A solvent evaporation route towards fabrication of hierarchically porous ZSM-11 with highly accessible mesopores

Article abstract of DOI:10.1039/c5ra02493e

A route to generate hierarchically porous zeolite ZSM-11 has been paved via solvent evaporation induced self-assembly assisted by hexadecyltrimethoxysilane to produce a preformed dry gel, followed by its subsequent transformation into zeolite via steam-assisted-crystallization. The crystallization in dry gel has been found to undergo an orientated attachment growth mechanism whereby hexadecyltrimethoxysilane directs the formation of auxiliary mesopores and inhibits the fusion of primary nucleates. Measurements such as XRD, SEM, TEM, N₂-physisorption, and TEM for an inverse replica of Pt derived from hierarchical ZSM-11 have been conducted to characterize the textural properties of the material. Ammonia temperature-programmed-desorption (NH₃-TPD) measurements and infrared spectra using probe molecules such as pyridine (Py-IR) and 2,4,6-collidine (Coll-IR) have been collected to investigate the acidic properties as well as the accessibility of the acid sites. The hierarchical ZSM-11 possesses more acid sites on the mesopore surfaces that are accessible towards large probe molecules such as 2,4,6-collidine. This improvement together with the enhanced pore-connectivity brings about an increase in 1,3,5-triisopropylbenzene cracking activity and benzene selectivity with respect to a conventional counterpart.

Full text of DOI:10.1039/c5ra02493e

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PAPER
A solvent evaporation route towards fabrication of
hierarchically porous ZSM-11 with highly accessible
mesopores
Cite this: RSC Adv., 2015, 5, 31195
Wen Song,a Zhiting Liu,a Liping Liu,b Anne Ladegaard Skov,c Nan Song,a
b
a
Guang Xiong, Kake Zhu and Xinggui Zhoua
*
*
A route to generate hierarchically porous zeolite ZSM-11 has been paved via solvent evaporation induced
self-assembly assisted by hexadecyltrimethoxysilane to produce a preformed dry gel, followed by its
subsequent transformation into zeolite via steam-assisted-crystallization. The crystallization in dry gel
has been found to undergo an orientated attachment growth mechanism whereby
hexadecyltrimethoxysilane directs the formation of auxiliary mesopores and inhibits the fusion of primary
nucleates. Measurements such as XRD, SEM, TEM, N2-physisorption, and TEM for an inverse replica of Pt
derived from hierarchical ZSM-11 have been conducted to characterize the textural properties of the
material. Ammonia temperature-programmed-desorption (NH3-TPD) measurements and infrared
spectra using probe molecules such as pyridine (Py-IR) and 2,4,6-collidine (Coll-IR) have been collected
to investigate the acidic properties as well as the accessibility of the acid sites. The hierarchical ZSM-11
possesses more acid sites on the mesopore surfaces that are accessible towards large probe molecules
such as 2,4,6-collidine. This improvement together with the enhanced pore-connectivity brings about an
Received 9th February 2015
Accepted 26th March 2015
DOI: 10.1039/c5ra02493e
increase in 1,3,5-triisopropylbenzene cracking activity and benzene selectivity with respect to
conventional counterpart.
a
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past 15 years or so, two major strategies have been used to
facilitate mass transport in zeolitic micropores: (1) downsize the
crystal of zeolites from microns to nanometers, viz., synthe-
sizing nanozeolites13–15 or zeolites with plate-like morphology of
nanometer thickness.16–19 (2) Introducing auxiliary meso- or
macro-pores in addition to the inherent micropores of zeolites,
i.e., generating hierarchically porous zeolites (HPZs) to take the
place of conventional zeolites that possess solely micro-
pores.20–24 Hard template route using carbon blacks,25 so
template route using organosilanes,26–31 cationic polymers,32
bifunctional structure directing agents,18,33,34 as well as post-
synthetic desilication35,36 routes have been established. Several
merits stems from prompted mass transport for instance, high
activity, occasional high primary product selectivity, improved
large molecule accessibility, as well as longer lifetimes, have
been achieved by replacing conventional zeolites with HPZs.37
More recently, there are several reports on the industrial
applications for this type of zeolitic catalysts,35,38,39 and more
examples are expected to come in the future.
1. Introduction
Zeolites are a family of microporous (<2 nm) crystalline solids
with ordered pore size, shape and connectivity that impart them
with unique size and shape selectivity to guest molecules that
can be accommodated in their pores.1,2 Articial zeolites are
nowadays mass produced on an industrial scale and nd broad
applications in areas such as ion-exchangers in laundry,
adsorbents for separation, and more importantly as solid acid
catalysts in oil renery and chemical industries.1,3 Moreover,
zeolites are also the preferred solid acid catalysts for a series of
important transformations using coal, natural gas or biomass
as feedstocks.4–7 Nowadays, the frequently used articial
zeolites are micronsized crystals, whereas their pore sizes are no
more than 2.0 nm. Consequently, the mass transport in the
micropores of zeolites are restricted, which undermines the
effectiveness and elds of applications for zeolites.8–12 In the
aUNILAB, State Key Lab of Chemical Engineering, School of Chemical Engineering, East
China University of Science and Technology, 130 Meilong road, Shanghai 200237, P. R.
