Mg2+-derived mesoporous ultra-high silica twelve-membered-ring basic zeolites: Straightforward synthesis and catalytic performance

Article abstract of DOI:10.1039/c5ce02075a

Mg²⁺-derived mesoporous ultra-high silica twelve-membered-ring zeolites with multiple topologies (MOR, BEA and MTW) were straightforwardly synthesized by a one-pot route, where the crucial step was the co-hydrolysis/condensation of silica source and magnesium salt under moderate acidic conditions. SiO₂/Al₂O₃ ratios can be adjusted from ～30 to as high as 410, thus generating superior basicity that was further improved by the incorporation of Mg species. A mesoporous structure was self-formed without the assistance of any template or special strategy. Catalysis tests showed high activity of these zeolites in a typical base reaction, Knoevenagel condensation, even for the bulky substrates due to the enhanced mass transfer arising from the mesopores. This methodology provides a promising approach towards target synthesis of valuable Mg²⁺-derived mesoporous ultra-high silica zeolites with tunable acid/base properties, which can even act as an efficient mesoporous zeolitic solid base.

Full text of DOI:10.1039/c5ce02075a

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Mg2+-derived mesoporous ultra-high silica twelve-
membered-ring basic zeolites: straightforward
synthesis and catalytic performance†
Cite this: CrystEngComm, 2016, 18,
1164
*
*
Jingyan Xie, Haimeng Wen, Wei Zhang, Yu Zhou and Jun Wang
Mg2+-derived mesoporous ultra-high silica twelve-membered-ring zeolites with multiple topologies (MOR,
BEA and MTW) were straightforwardly synthesized by a one-pot route, where the crucial step was the co-
hydrolysis/condensation of silica source and magnesium salt under moderate acidic conditions. SiO2/Al2O3
ratios can be adjusted from ∼30 to as high as 410, thus generating superior basicity that was further im-
proved by the incorporation of Mg species. A mesoporous structure was self-formed without the assis-
tance of any template or special strategy. Catalysis tests showed high activity of these zeolites in a typical
base reaction, Knoevenagel condensation, even for the bulky substrates due to the enhanced mass transfer
arising from the mesopores. This methodology provides a promising approach towards target synthesis of
valuable Mg2+-derived mesoporous ultra-high silica zeolites with tunable acid/base properties, which can
even act as an efficient mesoporous zeolitic solid base.
Received 23rd October 2015,
Accepted 7th January 2016
DOI: 10.1039/c5ce02075a
www.rsc.org/crystengcomm
additional acid or base solution that may increase the waste
release.10 Various zeolites with different topologies and SAR
1. Introduction
Zeolites are crystalline aluminosilicates with large surface
areas, well-defined uniform micropores, and excellent stabil-
ity. They have been extensively applied as adsorbents, ion ex-
changers and catalysts in numerous industrial processes,
such as petroleum refining and production of fine
chemicals.1–4 The actual performance of zeolites lies in their
topology, elemental composition, morphology, surface state,
etc. In this regard, SiO2/Al2O3 molar ratio (SAR) is one of the
most important parameters for the application of zeolites in
industry, because it significantly affects their properties, espe-
cially the acid/base properties.5–7 Therefore, it is always one
major research context to control SAR over the required zeo-
litic topology and composition.
SAR of zeolites can be basically tailored in two ways: (i)
during a straightforward synthesis and (ii) by post-synthesis
treatment. Although the post-synthesis treatment is an effi-
cient way of tailoring SAR,8,9 the control of SAR in the
straightforward synthesis is preferred because of the follow-
ing advantages: (1) simplification of the preparation pro-
cess;10 (2) excellent preservation of the crystallinity and
microporous channels;11,12 (3) avoidance of the usage of
have been straightforwardly synthesized by using organic
structure-direct-agents,13,14 seeded growth,15 or fluoride
growth media.16 One typical example is the ten-membered-
ring MFI topological zeolite, which can be synthesized in a
wide SAR range (up to pure silica, i.e. silicalite-1).17 However,
it is still a huge challenge for the direct synthesis of many
kinds of zeolites with widely tunable SAR, especially with
ultra-high SAR.18 According to Lowenstein's rule,19 SAR ≥ 2
is commonly seen in the synthesis of zeolites, but no SAR ≥
2 is achieved over LTA, except for the only one example of
pure silica LTA reported by Corma et al.,20 suggesting that it
is hard to adjust the SAR of LTA zeolites. For another exam-
ple, SAR of MOR zeolites was limited to lower than 74 from
the direct synthesis process.21 As a result, many efforts have
been made to develop new synthetic routes to control the
growth of zeolite crystals.22 One effectual way is the introduc-
tion of metal ions in the zeolite synthesis. The germano-
silicate molecular sieve is a successful example in this con-
text,23,24 which implies that some metal additives may favor
the formation of ultra-high silica zeolites.
During zeolite syntheses, NaOH frequently acts as a min-
eralizer;25 yet, the role of bivalent metallic cations is rarely
studied and limited to several isolated reports. The
pioneering work of an early patent described the introduction
of magnesium ions into directly one-pot synthesized zeolites,
which however involved no characterization proof and discus-
sion.26 Recently, alkaline earth metal ions have been incorpo-
rated into silicalite-1 by directly adding the corresponding
State Key Laboratory of Materials-Oriented Chemical Engineering, College of
Chemical Engineering, Nanjing Tech University (formerly Nanjing University of
Technology), Nanjing 210009, Jiangsu, PR China.
