Sorbate Transport in Carbon Molecular Sieve Membranes and FAU/EMT Intergrowth by Diffusion NMR.

Robert Mueller,Douglas B Galloway,Sesh Prabhakar,Amrish Menjoge,Sergey Vasenkov,Steven A Bradley,William J Koros,Mayumi Kiyono-Shimobe,Rohit Kanungo,John J Low

doi:10.3390/ma5020302

Abstract

In this paper we present and discuss selected results of our recent studies of sorbate self-diffusion in microporous materials. The main focus is given to transport properties of carbon molecular sieve (CMS) membranes as well as of the intergrowth of FAU-type and EMT-type zeolites. CMS membranes show promise for applications in separations of mixtures of small gas molecules, while FAU/EMT intergrowth can be used as an active and selective cracking catalyst. For both types of applications diffusion of guest molecules in the micropore networks of these materials is expected to play an important role. Diffusion studies were performed by a pulsed field gradient (PFG) NMR technique that combines advantages of high field (17.6 T) NMR and high magnetic field gradients (up to 30 T/m). This technique has been recently introduced at the University of Florida in collaboration with the National Magnet Lab. In addition to a more conventional proton PFG NMR, also carbon-13 PFG NMR was used.

Highlights

Detailed understanding of diffusion of guest molecules in microporous materials is important for numerous industrial applications, which include separations, catalysis, and detection of chemically or biologically important species
Methane self-diffusion was investigated by pulsed field gradient (PFG) NMR in the following three carbon molecular sieve membrane samples: 6FDA/BPDA(1:1)-DAM CMS pyrolized in an inert gas (6FDA/BPDA), Matrimid® CMS pyrolized in an inert gas (Matrimid Sample 1), and Matrimid® CMS pyrolized under vacuum (Matrimid Sample 2)
It was observed that the diffusion data for methane measured under the same conditions by 1H and 13C PFG NMR were identical within the experimental uncertainty

Summary

Introduction

Detailed understanding of diffusion of guest molecules in microporous materials is important for numerous industrial applications, which include separations, catalysis, and detection of chemically or biologically important species. In this work we review results of our recent studies of sorbate self-diffusion in two types of microporous materials, viz. Carbon molecular sieve (CMS) membranes and the intergrowth of FAU-type and EMT-type zeolites. Until now microscopic studies of diffusion of light gases in a broad range of length scales inside CMS membranes were not reported. To fill this gap, we review our first data on sorbate self-diffusion on the micrometer and sub-micrometer length scales inside. The other focus of this work is to review our recent results on sorbate self-diffusion in the intergrowth of FAU and EMT-type zeolites. The results reported below demonstrate that these measurements show high potential for uncovering the relationship between structural properties and transport in materials with a complex micropore structure

Details of PFG NMR Measurements and Data Processing

Methane Self-Diffusion in CMS Membranes

CMS Membranes

Zeolite Samples

Preparation of Samples for PFG NMR Studies

Conclusions