China. E-mail: kakezhu@ecust.edu.cn
bState Key Lab of Fine Chemicals, School of Chemical Engineering, Dalian University of
Technology, 2 Lingshui road, Dalian, Liaoning, 116024, P. R. China. E-mail: gxiong@
dlut.edu.cn
cCenter for Energy Resources Engineering, Department of Chemical and Biochemical
Engineering, Technical University of Denmark, Søltos Plads, DK-2800, Kgs. Lyngby,
Denmark
From the mass transport viewpoint, molecular transport
undergoes congurational diffusion in the micropores of
zeolites, which can be understood by taking into the Thiele
rffiffiffiffiffiffiffiffi
kin
Deff
modulus (f ¼ L
, whereby L, kin, Deff stands for diffusion
length, intrinsic reaction rate constant and effective diffusivity,
respectively).8 The larger the Thiele modulus, the more serious
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the effect of intra-particle diffusion over reaction. Advantages of small as ca. 35 to 60 nm and pore quality is high. The acidity and
nanozeolites or introducing auxiliary porosity originate from accessibility has been measured and catalytic performance has
the fact that the diffusion pathway (L) has been shortened, been probed using 1,3,5-triisopropylbenzene cracking as a
henceforth, the utility of zeolitic material is enhanced.6 On the model reaction.
other hand, it has been recognized that the extent to which
mass transport is improved is highly dependent on the “quality”
of meso- (or macro-)pores, as mesopores can enhance mass
transport only if they are connected with the exterior surface of
zeolite crystals.36,38,40 Weitkamp et al.41 have observed that the
2. Experimental section
2.1. Reagents
Hexadecyltrimethoxysilane (HTS, 85 wt%, Aladdin), aluminum
presence of intracrystal mesopores in USY has virtually no
impact on mass transport, a phenomenon that has also been
recently corroborated by Perez-Ramirez et al. using positron
annihilation lifetime spectroscopy.36,38 It is therefore desirable
to nd ways that can produce HPZs with small primary particle
size that can shorten the diffusion pathway, and at the same
time keep the mesopores connected to the external surfaces.
ZSM-11 possesses a MEL topology that is an important
member among the pentasil family of zeolites, the MEL struc-
ture consists of intersecting straight 10 member ring channels
tert-butoxide (ATTB, 99 wt%, J&K Chemical), tetrabutylammo-
nium hydroxide (TBAOH, 25 wt% in water, Aladdin), ethyl sili-
cate (TEOS, 28 wt% SiO2, Shanghai Lingfeng Chemical Reagent
Co., Ltd), ethanol (99.7%, Shanghai Titan Scientic Co., Ltd),
chloroplatinic acid hexahydrate (H2PtCl6$6H2O, 37.5 wt% Pt,
Aladdin), hydrogen uoride solution (HF, 40.6 wt%, Shanghai
Lingfeng Chemical Reagent Co., Ltd) were of analytical reagent
grade and used without further purication.
2.2. Synthesis
(5.3 ꢀ 5.4 A) that is similar to ZSM-5 (MFI topology).42 The rst
˚
The molar composition of the starting gel used for the synthesis
of hierarchical porous zeolite ZSM-11 was: 0.95SiO2/0.05HTS/
0.01Al2O3/0.20TBAOH/8.65H2O. In a typical synthesis, 0.29 g of
ATTB was dissolved in 10.39 g of TBAOH aqueous solution
followed by adding 100 ml of ethanol at room temperature
under stirring for 30 min to form an apparent clear mixture.
Another clear mixture of 10.72 g of TEOS, 1.06 g of HTS and
50.0 ml of ethanol was made by vigorously stirring for 2 h. The
above two mixtures were mixed and stirred for another 2 h to
afford a clear solution-like mixture. The mixture was transferred
into a Petri dish to allow the evaporation of solvent that
produced a transparent dry gel. The dry gel was ground to
powder by mortar and pestle before charging into a 10 ml Teon
cup placed inside Teon-lined stainless steel autoclave (100
cm3), and 5.0 g H2O was added outside the cup to create enough
steam for SAC treatment. Then the autoclave was kept in an
oven to carry out a SAC type treatment at 453 K up to 4 days. The
product was recovered by ltration, washing thoroughly with
deionized water, and dried at 373 K for 5 h. The organic
template was removed by calcination for 20 h in air with a ramp
of 2 K minꢁ1 to 823 K. Thus the nal product named ZSM-11-H
was produced. For comparison, conventional ZSM-11 was
synthesized by the same procedure in the absence of HTS, and
the starting gel composition of conventional porous zeolite
ZSM-11 was: 1.00SiO2/0.01Al2O3/0.20TBAOH/8.65H2O. The nal
obtained sample was labeled as ZSM-11-C, whereby C stands for
conventional microporous sample.