E-mail: njutzhouyu@njtech.edu.cn, junwang@njtech.edu.cn; Tel: +86 25 83172264
† Electronic supplementary information (ESI) available: Fig. S1–S5. See DOI:
10.1039/c5ce02075a
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metal ions into the initial synthetic gels, and the obtained ze-
olites have been systematically characterized and have
exhibited superior base properties due to the existence of
framework alkaline earth metal ions.27 It implies that magne-
sium ions as the metal additive benefited the synthesis of
high silica or pure silica zeolites and intrinsically altered the
acid/base properties for the obtained zeolites. Further, as
known, in order to improve the mass transfer in zeolitic
micro-channels, numerous approaches were proposed to di-
rectly fabricate a mesoporous structure over different zeo-
lites.28,29 Nonetheless, these syntheses usually relied on addi-
tional soft or hard templates, and the thus obtained zeolites
normally acted as solid acids rather than bases.30,31
into the above mixture one by one. HCl was neutralized by ex-
cessive TEAOH, varying the mixture from acidic to basic con-
ditions. The obtained slurry was aged at room temperature
under dynamic (stirring) conditions for 24 h. The final molar
composition of the gel was
1 SiO2 : xAl2O3 : 0.02MgO :
0.35TEAOH : 22.5H2O. After agitation, the pH of the gel was
adjusted by sodium hydroxide (NaOH, 99 wt%, Sinopharm
Chem. Reagent Co., AR) to the required value and then trans-
ferred into a Teflon-lined stainless steel autoclave and heated
statically at 140 °C. The solids were separated by centrifuga-
tion, washed with deionized water and dried at 100 °C for 12
h. Finally, they were calcined at 550 °C for 5 h in air to re-
move the structure directing agent.
In this work, we report the straightforward synthesis of
Mg2+-derived mesoporous ultra-high silica zeolites with tun-
able SAR and multiple topologies for twelve-membered-ring
zeolites, which are proved to be a new series of mesoporous
zeolitic solid bases. The synthesis is achieved via the uncon-
ventional pre-co-hydrolysis/condensation of a silica precursor
with Mg salt under acid conditions;32,33 this crucial early ge-
lation stage does not involve any other metal ions and the in-
troduced Mg ions contribute indispensably to the formation
of an ultra-high silica zeolitic framework. Zeolitic phases are
facilely controlled by the pH values employed for the followed
hydrothermal crystallization processes. Multiple Mg2+-derived
mesoporous zeolites with MOR, MTW or BEA topologies are
achieved with ultra-high SAR (up to 410). Abundant meso-
pores are self-formed during the synthesis without the aid of
any mesopore-formation template.34 Herein, it is the first oc-
casion to directly generate mesoporous zeolitic solid bases.
Systematical syntheses and characterization are performed,
and the catalytic performance is assessed in Knoevenagel
condensations, a series of typical base-catalyzed reactions.35
High conversion is achieved over Mg2+-derived ultra-high sil-
ica zeolites. Catalysis tests over the substrates with different
molecular sizes illustrate the promoted activity due to the en-
hanced mass transfer in mesopores.
For the preparation by base co-hydrolysis, TEOS was
added into the deionized water and stirred uniformly, and
then TEAOH was added dropwise and stirred for 10 min.
MgIJNO3)2·6H2O was added into the above mixture. After-
wards, a mixture of sodium aluminate and deionized water
was added. The obtained slurry was aged at room tempera-
ture under dynamic (stirring) conditions for 12 h. The final
molar composition of the gel was 1SiO2 : xAl2O3 : 0.02MgO :
0.35TEAOH : 22.5H2O. The pH of the gel was also adjusted by
sodium hydroxide (NaOH, 99 wt%, Sinopharm Chem. Re-
agent Co., AR) to the required value. Finally, the slurry was
transferred into a Teflon-lined stainless steel autoclave and
heated statically at 140 °C for 14 days. The product was sepa-
rated by centrifugation, washed with deionized water and
dried at 100 °C for 24 h. After that, they were calcined at 550
°C for 5 h in air.
