Direct quantification of surface barriers for mass transfer in nanoporous crystalline materials

Mingbin Gao,Zhongmin Liu,Peng Tian,Hua Li,Pengfei Wu,Miao Yang,Mao Ye,Shutao Xu,Shushu Gao

doi:10.1038/s42004-019-0144-1

Abstract

Mass transfer of guest molecules in nanoporous crystalline materials has gained attention in catalysis, separation, electrochemistry, and other fields. Two mechanisms, surface barriers and intracrystalline diffusion, dominate the mass transport process. Lack of methods to separately quantify these two mechanisms restricts further understanding and thus rational design and efficient application of nanoporous materials. Here we derive an approximate expression of uptake rate relying solely on surface permeability, offering an approach to directly quantify surface barriers and intracrystalline diffusion. By use of this approach, we study the diffusion in zeolitic materials, and find that the intracrystalline diffusivity is intrinsic to the topological structure of host materials at low molecular loading for the given guest molecules, while the surface permeability is sensitive to the non-ideality of a crystalline surface owing to the physical and chemical properties of the crystalline surface, host–guest interaction at the surface, and change of the environment.

Highlights

Mass transfer of guest molecules in nanoporous crystalline materials has gained attention in catalysis, separation, electrochemistry, and other fields
Studies based on the visualization methods such as interference microscopy (IFM) and infrared microscopy (IRM) discovered that, in addition to intracrystalline diffusion, surface barriers can dominate the mass transfer of guest molecules in some nanoporous materials[3,12,16]
Due to lack of methods to direct quantify these two mechanisms, surface barriers and intracrystalline diffusion are often undistinguished in mass transfer of guest molecules[5,6,22]

Summary

Introduction

Mass transfer of guest molecules in nanoporous crystalline materials has gained attention in catalysis, separation, electrochemistry, and other fields. Based on the transition state theory[23,38], the intracrystalline diffusivity is directly related to the properties of guest molecules and the structure of host nanoporous crystalline materials[14,38], and it should be independent of measurement techniques, crystal size, and external surface characteristics. Based on the uptake/release rate measurement, Heinke et al.[5] proposed to obtain the intracrystalline diffusivity and surface permeability using the relation between the characteristic time of overall mass transport and crystal size of materials. Such method requires the preparation of crystalline materials with a series of crystal sizes and is difficult to quantify the mass transport properties of a specific material in practice. Fasano et al.[6] derived the intracrystalline diffusivity by molecular dynamics (MD)

Methods

Results

Conclusion