Octahedra-based molecular sieve aluminoborate (PKU-1) as solid acid for heterogeneously catalyzed Strecker reaction

Article abstract of DOI:10.1016/j.catcom.2014.09.004

An aluminoborate (PKU-1) with octahedron-based framework was found to be a highly efficient solid acid catalyst for the rapid and convenient synthesis of α-aminonitriles from imines and TMSCN under mild reaction conditions. The catalytic active sites are

Full text of DOI:10.1016/j.catcom.2014.09.004

Catalysis Communications 58 (2015) 174–178
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Octahedra-based molecular sieve aluminoborate (PKU-1) as solid acid for
heterogeneously catalyzed Strecker reaction
⁎
⁎
Weilu Wang, Ying Wang, Bin Wu, Rihong Cong, Wenliang Gao , Bo Qin , Tao Yang
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 May 2014
Received in revised form 29 August 2014
Accepted 3 September 2014
Available online 16 September 2014
An aluminoborate (PKU-1) with octahedron-based framework was found to be a highly efficient solid acid
catalyst for the rapid and convenient synthesis of α-aminonitriles from imines and TMSCN under mild reaction
conditions. The catalytic active sites are related to the borate hydroxyl groups in the 18-ring structural channels.
The borate hydroxyl groups play a critical role as Brønsted acid to protonate imine and activate TMSCN, thus
providing the imine intermediate and producing CN anions, respectively.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Aluminoborate
Solid acid
α-Aminonitriles
TMSCN
Mechanism
1. Introduction
specific structure features [17]. As shown in Fig. 1, its backbone consists
of edge-sharing octahedra [AlO6] that form 18- and 10-ring windows.
The Strecker reaction is an important process in organic syntheses
and can grant access to a wide range of interesting compounds such
as α-aminonitriles [1]. α-Aminonitriles are key precursors as chiral
building blocks in the pharmaceutical industry, particularly in the
preparation of α-amino acids, and other various nitrogen or sulfur-
containing heterocycles [2]. α-Aminonitriles are usually prepared by
the nucleophilic addition of the cyanide anion to an imine in the
presence of a homogeneous acid catalyst [3]. Conventional catalytic
processes using homogeneous catalysts are highly efficient, however
they show several disadvantages, such as the use of expensive or toxic
catalytic systems, difficulty in separation, tedious work-up and waste
discarding [4]. From this aspect, heterogeneous catalysis has emerged
as a promising alternative, reducing the waste production, providing a
straightforward and simple separation/recovery of the catalysts. As far,
various heterogeneous systems either containing transition metal cen-
ters or strong acid sites have been introduced in the Strecker reaction,
and high yields of α-aminonitriles can also be achieved in the presence
of those solid catalysts. The representative catalysts are molecular sieves
[5,6], natural polymer sulfuric acid [7,8], nanosize materials [9,10],
sulfated tungstate [11], MOF[12], supported complexes [13–15], and
heteropolyacids [16].
Borate groups, in the forms of [BO3] or [B2O5] fragments, decorate to
the [AlO6] framework by sharing vertex oxygen atoms, and borate hy-
droxyl groups can provide theoretically Lewis and Brønsted acid sites.
These intrinsic features make PKU-1 an attractive candidate to catalyze
the synthesis of α-aminonitriles. More importantly, our investigations
indicate that the existence of Brønsted acid sites in PKU-1 is prerequisite
in the Strecker reaction, because Lewis acid sites from tri-coordinated
B(III) alone are not enough to promote the effective accomplishment
of the above model reaction. This result provides useful details for
Herein we report a novel octahedron-based molecular sieve PKU-1
(Al3B6O12(OH)5·nH2O) as a recoverable heterogeneous catalyst for the
Strecker reaction. PKU-1 is an open-framework aluminoborate with
⁎
Corresponding authors.
E-mail addresses: gaowl@cqu.edu.cn (W. Gao), qinbo1979@163.com (B. Qin).
Fig. 1. Projection of the PKU-1 structure along the c-axis.
http://dx.doi.org/10.1016/j.catcom.2014.09.004
1566-7367/© 2014 Elsevier B.V. All rights reserved.