2.2 Characterization
The crystalline structures of the prepared samples were char-
acterized by X-ray diffraction analysis (XRD) using
a
SmartLab diffractometer from Rigaku equipped with a 9 kW
rotating anode Cu source at 45 kV and 100 mA, from 5° to
50° with a scan rate of 0.2° s−1. The morphologies of the sam-
ples were tested using a field-emission scanning electron
microscope (SEM; HITACHI S-4800). The nitrogen sorption
isotherms and pore-size distribution curves were measured at
the temperature of liquid nitrogen (77 K) by using a
BELSORP-MINI analyzer with the samples degassed at 300 °C
for 3 h before analysis. Specific surface areas and pore vol-
umes of the calcined samples were obtained from N2-
adsorption–desorption isotherms using multipoint BET and
t-plot methods. The FTIR spectra were recorded on an Agilent
Cary 660 FT-IR instrument (KBr disks) in the 4000–400 cm−1
region. The solid UV-vis spectra were measured using a
SHIMADZU UV-2600 spectrometer, and BaSO4 was used as an
internal standard. Chemical compositions of zeolites were
analyzed by using a Jarrell-Ash 1100 inductively coupled
plasma (ICP) spectrometer. The acid and base sites were ana-
lyzed by using a Catalyst Analyzer BELCAT-B under NH3 and
CO2 atmospheres, respectively. The samples are pre-treated at
550 °C. The X-ray photoelectron spectra (XPS) were conducted
on a PHI 5000 Versa Probe X-ray photoelectron spectrometer
2. Experimental section
2.1 Materials synthesis
Mg2+-derived mesoporous ultra-high silica zeolites were syn-
thesized by an acidic co-hydrolysis route.36 Reaction mixtures
having batch molar compositions of 0.02MgO : xAl2O3 : SiO2 :
0.35TEAOH : 22.5H2O, where x = 0.01–0.0125, were prepared
by either an acidic or a base co-hydrolysis route. For the prep-
aration by the acidic co-hydrolysis route, a mixed aqueous so-
lution of MgIJNO3)2·6H2O (99 wt%, Sinopharm Chem. Reagent
Co., AR) and tetraethylorthosilicate (TEOS, 28.4 wt% SiO2,
Sinopharm Chem. Reagent Co., AR) was hydrolyzed under
moderately acidic conditions (pH ∼ 1.0) by dropwise addition
of hydrochloric acid (HCl) aqueous solution (0.30 M, Shang-
hai Chem. Reagent Co., AR) at 90 °C for 4 h. After that, tetra-
ethylammonium hydroxide (TEAOH, 35 wt% aqueous solu-
tion, Jintan Huadong, AR) and sodium aluminate (NaAlO2, 41
wt% Al2O3, Shanghai Chem. Reagent Co., AR) were added
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equipped with Al Kα radiation (1486.6 eV). The 29Si and 27Al
MAS NMR spectra were recorded on a Bruker AVANCE III 600
spectrometer. The 29Si MAS NMR spectra with high-power
proton decoupling were recorded on a 4 mm probe with a
spinning rate of 12 kHz, a π/4 pulse length of 2.6 μs, and a re-
cycle delay of 80 s. The chemical shift of 29Si was referenced
to TMS.
2.3 Catalysis test
Fig. 1 XRD patterns for as-calcined Mg2+-derived mesoporous ultra-
high silica zeolites synthesized by acidic pre-co-hydrolysis/condensa-
tion. (A) MOR; (B) BEA; (C) MTW.
A typical procedure for Knoevenagel condensation of benzal-
dehyde with ethyl cyanoacetate was performed as follows:
benzaldehyde (10 mmol) and ethyl cyanoacetate (10 mmol)
were added to a 25 mL round-bottomed-flask reactor at the
predetermined reaction temperature. After addition of 0.1 g
of catalyst, the reaction slurry was stirred under reflux for the
required reaction time. After reaction, the internal standard
n-dodecane was added, and the resulting mixture was diluted
with ethanol. The reaction mixture was centrifuged to remove
the solid catalyst, and the liquid was analyzed by GC using a
gas chromatograph (GC SP-6890) equipped with a flame ioni-
zation detector and a capillary column (SE-54; 30 m × 0.32
mm × 0.25 mm).
stands for the SiO2/Al2O3 molar ratio (SAR) in the starting re-
action mixture. Adjusting the value of n from 100 to 800 gives
the actual SAR from 32 to 274 in the final solid product
(Table 1). Further increasing the SAR in the gel only attains
an amorphous product, and even by extending the crystalliza-
tion time to 18 days, no crystal product is obtained. The
SiO2/MgO ratio for all final solid samples is roughly ∼100,
which is higher than the ratio (50) in the initial gels.
Two other zeolite phases of BEA and MTW are obtained by
keeping the pH value of crystallization at 12.5 0.1 (Fig. 1).
The corresponding samples are named as Mg-BEA(n) and Mg-
MTWIJn), respectively (Table 1). BEA and MTW zeolites possess
a very similar topology, in which their a–c projection is viewed
along and perpendicular to the 12R straight channels.27,34
Crystal phase transformation could occur between BEA and
MTW, because BEA can provide a specific growth surface for
the crystallization of the MTW phase through their structural
similarity (see Fig. S5† and the related additional explanation
text).29 Fixing the crystallization time at 14 days and varying
the SAR in the gel (n) cause a significant phase transfer from
BEA to MTW: a pure BEA phase is formed with n from 100 to
250 (Fig. 1B); a mixed phase of BEA and MTW is obtained
with a higher n value (300–550) (Fig. S1A†), and a pure MTW
phase is obtained with a more higher n (600–800) (Fig. 1C).
Further increasing the n value causes an amorphous struc-
ture. In addition, BEA–MTW inter-zeolite transformation oc-
curs when the crystallization time is varied from 6 to 18 days
(Fig. S1B†), suggesting that a higher crystallization time favors
the formation of the BEA phase. Correspondingly, two Mg2+-
derived ultra-high silica BEA zeolites, Mg-BEA(300) and Mg-
BEA(800), are synthesized by the 18 days crystallization, giving
the final SAR of 128 and 253 for solid products, respectively.
