Figure 2. (a) HAADF-STEM image and elemental distributions of H-MEL@Fe-20 by EDS mapping. HRTEM image of the single-crystalline hierarchical H-MEL@Fe-20 zeolite (b) containing faceted intracrystalline mesopores, and (c) containing pore walls along [100] direction. FFT pattern is embedded. (d) Contrast-transfer-function (CTF) corrected image (at a defocus of -230 nm) with overlaid simulated projected potential map (with a point-spread-function width of 1.8 Å) and structural projections of H-MEL-31 zeolite along [100] direction.
SEM images of as-synthesized MEL zeolites showed a uniform rugby-like morphology (Fig. S4), and P-MEL had a smooth surface. When Fe and Al species were incorporated into the zeolite framework, the samples exhibited a rough surface and some fragments appeared on the surface of zeolites (Fig. S4b-S4f), but the rugby-like structure was not destroyed. TEM images of zeolites were further carried out to investigate the framework structure of samples, as shown in Fig. S5. It was observed that P-MEL (Fig. S5a) and P-MEL@Fe (Fig. S5c) showed condensed structure, while the H-MEL-31 and H-MEL@Fe-x exhibited unconsolidated structure (Fig. S5b and S5d-S5f). Besides, electron diffraction pattern of a randomly selected MEL zeolite particle further confirmed its single-crystalline nature (Fig. S6).
The N2 adsorption-desorption isotherm was used to reveal the porosity of resultant MEL zeolites, as shown in Fig. S7. It was observed that H-MEL-31, H-MEL@Fe-34, H-MEL@Fe-23 and H-MEL@Fe-20 showed distinct Type-IV isotherms with H2 hysteresis loop corresponding to capillary condensation in the mesopores.30,31By contrast, the P-MEL and P-MEL@Fe didn’t show any hysteresis loops owing to sole microspore. The pore size distribution of obtained zeolite samples was analyzed by NLDFT (non-local density functional theory) and BJH (Barrett–Joyner–Halenda) methods respectively, and the data was shown in Fig. S8. All of samples exhibited a narrow peak at ~0.56 nm in Fig. S8a corresponding microspore size of MEL zeolite, while the H-MEL-31, H-MEL@Fe-34, H-MEL@Fe-23 and H-MEL@Fe-20 showed the additional broad peaks in Fig. S8b compared with P-MEL