Removal of Ammonia Emissions via Reversible Structural Transformation in M(BDC) (M = Cu, Zn, Cd) Metal–Organic Frameworks

Abstract

NH3 is the most important gaseous alkaline pollutant, which when accumulated at high concentrations can have a serious impact on animal and human health. More importantly, NH3 emissions will react with acidic pollutant gases to form particulate matter (PM2.5) in the atmosphere, which also poses a huge threat to human activities. The use of adsorbents for NH3 removal from emission sources or air is an urgent issue. However, there are difficulties in the compatibility between high adsorption capacity and recyclability for most conventional adsorbents. In this work, a structural transformation strategy using metal–organic frameworks (MOFs) is proposed for large-scale and recyclable NH3 adsorption. A series of M­(BDC) (M = Cu, Zn, Cd) materials can transform into one-dimensional M­(BDC)­(NH3)2 after NH3 adsorption, resulting in repeatable adsorption capacities of 17.2, 14.1, and 7.4 mmol/g, respectively. These MOFs can be completely regenerated at 250 °C for 80 min with no adsorption capacity loss. Besides, breakthrough and cycle tests indicate that Cu­(BDC) and Zn­(BDC) show good performance in the removal of low concentrations of NH3 from the air. Overall, combining the advantages of high adsorption capacity and recyclability due to the reversible structural transformation, Cu­(BDC) and Zn­(BDC) can be employed as ideal adsorbent candidates for NH3 removal.

Exceptional High and Reversible Ammonia Uptake by Two Dimension Few-layer BiI3 Nanosheets

“…To meet this demand, recent endeavors have focused on the development of highly efficient solid adsorbents, such as activated carbons, zeolites, mesoporous silica, Prussian blue analogues, metal–organic frameworks (MOFs), covalent organic frameworks (COFs) and porous organic polymers (POPs) for NH 3 capture. − However, it is difficult for these materials to possess high NH 3 absorption capacity and good recyclability at the same time. For example, NH 3 absorption by activated carbon and zeolite is poor even though acid modification is applied to enhance the uptake capacity, and these substances are hardly regenerated. , Although adsorption capacity of Mg 2 (dobpdc) MOFs (with the record NH 3 uptake capacity of 23.90 mmol/g at 1.0 bar and 25 °C) is much larger than that of activated carbon and zeolite, most of them have limited uptake capacity, higher regeneration temperature, and possible structural collapse after exposure to NH 3 due to the strong coordination of NH 3 to metal cations on the frameworks. ,,− As far as COFs are concerned, the highest uptake capacity is 19.8 mmol/g at 1.0 bar and 10 °C, and the desorption temperature of NH 3 is very high (about 200 °C). , Meanwhile, the synthesis procedure of COF is very complicated. Therefore, it is a great challenge to develop new solid adsorbents with both high NH 3 uptake capacity and good recyclability.…”

Defect-Rich Hierarchical Porous UiO-66(Zr) for Tunable Phosphate Removal

“…Metal−organic frameworks (MOFs) are organic−inorganic hybrid crystalline porous materials composed of a regular array of metal ions (or clusters) linked by organic ligands. 1−3 The intriguing properties of MOFs, including permanent porosity, high chemical stability, and excellent tunability, 4,5 have rendered them as highly attractive sorbents toward environmental remediation. 6,7 Compared with traditional adsorbents, such as activated carbon and metal oxides, MOFs exhibit highly tunable structures, compositions, and pore sizes (at atomic and molecular levels).…”

“…Metal–organic frameworks (MOFs) are organic–inorganic hybrid crystalline porous materials composed of a regular array of metal ions (or clusters) linked by organic ligands. − The intriguing properties of MOFs, including permanent porosity, high chemical stability, and excellent tunability, , have rendered them as highly attractive sorbents toward environmental remediation. , Compared with traditional adsorbents, such as activated carbon and metal oxides, MOFs exhibit highly tunable structures, compositions, and pore sizes (at atomic and molecular levels). − However, most reported MOFs are highly microporous (e.g., pore size <2 nm), which may block the molecules of interest (e.g., tetrahedral phosphate anions) from entering the cavities within the frameworks . For instance, the phosphate adsorption capacity of UiO-66­(Zr) has been reported to be rather limited (74.5 mg P/g) …”

Tailoring Multiple Sites of Metal–Organic Frameworks for Highly Efficient and Reversible Ammonia Adsorption

“…This feature of MOFs provides the possibility for their application in ammonia capture and storage. However, as far as NH 3 adsorption is concerned, − the framework collapse of MOFs is often reported because of the strong coordination of NH 3 to the metals on the frameworks. For example, structural collapse was observed during adsorption of NH 3 by typical MOF materials such as MOF-5, MOF-177, Cu-MOF-74, and Cu 3 BTC 2 , − owing to the strong interactions between NH 3 and the metal Zn/Cu sites.…”