W. Wang et al. / Catalysis Communications 58 (2015) 174–178
175
understanding the catalytic mechanism of the Strecker reaction in a
heterogeneous system.
2. Experimental sections
2.1. Catalyst syntheses and characterizations
PKU-1 was prepared as described in reference [17]. Powder X-ray dif-
fraction was performed on a PANalytical X'pert diffractometer equipped
with a PIXcel detector and Cu Kα radiation. Scanning electron microscopy
(SEM) was performed on a JSM-7800F electron microscope. Thermogra-
vimetric analysis (TGA) was performed on a NETZSCH STA449C instru-
ment under Ar flow. The acidity was measured with temperature-
Fig. 3. TG curve of PKU-1.
programmed desorption of NH3 (NH3–TPD) using a Quantachrome
ChemBET. Nuclear magnetic resonance (NMR) spectra were obtained
using Bruker Avance (500 MHz) or Agilent VNMRS 400 MHz at ambient
temperature. High resolution mass spectra (HRMS) were collected using
Bruker SolariX to determine the structure of some substrates and prod-
ucts. The structure views of PKU-1 (Figs. 1 and 4) were drawn using the
software Atoms 6.0.
2.2. Catalytic evaluation
Reaction solvents were purified by distillation method. Other
reagents were used as received. Thin layer chromatography (TLC) was
carried out using Merck 0.2 mm silica gel 60F-254 Al-plates (200–
300 mesh). 1H NMR spectra were recorded in dimethyl sulfoxide-d6
(DMSO) using tetramethylsilane (TMS) as an internal standard. The
following abbreviations are used to report NMR spectroscopic data:
s = singlet, d = doublet, t = triplet, m = multiplet, dt = doublet of
triplets. 1H NMR spectra of the products were used to determine the
yield of the obtained compounds. 13C NMR spectra were recorded
using DMSO solvent as an internal standard.
2.2.1. Synthesis of N-tosyl aldimine substrates
Scheme 1 gives the synthesis of N-tosyl aldimines, and the details
are provided in the Electronic Supplementary Information (ESI). 1H
NMR spectra and 13C NMR spectra of all substrates were collected
(Figs. S1–S10) and placed in ESI. All five substrates were proved to be
successfully produced, in which, 1a, 1d and 1e agreed well with refer-
ences[18] and [19], respectively. Other two substrates 1b and 1c were
identified with high resolution mass spectrum and NMR, and the corre-
sponding MS data (Figs. S21–S22) were also placed in ESI.
Fig. 2. a) Powder XRD of PKU-1 before and after five-run reaction; SEM images b) before
and c) after five-run reaction.
Fig. 4. Three types of borate hydroxyl groups in PKU-1, a) 18- and b) 10-ring windows.

176
W. Wang et al. / Catalysis Communications 58 (2015) 174–178
It should be mentioned that there are three types of borate hydroxyl
groups in the framework of PKU-1 [17], i.e. BO2(OH)/BO(OH)2 and
B2O4(OH). As shown in Fig. 4, the B2O4(OH) groups located within the
10-ring rectangular windows and almost completely block the channels
along the [100] directions. The BO2(OH) and BO(OH)2 groups locate
within the 18-ring windows and narrow (but not block) the channels
along the [001] direction. The existence of BO2(OH) and BO(OH)2
provides abundant Brønsted acid sites, while three-fold coordinated
boron atoms probably act as Lewis acid sites.
The observed low- and high-temperature bands centered at ~365 °C
and ~560 °C for NH3–TPD analyses correspond to moderate and strong
acidic sites, respectively (see Fig. 5). The quantitative analysis indicates
that the total acid amount is about 0.172 mmol/g. Since [AlO6] octahe-
dra in PKU-1 are covered by borate groups, it is reasonably speculated
that borate groups probably behave as the acid centers. Therefore, the
low-temperature desorption band probably comes from the ammonia
bonded with the weak-moderate Brønsted acidic sites inside the
cavities (for instance, BO2(OH) and BO(OH)2 in the 18-ring windows).
It should be noted that B2O4(OH) cannot provide Brønsted acid sites be-
cause B2O4(OH) groups block the 10-ring channels and NH3 molecules
cannot approach these acidic sites. As for high-temperature desorption
band, it should mostly originated from NH3 bonded with strong Lewis
acidic centers.
Fig. 5. Acidic measurements using NH3–TPD for PKU-1 catalyst.
2.2.2. Cyanation of N-tosyl aldimines catalyzed by PKU-1
Scheme 2 gives the cyanation of N-tosyl aldimines catalyzed by PKU-
1, and details are provided in ESI. 1H NMR spectra and 13C NMR spectra
of all products were collected (Figs. S11–S20) and placed in ESI. The
products 2a, 2b and 2d were in complete agreement with references
[20–22], respectively. Other two products 2c and 2e were identified
with high resolution mass spectrum and NMR, and the corresponding
MS data (Figs. S23–S24) were also placed in ESI.
3.2. Catalytic activity
We first performed the reaction using different imines as model
substrates to find the optimal condition, and the results are shown in
Table 1 (entries 1-5). Substrate 1a has shown a higher yield than
other imines ,therefore 1a was selected as model substrate to synthesize
α-aminonitriles.
3. Results and discussion
3.1. Catalyst characterizations
At room temperature, different solvent including acetone, dichloro-
methane, chloroform, and tetrahydrofuran, was used as a reaction me-
dium. The obtained results were summarized in Table 1 (entries 1, 6–
8). It indicates that chloroform solvent is the suitable medium and the
maximum catalytic activity of 91% was acquired at the given operational
conditions, whereas other solvents such as dichloromethane, tetrahy-
drofuran, and acetone were found to be relatively inferior.
Usually, the chemical equivalent of the used catalysts or cyanation
reagent TMSCN also has a key influence on the yield of aminonitriles.
Here, the yield decreased along with decline of the equivalent of the cat-
alyst. The use of 0.2 equiv. of catalyst was sufficient to produce an excel-
lent yield of the product (Table 1, entries 1, 9–11). As for the amounts of
TMSCN, a similar tendency was displayed and a distinct decrease in yield
was observed with reducing the equiv. of TMSCN. The use of 2 equiv. can
provide a prominent yield of the product (Table 1, entries 12–15).
Preliminary investigation also showed that the rates of the reaction
were significantly affected by the temperature, and a remarkable in-
crease in yields was observed at 35 °C compared to other temperatures
As shown in Fig. 2a, all the XRD peaks for PKU-1 can be indexed
using the trigonal cell lattice (a = 22.038, c = 7.026 Å). No impurity
peak was observed, which indicates the successful synthesis of pure
PKU-1. After five runs for catalytic reaction, the recycled PKU-1 has
the identical XRD pattern, and the sharp diffraction peaks suggest its
high durability. As shown in Fig. 2b and Fig. 2c, SEM images clearly indi-
cate that the fresh PKU-1 are aggregations of tiny rod-like particles, and
the average size of the nanoparticles is estimated to be ~0.2 μm in diam-
eter and ~2 μm in length. The recycled PKU-1 basically retains the
morphology.
The TGA curve shows two stages of weight loss (see Fig. 3). The first
one occurs below 160 °C and belongs to the removal of crystalline H2O
molecules; the second one is between 160 and 600 °C and related to the
dehydration of borate hydroxyl groups. Above 680 °C, the framework
eventually collapsed and transformed to the anhydrous compound
Al4B2O9.
Scheme 1. Synthesis of N-tosyl aldimines.

