Achieving highly selective CO 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 CO. However, SAPO-35 zeolites (LEV framework topology) synthesized using conventional templates are less reported for highly selective CO adsorption which might be due to inappropriate Si contents and acidities in the framework. In this work, by using -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 CO adsorption and separation properties of SAPO-35_ (: Si/(Si + P + Al) in molar ratio) were investigated, and a close relationship between the acidity, polarity and CO adsorption and separation capacity was revealed. SAPO-35_0.14 with the strongest acidity showed the highest CO uptake of 4.76 mmol g (273 K and 100 kPa), and appeared to be one of the best SAPO materials for CO adsorption. Moreover, increased Brønsted acidity can significantly enhance the adsorption selectivity of CO over N. At 298 K and 100 kPa, SAPO-35_0.14 showed the highest CO/N 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 CO adsorption performance. This work demonstrates that modulating the silicon content and acidity in SAPO zeolites a suitable choice of template, as well as polarity, is of great significance for the rational synthesis of zeolites with superior CO adsorption and separation abilities.