Continuous solid solutions constructed from two isostructural octahedron-based molecular sieves: preparation, acidity regulation and catalytic application in Strecker reactions

Article abstract of DOI:10.1039/c9nj04406j

Strecker reactions have an important application in the preparation of α-amino acids and biologically active molecules in homogeneous acid- or base-catalyzed systems. As novel solid acid catalysts, Al_xGa_1-x-PKU-1 (x = 0-1) solid solu

Full text of DOI:10.1039/c9nj04406j

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Continuous solid solutions constructed from two
isostructural octahedron-based molecular sieves:
preparation, acidity regulation and catalytic
application in Strecker reactions†
Cite this: New J. Chem., 2019,
43, 18184
a
a
Weilu Wang,a Chunmei Zeng,ab Yao Yang,a Pengfei Jiang,
Wenliang Gao,
*
a
a
Rihong Cong
and Tao Yang
*
Strecker reactions have an important application in the preparation of a-amino acids and biologically
active molecules in homogeneous acid- or base-catalyzed systems. As novel solid acid catalysts,
AlxGa1Àx-PKU-1 (x = 0–1) solid solutions constructed by two octahedron-based molecular sieves were
prepared by using a molten boric acid method and exhibited a continuously-adjustable acidity in
heterogeneously-catalyzed Strecker reactions under mild conditions. AlxGa1Àx-PKU-1 accommodated a
large number of Brønsted acid sites in the framework, and the acidic density and strength were
enhanced with the successive incorporation of Al atoms, thus the catalytic activity became superior in
the formation of a-aminonitriles. Three-fold coordinated boron atoms, despite the electron-deficient
configuration, seemed unable to behave as Lewis acid centers probably due to steric hindrance against
the adsorption of large organic substrates. This work illustrates, for the first time, possible application in
Strecker reactions using zeolite-like solid solutions, and will potentially provide an effective method and
a beneficial reference to control surface acidity for porous materials.
Received 26th August 2019,
Accepted 18th October 2019
DOI: 10.1039/c9nj04406j
rsc.li/njc
readily accessible cavities and extensive porosity, however, the main
drawbacks are their rather low thermal and chemical stability.4–6
Introduction
Strecker reactions have attracted considerable attention in The supported homogeneous catalysts have also been developed as
recent years because of their extensive implications in the solid acids for Strecker reactions, however because of the strong
preparation of a-amino acids, biologically active molecules, tendency of H2O coordination to acidic sites, the catalytic centers
and nitrogen or sulfur-containing heterocycles.1,2 Among them, gradually became inactive probably due to the formation of Lewis
homogeneous systems are commonly employed due to their acid–base adducts.7–9 In general, the common feature of these
excellent enantioselectivity as well as high yields, however the employed catalysts is to possess a certain amount of acidic and/or
major deficiency lies in the difficulty in catalyst separation and basic sites, and usually, some post-treatment methods, such as
reutilization, workup scalability and discarding waste.3 In order to surface sulfonation, ion exchange, dealuminization, metal doping,
avoid the above problems, various solid catalysts have been etc. are needed to enhance or regulate their surface acidity/basicity.
investigated in the past decades, including metal–organic frame-
In addition to the above-mentioned post-treatment modifi-
works (MOFs),4–6 supported complexes,7–9 solid bases,10 molecular cations, preparing solid solutions is also regarded as an effective
sieves,11,12 acid-modified catalysts,13,14 polymers,15,16 nano-sized method to control surface acidity/basicity for porous materials.
