Organic template-free synthesis of zeolite Y nanoparticle assemblies and their application in the catalysis of the Ritter reaction

Article abstract of DOI:10.1039/c6ra27129d

Zeolite Y nanoparticle assemblies (Y-NA) with a mesoporous structure were directly synthesized at 75?°C for 16 h without adding any organic templates. The changes in structure, morphology and textural parameters of the materials obtained after different crystallization times were investigated via powder X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N₂-sorption. The results show that Y-NA had a micro-mesoporous structure composed of highly crystalline particle assemblies with sizes of 400-900 nm. The H-form Y-NA (HY-NA) is strongly acidic, and exhibits a good catalytic performance in the Ritter reaction, as compared with the H-form microporous zeolite Y and mesoporous zeolite ZSM-5.

Full text of DOI:10.1039/c6ra27129d

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Organic template-free synthesis of zeolite Y
nanoparticle assemblies and their application in the
Cite this: RSC Adv., 2017, 7, 7711
catalysis of the Ritter reaction†
*
Ting Tang, Lei Zhang, Hai Dong, Zhongxue Fang, Wenqian Fu, Quanyong Yu
*
and Tiandi Tang
Zeolite Y nanoparticle assemblies (Y-NA) with a mesoporous structure were directly synthesized at 75 ꢀC for
16 h without adding any organic templates. The changes in structure, morphology and textural parameters
of the materials obtained after different crystallization times were investigated via powder X-ray diffraction
(XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), transmission electron microscopy
(TEM) and N2-sorption. The results show that Y-NA had a micro–mesoporous structure composed of
highly crystalline particle assemblies with sizes of 400–900 nm. The H-form Y-NA (HY-NA) is strongly
acidic, and exhibits a good catalytic performance in the Ritter reaction, as compared with the H-form
microporous zeolite Y and mesoporous zeolite ZSM-5.
Received 22nd November 2016
Accepted 6th January 2017
DOI: 10.1039/c6ra27129d
www.rsc.org/advances
transformations.9,10 Recently, we rst synthesized mesoporous
zeolite Y by directly templating with organosilane, and the
1. Introduction
supported Pd catalyst presented a good catalytic performance in
the hydrodesulfurization of bulk 4,6-dimethyldibenzothio-
Aluminosilicate zeolites are crystalline materials with a charac-
teristic well-dened pore structure, strong acidity, and large
surface area, and have been widely used as heterogeneous
catalysts in the petrochemical and ne chemical industries.1–4
Among them, zeolite Y is one of the most important solid acid
catalysts employed in hydrocracking, isomerization and
hydrotreating processes5–7 due to its three-dimensional pore
structure with 12-membered ring channels (0.74 nm).8,9 Despite
the relatively large micropores in zeolite Y, they do not allow the
diffusion of molecules of a size larger than 0.74 nm, greatly
hindering its industrial applications.
Conventionally, this issue can be overcome by introducing
mesopores into the zeolite crystals by the steam dealumination
process.8,10,11 However, the drawback of this method is that the
microporous framework can be destroyed.10 In the last decade,
several types of templating methods, including hard (nano-
structured carbons)12,13 and so templating (organic surfac-
tants),14,15 were developed and successfully applied to prepare
hierarchical porous and nanosized zeolites, such as ZSM-5,13,15
Beta,16 MOR17,18 and titanium silicalite zeolites.19,20 The hierar-
chical porous structure in zeolites and the relatively large outer
surface of nanozeolites can greatly promote molecule diffusion
and improve reaction rates and product selectivity in various
phene.21 Subsequently, zeolite
Y nanoparticles were also
synthesized, which could assemble to form ower-shaped card-
like nanosheets with a micro–mesoporous structure.22 These Y
nanoparticles-assembled Pd species show high activity and
product selectivity in the vinylation of thiophenes. Despite the
mesoporous zeolite Y and zeolite Y nanoparticles showing
a good catalytic activity for the conversion of the bulk mole-
cules, an organic template must be added to the initial
synthesis gel, and a longer crystallization time (4 days) is
required for the synthesis of zeolite Y nanoparticles. The use of
organic templates not only increases the synthetic cost but also
results in the production of environmentally unfriendly waste
gases, generated during the removal of the organic templates by
calcination at high temperatures.3,4 A three-stage temperature-
controlled process without organic template was developed by
Wang and coworkers for the synthesis of zeolite Y nano-
particles,23 but the method was complicated and the crystalli-
zation time was relatively long (96 h).23 Therefore, nding
organic template-free, rapid and simple processes for the
synthesis of mesoporous zeolite Y and zeolite Y nanoparticles is
still a challenge.