W. Wang et al. / Catalysis Communications 58 (2015) 174–178
177
Scheme 2. PKU-1 catalyzed Strecker reaction between N-tosyl aldimine and TMSCN.
(Table 1, entries 12, 16–19). It is worth noting that a further increase in
temperature has a detrimental influence in yields. Commonly speaking,
a higher reaction temperature is usually favorable in the promotion of
effective transformation of reactants. However, the used reactants
become unstable and easily decompose into other byproducts at high
temperatures, thus resulting into a significant decrease of the yield.
We also evaluate the reusability of the heterogeneous catalysts of
PKU-1. The solid catalysts were easily recovered from the reaction
mixture by simple filtration, and then used for a new batch with fresh
reagents. PKU-1 can be reused at least five times without any obvious
loss of catalytic activity (Table 2).
Owing to the vacant 2pπ orbital, tri-coordinated boron atoms are
commonly used as potential Lewis acid centers [9,12]. Herein, however,
PKU-1 exhibits a completely different behavior that Brønsted acid sites
are irreplaceable and function as active centers in the cyanosilylation of
imine. Without borate hydroxyl groups, Strecker reaction cannot pro-
ceed and no product will be observed. As shown in Table 1 (entries
20–21), when PKU-1 was thermally heated at 200 °C for 2 h, the yield
of product 2a decreased from 91% to 15%. If PKU-1 was further calcined
at a higher temperature of 400 °C, the yield of product 2a was b1%. This
result infers that borate hydroxyl groups probably function as Brønsted
acid centers and plays an irreplaceable role in the bond activation. If
heating PKU-1 at high temperatures, borate hydroxyl groups would be
gradually dehydrated and escaped from the framework in the form of
water, which is clearly evidenced by an in-situ infrared spectrum (IR)
[17] (see Fig. S25 in ESI). As a consequence, the amounts of active cen-
ters are significantly reduced along with the disappearance of Brønsted
acid sites. For the as-synthesized PKU-1, there clearly shows three dis-
tinct O–H vibrations at 3180, 3440 and 3650 cm−1, respectively, corre-
sponding to three types of hydroxyl groups in the structure, i.e.
BO2(OH), B2O4(OH) and BO(OH)2. With the heating temperature in-
crease, the O–H vibrations became weak, and eventually disappeared
at 400 °C, indicating the progressive loss of the Brønsted acid sites in
the dehydration process. Interestingly, If PKU-1 was calcined at 400 °C
and placed in air overnight, its catalytic activity can completely restore
the same level as before (the yield is 90%, Table 1, entry 22), which sug-
gests that the active acid centers can be well re-activated and PKU-1 cat-
alyst has the superior stability.
As for the catalytic mechanism of the Strecker reaction, it is reason-
ably speculated that the imine can be protonated to produce an active
imine intermediate. Then, this intermediate reacts with a trapped CN−
anion to produce the final product. It seems that Lewis and protonated
acidic sites of the PKU-1 catalyst synergistically affect the reactive sub-
strates and intermediates during the progress of the reaction. As a mat-
ter of fact, TMSCN substrate is also necessary to be activated to provide
CN− anion in the presence of hydroxyl groups, [9] otherwise the crucial
step of the nucleophilic addition would intend to be blocked up due to
the deficiency of CN− anion. Therefore, borate hydroxyl groups are
critical in the Strecker reaction and the individual Lewis acid sites
from tri-coordinated B(III) alone are not enough to promote the
effective accomplishment of the above reaction.
Table 1
Summary of the results obtained for Strecker reaction using PKU-1 catalyst.a
Entry Solvent
Imines Catalyst Time/h TMSCN Reaction Yield/
equiv.
equiv.
temp./°C (%)b
1
2
3
4
5
6
7
8
Chloroform
1a
1b
1c
1d
1e
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.05
0.02
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
3
3
3
3
3
3
3
3
2
2
2
2
2
2
2
2
2
2
2
2
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
0
91
90
83
71
85
51
59
86
56
14
14
23
15
12
11
11
32
20
13
15
b1
90
Chloroform
Chloroform
Chloroform
Chloroform
Tetrahydrofuran 1a
Dichloromethane 1a
4. Conclusions
Acetone
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
In summary, PKU-1, as an octahedron-based molecular sieve, has
abundant Brønsted and Lewis acid sites in its structural channels and
therefore can act as an efficient solid acid catalyst in the Strecker
reaction. Compared to homogeneous systems, the present procedure
9
Chloroform
Chloroform
Chloroform
Chloroform
Chloroform
Chloroform
Chloroform
Chloroform
Chloroform
Chloroform
Chloroform
Chloroform
Chloroform
Chloroform
3
3
3
10
11
12
13
14
15
16
17
18
19
20c
21d
22e
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
3
1
0.5
0.2
2
2
2
2
2
2
2
Table 2
Recyclability of PKU-1 catalyst for the Strecker reaction.a
35
50
60
R.T.
R.T.
R.T.
Cycle
Yield/(%)b
TON
1
2
3
4
5
91
90
88
89
91
4.55
4.5
4.4
4.45
4.55
3
3
a
Reaction conditions: 0.2 mmol imines, 1 mL solvent, PKU-1 catalyst, argon atmosphere.
Yield was determined by 1H NMR spectroscopy.
PKU-1 was treated at 200 °C for 2 h.
PKU-1 was treated at 400 °C for 2 h.
PKU-1 was treated at 400 °C for 2 h, then placed in ambient air overnight.
b
c
a
Reaction conditions: 0.2 mmol imines, 1 mL solvent, 2 equiv. TMSCN, 0.2 equiv. PKU-1
catalyst, 3 h, R. T., argon atmosphere.
d
e
b
Yield was determined by 1H NMR spectroscopy.

