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