Herein, zeolite Y nanoparticles (Y-NA) were synthesized via
ꢀ
a simple process without adding organic template at 75 C for
Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, School
of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, P.
R. China. E-mail: tangtiandi@wzu.edu.cn; fuwenqian@cczu.edu.cn; tangtiandi@
cczu.edu.cn
16 h. During the crystallization, the nanoparticles aggregated
into large assemblies with a size of 400–900 nm, which were
easily separated via ltration. In addition, these aggregates
exhibited a mesoporous structure. The H-form Y-NA sample
(HY-NA) was strongly acidic, similar to the microporous zeolite
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra27129d
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Y. Finally, HY-NA exhibited a good catalytic performance in the cooled to 120 ꢀC under a He ow, a NH3–He mixed gas (10 vol%
Ritter reaction of aromatic nitriles with tert-alcohols, NH3) ow was passed over the sample for 30 min. Aer
compared to H-form microporous zeolite Y and mesoporous removing physically adsorbed NH3 by owing He for 90 min, ꢁth1e
ꢀ ꢀ
sample was heated to 600 C at a heating rate of 10 C min .
zeolite ZSM-5.
The 27Al and 29Si solid-state NMR spectra of Y-NA were ob-
tained on a Bruker AVANCE III 400WB spectrometer operated at
9.4 T using 4 and 7 mm od rotors (zirconia), respectively. The
27Al solid-state NMR spectra was recorded using a pulse of 0.5
ms (p/18) and a repeat time of 0.1 s with a rotor spinning rate of
14 kHz. The 29Si solid-state NMR spectra was recorded using
a pulse of 4.5 ms (p/2) and a repeat time of 20 s with a rotor
spinning rate of 6 kHz.
2. Experimental
2.1. Synthesis of zeolite Y nanoparticle assemblies
Zeolite Y nanoparticle assemblies (Y-NA) were hydrothermally
synthesized from a starting gel with a molar composition of
Al2O3/14.4Na2O/9.8SiO2/590H2O. In a typical synthesis, 9.9 mL
of water glass, 5 g of NaOH and 25.4 mL of deionized water were
mixed at room temperature and stirred for 2 h. Subsequently, 27
mL of 3.9 wt% NaAlO2 aqueous solution was added slowly
under vigorous stirring. Aer stirring for 20 h, the obtained gel
2.3. Catalytic test
All reagents, unless otherwise noted, were obtained from
commercial sources (purity > 99%) and used without further
purication. The reagent p-tolunitrile (98%) was purchased
from Energy Chemical, and t-BuOH (99.5%) was purchased
from Aladdin. The typical experimental procedure for the Ritter
reaction was as follows: 100 mg of catalyst, p-tolunitrile (0.5
mmol) and t-BuOH (1.0 mL) were placed into a 10 mL glass
ꢀ
was crystallized at 75 C for the required time inside a Teon-
coated stainless-steel autoclave of 50 mL capacity. The
product was isolated by ltration, washed and dried at 120 C
ꢀ
overnight, and nally calcined at 300 ꢀC for 4 h to eliminate
crystal water. For reference, a conventional zeolite Y sample (Y)
and mesoporous zeolite ZSM-5 (ZSM-5-M) were synthesized
according to a literature procedure.24,25 The H-form of the
samples was obtained via ion exchange with 1 M NH4NO3
solution at 80 ꢀC for 4 h, followed by calcination at 500 ꢀC in air
for 4 h under a N2 stream; this process was repeated twice. The
obtained samples were designated as HY-NA, HY and HZSM-5-
M. The scale-up synthesis of Y-NA was performed in a 5 L
stainless steel (316L) autoclave, and denoted as Y-NAL. The
starting gel composition, synthetic procedure and reaction
conditions were similar to those described above.
