Synthesis of hierarchical ZSM-12 nanolayers for levulinic acid esterification with ethanol to ethyl levulinate

Article abstract of DOI:10.1039/c9ra03213d

Hierarchical ZSM-12 nanolayers have been successfully synthesized via a one-pot hydrothermal process using dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (TPOAC) as a secondary organic structure-directing agent (OSDA). The as-synthesized ZSM-12 samples were characterized by means of XRD, SEM, TEM, N₂ physisorption, and NH₃-TPD. This clearly demonstrates that the TPOAC content and the crystallization time are crucial parameters for the formation of nanolayered structures. The presence of such a structure significantly improves the mesoporosity of ZSM-12 by generating interstitial mesopores between nanolayers, eventually resulting in enhancing external surface areas and mesopore volumes, and subsequently promoting the molecular diffusion inside a zeolite framework. To illustrate its advantages as a heterogeneous catalyst, hierarchical ZSM-12 nanolayers were applied in the catalytic application of an esterification of levulinic acid with ethanol to ethyl levulinate. Interestingly, hierarchical ZSM-12 nanolayers exhibit an improvement of catalytic activity in terms of levulinic acid conversion (78.5%) and ethyl levulinate selectivity (98.7%) compared with other frameworks of hierarchical zeolite nanosheets, such as ZSM-5 and FAU. The example reported herein demonstrates an efficient way to synthesize a unidimensional pore zeolite with hierarchical nanolayered structure via a dual template method and also opens up perspectives for the application of different hierarchical porous systems of zeolites in the bulky-molecule reactions such as in the case of levulinic acid esterification with ethanol.

Full text of DOI:10.1039/c9ra03213d

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Synthesis of hierarchical ZSM-12 nanolayers for
levulinic acid esterification with ethanol to ethyl
levulinate†
Cite this: RSC Adv., 2019, 9, 18087
*
Pannida Dugkhuntod, Thidarat Imyen,
Wannaruedee Wannapakdee,
Thittaya Yutthalekha, Saros Salakhum and Chularat Wattanakit
Hierarchical ZSM-12 nanolayers have been successfully synthesized via a one-pot hydrothermal process
using dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (TPOAC) as a secondary organic
structure-directing agent (OSDA). The as-synthesized ZSM-12 samples were characterized by means of
XRD, SEM, TEM, N2 physisorption, and NH3-TPD. This clearly demonstrates that the TPOAC content and
the crystallization time are crucial parameters for the formation of nanolayered structures. The presence
of such a structure significantly improves the mesoporosity of ZSM-12 by generating interstitial
mesopores between nanolayers, eventually resulting in enhancing external surface areas and mesopore
volumes, and subsequently promoting the molecular diffusion inside a zeolite framework. To illustrate its
advantages as a heterogeneous catalyst, hierarchical ZSM-12 nanolayers were applied in the catalytic
application of an esterification of levulinic acid with ethanol to ethyl levulinate. Interestingly, hierarchical
ZSM-12 nanolayers exhibit an improvement of catalytic activity in terms of levulinic acid conversion
(78.5%) and ethyl levulinate selectivity (98.7%) compared with other frameworks of hierarchical zeolite
nanosheets, such as ZSM-5 and FAU. The example reported herein demonstrates an efficient way to
synthesize a unidimensional pore zeolite with hierarchical nanolayered structure via a dual template
method and also opens up perspectives for the application of different hierarchical porous systems of
zeolites in the bulky-molecule reactions such as in the case of levulinic acid esterification with ethanol.
Received 29th April 2019
Accepted 3rd June 2019
DOI: 10.1039/c9ra03213d
rsc.li/rsc-advances
desirable, as they provide many advantages, such as ease of
separation from reaction products, the efficient reusability of
Introduction
the catalysts, and less corrosion of reactor materials. Therefore,
various solid catalysts have been extensively developed over the
past decade as potential catalysts for this reaction, such as
zeolites,2 sulfated oxides (ZrO2, SnO2, TiO2),6–8 Amberlyst-15,9
SAC-13 (SiO2-supported Naon),10 heteropoly acids,11 and
sulfonated carbons.12
Ethyl levulinate (EL), a short-chain fatty ester, is an important
compound in the avouring and fragrance industries. In addi-
tion, this compound is attractive as a diesel miscible biofuel
(DMB), as it can be used up to 5 wt% for regular diesel car
engines.1 Typically, EL can be produced by the esterication of
levulinic acid (LA) with bioethanol, which is an attractive green
process, as LA and bioethanol are easily obtained from
biomass.2 Typically, the esterication of LA with ethanol can
proceed using both soluble and solid acid catalysts. For the
esterication reaction in liquid-phase, mineral acids such as
H2SO4,3 HCl,4 and H3PO4 (ref. 5) have been used as homoge-
neous acid catalysts. However, these catalysts are corrosive but
are cost-effective. On the other hand, solid catalysts are more
Among these solid catalysts, zeolites are crystalline alumi-
nosilicate materials, having the acidic properties and a well-
dened three-dimensional crystalline structure with uniformly
sized nano-channels. Owing to their outstanding properties,
such as high surface area, shape selectivity, high thermal/
chemical stability, as well as tunable acid properties, they are
indispensable materials for various applications ranging from
separation, adsorption to catalysis.13,14 Contrary to non-zeolitic
catalysts, the catalytic performance of zeolites for esterica-
tion of LA with ethanol not only depends on the acidity of
catalysts but also relates to the porous structure of zeolites.2
Hence, it is essential to use zeolites that have larger cavities to
improve the catalytic performance in the esterication of LA
with ethanol, involving a bulky molecule as a product. There-
fore, it is reasonable to conclude that the structure–property
Department of Chemical and Biomolecular Engineering, School of Energy Science and
Engineering, Nanocatalysts and Nanomaterials for Sustainable Energy and
Environment Research Network of NANOTEC, Vidyasirimedhi Institute of Science
and Technology, Rayong 21210, Thailand. E-mail: thidarat.i@vistec.ac.th
† Electronic supplementary information (ESI) available: NH3-TPD proles and
their deconvolution (Fig. S1), 29Si-NMR (Fig. S2), BJH pore size distribution
(PSD) (Fig. S3), acetic acid conversion and ethyl acetate selectivity (Table S1),
NH3-TPD data (Table S2) and textural properties (Table S3). See DOI:
10.1039/c9ra03213d
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relationship of a catalyst is a key factor for further improvement separation in a mixture solution of the OSDAs and the zeolite
in productivity and selectivity.2
gel. Recently, various types of nanosheet assemblies such as
ZSM-12 (MTW) is a silica rich zeolite, composing of one- FER and FAU were successfully prepared by utilizing an orga-
dimensional 12-membered ring channel system with unique nosilane with 18-carbon alkyl chains, namely dimethyloctadecyl
15
˚
˚
pore openings of 5.7 A ꢀ 6.1 A. Since its pore size is slightly [3-(trimethoxysilyl)propyl] ammonium chloride or TPOAC as the
higher than that of ZSM-5, it can be used for the shape selective mesopore template.33–35 However, to the best of our knowledge,
conversion of large molecules, which cannot easily penetrate it has not yet been reported in the use of this type of organo-
into MFI framework. In addition, due to its acidic properties, silanes as the co-SDA for the synthesis of unidimensional ZSM-
ZSM-12 has been suggested as a useful catalyst in many reac- 12 porous nanolayers.
