Preparation of hierarchically structured y zeolite with low Si/Al ratio and its applications in acetalization reactions

Article abstract of DOI:10.1039/c4ra11042k

Hierarchically structured Y zeolites were prepared by a post-synthetic strategy, where the as-made NaY zeolite was sequentially treated by a lactic acid solution and an alkaline solution containing cetyltrimethyl ammonium bromide (CTAB). Many techniques, including X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), N₂ adsorption-desorption, Fourier-transform infrared spectroscopy (FT-IR), Thermogravimetric analysis (TG-DTG) and NH₃ Temperature Programmed Desorption (NH₃-TPD), were applied to characterize the chemical and textural properties of the samples. The results show that lactic acid pre-modification of NaY zeolite may cause on the one hand Na⁺ cation removal by proton exchange and on the other hand the dealumination of the zeolite framework with the formation of amorphous silicon-rich species offering nutrients for the fabrication of mesoporosity. After alkaline treatment in the presence of surfactant CTAB, mesoporosity can be successfully introduced into the NaHY zeolites with the microporous structures well preserved. Furthermore, the hierarchically structured Y zeolites exhibit much better performances in the acetalization reactions with large sized molecules involved. This could be attributed to the enhanced diffusion ability of large sized guest molecules through the combination of mesoporosity and microstructures compared with pure microporous Y zeolites.

Full text of DOI:10.1039/c4ra11042k

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Preparation of hierarchically structured Y zeolite
with low Si/Al ratio and its applications in
acetalization reactions†
Cite this: RSC Adv., 2014, 4, 58596
*
Wen Qian Jiao, Wen Hua Fu, Xiao Min Liang, Yi Meng Wang and Ming-Yuan He
Hierarchically structured Y zeolites were prepared by a post-synthetic strategy, where the as-made NaY
zeolite was sequentially treated by a lactic acid solution and an alkaline solution containing cetyltrimethyl
ammonium bromide (CTAB). Many techniques, including X-ray diffraction (XRD), Scanning Electron
Microscopy (SEM), N2 adsorption–desorption, Fourier-transform infrared spectroscopy (FT-IR),
Thermogravimetric analysis (TG-DTG) and NH3 Temperature Programmed Desorption (NH3-TPD), were
applied to characterize the chemical and textural properties of the samples. The results show that lactic
acid pre-modification of NaY zeolite may cause on the one hand Na+ cation removal by proton
exchange and on the other hand the dealumination of the zeolite framework with the formation of
amorphous silicon-rich species offering nutrients for the fabrication of mesoporosity. After alkaline
treatment in the presence of surfactant CTAB, mesoporosity can be successfully introduced into the
NaHY zeolites with the microporous structures well preserved. Furthermore, the hierarchically structured
Y zeolites exhibit much better performances in the acetalization reactions with large sized molecules
involved. This could be attributed to the enhanced diffusion ability of large sized guest molecules
through the combination of mesoporosity and microstructures compared with pure microporous Y
zeolites.
Received 23rd September 2014
Accepted 29th October 2014
DOI: 10.1039/c4ra11042k
www.rsc.org/advances
reactants/products to approach or leave the active sites in the
zeolite structures and may result in lowering the conversion or
1. Introduction
further reaction with coke deposition and catalyst deactivation.
In order to circumvent the diffusional limitations of large-
sized molecules, several potential procedures have been
explored, including preparations of ordered mesoporous
materials, i.e. MCM-41 and SBA-15 etc., synthesis of extra-large
pore molecular sieves, and fabrication of nanosized zeolites,
mesostructured zeolites or zeolites with hierarchical porosity
and so on. The topics are also comprehensively reviewed
recently by different groups.13–20 Nevertheless, the inferior
hydrothermal stability and relative weak acidity of the meso-
porous materials, the limited pore widening of the extra-large
pore molecular sieves all constrain these materials' utilities in
large scale.15 Therefore, the creation of microporous zeolites
with intra-and/or inter-mesopores is a much more promising
way for improving the large-sized molecule transportation
abilities while reserving their zeolitic properties. Until now,
many approaches, classied as “bottom-up” and “top-down”,
have been described in the literature for the preparation of
Zeolites are crystalline microporous aluminosilicate materials
with even and tunable pore structures and have already found
applications in adsorption, separation, purication and catal-
ysis areas.1–5 Amongst others, zeolite Y with FAU topological
structures are most important and frequently utilized as the
active phase in both uid catalytic cracking (FCC) and hydro-
cracking catalysts due to the high acidity, large internal surfaces
and uniform channels (0.8 nm) and cavities (a cage, 1.3 nm).6–8
Besides, zeolite Y also nds applications in many other indus-
trial processes, such as aldol condensations9 and Knoevenagel
condensation processes10 of benzaldehyde with n-butyl alcohol
and malononitrile respectively, hydrogenation of aromatic,11
hydrodesulfurization of 4,6-Dimethyldibenzothiophene12 as
catalysts or catalyst supports because of its excellent chemical
and textural properties. However, the intrinsic microporous
nature of zeolite Y (<2.0 nm) always impose diffusional limita-
tions on large-sized organic molecules, which may hinder the
hierarchically structured
Y zeolites, among which most
Shanghai Key Lab of Green Chemistry and Chemical Processes, Department of
Chemistry, East China Normal University, Shanghai 200062, P. R. China. E-mail:
ymwang@chem.ecnu.edu.cn; jiaowenq@hotmail.com; Fax: +86-21-62232251; Tel:
+86-21-62232251
“bottom-up” approaches involve the addition of mesoscale
templates, either hard or so templates.21 For example, Zhu and
co-workers once reported a strategy for the preparation of
hierarchical mesoporous Y zeolite by assembling zeolite Y or
sodalite fragments around mesoscale cationic surfactant
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra11042k
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cetyltrimethylammonium bromide (CTAB) micelles with cosol- for the formation of hierarchically structured zeolite Y catego-
vent tertbutyl alcohol (TBA) and the 1,3,5-trimethylbenzene rized in “bottom-up” approach is the synthesis of nano-sized Y
(TMB) additive and the obtained sample exhibited superior zeolite particles and thereaer fabrication of inter-mesopores
adsorption capacity towards bulky molecule N-nitro- by self-assembly of the nanoparticles.20,25 So far, many
sonornicotine owing to its hierarchical mesostructure attempts have been devoted to synthesize zeolite Y nano-