ꢀ
vessel under N2 atmosphere. The reaction proceeded at 120 C
for 12 h. Subsequently, the catalyst was separated via centrifu-
gation and ltered to obtain the liquid phase that was analyzed
with an Agilent 7890B gas chromatograph equipped with
a ame ionization detector. The nal products were puried
using ash column chromatography on silica gel employing
petroleum ether and ethyl acetate as eluents. The 1H NMR (500
MHz) and 13C NMR (125 MHz) were recorded at 20 ꢀC in CDCl3.
Chemical shis were given in parts per million relative to TMS
as the internal standard at room temperature.
2.2. Characterization
The scale-up reaction and recycling experiments were per-
formed in a Parr 4560 autoclave of 300 mL capacity. Typically,
the synthetic procedure was: 10 g of catalyst, p-tolunitrile (50
mmol) and t-BuOH (100 mL) were heated at 120 ꢀC and stirred
for 12 h at 300 rpm. The used catalyst was carefully collected
and dried at 120 ꢀC for 10 h, and then calcined at 500 ꢀC in air
for the recycling experiment.
The X-ray diffraction (XRD) patterns of the sample were ob-
tained with a RIGAKU SmartLab diffractometer using Cu Ka
radiation. Nitrogen adsorption–desorption isotherms were
recorded on a Micromeritics ASAP 2020M apparatus at the
temperature of liquid nitrogen (ꢁ196 ꢀC). Samples were
degassed at 300 ꢀC for 8 h prior to the measurements. Specic
surface area was calculated using the Brunauer–Emmett–Teller
(BET) equation according to the adsorption data. The pore size
distribution was calculated according to the Barrett–Joyner–
Halenda (BJH) model.
3. Results and discussion
3.1. Characterization
IR spectra were recorded on a Nicolet iS50 instrument. Fig. 1 shows the XRD patterns of the samples obtained aer
Scanning electron microscopy (SEM) images were recorded on different crystallization times. When the sample was crystal-
a eld emission scanning electron microscope (SUPRA55) lized for 4 h, only a broad prole, attributed to an amorphous
operating at an acceleration voltage of 5 kV. Transmission structure, is present in the XRD pattern. When the crystalliza-
electron microscopy (TEM) images were obtained on a JEM- tion time increased to 8 h, the pattern displayed clear diffrac-
2100 microscope with a limited line resolution capacity of 1.4 tion peaks at 6.1ꢀ, 15.5ꢀ, 23.4ꢀ and 26.7ꢀ, ascribed to the
faujasite structure,21 indicating that crystalline zeolite Y had
ꢀ
A at a voltage of 200 kV.
The acidity of the zeolite was measured using temperature- been formed. Furthermore, prolonging the crystallization time
programmed desorption of ammonia (NH3-TPD) on a Micro- to 12 h led to a considerable increase in the intensity of the
meritics ASAP 2920 instrument equipped with a thermal characteristic faujasite peaks. The diffraction peak intensity was
conductivity detector. A quartz tubular reactor was loaded with slightly increased aer 16 h, and almost no change for the
200 mg of the sample and heated under a He stream at a heating sample crystallized for 20 h was observed. These results suggest
rate of 5 ꢀC minꢁ1 from room temperature to 450 ꢀC and that highly crystalline zeolite Y is rapidly formed within 16 h.
maintained at this temperature for 2 h. Once the sample was Compared with those of the microporous zeolite Y (Fig. S1†),
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Fig. 1 XRD patterns of the samples obtained after different crystalli-
zation times.
Fig. 3 TEM images of Y-NA samples obtained at different crystalli-
zation times (a) 4 h, (b) 8 h, (c) 12 h and (d) 16 h. Inset of (a) electron
diffraction pattern.
the diffraction peaks of the Y-NA sample were broader. The full
width half maximum (FWHM) is 0.289 at 23.4ꢀ for the Y-NA
sample, higher than that for the microporous zeolite Y sample
(0.207), suggesting that Y-NA could be composed of relatively present an irregular crystal morphology (Fig. 2b). However, the
smaller crystal particles.
TEM image shows that amorphous particles are still present,
To gain an insight into the evolution of the sample at and the amorphous and crystalline phases are interlaced
different crystallization times, the obtained samples were (Fig. 3b). When the crystallization time was prolonged to 12 h,
characterized via SEM and TEM techniques. Aer 4 h crystalli- the particles displayed
a crystal morphology and were
zation, the SEM image shows irregular shapes consisting of composed of aggregated disordered nanoparticles (Fig. 2c).
amorphous nanoparticles with sizes of 30–70 nm (Fig. 2a). The Mesopores (white zone) could be formed in these aggregates
nanoparticle morphologies are in agreement with the TEM (Fig. 3c). The TEM image also conrms that these disordered
image (Fig. 3a), where a hollow structure in the amorphous nanoparticles are highly crystalline (Fig. 3c). Furthermore,
nanoparticles was observed (arrows in Fig. 3a). Aer crystalli- prolonging the crystallization time to 16 h, the amorphous
zation for 8 h, most of the amorphous nanoparticles are analogues disappear and the particles present a clear crystal
aggregated into larger particles, and these particles seemingly outline with a particle size of 400–900 nm (Fig. 2d and 3d). From
the SEM and TEM images, the interstitial mesopores could be
observed in the nanoparticle aggregates.
Fig. S2† shows 29Si MAS NMR and 27Al MAS NMR spectra of
the Y-NA sample obtained aer a crystallization time of 16 h.
The Y-NA sample displays the 29Si signals for Q4 at ꢁ85.4, ꢁ89.3
and ꢁ93.1 ppm (Fig. S2a†), which could be assigned to Si(4Al),
Si(3Al) and Si(2Al), respectively.26,27 A very weak peak at
ꢁ102.1 ppm could be attributed to the presence of surface SiOH
(Q3) in the Y-NA sample.28 Interestingly, only one signal at
59.3 ppm, associated with tetrahedrally coordinated framework
aluminum species, was observed,29 while no signal was found at
0 ppm, which is characteristic of extra-framework aluminium
(Fig. S2b†).29,30 These results indicate that the Y-NA sample had
a good crystallinity.
Fig. 4 shows the IR spectra of the samples crystallized for
different times. Aer a crystallization time of 4 h, only an
absorption band at 1000 cmꢁ1 was observed, which should be
attributed to the asymmetric stretching vibration of tetrahedral
TO4 units (T ¼ Si or Al).23 For the crystallization time of 8 h, the
faujasite characteristic bands at 562, 691 and 757 cmꢁ1 appear,
which could be associated with the double ring vibration and
Fig. 2 SEM images of the samples obtained at different crystallization
times (a) 4 h, (b) 8 h, (c) 12 h and (d) 16 h.
symmetric stretching vibration of the Al–O–Si in the zeolite
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Fig. 4 IR spectra of the samples obtained at different crystallization
times.
framework,23 indicating that zeolite Y crystals were formed. As
crystallization time increases, these characteristic bands grad-
ually become more intense.
