One-pot synthesis of MWW zeolite nanosheets using a rationally designed organic structure-directing agent

Article abstract of DOI:10.1039/c5sc01912e

A new material MIT-1 comprised of delaminated MWW zeolite nanosheets is made in a one-pot synthesis using a rationally designed organic structure-directing agent (OSDA). The OSDA consists of a hydrophilic head segment that resembles the OSDA used to synthesize the zeolite precursor MCM-22(P), a hydrophobic tail segment that resembles the swelling agent used to swell MCM-22(P), and a di-quaternary ammonium linker that connects both segments. MIT-1 features high crystallinity and surface areas exceeding 500 m² g^-1, and can be synthesized over a wide synthesis window that includes Si/Al ratios ranging from 13 to 67. Characterization data reveal high mesoporosity and acid strength with no detectable amorphous silica phases. Compared to MCM-22 and MCM-56, MIT-1 shows a three-fold increase in catalytic activity for the Friedel-Crafts alkylation of benzene with benzyl alcohol.

Full text of DOI:10.1039/c5sc01912e

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One-pot synthesis of MWW zeolite nanosheets
using a rationally designed organic structure-
directing agent†
Cite this: DOI: 10.1039/c5sc01912e
Helen Y. Luo,a Vladimir K. Michaelis,b Sydney Hodges,a Robert G. Griffinb
a
*
and Yuriy Roman-Leshkov
´
A new material MIT-1 comprised of delaminated MWW zeolite nanosheets is made in a one-pot synthesis
using a rationally designed organic structure-directing agent (OSDA). The OSDA consists of a hydrophilic
head segment that resembles the OSDA used to synthesize the zeolite precursor MCM-22(P), a
hydrophobic tail segment that resembles the swelling agent used to swell MCM-22(P), and
di-quaternary ammonium linker that connects both segments. MIT-1 features high crystallinity and
surface areas exceeding 500 m2 ꢀ1, and can be synthesized over a wide synthesis window that includes
a
Received 27th May 2015
Accepted 22nd July 2015
g
Si/Al ratios ranging from 13 to 67. Characterization data reveal high mesoporosity and acid strength with
no detectable amorphous silica phases. Compared to MCM-22 and MCM-56, MIT-1 shows a three-fold
increase in catalytic activity for the Friedel–Crafts alkylation of benzene with benzyl alcohol.
DOI: 10.1039/c5sc01912e
www.rsc.org/chemicalscience
In recent years, layered zeolite precursors have garnered
As such, post-synthetic methods have been developed to
increased attention as a platform for developing new mate- prevent layer condensation and generate exfoliated MWW
rials.1–3 Through post-synthetic modications, these layered nanosheets with a large fraction of exposed cups. Corma et al.
zeolite precursors can be transformed into 2-dimensional (2D), developed ITQ-2 by swelling the layers of MCM-22(P) with a
zeolites with open architectures. These novel hierarchical quaternary ammonium surfactant and then delaminating the
microporous/mesoporous materials with exposed active sites swollen sheets by ultrasonication.19 The calcined material,
can facilitate the conversion of bulky substrates while main- comprised of disordered sheets, featured very high external
taining higher stability than amorphous mesoporous mate-
rials.4–7 An important aluminosilicate layered zeolite precursor
is MCM-22(P)8,9 (isostructural to SSZ-25,10,11 ERB-1,12,13 and
PSH-3 14,15), which forms in single unit-cell thick (ca. 2.5 nm)
layers with the MWW topology. These layers are arranged
perpendicular to the c-axis such that half of the 12-ring cage is
exposed to the crystal exterior, effectively forming “cups” of fully
connected tetrahedral atoms on each side of the layer (see
Scheme 1).16 In contrast to typical surface acid sites, the
Brønsted acid sites located in the cups are as strong as those
located inside micropores.17,18 Unfortunately, upon calcination,
the layers of MCM-22(P) condense topotactically to form the
microporous three-dimensional (3D) zeolite MCM-22 (12-ring
cages connected by 10-ring channels).
aDepartment of Chemical Engineering, Massachusetts Institute of Technology,
Cambridge, MA 02139, USA. E-mail: yroman@mit.edu; Tel: +1-617-253-7090
bDepartment of Chemistry and Francis Bitter Magnet Laboratory, Massachusetts
Institute of Technology, Cambridge, MA 02139, USA
† Electronic supplementary information (ESI) available: Experimental procedures,
tables of synthesis screen results, 1H and 13C liquid and MAS NMR spectra of
OSDA, 31P MAS NMR spectra of TMPO and TBPO titrations. See DOI:
10.1039/c5sc01912e
Scheme
strategy to create MIT-1.
