Pulsed field gradient nuclear magnetic resonance spectroscopy was used to measure and rationalize 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 X-ray diffraction measurements: [3.65 x 4.38 Å] for ZSM-58, [3.70 x 4.17 Å] for Si-CHA and [4.00 x 4.22 Å] 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 10−8, 10.7 x 10−8 and 142.0 x 10−8cm2/s in ZSM-58, Si-CHA and Si-LTA, respectively; an increase in self-diffusivity of nearly 2 orders of magnitude that was primarily due to window size effects. At 301K and 101.3kPa, the self-diffusivities of methane, ethylene, ethane and propylene in Si-LTA were 1.42 x 10−6, 2.14 x 10−7, 2.09 x 10−7 and 4.70 x 10−11cm2/s, respectively; a decrease in self-diffusivity with molecular size of more than 4 orders of magnitude. These findings contribute to a fundamental understanding of self-diffusion in microporous materials and have important implications for kinetic based separation schemes in which diffusion plays 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