Iron (III) dinuclear metallacycle cavity as an effective ammonia storage system

Ammonia has a large volume energy density, consequently it has a high potential to be used as fuel and energy reservoir in the future.1 In this work, the action of a dinuclear metallacycle of FeIII (Fe(opba)), which the crystalline structure is shown in Figure 1, with an outstanding capacity to adsorb ammonia (11.3 mmol g–1) was investigated, showing a strong preference by nitrogenous compounds, especially short chain amines (Figure 2). Adsorption results, suggest that the Fe(opba) cavity has an essential role in the adsorption mechanism, being the responsible by the size selectivity during the adsorption process. In order to better understand the ammonia adsorption mechanism, the Fe(opba) was fully characterized before and after the adsorption process. The results of thermogravimetric-mass spectrometry and ammonia isotherm experiments reveal that 8 ammonia molecules are physisorbed by the complex cavity (free ammonia) and 4 molecules are chemisorbed (coordinated ammonia) per dinuclear metallacycle unit. Others characterizations also confirmed the adsorption mechanism proposed confirming a structural modification to the compound before and after the adsorption. In addition, reuse tests have shown that despite some loss of efficiency the compound is capable of desorbing most of the ammonia and can be used in consecutive cycles, being a potential ammonia storage or adsorbent for the removal of ammonia in effluents.