materials,17 and solid oxides.18,19 For example, metal–organic Making solid solutions is very common in the synthesis of bulk-
frameworks (MOFs) provide a promising catalysis-friendly process type oxides, however it is rare when synthesizing molecular sieves
in Strecker synthesis, benefitting from their large surface areas, or zeolite-like ones. Herein, we intend to report continuous solid
solutions composed of two octahedron-based molecular sieves
a College of Chemistry and Chemical Engineering, Chongqing University,
Chongqing 401331, People’s Republic of China. E-mail: gaowl@cqu.edu.cn,
taoyang@cqu.edu.cn
b Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province,
College of Chemistry and Chemical Engineering, China West Normal University,
Nanchong 637002, People’s Republic of China
(Ga-PKU-1 and Al-PKU-1), featuring ultra-large channels with an
18-membered ring window.20,21 In particular, the surface acidity
was proven to be continuously tunable with successive metal
substitutions. As a matter of fact, during the last decades, several
octahedron-based aluminoborate molecular sieves (such as PKU-2
and PKU-3) have also been synthesized using the molten boric
acid method.22–26 However, thus far, only PKU-1 has been
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9nj04406j
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extensively studied in catalysis,27–32 benefitting from its easy pre- according to the fact that a poor catalytic activity was observed after
paration, easy metal substitution and high density of acidic sites or losing the B–OH groups (by heating AlxGa1Àx-PKU-1). This is pre-
redox centers. Because of the features in the structure and func- sumably due to the steric hindrance against the approach of large
tional diversity, PKU-1 is a good candidate to accommodate a large organic substrates. Nevertheless, when AlxGa1Àx-PKU-1 was calcined
number of Brønsted or Lewis acid sites to promote Strecker at a higher temperature to transform into another so-called mullite-
reactions. From this perspective, compared to other heterogeneous type structural phase, a large number of Lewis acid or Lewis base
catalysts, as-designed continuous solid solutions are a good alter- sites were created due to the unsaturated AlO5 or GaO5 groups in the
native to allow an environmentally friendly process under mild framework. At last, the reaction mechanism for Strecker reactions
conditions. As expected, the obtained catalytic results showed that was also proposed to illustrate the reaction paths catalyzed over
the reconstructed hybrid catalysts can efficiently control the AlxGa1Àx-PKU-1 solid solutions.
amounts and strength of the acid sites, and also proved to be
efficient catalysts in the Strecker reaction with excellent yields of
a-aminonitriles and an added advantage of catalyst recyclability.
Experimental section
Preparation
Single crystal X-ray diffraction revealed that the framework
of Ga-PKU-1 is exclusively constructed by edge-sharing GaO6
In a typical run for the preparation of Al0.5Ga0.5-PKU-1, 5 mmol
octahedra, and an 18-membered-ring channel is formed along
the [001] direction.21 In order to neutralize the negative charges
of the Ga–O framework, three types of borate hydroxyl groups
attach to the GaO6-framework in the form of BO(OH)2, BO2(OH)
and B2O4(OH) fragments. Further investigations indicated that
Al can replace Ga in the whole concentration range to form
complete solid solutions (denoted as AlxGa1Àx-PKU-1, 0 r x r 1).
Due to the obvious difference in atomic configuration between Ga
and Al, these as-prepared solid solutions tend to exhibit a tunable
acidity with the increase of x. As shown in Fig. 1, the framework of
PKU-1 seems to provide two kinds of acidic centers, one is the
Brønsted acid site coming from borate hydroxyl group B–OH, and
the other one is the Lewis acid site originating from the Al3+/Ga3+
ion or BO3 group. Structurally speaking, AlxGa1Àx-PKU-1 can be
considered as a solid acid, thus it has potential to heterogeneously-
catalyze the synthesis of a-aminonitriles.
of b-Ga2O3 and 10 mmol of Al(NO3)3Á9H2O were dissolved in
2 mL of concentrated HNO3 and heated in a sealed autoclave at
180 1C for 5 h. After cooling, the autoclave was opened to
evaporate the liquid slowly and 0.3 mmol of H3BO3 was added.
The autoclave was sealed again and heated at 220 1C for another
5 days. After the reaction, the obtained solid was washed exten-
sively with warm water until the residual boric acid was comple-
tely removed. The final product after drying at 80 1C was a white
powder with a high yield (490%) according to metal sources.
Other AlxGa1Àx-PKU-1 samples with different Al/Ga compositions
were prepared in a similar procedure. For simplicity, all these
as-synthesized samples were denoted as Ga-PKU-1, Al0.2Ga0.8-PKU-1,
Al0.5Ga0.5-PKU-1, Al0.8Ga0.2-PKU-1 and Al-PKU-1.