1 Schematic representation of the one-pot synthesis
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surface areas of ca. 700 m2 gꢀ1 and was shown to be active for quaternary ammonium surfactant typically used for the
the cracking of vacuum gas oil,19 decalin and tetralin,20 as well swelling step during post-synthetic delamination. MCM-22(P)
as the isomerization of m-xylene.21 Exfoliation was shown to be can be synthesized using hexamethyleneimine (HMI) or trime-
most effective over a specic set of conditions that include thyladamantylammonium hydroxide (Ada-OH), while swelling
using highly alkaline conditions (pH > 12.5 at 353 K)21 and is typically performed with hexadecyltrimethylammonium
precursor materials with Si/Al ratios >20.22 In search of less bromide (CTAB). As shown in Scheme 1, the novel OSDA, named
damaging post-synthetic treatments, Maheshwari et al. Ada-i-16 (where i ¼ 4, 5, or 6 –CH2– linker groups), has a
demonstrated that the swelling step could be done at room hydrophobic tail segment that resembles CTAB, a hydrophilic
temperature,23 and Maluangnont et al. produced a stable head segment that resembles Ada-OH, and a di-quaternary
colloidal suspension of MWW monolayers without ultra- ammonium linker that connects both segments. The linker’s
sonication.24 Varoon et al. synthesized highly crystalline MWW ammonium composition and chain length were tuned to ach-
nanosheets by melt blending layered precursors to produce ieve an effective C/N+ ratio ranging from 17–18, which is close to
polystyrene nanocomposites,25 while Ogino et al. exfoliated the optimal values (i.e., 10–15) previously identied for high
MCM-22(P) layers using surfactants at pH ¼ 9 without ultra- silica hydroxide syntheses,36,37 and which decreases the risk of
sonication, albeit producing sheets with lower mesoporosity solubility problems for the OSDA in water. We note that
than ITQ-2.26 For borosilicates, Ouyang et al. achieved a single- previous attempts to use surfactants with single ammonium
step delamination and isomorphous substitution of B with Al by moieties to crystallize zeolites have only resulted in ordered
treating ERB-1 with an aqueous aluminum nitrate solution at amorphous materials.38,39 We varied the linker size between 4
408 K.27 However, the resulting material required a nal deal- and 6 –CH2– units because the linker size can affect the mobility
umination step by acid treatment to remove extra-framework and interdigitation of the C16 tails, thereby inuencing the
species.
packing (i.e., unilamellar vs. multilamellar) of the layers.40
Although one-pot synthesis methods are preferable for Molecular dynamics simulations indicate that the structure-
process intensication, they have been largely unsuccessful in directing head sits inside of the cups with the diquaternary
creating materials with comparable properties to those ammonium moieties stabilizing the pore mouth (ESI, Fig. S1†).
obtained with multi-step, post-synthetic methods. For example, Details of the procedure to synthesize Ada-i-16 can be found in
zeolites MCM-56,28–30 ITQ-30,31 and EMM-10 32 exhibit disorder the ESI.†
in the stacking of layers perpendicular to the c-axis, but their
An initial screening to understand the effect of Ada-i-16
low mesoporosity indicates their structure more closely resem- composition on MIT-1 crystallization was performed using a
bles their 3D counterparts. Although very careful control of the synthesis gel of 1 SiO2/0.1 Ada-i-16/0.05 Al(OH)3/0.2 NaOH/45
MCM-56 synthesis has yielded a possible unilamellar structure, H2O at 433 K with rotation at 60 rpm. This gel composition
the material must be delaminated or pillared to maintain high resembles the one typically used to make MCM-22(P); while the
external surface areas aer calcination.33,34 Consequently, the Ada-i-16/Si ratio of 0.1 is similar to that used for the synthesis of
development of an effective and robust method to create high- MFI nanosheets with similar diquaternary surfactants.40
quality MWW nanosheets without additional post-synthetic Varying the linker size drastically affected the crystallization
treatments continues to be challenging.