Similarly, elongating the crystallization time to 18 days but
with an extremely high n value of 1000 produces a high-silica
MTW zeolite with a much higher SAR of 410 for the obtained
sample Mg-MTWIJ1000). An amorphous structure is obtained
with n > 1000 no matter what the crystallization time is.
After reaction, the catalyst was separated from the reacted
mixture by filtration, washed, and calcined at 550 °C for 5 h.
The recovered catalyst was then charged into the next run for
reuse.
3. Results and discussion
3.1 Formation of Mg2+-derived ultra-high silica MOR, BEA
and MTW zeolites
Scheme 1 shows the synthesis procedures for the Mg2+-de-
rived ultra-high silica zeolites. Three twelve-membered-ring
(12R) zeolitic topologies can be obtained by controlling the
pH value before crystallization, as well as the related SAR in
the initial gel (Fig. 1). Pure phase Mg2+-derived MOR zeolites
are obtained by controlling the pH value at 13.0 0.1 with
the crystallization at 140 °C for 14 days (Fig. 1A). The
obtained samples are denoted as Mg-MORIJn), in which n
Several synthesis parameters are explored and compared
to further understand the above synthesis. When the synthe-
sis is conducted under basic conditions throughout, in which
the silica precursor undergoes no acidic pre-co-hydrolysis/
condensation process but is directly hydrothermally
Scheme 1 Synthesis procedures for ultra-high silica zeolites of MOR,
BEA and MTW, via straightforward hydrothermal crystallization involv-
ing acidic pre-co-hydrolysis/condensation.
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Table 1 Textural properties of the as-calcined Mg2+-derived mesoporous ultra-high silica zeolite samples synthesized by acidic pre-co-hydrolysis/
condensation
SiO2/MgOa
SBET (m2 g−1
)
V (cm3 g−1
)
D (nm)
a
Samples
SiO2/Al2O3
Mg-MORIJ100)
Mg-MORIJ200)
Mg-MORIJ300)
Mg-MORIJ600)
Mg-MORIJ700)
Mg-MORIJ800)
Mg-BEA(100)
Mg-BEA(200)
Mg-BEA(300)b
Mg-BEA(800)b
Mg-MTWIJ600)
Mg-MTWIJ700)
Mg-MTWIJ800)
Mg-MTWIJ1000)b
32
58
70
102
106
111
111
105
112
128
123
118
125
125
111
125
100
291
296
304
312
317
324
333
427
343
356
197
211
333
289
0.23
0.23
0.24
0.24
0.24
0.24
0.36
0.41
0.47
0.53
0.22
0.59
0.58
0.57
2.91
2.91
2.90
2.89
2.90
2.89
2.83
3.84
4.59
6.23
4.47
11.3
6.95
7.89
96
160
274
31
82
128
253
127
160
348
410
a
b
SiO2/MgO and SiO2/Al2O3 ratios for the final solid products determined by ICP. The crystallization time is 18 days, and the crystallization
time for the other samples is 14 days.
crystallized under alkaline conditions, only an amorphous
structure can be observed (Fig. S1C†). All samples have high
crystallinity and reflect the prerequisite role of the initial
acidic co-hydrolysis/condensation stage. The high crystallinity
is obtained when high temperature (90 °C) acidic co-
hydrolysis of TEOS precedes other operations (cooling down
to room temperature, addition of sodium aluminate and
TEAOH, room-temperature aging and hydrothermal treat-
ment of the aged reaction mixture). On the other hand, keep-
ing the synthesis conditions unchanged, no solid (crystalline)
product is formed when co-hydrolysis of TEOS is performed
at room or low temperature. This clearly indicates that the
(high) temperature of TEOS co-hydrolysis is crucial for the
processes which occur during either room-temperature aging
or hydrothermal treatment of the reaction mixture. Thus, the
influence of the temperature of TEOS co-hydrolysis on the
course of crystallization may be, at the first instance,
explained as follows: fast and complete hydrolysis of TEOS at
high temperature causes the formation of silica nano-
particles,37 and then the formation of Al-enriched amorphous
silica and further formation of various silicate and alumino-
silicate nanosized species during either room-temperature ag-
ing and/or the initial stage of hydrothermal treatment of the
reaction mixture, which are precursor species for nucleation
and crystal growth of zeolites.38,39 Since the silica nano-
particles needed for the formation of silicate and aluminosili-
cate precursor species obviously do not form, or the rate of
their formation is not appropriate, during room-temperature
hydrolysis, a crystalline product cannot be obtained when hy-
drolysis of TEOS is conducted at room temperature. The de-
tailed investigation on the initial pH value (Fig. S1C†) shows
the formation of amorphous products with either high (>1.0)
or low (<0.7) pH values, revealing that the gel state after that
hydrolyzation should have affected the creation of the prelim-
inary zeolite phase. The pH value just before the onset of hy-
drothermal crystallization is also studied from 12.1 to 13.4.
No solid can be obtained at pH values of 12.1, 12.3 and 12.7,
and a much lower crystallinity of MOR is obtained at 13.4,
which indicates that the highly crystallized zeolite phases are
very sensitive to the pH conditions for the crystallization.
