The Important Role of Rubidium Hydroxide in the Synthesis of Hierarchical ZSM-5 Zeolite Using Cetyltrimethylammonium as Structure-Directing Agent

Article abstract of DOI:10.1002/ejic.201900324

Hierarchical ZSM-5 zeolite with uniform mesopores was synthesized with a simple cetyltrimethylammonium (CTA⁺) template, which acted as a bifunctional surfactant in a RbOH-based alkaline synthesis gel. Rb⁺ plays a key role in obtaining uniform mesopores within ZSM-5 crystals. The structural, textural properties and the acidity were characterized by XRD, Ar physisorption, TEM, as well as CO IR and ²⁷Al MAS NMR spectroscopy. These data point to partial retention of the initial mesoscale ordered texture of the precursor in the final zeolite. These textural properties result in a strongly improved catalytic performance in the methanol conversion reaction compared to bulk zeolite.

Full text of DOI:10.1002/ejic.201900324

DOI: 10.1002/ejic.201900324
Full Paper
Zeolite Synthesis | Very Important Paper |
The Important Role of Rubidium Hydroxide in the Synthesis of
Hierarchical ZSM-5 Zeolite Using Cetyltrimethylammonium as
Structure-Directing Agent
Wannaruedee Wannapakdee,[a] Lingqian Meng,[b] Arno J. F. van Hoof,[b]
Alexsei Bolshakov,[b] Chularat Wattanakit,[a] and Emiel J. M. Hensen*[b]
Abstract: Hierarchical ZSM-5 zeolite with uniform mesopores physisorption, TEM, as well as CO IR and 27Al MAS NMR spectro-
was synthesized with a simple cetyltrimethylammonium (CTA+) scopy. These data point to partial retention of the initial meso-
template, which acted as a bifunctional surfactant in a RbOH- scale ordered texture of the precursor in the final zeolite. These
based alkaline synthesis gel. Rb+ plays a key role in obtaining textural properties result in a strongly improved catalytic per-
uniform mesopores within ZSM-5 crystals. The structural, tex- formance in the methanol conversion reaction compared to
tural properties and the acidity were characterized by XRD, Ar bulk zeolite.
Introduction
agents (SDAs) for generating nanostructured ZSM-5 in a one-
pot synthesis. The DQAS has two roles in this approach. Its hy-
drophilic part controls formation of zeolite structures by inter-
action of the head group with silica leading to pre-structured
moieties that will later condense to zeolite MFI, while the hydro-
phobic hydrocarbon tail serves as a mesoporogen.[7] Despite
the aesthetic beauty of these materials and their improved cata-
lytic performance they remain mostly model catalysts, because
typical DQAS are too expensive to synthesize nanostructured
zeolites at an industrial scale.
Zeolites, which are microporous aluminosilicates, are widely
used as catalysts in the petroleum refining and petrochemical
industry.[1,2] Their crystalline framework displays unique proper-
ties such as high surface area, good chemical and thermal sta-
bility, strong Brønsted acidity and shape selectivity in catalytic
transformation of hydrocarbons due to the specific arrange-
ment of channels and cages.[3] On the other hand, the micro-
porous nature of the pore system can result in slow diffusion
of reactant and product molecules, which limits catalytic reac-
tivity and may shorten catalyst lifetime due to increased coking
rates.[4] In order to circumvent these problems, the introduction
of a secondary system of larger pores in the zeolite framework
can be considered. In practice, such hierarchical zeolites contain
usually micropores and mesopores.[4] The additional porosity
can be introduced by modifying existing zeolite crystals (e.g.,
dealumination or desilication) or by direct synthesis using hard
or soft templates. Ryoo and co-workers[5,6] explored diquater-
nary ammonium surfactants (DQAS) as structure-directing
In contrast to these DQAS, a single quaternary ammonium
surfactant cetyltrimethylammonium (CTA+) is a simple organic
chemical. Interest in porous silicas based on CTA salts is rooted
in the discovery of ordered mesoporous MCM-41, formed by
condensation of silica on micellar rod arrays of the surfactant
(e.g., CTA+Br–).[8,9] Beneficial properties of this class of mesopo-
rous silicas are the high surface area, uniform pore size and
high adsorption capacity.[10,11] But, they usually exhibit a low
hydrothermal stability and display low catalytic performance,
which is mainly due to the weak acidity of the protons arising
from introduction of Al in the amorphous silica network.[12]
Hence, many researchers have attempted to crystallize zeolites
using CTA+ with the idea to co-generate mesopores. Unfortu-
nately, this nearly always results in microporous zeolites without
additional mesoporosity.[13–16] Recently, our group presented a
one-step method for synthesizing mesoporous ZSM-5 zeolites
by a CTA+ template using KOH as basic source.[17] The CTA+
surfactant not only generates a secondary mesopore system,
but is also involved as an SDA in the nucleation of MFI zeolite.