time (Table S1, ESI†). Specically, fully crystalline MIT-1 was
Here, we demonstrate an effective one-pot synthesis method obtained in 14 and 22 days when using Ada-4-16 and Ada-6-16,
to generate exfoliated single-unit-cell thick MWW nanosheets. respectively. Interestingly, the C5 linker did not yield a crystal-
The new material, named MIT-1, is synthesized using a ratio- line product even aer 30 days. 13C magic-angle spinning
nally-designed OSDA and results in a material with high crys- nuclear magnetic resonance (MAS NMR) on the as-synthesized
tallinity, surface area, and acidity that does not require post- material conrms that the OSDA remains intact in the pores
synthetic treatments other than calcination. The OSDA is (see Fig. S2–S6, ESI†). Increasing the Ada-4-16/Si ratio up to 0.3
comprised of a diquaternary ammonium surfactant with did not affect the synthesis time or phase purity. Decreasing the
tailored structure-directing head, alkyl linker, and hydrophobic temperature from 433 K to 423 K doubled the synthesis time,
tail groups that direct the formation of the MWW topology (see but did not alter the product phase. Adding Ada-OH as a
Scheme 1). A parametric study of Al, Na, and water content co-template at Ada-OH/Ada-4-16 ratios ranging from 0.025 to
reveals that MIT-1 crystallizes over a wide synthetic window. 1 promoted the crystallization of 3D SSZ-13 (CHA topology) aer
Characterization data show that MIT-1 has high mesoporosity short times. Indeed, Ada-OH is a well-known OSDA for the
with an external surface area exceeding 500 m2 gꢀ1 and a high synthesis of SSZ-13 at these temperatures and gel composi-
external acid site density of 21 ꢁ 10ꢀ5 mol gꢀ1. Catalytic tests tions.41 At even lower Ada-OH contents, the co-template did not
demonstrate that MIT-1 has three-fold higher catalytic activity have a noticeable effect, and the MIT-1 phase was observed
for the Friedel–Cras alkylation of benzene with benzyl alcohol exclusively.
as compared to that of MCM-22 and MCM-56.
The powder X-ray diffraction (PXRD) patterns acquired aer
The strategy to design an OSDA that could produce MWW calcination of MIT-1 (synthesized with Ada-4-16) at 813 K for
nanosheets in one-pot is depicted in Scheme 1. Inspired by the 10 h conrm that the sample has the MWW topology (Fig. 1A).
recent work by Ryoo et al.,35 we surmised that a suitable OSDA The diffraction pattern features broader peaks than those
should combine the elements of the traditional OSDA used for observed for MCM-22. More specically, the pattern shows
the synthesis of the MWW layered zeolite precursor and the reections belonging to the (hk0) directions, indicating the
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absence of long-range order in the c-direction, as expected for one unit-cell thick along the c-axis are in good agreement with
exfoliated MWW layers.42 Simulated diffraction patterns the experimental PXRD patterns for MIT-1. Extending the
(obtained using powder pattern theorem for ultrasmall zeolite crystallization time by seven additional days yields a material
crystals implemented with UDSKIP)43,44 for MWW crystalline with additional diffraction peaks, consistent with the morden-
constructs that are 15 unit cells wide along the a- and b-axes and ite (MOR) topology as well as increased condensation of the
nanosheets into the 3D MWW structure as conrmed by
transmission electron microscopy (TEM) (Fig. S7 and S8, ESI†).
MOR impurities have been previously observed in MCM-22(P)
syntheses,45 and condensation of disordered sheets into a
multilamellar structure has been observed for MFI nanosheets
crystallized for longer periods of time.46
Scanning electron microscopy of MIT-1 reveal particles
composed of disordered platelets agglomerated into >10 mm
clusters (Fig. 2). No other morphologies were detected during
low magnication inspections. TEM conrmed the presence of
disordered nanosheets ca. 2.5 nm thick along the (001) direc-
tion and ca. 150 nm (spanning 50–200 nm) long along the (100)
and (010) directions. Selected area electron diffraction perpen-
dicular to the plane of the sheets (inset, Fig. 2) reveals the
expected hexagonal symmetry of MWW topology crystals.
