Adsorptive Nature of Surface Barriers in MFI Nanocrystals

Zeolite nanocrystals with characteristic diffusion lengths of nanometers are widely used in molecular applications to overcome diffusion limitations. However, with a large fraction of external surface area, mass transport in these materials is often limited by the presence of a surface barrier, which limits their overall potential in catalytic or separation applications. Herein, silicalite-1 crystals of varying sizes were synthesized, and the adsorption and diffusion characteristics of four molecules (ethylcyclohexane, methylcyclohexane, cyclohexane, and cis-1,4-dimethylcyclohexane) were measured to mechanistically evaluate the mass transfer surface barrier. The results observed in this study support the presence of a nonstructural surface resistance associated with the strong enthalpic interaction between the diffusing molecule and zeolite surface. Further analysis indicates that the contributions of structural and nonstructural surface barriers to the mass transport vary greatly with the heat of adsorption. This work suggests that when diffusing molecules have a weak heat of adsorption on the zeolite surface, strategies to mitigate the surface barrier should focus on the structural modification of the zeolite surface using methods such as surface etching to remove pore blockages. When the heat of adsorption is strong, strategies should focus on tuning the adsorbate/adsorbent surface interaction by methods such as depositing a mesoporous silica overlayer to reduce surface adsorption or adding a secondary external surface to minimize re-entering of the micropores.