(337f) Understanding Effective Diffusion Length Theory in Nanoscale Zeolites

Understanding
effective diffusion length theory in nanoscale zeolites

Xiaoduo
Qi, Vivek Vattipalli, Paul Dauenhauer and Wei Fan

 Department of Chemical Engineering, University
of Massachusetts Amherst, 686 N Pleasant Street, Amherst, 01002

Zeolites represent a class of
crystalline aluminosilicates with well-defined micropores in the framework. Due to their unique molecular
sieving and catalytic properties, zeolites have been extensively utilized as
catalyst to facilitate chemical reactions. However, as the micropore size is comparable to the molecular dimension, the catalytic
performance of the zeolites is often
limited by the slow intra-crystalline molecular transport within the micropore channel
when bulky molecules try to access the active site. To overcome such a
transport limitation, considerable efforts have been made with the aim of
reducing the contribution of molecular transport to the overall rate of
reaction.¹
Among all other methods, synthesizing zeolite nanoparticle has been considered
as one of the most promising strategy due to simple and well controlled
preparation procedure. As the radius of the zeolite crystal is minimized to
nanometers, it is expected that the characteristic diffusion length of such
material is extremely short, leading to a rapid micropore
diffusion process.

However, diffusion studies done by ZLC technique have shown that for zeolite nanoparticles, the
effective micropore diffusion length can be much
longer than the radius of the crystal due to a secondary diffusion step on the
zeolite surface which leads to the occurrence of pore re-entry². Despite
the fact that the effective diffusion length phenomena can greatly hinder the
mass transport within zeolite nanocrystals, the origin of such negative effect
has not been well studied. In this work, four adsorbates (Cyclohexane, Methylcyclohexane, Ethylcycllohexane,
and cis-dimethylcyclohexane) with different adsorption
energies to MFI zeolite were used as probing molecules to investigate the cause
of the effective diffusion length effect. It shows that the significance of
such effect strongly depends on the adsorption strength of diffusion molecule
to the zeolite surface. More specifically, as the adsorption becomes stronger,
it is harder for diffusing molecule to completely desorb from the zeolite
surface after it exits the micropores. As a result,
this molecule will continue to diffuse along the surface and repeatedly diffuse
within the pore, leading to a longer diffusion length. This work provides solid
experimental evidence suggesting that the effective diffusion length effect is
strongly related to guest/host adsorption. This fundamental understanding of micropore diffusion process within zeolite nanoparticles also
enables us to optimize the mass transport by enhancing the adsorptive property
of the zeolite.

[1]  
Chang, C.-C.; Teixeira, A. R.; Li, C.; Dauenhauer, P. J.; Fan, W., Enhanced Molecular Transport in
Hierarchical Silicalite-1. Langmuir 2013, 29, (45), 13943-13950.
[2]   Vattipalli, V.; Qi, X.; Dauenhauer,
P. J.; Fan, W., Long Walks in Hierarchical Porous
Materials due to Combined Surface and Configurational Diffusion. Chemistry of
Materials 2016, 28, (21), 7852-7863.