Enhanced Surface Activity of MWW Zeolite Nanosheets Prepared via a One-Step Synthesis

Article abstract of DOI:10.1021/jacs.9b13596

The synthesis of two-dimensional (2D) zeolites has garnered attention due to their superior properties for applications that span catalysis to selective separations. Prior studies of 2D zeolite catalysts demonstrated enhanced mass transport for improved catalyst lifetime and selectivity. Moreover, the significantly higher external surface area of 2D materials allows for reactions of bulky molecules too large to access interior pores. There are relatively few protocols for preparing 2D materials, owing to the difficultly of capping growth in one direction to only a few unit cells. To accomplish this, it is often necessary to employ complex, commercially unavailable organic structure-directing agents (OSDAs) prepared via multistep synthesis. However, a small subset of zeolite structures exist as naturally layered materials where postsynthesis steps can be used to exfoliate samples and produce ultrathin 2D nanosheets. In this study, we selected a common layered zeolite, the MWW framework, to explore methods of preparing 2D nanosheets via one-pot synthesis in the absence of complex organic templates. Using a combination of high-resolution microscopy and spectroscopy, we show that 2D MMW-type layers with an average thickness of 3.5 nm (ca. 1.5 unit cells) can be generated using the surfactant cetyltrimethylammonium (CTA), which operates as a dual OSDA and exfoliating agent to affect Al siting and to eliminate the need for postsynthesis exfoliation, respectively. We tested these 2D catalysts using a model reaction that assesses external (surface) Br?nsted acid sites and observed a marked increase in the conversion relative to three-dimensional MWW (MCM-22) and 2D layers prepared from postsynthesis exfoliation (ITQ-2). Collectively, our findings identify a facile and effective route to directly synthesize 2D MWW-type materials, which may prove to be more broadly applicable to other layered zeolites.

Full text of DOI:10.1021/jacs.9b13596

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Article
Enhanced Surface Activity of MWW Zeolite
Nanosheets Prepared via a One Step Synthesis
Yunwen Zhou, Yanyu Mu, Ming-Feng Hsieh, Bernd Kabius, Carlos
N Pacheco, Carol Bator, Robert M. Rioux, and Jeffrey D Rimer
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b13596 • Publication Date (Web): 11 Apr 2020
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Enhanced Surface Activity of MWW Zeolite Nanosheets Prepared via
a One Step Synthesis
Yunwen Zhou1, Yanyu Mu2, Ming-Feng Hsieh1, Bernd Kabius2, Carlos Pacheco3, Carol Bator4, Robert
M Rioux2,3,*, and Jeffrey D. Rimer1,*
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1 Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204, USA
2 Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16801, USA
3 Department of Chemistry, The Pennsylvania State University, University Park, PA 16801, USA
4 Huck Institutes of Life Sciences, The Pennsylvania State University, University Park, PA 16801, USA
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ABSTRACT: The synthesis of 2-dimensional (2D) zeolites has garnered attention owing to their superior properties for applications that
span catalysis to selective separations. Prior studies of 2D zeolite catalysts demonstrated enhanced mass transport for improved catalyst
lifetime and selectivity. Moreover, the significantly higher external surface area of 2D materials allows for reactions of bulky molecules too
large to access interior pores. There are relatively few protocols for preparing 2D materials owing to the difficultly of capping growth in one
direction to only a few unit cells. To accomplish this, it is often necessary to employ complex, commercially-unavailable organic structure-
directing agents (OSDAs) prepared via multistep synthesis. However, a small subset of zeolite structures exist as naturally layered materials
where post-synthesis steps can be used to exfoliate samples and produce ultrathin 2D nanosheets. In this study, we selected a common layered
zeolite, the MWW framework, to explore methods of preparing 2D nanosheets via a one-pot synthesis in the absence of complex organic
templates. Using a combination of high-resolution microscopy and spectroscopy, we show 2D MMW-type layers with an average thickness
of 3.5 nm (ca. 1.5 unit cells) can be generated using the surfactant cetyltrimethylammonium (CTA), which operates as a dual OSDA and
exfoliating agent to affect Al siting and eliminates the need for post-synthesis exfoliation, respectively. We tested these 2D catalysts using a
model reaction that assesses external (surface) Brønsted acid sites and observed a marked increase in the conversion relative to 3-dimensional
MWW (MCM-22) and 2D layers prepared from post-synthesis exfoliation (ITQ-2). Collectively, our findings identify a facile and effective
route to directly synthesize 2D MWW-type materials, which may prove to be more broadly applicable to other layered zeolites.
