Surface Pore Engineering of Covalent Organic Frameworks for Ammonia Capture through Synergistic Multivariate and Open...

Ammonia (NH₃) is a commonly used industrial gas, but its corrosiveness and toxicity are hazardous to human health. Although many adsorbents have been investigated for NH₃ sorption, limited ammonia uptake remains an urgent issue yet to be solved. In this article, a series of multivariate covalent organic frameworks (COFs) are explored which are densely functionalized with various active groups, such as -N-H, -C=O, and carboxyl group. Then, a metal ion (Ca²⁺, Mn²⁺, and Sr²⁺) is integrated into the carboxylated structure achieving the first case of an open metal site in COF architecture. X-ray photoelectron spectroscopy reveals conclusive evidence for the multiple binding interactions with ammonia in the modified COF materials. Infrared spectroscopy indicates a general trend of binding capability from weak to strong along with -N-H, -C=O, carboxyl group, and metal ion. Through the synergistic multivariate and open metal site, the COF materials show excellent adsorption capacities (14.3 and 19.8 mmol g^-1 at 298 and 283 K, respectively) and isosteric heat (Q_st) of 91.2 kJ mol^-1 for ammonia molecules. This novel approach enables the development of tailor-made porous materials with tunable pore-engineered surface for ammonia uptake.