes hinder the
structure determination.7,26−29 Therefore, combining the 3D
data feature of RED to partition the kinematic PXRD data is a
superior way, which would expect to reduce the difficulties
created by dynamical effects in RED and overlapping reflections
in PXRD. Here, we report the structure of aluminoborate PKU-3,
with the formula H24.3Al9B18O51·Cl3.3·6.8H2O, which contains
the light atoms down to B, solved by combining RED and PXRD.
The framework and even the ordered Cl− species in the channel
can be well resolved. Additionally, PKU-3 shows a rarely seen
acidic framework with inclusive HCl. Due to the similar Lewis
acidity to PKU-1 and PKU-2, PKU-3 can be used as a solid acid
catalyst in the Strecker reaction for the synthesis of α-
aminonitriles.
norganic microporous materials such as zeolites, phosphates,
borates, and germanates have attracted significant attentions
I
due to their diverse porous structure and wide applications in
adsorption, catalysis, and separation.1−9 These materials are
normally synthesized with structure directing agents (SDAs),
and their formation heavily relies on the chemical interaction,
such as hydrogen bonds and ionic interaction between the SDAs
and the framework. SDAs such as polyamines have strong
interactions with the framework, and the obtained materials
frequently show bigger crystal sizes but their open-frameworks
can hardly be retained after the calcination. A typical class of this
material is germanates. On the contrary, the frameworks
synthesized with the weakly interacted SDAs are much more
stable after calcination, such as silicate zeolites. Aluminoborates is
a type of materials where both strongly (the alkali/alkaline earth
metal ions, organic amines, etc.) and weakly interacted SDAs
(boric acid, water etc.) can be employed in the synthesis.10−19
When weakly interacted species such as H3BO3, H2O are
involved in the channels, the obtained aluminoborates are stable
after calcination and can be used as catalysts in different
reactions.13,16,20
PKU-3 was synthesized by the boric acid flux method with a
mixture of AlCl3·6H2O, H3BO3, and concentrated HCl solution
reacted in a 23 mL autoclave at 180 °C for 7 days. Needle-like
PKU-3 crystals with a typical size of 0.3 × 0.3 × 10 μm (Figure
S1) were harvested after washing with hot water at 80 °C. In
addition, the partially Cr-doped PKU-3 named as Cr-PKU-3 has
also been synthesized by using CrCl3·6H2O to replace part of
AlCl3·6H2O.
Unfortunately, aluminoborates synthesized with weakly
interacted guest species normally crystallized in small sizes, and
most of their structures were difficult to be elucidated by single
crystal X-ray diffraction (SC-XRD).16−19,21−24 Powder X-ray
Received: April 9, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b03685
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Journal of the American Chemical Society
Communication
The attempt of ab initio structure determination by high
resolution synchrotron PXRD data from the DIAMOND light
source was performed in EXPO 2009.30 Only three Al and a few
oxygen atoms could be directly located. The reasons for the
failure of obtaining light atomic positions include the complexity
of the diffraction pattern (such as the relatively large unit cell
dimensions (with a = 14.58438 (1) Å and c = 12.22262 (2) Å)
and the severe peak overlapping due to the trigonal symmetry
(86% within 0.2 fwhm below 1.0 Å resolution)),31 flexible
coordination environments for boron atoms, and the disordered
heavy chlorine anions in the channel. The RED method25,32 was
then employed to collect 3D electron diffraction data sets on the
crushed nanosized PKU-3 crystal. An overview of the RED data is
shown in Figure 1a, and a trigonal unit cell with the possible
results in a SC-XRD-like data for the structure solution by Shelxs.
After the structure model of PKU-3 was derived, all the non-
hydrogen atoms in the structure model were further refined
against the PXRD data by the Rietveld refinement in TOPAS 4.0
with Rp = 0.0471, Rwp = 0.0623, and Rexp = 0.0326 (Figure 2).35
The obtained crystallographic data of PKU-3 and refinement
details are listed in Table S2.
Figure 2. Rietveld refinement of powder X-ray diffraction for as-
synthesized PKU-3 (λ = 0.82613(1) Å). The curves from the top to
bottom in red, blue, and black are calculated, observed, and difference
curves, respectively.
PKU-3 is built with the structure building unit (SBU) of
[Al3B6O24] as shown in Figure 3a. This SBU includes three edge
Figure 1. Reconstructed 3D RED data. (a) Overview of the data and the
nanocrystal used to collect the RED data; due to the electron beam
damage, the reflections at high resolution following the clockwise
direction become weaker and weaker. (b−d) Selected planes in the
reciprocal lattice corresponding to hk0, h-hl, and hhl planes, respectively.