tions such as isomerization,16 hydroisomerization,17 and
Therefore, in order to compliment the use of organosilanes
hydrocracking.18 Nevertheless, there is no report according to for synthesizing other hierarchical zeolites nanolayers besides
using this zeolite as the catalyst for EL production so far. FAU and FER nanosheets, a simple and practical approach to
Therefore, with its larger pore dimension compared with other prepare hierarchical ZSM-12 using TPOAC as the co-SDA is
zeolites and suitable acidity, it would benet from using this demonstrated herein. Since cyclic ammonium compounds were
material as a novel and useful catalyst in esterication of LA also found as the effective SDA to generate MTW zeolite struc-
with ethanol.
tures with nanocrystalline morphologies, it is benecial to
Unfortunately, it is well-known that a microporous structure combine both cyclic ammonia compound and TPOAC as the
of zeolites restricts the diffusion of guest molecules within their primary and the secondary SDA for the synthesis of hierarchical
porous network, leading to the low accessibility of active sites, ZSM-12 nanolayer assemblies, respectively. Moreover, the roles
mass transport limitations, and catalyst deactivation.14 In order of synthesis parameters including TPOAC content and crystal-
to ease these problems, zeolites exhibiting hierarchical porous lization time on the product characteristic were investigated.
structures have been developed. These materials exhibit the Additionally, the exceptional catalytic efficiency of synthesized
presence of at least one additional porous system, such as hierarchical ZSM-12 nanolayer for bulky-molecules reaction,
mesoporous and macroporous systems.19 These materials may namely, levulinic acid esterication with ethanol to ethyl levu-
benet to improve the diffusion in the typical microporous linate was also demonstrated. To further investigate the effect of
network because the created mesoporosity can facilitate the one-dimensional 12-membered ring channel system of ZSM-12
mass transport and lead to a more efficient use for catalytic on the catalytic performance, different zeolite nanosheets with
applications.20 The hierarchical zeolites can be obtained by various framework structures such as ZSM-5 and FAU,
several approaches, such as desilication,19,21 dealumination,21 composing of different dimensional porous systems were used
steaming,22 recrystallization,23,24 and templating approach.25,26 for the comparison in terms of catalytic activity and selectivity.
Among them, the use of a template has been considered as one
of the most the effective approaches to synthesize hierarchical
zeolites.
Experimental
Materials
Recently, the hierarchical zeolite nanosheets have been
successfully prepared and attracted much attention. Since these
materials are composed of sheet-like crystals with the thickness
in the nanometer range, they provide large surface areas as well
as high pore volumes, which are derived from the presence of
interstitial mesopores between adjacent nanocrystals.27 One of
the most efficient approaches for the preparation of hierar-
chical zeolite nanosheets is based on the use of an organic
structure-directing agent (OSDA) to control microporous struc-
tures of zeolites and nanocrystals formation.28–32
Choi et al.28 utilized a diquaternary ammonia-type surfactant
(C22H45–N+(CH3)2–C6H12–N+(CH3)2–C6H13) for the synthesis of
MFI nanosheets. The diammonium head group performs as
a structure-directing agent for MFI structure, whereas the
hydrophobic long-chain tails forming a micellar structure
restrict the growth of zeolite and create 2D zeolite nanolayers.
Sodium silicate (Na2Si3O7: 26.5 wt% SiO2, and 10.6 wt% Na2O,
Merck), aluminium sulphate (Al2(SO4)3$18H2O, Univar, Ajax
Finechem), sulfuric acid (H2SO4: 96%, RCI Labscan), sodium
hydroxide (NaOH: 98%, Carlo Erba), a,a0-dichloro-p-xylene
(98%, TCI), N,N,N0,N0-tetramethyl-1,6-hexanediamine (98%,
TCI), and dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammo-
nium chloride (TPOAC: 42 wt% in methanol, Aldrich) were used
as starting materials for the synthesis of hierarchical ZSM-12
nanolayers. Levulinic acid (>97%, TCI), toluene ($99.9%,
Merck), ethanol (99.9%, QRec), and decane ($99.9%, TCI) were
used for the catalytic activity testing without any further
purication.
