Achieving highly selective CO₂ adsorption on SAPO-35 zeolites by template-modulating the framework silicon content

Small-pore silicoaluminophosphate (SAPO) zeolites with 8-ring pore windows and appropriate acidities/polarities, for example, SAPO-34 (CHA) and SAPO-56 (AFX), have proven to be potential adsorbing materials for selective adsorption of CO₂. However, SAPO-35 zeolites (LEV framework topology) synthesized using conventional templates are less reported for highly selective CO₂ adsorption which might be due to inappropriate Si contents and acidities in the framework. In this work, by using N-methylpiperidine (NMP) as a template, SAPO-35 zeolites with various Si contents were synthesized under hydrothermal conditions, which allowed SAPO-35 zeolites with modulated acidities and polarities. The CO₂ adsorption and separation properties of SAPO-35_x (x: Si/(Si + P + Al) in molar ratio) were investigated, and a close relationship between the acidity, polarity and CO₂ adsorption and separation capacity was revealed. SAPO-35_0.14 with the strongest acidity showed the highest CO₂ uptake of 4.76 mmol g^-1 (273 K and 100 kPa), and appeared to be one of the best SAPO materials for CO₂ adsorption. Moreover, increased Brønsted acidity can significantly enhance the adsorption selectivity of CO₂ over N₂. At 298 K and 100 kPa, SAPO-35_0.14 showed the highest CO₂/N₂ selectivity of 49.9, exhibiting potential for industrial processes. Transient binary breakthrough experiments on SAPO-35_0.14 further proved the efficient separation performance and stable circulation. The results of this study prove that the framework Si content of SAPO-35 zeolites is essential for regulating their CO₂ adsorption performance. This work demonstrates that modulating the silicon content and acidity in SAPO zeolites via a suitable choice of template, as well as polarity, is of great significance for the rational synthesis of zeolites with superior CO₂ adsorption and separation abilities.