cups of MWW-type zeolite exhibit strong acidity, similar to
INTRODUCTION
sites within the micropores,20 which has generated interest for
Zeolites are widely used as absorbents and heterogeneous
their use in catalytic reactions involving bulky molecules with
catalysts in a number of industrial applications.1-7 The
micropores of zeolites, among other unique properties such as
steric restrictions for accessing interior cages/channels such
as Friedel–Crafts alkylation21-22
,
hydrocracking23, and
their (hydro)thermal stability and tunable acidity, provide
confined environments for molecular reactions, leading to
shape-selective catalysis with products and associated
transition states determined by the cage and channel
geometries of a particular crystal structure.4, 8-10 Zeolites with
small-to-medium sized pores (3 ~ 7 Å) often encounter mass
transport limitations with limited access of bulky
molecules.11-14 Two-dimensional (2D) zeolites, on the other
hand, possess properties that enhance mass transport within
pores while providing accessible acid sites on external
surfaces for molecules sterically restricted from accessing
internal sites.15-17 MCM-22 (MWW type) is one of the most
investigated 2D zeolites due to its unique crystallographic
structure comprised of stacked, thin layers (ca. 2.5 nm
thick).18-19 The MWW layers have thicknesses equal to a
single unit cell dimension, and are aligned perpendicular to
the c-axis (Scheme 1). The internal pores of MWW are
comprised of supercages (boxed area of Scheme 1A)
connected by 10-membered ring (MR) apertures. These cages
are surrounded by sinuosoidal channels with narrower 10-MR
openings located in close proximity to each corner of the
supercage. The exterior surfaces of MWW layers are
comprised of 12-MR pockets (or cups) equal to approximately
one-half the volume of a supercage. Distinct from surface acid
sites on many zeolite catalysts, the acid sites in the external
acetalization of aldehydes24. The MWW structure is
comprised of eight unique tetrahedral (T) sites, which are
illustrated in Scheme 1B and C.
Post-synthetic approaches have been developed to
exfoliate MCM-22P, where P is used to denote layered
precursors that are not covalently connected. Calcination of
the
Scheme 1. MWW framework with its network of pores consisting of (A)
12-MR pockets on the exterior surface, 10-MR windows accessing
internal supercages (indicated by the black box), and narrower 10-MR
pores that give access to sinusoidal channels. (B) Top down view of an
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external pocket with unique T sites for Al incorporation, and (C) side view
along the a-direction (90 degrees relative to panel B).
In this study, we report a one-pot synthesis of disordered
MWW zeolite using a conventional route that is modified
through the addition of commercial surfactant,
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precursors leads to the direct condensation of MWW layers,
which can be avoided via post-synthesis processes to exfoliate
the layers, thereby producing disorder nanosheets with a high
surface area exposing 12-MR pockets. A notable example is
ITQ-2 prepared by swelling MCM-22P with an exfoliating
agent (e.g. surfactant) and subsequently exfolieated via
sonication.25 Calcined ITQ-2 exhibits very high external
surface area (~700 m2/g), indicating the material is composed
of disordered MWW layers.26-27 It has been reported that ITQ-
a
cetyltrimethylammonium (CTA). Our findings reveal CTA
acts as a secondary OSDA to alter Al siting without impacting
the overall Si/Al ratio of the final product. Parametric studies
of growth conditions identify regions of growth solution
composition resulting in disordered MWW. Textural analysis
reveals the final product exhibits a degree of disorder and
corresponding external surface area comparable to ITQ-2,
while solid state 27Al NMR reveals materials prepared by
direct synthesis contains fewer Al defect sites. We also report
the catalytic performance of these materials relative to 3D
MWW (MCM-22) using a model reaction to verify that higher
surface area leads to greater access to external acid sites (i.e.,
greater initial conversion), while the intrinsic activity (when
normalized by the number of external acid sites) of the
disordered MWW is identical to MCM-22. Collectively, these
studies highlight a potentially new, facile route to generate 2D
zeolites from naturally layered frameworks that may prove
relevant to a broader class of microporous materials.
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shows superior catalytic activity as compared to
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conventional MCM-22 zeolites in various applications, such
as cracking of vacuum gas oil.25 The original procedure and
conditions used to prepare ITQ-2 involved the use of harsh
conditions (i.e., high pH and temperature), leading to partial
amorphization.28 Several methods were established since the
initial reports of ITQ-2 synthesis to overcome these problems.