mesoporous ZSM-11 (Si/Al ¼ 50) has been obtained by Chris-
tensen et al.25 using carbon black as hard template, the
Cu-containing mesoporous ZSM-11 is more catalytically active
than the conventional counterpart or the mesoporous ZSM-5 in
NO decomposition. The improved accessibility in the straight
channels compared to the sinusoidal channels together with
the presence of auxiliary mesopores are responsible for the
elevation of activity.43 Xie et al.44 have reported the synthesis of
mesoporous ZSM-11 (Si/Al ¼ 19) through polyvinyl butyral (PVB)
templating method, and the mesoporous ZSM-11 overperforms
microporous ZSM-11 in cracking of both 1,2,4-trime-
thylbenzene and 1,3,5-trimethylbenzene. The generation of
mesoporous ZSM-11 has also been reported by using cetyl-
trimethylammonium tosylate and TBAOH in a dual-templating
synthetic system by a two-step preparation,45 but the competing
role between the two structure directing agents and conse-
quently the phase segregation can hardly be ruled out as found
for similar synthetic systems.24,46
In preceding articles, we have used an organosilane and dry
gel conversion combined strategy to generate HPZs.29–31 The
organosilane route has been initiated by Serrano et al.27,47,48 as
an effective route to control zeolite crystal size, but entails
surface modication of zeolitic embryos before a second crys-
tallization process. The dry gel conversion route has been
proposed by Xu et al.49 and later developed by Matsukata
et al.,50,51 Davis,52 and many others into an alternative route for
zeolites or zeotype material synthesis.2 By combining the two
approaches, it is possible to control the texture of the dry gel
and avoid the fusion of primary zeolitic particles into large
crystals. Meanwhile, the phase separation of organosilanes
from zeolite crystals is inhibited, that has been found to be
important to the creation of mesopores.30,47,53 Herein, we report
the synthesis of HPZ ZSM-11, by using solvent evaporation route
to generate a preformed dry gel and its subsequent steam-
assisted-crystallization (SAC) treatment to convert the dry gel
into zeolites. The primary crystal size has been found to be as
Pt replica synthesis: 0.5 g calcined ZSM-11-H was mixed with
0.30 g H2PtCl6$6H2O in a glass beaker, where it was kept stirring
at 363 K for 8 h, subsequently, the solid was obtained by
centrifugation and then heated at 673 K in H2 for 2 h. The
zeolite was dissolved by a hydrogen uoride solution (HF), and
the Pt product was repeatedly washed with water and ethanol.
2.3. Characterizations
Powder X-ray diffraction (XRD) patterns of samples were
recorded at a Rigaku D/Max 2550VB/PC powder diffractometer
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˚
with Cu Ka radiation (l ¼ 1.5418 A) at 40 kV and 100 mA.
Scanning electron micrographic (SEM) and transmission elec-
tron micrographic (TEM) images were obtained on a Nova
NanoSEM450 microscope and a JEM-2100 microscope, respec-
tively. To prepare the sample for TEM, the powder was sus-
pended in ethanol and supported onto a lacey carbon lm on a
copper grid. Nitrogen adsorption–desorption isotherms were
measured on Micromeritics ASAP 2020 analyzer at 77 K. Before
measurements, calcined samples were degassed at 553 K for
10 h. The specic surface areas (SBET) were calculated by the
multi-point Brunauer–Emmett–Teller (BET) method. The
external specic surface area and micropore pore volume were
evaluated by the t-plot method. The pore size distribution was
estimated by applying the NL-DFT (Non-Local Density Func-
tional Theory) method to the adsorption branches of the
isotherms, and the total pore volumes were estimated from the
adsorbed amount at a relative pressure P/P0 ¼ 0.99. Ammonia
temperature programmed desorption (NH3-TPD) were per-
formed on PX200A type TPD/TPR setup. A sample of 100 mg was
activated in helium ow at 873 K for 1 h and then cooled to
373 K for the saturation of NH3. The NH3-TPD prole was
recorded by heating the sample at a rate of 15 K minꢁ1 from
423 K to 923 K under a helium ow (30 cm3 minꢁ1). Trans-
mission FTIR spectra of the samples were recorded with a
Spectrum 100 FT-IR spectrometer. All samples were pressed in
self-supporting wafers (diameter: 1.4 cm, weight: 166 mg) and
were pre-treated at 773 K under vacuum for 1 h prior to the
adsorption of probe molecules (pyridine or 2,4,6-collidine).
The amount of adsorbed probe molecules was calculated from
the integrated area of given bands with their distinct
molar extinction coefficients. For 2,4,6-collidine, 3H-bond
(1632–1648 cmꢁ1) ¼ 10.1 cm mmolꢁ1 has been used to calcu-
lated the amount of acid sites that resides on the mesopore wall
or external surface of zeolites.54 For pyridine, 3Brønsted
(1540 cmꢁ1) ¼ 1.02 cm mmolꢁ1 and 3Lewis (1450 cmꢁ1) ¼ 0.89 cm
mmolꢁ1 has been employed to deduce the total amount of
Brønsted and Lewis acid sites, respectively, that are given in
literature.54
Fig. 1 XRD patterns of ZSM-11-C and ZSM-11-H.
XRD patterns of ZSM-11-C and ZSM-11-H. The feature XRD
reections of ZSM-11 are at 2q of 7.92, 8.78, 23.14, 23.98, and
45.2ꢂ, together with double peaks at 23–25ꢂ and the single peak
at 45ꢂ are used to distinguish MEL from that of MFI struc-
tures.25,42,44 Both samples exhibit reections typical of the MEL
topology and no impurities have been detected, denoting an
absence of amorphous gel or other phases. In addition,
broadening of diffraction peaks for ZSM-11-H is intuitively seen
which is ascribed to the smaller particle size respect to the
conventional hydrothermal method. The inference of particle
size is not possible by the XRD techniques using the Scherrer
equation,55 because of the intergrowth nature of the material
(see later).