It was found that replacing the usual NaOH base with KOH
results in more facile dissolution of silica at elevated tempera-
tures at which zeolite nucleation can occur. The resulting
zeolites contain mesopores that are not uniform nor ordered,
likely because KOH disturbs the formation of micellar rods of
[a] Department of Chemical and Biomolecular Engineering, School of
Energy Science and Engineering, Vidyasirimedhi Institute of Science and
Technology,
Rayong, 21210, Thailand
[b] Inorganic Materials and Catalysis, Schuit Institute of Catalysis,
Department of Chemical Engineering and Chemistry, Eindhoven University
of Technology,
P.O. Box 513, 5600 MB, Eindhoven, The Netherlands
E-mail: e.j.m.hensen@tue.nl
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201900324.
© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. ·
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs. License, which permits use and distri-
bution in any medium, provided the original work is properly cited, the use
is non-commercial and no modifications or adaptations are made.
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CTA+ surfactants.[18,19] In this case, the morphology of the final mesopores in the RbOH-mediated synthesis are much more or-
small zeolite crystals is not related to that of the original meso- dered.
porous silica precursor.
The differences between NaOH and KOH mediated syntheses
can be linked to the rate of silica dissolution, which depends on
the radius of the hydrated cation that stabilized the negatively
charged silica network at high pH.[20] The sodium cation is
known as a water former because of its large hydrated shell of
0.36 nm.[21] In contrast, the larger potassium cation binds water
molecules weaker, forming smaller and more diffuse hydration
shells.[21] When these cations compensate the negative charge
of amorphous silica precursors in zeolite synthesis gels, the
more diffuse water shell for potassium allows for a more direct
access to Si–O-Si bonds than the rigid shell around sodium.
While potassium may give rise to a too high silica dissolution
rate, thereby disrupting already formed micellar rod arrays in
CTA-templated silica precursors,[19] the lower silica dissolution
rate for rubidium with its slightly smaller hydration shell[18,19]
might provide a way to preserve the initial order of the silica
morphology during the transformation of amorphous silica to
crystalline zeolite. Based on this, the goal of this work was to
synthesize uniform mesoporous ZSM-5 zeolites using CTAOH as
a bifunctional template in alkaline RbOH gels. The materials are
extensively characterized for their physico-chemical and acidic
properties. Their catalytic performance was evaluated in the
methanol-to-hydrocarbon (MTH) reaction.
Figure 1. XRD patterns of wide angle (left) and small angle (right) of calcined
zeolites; (a) MCM-41 reference, (b) MFI(C16TA,0 d), (c) MFI(C16TA,1 d), (d)
MFI(C16TA,2 d), (e) MFI(C16TA,4 d), (f) MFI(C16TA,5 d), (g) MFI(C16TA,7 d) and
(h) MFI(C20TA,7 d).
Figure 2. shows Ar physisorption isotherms and pore size
distributions (PSDs) for calcined zeolite samples as a function
of the crystallization time. The corresponding textural proper-
ties are listed in Table S1. The isotherms for the samples ob-
tained after 2, 5 and 7 days crystallization exhibit a typical type
IV shape with a H2 hysteresis loop.[24,25] The steep Ar uptake at
a low pressure (P/P0 < 0.1) is due to the filling of micropores,
while the hysteresis loop at P/P0 > 0.4 points to the presence
of mesopores. According to literature, the H2 hysteresis loop is
characteristic for a narrow size distribution of mesopores.[25]
Results and Discussion
The synthesis procedures are described in detail in the SI. The
crystal structure of the zeolites was determined by XRD. Figure
S1 shows that the XRD patterns of MFI(C16TA, 7 d) before and
after calcination are the same. XRD is also used to determine
the ordering at different length scales of the calcined zeolites
(Figure 1). The XRD patterns indicate that zeolite crystallization
starts after 2 days and nearly fully crystalline zeolite is obtained
after 4 days. Prolonged hydrothermal synthesis did not change
the diffraction patterns. In the small-angle region (Figure 1), a
pronounced feature at 1.940 is visible after 2 days, which grows
and slightly shifts to higher angle during crystallization at
413 K. There is a weak shoulder at 2.960. These peaks relate to
the ordering of silica at the mesoscale, which is confirmed by
the finding that the use of C20TA instead of C16TA results in a
shift to lower angles (Figure 1) with a small loss of zeolite crys-
tallinity as can be seen from Figure 1h. Notably, the XRD pat-
terns of the C16TA- and C20TA-templated samples are different
from the pattern typically observed for MCM-41 (Figure 1). The
evolution of the patterns for MFI(C16TA) during hydrothermal
synthesis implies that mesoscale order of silica was established
before zeolite crystallization and also that this order was main-
tained during zeolite crystallization. Therefore, it is likely that
the mesopores are integrated in the crystalline zeolite sample.
