Porous Organic Polymers for high pressure methane uptake and storage

A series of porous organic polymers were synthetized through extensive cross-linking of aromatic monomers containing multiple reactive sites. The inefficient packing of monomeric units determined the formation of highly porous frameworks with surface areas up to 4800 m2/g and broad pore size distributions. Besides, the formation of strong covalent bonds accounted for high chemical and thermal stability. In depth solid state NMR analysis allowed us to check the purity of samples and to determine the connectivity and the microstructure of porous frameworks. Due to their possible application in Adsorbed Natural Gas technology (ANG) methane uptake was measured up to 180 bar at room temperature. The methane uptake at high pressure was related to the surface area and the total pore volume obtained by nitrogen adsorption isotherms at 77K. For example, triptycene-based material (TRIP) with surface area as high as 1600 m2/g could adsorb more than 400 cm3/g of methane at 180 bar. We also evaluated the volumetric methane uptake (cm3 of adsorbate per cm3 of adsorbent) at high pressure which is a critical parameter in methane transportation by ships. Moreover, the volumetric uptake allowed us to compare the results directly with compressed natural gas technology (CNG). At 180 bar the methane adsorption of TRIP sample reached a considerable value of 220 cm3/cm3. Furthermore, we could evaluate the gain in methane storage due to the presence of the porous material by comparing the total volumetric uptake of CH4 in presence of TRIP with pure compressed methane: a gain above 100% could be achieved up to 65 bar (Figure1). The isosteric heat of adsorption, as measured by the Clausius-Clapeyron equation, provided an insight into the strength of interactions between the methane molecules and the pore walls. At low coverage it ranged from 19 to 21 KJ/mol and it was among the highest value reported in literature. Such high values were attributed to multiple CH-π interactions between the methane molecules and the electron-rich aromatic rings. Lastly, we investigated carbon dioxide uptake up to 10 bar. All samples showed high CO2 uptake, isosteric heat of adsorption up to 30 KJ/mol and an excellent CO2/N2 selectivity ranging from 20 to 25 at room temperature (estimated by the Ideal Adsorbed Solution Theory IAST). These adsorption properties combined with the high chemical and thermal resistance and low hydrophilicity made porous organic polymers attractive for post-combustion treatment of industrial emissions. [1] Bracco, S.; Piga, D.; Bassanetti, I.; Perego J.; Comotti A.; Sozzani P. J. Mater. Chem. A 2017, 5, 10328-10337.