Silicoaluminophosphates as CO2 sorbents

Silicoaluminophosphates (SAPO-17, SAPO-35, SAPO-56 and SAPO-RHO) synthesised via hydrothermal means are tested for their abilities to adsorb carbon dioxide. These 8-ring microporous phosphates show high capacities to adsorb CO2. SAPO-RHO has a high uptake of CO2 and a very low uptake of N2 due to its narrow pore window aperture at 273 K. Its significant uptake of N2 at 77 K is rationalised by a temperature induced shrinkage effect. SAPO-56 has a slightly higher CO2 capacity (5.42 mmol/g, 273 K, 101 kPa) and is less water sensitive than zeolite 13X. Cyclic adsorption and in situ infrared spectroscopy (IR) reveal that SAPOs retain 95% of their original CO2 capacity after six cycles and that adsorption occurs via physisorption. The calculated heat of adsorption for CO2 (at 0.2–0.7 mmol/g loading) on SAPO-56 and SAPO-RHO lies in the physisorption range (∼35 kJ/mol). SAPOs, in particular SAPO-56 and SAPO-RHO, possess many desirable properties and are potentially good adsorbents for CO2 capture in swing adsorption processes.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► We synthesised four microporous SAPOs and tested them as CO2 sorbents. ► SAPO-56 showed very high CO2 uptake. ► SAPO-RHO showed high relative CO2-over-N2 uptake. ► We followed the molecular details of CO2 adsorption on SAPOs using in situ infrared spectroscopy. ► The SAPOs tested here are potentially good CO2 sorbents.