Characterization
In this study, AlxGa1Àx-PKU-1 (0 r x r 1) molecular sieves were Powder X-ray diffraction (PXRD) measurements were per-
synthesized using a molten boric acid method and were further formed using a PANalytical Empyrean powder diffractometer
evaluated in the Strecker reactions. The nucleophilic addition (Cu Ka radiation, l = 1.5406 Å) with an operating voltage of
of imines by cyanide ions was gradually strengthened with the 40 kV and a current of 40 mA. Le Bail fitting was performed
increase of Al concentration. A catalytic survey also evidenced that using the TOPAS software package.33 The surface morphology
AlxGa1Àx-PKU-1 only offers a single type of acidic site, i.e. Brønsted was determined by field emission scanning electron micro-
acid sites (B–OH). Three-fold coordinated boron atoms, despite their scopy (JEOL, JSM-7800F), which was equipped with an energy
electron deficiency, seemed not to behave as Lewis acid centers dispersive spectrometer (EDS) analyzer. The powder samples
were prepared by casting the suspension of material onto a
double-sided conductive adhesive tape. The thermal stability of
AlxGa1Àx-PKU-1 was studied using a Mettler-Toledo TGA/DSC1
instrument in high purity nitrogen from room temperature
to 900 1C with a ramping rate of 10 1C minÀ1. Temperature-
programmed desorption of ammonia (NH3-TPD) was per-
formed using a Quantachrome ChemBET Pulsar instrument
equipped with a TCD detector to test their acidic properties.
Typically, AlxGa1Àx-PKU-1 samples were treated with helium for
1 h before the adsorption experiment of NH3. Then, the sample
was saturated in 5% NH3/He-mixed gas (80 mL minÀ1) at 100 1C
for 60 min. Helium was used as a carrier gas at a constant flow
rate in the temperature range from 100 1C to 527 1C at a
ramping rate of 10 1C minÀ1 after removing the weakly-
physisorbed NH3 by flushing helium for 30 min. The desorbed
Fig. 1 Octahedron-based framework of PKU-1 with 18-membered ring
NH3 was absorbed by a standard solution of H2SO4 (0.01 M),
which was then titrated with NaOH (0.001 M) for ascertaining
windows along the [001] direction; Grass green octahedron, AlO6 or GaO6;
orange plane triangle, BO3; cyan sphere, O atom; grey sphere H atom.
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the amount of acidic sites in the presence of the indicator of
Typical procedure for the cyanation of N-tosyl aldimines
phenolphthalein. Fourier transformation infrared (FT-IR) spec- catalyzed by AlxGa1Àx-PKU-1 catalysts. Scheme 2 shows the
tra were collected using a NICOLET iN10 MX (in the range reaction equation for the cyanation of N-tosyl aldimines.
1000–3800 cmÀ1) to measure the framework vibration of the At room temperature (R.T.), a mixture of a specified solvent
as-prepared materials. 1H NMR and 13C NMR data were (1 mL), cyanotrimethylsilane (TMSCN) (0.4 mmol), 1a (1b, 1c,
recorded using a Bruker Avance (500 MHz) or Agilent VNMRS 1d or 1e, 0.2 mmol) and AlxGa1Àx-PKU-1 (0.2 equiv., x = 0, 0.20,
400 MHz at 25 1C with dimethyl sulfoxide-d6 (DMSO) as the 0.50, 0.80, 1.00) was stirred for 3 h under a N2 atmosphere.
solvent. High-resolution mass spectrometry was performed on After the reaction was completed, the mixture was first
a Bruker solarix 7.0 T and by the ESI-FTICR-MS method.
