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.