Synthesis of hierarchical ZSM-12 nanolayer
Meanwhile, Na et al.27 proposed the use of different cyclic Firstly, cyclic diquaternary ammonium (CDM) was synthesized
ammonium compounds as the OSDAs to synthesize BEA, MTW, and utilized as an SDA for the synthesis of ZSM-12 zeolite
and MFI zeolite structures with nanocrystalline morphologies. following a previous procedure27 with some modications.
Among various OSDAs utilized for the zeolite preparation, an Briey, N,N,N0,N0-tetramethyl-1,6-hexanediamine was mixed
organosilane surfactant is one of the outstanding candidates for with an equimolar amount of a,a0-dichloro-p-xylene in aceto-
directing the formation of zeolite with layered structure. The nitrile under reux at 60 ꢁC for 3 h. Then, the obtained product
strong interaction between the molecules of organosilane and was ltered and washed with acetonitrile followed by diethyl
ꢁ
the growing zeolite crystals is a key aspect to prevent a phase ether, and dried in a vacuum oven at 50 C.
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Subsequently, hierarchical ZSM-12 nanolayers were synthe-
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Temperature-programmed desorption of ammonia (NH3-
sized by using a one-pot hydrothermal method, in which TPD) was used to investigate the acid properties of the obtained
sodium silicate and aluminium sulphate were used as silica and products. The measurement was conducted using a BELCAT II
alumina sources, respectively. Meanwhile, CDM and TPOAC instrument equipped with thermal conductivity detectors
was used as an SDA and co-OSDA to control microporous ZSM- (TCD). Prior to the measurement, the sample was pre-treated
12 and nanosheets structures, respectively. The molar compo- under He ow at 500 ꢁC for 1 h. Aer that, the sample was
sition was 30Na2O : 0.625Al2O3 : 100SiO2 : 10CDM : 15H2SO4 : saturated with NH3 at 100 ꢁC for 30 min. Subsequently, the NH3
6000H2O : xTPOAC, where x was 0, 3, 9, 12, and 15. Typically, desorption was performed from 100 to 550 ꢁC with the heating
ꢂ1
ꢁ
the rst solution was prepared by diluting sodium silicate with rate of 10 C min
NaOH solution, while the second solution was prepared by
.
The coordination environment of Si atom was investigated
diluting aluminium sulphate with H2SO4 solution. The two using 29Si MAS NMR spectroscopy performed on an AVANCE III
solutions were mixed under vigorous stirring before adding HD (400 MHz) Digital NMR spectrometer (Bruker) operating at
CDM into the mixed solution. Aer stirring for 1 h, the resultant the magnetic eld of 9.4 T.
mixture was transferred to a Teon lined stainless steel auto-
clave for hydrothermal treatment at 170 ꢁC for 24 h. The ob-
Catalytic activity testing via an esterication of LA with
tained produꢁct was ltered and washed with deionized water,
ethanol
ꢁ
dried at 100 C overnight, and calcined at 550 C for 6 h. The
Esterication of LA with ethanol to EL was carried out using
solid product was converted to H+ form via an ion-exchange
0.3 g of the catalyst in a Parr reactor for 24 h. The catalytic
ꢁ
with 1 M NH4NO3 solution at 80 C for 2 h. The ion-exchange
activity of the catalysts was tested with an equimolar mixture of
LA and ethanol in toluene solvent under stirring (500 rpm) at
100 ꢁC and autogenous pressure. The reaction mixture was
sampled and analyzed by gas chromatograph (GC, Agilent,
7890B) equipped with a FID detector and an Elite-Wax column
process was repeated for three time. Then, the derived
product was washed with deionized water, dried at 100 ꢁC
ꢁ
overnight, and calcined at 550 C for 6 h. All the synthesized
ZSM-12 nanosheets samples are denoted as ZSM-12-y, where y
refers to the molar ratio of TPOAC/SiO2 in the starting gel.
(30 m ꢀ 0.25 mm) at a certain time of the reaction. For the
product analysis, decane was used as an internal standard.
Subsequently, LA conversion and EL selectivity were calculated
by the following equations:
Characterizations
X-ray diffraction (XRD) patterns were collected using a Bruker
D8 ADVANCE diffractometer with Cu Ka radiation (30 kV and 40
mole LAinitial ꢂ mole LAfinal
LA conversion ð%Þ ¼
EL selectivity ð%Þ ¼
ꢀ 100
mA) in the 2q range of 5ꢁ to 60ꢁ with a step size of 0.02ꢁ and scan
rate of 1ꢁ minꢂ1. The relative crystallinity of zeolite products
was determined by the following equation: % XRD relative
crystallinity of the desired sample ¼ (Hx/Hr) ꢀ 100, where Hx
and Hr are the heights of the main diffraction peaks of the
desired sample and the reference sample, respectively.
mole LAinitial
mole EL formed
ꢀ 100
mole all products formed
The nanolayered structure of the products was conrmed by
scanning electron microscopy (SEM, JEOL, JSM-7610F model).
Additionally, the morphology of the samples was also moni-
tored by transmission electron microscopy (TEM, JEOL JEM-
2010) operated with an acceleration of 200 kV. Prior each TEM
Results and discussions
Synthesis of hierarchical ZSM-12 zeolite nanosheets
Effects of the TPOAC content in the synthesis gel. X-ray
analysis, the sample was suspended in ethanol, followed by diffraction patterns of ZSM-12 nanolayers synthesized with
evaporation of ethanol on a copper grid coated with a carbon different TPOAC/SiO2 molar ratios are presented in Fig. 1. The
lm.
samples obtained with TPOAC/SiO2 ratio ranging from 0 to 0.12
The Si/Al ratio of the products was obtained by X-ray uo- exhibit the characteristic diffraction peaks of MTW (7.2ꢁ, 8.8ꢁ,
rescence analysis (XRF, Bruker model S8 TIGER sequential 20.7ꢁ, and 23.1ꢁ)15,36 without the interference of other crystalline
WDXRF).