comparing to conventional NaY zeolite.22 However, the meso- particles, either in the presence of structure directing agent of
structured Y zeolite owned very low micropore volume because tetramethyl ammonium cations (TMA+),26–30 by space-conned
of its relatively poor crystallinity. Inayat et al. rstly reported the synthesis using starch as a template31 or by tuning the crystal-
synthesis of assemblies of mesoporous FAU-type zeolite nano- lization parameters.32 However, the decreased particle size of
sheets with a low Si/Al molar ratio of 1.0–1.5 using 3-(trime- zeolite Y may cause some problems during usage, such as a
thoxysilyl)propyl hexadecyl dimethyl ammonium chloride descending of the micropore volume, recovery difficulties and
(TPHAC) as template.23 However, the low Si/Al molar ratio poorer stabilities etc.15 Therefore, more and more attentions are
largely limited the material's utility. More recently, Fu et al. drawn towards the preparation of hierarchically structured FAU
reported that mesoporous zeolite Y was successfully synthesized zeolites in “Top-Down” approaches, including steaming treat-
from a synthetic mixture containing water glass and silylated ment or steaming combining with acid leaching, alkaline
quaternary ammonium (N,N-dimethyl-N-octadecyl-N-(3-trie- leaching, desilication combining with re-crystallization in the
thoxysilylpropyl) ammonium [(C2H5O)3SiC3H6N(CH3)2C18H37]+ presence of
a mesopore-directing agent and surfactant-
cation) that was used as a mesoscale template. And the obtained templated structure arrangement.21 Lutz and co-workers thor-
mesoporous Y zeolite was used as a support for loading Pd and oughly studied the structural and textural changes of zeolite Y
displayed much higher activity in hydrodesulfurization (HDS) of during steam treatment process and pointed out that different
4,6-dimethyldibenzothiophene (4,6-DM-DBT, 97.3%) than the kinds of mesopores formed along with dealumination and
Pd catalysts supported on mesoporous ZSM-5, mesoporous Beta silicon species migration,33–35 where intracrystalline (closed)
zeolite, conventional microporous Y zeolite and alumina.12 mesopores occur in the nuclei while open pores at the crystal
Furthermore, Liu et al. reported the hydrothermal synthesis of surface due to decomposition of its corresponding frameworks,
hierarchical zeolite Y, actually a mixture of Y and P zeolite, using furthermore, a third type of mesopores formed by dehydrox-
an organosilane surfactant of [3-(trimethoxysilyl)propyl]octadecyl- ylation of the hydroxyl nests le by dealumination.33–35 Jassen
dimethylammonium chloride [(CH3O)3SiC3H6N(CH3)2C18H37]Cl et al. also investigated the pore structures of steamed zeolite Y
(TPOAC), which could be homogeneously dispersed in and imaged the mesopores through three-dimensional trans-
viscous gel for synthesizing zeolite Y, as mesopore-generating mission electron microscopy (3D-TEM) so as to gain insight into
agent and the prepared mesostructrued Y zeolite owned a the shape and three dimensional ordering of the pore struc-
sharp mesopore size distribution centered at 3.8 nm.9 The tures.36,37 They stated that besides some (small amount of)
mesostructured zeolite showed much higher activity and better cylindrical pores connecting the external surface with the inte-
resistance to deactivation as compared with conventional rior of the crystallite, most of the mesopores formed during
zeolite Y when used as catalysts in the aldol condensation of steam process are present as cavities which are blocked inner
benzaldehyde with n-butyl alcohol, attributing to the improved the zeolite particles and only connected with the outer surface
mass transportation ability of guest species through the inter- through micropore channels. Therefore, to what extent the
connected micropore-to-mesopore networks in the mesostruc- accessibility and diffusion can be enhanced by these cavities
tured Y zeolite sample. Besides, Jin et al. also presented an was questioned. Aer a special hydrothermal treatment the
approach for the synthesis of mesoporous, pure zeolite Y by cavities adjacent to one another in the zeolite may coalesce with
using the same organosilane surfactant as template, and the the formation of interconnected cylindrical mesopores, which
ratio of mesoporosity vs. microporosity can be adjusted by can apparently enhance the diffusion properties of zeolites.
varying the addition amount of the template. The obtained Based on this, de Jong and co-workers tried base leaching of the
mesostructured Y zeolite was used as catalyst additive (5%) steamed Y zeolites in dilute NaOH solution at room tempera-
combining with conventional USY zeolite (25%) for fabrication ture for a quite short duration of 15 min and they found that
of cracking catalyst and the catalyst was evaluated in heavy oil besides zeolitic micropores and large mesopores (ca. 30 nm)
catalytic cracking process. Comparing with the classical Y caused by steaming and acid leaching, another smaller sponge-
derived catalyst, the mesostructured Y zeolite contained catalyst like mesopores (ca. 3 nm) were formed by desilication during
displayed superior catalytic performance with a signicant this alkaline leaching.38,39 The tri-modal porosity was benecial
increase of gasoline yields and reduction of coke and dry gas for diffusion of the cracking products, which suppressed further
yields.24 Although many examples have been recorded for the cracking, increased diesel selectivity and decreased coke
preparation of mesostructured Y zeolites in the presence of formation in VGO cracking. However, the framework structures
organic template through “bottom-up” approach, it is still not of the zeolites were strongly destructed during direct alkaline
convenient to obtain hierarchically structured zeolite Y because leaching along with a substantial loss of microporosity. In this
of the difficulty of dispersing the organic templates into the context, Verboekend et al. proposed a procedure in which an
parent gel used for the preparation of zeolite Y, and only those organic pore-directing agent, TPA+ for instance, was added in
organic surfactants with specially designed structures, such as alkaline solution so as not only to tailor the mesopore structures
organosilane species, can be successfully utilized. Another way but also to prevent realumination and amorphization of the
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zeolite crystals.40,41 Aer this desilication treatment with the
assistance of organic pore-directing agent, mesoporosity was
introduced and the crystallinity and micropore volume were
better retained. Besides, Garcia-Martinez and co-workers also
developed a facile procedure for the preparation of mesostruc-
2.2 Material preparations
Hierarchically structured
Y
zeolites were prepared as
following: 1.0 g of NaY zeolite was added into 20.0 mL 0.625
mmol Lꢀ1 lactic acid aqueous solution with stirring at room
temperature for 1 h, and then the resultant slurry was trans-
ferred into a Teon vessel, sealed and heated at 100 ꢁC for 2 h.