N2 adsorption–desorption isotherms have been measured to
provide the texture properties of ZSM-11-H, that is juxtaposed
together with the data for ZSM-11-C for comparison, as exhibi-
ted in Fig. 2 and the derived data tabulated in Table 1. The
isotherm for ZSM-11-C can be ascribed to a type I according to
the IUPAC classications, the inherent micropores in zeolite
has been lled at relatively low pressures (P/P0 < 0.1). At P/P0
above 0.95, a small jump in N2 uptake is observable, that is due
to capillary effect caused by the presence of interparticular
voids. The isotherm for ZSM-11-H is a hybrid type I and IV
isotherm, besides the micropore lling at low relative pressures
(P/P0 < 0.1), the isotherm contains an addition N2 uptake at P/P0
> 0.80, which can be attributed to the capillary effect caused by
the presence of mesopores.25,44 The corresponding hysteresis
loop can be categorized to a type H3, implying an irregularly
shaped mesopore. The absence of a forced closure in the
hysteresis loop of the isotherm at P/P0 z 0.45 caused by tensile-
strength-effect (TSE) indicated the formation of accessible
mesoporosity.56,57 As shown in Table 1, the micropore volumes
for the two samples are comparable, suggesting that the intro-
duction of mesopores has negligible effect on the inherent
microporosity of MEL structure. The external surface area and
mesopore volume for ZSM-11-C are 160 m2 gꢁ1 and 0.11 cm3 gꢁ1
due to surface roughness and exterior surface, respectively,
2.4. Catalytic assessments
The 1,3,5-triisopropylbenzene cracking was chosen as a model
reaction because the substrate 1,3,5-triisopropylbenzene has a
larger dynamic size (9.5 A)44 than the micropore pore size of ZSM-
˚
˚
11 (5.3 ꢀ 5.4 A). The cracking reaction was carried out on a pulse
micro-reactor. 0.1 g catalyst (10–20 mesh) was packed between
quartz wool plugs in a U-shaped stainless tube (i.d. ¼ 4 mm) in a
furnace. About 0.4 mL of reactant was injected every time. The
products were analyzed using Gas Chromatograph (GC) with a
capillary column of OV-101 and a ame ionization detector (FID).
3. Results and discussion
3.1. Crystal and texture properties of ZSM-11-H
whereas for ZSM-11-H a high external surface area (252 m2 gꢁ1
)
and mesopore volume (0.54 cm3 gꢁ1) can be derived. It can be
deduced that the mesopore volume is attributed to the
The structure of the calcined ZSM-11-H obtained aer solvent
evaporation and SAC treatment has been characterized by a
combination of various techniques. Fig. 1 shows the powder
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Fig. 2 N2 adsorption–desorption isotherm of ZSM-11-C and ZSM-11-
H, and the corresponding pore-size-distribution pattern for ZSM-11-H
sample was shown in the inset.
Fig. 3 SEM images for calcined samples, ZSM-11-C (a and b) and ZSM-
11-H (c and d).
aggregation of primary particulates, which also lead to the
increase of surface area and mesopore volumes. A SBET of
363 m2 gꢁ1 and Langmuir surface area of 611 m2 gꢁ1 is derived
TEM images of the ZSM-11-H are shown in Fig. 4. ZSM-11-H
exhibits crystalline edges of MEL structure in some area with
uffy-like morphology, mesopore ranges appearing as white
areas can be visualized between the primary crystal particles.
The white elds as mesopores are randomly distributed
throughout the whole crystal, as a result of aggregation of
primary building blocks. Another feature is that the mesopores
are not intracrystal, but are connected to the external surfaces.
The TEM images also reveal that olive-like spheres ZSM-11-H
particles are composed of large numbers of nanosize zeolite
(from 35 to 60 nm), according well with the SEM results. The
selected area electron diffraction (SAED, in the inset Fig. 4b)
pattern for ZSM-11-H shows that each crystal possesses single-
crystal-like diffraction spots, conrming that the primary
zeolite particles are mutually aligned. The structure is therefore
similar to the diffraction patterns for mesocrystals, whereby
nanoparticles form superstructures when they are orientated
along the same crystalline axis.59 The lattice fringes could be
clearly distinguished at high magnication (Fig. 4d), showing
that zeolite ZSM-11-H is highly crystalline. The alignment of
these primary particulates are similar to ZSM-5 derived from
solvent-evaporation and dry gel conversion protocol,29 zeolite
beta with similar formation mechanism has also been unrav-
eled recently.46 The ordered micropores and channels arisen
from the crystalline structure are well arranged in the crystal.
To shed light on the mesopore connectivity, a nanocasting
approach has been used to prepare a Pt particle that is a replica
for ZSM-11-H, that is larger than that for ZSM-11-C (SBET
¼
317 m2 gꢁ1, SLangmuir ¼ 556 m2 gꢁ1). Considering the different
porosity of ZSM-11-C and ZSM-11-H, one can conclude that the
addition of HTS by solvent evaporation route has created mes-
oporous region in the material by expanding interparticular
spacing between primary particles. It is noteworthy that the
mesopore volume is larger than hierarchical ZSM-11 derived
from either hard template route (0.40 m2 gꢁ1
)
25 or PVB template
(0.23 m2 gꢁ1).44
SEM images have been collected to throw light on the effect
of organosilane HTS over the morphology of ZSM-11 zeolites.
