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

Abstract

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 cm 2/s, 10.7 × 10 −8 cm 2/s and 142.0 × 10 −8 cm 2/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 cm 2/s, 21.4 × 10 −8 cm 2/s, 20.9 × 10 −8 cm 2/s and 0.0047 × 10 −8 cm 2/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.