quenched with saturated NaHCO3 aq., then extracted with
dichloromethane three times, dried with MgSO4 and further
concentrated under a reduced atmosphere. The obtained pro-
ducts were labeled as 2a (2b, 2c, 2d or 2e), and their yields were
determined by 1H NMR of aldimines. The spectrum data of these
5 products agreed well with the reported ref. 27, indicating their
successful synthesis, and the 1H and 13C NMR data of all products
and the HRMS of the two compounds are also listed in the ESI.†
Experimental procedure for the synthesis of a-aminonitriles
Organic solvents were purified by the distillation method using
CaH2 or Na as a desiccant. Other reactants were used as
received. Analytical thin layer chromatography (TLC) was per-
formed to monitor the reaction process using Merck 0.2 mm
silica gel 60 F-254 Al-plates (200–300 mesh). The following
abbreviations are used to record NMR spectroscopic data: s =
singlet, d = doublet, t = triplet, m = multiplet, and dt = doublet
of triplets. 1H NMR spectra of the products were used to
determine the yield of the obtained compounds.
Results and discussion
Catalyst syntheses and characterization
Typical procedure for the syntheses of N-tosyl aldimine
precursors. Scheme 1 shows the reaction equation for the synthe- Powder XRD spectra of these as-synthesized AlxGa1Àx-PKU-1
sis of N-tosyl aldimine precursors. 4-Methyl-benzenesulfonamide materials are shown in Fig. 2a, and all curves exhibited similar
(12 mmol) was added into a 50 mL flask containing the mixed diffraction patterns with that of the parent sample (Ga-PKU-1).
solution of toluene (25 mL) and benzaldehyde (10 mmol). While All these patterns can be indexed using the trigonal cell lattice
refluxing the solution at 110 1C for 60 min, 2 mmol of BF3OEt2 in the space group R3, indicating that metal Al and Ga can
was slowly added into the reaction mixture. After quenching with mutually substitute to form continuous solid solutions in the
NaOH solution (1 M), the reaction solution was extracted with whole concentration range. The unit cell parameters including
ethyl acetate three times, washed with brine several times, and a, c and V of the other four samples were also calculated from
dried over anhydrous Na2SO4. Then, the mixture was concen- Le Bail refinements and exhibited a linear contraction along
trated to obtain the primary product. In some cases, this with the increase of Al content (see Fig. 2b), which coincides
primary product was purified using flash column chromato- with the fact that the ionic radius of Al3+ (0.535 Å, coordination
graphy to afford a pure substrate, 4-methyl-N-phenylmethylene- number (CN) = 6) is smaller than that of Ga3+ (0.620 Å, CN = 6).
benzenesulfonamide (1a).
As a representative, the XRD pattern of Al0.2Ga0.8-PKU-1 was
If using 2,4,6-trimethyl-benzaldehyde, 3,4-dimethyl-benz- used to calculate its cell parameters with TOPAS software and
aldehyde, 4-methyl-benzaldehyde or 4-(1-methylethyl)-benzaldehyde showed a good convergence (see Fig. 2c). For an exact observa-
as a starting reagent, the corresponding precursor noted as 1b, tion, when the strongest diffraction peaks of AlxGa1Àx-PKU-1 at
1c, 1d or 1e was obtained. The spectrum data of these 5 2y = B81 (Miller indices (110)) were enlarged, the gradual shift
precursors agreed well with the reported ref. 27, indicating to lower 2y angle can be clearly observed with the increase of
their successful synthesis, and the 1H and 13C NMR data of Al content (the inset of Fig. 2c). The above results provide
these 5 precursors and the HRMS of the two compounds are convincing evidence of the successful formation of continuous
listed in the ESI.†
solid solutions in the entire concentration range.
Scheme 1 Synthesis of 1a precursor using 4-methylbenzenesulfonamide and benzaldehyde as reactants.
Scheme 2 Strecker reaction between N-tosyl aldimines and TMSCN catalyzed over AlxGa1Àx-PKU-1.
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Fig. 2 (a) PXRD patterns of AlxGa1Àx-PKU-1; (b) calculated cell parameters a, c, and V by Le Bail refinements; (c) Le Bail fitting for Al0.2Ga0.8-PKU-1. The
blue circles J and red solid line represent the observed and calculated patterns respectively; the difference curve (in black) is also shown below the
diffraction curves. The inset gives the strongest peak (110), which shows a gradual shift to lower angles with the increase of x.
