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.