posted on 2019-09-13, 15:36authored byXiaoduo Qi, Vivek Vattipalli, Ke Zhang, Peng Bai, Paul J. Dauenhauer, Wei Fan
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.