We note however that there is less order than in a typical MCM-
41 sample (Figure S2b).[22,23] We also found that a similar ap-
proach where RbOH was replaced by LiOH, NaOH or CsOH led
Figure 2. Ar physisorption isotherms (left) and pore size distribution (right)
of the calcined ZSM-5 zeolites using C16TAOH and C20TAOH surfactants. The
pore size distributions were calculated by the NLDFT method using the ad-
sorption branch.
The X-ray amorphous sample obtained after 2 days hydro-
to amorphous samples (Figure S3). Compared to our earlier thermal synthesis contained only mesopores with a narrow PSD
work in which we used KOH to obtain hierarchical ZSM-5, the around 2.94 nm (Figure 2). The absence of zeolitic micropores
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in this material is consistent with the XRD patterns. In contrast, hydrothermal conditions, zeolite crystallization occurs after a
MFI(C16TA, 5 d) and MFI(C16TA, 7 d) obtained after longer crys- few days. In a comparable synthesis with NaOH, the amorphous
tallization contained micropores with a size characteristic for precursor is not dissolved, while with KOH most of the precur-
ZSM-5 zeolite. The amount of mesopores increased with crystal- sor dissolves, resulting in a mesoporous zeolite with another
lization time. The PSDs are centered around 3.43 nm. These morphology than obtained with RbOH. The RbOH case is inter-
changes go together with a strong increase of the BET surface mediate in the sense that the texture and morphology retains
area for the calcined materials, which is predominantly caused some of the features of the ordered mesoporous structure of
by the formation of micropores. Thus, the use of C16TA in the the amorphous precursor. The widening of the mesopores
RbOH-based alkaline synthesis mixture not only results in the shows the transformation from amorphous silica to crystalline
formation of uniform mesopores, but also crystalline ZSM-5 zeolite involves some silica dissolution, implying that this syn-
zeolite. We found that the size of the alkyl chain of the SDA thesis does not represent a case of a solid–solid transformation.
affects the mesopore size (Figure S2). Compared to the This is understandable as the zeolite crystal domains observed
C16TA-templated sample, the C20TA-templated zeolite exhibited by TEM are larger than the thin silica walls of the amorphous
a broader peak at a higher diffraction angle (Figure 1), which precursor. We also verified that in a synthesis without CTAOH
can be explained by larger mesopores. This is in line with the only amorphous silica was obtained (Figure S4).
larger on average 4.67 nm pores derived from Ar physisorption
data (Figure 2). The textural properties of the various materials
are more detailed in Table S1.
We then investigated the acid strength of the calcined and
ammonium-exchanged MFI(C16TA,7 d) and bulk ZSM-5 cata-
lysts. CO IR spectra measured at liquid nitrogen temperature
TEM images of materials obtained at various crystallization are shown in Figure 4a. The equal red shift of 316 cm–1 of the
times are shown in Figure 3. The samples differ significantly in bridging hydroxyl groups evidences that both samples contain
morphology. Briefly, the MFI(C16TA,2 d) sample contains regions similar strong Brønsted acid sites. Also the intensities of the
with uniform mesopores (Figure 3a). For the samples obtained relevant bridging hydroxyl groups are similar for both samples.
after crystallization over 5 and 7 days, we observe that there 27Al MAS NMR of the calcined samples confirms that most Al
are significant changes in the morphology. Clearly, rod-shaped atoms are present in the zeolite framework (Figure 4b).
zeolite crystals are formed, which are likely oriented along the
earlier direction of the mesopore channels of the amorphous
precursor materials. The crystalline rod-shaped domains are
separated by mesopores. Inspection of larger areas of the sam-
ple crystallized for 7 days shows that most of the amorphous
mesostructured precursor phase has been transformed into
crystalline zeolite. This conversion is in line with the high micro-
pore volume of 0.12 cm3 g–1 (NLDFT), comparable to the value
recorded for bulk ZSM-5 zeolite, and the absence of a broad
feature in the XRD pattern characteristic for amorphous silica.
Thus, although the precursor morphology is not fully retained,
the final crystalline mesoporous zeolite contains some of the
features of the precursor, as evident from the textural analysis
and TEM.
