Synthesis of MWW zeolite nanosheets
The dual template synthesis strategy with HMI and long chain quaternary
ammonium surfactant was used to delaminate the MWW zeolites, which can
expect to generate MWW zeolite nanosheets and expose more exterior
Brønsted acid sites for alkylation. It was reported that
cetyltrimethylammonium bromide (CTAB) could act as a positive counterion
role to balance negative charge of framework Al atoms at surface sites
on the MWW zeolite lamellas in the zeolite synthesis
gel,15 the accumulation
of long chain quaternary ammonium surfactants on exterior surfaces can
impede layer−layer interactions owing to the tangling effect of long
hydrophobic tail of surfactant, resulting in the separation between MWW
layers. Herein, the long chain quaternary ammonium surfactants with
different tails such as dodecyl trimethylammonium bromide (DTAB),
myristyltrimethylammonium bromide (MTAB) and
cetyltrimethylammonium
bromide (CTAB) were employed to tune the degree of delaminated MWW
nanosheets based on discrepant long tails that can generate the various
tangling effect due to the steric
hindrance.18 In
addition, the excess surfactant was added into synthesis gel to ensure
that the sufficient coverage of incorporated surfactant on the exterior
surface of MWW layers was achieved in order to effectively impede layer
alignment.
Fig. 1 gave the result of powder x-ray diffraction (PXRD) patterns of
multiple-layer and mono-layer MWW zeolite nanosheets, which was commonly
used to evaluate the arrangement of MWW layers in the 2θ range of
6-10˚.19 As shown in
Fig. 1, MCM-22 presented three peaks at ~7.2˚, 8.0˚ and
10.0˚, corresponding (100), (101) and (102) reflections, indicating
ordered staking of MWW
layers.20 When the long
chain quaternary ammonium surfactant was added into the gel, these
samples such as
MZN4-12,
MZN4-14 and MZN4-16 also exhibited the
peaks corresponding (101) and (102) reflections, but the bands assigned
to the (101) and (102) peaks became broad, suggesting that the degree of
long-rang order of MWW layer along the c axis was decreased and
the multilayered structure was
formed.21 Besides, when
the addition of MTAB was increased to the 8.0 wt %, the intensity of
(101) and (102) reflections for MZN8-14 was obviously
decreased, and these reflections were transformed into a broad peak
owing to disordered arrangement of MWW nanosheets along the cdirection and the mono-layer MWW zeolites may be
produced.15 In
addition, for all the multiple-layer and mono-layer samples, the (100)
reflection can be observed, indicating that crystalline structure of MWW
framework was
maintained.22 Moreover,
the (002) reflection can be observed in the as-synthesized
MZN4-12, MZN4-14 and
MZN4-16 (Fig. S1) owing to the presence of interlayer
structure.15 As the
concentration of surfactant was increased to 8.0 wt % in synthesis gel,
the (002) reflection of MZN8-14 dramatically decreased,
indicating the loss of long-range order along the c -axis (Fig.
S1).
To further investigate the delaminated degree of MWW zeolite nanosheets,
the N2 adsorption/desorption experiments were carried
out. It was observed from Fig. S2 that MCM-22 and
MZN4-12,
MZN4-14 and MZN4-16 presented a
characteristic of micropore materials at P/P0< 0.9, while the sharp increase in the relative pressure
ranges above 0.9 was attributed to the meso/macropores formed by MWW
layers stacking.23 For
MZN8-14, it exhibited a typical type-Ⅲ isotherm with an
obvious hydrolysis loop in the range of P/P0> 0.6, indicating the presence of mesopores arised from
intergrown and stacking of MWW
nanosheets.21 Table S1
summarized the textural parameters of resultant MWW zeolites, it was
noted that the micropore volume of MWW zeolite nanosheets was lower than
MCM-22, which was reasonably attributed to the fact that the MWW zeolite
nanosheets destroy the conventional stacking of MWW layer as like 3D
order in MCM-22, resulting in the loss of 10-membered ring (MR) channels
and 12-MR supercages along the caxis.15 However, the
MWW zeolite nanosheets showed an enhanced fraction of mesoporous surface
or volume compared with MCM-22 owing to the delaminated effect, which
was expected to expose more accessible Brønsted acid sites. In addition,
the concept of disorder index as
ratio between the external and total surface areas was employed to
identify the delaminated degree of MWW layers following Rimer et al’s
report,15 it was found
that the degree of disorder for MWW zeolite nanosheets increased in the
order of MZN4-12
< MZN4-14 < MZN4-16
< MZN8-14, indicating that the delaminated
degree of MWW zeolite nanosheets can be tailored by tuning the length of
long tail in surfactants due to distinct steric hindrance. Specially,
when the amount of MTAB was increased to 8.0 wt %, it greatly improved
the separation of MWW layers since the sufficient surfactant can graft
on every single MWW layer, which can inhibit the inductive effects
between HMI with surface
silanols,24 leading to
the formation of mono-layer and presence of the highest disorder index.
The morphology of MCM-22 and MWW zeolite nanosheets was investigated by
scanning electron microscope (SEM) images, as shown in Fig. 2 and
Fig.
S3. The MCM-22 showed bird’s nest-like shape (Fig. 2a), this was typical
morphology of
non-agitated
MCM-22 sample.25 While
the MZN4-12,
MZN4-14 and MZN4-16 exhibited a thick
plate-like morphology (Fig. S3). Particularly,
MZN8-14 presented an
interpenetrating network of nanosheets with ultrathin flake-like
morphology (Fig. 2b), demonstrating that the additional surfactant can
impede MWW layer alignment act as an exfoliating agent, and increasing
the amount of surfactant was a more effective method to achieve the
delaminated MWW zeolite with ultrathin nanosheets compared with
tailoring the tail length of surfactants.
The delaminated nature of resultant MWW zeolite nanosheets was further
investigated by transmission electron microscope (TEM) images, as
depicted in Fig. 3 and Fig. S4. It was observed from Fig. S3 that the
MZN4-12, MZN4-14 and
MZN4-16 exhibited the discrete lamellar feature, but the
thickness of these MWW layers was still thick, indicating that the
additional surfactant can delaminate the 3D MWW zeolite to generate the
lamellar structure on a certain degree. Specially, the
MZN8-14 present an
ultrathin nanosheets morphology (Fig. 3a-c) compared with conventional
3D MCM-22 with a thickness of ~23 nm (Fig. 3e-f), and
the thickness of the MWW zeolite nanosheets was measured by atomic force
microscope (AFM), which showed a uniform nanosheet thickness of 2.5 ±
0.4 nm (Fig. 3d and Fig. S5), the average content of ultrathin
nanosheets estimated from AFM images was about 73 ± 5%, suggesting that
the mono-layer MWW nanosheets with single-unit-cell thickness was
produced.22