Boosting\nAmmonia Uptake within Metal–Organic\nFrameworks by Anion Modulating Strategy

Abstract

Ammonia with toxic and corrosive\nfeatures needs advanced protective\nmaterials and removal tools, although it is a vital component in human\nfood supply processes. So, to satisfy these requirements, materials\nwith high adsorption capacity and affinity for ammonia should be developed.\nThe present research has been focused on a series zinc-based metal–organic\nframeworks (MOF) containing mixed ligands, biphenyl dicarboxylic acid\n(BPDA) and tris­(4-(4<i>H</i>-1,2,4-triazol-4-yl)­phenyl)­amine\n(TTPA), which are modulated by different anions including CH₃COO^–, CF₃COO^–, and\nCF₃SO₃^–. Ammonia uptake capacity\nwas measured via static and dynamic conditions under 50% relative\nhumidity. Among all compounds, CF₃SO₃^– anion could enhance the ammonia uptake capacity of MOFs up to 177.85\nand 349 mg/g during static and breakthrough measurements, respectively,\nso that 83.30% of the total uptake capacity (at <i>P</i>/<i>P</i>_o = 1.0 and 298 K) was achieved at\nlow relative pressure range (up to 0.1). The isosteric heats of ammonia\nadsorption on PFC-27 and derivatives were calculated in the range\nof 7.03–10.16 kJ mol^–1 so that they increased\nupon CF₃SO₃^–, CF₃COO^–, and CH₃COO^– ion\nincorporation. This is potentially beneficial for enhanced ammonia\nadsorption. Interestingly, adsorption capacities were retained with\nonly slight changes after five cycles and three regeneration temperatures,\n25 °C, 60 °C, and 120 °C, under vacuum. The special\naffinity for NH₃ adsorption and MOF phase stability after\ndesorption is clearly proved by FTIR spectra and PXRD analysis, respectively.\nGenerally, the results suggest that ion insertion modification is\nan efficient strategy for enhancement of MOF adsorption performance.