SEM was employed to observe the morphology of the as- Ga-rich samples, a slender fascicular-arranged crystalline mor-
prepared AlxGa1Àx-PKU-1 samples. As shown in Fig. 3, the type phology was thermodynamically preferred, while the densely-
and amount of metal element have an important influence on packed rod-liked crystals tend to be formed for aluminum-rich
the sample morphology, and Ga seems able to increase the PKU-1 samples. The relative metal content of as-synthesized
crystallization and gives a more orderly arrangement. As for AlxGa1Àx-PKU-1 was determined using energy dispersion spectro-
Fig. 3 SEM images of as-prepared AlxGa1Àx-PKU-1 samples. (a) x = 0; (b) x = 0.2; (c) x = 0.5; (d) x = 0.8; (e) x = 1; (f) EDS analysis of the Al0.8Ga0.2-PKU-1
sample. The values in red are the atomic ratios of Ga/Al determined by EDS analysis.
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H3BO3 dispersed inside the channels; the second one centered
at B372 1C or 364 1C, however, is probably attributed to the
dehydroxylation of borate hydroxyl groups located in the frame-
work. Despite the similarity in the decomposition temperature,
Al-PKU-1 has a larger total weight-loss percentage than Ga-PKU-1
(nearly twice), which denotes that the former has more hydroxyl
groups in the framework than the latter and therefore can provide
more sufficient Brønsted acid sites.
Catalytic performance
Hereinafter, AlxGa1Àx-PKU-1 solid solutions with different Al
contents were used as the solid acids to survey the catalytic
nucleophilic addition of the cyanide anion to imines in the
Strecker reactions. TMSCN was chosen as the cyanide source
because it is an effective, safe and easily-handled cyanation
reagent compared to HCN and alkali cyanides. Under the given
reaction conditions, imine 1a was first used as the model
reactant to react with TMSCN, and the obtained catalytic results
are given in entries 1–5 of Table 1. With the increase of Al
concentration, the yields of 2a grew quickly, and the completely
Al-substituted PKU-1 (Al-PKU-1) is nearly 4 times more active
than Ga-PKU-1. Such a catalytic activity tendency is obviously
related with the acidic properties of AlxGa1Àx-PKU-1. As indicated
by NH3-TPD, there was a successive increase in both acidic
strength and acidic amount along with the incorporation of Al
atoms; it seems that Al atoms can bring more active sites to
activate the nucleophilic addition of imines. In addition, the
scope of the Strecker reaction was extended to different imine
substrates, and the other 4 aromatic imines were employed to
react with TMSCN to synthesize a-aminonitrile. The results are
also summarized in Table 1 (entries 6–25). Similarly, the yields of
a-aminonitriles followed the same rules as using 1a imine, and
the highest yields were achieved in the presence of Al-PKU-1
catalyst.
Fig. 4 NH3-TPD measurements of the as-prepared AlxGa1Àx-PKU-1 solid
solutions.
scopy (EDS), as indicated in Fig. 3, where the measured molar
ratios of n(Ga)/n(Al) are in good agreement with the proportion
used in the starting materials.
NH3-TPD was employed to monitor the acidity evolution for
all AlxGa1Àx-PKU-1 catalysts. Generally speaking, the tempera-
ture of NH3 desorption is related to the acidic strength, while
the integration area below the curve corresponds to the amount
of acid sites.34 As shown in Fig. 4, these different AlxGa1Àx-PKU-1
solid solutions display very different acidic properties. In the
whole temperature range, all AlxGa1Àx-PKU-1 samples only give
a single desorption peak, and the NH3-desorption peaks shift
towards high temperatures with the increase of x. That is to
say, the acidic strength becomes stronger with the successive
incorporation of Al atoms. Fig. 4 also gives the amount of acidic
sites, and these quantitative data indicate that the introduction of
Al3+ results in a linear increase in the number of total acid sites.
PKU-1 with a higher aluminum content probably has a better
performance in catalytic Strecker reactions.
