Figure 5 . The crystal structure model of various T-sites in MEL
lattice.
3.3
Analysis of
diffusion
Over the above the effects arising from pore size, pore
interconnectivity and surface openness, the chemical environment of the
internal pore wall also plays a critical role determining the overall
molecular diffusion properties. The kinetic adsorption experiments over
diverse probe molecules allow the unambiguous discrimination between
intracrystalline and surface diffusion
events.38 Hereby, the
effects arising from electrostatic interaction between internal/external
surface and adsorbate were investigated over the non-polar benzene and
polar toluene molecules with a similar kinetic diameter
(~ 0.56 nm) close to the channel size of MEL (0.53 ×
0.54 nm) respectively. These two selected probe molecules were also free
of functional groups to avoid any potential chemical bonding with the
surface. Fig. 6(a,b) showed the plots of transient fractional uptake
[(Qt-Q0)/(Qe-Q0)]
of benzene and toluene versus square root of time in H-MEL-31, P-MEL@Fe
and H-MEL@Fe-20 at 35 oC, respectively. Briefly, the
logarithm of normalized transient fractional uptake
[(Qt-Q0)/(Qe-Q0)]
versus square root of time produce a straight line in a short time
period. The slope of the kinetic adsorption curve can be used to
calculate the diffusion time constant (D/r2), which is
based on the Fick’s law (see Supporting Information) and summarized in
Table S4. It was found that the diffusion time constants
(D/r2) for all investigated samples were well within the
scale of 10-3 s-1, indicating that
the diffusion process in these as-synthesized MEL zeolites was dominated
by a combination of both intracrystalline and surface
diffusion.39,40