Aer treatment, the mixture was ltrated and washed with hot
water and then dried, the obtained sample was denoted as
HLA-HT. 0.2 g NaOH or 3.4 g NH3$H2O (25 wt%) was added
into 30 mL of water containing 0.72 g of cetyltrimethyl
ammonium bromide (CTAB) with stirring. 1.00 g sample HLA-
HT was added into the clear solution with keeping stirring for
another 1 h. The mixture was transferred into a Teon-lined
vessel, sealed and heated at 150 ꢁC under autogenous pres-
sure for 1 day, aer which the mixture was ltrated, washed
with hot water and then dried. The obtained samples were
denoted as NaOH-CTAB-a and NH3-CTAB-a, respectively.
NaOH-CTAB-a and NH3-CTAB-a were calcined at 550 ꢁC for 5 h
to remove the occluded CTAB organic moieties, and
the resultant samples were denoted as NaOH-CTAB-c
and NH3-CTAB-c, respectively. Samples NaOH-CTAB-c and
NH3-CTAB-c were ion-exchanged with 1.0 M NH4Cl solution at
tured
Y zeolite in a surfactant-templated structure re-
arrangement process.21,42–45 Mesostructured Y zeolites with
secondary intracrystalline mesoporosity can be obtained in a
single-step process combing alkaline treatment with surfactant
templating while the intrinsic zeolitic microporosity was well
preserved. The resultant zeolite exhibited the improved catalytic
behavior in the cracking of vacuum gasoil due to enhanced
diffusion ability. Similar to Garcia-Martinez' report, Chal et al.
prepared mesostructrued Y zeolites in TMAOH solution in the
presence of CTAB. Two interconnected pore systems, micro-
and mesopores, in the zeolite crystals can be fabricated through
pseudomorphic transformation of parent Y zeolite. However,
the modication objects aforementioned are all zeolites with
high silica to alumina molar ratios in the framework structures
and examples of introduction of mesopores into Y zeolites with
low Si/Al molar ratios are very rare,41,43,46,47 mostly since the
abundant aluminum ions in the zeolite framework may hinder
the dissolution of the silicon species by the alkaline and thus
the formation of mesopores48,49 or since Si–O–Al bonds in high
aluminum zeolites are not as labile as the Si–O–Si under basic
conditions and cannot rearrange the structure so as to incor-
porate mesoporosity.
+
ꢁ
80 C for three times and then converted to H form by calci-
nation at 550 ꢁC for 5 h. The prepared samples were denoted as
NaOH-CTAB-cat and NH3-CTAB-cat, respectively.
2.3 Characterization
Herein, hierarchically structured Y zeolites were prepared in
top-down procedure assisted by surfactant CTAB in an alkaline X-ray diffraction patterns (XRD) of zeolite Y were collected by
solution using lactic acid pretreated NaY zeolite as starting Rigaku-Ultima diffractometer using a Cu Ka radiation source
material. Before alkaline treatment, NaY zeolite was pre- (l ¼ 0.15432 nm). The relative crystallinity was calculated by
modied by lactic acid in order to transform the zeolite comparing the eight peak intensities of the treated samples
framework to some extent to amorphous silicon-rich species. In with that of the parent NaY sample according to SH/T 0340-
the following, mesoporosity can be introduced with the assis- 92.50,51 The framework Si/Al ratios of Y zeolites were deter-
tance of surfactant CTAB, where the formed amorphous silicon- mined from the Y zeolite unit cell a0 detected according to
rich species play an essential role. Besides, the dissolution literature.1 In order to determine the bulk Si/Al molar ratios of
degree of the Si-rich non-framework species in different alka- the samples, Inductively Coupled Plasma Atomic Emission
line solutions greatly affected the mesoporosity formation Spectrometry (ICP-AES) were recorded on a Thermo IRIS
process. The hierarchically structured Y zeolites with meso- Intrepid II XSP atomic emission spectrometer aer dissolving
pores are expected to be of enhanced diffusion abilities for large the calcined samples in HF solution. Scanning electron
molecules in the catalytic reactions when used as catalysts, and microscope (SEM) was performed on a Hitachi S-4800 eld
thus their performances in acetalization reaction of cyclohexa- emission scanning electronmicroscope (FE-SEM, Hitachi,
none were investigated.
Japan) with an acceleration voltage of 3 kV. Fourier transform
infrared spectra (FTIR) were performed on a Nicolet Fourier
transform infrared spectrometer (NEXUS 670) combined with
the conventional KBr wafer technique. The spectra were
collected for the wavenumber range of 400–4000 cmꢀ1, with a
resolution of 4 cmꢀ1. Thermo gravimetric analysis (TG) was
performed using a NET2SCH STA449F3 TGA analyzer with
2. Experimental section
2.1 Reagents
All chemical reagents were of analytical grade and used as heating rate of 10 ꢁC minꢀ1 from 25 to 800 ꢁC under an air ow.
received without further purication. Lactic acid (HLA), NaOH, N2 adsorption–desorption isotherms were measured at
ꢁ
NH3$H2O (25 wt%), cetyltrimethyl ammonium bromide (CTAB) ꢀ196 C on a Quantachrome Autosorb-3B volumetric adsorp-
were provided by Sinopharm Chemical Reagent Co., Ltd. Si, Al tion analyzer. Before the measurements, the samples were out
and Na ions standard solutions were provided by Alfa-Asea. NaY gassed for 6 h in the degas port of the adsorption apparatus at
and HY zeolites were supplied by Huahua Catalyst 300 ꢁC. The BET specic surface area was calculated using
Manufacturing Company, China. Deionized water was used adsorption data acquired at a relative pressure (P/P0) range of
directly without further purication.
0.05–0.30 and the total pore volume determined from the
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amount adsorbed at a relative pressure of about 0.99. The pore standard method, where n-decane was applied as the internal
size distribution (PSD) curves were calculated from the anal- standard. The formation of diacetals of pentaerythritol was
ysis of adsorption branch of the isotherm using the Barrett– conrmed by GCMS. The acetalization reactions are illustrated
Joyner–Halenda (BJH) algorithm. Transmission electron in Scheme 1.
microscopy (TEM) experiments were conducted on TECNAI G2
F30 operating at 300 kV. For the TEM image, the specimens
were dispersed in ethanol and placed on holey copper grids.
3. Results and discussion
29Si and 27Al MAS NMR spectra were measured on a VARIAN
VNMRS 400WB NMR spectrometer under one-pulse condi-
tions. The spectra were obtained with a frequency of 79.43
MHz, a spinning rate of 3.0 kHz, and a recycling delay of 60 s.
The chemical shi was referred to as Q8M8
([(CH3)3SiO]8SiO12). The 27Al NMR spectra were recorded with
a frequency of 104.18 MHz, a spinning rate of 10.0 kHz, and a
recycling delay of 4 s. KAl(SO4)2$12H2O was used as the
reference for chemical shi. Temperature-programmed
desorption of NH3 (NH3-TPD) testing was applied to detect
the acidity properties of the samples and the tests were per-
formed using a TP-5080 chemisorption instrument (Xianquan
Co., Ltd, Tianjin, China) with a thermal conductivity detector
Fig. 1 exhibits the wide- and small-angle XRD patterns of parent
NaY zeolite and the resultant samples aer treatments. It shows
that the parent NaY zeolite sample displays a typical topological
structure of FAU zeolite with well-dened XRD patterns
matching well with the JCPDS powder diffraction pattern 43-
0168. Aer lactic acid modication, the diffraction pattern of
sample HLA-HT is almost the same with that of sample NaY.
Nevertheless, the intensities of the peaks ascribing to FAU
crystalline structures decrease to some extent, indicating partial
collapse of zeolite framework structures. The chemical proper-
ties of NaY zeolite precursor and the resultant products are
listed in Table 1. It can be clearly seen that, comparing with the
parent NaY zeolite, sample HLA-HT owns much lower crystal-
linity, meaning that zeolite framework structure destruction
occurred during lactic acid modication, mainly caused by
dealumination. Aer alkaline treatment in the presence of
CTAB, no matter in NaOH solution or NH3$H2O solution, the
resultant samples still display the typical diffraction patterns for
FAU-zeolite, indicating that the zeolite structures remained well
and no new crystalline phases emerged during the hydro-
thermal treatment. Fig. 1II is the small-angle X-ray diffraction
patterns of the samples before and aer treatment. For the nal
samples aer treatment in alkaline solutions assisted by
template CTAB, obviously a wide diffraction peaks in the range
of 2q 0.5–3.0ꢁ can be seen, indicative of the formation of
some organized mesopores.53 However, parent NaY zeolite and
HLA-HT sample exhibit no such peaks. Notably the sample
NaOH-CTAB-a shows two small diffraction peaks located in the
range of 2q 3.9–4.4ꢁ, possibly attributed to the high-indexed
facets for ordered mesoporous MCM-41 type structures. In
contrast to the pure ordered MCM-41 materials, the hierarchi-
cally structured zeolites show much broader and less specied
Bragg peaks at low angle region, indicating poor ordering of the
meso-structures.
ꢁ
(TCD). Aer pretreatment at 550 C under owing helium (25
mL minꢀ1) for 1 h, each sample (100 mg) was cooled to 100 ꢁC,
and then adsorbed to saturation by ammonia for 30 minutes.
Ammonia physically adsorbed on the catalyst was removed by
ushing the sample with helium (25 mL minꢀ1) for 1 h at the
adsorption temperature. Thermal desorption of ammonia was
carried out in the temꢀp1erature range of 100–550 ꢁC increasing
ꢁ
at a rate of 10 C min
.
2.4 Catalytic evaluation
Acetalization of cyclohexanone with glycol and pentaerythritol
were used as model reactions and carried out under slurry
phase reaction conditions using a conventional oil bath
attached to a condenser according to literature.52 A typical
reaction mixture, in a 10 mL round bottle ask, contained
cyclohexanone (20 mmol), glycol (32 mmol, excess amount) (if
pentaerythritol was used, addition amount was 10 mmol), n-
decane (10 mmol) and a freshly activated catalyst (50 mg).
Activation of the catalyst was achieved by calcination at 550 ꢁC
ꢁ
for 5 h. The reaction was carried out at 120 C for 2 h. Aer
cooling to room temperature, the ask was washed several
times with acetone. All the mixture was collected and sepa-
rated by centrifugation. The liquid supernatant was analyzed
using a Shimadzu GC-2014 gas chromatograph with an Rtx1-
Wax capillary column and FID detector. The conversion was
calculated based on cyclohexanone using the internal
Scheme 1 Acetalization of Cyclohexanone with glycol (I) and pen- Fig. 1 Wide- and small-angle XRD patterns of (a) parent NaY zeolite,
taerythritol (II).
(b) HLA-HT, (c) NH3-CTAB-a and (d) NaOH-CTAB-a.
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Table 1 Chemical properties of the precursors and the resultant
meso-structured Y zeolites prepared in different alkaline solutions in
the presence of CTAB
distribution curves. As shown in Fig. 2II, no obvious porous
structures with diameters larger than 2 nm can be observed in
sample NaY and HLA-HT, while clear mesostructures
with pore size centered at 2.4–2.7 nm can be seen for sample
NaOH-CTAB-c and NH3-CTAB-c, which is consistent with the
XRD results in the small-angle region. The textural properties
of different samples calculated from N2 adsorption–desorp-
tion isotherms are listed in Table 2. It exhibits that parent
sample NaY displays the highest microporous specic surface
area and volume due to its integral framework structures while
its external surface area is very limiting. Lactic acid modi-
cation towards NaY only results in decrease of the specic
surface area and micropore volume due to the partial collapse
of zeolite framework. As it has about a half of the relative
crystallinity of parent NaY calculated from XRD data, the
micropore volume decreases to 0.18 cm3 gꢀ1, about a half of
that for parent NaY zeolite (0.35 cm3 gꢀ1). Aer alkaline
treatment assisted by CTAB, samples NaOH-CTAB-c and NH3-
CTAB-c, both external surface areas and total pore volumes
improve, which is contributed to the formation of meso-
structures. However, the collapse of the zeolite framework to
some extent and reduced crystallinity result in the decrease of
micropore surface area and volume.
SiO2/Al2O3
Samples
Yield %
R. C. %
Framework
Bulk
Na2O %
NaY
HLA-HT
NaOH-CTAB-c
NH3-CTAB-c
—
—
67
95
100
47
77
4.91
5.30
5.51
5.53
5.34
9.98
6.66
8.50
13.1
3.63
6.26
2.60
58
Fig. 2 N2 adsorption–desorption isotherms (I) and pore size distri-
bution curves (II) of samples (a) NaY, (b) HLA-HT, (c) NH3-CTAB-c and
(d) NaOH-CTAB-c. The hollow dots denote desorption. The isotherms
for sample NH3-CTAB-c were offset vertically by 40 cm3 gꢀ1 (STP). The
PSD curves were calculated from the adsorption isotherms.
Besides the crystalline structures and textural properties,
the morphologies of the samples before and aer treatment
vary from one to another. The SEM images of the parent NaY
zeolite and the resultant hierarchically structured Y zeolite
samples aer hydrothermal treatment assisted by CTAB in the
alkaline solutions are shown in Fig. 3. For the parent NaY
zeolite, typical FAU zeolite crystallites with particle size of
about 500–1000 nm can be observed with the surface at and
smooth, and there is no other morphologies can be seen. As
for sample NH3-CTAB-c prepared in ammonium water solu-
tion in the presence of CTAB, almost identical morphology can
be observed, yet the surface of the particles is not as smooth as
that of parent NaY zeolite, which may be caused by the
formation of mesostructures. Otherwise, the morphology of
sample NaOH-CTAB-c is of much more differences comparing
with that of sample NaY zeolite, as shown in Fig. 3c. Besides
bulky zeolite crystallites, amount of debris without specied
morphologies co-exist in sample NaOH-CTAB-c, which is
assumed to be the formed amorphous mesoporous structures.
The morphologies and porous structures of the samples are
further studied by TEM as shown in Fig. 4. As can be seen in
the TEM images, lattice fringes of zeolite Y crystalline are well-
resolved in large scale through the zeolite particles in both
NH3-CTAB-c and NaOH-CTAB-c samples indicating the pres-
ervation of microporous structures. Along with that, amorph-
ization of the particles at the margins and the formation of
vacancies and mesopores in the zeolite particles can be
observed. In sample NH3-CTAB-c, clear organized mesoporous
structures can be clearly evidenced as marked in white circles
in Fig. 4c which is consistent with the small-angle XRD results
shown in Fig. 1b. Meanwhile, such organized mesopores also
can be viewed in debris in sample NaOH-CTAB-c as exhibited
in Fig. 4f.
Fig. 2 shows the N2 adsorption–desorption isotherms and
pore size distribution curves of samples before and aer
treatment. For parent NaY zeolite without any treatment, type I
isotherms for typical microporous structures can be observed
since obvious adsorption of nitrogen only occurs at very low
relative pressure, while the adsorption amount increases very
limitedly with raising the relative pressure P/P0. Aer lactic
acid modication, the resultant HLA-HT sample also displays
type I isotherms, which means no mesostructures/mesopores
formed during lactic acid pre-treatment. Furthermore, the
adsorption amount of nitrogen at very low relative pressure is
much less than that of parent sample NaY, attributing to the
decrease of microporosity caused by destruction of zeolite
framework. The samples of NaOH-CTAB-c and NH3-CTAB-c
obtained in alkaline solutions in the presence of CTAB
display quite different types of isotherms. Besides adsorption
of nitrogen at relatively low pressure P/P0 < 0.01 indicative of
the presence of micropores, further adsorption can be
observed in the isotherms for both samples in the range of P/P0
0.2–0.5, suggesting the presence of obvious mesoporous
structures in the alkaline-treated samples. Furthermore, the
adsorption amount changes very slowly with further
increasing relative pressure P/P0 > 0.5 and the isotherms end
with nearly horizontal plateau at high relative pressure,
showing that there is no large mesopores/macropores in the
samples and this could be further proved in the pore size
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Table 2 Textural properties of the obtained meso-structured Y zeolites
SBET m2 gꢀ1
VTotal cm3 gꢀ1
VMicro cm3 gꢀ1
SExternal m2 gꢀ1
NaY
797
382
861
767
779
0.40
0.25
0.53
0.53
0.45
0.35
0.18
0.23
0.18
0.33
39
48
377
416
57
HLA-HT
NaOH-CTAB-c
NH3-CTAB-c
HY
Fig. 3 SEM images of samples (a) NaY, (b) NH3-CTAB-c and (c)
NaOH-CTAB-c.
Further TEM images are given in Fig. 5 as to accurately
clarify the porosity distribution in the ultramicrotomed sample
NH3-CTAB-c. It clearly shows that small mesopores with
uniform size around 2–3 nm distribute evenly in large scale in
the zeolite particle (light dots), which is largely different from
those core–shell structures combining zeolite Y with meso-
porous MCM-41 or those meso-microporous composites.16
Besides the presence of uniform mesopores, distinct lattice
fringes corresponding to zeolite crystalline structures can also
be observed in the same particle from the magnied image in
Fig. 5b. The co-occurrence of mesoporosity and zeolite crystal-
line structure in the same particle also exhibits that sample
NH3-CTAB-c was not just composed of physical mixture
of mesoporous MCM-41 solids with microporous zeolite Y
particles.
Fig.
4 TEM images of samples (a–c) NH3-CTAB-c and (d–f)
NaOH-CTAB-c.
As recorded, hierarchically structured FAU zeolites can be
prepared by post-treatment in many approaches, such as
steaming treatment or steaming combining with acid leaching,
alkaline leaching, desilication combining with re-crystallization
in the presence of a pore-directing agent and surfactant-
Fig. 5 TEM images of the ultramicrotomed sample NH3-CTAB-c.