For example, post-synthesis swelling can be performed at
room temperature (compared to 353 K).29 Similarly, the
separation of MWW layers can be achieved without the use of
sonication.30 Potential amorphization of MWW layers can be
avoided when operating under pH 9 and via the use of
tetraalkylammonium ions to avoid energy-intensive
sonication.31 Roth et al. obtained the first pillared zeolitic
material, MCM-36, where swollen layers are intercalated by
silica species.32 Recently, Liu et al. developed a novel vapor-
phase pillarization (VPP) process to produce pillared 2D
zeolite materials with ~100% efficiency.33 Interestingly, post-
synthetic processes have been extended to borosilicates,
where single-step exfoliation along with isomorphous
substitution of boron with aluminum has been demonstrated
on ERB-1 (MWW type; borosilicate counterpart).34
Exfoliation has also been used in a related processes referred
RESULTS AND DISCUSSION
Preparation of disordered MWW-type nanosheets. We use
a modified protocol for conventional MCM-22 synthesis that
employs hexamethyleneimine (HMI) as an ODSA. To the
growth mixture we add a commercially available quaternary
ammonium surfactant, cetyltrimethylammonium (CTA).
Established post-synthesis methods (top-down approaches) of
MCM-22 exfoliation employ CTA as an exfoliating (or
swelling) agent, where it is presumed that the hydrophobic
tails of the surfactant aid in the separation of layers. Here, we
use a bottom-up approach where the surfactant, with its
positively-charged head group, has the ability to associate
with negatively-charged aluminates in solution and/or Al
tetrahedral sites within the MWW framework, thus creating
opportunities for CTA to act as a secondary OSDA. The
primary OSDA (HMI) is neutral in alkaline growth mixtures,
but prior studies have indicated HMI can adopt a positive
charge in confined channels of MWW zeolite, thereby acting
as an extra-framework cation to counterbalance the negative
charge of Al framework sites.52 Through parametric analysis
of synthesis mixtures prepared with a combination of HMI
and CTA, we explore the putative role of the latter as a
cooperative OSDA and dual exfoliating agent in the synthesis
of MWW type materials.
Powder X-ray diffraction (PXRD) patterns were used to
validate the successful synthesis of MWW type materials and
qualitatively assess differences in the degree of layer disorder,
which was specifically evaluated within the range 2θ = 6 ‒ 10°
based on common practice in literature47 to differentiate
interlayer distances along the c direction. As-synthesized
MCM-22 (Figure i) contains four peaks in this range. The
position of the (002) inter-layer reflection, purely related to
the layer thickness in the c direction, is located at 6.5°. Due to
the presence of inter-layer reflections, (101) and (102), two
discrete peaks appear at 7.8 and 9.6°, respectively. After
calcination, the (002) reflection of the MCM-22 sample
(Figure 1, i-c) shifts to 7.0° and overlaps with the (100)
reflection due to condensation of individual layers. As the
CTA concentration in the growth mixture is increased to 5.5
wt% (Figure 1, ii) and 8.0 wt% (Figure 1, iii), the intensity of
the (002) peak correspondingly decreases and the bands
to
as
ADOR
(assembly-disassembly-organization-
reassembly) to construct new frameworks.35-39 In these
processes, building units consisting of a chemical weakness
(i.e. Ge heteroatoms) are used to exfoliate (or disable) and
then reassemble via silica bridges to construct new
frameworks that are otherwise difficult to prepare directly.
Moreover, layered zeolites can be used as seeds for the
topotactic conversion between different 2D zeolitic
precursors through
a
3-dimensional germanosilicate
intermediate.40
It is highly desirable from both an economic and practical
standpoint to develop one-pot synthesis methods to yield
zeolite MWW nanosheets with large external surface areas,
while simultaneously avoiding loss of structure that is often
the result of post-synthesis treatments. To our knowledge,
there are few strategies reported in literature to achieve
disordered MWW-type materials with properties similar to
ITQ-2. Several approaches that come close include the
preparation of UZM-8,41 MCM-56,42-46 EMM-10,47 and ITQ-
30.48 Despite the fact these materials exhibit some degree of
disordered MWW layers, their relatively low mesoporosity
indicates MWW layers are not highly separated with respect
to a fully exfoliated MWW-type zeolite. Multiple groups have
reported49-51 the preparation of MWW-type materials with
single layers using a one-step method; however, these
methods required the use of custom organic structure-
directing agents (OSDAs) that were not commercially
available, and required multi-step synthesis. To this end, it has
remained elusive to design a one-step synthesis of exfoliated
MWW type zeolite through more practical pathways.
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assigned to the (101) and (102) peaks gradually coalesce into
channels and 12-MR supercages along the c direction (see
Scheme 1) with an increasing percentage of disordered layers.
