Pulsed field gradient nuclear magnetic resonance spectroscopy was used to

measure the intracrystalline self-diffusion coefficients of small hydrocarbons in

high-silica DDR (ZSM-58, Si/Al=190), pure silica chabazite (Si-CHA) and ITQ-29 (Si-LTA) structures. The self-diffusivities of methane, ethane, ethylene and

propylene were measured on these materials at 301K and 101.3kPa. A clear

correlation was shown between the size of the 8-ring windows and the size of the

molecules on the measured self-diffusivities. Window sizes were obtained from Xray

diffraction measurements: [3.65 x 4.38Å] for ZSM-58, [3.70 x 4.17Å] for Si-

CHA and [4.00 x 4.22Å] for Si-LTA. An increase in self-diffusivity with window

size and a decrease with molecular size were clearly observed. The magnitudes of

these effects were shown to be very large. For example, at 301K and 101.3kPa, the

self-diffusivities of methane were 1.6 x 10−8cm2/s, 10.7 x 10−8cm2/s and 142.0 x

10−8cm2/s in ZSM-58, Si-CHA and Si-LTA, respectively; an increase in selfdiffusivity

of nearly 2 orders of magnitude that was primarily due to window size

effects. Similarly, at 301K and 101.3kPa, the self-diffusivities of methane,

ethylene, ethane and propylene in Si-LTA were 142.0 x 10−8cm2/s, 21.4 x

10−8cm2/s, 20.9 x 10−8cm2/s and 0.0047 x 10−8cm2/s, respectively; a decrease in

self-diffusivity with molecular size of more than 4 orders of magnitude. These

findings contributed to a fundamental understanding of self-diffusion in

microporous materials and have important implications for kinetic based separation

schemes in which diffusion played a key role.

PFG NMR Self-Diffusion of Small Hydrocarbons in High Silica DDR, CHA and

LTA Structures. N.Hedin, G.J.DeMartin, W.J.Roth, K.G.Strohmaier, S.C.Reyes:

Microporous and Mesoporous Materials, 2008, 109[1-3], 327-34