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