More importantly, Mg-free synthesis is performed for com-
parison by controlling the similar pH values at 13.0 and 12.5
for the crystallization. In the case of pH = 13.0, analcite is the
main product in the absence of Mg2+ in the gel (Fig. S1D†),
indicating that magnesium is indispensable for the forma-
tion of the MOR phase. When pH = 12.5, the BEA phase
changes to MTW in the absence of Mg ions at n = 200 (Fig.
S1D†), suggesting that Mg ions may contribute to the forma-
tion of the BEA phase at the relatively low SAR in the gel. The
Mg-free system cannot produce any products at a high n
value (≥600). These results demonstrate that the synthesis of
these 12R ultra-high silica zeolites relies on the presence of
Mg ions. The reason may be due to that Mg ions function as
co-structure-directing sites, as performed by various other
metal ions in zeolite syntheses.
Under acidic conditions, silica species are the less con-
densed linear oligomers, while in alkaline solution the silica
species are the more cross-linked clusters.40 If the reaction
mixtures are adjusted to the alkaline system, hydrolysates
obtained by acid hydrolysis are easier to dissolve under hy-
drothermal conditions theoretically. More active components
can be formed to participate in the formation of the crystal
nucleus and growth of zeolite crystals, resulting in higher uti-
lization of Si, so as to improve SAR. Moreover, the acidic con-
ditions for hydrolysis and condensation of silica precursors
offer a lower rate than the basic one,41 allowing more oppor-
tunities for isolation and dispersion of metal ions and thus
promoting the bond formation between metal ions and
silanol groups.32,33 In this way, it is more likely to generate
the desirable Si–O–Mg–O–Si linkage (known as the primary
building blocks for further growth of the heteroatomic zeolite
crystals) in the gel mixture, and such interaction is crucial
for Mg ions to act as co-structure-directing agents for the for-
mation of zeolites. On the contrary, hydroxide or oxide
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precipitates can be generated quickly through the basic hy-
drolysis route, inhibiting the above interaction to perform
the function of Mg ions as co-structure-directing agents. This
is why the acidic co-hydrolysis route benefits the synthesis of
high or ultra-high silica zeolites (Fig. 2).
3.2 Mesoporosity and other characterization
Porosities of the above obtained Mg2+-derived ultra-high sil-
ica zeolites are measured by nitrogen sorption experiments
(Fig. 3 and S2†). The corresponding textural properties are
listed in Table 1. The N2 sorption isotherms of all the sam-
ples demonstrate a sharp uptake at low relative pressure (p/p0
< 0.05), index of typical zeolitic microporosity. The continu-
ous increase in N2 sorption at a relatively high pressure (p/p0
= 0.1–0.99) reveals the presence of mesopores, which is fur-
ther reflected by the apparent pore size distribution. The iso-
therms for Mg-MORIJ100–800) samples exhibit a similar shape
with a type H2 hysteresis loop, suggesting that these samples
possess similar mesopores and the variation of SAR rarely af-
fects the mesoporous structure. For Mg-BEA(n) series, they
display a type H4 hysteresis loop with a sharp step closure at
a relative pressure of about 0.6, and the larger SAR of these
BEA zeolites and the higher uptake at high relative pressure
(p/p0 > 0.6) suggested enhanced mesoporosity upon an in-
crease in SAR. In previous work, the formation of mesopores
on zeolites during a straightforward synthesis usually relies
on the utilization of additional soft/hard templates except for
the one example under dynamic crystallization;28,29,34 here,
mesoporous zeolites are obtained by using a special acidic
co-hydrolysis route without using any mesopore-formation
template. The present synthesis has the advantages of saving
raw materials and simplifying preparation procedures; for ex-
ample it requires no template removal process like a hard-
template pathway.
Fig.
3
N2 adsorption–desorption isotherms (left) and the
corresponding pore-size distributions (right) for Mg2+-derived meso-
porous ultra-high silica zeolites synthesized by acidic pre-co-hydroly-
sis/condensation: (A) MOR; (B) BEA; (C) MTW.
= 200, and their size is higher than ten micrometers, slightly
larger than that of Mg-MORIJ100). At higher SAR of n = 300, it
emerges as irregular particles derived from the aggregation of
small primary crystals rather than the terrace-like layers, and
their size is further expanded to tens of micrometers. Further
increasing the SAR to n = 600, the terrace-like layers disap-
pear and the morphology changes to fluctuating particles
with a smooth surface, and their sizes vary in a wide range
from several to tens of micrometers. Increasing n to 700 and
800, all the particles evolve into irregular aggregations with a
rough surface and wide size distributions. SEM images of all
the samples of Mg-BEA and Mg-MTW are presented in Fig. 4
and S3.† Their morphologies are all irregular bulks composed
of small crystals with the size of hundreds of nanometers.