The SEM images of ZSM-11-H and ZSM-11-C are displayed in
Fig. 3. The ZSM-11-C has typical olive-like shape of MEL zeolite
crystallites with the smooth surface and clear crystalline edges
and compact texture, indicating the solely presence of inherent
micropores in the sample.25 And the average particle size of
ZSM-11-C is approximately 450–500 nm as can be detected in
the SEM images at low magnications. In contrast, the crystal
size of ZSM-11-H lays in the range of 380–480 nm, and each
discrete crystal is made up of tiny primary particles between
35 to 60 nm. The crystal size for ZSM-11-H is smaller than that
of hierarchical ZSM-5 reported previously,29 presumably due to
the differences in crystalline structure and the structure-
directing-agents (SDAs). The surface of ZSM-11-H appears
rough and sponge-like, that is similar to other hierarchical
zeolites and strongly indicating the presence of additional
porosity within the crystal.25,33,44,46,58
Table 1 Structural properties of the calcined samples of ZSM-11-C and ZSM-11-H
SBETa (m2 gꢁ1
)
SLangmuir (m2 gꢁ1
)
Sext (m2 gꢁ1
)
Vtotal (cm3 gꢁ1
)
Vmicro (cm3 gꢁ1
)
Vmesod (cm3 gꢁ1
)
b
c
c
Sample
ZSM-11-C
ZSM-11-H
317
363
556
611
160
252
0.17
0.65
0.12
0.11
0.05
0.54
a
b
c
Calculated by the BET method in the P/P0 range of 0.05–0.20. Calculated using the Langmiur method. Calculated using the t-plot method.
d
Calculated using Vtotal ꢁ Vmicro
.
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Fig. 5 TEM image for Pt-replica derived from ZSM-11-H by a nano-
Fig. 4 TEM images for ZSM-11-H after calcinations, the SAED pattern
casting approach.
was shown in the insets.
For the same sample, it is found that attachment of primary
particles into agglomerates has occurred in most areas. Aer
48 h, majority of the starting gel has been converted into MEL
of the mesopores for ZSM-11-H, the TEM image for this Pt
particle is exhibited in Fig. 5. Pt derived from nanocasting using
silica as a hard template has been reported before by Fan et al.,60
as the Pt-replica honestly preserves the shape of the pores of
parent template, the Pt-replica can be employed to estimate the
pore connectivity for the parent hard template (mold). It is seen
from Fig. 5 that the Pt-replica appear as dark dense solids of
overlapping small particles of 2–5 nm, constituting aggregates
of varied bigger sizes, which is consistent with the broad mes-
opore size distribution deduced from N2 physisorption data.
Most of the aggregated areas are too thick to provide informa-
tion on pore-connectivity, but on the margins of some areas,
worm-like shaped Pt-replicas can be identied (as specied by
arrows), suggesting that the mesopores are worm-like in shape
and are interconnected to construct a porous system.
3.2. Formation process of ZSM-11-C and ZSM-11-H
The time-dependent structure evolution during steaming
treatment has been monitored by XRD and SEM for quenched
samples collected at varied time intervals for both ZSM-11-C
and ZSM-11-H, these results are displayed in Fig. 6 and 7,
with the intention to understand the crystallization process. For
ZSM-11-C, the initial dry gel aer solvent evaporation is amor-
phous in nature, as exhibited by the lump diffraction pattern
from 18 to 30ꢂ (Fig. 6a) and the corresponding SEM image
(Fig. 7a). The dry gel comprises of partly fused particulates
between 9 and 15 nm, with occasional bigger ones that can be
also be found in the sample. Aer 6 h of SAC treatment, XRD
detectable transformation from dry gel to MEL structure takes
place, when a noticeable diffraction out of MEL structure is
observable. The gel has decomposed into small particle of
10–20 nm aggregates. Prolonged crystallization for 12 h leads to
the formation of more MEL phase and fusion of these tiny
zeolitic embryo into bulky crystals. At this stage, particles with
an average size of 10–30 nm can be identied, whereby grain
Fig. 6 Time-dependent XRD patterns for dry gel after steaming
treatment at varied time in the synthesis of ZSM-11-C (a) and ZSM-11-
boundaries are seen more clearly due to ordering development. H (b).
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zeolites, when no amorphous diffraction patterns can be iden-
tied, and the typical round shape of 300–700 nm MEL crystal is
clearly visible. The disappearance of grain boundaries between
primary particulates can be visualized as well. The temporal
overlap of crystallization and morphology change from tiny
nanocrystals to aligned bulky crystal is related to the crystal
growth mechanism, which is frequently found for growth of
crystals undergoing an orientated attachment (OA) route.61,62
The formation and alignment of zeolitic crystals has been
proposed as a result of OA growth by Bein et al.,58 Matsukata
et al.51 and us46 in the case of beta zeolite formation in dry gel
system, similar phenomenon can occur to ZSM-11. Tsapatsis
et al.63 have also proposed that siliclite-1 (MFI) growth in solu-
tion system also takes place via attachment of primary building
units. It is speculated that OA growth can be the dominant route
in dry gel systems, as supersaturation ratio is high and less
water is available for the complete dissolving of precursor
ingredients. The high supersaturation ratio guarantees a high
nucleation rate that lead to the formation of large numbers of
nucleates at the early stage of crystallization, while the lack of
water does not favor the transport of starting materials and
further growth happens mainly through the alignment of
primary crystals. Based on the above observations, a possible
formation mechanism has been sketched in Scheme 1a.
Scheme 1 The proposed formation mechanism for ZSM-11-C (a) and
ZSM-11-H (b) in solvent evaporation and steam-assisted-crystalliza-
tion process.
