Role of hydrogen bonding in the capture and storage of ammonia in zeolites

Ammonia is an important chemical compound used in a wide range of applications. This makes its capture, purification and recovery necessary. We combine experimental and molecular simulation techniques to identify the molecular mechanisms ruling the adsorption of ammonia in pure and high silica zeolites. To reproduce accurately the interaction between ammonia and the zeolites the development of a transferable set of Lennard-Jones parameters was needed. Adsorption isotherms were measured and also calculated using the new set of parameters for several commercial pure silica zeolites, including MFI, FAU, and LTA topologies. We found an anomalous behavior of the adsorption isotherm of ammonia in MFI, which can be explained through a monoclinic to orthorhombic structural phase transition. We also found that low concentration of extra-framework cations favors the adsorption of ammonia in these high silica zeolites. Using radial distribution functions and hydrogen bond analyses we identified ammonia clusterization as the key mechanism involved in the adsorption. Based on it, hydrophobic zeolites with large pores could be used for ammonia sequestration with lower cost than the currently used techniques.