To illustrate the beneficial effect of the mesopores in the
ZSM-5 crystals, we compared the catalytic performance of the
mesoporous zeolites in the MTH reaction to a bulk ZSM-5 sam-
ple. Besides its practical relevance, this reaction also serves as
a model reaction for evaluating the influence of increased ac-
cessibility of the zeolite domains. In the MTH reaction, meth-
anol is converted to light olefins, mostly C3 and higher olefins,
as well as higher hydrocarbons. Competing is the formation of
coke deposits that block the zeolite micropores and deactivates
the catalyst. This type of coking deactivation is fast for bulk
ZSM-5 zeolite, where methanol diffusion is slow compared to
the reaction rate. At typical conditions (WHSV = 6 h–1; T =
673 K), the initially complete conversion of bulk ZSM-5 zeolite
decreases to 50 % in about 7 h (Figure 4c). The mesoporous
Based on these observations, we propose a mechanism of zeolites deactivate much slower: the time for MFI(C16TA,7 d) to
the formation of mesoporous ZSM-5 zeolites (Scheme 1). Ini- reach 50 % conversion is 36 h. Notably, the mesoporous zeolites
tially, an amorphous silica-surfactant assembly is formed at low display a slightly higher light olefins selectivity and lower aro-
temperature, which contains some ordered mesoporosity that matics selectivity than bulk ZSM-5 (Table S2), which is likely
resembles that in MCM-41. The lower mesoscale order relates due to the reduced average residence time in the mesoporous
to the use of RbOH as a base. When the mixture is brought to zeolite.
Figure 3. TEM images of (a) MFI(C16TA,2 d), (b) MFI(C16TA,5 d), (c) MFI(C16TA,7 d) and (d) HRTEM of MFI(C16TA,7 d) samples.
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Scheme 1. Proposed mechanism of the formation of mesoporous ZSM-5 zeolites: The amphiphilic surfactant self-assembles into micellar rods, with the length
of the hydrophobic tail of the surfactant determining the diameter of the micelles. Simultaneously, amorphous silica-alumina species aggregate around these
micelles by electrostatic interactions. During hydrothermal synthesis, the ammonium head group of CTA+ induces ZSM-5 crystal growth through interactions
with this amorphous silica-alumina phase, under retention of the mesoscale structure. A crucial role is played by Rb+, which compensates the charge of the
negatively charged surface of the amorphous silica-alumina precursor. Compensation by Rb+ instead of Na+ allows for dissolution of sufficient silicate species
to start zeolite nucleation close to the surface of the meso-structured silica-alumina precursor, leading to a final zeolite morphology akin to the mesostructured
precursor. After removal of the organic template by calcination in air, uniform mesopores remain between the obtained ZSM-5 zeolite crystals.Proposed
mechanism of the formation of mesoporous ZSM-5 zeolites.
Figure 4. (a) OH (3800 cm–1 – 3000 cm–1) and CO stretching regions (2280 cm–1 – 2050 cm–1) of IR spectra of bulk ZSM-5 (green) and mesoporous MFI(C16TA,7 d)
(red) zeolites recorded at 77 K. The bolder spectra represent the spectra at the highest CO coverage. (b) 27Al MAS NMR spectra. (c) Methanol conversion of
bulk and mesoporous ZSM-5 zeolites as a function of time-on-stream. The lifetime as defined by the time to reach a methanol conversion of 50 % is indicated.
Conclusions
orders the amorphous silica into a mesophase. The use of Rb
cations modifies the silica dissolution rate in such way that the
We achieved the synthesis of hierarchical ZSM-5 zeolite with mesoporous texture of the amorphous precursor is retained to
uniformly sized mesopores well integrated into the crystalline some extent. The resulting mesoporous zeolite contains acid
zeolite phase. Essential to this unusual synthesis is the combina- sites similar in density and strength as a bulk ZSM-5 reference
tion of RbOH as a base with an amphiphilic surfactant that pre- zeolite. As a result of the high interconnectivity of the micro-
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Received: March 22, 2019
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Zeolite Synthesis
ZSM-5 zeolite was prepared with cetyl-
trimethylammonium (CTA+) as tem-
plate. The use of RbOH for alkalinity
preserves much of the ordered meso-
porous silica-alumina precursor during
its (re-)crystallization. The mesopore
size can be tuned by changing the hy-
drophobic chain length of the surfac-
tant. The product performs better than
bulk ZSM-5 zeolite in the conversion of
methanol to hydrocarbons.
W. Wannapakdee, L. Meng,
A. J. F. van Hoof, A. Bolshakov,
C. Wattanakit, E. J. M. Hensen* ..... 1–6
The Important Role of Rubidium
Hydroxide in the Synthesis of Hier-
archical ZSM-5 Zeolite Using Cetyl-
trimethylammonium as Structure-
Directing Agent
DOI: 10.1002/ejic.201900324
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© 0000 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim