Metal chloride functionalized MOF-253(Al) for high-efficiency selective separation of ammonia from H2 and N2

Ammonia (NH3) is produced from nitrogen (N2) and hydrogen (H2) by the Harber-Bosch process at high pressure and high temperature. Due to the reversibility and low conversion efficiency of the reaction, the product contains a large amount of raw gas, which seriously affects the production efficiency of ammonia. Therefore, it is essential to exploit high-performance adsorbents for separating NH3 from NH3/N2/H2 mixtures. Herein, we designed and synthesized a kind of metal–organic framework (MOF) composites by anchoring metal chloride (NiCl2, CoCl2, or SnCl2) on the bipyridinium groups in the MOF-253(Al) pores. It is found that the optimal composite shows an NH3 uptake capacity of 18.0 mmol/g at 25.0 °C and 1.0 bar, which is 3.33 times that of the pristine MOF-253(Al), and the composite exhibits a much faster adsorption kinetics of NH3 than the pristine MOF-253(Al). Significantly, the excellent NH3 uptake capacity at low pressure allows selective trapping of NH3 from NH3/N2/H2 at 25.0 °C with the selectivity coefficients of 5708 to N2 and 2320 to H2. Spectral measurements reveal that the synergistic action of NH3 coordination to the metal in the MOF and the hydrogen bonding of NH3 with both O atoms of the carboxyl groups and Cl− of the guest molecules account for such an excellent capture and selectivity. In general, the strategy developed here is simple and effective for improving the selectivity of NH3 separation, which opens a new way for the design of high-performance solid adsorbents.