Figure 4 . (a) Positron annihilation lifetime spectroscopy lifetime spectra and (b) corresponding lifetime components and relative fractions for P-MEL and H-MEL-31 zeolites.
due to its unprecedented sensitivity towards bulk connectivity of the complex pore network. The normalized PALS spectra and corresponding lifetime components with relative fractions of P-MEL and H-MEL-31 were shown in Fig. 4a and Fig. 4b, respectively. It was observed from Fig. 4b that the P-MEL exhibited higher fractions of o-Ps annihilating in the micro- and mesopores than H-MEL-31, which was indicative of increased resistance of o-Ps migration from both micropores-to-mesopores and micropores/mesopores-to-vacuum.16While the high fraction of o-Ps annihilating in vacuum for the hierarchically structured H-MEL-31 compared with P-MEL suggested that H-MEL-31 possessed superior interconnectivity of the micro- and mesopore network and surface openness, which greatly promote the intracrystalline molecular transport.17
3.2 Acidity of zeolites
In general, the acidity of zeolites is from the Al species that are substituting for tetrahedral Si atoms in the framework, which gives the BrØnsted acid sites and Lewis acid sites depending on different Al coordinated environment.33 For MEL zeolite, it contains 7 crystallographically distinct T sites, as shown in Fig. 5 and Fig. S10. Here, the acidity of synthesized samples was evaluated by IR spectra of pyridine/2,6-di-tert-butyl pyridine adsorption on H+form of zeolites. The pyridine with a kinetic diameter of ~ 0.5 nm can enter the channel of MEL (0.53 × 0.54 nm).34 Thus, it can detect the total acidity of zeolites. Fig. S11a exhibited the IR spectra of adsorbed pyridine over the investigated samples, it was known that the absorption bands at ~ 1546 and 1446 cm-1 were attributed to Brønsted acid sites and Lewis acid sites, respectively,35,36and the corresponding acidity of zeolites were listed in Table S3. As can be seen from Table S3, P-MEL and P-MEL@Fe didn’t have any Brønsted acidity due to the absence of Al species in the framework of MEL zeolites. After incorporating Al into the zeolite lattice, the Brønsted acidity of samples gradually increased. For example, the Brønsted acidity of P-MEL@Fe and H-MEL@Fe-xincreased in the order of P-MEL@Fe < H-MEL@Fe-34 < H-MEL@Fe-23 < H-MEL@Fe-20, which indicated that Al species were preferred to exist in the form of Si‒OH+‒Al, resulting in the generation of Brønsted acid sites. In addition, P-MEL@Fe showed a higher Lewis acidity than that of P-MEL owing to the additional Fe species that can became the Lewis acid sites by accepting a pair of electrons from the adsorbed species.37
2,6-di-tert-butyl pyridine (2,6-DTBPy) with a kinetic diameter of ~0.8 nm was difficult to enter the channel of MEL zeolite, and therefore it can be used to test the acidity on the external surface of zeolites. As shown in Fig. S11b, the band at ~1616 cm-1 was characteristic of Brønsted acid sites, which can catalyze the alkylation between mesitylene (the kinetic diameter of ~0.87 nm) and benzyl alcohol. Table S3 gave the external Brønsted acidity of zeolites, it was observed that P-MEL and P-MEL@Fe didn’t show any external Brønsted acidity owing to the missing of Al species. However, the concentration of Brønsted acid of H-MEL@Fe-xon the external surface increased with decreasing Si/Al ratios, and the Brønsted acid/Lewis acid ratios of H-MEL@Fe-xwith Si/Al ratios also exhibited the similar trend, indicating that the Brønsted acid/Lewis acid ratios and external Brønsted acidity of H-MEL@Fe-x can be modulated by tuning the Si/Al ratios of zeolites, which is important for tailoring the catalytic properties of zeolite catalysts.