Introduction of SAPO based adsorbents for CO2 capture

https://doi.org/10.1039/D2SC00702A

“Silicoaluminophosphate (SAPO) zeolites, which are an important category of zeolites, provide equally high CO₂ adsorption capacity as adsorbents at corresponding pressures. Their weaker electrical field gradients lead to highly reversible CO₂ uptake.²⁴ For instance, SAPO-56 displayed a higher CO₂ adsorption uptake (5.42 mmol g⁻¹ at 273 K and 101 kPa) and less water sensitivity than aluminosilicate zeolite 13X. Cyclic adsorption and in situ infrared spectroscopy (IR) revealed that SAPO-56 retained 95% of its initial CO₂ capacity after six cycles and that adsorption occurred via physisorption.^24a

SAPO-35 with LEV topology will be a preferred candidate for CO₂ adsorption and separation due to its structural features. Levyne (LEV) is a typical small pore 8-ring window zeolite belonging to the ABC-6 family constructed by lev cages, single 6-rings and double 6-rings, whose window dimensions (0.36 × 0.48 nm) allowing the molecules to diffuse through are very suitable for the separation of CO₂ from N₂.^24a,25 However, few studies have focused on the investigation of Levyne and its analogues for CO₂ adsorption and separation. The SAPO-35 zeolites were typically synthesized by using hexamethyleneimine (HMI) as a template which showed a poor CO₂ adsorption capacity.^24a This might be due to the inappropriate Si acidities/polarities in the framework. Herein, by adopting N-methylpiperidine (NMP) as a template, a series of SAPO-35 zeolites were synthesized, which exhibited a wider range of Si content from 5% to 23%. The relationship between Si content and CO₂ adsorption and separation abilities was also investigated. By regulating the Si content in SAPO-35 zeolites synthesized using NMP, we found that the sample with moderate Si content showed the strongest Brønsted acidity and polarity, further aiding in CO₂ affinity and separation of the CO₂/N₂ mixture. “