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templated structure arrangement.21 Anyway, the formation of mesoporosity into low Si/Al Y zeolites. As demonstrated by the
mesopores always asks for high Si/Al molar ratio. In the present authors, individual acid or base treatment on low Si/Al Zeolite Y
procedure, lactic acid is utilized to dealuminate and increase does not lead to apparent development of mesoporosity, similar
the Si/Al molar ratio before alkaline treatment. As known, Lactic to the results of lactic acid pre-modication presented herein.
acid is a hydroxyl carboxylic acid and can coordinate with Notably, before desilication in alkaline medium, dealumination
aluminum ions in the framework just like some other chelating of low Si/Al Y zeolites by acid pre-treatment was indispensable
agent, e.g. EDTA,54,55 citric acid56 and tartaric acid.51 Aer lactic to make the bulk Si/Al molar ratio (detected by inductively
acid treatment, the resultant sample HLA-HT owns apparent coupled plasma optical emission spectroscopy, ICP-OES) exceed
higher Si/Al molar ratios, both framework or bulk Si/Al molar 4 and weaken the protection effect of the framework aluminum
ratios, than those of parent sample NaY, as listed in Table 1. ions toward silicon dissolution. Thus during the sequential
Nevertheless, the removal of aluminum ions from the frame- base treatment, mesoporosity can be introduced by desilication
work and subsequent formation of vacancies caused dramati- with extra-framework Al species le in the porosity and as
cally descending of the crystallinity. Furthermore, the bulk Si/Al claimed the removal of Al-rich debris was benecial for freeing
molar ratio of sample HLA-HT is much higher than its frame- the porosity and increasing the crystallinity. Although hierar-
work Si/Al molar ratio indicating that some silicon-rich extra- chically structured Y zeolites with low Si/Al molar ratio can be
framework species were occluded in the sample.57–59 Besides synthesized through the sequential acid and base treatments,
dealumination, exchange of extra-framework Na+ ions by severe decrease of the crystallinities (<20%) of the nal samples
protons takes place simultaneously. However, the removal of can be observed.
framework aluminum does not result in obvious emerge of
The dealumination–desilication process can also be inspec-
mesostructures or signicant increase in external surface area, ted through Solid 29Si and 27Al MAS NMR spectra as shown in
as listed in Table 2. Aer alkaline treatment at high temperature Fig. 6. The 29Si MAS NMR spectrum of the parent NaY sample
of 150 ꢁC, removal of amorphous Si-rich moieties would happen shows typical ve signals with Si(nAl) building units (n ¼ 0–4) in
to sample HLA-HT to varied extents in different alkaline solu- the range of chemical shis between ꢀ85 and ꢀ125 ppm5 while
tions, resulting in decrease of the bulk Si/Al molar ratio which in the 27Al MAS NMR spectrum, only one peak positioned at
can be detected by ICP-AES, whereas the crystallinities and about 61 ppm can be observed assigning to four-coordinated
framework Si/Al molar ratios determined through XRD patterns framework aluminum ions, which means that there are no
increase. This suggests that the decrease of bulk Si/Al molar extra-framework aluminum species in parent NaY. Aer lactic
ratio was fully caused by elimination of silicon-rich extra- acid leaching, sample HLA-HT displays a spectrum with drastic
framework species while the dissolving of zeolitic framework changes in the shape and intensity of 29Si MAS NMR signals
was inhibited since the aluminum ions in the framework may comparing with that of sample NaY. As can be seen that the
protect the zeolite crystalline structure from alkaline attack peaks of Si(4Al), Si(3Al), and Si(2Al) structure units decrease in
during the alkaline treatment.48 The minor increase of the intensity whereas the intensities of peaks corresponding to
framework Si/Al molar ratios for both of the samples are Si(1Al) and Si(0Al) units increase which means the removal of
ascribed to the partial structure recovery by the silicon-rich aluminum ions rstly occurred in Al-rich framework in Y
extra-framework species during the hydrothermal treatment at zeolite, especially in Si(2Al) and it is consistent with the results
high temperatures.
described by Yue et al.51 and Xie and co-workers56 when zeolite
Starting from Y zeolite with low Si/Al molar ratios, Garcia-
Martinez and co-workers43,46 once recorded an attempt to
introduce mesostructures into the zeolite in a two-step method,
namely citric acid washing pretreatment and alkaline treatment
in the presence of pore directing agent (CTAB). As reported, acid
washing pretreatment using dilute citric acid was essentially
important for the formation of mesopores in the subsequent
alkaline modication process. Varied from the lactic acid
pretreatment process described herein, dilute citric acid
washing just had some of the Si–O–Al bonds in zeolite frame-
work open with the formation of Si–OH and/or Al–OH ligands.
Minor removal of the aluminum ions and collapse of the zeolite
crystalline structures can be observed. The formed Si–OH and/
or Al–OH during acid washing pretreatment played a key role for
accommodating the surfactant micelles and zeolite structure re-
arrangement, thus formation of mesostructures. However, no
structural Si/Al evolution or textural property details were given
for the nal mesostructured Y zeolites prepared from low Si/Al Y
zeolites (CBV100 or CBV300) in the publications. Besides this,
Verboekend et al.41 also applied acid pretreatment or coordi-
Fig. 6 29Si and 27Al NMR of samples before and after treatment: (a)
nation combining alkaline leaching method to introduce NaY, (b) HLA-HT, (c) NH3-CTAB-c and (d) NaOH-CTAB-c.
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FAU or BEA was treated using hydrocarboxylic acids. Further-
more, broadening in the signal distributions also can be
observed and a signicant shoulder appears in the region
between ꢀ108 and ꢀ115 ppm in the sample HLA-HT, which is
corresponding to silicon-rich amorphous aluminosilicate and/
or silica species.60 All these can prove that internal changes of
the framework structure take place with the formation of new
siliceous-rich extra-framework species and vacancies in zeolite
structures when parent NaY was treated by lactic acid. Besides,
the aluminum ions extracted from zeolite framework are mostly
dissolved into the solution in the form of aluminum lactate
coordinates instead of extra-framework aluminum species
concluded in the pore structures of zeolite as the peak in 27Al
NMR spectrum located at about 0 ppm ascribing to six-
coordinated Al ions is very weak. Aer treatment in ammo-
nium water solution, the sample NH3-CTAB-c displays a spec-
trum very similar to that of sample HLA-HT but with the
intensity of peaks Si(3Al) and Si(2Al) further decreased whereas
intensity of peak Si(1Al) increased and this is consistent with
the increased framework Si/Al molar ratio as detected by XRD.