Textural details and statistically-relevant measurements
of the thickness of MCM-22 and d-MWW were assessed with
cryogenic transmission electron microscopy (cryoTEM). The
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a single broad peak. Both changes reflect the loss of long-
range order in the c direction. When the quantity of CTA is
further increased (e.g. 10 wt%),
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Figure 1. PXRD patterns of MWW type materials (gel Si/Al = 15)
prepared with (i) 0, (ii) 5.5, and (iii) 8 wt% CTA. The top (grey) lines are
as-synthesized solids extracted from growth solutions, whereas the
bottom (black) lines are the corresponding calcined (c) materials. The
MWW framework has a hexagonal crystal structure (P6/mmm) with unit
cell parameters a = 1.421, b = 1.421, and c = 2.495 nm.53
Figure 2. Scanning electron micrographs of calcined MWW type
materials (gel Si/Al = 15): (A and B) MCM-22; (C and D) d-MWW8.0
.
All scale bars are equal to 1 μm.
the PXRD pattern reveals a purely amorphous phase (Figure
S1), thus indicating an upper limit of CTA that seemingly
disrupts the nucleation of the MWW zeolite. For calcined
samples prepared with CTA concentrations below this
threshold (Figure 1, ii-c and iii-c), the single broad band at 2θ
= 7.8 ‒ 9.6° is preserved, which confirms the disordered layer
arrangement of CTA-derived materials (herein referred to as
d-MWWx where x = weight percentage of CTA).
collection of TEM images with the MWW samples cooled to
liquid nitrogen temperatures enabled high-resolution imaging
and long-time imaging of single samples to collect data over
hundreds of nanosheets to quantify average thickness
statistics. Contamination and radiation damage were
suppressed significantly due to the low temperature and the
use of the Falcon 3 direct electron detector, which allowed
observation at low electron dose. Figure 3 is a summary of
images collected with cryoTEM for the d-MWW8.0 sample.
Figure 3A is a bright field high-resolution TEM (HRTEM)
image collected at liquid nitrogen (LN2) temperature of d-
MWW8.0 zeolite sheets randomly oriented. The sheets, which
are visualized “edge-on” ([001] direction perpendicular to the
electron beam), appear dark and narrow. Figure 3B shows two
of these sheets at higher magnification. The one on the left-
hand side is a single unit cell thick, while the sheet on the right
is two unit cells thick. Both high resolution images of the
zeolite crystal structure in Figure 3B are along a low indexed
zone axis, type (xy0). A small deviation (~1°) from the
observed zone axis would destroy the high resolution pattern.
Therefore, these sheets are oriented nearly edge-on, which
makes thickness measurements of the sheets accurate. The
brightest areas in Figure 3A are “flat” sheets where the (001)
normal is oriented parallel to the electron beam. A flat area at
higher magnification (Figure 3C) shows a high-resolution
image of the crystal structure of MWW zeolite in the (001)
orientation. In Figure 3D, a model of the zeolite structure is
superimposed on a detail from Figure 3C under the
conventional assumption that the bright areas are identical
with the cages in the zeolite. A fast Fourier transform (FFT)
of a single crystalline portion
To further investigate the degree of disorder of MWW
type materials, scanning electron microscopy (SEM) and
nitrogen
adsorption/desorption
measurements
were
performed. Scanning electron micrographs of MCM-22
(Figure 2A and B) show aggregates of plate-like crystals. In
comparison, the disordered material d-MWW (Figure 2C and
D) is an interpenetrating network of nanosheets with apparent
uniform thickness, consistent with PXRD patterns indicating
CTA alters crystal morphology. Textural analysis (Table 1)
can be used to quantify the degree of disorder for which we
define a disorder index as the ratio between the external and
total surface areas. We introduce this parameter here as an
approximate measure of MWW layer exfoliation. The
addition of CTA from 0 to 8 wt% increases the disorder index
from 0.2 to 0.6, respectively, thus indicating CTA
quantitatively improves the separation of layers in MWW type
materials. Nitrogen physisorption on calcined MCM-22
reveals a type I isotherm (Figure S4) whereas d-MWW
samples exhibit a type IV isotherm. At the highest
concentration of CTA (8 wt%), the isotherm is characteristic
of a mesoporous material with a sharp increase in N2 uptake
at higher relative pressures and a large hysteresis loop,
indicating the presence of high external surface area and
mesoporosity (consistent with SEM images in Figure 2). At
low relative pressure in the isotherms (i.e. the micropore
filling region), the d-MWW materials prepared with 5.5 and 8
wt% CTA exhibit lower N2 uptake compared to MCM-22.
This is expected due to the loss of 10-membered ring (MR)
Table 1. Properties of MWW materials produced with and without CTA
b
b
BET SA
(m2/g)
External
Vmicro
sample
DIa
SAb (m2/g)
(ml/g)
MCM-22
0.2
634
558
149
164
0.19
d-MWW5.5 0.3
0.16
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