According to the analyses of the crystal growing process, it
can be concluded that zeolite Y nanoparticles were formed via
a solution-mediated crystallization mechanism.31 As the crys-
tallization time elapsed, the initial amorphous phase gradually
disappeared, leaving cavities (arrows in Fig. 3a) in the amor-
phous particles. The amorphous particles may have dissolved
into solution, and then relatively small zeolite crystals may have
formed and grown to afford bulk zeolite crystals using the dis-
solved species as the crystal seeds.
Fig. 5 shows the nitrogen adsorption isotherms and meso-
pore size distribution of the samples obtained aer different
crystallization times. For the sample crystallized for 4 h, the
adsorption is very low; the sample has a BET surface area of only
37 m2 gꢁ1 (Table 1). For the sample crystallized for 8 h, the
isotherms display a hysteresis loop at a relative pressure of P/P0
¼ 0.45–0.85 (Fig. 5a), indicating a mesoporous structure for this
sample with a BET surface area of 437 m2 gꢁ1 (Table 1) and
a pore size centered at 6.5 nm (Fig. 5b). These results indicate
that a highly porous structure was formed in this sample.
Furthermore, prolonging the crystallization time from 8 to 12
and 16 h, the isotherm shapes remained unchanged (Fig. 5a),
but the adsorption of the sample greatly increased, the meso-
Fig. 5 N2-Adsorption isotherms (a) and mesoporous size distribution
(b) of the Y-NA samples with different crystallization times (the
isotherm of the samples crystallized for 12 h and 16 h has been offset
by 30 and 95 cm3 ꢁ1, respectively, along the vertical axis, for clarity.
g
The pore size distribution of the samples crystallized for 8 h, 12 h and
16 h has been offset by 0.2, 0.55 and 0.9 cm3 ꢁ1, respectively, along
g
the vertical axis, for clarity).
that of Y-NA, obtained at 50 mL bench scale. Notably, Y-NAL has
a BET surface area, microporous volume and mesoporous
volume of 647 m2 gꢁ1, 0.23 and 0.15 m3 gꢁ1, respectively, similar
to the Y-NA sample (Table 1).
3.2. Catalyst performance
porous size decreased to 5.2 nm (Fig. 5b), and the BET surface The activity of HY-NA, HZSM-5-M and HY catalysts was rst
area increased from 437 m2 gꢁ1 (8 h-sample) to 608 and 687 m2 investigated in the Ritter reaction of the p-tolunitrile with t-
gꢁ1 for the 12 and 16 h-samples, respectively (Table 1). At the BuOH as the model reaction (Table 2). The HY-NA catalyst
same time, the microporous volume also increased from 0.17 to shows the highest activity and target product selectivity,
0.24 cm3 gꢁ1. These results indicate that the sample crystallinity affording the target product in 95% yield (Table 2, entry 2). As
gradually increases with the crystallization time.
a comparison, the p-tolunitrile conversions over HY and HZSM-
Fig. 6 shows the NH3-TPD curves of the HY-NA, HY and HZSM- 5-M catalysts are only 51% and 59%, respectively, much lower
5-M zeolites. It is clear that the HY-NA and HY samples present than that of the HY-NA catalyst (Table 2, entries 3, 4).
the same desorption prole. The low temperature desorption
The scope of the HY-NA-catalyzed Ritter reaction was inves-
prole with a peak at 225 ꢀC is related to moderately acidic sites, tigated with various nitriles and tertiary alcohols, and the
and the high temperature shoulder peak at 390 ꢀC corresponds to results are summarized in Table 3. Benzonitrile with either
strong acidic sites. Clearly, the acid site density of HY-NA and HY electron-rich or electron-withdrawing groups were successfully
is much higher than that of the HZSM-5-M zeolite.