In theory, AlxGa1Àx-PKU-1 can provide both Brønsted and
Lewis acid sites, the former coming from B–OH and the latter
from the electron-deficient B atoms (BO3 groups) owing to their
vacant 2pp orbitals. Three-fold coordinated B atoms are usually
considered as an efficient Lewis acid center, for example, BF3
was used as a strong Lewis acid in some homogeneously acid-
catalyzed organic reactions.35 As for AlxGa1Àx-PKU-1, however,
the further investigations gave the indication that BO3 groups
in the framework do not exhibit any acidic attributes. As shown
in Table 2, when these as-synthesized AlxGa1Àx-PKU-1 catalysts
(x = 0, 0.5, 1) were preheated at high temperatures, the yields of
2a all declined dramatically. Taking Al-PKU-1 as an example,
the Al-PKU-1 sample without the heating treatment gave the
yield as high as 91%, however, after heating at 523 and 673 K,
the yields of 2a declined to one eighth and one fifteenth of the
original values. The above results agree with the destruction of
Brønsted acid centers in AlxGa1Àx-PKU-1. As the Brønsted acid
centers, B–OH groups in the framework can effectively activate
the resolved reactants in the imine cyanation, however they are
forced to be released gradually in the form of H2O during high
temperature calcination; finally, the destruction of these active
sites will result in a rapid decline in the catalytic activity for
The thermal behaviors of Al- and Ga-PKU-1 catalysts were
measured by thermogravimetric analysis (TGA), and both TGA
curves show a two-step weight loss in the whole temperature
range from 27 to 900 1C. As shown in Fig. 5, the first step
centered at B110 1C or B120 1C should come from the release
of weakly-absorbed water molecules and the dehydration of
Fig. 5 Thermal behaviors of Al-PKU-1 and Ga-PKU-1.
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Table 1 AlxGa1Àx-PKU-1 (x = 0, 0.2, 0.5, 0.8, 1) catalyzed Strecker reactions using different aromatic imines
Entry
Imine
Product
Catalyst
Yield (%)
1
2
3
4
5
Ga-PKU-1
24
43
62
85
91
Al0.2Ga0.8-PKU-1
Al0.5Ga0.5-PKU-1
Al0.8Ga0.2-PKU-1
Al-PKU-1
6
7
8
9
Ga-PKU-1
16
40
57
80
90
Al0.2Ga0.8-PKU-1
Al0.5Ga0.5-PKU-1
Al0.8Ga0.2-PKU-1
Al-PKU-1
10
11
12
13
14
15
Ga-PKU-1
21
31
51
72
83
Al0.2Ga0.8-PKU-1
Al0.5Ga0.5-PKU-1
Al0.8Ga0.2-PKU-1
Al-PKU-1
16
17
18
19
20
Ga-PKU-1
20
24
48
60
71
Al0.2Ga0.8-PKU-1
Al0.5Ga0.5-PKU-1
Al0.8Ga0.2-PKU-1
Al-PKU-1
21
22
23
24
25
Ga-PKU-1
12
40
55
66
85
Al0.2Ga0.8-PKU-1
Al0.5Ga0.5-PKU-1
Al0.8Ga0.2-PKU-1
Al-PKU-1
imine cyanation. Based on the above experimental facts, it is
rationally inferred that the three-fold coordinated B atoms are
Table 2 Yields of 2a catalyzed by thermally-treated AlxGa1Àx-PKU-1 at unable to offer the Lewis sites to activate Strecker reactions.
different temperatures
Probably, these small-volume B atoms are difficult to be
approached efficiently by these large organic substrates owing
to steric hindrance.