phases, conrming that ZSM-12 zeolite was successfully
The textural properties of the products were determined synthesized. However, the intensities of these diffraction peaks
from nitrogen adsorption–desorption isotherm measurement decrease when TPOAC/SiO2 ratio was increased, indicating that
at ꢂ196 ꢁC by using a MicrotracBEL, BELSORP-max model. The the crystallinity of ZSM-12 decreases as a function of the TPOAC
specic surface area (SBET) was estimated by the Brunauer– content, in agreement with what have been reported in previous
Emmett–Teller (BET) method. Meanwhile, the micropore works.33,34,36 The reason for a decrease in crystallinity can be
surface area (Smicro), the external surface area (Sext), and the related to the reduced microporous feature, which is the main
microporous volume (Vmicro) were determined by the t-plot characteristic of zeolite. Typically, the relative crystallinity is
method. The total pore volume (Vtotal) was obtained from data at determined based on the summation of the heights of the main
P/P0 of 0.99. The pore size distribution was derived from the diffraction peaks of ZSM-12 (7.2ꢁ, 8.8ꢁ, 20.7ꢁ, and 23.1ꢁ) of the
desorption branch of the Barrett–Joyner–Halenda (BJH) desired sample with respect to those of the reference
method.
sample.36,37 In this case, the ZSM-12 sample synthesized in the
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Fig. 1 XRD patterns of ZSM-12 zeolites synthesized with different
TPOAC/SiO2 molar ratios: (a) ZSM-12-0, (b) ZSM-12-0.03, (c) ZSM-12-
0.09, (d) ZSM-12-0.12, and (e) ZSM-12-0.15.
absence of TPOAC as a secondary SDA (ZSM-12-0) was chosen as
the reference sample. For instance, the relative crystallinity of
ZSM-12 products decreases from 100 to 95.02, 85.07, and
66.32% for ZSM-12-0.03, ZSM-12-0.09, and ZSM-12-0.12,
respectively. Obviously, at the TPOAC/SiO2 molar ratio of 0.15,
there is no any characteristic peaks of MTW and only amor-
phous phase is observed. It is therefore reasonable to concern
that the optimum amount of TPOAC is a crucial parameter to
control the nanolayered conguration.
The morphology of the synthesized ZSM-12 was revealed by
SEM images as illustrated in Fig. 2. For the sample obtained
without TPOAC (ZSM-12-0), the agglomeration of nanosized
crystals without the formation of nanolayers of ZSM-12 was
detected and similar to what have been described in the
previous literature27 (Fig. 2(a)). In strong contrast to this, with
the addition of TPOAC as the co-SDA with the suitable amount,
Fig. 2 SEM images of ZSM-12 zeolites synthesized with different
TPOAC/SiO2 molar ratios: (a) ZSM-12-0, (b) ZSM-12-0.03, (c) ZSM-12-
0.09, (d) ZSM-12-0.12, and (e) ZSM-12-0.15.
morphology is completely destroyed (Fig. 2(e)), and the absence
of diffraction peaks in XRD patterns can allow us to assume that
the excess amount of TPOAC can generate an amorphous phase
instead of a crystalline zeolite (see Fig. 1). These observations
suggest that too high TPOAC content can inhibit crystal growth
and also disrupt the self-assembly of nanosheets.
The excessive amount of TPOAC can cause the adsorptive
competition of the organosilane surfactant on zeolite precursor
surfaces, eventually resulting in the loss of the long-range order
of zeolite structures.38 It is therefore reasonable to propose the
formation process of ZSM-12 nanolayer assemblies as
the assemblies of ZSM-12 nanolayers with
a crystalline
morphology were observed, indicating that the presence of
TPOAC is crucial for the formation of ZSM-12 nanolayered
structures.
However, the formation of such structures strongly depends
on the content of TPOAC in the synthesis gel. For example, as
the TPOAC/SiO2 ratio is changed from 0 to 0.03, the nanosized
crystals become the brick-like crystals (Fig. 2(b)). Moreover,
upon increasing the TPOAC/SiO2 ratio to 0.09, the average
particle size is increased and the well-dened nanolayer-
assembled ZSM-12 having the thickness of each nanolayer
around 20–25 nm is obtained (Fig. 2(c)), indicating that the
intergrowth of nanolayers is predominant to form nanolayer
assembly.33,34 Nevertheless, with further increase of the TPOAC/
SiO2 ratio to 0.12, nanolayer assemblies are revealed in the
simultaneous presence of the amorphous phases and crystal-
line phase of ZSM-12 (Fig. 2(d)). The presence of the amorphous
phase could be indicated by a signicantly lower crystallinity
and broad hump at 2q around 20 (see Fig. 1). Finally, when the
TPOAC/SiO2 molar ratio reaches 0.15, the nanolayered
Scheme 1 The formation of ZSM-12 nanolayer assemblies as a func-
tion of TPOAC concentration.
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demonstrated in Scheme 1. Since, the introduction of TPOAC
has a considerable inuence on the crystallization of ZSM-12
nanolayers, the suitable amount of TPOAC is required in
order to obtain homogeneous ZSM-12 nanolayers with the well-
dened structure as well as high crystallinity. Therefore, as
above-mentioned results, the most suitable TPOAC/SiO2 molar
ratio under the present synthesis circumstance is 0.09.
