Diffusion of Methane and Carbon Dioxide in Carbon Molecular Sieve Membranes by Multinuclear Pulsed Field Gradient NMR

Abstract

Carbon molecular sieve (CMS) membranes are promising materials for energy efficient separations of light gases. In this work, we report a detailed microscopic study of carbon dioxide and methane self-diffusion in three CMS membrane derived from 6FDA/BPDA(1:1)-DAM and Matrimid polymers. In addition to diffusion of one-component sorbates, diffusion of a carbon dioxide/methane mixture was investigated. Self-diffusion studies were performed by the multinuclear (i.e., (1)H and (13)C) pulsed field gradient (PFG) NMR technique which combines the advantages of high field (17.6 T) NMR and high magnetic field gradients (up to 30 T/m). Diffusion measurements were carried out at different temperatures and for a broad range of the root-mean-square displacements of gas molecules inside the membranes. The diffusion data obtained from PFG NMR are compared with the corresponding results of membrane permeation measurements reported previously for the same membrane types. The observed differences between the transport diffusivities and self-diffusion coefficients of carbon dioxide and methane are discussed.

Single-File Diffusion of Gas Mixtures in Nanochannels of the Dipeptide l-Ala-l-Val: High-Field Diffusion NMR Study

“…The 13-interval bipolar PFG NMR pulse sequence with longitudinal eddy current delay was used for all diffusion measurements. 8,13,15 The longitudinal eddy current delay was between 3.5 and 5 ms. Diffusion data were obtained by measuring the PFG NMR signal intensity (S) as a function of the magnetic field gradient (g) while keeping all other sequence parameters constant. For each of the studied gas molecule types, S was obtained by measuring the area under the single 13 C NMR line of these molecules.…”

MOF-Based Membranes for Gas Separations

“…304,305 Of note, PFG NMR is an equilibrium technique which yields self-diffusion data directly, as opposed to transport diffusivities which are commonly measured through steady-state experimental techniques such as permeation tests. Transport diffusivities, D T , and self-diffusion coefficients, D S , are correlated through a Darken-type relationship: 303,304,306,307…”

Hydrocarbon Behavior at Nanoscale Interfaces

“…Thus, PFG NMR may yield the probability distribution P(x,t) function that, during time t, a randomly selected molecule of the sample is shifted over a distance x in the direction of applied field gradient (Kärger 2008). Such capabilities of PFG NMR provides an opportunity to investigate diffusion behavior of molecules confined to both mesoporous and nanoporous systems (Stallmach et al 2000(Stallmach et al , 2001Kärger et al 2003Kärger et al , 2009, to explore surface permeability of nanoporous particles (Krutyeva et al 2007), to study transport properties of supercritical fluids in confined geometry (Dvoyashkin et al 2007), and to study diffusion of small molecules such as methane and carbon dioxide in carbon molecular sieve membranes (Mueller et al 2012). However, there are some problems in performing NMR experiments of diffusion of mixtures within porous materials since they represent heterogeneous systems (Pampel et al 2005;Fernandez et al 2008).…”