178
W. Wang et al. / Catalysis Communications 58 (2015) 174–178
represents a clean, practical, simple, mild, and time-saving method in
the synthesis of α-aminonitriles and high yields were obtained. Our
investigations also suggest that Brønsted acid sites are necessary for
the catalytic process and in fact, play a critical role in the Strecker
reaction using PKU-1 as a heterogeneous catalyst. This result may
provide an insight in the exploration of catalytic mechanism for the
Strecker reaction in heterogeneous systems.
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Acknowledgment
This work was supported by the Nature Science Foundation of China
(Grants 91222106, 21101175, 21171178, 21272293) and the Natural Sci-
ence Foundation Project of Chongqing (CSTC 2012jjA0438, 2012jjA50007,
2014jcyjA50036).
Appendix A. Supplementary data
Synthetic procedure for imine substrates and products; 1H and 13
C
NMR spectra of five substrates and five products; HRMS for substrates
(1b, 1c) and products (2c, 2e); infrared spectra of PKU-1 degassed at
200, 250, 300 and 400 °C (Fig. S25). This material is available free of
charge via the internet at http://www.elsevier.com. Supplementary
data associated with this article can be found, in the online version, at
doi: http://dx.doi.org/10.1016/j.catcom.2014.09.004. This data include
MOL file and InChiKey of the most important compounds described in
this article.