Seed-assisted, organic structure-directing agent-free synthesis of KFI-type zeolite with enhanced micropore volume and CO₂ adsorption capacity

ZK-5 (a KFI-type framework topology) is a “small-pore” zeolite with a three-dimensional channel system with eight-membered ring micropore openings (ca. 0.39 nm). Several recent studies have shown that ZK-5 has the potential to be applied in solid adsorbents or catalysts, particularly in CO₂ capture and NOx reduction in industry. However, the quality (e.g. micropore volume) of conventional ZK-5 (0.21–0.22 cm³ g⁻¹) remains smaller than the theoretical micropore volume of the KFI-type framework topology (0.32–0.34 cm³ g⁻¹). Improving the micropore volume of ZK-5 could therefore enhance its performance. In this study, we apply the seed-assisted method to obtain KFI-type zeolite starting by adding conventionally synthesized ZK-5 as a seed into organic structure-directing agent (OSDA)-free K,Sr-aluminosilicate reactants, followed by hydrothermal treatment. The series of experimental characterizations of the obtained products showed that under certain synthesis conditions, the addition of a seed could enhance the micropore volume of KFI-type zeolites to 0.26–0.28 cm³ g⁻¹, which is larger than previously reported for ZK-5. Such enhanced properties led to higher CO₂ adsorption capacities for the obtained KFI-type zeolites compared with conventional ZK-5 over a wide range of pressures. A series of experiments and comparison with previous literature, indicated that the crystallization behavior of the KFI-type zeolites did not follow typical seed-mediated growth. Instead, seed-dissolution–recrystallization behavior is thought to have occurred in the seeded-gel system.