Molecular sieve carbons for CO2 capture

With the current interest in controlling greenhouse gases, there is a new focus on materials that can discriminate between gas molecules by capturing those gases which must be curtailed and releasing those which pose no adverse effects to the atmosphere. Although zeolites can be applied to such tasks, molecular sieve (MS) carbons have an additional advantage in their ability to both adsorb and desorb molecules cyclically with comparatively little energy demand.This paper describes studies of commercial ATMI carbons obtained from polyvinylidene chloride pyrolysis that have experienced different levels of activation and additives to modify the sizes of pore ‘portals’ (i.e., those pores which orchestrate movement of molecules from the gas phase into the inner structure of the carbon). The structures are probed using molecules in the 0.3–0.68 nm size range. One significant feature of these materials is that total pore volume for smaller molecules is maintained during the ‘tailoring’ of the portal pore size. These carbons have additional benefits such as high strength and high density, becoming ideal candidates for capturing CO2. Data for select carbons indicate capacities at ambient conditions for CO2 of >140 v/v (∼20 wt.%) and, in simulated power plant exhaust, Henry’s Law Separation Factors >6. Through collaboration with SRI International, ATMI has developed carbons that exhibit >97% capture of CO2 in such streams with subsequent desorption of 98% purity CO2 in the stripper section of a pilot-scale unit. In this application, these carbons outperform the claims made for MOFs and other similar materials.

Molecular sieve carbon microspheres in a fluid flow pass down through a static mixer and capture CO2 from a countercurrent synthetic flue gas steam at 1 bar and ambient temperatures. The carbon then passes into a transition section and a steam-stripping section where the CO2 is desorbed and released as a 98% pure CO2 stream.Figure optionsDownload as PowerPoint slideHighlights
► Necessary properties for CO2 adsorbents include capacity, heat of adsorption.
► Separation Factor, Diffusivity, Selectivity and hydrothermal stability important.
► Tailoring the porosity of a molecular sieve carbon to get high capture is described.
► MS carbon shows significant improvement over MOFs.
► Trickle bed flow of MS microspheres into a synthetic flue gas achieves 99% capture.