PFG NMR self-diffusion of small hydrocarbons in high silica DDR, CHA and LTA structures

Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) 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 301 K and 101.3 kPa. A clear correlation is 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 × 4.38 Å] for ZSM-58, [3.70 × 4.17 Å] for Si-CHA and [4.00 × 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 are shown to be very large. For example, at 301 K and 101.3 kPa, the self-diffusivities of methane were 1.6 × 10−8 cm2/s, 10.7 × 10−8 cm2/s and 142.0 × 10−8 cm2/s in ZSM-58, Si-CHA and Si-LTA, respectively; an increase in self-diffusivity of nearly 2 orders of magnitude that is primarily due to window size effects. Similarly, at 301 K and 101.3 kPa, the self-diffusivities of methane, ethylene, ethane and propylene in Si-LTA were 142.0 × 10−8 cm2/s, 21.4 × 10−8 cm2/s, 20.9 × 10−8 cm2/s and 0.0047 × 10−8 cm2/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.