Controlling the uptake and regulating the release of nitric oxide in microporous solids

Representative compounds from three classes of microporous solids, namely metal-organic frameworks (MOFs), hybrid ultramicroporous materials (HUMs) and porous-organic polymers (POPs), were investigated for their nitric oxide gas uptake and release behavior. Low pressure sorption studies indicated strong chemisorption of NO on the free amine groups decorating the MOF UiO-66-NH2 when compared to its non-amine functionalized parent. The HUMs demonstrated reversible physisorption within the low pressure regime but interestingly in one case there was evidence for chemisorption following pressurization with NO at 10 bar. Significant release of chemisorbed NO from the UiO-66-NH2 and one of the HUMs was triggered by addition of acid to the medium, a pH change from 7.4 to 5.4 being sufficient to trigger NO release. An imidazole-based POP exhibited chemisorption of NO at high pressure wherein the ring basicity facilitated both NO uptake and spontaneous release upon contact with the aqueous release medium.