The reflection conditions can be obtained as h-hl, l = 2n, 00l, l = 2n. Due
to the dynamical effects, some weak reflections violate the reflection
conditions.
space group of P3c1 could index the whole data set as shown in
̅
Figure 1b−d. With the RED data, ab initio structure
determination by direct methods was successfully applied in
both SIR201133 and Shelxs 97.34 Although some oxygen atoms
were assigned as Al and the ordered Cl− ion was assigned as B,
the whole framework structure can be obtained by correcting the
element types and refined using Shelxl as shown in Table S1. The
wrong assignment of element types might be due to the residue
dynamical effects in RED data. Since the intensity summations
for overlapping reflections are very accurate in the synchrotron
data, we used RED data to repartition overlapping PXRD
reflections, and in such a combination, we took both advantages
of the nonoverlapping feature in RED and more accurate
intensities in PXRD. As expected, most framework atoms and an
ordered Cl− ion in the pores can be obtained directly with correct
element assignments (Al/Cl or B/O) by Shelxs 97. To the best of
knowledge, this is the first time that the atoms in the channel of
the open framework can be well resolved from RED data.
Besides, we can also conclude that even with a high Rint for RED
data (Rint = 0.48 in this case), its intensities can be used to
partially solve the overlapping problems in the PXRD data, which
Figure 3. Building unit of PKU-3. (a) [Al3B6O24] SBU; (b) Zig-zag
chain along the c-axis: AlO6 (pink octahedron), BO3 (orange triangle),
and BO4 (yellow tetrahedron).
shared AlO6 octahedra with two BO3 triangles and one BO4
tetrahedron on each side. The ordered BO3 and BO4 units were
confirmed by solid state 11B magic angle spinning (MAS) NMR
(peaks at 0.9 and 10.0 ppm, respectively, in Figure S2a),
obviously different from the disordered BO3 contained borates
such as NDTB-1.36 The 27Al MAS NMR showed that all Al
atoms are octahedrally coordinated with a peak at δ = −10.9 ppm
seen in Figure S2b, consistent with the structure model obtained
from diffraction experiments.16 Two different 3-rings exist in the
B
DOI: 10.1021/jacs.5b03685
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Journal of the American Chemical Society
Communication
[Al3B6O24] SBU: one is the AlO6−BO3−BO3 ring, which was
commonly seen in PKU-113 and PKU-2;16 the other is AlO6−
BO4−BO3 ring, which was also found in PKU-8. These
[Al3B6O24] SBUs are connected to each other to form a
composite columnar building unit along the c-axis by sharing the
oxygen atoms in AlO6 octahedra and BO3 triangles (Figure 3b).
These columns are further connected to each other through BO4
tetrahedra (Figure 3b), forming a three-dimensional (3D)
structure as shown in Figure 4a. Two one-dimensional channels
In order to explore the application of this framework material,
NH3·H2O and KOH mixed solution was used to wash the as-
synthesized PKU-3 in order to evacuate the channel, and part of
HCl can be removed with the surface area increasing from 42 to
172 m2/g compared to the as-synthesized PKU-3 (Figures S5−
S7). The EDS result for the washed PKU-3 showed Al/Cl molar
ratio of about 6.1 (6), indicating that all Cl− in the 12-ring
channel and part of Cl− in the 9-ring channel can be removed.
The in situ PXRD in Figure S3 shows that the framework of
PKU-3 is stable up to 300 °C in air, consistent with the
observation in the TG-DSC curve (Figure S4). Compared with
PKU-1 and PKU-2 where the framework is mainly built with
edge sharing AlO6, the lower thermal stability of PKU-3 may be
due to the corner sharing between the [Al3B6O24] SBUs or the
easy volatility of HCl in channels.
Similar to the Lewis acidity in PKU-1 and PKU-2,16,43 the
framework of PKU-3 with the BO3 (or B2O5, BO2OH) groups
potentially provides both Lewis and Brønsted acid sites. This
specific feature makes PKU-3 an attractive candidate as a solid
acid catalyst in the Strecker reaction for the synthesis of α-
aminonitriles (Scheme 1). As shown in Table 1 (entry 1−3), it
Scheme 1. Cyanosilylation of N-(Phenylmethylene)-
a
benzenamine Catalyzed by PKU-3
Figure 4. Three-dimensional framework of PKU-3: (a) view along c-
axis; (b) 9-ring and (c) 12-ring.
with different sizes are present in PKU-3 along the c-axis: the
small 9-ring channel with three SBUs in Figure 4b has a free
diameter of 0.47 × 1.96 Å; the large 12-ring channel with six
SBUs has a free diameter of 6.41 × 5.57 Å (Figure 4c), taking into
account of the van der Waals radius of oxygen 1.35 Å.
a
TMSCN: Trimethylsilyl cyanide.
a
Table 1. Yields of Cyanosilylation by PKU-3 and Cr-PKU-3
b
entry
catalyst
PKU-3
solvent
DCM
yield
Based on the refinement, EDS, and TGA-MS results, the
channels of PKU-3 are occupied by Cl− ions and water. In the
small 9-ring channel, one ordered Cl− ion was located directly
from the structure solution, while in the 12-ring channel, only
disordered Cl− and water molecules were found, and disordered
oxygen atoms with large ADPs were used to mimic their
scattering power in the refinement. The EDS results showed Al/
Cl molar ratio of about 2.7(6), and this additional 1.3 Cl− ion
should be located in the 12-ring channel in a disordered form.