These SEM images reveal that the formation of mesopores
over these ultra-high silica zeolites may be due to the small
Fig. 4 shows the SEM images of Mg-MORIJn) zeolites, dem-
onstrating significant variation of morphologies for this
series. Mg-MORIJ100) is composed of isosceles trapezoid flake
layers with the size of about several micrometers. As SAR in-
creases, the morphology evolves into terrace-like layers for n
Fig. 4 SEM images for as-calcined samples synthesized by acidic pre-
co-hydrolysis/condensation: (A) Mg-MORIJ100), (B) Mg-MORIJ200), (C)
Mg-MORIJ300), (D) Mg-MORIJ600), (E) Mg-MORIJ700), (F) Mg-MORIJ800),
(G) Mg-BEA(100), and (H) Mg-MTWIJ800); (I) TEM image for as-calcined
Mg-MTWIJ1000).
Fig. 2 Ternary phase diagram of topological structures obtained from
the straightforward synthetic system synthesized by acidic pre-co-
hydrolysis/condensation.
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intracrystalline voids inside the polycrystalline aggregates.
For Mg-MOR samples, they are in the form of single crystals
at SAR = 100 (Fig. 4A). These single crystals become more
and more irregular in shape as SAR increases (Fig. 4B–F),
which contain a small amount of mesopores. On the other
hand, Mg-BEA and Mg-MTW samples appear mainly in the
form of polycrystalline aggregates (Fig. 4G–I and S3†)39,42,43
and most of them exhibit mesoporosity. The TEM image of
Mg-MTWIJ1000) with the highest SAR shows that the primary
particles are highly crystallized nano-crystallites and denotes
that the nano-crystallites pack each other by intergrowth dur-
ing the crystallization process into bigger particles, forming
mesopores (Fig. 4I).
For Mg-MTWIJ1000), we further carry out 29Si and 27Al MAS
NMR analysis to study the chemical state of silica and alumi-
num. The 29Si NMR spectrum (Fig. 5A) presents two strong
signals at −103 and −111 ppm that can be assigned to the
SiIJOSi)4 (Q4) structure,44,45 suggesting that the majority of the
silicon atoms are located in four-connected sites as expected
for a framework silicate. The broad signal at −103 ppm very
likely contains not only Q4 species but also SiIJOSi)3IJOH) (i.e.
Q3 typically appearing around −102 ppm) and maybe also an-
other Q4 site SiIJOSi)3IJOMg). One additional peak emerges at
−94 ppm mainly attributable to the Q3 structure for Mg-
MTWIJ1000), which is derived from the formation of some de-
fects or silicon atoms representing the terminal (surface)
ones. Such signal may comprise SiIJOSi)2IJOH)2 (Q2) as well as
Si species bonded to OMg and OH at the same time. The 27Al
NMR spectrum (Fig. 5B) exhibits a single band at 54 ppm for
the tetrahedral coordinated aluminum species, while no sig-
nal at 0 ppm for the extra-framework octahedral coordinated
aluminium species is observed.46 Such phenomenon demon-
strates that the Al atoms are entirely incorporated into the
crystalline framework. The above NMR analysis further re-
veals the highly crystalline structure even for the sample with
extremely high SAR.
Fig. 6 IR spectra (A) and UV-vis spectra (B) for as-calcined MOR sam-
ples and (C) IR spectra and (D) UV-vis spectra for as-calcined BEA and
MTW samples, with different SiO2/Al2O3 molar ratios. All samples are
synthesized by acidic pre-co-hydrolysis/condensation.
high framework SAR for these MOR zeolites. Typical vibra-
tions for the Mg-BEA and Mg-MTW samples are seen in
Fig. 6C. The UV-vis spectra are shown in Fig. 6B. No absorp-
tion in the 220–800 nm range is observed over the Mg-free
sample, while all the Mg2+-derived zeolites exhibit a distinct
absorption band at 260 nm, the same as the Mg2+-incorpo-
rated silicate-1 sample.27 Such a signal can be assigned to the
excitation of O atoms around the framework Mg species,
which is highly distorted or possesses vacancies compared to
the regular MgO topology.48
Fig. 7 shows the XPS core-level spectra O 1s, Si 2p, and Mg
2p, for the selected ultra-high silica zeolites Mg-BEA(800),
Mg-MTWIJ1000) and Mg-MORIJ800). In the O 1s spectra, the
peak shifts to higher binding energies with the increase in
SAR (Fig. 7), due to reduction of the fraction of chemical oxy-
gen environments bonded to aluminum (Al–O).49 Mg-free
sample MTW(200) exhibits a signal at 534.3 eV, compared to
which, the peaks for Mg-BEA(800), Mg-MTWIJ1000) and Mg-
Fig. 6 presents the IR and UV-vis spectra of these zeolites.
Several characteristic vibrations for the MOR structure are
found in the IR spectra of Mg-MORIJn) samples (Fig. 6A). The
band observed at 580 cm−1 is characteristic of the stretching
vibration of the double five-membered rings, typically due to
the variation of the pentasil framework.47 The offset for the
peak at 1100 cm−1 with the increase in SAR agrees with the
Fig. 7 XPS O1s, Si 2p, and Mg 2p spectra for samples Mg-BEA(800),
Mg-MTWIJ1000) and Mg-MORIJ800). The samples are synthesized by
acidic pre-co-hydrolysis/condensation.
Fig. 5 (A) 29Si and (B) 27Al MAS NMR spectra for Mg-MTWIJ1000)
synthesized by acidic pre-co-hydrolysis/condensation.
