Modification of MFI membranes with amine groups for enhanced CO2 selectivity

Abstract

Much research on zeolite membranes has been devoted to MFI type zeolite and these membranes can be prepared reproducibly with high quality by our group. However, the MFI pores are too large for separation of small molecules such as CO₂, H₂ CH₄, and H₂O by molecular sieving and the CO₂ affinity is not sufficiently strong in our current membranes with quite high Si/Al ratio to achieve CO₂ selective membranes. In the present work, existing MFI membranes with high Si/Al ratio are modified with methylamine to increase the CO₂ affinity and thus increase the CO₂ selectivity. To the best of our knowledge, this is the first time this type of modification is reported for zeolite membranes. These membranes were then evaluated for separation of CO₂ from various mixtures of CO₂, H₂, CH₄ and H₂O. The modification had significant effects on both permeances and separation factors and the selectivity towards CO₂ was increased considerably for all the feed mixtures tested. The highest separation factor was observed for a CO₂/CH₄/H₂O mixture and α-CO₂/CH₄ was 12 at about 40 °C. At the same time, the CO₂ permeance was as high as 9 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹. The separation factor for the amine-modified silicalite-1 membranes was comparable to the highest reported separation factors for MFI membranes, while the CO₂ permeance was higher than reported for other selective MFI membranes.