The minor decrease of the bulk Si/Al listed in Table 1 may result
from the dissolving of small portion of silicon-rich species
attacked by ammonium water solution at 150 ꢁC. Concurrently,
silicon dissolution leads to more extra-framework aluminum
species le in the zeolite structures which is similar with that
claimed by Verboekend et al.41 and can also be proved by the
27Al NMR spectrum in Fig. 6II-c. It can be noticed that the
intensity of the peak located at 0 ppm corresponding to six-
coordinated aluminum ions is obviously strengthened
comparing with that for sample HLA-HT in Fig. 6II-b. As for
sample NaOH-CTAB-c prepared in NaOH solution, the spec-
trum (Fig. 6d) shows a largely different pattern comparing with
that of sample HLA-HT (Fig. 6b). In the 29Si NMR spectrum of
sample NaOH-CTAB-c, it clearly exhibits that the numbers of
Si(1A1) units apparently decrease, giving rise to Si(2A1) and
Si(3A1) units. Besides, differing from that in the spectrum for
sample NH3-CTAB-c in Fig. 6Ic, the shoulder in the region
between ꢀ108 and ꢀ115 ppm almost diminishes, suggesting
that severe dissolving of silicon-rich species takes place during
alkaline treatment in NaOH solution which results in distinct
decrease of bulk Si/Al molar ratio as listed in Table 1. Further-
more, since the alkalinity of NaOH solution is strong enough,
aluminum containing extra-framework species are also dis-
solved evidenced by the vanishing of the peak at about 0 ppm in
the 27Al NMR spectrum in Fig. 6IId and this is not the same with
that happened to sample NH3-CTAB-c. As the bulk Si/Al molar
ratio is still slightly higher than the framework Si/Al molar ratio
for the NaOH-CTAB-c as listed in Table 1, the silicon-rich
amorphous species were not entirely dissolved with a small
quantity still le in sample NaOH-CTAB-c, for example, the
formed silicon-rich mesostructured debris shown in Fig. 4f.
During the alkaline treatment, fabrication of mesostructures
by assembling amorphous Si-rich species around CTA+ micelles
also takes place along with dissolution. The occlusion of
organic CTA+ in the prepared samples can be proved by FT-IR
spectra. Fig. 7 gives IR spectra of the parent NaY and the
Fig.
7
FT-IR spectra of (a) parent NaY zeolite, (b) HLA-HT, (c)
NH3-CTAB-a and (d) NaOH-CTAB-a.
that all the samples display almost identical spectra which is
typical for FAU-type zeolite, in which peaks centered at 1022 and
1145 cmꢀ1 are ascribed to framework asymmetric stretching
vibrations while peaks located at 794 and 723 cmꢀ1 are corre-
sponded to framework symmetric stretching vibrations and
peaks at 582 cmꢀ1 and 489 cmꢀ1 to tetrahedral vibrations of
double six-membered rings (D6R) and O–T–O bending vibra-
tions, respectively.61 Besides the transmittance peaks attrib-
uting to FAU zeolite structures, some peaks positioning at 2854
and 2925 cmꢀ1 ascribed to –CH2 and –CH3 stretching vibra-
tions, peaks in the range of 1300–1500 cmꢀ1 ascribed to C–H
bending vibrations and weak peak around 910 cmꢀ1 to C–C
stretching vibrations can be observed only in the spectra of
sample NH3-CTAB-a and NaOH-CTAB-a, which are caused by
CTA+ moieties occluded in the samples. The absence of peaks in
such districts for sample HLA-HT further proves the formed
aluminum lactate coordinate was removed and little lactic acid
residues was included in the sample during treatment.
Aiming to estimate the amount of organic CTA+ moieties
embedded in different zeolite samples, TG-DTG curves were
recorded as depicted in Fig. 8. It shows that for sample NaY
zeolite without any treatment, the main weight loss, about 17–
Fig. 8 TG-DTG curves of (a) parent NaY zeolite and (b) NH3-CTAB-a
modied Y zeolite samples in acid/base solutions. It can be seen and (c) NaOH-CTAB-a.
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18% of the total weight ascribing to removal of the physical Martinez et al. once proposed a surfactant-assisted crystal
adsorbed water, takes place in the temperature range of rearrangement mechanism to illustrate the mesopores genera-
25–200 ꢁC. As the temperature increases, no further obvious tion process in which Si–O–Si bonds in the zeolite structures
weight loss can be detected. Whilst for sample NH3-CTAB-a and were attacked rstly by alkaline and broken with the formation
NaOH-CTAB-a, the thermal evolution processes are very similar of negatively charged sites. The resultant negatively charged
to each other. Combined with DTG curves, it is easy to divide the sites can react with positively charged surfactants through
weight loss process of the samples into three regions and the electrostatic interactions. And nally, the occluded surfactant
relative weight losses in each region are not identical. Taking cations assembled together with the formation of micelles
sample NaOH-CTAB-a as an example, it can be seen that the rst within the zeolite crystals and made the zeolite microstructures
weight loss step, about 10% of the total weight, also proceeds in rearrange so as to form mesopores around the micelles.43
the temperature range of 25–200 ꢁC. Furthermore, another two Otherwise, Ivanova and co-workers demonstrated another
weight losses in the range of 200–420 ꢁC and 420–700 ꢁC can be possibility for the formation of micro-mesoporous hierarchical
observed which can be ascribed to decomposition of organic materials through post-treatment of zeolite, in which zeolite
CTA+ clusters and combustion of the small chemical residuals dissolution and re-assembly were involved.16 As stated, zeolite
and dehydration by silanol polymerization. Amongst the three with high enough Si/Al molar ratio rstly dissolved under
samples, apparently more water is physically adsorbed in NaY alkaline circumstances to some extent leading to partial zeolite
zeolite because of its higher hydrophilic zeolite nature due to structure destruction and extraction of zeolitic fragments.