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MORIJ800) all shift to lower binding energies. This is attribut-
able to the introduction of the magnesium ions with weak
electronegativity that increases the electron density in the O
atom.50 The observed similar shifts towards higher binding
energies of Si 2p with the increase in SAR can be assigned to
the change in the polarizability of oxygen, indicating the for-
mation of a more stoichiometric SiO2 phase.51–53 Similar to
the variation in the O 1s spectra, the Si 2p signals in these
three Mg containing samples all shift to a lower binding en-
ergy than that (105.2 eV) for Mg-free MTW(200). Finally, the
binding energy of Mg 2p for Mg-BEA(800) is 51.5 eV and
shifts to 52.23 eV and 52.28 eV for Mg-MORIJ800) and Mg-
MTWIJ1000), respectively. These signals shift to a higher bind-
ing energy compared to the peak position of MgO (49.8 eV)
and MgIJOH)2 (49.7 eV),50 reflecting a strong binding of Mg
ions to the zeolitic framework, different from the Mg state in
MgO or MgIJOH)2.54
Temperature-programmed desorption (TPD) of CO2 and
NH3 is utilized to gain insight into the basic and acidic prop-
erties of these Mg2+-derived ultra-high silica zeolites.
Fig. 8A and C show their CO2-TPD profiles. All the samples
give only one peak ranging 100–200 °C, which is ascribed to
CO2 desorption from weak/medium base sites. As SAR in-
creases, no matter what the zeolite topology is, this peak
shifts to higher temperature accompanied by the enlarge-
ment of the peak area, revealing the enhanced base intensity
and increased concentration of basic sites. Fig. 8B and D are
the NH3-TPD profiles. All the samples show one NH3 desorp-
tion peak at 150 °C, which is attributable to weak acidic sites.
As SAR increases, this peak shifts to lower temperature with
the peak area dramatically decreasing, index of decreased
acidity. The above CO2- and NH3-TPD analyses indicate that
these Mg2+-derived ultra-high silica zeolites possess enhanced
basic sites, and the higher SAR, the more enhanced the basic
properties are with correspondingly decreased acidity. In
other words, with the increase in SAR, these samples demon-
strate an enhanced CO2 desorption amount but a decreased
NH3 desorption amount, index of increased basicity and de-
creased acidity. For example, the Mg-MORIJ800) sample ex-
hibits the largest CO2 desorption peak but the weakest NH3
desorption peak, suggesting that it possesses superior basic-
ity but inferior acidity. By contrast, Mg-MORIJ100) exhibits the
largest NH3 but the weakest CO2 desorption, showing its su-
perior acidity but inferior basicity.
The mediate sample such as Mg-MORIJ600) demonstrates
moderate CO2 and NH3 desorption, index of moderate acidity
and basicity. For more comparison, Fig. 8(C) and (D) are
CO2-TPD and NH3-TPD profiles for Mg-BEA(40) with a rela-
tively much lower SAR of 30 for the final solid product and
the Mg-free sample MTW(200), showing that their base inten-
sity are obviously weaker, which accounts that both higher
SAR and existence of magnesium ions are responsible for the
enhanced base properties of our Mg2+-derived high-silica
zeolites.
3.3 Catalytic performance
Catalytic performances are assessed in Knoevenagel conden-
sation, a typical probe reaction for evaluating base catalysts.
Knoevenagel reaction of aldehydes with compounds
containing activated methylene groups has been widely
employed in the synthesis of many fine chemicals including
heterocyclic compounds of biological significance.55 Fig. 9
shows the results of all the obtained Mg-MOR, Mg-BEA and
Mg-MTW zeolites for catalysing Knoevenagel condensation of
benzaldehyde and ethyl cyanoacetate. The conversion of ethyl
cyanoacetate is 38% (selectivity >99.9%) over the catalyst Mg-
MORIJ100). With the increase in SAR, the conversion continu-
ously increases and reaches the high level of 94% for Mg-
Fig.
9 Change in reactivity for Knoevenagel condensation over
Fig. 8 (A) CO2-TPD and (B) NH3-TPD curves for as-calcined ultra-high
silica MOR samples synthesized by acidic pre-co-hydrolysis/condensa-
tion and (C) CO2-TPD and (D) NH3-TPD curves for as-calcined ultra-
high silica BEA and MTW samples, with different SiO2/Al2O3 molar
ratios.
Mg2+-derived mesoporous ultra-high silica zeolites with the increase
in SiO2/Al2O3 molar ratios in final solids. (A) Mg-MOR, (B) Mg-BEA
(blue) and Mg-MTW (red). Reaction conditions: 0.1 g of zeolite catalyst,
10 mmol of benzaldehyde, 10 mmol of ethyl cyanoacetate, 5 mL of
C2H5OH as solvent, 70 °C, 4 h.