To further study the effects of TPOAC in the synthesis gel on
the textural properties of the crystalline ZSM-12 products, N2
adsorption–desorption experiments were carried out on the
crystalline ZSM-12 samples obtained with TPOAC/SiO2 molar
ratios of 0 to 0.09. The textural properties of the samples are
summarized in Table 1. As depicted in Fig. 3, both ZSM-12-
0 and ZSM-12-0.09 samples exhibit type-IV N2 isotherms with
a hysteresis loop,39 conrming the simultaneous presence of
micropores and meso/macropores in the obtained ZSM-12
products. The steep N2 uptakes at low P/P0 relates to the
microporous structure, while the hysteresis loop existing in the
region 0.45 < P/P0 < 0.95 corresponds to capillary condensation
in mesopores.27,40 Interestingly, ZSM-12-0.03 sample exhibits
type-I N2 isotherm, a typical of microporous materials, with
a thin hysteresis loop between 0.4 and 0.8, indicating to the
presence of some mesoporosity as shown in Table 1.36
Unsurprisingly, the mesoporosity of the sample synthesized
in the absence of TPOAC (ZSM-12-0) is created by the use of
a single SDA of cyclic diquaternary ammonium (CDM) to
control the MTW framework. The high affinity of the organic
cyclic diammonia with silicate species and the intergrowths
lead to the agglomeration of nanocrystalline ZSM-12, yielding
mesopores between adjacent nanoparticles.27 The introduction
of TPOAC as the secondary template directly affects the fraction
of mesopores in ZSM-12 products. Typically, the presence of
TPOAC can inhibit the crystal growth at specic direction,
resulting in the generation of 2D zeolite nanolayers and inter-
stitial mesopores between these nanolayers. However, as dis-
cussed above, the content of TPOAC should be suitable in order
to obtain well-dened and homogeneous ZSM-12 nanolayers
with high crystallinity. For instance, when the TPOAC/SiO2
molar ratio is changed from 0 to 0.03, the nanosized crystals
become the brick-like crystals (Fig. 2(a) and (b)). Subsequently,
with a bigger size of brick-like crystals (Fig. 2(b) and Scheme 1),
the interparticle voids become smaller and thus the fraction of
mesopores is lower. Hence, the microporous feature is
predominant, while only small fraction of mesopores is present
in the ZSM-12-0.03 sample, as evidenced by N2 isotherm and
textural properties shown in Fig. 3 and Table 1, respectively.
Fig.
3
N2 adsorption–desorption isotherms of ZSM-12 zeolites
synthesized with different TPOAC/SiO2 molar ratios: (a) ZSM-12-0, (b)
ZSM-12-0.03, and (c) ZSM-12-0.09.
On the other hand, with the addition of TPOAC with the
suitable amount (i.e. TPOAC/SiO2 molar ratio of 0.09), the
fraction of mesopores in the ZSM-12 product becomes higher,
as demonstrated by a manifest hysteresis loop presenting in the
N2 isotherms of ZSM-12-0.09 sample (Fig. 3). As expected, the
external surface area of ZSM-12 products increases with an
increasing of TPOAC/SiO2 ratio to 0.9, for example; the external
surface area increases to 278 m2 gꢂ1 for ZSM-12-0.09. In addi-
tion, the mesopore volume and the total pore volume of ZSM-12-
0.09 increase to 0.71 and 0.75 cm3 gꢂ1, respectively, compared
with ZSM-12 products obtained with lower TPOAC content. As
the TPOAC/SiO2 ratio is increased to 0.09, the crystal thickness
becomes smaller and nanolayer-assembled ZSM-12 is subse-
quently obtained (Fig. 2(c)). The above results conrm that the
introduction of TPOAC as the secondary template can improve
the mesoporosity of ZSM-12 zeolite. This behaviour relates to
the fact that the nanosized crystals become sheet-like and the
assembly of nanosized sheet-like crystals, which can enhance
the interstitial mesoporosity.
The acid properties of synthesized ZSM-12 were determined
by NH3-TPD measurement as shown in Fig. 4. NH3-TPD proles
of all samples were decomposed into three peaks at around
180–200 ꢁC, 250–310 ꢁC, and 360–400 ꢁC (Fig. S1†), corre-
sponding to weak, medium, and strong acid sites, respec-
tively.36,40 The desorption temperatures and the corresponding
Table 1 Textural properties and the Si/Al ratio of the ZSM-12 products
Si/Alh
a
b
c
d
e
f
g
Sample
SBET
Smicro
Sext
Vtotal
Vmicro
Vext
Vext/Vtotal
ZSM-12-0
ZSM-12-0.03
ZSM-12-0.09
484
393
421
307
278
143
176
115
278
0.54
0.33
0.75
0.11
0.10
0.04
0.43
0.23
0.71
0.80
0.70
0.95
52.5
58.7
69.7
a
b
c
d
e
SBET: BET specic surface area. Smicro: microporous surface area. Sext: external surface area. Vtotal: total pore volume. Vmicro: micropore
volume. Vext ¼ Vtotal ꢂ Vmicro; all surface areas and pore volumes are in the units of m2 gꢂ1 and cm3 gꢂ1, respectively. Fraction of mesopore
f
g
volume. h Si/Al ratio obtained by XRF.
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acid amounts of all samples are summarized in Table 2. a spongy-like morphology (Fig. 5(a)–(c)). The amorphous
Compared with ZSM-12-0 as the reference sample, the acid material obtained in this crystallization period was also
strength of ZSM-12 nanolayer assemblies is slightly weaker, as conrmed by XRD patterns aer 6 h of crystallization (Fig. 7).
