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.