FT-IR spectra can also display obvious changes of vibration
modes to evidence the above-mentioned dehydroxylation pro-
cess. As shown in Fig. 6, at room temperature, both Ga-PKU-1
and Al-PKU-1 give a broad and strong absorption peak in the
range of 3000–3800 cmÀ1, corresponding to O–H stretching
vibrations of weakly-absorbed water molecules and hydroxyl
groups in the framework. After heating at 250 1C, the above
absorption band tends to shrink until splitting into several
independent peaks. In this process, the O–H vibration peaks
originating from physically-absorbed water molecules become
weaker due to the progressive escape of water molecules; in the
meantime, hydroxyl groups in the framework are still thermo-
dynamically stable and therefore their vibration modes become
more visible. Namely, three independent O–H vibration peaks,
located at around 3175, 3434 and 3640 cmÀ1, appeared
Entry
Catalyst
Temperature (1C)
Yield (%)
1
2
3
4
5
6
7
8
Al-PKU-1
Al-PKU-1
As-synthesized
250
400
627
As-synthesized
250
400
91.0
10.9
6.4
17.0
62.0
7.4
3.6
24.0
5.4
Al-PKU-1
Al-PKU-1a
Al0.5Ga0.5-PKU-1
Al0.5Ga0.5-PKU-1
Al0.5Ga0.5-PKU-1
Ga-PKU-1
As-synthesized
9
10
11
Ga-PKU-1
250
400
627
Ga-PKU-1
3.5
85.0
Ga-PKU-1b
a
b
Al-PKU-1 has transformed into Al4B2O9 at this temperature. Ga-PKU-1
has transformed into Ga4B2O9 at this temperature.
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Fig. 6 FT-IR spectra of Ga-PKU-1 and Al-PKU-1 treated at different temperatures.
prominently after heating. Judging from the crystal microstruc- 5-coordinated Ga3+; these oxygen atoms seem to act as the
ture, these three vibration peaks coincide with three kinds of adsorption sites in base-catalyzed Strecker reactions.10
–OH groups in the framework, i.e. BO2(OH), B2O4(OH) and
Reusability is an important factor to value the performance
BO(OH)2. Upon further heating to a higher temperature, of heterogeneous catalysts, and therefore, the recycling experi-
Ga-PKU-1 and Al-PKU-1 will undergo a complete dehydration ments using three types of AlxGa1Àx-PKU-1 catalysts (x = 0, 0.5, 1)
process to lose most of the B–OH groups, thus eventually resulting were performed, and the heterogeneous nature of this reaction
in the collapse of the framework and the disappearance of allowed the quick recovery of the solid catalyst from the solution.
Brønsted acid sites. In addition, compared to the as-prepared After each catalytic run, the powder catalyst was separated by
PKU-1 sample, there appeared two new vibration peaks at around centrifugation followed by solvent-washing. The recovered catalyst
2340 and 1860 cmÀ1 at 250 1C, and these newly-emerged two was dried and activated at 100 1C for 60 min prior to the next run.
vibration peaks exactly reflected the transient changes of Fig. 7 reveals that no considerable activity loss for AlxGa1Àx-PKU-1
borates or hydroxyl groups in the PKU-1 framework during catalysts (x = 0, 0.5, 1) was observed for at least 4 successive runs.
the dehydroxylation process.
The XRD pattern for the recovered catalyst (after 4 runs) also
It is remarkable that Ga-PKU-1 and Al-PKU-1 exhibited exhibited no obvious decline of crystallinity. So, AlxGa1Àx-PKU-1
entirely different catalytic behaviors when they were further catalysts are stable and can maintain their framework structure
calcined at a higher temperature. As indicated in entries 4 and under the performed reaction conditions.