indicated by a peak shiing to lower desorption temperatures During the crystallization time of 12 h, the nanolayers start to
when TPOAC/SiO2 molar ratio is increased from 0 to 0.03 and form under the directing of TPOAC and gradually assemble to
0.09, respectively (Table 2). In addition, the total number of acid nanosheet assemblies as demonstrated by SEM and TEM
sites represented by the area of NH3-TPD peaks of ZSM-12 images (Fig. 5(d) and Fig. 6(d)). However, there is still amor-
products is signicantly decreased when TPOAC content is phous phase detected in this period (Fig. 5(d)). It is well known
increased from 0 to 0.03 and 0.09, respectively. A lower acidity of that the zeolite crystallization occurs aer the dissolution of
ZSM-12 nanolayer assemblies compared with that of the refer- amorphous phase.27
ence ZSM-12 is correlated with a higher Si/Al ratio caused by the
TEM images as illustrated in Fig. 6 also provide the infor-
presence of TPOAC in the synthesis gel. Since TPOAC can also mation regarding the formation of ZSM-12 nanolayers as
act as a part of silica source,40 the addition of TPOAC tends to a function of crystallization time. For instance, at the initial
introduce more silicon atoms in the zeolite framework, result- stage of the crystallization (0 to 6 h), the sample is present as an
ing in increasing the Si/Al ratio and thus decreasing the number amorphous aluminosilicate gel (Fig. 6(a)–(c)). As the crystalli-
of acidic sites. For instance, a Si/Al ratio increases from 52.5 to zation time is increased to 12 h, a crystalline phase starts to
69.7 when TPOAC/SiO2 ratio increases from 0 to 0.09 (see Table appear (Fig. 6(d)). Subsequently, aer the crystallization of 24 h,
1).
the amorphous phase is completely transformed to the multi-
Effects of the crystallization time. The formation of ZSM-12 layer splice of ZSM-12 nanosheets, which can also be conrmed
nanolayer assembly was investigated by varying the crystalliza- by high crystallinity (85.07%).
tion time from 2 to 24 h, while TPOAC/SiO2 ratio was xed at
0.09, as it is the most suitable ratio to obtain the well-dened
nanolayered structure with high crystallinity. A change of
morphology as a function of crystallization time was observed
by SEM and TEM images as illustrated in Fig. 5 and 6, respec-
tively. At the crystallization period of 2 to 6 h, the aluminosili-
cate species are initially formed as the amorphous gel with
Fig. 4 NH3-TPD profiles of ZSM-12 zeolites synthesized with different
TPOAC/SiO2 molar ratios: (a) ZSM-12-0, (b) ZSM-12-0.03, and (c) Fig. 5 SEM images of ZSM-12-0.09 samples obtained at different
ZSM-12-0.09.
crystallization time: (a) 2 h, (b) 4 h, (c) 6 h, (d) 12 h, and (e) 24 h.
Table 2 NH3-TPD data and the acid amount of synthesized ZSM-12 zeolites
Tpeak (ꢁC)
Acid amount (mmol gꢂ1
)
Sample
LT peak
MT peak
HT peak
Weak acidity
Medium acidity
Strong acidity
Total acidity
ZSM-12-0
ZSM-12-0.03
ZSM-12-0.09
191.3
185.5
182.7
309.7
285.4
254.2
395.3
387.7
366.6
0.081
0.058
0.041
0.074
0.055
0.022
0.098
0.109
0.098
0.253
0.222
0.161
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peak located in the range between ꢂ75 and ꢂ120 ppm was
observed for all samples. The broad peak can be divided into
three peaks centered at ꢂ111, ꢂ99, and ꢂ92 ppm, corre-
sponding to Q4, Q3, and Q2 species, respectively, where n in Qn
refers to the number of neighbouring Si atoms that are con-
nected with an oxygen bridge.41,42 For instance, the Q4-type Si
atoms are related to Si(OSi)4.43,44 The Q3-type Si atoms can be
referred to Si(OAl)(OSi)3 or terminal silanol groups in
Si(OH)(OSi)3,41,44 whereas the Q2-type Si atoms are referred to
Si(OAl)2(OSi)2 and geminal silanol groups in Si(OH)2(OSi)2.44–46
From the results, it is clearly observed that at the initial stage of
crystallization (2 h), the Q4 peak (ꢂ111 ppm) is very broad and
present as a shoulder peak of the Q3 peak (ꢂ99 ppm), sug-
gesting that the coordination environment of the correspond-
ing framework Si atoms is distorted.47 As conrmed by SEM
image (Fig. 5(a)) and XRD pattern (Fig. 8), the crystalline
structure is not formed at this early stage. However, as the
crystallization time is increased to 12 and 24 h, the Q4 peak
becomes more distinct and narrower, indicating that the sili-
cate network is gradually formed and the degree of Si–O–Si
networking becomes higher. Finally, the most distinct Q4 peak
is observed for ZSM-12 sample obtained at the crystallization
time of 24 h, conrming that the degree of Si–O–Si networking
of this sample is the highest compared with other samples
obtained at a shorter crystallization time.
As stated above, organosilane surfactants have been exten-
sively utilized to control the structure and the morphology of
hierarchical materials because of their amphiphilic properties,
allowing us to ne-tune their interaction with zeolitic precur-
sors.40,48,49 In order to minimize its surface free energy, the
organosilane surfactant creates new surfaces by their self-
assemblies, acting as a crystal face inhibitor, which can
obstruct the crystal growth in specic direction.50 Since TPOAC
is composed of hydrolysable methoxysilyl groups (Si–OCH3),
aluminosilicate species can react with the hydrolyzed part of
TPOAC (Si–OH) during the zeolite synthesis. The interaction
between the surfactant molecules and the zeolite precursors can
Fig. 6 TEM images of ZSM-12-0.09 samples obtained at different
crystallization time: (a) 2 h, (b) 4 h, (c) 6 h, (d) 12 h, and (e and f) 24 h.
Fig. 7 High-resolution TEM images of ZSM-12 nanolayer sample
(ZSM-12-0.09) obtained at crystallization time of 24 h.
In addition, as illustrated in Fig. 7, high-resolution TEM
images also conrm that ZSM-12-0.09 obtained at crystalliza-
tion of 24 h consists of a multilayer stacking of ZSM-12 nano-
layers with a layer thickness around 13–25 nm, which is in
accordance with SEM image (Fig. 2(c)).
The degree of networking of Si atoms in the synthesized
samples as a function of crystallization time can be reected by
29Si MAS NMR spectra. As demonstrated in Fig. S2,† a broad
Fig. 8 XRD patterns of ZSM-12-0.09 samples obtained at different
crystallization time: (a) 6 h, (b) 12 h, and (c) 24 h.