12 h SAC treatment, i.e., the crystallization has been slowed
down as inhibited by HTS. Even aer 48 h of SAC treatment, an
obvious broad diffraction out of amorphous gel can be found,
indicating that the majority of the starting gel has not been
transformed. For the 48 h treated sample, the gel comprises
particulates between 35–50 nm with open framework structures
that is rich in porous region. Compared with the nal products
(Fig. 3c and d), it is obvious that the ZSM-11-H has grown from
these discrete particulates, the crystallization of MEL structure
is accompanied with prominent morphology change at the
mesoscale. The role of HTS can be understood by comparing
with the crystallization of ZSM-11-C. HTS slows down the rate of
crystallization in a OA growth mechanism, meanwhile, the
aggregation of primary particulates into bulky crystals has been
inhibited. The alkyl tails of HTS form the interparticular voids
that form mesopores during SAC treatment. This process has
been depicted in Scheme 1b to highlight the role of HTS.
For ZSM-11-H, the dry gel aer solvent evaporation appears
to be amorphous (Fig. 6b) and more uniform in size by the
inclusion of HTS, and a particle size between 10 to 20 nm can be
inferred from SEM images (Fig. 7b). The gel becomes aggregates
made up of 15–30 nm particulates aer 6 h of SAC treatment
and remains amorphous. The crystallization starts only aer
3.3. Acid properties and catalytic performance
Fig. 8 presents the NH3-TPD proles of the ZSM-11-C and ZSM-
11-H. The acid concentrations of differing strengths derived
from the proles are formulated in the Table 2. Obviously, there
are two desorption peaks for the two samples, corresponding to
the weak and strong acid sites, respectively. The low tempera-
ture desorption of NH3 is attributed to the physically adsorbed
NH3, while the high temperature desorption peak is associated
with NH3 interacting with strong acid sites.44,64 The high
temperatures of desorption for ZSM-11-C and ZSM-11-H appear
at 682 and 676 K, the close in desorption temperature shows
that the acidic strength for the two samples are comparable, i.e.,
the integration of mesopores to the native micropores in MEL
structure by our method does not cause substantial decrease in
strong acidic strength. From the Table 2, it can be estimated
that the number of strong acid sites for ZSM-11-C and ZSM-11-H
are 0.58 and 0.62 mmol gꢁ1-cat., implying that the two samples
have similar numbers for strong acid sites. The ZSM-11-H
possesses less weak acid site (0.24 mmol gꢁ1-cat.) and more
strong acid site (0.62 mmol gꢁ1-cat.), that is different from
previously reported hierarchical ZSM-11 templated by PVB.44
FT-IR measurements were employed to distinguish the acid
type and the acidic accessibility of those samples by using
˚
adsorbed pyridine (Py, 5.7 A in diameter) and 2,4,6-collidine
Fig. 7 The SEM images for dry gel after steaming treatment for varied
˚
time in the synthesis of ZSM-11-C (a) and ZSM-11-H (b).
(Coll, 7.4 A in diameter) as probe molecules. As illustrated in
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Fig. 8 NH3-TPD profiles of ZSM-11-C and ZSM-11-H.
Fig. 9a, the adsorption bands at 1454 cmꢁ1 and 1542 cmꢁ1 are
assigned to the chemisorbed pyridine, which correspond to the
pyridine interacting with Lewis and Brønsted acid sites,
respectively.65 The peak at 1490 cmꢁ1 is synergetic result from
both Lewis and Brønsted acid sites.65 The areas of the peaks
decrease gradually with the raising of outgassing temperatures
from 473 K to 673 K, as a consequence of weakly adsorbed
pyridine desorption at elevated temperatures. The Py-IR and
Coll-IR results have been analyzed to generate information on
the type and accessibility of acid sites,57 with respect to previ-
ously established methods. The detailed results are compiled in
the Table 3. Despite the fact that the total acid sites are similar
for ZSM-11-C (0.64 mmol Py gꢁ1-cat.) and ZSM-11-H (0.56 mmol
Py gꢁ1-cat.), the number for strong Brønsted acid sites
(measured at 673 K) are 0.26 and 0.10 mmol Py gꢁ1-cat.,
respectively. Meanwhile, the measured numbers for strong
Lewis acid sites (measured at 673 K) are 0.01 and 0.11 mmol Py
gꢁ1-cat., for ZSM-11-C and ZSM-11-H, respectively. The Py-IR
data point out that the integrating of auxiliary porosity using
our method has altered the type of strong acid sites from a
Brønsted-acid-sites-rich type to a Lewis-acid-sites-rich type. The
International Zeolite Association (IZA) shows that the channel
Fig. 9 Pyridine (a) and 2,4,6-collidine (b) infrared spectra for ZSM-11-
C and ZSM-11-H.