applied to this transformation to generate the corresponding
Fig. S3† shows the XRD pattern and SEM image of the Y-NAL amides 3a–3h in high yields. The nitriles bearing 2-thienyl and
sample. It is clear that the morphology of Y-NAL is similar to 2-naphthyl afforded 3i and 3j in moderate yields. When
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Table 1 Texture parameters of the various zeolites
SBETa (m2 gꢁ1
)
Smicb (m2 gꢁ1
)
Vmic (cm3 gꢁ1
)
Sext (m2 gꢁ1
)
Vmese (cm3 gꢁ1
)
c
d
Sample
Y-NAf
Y-NAg
Y-NAh
Y-NAi
Y-NAL
Y
37
—
—
37
53
85
100
111
59
0.09
0.11
0.13
0.14
0.15
0.05
0.37
437
608
687
647
741
418
420
522
587
556
682
250
0.17
0.21
0.24
0.23
0.28
0.1
ZSM-5-M
168
a
b
c
d
BET surface area. Microporous surface area. Microporous pore volume. External surface area, including mesoporous surface area.
Mesoporous pore volume. f Sample crystallized for 4 h. g Sample crystallized for 8 h. h Sample crystallized for 12 h. i Sample crystallized for 16 h.
e
The reusability of the zeolite HY-NA catalyst for the Ritter
reaction of p-tolunitrile with t-BuOH was also tested under the
standard reaction conditions. Fig. S4† shows the time depen-
dence of p-tolunitrile conversion and the initial apparent reac-
tion rates (robs) over the HY-NA catalyst for the rst and fourth
runs (for details please see the ESI†). Table 4 shows the reus-
ability test results for the HY-NA catalyst. From Fig. S4a,† it can
be seen that the initial reaction activity of the HY-NA catalyst in
the rst run is slightly higher than that of the reused catalyst in
the fourth run, and the robs calculation results in Fig. S4b† also
demonstrate that the robs (4.4 ꢂ 10ꢁ2 mol kgꢁ1 sꢁ1) for the fresh
HY-NA catalyst in the rst run is much higher than that of the
reused HY-NA catalyst (3.5 ꢂ 10ꢁ2 mol kgꢁ1 sꢁ1) in the fourth
Fig. 6 NH3-TPD curves of the HY-NA, HY and HZSM-5-M samples.
run. Nevertheless, the p-tolunitrile conversions over fresh and
reused HY-NA catalysts at a reaction time of 12 h were similar.
cinnamonitrile was used as substrate, the amide 3k was ob- The data in Table 4 also shows that the HY-NA catalyst exhibits
tained with a good yield of 89%. To our delight, the aliphatic
phenylacetonitrile was also successfully transformed, gener-
ating the corresponding amide 3l in 97% yield. When isoamylol
Table 3 Scope of the HY-NA-catalyzed Ritter reaction with various
was used as a substrate, the yield of the obtained amide 3m was
74%. In addition, 1,3-benzenedicarbonitrile was efficiently
converted to the corresponding products 3n0 and 3n00 (molar
ratio 1 : 2). Remarkably, the benzonitrile containing a tri-
uoromethyl group also afforded the target product 3o in a 97%
yield.
nitriles and tertiary alcoholsa
Table 2 Ritter reaction of p-tolunitrile with t-BuOH over different
catalystsa
Entry
Catalyst
Conversion (%)
Selectivity (%)
Yieldb (%)
1
2
3
4
None
HY-NA
HY
—
95
51
59
—
100
90
—
95(93)c
46
HZSM-5-M
60
25
a
Reaction conditions: p-tolunitrile (0.5 mmol), t-BuOH (1 mL), catalyst
(100 mg), 120 ꢀC for 12 h. b Yield obtained by GC. c The yield in brackets
is the isolated yield.
a
Reaction conditions: p-tolunitrile (0.5 mmol), t-BuOH (1 mL), catalyst
(100 mg), 120 ꢀC.
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Table 4 Catalyst reusability testa
Entry
Recycle
Yield (%)
1
2
3
4
5
6
7
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Run 7
94
93
92
93
91
92
90
a
Reaction conditions: 10 g catalyst, p-tolunitrile (50 mmol), t-BuOH
(100 mL), 120 ꢀC for 12 h, 300 rpm.
