Separation of molecules based on molecular size in zeolites with appropriate pore aperture dimensions has given rise to the definition of "molecular sieves" and has been the basis for a variety of separation applications. We show here that for a class of chabazite zeolites, what appears to be "molecular sieving" based on dimension is actually separation based on a difference in ability of a guest molecule to induce temporary and reversible cation deviation from the center of pore apertures, allowing for exclusive admission of certain molecules. This new mechanism of discrimination permits "size-inverse" separation: we illustrate the case of admission of a larger molecule (CO) in preference to a smaller molecule (N(2)). Through a combination of experimental and computational approaches, we have uncovered the underlying mechanism and show that it is similar to a "molecular trapdoor". Our materials show the highest selectivity of CO(2) over CH(4) reported to date with important application to natural gas purification.

No abstract

Capture of CO 2 from flue gas streams using adsorption processes must deal with the prospect of high humidity streams containing bulk CO 2 as well as other impurities such as SO x , NO x , etc. Most studies to date have ignored this aspect of CO 2 capture. In this study, we have experimentally examined the capture of CO 2 from a 12% synthetic flue gas stream at a relative humidity of 95% at 30°C. A 13X adsorbent was used and the migration of the water and its subsequent impact on capture performance was evaluated. Binary breakthrough of CO 2 /water vapor was performed and indicated a significant effect of water on CO 2 adsorption capacity, as expected. Cyclic experiments indicate that the water zone migrates a quarter of the way into the column and stabilizes its position so that CO 2 capture is still possible although decreased. The formation of a water zone creates a "cold spot" which has implications for the system performance. The recovery of CO 2 dropped from 78.5% to 60% when moving from dry to wet flue gas while the productivity dropped by 22%. Although the concentration of water leaving the bed under vacuum was 27%(vol), the low vacuum pressure prevented condensation of water in this stream. However, the vacuum pump acted as a condenser and separator to remove bulk water. An important consequence of the presence of a water zone was to elevate the vacuum level thereby reducing CO 2 working capacity. Thus although there is a detrimental effect of water on CO 2 capture, long term recovery of CO 2 is still possible in a single VSA process. Pre-drying of the flue gas steam is not required. However, careful consideration of the impact of water and accommodation thereof must be made particularly when the feed stream temperature increases resulting in higher feed water concentration.