Initial optimization of the catalytic performance
Fig. 9 exhibited the catalytic performance of PMWW(31) and PMWW(74) in
the liquid alkylation between benzene with 1-dodecene. As shown in Fig.
9a, PMWW(31) presented the comparable conversion of 1-dodecene compared
with MZN4-14 since they had the similar concentration of
external
Brønsted
acid sites (Table S2). While PMWW(74) showed the best activity in all of
investigated MWW zeolites due to its largest concentration of external
Brønsted acid sites (Table S2, Fig. S10), and the fraction of external
Brønsted acid sites for PMWW(74) achieved 88.5 %, indicating that the
optimized pillaring process was an effective method to expose the
accessible Brønsted acid sites. Moreover, the 27Al
multiquantum magic angle spinning (MQMAS) NMR spectroscopy was further
conducted to reveal the potential differences in the spatial
distribution of Al sites between PMWW(31) and
PMWW(74)
samples, as shown in Fig. 10.
The framework of MWW zeolite was composed of eight different T sites and
three types of pores: 12-MR surface pockets, 10-MR windows accessing
internal supercages and 10-MR sinusoidal channels, which was depicted in
Fig. S11. It was noted from Fig. 10 that both PMWW(31) and PMWW(74)
exhibited two peaks at around ~56 ppm and 0 ppm, which
was assigned to the 4-coordinated framework (AlF) and
6-coordinated extra-framework aluminum (AlEF) species,
respectively.33According to previous
study,34 the broad peak
around ~56 ppm can be resolved into three overlapped
signals at 50, 56, and 61 ppm, which was attributed to
T6+ T7 (δ = 50 ppm), T1+ T3 + T4 + T5 +
T8 (δ = 56 ppm) and T2 (δ = 61 ppm),
respectively. Following this way, the broad peak around
~56 ppm was deconvoluted, as presented in Table S3 and
Fig. 10. It was observed that the proportion of AlF(50)species for PMWW(31) was about 49 % (the combination of δ = 50 and δ =
51), indicating the AlF species mainly located in the
supercages and surface
pockets,35 in which the
alkylation occurred as a result of accessible Brønsted acid sites arised
from the AlF(50) species. While the
AlF(61) species for PMWW(31) was missing, which
demonstrated that proportion of AlF(61) species in
PMWW(31) was negligible. However, AlF(61) species were
related to T2 sites that located on the exterior surface
of MWW nanosheets close to the outer edge of the 12-MR pockets, and the
T2 sites was the most accessible to bulky
adsorbates.15 In this
point of view, it was reasonable that PMWW(74) had an advantage over
PMWW(31) in the alkylation due to the presence of considerable
AlF(61) species (~13 %). In addition,
PMWW(31) and PMWW(74) possessed the similar quantity of
AlEF(0) species. Fig. 9b showed that PMWW(31) and
PMWW(74) had comparable selectivity of 2-LAB regardless of the
multilamellar arrangement of MWW zeolite nanosheets, because PMWW(31)
and PMWW(74) shared the same 12-MR hemicavities structure on the upper
and lower surface of MWW
layers,36 which was in
line with above computer calculation data (Fig. 5). These analyses
corroborated that the fabrication of order multilamellar arrangement of
MWW nanosheets and precise control of Al species distribution can
greatly expose the accessible Brønsted acid sites.
Stability, acidity and catalytic performance of
regularly spaced MWW
zeolite nanosheets
The stability, acidity and catalytic performance of optimized PMWW(74)
was further investigated in the alkylation between benzene with
1-dodecene. As shown in Fig. 11a, the conversion of 1-dodecene for
PMWW(74) and MCM-22 was 85 % and 40 % in the first cycle,
respectively. With the number of cycles increasing, the activity of
PMWW(74) and MCM-22 all displayed the tendency declining. For MCM-22,
the conversion dropped to 19 % after five cycles, while PMWW(74) still
maintained 68 % conversion even after five recycling experiments,
indicating that PMWW(74) possessed strong stability compared with MCM-22
due to the enhanced diffusion of PMWW(74) with open single-layered
structure.21 As shown
in Fig. S12 and Table S4, which clearly verified that the effective
diffusional time constants
(D eff/r2 ) of PMWW(74)
was higher than MCM-22. Interestingly, the selectivity of 2-LAB for MWW
zeolite catalysts was almost not influenced after cycle experiments
(Fig. 11b), because MWW zeolite possessed special 12-MR pockets on the
exterior surfaces that had the best matched steric configuration and
charge force with 2-LAB isomer.
To examine the structure of zeolite catalysts before and after cycles,
the corresponding XRD, N2 adsorption-desorption isotherm
and acidity of fresh and spent catalysts after five cycles were
performed, as given in Fig. 12, Fig. S13 and Fig. S14, respectively. It
was observed from Fig. 12 that both
PMWW(74) and MCM-22 exhibited the
robust stability with similar reflection patterns before and after
cycles. However, the coalescence of the (101) and (102) diffraction
peaks into a broadband reflection for spent PMWW(74) was observed,
indicating partial loss of vertical alignment order in the stackingc direction in the recycle
experiments,37 but
(001) and (002) reflections were still preserved, suggesting that
multilamellar structure of PMWW(74) was also totally
preserved.16 In
addition, the fresh PMWW(74) and spent
PMWW(74)
showed the comparable N2 adsorption-desorption isotherms
(Fig. S13) and textural parameters (Table S1), suggesting that
multilamellar
arrangement of MWW nanosheets over PMWW(74) was not destroyed, but the
spent MCM-22 exhibited a decreased BET surface area (557
m2g-1 to 451
m2g-1, Table S1), indicating that
the channels of MCM-22 was blocked on a certain degree due to diffusion
limitation under the sole micropores. Moreover, the total
Brønsted
acid sites and external Brønsted acid sites of PMWW(74) and MCM-22 after
five cycles were examined, as presented in Fig. S14 and Table S2. It was
observed from Table S2 that the
total
Brønsted acid sites and external Brønsted acid sites of spent MCM-22
dropped by 83.2% and 34.6 %, respectively. While the total Brønsted
acid sites and external Brønsted acid sites of spent PMWW(74) decreased
by 32.3 % and 30.6 %, respectively. These data confirmed that
multilamellar arrangement of MWW nanosheets for PMWW(74) exhibited the
evident advantage of architecture compared with conventional 3D MCM-22,
in which the sole micropores of MCM-22 suffered from serious diffusion
limitation during the alkylation and their narrow micropore entrance was
easily blocked by bulk alkylbenzene
products,21 resulting
in the concentration of total Brønsted acid sites drastically decreasing
after cycles.