Abscheidung von Kohlendioxid: Perspektiven für neue Materialien

Der stark ansteigende Kohlendioxidgehalt in der Atmosphäre ist eines der drängendsten Umweltprobleme unserer Zeit. Eine Option zur Verringerung anthropogener CO₂‐Emissionen ist die Abscheidung und Speicherung von Kohlendioxid (Carbon Capture and Storage, CCS) an Punktquellen wie Kraftwerken. Durch diese Sequestrierung steigt allerdings der Energiebedarf der Kraftwerke um 25–40 %. Wir berichten hier über die Technologien zur Abscheidung, die zur Verringerung der CO₂‐Emissionen wahrscheinlich am besten geeignet sind. Dazu zählen Postcombustion‐Verfahren, bei denen die Abscheidung nach der Verbrennung stattfindet (vor allem die CO₂/N₂‐Separation), Precombustion‐Verfahren, bei denen CO₂/H₂‐Gemische eingesetzt werden, und die Konditionierung von Erdgas (CO₂/CH₄). Der Schlüssel zu deutlichen Fortschritten sind bessere Trennmittel zur Separation. Wir werden hier aktuelle Entwicklungen und neuartige Konzepte zur CO₂‐Abtrennung durch Lösungsmittel‐Absorption, chemische und physikalische Adsorption und Membranen schildern und besonders auf Fortschritte auf dem wachsenden Gebiet Metall‐organischer Gerüste eingehen.     

All‐Nanoporous Hybrid Membranes: Redefining Upper Limits on Molecular Separation Properties

New membrane‐based molecular separation processes are an essential part of the strategy for sustainable chemical production. A large literature on “hybrid” or “mixed‐matrix” membranes exists, in which nanoparticles of a higher‐performance porous material are dispersed in a polymeric matrix to boost performance. We demonstrate that the hybrid membrane concept can be redefined to achieve much higher performance if the membrane matrix and the dispersed phase are both nanoporous crystalline materials, with no polymeric phase. As the first example of such a system, we find that surface‐treated nanoparticles of the zeolite MFI can be incorporated in situ during growth of a polycrystalline membrane of the MOF ZIF‐8. The resulting all‐nanoporous hybrid membrane shows propylene/propane separation characteristics that exceed known upper‐bound performance limits defined for polymers, nanoporous materials, and polymer‐based hybrid membranes. This serves as a starting point for a new generation of chemical separation membranes containing interconnected nanoporous crystalline phases.     

Growth of High‐Quality,Thickness‐Reduced Zeolite Membranes towards N2/CH4 Separation Using High‐Aspect‐Ratio Seeds

Greatly improved zeolite membranes were prepared by using high‐aspect‐ratio zeolite seeds. Slice‐shaped seeds with a high aspect ratio (AR) facilitated growth of thinner continuous SAPO‐34 membranes of much higher quality. These membranes showed N₂ permeances as high as (2.87±0.15)×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at 22 °C while maintaining a decent N₂/CH₄ selectivity (9–11.2 for equimolar mixture). On the basis of these thinner high‐quality SAPO‐34 membranes, fine‐tuning the local crystal structure by incorporating more silicon further increased the N₂ permeance by 1.4 times without sacrificing the N₂/CH₄ selectivity. We expect that application of large AR zeolite seeds might be a viable strategy to grow thin high‐quality zeolite membranes. In addition, fine‐tuning of the crystal structure by changing the crystal composition might be a feasible way for further improving the separating performance of high‐quality zeolite membranes.