Nitrogen adsorption studies demonstrate that MIT-1 has much
higher mesoporosity than MCM-22 or MCM-56 with a very
broad mesopore size distribution (see Fig. 1C and S9 ESI†). The
total pore volume and external surface area of MIT-1 aer
calcination are 1.014 cm3 gꢀ1 and 513 m2 gꢀ1, respectively (see
Table S2 and Fig. S10, ESI†). This surface area is very close to the
theoretical value of 517 m2 gꢀ1 calculated for 150 ꢁ 150 nm long
and 2.5 nm thick MWW sheets using geometric arguments (see
Table S3, ESI†). In contrast, the total pore volume and external
surface area of MCM-22 are three times lower at 0.289 cm3 gꢀ1
and 121 m2 gꢀ1, respectively. For MCM-56, the total pore volume
and external surface area are two times lower at 0.601 cm3 gꢀ1
and 219 m2 gꢀ1, respectively. A log-plot of the adsorption
isotherms (Fig. 1C, inset) shows that, in the pressure range of
10ꢀ7 to 10ꢀ3 P/P0, MIT-1 has a lower N2 uptake than MCM-22,
which is consistent with the loss of the 10-ring channels asso-
ciated with the 12-ring supercages along the c-axis.26 Taken
together, the characterization data conrm that MIT-1 is a
highly crystalline delaminated MWW material with high surface
area and mesoporosity.
The coordination environment of Al atoms was analyzed by
27Al MAS NMR (see Fig. 1B). MIT-1 with a Si/Altotal ¼ 16.2 (as
quantied from elemental analysis) features mainly tetrahe-
drally-coordinated framework Al species at 55 ppm with only a
small fraction (<8%) of octahedrally coordinated extra-frame-
work Al species present at 0 ppm. Note that the amount of extra-
framework Al increases to ca. 30% aer calcination (see
Fig. S11, ESI†), in agreement with previous reports by Corma
et al. for MCM-22.18 Calcination conditions require further
optimization to minimize dealumination.
The number of internal and external acid sites were inves-
tigated with 31P MAS NMR using trimethylphosphine oxide
(TMPO) and tributylphosphine oxide (TBPO), respectively, as
probe molecules. MCM-22, MCM-56, and MIT-1 have compa-
rable peak signals at 85, 72, 68, and 63 ppm (Table S4 and
Fig. S12, ESI†), which correspond to acid sites present in the
12-ring cages and 10-ring channels of MCM-22.47 These
Fig. 1 PXRD patterns (A) for calcined MCM-22 (a), MCM-56 (b), MIT-1
(c), simulated MWW nanosheets (d). 27Al MAS NMR spectra of
as-synthesized MIT-1 (B). N2 adsorption and desorption isotherms (C)
for calcined MCM-22 (a), MCM-56 (b), MIT-1 (c). Inset shows data on a
semi-log plot.
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Table 1 Reactivity and selectivity data for the Friedel–Crafts alkyation
of benzene with benzyl alcohola
Catalyst
Conversion (%)
Yield DP (%)
Yield DE (%)
MCM-22
MCM-56
MIT-1
40
44
98
49
100
2
19
19
65
23
99
<1
<1
18
20
26
21
<1
<1
2
MIT-1b
MIT-1c
Al-MCM-41
Al-MFI
3
a
Reaction conditions: BA/Al ¼ 200 mol molꢀ1, 6.5 wt% BA in benzene, 3
h, 358 K. b 1.5 h. BA/Al ¼ 100 mol molꢀ1, 5 h.
c
The Friedel–Cras alkylation of benzyl alcohol (BA) with
benzene was used as a model reaction to assess the catalytic
activity of MIT-1. Both the C-alkylation (diphenylmethane (DP))
and O-alkylation (dibenzyl ether (DE)) products are unable to t
inside 10-ring pores, thus limiting catalytic activity to the
external surface for regular 3D zeolites. As shown in Table 1,
MIT-1 converts 49% of BA aer 1.5 h at 358 K with a DP yield of
23%. Nearly full conversion is observed aer 3 h reaction time
with a 65% yield of DP. Aer 5 h, the yield of DP increases to
99% as DE is reversibly converted back to BA, which is C-alky-
lated into DP. In contrast, MCM-22 and MCM-56 only reach 40%
and 44% conversion aer 3 h, respectively, at comparable Al
loadings. Thus, Al-normalized rates at similar conversion levels
show that MIT-1 has a three-fold increase in activity. This
reactivity prole is proportional to the increase in external
surface area and external acid site concentration of MIT-1
compared to MCM-22. Bulk Al-MFI zeolites showed negligible
activity due to their low external surface areas. Al-MCM-41 also
showed low activity, in agreement with Na et al. who showed
that strong Brønsted acid sites are required to catalyze this
reaction.4
Fig. 2 Transmission electron microscopy images of MIT-1 (a and b),
with selected-area diffraction patterns perpendicular to the plane of
sheets (inset). Scanning electron microscopy images of MIT-1 (c and d).
