Synthesis of OSDA-free offretite in the presence of cesium

Zeolites containing 8-membered rings (8MRs) such as offretite have shown promising properties in selective adsorption of CO 2 . However, the necessity of organic structure-directing agents (OSDAs) in the conventional synthesis of offretite leads to additional costs and energy consumption which critically restricts its industrial application. Herein, we report a novel OSDA-free synthetic strategy of offretite by direct synthesis in a colloidal precursor suspension containing inorganic cations as the structure-directing agents (SDAs). For the first time Cs + was used as an SDA in a synthesis mixture to obtain offretite. Based on empirical evidence presented here and from the literature, we hypothesise that Cs + , in combination with K + and Na + cations, facilitates the formation of gme cages and avoids the formation of other crystalline phases such as LTL and PHI. By careful optimization, the effect of various parameters related to the purity and crystal sizes (from 15 μm to 2 μm) have been investigated, including synthesis temperature, composition of mixed cations, aging time, silica-to-aluminium ratio, aluminium source and water content. Offretite as pure phase, with a Si/Al ratio of 3.5, and tunable crystal sizes have been successfully prepared solely by using inorganic SDAs. The offretite exhibited a high sorption capacity for CO 2 (4.1 mmol/g at 0.95 bar and 0 °C), showing potential for selective CO 2 capture. • A green OSDA-free synthesis of offretite (OFF) was developed. • The role of alkali cations on OFF was elucidated. • Cs + facilitated the formation of gme cages and prevented competing phases. • Varying synthesis parameters enabled control of crystals purity, crystallinity, particle size. • Small offretite crystals showed high CO 2 uptake.