large amount of Al ions in the framework62 and the presence of Then, the zeolitic fragment dispersed in alkaline solutions may
CTA+ in the other samples may also inhibit the occlusion of re-assemble together around CTA+ micelles with the generation
water molecules in the zeolite pores. Comparing the weight of mesoporous structures. Furthermore, depending on the
losses of sample NH3-CTAB-a with that of sample NaOH-CTAB-a, zeolite dissolution degree, the formed mesostructures may
it can be noted that the amount of CTA+ moieties occluded in either just cover the zeolite surface to generate micro-
sample NaOH-CTAB-a is larger than that in sample NH3-CTAB-a, mesostructured composites, or exist apart from zeolite with
which does not match well with the results of N2 adsorption the generation of micro/mesoporous dual-phase mixtures, or
experiment that sample NH3-CTAB-a owns superior meso- completely immerses the residual zeolite. The dissolution and
structures, e.g. larger external specic surface area and higher re-assemble mechanism was also veried by Perez-Ramirez and
mesopore volume as shown in Table 2. So it may be deduced co-workers when CTA+ was used as pore-directing agent in the
that the proportion of mesostructures in the samples has no post-treatment of zeolites.40,63 According to the SEM images of
direct relationship with the occluded CTA+ amount. Yet, the sample NaOH-CTAB-c shown in Fig. 3c, displaying mixtures of
addition of CTAB in the alkaline treatment process, especially in large zeolite-like bulks and small pieces of debris, we can
NaOH solution, is necessary as to direct the formation of mes- conclude that the formation of mesostructures in sample
ostructures and inhibit the emerging of zeolite P (ESI Fig. S1†), NaOH-CTAB-c may proceed following the way proposed by Iva-
for zeolite P can easily form during the crystallization or post- nova and co-workers.16 On the other hand, the low solid yield
treatment process in high alkaline solutions.
(75%) aer alkaline treatment in NaOH solution without addi-
As recorded, the introduction of mesopores into the micro- tion of CTA+ also proves the dissolution process as listed in ESI
structured zeolite in the presence of CTAB always proceeds in Table S1.† When ammonium water solution was utilized
different ways according to different groups. Based on the facts instead in the absence of CTA+, high solid yield (97%) can be
that the preservation of zeolite framework Si/Al molar ratio and obtained due to the low alkalinity of the solution, that is to say,
almost 100% of solid recovery aer alkaline treatment, Garcia- the mesostructures in sample NH3-CTAB-c can not form in the
dissolution-reassembly way, at least not possible in large scale
as shown in Fig. 4c. Therefore, the mesostructures in zeolite
NH3-CTAB-c were alternatively directed in
a structure-
rearrangement procedure caused by the weaker silicon disso-
lution ability.
In order to elucidate the impact of the co-existence of micro-
mesoporous structures on the diffusion of large organic mole-
cules, acetalization of cyclohexanone with glycol or pentaery-
thritol were applied as probe reactions. As a comparison,
industrial H+-type Y zeolite, microporous HY (textural proper-
ties as listed in Table 2) was also used as a catalyst in the
reactions. As shown in Fig. 9 is the NH3-TPD curves of the three
different catalyst samples. Amongst them, sample HY owns the
largest amount of acid sites because of its highest crystallinity of
zeolite structures. Generally, the acidity decreases apparently
with a decrease in the zeolite crystallinity despite the similar Si/
Al molar ratio. When used in the acetalization reaction of
cyclohexanone with glycol, the three catalyst samples own
Fig.
9 NH3-TPD curves of samples (a) NH3-CTAB-cat, (b)
NaOH-CTAB-cat and (c) HY.
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NaOH-CTAB-cat and NH3-CTAB-cat lie in the same level with
cyclohexanone conversion almost the same, even though
catalyst NaOH-CTAB-cat owns a little higher cyclohexanone
conversion in the reaction as demonstrated in Fig. 10 and in
Table 3.
4. Conclusions
Hierarchically structured Y zeolites were prepared by post-
synthetic strategy, where parent NaY zeolite was sequen-
tially treated by lactic acid solution and an alkaline solution
containing cetyltrimethyl ammonium bromide (CTAB).
Lactic acid pre-modication of NaY is vitally important
because it may dealuminate the zeolite framework and result
in the formation of silicon-rich amorphous species offering
nutrients for the formation of mesoporosity. During alkaline
treatment in the presence of surfactant CTA+ micelles, mes-
oporosity can be successfully introduced into the NaHY
zeolites while the microporous structures are well preserved.
By varying the alkaline solution used, the textural properties
of the nal products are different because of different silicon
solubility and different mesopore formation mechanisms are
discussed: aer treated in ammonium solution, mesostruc-
tured zeolite was formed in structure re-arrangement mech-
anism proposed by Garcia-Martinez and co-workers,21,43
otherwise, mesostructures were introduced through
dissolution-reassembly procedure described by Ivanova's
group16 when treated in NaOH solution. The hierarchically
structured Y zeolites exhibit much better performances in the
acetalization reaction of cyclohexanone with pentaerythritol
than conventional Y zeolites while the conventions of cyclo-
hexanone are almost identical when reacted with glycol. This
phenomenon could be attributed to the enhanced diffusion
ability of large sized guest molecules through the combina-
tion of mesoporosity and microstructures comparing with
Fig. 10 Comparative catalytic performance of microporous HY
zeolites and hierarchically structured Y zeolites in Acetalization of
Cyclohexanone with (I) glycol and (II) pentaerythritol.
Table 3 Catalytic performances in acetalization of cyclohexanone
with glycol and pentaerythritol
Conv.a/%
0.5 h
Conv.b/%
Catalysts
1 h
2 h
1 h
2 h
4h
NaOH-CTAB-cat
NH3-CTAB-cat
HY
68.4
67.3
66.9
69.1
69.2
67.7
72.6
71.8
72.2
31.8
28.1
12.3
35.0
33.5
17.2
34.4 pure microporous Y zeolites.
33.7
23.6
Acknowledgements
a
Conversion of cyclohexanone aer acetalization with glycol.
Conversion of cyclohexanone aer acetalization with pentaerythritol.
b
This work is supported by National Key Technology R&D
Program (no. 2012BAE05B02) and National Science Foundation
of China (20890124). And Prof. Wang thanks the Fundamental
Research Funds for the Central Universities and the Program
for New Century Excellent Talents in University (NCET-11-0145),
Ministry of Education of China.
almost the same reactivity as shown in Fig. 10a, since the
product has a relative small molecular size and its diffusion in
zeolite Y is not constrained. With increasing the reaction
duration, the conversion of cyclohexanone is increased slightly
as shown in Table 3, regardless of the catalyst nature used.
When the catalytic reaction proceeds under the otherwise same
condition using pentaerythritol instead of glycol, the conver-
sion of cyclohexanone with pentaerythritol is decreased
comparing with that with glycol. But comparing the three
catalyst samples, HY displays the worst performance in acetal-
ization of cyclohexanone with pentaerythritol, because the
molecular size of the product is too large to diffuse out from the
inner reactive sites in the zeolite structures, consistent with the
results reported elsewhere.52 While the reactivities of catalysts
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