chemical shis are associated with strong Brønsted acid sites as
determined by theoretical calculations between proton affini-
31
ties and P chemical shis.47 Additional peaks at 53, 42, and
31 ppm correspond to TMPO adsorbed onto Lewis acidic extra-
framework Al, physisorbed TMPO, and crystalline TMPO,
respectively. The total number of acid sites were quantied
using spectra integration coupled with elemental analysis,
showing 46, 32, and 33 ꢁ 10ꢀ5 mol gꢀ1 for MCM-22, MCM-56,
and MIT-1, respectively (see Table S2, ESI†). Following the same
procedure, the external acid sites were probed with TBPO (ca.
0.8 nm), which cannot t inside 10-ring channels.48 MIT-1 had
21 ꢁ 10ꢀ5 mol gꢀ1 of external acid sites, which is approximately
three times more surface sites than those of MCM-22 (6 ꢁ 10ꢀ5
mol gꢀ1) and two time more surface sites than those of MCM-56
(13 ꢁ 10ꢀ5 mol gꢀ1). These values correspond well with the
three and two-fold increases in external surface area for MIT-1
compared to MCM-22 and MCM-56.
A parametric study of Al, Na, and water content was
conducted to determine the synthesis window for MIT-1. Table
S5 and Fig. S13 (ESI†) show the resulting phases formed at
different gel compositions. The synthesis space closely mirrors
the space for phase-pure MCM-22(P). At Si/Al ratios below 12,
only amorphous product is observed, while at Si/Al ratios above
70, competing MFI phases are observed. Materials synthesized
in the absence of Al consistently resulted in a disordered MRE
Conclusions
We present the rst one-pot synthesis of MWW zeolite nano-
sheets with high surface area and high crystallinity. Rational
design of OSDAs can be generalized for the synthesis of other
zeolite topologies with open architectures, which are needed to
address new challenges arising from our increasing need to
convert bulky substrates. Current efforts in our group are
focused on investigating other OSDA designs.
Acknowledgements
topology (ZSM-48).49–51 Increasing the NaOH/Si from 0.2 to 0.3 This work was sponsored by the Chemical Sciences, Geo-
decreased the crystallization time from 14 to 7 days. Decreasing sciences and Biosciences Division, Office of Basic Energy
the NaOH/Si to 0.1 increased the crystallization time to 30 days. Sciences, Office of Science, U.S. Department of Energy, under
This modulation of crystallization time likely arises from the Award No. DE-FG0212ER16352. We acknowledge the National
increase in OHꢀ content, since substituting NaCl as a sodium Institutes of Health for funding support of the MIT-Harvard
source resulted in an amorphous product. Increasing the Center for Magnetic Resonance Facility at the Francis Bitter
H2O/Si ratio above 30 did not inuence crystallization, but Magnet Laboratory (EB-002026). VKM is grateful to NSERC and
lower water contents generated only amorphous phases.
the Government of Canada for a Banting post-doctoral
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fellowship. We thank Sean Hunt for useful discussion about 25 K. Varoon, X. Zhang, B. Elyassi, D. D. Brewer, M. Gettel,
elemental analysis procedures. We thank the Center for Mate-
rial Science and Engineering for their microscopy facilities. SH
thanks MIT's Undergraduate Research Opportunities Program
S. Kumar, J. A. Lee, S. Maheshwari, A. Mittal, C. Y. Sung,
M. Cococcioni, L. F. Francis, A. V. McCormick,
K. A. Mkhoyan and M. Tsapatsis, Science, 2011, 334, 72–75.
for funding. We thank Alejandro Krauskopf and Thomas Norris 26 I. Ogino, M. M. Nigra, S.-J. Hwang, J.-M. Ha, T. Rea, S. I. Zones
for their work as undergraduate researchers.
and A. Katz, J. Am. Chem. Soc., 2011, 133, 3288–3291.
27 X. Ouyang, S.-J. Hwang, R. C. Runnebaum, D. Xie,
Y.-J. Wanglee, T. Rea, S. I. Zones and A. Katz, J. Am. Chem.
Soc., 2014, 136, 1449–1461.
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