Comparison of the catalytic properties of MWW zeolite
nanosheets
The catalytic properties of MWW zeolite nanosheets were evaluated in the
alkylation between benzene and 1-dodecene. It was noted from Fig. 4a
that the MWW zeolite nanosheets exhibited a higher conversion than
MCM-22 owing to the enhanced external Brønsted acid sites (Table S2 and
Fig. S6), which originated from the delaminated process that can
generate the MWW nanosheets and expose more accessible Brønsted acid
sites. To verify the external Brønsted acid sites was main active sites
for alkylation between benzene with 1-dodecene since it involves the
bulk molecules, the pyridine and 2,6-di-tert-butylpyridine (DTBP)
poisoning experiment of typical MZN4-14 were performed,
as shown in Fig. S7. It was found that the pyridine poisoning
MZN4-14 totally
lost the activity, and the DTBP poisoning MZN4-14
maintained ~8 % conversion, the conversion dropped by
82 % compared with fresh
MZN4-14,
indicating that the external Brønsted acid sites of MWW zeolites were
main active sites in alkylation since they can be easily accessed
compared with Brønsted acid sites located in the limited micropore
channels. In addition, the conversion of MWW zeolite nanosheets
increased in the order of MZN4-12 <
MZN4-14 < MZN4-16 <
MZN8-14, which was in agreement with concentration of
external Brønsted acid sites as shown in Table S2.
Moreover, the MWW zeolite nanosheets presented a comparable selectivity
of 2-LAB (> 70 %) compared with MCM-22, as listed in Fig.
4b. As is known to everyone, the shape-selective catalysis for zeolites
mainly
depends on zeolite confined environments. Here, the alkylation mostly
occurred on the external surface of MWW zeolites based on the
aforementioned analysis, and MWW zeolite is a class of layer zeolite
containing external isolated 12-MR hemicavities with free internal
diameter of ~0.71 nm, in which the alkylation takes
place under exposed acid sites to produce the
LAB.11 Therefore, the
interaction between LAB isomers with 12-MR hemicavities was calculated
by using Forcite module in Materials Studio in order to reveal why the
MWW zeolite possessed comparable selectivity of 2-LAB regardless of MWW
layers arrangement, as shown in Fig. 5. It was observed that 2-LAB had
the highest binding energy and
the
best matched steric configuration with 12-MR cage located on MWW
lamellar surface than other isomers, indicating that 2-LAB can be
steadily formed in the external confined 12-MR hemicavities of MWW
zeolites, resulting in high selectivity of 2-LAB. With the reaction time
increasing, the thermodynamic equilibrium mixture of LAB isomers was
formed by reverse alkylation and isomerization
steps,26 leading to the
selectivity of 2-LAB decreasing with reaction time increasing.
Fabrication of
regularly
spaced MWW zeolite
nanosheets
Comprehensive analysis of the above data, it was found that tuning the
tail length of long chain quaternary ammonium surfactant was not an
appropriate strategy to effectively delaminate the MWW zeolites to
produce the ultrathin MWW zeolite nanosheets. When excessive surfactant
was used, it can produce the mono-layer MWW zeolites, but the nucleation
of MWW zeolite was disrupted on a certain degree, resulting in low
crystallinity (Fig. 1 and Table S1). For alkylation of benzene and
1-dodecene, the external Brønsted acid sites were main active sites.
Thus, it’s necessary to develop a strategy to fabricate the ultrathin
MWW zeolite nanosheets without
destroying
their crystallinity in order to maximize the accessible Brønsted acid
sites. The desired ultrathin MWW
zeolite nanosheets not only have the single MWW unit cell thickness but
also possess enough stability that can efficiently inhibit the layers
stacking. Naturally, the pillaring ultrathin MWW zeolite nanosheets was
an ideal approach to produce ordered MWW lamellas that can satisfy the
above requirements.