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MORIJ800) (Fig. 9A). Parallel variation is observed over Mg-
BEA and Mg-MTW samples (Fig. 9B). A high conversion of
96% is achieved over Mg-MTWIJ1000). The results demon-
strate that SAR is the key factor for these zeolites, which sig-
nificantly affects their activity for this reaction: the higher
SAR, the better the performance. Such variation is well in ac-
cordance with the TPD results of Mg-zeolites, verifying that
they do act as solid base catalysts. Nevertheless, zeolites hav-
ing similar Mg content and SAR value but with different to-
pologies do not give a similar catalytic performance. For ex-
ample, Mg-MOR always presents the highest conversion,
while Mg-BEA gives the lowest one, in the case of the same
SAR. Such
a
phenomenon suggests that the micro-
environment on the surface of zeolitic channels, closely re-
lated to the topology, affects the actual performance of the
basic sites over these zeolites. Mg-MTWIJ1000) shows the
highest conversion due to its highest SAR. After reaction,
these zeolites can be directly separated by filtration and
reused in the next run. Recycling tests are carried out over
the selected ultra-high silica zeolites Mg-MORIJ800), Mg-
BEA(800) and Mg-MTWIJ1000) due to their superior activities
(Fig. S4†). After the five-run recycles, still considerably high
conversions of ethyl cyanoacetate are observed over these re-
peatedly used catalysts, more stable than the previously
reported zeolitic solid bases.52
For further understanding the catalytic behavior, two con-
trol samples, Mg-BEA(40) with a relatively low actual SAR of
30 and Mg-free high silica ZSM-12 (SAR = 96), are catalytically
assessed and compared in Knoevenagel condensation of
benzaldehyde and ethyl cyanoacetate. Our Mg2+-derived ultra-
high silica zeolites all show much higher catalytic conver-
sions than that of Mg-beta (the conversion is 40%), indicat-
ing that higher values of SAR favor the displaying of Mg2+-de-
rived basicity and thus promotes the catalytic activity.
Meanwhile, high silica ZSM-12 has a much lower conversion
of 51%, due to the absence of the magnesium-derived basic
sites.
Fig. 10 Comparison of activity for Knoevenagel condensation of ethyl
cyanoacetate with (A) benzaldehyde, (B) 1-naphthaldehyde, (C)
2-hydroxy-1-naphthaldehyde, and (D) 9-anthraldehyde, as a function
of reaction time.
77% is achieved over Mg-MTWIJ1000) within the same reac-
tion time, mostly also due to the enhanced mass transfer de-
rived from the mesopores. A more apparent tendency can be
observed over the condensation with the much larger sub-
strates of 2-hydroxy-1-naphthaldehyde and 9-anthraldehyde.
Mg-MTWIJ1000) with the most abundant mesopores can effi-
ciently convert the bulky substrates with a more rapid reac-
tion rate than Mg-MORIJ800) with less mesopores, and the lat-
ter mesoporous zeolite once more reacts more rapidly than
the nonporous MgO, further confirming the mesopore-
resulted enhanced mass transfer.56–59 This feature strongly
reveals the positive function of the mesoporous zeolites in
enhancing mass transfer for the liquid–solid heterogeneous
Knoevenagel condensations.
Fig. 10 shows the conversion vs. reaction time in the con-
densation between ethyl cyanoacetate and benzaldehyde,
1-naphthaldehyde, 2-hydroxy-1-naphthaldehyde, or 9-anthr-
aldehyde with different molecular sizes, catalyzed by Mg-
4 Conclusions
MORIJ800) and Mg-MTWIJ1000).
A parallel test is also
conducted on MgO, a commercially available solid base hav-
ing plenty of weak base sites as well as certain medium and
strong ones. Mg-MTWIJ1000) demonstrates the highest con-
version due to its abundant mesopores, while MgO shows the
lowest one, which is mainly attributed to its nonporous struc-
ture with a very small surface area of 7 m2 g−1. When benzal-
dehyde is reacted with ethyl cyanoacetate, a high conversion
of 96% is observed at 4 h over Mg-MTWIJ1000), higher than
94% over Mg-MORIJ800) and 76% over MgO, although the
conversions all reach 100% after 5 h. This implies that the ex-
istence of abundant mesopores favors a faster reaction rate
due to the enhanced mass transfer in the mesopores. For the
larger-sized 1-naphthaldehyde, the conversion over MgO is
only 45% at 1 h; in contrast, a much higher conversion of
Mg2+-derived mesoporous ultra-high silica zeolites are di-
rectly synthesized by a one-pot acidic co-hydrolysis route,
wherein the introduced Mg2+ is indispensible for achieving
the ultra-high SiO2/Al2O3 molar ratio (SAR). Zeolite phases
can be controlled to be MOR, BEA and MTW by finely
adjusting the crystallization pH value, and SAR can be en-
hanced up to 410. Abundant mesopores are self-formed over
these zeolites without using an additional mesopore-
formation template. The ultra-high silica framework favors
stronger basicity, which is further improved by the introduc-
tion of Mg ions. The obtained zeolites exhibit superior activ-
ity in the typical base reaction, Knoevenagel condensation,
and can efficiently convert various bulky substrates due to
the enhanced mass transfer arising from the mesopores. This
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work reports the straightforward synthesis of ultra-high silica
zeolites containing mesopores and Mg species, thus provid-
ing a new series of mesoporous zeolitic solid bases.
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Acknowledgements
The authors thank the National Natural Science Foundation
of China (No. 21136005, 21303084 and 21476109), the Jiangsu
Province Science Foundation for Youths (No. BK20130921),
the Specialized Research Fund for the Doctoral Program of
Higher Education (No. 20133221120002), and the Project of
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