Introduction

Ammonia is a toxic and corrosive gas, its strong causticity and toxicity can cause convulsions, coma, and death to all vertebrates. The U.S. Occupational Safety and Health Administration (OSHA) set a 15 min exposure limit for gaseous ammonia of 35 ppm, and an 8 hour exposure limit of 25 ppm. On the other hand, ammonia is widely used in industrial development such as pharmaceutical and chemical industries, especially in fertilizer production. It has also been explored as an on-board energy source for fuel cells. To eliminate such hazards or better application of ammonia in industrial production, engineered materials that can adsorb large amounts of ammonia are desperately needed. For ammonia gas storage and separation, materials with high porosity and high surface area, high gas adsorption capacity, powerful affinity to ammonia and high resistance are strictly desirable. MOFs and COFs fully meet the above requirements, and have been widely used in these filed recent years.

Ammonia Storage and Separation Using MOFs

MOFs, with highly tunable structure, have great potential in ammonia capture and storage, because their open metal sites (OMSs) can be effective ammonia adsorption sites. Rieth et al. studied a series of triazolate MOFs, named M₂Cl₂BBTA (M=Co, Ni, Cu; BBTA=1H,5H-benzo(1,2-d),(4,5-d')bistriazole) for ammonia capture and separation in 2018. The M₂Cl₂BBTA has high density of open metal sites which is beneficial for higher ammonia capacities. Under equilibrium conditions at 298 K and 1 bar, Cu₂Cl₂BBTA displayed a record static ammonia capacity of 19.79 mmol/g, which is more than twice the industry standard^[1]. Besides the OMSs of MOFs, functional groups in frameworks can also act as strong ammonia capture sites. Since ammonia is a Brønsted base, it has high affinity to carboxylic and sulfonic acids. Thus, many attempts were made to adsorb ammonia with 'acid-bearing' MOFs.

Ammonia Storage and Separation Using COFs

Desirable properties, including extensive pore surface area and tunable surface chemistry, attract great interest for the development of novel COF-based adsorbents for ammonia sorption. Yang et al. synthesized a series of multivariate COFs which are functionalized with various active groups such as N-H, -C=O, and carboxyl group, then, the metal ions (Ca²⁺, Mn²⁺, and Sr²⁺) are integrated into the functionalized structure. The results showed that through the synergistic multivariate and open metal site, the COF materials show excellent ammonia adsorption capacities (14.3 and 19.8 mmol/g at 298 and 283 K, respectively) and isosteric heat (Qst) of 91.2 kJ/mol for ammonia molecules^[2].

Fig.1 The schematic diagram of ammonia storage by COFs

Advantages

There are many advantages of ammonia storage and separation by using MOFs and COFs when compared with other methods, some of are listed below:

• A highly porous framework consisting of unique micro sized pores that can increase the storage ability for ammonia by phyisical absorption;
• Effective functional groups in the MOFs and COFs structure that can interact increase the storage ability for ammonia by chemical adsorption;
• Tunable structure and easy to modify that endow MOFs and COFs with high selectivity when absorbed ammonia;
• MOFs and COFs can be recycled and reused.

What Can Alfa Chemistry Do

Alfa Chemistry has a profound research foundation in the field of ammonia storage and separation by MOFs and COFs, and is committed to design and synthesis of various high-quality MOFs and COFs. In addition, we are committed to supporting customers a series of solutions in ammonia storage and separation by using MOFs and COFs. Alfa Chemistry will serve you with the most abundant experience and the most affordable price. If you have any problems, we will provide technical support for you. If you have special needs, we will develop a unique solution for you. Please don't hesitate to contact us.