Fig. 7 Plausible mechanism for the Ritter reaction of nitriles with
tertiary alcohols.
a high activity even aer the seventh run, indicating that the
HY-NA catalyst has a good reusability.
The superior catalytic performance of the HY-NA catalyst
should be attributed to its unique micro–mesoporous structure
and the abundant strongly acidic sites. HY and HY-NA have
a similar density of strong acid sites, but the p-tolunitrile
conversion over HY-NA is higher than that over the HY zeolite
catalyst. This could be due to the difference in the porous
structure between Y and Y-NA zeolites. The Y-NA catalyst has
a mesoporous surface area of 100 m2 gꢁ1 and mesoporous
volume of 0.14 cm3 gꢁ1, whereas Y catalyst only has a low
external surface area of 59 m2 gꢁ1. Thus, the mesopores in the
HY-NA catalyst could favor the reactant diffusion, enhancing
the reaction rate. Similar phenomenon was also reported by
Wu.32 He reported that microporous zeolite HY with a lower
external surface area inhibited the diffusion of the reactants,
resulting in an unsatisfactory catalytic activity and selectivity in
the Ritter reaction of acrylonitrile with t-butanol, compared to
that of ultrastable mesoporous zeolite. Compared with Y-NA,
although ZSM-5-M has a high mesoporous surface area (168
m2 gꢁ1) and mesopore volume (0.37 cm3 gꢁ1), the p-tolunitrile
conversion over HZSM-5-M is only 59% (Table 2). This is due to
the fact that the acid site density on the HZSM-5-M catalyst is
much lower than that on the HY-NA catalyst. Thereby, a high
density of strongly acidic sites is critical for this transformation.
On the other hand, Y-NA has a relatively large micropore size of
0.74 nm and a relatively high microporous surface area (587 m2
gꢁ1), while ZSM-5-M has a relatively small micropore (0.53–0.56
nm) and low micropore surface area (250 m2 gꢁ1). Therefore,
the HY-NA catalyst, with a relatively large micropore size and
high microporous surface area, could provide a higher number
of accessible acid sites compared to that of HZSM-5-M, resulting
in a higher catalytic activity for the HY-NA catalyst. Wu also
reported that because of the steric hindrance effect due to the
micropore size (0.55 nm) in mesoporous zeolite ZSM-5, t-
butanol is not easily accessible to the acidic sites in the
microporous channels, resulting in lower target product selec-
tivity over mesoporous zeolite ZSM-5 in the Ritter reaction of
acrylonitrile with t-butanol.32
The possible reaction mechanism over HY-NA is shown in
Fig. 7. The alcohol is easily protonated by the acidic sites on the
HY-NA catalyst to give the oxonium ion 4 that can eliminate one
water molecule to form the carbocation intermediate 5. This
carbocation is attacked by electron-rich p-tolunitrile nucleo-
philes to form the nitrilium ion 6,32 which is subsequently
attacked by a water molecule, followed by the elimination of
a proton to generate the nal compound.32
4. Conclusion
Crystalline zeolite Y nanoparticle assemblies (Y-NA) with micro–
mesoporous structure were directly synthesized via a one-step
crystallization method in the absence of an organic template.
This synthesis process is cost-effective, environmentally friendly
and capable of scale-up. The SEM and TEM characterization
results indicate that the Y-NA zeolite may be formed via a solu-
tion-mediated crystallization mechanism. The obtained zeolite
HY-NA catalyst not only is strongly acidic, but also has a micro–
mesoporous structure. The strongly acidic sites in the HY-NA
can facilitate the activation of the tertiary alcohols, and the
mesoporous structure in the HY-NA can favor the diffusion of
the reactants and products. Finally, the HY-NA catalyst could be
recycled up to seven times and still maintain a good catalytic
activity.
Acknowledgements
This study was supported by the National Natural Science
Foundation of China (U1463203, 21476030, U1662139) and the
Natural Science Foundation of the Jiangsu Province of China
(BK20150258).
Notes and references
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