Environmentally friendly synthesis of flexible MOFs M(NA)2 (M = Zn, Co, Cu, Cd) with large and regenerable ammonia capacity

Abstract

Two environmentally friendly and efficient synthesis methods, NH₃-assisted synthesis and solvent-evaporated conversion, have been developed for the synthesis of a series of M(NA)₂ (M = Zn, Co, Cu, Cd; NA = nicotinate) flexible MOFs. The two-dimensional M(NA)₂ (M = Zn, Co) and three-dimensional M(NA)₂ (M = Cu, Cd) materials exhibit peculiar structural transformation. These frameworks can be obtained from dehydration of M(NA)₂(H₂O)₄ (M = Zn, Co, Cu, Cd) which are zero-dimensional supramolecular structures. Interestingly, the structural transformation of M(NA)₂ (M = Zn, Co, Cd) is reversible after liquid water adsorption. Due to the flexible structure of these MOFs, they exhibit abnormal NH₃ adsorption properties. The two-dimensional Zn(NA)₂ shows a gate-opening behavior for NH₃ adsorption. Its layers opened at a pressure of 0.22 bar in the first cycle, resulting in a two-step NH₃ uptake with a capacity of 10.2 mmol g⁻¹ at 1 bar. The gate-opening pressure shifted to a lower value with cycles. Co(NA)₂ has a huge NH₃ uptake of 17.5 mmol g⁻¹, which is top-ranking among the reported values. For the NH₃ adsorption over Cu(NA)₂ and Cd(NA)₂, the adsorption rates increase and adsorption equilibrium is achieved faster after three cycles. Their maximum capacity at 1 bar is 13.4 and 6 mmol g⁻¹, respectively. More importantly, all MOFs can be regenerated under vacuum and heating conditions of 150 °C for 70 min, and they all retained the capacity. The advantages of environmentally friendly synthesis, large adsorption capacity, and regenerable properties indicate that M(NA)₂ (M = Zn, Co, Cu) are promising candidates for NH₃ adsorption.