Brønsted acid sites through larger auxiliary pores,57 that could
only be used to probe acid sites situated at auxiliary mesopores
and the external surfaces of MEL structure. As the intracrystal-
line mesopores can only be accessed through the micropores,
whereby the 2,4,6-collidine adsorption is normally denied, the
Coll-IR is henceforth speculated to only give the number of acid
sites sitting on mesopores that are accessible from exterior
surface of zeolite crystals.36 The vibrations at 1638 and 1633
cmꢁ1 have been assigned to 2,4,6-collidine molecule interacting
with Brønsted and Lewis acid sites according to Gilson et al.,57,68
and only 0.04 mmol gꢁ1-cat. is measurable on the exterior
surface of ZSM-11-C. As shown in Table 3, it can be noticed that
the 2,4,6-collidine adsorption on the hierarchical zeolite ZSM-
11-H (0.10 mmol Coll gꢁ1-cat.) is obviously higher than that of
conventional zeolite ZSM-11-C (0.04 mmol Coll gꢁ1-cat.), and
the ratio for 2,4,6-collidine accessible acid sites on ZSM-11-H to
ZSM-11-C at 673 K is 2.5. This result evidences that the
appearance of mesopores in ZSM-11-H zeolite leads to the
increment of available acid sites for bulky molecules. This
observation is in accordance with the reported trend that mes-
opore surface area increases hand-in-hand with the accessibility
of acidic sites.57 From the above acid property measurements, it
is concluded that ZSM-11-H has similar total strong acid sites to
˚
diameter of ZSM-11 zeolite is about 5.3 ꢀ 5.4 A, which is similar
˚
to the dynamic molecular size of pyridine (5.7 A (ref. 57)), and
pyridine can enter the micropores of MEL structure.66,67 There-
fore, the Py-IR results reect the overall acid properties without
discriminating acid sites located on zeolite external surfaces or
their internal micro-channels. 2,4,6-collidine, on the other
˚
hand, is bulkier (7.4 A) than pyridine and can only access to
Table 2 NH3-TPD data of ZSM-11-C and ZSM-11-H
Tpeak/K
LT-peaka HT-peakb Total acid Weak acid Strong acid
Acidity (mmol NH3 gꢁ1-cat.)
Sample
ZSM-11-C 498
ZSM-11-H 509
682
676
0.95
0.86
0.37
0.24
0.58
0.62
a
Low temperature. b High temperature.
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Table 3 Comparison of acid amount and Brønsted/Lewis type determined by pyridine-IR and 2,4,6-collidine-IR for ZSM-11-C and ZSM-11-H
Aciditya (mmol Py gꢁ1-cat.)
Acidityb (mmol Coll gꢁ1-cat.)
(1632–1648 cmꢁ1
)
Brønsted (1554 cmꢁ1
)
Lewis (1452 cmꢁ1
)
Sample
Total aciditya mmol gꢁ1
473 K
573 K
673 K
473 K
573 K
673 K
473 K
573 K
673 K
ZSM-11-C
ZSM-11-H
0.64
0.56
0.60
0.42
0.42
0.15
0.26
0.10
0.04
0.14
0.03
0.13
0.01
0.11
0.09
0.21
0.07
0.11
0.04
0.10
a
Calculated by Py-IR. b Calculated by Coll-IR.
Table 4 Catalytic performance of ZSM-11-C and ZSM-11-H in triisopropylbenzene cracking, mcatal. ¼ 0.10 g
Production distribution (%)
Sample
T (K)
Conv. (%)
Propylene
Benzene
Cumene
1,3-DIPB
1,4-DIPB
TetraIPBs
ZSM-11-C
ZSM-11-H
ZSM-11-C
ZSM-11-H
623
623
673
673
51.9
68.2
60.3
77.1
32.7
28.9
36.4
39.6
21.6
23.9
24.5
29.5
Trace
6.1
Trace
7.8
42.7
26.0
34.1
19.7
Trace
Trace
Trace
Trace
3.0
15.1
5.0
3.4
that of ZSM-11-C. The bulky molecule accessibility to acid sites benzene, with propylene as a co-product in each consecutive
has been improved with the integration of auxiliary mesopores. step. These molecules are small enough to penetrate the
There are more Lewis acids sites for ZSM-11-H than on ZSM-11- micropores system to undergo further cracking at reaction
C, as can be inferred from pyridine IR data in Table 3.
conditions, henceforth, the products distribution reect the
The ultimate goal of fabricating hierarchical zeolite is to effect out of diffusion rather than accessibility.69,70 In ZSM-11-H,
promote the catalytic performances, especially the accessibility as the diffusion pathway has been shortened as the crystal size
and diffusion pathway for bulky reactants. The catalytic activity is reduced, more primary product 1,3-diisopropylbenzene has
of ZSM-11-H has been investigated in 1,3,5-triisopropylbenzene been converted into cumene and benzene, resulting in a
(1,3,5-TIPB) cracking at 623 and 673 K, respectively. 1,3,5-TIPB detectable increase in their selectivities (Table 4). It should be
is regarded as a model compound for heavy gas oil molecules, noted that the selectivity to tetraisopropylbenzenes (TetraIPBs)
and is widely accepted as a probe reaction to estimate the effect is much higher over ZSM-11-H at 623 K. This is probably due to
of hierarchical porosity of the catalytic performance. The cata- the disproportionation reaction of 1,3,5-triisopropylbenzenes,
lytic results obtained over the two catalysts at 623 and 673 K are or the alkylation reaction between 1,3,5-triisopropylbenzene
presented in Table 4. The 1,3,5-TIPB conversion for conven- and propylene. The formation of more tetraisopropylbenzenes
tional ZSM-11-C is 51.9% and 60.3% at 623 K and 673 K, further proves the improved accessibility of ZSM-11-H. As the
respectively. Interestingly, the 1,3,5-TIPB conversion over hier- temperature rises the selectivity to tetraisopropylbenzene
archical ZSM-11-H is 68.2% and 77.1% at 623 K and 673 K, decreases from 15.1 to 3.4 over ZSM-11-H. This is because that
respectively. This is because the parent ZSM-11-C has only high temperature is conducive to cracking reaction, which leads
˚
native micropore diameter no larger than 5.6 A, that is signi- to the cracking of tetraisopropylbenzene and the formation of
˚
cantly smaller than the kinetic diameter of 1,3,5-TIPB (9.5 A) more benzene and cumene. The effect of hierarchical porous
and thus their catalytic conversion occurs exclusively at the structure on the cracking of 1,3,5-TIPB molecule is an increase
exterior surface of ZSM-11 crystal. The small external surface in activity stemming from more accessible acid sites, and an
area (Table 1) and the sterically hindered accessibility to acid increased selectivity towards consecutive de-alkylation products
sites (as probed by 2,4,6-collidine) strongly limits the effective- due to reduced diffusion length in micropores.