Note that the large standard deviation of Al/Cl molar ratio may
be due to occupancy fluctuation of the disordered Cl− in the
channel and the error of the EDS technique. Thus, the formula
could be written as H24.3Al9B18O51·Cl3.3·6.8H2O, where the
amount of water was determined from TGA and the amount of
hydrogen atoms were calculated based on the charge balance of
the whole structure. Ion-exchange experiments show that the H+
and Cl− in PKU-3 can be partially exchanged by K+ and Br−
without destroying the framework structure, indicating a special
HCl inclusive feature for PKU-3. Compared to other synthesized
microporous material structures with monocharged spe-
cies,3,36,37 neutral structure direct agents38 or salts39−41 included
in the structural channel, the H+ and Cl− inclusive feature in
PKU-3 is unique. The only known example with both protons
and acid anions in the channel, similar to PKU-3, is the natural
mineral akaganeite, which is constructed from FeO6 octahedra
with H+ and Cl− in its 8-ring channels.42
1
2
3
4
5
6
79%
87%
74%
99%
93%
92%
CF
acetonitrile
DCM
Cr-PKU-3
CF
acetonitrile
a
Reaction conditions: N-(phenylmethylene)-benzenamine (0.2
mmol), TMSCN (0.4 mmol), solvent (1 mL), catalyst (20% equiv),
b
1
3 h, RT. Yield was determined by H NMR.
shows the highest yield (87%) of α-aminonitriles in chloroform
(CF) within 3 h, which indicates that nonpolar solvent is
beneficial to this reaction. Note that the blank experiment in CF
(without adding any solid catalyst) shows no yield at all. It is
consistent with the results in literature,44,45 where the
uncatalyzed reactions show only trace conversion. It has been
proved that the Strecker reaction can be effectively catalyzed by
heterogeneous catalysts either containing solid acid sites or
transition metal centers,46 and therefore, chromium doped PKU-
3 (denoted as Cr-PKU-3) can be considered as a bifunctional
catalyst with more active centers. Indeed, Cr-PKU-3 has a
superior activity (Table 1, entry 4−6) in all used solvents; the
yield of α-aminonitriles reaches 99% in DCM within 3 h.
Moreover, we did not observe any appreciable loss in the catalytic
C
DOI: 10.1021/jacs.5b03685
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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approves the recyclability of PKU-3 in the Strecker reactions.
In summary, a novel aluminoborate framework denoted as
PKU-3 was successfully synthesized. The structure was
determined by the combination of RED and synchrotron
PXRD, and this combination was proved to be as powerful as
single-crystal X-ray diffraction. The ordered Cl− can be resolved
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ASSOCIATED CONTENT
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* Supporting Information
(25) Wan, W.; Sun, J.; Su, J.; Hovmoller, S.; Zou, X.; Hovmo, S. J. Appl.
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Crystallogr. 2013, 46, 1863−1873.
Experimental details and crystal structure information files. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/jacs.5b03685.
(26) Su, J.; Kapaca, E.; Liu, L.; Georgieva, V.; Wan, W.; Sun, J.;
Valtchev, V.; Hovmoller, S.; Zou, X. Microporous Mesoporous Mater.
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AUTHOR INFORMATION
A.; Zou, X.; Yaghi, O. M. J. Am. Chem. Soc. 2013, 135, 16336−16339.
(28) Willhammar, T.; Yun, Y.; Zou, X. Adv. Funct. Mater. 2013, 24,
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■
Corresponding Authors
*junliang.sun@pku.edu.cn
*jhlin@pku.edu.cn
(29) Martínez-Franco, R.; Moliner, M.; Yun, Y.; Sun, J.; Wan, W.; Zou,
X.; Corma, A. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 3749−3754.
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Notes
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The authors declare no competing financial interest.
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(32) Zhang, D.; Oleynikov, P.; Hovmoller, S.; Zou, X. Zeitschrift fur
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ACKNOWLEDGMENTS
Krist.−Cryst. Mater. 2010, 225, 94−102.
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This project is supported by the National Basic Research
Program and Nature Science Foundation of China (No.
2013CB933402, 91222106, 91222107, 21471009), the Swedish
Research Council (VR), the Swedish Governmental Agency for
Innovation Systems (VINNOVA), the Knut & Alice Wallenberg
Foundation through the project grant 3DEM-NATUR, and a
grant for purchasing the TEM. Diamond light source is
acknowledged for the beamtime.
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