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initiate the anisotropic crystal growth, eventually leading to the mesopore volume. At a lower range of mesopore volume (i.e.,
formation of the nanolayers of ZSM-12.31,40 During the zeolite smaller than 0.4 cm3 gꢂ1), when the mesopore volume increases
crystallization, ZSM-12 nanolayer assemblies are gradually for two times (from 0.23 to 0.43 cm3 gꢂ1), LA conversion is
formed. Finally, as the crystallization is prolonged to 24 h, these improved for only 4.7% (from 46.1% to 50.8%). On the other
ZSM-12 nanolayer assemblies become more uniform in hand, at a higher range of mesopore volume (i.e., larger than 0.4
company with a higher crystallinity.
cm3 gꢂ1), when the mesopore volume increases from 0.43 to
0.71 cm3 gꢂ1, LA conversion is signicantly enhanced from
50.8% to 78.5%. In agreement with the previous works,2,8,20,52
a larger pore volume as well as a bigger pore size can decrease
the mass transfer resistance, as a larger space is provided for
bulky molecules to diffuse inside the zeolite network during the
reaction. However, in order to greatly improve the catalytic
activity in the LA esterication, the mesopore volume should be
large enough to contribute to the mass transfer improvement
inside the zeolite structure. Therefore, the mesopores generated
from the interparticle voids of ZSM-12 nanolayer assemblies
could contribute to the exceptional performance of hierarchical
ZSM-12 nanolayer for the esterication involving a large mole-
cule in the reaction pathway.
Catalytic activity testing
To demonstrate the advantageous effect of hierarchical ZSM-12
nanolayers, we also performed additional experiments accord-
ing to the catalytic study of an esterication of levulinic acid
(LA) with ethanol to ethyl levulinate (EL). The catalytic perfor-
mance of synthesized hierarchical ZSM-12 zeolites for esteri-
cation of LA to EL was tested in terms of LA conversion (%) and
EL selectivity (%) achieved aer 24 h of reaction time as shown
in Table 3. For hierarchical ZSM-12 nanolayers synthesized in
the absence of TPOAC (ZSM-12-0), LA conversion of 50.6% with
EL selectivity of 96.8% was obtained. This catalyst exhibits
activity comparable to the other catalysts reported in the
previous literatures.1,8,51 Moreover, the catalytic activity of hier-
archical ZSM-12 nanolayer obtained with a TPOAC/SiO2 ratio of
0.09 (ZSM-12-0.09) is signicantly enhanced, as LA conversion
and EL selectivity are increased to 78.5 and 98.7%, respectively.
It is therefore reasonable to assume that the improved catalytic
performance of ZSM-12-0.09 sample is attributed to its nano-
layered structure. The formation of such structure can generate
interstitial mesopores between layers, enlarging the external
surface areas, and thus promoting the molecular accessibility to
active sites of zeolite.
As evidenced in Table 1, the external pore volume (due to
generated mesopores) is increased from 0.43 to 0.71 cm3 gꢂ1
when TPOAC/SiO2 ratio in the synthesis gel is increased from
0 to 0.09. Moreover, pore size distribution (PSD) of ZSM-12-0.09
sample also reveals the additional macropores with a pore
diameter around 80 nm (Fig. S3†). These larger pores are
created from the interparticle voids of ZSM-12 nanolayer
assemblies and could contribute to the exceptional perfor-
mance of hierarchical ZSM-12 nanolayer for the bulky-molecule
esterication.
To proof the concept, an esterication of small carboxylic
acid molecule with ethanol was also carried out under the
similar experimental condition. It clearly shows that the nano-
layered structure has less impact on the catalytic performances
in acetic acid esterication, as acetic acid conversion over both
ZSM-12-0 and ZSM-12-0.09 are comparable (Table S1†). This
result can be explained by the molecular size of the reactants
with respect to the pore size of the zeolite. As reported in the
literature,53 the kinetic diameter of acetic acid is 4.4 A, which is
˚
˚
˚
smaller than the micropore of ZSM-12 (5.7 A ꢀ 6.1 A). There-
fore, these small molecules can access to the active sites in
zeolite and a larger pore is not required for the mass transfer
inside the zeolite pores. On the other hand, the kinetic diameter
of levulinic acid is 5.7 A.54 Therefore, these observations conrm
˚
that the catalytic performance of a bulky-molecule esterica-
tion, which also produces a large molecule of a product, can be
greatly improved by the nanolayered structures, which can
greatly enhance the accessibility to Brønsted acid sites inside
microporous structures.
Furthermore, the catalytic performance for esterication of
LA was also evaluated on various zeolite nanosheets with
different frameworks. It is worth noting that the synthesis
procedures of both ZSM-5 and FAU nanosheets tested herein
and their corresponding textural properties were reported in
our previous works.34,55 For ZSM-5 and FAU nanosheets, they
were successfully fabricated by using a single SDA of tetrabu-
tylphosphonium hydroxide (TBPOH) and TPOAC, respectively.
The functions of these SDAs are to control the microporous
structure of zeolite and generate two-dimensional nanolayers.
In contrast, ZSM-12 nanosheet was synthesized by using dual-
templates of CDM and TPOAC. By using CDM as a single
template, hierarchical ZSM-12 zeolite could be obtained with
nanocrystalline morphology and intercrystalline mesoporosity
due to the high affinity of CDM structure with silicate species.