The structure of regularly spaced MWW zeolite nanosheets was identified
by PXRD technology, as presented in Fig. 6. It was noted that both of
PMWW(31) and PMWW(74) exhibited the (001) reflections in the low-angle
region of 1-5˚, and the corresponding (002) and (003) can be found in
PMWW(74), indicating that the resultant PMWW(74) possessed the
characteristic of long-rang ordered structure even after multi-pillaring
steps.16 For
PMWW(31),
the second-order reflection was missing due to mismatch between the
thickness of a single MWW nanosheet and one-half of the adjacent-layer
space,27 which
demonstrated that the PMWW(31) maintained the long range order, but the
degree of long range order was inferior compared with PMWW(74). By
comparation, the MCM-22 didn’t show any reflection in the low angle
region of 1-5˚ owing to its typical 3D structure. In addition, the (101)
and (102) reflections were obviously visible in PMWW(74) and PMWW(31),
which was different from the previous MCM-36 material that the (101) and
(102) reflections were transformed into a broad peak due to the partial
loss of the vertical alignment order of MWW layers along thec -axis.19 The
presence of these reflections indicated that the ordered lamellar
structure of MWW layers with the vertically aligned layers being ordered
perpendicular to the c axis in PMWW(74) and PMWW(31) was well
maintained even after
pillaring,28-30 which
was also the typical characteristic of long-range order of MWW
nanosheets in thec -direction.31Therefore, the regularly spaced MWW zeolite nanosheets with long range
order were fabricated based on the analysis of low-angle and high-angle
PXRD data. Moreover, the only difference between PMWW(31) and PMWW(74)
was discrepant Si/Al ratios, PMWW(31) had the same Si/Al ratio with
aforesaid MWW zeolite nanaosheets in the synthesis of MCM-22(P) that was
the precursor for fabrication of pillared MWW zeolites, and PMWW(74)
increased the Si/Al ratio.
For silica pillaring process, it can be performed through formation the
Si-O-Si bonds between silanol groups from the MWW layers with silica
pillar species,16 and
the high Si/Al ratio of MWW layers can definitely influence the
concentration of silanol groups located on the external surface of MWW
layers.32 It’s
postulated that the MWW layers with high Si/Al ratio have sufficient
opposing silanol groups in the neighboring layers, which is benefit to
formation of ordered silica pillars. While the
Si/Al
ratio of MWW layers decreases, there is an increased probability of Al
atoms becoming incorporated at external surface sites on the MWW layers,
it reduces the concentration of terminal silanol groups in the adjacent
layers, and further disrupts the pillaring process on a certain degree
due to the slight deviations or mismatch between the position of silanol
groups on each MWW
layers,15 resulting in
the low degree of long range order for PMWW(31), and the proposed
influence of Si/Al ratio on the pillaring process was illustrated in
Scheme 1.
The pillared MWW zeolites were further investigated by
N2 adsorption experiments, it was observed from Fig. S8a
that PMWW(31) and PMWW(74) exhibited an increased adsorption atP/P0 > 0.4, indicating the presence
of mesoporosity caused by pillaring process, which was in accordance
with previous report.16By comparison, MCM-22 achieved a saturation at a relative pressure of
0.1 owing to the sole micropores. The corresponding NLDFT (nonlocal
density functional theory) pore size distribution of PMWW(31) and
PMWW(74) (Fig. S8b) also presented a substantial increase in the range
of (1.2~3.3 nm) as a result of pillaring. Moreover, the
textural data (Table S1) further corroborated that PMWW(31) and PMWW(74)
had increased Sext/SBET andVmeso/Vtot owing to the effective
pillaring procedure. In addition, the PMWW(31) and PMWW(74) showed a
thin flake-like morphology with layers stacked in a lamellar arrangement
(Fig. 7), and the framework structure of PMWW(31) and PMWW(74) were
further investigated by TEM images, as shown in Fig. 8 and Fig. S9. It
was noted from Fig. 8 that both PMWW(31) and PMWW(74) exhibited an
ordered multilamellar architecture with a vertical distance of
~1.8 nm, the interlayer distance (~1.8
nm) between the MWW nanosheets was almost equal to the theoretical
length of MTAB surfactant (~1.75
nm),18 which was also
in agreement with previous XRD data (~1.6 nm), and the
thickness of regularly spaced MWW nanosheets was ~2.5 nm
corresponding to one unit cell of MCM-22 in the c -axis. However,
the multilamellar arrangement of PMWW(31) presented a litter deviation
compared with PMWW(74) due to the low Si/Al ratio, which was in
accordance with above analysis.