Surface Pore Engineering of Covalent Organic Frameworks for Ammonia Capture through Synergistic Multivariate and Open Metal Site Approaches

Abstract

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 (Qst) 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.

Opportunities of Covalent Organic Frameworks for Advanced Applications

“…[18] The material with no acidic groups has an NH 3 uptake capacity of 6.85 mmol g −1 at 1 bar, 298 K, which increases nearly 50% to 9.34 mmol g −1 for the material with 17% acid functionalization. To address this issue, the advantages of both MOFs and COFs can be combined, as the covalent bond constructed backbone provides stability, while the introduction of metal ions with accessible coordination sites confers a high affinity for ammonia.…”

Tailoring Covalent Organic Frameworks To Capture Water Contaminants

“…In addition to uniform designable pore surfaces, COFs feature high surface areas, low densities, and high thermal and chemical stability. Therefore, they have been explored for a wide range of applications, such as gas storage and separation, catalysis, optoelectronics, sensing, and drug delivery . For a comprehensive overview of COFs, the reader is referred to a recent review by Lohse and Bein …”

“…Therefore, by carefulm olecular design and selection,C OFs with tailor-madep ores can be obtained. In addition to uniform designable pore surfaces, COFs feature high surface areas, low densities, and high thermal and chemicals tability.T herefore, they have been exploredf or a wide range of applications, [6] such as gas storage [7][8][9][10] and separation, [11,12] catalysis, [13][14][15][16][17][18][19] optoelectronics, [20][21][22][23] sensing, [24][25][26][27] and drug delivery. [28][29][30] For ac omprehensive overview of COFs, the reader is referredt oarecent review by Lohse and Bein.…”

Ammonia Capture within Isoreticular Metal-Organic Frameworks with Rod Secondary Building Units

“…Although zeolites, activated carbon, and porous organic polymers have been used as ammonia sorbents, metal-organic frameworks (MOFs) have also been widely studied as potential NH3 capture materials on account of their high porosity, tunability and structural diversity, which often showed superior performance compared to the former. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Specialized molecular architectures are often needed for NH3 sorbents due to its ability to interact with Lewis and Brønsted acidic sites. [21][22][23][24] Consequently, NH3 sorption is affected by the nature and amounts of the specific adsorptive site rather than total surface area of the adsorbent.…”