However, the introduction of TPOAC as the co-SDA, the
morphology of ZSM-12 was transformed from nanocrystals to
nanosheet assemblies, and subsequently intercrystalline mes-
oporosity is improved. Hence, TPOAC is essential for the
The effect of the mesopore volume on the enhanced catalytic
activity of the LA esterication can also be conrmed by a plot of
LA conversion as a function of the mesopore volume of ZSM-12
(Fig. S4†). The results demonstrate that the effect of the meso-
pore volume on LA conversion is more pronounce at larger
Table 3 Levulinic acid conversion and ethyl levulinate selectivity ob-
tained on different catalysts after 24 h of reaction (reaction conditions:
T ¼ 100 ꢁC, LA : ethanol molar ratio of 1 : 1, catalyst loading of 0.3 g)
LA conversion
(%)
EL selectivity
(%)
Sample
Time (h)
ZSM-12-0
ZSM-12-0.09
ZSM-5 nanosheet
FAU nanosheet
24
24
24
24
50.6
78.5
52.6
58.0
96.8
98.7
98.7
95.4
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fabrication of hierarchical ZSM-12 nanosheet with enhanced with alcohols involves the protonation of carboxylic acid and it
mesoporous feature. can be assisted by the Brønsted acid zeolite. With coordination
As shown in Table 3, all catalysts show EL selectivity higher of carboxyl group with the acidic site, the substituent groups
than 95%, conrming that EL is the main product of this could interact with the framework structure.1 Hence, the
reaction. Interestingly, hierarchical ZSM-12 nanolayer (ZSM-12- formation of such a transition state might be obstructed by
0.09) exhibits the highest LA conversion, corresponding to steric hindrance. However, the steric repulsion is considerably
78.5%. The highest catalytic activity of hierarchical ZSM-12 less severe, and protonation of the carbonyl oxygen atom is
nanolayer could be attributed to the benet of its micropo- more favorable for zeolite a large pore system without forming
rous structure, composing of a large one-dimensional 12- large cavities, such as MTW zeolite.
membered ring channel system15 and the improved meso-
porous feature due to the presence of the secondary SDA.
Conclusions
Although both ZSM-12 and ZSM-5 compose of hierarchical
structures and have the similar acid properties (Table S2†), the The nanolayer-assembled ZSM-12 zeolite was successfully
reason of a higher catalytic activity of hierarchical ZSM-12 prepared by using an organosilane surfactant (TPOAC) as
nanolayer may relate to the benet of a one-dimensional 12- a secondary structure-directing agent. It is demonstrated that
membered ring network, which is also larger than that of ZSM-5 the formation of nanolayer structure depends on both TPOAC
microporous channels. It is well-known that ZSM-5 consists of content and crystallization time. By varying the molar ratio of
two types of interconnecting 10-membered ring channels TPOAC/SiO2 in the synthesis gel, the TPOAC/SiO2 molar ratio of
(straight and sinusoidal channels) with pore openings of ꢃ5.5 0.09 is the most suitable condition under our synthesis
˚
A. Regarding to the molecular dimension of related molecules circumstance, as it provides hierarchical ZSM-12 nanolayer
with respect to microporous structures of all zeolites, it is sug- assemblies with well-dened structure. In addition, uniform
˚
gested that the molecular size of levulinic acid (5.7 A) is close to ZSM-12 nanolayer assemblies were observed under the crystal-
˚
˚
the pore openings of ZSM-12 (5.7 A ꢀ 6.1 A) as shown in Scheme lization time of 24 h. Therefore, with the TPOAC/SiO2 molar
2. This indicates that the catalytic reaction of levulinic acid ratio of 0.09 and the crystallization time of 24 h, nanolayer-
esterication with ethanol to ethyl levulinate requires high assembled ZSM-12 with outstanding properties involving
catalytic spaces. Indeed, FAU exhibits a high acid density, which a high fraction of mesopore volumes (Vext/Vtotal ¼ 0.95) was
may have a benet for an esterication activity (Table S2†). obtained.
However, although its structure is constructed from a large 12-
Interestingly, our designed hierarchical ZSM-12 nanolayers
˚
membered ring channel system with pore opening of 7.4 A and can considerably enhance the catalytic activity for levulinic acid
56
˚
cavity of 13 A, it oen suffers from disadvantages of its esterication with ethanol in terms of levulinic acid conversion
supercage structure, which may inhibit a catalytic activity due to and ethyl levulinate selectivity. The improved activity of hier-
ease of further reactions in a cage-like space, resulting in archical ZSM-12 nanolayer is related to its enhanced meso-
generating the pore-blocking phenomenon.57 This behavior porosity and the presence of a large one-dimensional 12-
leads to a low catalytic activity even though it contains a high membered ring network, which can promote the accessibility of
portion of hierarchical porosity (Table S3†).
moleculars to active sites of zeolite. The approach reported
It was stated that the activity of zeolites can be associated herein demonstrates an efficient way to synthesize hierarchical
with their pore structure and the formation of the transition zeolite nanolayer assemblies with ZSM-12 structure via a dual
states of the LA esterication inside the channels.1 The mech- template method and also opens up the interesting aspects for
anism for the acid-catalyzed esterication of carboxylic acids the design of hierarchical zeolites as the heterogeneous catalyst
for bulky-molecule reactions like in the case of levulinic acid
esterication with ethanol. Moreover, this also extend the
design of zeolite nanosheets on other types of zeolites, which
has been previously limited to MFI, FAU, and FER types.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was nancially supported by the Vidyasirimedhi
Institute of Science and Technology, Frontier Research Center
(FRC), TTSF research project supported by Thailand Toray
Science Foundation, Thailand Research Fund (MRG6180099),
and the Office of Higher Education Commission. In addition,
this work has been partially supported by the National Nano-
technology Center (NANOTEC), NSTDA, Ministry of Science and
Scheme 2 The molecular size of levulinic acid compared to pore
channels of different zeolite frameworks.
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¨
Network NANOTEC.
R. Schlogl, T. Yokoi and T. Tatsumi, Angew. Chem., 2006, 118,
3162–3165.
26 M. B. Yue, M. N. Sun, F. Xie and D. D. Ren, Microporous
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