ness of ZSM-11-C for the reaction, consequently, the catalytic
reaction only occurs on the external surfaces and pore-mouth of
4. Conclusions
the ZSM-11-C crystals. On the other hand, for ZSM-11-H, the
presence of mesopores together with the improved pore
connectivity, enhanced bulky molecule accessibility towards
catalytic sites offer the opportunity of more available sites to
A new route to generate hierarchically porous ZSM-11 has been
paved by a organosilane assisted solvent evaporation route to
fabricate dry gel and its subsequent transformation into
participate the catalysis, leading to an increase in conversion.
zeolites. The hierarchical ZSM-11-H possesses both native
The primary product aer 1,3,5-triisopropylbenzene de-
micropores and auxiliary mesopores, where mesopores are
interconnected. The synthesis merits small primary particle size
and well mesopore-connectivity. The introduction of mesopore
˚
alkylation is 1,3-diisopropylbenzene (1,3-DIPB, 8.4 A), that in
˚
turn, can be de-alkylated to cumene (6.8 A) or further to produce
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also lead to the formation of more Lewis acid sites than 14 N. Eng-Poh, D. Chateigner, T. Bein, V. Valtchev and
Brønsted sites, while the total number of strong acid sites are S. Mintova, Science, 2012, 335, 70–73.
almost identical to that of microporous ZSM-11. Meanwhile, the 15 M. A. Camblor, A. Corma and S. Valencia, Microporous
accessibility to the otherwise sterically hindered Brønsted sites Mesoporous Mater., 1998, 25, 59–74.
has been enhanced. ZSM-11-H shows improved catalytic activity 16 L. Wang, Y. Wang, Y. Liu, L. Chen, S. Cheng, G. Gao, M. He
in probe catalytic assessment for 1,3,5-triisopropylbenzene
cracking reaction with respect to the conventional ZSM-11.
and P. Wu, Microporous Mesoporous Mater., 2008, 113, 435–
444.
It is observed that orientated attachment growth can be the 17 M. Choi, K. Na, J. Kim, Y. Sakamoto, O. Terasaki and R. Ryoo,
dominating crystallization route for ZSM-11 growth in a dry gel Nature, 2009, 461, 246–249.
system, thereby the method can be extrapolated to other dry gel 18 X. Zhang, D. Liu, D. Xu, S. Asahina, K. A. Cychosz,
systems. Applicability of such a strategy in synthesizing other
zeolites or zeotype materials is underway and will be reported in
the future.
K. V. Agrawal, Y. Al Wahedi, A. Bhan, S. Al Hashimi,
O. Terasaki, M. Thommes and M. Tsapatsis, Science, 2012,
336, 1684–1687.
19 E. Verheyen, L. Joos, K. Van Havenbergh, E. Breynaert,
N. Kasian, E. Gobechiya, K. Houthoofd, C. Martineau,
M. Hinterstein, F. Taulelle, V. V. Speybroeck,
M. Waroquier, S. Bals, G. V. Tendeloo, C. E. A. Kirschhock
and J. A. Martens, Nat. Mater., 2012, 11, 1059–1064.
Acknowledgements
KZ is grateful for the nancial support from Fundamental
Research Funds for the Central Universities (WB 1213004-1),
and New Century Excellent Talents in University (NCET-11- 20 K. Moeller and T. Bein, Chem. Soc. Rev., 2013, 42, 3689–3707.
0644). KZ is also indebted to ELECTRONANOMAT (Grant 21 K. Moeller, B. Yilmaz, R. M. Jacubinas, U. Mueller and
Agreement Number: PIRSES-GA-2012-318990) sponsored by the
Marie-Curie Act program of EU for the trip to Denmark. XZ is 22 D. P. Serrano, J. M. Escola and P. Pizarro, Chem. Soc. Rev.,
thankful for National Natural Science Foundation of China 2013, 42, 4004–4035.
(U1162112) for the nancial support. ZL is grateful for the 23 Z. K. Xie, Z. C. Liu, Y. D. Wang, Q. H. Yang, L. Y. Xu and
T. Bein, J. Am. Chem. Soc., 2011, 133, 5284–5295.
Fundamental Research Funds for the Central Universities (WA
1514011).
W. P. Ding, Int. J. Mol. Sci., 2010, 11, 2152–2187.
24 K. Na, M. Choi and R. Ryoo, Microporous Mesoporous Mater.,
2013, 166, 3–19.
25 M. Y. Kustova, P. Hasselriis and C. H. Christensen, Catal.
Lett., 2004, 96, 205–211.
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