11 of Table 2, the yields of 2a showed a significant increase
Catalytic mechanisms of imine cyanations promoted by
after a severe heat-treatment at 627 1C. In detail, when raising Brønsted acid sites have been discussed elsewhere.3,15 As illu-
the temperature from 400 to 627 1C, the catalytic activity of strated in Fig. 8, the imine was first activated by the massive B–OH
Al-PKU-1 and Ga-PKU-1 abnormally increased by 2 and more groups by protonation to produce an active imine intermediate;
than 20 times, respectively. In essence, although PKU-1 has thereafter, TMSCN was activated by the O2À site neighboring the B
completely lost its Brønsted acid sites, it was possible to atom to coordinate with O2À to form a 5-coordinated silicate
restructure a large number of new Lewis acid/base sites in its species so that the cyanide group was polarized to acquire more
framework to accelerate imine cyanation during the calcination reactivity. The resultant intermediate species of 5-coordinated
process. As a matter of fact, the framework of Al- or Ga-PKU-1 is silicate was proved elsewhere by solid state 29Si MAS-NMR
thermodynamically unstable at 627 1C and will transform into a spectra.37,38 In the meantime, the activated imine by Brønsted
mullite-type structure (Al4B2O9 or Ga4B2O9).36 Al4B2O9 and acid sites was attacked by these highly reactive cyanide ions to
Ga4B2O9 have a similar crystal structure, in which octahedral form an N-TMS intermediate, which was subsequently hydrolyzed
units (AlO6 or GaO6) share edges in a trans-manner forming to other products due to the cleavage of the rather labile N–Si
one-dimensional chains along the b-direction, and the chains bond by a trace amount of moisture present in the solvent, which
are further cross-linked into a mullite-type structure by BO3, is very important to accelerate the transformation of the substrate,
BO4, AlO4 and AlO5 in Al4B2O9 or BO3, BO4 and GaO5 as observed earlier by Feng et al.3
in Ga4B2O9. Theoretically, Al or Ga atoms with unsaturated
coordination are potential Lewis acid sites, and organic substrates
tend to bond with these unsaturated sites in the energetically-
favored configuration to be activated. The significant increase of
Conclusion
2a yield using the Ga4B2O9 catalyst is most likely from the In conclusion, complete solid solutions of AlxGa1Àx-PKU-1
generation of stronger basic centers, i.e. m3-O neighboring with (x = 0, 0.2, 0.5, 0.8, 1), composed of two molecular sieves with
18190 | New J. Chem., 2019, 43, 18184--18192 This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

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Fig. 7 Structural stability identified by PXRD patterns of Ga-PKI-1 (a), Al0.5Ga0.5-PKU-1 (b), Al-PKU-1 (c) after four cycles and the reusablity (d). Reaction
conditions: 0.2 mmol of imines (1a), 0.4 mmol of TMSCN, AlxGa1Àx-PKU-1 catalyst (0.2 equiv., x = 0, 0.5, 1), 1 mL of chloroform, N2 atmosphere, 3 h. The
yield was calculated according to 1H NMR measurements.
octahedron-based frameworks (Ga-PKU-1 and Al-PKU-1), have
been facilely synthesized by the molten boric acid method. The
as-synthesized AlxGa1Àx-PKU-1 solid acid catalysts become more
efficient in the cyanation of aldimines with the increase of x. The
enhancement in catalytic activity was largely contributed from the
stronger Brønsted acidic strength and more Brønsted acidic sites
generated with the incorporation of Al atoms, which was evidenced
with the NH3-TPD measurement. Despite their electron deficiency,
boron atoms in AlxGa1Àx-PKU-1 seem unable to act as Lewis acidic
centers to catalytically activate Strecker reactions probably due to the
steric hindrance against the approach of large organic substrates
according to the fact of the severe weakening of a-aminonitrile yields
after a dehydroxylation process at given temperatures. Despite the
great challenges in the design and preparation of solid solutions
in the field of porous materials, AlxGa1Àx-PKU-1 has potentially
provided an effective method and a beneficial reference to
control surface acidity or basicity of porous materials.
Conflicts of interest
Fig. 8 The proposed reaction mechanism of Strecker reactions over
AlxGa1Àx-PKU-1 solid acids.
There are no conflicts to declare.
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18 N. V. Costantini, A. D. Bates, G. J. Haun, N. M. Chang and
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21671028, 21771027, 21805020), Natural
Science Foundation of Chongqing (cstc2019jcyj-msxmX0312,
cstc2019jcyj-msxmX0330), Chongqing Special Postdoctoral
Science Foundation (XmT2018004) and Postdoctoral Research
Foundation of China (2018M643402). We also acknowledge the
support from the sharing fund of large-scale equipment of
Chongqing University (201712150075).
22 T. Yang, A. Bartoszewicz, J. Ju, J. L. Sun, Z. Liu, X. D. Zhou,
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